Nonconjugated radical-based materials are an emerging class of electrical conductors. However, most ongoing work is focused on developing amorphous radical polymers. Thus, the impact of molecular packing at the microscopic scale on the charge transport properties has received little attention. Herein, we design, synthesize, and crystallize two nitroxide radical-based small molecules, 4’-cyano-[1,1’-biphenyl]-4-yl 1-hydroxyl-2,2,6,6-tetramethylpiperidine-4-carboxylate (TEMPO-BPCN-1) and 1-hydroxyl-2,2,6,6-tetramethylpiperidin-4-yl 4'-cyano-[1,1'-biphenyl]-4-carboxylate (TEMPO-BPCN-2). These two molecules are structurally similar and only differentiated by the connectivity of the ester bond that between the biphenyl and TEMPO moiety. However, the spatial assemble of the radical moieties in their crystal structures shows a large difference. That is, the molecular packing of TEMPO-BPCN-1 results in much smaller mean separation distance between the radical groups compared with that in TEMPO-BPCN-2. Therefore, the room-temperature electrical conductivity in the single crystal of TEMPO-BPCN-1 (i.e., ~3 S m-1) is a 1,000-fold higher than that of TEMPO-BPCN-2 (i.e., ~10-3 S m-1). Kinetic Monte Carlo computations reveal that molecular packing in the crystal structure of TEMPO-BPCN-1 facilitates charge transport, while the molecular packing in TEMPO-BPCN-2 results in many charge traps that restrict the charge mobility. Furthermore, the electrical conductivity in the single crystals of both molecules is quantified across a temperature range of 10 K to 350 K, and the temperature dependence is fit to a variable range hopping model. In this way, the charge transport mechanism in these crystals is elucidated. In summary, open-shell materials are developed with superior charge transport capabilities, and a charge transport model is built for these materials. This effort provides clear insights into designing future nonconjugated radical electrical conductors.

Impact of molecular design on charge transport in nitroxide radical single crystals

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