Density functional study of the electronic and optical properties of fluorene-thieno[3,2-b ]thiophene-based conjugated copolymers

摘要

Quantum mechanical techniques are applied to investigate a family of 7r-conjugated copolymers: poly(9,9'_dimethylfluorene-alt-thiophene) (PFT), poly(9,9'-dimethylfluorene-alt-thieno[3,2-b ]-thiophene) (PFTT), poly(9,9`_dimethylfluorene-alt-bithiophene) (PFT2), and poly(9,9`-dimethylfluorene-alt-α,a`-bisthieno[3,2-b]-thiophene) (PFTT2). Linear extrapolation is employed to obtain polymers' properties from oligomer calculations. That is, the HOMO_LUMO gaps (Δ_H_L~S), band gaps (E_gs), ionisation potentials and electron affinities of the copolymers are obtained by plotting the corresponding quantities of the oligomers as a function of the inverse chain length (1) and extrapolating them to infinite chain length. The electronic properties of the neutral, positive and negative oligomers are determined using the density functional theory (DFT) at B3LYP/6-31G~* approximation. The lowest singlet excitation energies of the oligomers of PFT, PFTT, PFT2, and PFTT2 are also determined with the use of the time-dependent DFT again at B3LYP/6-31G~* approximation. Comparisons are made with experimental values when possible.