Time controlled structural/packing transformation and tunable luminescence of Cd(II)-chloride-triBZ-ntb coordination assemblies: an experimental and theoretical exploration

In order to explore the supramolecular solid-state structural and packing transformations and the property tuning as a function of reaction time, a tripodal triBZ-ntb (4,4′,4′′-(2,2′,2′′-nitrilotris(methylene)tris(1H-benzo[d]imidazole-2,1-diyl)tris(methylene))tribenzonitrile) ligand was self-assembled with cadmium chloride by applying a hydrothermal method for differing reaction times. Three coordination structures were obtained, namely, [Cd(triBZ-ntb)Cl]2(CdCl4)·2H2O·2DMF (Cd5), [Cd(triBZ-ntb)Cl2]·3H2O (Cd10), and [Cd(triBZ-ntb)Cl]2(CdCl4)·2H2O (Cd20), and characterized by IR, EA, single crystal and powder X-ray diffraction methods. Cd5 and Cd20 have identical coordination monomers, [Cd(triBZ-ntb)Cl]+ and (CdCl4)2− counter anions, with a total metal-to-ligand ratio of 3 : 2, but with slightly different packing states. Meanwhile, the intermediate compound Cd10 is composed of only a neutral coordination unit [Cd(triBZ-ntb)Cl2], with a metal-to-ligand ratio of 1 : 1, by the additional coordination of one Cl− to the Cd(II) metal center. During the structural transformation process, the formation of (CdCl4)2− counter anions in Cd5 and Cd20 serves as an auxiliary “cabinet” for the storage of surplus Cd2+ metal ions. Theoretical study reveals the relative energy changes in the transformation processes. Furthermore, the switchable structures and packing states in these complexes result in tunable luminescent properties.