Diffusion controlled reactions: fluorescence quenching of cationic dyes by charged quenchers

Abstract

Rate coefficients of diffusion-limited fast reactions of charged reactants are predicted to be time dependent or time independent in different theoretical approaches to solve the Debye-Smoluchowski equation. Different rate coefficient expressions are also predicted in different time domains. Fluorescence quenching experiments using two cationic dyes (rhodamine B and cresyl violet) and four efficient ionic quenchers (iron complexes) in aqueous solutions were carried out to verify the theoretical prediction. A detailed data analysis of the quenched fluorescence decay over a range of concentration of the quenchers supports the prediction that the "long" time rate coefficient is k(t)∼a+bt. Exact agreement with the theoretical predictions is not obtained in any of the fluorophore-quencher systems described in this work. Cresyl violet-ferrocyanide and cresyl violet-ferricyanide are two systems for which the experimental values of diffusion coefficient D and distance parameter RHN are justifiably close to theoretical expectation.