Predicting Molecular Design Features for Charge Transport in Radical Polymers

Conducting polymers based on open-shell radical moieties exhibit potentially advantageous processing, stability, and optical attributes compared with conventional doped conjugated polymers. However, reported radical conductors have been based almost exclusively on (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO), which raises fundamental questions regarding the ultimate limits of charge transport in these materials. Herein, we have performed a density functional theory (DFT) study of the charge transfer characteristics of a broad range of p-type, n-type, and ambipolar open-shell chemistries. We have determined that, far from being representative, TEMPO exhibits anomalously high reorganization energies, due to strong spin localization. This, in turn, limits charge transfer in TEMPO compared with more delocalized species. By comprehensively mapping the dependence of charge transfer on radical-radical orientation, we also have identified a large mismatch between the conformations that are favored by intermolecular interactions and the conformations that maximize charge transport. These results suggest that significant opportunities exist to exploit directing physical interactions at the molecular level such that charge transport is promoted in radical polymer conductors.