The hyperbranching in dendrimers suggests several purposes for this architecture in rational, molecular design schemes. To this end, an increasingly wide variety of dendrimers have been synthesized and studied. One role for dendrimers is that of encapsulation. As discussed in several previous reviews, [1±4] encapsulation in dendrimers can be studied in several contexts. These include small molecule guest encapsulation and covalent encapsulation of a core moiety. [5] In this latter class, dendrimers are arguably ideal as encapsulating agents for two reasons. First, they can be synthesized to give monodisperse macromolecules with well-defined primary structures. Second, the interior and exterior of the dendrimer may be tailored to control both the interior nano-environment and the external functionality presented by the molecule. This feature has the potential to lead to new, possibly unique physical properties in coreencapsulated dendrimers.Early evidence that dendrimers can influence physical properties through encapsulation came from observation of changes in the photophysical properties of dendrimers. FrØchet's group observed shifts in UV-vis absorbance maximum of p-dialkyl amino-nitrobenzene core dendrimers as the number of hyperbranches in the dendrimer increased. [6] Moore's group observed shifts in the fluorescence spectra of p-dimethoxybenzene core dendrimers with increasing generation. [7] These results led to the conclusion that dendrimers around approximately generation three or four (where generation refers to the number of hyperbranches in the molecule) adopted a sterically enforced globular shape that encapsulated the cores.To probe further the relationship between dendrimer generation, structure, and core encapsulation, several groups have studied redox-active core dendrimers. By examining the thermodynamic redox potential and/or the reversibility of the electrochemical charge transfer of these dendrimers, additional insight into structure±property relationships for encapsulation has been obtained. Studies of variations in the thermodynamic redox potential of an encapsulated core have been discussed previously [5,8] and are not further highlighted here. A decrease in the reversibility of the electrochemical charge transfer (or, more quantitatively, the heterogeneous electron transfer rate) has often been correlated with increasing dendrimer generation. Presumably this decrease arises due to steric shielding of the electroactive moiety from the electrode. This relationship between dendritic encapsulation and the attenuation of electron transfer rates can be illustrated by the following examples of electroactive core dendrimers.Diederich and co-workers observed the first example of electron transfer rate attenuation with increasing dendrimer size. [9] First, second, and third generation zinc porphyrin core dendrimers with four arms (Fig. 1A) were synthesized and their cyclic voltammograms measured. As the generation of the dendrimer increased, the voltammetric waves shifted and broadened, which indi...

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