Photoformation

Photodimerization of cinnamic acids and its derivatives generally proceeds with high efficiency in the crystal (176), but very inefficiently in fluid phases (177). This low efficiency in the latter phases is apparently due to the rapid deactivation of excited monomers in such phases. However, in systems in which pairs of molecules are constrained so that potentially reactive double bonds are close to one another, the reaction may proceed in reasonable yield even in fluid and disordered states. The major practical application has been for production of photoresists, that is, insoluble photoformed polymers used for image-transfer systems (printed circuits, lithography, etc.) (178). Another application, of more interest here, is the use that has been made of mono- and dicinnamates for asymmetric synthesis (179), in studies of molecular association (180), and in the mapping of the geometry of complex molecules in fluid phases (181). In all of these it is tacitly assumed that there is quasi-topochemical control in other words, that the stereochemistry of the cyclobutane dimer is related to the prereaction geometry of the monomers in the same way as for the solid-state processes. 

Uyeta, M., Taue, S., Chikasawa, K., and Mazaki, M. Photoformation of polychlorinated biphenyls from chlorinated benzenes, Nature (London), 264(5586) 583-584, 1976. 

The course of the photoformation of the tetrapyrrole ring-localized radicals (group 1 of the above classification) can be expressed by four equations... 

Back electron-transfer processes of n-anion and n-cation radicals with reversible electron donors or acceptors (e.g. aquated Fe3+, [Fe(CN)6]3, quinones) are fast reactions realized in nano- or picosecond time scale. In cases when irreversible redox partners are used (e.g. S20, CBr4, CC14, EDTA) tetrapyrrole ring ring localized radicals dimerize [193], decompose [212], undergo disproportionation [215] or other stabilization reactions. Photoformation of stable products will be discussed later. 

Fox, S. W., Adachi, T., Stillwell, W. A quinone-assisted photoformation of energy-rich chemical bonds, in Solar Energy International Progress, Vol. 2 (ed.) Veziroglu, T. N., p. 1056, New York, Pergamon Press 1980... 

Finally, note that the rate of photochemical reactions, including those involving photoformed OH, is maximum in the water surface layer and decreases with the water column depth because of decreasing sunlight intensity. Accordingly, the... 

Shiragami, T. Fukami, S. Pac, C. Yanagida, S. Semiconductor photocatalysis Quantised CdS-catalysed photoformation of 1 -benzyl- 1,4-dihydronicotinamide (BNAH) from 1-benzylnicotinamide (BNA+), 7. Chem. Soc., Faraday Trans. 1993, 89, 1857. 

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