Excited-state-dependent chiral pathways in the photoisomerization of cis-4,4'-azopyridine

As azobenzene derivatives, azopyridine molecules offer enhanced potential for functional material design. On-the-fly surface-hopping simulations were performed at the SA3-CASSCF(6,6)/6-31G level to systematically understand the excited-state-dependent photochemical reaction mechanisms and chirality preference of the P- and M-helical cis isomers of 4,4'-azopyridine upon S1 and S2 excitations. The trans-isomer quantum yields and lifetimes of the two cis isomers are found to be numerically comparable with those of cis-azobenzene under both excitation conditions. Surface-hopping events and key geometric parameters of simulated trajectories indicate that both S1 and S2 excitations drive photoisomerization along the central N-N torsional path, accompanied by the asymmetric N-N-C bending vibration mode. After being excited to the S1 state, each cis conformer adopts a pure chiral decay pathway, maintaining its initial helicity throughout the entire relaxation process and undergoing unidirectional torsion around the N-N bond for cis-trans photoisomerization. In contrast, S2 excitation utilizes both a pure chiral pathway and a helicity-mixed channel (92% vs 8%). This systematic understanding of the excited-state decay dynamics of 4,4'-azopyridine in the gas phase establishes a solid foundation for future investigations into related compounds under chiral constraints and offers valuable insights for the rational design of photoswitches and molecular motors.