The development of actuators based on materials that reversibly change shape and/or size in response to external stimuli has attracted interest for some time. A particularly intriguing possibility is offered by light-responsive materials, which allow remote operation without the need for direct contact to the actuator. The photo-response of these materials is based on the photoisomerization of constituent molecules (typically trans-cis isomerization of azobenzene chromophores), which gives rise to molecular motions and thereby deforms the bulk material. This effect has been used to create light-deformable polymer films and gels, but the response of these systems is relatively slow. Here we report that molecular crystals based on diarylethene chromophores and with sizes ranging from 10 to 100 micrometres exhibit rapid and reversible macroscopic changes in shape and size induced by ultraviolet and visible light. We find that on exposure to ultraviolet light, a single crystal of 1,2-bis(2-ethyl-5-phenyl-3-thienyl)perfluorocyclopentene changes from a square shape to a lozenge shape, whereas a rectangular single crystal of 1,2-bis(5-methyl-2-phenyl-4-thiazolyl)perfluorocyclopentene contracts by about 5-7 per cent. The deformed crystals are thermally stable, and switch back to their original state on irradiation with visible light. We find that our crystals respond in about 25 microseconds (that is, about five orders of magnitude faster than the response time of the azobenzene-based polymer systems) and that they can move microscopic objects, making them promising materials for possible light-driven actuator applications.
An excited state energy migration was found to compete with a photocyclization reaction in the single crystal of 1,2-bis(2,4-dimethyl-3-thienyl)perfluorocyclopentene.Photochromic diarylethenes belong to the most promising photochromic compounds because of thermal stability of both isomers and fatigue-resistant property.1 Some diarylethene derivatives can undergo photochromism even in the single-crystalline phase.2 The colored forms of diarylethenes are stable in the crystals even at 100 C. Such thermally irreversible crystalline photochromic materials are potentially applicable to various optoelectronic devices, such as holographic and three-dimensional optical recording memories and displays. [3][4][5][6][7] In the course of study on single-crystalline photochromism of diarylethenes, we prepared a binary co-crystal composed of two types of diarylethenes. 8,9 Both diarylethenes can undergo photochromism in the home-crystals phase. However, in the co-crystals only one diarylethene selectively undergoes a photocyclization reaction to give the closed-ring isomer upon irradiation with ultraviolet light. This suggests that an excited state energy migration takes place in the crystal. The purpose of this paper is to examine the excited state energy migration in a single-component crystal of 1,2-bis(2,4-dimethyl-3-thienyl)perfluorocyclopentene (1a) (Scheme 1).In solution, diarylethenes have two stable conformations with the aryl rings in mirror symmetry (named parallel conformation) and C 2 symmetry (named antiparallel conformation) in almost equal amounts, and they interconvert with each other in solution. The interconversion rate between the two conformations is much slower than the lifetime of photoexcited states. 10The conrotatory cyclization can proceed only from the antiparallel conformation.11 Diarylethenes in crystal cannot undergo the thermal interconversion between the two conformers. The photocyclization reactivity of diarylethenes in the crystalline phase depends on the distance between the reactive carbon atoms. When the distance is longer than 4.2 # A, the photocyclization cannot take place. When the distance is shorter than 4.2 # A, efficient photocyclization reactions take place.
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