Electroactive dendrimer

V vs. SCE in CH2CI2/TBAH). The electroactive dendrimer film behaved almost ideally with rapid chaise transfer kinetics. These voltammetric features unequivocally indicate the surface-confined nature of the electroactive ferrocenyl moieties in the dendrimer. Surface coverage of electroactive ferrocenyl sites in the film r(mol cm ) was determined from the integrated charge of the cyclic voltanunetric wave. The obtained results correspond to about a close-packed monolayer of... 

Since these organometallic dendrimers have non-interacting ferrocenyl redox centers, Moran and coworkers1401 successfully modified electrode surfaces by electrodeposition of these dendritic materials in their oxidized forms. Electrodeposition was accomplished by controlled potential electrolysis or by repetitive anodic and cathodic cycling the amount of electroactive dendrimer deposited could thus be regulated. 

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 application of dendrimers in electrochemical biosensors is an emerging area of research. The structural homogeneity, biocompatibility, internal porosity, high surface area and ease of functionalization of dendrimers make them very desirable for biosensor applications. In the past ten years, there has been a steady and gradual development in the evaluation of the bionanocomposites of dendrimers for electrochemical sensors. The expanding interest in the development of novel electroactive dendrimers has enabled their viability for this application. 

A number of reviews have appeared of topical interest. These include photoisomerism of azo dyes, ° photofunctional polysilanes,photochromic pigments, rare earth complexes, pressure sensitive paints, electron-transfer processes,electroactive dendrimers, chiral polyisocyanates, photodefin-able benzocyclobutene, photoconductive polymers,excited states in conjugated polyenes, photosensitive materials and polyazomethanes. ... 

As mentioned above, the position of EU in a dendrimer plays a key role in defining their electrochemical behavior. As expected, the electrochemical behavior of met-alloporphyrins in the core is completely different from those at dendrimers surface. These two cases will be compared with other pertinent electroactive dendrimers in order to have a more comprehensive view of the factors involved in the electron transfer process. 

Ceroni P, Venturi M (2011) Photoactive and electroactive dendrimers future trends and applications. Aust J Chem 64(2) 131-146... 

Dendritic molecules with electroactive units at either the focal point or core have been reported [92, 97]. There are, however, only a few examples of such moieties specifically pinned within cascade infrastructure. Our recent efforts in this direction [104-106] involve the incorporation of chromophoric 1,4-di-aminoanthraquinone (35) within the cascade infrastructure. Dendrimers based on a four-directional pentaerythritol core were synthesized using the extended 1 — 3 building block 36. A high dilution technique was applied to synthesize 36... 

Towards understanding biological electron transfer processes many researchers have reported the synthesis of dendrimers with electroactive cores (e.g., porphyrin). Dendrimers with organic dendrons attached tetrahedrally around an inorganic, electroactive iron-sulfur core were reported by Gorman and coworkers [109]. These are the first examples of dendrimers with a hybrid... 

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. 

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. 

The first attempt to construct a dendrimer with an electroactive Ru-polypyridi-ne core was based on the reaction of Ru(bpy)2Cl2 with a branched polyether-substituted phenanthroline ligand (11) [27]. In the potential window +2/-2V, this compound shows a one-electron oxidation process and three distinct one-electron reduction processes that, by comparison with the behavior of the... 

Porphyrin complexes are particularly suitable cores to construct dendrimers and to investigate how the behavior of an electroactive species is modified when surrounded by dendritic branches. In particular, dendritic porphyrins can be regarded as models for electron-transfer proteins like cytochrome c [42, 43]. Electrochemical investigation on Zn-porphyrins bearing polyether-amide branches has shown that the first reduction and oxidation processes are affected by the electron-rich microenvironment created by the dendritic branches [42]. Furthermore, for the third generation compound all the observed processes become irreversible. 

This review of the electroactive metal-containing dendrimers, not exhaustive for space reasons, clearly indicates that several interesting dendrimers have been synthesized, but in most cases their electrochemical properties have not been investigated in detail. A few carefully performed studies have shown that electrochemistry is a powerful technique (i) to elucidate the structure (and purity) of the dendrimers, a task not at all easy in the case of highly charged compounds,... 

Without doubt, the most noteworthy aspect of the redox behavior of the synthesized organometallic dendritic macromolecules 1-6, having a predetermined number of noninteracting ferrocenyl redox centers, is their ability to modify electrode surfaces. In this way, for the first time, electrode surfaces have been successfully modified with films of dendrimers containing reversible four- and eight-electron redox systems, resulting in detectable electroactive materials persistently attached to the electrode surfaces. ... 

The ferrocenyl dendrimers were electrodeposited in their oxidized forms onto the electrode surfaces (platinum, glassy-caibon, and gold) either by controlled potential electrolysis or by repeated cycling between the appropriate anodic and cathodic potential limits therefore the amount of electroactive material electrode-posited can be controlled with the electrolysis time or the number of scans. The electrochemical behavior of films of the polyfeirocenyl dendrimers was studied by cyclic voltammetry in fresh CH2CI2 and CHjCN solutions containing only supporting electrolyte. 

Figure 6. Cyclic voltammograms of a platinum disk-electrode modified with a film of the octanuclear dendrimer 2, measured in 0.1 M Bu NPFj/CHjClj. The surface coverage of electroactive ferrocenyl sites in the film is determined to be T = 2.01 x 10" mol cm . inset, scan rate dependence of the anodic peak current.

Ferrocenyl dendrimers also afford electroactive films on indium tin oxide (ITO) electrodes in the same manner as described above. UV-visible spectroelectro-chemical measurements of this modified electrodes on oxidation show changes characteristic for the formation of fenocenium cations. Thus, Figure 8 shows the UV-visible absorption spectrum of a film of 2 electrodeposited on a transparent ITO electrode, which exhibits a strong band at 260 nm and a weak absorption band centered at 600 nm, which agree with those observed for the cationic dendrimer [2 KPF j ]g in solution described above. 

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. 

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