Photomechanical Organic Crystals As Smart Materials for Advanced

DOI:10.1002/chem.201805382 Minireview

& Photochemistry |Reviews Showcase| Photomechanical Organic Crystals as Smart Materials for Advanced Applications Qi Yu,[a] Briana Aguila,[d] Jia Gao,[a] Peixin Xu,[a] Qizhe Chen,[a] Jie Yan,[a] DongXing,[a] YaoChen,[b] Peng Cheng,[a, c] Zhenjie Zhang,*[a, b, c] and Shengqian Ma*[d]

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Abstract: Photomechanical molecular crystals are receiving crystalsduring irradiationare triggered by solid-state photo- much attentiondue to their efficient conversion of light into chemicalreactions and accompanied by phase transforma- mechanical work and advantages including faster response tion. This Minireview summarizesrecent developments in time;higher Young’s modulus;and orderedstructure, as growingresearch into photoresponsive molecular crystals. measured by single-crystal X-ray diffraction. Recently,various The basic mechanisms of different kinds of photomechanical photomechanical crystals with different motions (contrac- materialsare described in detail;recent advances in photo- tion, expansion,bending, fragmentation, hopping, curling, mechanical crystalsfor promising applications as smart ma- and twisting) are appearing at the forefront of smart materi- terials are also highlighted. als research. The photomechanical motionsofthese single

1. Introduction Various types of photomechanical motionshave been observed in photomechanical crystals, including jumping, ex- The conversion of externalenergy into macro-, micro-, or panding, bending, twisting, crawling, rolling,and even walking, nanoscopicmechanical motion by stimuli-responsivematerials upon irradiation with UV or visible light.The mechanical is of great interest from both fundamental and technological motion of crystalsisusually directly driven by the transforma- standpoints. Stimuli-responsive materials can presentdramatic tion of their molecular structure and molecular packing upon mechanical motionsunder the stimuli such as light irradia- exposure to stimuli.Dramatic crystal shape changes always tion,[1] heating,[2] externalforce,[3] solvent,[4] electrical fields,[5] originate from strain accumulation in the crystal interior and so on.[6] Amongthese stimuli,light is currently attracting caused by crystal structural transformations. Therefore, differ- increasing interestdue to its superior advantages, including ent light-sensitivechromophores associate with different pho- noncontact control, easy to obtain, cost-effectiveness, and vari- toresponsive mechanisms, including trans–cis isomerization of ous manipulatingmethods.[7] Various photomechanical materi- azobenzene, photocyclization of diarylethenes, [2+2] cycload- als, such as polymers,[8] elastomers,[9] liquid-crystalline materi- dition of olefins, [4+4] photodimerization of anthracene, and als,[10,11] andmolecular crystals,[12] have been reported, to date. other cis–trans isomerismtransformations. Commonly,photo- In particular, photomechanical crystals have attracted increas- mechanical molecular crystals can be classified into two types, ing attention because they not only have great potential for according to the recoveryconditions from photogenerated applicationsinlight harvesting, mechanical actuators,molecu- products to original phases, namely,Ttype (thermally revers- lar machines, optical sensors, and smart switches,[13] but also ible) and Ptype (thermally irreversible, but photochemically re- provide the opportunity to understand their performance versible).[15] Most photomechanical molecular crystals are of T throughXRD methods by solving ordered molecular struc- type, which requireheating to recover their photomechanical tures.[14] Compared with polymers, elastomers, and liquid-crys- ability,such as azobenzene, olefins, and anthracene. Only a talline materials, molecular crystals possess many advantages, few familiesofmolecular crystals can undergo photochemical- such as faster response times,[12] shorter recovery times, a ly reversible reactions, such as diarylethenes. higher Young’smodulus,[15] and awell-definedcrystal struc- To date, photomechanical behavior has been observed in ture.[16,17] In addition, light and mechanical energy transfer rap- molecular crystalsoforganic compounds, coordination com- idly occurs in orderedmolecular single crystalswith less plexes, and organometallic compounds. It should be noted energy dissipation. that most studies relatedtophotomechanical crystalsfocus on organic crystalsdue to their modular design natureand con- [a] Q. Yu,J.Gao, P. Xu, Q. Chen, J. Yan, D. Xing, Prof. P. Cheng, Prof. Z. Zhang venient synthesis. Herein, recent progress and development in College of Chemistry, Nankai University,Tianjin, 300071 (P.R. China) photomechanical organic crystals are summarized,and remarks E-mail:[email protected] on existing challenges and possible future directions in this [b] Prof. Y. Chen,Prof. Z. Zhang State Key Laboratory of Medicinal Chemical biology field are given. Nankai University,Tianjin,300071 (P.R. China) [c] Prof. P. Cheng, Prof. Z. Zhang Key Laboratory of Advanced EnergyMaterialsChemistry 2. Classification of Photomechanical Organic Ministry of Education, Nankai University,Tianjin, 30007 (P.R. China) Crystals Based on Photoreactions [d] B. Aguila, Prof. S. Ma 2.1 Photomechanical organic crystals inducedbytrans–cis Department of Chemistry,University of South Florida 4202 E. Fowler Avenue, Tampa, FL 33620 (USA) isomerization E-mail:[email protected] 2.1.1 The trans–cis isomerization of azobenzene Supporting information and the ORCID identification number(s) for the author(s) of this articlecan be found under: Azobenzene, adiazene (HN=NH) derivativeinwhich both hy- https://doi.org/10.1002/chem.201805382.Table S1 summarizes the molecu- drogen atoms are replaced by phenylgroups, is aprominent lar structure, irradiationconditions,and photomechanical behavior of the reported photoresponsive crystals. family of photomechanical molecules based on trans–cis iso- [18] Selected by the EditorialOffice for our Showcaseofoutstanding Review- merization. The trans–cis isomerization phase transition is type articles (www.chemeurj.org/showcase). usually driven by UV-lightirradiation, which causes large-scale

Chem. Eur.J.2019, 25,5611–5622 www.chemeurj.org 5612 2019 Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim Minireview deformation of the crystal shape and size. The reverse reaction it possible for applications in remote-controlled microsized of cis to trans isomerization occurs spontaneously at room valves on solid substrates. temperature in the dark due to the thermodynamic stability of The photoinducedbending behavior of azobenzene derivate the trans isomer relative to that of the cis isomer.Geometry crystalshas been widely studied. Under continuous UV excita- changes to azobenzene result in crystal motion,owing to tion, crystals bend continuously with arange of curvatures; strain accumulated in the crystals. Azobenzene and its deriva- this provides abasis to study effects of deformation. Upon kin- tives exhibit remarkable photostability and faster responses ematic analysis, the authors disentangled the relationship be- under light irradiation (Figure 1). Photoinducedbending of the tween intrinsic factors(the size and initial shape of the crystal) azobenzene crystal under UV-light irradiation was first reported and externalfactors(excitation power, direction, andtime) in in 2009.[19] The development of azobenzene photomechanical detail.[21] The trans–cis isomerization of azobenzene crystals crystalshas boomed in the past few years. Through modulat- was usually stimulatedbyUVirradiation, which greatly limited ing the substituent sites of azobenzene and interactions be- their practical applicationsdue to the inconvenience and po- tween molecules, variousmechanical motionsare exhibited. tential dangers of UV light. Thus,the preparation of azobenzene crystals photoresponsive to visible light is of great signifi- cance. Barrett et al. reportedaseries of azobenzene compounds that responded to visible-light irradiation through modifying astrong electron-withdrawing “pull” group and a strong “push” donor in molecules of azobenzene.[22] This class

Figure 1. Isomerization of azobenzene. Qi Yu received her BSc and MS degrees in 2013 and 2016 from Harbin University of Sci- ence and Technology (advisor: Professor Xiao- Norikaneand co-workersreported an azobenzene derivative, jun Sun). She was then recommended as a the 3,3-dimethylazobenzene (DMAB) crystal, whichexhibited PhD candidate toNankai University under the directional and continuous motion on aglass surfaceduring si- supervision of Prof. Peng Cheng and Prof. Zhenjie Zhang. Her research is focused on l multaneousirradiation at two different wavelengths ( = 365 photoresponsive crystalline materials and [20] and 465 nm;Figure 2). Moreover,crystals can climb on verti- solid-state chemistry. cal surfaces at room temperature. This trans–cis conversion induced liquefaction of the crystals;inother words, caused the crystalline phasetotransform into aliquid. This finding makes

Dr.Zhenjie Zhang received his BSc and MS degrees from Nankai University(advisor:Pro- fessorPeng Cheng). In 2014 he obtained his PhD degree from the University of South Flori- da (advisor: Professor Michael J. Zaworotko). From 2014–2016, he was apostdoc at UC San Diego (Advisor:Seth Cohen). From 2016, he started his independentcareer atNankai University as afull professorofInorganic Chemistry. He is currently focusingonre- searchinseparation application using porous materials;phototriggeredartificial muscles.

Dr.ShengqianMaobtained his BS degree from Jilin University(China) in 2003, and graduatedfrom Miami University (Ohio) with aPhD degreein2008. After atwo-year Direc- tor’s Postdoctoral Fellowship at Argonne Na- tionalLaboratory,hejoined the Department of ChemistryatUniversity of South Florida (USF) as an Assistant Professor in August 2010. He was promoted to an Associate Pro- fessorwith early tenue in 2015 and to aFull Professor in 2018. His current research interest focuses on task-specific design and function- Figure 2. a) Molecular structure of DMAB;b)schematic illustration of the ex- alization of advanced porous materials for perimental setupand motion;c)microscopyimages of the motion of DMAB energy, biological, environmental-related ap- crystals after irradiation for 0(d), 3(e), 6(f), 10 (g), 15 (h), and 20 min (i). plications. (Copyright,2014,Springer Nature.)

Chem. Eur.J.2019, 25,5611–5622 www.chemeurj.org 5613 2019 Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim Minireview of photomechanical crystals not only responded to visible benzoxazole (NBO) crystal, in which molecules packed in apar- light, but also demonstrated spontaneousand rapid reverse allel fashion to form aherringbone-like shape.[25] The distance motion. between C=Cbonds was 6.107 ,which was too great to In addition to exploring new compounds with chromophore allow anormal [2+2] cycloaddition reaction. However, the groups,the cocrystal strategy,under the guidance of crystal large distance provided enough space for trans–cis photoiso- engineering, has proventobeanefficient approach to prepare merization.Notably,the needlelikecrystalscan bendback- an increasing number of photomechanical crystals. Crystal en- wards away from the UV light source. If the irradiation direc- gineering is an efficient methodtoallow modification of the tion was reversed, the crystalsbent back to the originalposi- solid-state properties through the formation of cocrystals, in- tions. This process could be repeated many times withoutfati- cluding multicomponent crystals. The formation of azobenzene gue (Figure4). cocrystalswas explored through formingweak interactions be- Bardeen and Al-Kays and co-workersreported adimethyl-2- tween different components,including p–p stacking, electro- (3-anthracen-9-yl)allylidenemalonate (DMAAM) divinylanthra- static interactions, and hydrogen-bonding interactions. Frisˇcˇic´ cene derivative.[26] The DMAAM crystal could undergo slight and Barrett et al. demonstrated the first example of generating bending upon exposure to visible light (l=450–500 nm) for new photomechanical azobenzene cocrystals.[23] After combin- 1min in water due to the E–Z isomerization of the vinyl group ing azobenzene derivatives and bis(4-pyridyl)ethylene (bpe) to- (Figure 5). However, if crystals were dispersed in CTAB, the gether in acrystal,the structureof(cis-2)(cis-bpe) was deter- crystalsalmost lost their crystallinity and dissolved in CTAB mined through single-crystal X-ray diffraction (SCXRD; after irradiation. The authors found that the crystalsinsurfac- Figure 3). Under light irradiation, the cocrystal of (cis-2)(cis- tants (CTAB) could help to accelerate the photochemical reac- bpe) transformed into the (trans-2)(cis-bpe) phase, which in- tion rate by at least afactor of 10 compared with that in duced the bending behavior. Unfortunately,the diffraction in- water.CTABprovided asurfaceenvironment that facilitated tensity of the (trans-2)(cis-bpe) cocrystal decreasedafter light molecular motion on the surface. The photochemical reaction irradiation. These resultsrevealed that the cocrystal synthetic rates can be accelerated and expose new regions of the crys- methodopened up anew opportunity to design more photo- tals to further catalyze the reaction. mechanical materials. Obtaining the crystal structure of the cis isomer of azobenzene is also helpfultodemonstrate the trans– cis phase transformation.[24] 2.2 Photomechanical organic crystals inducedbyphotocycli- zation of diarylethenes Although variousmolecules with photomechanical properties have been reported, to date, only limited numbersofthem can undergo photoreversible processes (most of them with thermoreversible processes) after irradiation with light of different wavelengths. A“diarylethene” family (P-type), derivatives of stilbene, appears to apotential and indispensable candidate for use in optical memory and molecular machines with good thermalstability. After irradiationand measurement by SCXRD, the crystallinephase revealed that the diarylethenemolecule could undergo aphotoreactionprocess from ring-opening isomers to ring-closing isomers.[27] The geometry of the diarylethene derivatives remained almostthe same after irradiation, which permitted the occurrenceofphotochromism in acrys- talline phase. After irradiation with UV light, the ring in the diarylethene isomers closes and returns to the ring-opened state upon exposure to visible light. Because molecules pack dense- ly in alattice, deformationofthe crystal on the macroscopic

Figure 3. a) Structures of the trans isomersof2 and bpe. b) Photomechani- scale is anticipated following the photochromic processofdi- cal bending of the (cis-2)(cis-bpe) cocrystalafter irradiation. c) Single-crystal arylethene molecules. structural changesduring the transformation of (cis-2)(cis-bpe) into (trans- In 2007, Irie et al. reportedamolecular crystal composed of 2)(cis-bpe). d) Composite diffraction images of the h0l plane for different adiarylethene compound (Figure 6a).[28] Thismolecular crystal processes. (Copyright,2014, The Royal Society of Chemistry.) exhibited rapid and reversible macroscopic changes in shape and size inducedbyUVand visible light. After irradiation with UV light (l= 365 nm), the single crystal of 1 changed from a 2.1.2 The trans–cis isomerization of vinyl compounds colorless square microcrystal to ablue lozenge-shaped one. Apart from trans–cis isomerization of azobenzene, trans–cis iso- The angle of crystal 1 decreased by as much as 5–68 (Fig- merization of vinyl groups also exhibited photomechanical ef- ure 6b). The deformed crystalscan easily return to their origi- fects. The group of Lu reported an example of anaphthylvinyl- nal status upon exposure to visible light (l> 500 nm).

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Figure 4. a) Crystal and b) chemical structure of NBO and the packing distance. c) Photomechanical bendingofNBO after increasing irradiation times, from right (top line) to left (bottom line).(Copyright,2013, Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim.)

Another cocrystal containing two-component diarylethene can endure bending motions of more than 1000 cycles without any cracking or flaws emerging;thus revealing outstanding fa- tigue resistance. The response speed of the photochromic crystal can be enhanced to 5 ms, which is superiortothat of existing polymer muscles. Photoresponsive crystalsofdiarylethene are promising functional materials in manufacturing light- driven actuators after they can overcome the imperfections of structuralfatigue.[29] Recently,Kobatake and Bardeen et al. found unique behavior of areversible photomechanical twisting motion upon irradiation with alight source.[30] Apart from the photomechanical properties being influenced by light wavelength, the irradiation direction also induced different photomechanical behavior.Ifthe ribbonlike crystal was irradiated fromthe top of the crystal (incident light angle q= 08), the crystal twisted into a helicoidshape (Figure 7a). As the irradiation angle increased, the crystal gradually transformed into acylindrical helix shape (Figure 7b–g). If the incident light angle wasabout 908 (Fig- ure 7h), the crystal exhibitedbending behavior rather than twisting. These results indicated that the stress tensor on the crystal surface could be tuned by the direction of light irradiation. Changes in the initial shape of the crystals will lead to differ- Figure 5. a) Photoisomerization of the DMAAM molecule. Optical microscopy ent photomechanical behavior. Uchida and Morimoto et al. re- imagesof(E)-DMAAMnanowires (NWs) in pure water before (b) and after (c) portedahollow crystal of diarylethene (Figure 8a,b,and e) irradiation for 1min.Photomechanical bending in 2mm cetyltrimethylam- [31] moniumbromide(CTAB) before (d) and after (e) irradiation for 5s.(Copy- that displayed aremarkable photosalientphenomenon. In- right, 2017, American Chemical Society.) terestingly,itwas observed that the flying debris released from

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Figure 6. a) Chemical structure and deformationofacrystal of 1,2-bis(2-ethyl-5-phenyl-3-thienyl)perfluorocyclopentene (1). b) Relationship between the angle of the single crystal and absorbance of the crystal.(Copyright,2007, Springer Nature.)

2.3 Photomechanicalorganiccrystals inducedbyphotocy- cloaddition reactions

Solid-state cycloaddition reactions, which include [2+2] and [4+4] reactions, have attracted increasing interestsince the groups of Schmidt andMacGillivray established rules of the topo-photochemical reactionfor cycloadditionreactions.[32–34] According to their rules, the challenge of obtaining aphoto- mechanical crystal not only lies in suitablestacking arrange- mentsoflight-sensitivechromophores, butalso in avery strict requirement for packing distances of chromophores. Photome- chanicalcrystals demonstrated various photomechanical motions, including bending, jumping, curling, and twisting, if the photocycloaddition reaction occurredupon light irradiation. Notably,cycloaddition reactions are reversibleand inclined to shift towards depolymerization under heating,which provides apathway from photomechanical motiontophotoactuators.

2.3.1 Photocycloaddition of olefins It is an advantage of photocycloaddition reactions that the reaction process can be tracked through SCXRD due to the reaction nature of photocycloaddition. The group of Naumov found that multiple mechanical responsescould be elicited in Figure 7. a–h) Differenttwisting motions dependingonthe angle of the in- the same material, and they reported on how to tune the pho- cident light. (Copyright, 2018,American Chemical Society.) tochemical responsethrough surface modification,deformation, and disintegration of crystals.[35] Under UV-light irradiation, 3-benzylidenedihydrofuran-2(3H)-one (Z-BDHF) first trans- the crystal exceeded several meters per second upon UV irradi- formed into E-BDHF through an isomerization reaction (Fig- ation. In addition, the hollow crystal first elongated along the ure 9a). Continuous irradiation would induce the [2+2] photo- photoirradiated surface(Figure 8f)upon light irradiation and cycloaddition reactions, as evidenced by single-crystal cracked perpendicular to the a axis on the surfaceasthe irradi- structuralchanges to the E-BDHF structure (Figure 9b). The ation time extended (Figure 8c and d). The four corners of the crystalsexhibitedvarious mechanical response motionsdue to hollow crystalswere fixed in the hollow structure, which result- different effects. For example, 1) the crystalscracked, while re- ed in breaking of the hollow structure instead of bending. maining still (Figure 9c); 2) they movedthrough displacement by hopping off the stage (Figure 9d); 3) some crystals split into pieces and flew off in opposite directions (Figure9e);and 4) some crystals would bend first and move or split into pieces subsequently (Figure 9f). Cracks appeared in responsetothe

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Figure 8. SEM images of ahollow crystal of diarylethene: a) side view; b) top view; c) standing hollow crystal, and d) upon exposure to l=313 nm light for 1min. e) Molecular structures of ring-opening and ring-closing isomers. f) Schematic illustration of the hollow crystal upon UV irradiation. (Copyright,2017, Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim.)

crystalswere soaked in methanolorganogel for irradiation, the nanofiberscurled drastically,even into acircle. Meanwhile,a small slice rolled in the organogel under UV light (Figure 10 b). After UV-light irradiation, the C···C length changed from 3.865 to 1.580 and 1.574 in the single-crystal structure of photodimers(Figure 10c–e). This structural change followed an accumulation of strain in crystals that would be released throughmechanical motion.

2.3.2 Photocycloaddition of anthracenes Apart from [2+2] photocycloaddition of olefins, double bonds at the 9,10-positions of anthracene molecules are also photo- active and can undergo [4+4] photocycloaddition reactions if anthracene molecules pack in parallel and the distance between two anthracene planes satisfies the Schmidt rules. The photoresponsive behavior of crystalsthrough the [4+4] reaction is alwaysdifferentfrom that of [2+2] cycloaddition. Bardeen et al. synthesized branched microcrystals of 4-fluo- roanthracenecarboxylic acid (Figure 11 a) through apH-driven precipitation method.[37] If the branched microcrystals were il- luminated with light of l= 405 nm, the individual branched crystalsunderwent acombination of twisting and bending. Figure 9. a) Scheme of the reaction process. b) Changes to the crystal struc- These deformations drove the rotation of branched crystals ture of E-BDHF.c–f) Different photomechanical motions under UV-lightirra- clockwise after each irradiation period(Figure 11 b). If the light diation.AFM images of the surface of the crystals before (g) and after (h) was off, the bending arms relaxed and returned to the original light irradiation. (Copyright, 2015,American Chemical Society.) shape within 30 s. The authors claimed that the ability of this crystal to undergoratchetlike rotationwas attributedtothe tendency of the crystal to relax differential anisotropybetween chiral shape. 9-tert-Butylanthracenecarboxylate nanorods were the surface and inner layers(Figure 9g and h). synthesized in Al2O3 templates.The nanocrystals expanded by The group of Lu discovered aphotosalient effect of the as much as 15%along the long axis under UV-light irradia- olefin crystal driven by the photocycloaddition reaction.[36] tion.[38] The same group also studied aseries of 9-anthroate Upon irradiationwith UV light from below,the thinner crystal esters as photomechanically responsive crystalline nanorods bent upward clearly,whereas the thicker one bent upward (200 nm in diameter).[39] slightly.The two bending needlelike crystalscould straighten Recently,wedesigned and synthesized aseries of novel an- after irradiation from opposite directions (Figure 10 a). If the thracene derivatives (BAnDA; n= 2–8). These crystalscan har-

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Figure 10. a) Images of needlelike crystals before and after irradiation;b)rolling and curling of the crystals in amethanol organogel;c)single-crystal structure viewed along the b axis;d)the distance between two adjacent parallel molecules;and e) structureofthe photodimer formed. (Copyright,2017, Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim.)

Figure 11. a) Reactionsequence andb)optical microscopy images of the clockwise rotation of X-shaped crystals. (Copyright,2016, Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim.) vest the energy of visible light and transform it into various motions, including bending, curling, jumping, andwriggling.[40] The crystal also exhibitedhigh elasticity and flexibility,even under mechanical force (Figure 12). These results provided a new avenue to design and synthesize new photomechanical Figure 12. a) Chemical and b) crystal structures of the BA2DA molecule. c) The bending behaviorofthe crystal under white-light irradiation.d)Elas- crystalsbased on anthracenederivatives. ticity under mechanical force. (Copyright,2018, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.) 2.4 Other types of photomechanical crystals In addition to the abovementioned photomechanical crystals, and co-workers reported aplatelike crystal with reversible there are also some photomechanical crystals based on rela- bending behavior upon alternating irradiation with UV and visi- tively less studied functional groups. For example, Asahi and ble light (Figure13c).[42] If the microcrystal was irradiated from Koshima et al. reportedplatelike crystalsofS and R enantio- the diagonal underside with UV light for 1s,the crystal curled mers of photochromic N-3,5-di-tert-butylsalicylidene-1-phenyl- from the upper-right corner towardthe light and reached a ethylamine.[41] The transformationbetweenenol-(S)-1 and maximum twisted curl after irradiation for 2s.The phase trans- enol-(R)-2 caused bending behavioralong with atwisting formation was always accompanied by acolor change from motion upon UV-light irradiation (Figure 13aand b). Koshima pale yellow to red (Figure 13d).

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Figure 13. a) Photoinduced hydrogen-transfer reaction of salicylidenephenylethylamines enol-(S)-1 and enol-(R)-2. b) Bending with atwisting motion of a wide andthin platelike crystal.(Copyright, 2016, American Chemical Society.)c)Photochromicreactionoffurylfulgide 4.d)The (101)face of aplatelike microcrystal before and after UV irradiation for 1and 2s.(Copyright,2011, The Chemical Society of Japan.)

3. Potential ApplicationsofPhotomechanical was in the ON state. This ON and OFF process can undergo Crystals more than 10 cycles of light irradiation. These findings not only provide anew strategy to design photomechanical actua- It is of great importance to study mechanical motion based on tors, but also reveal apotential practical use of photomechani- changes to the geometricalstructure of molecular crystals in- cal crystals. duced by light stimuli from scientific and technological points Another example of molecular crystals acting as actuators of view.The crucial function to convert chemical energy into was reported by Irie et al.[29] The rodlike crystal of diarylethene mechanical work has attracted increasing attention. Various bent towardsthe direction of the incident light. The bending types of molecularcrystalsfor potentialapplicationsinactua- crystal moved agold macroparticle with aweight 90 times tors, artificial muscles, photoswitches, and artificial robots have greater than that of the crystal over adistance of 30 mm; thus been studied. For instance, Kobatake andKitagawareported a the crystal proveditself apromising materialfor possible light- crystal of 1,2-bis-(5-methyl-2-phenyl-4-thiazolyl)perfluorocyclo- driven actuatorapplications. The rodlikecrystal can bring pentene covered with gold, which exhibited photoreversible about gearwheel rotation. Upon irradiation with UV light, the current ON/OFF switching behavior (Figure 14).[43] If the gold- crystal bent and hitthe gear to rotate the gearwheel coated diarylethenecrystal wasirradiated with UV light from (Figure 15). This is actual photomechanical work by molecular above,the crystal bent towards the light and the flow current crystals. The rodlikecrystal was also able to lift ametal weight was cutoff. Thereafter,the switch was in the OFF state. Once ir- (2.2710 mg) 908 times heavierthan that of the crystal;this can radiated withvisible light,the crystal started to have contact be ascribed to ahighYoung’s modulus of the crystalline mate- with the electrical wire and the current flowed, so the switch rial.

Figure 14. Images (a) and photoreversible currentswitching (b) upon alternatingirradiation with UV and visible light of acrystal of 1,2-bis-(5-methyl-2-phenyl- 4-thiazolyl)perfluorocyclopentene covered with gold. (Copyright,2015, The Royal Society of Chemistry.)

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Figure 17. Tris{4-[(E)-phenyldiazenyl]phenyl}benzene-1,3,5-tricarboxamide (Azo-1)and apolystyrene(PS) NW arms select, lift, and move amicroparticle on asilicon substrate. (Copyright,2015, The Royal SocietyofChemistry.) Figure 15. a) Movement of agold macroparticle by acrystal upon irradiation with UV light.b)Gearwheel rotationoperated by alight-driven molecular crystal actuator.(Copyright,2012, Wiley-VCH Verlag GmbH &Co. KGaA, and the PS NW experienced no photomechanicalresponse, Weinheim.) which allowed the device to grip amicro-object remotely. The light-driven NW actuators for manipulation of micro-/nano-ob- Irie and Morimoto reportedadiarylethene cocrystal with jects have general utility in biomedical science and engineer- perfluoronaphthalene.The crystal exhibited reversible bending ing. motion upon alternate irradiation with UV and visible light.[44] In 2017, the group of Bardeen and Al-Kaysi[46] synthesized The molecular crystal could lift ametal ball that was 200–600 branched crystalsof4-fluoroanthracene-9-carboxylic acid times heavier than itself upon UV irradiation(Figure 16). The through aslow-release methodwith aspecific combination of maximum generated stress was estimated to be 44 MPa after temperature (358C) and solution (phosphoric acid/sodiumdo- UV irradiation. The stress was about 100 times larger than that decyl sulfate (SDS)/1-dodecanol) conditions. Upon exposure to of muscles(0.3 MPa) andcomparable to that of piezoelectric UV light, the sheaflike microcrystals tended to partially twist crystals(50 MPa). and bend in acollective manner toward the center(Fig- Asingle NW with an azobenzene crystal derivative was syn- ure 18a–c). This photomechanical motion generated asweep- thesized by employing adirect writingmethod.[45] This NW ex- like behavior.After UV irradiation, about 75 %ofthe silica mi- hibited reversible omnidirectional bending and unbending crospheres wereswept out of the region to stir microenviron- with large displacements ( 1.7 mm) upon irradiation with UV ments and returned to the original shape in seconds (Fig- and visible light (Figure 17). The Azo-1 NW (yellow) wasused ure 18d–f). This result provided agood demonstration of the together with aPSNWarm (Figure 17, black). After irradiation use of photomechanical crystalsfor novel applications. with UV light, the azobenzene NW bent towards the UV light

Figure 18. Abranched microcrystal before(a) and after (b) exposuretoUV Figure 16. Adiarylethene cocrystal lifting aheavymetal ball. (Copyright, light;c)10safter the UV light is switchedoff;and d)–f) the sweeping action 2010, American Chemical Society.) of branched crystals. (Copyright,2017,The Royal SocietyofChemistry.)

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Figure 19. a) Optical images of crystal–polymer hybrid materials hanging ared object from an arm;b–e) grasping objectswith multiple arms;and f) crawling and swimmingbehavior.(Copyright, 2012,Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim.)

For the first time, we successfully employedamixed-matrix plicationassmart materials in the future, some requirements membrane (MMM) method to combine the motions of me- for mechanical crystals must be satisfied, including reversibility, chanical crystalsthrough connective polymers. The crystals rapid response rate, repeatability,high sensitivity,and good fa- were mixed with polyvinylidene fluoride(PVDF)polymer.The tigue resistance. The photomechanical organic crystals present- formed hybridmaterials can be controllably actuated by light ed herein illustrate growinginterest in solid-state chemistry on the macroscale and exhibit rapid, reversible, and fatigueless and their various mechanical effects. Transforming light energy mechanical actuation to mimic the motility of artificial muscles. into mechanical work with organic crystals is important in the Benefiting from the features of photomechanical crystals and developing area of mechanical actuators, artificial muscles, op- connective polymers, the hybrid materials were deemed to be tical sensors,and switches. However, the question remainson artificial muscles with extended elasticity andmotility.The how to transform this mechanicalwork into useful work on a hybrid materials demonstrated exceptionallycontrollable mo- macroscopic scale. The increasing number of reported photo- bility,and were able to reversibly lift objects (Figure 19 a), mechanical crystals promisesabright future in the field of grasp objects (Figure 19 b–e), andcrawl and swim (Figure 19 f); crystal engineering as smartmaterials. The outstanding per- all movements were triggeredbyvisible-light irradiation. Our formance of molecular crystalsrespondingtolight sources work thus not only openedupanew strategy to combine offers great potential for applications in photoelectric materi- photomechanical crystals with polymers as artificial muscles, als, biological science, and photoactuators. but also provided anew approachtothe design of thermo-/ photomechanical artificial muscles. Acknowledgements

4. Summary and Outlook We acknowledge financial support from the National Natural We discussed current progress in the synthesis and application ScienceFoundation of China (21601093). of photomechanical organic crystals, including derivatives of azobenzene, vinyl, diarylethene, olefins,and anthracene. Al- thoughmolecular crystals with photomechanical motions are Conflict of interest in the process of rapid development, there are still some un- solved challenges and potential opportunities.First, develop- The authors declare no conflict of interest. ing new materials or strategies to prepare macroscale photoresponsive devices based on photomechanical crystals is es- Keywords: crystal engineering · molecular machines · sential. Second, it is of great urgency to design and prepare photochemistry · photochromism · X-ray diffraction photoresponsive molecular crystals towardefficient, rapid, con- trollable, and fatigueless mechanical responses that are essen- [1] T. Kim, L. Zhu, R. O. Al-Kaysi, C. J. Bardeen, ChemPhysChem 2014, 15, tial for optical molecular actuators. Third, prior to realizing ap- 400–414.

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