In Situ Generation of Photosensitizer-Catalyst Pairs on a Lipid Membrane for Efficient CO₂ Photoreduction

Molecular-based photocatalytic CO₂ reduction in liposomes has been attracting renewed interest for emulating the thylakoid membrane in natural photosynthesis. Such reactions typically require an electron donor, a visible-light-absorbing photosensitizer (PS), and a CO₂-reduction catalyst; therefore, maximizing electron transfer (eT) efficiency between the PS and the catalyst is crucial. However, conventional liposome-preparation methods randomly distribute these components within the membrane, limiting control over their intermolecular proximity and therefore eT efficiency. Herein, we report a novel strategy for generating active PS-catalyst pairs in situ on lipid membranes. An Ir-(III)-based PS is first immobilized on the membrane surface, while a water-soluble Re-(I) catalyst precursor bearing triethylammonium substituents is dissolved in the outer aqueous phase. Upon photoirradiation in the presence of ascorbate as an electron donor, the catalyst precursor is reduced by the photogenerated reduced PS species and undergoes Hofmann-type elimination to afford a hydrophobic Re-(I) catalyst. This active species is trapped in the vicinity of the PS, forming PS-catalyst pairs on the membrane surface that drive photocatalysis. Photocatalytic assessments demonstrate that this system produces eight times more CO than previously reported liposomes prepared by a conventional method. The enhanced performance likely arises from the combined effects of efficient eT from the PS to the catalyst and a continuous supply of fresh catalyst from the aqueous phase. This proof of concept establishes a simple and generalizable approach for positioning PSs and catalysts in close proximity on lipid membrane surfaces, offering new opportunities for constructing efficient artificial photosynthetic systems in aqueous media.