The crystal structure of hexaphenylbenzene under high hydrostatic pressure

High-pressure single-crystal X-ray diffraction, intermolecular interaction energy calculations, and density functional theory are used to examine the structure and emission properties of the classical organic rotor hexaphenylbenzene (C6Ph6) during hydrostatic compression to 4.14 GPa. Under applied pressure, the inter-phenyl distance in intermolecular dimers with displaced-stacked conformations gradually shortens, indicating the likelihood for hexaphenylbenzene to undergo mechanofluorochromism, while the stability of the crystal is governed by T-shaped intermolecular phenyl dimers. The theoretical band gaps and UV absorption spectra of hexaphenylbenzene at ambient pressure and 1.05 GPa predict an absorption red-shift by about 50 nm, in agreement with shortening of the intermolecular displaced-stacked interactions in the crystal under pressure. Compression of the intermolecular interactions in hexaphenylbenzene prompts a non-centrosymmetric to centrosymmetric phase transition from orthorhombic Pna21 to monoclinic P21/c at 1.05 GPa, as the approximately hexagonally-packed layers of hexaphenylbenzene molecules adopt a perfect hexagonal arrangement. This pressure-stimulated phase transition involving gain of centrosymmetry is an interesting structural phenomenon that is previously unreported in organic crystals. High-pressure single-crystal X-ray diffraction, intermolecular interaction energy calculations, and density functional theory are used to examine the structure and emission properties of hexaphenylbenzene during hydrostatic compression to 4.14 GPa.