Stern-Volmer product quenching

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). 

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. 

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. 

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. 

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... 

Fig. 2. Stern-Volmer plot for the quenching of (+) cycloadduct 56, ( ) ene product 5 7, and (0) azepinone 58 formation from photoaddition of 3-ethoxy isoindolenone (50) to tetramethyl-ethylene by di-f- butyl nitroxide (DTBN)...

The Stern-Volmer plot for DTBN quenching of photoaddition to c/s-2-butene (Table 6) is consistent with the quenching of longer lived parallel intermediates, not the triplet state of 50, leading to ene products and cycloadducts, respectively. Quenching of a common intermediate such as the triplet state of 50, alone or together with one or more additional intermediates, would not give the linear, noncoincident Stern-Volmer plots. 

In equation (1) K y is referred to as the Stern-Volmer constant Equation (1) applies when a quencher inhibits either a photochemical reaction or a photophysical process by a single reaction. <1>° and M° are the quantum yield and emission intensity (radiant exitance), respectively, in the absence of the quencher Q, while <1> and M are the same quantities in the presence of the different concentrations of Q. In the case of dynamic quenching the constant K y is the product of the true quenching constant kq and the excited state lifetime, t°, in the absence of quencher, kq is the bimolecular reaction rate constant for the elementary reaction of the excited state with the particular quencher Q. Equation (1) can therefore be replaced by the expression (2)... 

Quenching of Ru(blpy)3 by Tl in aqueous solutions also occurs by direct electron transfer (242). Photolysis of Ru(bipy)3 in solutions containing Tl " " produced Ru(bipy)33 with the limiting quantum yield = 2.0 0.4. Stern-Volmer constants for the quenching of the emission from Ru (blpy) 3 and for the production of Ru(bipy)3 " were the same within... 

Equation 3.42 Stern Volmer equation for the appearance of product C by the two step reaction (Scheme 3.3) with quenching of both the lowest singlet and triplet state... 

Triplet sensitization (Section 2.2.2) can be used to determine 3A v for the quenching of a triplet reactant. A complete reaction scheme for sensitization of a reactant A yielding product B from the triplet state would lead to a rather complex Stern Volmer expression, if all steps are treated explicitly. However, if ISC of the sensitizer is very fast and efficient (< >T= 1) and if the concentration of A is chosen to be much higher than that of the quencher added, then we may assume that triplet energy transfer from the sensitizer to A is fast and efficient at all quencher concentrations, so that a plot of versus cq will... 

Incorporation of a I -cyanoethyl group occurs when indoles are irradiated with UV light in the presence of acrylonitrile [13b, 57,58]. With indole, the major product formed is 3-(r-cyanoethyl)indole [13b]. In the case of 3-methylindole, where the 3-position is blocked, the products formed in methanol are shown in Scheme 25 only the 5- and 7-positions of the indole ring are unreactive. The reaction is not quenched by piperylene [13b], while quenching of the indole fluorescence by acrylonitrile follows Stern-Volmer kinetics and yields a rate constant for quenching that is close to the rate... 

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