Two Zn(II) complexes based on bis(benzimidazole) and different dicarboxylate anions: synthesis, structures, and fluorescent properties

Two new zinc(II) complexes, [Zn2(PBM)2(terephthalate)(formate)2] (1) and {[Zn(PBM)(fumarate)]center dot H2O}n (2), were synthesized using 1,3-bis(1H-benzo[d]imidazol-2-yl)propane (PBM) and two different dicarboxylic acids under solvothermal conditions. Complexes 1 and 2 have been characterized by infrared spectroscopy, elemental analysis, and single-crystal X-ray diffraction analysis. The structural analysis showed that 1 was a binuclear structure and then formed a 3D supramolecular network structure through hydrogen bond and pi-pi conjugation, while 2 was a one-dimensional coordination polymer. In 1 and 2, the zinc ions were four-coordinate with an N2O2 donor set and a slightly distorted tetrahedral geometry. The solid-state fluorescence properties indicate that the fluorescence peaks belong to the pi*-pi transition of the ligand PBM, which has a significant red shift and enhancement with the order being 1 > 2 > PBM. The stronger fluorescence of the complexes than the ligand is due to the immobilization of the ligand through coordination, which reduces the energy loss of thermal vibration. The pi center dot center dot center dot pi conjugation in 1 results in a stronger fluorescence of 1 than 2. Therefore, coordination and pi center dot center dot center dot pi conjugation will affect the fluorescence of complexes.