Photoreductions, organic substrates

This paper deals with the mechanistic investigations of the photoreduction of CO2 using CdS nanocrystallites as photocatalysts in view of the changes in surface structures of CdS-DMF induced by the addition of excess Cd + and the interaction of adsorbed CO2 molecules with the surface. The work is extended to the achievement of photocatalytic CO2 fixation into organic substrates. 

Semiconductor particles have been used to induce efficient photoreactions of organic substrates for synthetic applications [4], Recently the study was extended to semiconductor nanoclusters [148-152]. The ZnS nanoclusters (2-5 nm) and their aggregates were found to be effective catalysts for the photoreduction of aliphatic ketones to alcohols [150], The coexistence of both S2 and SO3 is required for effective photocatalysis to occur. The observed overall reactions, using 2-butanone as an example, can be summarized as [150]... 

The first of the above classes comprises a large number of phototransformations of organic compounds (Table 7). Of the substrate photoreductions catalyzed by metallotetrapyrroles, the photoreduction of CC14 by alcohols has been studied in detail [139, 140, 261]. The reaction can be expressed as follows... 

The photochemist is rather familiar with the photoexcited triplet states and the associated intersystem crossing processes. It is well documented that the photoexcited triplet state plays an important role in organic photochemistry. It is thus conceivable that the electron spin polarization of the photoexcited triplet can be further transferred to a radical pair formed by the reactions of the triplet with a suitable substrate. Such a photoexcited triplet mechanism was first proposed by Wong and Wan in 1972 (135) to account for the "initial polarization" observed in the naphthosemiquinone radical formed in the photoreduction of the parent quinone in isopropanol. It was further considered that the triplet mechanism might also lead to CIDNP if such initially polarized radicals react rapidly to give products with nuclear spin polarization induced via the Overhauser mechanism. 

Although the primary photoprocess for alkyl halide photoreduction may not be atom transfer in all cases, 3(da pa) excited-state hydrogen atom transfer has been established as the mechanism of the reactions between several binuclear d3 complexes and a number of organic and organometallic substrates (Roundhill, D. M. Che, C.-M. Gray, H. B. Accts. Chem. Res., in press, 6-8). Initial work in this area focused on Pt2(P204H2)4 ", for which the catalytic conversion of isopropanol to acetone (Equation 1) had been first observed (18). 

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