Photochromic reactions of diarylethenes in single crystals with intermolecular O-H•••N hydrogen-bonding networks

The crystal structures and photochromic performance of a single crystal of a diarylethene derivative possessing carboxyl groups, 1,2-bis(5-car-boxyl-2-methyl-3-thienyl)perfluorocy- clopentene (1a), and cocrystals of 1a with 4,4 '-, 2,4 '-, and 2,2 '-bipyridines were examined. In crystal la, a discrete cyclic structure was observed, in which four 1a molecules are linked through hydrogen bonds between the carboxyl groups. In the homocrystal, photoreactive and photoinactive conformers of la exist in the ratio of 1:1. In the cocrystals of 1a with bipyridines, O-(HN)-N-...-type hydrogen bonds between 1a and pyridyl groups were formed, and all 1a molecules are fixed in a photoreactive conformation. Both the homocrystal 1a and the cocrystals showed photochromic performances, and color variation from bluish-violet to cyan was observed, depending on the conformation of the packed diarylethene molecules.