Elimination of the hydrogen bonding effect on the solvatochromism of 3-hydroxyflavones

We address the important, frequently discussed, and still unresolved question of the role of solute-solvent hydrogen bonding in modulating the excited-state intramolecular proton transfer (ESIPT) reaction in 3-hydroxyflavone (3HF) derivatives, which provides a dramatic variation of the relative intensities of normal (N*) and phototautomer (T*) emissive species. We synthesized a new 3HF derivative, 5,6-benzo-4'-diethylamino-3-hydroxyflavone (BFE), in which the additional benzene ring protects the 4-carbonyl from H-bonding with the protic solvents but allows the intramolecular bond with the 3-hydroxyl group, which is the pathway of ESIPT, to be maintained. The absorption and fluorescence properties of BFE and its parent analogue 4'-diethylamino-3-hydroxyflavone (FE) were studied in a set of 20 representative solvents. In aprotic media, these dyes in absorption and fluorescence spectra show similar solvatochromism, whereas in protic solvents dramatic differences are observed, which demonstrate that for BFE the effects of intermolecular H-bonding are eliminated. The elimination of the specific interaction of BFE with protic media in the ground state is shown by thin-layer chromatography. The most dramatic differences in spectroscopic properties between BFE and FE are observed for the intensity ratio of the two emission bands, I-N*/I-T*. Whereas for FE a linear correlation of log(IN*/IT*) with the solvent polarity function f(is an element of) exists only in aprotic media and in protic solvents strong systematic deviations are observed, for BFE this linear correlation extends to all of the studied solvents. Therefore, we suggest that the protic solvents modulate ESIPT in 3-hydroxyflavone derivatives exclusively by proton-donor intermolecular H-bonding with 4-carbonyl, which occurs with the preservation of the intramolecular H-bond. Our results show how molecular design allows the elimination of the strong intermolecular H-bonding perturbation of the solvent polarity-dependent ESIPT reaction, which provides the means for constructing fluorescent probes with strong selectivity to universal solvent effects.