Understanding Structure-Property Relationships in Coordination Polymers: A Comparative Study of Copper(II) and Zinc(II) Coordination mechanism

Abstract

Coordination polymers (CPs) are an interesting class of materials due to their tunable structure and electrical properties, where, however, the correlation between the former and latter are still not fully understood. Here we compare the structure and properties of CPs derived from copper(II) and zinc(II) ions coordinating a triphenylene derivative (OHPTP). To focus on the effect of the coordinating ion used and avoid possible differences due to the processing method, we synthesized the different CPs using a novel two-step technique, potentially scalable for applications in transistors, sensors, and photovoltaics: first, the organic ligand is deposited using a shear-coating technique which ensures uniform deposition on macroscopic scale. Then, in a second step, the sample is exposed to solutions of the metal ions, which can penetrate in the organic layer to coordinate with the ligand. Density functional theory (DFT) calculations show that Cu ions have a higher affinity for the ligand and form square-planar CP structures due to their d9 electronic configuration. Conversely, Zn ions can coordinate with the chelating ligands using only their empty 4s and 4p orbitals to achieve sp3 hybridisation, thus preferring to adopt a tetrahedral geometry and leading to less ordered structures with significantly hampered conductivity. FT-IR and UV-Vis spectra, XPS and conductive atomic force microscopy confirm the distinct coordination behaviour of Cu and Zn ions. Thermal stability analysis further shows that Zn-based CPs retain their structural integrity at temperatures up to 300°C, whereas Cu-based CPs degrade earlier. These results show how metal-ligand interactions impact CP properties, enhancing the understanding of structure-property relationships, and provide practical insights for designing CPs with desired electronic and thermal properties by varying the coordinating metal ions.