Probing the π → π* photoisomerization mechanism of trans-azobenzene by multi-state ab initio on-the-fly trajectory dynamics simulations

Global nonadiabatic switching on-the-fly trajectory surface hopping simulations at the 5SA-CASSCF(6,6)/6-31G quantum level have been employed to probe the photoisomerization mechanism of trans-azobenzene upon pi pi* excitation within four coupled singlet low-lying electronic states (S-0, S-1, S-2, and S-3). We have performed 586 sampling trajectories (331 starting from S-2 and 255 from S-3), and we found about half of the sampling trajectories staying on S-1 or S-2 states as resonances and the other half of them ending on the ground S-0 state as active trajectories. The present simulation has demonstrated that there are six distinct photoisomerization pathways which can be summarized as three categories; one is the newly opened inversion-inversion nonreactive isomerization pathway accounting for 40% (34%) of active trajectories at a time constant of 80 fs (320 fs), the other is the inversion-torsion reactive and nonreactive isomerization pathways accounting for 40% (20%) of active trajectories at a time constant of 880 fs (1700 fs), and the third is the torsion-torsion reactive and nonreactive isomerization pathways accounting for 20% (46%) of active trajectories at a time constant of 780 fs (1000 fs) upon S-2 (S-3) pi pi* excitation. The simulated total reactive quantum yield for trans-azobenzene photoisomerization upon S-2 (S-3) pi pi* excitation is about 0.11 (0.13) which is in good agreement with recent experimental results of 0.09-0.20. Furthermore, the newly opened inversion-inversion nonreactive isomerization pathway from the present simulation agrees well with cascade experimental measurements of the S-n -> S-1 -> S-0 relaxation mechanism in both branching ratio and time constant.