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Photoresponsive Supramolecular Amphiphiles forControlled Self-Assembly of Nanofibers and Vesicles

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By Chao Wang, Qishui Chen, Huaping Xu, Zhiqiang Wang, and Xi Zhang*

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Supramolecular amphiphiles refer to amphiphiles that aresynthesized on the basis of noncovalent interactions.[1]

The amphiphilicity of the supramolecular amphiphiles can betunable between hydrophilic and hydrophobic, thus allowing forcontrolled self-assembly and disassembly.[2] As a result, supra-molecular amphiphile is a useful building block to fabricate softmaterials with controlled structures and functions. Moreover,functional groups can be introduced directly by noncovalentinteractions, thus significantly reducing the need of time-consuming covalent synthesis.[3] Recently, a few nanoscale softmaterials have been prepared using the idea of supramolecularamphiphile, such as stimuli responsive vesicles, ultra-longnanofibers.[2,4] Among the self-assembly architectures, one-dimensional nanostructures have stimulated special interestfor their potential applications as nanowires and biomaterials.[5]

One dimensional nanostructures with stimuli responsive proper-ties may be further developed as candidates for smart softmaterials.[6] Therefore, the preparation of stimuli-responsive onedimensional nanostructures based on the concept of supramo-lecular amphiphile is of special importance.

Herein, we report a photoresponsive one-dimensional nano-fiber on the basis of a supramolecular amphiphile. The supra-molecular amphiphile consists of two components (Scheme 1).One component is a pyrene containing surfactant (PYN), and theother is a commercially available photoresponsive malachitegreen molecule (MGCB). Due to the hydrophobic interactionsbetween pyrenyl group and MGCB molecule, PYN and MGCBcan preassemble into a supramolecular amphiphile (PYN-MGCB).Moreover, theMGCBmolecule can change into a hydrophilic cationstate in response to UV irradiation, and the noncovalentinteractions between the MGCB molecule and pyrenyl groupwill decrease drastically.[7] Therefore, the PYN-MGCB supramo-lecular amphiphile will disassemble, further changing thesupramolecular nanostructures.

The PYN-MGCB complex was prepared by mixing PYN andMGCB with a molar ratio of 1:1 in THF. The solvent was thenremoved under reduced pressure. The dissolution-evaporationprocedure was repeated three times to ensure a completecomplexation. The resultant complex was finally dried at roomtemperature in vacuum. Interestingly, the preassembled complexcould readily disperse in water and yielded a transparent solutionwhich was stable for months. As MGCB itself cannot disperse in

[*] Prof. X. Zhang, C. Wang, Q. Chen, Dr. H. Xu, Prof. Z. WangKey Lab of Organic Optoelctronics and Molecular EngineeringDepartment of ChemistryTsinghua University, Beijing, 100084, ChinaE-mail: xi@mail.tsinghua.edu.cn

DOI: 10.1002/adma.200904334

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water, the observed solubility was endowed by its complexationwith PYN.

When dissolved in aqueous solution, PYN-MGCB self-assembles into an aggregate due to its amphiphilic nature. Theaggregation of PYN-MGCB was studied by transmission electronmicroscopy (TEM) at a concentration of 1.0� 10�4

M in aqueoussolution. As shown in Fig. 1, the supramolecular amphiphile ofPYN-MGCB self-assembles into a one dimensional fiberlikenanostructure with a uniform diameter. The length of thenanofibers reaches several micrometers. As PYN itself self-assembles into vesicles in water (Fig. S1), the formation of thenanofibers is a result of the complexation betweenPYN andMGCB.

To obtain a clear indication of the self-assembled nanostruc-tures, cryo-TEM observations were performed. As shown inFigure 1b, a one-dimensional fiberlike nanostructure is observed,which is consistent with the TEM results. It is also noted that noclear contrast is observed between the edge and the central parts,indicating that the one dimensional nanostructures are solidnanofibers. The diameter of the nanofiber is estimated to be about5 nm. It should be noted that the nanostructure is very stable ondifferent substrates. For example, on freshly prepared micasheets or on glasses, the nanofiber structure can be maintainedvery well (Supporting Information, Fig. S2), which provides thepotential possibility of its applications in organic electronics.

As MGCB can change its molecular state upon UV irradiation,we wonder if the co-assembly nanostructures can exhibitphotoresponsive properties. The transformation of MGCBmolecules upon UV irradiation was monitored by UV–visspectroscopy. As is shown in Figure 2a, a new peak at 617 nmappears, which represents the characteristic absorption ofcationic MGCB.[7] When the irradiation time reaches 50 s, theabsorbance at 617 nm does not change anymore, indicating thatthe MGCB molecules have transformed into the cation state(Fig. 2b). At the same time, the colour of the solution graduallychanges into dark green, which is the characteristic colour ofcationic MGCB molecules.

Remarkably, the self-assembled nanofibers transform intospherical objects upon 50 s UV irradiation of the PYN-MGCBaqueous solution. Cryo-TEM images show that the size of thespherical particles ranges from 10 to 30 nm (Fig. 3). Moreover, theclear contrast between the dark periphery and the gray central partreveals that the spheres are actually hollow in the centre, typicallyvesicular structures. The thickness of the bilayer is about 4–5 nm,about the length of two antiparallelly packed PYN molecules withthe pyrene rings overlapped. It can also be observed in Fig. 3b thatsome of the vesicles are still connected linearly, whichmay be dueto the incomplete evolution from one dimensional nanostructures.[8]

To reveal the position of MGCB molecules during thestructural transformation, we have used Fluorescence spectro-scopy and NMR measurements of the PYN-MGCB complex

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Scheme 1. Schematic illustration of the transformation from nanofibers tovesicles.

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Figure 2. a) UV–vis absorption of PYN-MGCB upon UV irradiation atdifferent irradiation time. b) UV–vis absorption changes of PYR-MGCBat 617 nm.

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before and after UV irradiation. As shown in Figure S3, for theself-assembly of PYN itself, except from the monomer emissionin 380–400 nm, an intense and wide pyrene excimer band at470 nm can be observed as well. The formation of the excimerindicates a close packing of the pyrenyl groups.[9] Aftercomplexation, the PYN-MGCB complex are shown to have aconsiderably different fluorescence pattern. Only the monomeremission can be observed while the pyrene excimer band nearlydisappears. A rational explanation is that the intercalation ofMGCB driven by interaction as well as hydrophobic interactiondestroys the close packing of the pyrenyl groups. However, uponUV-irradiation, the MGCB molecules become hydrophilic.Because of the better water-solubility of the MGCB cations andtheir repulsive electrostatic interactions with the positivelycharged PYN headgroups, the MGCB cations are driven out ofthe complex. As a result, the excimer band appears again,indicating the close packing of pyrenes is recovered. The positionof MGCBmolecules are further confirmed by NMR. As shown inFig. S4, in the PYN-MGCB complex before irradiation, thechemical shifts of the protons on MGCB (peaks between6.5–7.5 ppm) can hardly be observed, indicating that theMGCB molecules are trapped inside the aggregates. However,upon UV irradiation, the chemical shifts of MGCB can beobserved clearly, confirming that the MGCB molecules havecome out of the aggregates into aqueous solution.

The unique structural transformation from nanofibers tovesicles upon UV irradiation can be rationalized by considering

Figure 1. a) TEM and b) cryo-TEM images of the self-assembled nanofiber b

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the packing arrangements of the amphiphilic building blocks.PYN itself tends to form bilayer structures, due to the parallelpacking of the PYN molecules and the pyrene arrays. In theself-assembly of the supramolecular amphiphile, however, theunplanar MGCB molecules favor the intercalation betweenthe pyrenyl groups driven by hydrophobic and p�p interactions.Due to the stereo hindrance caused by the intercalation of theMGCB molecules, the parallel pyrene arrays are driven into atwist packing, which may favor the formation of a nanofiberstructure. Upon UV irradiation, MGCB molecules becomehydrophilic in aqueous solution and are driven out of theself-assemblies. As a result, the twisted structure is graduallyeliminated, and therefore the self-assembly structure changesback into the bilayer structure.

To further confirm the above de-twisting mechanism, we useTEM and cryo-TEM to monitor the transformation process fromnanofibers to vesicles. Samples of PYN-MGCB with differentphoto irradiation times were prepared. At this stage, only part ofthe MGCB molecules have leaked out of the nanofibers and theintermediate structure during the nanofiber-vesicle transforma-tion can be revealed. For PYN-MGCBwith only 5 s UV irradiation,some MGCBmolecules are expelled from the self-assemblies. Aspointed out by the red arrows in Figure S5a, the coexistence ofnanofiber and twisted ribbons is revealed, indicating ananofiber-nanoribbon transformation. Upon further irradiation,15 s, for example, when more MGCB molecules are driven out ofthe aggregates, vesicular structures also begin to appear, whichcoexists with the twisted nanoribbons, while the nanofibersdisappear. It should be noted that twisted nanoribbon is a typicalintermediate state between twisted nanofiber and untwisedbilayer structure, supporting that the detwisting mechanism isplausible.

In conclusion, we have demonstrated the fabrication of aphotoresponsive supramolecular amphiphile. The supramolecu-

y PYN-MGCB.

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lar amphiphile can self-assemble in water toform uniform nanofibers. Interestingly, thenanofiber can readily transform into vesicles inresponse to UV irradiation. Generally, covalentphotoresponsive amphiphiles require time-consuming synthesis. The photoresponsivesupramolecular amphiphile, however, can beobtained directly from an easily synthesizedamphiphile and a commercially availablephotoresponsive moiety. Considering the goodresponsiveness and economical preparation,the supramolecular amphiphile strategy wouldshow great potential in fabricating smart

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Figure 4. a) TEM and b) cryo-TEM images of PYN-MGCB upon 15 s irradiation.

Figure 3. a,b) Cryo-TEM images of PYN-MGCB after 50 s UV irradiation.

supramolecular soft materials. Moreover, the well-definedphotodegradable one-dimensional nanostructures, which canbe maintained very well on different substrates, are a promisingcandidate as nanowires for fabricating electronic nanodevices.

Experimental

Material Preparation: MGCB was bought from Alfa without furtherpurification. 11-bromo-N-((5a1,8a-dihydropyren-1-yl)methyl)undecanamide:11-bromoundecanoic acid and (5a1,8a-dihydropyren-1-yl)methanamine weredissolved in chloroform and mixed with N,N-Diisopropylethylamine (DIEA)and Benzotriazole-1-yl-o (BOP). The mixture was kept stirring in the Aratmosphere for 2 days. The yield was washed 3 times with dilute HCl andNaHCO3. Finally, the solutionwaswashedwith water and purified by silica gelcolumn chromatography to yield a white solid. (yield: 87%)

1-(11-((5a1,8a-dihydropyren-1-yl)methylamino)-11-oxoundecyl)pyridinium(PYN): The white solid was dissolved in pyridine. Themixture was then keptstirring for 2 days at 50 8C in Ar atmosphere. After evaporating the solvent,the residue was dissolved in 1mL methanol and then added dropwise to60mL diethyl ether to give a yellow solid precipitation. The crude productwas re-dissolved in methanol and precipitated twice in diethyl ether to givea white solid. 1H NMR spectroscopy (300 MHZ, DMSO d): 9.06 (d, 2H),4.99 (d, 2H), 4.56 (t, 2H), 2.16 (t, 2H). Mass spectrometry (ESI,m/z) calcdfor C33H37N2O

þ 477.29, found 477.28.TEM Experiments: TEM observation was performed on a JEMO 2010

Electron Microscopy operating at an acceleration voltage of 120 kV. Thesamples were prepared by drop-coating the aqueous solution on thecarbon coated copper grid and then negatively stained with phospho-tungstic acid solution.

1H NMR and ESI-MS: NMR spectra were recorded on a JOELJNM–ECA300 apparatus; ESI-MS spectra were recorded on a PE SciexAPI 3000 apparatus.

Adv. Mater. 2010, 22, 2553–2555 � 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Wein

UV–vis spectra: UV–vis spectra were obtainedusing a HITACHI U-3010 spectrophotometer.

Acknowledgements

This work was financially supported by the NationalBasic Research Program (2007CB808000), NSFC(50973051, 20974059), NSFC-DFG joint grant (TRR61), and the Tsinghua University Initiative ScientificResearch Program (2009THZ02-2). The authorsacknowledge Prof. Fei Sun at the Institute ofBiological Physics, CAS, for help with cryo-TEM.

Received: December 17, 2009

Published online: May 5, 2010

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