Kinetics of fluorescence quenching of S2-thioketones studied by the Smoluchowski–Collins–Kimball model: Standard systems

Diffusion-influenced excited-state reversible transfer reactions, A*+B⇌C*+D, with two different lifetimes: Theories and simulations

We report accurate Brownian simulation results for the kinetics of the pseudo-first-order diffusion-influenced excited-state reversible transfer reaction A(*) + Bright harpoon over left harpoonC(*) + D with two different lifetimes using two different propagation algorithms. The results are used to test approximate solutions for this many-particle problem. Available theories fail when one of the two reactions or (decay) rate constants is large. To remedy this situation, we develop two uniform approximations, which are based on introducing a generalized Smoluchowski term into the relaxation-time approximation. The best of these is the extended unified theory of reversible target reactions, which reduces correctly in all limits and exhibits superior agreement with simulations.

Kinetics of fluorescence quenching for electron transfer and for energy transfer: Molecular dynamics tests for spherical molecules

The Smoluchowski approach to description of fluorescence quenching is tested by comparing the theory with computer simulations for the case of spherical molecules. The distance dependent sink terms describing the electron transfer mechanism and the Forster model for the energy transfer are considered. It is shown that the agreement between the rate coefficient from the model and from simulations depends on the strength of the solute-solvent interactions as well as on the speed of reaction itself. Comparing results of simulations for different quencher concentrations we estimate the strength of quencher concentration dependence effect and the range of times the effect may be significant. In the long time limit the increase in quencher concentration decreased the rate coefficient.

Rapid fluorescence quenching of S2-xanthione by 3,3-diethylpentane in perfluorohydrocarbons

Rapid fluorescence quenching of S2-xanthione by 3,3-diethylpentane has been studied in three perfluorohydrocarbons of different viscosities. The donor fluorescence decay in the presence of a quencher was fitted using the Smoluchowski-Collins-Kimball (SCK) model. The molecular parameters, R (the sum of the molecular radii), D (the sum of diffusion coefficients), and the specific rate constant of the process, kappa, were determined. The values of parameter D for all systems studied differed from the sum of the macroscopic diffusion coefficients Dx measured independently. This behavior is explained by the dependence of the molecular diffusion coefficient (as determined from the SCK model) on the distance traveled by the donor and quencher molecules during the excited donor lifetime.