Conducting polymers prepared from symmetrically disubstituted bithiophenes - electrochemical and spectroelectrochemical behaviour

Abstract

The influence of side group distribution on the electrochemical and spectroelectrochemical properties of substituted polythiophenes has been studied. Two polymers were selected, whose chain geometry corresponds to head to head and tail to tail coupled 3-substituted thiophene rings, namely poly(4,4′-dibutyl-2,2′-bithiophene) (PBBT), and poly(3,3′-dimethoxy-2,2′-bithiophene) (PMBT). In cyclic voltammetry both polymers exhibit much narrower oxidation peaks as compared to the corresponding head to tail coupled polymers poly(3-alkyl-) and poly(3-alkoxy-thiophenes). In disubstituted poly(bithiophenes) the redox couples are less reversible. Quasi-static spectroelectrochemical experiments show that the polymer can be oxidized at significantly lower potentials than during voltammetric oxidation. All these observations indicate that in both polymers the oxidation is retarded. For PBBT this behaviour can be rationalized on the basis of spectroelectrochemical measurements. Although its π→π ∗ transition is blue shifted due to significant torsion of adjacent thiophene rings, the polaronic bands grow in the same spectral range as in poly(3-n-butyl-thiophene). Thus oxidative doping of PBBT must be preceded by structural rearrangements toward higher chain planarity.