Synthesis, structure and oxygen-sensing properties of Iridium(iii)-containing coordination polymers with different cations

Abstract

Four iridium(III)-containing coordination polymers 1–4 using Ir(ppy)₂(H₂dcbpy)PF₆ (L-H₂, ppy = 2-phenylpyridine, H₂dcbpy = 4,4′-dicarboxy-2,2′-bipyridine) as the bridging ligand, [ZnL₂]·3DMF·5H₂O (1), [CdL₂(H₂O)₂]·3DMF·6H₂O (2), [CoL₂(H₂O)₂]·2DMF·8H₂O (3) and [NiL₂(H₂O)₂]·3DMF·6H₂O (4), have been synthesized and structurally characterized. The emissions from 1–4 are ascribed to a metal-to-ligand charge transfer transition (MLCT). The absolute emission quantum yields for 1–4 in single crystals were measured in air to be 0.274, 0.193, 0.001 and 0.002, respectively. The noteworthy oxygen-sensing properties of 1–4 as well as L-H₂ in a single crystal were also evaluated. The Stern–Volmer quenching constant, K_SV values, of 1–4 and L-H₂ can be deduced to be 0.834, 2.820, 1.328, 1.111 and 2.476, respectively. The results show promising K_SV values (e.g.2) that are competitive or even larger than those of many known Ir-complexes. Moreover, the short response time (e.g. compound 2) and recovery times toward oxygen of 1–4 have been measured in their single crystal forms. The reversibility experiments for 1–4 were carried out for seven repeated cycles. As a result, >75% recovery of intensity for 1 and 2 on each cycle demonstrates a high degree of reproducibility during the sensing process. It should be noted that iridium(III)-containing coordination polymers with high emission intensity and notable oxygen sensing properties are obscure, especially in the single crystal form. This, in combination with its fine reversibility, leads to success in single crystal oxygen recognition based on photoluminescence imaging. The detection limit could be 0.50% for gaseous oxygen. Moreover, the temperature effect of compound 2 in a single crystal upon application as an oxygen sensor was expected.