Designing Polymorphic Bi3+-Containing Ionic Liquids for Stimuli-Responsive Luminescent Materials

Solid-state luminescent materials that possess reversible fluorescence changes toward external multistimuli are of immense interest because of their potential applications in data storage and sensors. While the recent developments in this field are mainly focused on the π-conjugated organic molecules. Herein two polymorphic luminescent ionic liquid (IL)-based stimuli-responsive materials were designed by the supramolecular assemblies of an organic-decorated chlorobismuthate anion and a rotationally flexible imidazolium cation, namely, α (1)/β (2)-[Bmmim][BiCl4(2,2'-bpy)] (Bmmim = 1-butyl-2,3-dimethylimidazolium; 2,2'-bpy = 2,2'-bipyridine). Because of the different conformations of the n-butyl chains on the imidazolium cations, tuning of the supramolecular packing structures as well as luminescent colors for 1 and 2 was realized. Single-crystal X-ray diffraction and Hirshfeld surface analyses disclose that the polymorphism-dependent emission may be attributed to the different weak interactions, especially to the π-π interactions between adjacent [BiCl4(2,2'-bpy)]- anions in two compounds. Additionally, compound 2 could be transformed into 1 spontaneously at ambient conditions, which could be triggered by the moisture in the air. Both of the title compounds could detect NH3 vapor selectively through the luminescence "turn-off" method rapidly and reversibly because of the destruction of intermolecular interactions, indicating their stimuli-responsive property toward NH3.