Monolayers and Langmuir–Blodgett films of Fe2+-mediated polyelectrolyte with viologen derivatives as linkers at the air–water interface

Monolayer behaviors of newly synthesized viologens (Vs) containing tolunitrile substituents have been investigated at the air–water interfaces. Surface pressure–area isotherms indicated that these viologens could not form stable monolayers on the surfaces of the subphases of pure water, Fe(BF4)2 and anionic polyelectrolyte of PSS (PSS: poly(styrenesulfonic acid-o-maleic) acid), while they could form insoluble monolayers on the surface of the subphase containing mixtures of Fe(BF4)2 and PSS, which was attributed to the formation of Fe–V metal–organic polyelectrolytes based on an interfacial coordinative reaction between the Fe2+ ions and viologen derivatives. With the use of Langmuir–Blodgett (LB) method, monolayers of Fe–V polyelectrolytes were transferred on the substrate surfaces. X-ray photoelectron spectra indicated that the LB film was composed of the elements of S, C, N, O and Fe, suggesting the formation of Fe–V/PSS hybrid multilayers. Cyclic voltammograms of the viologens revealed two reversible redox couples in the solutions, corresponding to two electron transfers of V2+ ↔ V+ and V+ ↔ V0, respectively. For the indium tin oxide electrode covered by the LB films of Fe–V/PSS, only one broad redox couple was recorded in the potential range of 0 and −1.0 V vs Ag/AgCl, which was attributed to the first redox couple of V2+ ↔ V+. The charger transfer process of viologens in the solutions and Fe–V/PSS multilayers was investigated by the potential chronocoulometry method.