Electronic and structural characteristics of five poly membered heterocycles (polythiophene, polypyrrole): An ultraviolet and x-ray photoelectron spectroscopy study

Abstract

The electronic and structural properties of electrochemically synthetized five poly membered heterocycles [polypyrrole (PP), poly-N-methylpyrrole (PNMeP), polythiophene (PT), polybithiophene (PBT), and poly-3-methylthiophene (PMeT)] in their doped conducting and undoped semiconducting states have been determined by x-ray photoelectron spectroscopy (XPS) and UV photoelectron spectroscopy (UPS). A systematic evolution of the π bonding orbitals is observed by going successively from thiophene to bithiophene and to terthiophene which leads finally to the formation of an intrinsic π bonding band for the undoped polythiophene, located at 1.05 eV below the Fermi level Ef. This π bonding band is also detected with PP, PBT, and PMeT in good agreement with theoretical band structure calculations and implies a long range order along the polymeric chains. In contrast, PNMeP valence spectra exhibit broad features connected with the existence of structural defects and/or torsion angle between the monomeric units. Shake-up satellites are generally observed on the high binding energy side of the C1s, S2p (or N1s) core levels related to π→π* transitions. The S/C (or N/C) ratio is always very close to the predicted theoretical value which means that the structural architecture of the polymeric backbone is very similar to the monomer one. Upon ClO−4 doping, the π bonding band is extended up to ∼0.2 eV below Ef. Simultaneously, the bonding band intensity decreases, and the C1s, S2p, (or N1s) core level peaks become broad and asymmetric (PP, PMeT, PT). These evolutions are completely reversible during the doping–undoping processes, independent of the anion used (BF−4,SO3CF−3) and of the thickness of the film (20–1000 Å). These modifications are dramatically reduced with PNMeP although its doping level is as high as 30%. These UV photoelectron spectroscopy and x-ray photoelectron spectroscopy results combined with the near edge x-ray absorption fine structure (NEXAFS) characteristics give insight into the conduction mechanism inside these conducting polymers: Appearance of a metallic-like behavior due to first the extraction of electrons from the π bonding band, and second, an extension of the π and π* bands towards Ef narrowing the band gap from 2.1 eV in the undoped state to 0.3–0.4 eV in the doped form.