Photoisomerization Dynamics and Pathways of <i>trans</i>- and <i>cis</i>-Azobenzene in Solution from Broadband Femtosecond Spectroscopies and Calculations

The photoisomerization of azobenzene in solution was studied experimentally and by calculations. trans-to-cis and cis-to-trans dynamics are described through broadband transient absorption, fluorescence, and stimulated Raman spectroscopy. Transient absorption was extended to cover not only the nπ* band but also the ππ* band in the ultraviolet. Isomerization yields are used for a quantitative comparison of trans and cis transient spectra under different excitation. For the trans-to-cis path upon nπ*(S(1)) excitation, the evolution develops with 0.3, 3, and 16 ps. The first two times reflect population relaxation to a local minimum S(1t )(L) and subsequent transition to a dark intermediate S(1t)(D) over an 8 kJ/mol barrier. The existence of stationary points S(1t)(L) and S(1t)(D) is confirmed by quantum-chemical calculations. The third time corresponds to S(1t) (D) → S0 relaxation to the ground state via an S1/S0 conical intersection over a 12 kJ/mol barrier. Thus, the 16 ps time constant is attributed to the isomerization process and not to vibrational cooling, contrary to the current view and in line with the previous interpretation by Lednev et al. (J. Phys. Chem. 1996, 100, 13338). The decay of the long-lived intermediate S(1t)(D) is consistent with the hula twist rather than with the inversion mechanism. For the cis-totrans reaction following nπ* excitation, signal decay is strongly nonexponential, with 0.1 and 1 ps. The latter (1 ps) is much shorter than the 16 ps decay of the trans isomer, implying different S1/S0 conical intersections and relaxation paths for the cis-totrans and trans-to-cis reaction. New results are also obtained with ππ*(Sn) excitation. Thus, for trans-azobenzene, 50% of the population relaxes to an S1 region, which is not accessible under nπ* excitation. For cis-azobenzene, up to 30% of the excited species isomerize to trans via an Sn/S1 intersection, resulting in a mixed cis/trans S1 population. The isomerization kinetics of azobenzene shows no viscosity dependence, putting into question the torsion mechanism and suggesting the hula-twist isomerization mechanism.