Photochemical reduction mechanism

An interesting alternative mechanism of activation is the photochemical reduction of Pt(IV) to Pt(II) (Fig. 3). In addition to photoreduction, photosubstitution and photoisomerization can also occur, making the photochemistry of Pt complexes difficult to predict and a careful analysis of the photoproducts imperative (21). We have been involved particularly in the development of photochemotherapeutic agents based on Pt(IV) and the study of their photodecomposition and (subsequent) interactions with... 

The following discussion begins by presenting an in-depth view of the mechanism for the photochemical reduction of benzophenone by N, iV-dimethyl-aniline. This discussion is followed by a presentation of the theoretical models describing the parameters controlling the dynamics of proton-transfer processes. A survey of our experimental studies is then presented, followed by a discussion of these results within the context of other proton-transfer studies. 

The reaction pathways by which the net transfer of a hydrogen atom from an amine to a photoexcited ketone has been extensively examined in the nanosecond [23, 25-30], picosecond [20, 22, 31-33], and femtosecond [24] time domains. The following mechanism, as it pertains to the photochemical reduction of benzophenone (Bp) by N, A-dimethylaniline (DMA), is derived from these numerous studies. Only an overview of the mechanism will be presented. The details of the studies leading to the mechanism will not be given for specifics, the reader is referred to the original literature. 

Both CIDNP and ESR techniques were used to study the mechanism for the photoreduction of 4-cyano-l-nitrobenzene in 2-propanol5. Evidence was obtained for hydrogen abstractions by triplet excited nitrobenzene moieties and for the existence of ArNHO, Ai N( )211 and hydroxyl amines. Time-resolved ESR experiments have also been carried out to elucidate the initial process in the photochemical reduction of aromatic nitro compounds6. CIDEP (chemically induced dynamic electron polarization) effects were observed for nitrobenzene anion radicals in the presence of triethylamine and the triplet mechanism was confirmed. 

Of all the mechanisms that should enhance the bioavailability of complexed Fe and P, the photochemical reduction of Fe and the subsequent release of P from the DOM-Fe-P complex have received the most attention... 

The irradiation of ammonium dichromate in gelatin results in crosslinking via two mechanisms. Oxidative crosslinking can occur, and the Cr +, which is formed by photochemical reduction, can also act as a crosslinking agent for the protein. The spectral sensitivity extends past 500 nm, and the exposure is typically a few mJ/cm. Resolution is very high, at least 5000 lines/mm. 

The Mehler reaction is a photochemical reduction of O2 to H2O2 or H2O in photosystem I (Box 35.2). Mehler activity is thought to be a mechanism for energy dissipation under high tight intensities or when carbon frxation is limited by supply... 

Laser flash photolysis time-resolved spectrophotometry, utilizing deazariboflavin-EDTA as a photochemical reductant, has been used with this system in order to characterize the initial step in the ET mechanism. Figure 3 shows examples of the type of data obtained in these studies. In the top panel, a transient is shown [54] that was obtained at 507 nm in 100 mM phosphate buffer, pH 7.0, containing 35 pM Fd, and in the middle panel, 10.3 pM FNR has been added to the solution prior to photolysis. This wavelength corresponds to an isosbestic point for the FAD cofactor of the reductase, and thus the absorbance change monitors the oxidation state of the [2Fe-2S] cluster of Fd (and also the formation and decay of the dRfH species). As is evident, immediately after the laser flash there is a rapid rise in absorbance due to dRfH formation. This is followed by a sharp absorbance decrease corresponding to Fd reduction and dRfH oxidation. The subsequent slow increase in absorption shown in the middle panel is a consequence of Fd reoxidation that is due to electron transfer to FNR. The latter is confirmed by measurement at 610 nm (bottom panel), a wavelength which monitors FAD neutral semiquinone formation the rate constant obtained from the 610 nm absorbance rise is the same as that obtained from the slow absorbance increase at 507 nm, consistent with this interpretation. 

Skuratova 97 has put forward an alternative mechanism for the primary step in the extensively studied photochemical reduction and hydroxylation of anthraquinone sulphonates. In distinction to the conventionally accepted mechanism it is proposed that hydroxyl radicals are involved. 

The interest in these highly reduced states stems from their potential use as catalysts for the electrochemical or photochemical reduction of COj to The mechanism involves binding of COj as an axial ligand to the reduced metalloporphyrin followed by two-electron transfer. Fe and Co porphyrins do not react with CO2 but when they are reduced to the M° oxidation state they react rapidly. 

For Co and Fe phthalocyanines, Grodowski and coworkers studied the electrochemical and photochemical reduction of CO2. The combined results of flash photolysis, chemical reduction, cyclic voltammetry and IR and UV-Vis spectroscopy allowed the authors to conclude that the cation radical [Co(I)Pc ] is the species responsible of the catalysis. However for Fe phthalocyanine the results are not very clear. Other contributions to the reduction mechanism with Co phthalocyanine have been report recently . The authors found a dependence of the substituents in the periphery of the ring with the mechanism of the reaction. The reaction studied by in situ potential-step chronoamperospec-troscopy (PSCAS) inferred the formation of different radical species . For Co-phthalocyanine and Co-octacyano-phthalocyanine the anion radical species is [Co(I)Pc(-3)X] (where X is the peripheric substituents of the phthalocyanine ring) 22, for Co-octabutoxy-phthalocyanine the responsible species is [Co(I)Pc (-3)X(H)] . The formation of the hydride intermediate is explained in terms of the electron donor properties of the butoxy substituents. This fact could affect the stability of the dianion species and for stabilization purposes the dianion formed needs atomic H before its coordinates to the CO2 molecule . ... 

Fig. 3.13 Proposed mechanism of the photochemical reduction of 1,2-dichloroethane by Mo2(02P(OCgH5)2)4, adapted from [64]...

Scheme 8.16 Reaction mechanism for the formation of CO and HCQj in the photochemical reduction of CO2. Two consecutive e (+H ) transfers of co-ordinated CO2 to the metal in a low oxidation state were proposed with an imusual OH elimination in the formation of CO. Adapted from [67]...

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