In this study, we utilize a photo-active Ir-metalloligand, Ir(C^N)2(L) (C^N = 2-(2,4-difluorophenyl) pyridine, L = [2,2'-bipyridine]-5,5'-dicarboxylic acid), to assemble with CdX2 under hydrothermal conditions, yielding highly emissive crystals of two-dimensional metal-organic frameworks (2D MOFs) (named Ir-Cd2X2, X = Cl, Br). The Ir-Cd2X2 MOFs exhibit μs-level phosphorescence lifetimes and more than 55% quantum yield (QY) at room temperature because of the rigid framework connected by Cd2X2 clusters. By immersing Ir-Cd2X2 in water solution for 5 min, a new MOF (Ir-Cd) was obtained, which is given a structure with hydrolyzed Cd-nodes by complete removal of halogen bridges as elucidated by single-crystal diffraction. Although the phosphorescence emission of pristine CdX2 MOFs exhibits oxygen quenching resistance, the converted Ir-Cd MOF possesses sensitively oxygen-responsive 3MLCT properties, showing a KSV value as high as 14.5 with strictly linear relation (R2 = 0.995). This work differs from the traditional method for improving oxygen-sensing metrics by enhancing QY and phosphorescence lifetime in Ir complexes, while also demonstrating that the transformation in the surrounding coordination environment on adjacent metal centers can also constitute key factors for improved photoluminescence stability or responsive properties in Ir-based heteronuclear MOFs, providing clues for the development of either oxygen quenching blockers or sensors suitable for different occasions.
Keywords: MLCT; MOFs; oxygen sensor; stimulus response; structural transformation.