Structure-Property Relationships for Alternatively Stiff and Flexible Redox-Active Core Dendrimers of the Type, [Fe4S4(S-Dend)4]2-

Abstract

The purpose of this work has been to establish structure-property relationships in novel redox-active core dendrimers. Alternatively stiff and flexible series of redox-active, iron-sulfur core dendrimers of the general structure (nBu4N)2[Fe4S4(S-Dend)4] (Dend = dendrons of generations 1 through 4) were studied. Molecular dynamics simulations were performed on dendrimer models to produce detailed pictures of three-dimensional structure. These simulations along with NMR experiments (Pulsed Field-Gradient Spin-Echo and Inversion Recovery) indicated that the flexible dendrimers are much more compact than the rigid dendrimers. In addition, the simulations indicated an offset and mobile iron-sulfur core. In contrast, the rigid dendrimers were open with a more central and relatively immobile iron-sulfur core. Heterogeneous electron transfer rate constants measured using cyclic voltammetry and Osteryoung square wave voltammetry, indicated that the rigid dendrimers were more effective at attenuating the rate of electron transfer than were the flexible dendrimers of comparable molecular weight. These key structural differences for alternatively stiff and flexible dendrimers turned out to play a critical role in rationalizing their electron transport properties. That is, the offset positioning of the core in the flexible dendrimers permits facile electron transfer to/from a poised platinum electrode compared to the rigid dendrimers, where the core is centrally positioned. While the dendrimers containing rigid ligands had better encapsulated redox cores for a given molecular weight, these molecules had higher electron transfer rates for a given molecular radius. Moreover, for the rigid dendrimer series, the attenuation of electron transfer was modest as the molecular size increased, indicative of a highly 'conductive' medium. This behavior was not observed in the flexible series. Here, a steeper attenuation of the electron transfer rate constant was observed as molecular size increased, indicative of a comparably more insulating electron transfer medium.