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 reduction of organic compounds such as aromatic aldehydes and alpha-ketoesters in organic solvents in which the reducing agent (e.g. Na2S2O4) is usually insoluble.