Crystal structure-reactivity correlation in single-crystalline photochromism of 1,2-bis(2-methyl-5-phenyl-3-thienyl)perfluorocyclopentene

Single-crystalline photochromism of 1,2-bis(2-methyl-5-phenyl-3-thienyl)per fluorocyclopentene la was followed in situ by X-ray crystallographic analys is. The crystal of la has two molecules with different conformations in the crystallographic asymmetric unit. The X-ray analysis of the photoirradiate d crystal showed that the photocyclization reaction of the molecule with th e shorter distance between the reactive carbons prevails over the reaction of the other molecule. The mechanism has been discussed based on the calcul ation of the electronic transitions of the two molecules in the different c onformations. The photocyclization quantum yield was determined to be 1 (10 0%) in the crystal.