Responsive molecular chromic materials have shown potential applications in molecular optical switches and sensor devices as they undergo reversible colour changes upon application of external stimuli. Herein, six host–guest coordination polymers 1–6 and one organic supramolecular compound 7 containing viologens have been synthesized under hydrothermal conditions. Compound 1 shows thermochromic behavior with a colour change from green to blue upon heating at 150 °C in air. Compounds 3, 4 and 7 display photochromic behaviors with a colour change from pale yellow to green under UV light, visible light or sunlight. The relationship between their structures and chromic behaviors has been discussed. Thermo/photochromic mechanisms have been investigated by infrared spectroscopy, powder X-ray diffraction analysis, UV-vis absorption spectroscopy, electron paramagnetic resonance spectroscopy, and X-ray photoelectron spectroscopy. The results demonstrate the generation of viologen radicals caused by thermo/photo-induced electron transfer. It is found that compound 4 can be used as an inkless and erasable printing medium. In addition, compounds 3, 4, and 7 can be used as anti-counterfeiting materials for QR codes. Our findings demonstrate the possibility of applying thermo/photochromic coordination polymers in molecular devices.

By using a simple room temperature solution reaction, we prepared a new type of one-dimensional (1D) hybrid lead halide [DMEDA]PbCl₄. The compound gives a bright yellow emission with efficient photoluminescence quantum efficiency (PLQE) and optical stability.

A series of coordination polymers, namely [Zn(μ₃-L)(H₂O)₂] (1), [Zn(μ₂-L)(2,2′-bpy)(H₂O)] (2), [Zn(μ₃-L)(μ₂-4,4′-bpy)_0.5] (3), [Cd(μ₃-L)(2,2′-bpy)] (4), and [Cd(μ₂-L)(phen)(H₂O)] (5) (H₂L = 3-carboxy-1-(3′-carboxybenzyl)-2-oxidopyridinium, 2,2′-bpy = 2,2′-bipyridine, 4,4′-bpy = 4,4′-bipyridine, phen = 1,10-phenanthroline), have been synthesized under hydrothermal conditions. These coordination polymers have been characterized by single-crystal X-ray diffraction, elemental analyses, infrared spectroscopy, thermogravimetric analyses and powder X-ray diffraction. The structure analysis by single crystal X-ray diffraction reveals that compound 1 shows a 1D double chain which can be expanded to a 2D supramolecular architecture through intermolecular π–π stacking interactions. In compound 2, the 1D wave-like chains are extended to a 3D supramolecular framework by intermolecular hydrogen bonds and π–π stacking interactions. In compound 3, the infinite rod-like metal ion chains are linked with L²⁻ anions and 4,4′-bpy to form a 3D framework. In compound 4, the adjacent 1D loop-like chains are interconnected to a 3D supramolecular framework by intermolecular hydrogen bonds and π–π stacking interactions. Compound 5 displays a 3D supramolecular architecture extended from the 1D zigzag chains by intermolecular hydrogen bonds and π–π stacking interactions. The result reveals that the organic ligand plays a key role in the self-assembly of coordination polymers. The luminescence sensing experiments reveal that compounds 1–5 can effectively detect Fe³⁺, Cr₂O₇ ²⁻, and NB. In particular, compound 1 shows a high quenching constant and a low limit of detection for Fe³⁺.