pH dependency of the structural and photophysical properties of the atypical 2′,3-dihydroxyflavone

Abstract

2′,3-Dihydroxyflavone (2′3HF) is a natural flavonol that has barely ever been studied, however the scarce studies of its physico-chemical properties have highlighted its atypical behaviour. We present a structural and spectral study of 2′3HF, performed using UV-visible absorption and fluorescence spectroscopies, coupled with DFT and TD-DFT calculations. Although its structure is close to that of 3-hydroxyflavone, 2′3HF shows a much lower pK_a value. We show that the origin of this particularity is the substitution by a hydroxyl group on position 2′, that induces a stronger inter-ring interaction weakening the bonding of the proton at position 3. The main absorption band of the is red-shifted upon deprotonation. The remaining proton is highly bonded in between oxygen atoms 3 and 2′, making the second deprotonation unattainable in methanol. The neutral form can undergo an excited-state intramolecular proton transfer to emit dual fluorescence by the normal and tautomer forms. We suggested five geometries to be the sources of the emission bands, and showed that the energy barriers to interconversions were almost null. The anion is also fluorescent. The Stokes shifts for the neutral normal and anion species are extremely high, that can be explained by the conformational rearrangement, as the species go from twisted in the ground-state, to planar in the excited-state. Finally, another emission band is evidenced when exciting in the vicinity of the absorption maximum of the anion species in acidic medium. We suggest an aggregate with the solvent to be the origin of the emission.