Abstract:
An organic material capable of generating luminescence by charge of an electric current includes a compound having a general formula (1):  
                         
 
     wherein Ar 1 , Ar 2  and Ar 3  may be the same or different and, respectively, represent a substituted or unsubstituted aryl group or heterocyclic aryl group and wherein R 1 , R 2 , R 3 , and R 4  may be the same or different and, respectively, represent a alkyl group, cycloalkyl group, a substituted or unsubstituted aryl group or heterocyclic aryl group.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates to an organic electroluminescent material and an organic electroluminescent device fabricated using the material. More specifically, the present invention relates to an organic luminescent material used for full color display and an organic electroluminescent device fabricated using the material.  
           [0003]    2. Description of the Related Art  
           [0004]    Electroluminescent devices using organic luminescent materials have been actively researched recently because of wider viewing angles and faster response time than conventional LCDs. More particularly, when using organic compounds as a luminescent material, it has been expected to realize a flat panel display, which makes use of spontaneous light and has a high response speed regardless of an angle of field. These organic electroluminescent devices when incorporated in consumer electronic devices such digital camera, PDA and videophones will offer several advantages such as low power consumption, high brightness, and light and thin design.  
           [0005]    Typically, the organic electroluminescent (EL) device has an organic thin film which contains a luminescent material capable of emitting light through the charge of an electric current and is formed between an optically transparent anode and a metallic cathode. For the production of full-color EL display panel, it is necessary to have efficient red, green and blue (RGB) EL materials with proper chromaticity and sufficient luminance efficiency.  
           [0006]    Currently, organic electroluminescent devices still have problems to solve. There has been difficulty in developing a stable red luminescent material with high luminance. For instance, red luminescent material DCJTB, disclosed in U.S. Pat. No. 5,935,720 is not satisfactory because its high molecular dipole moment causes self-quench and results in low luminance efficiency.  
         SUMMARY OF THE INVENTION  
         [0007]    It is an object of the present invention to provide a red luminescent material and an organic electroluminescent device fabricated using the material.  
           [0008]    To achieve the above listed and other objects, the present invention provides an organic material capable of generating luminescence by charge of an electric current. The organic material contains a compound represented by the following general formula (1):  
                         
 
           [0009]    wherein Ar 1 , Ar 2  and Ar 3  may be the same or different and, respectively, represent a substituted or unsubstituted aryl group or heterocyclic aryl group, and wherein R 1 , R 2 , R 3 , and R 4  may be the same or different and, respectively, represent a alkyl group, cycloalkyl group, a substituted or unsubstituted aryl group or heterocyclic aryl group.  
           [0010]    The compound represented by the general formula (1) and used as a luminescent material in the organic electroluminescent device of the invention may be available as at least one of the following structural formulas (2)-1 and (2)-2.  
                         
 
           [0011]    To achieve the above listed and other objects, the present invention further provides an organic electroluminescent device made of the compound represented by the general formula (1). 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    Other objects, advantages, and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings:  
         [0013]    [0013]FIG. 1 is a schematic sectional view of an organic electroluminescent device according to one preferred embodiment of the invention;  
         [0014]    [0014]FIG. 2 is a schematic sectional view of an organic electroluminescent device according to another preferred embodiment of the invention; and  
         [0015]    [0015]FIG. 3 is a schematic sectional view of an organic electroluminescent device according to still another preferred embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0016]    The present invention discloses a novel red luminescent material comprising a compound having a general formula (1):  
                         
 
         [0017]    In the general formula (1), Ar 1 , Ar 2  and Ar 3  may be the same or different and, respectively, represents an aryl group or a heterocyclic aryl group. The aryl group may be exemplified by a substituted or unsubstituted phenylene group, a substituted or unsubstituted naphthalene group, a substituted or unsubstituted anthracene group, a substituted or unsubstituted phenylene-vinylene group, a substituted or unsubstituted naphthalene-vinylene group, and a substituted or unsubstituted anthracene-vinylene group. The heterocyclic aryl group may be exemplified by a substituted or unsubstituted thiophene group, a substituted or unsubstituted carbazole group, a substituted or unsubstituted pyrrole group, a substituted or unsubstituted pyridine group, a substituted or unsubstituted thiophene-vinylene group, a substituted or unsubstituted carbazole-vinylene group, and a substituted or unsubstituted pyrrole-vinylene group. R 1 , R 2 , R 3 , and R 4  may be an alkyl group, a cycloalkyl group, or a substituted or unsubstituted aryl group or heterocyclic aryl group. The alkyl group may be exemplified by a methyl group, ethyl group, propyl group, isopropyl group, butyl group, isobutyl group and neobutyl group. The cycloalkyl group may be exemplified by a cyclopentyl group and cyclohexyl group. The substituted or unsubstituted aryl group may be exemplified by a substituted or unsubstituted phenylene group, a substituted or unsubstituted naphthalene group, and a substituted or unsubstituted anthracene group. The substituted or unsubstituted heterocyclic aryl group may be exemplified by a substituted or unsubstituted thiophene group, a substituted or unsubstituted carbazole group, and a substituted or unsubstituted pyrrole group.  
         [0018]    The compound represented by the general formula (1) has both electron transportability and hole transportability, and can be used as a luminescent layer serving as an electron transport layer, or as a luminescent layer serving as a hole transport layer in an organic electroluminescent device. Moreover, it is possible to provide a device wherein the compound represented by the general formula (1) is formed as a luminescent layer sandwiched between an electron transport layer and a hole transport layer.  
         [0019]    FIGS.  1  to  3  show embodiments of organic electroluminescent devices, respectively, according to the invention. In the figures, numeral  10  indicates a substrate for forming an organic electroluminescent device, which may be made of glass, plastics and other appropriate materials. Numeral  20  indicates a transparent electrode (anode), which can be made of ITO (indium tin oxide), SnO 2  or the like. Numeral  50  indicates an organic luminescent layer, which contains the above-mentioned compound represented by the general formula (1) as a luminescent material. The organic luminescent layer  50  may be formed on the upper surface of the anode  20  by physical vapor deposition (PVD) such as molecular beam deposition method, resistive heating method, and the like. Numeral  30  indicates a cathode formed by a metal thin film on the upper surface of the organic luminescent layer  50 . The cathode may be made of an alloy or metal, such as, aluminum, magnesium, aluminum-lithium, magnesium-sliver, and the like.  
         [0020]    In the organic electroluminescent device of the invention, the organic luminescent layer  50  may have an organic built-up structure. Referring to FIG. 2, the organic luminescent layer  50  includes an electron transport layer  50   a , a hole transport layer  50   b , and the compound represented by the general formula (1) may be contained in one or both of these layers to provide a luminescent electron transport layer  50   a  or hole transport layer  50   b . Alternatively, for improving the luminescent performance, it is preferred to provide a structure (see FIG. 3) wherein the luminescent layer  50   c  containing the compound represented by the general formula (1) is sandwiched between the electron transport layer  50   a  and the hole transport layer  50   b . The electron transport layer  50   a  may include tris (8-quinolinolat) aluminum (Alq 3 ), and the hole transport layer  50   b  may include Bis((N-(1-naphthyl-n-phenyl))benzidine (α-NPD).  
         [0021]    The compound represented by the general formula (1) of the present invention can be synthesized through the synthetic sequences outlined in Scheme 1.  
                         
 
         [0022]    According to the synthetic sequences outlined in Scheme 1, 10 mmol of compound 1 and 5 mmol of compound 2 are added to a 100 ml reaction flask. 10 ml of t-BuOH and 10 ml of THF are then added as solvents, and the compounds are dissolved by stirring. Then, the solution is heated under reflux for 16 hours after 10 mmol of KOBu t  is added. After reaction is complete, the solution is added dropwise to water so as to precipitate compound 3. Therefore, compound 3 is recovered as a solid product by suction filtration and is further purified through recrystallization using MeOH/CH 2 Cl 2  as a solvent.  
         [0023]    The compound represented by the general formula (1) of the invention can be also synthesized through the synthetic sequences outlined in Scheme 2.  
                         
 
         [0024]    According to the synthetic sequences outlined in Scheme 2, 20 mmol of compound 4, 20 mmol of compound 5, and 20 mmol of NaOBu t  are added to a 100 ml two-necked flask, and then 50 ml of o-xylene is added as a solvent. The compounds are dissolved by stirring. The solution is heated and refluxed for 24 hours after 1 mol % of Pd(OAc) 2  and 4 mol % of P(t-Bu) 3  are added as catalysts. The solvent is removed under reduced pressure and compound 6 is obtained by purification through column chromatography using acetic ester/n-hexane (1:10) as eluent.  
         [0025]    Thereafter, 10 mmol of compound 7 and 5 mmol of compound 2a are added to a 100 ml flask. 10 ml of t-BuOH and 10 ml of THF are added as solvents. The compounds are dissolved by stirring. The solution is heated under reflux for 16 hours after 10 mmol of KOBu t  is added. After reaction is complete, the solution is added dropwise to water so as to precipitate compound 8 which is recovered as the solid product by suction filtration and purified through sublimation under reduced pressure.  
         [0026]    Finally, 10 mmol of compound 6, 5 mmol of compound 8, and 20 mmol of NaOBu t  are added to a 100 ml two-necked flask, and 25 ml of o-xylene is added as a solvent. The reaction mixture is dissolved by stirring. The solution is heated under reflux for 24 hours after 1 mol % of Pd(OAc) 2  and 4 mol % of P(t-Bu) 3  is added as catalysts. When the solution cools down to room temperature, the solution is filtered and the filtrate is added to methanol so as to precipitate the solid product 9 which is obtained by suction filtration and purified through sublimation under reduced pressure.  
       EXAMPLES  
       [0027]    The invention is more particularly described by way of examples, which should not be construed to limit the invention thereto.  
       Example 1  
       [0028]    The compound represented by the structural formula (2)-1 is synthesized through the synthetic sequences outlined in Scheme 1.  
                         
 
         [0029]    (1) First, 5.44 g (40 mmol) of terephthaldicarboxaldehyde is added to a 250 ml two-necked flask, and 100 ml of 1,4-dioxane and 1.5 ml of water are added as solvents. Then, the solution is heated under reflux for 24 hours after 16.2 g (40 mmol) of Ph 3 P+CH 3 I— and 9.0 g of potassium carbonate are added. The solution is filtered to remove the solid therein and then the silica gel is added to the filtered solution. The solvent is removed under reduced pressure and compound 10 (2.64 g, yield=50%) is obtained by purification through column chromatography using 5% EA/Hexane as eluent.  
         [0030]    (2) 3.96 g (30 mmol) of compound 10 and 6.06 g (30 mmol) of 4-Bromo-N, N-dimethylaniline are added to a 500 ml two-necked flask, and 150 ml of DMF and 90 ml of triethyl amine are added as solvents. Then, the solution is heated for 24 hours under nitrogen atmosphere after 0.35 g (1.6 mmol) of Pd(OAc) 2  and 1.97 g (6.4 mmol) of P(o-toyl) 3  are added as catalysts. The solution is added dropwise to water so as to precipitate the solid. Thereafter, yellow solid la (6.1 g, yield=81%) is obtained by suction filtration and purified through recrystallization using MeOH/CH 2 Cl 2  as solvent.  
         [0031]    (3) 2.51 g (10 mmol) of compound la (Ar 4  is phenylene-vinylene group) and 0.81 g (5 mmol) of compound 2a (p-phenylene-diacetonitrile (Ar 1  is phenyl group)) are added to a 100 ml two-necked flask, and 10 ml of t-BuOH and 10 ml of THF are added as solvents. Then, the solution is heated under reflux for 16 hours. After reaction is complete, the solution is added dropwise to water so as to precipitate the compound represented by the structural formula (2)-1. The solid product 3a is obtained by suction filtration and purified through recrystallization using MeOH/CH 2 Cl 2  as solvent (yield=80%). The compound represented by the structural formula (2)-1 in CHCl 3  solution has a PL λmax=593 nm measured by fluorescent spectrometer.  
       Example 2  
       [0032]    The compound represented by the structural formula (2)-2 is synthesized through the synthetic sequences outlined in Scheme 2.  
                         
 
         [0033]    (1) First, 4.04 g (20 mmol) of 4-Bromo-N, N-dimethylaniline (R 5 , R 6  are hydrogen), 1.86 g (20 mmol) of aniline (R 7  is hydrogen) and 1.92 g (20 mmol) of NaOBu t  are added to a 100 ml two-necked flask and 50 mL of o-xylene is added as solvent. The reaction mixture is dissolved by stirring. Then, the solution is heated under reflux for 24 hours after 44 mg (1 mol %) of Pd(OAc) 2  and 0.16 g (4 mol %) of P(t-Bu) 3  are added as catalyst. The solvent is removed under reduced pressure and compound 6a is obtained by purification through column chromatography using acetic ester/n-hexane (1:10) as eluent (yield=50%).  
         [0034]    (2) Then, 1.86 g (10 mmol) of 4-bromobenzaldehyde and 0.81 g (5 mmol) of compound 2a are added to a 100 ml two-necked flask, and 10 ml of t-BuOH and 10 ml of THF are added as solvents. The reaction mixture is dissolved by stirring. Then, the solution is heated under reflux for 16 hours after 1.2 g (10 mmol) of KOBu t  is added. After reaction is complete, the solution is added dropwise to water so as to precipitate compound 8 which is recovered as the solid product by suction filtration and purified through sublimation under reduced pressure (yield=72%).  
         [0035]    (3) Finally, 2.12 g (10 mmol) of compound 6a, 2.45 g (5 mmol) of compound 8 and 2.4 g (20 mmol) of NaOBu t  are added to a 100 ml two-necked flask. 25 ml of o-xylene is added as solvent. The reaction mixture is dissolved by stirring. Then the reaction mixture is heated under reflux for 24 hours after 22 mg (1 mol %) of Pd (OAc) 2  and 80 mg (4 mol %) of P(t-Bu) 3  are added as catalysts. When the solution cools down to room temperature, the solution is filtered and the filtrate is added to methanol so as to precipitate the solid product 9a (represented by the structural formula (2)-2 which is obtained by suction filtration and purified through sublimation under reduced pressure (yield=75%). The compound represented by the structural formula (2)-1 in CHCl 3  solution has a PL λmax=593 nm measured by fluorescent spectrometer.  
         [0036]    Although the invention has been explained in relation to its preferred embodiment, it is to be understood that many other possible modifications and variations can be made without departing from the spirit and scope of the invention as hereinafter claimed.