Luminescence of lanthanide complexes: From fundamental to prospective approaches related to water- and molecular-stimuli

Luminescent lanthanide (Ln) complexes are attracted much attention because of their stable emission colors induced by the photo-antenna effect through the photo-excited energy transfer from aromatic ligands to Ln ions. Here, we will introduce some systems of luminescent Ln complexes with metastable states with the phase transition induced by water and other small molecules, the relative arrangement of hydrogel formation and Ln luminescence enhancement, and the diversity of the thin air-water interface. The energy donor levels in each system should be designed to sensitize Ln-luminescence with the consideration of media, interaction and assembling. Luminescence quenching of Ln complexes in water is a point that should be considered for the development of materials and for the purpose of bio-related materials. Then, the principle of the change in luminescence intensity by the effect of water molecules is described, and the estimation of a hydrated structure of the complex is estimated using the luminescence lifetimes in H₂O and D₂O. The molecular arrangement of these crystals changes under the vapor-stimuli, and the coloration and luminescence may be enhanced. Some interesting cases of luminescent Ln complexes with the crystal-to-crystal phase transitions will be introduced with the vapor adsorption. Hydrogels are mostly water by volume; a system in which Ln luminescence is maintained implies that Ln ions are placed in hydrophobic positions in self-assemblies with strong luminescence. The formation of thin films at the molecular level and their Ln luminescence properties are introduced. For example, when a monolayer of a surface-active Ln complex is formed at the air-water interface, the repeated accumulation of the flexible film forms a metastable structure with a regular structure different from that of a crystal, and no water is incorporated into the film. These can not only derive circularly or linearly polarized light, but also take in other molecules and change the emission. Finally, we will suggest the prospects for the development of Ln complexes.