Donor-acceptor compounds quinone acceptors

The interaction of the D1 and D2 proteins with other components of PS2 has been demonstrated by analysis of the composition of the isolated reaction centre preparation and reconstitution with the extrinsic 33 kD (manganese stabilising) protein, artifical electron donor and acceptor compounds, quinones and diacyl glycerolipids. The isolated reaction centre compiex was shown to contain three proteins in addition to D1 and D2 and these were identified as the products of the chloroplast genes psb E, F and 1. This preparation could bind the 33 kD protein under specific conditions and comparison with binding to a more complex PS2 core preparation indicated that binding is not greatly influenced by polypeptides other than those in the isolated reaction centre. 

Firstly, we reported the synthesis of 13,13,14,14-tetracyanobenzo[fe]naphto[2,3-e][l,4]dithiin-6,l 1 -qui-nodimethane (72) and 13,13,14,14-tetracyanoben-zo[b]naphto-[2,3-e][l,4]oxathiin-6,ll-quinodimelhane (73) and other methyl-substituted derivatives as novel single-component donor-acceptor compounds [26]. Their syntheses were carried out from the respective quinones by reaction with malononitrile in the presence of titanium tetrachloride and pyridine. The TCNQ derivatives were thus obtained as dark blue, high melting point solids in moderate yields. 

The ability of compounds with a quinonic structure to form donor-acceptor interactions and CT complexes is useful in regioselective halogenation of phenol (or naphthols and their derivatives). 

Several organofullerene donor-acceptor molecular material hybrid systems have been synthesized via 1,3-dipolar cycloaddition reactions of azomethine ylides, via Bingel cyclopropanation and methanofullerene formation intermediates as well as via cycloaddition reactions, that have already been discussed in previous sections. The majority of such hybrid systems possess always as acceptor unit the fullerene core and as donor moieties porphyrins, tetrathiafulvalenes, ferrocenes, quinones, or electron-rich aromatic compounds that absorb visible light [190-193]. The most active research topic in this particularly technological field relies (i) on the arrangement of several redox-active building blocks in... 

A comparison of electronic spectra of the pyrrolo-TTF derivatives 25 containing a quinone acceptor system with macrocyclic D-A systems with the compound 26 based on the 4,4 -bipyridinium dication showed that both compounds contained in their structures moderate acceptors and the same strong TTF donors. Direct Jt-overlap between donor and acceptor was evidenced by a strong band (e = 403) in the rigid structure of 26 < 1997JOC679>. In contrast, the UV-Vis spectrum of 25 did not exhibit CT bands between 500 and 820 nm <1998JOC1198>. 

Similar hydrogen exchange reactions take place over the electron donor-acceptor complexes of the metallocenes with various organic electron acceptors such as quinones, and nitro- and cyano-substituted compounds (7). The hydrogen exchange reaction between acetylene or molecular hydrogen, and various EDA complexes of metallocenes proceeded at room temperature via bonded hydrogen (HZ) in the complexes, but did not take place over either metallocenes or quinones alone under the same reaction conditions. 

Quinoid compounds are excellent acceptors of electrons and form electron donor-acceptor (EDA) complexes as a consequence of low-lying unoccupied electronic energy levels205. The EDA complexes may be easily formed in interactions with phenolic or amine components of a stabilizing mixture, with other additives which have reactive H atoms, with RO 2 radicals, or with some metallic impurities in polymers via rr-orbital interactions. Quinones efficiently participate in oxidation of polymers by virtue of these processes. 

Many compounds containing a donor and an acceptor joined by a flexible linker have been synthesized (see Ref. 185 for an extensive list of references). The difficulty in using such compounds to study the distance dependence of ET is that the flexibility of the tinker precludes knowing the donor-acceptor separation precisely. Distances have been estimated from fluorescence quenching volumes (61) and NMR conformational studies (60) with such compounds. NMR studies indicated that the methylene tinker between the porphyrin and the quinone in carotenoid-porphyrin-quinone triads is in an extended (all-anti) conformation (60). The calculated value of p, based on this conformation for the series of methylene linkers, was... 

As, with the exception of the mentioned oxidized compounds, the metallomacrocycles do not crystallize forming columnar quasi-one-dimensional structures, one of the prerequisites for conductivity is not met. In addition the stacking repeat distances and the donor-acceptor distances in doped compounds cannot be controlled in any way. By using oxidizing dopants other than halogens, e.g. quinones, integrated stacks can be formed leading to insulators. 

All complexes of this kind are quite polar and water soluble. The permanent dipole moments are caused by the fact that the olefinic ligands are predominantly donors and only weak acceptors so that the duroquinone molecule interacts with filled 3d orbitals of nickel even more strongly than in the case of bis(duroquinone)-nickel. Consequently, the quinone C=0 groups are more polarized than in the parent compound bis(duroquinone)-nickel (see Table I). The particularly high stability and polarity of the... 

One of the most versatile electron-accepting molecules is the quinonoid compound, and the redox reaction of the quinone-hydroquinone couple is one of the most thoroughly studied proton-coupled electron transfer systems of organic molecules. Quinones show the reversible two-step le reduction in aprotic organic solvents (Fig. 2). One-electron addition to quinone forms the semiquinone radical with five n electrons. The stability of the semiquinone form is affected by the existence of a minute amount of proton, which appears as the large shift of the reduction potentials in the positive direction. This implies that quinonoid compoimds are representative acceptor molecules of which redox properties are influenced by external perturbation, such as protonation and solvation (Fig. 2). They are employed in covalently and noncovalently linked donor-acceptor systems of particular interest in the study of proton-coupled electron transfer and photoin-duced electron transfer. ... 

The vast majority of colored organic compounds are based on donor-acceptor chromogens, and with the exception of the polycyclic quinones and the phthalo-cyanines, all the commercially important synthetic dyes are of this type [34, 35]. 

Since long retention times are often applied in the anaerobic phase of the SBR, it can be concluded that reduction of many azo dyes is a relatively a slow process. Reactor studies indicate that, however, by using redox mediators, which are compounds that accelerate electron transfer from a primary electron donor (co-substrate) to a terminal electron acceptor (azo dye), azo dye reduction can be increased [39,40]. By this way, higher decolorization rates can be achieved in SBRs operated with a low hydraulic retention time [41,42]. Flavin enzyme cofactors, such as flavin adenide dinucleotide, flavin adenide mononucleotide, and riboflavin, as well as several quinone compounds, such as anthraquinone-2,6-disulfonate, anthraquinone-2,6-disulfonate, and lawsone, have been found as redox mediators [43—46]. 

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