# Enhancement of the Luminescence Properties of Eu (III) Containing Paramagnetic Saponite Clays
## 3 Results and Discussion
Accordingly, it was found that is possible to modulate the emission color of the entire system by changing the amount of antenna groups introduced in the material. The results obtained from the photoluminescence measurements were further confirmed by the analysis of the parameters that describe the efficiency (\(E_{\rm{EnT}}\)) and the constant rate (\(k_{\rm{EnT}}\)) of the energy transfer process between the ligand and the lanthanide ions, as presented in Equations (1) and (2). [36; 37; 50; 60; 61]:  
\[E_{Ent}\left[\%\right]=\left(1-\frac{\tau_{DA}}{\tau_{D}}\right)\cdot 100 \tag{1}\]  
\[k_{EnT}\left[s^{-1}\right]=\left(\frac{1}{\tau_{DA}}-\frac{1}{\tau_{D}}\right) \tag{2}\]  
The parameters were derived from the time-resolved fluorescence spectroscopy analyses at solid state of the experimental lifetimes (\(\tau_{\rm{D}}\)) of the Donor system (Phen-SAP clays) and those (\(\tau_{\rm{DA}}\)) of the Donor-Acceptor (Phen-GdEuSAP clays), collecting the decay curves of the S\({}_{0}\) excited state of Phen at 415 nm [46; 47; 48; 49; 54] upon excitation at 295 nm (Figure S7A,B for Phen-GdEuSAP samples and Figure S8A,B for Phen-SAP samples). The curves were  
Figure 4: CIE 1931 _xy_ chromaticity diagrams of Phen–GdEuSAP_1 (**A**) and Phen–GdEuSAP_2 (**B**) samples (\(\lambda_{\rm{exc}}\) of 395 (a) and 290 nm (b)), derived from emission spectra reported in Figure 3.  
well fitted with a bi-exponential function, obtaining average \(\tau\) values for each one. The results, reported in Table 2, show that Phen-GdEuSAP_2 is characterized by the most efficient optical communication between Phen molecules and Eu\({}^{3+}\) centers, with the highest energy transfer efficiency of _ca._ 21% and a rate constant of 6.31 \(\times\) 10\({}^{7}\) s\({}^{-1}\). The values of the calculated rate constants suggest that the transfer process from Phen to the Eu\({}^{3+}\) sites occurs following the Forster resonance energy transfer (FRET) mechanism. The lifetime of the excited states of Eu\({}^{3+}\) for Na-GdEuSAP and the solids containing different amounts of 1,10-phenanthroline were also measured by irradiating the samples at 370 nm and monitoring the electronic transition at 615 nm. A gradual increase of the decay lifetime of Eu\({}^{3+}\) from 0.05 ms for Na-GdEuSAP to 0.11 and 0.28 ms for Phen-GdEuSAP_1 and Phen-GdEuSAP_2, respectively, was detected (Figure S9). This could suggest a stabilization of the excited state of Eu\({}^{3+}\) ions, promoted by the presence of the 1,10-phenanthroline molecules. Finally, the aqueous suspension of Phen-GdEuSAP_1 and Phen-GdEuSAP_2 clays were submitted to \({}^{1}\)H-NMR relaxometric analysis by measuring the 1/\(T_{1}\)\({}^{1}\)H nuclear magnetic dispersion (NMRD) profiles, collected in the magnetic field range of 0.01-120 MHz at 37 \({}^{\circ}\)C and neutral pH (Figure 5). The samples were measured in aqueous suspension, in the presence of 0.1 wt.% of xanthan gum in order to improve the suspension stability over time. The relaxivity parameter, expressed as \(r_{1}\), [mM\({}^{-1}\).s\({}^{-1}\)], was measured as a function of the applied magnetic field strength [41; 42; 62; 63], by evaluating the relaxation rate of bulk water protons (\(R_{1}\)) in the presence of the paramagnetic entity and dividing the obtained value by the concentration (expressed in mM) of Gd\({}^{3+}\) centers. The profiles of Phen-GdEuSAP_1 (b) and Phen-GdEuSAP_2 (c) exhibited a similar shape compared to that of Na-GdEuSAP measured in the previous study (Figure 5) [45]. This indicates that the relaxometric properties remain almost unchanged after introduction \begin{table}
\begin{tabular}{c c c} \hline \hline
**Sample** & **1** & **2** \\ \hline \(\tau_{\text{D}}\) [ns] & 3.82 & 4.28 \\ \(\tau_{\text{DA}}\) [ns] & 3.41 & 3.37 \\ \(k_{\text{EnT}}\) [s\({}^{-1}\)] & \(3.15\times 10^{7}\) & \(6.31\times 10^{7}\) \\ \(E_{\text{EnT}}\) [\%] & 10.73 & 21.26 \\ \hline \hline \end{tabular}
\end{table}