Sensitized photodegradation

Table 16.5 General structure of a sulfa drug, structure of five sulfa drugs containing six-mem-bered heterocyclic R substituents, and measured pK values pK values were calculated using a spectrophotometric titration. Errors represent the 95% confidence levels and both the values and the associated errors were obtained from fits of the data using Scientist for Windows. Reprinted with permission from Boreen AL, Arnold WA, McNeil K (2005) Triplet-sensitized photodegradation of sulfa drugs containing six-membered heterocyclic groups identification of an S02 extrusion photoproduct. Environ Sci Technol 39 3630-3638. Copyright 2005 American Chemical Society...

Cunningham, J. and Al-Sayyed, G., Factor influencing efficiencies of TiOj-sensitized photodegradation, /. Chem. Soc. Faraday Trans., 86, 3935, 1990. 

Erkan A, Bakir U, Karakas G (2006) Photocatalytic microbial inactivation over Pd doped SnOi and TiOi thin films. J Photochem Photobiol A Chem 184(3) 313-321 Evgenidou E, Fytianos K, Poidios I (2005) Semiconductor-sensitized photodegradation of dichlor-vos in water using TiOj and ZnO as catalysts. Appl Catal Environ 59(l-2) 81-89 Farooq S et al (1977) The effect of ozone bubbles on disinfection. Water Ozone Sci Eng 9(2) 233 Fiscetti M (2007) Fresh from the sea. Sci Am 297 118-119... 

The aim of the present contribution is first to investigate different dyes, besides of the mainly used Ru-complex (N3), to study chemical preconditions for improved stability and, further, to find out whether the excited or the oxidized sensitizer is a key problem in inducing photochemical side-reactions. A key question will be the role which Ti02 and its surface chemistry play for sensitizer photodegradation. For this reason, ZnO will be studied as a substrate in parallel. It should be investigated to what extent the chemical quality and surface morphology of nano-Ti02 do have an influence on the survival of oxidized sensitizer molecules. 

Deterioration. The causes of degradation phenomena in textiles (155—158, 164) are many and include pollution, bleaches, acids, alkaUes, and, of course, wear. The single most important effect, however, is that of photodegradation. Both ceUulosic and proteinaceous fibers are highly photosensitive. The natural sensitivity of the fibers are enhanced by impurities, remainders of finishing processes, and mordants for dyes. Depolymerization and oxidation lead to decreased fiber strength and to embrittlement. 

The process by which a solubility differential between exposed and unexposed areas occurs is well known (74). Photodegradation products of the naphthoquinone diazide sensitizer, eg, a l,2-naphthoquinonediazide-5-sulfonic acid ester (11), where Ar is an aryl group, to an indene carboxylic acid confers much increased solubility in aqueous alkaline developer solutions. 

PL can be used as a sensitive probe of oxidative photodegradation in polymers. After exposure to UV irradiation, materials such as polystyrene, polyethylene, polypropylene, and PTFE exhibit PL emission characteristic of oxidation products in these hosts. The effectiveness of stabilizer additives can be monitored by their effect on PL efficiency. 

The extent of the photodegradation reaction is measured by the photodegradation quantum yield, pd, which is defined as the fraction of molecules degraded in relation to how many photons were absorbed, and quantifies the light sensitivity of the molecule (Turro 1978). Usually, pd < 1 but values higher than unity can indicate more complex processes, such as radical chain reactions. 

Petersen, M., L. Wiking, and H. Stapelfeldt. 1999. Light sensitivity of two colorants for Cheddar cheese. Quantum yields for photodegradation in an aqueous model system in relation to light stability of cheese in illuminated display. J. Dairy Res. 66 599-607. 

Another mechanism for alkanone-sensitized photodehydrochlorination comprises Norrish type I scission of the ketone, followed by ground-state reactions of radicals (19). However, the evidence for such a mechanism is based on experiments that were carried out in the vapor phase (19). Initiation of the photodegradation of PVC by hexachloroacetone has been suggested to involve the abstraction of hydrogen from the polymer by radicals resulting from the photolysis of the ketone s carbon-chlorine bonds (22). 

As noted above, there is experimental evidence to indicate that the carbonyl groups of (vinyl chloride)-(carbon monoxide) copolymers are effective sensitizers for the photodegradation of these materials (9,10,11). A reasonable sensitization mechanism can be formulated for this system on the basis of the information now on hand. 

Catalytic hydrogenation of PVC causes a significant reduction in the rate of the polymer s subsequent photodegradation (32). Although this result is consistent with the occurrence of alkene sensitization (32), it can also be attributed, per se, to the removal of other possible sensitizers such as carbonyl groups and peroxide linkages. 

Tetrahydrofuran has been reported to exhibit an absorption maximum at 280 nm (52,56), but several workers have shown that this band is not produced by the purified solvent (30,41,57). Oxidation products from THF have been invoked in order to account for the appearance of the 280-nm band in PVC films that are solvent-cast from THF in air (57. 581. However, in some reported cases (56,59), this band was undoubtedly produced, at least in part, by a phenolic antioxidant (2.6-di-tert-butyl-p-cresol)(59) in the solvent. Since certain -alkylphenols have now been shown to be powerful photosensitizers for the dehydrochlorination of PVC (60), it is clear that antioxidant photosensitization might well have been responsible for some of the effects attributed previously (56) to THF alone. On the other hand, enhanced rates of photodegradation under air have also been observed for PVC films cast from purified THF (57), a result which has been ascribed to radical formation during the photooxidation of residual solvent (57,61). Rabek et al. (61) have shown that this photooxidation produces a-HOO-THF, a-HO-THF, and y-butyro-lactone, and they have found that the hydroperoxide product is an effective sensitizer for the photodehydrochlorination of PVC at X = 254 nm (61). 

In keeping with earlier observations (19,98), the nonoxidative thermal dehydrochlorination of PVC has been shown recently to be facilitated by preliminary photodegradation of the polymer (10,99). The thermal sensitivity enhancement increases with decreasing wavelength of irradiation (10) and undoubtedly results from the photolytic formation of thermally labile defect sites (10). 

Finally, it is worth noting that, in response to triplet sensitization, simple acyclic allylic chlorides may be converted into the corresponding chlorocyclopropanes (101). Perhaps this reaction can also occur during the photodegradation of PVC. 

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