Hydroxyl Group-Directed Solvation of Excited-State Intramolecular Proton Transfer Probes in Water: A Demonstration from the Fluorescence Anisotropy of Hydroxyflavones

Formation of a probe-solvent network resulting in unusually high fluorescence anisotropy (FA) of an excited-state intramolecular proton transfer (ESIPT) probe, 3-hydroxyflavone (3HF), in water prompted us to explore the solvation patterns on its 7-hydroxy (7HF) and 6-hydroxy (6HF) positional analogues. In the present study, it was observed that 7HF exhibits a lower FA than 3HF does in water, implying that the volume of the 7HF-water cluster is less than that of the 3HF-water cluster. Experimental and computational results led us to propose that 7HF forms its water cluster at the molecular periphery in contrast to the projected-out structure in case of the 3HF-water cluster. Density functional theory (DFT)-based quantum chemical calculations provide an approach for the differential solvation patterns of 3HF and 7HF. 6HF, a non-ESIPT probe, exhibits very low FA in water compared with both 3HF and 7HF. This study demonstrates that proper positioning of the hydroxyl group and its participation in the extended π-conjugation within the molecule dictate the formation of the solvated cluster endorsing directed solvation.

Origin of Unusually High Fluorescence Anisotropy of 3-Hydroxyflavone in Water: Formation of Probe-Solvent Cage-like Cluster

Based on the unusually high fluorescence anisotropy (FA) of 3-hydroxyflavone (3HF) in water medium in contrast to the very low FA of its methoxy counterpart (3MF), our proposition invoked formation of an intermolecular hydrogen-bonded cage-like probe-solvent cluster of 3HF in water. In the present work, ab-initio DFT-based quantum chemical calculations have been exploited to provide a foundation for our interpretation. Ground-state optimization of 3HF with varying numbers of water molecules leads to the formation of a cage-like or loop-like probe-water cluster. Our calculations reveal that the structures with four to five water molecules are stabilized to the maximum extent. Classical molecular dynamics simulations reveal that the rotational dynamics of 3HF is much slower in water compared to that in alkane medium, which also goes in favor of the probe-solvent cluster formation in water medium. Apart from the theoretical studies, an indirect experimental approach has been adopted to substantiate formation of the probe-water cluster. The atypical observation of reduced FA of 3HF entrapped in micelles relative to that of the fluorophore in water implies disruption of the probe-water cluster with the addition of micelles, corroborating our original proposition of formation of an intermolecularly hydrogen-bonded 3HF-water cluster.

Inhibition of the prototropic tautomerism in chrysazine by p-sulfonatocalixarene hosts

This study reveals the unusual inhibition of excited-state prototropic tautomerism of Chrysazine by p-sulfonatocalix[4,6]arene hosts.

Theoretical study on the photooxygenation and photorearrangement reactions of 3-hydroxyflavone

The mechanisms of three photodegradation reactions of 3-hydroxyflavone – its photosensitized oxygenation, photooxygenation with ³O₂ and photorearrangement into an indanedione derivative – have been investigated by computing the free energy profiles.