Charge transport and antiferromagnetic ordering in nitroxide radical crystals

Abstract

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. Therefore, this work provides an insight into a design strategy for radical-based electronic and magnetic materials through proper molecular structure modifications.