Dendrimers Containing Electroactive Units

Mesomorphic dendrimers containing electroactive units have potential for construction of dendrimer based molecular switches. Deschenaux et al. reported [154] the synthesis and liquid-crystalline properties of a novel dendrimer containing six mesomorphic ferrocene units. Apart from exhibiting a broad enantiotropic smectic A phase as determined by polarized optical microscopy, DSC, and XRD studies, thermogravimetry revealed the excellent thermal stability of the macromolecule. 

Of course, there are also dendritic structures that belong to more than one of these categories. The most significant examples of electron transfer reactions involving dendrimers containing electroactive units will be presented in Section 9.4, arranged on the basis of the electroactive unit involved. Moreover, there are examples where the dendrimer itself does not contain any electroactive unit, but is in-... 

Electroactive dendrimers are defined as those that contain functional groups capable of undergoing fast electron transfer reactions [85], The combination of specific electron transfer properties of redox active probes with the unique structural properties of dendrimers offers attractive prospects of their exploitation in electrocatalytic processes of biological and industrial importance [86], Further, the interest in dendrimers containing electroactive units also relies on the fact that electrochemistry is a powerful technique to elucidate the structure and purity of dendrimers, to evaluate the degree of electronic interaction of their chemically and/or topologically equivalent or non- equivalent moieties, and also to study their endo- and exo-receptor capabilities [87],... 

The electroactive units in the dendrimers that we are going to discuss are the metal-based moieties. An important requirement for any kind of application is the chemical redox reversibility of such moieties. The most common metal complexes able to exhibit a chemically reversible redox behavior are ferrocene and its derivatives and the iron, ruthenium and osmium complexes of polypyridine ligands. Therefore it is not surprising that most of the investigated dendrimers contain such metal-based moieties. In the electrochemical window accessible in the usual solvents (around +2/-2V) ferrocene-type complexes undergo only one redox process, whereas iron, ruthenium and osmium polypyridine complexes undergo a metal-based oxidation process and at least three ligand-based reduction processes. 

In this chapter, we will review recent advances in the field of dendrimers as multielectron storage devices that are dendrimers containing multiple electroactive units. Because of the very extensive literature and space reasons, only few selected examples, divided according to the chemical nature of the units used to functionalize the dendritic structure, will be described. 

Because of their reversible electrochemical properties, ferrocene [biscyclopentadie-nyl-iron(II), FeCp2 and cobaltocenium [biscyclopentadienyl-cobalt(III), CoC p2 1 I are the most common electroactive units used to functionalize dendrimers. Both metallocene residues are stable, 18-electron systems, which differ on the charge of their most accessible oxidation states zero for ferrocene and + 1 for cobaltocenium. Ferrocene undergoes electrochemically reversible one-electron oxidation to the positively charged ferrocenium form, whereas cobaltocenium exhibits electrochemically reversible one-electron reduction to produce the neutral cobaltocene. Both electrochemical processes take place at accessible potentials in ferrocene- and cobaltocenium-containing compounds. 

As far as electron transfer properties directly involving dendrimers are concerned, it can be generally considered that these reactions may be observed whenever the macromolecular structure contains one or more units featuring redox levels at accessible potentials. The first dendrimers prepared were purely organic macromolecules, with no unit appropriate for electron transfer reactions. Later, however, the introduction of metal and organometallic complexes in the dendritic structure opened new possibilities to the chemistry of dendrimers. Indeed, the incorporated metal units exhibit important properties such as absorption and emission of visible light (relevant for the construction of antenna systems see Volume V, Part 1, Chapter 7) and redox levels at accessible potential, which are necessary for electron transfer reactions. Successively, purely organic electroactive units have also been used to functionalize the dendrimers. 

This section presents a literature survey on dendrimers containing at least one electroactive unit. The arrangement is on the basis of the electroactive unit incorporated in the dendrimer structure. In a few cases, different electroactive units are present in the same dendrimer, so that the classification is somewhat arbitrary. 

Asymmetric functionalization was achieved in the two dendrimers 60 and 61, which contain two branches terminated with electron-donor TTF units and one branch terminated with electron-acceptor anthraquinone (AQ) units [131]. Cyclic voltammetric investigation in acetonitrile showed that each of these electroactive units are noninteracting, giving rise to two waves on oxidation (due to TTF) and to two waves on reduction (due to AQ). The number of electrons exchanged in each of... 

The naphthalene diimide electroactive unit was used to functionalize poly(amidoamine) dendrimers up to the sixth generation [134]. Compound 66 illustrates the first-generation dendrimer, containing six peripheral diimide units in the sixth generation dendrimer the peripheral units number 192. Cyclic voltamme-... 

Dendrimer 67 contains two naphthalene diimide electroactive units in the core [137]. Electrochemical investigation in DMF showed the presence of two reversible bielectronic reduction processes at the potential typically featured by other naphthalene diimide-containing compounds. Thus, the two units behave independently and can be readily accessed from the electrode surface. The lack of encapsulating... 

This section describes cases where the dendrimer itself does not contain any electroactive unit, but is instead used as a medium to host species involved in electron transfer reactions. 

Supramolecules containing metal-polypyridine units, especially the Ru(dpp)-based dendrimers, could be used as electron reservoirs or components of molecular-electronic devices. Supramolecules in which an electroactive M(N,N) group is attached to a receptor capable of molecular recognition (crown ethers, calixarenes, cryptands etc.) can work as electrochemical sensors. Electrochemical recognition of cations as well as anions has been reported [33-35, 257, 263]. 

In a few cases, interaction between equivalent electroactive ferrocene units has been observed. One example is the silicon-based dendrimer 6, containing 16 ferrocene units, which upon oxidation in CH2CI2 shows two well-separated reversible... 

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