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

• Published in: Photochemical & photobiological sciences 2012-07, Vol.11 (7), p.121-1219
• Main Authors: Bahrenburg, Julia, Röttger, Katharina, Siewertsen, Ron, Renth, Falk, Temps, Friedrich
• Format: Article
• Language: English
• Subjects: Biochemistry; Biomaterials; Chemistry; Physical Chemistry; Plant Sciences
• ISSN: 1474-905X; 1474-9092
• DOI: 10.1039/c2pp05400k

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, τ 1 = 0.08 ± 0.03 ps, τ 2 = 0.99 ± 0.02 ps, and τ 3 = 6.0 ± 0.1 ps. Fluorescence-time profiles were biexponential with time constants τ 1 ′ = 0.12 ± 0.06 ps and τ 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 λ 570 nm, and on the global kinetic analysis of the time profiles, τ 1 is assigned to an ultrafast transformation of the optically excited ππ* state to an intermediate state, which may be the nπ* state, τ 2 to the subsequent isomerisation and radiationless deactivation time to the S 0 electronic ground state, and τ 3 to the eventual vibrational cooling of the internally "hot" S 0 molecules. The ultrafast photoisomerisation and radiationless electronic deactivation of the push-pull azobenzene Disperse Red 1 excited at λ = 475 nm proceeds via a sequential mechanism in Δ t 1.0 ps.