Quenching Stern-Volmer plots

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). 

Figure 10 illustrates Stern-Volmer plots for the fluorescence quenching of APh-x by MV2+ and SPV in aqueous solution [74]. With MV2+, the quenching is so effective that it occurs at very low quencher concentrations (in the range of 10 6 M), whereas with SPV, it proceeds to about the same extent at two-orders of magnitude higher quencher concentration (in the range of 10 4 M). 

Table 2 lists the apparent Stern-Volmer quenching constants (Ksv) for APh-x, estimated from the initial slopes of the Stern-Volmer plots, along with the... 

Figure 11 shows Stern-Volmer plots for fluorescence quenching of the amphiphilic cationic copolymer QPh-x [74]. The quenching of QPh-x with MV2+ is expected to be much less effective than that of APh-x. The quenching data for the QPh-x system are presented in Table 3. For comparison, the data for a related... 

Fig. 1. a) UV-Vis absorption and fluorescence emission spectra of riboflavin (RF, 20 pM) and Gum Arabic aqueous solutions at pH 7 (phosphate buffer 100 mM). b) Transient absorption spectra of RF (35 pM) in N2-saturated MeOH-Water (1 1) solution. The insets show the transient decay at 720 nm for the RF species and the Stern-Volmer plot for the quenching of 3RF by GA, eqn 11. 

Figure 3. Stern-Volmer plots for quenching of yellowing following photolysis of PVCa solutions in methylene chloride by (a) piperylene and (b) naphthalene. Yellowing is measured as the increase in absorption at 390 nm.

Figure 4. Stern-Volmer plots and quenching constants derived from the fluorescence quenching of DMA (T), 1,2,3,4-tetra-hydro-BA ( ), 5,6-dihydro-BA (A), 8,9,10,11-tetrahydro-BA ( ) and anthracene ( ) by DNA in 15% methanol at 23° C. Emission and excitation wavelengths and details concerning the experimental conditions are given in refs. 12 and 14. The open symbols, o and V, show I /I for 1,2,3,4-tetrahydro-BA and DMA respectively in denatured DNA([P04"] 4.4 x 10 4 M).

Figure 6.1 A Stern-Volmer plot of fluorescence quenching...

Figure 10.3 Stern-Volmer plot for the quenching of the Norrish type 2 photoreaction of hexan-2-one by penta-1,3-diene...

A linear relationship is thus obtained, as in the case of the Stern-Volmer plot (Eq. 4.10), but there is no change in excited-state lifetime for static quenching, whereas in the case of dynamic quenching the ratio I0/I is proportional to the ratio to/t of the lifetimes. 

Figure 4.2 summarizes the various cases of quenching, together with the possible origins of a departure from a linear Stern-Volmer plot. 

Figure 5.3. Simulated Stern-Volmer plots of the ratio of the initial fluorescence intensity F0 to the intensity Fin the presence of quencher of concentration [Q] showing (a)static quenching, (b) dynamic quenching (linear), and (c) binding and/or inaccessible quenchers.

To verify the quenching interaction between the Re-complex and the di-methyl-/7-toluidine, a Stem-Volmer plot of the results of a concentration dependent study of Re-complex fluorescence intensity as a function of amine concentration in fluid MMA was prepared (Figure 3). The samples contained 1.6 X IQ- mol Re-complex, and up to a maximum of 2.6 X 10 mol of amine, in —2.5 g of MMA. Re-complex CT band peak heights at 612 nm were measured from uncorrected fluorescence spectra taken in single scans following excitation at 350 nm. The Stern-Volmer plot is linear over the range of amine concentrations studied. A linear Stem-Volmer plot. 

The transient phenomena of c-Sf that involved the isomerization and HT quenching of c-Sf using AN are shown in Scheme 12. According to Scheme 12, the linear Stern-Volmer plots of vs. [AN] afforded = 120 30 psec being one-half... 

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... 

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... 

Fig. 6. Stern-Volmer plot of the quenching of type II photoelimination of 2-pentanone and 2-hexanone.

The observed quenching effects suggest that the excitation energy of the reactive triplet may be close to the 71 kcal Zimmerman estimated from the phosphorescence spectrum of 35. On the other hand, lumi-santonin, also an enone, must have a relatively low-energy reactive triplet, since its rearrangement is sensitized by Michler s ketone.401 From a Stern-Volmer plot of the effect of naphthalene on the quantum yield for photorearrangement of 35, kjkr was measured to equal 7000.416 Thus if naphthalene quenches triplet 35 at the diffusion-con-... 

Figure 3.41 provides an example of a Stern-Volmer plot. In this case the triplet excited state Tj of an aromatic molecule is quenched by the external heavy atom effect. The Stern-Volmer plot must by definition have an... 

Figure 3.41 Example of the Stern- Volmer plot of dynamic quenching. The fluorescence of anthracene is quenched by bromobenzene (heavy atom effect)...

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