Photochemical reactions Stern-Volmer plot

As with other quantitative photochemical studies, it is important to design Stern-Volmer experiments carefully the quencher should be chosen to ensure that it interacts only with the excited state that is of interest, the extent of reaction should be small enough to ensure that substrate depletion does not affect the intensity of light absorbed, and the concentration of quencher should not be so small that it is significantly depleted by its sensitized reaction. Stern-Volmer plots may turn out to be non-linear, for example because the quencher interacts with more than one excited state on the reaction pathway, or because two different excited states lead to the same chemical. product, and such results are of value in unravelling the mechanism. 

Figure 5.34 shows the Stern-Volmer plot for triplet quenching of the photochemical addition of benzaldehyde to 2,3-dimethyl-2-butene (cf. Section 7.4.3) by piperylene. The ratio of the quantum yield 4> of oxetane formation without quencher to that with quencher is plotted against the quencher concentration [Q]. From the resulting straight line it may be concluded that the reaction proceeds via a single reactive state which is assigned as (n, r ). 

It should be noted that this expression is a general one that can be used for any photochemical reaction that can be quenched. It is commonly called the Stern-Volmer equation. This equation predicts that if the proposed mechanism is correct, the data, when plotted as 4>a0/4>a vs. [Q], should be linear with an intercept equal to unity and a slope equal to kqr. Linear plots were indeed observed out to large d>°/d> values. Assuming a value of 5 x 10 M 1 sec-1 for the quenching rate constant,(7) the data presented in Table 4.1 were obtained. 

Stern-volmer kinetic relationships This term apphes broadly to variations of quantum yields of photophysical processes e.g., fluorescence or phosphorescence) or photochemical reaction (usually reaction quantum yield) with the concentration of a given reagent which may be a substrate or a quencher. In the simplest case, a plot of (or /M for emission) vs. concentration of quencher, [Q], is linear, obeying the equation... 

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 ... 

Aromatic nitriles are strong oxidants in their excited states (see Table I). Since they fluoresce strongly, the involvement of the singlet states can be easily proved by application of fluorescence quenching techniques. In all of the tested cases, it has been found that the Stern Volmer constant obtained from fluorescence analysis and that obtained from the double reciprocal plots of reaction quantum yield vs. quencher concentration are nearly equal, thus proving that the singlet stale is actually involved In the photochemical reaction. Actually it has been observed that a AG < 0 and polar solvents are necessary (although not sufficient, see Section 3) conditions for the photochemical proc-... 

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