Verdazyl–Nitroxide Diradical with S = 1 Ground State: Observation of Long-Range Ordering and Haldane Gap in a Highly Isotropic S = 1 Antiferromagnetic Heisenberg Chain

We herein report the first observation of long-range ordering and excitation gaps in an organic S = 1 antiferromagnetic Heisenberg spin chain. Namely, the synthesis and characterization of a p-phenylene-linked verdazyl–nitroxide diradical are described. The diradical possesses a high-spin (S = 1) ground state and forms a 1D antiferromagnetic chain in the solid state with an intramolecular exchange interaction of JF/kB = +190 K and intermolecular exchange interaction of JAF/kB = −2.7 K determined by SQUID magnetometry and single crystal XRD studies. The 1D chain was highly isotropic, as evidenced by a local anisotropy |D/2JAF| of 1.9 × 10–3. The system exhibits canted Néel order below Tc = 3.4 K with a canting angle as small as 0.2°, indicating the formation of long-range ordering. Moreover, the magnetization of the diradical exhibited nonlinear field dependence with a bending point of 4000 Oe at 1.8 K, clearly demonstrating the existence of an energy gap to the excited state. Thermal robustness of the diradical up to 158 °C was revealed by the TG-DTA study, which found thermal decomposition upon melting at the temperature.