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

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−1/2. 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.