Guests electroactive

If we look at the mechanistic and crystallographic aspects of the operation of polycomponent electrodes, we see that the incorporation of electroactive species such as lithium into a crystalline electrode can occur in two basic ways. In the examples discussed above, and in which complete equilibrium is assumed, the introduction of the guest species can either involve a simple change in the composition of an existing phase by solid solution, or it can result in the formation of new phases with different crystal structures from that of the initial host material. When the identity and/or amounts of phases present in the electrode change, the process is described as a reconstitution reaction. That is, the microstructure is reconstituted. 

The second way in which an electroactive species such as lithium can be incorporated into the structure of an electrode is by a topotactic insertion reaction. In this case the guest species is relatively mobile and enters the crystal structure of the host phase so that no significant change in the structural configuration of the host lattice occurs. 

Figure 15. Possible mechanism of guest-induced permeability changes for electroactive markers by oriented membranes of the calix[61arene hexaester 28 (R = Bu ) in the cases that (a) anionic, (b) cationic, and (c) neutral markers are used.

Figure 19. Schematic representations of the permeation behaviors of electroactive markers through the condensed monolayer of p-cyclodextrin derivative 41 in the presence and absence of the guest, (a) Permeable markers, (b) Nonpermeable markers. ...

Typically in supramolecular chemistry, the term host refers to the larger and more structurally complex of two binding partners, while the term guest refers to the smaller, less complex binding partner. However, for the purpose of this discussion, the term host will be used to refer to the electroactive binding partner and guest to refer to the nonelectroactive binding partner, irrespective of their size or structural complexity. 

Guest-Induced Changes in Membrane Permeability. Calixarene derivatives are also used for sensing systems other than ISEs or optodes. Recently, a systematic investigation on the control of membrane permeability by use of oriented monolayers composed of calixarene esters was carried out.36 The hosts used were short alkyl chain esters of calix[6]arene [28 (R2 = Bu )] and calix[4]arene [26 (R2 = Bu ), 30 both cone conformers]. The permeabilities through the intermo-lecular voids of these monolayers were evaluated by cyclic voltammetry, as described earlier for oriented membranes of nucleobase derivatives. Cationic, anionic, and neutral electroactive compounds were used as the permeability markers. The voltammetric measurements were carried out either for a monolayer... 

Studies on the electrochemical behavior of ferrocene encapsulated in the hemi-carcerands 61 and 62, indicated that encapsulation induces substantial changes in the oxidation behavior of the ferrocene subunit [98]. In particular, encapsulated ferrocene exhibits a positive shift of the oxidation potential of c. 120 mV, probably because of the poor solvation of ferrocenium inside the apolar guest cavity. Lower apparent standard rate constants were found for the heterogeneous electron transfer reactions, compared to those found in the uncomplexed ferrocene under identical experimental conditions. This effect may be due to two main contributions (i) the increased effective molecular mass of the electroactive species and (ii) the increased distance of maximum approach of the redox active center to the electrode surface. 

The construction of macrocyles containing TTF units has received considerable attention since such molecules may act as a host in host-guest chemistry. Planar derivatives containing the TTF-crown ether 971 and TTF-crown thiaether 972 have been investigated for potential use as electroactive cation sensors <2000CSR153>. 

Equations (2.14) to (2.23) were obtained on the assumption that electroactive molecules are uniformly distributed in the entire volume of the solid. However, this is an unrealistic assumption in cases where bulky guest species are entrapped within the cavities of the porous material. Here, ship-in-a-bottle synthetic procedures most likely yield a nonuniform distribution of guest species in the network of the host porous material. Electrochemical data can then be used for obtaining information on the distribution of electroactive species. 

Voltammetric and coulometric data indicate that under ordinary experimental conditions, only a small percentage (lower than 1-2%) of guest molecules is electroactive. 

Voltammetric responses displayed by zeolite probes containing electroactive guests in contact with size-excluded electrolyte countercations such as... 

As previously noted (see Section 2.5), the depth reached in a given experiment can, in principle, be approached by the advance of the diffusion layer if the diffusion coefficient for the rate-determining charge transport is known. Then, at short times, only the electrochemical response of the guest molecules externally adsorbed or located in the more external layers of the crystals will be electroactive. At longer times, the observed response will reflect the contribution of molecules located deeper in the host crystals. 

In the above-studied systems, the aluminosilicate framework merely acts as a porous, electrochemically inert support embedding the redox-active guests. In contrast to this, the presence of electroactive atoms in the composition of the aluminosilicates renders the solid electroactive. This applies to the case of titanium... 

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