Chain-Length Dependence of Electrochemical and Electronic Properties of Neutral and Oxidized Soluble α,α-Coupled Thiophene Oligomers

Abstract

The electrooxidation of α,α-coupled thiophene oligomers with terminal α-(CH3)3Si groups (α-TMS) and β-CH3 groups was studied in methylene chloride at room temperature. The oligomers with four, five, six, seven, and eight thiophene rings undergo two stepwise oxidations to produce the radical cation and dication, respectively, as confirmed by the ESR spectra. The redox waves are chemically reversible and separated by 180 mV for the hexamer, heptamer, and octamer, suggesting that the same electrochemical behavior should be observed with the next higher oligomers or with the π-conjugated segments of polythiophene. Thus it is proposed that the broad, featureless voltammogram observed with films of polythiophene is not an inherent property of the polymer segments but may reflect complications from the solid-state nature of the film. Radical cations and dications of some of the oligomers were succesively generated with stoichiometric amounts of FeCl3 in CH2Cl2 and characterized by vis-near-IR. The energies of the sharp absorption bands in the visible and near-infrared of the neutral and oxidized oligomers are found to scale linearly with the inverse of the oligomer size. Extrapolation of the absorption energy to infinite chain length leads to excellent agreement with the values for neutral poly(methylthiophene), but significant differences are observed with those for the oxidized forms. The implications of this difference on the derealization length of the polymer radical cation and dication are discussed. Some of the electronic transition assigments for the oxidized thiophene oligomers which appear in previous reports are reinterpreted. © 1992, American Chemical Society. All rights reserved.