An investigation into the synthesis of conjugated bipyridinium-containing polymers
1,1'-dimethy1-4,4 1 -bipyridinium dication (methyl viologen) can be reversibly reduced to a blue radical cation, either electrically or photochemically. Other alkyl viologens behave similarly, but aryl viologens are more easily reduced, and this trend increases with the electron-withdrawing capability of the substituents. Polymers containing viologens have been made and shown to give a permanent blue colouration when irradiated, but the viologen units were always isolated electronically. The present work describes attempts made to produce polymers containing viologens linked by wholly conjugated chains, either from the 1- (N-) or 2- positions of the bipyridyl. The only polymer produced was made by the condensation of 2,2'-dimethy1-4,4'-bipyridyl with 1,4- diformylbenzene, and contained viologen units linked by "styryl" groups at the 2- positions. By condensation with other aldehydes, several new viologens were made. In addition, new compounds were made from diamines in which the delocalized system of the viologen was extended over up to six aromatic rings, from the nitrogen atoms. Model experiments were often performed using pyridinium salts rather than bipyridinium salts, and, because of the chemical differences between the two systems, interesting compounds were made with the former but not with the latter. Amongst these was the novel Wittig reaction of a 1-pyridinium ylid with aromatic aldehydes to give exclusively the unexpected Z-(cis)-olefin products, which are new compounds. The feasibility of synthesis of the various envisaged polymer types is discussed in general terms.