Synthesis and characterization of near-infrared electroluminescent fluorene-based conjugated copolymers

A series low band-gap conjugated copolymers (PFO-DTP) derived from 9,9-dioctylfluorene (DOF) and 5,7-dithien-2-ylthieno[3,4-b]pyrazine (DTP) is prepared by the palladium-catalyzed Suzuki coupling reaction with the feed ratios of DTP mole percents around 1%, 5%, 15%, 30%. The resulted polymers are soluble in common organic solvents. There are two absorbance peak around 380 nm and 632 nm in the spectra of copolymers. The absorbance peak around 632 nm increased as the DTP contents increased. Maximum external quantum efficiency reaches 0.1% in devices of copolymers with the DTP content at 5 mol %. In comparison with its very well studied isomer copolymer poly(2,7-(9,9-dioctyl- fluorene)-co-4,7-di-2-thienyl-2′,1′,3′-benzothiadiazole), the poly(2,7-(9,9-dioctylfluorene)-co-5,7-dithien-2-ylthieno[3,4-b]pyrazine) (PFO-DTP) shows a red shift of about 110 nm for photoluminescence (PL) and electroluminescence (EL) emissions, and the optical onset wavelength of the PFO-DTP is red shifted about 130 nm and their optical response is more matching the optical spectra of the solar terrestrial radiation. These copolymers are promising candidate materials for near-infrared light-emitting diodes and polymeric photovoltaic solar cell.