Abstract
Viologen units have been widely used to impart metal–organic frameworks (MOFs) with photochromic properties. However, construction of a stable photochromic system in viologen MOFs has not been fully explored. Herein, we report three examples of MOFs, namely, {[Cd(CEbpy)(m-BDC)(DMF)]·2H2O}n (1), {[Cd(CEbpy)(p-BDC)(H2O)]·H2O}n (2), and {[Zn(CEbpy)(p-HBDC)(p-BDC)0.5]·H2O}n (3) based on benzenedicarboxylic acids (m-H2BDC = 1,3-benzenedicarboxylic acid, p-H2BDC = 1,4-benzenedicarboxylic acid) and a viologen-derived ligand 1-carboxyethyl-4,4′-bipyridine (L = CEbpy). As expected, the incorporation of the viologen unit into the frameworks results in the predefined photochromism upon both sunlight and UV-light. Compounds 1–3 feature a two-dimensional (2D) layered structure and are all photochromic due to the formation of CEbpy radicals by photoinduced electron transfer (PET). The aggregates build an interesting stable crystalline framework that exhibits long-lived color constancy in the solid state.
Highlights
As a unique highly ordered and porous material, metal–organic frameworks (MOFs)1 have become increasingly popular due to their con gurable structure, high surface area and adjustable porosity
We propose a viologen-derived ligand, 1carboxyethyl-4,40-bipyridinine (L 1⁄4 CEbpy) with the terminal oxygen and nitrogen atoms serving as coordination sites, in
All reagents and solvents were commercially purchased and used without further puri cation. 1-Carboxyethyl-4,40-bipyridine (L 1⁄4 CEbpy) was synthesized according to the literature;17 Elemental analyses (C, H, and N) were conducted on a Vario EL III CHNOS elemental analyzer; X-ray powder diffraction (PXRD) data were obtained with a Rigaku Ultima IV-185 diffractometer; UV-vis diffuse re ectance spectra were performed at room temperature with a Varian Cary 5000 UVvisible spectrophotometer; electron paramagnetic resonance (EPR) spectroscopy were recorded at room temperature on a Bruker A300-10/12 electron resonance spectrometer; a ThermoFisher ESCALAB 250 X-ray photoelectron spectrometer by Al Ka radiation (l 1⁄4 8.357 A; spot size, 500 m) was used to perform X-ray photoelectron spectroscopy (XPS)
Summary
As a unique highly ordered and porous material, metal–organic frameworks (MOFs) have become increasingly popular due to their con gurable structure, high surface area and adjustable porosity. Because of the excellent electron acceptability and Lewis acidic sites, viologens are extensively studied in the eld of chromic materials such as photochromism, thermochromism and electrochromism.. Viologens are well known to be photo-reduced accompanied by obvious color changes.. Stable organic radicals obtained have been limited to a few systems with variable color behavior.. The preparation of cross-linked viologen MOFs is reported to be a good way to perform PET and to generate stable free radicals in air, accompanied by color changes.. To construct a stable photochromic system in viologen MOFs has not been fully explored.. To construct a stable photochromic system in viologen MOFs has not been fully explored.16 In this example, we propose a viologen-derived ligand, 1carboxyethyl-4,40-bipyridinine (L 1⁄4 CEbpy) with the terminal oxygen and nitrogen atoms serving as coordination sites, in. It is notable that the photoinduced free radicals of the three MOFs can keep stable in air for months at room temperature
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
