Manipulation of the Thermochromic Transition Temperature in a Classic Metal-Organic Complex by Selective Anion Doping

The origin of thermochromism displayed by the hybrid material [Ni(dieten)₂](BF₄)₂ (dieten = N,N-diethylethylenediamine) is explored by anion substitution of the tetrafluoroborate anions (BF₄^-) with varying percentages (0-25%) of bromide (Br^-). Differential scanning calorimetry and variable-temperature diffuse reflectance spectroscopy indicate that the yellow-orange to orange-red thermochromic transition inherent to undoped [Ni(dieten)₂](BF₄)₂ shifts from 100 to 90 °C as the doping concentration increases from 0 to 25%. Similarly, a 15 nm line broadening of the Kubelka-Munk transformed diffuse reflectance signal (proportional to the absorbance of the complex) and a broadening of the endothermic transition are observed with increasing Br^- doping. The structure of the undoped [Ni(dieten)₂](BF₄)₂, determined by single-crystal X-ray diffraction, is presented, and powder X-ray diffraction was used to confirm that the crystal structure and crystallinity of each doped sample remains unchanged from the BF₄^- phase. We provide evidence for an underlying mechanism of thermochromism that is linked to hydrogen bonding within the crystal structure and which can be manipulated via targeted modulation of lattice anions. The mechanism proposed here is likely applicable to other materials within the family of dieten complexes ([M(dieten)₂](X)₂, where M = Ni²⁺, Cu²⁺ and X = BF₄^-, ClO₄^-, NO₃^-).