Acetone triplet state, quenching

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. 

One example of an aromatic molecule with markedly different singlet and triplet-state photochemistry is provided by [2.2]paracyclo-phane.203 Irradiation at wavelengths above 2700 A or photosensitization with acetone yields p-ethylbibenzyl as the sole product. This must be a triplet product because the reaction is quenched by naphthalene. Direct irradiation of the paracyclophane at 2537 A and lower wavelengths produces other products. 

The quenching of chemically excited triplet acetone by biologically important compounds such as indoles, tyrosine derivatives, quinones, riboflavin, and xanthene dyes has been studied. Quenching occurs by electron and Forster transfer processes. Triplet-state parameters have been presented for the... 

Rebbert and Ausloos found C2H4 and C3H8 to be the main products of the acetone-photosensitized decomposition of n-butyraldehyde at 3130 A. Since triplet aldehyde molecules are formed in the energy transfer process, it follows that, under the given conditions, the triplet state can decompose both by processes I and III. This conclusion is supported by the observation that oxygen, which quenches the triplet aldehyde molecules, also suppresses the formation of C2H4 and C3H8. 

Finally, it should be mentioned that acetone itself quenches both its excited singlet and triplet states. A Stern-Volmer relation is obeyed by the self-quenching of phosphorence and probably of fluorescence as well . ... 

Larson and O Neal reported a value of = 3.5 x 10 sec at 44 °C. Investigating the competition between decomposition and reaction with HBr of the ther-malized triplet acetone, these authors found that k. = 1.8 x 10 exp (-9450/Rr) l.mole . sec. A value of k 8 x 10 I.mole . sec" was reported for the quenching of the triplet state by oxygen. (For the triplet energy transfer efficiencies of organic molecules, see the references cited earlier.)... 

Gorin ° determined the values of 1.0 (2537 A) and 0.85 (3130 A) for primary quantum yield i. Later investigations also confirmed the value at 2537 A however, a considerably lower value, < ch3I 0-2, was reported -at 3130 A and at temperatures of 100 °C and above. At 3130 A, the primary quantum yield is temperature dependent in the presence and absence of iodine Since the primary decomposition quantum yield is known to be near unity at 3130 A and above 100 °c - , one is inclined to interpret the low quantum yields, determined in the presence of iodine, as indications of the quenching effect of iodine. It is the triplet state of acetone which is likely to be quenched by Ij. Some of the results are, however, inconsistent with such a conclusion nevertheless, the quantum yields, determined in the iodine inhibition experiments, should be accepted with reserve. 

Primary step I The relative efficiencies of biacetyl and butene-2 in the quenching of reaction I were found to be not far from unity in the case of the ketones studied consequently, primary process I occurred from the high vibrational levels of the triplet state, possessing sufficient energy to induce efficiently the isomerization of butene-2. The efficiency of energy transfer to cw-butene-2 compared with that to biacetyl, measured in terms of reaction I, was found to be in the following order 2-pentanone > 2-butanone > acetone. It is possible that this order reflects the facility with which the vibrational energy of the donors may be made available to... 

Two additional complications may be present in photochemical reactions, the presence of hot molecules and hot radicals referred to earlier in thermal systems, and the possible physical and chemical primary processes that may occur. Compounds such as biacetyl, O2, NO and olefins are particularly efficient quenchers of electronically excited species. In accordance with the Wigner spin conservation rule that the total spin in a quenching process is conserved, triplet state acetone ( A) is quenched very efficiently to the ground state ( A) by olefins (O) . 

Rebbert, R. E., and P. Ausloos Quenching of the Triplet State of Acetone and Biacetyl by Various Unsaturated Hydrocarbons. J. Am. Chem. Soc. 87, 5569... 

The acetone-sensitized decarbonylation of -(lS),2(S) (125a) has been studied. The principal reaction is the formation of the 2(S),3(R)-cyclopropane (126a). Other products (127a), (128a) and (129a) are also formed in low yield. The reaction arises from the triplet state and this was confirmed by using Michler s ketone as the sensitizer and by quenching experiments. A similar selectivity is... 

The active vibration mode associated with CD group favors the non-radiative transition Tj->1Sq and shortens the intrinsic lifetime of the triplet state. This accounts for the lack of self-quenching reaction. Thus hexadeuterated acetone appeeurs to be a more specific photosensitizer than acetone in water (19). 

With regard to direct energy transfer to Indoles, the yields of Indole triplets In triplet acetone quenching were measured by Kasama et al. (18). The yields obtained for the test indoles were 0.2-0.43. The presence of chromophores in the aqueous CRH-1 solution, therefore, could sensitize the photolysis of Indoles directly by energy transfer to the triplet state. 

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