Exploration of the high-pressure behaviour of polycyclic aromatic hydrocarbons: naphthalene, phenanthrene and pyrene.

The structural response of three members of the family of polycyclic aromatic hydrocarbons (PAHs) to high-pressure recrystallization from dichloromethane solutions is presented. Recrystallization of naphthalene in the 0.2-0.6 GPa pressure range does not result in the formation of a new polymorph. Furthermore, direct compression of a single crystal to 2.1 GPa does not result in a phase transition. A density decrease of 18.2% over the 0.0-2.1 GPa pressure range is observed and the principal effect of pressure is to ;tighten' the herringbone structural motif and decrease the size of void regions. A new polymorph of pyrene, form III, has been crystallized at 0.3 and at 0.5 GPa. Structural investigation of this new polymorph by means of topological analysis and comparison of Hirshfeld surfaces and fingerprint plots shows that intermolecular interactions are substantially different from those found in the ambient-pressure structures, and do not fit a previously established packing model for PAHs. Similar discrepancies are found for the high-pressure polymorph of phenanthrene, which is here re-investigated in greater detail. The structures of these high-pressure polymorphs are dominated by pi...pi stacking with a limited contribution from C-H...pi (peripheral) interactions. It is perhaps not surprising that high-pressure polymorphs deviate from a model that has been devised for ambient-pressure structures, and this may be a direct consequence of the ability of pressure to modify and combine intermolecular interactions in ways that are not usually found at ambient pressure. This is achieved by modifying the relative orientations of molecules and by encouraging the formation of denser structures in which molecules pack together more efficiently.