CCS Chemistry: Iron–Alizarin Complexes for Stable Photocatalytic CO₂ Reduction

First authors: Yiting Wang and Kai Guo
Corresponding author: Prof. Zhiji Han
DOI: 10.31635/ccschem.026.202506925

Research highlight: Prof. Zhiji Han’s team at Sun Yat-sen University developed self-sensitized iron–alizarin molecular photocatalysts that integrate visible-light absorption and CO₂-reduction activity. The system achieved more than 360 hours of stability, a turnover number (TON) of 9930 and 96% selectivity. Li⁺ or Na⁺ spectator cations were used to accelerate formation of a key intermediate and improve catalytic efficiency.

Introduction

In 2026, CCS Chemistry published the team’s latest study on molecular photocatalytic CO₂ reduction. By coupling an earth-abundant iron center with the light-absorbing alizarin ligand, the researchers created a self-sensitized system that combines the photosensitizer and catalytic site within one molecular architecture.

Research Background

Artificial photosynthesis offers a route for converting CO₂ into useful chemicals, but molecular photocatalysts frequently suffer from limited long-term stability. Multicomponent systems and precious-metal catalysts can deliver high activity, yet catalyst degradation and component incompatibility remain obstacles. Designing a self-sensitized molecular catalyst from earth-abundant elements—without sacrificing activity, selectivity or stability—is therefore an important challenge.

Instrumentation Used in the Study

The photocatalytic experiments used the Perfectlight PCX-50C Discover Multi-Channel Photochemical Reaction System. The nine-position LED platform supports parallel catalyst and condition screening with selectable or customized wavelengths. In this study, reactions were performed under a 450 nm blue LED at 100 mW cm⁻².

Figure and Table Analysis

Figure 1. Synthesis, Crystal Structures and Molecular Structures

The iron–alizarin complexes were synthesized using sodium bicarbonate or lithium hydroxide as base, affording L₆Fe₂Li₃(TBA)₃ (1) and L₆Fe₂Na₃(TBA)₃ (2), where L is the alizarin dianion and TBA is tetrabutylammonium. Deep-purple single crystals were obtained by slow diffusion of diisopropyl ether into acetonitrile solutions. The Fe···Fe separation in complex 2 (5.90 Å) was longer than in complex 1 (5.43 Å), consistent with the larger ionic radius of Na⁺ relative to Li⁺.

Table 1. Photophysical Properties and Photocatalytic CO Production

Photocatalysis was evaluated in CO₂-saturated DMF with 2.0 μM iron photocatalyst and 50 mM BIH under 450 nm irradiation at 100 mW cm⁻². CO production, TON and turnover frequency were calculated over 360 hours for complexes 1 and 2 and over 120 hours for the iron–purpurin comparator. Error values were based on three independent experiments. The absence of a pronounced decline in CO-generation rate or TOF demonstrated the long-term stability of the iron–alizarin system.

Figure 2. Photocatalytic CO₂-Reduction Performance

The performance data show that integrating the chromophore and catalytic center within the same iron–alizarin complex can sustain CO evolution over an unusually long irradiation period. The system reached a TON of 9930 and CO selectivity of 96%. Comparison of the Li⁺- and Na⁺-containing complexes further indicates that chemically inert cations can still influence the local coordination environment and the kinetics of key intermediate formation.

Why This Study Matters

The study demonstrates a route to durable molecular CO₂ photoreduction using an earth-abundant metal and an inexpensive organic ligand. Its self-sensitized design reduces the number of independent light-harvesting and catalytic components, while the spectator-cation effect offers an additional handle for controlling reaction kinetics. The combination of long-term stability, high TON and high selectivity provides a useful design framework for future molecular artificial-photosynthesis systems.

Publication Information

Yiting Wang, Kai Guo, et al. “Iron–Alizarin Complexes for Highly Stable Photocatalytic CO₂ Reduction.” CCS Chemistry (2026). https://doi.org/10.31635/ccschem.026.202506925.

Related resources: Photocatalytic CO₂ Reduction Technical Resources · Gas–Solid vs Liquid–Solid CO₂ Photoreduction

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