The ultrafast dynamics of the push–pull azobenzene Disperse Red 1 following photoexcitation at λpump = 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 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 λ ≈ 570 nm, and on the global kinetic analysis of the time profiles, τ1 is assigned to an ultrafast transformation of the optically excited ππ* 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.
Fetching data from CrossRef. This may take some time to load.
Photochemical & Photobiological Sciences
- Information Point