Photodegradation reactions

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. 

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. 

Olefins are also susceptible to photodegradation reactions other than cycloaddition reactions (43). The olefin bond is susceptible to E(trans)-Z(cis) isomerization as well as oxidation (Fig. 107) (154). Photochemical E-Z isomerization has a major role in photobiological systems and has practical applications in vitamin A and vitamin D industrial processes... 

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. 

Fig. 6). This clearly proves the catalytic nature of the photodegradation reaction. 

Moore DE. Standardization of kinetic studies of photodegradation reactions. In Tonnesen HH, ed. The PhotostabUity of Drugs and Drug FormiUations, 2nd ed. CRC Press LL Boca Raton USA, 2004. 

Akimoto K, Akiko K, Ohya K, Sawada S, Aiyama R. Photodegradation reactions of GPT-11, a derivative of camptothecin 11 chemical structure of main degradation products in aqueous solution. Drug Stab 1997 1 118-122. 

Other more complicated kinetic relationships, such as second- or fractional-order reactions, arise very infrequently from complex reaction mechanisms, and do not need to be considered for photodegradation reactions. 

A major drawback of the spectrophotometric analysis is the fact that many photodegradation reactions provoke no or only negligible spectral changes. This leads to the fact that many photoreactions cannot be registered accordingly. 

Chatterji and Galleli (58) determined that the photodegradation reaction of methotrexate after an initial lag phase follows zero-order kinetics. The photolability of the drug increases at higher dilutions. Bicarbonate ion has also been identified as a catalyst for the photolysis of methotrexate. No photodegradation products were observed, in an infusion solution containing 2mg/mL methotrexate and 0.05 mol sodium bicarbonate, after 12 hours exposure to room radiation. 

Sodium nitroprusside in solution is extremely photosensitive, undergoing rapid and numerous photodegradation reactions (69). The deterioration of the product is evidenced by a color change from brown to blue, resulting from the reduction of the ferric ion to the ferrous ion. Hydrogen cyanide is one of the toxic degradation products formed. Therefore, reconstituted solutions should be stored protected from UV-VIS radiation by wrapping the container with aluminium foil or some other opaque material. Solutions with adequate photoprotection are stable for up to 24 hours (70-72). 

Photosensltlzatlon of polymers has been found to be Important for a variety of purposes such as 1) Photopolymerization, 2) PhotomodlfIcatlon (photo-grafting), 3) Photodegradation, and 4) Photostablllzatlon (13 In commercial polymers photodegradation reactions are usually photosensitized by the presence of alien groups In the chain or by the presence of trace Impurities (2,3). 

Approximately half of the papers in the book deal with photopolymerization reactions including the role of photoinitiators and photosensitizers. The other half of the papers deal with photodegradation reactions including the use of photostabilizers and photodegradable polymers. 

The method of simulation and fitting enables extraction of significant kinetic information even with more complicated photochromic systems, such as (hose which give several photoisomers or those in which there is a photodegradation reaction. Such systems generally involve three overall species A, B, and C and will thus be designated as ABC systems (Table 3). 

The main component of the mechanism is a thermally reversible photochromic process of the (AB, 14>, 1 ) type, supplemented by a couple of photodegradation reactions, one from A (<(>3) and the other from B (4). Without going into the details of the mechanisms, we can assume that both the closed and open forms (A and B) undergo photodegradation. No assumptions are made about the structures of the photodegradation products. We only assume that they partly catalyze the thermal decay (k5 and 4 are related to the catalysis by the photodegradation products of A and B, respectively).61-63 This mechanism is described by the following set of differential equations ... 

Photodegradation of pharmaceuticals has been known for decades. A complication encountered when studying photodegradation reactions is that there are many degradation pathways that each have the potential to yield different products. When oxidizers are present, photodegradation can accompany oxidation. 

When many polymers are exposed to UV levels similar to those on a sunny day near to the equator, photodegradation reactions proceed very rapidly and the rate of oxygen consumption is very high. With some polymers, oxygen diffusing into the material from the exposed surface is consumed by reaction before it can penetrate very far. Therefore, under a period of sustained UV exposure, any oxygen that had... 

The photodegradation reactions involve not only the photooxidation, but also the photoreduction. For example, Chu W. and Javert C.T. (1994) reported a photoreduction reaction for aromatic compounds in the presence of hydrogen sources in which high reaction quantum yields were observed. Many dyes are also noted for their ease of deco lour ization in the absence of oxygen when a suitable electron donor or hydrogen source is present. For example, in the scheme IV, formation of colourless leuco-form of anthraquinone dye is observed after photo-reduction that the main structural integrity of the dye molecule retains (Rys P. and Zollinger H., 1972). 

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