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

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/H-0 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.