Quenching collisional

Figure 7.3 Radiative (absorption, stimulated emission, fluorescence) and non-radiative (quenching, collisional energy transfer, elastic scattering) processes in a molecular system with electronic, vibrational and rotational energy levels...

During the past two decades, considerable effort has been expended in the development of phosphorimetric methods that can be carried out at room temperature. The first observations of room-temperature phosphorescence were made with the analyte bound to a solid support, such as filter paper or silica gel. In these applications, a solution of the analyte is dispersed on the solid, and the solvent is evaporated. The phosphorescence of the surface is then measured. The rigid matrix minimizes deactivation of the triplet state by collisional quenching. Collisional quenching has much more of an effect on phosphorescence than on fluorescence because of the much longer lifetime of the triplet state. 

Michaels C A, Lin Z, Mullin A S, Tapalian H C and Flynn G W 1997 Translational and rotational excitation of the C02(00°0) vibrationless state in the collisional quenching of highly vibrationally excited perfluorobenzene evidence for impulsive collisions accompanied by large energy transfers J. Chem. Phys. 106 7055-71... 

The acetone-sensitized photodehydrochlorination of 1,4-dichlorobutane is not suppressed by triplet quenchers (20), but the fluorescence of the sensitizer is quenched by the alkyl chloride (13). These observations imply the operation of a mechanism involving collisional deactivation, by the substrate, of the acetone excited singlet state (13,21). This type of mechanism has received strong support from another study in which the fluorescence of acetone and 2-butanone was found to be quenched by several alkyl and benzyl chlorides (24). The detailed mechanism for alkanone sensitization proposed on the basis of the latter work invokes a charge-transfer (singlet ketone)-substrate exciplex (24) and is similar to one of the mechanisms that has been suggested (15) for sensitization by ketone triplets (cf. Equations 4 and 5). 

In this group, there are collisional interactions, which are responsible for quenching of excited states by molecular oxygen, paramagnetic species, heavy atoms, etc. [1, 2, 13-15]. Probability of such quenching can be calculated as ... 

Anion concentrations can also be monitored through lanthanide luminescence. Once again, a wide range of pathways can be responsible for luminescence quenching. For instance, complex (60) with a pendant phenanthridinium group exhibits halide ion-dependent luminescence properties as a result of collisional quenching of the phenanthridinium-centered singlet state by halide ions.165... 

Quenching is the reduction in fluorescence intensity and can be caused by various processes. It occurs either during the lifetime of the excited state or in the ground state. Quenching processes that happen in the excited state are collisional quenching, charge transfer reactions, or energy transfer. The latter is the basis for FRET probes but the other events happen as well under certain conditions and it is important to consider them. 

Energy transfer, as described by Forster [78], requires a long range dipole-dipole interaction between the donor and the acceptor fluorophore. This energy transfer is possible at distances between 2 and 10 nm. Contrary to what happens in collisional quenching, there is no need for physical contact between the two molecules. 

Quenching of a photoluminescent dye (D) by molecular oxygen (O2) is a non-chemical collisional process which can be presented as follows ... 

This hybrid approach can significantly extend the domain of applicability of the AIMS method. The use of interpolation significantly reduces the computational effort associated with the dynamics over most of the timescale of interest, while regions where the PESs are difficult to interpolate are treated by direct solution of the electronic Schrodinger equation during the dynamics. The applicability and accuracy of the method was tested using a triatomic model collisional quenching of Li(p) by H2 [125], which is discussed in Section III.A below. 

The fluoresence lifetimes calculated for TIN in low viscosity alcohols are approximately proportional to the solvent viscosity (12) which suggests that in these solvents there is a non-radiative process related to. the rotational diffusion mobility of the TIN molecule. The observed extent of the quenching, however, is significantly greater than that expected due to viscosity effects alone and cannot be explained by a collisionally-induced, Stern-Volmer type process involving methanol molecules (25.) as the appropriate plot is non-linear. 

The methods of anion detection based on fluorescence involve quenching, complex formation, redox reactions and substitution reactions (Fernandez-Gutierrez and Munoz de la Pena, 1985). This chapter will be restricted to anion molecular sensors based on collisional quenching (in general, they exhibit a poor selectivity) and on recognition by an anion receptor linked to a fluorophore (fluoroionophore). 

Many fluorescent molecular sensors for halide ions (except F ) are based on collisional quenching of a dye. In particular, the determination of chloride anions in living cells is done according to this principle. Examples of halide ion sensors are given in Figure 10.29. 

It is important to notice that a change in lifetime is not a necessary result of a change in fluorescence intensity. For instance, the Ca2+ probe Fluo-3 displays a large increase in intensity on binding Ca2+, but there is no change in lifetime. This is because the Ca-free form of the probe is effectively nonfluorescent, and its emission does not contribute to the lifetime measurement. In order to obtain a change in lifetime, the probe must display detectable emission from both the free and cation-bound forms. Then the lifetime reflects the fraction of the probe complexed with cations. Of course, this consideration does not apply to collisional quenching, when the intensity decay of the entire ensemble of fluorophores is decreased by diffusive encounters with the quencher. 

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