Redox of Viologen for Powering and Coloring

Abstract Viologen is among the most attractive and easiest‐to‐use organic redox active group in many functional molecular assemblies. It plays crucial roles as an electron transfer mediator in the artificial photo‐energy conversion systems and electron‐transfer protein assemblies and as a building block of supramolecules. Its features include electrochemically reversible redox activity and stability. Strong blue color and tendency to dimerization of the one‐electron reduced form, viologen mono‐radical mono‐cation, are remarkable. In this Account, we describe the use of viologen to give a powered movement of small molecules and motion of millimetre‐sized macroscopic soft‐matters and the use of viologen ionic liquid as electrochromic materials. Attractivities of the use of viologen units for powering and coloring are demonstrated and discussed. In particular, we highlight driving of mechanical movements by π–π stacking dimerization, incorporation in a hydrogel to attain highly deformable material, induction of 2D phase transformation, and sharp color change of very thin ionic liquid layer in a compartment‐less electrochromic display.