Dendrimer redox potentials

As larger and larger dendrimer units are placed around the core, they influence the environment around the core and thus its chemical (and redox) potential. This section gives several examples (and two counterexamples) of this behavior. Furthermore, a general model to explain how environment influences redox potential is provided. 

Nierengarten et al. expanded upon their Ceo-based dendrimers to make a series of copper bis-phenanthroline core dendrimers of the form Core3-Rpt3-Periph4.80 In this series, the redox potential of the core did not vary notably with generation, but the kinetics slowed down as the generation increased. The CV wave of the second- and third-generation dendrimers could not be observed. 

The redox potentials of Co(II) bound tyrosine residues in dendrimers of the form Core5-Rpt2-Periph3 were assessed using CV.82 The value of Eyl for the Co(II) to Co... 

I) reduction initially decreased in magnitude from —780 to —580 mV in going from n = 0 to n = 2 (versus Ag wire quasi-reference, 100 mM Bu4NC104 in DMF (dimethyl formamide), GCE). It then was reported to increase in magnitude to —830 mV for n — 3 and —880 mV for n — A. However, from the narrative, it was unclear why the authors were able to measure a thermodynamic redox potential as they commented on the broadness of their CV data. At least for the first few points, the trend in the redox potential follows the established paradigm. As the charge upon reduction decreases, the relatively less polar environment of the dendrimer compared to the solvent destabilizes the reduced form less than the oxidized form. 

Liwpomcharoenvong and Luck showed perhaps the most dramatic redox potential shift in a series of Mo2 tetrabenzoate cluster core dendrimers of the form... 

In a second example, Astruc et al. showed that the redox potential of FeCp(rjb-arene) core dendrimers (molecule 2 is the largest example studied) was similar to smaller molecules.86 The degree of hyperbranching is minimal in this molecule. Moreover, the authors also pointed out that the redox orbital is quite well buried at... 

Chow et al. prepared a set of dendrimers of the form Core12-Rpt7-Periph10, M = Ru and observed a similar trend in redox potential changes to that reported by Newkome et al. above.106 They also graphed peak separation between the anodic... 

These examples illustrate a broad range of interesting behaviors for surface-confined, redox-active dendrimers. However, let us return to our original questions How does the generation of a dendrimer influence the rate and redox potential for heterogeneous electron transfers Now we can ask this question with regard to surface-confined dendrimers. 

In solution, it was solvent, not dendrimer environment, that dominated the environment and thus the redox potential. The same conclusions were made when we studied similar films of iron-sulfur cluster dendrimer isomers of the form [Fe4S4(Lig)4]2 described above.187... 

Modification of the electrochemical properties of a redox centre surrounded by dendritic fragments [93] can lead to two different dendritic effects. The first one is manifested in a shift of the redox potentials, the extent and direction depending upon the dendritic architecture and the solvent. Such behaviour was observed in dendritic iron-porphyrins [94]. The second effect is apparent in a delay of redox transfer kinetics and is characterised by a stepwise increase in the distance between the peaks in a cyclovoltammogram with increasing dendrimer generation number. 

Diederich et al.11921 reported the divergent synthesis of dendrimers possessing porphyrin cores with the aim of modeling redox potentials of electroactive chromophores via environmental polarity modification. The dendrimers thus can be considered as electron-transfer protein mimics for such proteins as cytochrome c oxidation potentials for cytochrome c in aqueous solution are known to be 300-400 mV more positive than those reported for similarly ligated heme mimics lacking hydrophobic peptide encapsulation J193a ... 

These amidometallocenes do not significantly recognize halides, however, the redox potential shifts being very small [372]. Chloride and bromide were specifically recognized by 24-cationic dendrimers bearing iron sandwich units at the termini of the branches [376, 377]. The sandwich units was [FeCp ( / -A-alkylaniline)]+. 

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