Sequential photoisomerisation dynamics of the push-pull azobenzene Disperse Red 1

The ultrafast dynamics of the push-pull azobenzene Disperse Red 1 following photoexcitation at lambda(pump) = 475 nm in solution in 2-fluorotoluene have been probed by broadband transient absorption spectroscopy and fluorescence up-conversion spectroscopy. The measured two-dimensional spectro-temporal absorption map features a remarkable fast excited-state absorption (ESA) band at lambda approximate to 570 nm appearing directly with the excitation laser pulse and showing a sub-100 fs lifetime with a rapid spectral blue-shift. Moreover, its ultrafast decay is paralleled by rising distinctive ESA at other wavelengths. Global fits to the absorption-time profiles using a consecutive kinetic model yielded three time constants, tau(1) = 0.08 +/- 0.03 ps, tau(2) = 0.99 +/- 0.02 ps, and tau(3) = 6.0 +/- 0.1 ps. Fluorescence-time profiles were biexponential with time constants tau(1)' = 0.12 +/- 0.06 ps and tau(2)' = 0.70 +/- 0.10 ps, close to the absorption results. Based on the temporal evolution of the transient spectra, especially the fast excited-state absorption band at lambda approximate to 570 nm, and on the global kinetic analysis of the time profiles, tau(1) is assigned to an ultrafast transformation of the optically excited pi pi* state to an intermediate state, which may be the n pi* state, tau(2) to the subsequent isomerisation and radiationless deactivation time to the S-0 electronic ground state, and tau(3) to the eventual vibrational cooling of the internally hot S-0 molecules.