Retrieving the Stability and Practical Performance of Activation-Unstable Mesoporous Zr(IV)-MOF for Highly Efficient Self-Calibrating Acidity Sensing

Abstract

The practical applications of activation-unstable mesoporous metal-organic frameworks (MOFs) are often constrained by their structural instability. However, enhancing their stability could unlock valuable functionalities. Herein, we stabilized the otherwise unstable, post-activated structure of a novel mesoporous Zr(IV)-MOF, NKM-809, which uses a pyridine-containing amphiprotic linker (PPTB). We applied two strategies: mixed-linker synthesis and linker installation. In the mixed-linker approach, we incorporated an auxiliary linker, TPTB, which resembles PPTB, during synthesis to improve the framework's stability. In the linker installation approach, we introduced a ditopic carboxylate linker (BPDC) into the coordination-unsaturated sites of NKM-809. These strategies produced stabilized derivatives, named NKM-808.X (X = χPPTB) and NKM-809-BPDC, which exhibit pH-responsive dual-wavelength fluorescence at distinct emission wavelengths. Remarkably, these emissions shift oppositely upon protonation and dissociation, distinguishing them as highly sensitive, self-calibrating acidity sensors. In NKM-809-BPDC, an additional quenching of the linker-emission (419 nm) minimizes inherent interference, enabling integrated quality and lifespan self-monitoring. Theoretical calculations identified transitions between (n, π*) and (π, π*) emission states during the sensing process and highlighted the role of a stable mesoporous network in achieving stronger protonation response. These findings showcase the potential of stabilized mesoporous MOFs for practical applications, alongside valuable insights into strategies for optimizing such materials.