Interhalogen 1,2,3-Triazolium Ionic Liquid: Synthesis, Crystal Structure, Hirshfeld Surface Analysis and Photophysical Properties

A new interhalogen ionic liquid [C17H25N3Br]+[IBr2]− (IL-1) was synthesized from its triazolium iodide precursor [C17H26N3+I−] in THF under cold and basic conditions. X-ray diffraction data showed that IL-1 is composed of two moieties contained in a triclinic unit cell; the five-membered triazolium cation and an almost linear iodidobromide anion, IBr2−. The Br atoms of the anion form quadfurcated C–H···Br hydrogen bonds with neighbouring cationic halo-1,2,3-triazolium H-species. Intermolecular Br···Br halogen bonds and I···πtriazole interactions form a distinctive ring-like pattern that links together four molecular units in the crystal packing. Hirshfeld surface analysis revealed that the most significant dnorm surface contribution at 59% is due to H···H and reciprocal C···H contacts, while the Br···Br contacts only contributed 3%. The prevalence of extensive H···H and C···H contacts is potentially due to the linear aliphatic chain, the N-octyl wingtip substituent of the triazolyl moiety. The Hirshfeld surface mapping also shows the contribution of intermolecular C–H···Br interactions at 26% of all contacts. The title compound (IL-1) showed interesting photophysical properties in MeCN solution, with an absorption band at 254 nm and two-shoulder emission bands due to strong π → π∗ transitions from the triazolium moiety, indicating the presence of two energetically associated species. Structural and photophysical studies on a new ionic liquid compound containing a mixed halide anion [C17H25N3Br]+[IBr2]− have yielded important results on intermolecular interactions between the triazolium cation and the iodidobromide counterion.