Stoehr, Jacob F. | Search NASA

DOE OSTI2023

Significant charge transport effects due to subtle molecular changes in nitroxide radical single crystals

Radical-containing materials have received renewed interest due to their charge-conducting properties. However, most work focuses on macromolecules, while less attention has been paid to radical-containing small molecules. Here, we quantify the electrical conductivity in two organic radical single crystals and demonstrate that a subtle change in atomic connectivity drastically alters the macroscopic electronic properties of the materials. One radical crystal has an electrical conductivity of 3S m –1 , which is the among the highest values for nonconjugated radical conductors over a 1 μm scale. However, the other radical crystal has a 1,000-fold lower conductivity, despite their extremely similar molecular structures. The temperature-dependent conductivity follows the variable range hopping mechanism for both radical crystals, and the difference in effective charge mobility is the reason for the conductivity difference. These results present a clear picture of the design rules and charge transport mechanism for radical-based small-molecule materials.

36 MATERIALS SCIENCE↗

DOE OSTI2022

Charge transport and antiferromagnetic ordering in nitroxide radical crystals

Nonconjugated radical polymers and small molecules are employed as functional materials in organic electronic devices. Furthermore, the unpaired electrons on these materials have permanent magnetic moments, making these materials promising candidates for organic magnets. Through molecular design, strong antiferromagnetic and ferromagnetic ordering have been achieved in conjugated materials. However, the magnetic properties of nonconjugated radical polymers have only shown weak magnetic interactions among the open-shell sites due to the large mean separation between radicals in typical materials. Here, we have designed, synthesized, and crystalized two open-shell molecules that used molecular engineering to control the assembly of the open-shell sites into a strong antiferromagnetically ordered network. The strong antiferromagnetic interaction is evidenced by a high paramagnetic-to-antiferromagnetic transition temperature of ~40 K. This high transition temperature was a result of a high spin exchange coupling constant J of about –20 cm –1 , which was suggested by both experimental and computed coupling parameters given by the energy difference between high-spin and low-spin broken-symmetry structures. In addition, a single-crystal electrical conductivity of ~10 –3 S m –1 was achieved, which indicated the potential of this material in electronic applications. As a result, this work provides an insight into a design strategy for radical-based electronic and magnetic materials through proper molecular structure modifications.

36 MATERIALS SCIENCE↗