Molecular photo-charge-separators enabling single-pigment-driven multi-electron transfer and storage leading to H₂ evolution from water

Single-chromophore-driven single-electron-pumping processes leading to multi-electron transfer and storage are effectively promoted in natural photosynthesis, generating photocurrent at the molecular level. Moreover, these single-electron-pumping events are converted into double-electron-pumping events by releasing multiple plastoquinol molecules without releasing reactive semiquinone radicals, thereby enabling storage of two-electron-reduced molecules within the lipid bilayer constructing the thylakoid membrane. Here we report new unimolecular architectures that enable these highly sophisticated light-driven multi-electron transfer and storage processes. The photo-charge-separators (PCSs) reported herein possess a single Ru(bpy)₃ ²⁺ fragment with each bpy derivatized with four dicationic viologen acceptors, abbreviated as [Ru^II(bpy)₃ ²⁺-(MV²⁺)₁₂]²⁶⁺ (MV²⁺ is a viologen unit). These highly positively charged PCSs form stable ion pairs with anionic electron donors, enabling consecutive multi-electron transfer processes from the donors to the pendant viologen acceptors. The multiple transferred electrons are collected over twelve pendant viologen acceptors, leading to storage of 7-8 electrons per molecule. The resulting organic radicals show a strong preference to form diamagnetic π-dimers, which suppress reactive radical formation. These reducing equivalents can then be efficiently consumed in catalytic H₂ evolution from water in the presence of a colloidal platinum catalyst.