Influence of the hydrogen-bond interactions on the excited-state dynamics of a push-pull azobenzene dye: the case of Methyl OrangeElectronic supplementary information (ESI) available: Quantum chemical calculations, time-resolved fluorescence data, transient electronic absorption spectra, stationary and transient vibrational absorption spectra, analysis of the continuous illumination data. See DOI: 10.1039/c7cp08390d

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. H-bonding with the solvent affects significantly the photoisomerisation of Methyl Orange.