First Authors: Siming Wang and Qi Zhang
Corresponding Authors: Jiaming Zhang, Enwei Zhu, Xiaolin Zhu, and Yongfa Zhu
DOI: 10.1002/anie.5791449
1. A linker-nitrogen-engineering strategy was developed to precisely tune charge polarity and electrostatic-potential distribution within COF frameworks by varying the number of nitrogen atoms in the linker.
2. Increasing the nitrogen content substantially strengthens the built-in electric field, lowers the exciton binding energy, and promotes exciton dissociation and photogenerated charge separation.
3. The optimized TAPP-Bpy-COF delivers efficient visible-light-driven organic oxidation, achieving greater than 99% conversion and greater than 99% selectivity within 1 h in both model reactions.
Angewandte Chemie International Edition recently published new research from Professor Yongfa Zhu’s team at the Department of Chemistry, Tsinghua University, on photocatalytic oxidation using covalent organic frameworks (COFs). The study introduces a linker-nitrogen-engineering strategy for regulating the built-in electric field of COFs. By progressively changing the number of nitrogen atoms in the bridging linker while retaining the same porphyrin node, imine-linkage chemistry, and closely related framework structure, the researchers systematically tuned framework charge polarity, electrostatic-potential distribution, and built-in electric-field strength. The enhanced built-in electric field lowers the exciton binding energy, promotes photogenerated charge separation and interfacial O₂ activation, and consequently improves metal-free visible-light-driven organic oxidation.
Photocatalytic organic synthesis offers a green, mild route for using solar energy to produce high-value chemicals. Its performance depends on the efficient coupling of light absorption, excited-state evolution, charge separation, and interfacial reactions. With designable molecular structures, ordered pores, and tunable π-conjugated frameworks, COFs have emerged as an important platform for heterogeneous photocatalysis.
Organic framework materials generally have low dielectric constants, which intensify Coulombic attraction between photogenerated electrons and holes. The resulting high exciton binding energies hinder exciton dissociation, increase charge recombination, and limit interfacial charge utilization. This study introduces a structurally defined, countable, and tunable electronic parameter into a closely related COF platform to precisely regulate the built-in electric field, exciton dissociation, and charge-separation processes.
The study used an earlier-generation Perfectlight trace-gas reaction evaluation system. Its current-generation successor, the μGAS1001 Trace Gas Reaction Evaluation System, is designed for catalytic reactions that generate trace amounts of gas, including photocatalytic overall water splitting and photocatalytic CO₂ reduction. The integrated platform combines a control unit, gas-circulation module, automated sampling and injection module, and reactor module.
Porphyrin units serve as photoactive nodes, while the number of nitrogen atoms in the bridging linker is varied to construct three structurally related COFs: TAPP-BD-COF, TAPP-py-COF, and TAPP-Bpy-COF. As the nitrogen content of the linker increases, the electron-accepting ability and framework polarity rise progressively, favoring built-in electric-field formation, exciton dissociation, and photogenerated charge separation.
The experimental PXRD pattern agrees with the simulated result. FT-IR spectroscopy and solid-state ¹³C NMR confirm successful formation of imine linkages, while N 1s XPS identifies pyridinic, imine, and pyrrolic nitrogen species. HRTEM lattice fringes further demonstrate the ordered layered structure of the COF.
As the number of nitrogen atoms in the linker increases, the molecular dipole moment rises from 1.83 to 4.03 D. Nanoscale SPV and KPFM measurements show that TAPP-Bpy-COF exhibits the strongest surface-photovoltage response and the largest contact-potential-difference shift under illumination, corresponding to a built-in electric field of approximately 368 kV cm⁻¹.
Time-resolved photoluminescence and femtosecond transient-absorption measurements reveal more favorable excited-state behavior in TAPP-Bpy-COF. Temperature-dependent PL combined with Arrhenius fitting gives the lowest exciton binding energy—only 32.6 meV—demonstrating that the stronger built-in electric field weakens electron–hole binding and promotes exciton dissociation.
TAPP-Bpy-COF shows the highest activity for oxidative dehydrogenation and aromatization of 1,2,3,4-tetrahydroquinoline. Cycling and substrate-scope tests demonstrate good stability and broad applicability. The material also efficiently catalyzes the oxidative coupling of benzylamine.
Scavenger experiments, EPR, NBT-probe tests, O₂-TPD, and Gibbs-free-energy calculations identify •O₂⁻ and photogenerated holes as the principal reactive species. Under visible-light irradiation, electrons reduce O₂ to •O₂⁻, while holes oxidize the organic substrate; these processes work cooperatively to drive the oxidation reaction.
The study demonstrates that varying the number of nitrogen atoms in the linker strengthens framework polarization, lowers the exciton binding energy, and promotes photogenerated charge separation and oxygen activation, thereby improving visible-light-driven organic oxidation. Linker-nitrogen regulation therefore provides a powerful design parameter connecting molecular structure, built-in electric fields, and photocatalytic performance.
Yongfa Zhu is a professor and doctoral supervisor in the Department of Chemistry at Tsinghua University and a recipient of the National Science Fund for Distinguished Young Scholars. His research focuses on photocatalysis for energy conversion, environmental photocatalysis, and photocatalytic health applications. Research group website: https://www.yfzhugroup.com/
Siming Wang, Qi Zhang, Chou-Hung Hsueh, Yujia Li, Hang Su, Meichi Chong, Jingyi Xu, Jiaming Zhang, Enwei Zhu, Junshan Li, Xiaolin Zhu, and Yongfa Zhu. “Linker Nitrogen Tunes Charge Polarity to Strengthen Built-In Electric Fields in Covalent Organic Frameworks for Photocatalytic Oxidation.” Angewandte Chemie International Edition, 2026, e5791449.
