Superoctazethrene: An Open-Shell Graphene-like Molecule Possessing Large Diradical Character but Still with Reasonable Stability

Laterally extended zethrenes can be regarded as Z-shaped nanographenes with four zigzag edges, but their synthesis is very challenging. Herein, we report the successful synthesis of by far the largest zethrene molecule, a superoctazethrene (SOZ) derivative SOZ-Cl, in crystalline form. Although the parent SOZ is calculated to have a very large diradical character (y₀ = 81.0%), SOZ-Cl shows reasonable stability (t_1/2 = 64 h under ambient conditions) and can be purified by silica gel column chromatography. It exhibits a small electrochemical energy gap (E_g = 1.01 eV) and characteristics for open-shell singlet diradicaloids. Compared with a previously reported octazethrene derivative (OZ-TIPS), SOZ-Cl shows much larger diradical character (y₀ = 76.3% vs 43.4%) and smaller singlet–triplet gap (ΔE_S‑T = −2.30 kcal/mol vs −3.87 kcal/mol). Calculations also demonstrate global aromatic character of SOZ, but the smaller size octazethrene shows local aromaticity.