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, S-3(pi pi* L-b), S-2(n pi*, pi pi* L-a) and S-1(pi pi* L-a, n pi*), and the ground state S-0 at the four stateaveraged complete active space self-consistent field, SA(4)-CASSCF(12,10)/6-31G* level of theory. Upon photoexcitation to the third excited state S-3(pi pi* L-b) in the Franck-Condon region, 80% sampling trajectories decay to the dark S-2(n pi*) state within an average of 5 fs via the conical intersection S-3(pi pi* L-b)/S-2(n pi*), while 20% decay to the S-2(pi pi* L-a) state within an average of 11 fs via the conical intersection S3(pi pi* L-b)/S-2(pi pi* L-a). Then, sampling trajectories via S-2(n pi*)/S-1(pi pi* L-a) continue with ultrafast decay processes to give a final distribution of quantum yields as follows: 42% stay on the dark S-1(n pi*) state, 43.3% go back to the ground S-0 state, 12% undergo a ring-opening reaction to the Z-form S-0(Z) state, and 2.7% go to the E-form S-0(E) state. The lifetimes of the excited states are estimated as follows: the S-3 state is about 12 fs on average, the S-2 state is about 80 fs, and the S-1 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.