Photochromic naphthalene diimide Cd-MOFs based on different second dicarboxylic acid ligands

Three novel 3D calcium-based metal–organic frameworks (FJU-67, FJU-68, and FJU-69) established on naphthalene diimide chromophores have been synthesized, which exhibit unique multiple interpenetrated networks with dia net topologies. It is noticeable that the shape and size of the second carboxylic ligands can directly influence the mode of the interpenetrated frameworks. Linear dicarboxylic acid H2BDC or NH2-H2BDC ligands can yield 5-fold interpenetrated frameworks, while 2,6-naphthalenedicarboxylic acid (H2NDC) ligand with a longer length and slight angle can produce a 4-fold (2 + 2) framework. Among the three Cd-MOFs, FJU-67 and FJU-69 with a relatively pale color exhibit reversible photochromism via the radical formation of NDI units and interpenetrating packing has a definite impact on their photochromic behaviors.