Toward Tetraradicaloid: The Effect of Fusion Mode on Radical Character and Chemical Reactivity

Open-shell singlet diradicaloids
display unique electronic, nonlinear
optical, and magnetic activity and could become novel molecular materials
for organic electronics, photonics, and spintronics. However, design
and synthesis of diradicaloids with a significant polyradical character
is a challenging task for chemists. In this Article, we report our
efforts toward a tetraradicaloid system. A series of potential tetraradicaloids
by fusion of two p-quinodimethane (p-QDM) units with naphthalene or benzene rings in different modes
were synthesized. Their model compounds containing one p-QDM moiety were also prepared and compared. Their ground-state structures,
physical properties, and chemical reactivity were systematically investigated
by various experimental methods such as steady-state and transient
absorption, two-photon absorption, X-ray crystallographic analysis,
electron spin resonance, superconducting quantum interference device,
and electrochemistry, assisted by density functional theory calculations.
It was found that their diradical and tetraradical characters show
a clear dependence on the fusion mode. Upon the introduction of more
five-membered rings, the diradical characters greatly decrease. This
difference can be explained by the pro-aromaticity/antiaromaticity
of the molecules as well as the intramolecular charge transfer. Our
comprehensive studies provide a guideline for the design and synthesis
of stable open-shell singlet polycyclic hydrocarbons with significant
polyradical characters.

开壳层单线态双自由基类化合物（Open-shell singlet diradicaloids）展现出独特的电子学、非线性光学与磁学特性，有望成为有机电子学、光子学以及自旋电子学领域的新型分子材料。然而，设计并合成具有显著多自由基特征的双自由基类化合物，对化学家而言仍是一项极具挑战性的工作。
本文中，我们汇报了针对四自由基类体系的研究工作。通过不同的融合方式将两个对苯二醌二甲烷（p-quinodimethane, p-QDM）单元与萘环或苯环相结合，我们合成了一系列潜在的四自由基类化合物。同时，我们还制备了仅含一个p-QDM结构单元的模型化合物并进行了对照分析。
我们借助稳态与瞬态吸收光谱、双光子吸收光谱、X射线晶体结构分析、电子自旋共振、超导量子干涉器件测试以及电化学测试等多种实验手段，并结合密度泛函理论计算，系统研究了这类化合物的基态结构、物理性质与化学反应活性。
研究发现，它们的双自由基与四自由基特征与融合方式存在显著关联。当引入更多五元环时，双自由基特征会显著减弱。该差异可通过分子的前芳香性/反芳香性以及分子内电荷转移效应加以解释。
我们的系统性研究为设计与合成具有显著多自由基特征的稳定开壳层单线态多环烃类化合物提供了指导思路。