Graduate School of Material Science, University of Hyogo^* Graduate School of Natural Science and Technology, Kanazawa University^**
○Yoshiki Ozawa^* Takae Yonezawa^* Tomomi Yokoyama^* Minoru Mitsumi^* Koshiro Toriumi^* Hidetaka Nakai^** Yoshihito Hayashi^** Kiyoshi Isobe^**

An orgarno-dirhodium dithionite complex [(C₅Me₅)(mu-CH₂)₂Rh(O₂SSO₂)](1) shows fully reversible photochoromism in which one of four terminal oxygen atoms in the side-on type dithionite mu-O₂SSO₂ moiety is converted to a bridged one to produce geometrical isomer form mu-OSOSO₂ by visible light illumination in crystalline-state. The reverse reaction occurs thermally at room temperature. During the photoreaction, positional disorders of oxygen atoms were found which indicated geometrical isomers of photoproducts exist. We have tried to analyze time and temperature dependencies of the photo-isomerization process in crystalline state by x-ray structure analyses of the photo illuminated single crystal in order to investigate photo isomerization mechanism.
There are four configurational or conformational isomers of photoproducts (2a, 2b, 2c, 2d) can be generated from the starting complex (1) in the crystal due to combination of geometries of bridged and terminal oxygen atoms. Though all four isomers were found at the initial stage of the reaction, only one isomer (2a) was survived with almost 100% population when the reaction completed after 150 h photo irradiation at 293K. When the photo irradiation was performed at 110 K, all four isomers were generated (but not evenly distributed) and no decreases of populations were observed even after the photoreaction was completed over 90%. These phenomena indicate that conversions between the photoproduct isomers occur with the increase of the population of the most thermally stable isomer (2a) in the crystal cavity.