Nonradiative deactivation in benzylidene malononitriles

Electronic structure calculations are used to examine the nuclear motions responsible for ultrafast internal conversion in the benzylidene malononitriles DMN (4-N,N-dimethylaminobenzylidenemalononitrile) and JDMN (julolidinemalononitrile). Gas-phase B3LYP and RI-CC2 calculations using tripleζ valence polarized basis sets reproduce the structural features measured in the crystalline state and the solution-phase dipole moments of these molecules in their ground states with reasonable accuracy. Most properties of the vertical S₀ → S₁ transition, which is well separated from other transitions, are also reasonably reproduced by both types of calculation. The large change in dipole moment (8-9 D) upon excitation is grossly underestimated by TDDFT calculations, despite the fact that such calculations predict the transition energies to within experimental uncertainties. Exploration of the S₁ potential energy surface of DMN using DFT, RI-CC2, and CASSCF methods indicates that the internal conversion pathway is double-bond isomerization, not the TICT process often assumed. Preliminary classical molecular dynamics simulations of DMN in acetonitrile using the ab initio S₁ surface support this assignment.