A Piezoelectric Molecular Cocrystal with Unconventional π-Stacking

We demonstrate the crystallization of a polar octafluoronaphthalene (OFN, C₁₀F₈)–phthalazine (Phth, C₈H₆N₂) cocrystal formed in a 1:2 ratio by slow evaporation. The crystal structure and vibrational properties of the cocrystal were determined using powder/single-crystal X-ray diffraction (XRD) and Fourier-Transform Infrared (FTIR) spectroscopy, and confirmed with density functional theory (DFT) and density functional perturbation theory (DFPT) calculations. The molecular π-stacking of aromatic rings is unconventional compared with that of other arene–perfluoroarene cocrystals. Phth molecules are offset and misaligned with respect to the major axis of OFN due to combined π–π and dipole–dipole interactions, enabling overall electric polarization attributed to the dipole moment of Phth. Our calculations show that OFN:2Phth is an insulator with a band gap >2.4 eV. The electric polarization was calculated to be 7.1 μC cm^–2, while the shear piezoelectric coefficient (d₃₄) may be as large as 11.4 pC N^–1.