4 '-Methoxy-3-hydroxyflavone excited state intramolecular proton transfer reaction in alcoholic solutions: Intermolecular versus intramolecular hydrogen bonding effect

摘要

Influence of protic surrounding on the excited state intramolecular proton transfer (ESIPT) reaction was studied both experimentally and theoretically on the example of 4'-methoxy-3-hydroxyflavone (4M3 H F), a model fluorescent dye possessing multi-banded emission spectra. Experimentally determined 4M3 H F ESIPT rates in alcohol solutions correlate with the general acidity of a protic solvent and steric hindrance to intermolecular H-bonding caused by the space dimensions and branching of an alcohol molecule hydrocarbon residue. The role of intermolecular hydrogen bonds was elucidated by quantum-chemical and molecular dynamics modeling: several types of H-bonded complexes were examined systematically aiming to outline their accelerating or retarding effects on ESIPT reaction rate. An intermolecular hydrogen bond to the lone electron pair of the carbonyl group oxygen atom, which is not involved in the intramolecular H-bonding, was shown to be the most important factor for regulation of ESIPT rate in protic or aprotic humid surrounding.