Comparison of the electrochemical and luminescence properties of two carbazole-based phosphine oxide Eu^III complexes: Effect of different bipolar ligand structures

The photoluminescence (PL), electrochemical, and electroluminescence (EL) properties of Eu^III complexes, [Eu(cppo)₂(tta) ₃] (1) and [Eu(cpo)₂(tta)₃] (2; TTA = 2-thenoyltrifluoroacetonate) with two carbazole-based phosphine oxide ligands, 9-[4-(diphenylphosphinoyl)phenyl]-9H-carbazole (CPPO) and 9-(diphenylphosphoryl) -9H-carbazole (CPO), which have different bipolar structures, donor-π-spacer-acceptor (D-π-A) or donor-acceptor (D-A) systems respectively, are investigated. The CPPO with D-π-A architecture has improved PL properties, such as higher PL efficiency and more efficient intramolecular energy transfer, than CPO with the D-A architecture. Gaussian simulation proved the bipolar structures and the double-carrier injection ability of the ligands. The carrier injection abilities of triphenylphosphine oxide, CPO, and CPPO are gradually improved. Notably, the Gaussian and electrochemical investigations indicate that before and after coordination, the carrier injection ability of the ligands show remarkable changes because of the particularity of the D-π-A and D-A systems. The electrochemical studies demonstrate that coordination induces the electron cloud to migrate from electron-rich carbazole to electron-poor diphenylphosphine oxide, and consequently increases the electron-cloud density on diphenylphosphine oxide, which weakens its ability for electron affinity and induces the elevation of LUMO energy levels of the complexes. Significantly, the π-spacer in the D-π-A system exhibits a distinct buffer effect on the variation of the electron-cloud density distribution of the ligand, which is absent in the D-A system. It is demonstrated that the adaptability of the D-π-A systems, especially for coordination, is stronger than that of D-A systems, which facilitates the modification of the complexes by designing multifunctional ligands purposefully. 1 seems favorable as the most efficient electroluminescent Eu^III complex with greater brightness, higher efficiencies, and more stable EL spectra than 2. These investigations demonstrate that the phosphine oxide ligands with D-π-A architecture are more appropriate than those with D-A architecture to achieve multifunctional electroluminescent Eu^III complexes.