Continuous and reversible solar-responsive thermochromism in [BMIM]₄[Ni(NCS)₆], an ionic liquid that withstands numerous heating–cooling cycles.

The material dispersion of the [C_kmim][BF₄] (k = 2, 3, 4, 6, 7, 8, 10) family of ionic liquids is measured at several temperatures over a broad spectral range from 300 nm to 1550 nm. The experimental curves are fitted to a modified three-resonance Sellmeier model to understand the effects of temperature and alkyl chain length on the dispersion behaviour. From the parameters of the fitting, we analyze the influence that the different constituents of these ionic liquids have on the dispersion behaviour. In addition, a semi-empirical approach combining simulated electronic polarizabilities and experimental densities for predicting the material dispersion is successfully tested by using a direct comparison with the experimental results. The limitations of this method are analyzed in terms of the molecular structure of the ionic liquids. The results of this work aim to increase our knowledge about how the molecular structure of an ionic liquid influences its material dispersion. Understanding this influence is fundamental to producing ionic liquids with tailored optical properties.

We investigated the influence of transition metal coordination on the optical dispersion and thermo-optic behavior of the ionic liquid 1-butyl-3-methylimidazolium thiocyanate ([BMIM][SCN]). Refractive index measurements in the visible–near-infrared range (400–1000 nm), combined with temperature-dependent characterization (298–323 K), demonstrate that coordination with Al³⁺, Cd²⁺, Zn²⁺, and Mn²⁺ consistently increases the refractive index relative to the neat ionic liquid. All systems exhibit normal dispersion, following the hierarchy n(Al) > n(Cd) ≳ n(Zn) > n(Mn) > n([BMIM][SCN]), which reflects cooperative contributions from metal-centerd polarizability and coordination-induced modifications to density and electronic structure. Negative thermo-optic coefficients are measured for all samples, with [BMIM]₃[Al(SCN)₆] displaying the highest temperature sensitivity. Abbe diagrams and group-velocity dispersion analyses confirm a predictable index–dispersion trade-off and show that dispersion-related transport parameters are less temperature dependent than n(T). Collectively, these findings establish a structure–property framework for tuning refractive index, chromatic dispersion, and thermo-optic response via coordination chemistry, supporting the targeted design of thiocyanate-based ionic liquids for photonic components, thermal lenses, and dispersion-managed optical devices.