Synthesis and characterization of phenanthroimidazole derivatives for applications in organic electroluminescent devices

A series of phenanthroimidazole derivatives have been synthesized, characterized and applied as non-doped emitters in organic light-emitting devices. The compounds show high fluorescent quantum yields up to 0.75 as well as good thermal and film-forming abilities. Crystal structures of the compounds were determined by X-ray crystallography. Correlations between optoelectronic properties, energy levels and molecular structures of the materials were investigated. It was found that introduction of a thiophene ring on the C2-position of phenanthroimidazole can effectively decrease the ionization potentials (I-p) of the compounds. Fluorescent properties of the materials were found to be related to the dihedral angles in the compounds with different substituents. The low I-p (from 5.00 to 5.21 eV) of the materials enables efficient hole-injection from the hole-transporting layer and results in low turn-on (<2.7 V) and operation voltages (<5.5 V to give 1000 cd m(-2)).