Stern-Volmer quenching rates

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... 

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... 

Stern-Volmer quenching constant (iTsv) Constant for a given quenching process it is the product of the bimolecular rate constant for the reaction of quencher and the luminescent species and the luminescence lifetime in the absence of quencher... 

Another useful technique for measuring the rates of certain reactions involves measuring the quantum yield as a function of quencher concentration. A plot of the inverse of the quantum yield versus quencher concentration is then made Stern-Volmer plot). Because the quantum yield indicates the fraction of excited molecules that go on to product, it is a function of the rates of the processes that result in other fates for the excited molecule. These processes are described by the rate constants (quenching) and k (other nonproductive decay to ground state). 

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. 

Dynamic quenching of fluorescence is described in Section 4.2.2. This translational diffusion process is viscosity-dependent and is thus expected to provide information on the fluidity of a microenvironment, but it must occur in a time-scale comparable to the excited-state lifetime of the fluorophore (experimental time window). When transient effects are negligible, the rate constant kq for quenching can be easily determined by measuring the fluorescence intensity or lifetime as a function of the quencher concentration the results can be analyzed using the Stern-Volmer relation ... 

This is the Stern-Volmer relationship with = k /k(j, and is an important basis for determining quenching rate constants after pulsed excitation. The quantum yield of (pro-duct)o can be measured without (0q) and with (0) quencher under continuous excitation (0 = moles of product/einsteins of light absorbed by system). Assuming that a steady state concentration of S exists in both cases. 

A representative plot is shown in Figure 1.15 this is known as a Stern-Volmer plot, and 0.16) as a Stern-Volmer equation. This method for obtaining reaction rate constants is again a comparative one, since there is competition between the primary reaction step and the quenching process. A value for the quenching rate constant needs to be known, but in many cases this is independent of the substrate and quencher because triplet quenching is controlled by diffusional collision of the two species. So for a particular solvent at a given temperature K, values are available in the literature as an... 

Wilkinson has recently described a novel approach.113 It has long been known that solvents containing heavy atoms markedly quench the fluorescence of aromatic hydrocarbons, and it has been shown that this effect arises from an enhancement of the rate of intersystem crossing. Thus the ratio of phosphorescence to fluorescence for naphthalene irradiated at 77°K can be increased more than a hundredfold upon addition of some isopropyl iodide to the solvent.114 The same effect has been noted upon changing from hydrocarbon glasses to frozen krypton and xenon matrices.115 Wilkinson found that the decrease in fluorescence intensity from irradiated solutions of anthracene and some of its derivatives upon addition of bromobenzene is attended by an increase in T-T absorption intensity.116 The fluorescence quenching follows the Stern-Volmer law ... 

The problem of accurately determining rates of quenching is important not only for understanding energy transfer but also for estimating rates of physical and chemical reactions of excited triplet species. Quenching studies of the Stern-Volmer type184 yield values of kQrT, where rT is the lifetime of the triplet species and kq is the rate constant with which some compound quenches it. Since quantum-yield and product-yield measurements allow rT to be factored into rate constants for individual reactions, absolute values of these reaction rate constants can be determined provided that the absolute value of... 

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