# Enhancement of the Luminescence Properties of Eu (III) Containing Paramagnetic Saponite Clays
## 3 Results and Discussion
Na\({}^{+}\)-exchanged saponites intercalated with Phen molecules were also studies as reference materials. All the samples were studied at the solid state. Figure 1: (A) X-ray powder diffraction profiles of Na–GdEuSAP (a), Phen–GdEuSAP_1 (b) and Phen–GdEuSAP_2 (c). Diffractograms collected at low angles (2\({}^{\circ}\)–15\({}^{\circ}\)2\(\theta\)) are reported in (**B**). The excitation spectrum of the Phen-GdEuSAP_1 sample collected at 615 nm (Figure S4) shows the characteristic narrow peaks of the intra-4\({}^{6}\) electronic transitions of Eu\({}^{3+}\) (\({}^{5}\)D\({}_{0}\)-\({}^{7}\)F\({}_{I}\), \(J\) = 0-4; \(\lambda_{\text{max}}\) = 395 nm, \({}^{7}\)F\({}_{0}\)-\({}^{5}\)L\({}_{6}\)) [52] and Gd\({}^{3+}\) (\({}^{8}\)S\({}_{7/2}\)-\({}^{6}\)Ij; \(\lambda_{\text{max}}\) = 272 nm, \({}^{8}\)S\({}_{7/2}\)-\({}^{6}\)Pj) [53] together with the S\({}_{0}\)\(\rightarrow\) S\({}_{1}\) (\({}^{1}\pi\)\(\rightarrow\)\({}^{1}\pi^{*}\)) electronic transition of the Phen, located at 290 nm [46, 47, 48, 49, 54] (Figure S5A). The emission spectra of both Phen-GdEuSAP samples at solid state upon direct excitation of Eu\({}^{3+}\) at \(\lambda_{\text{max}}\) = 395 nm were found to be comparable with that of the starting Na-GdEuSAP clay measured in the same experimental conditions (Figure S6) [45]. The emission spectra of Phen-GdEuSAP samples, collected under irradiation at both 290 (\(\lambda_{\text{exc}}\) of Phen) and 395 nm (\(\lambda_{\text{exc}}\) of Eu\({}^{3+}\)) (Figure 3), showed that upon direct excitation of the antenna group there is a marked increase in the intensity of the hypersensitive \({}^{5}\)D\({}_{0}\)-\({}^{7}\)F\({}_{2}\) transition at 615 nm of Eu\({}^{3+}\) compared to the direct excitation of the lanthanide, thus confirming the interaction of Phen molecules with Eu\({}^{3+}\) sites and the occurrence of the ligand-to-metal energy transfer phenomenon [36, 37, 50]. The emission spectra of both hybrid materials excited at 290 nm (\(\lambda_{\text{max}}\) of Phen) shown in Figure 3 also show a broad emission band centered at 415 nm associated with the residual T\({}_{1}\)\(\rightarrow\) S\({}_{0}\) (\({}^{3}\pi^{*}\)\(\rightarrow\)\({}^{1}\pi\)) electronic transition of Phen [46, 47, 48, 49, 54  
The best optical improvement under the energy transfer condition, related to the sensitization of the intensity emission of the 615 nm band of europium, was observed for the Phen-GdEuSAP_2 sample, with an increase of +1051% (Figure 3B(b)) compared to the direct excitation of europium at 395 nm (Figure 3B(a)). This enhancement is considerably higher than the value observed by exploiting the Gd\({}^{3+}\)\(\rightarrow\)Eu\({}^{3+}\) energy transfer mechanism (+240%) in the Na-GdEuSAP clay at solid state [45]. Moreover, Eu\({}^{3+}\) centers located in the inorganic framework experienced a local low symmetry environment after the intercalation of Phen molecules. The asymmetry factor (R) [55; 56; 57; 58] for the two intercalated samples is in the 5 to 6 range, higher than that calculated for Na-GdEuSAP (R = 3.10). The R parameter was extrapolated from the intensity ratio of electric-dipole \({}^{5}\)D\({}_{0}\)\(\rightarrow\)\({}^{7}\)F\({}_{2}\) on magnetic-dipole \({}^{5}\)D\({}_{0}\)\(\rightarrow\)\({}^{7}\)F\({}_{1}\) transitions in the emission spectra (Figure 3). An evaluation of the colorimetric features of Phen-GdEuSAP clays was also carried out by calculating the chromaticity coordinates (_xy_) and related RGB and Hex parameters according to CIE 1931 color spaces [59]. The chromaticity diagrams were extrapolated from the emission spectra collected at solid state of Phen-GdEuSAP samples shown in Figure 3. As reported in Figure 4, and from the calculated parameters of Table S1, the Phen-to-Eu\({}^{3+}\) energy transfer process led to final emissions in the blueish-violet visible region of the electromagnetic spectrum (b), especially visible for Phen-GdEuSAP_2, compared to the pure red emission of the direct excitation of Eu\({}^{3+}\) (a).