Dendrimers redox systems

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. ... 

Casado C, Gonzalez B, Cuadrado I, Alonso B, Moran M, Losada J (2000) Mixed ferrocene-cobaltocenium dendrimers the most stable organometallic redox systems combined in a dendritic molecule. Angew Chem Int Ed 39 2135-2138... 

Electron-hopping is the main charge-transport mechanism in ECHB materials. There is precedence in the photoconductivity Held for improved charge transport by incorporating a number of redox sites into the same molecule. A number of attempts to adapt this approach for ECHB materials have been documented. Many use the oxadiazole core as the electron-transport moiety and examples include radialene 40 and dendrimer 41. However, these newer systems do not offer significant improvements in electron injection over the parent PBD. 

Ferrocenyl-based polymers are established as useful materials for the modification of electrodes, as electrochemical biosensors, and as nonlinear optical systems. The redox behavior of ferrocene can be tuned by substituent effects and novel properties can result for example, permethylation of the cyclopentadienyl rings lowers the oxidation potential, and the chaige transfer salt of decamethylfer-rocene with tetracyanocthylene, [FeCpJ]" (TCNE], is a ferromagnet below = 4.8 K, and electrode surfaces modified with a pentamethylferrocene derivative have been used as sensors for cytochrome c These diverse properties have provided an added impetus to studies on ferrocene dendrimers. 

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. 

Yamamoto also explored triphenylamine core dendrimers of the form Corei7-Rpti2-Periph15.125 In this system, the redox process studied was the oxidation of this core moiety. They showed that as the generation of the dendrimer increased from 1 to 4, the shape of the CV broadened, indicating slowing electron transfer kinetics. 

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. 

---