Abstract:
An organic EL device which contains an anode, a cathode, and at least one organic thin-file layer including a light emitting layer which contains a compound represented by the following general formula (1):  
                         
 
     wherein Ar 1  represents a substituted or unsubstituted aromatic hydrocarbon group or a substituted or unsubstituted aromatic heterocyclic group. X represents a single bound or methylene group. R1 to R8 each independently represents a hydrogen atom, a halogen atom, a nitro group, a cyano group, a substituted or unsubstituted alkyl group, a substituted or unsubstituted amino group, a substituted or unsubstituted alkoxy group a substituted or unsubstituted aryloxy group or a substituted or unsubstituted alkoxycarbonyl group, a substituted or unsubstituted aromatic hydrocarbon group or a substituted or unsubstituted aromatic heterocyclic group. Any two of R1 to R8 may form a ring.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention generally relates to a light emitting device, and more specifically to an organic electroluminescent device of a blue luminescent material having an isoindolo [2,1-a] indole skeleton and good light-emitting properties.  
           [0003]    2. Description of the Related Art  
           [0004]    The organic electroluminescent device (organic EL device) is a light emitting device, containing a fluorescent material which emits light in response to the recombination of hole and electron injected from anode and cathode (C. W. Tang et al. Applied Physics Letters, 51,913 (1987)). Luminescence efficiency can be improved through a method of doping a fluorescent dye. An organic EL device with a coumarin dye as the doping material (Applied Physics Letters, 65,3610 (1989)) can be used to greatly improve the luminescence efficiency. A C-545T (U.S. Pat. No. 4,769,292), which is a well-known coumarin dye, has the following structure:  
                         
 
           [0005]    For improving the recombination efficiency of the injected hole and electron, multi-layered devices have been introduced. A hole transporting layer (HTL) contains hole transporting material (HTM) is used to improve the hole injection and transporting from the anodeinto the organic layer. An NPB (4-4′-bis [N-(1-naphthyl)-N-phenyl-amino-] bisphenyl), which is a well-known HTM, has the following structure:  
                         
 
           [0006]    An electron transporting layer (ETL) consisting of an electron transporting material (ETM) is used to improve the electron injection from the cathode into the organic layer. An Alq 3  (aluminum tris(8-hydroxyquinolate)), which is a typical ETM, has the following structure:  
                         
 
           [0007]    Other materials such as oxadiazole compounds, triazine compounds and triazole compounds also can be used as ETM.  
           [0008]    Armoatic dimethylidyne compounds have been used as the blue light emissive material for the organic EL device (U.S. Pat. No. 6,093,864). One example is DPVBi (1,4-bis (2,2-di-phenylvinyl)) biphenyl with an EL peak at about 485 nm, having the following structure:  
                         
 
           [0009]    Other blue light emissive materials for organic EL device include:  
           [0010]    1,3-Dibenzyldieneindane compound (U.S. Pat. No. 6,180,267), an example of them has the following structure:  
                         
 
           [0011]    o-(N-aryl-2-benzimidazolyl)phenol organometallic complex (U.S. Pat. No. 5,755,999), an example of them has the following structure:  
                         
 
           [0012]    wherein M is a divalent or a trivalent metal.  
           [0013]    Heterocyclic compounds (JP-2000-299186, JP-2001-23777, JP-2001-35664, JP-2001-118683, JP-2001-196181, JP-2001-196182), some examples of these compounds are shown with the following structures:  
                         
 
         SUMMARY OF THE INVENTION  
         [0014]    An object of the present invention is to provide a material having an isoindolo[2,1-a]indole skeleton and to provide an organic EL device having blue luminescence. The organic EL device comprises an anode, cathode, and one or more organic thin film layers which contain, either singly or as a mixture, an indole compound represented by the following general formula (1):  
                         
 
           [0015]    wherein Ar 1  represents a substituted or unsubstituted aromatic hydrocarbon group or a substituted or unsubstituted aromatic heterocyclic group. X represents a single bound or methylene group. R1 to R8 each independently represents a hydrogen atom, a halogen atom, a nitro group, a cyano group, a substituted or unsubstituted alkyl group, a substituted or unsubstituted amino group, a substituted or unsubstituted alkoxy group a substituted or unsubstituted aryloxy group or a substituted or unsubstituted alkoxycarbonyl group, a substituted or unsubstituted aromatic hydrocarbon group or a substituted or unsubstituted aromatic heterocyclic group. Any two of R1 to R8 may form a ring  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    [0016]FIG. 1 illustrates a construction of an organic EL element of the present invention;  
         [0017]    [0017]FIG. 2 illustrates another construction of the organic EL element of the present invention;  
         [0018]    [0018]FIG. 3 illustrates the  1 H-NMR of compound (A);  
         [0019]    [0019]FIG. 4 illustrates the Photoluminescence spectrum of compound (A);  
         [0020]    [0020]FIG. 5 illustrates the EL spectrum of device example 1; and  
         [0021]    [0021]FIG. 6 illustrates the EL spectrum of device example 2. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0022]    The present invention will hereinafter be described in detail. In the present invention, an organic EL element contains light emitting material having isoindolo [2,1-a] indole, represented by the following general formula (1):  
                         
 
         [0023]    wherein Ar 1 , X, R1 to R8 have the same meaning as above.  
         [0024]    Referring to the following reaction schemes, synthesis methods to obtain the isoindolo [2,1-a] indole compound represented by the formula (1) of the present invention will be described.  
         [0025]    The main skeleton of the isoindolo [2,1-a] indole compound can be formed by the following scheme (1):  
                         
 
         [0026]    wherein R1 to R8 are the same as defined above.  
         [0027]    As shown in the following scheme (2), formylation of isoindolo [2,1-a] indole skeleton compound represented by formula (2) of the present invention can obtain isoindolo [2,1-a] indole 3-aldehyde compound represented by the formula (3):  
                         
 
         [0028]    wherein R1 to R8 are the same as defined above.  
         [0029]    Finally, isoindolo [2,1-a] indole compound represented by the formula (1) of the present invention can be obtained by the following reaction scheme (3):  
                         
 
         [0030]    wherein R1 to R8, X and Ar are the same as defined above.  
         [0031]    If the aldehyde represented by the general formula (3) reacts with a Witting reagent having the following general formula:  
                         
 
         [0032]    wherein Ar1 is the same as Ar.  
         [0033]    The isoindolo [2,1-a] indole skeleton compound represented by the general formula (1) with X is the methylene group, if the aldehyde represented by the general formula (3) reacts with amino aromatic compounds having the following general formula:  
                         
 
         [0034]    wherein R9 to R12 are the same as defined R1 to R4.  
         [0035]    The isoindolo[2,1-a]indole skeleton based compound represented by the general formula (1) with X as a single bond.  
         [0036]    Representative examples of these isoindolo [2,1-a] indole skeleton based compounds are shown below.  
                         

                         

                         
 
         [0037]    The organic EL device according to the present invention has a multi-layered structure including a light emitting layer, hole transporting layers, and electron transporting layers.  
         [0038]    Hole transporting layer contains one or more organic layers including a hole injection layer. A hole injection layer increases the light emitting performance by improving the hole injection from the anode into the organic layers, and improving the contact of anode with organic layers. Typical compounds for the hole injection materials include porphyrin compounds (U.S. Pat. Nos. 3,935,031 or 4,356,429) having the example structure:  
                         
 
         [0039]    wherein M is a metal, metal oxide, or metal halide. Aromatic terti amine compounds (U.S. Pat. Nos. 4,127,412, 6,047,734) include diarylamine or triarylamine having the example structures:  
                         
 
         [0040]    These compounds are suitable for both hole injection materials and hole transporting materials.  
         [0041]    Electron transporting layer contains one or more organic layers to inject and transport electron from cathode into organic layer. An electron injection layer increases the light emitting property by improving the electron injection performance from the cathode into the organic layers. Typical compounds for the electron injection materials include oxadiazole compounds, triazine compounds and triazole compounds. Examples of these compounds are shown below:  
                         
 
         [0042]    Cathode for an organic EL device can be form by vacuum deposition a single metal or two kinds of metal. Typical examples of using single metal as cathode include aluminum (Al), magnesium (Mg), calcium (Ca) and lithium (Li). Common examples of using two kinds of metal as cathode include aluminum-lithium (Al—Li) and magnesium-silver (Mg—Ag). In this present invention, Al is chosen to be a single metal cathode.  
         [0043]    Anode for an organic EL device can be form by coating a conducting material on a substrate. Glass is a common and widely used substrate. In this present invention, conducting material indium-tin-oxide (ITO) on glass substrate is used to be the anode.  
         [0044]    In this present invention, an organic EL device is manufactured by vacuum deposition of organic materials and cathode compressed ITO (anode)/organic layers/Al (cathode). Organic layers include hole injection layer, hole transporting layer, emitting layer and electron injection layer. The total thickness of organic layers in this present invention ranges from 5 nm to 500 nm. And the thickness of the cathode is preferably 150 nm to 250 nm. When a DC power of 5 to 25 voltage is applied to the organic EL device in this present invention, blue light emission is obtain.  
         [0045]    The present invention will hereafter be described in detail with reference to examples, but the present invention is not limited only to the following examples.  
       EXAMPLE I  
       [0046]    Synthesis of indole based compound (A) having the structure:  
                         
 
         [0047]    0.011 mole of indole and 0.011 mole of KOH were stirred homogenous in DMF for 1 hour. 0.01 mole of 2-Bromobenzyl bromide was added into the reaction mixture. The mixture was stirred for 3 hours. Water was added to quench the reaction and extracting with ether. Organic solution was evaporated under reduced pressure and further purification by column chromatography on silica gel with hexane as an eluent gave a pale yellow solid of N-o-bromobenzylbromide indole. (80% yield). 0.01 mole of the indole product, KOAC, DMA, Pd(Ph 3 P) 4  were stirred at 160° C. for 16 hours. After cooling down, the reaction mixture was evaporated under reduced pressure to remove DMA. MeOH was added to get the solid product (A-1). (70%).  1 H-NMR (CDCl 3 , TMS) δ(ppm)=5.1 (s, 2H), 6.7 (s, 1H), 7.1-7.8 (m, 8H, aromatic H).  
                         
 
         [0048]    0.01 mole of A-1 was dissolved in DMF (10 ml), and a mixture of POCl 3  (0.011 mole) and DMF (0.011 mole) was added drop-wise. After stirring at 75° C. for 1 h, the reaction mixture was added into saturate NaHCO 3  solution. The precipitated solid was collected by filtration and washed with ethanol to give a pale gray solid of compound (A-2) (85% yield).  1 H-NMR (CDCl 3 , TMS) δ(ppm)=5.1 (s, 2H), 7.1-7.8 (m, 7H, aromatic H), 8.3 (d, 1H, aromatic H), 10.3 (s, 1H, aldehyde H).  
                         
 
         [0049]    A dry DMF solution of the compound (A-2) (0.01 mole) was added o-aminothiophenol (0.013 mole) and Ac 2 O (0.5 ml) at room temperature. The mixture was stirred at 60° C. for 3 h. The reaction mixture was poured into stirred benzene and the precipitated solid was collected by filtration. The crude solid was washed with MeOH and recrystallized from benzene to afford target compound (A) (32% yield).  1 H-NMR spectrum is shown in FIG. 3. Photoluminescencs spectrum is shown in FIG. 4.  
         [0050]    The organic EL device in the example uses the compound (A) as the light emitting material. The example uses the glass substrates with ITO electrode having a surface resistance of 20 (Ω/□) as the anode.  
       DEVICE EXAMPLE 1-1  
       [0051]    As shown in FIG. 1, a 60 nm organic layer  12  is formed on the ITO  11  as the hole-transporting layer by vacuum deposition NPB having the following structure:  
                         
 
         [0052]    Over the hole-transporting layer  12 , a 30 nm emitting layer  13  is formed by vacuum deposition carbazole biphenyl (CBP) as the host having the following structure:  
                         
 
         [0053]    and 2% weight of compound (A) as the dopant on the hole-transporting layer  12 . Then, a 20 nm second electron-transporting layer  14  is formed on the emitting layer  13  by vacuum deposition Alq 3  having the following structure:  
                         
 
         [0054]    A 0.7 nm first electron transporting layer  15  is form on second the electron transporting layer  14  by vacuum deposition LiF. Finally, a 200 nm aluminum cathode  16  is formed by vacuum deposition on the first electron-transporting layer  15 . When a dc voltage of 15 V is applied to the resulting device, a 2700 cd/m 2  brightness blue light emission is obtained. EL spectrum is shown in FIG. 5.  
       DEVICE EXAMPLE 1-2  
       [0055]    As shown in FIG. 2, a 30 nm organic layer  22  is formed on the ITO 21 as a first hole transporting layer by vacuum deposition CuPC having the following structure:  
                         
 
         [0056]    Over the first hole transporting layer  22 , a 40 nm second hole-transporting layer  23  is formed by vacuum deposition NPB. Over the second hole-transporting layer  23 , a 30 nm emitting layer  24  is formed by vacuum deposition CBP and 2% weight of compound (A) on the hole-transporting layer  23 . Then, a 20 nm second electron-transporting layer  25  is formed by vacuum deposition Bphen having the following structure:  
                         
 
         [0057]    A 0.7 nm first electron injection layer  26  is form by vacuum deposition LiF on the second electron transporting layer  25 . Finally, a 200 m aluminum cathode  27  is formed by vacuum deposition on the first electron-transporting layer  26 . When a dc voltage of 15 V is applied to the resulting device, a 3800 cd/m 2  brightness blue light emission is obtained. EL spectrum is shown in FIG. 6. The spectrum is quite the same as the spectrum of example 1.  
       COMPARED DEVICE EXAMPLE 1-1  
       [0058]    In the similar manner as applied to device example 1, an emitting layer is form by vacuum deposition DPVBi having the structure:  
                         
 
         [0059]    When a dc voltage of 15 V is applied to the resulting device, a 1750 cd/m 2  brightness blue light emission is obtained.  
         [0060]    In comparison with DPVBi as light emitting material, an organic EL device using compound (A) as light emitting material has higher brightness.