Stem-Volmer quenching rate

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... 

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... 

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 ... 

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

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

Stem-Volmer quenching of the fluorescence of the substrates (D) by an electron acceptor (A) occurred at a nearly diffusion-controlled rate. 

In the case of redox photosensitization (Scheme 6.9B), the Stem-Volmer quenching of fluorescence of the sensitizer (S) at a nearly diffusion-controlled rate and negative free-energy changes from the excited singlet state of S to A is required for the confirmation of the electron transfer from the exited singlet of 5 to A. The hole transfer from the resulting cation radical of S to D depends on the difference between the sensitizer and the substrates in the oxidation potentials. 

The quenching of the trans dimer with oxygen and ferrocene indicates that this product is formed almost entirely from the triplet state. It is possible to calculate the amount of triplet-derived product in benzene by subtracting the amount of product obtained in the presence of oxygen from the amount of product obtained in the absence of oxygen. Such a calculation indicates that acenaphthylene triplets in benzene give both trans and cis dimers in the ratio of 74 26. The triplet state accounts for almost all of the trans product and about 10% of the cis product. The break in the slope of the Stem-Volmer plot for the trans dimer (Figure 10.3) may be attributed to the presence of two excited species which are quenched at different rates. These two species could be (a) two different monomeric acenaphthylene triplet states 7 and T2 or (b) a monomeric acenaphthylene triplet state 7 and a triplet excimer. This second triplet species is of relatively minor importance in the overall reaction since less than 5% of the total product in an unquenched reaction is due to this species. 

Dimers (73) and (74) were formed in approximately equal amounts in all cases, although, as in the cases of 2-cyclopentenone and 2-cyclohexenone, the relative amount of (72) (either cis-syn-cis or cis-anti-cis) was found to vary substantially with solvent polarity. As in 2-cyclopentenone, this increase in the rate of head-to-head dimerization was attributed to stabilization of the increase in dipole moment in going to the transition state leading to (72) in polar solvents. It is thought that the solvent effect in this case is not associated with the state of aggregation since a plot of Stem-Volmer plot and complete quenching with 0.2 M piperylene indicate that the reaction proceeds mainly from the triplet manifold. However, the rates of formation of head-to-head and head-to-tail dimers do not show the same relationship when sensitized by benzophenone as in the direct photolysis. This effect, when combined with different intercepts for head-to-head and head-to-tail dimerizations quenched by piperylene in the Stem-Volmer plot, indicates that two distinct excited triplet states are involved with differing efficiencies of population. The nature of these two triplets has not been disclosed. 

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

Whereas a straight Stem-Volmer plot could not be obtained in quenching studies with octafluomaphthalene, the orders of magnitude of the rate constants of triplet decay ka) and of reaction with olefin ( r) could still be estimated Ad = 6 X 10 s i and ki =2 X 10 1 mole is i >). In view of this rapid decay, a high concentration of olefin is evidently necessary for effective addition. 

Figure 3.62. The light dependence of the Stem—Volmer constant K, i (/, i for diffusional quenching with given exponential rate (Wq — 103 exp[—2(r — cr)/L] ns-1), but at different diffusion in pairs containing the excited molecules (a) D =0.1 Dd, (6) D = D= 10 5cm2/s (dashed line—the same, but in contact approximation), (c) D = WD. Other parameters a = 5A, L = 1.0 A, i = 10ns, ko = f Wq(r)d3r = 1.9 x 105 A3/ns. (From Ref. 200.)...

Figure 3.85. The Stem—Volmer constant of reversible energy quenching at zB = oo, A -/ . related to its irreversible analog K as a function of backward energy transfer rate constant kb related to the forward one, ka. The thick line is an IET result, while the thin lines are obtained with MET at different concentrations of A 4na3NA/3 = 0.05,0.15,0.3 (from bottom to top). The remaining parameters are 47ia3Afi/3 = 0.15, = 2t(/, and kD

TABLE 1 Emission lifetimes (in aerobic aqueous solution), Stem Volmer constants and rate constants for quenching by MV2+ of the excited [Ru(bpy3]2+—like centre of the N-ethylated vinylpyridine copolymer at various salt concentrations. 

It is clear that, by changing the experimental conditions and/or detection wavelength, limiting values can be found for all of the quantities mentioned above from measurements of the fluorescence decay time. The effects of collisional and spontaneous processes can be separated by conventional Stem—Volmer analysis [36]. The concentration, [M], of quenching molecules is varied and the reciprocal of the observed lifetime is plotted against the concentration of M. The quenching rate coefficient is thus obtained from the slope and the intercept gives the rate coefficient for the spontaneous relaxation processes, which is usually the natural lifetime of the excited state. In cases where the experiment cannot be carried out under collision-free conditions, this is the only way to measure the natural lifetime from observation of the fluorescence decay. 

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