Aggregation-Induced Emission-Responsive Metal-Organic Frameworks

Although many studies on luminescent metal-organic frameworks (MOFs) have been reported for chemical sensing applications, it has yet to be realized in MOFs the precise linearity control over photophysical characteristics and sensing sensitivity at the molecular level for a fundamental understanding of the structure-property relationships. Here we demonstrate the first example of aggregation-induced emission (AIE)-responsive MOFs with precise linearity control of photophysics and chemical sensing. We employ a multivariate strategy to tune the number of AIE molecular rotors (dynamic phenyl rings) in a MOF system by varying the ratio of tetraphenylethylene (TPE)-based organic linker, leading to highly tunable photophysical characteristics (e.g., maximum emission peak, quantum yield, and optical band gap) featuring linear correlations with linker content. Importantly, the sensing sensitivity of these dynamic MOFs can be enhanced by increasing the number of AIE molecular rotors with perfect linearity control, as systematically investigated by fluorescence responsive to temperature, viscosity, guest molecular size, as well as theoretical calculations. Our study shows that the sensing sensitivity of the AIE-responsive MOF in this study (termed as NUS-13-100%) is better than those of our previously reported materials. Significantly, the observed linear relationship between emission intensity and molecular weight of polystyrene as the analyte suggests that such AIE-responsive MOFs could be used as molecular sensors for fluorescence-based determination of polymer molecular weight. Eventually, the optical sensing device containing NUS-13-100% shows a perfect linearity response with high sensitivity for the detection of trace toxic benzene vapor. In short, our work paves the way toward porous MOFs containing ALE molecular rotors with a versatile responsive emission mechanism and suitable pore size/geometry for broad applications in chemical sensing and environmental monitoring.