Constructing asymmetric D'-A-D" imine covalent organic frameworks to optimize exciton effect and enhance photocatalytic reduction of U(VI)

Abstract

Imine-based covalent organic frameworks (COFs) exhibit high exciton binding energies due to their weaker electronic delocalization. Herein, we report an asymmetric ternary imine COF (NCOF-3) bridged by D_{2 h} and C_{2 h} units, which is synthesized via an aldol-type condensation reaction mediated by amine monomers. Compared to symmetric NCOF-1 and NCOF-2, the asymmetric architecture of NCOF-3 maximizes interlayer interactions, leading to enhance crystallinity. Moreover, by incorporating two electron-donating units into the triazine framework, an asymmetric D'-A-D'' topological structure is formed, which facilitates the forward dissociation of photogenerated excitons, achieving a high carrier separation efficiency. Temperature-dependent fluorescence spectroscopy reveals that NCOF-3 possesses a smaller exciton binding energy (40.15 meV) compared to those of symmetric imine-based COFs. Under illumination conditions, NCOF-3 exhibits uranium removal capacity of 1084 mg g^-1, significantly surpassing those of binary COFs containing imine bonds. Additionally, anti-interference experiments demonstrate that NCOF-3 maintains a uranium removal efficiency of over 90 % even in the presence of excess competing metal ions, confirming its high selectivity. Density functional theory (DFT) calculations indicate that lattice dipole asymmetry plays crucial roles in reducing the electronic bandgap of NCOF-3. This work highlights the importance of designing and fabricating imine-based COFs with dipole asymmetry of the connecting components to effectively manipulate the internal charge distribution, ultimately enhancing photocatalytic activity.

Keywords: Asymmetric; Covalent organic frameworks; Photocatalysis; Tri-component; Uranium.