Symmetry-Breaking Modulated Zr-Metal–Organic Framework Dimensionality for Gradient Acid Sensing with Dual-Phase Optical Response

The efficient and precise detection of volatile acidic gases and acidic solutions is of significant importance for industrial safety, real-time environmental monitoring, and chemical process control. However, existing metal-organic framework (MOF) sensing materials face bottlenecks such as difficulties in dual-phase response, weak acid discrimination capabilities, and unclear microscopic mechanisms, which severely restrict the rational design and practical application of high-performance sensing materials. In this study, starting from molecular design, a quinoxaline-based asymmetric tricarboxylate ligand was employed to successfully construct a two-dimensional (2D) layered Zr-MOF. The 2D open structure effectively overcomes the limitations of buried active sites and slow mass transport prevalent in traditional three-dimensional MOFs, providing excellent accessibility for substrate molecules. Benefiting from these structural advantages, the material establishes a unique dual-phase/dual-mode sensing platform. In the gas phase, it exhibits a second-level response to strong acid vapors such as HCl and CF3COOH, accompanied by a naked-eye visible white-to-yellow colorimetric change. In the liquid phase, it achieves semi-quantitative gradient discrimination of halogen acid strength through significant differences in fluorescence quenching efficiency (HCl < HBr < HI). DFT calculations revealed the response mechanism at the electronic structure level: the introduction of acidic molecules induces frontier orbital reconstruction of the system. Halide ions not only enhance non-radiative transitions via the heavy atom effect but also open a new "halide-to-ligand" charge transfer pathway. This pathway competes with the inherent ligand-centered luminescence pathways, synergistically modulating the decay modes of excited-state electrons, thereby leading to a gradient fluorescence quenching effect that increases with the halogen atomic number. Furthermore, a flexible Zr-MOF@MF sensing device was developed, which maintained excellent sensing performance and structural integrity over more than 15 reversible cycles. This work provides new insights for developing next-generation intelligent sensing materials that combine precise recognition, clear mechanisms, and practical potential.