Proton transfer reactions in nanoscopic polar domains: 3-hydroxyflavone in AOT reverse micelles

A dramatic reduction in the excited-state intramolecular proton transfer (ESIPT) rate is observed for 3-hydroxyflavone (3-HF) within the nanoscopic polar domains of Aerosol-OT (AOT)/n-heptane reverse micelle solutions. It is attributed to the formation of intermolecularly hydrogen-bonded 3-HF:AOT complexes, which cause a significant disruption of intramolecular hydrogen bonding within the complex-bound 3-HF molecules, thereby limiting the overall rate of the ESIPT process. Introduction of strong hydrogen-bonding polar solvents like water or methanol into the reverse micelles causes extensive solvation of the AOT head groups, leading to the collapse of the 3-HF:AOT complex and eventual release of intramolecularly hydrogen-bonded 3-HF molecules which are then able to undergo ultrafast ESIPT. With increasing W-value (W=[polar solvent]:[AOT]), a larger number of 3-HF:AOT complexes are decimated, thus accelerating the overall ESIPT process. In contrast, in presence of solvents like acetonitrile, whose hydrogen-bonding power is inherently weak, the AOT head groups are poorly solvated, so that the 3-HF:AOT complexes are hardly affected at any W-value. Consequently the ESIPT dynamics of 3-HF in acetonitrile-containing AOT reverse micelles is nearly independent of the W-value, and always slower compared to that in water- or methanol-containing AOT reverse micelles. The results highlight the importance of hydrogen-bonding property of the polar solvent on the ESIPT of 3-HF within a nanoscopic domain.