Effect of the mixture composition of Acetonitrile/Benzene on excited state intramolecular proton transfer in 3-hydroxyflavone, theoretical insights: QTAIM, NBO, NLO behavior, thermodynamic and kinetic aspects.

The effect of the Acetonitrile/Benzene mixture composition on excited state intramolecular proton transfer in 3-hydroxyflavone (3HF) was studied by means of density functional theory (DFT) and time-dependent DFT (TDDFT). The static dielectric constant εm is considered as a polarity parameter to characterize the composition of the two mixed solvents. We used the B3LYP/6-31G(d) level of theory for all the calculations of various properties (structural, electronic, spectroscopic, photophysical) in both ground and excited states. Our initial findings show that the mixture composition has a negligible effect on the geometry parameters. The computation of the maximum absorption and fluorescence wavelengths for 3HF and its tautomer correlates well with their corresponding experimental counterparts. The QTAIM and NBO tools were employed to estimate the strength of intramolecular hydrogen bond IHB. It reveals that EHB is weakened with the increase of εm, due to the existence of electrostatic interactions. The title molecule is prone to exhibit remarkable NLO response. Secondly, the kinetic and thermodynamic outcomes, together with all former results, strongly support the occurrence of ESIPT phenomenon. Meanwhile, they also highlight the limits of PCM model in accurately describing solute-solvent interactions. Finally, the estimated ESIPT time is very close to the experimental values, which validates our level of theory.