Structure-directing effect of terephthalate in bridging Zn(ii)- and Cd(ii)-based coordination polymers towards application in the detection of trace quantities of Pd2+ in aqueous media and their electrical conductivities

Abstract

Energy crisis and environmental pollution are two central themes of contemporary research towards achieving sustainable development goals (SDGs). Material chemistry is the chief discipline that can resolve glitches in these areas through the appropriate design of chemical compounds with multifunctional properties. In this regard, two stable coordination polymers (CPs) were synthesised in this work using Zn(II) (3d¹⁰) and Cd(II) (d¹⁰) metal nodes with 1,4-benzenedicarboxylate (bdc²⁻) as the bridging ligand and monodentate pyridyl-N coordinated 9H-fluoren-2-yl-pyridin-4-ylmethylene-amine (flpy) as the fluorogenic partner. The structures of the polymers [Zn₂(bdc)₄(flpy)₂]_n (CP1) and [Cd(bdc)₂(flpy)₂(H₂O)]_n·(flpy) (CP2) were confirmed via single-crystal X-ray diffraction measurements. In CP1, the paddle-wheel coordination unit [Zn₂(bdc)₄] was propagated to constitute a 2D polymer, while in CP2, the capped octahedron motif CdN₂O₅ generated a 1D chain. Both CP1 and CP2 were strongly emissive, and the emission could be quenched selectively by Pd²⁺ in aqueous solutions in the presence of as many as twenty other metal ions. Pd(II) is the most toxic in its three oxidation states of 0, II, and IV, and the limit of detection of Pd²⁺(aq) was 79.1 nM (CP1) and 89.2 nM (CP2), which were much below the toxicity limit of Pd²⁺ recommended by WHO (the tolerance limit of Pd²⁺ in water is 3.97–46.98 μM). Based on the Tauc plots of the ITO/(CP1 or CP2)/Al thin films, the bandgaps were determined as 3.63 eV for CP1 (theoretical value = 3.28 eV) and 3.55 eV for CP2 (theoretical value = 3.21 eV). Moreover, the electrical conductivity values of the Schottky semiconducting devices fabricated using these polymers at ambient conditions were 1.285 × 10⁻⁴ (CP1) and 2.399 × 10⁻⁴ S m⁻¹ (CP2). Therefore, the application of these two CPs can accomplish sustainability goals for future generations.