Electrochemical and Guest Binding Properties of Fréchet- and Newkome-Type Dendrimers with a Single Viologen Unit Located at Their Apical Positions

Several new series of dendrimers containing a single redox-active 4,4'-bipyridinium (viologen) residue were synthesized and characterized. In these dendrimers, the viologen group is covalently attached to the apical position of a Newkome- or Frechet-type dendron, ranging in size from first to third generation of growth. The half-wave potentials corresponding to the two consecutive one-electron reductions of the viologen residue are affected by the size of the dendritic component. The size effects are more pronounced in the Newkome-type dendrimers and seem to result from the polarity contrast between the microenvironments provided by the solution and the internal phase of the dendrimer. Unlike in many other dendrimers having a redox-active core, the voltammetric behavior remains fast (reversible) even in third generation dendrimers. Pulse gradient stimulated echo NMR diffusion coefficient measurements on the Newkome-type dendrimers reveal that their hydrodynamic radii are relatively invariant in solvents of widely different polarities (dichloromethane to dimethyl sulfoxide). The host-guest binding interactions between the viologen residue in these dendrimers and the crown ether host bis-p-phenylene-34-crown-8 were also investigated. While in Newkome-type dendrimers the growth of the dendron caused a substantial attenuation of the binding constant values, this size effect was not observed in the Frechet-type dendrimers. These electrochemical and binding measurements underscore some of the structural differences between these two common types of dendritic architectures.