Diamagnetic Precursor to Biradical by Means of a Thermal Trigger: A Head-to-Tail (N–O)₂ Four-Membered Ring Formed in Naphthalene-1,8-diyl Bis(tert-butyl Nitroxide)

The title compound was synthesized, and ESR and magnetic analysis revealed diamagnetic properties below 400 K. The crystal structure analysis clarified that the molecule possesses an approximate C₂ symmetry with a head-to-tail (N–O)₂ four-membered ring. The N···O distances are 2.349(4) – 2.374(4) Å, belonging to the class found in diamagnetic phases of dimerized nitroxide materials. A zero-field splitting pattern appeared in the solid-state ESR around 440 K on heating. The density functional theory (DFT) calculations supported the formation of a biradical.