Quantum yield of photodegradation

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. 

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. 

In an earlier paper (3), we reported that the quantum yield of photodegradation and the triplet state lifetime of I are both significantly higher in a protic solvent. As well, we presented evidence that significant reaction can occur from the first excited singlet state of I, a phenomenon previously unobserved for aromatic ketones. Here... 

TABLE I Quantum Yield of Photodegradation ( and Triplet Lifetime (x) for a-Guaiacoxyacetoveratrone... 

Figure 1 Quantum yield of photodegradation for I in dioxane, as a function of mole fraction of added protic solvent.

The photoisomerization of all types of azobenzenes is a very fast reaction on either the singlet or triplet excited-state surfaces according to the preparation of the excited state, with nearly no intersystem crossing. Bottleneck states have lifetimes on the order of 10 ps. The molecules either isomerize or return to their respective ground states with high efficiency. So photoisomerization is the predominant reactive channel, and the azobenKnes are photochemically stable. Only aminoazobenzene-type molecules and pseudo-stilbenes have small quantum yields of photodegradation. 

Figure 5.34 Spectral dependencies of the quantum yield (/> of photodegradation of phenol (1) and 4-chlorophenol (2) over Ti02 (P-25). Reprinted with permission from Emeline et al. (2000c). Copyright (2000) American Chemical Society.

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. 

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). 

The deficiency of tlie LH equation was demonstrated also for pentachlorophenol [25] and by Cunningham [26-28]. The quantum yields for photodegradation of salicylate and other strongly sorbing substituted benzenes were less than those measured for nonsorbing chlorophenols. For salicylic acid, which chemisorbs, the photo-oxidation rates in air-saturated solutions are independent of the salicylic acid concentration, although the surface excess increases with the concentration [29]. 

The molecular absorber must remain robust through millions of cycles. The quantum yield for photodegradation reactions must be lower than 10 6. This is a very severe constraint. 

Choudhary, G.G., Webster, G.R.B. (1985a) Environmental photochemistry of PCDDs. Part I. Kinetics and quantum yields of the photodegradation of 1,2,3,4,7-penta- and 1,2,3,4,7,8-hexachlorodibenzo-p-dioxin in aqueous acetonitrile. Chemosphere 14, 9-26. 

Indolinospirooxazines are inherently more fatigue resistant than the spiropyr-ans. As measured by the quantum yield for photodegradation, the spironaphthox-azines are two or three orders of magnitude more photostable than the spirobenzopyrans. 37 From the point of view of industrial applications, spirooxazines are reported to be stabilized even further by various protective methods. 

N. Y. C. Chu, in Proceedings of the 10th IUPAC Symposium on Photochemistry, Quantum yield for photodegradation, Interlaken, Switzerland (1984). 

Quantum yields of phototransformations of phenoxy-substituted naphthacene-quinones depended on the nature of the solvent.59 One can see from Table 7.7 that the replacement of benzene by ethanol led to a decrease in the quantum yield of the ana form. At the same time, the quantum yield of the photodegradation of the ana form increased. 

Further studies on the photodegradation of 4-chlorophenol were conducted, respectively, by combinations of flash irradiation and product analysis [25], and by fluorescence [26]. In the former study, a degradation quantum yield of = 0.24 was reported. 

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. 

Dicamba is a dichlorinated 2-methoxybenzoic acid derivative. It was found to be photodegraded in aqueous solution with a quantum yield of 0 = 0.0215 at A, = 275nm [99]. The photoproducts were the dihydroxylated derivative and an unexpected dicyclic compound, only formed in the presence of O2 (Scheme 10). 

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