Primary organic photochemical processes

Numerous possible reaction routes are known from both singlet and triplet photo-excited state manifolds of organic molecules (Fig. 4). Although the excited state of a specific compound may react by more than one chemi- 

Photoj roc esses Commonlv Initiating Se -ondarv Reartifuis  

Ensuing reactions of the oxidized ligand molecule then lead to the observed products. These processes are likely to occur for compounds containing two or more functional groups that can form relatively stable chelates with transition metals existing in their higher oxidation states, but having an accessible lower oxidation state (e.g., Fe, Cu, Cr, V, Hg, Ce, and U). 

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Stereo-isomerizations are quite common photochemical processes with unsaturated organic molecules (the primary photochemical reaction of vision is of this type). 

The primary photochemical processes of photosynthesis take place within membrane bound complexes of pigments and protein, reaction centers (Shuvalov and Krasnovsky, 1981 Deisenhofer et al., 1986, Rees et al., 1989 Norris and Shiffer, 1990 Kirmaier and Holten, 1991 Feher et al, 1992 Stowell et al, 1997). One mole of a reaction center from different bacteria contains 4 moles of bacteriochlorophyl (Bchl), 2 moles of bacteriopheophytin (Bph), two moles of ubiquinone (Q) and a non-heme Fe atom. In RC from Rhodobacter speroides, a total of 11 hydrophobic a-helixes create a framework that organizes the cofactor and a hydrophobic band approximately 35 A wide. RC from Rhodopseudomonus viridus has three polypeptides having pronounced hydrophobic properties. The molecular mass of the polypeptides are 37 571 (L), 35902 (M) and 28902 (H). The H subunit does not carry pigments but it is sufficient for the photochemical activity. The protein components of reaction centers from different-bacteria are similar. 

The redefinition of some terms and the creation of new definitions will facilitate the expression of ideas that are currently of greatest interest. As far as the chemical aspect of the problem is concerned, especially with regard to organic photochemistry, the nomenclature used is largely that of ordinary reactions. For that reason, the development here will be concerned mainly with what Noyes, Porter, and Jolley (12) referred to as the "primary photochemical process." In addition, because of the growing importance of energy transfer in the study of photochemical systems and the almost complete lack of a coherent nomenclature, this aspect will be considered in the following treatment. 

In contrast to the typical behavior of organic compounds discussed above, many photoreactions of transition metal complexes have wavelength-dependent quantum yields (7). Generally, these wavelength effects have been interpreted in terms of more than one reactive excited state of the photolyzed species. The photoreactivity of V(CO) L (L = amine), for example, has been interpreted in this manner with the previously mentioned model of substitutional photoreactivity proposed by Wrighton et al. (42, 49,73). Assuming ligand dissociation to be the only primary photochemical process (Section III-B-1), photolysis of W(C0)5L could produce three primary products ... 

Biophysical chemical studies have suggested that the thylakoid membrane of the chloroplast consists of an ultrathln layer of lipids (presumably a lipid bllayer) with sorbed proteins nd pigments organized in a lamellar structure approximately 100 A thick (Figure 4). Although the precise functions of the thylakoid membrane are still obscure, it is believed that the membrane is the locus of the primary photophyslcal and photochemical 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. 

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. 

To gain a more indepth insight into the primary photochemical process, Ce(IV) photoreactions have been studied in the presence of various organic substrates using EPR spectroscopy of frozen solutions at 77 Thus, photolysis of 4M HCIO solutions of 0.1 M Ce(ClO ) and 0.1 M RCO2H produces the corresponding alkyl radical R. The primary photochemical event is a concerted oxidative decarboxylation ... 

The photochemistry of phenyl azide and its simple derivatives have received the most attention in the literature. The results of early studies were summarized in a number of reviews. " Over the last decade, modem time-resolved spectroscopic techniques and high level ab initio calculations have been successfully applied and reveal the detailed description of aryl azide photochemistry. This progress was analyzed in recent reviews. Femtosecond time resolved methods have been recently employed to study the primary photophysical and photochemical processes upon excitation of aryl azides. The precise details by which aryl azide excited states decompose to produce singlet arylnitrenes and how rapidly the seminal nitrenes lose heat to solvent and undergo unimolecular transformations were detailed. As a result of the application of modem experimental and theoretical techniques, phenylnitrene (PhN) - the primary intermediate of phenyl azide photolysis, is now one of the best characterized of all known organic nitrenes. " 5 "-2° - ... 

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