@Article{PhysChemChemPhys_20_7254, author = {C. Nan{\c{c}}oz and G. Licari and J.S. Beckwith and M. Soederberg and B. Dereka and A. Rosspeintner and O. Yushchenko and R. Letrun and S. Richert and B. Lang and E. Vauthey}, title = {{Influence of the hydrogen-bond interactions on the excited-state dynamics of a push-pull azobenzene dye: the case of Methyl Orange}}, journal= {Phys. Chem. Chem. Phys.}, ISSN = {1463-9076}, volume= {20}, number= {10}, pages = {7254-7264}, url = {http://xlink.rsc.org/?DOI=C7CP08390D}, doi= {10.1039/C7CP08390D}, abstract = {{The excited-state dynamics of the push{\frac{ }{ }}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 {\em trans} form results in the population of the S$_2$ $\pi\pi$* state and is followed by internal conversion to the S$_1$ n$\pi$* state in ∼50 fs. The population of this state decays on the sub-picosecond timescale by both internal conversion to the {\em trans} ground state and isomerisation to the {\em cis} ground state. Finally, the {\em cis} form converts thermally to the {\em 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 {\em $\alpha$}, were observed: compared to the other solvents, in highly protic solvents ({\em $\alpha$} > 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 {\em trans}-to-{\em cis} photoisomerisation efficiency is smaller, and (iv) the thermal {\em cis}-to-{\em 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.}}, year = {2018} }