Mechanistic Insights on Charge Transfer Dynamics of Photocatalytic Water Oxidation at the Lipid Bilayer-Water Interface

Photosystem II is a large protein complex embedded in a phospholipid membrane. Photocatalytic water oxidation in liposomes using amphiphilic ruthenium(II)-trisbipyridine photosensitizer (PS) and amphiphilic 6,6’-dicarboxylato-2,2’-bipyridine-ruthenium(II) catalysts (Cat) with a water-soluble sacrificial electron acceptor (Na₂S₂O₈) forms much simpler system, but the effect of embedding photocatalytic system in liposome membranes on the mechanism of photocatalytic water oxidation was not well understood. Several phenomena have been identified by spectroscopy tool, which gives quite different results from those of the corresponding homogeneous system. First, the oxidative quenching of photoexcited *PS by S₂O₈^2- at the liposome surface occurs solely via static quenching, while dynamic quenching is observed for the homogeneous system. Moreover, the charge separation efficiency after the quenching reaction is much smaller than unity, in contrast to the quantitative generation of PS⁺ in homogeneous solution. In parallel, the high local concentration of the membrane-bound PS induces self-quenching at 10:1 – 40:1 molar lipid–PS ratios. Finally, while the hole transfer from PS⁺ to catalyst is rather fast in homogeneous solution (k = 3.2 ×10⁸ M^-1 s^-1), in liposomes at pH = 4 the reaction is rather slow (k_obs » 17 s^-1  for 5 mM catalyst in 100 mM DMPC lipid). Overall, understanding these productive and unproductive pathways explains why photocatalytic water oxidation is limited by different factors in homogeneous and liposomal systems, and allows rational optimization that might be of general importance for light-driven catalysis within self-assembled lipid interfaces.