Quenching by Amines

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... 

Fluorescence of PDC is also quenched by amines. The ordering of reactivity is tertiary > secondary > primary, which follows inversely the ionization potential (Table 9.13). The results are explained as indicating that PDC undergoes photoreduction by amines, thereby forming triplet charge-transfer intermediates as the primary step in quenching. Therefore, the mechanism of the PDC reaction is not the same as the proposed mode of reaction of PDC, which involves direct formation of an yhde intermediate by electrophilic attack on the lone-pair electrons of the amine (Table 9.13). ... 

To circumvent any problems caused by cluster quenching by amines on the polymer, a preparation of pyrenylmethyl polyethylenimine was made in which all the primary and almost all the secondary amines were acetylated. Steric factors in the polymer should block quenching interactions with the tertiary amines. In addition, these samples were examined... 

Another physical study which used flash photolysis relates directly on the MB/amine system. Kayser and Young (36) examined a more extensive series of amines, both aromatic and aliphatic, than Steiner (33). Their results are shown in Table 3. Excellent correlation was obtained between the amine ionization potential and the rate constant for MB quenching the slope of the logarithmic plot was -1.75 eV-1- This value is relatively small compared to some oxidizing excited states (e.g. hydrocarbons, -17 eV l (37)), but it is similar to the value observed for ketone triplet quenching by amines (-1.5 eV l (38)), and does indicate that the quenching interaction becomes more facile as... 

Some data for the rate constants of the triplet-state quenching by amines are given in Table 2. As expected, a correlation between the 3k H values and the ionization potential of the amine exists. However, the influence of the spectroscopic nature of the excited state (e.g., nre or nn ) is not adequately understood, so far. 

Wubbeis, G. G., Sevetson, B. R., Sanders, H. Competitive cataiysis and quenching by amines of photo-Smiies rearrangement as evidence for a zwitterionic tripiet as the proton-donating intermediate. J. Am. Chem. Soc. 1989, 111, 1018-1022. 

Figure 4. Response of porous silicon PL to SO2 in Ar. As in the case of quenching by amines, SO2 quenching was found to be unaffected by silylation of the PS surface. This again rules out a role for surface silanols. Inset- Initial response of the detector showing the bi eak-in behavior.

The luminescence of Re2Cl also is quenched by secondary and tertiary aromatic amines in acetonitrile solution(17). 

The mechanistic basis of 3 and 4 is a double application of the argument used for the YES operation with 1. Each receptor in 3 is capable of launching a PET process if the lumophore is powered up by ultraviolet excitation. In other words, the fluorescence of the 9-cyanoanthracene unit is efficiently quenched by either the amine or the 1,2-dioxybenzene group within the benzo-15-crown-5 ether unit. These quenching processes are predictable from thermodynamic calculations or from related bimolecular quenching experiments in the literature. The small separation of the amine from the lumophore ensures rapid PET kinetics. On the other hand, the presence of four bonds between the benzo-15-crown-5 ether and the lumophore is probably responsible for the incomplete quenching seen between this pair. No information is yet available regarding possible folded conformations of 3. 

The photohydrolysis of 2-fluoro-4-nitroanisole to 2-methoxy-5-nitrophenole is sensitized by benzophenone and completely quenched by sodium sorbate The excited state multiplicity in photoaminations has also been studied. Photolysis of mNA in liquid ammonia yields m-nitroaniline. If the amination is carried out in a large excess of benzophenone, 2-methoxy-4-nitroaniline is formed instead and thus an excited singlet state as reacting species is envisaged in the unsensitized photoamination loo.ioi). it may well be that uptake of the nucleophile present in high concentration successfully competes with intersystem crossing. 

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