Experimental and theoretical investigation on the hydrochromic property of a Ni(II)-containing coordination polymer with an inclined 2D → 3D polycatenation architecture

Abstract

In recent years, hydrochromic materials have been widely used for visual inspection, rewriting papers, humidity sensors, and the paint and coating industry. However, the investigation on hydrochromic coordination polymers is relatively rare. In this work, a hydrochromic coordination polymer [Ni(L1)(L2)(H2O)(DMF)]·DMF (1) (L1 = N,N'-di(4-pyridyl)-1,4,5,8-naphthalene diimide, H2L2 = 4,4′-stilbenedicarboxylic acid, and DMF = N,N'-dimethylformamide) is synthesized by employing the π-electron-deficient L1 as the main ligand under the solvothermal condition. Compound 1 displays a rare inclined 2D → 3D polycatenation framework, resulting in extraordinary flexibility and stability that enables compound 1 to undergo cell compression or expansion after water loss or absorption, thereby altering the distance between the electron donor and the acceptor, causing the generation or quenching of radicals and hydrochromic phenomenon. Compound 1 undergoes a color change from yellow to green upon heating at 150 °C within five minutes or placing in a desiccator with concentrated sulfuric acid as desiccant for several hours. The green samples can be restored to yellow after being placed in the air for about two minutes. If the green samples contact with water directly, they can immediately return to yellow. The hydrochromic mechanism is investigated by thermogravimetric analysis, UV–Vis, infrared spectra, electron paramagnetic resonance spectra, powder X-ray diffraction spectra and density functional theory simulation.