Multifunctional Humidity and Chemosensitive Thiazolothiazole-Based Metal-Organic Framework Sensor

Thiazolo[5,4-d]thiazole (TTz) is a rigid, planar, π-conjugated heterocycle that has been widely used as a versatile sensing unit. To exploit the properties of the TTz unit within a metal-organic framework, we synthesized an electroactive Cd-MOF derived from thiazolo[5,4-d]thiazole, [Cd₂(DPTTZ)₂(BDC)₂]·DMF, where BDC = 1,4-benzenedicarboxylate, DPTTZ = 2,5-di(pyridine-4-yl)thiazolo[5,4-d]thiazole, and DMF = N,N-dimethylformamide, via a mixed-ligand solvothermal strategy. Single-crystal analysis reveals a three-dimensional framework crystallizing in the orthorhombic system with space group Iba2. We investigated the sensitivity and electrical conductivity (EC) of the synthesized metal-organic framework incorporating the DPTTZ ligand under exposure to guest molecules such as water and other volatile solvents. These guests can modify MOF conductivity through interactions with ligands and metal centers or via their redox properties. The MOF was processed as an electrode-supported sensing layer and evaluated by complex impedance spectroscopy under controlled relative humidity (RH) cycling. The material's sensing performance at a 3 kHz operating frequency exhibits a broad dynamic sensing range, high sensitivity to RH variations (11-95%), rapid response and recovery times (under 10 and 7 s for RH 33% and under 25 and 20 s for RH 95%), minimal hysteresis, and a high coefficient of determination (R² = 0.97), which confirms an excellent linear correlation and suggests the sensor delivers reliable, predictable performance over the measured humidity range. Mechanistic analysis attributes the impedance changes to stepwise water uptake into pore channels and interaction with polar functional sites, which enhance ionic/protonic conduction pathways. These results demonstrate that electroactive TTz-derived linkers can impart mixed-conducting behavior in MOFs and provide a viable platform for impedance-based humidity and chemosensitive sensors amenable to on-chip integration and field deployment.