Examination of the viability of the Smoluchowski–Collins–Kimball model in fluorescence quenching of S2-xanthione

Abstract

The fluorescence quenching of S2-xanthione by five different hydrocarbon quenchers in perfluoro-1,3-dimethylcyclohexane has been studied assuming the Smoluchowski–Collins–Kimball (SCK) model. The fluorescence decay curves were measured by means of a picosecond time-resolved, single-photon counting technique. The model used has been shown to satisfactorily describe the process of S2-thioketones fluorescence quenching for six donor–quencher–solvent systems best fitting the model assumption [Krystkowiak and Maciejewski, J. Chem. Phys. 117, 2246 (2002)]. The systems studied were characterized by a short lifetime of an excited donor (equal to 152±2 ps in perfluoro-1,3-dimethylcyclohexane), the quenching process mechanism demanding a direct contact of reacting molecules and at the end the inert solvent. The influence of the shape, size, reactivity anisotropy of quencher molecules on the possibility of the SCK model application, and the values of molecular parameters: R (the sum of the molecular radii), D (the sum of diffusion coefficients), and the specific rate constant of the process (κ) was studied for S2-xanthione–n-hexane, S2-xanthione–n-decane, S2-xanthione–n-hexadecane, and S2-xanthione–longicyclene systems. The system with benzene as a quencher was studied for comparative reasons because of a different mechanism of the S2-xanthione fluorescence quenching process, which does not need a direct contact of reagent molecules. We have confirmed that the fluorescence quenching process of S2-xanthione by hydrocarbons could be described by a Smoluchowski–Collins–Kimball model only when the quencher molecules are characterized by a well-defined shape (not necessarily spherical) and isotropic reactivity and when the excited donor and quencher molecules are in direct contact within the contact pair. A satisfactory fit of the Smoluchowski–Collins–Kimball function the donor fluorescence decay in the presence of the quencher cannot be a sufficient criterion of the correctness of the model application. The parameters R, D, and κ determined for electronically excited donor–quencher pairs should be compared with these obtained from an independent study.