Optimizing OLED Efficacy of 2,7-Diconjugated 9,9-Dialkylfluorenes by Variation of Periphery Substitution and Conjugation Length

The photoluminescence (PL) and electroluminescence (EL) of four 2,7‐bis(phenylethenyl)fluorenes (OFPVs) and two 2,7‐diphenylfluorenes (OFPhs) are compared to evaluate effects of nonconjugating peripheral substitution and conjugation length on their EL emissions. The OFPVs exhibit very similar PL spectra with 460–480 nm emission maxima but show large variation in the organic light‐emitting diode (OLED) efficacy: from a material that does not give persistent emission in test OLEDs (9,9‐diheptyl substitution on the fluorene ring) to materials with luminance efficiencies of 0.5 cd A–1 and greater (9,9‐diethyl substitution on the fluorene ring, methoxy and methoxy/heptyloxy substituents on the phenylethenyl rings). The best OFPV in an ITO/PEDOT:PSS/(emitter)/Ca–Al (ITO: indium tin oxide; PEDOT: poly(ethylenedioxythiophene); PSS: poly(styrene sulfonate)) OLED configuration has 9,9‐diethyl substitution and terminal heptyloxy substitution (maximum luminance of 1500 cd m–2 at 12 V). Unlike the OFPVs, the neat OFPhs show not only EL at the desired blue output of ca. 400–410 nm emission maxima but also an undesired green emission component at 500–550 nm. Blending the OFPhs with poly(methyl methacrylate) eliminates the long‐wavelength component when the emitter load is 10–25 %, but the OFPh luminance efficiencies, turn‐on voltages, and maximum luminance tend to be poorer than those of the OFPVs. The deficiencies of the OFPhs appear to be attributable to thermal degradation and oxidative reactivity, although solid‐state annealing and a nonoptimal bandgap match to the OLED device configuration may also contribute. Electroluminescence efficacy of 2,7‐ bis(phenylethenyl)fluorenes is strongly affected by the choice of substituents at the 9‐position of the fluorene unit, and upon the terminal phenyl groups. Shortening the conjugation length by removal of the ethenyl units to give 2,7‐diphenyl‐9,9‐dialkylfluorene (OFPh) derivatives decreases both emission intensity and color purity (see figure), although color purity in the OFPhs could be improved by dilution in poly(methylmethacrylate) matrices.