Benzoyl chromophore

Recently, however, examples of intramolecular triplet sensitization have been described. Alkyl aryl ketones with pendant azido groups in the alkyl moiety were irradiated into the benzoyl chromophore. The excited alkylphenyl ketones undergo intersystem crossing to their triplet states within picoseconds and they can then transfer their triplet energy to the nearby alkyl azido group. This process... 

From his results he concluded that ketones such as the p-methoxy-benzoyl chromophore react from equilibrium concentrations of their n,iT triplets. Support for this view is found in the fact that reversible Internal conversion of the two triplets is much faster than any irreversible decay modes of either individual triplet. Furthermore, the observation that interchromophore energy transfer is sufficiently rapid to allow almost complete equilibration before any chemical reaction, certainly lends support to the supposition of rapid intrachromophore energy transfer. 

In a general way, an adequate substitution at the Rj and R2 moieties of a R j-Ry photoinitiator has led to a large variety of compounds. Most cleavable compounds are based on the benzoyl chromophore Rj. 

SulfonylKetones Sulfony 1 ketones (10.20) undergo a (3-cleavage process that occurs in the triplet state [2i,35,36], A large variety of compounds have been developed [37], The properties of the benzoyl chromophore are strongly modified by the presence of a sulfoxide, a thiother or a phenyl substituent at the para position. 

Various hydroxylactams, for example (10), have been synthesised from w-phthalimidoalkanoates (11) by triplet-sensitized photodecarboxylation in a process whose crucial step in this macrocyclisation is protonation of the intermediate ketyl radicals. P-Aminopropiophenones (12) can be regioselec-tively photolysed to 2-aminocyclopropanols (13) which can subsequently undergo photooxidative ring opening to the enaminones (14). The regioselectivity is controlled by the preferred charge transfer interaction between the photoexcited benzoyl chromophore and the amino group. 

Cleavage occurs predominately in a-halogenoacetophenones however, it is also seen in photoinitiators with a C—S or C—O bond adjacent to the benzoyl chro-mophore, such as /3-ketosulfoxides (3,11). Equation 2 demonstrates this cleavage mechanism for the photoinitiator 2,2,2-trichloro-4 - ert-butylacetophenone (the dashed box highlights the benzoyl chromophore). 

Water-Soluble Photolnitlators. Water-soluble photoinitiators are needed for systems snch as printing inks and emulsion processes, where the reaction system is an aqneons solution rather than simply a monomer. Since free radical photoinitiators are based on the aromatic benzoyl chromophore, the molecules are generally nonpolar and therefore incompatible with water. Research has fo-cnsed on developing photoinitiators with hydrophilic substituents that increase water solnbility (3,25). Both unimolecular and bimolecular systems have been suc-cessfnlly demonstrated. An example of a imimolecular photoinitiator is sodium 4-benzoylbenzenemethane sulfonate (13). 

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