Enhanced piezo-photocatalytic water splitting activity via engineering robust dipole moments in covalent organic frameworks

Covalent organic frameworks with tunable optical bandgaps and notable piezoelectricity enable sustainable piezo-photocatalytic production of H₂ and H₂O₂ via pure water splitting. Strong dipole moments can significantly boost piezoelectric properties, necessitating a systematic exploration of their structure-property relationships. Here, we synthesize a series of β-ketoenamine/imine-linked covalent organic frameworks, such as TP-BT-0F/1F/2F-COF, BTA-BT-0F-COF, and TP-SB-COF, through molecular design engineering. The combination of symmetry-breaking benzothiadiazole units and in-plane polarized β-ketoenamine linkages creates a robust dipole moment in TP-BT-0F-COF. This material demonstrates a high piezoelectric coefficient and bandgap narrowing, achieving H₂ and H₂O₂ production rates of 1501.4 and 1435.8 μmol g^-1 h^-1 under co-exposure to ultrasound (60 W, 40 kHz) and visible light. Density functional theory identifies N⁸ and C⁶ sites on benzothiadiazole units as potential catalytic sites for H₂ and H₂O₂ evolution, respectively. Molecular design of covalent organic frameworks with prominent dipole moments advances mechano-optical energy conversion technologies.