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Influence of the hydrogen-bond interactions on the excited-state dynamics of a push–pull azobenzene dye: the case of Methyl Orange
EuXFEL staff, Other
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Instrument SPB/SFX
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The excited-state dynamics of the push–pull azobenzene Methyl Orange (MO) were investigated in several solvents and water/glycerol mixtures using a combination of ultrafast time-resolved fluorescence and transient absorption in both the UV-visible and the IR regions, as well as quantum chemical calculations. Optical excitation of MO in its trans form results in the population of the $S_{2} ππ*$ state and is followed by internal conversion to the $S_{1} nπ*$ state in ∼50 fs. The population of this state decays on the sub-picosecond timescale by both internal conversion to the trans ground state and isomerisation to the cis ground state. Finally, the cis form converts thermally to the trans form on a timescale ranging from less than 50 ms to several minutes. Significant differences depending on the hydrogen-bond donor strength of the solvents, quantified by the Kamlet Taft parameter α, were observed: compared to the other solvents, in highly protic solvents (α > 1), (i) the viscosity dependence of the $S_{1}$ state lifetime is less pronounced, (ii) the $S_{1}$ state lifetime is shorter by a factor of ≈1.5 for the same viscosity, (iii) the trans-to-cis photoisomerisation efficiency is smaller, and (iv) the thermal cis-to-trans isomerisation is faster by a factor of $≥10^{3}$. These differences are explained in terms of hydrogen-bond interactions between the solvent and the azo nitrogen atoms of MO, which not only change the nature of the $S_{1}$ state but also have an impact on the shape of ground- and excited-state potentials, and, thus, affect the deactivation pathways from the excited state.
Royal Society of Chemistry, 2018
Journal Information:
Physical Chemistry Chemical Physics, 20 (10), 7254-7264 (2018)
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 Record created 2019-03-01, last modified 2019-03-04

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