Phenyl ketones, triplet state

The homolysis of iodonaphthalene can be photosensitized in the presence of ketones.216 Homolysis of phenyl-iodine and phenyl-bromine bonds is a general photochemical reaction217,218 but even in the case of aromatic ketones,217 it is not as yet known whether the triplet state is involved in the direct photolysis. 

It is known that for phenyl alkyl ketones both the Type I and Type II reactions occur only from triplet state. The radical pair formed from a-cleavage undergo disproportionation resulting in Type I products or recombine to regenerate the ground state ketone. The latter process reduces the efficiency of Type I product formation. 

Flash photolysis and laser flash photolysis are probably the most versatile of the methods in the above list they have been particularly useful in identifying very short-lived intermediates such as radicals, radical cations and anions, triplet states, carbenium ions and carbanions. They provide a wealth of structural, kinetic and thermodynamic information, and a simplified generic experimental arrangement of a system suitable for studying very fast and ultrafast processes is shown in Fig. 3.8. Examples of applications include the keton-isation of acetophenone enol in aqueous buffer solutions [35], kinetic and thermodynamic characterisation of the aminium radical cation and aminyl radical derived from N-phenyl-glycine [36] and phenylureas [37], and the first direct observation of a radical cation derived from an enol ether [38],... 

Inasmuch as naphthyl ketones are not totally unreactive at hydrogen abstractions 54,60), there must be a small amount of reactivity intrinsic to n,n states. Such reactivity is actually known for 71,71 olefin triplets 81> and for enone triplets 6 2). In the case of phenyl ketones the carbonyl and aryl jr-systems are mixed, such that a locally excited carbonyl n,n triplet contributes a small fraction to the overall wave function for the triplet. Given this intrinsic reactivity, which is some 10-4—10-6 that of an n,n triplet, the observed reactivity of 71,71 triplets with nearby n,7t triplets can be expressed as follows, where k% is the rate constant... 

The only studies specifically designed to differentiate between vibronic mixing and equilibration as the source of ,jr -reactivity in ketones with 71,71 lowest triplets have been performed here at Michigan State University. Perhaps the best evidence that equilibration is the major mechanism involves a comparison of intramolecular triplet state hydrogen abstraction by a series of phenyl ketones hi,71 lowest) and an analogous series of p-methoxyphenyl ketones (37i,n lowest). 

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