Four new lead(ii)–iridium(iii) heterobimetallic coordination frameworks: synthesis, structures, luminescence and oxygen-sensing properties

Abstract

Four new lead(II)–iridium(III) heterobimetallic coordination frameworks, i.e., [Pb₂(L)₄(DMF)₂]·ClO₄·2DMF·13H₂O (1·DMF), [Pb(L)₂·H₂O]·ClO₄·3C₃H₆O·3H₂O (1·Acetone), [Pb(L)₂·H₂O]·3C₃H₆O·3H₂O·CH₃CN (2·Acetone), and [Pb₄(L)₄I₄·(DMF)₂]·10H₂O (3·DMF), where is L-H₂ = Ir(ppy)₂(H₂dcbpy)PF₆, ppy = 2-phenylpyridine, and H₂dcbpy = 4,4′-dicarboxy-2,2′-bipyridine, have been synthesized and structurally characterized using elemental analyses, IR spectroscopy, optical spectroscopy, and single-crystal X-ray diffraction. Heavy atoms, i.e. lead(II) atoms, were introduced to the frameworks and coordinated with the chromophore to promote an efficient intersystem crossing from the singlet to the triplet ³MLCT excited state and further promote sensitivity to oxygen. The emissions of 1·DMF, 1·Acetone, 2·Acetone and 3·DMF (referred to hereafter as 1·DMF–3·DMF) were ascribed to metal-to-ligand charge transfer transitions (MLCTs). These four compounds, using phosphorescence based detection, were able to detect oxygen sensitively in real gas, and their noteworthy oxygen-sensing properties were also evaluated. The quenching constants, or K_SV values, between 1·DMF–3·DMF and oxygen, can be deduced to be 1.44, 1.60, 2.85, and 5.11, respectively. The minimum detectable concentrations (LODs) of oxygen for 1·DMF–3·DMF were calculated to be 0.65%, 0.70%, 0.61%, and 0.52%, respectively, from three times the signal to noise ratio. Moreover, taking 3·DMF as an example, short response (50 s) and recovery times (24 s) toward oxygen have been measured. It should be noted that the evaluated recovery time for 3·DMF is even shorter than those reported for rare-earth MOF films. The gas sensing properties, including sensitivity, sensing linearity, reproducibility, matrix effect, cross-sensitivity effect, temperature effect, and long-term stability, were investigated. Finally, in order to spark a broad spectrum of interest in terms of an application, 3·DMF was mixed with a commercial dye, Coumarin 480 (C480), to form a ratiometric oxygen sensor for the analysis of real gas in air.