Molecular cavities electron transfer

Some of the materials highlighted in this review offer novel redox-active cavities, which are candidates for studies on chemistry within cavities, especially processes which involve molecular recognition by donor-acceptor ii-Jt interactions, or by electron transfer mechanisms, e.g. coordination of a lone pair to a metal center, or formation of radical cation/radical anion pairs by charge transfer. The attachment of redox-active dendrimers to electrode surfaces (by chemical bonding, physical deposition, or screen printing) to form modified electrodes should provide interesting novel electron relay systems. 

While powerful contemporary techniques permit the use of molecular cavities of complex shape [3], it is instructive to note a few cases based on idealized representations of solute cavity and charge density. Cavities are typically constructed in terms of spherical components. Marcus popularized two-sphere models, [5,38] which can be used to model CS, CR, or CSh processes (see Section 3.5.2), where the two spheres are associated with the D and A sites, and initial and final charge densities are represented by point charges (qD and qA) at the sphere origins. If a single electron is transferred, Ap corresponds to A = 1 in units of electronic charge (e), and Aif is given by [5,38]... 

Theoretical formulations of reorganization in the course of electron-transfer processes have undergone a number of advances in recent years. The relative importance of various solvent contributions (including translational as well as orientational response, and inductive and dispersion as well as elecrostatic interactions) can depend strongly on the polarity (i.e., dipolar, higher multipolar, or nonpolar) as well as other molecular features of the solvent [21, 47-49]. Molecular-level perspectives on solvent response are of great utility in helping to parameterize effective cavity models (e.g., in conjunction with conventional [50] or spatially nonlocal [47] dielectric models). Additivity relationships traditionally assumed to pertain to sol-... 

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. 

Various recent papers have discussed possible applications of dendrimers [3, 4, 11, 14, 20, 23]. From the point of view of electron transfer, the interest in highly branched structures such as dendrimers is related not so much to their size, but rather to the presence of different components. In fact, an ordered array of different components can generate valuable properties, such as the presence of cavities with different size, surfaces with specific functions, gradients for photoinduced energy and electron transfer, and sites for multielectron transfer catalysis. At this point it is perhaps worth summarizing the practical applications of dendrimers encountered in this chapter. Dendrimers have already been used (i) in catalytic processes [122, 149, 172] (ii) in the functionalization of electrodes [48, 63, 172-175] which can also work as sensors [44, 48, 63] (iii) in the fabrication of nanoparticles [170-172] and (iv) in other devices such as LEDs [140-148] and molecular gates [173, 174], Moreover, as many different exciting applications will certainly be added to this list in the future, it is easy to foresee a remarkable expansion in dendrimer related research. 

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