Red organic electroluminescent materials

Details Red organic electroluminescent materials Red organic electroluminescent materials

2003-01-20

2003-11-18

Aulakh, Charanjit S. (Department: 1625)

Compositions

Organic luminescent material containing compositions

C546S094000, C546S095000, C428S690000, C428S917000, C252S301220, C252S301260, C252S301320

Reexamination Certificate

active

06649089

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to red fluorescent materials that can be used as dopants in organic electroluminescent devices.
BACKGROUND OF THE INVENTION
One of the applications for using red fluorescent materials is in organic EL (electroluminescent) devices. Organic EL devices are known to be highly efficient and are capable of producing a wide range of colors. Useful applications such as flat-panel displays have been contemplated and commercialization is well underway. 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
, Vol. 30, pp. 322-334, 1969; and Dresner,. U.S. Pat. No. 3,710,167, issued Jan. 9, 1973. Typical organic emitting materials were formed of a conjugated organic host material and a conjugated organic activating agent having condensed benzene rings. The organic emitting material was present as a single layer medium having a thickness much above 1 micrometer. Thus, this organic EL medium was highly resistive and the EL device required a relatively high voltage (>100 volts) to operate.
The most recent discoveries in the art of organic EL device construction have resulted in devices having the organic EL medium consisting of extremely thin layers (<1.0 micrometer in combined thickness) separating the anode and cathode. Herein, the organic EL medium is defined as the organic composition between the anode and cathode electrodes. Typically, organic EL devices are composed of three layers of organic molecules that are interposed between a transparent electrode and a metallic electrode. The three layers include an electron-transporting layer, a luminescent layer and a hole-transporting layer. The luminescent layer between the electron-transporting and hole-transporting layers provides an efficient site for the recombination of the injected hole-electron pair and resultant electroluminescence. A hole-injecting layer may be added to reduce the driving voltage. Optionally a hole- or electron-blocking layer may be added to improve the luminance efficiency. An organic EL device comprising four to six layers of organic molecules is thus obtained.
The extremely thin organic EL medium offers reduced resistance, permitting higher current densities for a given level of electrical bias voltage. Since light emission is directly related to current density through the organic EL medium, the thin layers coupled with increased charge injection and transport efficiencies have allowed acceptable light emission levels to be achieved with low applied voltages in ranges compatible with integrated circuit drivers, such as field effect transistors.
Organic EL devices have lower driving voltages and have been shown applicable for use in full-color flat-panel displays. The investigation in organic EL devices and materials has attracted a lot of worldwide attention and investment. The improvements in organic EL devices such as color, stability, efficiency and fabrication methods have been disclosed in U.S. Pat. Nos.: 5,151,629; 5,150,006; 5,141,671; 5,073,446; 5,061,569; 5,059,862; 5,059,861; 5,047,687; 5,104,740; 5,227,252; 5,256,945; 5,069,975; 5,122,711; 5,366,811; 5,126,214; 5,142,343; 5,389,444; 5,458,977; 5,908,581; 5,935,720; 6,091,195.
One of the ultimate objectives of the developments in organic light-emitting diodes (OLED) is the application in full color flat-panel displays. Therefore, obtaining three primary colors of red, green and blue that meet commercial requirements is critical in the product applications of OLED. In current developments, one of the most commonly used methods to modify the color and luminance efficiency is by doping a small amount of a highly fluorescent material into the main EL emitter as a dopant. In this manner, lights of three primary colors of red, green and blue can be obtained. Accordingly, seeking perfect doping materials of red, green and blue (RGB) and improving the luminous efficiency, luminance and chromaticity of the RGB colors have become one of the most important research objectives. Among the three primary colors of lights, the research for a primary red light is most problematic.
The organic EL mechanism utilizes the energy transfer between the host and guest materials. For example, a red organic EL emission can be obtained by doping a small amount of a red fluorescent material (a guest) into tris-(8-hydroxyquinolinato)aluminum (Alq
3
) (the host) to transfer the green luminescent energy of Alq
3
to the red guest (see U.S. Pat. No. 4,769,292). In the current red fluorescent dopants, a series of materials developed by Eastman Kodak Company of the U.S. are most notable (see U.S. Pat. No. 5,908,581). Alq
3
is a suitable host for red EL emitters since its emission at 530 nm is adequate to sensitize guest EL emission in the red spectral region.
The photoluminescent efficiency of the earliest red guest dye, 4-(dicyano-methylene)-2-methyl-6-(p-dimethylaminostyryl)4-pyran (DCM), (Formula A) is 78% at &lgr;
max
=596 nm in a reasonable doping concentration (about 0.5%). The light emitted is a yellow-biased red. Subsequently, Kodak developed a julolidyl derivative DCJ red guest material (Formula B) which emits light with a peak wavelength within the red-range of 610~690 nm. The Alq
3
-based EL emitter has a maximum luminous intensity when doped with 0.57% of DCJ. However, the color of the light emitted is reddish orange due to the green luminescence of Alq
3
that was not totally quenched. While more saturated red luminescence can be obtained when DCJ doped into the emitter is >4% of the emitter, the luminance efficiency drops to 50% of that with the maximum luminous intensity. This is because the red guest dyes aggregate and influence each other at high concentrations, which leads to the quenching of the luminescence.
Other considerations in the fields of fluorescence and electroluminescence applications are the purity of fluorescent materials and the degree of synthetic complexities, including consideration of yield loss due to post-process purification procedures. In the aforementioned patents, the preparation and subsequent purification of both DCM and DCJ are complicated by the inevitable generation of a significant amount of an unwanted corresponding bis-condensed dye caused by the further reaction of the “active” methyl group present in the fluorescent dye molecules (see Hammond,
Optics Comm
., 1989, 29, 331). Furthermore, once the bis-condensed dye is formed in the reaction mixture, it is difficult to be removed and tends to diminish or extinguish DCM (or DCJ) fluorescence.
To reduce the quenching of the guest dye per se and correspondingly raise the luminance efficiency, Kodak developed derivatives which are based on tetra-methyljulolidine (DCJT) (Formula C) (see Chen et al.,
Proc
. 2
nd
Internat. Sym. Chem. Functional Dyes
. 1992, 536). They have four more methyl groups than DCJ. Thereby, the methyl groups result in a steric hindrance between dye molecules, and thus reduce the quenching effect caused by the formation of aggregate due to high concentration.
While DCJT has a good luminance efficiency, the controlling of synthesis and purification remains a problem. The reactive methyl group at the C-6 position of pyran moiety of the DCJT product may further react with the starting material tetramethyljulolidyl aldehyde to produce the by-product, bis-DCJT (Formula D). (See C. H. Chen, C. W. Tang, J. Shi and K. P. Klubek,
Macromol. Symp
. 125, 49 1997.) Therefore, the preparation and subsequent purification of DCJT are also complicated by the inevitable generation of significant amount of the unwanted bis-condensed dye that tends to decrease the fluorescence efficiency of DCJT.
To overcome this problem, C. H. Chen et al. (see U.S. Pat. No. 5,908,581, issued Jun. 1, 1999, entitled “Red Organic Electroluminescent Materials” and U.S. Pat. No. 5,935,720, issued Aug. 10, 1999, entitled “Red Organic Electroluminescent Devices”)

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