In this work, we report a new microporous negatively charged metal–organic framework, and its utilization as a platform for the highly selective adsorption and separation of organic dyes through an ion-exchange process.

Four coordination polymers, namely, [Zn(HL1)(L2)]·HO (1), [Cd(HL1)(L2)]·HO (2), [Zn(L1)(L3)]·HO (3), and [Cd(L1)(L3)] (4) (HL1 = (3,5-dicarboxyl-phenyl)-(4-(2'-carboxyl-phenyl)-benzyl)ether, HL2Cl = 1,1'-bis(4-carboxy-benzyl)-4,4'-bipyridinium dichloride, and L3Cl = 1,1'-dimethyl-4,4'-bipyridylium dichloride), have been synthesized hydrothermally. The structures of compounds 1-4 have been determined by single-crystal X-ray diffraction analyses, and further characterized by elemental analyses, infrared (IR) spectra, powder X-ray diffraction (PXRD) analyses, and thermogravimetric analyses. Compounds 1 and 2 display three-dimensional 2-fold interpenetrating frameworks, whereas compounds 3 and 4 exhibit two-dimensional layer structures. These compounds display photochromic behaviors from pale yellow to green under UV light, visible light, or sunlight. The photochromic mechanisms of these compounds have been studied by IR spectra, PXRD analyses, UV-vis absorption spectra, electron paramagnetic resonance spectra, density functional theory calculations, and X-ray photoelectron spectroscopy. The capabilities of compounds 1 and 2 as inkless and erasable printing media have also been tested. Moreover, the photomodulated fluorescence of these compounds has also been investigated.

Solvothermal reaction of multidentate organic ligand, 5,5′,5″,5‴,5],5]′-[1,2,3,4,5,6-phenylhexamethoxyl]hexaisophthalic acid (H12L), with Cd(NO3)·4H2O produced an anionic 3D metal–organic framework [(CH3)2NH2]4[Cd3(H2L)] (1), which features a novel cage-to-cage connection. Interestingly, the 1D channel, which is stacked by cages, is divided into uniform segments by molecular-scale bricks. The luminescent properties of compound 1 have been explored, which shows that 1 is a potential luminescent sensory material for nitrobenzene.

Some viologen-containing compounds displaying thermo/photochromic behaviors which can be used as inkless print media and anti-counterfeiting materials have been reported.