Functionalized viologens for novel electron relay systems

Molecular electron relay systems based on viologens substituted with, for example, crown ethers offer the opportunity to combine molecular recognition/complexation with electron transfer. Recent progress in the synthesis of such materials is reported, together with examples of their application as electron‐transfer mediators in biomimetic electron transfer systems, and the controlled reduction of organic compounds such as aromatic aldehydes and α‐ketoesters in organic solvents in which the reducing agent (e.g. Na₂S₂O₄) is usually insoluble.