Angew: Electrostatic-Field-Enabled Ionic COFs for Photocatalytic CO₂ Reduction

First author: Qiang Xu
Corresponding author: Prof. Heng Rao
DOI: 10.1002/anie.1518521

Research highlight: Prof. Heng Rao’s team at Jilin University designed a family of ionic porphyrin covalent organic frameworks (iCOFs) with local electrostatic-field effects. Tetraalkylammonium cationic chains of different lengths were introduced into the COF channels to coordinate photocarrier separation, CO₂ enrichment and stabilization of key reaction intermediates. The best-performing CoTph-3C-N⁺ COF delivered a CO evolution rate of 1006 mmol g_Co⁻¹ h⁻¹, while the combined experimental and theoretical analysis clarified how electrostatic fields promote electron transfer and proton-coupled electron transfer (PCET).

Introduction

In 2026, Angewandte Chemie International Edition published the team’s work on highly efficient photocatalytic CO₂-to-CO conversion using electrostatic-field-enabled ionic porphyrin COFs. The study combines catalyst synthesis, in situ spectroscopy, carrier-dynamics measurements, molecular simulation and density functional theory (DFT) to explain the origin of the enhanced activity.

Research Background

Solar-driven conversion of CO₂ into fuels and value-added chemicals is an important route toward carbon-neutral chemical production. Covalent organic frameworks offer designable structures, ordered channels and useful optical properties, making them promising platforms for photocatalytic CO₂ reduction. Metalloporphyrin COFs are particularly attractive because their M–N₄ motifs provide well-defined catalytic sites.

Two limitations remain common: rapid electron–hole recombination limits charge separation, while insufficient CO₂ adsorption and activation raises the energy required to form the key *COOH intermediate. Enzymes frequently use local electrostatic fields to stabilize intermediates and lower reaction barriers. Inspired by this principle, the researchers introduced tetraalkylammonium side chains into a porphyrin COF through post-synthetic modification, creating a tunable ionic microenvironment with a built-in electrostatic field.

Instrumentation Used in the Study

The experiments used the Perfectlight PCX-50C Discover Multi-Channel Photochemical Reaction System. The platform provides nine parallel LED irradiation positions for catalyst screening, reaction-condition optimization and substrate-scope studies. Selectable and customizable wavelengths span the ultraviolet to near-infrared regions, improving throughput while maintaining comparability among parallel reactions.

Figure-by-Figure Analysis

Figure 1. Construction and Structural Characterization of Ionic COFs

A two-dimensional CoTph-OH COF was assembled from a cobalt-porphyrin unit and 2,5-dihydroxyterephthalaldehyde. Tetraalkylammonium side chains of different lengths were then introduced through Williamson etherification to obtain CoTph-1C-N⁺, CoTph-2C-N⁺ and CoTph-3C-N⁺.

PXRD showed that the crystalline COF framework remained intact after post-functionalization. TEM and HRTEM confirmed a highly ordered structure and good crystallinity. XPS, XANES and EXAFS indicated that the cobalt sites remained highly dispersed in Co–N₄ coordination environments, providing stable active centers for CO₂ reduction.

Figure 2. Electrostatic-Field-Enhanced Photocatalytic CO₂ Reduction

Photocatalytic performance was evaluated in the presence of Ru(bpy)₃²⁺ as photosensitizer and BIH as sacrificial donor. CoTph-OH produced CO at only 2.9 mmol g_COF⁻¹ h⁻¹. Introducing the cationic side chains markedly increased activity, with CoTph-3C-N⁺ reaching 50.3 mmol g_COF⁻¹ h⁻¹. When normalized to cobalt active sites, the initial rate reached the molar scale. The catalyst also maintained stable activity during a continuous 16-hour test.

Figure 3. Improved Charge Separation and Transport

Fluorescence-quenching measurements showed that the ionic COFs captured electrons from excited Ru(bpy)₃²⁺ more effectively. CoTph-3C-N⁺ produced the strongest transient photocurrent and the lowest electrochemical charge-transfer resistance. Femtosecond transient-absorption spectroscopy further showed longer-lived excited-state carriers, providing more electrons for CO₂ reduction. Together, these measurements support the conclusion that the local electrostatic field promotes photocarrier separation and migration.

Figure 4. CO₂ Enrichment and Reaction Kinetics

Molecular-dynamics simulations indicated that longer cationic chains slowed CO₂ diffusion but substantially increased residence time inside the channels. CoTph-3C-N⁺ showed the highest local CO₂ population, increasing the probability that CO₂ would reach Co–N₄ active sites. The electrostatic field therefore affected not only electronic structure but also substrate transport and enrichment.

Figure 5. In Situ Evidence and DFT Mechanism

In situ ATR-FTIR detected a growing band at 1515 cm⁻¹ under illumination, assigned to the key *COOH intermediate. DFT calculations identified *COOH formation as the rate-determining step in each catalyst. The free-energy requirement fell from 1.40 eV on CoTph-OH to 0.77 eV on CoTph-3C-N⁺, while the proton-transfer barrier decreased from 0.71 to 0.52 eV.

Differential charge-density analysis showed that the electrostatic field increased electron density at the cobalt center and promoted electron transfer into *COOH. The field consequently enhances CO₂ photoreduction through three mutually reinforcing effects: faster electron transfer, accelerated proton transfer and stronger intermediate stabilization.

Summary

This work establishes electrostatic-field engineering as a practical strategy for controlling photocatalytic CO₂ reduction in COFs. Post-synthetic introduction of tetraalkylammonium chains synchronized carrier dynamics, CO₂ adsorption and reaction energetics. CoTph-3C-N⁺ achieved a CO evolution rate of 1006 mmol g_Co⁻¹ h⁻¹ and an apparent quantum yield of 5.67%, while maintaining activity for 16 hours.

More broadly, the study provides a mechanistic framework for how a local electrostatic field can promote electron transfer, PCET and intermediate stabilization in an artificial photosynthetic system.

About the Corresponding Author

Heng Rao is a professor at the State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, and a member of the International Center of Future Science at Jilin University. His research focuses on photo- and electrocatalytic CO₂ reduction, water splitting and energy catalysis.

Publication Information

Qiang Xu, Jingwei Han, Hai Sun, et al. “Electrostatic Field Effects in Covalent Organic Frameworks for Photocatalytic CO₂-to-CO Conversion beyond 1000 mmol g_Co⁻¹ h⁻¹.” Angewandte Chemie International Edition (2026), e1518521. https://doi.org/10.1002/anie.1518521.

Related resources: Photocatalytic CO₂ Reduction Technical Resources · Characterization Methods for Photocatalytic CO₂ Reduction

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