Quenching triplet

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... 

The photodecarbonylation of a series of dibenzyl ketones was studied by Robbins and Eastman/63 The results of this study are presented in Table 4.5. The data in Table 4.5 indicate that the presence of a p-methyl or a p-methoxy group has little effect on the quantum yield for this reaction. p-Cyano groups, on the other hand, essentially totally eliminated the decarbonylation. Since the reaction could also be quenched (inefficiently) by benzonitrile or biphenyl, it was concluded that the decarbonylation occurs from a short-lived triplet state. The effect of the p-cyano groups then could result from internal triplet quenching. 

A nonlinear plot of loge[ecd vs. T indicates that bimolecular processes such as triplet-triplet annihilation or triplet quenching are contributing to triplet state deactivation. 

Hence the steady-state population of triplets should increase under heavy-atom perturbation. However, this conclusion is valid only if unimolecular decay is the main route leading to triplet state depopulation. If bimolecular triplet quenching as shown below is more important than unimolecular decay by several orders of magnitude, kd could be increased as much or more than klte without decreasing the steady state triplet population<136) ... 

Triple-layer ODRs, 74 858-859 Triple point of water, 24 439-440 Triple-point temperature, 24 440 Triple porphyrin receptor, 76 787 Triple superphosphate, 77 119-120 Triplet multiplicity, 79 109 Triplet quenching additives, 74 703 Tripodal coelenterands, 24 45... 

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. 

Flash spectroscopy techniques have been applied in order to determine triplet lifetimes (1 /ks) and the quenching constant, kl0, for the triplet-quenching reaction. 

Thus, the fraction of benzophenone triplets quenched by naphthalene (Fq) is given by... 

C. Triplet-Quenching as a Source of High Chemical Potential. 376... 

The ratio of the intensities of the two delayed emission bands should thus be completely independent of 4>t and of all triplet quenching processes. Since ke represents a thermally activated process,... 

Approximate values of v and r for proflavine hydrochloride are shown in Table III. As judged by both quantities, triplet quenching processes appear to have largely disappeared at a temperature as high as —58 °C. This is in marked contrast to eosin (see Table II). The value of... 

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