We report relatively persistent, open-shell thiophene-based double helices, radical cations 1^•+-TMS₁₂ and 2^•+-TMS₈. Closed-shell neutral double helices, 1-TMS₁₂ and 2-TMS₈, have nearly identical first oxidation potentials, E ^+/0 ≈ +1.33 V, corresponding to reversible oxidation to their radical cations. The radical cations are generated, using tungsten hexachloride in dichloromethane (DCM) as an oxidant, E ^+/0 ≈ +1.56 V. EPR spectra consist of a relatively sharp singlet peak with an unusually low g-value of 2.001–2.002, thus suggesting exclusive delocalization of spin density over π-conjugated system consisting of carbon atoms only. DFT computations confirm these findings, as only negligible fraction of spin density is found on sulfur and silicon atoms and the spin density is delocalized over a single tetrathiophene moiety. For radical cation, 1^•+-TMS₁₂, energy level of the singly occupied molecular orbital (SOMO) lies below the four highest occupied molecular orbitals (HOMOs), thus indicating the SOMO–HOMO inversion (SHI) and therefore, violating the Aufbau principle. 1^•+-TMS₁₂ has a half-life of the order of only 5 min at room temperature. EPR peak intensity of 2^•+-TMS₈, which does not show SHI, is practically unchanged over at least 2 h.

中文翻译：

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基于噻吩的双螺旋：具有 SOMO–HOMO 能级反转的自由基阳离子†

我们报告了相对持久的、基于噻吩的开壳双螺旋、自由基阳离子 1 ^•+ -TMS ₁₂和 2 ^•+ -TMS ₈。闭壳中性双螺旋 1-TMS ₁₂和 2-TMS ₈具有几乎相同的第一氧化电位E ^+/0  ≈ +1.33 V，对应于其自由基阳离子的可逆氧化。使用二氯甲烷 (DCM) 中的六氯化钨作为氧化剂，生成自由基阳离子，E ^{+/0 ≈ +1.56 V。EPR 光谱由一个相对尖锐的单峰组成，} g 值异常低- 2.001–2.002 的值，因此表明自旋密度在仅由碳原子组成的 π 共轭系统上的排他性离域。DFT 计算证实了这些发现，因为在硫和硅原子上仅发现了可忽略不计的自旋密度部分，并且自旋密度在单个四噻吩部分上离域。对于自由基阳离子，1 ^•+ -TMS ₁₂，单占据分子轨道（SOMO）的能级低于四个最高占据分子轨道（HOMO），因此表明SOMO-HOMO反转（SHI），因此违反了Aufbau原则。1 ^•+ -TMS ₁₂在室温下的半衰期仅为 5 分钟。EPR 峰值强度为 2 ^•+ -TMS ₈，不显示 SHI，至​​少在 2 小时内几乎没有变化。