Factors solid-state photoreactivity

The purpose of this review is to examine the recent progress in the field of asymmetric photoreactions involving a chiral reactant, and to discuss the factors that control the diastereodifferentiation. In the excited state as in the ground state, the level of asymmetric induction is strongly related to the conformational flexibility of the reactants. When carried out in the solid state, in a confined cavity of zeolites or in supramolecular scaffoldings, restrictions of the mobility occur, and photoreactions can become highly stereoselective. These aspects of asymmetric induction will be covered in separate chapters, and this review will consider only diastereoselective photoreactions carried out in solution. 

Photoreactivity of carbonyl compounds in the solid state is little understood. Many factors such as rates of elementary processes, lifetimes of intermediates, crystal structure, and looseness of crystal packing must be considered to elucidate the mechanism (cf. Fig. 3). In addition to the traditional X-ray crystallography, the usefulness of solid-state quantum yield measurements is exemplified. 

Since organogelators are in crystalline or lyotropic aggregate states in a gel. the nature of their intermolecular interactions becomes a factor of paramount importance in determining the nature and the intensity of emitted radiation from a gelled sample. Molecular proximity opens possible reactive channels for the excited states that are not available in dispersed solutions. For example, it has been shown that CAB dimerizes in its neat solid, liquid-crystalline, and gelled (fiber) states when exposed to UV radiation [47,48]. (See Structure I.) In dilute isotropic solutions, no photoreaction is observed because the time required for an... 

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