Tuning the electronic structures and optical properties of fluorene-based donor-acceptor copolymers by changing the acceptors: a theoretical study

Five fluorene-based conjugated copolymers were studied to explore the effect of acceptor on the electronic and optical properties. Their ground-state, excited-state electronic structures and the tunable optical properties were theoretically investigated using density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods. The acceptors including quinoxaline (Q), 2,1,3-benzothiadiazole (BT), thieno[3,4-b]pyrazine (TP), 2,1,3-benzooxadiazole (BO), and pyridopyrazine (PP) can significantly influence the copolymers' electronic structures, molecular orbitals, geometric conformations, and optical properties. Calculations were made on systems containing one, two, three, and four oligomers in the neutral, cationic, and anionic structures, which can be extrapolated to infinite chain length polymers. The result indicated that the sequence of the band gap was on the reverse trend of emission wavelength. The strong electron-withdrawing strength of TP unit and coplanar backbone in poly[2,7-(9,9'-dihexylfluorene)-alt-2,3-dimethyl-5,7-dithien-2-yl-thieno[3,4-b] pyrazine] resulted in the enhanced degree of intramolecular charge transfer (ICT) and lowest band gap. The contribution of acceptors to IP was also found to follow the sequence of TP < Q < PP < BT < BO. The absorption and emission spectra exhibited red-shift with increasing the conjugation lengths. The present study suggested that the electronic and optical properties of donor-acceptor conjugated copolymers were affected by the acceptor structure.