Photo-oxidation-reduction reactions

Intermolecular photo-oxidation-reduction reactions involve a light initiated electron transfer between a complex and any other suitable molecule available in the medium. An oxidized or a reduced form of the complex may be obtained. 

C, Photo-oxidation-reduction or Redox-reactions. A photo-oxidation-reduction process in solution may be intramolecular when the redox reaction occurs between the central metal atom and one of its ligands or intermolecular when the complex reacts with another species present in the solution. 

A large variety of organic oxidations, reductions, and rearrangements show photocatalysis at interfaces, usually of a semiconductor. The subject has been reviewed [326,327] some specific examples are the photo-Kolbe reaction (decarboxylation of acetic acid) using Pt supported on anatase [328], the pho-... 

Platinum-loaded Ti02 systems can be considered as a short-circuited photo-electrochemical cell where the Ti02 semiconductor electrode and metal Pt counterelectrode are brought into contact [159]. Light irradiation can induce electron-hole (e -h +) pair formation and surface oxidation and also reduction reactions on each Pt/Ti02 particle (Figure 4.11). These powder-based systems lack the advantage of... 

The majority of inorganic reactions can be placed into one of two broad classes (1) oxidation-reduction (redox) reactions including atom and electron transfer reactions and (2) substitution reactions. Terms such as inner sphere, outer sphere, and photo-related reactions are employed to describe redox reactions. Such reactions are important in the synthesis of polymers and monomers and in the use of metal-containing polymers as catalysts and in applications involving transfer of heat, electricity, and light. They will not be dealt with to any appreciable extent in this chapter. 

Similar to the molecular photosensitizers described above, solid semiconductor materials can absorb photons and convert light into electrical energy capable of reducing C02. In solution, a semiconductor will absorb light, and the electric field created at the solid-liquid interface effects the separation of photo-excited electron-hole pairs. The electrons can then carry out an interfacial reduction reaction at one site, while the holes can perform an interfacial oxidation at a separate site. In the following sections, details will be provided of the reduction of C02 at both bulk semiconductor electrodes that resemble their metal electrode counterparts, and semiconductor powders and colloids that approach the molecular length scale. Further information on semiconductor systems for C02 reduction is available in several excellent reviews [8, 44, 104, 105],... 

The photo-oxidation of PE sensitized by DC A in homogeneous solution followed by reduction of the reaction mixture with sodium sulfite solution gave the ene product pinocarveol 14 and the non-ene products myrtenal 15, epoxide 16 and aldehyde 17, as shown in Fig. 21. The ene product and the non-ene products have been proposed to be derived from the energy-transfer and electron-transfer pathways, respectively [177-181], The product distributions in acetonitrile is given in Fig. 21. 

Protopine has been isolated from Bocconia frutescens,110 Fumaria judaica,111 F. schleicheri,112 and Papaver bracteatum,146 cryptopine from F. schleicheri,112 and allocryptopine from B. frutescens110 and Zanthoxylum nitidum.141 The protopine ring-system has been prepared from tetrahydrobenzindenoazepines (75) by photo-oxidation to the amides (76) followed by reduction with lithium aluminium hydride and re-oxidation with manganese dioxide.148-150 The tetrahydrobenzindenoazepines have been prepared from A-chloroacetyl-/ -phenylethylamines (73) by cyclization to the lactam (74) followed by reaction with a benzyl bromide and phosphorus oxychloride. -Protopine (77 R R2 — CH2)148 and fagarine II (77 R1 = R2 = Me)149 have been synthesized in this way. 

In order to account for such a mechanism, photochemical excitation of a semiconductor surface might induce the promotion of an electron from the valence band to the conduction band. Since relaxation of the high-energy electron is inhibited by the absence of intra-states, if the lifetime of this photo generated electron-hole pair is sufficiently long to allow the interfacial electron transfer from an accumulation site to an electron acceptor, as well as the interfacial electron transfer from an adsorbed organic donor to the valence-band hole, the irradiated semiconductor can simultaneously catalyze both oxidation and reduction reactions in a fashion similar to multifunctional enzymes reactions [232]. 

Figure 19.4 shows a schematic representation of a photoelectrochemical cell. Photo-generated electrons are driven through the external circuit as a result of the potential difference across the cell. Hence, oxidation and reduction reactions occur at different electrodes of the device. Photoelectrochemically active semiconductor devices have been prepared from colloidal ZnO, Ti02, Sn02, and WO3 [95]. In a photoelectrochemical cell the photocurrent observed provides a direct measurement of the reaction rate. 

As with Ti02, CdS can be used to photocatalyse reactions other than water cleavage. Oxidation of halide ions " proceeds smoothly at chalcogenide electrodes and n-type CdS can be used to photo-oxidize NO in the presence of iron(n) complexes. Similar studies have described the photoassisted reduction of CO2 to CO and the photo-oxidation of formic acid, formaldehyde, and methanol. ... 

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