The pure red light-emitting fluorene-based copolymers by introducing low energy band-gap dyes

Four novel fluorene-based light-emitting polymers (PFR1, PFR2, PFR3, and PFR4) containing different comonomers have been designed, synthesized, and characterized. These polymers were thermally stable and readily soluble in organic solvents. The single layer LED devices fabricated from these polymers emitted light from bluish green to pure red. The results show that the color of the light emitted by the homopolymer, poly(9,9-bis(2′-ethylhexyl)fluorene), could be tuned by incorporating comonomers (R1, R2, R3, and R4) with narrower band-gaps. The energy band-gap of the copolymers with those comonomers varied according to the position of the cyano group in the vinylene unit and the type of aromatic group (thiophene or phenylene) incorporated. Notably, PFR4 showed pure red emission (chromaticity values x = 0.67, y = 0.34) that is almost identical to the standard red (0.66, 0.34) demanded by the national television system committee (NTSC) and also exhibited a low turn-on voltage.