Charge transfer complexes viologens

Benzpinacols (or their trimethylsilyl ethers) are effective electron donors and readily form vividly colored charge-transfer complexes with common electron acceptors such as chloranil (CA), dichlorodicyanobenzoquinone (DDQ), tetracyanobenzene (TCNB), methyl viologen (MV2+), and nitrosonium (NO+) cation.191-194 For example, the exposure of a silylated benzpinacol to chloranil... 

There is no evidence for ground state charge-transfer complex formation between stilbenes and neutral amines. Amine cations and dications are powerful electron acceptors and can form ground state complexes in which t-1 serves as the electron donor. Complex formation between t-1 and the organic dication methyl viologen is responsible for quenching of the fluorescence of surfactant stilbenes in organized assemblies (112). 

Control of self-assembly by switchable redox processes in complexes between dendrimers with re-donor and re-acceptor units permitted specific prediction of the space occupied by dendritic aggregates. Use of cucurbit[8]uril (see Fig. 6.9) as donor-host compound for viologen guests (dendronised 4,4 -bipyridines as acceptors) promotes formation of such charge-transfer complexes [24]. 

Spectroscopic methods can be used to specify the position of donors and acceptors before photoexcitation [50]. This spatial arrangement can obviously influence the equilibrium eomplexation in charge transfer complexes, and hence, the optical transitions accessible to such species [51]. This ordered environment also allows for effective separation of a sensitizing dye from the location of subsequent chemical reactions [52], For example, the efficiency of cis-trans isomerization of A -methyl-4-(p-styryl)pyridinium halides via electron transfer sensitization by Ru(bpy) + was markedly enhanced in the presence of anionic surfactants (about 100-fold) [53], The authors postulate the operation of an electron-relay chain on the anionic surface for the sensitization of ions attached electrostatically. High adsorptivity of the salt on the anionic micelle could also be adduced from salt effects [53, 54]. The micellar order also influenced the attainable electron transfer rates for intramolecular and intermolecular reactions of analogous molecules (pyrene-viologen and pyrene-ferrocene) solubilized within a cationic micelle because the difference in location of the solubilized substances affects the effective distance separating the units [55]. 

Sulfur- and selenium-donor ligands stabilize several polynuclear zinc complexes, for example, the [E4Zii4(SPh)i6] (E = S or Se) anions, prepared as their Mc4N+ salts.The complex [Zn4(SPh)io] forms a ground-state charge transfer complex with methyl viologen. These... 

Fig. 15A-C. Mechanical switching in catenanes. A The individual rings of catenanes may be unsymmetrical. A [2]catenane with two distinct sites on each ring may exist in four different states. B A switchable catenane based on the competitive binding of TTF and dioxynaph-thalene residues on one ring inside the cavity of the second ring. The TTF moiety is bound preferentially in the reduced state, but is repelled in the oxidized state and replaced with the dioxynaphthalene unit. C A proton-switchable [2]catenane. The phenanthroline units in the rings of 37 form a complex around H+, but, when H+ is not present, a charge-transfer complex is formed between the viologen units in one ring and the dioxynaphthalene unit in the other. The absorbance spectra are adapted from [64] with permission...

Figure 5 Vesicles composed of a ternary complex of cucurbit[6]uril, n-alkyl viologen, and dihydroxynaphtalene. (A) Formation of a charge transfer complex of n-alkyl viologen and dihydroxynaphtalene in the cavity of cucurbit[6]uril. (B) TEM and SEM images of complex 2 and 3. (Reproduced from Ref. 31. Wiley-VCH, 2002.)...

Similarly, intramolecular charge-transfer complexation was obtained by alternating copolymerization of tetracyanoquinodimethane (TCNQ) with ferrocene into one chain [91, 111]. Spectroscopic and electrochemical analyses of mixtures of the monomeric units showed clear intramolecular charge-transfer complexation. In a related study, poly(methylated ferrocenylsilanes) were shown to interact with tetracyanoethylene [262]. A similar concept is valid for viologen-based polymers [55]. 

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