Redox-switching

Baumann, A. Livoreil, W. Kaim and J.-P. Sauvage, Chem. Commun., 1997, 35. 

A similar concept has been applied to the switching of a macrocyclic component 3 between two different co-ordination sites on the linear component 26 to yield the corresponding pseudo-rotaxane. A subsequent report describes parallel studies involving an analogous [2]-rotaxane (incorporating bulky terminal mesylate 

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Redox switching, 126 Reference electrodes, 100, 105, 142 Reflectance spectroscopy, 44 Resistance, 22, 105 Resolution 50, 71 Reverse pulse polarography, 68 Reversible systems, 4, 31 Reticulated vitreous carbon, 114, 115 Riboflavin, 37... 

Figure 19. Scheme describing the redox switch, which is based on a viologen redox center incorporated within the nanoclusters ligand shell. For simplification the counterelectrode is not shown. (Adapted with permission from Ref [34], 2000, Nature Publishing Group.)... 

Following the discovery of the unique electronic properties of polypyrrole, numerous polymers of pyrrole have been crafted. A copolymer of pyrrole and pyrrole-3-carboxylic acid is used in a glucose biosensor, and a copolymer of pyrrole and A-methylpyrrole operates as a redox switching device. Self-doping, low-band gap, and photorefractive pyrrole polymers have been synthesized, and some examples are illustrated [1,5]. 

Hembre, R. T., McQueen, J. S. and Day, V. W. (1996) Coupling H2 to electron transfer with a 17-electron heterobimetallic hydride A Redox Switch model for the H2-activating center of hydrogenase./. Am. Chem. Soc., 118, 798-803. 

Electrochromic materials are electroactive compounds whose visible spectra depend on the oxidation state. Possible applications are smart windows, displays, mirrors, and so on. Among the most important performance aspects in electrochromic materials, the reversibility and lifetime of the material to repeated cycles, the time of response (usually in order of seconds), the colors of the oxidized/reduced forms and the change in absorbance upon redox switching (contrast) are of interest. 

As mentioned earlier in Chapter 5, there are ion-radicals capable of forming hydrogen-bond complexes with neutral molecules. Such complexation significantly changes the redox potential comparatively to that of an initial depolarizer. Of most importance is that the formation of ion-radicals is a reversible process. In other words, the redox-switched effect operates in this host-gnest systems. Scheme 8.5 illnstrates the effect realized in the systems of ferrocene/ferrocenium (Westwood et al. 2004) and of nitrobenzene/the nitrobenzene anion-radical (Bn et al. 2005). 

A first example is represented by the Mn(III)/Mn(II) redox switch. The complexes of Mn(II) and Mn(III) with the water-soluble tetraphenylsulpho-nate porphyrin (TPPS, Chart 13) display significantly different ri values at low magnetic field strength (lower than 1 MHz), but very similar values at the fields currently used in the clinical practice (> 10 MHz) (141). However, the longer electronic relaxation rates of the Mn(II) complex makes its relaxivity dependent on the rotational mobility of the chelate. In fact, upon interacting with a poly-p-cyclodextrin, a 4-fold enhancement of the relaxivity of [Mn(H)-TPPS(H20)2] at 20 MHz has been detected, whereas little effect has been observed for the Mn(III)-complex. The ability of the Mn(II)/Mn(III)... 

An ab initio study using the 6-31G basis set was used to investigate the vibrational and electronic structures of a benzodithiophene and benzodifuran quinone as potential redox switches. The calculated and experimental data were... 

Figure 2.2 The redox switched translocation of a metal ion within a two-compartment ligand.

Fluorescent redox switches based on compounds with electron acceptors and fluorophores have been also reported. For instance, by making use of the quinone/ hydroquinone redox couple a redox-responsive fluorescence switch can be established with molecule 19 containing a ruthenium tris(bpy) (bpy = 2,2 -bipyridine) complex.29 Within molecule 19, the excited state of the ruthenium center, that is, the triplet metal-to-ligand charge transfer (MLCT) state, is effectively quenched by electron transfer to the quinone group. When the quinone is reduced to the hydroquinone either chemically or electrochemically, luminescence is emitted from the ruthenium center in molecule 19. Similarly, molecule 20, a ruthenium (II) complex withhydroquinone-functionalized 2,2 6, 2"-terpyridine (tpy) and (4 -phenylethynyl-2,2 6, 2"- terpyridine) as ligands, also works as a redox fluorescence switch.30... 

Molecular Redox Switches Based on Metal Ions... 

Fe(II)/Fe(III) is another well-known metal ion redox couple. By making use of the different coordination preferences of Fe(II) and Fe(III), molecular redox switches... 

The diffusion of the charge-compensating counterions through the thin films determines the response time of the systems during redox switching. A more open polymer morphology therefore enhances the ionic mobility and yields a faster response [40]. 

The presence of Au(I) may likewise catalyze the reaction of Au(III) with methylcob(III)alamin Wood has included this couple under his Redox Switch category. Like PtCl l-, the reaction of AuC14 with CH3B12 is enhanced by the presence of Br" (46). It remains to be proven that the Au reaction does involve methyl radical transfer, presumably with subsequent oxidation of B12r [a cob(II)alamin compound] to the observed aquocobalamin. However, the available evidence for the formation of Au(II) intermediates is more extensive and convincing (200) than for Pt(III) intermediates (201). 

Polythiophenes (PTs) have received a great deal of attention due to their electrical properties, environmental stability in doped and undoped states, non-linear optical properties, and highly reversible redox switching [1]. Thiophene possesses a rich synthetic flexibility, allowing for the use of several polymerization methods and the incorporation of various side chain functionalities. Thus, it is of no great surprise that PTs have become the most widely studied of all conjugated polyheterocycles [184]. 

The insertion of a photosensitive group or of a redox active unit into the push-pull system 1 yields switchable molecular wires and push-pull molecules that contain a photo-switch or a redox switch S, as represented in 4. Compounds of such type containing for instance electroactive ferrocene groups and photosensitive metal complexes, have been synthesized. Some of them are shown in series 5 (Marczinke, B. Przibilla, K.J. Lehn, J.-M., unpublished data). 

Goulle, V., Harriman, A., Lehn, J. M., An electro-photoswitch - redox switching of the luminescence of a bipyridine metal-complex. J. Chem. Soc., Chem. Commun. 1993, 1034-1036. 

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