High-Pressure Reactivity of Triptycene Probed by Raman Spectroscopy

The high-pressure reactivity of caged olefinic carbons and polyatomic aromatic hydrocarbons (PAHs) are of interest because of their ability to produce unique C-H networks with varying geometries and bonding environments. Here, we have selected triptycene to explore the creation of pores via high-pressure polymerization. Triptycene has internal free volume on a molecular scale that arises due to its paddle wheel-like structure, formed via fusion of three benzene rings via sp³-hybridized bridgehead carbon sites. At 25 GPa and 298 K, triptycene polymerizes to yield an amorphous hydrogenated carbon, with FTIR indicating an sp³ C-H content of approximately 40%. Vibrational spectroscopy conclusively demonstrates that triptycene polymerizes via cycloaddition reactions at the aromatic sites via a ring opening mechanism. The bridgehead carbons remain intact after polymerization, indicating the rigid backbone of the triptycene precursor is retained in the polymer, as well as molecular-level (∼1-3 Å) internal free volume. High resolution transmission electron microscopy, combined with dark field imaging, indicates the presence of ∼10 nm voids in the polymer, which we attribute to either polymeric clustering or a hierarchical tertiary porous network. Creation of a polymerized network that retains internal voids via high-pressure polymerization is attributed to the presence and retention of the bridgehead carbons.