Electroluminescent zinc(II) bis(8-hydroxyquinoline): Structural effects on electronic states and device performance

We present direct evidence for stable oligomers in vacuum-deposited thin films of zinc(II) bis-(8-hydroxyquinoline) (Znq(2)). The tetramer [(Znq(2))(4)] is the energetically favored configuration in both the single crystal and the vacuum-deposited thin film. Oligomerization leads to distinct, symmetry-driven differences between the electronic states in Znq(2) and those in the archetypal organic electroluminescent molecule tris(8-hydroxyquinoline) aluminum (Alq(3)). In the case of the Znq(2) tetramer, symmetry leads to an extended network of overlapping pyridyl and phenolato moieties in the solid film. Analysis of the electronic structure of (Znq(2))(4) calculated by ab initio Hartree-Fock (HF) methods reveals a localization and energy shift of high-lying occupied and low-lying unoccupied states on symmetry related ligands located on opposite sides of the supramolecular structure resulting in a dipole moment for (Znq(2))(4) tetramer close to zero. The optimal pi-overlap pathways, altered charge distributions, and extended electronic states of tetrameric Znq(2) may be expected to enable low operating voltage organic light-emitting devices (OLEDs) based on Znq(2). We present preliminary evidence that the operating voltage of (Znq(2))(4)-based OLEDs is indeed lower than that of identical devices made with Alq(3). Strategic substitution of 8-hydroxyquinoline ligands and control of the structural symmetry of the corresponding metal chelates may offer a route to high efficiency and low operating voltage small molecule OLEDs.