Quenching static/dynamic

Besides the quenching in the excited state dynamic quenching), there exists another type of quenching (static), which takes place in the ground state and occurs due to the formation of nonemitting complexes. 

Figure 10.9 shows the intensity versus [O2] quenching curve for three complexes in Cab-O-Sil disks. The solid lines are the best fits to a Freundlich adsorption model (see below). All quenching is dynamic with no static component. 

There are two mechanisms of quenching, static and dynamic. Static quenching is the nonradiative return of an excited state to the ground state,... 

Figure 5.3. Simulated Stern-Volmer plots of the ratio of the initial fluorescence intensity F0 to the intensity Fin the presence of quencher of concentration [Q] showing (a)static quenching, (b) dynamic quenching (linear), and (c) binding and/or inaccessible quenchers.

Figure 3.42 Kinetics of dynamic (diffusional), D, and static, S, quenching. In dynamic quenching the excited state lifetime gets shorter with increasing quencher concentration, from to with no quencher to t1 t2 with added quencher. In static quenching excited state lifetime remains unchanged but the initial concentration of excited states is reduced...

As far as deactivation by T1+ is concerned, a fluorescent label attached to PMAA is considered [95,96] to undergo a mixture of static and dynamic quenching. (Static quenching [1] can be defined as a process that occurs too fast to resolve within the timescale of the experiment. In other words, a ground-state interaction or complex forms between the quencher and the fluorophore before excitation. Such a situation would perhaps be not unexpected when counterions condense in high concentrations to a polyelectrolyte backbone in close proximity to a fluorescent label.)... 

Short Range Quenching Static and Dynamic Quenching, Perturbation, Electron Transfer and Dexter Quenching... 

The DPA moiety is less active in forming the CT complex with viologens than the pyrene moiety e.g., for PMAvDPA the KCT values with MV2+ and SPV are 1.3 x 103 M 1 and almost zero, respectively, at pH 8-9 [60, 77], whereas for PMAvPY they are 7.8 xlO4 and 6.3 x 102 M, respectively, at pH 11 [77]. Therefore, the polymer-bound pyrene system undergoes much more static quenching than the polymer-bound DPA system. As will be discussed in Chapter 6, it is very important for charge separation whether the fluorescence quenching is static or dynamic. 

The data for sodium 9-anthroate in benzonitrile do not fit the pattern of the other derivatives since in this case kD + fccq > kMThis effect cannot be due to kD since this value is less than those of the other derivatives. Therefore cq must be greatly increased for the salt. This effect is thought to arise from both dynamic quenching and static quenching due to ion pairs. 

Fluorescence quenching may be dynamic, if the photochemical process is the result of a collision between the photoexcited indicator dye and the quencher species, or static, when the luminophore and the quencher are preassociated before photoexcitation of the former20. It may be easily demonstrated that dynamic quenching in isotropic 3-D medium obeys the so-called Stem-Volmer equation (2)21 ... 

It may also happen that an association equilibrium exists between the luminescent indicator and the quencher. Non-associated indicator molecules will be quenched by a dynamic process however, the paired indicator dye will be instantaneously deactivated after absorption of light (static quenching). Equation 2 still holds provided static quenching is the only luminescence deactivation mechanism (i.e. no simultaneous dynamic quenching occurs) but, in this case, Ksv equals their association constant (Kas). However, if both mechanisms operate simultaneously (a common situation), the Stem-Volmer equation adopts more complicated forms, depending on the stoichiometry of the fluorophore quencher adduct, the occurrence of different complexes, and their different association constants. For instance, if the adduct has a 1 1 composition (the simplest case), the Stem-Volmer equation is given by equation 3 ... 

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) ... 

Fluorescence quenching is described in terms of two mechanisms that show different dependencies on quencher concentration. In dynamic quenching, the quencher can diffuse at least a few nanometers on the time scale of the excited state lifetime (nanoseconds). In static quenching, mass diffusion is suppressed. Only those dye molecules which are accidentally close to a quencher will be affected. Those far from a quencher will fluoresce normally, unaware of the presence of quenchers in the system. These processes are described below for the specific case of PMMA-Phe quenched by MEK. 

At an MEK concentration greater than 1 M, both the dynamic and the static quenching mechanisms have to be taken into account. Therefore, Frank and Vavilov s model of combined static and dynamic quenching model (28),... 

Figure 6. Calculated PMMA-Phe Fluorescence Intensity from Static and Dynamic Quenching Theory as a Function of MEK Concentration.

Following an external perturbation, the fluorescence quantum yield can remain proportional to the lifetime of the excited state (e.g. in the case of dynamic quenching (see Chapter 4), variation in temperature, etc.). However, such a proportionality may not be valid if de-excitation pathways - different from those described above - result from interactions with other molecules. A typical case where the fluorescence quantum yield is affected without any change in excited-state lifetime is the formation of a ground-state complex that is non-fluorescent (static quenching see Chapter 4). 

Case C Q is not in large excess and mutual approach of M and Q is possible during the excited-state lifetime. The bimolecular excited-state process is then diffusion-controlled. This type of quenching is called dynamic quenching (see Section 4.2.2). At high concentrations of Q, static quenching may occur in addition to dynamic quenching (see Section 4.2.4). 

The excited-state lifetime of the uncomplexed fluorophore M is unaffected, in contrast to dynamic quenching. The fluorescence intensity of the solution decreases upon addition of Q, but the fluorescence decay after pulse excitation is unaffected. Quinones, hydroquinones, purines and pyrimidines are well-known examples of molecules responsible for static quenching. 

A linear relationship is thus obtained, as in the case of the Stern-Volmer plot (Eq. 4.10), but there is no change in excited-state lifetime for static quenching, whereas in the case of dynamic quenching the ratio I0/I is proportional to the ratio to/t of the lifetimes. 

Static and dynamic quenching may occur simultaneously, resulting in a deviation of the plot of Io/I against [Q] from linearity. 

Let us consider first the case of static quenching by formation of a non-fluorescent complex. The ratio I0/I obtained for dynamic quenching must be multiplied by the fraction of fluorescent molecules (i.e. uncomplexed)... 

It should be emphasized that time-resolved experiments are required for unambiguous assignment of the dynamic and static quenching constants. 

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