Effect of End Groups on Optoelectronic Properties of Poly(9,9-dioctylfluorene): A Study with Hexadecylfluorenes as Model Polymers

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Abstract

Monodisperse hexadecyl(9,9-dioctylfluorene)s F16, BrF16, and BF16, which comprise H, Br, and boronic pinacol ester end groups, respectively, were synthesized, and the effect of these end groups on the photophysical, electrochemical, and electroluminescent properties of the polymers were investigated. The end groups have no noticeable effect on the energy levels of the polymers. F16 and BrF16 show high photoluminescence (PL) quantum yields (Φs) in both solution and film states. Film PL Φs (Φfilms) of F16 and BrF16 are 0.72 and 0.79, respectively. These values are about 1.4–1.5 times of that of BF16 and poly(9,9-dioctylfluorene) (PFO) prepared by Suzuki polycondensation. Moreover, Φfilms of F16 and BrF16 were kept almost the same after thermal annealing at 120 °C in both air and argon atmospheres. By contrast, the Φfilms of BF16 and PFO dramatically decreased to 0.25 and 0.29 after thermal annealing at 120 °C in air. Polymer light-emitting diodes (PLEDs) were fabricated. F16, BrF16, and PFO exhibited maximum luminance efficiencies of 0.60, 0.27, and 0.39 cd·A–1, respectively, while that of BF16 was as low as 0.024 cd·A–1. Meanwhile, green emission was observed for devices of BrF16, BF16, and PFO, but not for devices of F16. Electron- and hole-only devices were fabricated for elucidating the effect of end groups on charge transporting properties of the polymers. While Br is a mild hole-trapper, boronic ester group traps both holes and electrons, resulting in severely unbalanced hole- and electron-transport. Our results indicate that boronic ester end groups have a detrimental effect on the optoelectronic properties of polyfluorenes larger than Br.

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