The ultrafast dynamics of the push-pull azobenzene Disperse Red 1 following photoexcitation at λ_(pump) = 475 nm in solution in 2-fiuorotoluene 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 λ ≈ 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, τ1 = 0.08 ± 0.03 ps, τ2 = 0.99 ± 0.02 ps, and τ3 = 6.0 ± 0.1 ps. Fluorescence-time profiles were biexponential with time constants τ1' = 0.12 ± 0.06 ps and τ2' = 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 X ≈ 570 nm, and on the global kinetic analysis of the time profiles, T1 is assigned to an ultrafast transformation of the optically excited rot* state to an intermediate state, which may be the nπ* state, τ2 to the subsequent isomerisation and radiationless deactivation time to the S0 electronic ground state, and τ3 to the eventual vibrational cooling of the internally "hot" S0 molecules.
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