Near-Flat D–A-Typed Triphenylamine-COF toward Efficient Charge Migration and Enhanced Oxygen Activation for Boosting Photoredox Reactions

High energy level difference between electron donor (D) and electron acceptor (A) and low energy loss during charge transfer are prerequisites for the construction of efficient D–A-type covalent-organic framework (COF) photocatalysts. However, the spatial structure characteristics of the building blocks inevitably lead to the torsion of the D–A COF skeleton during the construction process and thus prevent effective charge migration. Herein, with the high-energy-level triphenylamine (TPA) as a model electron donor, the formyl positional isomers of tris(3-formylbiphenyl) amine (NDC) and tri(4-formylbiphenyl) amine (NBC) react with an acceptor 1,3,5-tris(4-aminophenyl) triazine (TTA), producing both D–A COFs for photocatalytic CO2 reduction and benzylamine coupling reactions. Structural investigations evidence that the formyl positional isomerism imparts the dihedral angle of TPA single arm reducing from 51.09 to 2.12°, the interlayer spacing from 3.89 to 3.55 Å, and the exciton binding energy from 97.25 to 85.86 meV in TTA-COFs. Such the “near-flat” TTA-NDC structure with confined clover-like channels dramatically accelerates the intralayer and interlayer charge migration efficiency and improves CO2 adsorption and oxygen activation ability (O2•– concentration of 48.01 μmol·L–1 in 3 h). To the best of our knowledge, TTA-NDC exhibits the highest benzylamine coupling efficiency with a turnover frequency value of 518.4 h–1 among the pristine COF photocatalysts, and its electron consumption rate for photocatalytic CO2 reduction reaches 1632.2 μmol·g–1·h–1, 3.85-fold higher than TTA-NBC, far surpassing most COF-based photocatalysts. Parallelly, the protonated H-TTA-NDC shows high CO2 reduction to CH4 yield (310.1 μmol·g–1·h–1) and selectivity (85.1%) due to the improved proton conductivity (σ = 1.07 × 10–2 S·cm–1). Additionally, the durability, stability, and possible photocatalytic mechanisms of TTA-NDC were systematically investigated.