Structural Effects on Encapsulation as Probed in Solution - Based and Surface - Confined Redox - Active Core Dendrimers

Abstract

The purpose of this research was to study structure — property relationships of iron sulfur core [Fe4S4(S-Dend)4]2- dendrimers. Previous studies have demonstrated that biasing dendrimer architecture increases the effective encapsulation of redox-active, paramagnetic, Fe4S4 clusters. To further examine structure-property relationships of iron-sulfur core dendrimers, studies were carried out to 1) probe the relationship between dendritic architecture and encapsulation via the study of solution-based and surface-confined constitutional isomers differing only in their benzyl substitution patterns, and 2) studying the effects of counterion concentration and permeability on the electronic properties of iron-sulfur core dendrimer thin films.

Three pairs of isomeric, iron-sulfur core dendrimers were synthesized. Each isomer pair was distinguished by a 3,5-aromatic substitution pattern (extended) versus 2,6-aromatic substitution pattern (backfolded). Several observations were made supporting the hypothesis that the iron-sulfur cluster cores were encapsulated more effectively in the backfolded isomers as compared to their extended counterparts. The backfolded isomers were more difficult to reduce electrochemically, consistent with encapsulation in a less polar 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 dendrimer size was not the major factor in determining electron-transfer rate. Instead, the effective electron-transfer distance, determined by the relative core position and mobility, is most relevant for encapsulation.

In addition to solution studies, the electrochemical behavior of thin films composed of redox-active, iron-sulfur core dendrimers were studied as a function of the type of counterion available during reduction and re-oxidation. The rate of permeation/migration of counterions into the film appeared to be the bottleneck to electron transfer through the film. As the dendrimer is essentially non-polar, decreasing the relative polarity of the counterion increased the rate and extent of electron hopping within the films.