Thioxanthone triplet formation

As with arene-amine radical ion pairs, the ion pairs formed between ketones and amines can also suffer a-deprotona-tion. When triplet benzophenone is intercepted by amino acids, the aminium cation radical can be detected at acidic pH, but only the radical formed by aminium deprotonation is detectable in base (178). In the interaction of thioxanthone with trialky lamines, the triplet quenching rate constant correlates with amine oxidation potential, implicating rate determining radical ion pair formation which can also be observed spectroscopically. That the efficiency of electron exchange controls the overall reaction efficiency is consistent with the absence of an appreciable isotope effect when t-butylamine is used as an electron donor (179). 

Evidence for the formation of triplet benzoyl nitrene on irradiation of 1-benzyl-l,2,4-triazolio-4-benzoylamidate (109) in the presence of benzophenone or thioxanthone has been described. 

In the latter, the chromophore itself does not initiate the polymerization. Instead, a coinitiator should be present in the medium as the source of initiating radicals. The chromophore, also termed as sensitizer, absorbs UV radiation and is excited to a triplet state. This stage is accompanied by abstraction of labile hydrogen from the coinitiator to form two radical species. The radical formed on the sensitizer is usually unreactive and does not take part in initiation. (Scheme 13.2) This way homopolymer formation can be prevented. The most common sensitizers are aromatic ketones such as benzophenones with maximum absorbance at 254 nm. Others include thioxanthones... 

There are a series of photochemical investigations in which product ratios indicate that, in some cases, it is neither the singlet nor the triplet carbene in their ground states that forms certain products with a given substrate. The results indicate that photoexcited diazo compounds or carbenes in excited states may react. Evidence for such mechanisms came hitherto only from product studies, some of these cases will be mentioned only in the sections on the corresponding products. Potential reactive intermediates in carbene formation are summarized in Scheme 8-11. The latter includes the excited singlet and triplet carbenes, as well as the formation of triplets by triplet sensitizers such as benzophenone or thioxanthone ( Sens) and the direct formation of products from the excited diazo compound with the substrate S . Product... 

It consists of thioxanthone or 2-isopropylthioxanthone in combination with (2,4,6-trimethylbenzoyl)-diphenyl-phosphine oxide or bis(2,4,6-trimethyl-benzoyl)-phenylphosphine oxide. The mechanism of sensitization was reported to involve triplet-triplet energy transfer from the thioxanthones to die phosphine oxides. That is followed by formation of radicals through a-cleavage of the photoinitiators. Direct photolysis of the phosphine oxides results in an absorptive, chemically induced, d)mamic electron polarization due to the triplet mechanism of polarization of the substituted benzoyl. The phosphoras-centered radicals produced by a-cleavage of the photoinitiators are the same radicals that... 

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