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Photomechanical Azobenzene Crystals

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Photomechanical Azobenzene Crystals crystals Review Photomechanical Azobenzene Crystals Takuya Taniguchi 1 , Toru Asahi 2,3 and Hideko Koshima 3,* 1 Center for Data Science, Waseda University, 27 Waseda-cho, Shinjuku-ku, Tokyo 1620042, Japan 2 Department of Advanced Science and Engineering, Graduate School of Advanced Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 1698555, Japan 3 Research Organization for Nano & Life Innovation, Waseda University, 513 Wasedatsurumaki-cho, Shinjuku-ku, Tokyo 1620041, Japan * Correspondence: [email protected]; Tel.: +81-3-5286-8307 Received: 12 August 2019; Accepted: 20 August 2019; Published: 22 August 2019 Abstract: Photomechanically responsive materials are promising candidates for future smart actuator applications. The photo-responsive behaviors originate from the photoisomerization of photochromic molecules. A typical photochromic compound, azobenzene, has been studied extensively in the solution state and has played a crucial role in the photomechanical behaviors of materials such as polymers and gels, via chemical bridging with their matrix. In contrast to polymers and gels, the photomechanical attributes of molecular crystals have not progressed to the same degree, due to their rigidity and fragility. However, the past decade has witnessed an increasing number of reports of the photomechanical motion of molecular crystals, including azobenzene crystals. This paper reviews the current state-of-the-art of mechanically responsive azobenzene crystals, including the history, crystal design strategy, and future promising applications. Keywords: azobenzene; photomechanical motion; trans-cis photoisomerization; molecular crystal; actuation 1. Introduction to Photomechanical Materials 1.1. Actuator Materials Actuation, which refers to mechanical motion or operation resulting from an energy conversion process, is prevalent throughout nature and characterizes some human activities. Animals and plants consume energy to create the mechanical motion required for survival and reproduction. Humans have developed mechanical systems for many purposes, and related to a wide range of actuation functions, to meet various needs. As to the specific materials used for actuation, inorganic materials that respond mechanically to external stimuli have historically played crucial roles in advancing actuation technology. For instance, a small piece of quartz, i.e., a crystal of silicon dioxide, ticks every second in an electric watch by oscillating at a constant frequency under an applied voltage. Shape memory alloys have been developed for many applications due to their shape recovery effect in response to a temperature change. Other examples of material actuation include those based on the piezoelectric effect, magnetostrictive effect, shape recovery, and thermal expansion [1]. The development of organic materials, mainly polymeric materials, for actuation has advanced steadily. Responses to various kinds of stimuli, including light, heat, voltage, magnetic fields, pressure, humidity, and biological enzymes have been achieved [2,3]. In addition, organic materials are generally softer, lighter, and cheaper than inorganic materials. Therefore, mechanically responsive organic materials are expected to come to the forefront in next-generation smart actuators. Potential future applications include soft robots, mechanical devices, active catheters, wearable applications, and so on [4,5]. Crystals 2019, 9, 437; doi:10.3390/cryst9090437 www.mdpi.com/journal/crystals Crystals 2019, 9, 437 2 of 14 Crystals 2019, 9, x FOR PEER REVIEW 2 of 14 1.2. Azobenzene Compounds for Actuator Materials The photomechanical response upon light irradiationirradiation has advantages for remote control and diverse actuation actuation via via focused focused light light or orselective selective wavelengths wavelengths [6]. [Photomechanical6]. Photomechanical polymers polymers have havebeen beenwell developed well developed by crosslinking by crosslinking the polymeric the polymeric main chain main with chain photochromic with photochromic compounds, compounds, which whichconvert convert their molecular their molecular geometry geometry and andelectronic electronic state state upon upon photoirradiation photoirradiation [7]. [7]. The The typical photochromic compound compound is isazobenzene, azobenzene, which which undergoes undergoes transtrans-to-cis-to- photoisomerizationcis photoisomerization and cis and-to- cistrans-to- transback-isomerizationback-isomerization reversibly reversibly (Scheme (Scheme 1). 1The). The physicochemical physicochemical properties properties of azobenzeneazobenzene compounds havehave been been researched researched extensively extensively in the in solution the solution state [8 ].state In polymeric [8]. In polymeric networks, networks, molecular geometrymolecular changesgeometry due changes to trans due-to- tocis transphotoisomerization-to-cis photoisomerization result in photo-actuation. result in photo-actuation. When a polymer When crosslinkeda polymer withcrosslinked azobenzene with is azobenzene irradiated by is light irradiated at wavelengths by light suitable at wavelengths for photoisomerization, suitable for cisphotoisomerization,-isomer photoproducts cis-isomer are generated, photoproducts mostly on are the generated, irradiated mo surface.stly on The the number irradiated of photoproducts surface. The decreasesnumber of according photoproducts to the penetration decreases according depth of the to light. the penetration The gradient depth distribution of the oflight.cis-isomer The gradient results indistribution a bilayer-like of structurecis-isomer in results the material, in a bilayer-like inducing macroscopic structure in deformations the material, such inducing as bending, macroscopic twisting, anddeformations helicoidal such motions as bending, [9–11]. This twisting, actuation and helico mechanismidal motions can also [9–11]. be applied This actuation to azobenzene-containing mechanism can gels.also be Gels applied are generally to azobenzene-containing much softer than gels. polymers Gels are due generally to the existence much softer of solvents, than polymers and result due in to a photomechanicalthe existence of solvents, response and with resu smallerlt in a photomechanical output force [12,13 response]. with smaller output force [12,13]. Scheme 1.1. Trans-to--to-cis photoisomerizationphotoisomerization ofof azobenzeneazobenzene uponupon photoirradiationphotoirradiation (h(hνν).). CisCis-to--to-trans back-isomerization undergoesundergoes by by thermal thermal energy energy (kT) (kT) or lightor light irradiation irradiation at di ffaterent different wavelengths wavelengths (hν’). (hν’). 1.3. Photomechanical Crystals 1.3. PhotomechanicalIn contrast to soft Crystals materials such as polymers and gels, molecular crystals are considered a more rigidIn and contrast fragile to material. soft materials For example, such as polymers when strain and is gels, applied molecular to a single crystals crystal are viaconsidered photoproducts a more orrigid mechanical and fragile stress, material. the periodicallyFor example, ordered when strain alignment is applied of molecules to a single results crystal in via the photoproducts collapse of a singleor mechanical crystalline stress, state the into periodically a polycrystalline ordered or alignment amorphous of state molecules [14]. However, results in inthe contrast collapse to of this a stereotypicalsingle crystalline characterization, state into a polycrystalline a landmark report or amorphous published in state 2007 [14]. demonstrated However, thatin contrast photochromic to this crystalsstereotypical of diarylethene characterization, compounds a landmark bent at anreport observable published scale in upon2007 lightdemonstrated irradiation that [15 photochromic]. This finding hascrystals had aof significant diarylethene impact compounds on the research bent at community. an observable A decade scale later,upon interestlight irradiation in photomechanical [15]. This crystalsfinding ofhas various had photochromica significant compoundsimpact on the continues research togrow, community. as evidenced A decade by the numerouslater, interest reports in inphotomechanical the literature [16 crystals–21]. of various photochromic compounds continues to grow, as evidenced by the numerousRegarding reports typical in photochromic the literature compound [16–21]. azobenzenes, dozens of crystals have been reported to enableRegarding various typical photomechanical photochromic behaviors compound based azobenz on theirenes,trans dozens-cis photoisomerization of crystals have been (Figure reported1). Thus,to enable azobenzenes various photomechanical have played a rolebehaviors in demonstrating based on their the trans photo-responsive-cis photoisomerization functionality (Figure ofthe 1). crystallineThus, azobenzenes state, prompting have played this reviewa role in of demonstra its potentialting contribution the photo-responsive to future developments functionality inof thisthe researchcrystalline area. state, prompting this review of its potential contribution to future developments in this researchBefore area. moving to the next section, we briefly discuss the mechanical properties of mechanically responsiveBefore molecular moving to crystals the next by section, comparing we briefly them to discuss other actuator the mechanical materials properties (Figure2)[ of22 mechanically]. Molecular crystalsresponsive possess molecular a Young’s crystals modulus by ofcomparing
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