Nonadiabatic Ensemble Simulations of cis-Stilbene and cis-Azobenzene Photoisomerization

Structurally, stilbene and azobenzene molecules exist in closed and open cis and trans forms, which are able to transform into each other under the influence of light (photoisomerization). To accurately simulate the photoisomerization processes, one must go beyond ground-state (Born Oppenheimer) calculations and include nonadiabatic coupling between the electronic and vibrational states. We have successfully implemented nonadiabatic couplings and a surface-hopping algorithm within a density functional theory approach that utilizes local orbitals. We demonstrate the effectiveness of our approach by performing molecular dynamics simulations of the cis trans photoisomerization in both azobenzene and stilbene upon excitation into the Si state. By generating an ensemble of trajectories, we can gather characteristic transformation times and quantum yields that we will discuss and compare with ultrafast spectroscopic experiments.