Quinone-type groups

As shown in Chapter 5, functional groups are always present on the surface of nanoporous carbons. Some of them, as the quinone-type groups, may be involved in reversible redox reactions giving a... 

XPS studies of surface oxide groups obtained by electrochemical oxidation of carbons showed that carbonyl type and quinone type groups become tlte major oxide components fonned during dectrochemical (Kidation of carbcais [187]. 

Quinones, lipid-soluble substances involved in electron transport, can also be used as biomarkers. Lipski et al. (155) u.sed quinone analyses, physiological tests, and fatty acid profiles to differentiate Gram-negative non-fermentative bacteria isolated from biofilters. Quinone type was found to be an efficient method to group isolates prior to the analysis of results from the physiological tests. The detection of quinones appears to be restricted to the discrimination of isolated colonies and has limited potential to the analysis of mixed populations. 

Among the many other substances 1 showing resonance of the semi-quinone type mention may be made of those of the dipyridyl group. 

Quinone type carriers perform the cotransport of two protons and two electrons (2e, 2H+ symport) [6.48, 6.49] and take part in mitochondrial and photosynthetic electron transport. Cation receptor sites such as crown ethers or cryptands bearing a quinone [6.50a] or a ferrocene [6.50b] group (see also Section 8.3.1), bind and carry cations with redox coupling through switching between a low affinity state (quinone, ferricinium) and a high affinity state (reduced quinone, ferrocene). 

Substitution of phenolic groups in 1,4-benzoquinone also leads to a decrease in formal potentials, as can be seen in the 2-hydroxo-l,4-benzoquinone/1,2,4-trihydroxybenzene system (Table 8). However, the addition of a carboxyl group has no such effect. Therefore, the peak potential in neutral aqueous solution (0.1 M NaiS04) determined for oxidized carbon samples, set out in Table 10, confirms the presence of surface quinone-type structures conjugated with hydroxyl functionalities and 7t-electrons of the aromatic system. 

The distance dependencies of photoinduced electron transfer rates have been examined in anthracene-spacered porphyrin-quinone cyclophanes, and the same authors have also discussed the distance dependencies of photo-induced electron-transfer rates in benzene-, naphthalene-, and anthracene-spacered porphyrin-quinone cyclophanes and biphenylene-spacered porphyrin-quinone cyclophanes. Photoelectron transfer reactions of the porphyrin-quinone cyclophanes (3) and their zinc complexes have been examined, and in some cases at least interaction of the quinone carbonyl group with the zinc atom may be an alternative to through-space electron transfer. A study of intramolecular photoinduced electron transfer for the quinone-porphyrin cyclophane type (4) containing the especially strong acceptor 7,7,8,8-tetracyanoquinodimethane (TCNQ) has appeared." The distance dependence of the TCNQ and porphyrin is of particular interest, and to this end the corresponding 2,8-naphthalenediyl-TCNQ-porphyrin has been synthesised. 

It has been previously reported that l,T-binaphthalene undergoes racemization through the triplet state,whereas this process for the 2,2 -diol derivative is proposed to involve an intermediate arising from intramolecular addition of a hydroxy group to the C2-C3 bond. Nevertheless, neither of these routes is considered to operate for the recently described efficient photoracemization of the naturally occurring l,r-biphenanthrene, blestriarene (200) in ambient light. Instead it is proposed that quinone-type species are the likely intermediates in this case. 

The RCs in photosynthetic organisms have been classified into two groups, i.e., quinone-type and iron sulfur-type. The quinone-type RCs are present in purple non-sulfur bacteria as well as in the PS II of cyanobacteria and chloroplasts (of algae and higher plants), whereas the iron sulfur-type RCs are present in green and purple sulfur bacteria as well as in the PS I of cyanobacteria and chloroplasts (Blankenship, 1992 HauskaetaL, 1995). 

The oxidation of formyl groups accompanied by subsequent functionalization has showcased the utility of the Baeyer-Villiger oxidation. Franck reported the Baeyer-Villiger oxidation of naphthaldehydes as precursors to pharmacologically active quinone type compounds. This was exemplified by the conversion of aldehyde 104 into formate 105, which after formate cleavage, was further oxidized to the quinone system 106.55... 

The experimental results were well described by a kinetic model based on a redox mechanism of the Mars-van Krevelen type, where the quinone surface groups are reduced to hydroquinone by adsorbed ethylbenzene, and reoxidized back to quinone by oxygen [46,63,64], as shown in Figure 6.3. A recent... 

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