A New Class of Blue-Emitting Materials Based on 1,3,5-Oxadiazole Metal Chelate Compounds for Electroluminescent Devices

Abstract

Since the pioneering work of Tang et al., [1] which first demonstrated efficient electroluminescent (EL) devices based on organic materials, many efforts have been devoted to exploring new EL materials for display applications. Two distinct classes of organic materials, namely small organic molecules [1,2] and conjugated polymers, [3] have been developed for this purpose. Motivated by the success of tris(8-hydroxyquinoline) aluminum (Alq 3 ) in vacuum-deposited EL devices, organic metal±chelate compounds in particular have attracted a lot of attention. This class of compounds generally offers many attractive properties such as displaying a double role of electron transport and light emission, high thermal stability, and ease of sublimation. The major drawback of metal±chelate compounds, however, is the lack of suitable blue-emitting molecules.Only a few types of blue-emitting metal±chelate compounds, such as metal complexes based on 2-methylquinolin-8-ol, [4] azomethine, [5] and 7-azaindole, [6] have been previously reported. In this communication, we present a new class of blue-emitting compounds for EL applications based on oxadiazole ligands. Oxadiazole derivatives, such as 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,5-oxadiazole (PBD), have shown promising electron transport and emission characteristics when used in organic EL devices. [7] Oxadiazole units have also been used to design photoluminescent polymers with improved electron transport properties. [8] The new class of oxadiazole metal±chelate compounds is expected to offer the combined properties of metal±chelates and oxadiazoles in EL application. Two beryllium chelate compounds, 2a and 2b, have been synthesized for the present study. As shown in Scheme 1, they can be readily obtained by the direct metal chelation of oxadiazole ligand, 1, with beryllium sulfate in the presence of sodium hydroxide, and purified by the train sublimation technique.[9] Compound 1 was prepared from a reaction between acetylsalicyloyl chloride and an aryltetrazole derivative. We have also synthesized the zinc metal complexes of 1, which gave poor sublimation yields and were less fluorescent compared to the beryllium compounds. All metal complexes possessed very low solubility in most organic solvents. The formula of 2a and 2b were confirmed by elemental analysis. Figure 1 illustrates the molecular structure of 2b as determined by X-ray diffraction analysis. All metal complexes were reasonably stable upon exposure to air, and exhibited high thermal stability. The melting points of 2a and 2b as measured by differential scanning calorimetry (DSC) were 433 and 404 C, respectively. Both beryllium chelate compounds can form smooth and uniform films by vacuum evaporation. As anticipated, 2a and 2b display intense photoluminescence (PL) in the solid state when irradiated with UV light. Figure 2 illustrates their PL spectra recorded from vacuum-deposited thin films. 2a and 2b provided very similar spectra with emission peaks at 452 and 458 nm, respectively. Substituting a nap...

Synthesis, photophysical and electrochemical properties of 2,8-diaryl-dibenzothiophene derivatives for organic electronics

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Synthesis and electroluminescent property of dinuclear aluminum 8-hydroxyquinoline complex

“…Especially, the use in light-emitting diodes has shown to be a field of potential application [1][2][3][4][5]. Aluminum 8-hydroxyquinoline (Alq 3 ) is a typical representative with high emitting efficiency and excellent electron-transporting ability [1].…”

Real blue-emitting light-emitting diodes based on fluorene derivative polymers

“…Real blueemitting polymers are especially interesting because of realizing the full color PLEDs. Recently, blue-emitting metal chelate compounds, such as metal complexes based on oxadiazole and polyfluorene with maximum luminances of 750 cd/m 2 and near 1000 cd/m 2 at 8 V, respectively, have been reported [1]. Some others that emit near the blue include an unidentate organometallic complex containing lithium as the emitting layer with a broad emission spectrum from 400 to 550 nm, a lithium tetra-(8-hydroxyquinolinato) boron with emission at 475 nm, a power efficiency of 0.8 lm/W, and a maximum luminance of 3860 cd/m 2 at an applied voltage of 10 V, and a distyrylarylene (DSA) doped with DSA-amine with a maximum power efficiency of 1.5 lm/W at a voltage and luminance of 7 V and 277 cd/m 2 , respectively [2][3][4][5][6][7].…”