Proton transfer dynamics in substituted 3-hydroxyflavones: Solvent polarization effects

The spectroscopy and excited state proton transfer (ESPT) dynamics of 4′-N,N-dimethylamino-3HF (I) and 4′-N,N-diethylamino-3HF (II) have been studied in acetonitrile/benzene solvent mixtures. Solvent composition-dependent spectral shifts are observed and can be understood in terms of an Onsager cavity model. Analysis of these spectral shifts accurately predicts solvent composition-dependent excited state equilibrium constants, which are also experimentally determined. The ESPT rates are analyzed within the framework of a transition state theory treatment of solvent polarization-mediated proton transfer. This treatment is analogous to electron transfer theory. In this treatment, the energetics of the transition state are largely determined by known solvent properties and the solvent-dependent spectroscopy. This analysis yields solvent-dependent ESPT activation energies. The corresponding calculated ESPT rates are in excellent agreement with the experimentally determined rates.