Synthesis and optical properties of a fluorene-benzothiadiazole anthracene copolymer

Abstract

A novel green-emitting terpolymer of Poly[2,7-(9,9-dihexylfluorene)-co-4,7-(2,1,3-benzothiadiazole)-co-9,10-(anthracene)] (PFABT), containing 9,9-dihexylfluorene and anthracene as electron-pushing unit and benzothiadiazole as electron-pulling unit, was synthesized via Suzuki coupling reaction. Poly[2,7-(9,9-dihexylfluorene)-co-9,10-(anthracene)] (PFA) and Poly[2,7-(9,9-dihexylfluorene)-co-4,7-(2,1,3-benzothiadiazole)] (PFBT) copolymers were also synthesized for comparative purposes. These copolymers were characterized by GPC, FTIR and 13C NMR and computational calculations were performed using density functional theory (DFT) and its time-dependent version (TDDFT) at CAM-B3LYP/6–311 +G(d,p) level. Natural transition orbitals (NTO) and vertical excitation energies were obtained from oligomer models that represent the PFA, PFBT and PFABT polymers. The NTO densities showed that the excitation of the anthracene moiety is responsible for the S1 state on both PFA and PFABT, and that the S2 state of PFBT and PFABT is a consequence of the excitation of the benzothiadiazole-anthracene moiety. We also studied polymer light-emitting diodes (PLEDs) of blends of poly(9-vinylcarbazole) (PVK) as a host matrix with PFA, PFBT or PFABT as guests. The concentrations of the copolymers in the PVK were varied at 5, 25 and 50 wt%. The PL spectra for all blends exhibited one band at 400 nm attributed to PVK and another centered at 535 nm assigned to the PFO derivatives emission. We observed an increase in energy transfer efficiency, E < FRET> , with an increase in the weight ratio of the copolymer (PFA, PFBT or PFABT) into the PVK matrix. All the PLEDs emitted a greenish color, and the concentration of the polymer guest impacted on the performance of the diode.