A theoretical investigation on the electronic and optical properties of π-conjugated copolymers with an efficient electron-accepting unit bithieno[3,2-b:2′3′-e]pyridine

One of the drawbacks for light-emitting diodes based on polyfluorene and derivatives (PFs) is the injection of electrons from the cathode due to the low electron affinity (EA) of most derivatives. Substitution by electron-accepting charge carriers on the conjugated polymerʹs backbone produces a remarkable influence on its electronic and optical properties. In this contribution, we apply quantum-chemical techniques to investigate a family of π-conjugated polymers poly(fluorene-alt-co-bithieno[3,2-b:2′3′-e]pyridine) (PFBTP) and poly(indenofluorene-alt-co-bithieno[3,2-b:2′3′-e]pyridine) (PIFBTP). The electronic properties of the neutral molecules, HOMO–LUMO gaps (ΔH–L), in addition to the positive and negative ions, are studied using B3LYP functional. The lowest excitation energies (Eg) and the maximal absorption wavelength (λabs) of PFBTP and PIFBTP are studied employing the time dependent density functional theory (TD-DFT) and semiempirical method ZINDO. The IP, EA and λabs of the polymers were also obtained by extrapolating those of the oligomers to the inverse chain length equal to zero (1/n=0). Especially, the influence of the presence of bithieno[3,2-b:2′3′-e]pyridine (BTP) groups on to the fluorene or indenofluorene moieties on the electron-accepting and -transporting is emphasized. As shown the BTP is a good electron-accepting moiety for electronic materials due to the presence of the three electron negative heteroatoms. For both PFBTP and PIFBTP, the LUMOs are significantly lower about 0.6 eV than that of their corresponding polyfluorene (PF), which results in the increasing of EAs by about 0.6 eV than PF, indicating that the bithieonpyridine units have significantly improved the electron-accepting properties of the copolymers. These cause the band gap narrower and the maximal absorption red-shifted comparing with PF.