Photochemical secondary processes

While primary photochemical processes are restricted to those compounds which are excited by the direct absorption of radiation, secondary processes of one type or another can involve all organic compounds. The reactivity, the concentration, and the lifetime of secondary transient reactants will be determining factors in what types of organic compounds are susceptible to attack and what predominant modes of reactions are occurring. Because so many of the constituents of seawater may be involved, the number of possible alternative reactions occurring is potentially very complex. Therefore, only the immediate reactions of three initiating sources of secondary reactions will be discussed. 

Since organic-free radiceils (R- and ROO ) should be present at low steady-state concentrations in seawater, the termination reactions should be un- 

Organic compounds can generate the initiators of free radical sequences through the primary photochemical processes homolytic dissociation into radicals, hydrogen-atom abstraction, photoionization, and electron transfer reactions. The homolytic dissociation reactions are limited to compounds containing relatively weak bonds ( 98 kcal), such as sulfides, peroxides, and some halides and ethers. Representatives of all of these classes of compounds are certainly present in seawater, but the limited information on the qualitative and quantitative aspects of their occurrence does not allow for an estimate of their importance in the promotion of free radical reactions. The same is true for electron transfer reactions, which may be an important photochemical process for organic transition metal complexes. 

The measurement of significzmt concentrations of hydrogen peroxide in seawater (Van Baalen and Marler, 1966) and the demonstration of its photochemical production (Zika, 1978) in seawater support the occurrence of such processes. If hydrogen peroxide generation is ubiquitous over the oceans then through its spontaneous degradation pathways it represents an important secondary reaction process. 

Aside from promoting energy transfer processes, there are other possible alternative routes available to photosensitizers (Fig. 5). The excited states of many photosensitizers act as oxidizing and reducing agents as the result of the change in electron distribution brought about by promotion of an elec- 

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Secondary Photochemical Processes. While the nature of the primary photochemical step may be described as still uncertain, the nature of the subsequent secondary steps is best characterized as obscure. A previous trapping study during exhaustive irradiation (30) demonstrated that silylenes are formed somewhere along the line and implicated silyl radicals as well since the formation of Si-H bonds was observed, presumably by hydrogen atom abstraction. 

In some physical chemistry texts, the primary photochemical process is incorrectly considered to be no more than the absorption of radiation. Such a definition is not acceptable because absorption is not a chemical transformation and, more important, because it does not correspond to current usage in photochemistry. These texts then label such diverse processes as fluorescence, dissociation of an excited molecule, and chain reactions, all as different types of secondary photochemical processes. This is also unacceptable because "secondary" has come to have a specific meaning (as is discussed in Section III.A.3) which does not apply to all of these transformations, and also because some of them are not chemical. Photochemists instead use primary and secondary in the original sense of Bodenstein. 

This chapter deals with the characteristics of the photolyses of alkyl ketones adsorbed on porous Vycor glass which arises from the electronic perturbation and the steric hindrance effects of the surfaces upon the primary and secondary photochemical processes. The effects of surface hydroxyl groups upon the primary and secondary photochemical processes are also discussed, since surface hydroxyl groups have been found to play a significant role in the photochemistry of the adsorbed... 

Steglich52 obtained the 2-azabicyclo[3.1.0]hex-2-en system (45) stereospe-cifically by thermolysis of the oxazolin-5-one 43, a method known to produce nitrile ylides. The intermediate 44 could be trapped with dimethyl acetylene-dicarboxylate. Brief photolysis (4 min) of 45 caused epimerization to 47 (Scheme 9).52 Evidently, diradicals (or zwitterions) such as 46 are formed in a secondary photochemical process, which would also appear to apply to Scheme 8. 

Various types of photochemical reactions such as addition or substitution reactions, atom abstractions or rearrangements can arise from either singlet or triplet excited states of molecules. They can be described as secondary photochemical processes. Many examples of these are to be found in works concerned with the mechanistic aspects of photochemistry [4, 5]. 

Obviously if a secondary photochemical process is to arise from the excited singlet state of a molecule it must be sufficiently fast to compete with the other deactivation processes. Since excited triplet states have much longer lifetimes, more secondary photochemical processes are likely to occur from levels of that multiplicity. 

Main chain scission can occur as a consequence of primary or secondary photochemical processes or even of subsequent thermal reactions. It results in a decrease in the average molecular weight and can be represented as in Fig. 6. 

Exposure of polymers to sunlight or to artificial light sources results in a more or less rapid deterioration of their physical and mechanical properties as a consequence of either primary and secondary photochemical processes or of subsequent photo-initiated thermal reactions. 

Secondary photochemical processes are chemical reactions involving excited species (in either the triplet or singlet state) produced by electronic excitation of the molecule ... 

Photo-initiated thermal reactions can follow either primary or secondary photochemical processes. They usually involve free radicals produced by photodissociation or other unstable intermediates ... 

Deactivation of excited states by transfer of energy to a suitable acceptor can efficiently inhibit secondary photochemical processes. The reaction can be represented schematically as... 

The transfer of energy can be practically quantitative and result in a complete inhibition of secondary photochemical processes when excited triplet states only are involved. This is illustrated by the photo-reticulation... 

After initiation through this path, polymer degradation usually continues via secondary photochemical processes at which participate both initial macromolecules and products resulted from the primary photochemical process. 

Polymer photodegradation continues through secondary photochemical processes with participation of both macromolecules and products resulted during primary photochemical processes. Afterwards, the excited macromolecule returns to its fundamental energetic state via different processes such as heat release, physical radiative photoluminiscence phenomena, nonradiative transitions or energy transfer towards another acceptor molecule in the system. 

The quantum efficiency of secondary photochemical processes varies from fractional value to several millions. This is due to the fact that initially excited molecule starts a chain of excitation (like photo degradation of ozone by chloro-floro hydrocarbons). Those reactions which are monophotonic occur from singlet excited state... 

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