Supramolecular Architectures, Photophysics, and Electroluminescence of 1,3,4-Oxadiazole-Based Iridium(III) Complexes:  From μ-Dichloro Bridged Dimer to Mononuclear Complexes

One μ-dichloro bridged diiridium complex and three mononuclear iridium(III) complexes based on the 1,3,4-oxadiazole derivatives as cyclometalated ligands and acetylacetonate (acac) or dithiolates O,O‘-diethyldithiophosphate (Et₂dtp) or N,N‘-diethyldithiocarbamate (Et₂dtc) as ancillary ligands have been synthesized and systematically studied by X-ray diffraction analysis. The results reveal that three mononuclear complexes all adopt distorted octahedral coordination geometry around the iridium center by two chelating ligands with cis-C−C and trans-N−N dispositions, which have the same coordination mode as the diiridium dimer. The dinuclear complex crystallizes in the monoclinic system and space group C2/c, whereas three mononuclear iridium complexes are all triclinic system and space group P1̄. In the stacking structure of the dimer, one-dimensional tape-like chains along the b-axis are formed by hydrogen bondings, which are strengthened by π stacking interactions between phenyl rings of 1,3,4-oxadiazole ligands. Then these chains assemble a three-dimensional alternating peak and valley fused wave-shape structure. In each stacking structure of three mononuclear complexes, two molecules form a dimer by the C−H···O hydrogen bondings, and these dimers are connected by π stacking interactions along the b-axis, constructing a zigzag chain. Then these zigzag chains are interacted by π stacking along the a-axis, building up a two-dimensional structure. All complexes emit green with emission wavelengths in the range of 501−535 nm, depending on the structures of cyclometalated ligands and ancillary ligands. Electroluminescent devices using complexes 2−4 as phosphorescent dopants have been fabricated. A high-efficiency green emission device with a maximum luminous efficiency of 5.26 cd/A at a current density of 1.38 mA/cm² and a maximum brightness of 2594 cd/m² at 15.5 V has been achieved using 2 as the emitter.