Stem-Volmer quenching rate constants

In fact, quenching effects can be evaluated and linearized through classic Stem-Volmer plots. Rate constants responsible for dechlorination, decay of triplets, and quenching can be estimated according to a proposed mechanism. A Stern-Volmer analysis of photochemical kinetics postulates that a reaction mechanism involves a competition between unimolecular decay of pollutant in the excited state, D, and a bimolecular quenching reaction involving D and the quencher, Q (Turro N.J.. 1978). The kinetics are modeled with the steady-state approximation, where the excited intermediate is assumed to exist at a steady-state concentration ... 

The Stem-Volmer(52) equation relates fluorescence intensity and the quenching rate constant, kq ... 

The constant K is known as the Stem-Volmer quenching constant /cQ is the rate constant for the quenching reaction, and t0 the lifetime in the absence of quencher. Fluorescence quenching of tryptophan in proteins by acrylamide or 02 has been used to determine whether tryptophan side chains are accessible to solvent or are "buried" in the protein.141 142 The long-lived phosphorescence of tryptophan can be studied in a similar... 

An important aspect of the photophysics of the Pt(diimine)(dithiolate) photochemistry that has received increasing attention is the ability of the excited-state complexes to undergo self-quenching. Initial work by Connick and Gray (111) showed that the lifetime of the complex Pt(bpy)(bdt) (bdt = benzene-1,2-dithiolate, 31) decreased with increasing solution concentration. The bimolecular self-quenching rate constant, calculated from a Stem-Volmer quenching analysis, was found to be 9.5 x 109 A/-1 s-1 in acetonitrile and 4 x 109 M 1 s 1 in chloroform. However, no evidence of excimer formation... 

A representative plot is shown in Figure 1.15 this is known as a Stem-Volmer plot, and (1.16) as a Stern-Volmer equation. This method for obtaining reaction rate constants is again a comparative one, since there is competition between the primary reaction step and the quenching process. A value for the quenching rate constant needs to be known, but in many cases this is independent of the substrate and quencher because triplet quenching is controlled by diftusional collision of the two species. So for a particular solvent at a given temperature kf values are available in the literature as an... 

Strong evidence for this sensitization mechanism came from a comparison of the excited state quenching rate constant and the photocurrent enhancement by the same donor. Photoelectrochemical Stem Volmer constants were measured with the aid of Eq. 19 ... 

Example The quenching rate constant can be calculated using the Stem-Volmer equation ... 

The rate constants for hydrogen atom transfer can be measured by Stem-Volmer quenching and have been studied for a wide range of substrates, such as alcohols, hydrocarbons, and tin hydrides 134). The rate constants fall in the range 10 -10 s and qualitatively... 

The accessibility of quenchers to probes in supramolecular structures can be inferred from bimolecular quenching rate constants. The necessary assumptions have been discussed in Section II.B. Quenching can be studied by the decrease of the fluorescence intensity with the addition of a quencher and the data are treated by employing the Stem-Volmer equation ... 

K Rate constant for Stem-Volmer quenching (equal to Tq)... 

Whereas the observed decay profile no longer is characterized by a single decay rate, the steady-state fluorescence intensity becomes dependent on both yobs and fcobs- The typical Stem-Volmer plot is no longer represented by Equation (30.7a), but rather by Equation (30.7b), where obs is defined by Equation (30.6b), fcq is the bimolecular quenching rate constant, ko is the probe s mean excited-state unimolecular decay rate constant, fcobs is the mean observed decay rate constant, yo is the distribution parameter of the Gaussian for the unimolecular decay, and yobs is the distribution parameter for the observed unimolecular decay rate. 

Bimolecular quenching rate constant Radiative rate constant Stem-Volmer constant Rate constant for vibrational relaxation... 

What is quite surprising in these plots is not the (solution-like) Stem-Volmer behavior that is found, but the diffusion coefficients which can be calculated for MV2+. From the luminescence lifetime of Ru(bpy)32+ in the absence of quencher, the kgv values can be converted to bimolecular quenching rate constants kq. The diffusion coefficient is derived from kq by means of a modified Smoluchowsky equation (equation 1), where Dq is the diffusion coefficient of MV2+, Rq is its radius (taken to be 6.7 A), R is the average... 

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