Photoredox process

In this paper we will describe and discuss the metal-to-metal charge-transfer transitions as observed in optical spectroscopy. Their spectroscopic properties are of large importance with regard to photoredox processes [1-4], However, these transitions are also responsible for the color of many inorganic compounds and minerals [5, 6], for different types of processes in semiconductors [7], and for the presence or absence of certain luminescence processes [8]. 

Fig. 5.58 Scheme of a photogalvanic cell. The homogeneous photoredox process takes place in the vicinity of the optically transparent anode (a) or cathode (b)... 

A regenerative photogalvanic cell with oxidative quenching (Fig. 5.58b) is based, for example, on the Fe3+-Ru(bpy)2+ system. In contrast to the iron-thionine cell, the homogeneous photoredox process takes place near the (optically transparent) cathode. The photoexcited Ru(bpy)2+ ion reduces Fe3+ and the formed Ru(bpy)3+ and Fe2+ are converted at the opposite electrodes to the initial state. 

Photoredox processes with colloidal semiconductor particles as photo-catalyst, e.g. degradation of refractory organic substances... 

Rate of the photochemical reductive dissolution of hematite, = d[Fe(II)]/dt, in the presence of oxalate as a function of the wavelength at constant incident light intensity (I0 = 1000 peinsteins "1 lr1). The hematite suspensions were deaerated initial oxalate concentration = 3.3 mM pH = 3. (In order to keep the rate of the thermal dissolution constant, a high enough concentration or iron(II), [Fe2+] = 0.15 mM, was added to the suspensions from the beginning. Thus, the rates correspond to dissolution rates due to the surface photoredox process). 

The F ligand in a series of complexes tra .s-[Fe(R-salen)(MeOH)F] stabilizes the Fe with respect to photoredox processes in comparison with CU, Br, or 1 analogues quantum yields are lower for the F complexes. 

This section will deal with the first class of photoredox processes, two latter groups being the subject of Sect. 7. 

Examples of photoredox processes belonging to each of the above groups are given in Table 4. The first three types of photoredox processes (a, b, c) are mostly intracomplex monomolecular reactions (photoredox additions are bimolecular processes) which usually do not occur when a complex is in its ground state. Photoinduced long-range electron transfer reactions can be understood as a boundary between outer-sphere processes (due to a great distance between the reaction sites) and inner-sphere ones (they are, in fact, realized in one molecule). 

The third class (c) of photoredox processes, namely electron transfer from the coordinated tetrapyrrole ring (its jr-system) to the central atom or vice versa, comprises a few cases, represented in Table 4 by the neutral complex Sn(Pc)2... 

Lastly, inorganic based photoredox processes on semiconductor electrodes has recently sparked considerable interest in that these can be coupled to important redox... 

Photoredox processes include both photoreduction and photo-oxidation of the excited species. An electron transfer that results from an electronic state produced by the resonant interaction of electromagnetic radiation with matter is called photoinduced electron transfer (PET) [30-32]. This can be done by either a direct or a photosensitized process (see below and Figure 6.6). 

Mytych P, Ciesla P, Stasicka Z. Photoredox processes in the Cr(VI)-Cr(III)-oxalate system and their environmental relevance. Appl Catalysis B Environ 2005 59 161-70. 

Inorganic NO donors are good examples of prospective pharmaceuticals activated via their photodissociation, photosubstitution, and photoredox processes. 

The time-resolved spectroscopic studies of the transient intermediates provide the keys to understanding the charge-transfer photochemistry of contact ion pairs. Thus, the unambiguous identification of the carbonylmetal radical Mn(CO)ja immediately following the 10-nsecond laser-flash excitation of the contact ion pair Cp2Co+ Mn(CO)s- at A = 532 nm relates directly to the photoredox process,... 

The importance of such a redox property is also illustrated in the photodissociation (59) of Co2(CO)6(PBu3)2 in the presence of both Q+ PF6" and PBu3. Separately, neither Q+ nor PBu3 has any perceptible effect on the photostationary state of Co2(CO)6(PBu3)2 on irradiation of the a,a band (60) at wavelengths of approximately 380 nm. However, when both are present, essentially quantitative yields of Co(CO)3(PBu3)2+ PF6 are obtained by a photoredox process that can be described as... 

Natural processes involving redox reactions are very frequent. Examples include the oxidation of aqueous ions [like Fe(II) to Fe(III)], oxidation of solids (like pyrite, FeS2 to SO2-), corrosion of metals, production of H2S by sulfate-reducing bacteria, photoredox processes in the atmosphere, water, and soil, etc. Therefore, it is important to understand the principles underlying redox chemistry and to organize them in the form of tables and diagrams such as those discussed below. 

Another very interesting example of a photochemical reaction directly involving the two chromophores 5 and 6 (Figs. 3 and 4) has recently been described (37,38). The spectral changes of the corresponding photoredox process are shown in Fig. 5. 

Fig. 11. Schematic illustration of a net two-electron transfer photoredox process. Excitation of reactants (R) forms a metastable one-electron intermediate (I), which finally can yield an energy-rich permanent product (P) after a second electron transfer step. Adapted from Ref (8).

Fig. 19. Different ways to introduce oxyl radical reactivity nature employs metal bound tyrosyl radicals (19) or high-valent metal oxo fragments in many active sites (65,153). Nitroxyl radicals such as 2,2,6,6,-tetramethylpiperidin-l-oxyl (TEMPO, 20) are reactive species used in organocatalysis (154). The excited states of carbonyl functional groups (21) and metal oxo-fragments (22) display a radical pair character, which may become very attractive for biomimetic photoredox processes upon spectral sensitization (3,5).

The selective activation of compounds with potential therapeutic effects and the controlled delivery of bioactive molecules triggered by light are topics of intensely growing interest (6,197-200). Besides the search for light-sensitive prodrugs activated by photochemical cleavage, isomerization or photoredox processes (201-203), especially the release of small molecules such as NO, CO, CS2, and H2S, have attracted a lot of interest in the past years (204-208). 

The photoredox reaction of chromimn(III) complexes with ligands such as oxalate, citrate, or edta can proceed under environmental conditions. In consequence, these pollutants imdergo oxidative degradation, but simultaneously the harmful and toxic chromate(VI) is generated. To close the photocatal5rtic cycle, Cr(VI) had to vmdergo successive photoredox processes. 

The photophysics and photochemistry of gaseous PuFe have been ex-amined. Studies involving zinc porphyrins have been reported and include photo-oxidations in aqueous solution, photoreductions of Zn-TPP with hydrazines, and the role of Zn-TPPSa/ethyl viologen in photoredox processes. The mechanism of the photo-oxidation of water to oxygen with silver chloride has been discussed, and the synthesis of bis(chlorosilyl)-mercury compounds described. Colloidal CdSe has been shown to sensitize the photoreduction of O2 and of methyl viologen by cysteine. ... 

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