Stern-Volmer constants

Figure 13.11 Stern-Volmer constant Ksv for the quenching of Ru(bpyh2+ (5 juW ) by MV2+ in air-saturated aqueous solutions of PAMAM dendrimers. Adapted from ref. 23...

The drawback of these molecular sensors is their lack of selectivity, as shown by the Stern-Volmer constants (Table 10.4). For instance A-l, 6-methoxy-N-(3-sulfopropyl)quinolinium (SPQ) is mainly used as a Cl -sensitive fluorescent indicator, but its fluorescence is also quenched by several other anions (I-, Br and SCN-, but not by NO ). 

Tab. 10.4. Stern-Volmer constants (M 1) of halide molecular sensors in aqueous solutions (see chemical formulae in Figure 10.29) (data from Biwersi et al., 1994)...

A basic problem of the energy-transfer experiments is that the quenchers used may also react with electron or cationic species, in addition to the excited molecules. If the excited molecules also form after charge recombination (which is now unambiguously established), the quenchers may considerably hinder the formation of excited molecules [8]. Comparison of the results obtained in this manner with those obtained via other techniques shows that the solute technique strongly underestimates the G(Si) value. It should be mentioned that in most of the sensitization experiments unrealistically high K Stern-Volmer constants were obtained [141,142]. 

This expression is known as the Stern-Volmer equation and Ksv as Stern-Volmer constant. Ksv is the ratio of bimolecular quenching constant to unimolecular decay constant and has the dimension of litre/mole. It implies a competition between the two decay pathways and has the ch".acter of an equilibrium constant. The Stern-Volmer expression is linear in quencher concentration and Ksv is obtained as the slope of the plot of 4>f°If vs [Q], if the assumed mechanism of quenching is operative. Here, t is the actual lifetime of the fluorescer molecule in absence of bimolecular quenching and is expressed as... 

Stern-Volmer constant from fluorescence quenching data. 

Stern-Volmer constant from product quantum yield data and limiting quantum yield for triplet sensitized reactions. 

Table II. Stern-Volmer Constants for Quenching of SNA and MSNA Fluorescence by MV in 0.028 M SDS at 25°C.

If the Stern-Volmer constants for oxygen for both sensors are the same, simplifies to... 

Fast generation of the radical ions can be attributed to electron-transfer reaction from the singlet excited state and slow radical generation to that from triplet excited state. Fluorescence of both 326 and 327 was quenched in the presence of CCI4 according to the Stern-Volmer equation. The Stern-Volmer constants were estimated to be 1.55 and 17.7 M 1 for 326 and 327, respectively, and quenching rate constants were estimated to be 1010 and 2.7 x 1010 M 1 s 1. 

In 1977, Scharf and Mattay [123] found that benzene undergoes ortho as well as meta photocycloaddition with 2,2-dimethyl-1,3-dioxole and, subsequently, Leismann et al. [179,180] reported that they had observed exciplex fluorescence from solutions in acetonitrile of benzene with 2,2-dimethyl-l,3-dioxole, 2-methyl-l,3-dioxole, 1,3-dioxole, 1,4-dioxene, and (Z)-2,2,7,7-tetram-ethyl-3,6-dioxa-2,7-disilaoct-4-ene. The wavelength of maximum emission was around 390 nm. In cyclohexane, no exciplex emission could be detected. No obvious correlation could be found among the ionization potentials of the alkenes, the Stern-Volmer constants of quenching of benzene fluorescence, and the fluorescence emission energies of the exciplexes. Therefore, the observed exciplexes were characterized as weak exciplexes with dipole-dipole rather than charge-transfer stabilization. Such exciplexes have been designated as mixed excimers by Weller [181],... 

The overall oxygen sensitivity exhibited by an optical sensor is basically predefined by the Stern-Volmer constant Ksv. The sensitivity of the final optical oxygen sensor increases with Ksv [65]. Generally, high Ksv values are provided by the Pd- and Pt-porphyrin complexes, by Ru(dpp)3, and by pyrene. Fluorescence quenching by oxygen not only affects the fluorescence intensity of the dye, but also has an influence on its lifetime r (Fig. 6) ... 

Stern-Volmer constant Ksv 15 (in acetonitrile at room temperature) that is clearly not compatible with dynamic quenching [14-16], Close spectroscopic examination of solutions having high quencher concentrations shows that there is a weak complex formed between the stilbene and viologen that is characterized by small changes in the stilbene transitions and a weak new transition in the visible... 

This shows that the luminescence intensity decreases as a function of [Q] (Q is called a luminescence inhibitor or quencher ), in parallel to the lifetime variation no additional information is obtained by measurement of I as compared to lifetime measurements. In particular, the emission spectrum of M is not distorted (no spectral shift) when Q is added to the solution. The exact chemical nature of Z cannot be deduced from the luminescence measurement of M because the spectroscopic characteristics of Z do not appear in eqs. (15)—(18). Finally, it should be noted that, strictly speaking, the term luminescence inhibition and the Stern-Volmer equation should apply solely to experiments for which both the lifetime and the luminescence intensity decrease in parallel as a function of the concentration of a quencher . Finally, note that Stern-Volmer constants having a negative value have no physical meaning (Kessler, 1998). 

By substituting this expression into Eq. (3.13), we obtain the ideal Stern-Volmer constant for contact quenching [25,64]. 

At weak quenching (cfc T 1) the expression (3.31) can be subjected to concentration expansion that brings the Stern-Volmer constant to the form... 

Using this expression in Eq. (3.4) for R(t) and integrating the latter in Eq. (3.10), one can get the general contact q and the corresponding Stern-Volmer constant, which is an increasing function of quencher concentration c. There are also a number of competing contact theories that do the same but with slightly different results. They were compared in Ref. 46, reviewed in Section XII. 

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