Electroluminescent Zinc(II) Bis(8-hydroxyquinoline):  Structural Effects on Electronic States and Device Performance

Abstract

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.

Photostable BF2-Chelated Fluorophores Based on 2-(2'-Hydroxyphenyl)benzoxazole and 2-(2'-Hydroxyphenyl)benzothiazole

“…Among them, aluminum (III) complex of 8-hydroxyquilonine (Alq3) has been used as a green emitter and one of the stable electron transport materials with high emission efficiency. 3 Similarly, zinc (II) complex 4 and boron (III) complex 5 of 8-hydroxyquilonine have been investigated as an EL materials in vapor-deposited films. In general, boron compounds are more stable than corresponding aluminum compounds, and boron difluoride dyes such as boron dipyrromethene (BODIPY) and boron diketonates possess remarkable properties such as large molar extinction coefficients, high fluorescence quantum yields and excellent photostability.…”

Surface-Modified Zinc Oxide Nanoparticles for Electron Injection Layers in Organic Light-Emitting Devices

“…It is important to take into account the LUMO levels of the surface modifiers. The LUMO levels were estimated to be −2.65 eV for BTz and −3.2 eV for 8Qn based on the corresponding LUMO levels of the metal complexes previously reported [19][20][21]. The 8Qn LUMO level matches the F8BT LUMO level of 3.5 eV, whereas the BTz LUMO level is too shallow to match the F8BT LUMO level.…”

Organic Light Emitting Diodes Based on Novel Zn and Al Complexes

“…Among these materials, Zn complexes have been especially important because of the simplicity in synthesis procedures and wide spectral response. Extensive research work is going on in various laboratories to synthesize new Zn complexes containing new ligands to produce a number of novel luminescent Zn complexes as emitters and electron transporters (Sapochak et al, 2001(Sapochak et al, , 2002Hamada et al, 1996;Sano et al, 2000;Kim et al, 2007;Rai et al, 2008). Zinc(II) bis[2-(2-hydroxyphenyl) benzothiazole] (Zn(BTz) 2 ) has been studied as an effective white light emissive and electron transporting material in OLED.…”