Ab initio study on the substituent effect of the S1/S0 conical intersection of styrene derivatives

Ab initio complete active space self-consistent field calculations have been performed to determine the conical intersections (CIXs) of the styrene derivatives where the radiationless relaxation from S-1 into S-0 take place. All the S-1/S-0-CIXs of styrene derivatives studied here are characterized as a crossing region between zwitterionic and covalent diradical states, which has already been found in styrene. The zwitterionic state is formed by electron transfer from the methylene to the benzylidene. The geometrical features of the styrene part are also very similar to those of unsubstituted styrene. However, the zwitterionic state of the styrene derivatives with electron withdrawing group has a much larger dipole moment than those of styrene and its derivatives with electron donating group. This is ascribed to a stabilization of the electron rich benzylidene part by the electron withdrawing group. It has been also examined how the truncation of the active space affects the optimized geometry of the S-1/S-0-CIX for each styrene derivatives. (C) 2003 Elsevier Science B.V. All rights reserved.