Quenching rate constants

In the present case, = 1.1x10 M- s i, a typical value for the reaction of aminoacid moieties with 1O2 (Michaeli Feitelson, 1994 Bisby et al., 1999). By comparison with the total quenching rate constant, kfi = 2.7x10 M- s-i, it can be concluded that almost 60% of the interaction with 1O2 is through physical quenching and about 40% of the reactive moieties of GA are oxidized by 1O2. 

Porter and Wilkinson(56) measured the rates of quenching for a variety of triplet donors with triplet acceptors at room temperature in fluid solution by flash photolysis. The appearance of the triplet-triplet absorption spectrum of the acceptor and the simultaneous disappearance of the donor triplet-triplet absorption spectrum provided unequivocal evidence for the triplet-triplet energy transfer process. Table 6.5 provides some of the quenching rate constants reported in this classic paper. 

It should be noted that this expression is a general one that can be used for any photochemical reaction that can be quenched. It is commonly called the Stern-Volmer equation. This equation predicts that if the proposed mechanism is correct, the data, when plotted as 4>a0/4>a vs. [Q], should be linear with an intercept equal to unity and a slope equal to kqr. Linear plots were indeed observed out to large d>°/d> values. Assuming a value of 5 x 10 M 1 sec-1 for the quenching rate constant,(7) the data presented in Table 4.1 were obtained. 

Quenching rate constants for dienes and quadricyclenes have similar sensitivities to the electronic and structural features of the excited aromatic hydrocarbon. However, during this process quadricyclene isomerizes to nor-boraadiene with a quantum yield of 0.52, whereas dienes usually remain unchanged/10 Hammond has suggested that vibrational energy which is partitioned to the acceptor upon internal conversion of the exciplex can lead to isomerization(10a,103) ... 

Singlet Oxygen Quenching Rate Constants for Carotenoids in Benzene... 

Second-Order Quenching Rate Constants for the Quenching of 102 by Carotenoids in Unilamellar DPPC Liposomes, Benzene, and Triton X-100/405 Micelles... 

Kuimova, MK, Yahioglu, G, and Ogilby, PR, 2009. Singlet oxygen in a cell Spatially dependent lifetimes and quenching rate constants. J Am Chem Soc 131, 332-340. 

In eq. 8 are shown the results of a kinetic analysis of the series of reactions in Scheme 1. The analysis is based on the quenching rate constant k, corrected for diffusional effects, which would be measured for the quenching of Ru(bpy>3 + by PQ +. 

In the equations, k (0) is the hypothetical quenching rate constant when AE ( AG) = 0 and AG is the free energy change on quenching. 

Eqs. 9 and 10 make clear predictions about the dependence of quenching rate constants on the free energy change in the quenching step. One way of testing the theory is to observe the quenching of the excited state by a series of related quenchers where the parameters kq(0), K, and k j) should remain sensibly constant and yet where the potentials of the quenchers as oxidants or re-ductants can be varied systematically. Such experiments have been carried out, most notably with the MLCT excited state, Ru(bpy)3 + (1). The experiments have utilized both a series of oxidative nitroaromatic and alkyl pyridinium quenchers, and a series of reductive quenchers based on aniline derivatives. From the data and known redox potentials for the quenchers, plots of RTlnk q vs. 

Global compartmental analysis can be used to recover association and dissociation rate constants in some specific cases when the lifetimes are much shorter than the lifetimes for the association and dissociation processes. An example is the study for the binding dynamics of 2-naphthol (34, Scheme 14) with / -CD.207 Such an analysis is possible only if the observed lifetimes change with CD concentration and at least one of the decay parameters is known independently, in this case the lifetime of the singlet excited state of 33 (5.3 ns). From the analysis the association and dissociation rate constants, as well as intrinsic decay rate constants and iodide quenching rate constants, were recovered. The association and dissociation rate constants were found to be 2.5 x 109M-1 s 1 and 520 s 1, respectively.207... 

As a first approach, the experimental quenching rate constant fcq is assumed to be time-independent. According to the simplified Scheme 4.1, the time evolution of the concentration of M following a b-pulse excitation obeys the following differential equation ... 

The crucial requirement of excited-state proton transfer (ESPT) is suggested by the failure of 1-naphthyl methyl ether to undergo self-nitrosation under similar photolysis conditions. The ESPT is further established by quenching of the photonitrosation as well as 1-naphthol fluorescence by general bases, such as water and triethylamine, with comparable quenching rate constants and quantum yield. ESPT shows the significance in relation to the requirement of acid in photolysis of nitrosamines and acid association is a photolabile species. 

The quenching rate constants do not follow the expected electron transfer patterns and no oxidized or reduced species have been detected. Furthermore, k does not appear to be... 

Figure 2. The logarithm of kqc (the diffusion-corrected quenching rate constant) vs. — (the reduction potential for the RuLs3 / RuLs2 couple) for quenching (left side, by Rh(4f4 -(CHs)ibpy)s3+ (24) and (right side, + by Euaq3 (22). (Copyright 1982, American Chemical Society.)...

Where quenching is present, the bimolecular quenching rate constant, fa, is assumed constant across the distributions, although for different classes of sites the rate constant may vary. For decay curves for a single site, Eq. (4.5) is used but r in the exponential decay part depends on quencher concentration and fa. 

Figure 4.13. Intensity Stem-Vo I met plot for a double Gaussian distribution of lifetimes as a function of distribution width K. The two lifetimes are 5 and 15 with bimolecular quenching rate constants of 0.30 and 0,01, respectively. Theunqucnchcd intensity contribution is 0.5 for each lifetime. The R for each distribution is indicated next to the curve. (Adapted from Ref. 54.)...

Other groups may cause shortening of the lifetime. The phosphorescence of parvalbumin is quenched by free tryptophan with a quenching rate constant of about 10s M i s l (D. Calhoun, unpublished results). A more extensive survey of proteins or model compounds with known distances between tryptophans is needed to study how adjacent tryptophans affect the lifetime. It should be noted that at low temperature the phosphorescence lifetime of poly-L-tryptophan is about the same as that of die monomer.(12) This does not necessarily mean that in a fluid solution tryptophan-tryptophan interaction could not take place. Thermal fluctuations in the polypeptide chain may transiently produce overlap in the n orbitals between neighboring tryptophans, thus resulting in quenching. 

The Stem-Volmer(52) equation relates fluorescence intensity and the quenching rate constant, kq ... 

The observed quenching rate constant, kq, according to this model will be a function of the internal diffusion of the quencher, fcrf(int), and is given by... 

Triplet decay in the [Mg, Fe " (H20)] and [Zn, Fe (H20)] hybrids monitored at 415 nm, the Fe " / P isosbestic point, or at 475 nm, where contributions from the charge-separated intermediate are minimal, remains exponential, but the decay rate is increased to kp = 55(5) s for M = Mg and kp = 138(7) s for M = Zn. Two quenching processes in addition to the intrinsic decay process (k ) can contribute to deactivation of MP when the iron containing-chain of the hybrid is oxidized to the Fe P state electron transfer quenching as in Eq. (1) (rate constant kj, and Forster energy transfer (rate constant kj. The triplet decay in oxidized hybrids thus is characterized by kp, the net rate of triplet disappearance (kp = k -I- ki -I- kj. The difference in triplet decay rate constants for the oxidized and reduced hybrids gives the quenching rate constant, k = kp — kj, = k, -I- k , which is thus an upper bound to k(. 

Fig. 7. Temperature dependence of the triplet-state quenching rate constant (k,) for the [a(Zn), PiFe +HjO)] hybrid. Adapted from Ret [7d]...

This is the Stern-Volmer relationship with = k /k(j, and is an important basis for determining quenching rate constants after pulsed excitation. The quantum yield of (pro-duct)o can be measured without (0q) and with (0) quencher under continuous excitation (0 = moles of product/einsteins of light absorbed by system). Assuming that a steady state concentration of S exists in both cases. 

This represents a competitive, non-kinetic, method for determining relative rate constants for the photochemical system. The value of may be obtained after one direct determination of has been made. For an extensive compilation of quenching rate constants for excited states of metal complexes see Ref. 358. 

Emission quenching is also observed with mononucleotides. In that case the quenching efficiency decreases from GMP (guanosine 5 monophosphate) to AMP (adenosine 5 monophosphate) i.e. it also follows the redox potentials of the bases, as G is more easily oxidisable than A, although the oxidation potential valura reported in the literature are rather different from one author to the other [101-104], Moreover the quenching rate constant by GMP in a Kries of different TAP and HAT complexes plotted versus the reduction potential of the excited state (Fig. 12) [95] is consistent with an electron transfer process. Indeed, as will be demonstrated in Sect. 4.3.1, these quenchings (by the mono-and polynucleotides) originate from such processes. 

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