Structural and physical properties of a new reversible and continuous thermochromic ionic liquid in a wide temperature interval: [BMIM] 4 [Ni(NCS) 6 ]

We report spectroscopic, structural, optical and magnetic characterization of tetra(1-butyl-3-methylimidazolium)hexaisothiocyanatonickelate. This paramagnetic ionic liquid exhibits reversible and continuous thermochromism from 298 K up to 400 K and it is solar responsive resisting numerous heating–cooling cycles. Its appearance changes from pale blue to grass green from 298 K to 343 K. Well above these temperatures it becomes brown and gray. Thermochromism is observed both in the solid and liquid phase.