Synthesis and properties of electron accepting star-shaped phosphaviologen oligomers

Viologens, with their three stable redox states, are highly valuable organic compounds and currently being sought out as an electroactive component for new battery technologies. In this contribution we report the synthesis and characterization of two multicationic star-shaped species with three or four phosphoryl-bridged viologen units as pendant, electroactive functional groups, respectively. The new, water-soluble species show enhanced electronic features over those of the parent viologens with considerably lowered reduction thresholds. Extensive electrochemical and optical studies confirm that the species are indeed stable in their three electronically accessible states, but also that their large size and the molecular architecture that brings the phosphaviologens into close proximity, give rise to slow diffusion rates in solution, as well as intramolecular interactions that lead to slower electron-transfer processes when compared to the monomeric relatives.