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 pi-conjugation within the molecule dictate the formation of the solvated cluster endorsing directed solvation.

Automated summary

The study shows that the differences in fluorescence anisotropy between 3HF and 7HF in water are mainly due to the volume difference of their solvated clusters, while 6HF exhibits very low fluorescence anisotropy. This research reveals the significant roles of the positioning of hydroxyl groups and their participation in extended π-conjugation within the molecule in determining the formation of solvated clusters.