Nakano et al. reported that the antiaromatic indenofluorene (IF) isomers are diradicaloid molecules having varying degrees of open-shell character, with indeno[1,2-b]fluorene displaying a weaker diradical character index (y₀ = 0.072). Unlike 6,12-trimethylsilylethynyl disubstituted [1,2-b]IF, the 6,12-aryl disubstituted [1,2-b]IF derivatives did not show any experimental evidence of diradical properties. This raised the question of whether an [1,2-b]IF dimer would prefer a closed-shell or an open-shell ground state. To address it, herein we report the synthesis of a 6,6'-biindeno[1,2-b]fluorene derivative, which is an [1,2-b]IF dimer, constructed by linking two [1,2-b]IF units with a C-C single bond at carbons 6 and 6' bearing the largest orbital coefficients for the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO). The C6-C6' linkage effectively narrowed the HOMO-LUMO gap while the strong desire to avoid s-indacene antiaromaticity restored two Clar sextets in two proaromatic para-quinodimethane subunits, resulting in an open-shell bifluorenylidene-type diradicaloid (y0 = 0.268) ground state with minor tetraradical character index (y₁ = 0.007). The open-shell nature was confirmed by single crystal X-ray and electron paramagnetic resonance analyses, and supported by theoretical calculations.

中文翻译：

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6,6'-Biindeno[1,2-b]芴：一种开壳茚芴二聚体


Nakano 等人报道，反芳香族茚芴 （IF） 异构体是具有不同程度开壳特性的双自由基分子，其中茚并[1,2-b]芴表现出较弱的双自由基特性指数 （y₀ = 0.072）。与 6,12-三甲基硅烷基乙炔基二取代 [1,2-b]IF 不同，6,12-芳基二取代 [1,2-b]IF 衍生物未显示出任何双自由基性质的实验证据。这就提出了一个问题，即 [1,2-b]IF 二聚体更喜欢闭壳层还是开壳层基态。为了解决这个问题，我们在此报道了 6,6'-联同啶[1,2-b]芴衍生物的合成，它是一种 [1,2-b]IF 二聚体，通过将两个 [1,2-b]IF 单元与碳 6' 处的 C-C 单键连接起来而构建，碳 6' 处具有最高的占据和最低未占据的分子轨道（HOMO 和 LUMO）具有最大的轨道系数。C6-C6' 键有效地缩小了 HOMO-LUMO 差距，而避免 s-吲二烯抗芳烃抗芳烃性的强烈愿望恢复了两个原芳香族对喹二甲烷亚基中的两个 Clar 六重体，导致具有次要四自由基特征指数 （y₁= 0.007）。单晶 X 射线和电子顺磁共振分析证实了开壳性质，并得到了理论计算的支持。