Structures and photoluminescence properties of two cadmium(II) coordination polymers synthesized by flexible and rigid ligands

Two new Cd(II) coordination polymers, [Cd(BDC)(m-bitmb)(H2O)] (1) and [Cd2(BDC)2(p-bitmb)1.5(H2O)2] (2), have been solvothermally synthesized by reacting cadmium sulfate and 1,4-benzenedicarboxylic acid (H2BDC) with flexible ligands 1,3-bis(1-imidazol-1-ylmethyl)-2,4,6-trimethylbenzene (m-bitmb) or 1,4-bis(1-imidazol-1-ylmethyl)-2,3,5,6-tetramethylbenzene (p-bitmb), respectively. Compound 1 shows an uncommon 3D 4-connected 66 topology, a distorted diamond structure, while compound 2 is a novel 2D (3,4)-connected W-shape-like layer framework with a novel (63·66) topology. 1 and 2 have different fluorescence and thermal properties. Compound 1 shows a ligand based fluorescence, while compound 2 shows a red-shifted emission compared to the ligands and is assigned to the ligand-to-ligand charge transfer (LLCT) emission. These results demonstrated the structure–property relationship of MOF material.