A novel Cd(II) coordination polymer: Dual photochromism, photo-controlled luminescence, and application in anti-counterfeiting

Abstract

The viologen ligand, 1,1′-bis((3-carboxylatobenzyl)-4,4′-bipyridinium) (H₂bcbpy) contains rich carboxylate groups which provide abundant coordination sites and act as electron donors. The 4,4′-bipyridinium moiety of H₂bcbpy could act as an electron acceptor. To promote the photochromic process and photoluminescence property, terephthalic acid (PTA) was selected as a secondary ligand, and a novel coordination polymer {[Cd(bcbpy)_0.5(PTA)(H₂O)]·0.5(C₂H₇N)}_n (1) with lattice molecule dimethylamine (C₂H₇N) was constructed. Coordination polymer 1 displayed dual UV/X-ray photochromic and photo-controlled luminescence properties. Compared to previously reported bcbpy-based complexes, coordination polymer 1 exhibited a fast colouration response to UV irradiation changing from colourless to light blue in 0.5 s, however the decolouration reaction took up to 60 days. After 5 s of illumination, the color saturated and coordination polymer 1 turned into a deep blue product, 1P. Coordination polymer 1 was also sensitive to exposure to X-rays. Based on the results of single-crystal X-ray diffraction and theoretical calculation, the lattice molecule was involved in the electron transfer. The photochromic performance of coordination polymer 1 was attributed to the electron transfer from different carboxylate oxygen atoms and dimethylamine nitrogen atom to the pyridinium ring via C–H⋯O and C–H⋯N hydrogen bonds, leading to the formation of radicals. Additionally, the decay curve of emission at 530 nm ascribed to delayed fluorescence showed that the lifetime decreased from 113.26 to 0.88 ms with increasing temperature from 77 to 297 K. Compared with bcbpy, the construction of coordination polymer 1 prolongs the lifetime and afterglow, together with fast response and high stability. Given the excellent optical properties, coordination polymer 1 was developed into inkless, erasable printing materials, information encryption and anti-counterfeiting with potential applications.