Decoupling high-temperature phase transition and charge-transfer emission in a crown ether-based crystal.

Area of Science:

• Supramolecular Chemistry
• Crystal Engineering
• Materials Science

Background:

• Crown ether-ammonium inclusion compounds are key in supramolecular chemistry, offering tunable dynamics and phase transitions.
• Existing systems often transition below room temperature, limiting practical use, and their luminescence is understudied.

Purpose of the Study:

• To synthesize and characterize a novel host-guest inclusion compound with a high-temperature phase transition.
• To investigate the influence of fluorine substitution on phase transition and luminescence properties.
• To explore the decoupling of thermal and optical properties in multifunctional molecular crystals.

Main Methods:

• Synthesis of [(2-fluorophenethylaminium)(18-crown-6)][PF6] and its regioisomers.
• Single-crystal X-ray diffraction to determine crystal structure and hydrogen bonding.
• Thermal analysis to identify phase transition temperatures.
• UV-Vis spectroscopy and luminescence measurements.
• Density Functional Theory (DFT) calculations for electronic structure analysis.

Main Results:

• A reversible phase transition was observed at 404 K in the 2-fluoro analogue, one of the highest for crown ether systems.
• The 3-fluoro and 4-fluoro isomers showed no detectable phase transition.
• All three isomers exhibited intense blue luminescence under UV irradiation.
• DFT calculations indicated an intermolecular charge-transfer mechanism for luminescence, with HOMO on crown ether and LUMO on the cation.

Conclusions:

• The regiochemistry of fluorine substitution is critical for achieving high-temperature phase transitions in these compounds.
• Luminescence is governed by supramolecular confinement effects and is independent of fluorine position.
• Decoupling of thermal and optical properties is achievable, providing a design strategy for multifunctional molecular crystals.