Reversible Thermochromism of Nickel(II) Complexes and Single-Crystal-to-Single-Crystal Transformation

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^-).

Mechanical Actuation and Patterning of Rewritable Crystalline Monomer-Polymer Heterostructures via Topochemical Polymerization in a Dual-Responsive Photochromic Organic Material

The dark-orange monomer single crystals of 1,1'-dioxo-1H-2,2'-biindene-3,3'-diyldidodecanoate (BIT-dodeca₂) convert to a transparent single-crystalline polymer (PBIT-dodeca₂) material via a single-crystal-to-single-crystal (SCSC) polymerization reaction under sunlight, which then undergoes reverse thermal transformation into BIT-dodeca₂ single crystals, leading to reversible photo-/thermochromism, coupled with mechanical actuation. We exploit the properties of this unique material to demonstrate the formation of monomer-polymer heterostructures in selected regions of single crystals with micrometer-scale precision using a laser. This is the first example of heterostructure patterning involving monomer-polymer domains in single crystals. We reveal that the speed of photomechanical bending induced by the polymerization reaction in this example is comparable to those of the well-known diarylethene derivatives, in which electrocyclic ring-closing-ring-opening reactions operate. Furthermore, we characterize the distinct mechanical properties of the monomer and polymer using a quantitative nanoindentation technique as well as demonstrate photopatterning on a monomer-coated paper for potential use in security devices. These crystals with several advantages, such as photomechanical bending (weight lifting) even when the crystal size is large, responsiveness to both UV and visible light, distinct solubilities (the polymer is insoluble, whereas the monomer is soluble in most organic solvents) and colors, provide unique opportunities for their use at different length scales of the sample (μm to mm) for various purposes.

Thermo-responsive light-emitting metal complexes and related materials

This review discusses the fundamentals and design strategies for the development of thermo-responsive metal–ligand coordination materials and the applications of these materials in temperature sensing, bioimaging, information security, etc.

A square-pyramidal coordinated copper(ii) hydrazine dimeric complex showing reversible phase transition, dielectric anomaly and thermochromism

A square-pyramidal coordinated copper(ii) hydrazine dimeric complex was prepared by reaction of CuCl₂ with trimethyl hydrazine showing reversible phase transition, dielectric anomaly and thermochromism.