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388 Handbook on the Physics and Chemistry of Rare Earths the triplet energy states T1 and T01 and the complexes were processed as dou- ble salts. The preparation method is very simple, it just requires the dissolu- tion of ligands and Ln3+ salts in a solvent, usually an alcohol, and the evaporation of this solvent on a substrate to form a film. However, the pro- cessability and the practical application of salt powder films are quite reduced, limited to thermometry of some surfaces with salt adherence capac- ity, low mechanical requirements poor homogeneity, and low spatial resolu- tion. The working range for a given composition is not wide but it can be tuned with the Tb/Eu ratio (from 4:1 to 49:1) and the type of ligand (btfa and hfa) in the region from 73 K to near 373 K. The sensitivity is not provided by the authors. Another interesting example is the cryogenic thermometer recently reported by Ren et al., the first example of a ratiometric Eu3+/Tb3+ thermometer based on a lanthanide phosphonate (Ren et al., 2015). The com- pound presents a temperature-dependent emission under 393 nm excitation that enabling its use as a molecular thermometer with Sm1⁄43.90%K1 and dT 1⁄4 0.15 K, both at 38 K. There is another type of thermometer also based on the use of organic complexes, but operating with a very different mecha- nism of emission temperature dependence (Yuasa et al., 2014). In aqueous solution, the [(Eu)2(oda)3(bp)] dinuclear complex displays two different struc- tures in equilibrium. Consequently, the strongest 5D0 ! 7F2 transition is split into two levels at 613 and 616 nm. The thermometric parameter is the inten- sity ratio of these emissions that changes with the temperature in the physiological range. 5.2 Molecular Thermometers In our classification, the luminescent thermometers based on the emission of discrete molecules of Ln3+ ions with organic ligands are referred to as molec- ular thermometers. This concept of molecular thermometry has been extended to several kinds of matrices (Fig. 22) (eg, polymers and organic–inorganic hybrids), and to a wide variety of forms, such as bulky materials, films, and NPs, being thus very versatile. They have even been incorporated in nonvola- tile liquids such as anthracenes to make thermoresponsive liquids (Babu et al., 2013). Moreover, only a few molecules are necessary to give a thermometric response. These facts are determinant in the performance and processability of the thermometer. The molecular nature of the system introduces also another important advantage: the possibility to achieve a very high spatial resolution. Actually, it has been recently demonstrated that Ln3+ molecular thermometers can also be implemented as a self-assembled polymer monolayer on a Si sur- face (Rodrigues et al., 2016). This monolayer of Tb3+ and Eu3+ complexes behaves as a ratiometric thermometer with Sm1⁄41.45%K1, a cycle–recycle reliability R1⁄498.6%, and dT1⁄40.3 K. Moreover, this thermometer presents a unique behavior consisting in reversible bistability permitting that the Si-functionalized surface can operate as an optically active two-modulePDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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