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

Abstract

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.

A Picosecond Time‐Resolved Spectroscopic Investigation of the Effect of pH on Morin Fluorescence

“…But the excitation spectrum performed at 496 nm (Figure 4) evidenced that the origin of the fluorescence was not the fully protonated morin (absorption maximum at 357 nm) but another species absorbing at 423 nm. Supported by the two former works, [11,46] by the fact that the fully protonated morin was virtually not fluorescent and by the excitation spectrum, we put forward the hypothesis that the fluorescence centred at 500 nm for a pH 2.01 (and more generally in acidic MeOH) was coming from a solvated neutral morin-MeOH intermolecular H-bonded cage-like complex.…”

“…Another recent work combining experiments and theoretical quantum chemical calculations brought new insights about the origins of the LW absorption band of 3HF: Chattopadhyay et al evidenced that in protic solvents, especially in water, 3HF exhibited a high fluorescence anisotropy contrarily to 3-methoxyflavone. [46] In nonpolar aprotic solvents, they observed that the fluorescence anisotropy of 3HF was low. Using ab-initio quantum chemical calculations, they found that 3HF formed a solvated cluster in aqueous solution.…”

pH dependency of the structural and photophysical properties of the atypical 2′,3-dihydroxyflavone

“…It is suspected that 3HF forms a stable complex with the solvent, of red-shied absorption energy compared to the lone neutral 3HF, explaining the high dependence on the medium of the emission. [69][70][71] Most research groups concluded however on the emission from the anion aer an ESPT (intermolecular) to the solvent. [72][73][74][75] The situation is more complicated in the case of 2 0 3HF, as band X and A are very different in intensities, energies and shape, a result that cannot be explained currently.…”

Effect of Water Microsolvation on the Excited-State Proton Transfer of 3-Hydroxyflavone Enclosed in γ-Cyclodextrin

“…3HF exhibits ESIntraPT and gives dual fluorescence corresponding to its E* and K* forms with a large Stokes shift and photostability [28][29][30]. Thus, 3HF has been used as a prototype for the ESIntraPT processes and as sensitive fluorescence probes for discovering binding sites in various bio-relevant targets such as DNA, protein, and biomembranes [31][32][33]. The photophysical properties and ESPT processes of 3HF in organic solvents have been extensively studied [34][35][36][37][38][39][40][41][42].…”