Azobenzene photoisomerization dynamics: Revealing the key degrees of freedom and the long timescale of the trans-to-cis process

The photoisomerization reaction of azobenzene in both directions have been investigated with a density functional theory based approach using the surface hopping procedure with forced jumps. While the cis-to-trans isomerization was found to be a stepwise reaction along the CNNC dihedral angle, the trans-to-cis isomerization was observed to be one smooth step. The further unbiased full-dimensional analysis of the cis-to-trans isomerization revealed that, while the CNNC dihedral angle is an important degree of freedom for describing the reaction, it is insufficient for describing all of the dynamics. For a fuller picture two coupled modes must be considered. The trans-to-cis isomerization on the other hand was found to be well described along only the CNNC dihedral angle, and its longer timescale could be ascribed to the slow oscillations of this degree of freedom rather than a potential energy barrier in the excited state. The timescales observed in this study was found to be in good agreement with experiment, and thus this work provides insights into the interpretation of experimental observations. Finally, investigations of the structures of the CIs for both reactive and non-reactive trajectories showed a heavy functional dependency.