HIGHLY EFFICIENT BLUE ELECTROLUMINESCENCE OF LITHIUM TETRA-(2-METHYL-8-HYDROXY-QUINOLINATO) BORON
HIGHLY EFFICIENT BLUE ELECTROLUMINESCENCE OF LITHIUM TETRA-(2-METHYL-8-HYDROXY-QUINOLINATO) BORON
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DOI：
10.1021/ja9924142
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发表时间：
1999-09
影响因子：
15
通讯作者：
X. Tao;H. Suzuki;T. Wada;S. Miyata;H. Sasabe
中科院分区：
文献类型：
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作者：
X. Tao;H. Suzuki;T. Wada;S. Miyata;H. Sasabe
蓝色电致发光体是开发基于“变色介质”技术或RGB滤色白色发光的全色显示器所必需的。然而，通常，由于发射材料的较大带隙能量，蓝色发射有机发光二极管（OLED）具有比绿色或红色器件更低的效率，这抑制了阴极处的电子注入，并且有效的有机或有机金属蓝色发射体仍然非常罕见。1-5自从Tang等人首次报道了使用三-（8-羟基喹啉）铝（Alq 3）作为电子转移和发光材料的双层器件的高效电致发光（EL）以来，6这种材料由于其良好的稳定性和发光性能而成为应用最广泛和研究最深入的分子发射材料之一。Alq 3是峰值波长在530 nm左右的绿色发光体。为了使Alq 3发光区向蓝区移动，人们做了大量的研究，如在喹啉配体的2位或4位引入给体或在5位引入受体。7-9与此同时，所有其他的羟基喹啉的13族元素配合物也被报道用于EL应用。在这些配合物中，喹啉都起螯合配体的作用，并通过氮和氧供体原子与中心原子结合，配位数为6。Mq 3（M）（Al，Ga，In）配合物的发射波长主要由金属-氮键的共价性质决定：金属-氮键越弱，发射波长越短。7硼也属于第13族，但它是一种准金属元素，具有非金属和金属特性。此外，硼具有小尺寸，并且硼络合物的配位数通常为4或3。我们尝试以羟基喹啉为配体，以氧原子为中心金属配位，合成了一些新颖的配合物。这类配合物的发射波长应明显比Alq 3的蓝移。在这篇文章中，我们报道了第一个没有金属-氮键合的羟基喹啉配合物用于亮蓝色发射应用的例子。四-（2-甲基-8-羟基-喹啉）硼锂[LiB-（qm）4]通过硼氢化锂（LiBH 4）与四倍量的2-甲基-8-羟基-喹啉在乙醇中在室温下反应以接近定量的产率制备，如方案1所示。产物作为白色粉末从溶液中沉淀。在10-4托或更低的压力下通过升华法进一步纯化产物。通过元素分析和核磁共振分析证实了该配合物的组成。图1示出了LiB（qm）4的吸收、光致发光（PL）光谱以及LiB（qm）4的电致发光光谱。
Blue electroluminescent emitter is essential for the development of a full-color display based either on the “color changing medium” technology or the RGB filtered white emission. However, in general, blue emitting organic light emitting diodes (OLEDs) have lower efficiencies than those of green or red devices due to the larger band-gap energy of the emission material, which inhibits the injection of electrons at cathode, and efficient organic or organometallic blue emitters are still very rare. 1-5 Since the first report of high-efficiency electroluminescence (EL) of a bilayer device using tris-(8-hydroxyquinoline) aluminum (Alq3) as electron transfer and luminance material by Tang et al., 6 this material has become one of the most widely used and thoroughly studied molecular emitter materials because of its good stability and luminescence properties. Alq3 is a green emitter with peak wavelength around 530 nm. To shift the luminance of Alq3 to blue region, a lot of research has been done, such as, introducing donors at the positions 2 or 4 or acceptors at position 5 of the quinoline ligand. 7-9 Meanwhile all other group 13 element complexes of hydroxyquinolines have also been reported for EL applications. 10 In these complexes, the quinolines all function as chelating ligands and bind to the central atom through both nitrogen and oxygen donor atoms, and the coordination numbers are 6. The emission wavelengths of the Mq3 (M) Al, Ga, In) complexes are primarily determined by the covalent nature of the metal-nitrogen bond: the weaker the metal-nitrogen bonding, the shorter the emission wavelength. 7 Boron also belongs to group 13, but it is a metalloid element and has both nonmetallic and metallic characters. Moreover, boron has small size, and the coordination numbers of boron complexes are generally 4 or 3. We sought to synthesize some novel complexes using hydroxyquinolines as ligands, and only oxygen atoms coordinated to the central metals. Such kinds of complexes should have apparently more blue-shifted emission wavelengths than that of Alq3. In this communication, we report the first example of hydroxyquinoline complexes without metal-nitrogen bonding for bright blue emission applications. Lithium tetra-(2-methyl-8-hydroxy-quinolinato) boron [LiB-(qm) 4] was prepared in near quantitative yields by reaction of lithium borohydride (LiBH4) with four times of 2-methyl-8-hydroxy-quinoline in ethanol at room temperature as shown in Scheme 1. The product precipitated from solution as a white powder. The products were further purified by sublimation method at a pressure of 10-4 Torr or lower. The composition of this complex was verified by elemental and NMR analysis. Figure 1 shows the absorption, photoluminescence (PL) spectra of the LiB (qm) 4 as well as the electroluminescence spectrum of a