Evidence of quinonoid structures in the vibrational spectra of thiophene based conducting polymers: Poly(thiophene), poly(thieno[3,4-b]benzene), and poly(thieno[3,4-b]pyrazine)

Abstract

By combining vibrational spectra and ab initio calculations, we obtained a consistent description of the IR and nonresonant Raman spectra, including intensities, of four thiophene based polymers—undoped and heavily doped poly(thiophene) (PTh), undoped poly(thieno[3,4-b]benzene) (PITN), and poly(thieno[3,4-b]pyrazine) (PThP) for the first time. Predicted spectra for poly(thiophene) agree with experiment very well. Based on the calculated force constants and Badger’s rule, we also estimated the average inter-ring bond lengths of undoped and doped PTh to be 1.47 and 1.42 Å, respectively. The latter leads to an estimated 33% quinonoid character on average for heavily doped PTh. The average inter-ring bond lengths of undoped PITN and PThP, that are consistent with their vibrational spectra, are estimated to be 1.41, and 1.42 Å, respectively. These values showed that undoped PITN and PThP have quinonoid character close to that of heavily doped PTh. Further, we also estimated that, upon doping the average bond lengths of PTh changed by −0.01, 0.11, and −0.05 Å for intra-ring Cβ–Cβ, Cα–Cβ, and inter-ring bonds, respectively. These bond length changes are significantly different from those of Hartree–Fock-type calculations, reflecting significant correlation contributions and are also in conflict with earlier empirical fits of the vibrational spectrum of the highly doped phase of PTh. However, our results are more in line with the generally accepted picture of an aromatic to quinonoid “transition” of the doping process. Furthermore, the counterintuitive downward frequency shifts in the vibrational spectra of PTh upon doping can be explained by the structural change from an essentially aromatic to a partially quinonoid form.