Quenching mechanism Stem-Volmer Plots

In fact, quenching effects can be evaluated and linearized through classic Stem-Volmer plots. Rate constants responsible for dechlorination, decay of triplets, and quenching can be estimated according to a proposed mechanism. A Stern-Volmer analysis of photochemical kinetics postulates that a reaction mechanism involves a competition between unimolecular decay of pollutant in the excited state, D, and a bimolecular quenching reaction involving D and the quencher, Q (Turro N.J.. 1978). The kinetics are modeled with the steady-state approximation, where the excited intermediate is assumed to exist at a steady-state concentration  

Photochemcial reaction, in fact involved several complex reaction. However, several assumptions may be made based on earlier observations or hypotheses regarding the specific mechanisms of transformation, resulting in consideration simplification of the kinetic expression. Nevertheless, if the singlet state is responsible for the reaction, a simpler mechanism may be proposed  

In the presence of hydrogen sources and electron donors, the reaction mechanisms for photodechlorination of aryl halides may follow two general mechanisms, i.e., homolysis and electron transfer processes. In the homolysis process, C-Cl bond photolytic cleavage is the primary reaction, forming an aryl radical, which abstracts a hydrogen from the hydrogen source, and gives the product ArH. 

Hawari J. et al. (1992) reported that aryl halides are photo-dehalogenated in the presence of the hydrogen source, 2-propanol (in the presence of ionic metal alkaoxides) through 

Upon irradiation, eq 36 is the initiation step for free radical formation, while in later steps, 2-propanol has been used as both hydrogen source (eq 38) and an electron donor (eq 40). Upon inspection, eqs 36, 37 and 38 represent the homolysis process, and eqs 38, 39, 40 and 41 represent at the electron transfer process mentioned previously. 

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The commercialization of inexpensive robust LED and laser diode sources down to the uv region (370 nm) and cheaper fast electronics has boosted the application of luminescence lifetime-based sensors, using both the pump-and-probe and phase-sensitive techniques. The latter has found wider application in marketed optosensors since cheaper and more simple acquisition and data processing electronics are required due to the limited bandwidth of the sinusoidal tone(s) used for the luminophore excitation. Advantages of luminescence lifetime sensing also include the linearity of the Stem-Volmer plot, regardless the static or dynamic nature of the quenching mechanism (equation 10) ... 

Shown in Figure 7 are the Stem-Volmer plots of emission intensities and lifetimes, monitored at 630 nm, as a function of MDESA concentration. Both the static (intensity) and the dynamic (lifetime) components are nonlinear and indicate that the quenching mechanism is comphcated. The extent of the static reaction (attributed to MDESA " anions associated with Ru(bpy)3 cations)... 

Decisive evidence on whether static quenching is appreciable in a particular system can come from fluorescence lifetime measurements. If the mechanism represented by Equation (6.24) is correct, fluorescence emission occurs only from free A molecules, so the lifetime is unaffected by static quenching and is given by Equation (6.16) above, which may be written in the form of a lifetime Stem-Volmer relation (i.e., to/t[q] = 1 -f- fesroiQ]). A plot of to/t[q] against quencher concentration [Q] will be linear (despite the non-linearity of the corresponding plot of /o//[Q]) with unit intercept and slope whence the value of... 

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