Patent Publication Number: US-2015060818-A1

Title: Organic Compound, Light-Emitting Element, Light-Emitting Device, Electronic Device, and Lighting Device

Description:
TECHNICAL FIELD 
     The present invention relates to a semiconductor device, a display device, a light-emitting device, a lighting device, a driving method thereof, and a manufacturing method thereof. Specifically, one embodiment of the present invention relates to a novel organic compound and a light-emitting element including the organic compound. One embodiment of the present invention relates to a light-emitting element using organic electroluminescence (EL). One embodiment of the present invention also relates to a light-emitting device, an electronic device, and a lighting device each including the light-emitting element. 
     BACKGROUND ART 
     In recent years, research and development have been extensively conducted on light-emitting elements using electroluminescence. In a basic structure of such a light-emitting element, a light-emitting layer containing a light-emitting substance is interposed between a pair of electrodes. By applying voltage to this element, light emission from the light-emitting substance can be obtained. 
     Such a light-emitting element is of self-luminous type, and thus has advantages over a liquid crystal display in that visibility of pixels is high, a backlight is not needed, and so on. Therefore, such a light-emitting element is regarded as being suitable as a flat panel display element. Besides, such a light-emitting element has advantages in that it can be manufactured to be thin and lightweight, and has very fast response speed. 
     Since such light-emitting elements can be formed in a film form, they make it possible to provide emission from a planar surface. Thus, a large-area element having a planar emission surface can be easily formed. This is a feature that is difficult to obtain with point light sources typified by an incandescent lamp and an LED or linear light sources typified by a fluorescent lamp. Therefore, the light-emitting element is very effective for use as a surface light source applicable to lighting and the like. 
     Such light-emitting elements utilizing electroluminescence can be broadly classified according to whether a light-emitting substance is an organic compound or an inorganic compound. In the case of an organic EL element in which a layer containing an organic compound used as a light-emitting substance is provided between a pair of electrodes, application of voltage to the light-emitting element causes injection of electrons from a cathode and holes from an anode into the layer containing the organic compound having a light-emitting property and thus current flows. The injected electrons and holes then lead the organic compound to its excited state, whereby light emission is obtained from the excited organic compound. 
     Note that excited states of the organic compound include a singlet excited state and a triplet excited state. Light emission from the singlet excited state (S*) is called fluorescence, and light emission from the triplet excited state (T*) is called phosphorescence. The statistical generation ratio thereof in the light-emitting element is considered to be S*:T*=1:3. 
     At room temperature, a compound capable of converting a singlet excited state into luminescence (hereinafter, referred to as a fluorescent compound) generally exhibits only luminescence from the singlet excited state (fluorescence), and luminescence from the triplet excited state (phosphorescence) cannot be observed. Accordingly, the internal quantum efficiency (the ratio of the number of generated photons to the number of injected carriers) of a light-emitting element including the fluorescent compound is assumed to have a theoretical limit of 25%, on the basis of S*:T*=1:3. 
     In contrast, a compound capable of converting a triplet excited state into luminescence (hereinafter, referred to as a phosphorescent compound) exhibits luminescence from the triplet excited state (phosphorescence). Further, since intersystem crossing (i.e., transition from a singlet excited state to a triplet excited state) easily occurs in a phosphorescent compound, the internal quantum efficiency can be theoretically increased to 100%. That is, higher emission efficiency can be achieved than the case of using a fluorescent compound. For this reason, light-emitting elements using a phosphorescent compound have been under active development recently so that high-efficiency light-emitting elements can be achieved. 
     When a light-emitting layer of a light-emitting element is formed using the phosphorescent compound described above, in order to suppress concentration quenching or quenching due to triplet-triplet annihilation of the phosphorescent compound, the light-emitting layer is usually formed such that the phosphorescent compound is dispersed in a matrix of another compound. Here, the compound serving as the matrix is called host material, and the compound dispersed in the matrix like the phosphorescent compound is called guest material. 
     When the phosphorescent compound is used as the guest material, the host material is required to have a higher triplet excitation energy level (difference in energy between the ground state and the triplet excited state, which is also referred to as T 1  level) than the phosphorescent compound. 
     Since the singlet excitation energy level (difference in energy between the ground state and the singlet excited state, which is also referred to as S 1  level) is higher than a T 1  level, a substance that has a high T 1  level also has a high S 1  level. Therefore, the above substance that has a high T 1  level is also effective in a light-emitting element using a fluorescent compound as a light-emitting substance. 
     For example, compounds having a dibenzo[f,h]quinoxaline skeleton have been studied as examples of a host material used when a phosphorescent compound is a guest material (see Patent Documents 1 and 2). In addition, a quinoxaline-based compound that is used in a light-emitting layer, an electron-transport layer, or an electron-injection layer has been studied (see Patent Document 3). 
     REFERENCE 
     Patent Document 
     [Patent Document 1] PCT International Publication No. 03/058667 
     [Patent Document 2] Japanese Published Patent Application No. 2007-189001 
     [Patent Document 3] Japanese Published Patent Application No. H09-188874 
     DISCLOSURE OF INVENTION 
     As reported in Patent Document 1 or 2, although host materials of phosphorescent compounds have been developed, there is room for improvement in tennis of emission efficiency, reliability, light-emitting characteristics, synthesis efficiency, cost, or the like, and further development is required for obtaining more excellent phosphorescent compounds. 
     As reported in Patent Document 3, a compound in which a biphenyldiyl group has two quinoxaline moieties has been studied. That is, a compound in which a p-phenylene group has two dibenzo[f,h]quinoxaline skeletons is synthesized, but the element characteristics, reliability, or the like of a light-emitting element including the compound are not mentioned. The use of such a quinoxaline-based compound causes some problems. For example, a material including the quinoxaline-based compound has low heat resistance, or crystallization readily occurs in a thin film state. phosphorescent compound 
     In view of the above problems, an object of one embodiment of the present invention is to provide a novel organic compound. Another object of one embodiment of the present invention is to provide a novel organic compound that can be used in a light-emitting element as a host material of a light-emitting layer in which a light-emitting substance is dispersed. In particular, the object is to provide a novel organic compound that can be suitably used as a host material in the case where a phosphorescent compound is a light-emitting substance. Another object of one embodiment of the present invention is to provide a novel organic compound that has a high electron-transport property and suitably used in an electron-transport layer in a light-emitting element. 
     Another object of one embodiment of the present invention is to provide a novel light-emitting element. Another object of one embodiment of the present invention is to provide a light-emitting element which is driven at a low voltage and has high current efficiency. Another object of one embodiment of the present invention is to provide a light-emitting element having a long lifetime. Another object of one embodiment of the present invention is to provide a light-emitting device, an electronic device, and a lighting device each having reduced power consumption by using the above light-emitting element. 
     Note that the description of these objects does not disturb the existence of other objects. In one embodiment of the present invention, there is no need to achieve all the objects. Other objects will be apparent from and can be derived from the description of the specification, the drawings, the claims, and the like. 
     One embodiment of the present invention is an organic compound represented by General Formula (G1-1). 
       Ar 1 A) n   (G1-1)
 
     In General Formula (G1-1), A represents a substituted or unsubstituted dibenzo[f,h]quinoxalin-yl group; A 1  represents a substituent formed by 1 to 4 rings; the ring is a substituted or unsubstituted benzene ring or a substituted or unsubstituted fluorene ring; and n represents 2 or 3. 
     Another embodiment of the present invention is an organic compound represented by General Formula (G2-1). 
     
       
         
         
             
             
         
       
     
     In General Formula (G2-1), Ar 1  represents a substituent formed by 1 to 4 rings; the ring is a substituted or unsubstituted benzene ring or a substituted or unsubstituted fluorene ring; R 1  to R 9  independently represent any of hydrogen, an alkyl group having 1 to 6 carbon atoms, a cycloalkyl group having 3 to 6 carbon atoms, and a substituted or unsubstituted aryl group having 6 to 13 carbon atoms; and n represents 2 or 3. 
     Another embodiment of the present invention is an organic compound represented by General Formula (G3-1). 
     
       
         
         
             
             
         
       
     
     In General Formula (G3-1), Ar 1  represents a substituent formed by 1 to 4 rings; the ring is a substituted or unsubstituted benzene ring or a substituted or unsubstituted fluorene ring; R 11  to R 18  and R 20  each independently represent any of hydrogen, an alkyl group having 1 to 6 carbon atoms, a cycloalkyl group having 3 to 6 carbon atoms, and a substituted or unsubstituted aryl group having 6 to 13 carbon atoms; and n represents 2 or 3. 
     Another embodiment of the present invention is an organic compound represented by General Formula (G4-1). 
     
       
         
         
             
             
         
       
     
     In General Formula (G4-1), Ar 1  represents a substituent formed by 1 to 4 rings; the ring is a substituted or unsubstituted benzene ring or a substituted or unsubstituted fluorene ring; R 11  to R 17 , R 19 , and R 20  each independently represent any of hydrogen, an alkyl group having 1 to 6 carbon atoms, a cycloalkyl group having 3 to 6 carbon atoms, and a substituted or unsubstituted aryl group having 6 to 13 carbon atoms; and n represents 2 or 3. 
     Another embodiment of the present invention is an organic compound represented by General Formula (G1-2). 
       Ar 2 A) n   (G1-2)
 
     In General Formula (G1-2), A represents a substituted or unsubstituted dibenzo[f,h]quinoxalin-yl group; Ar 2  represents a substituent formed by three or four substituted or unsubstituted benzene rings; and n represents 2 or 3. 
     Another embodiment of the present invention is an organic compound represented by General Formula (G2-2). 
     
       
         
         
             
             
         
       
     
     In General Formula (G2-2), Ar 2  represents a substituent formed by three or four substituted or unsubstituted benzene rings; R 1  to R 9  each independently represent any of hydrogen, an alkyl group having 1 to 6 carbon atoms, a cycloalkyl group having 3 to 6 carbon atoms, and a substituted or unsubstituted aryl group having 6 to 13 carbon atoms; and n represents 2 or 3. 
     Another embodiment of the present invention is an organic compound represented by General Formula (G3-2). 
     
       
         
         
             
             
         
       
     
     In General Formula (G3-2), Ar 2  represents a substituent formed by three or four substituted or unsubstituted benzene rings; R 11  to R 18  and R 20  each independently represent any of hydrogen, an alkyl group having 1 to 6 carbon atoms, a cycloalkyl group having 3 to 6 carbon atoms, and a substituted or unsubstituted aryl group having 6 to 13 carbon atoms; and n represents 2 or 3. 
     Another embodiment of the present invention is an organic compound represented by General Formula (G4-2). 
     
       
         
         
             
             
         
       
     
     In General Formula (G4-2), Ar 2  represents a substituent formed by three or four substituted or unsubstituted benzene rings; R 11  to R 17 , R 19 , and R 20  each independently represent any of hydrogen, an alkyl group having 1 to 6 carbon atoms, a cycloalkyl group having 3 to 6 carbon atoms, and a substituted or unsubstituted aryl group having 6 to 13 carbon atoms; and n represents 2 or 3. 
     In each of the above structures, the benzene rings are preferably meta-substituted or ortho-substituted. 
     Another embodiment of the present invention is an organic compound represented by any one of Structural Formulae (101), (102), (103), (105), and (106). 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     Another embodiment of the present invention is a light-emitting element including any of the above organic compounds. Another embodiment of the present invention is a light-emitting device including the light-emitting element. Another embodiment of the present invention is an electronic device and a lighting device each including the light-emitting device. 
     One embodiment of the present invention can provide a novel organic compound that can be used in a light-emitting element as a host material of a light-emitting layer in which a light-emitting substance is dispersed. In particular, a novel organic compound that can be suitably used as a host material in the case where a phosphorescent compound is a light-emitting substance can be provided. Furthermore, a novel organic compound that has a high electron-transport property and suitably used in an electron-transport layer can be provided. Note that effects of one embodiment of the present invention are not limited to the above. Depending on circumstances or conditions, one embodiment of the present invention might produce another effect. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIGS. 1A to 1C  each illustrate a light-emitting element of one embodiment of the present invention. 
         FIG. 2  illustrates a light-emitting element of one embodiment of the present invention. 
         FIGS. 3A and 3B  illustrate a light-emitting device of one embodiment of the present invention. 
         FIGS. 4A and 4B  illustrate a light-emitting device of one embodiment of the present invention. 
         FIGS. 5A to 5E  each illustrate an electronic device of one embodiment of the present invention. 
         FIGS. 6A and 6B  illustrate lighting devices of embodiments of the present invention. 
         FIGS. 7A and 7B  are  1 H NMR charts of 2,2′-(1,1′-biphenyl-3,3′-diyl)di(dibenzo[f,h]quinoxaline) (mDBq2BP). 
         FIGS. 8A and 8B  show an emission spectrum and an absorption spectrum of a toluene solution of mDBq2BP. 
         FIGS. 9A and 9B  show an emission spectrum and an absorption spectrum of a thin film of mDBq2BP. 
         FIGS. 10A and 10B  are  1 H NMR charts of 2,2′,2″-[(1,3,5-benzene-triyl)tri(3,1-phenylene)]tri(dibenzo[f,h]quinoxaline) (mDBqP3P). 
         FIGS. 11A and 11B  are  1 H NMR charts of 2,2′-(1,1′:3′,1″:3″,1′″-quaterphenylene-3,3′″-diyl)di(dibenzo[f,h]quinoxaline) (mDBqP2BP). 
         FIGS. 12A and 12B  are  1 H NMR charts of 2,2′-[(9,9-dimethyl-9H-fluorene-2,7-diyl)di(3,1-phenylene)]di(dibenzo[f,h]quinoxaline) (mDBqP2F). 
         FIGS. 13A and 13B  show an emission spectrum and an absorption spectrum of a dimethylformamide solution of mDBqP2F. 
         FIGS. 14A and 14B  show an emission spectrum and an absorption spectrum of a thin film of mDBqP2F. 
         FIGS. 15A and 15B  are  1 H NMR charts of 2,2′-(1,1′:3′,1″-terphenylene-3,3″-diyl)di(dibenzo[f,h]quinoxaline) (mDBqP2P). 
         FIGS. 16A and 16B  show an emission spectrum and an absorption spectrum of a dimethylformamide solution of mDBqP2P. 
         FIGS. 17A and 17B  show an emission spectrum and an absorption spectrum of a thin film of mDBqP2P. 
         FIGS. 18A and 18B  are  1 H NMR charts of DBt-mDBqP2P. 
         FIG. 19  illustrates a light-emitting element of Example. 
         FIG. 20  shows current density-luminance characteristics of Light-emitting element 1 (element 1) of Example. 
         FIG. 21  shows voltage-luminance characteristics of Light-emitting element 1 of Example. 
         FIG. 22  shows luminance-current efficiency characteristics of Light-emitting element 1 of Example. 
         FIG. 23  shows voltage-current characteristics of Light-emitting element 1 of Example. 
         FIG. 24  shows an emission spectrum of Light-emitting element 1 of Example. 
         FIG. 25  shows current density-luminance characteristics of Light-emitting element 2 (element 2) of Example. 
         FIG. 26  shows voltage-luminance characteristics of Light-emitting element 2 of Example. 
         FIG. 27  shows luminance-current efficiency characteristics of Light-emitting element 2 of Example. 
         FIG. 28  shows voltage-current characteristics of Light-emitting element 2 of Example. 
         FIG. 29  shows an emission spectrum of Light-emitting element 2 of Example. 
         FIG. 30  shows current density-luminance characteristics of Light-emitting element 3 (element 3) of Example. 
         FIG. 31  shows voltage-luminance characteristics of Light-emitting element 3 of Example. 
         FIG. 32  shows current luminance-efficiency characteristics of Light-emitting element 3 of Example. 
         FIG. 33  shows voltage-current characteristics of Light-emitting element 3 of Example. 
         FIG. 34  shows an emission spectrum of Light-emitting element 3 of Example. 
         FIG. 35  shows current density-luminance characteristics of Light-emitting element 4 (element 4) of Example. 
         FIG. 36  shows voltage-luminance characteristics of Light-emitting element 4 of Example. 
         FIG. 37  shows luminance-current efficiency characteristics of Light-emitting element 4 of Example. 
         FIG. 38  shows voltage-current characteristics of Light-emitting element 4 of Example. 
         FIG. 39  shows an emission spectrum of Light-emitting element 4 of Example. 
         FIG. 40  shows results of a reliability test of Light-emitting element 1 of Example. 
         FIG. 41  shows results of a reliability test of Light-emitting element 2 of Example. 
         FIGS. 42A and 42B  show an emission spectrum and an absorption spectrum of a toluene solution of mDBqP2BP. 
         FIGS. 43A and 43B  show an emission spectrum and an absorption spectrum of a thin film of mDBqP2BP. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Embodiments and examples of the present invention will be described in detail with reference to the accompanying drawings. Note that the present invention is not limited to the description below, and it is easily understood by those skilled in the art that a variety of changes and modifications can be made without departing from the spirit and scope of the present invention. Therefore, the present invention should not be construed as being limited to the descriptions of the embodiments and the examples below. 
     The light-emitting device in this specification includes, in its category, an image display device that uses a light-emitting element. Further, the category of the light-emitting device includes a module in which a light-emitting element is provided with a connector, an anisotropic conductive film, or a TCP (tape carrier package); a module in which the end of the TCP is provided with a printed wiring board; and a module in which an IC (integrated circuit) is directly mounted on a light-emitting element by a COG (chip on glass) method. Furthermore, the category includes light-emitting devices that are used in lighting devices or the like. 
     Embodiment 1 
     In this embodiment, an organic compound of one embodiment of the present invention will be described. 
     One embodiment of the present invention is an organic compound represented by General Formula (G1-1). 
       Ar 1 A) n   (G1-1)
 
     In General Formula (G1-1), A represents a substituted or unsubstituted dibenzo[f,h]quinoxalin-yl group; Ar 1  represents a substituent formed by 1 to 4 rings; the ring is a substituted or unsubstituted benzene ring or a substituted or unsubstituted fluorene ring; and n represents 2 or 3. 
     Another embodiment of the present invention is an organic compound represented by General Formula (G2-1). 
     
       
         
         
             
             
         
       
     
     In General Formula (G2-1), Ar 1  represents a substituent formed by 1 to 4 rings; the ring is a substituted or unsubstituted benzene ring or a substituted or unsubstituted fluorene ring; R 1  to R 9  each independently represent any of hydrogen, an alkyl group having 1 to 6 carbon atoms, a cycloalkyl group having 3 to 6 carbon atoms, and a substituted or unsubstituted aryl group having 6 to 13 carbon atoms; and n represents 2 or 3. 
     Another embodiment of the present invention is an organic compound represented by General Formula (G3-1). 
     
       
         
         
             
             
         
       
     
     In General Formula (G3-1), Ar 1  represents a substituent formed by 1 to 4 rings; the ring is a substituted or unsubstituted benzene ring or a substituted or unsubstituted fluorene ring; R 11  to R 18  and R 20  each independently represent any of hydrogen, an alkyl group having 1 to 6 carbon atoms, a cycloalkyl group having 3 to 6 carbon atoms, and a substituted or unsubstituted aryl group having 6 to 13 carbon atoms; and n represents 2 or 3. 
     Another embodiment of the present invention is an organic compound represented by General Formula (G4-1). 
     
       
         
         
             
             
         
       
     
     In General Formula (G4-1), Ar 1  represents a substituent formed by 1 to 4 rings; the ring is a substituted or unsubstituted benzene ring or a substituted or unsubstituted fluorene ring; R 11  to R 17 , R 19 , and R 20  each independently represent any of hydrogen, an alkyl group having 1 to 6 carbon atoms, a cycloalkyl group having 3 to 6 carbon atoms, and a substituted or unsubstituted aryl group having 6 to 13 carbon atoms; and 17 represents 2 or 3. 
     Another embodiment of the present invention is an organic compound represented by General Formula (G1-2). 
       Ar 2 A) n   (G1-2)
 
     In General Formula (G1-2), A represents a substituted or unsubstituted dibenzo[f,h]quinoxalin-yl group; Ar 2  represents a substituent formed by three or four substituted or unsubstituted benzene rings; and n represents 2 or 3. 
     Another embodiment of the present invention is an organic compound represented by General Formula (G2-2). 
     
       
         
         
             
             
         
       
     
     In General Formula (G2-2), Ar 2  represents a substituent formed by three or four substituted or unsubstituted benzene rings; R 1  to R 9  each independently represent any of hydrogen, an alkyl group having 1 to 6 carbon atoms, a cycloalkyl group having 3 to 6 carbon atoms, and a substituted or unsubstituted aryl group having 6 to 13 carbon atoms; and n represents 2 or 3. 
     In General Formula (G2-1) or (G2-2), when R 1  represents an aryl group, R 1  and Ar 1  or R 1  and Ar 2  might undergo photo-cyclization, which significantly decreases the T 1  level. The significant decrease in the T 1  level might reduce the driving lifetime of a light-emitting element. Accordingly, in General Formula (G2-1) and (G2-2), R 1  preferably represents an alkyl group or hydrogen. For easier synthesis, R 1  preferably represents hydrogen. 
     Another embodiment of the present invention is an organic compound represented by General Formula (G3-2). 
     
       
         
         
             
             
         
       
     
     In General Formula (G3-2), Ar 2  represents a substituent formed by three or four substituted or unsubstituted benzene rings; R 11  to R 18  and R 20  each independently represent any of hydrogen, an alkyl group having 1 to 6 carbon atoms, a cycloalkyl group having 3 to 6 carbon atoms, and a substituted or unsubstituted aryl group having 6 to 13 carbon atoms; and n represents 2 or 3. 
     In General Formula (G3-1) or (G3-2), when each of R 11  and R 12  represents an aryl group, R 11  and R 12  might undergo the photo-cyclization, which might significantly decrease the T 1  level of a substance. The significant decrease in the T 1  level might reduce the driving lifetime of a light-emitting element. Accordingly, in General Formula (G3-1) and (G3-2), it is preferable to prevent the case where each of R 11  and represents an aryl group. In other words, it is preferable that R 11  and/or R 12  represent hydrogen. For easier synthesis, each of R 11  and R 12  preferably represents hydrogen. 
     Another embodiment of the present invention is an organic compound represented by General Formula (G4-2). 
     
       
         
         
             
             
         
       
     
     In General Formula (G4-2), Ar 2  represents a substituent formed by three or four substituted or unsubstituted benzene rings; R 11  to R 17 , R 19 , and R 29  each independently represent any of hydrogen, an alkyl group having 1 to 6 carbon atoms, a cycloalkyl group having 3 to 6 carbon atoms, and a substituted or unsubstituted aryl group having 6 to 13 carbon atoms; and n represents 2 or 3. 
     In General Formula (G1-2), (G2-2), (G3-2), or (G4-2), Ar 2  is a substituent formed by three or four substituted or unsubstituted benzene rings, which is preferable because the heat resistance of the organic compound is improved and the quality of a thin film including the organic compound is stabilized. 
     In General Formula (G1-1), (G2-1), (G3-1), (G4-1), (G1-2), (G2-2), (G3-2), and (G4-2), the benzene rings are preferably meta-substituted or ortho-substituted. This is because when the benzene rings are meta-substituted or ortho-substituted, crystallization of the organic compound is less likely to occur, the heat resistance of the organic compound is improved, and the quality of a thin film including the organic compound is stabilized. Furthermore, the organic compound in which the benzene rings are meta-substituted or ortho-substituted has a higher T 1  level than an organic compound in which the benzene rings are para-substituted. The use of the organic compound can achieve a phosphorescent element with high emission efficiency. Since the organic compound has a high T 1  level, the organic compound can be suitably used in a phosphorescent element with a shorter emission wavelength. 
     In General Formula (G1-1), (G2-1), (G3-1), (G4-1), (G1-2), (G2-2), (G3-2), or (G4-2), each of Ar 1  and Ar 2  is a substituted or unsubstituted benzene ring or a substituted or unsubstituted fluorene ring. When any of Ar 1  and Ar 2  has a substituent, examples of the substituent include an alkyl group having 1 to 6 carbon atoms, a cycloalkyl group having 3 to 6 carbon atoms, an aryl group having 6 to 13 carbon atoms, a carbazolyl group, a dibenzothiophenyl group, and a dibenzofuranyl group. The substituent is not limited to these examples, and the substituent may further have a substituent. For example, the aryl group having 6 to 13 carbon atoms may have a carbazolyl group, a dibenzothiophenyl group, or a dibenzofuranyl group. 
     Examples of the carbazolyl group include a 9H-carbazol-9-yl group, a 9H-carbazol-3-yl group, a 9H-carbazol-2-yl group, a 9-phenyl-9H-carbazol-3-yl group, a 9-phenyl-9H-carbazol-2-yl group, and a 2,8-diphenyl-9H-carbazol-9-yl group. Examples of the dibenzothiophenyl group include a 1-dibenzothiophenyl group, a 2-dibenzothiophenyl group, a 3-dibenzothiophenyl group, a 4-dibenzothiophenyl group, a 2,8-diphenyl-4-dibenzothiophenyl group, and 2,6,8-triphenyl-4-dibenzothiophenyl group. Examples of the dibenzofuranyl group include a 1-dibenzofuranyl group, a 2-dibenzofuranyl group, a 3-dibenzofuranyl group, a 4-dibenzofuranyl group, a 2,8-diphenyl-4-dibenzofuranyl group, and a 2,6,8-triphenyl-4-dibenzofuranyl group. 
     Examples of the alkyl group having 1 to 6 carbon atoms and the cycloalkyl group having 3 to 6 carbon atoms include a methyl group, an ethyl group, an n-propyl group, an isopropyl group, a n-butyl group, a sec-butyl group, an isobutyl group, a tert-butyl group, an n-pentyl group, an isopentyl group, a sec-pentyl group, a tert-pentyl group, a neopentyl group, an n-hexyl group, an isohexyl group, a sec-hexyl group, a tert-hexyl group, a neo-hexyl group, a cyclohexyl group, a 3-methylpentyl group, a 2-methylpentyl group, a 2-ethylbutyl group, a 1,2-dimethylbutyl group, and a 2,3-dimethylbutyl group. 
     Examples of a substituted or unsubstituted aryl group having 6 to 13 carbon atoms include a phenyl group, a 1-naphthyl group, a 2-naphthyl group, an ortho-tolyl group, a meta-tolyl group, a para-tolyl group, an ortho-biphenyl group, a meta-biphenyl group, a para-biphenyl group, a 9,9-dimethyl-9H-fluoren-2-yl group, a 9,9-diphenyl-9H-fluoren-2-yl group, a 9H-fluoren-2-yl group, a para-tert-butylphenyl group, a mesityl group, a 2-(9H-carbazol-9-yl)phenyl group, a 3-(9H-carbazol-9-yl)phenyl group, a 4-(9H-carbazol-9-yl)phenyl group, a 2-(4-dibenzothiophenyl)phenyl group, a 3-(4-dibenzothiophenyl)phenyl group, a 4-(4-dibenzothiophenyl)phenyl group, a 2-(2-dibenzothiophenyl)phenyl group, a 3-(2-dibenzothiophenyl)phenyl group, a 4-(2-dibenzothiophenyl)phenyl group, a 2-(4-dibenzofuranyl)phenyl group, a 3-(4-dibenzofuranyl)phenyl group, a 4-(4-dibenzofuranyl)phenyl group, a 2-(2-dibenzofuranyl)phenyl group, a 3-(2-dibenzofuranyl)phenyl group, and a 4-(2-dibenzofuranyephenyl group. 
     The substituent is not limited to these examples, and the substituent may further have a substituent. 
     Another embodiment of the present invention is an organic compound represented by any one of Structural Formulae (101), (102), (103), (105), and (106). 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     Specific examples of the organic compounds represented by General Formulae (G1-1), (G1-2), (G1-3), (G1-4), (G2-1), (G2-2), (G2-3), and (G2-4) are organic compounds represented by Structural Formulae (100) to (150). Note that one embodiment of the present invention is not limited thereto. 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     Next, an example of a method of synthesizing the organic compound represented by the General Formula (G1-1) will be described. A variety of reactions can be applied, and they can be synthesized through synthesis reactions described below, for example. Note that the synthetic method is not limited to the following reaction. 
     &lt;&lt;Method of Synthesizing Organic Compound Represented by General Formula (G1-1)&gt;&gt; 
     The organic compound represented by General Formula (G1) can be synthesized as in Synthesis Scheme (A-1) shown below. Through coupling of a compound having the rings selected from benzene rings and fluorene rings (Compound 1) and the dibenzo[f,h]quinoxaline compound (Compound 2), a compound represented by General Formula (G1-1) can be obtained. 
     
       
         
         
             
             
         
       
     
     In Synthesis Scheme (A-1), A represents a substituted or unsubstituted dibenzo[f,h]quinoxalin-yl group; Ar 1  represents a substituent Ruined by 1 to 4 rings; the ring is a substituted or unsubstituted benzene ring or a substituted or unsubstituted fluorene ring; one of X 1  and X 2  represents halogen or a trifluoromethanesulfonyloxy group, and the other of X 1  and X 2  represents a boronic acid, a boronic ester, a halogenated magnesium group, or an organotin group; and n represents 2 or 3. 
     In Synthesis Scheme (A-1), when the Suzuki-Miyaura coupling using a palladium catalyst is performed, it is preferable that one of X 1  and X 2  represent halogen or a trifluoromethanesulfonyloxy group, and the other represent a boronic acid or a boronic ester. The halogen is preferred to be iodine, bromine, or chlorine. In the reaction, a palladium compound such as bis(dibenzylideneacetone)palladium(0) or palladium(II) acetate and a ligand such as tri(tert-butyl)phosphine, tri(n-hexyl)phosphine, tricyclohexylphosphine, di(1-adamantyl)-n-butylphosphine, 2-dicyclohexylphosphino-2′,6′-dimethoxybiphenyl, or tri(ortho-tolyl)phosphine can be used. In the reaction, an organic base such as sodium tert-butoxide, an inorganic base such as potassium carbonate, cesium carbonate, or sodium carbonate, or the like can be used. In the reaction, toluene, xylene, benzene, tetrahydrofuran, dioxane, ethanol, methanol, water, or the like can be used as a solvent. Note that reagents that can be used in the reaction are not limited thereto. 
     The reaction performed in Synthesis Scheme (A-1) is not limited to the Suzuki-Miyaura coupling reaction. For example, the Migita-Kosugi-Stille coupling using an organotin compound, the Kumada-Tamao-Corriu coupling using the Grignard reagent, the Negishi coupling using an organozinc compound, or a reaction using copper or a copper compound can be employed. 
     The organic compound represented by General Formula (G2-1) can be obtained through Synthesis Scheme (A-1) where a compound formed by benzene rings is used as Compound 1. 
     Through the above-described steps, the organic compound of this embodiment can be synthesized. 
     Since the organic compound of one embodiment of the present invention has a high S 1  level, a high T 1  level, and a wide energy gap (Eg) between the HOMO level and the LUMO level, high current efficiency can be obtained by using the organic compound in a light-emitting element as a host material of a light-emitting layer in which a light-emitting substance is dispersed. In particular, the organic compound of one embodiment of the present invention is suitably used as a host material in which a phosphorescent compound is dispersed. Further, since the organic compound of one embodiment of the present invention is a substance having a high electron-transport property, it can be suitably used as a material for an electron-transport layer in a light-emitting element. By using the organic compound of one embodiment of the present invention, a light-emitting element with low driving voltage and high current efficiency can be obtained. Furthermore, by using this light-emitting element, a light-emitting device, an electronic device, and a lighting device each having reduced power consumption can be obtained. 
     Note that the structure described in this embodiment can be combined as appropriate with any of the structures described in the other embodiments. 
     Embodiment 2 
     In this embodiment, a light-emitting element in which an organic compound of one embodiment of the present invention is used for a light-emitting layer will be described with reference to  FIGS. 1A to 1C . 
     In the light-emitting element of this embodiment, the EL layer having at least a light-emitting layer is interposed between a pair of electrodes. The EL layer may also have a plurality of layers in addition to the light-emitting layer. The plurality of layers has a structure in which a layer containing a substance having a high carrier-injection property and a layer containing a substance having a high carrier-transport property are combined and stacked so that a light-emitting region is formed in a region away from the electrodes, that is, so that carriers recombine in a region away from the electrodes. In this specification, the layer containing a substance having a high carrier-injection or a high carrier-transport property is also called functional layer which functions, for instance, to inject or transport carriers. As the functional layer, a hole-injection layer, a hole-transport layer, an electron-injection layer, an electron-transport layer, or the like can be used. 
     In the light-emitting element of this embodiment illustrated in  FIG. 1A , an EL layer  102  is provided between a pair of electrodes, a first electrode  101  and a second electrode  103 , which is located over a substrate  100 . The EL layer  102  includes a hole-injection layer  111 , a hole-transport layer  112 , the light-emitting layer  113 , an electron-transport layer  114 , and an electron-injection layer  115 . Note that, in the light-emitting element described in this embodiment, the first electrode  101  functions as an anode and the second electrode  103  functions as a cathode. 
     A substrate  100  is used as a support of the light-emitting element. For example, glass, quartz, plastic, or the like can be used for the substrate  100 . A flexible substrate may be used. The flexible substrate is a substrate that can be bent, such as a plastic substrate made of, for example, a polycarbonate, a polyarylate, or a poly(ether sulfone). Alternatively, a film (made of polypropylene, a polyester, poly(vinyl fluoride), poly(vinyl chloride), or the like), a film on which an inorganic substance is deposited by evaporation, or the like can be used. Note that a different substrate can be used as long as it can function as a support in a process of manufacturing the light-emitting element. 
     For the first electrode  101  and the second electrode  103 , a metal, an alloy, an electrically conductive compound, a mixture thereof, and the like can be used. Specifically, indium oxide-tin oxide (ITO: indium tin oxide), indium oxide-tin oxide containing silicon or silicon oxide, indium oxide-zinc oxide, indium oxide containing tungsten oxide and zinc oxide, gold (Au), platinum (Pt), nickel (Ni), tungsten (W), chromium (Cr), molybdenum (Mo), iron (Fe), cobalt (Co), copper (Cu), palladium (Pd), and titanium (Ti) can be used. In addition, an element belonging to Group 1 or Group 2 of the periodic table, examples of which are an alkali metal such as lithium (Li) or cesium (Cs) and an alkaline earth metal such as calcium (Ca), or strontium (Sr), an alloy containing such an element, a rare earth metal such as europium (Eu) or ytterbium (Yb), an alloy containing such an element, magnesium (Mg), graphene, and the like can be used. The first electrode  101  and the second electrode  103  can be formed by, for example, a sputtering method, an evaporation method (including a vacuum evaporation method), or the like. 
     The EL layer  102  formed over the first electrode  101  includes at least the light-emitting layer  113 , and part of the EL layer  102  is formed using the organic compound of one embodiment of the present invention. For the EL layer  102 , a variety of substances can be used, and either a low molecular compound or a high molecular compound can be used. Note that the substance used for forming the EL layer  102  may have not only a structure formed of only an organic compound but also a structure in which an inorganic compound is partially contained. 
     As a substance with a high hole-transport property that is used for the hole-injection layer  111  and the hole-transport layer  112 , a π-electron rich heteroaromatic compound (e.g., a carbazole derivative or an indole derivative) or an aromatic amine compound can be used. For example, the following substances can be used: a compound having an aromatic amine skeleton such as N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine (abbreviation: TPD), 4,4′-bis[N-(Spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl (abbreviation: BSPB), 4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine (abbreviation: BPAFLP), 4-phenyl-3′-(9-phenylfluoren-9-yl)triphenylamine (abbreviation: mBPAFLP), 4-phenyl-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation: PCBA1BP), 4,4′-diphenyl-4″-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation: PCBNBB), 4-(1-naphthyl)-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation: PCBANB), 4,4′-di(1-naphthyl)-4″-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation: PCBNBB), 9,9-dimethyl-N-phenyl-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]fluoren-2-amine (abbreviation: PCBAF), or N-phenyl-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]spiro-9,9′-bifluoren-2-amine (abbreviation: PCBASF); a compound having a carbazole skeleton such as 1,3-bis(N-carbazolyl)benzene (abbreviation: mCP), 4,4′-di(N-carbazolyl)biphenyl (abbreviation: CBP), 3,6-bis(3,5-diphenylphenyl)-9-phenylcarbazole (abbreviation: CzTP), or 3,3′-bis(9-phenyl-9H-carbazole) (abbreviation: PCCP); a compound having a thiophene skeleton such as 4,4′,4″-(benzene-1,3,5-triyl)tri(dibenzothiophene) (abbreviation: DBT3P-II), 2,8-diphenyl-4-[4-(9-phenyl-9H-fluoren-9-yl)phenyl]dibenzothiophene (abbreviation: DBTFLP-III), or 4-[4-(9-phenyl-9H-fluoren-9-yl)phenyl]-6-phenyldibenzothiophene (abbreviation: DBTFLP-IV); and a compound having a furan skeleton such as 4,4′,4″-(benzene-1,3,5-triyl)tri(dibenzofuran) (abbreviation: DBF3P-II) or 4-{3-[3-(9-phenyl-9H-fluoren-9-yl)phenyl]phenyl}dibenzofuran (abbreviation: mmDBFFLBi-II). 
     In the above-mentioned substances, a compound having a carbazole skeleton is preferable because the compound is highly reliable and has a high hole-transport property to contribute to a reduction in drive voltage. 
     Furthermore, as a material that can be used for the hole-injection layer  111  and the hole-transport layer  112 , a high molecular compound such as poly(N-vinylcarbazole) (abbreviation: PVK), poly(4-vinyltriphenylamine) (abbreviation: PVTPA), poly[N-(4-{N′-[4-(4-diphenylamino)phenyl]phenyl-N-phenylamino}phenyl)methacrylamide](abbreviation: PTPDMA), or poly[N,N′-bis(4-butylphenyl)-N,N-bis(phenyl)benzidine](abbreviation: Poly-TPD) can be used. 
     A layer in which any of the substances with a high hole-transport property given above and a substance with an acceptor property are mixed is preferably used as the hole-injection layer  111  and the hole-transport layer  112 , in which case a favorable carrier-injection property is obtained. Examples of the acceptor substance to be used include an oxide of a metal belonging to any of Groups 4 to 8 of the periodic table. Specifically, molybdenum oxide is particularly preferable. 
     The light-emitting layer  113  preferably contains, for example, an electron-transport material as a host material (a first organic compound), a fluorescent compound as a guest material (a second organic compound), and a hole-transport material as an assist material (a third organic compound). Note that a relation regarding the carrier-transport property between the host material and the assist material is not limited to the above; an electron-transport material may be used as the assist material and a hole-transport material may be used as the host material. 
     The organic compound of one embodiment of the present invention described in Embodiment 1 can be used as a host material in the light-emitting layer  113 . 
     Note that the organic compounds of one embodiment of the present invention have a high T 1  level and thus also have a high S 1  level. Thus, they can also be used as a host material for a fluorescence compound. 
     As examples of the guest material, a phosphorescent compound and a thermally activated delayed fluorescent (TADF) material can be given. 
     As the phosphorescent compound, for example, a phosphorescent compound having an emission peak between 440 nm to 520 nm is given, examples of which include organometallic iridium complexes having 4H-triazole skeletons, such as tris{2-[5-(2-methylphenyl)-4-(2,6-dimethylphenyl)-4H-1,2,4-triazol-3-yl-κN2]phenyl-x C}iridium(III) (abbreviation: Ir(mpptz-dmp) 3 ), tris(5-methyl-3,4-diphenyl-4H-1,2,4-triazolato)iridium(III) (abbreviation: [Ir(Mptz) 3 ], and tris[4-(3-biphenyl)-5-isopropyl-3-phenyl-4H-1,2,4-triazolato]iridium(III) (abbreviation: Ir(iPrptz-3b) 3 ); organometallic iridium complexes having 1H-triazole skeletons, such as tris[3-methyl-1-(2-methylphenyl)-5-phenyl-1H-1,2,4-triazolato]iridium(III) (abbreviation: [Ir(Mptz1-mp) 3 ]) and tris(1-methyl-5-phenyl-3-propyl-1H-1,2,4-triazolato)iridium(III) (abbreviation: Ir(Prptz1-Me) 3 ); organometallic iridium complexes having imidazole skeletons, such as fac-tris[1-(2,6-diisopropylphenyl)-2-phenyl-1H-imidazole]iridium(III) (abbreviation: Ir(iPrpmi) 3 ) and tris[3-(2,6-dimethylphenyl)-7-methylimidazo[1,2-f]phenanthridinato]iridium(III) (abbreviation: Ir(dmpimpt-Me) 3 ); and organometallic iridium complexes in which a phenylpyridine derivative having an electron-withdrawing group is a ligand, such as bis[2-(4′,6′-difluorophenyl)pyridinato-N,C 2′ ]iridium(III) tetrakis(1-pyrazolyl)borate (abbreviation: FIr6), bis[2-(4′,6′-difluorophenyl)pyridinato-N,C 2′ ]iridium(III) picolinate (abbreviation: FIrpic), bis{2-[3′,5′-bis(trifluoromethyl)phenyl]pyridinato-N,C 2′ }iridium(III) picolinate (abbreviation: Ir(CF 3 ppy) 2 (pic)), and bis[2-(4′,6′-difluorophenyl)pyridinato-N,C 2′ ]iridium(III) acetylacetonate (abbreviation: FIr(acac)). Among the materials given above, the organometallic iridium complex having a 4H-triazole skeleton has high reliability and high emission efficiency and is thus especially preferable. 
     Examples of the phosphorescent compound having an emission peak between 520 nm to 600 nm include organometallic iridium complexes having pyrimidine skeletons, such as tris(4-methyl-6-phenylpyrimidinato)iridium(III) (abbreviation: [Ir(mppm) 3 ]), tris(4-t-butyl-6-phenylpyrimidinato)iridium(III) (abbreviation: [Ir(tBuppm) 3 ]), (acetylacetonato)bis(6-methyl-4-phenylpyrimidinato)iridium(III) (abbreviation: [Ir(mppm) 2 (acac)]), (acetylacetonato)bis(6-tert-butyl-4-phenylpyrimidinato)iridium(III) (abbreviation: [Ir(tBuppm) 2 (acac)]), (acetylacetonato)bis[4-(2-norbornyl)-6-phenylpyrimidinato]iridium(III) (endo- and exo-mixture) (abbreviation: Ir(nbppm) 2 (acac)), (acetylacetonato)bis[5-methyl-6-(2-methylphenyl)-4-phenylpyrimidinato] (abbreviation: [Ir(mpmppm) 2 (acac)]), and (acetylacetonato)bis(4,6-diphenylpyrimidinato)iridium(III) (abbreviation: [Ir(dppm) 2 (acac)]); organometallic iridium complexes having pyrazine skeletons, such as (acetylacetonato)bis(3,5-dimethyl-2-phenylpyrazinato)iridium(III) (abbreviation: [Ir(mppr-Me) 2 (acac)]) and (acetylacetonato)bis(5-isopropyl-3-methyl-2-phenylpyrazinato)iridium(III) (abbreviation: [Ir(mppr-iPr) 2 (acac)]); organometallic iridium complexes having pyridine skeletons, such as tris(2-phenylpyridinato-N,C 2′ )iridium(III) (abbreviation: [Ir(ppy) 3 ]), bis(2-phenylpyridinato-N,C 2′ )iridium(III) acetylacetonate (abbreviation: [Ir(ppy) 2 (acac)]), bis(benzo[h]quinolinato)iridium(III) acetylacetonate (abbreviation: [Ir(bzq) 2 (acac)]), tris(benzo[h]quinolinato)iridium(III) (abbreviation: [Ir(bzq) 3 ]), tris(2-phenylquinolinato-N,C 2′ )iridium(III) (abbreviation: [Ir(pq) 3 ], and bis(2-phenylquinolinato-N, acetylacetonate (abbreviation: [Ir(pq) 2 (acac)]); and a rare earth metal complex such as tris(acetylacetonato) (monophenanthroline)terbium(III) (abbreviation: [Tb(acac) 3 (Phen)]). Among the materials given above, the organometallic iridium complex having a pyrimidine skeleton has distinctively high reliability and emission efficiency and is thus especially preferable. 
     Examples of the phosphorescent compound having an emission peak between 600 nm to 700 nm include organometallic iridium complexes having pyrimidine skeletons, such as (diisobutyrylmethanato)bis[4,6-bis(3-methylphenyl)pyrimidinato]iridium(III) (abbreviation: [Ir(5mdppm) 2 (dibm)]), bis[4,6-bis(3-methylphenyl)pyrimidinato](dipivaloylmethanato)iridium(III) (abbreviation: [Ir(5mdppm) 2 (dpm)]), and bis[4,6-di(naphthalen-1-yl)pyrimidinato](dipivaloylmethanato)iridium(III) (abbreviation: [Ir(dlnpm) 2 (dpm)]); organometallic iridium complexes having pyrazine skeletons, such as (acetylacetonato)bis(2,3,5-triphenylpyrazinato)iridium(III) (abbreviation: [Ir(tppr) 2 (acac)]), bis(2,3,5-triphenylpyrazinato) (dipivaloylmethanato)iridium(III) (abbreviation: [Ir(tppr) 2 (dpm)]), and (acetylacetonato)bis[2,3-bis(4-fluorophenyl)quinoxalinato]iridium(III) (abbreviation: [Ir(Fdpq) 2 (acac)]); organometallic iridium complexes having pyridine skeletons, such as tris(1-phenylisoquinolinato-N,C 2′ )iridium(III) (abbreviation: [Ir(piq) 3 ]) and bis(1-phenylisoquinolinato-N,C 2′ )iridium(III) acetylacetonate (abbreviation: [Ir(piq) 2 (acac)]); a platinum complex such as 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrin platinum(II) (abbreviation: PtOEP); and rare earth metal complexes such as tris(1,3-diphenyl-1,3-propanedionato)(monophenanthroline)europium(III) (abbreviation: [Eu(DBM) 3 (Phen)]) and tris[1-(2-thenoyl)-3,3,3-trifluoroacetonato](monophenanthroline)europium(III) (abbreviation: [Eu(TTA) 3 (Phen)]). Among the materials given above, the organometallic iridium complex having a pyrimidine skeleton has distinctively high reliability and emission efficiency and is thus especially preferable. Further, the organometallic iridium complex having a pyrazine skeleton can provide red light emission with excellent chromaticity. 
     As the assist material, a substance with a high hole-transport property which can be used for the hole-injection layer  111  and the hole-transport layer  112  may be used. 
     Specifically, a compound having a carbazole skeleton is preferably used as the assist material because the compound is highly reliable and has a high hole-transport property to contribute to a reduction in drive voltage. 
     It is preferable that each of the host material and the assist material do not have an absorption in a wavelength range of blue light. Specifically, an absorption cutoff is preferably at 440 nm or less. 
     The electron-transport layer  114  is a layer containing a substance with a high electron-transport property. The organic compound of one embodiment of the present invention can be used for the electron-transport layer  114  due to its high electron-transport property. In addition to the organic compound of one embodiment of the present invention, a metal complex such as tris(8-quinolinolato)aluminum (abbreviation: Alq 3 ), tris(4-methyl-8-quinolinolato)aluminum (abbreviation: Almq 3 ), bis(10-hydroxybenzo[h]quinolinato)beryllium (abbreviation: BeBq 2 ), bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum (abbreviation: BAlq), bis[2-(2-benzoxazolyl)phenolato]zinc (abbreviation: Zn(BOX) 2 ), or bis[2-(2-hydroxyphenyl)benzothiazolato]zinc (abbreviation: Zn(BTZ) 2 ) can be used for the electron-transport layer  114 . Furthermore, a heteroaromatic compound such as 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation: PBD), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene (abbreviation: OXD-7), 3-(4′-tert-butylphenyl)-4-phenyl-5-(4″-biphenyl)-1,2,4-triazole (abbreviation: TAZ), 3-(4-tert-butylphenyl)-4-(4-ethylphenyl)-5-(4-biphenylyl)-1,2,4-triazole (abbreviation: p-EtTAZ), bathophenanthroline (abbreviation: BPhen), bathocuproine (abbreviation: BCP), or 4,4′-bis(5-methylbenzoxazol-2-yl)stilbene (abbreviation: BzOs) can be used. Further, a high molecular compound such as poly(2,5-pyridinediyl) (abbreviation: PPy), poly[(9,9-dihexylfluorene-2,7-diyl)-co-(pyridine-3,5-diyl)] (abbreviation: PF-Py) or poly[(9,9-dioctylfluorene-2,7-diyl)-co-(2,2′-bipyridine-6,6′-diyl)](abbreviation: PF-BPy) can be used. The substances given here are mainly ones having an electron mobility of 10 −6  cm 2 /Vs or higher. Note that any other substance may be used for the electron-transport layer  114  as long as the substance has an electron-transport property higher than a hole-transport property. 
     The electron-transport layer  114  is not limited to a single layer, and may be a stack of two or more layers containing any of the above substances. 
     The electron-injection layer  115  is a layer containing a substance with a high electron-injection property. For the electron-injection layer  115 , an alkali metal compound or an alkaline earth metal compound, such as lithium fluoride (LiF), cesium fluoride (CsF), calcium fluoride (CaF 2 ), or lithium oxide (LiO x ) can be used. A rare earth metal compound such as erbium fluoride (ErF 3 ) can also be used. In addition, an electride may be used for the electron-injection layer  115 . As an example of electride, a substance in which electrons are added to a mixed oxide containing calcium and aluminum is given. Any of the substances for forming the electron-transport layer  114 , which are given above, can also be used. 
     Alternatively, a composite material in which an organic compound and an electron donor (donor) are mixed may be used for the electron-injection layer  115 . Such a composite material is excellent in an electron-injection property and an electron-transport property because electrons are generated in the organic compound by the electron donor. In this case, the organic compound is preferably a material excellent in a property of transporting the generated electrons. Specifically, for example, the substances for forming the electron-transport layer  114  (e.g., a metal complex and a heteroaromatic compound), which are given above, can be used. As the electron donor, a substance exhibiting an electron-donating property to the organic compound may be used. Specifically, an alkali metal, an alkaline earth metal, and a rare earth metal are preferable, and lithium, cesium, magnesium, calcium, erbium, and ytterbium are given. Further, an alkali metal oxide or an alkaline earth metal oxide is preferable and for example, lithium oxide, calcium oxide, barium oxide, and the like can be given. A Lewis base such as magnesium oxide can also be used. An organic compound such as tetrathiafulvalene (abbreviation: TTF) can also be used. 
     Note that each of the above-described hole-injection layer  111 , hole-transport layer  112 , light-emitting layer  113 , electron-transport layer  114 , and electron-injection layer  115 , can be formed by a method such as an evaporation method (e.g., a vacuum evaporation method), an inkjet method, or a coating method. 
     In the above-described light-emitting element, current flows due to a potential difference applied between the first electrode  101  and the second electrode  103  and holes and electrons recombine in the EL layer  102 , whereby light is emitted. Then, the emitted light is extracted outside through the first electrode  101 , the second electrode  103 , or both. Thus, the first electrode  101 , the second electrode  103 , or both are electrodes having light-transmitting properties. 
     A structure of a layer provided between the first electrode  101  and the second electrode  103  is not limited to the above. A structure other than the above may be employed as long as a light-emitting region in which holes and electrons recombine is provided in a portion away from the first electrode  101  and the second electrode  103  in order to prevent quenching due to proximity of the light-emitting region to a metal. 
     In other words, there is no particular limitation on a stack structure of the layers. A layer containing a substance with a high electron-transport property, a substance with a high hole-transport property, a substance with a high electron-injection property, a substance with a high hole-injection property, a bipolar substance (substance having a high electron-transport property and a high hole-transport property), a hole-blocking material, or the like may freely be combined with a light-emitting layer containing the organic compound of one embodiment of the present invention. 
     When the organic compound of one embodiment of the present invention is used in both the light-emitting layer (particularly, as a host material for the light-emitting layer) and the electron-transport layer, extremely low driving voltage can be achieved. 
     Next, the light-emitting elements illustrated in  FIGS. 1B and 1C  will be described. 
     The light-emitting element illustrated in  FIG. 1B  is a tandem light-emitting element including a plurality of light-emitting layers (a first light-emitting layer  311  and a second light-emitting layer  312 ) between a first electrode  301  and a second electrode  303 . 
     The first electrode  301  functions as an anode, and the second electrode  303  functions as a cathode. Note that the first electrode  301  and the second electrode  303  can have structures similar to those of the first electrode  101  and the second electrode  103 . 
     The first light-emitting layer  311  and the second light-emitting layer  312  can have a structure similar to that of the light-emitting layer  113 . Note that the structures of the first light-emitting layer  311  and the second light-emitting layer  312  may be the same or different from each other as long as at least one of the first light-emitting layer  311  and the second light-emitting layer  312  has a structure similar to that of the light-emitting layer  113 . Further, in addition to the first light-emitting layer  311  and the second light-emitting layer  312 , the hole-injection layer  111 , the hole-transport layer  112 , the electron-transport layer  114 , and the electron-injection layer  115  which are described above may be provided as appropriate. 
     A charge-generation layer  313  is provided between the first light-emitting layer  311  and the second light-emitting layer  312 . The charge-generation layer  313  has a function of injecting electrons into one of the light-emitting layers and injecting holes into the other of the light-emitting layers when voltage is applied between the first electrode  301  and the second electrode  303 . In this embodiment, when voltage is applied such that the potential of the first electrode  301  is higher than that of the second electrode  303 , the charge-generation layer  313  injects electrons into the first light-emitting layer  311  and injects holes into the second light-emitting layer  312 . 
     Note that in terms of light extraction efficiency, the charge-generation layer  313  preferably has a light-transmitting property with respect to visible light (specifically, the charge-generation layer  313  has a visible light transmittance of 40% or more). The charge-generation layer  313  functions even if it has lower conductivity than the first electrode  301  or the second electrode  303 . 
     The charge-generation layer  313  may have either a structure in which an electron acceptor (acceptor) is added to an organic compound having a high hole-transport property or a structure in which an electron donor is added to an organic compound having a high electron-transport property. Alternatively, both of these structures may be stacked. 
     In the case of the structure in which an electron acceptor is added to an organic compound having a high hole-transport property, as the organic compound having a high hole-transport property, for example, an aromatic amine compound such as NPB, TPD, TDATA, MTDATA, or 4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl (abbreviation: BSPB), or the like can be used. The substances given here are mainly ones having a hole mobility of 10 −6  cm 2 /Vs or higher. However, substances other than the above substances may be used as long as they are organic compounds having a hole-transport property higher than an electron-transport property. 
     Further, as the electron acceptor, 7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (abbreviation: F 4 -TCNQ), chloranil, and the like can be given. In addition, an oxide of metals that belong to Group 4 to Group 8 of the periodic table can be given. Specifically, vanadium oxide, niobium oxide, tantalum oxide, chromium oxide, molybdenum oxide, tungsten oxide, manganese oxide, and rhenium oxide are preferable because of their high electron-accepting properties. Among these metal oxides, molybdenum oxide is especially preferable since it is stable in the air, has a low hygroscopic property, and is easily handled. 
     In the case of the structure in which an electron donor is added to an organic compound having a high electron-transport property, as the organic compound having a high electron-transport property, for example, a metal complex having a quinoline skeleton or a benzoquinoline skeleton, such as Alq, Almq 3 , BeBq 2 , or BAlq, or the like can be used. Alternatively, a metal complex having an oxazole-based ligand or a thiazole-based ligand, such as Zn(BOX) 2  or Zn(BTZ) 2 , or the like can be used. Other than metal complexes, PBD, OXD-7, TAZ, BPhen, BCP, or the like can be used. The substances given here are mainly ones having an electron mobility of 10 −6  cm 2 /Vs or higher. Note that substances other than the above substances may be used as long as they are organic compounds having an electron-transport property higher than a hole-transport property. 
     As the electron donor, an alkali metal, an alkaline earth metal, a rare earth metal, a metal belonging to Group 13 of the periodic table, or an oxide or carbonate thereof can be used. Specifically, lithium (Li), cesium (Cs), magnesium (Mg), calcium (Ca), ytterbium (Yb), indium (In), lithium oxide, cesium carbonate, or the like is preferably used. An organic compound such as tetrathianaphthacene may also be used as the electron donor. 
     Although the light-emitting element having two light-emitting layers is illustrated in  FIG. 1B , the present invention can be similarly applied to a light-emitting element in which n light-emitting layers (n is three or more) are stacked as illustrated in  FIG. 1C . In the case where a plurality of light-emitting layers are provided between a pair of electrodes as in the light-emitting element of this embodiment, by providing the charge-generation layer  313  between the light-emitting layers, the light-emitting element can emit light in a high luminance region while the current density is kept low. Since the current density can be kept low, the element can have a long lifetime. Moreover, a light-emitting device which can be driven at low voltage and has low power consumption can be achieved. 
     By making emission colors of the light-emitting layers different, light of a desired color can be obtained from the light-emitting element as a whole. For example, by forming a light-emitting element having two light-emitting layers such that the emission color of the first light-emitting layer and the emission color of the second light-emitting layer are complementary colors, the light-emitting element can provide white light emission as a whole. Note that “complementary colors” refer to colors which produce an achromatic color when mixed. In other words, emission of white light can be obtained by mixture of light emitted from substances whose emission colors are complementary colors. 
     Further, the same can be applied to a light-emitting element having three light-emitting layers. For example, the light-emitting element as a whole can provide white light emission when the emission color of the first light-emitting layer is red, the emission color of the second light-emitting layer is green, and the emission color of the third light-emitting layer is blue. 
     As described above, the organic compound of one embodiment of the present invention is used in the light-emitting layer of the light-emitting element of this embodiment. Since the organic compound of one embodiment of the present invention has a wide energy gap, high current efficiency can be obtained by using the organic compound in a light-emitting element as a host material of a light-emitting layer in which a light-emitting substance is dispersed. In particular, the organic compound of one embodiment of the present invention is suitably used as a host material in which a phosphorescent compound is dispersed. 
     Furthermore, a light-emitting element which includes the above organic compound in a light-emitting layer can be driven at low voltage. The light-emitting element can also have a long lifetime. 
     Note that the structure described in this embodiment can be combined as appropriate with any of the structures described in the other embodiments. 
     Embodiment 3 
     In this embodiment, a light-emitting element in which the organic compound of one embodiment of the present invention is used for a light-emitting layer will be described with reference to  FIG. 2 . The light-emitting element of this embodiment includes an EL layer between a pair of electrodes, and a light-emitting layer in the EL layer contains an organic compound of one embodiment of the present invention and two or more kinds of organic compounds. 
     A light-emitting element described in this embodiment includes an EL layer  203  between a pair of electrodes (a first electrode  201  and a second electrode  202 ) as illustrated in  FIG. 2 . The EL layer  203  includes at least a light-emitting layer  204  and may include a hole-injection layer, a hole-transport layer, an electron-transport layer, an electron-injection layer, a charge generating layer, and the like as appropriate between the first electrode  201  and the light-emitting layer  204  and between the second electrode  202  and the light-emitting layer  204 . As substances for the hole-injection layer, the hole-transport layer, the electron-transport layer, the electron-injection layer, and the charge-generation layer, the substances described in Embodiment 2 can be used. Note that the first electrode  201  is used as an anode and the second electrode  202  is used as a cathode in this embodiment. 
     The light-emitting layer  204  described in this embodiment contains, as a first organic compound  206 , the organic compound of one embodiment of the present invention described in Embodiment 1 in addition to a second organic compound  207  and a phosphorescent compound  205 . The phosphorescent compound  205  is a guest material, and one of the first organic compound  206  and the second organic compound  207 , the content of which is higher than that of the other in the light-emitting layer  204 , is a host material. Here, a structure in which the first organic compound  206  is used as a host material is described. 
     When the light-emitting layer  204  has the structure in which the guest material is dispersed in the host material, crystallization of the light-emitting layer can be suppressed. Further, it is possible to suppress concentration quenching due to high concentration of the guest material, and thus the light-emitting element can have higher emission efficiency. 
     It is preferable that a T 1  level of each of the first organic compound  206  and the second organic compound  207  be higher than that of the phosphorescent compound  205 . This is because, when the T 1  level of the first organic compound  206  (or the second organic compound  207 ) is lower than that of the phosphorescent compound  205 , the triplet excitation energy of the phosphorescent compound  205  which contributes to light emission is quenched by the first organic compound  206  (or the second organic compound  207 ) and accordingly the emission efficiency is decreased. 
     Here, for improvement in efficiency of energy transfer from a host material to a guest material, it is preferable that an emission spectrum of a host material (fluorescence spectrum in energy transfer from a singlet excited state, phosphorescence spectrum in energy transfer from a triplet excited state) largely overlap with an absorption spectrum of a guest material (specifically, an absorption band on the longest wavelength side). However, in the case of a general phosphorescent guest material, it is difficult to obtain an overlap between a fluorescence spectrum of a host material and an absorption band on the longest wavelength side of a guest material. The reason for this is as follows: if the fluorescence spectrum of the host material overlaps with the absorption band on the longest wavelength side of the guest material, since a phosphorescence spectrum of the host material is located on a longer wavelength side than the fluorescence spectrum, the T 1  level of the host material cannot be higher than the T 1  level of the phosphorescent compound and the above-described quenching occurs; yet, when the host material is designed in such a manner that the T 1  level of the host material is higher than the T 1  level of the phosphorescent compound to avoid the quenching, the fluorescence spectrum of the host material is shifted to the shorter wavelength side, and thus the fluorescence spectrum negligibly overlaps with the absorption band on the longest wavelength side of the guest material. For that reason, in general, it is difficult to obtain an overlap between a fluorescence spectrum of a host material and an absorption band on the longest wavelength side of a guest material so as to maximize energy transfer from a singlet excited state of a host material. 
     Thus, in this embodiment, a combination of the first organic compound  206  and the second organic compound  207  preferably forms an exciplex. In that case, the first organic compound  206  and the second organic compound  207  form an exciplex at the time of recombination of carriers in the light-emitting layer  204 . Thus, the light-emitting layer  204  gives an emission spectrum of the exciplex on a longer wavelength side compared with those of the first organic compound  206  and that of the second organic compound  207 . Moreover, when the first organic compound  206  and the second organic compound  207  are selected in such a manner that the emission spectrum of the exciplex largely overlaps with the absorption spectrum of the guest material, energy transfer from a singlet excited state can be maximized. Note that also in the case of a triplet excited state, energy transfer from the exciplex is assumed to occur instead of the energy transfer from the first organic compound  206  or the second organic compound  207 . 
     As the phosphorescent compound  205 , for example, the phosphorescent compound described in Embodiment 2 can be used. It is also possible to use the thermally activated delayed fluorescent material instead of the phosphorescent compound. As the first organic compound  206 , for example, the organic compound of one embodiment of the present invention can be used. The organic compound of one embodiment of the present invention is a compound that easily accepts electrons (an electron-trapping compound). As the second organic compound  207 , for example, a compound that easily accepts holes (a hole-trapping compound) can be used. 
     As a compound which is likely to accept holes, it is possible to use, for example, PCBA1BP, 3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole (abbreviation: PCzPCN1), 4,4′,4″-tris[N-(1-naphthyl)-N-phenylamino]triphenylamine (abbreviation: 1′-TNATA), 2,7-bis[N-(4-diphenylaminophenyl)-N-phenylamino]spiro-9,9′-bifluorene (abbreviation: DPA2SF), N,N′-bis(9-phenylcarbazol-3-yl)-N,N′-diphenylbenzene-1,3-diamine (abbreviation: PCA2B), N-(9,9-dimethyl-2-N,N′-diphenylamino-9H-fluoren-7-yl)diphenylamine (abbreviation: DPNF), 4-phenyldiphenyl-(9-phenyl-9H-carbazol-3-yl)amine (abbreviation: PCA1BP), N′,N″-triphenyl-N,N′,N″-tris(9-phenylcarbazol-3-yl)benzene-1,3,5-triamine (abbreviation: PCA3B), 2-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]spiro-9,9′-bifluorene (abbreviation: PCASF), 2-[N-(4-diphenylaminophenyl)-N-phenylamino]spiro-9,9′-bifluorene (abbreviation: DPASF), N,N-di(biphenyl-4-yl)-N-(9-phenyl-9H-carbazol-3-yl)amine (abbreviation: PCzBBA1), N,N′-bis[4-(carbazol-9-yl)phenyl]-N,N′-diphenyl-9,9-dimethylfluorene-2,7-diamine (abbreviation: YGA2F), TPD, 4,4′-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl (abbreviation: DPAB), N-(9,9-dimethyl-9H-fluoren-2-yl)-N-{9,9-dimethyl-2-[N′-phenyl-N′-(9,9-dimethyl-9H-fluoren-2-yl)amino]-9H-fluoren-7-yl}phenylamine (abbreviation: DFLADFL), 3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole (abbreviation: PCzPCA1), 3-[N-(4-diphenylaminophenyl)-N-phenylamino]-9-phenylcarbazole (abbreviation: PCzDPA1), 3,6-bis[N-(4-diphenylaminophenyl)-N-phenylamino]-9-phenylcarbazole (abbreviation: PCzDPA2), 4,4′-bis(N-{4-[N′-(3-methylphenyl)-N′-phenylamino]phenyl}-N-phenylamino)biphenyl (abbreviation: DNTPD), 3,6-bis[N-(4-diphenylaminophenyl)-N-(1-naphthyl)amino]-9-phenylcarbazole (abbreviation: PCzTPN2), 3,6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole (abbreviation: PCzPCA2), or the like. 
     The above-described first and second organic compounds  206  and  207  are not limited to the above examples. The combination is determined so that an exciplex can be formed, the emission spectrum of the exciplex overlaps with the absorption spectrum of the phosphorescent compound  205 , and the peak of the emission spectrum of the exciplex has a longer wavelength than the peak of the absorption spectrum of the phosphorescent compound  205 . 
     Note that in the case where a compound which is likely to accept electrons and a compound which is likely to accept holes are used for the first organic compound  206  and the second organic compound  207 , carrier balance can be controlled by the mixture ratio of the compounds. Specifically, the ratio (weight ratio) of the first organic compound  206  to the second organic compound  207  is preferably 1:9 to 9:1. 
     In the light-emitting element described in this embodiment, energy transfer efficiency can be improved owing to energy transfer utilizing an overlap between an emission spectrum of an exciplex and an absorption spectrum of a phosphorescent compound; accordingly, it is possible to achieve high external quantum efficiency of a light-emitting element. 
     Note that the structure described in this embodiment can be combined as appropriate with any of the structures described in the other embodiments. 
     Embodiment 4 
     In this embodiment, a light-emitting device which includes the light-emitting element of one embodiment of the present invention will be described with reference to  FIGS. 3A and 3B .  FIG. 3A  is a top view illustrating the light-emitting device, and  FIG. 3B  is a cross-sectional view taken along lines A-B and C-D of  FIG. 3A . 
     The light-emitting device of this embodiment includes a source side driver circuit  401  and a gate side driver circuit  403  which are driver circuit portions, a pixel portion  402 , a sealing substrate  404 , a sealing material  405 , a flexible printed circuit (FPC)  409 , and an element substrate  410 . A portion enclosed by the sealing material  405  is a space  407 . 
     A lead wiring  408  is a wiring for transmitting signals that are to be input to the source side driver circuit  401  and the gate side driver circuit  403 , and receives a video signal, a clock signal, a start signal, a reset signal, and the like from the FPC  409  which serves as an external input terminal. Although only the FPC is illustrated here, a printed wiring board (PWB) may be attached to the FPC. The light-emitting device in this specification includes not only a light-emitting device itself but also a light-emitting device to which an FPC or a PWB is attached. 
     The driver circuit portion and the pixel portion are formed over an element substrate  410  illustrated in  FIG. 3A . In  FIG. 3B , the source side driver circuit  401  which is the driver circuit portion and one pixel in the pixel portion  402  are illustrated. 
     Note that the source side driver circuit  401  includes an FET  423  and an FET  424 . The source side driver circuit  401  that includes the FET  423  and the FET  424  may be formed with a circuit including transistors having the same conductivity type (either an n-channel transistor or a p-channel transistor) or a CMOS circuit including an n-channel transistor and a p-channel transistor. Although a driver-integration type in which the driver circuit is formed over a substrate is described in this embodiment, one embodiment of the present invention is not limited to this type, and the driver circuit can be formed outside the substrate. 
     The pixel portion  402  is formed of a plurality of pixels each of which includes a switching FET  411 , a current control FET  412 , and a first electrode  413  electrically connected to a wiring (a source electrode or a drain electrode) of the current control FET  412 . In this embodiment, the pixel portion  402  includes two FETs, the switching FET  411  and the current control FET  412 , but one embodiment of the present invention is not limited thereto. The pixel portion  402  may include, for example, three or more FETs and a capacitor in combination. 
     As the FETs  411 ,  412 ,  423 , and  424 , for example, a staggered transistor or an inverted staggered transistor can be used. Examples of a semiconductor material that can be used for the FETs  411 ,  412 ,  423 , and  424  include Group W semiconductors (e.g., silicon and gallium), compound semiconductors, oxide semiconductors, and organic semiconductors. In addition, there is no particular limitation on the crystallinity of the semiconductor material, and an amorphous semiconductor or a crystalline semiconductor can be used. In particular, an oxide semiconductor is preferably used for the FETs  411 ,  412 ,  423 , and  424 . Examples of the oxide semiconductor include an In—Ga oxide and an In-M-Zn oxide (M represents Al, Ga, Y, Zr, La, Ce, or Nd). For example, an oxide semiconductor that has an energy gap of 2 eV or higher, preferably 2.5 eV or higher, further preferably 3 eV or higher is used for the FETs  411 ,  412 ,  423 , and  424 , so that the off-state current of the transistors can be reduced. 
     An insulator  414  is formed to cover an end portion of the first electrode  413 . Here, the insulator  414  is formed using a positive photosensitive acrylic resin. The first electrode  413  is used as an anode in this embodiment. 
     The insulator  414  preferably has a curved surface with curvature at an upper end portion or a lower end portion thereof. This enables the coverage with a film to be formed over the insulator  414  to be favorable. The insulator  414  can be formed using, for example, either a negative photosensitive resin or a positive photosensitive resin. The material of the insulator  414  is not limited to an organic compound, and an inorganic compound such as silicon oxide, silicon oxynitride, or silicon nitride can also be used. 
     An EL layer  416  and a second electrode  417  are stacked over the first electrode  413 . The EL layer  416  is provided with at least a light-emitting layer. In addition to the light-emitting layer, a hole-injection layer, a hole-transport layer, an electron-transport layer, an electron-injection layer, a charge-generation layer, and the like can be provided as appropriate in the EL layer  416 . Note that in this embodiment, the second electrode  417  is used as a cathode. 
     A light-emitting element  418  has a stacked structure of the first electrode  413 , the EL layer  416 , and the second electrode  417 . For the first electrode  413 , the EL layer  416 , and the second electrode  417 , the materials described in the above embodiments can be used. Although not illustrated, the second electrode  417  is electrically connected to the FPC  409  that is an external input terminal. 
     Although the cross-sectional view of  FIG. 3B  illustrates only one light-emitting element  418 , a plurality of light-emitting elements are arranged in a matrix in the pixel portion  402 . Light-emitting elements that emit light of three kinds of colors (R, G, and B) are selectively formed in the pixel portion  402 , whereby a light-emitting device capable of full color display can be fabricated. In addition to the light-emitting elements that emit light of three kinds of colors (R, G, and B), for example, light-emitting elements that emit light of white (W), yellow (Y), magenta (M), and cyan (C) may be formed. In that case, advantages of high color purity and low power consumption can be obtained. Alternatively, a light-emitting device capable of full color display may be fabricated by a combination with color filters. 
     The sealing substrate  404  is attached to the element substrate  410  with the sealing material  405 , so that a light-emitting element  418  is provided in the space  407  enclosed by the element substrate  410 , the sealing substrate  404 , and the sealing material  405 . The space  407  may be filled with an inert gas (such as nitrogen or argon) or the sealing material  405 . 
     An epoxy-based resin is preferably used as the sealing material  405 . Such a material preferably allows as little moisture and oxygen as possible to pass therethrough. As a material used for the sealing substrate  404 , a plastic substrate formed of fiber-reinforced plastics (FRP), poly(vinyl fluoride) (PVF), a polyester resin, an acrylic resin, or the like can be used other than a glass substrate or a quartz substrate. 
     As described above, the active matrix light-emitting device including the light-emitting element of one embodiment of the present invention can be obtained. 
     Further, a light-emitting element of one embodiment of the present invention can be used for a passive matrix light-emitting device as well as the above active matrix light-emitting device.  FIGS. 4A and 4B  illustrate a perspective view and a cross-sectional view of a passive matrix light-emitting device including a light-emitting element of one embodiment of the present invention. Note that the cross-sectional view of  FIG. 4B  is taken along line X-Y of  FIG. 4A . 
     In  FIGS. 4A and 4B , an EL layer  504  is provided between a first electrode  502  and a second electrode  503  over a substrate  501 . An end portion of the first electrode  502  is covered with an insulating layer  505 . In addition, a partition layer  506  is provided over the insulating layer  505 . The sidewalls of the partition layer  506  slope so that a distance between both the sidewalls is gradually narrowed toward the surface of the substrate. In other words, a cross section taken along the direction of the short side of the partition layer  506  is trapezoidal, and the base (side in contact with the insulating layer  505 ) is shorter than the upper side (side not in contact with the insulating layer  505 ). With the partition layer  506  provided in such a way, a defect of a light-emitting element due to crosstalk or the like can be prevented. 
     Thus, the light-emitting device which includes the light-emitting element of one embodiment of the present invention can be obtained. 
     The light-emitting devices described in this embodiment are formed using the light-emitting element of one embodiment of the present invention, and accordingly, the light-emitting devices can have low power consumption. 
     Note that this embodiment can be implemented in appropriate combination with any of the other embodiments. 
     Embodiment 5 
     In this embodiment, with reference to  FIGS. 5A to 5E  and  FIGS. 6A and 6B , examples of a variety of electronic devices and lighting devices that are each completed by the use of a light-emitting device of one embodiment of the present invention will be described. 
     Examples of the electronic devices are television devices (also referred to as TV or television receivers), monitors for computers and the like, cameras such as digital cameras and digital video cameras, digital photo frames, cellular phones (also referred to as portable telephone devices), portable game machines, portable information terminals, audio playback devices, large game machines such as pachinko machines, and the like. 
     An electronic device or a lighting device that has a light-emitting portion with a curved surface can be obtained with a light-emitting element including any of the organic compounds of embodiments of the present invention, which is fabricated over a substrate having flexibility. 
     In addition, an electronic device or a lighting device that has a see-through light-emitting portion can be obtained with a light-emitting element including any of the organic compounds of embodiments of the present invention in which a pair of electrodes are formed using a material having a property of transmitting visible light. 
     Further, a light-emitting device to which one embodiment of the present invention is applied can also be applied to lighting for motor vehicles, examples of which are lighting for a dashboard, a windshield, a ceiling, and the like. 
     In  FIG. 5A , an example of a television device is illustrated. In a television device  7100 , a display portion  7103  is incorporated in a housing  7101 . The display portion  7103  is capable of displaying images, and the light-emitting device can be used for the display portion  7103 . In addition, here, the housing  7101  is supported by a stand  7105 . 
     The television device  7100  can be operated with an operation switch of the housing  7101  or a separate remote controller  7110 . With operation keys  7109  of the remote controller  7110 , channels and volume can be controlled and images displayed on the display portion  7103  can be controlled. Furthermore, the remote controller  7110  may be provided with a display portion  7107  for displaying data output from the remote controller  7110 . 
     Note that the television device  7100  is provided with a receiver, a modem, and the like. With the receiver, general television broadcasting can be received. Furthermore, when the television device  7100  is connected to a communication network by wired or wireless connection via the modem, one-way (from a transmitter to a receiver) or two-way (between a transmitter and a receiver, between receivers, or the like) data communication can be performed. 
     In  FIG. 5B , a computer is illustrated, which includes a main body  7201 , a housing  7202 , a display portion  7203 , a keyboard  7204 , an external connection port  7205 , a pointing device  7206 , and the like. This computer is manufactured with the use of the light-emitting device for the display portion  7203 . 
     In  FIG. 5C , a portable amusement machine is illustrated, which includes two housings, a housing  7301  and a housing  7302 , connected with a joint portion  7303  so that the portable amusement machine can be opened or closed. A display portion  7304  is incorporated in the housing  7301  and a display portion  7305  is incorporated in the housing  7302 . In addition, the portable amusement machine illustrated in  FIG. 5C  includes a speaker portion  7306 , a recording medium insertion portion  7307 , an LED lamp  7308 , input means (an operation key  7309 , a connection terminal  7310 , a sensor  7311  (a sensor having a function of measuring or sensing force, displacement, position, speed, acceleration, angular velocity, rotational frequency, distance, light, liquid, magnetism, temperature, chemical substances, sound, time, hardness, electric field, current, voltage, electric power, radiation, flow rate, humidity, gradient, oscillation, odor, or infrared rays), and a microphone  7312 ), and the like. It is needless to say that the structure of the portable amusement machine is not limited to the above as long as the light-emitting device is used for at least either the display portion  7304  or the display portion  7305 , or both, and may include other accessories as appropriate. The portable amusement machine illustrated in  FIG. 5C  has a function of reading out a program or data stored in a storage medium to display it on the display portion, and a function of sharing information with another portable amusement machine by wireless communication. Note that functions of the portable amusement machine illustrated in  FIG. 5C  are not limited to the above, and the portable amusement machine can have a variety of functions. 
     In  FIG. 5D , an example of a cellular phone is illustrated. A cellular phone  7400  is provided with a display portion  7402  incorporated in a housing  7401 , operation buttons  7403 , an external connection port  7404 , a speaker  7405 , a microphone  7406 , and the like. Note that the cellular phone  7400  is manufactured with the use of the light-emitting device for the display portion  7402 . 
     When the display portion  7402  of the cellular phone  7400  illustrated in  FIG. 5D  is touched with a finger or the like, data can be input to the cellular phone  7400 . Further, operations such as making a phone call and creating e-mail can be performed by touch on the display portion  7402  with a finger or the like. 
     There are mainly three screen modes of the display portion  7402 . The first mode is a display mode mainly for displaying an image. The second mode is an input mode mainly for inputting information such as a character. The third mode is a display-and-input mode in which two modes of the display mode and the input mode are mixed. 
     For example, in the case of making a phone call or creating e-mail, the character input mode mainly for inputting a character is selected for the display portion  7402  to input a character on a screen. In this case, it is preferable to display a keyboard or number buttons on almost the entire screen of the display portion  7402 . 
     When a detection device including a sensor for detecting inclination, such as a gyroscope or an acceleration sensor, is provided inside the cellular phone  7400 , display on the screen of the display portion  7402  can be automatically changed by determining the orientation of the cellular phone  7400  (whether the cellular phone is placed horizontally or vertically for a landscape mode or a portrait mode). 
     The screen modes are switched by touch on the display portion  7402  or operation with the operation buttons  7403  of the housing  7401 . The screen modes can also be switched depending on kinds of images displayed on the display portion  7402 . For example, when a signal for an image to be displayed on the display portion is for moving images, the screen mode is switched to the display mode; when the signal is for text data, the screen mode is switched to the input mode. 
     Moreover, in the input mode, if an optical sensor in the display portion  7402  determines that touch on the display portion  7402  is not performed for a certain period, the screen mode may be changed from the input mode to the display mode. 
     The display portion  7402  may function as an image sensor. For example, an image of a palm print, a fingerprint, or the like is taken by touch on the display portion  7402  with the palm or the finger, so that personal identification can be performed. Furthermore, by provision of a backlight or a sensing light source emitting near-infrared light to the display portion, an image of a finger vein, a palm vein, or the like can also be taken. 
     In  FIG. 5E , a desk lamp is illustrated, which includes a lighting portion  7501 , a shade  7502 , an adjustable arm  7503 , a support  7504 , a base  7505 , and a power switch  7506 . The desk lamp is manufactured with the use of the light-emitting device for the lighting portion  7501 . Note that the “lighting device” also includes ceiling lights, wall lights, and the like. 
     In  FIG. 6A , an example in which the light-emitting device is used for an interior lighting device  601  is illustrated. Since the light-emitting device can have a larger area, it can be used as a lighting device having a large area. Furthermore, the light-emitting device can be used as a roll-type lighting device  602 . As illustrated in  FIG. 6A , the desk lamp  603  described with reference to  FIG. 5E  may also be used in a room provided with the interior lighting device  601 . 
     In  FIG. 6B , an example of another lighting device is illustrated. A table lamp illustrated in  FIG. 6B  includes a lighting portion  9501 , a support  9503 , a support base  9505 , and the like. The lighting portion  9501  includes any of the organic compounds of embodiments of the present invention. Thus, a lighting device that has a curved surface or a lighting portion that can be flexibly bent can be provided by fabrication of a light-emitting element over a substrate having flexibility. Such use of a flexible light-emitting device for a lighting device enables a place having a curved surface, such as the ceiling or dashboard of a motor vehicle, to be provided with the lighting device, as well as increases the degree of freedom in design of the lighting device. 
     In the above-described manner, electronic devices or lighting devices can be obtained by application of the light-emitting device. Application range of the light-emitting device is so wide that the light-emitting device can be applied to electronic devices in a variety of fields. 
     Note that the structure described in this embodiment can be combined with any of the structures described in the other embodiments as appropriate. 
     Example 1 
     Synthesis Example 1 
     In this example, a method of synthesizing 2,2′-(1,1′-biphenyl-3,3′-diyl)di(dibenzo[f,h]quinoxaline) (abbreviation: mDBq2BP) represented by Structural Formula (101) in Embodiment 1 will be described in detail. The structure of mDBq2BP is shown below. 
     
       
         
         
             
             
         
       
     
     A synthesis scheme of mDBq2BP is shown below. 
     Step 1: Synthesis of 2-[3-(dibenzo[f,h]quinoxalin-2-yl)phenyl]-4,4,5,5-tetramethyl-1,3,2-dioxaborolane 
     Into a 200-mL three-neck flask were put 1.9 g (4.8 mmol) of 2-(3-bromophenyl)dibenzo[f,h]quinoxaline, 2.0 g (7.7 mmol) of bis(pinacolato)diboron, 0.12 g (0.14 mmol) of [1,1-bis(diphenylphosphino)ferrocene]palladium(II)dichloride (abbreviation: Pd(dppf)Cl 2 ), 2.0 g (20 mmol) of potassium acetate (abbreviation: AcOK), and the atmosphere in the flask was replaced with nitrogen. To this mixture was added 17 mL of 1,4-dioxane. This mixture was stirred at 90° C. for 8 hours. After the stirring, 50 mL of water and 50 mL of toluene were added to this mixture. An aqueous layer of the resulting mixture was subjected to extraction with toluene. The extracted solution and the organic layer were combined, the mixture was washed with saturated saline, and then magnesium sulfate was added thereto. The mixture was gravity-filtered, and then the obtained filtrate was concentrated to give a black solid. A synthesis scheme (a-1) of Step 1 is shown below. 
     
       
         
         
             
             
         
       
     
     Step 2: Synthesis of mDBq2BP 
     Into a 300-mL three-neck flask were put 2-[3-(dibenzo[f,h]quinoxalin-2-yl)phenyl]-4,4,5,5-tetramethyl-1,3,2-dioxaborolane obtained through Synthesis Scheme (a-1), 1.7 g (4.4 mmol) of 2-(3-bromophenyl)dibenzo[f,h]quinoxaline, 0.16 g (0.53 mmol) of tris(2-methylphenyl)phosphine (abbreviation: P(o-tolyl) 3 ), 22 mL of toluene, 24 mL of ethanol, and 6.6 mL of an aqueous potassium carbonate solution (2.0 mol/L). The mixture was degassed by being stirred under reduced pressure. After the degassing, the atmosphere in the flask was replaced with nitrogen. To this mixture was added 30 mg (0.13 mmol) of palladium(II) acetate (abbreviation: Pd(OAc) 2 ). The mixture was stirred at 80° C. under a nitrogen stream for 4 hours. After a predetermined time, a solid precipitated in the flask was collected by suction filtration. The obtained solid was washed with water, ethanol, and toluene in this order, and then collected by suction filtration to give 2.6 g of a brown solid in a yield of 90%. Synthesis Scheme (a-2) of Step 2 is shown below. The yield is the total of the two steps, Synthesis Scheme (a-1) and Synthesis Scheme (a-2). 
     
       
         
         
             
             
         
       
     
     Next, 2.5 g of the obtained brown solid was purified by train sublimation. In the purification by sublimation, the solid was heated at 340° C. under a pressure of 2.6 Pa with a flow rate of argon of 5.0 mL/min for 46 hours. After the heating, 2.1 g of the target light brown solid was collected in 84%. 
     This compound was identified as mDBq2BP, which was the target substance, by nuclear magnetic resonance ( 1 H NMR). 
       1 H NMR data of the obtained compound is shown below.  1 H NMR (CDCl 3 , 500 MHz): δ 7.75-7.85 (m, 10H), 7.96 (d, J=4.8 Hz, 2H), 8.41 (d, J=4.2 Hz, 2H), 8.68 (d, J=5.1 Hz, 4H), 8.75 (s, 2H), 9.30 (d, J=4.8 Hz, 2H), 9.47 (d, J=4.5 Hz, 2H), 9.53 (s, 2H). 
     The  1 H NMR charts are shown in  FIGS. 7A and 7B .  FIG. 7B  is an enlarged chart showing a range of from 7.5 ppm to 10.0 ppm in  FIG. 7A . 
       FIG. 8A  shows the emission spectrum of mDBq2BP in toluene, and  FIG. 8B  shows the absorption spectrum thereof.  FIG. 9A  shows the emission spectrum of a thin film of mDBq2BP, and  FIG. 9B  shows the absorption spectrum thereof. In FIG.  8 A and  FIG. 9A , the horizontal axis represents wavelength (nm) and the vertical axis represents emission intensity (arbitrary unit). In  FIG. 8B  and  FIG. 9B , the horizontal axis represents wavelength (nm) and the vertical axis represents absorption intensity (arbitrary unit). In the case of the toluene solution, emission peaks are observed at 387 nm and 405 nm (excitation wavelength: 365 nm), and absorption peaks are observed at 303 nm, 366 nm, and 375 nm. In the case of the thin film, emission peaks are observed at 406 nm, 440 nm, 456 nm, and 492 nm (excitation wavelength: 369 nm) and absorption peaks are observed at 311 nm, 369 nm, and 384 nm. 
     The absorption spectra were measured with an ultraviolet-visible spectrophotometer (V-550, produced by JASCO Corporation). The measurement of emission spectra and absorption spectra was performed with samples prepared in such a manner that the solution was put in a quartz cell and the thin film was obtained by evaporation onto a quartz substrate. Note that the absorption spectrum of the solution was obtained by subtraction of the absorption spectra of the quartz cell and toluene or dimethylformamide from the measured spectrum, and the absorption spectrum of the thin film was obtained by subtraction of the absorption spectrum of the quartz substrate from the measured spectrum. 
     The thermogravimetry-differential thermal analysis of mDBq2BP prepared in Synthesis example 1 was performed. For the measurement, a high vacuum differential type differential thermal balance (TG-DTA 2410SA, produced by Bruker AXS K.K.) was used. The measurement was performed under normal pressure under a nitrogen stream (at a flow rate of 200 mL/min) at a rate of temperature increase of 10° C./min. From the relationship between the weight and the temperature (thermogravimetry), it was found that a 5% weight loss of mDBq2BP was seen at a temperature of 460° C. 
     Example 2 
     Synthesis Example 2 
     In this example, a method of synthesizing 2,2′,2″-[(1,3,5-benzene-triyl)tri(3,1-phenylene)]tri(dibenzo[f,h]quinoxaline) (abbreviation: mDBqP3P) represented by Structural Formula (106) in Embodiment 1 will be described in detail. The structure of mDBqP3P is shown below. 
     
       
         
         
             
             
         
       
     
     A synthesis scheme of mDBqP3P is shown below. 
     Step 1: Synthesis of mDBqP3P 
     Into a 200-mL three-neck flask were put 2.2 g (5.0 mmol) of 2-[3-(dibenzo[f,h]quinoxalin-2-yl)phenyl]-4,4,5,5-tetramethyl-1,3,2-dioxaborolane synthesized in a method similar to Synthesis Scheme (a-1) of Step 1 in Example 1, 0.50 g (1.6 mmol) of 1,3,5-tribromobenzene, and 3.2 g (10 mmol) of cesium carbonate, and the atmosphere in the flask was replaced with nitrogen. To this mixture was added 100 mL of mesitylene, and the resulting mixture was degassed by being stirred while the pressure in the flask was reduced. After the degassing, the atmosphere in the flask was replaced with nitrogen, and 0.11 g (0.10 mmol) of tetrakis(triphenylphosphine)palladium(0) was added thereto. The resulting mixture was stirred at 150° C. for 7.5 hours. A precipitate was collected by suction filtration, and washed with water, ethanol, and toluene. After the washing, the obtained solid was collected by suction filtration, whereby 1.3 g of the target brown solid was obtained in a yield of 82%. Synthesis Scheme (b-1) of Step 1 is shown below. 
     
       
         
         
             
             
         
       
     
     This compound was identified as mDBqP3P, which was the target substance,  1 H NMR. 
       1 H NMR data of the obtained compound is shown below.  1 H NMR (CDCl 3 , 500 MHz): δ=7.71-7.83 (m, 15H), 8.03 (d, J=4.5 Hz, 3H), 8.22 (s, 3H), 8.43 (d, J=4.8 Hz, 3H), 8.61 (d, J=4.8 Hz, 3H), 8.65 (d, J=4.8 Hz, 3H), 8.82 (s, 3H), 9.29 (d, 4.8 Hz, 3H), 9.45 (d, J=4.2 Hz, 3H), 9.55 (s, 31-1). 
     The  1 H NMR charts are shown in  FIGS. 10A and 10B .  FIG. 10B  is an enlarged chart showing a range of from 7.5 ppm to 10.0 ppm in  FIG. 10A . 
     Example 3 
     Synthesis Example 3 
     In this example, a method of synthesizing 2,2′-(1,1′:3′,1″:3″,1′″-quaterphenylene-3,3′″-diyl)di(dibenzo[f,h]quinoxaline) (abbreviation: mDBqP2BP) represented by Structural Formula (103) in Embodiment 1 will be described in detail. The structure of mDBqP2BP is shown below. 
     
       
         
         
             
             
         
       
     
     A synthesis scheme of mDBqP2BP is shown below. 
     Step 1: Synthesis of 2-(3′-bromo-1,1′-biphenyl-3-yl)dibenzo[f,h]quinoxaline 
     Into a 300-mL three-neck flask were put 10.0 g (23 mmol) of 2-[3-(dibenzo[f,h]quinoxalin-2-yl)phenyl]-4,4,5,5-tetramethyl-1,3,2-dioxaborolane, 6.5 g (23 mmol) of 1-bromo-3-iodobenzene, 0.38 g (1.3 mmol) of P(o-tolyl) 3 , 115 mL of toluene, 10 mL of ethanol, and 35 mL of an aqueous potassium carbonate solution (2.0 mol/L). This mixture was degassed by being stirred while the pressure in the flask was reduced. After the degassing, the atmosphere in the flask was replaced with nitrogen, and the mixture was heated to 80° C. At the same temperature, 70 mg (0.31 mmol) of Pd(OAc) 2  was added, and the resulting mixture was stirred at 80° C. for 2 hours. After the stirring, the mixture was cooled down to the room temperature and degassed under reduced pressure. Then, the atmosphere in the flask was replaced with nitrogen again, and the mixture was heated to 80° C. At the same temperature, 60 mg (0.27 mmol) of Pd(OAc) 2  was added, and the mixture was heated for 8 hours. A precipitate was collected by suction filtration. The obtained residue was suspended in toluene, and the mixture was subjected to hot filtration. The obtained filtrate was suction-filtered through Celite (produced by Wako Pure Chemical Industries, Ltd., Catalog No. 531-16855) and alumina. The obtained filtrate was concentrated to give a white solid. The obtained solid was recrystallized with toluene and hexane, whereby 5.1 g of the target white powder was obtained in a yield of 48%. Synthesis Scheme (c-1) of Step 1 is shown below. 
     
       
         
         
             
             
         
       
     
     Step 2: Synthesis of 2-[3′-(dibenzo[f,h]quinoxalin-2-yl)biphenyl-3-yl]-4,4,5,5-tetramethyl-1,3,2-dioxaborolane 
     Into a 100-mL three-neck flask were put 5.1 g (11 mmol) of 2-(3′-bromo-1,1′-biphenyl-3-yl)dibenzo[f,h]quinoxaline, which was obtained through Synthesis Scheme (c-1), 2.8 g (11 mmol) of bis(pinacolato)diboron, and 3.2 g (33 mmol) of AcOK, and the atmosphere in the flask was replaced with nitrogen. To this mixture was added 30 mL of 1,4-dioxane, and the mixture was heated to 100° C. After that, at the same temperature, 90 mg (110 μmol) of Pd(dppf)Cl 2  was added. The resulting mixture was stirred at 100° C. for 2 hours. After the stirring, 90 mg (110 μmmol) of Pd(dppf)Cl 2  was further added and the mixture was stirred at 100° C. for 7.5 hours. After the stirring, the obtained mixture was suction-filtered and the residue was washed with water and ethanol. The obtained residue was recrystallized with hexane and toluene to give a solid. To the obtained solid were added hexane and toluene, irradiation with ultrasonic waves was performed, and the solid was collected by suction filtration, whereby 4.4 g of the target brown powder was obtained in a crude yield of 79%. Synthesis Scheme (c-2) of Step 2 is shown below. 
     
       
         
         
             
             
         
       
     
     Step 3: Synthesis of mDBqP2BP 
     Into a 200-mL three-neck flask were put 1.5 g (3.2 mmol) of 2-(3′-bromo-1,1′-biphenyl-3-yl)dibenzo[f,h]quinoxaline, 1.6 g (3.2 mmol) of 2-[3′-(dibenzo[f,h]quinoxalin-2-yl)-1,1′-biphenyl-3-yl]-4,4,5,5-tetramethyl-1,3,2-dioxaborolane, 40 mg (131 μmol) of P(o-tolyl) 3 , 16 mL of toluene, 2 mL of ethanol, and 5 mL of an aqueous potassium carbonate solution (2 mol/L). The mixture was degassed by being stirred while the pressure in the flask was reduced. After the degassing, the atmosphere in the flask was replaced with nitrogen, and the mixture was heated to 80° C. Then, at the same temperature, 10 mg (45 μmol of Pd(OAc) 2  was added to this mixture. The resulting mixture was stirred at 80° C. for 6 hours. After the stirring, the obtained mixture was suction-filtered to give a residue. The residue was washed with water, ethanol, and then mesitylene to give 1.6 g of the target powder in a yield of 73%. Synthesis Scheme (c-3) of Step 3 is shown below. 
     
       
         
         
             
             
         
       
     
     Next, 1.5 g of the obtained solid was purified by train sublimation. In the purification by sublimation, the solid was heated at 400° C. under a pressure of 5.0 Pa with a flow rate of argon of 15 mL/min for 16 hours. After the purification by sublimation, 1.3 g of powder was collected in 87%. Then, 1.3 g of the solid obtained by the sublimation purification was further purified by train sublimation. In the purification by sublimation, the solid was heated at 400° C. under a pressure of 5.0 Pa with a flow rate of argon of 15 mL/min for 17.5 hours. After the purification by sublimation, 1.1 g of powder was collected in 85%. 
     Next, this compound was identified as mDBqP2BP, which was the target substance, by  1 H NMR. 
       1 H NMR data of the obtained compound is shown below.  1 H NMR (1,1,2,2-tetrachloroethane-d 2 , 500 MHz): δ=7.71-7.85 (m, 16H), 7.93 (d, J=7.5 Hz, 2H), 8.18 (s, 2H), 8.38 (d, J=7.5 Hz, 2H), 8.62 (d, J=7.0 Hz, 4H), 8.76 (s, 2H), 9.25 (d, J=7.0 Hz, 2H), 9.42 (d, J=8.0 Hz, 2H), 9.49 (br, 2H). 
     The  1 H NMR charts are shown in  FIGS. 11A and 11B .  FIG. 11B  is an enlarged chart showing a range of from 7.5 ppm to 10.0 ppm in  FIG. 11A . 
       FIG. 42A  shows the emission spectrum of mDBqP2BP in toluene, and  FIG. 42B  shows the absorption spectrum thereof  FIG. 43A  shows the emission spectrum of a thin film of mDBqP2BP, and  FIG. 43B  shows the absorption spectrum thereof. In  FIG. 42A  and  FIG. 43A , the horizontal axis represents wavelength (nm) and the vertical axis represents emission intensity (arbitrary unit). In  FIG. 42B  and  FIG. 43B , the horizontal axis represents wavelength (nm) and the vertical axis represents absorption intensity (arbitrary unit). In the case of the toluene solution, emission peaks are observed at 393 nm and 406 nm (excitation wavelength: 377 nm), and absorption peaks are observed at 359 nm and 375 nm. In the case of the thin film, an emission peak is observed at 427 nm (excitation wavelength: 382 nm) and absorption peaks are observed at 262 nm, 313 nm, 368 nm, and 384 nm. 
     Note that the absorption spectra were measured with the apparatus described in Example 1, and the emission spectra and absorption spectra were measured by the method described in Example 1. 
     The thermogravimetry-differential thermal analysis of mDBqP2BP prepared in Synthesis example 3 was performed. The relationship between weight and temperature (thermogravimetry) shows that the 5% weight loss temperature of mDBqP2BP is 500° C. or higher. Note that the thermogravimetry-differential thermal analysis was performed using the apparatus and the method which are described in Example 1. 
     Furthermore, differential scanning calorimetry of mDBqP2BP prepared in Synthesis example 3 was performed. For the measurement, a differential scanning calorimeter (Pyris 1, produced by PerkinElmer Japan Co., Ltd.) was used. One cycle in the measurement was as follows: the temperature was increased from 30° C. to 500° C. at a rate of 50° C./min, kept at 500° C. for 1 minute, and decreased from 500° C. to 30° C. at a rate of 50° C./min. In this measurement, two cycles were performed. From the result at the rising temperature in the second cycle, it was found that the melting point (Tm) was 364° C. Therefore, mDBqP2BP has high heat resistance. 
     Example 4 
     Synthesis example 4 
     In this example, a method of synthesizing 2,2′-[(9,9-dimethyl-9H-fluorene-2,7-diyl)di(3,1-phenylene)]di(dibenzo[f,h]quinoxaline) (abbreviation: mDBqP2F) represented by Structural Formula (105) in Embodiment 1 will be described in detail. The structure of mDBqP2F is shown below. 
     
       
         
         
             
             
         
       
     
     A synthesis scheme of mDBqP2F is shown below. 
     Step 1: Synthesis of mDBqP2F 
     First, into a 200-mL three-neck flask were put 4.7 g (11 mmol) of 2-[3-(dibenzo[f,h]quinoxalin-2-yl)phenyl]-4,4,5,5-tetramethyl-1,3,2-dioxaborolane, 1.8 g (5.0 mmol) of 2,7-dibromo-9,9-dimethyl-9H-fluorene, 0.15 g (0.49 mmol) of P(o-tolyl) 3 , 25 mL of toluene, 2 mL of ethanol, and 16 mL of an aqueous potassium carbonate solution (2 mol/L). This mixture was degassed by being stirred while the pressure in the flask was reduced. After the degassing, the atmosphere in the flask was replaced with nitrogen, and the mixture was heated to 80° C. After the temperature in the flask became 80° C., 20 mg (90 μmol) of Pd(OAc) 2  was added to this mixture, and the resulting mixture was stirred at the same temperature for 3.5 hours. After the stirring, a precipitate was collected by suction filtration. The obtained solid was washed with water, ethanol, and then dimethylformamide, whereby 2.4 g of the target light brown solid was obtained in a yield of 59%. Synthesis Scheme (d-1) of Step 1 is shown below. 
     
       
         
         
             
             
         
       
     
     Next, 0.70 g of the obtained solid was purified by train sublimation. In the purification by sublimation, 0.70 g of the solid was heated at 395° C. under a pressure of 2.5×10 −2  Pa for 15 hours. After the heating, 0.40 g of the target light yellow powder was collected in 58%. 
     This compound was identified as mDBqP2F, which was the target substance, by  1 H NMR. 
       1 H NMR data of the obtained compound is shown below.  1 H NMR (1,1,2,2-tetrachloroethane-d 2 , 500 MHz): δ=1.72 (s, 6H), 7.77 (t, J=7.8 Hz, 2H), 7.80-7.88 (m, 12H), 7.92 (d, J=7.5 Hz, 2H), 7.97 (d, J=8.0 Hz, 2H), 8.38 (d, J=8.0 Hz, 2H), 8.68 (s, 2H), 8.71 (d, J=8.0 Hz, 4H), 9.28 (d, J=8.0 Hz, 2H), 9.47 (d, J=1.5 Hz, 2H), 9.54 (s, 2H). 
     The  1 H NMR charts are shown in  FIGS. 12A and 12B .  FIG. 12B  is an enlarged chart showing a range of from 7.5 ppm to 10.0 ppm in  FIG. 12A . 
       FIG. 13A  shows the emission spectrum of mDBqP2F in dimethylformamide, and  FIG. 13B  shows the absorption spectrum thereof.  FIG. 14A  shows the emission spectrum of a thin film of mDBqP2F, and  FIG. 14B  shows the absorption spectrum thereof. In  FIG. 13A  and  FIG. 14A , the horizontal axis represents wavelength (nm) and the vertical axis represents emission intensity (arbitrary unit). In  FIG. 13B  and  FIG. 14B , the horizontal axis represents wavelength (nm) and the vertical axis represents absorption intensity (arbitrary unit). In the case of the dimethylformamide solution, an emission peak is observed at 497 nm (excitation wavelength: 375 nm), and absorption peaks are observed at 305 nm, 328 nm, 359 nm, and 374 nm. In the case of the thin film, emission peaks are observed at 460 nm and 487 nm (excitation wavelength: 370 inn) and absorption peaks are observed at 312 nm, 329 nm, 366 nm, and 382 nm. 
     Note that the absorption spectra were measured with the apparatus described in Example 1, and the emission spectra and absorption spectra were measured by the method described in Example 1. 
     The thermogravimetry-differential thermal analysis of mDBqP2F prepared in Synthesis example 4 was performed. The relationship between weight and temperature (thermogravimetry) shows that the 5% weight loss temperature of mDBqP2F is 500° C. or higher. Note that the thermogravimetry-differential thermal analysis was performed using the apparatus and the method which are described in Example 1. 
     Differential scanning calorimetry of mDBqP2F prepared in Synthesis example 4 was performed. One cycle in the measurement was as follows: the temperature was increased from −10° C. to 365° C. at a rate of 50° C./min, kept at 365° C. for 1 minute, and decreased from 365° C. to −10° C. at a rate of 50° C./min. In this measurement, two cycles were performed. From the result at the rising temperature in the second cycle, it was found that the glass transition temperature (Tg) was 166° C. Therefore, mDBqP2F has high heat resistance. Note that the differential scanning calorimeter was the same as that described in Example 3. 
     Example 5 
     Synthesis Example 5 
     In this example, a method of synthesizing 2,2′-(1,1′:3′,1″-terphenylene-3,3″-diyl)di(dibenzo[f,h]quinoxaline) (abbreviation: mDBqP2P) represented by Structural Formula (102) in Embodiment 1 will be described in detail. The structure of mDBqP2P is shown below. 
     
       
         
         
             
             
         
       
     
     A synthesis scheme of mDBqP2P is shown below. 
     Step 1: Synthesis of 2-(3′-bromo-1,1′-biphenyl-3-yl)dibenzo[f,h]quinoxaline 
     Into a 200-mL three-neck flask were put 3.1 g (7.2 mmol) of 2-[3-(dibenzo[f,h]quinoxalin-2-yl)phenyl]-4,4,5,5-tetramethyl-1,3,2-dioxaborolane, 4.0 g (14 mmol) of 1-bromo-3-iodobenzene, 0.22 g (0.70 mmol) of P(o-tolyl) 3 , 70 mL of toluene, 8 mL of ethanol, and 20 mL of an aqueous potassium carbonate solution (2.0 mol/L). This mixture was degassed by being stirred while the pressure in the flask was reduced. After the degassing, the atmosphere in the flask was replaced with nitrogen, 30 mg (0.13 mmol) of Pd(OAc) 2  was added, and the mixture was stirred at 80° C. for 1 hour. After the stirring, a precipitate was collected by suction filtration, and a residue was washed with water and ethanol. The obtained residue was suspended in toluene, and the mixture was subjected to hot filtration. The obtained filtrate was suction-filtered through Celite (produced by Wako Pure Chemical Industries, Ltd., Catalog No. 531-16855) and alumina. The obtained filtrate was concentrated to give a white solid. The obtained solid was recrystallized with toluene, whereby 1.8 g of the target white powder was obtained in a yield of 54%. Synthesis Scheme (e-1) of Step 1 is shown below. 
     
       
         
         
             
             
         
       
     
     Step 2: Synthesis of mDBqP2P 
     Into a 200-mL three-neck flask were put 1.0 g (2.6 mmol) of 2-(3′-bromo-1,1′-biphenyl-3-yl)dibenzo[f,h]quinoxaline, 1.0 g (2.4 mmol) of 2-[3-(dibenzo[f,h]quinoxalin-2-yl)phenyl]-4,4,5,5-tetramethyl-1,3,2-dioxaborolane, 34 mg (0.11 mmol) of P(o-tolyl) 3 , 10 mL of toluene, 1 mL of ethanol, and 4 mL of an aqueous potassium carbonate solution (2.0 mol/L). This mixture was degassed by being stirred while the pressure in the flask was reduced. After the stirring, the atmosphere in the flask was replaced with nitrogen, and this mixture was heated to 80° C. After the heating, 5.0 mg (22 mmol) of abbreviation: Pd(OAc) 2  was added to this mixture, and the resulting mixture was stirred at 80° C. for 5 hours. After the stirring, the obtained mixture was suction-filtered, and the obtained residue was washed with water, ethanol, and then mesitylene to give 1.1 g of the target solid in a yield of 67%. Synthesis Scheme (e-2) of Step 2 is shown below. 
     
       
         
         
             
             
         
       
     
     Next, 0.76 g of the obtained solid was purified by train sublimation. In the purification by sublimation, the solid was heated at 345° C. under a pressure of 2.7 Pa with a flow rate of argon of 5 mL/min for 1 hour. After the purification by sublimation, 0.63 g of powder was collected in 83%. 
     This compound was identified as mDBqP2P, which was the target substance, by  1 H NMR. 
       1 H NMR data of the obtained compound is shown below.  1 H NMR (1,1,2,2-tetrachloroethane-d 2 , 500 MHz): δ=7.73-7.89 (m, 13H), 7.95 (d, J=7.5 Hz, 2H), 8.18 (s, 1H), 8.42 (d, J=8.0 Hz, 2H), 8.68 (t, J=7.8 Hz, 4H), 8.74 (s, 2H), 9.33 (d, J=8.0 Hz, 2H), 9.48 (d, J=7.5 Hz, 2H), 9.55 (s, 2H). 
     The  1 H NMR charts are shown in  FIGS. 15A and 15B .  FIG. 15B  is an enlarged chart showing a range of from 7.5 ppm to 10.0 ppm in  FIG. 15A . 
       FIG. 16A  shows the emission spectrum of mDBqP2P in dimethylformamide, and  FIG. 16B  shows the absorption spectrum thereof.  FIG. 17A  shows the emission spectrum of a thin film of mDBqP2P, and  FIG. 17B  shows the absorption spectrum thereof. In  FIG. 16A  and  FIG. 17A , the horizontal axis represents wavelength (nm) and the vertical axis represents emission intensity (arbitrary unit). In  FIG. 16B  and  FIG. 17B , the horizontal axis represents wavelength (nm) and the vertical axis represents absorption intensity (arbitrary unit). In the case of the dimethylformamide solution, an emission peak is observed at 408 nm (excitation wavelength: 377 nm), and absorption peaks are observed at 300 nm, 361 nm, and 375 nm. In the case of the thin film, emission peaks are observed at 439 nm (excitation wavelength: 370 nm) and absorption peaks are observed at 311 nm, 369 nm, and 384 nm. 
     Note that the absorption spectra were measured with the apparatus described in Example 1, and the emission spectra and absorption spectra were measured by the method described in Example 1. 
     The thermogravimetry-differential thermal analysis of mDBqP2P prepared in Synthesis example 5 was performed. The relationship between weight and temperature (thermogravimetry) shows that the 5% weight loss temperature of mDBqP2P is 482° C. Note that the thermogravimetry-differential thermal analysis was performed using the apparatus and the method which are described in Example 1. 
     Furthermore, differential scanning calorimetry of mDBqP2P prepared in Synthesis example 5 was performed. One cycle in the measurement was as follows: the temperature was increased from 30° C. to 330° C. at a rate of 50° C./min, kept at 330° C. for 1 minute, and decreased from 330° C. to 30° C. at a rate of 50° C./min. In this measurement, one cycle was performed. From the result at the rising temperature in the second cycle, it was found that the glass transition temperature (Tg) was 138° C. and the melting point (Tm) was 302° C. Therefore, mDBqP2P has high heat resistance. Note that the differential scanning calorimeter was the same as that described in Example 3. 
     Example 6 
     Synthesis example 6 
     In this example, a method of synthesizing 2,2′-[5′-(dibenzothiophen-4-yl)-1,1′:3′,1″-terphenylene-3,3″-diyl]di(dibenzo[f,h]quinoxaline) (abbreviation: DBt-mDBqP2P) represented by Structural Formula (120) in Embodiment 1 will be described in detail. The structure of DBt-mDBqP2P is shown below. 
     
       
         
         
             
             
         
       
     
     A synthesis scheme of DBt-mDBqP2P is shown below. 
     Step 1: Synthesis of 5-(dibenzothiophen-4-yl)-1,3-dihydroxybenzene 
     Into a 200-mL three-neck flask were put 2.3 g (12 mmol) of 5-bromoresorcinol, 3.0 g (13 mmol) of dibenzothiophene-4-boronic acid, 0.19 g (0.62 mmol) of P(o-tolyl) 3 , 60 mL of toluene, 6 mL of ethanol, and 40 mL of an aqueous potassium carbonate solution (2 mol/L). This mixture was degassed by being stirred while the pressure in the flask was reduced. After the degassing, the atmosphere in the flask was replaced with nitrogen, and this mixture was heated to 80° C. Then, 30 mg (0.13 mmol) of Pd(OAc) 2  was added to this mixture at the same temperature, and the resulting mixture was stirred at the same temperature for 7 hours. After that, the mixture was cooled down to room temperature and degassed again. The atmosphere in the flask was replaced with nitrogen, and this mixture was heated to 80° C. To this mixture was added 30 mg (0.13 mmol) of Pd(OAc) 2  at the same temperature, and the mixture was stirred at the same temperature for 1 hour. Then, ethyl acetate and water were added to the obtained mixture to separate an organic layer and an aqueous layer, and the aqueous layer was subjected to extraction with ethyl acetate three times. The extracted solution and the organic layer were combined and the mixture was washed with saturated saline, and magnesium sulfate was added thereto. This mixture was gravity-filtered, and the obtained filtrate was concentrated to give a residue. The residue was purified by column chromatography (developing solvent: a mixed solvent of hexane and ethyl acetate in a ratio of 4:1). The obtained fraction was concentrated to give a black oily substance. To this oily substance was added chloroform, and the mixture was subjected to irradiation with ultrasonic waves. The obtained mixture was suction-filtered, whereby 1.3 g of the target white powder was obtained in a yield of 36%. Synthesis Scheme (f-1) of Step 1 is shown below. 
     
       
         
         
             
             
         
       
     
     Step 2: Synthesis of 5-(dibenzothiophen-4-yl)phenyl 1,3-bistrifluoromethansulfonate 
     Into a 100-mL three-neck flask was put 1.3 g (4.3 mmol) of 5-(dibenzothiophen-4-yl)-1,3-dihydroxybenzene. The pressure in the flask was reduced while stirring, and the atmosphere in the flask was replaced with nitrogen. Then, 20 mL of dehydrated dichloromethane was added under nitrogen, and this solution was cooled to −20° C. Then, at the same temperature, 2.5 mL (18 mmol) of triethylamine and 2.0 mL (12 mmol) of trifluoromethanesulfonic anhydride was added, and the mixture was stirred for 1.5 hours. The temperature was raised to 0° C. and stirring was performed for 5 hours. The temperature was further raised to room temperature and stirring was performed for 15 hours. To the obtained mixture were added water and an aqueous sodium carbonate solution, the mixture was stirred, and it was checked that its pH became 8. An aqueous layer of this mixture was subjected to extraction with dichloromethane three times. The extracted solution and an organic layer were combined and this mixture was washed with saturated saline, and magnesium sulfate was added thereto. The resulting mixture was gravity-filtered and the obtained filtrate was concentrated to give a black oily substance. The obtained oily substance was purified by silica gel column chromatography (developing solvent: a mixed solvent of hexane and ethyl acetate in a ratio of 10:1), whereby 2.2 g of the target yellow oily substance was obtained in a yield of 91%. Synthesis Scheme (f-2) of Step 2 is shown below. 
     
       
         
         
             
             
         
       
     
     Step 3: 3-[3-(dibenzo[f,h]quinoxalin-2-yl)phenyl]-5-(dibenzothiophen-4-yl)phenyl trifluoromethansulfonate 
     Into a 200-mL three-neck flask were put 2.2 g (4.0 mmol) of 5-(dibenzothiophen-4-yl)phenyl 1,3-bistrifluoromethansulfonate, 3.8 g (8.9 mmol) of 2-[3-(dibenzo[f,h]quinoxalin-2-yl)phenyl]-4,4,5,5-tetramethyl-1,3,2-dioxaborolane, 0.12 g (0.39 mmol) of P(o-tolyl) 3 , 20 mL of toluene, 2 mL of ethanol, and 13 mL of an aqueous potassium carbonate solution (2 mol/L). This mixture was degassed by being stirred while the pressure in the flask was reduced. After the degassing, the atmosphere in the flask was replaced with nitrogen, and this mixture was heated to 80° C. Then, 20 mg (0.09 mmol) of Pd(OAc) 2 ) was added to this mixture at the same temperature, and the mixture was stirred at the same temperature for 4.5 hours. A precipitate was collected by suction filtration to give a filtrate and a residue. The obtained filtrate was separated into an organic layer and an aqueous layer, and the aqueous layer was subjected to extraction with toluene. The organic layer obtained by suction filtration and the extracted solution were combined and the mixture was washed with saturated saline. To this solution was added anhydrous magnesium sulfate. This mixture was gravity-filtered, whereby a filtrate A was obtained. Furthermore, the residue obtained by the suction filtration performed after the stirring was washed with water and ethanol. After the washing, toluene was added to the obtained solid and the mixture was subjected to hot filtration, whereby a black powder was removed. The obtained filtrate by the hot filtration was concentrated, chloroform was added thereto, and this solution was filtered, whereby a filtrate B was obtained. The filtrate A and the filtrate B were combined and the mixture was concentrated to give a brown solid. The obtained solid was purified by high performance liquid chromatography (developing solvent: chloroform) to give 1.3 g of the target yellow oily substance in a yield of 44%. Synthesis Scheme (f-3) of Step 3 is shown below. 
     
       
         
         
             
             
         
       
     
     Step 4: Synthesis of DBt-mDBqP2P 
     Into a 200-mL three-neck flask were put 1.3 g (1.8 mmol) of 3-[3-(dibenzo[f,h]quinoxalin-2-yl)phenyl]-5-(dibenzothiophen-4-yl)phenyl trifluoromethansulfonate, 0.83 g (1.9 mmol) of 2-[3-(dibenzo[f,h]quinoxalin-2-yl)phenyl]-4,4,5,5-tetramethyl-1,3,2-dioxaborolane, 30 mg (0.07 mmol) of 2-dicyclohexylphosphino-2′,6′-dimethoxybiphenyl (abbreviation: SPhos), 9 mL of toluene, 1 mL of ethanol, and 3 mL of an aqueous potassium carbonate solution (2 mol/L). This mixture was degassed by being stirred while the pressure in the flask was reduced. After the degassing, the atmosphere in the flask was replaced with nitrogen, and this mixture was heated to 80° C. After that, 8.0 mg (36 μmol) of Pd(OAc) 2  was added to this mixture at the same temperature, and the resulting mixture was stirred at the same temperature for 8 hours. A precipitate was collected by suction filtration to give a residue. The obtained residue was washed with water, ethanol, and then mesitylene to give 1.1 g of a light black powder in a yield of 72%. Synthesis Scheme (f-4) of Step 4 is shown below. 
     
       
         
         
             
             
         
       
     
     Next, 0.87 g of the obtained solid was purified by train sublimation. In the purification by sublimation, the solid was heated at 420° C. under a pressure of 3.2 Pa with a flow rate of argon of 5 mL/min for 20 hours. After the purification by sublimation, 90 mg of powder was collected in 10.1%. 
     This compound was identified as DBt-mDBqP2P, which was the target substance, by  1 H NMR. 
       1 H NMR data of the obtained compound is shown below.  1 H NMR (tetrachloroethane-d 2 , 500 MHz): δ=7.50-7.58 (m, 2H), 7.70-7.90 (m, 13H), 8.05 (d, J=8.0 Hz, 2H), 8.27-8.30 (m, 5H), 8.45 (d, J=8.5 Hz, 2H), 8.68 (t, J=8.5 Hz, 4H), 8.86 (s, 2H), 9.32 (d, J=8.0 Hz, 2H), 9.49 (d, J=8.0 Hz, 2H), 9.57 (s, 2H). 
     The  1 H NMR charts are shown in  FIGS. 18A and 18B .  FIG. 18B  is an enlarged chart showing a range of from 7.5 ppm to 10.0 ppm in  FIG. 18A . 
     Example 7 
     In this example, light-emitting elements (Elements 1 to 4) of one embodiment of the present invention will be described with reference to  FIG. 19 . Chemical formulae of materials used in this example are shown below. 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     Methods of manufacturing Light-emitting elements 1 to 4 of this example will be described below. 
     (Light-Emitting Element 1) 
     First, over a substrate  1100 , an indium oxide-tin oxide containing silicon or silicon oxide (ITO-SiO 2 , hereinafter abbreviated to ITSO) was deposited by a sputtering method, whereby a first electrode  1101  was formed. Note that the composition ratio of In 2 O 3  to SnO 2  and SiO 2  in the target used was 85:10:5 [wt %]. The thickness of the first electrode  1101  was 110 nm and the electrode area was 2 mm×2 mm. Here, the first electrode  1101  is an electrode that functions as an anode of the light-emitting element. 
     Next, in pretreatment for forming the light-emitting element over the substrate  1100 , UV ozone treatment was performed for 370 seconds after washing of a surface of the substrate with water and baking at 200° C. for 1 hour. 
     After that, the substrate was transferred into a vacuum evaporation apparatus where the pressure had been reduced to approximately 10 −4  Pa, and subjected to vacuum baking at 170° C. for 30 minutes in a heating chamber of the vacuum evaporation apparatus, and then the substrate  1100  was cooled down for about 30 minutes. 
     Then, the substrate  1100  over which the first electrode  1101  was formed was fixed to a substrate holder provided in a vacuum evaporation apparatus so that the surface on which the first electrode  1101  was formed faced downward. The pressure in the vacuum evaporation apparatus was reduced to about 10 −4  Pa. After that, over the first electrode  1101 , 4,4′,4″-(benzene-1,3,5-triyl)tri(dibenzothiophene) (abbreviation: DBT3P-II) and molybdenum oxide were co-deposited by a co-evaporation method, whereby a hole-injection layer  1111  was formed. The thickness of the hole-injection layer  1111  was set to 20 nm, and the weight ratio of DBT3P-II to molybdenum oxide was adjusted to 2:1 (=DBT3P-II: molybdenum oxide). Note that the co-evaporation method refers to an evaporation method in which evaporation is carried out from a plurality of evaporation sources at the same time in one treatment chamber. 
     Next, on the hole-injection layer  1111 , 4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine (abbreviation: BPAFLP) was deposited to a thickness of 20 nm, whereby a hole-transport layer  1112  was formed. 
     Next, mDBq2BP synthesized in Example 1, N-(1,1′-biphenyl-4-yl)-9,9-dimethyl-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]-9H-fluor en-2-amine (abbreviation: PCBBiF), and (acetylacetonato)bis(6-tert-butyl-4-phenylpyrimidinato)iridium(III) (abbreviation: Ir(tBuppm) 2 (acac)) were co-deposited by evaporation, whereby a first light-emitting layer  1113   a  was foamed over the hole-transport layer  1112 . Here, the weight ratio of mDBq2BP to PCBBiF and Ir(tBuppm) 2 (acac) was adjusted to 0.7:0.3:0.05 (=mDBq2BP: PCBBiF: Ir(tBuppm) 2 (acac)). The thickness of the first light-emitting layer  1113   a  was set to 20 nm. 
     Next, mDBq2BP, PCBBiF, and Ir(tBuppm) 2 (acac) were co-deposited by evaporation over the first light-emitting layer  1113   a , whereby the second light-emitting layer  1113   b  was formed. Here, the weight ratio of mDBq2BP to PCBBiF and Ir(tBuppm) 2 (acac) was adjusted to 0.8:0.2:0.05 (=mDBq2BP: PCBBiF: Ir(tBuppm) 2 (acac)). The thickness of the second light-emitting layer  1113   b  was set to 20 nm. 
     Note that in the first light-emitting layer  1113   a  and the second light-emitting layer  1113   b , mDBq2BP has an electron-transport property and serves as a host material; PCBBiF has a hole-transport property and serves as an assist material; and Ir(tBuppm) 2 (acac) converts triplet excitation energy into light emission and serves as a guest material. 
     Next, mDBq2BP was deposited by evaporation to a thickness of 15 nm on the second light-emitting layer  1113   b , whereby a first electron-transport layer  1114   a  was formed. 
     Then, bathophenanthroline (abbreviation: BPhen) was deposited by evaporation to a thickness of 10 nm on the first electron-transport layer  1114   a , whereby a second electron-transport layer  1114   b  was formed. 
     Next, lithium fluoride (LiF) was deposited by evaporation to a thickness of 1 nm on the second electron-transport layer  1114   b , whereby an electron-injection layer  1115  was formed. 
     Then, aluminum was deposited by evaporation to a thickness of 200 nm over the electron-injection layer  1115 , whereby a second electrode  1103  serving as a cathode was formed. Thus, Light-emitting element 1 of this example was fabricated. 
     Note that in all the above evaporation steps, evaporation was performed by a resistance heating method. 
     (Light-Emitting Element 2) 
     In Light-emitting element 2, materials of the first light-emitting layer  1113   a , the second light-emitting layer  1113   b , and the first electron-transport layer  1114   a  are different from those in Light-emitting element 1. Components of Light-emitting element 2 which are different from those of Light-emitting element 1 are described below. 
     Over the hole-transport layer  1112 , mDBqP2F synthesized in Example 4, PCBBiF, and (acetylacetonato)bis(4,6-diphenylpyrimidinato)iridium(III) (abbreviation: Ir(dppm) 2 (acac)) were co-deposited by evaporation, whereby the first light-emitting layer  1113   a  was formed. Here, the weight ratio of mDBqP2F to PCBBiF and Ir(dppm) 2 (acac) was adjusted to 0.7:0.3:0.05 (=mDBqP2F:PCBBiF:Ir(dppm) 2 (acac)). The thickness of the first light-emitting layer  1113   a  was set to 20 nm. 
     Next, mDBqP2F, PCBBiF, and Ir(dppm) 2 (acac) were co-deposited by evaporation over the first light-emitting layer  1113   a , whereby the second light-emitting layer  1113   b  was formed. Here, the weight ratio of mDBqP2F to PCBBiF and Ir(dppm) 2 (acac) was adjusted to 0.8:0.2:0.05 (=mDBqP2F: PCBBiF: Ir(dppm) 2 (acac)). The thickness of the second light-emitting layer  1113   b  was set to 20 nm. 
     Note that in the first light-emitting layer  1113   a  and the second light-emitting layer  1113   b , mDBqP2F has an electron-transport property and serves as a host material; PCBBiF has a hole-transport property and serves as an assist material; and Ir(dppm) 2 (acac) converts triplet excitation energy into light emission and serves as a guest material. 
     Next, mDBqP2F was deposited by evaporation to a thickness of 20 nm on the second light-emitting layer  1113   b , whereby the first electron-transport layer  1114   a  was formed. 
     (Light-Emitting Element 3) 
     In Light-emitting element 3, materials of the first light-emitting layer  1113   a , the second light-emitting layer  1113   b , and the first electron-transport layer  1114   a  are different from those in Light-emitting element 1. Components of Light-emitting element 3 which are different from those of Light-emitting element 1 are described below. 
     Over the hole-transport layer  1112 , mDBqP2P synthesized in Example 5, PCBBiF, and Ir(tBuppm) 2 (acac) were co-deposited by evaporation, whereby the first light-emitting layer  1113   a  was formed. Here, the weight ratio of mDBqP2P to PCBBiF and Ir(tBuppm) 2 (acac) was adjusted to 0.7:0.3:0.05 (=mDBqP2P: PCBBiF: Ir(tBuppm) 2 (acac)). The thickness of the first light-emitting layer  1113   a  was set to 20 nm. 
     Next, mDBqP2P, PCBBiF, and Ir(tBuppm) 2 (acac) were co-deposited by evaporation over the first light-emitting layer  1113   a , whereby the second light-emitting layer  1113   b  was formed. Here, the weight ratio of mDBqP2P to PCBBiF and Ir(tBuppm) 2 (acac) was adjusted to 0.8:0.2:0.05 (=mDBqP2P: PCBBiF: Ir(tBuppm) 2 (acac)). The thickness of the second light-emitting layer  1113   b  was set to 20 nm. 
     Note that in the first light-emitting layer  1113   a  and the second light-emitting layer  1113   b , mDBqP2P has an electron-transport property and serves as a host material; PCBBiF has a hole-transport property and serves as an assist material; and Ir(tBuppm) 2 (acac) converts triplet excitation energy into light emission and serves as a guest material. 
     Next, mDBqP2P was deposited by evaporation to a thickness of 20 nm on the second light-emitting layer  1113   b , whereby the first electron-transport layer  1114   a  was formed. 
     (Light-Emitting Element 4) 
     In Light-emitting element 4, materials of the first light-emitting layer  1113   a , the second light-emitting layer  1113   b , and the first electron-transport layer  1114   a  are different from those in Light-emitting element 1. Components of Light-emitting element 4 which are different from those of Light-emitting element 1 are described below. 
     Over the hole-transport layer  1112 , 2-[3′-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline (abbreviation: 2mDBTBPDBq-II), PCBBiF, and Ir(tBuppm) 2 (acac)) were co-deposited by evaporation, whereby the first light-emitting layer  1113   a  was formed. Here, the weight ratio of 2mDBTBPDBq-II to PCBBiF and Ir(tBuppm) 2 (acac) was adjusted to 0.7:0.3:0.05 (=2mDBTBPDBq-II: PCBBiF: Ir(tBuppm) 2 (acac)). The thickness of the first light-emitting layer  1113   a  was set to 20 nm. 
     Next, 2mDBTBPDBq-II, PCBBiF, and Ir(tBuppm) 2 (acac) were co-deposited by evaporation over the first light-emitting layer  1113   a , whereby the second light-emitting layer  1113   b  was formed. Here, the weight ratio of 2mDBTBPDBq-II to PCBBiF and Ir(tBuppm) 2 (acac) was adjusted to 0.8:0.2:0.05 (=2mDBTBPDBq-II: PCBBiF: Ir(tBuppm) 2 (acac)). The thickness of the second light-emitting layer  1113   b  was set to 20 nm. 
     Note that in the first light-emitting layer  1113   a  and the second light-emitting layer  1113   b , 2mDBTBPDBq-II has an electron-transport property and serves as a host material; PCBBiF has a hole-transport property and serves as an assist material; and Ir(tBuppm) 2 (acac) converts triplet excitation energy into light emission and serves as a guest material. 
     Next, mDBqP2P synthesized in Example 5 was deposited by evaporation to a thickness of 20 nm on the second light-emitting layer  1113   b , whereby the first electron-transport layer  1114   a  was formed. 
     As described above, the organic compound of one embodiment of the present invention can be used in only an electron-transport layer in a light-emitting element. 
     Table 1 shows element structures of Light-emitting elements 1 to 4 obtained as described above. 
     
       
         
           
               
             
               
                 TABLE 1 
               
               
                   
               
             
            
               
                 Structure of Light-emitting elements (Elements) 1 to 4. 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 First 
                   
                   
                 First 
                 Second 
                 First 
                 Second 
                   
                 Second 
               
               
                 Element 
                 electrode 
                 HIL a   
                 HTL b   
                 LEL c   
                 LEL c   
                 ETL d   
                 ETL e   
                 EIL f   
                 electrode 
               
               
                   
               
               
                 1 
                 ITSO 
                 DBT3P-II:MoO x   
                 BPAFLP 
                 v.i. 
                 v.i. 
                 mDBq2BP 
                 Bphen 
                 LiF 
                 Al 
               
               
                   
                 110 nm 
                 (=2:1) 
                 20 nm 
                   
                   
                 15 nm 
                 10 nm 
                 1 nm 
                 200 nm 
               
               
                   
                   
                 20 nm 
               
               
                 2 
                 ITSO 
                 DBT3P-II:MoO x   
                 BPAFLP 
                 v.i. 
                 v.i. 
                 mDBqP2F 
                 Bphen 
                 LiF 
                 Al 
               
               
                   
                 110 nm 
                 (=2:1) 
                 20 nm 
                   
                   
                 20 nm 
                 10 nm 
                 1 nm 
                 200 nm 
               
               
                   
                   
                 20 nm 
               
               
                 3 
                 ITSO 
                 DBT3P-II:MoO x   
                 BPAFLP 
                 v.i. 
                 v.i. 
                 mDBqP2P 
                 Bphen 
                 LiF 
                 Al 
               
               
                   
                 110 nm 
                 (=2:1) 
                 20 nm 
                   
                   
                 20 nm 
                 10 nm 
                 1 nm 
                 200 nm 
               
               
                   
                   
                 20 nm 
               
               
                 4 
                 ITSO 
                 DBT3P-II:MoO x   
                 BPAFLP 
                 v.i. 
                 v.i. 
                 mDBqP2P 
                 Bphen 
                 LiF 
                 Al 
               
               
                   
                 110 nm 
                 (=2:1) 
                 20 nm 
                   
                   
                 20 nm 
                 10 nm 
                 1 nm 
                 200 nm 
               
               
                   
                   
                 20 nm 
               
               
                   
               
            
           
           
               
            
               
                 Structure of Light-emitting elements 1 to 4. 
               
            
           
           
               
               
               
            
               
                 Element 
                 First Light-emitting layer 
                 Second Light-emitting layer 
               
               
                   
               
               
                 1 
                 mDBq2BP:PCBBiF:Ir(tBuppm) 2 (acac) 
                 mDBq2BP:PCBBiF:Ir(tBuppm) 2 (acac) 
               
               
                   
                 (=0.7:0.3:0.05) 
                 (=0.8:0.2:0.05) 
               
               
                   
                 20 nm 
                 20 nm 
               
               
                 2 
                 mDBqP2F:PCBBiF:Ir(dppm) 2 (acac) 
                 mDBqP2F:PCBBiF:Ir(dppm) 2 (acac) 
               
               
                   
                 (=0.7:0.3:0.05) 
                 (=0.8:0.2:0.05) 
               
               
                   
                 20 nm 
                 20 nm 
               
               
                 3 
                 mDBqP2P:PCBBiF:Ir(dppm) 2 (acac) 
                 mDBqP2P:PCBBiF:Ir(tBuppm) 2 (acac) 
               
               
                   
                 (=0.7:0.3:0.05) 
                 (=0.8:0.2:0.05) 
               
               
                   
                 20 nm 
                 20 nm 
               
               
                 4 
                 2mDBTBPDBq-II:PCBBiF:Ir(dppm) 2 (acac) 
                 2mDBTBPDBq-II:PCBBiF:Ir(tBuppm) 2 (acac) 
               
               
                   
                 (=0.7:0.3:0.05) 
                 (=0.8:0.2:0.05) 
               
               
                   
                 20 nm 
                 20 nm 
               
               
                   
               
               
                   a Hole-injection layer. 
               
               
                   b Hole-transport layer. 
               
               
                   c Light-emitting layer. 
               
               
                   d Electron-transport layer. 
               
               
                   f Electron-injection layer. 
               
            
           
         
       
     
     In a glove box containing a nitrogen atmosphere, Light-emitting elements 1 to 4 were each sealed with a glass substrate so as not to be exposed to the air (specifically, a sealant was applied onto an outer edge of the element and heat treatment was performed at 80° C. for 1 hour at the time of sealing). Then, operation characteristics of the light-emitting elements were measured. Note that the measurements were carried out at room temperature (in the atmosphere kept at 25° C.). 
       FIG. 20 ,  FIG. 25 ,  FIG. 30 , and  FIG. 35  show current density-luminance characteristics of Light-emitting elements 1 to 4, respectively. In each of  FIG. 20 ,  FIG. 25 ,  FIG. 30 , and  FIG. 35 , the horizontal axis represents current density (mA/cm 2 ) and the vertical axis represents luminance (cd/m 2 ).  FIG. 21 ,  FIG. 26 ,  FIG. 31 , and  FIG. 36  show voltage-luminance characteristics of Light-emitting elements 1 to 4, respectively. In each of  FIG. 21 ,  FIG. 26 ,  FIG. 31 , and  FIG. 36 , the horizontal axis represents voltage (V) and the vertical axis represents luminance (cd/m 2 ).  FIG. 22 ,  FIG. 27 ,  FIG. 32 , and  FIG. 37  show current luminance-efficiency characteristics of Light-emitting elements 1 to 4, respectively. In each of  FIG. 22 ,  FIG. 27 ,  FIG. 32 , and  FIG. 37 , the horizontal axis represents luminance (cd/m 2 ) and the vertical axis represents current efficiency (cd/A).  FIG. 23 ,  FIG. 28 ,  FIG. 33 , and  FIG. 38  show voltage-current characteristics of Light-emitting elements 1 to 4, respectively. In each of  FIG. 23 ,  FIG. 28 ,  FIG. 33 , and  FIG. 38 , the horizontal axis represents voltage (V) and the vertical axis represents current (mA). 
     Table 2 shows voltage (V), current density (mA/cm 2 ), CIE chromaticity coordinates (x, y), current efficiency (cd/A), and external quantum efficiency (%) of each light-emitting element at a luminance of around 1000 cd/m 2 . 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Characteristics of Light-emitting elements 1 to 4. 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                   
                 External 
               
               
                   
                   
                 Current 
                 chromaticity 
                   
                 current 
                 quantum 
               
               
                   
                 Voltage 
                 density 
                 coordinates 
                 Luminance 
                 efficiency 
                 efficiency 
               
               
                 Element 
                 (V) 
                 (mA/cm 2 ) 
                 (x, y) 
                 (cd/m 2 ) 
                 (cd/A) 
                 (%) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 1 
                 2.6 
                 0.9 
                 (0.41, 0.58) 
                 830 
                 95 
                 25 
               
               
                 2 
                 2.8 
                 1.1 
                 (0.54, 0.45) 
                 760 
                 69 
                 18 
               
               
                 3 
                 2.7 
                 1.2 
                 (0.42, 0.58) 
                 1200 
                 100 
                 27 
               
               
                 4 
                 3.0 
                 1.0 
                 (0.42, 0.57) 
                 990 
                 96 
                 26 
               
               
                   
               
            
           
         
       
     
       FIG. 24 ,  FIG. 29 ,  FIG. 34 , and  FIG. 39  show emission spectra of Light-emitting elements 1, 2, 3, and 4, respectively, at a current density of 2.5 mA/cm 2 . As shown in  FIG. 24 , the emission spectrum of Light-emitting element 1 has a peak at 547 nm. As shown in  FIG. 29 , the emission spectrum of Light-emitting element 2 has a peak at 579 nm. As shown in  FIG. 34 , the emission spectrum of Light-emitting element 3 has a peak at 546 nm. As shown in  FIG. 39 , the emission spectrum of Light-emitting element 4 has a peak at 546 nm. 
     Next, a reliability test was performed on each of Light-emitting elements 1 and 2.  FIG. 40  and  FIG. 41  show results of the reliability tests. 
     In the reliability test, each of Light-emitting elements 1 and 2 was driven under the conditions where the initial luminance was 5000 cd/m 2  and the current density was constant. In  FIG. 40  and  FIG. 41 , the horizontal axis represents driving time (h) of the element, and the vertical axis represents normalized luminance (%) with the initial luminance of 100%.  FIG. 40  shows that the normalized luminance of Light-emitting element 1 after 335 hours is 82%.  FIG. 41  shows that the normalized luminance of Light-emitting element 2 after 390 hours is 65%. 
     The results of  FIG. 20  to  FIG. 39  indicate that Light-emitting elements 1 to 4, each of which is one embodiment of the present invention, have excellent element characteristics (voltage-luminance characteristics, luminance-current efficiency characteristics, and voltage-current characteristics). In each of Light-emitting elements 1 to 3, the organic compound of one embodiment of the present invention is used as host materials in the first light-emitting layer  1113   a  and the second light-emitting layer  1113   b , and also used in the first electron-transport layer  1114   a . As compared with Light-emitting element 4 in which the organic compound of one embodiment of the present invention is used only in the first electron-transport layer  1114   a , Light-emitting elements 1 to 3 are driven at an extremely low voltage. This low voltage driving is achieved probably because the host material in the light-emitting layer (the first light-emitting layer  1113   a  and the second light-emitting layer  1113   b ) has two or more dibenzo[f,h]quinoxaline skeletons intramolecularly. Furthermore, in each of Light-emitting elements 1 to 3, an electron-transport material (mDBq2BP, mDBqp2F, or mDBqP2P), which serves as a host material and is one embodiment of the present invention, and a hole-transport material (PCBBiF), which serves as an assist material, form an exciplex. Energy is efficiently transferred from the exciplex to a light-emitting substance (Ir(tBuppm) 2 (acac) or Ir(dppm) 2 (acac)); thus, Light-emitting elements 1 to 3 can have high efficiency. Therefore, a more significant effect can be obtained in the case where the organic compound of one embodiment of the present invention is used in both a light-emitting layer and an electron-transport layer. Note that, like Light-emitting element 4, the organic compound of one embodiment of the present invention can be also applied to a light-emitting element in which the organic compound is used only in an electron-transport layer. 
     The results of  FIG. 40  and  FIG. 41  indicate that Light-emitting element 1 and Light-emitting element 2, each of which is one embodiment of the present invention, have a long lifetime. In each of the organic compounds used in Light-emitting elements 1 and 2, the linker of two dibenzo[f,h]quinoxaline skeletons includes at least two metaphenylene groups. Such a molecular structure leads to a stable film quality and a long lifetime of the light-emitting element. In particular, in each of the organic compounds included in Light-emitting elements 1 and 2, a phenyl group (an aryl group) is not bonded to any of the 3-positions of the dibenzo[f,h]quinoxaline units; thus, Light-emitting elements 1 and 2 can have an extremely long lifetime. 
     Note that the structure described in this example can be combined as appropriate with any of the structures described in the embodiments or the other examples. 
     Example 8 
     In this example, T 1  levels of the organic compound represented by Structural Formula (101), which is one embodiment of the present invention, and the comparative organic compounds represented by Structural Formulae (200) and (201) were calculated. Chemical formulae of the materials used in this example are shown below. 
     
       
         
         
             
             
         
       
     
     The calculating method is as follows. Gaussian 09 was used as the quantum chemistry computational program. A high performance computer (Altix 4700, produced by SGI Japan, Ltd.) was used for the calculations. 
     First, the most stable structure in the singlet ground state was calculated using the density functional theory. As a basis function, 6-311G (a basis function of a triple-split valence basis set using three contraction functions for each valence orbital) was applied to all the atoms. By the above basis function, for example, is to 3s orbitals are considered in the case of hydrogen atoms, while is to 4s and 2p to 4p orbitals are considered in the case of carbon atoms. To improve calculation accuracy, the p function and the d function as polarization basis sets were added respectively to hydrogen atoms and atoms other than hydrogen atoms. As a functional, B3LYP was used. 
     Next, the most stable structure in the lowest excited triplet state was calculated. Then, vibration analysis was conducted on the most stable structures in the singlet ground state and in the lowest excited triplet state, and a zero-point corrected energy difference was obtained. The T 1  level was calculated from the zero-point corrected energy difference. As a basis function, 6-311G (d, p) was used. As a functional, B3LYP was used. 
     Table 3 shows the results of the calculations performed in the above manner. 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 T 1  levels of compounds represented by 
               
               
                 formulae (101), (200), and (201). 
               
            
           
           
               
               
               
            
               
                   
                 Formula 
                 T 1 level (eV) 
               
               
                   
                   
               
               
                   
                 (101) 
                 2.42 
               
               
                   
                 (200) 
                 2.20 
               
               
                   
                 (201) 
                 2.26 
               
               
                   
                   
               
            
           
         
       
     
     As shown in Table 3, the T 1  level of the organic compound represented by Structural Formula (101), which is one embodiment of the present invention, is higher than the T 1  level of each of the comparative organic compounds represented by Structural Formulae (200) and (201). This difference in T 1  level results from the structure of the linking group which links the dibenzo[f,h]quinoxaline skeletons. Specifically, in the organic compound represented by Structural Formula (101), the linking group which links the dibenzo[f,h]quinoxaline skeletons to each other includes meta-substituted phenylene groups. On the other hand, in each of the organic compounds represented by Structural Formulae (200) and (201), the linking group which links the dibenzo[f,h]quinoxaline skeletons to each other includes para-substituted phenylene groups. Thus, it is preferable that linking groups bonded to the dibenzo[f,h]quinoxaline skeletons contain meta-substituted arylene groups. 
     Note that the structure described in this example can be combined as appropriate with any of the structures described in the embodiments or the other examples. 
     EXPLANATION OF REFERENCE 
       100 : substrate,  101 : electrode,  102 : EL layer,  103 : electrode,  111 : hole-injection layer,  112 : hole-transport layer,  113 : light-emitting layer,  114 : electron-transport layer,  115 : electron-injection layer,  201 : electrode,  202 : electrode,  203 : EL layer,  204 : light-emitting layer,  205 : phosphorescent compound,  206 : organic compound,  207 : organic compound,  301 : electrode,  303 : electrode,  311 : light-emitting layer,  312 : light-emitting layer,  313 : charge-generation layer,  401 : source side driver circuit,  402 : pixel portion,  403 : gate side driver circuit,  404 : sealing substrate,  405 : sealing material,  407 : space,  408 : wiring,  409 : FPC,  410 : element substrate,  411 : FET,  412 : FET,  413 : electrode,  414 : insulator,  416 : EL layer,  417 : electrode,  418 : light-emitting element,  423 : FET,  424 : FET,  501 : substrate,  502 : electrode,  503 : electrode,  504 : EL layer,  505 : insulating layer,  506 : partition wall layer,  601 : lighting device,  602 : lighting device,  603 : desk lamp,  1100 : substrate,  1101 : electrode,  1103 : electrode,  1111 : hole-injection layer,  1112 : hole-transport layer,  1113   a : light-emitting layer,  1113   b : light-emitting layer,  1114   a : electron-transport layer,  1114   b : electron-transport layer,  1115 : electron-injection layer,  7100 : television device,  7101 : housing,  7103 : display portion,  7105 : stand,  7107 : display portion,  7109 : operation key,  7110 : remote controller,  7201 : main body,  7202 : housing,  7203 : display portion,  7204 : keyboard,  7205 : external connection port,  7206 : pointing device,  7301 : housing,  7302 : housing,  7303 : joint portion,  7304 : display portion,  7305 : display portion,  7306 : speaker portion,  7307 : recording medium insertion portion,  7308 : LED lamp,  7309 : operation key,  7310 : connection terminal,  7311 : sensor,  7312 : microphone,  7400 : mobile phone device,  7401 : housing,  7402 : display portion,  7403 : operation button,  7404 : external connection port,  7405 : speaker,  7406 : microphone,  7501 : lighting portion,  7502 : shade,  7503 : adjustable arm,  7504 : support,  7505 : base,  7506 : power supply,  9501 : lighting portion,  9503 : support,  9505 : support base 
     This application is based on Japanese Patent Application serial no. 2013-179345 filed with Japan Patent Office on Aug. 30, 2013 the entire contents of which are hereby incorporated by reference.