Excited-state-dependent chiral pathways in the photoisomerization of <i>cis</i> -4,4′-azopyridine

Abstract

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.