Structural control of conjugation in functionalized polythiophenes

Abstract

A comparative analysis of the electrochemical and optical properties of 3-substituted poly- (thiophenes) has been carried out on four polymer structures, poly(3-methylthiophene) (PMT), poly(3- isoamylthiophene) (PiAT), poly(3-nonylthiophene) (PNT), and poly(3-(3,6-dioxyheptyl)thiophene) (PDHT). The spectroelectrochemical properties of these polymers have been investigated by cyclic voltammetry (CV), derivative cyclic voltammetric absorptometry (DCVA), and spectroelectrochemistry. CV shows that the symmetry of the anodic and cathodic waves increases from PMT to PDHT; furthermore in this latter case, a second redox system appears in the CV. The DCVA responses of the polymers indicate that the oxidation potential decreases in the order PiAT > PMT > PNT > PDHT whereas the increasing symme- try of the curves suggests an increasing electrochemical reversibility. UV-visible absorption spectra have been recorded in situ at various applied potentials. In the undoped state, the absorption maximum of the polymers shifts bathochromically from PiAT to PDHT and the absorption spectrum of PDHT exhibits several defined structures resembling those previously observed at low temperature on poly(3- alkylthiophenes). Upon electrochemical doping, the spectroscopic behavior of PiAT, PNT, and PDHT differs from that of PMT by a more intense absorption at 1.5 eV at the intermediate doping level and by a more pronounced narrowing of the band gap with the appearance of metallic-like behavior at high applied potential. These effects that are particularly evident in PDHT are interpreted by an extension of the effective mean conjugation length resulting from the stabilization of more planar conformations.