Sheet integrated rare earth complex and use thereof

A rare earth complex having a crosslinked structure, comprising (a) phosphine oxide compounds each denoted by formula (1):

• where X denotes an atom having three-fold symmetry in a plan view of the chemical structural formula containing the connectors of the atom, or a group of atoms having three-fold symmetry in a plan view of the chemical structural formula, wherein the group of atoms is an aryl group which may comprise one or more substituents, or a heteroaryl group which may comprise one or more substituents;
• where each of Ar1 to Ar9 independently denotes an aryl group which may comprise one or more substituents;
• (b) rare earth ions selected from the group consisting of Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu;
• and
• (c) diketo compounds each denoted by formula (2);
• where A independently denotes a hydrogen atom, an alkyl group with 1 to 6 carbon atoms, or a halogen atom, and Z denotes a hydrogen atom or a deuterium atom;
• wherein a coordination number of the diketo compound and the phosphine oxide compound relative to the rare earth ions is 8 to 10;
• wherein each of three phosphine oxide groups of the phosphine oxide compound bonds to a different rare earth ion whereby the phosphine oxide compounds and the rare earth ions form a crosslinked structure.