Viologen units

They found almost complete quenching of the emission from the ruthenium complex and in addition the covalently linked compound considerably enhanced electron transfer to relay systems of aligned viologen units on micelles and polymers. 

K. A. Jolliffe, T. D. M. Bdl, K. P. Ghiggino, S. J. Langford, M. N. Paddon-Row, Efficient Photoinduced Electron Transfer in a Rigid U-Shaped Tetrad Bearing Terminal Porphyrin and Viologen Units , Angew. Chem, Int. Ed. 1998,37, 916-919. 

Ru(bpy)j+ was covalently linked to viologen units to give a model of photoreaction center (11-13)35). 

The emission from Ru(bpy)2+ of these compounds was extremely weak because of the rapid quenching by the adjacent viologen unit. The systems of mixed micells composed... 

The photoreduction of polymer pendant viologen by 2-propanol was reported to proceed by the successive two-electron transfer processes between the adjacent viologen units and the propanol which is a two-electron reducing agent44). Preferential formation of a dimeric cation radical of viologen observed was ascribed to the polymeric structure and the two-electron process. These fundamental studies on polymeric electron mediators contribute to the construction of solar energy conversion systems. 

It is also interesting to notice that these two families of poly viologen dendrimers can also act as polytopic receptors toward electron donor substrates, hosting a number of eosin dianions equal to the number of viologen units present in their branches. The data... 

Very high quenching rate constants are obtained if the nitrogen derivative is attached directly to [Ru(bipy)3]2+ making it amphiphilic, however, very low yields of redox products are achieved even in the presence of EDTA, since back electron transfer is also extremely efficient.149 Addition of CTAC above the CMC makes little difference but if didodecylviologen is also added, the rate of cation radical formation increases five-fold. Still higher rates of cation radical production (55x) are observed if a polysoap with pendant viologen units is added.149... 

Attachment of the ruthenium complex to the polysoap leads to even greater rate enhancements and it is believed that sequential passage of the electron from the ruthenium attached viologen after the photochemical electron transfer to neighbouring viologen units and then on down the chain of local viologens leads to efficient charge separation.149... 

Using the polymerized viologen bilayers, the viologen units were removed from the outer surface to which they were attached by ester linkages by reaction with imino ethanol. Photolysis with [Ru(bipy)3]2+ in the bulk phase then led to viologen cation radical formation on the inner surface. Presumably the iminoethanol groups acted as electron donors.342... 

Very recently [88] a synthetic derivative of Vitamin B12 where five viologen units have been covalently linked to the 3 side chains of the corrin nucleus has been reported in the literature (Fig. 36). 

As a different type of photoswitchable rotaxane, Murakami et al. reported on the rotaxane, in which a-CD is threaded by an azobenzene-containing chain with two methylenes as the spacer between azobenzene and viologen units (109) [97],... 

Fig. 14 Structural formulas of two families of dendrimers containing a 1,3,5-trisubstituted benzenoide core and 9 (A918+ and B918+) and 21 (A2142+ and B2142+) viologen units in their branches [69-71]...

Ong W, Grindstaff J, Sobransingh D, Toba R, Quintela JM, Peinador C, Kaifer AE (2005) Electrochemical and guest binding properties of Frechet- and Newkome-type dendrimers with a single viologen unit located at their apical positions. J Am Chem Soc 127 3353-3361... 

Dendrons have also been used as the capping moieties for rotaxane cores (Figure 8). Amabilino et al. reported the Williamson coupling of the third generation Frechet-type dendrons 6 to cap polyether macrocycles threaded around linear bipyridinium oligomers.241 When the linear component consisted of multiple viologen units, 171, shuttling of the... 

Polymers incorportating viologen units in their main chains were also prepared (Figure 6 see Appendix for details of the procedure).46,47 The effects of counterions and additives on the photochromism of viologen ionene polymers... 

T. Hasimoto, S. Kohjiya, S. Yamashita, and M. Me, Photochromic and photochemical ionene elastomer containing poly(tetrahydrofuran) segments and viologen units. J. Polym. Sci., Polym. Chem. 29, 651-665 (1991). 

Upon reduction in the region where the cation radical PTV+ was formed, a new signal with hyperfine structure, due to the reduced viologen moiety, could be seen superimposed on the previous one.26 The presence of the hyperfine structure indicated the high flexibility of the alkyl-chain-bound viologen units. 

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...

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