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Link to original content: http://pubmed.ncbi.nlm.nih.gov/35515691/
pH dependency of the structural and photophysical properties of the atypical 2',3-dihydroxyflavone - PubMed Skip to main page content
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. 2020 Sep 22;10(58):35017-35030.
doi: 10.1039/d0ra06833k. eCollection 2020 Sep 21.

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

Luc Labarrière  1 , Aurélien Moncomble  1 , Jean-Paul Cornard  1
Affiliations

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

Luc Labarrière et al. RSC Adv. .

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.

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Conflict of interest statement

There are no conflicts to declare.

Figures

Fig. 1
Fig. 1. Structures of flavone, 3HF, 2′3HF and morin, with IUPAC atomic numbering and ring labelling.
Fig. 2
Fig. 2. Equilibrium between the two conformers A (left), and B (right).
Fig. 3
Fig. 3. Optimized A, A′, B, and B′ conformers (top). Evolution of the molecular energy with the variation of the C3C2C1′C2′ dihedral angle (τ) (bottom).
Fig. 4
Fig. 4. Evolution of the UV-visible absorption spectrum (left), along with the increase in pH, starting from 2 to 12, in methanol. Evolution of the absorbance at λabs = 387 nm, plotted against pH (right). The red points are experimental values whereas the blue line is the model.
Fig. 5
Fig. 5. Optimized geometries depicting the possible positions for the remaining proton, after the removal of the one of the two.
Fig. 6
Fig. 6. Comparison of the UV-visible absorption spectrum (red) of 2′3HF with the computed electronic transitions (blue) for the proposed species. Left: spectrum in methanol + HCl medium and electronic transitions of A (solid) and B (dashed). Right: spectrum in methanol + NaOH medium and electronic transitions of dep2′ (solid) and dep3 (dashed).
Fig. 7
Fig. 7. HOMO – 3, 2, 1, HOMO and LUMO of A, dep2′, and dep3 species, The arrows indicate the similarities of the MOs between the different structures.
Fig. 8
Fig. 8. Normalized UV-visible absorption (black, dashed) spectrum compared to the fluorescence emission and excitation spectra of 2′3HF in methanol + HCl medium. (a): λexc = 335 nm (blue), λexc = 395 nm (red). (b): λexc = 335 nm (black), λem = 430 nm (blue), λem = 550 nm (red).
Fig. 9
Fig. 9. A*, T3*, T2′*, B* and BT3* optimized geometries.
Fig. 10
Fig. 10. Absorption (black, dashed), excitation (blue, λem = 570 nm), emission (black, solid, λexc = 395 nm), spectra of 2′3HF in basic medium.
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