Excited state intramolecular proton transfer reactions (ESIPT) of 4'-N, N-diethylamino-3-hydroxyflavone (DEAHF) in ionic liquids were studied by steady-state and time-resolved fluorescence measurements at different excitation wavelengths. Steady-state measurements show the relative yield of the tautomeric form to the normal form of DEAHF decreases as excitation wavelength is increased from 380 nm to 450 nm. The decrease in yield is significant in ionic liquids that have cations with long alkyl chains. The extent of the decrease correlated with the number of carbon atoms in the alkyl chains. Time-resolved fluorescence measurements using optical Kerr gate spectroscopy show that ESIPT rate has a remarkable excitation wavelength dependence. There was a large difference between the spectra at a 200 ps delay from different excitation wavelengths in each ionic liquid. The difference was pronounced in ionic liquids having a long alkyl chain. The equilibrium constant in the electronic excited state obtained at a 200 ps delay and the average reaction rate were also correlated with the alkyl chain length. The excitation wavelength dependence of the solvation dynamics of the normal form and related compounds of DEAHF indicated that the initial solvation state is strongly dependent on the excitation wavelength. RISM-SCF-SEDD calculations on the electronic state of 4'-N, N-dimethylamino-3-hydroxyflavone also indicated that the reaction potential is strongly dependent on the solvation state of normal form. Considering these facts, the excitation wavelength dependence of ESIPT was ascribed to the state selective excitation due to the difference of the solvation, and the number of alkyl chain carbon atoms was found to be a good indicator of the effect of inhomogeneity for this reaction.

Excitation wavelength dependence of the excited state intramolecular proton transfer reaction in ionic liquids

248th National Meeting (2014)

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