Redox macrocyclic receptor molecules

Beer, P.D. Redox responsive macrocyclic receptor molecules containing transition-metal redox centers. Chem. Soc. Rev. 1989. 18. 409-450. 

The prospect of advancing chemical sensor technology, modelling electron-transfer processes in biological systems and producing new redox catalysts has led to considerable interest in the design and syntheses of redox-active macrocyclic receptor molecules that contain a redox centre in close proximity to a host binding site. ... 

The electrochemical properties of redox-active substrates are also affected on binding to a receptor molecule. This is the case for the complexation of metal hexacyanides by polyammonium macrocycles where the shift of the redox potential depends on the binding constants and the oxidation or reduction of the substrates leads to pronounced changes in stability (see Chapter 3) [3.21, 3.22]. 

A method for the preparation of thin films of Fe4[Ru(CN)6]3 ( ruthenium purple ) involving electrochemical reduction of K3[Ru(CN)6] in a solution of Fe2(S04)3 has been developed.28 This ruthenium purple modified electrode is claimed to be one of the best catalysts for evolution of oxygen and chlorine. Electrochemical studies on polyammonium macrocyclic complexes of [Ru(CN)6]4 indicate a 1 1 stoichiometry with a monoelectronic, reversible, oxidation for these complexes this illustrates the control of redox potential of anions by complexation with appropriate receptor molecules.29 The kinetics of oxidation of [Ru(CN)6]4 by [Mn04] in HC104 have been investigated by stopped-flow techniques. It is found that [Ru(CN)6]4" is quantitatively oxidized to [Ru(CN)6]3 in accordance with equation (1) and that two protonated intermediates [RuH(CN)6]3 and [RuH2(CN)6]3 are involved in the oxidation process.30... 

Exceptionally stable and electrochemically well-behaved SAMs bearing dithia-crown-annelated TTF derivatives were prepared using bipodal binding of the receptor molecules to Au electrodes [73, 74]. The CVs in Fig. 9 are of much better quality compared to those in Fig. 8, indicating that formation of TTF" " crown after the first oxidation does not repel the metal ion from the complex. The interplay between macrocycle size and cation size influences the shift of the redox waves upon complexation. The redox potentials of the SAM of la are not sensitive to the presence of li" " and K+ in solution, while Na+ causes a small shift of 10 mV in the first wave. The SAM of lb shows high sensitivity to Na+ (55-60 mV, both waves) and a moderate sensitivity to K+ (20-30 mV), but li+ does not influence the signal. 

Protons are relatively simple targets for sensor molecules and do not require engineered receptors, however, achievement of selective interactions with other chemical species requires much more elaborate receptors. In the most cases cations are bound via electrostatic or coordinative interactions within the receptors alkali metal cations, which are rather poor central ions and form only very weak coordination bonds, are usually bound within crown ethers, azacrown macrocycles, cryptands, podands, and related types of receptor moieties with oxygen and nitrogen donor atoms [8], Most of the common cation sensors are based on the photoinduced electron transfer (PET) mechanism, so the receptor moiety must have its redox potential (HOMO energy) adjusted to quench luminescence of the fluorophore (Figure 16.3). 

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