Quencher association constants

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 ionic strength effect is not limited to ksv variation as described by eq. (22). The addition of large amounts of electrolytes may also modify the quencher solubility and thus its efficiency. This effect has been used by some authors, in systems very different from those examined in this work, in order to determine the association constant of the inhibitor salt (Mac, 1997 Mac and Tokarczyk, 1999) as the electrolyte concentration is increased, the quencher ion associates, so that the effective concentration of the inhibitor ion decreases, leading to a downward curvature of the Stern-Volmer plot. Such a curvature can be quantitatively related... 

Models with increasing sophistication for the analysis of dynamic processes in supramolecular systems, notably micelles, as well as for the determination of other parameters have been developed over the past two decades. The basic conceptual framework has been described early on [59,60,95,96] and has been classifred into different cases which take into account the extent of quencher mobility and the mechanism of quenching [95]. Two of those cases lead to information about mobility and will be discussed. It is important to emphasize that this analysis is only applicable to self-assembled system such as micelles and vesicles it cannot be applied to host-guest complexes. This model assumes that the probe is exclusively bound to the supramolecular system and that no probe migration occurs during its excited state lifetime. The distribution of probe and quencher has been modeled by different statistical distributions, but in most cases, data are consistent with a Poisson distribution. The Poisson distribution implies that the quencher association/dissociation rate constants to/from the supramolecular system does not depend on how many... 

In addition to comparing overall quenching rate constants, it is also possible to recover the values of the quencher association and dissociation rate constants from quenching experiments. The same model that was employed for fluorescent probes can be employed. This model considered that the probe was immobile. The general solution to this model is given by Eq. (8), which has four parameters defined by the rate constants for the processes described in Fig. 1. However, the experimental results showed that the triplet state decayed by pseudo-first-order kinetics, suggesting that once the quenchers enter the supramolecular system, quenching occurred with an efficiency of unity. Under these conditions, Eq. (5) can be applied. In addition, if the condition that [H] holds, Eq. (5) can be reduced to... 

The dependence of the fluorescence intensity upon quencher concentration for static quenching is easily derived by consideration of the association constant for complex formation. This constant is given by... 

When the quenching is dominated by either a purely static or dynamic pathway, the quenching behavior follows Equation 14.2 and consequently SV plots of I°/Ivs. [Q] are linear. However, in many situations (as shown below) the SV plots are curved upward (i.e., superlinear). Superlinear SV plots can arise from a variety of processes, including mixed static and dynamic quenching, variation in the association constant with quencher concentration, and chromophore (or polymer aggregation). 

The dependence of the luminescence intensity on the quencher concentration can be derived by considering the association constant for the formation of the L- Q complex, IQ ... 

When monitoring the transient due to triplet carbenes is difficult because of the inherent weak nature of the bands and/or severe overlapping with the absorption bands of the parent diazo compounds, it is more convenient to follow the dynamics of the triplet carbene by measuring the rate of the products formed by reaction of triplet carbenes with quenchers such as radicals (Section 5.3) and carbonyl oxides (Section 6.5). In this case, note that the observed rate constant (feobs) of a triplet carbene reaction is the sum of the decay rate constants of the triplet. These may include decay via an associated but invisible singlet with which the triplet is in rapid equilibrium. Thus in general. 

NaCl above 0.1 M at lower salt concentrations the intensity plots show upwards curvature indicating a slight amount of association between 5 and the quencher. The dynamic quenching constant obtained is 5.6 X 10 M i s l. A similar sttidy with Cu as a quencher leads to linear lifetime Stem-Volmer plots giving a quenching constant, kq = 5.8 x 10 M 1 s l... 

---