High spin (S = 1) organic diradicals may offer enhanced properties with respect to several emerging technologies, but typically exhibit low singlet triplet energy gaps and possess limited thermal stability. We report triplet ground state diradical 2 with a large singlet-triplet energy gap, ΔE ST ≥ 1.7 kcal mol −1 , leading to nearly exclusive population of triplet ground state at room temperature, and good thermal stability with onset of decomposition at ~160 °C under inert atmosphere. Magnetic properties of 2 and the previously prepared diradical 1 are characterized by SQUID magnetometry of polycrystalline powders, in polystyrene glass, and in other matrices. Polycrystalline diradical 2 forms a novel one-dimensional (1D) spin-1 (S = 1) chain of organic radicals with intrachain antiferromagnetic coupling of J′/k = −14 K, which is associated with the N•••N and N•••O intermolecular contacts. The intrachain antiferromagnetic coupling in 2 is by far strongest among all studied 1D S = 1 chains of organic radicals, which also makes 1D S = 1 chains *

We report a relatively persistent, open-shell aza-thia[7]­helicene with cross-conjugated electron-rich π-system. The singly occupied molecular orbital (SOMO) energy levels of both radical cation and neutral radical of the [7]­helicene are below the highest occupied molecular orbital (HOMO) energy levels, thereby violating the Aufbau principle. The aza-thia[7]­helicene is prepared from β-hexathiophene by a three-step one-pot reaction, in which the pyrrole ring is constructed by two consecutive C–N bond formations. Chemical oxidation converts the helicene to its radical cation, while in the presence of base (Cs2CO3), the oxidation gives neutral aminyl radical, likely via proton dissociation from the aminium radical cation with a low pK a. Reaction of the aza-thia[7]­helicene with NaH provides the corresponding anion, which shows characteristic cyclic voltammetry wave at anodic peak potential E p a ≈ +0.2 V. Chemical oxidation of the anion with ferrocenium hexafluorophosphate at room temperature gives persistent neutral aminyl radical. Structure of the aza-thia[7]­helicene is supported by NMR, IR, X-ray crystallography, and cyclic voltammetry. The radical cation and neutral radical are characterized by EPR and UV–vis-NIR spectroscopies. DFT computations of the radical cation and neutral radical predict the SOMO–HOMO energy level inversion, which is supported experimentally by electrochemical data for the radical cation.