Quenching process

Quenching Processes.—Complexes formed between excited states and ground states are now recognized to be very widespread in all aspects of photochemistry. Consequently they are the subject of numerous papers. 

Cline Love and L. M. Upron, Anal. Cliim. Acta, 1980, 118, 325. 

Tashita, and N. Malaga. Chem. Phys. Lett.. 1980, 75, 220. 

Picosecond spectroscopy has been used to study the photodissociation of Ij-aromatic complex and the formation of iodine atom-aromatic complexes. Shizuka, Nakamura, and Morita have studied the anion-induced fluorescence of aromatic molecules. Electron transfer from the anion to the excited aromatic seems to be the key step (Table 20). Triplet formation has also been studied by nanosecond laser spectroscopy. The rate constants are shown in Table 21. 

The fluorescence of 2-ethoxynaphthalene is quenched by methyl benzoate through an exciplex. The importance of the rate of formation of exciplexes and radical ions in the encounters of excited aromatic esters with aliphatic amines has been stressed by Costa and Macanita. A charge-transfer mechanism is clearly demonstrated in the quenching of excited aromatic esters by triethylamine.  

In bimolecular processes, the lowest excited state of these complexes can act as an energy donor [Eq. (56)], electron acceptor [Eq. (57)] and electron donor [Eq. (58)]. 

Consider, for example, the quenching of Cr(bpy)3+, Ru(bpy)3+ and Os(bpy)2+ by Fe(CN)6- 16°). In principle, the quenching may occur, as we have seen before, by energy transfer or by reductive or oxidative electron transfer  

Other interesting studies on the quenching of polypyridine complexes are reported in Refs.171-180  

The intramolecular processes responsible for radiative and radiationless deactivation of excited states we have considered so far have been uni-molecular processes that is, the processes involve only one molecule and hence follow first-order kinetics. 

We now look at the intermolecular deactivation of an excited molecule by another molecule (of the same or different type), a process called quenching. Any substance that increases the rate of deactivation of an electronically-excited state is known as a quencher and is said to quench the excited state. 

By measuring the reduction in fluorescence intensity in the presence of a quencher it is possible to determine the effect of quenching on the Si state. 

Fluorescence quantum yields approach their maximum values when measured in very dilute solution from which all impurities are rigorously excluded. If the concentration of the fluorescent compound is increased, or if other substances are present in the solution, the fluorescence quantum yield will be reduced. 

Consider the deactivation processes of the excited Si state in the presence of a quencher, Q  

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The interpretation of emission spectra is somewhat different but similar to that of absorption spectra. The intensity observed m a typical emission spectrum is a complicated fiinction of the excitation conditions which detennine the number of excited states produced, quenching processes which compete with emission, and the efficiency of the detection system. The quantities of theoretical interest which replace the integrated intensity of absorption spectroscopy are the rate constant for spontaneous emission and the related excited-state lifetime. 

Nonradiative reiaxation and quenching processes wiii aiso affect the quantum yieid of fluorescence, ( )p = /cj /(/cj + Rsiative measurements of fluorescence quantum yieid at different quencher concentrations are easiiy made in steady state measurements absoiute measurements (to detemrine /cpjj ) are most easiiy obtained by comparisons of steady state fluorescence intensity with a fluorescence standard. The usefuiness of this situation for transient studies... 

Samples are usually in solution, but soHds (often fro2en solutions) yield narrow-line spectra that are useful in distinguishing components of mixtures. In phosphorimetry soHd sampling may be necessary to minimi2e quenching processes. 

X-ray diffraction peaks were rather broad with coherence lengths as low as 20 nm and this was attributed to rapid quenching. It was proposed that the carbon atoms are arranged in polyyne chains (n = 4) along the c-axis. The density of Carbolite (1.46 g-cm ) is lower than values for other carbynes and for diamond and graphite - hence the name - and this was attributed to a rapid quenching process. 

This effect, which can also be produced if fluorescent substances are applied to the chromatogram by spraying or dipping after development, is an absorption effect and not a quenching process in the true sense of the word. It is correct to refer to fluorescence or phosphorescence diminishing. The more absorbant sample molecules there are present in the zone the darker this will appear (Fig. 4B). 

Heterocycles via Pd-catalyzed molecular quenching process 98PAC1047. 

In turn, 1O2 is a very electrophilic excited state species of molecular oxygen that interacts efficiently with electron-rich molecules, such as aminoadd residues of proteins like histidine, metionine, tryptophan, tyrosine, etc., by both physical and chemical quenching processes, eqns. 9 and 10 (Davies, 2003 Bisby et al., 1999). 

Quenching process of a flat hmit flame, propagating downward from the open end of the tube in mixture with 2.20% CjHg observed by schlieren system with superimposed direct photography. Square tube 125 mm x 125 mm x 500mm. Time interval between frames 0.3 s. 

Poinsot, T., Veynante, D., and Candel, S., Quenching processes and premixed turbulent combustion diagrams, /. Fluid Mech., 228, 561, 1991. 

Fig. 3a, b. Schematic representation of (a) conventional fluorescent sensor and (b) fluorescent sensor with signal amplification. Open rhombi indicate coordination sites and black rhombi indicate metal ions. The curved arrows represent quenching processes. In the case of a den-drimer, the absorbed photon excites a single fluorophore component, which is quenched by the metal ion regardless of its position... 

The same arguments as for Tb(III) hold for Ce(III) and Pr(III). The species PrWioOfg shows only weak luminescence due to this quenching process [40]. In YVO4 these ions behave as killers of luminescence. 

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]). 

For anthracene the concentration quenching process could be the decomposition of the excimer to two ground state anthracenes, as shown in... 

We should now examine the nature of the concentration quenching process that we proposed in our singlet dimerization mechanism. There are a number of possibilities, as follows. 

Avital, S., V. Brumfeld, and S. Malkin. 2006. A micellar model system for the role of zeaxanthin in the non-photochemical quenching process of photosynthesis—Chlorophyll fluorescence quenching by xan-thophylls. Biochim. Biophys. Acta 1757 798-810. 

Perhaps the most obvious method of studying kinetic systems is to periodically withdraw samples from the system and to subject them to chemical analysis. When the sample is withdrawn, however, one is immediately faced with a problem. The reaction will proceed just as well in the test sample as it will in the original reaction medium. Since the analysis will require a certain amount of time, regardless of the technique used, it is evident that if one is to obtain a true measurement of the system composition at the time the sample was taken, the reaction must somehow be quenched or inhibited at the moment the sample is taken. The quenching process may involve sudden cooling to stop the reaction, or it may consist of elimination of one of the reactants. In the latter case, the concentration of a reactant may be reduced rapidly by precipitation or by fast quantitative reaction with another material that is added to the sample mixture. This material may then be back-titrated. For example, reactions between iodine and various reducing agents can be quenched by addition of a suitably buffered arsenite solution. 

The readily available, nonracemic indoloquinolizidine template 471 has been studied as a substrate for the construction of frameworks related to bioactive natural products. The lithiated dithiolane 472 served a dual role in its reaction with 471, both as a nucleophile giving the non-isolated intermediate 473, and, in the same pot, as an electrophile during the quench process. This reaction afforded compound 474 as a single diastereomer <2006TL1961>. 

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