Synthetic chlorophylls for red-light-driven CO2 reduction

Solar energy conversion has been considered as one of the most promising strategies for obtaining renewable chemicals. However, it is difficult to harvest all energy from the sun because sunlight is composed of a series of photons with very different energy/wavelengths (ranging from ~200 to 2500 nm).  
In the past half-century, research in light-driven reactions has been very active in the chemical society. In the early stage, majority of the works were conducted using high-energy photons (UV light). Recently, more and more reactions driven by blue and green portions of sunlight have been realized. However, the development of catalytic systems performing under the irradiation of low-energy light (red and near-infrared) remains a significant challenge. Under AM1.5G, the maximum harvestable photons below 600 nm are less than 20%. Thus, there is increasing enthusiasm in seeking light-driven systems for the utilization of low-energy photons in solar energy conversion.
In photosynthesis, chlorophylls (forms of porphyrin) in plants and bacteria are responsible for harvesting red-light and ultimately drive the reduction of proton and CO2, which are key steps of energy conversion in nature. Although numerous artificial photosynthetic studies have been reported, catalytic systems with high activity and efficiency under red and near-infrared light irradiation are extremely rare. Our group has recently developed a series of highly active red-light-driven systems for hydrogen production, using either simple anthraquinone or CdSe quantum dots as the chromophores. However, none of these systems were able to activate CO2 and reduce it into useful forms. In the literature, there was only one precious-metal-free molecular system demonstrated to reduce CO2 under red-light irradiation, although with turnover number (TON) lower than 1.
   
In this work, we decided to mimic the functionality of chlorophylls by examining a series of structurally relevant porphyrin chromophores for red-light-driven CO2 reduction. We noticed that a porphyrin in its commonly oxidized form (TPP) was a poor chromophore under red-light-driven conditions, giving a TON of 4 in CO2 reduction to CO. We discovered two ways that can significantly improve the activity of the chromophore: (1) introducing fluorine substituents to the backbone of the porphyrin; (2) using a 2e–/2H+ reduced porphyrin–chlorin (Ch) as adapted by most of the chlorophylls in nature. Moreover, we found that more fluorine substituents on both TPP and Ch chromophores further promoted their red-light-driven activity. The perfluorinated chlorin (F20Ch) realized a high turnover number of 1790 with selectivity of CO nearly 100%. Under appropriate conditions, the system lasted over 240 h and stayed active under low concentration (1%) of CO2.
To understand the very different activity between TPP, Ch, and their fluorinated forms, we have performed detailed mechanistic studies to reveal key intermediates and their interconversion during red-light-driven CO2 reduction. For more details on the results, please read our paper: doi.org/10.1038/s41467-024-50084-8
Overall, there are two surprising aspects of our results that will guide the development of chromophores for utilization of low-energy light. First, in contrast to all previous porphyrin chromophores which are in their oxidized forms, the more active ones have to be generated through proton-coupled electron transfer to get to the reduced forms–chlorins and chlorinphlorins. Second, we show that fluorination of chromophore is an effective strategy both in facilitating such conversion and leading to higher red-light-driven activity.