Femtosecond fluorescence dynamics of trans-azobenzene in hexane on excitation to the S₁(n,π*) state

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).