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.

侧向延伸的泽特烯（zethrenes）可被视作带有四条锯齿形边缘的Z型纳米石墨烯，但其合成极具挑战性。本研究中，我们成功合成了迄今为止尺寸最大的泽特烯分子——结晶态的超八泽特烯（superoctazethrene, SOZ）衍生物SOZ-Cl。尽管理论计算表明母体SOZ拥有极高的双自由基特征（y₀=81.0%），但SOZ-Cl却表现出良好的稳定性（室温环境下半衰期t₁/₂=64 h），且可通过硅胶柱层析进行纯化。该化合物展现出较小的电化学能隙（E_g=1.01 eV）以及开壳层单线态双自由基类化合物的特征。与此前报道的八泽特烯（octazethrene）衍生物OZ-TIPS相比，SOZ-Cl拥有更强的双自由基特征（y₀=76.3% 对比43.4%），且单线态-三线态能隙更小（ΔE_S-T=-2.30 kcal/mol 对比-3.87 kcal/mol）。计算结果同时证实了SOZ具有全局芳香性，而尺寸更小的八泽特烯则仅表现出局部芳香性。