18]Annulene: a benzene-like S0/S1 conical intersection located with CASSCF + GVB

Abstract

A further parallel between the planar [18]annulene and benzene has been demonstrated: the existence of a topologically equivalent S0/S1 conical intersection. The lowest energy point on the crossing has been optimized with the MMBV and CASSCF+GVB/4-31G methods. These calculations suggest that fluorescence might be observed from the S1 state of [18]annulene, and that this would disappear at higher vibrational excess energies as a route to efficient non-radiative decay via the conical intersection became accessible (the 'channel 3' mechanism for benzene).