Efficient 9-alkylphenyl-9-pyrenylfluorene substituted pyrene derivatives with improved hole injection for blue light-emitting diodes

Two efficient blue-light-emitting materials with stable diarylfluorene and hole-transporting pyrene groups, 9-alkoxyphenyl-9-pyrenylfluorene substituted pyrenes (EHOP1 and EHOP2), have been synthesized by Suzuki coupling reaction and have been fully characterized. They have extreme thermal stability in nitrogen with onset decomposition temperature over 430 °C. They are amorphous materials with T_g of 103 and 191 °C for EHOP1 and EHOP2, respectively. They show stable bright blue emission in the solid state. Due to the non-conjugated pyrene groups at C9 of the fluorene moieties, the absorption spectra of the two compounds showed some characteristic peaks and they showed improved hole-injection ability than conjugated fluorene derivatives. A preliminary simple three-layer blue light-emitting diode, i.e. ITO/TCTA (8 nm)/EHOP1 (30 nm)/BCP (45 nm)/Mg:Ag, was obtained without the need for a hole-injection layer, with high luminance of 11620 cd m^-2, turn-on voltage of 4.0 V and current efficiency of 3.04 cd A^-1. Furthermore, the device still had a considerable efficiency of 1.74 cd A^-1, when the luminescence reaches its maximum.