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.