Evolution of structure and spectroscopic properties of a new 1,3-diacetylpyrene polymorph with temperature and pressure

A new polymorph of 1,3-diacetylpyrene that is luminescent in the solid state and a prominent negative thermal expansion material has been obtained from its melt. A thorough structural characterization of this new crystal form was performed in a wide temperature and pressure range using single-crystal X-ray diffraction. Structural studies have been combined with steady-state UV–Vis spectroscopy and periodic density functional theory calculations. A previously published methodology of crystal placement in a diamond anvil cell has been successfully applied in predicting optimal 2°AP-β sample orientation, ensuring >80% data coverage and enabling unrestrained Hirshfeld atom refinements for high-pressure structures as well as analysis of anharmonic oscillations.

A new polymorph of 1,3-diacetylpyrene has been obtained from its melt and thoroughly characterized using single-crystal X-ray diffraction, steady-state UV–Vis spectroscopy and periodic density functional theory calculations. Experimental studies covered the temperature range from 90 to 390 K and the pressure range from atmospheric to 4.08 GPa. Optimal sample placement in a diamond anvil cell according to our previously presented methodology ensured over 80% data coverage up to 0.8 Å for a monoclinic sample. Unrestrained Hirshfeld atom refinement of the high-pressure crystal structures was successful and anharmonic behavior of carbonyl oxygen atoms was observed. Unlike the previously characterized polymorph, the structure of 2°AP-β is based on infinite π-stacks of antiparallel 2°AP molecules. 2°AP-β displays piezochromism and piezofluorochromism which are directly related to the variation in interplanar distances within the π-stacking. The importance of weak intermolecular interactions is reflected in the substantial negative thermal expansion coefficient of −55.8 (57) MK⁻¹ in the direction of C—H⋯O interactions.