α- and β-[Bmim][BiCl₄ (2,2'-bpy)]: Two Polymorphic Bismuth-Containing Ionic Liquids with Crystallization-Induced...

Area of Science:

• Materials Science
• Inorganic Chemistry
• Supramolecular Chemistry

Background:

• Ionic liquids (ILs) are versatile materials with tunable properties.
• Bismuth(III) halide compounds are explored for their unique electronic and optical characteristics.
• Polymorphism, the ability of a solid material to exist in more than one crystal form, can significantly impact material properties.

Purpose of the Study:

• To synthesize and characterize two novel supramolecular polymorphic compounds of bismuth(III) chloride and an imidazolium cation.
• To investigate the structural differences and intermolecular interactions in the two polymorphs.
• To study the luminescence properties and their dependence on crystal packing.

Main Methods:

• Synthesis of hybrid bismuth(III) chloride compounds with imidazolium cations.
• Single-crystal X-ray diffraction for structural analysis.
• Hirshfeld surface and 2D fingerprint analyses for intermolecular interactions.
• Photoluminescence spectroscopy to investigate luminescence properties.

Main Results:

• Two polymorphic compounds, α- (1) and β- (2) [Bmim][BiCl4(2,2'-bpy)], were successfully synthesized and structurally characterized.
• The different packing modes were attributed to the rotational flexibility of the butyl group in the 1-butyl-3-methylimidazolium ([Bmim]+) cation.
• Both compounds exhibited strong greenish-yellow phosphorescence upon crystallization, with quantum yields up to 36.59%.
• The observed differences in phosphorescence were linked to variations in weak intermolecular interactions, particularly π-π contacts.

Conclusions:

• This study presents the first examples of Bi3+-containing ionic liquids with distinct polymorphic forms.
• The findings highlight the significant influence of polymorphism and intermolecular interactions on the luminescence properties of these hybrid materials.
• This work provides a foundation for designing novel polymorphism-dependent luminescent materials by combining rotationally isomeric IL cations with functionalized bismuth(III) chloride anions.