Dispersive Optical Properties and Refractive Index of BMIMSCN Ionic Liquids with Transition Metal Coordination

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 (BMIMSCN). 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(BMIMSCN), 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 BMIM3Al(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.