Fluorescence quenching of boronic acid derivatives by aniline in alcohols - A Negative deviation from Stern-Volmer equation

The fluorescence quenching study of two boronic acid derivatives 5-chloro-2-methoxy phenyl boronic acid (5CMPBA) and 4-fluoro-2-methoxyphenyl boronic acid (4FMPBA) in alcohols of varying viscosities is carried out at room temperature by steady state fluorescence measurements. Aniline is used as quencher. The negative deviation in the Stern-Volmer (S-V) plots has been observed for both the molecules with moderate quencher concentration. The downward curvature in the S-V plot is interpreted in terms of existence of different conformers of the solutes in the ground state. The formation of intermolecular and intramolecular hydrogen bonding in alcohol environments is taken to be responsible for the conformational changes in the ground state of the solutes. The modified Stern-Volmer equation or Lehrer equation is used to calculate Stern-Volmer constant (KSV) and it is found to be above 100 for most of the solvents used.