Photophysical and photochemical properties of 3-hydroxyflavone in ethanol solution: Implicit vs explicit solvent models

Photophysical and photochemical properties of 3-hydroxyflavone in gas-phase and solution were studied using implicit (Polarizable Continuum Model, PCM) and explicit solvent (Averaged Solvent Electrostatic Potential from Molecular Dynamics calculations, ASEP/MD) models. The conformational equilibrium in the excited state between the normal (N*) and tautomeric (T*) forms and their absorption and fluorescence spectra was studied with Time-Dependent Density Functional Theory (TD-DFT) and three different functionals. The calculated transitions are in good agreement with the spectroscopic data. It was found that solvent effects on the absorption and fluorescence spectral bands are negligible. Still, they can modify appreciably the relative stability of the N* minimum with respect to the Franck-Condon position, which affects the kinetics of the reaction. This fact, together with the increase in the barrier height in protic solvents, permits us to explain the reduction of the emission signal from T* and the increase of the N* signal in ethanol solution.