ANALYSIS OF FLUORESCENCE QUENCHING IN SOME ANTIOXIDANTS FROM NONLINEAR STERN-VOLMER PLOTS

The fluorescence quenching of 6-hydroxy-1,4-dimethylcarbazole (HDC) an d alpha-tocopherol or vitamin E (alpha-toc) by methylene bromide (CH2B r2) in n-butanol was investigated by both stationary and time-dependen ce measurements. Nonlinear Stern-Volmer behavior was observed in both cases at higher quencher concentrations for these highly exothermic el ectron transfer (ET) intermolecular reactions. Both static and dynamic quenching models can fit the experimental data, but only the finite s ink approximation model was able to recover both the mutual diffusion coefficient and the distance-averaged 'activation energy controlled' r ate constant (k(a)). The two ET reactions investigated are diffusion-c ontrolled (alpha-toc) (k(a) much-greater-than k(d)) and diffusion-infl uenced (HDC) (k(a) congruent-to k(d)) respectively.