Theoretical electronic structure and properties of alternating fluorene-acceptor conjugated copolymers and their model compounds

The theoretical geometries and electronic properties of fluorene (F) based alternating donor-acceptor conjugated copolymers and their model compounds were studied by the density function theory (DFT) at the B3LYP level with 6-31G or 6-31G** basis set. The acceptors investigated in this study include thiazole (TZ), thiadiazole (TD), thienopyrazine (TP), thienothiadiazole (TT), thiadiazolothienopyrazine (TTP), quinoxaline (Q), benzothiadiazole (BT), pyrazinoquinoxaline (PQ), thiadiazoloquinoxaline (TQ), and benzobisthiathiadiazole (BB). The torsional angle, bridge bond length, bond length alternation, and intramolecular charge transfer were simulated and correlated with the electronic properties, i.e., HOMO, LUMO level, and band gap. The geometries of fluorene-based donor-acceptor alternating copolymers and their model compounds are significantly affected by the structure of acceptors, particularly the ring size on the backbone. The electronic properties of the polymers and their model compounds are well correlated with the acceptor strength, coplanarity of the backbone, and intramolecular charge transfer. The theoretical study suggests that the electronic properties of alternating fluorene-acceptor conjugated copolymers could be tuned by the geometries or acceptor strength. Hence, these proposed copolymers could have potential applications as light-emitting diodes (LEDs), transparent conductor, or photovoltaic devices.