Supramolecular Chirality in Self-Assembled SystemsMinghua Liu,* Li Zhang, and Tianyu Wang

Beijing National Laboratory for Molecular Science (BNLMS), CAS Key Laboratory of Colloid, Interface and ChemicalThermodynamics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China

CONTENTS

1. Introduction 73052. Basic Concepts Related to Molecular and Supra-

molecular Chirality 73052.1. Configuration and Conformation Chirality 73062.2. Induced Chirality 73062.3. Helicity or Helical Chirality 7307

3. Characterization of Supramolecular Chirality 73083.1. Morphology Observation 73083.2. Spectroscopic Methods for Characterization

of Chirality 73093.2.1. CD Spectra of Supramolecular Systems 73093.2.2. Measurement Aspects 73093.2.3. CD Spectra and Interpretation 7309

4. Supramolecular Chirality in Self-Assembled Sys-tems Containing Chiral Molecular Components 73104.1. Supramolecular Chirality in Assemblies of

Chiral Components 73104.1.1. Amphiphiles 73104.1.2. C3-Symmetric Molecules 73134.1.3. π-Conjugated Molecules 73164.1.4. Molecules with Multiple Chiral Centers 7324

4.2. Chirality Transfer in Systems ContainingChiral and Achiral Molecules 7325

4.2.1. Chirality Transfer through NoncovalentBonds 7325

4.2.2. Chirality Transfer from Solvent to Assem-blies 7329

4.2.3. Chirality Transfer from Low MolecularWeight Molecules to Macromolecules 7330

4.3. Dynamic Features and Regulation of Supra-molecular Chirality 7331

4.3.1. Solvents 73324.3.2. Temperature 73354.3.3. Redox Effect Chirality 73364.3.4. Photoirradiation 73364.3.5. Chemical Additives 7336

4.3.6. Sonication 73394.3.7. pH Value 7340

4.4. Chiral Amplification in Supramolecular Sys-tems 7340

4.4.1. Analogue-Induced Chiral Amplification 73404.4.2. Chiral Amplification in Binary Systems 73434.4.3. Chiral Amplification to Nanoscale 73434.4.4. Unexpected Amplification in Racemate

Assemblies 73444.5. Chiral Memory in Supramolecular Systems 7344

4.5.1. Helicity Memory in Noncovalently-In-duced Helical Polymers 7344

4.5.2. Chiral Memory in Aggregates Such as Jand H Aggregates 7346

4.5.3. Helicity Memory in Chiral Cages fromCoordination Compounds 7347

5. Spontaneous Symmetry Breaking and Emergenceof Supramolecular Chirality in Self-AssembledSystems from Exclusively Achiral Molecules 73475.1. Liquid-Crystal and Banana-Shaped Mole-

cules 73485.2. Solution Systems, Micelles 73495.3. Gel Systems 73535.4. Air/Water Interface and LB Films 73545.5. Controlling Handedness of Supramolecular

Chirality 73585.5.1. Vortices and Spin Coating 73585.5.2. Circularly-Polarized Light 73615.5.3. Surface Pressure 7362

5.6. Self-Assembly of Racemic Systems 73626. Applications of Supramolecular Chirality 7365

6.1. Supramolecular Chiral Recognition and Sens-ing 7366

6.2. Supramolecular Chiroptical Switches 73726.3. Supramolecular Chiral Catalysis 73746.4. Optics and Electronics Based on Supra-

molecular Chiral Assembly 73816.5. Circularly Polarized Luminescence (CPL)

Based on Chiral Supramolecular Assemblies 73816.6. Biological Applications of Supramolecular

Chirality 73837. Conclusions 7384Author Information 7384

Corresponding Author 7384Notes 7384Biographies 7384

Special Issue: 2015 Supramolecular Chemistry

Received: December 8, 2014Published: July 20, 2015

Review

pubs.acs.org/CR

© 2015 American Chemical Society 7304 DOI: 10.1021/cr500671pChem. Rev. 2015, 115, 7304−7397

Acknowledgments 7385References 7385

1. INTRODUCTION

Chirality is a basic characteristic of living matter and nature.During the evolution of life on our planet, nature has favored onekind of chirality, thereby selecting the L-amino acids (with theexception of glycine) as the main component of proteins andenzymes and D-sugars as the main components of DNA andRNA. In addition, chirality is universal and can be observed atvarious hierarchical levels from subatomic and molecular tosupramolecular, nanoscopic, macroscopic, and galactic scales.1

Figure 1 illustrates some typical chiral substances and objects atthese various scales.At a subatomic level, chirality is connected to parity

conservation. Therefore, only left-handed helical neutrinos arefound. At a molecular level, there are a huge number of chiralmolecules in natural system such as amino acids, sugars, andterpenes, and many synthetic compounds are also chiral.Furthermore, there are many biological macromolecules orsupramolecular systems with chirality, microorganisms withhelix-shaped viruses, and bacteria such as tobacco mosaic virusand Helicobacter pylori, respectively, and macroscopic livingsystems such as snails. On a larger scale, one finds that manyplants express chiral sense, such asmountain climbing vines. On alight-year scale, our galaxy system is also chiral.Among these various levels, chirality at a molecular and

supramolecular level is of vital importance since it is stronglyrelated to chemistry, physics, biology, materials, and nano-science, which treat the matter in scales from atomic to molecularand supramolecular.2 The concept of molecular chirality has longbeen recognized and provided guidance in the design of drugsand functional molecules, while chirality at a supramolecular levelis currently attracting great attention due to rapid developmentsin supramolecular chemistry and molecular self-assembly.Supramolecular chemistry is the chemistry beyond molecules

or the chemistry of entities generated by intermolecularnoncovalent interactions.3,4 Supramolecular chemistry isstrongly related to self-assembly, which has been defined as theautonomous organization of components into patterns orstructures without human intervention.5 Both molecular self-assembly and supramolecular chemistry are connected bynoncovalent bonds and/or certain nano/microsized architec-tures. Molecular self-assembly plays an important role inbiological systems, the transfer and storage of geneticinformation in nucleic acids, and the folding of proteins intoefficient molecular machines.6 During such biological processes,supramolecular chirality, which can be simply regarded aschirality at a supramolecular level, is the result of biologicalmolecular self-assembly. A typical example is the secondarystructures of proteins, which can exhibit various conformationssuch as α-helix, β-sheet, and random coil structures with differentsupramolecular chirality. During the molecular self-assembly,supramolecular chirality is also the result of the special spatialarrangements of the molecules. Although supramolecularchirality is strongly related to the chirality of the componentchiral molecules, it is not necessary that all components be chiral.To this end, achiral molecules can also possibly producesupramolecular chirality in a self-assembled system. Therefore,a deeper exploration of chirality at the molecular andsupramolecular level will provide a better understanding of

biological systems and self-assembly, and many assist indeveloping new drugs and materials.In this review, we present an overview of the progress in

supramolecular chirality in self-assembly systems, which mainlyinclude self-assembly in solution or in dispersion systems,supramolecular gels, organized molecular films such as Langmuirand Langmuir−Blodgett films, and others. Although there areseveral reviews detailing supramolecular chirality, self-assembly,as well as chiral nanomaterials and nanostructures,7−13 the fieldhas grown rapidly, and many new exciting results andphenomena have emerged. Furthermore, a general view of thechirality issue through the prism of supramolecular chirality willbe helpful in better understanding many emergent chiralphenomena. In this review, we try to provide a comprehensiveunderstanding of various organized chiral systems from theperspective of supramolecular chirality, with reference mainly towork published after 2010. First, we will provide a generaloverview of the supramolecular chirality, its definition, andspecial features through a comparison with the molecularchirality. Second, we will simply introduce the various moderntechniques of characterization used in supramolecular chirality.Third, we will show in relative detail how molecular chiralitycould be transferred or related to supramolecular chirality in self-assembled systems containing chiral molecular components.Here, we will further show some special features of supra-molecular chirality such as dynamic chirality, the principlesgoverning the chiral amplification, and chiral memory. In thefourth part, we will discuss how achiral molecules can self-assemble into a chiral system, i.e., symmetry breaking and theemergence of supramolecular chirality in systems containingexclusively achiral molecules. A great challenge in the supra-molecular chiral systems constructed from achiral molecules isthe control of the chirality of system. Thus, we will discuss themanner of controlling the supramolecular chirality in systemscomposed of achiral molecules. Finally, we will show sometypical applications of supramolecular chiral systems in electro-optics, sensing, asymmetric catalysis, biological applications,among others. In this portion, we will focus on the uniqueness ofchiral supramolecular systems, how they differ from molecularchiral systems, and the new properties that emerge fromsupramolecular chirality.Currently, with the rapid development of supramolecular

chemistry, self-assembly, and nanoscience, chirality has becomean important issues, and many new chirality-related topics haveappeared, such as chirality at a surface,14−16 chirality in acoordination system,17−22 and plasmonic chirality.23 Thesetopics have been discussed and reviewed but are beyond thescope of this review.

2. BASIC CONCEPTS RELATED TO MOLECULAR ANDSUPRAMOLECULAR CHIRALITY

Chirality is used to describe an object that cannot besuperimposed on its mirror image. When a molecule is notsuperimposable on its mirror image, then the molecule can betermed a chiral molecule. However, in practice, when judgingwhether a molecule is chiral, it is preferable to see if there is anasymmetric carbon atom in the molecule. An asymmetric carbonatom or chiral carbon is a sp3 carbon atom that is attached to fourdifferent types of atoms or four different groups of atoms. Inaddition, if a molecule possesses two noncoplanar rings that aredissymmetrically connected and cannot easily rotate about thechemical bond connecting them or the molecule possesses anaxis about which a set of substituents is held in a spatial

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arrangement that is not superimposable on its mirror image, themolecule can also be chiral even if it lacks an asymmetric carbonatom. Such chirality is termed planar chirality and axial chirality,respectively. Thus, molecular chirality can be essentially classifiedas point, plane, and axis chirality.Since supramolecular chemistry is based on the chemistry of a

noncovalent bond, supramolecular chirality is produced bynonsymmetric arrangement of molecules through a noncovalentbond. Therefore, supramolecular chirality can be produced fromchiral component molecules, the combination of chiral andachiral molecules, or exclusively achiral molecules. Supra-molecular chirality is largely dependent on the manner ofassembly of the molecular components, but the chirality of thecomponent molecule plays an important role in determining thismanner in supramolecular systems. Generally, chiral moleculestend to form specific chiral structures with determinedsupramolecular chirality. In the combination of chiral and achiralmolecules, achiral molecules can be induced into chiralassemblies if there is a strong interaction between the chiraland the achiral molecules. In most cases, the supramolecularchirality of the system is also determined and may follow thechirality of the chiral molecules. In the case of exclusively achiralcomponents, supramolecular chirality can result from theformation of supramolecular systems but in general will beracemic in the resulting macroscopic system.Table 1 lists a simple comparison between molecular and

supramolecular chirality. Both share some common features andare strongly related. When we speak of supramolecular chirality,molecular chirality should be frequently considered. Forexample, in the case of peptides, the chiral monomers covalently

polymerize into polymers to form chiral primary structures andthen self-assemble into secondary and tertiary structures throughnoncovalent bonds, where both molecular and supramolecularchirality are involved. The key to their difference originates fromthe differences in the covalent and noncovalent bonds. There aresome unique features of supramolecular chirality, as shown inTable 1. For example, supramolecular chirality is generallydynamic and changes in response to external stimuli and theenvironment. Chiral memory effects can also be seen in manysupramolecular systems. Molecular chirality can originate fromthe tetrahedral geometry of certain atoms or the asymmetric axesand planes, while supramolecular chirality can be due to self-assembled helical, spiral structures and chiral sheets or chiraldomain structures on surfaces. It should be noted that herein wemean molecular chirality generally refers to that of smallmolecules. If we consider the chirality of polymer systems, thedistinctions between this and supramolecular chirality are lessobvious. For example, the sergeant−soldier rule and the majorityrule of chirality were originally proposed based on polymers andare also applicable to supramolecular systems.Below are some general terms related to molecular and

supramolecular chirality.

2.1. Configuration and Conformation Chirality

Configuration refers to the permanent geometry resulting fromthe spatial arrangement of a system’s bonds. Conformation refersto the spatial arrangement of substituent groups that are free toassume different positions in space without breaking any bonds,because of the freedom of bond rotation. While configurationchirality is generally used in the case of molecular chirality, suchas the absolute configuration of a chiral molecule, conformationchirality usually refers to supramolecular chirality in systems suchas the secondary and tertiary structures of proteins. However,this term has not always been rigorously used based on thisdefinition.

2.2. Induced Chirality

Induced chirality generally refers to those chiral supramolecularsystems where chirality is induced in an achiral guest molecule asa result of asymmetric information transfer from a chiral host orvice versa. This host could be a chiral molecule, chiral pocket,cavity, or chiral nanostructure. In order to produce the inducedchirality, it is necessary for the achiral molecule to have a stronginteraction with the chiral host through a noncovalent bond. Atypical example of induced chirality is the encapsulation of achromophore into the cavity of cyclodextrin.24

Figure 1. Chiral architectures at various scales, from neutrinos to enantiomeric molecules, nanosized biomacromolecules with chiral structures (DNAand proteins), self-assembled micrometer-sized helical ribbons, microorganisms (helix-shaped bacteria), macroscopic living systems (seashells andplants), and galaxies. SEM image showing a helix is reprinted with permission from ref 285. Copyright 2014 JohnWiley & Sons. The picture of a proteinstructure was obtained from Wikipedia (http://upload.wikimedia.org/wikipedia/commons/f/f3/T7RNA_polymerase_at_work.png) and reprintedunder the “fair use” underWikipedia’s license. Pictures are obtained from the followingWeb sites and apply to “fair use”: bacteria (http://tech.sina.com.cn/d/2010-01-29/10113816919.shtml), seashell (http://news.hainan.net/hainan/yaowen/tupian/2014/08/14/2016639.shtml), and flower (http://www.chla.com.cn/htm/2011/0403/80069.html). The picture of a galaxy is a free stock graphic obtained from http://www.rgbstock.com/bigphoto/mVErmjU%2FSpiral+Galaxy.

Table 1. Comparison between Molecular and SupramolecularChirality

molecularchirality supramolecular chirality

composition atom molecule, building block, tactonbond covalent bond noncovalent bondchiral geometry tetrahedron, axis,

planehelical, spiral, chiral sheet, chiral domain

manifestation ofchirality

point, axis, andplane

conformation, secondary and tertiarystructures, helicity, induced chirality,etc.

namingconvention

R/S, L/D, M/P M/P

special feature fixed chirality,recognition

dynamic, sergeant−soldier rule, majorityrule, chiral memory, recognition

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2.3. Helicity or Helical Chirality

Helicity is a special form of axial chirality, which is defined as anentity that has an axis about which a set of substituents is held in aspatial arrangement that is not superimposable on its mirrorimage. If these substituents are atoms of molecular groupscovalently attached to the axis then it can be classified as a chiral

molecule with axial chirality. If the substituents are molecules

held together along the axis by noncovalent bonds then the

assemblies can be regarded as helical and have helical chirality.

Helicity is very common in the supramolecular systems, and in

particular, such chirality can often be visualized through AFM,

Figure 2. Some typical chiral molecules and their corresponding naming conventions.

Figure 3. (Top) Comparison of various microscopies used to characterize the chiral architectures. (Bottom) (A) AFM images of xerogels self-assembledfrom L- and D-HDGA (N,N-hexadecanedioyl-diglutamic acid). Reprinted with permission from ref 27. Copyright 2010 Royal Society of Chemistry. (B)STM images of the chiral twin chains from PVBA. Reprinted with permission from ref 28. Copyright 2001 The American Physical Society. (C) Mirror-imaged nanorods self-assembled from TPPS and (1R,2R)- or (1S,2S)-1,2-diaminocyclohexane. Reprinted with permission from ref 29. Copyright 2013Royal Society of Chemistry. (D) TEM image of a chiral twist self-assembled from pyridine-containing L-glutamide. Reprinted with permission from ref30. Copyright 2011 Royal Society of Chemistry.

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SEM, and TEM observations. Helicity can be classified asM or Phelicity, which will be discussed later.Besides these chirality concepts, the naming conventions of

chirality are complicated, and the detailed descriptions have beenpublished.25−27 For the reader’s convenience, Figure 2 illustratessome typical chiral compounds or assemblies with certain typesof chiral conventions.The R/S system is the most important and general

nomenclature system for denoting enantiomers. Hereby, eachchiral center is labeled R or S according to a system where itssubstituents are each assigned a priority, according to the Cahn−Ingold−Prelog priority rules (CIP). The D/L system (coinedfrom the Latin dexter and laevus, right and left) is related toglyceraldehyde, whose two chiral isomers are labeled D and L. Inthis system, compounds are named by analogy to glyceraldehyde.Many biological molecules are labeled using this method. TheM/P chirality generally refers to a supramolecular system or theaxial or planar molecular chirality. Viewing from either end of amolecule or supermolecule downward along the helical axis, thesystem has P helicity if the rotation is clockwise andM helicity ifthe rotation is anticlockwise. The Λ and Δ chirality terms areused for defining coordination compounds. The enantiomers canbe designated as Λ for a left-handed twist of the propellerdescribed by the ligands and Δ for a right-handed twist, asillustrated in Figure 2.

3. CHARACTERIZATION OF SUPRAMOLECULARCHIRALITY

An important step in the research of supramolecular chirality isthe characterization of the chirality. Although there are manyways to characterize the chiral features of a supramolecularsystem, two classes of characterization of supramolecularchirality are usually applied. One is the morphologicalobservation by various microscopes, with which one can directlyobserve the chiral molecules and chiral structures. With the rapiddevelopment of STM, AFM, SEM, and TEM technologies, directobservation of chiral structures has been made possible, andthese techniques have significantly assisted in the development ofresearch on chirality, especially in the self-assembled supra-molecular systems. The other class of characterization isspectroscopy techniques such as circular dichroism (CD),vibrational CD (VCD), and Raman optical activity (ROA)spectroscopy. With these techniques, the dynamic features of thesupramolecular chirality can be followed and the self-assemblyprocess can be unveiled. Although X-ray structural analysis isuseful in determining the absolute configuration of the chiralmolecules, it requires that samples form into crystals. Self-assembly systems are generally not crystallized and thus notapplicable for X-ray crystallography. However, the molecularconfiguration and packing information from the X-ray studiescan help in understanding the self-assembly process. Herein, wedo not attempt to explain all of the possible characterizationmethods in detail but provide a general introduction as to howthese techniques are applied to the study of supramolecularchirality.

3.1. Morphology Observation

Seeing is believing. The rapid development of research insupramolecular chiral systems is largely dependent on thosetechniques that lead to direct visualization of the chiralnanostructures. Figure 3 shows typical chiral images obtainedby these techniques and a comparison between these techniques.

Atomic force microscopy (AFM) is based on measurement ofthe force between a sharp tip and a sample’s surface. The sampleis mounted on a piezoelectric scanner that moves the samplebeneath a tip mounted on a soft cantilever. As the sample passesbeneath the tip, the force between the tip and the surface can bemeasured, which forms an AFM image. AFM has been usedsuccessfully to probe the surfaces at scales down to the atomiclevel in vacuum, air, or other environments. The sample isgenerally fabricated on a very flat surface such as those of silica ormica. For example, AFM was used to observe self-assembledchiral nanotubes obtained through the gelation of bolaamphi-philes terminated with L- or D-glutamic acids.27 On the basis ofthe AFM observation, we can directly judge the supramolecularchirality of the nanotube. L-HDGA formed a right-handed helicalnanotube, while D-HDGA formed a left-handed one.Scanning tunneling microscopy (STM) technology is a

technology based on quantum tunneling. When a conductingtip is brought very close to a conductive surface and a bias voltageis applied, a tunneling current flows between the tip and thesurface. The resulting tunneling current is a function of the gapbetween the tip and the surface.31 If the tunneling current ismonitored and kept constant by adjusting the gap, the elevationof the surface can be traced and thus displayed an STM image.This technique provides an excellent means for controlling thedistance between the probe and the surface and a very highresolution image of the samples mounted on an atomically flatconductive substrate such as HOPG. The STM techniqueprovides a molecular level resolution and is used to directlydiscriminate the absolute configuration of chiral molecules.32

Further, the technique is also applicable to observation of thesupramolecular chirality at surfaces.14 For example, Figure 3Bshows mirror-imaged chiral twin chains that were self-assembledfrom PVBA (4-[trans-2-(pyrid-4-vinyl)]benzoic acid) adsorbedon a palladium substrate. The twin chains display supramolecularchirality.28

The scanning electron microscope (SEM) is the most widelyused electron microscope for investigating the surface features ofmaterials. When electrons interact with atoms in the sample theyproduce various signals that can be detected including scatteredelectrons and X-rays. SEM uses electron illumination to formimages from the reflected electrons. SEM is critical in all fieldsthat require characterization of solid materials. The SEM image isseen in three dimensions, but the result is a two-dimensionalphotograph; thus, it is especially useful for detecting chiralstructures such as helices or twists. Figure 3C shows our resultswith self-assembled twisted nanorods by treating water-solubleTPPS with (1R,2R)- or (1S,2S)-1,2-diaminocyclohexane inwhich the mirror-imaged left-handed and right-handed heliceswere formed, respectively.29

Transmission electron microscopy (TEM) is a microscopytechnique in which a beam of electrons is transmitted through anultrathin specimen, interacting with the specimen as it passesthrough. A TEM image is formed from the interaction of theelectrons transmitted through a very thin specimen (<200 nm).In addition, using TEM, a diffraction pattern can also beobtained. TEM provides very high resolution (∼0.1 nm) and isuseful in determining the structures of nanomaterials. It is alsouseful for observing chiral structures. For example, using TEM, aself-assembled helical twist structure formed by a pyridine-containing amphiphilic L-glutamide was clearly observed.30

Although all of the above techniques can be used to observechiral nanostructures, not all of the techniques are suitable forthese observations. It is important to select the characterization

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method based on the samples and the self-assembled systems.Taking self-assembled gel systems as an example, AFM isgenerally applicable for investigating transparent gels, while SEMis preferable for observation of white gels, since the sizes of thenanostructures are different. For use of STM, a conductivesample is generally needed, while for TEM observation, materialswith heavy metals or metal ions are relatively easily observed.

3.2. Spectroscopic Methods for Characterization of Chirality

Spectroscopy provides a powerful method for detecting thechiral characteristics of supramolecular systems. Generally,spectroscopy methods used for characterization of molecularchirality in solution can also be used for characterizingsupramolecular systems. Depending on the light source,measured physical variables, and principles, the measurementmethods can be divided into linear and nonlinear opticalmethods. Linear optical methods include the CD, VCD, andVOA spectroscopies apparatus, which are commerciallyavailable. Nonlinear optical methods include SHG and SFGmethods, which are not available as commercial instruments.Many of these techniques have been introduced in detail in booksor reviews,33−36 so they will not be described here in detail.Among these techniques, CD spectroscopy is the most widelyused for chirality characterization. We will take CDmeasurementas an example to show how supramolecular chirality is most oftencharacterized.3.2.1. CD Spectra of Supramolecular Systems. Circular

dichroism (CD) is the differential absorption of left versus rightcircularly polarized light. A CD spectrum records the circulardichroism as a function of wavelength. Thus, a CD spectrum isstrongly related to the absorption spectrum of certaincompounds. If the molecules do not contain any chromophoreor the absorption is outside the wavelength region then no CDsignal can be detected. For this reason, the two types of spectra,UV−vis and CD spectroscopies, should be considered togetherin most cases.Circular dichroism spectroscopy was developed to study

molecular chirality. However, through a series of detailedinvestigations using CD spectroscopy in supramolecular systems,it has been found that this technique is particularly useful formonitoring self-assembled systems for reasons described below.First, self-assembly is usually a dynamic process where

assembly and disassembly occurs simultaneously. Thesedynamics generally occur in the time scale of CD measurements.Thus, CD spectroscopy is very useful in obtaining information,such as the formation dynamics of the chiral nanostructures, aswell as the interaction between the chiral species and the chiralsubstrates.Second, CD signals originate from the electronic transitions of

the chromophore and are generally strong and sensitive to thechromophores as well as the chromophore packing. During self-assembly, molecular packing plays an important role indetermining the chiral nanostructures, and the CD spectrumcan give detailed information on packing. In many cases, we candetect exciton coupling in the self-assembled systems. Theexciton CD can experimentally assign molecular conformations,absolute configurations, and molecular interactions, even forrather complicated supramolecular systems. In many cases, it ismore sensitive than UV−vis spectra and gives more detailedinformation on chiral interactions.Third, CD spectroscopy is also useful for detecting the chiral

perturbations of the assemblies. Many self-assembled systems aredynamic and responsive to external stimuli; thus, CD spectra will

be very useful in monitoring dynamic processes if the systemcontains chiral elements.As a result, CD spectroscopy has developed rapidly because of

its application in supramolecular systems and been extended toinvestigating nanosystems such as plasmonic chirality research.This has become the most important technique for character-izing molecular, supramolecular, and nanoassembly systems.With respect to CDmeasurements of supramolecular systems,

there exist excellent reviews on this technique.34,37 Thus, herein,only a few important measurement techniques will bementioned, together with an explanation of the use of CDspectroscopy in the measurement of supramolecular chirality.

3.2.2. Measurement Aspects. The CD technique istypically suitable for viewing samples in isotropic solutions.Appropriate selection of the solvent, solute concentration, andmeasurement cell can facilitate measurement of the CDspectrum of a system. However, when measuring a supra-molecular system such as solid or colloid dispersions, membranesor films, gels, or liquid crystals, many factors such as the turbidity,birefringence, and anisotropic chiral nanostructures will seriouslyaffect the data collection and subsequent data interpretation. Amajor interference can result from linear dichroism (LD). Auseful method for the CD measurement of films and to excludethe effect of LD has been proposed by Spitz et al. to distinguishintrinsic chirality from possible parasitic artifacts, and thismethod was applied to Langmuir−Blodgett (LB) films.38,39 Insome self-assembly systems, the LD could be enlarged if there isan ordered arrangement of the molecules. Thus, simultaneousmeasurement of the LD spectra may give clear warnings aboutthe accuracy of the CD spectra. In addition, the contributionfrom linear and circular birefringence (CB) cannot be neglectedin some anisotropic liquid systems. A more powerful trans-mission two-modulator generalized ellipsometry is proposed tocompletely determine the linear and circular birefringence (CB),the linear dichroism and circular dichroism, and the depolariza-tion of the sample.40 This measurement is particularly importantin symmetry-breaking systems where achiral molecules orbuilding blocks produce macroscopic chirality. Althoughrelatively little attention has been paid to these measurementsin the past, more recent literature provides a deeper under-standing of these effects.

3.2.3. CD Spectra and Interpretation. In CD spectralmeasurement, two types of spectra are generally obtainable, asillustrated in Figure 4A.

Figure 4. (A) Typically classified CD spectra in supramolecular systems.(B) Qualitative Kramers−Kronig-consistent transformation of a CDbisignate band.

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Observation of a peak or valley in the CD spectrum is referredto as the Cotton effect, which is the characteristic change incircular dichroism in the vicinity of an absorption band of asubstance. The Cotton effect is deemed positive if the circulardichroism first increases as the wavelength decreases (as firstobserved by Cotton) and negative if the CD decreases first.When measuring the CD spectrum, the Cotton effect generallycorresponds to the absorption maximum in the UV−visspectrum, while the sign of the CD is determined by thehandedness of the supramolecular assemblies. If the enantiomersare measured, mirror images should result.Another technique is the exciton-type CD spectrum, as shown

in Figure 4A with dashed lines. This CD spectrum is commonlyreferred to as a bisignate band, which is Kramers−Kronigconsistent with the measured CB, as illustrated in Figure 4B.40 Ingeneral, in the case of solutions, there is a direct correlationbetween the regions of absorption and the CD. In the case of anoncoupled chromophore, the shapes of the two spectraobserved for the enantiomers are similar, although the vibrationalfine structure could be different. If two or more stronglyabsorbing chromophores are oriented chirally with respect toeach other, an exciton spectrum is observed and characterized bythe presence of two bands with opposite signs. The zero CDbetween the valley and the peak is the crossover, which is usuallyin the position of the absorption maximum in the UV−visspectra. In supramolecular systems, the exciton CD is frequentlyobserved. While the exciton CD spectrum is generally sym-metrical in solutions, it is quite common that the two bands of anexciton spectrum do not have the same intensity in thesupramolecular system. This is because there are manyaggregated or cooperatively interacting chromophores, whichcan lead to nondegenerate couplings of the same chromophore,which alters the relative intensity of the two bands. In addition, insome cases, the scattering effect may affect the shape of the CDspectra.41

4. SUPRAMOLECULAR CHIRALITY INSELF-ASSEMBLED SYSTEMS CONTAINING CHIRALMOLECULAR COMPONENTS

During the chiral self-assembly, an important issue is how thesupramolecular chirality or the chiral nanostructures areproduced. Propagation of chiral information through specificinteractions and organization in materials or supramolecularassemblies is generally called chirality transfer. The chiralitytransfer from the molecules to the supramolecular systemrepresents an important origin of supramolecular chirality. Thischirality transfer in self-assemblies containing chiral molecularcomponents can be divided into two cases: (1) the chiralinformation on a chiral center (generally an asymmetric carbonatom or axial chirality) is imposed to the whole aggregate orassemblies containing a chromophore, which usually can bedetected by CD spectroscopy, and (2) the chiral sense of onecomponent is transferred to achiral components to form acomplex system exhibiting supramolecular chirality. The bridgesfor this chiral transfer are various noncovalent interactions, suchas hydrogen bonding, electrostatic interactions, metal−ioncoordination, donor−acceptor interactions, host−guest inter-actions, and van der Waals interactions. In this section, we willdiscuss the chirality transfer in systems composed of chiralmolecules and in those containing mixed chiral/achiralmolecules. Chiral systems composed exclusively from achiralmolecules will be the topic of the next section.

4.1. Supramolecular Chirality in Assemblies of ChiralComponents

The transfer of chirality from a chiral center to aggregates orassemblies is widely found in molecular self-assemblies. It isgenerally accepted that chiral molecules easily form chiralsupramolecular systems. However, such a transfer, in fact,depends on a number of factors including the distance of thechiral center to the assembly site, the strength of the noncovalentbonds, and the competition of the chiral and achiral interactionsto name just a few. The supramolecular chirality of the assembliescan be determined by their CD spectral measurement and/ormorphological observation using AFM, SEM, and TEM.Generally, in their monomeric or free state, the CD signal ofthe compounds is silent in the chromophore portion if thechromophore is located far from the chiral center. However,through self-assembly, the entire assembly becomes chiral andsupramolecular chirality can be produced and thereby detectedby CD spectroscopy. During the self-assembly of the chiralcomponents, the chromophores are held together by variousnoncovalent bonds and tend to adopt a spatial nonsymmetric orchiral arrangement that lowers the energy of the system. In manycases, such assemblies appear as chiral nanostructures, which canbe easily visualized using AFM or SEM observation. However, itis not necessary that a chiral system that exhibits a CD signalshould always be composed of chiral nanostructures. Thechirality transfer will depend on the structure of the chiralmolecules. Here, we summarize the correlation between thesupramolecular chirality of typical chiral supramolecular systemsbased on their component molecules.

4.1.1. Amphiphiles. Molecules that contain both hydro-philic and hydrophobic groups are called amphiphiles. A typicalamphiphile consists of a polar hydrophilic group, usually calledthe head, which is joined to a nonpolar hydrophobic moiety,referred to as the tail. Amphiphiles are some of the mostextensively investigated building blocks in self-assembly systems.According to the number of polar head(s) and hydrophobictail(s) and the connection between them, amphiphiles are usuallyclassified as (1) single head/single or double tail amphiphiles, (2)bolaamphiphiles, in which two hydrophilic heads are connectedby a hydrophobic skeleton group, (3) Gemini or “dimeric”amphiphiles made up of two hydrocarbon tails and two ionicgroups linked by a spacer, or (4) dendritic amphiphiles, wherethe headgroup or the hydrophobic chain is dendritic.Amphiphilic molecules self-assemble in water or in an organic

phase to form various kinds of ordered structures includingmicelles, vesicles, microemulsions, and liquid-crystalline meso-morphic phases. In addition, chiral self-assembly can hierarchi-cally lead to a rich variety of more organized nanostructures suchas fibers, ribbons, helices, “superhelices”, and tubes when theamphiphiles are endowed with chiral elements. During thisprocess, supramolecular chirality is often, but not always,expressed in the morphology of these aggregates at a lengthscale of nanometers or micrometers.

4.1.1.1. Conventional Amphiphiles. Over the past threedecades, a number of synthetic chiral amphiphiles with diversemolecular structures have been designed to form supramolecularchiral nanostructures. In the design of amphiphiles, both theposition of the chiral center and the noncovalent bonding sitesshould be considered. When chiral amphiphiles with long alkylchains self-assemble, a basic bilayer structure is generally formedin the initial stage, which can further stack into multibilayerstructures. With the help of chiral sense in the amphiphiles, thesesheet-like bilayer membranes (single or multiple) often distort to

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form chiral nanostructures with a large curvature and/or a highaspect ratio, leading to twists, helices, superhelices, tubes, and soon. In 1984, Kunitake et al. reported the first evidence of theformation of nanoassemblies with a helical sense from a bilayer oftotally synthetic dialkyl amphiphiles.42,43

To date, a large number of amphiphiles have been reported toform chiral fibrous aggregates based on distorted bilayers.44

These amphiphiles include diacetylenic phospholipids, glyco-lipids with open or cyclic sugars and various quantities and typesof unsaturation in the tail (double bonds, diacetylenicmoieties),45,46 amino-acid-based amphiphiles,47 as well ashexamethylenediamine-based amphiphiles. More detailed re-views can be found elsewhere.13,48−50 Herein, we show recentexamples of self-assemblies of the L-glutamic-acid-basedamphiphiles. Liu’s group and the Ihara group designed a seriesof chiral amphiphiles based on L- or D-glutamide, and Liu et al.investigated the chirality transfer from the molecular tosupramolecular level or nanostructures, as shown in Figure5.30,51−66 Self-assembled chiral nanostructures are generally

obtained by a supramolecular gelation procedure in which theamphiphiles are dispersed into a solvent (water or an organicsolvent) by heating and/or sonication, and then the transparentsolution is allowed to cool to room or a designated temperature,yielding supramolecular gels with an expression of supra-molecular chirality and various chiral nanostructures.For instance, three isomeric pyridine-containing (ortho, meta,

and para isomers) L-glutamic amphiphiles can easily formorganogels with DMSO (3a−c).30 Although none of the gelatormolecules exhibited CD signals in solution, when they formedthe organogels, the assemblies based on the m-pyridine-substituted (3b) and p-pyridine-substituted (3c) gelatorsexhibited CD signals. However, the o-pyridine gels (3a) didnot. This difference in the chiral transfer from themolecular scale

to the superassemblies resulted from their differences in thehydrogen bonding between the gelator molecules. Both the 3band the 3c gelators self-assembled through strong intermolecularH bonds and π−π stacking, while 3a tended to form anintramolecular H bond. Such differences in the H bonding couldfurther lead to varying morphologies, in which the gels from 3band 3c formed nanotwists and nanotubes, respectively, exhibitinga clear chiral feature in their morphologies, while 3a formedconventional nanofibers without a clear chiral feature.

4.1.1.2. Gemini Amphiphiles and Bolaamphiphiles. Incontrast to conventional amphiphiles with only one headgroup,both Gemini amphiphiles and bolaamphiphiles have twoheadgroups covalently connected by an alkyl spacer and, assuch, have attracted great interest in their manner of self-assembly and structure. During self-assembly, the cooperation ofthese two headgroups together with the covalently linked spacerplays an important role.Oda et al. pioneered work on the helical self-assemblies

through gelation of cationic Gemini surfactants with chiralcounterions.67−69 They found that the coassembly of cationicGemini amphiphiles with chiral tartrate counterions in chloro-form leads to the formation of stable twist ribbons. L-Tartrateproduced exclusively right-handed helices, while the D-enantiomer produced left-handed helices.Bolaamphiphiles are molecules that contain two hydrophilic

groups covalent linked by a hydrophobic skeleton (e.g., one, two,or three alkyl chains, a steroid, or a porphyrin).70 Bolaamphi-philic molecules have attracted much research interest due to thefact that they can be found in archaebacteria, which are able tosurvive in a volcanic environment, i.e., in hot sulfuric acid. It isbelieved that the monolayer structure of a membrane togetherwith its helix formation can stiffen the cell membrane, enablingthe archaebacteria to survive in the hostile volcanic conditions.The most important feature of assemblies formed bybolaamphiphiles is their monolayer lipid membrane (MLM)intermediate, which is very similar to a bilayer structure but thetwo polar layers are covalently connected. The Shimizu researchgroup conducted a series of studies on the self-assembly of thebolaamphiphiles that revealed their manner of assemblysystematically.71 During the self-assembly process, many ofthese molecules formed into single- or multi-MLMs and thenself-assembled into nanotubes containing a variety of wallthicknesses, lengths, and exterior/interior diameters.71 If the

Figure 5. Structure of glutamide amphiphiles (gelators) that formedsupramolecular chiral systems through gelation.

Figure 6. (A) Illustration of the self-assembly manner of differentisomeric molecules. Bilayer units were first formed for 3a and 3c andthen stacked into multibilayers to form nanofibers and nanotubes,respectively. Only one bilayer unit is shown for clarity. 3b stacked intosquare columns and then into a nanotwist. TEM images obtained fromvarious DMSO gels: (B) 3a, (C) 3b, and (D) 3c. Reprinted withpermission from ref 30. Copyright 2011 Royal Society of Chemistry.

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headgroup is modified and the MLM is allowed to stabilize,single-walled chiral nanotubes could possibly be fabricated fromthe bolaamphiphiles. For example, Liu and co-workers designeda simple bolaamphiphile containing two L-glutamic acids as thehydrophilic head (15a, 15b) and found that this molecule canself-assemble into a helical nanotube.27,72 The wall of thenanotube consisted of a single molecular layer, and thenanotubes were sufficiently long to form entanglements betweenadjacent tubes. However, introduction of a rigid benzenesegment into the hydrophobic chain, in molecule 17, changedthe morphology of the resulting assemblies to nanofibers ornanoribbons.73 Another bolaamphiphile containing a methylester of L-histidine (18) as the terminal group was also found toform single-wall nanotubes at a low pH.74

Interestingly, when compound 18 was hydrolyzed to itscarboxylic acid, 19,75 it exhibited a hierarchical self-assembly intomultiwalled ultralong supramolecular nanotubes in slightlyalkaline aqueous solution (pH 8−9) as a result of hydrogenbonding and electrostatic interaction. Of greater interest, thesenanotubes then formed bundles consisting of thousands ofultralong supramolecular nanotubes packed in a parallel manner

and could be spun unto yarns using an automatic spinningmachine.75 The self-assembled supramolecular nanotube yarnshad a nominal tensile strength of 45−60 MPa with Young’smoduli as high as 6.8−9.9 GPa, which is comparable to manycovalently linked polymers. This result illustrates that a complexsupramolecular polymer with relative strong mechanical proper-ties can be obtained through hierarchical self-assembly of a smallmolecule. Here, use of the chirally terminated amino acids mayplay an important role.While the design of symmetric bolaamphiphiles is relative

simple, the self-assembly of unsymmetrical bolaamphiphilesprovides more diversity in the control of the inner and outersurfaces of the resulting nanotube. Shimizu et al. designedunsymmetrical L-glucosamide bolaamphiphiles that possessedheadgroups varying in size or properties, and these were used toconstruct a lipid nanotube and an unsymmetrical lipidmembrane. Unsymmetrical MLM-based nanotubes possessboth an outer surface containing sugar hydroxyl groups and aninner surface containing carboxylic acid groups. They were ableto control the inner diameters of the unsymmetrical MLMnanotubes by varying the length of the spacer chain.76 Suchnanotubes offer two advantages when compared with thoseprepared from the monopolar amphiphiles: (i) they possessdistinctive inner and outer surfaces, which can be used forefficient encapsulation of materials, in particular, for selectivesurface functionalization, and (ii) the diameter of thesenanotubes can be controlled by changing the molecular shape.It is common practice to attach the chiral centers as the

headgroup when designing a chiral amphiphile. Thereby,bolaamphiphiles with two headgroups of the same chirality aregenerally designed. However, it would be interesting to observethe chiral assembly process when the bolaamphiphiles containtwo headgroups with opposite chirality.

4.1.1.3. Peptide Amphiphiles. Among the many availableheadgroups in an amphiphile, peptide amphiphiles (abbreviatedas PA) possessing peptides as the headgroup have attractedincreasing attention recently owing to their close relationship tobiological systems. Many types of peptides can be introducedinto the amphiphilic molecules ranging from simple amino acidunits to complicated peptide sequences with bioactive functions.The group of Stupp77,78 developed a class of peptide

amphiphiles capable of self-assembly into cylindrical nanofiberswith high aspect ratios. The chiral sense of the peptide offers anopportunity to fabricate cylindrical intertwining nanofibers thatform multiple helices or superhelices for biomedical applica-tions.77,78

In the systematic study of peptide amphiphiles containingvaline−glutamic acid dimers, Stupp et al. found that the dimericrepeat unit promoted self-assembly into belt-like flat assem-blies.79 The lateral growth of these assemblies can be controlledin the range from 100 nm to as little as 10 nm as the number ofdimeric repeat units increased from two to six. With the growthof the peptide sequence, these flat β-sheet assemblies appeared totwist (Figure 9). Experimental results demonstrated that thepeptide amphiphile sequences can profoundly affect thesubsequent supramolecular morphologies in water.Of greater interest was the finding that the sequence of the

amino acids in the peptide amphiphiles could significantlyinfluence the one-dimensional (1D) chiral nanostructures. It wasfound that four peptide amphiphile isomers, with identicalcomposition but varying sequences of four amino acids, had adrastic effect on the resulting 1D nanostructures under identicalenvironmental conditions.80 The molecules with a peptide

Figure 7. Structures of bolaamphiphiles that were found to form chiralnanotubes or twists.

Figure 8. AFM images of hydrogels of (A) 15c, (B) 18, and (C) 19.Reprinted with permission from refs 72, 74, and 75. Copyright 2005 and2013 Royal Society of Chemistry and Copyright 2013 John Wiley &Sons.

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sequence of alternating hydrophobic and hydrophilic aminoacids, such as VEVE and EVEV, are able to self-assemble into aflat nanostructure. EVEV was shown to form 3D twisted ribbonsor helical ribbons. In addition, occasional nanotubes can coexistwith the twisted and helical ribbons. By contrast, nonalternatingisomers such as VVEE and EEVV result only in the formation ofcylindrical nanofibers that lack a chiral sense. This result is anexcellent demonstration that the peptide sequences candetermine the supramolecular chirality as well as the possiblechiral architectures (Figure 10).80

4.1.1.4. Dendritic Amphiphiles. Although dendritic amphi-philes have not been frequently reported in the past decade, theyexhibit some interesting assembly features with regard to theexpression of chirality.81−86 For example, aromatic rings attachedto the core of a dendron with chiral glutamic acid substituents can

show strong CD signals in their absorption regions via self-assembly, which means the chirality can be transferred from theperiphery to the core and the whole assembly. A helical tubefabricated from a dendritic glutamide lipid87 (21) exhibited a wallwith a thickness of 4 nm, which corresponds to a bilayer thickness(Figure 11). While many other compounds generally formmultiwalled nanotubes, these dendron molecules appeared toform a double-walled nanotube, because of the multiple H bondsbetween the dendron heads that stabilize the bilayer structures.More interestingly, the nanotube structure is very stable in a widepH range.

4.1.2. C3-Symmetric Molecules. Among the many supra-molecular building motifs, C3-symmetric molecules have gainedspecial interest in forming organized nanoassemblies eitherchirally or nonchirally.88,89 C3-symmetric building blocks, usuallyconsisting of a central aromatic or cyclohexane ring function-alized at the 1, 3, and 5 positions, have been frequently exploited.In the design of the C3-symmetric molecules, other functionalgroups can be attached via amide or urea bonds with tricarboxylicacids or triamines. If chiral elements are introduced into thesemolecules, chiral supramolecular assemblies can be easilyobtained. The C3-symmetric molecules often adopt a propeller-like conformation, because of the steric hindrance surroundingthe central aromatic core and the wedged substituent groupwhich causes rigidity through intramolecular interactions(mainly hydrogen bonding). During stacking, the chiral elementsplay an important role in determining the chiral sense of theassemblies. The benzene-1,3,5-tricarboxamide (BTA) motifcomprising either three N-centered or three CO-centeredamides attached to a benzene core (Figure 12) has been widelyemployed in the assembly of chiral supermolecules. The threeamide bonds in these molecules form H bonds, which contributeto the one-dimensional growth of the monomers into acolumnar-type supramolecular polymer. The chiral informationat the substituents of the molecule can be transferred to thecentral aromatic rings and induce preferential formation of asingle helicity in their supramolecular architectures. During thisprocess, chirality transfer is largely dependent on the size of theC3 core and the connection between the wedged substituentsand the core.

Figure 9. (A) Structures of 20 (VE)2, (VE)4, and (VE)6. (B−D)Cryogenic transmission electron micrographs of (VE)2, (VE)4, and(VE)6 nanostructures formed in 5 mM solutions of PA. Scale bars: 200nm. Reprinted with permission from ref 79. Copyright 2013 AmericanChemical Society.

Figure 10. Structures of the isomeric peptide amphiphiles, and a schematic illustration of the nanostructures formed. Reprinted with permission from ref80. Copyright 2014 American Chemical Society.

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Meijer et al. presented pioneering work on the chiral self-assembly of BTAmolecules.90−93 The self-assembly of the parentcompound, CO-centered BTA equipped with chiral aliphaticside chains, has been found to self-assemble into helical, one-dimensional aggregates by means of strong, 3-fold H bonding,which was confirmed by a strong Cotton effect centered around220 nm. Later, several groups extended these studies bysynthesizing BTA derivatives with increased π−π surfaces.Examples include bipyridine,94,95 tetrathiafulvalene (TTF),96

NDI,97and porphyrins98,99 substituted with chiral side chains,which were connected to the BTA core. Due to the largeconjugated surface, intermolecular self-assembly in solution takesplace via strong π−π interactions and solvophobic effects.100

The self-assembly of N-BTAs shows self-assembly behaviorthat is similar to that of their CO-centered counterparts, butthe amplification of chirality is less pronounced as the result ofthe relatively weaker aggregation. A theoretical simulation revealsthat there is a higher energy penalty for rotation around the Ph−

NH bond compared to the Ph−CO bond in the monomer.Therefore, N-centered BTAsmay exhibit less cooperation in self-assembly relative to that of the CO-centered BTAs, whichresults in a lower degree of amplification of chirality.101

To rationalize the chirality transfer mechanism in the self-assembly of C3 molecules, variations of the molecular structure,including the distance of the chiral center to the central cores, thenumber of chiral side chains, as well as the central core, have beenconducted. When replacement of a hydrogen atom by deuteriumwas used as the source of chiral information, only a small energydifference between the diastereomerically related right- and left-handed helical aggregates was observed (Figure 14).92 Theauthors concluded that the value of the molar CD effect wasapproximately three times lower than that of 34a, although thesign and shape of the CD spectrum of the deuterated isomer 33awas similar to that of 34a.The structure in the central core has been found to have a

profound effect on the chirality transfer. Luis Sanchez et al.102,103

utilized oligo(phenyleneethynylene)-based tricarboxamides(OPETAs) as the central core of the C3-symmetric discotics.They compared the self-assembly of two series of C3-symmetricdiscotics based on benzene-1,3,5-tricarboxamides (35) andoligo(phenyleneethynylene)-based tricarboxamides (OPETAs)(36), in which the peripheral groups were decorated with chiralN-(2-aminoethyl)-3,4,5-trialkoxybenzamide units (35b and36b) (Figure 15). Unexpectedly, the chiral BTAs here werepractically CD silent, which differed from previously reportedBTA derivatives. The authors speculated that the outer amidefunctionalities of one molecule were too far apart to formintermolecular, helical supramolecular polymers. However, theCD spectrum of chiral OPE−TA 36b at a concentration of 1 ×10−5 M in MCH exhibited an intense bisignated Cotton effect,which suggested that this compound had self-assembled into aleft-handed helical structure. These results confirmed thecombinatory influence of the π surface of the central aromaticcore and the branched nature of the peripheral side chains on thechirality transfer.

Figure 11. Structure of dendron amphiphiles (left) and height AFM images of hydrogels of 21 (right): (A) pH 3, (B) pH 7, (C) pH 10, (D) pH 11. (E)AFM image analysis of the helical pitch. (F) TEM image of a hydrogel obtained at pH 7. AFM and TEM images reprinted with permission from ref 87.Copyright 2011 John Wiley & Sons.

Figure 12. (Top) Propeller-like shape of C3-symmetric molecules andtheir assembly into a helical superstructure upon stacking in columns.(Bottom) Structure of CO-centered and N-centered BTA. Reprintedwith permission from refs 101 and 222. Copyright 2010 and 2007 JohnWiley & Sons.

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Two series of oligo(phenyleneethynylene) (OPE)-based C3

molecules have been synthesized to study the influence ofstructural features of the OPE discotics on chiroptical properties(37, 38). Initially, the author compared the effect of the groupslinked between the aromatic core and the peripheral side chainson the supramolecular chirality. The OPE-based trisamides witha variable number of chiral side chains (compounds 37b−d) self-assembled into helical aggregates. By contrast, the triangle-shaped OPEs with ether and amide functional groups did notexhibit effective chirality transfer (compounds 38), as demon-strated by the corresponding CD studies, even though there wasan absolute configuration of the stereogenic centers at all of theperipheral chains. Therefore, the cooperation of the three highlydirectional H bonds between the amide functional groups playsan important role in the hierarchical self-assembly andcorresponding chirality transfer. Second, the CD spectra of theOPE-based trisamides with a variable number of chiral sidechains demonstrated that only one stereogenic center wasneeded to achieve a helical organization with a preferredhandedness. In addition, the ability to amplify the chiralityincreased with an increase in the number of stereocenters at theperipheral side chains. The study presented herein improved theunderstanding of the structural rules that regulate the chiralsupramolecular organization of discrete molecules.104

Meijer et al. designed a C3-symmetric molecule that combinedthe conjugated structures of OPEs with amide groups torebalance the π−π stacking and H-bonding interactions.105 Forthe chiral compound 40 at a concentration of 8 × 10−6 M inchloroform, no CD effect was detected, suggesting that 40 wasmolecularly dissolved at this concentration. However, inmethylcyclohexane (MCH), a bisignate Cotton effect wasobserved, which indicated a preferred helicity in the columnaraggregates formed by 40, induced by the optical activity.Interestingly, when the authors changed the N-centered OPE-based discotics to CO-centered OPE-based discotics, the CDspectra of the supramolecular polymers were opposite in signwhen keeping with the configuration of the identical stereogeniccenter. This suggests that the helical preference of OPE-basedC3-symmetric molecules is governed by both the configuration ofthe stereogenic center and the manner of the amide connectivity,which affects the conformation of the amides with respect to theπ-conjugated core. Furthermore, the authors evaluated theimpact of symmetrization of the discotic structure on the chiralityof the supramolecular polymer when C2-symmetric discotic tris-amide 41 was synthesized. Despite 40 and 41 having an identicalconfiguration of the stereogenic centers, opposite chiralinformation for the helical aggregates of 40 and 41 was observed.Temperature-dependent CD measurements directly reflectedweaker noncovalent interactions for the C2-symmetric 41 than

Figure 13. Structures of CO-centered BTAs having intramolecular hydrogen bonds with increased π−π surface.

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for 40. While the number of amide bonds was the same in bothsystems, the strength of the hydrogen bonding varied. Combinedwith the weaker π−π stacking interactions in the C2-symmetricdiscotics relative to the C3-symmetric discotics, the chiralitytransfer may have occurred in a different manner.The chirality of the chiral center can further be transferred to

determine the supramolecular chirality of the nanostructures.For example, enantiomeric C3 compounds containing π-functional tetrathiafulvalene units (compound 31) self-assembled into helical aggregates showing a preferential helicitytwist over several length scales (Figure 17).96 The formation ofprimary helices as twisted stacks was investigated by CDmeasurements and further confirmed by the theoreticalcalculations. molecular mechanics (MM) and moleculardynamics (MD) simulations were used to evaluate the relativestability of the P and M conformations of the stacks, and theresults indicated that the (S)-31 enantiomer provided the Mhelix, which showed greater stability (2 kcal mol−1) per moleculethan the P helix, which was in agreement with the optical activityobserved in the CD spectra. However, it was a surprise to obtainmesoscopic-size chiral fibers of this compound in higherconcentrations, which exhibited inverted helicity, i.e., P helicesfor the S enantiomer and M helices for the R one. Although theinversion of helicity between the primary twisted stack insolution and the secondary helical aggregates for the solid fibers,which can be seen as superhelices from hierarchical assembly,seems to be incomprehensible, it is a common phenomenon thathas been reported in many supramolecular systems. A moredetailed and exhaustive study needs to be conducted to explorethe link between molecular chirality, supramolecular chirality,and the higher order of chiral expression or chiral nanostructures.C3-symmetric benzene- or cyclohexane-centered chiral mole-

cules often self-assemble into nanotube structures by columnstacking of the assembly unit. When chiral units are introduced,the self-assembly process and formation of the nanotubes seemsto be easier. For example, a C3-symmetric L-glutamic acid ethyl

ester gelator was found to form hexagonal tubes ranging fromnano- to micrometer scale depending on the solvents used.106 Inaddition, through antisolvent gelation in a wide range of mixedsolvents, hexagonal nanotubes are formed instantly upon mixingat room temperature.

4.1.3. π-Conjugated Molecules. π-Conjugated moleculesoccupy a very important position in the research on supra-molecular chirality of self-assembled systems. For this there areseveral reasons. First, π-conjugated molecules possess inherentelectronic properties. They are unique because of their potentialuse in organic electronic devices, such as organic solar cells, field-effect transistors (FETs), light-emitting diodes (LEDs),etc.107,108 If endowed with chirality, these molecules mayrepresent new structures with novel properties. Second, π−πstacking is one of the most important forms of noncovalentbonding, which frequently determines if a system can performself-assembly and also determines the self-assembly path-way.109,110 Third, π-conjugated molecules have strong absorp-tion in the UV−vis region, which allows their chiral assemblyprocesses to be easily characterized with CD spectra and othermorphological observations.In order to realize chiral self-assemblies based on π-conjugated

molecules, it is important to introduce chiral elements into the π-conjugated molecules. This can generally be done throughfunctional group substitutions at different positions on thearomatic rings or in the substituted alkyl chains. In addition, the

Figure 14. Structures of N,N′,N″-trialkylbenzene-1,3,5-tricarboxamides(BTAs) (33, 34). Reprinted with permission from ref 92. Copyright2010 Nature Publishing Group.

Figure 15. (A) Structures of C3-symmetric BTAs (35) and triangle-shaped OPETAs (36). (B) CD spectra of compound 36b (2.5 × 10−6 Min MCH) at room temperature and 90 °C. Insets depict the coolingcurves of compound 36b from 363 to 288 K at intervals of 0.5 K min−1.Reprinted with permission from ref 102. Copyright 2013 John Wiley &Sons.

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chiral feature of the building unit itself, the substituent group, thenature of the π-conjugated backbone, solvent properties,temperature, and even the stimuli factors such as light, heat,sonication, magnetic fields, etc., can have a significant influenceon the chiral assemblies. Herein we demonstrate the typical chiralassembly features for important π-conjugated molecules such aspolycyclic aromatic hydrocarbons, thiophene and its derivatives,oligo(p-phenylenevinylene) (OPV) and its derivatives, andperylene bisimide (PBI) and its related compounds. Since theself-assembly of a π-conjugated molecule in a gel has beenreviewed,111,112 herein we focus on those materials withsupramolecular chirality.4.1.3.1. Pyrene (a Polycyclic Aromatic Hydrocarbon). The

pyrene moiety is well known for its ability to form excimers atspecified concentrations in solution. In addition, the strong π−πstacking of the chromophore makes it easy to perform self-assembly, during which supramolecular chirality can also beobserved in the CD spectra if the chiral unit is introduced in thevicinity of the chromophore. For example, compounds 42 and 43contain a pyrene moiety covalently connected to the chiral unitthrough the urethane moiety, forming organogels in isooctaneand n-dodecane, respectively. Although the solution of thecompounds did not show a CD signal, they exhibited CD signalsat gel states in the region of pyrene chromophores, indicatingthat the chirality was transferred from the asymmetric carbonatom to the whole assembly. A temperature-dependent CDspectrum indicated that as the gel was progressively heated, theCD signal decreased and completely disappeared as the gelmelted (Figure 18b), suggesting that chiral supramolecularassembly was responsible for the CD signal.113

Two pyrene-conjugated glutamide derivatives were designedto investigate the effect of the link spacer on the chirality transfer.The pyrene moiety was linked to amphiphilic L-glutamidedirectly (4a) or with three methylene spacers (4b). In both

assemblies, the chirality in the L-glutamide moiety can betransferred to the self-assembled nanostructures, but theexpression of the chirality at a supramolecular level appearedto be dependent on the spacer and solvents. The 4b gels showedthe same P chirality both in polar and in nonpolar solvents;however, the gel of 4a displayed an interesting chiral inversioninduced by solvent polarity, i.e.,M chirality in nonpolar solventsand P chirality in polar solvents (Figure 19). It was concludedthat the spacer between the amide groups and the pyrene ring

Figure 16. Structure of OPEs derivatives. Reprinted with permissionfrom refs 103 and 104. Copyright 2011 and 2012 American ChemicalSociety.

Figure 17.Molecular structure of C3-symmetric compounds containingtetrathiafulvalene units (31), and an SEM image of helical aggregates.Reprinted with permission from ref 96. Copyright 2011 AmericanChemical Society.

Figure 18. (a) CD spectra of isooctane gels of 42R (solid line) and 43R(dotted line) at 293 K (1 mm path length). (b) Variable-temperatureCD spectra on an isooctane gel of 43R (31 mM). Reprinted withpermission from ref 113. Copyright 2010 American Chemical Society.

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effectively regulated the hydrogen bonding and π−π interactionsand then influenced the assembly mode as well as thecorresponding supramolecular chirality.4.1.3.2. Hexa-peri-hexabenzocoronene (HBC). Amphiphilic

hexa-peri-hexabenzocoronenes (HBC) developed by Aida etal.114 are a unique class of amphiphilic aromatic molecules thatexhibit excellent self-assembly behavior. Although the achiralHBC was found to self-assemble into chiral nanotubes, itsimultaneously formed two mirrored chiral assemblies of equalquantity. Therefore, they canceled each other, and no macro-scopic chirality could be detected. However, if some chiralelements were introduced into the HBC molecules, a particularsupramolecular chirality can be obtained. For example, Aida et al.found that a hexa-peri-hexabenzocoronene having two chiraloxyalkylene side chains along with two lipophilic side chains(denoted HBC 44) yielded graphitic nanotubes in MeTHF (2-methyltetrahydrofuran).114 The CD spectra measurementsrevealed that the solution of (S)-44 at 50 °C was CD silent.Cooling of the solution resulted in the appearance of positive CDbands at 389, 400, and 423 nm, which gradually intensified withtime. This result suggested that the tubular assembled 44 mostlikely contained a helical molecular arrangement of the π-stackedHBC units, whose helical sense was determined by the absoluteconfiguration of the chiral centers in the hydrophilic side chains.Thus, the molecular chirality was successfully transferred into theresulting supramolecular helical nanotubular assembly (Figure20). By contrast, the HBC amphiphile 45 bearing branchedasymmetric centers in the paraffinic side chains produced fewnanotubular assemblies, which may be due to its branchedparaffinic side chains that prevent the formation of a bilayer tapeand further the chiral nanostructures. This indicated that not allof the molecular chirality can be transferred to the supra-molecular system.4.1.3.3. Perylenebisimide (PBI). Perylenebisimide (PBI) is a

well-known dye which displays photostability and outstandingoptical and electronic properties as well as strong hydrophobicinteractions and π−π stacking, which are of potential use in

electronic and photonic devices through the formation of self-assembled structures. There are many reports on the emergenceof supramolecular chirality based on PBIs as building units.115,116

For example, the aggregates of dipeptides and perylenebisimide conjugates (glycine-tyrosine, GY, or glycine-asparticacid, GD) have been reported to show different chiral self-assembly depending on the nature of the peptide used. There is acompetition between H bonding among the peptides and thearomatic π−π stacking of PBI. Most interestingly, the peptidesequence has a profound effect on the chirality transfer. In anaqueous buffer, PBI-[GY]2 formed chiral nanofibers. In thecorresponding CD spectra of PBI-[GY]2 aggregates, a negativeCotton effect at 447 nm and two positive Cotton effects wereobserved at approximately 515 and 552 nm, indicating that thechiral sense of the peptide was transferred to the PBI moiety. Bycontrast, the PBI-[GD]2, which formed spherical aggregates

Figure 19. Possible packing mode in the organogels of 4a (top) and 4b(bottom). For the 4b organogel, the steric hindrance between thepyrenemoieties was reduced by the spacer. The units packed in the samedirection both in polar and in nonpolar solvents. For the 4a gel, similarpacking as in the 4b gel is observed in polar solvents. In nonpolarsolvents, due to the large steric hindrance and the stronger H bondbetween the conjugated amide, opposite packing was adopted and thechirality was inverted. Reprinted with permission from ref 60. Copyright2013 Royal Society of Chemistry.

Figure 20. Structures of HBC derivatives 44 and 45 and formation ofself-assembled graphitic nanotubes. (a) Schematic illustrations of thestructure of self-assembled graphitic nanotubes consisting of HBCamphiphiles. (b) Formation of chiral graphitic nanotubes with one-handed helical arrays of π-stacked HBC units through translation ofmolecular chirality into supramolecular helical chirality. Reprinted withpermission from ref 114. Copyright 2005National Academy of Sciences,U.S.A.

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rather than fibers, was CD silent. This work established anassociation of the chiral transfer from a molecularly chiralcomponent to supramolecular chirality as well as themorphologyof the assemblies. It further provides a strategy for using shortpeptides and specifically their sequence structure to manipulatethe PBI chiral nanostructure through chirality transfer.117

Many PBI helical assemblies are formed with the help ofintermolecular π−π interactions because of their large planarstructure. However, Zhu et al.118 found that chiral carboxylic-acid-functionalized PBI systems spontaneously self-assembledinto supramolecular helices via intermolecular hydrogen bondingrather than π−π stacking based on fluorescence spectrameasurements. The fluorescence of the PBI unit herein did notchange much upon formation of supramolecular assemblies,which was in contrast to the π−π arrangement, which led tosignificant fluorescence quenching.In addition to the chiral transfer from the chiral centers, the

axial chirality of binap was also found to be efficiently transferredto perylenebisimides (PBIs) with the help of molecular stacking.The supramolecular chirality of the PBI assemblies was opticallyprobed by circular dichroism (CD), vibrational circulardichroism (VCD), and circularly polarized luminescence(CPL). The biPBI derivatives 46 formed one-dimensionalaggregates in methylcyclohexane (MCH) and sphericalaggregates in chloroform at a higher concentration.119 Theone-dimensional aggregates exhibited twice the value of theluminescence dissymmetry factor (glum) when compared with thespherical aggregates. The sum of excitonic couplings between theindividual chromophore units contributed to the high CPLdissymmetry of the nanostructures.Frauenrath and co-workers found an unprecedented 2-fold

odd−even effect during the investigation of oligopeptide−polymer-substituted perylene bisimides comprising a varyingnumber of L-alanines.120 Depending upon the number of L-alanine units, the observed CD activity was alternatively strongand weak. The molecules bearing an odd number of L-alanines inthe side chains exhibited well-defined spectra with molarellipticities that increased directly with the number of L-alanineresidues. By contrast, the CD spectra of the compounds bearing

an even number of L-alanine groups in the side chains wereconsiderably less well defined and exhibited much smaller molarellipticities. Moreover, they observed a 2-fold odd−even effect,i.e., on one hand, expressed by an alternating reversal of theCotton effect upon increasing the number of L-alanine unitswithin a series of molecules with the same spacer length. On theother hand, the sign of the Cotton effect also alternated in reversewith the increase in the spacer length for molecules with anidentical number of L-alanine units.

4.1.3.4. Phenylenes. Phenylenes are a typical class of rigid-rodmolecules that has been studied in the area of self-assembly aswell as optical properties. By introducing a chiral group in the coilmoiety of phenylenes, the chiral information can be transferredto the phenylene moieties through the self-assembly process.121

For example, the bent rod-shaped rod molecule 47 was found toself-assemble into the hollow tubules in dilute aqueous solutions.Circular dichroism (CD) spectra of the aqueous solutionsshowed a significant Cotton effect above certain concentrations(0.002 wt %) in the region of the aromatic chromophore,indicating that the tubules adopted a one-handed helicalstructure. Combining vapor pressure osmometry (VPO)measurements and CPK models, Lee et al.122 proposed thatcompound 47 self-assembles via a fully overlapped packingarrangement into the hexamericmacrocycles, which, in turn,stack on top of each other with mutual rotation in a singledirection to form helical tubules (Figure 23). When a pyridineunit was introduced into the concave side of the apex of the bent-shaped aromatic compound, 48, the pulsating motions of thetubules were found to show a chiral inversion by virtue of thepyridine forming water clusters through hydrogen bonding andresulting in adjacent molecules that slide into a looser packingarrangement.122

Another interesting example was taken from the helical self-assembly of oligo-p-phenylene-based organogelators 49−52(Figure 24), which has been found to be dependent on the

Figure 21. Circular dichroism spectra of PBI-[GY]2 and PBI-[GD]2aggregates in buffer solution (pH 10.8), concentration 1 × 10−3 M.Reprinted with permission from ref 117. Copyright 2014 AmericanChemical Society.

Figure 22.Chemical Structures of the S and R isomers of the compound46 [R = CH(C6H13)2] and illustration of the self-assemblies structureson the value of luminescence dissymmetry factor (glum). Reprinted withpermission from ref 119. Copyright 2014 American Chemical Society.

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phenyl ring in the core.123 The OPPS with more than two phenylrings in the core self-assembled into left-handed helices, butwhen it contained a biphenyl core the resulting molecule showedexchangeable helicity depending on the reaction time, temper-ature, and concentration, exhibiting a competitive modulation ofsupramolecular helicity controlled by a kinetic versus thermody-namic process. At low temperature, low concentration, and shortassembling times, compound 50 formed right-handedmetastablesupramolecular helices under kinetic control. In this case, thesupramolecular chirality was determined by the axial chirality ofthe biphenyl core. On the contrary, at higher temperature, higherconcentration, and longer reaction times, the external S

stereocenters determined the left-handed supramolecular helixthrough thermodynamic control. By contrast, compounds 50and 51, with three and four phenyl rings, respectively, self-assembled exclusively into left-handed aggregates, with noinversion of the helicity. This indicated that the influence ofthe oligophenyl atropisomerism is much weaker than that of theexternal stereocenters. This study provides not only a goodexample of kinetically controlled modulation of supramolecularchirality but also a better understanding of the chemical andtopological control in the generation of helical supramolecularstructures and the impact of the synergy between different chiralelements.

Figure 23. (A) Structures of 47 and 48. (B) CD spectra of S-48 in aqueous solution at various concentrations. (C) Temperature-dependent CD spectraof S-48 (0.01 wt %) in aqueous solution. (D) Schematic representation of reversible switching of the tubules between expanded and contracted stateswith chirality inversion. Reprinted with permission from ref 122. Copyright 2012 American Association for the Advancement of Science.

Figure 24. Structures of 49−52, and schematic illustration of the aggregation pathways. At low concentrations, low temperatures, and short times, thehelical organization of 50 is dominated by the atropisomerism of the central biphenyl unit and metastable P-type helices are formed. 51 and 52, and also50 at higher concentrations, higher temperatures, and longer times, self-assemble into supramolecular structures of the opposite helicity (M-type).Reprinted with permission from ref 123. Copyright 2013 John Wiley & Sons.

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4.1.3.5. Porphyrin. Porphyrin is one of the most extensivelyinvestigated π-conjugated compounds and exhibits excellentassembly capability and biocomptability. Like other π-conjugatedmolecules, the introduction of chiral elements into themacrocyclic ring induces chiral self-assembly and producessupramolecular chirality. Porphyrin derivatives based on asymmetrical amide-substituted discotic with chiral hydrocarbonside chains were designed by Meijer et al.124 (Figure 25). At

room temperature, 53 was molecularly dissolved in chlorofor-mand. It had a sharp Soret band at λmax = 422 nm but lacked a CDsignal. In MCH, porphyrin 53 exhibited a large blue shift to abroadened band at λmax = 390 nm, which is a typical bandobserved for cofacially arranged porphyrins or H aggregates.125

The CD measurements revealed an intense bisignate Cottoneffect with a crossover at 390 nm, indicating a helicalarrangement of the chromophores in the aggregate. Theporphyrin assemblies were disrupted by heating, accompaniedby a disappearance of the CD response. Upon cooling at aconcentration of 5.0× 10−5 M, the CD effect reappeared at 69 °C(Figure 25).A unique property of porphyrin compounds is their axial

coordination ability. Changes of the axial ligand will cause thesupramolecular chirality of the system to be regulated. Forexample, the addition of the axial ligand pyridine to aggregates of53 was found to alter the supramolecular chirality of the system.Without pyridine, the porphyrin Soret band appears at 390 nm.When 40 equiv of pyridine was added, a new red-shifted and splitband at 418 and 427 nm appeared. The exciton splitting energyof 500 cm−1 is indicative of a dimeric porphyrin pyridineadduct.126 With the dissociation of porphyrin aggregates, the CDspectrum in the exciton split band region became weak.Ultimately, at a pyridine molar excess of 80 000, this split Soretband gradually converted into a single, narrow, CD-silent band at430 nm. This band was identical in shape and position to amonomeric porphyrin pyridine adduct,127 which suggested thatat this rather high pyridine concentration, the porphyrinaggregates have dissociated.Another feature of the porphryin derivatives is their

modification in the central core by metal ions. A porphyrinatozinc complex covalently linked with a peptide was designed and

synthesized by Jiang et al.128 The self-assembly of this novelporphyrin−pentapeptide conjugate in THF/hexane and THF/water was comparatively investigated to illustrate the effect of thepeptide’s second conformation on the helical arrangement ofporphyrin in the assemblies. The positive chirality of porphyrinpentapeptide aggregates was observed from conjugates in THF/hexane, suggesting the helical arrangement of the porphyrinchromophore with a P helicity. The negative chirality was foundfor the aggregates fabricated in THF/water, which was oppositethat observed in THF/hexane. This work confirmed that thesecondary conformation of the peripheral peptide tuned bysolvent polarity further influenced the porphyrin chromophorepacking mode and supramolecular chirality in aggregates. Thisresult not only represented an example of organic nanostructuresself-assembled from a covalently linked porphyrin−pentapeptideconjugate but provided a strategy for controlling and tuning themorphology and, in particular, the supramolecular chirality ofporphyrin nanostructures.

4.1.3.6. Oligo(p-phenylenevinylenes). Oligo(p-phenylenevi-nylenes) (OPVs) are another class of linear π-conjugatedmolecules which are widely used in the fabrication of chiralsupermolecules, due to the electronic properties of the π system,which is sensitive to intermolecular interactions, particularly theway in which the chromophores are organized.Ajayaghosh and co-workers reported many helical nanostruc-

tures composed of OPV assemblies that are formed by attachinghydrocarbon chains with asymmetric carbons to the OPVbackbone, which have been reviewed recently.111 They mainlyfocused on the emergence of supramolecular chirality during theOPV organogelation, and a detailed description of the processcan be found in his review.George and co-workers129 studied the self-assembly of OPVs

bearing a chiral side chain and obtained two different kinds ofassemblies which depended on the solvents employed and thesystem temperature. Two aggregates were found, correspondingto State A (2.5% THF) with sheet morphology and State B (10%THF) with rolled nanotube structure. Remarkably, circulardichroism (CD) studies performed on State B showed that theseassemblies were CD silent, while State A was marked by theappearance of a bisignated CD signal with a positive Cottoneffect at 415 nm and a negative Cotton effect at 375 nm which ischaracteristic of exciton-coupled OPV chromophores. Thechiroptical properties could be due to the different packing ofthe chromophores. Notably, in State B, an annealing process wasfound to improve the molecular ordering in the assemblies andconvert tubes to the nanosheets (similar to State A), which wasconfirmed by a sudden appearance of the bisignated CD signalsduring cooling, characteristic of excitonically coupled chromo-phores. Thus, the supramolecular chirality in the self-assembledsystems of the π-conjugated molecules is strongly related to theirπ−π stacking even when they had chiral substituents.Meijer, Schenning, and co-workers investigated the chiral

assembly of two OPVs through chiral peptide segmentscomposed of either a glycinyl-alanyl-glycinyl-alanyl-glycine(GAGAG), silk-inspired β-sheet, or a glycinyl-alanyl-asparagyl-prolyl-asparagyl-alanyl-alanyl-glycine (GANPNAAG), β-turn-forming oligopeptide sequence.130 Due to the different natureof the two peptides, OPV-GAGAG dissolvedmolecularly in THFand could only form a left-handed helix in water andMCH, whileOPV-GANPNAAG formed a left-handed helix in THF andchloroform but a right-handed helix in water. In addition, thestability of the formed chiral structures was remarkable. Thetemperature-dependent CD spectra in water showed that the

Figure 25. (Top) Structure and temperature-dependent CD spectrumof 53 in methylcyclohexane between 20 and 90 °C with 10 °C intervals.(Bottom) Amodel in which aggregates, monomers, andmonomeric anddimeric porphyrin pyridine adducts are connected by equilibriumconstants. Reprinted with permission from ref 124. Copyright 2010John Wiley & Sons.

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helical aggregates of OPV-GAGAGwere completely destroyed atabove 20 °C, while CD could still be observed at 90 °C for OPV-GANPNAAG. The chiral assembly of OPV-peptide conjugatesdepended greatly on solvent polarity, temperature, and, inparticular, the peptide nature. This was a good example ofregulation of supramolecular chirality of π-conjugated systemsusing peptide secondary structures.Oxadiazole-containing OPVs have been chemically attached to

an α-helical peptide, and the effect of the relative spacing andorientation of the chromophores in the peptide on the chiralassembly of the OPVs was explored (Figure 26).131 Oxa-6D

mono (equipped with a single Oxa-OPV) showed no exciton-coupled CD signal. Oxa-6D and Oxa-7O, both of which have twoOxa-OPVs with different orientations, generated very similarnegligible positive split Cotton effects. The weak Cotton effectmight be due to the high degree of overlap of the sidechromophores for Oxa-6D and the isolated Oxa-OPVs for Oxa-7O.Oxa-11D equipped with two chromophores on the same sidewith a lager spacing than Oxa-6D showed negative Cotton effectswith the greatest intensity among the three molecules. Thisstrong CD intensity suggested that the side chains of Oxa-11Dwere in sufficient proximity to one another for exciton coupling.Chiral assemblies obtained fromOPV polymers have also been

investigated.132 It was found that the chiral polymers (Rac and R)with both high and low molecular weight could form helical

suprastructures with weak CD signals: this was positive for thehigh molecular weight polymer and negative for the lowmolecular weight polymer. On increasing the poor solvent inthe solution, the chirality of the high molecular weight polymershowed no distinct changes in its CD spectrum, while for the lowmolecular weight polymer, the single Cotton effect became abisignate Cotton effect with increased intensity. It was suggestedthat the chiral centers can only offer small chiral perturbation tothe highmolecular weight polymer systems but can impart helicalsuprastructures to the low molecular weight polymer systems.OPV has been incorporated into the main chain of chiral

poly(L-lactic acid)(PLLA), and the chirality transfer from PLLAto OPV was investigated in the solid state.133 Under thesecircumstances, the chirality information on PLLA was found tobe transferred to the self-assembled OPV chromophores. Thissuggests that the molecular packing and supramolecular chiralityof OPV in the aggregates can be tuned by the PLLA. It wasobserved that the mole percent incorporation of the OPVchromophore can greatly affect the chirality transfer from PLLA,and 3 mol % proved to be the incorporation ratio where thestrongest CD intensity was observed.

4.1.3.7. Oligo- and Poly(thiophenes). Thiophene and itsderivatives are another class of π-conjugated molecules whosechiral packing has been extensively studied. Chiral sexithio-phenes can form helical aggregates in water, butanol, and thesolid state. In water and butanol, the chiral assemblies showdifferent “melting transition” temperatures. Furthermore, chiralsexithiophenes have been found to induce the chiral packing ofachiral sexithiophenes. Interestingly, both thermodynamicallystable and kinetically favored mixed aggregates with oppositesupramolecular chirality were obtained. Later, Schenning and co-workers investigated the odd−even effect of the chiral centerposition on the oligo(ethylene oxide) chains away from thethiophene backbone and the number of thiophene rings on thesupramolecular chirality of the assemblies (Figure 27).134 It wasfound that bisignate Cotton effects were observed for all thesemolecules. As expected, the sign of the Cotton effect was reversedfor the aggregates of T6βS and T6βR due to the oppositeconfiguration of the stereocenter. Notably, the Cotton effectshowed positive signs for T5βS and T7βS but a negative sign forT6βS. In addition, when the chiral center was moved from the αto the ε position, there was a positive CD signal for T6αS andT6εS and a negative CD signal for T6βS and T6σS. Therefore,both the number of thiophene moieties and the chiral center on

Figure 26. (Left) Structures of Oxa-OPV, Oxa-6D, Oxa-7O, and Oxa-11D. (Right) CD spectra of Oxa-6D, Oxa-7O, Oxa-11D, and Oxa-6Dmono. A representative absorption spectrum for Oxa-11D is also shown.Reprinted with permission from ref 131. Copyright 2008 AmericanChemical Society.

Figure 27. Structures of chiral oligothiophenes, and (a) CD spectra of T5βS, T6βS, and T7βS in butanol (2.6 × 10−5 M) at 283 K. (b) CD spectra for allchiral T6 derivatives (8 × 10−5 M) at 283 K. Reprinted with permission from ref 134. Copyright 2006 American Chemical Society.

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the oligo(ethylene oxide) chains effected the chiral assembly inan obvious odd−even manner.Bauerle et al.135 explored how the chirality of the biomolecules

affected the chiral self-assembly of oligothiophenes. Initially, hedecorated the tetrathiophenes with carbohydrates, whichresulted in the chirality of the carbohydrate directing the chiralityof the assemblies of the carbohydrate−thiophene. The self-assembly of thiophenes into chiral superstructures can be tunedby the choice of saccharidic building blocks with suitablestereochemistry. The authors further synthesized thiophenescontaining a single chiral amino acid (proline) to study the chiralassembly of the conjugates.135 It was demonstrated that prolinewith two chiral centers could induce a defined helical packing ofthe conjugates, whose helicity was controlled by the config-uration of the amino acid moiety. Since proline has two chiralcenters, the authors also studied how the proline with oppositechirality affects the self-assembly of proline−thiophene.136 Thethiophene functionalized with a proline unit of opposite chiralityshowed a reversed Cotton effect in the region of the π−π*transition of thiophene, which indicated that the chirality wastransferred to the thiophene stacks and that the supramolecularchirality was related to the stereochemistry of the proline residue.The thiophene containing diastereomeric proline showed a silentCD spectrum due to the lack of chirality in the formedaggregates. The mixture of the two enantiomers showed a CDspectrum in which the CD signal nearly vanished.Besides thiophene oligomers, polythiophenes have also been

attracting much attention. Inganas et al.137 investigated the chiralassembly of polythiophenes with synthetic peptides. In this workit was observed that positively and negatively charged peptides

have different effects on the self-assembly of chiral polythio-phenes.Thiophene block copolymers equipped with a chiral side chain

on one or both of the blocks were synthesized and investigated todetermine which chiral side chain played a key role in the chiralitytransfer. Koeckelberghs et al.138 synthesized P3AT(S*)-b-P3AOT, P3AT(R*)-b-P3AOT(S*), and P3AT(S*)-b-P3AOT-(S*) (54−57) composed of an alkyl- and an alkoxy-substitutedpolythiophene block and investigated their aggregation behaviorusing UV−vis, CD, and emission spectroscopies. Through theintroduction of poor solvent, the chiral aggregation of theP3AOT block occurred first owing to its lower solubility in thechosen solvent than that of P3AT. Upon further addition of thepoor solvent, the P3AT block also aggregated, thereby adoptingthe same helical supramolecular organization as the P3AOTblock. The results obtained from CD spectroscopy suggestedthat the P3AOT block transferred its helical supramolecularstructure to the P3AT block, because the chiroptical behavior ofP3AT(S*)-b-P3AOT significantly contrasted with that of theother three polymers. The achiral P3AOT block aggregated in anachiral way regardless of the chirality of the P3AT block.However, the substituent on the P3AT unit can complicate thestacking, as expressed by the intensity of the CD spectra. Thesestudies revealed that for all block copolymers the initial blockaggregation addition of a nonsolvent has a major influence on thestacking and the chiroptical behavior of the other block.

4.1.3.8. Alkynylmetal. Yam et al.139 reported an example ofthe control of the chiral supramolecular structures exerted byvariation of the counteranions in a single gelator molecule of aluminescent chiral alkynylplatinum(II)−terpyridyl. Through

Figure 28. Influence of the addition of methanol to a chloroform solution of the block copolymers 54−57. (A−D) CD spectra of compounds 54−57 inmixtures of methanol and chloroform. Reprinted with permission from ref 138. Copyright 2010 American Chemical Society.

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metal−metal and π−π interactions, molecule 58 can formmetallogels in DMSO and CD signals were obtained in the gels.This result together with the lack of activity in the CD spectrumof a solution of 58−OTf in dichloromethane at the sameconcentration indicated that the CD signals observed in themetallogels originated from the helical chirality of the self-assembled chromophores, transferred from the chiral group,rather than from the intrinsic chirality of the gelator molecules.The chiral supramolecular structures of the metallogels areinfluenced by varying the counteranions. The metallogels withvarious counteranions in DMSO showed different CD spectralpatterns with a negative Cotton effect in the region of theMMLCT transition. The CD signal of 58−OTf was muchstronger than that of other counteranions. The authors inferredthat 58−OTf formed highly ordered helical supramolecularstructures. The different CD spectral patterns of 58 associatedwith the different counteranions suggested that the variation ofthe counteranions would give rise to different chiral supra-molecular structures in the gel phase by varying the degree ofaggregation through Pt···Pt and π−π interactions.Yam et al.140 also incorporated alkynylplatinum(II) terpyr-

idine units into the single-turn backbone of a binaphtholderivative, 59 (Figure 29). The complex experienced a transition

from random coils to single-turn helical strands in which theconformational transition was controlled by the Pt···Pt and π−πinteractions of alkynylplatinum(II) terpyridine moieties basedon the solvents used and temperature. The bisignate Cottoneffect in the circular dichroism spectra was indicative of thecooperative transformation from a random coil state to acompact single-turn M or P helix. The metal···metal and π−πinteractions of the alkynylplatinum(II) terpyridine moieties weresupposed to stabilize the metallofoldamers, as defined by densityfunctional theory calculations.140

4.1.4. Molecules with Multiple Chiral Centers. Thus far,supramolecular chirality based onmolecules with one or multiplehomochiral centers has been the central topic of this review.However, how a molecule with two or more opposite chiral

centers (heterochiral) can affect the supramolecular chirality ofthe assemblies is a very interesting consideration. The importantquestion here is which chiral center will control the supra-molecular chirality? Homochiral, heterochiral, and achiralpeptide auxiliaries appended with naphthalenediimides (NDIs)were designed and synthesized, as shown in Figure 30. It was

found that in the case of the heterochiral peptide conjugates (LDand DL) the chirality of the first stereocenter (irrespective of thestereochemistry of the second stereocenter) adjacent to the NDIcore determined the supramolecular helicity. Remarkably,homochiral LL and DD peptide-modified NDIs self-assembledinto 1D hierarchical supramolecular polymers with oppositehelicity, while the heterochiral peptide conjugates LD and DL

formed microspheres.141

Yang et al.142 reported an interesting control of handedness ofthe alanine dipeptide self-assemblies by the chirality of the

Figure 29. Structure of alkynylplatinum(II) derivatives 58 and 59.Reprinted with permission from refs 139 and140. Copyright 2009 JohnWiley & Sons and Copyright 2013 National Academy of Sciences,U.S.A.

Figure 30. Structures of compound 60, homochiral (LL and DD),heterochiral (LD and DL), and achiral (AA) peptide conjugates of NDI.Proposed models: (a) Schematic representation of left-handed (LL andLD) and right-handed (DD and DL) chiral supramolecular assemblies; (b)schematic illustration of sergeants-and-soldiers effect in which theachiral soldier AA follows the chiral sergeant, LL or DD. Reprinted withpermission from ref 141. Copyright 2012 John Wiley & Sons.

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alanine moiety adjacent to the hydrophobic headgroups.142 Thepeptide amphiphiles termed (L,L)-61 and (L,D)-61 showednegative CD signals, and (D,D)-61 and (D,L)-61 exhibitedopposite signals. Meanwhile, the handedness of the nanoribbonsof (L,L)-61 and (D,L)-61 were left-handed. The nanoribbons of(L,D)-61 and (D,D)-61 were right-handed. The morphologies ofthe assemblies were consistent with the CD spectral data, whichalso indicated that the handedness of these organic self-assemblies was controlled by the chirality of the alanines at theterminals.

4.2. Chirality Transfer in Systems Containing Chiral andAchiral Molecules

Another important system of chirality transfer is that of a chiralcomponent to an achiral one and the extension over the wholesystem. Here the induction of the chirality in the achiralcomponents is of utmost importance. In order to induce thechirality of the achiral components, the interaction between thechiral molecules and the achiral molecules plays a very importantrole. Therefore, the design of molecules with matched bondingsites and their cooperations are of utmost importance. All of thenoncovalent bondshydrogen bonds, electrostatic interactions,host−guest interactions, as well as hydrophobic interactionscould be utilized to perform the chirality transfer between thechiral and the achiral molecules. In some cases, chiral spaces orenvironments can also endow achiral components with chirality.An obvious merit of the chiral transfer in these system is thatinstead of the tedious organic synthesis required to introducechiral units, a simple mixing of the functional achiral units withthe commercially available chiral molecules can producefunctional supramolecular chiral assemblies.4.2.1. Chirality Transfer through Noncovalent Bonds.

4.2.1.1. H-Bond-Directed Chirality Transfer. Hydrogen bond-ing, which is directional and relatively strong, is the mostimportant interaction in self-assembly. Hydrogen bonding is

often used for translation of chiral information from onecomponent to others in multicomponent assemblies.143−149

For example, the pyridine-ended OPV 62 is a pregelator,which can form supramolecular organogels via molecularrecognition of the chiral forms of tartaric acid (TA).147 Themolecule 62 with M-TA (meso-tartaric acid) did not form a gel,suggesting that the spatial orientation of the −COOH moiety inthe chiral TA played a key role in determining the gel formationmode. The chiral information on the TA enantiomer led to theformation of P- and M-helical fibers in complexes containing D-TA and L-TA, respectively, while both helices existed in thecomplexes upon induction by M-TA. This result was consistentwith CD spectra measurements, in which the combination of 62and enantiomeric TA revealed a bisignated Cotton effect. Avirtually mirror-image spectrum was observed for the complexeswith two enantiomers (L- and D-TA), indicating the transfer ofthe chiral information on TA to the self-assembled chromo-phores in a helical sense. The complexes containing M-TAshowed no helical bias.Chiral diamines were successfully employed as triggers to

transfer their chiral information to achiral tetracarboxymetal-lophthalocyanine 63 in DMSO/CHCl3 through hydrogenbonding between carboxylic acid and amine.149 The sign andamplitude of the supramolecular chirality was effected by thestructures of the aminemolecules, volume ratio of the poor/goodcosolvents, type of poor solvents, molar ratio of chiral moleculardiamine to tcPcM, cavity metal of phthalocyanine, and additionorder of the amines.Chirality transfer through hydrogen bonding between pyridine

and carboxylic acid was reported by the Liu group.150 As shownin Figure 34, a simple supramolecular approach has beenproposed to achieve chirality transcription and resulted intwisted nanostructures in a two-component system consisting ofL-glutamic-acid-based amphiphiles 64 and bipyridines 4Py.Compound 64 can self-assemble into nanofibers in water.Upon coassembly with bipyridine, the nanostructures underwentexciting changes to chiral twists due to strong hydrogen bondingbetween the carboxylic acid and the pyridyl nitrogen atoms. Themolecular chirality of gelator molecules can be transferred to thebipyridine aggregates by strong hydrogen bonding. Supra-molecular chirality is expressed not only by the CD signals inthe corresponding absorption band of bipyridine but also by thechiral twist structures.De Feyter, Schenning, and Lazzaroni et al. reported the chiral

assembly of OPVs assisted by nucleobases and nucleo-sides.151−153 Both achiral and chiral OPVs can form chiralrosette structures. After addition of thymidine molecules, themorphology of the OPVs transformed from rosettes to lamellastructures composed of dimers. The chirality of the lamelladepended on the chirality of the thymidine even for the chiralOPVs. In addition, the OPVs can coassemble with thymine intochiral patterns. It was found that the achiral guest moleculethymine can induce the formation of diastereomers from anenantiomeric OPV. The chirality of OPVs can then be tuned bycoadsorption with nucleobases or nucleosides.

4.2.1.2. Electrostatic Interaction. Electrostatic interactionsplay an essential role in specific molecular recognition andmolecular assembly.154 An anionic chiral compound can induce acationic π-conjugated polymer to form an interchain helically π-stacked assembly that is stabilized by both electrostatic and π−πinteractions, which hierarchically self-organize into super-molecules with circularly polarized blue luminescence. A water-soluble poly(p-phenylene) derivative (PPP, 66) was synthesized

Figure 31.Molecular structures of compound 61; CD andUV spectra ofthe hydrogels at a concentration of 30.0 g L−1. Reprinted withpermission from ref 142. Copyright 2013 American Chemical Society.

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by introducing tetraalkylammonium cations at the terminal sitesof the side chains, forming a complex with a water-solublediaxially chiral binaphthyl derivative (BNP) bearing twosulfonate anions at terminal sites of the substituents, as shownin Figure 36.155 A complex of 66 (PPP) andBNP exhibited a CDband in the π−π* transition region of the π-conjugated backbone(Figure 36a). The strong bisignate Cotton effects observed at376 and 341 nm implied the presence of an exciton-couplingphenomenon between the main chains. These results confirmedthat the assembly showed induced chirality of the polymermoiety, which was caused by chirality transfer from the axiallychiral compound (BNP) to the achiral polymer (66). Theelectrostatic interactions here are essential to the chiralityimposed on the polymers, resulting in large dissymmetry factors

of |10−2|−|10−1| in luminescence. Further, the polymerassemblies gathered to form spherulites, which can be regardedas semicrystalline nanospheres, and the spherulites exhibitedcircularly polarized blue luminescence. This work provides asimple way to fabricate chiral spherulites, which may findapplication in novel chiral nanomaterials for the next generationof plastic optoelectronics.Liu and co-workers reported that the enantiomer of

diaminocyclohexane induced a water-soluble porphyrin(TPPS) to form helical nanorods in organic solvents.29 Mirror-imaged helical nanorods were observed in these systems,indicating that the transferred chirality or induced chirality ofachiral π-conjugated molecules can manifest not only via CDspectra but also via direct helical nanostructures. Such chirality

Figure 32.Molecular structure of pyridine-end OPV 62, AFM topographic images showing (i)M helix for 62 + L-TA, (ii) P helix for 62 + D-TA, and (iii)a mixture of M and P helices for 62 + M-TA. Reprinted with permission from ref 147. Copyright 2012 Royal Society of Chemistry.

Figure 33. Structures of tetracarboxymetallophthalocyanine 63 and chiral amines. Reprinted with permission from ref 149. Copyright 2011 JohnWiley& Sons.

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control over a large length scale from molecules to nanostruc-tures could have implications in the design of asymmetricnanocatalysts.4.2.1.3. Metal−Ligand Coordination. The interactions

between metals and ligands are at the heart of a wide variety ofchemical, physical, and biological phenomena. Metal−ligandinteractions allow the design of materials with controlledtopology and with specific physical properties such as redox,magnetic, or photochemical properties. Combined with a chiralsense, metal-based materials can be designed that have uniqueproperties including chiroptical properties (circular dichroism

(CD), circularly polarized luminescence (CPL), opticalrotation), chiroptical switching processes, and nonlinear optical(NLO) activity.A supramolecular assembly of compound 68 (perylenebisi-

mide (PBI) functionalized with dipicolylethylenediamine−zinc(DPA−Zn) binding sites), which can specifically bind tophosphates, showed an induced chirality when binding withATP (Figure 38).156 When DPA−Zn was mixed with 1 equiv ofATP, a positive bisignate CD signal was observed, i.e., positive at518 nm and then negative at 480 nm, in the PBI absorptionregion with a zero crossing at 507 nm. This is characteristic of anexcitonically coupled right-handed helical organization of thePBI chromophores, indicating the efficient chirality induction toan achiral chromophoric assembly through the specific binding ofthe phosphate guest molecules to the DPA−Zn sites.Kleij described trinuclear Schiff base host complexes in which

the conformation was rigidified by a central Zn ion.157 Thecoordination of a series of suitable monotopic ligands to thiscentral Zn ion caused the effective chirality transfer to the host ascharacterized by circular dichroism (CD) spectroscopy. Thechirality transfer provides the possibility for the development ofsubstrate-specific host systems that are useful for determinationof the absolute configuration of various types of organicmolecules.Metal−ligand interactions were also attributed to express

chirality at the nanoscale via extending the π-conjugated systemand enhancing the molecular interactions. Liu et al. reported thatthe Cu2+ ions triggered the amphiphilic Schiff base assemblies toform twist nanofibers.158 The square-planar coordinationbetween Cu2+ and the Schiff base was attributed to the extensionof π-conjugated system and further enhancement of theintramolecular interactions, leading to the chirality beingexpressed on the nanoscale, because the chiral interactionaccumulated in a confined space. Another example is aterephthalic-acid-substituted amphiphilic L-glutamide (com-pound 6) gel in DMSO.57 A left-handed uniform helical twistwas obtained in the presence of a wide range of metal ions,including Na+, Li+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Mn2+,Mg2+, Ca2+, Ag+, Eu3+, and Tb3+ (Figure 39). First, the ligand

Figure 34. Structures of compounds 64 and bipyridines (xPy).Morphologies of coassembled 64/4Py (a, b, c), 64/4ePy (d), and 64/2Py (e) at molar ratios of 1:2. Insets are photographs of the samples.Reprinted with permission from ref 150. Copyright 2011 John Wiley &Sons.

Figure 35. Structures of OPVs 65, thymidine, and thymine. Reprinted with permission from refs 151 and152. Copyright 2011 and 2013 AmericanChemical Society. Reprinted with permission from ref 153. Copyright 2014 Royal Society of Chemistry.

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molecules formed a flat multibilayer structure through π−πstacking between the benzene rings and theH bonds between theamide groups as well and the carboxylic acids. The interaction ofthe metal ions with the carboxylic acid may change the flatmultibilayer structure to a left-handed chiral twist due to thechiral nature of the gelator molecules, providing an easy way totune the chiral twist by simply changing the metal ions.4.2.1.4. Host−Guest Interaction. The binding or encapsula-

tion of a chiral guest in an achiral cavity has been proven to be aneffective method for chirality transfer. Rebek and co-workersdeveloped a hydrogen-bonded dimeric capsule composed of twoachiral monomers that produced a dissymmetric space in which asmall chiral guest can be encapsulated in a diastereoselectivefashion.159,160 This capsule was generally expressed as racemic,because the enantiomers of the capsule are dynamicallyinterconvertible through dissociation and recombination. Theenergy between the diastereomeric complexes would be differentas a result of the chiral interior recognizing the shape of the chiralguest when a chiral guest is captured. The group of Hainodeveloped a calixarene-based capsule, which can encapsulate avariety of guest molecules and heterodimeric hydrogen-bondedpairs of carboxylic acids.161 When the chiral guest was

encapsulated, two diastereomeric isomers were formed,suggesting that the P and M helicities of the capsule can bebiased by the chiral guest encapsulation.A class of oligothiophene-based organogelator bearing two

crown ethers at both ends was found to gelatinize several organicsolvents in the presence of ammonium, forming one-dimensionalfibrous aggregates (Figure 40).162 The helical one-dimensionalassemblies were induced by the chirality of 1,2-bisammoniumguests through host−guest interactions. It was interesting to notethat the chirality of an oligothiophene-based organogel can becreated by thermal gelation, whereas it was silent in thixotropicgelation.

4.2.1.5. Hydrophobic Interactions. Chirality transfer basedon hydrophobic interactions is rarely reported, which may be dueto the weakness of this interaction relative to other noncovalentinteractions. However, this is possible in gel systems where alkylchains of a chiral gelator and achiral guest molecules can entangleeach other to form chiral assemblies. We have presented chiralitytransfer by taking advantage of this concept.51 The chirality canbe transferred to porphyrin chromophores through interchaininteraction between the alkyl chains of both the porphyrin andthe gelator during the coassembly. By contrast, when porphyrins

Figure 36. Poly(p-phenylene) derivative (PPP) 66 and binaphthyl derivatives (BNPs) with (R)- and (S)-configurations. A plausible model of theelectrostatic and π−π interactions between two 66 repeating units and one (R)-BNPmolecule. (a) UV−vis absorption, CD, and gabs spectra, and (b) PL,CPL, and glum spectra of PPP, BNP, and a mixture (PPP−BNP) (1.0:2.0 mol/mol) in methanol−water (50:50 v/v). Reprinted with permission from ref155. Copyright 2012 John Wiley & Sons.

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without long alkyl chains were used in the gels, no CD signalswere observed in either the mixed solutions or the organogels.This fact indicated that the entanglements or hydrophobicinteractions of long alkyl chains played an important role inchirality transfer. In this case, the situation is more akin to achiralmolecules doped in the chiral liquid crystals. The chiral three-dimensional microenvironment provided by the chiral gels isbelieved to be responsible for the induction of the chirality ofTPPOC12H25 assemblies. Besides this approach, when an achiralSchiff base bearing long alkyl chains (70) was mixed with chiralgels formed by 2L and 2D, the chiral information in the gelatormolecules was transferred to the Schiff base chromophore andsupramolecular chirality was obtained.163 On the basis of thedynamic covalent chemistry of the imine, the pH-responsiveproperty of the supramolecular chirality was explored and a pH-driven chiroptical switch was obtained upon treatment with acidand base alternatively. Thus, a supramolecular chiroptical switchwas established based on supramolecular chirality transfer anddynamic covalent chemistry.

An interesting example of the chirality transfer throughhydrophobic interactions was reported by Ghosh and co-workers. They investigated an H-bonding-mediated assemblyin bis-amide-functionalized chiral acceptor (NDI) and achiraldonor (DAN) molecules.164 Two types of homoaggregatedfibers were obtained due to themismatch in the distance betweenthe two amide groups. CD experiments revealed a helicalassembly for both the donor and the acceptor stacks, although achiral center was present only in the acceptor building block. Theauthors suggested that the induction of helical bias was from theacceptor stack to the donor stack via hydrophobic interactionamong the peripheral alkyl chains.

4.2.2. Chirality Transfer from Solvent to Assemblies.The solvent, as the second supramolecular partner of each softself-assembly system, is crucial in determining the thermody-namic process of a self-assembly system. There are someinteresting cases that illustrate the chirality transfer from solventsto supramolecular assemblies. The first chiral solvent effect was

Figure 37. (Top) Structures of TPPS (67) and chiral amines. (Bottom)(A and B) Typical SEM images of the 67 (TPPS) nanostructuresassembled with the assistance of R-DAC (A) and S-DAC (B). (C) UV−vis (bottom) and CD (top) spectra of the dispersion of TPPS obtainedin the presence of R-DAC (black) and S-DAC (red). (D−G) TypicalTEM and HRTEM images of the TPPS nanostructures assembled withthe assistance of R-DAC (D and E) and S-DAC (F and G). (Inset inpanels E and G) FFT of the corresponding HRTEM image. TPPS/DACratio is 1/4. At the bottom is a schematic illustration of the formation ofmirror-imaged 67 nanorods with the assistance of DAC molecules.Reprinted with permission from ref 29. Copyright 2013 Royal Society ofChemistry.

Figure 38. Structure of molecule 68 (PDPA), and schematic of theguest-induced regulation of supramolecular chirality in PDPAassemblies. Reprinted with permission from ref 156. Copyright 2014Royal Society of Chemistry.

Figure 39. Illustration of the self-assembly of 6 in DMSO. In the absenceof the metal ions, the bilayer structure was initially formed, many ofwhich further assembled into nanofiber structures. When metal ionswere present, they reacted with the headgroup and caused a twist of themultibilayer structure. When Eu3+ and Tb3+ were added, red and greenemissive chiral twists were formed. For the sake of simplicity, only onebilayer is shown. Reprinted with permission from ref 57. Copyright 2012Royal Society of Chemistry.

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reported for the emergence of Cotton CD signals due to thetwisted form of CD-silent benzyl molecules dissolved in (2S,3S)-butanediol.165 The second example was a helical preferencerevealed by a study of the CD characteristics of poly-(hexylisocyanate) in nonracemic chlorinated chiral solvents.166

A further study showed that helix formation can be induced in acosolvent containing chiral and achiral solvents.167,168

Recently, it was reported that the self-assembly of an achiralperylenebisimide (PBI) organogelator (73) with two 3,4,5-tridodecyloxybenzoylaminoethyl substituents at the imidepositions was chiroptically silent in achiral solvents. However,in reality, it was found that this system formed both left- andright-handed helices in equal amounts, which canceled anychiroptical signal.169 When (R)- or (S)-limonene was used as achiral solvent, it was shown that a preferential population of acertain handedness of the helical assemblies can be selected bychoosing a chiral solvent. Interestingly, the enantiomericselectivity depended on the assembly process. With dilutesolutions and sufficient equilibration time (thermodynamicconditions), the enantiomeric excess was close to 100%, whereasfor the assemblies fabricated by a controlled kinetic self-assemblyprocess (higher concentration), the enantiomeric excess wasonly 20%. It was inferred that the fast gelation process at highconcentration was controlled by nonequilibrated nuclei in akinetic rather than a thermodynamic self-assembly process.Under these conditions the chiral induction from the homochiralsolvent may not be adequate to effectively impose a singlehandedness on helices.The solvents not only induced the chirality transfer to

supermolecules assembled from low-mass molecules but canimpose the chiral information on solvents into the polymer oroligopolymer assemblies.170,171 Achiral oligo(p-phenyleneviny-lene) (OPV) derivatives equipped with either ureidotriazine (A-OPVUTs) or diaminotriazine (A-OPVTs) H-bonding arrayswere found to self-assemble into columnar stacks in apolarsolvents. When using enantiomerically pure R- and S-citronellolas solvents, circular dichroism spectroscopy (CD) of A-

OPV4UT in S-citronellol at room temperature showed a strongbisignated Cotton effect, which is characteristic of the exciton-coupled helically ordered chromophore, similar to that reportedfor homochiral OPV4UT analogues and the induced chiralityfrom citronellic acid guest molecules. This indicated the transferof chirality from the chiral solvent molecules to the racemicstacks of achiral OPV molecules. The mirror image CD spectraobtained for A-OPV4UT in the other enantiomeric chiralsolvent, R-citronellol, provided proof of chirality transfer. Similarmirror-image, bisignated CD spectra and morphologies wereobserved for A-OPV3UT in enantiomerically pure chiralalcohols, showing that chiral induction in supramolecular stacksthrough chiral solvents is possible.144

Zhang et al.172 synthesized an azo-containing π-conjugatedpolymer poly[(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-4,40-azobenzene](F8AZO), 74, and found that the solvent chiralityof (S)- and (R)-limonenes was successfully transferred to main-chain polymers, which generated optically active 74 aggregates.The intense circular dichroism (CD) signals corresponding to 74in the visible region confirmed the chirality transfer from solventsto polymer. More interestingly, the reversible chiroptical switchwas achieved upon alternating photoirradiation at 405 (transform) and 546 nm (cis form).In general, the helical bias induced by a chiral solvent is not as

strong as the helical bias induced by a chiral monomer. Forexample, chiral solvents (limonene) triggered assembly of aracemic bisurea into a helical nanotube, which was characterizedby its circular dichroism signature. However, the helical bias wasonly 33%, much lower than that induced by a chiral monomer.However, this method offers a simple way to impose chirality onsupramolecular assemblies without introduction of a chiralmatrix or auxiliary.173

4.2.3. Chirality Transfer from Low Molecular WeightMolecules to Macromolecules. While the synthesis of themain-chain chiral polymers is an important topic, the regulationor control of chirality of the polymer main chain through the

Figure 40. Structure of oligothiophene derivative 69; CD spectra of the gel phase of 69 (straight line), 69·(R,R)-diammonium (dashed line; triangle),and 69·(S,S)-diammonium (dotted line; circle). (a) ICD spectra of the 1:1 mixture. (b) CD titration study at 370 nm. Reprinted with permission fromref 162. Copyright 2012 John Wiley & Sons.

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interaction with a chiral unit in their side chain provides a newarena for study (Figure 46).The pioneering studies in this field have been reviewed

thoroughly by Yashima et al.174 The first helical polymer inducedby chiral amines through acid−base interactions was reported at1995.175 Upon interacting with chiral amines in DMSO, apreferred helical handedness of a cis-transoidal, stereoregularpoly((4-carboxyphenyl) acetylene) (75-f) is instantaneouslyinduced in the polymer, showing a characteristic ICD in the π-conjugated polymer backbone region, indicating that the chiralityof the amines was imposed into the main-chain polymers.175

Later, this helical sense induction concept through noncovalentchirality transfer was applied to the synthesis of a variety ofchirality-responsive PPAs by introducing a specific functionalgroup as the pendant group.176−183 Yashima and co-workerssystematically investigated the formation of one-handed helicesfrom achiral polymers by acid−base complexation of chiralamines, amino alcohols, and amino acids with organic acidfunctions in their side chains.175−180 During this process, therather irregular twist of the adjacent double bonds around asingle bond is transformed into a helical conformation with apredominant handedness. This was confirmed by bisignateCotton effects in the polymer backbone absorption, whichshowed a mirror-image relationship between two enantiomers of

chiral small molecules. Various acyclic and cyclic, primary,secondary, and tertiary amines were introduced in the side chainsof the poly(acetylene)s. The same acid−base chemistry as aboveholds for the interaction of these polymers with chiral carboxylicacids. In addition, the resulting optical activity is strongly relatedto the structure of the amine-functionalized polymer.Other noncovalent interactions, such as metal−ligand

interactions, host−guest interactions, and electrostatic inter-actions, also contribute a great deal to inducing the formation ofone-handed helices from achiral polymers. For example, whencrown ethers were present in the pendants,184 a predominantlyone-handed helical conformation was formed in aqueoussolutions (HClO4) upon complexation with various chiralcompounds, such as amino acids, peptides, amino, sugars,amines, and amino alcohols. Addition of NaCl or KCl decreasedthe magnitude of the CD spectra, which suggested theimportance of the crown ether−ammonium complexation inacidic water.Interactions between chiral molecules and the functional

group in the polymer side chains were also observed for otherpolymer backbones. Other achiral polymers, including poly-isocyanates, poly(phenylisocyanide), poly(thiophene), and poly-(guanidine), can also be endowed with dynamic chirality byvarious noncovalent interactions.Vandeleene et al. reported poly(phenyleneethynylene-alt-

bithiophene) copolymers with chiral pendants and pendantsbearing carboxylic acid groups in solution and in films.185 Here, itwas noticed that the addition of chiral primary amines resulted inchiral aggregation of the polymers. When the chiral centers of thependants and amines were the same, cooperation between themin helical stacks was observed by CD spectroscopy. The oppositesituation holds when they are opposed.Inoue and co-workers synthesized a poly(m-ethynylpyridine)

polymer that comprised at least 72 pyridine moieties with amolecular weight of ca. 4500.186 When a chiral saccharide wasenclosed in the inner sphere of the polymers, helical structures ofpolymers were guided by uncharged hydrogen-bondinginteractions with saccharides. Circular dichroism studies revealedthe nature of the chirality induction and how the achiral hostsenses differently the chiral structure of a range of saccharides.

4.3. Dynamic Features and Regulation of SupramolecularChirality

Dynamic exchanges and rearrangements of building blocks inassemblies present challenges in supramolecular chemistry. Thisis also true for supramolecular chirality. In contrast to a systemunder thermodynamic control, which often exhibits a single,simple assembly route, supramolecular chirality based onsupramolecular chemistry also shows the complexity anddiversity of kinetic direction. Various noncovalent interactionsmay result in nonequilibrium self-assembly, in which structuraldiversity is achieved by forming several kinetic products based ona single covalent building block. The multiple availableinteraction sites and the flexibility of the interaction modesmake the supramolecular chirality dependent on the kinetics ofself-assembly.Stupp and co-workers187 demonstrated that the preparation

protocols of the peptide amphiphiles self-assembling in water canresult in the formation of different supramolecular morphologies,either long filaments containing β-sheets or smaller aggregatescontaining peptide segments in random coil conformations. Thepeptide amphiphiles (PA) were found to exist as monomers inthe good solvent HFIP and formed assemblies upon addition of

Figure 41. Structures of (A) dopant Schiff base compound 70 and (B)2L and 2D. (C) Schematic illustration of their coassembly. (a) A Schiffbase based on a dynamic covalent bond. (b) In the coassembly of 70 and2, 70 can be inserted into the alkyl chains of 2molecules. On the basis ofhydrophobic interactions, a supramolecular assembly with twiststructure can be formed, and the supramolecular chirality can betransferred from 2 to the Schiff base moiety. The supramolecularchirality showed “on” and “off” states through the alternate treatment ofacid and base. Reprinted with permission from ref 163. Copyright 2013Royal Society of Chemistry.

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nonsolvent−water. Two peptide amphiphile assemblies of thesame composition were prepared in two different ways, as shownin Figure 47.187 Although the two systems have the same HFIPand PA content, clear differences can be observed between theCD spectra of solutions 1 and 2 (Figure 47b). Solution 1exhibited a random coil CD spectrum, whereas the spectrum ofsolution 2 had β-sheet character. The presence of β-sheets insolution 2 can be rationalized by the fact that for this solution thestock solution of PA1 in HFIP is initially added to water. Insolution 2, although the amount of HEIP reached 20%, these β-sheets do not disassemble completely, because there was notransition back to the random coil conformation. These resultsdemonstrate that β-sheet assemblies have high kinetic stabilityand, once formed, do not readily disassemble. It is evident thatinsights into the characteristic dynamics of a supramolecularsystem can provide an efficient way to select the optimumassembly pathway necessary for function.Zhang and Liu investigated the aggregation of an anionic

porphyrin (TPPS) (compound 67) on the a cationic polypeptide(poly(lysine)) controlled by dynamic assembly.188 Throughsimple adjustment of the mixing sequences of TPPS withpoly(lysine), opposite CD signals from TPPS J aggregation wereobtained. When PLL was dropped into the TPPS solution(process I), a negative CD signal was observed. By contrast, apositive Cotton effect was found when TPPS was added to thePLL solution (process II). The time scan of the CD spectrarevealed that process II was controlled by thermodynamics, whileprocess I was controlled by kinetics. The negative supra-

molecular chirality that occurred in process I can be transferredto a positive one, which is similar to that of process II.In comparison with molecular chirality, supramolecular

chirality can be easily altered by external factors such as solvent,temperature, sonication, photoirradiation, redox potential, andchemical additives. This provides an opportunity to regulate thesupramolecular chirality in self-assembled systems.

4.3.1. Solvents. Solvent is the medium for self-assemblyprocesses and can strongly influence self-assembly via the specificinteractions between solvent and solute. The basic features ofsolvents such as polarity, viscosity, and solubility for the soluteand other compounds could also affect the supramolecularchirality of a supramolecular system. The majority of reports inthe field of supramolecular chirality focus on the influence of themolecular structure on assembly and largely ignore the role of thesolvent. However, understanding how solvent propertiesinfluence chiral structures can help provide a deep understandingof how supramolecular chirality is produced.For example, an L-glutamate-based amphiphilic gelator bearing

an azobenzene segment 5 formed organogels that showed anexcellent photoregulated gel−sol transition.56 It was found thattotally opposite CD signals were observed in DMSO and toluenegels. The DMSO gel exhibited a positive Cotton effect, while anegative Cotton effect was observed in the toluene gel, as shownin Figure 49. The opposite Cotton effect obtained from differentsolvents implied that the supramolecular chirality was reversed asa result of different molecular orientations at the molecular level.According to the results of XRD and temperature-dependentUV−vis spectroscopy, two kinds of molecular stacking models

Figure 42. Schematic illustration of chiral induction by helical neighbors. Reprinted with permission from ref 164. Copyright 2011 Royal Society ofChemistry.

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Figure 43.Molecular structure of PBI derivative 73. AFM images of a film spin coated (2000 rpm) from a solution of PBI 1 in (S)-limonene (c = 1× 10−4

M) ontoHOPG. (a and b)Height images. (c) Phase image. Scale bars in a and c correspond to 450 nm; the z scale in a and b is 9 nm. The statistical graphofM and P helices is derived from image a. (a−b and c−d) Cross-section analyses of the fibers. Reprinted with permission from ref 169. Copyright 2013John Wiley & Sons.

Figure 44. Schematic illustration of the preferential chiral solvation in OPVUT self-assembled stacks. Reprinted with permission from ref 144.Copyright 2011 Royal Society of Chemistry.

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have been proposed: the azobenzene groups packed face to face(π−π stacking), and the amino acids packed by formingH bonds,as shown in Figure 49.In the case of a pyridylpyrazole-linked L-glutamide organo-

gelator,58 diverse nanostructures over a wide scale range fromnanofiber to nanotube and microtubes were obtained based onthe polarity of the solvent. The nanofiber, nanotwist, nanotube,and microtube structures of 9 were obtained in toluene,chloroform, DMF, andDMSO, respectively. Suchmorphologicalchanges can also occur with xerogels in the solvent vapors. It has

been suggested that the interaction between the pyridylpyrazoleheadgroup and the solvents may subtly change the stacking of themolecules and thus their self-assembled nanostructures. Thus, bychoosing appropriate solvents, a transition in morphology fromnanofibers to chiral twists to nanotubes and to microtubes can beachieved.Solvent-driven morphological transitions may dominate

supramolecular self-assembly in many cosolvent mixtures.189

For example, Liu et al. found that the addition of a small amountof water to organic solvents, either water miscible or immiscible,

Figure 45. Schematic illustration of unpolarized-light-driven chiroptical switching in CHCl3/(1R or 1S)/IPA (0.3/1.5/1.2, v/v/v). In 74-trans, chiralaggregation manifested as a bright yellow and turbid solution, while nonaggregated 74-cis formed a yellow transparent solution. Reprinted withpermission from ref 172. Copyright 2011 American Chemical Society.

Figure 46. Schematic illustration of helical polymers obtained from achiral polymers induced by achiral guests and structures of poly(phenylacetylene)s,poly(thiophene)s, and poly(phosphazene)s. Reprinted with permission from ref 174. Copyright 2009 American Chemical Society.

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can trigger the formation of chiral nanostructures of a cationicamphiphile (7).61 In ethanol, nanofibrous structures without anychiral sense evolved into the helical nanostructures, andfurthermore, the helical pitch could be tuned by the amount ofwater present. In nonpolar solvents, helical tube structures wereproduced upon the addition of water.The supramolecular interactions of the PBI derivatives were

successfully modulated by solvents,190,191 which not onlyinduced a CD signal inversion but also the macroscopicproperties could be modulated by the solvent, from a left-

handed helix in nonpolar solvents to a right-handed helix in polarsolvents.

4.3.2. Temperature. As mentioned above, hydrogenbonding is the most important interaction in self-assembly: thestrength of this interaction decreases with increasing temper-ature. Therefore, chiral assemblies based on hydrogen bondingare especially sensitive to the adjustment of temperature. Themost popular instances of this are reported in the supramoleculargels based on H bonds, in which CD signals are silent in solutionor in the monomer state as the system temperature is increased.Supramolecular chirality is induced by the gel formation process.In other words, gelation-induced supramolecular chirality is quitesensitive to the temperature-regulated sol−gel transforma-tion.192

Meijer and co-worker investigated a series of supramolecularpolymers based on C3-symmetric molecules.90−95 In thesesystems, temperature-dependent supramolecular chirality is

Figure 47. Assembly of PA1 is dependent on the preparation protocol.(a) Two PA1 solutions (50 μg mL−1) in 20% HFIP were prepared viatwo methods that differ by the order in which pure HFIP and the PA/HFIP stock solution were added to water. Even though both solutions 1and 2 contain the same PA concentration and HFIP content, cleardifferences can be observed in CD (b) and DLS (c). Time-dependentCD (200 nm) acquired on solutions 1 and 2, shown in the inset of panelb, demonstrates the large hysteresis involved. Reprinted with permissionfrom ref 187. Copyright 2014 American Chemical Society.

Figure 48. (A) CD spectra of a PLL/TPPS (67) mixture in differentmixing sequences: (a) PLL was added to TPPS (process I) and (b)TPPS was added to PLL solution (process II). (B) Time-dependent CDspectra of the PLL/TPPS mixture. Illustration of the formation of TPPSaggregate on polymer. The green block represents TPPS, and its chargeswere omitted. (a) Pending-type aggregate in which one site of TPPSbinds on the PLL, while the other unit is stacked on the first in a head-to-tail manner as a J aggregate. When less PLL presented in the solution orPLL was added into TPPS, such aggregates were predominantly formedand the process is a dynamic one. (b) Wrapping-type aggregation inwhich every TPPS unit is wrapping around the polymer chain, whilethese units formed head-to-tail stacks as J aggregates. Reprinted withpermission from ref 188. Copyright 2009 American Chemical Society.

Figure 49. (A) CD spectra of a gel of 5 in various states: DMSO gel (▲)with a concentration of 4.3 mM, and toluene gel (▽) with aconcentration of 5.0 mM. (B) Schematic illustration of molecularpacking in DMSO and toluene. Reprinted with permission from ref 56.Copyright 2011 American Chemical Society.

Figure 50. Chemical structure of the PTCDI−HAG amphiphilemolecule 81. (a, c) CD spectra and (b, d) TEM images of thePTCDI−HAG molecules 81 in different volume ratios of CHCl3/n-C8H18 or THF/H2O. Reprinted with permission from ref 190.Copyright 2011 Royal Society of Chemistry.

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widely found. In general, at higher temperatures, the moleculesexist in the monomeric state, in which the CD is silent. Uponcooling, the chirality appears gradually during the self-assembly.Thus, temperature-dependent CD spectroscopy is oftenperformed to illustrate the mechanism of self-assembly.With the exception of the thermally dependent change of

chirality in the supramolecular assemblies based on hydrogenbonding, a thermally reversible method for the inversion ofchirality was developed by changing of the lattice symmetry. Athermally reversible inversion of chirality was discovered inhelical supramolecular columns formed by C3-symmetric self-assembling dendrimers based on dendrons connected at theirapex via trisesters and trisamides of 1,3,5-benzenetricarboxylicacid.86 The authors demonstrated a change in the latticesymmetry as follows: negative chirality for 2D and 3D phaseswith triangular symmetry (columnar hexagonal) and positivechirality for 2D and 3D phases with rectangular symmetry(columnar rectangular and orthorhombic) which was attributedto a thermally-induced inversion. This is the first example of theelucidation of the mechanism of reversible inversion of helicalchirality in supramolecular dendrimers. The structural changesreported can be used to design complex functions based onhelical supramolecular dendrimers with different degrees ofpacking on their periphery.86

4.3.3. Redox Effect Chirality.When metal ions or moietieswith variable valence states are inserted into a molecular buildingblock, the redox chemistry of this moiety will cause the variationof supramolecular chirality.193,194 Liu et al. reported redox-responsive chiral organogels based on a Cu(II)−quinolinolderivative (10).54 With the gels of the Cu(II)−quinolinolderivative, a positive Cotton effect at 280 nm and a negative bandat 240 nmwith a crossover at 260 nm and a single positive band at351 nm and a negative band at 475 nm were present, whichsuggested that the chirality was transferred onto the metallogelassemblies. Upon reduction of Cu(II) to Cu(I) by ascorbic acid,the signals at around 475 and 351 nm disappeared, whereas theexciton band at around 260 nm was retained. This result showedthat the reduction was mainly localized on the central Cu(II)ions, whereas the π−π stacking of the aromatic rings, which werebrought proximal to the amide groups, was not destroyed. Owingto the formation of this different structure, the chirality could notpropagate the L−L band; thus, the bands at 351 and 475 nmdisappeared. Subsequently, as the system was oxidized by O2, thenegative Cotton effect at around 475 nm and the positive band at351 nm reappeared. These results revealed that the supra-molecular chirality could be tuned by redox chemistry and aredox-driven chiroptical switch could be realized.54

4.3.4. Photoirradiation. Photoirradiation is a noninvasive,easy, and fast external stimulus that is often utilized to adjust thestructures of supramolecular assemblies.56,195−199 Photoinducedisomerization of azobenzenes,200,201 dithienylethenes,202 andspiropyrans is the most often-used strategy for developingphototriggered chiroptical switches. An azobenzene-linkedphenyleneethynylene bearing chiral groups (82) showed anintense positive signal at 464 nm with two negative signals at 407and 322 nm with a zero crossing at 421 nm corresponding to theπ−π* transition of the PEmoiety. Surprisingly, the CD spectrumafter UV irradiation at 323 K, followed by cooling the solution toa lower temperature, showed a reversal of the CD spectrum;however, there was a lower intensity in the CD bands. In thiscase, photoirradiation resulted in a reversal of the CD signal.203

SEM analysis of (S)-83 before photoirradiation showedentangled right-handed (P) helical ropes of diameters ranging

from 50 nm to 1 mmwith lengths of several micrometers (Figure52a). After irradiation at 323 K followed by cooling, the helicityof the fibers was found to be left handed (M), as shown in Figure52b. AFM analysis of (S)-83 revealed reversal of their nativehelicity to the induced opposite screw sense after irradiation.In the absence of heating, the helicity did not change even after

irradiation for several hours. Even if there could be slowisomerization of the molecules within the helical fibers, thehelical twist did not change significantly. This work demon-strated that the handedness of a photoresponsive supramolecularobject can be tuned with the cooperation of light and heat,without changing the inherent molecular chirality of theindividual building blocks.Cone-shaped alkoxyazobenzenes dimers functionalized with

amide groups were synthesized, and the presence of amidehydrogen-bonding sites in one side of the folded moleculesprevented the antiparallel stacking favored by asymmetricstructures, facilitating the formation of toroidal aggregates, inwhich the chirality was transferred to supramolecular chirality.The resulting toroidal structures have large π surfaces on theirtop and bottom and can hierarchically organize into tubularnanostructures. Irradiation of 84 with UV light quantitativelyconverted aggregative trans-azobenzene moieties to nonaggre-gative cis isomers (Figure 53a),204 collapsing nearly all aggregatesto monomers (DLS study). However, a different situation wasachieved when 84 was irradiated with visible light at 470 nm: thereversible trans−cis isomerization of the azobenzene moietiescould be promoted because both isomers have absorptions at 470nm, while the fraction of the aggregative trans isomer is kept inlarge excess because the cis isomer has a greater molar extinctioncoefficient at this wavelength (Figure 53b). In this situation, arapid generation and evanescence of polar cis-azobenzenemoieties occurred within the toroidal aggregates. This is capableof promoting hierarchical growth of the aggregates by increasingnanostructure surface polarity in nonpolar environments.

4.3.5. Chemical Additives. The addition of guest moleculesand metal ions was found to be effective in adjusting the chiralsuperstructures of assemblies through guest−host interaction

Figure 51. (Top) Cu(10)2 molecules self-assembled into gels throughcoordination, hydrogen bonding, and hydrophobic interactions, as wellas through π−π stacking. On reduction, this π−π stacking was impeded,and accordingly, the gels changed into a sol. (Bottom) CD spectra of 10(a) and Cu(10)2 gels (b), the sol after reduction (c), and the revived gelafter redox (d). (Inset) Enlargement of the spectra in the range 400−500 nm. Reprinted with permission from ref 54. Copyright 2013 JohnWiley & Sons.

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and metal−ligand cooperation.57,158,205−208 For example, anachiral-guest-triggered chiral inversion in a novel supramolecularassembly fabricated by pillar[5]arenes has been reported.209 Theplanar chirality of pillar[5]arenes is caused by the substitutionposition of the alkoxy groups, which have two equivalent stableconformations (pS and pR), as shown in Figure 54B(a). In orderto isolate the two enantiomers, the rotation of these units shouldbe inhibited because the interconversion between pS and pRoccurs by rotation of these units in solution. General approachesto hinder this rotation are to modify both rims with bulkysubstituents or form a rotaxane consisting of a pillar[5]arenewheel and a guest axle.

Yamagishi et al.209 reported a highly stable 1:1 host−guestcomplex formed by pseudo[1]catenane 1,1,4-dicyanobutane(G1) as a guest and pillar[5]arenes as a host. Inclusion of G1 inthe cavity of pillar[5]arenes causes dethreading of the alkyl chainmoiety from the cavity of pillar[5]arenes, which caused theachiral guestG1 to induce the planar chiral inversion from in-pS-1 to out-pR-1. The chiral inversion was characterized by theobserved CD spectra, which changed dramatically from positiveto negative with increasing G1 concentration. An ammoniumcation (G2) was another guest that could be added to the secondfraction (in-pR-1), and a decrease in the CD intensities wasobserved. The authors concluded that the absence of chiralinversion was due to the weaker association constants between

Figure 52. Photoisomerization of the azobenzene-linked phenyleneethynylene derivatives 82 and 83. A mixture of E,E, E,Z, and Z,Z isomers is possible.SEM images of (S)-83 (a) before and (b) after photoisomerization; AFM images for (S)-83 (c) before and (d) after photoisomerization. Reprinted withpermission from ref 203. Copyright 2012 John Wiley & Sons.

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Figure 53. (A) Schematic illustration of the self-assembly of 84. (B) Photoinduced (a and b) UV−vis and (c and d) CD spectral change of 84 (c = 3.0 ×10−4 M) in MCH at 20 °C. (a and c) Changes upon irradiation of a trans-rich solution with 365 nm UV light. (b and d) Changes upon irradiation of thecis-rich solution with 470 nm visible light. (e) Plots of the fraction of cis-azobenzenemoieties (redmarks, left axis), andmaximumΔε values (bluemarks,right axis) versus irradiation time of UV (left side) and visible (right side) light. Aggregation states are shown with graduated background colorsrepresenting monomeric (water blue) and aggregated (orange) states. Reprinted with permission from ref 204. Copyright 2012 American ChemicalSociety.

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G2 and pillar[5]arenes than those between G1 and pillar[5]-arenes. More interestingly, G2 could be removed from the cavityof pillar[5]arenes with the assistance of the crown ether[24]crown-8 ([24,8]), resulting in CD intensities that nearlyrecovered to their initial state. This result displayed the host−guest complexation as a valid driving force for the chiralinversion, and this guest-triggered chiral inversion system will beuseful for chiral switching or sensing systems.Naphthalene diimide amphiphiles functionalized with the

dipicolylethylenediamine−Znmotif were synthesized in order topromote a guest-induced self-assembly and chiral inductionthrough specific binding interactions.210 Titration of NDPAamphiphiles with increasing molar ratios of ADP resulted in thegradual evolution of strong Cotton effects, indicating that ADPbinding induced a preferred helical handedness to the resultingassemblies of achiral NDIs. The binding of ATP inducedopposite handedness to NDI assemblies, as evident from thepositive bisignated CD signal, with positive and negative maximaat 390 and 359 nm, respectively. The mirror-image Cottoneffects of NDPA-Bola assemblies obtained with ADP and ATPindicated the induction of chirality with opposite handedness.Interestingly, addition of 0.5 equiv of ATP to NDPA-Amph/ADP assemblies resulted in positive bisignated CD signals whichexactly match with those of NDPA-Amph/ATP stacks alone.This clearly suggested the competitive replacement of ADP byATP from the assemblies as expected and an instantaneousreversal of its helical handedness. The authors also revealed adynamic helix reversal procedure through an intrastackmechanism.4.3.6. Sonication. Ultrasound is often used as a source of

energy to cleave and homogenize H-bonding, π−π stacking, andhydrophobic interactions of molecular building blocks and toreshape the packing mode and the morphology.211−216

The self-assembly of bichromophoric perylene bisimide intochiral nanostructures, and the supramolecular helicity of thenanostructures could be controlled by varying the method of

preparation.217 The aggregates prepared by the heating−coolingmethod possessed ordered molecular packing and enhancedoptical chirality. In contrast, ultrasonication resulted in molecularaggregates with less ordered packing and opposite supra-molecular chirality to the sample prepared via a heating−coolingmethod. This heating−cooling method caused the nanofibers tohave extended length and a prominent helical twist. The S isomergave left-handedM helices, and R isomers provided right-handedchiral sense. In contrast, the assemblies prepared by theultrasonication method exhibited thinner fibers (10−15 nm)with the opposite twist in helices for the corresponding isomers,left- (M) and right-handed (P) twists for the R and S isomers,respectively. The procedure which uses a heating−cooling cycleis thermodynamically driven and results in the formation of morestable nanostructures. In contrast, ultrasonication is a fast processleading to a kinetically stable product in a shorter time frame. Thetunable chiroptical properties in these supramolecular systemsmake them potential candidates for applications in the field ofoptical and electronic device fabrication based on organicnanostructures.

Figure 54. (A) Molecular structure of pillar[5]arene, competitive guests (G1 andG2), and a competitive host ([24,8]). (B) Representations of (a) theplanar chiral inversions triggered by achiral guestG1 and (b) alternating addition of achiral guestG2 and host [24,8]. Reprinted with permission from ref209. Copyright 2013 John Wiley & Sons.

Figure 55. (a) CD spectra and (b) schematic of the dynamic helicalreversal of NDPA-Amph/ADP assemblies upon competitive guestbinding experiments with ATP (c = 7× 10−5M, 70% aqHEPES buffer inTHF). Reprinted with permission from ref 210. Copyright 2012 RoyalSociety of Chemistry.

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4.3.7. pH Value. The helicity inversion can often be found tobe triggered by the change of pH value through the change ofconformation of peptide and the interaction between buildingblocks that are based on amino acids.218−220

Lednev et al. reported that a small pH change initiatedspontaneous transformation of insulin fibrils from onepolymorph to another.220 These authors found that the sign ofthe VCD band pattern from filament chirality can be controlledby adjusting the pH of the incubating solution, above pH 2 for“normal” left-hand helical filaments and below pH 2 for“reversed” right-hand helical filaments. Later, they extendedthis to other proteins and peptide fragments and again found thatpH variation triggered filament chirality change.

4.4. Chiral Amplification in Supramolecular Systems

Amplification of chirality is a well-known phenomenon inclassical covalent polymers, the pioneering studies of which wereperformed by Green and co-workers using the poly-(alkylisocyanates) system.221 They defined two effects thatinfluence the amplification of chirality and named them the“sergeants-and-soldiers” principle and the “majority-rules”effects. In recent decades, interest in the amplification of chiralityhas broadened to supramolecular polymer systems based onnoncovalent interactions. In the supramolecular assemblies,chiral amplification has been described as a phenomenon wherelocal chirality of a small fraction of chiral bias decides the chiralsense of the entire assembly and is in general followed bymanifestation in the CD signals (Figure 58). The two principles,sergeants-and-soldiers and majority rules, often describe thestrong amplification of a small chiral imbalance at the molecularlevel to a supramolecular chirality. The basic concepts ofamplification of chirality in the self-assembled systems areillustrated in Figure 58.The key challenge in the case of the sergeant-and-soldiers rule

is how small can the amount of sergeant molecules be while still

controlling the supramolecular chirality of the system, and in thecase of majority rule, how small can a chiral bias be while stilldetermining the chirality of an entire system.In the first case, a large amount of achiral units (the soldiers)

obey the rule of a small number of chiral molecules (thesergeants). Majority rules refers to a slight initial excess of a singleenantiomer leading to a strong bias toward the same helical sensein the whole aggregate.

4.4.1. Analogue-Induced Chiral Amplification.Meijer etal.94 first attempted to apply these two rules to explain the chiralamplification in supramolecular systems in solutions. When asmall amount of a chiral disc-shaped molecule was added to asolution of an achiral analogue in hexane, the CD effect of thebipyridine transition showed a nonlinear response to the amountof chiral sergeant.94,96,127,222 Fitting the data to a theoreticalmodel showed that on introducing on average one molecule ofchiral 32a per 80 molecules of achiral 32b, the chiral component(the sergeant) dictated the helical sense of the total stack (ofsoldiers). Themixtures of the enantiomers 32a and 32c showed anonlinear response of the CD effect on the enantiomeric excess,indicating that chiral amplification in these systems correspondsto the majority-rules effect.Many of the supramolecular systems were found to obey these

rules. Coronenebisimides (CBIs), as potential candidates fornovel liquid-crystalline materials and active n-type semi-conductor molecules in organic electronics, were assembled innonpolar methylcyclohexane.223 Derivatives of CBIs bearingchiral and achiral 3,4,5-trialkoxyphenyl groups at the imideposition (85 and 86) self-assembled mainly through π-stackingand van derWaals interactions inmethylcyclohexane, resulting inlong 1D fibrillar stacks. Different amounts of 85 werecoassembled with 86 (c = 2.5 × 10−5 M), and their chiropticalproperties were probed. Even with a small amount (3%) of thechiral derivative (85) as the sergeant the CD spectrum of thecoassembly showed a bisignated Cotton effect (Figure 60a). Theanisotropy factor or g value Δε/ε monitored at λ = 320 nmshowed nonlinear behavior (Figure 60b), which reached thecorresponding value of the pure chiral assembly at around 50% ofthe sergeant. This result suggested a chiral amplification based onsergeants-and-soldiers rule in this system.The effect of chemical structure on the amplification of

chirality was studied by systematic variation of the chemicalstructure of benzene-1,3,5-tricarboxamide derivatives 87−92(Figure 61).224 Since each BTA comprises three side groups,

Figure 56. Schematic illustration of the difference in molecular packingleading to reversal of supramolecular chirality of the aggregates formedby the twomethods. Reprinted with permission from ref 217. Copyright2011 American Chemical Society.

Figure 57. AFM images of prion fibrils grown in pH 2.0 (a) and 3.9 (b).Reprinted with permission from ref 220. Copyright 2014 AmericanChemical Society.

Figure 58. Illustration of chiral amplification and the model of twoprinciples: sergeants-and-soldiers rule and majority rule.

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asymmetrically substituted monomers have been synthesized tostudy the effect of the number of stereocenters and the positionof the stereogenic center on the degree of chiral amplification.First, when the position of the stereogenic center in asymmetri-cally substituted BTAs is varied, an odd−even effect can bediscerned, which was characterized by the appearance of apositive Cotton effect as for (R)-89 and (R)-91 and a negativeCotton effect for (R)-90. For the (R)-89:88 and (R)-91:88mixtures, a net helicity of 1 is obtained at a sergeant fraction of0.15 in both of these systems, while more than 30% sergeant (R)-90 is needed to obtain a net helicity of 1 in the (R)-90:88mixture.Along with sergeants-and-soldiers experiments, the authors

further performed majority-rules experiments. In contrast withthe weaker amplification of chirality of (R)-90 in sergeants-and-

soldiers experiments, it was found that 90 showed a strongeramplification of chirality in majority-rules experiments, i.e., a nethelicity of 1 was reached at lower ee for 90.In the study of the influence of the number of stereogenic

centers on the chiral amplification, the results suggested thatlowering of the number of stereocenters caused an enhancementof the degree of chiral amplification.224

To quantify the majority-rules and sergeants-and-soldiers data,Meijer et al. proposed two free energy penalties, i.e., HRP andMMP, in the chiral amplification in supramolecular assemblies.Herein, the HRP (helix reversal penalty) describes the energypenalty of a helix reversal in the aggregate.225,226 This energypenalty is paid when in a helical stack of these building blocks thehandedness of the stack is reversed, i.e., going from a left-handedto a right-handed helical segment or vice versa. The HRP value is

Figure 59. (a) Hydrophobic disc-shaped compounds with bipyridine units (32a−c). (b) Amplification of chirality observed upon mixing solutions of32a and 32b in hexane results in a nonlinear relationship between the CD effect and the amount of chiral 32a added to achiral 32b. (c) Net helicity as afunction of enantiomeric excess measured by CD spectroscopy of mixtures of 32a and 32c in n-octane. The line indicates the theoretical result that givesthe closest agreement with the experiment. Reprinted with permission from ref 96. Copyright 2011 American Chemical Society.

Figure 60.Coronene bisimide molecules 85 and 86 and coassembly of 85 and 86 and resultant chiral amplification. All experiments were done inMCH(c = 2.5 × 10−5 M). (a) CD spectra of the coassembly at different percentages of 85 in a 1 cm cuvette at 20 °C. The arrow indicates the spectral changewith an increase in the percentage of the sergeant. (b) Anisotropy value or g value monitored at λ = 320 nm as a function of the percentage of 85. Thedashed line that connects the fraction of 85 indicates the linear variation of the g value in the absence of any chiral amplification. Reprinted withpermission from ref 223. Copyright 2013 John Wiley & Sons.

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related to intermolecular interaction, as once a handedness ischosen strong intermolecular interactions are favorable tomaintain this handedness throughout the stack. The MMP(mismatch penalty) is related to the incorporation of a chiralmonomer in a helical aggregate of its unpreferred helicity. Inmajority-rules and sergeants-and-soldiers experiments, the HRPwould be very similar, since it is related to the intermolecularinteractions. The MMP, on the other hand, has different physicalmeaning in the two types of experiments. For the sergeants-and-soldiers experiment, a MMP arises when the chiral sergeant isincorporated in a stack of achiral molecules of its unpreferredhelicity. For the majority-rules experiment, theMMP arises whenone chiral enantiomer is incorporated in a stack formed fromchiral monomers of opposite stereoconfiguration with corre-sponding opposite helicity. For the BTA derivatives discussedabove, the HRP value is similar in all systems, but the MMP isdirectly related to the number of stereocenters present in themolecules. Increasing this number from one to three resulted inan increase in this energy penalty while leaving the HRPunaffected. These findings can help gain a better understandingof the ultimate limits of chiral amplification.Elemans et al. further illustrated the effect of molecular

structure on chiral amplification through the synthesis of ananalog of porphyrin trimers based on benzenetricarboxyamide(BTA), 93, 94, 95, and 96.99 In this case, the chiral porphyrin

trimer 94 would be able to act as a sergeant and control theoverall helicity of a columnar stack consisting of soldiers of 93.However, it was surprising to find that a total absence ofamplification of chirality was found in this system. The authorproposed that the interactions between 93 and 94were not in thecorrect balance to express chirality from the sergeants to thesoldiers. For porphyrin trimers 95 and 96, which, in comparisonto trimers 93 and 94, lack three of the four meso-phenyl rings atthe porphyrin moieties, intermolecular interactions betweenthese molecules in the columnar assemblies would be strongerthan between the molecules of 93 and 94 in their respectivestacks. This is because 95 and 96 can approach each other as theresult of the reduced steric hindrance and thus enhance theintermolecular π−π stacking interactions. The amplification ofchirality in toluene was thus very successful, while in n-heptane itwas completely absent. This might be due to the columnar stacksof porphyrintrimers in n-heptane which are kinetically inertassemblies, and as a result, the chiral and achiral stacks cannotdynamically exchange their building blocks. The sergeant andsoldiers experiments have proven to be an excellent method forrevealing this behavior. The work presented here shows theinfluence of variations in the molecular structure and the choiceof solvent amplification of chirality on system chiralamplification.

Figure 61. CD spectra of mixtures of (R)-89:88 (A), (R)-90:88 (B), and (R)-91:88 (C). Net helicity versus fraction of sergeant for mixtures of (R)-89:88, (R)-90:88, and (R)-91:88 (D). Reprinted with permission from ref 224. Copyright 2010 American Chemical Society.

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4.4.2. Chiral Amplification in Binary Systems. Chiralamplification was also studied in binary complex systems.227 Inthis case, melamines equipped with two PBI chromophores andtwo 3,7-dimethyloctyl chiral handles were mixed with cyanuricacid to form a discotic supramolecular complex (CA).228 It wasfound that the sergeants-and-soldiers effect where a few chiralbuilding blocks can control the helical sense of the large numberof structurally related achiral ones was not applicable for thissystem. When mixed chiral 97S was used as sergeant with theoptically inactive achiral 97A as the soldiers, plots of Δε versusthe amount of 97S for these ternary mixtures (97S/97A/CA)showed a decrease in their optical activities compared to thechiral complex, indicating the absence of any sergeant-and-soldiers effect. In contrast to the sergeants-and-soldiers effect, theother chiral amplification effect, the majority-rules effect,occurred at the level of the hydrogen-bonded complexes.Enantiomeric mixtures of 97S and 97R showed almost lineardependence of Δε on ee in the absence of CA (recipe i),suggesting that the self-aggregation of the enantiomers (self-sorting) or coaggregation occurs; they obey their own preferredhelicities. In the presence of CA (recipe ii), the Δε/ee plots

indeed diverged from linearity, indicating that the amplificationof chirality obeyed the majority rules and occurred at the level ofthe hydrogen-bonded complexes.

4.4.3. Chiral Amplification to Nanoscale. Chiralamplification can be expressed in the form of nano/micro-structures. Compound 98 is an achiral molecule and the analogueof compound 99, which has four chiral centers in its alkyl chains.The self-assembly of 98 could only give a flat nanostructure.However, when 98 was coassembled with 99, the flat lamellaewere transformed into twisted ribbons.229 The presence of only 5mol % of the sergeant 99 was able to transfer the chiralityembedded in the peripheral chains to the remaining 95% of thesoldiers (98), as revealed by the corresponding SEM images, inwhich the micrometer-long twisted ribbons of high aspect ratioappeared. The presence of stereogenic centers in the coassemblyof achiral 98 with chiral 99 provoked the chiral propagation ofthe H bonding of the amide functionalities reinforcing theformation of twisted ribbons. Increasing the percentage of chiralsergeant 99 in the coassembly caused the formation of twistedribbons to increase. The results represent an excellent example ofthe study of homochirality on surfaces and, at the same time,

Figure 62. (A) Molecular structures of the porphyrin trimers 93−96. (B) Schematic representation of the self-assembly of porphyrin trimers in helicalcolumnar stacks. Reprinted with permission from ref 99. Copyright 2012 Royal Society of Chemistry.

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contribute to the knowledge about the set of rules governing thegeneration of chiral objects that hold great potential for thedevelopment of supramolecular devices.4.4.4. Unexpected Amplification in Racemate Assem-

blies. Liu et al. found an interesting chiral amplification in self-assembled systems based on L- and D-alanine derivativescontaining an N-fluorenyl-9-methoxycarbonyl (Fmoc) moietyand a long alkyl chain.230 It has been found that both theenantiomeric and the racemic assemblies showed CD signals.The enantiomer 100 showed mirror-imaged CD spectra, and thesign of the CD spectra for the assemblies followed the molecularchirality. However, the supramolecular chirality of the racemateassembly was not certain. It was revealed that the slight excess ofone enantiomer in the racemic mixtures may result in an activeCD signal since the exact 1:1 mixture at a molecular level cannotbe reached. When mixing the two enantiomers (100L/100D)with different molar ratios it was found that an excess of 100Lresulted in a negative Cotton effect, whereas mirror-imaged CDspectra were obtained for mixtures with an excess of 100D.Furthermore, the CD signals observed for a nonequimolarmixture of the enantiomers were more intense than those for thepure enantiomers. For the system obeying the majority-rulesprinciple, the CD intensity generally decreased when theirmixing ratio deviated from the pure one. However, in the case ofthe mixed 100L/100D system, the CD signals intensified whenthe mixing ratio approached 1:1, as shown in Figure 65. Thisindicated that the self-assembly of the racemic mixture is verysensitive to a slight enantiomeric excess and the system could beused for the detection of a broad range of chiral amino acidderivatives.

4.5. Chiral Memory in Supramolecular Systems

The phenomenon of chirality memory describes a supra-molecular system in which chirality is first induced and thenmaintained after the chiral source is erased or replaced by anachiral component. Thus, these complexes have a memory forthe chirality of the species that induced the system’s asymmetryafter the removal of these inductor molecules.Generally, chiral memory is difficult to induce in noncovalent

supramolecular assemblies, partly because additives ofteninterfere with the noncovalent interactions that hold theassembly together. However, in recent decades, more efforts toexplore supramolecular chirality memory systems have proven tobe successful through the efforts to design the chiral and achiralunits and control the dynamics. In order to realize the chiralmemory, there are several important elements. First, the inducedchiral nanostructure should be generally stable. Thus, even whenyou remove the chiral species, the chirality is maintained. Second,a small amount of the chiral substance should be able to inducechirality in the system. A successful chiral memory system maycontain (1) noncovalently induced helical polymers, (2) strongaggregates from achiral building blocks such as J or H aggregates,or (3) chiral cages from coordination compounds.

4.5.1. Helicity Memory in Noncovalently-InducedHelical Polymers. Helicity memory in the noncovalentlyinduced helical polymers has been thoroughly investigated byYashima and co-workers.174 On the basis of a chiral memorysystem of noncovalent helicity induction in optically activepolymers, Yashima et al. developed an excellent macromolecularmemory system of a helical polyacetylene in the solid state.231

They synthesized a polyacetylene derivative, 101, and induced itspreferred-handed helicity in the presence of (S)-phenylethanol inn-hexane through weak hydrogen-bonding interactions. 101 was

Figure 63. (a) Chemical structures of 97 and CA. (b) Proposed structure of 3:1 hydrogen-bonded complex 97S·CA. (c) Schematic illustration of helicalcolumns. Reprinted with permission from ref 228. Copyright 2011 John Wiley & Sons.

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then recovered by filtration followed by washing with methanolto completely remove (S)-phenylethanol. Upon furtherdissolving 101 in n-hexane at 20 °C, an apparent ICD bandexhibited an intensity that increased with time. This intensityfinally became nearly equal to that induced in an n-hexanesolution in the presence of (S)-phenylethanol after 1 h. Themacromolecular helicity memory in the solid state was morestable than in solution, because the ICD intensity in the solidstate persisted for at least 11months at 25 °C.More interestingly,by simply immersing the polymer with macromolecular helicitymemory by (S)-phenylethanol in a solution containing (R)-phenylethanol followed by washing with methanol, the helicity of101was completely inverted and memorized. Thus, there was nodoubt that a preferred-handed helix of the optically inactivepolymer 101 was induced via weak noncovalent bondinginteractions in the solid state simply by immersing 101 in anonracemic liquid, and this helicity could be memorized andswitched automatically. This chiral memory and switchablesystem in the solid state will be beneficial to the development of achiral stationary phase for the separation of enantiomers.Inoue and Takashima et al. designed a m-ethynylpyridine

polymer that has a metal coordination site at the 4 position ofeach pyridine unit, which showed a chiral memory effect on a m-ethynylpyridine oligomer.232 It is found that the polymer canform CD-active helical complexes with various kinds of guestsaccharides by the interaction of hydrogen bonds between the

Figure 64. (Top) Structure of bisamides 98 and 99, schematicillustration of the self-assembly of 98 into sheets, and the coassembly of98 and 99 into twisted ribbons. (Bottom) SEM images of twistedribbons formed by the coassembly of the achiral soldier 98 and chiralsergeant 99 at different percentages of chiral component: 95/5 (a andb), 90/10 (c), and 85/15 (d). Reprinted with permission from ref 229.Copyright 2010 Royal Society of Chemistry.

Figure 65. (A)G values centered at 309 and 255 nm as a function of theee value of nonequimolar mixtures of 100L and 100D. The G value ofthe racemate was set to zero. (B) SEM images of the nanostructuresformed from mixtures of 100L and 100D with various ee values.Reprinted with permission from ref 230. Copyright 2013 John Wiley &Sons.

Figure 66.Reversible switching andmemory of macromolecular helicityof 101 in the solid state. Preferred-handed macromolecular helicity of101 is induced and subsequently memorized in the optically inactive101 via noncovalent interactions with a nonracemic alcohol (S- or R-phenylethanol) followed by complete removal of phenylethanol in thesolid state. The polymer’s helical handedness and axial twist sense areswitched reversibly in the solid state in the presence of the oppositeenantiomeric alcohol. Reprinted with permission from ref 231.Copyright 2014 Nature Publishing Group.

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nitrogen atoms of the pyridine rings and the hydroxy groups ofthe saccharides. Moreover, the ICD band was remarkablyenhanced by the addition of Cu(OTf)2 (0.5 equiv to pyridineunits) and o-phenanthroline (phen) as a result of stabilization ofthe helical structure of the polymer. Even when an equimolaramount of another enantiomer of glucopyranoside was added tomake the whole system apparently racemic, the ICD of theethynylpyridine polymer was memorized and remained forseveral weeks.

4.5.2. Chiral Memory in Aggregates Such as J and HAggregates. Purrello et al. reported that a number of self-assembly systems could memorize the chirality of complexeswhen the chiral auxiliary had been removed. They first inducedthe formation of aggregates between the cationic porphyrinCuTMP and anionic porphyrin H4TPPS, 63, with the chiralmatrix of either L- or D-polyglutamic acid.233 Upon the formationof the ternary complex, the induced CD of binary CuTMP andTPPS was obtained in the α-helix structure of polyglutamic acid.Interestingly, the induced CD signals of the porphyrin complexesremained even when the conformation of the polyglutamic acidwas switched to a random coil by increasing the pH to 12. Thissuggested that during the formation of the ternary complex thechirality of the polymer was transferred to and memorized by theporphyrin complexes. Interestingly, the chiral complex was stableand maintained even when a 5-fold excess of a competing andantipodal chirality source; namely, poly-D-glutamate, was addedto the CuTMP/TPPS/poly-L-glutamate system. A similar self-assembly memory system based on the aggregation of theoppositely charged CuTMP and TPPS in the presence ofenantiopure aromatic amino acids was also found.234−238

Further, Shi et al. and He et al. found simple TPPS J aggregatesalso showed chiral memory effects.239−242 For instance, He etal.242 found that with the existence of L- or D-enantiomers of cis-[CoBr(NH3)(en)2]Br2 as chiral triggers for the J aggregates ofachiral 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin(TPPS), 67 could be fabricated into chiral assemblies, duringwhich the metal-centered chirality can be transferred to the Jaggregates. In addition, the chirality was memorized in theporphyrin J aggregates. These authors initially synthesized theporphyrin aggregates with the L-Co(III) complex, and then the D-Co(III) complex was added to the system. During the addition ofthis opposite D-Co(III) chiral species, the UV absorption at theTPPS Soret band (434 nm) gradually decreased; however, theinduced CD signal of the porphyrin aggregates increased (ratherthan inverting) after the addition of excess D-Co(III) complex.This suggested that the chirality of TPPS J aggregates induced byone enantiomer of Co(III) coordination was maintained evenwith the addition of an excess of the opposite enantiomer of theCo(III) complex.Meijer et al. developed a class of highly tunable porphyrin-

based chiral memory system based on the chiral Zn porphyrins

and achiral Cu porphyrins through a dynamic control ofassemblies.243,244 The chiral Zn porphyrins were used as asergeant to transfer their chirality to the achiral Cu−porphyrinsoldiers. After the sergeant was removed from the coaggregatesby axial ligation with a Lewis base (quinuclidine), the chiralinformation in the remaining aggregate was preserved as a resultof slow conformational dynamics, which revealed a chiralmemory effect.Such chiral memory based on the aggregation has been

expanded to other systems. Jiang et al. established a J aggregate ofan achiral perylene dianhydride (PDA) in CTABmicelle solutionand employed small molecule D- and/or L-tartaric acid as thechiral auxiliary to induce transfer of the chirality to PDAaggregates.245 The ICD signal of the J aggregates was also foundto remain unchanged upon addition of a large excess of thealternative enantiomer of tartaric acid, implying the imprinting ofthe chirality in the J aggregates.Aida et al. synthesized an elaborate “nanotubular” helical

architecture with 60% de (80:20 diastereomeric ratio) by the self-assembly of a hexabenzocoronene derivative, HBCPy, carrying achiral (BINAP)Pt(II) moiety as a detachable chiral auxiliary. Theoptically active nanotubes did not racemize after removing thechiral auxiliary through the addition of ethylenediamine. Oncethe helical tubular structure of HBC formed, the addition of(BINAP)Pt(II) with an absolute configuration opposite to theoriginal one did not cause the helical inversion. These results

Figure 67. Assumed mechanism of helix stabilization of m-ethynylpyridine host oligomer 102 by coordination of copper andphen inside the helix. Reprinted with permission from ref 232.Copyright 2012 Royal Society of Chemistry.

Figure 68. Schematic structures of H2TPPS and Δ- or Λ-Co(III)complexes and Schematic illustration of the induction, memory, andamplification of chirality in H2TPPS with Λ- or Δ-Co(III) complexes.Reprinted with permission from ref 242. Copyright 2010 Royal Societyof Chemistry.

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demonstrated a good example of a stereochemical memory effectin HBC nanotubular helical architecture.246

4.5.3. Helicity Memory in Chiral Cages from Coordina-tion Compounds. A class of FeII4L4 capsules with chirotopiccavities has been prepared by the in situ metal-templated iminecondensation between tris(formylpyridyl) benzene and a chiralamine.247 This capsule maintained the stereochemistry of thecage framework (99% ee) even when the chiral amine wasreplaced by an achiral one. The cage retained its stereochemistryafter 4 days at 90 °C. The author inferred that the strongcooperative stereochemical coupling between the iron(II)

stereocenters of the structure enabled retention of configurationupon replacement of the chiral subcomponents. This memoryeffect allows for the stereoselective preparation of a metal−organic capsule that ultimately contains only achiral subcompo-nents, which can extend the application of metal−organiccapsules in stereoselective guest recognition and sensing and asasymmetric reaction vessels.

5. SPONTANEOUS SYMMETRY BREAKING ANDEMERGENCE OF SUPRAMOLECULAR CHIRALITY INSELF-ASSEMBLED SYSTEMS FROM EXCLUSIVELYACHIRAL MOLECULES

Through self-assembly, not only chiral molecules but alsocompletely achiral molecules can form chiral supramolecularassemblies. This situation results from spontaneous symmetrybreaking,10 which is one of the most important issues inobtaining assemblies with macroscopic chirality248 or opticalactivity249 instead of producing the same amount of theenantiomeric nanostructures. On the other hand, there is a realpossibility that the origin of life could have depended onmolecular chirality and supramolecular chirality.250 Although westill do not clearly know the origin of natural homochirality,supramolecular chirality can now be created from achiralmolecular building blocks. In general, asymmetric environmentsare necessary for symmetry breaking, which provide thesupramolecular chirality to the assemblies of achiral mole-cules.236,251−253 However, the most intriguing possibility wouldbe symmetry breaking without discernible chiral conditions. Inthis section, we will show examples where supramolecularchirality emerged from achiral building blocks.

Figure 69. (A) Chiral/achiral amide-functionalized zinc/coppertetraphenylporphyrins. (B) Schematic depiction of selective depolyme-rization with chirality retention and temperature-induced switching ofthe chiral memory. Reprinted with permission from ref 244. Copyright2010 American Chemical Society.

Figure 70. Schematic illustration of a series of experiments for investigating the dynamic nature of nanotubularly assembled HBCPy. Reprinted withpermission from ref 246. Copyright 2013 American Chemical Society.

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5.1. Liquid-Crystal and Banana-Shaped Molecules

Even though the origin of chirality and natural homochirality hasattracted much attention over the past decades, supramolecularchirality resulting from the self-assembly of achiral molecularbuilding blocks has been of major interest as a result of the earlywork conducted on liquid crystals. The first report of this can befound in the work of Young and co-workers.254 These authorsdesigned and synthesized a series of stilbene derivatives andstudied the possibility of forming nematic liquid crystals from theassembly of these molecules. Interestingly, with the racemicstilbene mixtures they observed small cholesteric liquid-crystalline regions in the nematic field under polarized lightmicroscopy.In the use of achiral molecules to form chiral liquid crystals, the

most famous building blocks are the banana-shaped or bent-coreachiral molecules.255 In the field of liquid crystals, the self-assembly of banana-shaped achiral molecules has been widelyinvestigated. For example, Tschierske and colleagues studied theliquid-crystalline phases formed by silicon-containing polyphilicbent-core achiral molecules 104. These authors found thattemperature-induced inversion of chirality in a supramolecularsystem formed by achiral molecules can be established (Figure72).256

The research group of Cheng constructed chiral propellersfrom the self-assembly of achiral molecules (BPCA-Cn-PmOH),which were composed of 4-biphenylcarboxylic acids connectedwith phenol via alkoxyl chains 105. The achiral BPCA-Cn-PmOH molecules can form individual head-to-head dimers, andthe twisting of the dimers can lead to chiral N phases. Theseresults demonstrate that neither molecular chirality nor amolecular bend is necessary to form a chiral phase (Figure 73).257

For chiral assemblies formed by banana-shaped (bent-core)achiral molecules, a notable work is that of Hough, Clark, and co-

workers (Figure 74).258 In this instance, the self-assembly ofbanana-shaped (bent-core) achiral molecules 106 can result instrong supramolecular chirality. Most interestingly, theseassemblies do not exhibit anisotropy at the macroscopic scale.These assemblies are macroscopically isotropic fluids thatpossess only short-range orientation and positional order, justlike a true liquid. Therefore, the self-assembly of achiralmolecules was found to form isotropic fluids with supramolecularchirality. In particular, the assembly of bent-core achiralmolecules can form a phase that exhibits smectic layering withfluid order within the smectic layers. Thus, the coherent length ofthe smectic layering is very short, smaller than ca. 100 nm. Themechanism of formation of this property has been attributed tothe formation of saddle−splay deformations involving the elasticconstant K24. Saddle−splay director fields are not space filling,and smectic layers are unstable in saddle−splay deformations andthus incompatible with long-range order.With respect to the banana-shaped (bent-core) achiral

molecules forming supramolecular chirality, Hough, Clark, andco-workers (Figure 75) also studied helical nanofilament phases,in which a local chiral structure is expressed as twisted layers.259

Although composed of achiral molecules 107, the layers in thesefilaments are twisted and rigorously homochiral, demonstratingbroken symmetry. This work was published in the same issue ofScience as that of isotropic chiral fluids. In contrast to isotropic

Figure 71. Route i: formation of racemic cage 2 through subcomponentself-assembly. Route ii then route iii: enantioselective formation of cage2 through subcomponent substitution. Reprinted with permission fromref 247. Copyright 2013 American Chemical Society.

Figure 72. Molecular structures of silicon-containing bent-core achiralmolecules, and the temperature-induced inversion of supramolecularchirality. Reprinted with permission from ref 256. Copyright 2006American Chemical Society.

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chiral fluids assembled by achiral building blocks, these helicalnanofilament phases exhibit birefringence, indicating long-scaleordering.In the production of helical nanofilament phases from the

assembly of banana-shaped (bent-core) achiral molecules,Takezoe and co-workers developed a mixture system containing

both bent-core molecules and rod-like molecules.260 Thisprocess can be considered a general method for producinghomochiral helical nanofilaments. Banana-shaped molecules(108, 109) are in the B4 phase, while rod-like molecules (110,111) can form a nematic phase. The mixture of differentmolecules has the phase sequence N−Bx(B4/N), and homo-chiral helical nanofilaments can be obtained upon cooling(Figure 76).261

To understand the helical nanofilament phases assembled bybanana-shaped (bent-core) achiral molecules, the correspondinghierarchical nanostructures have attracted much attention. It wasfound that the arrangement of different helical nanofilaments canbe very important to the assembly process. Jakli et al. found thatsome properties, especially structural color, of the B4-phaseliquid crystals formed by bent-core achiral molecules 112 cannotbe explained by the nanostructures of nanofilaments alone. Intheir study, they found that different helical nanofilaments do notform parallel packing. Instead, these helical nanofilaments werearranged at an angle of 35−40° with respect to each other,forming a doubly twisted nanostructure, which caused theunusual structural color of these liquid crystals (Figure 77).262

5.2. Solution Systems, Micelles

Asymmetric environments or conditions are usually necessary forsymmetry breaking within supramolecular systems, in which theaggregation of achiral molecular building blocks can lead tosupramolecular chiral assemblies. Thus, chiral dopants202 or achiral matrix263 can assist achiral molecules to assemble intochiral nanostructures. Solid surfaces264,265 also can provide two-dimensional confined environments that lead to the symmetrybreaking.

Figure 73. (A) Structure of the dimeric building block of BPCA-C7-PmOH 105. (B) Chiral propellers from the self-assembly of this achiralmolecule 105 (BPCA-Cn-PmOH). Reprinted with permission from ref257. Copyright 2006 John Wiley & Sons.

Figure 74. (A) Structures, phase sequences, layer spacing, andcorrelation lengths (as measured by XRD and freeze−fracture TEM)of achiral 106. Freeze−fracture TEM images of the dc phase of 106. (B)Frustration between molecular fragments in a tilted bent-core moleculecan be relieved by saddle-splay curvature of the layers. Reprinted withpermission from ref 258. Copyright 2009 The American Association forthe Advancement of Science.

Figure 75. (A) Structure and phase sequence upon cooling of the fourB4 phase-forming compounds studied. (B) Mechanism and TEMimages of hierarchical self-assembly of the nanofilament (NF) phasestarting with bent-coremesogenic molecules. Reprinted with permissionfrom ref 259. Copyright 2009 The American Association for theAdvancement of Science.

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In a homogeneous solution, the birth of chiral informationfrom totally achiral systems can be very subtle, so there are no

obvious approaches to make the achiral molecules form chiralassemblies in organic or aqueous solution. Indeed, there are somereports showing that achiral molecules self-assemble into helicalnanostructures. However, within these systems, the mixture ofboth left- and right-handed helices in equal amounts does notfulfill the requirements for true supramolecular chirality.266−268

In this case, the assemblies are overall racemic mixtures and showno macroscopic optical activity with a silent CD.Researchers in the field of supramolecular chirality are very

interested in the formation of assemblies with unequal amountsof left- and right-handed helices using purely achiral molecularbuilding blocks in solution. This type of symmetry breakingshould be able to be detected by CD spectral measurements.There are only a few published reports of the aggregation of

achiral molecules in solution to form supramolecular chirality.Amazingly, the first paper on this issue was published in 1996.269

Dahne and co-workers found that one achiral charged dyemolecule containing long alkyl chains (113) can form supra-molecular chiral assemblies in solution. In this system, two typesof noncovalent interactions play very important roles. Thus, boththe self-association of organic dyes and hydrophobic interactionsbetween long alkyl chains cause the achiral molecules to undergoJ aggregation with supramolecular chirality, which can beconfirmed by CD spectra. It is suggested that the dye moleculecan form twisted herringbone-type assemblies, which maypossibly lead to supramolecular chirality (Figure 78).The Liu group studied the aggregation of achiral molecular

building blocks for forming chiral assemblies in oil/water mixedmicelle dispersions.270 The work is based on a surfactant-assistedself-assembly (SAS). In this work, an aqueous solution ofcetyltrimethylammonium bromide (CTAB) and a chloroformsolution of another achiral molecular building block was added

Figure 76. (A) Chemical structures of bent-core and rod-likecomponents of the mixtures: (a) 12OAzo5AzoO12 (108), (b) P8-O-PIMB (109), (c) 5CB (110), and (d) 5PCB (111). (B) AFM image ofBx surface. (C) (a) UV−vis absorption spectra of 12OAzo5AzoO12(108) (bent-core) and ZLI2293 (rod-like). (b) CD spectra of the Bxphase. (c) CD spectra showing a longer wavelength peak made fromTNcells of various cell thicknesses: 0.4, 0.6, 0.7, and 0.9 μm. (Inset) CDintensity as a function of cell thickness. Reprinted with permission fromref 261. Copyright 2013 John Wiley & Sons.

Figure 77. (A) Molecular structure and phase sequence of PnOPIMB (112) (n = 7, 8, 9 and 12) during cooling at 1 °C min−1. (B) TEM image andmodels showing the double-twist structure. Reprinted with permission from ref 262. Copyright 2014 Nature Publishing Group.

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dropwise into the aqueous solution with stirring. This method isrelated to the formation of a microemulsion, where thechloroform evaporates during the self-assembly process. Usingthe SAS method via an oil/aqueous medium, supramolecularassemblies with different nanostructures and properties could beobtained in a controllable manner from very simple achiralmolecular building blocks. For example, when a chloroformsolution of zinc 5,10,15,20-tetra(4-pyridyl)-21H,23H-porphine(ZnTPyP) (114) was added dropwise to a cetyltrimethylammo-nium bromide (CTAB) aqueous solution, different nanostruc-tures with varying supramolecular chirality can be obtaineddepending on the concentration of CTAB and the aging time.Using the 0.9 mM CTAB system for assembly with subsequentaging for 3 days, chiral nanorods can be produced by the CTAB-assisted self-assembly of ZnTPyP. The supramolecular chiralitywas proven by CD spectral measurements. However, when theconcentration of CTAB and the time for aging were changed,different supramolecular nanostructures, such as nanotubes andlong nanofibers, were produced by the CTAB-assisted self-

assembly of ZnTPyP. Interestingly, no supramolecular chiralitywas detected in these nanotubes or nanofibers (Figure 79).With regard to symmetry breaking upon self-assembly,

different achiral porphyrins have been used as model molecularbuilding blocks. In aqueous solution, the self-assembly of somewater-soluble porphyrins (such as tetrakis(4-sulfonatophenyl)-porphine, TPPS, 67) has been widely investigated and is furtherdiscussed below. On the other hand, the uncharged achiral water-soluble porphyrin was also found to form chiral assemblies inaqueous solution.Mineo et al. synthesized a TPP-based porphyrin containing

polyethyleneoxy substituents (PPeg4, 115).271 This unchargedporphyrin can spontaneously form self-assembled structures inwater. Although PPeg4 is achiral, this molecule forms a chiralassembly in water. Most interestingly, the supramolecularchirality from the assembly of PPeg4 was found to be inducedby a weak thermal force. In this case, the intensity of the CDsignal of the PPeg4 assemblies could be enhanced by increasingthe temperature (Figure 80).Meijer and co-workers recently investigated the symmetry

breaking of an assembly of achiral molecular building blocks inorganic solvents.272 They found that the self-assembly of achiralpartially fluorinated benzene-1,3,5-tricarboxamide molecules116 could produce supramolecular chirality (Figure 81A). Forthese systems, they carefully studied the kinetics of the self-assembly, and the unique two-step self-assembly behavior wasconfirmed. Moreover, they found that true symmetry breakingcould happen during a kinetically controlled secondarynucleation (Figure 81B). Thus, within the initial self-assemblyprocess, achiral partially fluorinated benzene-1,3,5-tricarboxa-mide molecules could assemble into equal amounts of one-dimensional left- (M) and right-handed (P) helical aggregates,whereas the systems as a whole did not show any optical activity.During kinetically controlled secondary nucleation, these one-dimensional helical aggregates could bundle into nanofibers,which could be optically active.272 Here, the partially fluorinatedmolecular structures are important, which produced the chiralbias that was then amplified by different noncovalentinteractions. Hydrogen bonding was considered to dominate

Figure 78. Spontaneous formation of supramolecular chirality in Jaggregates.

Figure 79. Schematic illustration showing the controlled synthesis of various porphyrin nanostructures with varied supramolecular chirality by means ofan SAS, where an oil/aqueous mediumwas employed. CD spectra of ZnTPyP (114) nanorods fabricated in sample B that was aged for 3 days. Black andred curves are the results detected from the samples prepared in different batches. Reprinted with permission from ref 270. Copyright 2010 AmericanChemical Society.

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the formation of the helical aggregates, while dipole−dipoleinteractions dominate the secondary nucleation.Hao and Sun et al. found that achiral bolaamphiphilic

azobenzene 117 was able to form chiral assemblies in aqueoussolution.199 It was shown that the precipitation process wasimportant. The achiral bolaamphiphilic azobenzene containingmany carboxyl groups (Figure 82A) was found to dissolve inwater at pH 6.56, but when the pH value was adjusted to 2.77 byadding HCl solution, the bolaamphiphilic azobenzene formedprecipitates. TEM and SEM measurements of the materialshowed that these precipitates consisted of nanotubes withsupramolecular chirality (Figure 82B), which was detected byCD spectral measurements (Figure 82C). Furthermore, since theazobenzene derivatives are photosensitive, the morphology andchirality of the self-assembled nanostructures derived from theachiral bolaamphiphilic azobenzene could change in response toexternal stimuli such as light and heat.An acidification process was also implemented for the self-

assembly of TPPS (67) in aqueous solution. TPPS is a well-known building block for the occurrence of symmetry breakingduring the self-assembly. The acidification of TPPS could help inthe formation of J aggregates with supramolecular chirality.Interestingly, the emergence of supramolecular chirality onTPPS assemblies in aqueous solution can be controlledkinetically by modulating the speed of the acidification of theporphyrin. In a recently published paper by Scolaro et al., detailedkinetic investigations of the self-assembly of acidified TPPSdemonstrate that the rate of the aggregation process stronglyaffects the chiral induction.273 In this study, the aggregationprocess in aqueous solution could be changed by adding theporphyrin as the first (PF) or last reagent (PL). PF represents theslow aggregation process, while PL represents the fast acid-ification and aggregation process. The CD spectral measure-

ments show that only the slow aggregation process can lead tosupramolecular chiral assembly (Figure 83).For symmetry breaking upon the self-assembly of achiral

building blocks, a recently published THF/water system is worthmentioning. Since THF and water are not completely miscible, aliquid/liquid interface is present in THF/water mixtures withvolume ratios of 1/1. When a THF solution of achiral carboxylazobenzene derivatives (Figure 84A) was added dropwise to anaqueous solution of melamine, supramolecular chiral assembliescan be obtained at the THF/water interface along with thepermeation and volatilization of THF (Figure 84C).274 For theformation of coassemblies, hydrogen bonding between carboxylazobenzene derivatives and melamine plays a very importantrole. TEM and AFM measurements show that these assembliesare long and helical fibers with intrinsic conformational chirality(Figure 84B). Furthermore, the morphology and chirality of thesupramolecular assemblies are photoresponsive, which isinduced by the photoisomerization of the azobenzenecomponents within the self-assembled nanostructures.For supramolecular chirality from an assembly of achiral

molecules in solution, it seems that π-conjugated molecules arealways necessary. Within these self-assemblies, the aromatic ringstend to overlap through π−π stacking. Thus, the displacement ofaromatic rings within the molecular packing shows a slight anglebetween neighboring rings, which will produce a chiral bias. Ifthis bias is repeated by forming a helix in a certain direction,supramolecular chirality would emerge and be amplified.

Figure 80. (A) Structure of PPeg4 (115). (B) Circular dichroismspectra at T = 26 °C (full line), 30 °C (dotted line), and 36 °C (dashedline) of the PPeg4 aqueous solution. Reprinted with permission from ref271. Copyright 2014 The Royal Society of Chemistry.

Figure 81. (A) Chemical structures of the partially fluorinated BTAs(116). (B) (a) AFM image of drop-casted solutions of BTA-F8H onmica (scale bar = 0.5 mm). (b and c) Results of stopped-flowexperiments for solutions of BTA-F8H. (d) Self-assembly mechanism of116. Reprinted with permission from ref 272. Copyright 2012 JohnWiley & Sons.

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5.3. Gel Systems

As a very important form of soft matter with extensive potentialapplications, supramolecular gels fabricated by noncovalentbonds have attracted great interest recently.275−280 Chiralsupramolecular gels are usually fabricated by chiral gelators andshow macroscopic optical activity and/or helical nanostructures.A few achiral gelators have been found to self-assemble intohelical nanostructures with an equivalent amount of left- andright-handedness,266,268,281 but macroscopic optical activity wasbarely detected from these supramolecular gels. However, thereare really some examples of optically active gels obtained fromachiral gelators.Kimura et al. synthesized an achiral disk-shaped molecule

having one imidazole unit (Figure 85A). This achiral molecule118was found to form a supramolecular gel in 2-methoxyethanoland assemble into long and twisted nanoscopic fibers (Figure85C). Although the molecular building block is achiral, it formssupramolecular gels with optical activity, which can bedemonstrated by CD spectral measurements (Figure 85B).During the emergence of the optical activity, it was found that theimidazole substituent is very important, because it increases theasymmetrical characteristics of the molecular building block and

helps the formation of hydrogen bonding. Thus, spontaneouschiral symmetry breaking through the steric effect of imidazoleupon gelation was achieved.282

Another chiral supramolecular gel assembled by achiralmolecules is also related to the complexation of the imidazoleunit with metal ions. You and co-workers demonstrated thatsimple achiral molecules containing an imidazole unit could formsupramolecular polymers by complexation with Ag+, which canfurther gel in a variety of solvents. The supramolecular metal gelsformed by simple achiral molecules can be optically active.283

Recently, Liu and Wang et al. synthesized an achiral C3-symmetric benzene-1,3,5-tricarboxamide substituted with ethylcinnamate (BTAC, 119) and studied its supramolecular gelationand macroscopic chirality from the self-assembly of BTAC in theDMF/H2O mixture. They found that upon gelation this achiralcompound can simultaneously self-assemble into unequalamounts of left- and right-handed twists, thus resulting inmacroscopic chirality without any chiral additives (Figure 86).The symmetry breaking and formation of macroscopic chiralityfrom the assembly of this type of molecules is quite rare. Thehierarchical self-assembly of an uneven number of different chiralassemblies produces the unbalanced left- and right-handed

Figure 82. (A) Molecular structure of achiral bolaamphiphilic azobenzene 117 and its packing mode within the self-assembly. (B) (a) TEM and (b)SEM observation of the self-assembled coiled and tubular nanostructures of 117 from water at pH 2.77. (c) TEM image of the twisted ribbon. (d) HR-TEM image of the nanotube. (C) CD spectra of the self-assembled suspension 117 at pH 2.77 (solid line) and 117 itself (dashed line). Reprinted withpermission from ref 199. Copyright 2012 The Royal Society of Chemistry.

Figure 83. (A) Molecular structure of TPPS (67). (B) Schematic illustration showing kinetic control of the supramolecular chirality of the assembly ofacidified TPPS with J aggregation. Reprinted with permission from ref 273. Copyright 2014 American Chemical Society.

Figure 84. (A) Chemical structures of carboxyl azobenzene derivatives and melamine, and the schematic representation of their complex. (B) (a, b)TEM and (c and d) AFM observation of the self-assembled helixes and supercoils with labeled handedness. (C) CD measurements of the assemblies.Reprinted with permission from ref 274. Copyright 2014 The Royal Society of Chemistry.

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twists, as confirmed by a series of CD spectral measurements andSEM studies. Importantly, the overcrowded stacking of thecinnamate rings within the assembly plays a very important rolein the spontaneous symmetry breaking and production of anuneven number of helical nanostructures with different handed-ness.284 Furthermore, this phenomenon may generally occur inthe gel systems of BTA derivatives. It is expected that many of theother BTA derivatives will show similar properties.5.4. Air/Water Interface and LB Films

The aggregation of adjacent molecules plays an important roleduring the self-assembly of achiral molecules with the emergenceof the supramolecular chirality. The initial stacking at a certainangle from the neighboring molecules can produce a chiral bias,and this dislocation can be left-handed or right-handed. If thischiral bias can be further grown or amplified, chiral structureswith left- and right-handedness are obtained. Moreover, mostimportantly, if the growth rate of the two biases is different duringthe self-assembly, optical activity of the system can be expected,with an unequal amount of left-handed or right-handed chiralnanostructures. This type of symmetry breaking is sometimesobserved in the aforementioned processes of self-assembly insolution and gel systems. However, when the assembly of achiralmolecules was studied at the air/water interface, it was found thatthis kind of symmetry breaking is quite general in nature. Thus,achiral molecules could be fabricated into optically active

Langmuir−Blodgett (LB) films through the air/water interfacialself-assembly.The Liu group first reported air/water interfacial chiral

assemblies from achiral molecular building blocks in 2003.39 Inthis work, it was found that when an achiral amphiphilic moleculecontaining a naphtha[2,3]imidazole headgroup and long alkylchain (NpImC17, 120) was assembled with Ag(I) ions at the air/water interface, the compound could coordinate with AgNO3 toform a stable monolayer, which can be then further transferredonto a solid substrate to produce a Langmuir−Blodgett (LB)film. The CD spectra of these LB films showed a strong Cottoneffect after removing the possible LD effects, which suggestedthat the supramolecular chirality resulted from the assembly ofachiral molecular building blocks. It was suggested that theovercrowded stacking of the achiral chromophores in a helicalsense would produce such macroscopic supramolecularchirality.40 Interestingly, in this system containing only achiralmolecules, the sign of the supramolecular chirality cannot bedetermined in the air/water interfacial assembly. One can obtainM- or P-chiral assemblies in different batches. It is worthmentioning that air/water interfacial chiral assemblies obtainedfrom achiral molecules are totally different from those obtainedfrom racemic mixtures, in which no optical activity can bedetected. In contrast, these LB films are optically active, eventhough the handedness of the supramolecular chirality canchange from batch to batch.With air/water interfacial assembly, it is not rare to obtain the

supramolecular chirality from achiral molecular building blocks.For many achiral molecules, this process of making supra-molecular chiral assemblies is feasible. For example, Liu et al.further designed an achiral amphiphilic barbituric acid 121 andobtained supramolecular chiral assemblies with optical activity atthe air/water interface (Figure 88A). These molecules furtherself-assembled into spiral nanoarchitectures (Figure 88B). Thehydrogen bonding between the headgroups and the over-crowded arrangement of the aromatic ring within the assemblyplayed very important roles.285

The relationship between molecular structures of achiralmolecules and the supramolecular chirality of their assemblies atthe air/water interface has been thoroughly investigated. It wasfound that many achiral molecules with larger steric hindranceduring self-assembly can form chiral assemblies on the surface ofwater. For example, some achiral arylbenzimidazoles with 2-substituted anthryl groups were found to interact with AgNO3 inthe subphase and form chiral assemblies.286 In the case of twoachiral coumarin derivatives, 7-octadecyloxylcoumarin (7-CUMC18, 122) and 4-octadecyloxylcoumarin (4-CUMC18,123) substituted at different positions, the Langmuir−Schaefer(LS) film of 4-CUMC18, which has larger steric hindrance,

Figure 85. (A) Molecular structures of an achiral disk-shaped moleculehaving one imidazole unit (118). (B) CD spectra of the supramoleculargels obtained in different batches at 20 °C. (C) (a) TEM and (b) AFMimages of twisted nanofibers formed from the assembly of 118.Reprinted with permission from ref 282. Copyright 2010 AmericanChemical Society.

Figure 86. Formation of optically active supramolecular gels with chiral nanostructures and optical activity by the hierarchical self-assembly of achiral119. Reprinted with permission from ref 284. Copyright 2014 John Wiley & Sons.

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showed supramolecular chirality, while 7-CUMC18 did not(Figure 89).287

The azobenzene derivatives can easily undergo trans−cisisomerization; thus, the aggregation behavior of two azobenzeneisomers is often different. Liu et al. found that the trans form ofthe achiral azobenzene derivative 4-octyl-4′-(5-carboxypentame-thyleneoxy) azobenzene (trans-C8AzoC5, 124) showed a strongCotton effect in its LB films, while the assemblies of cis-C8AzoC5did not (Figure 90).288

The aggregation of porphyrin derivatives can be one of themost attractive models for understanding supramolecularassemblies. Different molecular packing of porphyrins can leadto H or J aggregation, which have different properties. Achiralporphyrins can also assemble into helical nanostructures withsupramolecular chirality. For example, through the air/waterinterfacial assembly of TPPS (67) with achiral positively chargedamphiphiles, Zhang and Liu et al. obtained chiral assemblies, asshown in Figure 91. The emergence of the supramolecularchirality could be due to the helical stacking of the TPPS units ina confined two-dimensional interface.289

The packing mode of the supramolecular assembly of achiralporphyrins can be modulated upon protonation of their centralnitrogen atoms. If such protonation was performed in situ at theair/water interface, many achiral porphyrins can assemble into

Figure 87. (A) Chemical structures of NpImC17 (120), and the model for the formation of the chiral NpImC17−Ag(I) coordination assemblies. (B)Circular dichroism (CD) (A) and UV (B) spectra of a 10-layer NpImC17 LB film transferred from the pure water surface (a) and a 20-layer Ag(I)−NpImC17 LB film (b). Reprinted with permission from ref 39. Copyright 2003 American Chemical Society.

Figure 88. (A) Structure of achiral amphiphilic barbituric acid 121. (B) AFM images of one-layer LB films deposited at various surface pressures at 20 °C(a) and (b) 7, (c) 20, and (d) 30 mN/m after the inflection point. Reprinted with permission from ref 285. Copyright 2004 American Chemical Society.

Figure 89. (A)Molecular structures of achiral coumarin derivatives. (B)CD spectra of 4-CUMC18 (a, c) and 7-CUMC18 (b) LS filmstransferred from the water surface at 20 mN/m. Reprinted withpermission from ref 287. Copyright 2007 American Chemical Society.

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optically active LB films, as illustrated in Figure 92.290 Similarly tothese protonated porphyrins, introduction of the metal ions intothe centers of the porphyrin can also cause some of the achiralporphyrins to form chiral assemblies at the air/water interface.291

Generally, the chiral assemblies obtained from achiralmolecules at the air/water interface are not so stable. Thus, ifthe air/water interface is used as the platform to form the chiralassemblies first and then introduce more covalent bonds to thesystems, the stability can be increased. One of the best ways is thetransform through the topochemical polymerization. Forexample, Liu at al. found that achiral amphiphilic diacetylenederivatives 125 can form chiral assemblies at the air/waterinterface, and these diacetylene derivatives can be polymerizedupon photoirradiation. Interestingly, the photopolymerizedorganized molecular films of polydiacetylene showed strongoptical activity as well as helical nanostructures (Figure 93).292

Zou et al. synthesized different achiral azobenzene-substituteddiacetylene monomers (Figure 94) and studied the air/waterinterfacial assembly of these molecules.293 The results show thatonly the achiral diacetylene monomer containing one hydro-phobic chain can form chiral assemblies at the air/water interface.When these LB films of compounds 126 and 127 were irradiated

Figure 90. (A) Structure of 4-octyl-4′-(5-carboxypentamethyleneoxy)-azobenzene (trans-C8AzoC5, 124). (B) Formation of optically activeand inactive supramolecular assemblies from trans-C8AzoC5 (top, 124)and cis-C8AzoC5 (bottom), respectively. Reprinted with permissionfrom ref 288. Copyright 2006 American Chemical Society.

Figure 91. (A) CD spectra of 20-layer LS films of (a) ODA, (b) CTAB,and (c) DOAB with TPPS transferred at 30 mN/m. (B) Schematicillustration of TPPS/amphiphile coassemblies and the J aggregation ofTPPS. Reprinted with permission from ref 289. Copyright 2003American Chemical Society.

Figure 92. Protonation of achiral water-insoluble free-base porphyrins can lead to optically active supramolecular assemblies at the air/water interface.Reprinted with permission from ref 290. Copyright 2007 John Wiley & Sons.

Figure 93. (A) Structure of amphiphilic diacetylene derivative (tricosa-10,12-diynoic acid, TDA, 125). (B) UV−vis and CD spectra of LB filmsdeposited from in situ photopolymerized PDA films (a) from pure waterand (b) from Cu(NO3)2. (C) TEM images of PDA films in situpolymerized on pure water. Reprinted with permission from ref 292.Copyright 2002 The Royal Society of Chemistry.

Figure 94. Molecular structures of the three azobenzene-substituteddiacetylene monomers.

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by left- or right-handed circularly polarized UV light (CPUL),supramolecular chirality with polymerization of diacetylenegroups can be detected. Interestingly, for the LB films formedby compound 128 containing two hydrophobic diacetylenechains, polymerization cannot be achieved, even thoughsupramolecular chirality can be detected from the CD signalsof azobenzene chromophores.In addition to amphiphilic diacetylenes, the chiral polymer

from achiral phthalocyanine derivatives was also achieved. Thea ch i r a l p h t h a l o c y an i n e bu i l d i n g b l o c k s i l i c on2,3,9,10,16,17,23,24-octakis(octyloxy)-29H,31H-phthalocya-nine dihydroxide (Pc 1) was spreading at the air/water interfaceto form a monolayer and subsequently transferred onto solidsubstrates to produce LS films with optical activity. Interestingly,upon heating at 180 °C in a high vacuum for 10 h, the chiralassemblies within the LS films could be converted into chiralcovalent polymers with the CD signals increased significantly.294

This result indicated that the transform from the noncovalent tothe covalent bond not only increased the stability of the chiralassemblies but also amplified the supramolecular chirality of thesystems in these interfacial films.In these above examples, for the supramolecular chirality

formed by air/water interfacial assembly of achiral molecules, theCD spectra have always been measured in the corresponding LBfilms. Thus, one may doubt whether the supramolecular chiralityoriginated from the deposition process for making LB films ormerely originates from the assembly of achiral molecules on thewater surface. Therefore, when achiral molecules were placed onthe water surface, the in situ measurement of the supramolecularchirality could be very important. Recently, the SHG-LDtechnique has been developed for detecting supramolecularchiral assemblies in situ on water surfaces.295 It was confirmedthat the supramolecular chirality is produced from the assemblyof achiral molecules at the air/water interface. In addition, somesubtle effects of the chirality change of the systems can also bedetected from in situ measurements. For example, Liu, Wang,

and Guo et al. designed a series of TPP-based achiral porphyrinswith different alkyl chains and hydrophilic substituents andinvestigated the supramolecular chirality within the LB films aswell as in situ in the monolayers. Depending on the number ofalkyl chains, different supramolecular chirality can be detectedfrom the LB films of these porphyrin derivatives (Figure 95).296

Furthermore, the production of the supramolecular chirality inthe monolayers of these porphyrins at the air/water interface wasstudied by means of second-harmonic generation lineardichroism (SHG-LD). Interestingly, achiral TPPA2a (131) wasdemonstrated to form chiral assemblies by in situ SHG-LDmeasurements even though the LS film did not have clear CDsignals. This is because the SHG-LD can provide more sensitivesignals than conventional CD spectra. In addition, a subtleinteraction at the air/water interface can affect the supra-molecular chirality in the monolayers, which can be monitoredby the SHG-LD. For instance, using the SHG-LD technique, itwas found that Cu2+ ions in the subphase enhance thesupramolecular chirality of the TPPA2a monolayer, while Zn2+

ions inhibit the formation of chiral assemblies (Figure 96).297

Figure 95. (A)Molecular structures of different porphyrin derivatives. (B) Possible stacking of the porphyrin TPPA3 (133) and TPPA0 (129) assemblyon the air/water interface. Reprinted with permission from ref 296. Copyright 2008 The Royal Society of Chemistry.

Figure 96. In situ supramolecular chirality from the assembly of achiralTPPA2a (131), which can be modulated by adding different metal ionsto the subphase. Reprinted with permission from ref 297. Copyright2014 American Chemical Society.

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5.5. Controlling Handedness of Supramolecular Chirality

Constructing supramolecular chiral assemblies from achiralmolecular building blocks is a fascinating possibility. A furtherstep would be to control the handedness of supramolecularchirality containing only achiral molecular building blocks.Unfortunately, there are still very few methods available forcontrolling handedness. However, we can gain hints from severalimportant cases. Herein, we focus on the effects of mechanicalforce and circularly polarized light.5.5.1. Vortices and Spin Coating. For the formation of

supramolecular chiral assemblies from achiral molecular buildingblocks, the macroscopic force field may play a very importantrole. It is worth mentioning that the handedness of chiralassemblies formed by achiral molecules can be controlled by thedirection of vortex stirring. This is the advantage of symmetrybreaking with vortex stirring or spin coating. Considering that thechiral information is exhibited below the nanoscale, whilemechanical perturbations occur on macroscopic scales, themodulation of supramolecular chirality is remarkable.298

With regard to this issue, the first significant work waspublished by Ribo and co-workers.249 The molecular buildingblocks for these studies were various achiral diprotonated meso-sulfonatophenyl-substituted porphyrins. A vortex motion witheither clockwise or anticlockwise direction was generated using arotary evaporator with different directions of rotation. Uponrotary evaporation, the achiral diprotonated meso-sulfonato-phenyl-substituted porphyrins could form assemblies withoptical activity. Interestingly, the chirality sign of theseassemblies can be selected by vortex motion during theaggregation process, as confirmed by circular dichroism (CD)spectra. In this context, the aggregation of the porphyrins couldbe modulated to show different supramolecular chirality bychanging the rotational direction of the rotary evaporator (Figure97).The supramolecular chirality from the assembly of achiral

meso-sulfonatophenyl-substituted porphyrins under vortex mo-tion or stirring can be detected from the CD spectra andobserved directly from TEM and AFMmeasurements. Ribo et al.studied stirred solutions of 5-phenyl-10,15,20-tris(4-

sulfophenyl)porphyrin (135). Within the assemblies, the achiralporphyrins were found to undergo J aggregation. In addition,AFM measurements on the stirred solutions showed helicalnanoribbons, which indicated supramolecular chirality at thenanoscale (Figure 98).299

With regard to vortex motion or stirring introduced into thesupramolecular chiral assemblies, achiral porphyrins areimportant model molecules. In this context, the π−π interactionsbetween the aromatic rings play very important roles for formingchiral assemblies. Aida and co-workers studied dendritic zincporphyrins with two carboxylic acid groups (136). Theseporphyrins could form J aggregates in CHCl3 through π−πinteractions from the porphyrin rings and also from the aromaticrings within their dendritic substituents. Moreover, hydrogenbonding between the carboxylic acid groups can also be a veryimportant driving force for forming J aggregates. The spincoating of the J-aggregate solutions could produce very stableoptically active films. Interestingly, the handedness of thesupramolecular chirality of the films can be controlled bychanging the direction of the spin coating (Figure 99).248

Aside from spin coating, the stirring of benzene solutions ofachiral dendritic zinc porphyrins with two carboxylic acid groups(136) can also produce supramolecular chirality. Strong CDsignals can be detected from the porphyrin solution uponstirring. Interestingly, when the direction of stirring was changedfrom clockwise (CW) to counterclockwise (CCW), the oppositeCD signals were detected. For this system, the nanofibers formedfrom the assembly of porphyrins via J aggregation play veryimportant roles. Some of the observed chiroptical activity couldalso originate from the macroscopic helical alignment ofnanofibers (Figure 100).300

For achiral porphyrins in aqueous solution under stirring,Purrello et al. carefully studied the changes of CD spectra ofTPPS (67) aqueous solutions in cuvettes upon stirring.301 Theseauthors found that by stirring in different directions theprotonated TPPS could form two different J aggregates. Theresults showed that the CD signal of the solution inverted with anincreasing in stirring intensity when the direction of stirring wasaltered (Figure 101B). Remarkably, with stirring, assemblies ofthe porphyrin were deposited on the cuvette wall, and thechirality of the assemblies on the solid surface were different thanthose in solution. However, the directions of the CD signals ofthe porphyrin aggregations on the cuvette wall were also found tobe dependent on the stirring direction.301

In some cases, strong CD signals can be detected fromassemblies of achiral molecules, but the supramolecular chiralityof these assemblies is still in doubt. Linear polarizationproperties, such as linear dichroism (LD) and linearbirefringence (LB), can hamper the actual circular dichroism(CD). Images of the true CD signal can be obtained usingMueller matrix spectroscopy (MMS). For some assemblies,formed by using achiral molecular building blocks, using MMSmay overcome the problems related to linear polarizationproperties. Okano and Yamashita et al. developed a two-component system to investigate the supramolecular chiralitythat originates from the assembly of achiral molecules.303 In thisstudy, the achiral ionic oligomer (poly[pyridinium-1,4-diylimi-no-carbonyl-1,4-phenylene-methylene chloride], 138) formedhydrogels at high concentration. At lower concentrations inwater, the achiral ionic oligomer could form fibrous nanostruc-tures with supramolecular chirality with vortex stirring. Toremove the influence of the linear polarization properties foridentifying the supramolecular chirality, another achiral dye

Figure 97. Achiral diprotonated meso-sulfonatophenyl-substitutedporphyrins form optically active assemblies upon rotary evaporation.Reprinted with permission from ref 249. Copyright 2001 The AmericanAssociation for the Advancement of Science.

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molecule, Congo Red (CR, 137), was added to the system. As aresult, the real supramolecular chirality of the assemblies ofachiral ionic oligomers, i.e., the chiral information, wastransferred to the Congo Red. In this context, a strong CDsignal from the Congo Red can be detected by Mueller matrixspectroscopy. Amazingly, due to the monosignate shape of theUV−vis spectra of Congo Red, which suggested the absence ofchromophore coupling, the chirality of Congo Red wasattributed to its single chromophore, instead of its aggregates.302

These results suggest that the supramolecular chirality can in factbe generated from an assembly of achiral molecules (Figure 102).Similar chiral assemblies from achiral two-component

molecular building blocks were studied using circularly polarizedluminescence. For this study, the concentration of the achiralionic oligomer (138) was increased to form hydrogels, andRhodamine B dye (139) was embedded into the hydrogel.Vortex stirring was performed during the slow cooling from thesol to the gel. The supramolecular chirality was studied bycircularly polarized luminescence (CPL) fromRhodamine B dye,and supramolecular chirality was detected in these hydrogels.When the samples were heated to change the gel to a solution,the resulting supramolecular chirality disappeared. Interestingly,these results also show that the sense of the CPL could becontrolled by switching the stirring direction from clockwise(CW) to counterclockwise (CCW) (Figure 103).303

The mechanism of the formation of chiral assemblies formedby achiral molecular building blocks with vortex stirring has been

Figure 98. (A) Structure of 5-phenyl-10,15,20-tris(4-sulfophenyl)porphyrin (135). (B) AFM images of assemblies of 135 showing the onset of foldingin stagnant and stirred solutions. Reprinted with permission from ref 299. Copyright 2006 John Wiley & Sons.

Figure 99. (A) J aggregation of achiral dendritic zinc porphyrin (136) and macroscopic chirality from spinning. (B) Mechanism for the formation ofchiral assemblies. Reprinted with permission from ref 248. Copyright 2004 John Wiley & Sons.

Figure 100. Molecular structure of achiral dendritic zinc porphyrin(136) and the corresponding supramolecular chiral assembly generatedfrom vortex flows. Reprinted with permission from ref 300. Copyright2007 John Wiley & Sons.

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studied by Ribo et al. Their results show that the assembly ofdifferent achiral diprotonated meso-sulfonatophenyl-substituted

porphyrins (Figure 104A) can be a hierarchical noncovalentpolymerization process preceded by a critical nucleation stage. Inthis case, the primary nucleation is a very slow process, while thesecondary nucleation stage is much faster. During the primarynucleation process, significant enantiomeric excesses can beobtained from the formation of a few primary nuclei, and thevortex stirring controlled the secondary nucleation by assisting

Figure 101. (A) Schematic structure of the protonation of TPPS. (B)Schematic representation of the possible effects of the stirring on aracemate (A). CW (B) and CCW (C) stirring favor Δ and Λ Jaggregates, respectively. Reprinted with permission from ref 301.Copyright 2010 John Wiley & Sons.

Figure 102. Chiral induction to an achiral molecule (Congo Red dye,137) from the chiral assembly induced by an achiral oligomer (138) wasdemonstrated by Mueller matrix spectroscopy analysis of the opticalpolarization properties. The true CD image of the clockwise (top) andcounterclockwise (bottom) stirred solution of achiral ionic oligomer138 are shown. Reprinted with permission from ref 302. Copyright 2011John Wiley & Sons.

Figure 103. (A) Structure of achiral ionic oligomer (138) andRhodamine B dye (139). (B) (a) Photoluminescence (PL) spectra(λex = 520 nm) of Rhodamine B (139) (1.6 × 10−5 M) in an aqueoussolution of 138 (0.6 wt %). (B) Circularly polarized luminescencespectra (CPL) of the solution with clockwise (CW, red) andcounterclockwise (CCW, blue) stirring at 1000 rpm and unstirred(black). Reprinted with permission from ref 303. Copyright 2011 JohnWiley & Sons.

Figure 104. (A) Molecular structures of different achiral diprotonatedmeso-sulfonatophenyl-substituted porphyrins. (B) CD spectra ofH4TPPF5S3 (140) (10 μm, HCl 0.1 M) J aggregates obtained undervigorous-shaking conditions (6 h, full line) and in magnetically stirredconditions (24 h, dashed line). Reprinted with permission from ref 304.Copyright 2012 John Wiley & Sons.

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the growth of the primary chiral nuclei.304 This mechanism canbe demonstrated from the difference between the CD spectrameasured under vigorous shaking and magnetic stirring (Figure104B). Thereby, chiral assemblies can be obtained from achiralbuilding blocks.Although vortex stirring can help achiral molecules to form

chiral assemblies, both rotational and magnetic forces providedmore efficient modulation of the supramolecular chirality duringthe assembly of achiral molecules. This issue is fullydemonstrated by Scolaro et al. using the aggregation of anachiral porphyrin, tris(4-sulfonatophenyl) phenylporphyrin(TPPS3, 135), to form chiral assemblies under both rotationaland magnetic forces.305 Interestingly, the influence of themagnetic forces can be tuned to an effective gravity whichplays a very important role, while the magnetic orientation of theaggregates is also essential (Figure 105). Moreover, this studyalso suggested that application of rotational and magnetic forcesduring the primary nucleation process is sufficient to form chiralassemblies.

Nevertheless, supramolecular chirality originating fromassemblies of achiral molecular building blocks upon vortexstirring has been generally acknowledged. On the other hand, it iswell known that some chiral dopants can coassemble with achiralmolecular building blocks to form chiral supramolecularaggregations. When the chiral species and the vortex stirringmeet together, which effect will be dominate? Sagues and co-workers studied the chiral competition between vortex rotatingand chiral dopants.306 In their study, achiral trans-azobenzenesurfactant 141 was used as the primary building block, while itschiral analogues 142 and 143 were the chiral dopants. Vortexrotating was performed to form monolayer assemblies, and thesupramolecular chirality was studied using Brewster anglemicroscope (BAM) measurements. The results showed thatthe influence of the vortex rotation can be comparable to that ofthe chemical induction processes. However, the influence of thevortex rotation can be more easily controlled by changing thedirection or speed of stirring (Figure 106).5.5.2. Circularly-Polarized Light. Circularly polarized light

(CPL) can be regarded as a type of energy that contains chiralinformation. When achiral supramolecular assemblies areirradiated by circularly polarized light, chiral information fromthe illumination can be transferred to the systems to formsupramolecular chiral assemblies.For more than a century, scientists have considered CPL as a

possible cause of natural homochirality. Because CPL has beenobserved in star formation, CPL could be the source of chiralinformation during the origins of life.307,308 When supra-

molecular assemblies containing only achiral molecular buildingblocks were irradiated by circularly polarized light, chirality canbe introduced into the systems.Oriol et al. studied liquid crystals with chiral organization

formed by achiral molecules upon irradiation with circularlypolarized light (Figure 107). The achiral building block used wasa polymer with methoxyazobenzene groups in the side chain.When thin films of the nematic glassy phase of this polymer wereirradiated with 488 nm circularly polarized light (CPL), a chiralarrangement of the azobenzene groups with helical nanostruc-tures resulted from the selective reflection of visible light.Furthermore, the CPL-induced supramolecular chirality in thepolymer was confirmed by CD spectra and vibrational circulardichroism (VCD) spectra.201

Zou et al. synthesized an achiral amphiphilic azobenzenederivative (144), which can assemble into a monolayer on thesurface of water (Figure 108A). Supramolecular chirality withhelical packing of azobenzene derivatives can be introduced intothese monolayers by irradiation with CPL. The resultinghandedness of the supramolecular chirality of these monolayerswas controlled by the handedness of the CPL. This is an excellentexample that supramolecular chirality from an achiral componentbe controlled by CPL. Furthermore, these chiral azobenzenemonolayers can be used as a chiral template, which supports thecontention that the diacetylene derivatives can form chiralpolydiacetylene under normal UV irradiation (Figure 108B).309

Moreover, Zou et al. also prepared discotic hydrogen-bondingcomplexes of diacetylene (DA) units. When these complexeswere irradiated by circularly polarized ultraviolet light within theliquid-crystal phase, helical polydiacetylene can be obtained(Figure 109A).310 Most interestingly, by using linearly polarizedlight irradiation together with a magnetic field, the enantiose-lective polymerization of these complexes can also be achievedwithin the liquid-crystal phase (Figure 109B).311 In this case, the

Figure 105. Chiral self-assembly of TPPS3 under rotational andmagnetic forces, showing the relationship between the observed chiralityand the applied physical forces. Reprinted with permission from ref 305.Copyright 2012 Nature Publishing Group.

Figure 106. (a−d) Self-assembly leading to the formation of a two-dimensional emulsion featuring chirally resolved domains. trans-Azobenzene surfactant molecules (b, 141), resulting from thespontaneous isomerization of the cis isomer (a), self-assemble at theair/water interface into segregated chirally resolved circular domainssurrounded by the cis-rich matrix (d); (e) molecular structures of achiraltrans-azobenzene surfactant 141 and its chiral analogues 142 and 143.(f) Coupling between the chiral modifier and the vortical flow.Reprinted with permission from ref 306. Copyright 2012 NaturePublishing Group.

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handedness of the helical polydiacetylene chains can becontrolled by changing the orientation of the linearly polarizedlight and the magnetic field. The enantioselective recognition ofD- or L-lysine using these helical polydiacetylene assemblies wasachieved. Combination of these physical vectors showed effective

control on the supramolecular chirality in self-assembledsystems.

5.5.3. Surface Pressure. Indeed, many supramolecularchiral assemblies can be obtained from different achiral molecularbuilding blocks via air/water interfacial assembly. However, forthis method, a great remaining challenge is the control of themacroscopic chirality of these systems. Normally, the handed-ness of the chirality of the assemblies obtained from the air/waterinterfacial assembly from achiral molecular building blocks israndom. In order to control the macroscopic chirality of air/water interfacial assembly from achiral molecules, a new methodusing unilateral compression geometry has been developed. Inthis approach, only one of the two movable Langmuir barrierswas used for the compression. Using this kind of unidirectionalcompression, Chen and Liu et al. found that the assembliesdeposited from the mirror regions of the LB trough can displaymirror-image macroscopic chirality.312 During the compression,vortex-like flows are suggested to be generated, and the directionof this compression-generated vortex-like flow can determine themacroscopic chirality of the formed assemblies (Figure 110).

5.6. Self-Assembly of Racemic Systems

Molecular or supramolecular chirality plays a very important rolein many self-assembly processes. In contrast to constructingchiral assemblies from pure achiral building blocks, the self-assembly manner of racemic systems is unique and largelydependent on the mixing of chiral enantiomers with differenthandedness.First, the mixing of an enantiomer and its mirror image may

meet with problems such as phase separation, so that thesituation of these systems can be very complicated. In general, forself-assembly of racemic systems, regardless of the chiralmolecules or the chiral nanostructures, some enantiomers canpreferentially aggregate with themselves, while other enan-tiomers might prefer to form complexes with their mirror images.Therefore, self-recognition can be established when someenantiomers recognize themselves and form homochiralassemblies, while in the case of an R enantiomer forming acomplex with its S enantiomer, this kind of self-sorting has beenregarded as self-discrimination to form heterochiral coassem-blies.313−317

For the self-assembly of racemic systems, in most cases, thereare much more possibilities for forming homochiral assembliesthan that of forming heterochiral assemblies. However, becausethe energy difference between the enantiomers with differenthandedness is quite small, phase separation in racemic systems isnot very significant.72

Wurthner et al. studied the chiral self-sorting of perylenebisimide (PBIs) assemblies.318 In this case, different chiral PBImolecules containing oligoethylene glycol bridges have beensynthesized, and the coassembly of these enantiomers withdifferent handedness was investigated. The results showed thatchiral self-recognition always prevails over self-discrimination forthese PBIs assemblies. The coassembly of this system tends toform homochiral aggregates (Figure 111).Percec et al. synthesized chiral hat-shaped dendronized

cyclotriveratrylene (CTV) and studied the racemic assembly ofthese molecules. The results showed that these molecules canform heterochiral assemblies at low temperature. When thesesystems were heated below 60 °C for 2 h, homochiral assembliesformed upon self-sorting (Figure 112).319

Although chiral self-sorting can be very significant for aracemic assembly, systems containing enantiomers with different

Figure 107. Model for circularly polarized-light introduced chirality inazomaterials. Reprinted with permission from ref 201. Copyright 2007John Wiley & Sons.

Figure 108. (A)Molecular structures of achiral amphiphilic azobenzenederivative 144 and achiral diacetylene derivative 125. (B) Schematicillustrations of the generation of chirality from PTDA/DBA hybrid filmswith the azobenzene-containing monolayer irradiated by left- and right-handed CPL. Reprinted with permission from ref 309. Copyright 2011The Royal Society of Chemistry.

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handedness can still show many new features, of which thenanostructures and properties are different from correspondingpure chiral assemblies.67 For example, for the formation ofsupramolecular gels, the racemates are usually believed to bepoor gelators. However, in some special cases, only racemateswere found to form supramolecular gels while the correspondingenantiomer cannot.320

For example, Yamaguchi et al. found that homochiralpseudoenantiomeric ethynylhelicene oligomers (147) cannotform organogels, while the mixture of P oligomer and Moligomer can form two-component organogels in toluene.

Changing the ratio of the pseudoenantiomers produced variousgel systems (Figure 113).321

With the exception of the formation of supramolecular gels,the self-assembly of racemic molecular building blocks can alsoimprove the macroscopic properties of soft matter. For example,the mechanical properties of peptide hydrogels can be improvedupon racemic self-assembly. Schneider et al. found that thehydrogels prepared from racemic β-hairpins showed non-additive, synergistic enhancement in material rigidity comparedto gels prepared from either pure enantiomer. Therefore, racemic

Figure 109. (A) Structure of discotic hydrogen-bonding complex of diacetylene (DA) units. (B) Asymmetric polymerization of the correspondingcomplex by using linearly polarized light irradiation and a magnetic field. Reprinted with permission from ref 310. Copyright 2014 The Royal Society ofChemistry. Reprinted with permission from ref 311. Copyright 2014 Nature Publishing Group.

Figure 110. Schematic illustrations of the chirality selectionphenomenon induced by compression-generated vortex-like flow.Reprinted with permission from ref 312. Copyright 2011 John Wiley& Sons.

Figure 111. (A) Structures of chiral perylene bisimides (PBIs)containing oligoethylene glycol bridges (146). (B) Energy diagram ofthe different supramolecular diastereomers of PBIs. Reprinted withpermission from ref 318. Copyright 2011 American Chemical Society.

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self-assembly can be a powerful tool for developing novelfunctional soft matters (Figure 114).322

Wang and Liu et al. studied the coassembly of the glutamic-acid-based bolaamphiphile racemates with melamine. In thissystem, the coassembly of melamine with pure enantiomericglutamic-acid-based bolaamphiphile (HDGA, 15b) cannot formgels. The assembly of the glutamic-acid-based bolaamphiphileracemate produced only precipitates. Mixing the glutamic-acid-based bolaamphiphile racemate with melamine produced goodsupramolecular gels. Remarkably, the racemic hydrogels showeda lower CGC value, enhanced mechanical rigidity, and dual pH-responsive ability compared to the pure enantiomer hydrogels.

The gelation properties, supramolecular chirality, and nano-structures of the racemic hydrogels can be regulated by changingmolar ratios of different molecular building blocks (Figure115).323

In addition to forming supramolecular gels, the mixing ofenantiomers has also been used to tune the properties of differentsupramolecular assemblies. Oda et al. studied the twists andnanotubes formed from the coassembly of nonchiral dicationic n-2-n Gemini amphiphiles with chiral tartrate anions. They foundthat the morphologies of the assemblies, such as the twist pitch ofthe ribbons, can be continuously modulated by varying theenantiomeric excess of tartrate anions. For instance, adding 10mol % of the opposite enantiomer of tartrate anions led to a 15%

Figure 112. Chiral self-sorting during supramolecular helical organization of hat-shaped molecules. Reprinted with permission from ref 319. Copyright2014 American Chemical Society.

Figure 113. (A) Structures of pseudoenantiomeric ethynylheliceneoligomers. (B) Minimal thermoreversible gelation concentration inmillimolar for 1.1 mixtures of (M)-n and (P)-n (n = 1−6) in toluene. S =soluble at room temperature (solubility > 5 mm), C = crystallization.Reprinted with permission from ref 321. Copyright 2010 John Wiley &Sons.

Figure 114. (A) (a) Assembly mechanisms for enantiomeric peptidesleading to the formation of a fibrillar network that defines hydrogelation.(b) Sequences of enantiomers MAX1, DMAX1, and the nonisomericcontrol peptide. D-Amino acid residues are italicized. (B) Dynamic timesweep rheological data measuring the storage moduli of 1 wt %hydrogels containing pure MAX1 (□), 3.1 MAX1.DMAX1 (▲), 1.1MAX1.DMAX1 (●), 1.3 MAX1.DMAX1 (▼), and pure DMAX1 (⧫).Reprinted with permission from ref 322. Copyright 2011 AmericanChemical Society.

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increase in the diameter of assembled supramolecular nanotubes(Figure 116).69

Liu et al. synthesized enantiomeric L- or D-glutamic-acid-basedlipids (1) and investigated the self-assembly of these chiralenantiomers. Although both L- and D-enantiomeric moleculesself-assembled into ultralong nanotubes, mixing D- and L-enantiomers with different molar ratios further changed thenanostructures consecutively from helical nanotubes to nano-twists to flat nanoplates (Figure 117).53

In most cases, the self-assembly of chiral molecules can lead tohelical nanostructures, while racemates usually assemble into flatnanostructures. However, this situation has exceptions. Liu et al.synthesized the enantiomeric L- or D-alanine derivatives (AlaC17,100), and the self-assembly and gelation properties of thecorresponding individual enantiomers and the racemates wereinvestigated. The self-assembly of individual enantiomers ofAlaC17 can form only flat nanostructures, even though both L-AlaC17 and D-AlaC17 can form gels in different organic solvents.Interestingly, racemic AlaC17 was found to self-assemble intobeautiful twisted ribbons. Moreover, these twists are verysensitive to a slight enantiomeric excess of many other aminoacids, showing remarkable macroscopic chirality. Therefore,these racemic assemblies can be used for the discrimination ofvarious amino acid derivatives (Figure 118).230

6. APPLICATIONS OF SUPRAMOLECULAR CHIRALITYRecently, self-assembled chiral supramolecular systems haveattracted greater attention due to the many potential applicationsof forms of soft matter. Such applications can further enhance ourunderstanding of supramolecular chirality. In particular, newproperties that single chiral molecules do not have emerge fromsupramolecular chiral systems. Certainly, for many applicationsof functional soft matters, supramolecular chirality plays thecritical role, which is also dependent on the properties andcharacteristics of the supramolecular chiral information ex-

pressed on different materials within diverse scales.11,324−326

Herein, we address the most prominent fields for the applicationof supramolecular chirality, such as chiral recognition, sensing,and catalysis. Some optical devices based on supramolecular

Figure 115. (A) Molecular structure of the glutamic-acid-basedbolaamphiphile (HDGA, 15b). (B) Schematic presentation of theracemic hydrogels formed by the coassembly of HDGA racemate andmelamine. (C) Gelation properties of the (L + D)-HDGA mixtures andthe (L + D)-HDGA/melamine mixtures with different ee values.Reprinted with permission from ref 323. Copyright 2014 AmericanChemical Society.

Figure 116. (A) Coassembly of nonchiral dicationic n-2-n Geminiamphiphiles with chiral tartrate anions. (B) Transition from twistedribbons to helical ribbons and then to tubules observed by TEMmeasurement. Reprinted with permission from ref 69. Copyright 2007American Chemical Society.

Figure 117. (A) Correlative plot of the vibration bands of N−H, amideI, and amide II and the d spacing of the nanostructures in the mixed gelsagainst the enantiomeric excess value (ED/L). (B) Proposed mechanismfor the formation of various nanostructures uponmixing enantiomeric L-or D-glutamic-acid-based lipids. Reprinted with permission from ref 53.Copyright 2010 John Wiley & Sons.

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chirality and biological applications of chiral soft matters will alsobe discussed.

6.1. Supramolecular Chiral Recognition and Sensing

Chiral recognition is a very important issue in supramolecularchemistry. In general, interactions between different enantiomersand another chiral molecule can produce totally different results.These different interactions can be detected by spectralmeasurements or determination of their crystal struc-tures,318,327−329 representing the chiral recognition. Thus far,studies on host−guest chemistry related to chirality have focusedon chiral recognition,330−337 and many excellent works havebeen published. For example, work on chiral recognition usingcyclodextrins is very famous and has been extensivelyreviewed.338−346

On the other hand, chiral recognition not only describes theinteractions between different chiral molecules but also canrepresent the interactions between chiral supramolecularassemblies and chiral molecules. Herein, we focus on the chiralrecognition of supramolecular assemblies.347−349 Thus, whenmolecules with contrary chirality interact with chiral supra-molecular assemblies, the chiral recognition shows some newfeatures, which are different from the case of chiral recognitionbetween different chiral molecules. For example, whenmoleculeswith different chirality are mixed with chiral supramolecular gels,chiral recognition can be achieved from the change in appearanceor rheological properties of the related supramolecular gels.More importantly, these chiral recognitions, detected as sol−geltransformation or color changes of the supramolecular gels, arevisible to the naked eye.350,192

Although chiral recognition related to the host−guestchemistry of cyclodextrin has been widely investigated, chiralrecognition based on highly symmetrical cucurbituril moleculesis still intriguing. Thus, when achiral cucurbiturils form a verystable complex with chiral molecules, the correspondingassemblies are able to completely discriminate other enan-tiomers. This work has been reported by Inoue and co-workers.351 In this study, achiral cucurbiturils (CBs) wereincorporated into (R)- or (S)-2-methylpiperazine, and theresulting complex showed significant enantiomeric discrim-ination of various chiral organic amines, such as (S)-2-methylbutylamine (Figure 119). The chiral recognition could

be demonstrated by NMR, MS, light-scattering, and calorimetricmeasurements.

Although chiral recognitions based on cyclodextrins have beenwidely investigated, supramolecular polymers constructed fromcyclodextrins also showed interesting recognition properties. Forexample, Yashima et al. synthesized polymers containing β-cyclodextrin substituents and studied enantioselective gelation ofthese polymers in response to the chirality of a chiral amine. It hasbeen found that (S)-1-phenylethylamine can help the polymersto form organogels but that (R)-1-phenylethylamine cannot.352

Shinkai et al. incorporated 4,4′-biphenyldicarboxylic acid intocyclodextrin (CD) as a bridging ligand, which further interactedwith TbIII to form polyrotaxane-type metallosupramolecularpolymers. Due to the chirality of cyclodextrin as well as the strongfluorescence of rare earth complexes, the recognition of smallchiral molecules by this supramolecular polymer was achievedbased on both fluorescence and circular dichroism spectralchanges.353

One of the most important advantages of the chiralrecognitions from different chiral soft matters can be thealteration of their macroscopic properties, which also can be

Figure 118. (A) Molecular structure of the enantiomeric alanine derivatives AlaC17 (100). (B) Schematic illustration of the molecular packing for asingle enantiomer and the racemate. The backgrounds are the SEM images of the transparent hexane gel formed by L-AlaC17 and the twisted ribbonsformed by the racemic mixture. (C) Intensity of the CD signals (centered at 309 nm) of the racemate upon addition of 2 mol % of various amino acidderivatives. LBG is a glutamic acid derivative with two long alkyl chains. Reprinted with permission from ref 230. Copyright 2013 John Wiley & Sons.

Figure 119. Chiral recognition of (S)-2-methylbutylamine from achiralcucurbiturils (CBs) incorporated with chiral 2-methylpiperazine.Reprinted with permission from ref 351. Copyright 2006 AmericanChemical Society.

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identified by the naked eye. Therefore, visual chiral recognitionscan be realized. For example, in the case of supramolecular gels,the recognition of chiral gelators can be achieved from simplyidentifying whether the systems are a “gel”.For supramolecular gel systems, chiral recognition by the

naked eye may be simply achieved from the sol−gel trans-formation. The work of Pu et al. is the first report of thisphenomenon. They synthesized a Cu(II) terpyridine complexcontaining 1,1′-bi-2-naphthol (BINOL) substituents (148),which can form stable supramolecular gels in CHCl3 uponsonication. Interestingly, some chiral amino alcohols were foundto change these gels into sols, while their enantiomers failed.Therefore, the chiral recognition of this organogel to some chiralamino alcohols can be achieved from the sol−gel transformation,which can be recognized by the naked eye. In this context, (S)-phenylglycinol (0.10 equiv) can break the CHCl3 gel network,while (R)-phenylglycinol cannot. With chiral 1-amino-2-propanol, the same enantioselective gel-collapsing process canalso be observed (Figure 121).354

Tu et al. synthesized a gelator containing steroidal substituentsand a platinum complex (149) and prepared metallogels fromthe assembly of these gelators. A visual chiral recognition can berealized from the sol−gel transformation of the metallogels.When (R)-binap was introduced into the system, the metallogelswere destroyed. In contrast, (S)-binap cannot change thesituation of metallogels (Figure 122).355

Liu et al. synthesized novel L-glutamide-based amphiphilicgelators containing Schiff base moieties and long alkyl chains (o-SLG, 8a and p-SLG, 8b). The self-assembly and gelationproperties of these amphiphiles with different metal ions havebeen investigated in many organic solvents. In particular, addingmetal ions to the organogels can significantly change the self-assembled nanostructures and the spectral characteristics ofthese systems. Thus, Cu2+ can transform the nanofiber gel into achiral twist, while adding Mg2+ ions enhanced the fluorescence ofthe gels. Remarkably, organogels containing Mg2+ ions have verygood chiral recognition ability. For example, when D- or L-tartaricacid was introduced into the system, the fluorescence quenchingprocesses of these organogels can be totally different (Figure123).158

Liu et al. designed and synthesized a series of amphiphilicmolecules containing both glutamide moieties and long alkylchains with different hydrophobic headgroups (Figure 5). Thesupramolecular assemblies of some of these amphiphiles can beused for the recognition of chiral molecules. Except for theorganogels prepared from o-SLG and p-SLG, the assembliesbased on quinolinol-functionalized L-glutamides (HQLG, 10)and metal ions were also found to have good chiral recognitioncapability.55

HQLG (10) can form complexes with different metal ions,such as Li+, Zn2+, and Al3+. Although these chiral complexes donot show a CD signal or chiral recognition properties in solution,they can form fluorescent metallogels with optical activity upongelation in several organic solvents. The recognition of the smallchiral organic molecules by these fluorescent metallogels can bedetected by the changes in their CD spectra or fluorescencespectra. For example, metallogels formed by Zn2+/HQLG (10)complex showed a totally different fluorescent color whentreated with (R,R)- or (S,S)-1,2-diaminocyclohexane. Therefore,using metallogels, chiral recognition can be achieved by thenaked eye. It is believed that the self-assembled nanostructuresplay very important roles for chiral recognition (Figure 124).Porphyrins are very important molecular building blocks for

the study of chiral supramolecular assembly. Ihara et al.synthesized an L-glutamide-functionalized zinc porphyrin (g-TPP/Zn, 150) and investigated the enantioselective recognitionof different amino acids by assemblies thereof (Figure 125A). g-TPP/Zn (150) can form organogels in different organic solvents.

Figure 120. (A) Schematic illustration of the fabrication of a TbIII-basedsupramolecular polymer. (B) Fluorescence (ex. 270 nm) and CDspectra before and after adding D- or L-tartaric acid (final concentration;L, TbIII, and D/L-tartaric acid, 10 μm, α-CD, 300 μm). (dotted line) L +α-CD + TbIII, (dashed line) D-tartaric acid, and (solid line) L-tartaricacid. Reprinted with permission from ref 353. Copyright 2013 JohnWiley & Sons.

Figure 121. Chiral recognition from enantioselective gel collapsing,which formed from a Cu(II) terpyridine complex. Reprinted withpermission from ref 354. Copyright 2010 American Chemical Society.

Figure 122. Visual chiral recognition of binap through enantioselectivemetallogel collapsing. Reprinted with permission from ref 355.Copyright 2011 John Wiley & Sons.

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TheCD spectra of the self-assembly of g-TPP/Zn in cyclohexaneshowed strong optical activity. Interestingly, L- and D-enantiomers of many different α-amino acid derivatives can bedifferentiated by organogels of g-TPP/Zn. For example, when L-histidine methyl ester (L-His-OMe) or D-histidine methyl ester(D-His-OMe) was mixed with g-TPP/Zn cyclohexane gels, verydifferent CD and fluorescence spectra were obtained (Figure125B and 125C).356

Smith and co-workers thoroughly investigated two-compo-nent organogels based on the coassembly of an L-lysine dendron(151) with different amines (Figure 126A). For these systems,when the chiral amines were used for the coassembly, veryinteresting chiral recognition phenomena can be detected. The

results show that the coassembly of L-lysine dendrons with chiralamines can form supramolecular gel fibers, and the chirality ofthe amine could control the corresponding diastereomericcomplexes. When both R and S amines were incorporated intothe systems, the L-lysine dendrons could selectively coassemblewith the R enantiomer, because the L-lysine dendron/R aminecomplexes formed the most stable gel. Moreover, when R amineenantiomers were added to the supramolecular gels formed bythe L-lysine dendron and pure S amines, the diffusion of R aminesand displacement of the original S amines from the “solid-like”fibers can be detected (Figure 126B).357 These results suggestthat the two-component organogels are very sensitive to themolecular chirality of gelators and have great potential for chiralrecognitions.

Figure 123. (A) Molecular structures and self-assembly of theamphiphilic Schiff bases with metal ions. (B) (a) Assembly mechanismof o-SLG (8a) and the chiral twist in the presence of Cu2+ ions. (b) o-SLG (8a) formed a complex with Mg2+ ions and transferred the chiralityto the whole assembly. When D-tartrate approached the Mg2+ ion, the D-enantiomer was favored. (c) Cu2+ ions reacted with p-SLG (8b) andcaused gelation. Reprinted with permission from ref 158. Copyright2012 John Wiley & Sons.

Figure 124. (A) Molecular structures of the HQLG ligand molecule (10) and its metal complexes. (B) Chiral recognition brought about by themetallogels. Reprinted with permission from ref 55. Copyright 2013 American Chemical Society.

Figure 125. (A) Schematic image of enantioselective recognitionthrough chirally ordered porphyrin assembly. (B) CD spectra of g-TPP/Zn (150) (50 μM) with and without L- and D-His-OMe (50 μM) incyclohexane at 20 °C. (C) Fluorescence spectra of the g-TPP/Zn (150)(50 μM) assembly with and without L- and D-His-OMe (50 μM) incyclohexane at 20 °C. Reprinted with permission from ref 356.Copyright 2012 The Royal Society of Chemistry.

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In principle, if the chiral recognition can be triggered by a verysmall amount of chiral organic molecules, these assemblies withchiral recognition ability can be used as a chiral sensor.Compared with the recognition of chiral molecules, chiralsensors based on supramolecular assemblies can bemore difficultto prepare. On the other hand, some wide-sense chiral sensorsare available based on organic molecules, polymers, andsupramolecular assemblies.192,358,359 Herein, we discuss sometypical examples of chiral sensors that have been recentlyreported.Very simple but efficient chiral sensors can be prepared from

the biaryls, which have been developed mainly by Wolf etal.360,361 For example, naphthalene derivatives containingsalicylaldehyde units and pyridyl N-oxide fluorophores havebeen synthesized. This is the result of the salicylaldehyde unitwhich can form a complex with amino alcohols with differentchirality and subsequently change the conformation of themolecules. The chiral sensing can be determined from thechanges in the CD and fluorescence spectra. By using this system,the absolute configuration of many amino alcohols can beanalyzed.42

Covalently connected porphyrin dimers, which were namedporphyrin tweezer systems, have been thoroughly studied byNakanishi, Berova, and co-workers. These porphyrin tweezersystems can be used to determine the stereochemistry of manysmall chiral organic molecules.362−367

Indeed, some of the porphyrin tweezers can be very goodchiral sensors with determination of the absolute chirality of theguest molecules. For example, Borhan et al. designed andsynthesized an electron-deficient fluorinated porphyrin tweezer(152) and demonstrated that it is a good chiral sensor for thediscrimination of many different small organic moleculescontaining two chiral centers. Depending on the chirality ofthe small organic substrates, the changes in the CD spectrum ofthe supramolecular assemblies formed by the porphyrin tweezersand chiral guests can be detected. In this case, the absolutestereochemical configuration of a variety of erythro and threo

substrates was determined using this porphyrin tweezer (Figure127).368

Suzuki et al. synthesized secondary terephthalamide deriva-tives containing four aryl blades (153a-H), which can be used asthe host molecule for chiral sensing. The conformation of thishost can be changed from a nonpropeller anti form to a propeller-shaped syn form with the formation of a complex with somechiral molecules, such as p-xylylenediammonium derivatives.Moreover, depending on the chirality of the guest enantiomerscoassembled with secondary terephthalamide derivatives, thecomplex can be biased to prefer a particular handedness, with theenhancement of CD signal. This secondary terephthalamidederivative can be used as chiral sensor for the discrimination ofthe very important neurotransmitter, (−)-phenylephrine (Figure128).369

Some biomacromolecules can also be used for chiralrecognition and sensing systems. As one of the most importantbiomacromolecules, DNA contains chiral information from themolecular to supramolecular level. Moreover, the stability ofDNA can be a huge advantage for building different devices forchiral sensing.Qu et al. constructed electrochemical DNA sensors for the

chiral sensing. In this system, DNAmolecules modified with thiolgroups were covalently bonded to a gold electrode. When smallchiral molecules interacted with DNA, changes in the electro-chemical characteristics of the gold electrode could be detected,thus demonstrating chiral sensing ability. It is worth mentioningthat this system offers great advantages for distinguishing chiralmetallosupramolecular complexes. For example, the authorsreported a three-way junction based on an E-DNA sensor. In thisstudy, the same palindromic DNA labeled with a redox-activemethylene blue (MB) tag at the 5′-terminus was used as thesupport. Discrimination of chiral metallosupramolecular com-plexes with an enantioselective recognition ratio of about 3.5 wasrealized (Figure 129A and 129B).370 In another case, the authorsreported a similar electrochemical DNA (E-DNA) chiral sensorbased on the human telomeric G-quadruplex formation. Anenantioselective recognition on zinc-finger-like chiral metal-losupramolecular assemblies reaches ratios higher than 5 (Figure129C).371

As previously described, supramolecular chiral assemblies havegood capabilities for the recognition of low molecular weightchiral organic molecules. Worthy of note are the chiralsupramolecular assemblies that can recognize very small amountsof chiral molecules. In this context, we will show somerepresentative examples of chiral sensing based on supra-molecular assemblies.

Figure 126. (A) Chiral gelation system of an L-lysine dendron (G2-Lys,151) and chiral amines (C6R/S). (B) Schematic of thermodynamicallycontrolled gel evolution upon addition of C6R to a gel made from G2-Lys (151) and C6S. Reprinted with permission from ref 357. Copyright2014 American Chemical Society.

Figure 127. Fluorinated porphyrin tweezer working as chiral sensor forthe discrimination of the absolute configurations of amino alcohols.Reprinted with permission from ref 368. Copyright 2008 AmericanChemical Society.

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Porphyrins are very important building blocks for the study ofsupramolecular chirality. Tsuda and Aida synthesized a zincporphyrin dimer containing a rigid linker and pyridine

substituents (154). This zinc porphyrin dimer can self-assembleinto box-shaped tetramers. In a solution of asymmetrichydrocarbons, such as limonene, the self-assembly of the zinc

Figure 128. (A) Molecular structures of the secondary terephthalamide derivatives containing four aryl blades (153) and their different conformations.(B) Schematic image showing the conformational changes from a nonpropeller anti form to a propeller-shaped syn form upon forming complexes withchiral guests. Reprinted with permission from ref 369. Copyright 2009 American Chemical Society.

Figure 129. (A) Schematic illustration of a three-way junction based on E-DNA for distinguishing chiral metallo-supramolecular complexes. (B)Changes of current from the E-DNA sensor showing discrimination of chiral metallosupramolecular complexes. (C) Schematic representation of thehuman telomeric DNA-based electrochemical DNA (E-DNA) sensor. Reprinted with permission from refs 370 and 371. Copyright 2012 The RoyalSociety of Chemistry.

Figure 130. Self-assembly of zinc porphyrin dimers can lead to box-shaped tetramers, which shows chiroptical sensing for limonene. Reprinted withpermission from ref 372. Copyright 2007 John Wiley & Sons.

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porphyrin dimer can lead to a homochiral box-shaped tetramericassembly. This self-assembled porphyrin box is enantiomericallyenriched and optically active, showing that chiroptical sensingcan be realized (Figure 130). From the CD spectra of theporphyrin box, the absolute configuration of limonene can bedetermined. Interestingly, in the case of very small enantiomericenrichment of limonene, extremely large molecular ellipticities ofthe porphyrin boxes were detected.372

Chiral sensing based on multicomponent assemblies contain-ing reversible covalent bonding are also worth mentioning.Anslyn et al. constructed a four-component reversible assemblycontaining carbonyl activation and hemiaminal ether stabiliza-tion (155) (Figure 131A). In this system, reversible binding ofthe monoalcohol has been achieved. Moreover, the tetradentateligand of the assembly renders close incorporation of secondaryalcohols. By binding and exchange of chiral alcohols, chiralsensing can be demonstrated by CD spectral measurements(Figure 131B). This chiral sensor can be used for determinationof the enantiomeric excess of mixed chiral alcohols with differentee values (Figure 131B).373

A chiral sensor with the ability to differentiate many differentchiral molecules, such as amino acids, peptides, proteins, andeven some aromatic drugs, has been developed by Biedermannand Nau. These systems are based on ternary complexes formedbetween the macrocyclic host cucurbit[8]uril, dicationic dyes,and chiral aromatic analytes (Figure 132A). Although bothcucurbit[8]uril and dicationic dyes are achiral, the chirality of thearomatic analytes with low micromolar concentrations in watercan be detected by the changes in the CD spectra. Remarkably,by using these chiral sensors, peptide sequences can also berecognized. Most interestingly, since the chiral sensors areconstructed via noncovalent interactions with good reversibility,real-time monitoring of the chirality of analytes can be achieved.Therefore, for certain enzyme-catalyzed chemical reactions, therate of reactions, the yield of products, and the ee values of theproducts can be detected in real time by measuring the CDspectra (Figure 132B).374

Although we would like to focus on supramolecular chiralitywithin self-assembled systems in this review, we still need tocover some of the chiral metal nanomaterials in this portion tofully address chiral sensing. Because of the free electrons on thesurface, the plasmonic absorption or CD spectra from the chiralmetal nanomaterials can be very sensitive for detecting theinteractions with other chiral molecules,375−380 and chiral metalnanomaterials can be constructed as ultrasensitive chiral sensors.

For example, Kadodwala et al. built an ultrasensitive chiral sensorbased on chiral metamaterials, which had the potential todiscriminate between large biomolecules with similar levels ofsensitivity and subtle structural differences at pictogramquantities. In this study, the optical excitation of plasmonicplanar chiral metamaterials can generate superchiral electro-magnetic fields, which are highly sensitive for the detection ofchiral peptide nanostructures. The sensitivity of this chiral sensorwas found to be up to 106 times greater than that of the opticalpolarimetry measurements. The largest differences wereobserved for proteins with high β-sheet content. The system

Figure 131. (A) Four-component reversible covalent assembly for secondary alcohol binding. OTf is trifluoromethanesulfonate (triflate). (B)Exploration of four-component assembly for chirality sensing and ee determination. CD spectra of 1-phenylethanol-induced assembly with different ee’sof the alcohol (from top to bottom −100%, −80%, −60%, −40%, −20%, 0%, 20%, 40%, 60%, 80%, 100%). Reprinted with permission from ref 373.Copyright 2011 Nature Publishing Group.

Figure 132. (A) Schematic illustration of the chiral sensing based on theternary complexes between the macrocyclic host cucurbit[8]uril,dicationic dyes, and chiral aromatic analytes. (B) Examples of reactionmonitoring. Reprinted with permission from ref 374. Copyright 2014John Wiley & Sons.

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has great potential for detecting amyloid diseases and certaintypes of viruses (Figure 133).381

6.2. Supramolecular Chiroptical Switches

In the field of supramolecular chemistry and nanotechnology,constructing assemblies that act like a “switch” is one of the mostinteresting topics. In general, any assembly in which specificproperties can be reversibly changed with an external stimuluscan be regarded as a supramolecular switch. A supramolecularchiroptical switch is based on reversible changes of supra-molecular chirality, which is seen as externally stimulated opticalactivity.382−384 The changes of supramolecular chirality can beachiral to chiral, either reversible or reversible from left-handedchirality to right-handed chirality.Various supramolecular assemblies, such as liquid crystals,385

host−guest complexes,386 LB films,387−389 and supramoleculargels,51,56 have been developed as supramolecular chiral switches.In addition, some small organic molecules or polymers can alsobe used as chiroptical switches based on photoirradiation orinteraction with other small molecules.391−396

A supramolecular chiroptical switch based on an amorphousazobenzene polymer (156) has been constructed by Kim et al.When the thin films of an achiral epoxy-based polymercontaining photoresponsive azobenzene groups were irradiatedby elliptically polarized light (EPL), supramolecular chirality wasintroduced into the system, which was confirmed by the CDspectral measurements. The helical arrangement of theazobenzenes plays a very important role in the photoinducedsupramolecular chirality. When the irradiation at 488 nm waschanged from right-handed elliptical polarization to left-handedelliptical polarization, the handedness of the chiral supra-molecular assembly also changed reversibly. This supramolecularchiroptical switch was able to operate several times before fatigueresistance occurred (Figure 134).391

Polythiophene is a very important conductive polymer.Yashima et al. constructed the first reversible supramolecularchirality switch based on chiral polythiophene aggregates. Whencopper(II) trifluoromethanesulfonate [Cu(OTf)2] was added tochiral aggregates of chiral regioregular polythiophene in achloroform−acetonitrile mixture, the CD signal of the assembliesdisappeared due to the oxidative doping of the polymer mainchain. However, when amines, such as triethylenetetramine(TETA), were added to the system to undope the polymer, theCD signals reappeared. Thus, a supramolecular chiral switch canbe prepared from the assembly of chiral polythiophenes with the

addition or removal of an electron from the correspondingpolymer main chain.397

Chiral LS films constructed from the air/water interfacialassembly of achiral molecular building blocks can be used assupramolecular chiral switches. In this context, the handedness ofthe supramolecular chirality of assemblies constructed by achiralmolecules can be changed by an external stimulus. This flexibility

Figure 133.Changes induced in the chiral plasmonic resonances of the planar chiral metamaterials (PCM) are readily detected using CD spectroscopy.(A) CD spectra collected from PCMs immersed in distilled water. (B) Influence of the adsorbed proteins hemoglobin, β-lactoglobulin, and thermallydenatured β-lactoglobulin on the CD spectra of the PCMs. (C) Hemoglobin (top) and β-lactoglobulin (bottom) (α-helix, cyan cylinder; β-sheet,ribbons), shown adopting a well-defined arbitrary structure with respect to a surface. The figure illustrates the more anisotropic nature of adsorbed β-lactoglobulin. Reprinted with permission from ref 381. Copyright 2010 Nature Publishing Group.

Figure 134. (A) Chemical structure of an achiral epoxy-based polymercontaining photoresponsive azobenzene groups (156) and the helicalarrangement of the azobenzenes with different handedness. (B) (a)Chiroptical switching of the CD spectra by alternating irradiation with r-and l-EPL. (b) Intensity of the CD signal at 410, 510, and 700 nm.Reprinted with permission from ref 391. Copyright 2006 John Wiley &Sons.

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is a typical characteristic of assemblies based on noncovalentinteractions, and supramolecular chiral switches have beenachieved from the assembly of amphiphilic molecules on a watersurface. For example, achiral 5-(octadecyloxy)-2-(2-thiazolylazo)phenol (TARC18, 157) can form chiral LS filmsvia an air/water interfacial assembly. Supramolecular chiralswitches based on the LS films of achiral TARC18 werefabricated by alternately exposing the film to HCl gas and to air(Figure 135).387

Chiral thin films that were constructed from achiralphthalocyanine derivatives via air/water interfacial assemblycan also be used as supramolecular chiral switches.382 Inparticular, supramolecular chiroptical switches based on theassembly of achiral phthalocyanine derivatives were found to bevery stable. Certainly, the π−π interactions between phthalo-cyanine rings can increase the stability of the assembly. Mostimportantly, the polymerization of achiral phthalocyaninederivatives can produce chiral assemblies based on covalentbonds. When LS films containing polymerized chiral assembliesof achiral phthalocyanine derivatives were alternately exposed toHCl and NH3, a reversible change in the CD spectra wasdetected. This process can be repeated many times without anydecrease in CD signal intensity.Zou et al. constructed LB films of azobenzene-substituted

diacetylene (NADA, 158). Although NADA is achiral, theNADA LB films show supramolecular chirality. Moreover, theassemblies within the LB films can be polymerized withphotoirradiation. When left- and right-handed circularlypolarized ultraviolet light (CPUL) was applied, polymerizedNADA (PNADA) LB films with different chirality were obtained,as confirmed by the corresponding CD spectra from bothazobenzene chromophores and polydiacetylene (PDA) chains.Interestingly, these polymerized LB films NADA (PDA LB films)can be used as chiroptical switches after irradiation using left- andright-handed circularly polarized lasers (CPL, 442 nm), due toalternation of the stereoregular packing of azobenzenechromophores.388

The self-assembly of banana-shaped achiral molecules wasfound to lead to chiral liquid crystals, and chiroptical switchesbased on liquid crystals containing only achiral molecularbuilding blocks were also achieved. Tschierske et al. synthesizedbent-core mesogens carrying branched oligosiloxane units. Achiroptical switch based on the phase transition was obtained byapplying an electric field or changing the temperature.398

An elegant light-driven supramolecular chiroptical switch wasdeveloped by Feringa and co-workers. This work was based on

their well-known work on light-driven molecular motors.399−401

In this context, the modified light-driven molecular motor wascovalently connected to the terminus of poly(n-hexyl isocyanate)(PHIC), and the chiral information from the headgroups of thepolymer was found to be expressed at both the supramolecularand the macromolecular levels. Upon irradiation with twodifferent wavelengths of light, a chiroptical switch has beendemonstrated, as evidenced by the CD spectral measurementsand images obtained from an optical microscope equipped withcrossed polarizers (Figure 137).385

Chiral polymers containing azobenzene groups were alsofound to form liquid crystals. Chiral switches based on thesechiral liquid-crystalline polymers were investigated by Angioliniand co-workers. It was found that the chiral polymers containingazobenzene groups and L-lactic acid formed a smectic A1/2 (fullyinterdigitated) liquid-crystalline phase. The resulting chiropticalswitching of the system was achieved by irradiation withcircularly polarized light (CPL) with different handedness.402

Chiroptical switches based on some soft matters, such assupramolecular gels or supramolecular assemblies in solution,have attracted increased attention recently. Cucurbituril is a veryimportant building block for preparing chiral supramolecularassemblies. Supramolecular chiroptical switches based on thecoassembly of chiral binaphthalene−bipyridinium guests togeth-er with cucurbituril hosts have been developed by Venturi and

Figure 135. (A) Molecular structure of TARC18 (157). (B) Schematicillustration of the possible helical stacking of the TARC18 (157)molecules in the LS films.M and P chiralities of the films were formed bychance. Reprinted with permission from ref 387. Copyright 2006 JohnWiley & Sons.

Figure 136. (A) Molecular structure of azobenzene-substituteddiacetylene (NADA, 158). (B) Schematic illustrations of (a)enantioselective polymerization with CPUL irradiation and (b) chiralitymodulation for NADA LB films with CPL treatment. Reprinted withpermission from ref 388. Copyright 2009 The Royal Society ofChemistry.

Figure 137. Schematic representation of the full photocontrol of themagnitude and sign of the supramolecular helical pitch of a cholestericLC phase generated by a polyisocyanate with a single chiropticalmolecular switch covalently linked to the polymer’s terminus. Reprintedwith permission from ref 385. Copyright 2008 American ChemicalSociety.

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Tian et al.403 In this study, three molecular tweezers containing4,4′-bipyridinium (BPY2+) and (R)-2,2′-dioxy-1,1′-binaphthyl(BIN) units with different length alkyl chains were designed andsynthesized (159). In aqueous solution, these chiral binaph-thalene−bipyridinium guests formed complexes with cucurbitur-il hosts. In this system, chiroptical switching was achieved fromthe reversible changes of the helicity of the BIN units, which wastriggered by reduction of the BPY2+ units. In addition, the alkyllinkers also play an important role in association to cucurbiturils.Thus, changing the length of the alkyl chains modulated theproperties of the chiral switches, such as the molar ratio of thecomplex and the dihedral angle of BIN (Figure 138).

The very good stimulus-responsive properties of self-assembled systems based on noncovalent interactions can helpthese systems form chiral switches with unique performance. Forexample, Liu et al. synthesized a glutamic-acid-based lipidcontaining an azobenzene headgroup (azo-LG2C18, 5), whichcan form organogels with supramolecular chirality in differentorganic solvents. Remarkably, the resulting organogels can beused as chiroptical switches with multiresponsibility. Thus, thesupramolecular chirality can be changed reversibly by photo-irradiation, temperature variation, or solvent polarity (Figure139).56

A supramolecular chiroptical switch based on multicompo-nent self-assembled soft matters has also been developed by Liugroup. The self-assembly of chiral glutamic-acid-based bolaam-phiphiles (HDGA, 15b) can lead to hydrogels and chiralsupramolecular nanotubes. For the construction of multi-component soft matters, the azobenzene derivative 4-(phenylazo)benzoic acid sodium salt (Azo) can be coassembledwith either HDGA molecules (15b) or chiral supramolecularnanotubes. Although very strong supramolecular chirality can bedetected from the coassembly of Azo with the preformed chiral

nanotubes, this hierarchical assembly cannot be used as achiroptical switch. Only when Azo was coassembled with thebolaamphiphile at the molecular level could the correspondingassemblies show reversible changes both in the UV−vis and inthe CD spectra upon alternative UV−vis irradiation of Azo(Figure 140).404

Chiral supramolecular nanotubes assembled by L- or D-glutamic-acid-based bolaamphiphiles (HDGA, 15b) in water canalso be used as templates to produce silica nanotubes. Mostinterestingly, only the inner walls of the formed silica nanotubeswere found to have supramolecular chirality. When thephotoactive azobenzene moieties were loaded onto the innerchiral silica nanotubes, chiroptical switches based on inorganicnanomaterials resulted (Figure 141).27

6.3. Supramolecular Chiral Catalysis

In synthetic chemistry, construction of chiral molecules has beenfound to be extremely important. Therefore, developing differentchiral homogeneous catalysts and heterogeneous catalysts hasbecome a significant research goal.405−411 For example,coordination complexes containing metal ions and chiral ligandshave been widely investigated as highly efficient chiral catalysts.Many outstanding reviews have been published examining thisissue.410−416 In particular, as a class of very importantcoordination complexes with crystalline structures, metal−organic frameworks (MOFs), which have infinite networkstructures built with multitopic organic ligands and metal ions,have been thoroughly studied as potential asymmetric catalysts.This topic has also been extensively reviewed recently.417

Similarly, the catalytic properties of cyclodextrin derivativeshave also attracted increased attention recently.418,419 As thechiral host, cyclodextrin derivatives can provide a suitable chiralenvironment, and reactions at the guest molecules can providereaction products with chirality. Moreover, the molecularstructures of cyclodextrin derivatives can be further modifiedto obtain more efficient chiral catalysts.420 There are also manygood review articles concerning chiral catalysis based oncyclodextrin derivatives. For example, Inoue and co-workerssummarized supramolecular photochirogenesis based on cyclo-dextrin derivatives.421,422

In a general sense, “supramolecular chiral catalysis” is presentlya major topic of research interest, but we cannot address everyaspect of this field of research. In this review, we concentrate onthe catalytic properties of some chiral supramolecular assemblies.Although the catalytic properties of metal complexes have been

Figure 138. (A) Structure of chiral binaphthalene−bipyridinium guests(159). (B) Pictorial representation of the conformational rearrange-ments of 159 in response to two-electron reduction and/or complex-ation with either CB[8] or CB[7]. Reprinted with permission from ref403. Copyright 2012 John Wiley & Sons.

Figure 139. Schematic illustration of the self-assembled azobenzene-containing lipid showing multiresponsibility for chiral switching.Reprinted with permission from ref 56. Copyright 2011 AmericanChemical Society.

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well investigated, supramolecular polymers based on coordina-tion interactions have very special characteristics for supra-molecular chiral catalysis.For example, Ding et al. constructed polymeric supra-

molecular chiral catalysts based on self-assembly. The authorssynthesized an organic ligand containing ureido-4[1H]-ureidopyrimidone (UP) and Feringa’s MonoPhos motifs. Bymixing the organic ligand with [Rh(cod)2]BF4, supramolecularpolymers can be produced by orthogonal self-assembly viahydrogen-bonding and ligand-to-metal coordination interac-tions. This supramolecular polymer shows excellent asymmetric

induction and reusability in the catalysis of asymmetrichydrogenation of dehydro-α-amino acid and enamide derivatives(Figure 142).423

Furthermore, Ding et al. synthesized a heteroditopic ligandcontaining a 2,2′:6′,2″-terpyridine (tpy) unit and Feringa’sMonoPhos (160). The selective coordination of this ligand withFeII and RhI ions can produce chiral supramolecular polymers,which can be used as chiral bimetallic self-supported catalysts. Inthe hydrogenation of α-dehydroamino acid, enamide, anditaconic acid derivatives, these reusable heterogeneous asym-metric catalysts can lead to very high reaction rates with excellentenantioselectivity (90−97% ee) (Figure 143).424

Among various supramolecular chiral catalysis from self-assembly, ion pair catalysts, which have been developed in recentyears, are very important systems with many distinctivecharacteristics. Ooi et al. developed supramolecular assembliescontaining ion pairs through intermolecular hydrogen bonding.This system was formed by the coassembly of a chiraltetraaminophosphonium cation, two phenols, and a phenoxideanion and was found to have chiral catalytic activity (161). Insolution, this ion pair complex promotes a highly stereoselectiveconjugate addition of acyl anion equivalents to α,β-unsaturatedester surrogates with a broad substrate scope (Figure 144).425

Ishihara et al. also produced self-assembled chiral catalystswithout metal ions. These supramolecular catalysts are based onin situ coassembly from chiral diols, arylboronic acids, andtris(pentafluorophenyl)borane (162). In the Diels−Alderreactions of cyclopentadiene with different acroleins, these

Figure 140. (A) Structures of the hydrogelators HDGA (15b) and Azo. (B) Gels formed by L-HDGA in water in the absence and presence of Azo. (C)Illustration of the coassembly of Azo with HDGA. Reprinted with permission from ref 404. Copyright 2011 The Royal Society of Chemistry.

Figure 141. Creating chirality in the inner walls of silica nanotubesthrough a hydrogel template, and the chiroptical switching of thesenanotubes. Reprinted with permission from ref 27. Copyright 2010 TheRoyal Society of Chemistry.

Figure 142. (A) Polymeric supramolecular chiral catalyst based on self-assembly. (B) Asymmetric hydrogenation of dehydro-α-amino acid and enamidederivatives. Reprinted with permission from ref 423. Copyright 2006 John Wiley & Sons.

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catalysts have very good endo/exo selectivities and highenantioselectivities (Figure 145).426

Supramolecular catalysts based on the coassembly of chiralamines and poly(alkene glycol)s have been reported by Xu et al.These systems were found to be highly efficient in theasymmetric catalysis of the unusual Diels−Alder reactionbetween cyclohexenones and nitrodienes, nitroenynes, ornitroolefins, providing excellent chemo-, regio-, and enantiose-lectivities (Figure 146).427

Although many chiral supramolecular catalysts have beendeveloped via self-assembly, the chiral catalysis characteristics ofsystems are still largely dependent on chirality at the molecularlevel. By contrast, even if many chiral nanostructures have beenconstructed via self-assembly, the relationship between chiralityat the nanoscale and chiral catalysis at the molecular level is stillrarely discussed.As mentioned previously, the self-assembly of chiral glutamic-

acid-based bolaamphiphiles (HDGA, 15b) led to hydrogels andchiral supramolecular nanotubes. When Cu2+ ions were added tothe system, a monolayer nanotube was transformed into a

Figure 143. (A) Molecular structure of a ligand containing the 2,2′:6′,2″-terpyridine (tpy) unit and Feringa’s MonoPhos (160), and schematicillustration of supramolecular catalysts through orthogonal coordination of two different metal ions with a single ditopic ligand. (B) Asymmetrichydrogenation of dehydroamino acid, enamide, and itaconic acid derivatives using a catalyst with high reaction rate and excellent enantioselectivity.Reprinted with permission from ref 424. Copyright 2010 John Wiley & Sons.

Figure 144. (A) Structures of chiral tetraaminophosphonium cations.(B) Oak Ridge thermal ellipsoid plot diagram of 161a·(OPh)3H2. (C)Scope of α,β-unsaturated acylbenzotriazole. Reprinted with permissionfrom ref 425. Copyright 2009 The American Association for theAdvancement of Science.

Figure 145. (A) Structures of chiral supramolecular catalyst (162). (B) Enantioselective Diels−Alder reactions with anomalous endo/exo selectivitiesusing chiral supramolecular catalysts. Reprinted with permission from ref 426. Copyright 2011 John Wiley & Sons.

Figure 146. Coassembly of chiral amines and poly(alkene glycol)sshowing highly efficient asymmetric catalysis of Diels−Alder reactions.Reprinted with permission from ref 427. Copyright 2011 John Wiley &Sons.

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multilayer nanotube with a tubular wall thickness of about 10 nm.Interestingly, the resulting Cu2+-containing supramolecularnanotubes were useful as an asymmetric catalyst for the Diels−Alder reaction between cyclopentadiene and aza-chalcone, whichaccelerates the reaction rate and enhances enantiomericselectivity. Thus, asymmetric catalysis of the molecular reactioncan be achieved by chiral nanostructures. It was suggested thatthrough the Cu2+-mediated nanotube formation the substratemolecules could be anchored on the nanotube surfaces toproduce a stereochemically favored alignment (Figure 147).Therefore, when adducts reacted with the substrate, both theenantiomeric selectivity and the reaction rate were found toincrease.428

Besides catalytic nanotubes, chiral catalysis based on supra-molecular nanostructures has also been observed using vesicles.Liu et al. synthesized amphiphilic molecules containing a prolineheadgroup (PTC12, 163). The self-assembly of PTC12 in waterunder compressed CO2 can produce vesicles. These assemblieswere found to catalyze the asymmetric aldol reaction with highenantiomeric selectivity without any additives. Importantly, thesize of the PTC12 assemblies and subsequently catalyst activityand stereoselectivity can be dynamically modulated by changingthe status of the compressed CO2. Moreover, because CO2 canbe easily removed from the system, it is very convenient for theseparation and purification of products, as well as the reuse of thechiral supramolecular catalysts (Figure 148).429

In the development of supramolecular chiral catalysis, chiralcovalent polymers, including some biomacromolecules, such asDNA and polypeptides, can be used as building blocks. Thecatalytic capability of these polymers may originate from themolecular chiral centers within these polymers but may alsoresult from their folding characteristics and hierarchicalnanostructures.Meijer and co-workers synthesized water-soluble segmented

terpolymers containing PEG and chiral benzene-1,3,5-tricarbox-amide side chains as well as a ruthenium complex (164). Due tothe chiral self-assembly of the benzene-1,3,5-tricarboxamide sidechains, the folding of these polymers can produce a helicalstructure in the apolar core around a ruthenium-based catalyst.This catalyst, resulting from the folding of polymers, was foundto catalyze the transfer hydrogenation of ketones (Figure149).430

Yashima et al. synthesized a chiral polymer containingriboflavin units as the main chain (165). Within the polymers,the 5-ethylriboflavinium cations can be reversibly transformedinto 4a-hydroxyriboflavins upon hydroxylation/dehydroxylation,which renders significant changes in the absorption and circulardichroism (CD) spectra of the polymers. It is believed that theface-to-face stacking of the intermolecular riboflavinium unitswithin the polymer produced twisted helical nanostructures withsupramolecular chirality. This optically active polymer contain-ing 5-ethylriboflavinium cations was found to efficiently catalyzethe asymmetric organocatalytic oxidation of sulfides withhydrogen peroxide, yielding optically active sulfoxides with upto 60% ee (Figure 150).431

DNA is a very important genetic material of living organisms.However, it can also be regarded as a very useful chiral functionalpolymer for different applications. As a polymer, DNA has manydifferent molecular chiral centers and charged substituents,which can increase its solubility in water. Moreover, the folding ofDNA can produce very ordered nanostructures via hydrogenbonding, which can also be modulated by changing thesequences in the DNA. It should be noted that DNA is relativelystable in comparison to other biomacromolecules, such as RNA.Therefore, developing DNA-based supramolecular chiral cata-lysts has recently attracted interest.The first DNA-based asymmetric catalyst containing copper

ions was reported by Roelfes and Feringa. In this study, theauthors synthesized an achiral ligand containing a DNA-intercalating moiety (9-aminoacridine), alkyl chain spacer, andmetal-binding group (166). A DNA-based asymmetric catalystwas fabricated from the coassembly of a copper(II)-enclosingligand and DNA. In the Diels−Alder reaction, this chiral catalystcould transfer the chirality of the DNA into the products, withthe an ee value up to 90% (Figure 151A).432

For many other organic chemical reactions, a DNA-basedchiral catalyst containing copper(II) was also found to be veryuseful. For example, Roelfes et al. developed a DNA-basedasymmetric catalyst containing copper(II) and an achiral ligandfor catalyzing the Michael reaction in water to achieve highenantioselectivity. These reactions can be performed on arelatively large scale, allowing recycling of the supramolecularchiral catalyst. For this system, many simple achiral ligands, such

Figure 147. Illustration of the assembly mechanism of the Cu2+−L-HDGA nanotubular structure and its asymmetric catalysis of the Diels−Alder reaction of aza-chalcone with cyclopentadiene. Reprinted withpermission from ref 428. Copyright 2011 American Chemical Society.

Figure 148. Self-assembly of vesicles regulated by compressed CO2 andthe proposed transition-state model for the direct asymmetric aldolreaction. Reprinted with permission from ref 429. Copyright 2013 JohnWiley & Sons.

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as dipyridine, can be used for coassembly with DNA and copperions (Figure 151B). The reactants in this study included α,β-unsaturated 2-acylimidazoles working as the Michael acceptorsand nitromethane and dimethyl malonate as the nucleophiles.Upon chiral catalysis by the DNA-based assemblies containingcopper(II) and achiral ligand, the enantioselectivities of theMichael reaction were found to be up to 99% ee.433

Furthermore, Feringa and Roelfes also studied asymmetricFriedel−Crafts alkylation with olefins in water catalyzed by aDNA-based chiral catalyst containing copper(II). In this system,4,4′-dimethyl-2,2′-bipyridine (dmbpy) was used as the achiralligand for the complex with copper(II) and coassembly withDNA. For the asymmetric Friedel−Crafts reaction of α,β-unsaturated 2-acylimidazoles with heteroaromatic π nucleo-philes, good yields and high enantioselectivities were obtainedusing a very small of amount of DNA-based chiral catalysts(Figure 151C). In this study, the catalytic efficiency of bothdouble-stranded DNA and single-stranded DNA with differentsequences was investigated. The results showed that only thechiral catalysts assembled by the double-stranded DNA canintroduce high enantioselectivities by catalyzing the asymmetricFriedel−Crafts alkylation. In addition, the highest enantiose-lectivities (up to 93%) were obtained by the supramolecularcatalysts assembled using d(TCAGGGCCCTGA)2 DNA.

434

A DNA-based chiral catalyst containing copper(II) and achiralligand was also studied in the reaction of asymmetricintramolecular cyclopropanation (Figures 151−155). For theasymmetric intramolecular cyclopropanation of α-diazo-β-keto

sulfones in water, high enantioselectivities with ee values of up to84% were achieved.435

Most interestingly, use of DNA-based chiral catalystscontaining copper(II) for chemical reactions in biologicalsystems, such as enantioselective addition of water to olefins inan aqueous environment, resulted in good efficiency under thelaboratory conditions. For the enantioselective hydration ofenones, the chiral β-hydroxy ketone product can be obtainedwith an ee up to 82% upon catalysis with DNA-based assemblies(Figure 151D). Moreover, the reaction was also found to bediastereospecific, with the formation of only the syn hydrationproduct.436

The folding of DNA can produce different nanostructures,which can also be modulated by changing the DNA sequences orother conditions for assembly. Besides double-helix DNA,telomeric G-quadruplex DNA was also studied to constructDNA-based chiral catalysts. Moses et al. constructed supra-molecular chiral catalysts based on the assembly of telomeric G-quadruplex DNA, achiral ligands, and copper(II). These catalyticsystems were found to catalyze Diels−Alder reactions success-fully with modest enantioselectivities.437

Another class of G-quadruplex-DNA-based chiral catalysts wasdeveloped by Li and co-workers. These supramolecular chiralcatalysts were constructed by self-assembly of human telomericG4DNA and different metal ions. In this case, additional achiralligands were not needed for building the catalysts. In anasymmetric Diels−Alder reaction, the complex of humantelomeric G4DNA and Cu2+ ions provided a significantenhancement in the reaction rate with good enantioselectivity

Figure 149. (A) Water-soluble segmented terpolymer containing PEG and chiral benzene-1,3,5-tricarboxamide side chains as well as a rutheniumcomplex (164). (B) Supramolecular single-chain folding of polymers in water affording a compartmentalized catalyst for the transfer hydrogenation ofketones. Reprinted with permission from ref 430. Copyright 2011 American Chemical Society.

Figure 150. Optically active polymers consisting of riboflavin units catalyze the asymmetric organocatalytic oxidation of sulfides. Reprinted withpermission from ref 431. Copyright 2012 American Chemical Society.

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(74% ee). In addition, the rate and enantioselectivity of thereaction can be modulated by changing the DNA sequence andmetal ions used to form the complex. Interestingly, the absoluteconfiguration of the products can be controlled by the assemblyof chiral catalysts (Figure 152). Thus, when the conformation ofthe G4DNA was switched from antiparallel to parallel, theabsolute configuration of the products obtained from Diels−Alder reactions could be reversed.438

For the DNA-based chiral catalysis, the relationship betweenthe handedness of the DNA helix and the molecular chirality ofproducts was investigated by Smietana and Arseniyadis et al.They constructed different DNA-based supramolecular chiralcatalysts from the assembly of both L-DNA and D-DNA. The L-DNA, which contains deoxyribose with an L-conformation, canself-assemble into left-helical nanostructures, while the folding ofnormal DNA only produces right-helical nanostructures. There-fore, the L-DNA-based and D-DNA-based supramolecular chiral

Figure 151. (A) Asymmetric Diels−Alder reaction of cyclopentadiene with aza-chalcone, catalyzed by copper complexes of the ligand in the presence ofDNA. (B) Asymmetric Michael addition reaction catalyzed by complexes formed between copper(II) ions and achiral ligands in the presence of DNA.(C) Cu−dmbpy/st-DNA-catalyzed Friedel−Crafts alkylation. Reprinted with permission from refs 432, 433, and434. Copyright 2005, 2007, and 2009John Wiley & Sons. (D) DNA-based catalyst and general reaction scheme of the catalytic enantioselective hydration of a variety of α,β-unsaturated 2-acyl-(1-alkyl)imidazole substrates, and overview of ligands used in this study. Reprinted with permission from ref 436. Copyright 2010 NaturePublishing Group. (E) Intramolecular cyclopropanation of α-diazo-β-keto sulfones in water using a DNA-based catalyst. Reprinted with permissionfrom ref 435. Copyright 2013 The Royal Society of Chemistry.

Figure 152. Enantioselective Diels−Alder reactions with G-quadruplex-DNA-based catalysts; the absolute configuration of the products can be reversedwhen the conformation of the G4DNA is switched from antiparallel to parallel. Reprinted with permission from ref 438. Copyright 2012 John Wiley &Sons.

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catalysts have totally different supramolecular chirality (Figure153). In the case of Friedel−Crafts reactions and Michaeladditions using many different substrates, enantiomers of theproducts can be obtained by the catalysis of L-DNA- or D-DNA-based supramolecular chiral catalysts.439

In the construction of DNA-based chiral catalysts, even thoughmetal ions are very important, they are not always necessary.Andreasson et al. studied the asymmetric closing reaction ofdithienylethene derivatives by complexion with DNA uponphotoirradiation. In this study, fluorinated dithienylethenederivatives containing methylpyridinium and methylquinoliumsubstituents were bound to DNA in both the open and the closedforms. Cyclization of dithienylethene derivatives upon photo-irradiation could produce the closed form of these moleculeswith significant enantioselectivity (Figure 154). In this case,chirality was transferred from the DNA to the products.440

Although the most popular natural chiral catalysts (enzymes)are polypeptides, constructing artificial enzymes via thecoassembly of polypeptides with other molecules has beenonly partially successful. Polypeptides are generally much morecomplicated biomacromolecules than DNA, due to the greaterarray of molecular building blocks available for the formation ofpolypeptides and the relatively weak but complicated non-

covalent interactions between amino acids during the folding ofpolypeptides, which produce more flexible nanostructures. Forsupramolecular chiral catalysis under laboratory conditions, suchcomplicated and often unstable nanostructures are not easy tohandle. Therefore, even though nature has used enzymes forcatalyzing many very subtle chemical reactions for billions ofyears in catalyzing chemical reactions under artificial conditions,DNA is still a better candidate.Nevertheless, polypeptide-based chiral catalysts have also been

constructed recently. For example, Roelfes et al. modulatednatural bovine pancreatic polypeptides with nonproteinogenicamino acids for binding Cu2+ ions. The resulting metalloenzymescatalyze Diels−Alder and Michael addition reactions in waterwith high enantioselectivities (Figure 155).441

Herrmann et al. developed chiral catalysts based on naturalpolypeptides without many chemical modifications. Thesepolypeptides are cyclic peptides formed by intramoleculardisulfide linking of cysteine residues at both ends of the peptide.The chiral catalysts were constructed by binding the cyclicpeptide with Cu2+ ions. The advantage of this system is small-sequence constriction and flexibility in the amino acids of thepolypeptides. In catalyzing Diels−Alder and Friedel−Craftsreactions, these cyclic-peptide-based chiral catalysts achievedhigh enantioselectivities of up to 99% ee and 86% ee, respectively(Figure 156). Furthermore, in this work, Herrmann et al. also

Figure 153.Tuning the absolute configuration in DNA-based asymmetric catalysis. Reprinted with permission from ref 439. Copyright 2013 JohnWiley& Sons.

Figure 154. Enantioselective cyclization of photochromic dithienyle-thenes bound to DNA. Reprinted with permission from ref 440.Copyright 2013 John Wiley & Sons.

Figure 155. Enantioselective artificial metalloenzymes based on a bovine pancreatic polypeptide scaffold, showing catalytic Diels−Alder and Michaeladdition reactions in water with high enantioselectivities. Reprinted with permission from ref 441. Copyright 2009 John Wiley & Sons.

Figure 156. (a) Cyclic peptide ligand with constrained conformationthrough an intramolecular disulfide bridge. (b) D−A reaction catalyzedby cyclic peptide ligand and Cu2+. Reprinted with permission from ref442. Copyright 2014 John Wiley & Sons.

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systematically studied the relationship between amino acidsequences and the corresponding enantioselectivities of thecatalytic reactions. The results show that the position of alaninewithin the sequences plays a very important role.442

6.4. Optics and Electronics Based on Supramolecular ChiralAssembly

Development of functional devices is one of the main objectivesof research on supramolecular assembly. In the application ofsupramolecular chirality from self-assembled systems, chiralelectronic or optical devices are also worth discussing. Recently,these issues have attracted increasing interest.443−448 Chiraloptics and electronics are directly dependent on many differentfunctions of supramolecular chiral assemblies. For example,chiral sensors can be used to construct chiral electronic devices,as shown by Wei and co-workers. They prepared ultraorderedsuperhelical microfibers with clear screws and favorablemonodispersity from chiral polyaniline (PANI). When thesesuperhelical microfibers were treated with chiral aminohexanevapor with different handedness, very different electricalconductivity in these microfibers was detected (Figure156).449,450

In the development of chiral electronic or optic devices, themost important aspect is the relationship between the optical/electrical properties of the materials and the supramolecularchirality. The first important work concerning chiral optics waspublished by Verbiest et al. in 1998. The authors prepared LBfilms of helicene with different supramolecular chirality andstudied the second-order nonlinear optical (NLO) properties ofthese LB films. The results show that the second-order NLOsusceptibility of the chiral assemblies can be 30 times larger thanthat of the racemic material with the same chemical structure.451

Except for optical properties, the electrical properties of somesupramolecular assemblies were also found to be dependent onthe material’s chirality. For example, Fourmigue et al. studied theelectronic conductivity of chiral salts of tetrathiafulvalenemethyl−oxazoline derivatives. The results showed that theconductivity of the pure enantiomeric salts can be an order ofmagnitude higher than the conductivity of the racemic salts.452

Wei et al. fabricated hierarchical chiral assemblies of theconducting polyaniline (PANI) with different nanostructuresand superstructures by controlling the interactions betweenmolecules. The anisotropic electrical transport properties basedon the arrangement of molecules and nanostructures wereprobed.453

The semiconductor properties of supramolecular assemblieshas always been very interesting. Remarkably, supramolecularchirality was also found to play a very important role in this issue.Mullen et al. studied field-effect transistor devices based on theassembly of coronene (168). In this system, discotic liquid

crystals, which formed from the assembly of coronene, are at thecore of thematter. Thus, stacking at certain twisting angles withinthe discotic liquid crystals increased charge carrier mobility ofthese devices.454

The photocurrent properties of the supramolecular chiralassemblies formed at the air/water interface were investigated bythe Liu group. They found that an anthracene derivative (AN)could be controllably assembled to nanocoils and straightnanoribbons on water surfaces depending on the differentsurface pressures. Most interestingly, the nanoribbons exhibiteda switchable photocurrent, while the nanocoils did not show aphotocurrent response (Figure 159).455

Not only can a photocurrent be generated from the chiralsupramolecular assemblies, devices based on chiral supra-molecular assemblies can also be used as sensors for detectingcircularly polarized light. These results were reported by Fuchterand Campbell et al. In this study, they constructed organic fieldeffect transistors from the assembly of helicene (Figure 160A),and a highly specific photoresponse to circularly polarized lightwas detected. Importantly, the photoresponse to circularlypolarized light was found to be directly related to the handednessof the helicene molecule (Figure 160B).456

6.5. Circularly Polarized Luminescence (CPL) Based on ChiralSupramolecular Assemblies

Circularly polarized light is inherently chiral and has beenregarded as one possible origin of natural homochirality457 andthe source of chiral information during the emergence oflife.308,309 As we described previously, supramolecular chiralitywith controlled handedness can be introduced into the

Figure 157. Superhelical conducting microfibers with homochirality forenantioselective sensing. Reprinted with permission from ref 449.Copyright 2013 American Chemical Society.

Figure 158. (A) Molecular structure of coronene (168). (B) Chargemobilities as a function of temperature as measured by the pulseradiolysis time-resolved microwave conductivity (PR-TRMC) techni-que. Reprinted with permission from ref 454. Copyright 2009 NaturePublishing Group.

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supramolecular assemblies containing only achiral molecularbuilding blocks via irradiation with circularly polarized light(CPL). Among the different circularly polarized light, thecircularly polarized luminescence from chiral assemblies,abbreviated CPL, can be extremely important and is attractingincreasing research interest.In the generation of circularly polarized luminescence (CPL)

from chiral supramolecular assemblies, the chiral arrangement ofthe luminescent chromophores is an essential prerequisite. Whenluminophores exist in a dissymmetric environment within thephotoexcited state, circularly polarized luminescence (CPL) isgenerated. The study of the circularly polarized luminescence(CPL) from chiral assemblies has been widely dominated bylanthanide complexes owing to their ability to exhibit high CPLdissymmetry.458,459

On the other hand, many different chiral supramolecularassemblies resulting from organic molecular building blocks havealso been recently found to be very important sources forgenerating circularly polarized luminescence (CPL). Thissituation can be another important application of chiralsupramolecular assemblies. The most prominent efforts arebased on the π-conjugated polymers with chiral side chains orhelical aggregated nanostructures that have been reported toshow intense CPL signals.460−465

For example, Swager et al. synthesized chiral poly(p-phenylenevinylene) derivatives and studied the circularlypolarized luminescence (CPL) spectroscopy of the assembliesfrom this polymer. Interestingly, using the same polymer withsame molecular chirality, different supramolecular assemblieswere found to produce opposite CPL spectra.463

Akagi et al. synthesized chiral polythiophenes and chiralthiophene−phenylene copolymers and found that thesepolymers, with different molecular structures and aggregationstates, could exhibit red, green, and blue fluorescent. Remarkably,mixing these different fluorescent polymers generated a unique,circularly polarized white luminescence.465

Another type of important chiral supramolecular system forgenerating circularly polarized luminescence is the assembly ofhelicenes. The aggregation of chiral helicenes was found to showlarge CPL dissymmetry owing to the strong helical distortion of πsystems.466−469

Maeda et al. introduced the BINOL−boron moiety todipyrrolyldiketones and fabricated the chiral conformation ofπ-conjugated system. The anions-triggered strong circularlypolarized luminescence (CPL) was observed from theseassemblies.468

In addition, Nakashima and Kawai et al. reported chiralbichromophoric perylene bisimides as active materials forcircularly polarized emission. They found that the compoundsformed chiral aggregates with solvent variations. A largeenhancement in the dissymmetry of circularly polarizedluminescence was achieved by the aggregated structures. It wasfurther found that the spacer between the chiral center and the

Figure 159. Controllable fabrication of supramolecular nanocoils and nanoribbons and their morphology-dependent photoswitching. Reprinted withpermission from ref 455. Copyright 2009 American Chemical Society.

Figure 160. (A) Molecular structure and device architecture of thecircularly polarized light-detecting helicene OFETs. (B) Response ofhelicene OFETs to circularly polarized light. Reprinted with permissionfrom ref 456. Copyright 2013 Nature Publishing Group.

Figure 161. Mixture of red, green, and blue fluorescent polymersgenerated a unique, circularly polarized white luminescence. Reprintedwith permission from ref 465. Copyright 2012 American ChemicalSociety.

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chromophoric units played a crucial role in the effectiveenhancement of chiroptical properties in these self-assembledstructures.470

6.6. Biological Applications of Supramolecular Chirality

The existence of life and biological evolution directly depend onboth molecular chirality and supramolecular chirality. Thissituation can be demonstrated from the homochirality of aminoacids, nucleic acids, and many other biomolecules as well as thehelical nanostructures formed by the folding of DNA andproteins. Many biomedical applications are also closely related tomolecular chirality and supramolecular chirality. For example,most drugs used for clinical application are chiral molecules.In general, every aspect of biological applications is dependent

on chirality. We cannot address all of these issues in this review.For a further understanding of supramolecular chirality from self-assembled systems, we will discuss some aspects of surpramo-lecular chirality effects on cell adhesion.Supramolecular hydrogels formed from the self-assembly of

peptide derivatives or nucleic acid derivatives have been studiedfor different biological applications.471−473 Certainly, thechirality always plays a very important role. For example, forthe hydrogels formed by short peptides, L-peptides have beenfound to be labile to proteases.474 Marchesan et al. recentlystudied the effects of amino acid chirality on tripeptide self-assembly and hydrogelation at physiological pH and cytocom-patibility in fibroblast cell culture. In this study, differentuncapped hydrophobic tripeptides with all combinations of D-and L-amino acids were prepared. The self-assembly andhydrogelation was found to be dependent on the chirality ofthe amino acids, and combinations of D, L-amino acids are veryuseful for maintaining the viability and proliferation of fibroblastsin vitro.475

Interestingly, the cell adhesion in the supramolecular hydro-gels was found to be dependent on the handedness of the self-assembled nanofibers. Feng et al. used the two enantiomers of a

1,4-benzenedicarboxamide phenylalanine derivative (170) assupramolecular gelators to construct different supramolecularhydrogels, and the cell adhesion within these supramolecularhydrogels was studied. It was found that cell adhesion andproliferation can be influenced by the chirality of the nanofibers.Thus, the left-handed helical nanofibers increased cell adhesionand proliferation, while the right-handed nanofibers decrease celladhesion and proliferation. The stereospecific interactionbetween chiral nanofibers and fibronectin plays a critical role inthese effects (Figure 163).476

As mentioned above, the handedness of self-assemblednanostructures can influence cell proliferation. However, Zouaniet al. demonstrated that helical nanostructures with the samehandedness, but different shapes and periodicities show totallydifferent capabilities for inducing human mesenchymal stem cell(hMSCs) adhesion and commitment into osteoblast lineage. Inthis study, mineralization of helical organic nanoribbons, whichformed from the self-assembly of Gemini-type amphiphiles,could produce chiral silica nanoribbons with two different shapesand periodicities. Interestingly, helical silica nanoribbons with aspecific periodicity of 63 nm (±5 nm) helped the specific celladhesion and stem cell differentiation, while silica twists with aspecific periodicity of 100 nm (±15 nm) did not (Figure 164).These results indicate that stem cells could interpret helicalnanostructures with supramolecular chirality.477

Recently, Liu et al. synthesized gelators bearing amphiphilic L-glutamide and D- or L-pantolactone (abbreviated as DPLG andLPLG, 171). The self-assembly of DPLG and LPLG producednanostructures with opposite supramolecular chirality. Theability of proteins to adhere to these nanostructures was foundto be dependent on their supramolecular chirality, asdemonstrated from quartz crystal microbalance measurements.Thus, the supramolecular nanostructures formed by DPLG havestronger adhesive ability to human serum albumin. Interestingly,the distinction of protein adhesion ability was only found at thesupramolecular level. At the molecular level, however, no cleardifference could be detected (Figure 163).478

Figure 162. Enhancement in the dissymmetry of circularly polarizedluminescence from the assembly of chiral bichromophoric perylenebisimides. Reprinted with permission from ref 470. Copyright 2013 JohnWiley & Sons.

Figure 163. Schematic representation of the culture of cells insupramolecular hydrogels and the different cell-adhesion and cell-proliferation behavior in the enantiomeric nanofibrous hydrogels (d:right-handed helical nanofibers; l: left-handed helical nanofibers). Themolecular structure of the gelator enantiomers (170) is shown.Reprinted with permission from ref 476. Copyright 2014 John Wiley& Sons.

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7. CONCLUSIONSChiral self-assembly from the molecular to the supramolecularlevel represents one of the most attractive and promising areas insupramolecular chemistry and self-assembly. The supramolecu-lar chirality in these self-assembled systems is the expression ofthe noncovalent interactions between the component molecules,where chiral transfer from a chiral component to the wholeassembly plays an important role. In addition, supramolecularchirality can also emerge through symmetry breaking even whenonly achiral molecules are involved. Due to the dynamic featuresof the self-assembly system, the supramolecular chirality can beregulated through the design of the chiral molecules themselves,external conditions such as pH, metal ions, photoirradiation,solvents, temperature, sonication, and so on. Different frommolecular chirality, supramolecular chirality can exhibit uniqueproperties such as the sergeant-and-soldier principle, themajority-rule principle, and chiral memories in several systems.Supramolecular chirality in self-assembled systems has beenfound to be useful in chiral sensing, chiral molecular recognition,and asymmetric catalysis. Some new functions such as chiropticalswitching, chiroptics, and CPL have also been observed.Furthermore, chiral nanostructures showed some interestingproperties when interacting with the biological systems. Thisreview has described many examples of the emergence,regulation, and unique features or functions of the supra-molecular chirality; however, there are still many unknownsrelated to supramolecular chirality.

Supramolecular chirality can be produced in systemscontaining chiral and/or achiral molecules. In contrast to themolecular chirality, it is still difficult to quantitatively evaluate thepurity of supramolecular chirality. In a system containing chiralmolecules, the main questions are do they form only one kind ofsupramolecular chirality or does there exist a percentage ofassemblies with the opposite chirality? Occasionally, supra-molecular chirality has emerged from assemblies based on achiralmolecules. Even though one can observe microscopic chirality,two enantiomers coexisted. Understanding the emergence ofchirality and how to evaluate the enantiomeric excess of a chiralassembly remains difficult. Supramolecular chirality is generallydynamic and strongly related to the self-assembly process. Whilewe achieved many controls over supramolecular chirality, thecharacterization techniques of the dynamic processes of thechiral assemblies in particular, the development of time-dependent spectroscopy and imaging technology is urgentlynecessary. Although we acquired much knowledge aboutsupramolecular chirality in self-assembly systems related tointermolecular interactions, structural control, and functiondevelopment, many of these are limited to one to twocomponents. There is a lack of understanding of how to tunemany different molecules into a complex chiral system in acooperative or syndetic way as is accomplished in a living cell.Nanostructured chiral materials offer many opportunities todevelop entirely new functional materials, which justifies researchinto supramolecular chirality. In this regard, can new catalytic,optical, opto-electrical, and magnetic materials result from workon chiral self-assembly systems? Chirality effects are fundamentalto biological systems, such as different enantiomers that can be auseful drug and or poisonous depending on their chirality. Howto construct the chiral/biointerface? Therefore, further efforts onsupramolecular chirality research should integrate new ideasfrom supramolecular chemistry, biology, medical science,pharmacology, and material and nanosciences.

AUTHOR INFORMATIONCorresponding Author

*Phone: +86 10 82615803. E-mail: liumh@iccas.ac.cn.Notes

The authors declare no competing financial interest.

Biographies

Minghua Liu, born in 1965 in China, is a Professor at the Institute ofChemistry of the Chinese Academy of Sciences (CAS). He graduatedfrom Nanjing University in 1986 and received his Ph.D. degree in 1994in Materials Science from Saitama University, Japan, under thesupervision of Prof. Kiyoshige Fukuda. He then joined the Institute of

Figure 164. SEM images of helical silica nanoribbons and silica twists;adhesion and differentiation of stem cells on helical silica nanoribbonsubstrates. Reprinted with permission from ref 477. Copyright 2013American Chemical Society.

Figure 165. Self-assembled nanostructures with opposite supra-molecular chirality showing different adhesive ability to human serumalbumin. Reprinted with permission from ref 478. Copyright 2014American Chemical Society.

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Physical and Chemical Research (RIKEN) as a Special PostdoctoralResearcher from 1994 to 1997. He joined the Institute of PhotographicChemistry, CAS, in 1998 and then the Institute of Chemistry, CAS, from1999. His research interests cover the colloid and interface sciences, self-assembly, supramolecular chemistry, and soft materials, particularly thechirality problems in those systems including monolayers, Langmuir−Blodgett films, supramolecular gels, and soft nanomaterials.

Li Zhang received her B.S. degree in Physical Chemistry from ShandongUniversity in China (1998) and Ph.D. degree in Physical Chemistryfrom the Institute of Chemistry of the Chinese Academy of Sciences(2004) under the supervision of Prof. Minghua Liu. Followinggraduation, she worked for 2 years at Tohoku University as aPostdoctoral Fellow. She has carried out research into chiralsupramolecular assemblies formed by achiral porphyrins and asym-metric catalysis of chiral assemblies. She is currently an AssociateProfessor at the Institute of Chemistry of the Chinese Academy ofSciences.

Tianyu Wang received his B.Sc. and M.Sc. degrees in Chemistry fromTianjin University, China. He received his Ph.D. degree in OrganicChemistry from the Institute of Chemistry of the Chinese Academy ofSciences in 2001. After that he worked as a Postdoctoral Researcher atthe Free University of Berlin in Germany with Prof. Dr. J.-H. Fuhrhop.Since 2007 he has been working as an Associate Professor at the Instituteof Chemistry, CAS. His research interests are supramolecular assembliesand soft matters.

ACKNOWLEDGMENTS

This work was supported by the Basic Research DevelopmentProgram (2013CB834504), the National Natural ScienceFoundation of China (Nos. and 21321063, 21473219,21474118, and 91427302), and the Fund of the ChineseAcademy of Sciences (No. XDB12020200).

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