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] (<b>1</b>) and {[Zn(PBM)(fumarate)]∙H₂O}_n (<b>2</b>), were synthesized using 1,3-bis(1H-benzo[d]imidazol-2-yl)propane (PBM) and two different dicarboxylic acids under solvothermal conditions. Complexes <b>1</b> and <b>2</b> have been characterized by infrared spectroscopy, elemental analysis, and single-crystal X-ray diffraction analysis. The structural analysis showed that <b>1</b> was a binuclear structure and then formed a 3D supramolecular network structure through hydrogen bond and π-π conjugation, while <b>2</b> was a one-dimensional coordination polymer. In <b>1</b> and <b>2</b>, 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 π*-π transition of the ligand PBM, which has a significant red shift and enhancement with the order being <b>1 </b>&gt;<b> 2 </b>&gt;<b> </b>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 π∙∙∙π conjugation in <b>1</b> results in a stronger fluorescence of <b>1</b> than <b>2</b>. Therefore, coordination and π∙∙∙π conjugation will affect the fluorescence of complexes.