# Enhancement of the Luminescence Properties of Eu (III) Containing Paramagnetic Saponite Clays
## 1 Introduction
These drawbacks can be overcome by exploiting a process called sensitization, in which a chemical entity (donor group) can absorb and transfer energy to excited states of the \(Ln^{3+}\) ion (acceptor group), with a much increased enhancement of the resulting photoluminescence [30; 31]. Different sensitization mechanisms have been studied over the years: (\(i\)) by adding another luminescent metal in close proximity to the \(Ln^{3+}\) (metal-to-metal energy transfer (MMET)) [32; 33]; (\(ii\)) via charge-transfer states (i.e., metal-to-metal (MMCT), metal-to-ligand (MLCT) and ligand-to-metal (LMCT)) [34; 35]; and (\(iii\)) by using specific organic ligands (chromophores) with intense absorption bands, which can funnel energy to the \(Ln^{3+}\) by an intramolecular energy transfer process (ligand-to-metal energy transfer (LMET), also called the antenna effect) [36; 37]. The development of novel magnetic/optical lanthanide systems represents an ongoing and exciting branch of research, especially through the encapsulation of lanthanides into inorganic materials, which can overcome the weakness of molecular complexes such as the weak thermal stability and low mechanical strength [38]. On this topic, in recent years, our research group has focused its attention on the combination of diverse paramagnetic and luminescent \(f\)-block elements with synthetic layered smectite clays. This class of materials represents an ideal host for the embedding of lanthanides, owing to its compelling physico-chemical characteristics such as high thermal stability, tuneable acidity, specific surface area, low production costs and outstanding chemical versatility [39; 40]. Particularly, synthetic saponites can be easily modified to have different particle size and chemical composition of the lamellae and of the interlayer space, achieved by choosing specific synthetic one-pot and post-synthesis procedures [39]. Lanthanides such as Gd\({}^{3+}\), Eu\({}^{3+}\) and Tb\({}^{3+}\) were included both in the form of complexes through intercalation procedures [41; 42], and as ions by adding them in the clay synthesis gel [43; 44; 45], thus obtaining novel layered materials through a modified one-pot hydrothermal procedure in which the metal ions are directly placed in structural positions. This procedure, compared to post-synthesis intercalation, has the advantage to limit the production costs, reduce the synthesis times and increase the chemical stability of the modified material. Among the saponite clays containing lanthanides, studied in the literature, the Na-GdEuSAP, functionalized with both Gd\({}^{3+}\) and Eu\({}^{3+}\) ions in tetrahedral layers, exhibited noteworthy paramagnetic and photophysical properties, with the latter due to the existence of the Gd\({}^{3+}\)-Eu\({}^{3+}\) energy transfer which led to an enhancement of the europium photoluminescence [45]. However, there is room for improvement on this topic, because the \(Ln^{3+}\)-\(Ln^{3+}\) energy transfer is not the only possible sensitization route and the chemical composition of the clay interlamellar region can be modified through intercalation of chromophores that can interact with the Eu\({}^{3+}\) and greatly enhance its optical properties. Following these ideas, in this work we investigated the enhancement of the luminescence of the Eu\({}^{3+}\) centres in the Na-GdEuSAP layered material, recently reported by our research group [45], with the use of an antenna group (1,10-phenanthroline, Phen), which was introduced in the interlayer space of the bifunctional synthetic saponite at two different loadings, with respect to the cation exchange capacity (C.E.C.) of the clay. Two final hybrid materials were prepared, hereafter called Phen-GdEuSAP_1 (lowest loading) and Phen-GdEuSAP_2 (highest loading) (Scheme S1). Pristine Na\({}^{+}\)-exchanged saponite (Na-SAP) intercalated with Phen molecules was also prepared as the reference material (Phen-SAP).