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 (T1) 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.