Enhancement of intersystem crossing by substitution: assignment of a long-lived triplet state by magnetic field effects in a non-viscous medium

The photoinduced electron transfer reactions in four similar donor-acceptor systems, N-ethyl carbazole (ECZ)-1,4-dicyanobenzene (DCB), 1,4,5,8,9-pentamethyl carbazole (PMC)-DCB, ECZ-1,2,4,5-tetracyanobenzene (TCNB) and PMC-TCNB have been studied to establish the effect of substitutions on the mechanistic pathways of electron transfer (ET) reactions. The initial spin states of the transients have been characterized by laser flash photolysis and magnetic field effect studies. The presence of more -CN groups in TCNB compared to DCB alters the ET pathways, by enhancing intersystem crossing. The temperature and free energy dependencies of these systems have been utilized to evaluate the process from the theoretical viewpoint. (C) 1997 Elsevier Science B.V.