The triply fluorescent 2',3-dihydroxyflavone computationally and spectroscopically investigated

Flavonoids are secondary metabolites found in plants, with interesting properties for human health. Among this family, 3-hydroxyflavone (3HF) exhibits an intramolecular proton transfer in the excited-state (ESIPT), that has been the subject of many studies. The phenomenon is at the origin of a dual fluorescence: one emission from the normal species, and the other one, red-shifted, from the proton-transferred tautomer. It is interesting to note that the substitution by hydroxyl groups on the backbone changes drastically the photophysical properties and morin (2’,3,4’,5,7-pentahydroxyflavone) is an example of a non-fluorescent derivative.
Using steady-state and time-resolved electronic spectroscopies, coupled with DFT and TD-DFT calculations, we investigated the effect of the 2’-hydroxyl group on the properties of the ground and excited states of 2’,3-dihydroxyflavone (2’3HF). We show that the hydroxylation in position 2’ modifies drastically the acid-base properties of the ground state of 3HF derivatives. Geometrical changes occur as well, with a non-zero value of the dihedral angle between the two moieties of the molecule. In contrast with morin, 2’3HF displays a triple fluorescence. Upon excitation, large geometry rearrangements occur, inducing a greatly Stokes shifted violet fluorescence for the normal species. The second fluorescence is attributed to the proton-transferred tautomer in analogy with 3HF that is supported by the calculations. Finally, the remaining fluorescence is evidenced when exciting in the low energy region of the absorption spectrum, which could arise from the formation of a stable solvent complex.

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