Effect of Liposomes on the Kinetics and Mechanism of the Photocatalytic Reduction of Methyl Viologen.

Liposomes are interesting scaffolds for photocatalysis. In particular, charged liposomes were shown to increase the quantum efficiency of photocatalytic reactions involving charged porphyrin photosensitizers and charged electron acceptors. In this work, the effects of adding positively charged liposomes (DMPC/eDMPCCl 1:1) on the mechanism of the photocatalytic reduction of methyl viologen (MV(2+)) by cysteine in the presence of sodium meso-tetra-(4-sulfonato)porphyrinatozinc (Na41) were probed by modeling UV-vis spectroscopy data using a steady-state approximation. By varying the concentration of methyl viologen, we found that the liposomes not only prevent the formation of a 1:1 complex between ground-state photosensitizer 1(4-) and MV(2+) but also that they increase the cage-escape yield in the excited state. Furthermore, the electrostatic repulsion between the liposomes and MV(2+) diminishes by 1 order of magnitude the rate of oxidative quenching of the photosensitizer triplet excited state ((T)1(4-)) by MV(2+). By varying the amount of sacrificial electron donor (cysteine), the effect of liposome addition on the charge recombination reactions could also be studied. Because of the positive charge borne by the photoproduct MV(•+), it was also repelled from the membrane, which significantly slows charge recombination at the surface of the liposome. Overall, compared to a liposome-free solution, the rates of most elementary steps of the photocatalytic reduction of MV(2+) by cysteine are strongly modified when the negative photosensitizer is adsorbed on a positively charged liposome surface. These results not only explain the much higher efficiency of the liposome-containing system but also illustrate the power of supramolecular chemistry for the tuning of photocatalysis.