Static quenching

From the relation between the quenching efficiency and the charge separation efficiency Webber et al. [77] and Morishima et al. [76, 119, 120] have reached the same conclusion that efficient fluorescence quenching static in nature does not lead to efficient charge separation. This conclusion seems to apply generally to... 

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. 

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

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. 

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.)... 

FIGURE 11.1 Polarizing micrographs of crack behavior of PLA (a) quenched static, (b) quenched impact, (c) annealed static, (d) annealed impact [22]. 

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

The values of K0 and Ksv obtained by using eq 9 are given in Table 4. The ground-state complex considered in eq 7 includes not only the CT complex but all kinds of complexes that may lead to apparent static quenching. Therefore, usually K0 is larger than KCT as can be seen from Table 4 (although there are a few exceptions). 

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. 

This means that if the intensity of one of the forms is zero (static quenching), such anisotropy sensor is useless since it will show anisotropy of only one of the forms. The account of fractional intensity factor R Fb // , (the ratio of intensities of bound and free forms) leads to a more complicated function for the fraction of bound target,/ ... 

A number of systems which in polymer literature are normally referred to as mesophases are obtained under kinetic control. Examples are the smectic phase of isotactic polypropylene [18,19], mesomorphic syndiotac-tic polypropylene [20-22], mesomorphic PET [23,24], and other instances where intermediate degrees of order result after quenching polymers from the melt to temperatures often close to Tg. In these cases disorder is plausibly more static than in bundles close to T0 and these phases usually crystallize upon heating to an appropriate temperature in the stable crystal phases. 

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) ... 

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