Critical assessment of solvent effects on absorption and fluorescence of 3HF in acetonitrile in the QM/PCM framework: A synergic computational and experimental study

Absorption and fluorescence properties of 3-hydroxyflavone (3HF) dissolved in the polar aprotic solvent acetonitrile have been investigated by electronic spectroscopies, associated to mixed quantum mechanical (QM)/classical calculations, where the effect of the solvent is included at the classical level by means of the polarizable continuum model (QM/PCM). Whereas absorption and fluorescence (λ exc = 342 nm) spectra confirmed previous results, a detailed spectrofluorimetric investigation of the anion, deriving from solvent-induced deprotonation of the OH group of 3-hydroxyflavone, showed for the first time that a single anionic form is present in solution, differently from what observed in another polar aprotic solvent, DMSO. A detailed computational study based on Density Functional Theory (DFT), using five different functionals, has been carried out in order to reproduce and better understand absorption and emission bands on the different existing forms for 3HF in solution: Normal (N), Tautomer (T), Anion (A). The results are discussed in terms of solvent effects on 3HF spectroscopic properties. It was found that simulations are in good agreement with the spectroscopic data for the N and T forms, whereas for the A form larger discrepancies are observed, especially for absorption properties. The effect of specific solute solvent hydrogen bond interactions involving the C[dbnd]O and OH moieties of 3HF was also explored, aiming to better simulate anionic spectroscopic properties. Results suggest that a more complete description of the whole solvation shell is necessary.