Abstract: Bianthrone [10(10-oxoanthracen-9-ylidene)anthracen-9-one] consists of two tricyclic anthraceneone units connected by a carbon?carbon double bond. Crystals of the form obtained under ambient conditions are yellow and contain folded centrosymmetric conformers in which the central ring of the anthraceneone unit is non-planar. When hydrostatic pressure is applied the crystals assume a red colouration which gradually deepens as pressures increases. The colour change is limited in extent to the surface of the crystals, the bulk remaining yellow. Comparison of high-pressure, single-crystal UV?vis spectra and powder diffraction data demonstrate that the colour change is associated with the formation of a polymorph containing a conformer in which the tricyclic fragments are planar and the molecule is twisted about the central C?C bond. Single-crystal diffraction data collected as a function of pressure up to 6.5?GPa reveal the effect of compression on the yellow form, which consists of layers of molecules which stack along the [010] direction. The structure remains in a compressed form of the ambient-pressure phase when subjected to hydrostatic pressure up to 6.5?GPa, and the most prominent effect of pressure is to push the layers closer together. PIXEL calculations show that considerable strain builds up in the crystal as pressure is increased with a number of intermolecular contacts being pushed into destabilizing regions of their potentials.

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