7-Hydroxyflavone Revisited: Spectral, Acid–Base Properties, and Interplay of the Protolytic Forms in the Ground and Excited States

Abstract

Spectral and acid-base properties of 7-hydroxyflavone (7HF) in the ground and excited states were investigated with a purpose to enable reasonable application of this dye and its derivatives as fluorescent probes. Analysis of solvatochromic and solvatofluorochromic ability of 7HF in 20 solvents, investigations of 7HF spectral properties in the frozen solvents, spectrophotometric and spectrofluorimetric titrations in methanol-water (4:1 v/v) in the wide pH/H0 range (from pH = 11.0 to H0 = -4.5), analysis of the 3D fluorescence and time-resolved spectra, as well as quantum-chemical calculations were carried out. It has been found that 7HF can exist in three protolythic forms-neutral, anion, and cation-depending on the environment acidity or basicity. In the excited state, in methanol-water solutions, there are four forms: neutral, cation and anion, which can be formed by direct excitation of the ground-state anion or by photodissociation of the neutral form depending on pH, and only one phototautomer, which appears in the H0 range from 1.3 to -4.5. It has been shown that the mechanism of the phototautomer formation depends on medium acidity. The photoautomer can be formed by cation photodissociation as well as by photoanion protonation. Finally, it was concluded which of the 7HF protolytic forms can be used for fluorescent probing.

Single and double intramolecular proton transfers in the electronically excited state of flavone derivatives

“…Compared to anion of 7-hydroxyavone 18 this corresponds to the batochromic shi values of 3240 and 2000 cm À1 , respectively, due to electron-withdrawing effect of the carbonyl substituents. More than 1.5 lower value of the shi in the case of 2b compared to 1b indicates less effective stabilization of A 7 by the carbonyl substituent in position 8 compared to position 6.…”

“…1a and b). Compared to 7-hydroxyavone, whose absorption band at the same conditions is positioned at 291 nm, 18 1b and 2b exhibit batochromic shi, caused by polarization of the hydroxyl group at position 7 due to IHB. Compared to 1b, the long-wavelength absorption band of 1c is batochromically shied to 334 nm (Table 1, Fig.…”

Origin of Spectral Features and Acid–Base Properties of 3,7-Dihydroxyflavone and Its Monofunctional Derivatives in the Ground and Excited States

“…Fluorescence from the N * form is, however, weak under these conditions (Figure 4b), but upon a rise of H 0 , new emission appears which can be ascribed to the neutral T 7 * form (at 568 nm) (Figure 4a), which is most likely produced via protolytic dissociation: C *  T 7 * + H + , as was reported previously for 7-hydroxyflavone. 38 Emission from T 7 * declines together with the disappearance of the C * form. The single band (at 481 nm) in the steady-state fluorescence spectra of 1 (Figure 5a,b) appearing in strongly acidic media (H 0 < -4.0) arises from the electronically excited cationic forms C * or C(1) * (fluorescence decay of which is shown in Figure S6a, Supporting Information) which are generated via the absorption of radiation by the ground state forms C or 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 The most distinguishable feature of 1 is its relatively intense dual fluorescence in basic solutions, a characteristic that has not been reported for other flavonolic systems so far.…”

Solvatochromism of naringenin in aqueous alcoholic mixtures

“…28 However, according to Serdiuk et al, there was no significant correlation between the position of the maximum in the spectra of 7-hydroxyflavone and the solvent parameters in pure alcoholic solvents. 29 In addition, in aprotic solvents, the solvatochromism of 7-hydroxyflavone depends on π* and β. 29 Such results, in accordance with the present findings, show the importance of specific interactions for the absorption spectra of flavonoids.…”

“…29 In addition, in aprotic solvents, the solvatochromism of 7-hydroxyflavone depends on π* and β. 29 Such results, in accordance with the present findings, show the importance of specific interactions for the absorption spectra of flavonoids. Further quantitative comparisons cannot be made with the present data because no literature data are available for aqueous binary mixtures.…”