Synthesis of Novel Luminescent Double-Decker Silsesquioxanes Based on Partially Condensed TetraSilanolPhenyl POSS and Tb 3+ /Eu 3+ Lanthanide Ions

: In this study, novel lanthanide-containing double-decker polyhedral oligomeric silsesquioxanes (POSS) were prepared by combining the partially condensed TetraSilanolPhenyl POSS with terbium (Tb 3+ ) and europium (Eu 3+ ) ions. This open-cage POSS possesses four diametrically opposite silanol groups that are able to coordinate, under mild conditions, different luminescent ions through a simple corner-capping method. The two metal-containing POSS functionalized with Tb 3+ and with an equimolar combination of Tb 3+ and Eu 3+ show a completely condensed structure with different luminescent properties. Their emission features depend on the chemical nature of the metal ions incorporated in the framework. An improved stokes shift was detected in the bimetallic compound containing both the Tb 3+ and Eu 3+ ions, promoted by the occurrence of a Tb 3+ → Eu 3+ energy transfer mechanism. These characteristics identify this metal-functionalized silica platform as a potential candidate for the development of novel luminescent devices.


Introduction
Metal-containing polyhedral oligomeric silsesquioxanes (M-POSS) [1][2][3][4][5][6] are an important class of inorganic compounds consisting of a condensed oligomeric silica structure with a typical cage shape and bearing a metal ion in one vertex of the structure [7,8]. The remaining edges of the cage are occupied by silicon atoms that are covalently bonded to oxygen and to organic arms, thus generating a three-dimensional Si-O-Si skeleton with tetrahedral units ((RSiO 1.5 ) n ) (n = 4, 6, 8, 10, 12; R = H or organic substituents) [9][10][11][12][13]. In addition to fully condensed POSS, which is commonly identified by a R 8 Si 8 O 12 cubic structure [9][10][11][12], the most common cage-like silsesquioxanes used for the synthesis of M-POSS are the partially condensed TriSilanol POSS with the R 7 Si 7 O 9 (OH) 3 molecular formula [14][15][16]. The latter shows a siloxane structure containing three reactive silanol groups that are able to coordinate a broad spectrum of metal centres through a cornercapping procedure [12,17]. Following this synthetic approach, the metal ions are directly incorporated within the POSS inorganic core [6,12,17]. Another interesting strategy for the preparation of M-POSS is based on the chelation of the metal precursors, which is promoted by the presence of ligand arms attached to the silicon vertices [18][19][20].
POSS and their metal-functionalized derivatives typically exhibit molecular sizes, high chemical versatility and good mechanical and thermal stabilities [8,21]. Furthermore, these solids attracted an increasing interest in several scientific and technological fields, finding applications as heterogeneous catalysts [5,6,22,23] for biomedical and diagnostic uses [18,21,[24][25][26][27], for the fabrication of 3D nanostructured materials with well-defined in 90 mL of anhydrous tetrahydrofuran (THF; ≥99%, Sigma-Aldrich, Burlington, MA, USA) in the presence of 272 µL of triethylamine (TEA; 4.17 mmol; ≥99%, Sigma-Aldrich, Burlington, MA, USA). The reaction was carefully purged with nitrogen for 10 min. After the addition of the reactants, the temperature was fixed to 50 • C, and the mixture was stirred for 24 h. Then, the solvent was removed. The powder was washed with THF and ultrapure water to remove unreacted compounds and by-products, thus obtaining the Tb-PhPOSS (1) sample. (2) The TbEu-PhPOSS sample was prepared following the same synthetic protocol described before for sample (1) with a relative molar ratio of the reactants of 2:0.5:0.5 (POSS:Tb 3+ :Eu 3+ ).

Analytical Methods
Elemental analyses were carried out on a Thermo Fisher Scientific X5 Series Inductively Coupled Plasma Mass Spectrometer (ICP-MS; Waltham, MA, USA). Before the analyses, the compounds were carefully mineralized by treating them with a mixture of nitric acid (5 mL) and hydrofluoric acid (5 mL) at 100 • C for 8 h.
Fourier-transform infrared spectroscopy (FT-IR) measurements were performed on a Bruker Equinox 55 Spectrometer operating in the 4000-400 cm −1 range with a resolution of 4 cm −1 . IR spectra of the solids mixed with potassium bromide (KBr) pellets were measured in absorbance mode at room temperature. The absorbance values are reported in absolute unit [a.u.].
X-ray powder diffractograms (XRPD) were collected on unoriented ground powders on a Bruker D8 Advance Powder Diffractometer, equipped with a Linxeye XE-T detector. The X-ray tube of the instrument operates with a Cu-K α1 monochromatic radiation (λ = 1.54062 Å), with the current intensity and operative electric potential difference set to 40 mA and 40 kV, respectively, and with variable primary divergent slits and primary and secondary Soller slits of 2.5 • . The X-ray profiles were recorded at room temperature in the 5-70 • 2θ range with a coupled-2θ-θ method, fast and continuous scan mode, time per step/rate of 0.05 • /min, increment/step size of 0.02 • and with automatic air scatter and slits.
Photoluminescence (PL) spectra (excitation and emission) of the compounds at solidstate were recorded on a Horiba Jobin-Yvon Model IBH FL-322 Fluorolog 3 Spectrometer, equipped with a 450 W Xenon arc lamp, double grating excitation and emission monochromators (2.1 nm·mm −1 dispersion; 1200 grooves per mm) and a Hamamatsu Model R928 photomultiplier tube. Time-resolved measurements were carried out, instead, by using the time-correlated single-photon counting (TCSPC) option. A 370 nm SpectraLED laser was employed to excite the samples at solid-state and in H 2 O and D 2 O. The signal was collected using an IBH DataStation Hub photon counting module, while data analysis was accomplished with the commercial DAS6 software (HORIBA Jobin Yvon IBH Ltd., Glasgow, UK).

Results and Discussion
The two luminescent POSS, incorporating Tb 3+ and a combination of Tb 3+ and Eu 3+ ions, were prepared by a one-step corner-capping reaction between the PhPOSS and the lanthanide precursors. The reaction was performed under simple conditions, in anhydrous solvent THF, in the presence of a small amount of TEA at 50 • C for 24 h. During the reaction, the four reactive Si-OH groups of PhPOSS reacted with lanthanide ions' reactants.
The Tb 3+ and Eu 3+ contents in both samples were determined by an inductively coupled plasma mass spectrometry (ICP-MS), while the amount of carbon was quantified by a CHN analysis ( Table 1). The amount of Tb 3+ ions was found to be 0.63 mmol/g and 0.26 mmol/g for Tb-PhPOSS and TbEu-PhPOSS, respectively, while the Eu 3+ loading was equal to 0.30 mmol/g in the bimetallic POSS. The carbon content was equivalent to 42.24 mmol/g and 41.30 mmol/g for Tb-PhPOSS and TbEu-PhPOSS, respectively, in agreement with the chemical composition of the reactant PhPOSS. By combining the data obtained from the ICP and CHN analyses, it was possible to extrapolate the number of lanthanide ions per POSS cage. In detail, for Tb-PhPOSS, we calculated a molar ratio between the number of cages and the Tb 3+ ion of 1.4. This could suggest that Tb-PhPOSS is composed by a polycondensed structure in which each Tb 3+ is shared by two coordinated POSS cages ( Figure 1). A similar result was obtained for TbEu-PhPOSS, considering for the quantification the total amount of the two lanthanide ions embedded in the structure ( Figure 1).
Additional structural information on the metal-containing POSS samples was obtained by infrared spectroscopy. IR spectra of the samples, recorded in the 4000-400 cm −1 range, are reported in Figure 2. The vibrational profiles of Tb-PhPOSS and TbEu-PhPOSS are compared with the spectrum of PhPOSS sample. The IR spectrum of PhPOSS ( Figure 2, a) shows the typical signals of phenyl groups at 3100-3000 cm −1 (stretching of C-H groups) and at 1595, 1490 and 1430 cm −1 , assigned to the stretching modes of C=C [100,106]. The main absorption of the POSS cage is detectable at 1100 cm −1 , and it is ascribed to the asymmetric stretching of the siloxane (Si-O-Si) groups of the inorganic framework [100,106,107]. Finally, the absorptions at 3250 and 895 cm −1 are characteristics of the stretching modes of OH and Si-OH functionalities, which are typical of partially condensed POSS molecules [100,106,107].
After reaction with the Tb 3+ precursor (Figure 2, b) or the mixture of terbium and europium salts (Figure 2, c), the IR spectra of both solids show a complete erosion of the bands located at 3250 and 895 cm −1 , as a consequence of the condensation of Si-OH groups with the lanthanide ions. The bands of phenyl groups are still visible, proving that the organic groups attached to the POSS cage are preserved during the corner-capping reaction. Finally, it is worth noting that the band at 1100 cm −1 appears more defined and narrower with respect to the POSS reactant. This is undoubtedly indicative of an orderly and regular organization of the Si-O-Si network, promoted by the presence of the lanthanide ions during the condensation reaction. This comment is qualitatively supported by the X-ray powder diffraction (XRPD) data shown in Figure 3. Additional structural information on the metal-containing POSS samples was obtained by infrared spectroscopy. IR spectra of the samples, recorded in the 4000-400 cm −1 range, are reported in Figure 2. The vibrational profiles of Tb-PhPOSS and TbEu-PhPOSS are compared with the spectrum of PhPOSS sample.
Further confirmation of the energy transfer process was obtained by collecting the solid-state emission spectra of TbEu-PhPOSS under irradiation at the λmax of Eu 3+ at 395 nm ( Figure 4C, b) and of Tb 3+ at 285 nm ( Figure 4C, c). The comparison with the photoluminescence spectrum of Tb-PhPOSS collected under excitation at 285 nm is reported in Figure 4C. The emission spectrum measured under excitation at 285 nm ( Figure 4C, c) showed a very weak and broad emission peak at 545 nm, typical of Tb 3+ ( 5 D4-7 F5) [108][109][110], together with the intense and characteristic emission bands of Eu 3+ ( 5 D0-7 FJ, J = 0-4), with the most intense line centred at 615 nm ( 5 D0-7 F2) [108][109][110][111]. The photoluminescence of Eu 3+ under excitation of Tb 3+ , coupled with a marked decrease in the intensity of the main emission band of Tb 3+ at 545 nm, testifies to the occurrence of the Tb 3+ →Eu 3+ energy transfer process because the two metals are in close proximity [110,[112][113][114].
The nature of the local chemical environment around the lanthanide centres was extrapolated from the emission spectra shown in Figure 4C. First, the Eu 3+ sites appear to be heterogeneously distributed in the TbEu-PhPOSS sample, due to the presence of two weak peaks assigned to the 5 D0-7 F0 electronic transition at 579 and 580 nm ( Figure S4). This assumption is based on the fact that the number of distinct chemical domains around Eu 3+ can be correlated to the multiplicity of the 5 D0-7 F0 band, since its initial and final energy states are both non-degenerated [115]. In addition, the Eu 3+ and Tb 3+ ions in both solids are placed in a low symmetry environment with no inversion centre, as extrapolated by the high asymmetry factor (R) values between 1.4 ÷ 1.8 for europium and 1.0 ÷ 4.3 for terbium The diffractogram of the PhPOSS is composed by well-resolved peaks in the 5-70 • 2θ range, as is typically observed for crystalline POSS (Figure 3, a). After the cornercapping reaction, the X-ray profiles of Tb-PhPOSS and TbEu-PhPOSS show new welldefined peaks, and both profiles differ from that of PhPOSS and the analogous completely condensed OctaPhenyl POSS (Figure 3, b and Figure S2). The coordination of Tb 3+ and Eu 3+ ions promoted a different ordered structural arrangement of the POSS molecules with respect to the PhPOSS (Figure 3, c and d). Furthermore, the two diffraction patterns are comparable to each other, thus testifying that both Tb-PhPOSS and TbEu-PhPOSS have a similar crystallographic arrangement. More structural information can be obtained by single crystal X-ray diffraction, but, unfortunately, we did not obtain suitable crystals for the analysis. Furthermore, NMR analysis is difficult to apply to samples containing "shift agents".
from their respective emission spectra, calculating the xy chromaticity coordinates and related Hex and RGB parameters according to CIE 1931 colour spaces ( Figure 4D and Table S2) [119]. The direct excitation of Eu 3+ sites at 395 nm in TbEu-PhPOSS led to a red emission colour (point b in Figure 4D), and, after excitation at 285 nm (λmax of Tb 3+ ), the bimetallic sample showed a very small blue-shift of its emission towards a slightly more orange-red colour (point c in Figure 4D). For reference, the colour associated with the emission of Tb-PhPOSS is light green (point a in Figure 4D).   Table S2).
The average hydration state of Tb 3+ ions (q Tb ) in the Tb-PhPOSS and of Eu 3+ ions (q Eu ) in the TbEu-PhPOSS were estimated by measuring, via time-resolved fluorescence spectroscopy, the experimental lifetimes (τ ) associated with the transitions at 545 and 615 nm, respectively, in H2O and D2O under excitation at 370 nm. The q Ln values for Tb 3+ and Eu 3+ were calculated by applying the following Equations (1) and (2) Table S2).
Further confirmation of the energy transfer process was obtained by collecting the solid-state emission spectra of TbEu-PhPOSS under irradiation at the λ max of Eu 3+ at 395 nm ( Figure 4C, b) and of Tb 3+ at 285 nm ( Figure 4C, c). The comparison with the photoluminescence spectrum of Tb-PhPOSS collected under excitation at 285 nm is reported in Figure 4C. The emission spectrum measured under excitation at 285 nm ( Figure 4C, c) showed a very weak and broad emission peak at 545 nm, typical of Tb 3+ ( 5 D 4 -7 F 5 ) [108][109][110], together with the intense and characteristic emission bands of Eu 3+ ( 5 D 0 -7 F J , J = 0-4), with the most intense line centred at 615 nm ( 5 D 0 -7 F 2 ) [108][109][110][111]. The photoluminescence of Eu 3+ under excitation of Tb 3+ , coupled with a marked decrease in the intensity of the main emission band of Tb 3+ at 545 nm, testifies to the occurrence of the Tb 3+ →Eu 3+ energy transfer process because the two metals are in close proximity [110,[112][113][114].
The nature of the local chemical environment around the lanthanide centres was extrapolated from the emission spectra shown in Figure 4C. First, the Eu 3+ sites appear to be heterogeneously distributed in the TbEu-PhPOSS sample, due to the presence of two weak peaks assigned to the 5 D 0 -7 F 0 electronic transition at 579 and 580 nm ( Figure S4). This assumption is based on the fact that the number of distinct chemical domains around Eu 3+ can be correlated to the multiplicity of the 5 D 0 -7 F 0 band, since its initial and final energy states are both non-degenerated [115]. In addition, the Eu 3+ and Tb 3+ ions in both solids are placed in a low symmetry environment with no inversion centre, as extrapolated by the high asymmetry factor (R) values between 1.4 ÷ 1.8 for europium and 1.0 ÷ 4.3 for terbium (Table S1). The decrease in the R parameter from ca. 4.3 for Tb-PhPOSS ( Figure 4C, a) to ca. 1.1 of TbEu-PhPOSS ( Figure 4C, c), both under excitation at 285 nm, represents other evidence of the energy transfer process. The R factor was calculated from the intensity ratio of the electric-dipole 5 D 0 → 7 F 2 on the magnetic-dipole 5 D 0 → 7 F 1 for europium [116,117], while for terbium the ratio is 5 D 4 → 7 F 5 / 5 D 4 → 7 F 6 [118].
The photometric features of the Tb-PhPOSS and TbEu-POSS solids were obtained from their respective emission spectra, calculating the xy chromaticity coordinates and related Hex and RGB parameters according to CIE 1931 colour spaces ( Figure 4D and Table  S2) [119]. The direct excitation of Eu 3+ sites at 395 nm in TbEu-PhPOSS led to a red emission colour (point b in Figure 4D), and, after excitation at 285 nm (λ max of Tb 3+ ), the bimetallic sample showed a very small blue-shift of its emission towards a slightly more orange-red colour (point c in Figure 4D). For reference, the colour associated with the emission of Tb-PhPOSS is light green (point a in Figure 4D).
The average hydration state of Tb 3+ ions (q Tb ) in the Tb-PhPOSS and of Eu 3+ ions (q Eu ) in the TbEu-PhPOSS were estimated by measuring, via time-resolved fluorescence spectroscopy, the experimental lifetimes (τ) associated with the transitions at 545 and 615 nm, respectively, in H 2 O and D 2 O under excitation at 370 nm. The q Ln values for Tb 3+ and Eu 3+ were calculated by applying the following Equations (1) and (2) [120][121][122][123].
The photoluminescence decay curves of the 5 D 4 excited state of Tb 3+ (Figure S5A,B) and 5 D 0 excited state of Eu 3+ ( Figure S5C,D), fitted with a bi-exponential function, suggest the presence of ca. five water molecules in the first coordination sphere of both lanthanide ions (Table S3). The results are consistent with the hypothetical structure of the samples described before, considering that the two lanthanide ions can easily expand their coordination sphere up to coordination number values of 8-9 [108,124].
The Tb 3+ -Eu 3+ energy transfer mechanism was analysed in more detail by collecting the lifetime values (τ) at solid-state of Tb-PhPOSS (donor system, D) and TbEu-PhPOSS (donor-acceptor system, DA). The intensity decay curves of 5 D 4 excited state of Tb 3+ were measured for both samples at λ em of 545 nm, under excitation at 370 nm ( Figure S6). The curves were well fitted with a bi-exponential function, and, from these, the average τ values were derived. The energy transfer efficiency (E EnT ) and rate (k EnT ) parameters were then Processes 2022, 10, 758 9 of 15 calculated by using the average lifetimes measured at 545 nm of the donor ion (Tb 3+ ) in the presence (τ DA , Figure S6B) or absence (τ D , Figure S6A) of the acceptor one (Eu 3+ ), by using Equations (3) and (4) [112,113,125,126].
The results, reported in Table 2, showed a Tb 3+ →Eu 3+ energy transfer efficiency value of ca. 62%, with a rate constant of the process equal to 4.09 × 10 3 s −1 , comparable to those observed for other inorganic materials functionalized with both Tb 3+ and Eu 3+ ions [110,112,114,127].

Conclusions
In this study, novel completely condensed luminescent double-decker POSS solids functionalized with Tb 3+ and a combination of Tb 3+ and Eu 3+ ions were prepared by a simple one-pot corner-capping reaction under mild conditions. The synthetic protocol adopted in this work allows for obtainment of a polycondensed POSS with the following properties: (i) ordered architecture of the POSS structure; (ii) a well-defined molar ratio of cage units and the lanthanide ions in both samples; (iii) each lanthanide ion is connected to different POSS cages with an expansion of the coordination state with five inner sphere water molecules; (iv) tunable emission properties for the bimetallic POSS and (v) remarkable energy transfer from the Tb 3+ to Eu 3+ centers in terms of efficiency and the process rate. The optical properties of both Tb-PhPOSS and TbEu-PhPOSS solids make them ideal candidates for the fabrication of novel optical thermometers and sensor devices. Further studies are currently underway to evaluate the effect of the reaction's stoichiometry on the structure and chemical composition of the final samples.
Supplementary Materials: The following supporting information can be downloaded at https: //www.mdpi.com/article/10.3390/pr10040758/s1. Figure S1: Schematic view of the structure of the partially condensed PhPOSS.; Figure S2: Schematic view of the structure of the completely condensed OctaPhenyl POSS.; Figure S3: Excitation spectrum at solid-state of Tb-PhPOSS monitored at 545 nm.; Figure S4: Magnification of the region associated with the 5 D 0 -7 F 0 electronic transition of Eu 3+ in the solid-state emission spectra of TbEu-PhPOSS reported in Figure 4C Table S1: Asymmetry factors (R) of Tb 3+ and Eu 3+ for Tb-PhPOSS and TbEu-PhPOSS, under irradiation at 285 nm (Tb 3+ ) and 395 nm (Eu 3+ ), calculated from the solid-state emission spectra reported in Figure 4C.; Table S2: Photometric data, in accordance with CIE 1931 colour spaces, for Tb-PhPOSS and TbEu-PhPOSS, extrapolated from the solid-state emission spectra shown in Figure 4C.; Table S3: Hydration state of Tb 3+ (q Tb ) and Eu 3+ (q Eu ) in Tb-PhPOSS and TbEu-PhPOSS, respectively,