Multi-state nonadiabatic deactivation mechanism of coumarin revealed by ab initio on-the-fly trajectory surface hopping dynamic simulation

Abstract

An on-the-fly trajectory surface hopping dynamic simulation has been performed for revealing the multi-state nonadiabatic deactivation mechanism of coumarin. The mechanism involves three adiabatic excited states, S₃(ππ*L_b), S₂(nπ*, ππ*L_a) and S₁(ππ*L_a, nπ*), and the ground state S₀ at the four state-averaged complete active space self-consistent field, SA4-CASSCF(12,10)/6-31G* level of theory. Upon photoexcitation to the third excited state S₃(ππ*L_b) in the Franck–Condon region, 80% sampling trajectories decay to the dark S₂(nπ*) state within an average of 5 fs via the conical intersection S₃(ππ*L_b)/S₂(nπ*), while 20% decay to the S₂(ππ*L_a) state within an average of 11 fs via the conical intersection S₃(ππ*L_b)/S₂(ππ*L_a). Then, sampling trajectories via S₂(nπ*)/S₁(ππ*L_a) continue with ultrafast decay processes to give a final distribution of quantum yields as follows: 42% stay on the dark S₁(nπ*) state, 43.3% go back to the ground S₀ state, 12% undergo a ring-opening reaction to the Z-form S₀(Z) state, and 2.7% go to the E-form S₀(E) state. The lifetimes of the excited states are estimated as follows: the S₃ state is about 12 fs on average, the S₂ state is about 80 fs, and the S₁ state has a fast component of about 160 fs and a slow component of 15 ps. The simulated ultrafast radiationless deactivation pathways of photoexcited coumarin immediately interpret the experimentally observed weak fluorescence emission.