Reversible diastereoselective photocyclization of a diarylethene in a single-crystalline phase

Optically active photochromic (S)- and (R)-1-(2-methyl-5-phenyl-3-thienyl)-2-[2-(3-methyl-1-penten-1-yl)-5-phenyl-3- thienyl]-3,3,4,4,5,5-hexafluorocyclopentenes ((S)-1a and (R)-1a) were synthesized. X-ray crystallographic measurement showed that (S)-1a and (R)-1a single crystals adopted orthorhombic chiral space group P2₁2₁2₁ and the open-ring isomers were packed in the crystals in only one conformer of the possible two atropisomers. Upon irradiation with 366 nm light (S)-1a and (R)-1a underwent reversible photocyclization reactions both in solution and in the single-crystalline state, and in the crystalline phase almost only one closed-ring diastereomer was produced, while the diastereoselection was not observed in solution. The dominant diastereomer produced from (S)-1a was determined to be (S,R,R)-1b by X-ray crystallographic analysis.