Redox of Viologen for Powering and Coloring

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.

π-Donor/π-Acceptor Interactions for the Encapsulation of Neurotransmitters on Functionalized Polysilicon-Based Microparticles

Bipyridinium salts, commonly known as viologens, are π-acceptor molecules that strongly interact with π-donor compounds, such as porphyrins or amino acids, leading their self-assembling. These properties have promoted us to functionalize polysilicon microparticles with bipyridinium salts for the encapsulation and release of π-donor compounds such as catecholamines and indolamines. In this work, the synthesis and characterization of four gemini-type amphiphilic bipyridinium salts (1·4PF₆–4·4PF₆), and their immobilization either non-covalently or covalently on polysilicon surfaces and microparticles have been achieved. More importantly, they act as hosts for the subsequent incorporation of π-donor neurotransmitters such as dopamine, serotonin, adrenaline or noradrenaline. Ultraviolet-visible absorption and fluorescence spectroscopies and high-performance liquid chromatography were used to detect the formation of the complex in solution. The immobilization of bipyridinium salts and neurotransmitter incorporation on polysilicon surfaces was corroborated by contact angle measurements. The reduction in the bipyridinium moiety and the subsequent release of the neurotransmitter was achieved using ascorbic acid, or Vitamin C, as a triggering agent. Quantification of neurotransmitter encapsulated and released from the microparticles was performed using high-performance liquid chromatography. The cytotoxicity and genotoxicity studies of the bipyridinium salt 1·4PF₆, which was selected for the non-covalent functionalization of the microparticles, demonstrated its low toxicity in the mouse fibroblast cell line (3T3/NIH), the human liver carcinoma cell line (HepG2) and the human epithelial colorectal adenocarcinoma cell line (Caco-2).

Synthesis and thermotropic properties of new green electrochromic ionic liquid crystals

New liquid crystal viologens, which exhibit green electrochromic behavior in the LC phase, rarely observed for small molecules, have been developed.