Fig. 2. (a) ON form at 0.02 and 4.18 GPa, coloured orange and red, respectively. The left structure shows the view along the a-axis, right—view along the c-axis. (b) Colour progression in ON at select pressures. The dark red colour of the crystal is obscured at 4.18 GPa by crystallisation of the pressure medium. Decompression to ambient pressure shows complete reversibility of the colour change. (c) π⋯π-stacking motifs responsible for the colour change. The corresponding θ angle between mean planes shows negligible change between the lowest and highest-pressure structures. Hydrogen atoms in (a) and (c) are omitted for clarity.