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.