Stem-Volmer quenching analysis

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... 

This result and the fact that a Stem-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 /i tt triplet products and (77), (78), and (79) were w -> w triplet products. 

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. 

It is seen from Fig. 2 that at the same molar ratios x the PL quenching is more effective for smaller QDs. For the analysis of the PL quenching curves as a function ofx, we have modified Stem-Volmer formalism as follows... 

As we shall see in the next section, Stem Volmer analysis of triplet sensitization or quenching experiments can lead to a determination of ISC efficiencies. 

Two self-calibrated methods are available that do not rely on the knowledge of of a reference compound. Horrocks et al. described an accurate method based on the measurement of triplet triplet absorption by flash photolysis, in combination with Stem Volmer analysis of fluorescence quenching (Section 3.9.8).238 Bromobenzene was used as a heavy-atom quencher of the fluorescence of 9-phenylanthracene. More recently, time-resolved measurements of delayed fluorescence (Section 2.2.4) were analysed to give accurate triplet quantum yields.239... 

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 effect of humic materials on the photolytic micellar system was evaluated in DR s photodegradation. DR solubilized within Tween 80 micellar solution with or without humic materials was determined. In order to calculate the quantum yield, the molar absorptivity of DR was determined by spectrophotometry. The determination of the quantum yield and reaction rates was examined through a pseudo first-order decay rate expression. Quenching and catalytic effects resulting from the humic substances were examined through Stem-Volmer analysis. A reaction mechanism of photolytic decay of DR solubilized within surfactant micelles in the presence of various amount of humic materials was proposed for this purpose. The effect of high and low concentration of humic materials has been accounted for by a designed model. 

In order to quantitatively characterize the photochemical reaction, several mathematical procedures were performed. These included I) Determination of pseudo first-order razte constants and quantum yields and 2) Stem-Volmer analysis of photochemical kinetiacs (quenching and rate enhancement study by humic materials). 

The Combined Stern-Volmer and Perrin Model A model has been proposed by Morishima et al. [97] which takes account of Manning s theory [98] of polyelectrolytes and introduces a modification into the Stem-Volmer equation to describe sphere-of-action (Perrin) quenching this has been termed combined Stern-Volmer and Perrin Analysis and has been adopted [95,96] in an effort to describe quenching of fluorescence from labeled PMAA by T1+ ions, for example. 

However, it is in xwtant to mentkm here that the presence of 5 Trp readues makes the analysis by tbe mo fied Stem-Volmer equation very approxiuate Selective quenching cannot in no way resolve the fluoiEScaice emission spectrum of each Tip residue. However, it allows quantifying the percentage of accessible fluorophores to the quencher. 

Figure 9.19 shows the analysis of the quenching data of Mh (Figure 9.18) in terms of static and dynamic quenching, at 15 and 35°C. In this case, the Stem-Volmer equation can be written as (Eftiiik and Ghiron, 1976 Narasimliulu. 1988) ... 

It is valuable to notice a difference in the method of data analysis for the modified Stem-Volmer plots (Section 8.8.A) and for the quenching-resi vedeiiussion spectra. In analyzing a modified Stem-Volmer plot, one assumes that a fraction of the fluorescence is totally inaccessible to quenchers. This may not be completely true because one component can be more weakly quenched, but still quenched to some extent. If possible, it is preferable to analyze the Stem-Volmer plots by nonlinear least-squares analysis when the/ and K/ values are variable. With this approach, one allows each component to contribute to the data according to its fractional accessibility, instead of forcing one component to be an inaccessible fraction. Of course, such an analysis is more complex, and the data may not be adequate to recover the values of fi and Kj at each wavelength. 

Modification of the Stem-Volmer Analysis for the Study of the Mercury Quenching Effect in Dunaliella tertiolecta 647... 

Initial investigations of the photoinduced electron transfer events were carried out In solution. The conjugated polymers PANi(EB> and PDMA(eb) were readily synthesized and characterized according to literature methods [131,132]. Stem-Volmer analysis of the emission spectrum of [Ru(dmb)3] at 612 nm as a function of increasing PDMA(eb) concentration demonstrated that the conjugated polymer system was capable of quenching the MLCT state (Fig. 13). In this experiment, the emission intensities were corrected for the small amount of competitive absorption at 458 nm from the polymer, and pure polymer samples dem-... 

However, if dynamic quenching is suspected to play a role, we need to modify this analysis. We start by noting that pure dynamic quenching is usually described by the Stem-Vohner relation, which states that the fluorescence intensity ratio Fo/Fob = 1-1- sv[Q] where Ks is the Stem-Volmer constant and as always, [Q] =free concentration of the quencher. If we wanted to describe static quenching or binding in the same way, that is, as a function of the guest/quencher concentration and the fluorescence intensity ratio Fo/F hs, we would get an almost identical expression in the form (34). 

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