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.

Structural phase transitions induced by external stimuli such as temperature, pressure, electromagnetic fields, and light play crucial roles in controlling the functions of solid-state materials. Here we report a new phase transition, referred to as the photo-triggered phase transition, of a photochromic chiral salicylideneamine crystal. The crystal, which exhibits a thermal single-crystal-to-single-crystal phase transition which is reversible upon heating and cooling, transforms to the identical phase upon light irradiation at temperatures lower than the thermal transition temperature. The photo-triggered phase transition originates from the strain of trans-keto molecules produced by enol-keto photoisomerization owing to the small energy barrier associated with changes in the crystal structure. The photo-triggered phase is metastable and returns to the initial stable phase via back isomerization from the trans-keto to enol form.

The mechanical motion of materials has been increasingly explored in terms of bending and expansion/contraction. However, the locomotion of materials has been limited. Here, we report walking and rolling locomotion of chiral azobenzene crystals, induced thermally by a reversible single-crystal-to-single-crystal phase transition. Long plate-like crystals with thickness gradient in the longitudinal direction walk slowly, like an inchworm, by repeated bending and straightening under heating and cooling cycles near the transition temperature. Furthermore, thinner, longer plate-like crystals with width gradient roll much faster by tilted bending and then flipping under only one process of heating or cooling. The length of the crystal is shortened above the transition temperature, which induces bending due to the temperature gradient to the thickness direction. The bending motion is necessarily converted to the walking and rolling locomotion due to the unsymmetrical shape of the crystal. This finding of the crystal locomotion can lead to a field of crystal robotics.

Photomechanical crystals are interesting from both basic and applied perspectives, and thus it is important to develop new examples. We investigated the photomechanical bending behaviour of a photochromic crystal of a dibenzobarrelene derivative. When a plate-like crystal was irradiated with ultraviolet (UV) light at 365 nm, two-step bending was observed. In the first step, the crystal quickly bent away from the light source, with an accompanying crystal colour change from colourless to purple. In the second step, under prolonged UV light, the bending returned slowly and then the crystal bent up towards the opposite direction, accompanied by an additional colour change to light yellow. Spectroscopic measurements and X-ray crystallographic analysis suggested that a long-lived biradical species is generated immediately upon UV light irradiation via a Norrish type II intramolecular hydrogen abstraction, and then the final photoproducts are formed under continuous UV exposure. X-ray crystallographic analysis before and after UV light irradiation for a few seconds revealed that the longitudinal axis (a axis) of the crystal elongated slightly after irradiation, which is consistent with the direction of the first-step bending. Based on these results, we propose that first-step bending could be induced by a biradical species, generated via a Norrish type II intramolecular hydrogen abstraction, and the second-step bending could originate from the formation of a mixture of final photoproducts under prolonged light irradiation.

The photomechanical motion of chiral crystals of trans-azobenzene derivatives with an (S)- and (R)-phenylethylamide group was investigated and compared with a racemic crystal. Changes in the UV/Vis absorption spectra of the powdered crystals before and after UV irradiation were measured by using an optical waveguide spectrometer, showing that the lifetime of the cis-to-trans thermal backisomerization of the chiral crystals was faster than that of the racemic crystals. Upon UV irradiation, a long plate-like chiral microcrystal bent away from the light source with a twisting motion. A square-like chiral microcrystal curled toward the light with some twisting. Reversible bending of a rod-like chiral microcrystal was repeatable over twenty-five cycles. In contrast, bending of a plate-like racemic microcrystal was small. A possible mechanism for the bending and twisting motion was discussed based on the optimized cis conformer determined by using calculations, showing that the bending motion with twisting is caused by elongation along the b axis and shrinkage along the a axis.

The photomechanical bending of crystals of a stilbene-type compound substituted with anthracene and indanone groups, (E)-2-(9-anthrylmethylene)-1-indanone (trans-1), was investigated. When a narrow plate-like microcrystal was irradiated with ultraviolet (UV) light at 365 nm, the crystal gradually bent away from the light source and finally reached a semicircular shape after more than 10 min. A larger rod-like crystal, approximately 10 mm in length, also exhibited a slight bending motion. The cessation of UV irradiation caused the bent crystals to return very slowly to their straight form. In the crystals, the anthracene planes of two neighbouring trans-1 molecules are arranged in a head-to-tail parallel manner, with a short plane-to-plane distance of only 3.72 angstrom. The H-1 nuclear magnetic resonance spectra of trans-1 crystals before and after UV irradiation revealed the intermolecular [4 + 4] photodimerisation of the two anthracene planes, while the trans-to-cis photoisomerisation was not significant. The UV-vis absorption spectra of the trans-1 powder crystals, obtained using a diffuse reflectance spectrophotometer, showed a gradual increase in absorbance between 200 and 500 nm with increasing UV irradiation time, reaching a maximum after 1 h. Thermal back-monomerisation was very slow in the dark, not recovering the initial spectrum even after 25 days. The fluorescence spectra at 570 nm, derived from the anthracene excimer, decreased in intensity with increasing UV irradiation time due to a decrease in the amount of anthracene chromophore via photodimerisation. In situ X-ray measurements revealed that the bending of the crystals was caused by slight elongation of the b axis of the unit cell, corresponding to the long axis of the rod-like crystals. Calculations revealed that the observed crystal elongation could be explained by an optimised head-to-tail [4 + 4] orientation of the anthracene dimer.

当該年度までに、光トリガー相転移という興味深い現象を見出していたため、当該年度はその光トリガー相転移について詳細に調べ、発現機構を明らかにした。光トリガー相転移は、熱的な構造相転移が起きる光反応性結晶において発見された現象である。この結晶は2度の構造相転移が起こり、温度変化に応じてα、β、γ相の間を変化できる。
これまでに-50℃で結晶に光照射すると光トリガー相転移が起きることは見出していた。これはβ相の結晶に光を当てるとγ相の結晶構造に変化することを意味する。新たに、α相およびγ相の結晶に光照射したところ、光トリガー相転移は起こらず、結晶相は変化しなかった。したがって、光トリガー相転移はβ相において特有の現象であることがわかった。
このβ相からγ相への光トリガー相転移の発現機構を考察するため、光異性化による生成物trans-keto体の分子構造を量子化学計算により最適化した。その結果、元のenol体とは二面角が大きく異なり、光異性化による構造変化が大きいことがわかった。この分子構造変化が結晶中で起きると、trans-keto体は周りの分子にひずみを与えると考えられる。実際の結晶において、光照射前は全ての分子がEnol体である。光照射すると、照射面から光異性化が進行し、trans-keto体ができ、ひずみを与え始める。ある程度trans-keto体が生成すると、そのひずみを解消するように結晶全体の構造がドミノ倒しのように進む。これが、光トリガー相転移の発現機構である。この時、β相からγ相へのエネルギー障壁が0.2 kJ/molと小さいことも光トリガー相転移の発現に寄与していると考えられる。
光照射によって、光異性化と光トリガー相転移を起こすことができ、それらを組み合わせた複雑なメカニカル機能を創出することができた。