Photochemical dynamics simulations for trans–cis photoisomerizations of azobenzene and bridged azobenzene

Surface hopping dynamics simulations based on the Zhu–Nakamura theory were performed to investigate the trans–cis photoisomerization mechanisms of azobenzene and bridged azobenzene excited to S1 state. In geometry optimization, both for the two compounds, two minimum-energy conical intersections between the ground state and the lowest excited state are located. Two conical intersections are confirmed to be decay funnels in the trans–cis photoisomerization processes in azobenzene but only one plays important parts in the photoisomerization of bridged azobenzene. Due to the smaller slope of potential energy surface in the S1 state, the lifetime of the S1 state of azobenzene in our work is much longer than that of bridged azobenzene. We show that the torsion around the central NN bond is the preferred reaction mechanism in the isomerization of two molecules. Rotation around the central NN bond and twisting of phenyl rings around their NC bonds allows the molecule to move to a minimum-energy conical intersection, after which surface hopping from S1 to S0 occurs. In the ground state, further rotation occurs around the NN bond and two NC bonds until the azo moiety and phenyl rings complete their isomerization. The additional CH2CH2 bridge in bridged azobenzene starts to rotate toward the cis form after the azo moiety and two phenyl rings complete their reorientation. The bridge structure in bridged azobenzene makes the rotation of the azo moiety faster and the torsion of two phenyl rings slower.