Carbonyl triplets, quenching

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... 

With aromatic carbonyls, oxetane formation appears to arise from the carbonyl triplet state, as evidenced by quenching studies. For example, benzaldehyde irradiated in the presence of cyclohexene yields products indicative of hydrogen abstraction reactions and an oxetane ... 

The decarbonylation of dibenzyl ketone has been shown to result from the carbonyl triplet state by its ability to be quenched by 1,3-cyclohexadiene or l,3-pentadiene.<66) Using 1,3-cyclohexadiene as quencher, photodimers of the cyclohexadiene were obtained. Since these are formed only by triplet sensitization,<66) the quenching of ketone triplet states, rather than their excited singlets, was assured. Further evidence for a triplet reaction follows from the fact that decarbonylation could be sensitized by acetone under conditions where the sensitizer absorbed 93% of the light. 

Irradiation of carbonyl compounds in the presence of dienes usually leads to the isomerization38 and dimerization39 (in concentrated diene solutions) of the dienes, but no photocycloaddition products. These reactions indicate that the dienes have quenched the carbonyl triplet. It is also known from spectroscopic studies that the triplet... 

Thus, the observations that (a) dienes quench the photoreduction reaction (b) the isomerization and dimerization of dienes is sensitized by the it,n carbonyl triplet and (c) there is a lack of photocycloaddition products with dienes, taken in conjunction with the relative energy levels of carbonyl compounds and dienes, form a consistent picture. 

Irradiation of olefins and dienes in the presence of oxygen and various sensitizers produces hydroperoxides and cyclic peroxides. Effective sensitizers include not only high-energy carbonyl triplets, but also low-energy organic dyes such as methylene blue, rose bengal, chlorophyl, and riboflavin. The same species that quench the phosphorescence of these complex molecules also quench their photosensitizing ability. 

Quenching by butadiene. Dienes are known to be excellent quenchers of carbonyl triplets (25) in homogeneous solution the rate constants for triplet quenching frequently approach the diffusion controlled limit. 

It is well-known that the carbonyl triplets of aromatic ketones are efficiently deactivated by p-aryl [7a,32] or p-vinyl [33] groups. This quenching process requires a n,n triplet state and typically leads to triplet lifetimes of about 1 ns at room temperature and is rather insensitive to substituent effects. It is suggested that this quenching process involves charge transfer interactions [32a]. 

Ferrocene does not react with iron carbonyls under irradiation. Its low ionization potential [ 7—7.5 eV 387>] and its high triplet quenching efficiency 185> would cause a strong decrease of G pea(co)9 and G-[Fe(C0)4]3 if benzene triplets or any ions were involved in their formation. This is not the case. 0.54 mole/1 ferrocene causes Gj e2(co)9 to decrease by only 20%, and has no effect on G[Fe(CO)4]3 [0-21 mole Fe(CO)s/l benzene] 289>. This observation suggests that energy transfer from excited benzene singlets could play a role. 

Environmental exposure Polycyclic hydrocarbons (atmospheric pollution) Carbonyl (by photolysis of hydroperoxides) Unsaturation (by photolysis of ketones) Singlet oxygen and derived hydroperoxides (by quenching of carbonyl triplet etc with triplet oxygen) Transition metal ions (particularly iron and copper)... 

Singlet oxygen formed by the quenching of excited carbonyl triplets... 

It was recently found (300) that quenching of carbonyl triplets by anions (see Table 11, column 2, for acetone) follows the pattern... 

Formation of Oxygen Molecules in the Singlet State by Quenching of Carbonyl Triplets.89... 

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