Faraday effect and refractive index of some imidazolium-based room-temperature ionic liquids and magnetic ionic liquids

Abstract

Knowledge of the Faraday effect (FE) is very important in fundamental research and applications. Here, we report FE and refractive index studies for two magnetic ionic liquids (MILs), namely 1-ethyl- and 1-butyl-3-methylimidazolium tetrachloroferrate. Similar studies are also conducted for ionic liquids (ILs) that have a diamagnetic counteranion in place of [FeCl4]. The magnitudes of the Verdet constants of MILs are comparable and exhibit a negative sign. Furthermore, they are about one order larger than those of ILs with a diamagnetic counteranion of a positive sign. The Verdet constant shows monotonic variations as a function of wavelength in the range from 450 to 650 nm and is mainly related to charge transfer transitions in [FeCl4], with bands located in the UV region, though the contribution of d-d transitions in Fe+3 is negligible. A model for describing the measured Verdet constant data as a function of wavelength and temperature is given with respective values of effective Faraday A-, B-, and d-terms. The introduction of an empirical rule allows the estimation of Verdet constants of iron and imidazolium derivative based MILs, which are in some cases very high, thus indicating their potential in photonics applications. Imidazole shows a positive and nearly-two orders lower magnitude of the Verdet constant. The mixture of an MIL and acetonitrile allows the tuning of both the Verdet constant and the refractive index. The refractive indices, thermo-optic coefficients, and electronic polarizabilities of MILs are higher than those of ILs with a diamagnetic counteranion.