Effect of hydrogen bonding and solvent polarity on the fluorescence quenching and dipole moment of 2-methoxypyridin-3-yl-3-boronic acid

Abstract

Two photophysical properties namely, fluorescence quenching and dipole moment (both ground state and excited state) of 2-methoxypyridin-3-yl-3-boronic acid (2MPBA) have been investigated in alcohol environment using steady state fluorescence technique at 300 K. In quenching studies, a rare but not unusual observation; negative Stern-Volmer (S-V) deviation has been noticed. It has been explained using the concept of solute’s conformational changes in the ground state due to inter-molecular and intra-molecular hydrogen bonding in alcohol environment. The spectroscopic data has been processed using Lehrer equation and thereby Stern-Volmer constant (<em>K</em>_SV) has been evaluated. It has been found to be above 100 for most of the solvents used. The data related to dipole moment has been examined using different solvent polarity functions. Theoretical calculation of dipole moment in the ground state has been done using Gaussian software. The general solute–solvent interactions and hydrogen bond interactions have been found to be operative. An appreciable red shift of about 25 nm in the emission spectra has been identified with the rise in solvent polarity and decrease in molar mass of alcohols. It confirms the π→π^* transition as well as the possibility of intra-molecular charge transfer (ICT) character in the emitting singlet state of 2MPBA.