Metal-Center-Regulated Ratiometric Glutathione Sensing and Proton Conduction in Isostructural Metal–Organic Frameworks

Two isostructural three-dimensional metal–organic frameworks (MOFs), [Co(TTz)(IPA)]·0.5(TTz)·0.5H2O (HC-1) and [Cd(TTz)(IPA)]·0.5(TTz)·H2O (HC-2), were constructed via a dual-ligand strategy using ligands 2,5-bis(pyridin-4-yl)thiazolo[5,4-d]thiazole (TTz) and isophthalic acid (H2IPA). Both MOFs display selective, intrinsic ratiometric fluorescence responses toward glutathione, arising from pronounced emission enhancement accompanied by red-shifted maxima, enabling selective and sensitive GSH detection with limits of detection of 1.28 μM for HC-1 and 0.30 μM for HC-2. Notably, the ratiometric response is achieved within single-component MOFs without the need for composite architectures. In addition, the materials display humidity-dependent proton conduction behavior. HC-1 achieves a high proton conductivity of 1.80 × 10–4 S·cm–1 at 98% relative humidity and 338 K, whereas HC-2 shows a much lower proton conductivity of 5.30 × 10–6 S·cm–1 under identical conditions. Comparative studies reveal that the metal ion plays a decisive role in regulating both fluorescence signaling and proton transport, despite minimal structural perturbation. These findings demonstrate that metal-ion identity serves as a decisive regulator of fluorescence signaling and proton conduction in isostructural MOFs, providing mechanistic insight into the design of multifunctional crystalline materials.