Ultrathin polyelectrolyte multilayers: in situ ESR/UV-Vis-NIR spectroelectrochemical study of charge carriers formed under oxidation

Abstract

Redox cycling of self-assembled polyelectrolyte multilayers, consisting of well-defined water-soluble electronically conducting poly-3-(3′-thienyloxy)propyltriethylammonium (P3TOPA) and insulating PSS (poly(sodium-4-styrenesulfonate)) on indium–tin oxide (ITO) electrode, or poly-3-(3′-thienyloxy)propanesulfonate) (P3TOPS) and insulating poly(diallyldimethylammonium chloride) (PDADMA) on Au electrode, were investigated using in situ ESR/UV-Vis-NIR cyclovoltammetry and modulated electroreflectance. The spectroelectrochemical responses of (P3TOPA/PSS)n multilayers on ITO were compared with the spectroelectrochemistry of P3TOPA dissolved in aqueous or DMSO solutions. We found that both in solution and in the film the radical cationic structures (polarons) were primarily formed upon oxidation in chains of high conjugation length. However, they fully disproportionate into neutral and dicationic segments and the spinless charge carriers dominate, similarly to recent results with well defined long chain oligothiophenes and ladder type indenofluoranthene oligomers. A similar spectroelectrochemical behaviour was observed for the (P3TOPS/PDADMA)n system and its electroreflectance data indicate that the generation of polarons was markedly slower than the production of bipolarons. We compare these new results on well-defined polymer chains with the previous in situ spectroelectrochemical studies of the electrochemically prepared conducting polymer layers (PT, PMeT, PPy, PANI) in order to find a general scheme to describe the redox behaviour of different films with various degree of structure homogenity.