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

This work reports the syntheses, structures, and luminescence properties of two supramolecular polymorphic compounds composed of a hybrid bismuth(III) chloride anion and an imidazolium cation, namely α- (1) and β- (2) [Bmim][BiCl₄ (2,2'-bpy)] (Bmim=1-butyl-3-methyl imidazolium; 2,2'-bpy=2,2'-bipyridine). They are the first two examples of Bi³⁺ -containing ionic liquids (ILs). Their different packing modes may be ascribed to the rotational flexibility of the butyl group on the [Bmim]⁺ cation. A comparative study of the weak intermolecular interactions present in the two polymorphs has been performed by Hirshfeld surface and two-dimensional fingerprint analyses. Investigations of the luminescence properties revealed that crystallization induced greenish-yellow phosphorescence with quantum yields of 26.07 % for 1 and 36.59 % for 2, which are among the highest hitherto reported for hybrid halobismuthate compounds. The difference in phosphorescence may be attributed to the different weak interactions in 1 and 2, especially π-π contacts. This work opens the way to further research on new types of polymorphism-dependent luminescent materials based on a combination of rotationally isomeric IL cations with organic decorated bismuth(III) chloride anions.