Tuning of HOMO levels of carbazole derivatives: New molecules for blue OLED

The Suzuki–Miyaura coupling between 3,6-dibromo-N-hexylcarbazole and aryl-boronic acids catalyzed by Pd(PPh3)4 afforded blue emitting molecules, 3,6-diaryl-N-hexylcarbazole: aryl = phenyl (1), p-methoxyphenyl (2), p-cyanophenyl (3), p-acetophenyl (4), pyrenyl (5), biphenyl (6), difluorophenyl (7) and p-nitrophenyl (8), in good yield. The HOMO level of the carbazole derivative varied between −5.67 and −6.02 eV due to the electron-withdrawing or donating substituent in the aryl group. This is confirmed by the linear correlation of the Hammett parameter of the substituent in the phenyl group with oxidation potential or energy levels. Density functional theory calculation (B3LYP/6-31G*) of carbazole derivatives confirmed the experimentally observed trend. Blue fluorescence quantum yield of these molecules is high (0.19–0.96), except for the nitrophenyl derivative (<0.1). Single layer organic light emitting devices (OLEDs) using spin-coated films of 2–5 showed blue electroluminescence (EL) with a turn-on voltage at ∼6–7 V. The performance of blue EL of molecules 3 and 4 improved dramatically (turn-on at ∼3 V, brightness at ∼1000 cd/m2, efficiency at ∼1 cd/A) in a multilayer OLED designed using standard molecules as electron and hole transport layers for efficient and balanced carrier injection.