# Enhancement of the Luminescence Properties of Eu (III) Containing Paramagnetic Saponite Clays
## 3 Results and Discussion  
As reported in the experimental section, Phen-GdEuSAP_n materials (\(n\) = 1, 2) were prepared by the intercalation process of 1,10-phenanthroline (Phen) in the interlayer space of Na-GdEuSAP clay, recently studied in the literature by our research group [45]. Since Phen was intercalated in acidic pH conditions (between 4 and 5), below its pKa (equal to 4.8 at 298 K) [51], the nitrogen atoms of the molecule are protonated and then suitable for the cation exchange process. Phen was added in a molar quantity equal to 1 and 2 times with respect to the cation exchange capacity of Na-GdEuSAP (C.E.C. = 44.7 \(\pm\) 7.9 meq/100 g) (see Figure S1) [45], in order to have a sufficient amount of the chromophore in close proximity to the structural europium sites, thus ensuring an efficient energy transfer from the antenna group to Eu\({}^{3+}\). The same procedure was adopted for the preparation of the reference Phen-SAP. The Gd\({}^{3+}\) and Eu\({}^{3+}\) contents in the Na-GdEuSAP, determined by inductively coupled plasma mass spectrometry (ICP-MS), were previously found to be 0.026 mmol/g for both metals [45]. The X-ray powder diffraction (XRPD) analyses show that the typical crystallographic reflections of the saponite clay are maintained after the intercalation procedure (Figure 1A,B) [39; 40]. Furthermore, the analyses suggested the intercalation of the Phen in the Na-GdEuSAP, as indicated by the marked shift of the basal plane (001) reflection in the modified clays with respect to un-modified Na-GdEuSAP, from 7.3\({}^{\circ}\) 2\(\theta\) (1.21 nm) to 6.40\({}^{\circ}\) 2\(\theta\) (1.38 nm) (Figure 1B). No traces of well resolved peaks associated to the crystalline Phen (Figure S2) were detected in XRD patterns of the hybrid materials, thus demonstrating the absence of extra-phase Phen species. Thermogravimetric analyses (TGA) were performed to (_i_) further confirm the intercalation of the Phen in the Na-GdEuSAP clay, (_ii_) evaluate its thermal stability in the hybrid materials and (_iii_) quantify the amount of Phen confined in the clays. The TGA profile of Na-GdEuSAP, collected under oxygen flow, shows a first weight loss at 100 \({}^{\circ}\)C due to the presence of physisorbed water and a second one in the 150-750 \({}^{\circ}\)C range, associated to a gradual release of interlamellar water and the initial dehydroxylation of the layered structure (Figure 2A). The thermal profiles of the intercalated saponites are mainly characterized by the presence of a relevant weight loss in the 430-560 \({}^{\circ}\)C range, attributed to the degradation of the Phen molecules (Figure 2B). The degradation of the intercalated molecules appears delayed by _ca_. 100\({}^{\circ}\)C with respect to the pure compound analyzed in the same conditions (Figure S3). The increase in the thermal stability of the Phen in Phen-GdEuSAP samples confirms its presence in the interlayer space [41; 42]. The intercalated amount of Phen, expressed as mmol of Phen per gram of saponite, was extrapolated from the TGA curves (Figure 2A) and the results are reported in Table 1. The Phen-GdEuSAP_1 contains 0.29 mmol/g of Phen, lower than that of the C.E.C. of the clay (0.447 mmol/g), suggesting that the interlayer space is partially occupied by the organic molecules. For Phen-GdEuSAP_2, the amount of Phen was 0.50 mmol/g, comparable to the C.E.C. of the clay. The photophysical properties of Phen-GdEuSAP samples were thoroughly investigated by photoluminescence spectroscopy (PL) in order to (_i_) evaluate the occurrence of Phen \(\rightarrow\) Eu\({}^{3+}\) energy transfer and the related optical enhancement of Eu\({}^{3+}\) centers, (_ii_) obtain information on the dependence of the energy transfer on the Phen loading and (_iii_) determine the parameters that define the energy transfer mechanism [36; 37; 50]. The Phen acted as an energy donor species/antenna group (donor, D) to the europium ion (acceptor, A), leading to the formation of a 'D-A complex' that greatly enhances the photoluminescence properties of europium.