Photoisomerization Dynamics and Pathways of trans- and cis-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 n pi* band but also the pi pi* 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 n pi*(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-lt(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) -> S-0 relaxation to the ground state via an S-1/S-0 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-to-trans reaction following n pi* 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 S-1/S-0 conical intersections and relaxation paths for the cis-to-trans and trans-to-cis reaction. New results are also obtained with pi pi*(S-n) excitation. Thus, for trans-azobenzene, 50% of the population relaxes to an S-1 region, which is not accessible under n pi* excitation. For cis-azobenzene, up to 3096 of the excited species isomerize to trans via an S-n/S-1 intersection, resulting in a mixed cis/trans S-1 population. The isomerization kinetics of azobenzene shows no viscosity dependence, putting into question the torsion mechanism and suggesting the hula-twist isomerization mechanism.