A Stable Tetraphenylethylene-Based Charge-Assisted Hydrogen-Bonded Organic Framework for Turn-On Fluorescence Sensing of Al³⁺ Ions

The development of stable and sensitive fluorescent sensors for metal ion detection remains a challenge in materials chemistry. Although hydrogen-bonded organic frameworks (HOFs) have shown great potential in luminescent applications, their practical use is often limited by structural instability. In this work, we present a novel charge-assisted HOF, termed FDU-HOF-21 ([H(NH₂Bpy)]₂(TPE)), constructed from a tetraphenylethylene (TPE)-based carboxylic acid ligand (H₄TCPE) and 2,2′-bipyridine-5,5′-diamine (NH₂Bpy). Single-crystal X-ray diffraction (SCXRD) reveals a stable three-dimensional framework stabilized by an extensive hydrogen-bonding network and reinforced by charge-assisted hydrogen bonds (CAHBs), and it exhibits exceptional stability across various solvents and pH conditions. Moreover, FDU-HOF-21 serves as a highly sensitive and selective fluorescent turn-on sensor for Al³⁺ ions, with a lowest limit of detection (LOD) of 1.7 × 10⁻⁶ M. Characterization and time-dependent density functional theory (TDDFT) calculations reveal that the fluorescence enhancement originates from the suppression of non-radiative decay likely due to the reduction in intermolecular charge transfer (Inter-CT) during the emission process, coupled with the restricted intramolecular rotation upon Al³⁺ chelation.