Probing diffusional transport in redox-active dendrimers

The diffusion coefficients of polyamidoamine (PAMAM) dendrimers (generations 0-4) surface-modified with terpyridyl and bipyridyl ruthenium complexes were determined using cyclic voltammetry (CV), voltammetry at a rotating disk electrode (RDE), and pulsed field gradient spin-echo nuclear magnetic resonance (PFGSE-NMR). In all experiments, the magnitude of the diffusion coefficient decreased with increasing dendrimer size, but each experimental technique led to dramatically different experimental results in terms of the value for a particular molecule. The diffusion, coefficients determined by PFGSE-NMR were larger than those determined by RDE, and the diffusion coefficients determined by CV were smaller still. The Stokes-Einstein equation was used to estimate the size of the dendrimers; from the measured diffusion coefficients and the results from CV and RDE experiments were found to predict unphysically large diameters for the dendrimer molecules. On the other hand, the diffusion. coefficients determined by PFGSE-NMR led to physically reasonable estimations of the dendrimers' sizes. It is proposed that, for macromolecules containing multiple redox centers, electrochemical methods may not provide an accurate measure of diffusion coefficients due to incomplete sampling of all redox sites. Unlike the electrochemical methods, PFGSE-NMR does not rely on electron-transfer occurring, and it appears to be superior to those electrochemical methods for measuring the diffusion coefficients, of redox-active macromolecules.