Enhanced infrared properties of regioregular poly(3-alkylthiophenes)

Regiorandom and regioregular head to tail coupled poly(3-butylthiophenes), poly(3-octylthiophenes) and poly(3-dodecylthiophenes) have been synthesized by direct oxidation of 3-n-alkylthiophenes using FeCl3 as oxidant and by regiospecific dehalogenation of 2-bromo-3-n-alkylthiophenes. NMR characterizations indicated 70% of head to tail couplings in the polymers synthesized by FeCl3 route, and more than 99% for regioselective polymerizations. The results of both the solution and the solid state UV-vis spectroscopic studies showed that all the regioregular poly(3-alkylthiophenes) had a lower pi-pi* transition energy, characteristic of a longer conjugation length. A series of visible-near-IR spectra of chloroform solution of FeCl3-doped regioregular P3ATs is reported and shows an increase of the maximum absorption energy of doped P3ATs as a function of alkyl side chain; i.e., longer conjugation lengths are observed in the P3AT possessing the longer alkyl side chain. Moreover, the effect of solvent on electronic spectra is dramatic. The spectrum in a polar solvent, nitrobenzene, is significantly shifted toward low energy compared with spectra measured in relatively non polar solvent, such as dichloromethane, chloroform or toluene. Even after complete removal of the solvent, the more expended coil conformation persists in the cast film since infrared reflectance feature is more metal like. This study allows us to generalize the concept of secondary dopant to poly(3-alkylthiophene)/FeCl3/nitrobenzene systems. Finally, reflectance spectra of regiorandom and regioregular P3ATs are compared. They are the largest values ever published for P3ATs and are comparable to highly conductive polyaniline or polypyrrole reflectances.