Illuminating the photoisomerization of a modified azobenzene single crystal by femtosecond absorption spectroscopy

The mechanism of isomerization for azobenzene is a topic still to be completely elucidated. Here, we describe the ultrafast dynamics of a brominated dioxane-methoxy-azobenzene under single crystal conditions by means of femtosecond transient absorption (TA) spectroscopy. Upon excitation with 400 nm light, spectral components with decays of 0.72, 2.9, and >10 ps are observed. The fast components of the system correspond to vibrational cooling of the population on the S-1 excited state, with a decay to a local minimum in the reaction coordinate, followed by a longer evolution to a dark intermediate state prior to relaxing to the ground state, S-0. The long time constant can be used to describe the isomerization process, returning excited population to the ground state. Spectral frequencies observed at 33 and 82 cm(-1) suggest that both rotation and inversion occur in the system, with a stronger contribution coming from the latter due to a weakened N-N double bond in the excited state. This information provides insight into the structural nature of modified azobenzene systems and sets the stage for future structural studies of the molecule's isomerization dynamics.