Semiconductor Photocatalysis Type

Thus, in general, a reduced and an oxidized product are obtained, in complete analogy with a classical electrochemical reaction. Except for a very few examples, almost all reactions photocatalyzed by semiconductors fall within this scheme and we have proposed to classify such reactions as semiconductor photocatalysis type A [45]. 

Stimulated by the early work of Bard et al. on the Ti02-catalyzed photo-Kolbe reaction [36], many papers appeared in subsequent years dealing with photocatalytic organic reactions [115] in the presence of colloidal or suspended semiconductor particles. They include cis-trans isomerizations [68, 93, 116-119], valence isomer-izations [120, 121], substitution and cycloaddition reactions [73, 80, 122-125], oxidations [126, 127], and reductions [128-130]. Characteristic of all these reactions is that in almost all cases well known compounds were formed, which were not isolated but only characterized by spectroscopic methods. The nature of the products can be rationalized within the mechanistic scheme of semiconductor photocatalysis type A, which means that at least one reduced and one oxidized compound are... 

Hydrazone cyclization and hydroalkylation [138-140] are rare examples of reactions conducted on a preparative scale, since the products were isolated in milligram amounts and not just identified in solution. As already mentioned in Section 6.2.5, photocorrosion of the semiconductor photocatalyst often prevents its use in preparative chemistry. This is very true also for colloidal semiconductors although the pseudo-homogeneous nature of their solutions allows one to conduct classical mechanistic investigations, until now they were too labile to be used in preparative chemistry [107, 141, 142]. In contrast to the above-mentioned reactions, in recent years we have isolated novel compounds on a gram-scale employing photostable zinc and cadmium sulfide powders as photocatalysts [97, 107, 143-145]. During this work we found also a new reaction type which was classified as semiconductor photocatalysis type B [45]. In contrast to type A reactions, where at least one oxidized and one reduced product is formed, type B reactions afford only one unique product, i.e., the semiconductor catalyzes a photoaddition reaction (see below). 

Hopfner, M., Weiss, H., Meissner, D., Heinemann, F. W., Kisch, H., Semiconductor Photocatalysis Type B Synthesis of Unsaturated a Amino Esters from Imines and Olefins Photocatalyzed by Silica supported Cadmium Sulfide, Photochem. Photobiol. Sci. 2002, 696 703. 

Figure 3.1. Schematic description of the photoredox reaction A+D = Ared + Dox catalyzed by a platinized semiconductor, a typical example of semiconductor photocatalysis type A for secondary back electron transfer, see Equations 7 and 8.

---