Synthesis of ferrocene-grafted poly(p-phenylene-ethynylenes) and control of electrochemical behaviors of their thin films

New ferrocene-coated poly(p-phenylene-ethynylenes) (PPEs) with end capping groups of protected thiol were prepared by a palladium-catalyzed Sonogashira coupling reaction. Ferrocene groups were covalently attached to polymers A and B through ethylene oxide tethers and to polymer C through methylene tethers. Polymers A and B are soluble in common solvents such as tetrahydrofuran (THF), chloroform, methylene chloride, acetone, dimethylformamide (DMF), and dimethyl sulfoxide (DMSO), and polymer C is soluble in toluene, THF, chloroform, and methylene chloride. Polymers A-C display low quantum yield, caused by electron-transfer quenching of ferrocene groups as electron donors. The polymer thin films were prepared through incubation of gold electrodes in THF solutions containing the polymers for 2 days. Ferrocene in thin films of polymers A and B display significantly faster electron-transfer rate than that of polymer C. Hydrophilic ethylene oxide side chains of polymers A and B decrease formal potential of tethered ferrocene groups because of electron-donating effect from ethylene oxide side chains, which stabilizes the ferrocenium ion and leads to a cathodic shift of the redox wave.