Structural effects on encapsulation as probed in redox-active core dendrimer isomers

Three pairs of isomeric, iron-sulfur core dendrimers were prepared. Each isomer pair was distinguished by a 3,5-aromatic substitution pattern (extended) versus 2,6-aromatic substitution pattern (backfolded). Several observations were made that supported the hypothesis that the iron-sulfur cluster cores were encapsulated more effectively in the backfolded isomers as compared to their extended isomeric counterparts. The backfolded isomers were more difficult to reduce electrochemically, consistent with encapsulation in a more hydrophobic microenvironment. Furthermore, heterogeneous electron-transfer rates for the backfolded molecules were attenuated compared to the extended molecules. From diffusion measurements obtained by pulsed field gradient spin-echo NMR and chronoamperometry, the backfolded dendrimers were found to be smaller than the extended dendrimers. Comparison of longitudinal proton relaxation (T-1) values also indicated a smaller, more compact dendrimer conformation for the backfolded architectures. These findings indicated that the dendrimer size was not the major factor in determining electron-transfer rate attenuation. Instead, the effective electron-transfer distance, as determined by the relative core position and mobility in a dendrimer, is most relevant for encapsulation.