Dendrimer redox-active units

An important field where the cyclopropanation reaction finds growing application is the construction of dendrimers possessing fullerenes either as functional cores [26-33] or branches [34-38]. Dendrimers can serve as building blocks for the construction of organized materials with nanosize precision due to the well-defined three-dimensional structure they possess. An issue of great importance is to incorporate photoactive and/or redox-active units at the center of the dendrimer in order to establish these types of materials as molecular devices. An example of an organofullerene material that has the potential to serve as a core building block for the construction of dendrimeric compounds... 

A dendrimer consisting of multiple identical and non-interacting redox units, able to reversibly exchange electrons with another molecular substrate or an electrode, can perform as a molecular battery [64, 65]. The redox-active units should exhibit chemically reversible and fast electron transfer processes at easily accessible potential difference and chemical robustness under the working conditions. 

PBE dendrons bearing a focal bipyridine moiety have been demonstrated to coordinate to Ru + cations, exhibiting luminescence from the metal cation core by the excitation of the dendron subunits [28-30]. The terminal peripheral unit was examined (e.g., phenyl, naphthyl, 4-f-butylphenyl) to control the luminescence. The Ru +-cored dendrimer complexes are thought to be photo/redox-active, and photophysical properties, electrochemical behavior, and excited-state electron-transfer reactions are reported. 

Our aim has been the construction of dendrimers that incorporate in their building blocks specific pieces of information such as the capability to absorb and emit visible light and to reversibly exchange electrons.To pursue this aim, we have designed a synthetic strategy to build up dendrimers based on luminescent and redox-active transition metal complexes. Species containing 4, 6, iP 10,2W7 28 gjjj 222930 metal-based units have already been obtained. We will see... 

Dendrimers are ideal scaffolds to construct these devices since (i) a large number of proper redox units can be placed in the periphery and/or branching points of their structure with the possibility of tuning their distance, (ii) the dendrimer skeleton can be designed to minimize electronic interaction between the redox centers, and (iii) the dendrimer periphery can be optimized to get the desired solubility properties and processability to eventually deposit the dendrimers on a surface. Other possible scaffolds are constituted by polymers11 and nanoparticles,12 which are easier to synthesize, but they do not enable control and tuning of the number, position, and distance of the active units. 

Poly(propylene amine) dendrimers containing 4, 8, and 64 amidoferrocene peripheral units have also been incorporated in the highly ordered channels of mesoporous silica obtaining a novel type of redox-active materials. One significant feature of these new composite materials is that the ferrocene units of the guest dendrimers are easily accessible to electrochemical oxidation, as revealed by studies carried out in MeCN solutions by using Pt electrodes derivatized with films of such dendrimer-matrix complexes.33... 

A wide range of dendrimers with functional core is described in the literature. Thus chromophores [7], electrochemically active, redox active [8], and catalyti-cally active [9] or also self-associating and chiral units as well as polymerisable monomers and polymers have been successfully introduced into the centre of dendrimers. However, the core unit not only has a determining effect on the function, but also has a decisive influence on the multiplicity, size, and shape of the dendrimer. 

When the electroactive units are used for peripheral functionalization (Figure 2b) and/or are located along to the branches (Figure 2c) of a dendrimer, a number of equivalent redox-active centers are present, since dendrimers are usually highly symmetric species by their own nature. The active centers may or may not interact, depending on distance and nature of the connector units. Multielectron redox processes can therefore be observed, the specific patterns of which are related to the degree of interaction among the various units. 

Fullerene has also been incorporated in redox-active dendrimers. In 62 the Ceo unit has been modified by connecting a single polyaryl-ether dendritic branch [132]. Electrochemical reduction in CH2CI2 of 62 features three reversible waves, which occur at a potential more negative than that of the free Cso molecule. This shift was ascribed to an insulating effect on the connected dendritic structure. 

Dendrimers with up to 96 redox-active TTF moieties on the periphery, which allow the generation of polycationic species bearing up to 192 positive charges on the surface, were incorporated into electrodes. These dendrimer-modified electrodes find application in electrochemical sensing of metal cations (i.e., Ba ), thanks to the grafting of crown ether/TTF units on the periphery of dendrimers < 2001AGE224>. 

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],... 

If the hexafunctionalization of hexamethylbenzene leads to stars, the octafunctionalization of durene leads to dendritic cores. The first of these octaalkylation reactions was reported as early as 1982, and led to a primitive dendritic core containing a metal-sandwich unit. Thus, as the hexafunctionalization, this reaction is very specific. Two hydrogen atoms in each methyl group are now replaced by two methyl, allyl, or benzyl groups (Scheme 11.4).19 Applications to the synthesis of dendrimers containing 8 or 24 redox-active groups have recently been reported.20... 

For the formation of metallodendrimers of precise nature, a second favorable position in the overall structure for complexation can obviously be at the periphery. Excellent examples of such systems have been reported that include a silicon dendrimer decorated with 243 ferrocenyl units at the periphery with stable redox activity [58]. Catalytic activity of dendrimers with metals located at the periphery has also proven to be of great interest as it has been recently reviewed by several authors [59,60]. Placing photoactive centers at this specific location can nonetheless be more intricate in this case, as demonstrated by the limited number of reported examples. 

Ferrocene dendrimers are also of interest for reasons other than their redox activity. For example, metalloden-drimer 242 possesses planar chiral ferrocene units that make the bidentate phosphine ligation sites of potential interest for applications in asymmetric catalysis. Indeed, asymmetric hydrogenations of dimethyl itaconate catalyzed by Rh complexes of 242 showed impressive ee values of 98%. " ... 

Serroni S, Juris A, Venturi M, Campagna S, Resino IR, Denti G, Credi A, Balzani V (1997) Polynuclear metal complexes of nanometre size. A versatile synthetic strategy leading to luminescent and redox-active dendrimers made of an osmium(II)-based core and ruthenium (Il)-based units in the branches. J Mater Chem 7 1227-1236... 

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