The design and synthesis of crystalline porous framework materials, such as covalent organic frameworks (COFs), metal-organic frameworks (MOFs), and hydrogen-bonded organic frameworks (HOFs), are of significant interest due to their unique properties and diverse applications. COFs are built upon strong covalent bonds, offering high stability. MOFs utilize coordination bonds between metal nodes and organic linkers, but often suffer from low toughness. HOFs, stabilized by hydrogen bonds, offer advantages like mild synthesis conditions and solvent processability. While N−H and O−H hydrogen bonds are commonly used, halogen bonds, particularly the D…X…D type, represent an underexplored area. Halogen atoms, being highly electronegative, can act as connection nodes, potentially creating frameworks with unique properties. The choice of organic building blocks also plays a crucial role; suitable ligands can provide enhanced light absorption and charge carrier mobility. Singlet open-shell diradicals, such as those found in quinoid structures, are attractive due to their interesting electronic properties and potential applications in photocatalysis. However, their instability poses a challenge. Uranium contamination is a significant environmental concern due to its toxicity and radioactivity, and efficient removal methods are needed. This research explores the synthesis and application of a novel halogen hydrogen-bonded organic framework material for uranium immobilization.

The existing literature extensively covers COFs, MOFs, and HOFs, highlighting their unique structural features and applications. COFs, with their covalent bonding, demonstrate high stability and are explored for various applications, including gas storage and catalysis (references 3-5). MOFs, characterized by metal-organic coordination bonds, offer tunable pore structures but can exhibit low toughness (references 6-8). HOFs, relying on hydrogen bonding, are synthesized under milder conditions and show potential for diverse applications (references 9-12). However, the exploration of halogen-bonded frameworks, especially those using the D…X…D type of hydrogen bonds, is limited (references 13-14). The literature also discusses the use of organic building blocks to tailor the properties of framework materials (references 22-25) including the potential of open-shell diradicals in photocatalysis (references 26-35). The need for efficient uranium removal techniques has driven research into various materials and strategies (references 37-46), including those employing photoreduction (references 53-56).

The research involved the synthesis of 7,7,8,8-tetraaminoquinodimethane (TAQ) ligand and a novel halogen hydrogen-bonded organic framework (XHOF-TAQ). Terephthalamide oxime (TPAO) was first synthesized via oximation of terephthalonitrile (TPN). XHOF-TAQ was then prepared solvothermally using TPAO and CuCl₂·2H₂O. The characterization of XHOF-TAQ involved various techniques, including Fourier Transform Infrared (FTIR) spectroscopy, Mass Spectrometry (MS), Energy-Dispersive X-ray Spectroscopy (EDS) coupled with Scanning Electron Microscopy (SEM), ¹H Nuclear Magnetic Resonance (NMR), Single-Crystal X-ray Diffraction (SC-XRD), X-ray Photoelectron Spectroscopy (XPS), Powder X-ray Diffraction (PXRD), Thermogravimetric Analysis (TGA), and Electron Paramagnetic Resonance (EPR). Density Functional Theory (DFT) calculations were employed to study the electronic structure of TAQ. The photoreduction ability of XHOF-TAQ towards U(VI) was evaluated by monitoring the decrease in U(VI) concentration under light irradiation using Inductively Coupled Plasma Mass Spectrometry (ICP-MS). The influence of various factors, including XHOF-TAQ dosage, coexisting anions and cations, and mixed interfering metal ions, on the photoreduction efficiency were investigated. UV-Visible diffuse reflectance spectroscopy (DRS) was used to determine the band gap of XHOF-TAQ.

The study successfully synthesized a new type of crystalline framework material, XHOF-TAQ, using Cl⁻ ions as connecting nodes and TAQ as organic ligands. Structural analysis revealed a 3D structure where each Cl⁻ ion forms three hydrogen bonds with three TAQ ligands. XHOF-TAQ exhibited high stability in various solvents and excellent air stability, which is attributed to the strong hydrogen bonds and the electron sharing between Cl⁻ and TAQ. The material displayed a high capacity for U(VI) photoreduction (1708 mg-U g⁻¹-material) with a fast immobilization speed (34.16 mg g⁻¹ min⁻¹). XPS analysis confirmed the reduction of U(VI) to U(IV) during the photocatalysis. EPR spectroscopy and DFT calculations demonstrated the formation of a singlet diradical state in TAQ under light irradiation, providing excited electrons for the photoreduction process. The band gap of XHOF-TAQ was determined to be 2.43 eV, consistent with DFT calculations. The photoreduction efficiency was optimized by adjusting the XHOF-TAQ dosage and was found to be influenced by coexisting anions and cations. Even in the presence of interfering metal ions, XHOF-TAQ maintained significant photoreduction efficiency for uranium.

The findings demonstrate the successful synthesis and application of a novel halogen hydrogen-bonded organic framework (XHOF-TAQ) for the efficient photoreduction of U(VI). The high uranium immobilization capacity and fast reaction rate of XHOF-TAQ offer a promising solution for uranium remediation. The unique structural features of XHOF-TAQ, including the use of Cl⁻ as a connecting node and the presence of a singlet open-shell diradical in TAQ, contribute to its high performance. The study expands the family of crystalline framework materials and opens up new avenues for designing highly efficient photocatalysts for environmental applications. The selectivity shown by XHOF-TAQ towards uranium, even in the presence of other metal ions, is particularly significant. This work highlights the potential of combining unique bonding motifs with suitable organic ligands to create materials with tailored properties for specific applications.

This research successfully synthesized a novel halogen hydrogen-bonded organic framework (XHOF-TAQ) that exhibits high efficiency in photoreducing soluble U(VI) to insoluble U(IV). The material’s stability, high uranium immobilization capacity, and selectivity make it a promising candidate for environmental remediation. The successful integration of a chemically unstable singlet open-shell diradical into a stable framework through a solvothermal synthesis process is a significant advancement in materials science. Future research could explore the use of other halogen ions and organic ligands to further optimize the performance of XHOF materials and investigate their applicability in different environmental scenarios.

The study primarily focused on the photocatalytic reduction of U(VI) in a model system using DMF as a solvent. Further research is needed to evaluate the performance of XHOF-TAQ in real-world environmental samples with complex matrices and varying conditions. The long-term stability of XHOF-TAQ under continuous irradiation and its scalability for industrial applications also warrant further investigation.