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

Abstract

Comprehensive spectral investigations of 3,7-dihydroxyflavone and its two derivatives, which each contain a methyl-blocked hydroxyl group, reveal complex radiation absorption in the 300-450 nm range and emission in the 370-650 nm range. The absorption and fluorescence characteristics of these compounds depend on the pH/H0 of the water/methanol media, which is caused by the existence of the compounds in various protolytic (cationic, neutral, anionic) and tautomeric forms. Combined analysis of steady-state, time-dependent and fluorescence decay spectral data enabled the identification of the emitting species, determination of their lifetimes with respect to radiative and nonradiative deactivation processes, fluorescence quantum yields, protolytic and tautomeric abilities under various conditions, and acidic dissociation constants of the cationic, neutral, and anionic forms of the compounds. Results of calculations carried out at the DFT and TD DFT levels of theory generally confirmed the experimental findings concerning tautomeric/protolytic transformations and equilibria. Computational methods also provided insight into possible tautomerization pathways. Electronically excited molecules are generally much more susceptible to tautomerization and acidic dissociation than ground-state ones. 3,7-Dihydroxyflavone exhibits distinguishable features among the compounds investigated and can be considered as potential spectral indicator of properties (polarity, hydrophobicity, hydrogen-bonding ability) and acidity/basicity of liquid environments.

Design and Emissive Features of Ionic White-Light Fluorophore

“…To evaluate at which position the PT site activates ESIPT, Gibbs free energies of cationic ( C* ) and cation-tautomeric forms ( CT* ) forms were calculated for the S 1 state of compounds 1–6 (Figure A) using TDDFT method with a B3LYP functional and a cc-pVDZ basis set. Calculations on such a level of theory proved to be relatively low time-consuming and give results in good correlation with the experiment, also for organic cations . According to thus obtained data, the Δ 298 G C* → CT* ° values of compounds 1 , 2 , 4 , and 5 are positive (Figure B), which indicates that ESIPT is thermodynamically unfavored.…”

“…Calculations on such a level of theory proved to be relatively low time-consuming and give results in good correlation with the experiment, also for organic cations. 20 According to thus obtained data, the Δ 298 G C*→CT* °values of compounds 1, 2, 4, For further investigations, namely, on the stage of preparation, formyl group in 3 was changed to the 3phenylpropanoyl one (MSF, Figure 1A) because of the similar electronic and proton-accepting effects of both these groups but higher chemical stability of the latter. The 3-phenyl-Scheme 1.…”

“…To date, however, the reports on the charged fluorophores which undergo the above-mentioned photochemical or photophysical processes (ESIPT, ICT, ISC, and AIE) affording multiband emission are very scarce, thus possibility of WL generation by such systems is rather questionable. Few reports on ESIPT dual emission of charged fluorophores concerned anionic species of 2-(2′-acetamidophenyl)­benzimidazole, 5-aminosalicylic acid, and 3,7-dihydroxyflavone in basic aqueous solutions. On the example of flavonol derivatives, it was proved that ionic groups not conjugated with the chromophore fragment can noticeably modify spectral features and ESIPT thermodynamics because of the electrochromic effect .…”

White Light from a Single Fluorophore: A Strategy Utilizing Excited-State Intramolecular Proton-Transfer Phenomenon and Its Verification

“…The above-mentioned relative intensity seems to be the most difficult parameter to design, as it depends on various factors, namely, ESIPT thermodynamics, kinetics, and medium effects. It can be noticed, that substituents with strong electron-releasing effect (substituted amino groups and deprotonated hydroxyl groups) and extended π-system enable close intensities of the N* and T* bands. Because reasons favoring close intensities of the blue and yellow components seem to be poorly understood, which impedes target design of WL ESIPT fluorophores, this article focuses on investigations of these reasons in a particular example.…”