Photoreactions of crystalline charge-transfer complexes

Abstract

Bis[1,2,5]thiadiazolotetracyanoquinodimethane 1 formed stable charge-transfer (CT) crystals with arylolefins. Upon photoirradiation of these CT crystals the [2+2]-type cycloadducts 2 were produced via single electron-transfer in the solid state. In contrast to the close similarity in solution photochemistry, the solid-state reactivities were quite different for the isomeric divinylbenzenes (DVs). Such differences were induced by incorporating differently shaped DV molecules in the similar cavities formed by S ·· N≡C interaction of 1. This interaction was suggested to take an important role in maintaining the topotaxy during the cycloaddition of the o-DV·1 CT crystal. Because of the single crystal-to-single crystal transformation as well as the predominant cycloaddition via one of the two overlaps, the crystal chirality in o-DV·1 could be transferred as the molecular chirality in the product, thus forming the cycloadduct 2 o in high optical yield (95% ee).