Nanoarchitecture through Strained Molecules: Cubane-Derived Scaffolds and the Smallest Carbon Nanothreads

Relative
to the rich library of small-molecule organics, few examples
of ordered extended (i.e., nonmolecular) hydrocarbon networks are
known. In particular, sp3 bonded, diamond-like materials
represent appealing targets because of their desirable mechanical,
thermal, and optical properties. While many covalent organic frameworks
(COFs)extended, covalently bonded, and porous structureshave
been realized through molecular architecture with exceptional control,
the design and synthesis of dense, covalent extended solids has been
a longstanding challenge. Here we report the preparation of a sp3-bonded, low-dimensional hydrocarbon synthesized via high-pressure,
solid-state diradical polymerization of cubane (C8H8), which is a saturated, but immensely strained, cage-like
molecule. Experimental measurements show that the obtained product
is crystalline with three-dimensional order that appears to largely
preserve the basic structural topology of the cubane molecular precursor
and exhibits high hardness (comparable to fused quartz) and thermal
stability up to 300 °C. Among the plausible theoretical candidate
structures, one-dimensional carbon scaffolds comprising six- and four-membered
rings that pack within a pseudosquare lattice provide the best agreement
with experimental data. These diamond-like molecular rods with extraordinarily
small thickness are among the smallest members in the carbon nanothread
family, and calculations indicate one of the stiffest one-dimensional
systems known. These results present opportunities for the synthesis
of purely sp3-bonded extended solids formed through the
strain release of saturated molecules, as opposed to only unsaturated
precursors.

相较于丰富的有机小分子库，已知有序扩展（即非分子）碳氢化合物网络实例寥寥无几。尤其是，sp3键合的类钻石材料因其理想的机械性能、热性能和光学性能而备受瞩目。尽管许多共价有机框架（COFs，即通过分子架构实现、具有扩展性、共价键合和多孔结构的材料）已经通过卓越的分子设计实现了，但密集、共价扩展固体的设计与合成一直是一项长期挑战。本研究中，我们报告了一种通过高压、固态双自由基聚合立方烷（C8H8）制备的sp3键合、低维碳氢化合物。立方烷是一种饱和且极度拉伸的笼状分子。实验测量结果表明，所得产物为结晶态，具有三维有序性，在很大程度上保留了立方烷分子前体的基本结构拓扑，并表现出极高的硬度（与熔融石英相当）和高达300°C的热稳定性。在众多可能的理论候选结构中，由六元环和四元环组成的一维碳支架，这些环在准方形晶格中堆积，与实验数据吻合最佳。这些具有异常微小厚度的类钻石分子棒是碳纳米纤维家族中最小的成员之一，计算表明它们是一维系统中最刚硬的之一。这些结果为通过饱和分子的应变释放来合成纯sp3键合扩展固体提供了机会，这有别于仅以不饱和前体为基础的合成方法。