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Crystals of diarylethenes having urethane bonding exhibited photoinduced rapid and explosive fragmentation according to the photochromic reaction. The speed of the photogenerated crystal fragments was as fast as a few meters per second. The kinetic energy was independent of size, thickness, and volume of crystals, and depended on the crystal characters, such as crystal density and strength of intermolecular hydrogen bonding. Mechanically responsive materials induced by light, heat, mechanical force, or other stimuli are of great interest to material science to construct molecular machines, molecular rotors, molecular robots, and artificial muscles. Such mechanically responsive materials, especially induced by light, have been investigated using liquid crystalline polymers and molecular crystals containing photoresponsive molecules, such as azobenzene, spiropyran, anthracene, diarylethene, and so on. In the research of photoresponsive molecular crystals, we have focused on the photomechanical response of diarylethene crystals. Diarylethene can reversibly undergo photoisomerization between the open-ring isomer and the closed-ring isomer upon alternating irradiation with ultraviolet (UV) and visible light. There are two stable conformations in the open-ring isomer, parallel and antiparallel conformations. Only from antiparallel conformation, the photocyclization reaction from the open-ring isomer to the closed-ring isomer can proceed. When diarylethene molecules exist in an antiparallel conformation and the distance between the reactive carbons is less than 4.2 Å, diarylethenes can undergo a photochromic reaction even in the crystalline phase. The geometrical structure change due to the photoisomerization causes the strain in crystal to result in the macroscopic crystal shape change of the bulk crystal. Various types of photomechanical motion in diarylethene crystals, such as contraction, expansion, bending, twisting, curling, and separation, have been reported. In particular, the bending is the most common motion and the mechanical works using bending motion, such as a gearwheel rotation, lifting up heavy metal balls having the 200~600 times weight of the crystal, and current switching, have been reported. On the other hand, recently, the “photosalient effect” that means crystal jumping like popping of corn has been reported in other molecular crystals. The photosalient effect is well investigated by Naumov group and the phenomenon is seemed to be very rare. As one of the molecules that exhibit the photosalient effect, crystals of diarylethene, 1,2-bis(5-methyl-2-(2-pyridyl)-3-thiazolyl)perfluorocyclopentene, have been reported. However, further studies have not been so far performed. In the course of study on the photomechanical behavior of various diarylethene crystals, we found that diarylethenes having urethane bonding, 1-(2-methyl-5-(4(phenylcarbarylmethyl)phenyl)-3-thienyl)-2-(2-methyl-5phenyl-3-thienyl)perfluorocyclopentene (1a) and 1-(2methyl-5-(4-(benzylcarbarylmethyl)phenyl)-3-thienyl)-2(2-methyl-5-phenyl-3-thienyl)perfluorocyclopentene (2a), Scheme 1. Diarylethenes used in this work. exhibit the photosalient effect; the photoinduced rapid and explosive fragmentation upon irradiation with 365 nm light. In this work, we investigated the influences of size (area), thickness, and volume of crystals on the fragmentation. Furthermore, the initial speed of fragmentation and the kinetic energy were estimated by the observation of the photoinduced crystal fragmentation behavior using a high-speed camera. The conversion from light energy to kinetic energy was also estimated. Diarylethenes 1a and 2a underwent reversible photoisomerization between the colorless open-ring isomer and the colored closed-ring isomer upon alternating irradiation with ultraviolet (UV) and visible light, as shown in Scheme 1. Figure 1 shows the absorption spectral changes of 1a and 2a in dichloromethane. Upon irradiation with 313 nm light, the colorless solutions of 1a and 2a turned blue, in which the visible absorption maxima were observed at 587 nm for both 1b and 2b. These spectral changes are ascribed to the photoisomerization from the open-ring isomer to the closed-ring isomer. The blue solution returned to the initial colorless solution upon irradiation with visible light (> 500 nm) and the absorption spectra also returned to those of 1a and 2a. The openand closed-ring isomers were stable at room temperature in the dark. The molar absorption coefficient (), the photocyclization quantum yield (Фo→c), the photocycloreversion quantum yield (Фc→o), and the conversion from the openring isomer to the closed-ring isomer upon irradiation with 313 nm light were determined as shown in Table 1. The photochromic performance of crystal 1a and 2a was examined. The crystals of 1a and 2a were prepared by recrystallization from acetone/n-hexane. To know molecular structure and packings in crystals, single crystal X-ray crystallographic analysis was performed. The crystallographic data are summarized in Table S1. The crystal systems of crystals 1a and 2a are orthorhombic and monoclinic, respectively. The space groups of 1a and 2a are Pbca and P21/c, respectively. Both crystals have one molecule in the asymmetric unit. The diarylethene molecules exist in an antiparallel conformation in both crystals and the distances between the reactive carbons in crystals 1a and 2a are 3.537 and 3.567 Å, respectively, which are sufficiently short for photocyclization to take place in the crystalline phase. Furthermore, in both crystals, there is intermolecular hydrogen bonding between the urethane units, as shown in Figure 2. In crystal 1a, the intermolecular hydrogen bonding extends along to the a-axis. The distance of O···HN between the urethane units is 1.997 Å. On the other hand, in crystal 2a, the intermolecular hyTable 1. Optical properties of diarylethenes in dichloromethane open-ring isomer (a) closed-ring isomer (b) Φo→c a) Φc→o b) Conv./% c) λmax/nm ε/M−1cm−1 λmax/nm ε/M−1cm−1 1 289 45000 587 22000 0.45 0.0053 95 2 290 42000 587 20000 0.51 0.0043 92 a) Upon irradiation with 313 nm light. b) Upon irradiation with 587 nm light. c) Conversion from the open-ring isomer to the closed-ring isomer upon irradiation with 313 nm light. Figure 1. Absorption spectral change of diarylethenes 1a (7.1 × 10−6 mol L−1) (a) and 2a (5.7 × 10−6 mol L−1) (b) in dichloromethane: the open-ring isomer (black line), the closed-ring isomer (blue line), the photostationary state upon irradiation with 313 nm light (sky blue). 0.0 0.1 0.2 0.3 0.4 300 400 500 600 700 800 A b s o rb a n c e

Photoinduced Rapid and Explosive Fragmentation of Diarylethene Crystals Having Urethane Bonding