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.