Ab Initio QM/MM Excited-State Molecular Dynamics Study of Coumarin 151 in Water Solution

Ab initio molecular dynamics (AIMD) Simulations are performed to investigate the excited state dynamics of coumarin 151 (C151) in the gas phase and in water solution at the CASSCF level of theory with segmented DZP basis sets, where in the latter case effective fragment potentials (EFP) are used. The dipole moment of an isolated C151 molecule increases considerably upon electronic vertical excitation, from 5.0 D (S-0 state) to 11.1 D (S-1 state). Two equilibrium structures have been identified in the S, state, i.e., a charge-transfer state with a planar amino group and a deformed structure of the six-membered ring with the carbonyl group, and a structure that is similar to the S, equilibrium structure. In AIMD simulations for an isolated C151 molecule (presumably similar to dynamics in nonpolar solvents), C151 decays from S, to S, via a crossing point of the charge-transfer state in some trajectories, while in the AIMD simulations for C151-EFP (including solvent), the S, and S, energies show an almost parallel energy variation with structural changes, and no crossing point is observed. This result is in good agreement with the experimental observation. (C) 2009 Wiley Periodicals, Inc. Int J Quantum Chem 109: 2308-2318, 2009