Stern—Volmer quenching

As already intimated, the effect of a quencher on the intensity of fluorescence stimulated by steady-state illumination gives information on the rate of the quenching reaction between fluorophor, A, and quencher, Q. Consider the reaction scheme 

These reactions are, respectively, photostimulation [at a rate F(f) species per second], fluorescence from the excited fluorophor, non-radiative de-excitation, and fluorescence quenching. Consider, for instance, that the photostimulation only occurs at time t — f0 that is, F(t) = F05(f — f0). Then, the concentration of the excited fluorophor [A ] varies according to 

If the quenching of fluorescence occurs at a diffusion-limited rate, then k(t) is given by the Smoluchowski expression [3] (19). Equation (35) is then equivalent to eqn. (31) providing t0 = 0. Pursuing this a little further, even if the illumination of the sample is with a constant intensity source, 

The last term is a small correction ( 5%) and is probably beyond experimental detection. As more quenching of fluorescence occurs by quenchers which were in excess of the steady-state concentration, the departures from the Stern—Volmer limiting law becomes more marked. Nevertheless, determining the slope of T0/T([ Q]) versus the quencher concentration and the lifetime of the excited state of the fluorophor in 

There have been many experimental studies of the quenching of fluorescence from aromatic molecules by such species as CC14, CBr4,02. The ratio between fluorescence intensity and (constant) photostimulation intensity is proportional to the fluorescence quenching quantum efficiency. Alternatively, the inverse fluorescence intensity should be linearly related to the quencher concentration the Stern—Volmer law [47]. Birks [6] has 

Collins and Kimball rate coefficient (27). This is the expression discussed by Birks [6] and Nemzek and Ware [7]. At low quencher concentration, the denominator is approximately unity so that the ratio /o//([Q]) is linear in [Q] the Stern—Volmer law [47]. At higher quencher concentration, this rate /o//([Q]) increases more than linearly with [Q]. At very large quencher concentration (experimentally almost unattainable), it increases approximately as 2K To[Q]. This behaviour is summarised in Fig. 8. To a good approximation, at quencher concentrations [Q]  

1l4 TTRD R/nDToy where little fluorescence quenching occurs before the steady-state is established, this ratio is given by 

---

Table 2 lists the apparent Stern-Volmer quenching constants (Ksv) for APh-x, estimated from the initial slopes of the Stern-Volmer plots, along with the... 

The quenching effects of these esters and the phenolic products were also measured using standard Stern-Volmer quenching procedures. 

This result and the fact that a Stern-Volmer quenching plot for (75) and (77) had somewhat different slopes (no statistical analysis given) led the author to propose that the (75) and (76) were n -> n triplet products and (77), (78), and (79) were triplet products. 

In some cases it is possible to obtain a measure of the association constant for intercalation directly from fluorescence quenching data. This method is applicable when the dynamic quenching of the hydrocarbon fluorescence by DNA is small and when the intercalated hydrocarbon has a negligible fluorescence quantum yield compared to that of the free hydrocarbon. If these conditions are met, the association constant for intercalation, Kq, is equal to the Stern-Volmer quenching constant Kgy (76) and is given by Equation 1. 

Table II. Intercalation Association Constants and Stern-Volmer Quenching Constants for Benzo[a]pyrene Metabolites and Metabolite Model Compounds3 with Calf Thymus DNA...

Stern-Volmer Quenching Constants. d Measured in 15% methanol. Taken from refs. 15 and 18. e Measured in 2.5% DMSO. Taken from ref. 9. 

The apparent binding constant K y obtained by Stern-Volmer quenching studies is the product of the number of receptors visited by the exciton and the binding constant of to the cyclophane receptor. For this reason polymer 3 and its monoreceptor model 2 were designed so that the binding constant for methyl viologen to the receptor was known for both systems. This allowed the calculation of the true amplification factor of 67. 

As in the former cases, k2 was calculated from the integrated extinction coefficients,149 k3 + kt was derived from fluorescence quantum yields,149 while k3 and k4 were separately estimated from the maximum quantum yields of photooxygenations at high oxygen concentrations.150 Flash spectroscopy techniques were used in order to determine k5 and k7, while kB was obtained from the Stern-Volmer quenching constant of oxygen.149 The ratio ke/kg was determined from the variation of AOz with the concentration of the anthracene.71 When photodimerization occurred, k13l(kia + k13) was calculated from the maximum yield of... 

As just described, Zimmerman has reported one instance of a dienone rearrangement which definitely does not fit Chapman s general picture. Schuster has provided two reports410,411 of cross-conjugated cyclohexadienones which eliminate radical species. With the trichloro-methyl-substituted ketone 34, both cleavage to the cresol and rearrangement to lumiproduct are quenched by dienes.411 Stern-Volmer quenching plots indicate that the rate at which the excited triplet reacts exceeds 10° sec"1 for both reactions.412... 

Figure 4. "Stern-Volmer" quenching plot of the phosphorescence emission of B phenylpropiophenone according to eq. 4 ...

Equation (3) is the well-known Stern-Volmer equation and kgv = kq r° is the Stern-Volmer quenching constant. In practice, the ratio of lifetime without to that with quencher is plotted against [B] and kq is obtained by dividing the slope by r°. Analogous Stem-Volmer equations can be obtained for the emission intensity and the reaction quantum yield... 

Figure 3.13. The Stern-Volmer quenching constant Ko obtained in Ref. 53 versus the free energy of ionization AG . The open circles are the data borrowed from another work [106], The free-energy dependence shown by the solid line represents the theoretical expectation of Rehm and Weller deduced from their original approach to the simplest reaction mechanism of quenching [53], (From Ref. 107.)...

Table I. Stern-Volmer quenching rate constants for lr2(TMB)42+ and Pt2(P205H2)4 ...

The Stern-Volmer quenching rate constants were determined for all our knots... 

N. Suzuki, I. Mizumoto, Y. Toy a, T. Nomoto, S. Mashiko, H. Inaba (1990). Steady-state near-infrared detection of singlet molecular oxygen A Stern-Volmer quenching experiment with luminol, superoxide dismutase, and Cypridina luciferin analogs. Agric. Biol. Chem., 54,2783-2787. 

---