It is well known that alkali metal salts such as lithium and sodium of aromatic phenols can form discrete cluster compounds in which multiple alkali metal atoms are coordinated by multiple ligands in an organized fashion. For example, see “Lithium Chemistry: A Theoretical and Experimental Overview”, A-M. Sapse and P. Von Rague Schleyer, Eds., J. Wiley & Sons, NY, 1995, Chapters 7-9 and Kissling et al, J. Org. Chem, 66(26), 9006 (2001).
Such cluster compounds are stable, neutral in overall charge and can form spontaneously and reproducibly from mixtures. The number of alkali metal atoms and ligands present in the cluster compound can vary depending on the nature of the ligand. In the case of lithium, examples of cluster compounds with 2, 3, 4 and 6 lithium atoms are known. For examples, see Fenton et al, JCS, Chem Comm, (23), 1303 (1972); Hao et al, Fagang Xuebao, 25(4), 419-424 (2004); Baker et al, Organometallics, 13(11), 4170-2 (1994) and Prakash et al, J Indian Chem Soc, 62(6), 424-5 (1985).
Metal salts of 2-(2-hydroxyphenyl)pyridine derivatives are well known in the art and their use in electroluminescent devices have been described. For example, see CN1245822, CN1544574 and US2005/0019605A1. However, it has not been reported whether these materials form cluster compounds.
The cluster compound of lithium sodium di-(8-hydroxyquinolate) has been described as useful in electroluminescent devices in CN1900213.
Banerjee et al, JCS, Inorg, Phys and Theor. (17), 2536-43 (1969) describes the isolation of a charged species of lithium 8-hydroxyquinolate complexed with an additional molecule of 8-hydroxyquinoline. A crystal structure was not reported.
Lithium 8-hydroxyquinolate (LiQ) is a well known compound that has been used in electroluminescent devices. As disclosed in M. Rajeswaran et al, Polyhedron, 26(14), 3653-3660 (2007) and W. Begley et al, Acta Crystallographica, Section E: Structure Reports Online (2006) E62(6), m1200-m1202, LiQ is a discrete cluster compound and is sometimes correctly referred to as [Li3Q3]2 or Li6Q6, which is a dimer of a timer. It is likely that the material described as LiQ and used in electroluminescent devices is in fact the cluster form of the compound and not the monomer.
LiQ has been reported in be useful in emitting layers of electroluminescent devices; for example, see US20060003089A1; US2005016412A1; WO2003080758A2; EP1458834A1; Zhao et al, Guangxue Xuebao, 20(2), 288 (2000) and Zhu et al, Bandaoti Guardian, 22(4), 279-281 (2001). LiQ has been reported as useful in electron-injecting layers; for example, see Liu et al, Synthetic Metals, 128(2), 211-214 (2002); Wu et al, Faguang Xuebao, 24(5), 473-476 (2003); Zheng et al, Thin Solid Films, 478(1-2), 252-255 (2005) and Schmitz at al, Chemistry of Materials, 12(10), 3012-3019 (2000). The use of LiQ and other organic lithium salts in electron-transporting layers has also been reported in US20060286405, US20020086180, US20040207318, U.S. Pat. No. 6,396,209, JP2000053957, WO9963023 and U.S. Pat. No. 6,468,676.
Various uses of organic alkali metal salts in OLED devices have also been disclosed in US20060286402, US20070092753, US20070207347, US20070092754, US20070092756 and US20070092755.
All of the above references disclose only organic alkali metal salts with only one kind of ligand present.
Banerjee et al, J Indian Chem Soc, 50(10), 691-3 (1973) describes a compound formed between lithium 8-hydroxyquinolate and a 1,10-phenanthroline ligand which is not an anion. The crystal structure was not reported. Prakash et al, J Indian Chem. Soc., 62(6), 424-5 (1985) describes charged complexes of LiQ and picolinic or quinaldinic acid. The crystal structure was not reported. It is generally recognized that when materials such as these are sublimed, they sublime separately as their individual component parts.
While organic electroluminescent (EL) devices have been known for over two decades, their performance limitations have represented a barrier to many desirable applications. In simplest form, an organic EL device is comprised of an anode for hole injection, a cathode for electron injection, and an organic medium sandwiched between these electrodes to support charge recombination that yields emission of light. These devices are also commonly referred to as organic light-emitting diodes, or OLEDs. Representative of earlier organic EL devices are Gurnee et al. U.S. Pat. No. 3,172,862, issued Mar. 9, 1965; Gurnee U.S. Pat. No. 3,173,050, issued Mar. 9, 1965; Dresner, “Double Injection Electroluminescence in Anthracene”, RCA Review, 30, 322, (1969); and Dresner U.S. Pat. No. 3,710,167, issued Jan. 9, 1973. The organic layers in these devices, usually composed of a polycyclic aromatic hydrocarbon, were very thick (much greater than 1 μm). Consequently, operating voltages were very high, often greater than 100V.
More recent organic EL devices include an organic EL element consisting of extremely thin layers (e.g. <1.0 μm) between the anode and the cathode. Herein, the term “organic EL element” encompasses the layers between the anode and cathode. Reducing the thickness lowered the resistance of the organic layers and has enabled devices that operate at much lower voltage. In a basic two-layer EL device structure, described first in U.S. Pat. No. 4,356,429, one organic layer of the EL element adjacent to the anode is specifically chosen to transport holes, and therefore is referred to as the hole-transporting layer, and the other organic layer is specifically chosen to transport electrons and is referred to as the electron-transporting layer. Recombination of the injected holes and electrons within the organic EL element results in efficient electroluminescence.
There have also been proposed three-layer organic EL devices that contain an organic light-emitting layer (LEL) between the hole-transporting layer and electron-transporting layer, such as that disclosed by C. Tang et al. (J. Applied Physics, Vol. 65, 3610 (1989)). The light-emitting layer commonly consists of a host material doped with a guest material, otherwise known as a dopant. Still further, there has been proposed in U.S. Pat. No. 4,769,292 a four-layer EL element comprising a hole injecting layer (HIL), a hole-transporting layer (HTL), a light-emitting layer (LEL) and an electron-transporting/injecting layer (ETL). These structures have resulted in improved device efficiency.
EL devices in recent years have expanded to include not only single color emitting devices, such as red, green and blue, but also white-devices, devices that emit white light. Efficient white light producing OLED devices are highly desirable in the industry and are considered as a low cost alternative for several applications such as paper-thin light sources, backlights in LCD displays, automotive dome lights, and office lighting. White light producing OLED devices should be bright, efficient, and generally have Commission International d'Eclairage (CIE) chromaticity coordinates of about (0.33, 0.33). In any event, in accordance with this disclosure, white light is that light which is perceived by a user as having a white color.
Since the early inventions, further improvements in device materials have resulted in improved performance in attributes such as color, stability, luminance efficiency and manufacturability, e.g., as disclosed in U.S. Pat. No. 5,061,569, U.S. Pat. No. 5,409,783, U.S. Pat. No. 5,554,450, U.S. Pat. No. 5,593,788, U.S. Pat. No. 5,683,823, U.S. Pat. No. 5,908,581, U.S. Pat. No. 5,928,802, U.S. Pat. No. 6,020,078, and U.S. Pat. No. 6,208,077, amongst others.
Notwithstanding all of these developments, there are continuing needs for organic EL device components, such as light-emitting materials, electron transporting materials and electron injecting materials, that will provide even lower device drive voltages and hence lower power consumption, while maintaining high luminance efficiencies and long lifetimes.