Decoupling High-Temperature Phase Transition and Charge-Transfer Emission in a Crown Ether-Based Crystal

Abstract

Crown ether-ammonium inclusion compounds have attracted great interest in the fields of supramolecular chemistry, crystal engineering, and molecular switches due to their tunable host-guest dynamics and stimuli-responsive phase transitions. However, most reported systems exhibit phase transitions only below room temperature, limiting practical applications, and their luminescence properties remain largely unexplored. Herein, we report a host-guest inclusion compound, [(2-fluorophenethylaminium)(18-crown-6)][PF₆] (1) that undergoes a reversible phase transition at 404 K, approaching the highest values reported in crown ether-based systems. Single‑crystal X‑ray diffraction confirms that the ammonium cation is tightly anchored within the 18‑crown‑6 cavity through N–H···O hydrogen bonds. Remarkably, the regioisomeric 3‑fluoro (2) and 4‑fluoro (3) analogues show no detectable phase transition, yet all three isomers display intense blue emission under 365 nm UV irradiation. This contrast demonstrates that 2‑fluorine substitution is essential for the high‑temperature transition, while luminescence arises from supramolecular confinement and does not depend on the fluorine substitution position. Density functional theory calculations support an intermolecular charge‑transfer mechanism: the HOMO is localized on the crown ether oxygen atoms, the LUMO on the aromatic ring of the cation, with a calculated HOMO–LUMO gap of 4.49 eV. Collectively, these results demonstrate decoupling of the two functions, where the phase transition is governed by fluorine regiochemistry and emission is controlled by confinement effects. This decoupling provides a design principle for independently tuning thermal and optical properties in multifunctional molecular crystals.