Femtosecond fluorescence dynamics of trans-azobenzene in hexane have been investigated on excitation to the S1(n,π*) state at 432 nm using the up-conversion technique. Two transient components were observed to represent the fast and the slow S1 fluorescence dynamics in the wavelength range of 550-732 nm. Based on the results obtained from recent ab initio calculations (Ishikawa, T.; Noro, T.; Shoda, T., J. Chem. Phys. 2001, 115, 7503), a dynamical picture is given in the following. Upon initial excitation to the S1 state, the excited molecule is moving away from the first detection window within the observed 200-300 fs. The structural relaxation from the Franck-Condon region may be responsible for the observed fast S1 dynamics with the driving force being the CNNC twisting motion along the rotational coordinate. For the rest of the motion on the S 1 global potential surface, the excited molecule may search for the S0/S1 conical intersection for an efficient internal conversion to the ground state. The nuclear motions for the observed slow S 1 dynamics not only involve the CNNC torsional coordinate but also the other degrees of freedom such as the CNN bending coordinate on the multidimensional S1 potential energy surface. The whole electronic relaxation process occurs within the observed 1-2 ps. The slow S1 dynamics were found to vary with the fluorescence wavelengths due to the influence of the solvent-induced vibrational relaxation in the S1 state; the vibrational relaxation should occur on a time scale comparable to the time scale of the electronic relaxation (S1→S0 internal conversion).
- Conical intersection