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.