Quenching action

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]). 

The study of the mechanism of photoinitiation is complicated by the quenching action of dioxygen (see page 122). The values of the triplet state of selected compounds used as photosensitizers are given in Table 3.17. 

Quenching methods, in which the fluorescence of a substance is reduced by the quenching action of an analyte, can also be used in fluorescence analysis. Such methods are particularly well suited to the analysis of gases. 

Various kinds of information can be expected from the high pressure combustion and flame experiments Reaction kinetics data for conditions of very high collision rates. Results about combustion products obtained at high density and with the quenching action of supercritical water, without or with flame formation. Flame ignition temperatures in the high pressure aqueous phases and the ranges of stability can be determined as well as flame size, shape and perhaps temperature. Stationary diffusion flames at elevated pressures to 10 bar and to 40 bar are described in the literature [12 — 14]. 

This assertion of experimental fact is not extracted simply from superficial examination of experimental evidence. Emission yields do vary considerably, but the effects can usually be associated with quenching by other solute molecules in condensed systems. Since a variety of common solvents do not function as quenchers in this sense, the quenching phenomenon is usually associated with special effects. The commonest explanations are transfer of electronic excitation to the quencher and some kind of chemical reaction. However, other more subtle quenching action has also been observed. 

Optically active aryl sulfoxides undergo facile photoracemiza-tion.472 The photo-inversion is sensitized by naphthalene but only partially quenched by piperylene.473 The quenching action was originally interpreted as indicating a triplet mechanism for isomerization. 

Since any quenching action is a bimolecular process, it is essential that the molecules M and Q should be in relatively close contact, but not necessarily in hard sphere (van der Waals) contact. Theoretical models lead to the distance dependence of the quenching rate constants as exponentials or sixth powers of r, the centre-to-centre distance of M and Q. Since these distance dependences are very steep, it is convenient to define a critical interaction distance r at which the quenching efficiency is, this being the distance at which 50% of the molecules M decay with emission of light (or undergo a chemical reaction) and 50% are quenched by some nearby molecule Q. 

Since the transition Si — T0 is strongly spin forbidden in such a light molecule, the gas phase lifetime of 02 reaches some 45 min ( ) and is then still shorter than the natural radiative lifetime. In solution the lifetime decreases as a result of quenching actions to somewhere between a few ps and a few ms, depending on the solvent water 2 ps ethanol 5 ps cyclohexane 15 ps chloroform 60 ps. 

These molecules have no absorption in the near UV, so they cannot act as internal filters. There is no evidence that they could act as quenchers of the excited impurity chromophores in common polymers, and it has been mentioned already that such quenching action would be, in any case, unlikely to be important in relatively rigid systems such as polymers. This remark does not apply to free radical scavengers, because a free radical has an unlimited lifetime since it can disappear only through a chemical reaction with another open-shell molecule. 

Figure 8.21 gives a schematic representation of the type of photophysical experiments which can be performed in L-B films. The distance dependence of a quenching action through energy transfer or electron transfer can be... 

The extra w initially present in Slurries will drive equilibrium (1) towards C02 and H2. This effect is somewhat counteracted by the reduction in detonation temp because of the quenching action of w contained in the undetonated Slurry, Lower temps mildly favor an equilibrium shift to the left... 

Sripriya, G. K., Chandrashekharan, K., Murthy, V. S., and Chandra, T. S. (1996). ESR spectroscopic studies on free radical quenching action of finger millet (Eleusine coracana). Food Chem. 57, 537-540. 

Learmonth, G. S. Thwaite, D. G. Flammability of plastics IV. An apparatus for investigating the quenching action of metal halides and other materials on premixed flames, British Polymer Journal, 1970, 2, 249-253. 

Figure 3 shows the quenching action of the viologen on cyanine dye fluorescence, obtained by varying the ratio of arachidic acid to acceptor in the contact layer. This allows the average distance between viologen molecules to be calculated. 

Directly measured rate constants for 2 quenching by a series of substituted TWV-dimethylanilines indicate that a partial charge-transfer interaction may be responsible for the quenching action.274... 

The protection offered by CD against the quenching action of foreign substances depends mainly on the geometry of the complex and information about the degree of the protection is scattered in various papers (vide infra). 

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