Cyclic transmembrane charge transport by pyrylium ions in a vesicle-based photocatalytic system

Solar energy is being used for power generation, but also attracts increasing interest as a renewable energy source for the photocatalytic production of useful chemicals. Simple systems based on vesicles with transmembrane redox mediators have been used to transform photon energy into long-lived, membrane-separated photoredox products(1-6). However, these systems are not suitable for high-throughput applications because the transmembrane electron carriers are oxidized inside the vesicle into charged species that are no longer able to readily traverse the membrane bilayer. This leads to continuous trapping of these carriers during photolysis and, ultimately, to the termination of the redox reaction due to accumulation of the available carriers within the vesicle interior. Living cells circumvent this problem by using quinones to simultaneously transport electrons and protons, thus allowing the carrier to remain neutral in its reduced and oxidized states and so retain the ability to undergo transmembrane diffusion throughout the redox cycle. But the incorporation of quinones into artificial systems is not practical because of their susceptibility to oxidative degradation and slow transmembrane diffusion(7). Here we describe an alternative mechanism for rapid electroneutral charge transport across vesicle membranes: we use pyrylium cations as the electron carrier, which undergo reversible ring-opening hydrolysis to form neutral diketones after deposition of the electron inside the vesicle. As the pyrylium cations are also the primary accepters for the photoproduced electrons, our approach greatly simplifies the design of vesicle-based photocatalytic devices.