Nonadiabatic Decay Dynamics of a Benzylidene Malononitrile

The photoinduced nonadiabatic decay dynamics of 2[4-(dimethylamino)benzylidene]malononitrile.(DMN) in the gas phase is investigated at the semiempirical OM2/MRCI level using surface hopping simulations. A lifetime of 1.2 ps is predicted for the S, state, in accordance with experimental observation. The dominant reaction coordinate is found to be the twisting around the C7=C8 double bond accompanied by pronounced pyramidalization at the C8 atom. Motion along this coordinate leads to the lowest-energy conical intersection (CI01 alpha). Several other S-0/S-1 conical intersections have also been located by full optimization but play no role in the dynamics. The time-resolved fluorescence spectrum of DMN is simulated by computing emission energies and oscillator strengths along the trajectories. It compares well with the experimental spectrum. The use of different active spaces in the OM2/MRCI calculations yields similar results and thus demonstrates their internal consistency.