RESUMO
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.
RESUMO
The chemical stability of solid cubane under high-pressure was examined with in situ Raman spectroscopy and synchrotron powder X-ray diffraction (PXRD) in a diamond anvil cell (DAC) up to 60 GPa. The Raman modes associated with solid cubane were assigned by comparing experimental data with calculations based on density functional perturbation theory, and low-frequency lattice modes are reported for the first time. The equation of state of solid cubane derived from the PXRD measurements taken during compression gives a bulk modulus of 14.5(2) GPa. In contrast with previous work and chemical intuition, PXRD and Raman data indicate that solid cubane exhibits anomalously large stability under extreme pressure, despite its immensely strained 90° C-C-C bond angles.