Photo-sensitisers

A detailed study of the photostability of the DAST-type agent Cl Fluorescent Brightener 85 on cellophane film was carried out recently. The initial fading reaction is a photo-sensitised trans-cis isomerisation of the stilbene grouping. The subsequent oxidative attack on the molecule is concentrated on the vulnerable ethene linkage at the centre of this moiety [39]. 

Mei and Barton [66, 67] have used photo-sensitised damage to DNA by /1-Ru(TMP)3 (induced by 02 formation) as a means of probing A-DNA conformations, and they have compared the damage so caused on a linear... 

In like manner, inclusion of mineral acid in the photo-sensitised grafting of styrene in methanol to cellulose enhances the polymerisation yields significantly (Figure 2). The results obtained with phosphoric and sulfuric acids were consistent with earlier data (13,14). Dioxan is also a useful solvent for these reactions when uranyl ion is used as sensitiser. However dioxan is not as efficient as methanol for the grafting reaction. The grafting in dioxan exhibits a Trommsdorff peak at 60% monomer concentration. The results in Figure 2 also show that the unsensitised process does occur in dioxan as in other solvents, but... 

Hammarstrom, L., Sun, L., Akermark, B., Styring, S. 2001. A biomimetic approach to artificial photosynthesis Ru(II)-polypyridine photo-sensitisers linked to tyrosine and manganese electron donors. Spectrochim. Acta Part A 37, 2145-2160. 

Excited singlet oxygen generated in situ by oxidising hydrogen peroxide with sodium hypochlorite, produces results chemically indistinguishable from photo-sensitised auto-oxidation of olefins [70]. 

A recent photo-sensitised oxygenation of Ai 20) pi-egnene derivatives (i8) offers a useful route to corticosteroid intermediates [78]. The reaction (19) involves stereospecific a-face attack by excited singlet oxygen and affords 20-hydroperoxy-pregn-16-enes (20) which may either be reduced to 20-alcohols (21) or be dehydrated by acetic anhydride in pyridine to give 20 ketones (22). The stereospecificity is such that a m-pregn-... 

Photo-sensitised oxygenation of mono-olefins gives hydro-peroxy-olefinic products, with double-bond migration. Irradiation of a solution of cholesterol (3) containing haemato-porphyrin gave the 5a-hydroperoxy-A -olefin (4) [73]. Cholest 5-en 3-one (5) gave 5a-hydroperoxy-cholest-6-en-3-... 

The polyolefins are particularly suitable subjects for study since formally they should not absorb in the uv above 285 nm which is the effective cut-off point for uv light coming through the earth s atmosphere. In practice they do absorb and it has been a matter of conjecture since polyethylene was first discovered as to which of the very minor chromophoric constituents of commercial polyolefins are responsible for their photo-sensitisation. 

Resonance lamps of the alkali metals, zinc and cadmium have been constructed. Laidler shows how Hg, Zn and Cd lamps may be put to very good use to place limits on bond energies and to determine the mechanisms of some photo-sensitised reactions. However, none of these lamps has been used extensively and have limited (or no) photochemical use. 

Other studies may be required such as local toxicity and photo-sensitisation, for excunple ocular irritation for nebulised products, and mechanistic studies where relevant to assess safety in humans. 

Burnside S., Moser J. E., Brooks K., Gratzel M. and Cahen D. (1999), Nanocrystalline mesoporous strontium titanate as photoelectrode material for photo-sensitised solar devices increasing photo voltage through flatband potential engineering , J. Phys. Chem. B 103, 9328-9332. 

When a diene is involved, there is often competition between ring closure to 1,2- and 1,4-cycloadduct (see also Section 4.2). For instance, the photo-sensitised reactions of l,l-dichloro-2,2-difluoroethylene, 1,1-dichloroethylene and tetrachloroethylene with cyclopentadiene at 0°C give the following products ... 

In separate work aimed at elucidating filler effects on surface, oxidation Vesely and coworkers [39] have examined the oxidation of polyethylene and polypropylene containing high levels of calcium carbonate. The grade of filler used was found to decrease polymer thermal stability, an effect they associated with transition metal impurities. They also believe that, in the presence of fillers, heteregeneous oxidation reactions become more important. They observed black polyenes to be formed above about 320 °C and postulated that these could be acting as photo-sensitisers. 

Friso, E. Roncucci, G. Dei, D. Soncin, M. Fabris, C. Chiti, G. Colautti, R et al., A novel B-10-enriched carboranyl-containing phthalocyanine as a radio- and photo-sensitising agent for boron neutron capture therapy and photodynamic therapy of tumours in vitro and in vivo studies. Photochem. Photobiol. Sci. 2006, 5, 39-50. 

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