Quantum yields photodegradation

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. 

The phototransformation of aqueous 2-chloroaniline was studied by Oth-men and Boule by means of product analysis [47] these authors later deepened and extended their work, including also 2-fluoro- and 2-bromo-aniline [48]. The range of photodegradation quantum yields was

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A number of other, but minor primary photoproducts was also found, among them the products expected from a radical (photo-Claisen) rearrangement and from photohydrolysis of the ortho chlorine 2- and 4-chlorophenol were detected too, but their formation remained unexplained. The photodegradation quantum yield of dichlorprop did not depend on pH and was 50 times smaller than that of the anionic form of the related monohalo-genated compound mecoprop (see above) [77]. This is another example of the marked influence of the pattern of ring halogen substitution on the course and on the efficiency of photodegradation. 

In a quantitative study of the photostability of spiropyrans, Malkin et al.11 concluded that the photodegradation quantum yield d is independent of the heterocyclic part and is determined instead by the structure of the chromene part of SPs. According to these authors, photodegradation of nitro-substituted SPs originates from the reaction of the cis-cisoid X isomer with solvent or impurities (Scheme 6) ... 

Obviously, low concentration of humics (Ha < 0.0333 pmol/L) can increase the photodegradation quantum yield (O) by about 16% from 1.068 x 10"4 (surfactant only without humic) to 1.238 x Hf1 (surfactant with 0.0333 pmol/L Ha). Likewise, in the presence of low concentration of Soil Extracted Materials (surfactant with 3.96 pmol/L SEM), the quantum yield of DR decay was significantly increased by 239% to 2.554 x 10"4. The quantum yield improvement is likely due to the increment of available hydrogen sources in the solution, which was known critical to the photoreduction reactions. 

These quantum yield values appear to be much higher than unity and therefore demonstrate that carbonization occurs by a chain reaction process. The mechanism of the laser-induced dehydrochlorination of photodegraded C-PVC can be schematically represented by the follo-... 

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. 

Frequently B will also undergo a back hydrogen transfer which regenerates the parent ketone, as well as cyclization (in most cases a minor reaction) as a result of this competition the quantum yields of fragmentation are typically in the 0.1-0.5 range in non-polar media. When the Norrish Type II process takes place in a polymer it can result in the cleavage of the polymer backbone. Poly(phenyl vinyl ketone) has frequently been used as a model polymer in which this reaction is resonsible for its photodegradation, reaction 2. 

Perhaps the most interesting feature is exhibited by the copolymers, rather than the homopolymers. We find that the reduction in the quantum yield of photodegradation always exceeds the abundance of o-methylbenzoyl chromophores (5.) Table I illustrates this effect. 

Quantum yields of photodegradation were measured using an automatic viscosity timer modified so that the samples could be irradiated and deaerated in situ. 

Quantum yields and lifetimes of emission (fluorescence) as well as other principal rates of deactivation have been measured on 2-hydroxy benzophenone and 2-hydroxyphenyl benzotriazole derivatives. Polymerizable UV screening agents have been prepared and copolymerized with acrylics in order to obtain transparent films containing nonfugitive UV screening agents. Preliminary results of studies of photodegradation on these copolymers are also reported here. 

During photolysis, the double bond content of the polysilane(P-l)(15mol% in this experiment) decreased to 10mol%, as measured by 1H-NMR spectroscopy. However, the ratio, quantum yield of scission(Q(S))/quantum yield of crosslinking(Q(X)), was not affected by the reaction of the double bond. West and his coworkers have reported that poly((2-(3-cyclohexenyl)-ethyl)methylsilane-co-methylphenylsilane) crosslinked upon irradiation(55). The difference between our results and West s may lie in the amount of the double bond and inhibitation of the radical closslinking by the phenol moiety. Polysilane with a halogen moiety, P-8, photodecomposed rapidly, compared with P-1 or P-3. The introduction of a chloride moiety was effective for the sensitization of the photodegradation. Similar results has already been reported(55). 

Photolytic. Dalapon (free acid) is subject to photodegradation. When an aqueous solution (0.25 M) was irradiated with UV light at 253.7 nm at 49 °C, 70% degraded in 7 h. Pyruvic acid is formed which is subsequently decarboxylated to acetaldehyde, carbon dioxide, and small quantities of 1,1-dichloroethane (2-4%) and a water-insoluble polymer (Kenaga, 1974). The photolysis of an aqueous solution of dalapon (free acid) by UV light (X = 2537 A) yielded chloride ions, carbon dioxide, carbon monoxide, and methyl chloride at quantum yields of 0.29, 0.10, 0.02, and 0.02, respectively (Baxter and Johnston, 1968). 

Oxygen substituted nitrenes have received much less study than aminonitrenes, but there have been some noteworthy recent developments. Toscano and co-workers " studied the photochemistry of diazenium diolates. The quantum yield of photodegradation was 0.10. 

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