Embed Size (px)
Citation preview
Mechanically Responsive Materials for Soft Robotics
Mechanically Responsive Materials for SoftRobotics
Edited byHideko Koshima
Editor
Prof. Hideko KoshimaWaseda UniversityResearch Organization for Nano & LifeInnovation513 Wasedatsurumaki-cho, ShinjukuTokyo 162-0041Japan
All books published by Wiley-VCHare carefully produced. Nevertheless,authors, editors, and publisher do notwarrant the information contained inthese books, including this book, tobe free of errors. Readers are advisedto keep in mind that statements, data,illustrations, procedural details or otheritems may inadvertently be inaccurate.
Library of Congress Card No.:applied for
British Library Cataloguing-in-PublicationDataA catalogue record for this book isavailable from the British Library.
Bibliographic information published bythe Deutsche NationalbibliothekThe Deutsche Nationalbibliothek liststhis publication in the DeutscheNationalbibliografie; detailedbibliographic data are available on theInternet at <http://dnb.d-nb.de>.
© 2020 Wiley-VCH Verlag GmbH &Co. KGaA, Boschstr. 12, 69469Weinheim, Germany
All rights reserved (including those oftranslation into other languages). Nopart of this book may be reproduced inany form – by photoprinting,microfilm, or any other means – nortransmitted or translated into amachine language without writtenpermission from the publishers.Registered names, trademarks, etc. usedin this book, even when not specificallymarked as such, are not to beconsidered unprotected by law.
Print ISBN: 978-3-527-34620-2ePDF ISBN: 978-3-527-82219-5ePub ISBN: 978-3-527-82221-8oBook ISBN: 978-3-527-82220-1
Typesetting SPi Global, Chennai, IndiaPrinting and Binding
Printed on acid-free paper
10 9 8 7 6 5 4 3 2 1
v
Contents
Preface xiii
Part I Mechanically Responsive Crystals 1
1 Photomechanical Behavior of Photochromic DiaryletheneCrystals 3Seiya Kobatake and Daichi Kitagawa
1.1 Introduction 31.2 Crystal Deformation Exhibiting Expansion/Contraction upon
Photoirradiation 61.3 Photoresponsive Bending 71.4 Dependence of Bending Behavior on Irradiation Wavelength 111.5 Photomechanical Work of Diarylethene Crystals That Exhibit
Bending 131.6 New Types of Photomechanical Motion 151.7 Photosalient Effect 201.8 Summary 22
References 23
2 Photomechanical Crystals Made from AnthraceneDerivatives 29Fei Tong, Christopher J. Bardeen, and Rabih O. Al-Kaysi
2.1 Introduction 292.2 Elements of Photomechanical Molecular Crystals 302.3 The Advantage of Using Anthracene Derivatives in Photomechanical
Crystals 332.4 Types of Anthracene Photomechanical Crystals 342.4.1 NR-Type Anthracene Derivatives 342.4.1.1 9-Anthracene Carboxylate Ester Derivatives 342.4.1.2 9-Methylanthracene 362.4.1.3 9-Cyanoanthracne, 9-Anthealdehyde, and 9,10-Dinitroanthracene 372.4.1.4 Conjugated Anthracene Derivatives with Trans-to-Cis
Photochemistry 382.4.2 T-Type Photomechanical Crystals Based on Reversible 4π+ 4π
Photodimerization 39
vi Contents
2.4.3 P-Type Anthracene Derivatives 442.5 Synthesis of Anthracene Derivatives 462.6 Future Direction and Outlook 472.6.1 Modeling Reaction Dynamics in Molecular Crystals 472.6.2 New Anthracene Derivatives and Crystal Shapes 482.6.3 Interfacing Photomechanical Molecular Crystals with Other
Materials 492.7 Conclusion 50
Acknowledgments 50References 50
3 Mechanically Responsive Crystals by Light and Heat 57Hideko Koshima, Takuya Taniguchi, and Toru Asahi
3.1 Introduction 573.2 Photomechanical Bending of Crystals by Photoreactions 593.2.1 Azobenzene 593.2.1.1 Bending 593.2.1.2 Twisted Bending 613.2.2 Salicylideneaniline and Analogues 613.2.2.1 Bending and the Mechanism 633.2.2.2 Comparison of Chiral and Racemic Crystals 643.2.3 Fulgide 643.2.4 Carbonyl Compounds 663.3 Locomotion of Crystals by Thermal Phase Transition 673.3.1 Inchworm-Like Walking 703.3.2 Fast Rolling Locomotion 713.4 Diversification of Mechanical Motion by Photo-triggered Phase
Transition 723.4.1 Discovery and the Mechanism of Photo-triggered Phase
Transition 723.4.2 Stepwise Bending 753.5 Why Crystals? 753.6 Summary and Outlook 77
References 77
4 Crawling Motion of Crystals on Solid Surfaces byPhoto-induced Reversible Crystal-to-Melt Phase Transition 83Yasuo Norikane and Koichiro Saito
4.1 Introduction 834.2 Isomerization of Azobenzene 844.3 Phase Transitions in Liquid Crystals (Liquid-Crystal-to-Isotropic) 864.4 Phase Transitions in Crystal Phase (Crystal-to-Melt) 874.4.1 Characteristics of the Crystal-to-Melt Phase Transition 874.4.2 Potential Applications of Crystal-to-Melt Transition 894.4.3 Mechanical Motions Derived from the Crystal-to-Liquid Phase
Transition 924.5 Photo-induced Crawling Motion of Azobenzene Crystals 944.5.1 Discovery of the Crawling Motion of Crystal on Solid Surface 94
Contents vii
4.5.2 Characteristics of the Crawling Motion of Crystals 954.5.3 Mechanism of the Crawling Motion 984.5.4 Crawling Motion of Azobenzene Crystals 984.6 Conclusion 98
References 99
5 Bending, Jumping, and Self-Healing Crystals 105Pance Naumov, Stanislav Chizhik, Patrick Commins, and Elena Boldyreva
5.1 Bending Crystals 1055.1.1 General Mechanism of Crystal Bending 1055.1.2 Kinetic Model of the Transformation 1085.1.3 Mechanical Response of a Crystal to Irradiation 1125.1.4 A Case Study, Linkage Isomerization of [Co(NH3)5NO2]Cl(NO3) 1165.1.5 Concluding Remarks 1175.2 Salient Crystals 1185.2.1 Salient Effects 1185.2.2 Mechanism of the Thermosalient Transition 1205.2.3 Thermal Signature of the Thermosalient Effect 1235.2.4 Directionality of Motion 1245.2.5 Effect of Intermolecular Interactions 1255.2.6 Effect of Crystal Habit 1275.2.7 Photosalient and Mechanosalient Effects 1285.2.8 Applications of the Salient Effects 1305.3 Self-healing Crystals 131
References 133
6 Shape Memory Molecular Crystals 139Satoshi TakamizawaIntroduction 139
6.1 Discovery of Organosuperelasticity 1416.2 Twinning Organosuperelasticity 1496.3 Organosuperplasticity Through Multilayered Sliding 1566.4 Twinning Ferroelasticity 1586.5 Summary 173
References 173
Part II Mechanically Responsive Polymers andComposites 177
7 Mechanical Polymeric Materials Based on Cyclodextrins asArticial Muscles 179Akira Harada, Yoshinori Takashima, Akihito Hashidzume, and HiroyasuYamaguchi
7.1 Introduction 1797.2 Artificial Muscle Regulated by Cross-Linking Density 1807.2.1 A Host–Guest Gel with αCD and Azo 180
viii Contents
7.2.2 Photo-Responsive Volume Change of αCD-Azo Gels 1817.2.3 Photo-Responsive Property of αCD-Azo Gels 1847.3 Artificial Muscle Regulated by Sliding Motion 1877.3.1 Preparation of a Topological Hydrogel (αCD-Azo Hydrogel) 1887.3.2 Mechanical and Photo-Responsive Properties of the αCD-Azo
Hydrogel 1887.3.3 UV and Vis Light-Responsive Actuation of the αCD-Azo Xerogel 1927.4 An Artificial Molecular Actuator with a [c2]Daisy Chain
([c2]AzoCD2) 1927.4.1 Photo-Responsive Actuation of the [c2]AzoCD2 Hydrogel 1947.4.2 Photo-Responsive Actuation of the [c2]AzoCD2 Xerogel 1967.5 Supramolecular Materials Consisting of CD and Sti 1997.5.1 (αCD-Sti)2 Hydrogel 1997.5.2 (αCD-Sti)2 Dry Gel 2027.6 Concluding Remarks 204
References 205
8 Cross-Linked Liquid-Crystalline Polymers as PhotomobileMaterials 209Toru Ube and Tomiki IkedaIntroduction 209
8.1 Structures and Functions of Photomobile Materials Based onLCPs 211
8.1.1 Polysiloxanes 2118.1.2 Polyacrylates 2138.1.3 Polyacrylate Elastomers Prepared from LC Macromers 2188.1.4 Systems with Multiple Polymer Components 2188.1.5 Composites 2208.1.6 Linear Polymers 2228.1.7 Rearrangeable Network with Dynamic Covalent Bonds 2248.2 Summary 226
References 226
9 Photomechanical Liquid Crystal Polymers and Bioinspired SoftActuators 233Chongyu Zhu, Lang Qin, Yao Lu, Jiahao Sun, and Yanlei Yu
9.1 Background 2339.2 Actuation Principles 2349.2.1 Photochemical Phase Transition 2359.2.2 Weigert Effect 2379.2.3 Photothermal Effect 2399.3 Bioinspired Actuators and Their Applications 2429.3.1 Soft Actuators Driven by Photothermal Effect 2439.3.2 Photoinduced Actuation of Soft Actuators 2459.4 Conclusion 251
References 253
Contents ix
10 Organic–Inorganic Hybrid Materials with PhotomechanicalFunctions 257Sufang Guo and Atsushi Shimojima
10.1 Introduction 25710.2 Azobenzene as Organic Components 25810.3 Siloxane-Based Organic–Inorganic Hybrids 25810.4 Photoresponsive Azobenzene–Siloxane Hybrid Materials 26110.4.1 Nanostructural Control by Self-Assembly Processes 26110.4.2 Lamellar Siloxane-Based Hybrids with Pendant Azobenzene
Groups 26210.4.3 Lamellar Siloxane-Based Hybrids with Bridging Azobenzene
Groups 26410.4.4 Photo-Induced Bending of Azobenzene–Siloxane Hybrid Film 26510.4.5 Control of the Arrangement of Azobenzene Groups 26810.5 Other Azobenzene–Inorganic Hybrids 27010.5.1 Intercalation Compounds 27010.5.2 Hybridization with Carbon-Based Materials 27010.6 Summary and Outlook 272
References 272
11 Multi-responsive Polymer Actuators by Thermo-reversibleChemistry 277Antoniya Toncheva, Loïc Blanc, Pierre Lambert, Philippe Dubois, andJean-Marie Raquez
11.1 Introduction 27711.2 Covalent Adaptive Networks 27911.2.1 Associative CANs 27911.2.2 Dissociative CANs 28011.3 Thermo-reversible Chemistry 28011.4 DA Reactions for Thermo-reversible Networks 28211.4.1 Basic Definitions 28211.4.2 DA Reactions for Polymer Synthesis 28211.4.3 DA Reactions for Thermo-reversible Polymer Network 28311.4.3.1 Self-healing Materials 28311.4.3.2 Hydrogels 28711.5 Soft Actuators 28911.6 DA-based SMPs for Soft Robotics Application 29211.7 On the Road to 3D Printing 29311.8 Perspectives and Challenges 295
Acknowledgments 298References 298
12 Mechanochromic Polymers as Stress-sensing SoftMaterials 307Daisuke Aoki and Hideyuki Otsuka
12.1 Introduction 30712.2 Classification of Mechanochromic Polymers 307
x Contents
12.3 Mechanochromophores Based on Dynamic Covalent Chemistry 30912.4 Mechanochromic Polymers Based on Dynamic Covalent
Chemistry 31012.4.1 Polystyrenes with Mechanochromophores at the Center of the
Polymer Chain 31012.4.2 Polyurethane Elastomers with Mechanophores in the Repeating
Units 31012.4.3 Mechanochromic Elastomers Based on Polymer–Inorganic
Composites with Dynamic Covalent Mechanochromophores 31212.5 Mechanochromic Polymers Exhibiting Mechanofluorescence 31512.6 Rainbow Mechanochromism Based on Three Radical-type
Mechanochromophores 31612.7 Multicolor Mechanochromism Based on Radical-type
Mechanochromophores 31812.8 Foresight 321
References 323
Part III Application of Mechanically Responsive Materials toSoft Robots 327
13 Soft Microrobots Based on Photoresponsive Materials 329Stefano Palagi
13.1 Soft Robotics at the Micro Scale 32913.2 LCEs for Microrobotics 33013.2.1 Thermal Response of LCEs 33013.2.2 Photothermal Actuation of LCEs 33113.3 Light-Controlled Soft Microrobots 33513.3.1 Structured Light 33713.3.2 Controlled Actuation 33813.3.2.1 Role of Control Parameters 33813.3.3 Swimming Microrobots 34113.4 Outlook 344
References 344
14 4D Printing: An Enabling Technology for Soft Robotics 347Carlos Sánchez-Somolinos
14.1 Introduction 34714.2 3D Printing Techniques 34814.2.1 Material Extrusion-Based Techniques 34914.2.2 Vat Photopolymerization Techniques 35014.3 4D Printing of Responsive Materials 35214.3.1 Shape Memory Polymers 35214.3.2 Hydrogels 35514.3.3 Liquid Crystalline Elastomers 35614.4 4D Printing Toward Soft Robotics 358
Contents xi
14.5 Conclusions 359Acknowledgments 360References 360
15 Self-growing Adaptable Soft Robots 363Barbara Mazzolai, Alessio Mondini, Emanuela Del Dottore, and Ali Sadeghi
15.1 Introduction 36315.2 Evolution of Growing Robots 36515.3 Mechanisms for Adaptive Growth in Plants 36715.4 Plant-Inspired Growing Mechanisms for Robotics 36915.4.1 Challenges in Underground Exploration 36915.4.2 The “Evolution” of Plantoids 36915.4.3 Sloughing Mechanism 37115.4.4 First Growing Mechanism 37115.4.5 Artificial Roots with Soft Spring-Based Actuators 37315.4.6 Growing Robots via Embedded 3D Printing 37515.4.6.1 Deposition Strategies 37615.5 Adaptive Strategies in Plant for Robot Behavior 37915.5.1 A Plant-Inspired Kinematics Model 38015.5.2 Plant-Inspired Behavioral Control 38215.5.3 Circumnutation Movements in Natural and Artificial Roots 38515.6 Applications and Perspective 387
Acknowledgments 388References 388
16 Biohybrid Robot Powered by Muscle Tissues 395Yuya Morimoto and Shoji Takeuchi
16.1 Introduction 39516.2 Muscle Usable in Biohybrid Robots 39616.2.1 Cardiomyocyte and Cardiac Muscle Tissue 39716.2.2 Skeletal Muscle Fiber and Skeletal Muscle Tissue 39816.2.3 Cell and Tissue Other Than Mammals 39916.3 Actuation of Biohybrid Robots Powered by Muscle 40016.3.1 Biohybrid Robot with a Single Muscle Cell 40116.3.2 Biohybrid Robot with Monolayer of Muscle Cells 40216.3.3 Biohybrid Robot with Muscle Tissues 40616.4 Summary and Future Directions 410
References 411
Index 417
xiii
Preface
Robots are playing an increasingly bigger role in society. We must considerthe symbiotic relationship between humans and robots, as robots may help toimprove our lives in the near future. However, conventional robots composedof metals have the disadvantage of being rigid and heavy. Soft robots made oforganic materials have attracted much attention recently, as they tend to be softand light and, therefore, suitable for daily interactions with humans.
Mechanically responsive materials that can move macroscopically by externalstimuli, such as light, heat, electricity, chemical reactions, and others, havebeen studied extensively in materials chemistry over the past two decades.Many mechanically responsive materials such as crystals, polymers, gels, andcomposites have been developed. The next step is the practical applicationof these mechanical materials. Specifically, mechanical materials that canmove autonomously by external stimuli are promising for soft robots withimproved safety and comfort. Arguably, soft robots may be the best applicationof mechanically responsive materials.
The purpose of this book is to bring readers to the forefront of the currentstatus of mechanically responsive materials for soft robotics. This book consistsof three parts: mechanically responsive crystals (Part I), mechanically respon-sive polymers and composites (Part II), and the application of mechanicallyresponsive materials to soft robotics (Part III). Despite the fact that the history ofresearch on mechanical molecular crystals is as short as 10 years, approximately,many excellent mechanical crystals that show bending, twisting, rotation,jumping, locomotion, self-healing, and shape memory have been developed,as described in Chapters 1–6 of Part I. Although currently limited to basicresearch, practical application to soft robots is expected in the near future. Incontrast, research on mechanical polymer materials precedes crystals and hasbeen conducted for several decades. Recently, mechanical polymer materialshave evolved into artificial muscles, photomobile materials, bioinspired softactuators, inorganic–organic hybrid materials, multi-responsive compositematerials, and strain sensor materials, as discussed in Chapters 7–12 of PartII. The application of mechanical materials to soft robots is just the beginning.In Chapters 13–16 of Part III, challenging and versatile applications, such assoft microrobots made from photoresponsive elastomers, four-dimensionalprinting for assembling soft robots, self-growing of soft robots like plants, andbiohybrid robots using muscle tissue, are presented. The history, development,
xiv Preface
and practical application of the research areas described are expected to be ofgreat interest to readers.
Many people, including robotics and materials researchers, as well as industryand others in the scientific community, are very excited about the recent advancesin soft robotics. However, further advances in this field require a hybrid under-standing of soft robotics and mechanical materials. It is our hope that this bookwill provide a bridge between these two research areas for academia and industry,enabling continued development of this exciting field.
Hideko KoshimaTokyo, JapanMarch 2019
1
Part I
Mechanically Responsive Crystals
3
1
Photomechanical Behavior of Photochromic DiaryletheneCrystalsSeiya Kobatake and Daichi Kitagawa
Department of Applied Chemistry, Graduate School of Engineering, Osaka City University, 3-3-138 Sugimoto,Sumiyoshi-ku, Osaka 558-8585, Japan
1.1 Introduction
Photochromism is defined as a reversible transformation reaction between twoisomers having different absorption spectra, which is induced in one or bothdirections by photoirradiation [1]. Among many photochromic compounds,diarylethenes with heteroaryl groups including thiophene, furan, thiazole,and oxazole rings have excellent properties, such as thermal stability of bothisomers, fatigue resistance, high coloration quantum yield, rapid response,and high reactivity even in the crystalline phase [2]. Such diarylethenes havepotential applications in ultraviolet (UV) sensors, photoswitches, displays,optical waveguides, optical memories, holographic recording media, nonlinearoptics, and actuators. Upon UV light irradiation, diarylethenes exhibit colorchanges because of a molecular structure change from the open-ring isomerform to the closed-ring isomer form. The colors remain stable in the dark atroom temperature. The colored isomers revert to their original colorless isomerforms by irradiation with visible light. The reversible color changes can berepeated many times.
Photochromic compounds that undergo a photochromic reaction in thecrystalline phase are known for paracyclophanes, triarylimidazole dimer,diphenylmaleronitrile, aziridines, 2-(2,4-dinitrobenzyl)pyridine, N-salicylide-neanilines, triazenes, and diarylethenes. The large change in geometricalstructures prohibits photochromic reactions in the crystalline phase. Even inthe crystalline phase, diarylethenes can undergo thermally irreversible andfatigue-resistant photochromic reactions when diarylethene molecules are fixedin the antiparallel conformation and the distance between the reactive carbonsis less than 4.2 Å [3]. The photocyclization reaction results in a color change inthe crystals from colorless to yellow, red, blue, or green, as shown in Figure 1.1.The color of the crystals can be maintained if they are stored in the dark. Thecolored crystals return to the initial colorless ones by irradiation with visiblelight. In the crystalline phase, the photocyclization quantum yield is close tounity and the coloration/decoloration cycles can be repeated more than 104
Mechanically Responsive Materials for Soft Robotics, First Edition. Edited by Hideko Koshima.© 2020 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2020 by Wiley-VCH Verlag GmbH & Co. KGaA.
4 1 Photomechanical Behavior of Photochromic Diarylethene Crystals
R2F F
UV
Vis.
Open-ring isomer Closed-ring isomer
Colorless to yellow upon photoirradiation
Colorless to red upon photoirradiation
Colorless to blue upon photoirradiation
Colorless to green upon photoirradiation
F FF F
F
S Me
Me
S
1 (450 nm)
4 (500 nm)
9 (546 nm)
14 (550 nm)
19 (600 nm) 20 (650 nm) 21 (655 nm)
18 (630 nm)
12 (560 nm) 13
15: R = Me (589 nm)
16: R = Et (630 nm)
10: R = Me (547 nm)
11: R = Et (550 nm)
17: R = iPr (650 nm)
5 (504 nm) 6 (510 nm) 7 (535 nm) 8 (545 nm)
2 (450 nm) 3 (450 nm)
F
F FF F
F
S Me
MeMe Me
S
F
F FF F
F
Me
MeMe MeOO
F
F FF F
R3
R1
R5
R6R4 SS
R2F F
F FF F
R3
R1
R5
R6R4 SS
F
N
O O
Et
Et
N
F
F FF F
F
O O
Me
Me
F
F FF F
F
S S
R
R
F
F FF F
F
S S
Me
Me
F
F FF F
F
S
Me Me
MeO OMe
S
iPr
iPr
F
F FF F
F
S S
Me
Me
MeMeF
F FF F
F
S
N R
R
F
F FF F
S S
N N
F
S
N Me
Me
F
F FF F
S
N
S
N
N N
F
O O
Me
Me
F
F FF F
F
S O
Me
Me
F
F FF F
F
S S
Et
Et
F
F FF F
F
S
O2N NO2
S
Me
Me
F
F FF F
F
S SMe
Me
Me
F
F FF F
F
S S
Me
Me MeMe
F
F FF F
F
S S
Me
Me
MeMeF
F FF F
Figure 1.1 Typical examples of diarylethenes that underwent photochromism in the singlecrystalline phase. Maximum absorption wavelength of the photogenerated closed-ringisomers in crystals is shown in parentheses. When exposed to UV radiation crystals 1–3 turnedto yellow, crystals 4–13 to red, crystals 14–16 to blue, and crystals 19–21 to green.
times [2]. There are many studies describing the photochromism of diarylethenecrystals, including investigations that report multicolor photochromism [4],dichroism under polarized light [5], fluorescence [6], three-dimensional opticalmemory [7], diastereoselective cyclization [8], selective photochromic reactionunder polarized light [9], theoretical studies [10], Raman spectroscopic studies[11], nanostructures [12], supramolecular architectures [13], nanocrystals [14],
1.1 Introduction 5
(a)
(b)
1.01 nm 0.90 nm
0.49 nm 0.56 nm
Figure 1.2 (a) Top and (b) side views of the geometrical structures of the open- andclosed-ring isomers of 1,2-bis(2,5-dimethyl-3-thienyl)perfluorocyclopentene (7) in crystals. Thetwo isomers were isolated and independently recrystallized. Source: Irie et al. 2014 [2b].Adapted with permission from American Chemical Society.
polymorphism [9a, 15], phase transitions [15b, c], surface wettability [15a, 16],and molecular motion observed by X-ray crystallography [17]. The research onmolecular motion observed by X-ray crystallography demonstrated that pho-tochromic reactions of diarylethene molecules in the crystals are accompaniedby a change in the unit cell dimensions because of a decrease in the molecularvolume resulting from photoisomerization of the open-ring isomer to yieldthe closed-ring isomer as shown in Figure 1.2 [2b]. The height of the triangleshape increases from 0.49 to 0.56 nm and the base width decreases from 1.01 to0.90 nm. The side view indicates that the thickness of the molecule is reduced.The change in the geometrical structure of diarylethene molecules plays animportant role in photomechanical phenomena.
In 2001, the crystal surface of diarylethene 18 was found to exhibit a pho-toreversible surface morphology change [18]. The flat crystal surface formed astep with a height of approximately 1 nm upon UV light irradiation. The stepwas erased by irradiation with visible light. The crystal thickness decreased as aresult of the photochromic isomerization of the open-ring isomer to yield theclosed-ring isomer. Another surface, which is perpendicular to the surface thatformed the step, exhibited a photoreversible valley formation. These reversiblesurface morphology changes are ascribed to photoinduced contraction in thedirection of the long axis of each diarylethene molecule regularly packed withinthe single crystal. These results indicate that the molecular-scale structuralchange of individual molecules may induce the macroscopic mechanicalmovement of materials.
In this chapter, recent developments in the light-driven actuators based on pho-tochromic diarylethene crystals are described.
6 1 Photomechanical Behavior of Photochromic Diarylethene Crystals
(a)
(b)
86 nm
82 nm50 μm
Figure 1.3 (a) Digital microscopic and (b) atomic force microscope (AFM) images for thinmicrocrystals of 16.
1.2 Crystal Deformation ExhibitingExpansion/Contraction upon Photoirradiation
A first example of photoreversible macroscopic crystal deformation was a thinmicrocrystal of 1,2-bis(2-ethyl-5-phenyl-3-thienyl)perfluorocyclopentene (16)[19]. The microcrystals were prepared by sublimation on a thin glass plate underatmospheric pressure at 144 ∘C. A photograph of microcrystals composed of 16is shown in Figure 1.3. The crystals have several tens of micrometers square insize with thickness of a few hundred nanometers. Upon irradiation with 365 nmlight, the crystal turned blue and the blue-colored crystal returned to the initialcolorless crystal form after irradiation with visible light. The conversion ratioin the crystal from the open- to the closed-ring isomers was followed by aninfrared (IR) absorption microspectroscopy. IR absorption spectra for thin singlecrystal 16 were taken under polarized IR light to avoid an overlap of peaks.Figure 1.4 shows the IR spectral changes of crystal 16 upon irradiation with365 nm light. The open-ring isomer in the crystal has two characteristic bandsat 1260 and 1350 cm−1. The band at 1350 cm−1 was split into two peaks uponUV light irradiation, whereas the band at 1260 cm−1 monotonously decreased.The closed-ring isomer has no absorption around 1260 cm−1. The conversionratio from the open- to the closed-ring isomers can be determined from thedecrease of the band at 1260 cm−1. Almost 70% conversion was observed at thephotostationary state under irradiation with 365 nm light.
In general, crystals of different molecules have different unit cell parameters,space group, and packing in the lattice. Figure 1.5 shows the photographs of crys-tals for diarylethenes 16, 11, and 17 [19, 20]. A single crystal of 16 with a thicknessof 570 nm was reversibly changed from a square-like shape with corner anglesof 88∘ and 92∘ to a lozenge-like shape with corner angles of 82∘ and 98∘ uponalternating irradiation with UV and visible light. The photochromic reaction tookplace homogeneously in crystals because of their thin crystallized forms with athickness of several hundred nanometers. Crystals of 11 and 17 have unit cell
1.3 Photoresponsive Bending 7
Figure 1.4 (a) IR spectraland (b) conversion changefor thin microcrystal of 16upon irradiation with365 nm light. IR spectrawere detected underpolarized IR light. Thepolarization direction wasset to the short axis in themolecule. Source: Kobatakeet al. 2007 [19]. Adaptedwith permission of SpringerNature.
0.4Time
(s)
05
1015202530405060
0335057616366707172
0 s
5 s
10 s
15 s
20 s
25 s
30 s
50 s
40 s
60 s
Conversion
(%)
0.3
0.2
0.1
(a)
(b)
01800 1600 1400
Wavelength (nm)
1200 1000 800
00
20
40
Convers
ion (
%)
Absorb
ance
60
80
100
10 20
Time (s)
30 40 50 60 70
parameters similar to 16. These diarylethenes have the same space group, i.e.Pbcn, as shown in Table 1.1. The molecular packings of 11 and 17 are also similarto that of 16, as shown in Figure 1.6. Crystal 11 changed its color from color-less to red and its corner angles from 90∘ and 90∘ to 86∘ and 94∘, and hence itsshape from square to lozenge. Crystal 17 changed its color from colorless to blueand the corner angles from 83∘ and 97∘ to 81∘ and 99∘ upon irradiation with UVlight. Crystals of 16, 11, and 17 exhibited a similar crystal shape deformation.These results indicate that the crystal shape deformation depends on the molec-ular packing of the molecules in the crystal and was not affected by difference inmolecular structure.
1.3 Photoresponsive Bending
A first example of photoresponsive bending of diarylethene crystals was a rod-like crystal of 1,2-bis(5-methyl-2-phenyl-4-thiazolyl)perfluorocyclopentene (10)[19]. The crystal bent toward the incident UV light source when irradiated withUV light. The bending is attributed to a contraction in the direction of the crystallong axis and a gradient in the extent of the photoisomerization reaction at thecrystal surface caused by high light absorbance. The rodlike crystal bent fast in
8 1 Photomechanical Behavior of Photochromic Diarylethene Crystals
(a)
UV
Vis.
UV
Vis.
UV
Vis.
(b)
(c)
b c
b c
b c
Figure 1.5 Photoinduced crystal shape deformation of crystals (a) 16, (b) 11, and (c) 17. Scalebar is 10 μm. Source: Kobatake et al. 2007 [19, 20]. Adapted with permission of Springer Natureand the European Society for Photobiology, the European Photochemistry Association, andThe Royal Society of Chemistry.
a few seconds timescale and could move a gold micro-particle that was 90 timesheavier than the single crystal. Moreover, it could launch a tiny silica particle likea tennis ball. Thus, scaling down the crystal size allows achieving macroscopicmechanical responses.
The rodlike crystal of 10 (53 μm× 3 μm× 3 μm) was also used for measuringthe response time of the bending because even low power single pulsed lasercan induce the bending of the rodlike crystal [19]. As a light source, the pulsedlaser of the third harmonics of Nd-YAG laser (355 nm, pulse width = 8 ns,power = 60 mJ/pulse) was used. The bending behavior induced by the singlepulsed laser was measured using a high-speed camera with an image intensifier.The exposure time of each frame was 25 μs (40 000 frames s−1). After irradiationwith the single pulsed laser, the straight rodlike crystal bent, and the bendingprocess was almost completed in one frame. This means that the response timeof the bending shape change is around 25 μs or faster.
Various types of rodlike crystals of diarylethenes have been reported on pho-toresponsive bending. There are two types of the photoinduced bending behav-ior: bending away from the incident light and bending toward the incident light[21]. Figure 1.7 shows the molecular structures of crystals that exhibit photoin-duced bending [19, 22–31]. When the long axis of the rodlike crystal expandsupon UV light irradiation, the crystal bends away from the incident light. On theother hand, when the long axis of the rodlike crystal contracts upon UV lightirradiation, the crystal bends toward the incident light.
1.3 Photoresponsive Bending 9
(a)
b
c
b
c
b
c
c(100)
Expansion
Contraction
92°
88°
(100)
Expansion
Contraction90°
90°
(100)
Expansion
Contraction82°
98°
a
c
a
c
a
(b)
(c)
Figure 1.6 Molecular packing of crystals (a) 16, (b) 11, and (c) 17 viewed from (100) (left) and(010) faces (right). The arrows indicate the direction of contraction and expansion of thecrystals upon UV irradiation. Source: Kobatake et al. 2007 [19, 20]. Adapted with permission ofSpringer Nature and the European Society for Photobiology, the European PhotochemistryAssociation, and The Royal Society of Chemistry.
10 1 Photomechanical Behavior of Photochromic Diarylethene Crystals
Table 1.1 X-ray crystallographic data for 16, 11, and 17.
16 11 17
Empirical formula C29H22F6S2 C27H20F6N2S2 C31H26F6S2
Formula weight 548.61 550.59 576.66T (K) 123 123 123Crystal system Orthorhombic Orthorhombic OrthorhombicSpace group Pbcn Pbcn Pbcna (Å) 22.332(5) 21.5461(15) 21.307(6)b (Å) 10.991(2) 10.8096(8) 12.193(4)c (Å) 10.601(2) 10.8098(8) 10.669(3)V (Å3) 2602.0(9) 2517.7(3) 2771.8(14)Z 4 4 4Density(cal.) (g cm−3) 1.400 1.453 1.382Corner angles before UVirradiation (∘)
88, 92 90, 90 83, 97
Corner angles after UVirradiation (∘)
82, 98 86, 94 81, 99
The bending velocity of the photoresponsive bending crystals dependson the molecular structure and the crystal structure. Rodlike crystals of1,2-bis(2-methyl-5-(4-(1-naphthoyloxymethyl)phenyl)-3-thienyl)perfluorocycl-opentene (22) bends away from the incident light upon UV light irradiation [22].It was found that the bending velocity depended on crystal faces subjected to UVlight irradiation. Figure 1.8 shows the photoinduced bending behavior of crystal22 upon irradiation with 365 nm light. When the (001) face was irradiated,the crystal bent slowly. In contrast, when the (010) face was irradiated, thecrystal bent significantly fast. This is ascribed to the difference in the absorptioncoefficients and the photoreacted thickness of the different faces. The depthof the photoreaction in the vicinity of the crystal surface affects the bendingvelocity. Therefore, this result suggested that crystal thickness is a very importantfactor in bending behavior.
The dependence of the bending velocity on UV irradiation power was examinedusing a diarylethene crystal of 22 [23]. For their rodlike crystals, it was revealedthat the initial velocity of curvature change (V init) increased in proportion to thepower of the incident UV light as shown in Figure 1.9. This result also suggeststhat the local strain owing to the individual diarylethene molecules structuralchanges acts cumulatively for the photomechanical bending behavior.
To understand the dependence of the bending velocity on crystal thickness,the bending behaviors of crystals 22, 24, and 34 with different thicknesses wereinvestigated [23]. Figure 1.10 shows V init as a function of the crystal thickness.The crystal bent significantly when it was thin. However, when the crystal thick-ness was 0.62 μm, the crystal could not bend. To explain this relationship, Timo-shenko’s bimetal model was introduced. The relation between V init and the crystal
1.4 Dependence of Bending Behavior on Irradiation Wavelength 11
O
O
O
O
O
Me
O
O
O
S
22[22,23]
24[23]
10[19,23a,24,27]
25[25] 26[25] 27[28]
29[29]28[23a]
30[26] 32[30]
31[26]33[30]
35[23a]34[23] 36[28]
23[31]
S
Me
Me
F F
F F
F F
S S
Me
Me
OMe
(a)
(b)
F F
F FF F
S S
Me
Me
NN
F F
F FF F
S S
Me
Me
Me
NN
F F
F FF F
S S
Me
Me
F F
O
O
F FF F
S S
Me
Me
F F
F FF F
S S
Me
Me
Me Me
NN
F F
F FF F
S S
Me
Me
MeO OMe
F F
F FF F
S S
NN Me
Me
MeO OMe
F F
F FF F
S
O O
S
Me
Me
F F
F FF F
S S
O
O
Me
Me
NN
F F
F FF F
S S
OMe
MeO
F F
F FF F
S S
Me
Me
F F
F FF F
S
O O
HN
HN
S
Me
Me
F F
F FF F
F F
F F
F F
F F
S S
Me
Me
F F
F FF F
Me
HN
HN
Figure 1.7 Photochromic diarylethene derivatives that exhibit the photoinduced crystalbending: (a) bending away from the incident UV light, (b) bending toward the incident UVlight.
thickness was well explained by Timoshenko’s bimetal model using two parame-ters. The depth of the photocyclization reaction in the crystal was defined to beh2. The initial change in the actuation strain per second in a UV light intensityunder the same condition was expressed as 𝛼2,init. Timoshenko’s bimetal modelwas applied to various diarylethene crystals and enabled quantitative evaluationof the potential photoinduced bending velocity among different crystals. Thus, itwas revealed that h2 and 𝛼2,init played important roles in determining the bendingvelocity.
1.4 Dependence of Bending Behavior on IrradiationWavelength
Utilizing UV light with a different wavelength is expected to change h2. To assessthe effect of a change in h2 for the photomechanical behavior, a rodlike crystal
12 1 Photomechanical Behavior of Photochromic Diarylethene Crystals
(a)
(001)
(010)
0 s 1 s 3 s 5 s 7 s
0 s 1 s 3 s 5 s 7 s
Cross-sectional
viewIrradiation to (010) face
Irradiation to
(001) face50 μm
(010)
(001)(001)
(010)
(010)
(001)(001)
(010)
a
b
c
(c)
(d)
(b)
Figure 1.8 (a) Crystal shape and (b) photoinduced bending behavior of crystal 22 uponirradiation with 365 nm light to (c) the (001) face and (d) the (010) face. (b) Superimpose of thephotographs observed from tip of the crystal. Source: Kitagawa and Kobatake 2013 [22].Reproduced with permission of American Chemical Society.
of 10 was irradiated with different wavelengths of 365 and 380 nm light [24].Figure 1.11 shows photographs of the bending behavior of the crystal. Whenirradiated with 365 nm light, the crystal bent toward the UV light source. Incontrast, when irradiated with 380 nm light, the crystal first bent away from theincident light and then bent toward the light source. This is ascribed to the differ-ence in the depth of the photochromic reaction from the crystal surface. Whenirradiated with 365 nm light, the photoisomerization of diarylethene moleculestook place on only the crystal surface because of its high absorbance capacity. Thisindicates that the depth of photoisomerization from the crystal surface is verysmall relative to the crystal thickness and that the conversion of the photoreactedlayer increases significantly. In contrast, when irradiated with 380 nm light, thephotoisomerization of diarylethene molecules took place deep within the crys-tal because diarylethene has a low absorption at 380 nm compared with that at365 nm. This indicates that, in this case, the depth of photoisomerization from thecrystal surface was close to the crystal thickness, and the conversion of the pho-toreacted layer increased slowly. Upon UV light irradiation, photoisomerization
1.5 Photomechanical Work of Diarylethene Crystals That Exhibit Bending 13
Figure 1.9 (a) Curvature changeagainst UV irradiation time and (b)the initial velocity of curvaturechange (V init) against power of theincident UV light for crystals 22(crystal thickness: 6.7 μm). Power ofthe incident UV light is 174 (•), 132(◾), 91.0 (⧫), 70.2 (▴), 52.7 (▾), 44.8(○), 35.1 (◽), 26.3 (◊), 17.6 (Δ), and8.17mWcm−2 (∇) for (a). Source:Hirano et al. 2017 [23b]. Reproducedwith permission of AmericanChemical Society.
0.6
0.5
0.4
0.3
0.2
0.1
0
5.0
4.0
3.0
2.0
1.0
0
0
0 50 100 150 200
0.05
(a)
(b)
0.10 0.15 0.20 0.25 0.30
Irradiation time (s)
Cu
rva
ture
(m
m–1)
Vin
it (
mm
–1 s
–1)
Irradiation power (mW cm–2)
of diarylethene molecules in the crystalline phase from the open-ring isomerto the closed-ring isomer occurs randomly at first. As this occurs, there couldbe repulsion between the open-ring isomer and the photogenerated closed-ringisomer, which induces the expansion of the photoreacted layer. As the photocon-version of the diarylethene molecules increases, this repulsion disappears and vander Waals interactions between the closed-ring isomers induce the contractionof the photoreacted layer. When the depth of photochromic reaction is very smallrelative to the crystal thickness, the crystal cannot bend to a significant extent. Incontrast, when the depth of the photochromic reaction is approximately half ofthe crystal thickness, the crystal can bend to a large extent. Therefore, upon irra-diation with 365 nm light, the initial bending away from the light source could notbe observed. On the other hand, upon irradiation with 380 nm light, the initialbending away from the light source was clearly observed. The proposed mecha-nism, shown in Figure 1.12, was well supported by the experimental results withrespect to the crystal thickness and the change in the unit cell dimensions asso-ciated with the photochromic reaction.
1.5 Photomechanical Work of Diarylethene CrystalsThat Exhibit Bending
As mentioned previously, how to apply the photomechanical behavior topractical use turns out to be an important task. In this section, several
14 1 Photomechanical Behavior of Photochromic Diarylethene Crystals
h2 = 1.5 μm
α2, init P–1 = 4.5 × 10–6 J–1 m2
(b)
0 5 10
Crystal thickness (μm)
15 20 25 30 35
3
2
1
0
Vin
it P
–1 (J
–1
m)
6
4
h2 = 1.2 μm
α2, init P–1 = 7.7 × 10–6 J–1 m2
2
00
(a)
5 10
Crystal thickness (μm)
15 20
Vin
it P
–1 (J
–1
m)
h2 = 2.8 μm
α2, init P–1 = 7.3 × 10–6 J–1 m2
(c)
0 5 10
Crystal thickness (μm)
15 20
3
2
1
0
Vin
it P
–1 (J
–1
m)
Figure 1.10 Initial velocity of curvature change (V init) relative to the crystal thickness uponirradiation with UV light for diarylethene crystals (a) 22, (b) 24, and (c) 34. The initial velocity isnormalized according to the power of the light (P). Source: Hirano et al. 2017 [23b].Reproduced with permission of American Chemical Society.
demonstrations using photomechanical behavior of diarylethene crystals weredescribed.
Cocrystals composed of 1,2-bis(2-methyl-5-(1-naphthyl)-3-thienyl)perfluoro-cyclopentene (25) and perfluoronaphthalene (26) with a length of 1–5 mmexhibited photoreversible bending behavior over 250 times upon alternatingirradiation with UV and visible light [25]. The crystals bent away from theincident UV light as a result of the elongation of the UV-irradiated thin surfacelayer, which corresponded to a change in the geometrical structure of the shortaxis of the diarylethene molecules from the open-ring isomer to the closed-ringisomer. The crystals were able to lift a heavy metal that was 200–600 timesheavier than the crystals, as shown in Figure 1.13. The maximum stress in thecrystals by photoirradiation was estimated to be 44 MPa, which is 100 timeslarger than that of muscle (∼0.3 MPa). The Young’s modulus of the crystalwas measured to be 11 GPa. The relatively large Young’s modulus enabled thecrystals to carry out large mechanical work. Thus, the crystals could be used as“molecular crystal cranes.”
Mixed crystals composed of 1-(5-methyl-2-phenyl-4-thiazolyl)-2-(5-methyl-2-p-tolyl-4-thiazolyl)perfluorocyclopentene (30) and 1,2-bis(5-methyl-2-p-tolyl-4-thiazolyl)perfluorocyclopentene (31) also exhibited more than 1000 reversiblebending cycles upon alternating irradiation with UV and visible light without
