Energy Transfer, Electron Transfer, and Addition Reactions of Triplet State of 1,3-Dihydroimidazole-2-thiones Investigated by Laser Flash Photolysis

Abstract

Abstract The transient absorption bands observed at ca. 400 and 550 nm for 1,3-dihydro-2H-imidazole-2-thione (IT), 1,3-dihydro-1-methyl-2H-imidazole-2-thione (MIT), and 1,3-dihydro-2H-benzimidazole-2-thione (BIT) have been assigned to their triplet states (3IT* ’s) by the quenching and sensitizing experiments using laser flash photolysis technique. Short intrinsic triplet lifetimes and relatively high triplet energies have been evaluated as characteristics of 3IT* ’s. For electron acceptors, photoinduced electron-transfer reaction occurs via 3IT* ’s in the diffusion controlled limit. The nucleophilic character of 3IT* ’s was confirmed by changing the substituents of alkenes on the addition reactions. MO calculations indicate that the lowest electronic configurations of both 3IT* and 3MIT* have 3(n,π*) character, while 3BIT* has 3(π,π*) character. The addition reactivity of 3BIT* is slightly higher than those of 3IT* and 3MIT*, which is opposite to the general tendency.