(404c) The Modular Design of Charge Transport in Radical Polymers

Radical polymers (i.e., macromolecules bearing open-shell radical moieties at pendant sites along nonconjugated backbones) exhibit unique redox and optoelectronic properties that are promising for many organic electronic applications. Nevertheless, understanding gaps remain between the fundamental molecular design and charge transport behaviors of these materials, which has slowed progress in this nascent field. To address this opportunity, we have performed the first quantum chemical study on radical polymers that is focused on establishing the correlations and complementary influence of several designable components, including backbone units, open-shell chemistries, and spacing units between the backbone and the radical groups. For each radical polymer, with hundreds of atoms and large orientational variability, comprehensive conformer sampling was implemented to characterize expected properties and their distributions under amorphous conditions. Charge transport analysis was performed on conformational ensembles of low-energy structures, in which multisite radical interactions were modeled based on pairwise radical charge transfer. For a given radical pair, we have determined a linear relationship between the log-log transformation of overlap integrals (S_ab, xTB level) and transfer integrals (H_ab, DFT level), which enabled an ultrafast and accurate estimation of electronic coupling values for large-scale conformationally sampled structures. Although rigid polymer backbones are not poor in terms of their average electronic couplings, the local traps they may induce are detrimental to the final rate of charge transport. For different open-shell chemistries, the delocalized feature-induced aggregation favors intramolecular charge transport. However, such aggregation may be attenuated when the backbone exhibits conformational freedom. Furthermore, the addition of spacer units linking the backbones to the radical groups has no consistent advantage to facilitate charge transport, as it does not significantly promote radical alignment, and generally results in a wider distribution of electronic couplings. This rationale of directing radical interactions through modular design creates enormous opportunities for the development of promising open-shell macromolecules.