Highly crystalline and efficient red-emissive π-conjugated polymer film: tuning of macrostructure for light-emitting properties

Abstract

Solid-state red-emissive π-conjugated molecules and polymers have attracted significant attention. In this work, the optical properties of alternating π-conjugated copolymers based on phenylene–bithiophene were investigated. It was found that the alternating 3,3′-dihexylbithiophene and tetrafluorophenylene structure was the primary one in the highly crystalline polymer film, while the other bithiophene–phenylene alternating copolymers did produce crystalline films. The number of fluorine atoms on the phenylene unit critically affected the film crystallinity; this could be attributed to the rigidity of the π-structure based on intramolecular H–F and S–F interactions. The crystalline polymer films demonstrated efficient red-emission. A red-shift of the emission band (+∼150 nm) and a twofold increase of the quantum yield for films compared with the 1,2-dichloroethane solution state were observed. This is likely due to crystallization-induced enhanced emission by the π-conjugated polymers. Additionally, significant tuning of the light-emission (from yellow, orange to red-colored emission) of the films was realized by controlling the crystallinity. The casting solvents and annealing treatment for film formation strongly affected the macrostructure (crystalline domain). X-ray diffraction patterns and the optical properties of the π-conjugated polymer films were used to investigate the dependence of the macrostructure on the fabrication parameters.