Photosensitization with Halogens

In a sensitized photochemical reaction, light acts directly on a molecule (or atom) of the sensitizer. The primary photochemical process can be either the dissociation of the sensitizer molecule to its atomic constituents or its excitation. An example of the first path is the photochemical decomposition of ethylene iodide in the presence of iodine. This reaction has been repeatedly studied [75, 400]. 

The following mechanism has been derived from experimental data I2 + hv = 21 (kg) 

The C2H4I decomposition rate is accordingly w = ki(C2H4l2) (I). From the stationarity condition for C2H4I and I this expression transforms to 

It is very likely that an excited halogen molecule suffers induced predissociation which is the first (initial) step of the given elementary reaction. Por this reason it may be considered that the halogen atoms are the primary active centers of halogen-sensitized photochemical reactions in virtually every case. A peculiar case of photosensitization with bromine has been observed for carbon monoxide oxidation [278]. The process Brg + hv - 2Br has been found to be followed by Br + O2 + CO - BrO + CO2, etc. 

It will be noted in connection with the halogen-induced photochemical sensitization that the reactions in which the sensitizer is not consumed seem to be possible only in the case of iodine. Bromine and chlorine atoms possess a higher chemical activity than the iodine atoms and under the experimental conditions used a certain amount of the sensitizer becomes irreversibly lost in virtually all the reactions studied. 

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Irradiation of oxadisilirane 82c in an acetonitrile-methylene chloride solvent mixture in the presence of 9,10-dicyanoanthracene (DCA) as photosensitizer with a tungsten-halogen lamp under an oxygen flow resulted in the formation of 83c in 69% yield. In the absence of the sensitizer, no reaction occurred and 82c was recovered quantitatively. A similar result was also obtained in the reaction of 82c with 302 in the presence of 10 mol% of tris(p-bromophenyl)aminium hexachloroantimonate [ /j-BrCr,H/ (N+SbClfU (BPHA) as the single ET reagent, which gave 83c in 58% yield (Table 17)97. 

Metalloporphyrins and related compounds continue to attract attention as photosensitizers and as building blocks for construction of artificial lightharvesting arrays. The photophysical properties of several structurally modified porphyrins have been measured with a view to identifying new sensitizers for use in photodynamic therapy. The triplet state properties of porphyrins adsorbed onto the outer surface of vesicles have been describedand the fluorescence spectral properties of some amphiphilic porphyrins have been recorded. Similar studies have been carried out with halogenated tetraar-... 

I. Yu., Grineva, L.G., Polishchuk, A.P., and Chernega, A.N., Products of quaternization of 4,4 -bipyridine with halogenated carboxylic acids. Synthesis, structure and photoreduction in the crystalline state, Russ. J. Gen. Chem., 68, 609, 1998 (d) Suzuki, M., Kimura, M., Hanabusa, K., and Shirai, H., Reversible color changes induced by photosensitized charge separation in partially quaternized poly(l-vinylimidazole)-bound ruthenium(ll) complex and viologen hlms, Eur. Polym. 

The gas-phase photochemistry of haiogenated ethenes has been studied in the UV and VUV [60, 61], as well as in the infrared, using multiple-photon-absorption excitation with powerful CO2 laser sources [62-66]. Also, sensitized decompositions, for example using electronically excited Hg( P) atoms, have also been reported [67-69]. The net gas-phase photochemistry of these systems usually involves hydrogen halide elimination via three-and/or four-center transition states, with some evidence for simple bond fission producing halogen atoms in the case of Hgf Pj) photosensitization [70]. 

Lomefloxacin reportedly causes the highest incidence of phototoxicity (photosensitivity) of the currently available quinolones. The presence of a halogen atom (fluorine, in this case) at the 8 position has been correlated with an increased chance of phototoxicity in the quinolone.s. ... 

The halogenated salicylanilides (tetrachlorosalicylanilide, brominated salicylanilides) and related compounds caused an epidemic of adverse reactions to sunlight between 1960 and 1970, when they were incorporated as antibacterials in soaps and cosmetics (Wilkinson 1961 Calnan et al. 1961 Epstein et al. 1968). After 1968 a striking reduction in numbers of patients with positive photopatch tests to these compounds has been noted (Smith and Epstein 1977), and explained partly by removal from the market of the more potent photosensitizing chemicals, partly by increased awareness by physicians. 

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