A previously reported thermochromic complex, [Ni(dieten)2] (BF4)2 (dieten = N,N-diethylethylenediamine) was synthesized and the percentage of tetrafluoroborate (BF4-) anion varied by systematic replacement with bromide (Br-). Powder X-ray diffraction confirmed the bulk crystalline phase of each sample remains unchanged relative to the BF4 phase, regardless of doping percentage in the range of 5-25%. Differential scanning calorimetry (DSC) and variable temperature diffuse reflectance spectroscopy (DRS) were used to characterize and compare the thermochromic transition of these doped phases. Rapid-onset and reversible thermochromism is observed in all doped samples, as commonly observed within this class of dieten materials. The yellow-orange to orange-red thermochromic transition inherent to the un-doped nickel complex shifts to lower temperatures as the dopant concentration increases. This suggests that the reduced density of the BF4 anion-to-complex hydrogen bonds may have an impact on the thermochromic transition and that the thermochromic mechanism in this material is likely the consequence of thermal disruption of the anion-to-complex hydrogen bonding. Additionally, we present for the first time the crystal structure of the un-doped phase of [Ni(dieten)2] (BF4)2 obtained by single crystal X-ray diffraction. This structure is consistent with similar previously reported materials in this family of thermochromic compounds.

Altering thermochromic transition temperatures in a classic example of metal-organic hybrid complex thermochromism

ACS Spring 2022

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