Dual-Functional Viologen-Based Metal–Organic Framework for Thermochromic and Fluorescence Detection of Fe³⁺, Cr₂O₇^2–, and Tetracycline

The development of dual-functional viologen-based metal-organic frameworks (MOFs) combining thermochromic properties with fluorescence sensing capabilities remains a significant challenge in materials science. This study addresses this gap by designing a novel viologen-incorporated MOF for simultaneous thermochromic behavior and multianalyte detection. We synthesized a viologen-based MOF (CUST-851) through solvothermal assembly of Cd(II) ions with 1,1-bis(4-carboxylbenzyl)-4,4-bipyridine dichloride (H₂bpybcCl₂) and 1,4-naphthalenedicarboxylic acid (1,4-ndc). Structural characterization employed single-crystal X-ray diffraction, thermogravimetric analysis, and powder X-ray diffraction. Electron paramagnetic resonance (EPR) and X-ray photoelectron spectroscopy (XPS) elucidated the thermochromic mechanism. CUST-851 exhibits remarkable reversible thermochromism (yellow ↔ dark blue) mediated by viologen radical formation. The material demonstrates exceptional fluorescence sensing performance, with high sensitivity and selectivity toward Fe³⁺ (LOD = 1.79 × 10^-4 M), Cr₂O₇^2- (LOD = 1.33 × 10^-4 M), and tetracycline (LOD = 1.81 × 10^-5 M), surpassing existing MOF-based sensors. This work establishes a new paradigm for multifunctional smart materials by successfully integrating thermochromic behavior with multianalyte detection in a single viologen-based MOF platform. The findings open promising avenues for environmental monitoring and biomedical sensing applications.