Switching from Oxygen Quenching Resistance to Linear Response by Smart Luminescent Iridium(III)-Based Metal–Organic Frameworks

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.