Rates triplet quenching

As with arene-amine radical ion pairs, the ion pairs formed between ketones and amines can also suffer a-deprotona-tion. When triplet benzophenone is intercepted by amino acids, the aminium cation radical can be detected at acidic pH, but only the radical formed by aminium deprotonation is detectable in base (178). In the interaction of thioxanthone with trialky lamines, the triplet quenching rate constant correlates with amine oxidation potential, implicating rate determining radical ion pair formation which can also be observed spectroscopically. That the efficiency of electron exchange controls the overall reaction efficiency is consistent with the absence of an appreciable isotope effect when t-butylamine is used as an electron donor (179). 

Measurements of Relative Triplet Quenching Rate Constants. 

The rate constants of various reagents quenching a photoexcited triplet are important parameters in both CIDEP and CIDNP observations. Knowledge of these absolute and relative rate constants is obviously important in photochemistry. We shall illustrate in this section and with some details how CIDEP experiments using a slow-response conventional spectrometer and a rotating sector can lead to measurements of relative triplet quenching rate constants. 

Table 3. Free energy change, AG°ct, and rate constants, of photoinduced electron transfer from group 14 organometallic electron donors to C, observed rate constants, obs, triplet quenching rate constants, kq, and limiting quantum yields, Oco, in the photoaddition of the donors to in benzonitrile at 298 K [212].

The photoreduction of triplet duroquinone by the antioxidant vitamin C has been studied by time-resolved EPR in homogeneous ethylene glycol-water solutions and in AOT and SDS micelles. The triplet quenching rate constant is lower in the latter medium, due to slower diffusion. ... 

The reaction between the photoexcited carbonyl compound and an amine occurs with substantially greater facility than that with most other hydrogen donors. The rate constants for triplet quenching by amines show little dependence on the amine a-C-H bond strength. However, the ability of the amine to release an electron is important.- - This is in keeping with a mechanism of radical generation which involves initial electron (or charge) transfer from the amine to the photoexcited carbonyl compound. Loss of a proton from the resultant complex (exciplex) results in an a-aminoalkyl radical which initiates polymerization. The... 

Both energy and electron transfer quenchers have been employed to show that the quenching rates of the fullerene triplet state are decreased as a function of the size of the dendrimer shell [36]. These results further demonstrate that fullerene is an excellent functional group to probe the accessibility of a dendrimer core by external molecules. 

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. 

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]...

Equation (4) describes a partition between two forms of the complex one, with fraction f, that exhibits quenching and one that does not. Equation (5) corresponds to a distribution of forms with a range of quenching rates whose breadth varies inversely with n the occurrence of distributions is well-established for proteins at low temperatures [29]. In either case, the non-exponential kinbtics necessitate that conformational interconversion in the transition range is slow compared to the lifetime of the triplet state and the gating limit is applicable. 

Quantitative data on rates of reaction have been obtained for some of the triplet reactions. Assuming triplet quenching to be approximately diffusion controlled, the rate constants for the reactions between excited species and nucleophile are 10 -10 1 mole s . The data show that in comparing and interpreting quantum yields—even in the case of related systems—one should proceed to determine separately rate constants as well as intersystem crossing efficiencies and lifetimes of the reacting excited species. 

One possible explanation for the lack of correspondence between emitting and reacting states is reaction of the singlet. In the case of benzophenone, there is little question that the reaction involves only the triplet state, since triplet quenchers can effectively inhibit the reaction. This need not be the case with all carbonyl compounds. For example, it has been shown that both the n,ir singlet and triplet states can be involved in the Norrish type II cleavage of alkyl ketones (25-27). At high concentrations, piperylene quenches only that part of the 2-hexanone cleavage which arises from the triplet. The rate constants for... 

The LFP of diphenyldiazomethane ( DDM ) in a variety of solvents produces triplet diphenylcarbene ( DPC, 14a), whose transient absorption is readily monitored. The optical absorption spectrum of DPC is quenched by methanol and yields the product of O—H insertion, suggesting that DPC is quenched by the O—H bond of methanol. The quenching rate constant (fex) is determined to be 6.8 X 10 M s in benzene. ... 

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