Benzophenone sensitiser

Interest continues in the photochromic behaviour of the spiropyran and related systems both in the patent and the basic research literature. Nanosecond laser kinetic spectroscopy has been used to investigate the intermediates involved in the interconversions between the colourless spiropyrans (35) and (36) and the coloured merocyanines present in solution at room temperature as the -isomers (37) and (38). On excitation (530 nm) of nitro derivative (37) in acetone, two transients were observed, the triplet excited state (lifetime 6ps) and the Z-isomer (lifetime 0.3ms). The same transients were observed on excitation (353 nm) of the spiropyran form (35). Naphthalene or benzophenone sensitisation of the photo-... 

Cyclopropane derivatives are the major products of photoelimination of nitrogen from 1-pyrazolines. Conversion of the fused pyrazoline (10) into the cyclopropane (11) was achieved in this way in 85% yield by irradiation in the presence of acetophenone and constitutes a valuable step in a synthesis of a diquinane alcohol. The short-lived triplet 1,3-cyclopentadiyl biradical, generated by benzophenone-sensitised irradiation of diazabicyclo-(2.2.1]hept-2-ene, has been trapped as a Jbis-alkoxyamine by a nitroxide. The diazabicycloheptenes (12) gave in a similar fashion the bicydo[2.1.0]pentanes (13) on triplet-sensitised photolysis, whereas laser/liquid jet excitation of the same compounds gave in addition the cyclopentenes (14), derived by 1,2-hydrogen shift evidence for a two-photon process is described. The results of a time-resolved spectroscopic study of the photodecomposition of 2,3-diazabicyclo[2.2.l]hept-2-ene in the vapour phase have also been reported.Photolysis of 2,3-diaza-5-methylenebicyclo[2.2.l]hept-2-ene affords the semi-localised... 

Odm Processes - The benzophenone sensitised reaction of the N,0-diacyl N-phenylhydroxylamine (189) in hexadecyltrimethylanunonium chloride micelles yields benzoyloxy-migrated products (190) and (191) derived from amidyl-benzoyloxyl radical pairs located at the micellar surface, whereas amidyl-phenyl radical pairs that penetrate more deeply into the micelle are responsible for phenyl-migrated products (192) and (193). Irradiation of ethanolic solutions of N-aryl-N-nitrosohydroxylamine ammonium salts produces azoxy compounds. 

Other Elements - Further investigations of the benzophenone sensitised reactions of methyl-substituted selenophenes and tellurophenes with a range of maleic anhydride derivatives have been reported. Cleavage of the acyl-tellurium... 

The aromatics listed above each give two structurally isomeric products in benzophenone-sensitised reactions at 20°C, 1,2-cycloaddition is exclusively or predominantly at unsubstituted positions on the ring, but at 100°C the proportions of products are changed andp-xylene is preferentially (10 l)attacked at the 1,2-positions . Steric effects and effects of temperature on equilibria involving charge-transfer complexes are suggested - . 

Ultraviolet irradiation of oxadiazoline (38d) at 333.6 nm (or irradiation using benzophenone as a triplet sensitiser) gave 2-diazopropane and methyl acetate. A triplet biradical intermediate formed by cleavage of the C(OMe)—N bond was postulated <90TL863>. Oxadiazolinone (42) underwent nucleophilic attack at the carbonyl group by methyllithium to give acetate (41) after treatment of the product with acetyl chloride <89CJC1753>. 

Beeby, A. Bushby, L. M. Maffeo, D. Williams, J. A. G. The efficient intramolecular sensitisation of terbium(III) and europium(III) by benzophenone-containing ligands. J. Chem. Soc., Perkin Trans. 22000,1281-1283. 

This method has been applied to the rigid 3-bond dyad, DNM[3]M, (Fig. 37), in which the maleate ester group (M) serves as the acceptor (the DCV acceptor group cannot be used because the energies of the singlet and triplet +DMN[3]DCV CS states all lie above the 3[ DMN[3]DCV] locally excited triplet state, see Fig. 29). Benzophenone (BP) was used as the sensitiser, and a highly polar solvent... 

Fig. 37 (a) Depiction of the triplet sensitisation of DMN[3]M by benzophenone, to give the locally excited donor triplet state, 3( DMN[3]M),... 

Benzophenones have been described as useful sensitisers for PET catalysed conjugate addition reactions of a-amino alkyl radicals to enones (Bertrand et al. 2000). We tried to modify this reaction and synthesised the pyrrolidinylethyl-substituted quinolone 35 from the known bromide (Bauer et al. 2005). Upon electron transfer from the pyrrolidine to a given acceptor, a radical cyclisation occurs (Scheme 15), which after electron and proton transfer generates a pyrrolizidine. We found 4,4/-dimethoxybenzophenone to be a suitable catalyst for this reaction. Remarkably, the reaction proceeded with excellent simple diastereos-electivity and a single diastereoisomeric product rac-36 was obtained. With 10 mol% of the catalyst, a chemical yield of 71% was achieved. 

They also obtained a similar CIDEP spectrum by triplet sensitisation with triplet benzophenone [13]. The polarization was found to consist of two net component, one reflecting the polrization of the triplet sensitizer due to the p-type TM, and the other being attributable to the d-type TM. 

Although benzophenone derivatives have found application in a number of photopolymerization processes (10), these have usually involved cross-linking reactions induced by energy transfer from photo-exdted benzophenones to groups (e.g. cinnamate) attached to the polymer chains. In such cases, the benzophenones act as sensitisers and not as photoinitiators. An e>fample in which hydrogen abstraction by photoexdted benzophenone has been utilized for photoinitiation is the ultra-violet induced grafting of styrene on to polyethylene (8,45). Recently, extensive studies concerned with photoinitiation of vinyl polymerization by benzophenone derivatives in homogeneous media have been carried out, and are described in detail below. 

The alkyl aryl ketone sensitised photolysis (313 nm) of hexane solutions of t-butyl Cl reported by HarrimaniZ also gave HCl as a major product together with 2-methyl propene. Quantum yields ranged from 0.19 when benzophenone was the sensitiser to 0.313 for EtC=0 and the involvement of the triplet state was confirmed by the quenching of the reaction by 2,5-dimethyl-hexa-... 

The activities of various alkanolamines have been examined in conjunction with the 2-(2-chlorophenyl)-4,5-diphenylimidazolyl radical and benzophenone. In the case of the former the ability of the radical to abstract a hydrogen atom from the amine determines the extent of the cure while for benzophenone it is the degree of hydrogen atom abstraction, and hence cure is the same. Polymeric and monomeric sensitisers have been compared and the influence of initiator... 

Poly(vinyl halides) - The photocatalysed oxidation of PVC has been undertaken in the presence of titanium dioxide and zinc oxide pigments and the extent of dehydrochlorination measured. Acetic and formic acids were major products along with carbon dioxide. Copper(II) dialkyldithiocarbamate complexes are also sensitisers. Photodegradable PVC has also been developed by grafting with benzophenone chromophores. Plasticised PVC also degrades and discolours on irradiation but this is due mainly to the plasticiser. ... 

Photochemical grafting of monomers/molecules onto substrates continues to show interest in terms of improving property requirements. Dyes have been effectively photografted onto polypropylene and nylon 6 films using 1,2-diphenyl-2,2-dimethoxyethanone while methacrylic acid and acrylamide have been successfully photografted onto polypropylene sheets using benzophenone as a sensitiser in order to modify the surface of the polymer from a hydrophobic to a hydrophilic one. A styrene-2,2,6,6-tetramethyl-4-piperidinyl... 

Coumarin has been studied extensively in this context in the absence of a sensitiser, it gives a syn head-to-head dimer in the presence of benzophenone as sensitiser, the anti isomer is formed the syn head-to-tail dimer is obtained by irradiation in acetic acid. Cyclobutane-containing products are obtained in modest yields by sensitiser-promoted cycloadditions of coumarins or 3-acyl-oxycoumarins, with alkenes, ketene diethyl acetal or cyclopentene. ... 

The formation of benzpinacol 8.210 by the action of light on benzophenone 8.43 is one of the earliest organic photochemical reactions to have been discovered.1182 It is now known that the reaction takes place from the 11-71 triplet state. Subsequently, and rapidly, the spin of one of the electrons inverts to give the triplet excited state 8.208 that is so often used to sensitise photochemical reactions. With two unpaired electrons, there is a question about which abstracts the hydrogen atom from the solvent. It is almost certainly the electron left behind in the p orbital on oxygen, which will be low in energy, and therefore close to the HOMO of the C—H bond of the solvent, B in Fig. 8.13. The other odd electron will be in the carbonyl n orbital, which is... 

---