An organic electroluminescence (hereinafter, “electroluminescence” will be referred to as “EL”, occasionally) device is a spontaneous light emitting device which utilizes the principle that a fluorescent substance emits light by energy of recombination of holes injected from an anode and electrons injected from a cathode when an electric field is applied. Since an organic EL device of the laminate type driven under a low electric voltage was reported by C. W. Tang et al. of Eastman Kodak Company (C. W. Tang and S. A. Vanslyke, Applied Physics Letters, Volume 51, Pages 913, 1987), many studies have been conducted on organic EL devices using organic materials as the constituting materials. Tang et al. used tris(8-hydroxyquinolinolato)aluminum for the light emitting layer and a triphenyldiamine derivative for the hole transporting layer. Advantages of the laminate structure are that the efficiency of hole injection into the light emitting layer can be increased, that the efficiency of forming excited particles which are formed by blocking and recombining electrons injected from the cathode can be increased, and that excited particles formed within the light emitting layer can be enclosed. As the structure of the organic EL device, a two-layered structure having a hole transporting (injecting) layer and an electron transporting and light emitting layer and a three-layered structure having a hole transporting (injecting) layer, a light emitting layer and an electron transporting (injecting) layer are well known. To increase the efficiency of recombination of injected holes and electrons in the devices of the laminate type, the structure of the device and the process for forming the device have been studied.
As the light emitting material, chelate complexes such as tris(8-quinolinolato)aluminum complex, coumarin derivatives, tetraphenylbutadiene derivatives, distyrylarylene derivatives and oxadiazole derivatives are known. It is reported that light in the visible region ranging from blue light to red light can be obtained by using these light emitting materials, and development of a device exhibiting color images is expected (for example, refer to Patent References 1 to 3). It is recently proposed that an organic phosphorescent material other than the fluorescent material is used in the light emitting layer of an organic EL device (for example, refer to Non-Patent Reference 1 and Non-Patent Reference 2). As described above, a great efficiency of light emission is achieved by utilizing an organic phosphorescent material excited to the singlet state and the triplet state in the light emitting layer of an organic EL device. It is considered that singlet excimers and triplet excimers are formed in relative amounts of 1:3 due to the difference in the multiplicity of spin when electrons and holes are recombined in an organic EL device. Therefore, it is expected that an efficiency of light emission 3 to 4 times as great as that of a device utilizing fluorescence alone can be achieved by utilizing a material emitting phosphorescent light.
In the organic EL devices described above, a construction formed by successively laminating layers such as an anode, a hole transporting layer, an organic light emitting layer, an electron transporting layer (a hole barrier layer), an electron transporting layer and a cathode is used so that the excited state of the triplet or excimers of the triplet do not fail to emit light, and a host compound and a phosphorescent compound are used for the organic light emitting layer (for example, Patent References 4 to 8). In these patent references, host materials having a skeleton structure of dibenzofuran or dibenzothiophene are described. However, it is not described whether a device using the above host material exhibits more excellent performance than devices using other materials having the carbazolyl skeleton structure. No descriptions can be found on combinations having silicon atom or germanium atom, either.
In Patent Reference 9 and 10, compounds having substituents such as arylsilyl groups are described. However, the compounds described in the present invention are not described. Moreover, no descriptions are found on the useful effect as the material for organic EL devices, in particular, as the phosphorescent material for organic EL devices emitting bluish light, such as the effect that the energy gap of the triplet state can be kept broad.
In Patent Reference 11 to 18, arylsilane-based compounds and arylgermane-based compounds are described, and the use of these compounds as the host material for a phosphorescent device emitting bluish light is described in the examples. However, the compounds described in the present invention are not described, and the effects of the compounds are not known.    [Patent Reference 1] Japanese Patent Application Laid-Open No. Heisei 8 (1996)-239655    [Patent Reference 2] Japanese Patent Application Laid-Open No. Heisei 7 (1995)-138561    [Patent Reference 3] Japanese Patent Application Laid-Open No. Heisei 3 (1991)-200889    [Patent Reference 4] International Patent Application Laid-Open No. WO05/101912    [Patent Reference 5] Japanese Patent Application Laid-Open No. Heisei 5 (1993)-109485    [Patent Reference 6] Japanese Patent Application Laid-Open No. 2004-002351    [Patent Reference 7] International Patent Application Laid-Open No. WO04/096945    [Patent Reference 8] Japanese Patent Application Laid-Open No. 2002-308837    [Patent Reference 9] Japanese Patent Application Laid-Open No. 2003-138251    [Patent Reference 10] Japanese Patent Application Laid-Open No. 2000-351966    [Patent Reference 11] International Patent Application Laid-Open No. WO04/095598    [Patent Reference 12] United States Patent No. 2004-209115    [Patent Reference 13] Japanese Patent Application Laid-Open No. 2004-103463    [Patent Reference 14] Japanese Patent Application Laid-Open No. 2005-183303    [Patent Reference 15] Japanese Patent Application Laid-Open No. 2005-317275    [Patent Reference 16] Japanese Patent Application Laid-Open No. 2004-200104    [Patent Reference 17] Japanese Patent Application Laid-Open No. 2005-310672    [Patent Reference 18] Japanese Patent Application Laid-Open No. 2005-306864    [Non-Patent Reference 1] D. F. O'Brien and M. A. Baldo et al. “Improved energy transferring electrophosphorescent devices” Applied Physics letters Vol. 74 No. 3, pp 442-444, Jan. 18, 1999    [Non-Patent Reference 2] M. A. Baldo et al. “Very high-efficiency green organic light-emitting devices based on electrophosphorescence” Applied Physics letters Vol. 75 No. 1, pp 4-6, Jul. 5, 1999