Picosecond time-resolved fluorescence studies are consistent with reversible excited-state intramolecular proton transfer in 4′-(dialkylaminoe)-3-hydroxyflavones

Picosecond time-resolved fluorescence spectroscopy has been applied to the studies of excited-state intramolecular proton transfer (ESIPT) dynamics in two 4'-(dialkylamino)-3-hydroxyflavone derivatives (unsubstituted and substituted at the 6-position) in ethyl acetate and dichloromethane. In all the studied cases, the fluorescence decay kinetics of both short-wavelength normal (N*) and long-wavelength tautomer (T*) bands can be characterized by the same two lifetime components, which are constant over the all wavelength range of the emission. In the meantime, the preexponential factor of the short-lifetime component changes its sign, being positive for the N* and negative for the T* emission band. Moreover, the two preexponential factors of the T* emission decay are the same in magnitude but opposite in sign. These features are characteristic of a fast reversible two-state ESIPT reaction. Reconstruction of time-resolved spectra allows observing the evolution of these spectra with the appearance, rapid growth, and stabilization (in less than 200 ps) of the relative intensities of the two emission bands. A detailed kinetic model was applied for the analysis of these data, which involved the determination of radiative and nonradiative decay rate constants of both N* and T* forms and of forward and reverse rate constants for transitions between them. We show that ESIPT reaction in the studied conditions occurs on the scale of tens of picoseconds and thus is uncoupled with dielectric relaxations in the solvent occurring at subpicosecond times. Moreover, the radiative and nonradiative deactivation processes were found to be much slower than the ESIPT reaction, suggesting that the relative intensities of the two emission bands are mainly governed by the ESIPT equilibrium. Therefore, both electrochromic and solvatochromic effects on the relative intensities of the two emission bands in 4'-(dialkylamino)-3-hydroxyflavones result from the shifts in the ESIPT equilibrium.