Kinetics of Photocatalytic Water Oxidation at Liposomes: Membrane Anchoring Stabilizes the Photosensitizer

A tris­(bipyridine)­ruthenium­(II) photosensitizer (PS) was anchored to the lipid bilayer of liposomes together with Ru-, Co-, or Ir-based water-oxidation catalysts in order to study the effect of liposomes on photocatalytic water oxidation in the presence of Na2S2O8. The Ru-based and Co-based systems both showed O2 production upon light irradiation, whereas the Ir-based system did not. Membrane anchoring caused a large shift in the quantum yield of oxidative quenching of the photosensitizer excited state by peroxodisulfate, which decreased from 180% in homogeneous solution to 7.3% at the surface of liposomes. For the Ru-based system the electron-transfer rate between the photosensitizer PS+ and the water-oxidation catalyst was increased relative to oxidative quenching. Consequently, the rate-limiting step of the photocatalytic water oxidation reaction at liposomes was oxidative quenching, whereas previous work showed that in homogeneous solution it is the reduction of the oxidized photosensitizer PS+ by the catalyst that limits O2 production. Overall, a lower dioxygen production rate was observed when photocatalytic water oxidation occurred at liposomes, but the stability of the liposomal system increased in comparison to that of the homogeneous system. Such stabilization is caused by the decreased concentration of the unstable PS+ species at liposomes, whereas this species accumulates in homogeneous solution, leading to faster degradation. Overall, liposomal water oxidation was found to be more tolerant to changes in light intensity and electron acceptor concentration, which is an interesting property for the building of solar fuel production devices.