Excited-state structure and dynamics of cis- and trans-Azobenzene from resonance Raman intensity analysis

Resonance Raman intensity analysis was used to investigate the initial excited-state nuclear dynamics of cis-and trans-azobenzene following S ₁ (nπ*) excitation, and fluorescence quantum yield measurements were used to estimate the excited-state lifetimes. trans-Azobenzene exhibits the strongest Raman intensities in its skeletal stretching and bending modes, while torsional motions dominate the nuclear relaxation of cis-azobenzene as indicated by intense Raman lines at 275, 542, 594, and 778 cm ^-1. The very weak fluorescence quantum yield for cis-azobenzene is consistent with its ∼100 fs electronic lifetime while trans-azobenzene, with a fluorescence quantum yield of 1.1 × 10 ^-5, has an estimated Si lifetime of ∼3 ps. The absorption and Raman cross-sections of both isomers were modeled to produce a harmonic displaced excited-state potential energy surface model revealing the initial nuclear motions on the reactive surface, as well as values for the homogeneous and inhomogeneous linewidths. For cis-azobenzene, this modeling predicts slopes on the S ₁ potential energy surface that when extrapolated to the position of the harmonic minimum give excited-state changes of ∼6-20° in the CNNC torsion angle and a ≤3° change in the CNN bending angle. The relatively large excited-state displacements along these torsional degrees of freedom provide the driving force for ultrafast isomerization. In contrast, the excited-state geometry changes of trans-azobenzene are primarily focused on the CNN bend and CN and NN stretches. These results support the idea that cis-azobenzene isomerizes rapidly via rotation about the NN bond, while isomerization proceeds via inversion for trans-azobenzene.