Product redox-pair

The methano-dimer of a-tocopherol (28)50 was formed by the reaction of o-QM 3 as an alkylating agent toward excess y-tocopherol. It is also the reduction product of the furano-spiro dimer 29, which by analogy to spiro dimer 9 occurred as two interconvertible diastereomers,28 see Fig. 6.23. However, the interconversion rate was found to be slower than in the case of spiro dimer 9. While the reduction of furano-spiro dimer 29 to methano-dimer 28 proceeded largely quantitatively and independently of the reductant, the products of the reverse reaction, oxidation of 28 to 29, depended on oxidant and reaction conditions, so that those two compounds do not constitute a reversible redox pair in contrast to 9 and 12. 

Back electron transfer takes place from the electrogenerated reduc-tant to the oxidant near the electrode surface. At a sufficient potential difference this annihilation leads to the formation of excited ( ) products which may emit light (eel) or react "photochemical ly" without light (1,16). Redox pairs of limited stability can be investigated by ac electrolysis. The frequency of the ac current must be adjusted to the lifetime of the more labile redox partner. Many organic compounds have been shown to undergo eel (17-19). Much less is known about transition metal complexes despite the fact that they participate in fljjany redox reactions. 

The choice of new complexes was guided by some simple considerations. The overall eel efficiency of any compound is the product of the photoluminescence quantum yield and the efficiency of excited state formation. This latter parameter is difficult to evaluate. It may be very small depending on many factors. An irreversible decomposition of the primary redox pair can compete with back electron transfer. This back electron transfer could favor the formation of ground state products even if excited state formation is energy sufficient (13,14,38,39). Taking into account these possibilities we selected complexes which show an intense photoluminescence (0 > 0.01) in order to increase the probability for detection of eel. In addition, the choice of suitable complexes was also based on the expectation that reduction and oxidation would occur in an appropriate potential range. 

The reaction of thioxanthone with various 3-thienyllithium compounds is the initial step in the synthesis of the diols such as 602 from which the bis(thioxanthylium) dication 603 is obtained. This species functions as a reversible redox pair with its reduction product, the hexaarylethane, creating an electrochromic system in which electron transfer brings about bond making and bond breaking. These oligomers 604 may be considered to be a new class of molecular wires (Scheme 238) <2004OL2523>. 

In contrast to standard borohydride reductive nanoparticle synthesis, we have developed an alternative strategy to amino acid encapsulated nanoparticles by utilizing a metal nanoparticle (M°-(Ligand))/metal ion (M"+) precursor redox pair with matched oxidation/reduction potentials. Simply, a metal nanoparticle such as Pt°-(Cys) acts as the principal reductant to a complimentary selected metal ion of Au + resulting in a new stabilized metal nanoparticle of Au°-(Cys) and the oxidation product of the original nanoparticle Pt"+. Malow et al. have reported a metathesis/transmetallation type reaction between a platinum colloid and a Au cyanide compound. Similarly, we employed a Pt°-(Cys)/AuCl4 pair and 0.5-2.0 equivalents of Au to Pt -(Cys). XRD analysis of the nanoparticle products revealed differences in crystallinity... 

We next discuss photoinitiating systems that contain a photo-redox pair but which consist of one neutral (dye) and a second charged (electron donor) component. After electron transfer a charged and a second neutral product are formed. There is... 

Hydrogen production by a 2-step water splitting thermochemical cycle can be based on metal oxides redox pairs. A two-step, water-split-ting cycle, based on metal oxides redox pairs bypasses the separation hurdle. Multi-step thermochemical cycles can allow the use of more moderate operating temperatures, but their efficiency is still limited by the irreversibility associated with heat transfer and product separation. 

Tetraalkylborate anion is oxidized into tetraalkylborate radical at 0.60 V (Scheme 12), which is then transformed into Bu that can react with 1,3,5-trinitrobenzene. The resulting radical species eliminate a proton, thus giving the corresponding nitroaromatic radical-anion. The latter is oxidized by the cyclohexadienyl radical (according to the standard potentials of redox pairs) [38, 66]. This reaction is somewhat similar to the termination step in the SrnI aromatic substitution reactions [64,67,68]. When the electrolysis is carried out at 1.06 V, the o -complexes are oxidized (path A, Scheme 12), as well as tetraalkylborate anions (path B, Scheme 12). The generation of Bu allows to improve yields of the Sn products, but Bu can also attack the Sn product to form dialkyl trinitrobenzene. This process appears to be more important at the final stage of the reaction, when... 

Another route to productive CT photochemistry can involve proton transfer reactions between A and D +, which are favored in the redox pair arising from the enhanced acidity of D + (relative to D) and basicity of A (relative to A). Numerous examples of these reactions exist in the organic literature [240-241], and such a pathway should be particularly important for transition-metal hydrides with significantly enhanced acidities of their (metastable) cation radicals [242]. Thus irradiation of the EDA complex of fumaronitrile (as acceptor) with the hydridic donor (CP2M0H2) [35] leads to CT hydro-metallation. 

Charvin, P., Abanades, S., Flamant, G., Lemort, F., 2007. Two-step water splitting thermo-chemical cycle based on iron oxide redox pair for solar hydrogen production. Energy 32 (7), 1124-1133. 

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