Synthesis, characterization, and electroluminescence of new conjugated polyfluorene derivatives containing various dyes as comonomers

Four new fluorene-based alternating polymers (PFR1-S, PFR2-S, PFR3-S, and PFR4-S) containing different comonomers (R1, R2, R3, and R4) have been designed, synthesized, and characterized. These polymers were found to be thermally stable and readily soluble in common organic solvents. Single layer LED devices fabricated from these polymers emitted bluish green to pure red light. Our results show that the color of the light emitted by the homopolymer, poly(9,9-dioctylfluorene-2,7-diyl) (PDOF), can be tuned by incorporating R1, R2, R3, and R4 comonomers, which have narrower band gaps. The absorption and emission maxima of the copolymers varied according to the position of the cyano group in the vinylene unit (α- or β-position) and the type of incorporated aromatic group (thiophene or phenylene). Notably, PFR4-S showed pure red emission (CIE coordinate values x = 0.66, y = 0.33) that is almost identical to the standard red (0.66, 0.34) demanded by the National Television System Committee. PFR3-S also exhibited pure red emission (chromaticity values x = 0.63, y = 0.38), and its maximum luminance and maximum external quantum efficiency were approximately 3100 cd/m² at 6 V and 0.46% at 4 V, respectively.