Collisional fluorescence quenching

The collisional fluorescence quenching and phosphorescence induction processes have been later observed for a large number of small and medium-size molecules. The interpretation of these results was however rather confusing collision-induced intersystem crossing considered as a transition from the pure singlet to the pure triplet state is in apparent contradiction with the Wigner rule of spin conservation, at least in the case of light collision partners that cannot affect the intramolecular spin-orbit interaction. 

The intensity of fluorescence can be decreased (quenched) by several processes, such as collisions (in solution), excimer or exdplex formation, and energy, electron or proton transfer [5]. In the context of polymer blends studies, the process of collisional fluorescence quenching, either static or dynamic (as described by the Stern-Volmer quenching), is not particularly relevant. 

The data for viscosities of solutions are usually given in the literature in terms of poise or centipoise (cp). For practical reasons cgs units are generally used and poise has the units dyne s cm (the dimensions of viscosity are ML" t ). Comprehensive tables of viscosities are found in the Handbook ofPhysics and Chemistry. In terms of cp the viscosity of water is 0.890,1.002 and 1.787 at 25,20 and 0 C respectively. An example of the large temperature dependence of the viscosity of water is presented by the decrease in collisional fluorescence quenching on cooling of a solution (see section 8.2). 

The Dp and Dq are the diffusion coefficients of probe and quencher, respectively, N is the number molecules per millimole, andp is a factor that is related to the probability of each collision causing quenching and to the radius of interaction of probe and quencher. A more detailed treatment of fluorescence quenching including multiexponential intensity decays and static quenching is given elsewhere/635 There are many known collisional quenchers (analytes) which alter the fluorescence intensity and decay time. These include O2/19 2( 29 64 66) halides,(67 69) chlorinated hydrocarbons/705 iodide/715 bromate/725 xenon/735 acrylamide/745 succinimide/755 sulfur dioxide/765 and halothane/775 to name a few. 

Since iodide is known to act in phospholipid vesicles as a collisional quencher for AF probes [6,9] by a diffusive process, fluorescence quenching was described by the Stem-Volmer relationship [6] ... 

Kasha M (1952) Collisional perturbation of spin-orbital coupling and the mechanism of fluorescence quenching—a visual demonstration of the perturbation. J Chem Phys 20 (l) 71-74. doi 10.1063/1.1700199... 

Mechanisms of fluorescence quenching are usually divided into two groups collisional and complex formation these are also referred to as dynamic and static mechanisms, respectively. The distinction between dynamic and static mechanisms can get blurred, as can the distinction between collision complexes and subsequent more specific interactions (see discussion of diffusion controlled reactions in section 7.4). In addition to these mechanisms there can be internal quenching if the optical density of the solution is high. This last phenomenon, which is the so-called inner filter effect, is only a nuisance and provides no useful information. 

Fluorescence quenching occurs by a variety of mechanisms. Static quenching describes the situation when the interaction between the fluorescent molecule (F) and the quencher (Q) takes place in the ground state. Dynamic quenching occurs when they undergo a collisional process during the lifetime of the excited state of the fluorescent molecule, as shown below ... 

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). 

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. 

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