Homoleptic Al(III) Photosensitizers for Durable CO2 Photoreduction

Exploiting noble-metal-free systems for high-perform-ance photocatalytic CO2 reduction still presents a key challenge, partially due to the long-standing difficulties in developing potent and durable earth-abundant photosensitizers. Therefore, based on the very cheap aluminum metal, we have deployed a systematic series of homoleptic Al(III) photosensitizers featuring 2-pyridylpyrrolide ligands for CO2 photoreduction. The combined studies of steady-state and time-resolved spectroscopy as well as quantum chemical calculations demonstrate that in anerobic CH3CN solutions at room temperature, visible-light excitation of the Al(III) photosensitizers leads to an efficient population of singlet excited states with nanosecond-scale lifetimes and notable emission quantum yields (10-40%). The results of transient absorption spectroscopy further identified the presence of emissive singlet and unexpectedly nonemissive triplet excited states. More importantly, the introduction of methyl groups at the pyrrolide rings can greatly improve the visible-light absorption, reducing power, and durability of the Al(III) photosensitizers. With triethanolamine, BIH (1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole), and an Fe(II)-quaterpyridine catalyst, the most methylated Al(III) photosensitizer achieves an apparent quantum efficiency of 2.8% at 450 nm for selective (> 99%) CO2-to-CO conversion, which is nearly 28 times that of the unmethylated one (0.1%) under identical conditions. The optimal system realizes a maximum turnover number of 10250 and higher robustness than the systems with Ru(II) and Cu(I) benchmark photosensitizers. Quenching experiments using fluorescence spectroscopy elucidate that the photoinduced electron transfer in the Al(III)-sensitized system follows a reductive quenching pathway. The remarkable tunability and cost efficiency of these Al(III) photosensitizers should allow them as promising components in noble-metal-free systems for solar fuel conversion.

Automated summary

Exploiting noble-metal-free systems for efficient photocatalytic CO2 reduction remains challenging due to the difficulties in developing earth-abundant photosensitizers. This study focuses on a series of homoleptic Al(III) photosensitizers with 2-pyridylpyrrolide ligands for CO2 photoreduction using inexpensive aluminum metal. By introducing methyl groups at the pyrrolide rings, the visible-light absorption, reducing power, and durability of the Al(III) photosensitizers are greatly improved. The optimized system achieves a high quantum efficiency and CO2-to-CO conversion selectivity, making it a promising component in noble-metal-free systems for solar fuel conversion.