Photochemistry of nonconjugated diketones: internal self-quenching and energy transfer

The triplet state behavior of nine α,ω-dibenzoylalkanes indicates the occurrence of a rapid quenching interaction between the two carbonyl groups. This quenching is fastest (k = 3 × 107 s−1) in dibenzoylbutane, is slightly slower (~107 s−1) in dibenzoylethane, dibenzoylpentane, and 2,2-dibenzoylpropane, but is absent in 1,3-dibenzoylpropane. It also occurs in several "mixed" 1,4-diaroylbutanes incorporating p-ethylbenzoyl or p-methoxybenzoyl chromophores This internal self-quenching is interpreted as the intramolecular counterpart of the well-known bimolecular self-quenching of aryl ketones, although no exact mechanism can be proposed. Such internal quenching does not occur as rapidly, if at all, in three "turned around" diketones: δ-(p-acetylphenyl)valerophenone, δ-(p-acetylphenoxy)valerophenone, and γ-(p-acetylphenoxy)butyrophenone. This fact, together with the varying rates of internal self-quenching in the dibenzoylalkanes, indicates the necessity for a very specific and close orientation of the two carbonyl groups for self-quenching. In the mixed diketones containing a p-alkylbenzoyl group, triplet excitation appears to be fully equilibrated between the two chromophores. However, in those containing a p-methoxybenzoyl group, excitation does not fully equilibrate before triplet decay, as evidenced by different quenching efficiencies for products from the two carbonyls. Analysis indicates intramolecular energy transfer rate constants ≤ 108 s−1. These are sufficiently lower than in other bichromophoric systems to suggest relatively slow energy hopping in the polymers of phenyl vinyl ketone. Keywords: nonconjugated diketones, dibenzoylalkanes, self-quenching, energy transfer, triplet ketones.