TEMPO-Labeled Viologen Dendrimers: Synthesis, Characterization, and Preliminary Distance Measurements

Abstract

Viologen dendrimers (G(0)* to G(2)*) bearing (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) spin labels are successfully synthesized via a divergent approach. 4-amino TEMPO is successfully grafted onto the periphery of the dendrimer moieties using Zincke reaction under dark conditions. The final products are characterized by cyclic voltammetry and the spin-label efficiencies are characterized by electron paramagnetic resonance (EPR) measurements. Cyclic voltammetric studies show the coexistence of both viologen as well as TEMPO electroactive subunits, and evidence for the intramolecular electrocatalytic reduction of TEMPO by viologen subunit is presented. EPR measurements show moderate to good spin-label efficiencies (72-85%), but the spin-labeled samples are not stable in the presence of air and light. The measured spin-label efficiency decreases with increasing dendrimer generation. Double electron resonance measurements provide interspin distances for zeroth generation G(0)* dendrimers. MM+ calculations perform using Hyperchem show an averaged single spin distance (2.34 nm) for G(0)* dendrimers and a more complex scenario of several possible interspin distances in the case of G(1)* and G(2)* dendrimers.