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

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 1,2-dichloroethane films compared with the 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.