Single-crystalline photochromism of diarylethene mixtures

Crystal structures of 1,2-bis (2,4-dimethyl-3-thienyl)perfluorocyclopentene 1a, 1,2-bis(2,4,5-trimethyl-3-thieny)perfluorocyclopentene 2a and their mixture were determined by X-ray crystal structure analysis and their photochromic reactivities were compared. In the crystal of 1a, 17 mol% of molecules adopted a photoactive anti-parallel conformation and underwent photochromism. The crystal of 2a was not photochromic, because the dithienylethenes are packed in a photochemically inactive anti-parallel conformation. In the co-crystal of 1a and 2a, 2a adopted a photoactive anti-parallel conformation and the ratio of molecules in a photoactive conformation increased to 27 mol%. In the co-crystal, the spectrum of the closed-ring form isomers, 1b and 2b, generated by UV irradiation was broader and blue-shifted by ca. 10 nm in comparison with that of the 1a homocrystal.