Wavelength-dependent photoisomerization of trans-4,4'-azopyridine: Nonadiabatic dynamics simulation

The trans-cis photoisomerization processes of 4,4'-azopyridine upon S₁ and S₂ excitations have been investigated by nonadiabatic dynamics simulations based on multi-reference CASSCF calculations. 119 sampling trajectories were simulated starting from the trans form excited to the S₁ (S₂) state and the cis-isomer quantum yield is evaluated to be (3 ± 2)% ((18 ± 4)%), which is qualitatively in agreement with the recent experimental results in ethanol. We found that rotation around the central N-N bond accompanied by the N-N-C symmetrical bending vibrations is the main mechanism in photoisomerization of the target molecule excited to the S₁ and S₂ states. Upon S₁ excitation, S₁-S₀ transition occurs earlier along the C-N-N-C torsional coordinate, leading to a low cis-isomer quantum yield. Upon S₂ excitation, half of the simulated trajectories are trapped in a potential well on the S₂ state, from which the twisted conical intersections are more easily reached in the internal conversion, resulting in a higher cis-isomer quantum yield.