title: Electrochemical and guest binding properties of Frechet- and Newkome-type dendrimers with a single viologen unit located at their apical positions.
abstract: 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.
authors.affiliation: Center for Supramolecular Science and Department of Chemistry, University of Miami, Coral Gables, Florida 33124-0431, USA.
authors.forename: Winston
authors.initials: W
authors.lastname: Ong
authors.forename: Jodi
authors.initials: J
authors.lastname: Grindstaff
authors.forename: David
authors.initials: D
authors.lastname: Sobransingh
authors.forename: Rosa
authors.initials: R
authors.lastname: Toba
authors.forename: José María
authors.initials: JM
authors.lastname: Quintela
authors.forename: Carlos
authors.initials: C
authors.lastname: Peinador
authors.forename: Angel E
authors.initials: AE
authors.lastname: Kaifer