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

摘要

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(ππ~*), while the dipole-forbidden ~1(nπ~*) 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.