Photochromism of diarylethenes in nanolayers of a single crystal

1,2-Bis(2-methyl-5-pentafluorophenyl-3-thienyl)perfluorocyclopentene (1a) and 1,2-bis(2-ethyl-5-phenyl-3-thienyl)-perfluorocyclopentene (2a) were found to form a 1:1 stoichiometric co-crystal. The crystal is composed of alternate nanolayers of 1a and 2a molecular sheets. pi-pi Stacking of 1a and 2a molecules by aryl-perfluoroaryl interactions was not discerned. One-component 1a and 2a crystals and the co-crystal showed a reversible colour change from colourless to blue upon irradiation with ultraviolet and visible light. Although both 1a and 2a molecules underwent efficient photochromic reactions in the one-component crystals upon photoirradiation, only 2a underwent the reaction up to a conversion of 20% in the co-crystal, as confirmed by HPLC and X-ray crystallography. This indicates that excited energy transfer occurs from 1a to 2a in the crystal.