Photochemistry in organized media

Ever since chemists made use of light in a scientific way, the control of distance, geometry, diffusion etc. remain the main challenges. Therefore it is not surprising that over the last one to two decades photochemistry partly migrated to an area which is linked to a discipline often referred to as photochemistry in organized and constrained media . The main aim in this field has been the discovery or development of reaction cavities that can be utilized by chemists to make molecules behave (chemically and physically) in the way chemists would like them to. There is a wealth of information on this subject and the interested reader is recommended to refer to V. Ramamurthy s monograph which comprehensively covers all photophysical and photochemical aspects up to 1991. 

However, applications to modem organic synthesis are still very seldom and therefore the editors are very grateful to the two authors who contributed examples to this book. 

In the di-7C-methane rearrangement example presented by M. Demoth the normal equipment of preparative laboratories is used, i.e. the suspension of chiral crystals ate irradiated in a Rayonet reactor leading to two isomers both in optically pure form. 

TTiese examples illustrate the high synthetic potential of photochemistry in organized media. Future developments will certainly lead to more applications utilizing the special features of other microenvironments such as host-guest complexes, molecules absorbed on silica, alumina and other surfaces, molecules as inclusion complexes in zeolites, porous materials, liquid crystals, micelles, vesicles, and even in biological media. 

The following recommended reading list may also stimulate future applications. 

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An excellent review on organic photochemistry in organized media, including aqueous solvent, has been reported.178 The quantum efficiency for photodimerization of thymine, uracil, and their derivatives increased considerably in water compared with other organic solvents. The increased quantum efficiency is attributed to the preassociation of the reactants at the ground state. 

For reviews, see (a) Ramamurthy, V. (1986) Organic photochemistry in organized media, Tetrahedron, 42,... 

Ramamurthy, V. 1986. Organic Photochemistry in Organized Media. Tetrahedron 42, 5753... 

Ramamurthy, V., Organic photochemistry in organized media. Tetrahedron, 42, 5753,1986. Ramamurthy, V., Photochemical reactions in oriented systems, ACS Symp. Ser., 278, 267,1985. Ramamurthy, V., Weiss, R.G., and Hammond, G.S., A model for the influence of organized media on photochemical reactions, Adv. Photochem., 18, 67,1993. 

Hwu, J. R., Chen, B. L., Huang, L. W., and Yang, T. H., Influence of beta-silyl groups in cycloal-kanones on the Norrish type I and type II cleavages, /. Chem. Soc., Chem. Commun., 299, 1995. Ramamurthy, V., Organic-photochemistry in organized media. Tetrahedron, 42, 5753, 1986. Turro, N. J., Liu, K.-C., and Chow, M.-R, Solvent sensitivity of type II photoreaction of ketones as a device to probe solute location in micelles, Photochem. Photobiol., 26, 413, 1977. 

Ramamurthy V. (Ed.) (1991) Photochemistry in Organized and Constrained Media, VCH, Weinheim. 

L. Lahav, M. Probing reaction pathways via asymmetric transformations in third and centro-symmetric crystals in Photochemistry in Organized and Constrained Media Ramamurthy, V., Ed. VCH Publishers New York, New York, 1991, pp 248-302. 

VaidaM,Popovitz-Biro R,Leiserowitz L,LahavM (1991) In Ramamurthy V (ed) Photochemistry in organized and constrained media. VCH, New York, p 249... 

Vaida, M., Popovitz-Biro, R., Leiserowitz, L., and Lahav, M. (1991). Probing reaction pathways via asymmetric transformations in chiral and centrosymmetric crystals. In Photochemistry in Organized and Condensed Media, ed. V. Ramamurthy. VCH, pp. 248-302. 

Figure 9. Schematic representation of active groups on silica surfaces (A) isolated silanols (B) siloxane bonds (C) geminal silanols (D) hydrogen-bonded silanols ( ) hydrogen-bonded water. (Reproduced with permission from L. Johnston in Photochemistry in Organized and Constrained Media, V. Ramamurthy, Ed., VCH, New York, 1991, p. 359.)...

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