Temperature-Dependent Modulation of Short-Wave-Infrared Light Transparency Based on Associated Structures of a Liquescent Nickel(III) Complex

Abstract

A liquescent bis(malononitriledithiolato)nickel(III) complex with a bis(methoxyethyl)imidazolium cation, 1[Ni(mnt)₂ ], exhibits three-stage thermochromic modulation of transparency/absorption in the short-wave-infrared (SWIR) region (1000-2500 nm), driven by associated structural changes. Upon heating, the electronic spectra of 1[Ni(mnt)₂ ] in the SWIR region shift to shorter wavelengths accompanying with the solid-liquid phase transition at 76 °C. Further heating to over 109 °C induces a second transition of the electronic spectra, characterized by a blue-shift of the SWIR absorption in the liquid phase. The results of temperature-dependent electronic spectra and magnetic susceptibility indicated that the thermochromic changes can be attributed to the two-step dissociation of the associated structures of [Ni(mnt)₂ ]^- , occurring during the solid-liquid phase transition and the shift of dimer-monomer equilibrium in the liquid state. These changes can be visualized using an SWIR imaging camera under appropriate SWIR lights.

Keywords: ionic liquid; magnetic property; nickel complex; phase transition; short-wave-infrared light transparency.