Local Environmental Effects on Light-Driven CO2 Reduction in Liposomes

We report the governing principles that regulate the activity of light-driven CO2 reduction by a molecular photosensitizer bis(2,2′-bipyridine)-(4,4′-dinonyl-2,2′-bipyridine)-ruthenium(II) (RuC9) and a molecular catalyst (5,10,15,20-tetra(4-methylphenyl)porphinato)cobalt(II) (CoTTP) in supramolecular assembly within the lipid bilayers of liposomes suspended in water. We tested six different lipids with membranes in either the gel phase, fluid phase, or at the transition between both states, as well as zwitterionic or negatively charged headgroups. The correlation of the membrane rigidity with light-driven catalysis performance is not conclusive for the investigated set of lipid membranes, but molecular dynamics simulations elucidate how catalyst efficiency increases with the distance from the membrane center as well as their calculated vertical reduction energies. Luminescence quenching studies revealed that mainly dynamic quenching was observed with the highest quenching efficiency found with 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and 1,2-dipalmitoyl-sn-glycero-3-phospho-(1′-rac-glycerol)(sodium salt) (DPPG)-based liposomes, in agreement with the results of the best performance in photocatalysis and the computational insights. A variation of cations did not show any significant influence on the performance, as opposed to electrochemical studies. The overall mechanistic findings of this study provide design principles for light-driven CO2 reduction by molecular components in liposomes.