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In aromatic molecules, the carbon atoms are sp2-hybridised and form three s-bonds with neighbouring atoms in a planar configuration. The application of pressure leads to the reduction of the intermolecular interplanar stacking distances in these crystals until the p-bonding p-orbitals of the molecules interact and intermolecular s-bonds are formed. As a result of the breakdown of aromaticity, the carbon atoms become sp3-hybridised and 4-coordinate. The pressure-induced polymerisation (PIP) reaction provides an alternative method for the synthesis of novel carbon-based sp3 nanomaterials [1,2].

The inverse charge distribution in aromatic and perfluorinated aromatic molecules (i.e., arenes and perfluoroarenes) leads to attractive interactions between these molecules. This makes the arene-perfluoroarene motif a robust supramolecular synthon, which is used for the development of highly oriented, stacked p-systems (Figure 10, left) [3]. This study reports the accurate pressure-evolution of the intermolecular distances of two arene-perfluoroarene co-crystals up to the onset

of the polymerisation reaction (Figure 11a) and their unit-cell parameters across the reaction, characterises the reaction kinetics, provides the computationally optimised crystal structures of the resulting polymers (Figure 10, right) and, finally, it proposes reaction mechanisms based on the molecular and polymer structures (Figure 11b).

Single-crystalline 1:1 co-crystals of anthracene or naphthalene with perfluoronaphthalene were pressurised in diamond anvil cells and loaded with helium as the pressure-transmitting medium. The crystal structures were investigated in the 0.0001 30 GPa range at beamline ID15B using single-crystal X-ray diffraction. Often, the low symmetry of crystals loaded in a high-pressure cell hinders structural characterisation due to the restricted access to the reciprocal space. However, the high energy (30 keV) and brilliance of the synchrotron beam at ID15B and the combined loading of the monoclinic crystals in different orientations resulted in very high-quality data and reliable structure refinements.

Fig. 10: Experimentally determined molecular (left) and theoretically computed (right) polymerised high-pressure structures of naphthalene:perfluoronaphthalene (top) and anthracene:perfluoronaphthalene (bottom).