Can Fulvene S1Decay Be Controlled? A CASSCF Study with MMVB Dynamics

CASSCF and CASMP2 calculations show that the minimum on the fulvene S1 potential energy surface is an S0/S1 conical intersection with a 90° twisted methylene group. We have also located a distinct planar azulene-like crossing point at higher energy, where the methylene is free to twist. The fulvene intersection-which exists for all twist angles-leads to efficient, irreversible radiationless decay and explains the lack of observed S1 fluorescence. We have modeled the femtosecond excited state motion leading to ultrafast decay that would be initiated by exciting the 0-0 and higher vibrational levels using semiclassical molecular dynamics with a hybrid quantum-mechanical/force-field potential (MMVB). Our simulation suggests that, with increased vibrational energy, decay occurs in the planar crossing region before relaxation by twisting can take place, and that isomerization might only be seen by pumping the 0-0 transition in laser studies.