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Lanthanides in Luminescent Thermometry Chapter 281 385 FIG. 20 Mechanisms of light harvesting in Ln3+-based luminescent thermometers: (A) complexing with organic ligands and (B) doping in host crystals. The solid arrows represent light absorption or emission whereas the interrupted arrows sign the energy transfer within the materials. Adapted with permission from Ding, M.Y., Chen, D.Q., Yin, S.L., Ji, Z.G., Zhong, J.S., Ni, Y.R., Lu, C.H., Xu, Z.Z. 2015. Simultaneous morphology manipulation and upconversion luminescence enhancement of b-NaYF4:Yb3+/Er3+ microcrystals by simply tuning the KF dosage. Sci. Rep.-UK 5, Article number: 12745. © 2015 Nature Publishing Group. Aromatic organic ligands serve as antennas for light harvesting and can multiply the photoluminescence intensity of Ln3+ ions by two orders of mag- nitude, at least (Shahi et al., 2015). Ligands and Ln3+ ions can be arranged in discrete molecules or in crystal lattices, such as MOFs. The ligands in discrete molecular compounds have b-diketonates (Binnemans, 2005), or polydentate azo compounds as binding groups (Fig. 21A), and residues with heterocyclic polycyclic aromatic rings (Fig. 21B), or N-base heterocycles (Fig. 21C) for light harvesting, and perfluoroalkyl residues to increase Lewis acidity (Binnemans, 2005). The residues also determine the energy of the singlet and triplet states of the ligand and, consequently, the luminescence properties of the Ln3+-based materials. On the other hand, ligands in MOFs have dicar- boxylic acids, as linking groups (Fig. 21A), and similar aromatic compounds for light harvesting (Fig. 21D). A scheme of the different types of thermometric materials that are based on coordination compounds with organic ligands is shown in Fig. 22. It includes (i) micro and macrocrystals of lanthanide salts; (ii) discrete molecu- lar complexes (molecular thermometers) embedded in amorphous matrices, such as polymers and organic–inorganic hybrids; and (iii) MOF micro and nanocrystals. In addition, we have to mention two other categories of lumines- cent thermometers: UCNPs and NIR-emitting NPs, which have also a crystal- line nature.PDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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