From Paramagnetic to Superparamagnetic Ionic Liquid/Poly(ionic liquid): The Effect of π-π Stacking Interaction.

Magnetic ionic liquids (MILs) and poly(magnetic ionic liquids) (PMILs) with FeCl 4 - as anions usually show weak magnetism, such as paramagnetism or antiferromagnetism, at room temperature. Inspired by the natural inorganic ferromagnet with ordered crystal structures, a soft superparamagnetic ionic liquid (TMBBDI[FeCl 4 ]) and corresponding poly(ionic liquid) (PTMBBDI[FeCl 4 ]) were prepared by introducing π-π stacking biphenyl groups into the organic cations. Both of the compounds exhibited superparamagnetism from 100 to 300 K, while a ferromagnetic hysteresis loop was found at 300 K. Ferromagnetic interactions were observed from zero field cooling and field cooling studies for both TMBBDI[FeCl 4 ] and PTMBBDI[FeCl 4 ]. However, the MIL and PMIL without π-π stacking interaction were paramagnetic without ferromagnetic interaction. The superparamagnetism of the TMBBDI[FeCl 4 ] and PTMBBDI[FeCl 4 ] was ascribed to the π-π stacking interactions between biphenyl groups, which not only shortened the Fe-Fe distance to the ferromagnetic interaction range but also increased the order degree of the molecules.