Transient Effects in Quenching

The theory for transient effects is complex and has been presented in a monograph on diffijsion-controlled reactions. Transient effects were first identified by Smoluchowski, who considered diffiision-controlled reactions between particles in solution. The rate constant for reaction between the particles was shown to be time-dependent  

Rgui 9.26. Coo erisooofdieRBCiedODQaioiidsfircollisioml qaenchinf of fluofescence. Revised ttMlicprimed. widtpennsaao. fron Rrf. 100. Copyri C 1996. AmMicttiOiefDic Sodety. 

Unfortunately tiie use of these queadiing models requires rather complex theoiy and andysis. Fcm tiie Smotuchowsld, RBQ or DDQ models, tiie intensity dec is given by 

The function erfe(y) is the cooqilemeiti of tiie oror function. 

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In this chapter we describe more advanced topics in quenching. Quenching in membranes is desoibed in some detail because of the numerous applications. These include localization of membrane-bound probes, estimation of diffusion coefficients in membranes, and the effect of quencher partitioning into membranes. We also describe transient effects in quenching, which result in nonexponential decays whenever diffusive quenching occurs. These effects can complicate the interpretation of the time-resolved data, but they also provide additional information about the diffusion coefficient of die quencher, the interaction radius, and the medianism of quendiing. For those interested in an introduction to fluorescence, the reading of this chi ter can be postponed. 

Lakowicz JR, Johnson ML, Joshi N et al (1986) Transient effects in quenching detected by harmonic-content frequency-domain fluorometry. Chem Phys Lett 131 343-348... 

R. W. Wijnaendts van Resandt, Picosecond transient effect in the fluorescence quenching of tryptophan, Chem. Rhys. Lett. 95, 205-208 (1983). 

The last term in this expression represents the transient effect in the long-time asymptote of quenching, which makes it nonexponential. 

Neglecting fast transient effects, the quenching reaction in a homogeneous liquid phase follows quasi-first-order kinetics ... 

The simplest correction to single-exponetial decay laws occurs as a result of transient effects in translational diffusional processes. An extensive review of the causes and consequences of these transient effects has been given elsewhere, and would be out of place here. In the diffusional quenching of molecule A by B assuming the simple scheme below... 

The stereochemistry of dienes has been found to have a pronounced effect in the concerted cyclo-additions with benzyne 64>65h A concerted disrotatory cyclo-addition of tetrafluorobenzyne, leading for example with trans- (3-methylstyrene to (63, R = Me), is likely and in accord with the conservation of orbital symmetry 68>. However while the electro-cyclic rearrangement of (63, R = H) to (65, R = H) is not allowed, base catalysed prototropic rearrangement is possible. A carbanion (64, R = H) cannot have more than a transient existence in the reaction of tetrafluorobenzyne with styrene because no deuterium incorporation in (65) was detected when either the reaction mixture was quenched with deuterium oxide or when the reaction was conducted in the presence of a ten molar excess of deuteriopentafluorobenzene. 

Dynamic quenching of fluorescence is described in Section 4.2.2. This translational diffusion process is viscosity-dependent and is thus expected to provide information on the fluidity of a microenvironment, but it must occur in a time-scale comparable to the excited-state lifetime of the fluorophore (experimental time window). When transient effects are negligible, the rate constant kq for quenching can be easily determined by measuring the fluorescence intensity or lifetime as a function of the quencher concentration the results can be analyzed using the Stern-Volmer relation ... 

We have also confirmed our previous results on the effect of methoxylation of phenolic hydroxyl groups within lignin. On the basis of our model studies, we suggest that the most likely explanation for this is that triplet carbonyl groups are quenched statically by hydroxyl groups within the lignin structure on timescales less than 20 ns thus reducing the amount of transient detected in our laser photolysis experiments. 

Non-exponential phosphorescence decay is frequently observed for various aromatic chromophores molecularly dispersed in polymer matrices. Various possible mechanisms for non-exponential decay are reviewed, and a dynamic quenching mechanism by polymer matrices including the effect of a time-dependent transient term in the rate coefficient is discussed in some detail. The biphotonic triplet-triplet annihilation mechanism is also introduced for the non-exponential decay under high-intensity and/or repeated laser irradiation. 

An additional source of non-exponential emission decays is the transient effect that might appear at short times following excitation. This effect is frequently found in collisional quenching controlled by diffusion. In this situation, the quenching rate depends on the encounter probability between the fluorophore and the quencher, which is obviously also dependent on the diffusion coefficient and on quencher accessibility at early times, fluorophores that have quenchers located at short distances will react almost immediately , leaving behind just those that have to diffuse to encounter a quencher centre. The phenomenon is going to be perceived as a quenching rate that is time dependent at early times, and results in a faster decay component of the fluorophore emission [93]. 

In some real systems, it is possible to encounter a combination of both types of quenching. The Stem-Volmer plots are frequently not linear because various transient effects may cause the upward curvature of the plot. On the other hand, downward curvature and leveling-off of the plots can be a result of uneven (hindered) accessibility of a fraction of fluorophores in micro-heterogeneous media. A number of specific models for analyzing fluorescence decays affected by quenching have been proposed in the literature [8, 15]. 

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