Photoreactions of carbonyl groups

Absorption of light by carbonyl groups produces two excited states the singlet (S ) (n - n transition) and by intersystem crossing (ISC) the triplet state (T ) n n transition). Ketones attached to the aromatic nucleus can produce the triplet state (T ) hy n- n transition. The most reactive is the triplet state (T ) in which carbonyl groups exists as a biradical (2.6)  

Triplet states T(n,n ) and T(n,7i ) have quite different physical and chemical properties. Difference in reactivity towards hydrogen abstraction, e.g. from a polymer molecule (PH) and formation of a ketyl radical (2.7)  

According to the Norrish type I reaction, a carbon-carbon bond in a position a to the carbonyl group is cleaved. The resulting ketyl radical is very likely to release carbon monoxide [Eq. (7-2)]. 

The Norrish type 11 process refers to a C-C bond cleavage initiated by the abstraction of a hydrogen in a y-position with respect to the carbonyl group. 

Note that Norrish-type reactions are not only of importance in relation to various polymers containing ketonic impurities, but they also play a dominant role in the photolysis of all polymers containing carbonyl groups as constituent moieties, such as polyacrylates, polymethacrylates, poly (vinyl acetate), polyesters, and polyamides. 

The formation of intermolecular cross-links, i.e. covalent bonds between different polymer chains, causes an increase in the average molar mass and eventually combines all of the macromolecules into a three-dimensional insoluble network. Cross-linking can be accomplished in various ways. Several methods rely on reactions of electronically excited pendant groups on the polymer chains, others on reactions of various kinds of reactive species in the ground state that are photogenerated in polymeric systems. Typical of the former reaction type are [2-f-2] cycloadditions that occur in the case of linear polymers bear- 

The reaction of an excited alkene molecule in its Si or Ti state with an alkene molecule in its ground state produces a cyclobutane derivative [Eq. (7-3)]. 

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