Redox controlled interaction of biferrocenyl-terminated dendrimers with Beta-cyclodextrin molecular printboards

This paper describes the synthesis and electrochemistry of biferrocenyl-terminated dendrimers and their β-cyclodextrin (β-CD) inclusion complexes in aqueous solution and at surfaces. Three generations of poly(propylene imine) (PPI) dendrimers, decorated with 4, 8, and 16 biferrocenyl (BFc) units, respectively, were synthesized. A water-soluble BFc derivative forms stable inclusion complexes with β-CD. The intrinsic binding constant is Ki=2.5×104 M-1. The BFc dendrimers were solubilized in water by complexation of the end groups with β-CD, resulting in large water-soluble supramolecular assemblies. Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) showed that all the end groups are complexed to β-CD. Adsorption of the dendrimers at self-assembled monolayers (SAMs) of heptathioether-functionalized β-CD on gold ("molecular printboards") resulted in stable monolayers of the dendrimers due to the formation of multivalent host-guest interactions between the BFc end groups of the dendrimers and the immobilized β-CD molecules. The number of interacting end groups is 3, 4, and 4 for dendrimer generations 1, 2, and 3, respectively. The complexation of BFc to β-CD is sensitive to the oxidation state of the BFc unit. Oxidation of neutral BFc-Fe2(II,II) to the cationic, mixed-valence biferrocenium BFc-Fe2(II,III)+ resulted in dissociation of the host-guest complexes. Scan-rate-dependent CV and DPV analyses of the dendrimer-β-CD assemblies immobilized at the β-CD host surface and in solution revealed that the dendrimers are oxidized in three steps. First, the surface-β-CD-bound BFc moieties are oxidized to the mixed-valence state, Fe2(II,III)+, followed by the oxidation of the non-surface-interacting BFc groups to the Fe2(II,III)+ state. The third step involves the oxidation of all the BFc moieties to the Fe2(III,III)2+ state.