DOI: 10.1002/cphc.201300906

Organic Photomechanical MaterialsTaehyung Kim,[a] Lingyan Zhu,[a] Rabih O. Al-Kaysi,*[b] and Christopher J. Bardeen*[a]

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1. Introduction

An important goal of nanotechnology is to develop structuresthat can manipulate nanometer-scale objects with high accura-cy and precision. Ideally, such actuators would function with-out being in physical contact with the controlling apparatus.Non-contact actuators can be completely immersed in thesample without physical contact with the controller, and multi-ple actuators can be operated in parallel as they do not needto be tethered to a central controller. Photons are in manyways the ideal tools for controlling nanoscale non-contact ac-tuators, as they can penetrate into a wider variety of mediaand transport both energy and information. Photomechanicalactuators are attractive because they do not require the pres-ence of a second chemical species, and because external con-trol can be achieved by manipulating the illumination condi-tions (light intensity, frequency, and polarization) inducing ac-tuator motion while leaving the rest of the system unpertur-bed.

Currently, there is much effort being devoted to transform-ing photon energy into electrical and electrochemical poten-tials (photovoltaics), but less attention has been paid to thetransformation of photons directly into mechanical work. It haslong been known that photons can generate mechanical ef-fects. Momentum transfer between photons and neutral atomsand molecules is the basis of laser cooling,[1] laser trapping,[2]

and has been used for molecular separations.[3] But these ef-fects tend to be very weak as they rely on momentum transferand do not convert the entire photon energy into work. Mole-cules that directly absorb the photon and convert its energy

into a chemical reaction are ideal transducers of light intomotion because the chemical change is usually accompaniedby a geometrical rearrangement. In many cases, photochemicalreactions can be reversed by heating or by photoexcitation ata different wavelength, so that the process can be repeated.

This Review focuses on molecular systems in which photo-chemical changes generate mechanical motion on lengthscales greater than the molecular dimensions. Volume and ge-ometry changes generated by individual molecular-level pho-tochemical reactions couple together to drive meso- to macro-scopic deformations in materials that contain the photoactivemolecules. Such photomechanical materials represent a way todirectly convert photon energy into mechanical motion andform the active elements for photomechanical actuator devi-ces. Although this Review emphasizes mechanical effects aris-ing from photochemical changes, we also briefly describe me-chanical motions arising from other effects, such as nonequili-brium heating and charge separation. After summarizingrecent progress and accomplishments in the field, we then tryto outline some challenges and possible future directions.

2. General Principles of PhotochemicalMaterials

The study of molecular photochemistry is a mature field that isalready covered by many review articles and several books. Amolecule designed to undergo reversible chemical changesunder light exposure, accompanied by a color change, istermed photochromic and there is a large field of effort dedi-cated to using such molecules in optical data storage materi-als.[4] Some representative molecular photochromic and photo-reactive systems are depicted in Scheme 1, and it should beemphasized that the capability of chemists to design new pho-toreactive compounds is essentially limitless. Although a photo-chemical reaction leads to a change in the geometry of an in-dividual molecule, in most cases the effects of its motioncannot be observed directly, as the molecules are dilute in anelastic medium (e.g. a liquid) and are randomly oriented, asshown in Figure 1 a. Changes in spectroscopic properties arethe only experimental indication of a structural change. Theproblem is how to harness the molecular-level motions in

[a] Dr. T. Kim, Dr. L. Zhu, Prof. Dr. C. J. BardeenDepartment of Chemistry, University of California, Riverside501 Big Springs Rd. , Riverside, CA 92521 (USA)Fax: (+ 1) 951-827-2435E-mail : christopher.bardeen@ucr.edu

[b] Prof. Dr. R. O. Al-KaysiDepartment of Basic SciencesCollege of Science and Health Professions-3124King Saud bin Abdulaziz University for Health Sciencesand King Abdullah International Medical Research CenterRiyadh 11426 (Kingdom of Saudi Arabia)E-mail : kaysir@ksau-hs.edu.sa

Organic molecules can transform photons into Angstrom-scalemotions by undergoing photochemical reactions. Orderedmedia, for example, liquid crystals or molecular crystals, canalign these molecular-scale motions to produce motion onmuch larger (micron to millimeter) length scales. In thisReview, we describe the basic principles that underlie organicphotomechanical materials, starting with a brief survey of mo-lecular photochromic systems that have been used as ele-ments of photomechanical materials. We then describe variousoptions for incorporating these active elements into a solid-state material, including dispersal in a polymer matrix, covalentattachment to a polymer chain, or self-assembly into molecularcrystals. Particular emphasis is placed on ordered media, such

as liquid-crystal elastomers and molecular crystals, that havebeen shown to produce motion on large (micron to millimeter)length scales. We also discuss other mechanisms for generat-ing photomechanical motion that do not involve photochemi-cal reactions, such as photothermal expansion and photoin-duced charge transfer. Finally, we identify areas for future re-search, ranging from the study of basic phenomena in solid-state photochemistry, to molecular and host matrix design,and the optimization of photoexcitation conditions. The ulti-mate realization of photon-fueled micromachines will likely in-volve advances spanning the disciplines of chemistry, physicsand engineering.

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order to generate a mechanical response on larger scales. Pho-tochemical events at the Angstrom scale must be amplified orsynchronized in some way to give rise to observable motionsat larger scales. Typically, the reactive molecules must be or-ganized, as shown in Figure 1 b, so that they all push in thesame direction. Common strategies for organizing photoreac-tive molecules include using an ordered host material (e.g.a liquid crystal polymer) or using ordered self-assembly of themolecules into a crystal. To sum up, most photomechanicalmaterials require two ingredients: a molecular photochemicalelement and some way to order these elements within a solid.

Once the chemical aspects of a material have been deter-mined, one can turn to the engineering question of how mo-lecular-level reactions couple together to drive large-scaleshape changes. Although the details of this coupling are com-plex and not well-understood, we can consider two generaltypes of actuator mechanisms illustrated in Figure 2. In a bi-

morph actuator, bending or twisting is induced by strain thatarises at the interface between two distinct chemical phases.In most systems, these two phases are comprised of the reac-tant (A) and product (B) molecules. For a large structure wherethe incident light experiences significant attenuation as it tra-verses the structure, one ends up with a gradient of reactedand unreacted molecules, as shown in Figure 2 a.[5] The stressbetween these two regions is what provides the energy todrive a large-scale deformation of the structure, for examplethe bending shown in Figure 2 a. In most (but not all) cases,the location of the strain interface that drives the motion isdictated by the illumination conditions. In this case, themotion depends on the direction, intensity, and duration ofthe light exposure. The advantage of this strategy is that itprovides a straightforward way to control the direction andmagnitude of the mechanical motion by controlling the light.One disadvantage is its sensitivity to the illumination condi-

Taehyung Kim received his B.Sc. (1994)

and M.Sc. degree (1996), under the su-

pervision of Prof. Won Ho Jo in Fiber

and Polymer Science from Seoul Na-

tional University, Korea, and his Ph.D.

degree in Polymer Science and Engi-

neering from the University of Massa-

chusetts, Amherst, under the supervi-

sion of Prof. Thomas J. McCarthy.

(2007) He started his post-doctoral

career in Florida International Universi-

ty, Miami, FL, under the supervision of

Prof. Wonbong Choi, and moved to the department of Chemistry

in the University of California, Riverside (2009), where he is an aca-

demic lecturer under the supervision of Prof. Christopher J.

Bardeen. His research interest covers self-assembled organic and

polymeric materials, CNT-based sensors, and organic photome-

chanical molecular crystals.

Lingyan Zhu received her B.Sc. degree

in Materials Chemistry from Nankai

University, Tianjin, China in 2006. She

did her undergraduate research with

Prof. Xianghai Tang preparing size-con-

trolled polymer nanospheres by ultra-

sonic emulsion polymerization. She

obtained her Ph.D. degree in Chemis-

try from the University of California,

Riverside in 2011, under the mentor-

ship of Prof. Christopher J. Bardeen.

Her dissertation was on the solid-state

photochemical and photomechanical studies of nanostructures

and microstructures of anthracene derivatives. She is currently

working as a junior specialist at the University of California, River-

side. Her research interests include stimuli-responsive materials,

functional material design, synthesis and characterization.

Rabih O. Al-Kaysi, received his B.Sc.

degree in chemistry from the Ameri-

can University of Beirut in 1995 and

a Ph.D. in organic photochemistry,

under the supervision of Prof. D.

Creed, from the University of Southern

Mississippi in 2002. He was a postdoc-

toral fellow at the University of Ro-

chester in NY from 2002–2005 with

Prof. J. L. Goodman. He then joined

the University of California Riverside as

a postdoctoral fellow with Prof. C.

Bardeen (2005–2007). In 2007 Dr. Al-Kaysi became an assistant Pro-

fessor of Chemistry at the King Saud Bin Abdulaziz University for

Health Sciences-National Guard Health Affairs in Riyadh Saudi

Arabia. In 2012 he was promoted to associate professor. His re-

search centers on the preparation of novel organic micro and

nano-materials and the synthesis of zwitterionic Meisenheimer

complexes.

Professor Bardeen received his B.Sc. in

chemistry from Yale University in 1989,

and a Ph.D. in chemistry from U. C.

Berkeley in 1995 under the guidance

of Prof. Charles V. Shank. After a post-

doctoral fellowship with Prof. Kent

Wilson at U. C. San Diego, he became

an assistant professor at U. Illinois,

Urbana-Champaign in 1998. In 2005 he

moved to the University of California,

Riverside. Prof. Bardeen’s research cen-

ters on the experimental study of

basic excited state properties and dynamics in molecular crystals

and organic photovoltaic materials. A second area of interest is the

use of these excited states to initiate photochemical reactions that

can power photomechanical shape changes and motion in molec-

ular crystal nanostructures.

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tions, which cannot be easily controlled when the actuator islocated in a scattering medium. A second more importantissue is that this strategy may fail when applied to sub-wave-

length structures within which light attenuation is negligible.An alternate strategy is shown in Figure 2 b, where the photo-chemical reaction A!B goes to 100 % completion. In this case,different packing interactions between the B molecules lead toa reconstruction of the entire structure. This approach mightbe expected to be useful for very small actuators, for whichlight penetration is more uniform. The shape change is intrinsi-cally determined by the crystal shape and the molecular orien-tations, and not the illumination conditions. Often, the detailedmechanism that causes an observed mechanical response isleft uninvestigated, although most studies assume that the bi-morph mechanism is dominant. In Section 5 we present evi-dence that it is possible to generate a bimorph structure with-out an irradiation gradient, thus exploiting the best aspects ofboth strategies.

3. Polymeric Photomechanical Materials

The most straightforward way to make a solid-state photome-chanical material is to simply embed a photoactive molecule ina polymer host. Obvious limitations of this approach includethe random orientations of the photochromes within the host,resulting in the averaging of the molecular displacements overall directions as illustrated in Figure 1 a, and the ability of theamorphous host to locally deform to accommodate the photo-product. Nevertheless, there exist some recent examples ofphotodriven bending of doped amorphous polymers. Athana-siou and co-workers studied the photoresponse of a low-glass-transition-temperature polymer PEMMA doped with a spiryo-pyran (SP) derivative.[6] The photoresponse of this compositewas quite complicated, and to generate bending in a strip ofpolymer, the UV-induced conversion of the nonpolar SP to itspolar merocyanine (MC) form had to be followed by exposureto 532 nm light, which led to photoisomerization between dif-ferent MC isomers. They concluded that aggregation of theseMC isomers led to volume changes in the polymer and finally

Scheme 1. Examples of reversible photochemical reactions.

Figure 1. a) How a disordered system of photoreactive molecules preventstheir geometrical distortion from generating a macroscopic response. b) Or-dered array of photoreactive molecules all push in the same direction, gen-erating a macroscopic shape change in the material.

Figure 2. a) Attenuation of exciting light leads to a gradient of reacted andunreacted molecules. This makes a bimorph-type actuator, the motion ofwhich is driven by strain between the different phases. b) Complete reactionof the crystal leads to a reconstruction to accommodate new packing ar-rangements of the product molecules.

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to a transient bending. Kuzyk and co-workers showed that ifthe azobenzene derivative Disperse Red 1 was doped intoa PMMA optical fiber, the fiber could bend in response to lightlaunched off-axis, which resulted in a spatially asymmetric exci-tation profile.[7] One notable aspect of this work is that thecontrol light was contained within the actuator structure itself,rather than being applied externally. In both these examples,the photomechanical response is not particularly dramatic.Given the complexity of the physical processes involved, it wasnot clear how to optimize the materials to improve the re-sponse, and the molecular doping approach has not been vig-orously pursued.

To simplify the material and achieve the maximum photo-mechanical response, one could utilize a polymer that is itselfcomposed of photoactive units. An interesting example of thisapproach is a polymer in which photoisomerizable azobenzenemoieties are covalently linked together. Such a polymer wasstudied by Gaub and co-workers at the single molecule level.[8]

By measuring the changes generated by photoisomerization ofsingle polymer chains attached to an atomic force microscope(AFM) tip, they were able to deduce quantities like the net mo-lecular displacement due to photoisomerization, the force gen-erated, and the free energy change. Although extension of thiswork on single chains to bulk materials is not straightforward,the experiment did illustrate the ability of covalently linkedphotochemical reactions to do measurable work and generatesignificant forces. A more synthetically tractable approach tomake photomechanical polymers is to attach the photochemi-cally active units as pendant groups on a non-reactive polymerbackbone. Work by Eisenbach,[9] Barrett,[10] and others[11]

showed that incorporation of an azobenzene moiety into a co-polymer could lead to polymers that exhibited measurable (~1 %) expansion or contraction that were at least somewhat re-versible. More recently, different photochemical reactions havebeen utilized that have generated larger motions. Kondo et al.have shown that large bends can be induced by [4 + 4] photo-cyclization reactions occurring in an anthracene copolymer.[12]

Trentjev and co-workers have shown that near infrared lightabsorption by carbon nanotubes (CNTs) can generate a me-chanical response in polydimethylsiloxane.[13] Lu and co-work-ers have prepared a multiblock copolymer that responds rever-sibly to near-infrared light that does not incorporate CNTs.[14]

Rack and co-workers have demonstrated photoreversiblebending motions that are driven by a photoinduced linkageisomerization in a ruthenium organometallic complex tetheredto a norborene backbone (Figure 3).[15] These encouraging re-sults suggest that going beyond the standard azobenzene cis–trans isomerization reaction may hold the key to making effec-tive amorphous polymer photomechanical materials.

We so far considered only polymer materials in which expo-sure to light generates strain and this drives a deformation inshape. However, it is also possible to design polymer materialsfor which light exposure releases strain and leads to a changein shape. This preparation usually involves locking in a strainedconformation by crosslinking, which can be relaxed througha photoinduced cleaving of the crosslinks.[16] Langer and co-workers introduced the concept of “light-induced shape-

memory polymers” and similar approaches have been exploit-ed by other workers to make photoresponsive polymer struc-tures.[17] Recent work by Dunn and co-workers has exploredthe concept of photo-origami in which directional strain islocked in at specific locations in a polymer sheet using spatial-ly localized photocrosslinking reactions (Figure 4).[18] Afterpreparation, when the entire sheet is irradiated, photoinducedstress relaxation can drive bending along prearranged hinges.The authors demonstrated the photoinitiated bending ofa sheet into complex shapes, including a cube. In all theseschemes, the shape change is a one-time event that is un-locked by an irreversible photochemical reaction, and thus thematerials are not generally considered actuator materials, al-though they are clearly related.

While amorphous polymers show promise as photomechani-cal materials, currently most research in the field focuses onmore highly ordered polymer systems. In ordered materials,alignment of the photoactive molecules ensures that theirphotomechanical displacements push in the same direction. A

Figure 3. A) Diagram depicting a polymer film composed of a ruthenium or-ganometallic complex tethered to a norborene backbone, with dimensions(1 � 5 � 0.002 mm3) and showing the area of irradiation. B) Diagram depictingside view of film and irradiation area. C) Image of the film before irradiation.Images (D)–(H) show the film after alternating 370 and 470 nm irradiationperiods. In each panel, the dashed line represents the position of the film inthe previous panel. After a large initial bend (D) due to 370 nm illumination,the film oscillates back and forth with alternating wavelength. Reprintedwith permission from reference [15] .

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high degree of order also ensures that there are fewer voidspaces to accommodate local deformations, allowing for localevents to add together and generate longer range motions.The key to achieving order in a polymeric system is to inte-grate liquid crystal mesogens with photochromic molecules.Liquid crystal elastomers (LCEs) comprise a large family of or-dered polymer solids whose applications as sensors and actua-tors have recently been reviewed.[19] Finkelmann and co-work-ers were the first to prepare a highly ordered photoactivesystem by using a polysilane LCE host doped with azobenzenederivatives.[20] That work demonstrated expansions on theorder of 40 % after light exposure, more than an order of mag-nitude greater than what had been observed previously. Sincethat seminal work, many other groups have used LCEs coupledwith azobenzene photoisomerization to make a variety of re-versible, photomechanical systems.[21] Much of this work up to2007 has been summarized in a comprehensive review byIkeda,[22] and for a detailed overview of photomechanical poly-mers, the interested reader is referred to that paper. Below webriefly summarize some notable research accomplishments inthis area.

In 2003 Ikeda and co-workers provided a dramatic illustra-tion of how light polarization could control the bending direc-tion in an azobenzene-based LCE sheet (Figure 5).[23] A yearlater, Palffy–Muhoray and co-workers showed that similar pho-toinduced bending in a LCE doped with the azo dye DisperseOrange 1 could propel a sheet of polymer through water byinducing a flapping motion similar to that used by sea crea-tures like rays (Figure 6).[24] Steady improvement in azoben-zene-containing LCEs by the group of Ikeda resulted in morestable, faster-responding materials that could move in all threedimensions.[25] This work culminated in the demonstration ofa macroscopic motor powered solely by light.[25a] Bunning andco-workers have demonstrated a photodriven polymer oscilla-tor that can operate at frequencies of up to 30 Hz for extendedperiods of time.[26] The use of photopatterning has allowed thecreation of arrays of pillars and strips composed of photome-

chanical LCEs.[27] A notable advance in this area was the workof Broer, who used inkjet printing to create artificial light-driven cilia networks for possible lab-on-a-chip applications.[28]

Recently, Aida and co-workers demonstrated a new conceptfor making large area ordered photomechanical materials bysynthesizing polymer brushes containing azobenzenegroups.[29] These polymers self-assemble as cylinders on top ofa Teflon sheet, where the azobenzene sidegroups exhibita high degree of three-dimensional order. The use of alternat-ing ultraviolet and visible light drives the bending and un-bending of the Teflon-polymer composite films. It is likely thatother creative ways to order photoactive molecules withinpolymers will be discovered, further expanding the array ofmaterials that can be used in this application.

Finally, when LCE materials are doped with absorbers likeCNTs, photoinduced heating can drive shape changes and me-

Figure 4. A six-sided box fabricated by photo-origami in which multiplehinges have been created by multiple straining and irradiation steps:a) photo-origami protocol, b) fabricated closed box after the pre-strainedsheet has been irradiated, c) simulated closed box, d) simulated box with anopen top, and e) simulated box with a partially open top. Reprinted withpermission from reference [18] . Figure 5. Azobenzene LCE polymer film where the bending direction is con-

trolled by linearly polarized light. The film bends in different directions in re-sponse to irradiation by linearly polarized light of different angles of polari-zation (white arrows) at 366 nm, and is flattened again after irradiation byvisible light longer than 540 nm. The flat film (4.5x3x7 mm3) rests ona copper pillar on a plate with temperature at 85 8C. The bending time forthe four different bending directions was within 10 s, when the light intensi-ty was 3.5 mW cm�2. Reprinted with permission from reference [23] .

Figure 6. A 5 mm-diameter disk of dye-doped LCE sample (0.32 mm thick)floats motionless on the surface of a water reservoir (roughly 2 cm in depth)when illuminated from above by an argon-ion laser with peak intensity of1.1 W cm�2 and beam waist of 3 mm. A) A series of video frames shows theLCE sample moving away from the area of sustained illumination, seen asthe bright disk in the center of each photo. B) Irregular rectangular LCEsample floating on ethylene glycol first folds then swims away when illumi-nated. Reprinted with permission from reference [24] .

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chanical response by initiating rearrangements in the polymerordering rather than actually breaking or remaking bonds.[30]

Heating also occurs in the case of azobenzene-functionalizedLCEs. In general, the underlying mechanisms that drive thephotomechanical response of these polymer systems may in-volve a complex interplay of chemical changes, heating, andorder-disorder transitions. All these phenomena can give riseto an asymmetric contraction/expansion of the material on theside of the sheet facing the light source, creating a bimorphstructure such as that illustrated in Figure 1 a. Experimental evi-dence that both photochemical and photothermal effects, am-plified by the ordered nature of the material, play a role in theoverall photomechanical response of azobenzene-functional-ized LCEs was provided by recent studies.[31] Unraveling the rel-ative importance of these effects is not trivial in polymers,since they are not sufficiently ordered for x-ray diffraction todetermine atomic positions and changes in chemical bonding.To obtain precise information on how molecular level changeslead to shape changes, even more ordered systems, such assingle crystals, are required.

4. Photomechanical Molecular Crystals

Rather than using covalent attachment of photoactive units tosynthesize a photomechanical polymer, it is also possible touse noncovalent self-assembly of photoactive units to gener-ate a photomechanical crystal. But replacing a soft polymerhost with a stiffer crystal matrix brings new challenges. This isbecause the volume changes associated with photoreactiontend to create phase-separated regions within the reactingcrystal.[32] The internal strain resulting from the interface be-tween different phases often leads to fracture and disintegra-tion of the original crystal, as opposed to elastic deformation.Strategies such as using low intensities, irradiation in the long-wavelength tail of the absorption,[33] and two-photon excita-tion (2PE) can alleviate the problem of domain formationwithin a single crystal,[34] but examples of the quantitative con-version of reactant into product while preserving the crystalintact remain rare. A second issue is that the dense, orderedpacking in the crystal may inhibit geometry changes, andthere is no guarantee that photoreactive molecules will retaintheir reactivity in the solid state. Stilbene and azobenzene areexamples of molecules that easily photoisomerize in solutionbut are unreactive in crystal form due to steric constraints im-posed by surrounding molecules. The effects of crystal packingon photochemical reactivity have been the subject of exten-sive study.[35]

Despite these complications, it is known that organic crystalscan be very dynamic. For example, thermally induced structur-al changes in crystals can induce mechanical motion (the ther-mosalient effect) that causes crystals to jump up to severalcentimeters in the air.[36] There exist many examples of photo-chromic molecular crystals,[4a, 37] and several of these also exhib-it a photomechanical response.[38] Most of these cases appearto belong to the class of actuators illustrated in Figure 2 a,where the photochemical reaction forms a bimorph structurewithin the crystal. The first example of a molecular crystal that

could reversibly bend in response to light involved a set ofrhodium semiquinone complexes studied by Lange et al.[39]

Thin crystalline needles were observed to reversibly bend byas much as 458 upon irradiation of a charge-transfer band inthe near infrared centered at 1600 nm. The bending actionwas ascribed to the formation of dimerized Rh�Rh bonds be-tween stacked complexes within the crystal. The reverse reac-tion required the use of a separate chemical species such asoxygen (as opposed to light or thermal fluctuations), makingthe reversibility less than ideal. If exposed to air over thecourse of a day, the crystals underwent an uncharacterizedchemical change that resulted in the loss of the bending abili-ty.

After the 1992 results of Lange et al. , little attention waspaid to the potential of molecular crystals as photomechanicalmaterials for more than a decade. In the meantime, work onsolid-state photochromic materials proceeded, with notableprogress in Irie’s group on the photoinduced ring-opening/closing reaction of the diarylethene family (Scheme 1 c).[40] Sev-eral derivatives were found that could undergo the ring-open-ing/closing reaction in crystalline form.[41] Experiments demon-strated that crystal surface features could be switched backand forth by exposure to UV followed by visible light.[42] Then,in 2006, our group showed that crystalline nanorods com-posed of an anthracene derivative could expand up to 15 %driven by a crystal-to-crystal photodimerization reaction.[43]

This result suggested that such crystals could generate largemechanical displacements, possibly useful for photomechanicalapplications. Our work nanostructured molecular crystals willbe described in more detail in the following section. In 2007,Irie’s group demonstrated that a molecular crystal composedof the diarylethene derivative shown in the inset of Figure 7could undergo reversible deformations upon exposure to dif-ferent wavelengths of light.[44] Although the magnitude of theshape changes was small (<1 %), if vacuum evaporation wasused to generate microcrystals on a surface, more dramatic ac-tuation behavior could be observed. Figure 8 shows an exam-

Figure 7. Open- and closed-ring chemical structures and reaction schemefor 1,2-bis(2-ethyl-5-phenyl-3-thienyl)perfluorocyclopentene. The images il-lustrate the deformation of a single crystal that can be switched back andforth using ultraviolet (365 nm) and visible (500 nm) light. A square singlecrystal of 1 with corner angles of 888 and 928 reversibly changes to a shapewith corner angles of 828 and 988. The crystal thickness was 570 nm. Re-printed with permission from reference [44] .

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ple of a 10 micron diameter molecular crystal rod grown inthis manner interaction with a gold microsphere. Upon photo-excitation, the rod flexes, pushing the sphere as far as 30 mm,indicating that a large force has been generated. The responsetime of the diarylethene crystal deformation was also veryrapid, in the microsecond range when high-power laser pulseswere used, and independent of temperature,[45] unlike poly-mer-based systems. Irie and other workers in the field havecontinued to exploit the diarylethene ring-opening/closing re-action for photomechanical applications.[46] An interesting ap-plication is the reversible creation of microfibrils on a crystalsurface that affect its wetting properties.[47] Another example isthe work by Feringa and co-workers, who synthesized chiralderivatives of diarylethenes and made rod-like crystals withthicknesses less than 1 micron by sublimation.[48] These chiralcrystals responded to 366 nm light by rolling instead of bend-ing as seen in Irie’s case. These same crystals could be unrolledto the flat form when exposed to >500 nm light. Diarylethenecrystals have also been observed to undergo twisting[49] andphotosalient (light induced jumping) behavior.[50] The combina-tion of reversibility, fast response time, and large force genera-tion would appear to make the diarylethenes promising forfurther study.

Other intramolecular photochromic reactions have beenused to generate photoresponsive materials as well. Microcrys-tals of furylfulgides undergo a UV-induced ring-closing reactionthat drives bending or curling motion.[51] As in the diaryle-thenes, this motion can be reversed by visible light irradiation,which induces the ring-opening reaction. The cis-trans photoi-somerization reaction, while not operative in crystals of unsub-stituted azobenzene, has been shown to generate reversiblebending in crystals of amino-azobenzene derivatives by Koshi-ma and co-workers.[52] The salicylideneanilines are anotherclass of compounds that undergo a photoinduced protontransfer followed by a cis-trans isomerization. Koshima et al.showed that this reversible solid-state photochemical reactioncan also drive crystal bending and straightening.[53] Recently,a cobalt-based coordination salt showed dramatic crystaljumping behavior that is driven by a photoinduced linkage iso-merization reaction.[54]

Intermolecular photochemical reactions have also been usedto drive crystal deformations. Much work in our group has cen-

tered on using the [4+4] photdimerization of anthracene de-rivatives, as shown in Scheme 1 g, and is also discussed inmore detail in Section 5. Naumov et al. showed that the benzy-lidinedimethylimidazoliones can undergo an intermolecular[2+2] photodimerization that causes crystal plates to bend byremarkably large (>908) without fracturing.[55] Our group hasdemonstrated that [2+2] photocyclization of 4-chloro-cinnamicacid (4Cl-CA) can drive twisting in microribbons.[56] Zhang andco-workers demonstrated the photoinduced bending of verylarge (~1 cm) crystals composed of trans-1,2-bis(4-pyridyl)ethy-lene.[57] The [2+2] photodimerization reaction is a very well-studied reaction in solid-state chemistry, and was the basis forforming the topochemical rules of reactivity.[58] However, thisphotoreaction is not easily reversible and thus is probably oflimited utility for actuator applications.

5. Photomechanical Nanocrystals andMicrocrystals

The results described in Section 4 demonstrate the potential ofmolecular crystal photomechanical materials. In many cases,microcrystals are used to demonstrate the phenomenon, andit is unclear if the same response would be observed for largeror smaller crystals. In our experience, the crystal disintegrationproblem discussed at the beginning of Section 4 is a generalproblem for larger crystals. It may be that the key to exploitingthe solid-state photomechanical properties of many types ofmolecules is to reduce the crystal dimensions. In 2002, Naka-nishi and co-workers showed that while the photopolymeriza-tion of a diolefin derivative resulted in the disintegration ofbulk crystals, the same photoreaction did not destroy nano-crystals.[59] The groups of MacGillivray and Garcia-Garibay havealso reported the ability of nano- and microcrystals to survivephotochemical transformations with their morphologyintact.[60] The ability of small crystals to withstand chemical re-actions is usually ascribed to the high surface-to-volume ratioof the nanostructures.[43] The idea is that any interfacial strainbuild-up in the interior of the particle can be relieved ata nearby surface, as opposed to fracture. A second possiblefactor is that the very small optical path lengths through theseparticles prevent the formation of large gradients in reactedversus unreacted molecules. A third factor that should be con-sidered is that there is now evidence that the mechanicalproperties of a molecular microcrystal can change in non-trivialways during photoreaction.[61] It should be noted that a com-prehensive theory of how fracture depends on crystal sizedoes not exist, and that any conclusions we draw are qualita-tive at best.

Our group has focused on the use of nano- and micro-struc-tured molecular crystals as photomechanical elements for tworeasons. First, as described above, these structures are morerobust and are able to survive photochemical reactions thatlead to disintegration of larger crystals. Second, we think thatthe advantages of photomechanical structures are most pro-nounced for small-scale applications where electrical connec-tions are impractical, for example, for structures functioninginside biological cells. The key to this approach is to develop

Figure 8. Movement of a gold micro-particle by the rod-like diarylethenecrystal after irradiation with ultraviolet (365 nm) light. The gold micro-parti-cle is 90 times heavier than the rod-like crystal (250 � 5 � 5 mm) and appearsin the images as a black spot. The ultraviolet light-induced bending of thecrystal could push the gold micro-particle as far as 30 mm. The exposuretime of each frame was 500 ms (2000 frames sec�1) and the numbers abovethe images are frame numbers. Reprinted with permission from refer-ence [44] .

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ways to grow small molecular crystals with reasonably well-de-fined shapes and sizes. We began by using solvent annealingin anodic aluminum oxide (AAO) templates to fabricate molec-ular crystal nanorods that exhibited large photomechanical re-sponses.[62] Work in our lab centered on anthracene derivativesthat undergo an intermolecular [4+4] photodimerization toform a bridged photodimer with new bonds formed betweenthe C9 and C10 carbons. An example is the molecule 9-tert-bu-tylanthroate (9TBAE), for which the photodimerization involveslarge-scale changes in the molecular geometry. Irradiation re-sults in fragmentation of bulk crystals, but 200 nm diameternanorods or nanowires are very robust under a variety of irra-diation conditions, even after undergoing very large (15 %) ex-pansions upon exposure to 365 nm light, as shown inFigure 9.[43] This work demonstrated that solid-state photo-chemical reactions could be harnessed to drive micron-scaledisplacements in molecular crystal nanostructures without the

fragmentation usually observed in larger crystals. Detailed in-vestigation of a family of 9-substituted anthracene esters re-vealed an interesting aspect of their solid-state photochemis-try.[63] After the monomer crystal is reacted, the photodimersrearrange into a metastable crystal form, of which the structureis different from that formed if the dimers are crystallized fromliquid solution. Over the course of months, the intermediatesolid-state reacted dimer crystal slowly converts into the lowerenergy solution-grown form. Since the photomechanical re-sponse is dictated by the metastable crystal structure, this re-search illustrated that the photomechanical response can arisefrom the formation of nonequilibrium structures and can bedifficult to predict based on equilibrium crystal structures. Thepossible role of non-equilibrium crystal structures in solid-statephotoreactions has not been extensively examined.

Although the 9TBAE [4+4] photodimerization can in princi-ple be reversed by thermal activation or short-wavelength irra-diation (254 nm), in practice thermal unzipping is detrimentalto the crystal structure of the nanorod and 254 nm irradiationcannot achieve 100 % back reaction due to the formation ofa photostationary state. To make a molecular crystal photoac-tuator that can be used multiple times, a reversible solid-statephotodimerization reaction is desirable. Fortunately, organicchemistry provides many opportunities to modulate the solid-

state photochemical reactivity of molecules. Simply removingthe tert-butyl group from 9TBAE yields 9-anthracene carboxylicacid (9AC), which crystallizes in a head-to-head syn arrange-ment,[64] rather than the head-to-tail anti arrangementcommon in most 9-substituted anthracenes.[65] Although thesyn arrangement is often assumed to prevent the [4+4] photo-cycloaddition reaction due to topochemical and steric factors,solid state NMR measurements showed that 9AC does in factundergo the [4+4] cycloaddition reaction characteristic of an-thracenes in the solid state.[66] The photodimer is unstable atroom temperature, however, and spontaneously reverts backto the monomer state within a few minutes. We monitored theexpansion and subsequent contraction of individual 9AC rodsusing AFM.[67] After light exposure, an individual rod briefly ex-pands by 1–3 % but then returns to its original shape. Toinduce more useful types of motion, we used spatially local-ized photoexcitation on isolated rods in aqueous solution. Wefound that a single rod, irradiated in its central region, instantlybends under the influence of the light beam (Figure 10). After2–5 min in the dark, the bent rod returned to its originalshape. This sequence could be repeated for multiple cycles,

the rod bending under illumination and then straightening inthe dark. The photobleaching of the monomeric 9AC could bemonitored by the loss of monomer crystal fluorescence aftereach illumination cycle. After five cycles of 15 s illumination,the amount of monomer emission decreased by about 20 %.Even though the fluorescence recovery indicated that somemonomer was lost during every illumination cycle, this photo-degradation did not appear to prevent the associated shapechanges. Thus the photomechanical response appears to bereasonably robust with respect to photobleaching. It is likelythat this could be improved by more rigorous exclusion of O2

from the sample, since electronically excited polyacenes areknown to undergo photoperoxidation. The photomechanicalbending was also observed for 9AC nanowires coated witha thin layer of silica, although the response time was signifi-cantly slower.[68] This result suggests that it is possible to havephotomechanical elements encased in a protective shell, whichmay be necessary for some practical applications.[69]

Figure 9. a) AFM image of a single nanorod before illumination and b) afterillumination with 365 nm. Scale bar is 6 mm. Note that the diameter of therod in the xy plane appears greater than 200 nm due to its convolution withthe broad AFM tip. Reprinted with permission from reference [43] .

Figure 10. Single 200 nm diameter nanorod of 9AC (~60 mm long) briefly ex-posed to 365 nm light in a 50 % solution of phosphoric acid in water. Thedotted circle shows the illuminated region (35 mm in diameter). The nanorodrepeatedly flexes back and forth (after UV illumination = panels b, d, f, h;after dark period = panels a, c, e, g). The time required to revert back isaround 2 min at room temperature. Scale bar = 20.7 mm. Reprinted with per-mission from reference [67] .

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To expand the capabilities of our photomechanical molecu-lar crystal nanostructures, we took several different ap-proaches. The first involved modifying the photoactive mole-cule using chemical synthesis. We systematically modified the9AC molecule by adding CH3, F, Cl, and Br substituents to the10 position, directly across from the COOH group, in an at-tempt to accelerate the photomechanical response.[64] Thiswork illustrated the challenges involved in the chemical modi-fication approach. 10CH3-9AC crystallized in a completely dif-ferent packing structure, for which the [4+4] dimerizationbecame geometrically impossible. 10Cl-9AC and 10Br-9AC didcrystallize into the same head-to-head stacking motif as 9AC,but showed no photochemical reactivity. 10F-9AC also crystal-lized in the head-to-head stacks, and only this molecule exhib-ited the same reversible photochemistry as 9AC. Unfortunately,the rate of dimer dissociation was at least one order of magni-tude slower in the 10F-9AC crystals, leading to a slower overallcycling time for actuation. We are continuing to explore thespace of substituted anthracenes to find improved photome-chanical crystals. Viable candidates must combine synthetic ac-cessibility and photochemical reactivity, while at the same timecrystallizing in a way that allows the photochemistry to pro-ceed. At this point in time, it is safe to say that the field reliesmore on luck than on rational molecular design and crystal en-gineering.

A second strategy involved changing the photoexcitationconditions. To achieve a higher degree of spatio-temporal con-trol over the nanorod motion, we combined our reversible9AC nanorods with 2PE methods.[70] 2PE allowed us to precise-ly localize the region of reacted molecules in the three-dimen-sional space and to control the location and magnitude of thebend. It also benefits from the superior penetration character-istics of the near infrared as opposed to the ultraviolet lightused for one-photon excitation. Figure 11 shows the results ofexciting different 1 micron diameter spots along a single 50micron long nanorod. Reversible bends, all of the same magni-

tude and duration, could be induced at any point along therod. In addition, the bend angle could be described in terms ofa simple kinetic model, for which the induced angle is propor-tional to the fraction of reacted monomers at the bend point.Thus it is possible to control both the rate and magnitude ofthe bend by controlling the duration and intensity of light ex-posure. This work demonstrated reversible bending in rodswith diameters as small as 35 nm, as well as the use of bend-ing to induce translational motion of the rods across a surface.

A third way to modify the photomechanical response is tovary the crystal morphology by using different growth condi-tions. One example of this has already been discussed: reduc-ing crystal dimensions to avoid fracture. But this concept is ac-tually much more general. In the case of 9AC, we used a modi-fied floating-drop method to grow microribbons instead ofnanorods.[71] Instead of bending or expanding under uniform il-lumination, these crystals could reversibly twist, as shown inFigure 12. The dependence of the twist period on ribbonheight and width could be described reasonably well by classi-cal elasticity theory, an encouraging sign that the mechanical

properties of these crystals may be understood within theframework of traditional engineering concepts. A second ex-ample of how crystal size and shape affect its photomechanicalresponse is provided by the molecule 4-chloro-cinnamic acid(4Cl-CA).[56] Like 9AC, this molecule crystallizes into hydrogen-bonded stacks, but undergoes a [2+2] photocycloaddition re-action across its double bond, rather than a [4+4] dimerizationbetween the aromatic rings. Large crystals of 4Cl-CA clearly un-dergo this photoreaction when irradiated with 365 nm light,but do not show any mechanical response. In general, we in-terpret the absence of a mechanical response in bulk crystalsto mean that the material has negligible photomechanical ac-tivity. But when crystalline microribbons of this molecule wereexamined, they showed the same type of twisting as 9AC(Figure 13). In this case, the motion was not reversible and thedimer crystal product eventually decomposed into an amor-phous phase. The lack of response in the larger crystals was ex-plained qualitatively in terms of the larger crystals being lessflexible and having greater resistance to distortion. Neverthe-less, 4Cl-CA is an example of a molecule the size and shape of

Figure 11. Single 9AC rod 200 nm in diameter exposed to800 nm light (fo-cused into a 1.5 mm spot) in phosphoric acid solution with surfactant. Therod in (a) is irradiated for 2 s in a spot near the top (hollow white circle indi-cates location and diameter of laser spot), resulting in a bend near the topof the rod in (b). After 2 min in darkness it relaxes to its former configuration(c). This cycle is then continued with irradiation in one spot (d, f), where thebend is indicated by the white circle, followed by relaxation (e, g). Scalebar = 15 mm. Reprinted with permission from reference [70] .

Figure 12. Optical microscopy images of a 9AC ribbon’s reversible twistingbehavior: a) before irradiation; b) immediately after irradiation; and c) 9ACbelt recovered after 9 min in the dark. The scale bar is 20 mm. Note that the9AC ribbon on the bottom right of the frame has a larger width and frac-tures when exposed to the UV light. Reprinted with permission from refer-ence [71] .

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the crystal of which determines whether a photomechanicalresponse can even be observed at all.

We end this section by returning to some of the ideas out-lined in Figure 1. In Section 2, we discussed how the bendingmotion induced by a bimorph structure requires an asymmet-ric distribution of reactant and product molecules. The mostcommon way to achieve this is by having a thick slab of mate-rial and relying on Beer’s Law to attenuate the excitation lightwithin the crystal. But this may not be the only way to gener-ate a mixture of reactant and product phases, for which the in-terface strain can drive crystal deformations. If a photoreactionis somehow self-limiting, one can still end up with a mixture ofreactants and products, even under uniform illumination con-ditions and in very thin crystals. In fact, we believe that thismechanism underlies the twisting observed in 9AC and 4Cl-CAmicroribbons.[56, 71] Although the reaction kinetics can be com-plex,[72] in both crystals the one-dimensional stacks imposea statistical limit on the ultimate dimerization yield, guarantee-ing a mixture of dimer and monomers within a reacted crys-tal.[73] If these two species phase separate, they can providea sort of built-in bimorph structure even in very small crystals.A second example of how reactant-product mixtures can drivecomplex motions is the recent observation of photoinducednanowire curling in our lab.[74] Using either the cis or transisomer of a novel anthracene-9-(1,3-butadiene) derivative, di-methyl-2(3-(anthracen-9-yl)allylidene)malonate (DMAAM) thatcan be isomerized using visible light, we grew single crystalnanowires. A single burst of visible light (475 nm), isomerizing20–40 % of the molecules within the nanowire, initiateda rapid collapse of the initially straight nanowires into a tightlycoiled ball (Figure 14). This dramatic photoinitiated shapechange does not rely on the details of the crystal structurewithin the nanowire, but rather on the generation of a mixedcrystal/amorphous phase that provides the internal energy todrive the shape change. While the mechanistic details of thisphotomechanical response remain to be worked out, it doesshow that the use of mixed phases can lead to unexpectedly

large and complex shape changes starting with relativelysimple crystal shapes.

6. Photomechanical Motion Induced by Heatand Charge Transfer

In the previous sections, we have described how photoin-duced changes in chemical bonding can induce larger scalephotomechanical motions. But it is not always necessary forchemical bonds to be broken and reformed. In this section, webriefly consider two other mechanisms, heating and chargeseparation, which can transform light into motion.

Light-induced heating can lead to localized expansion anda mechanical response. However, most materials have low co-efficients of thermal expansion and require large temperaturegradients to generate appreciable motion. Rapid heat dissipa-tion in condensed-phase system means that large localizedtemperature gradients must be sustained within a region ofa few microns, and this typically requires high light intensities.Since the actuator is in direct thermal contact with its local en-vironment, some heating of the experimental system is un-avoidable. For larger systems, thermal diffusion is less ofa problem, and there are many examples of macroscopic pho-tothermal actuators. A good example is the use of different al-lotropes of carbon as light absorbers that effectively convert100 % of the photon energy into heat. Using the uneven heat-ing caused by CNT or graphene absorption in layered polymercomposites, millimeter-scale tweezers and photoresponsiveplastic films can be fabricated.[75] Another example is the useof graphene-doped polyurethane composites as infrared trig-gered actuators.[76] Photoinduced heating can drive molecularrearrangements or phase transitions, and the cyclooctadienepolymers and CNT-doped LCE’s described in Section 3 can alsobe thought of as operating via a photothermal mechanism.But in these examples the light-induced heating generatesa distinct chemical species or conformer, rather than simply in-creasing the level of vibrational excitation. In practice, it canbe hard to distinguish between the two modes.

Rather than converting light into heat, one can also use thephoton energy to generate spatially separated electron–holepairs. Structural changes arise from the modified Coulomb in-

Figure 13. Optical images of a) a 4-chloro-cinnamic acid rectangular prismcrystal prepared by solvent evaporation, b) a 4-chloro-cinnamic acid platecrystal prepared by slow precipitation, and c) a 4-chloro-cinnamic acid mi-croribbon prepared by the floating drop method. The top images was takenbefore UV (365 nm) exposure, the middle images were taken after 5 min UVexposure, and bottom images were taken 1 h after turning off the UV. Scalebars : 50 mm. Reprinted with permission from reference [56] .

Figure 14. Snapshots showing coiling of a nanowire composed of (E)-DMAAM after 1 s of light exposure at time 0 s. Scale bars : 10 mm. Reprintedwith permission from reference [74] .

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teractions between the photoexcited molecules and their sur-roundings, rather than to rearrangement of the nuclei withina photoreacted molecule. Such deformations persist duringthe lifetime of the charge-separated state. Ijima and co-workersused laser light at 636 nm to induce reversible elastic deforma-tions in long fibers consisting of bundles of multi-walledcarbon nanotubes.[77] The detailed mechanism of the light-in-duced bending has yet to be elucidated but may be mediatedby photoinduced charge separation between nanotubes withdifferent electron affinities. Zewail and co-workers utilized ul-trafast electron microscopy to visualize nanoscale motion ofphotoexcited Cu-TCNQ charge-transfer crystals after pulsed ex-citation.[78] This material is known to undergo a transition froma high to low impedance state under the influence of both DCelectric fields and optical excitation. It appears that the lowand high impedance forms are structurally different and thatswitching between them can induce nanometer to microme-ter-scale expansion of the crystals (Figure 15). A related phe-nomenon is the transient expansion or deformation of a materi-

al due to the creation of a high-density electron–hole popula-tion. High intensity femtosecond excitation has been observedto cause transient deformations in nanostructures composedof ZnO, Au, and graphene.[79] This laser-induced deformation il-lustrates how nonequilibrium carrier heating can lead to largerphysical changes than would be expected based on equilibri-um parameters like the coefficient of thermal expansion. But itshould be emphasized that these nanoscale expansions existonly during irradiation and require high laser intensities.

Photoinduced charge transfer has also been invoked to ex-plain the photomechanical response of polyvinyldifluoride.[80]

When films of this piezoelectric polymer are coated witha metal, it has been observed that they undergo small me-

chanical motions when exposed to light. In this material, how-ever, it has also been postulated that photothermal effectsplay a role,[81] providing another example of how difficult it canbe to pin down the physical origin of the photoresponse inpolymer composites.

7. Future Directions

7.1. Solid-State Chemical-Reaction Dynamics

Interest in photomechanical materials is driven by their poten-tial applications, but there are a host of fundamental sciencequestions that arise in their study. While the study of molecularphotochemical reaction dynamics has reached an advancedstate, much less is known about how these dynamics changein a solid host material. Many groups have studied how thesurrounding medium affects molecular reactivity, with the top-ochemical postulate being the most well-known outcome ofthese studies.[35, 58a] But in photomechanical materials, the pho-toreaction of a single molecule is only the first step. Openquestions include whether reaction at a single site createslocal strain that further catalyzes the reaction, leading to spa-tially heterogeneous regions of reactant and product mole-cules within the solid, or whether the reaction proceeds ina spatially homogeneous manner with reactant and productuniformly mixed. After a significant number of molecules havereacted, how does their motion synchronize to drive large-scale shape changes? For materials based on LCE’s, these ques-tions are beginning to be addressed from an engineering per-spective.[82] The more recently observed photomechanical ef-fects observed in molecular crystals are not well understood inquantitative terms. One important question concerns the sizethreshold where a crystal fractures instead of deforming.[83]

Given that these systems can be characterized using x-ray dif-fraction, a realistic goal would be to achieve a predictive un-derstanding of how crystal size, shape and orientation deter-mine its photomechanical response. There is clearly room forboth experimental and theoretical physical chemists to studythese questions, although answering them may require consid-eration of many-body systems containing thousands of mole-cules. In the long run, a quantitative understanding of howmicron-scale motions arise from molecule-scale geometrychanges will be crucial for the rational design of new photo-mechanical materials.

7.2. Molecular Design

As we have seen, there already exist a variety of photochemi-cal reactions that can drive mechanical motion in the solidstate. In general, one wishes to have a reaction that 1) pro-ceeds rapidly and in high yield (to efficiently use the photonenergy) ; 2) generates significant force; and 3) can be repeatedmany times without losing efficiency due to secondary photo-chemical reactions. A novel strategy to increase the yield ofthe reaction is to design a self-propagating chemical reactionwith an effective quantum yield >1. An example of such a re-action in the solid state is the molecular decomposition of di-

Figure 15. Ultrafast electron microscopy images of [Cu(TCNQ)] single crystalin the “off” structure (a, c; no pulsed-laser irradiation) and in the “on” struc-ture (b, d; pulsed-laser irradiation). (scale bar = 500 nm); e) Higher-resolutionimage of the crystal in the “off” structure illustrating the gap that is closedby laser irradiation. f) Plot showing the results of a sequence of “on” and“off” cycles. The channel width W changed from 0 (pulsed-laser irradiation)to 140�5 nm (no pulsed-laser irradiation). Reprinted with permission fromreference [78a] .

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phenyl-cyclopropenone derivatives to yield CO and diphenyl-acetylene, which was found to have a photochemical quantumyield of decomposition >4.[84] Plasmonic effects can also beused to enhance photoreaction yields.[85] The photomechanicalreaction should also generate a large mechanical force. Asimple way to estimate the force is to use the relation[Eq. (1)]:[86]

DG ¼ useful work ¼ Force � Distance ð1Þ

where the distance is proportional to the change in moleculardimensions due to the photoreaction. This equation providesa starting point for the design of photoactive molecules thatgenerate large forces or displacements; large DG values aredesirable. A final area of concern is molecular photostability.The most robust photochromes (diarylethenes and fulgides) in-volve intramolecular ring-opening/closing reactions. Reactionsthat generate larger distortions upon photoexcitation, such asthe azobenzene cis-trans isomerization and the anthracene[4+4] photodimerization, appear to be more susceptible toside reactions. A key challenge for chemists is to design mole-cules that exhibit large geometry changes upon photoexcita-tion but are sufficiently robust to survive over the course ofmany photocycles. Photooxidation remains a problem whendealing with photoreversible reactions involving [4+4] photo-reactions in anthracene derivatives. One strategy is to lowerthe oxidation potential of these chromophores by adding elec-tron-withdrawing groups such as fluorine. An alternatemethod is to coat the surface of the molecular crystal witha layer of silica or other inert materials to protect the structurefrom the elements.

7.3. Solid Matrix Engineering

It should be emphasized that the photochemical reaction isnecessary but not sufficient to generate a photomechanical re-sponse. The photoreactive molecule must reside in an environ-ment that not only permits the reaction to proceed but alsoenables the generation of a mechanical response on largerscales. In general, we desire an actuator material with a highelastic modulus so that it can apply pressure to an object with-out deforming itself.[87] This consideration may favor molecularcrystals, which typically have higher elastic moduli, for photo-mechanical applications. On the other hand, the elasticity mustbe large enough that an actuator can respond to the internalstrain generated by the photoreactions without fracturing. Inpractice, the requirements of a specific application may deter-mine the nature of the solid-state matrix to be used.

Shape is a second parameter that can be tuned. Most photo-responsive elastomers exist as macroscopic sheets, while mo-lecular crystal structures tend to be needle or plate-like. Theseform a very limited subset of possible shapes for photome-chanical actuators. For liquid crystal elastomers, the use ofphotolithography may enable the manufacture of more com-plicated shapes.[88] For molecular crystal structures, there hasbeen encouraging progress in the creation of arrays in whichboth the crystal size and orientation can be controlled.[89]

There is clearly a need for better tools for shaping organicphotoactive materials.

A final challenge is to incorporate the photoactive materialsinto larger composite structures. Many workers in the fielddraw analogies between photomechanical organic materialsand biological muscle tissue, but muscles are usually attachedto some framework (e.g. bones in the skeleton) in order to pro-vide a point of leverage. The integration of organic photome-chanical structures into larger assemblies is a largely unex-plored area. We have already mentioned examples for whichphotomechanical LCEs have been used in devices,[25a, 28] and re-cently a novel polymer-organic nanocrystal composite actuatorhas been reported.[90] In the end, one would like to incorporatevarious photomechanical structures into a larger structurewhere they can truly act as artificial muscles.

7.4. Photoexcitation Conditions

In order to use the bimorph strategy (Figure 2 b) to drive thephotomechanical response, it is necessary to control the loca-tion and extent of photoexcitation within the photomechanicalstructure. The ability to selectively convert any volume elementwithin a structure at any time would enable precise control ofthe deformation of a single nanostructure. There now existmultiple strategies for localizing the photoexcitation to sub-wavelength regions in a three-dimensional sample, mainly forthe purposes of high-resolution fluorescence microscopy.[91]

Methods such as multi-photon absorption[92] and stimulatedemission depletion[93] have also been used to drive photo-chemical reactions such as polymerization, and it is not a largestep to envision using the same strategies to initiate photome-chanical reactions. The use of 2PE to induce bends at differentlocations within a single nanorod provided a preliminary exam-ple of how this might be accomplished.[70] The application ofsophisticated microscopy and photolithography excitationmethods to photoreactive systems should enable the genera-tion of more precise and complex motions in photomechanicalstructures.

Even if we can localize the exciting photons to a specificregion within the photomechanical structure, they still need todrive a photochemical reaction. Ideally, photoexcitation wouldconvert 100 % of the molecules from one form to the other inas short a time as possible. But for molecules that interconvertbetween short- and long-wavelength-absorbing species (diary-lethenes, azobenzenes), 100 % conversion from one state toanother can be difficult to achieve. This is because the long-wavelength-absorbing photoproduct usually has some absorp-tion at shorter wavelengths as well, leading to a photostation-ary state for which an equilibrium is established between reac-tant and product. The development of techniques for therapid and selective inversion of molecular populations isa problem in the field of quantum control.[94] There have al-ready been some intriguing results in the use of chirped fem-tosecond pulses to increase photoreaction yields in spiropyr-ans,[95] as well as using sequential absorption phenomena todrive spirooxazine reactions that do not occur naturally in thecrystalline state under low fluence excitation.[96] It is likely that

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novel laser excitation schemes can increase the rate and selec-tivity of the molecular photoreaction that powers the materi-al’s photomechanical response.

8. Conclusions

It is hoped that this Review provides the reader with an intro-duction to the field of photomechanical materials. It is an areaof research that lies at the intersection of materials engineer-ing, synthetic and physical chemistry, and optical physics.While it may be some time before the dream of microma-chines fueled by photons can be realized, it is clear that thereis much interesting science to be done along the way.

Acknowledgements

This research was supported by the National Science Foundationgrant DMR-1207063. R.O.A.-K. acknowledges the support of KingSaud bin Adbulaziz University for Health Sciences/King AbdullahInternational Medical Research Center through grants RC10/104and King Abdulaziz City for Science and Technology (KACST)through Grant AT-30-435.

Keywords: molecular crystals · nanostructures ·photochemistry · photochromism · photomechanics

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Received: October 4, 2013Published online on January 27, 2014

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