Benzophenones as photosensitizers

Since homopol)rmer had a marked effect on the aesthetic properties of grafted textiles, we considered methods whereby homopol3mier formation could be minimized. Earlier Howard, Kim, and Peters had successfully photografted methacrylic acid vapors on nylon 6 film using benzophenone as photosensitizer (24). They found that the technique was efficient only at low concentrations... 

A soln. of N-benzylidene-ferr-butylamine and an equimolar amount of benzophenone as photosensitizer in benzene irradiated under Ng with a high-pressure Hg-lamp -> benzonitrile. Y 50% based on startg. m. consumed. F. e. s. H. Ohta and K. Tokumaru, Chem. Ind. 1971, 1301. 

Using benzophenone as photosensitizer, radical alkenylation of unactivated C (sp )-H bond was developed with 1,2-bis(phenylsulfonyl)ethylene as coupling partner (Scheme 3.28) [80]. A variety of substrates were compatible, including amides, ethers, alcohols, and cycloalkanes. Moreover, the obtained siflfonylalkenes are useful building blocks for the construction of prenyl and pyrrole derivatives. 

Photo-induced grafting onto wood cellulose with several vinyl monomers using phenyl acetophenone and benzophenone derivatives as photosensitizers has been reported [72]. A fast increase in grafting was observed for short irradiation times, whereas longer ones resulted in a decreased percent grafting. 

The reaction between Cell—O radical and vinyl monomers leads to the formation of grafted cellulose. In the presence of photosensitizers generally used as photoinitiators, such as benzophenone and phenylace-tophenone derivatives, the photoinitiator absorbs the UV radiation and transforms to its singlet (S ) and then triplet (T ) states. After that it may decompose into free radicals or transfer its energy to cellulose or any other molecules in the system. Take benzophenone as an example ... 

Although this fact alone does not demand that the species be a triplet, presumptive evidence for the triplet mechanism is provided by the demonstration that the reaction can be carried out using benzophenone as a photosensitizer and yields the same product distribution. 

Schenck and Steinmetz [26] have found that the addition of maleic anhydride to benzene can be photosensitized by benzophenone. The photosensitized reaction also works with toluene, o-xylene, and chlorobenzene. Hardham and Hammond [27,28] have also studied the photosensitized addition and they obtained definitive evidence that this reaction proceeds via the triplet state. They also proposed an ortho adduct as intermediate but were unable to isolate it. Bryce-Smith and Vickery [29] found that phenanthrene reacts with maleic anhydride at the 9,10-positions to give an adduct that does not further react with a molecule of maleic anhydride. This reaction can also be sensitized by benzophenone. 

As indicated above, it is unlikely that free radical ions are formed in these CT quenching processes. If they were, protons more acidic than benzylic ones could be lost. For example, the radical cation of phenoxyacetic acid readily decarbonylates. Thus, the report that benzophenone can photosensitize the... 

Methylenecyclopropane undergoes photosensitized [2 + 2] cycloaddition with various substituted alkenes. Irradiation of acetone solutions of maleic anhydride or substituted derivatives and methylenecyclopropane in the presence of benzophenone as a sensitizer gave spiro[2.3]hexa-nes 24 in good yields. 

The same substances that are used as thermal initiators (I), e.g., azo compounds and peroxides, are also used often as photosensitizers. However, many other substances can be used as photosensitizers even though they do not undergo thermal dissociation to act as thermal initiators. Benzophenone and acetophenone and their derivatives are examples of such substances most commonly used as photosensitizers. When irradiated, these ketones undergo fragmentation generating radicals. For example, benzophenone dissociates as O O O... 

The use of these monomers for radiation cross-linking of polyethylene has been suggested [66]. With benzophenone as a photosensitizer, atactic as well as isotactic polypropylene is crosslinked with allyl acrylate by UV radiation. In this process both types of double bonds react [67]. Elastomers such as ethylene-vinyl acetate copolymer have been cross-linked with this monomer on a roller mill at 150°C using dicumyl peroxide as the initiator. Such cross-linked elastomers exhibit little or no swelling with aromatic solvents or chloroform after 24 hr at 30°C, conditions under which the uncured elastomers ordinarily dissolve [68]. Despite these interesting applications for such monomers, the bulk of the commercially produced allyl methacrylate finds application as a synthetic intermediate rather than as a monomer. 

An alkynyliodonium salt, namely, phenyl(phenylethynyl)iodonium hexafluorophosphate, has been tested for application as cationic photoinitiator [38]. The high activity of phenyl(phenylethynyl)iodonium salt as a photoinitiator was verified by photo differential scanning calorimetry (photo-DSC) experiments in direct irradiation and in photosensitized initiation using 9,10-dibutylanthracene, 2-isopropylthioxanthone and benzophenone as sensitizers [38]. 

Many organic substances have been found to act as photosensitizers such as phenones, porhyrines, naphtalenes, pyrenes, benzophenones and cyano compounds as well as inorganic species such as transition metal complexes and many others (Kavarnos and Turro 1987). It is well known that the formation of singlet dioxygen in biochemistry (which can afterwards add on olefins) generates ... 

Benzophenone was used as photosensitizer in a study of the photo-decomposition of benzene solutions of polystyrene and poly(a-methyl styrene). Laser photolysis (A 265 nm) of poIy(a-methyl styrene) in chlorofonn and carbon tetrachloride proceeded by attack of solvent radicals in the polymer molecules resulting in main chain cleavage. " A series of papers have been devoted to the study of the photo-induced decomposition of poly(a-methyl styrene) in benzene solutions, using a,a -bisisobutyronitrile, benzophenone, and triphenyl-methyl cations as sensitizers. Spin-trapping techniques were used to detect transient radicals formed during benzophenone-sensitized degradation of poly-(a-methyl styrene). The rate constant for intramolecular reaction of the macroradical was estimated. 

The reduction potentials for S,S-dialkyl-S-(hydroxyphenyl) sulfonium salts do not appear to have been measured. These sulfonium salts do not undergo electron transfer photosensitization. Rather, photosensitization has been achieved using typical diaryl ketones such as benzophenone, Michler s ketone, and thioxanthone as photosensitizers. Strongly acidic solutions are produced during photosensitized photolysis, suggesting the formation of a protonic add as one of the products. 

Benzophenones are photosensitizers for use in UV curing systems such as printing inks and wood and metal finishes. Purity 99.0% min. 

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