Quenching curves, Stern-Volmer

In a protein containing more than one Trp residue the Stern Volmer quenching curve often shows downward curvature (Pigure hi). This would result because the less accessible Trp residues will be quenched after the more exposed residue, and will have a lower value of k. However, the direction of curvature will depend on the relative values of and tj, especially in the case of a single chromophore when upward curvafttre is observed. This is due to the fact that at a high quencher concentration diffusion controlled kinetics are not observed, and there will be an appreciable fraction of the chromophore that is surrounded by the quencher at the instant of excitation. That molecule will be immediately quenched and F will be less than that expected for a diffusion controlled process. 

A single Gaussian distribution of conformations (unless pathologically wide (R > 1)) shows little detectable nonlinearity in Stern-Volmer intensity quenching curves. 

Work of Yang242 and of Warwick and Wells243 has shown that two excited states are involved in the photochemical addition of 9-anthraldehyde (56) to 2,3-dimethyl-2-butene. Quenching by di-/-butylnitroxide gave a curved Stern-Volmer plot that could be analyzed to yield lifetimes of 1.0 and... 

A Stern-Volmer plot obtained in the presence of donors for the stilbene isomerization has both curved and linear components. Two minimal mechanistic schemes were proposed to explain this unforeseen complexity they differ as to whether the adsorption of the quencher on the surface competes with that of the reactant or whether each species has a preferred site and is adsorbed independently. In either mechanism, quenching of a surface adsorbed radical cation by a quencher in solution is required In an analogous study on ZnS with simple alkenes, high turnover numbers were observed at active sites where trapped holes derived from surface states (sulfur radicals from zinc vacancies or interstitial sulfur) play a decisive role... 

The curved Stern-Volmer plot for the HH isomer suggests that it arises from both an excited singlet and an excited triplet state. This suggests that about 3, 1.5, and 3% respectively of the HH isomer is singlet derived in each solvent. The 360-nm irradiation of MeTND and benzophenone was explored in a solution of benzene, dibromomethane, and 1-bromobutane. The very efficient quenching of the HH isomer formation by COT implies that the majority of this isomer is formed from a triplet-state precursor. The unquenchable part of the HH isomer appears to be derived from singlet-state precursors, possibly singlet excimers. 

Figure 3.90. The experimental concentration dependence of the Stern-Volmer constant for three different temperatures (points) fitted by DET with the single-channel Marcus transfer rate (thick lines). The thin lines represent the contact analogs of the curves above for the same temperatures (decreasing from top to bottom) and the diffusional control of quenching ( o = oo). (From Ref. 46.)...

More complex chemical species have been determined using different cappings on the QDs surfaces. Shi et al. have recently demonstrated that QDs coated with oleic acid were efficiently quenched by a diversity of nitroaromatic explosives, such as 2,4,6-trinitrotoluene (TNT) or nitrobenzene.49 Different quenching behaviors were observed for the different molecules. Nevertheless, modified Stern-Volmer equations could be used, in most cases, to provide linear calibration curves. Time domain measurements showed that static quenching was the dominant process as no change in luminescence lifetime was observed in the presence of the analyte. 

Finally, a phenomenon called concentration quenching or static quenching can lead to upward curvature of Stern Volmer plots even at moderate quencher concentrations (c q > 0.01 M). Molecules that are located next to a quencher at the time of excitation will be quenched immediately. Therefore, the fluorescence decay curve will be nonexponential initially, exhibiting a very fast initial component. Moreover, the initial depletion of these molecules will result in an inhomogeneous distribution of the remaining excited molecules around the quenchers. As a result, the diffusion coefficient kA is no longer a constant, but becomes a function of time, kd(t), until the statistical distribution of excited molecules is re-established. The impact of these effects has been analysed in detail.231 Intrinsic rates of electron transfer in donor acceptor contact pairs can be extracted from the resulting curvature in Stern Volmer plots.232... 

Although dynamic Stern-Volmer plots for pyrene fluorescence quenching by CA, were curved, activation energies could be derived from the limiting slopes which yield k5r (Eqn. 11b). Activation parameters obtained from the Stern-Volmer treatments agree well with those which assume k3+k2 for pyrene in M is the same as in liquid paraffin(33) and which take k3 from the limiting slopes of Fig. 2a. In both, the activation energy for... 

The interest in the kinetic analysis of bichromophoric systems and of mixtures of aryl ketones having similar triplet excitation energies has continued. Wagner and Nakahira11 have shown, in this context, that in the study of the kinetics of the nonaphenone-p-methoxynonaphenone system, curved Stern-Volmer plots are obtained. They have observed that the presence of the second ketone drastically affects the quenching behaviour. 

For these quenchers, DeSchryver et al. analysed the fluorescence-decay curves numerically. The fluorescence decay is monoexponential at quenching by cesium ion. Here, the inequality (40) has the opposite sense with the right-hand part being large as compared with the deactivation rate of excited pyrene molecules. The quenching obeys the Stern-Volmer law, but the quenching constant depends upon the concentration of micelles ... 

Additional experimental evidence for triplet state involvement is provided by the quenching of both phosphorescence and delayed fluorescence by piperylene, a well known triplet quencher. In fact a study of its quenching effects on the delayed emission demonstrates that at least one of the triplet excitons involved in the fusion process is mobile. This is evident from the phosphoresence quenching curves (Stern-Volmer graphs) of PIVN and 1-ethylnaphthalene illustrated in Fig. 3. 

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