Effect of isomerism, inter-ring torsion angle and chain length in structural and electronic properties of oligothiophenes: computational study

Abstract

Density Functional Theory (DFT) and Time-Dependent DFT (TD-DFT) methods are employed to investigate the influence of isomerism and inter-ring torsion angle on the energetic, electronic, and structural properties of bithiophenes. Furthermore, the effect of the chain length of polythiophene (1–24 units) on their structural and electronic properties has been established.

The anti-gauche rotamer represents the most energetically favorable and reactive conformation in 2,2′-bithiophene, while the syn-gauche is the less stable rotamer in 3,3′-bithiophene. The inter-ring torsion energy profile for 2,2′-bithiophene reveals that the most stable conformer is twisted, exhibiting an optimized dihedral angle of 146.27° and a rotational barrier of approximately 1.48 kcal/mol. Increasing the length of the thiophene chain from one to 24 rings induces significant changes in their structural and electronic properties. The curvature behavior of oligothiophenes decreases as the oligomer size increases, from 0.0359 Å⁻¹ for T2 to 0.0270 Å⁻¹ for T24. The maximum absorption wavelengths of the oligothiophenes are red-shifted from 216.26 nm for T1 to 527.96 nm for T24. The orbital and optical band gaps of these oligothiophenes were calculated and plotted against 1/Tn. Linear relationships were obtained in both cases, with the optical band gap showing better agreement with experimental data. The extrapolated energy gap for infinitely long polythiophene is 2.15 eV in chloroform and 2.20 eV in the gas phase, consistent with the experimental values of 2.0–2.20 eV.