Quenching kinetics

According to Ludwig (1968), there is a some similarity between UV- and high-energy-induced luminescence in liquids. In many cases (e.g., p-ter-phenyl in benzene), the luminescence decay times are similar and the quenching kinetics is also about the same. However, when a mM solution of p-terphenyl in cyclohexane was irradiated with a 1-ns pulse of 30-KeV X-rays, a long tail in the luminescence decay curve was obtained this tail is absent in the UV case. This has been explained in terms of excited states produced by ion neutralization, which make a certain contribution in the radiolysis case but not in the UV case (cf. Sect. 4.3). Note that the decay times obtained from the initial part of the decay are the same in the UV- and radiation-induced cases. Table 4.3 presents a brief list of luminescence lifetimes and quantum yields. 

Reactivity differences between 2P + 2S photoexcited Cu and Ag atoms have also been observed with CHi (59). For example,comparative matrix quenching kinetic measurements for Cu and Ag atoms in solid CHi show that the first order rate constant is considerably larger for Cu Fig.7. Detailed studies on the Cu system... 

Our simplest continuous microheterogeneous model assumes that the luminophore exists in a distribution of spectroscopically different environmental sites. For a tractable, yet plausible, model each site is assumed to be quenched by normal Stem-Volmer quenching kinetics. For luminescence decays each individual component is assumed to give a single exponential decay with the following impulse response ... 

Both stopped-flow and rapid freeze quench kinetic techniques show that the substrate reduces the flavin to its hydroquinone form at a rate faster than catalytic turnover Reoxidation of the flavin hydroquinone by the oxidized Fe4/S4 center leads to formation of a unique spin-coupled species at a rate which appears to be rate limiting in catalysis. Formation of this requires the substrate since dithionite reduction leads to flavin hydroquinone formation and a rhombic ESR spectrum typical of a reduced iron-sulfur protein . The appearance of such a spin-coupled flavin-iron sulfur species suggests the close proximity of the two redox centers and provides a valuable system for the study of flavin-iron sulfur interactions. The publication of further studies of this interesting system is looked forward to with great anticipation. 

The kinetics of concentration quenching in vesicles can be described quantitatively in terms of the stochastic approach. This approach takes into account, first, the small (1-100) number of photosensitizer molecules in one vesicle and, second, the statistical character of their distribution in the vesicles. The small number of the photosensitizer molecules in a vesicle leads to a significant influence of the fluctuations of this number on the quenching kinetics. As shown in Phe-containing vesicles [134], under these conditions the stochastic approach describes the... 

The usual subject of experimental study with time-resolved methods is the nonexponential quenching kinetics, which is system response to 8-pulse... 

The failure of the contact and other models to fit the entire quenching kinetics does not make the problem unsolvable. The situation reverses if one turns to the asymptotic expression (3.56), which is nonmodel and allows extracting the true value of Rq. In fact, not the Rq value itself but its dependence on diffusion and the parameters of electron transfer is really informative. 

In order to fit the kinetic and stationary data together to get both Rq and D in one stroke, they were proposed by Costa et al. [16]. The authors stated that the quenching kinetics that they detected is almost an ideal exponent ... 

Figure 3.53. The quenching kinetics at long times with and without bulk recombination of ions (solid and long dashed lines, respectively). The false IET asymptote p- 5 2) is indicated by a dotted line, while the true asymptotic behavior of delayed fluorescence (t 2) is shown by a short dashed line. All the parameters are the same as for Figure 3.52. (From Ref. 189.)...

Figure 3.56. Semilogarithmic plot of the irreversible quenching kinetics with a large excess of acceptors [JV (0) = 10 4 M -C 10 2 M — c] calculated with IET (thick line), UT (thin line), and Markovian theory (dashed line). The remaining parameters are a = 6 A the preexponent for the ionization rate w = 1000 ns-1, D = 1.2 x 10 6cm2/s,/= 1.0A, k, = 1271 A3/ns. The initial nonstationary quenching is shown in the insert. (From Ref. 195.)...

After 5-pulse excitation there is no more pumping and the quenching kinetics can be described by the IET equations (3.162) with Wg = S) = 0 and the proper simplification of (R). However, in presence of permanent pumping, (S) / 0, and both kernels have to be redefined starting from the general IET equation (3.103a). It should be adopted for intramolecular relaxation catalyzed by inert particles whose concentration c = [Q] remains invariable. Substituting c for Ng and... 

A similar correction to IET is inherent in MET as well. For irreversible transfer (Ay, P 0), the quenching kinetics represented by P A (t) was obtained in Ref. 203 with the original program designed to solve the differential form of IET and MET equations. As seen from Figure 3.84, the difference between the curves representing these solutions is insignificant within the validity limits for IET established in Ref. 39... 

Figure 3.84. The quenching kinetics obtained with IET (dashed line) and MET (solid line). The vertical strip denotes the upper boundary of the region of IET validity. Parameters of exponential transfer rate Wc = 100ns 1, / = 1A, a = 5A. The remaining parameters are Nb = 0.1 M, D = 10"5 cm2/s. (From Ref. 203.)...

S A was also applied to the reversible reactions considered in Section XII.C.4. Since the results were not satisfactory, an extended superposition approach (ESA) was developed, then linearized, and later known as LESA [241]. Independently, a similar linearization over deviations from equilibrium was also made in Ref. 242. Although the asymptotic description of the quenching kinetics is improved, it was recognized [242] that LESA is not valid with a large equilibrium constant K because the superposition approach worsens when K increases [241]. This is especially true at earlier times when the deviations from equilibrium are not small. However, the authors who constructed LESA claimed that it is applicable at all times [241]. Therefore, it was taken for comparison with other approximations. In the irreversible limit (K —> oo), the kernels obtained in both works [241,242] coincide with that listed as LESA in Table V. 

As has been proved in Eq. (3.700), the lowest limit of the Stern-Volmer constant Ko = k(0) is the same for all contact theories. However, there is also the upper limit of k(c) reached at largest c. At the very beginning the quenching kinetics is always exponential... 

Johnson, K.A. (1995) Rapid quenching kinetics analysis in polymerases, adenosin tri phosphotase, and enzyme intermediates, in Purich, D. L. (eds.), Methods in Enzymology 249, Enzyme Kinetics and Mechanism, Part D, Academic Press, San Diego, pp. 3-37. 

When samples are unstable, or intermediates need to be studied, fast freeze techniques have been developed to enable the study of reactive and unstable systems (see Freeze-Quench Kinetics). 

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