New Carbazole-Oxadiazole Dyads for Electroluminescent Devices: Influence of Acceptor Substituents on Luminescent and Thermal Properties

A series of carbazole-oxadiazole dyads linked by amino functionality is prepared in good yields by C-N coupling reactions catalyzed by Pd(dba)2/P(t-Bu)3 under basic conditions in toluene. The compounds possess additional electron-withdrawing groups such as CF3 and CN either on oxadiazole or on carbazole nucleus. The placement of CF3 on the oxadiazole end enhances the electron deficiency of the oxadiazole unit, while the CN substituent at the carbazole nucleus decreases the donor strength of carbazole. This results in slight alterations in the oxidation potentials and thermal properties of the resulting dyads. This also leads to a pathway for fine-tuning the energy levels and amorphous morphology in these dyads. While CN groups alter by ~0.2 eV the energy levels, a counterproductive Tg reduction/thermal instability is observed for the CF3 derivatives. All of these derivatives display solvent-dependent emission profiles with the solid-state emission occurring in the cyan region. Electroluminescent devices fabricated using these compounds as hole-transporting layer and Alq3 or TPBI as the electrontransporting layer emit cyan color. The emission in most cases arises from the HTL layer. However, slight distortions in shape and peak position of the EL spectra were noticed, which were attributed to either the mixing of emissions from HTL and ETL layer or the complex formation between the HTL and ETL materials. Energetics governing the confinement of excitons in the emissive layer is critically analyzed.