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.

Modulation of the electronic structure of polyconjugated organic molecules by geometry relaxation: A discussion based on local Raman parameters

“…In this concern, it is worth mentioning the paper by Cuff and Kertesz [47], which provides a nice evidence of the fact that the Raman spectrum knows intra-molecular and inter-molecular interactions (included charge injection), through the resulting relaxation of the equilibrium BLA parameter. The authors [47] were indeed able to predict (by DFT simulations) the Raman response of doped polythiophenes, considering simple neutral model molecules characterized by a quinoidal ground state structure, induced by properly chosen end groups.…”

“…The authors [47] were indeed able to predict (by DFT simulations) the Raman response of doped polythiophenes, considering simple neutral model molecules characterized by a quinoidal ground state structure, induced by properly chosen end groups.…”

Response Equation Based Thermochemical Analysis of Singlet Bipolaron Structures in Oligo(3-Methyl-Thiophenes)

“…Our current report serves as a proof-of-concept for the response equation based quantitative structure-property relationship (REQ-QSPR) protocol presented, justifying the use of higher-level quantum chemical methods in refining this simple model developed. Besides this principal goal, the results also demonstrate that -at least at the RHF/PM3 level of theory -relatively short P3ATs already allow, by spontaneous change of their Cα' Cα bond-length alteration pattern, to develop aromatic-quinoidal transitions along the chain [17][18][19]. Therefore, these oligomers are capable of supporting diverse stable structures with both aromatic and quinoidal sections.…”

“…The changes in the electrical conductivity and in the coupled physical properties upon the redox transformation of the conjugated polymers have been generally interpreted on the basis of the polaron/bipolaron model [6,[9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25]. Despite the tremendous research done on the electronic properties of these molecules, however, less attention has been paid to the relative thermodynamic stability of their various possible structures [26,27].…”

A Benchmark Study on the Properties of Unsubstituted and Some Substituted Polypyrroles

“…Inter-ring bond length: The inter-ring bond length has been also determined using Badger's rule [23]. The estimated inter-ring bond length based on Badger's rule is 1.42Å indicating that the average structure is about 30% quinoid like.…”