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.
All Science Journal Classification (ASJC) codes
- Electronic, Optical and Magnetic Materials
- Physical and Theoretical Chemistry
- Surfaces, Coatings and Films