Quencher ions

The ionic strength effect is not limited to ksv variation as described by eq. (22). The addition of large amounts of electrolytes may also modify the quencher solubility and thus its efficiency. This effect has been used by some authors, in systems very different from those examined in this work, in order to determine the association constant of the inhibitor salt (Mac, 1997 Mac and Tokarczyk, 1999) as the electrolyte concentration is increased, the quencher ion associates, so that the effective concentration of the inhibitor ion decreases, leading to a downward curvature of the Stern-Volmer plot. Such a curvature can be quantitatively related... 

In iron bearing clays the time law of the luminescence decay of Ru(byp)3 + is multiexponential (29). In the long time limit (>1 /is) the decay approaches an exponential law, but at shorter time ( 300 ns), a more complex decay has been observed. The decay mechanism has been explained by a simple localized model based on the assumption that on each particle the quenching process can be described as occurring in an ensemble of small independent subsystems. Each subsystem is composed of an excited probe system (with a very reduced mobility) and of the nearest lattice sites of the solid which may be occupied by the quencher ions. For the sake of simplicity, it has been assumed that the adsorbed probe molecules occupy the sites of a superlattice which matches the lattice containing randomly distributed quenchers (Fe, for instance)... 

Figure 5. Sketch of the model suggested for the multi-exponential decay observed for the Ru(bpy)32+ luminescence probe molecule decay in the interlamelar space. To a square lattice for quencher ions is superimposed a (1/2, 1/2) translated square lattice for the probe molecules. (Reprinted with permission from ref. 29. Copyright 1984 Royal Society of Chemistiy.)...

The ability of a luminescence quencher molecule to adsorb on the surface of ultradispersed colloidal semiconductor in aqueous solutions was shown to depend mainly on the charge of the colloidal particle surface and the charge of quencher ions. 

PMAA and Tl+ from samples labeled at various cites within the macromolecule. They reasoned that as a population of fluorophores is isolated from the quencher ions, this requires use of a two-state model in which the probes are divided into an accessible fraction,/a, and an inaccessible fraction,/b, (/b 1 fj- If it is assumed that... 

Steady-state and time-resolved studies of the excited properties of the [Ru(bpy)3] adsorbed on a variety of clay minerals have been carried out by some research groups in order to understand the adsorbed states of [Ru(bpy)3] more clearly (77-87). Habti et al. have reported nonexponential decay of the excited state of [Ru(bpy)3] adsorbed on a variety of clay minerals with different iron contents (78). From the effects of iron content on the decay profiles, they point out the quenching effect of the irons within the lattice of the minerals and the essentially immobile character of adsorbed [Ru(bpy)3] on the microsecond time scale. Each adsorbed probe ion is able to interact with a vCTy limited number of neighboring quencher ions around the adsorption sites. The total quenching probability for a particular probe is determined by the quencha- concentration in the solid and by the number of sohd particles in contact with the probe. They have also mentioned that the degree of swelling affects the quenching. 

Fluorescence quenching is due to non-radiative loss of energy from the excited state as a consequence of either collision with a quencher ion (or molecule) in solution or by formation of a non-fluorescent or poorly fluorescent fluorophore-quencher complex. In both cases the quenching process follows the Stem-Volmer equation ... 

Here and (pi are the activity coefficients for the quencher ion in the aqueous phase and the micellar potential at any given ionic strength The value of the ionic strength can... 

After examining various possibilities, we ascribe this new component to the geminate ionpair of the polymer and quencher ion radicals. Some of the quencher ion radicals fail to diffuse out from the microdomain around polymers into the bulk and recombine with the original counter ions (31). The relative yield and the lifetime of this ionpair increase as the n-value does, independent upon the kind of quencher. In the case of PVCz-3700, the whole decay was analyzed as the first order decay and its time constant was obtained to be 42 ys (32). A similar behavior was also observed in a polymer having crowded chromophores (33). We concluded that the geminate recombination is a quite general behavior of polymer systems. 

Factors such as dissociation, association, or solvation, which result in deviation from the Beer-Lambert law, can be expected to have a similar effect in fluorescence. Any material that causes the intensity of fluorescence to be less than the expected value given by equation (2) is known as a quencher, and the effect is termed quenching it is normally caused by the presence of foreign ions or molecules. Fluorescence is affected by the pH of the solution, by the nature of the solvent, the concentration of the reagent which is added in the determination of inorganic ions, and, in some cases, by temperature. The time taken to reach the maximum intensity of fluorescence varies considerably with the reaction. 

Even couples of lanthanide ions show this quenching process. The Ce(III) and Eu(III) ions, for example, quench each other s luminescence [127]. Here a MMCT state with Ce(IV)-Eu(II) character is responsible. In solid [Ce <= 2.2.1] cryptate there occurs energy migration over the cryptate species. Also here [Eu c 2.2.l] acts as a quencher [128]. The quenching action is restricted to short distances (about 12 A [129]). 

It is logical that tri-p-tolylamine should be a better quencher than triphenylamine since it is better able to stabilize the resulting radical cation. Likewise, a more polar solvent would tend to stabilize the ion pair.<71>... 

The photophysical properties of lanthanide ions are influenced by their local environment, the nature of the quenching pathways available to the excited states of sensitizing chromophores, and the presence of any available quenchers (as we have seen when discussing bioassay). All of these factors can be exploited for the sensing of external species. 

Chemically inert triplet quenchers e.g. trans-stilbene, anthracene, or pyrene, suppress the characteristic chemiluminescence of radical-ion recombination. When these quenchers are capable of fluorescence, as are anthracene and pyrene, the energy of the radical-ion recombination reaction is used for the excitation of the quencher fluorescence 15°). Trans-stilbene is a chemically inert 162> triplet quencher which is especially efficient where the energy of the first excited triplet state of a primary product is about 0.2 eV above that of trans-stilbene 163>. This condition is realized, for example, in the energy-deficient chemiluminescent system 10-methyl-phenothiazian radical cation and fluoranthene radical anion 164>. 

Neither the electronic absorption nor the emission spectrum of Re2Cl8 changes in the presence of the quenchers, and no evidence for the formation of new chemical species was observed in flash spectroscopic or steady-state emission experiments. The results of these experiments suggest that the products of the quenching reaction form a strongly associated ion pair, Re2Cl8 D+. 

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