Pressure-Induced Polymerization of Polycyclic Arene–Perfluoroarene Cocrystals: Single Crystal X‑ray Diffraction Studies, Reaction Kinetics, and Design of Columnar Hydrofluorocarbons

Pressure-induced
polymerization of aromatic compounds leads to
novel materials containing sp3 carbon-bonded networks.
The choice of the molecular species and the control of their arrangement
in the crystal structures via intermolecular interactions, such as
the arene–perfluoroarene interaction, can enable the design
of target polymers. We have investigated the crystal structure compression
and pressure-induced polymerization reaction kinetics of two polycyclic
1:1 arene–perfluoroarene cocrystals, naphthalene/octafluoronaphthalene
(NOFN) and anthracene/octafluoronaphthalene (AOFN), up to 25 and 30
GPa, respectively, using single-crystal synchrotron X-ray diffraction,
infrared spectroscopy, and theoretical computations based on density-functional
theory. Our study shows the remarkable pressure stability of the parallel
arene–perfluoroarene π-stacking arrangement and a reduction
of the interplanar π-stacking separations by ca. 19–22%
before the critical reaction distance is reached. A further strong,
discontinuous, and irreversible reduction along the stacking direction
at 20 GPa in NOFN (18.8%) and 25 GPa in AOFN (8.7%) indicates the
pressure-induced breakdown of π-stacking by formation of σ-bonded
polymers. The association of the structural distortion with the occurrence
of a chemical reaction is confirmed by a high-pressure kinetic study
using infrared spectroscopy, indicating one-dimensional polymer growth.
Structural predictions for the fully polymerized high-pressure phases
consisting of highly ordered rods of hydrofluorocarbons are presented
based on theoretical computations, which are in excellent agreement
with the experimentally determined unit-cell parameters. We show that
the polymerization takes place along the arene–perfluoroarene
π-stacking direction and that the lateral extension of the columns
depends on the extension of the arene and perfluoroarene molecules.