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Photomechanical Behavior of Photochromic Diarylethene Crystals

Seiya Kobatake and Daichi Kitagawa

Department of Applied Chemistry, Graduate School of Engineering, Osaka City University, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka, 558-8585, Japan

1.1 Introduction

Photochromism is defined as a reversible transformation reaction between two isomers having different absorption spectra, which is induced in one or both directions by photoirradiation [1]. Among many photochromic compounds, diarylethenes with heteroaryl groups including thiophene, furan, thiazole, and oxazole rings have excellent properties, such as thermal stability of both isomers, fatigue resistance, high coloration quantum yield, rapid response, and high reactivity even in the crystalline phase [2]. Such diarylethenes have potential applications in ultraviolet (UV) sensors, photoswitches, displays, optical waveguides, optical memories, holographic recording media, nonlinear optics, and actuators. Upon UV light irradiation, diarylethenes exhibit color changes because of a molecular structure change from the open-ring isomer form to the closed-ring isomer form. The colors remain stable in the dark at room temperature. The colored isomers revert to their original colorless isomer forms by irradiation with visible light. The reversible color changes can be repeated many times.

Photochromic compounds that undergo a photochromic reaction in the crystalline phase are known for paracyclophanes, triarylimidazole dimer, diphenylmaleronitrile, aziridines, 2-(2,4-dinitrobenzyl)pyridine, N-salicylideneanilines, triazenes, and diarylethenes. The large change in geometrical structures prohibits photochromic reactions in the crystalline phase. Even in the crystalline phase, diarylethenes can undergo thermally irreversible and fatigue-resistant photochromic reactions when diarylethene molecules are fixed in the antiparallel conformation and the distance between the reactive carbons is less than 4.2?Å [3]. The photocyclization reaction results in a color change in the crystals from colorless to yellow, red, blue, or green, as shown in Figure 1.1. The color of the crystals can be maintained if they are stored in the dark. The colored crystals return to the initial colorless ones by irradiation with visible light. In the crystalline phase, the photocyclization quantum yield is close to unity and the coloration/decoloration cycles can be repeated more than 104 times [2]. There are many studies describing the photochromism of diarylethene crystals, including investigations that report multicolor photochromism [4], dichroism under polarized light [5], fluorescence [6], three-dimensional optical memory [7], diastereoselective cyclization [8], selective photochromic reaction under polarized light [9], theoretical studies [10], Raman spectroscopic studies [11], nanostructures [12], supramolecular architectures [13], nanocrystals [14], polymorphism [9a, 15], phase transitions [15b, c], surface wettability [15a, 16], and molecular motion observed by X-ray crystallography [17]. The research on molecular motion observed by X-ray crystallography demonstrated that photochromic reactions of diarylethene molecules in the crystals are accompanied by a change in the unit cell dimensions because of a decrease in the molecular volume resulting from photoisomerization of the open-ring isomer to yield the closed-ring isomer as shown in Figure 1.2 [2b]. The height of the triangle shape increases from 0.49 to 0.56?nm and the base width decreases from 1.01 to 0.90?nm. The side view indicates that the thickness of the molecule is reduced. The change in the geometrical structure of diarylethene molecules plays an important role in photomechanical phenomena.

Figure 1.1 Typical examples of diarylethenes that underwent photochromism in the single crystalline phase. Maximum absorption wavelength of the photogenerated closed-ring isomers in crystals is shown in parentheses. When exposed to UV radiation crystals 1-3 turned to yellow, crystals 4-13 to red, crystals 14-16 to blue, and crystals 19-21 to green.

Figure 1.2 (a) Top and (b) side views of the geometrical structures of the open- and closed-ring isomers of 1,2-bis(2,5-dimethyl-3-thienyl)perfluorocyclopentene (7) in crystals. The two isomers were isolated and independently recrystallized.

Source: Irie et al. 2014 [2b]. Adapted with permission from American Chemical Society.

In 2001, the crystal surface of diarylethene 18 was found to exhibit a photoreversible surface morphology change [18]. The flat crystal surface formed a step with a height of approximately 1?nm upon UV light irradiation. The step was erased by irradiation with visible light. The crystal thickness decreased as a result of the photochromic isomerization of the open-ring isomer to yield the closed-ring isomer. Another surface, which is perpendicular to the surface that formed the step, exhibited a photoreversible valley formation. These reversible surface morphology changes are ascribed to photoinduced contraction in the direction of the long axis of each diarylethene molecule regularly packed within the single crystal. These results indicate that the molecular-scale structural change of individual molecules may induce the macroscopic mechanical movement of materials.

In this chapter, recent developments in the light-driven actuators based on photochromic diarylethene crystals are described.

1.2 Crystal Deformation Exhibiting Expansion/Contraction upon Photoirradiation

A first example of photoreversible macroscopic crystal deformation was a thin microcrystal of 1,2-bis(2-ethyl-5-phenyl-3-thienyl)perfluorocyclopentene (16) [19]. The microcrystals were prepared by sublimation on a thin glass plate under atmospheric pressure at 144?°C. A photograph of microcrystals composed of 16 is shown in Figure 1.3. The crystals have several tens of micrometers square in size with thickness of a few hundred nanometers. Upon irradiation with 365?nm light, the crystal turned blue and the blue-colored crystal returned to the initial colorless crystal form after irradiation with visible light. The conversion ratio in the crystal from the open- to the closed-ring isomers was followed by an infrared (IR) absorption microspectroscopy. IR absorption spectra for thin single crystal 16 were taken under polarized IR light to avoid an overlap of peaks. Figure 1.4 shows the IR spectral changes of crystal 16 upon irradiation with 365?nm light. The open-ring isomer in the crystal has two characteristic bands at 1260 and 1350?cm-1. The band at 1350?cm-1 was split into two peaks upon UV light irradiation, whereas the band at 1260?cm-1 monotonously decreased. The closed-ring isomer has no absorption around 1260?cm-1. The conversion ratio from the open- to the closed-ring isomers can be determined from the decrease of the band at 1260?cm-1. Almost 70% conversion was observed at the photostationary state under irradiation with 365?nm light.

Figure 1.3 (a) Digital microscopic and (b) atomic force microscope (AFM) images for thin microcrystals of 16.

Figure 1.4 (a) IR spectral and (b) conversion change for thin microcrystal of 16 upon irradiation with 365?nm light. IR spectra were detected under polarized IR light. The polarization direction was set to the short axis in the molecule.

Source: Kobatake et al. 2007 [19]. Adapted with permission of Springer Nature.

In general, crystals of different molecules have different unit cell parameters, space group, and packing in the lattice. Figure 1.5 shows the photographs of crystals for diarylethenes 16, 11, and 17 [19,20]. A single crystal of 16 with a thickness of 570?nm was reversibly changed from a square-like shape with corner angles of 88° and 92° to a lozenge-like shape with corner angles of 82° and 98° upon alternating irradiation with UV and visible light. The photochromic reaction took place homogeneously in crystals because of their thin crystallized forms with a thickness of several hundred nanometers. Crystals of 11 and 17 have unit cell parameters similar to 16. These diarylethenes have the same space group, i.e. Pbcn, as shown in Table 1.1. The molecular packings of 11 and 17 are also similar to that of 16, as shown in Figure 1.6. Crystal 11 changed its color from colorless to red and its corner angles from 90° and 90° to 86° and 94°, and hence its shape from square to lozenge. Crystal 17 changed its color from colorless to blue and the corner angles from 83° and 97° to 81° and 99° upon irradiation with UV light. Crystals of 16, 11, and 17 exhibited a similar crystal shape deformation. These...