Sequential photoisomerisation dynamics of the push–pull azobenzeneDisperse Red 1

Abstract

The ultrafast dynamics of the push–pull azobenzene Disperse Red 1 following photoexcitation at λ_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 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.