Engineering Polar Nanoregions in COFs via Controlled Symmetry Breaking Toward Piezocatalysis.

To address the limited piezocatalytic activity of covalent organic frameworks (COFs) arising from high structural symmetry and rapid charge recombination, an imine-linked COFs (TFB-TAPT) is synthesized by Schiff-base condensation of 1,3,5-triformylbenzene (TFB) and 2,4,6-tri(4-aminophenyl)-1,3,5-triazine (TAPT), followed by partial substitution of amino ligands with 1H,1H-undecafluorohexylamine (UFHA) through a building-block exchange strategy to afford TFB-TAPT-FA. This approach achieves controlled symmetry breaking with precise polar group distribution, without the lattice disruption caused by asymmetric monomers in one-pot methods. The resulting randomly oriented polar nanoregions exhibit relaxation behavior, collectively generating a strong piezoelectric response under external stimuli. Piezoresponse force microscopy (PFM) measurements indicate that TFB-TAPT-FA has a piezoelectric coefficient of 140 pm/V, 2.5 times that of TFB-TAPT. This amplification of the electromechanical response arises from the polar nanoregions, as evidenced by its higher calculated dipole moment (5.68 Debye). Consequently, TFB-TAPT-FA achieves an impressive piezocatalytic hydrogen production rate of 3.8 mmol g-1 h-1, marking a 17-fold improvement over TFB-TAPT. Notably, TFB-TAPT-FA enables efficient hydrogen production without cocatalysts or heterojunctions. More broadly, the introduction of polar nanoregions similarly enhances the performance of other imine-linked COFs, demonstrating a potentially general strategy for designing piezocatalysts through nanoscale polarization engineering in COFs.