Methyl ketones, quantum efficiency

The efficiency of the type I reaction usually depends on the relative stability of the radicals produced. For this reason, higher type I quantum efficiencies would be expected for methyl isopropenyl ketone (MIPK) styrene copolymers because the polymer chain radical formed is tertiary rather than secondary, i.e.,... 

The quantum efficiencies of NBD-hydroxyproline range from 0.01 in water (pH 9) to 0.80 in isobutyl methyl ketone [204]. Tryptophan does not yield a fluorescent product [205]. 

The quantum yields of the Norrish Type II process depend on the chain length and structure of the ketone carbonyl group (Table 3.7) [859]. For a methyl or phenyl ketone, the quantum yield remains almost independent of chain length, regardless of whether it is in a polymer or a small molecule whereas, if it is in the centre of a chain, the quantum yield decreases with chain length. A side chain ketone shows a considerably higher efficiency, which is more or less independent of molecular weight. 

The expected correlation between photodecarbonylation efficiencies and the stability of the radicals formed can be illustrated with symmetric ketones giving rise to primary, secondary, and tertiary radicals in solution (Scheme 4). Results from irradiation of diethylketone (3-pentanone) in the gas and in solution are similar to those obtained with acetone. -"" In contrast, with two methyl groups on each a-carbon, the excited state of di-iso-propylketone 7 a-cleaves and loses CO to give decarbonylated products in 51% chemical yield. Finally, as expected from the presence of three radical-stabilizing methyl groups on each a-carbon, di-f-butylketone 8 was reported to decarbonylate with a quantum yield of = 0.7 from both singlet (3 = 0.4) states and a yield of CO formation of 90%. ... 

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