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 (t1/2 = 64 h under ambient conditions) and can be purified
by silica gel column chromatography. It exhibits a small electrochemical
energy gap (Eg = 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 (ΔES‑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.