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Luminescent Ln-Ionic Liquids beyond Europium
What is called an ionic liquid (IL) has a very broad definition, comprising multiple substances possessing a wide diversity of structures and properties. An IL consists of both organic and inorganic ions, and may contain more than one cation or anion. Normally, a substance is considered to be an IL if completely composed of ions, with a melting point below 100 °C. Ionic liquids containing lanthanides or lanthanide compounds in ionic liquids are very important in the field of soft luminescent materials.
Within ILs, there are electrostatic and dispersive interactions at different length scales, leading to a highly anisotropic character. The ions have a large structural diversity which varies from inorganic to organic, simple to complex, including fully or partially ionized acid or base, organic polymeric metal ions, or metalated coordination polymers , giving a boundless variety of cation/anion combinations, estimated around the order of 1019 .
2. Luminescent Ln-Ionic Liquids beyond Europium
To obtain highly efficient Ln molecular light-conversion devices, it is necessary to optimize several parameters: avoid self-quenching channels, use chromophores with high molar absorbance and ideal energy positions of singlet and triplet states (for an efficient energy transfer to Ln3+ ions), while avoiding competitive non-radiative pathways such as multiphonon relaxation to high-energy vibrations (e.g., O–H, C–H, and N–H stretching modes). With this context in mind, the combination of lanthanides and ionic liquids began by using ILs as matrices to protect Ln3+ ions from vibration-induced deactivation processes—mainly from the ever-present water adsorbed in organic solvents. Although ILs proved to provide good protection against the presence of water within the first and/second coordination spheres of the Ln centers, due to the low solubility of the Ln salts, this method usually enabled low concentrations of Ln ions, although higher concentrations could be achieved by the use of the same anionic moieties as both the ligand (of the Ln complex) and the anion (of the ILs). This shortcoming was circumvented by preparing Ln-based ionic liquids, either via direct preparation by metathesis, or by dissolving Ln salts in ILs with anions with coordinating capabilities. Ln-containing ionic liquids proved to be promising materials because, although liquids, they provide a low-phonon environment for the Ln3+ center, leading to appreciable excitation state lifetimes. Typically, liquid-state lanthanide compounds present lower emission quantum yields (Φ) than those in the solid state, due to a less rigid environment and energy loss from collisions. As such, it is not surprising that emission quantum yields for Ln@ILs are very low. The same reasoning is applicable to Ln-ILs although, surprisingly, out of the 58 Ln-ILs presented in our recent review paper (see reference at the end of this entry), only one RTIL—[C6mim]3[Sm(NO3)6]—had its emission quantum yield determined, with a value of 2.73% . Another important aspect to stress is that since Ln@ILs are liquids, no structural characterization was available for the majority of these compounds.
It is worth mentioning that many of the Ln-ILs were studied not only as phosphors, but also as paramagnetic liquids, opening avenues for multifunctional applications. Additionally, the combination of Ln and ILs has aroused so much interest that it led to the emergence of a new field of research, focused specifically on soft materials. In this area, new ionogels have been developed through covalently grafting—or simply dispersing—Ln complexes into silica-based materials, polymer matrices, liquid crystals, etc.
It was not intended to include ionogels in this entry but, since they are becaming more and more importent and just as an example, a simple and environmentally friendly (solvent-free) preparation of ionogels via the incorporation of Ln-ILs within poly(methyl methacrylate) (PMMA) was reported by Wang et al. as early as 2013 . In that work, the ILs Tb(sal)@[Carb-mim][Tf2N] and Eu(tta)@[Carb-mim] [Tf2N] were directly dissolved into MMA monomers with azodiisobutyronitrile (polymerization initiator), with stirring at 80 °C, yielding a yellowish liquid that was then cast into glass slides or glass bottles. After drying, ionogels in the form of monoliths, films, and flexible self-standing films could be obtained ( Figure 4 ).
Figure 4. Digital photos of the Ln-ILs-PMMA: (a–c) Eu(tta)–[Carb-mim][Tf2N]@PMMA under daylight (right) and UV light (left); (d–e) Tb(sal)–[Carb-mim][Tf2N]@PMMA under daylight (right) and UV light (left). The scale bar is 1.0 cm. Reproduced with permission from .
The accomplishments described in this entry prove Ln-ILs to be outstanding and promising optical materials. However, this field of research is still underdeveloped when compared with other fields of ionic liquid chemistry. Therefore, new studies focusing on different combinations of Ln ions and new ligands will certainly lead to more efficient luminescent molecular devices, paving the way for practical applications as varied as catalysis, biochemical analysis, energy production, and non-invasive diagnostics, such as biolabels.
The entry is from 10.3390/molecules26164834
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