Dielectric response and density functional theory assessment of fluorinated dicationic pyridinium ionic liquids

This study represent the synthesis and electrical conductivity of two focused dicationic pyridinium ionic liquids (DiILs) carrying tetrafluoroborate and/or hexafluorophosphate as counter anion. Thus, the synthesis required quaternization of the pyridine nitrogen atoms first, followed by a metathetical exchange reaction to produce the task-specific ILs bearing tetrafluoroborate and hexafluorophosphate as counter anions. The resulting DiILs were characterized by NMR and MS spectroscopy. The dielectric constant and the conductivity of the targeted DiILs were analyzed in detail, within a frequency-range of 1 KHz to 0.3 MHz and a temperature range of 25–100°C. Density functional theoretical (DFT) calculations were also carried out at the base site, B3LYP 6–311 g (d,p), for the Syn/E isomers of the investigated ionic liquids (ILs). We found non-coplanar molecular geometries with a twist angle of the Schiff base, dependent on the type of counter ion present. The energy levels and differences of the frontier molecular orbitals (FMOs) were also predicted using DFT calculations. The results revealed that the ILs incorporating BF4− as counter anions showed high HOMO and LUMO values with a higher energy gap, while low FMOs with a lower energy gap were recorded for the derivative encompassing PF6− counter anions. These findings were used to help explain the dielectric properties of the investigated DiILs.