Ultrafast cis-to-trans photoisomerization of a bridged azobenzene through nπ∗ excitation: Rotational pathway is not restricted

A bridged azobenzene was recently found by Siewertsen et al. to exhibit highly efficient photoisomerization through nπ∗ absorption. In the work of the present Letter, cis-to-trans photoisomerization of this molecule in the gas phase was examined in detail via density-functional-based dynamical simulations. Our results indicate that rotation around the central NN bond is the dominant mechanism when this reaction proceeds via nπ∗ excitation, and the bridging feature was found not to hinder the rotation. Nonadiabatic de-excitation occurred at an avoided crossing between the ground state and lowest singlet excited state near the midpoint of the rotational pathway.