Design, Synthesis and Characterization of Lanthanide Organic Cages for Molecular Sensing and Energy Conversion Applications

Coordination-driven lanthanide (Ln) organic polyhedral cages represent a fascinating group of supramolecular compounds which have emerged as significant systems for both fundamental research and various applications such as sensing, catalysis, and magnetism. On the other hand, the design, synthesis and control on the properties of these systems are complex and demanding tasks. This thesis aims to address the challenge by focusing on the synthesis of Lanthanide Organic Cages (LOCs), investigating their formation process, and exploring their potential applications as molecular sensors and Luminescent Solar Concentrators (LSCs). Initially, a set of LOCs with different geometries were synthesized, using bis- and tris-β-diketones. When combined with Ln3+ ions, these ligands form lantern-like supramolecular helicates of formula {[Ln2L4]Z2}, tetrahedra [Ln4L4] or octahedra {[Ln6L8]Z6}, where Ln is a lanthanide ion, L is the ligand and Z is the counterion. Consequently, a systematic investigation was conducted to unravel the formation process of lantern-like cages {[Ln2L4]Z2}. The consecutive and competitive formation of four different species, [La2L]4+, [La2L2]2+, [La2L3], [La2L4]2- has been determined. The equilibrium between the triple-stranded [La2L3] and the quadruple-stranded [La2L4]2- cage (the two most stable species) has been further analyzed, indicating that the interconversion is exergonic and endothermic, and mainly entropy driven. The electronic structure of Ln elements is at the origin of their very rich optical and chemical properties, and their potential applications across various fields. Some advanced applications rely on interactions with other metals in either homo- or heterometallic polynuclear molecular entities. Nonetheless, synthesizing heterometallic 4f – 4f’ molecular assemblies is particularly demanding because of the peculiar nature and chemistry of Ln ions. Post-synthetic ion exchange is one of the methods used to obtain heterometallic supramolecular systems. Hence, seven distinct [Ln2L4]2− (Ln = La, Nd, Eu, Tb, Er, Tm, and Lu) homometallic lanthanide-based quadruple-stranded helicates are reported, along with a study on transmetalation between two pre-assembled cages. Mixing two homonuclear helicates, [LnA2L4]2− and [LnB2L4]2−, leads to a mixture of homo- and heteronuclear systems due to ion exchange. Furthermore, as confirmed by SC-XRD, these cages are present both in solid state and in solution as a racemic mixture of left- and right-handed helicates. This property, along with the ability to encapsulate guests, allows the cages to function as chiral sensors. They can adapt their helical conformation to accommodate a chiral guest, enabling also straightforward enantiomeric excess determination, as shown through Circular Dichroism (CD). A significant challenge lies in the controlled release of the guest molecule after encapsulation, which is addressed through an innovative approach involving an external stimulus, specifically light. The metastable-state merocyanine-type photoacid (PAH) undergoes structural changes when subjected to light, releasing a proton in solution. This protonates one ligand of the quadruple-stranded cage, which converts into the triple-stranded one and releases the guest. The process is reversible when the system is left in the dark. Finally, the photophysical properties of the previously synthesized LOCs are discussed. In particular, an application of luminescent super-bright Eu3+ based LOCs is discussed, illustrating their use as fluorophores in LSCs. These devices, used for transparent or semi-transparent Building Integrated Photovoltaics, contribute to energy-efficient building designs. A series of highly transparent LSCs were developed by incorporating the family of binuclear Eu3+ quadruple-stranded cages into poly(methyl methacrilate) (PMMA). The proposed LSCs possess excellent transparency and aesthetic quality suitable for neutral colour applications in glass.