Self-assembled nickel(II)-centered metal-organic square grid complexes for CO₂ sensing

Detection and monitoring of hazardous CO₂ gas are essential for ensuring human health and environmental safety, which form part of Goal 13 of the United Nations Sustainable Development Goals (SDGs). Metal-organic square complexes in thin-film forms are explored in this work for the first time as CO₂ gas sensors. Two new Ni(II) metal-organic square grid complexes [Ni(HL)]₄Cl₄·26H₂O (1) and [Ni(HL)]₄ (BF₄)₄·20H₂O (2) were achieved by self-assembly, utilizing 1,5-bis(2-benzoylpyridine) thiocarbohydrazone (H₂L) as building blocks. The formation of a molecular square [NiHL]₄⁴⁺ unit containing four octahedrally coordinated Ni(II) centers in complex 1 was confirmed by single crystal X-ray diffraction (SCXRD). Complex 2, conversely, yielded single crystals of a new complex, [NiL]₄ (2a), from its DMF and ethanol mixture. The MALDI mass spectral study indicates that the metallosupramolecular square grid units are stable in solution, while the thermogravimetric study reveals the rigidity of the cationic grid structures in their solid states. The solvent accessible pore volume of 1952 ų (≈18.2%) for complex 1 was in accordance with the TG result of 18.1% weight loss up to 272 °C. Hirshfeld surface study was used for void surface analysis. The BET surface areas of complexes 1 and 2 were observed to be 2.390 and 4.803 m² g^-1, respectively. The experimentally observed solid-state band gap energy for complexes 1 and 2 was ∼1.45 eV, revealing semiconductor characteristics. Thin films of both these complexes were then developed using a drop casting process and were employed for CO₂ sensing, as these metallosupramolecular hosts consist of many secondary amine groups. Both complexes 1 and 2 demonstrated sensing capability towards hazardous CO₂ gas, which was found to improve further under white light illumination. The responses to CO₂ gas were found to be 31% and 59% for the sensors engineered using complexes 1 and 2, respectively. The limit of detection for reliable CO₂ gas sensing with complex 2 was found to be 500 ppm. The thin film gas sensor working in the chemiresistive mode using complex 2 outperformed its counterpart when operated as a CO₂ sensor.