Electron transfer reaction, radicals with diphenyliodonium salts

TABLE 5. ELECTRON TRANSFER REACTION OF RADICALS WITH DIPHENYLIODONIUM SALTS... 

DeVoe et al. have reported quantitative aspects of photosensitization of diphenyliodonium salt and bis(4-dimethylaminobenzylidene)acetone (DMBA) [101]. This ketone is a bis-vinylog of Michler s Ketone, which is a well-known sensitizer for onium salt initiated free radical polymerizations [102,103], The reaction with DMBA is an example of electron transfer sensitization gated by conformational relaxation of the sensitizer. The ratio of iodonium salt consumption to aminoketone consumption is two, the second iodonium salt equivalent is consumed by a second reducing equivalent from the aminoalkyl radical on the oxidized photosensitizer. 

Still another example is an initiating system composed of 7-diethylamino-3 -(2 -N-methyl-benzimidazolyl)-coumarin and diphenyliodonium hexafluorophosphate. This composition initiates the polymerization of methyl methacrylate in visible light. After the dye absorbs flic light energy, quick electron transfer takes place from the dye to the iodonium salt to produce free radicals. " The light induced reaction is claimed to occur mainly through the excited singlet state of the coumarin and results in low sensitive to O2. The fluorescence of the coumarin compound was reported to be quenched efficiently by the iodonium salt. " The reaction was observed to be in accord with the Stem-Volmer equation. The influence of the concentration of coumarin on the polymerization rate of methyl methacrylate led to the conclusion that the free radicals from coumarin act mainly as chain terminators. ... 

A particularly graphic example of electron-transfer photosensitization is the interaction of diphenyliodonium salts with perylene. Exposure of a dichloromethane solution of a mixture of these two compounds to UV light for 2 s results in the formation of the intensely blue-colored and relatively stable perylene eation-radical. The blue color remains in solution in air for a considerable time until it slowly fades. When this photolysis reaction is conducted in the presence of a cationically polymerizable monomer, the blue color of the perylene cation-radical is very transient. 

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