Photo-induced isomerization of ethylene-bridged azobenzene explored by ab initio based non-adiabatic dynamics simulation: A comparative investigation of the isomerization in the gas and solution phases

Azobenzene is one of the most widely used photoactive units and recently an ethylene-bridged azobenzene (BAB) was reported to have greatly enhanced conversion efficiency, quantum yield, and other favorable properties. As the first step towards exploring its photo-switchable character in real systems, we report here a systematic study on the photoisomerization dynamics between trans (E) and cis (Z) isomers in the gas phase and the CH3OH solution, using ab initio based surface hopping and molecular dynamics, which is the first report of dynamics simulation to reveal the environmental effects on BAB photoreactions. Results show that while the relatively faster S-1 relaxation of the photo-induced E -> Z process is only mildly affected by the solvent effect, the relatively slower S-1 relaxation of the reverse reaction becomes even slower in the solution compared to the gas phase. The subsequent S-0 dynamics from the conical intersection between S-1 and S-0 (CI_E) to Z is accelerated in solution compared to the gas phase because of avoided re-crossing to the S-1 state, while the S-0 dynamics from the conical intersection between S-1 and S-0 (CI_Z) to E are basically the same in both phases. Overall, the solvent effect was found to enhance the back-and-forth photo-switch efficiency between the Z and E isomers compared to the gas phase, while the quantum yields are reduced. But the solution yields of both the forward and backward photoreactions are still around 0.4. Therefore, BAB may have good photo-responsive properties if used as a photoactive unit in real systems. These results will facilitate future experimental and theoretical studies in this area to help design new azobenzene derivatives as photoactive units in biological processes, nanoscale devices, and photo-responsive materials. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4798642]