Stereoselective photodimerization of (E)-stilbenes in crystalline gamma-cyclodextrin inclusion complexes

Solid-state irradiation of the crystalline inclusion complex of (E)-stilben e in gamma-cyclodextrin (gamma-CD) yields a single isomer of syn-tetrapheny lcyclobutane stereoselectively in high yield. In contrast, the photodimeriz ation of stilbene in solution is very inefficient and unselective, and no p hotodimer is observed even upon prolonged irradiation of pure crystals. The monosubstituted stilbenes form a pair of photodimers stereoselectively, vi z. the syn head-to-head and syn head-to-tail isomers, in comparable yields. The photodimer yields of about 70% and the biphasic decay kinetics of the excited stilbene (as established by picosecond dime-resolved diffuse-reflec tance spectroscopy) indicate that the stilbene guests are located in at lea st two distinct sites in the gamma-CD crystal lattice, i.e., a dimerization site where excited stilbene is in close reach of another stilbene guest mo lecule and an isomerization site where excited stilbene does not find a clo se neighbor for dimerization and thus undergoes trans --> cia isomerization only.