It is known that there are some rare earth ions which emit light of a broad wavelength area, in a range from ultraviolet to infrared rays. This emission of light is based on electron transition derived from an f orbit which cannot be easily affected by the external field, such as a ligand field. Therefore, a wavelength interval of an emission band is extremely narrow as compared to that of an organic fluorescent substance and like substance, and color purity is theoretically high. The rare earth ion compares favorably with an organic fluorescent substance, in terms of heat, light, and stability over excitation. Furthermore, the rare earth ion is nonpoisonous, and therefore can be used easily industrially.
Since the rare earth ion has excellent characteristics as such, a rare-earth complex in which various ligands are coordinated to a rare earth ion are used for various purposes. More specifically, the rare-earth complex is used for various uses such as luminous ink and organic electroluminescence elements.
In order to enable application of the rare earth ion having such an excellent characteristic in further various fields, various researches have been performed regarding the rare earth ion and the rare-earth complex.
For example, the inventors of the present invention have developed several rare-earth complexes until now (for example, see Patent Literatures 1 to 4, and Non Patent Literature 1).