Molecular Design Features for Charge Transport in Nonconjugated Radical Polymers

Conducting polymers based on open-shell radical moieties exhibit potentially advantageous processing, stability, and optical attributes compared with conventional doped conjugated polymers. Despite their ascendance, 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 and whether some of the deficiencies exhibited by contemporary materials are due to the choice of radical chemistry. To address these questions, we have performed a density functional theory (DFT) study of the charge transfer characteristics of a broad range of open-shell chemistries relevant to radical conductors, including p-type, n-type, and ambipolar open-shell chemistries. We observe that far from being representative, TEMPO exhibits anomalously high reorganization energies due to strong charge localization. This, in turn, limits charge transfer in TEMPO compared with more delocalized open-shell species. By comprehensively mapping the dependence of charge transfer on radical-radical orientation, we have also identified a large mismatch between the conformations that are favored by intermolecular interactions and the conformations that maximize charge transfer in all of the open-shell chemistries investigated. These results suggest that significant opportunities exist to exploit directing interactions to promote charge transport in radical polymers.