Probing Highly Efficient Photoisomerization of a Bridged Azobenzene by a Combination of CASPT2//CASSCF Calculation with Semiclassical Dynamics Simulation

Mechanism of phototriggered isomerization of azobenzene and its derivatives is of broad interest. In this paper, the S-0 and S-1 potential energy surfaces of the ethylene-bridged azobenzene (1) that was recently reported to have highly efficient photoisomerization were determined by ab initio electronic structure calculations at different levels and further investigated by a semiclassical dynamics simulation. Unlike azobenzene, the cis isomer of 1 was found to be more stable than the trans isomer, consistent with the experimental observation. The thermal isomerization between cis and trans isomers proceeds via an inversion mechanism with a high barrier. Interestingly, only one minimum-energy conical intersection was determined between the S-0 and S-1 states (CI) for both cis -> trans and trans -> cis photoisomerization processes and confirmed to act as the S-1 -> S-0 decay funnel. The S-1 state lifetime is similar to 30 fs for the trans isomer, while that for the cis isomer is much longer, due to a redistribution of the initial excitation energies. The S-1 relaxation dynamics investigated here provides a good account for the higher efficiency observed experimentally for the trans -> cis photoisomerization than the reverse process. Once the system decays to the S-0 state via CI, formation of the trans product occurs as the downhill motion on the S-0 surface, while formation of the cis isomer needs to overcome small barriers on the pathways of the azo-moiety isomerization and rotation of the phenyl ring. These features support the larger experimental quantum yield for the cis -> trans photoisomerization than the trans -> cis process.