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.