Spectroscopic study on the reaction of singlet-excited nile blue with certain catecholic antioxidants

The photoinduced interaction of nile blue (NB) with various antioxidant molecules was investigated by fluorescence quenching technique and lifetime measurements. The various substituted catecholic compounds are employed as quenchers to evaluate their antioxidant activity. Formations of ground state complex between NB and quencher molecules was observed from the UV-Visible spectroscopy. Bimolecular quenching rate constants (k_q) values depend on presence of substituent and its electronic properties of quencher molecules. Fluorescence quenching experiments have been performed at three different temperatures to assess the thermodynamic parameters. Time resolved fluorescence measurements suggest that the fluorescence quenching of NB with antioxidant molecules undergoes static quenching mechanism. Bond dissociation enthalpy (BDE) values reveal the discharge of H* from the antioxidant molecules. The electronic properties play an important role in the antioxidant activity of quencher molecules. Mechanism of fluorescence quenching between NB and quencher molecules are analysed based on the fluorescence quenching, cyclic voltammetry experiments and BDE calculations.