Metal(II) Complexes Based on Imidazo[4, 5-f]-1, 10-phenanthroline and Bridging Dicarboxylato Ligands: Synthesis, Characterization and Photoluminescence†

Eight metal(II) complexes based on imidazo[4, 5-f]-1, 10-phenanthroline (HIMP) and bridging dicarboxylato ligands such as 4, 4′-biphenyldicarboxylic acid (H2BPDC), 1, 4-benzenedicarboxylic acid (H2BDC), thiophene-2, 5-dicarboxylic acid (H2TDC), and 2, 6-naphthalenedicarboxylic acid (H2NDC) were hydrothermally synthesized and structurally characterized by single-crystal X-ray diffraction. Complexes 1, 3, 6, and 7 are molecular dinuclear metal complexes. Complexes 2, 4, and 5 exhibit chain-like structures. Compound 8 shows a novel 3D architecture, in which ZnII dimers are connected by four NDC2– anions. In the metal(II) complexes, HIMP exhibits a similar chelating coordination mode. Different π···π stacking interactions are observed in the complexes. The emission of HIMP is completely quenched in complexes 1–4 due to the strong π···π stacking interactions in the structures. Complexes 5–8 exhibit different photoluminescence properties. Firstly, we quantitatively investigated the effect of the strong HIMP–HIMP stacking interactions on the emission quenching of HIMP in the metal complexes. It was found that a higher extent of π···π stacking interactions in the complexes resulted in a higher extent of the emission quenching of HIMP. The introduction of aromatic conjugated carboxylate groups into metal(II)-HIMP complexes changed the extent of the strong π···π stacking interactions in the structures and thus the photoluminescence properties of the complexes.