A time-dependent density-functional theory and complete active space self-consistent field method study of vibronic absorption and emission spectra of coumarin

Time-dependent density-functional theory (TD-DFT) and complete active space multiconfiguration self-consistent field (CASSCF) calculations have been used to determine equilibrium structures and vibrational frequencies of the ground state and several singlet low-lying excited states of coumarin. Vertical and adiabatic transition energies of S-1, S-2, and S-3 have been estimated by TD-B3LYP and CASSCF/PT2. Calculations predict that the dipole-allowed S-1 and S-3 states have a character of (1)(pi pi*), while the dipole-forbidden (1)(n pi*) state is responsible for S-2. The vibronic absorption and emission spectra of coumarin have been simulated by TD-B3LYP and CASSCF calculations within the Franck-Condon approximation, respectively. The simulated vibronic spectra show good agreement with the experimental observations available, which allow us to reasonably interpret vibronic features in the S-0 -> S-1 and S-0 -> S-3 absorption and the S-0 <- S-1 emission spectra. Based on the calculated results, activity, intensity, and density of the vibronic transitions and their contribution to the experimental spectrum profile have been discussed. (C) 2014 AIP Publishing LLC.