Sensitisation of the Luminescence of Trivalent Lanthanides by Non-Radiative Energy Transfer Processes: An Intelligent Strategy for Designing Optical Materials

In this thesis, various luminescent materials were synthesized and characterized to assess their potential application as phosphors for different applications (i.e. for anti-counterfeiting purposes). For all the synthesized samples, the luminescent properties of Tb3+ and Eu3+ ions at room temperature were investigated, along with their structural features through powder X-ray diffraction and IR spectroscopy. The studied samples belong to two different families of inorganic matrices: 1. Huntite - Aluminium Borates (Tb1-xEuxAl3(BO3)4). 2. Nanocrystalline Phosphates (Tb1-xEuxPO4.0.67H2O). 1) The Huntite family [Tb1-xEuxAl3(BO3)4] exhibits a tunable Tb3+ → Eu3+ energy transfer efficiency, dependent on the acceptor ion concentration. Furthermore, the decay curves show non-monoexponentially behaviour, which was described through a combination of two mathematical models: I. Inokuti-Hirayama for the TbAl3(BO3)4 sample. II. Adapted Parent model for Eu3+-doped samples. Additionally, the luminescence properties were studied as a function of temperature (12–310 K) for Tb0.99Eu0.01Al3(BO3)4. The results show that the Tb3+ → Eu3+ energy transfer efficiency increases with rising temperature in the 100–310 K range, while it remains essentially unchanged between 12–100 K. 2) The nanocrystalline phosphate family selectively crystallizes in one of two trigonal, enantiomorphic space groups, P3121 e P3221, when the crystal growth is directed by the chirality of L- or D-tartaric acid. Spectroscopic analysis revealed a good Tb3+ → Eu3+ energy transfer efficiency and confirmed the chiral nature of the samples through CPL (circularly polarized luminescence, a chiroptical feature) measurements. By varying the Tb3+/Eu3+ ratios, it was shown that while the unit cell volume remains in practice constant, the average crystal size changes in the range of 13–30 nm. A surface functionalization protocol was then developed using dicarboxylic acids: I. Dipicolinic Acid. II. Phtalic Acid. These acids provide an efficient ligand-to-metal energy transfer (antenna effect) for Tb³⁺ and Eu³⁺ ions, enhancing their emission intensity upon excitation at wavelengths close to 300 nm. Furthermore, changes in the CPL activity were observed depending on the chiral dicarboxylic acid used for surface coating. Currently, the scientific literature reports very few examples of chiral inorganic phosphors, especially when compared to MOFs or metal–ligand complexes. CPL activity could be conveniently applied to the field of anti-counterfeiting applications.