Eu3+ and Tb3+ @ PSQ: Dual Luminescent Polyhedral Oligomeric Polysilsesquioxanes

The synthesis and characterization of novel luminescent amorphous POSS-based polysilsesquioxanes (PSQs) with Tb3+ and Eu3+ ions directly integrated in the polysilsesquioxane matrix is presented. Two different Tb3+/Eu3+ molar ratios were applied, with the aim of disclosing the relationships between the nature and loading of the ions and the luminescence properties. Particular attention was given to the investigation of site geometry and hydration state of the metal centers in the inorganic framework, and of the effect of the Tb3+ → Eu3+ energy transfer on the overall optical properties of the co-doped materials. The obtained materials were characterized by high photostability and colors of the emitted light ranging from orange to deep red, as a function of both the Tb3+/Eu3+ molar ratio and the chosen excitation wavelength. A good energy transfer was observed, with higher efficiency displayed when donor/sensitizer concentration was lower than the acceptor/activator concentration. The easiness of preparation and the possibility to finely tune the photoluminescence properties make these materials valid candidates for several applications, including bioimaging, sensors, ratiometric luminescence-based thermometers, and optical components in inorganic or hybrid light-emitting devices.


Introduction
Polyhedral oligomeric silsesquioxanes (or POSS) represent a unique class of threedimensional organo-silicon compounds with a generic molecular structure consisting of an inorganic cage having different degrees of symmetry and hydrogen atoms or organic pendant arms bound to the apexes of the core framework. The term "silsesquioxane" derives from their intimate chemical nature, in which silicon atoms are covalently bounded to one-half oxygen (sesqui-) and hydrocarbon (or hydrogen) units (-ane), thus generating a 3D siloxane (Si-O-Si) skeleton of tetrahedral units with a base chemical formula (RSiO 3/2 ) n (n = 4-12; R = H or organic substituents) [1][2][3].
When multiple POSS are connected together, they can act as structural nodes in the establishment of a polymeric-like network, which falls within the category of materials known as POSS-based polysilsesquioxanes (or simply PSQs) [31][32][33]. In addition, POSS units can be further modified with specific functionalities (i.e., magnetic, luminescent, or catalytic species) to design innovative hybrid 3D silica-based materials. POSS-based PSQs have been prepared in two distinct architectures: (i) hierarchical hybrid materials with high specific surface area and hydrothermal stability and variable micro-/mesoporosities, obtained from template-assisted condensation of POSS cages under acidic or basic pH [22,[34][35][36][37]; (ii) amorphous or ladder-like PSQ structures, attained from long thermal treatments. New sustainable procedures have also been proposed to design amorphous PSQs, i.e., by self-condensation of open-corner T 7 -POSS [32].
Rare-earth metals, especially of the lanthanide series (Ln), represent critical elements for the scientific and technological development of humankind [38] due to their unique magnetic, electronic, optical, and catalytic properties [39][40][41][42][43]. Indeed, they are widely known for their sharp and well-defined absorption and emission bands in the UV-Vis-NIR spectral region, long-lived excited state lifetimes, large Stokes shift, and good photostability, as well as for variable electronic relaxation times and high effective magnetic moments [39,40]. Owning to these features, they have aroused great interest in many fields, i.e., for optical sensors, lightemitting diodes (LED), and in optical imaging and diagnostics [41,[43][44][45][46]. The incorporation of luminescent lanthanides in molecular POSS and polymeric PSQ structures, for example, can be extremely beneficial to isolate these metal centers in complex matrices, in order to confer them high thermal, mechanical, and chemical stabilities and modular optoelectronic properties [47].
In recent years, the research on new rare-earth (RE)-doped silsesquioxanes has been increasing, attracting growing attention for their potential applications, although the number of articles is still relatively small. Most studies have focused on the design of molecular RE-POSS, i.e., with Ce 3+ [48], Gd 3+ [49], Dy 3+ and Y 3+ [50], Er 3+ [51], Tb 3+ and Eu 3+ centers [19,52,53]. The incorporation of Eu 3+ ions into a polysilsesquioxane network based on octa(tetramethylammonium)-POSS cages (OctaTMA POSS, a fully-closed T 8 -POSS) instead, led to a stable POSS-based mesoporous material with intrinsic optical properties [22]. Recently, our group proposed new luminescent and water-stable amorphous PSQs derived from the condensation of anionic OctaTMA POSS units and Eu 3+ ions in simple and sustainable conditions, in which the metal acted as both a structural and functional agent [24]. Nonetheless, very few examples of silsesquioxanes simultaneously hosting different lanthanide ions have been reported so far [25,54,55]. Indeed, the interest towards co-doped lanthanide-based systems is continuously rising, since different properties can be inferred to the resulting materials by appropriate selection of the ion pairs [56], such as upconversion or tuneable magnetic and luminescent features [57].
Based on these considerations, in this study we describe the synthesis of novel codoped luminescent amorphous PSQs with both Tb 3+ and Eu 3+ ions directly integrated in the POSS-based polysilsesquioxane matrix, following an optimized version of our previous synthetic protocol [24]. Two co-doped solid materials were prepared in water at room temperature and slightly acidic pH, followed by simple water evaporation, from the combination of completely condensed anionic OctaTMA POSS with lanthanide acetate precursors, using different Tb 3+ :Eu 3+ molar ratios. The Si-O − groups of the T 8 -POSS coordinate the Ln 3+ ions, which act as metal bridges between the POSS cages. A mono-doped Tb-PSQ was also prepared as a reference solid. The effect of the different stoichiometric ratios of metal ions was then evaluated by multi-technique physico-chemical characterization to better understand the structure-property relationship of the final materials.

Synthesis of Tb-PSQ
Tb-PSQ was prepared by condensation reaction of OctaTMA-POSS with terbium acetate, following the synthetic protocol adopted in the literature by S. Marchesi et al. for the preparation of mono-lanthanide doped polysilsesquioxanes through the combination of OctaTMA-POSS and lanthanide precursor [24]. The mol/mol ratio of POSS and terbium precursors was fixed at 1:1.
In detail, anhydrous terbium acetate (Tb(CH 3 COO) 3 ; 151 mg, 0.451 mmol; Sigma Aldrich) was added with vigorous stirring to a solution of OctaTMA-POSS (POSS; 1 g, 0.451 mmol; Hybrid Plastics) in 20 mL of MilliQ ® water, previously adjusted to pH 5.5-5.6 with the addition of 1 M HCl solution. After addition of the reactants, the mixture was stirred at room temperature for 10 min and then transferred in an oven and heated at 80 • C until complete water evaporation (ca. 24 h). The powder was washed with pure water to remove unreacted compounds and by-products, obtaining the Tb-PSQ sample. In detail, a solid mixture of anhydrous terbium acetate (Tb(CH 3 COO) 3 ; 106 mg, 0.315 mmol for TbEu-PSQA and 45 mg, 0.134 mmol for TbEu-PSQB) and europium acetate (Eu(CH 3 COO) 3 ; 44 mg, 0.134 mmol for TbEu-PSQA and 104 mg, 0.316 mmol for TbEu-PSQB) was added to a solution of OctaTMA-POSS (POSS; 1 g, 0.451 mmol) in 20 mL of MilliQ ® water, previously adjusted to pH 5.5 by the addition of 1 M HCl solution. The solution was stirred for 10 min at room temperature (25 • C), and then it was transferred to an oven and heated at 80 • C until complete water evaporation. The final powder was washed with pure water in order to remove soluble reactants and by-products (POSS and lanthanide precursors).

Characterization
The washing of the three lanthanide-PSQs was carried out by redispersing the solid in water, followed by centrifugation (8000 rpm, 3 min); the supernatant was tested with the xylenol orange test in order to verify the presence of unreacted lanthanide precursors [58]. Briefly, an aliquot (200 µL) of supernatant was treated with 2 mL of xylenol orange solution, previously prepared by dissolving 3 mg of the chromophore in 250 mL of acetate buffer solution (50 mM, pH = 5.8) and characterized by UV-Visible spectroscopy. UV-Visible spectra in the 300-800 nm range were collected by using a Perkin Elmer Lambda 900 spectrometer (Waltham, MA, USA).
Energy dispersive X-ray (EDX) spectroscopy analyses were carried out on a FEG-SEM TESCAN S9000G instrument (Brno, Czech Republic) equipped with Oxford microanalysis detector Ultim Max and AZtec Software (version 6.0).
Fourier-transform infrared spectroscopy (FTIR) measurements were performed on a Nicolet 57,700 Spectrometer (Thermo Optics; Waltham, MA, USA), 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 (0.5 wt.%) were measured in the absorbance mode at room temperature.
X-ray powder diffractograms (XRPDs) were collected on unoriented ground powders on a Bruker D8 Advance Powder Diffractometer (Karlsruhe, Germany), operating in Bragg-Brentano geometry, with a Cu anode target equipped with a Ni filter (used as X-ray source) and with a Lynxeye XE-T high-resolution position-sensitive detector. Trio and Twin/Twin optics are mounted on the DaVinci.Design modular XRD system. 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 automatic 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 • -60 • 2θ range with a coupled 2θ−θ method, continuous PSD fast scan mode, time per step (rate or scan speed) of 0.100 s/step, and 2θ step size (or increment) of 0.01 • , with automatic synchronization of the air scatter (or anti-scatter) knife and slits and with the fixed illumination sample set at 15 mm.
Dynamic light scattering (DLS) analysis was carried out in aqueous solution by using a Malvern Zetasizer NanoZS instrument (Malvern, UK) equipped with a He-Ne laser (λ = 633 nm). The DLS measures were performed at neutral pH and 25 • C. The sample suspensions were prepared by dispersing 1 mg of solid compound in 1 mL of ultrapure water and sonicating at 59 kHz for 5 min. Surface ζ-potential was quantified in water at 25 • C and neutral pH.
Photoluminescence (PL) spectra (excitation and emission) of the compounds in the solid state were recorded on a Horiba Jobin-Yvon Model IBH FL-322 Fluorolog 3 Spectrometer (Northampton, UK) 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 (version 6.6, HORIBA Jobin Yvon IBH).

Results and Discussion
Two co-doped samples defined as TbEu-PSQA and TbEu-PSQB were prepared by coordination of anionic OctaTMA-POSS in the presence of two diverse molar ratios of Eu 3+ and Tb 3+ precursors (TbEu-PSQA: Tb 3+ = 0.7, Eu 3+ = 0.3; TbEu-PSQB: Tb 3+ = 0.3, Eu 3+ = 0.7). A 1:1 molar ratio of POSS and total lanthanide precursors was applied in both preparations. The general synthetic procedure is an optimized co-doped variant of our previously published synthesis regarding the preparation of mono-lanthanide doped PSQs with structural POSS units and Eu 3+ ions [24]. During the reaction, both Tb 3+ and Eu 3+ metal ions coordinate the Si-O − groups of anionic POSS molecules in different positions, thus forming a polycondensed network of POSS cages connected by lanthanide centers. A Tb 3+ -containing PSQ solid, hereafter named Tb-PSQ, was also prepared as reference following the same experimental procedure. When necessary, a comparison was made of the data of the co-doped materials with those of the starting OctaTMA-POSS molecule and of the metal-free PSQ obtained in the previous study [24], in which the POSS cages co-condensed through simple siloxane bonds. The reaction was performed in a slightly acidic aqueous medium (pH 5.5) to avoid metal hydroxide precipitation [59], for 10 min at room temperature. It was subsequently followed by an aging step at 80 • C to allow slow water evaporation. The obtained solids were washed with water to remove any unreacted species (both POSS and lanthanide precursors); after each washing step, the amount of free metal ions present in the supernatant of each sample was examined with UV-Visible spectroscopy in the presence of a complexometric probe (xylenol orange). The chromophore shows a different color when it is in interaction with free metal ions, and it is one of the most common 'indicators' for free lanthanide ion detection adopted in the literature [58]. The UV-Vis spectrum of the free dye in acetate buffer at pH = 5.8 is defined by two separate absorptions at 432 and 578 nm, which are attributed to π → π* electronic transitions ( Figure S1) [58]. The absorbance ratio of the two bands is sensitive to pH and to the presence of free lanthanide ions; thus, it is used to monitor the presence of free lanthanide ions in solution [58]. Tb-PSQ, TbEu-PSQA and TbEu-PSQB were washed until no perturbation of the intensity ratio of xylenol orange absorption bands was detected, confirming the absence of free Tb 3+ and/or Eu 3+ ions in the supernatant.
The actual loading of lanthanide dopants (in particular, the Tb 3+ to Eu 3+ ratio) was evaluated by EDX (Energy Dispersive X-ray) analysis ( Figure S2), confirming that the desired stoichiometry was achieved, as reported in Table 1. In the Tb-PSQ reference sample, only Tb 3+ was detected as the dopant, as expected. The structural features of TbEu-PSQ samples were analyzed by the X-ray powder diffraction technique and compared to those of the reference Tb-PSQ solid, the metal-free PSQ material [24] Figure 1, curve a) are completely lost after condensation of the anionic POSS cages. This indicates that the ordered organization of the silsesquioxane molecules is totally lost, suggesting an asymmetric and irregular re-organization of the POSS cages in polysilsesquioxane chains without the presence of ladder-like domains [60]. These observations are in agreement with what was already observed in the case of Eu-PSQ systems [24].
Further information on the local structure of the final samples was obtained through FTIR spectroscopy with the powders dispersed in KBr matrix (0.5 wt.%). FTIR spectra of Tb-PSQ, TbEu-PSQA and TbEu-PSQB are reported in Figure 2 and compared to the FTIR spectrum of OctaTMA-POSS. The FTIR spectrum of OctaTMA POSS ( Figure 2, curve a) presents two main absorptions at ca. 1100 and 1000 cm −1 of the asymmetric stretching of Si-O-Si and Si-O − groups of the POSS cage [61], and two narrow peaks at 1490 and 950 cm −1 due to bending modes of -CH 3 and symmetric stretching of the C-N bond of tetramethyl ammonium (TMA) groups, which are in interaction with O − atoms of the inorganic cage [62]. After complexation of the POSS units with Tb 3+ (Figure 2, curve c) or both Tb 3+ and Eu 3+ ions in different stoichiometric ratios (Figure 2, curves d and e), a coalescence of the bands at 1000 and 1100 cm −1 is observed, leading to a wide multicomponent absorption in the 1200-1000 cm −1 range of the spectrum. This feature can be ascribed to different inorganic architectures of the Si-O-Si groups of POSS cages after the polycondensation mechanism [24,32]. The same characteristics are found in the self-condensed reference PSQ material (Figure 2, curve b). Moreover, the signals of the TMA groups at 1490 and 950 cm −1 are almost completely eroded in the mono and co-doped samples, while some residuals are still visible in PSQ; this behavior is due to replacement of the organic groups in the presence of Tb 3+ or both Tb 3+ /Eu 3+ metal ions in the final polycondensed inorganic network, as previously observed in the literature for parent Eu-PSQ materials [24].  [61], and two narrow peaks at 1490 and 950 cm −1 due to bending modes of -CH3 and symmetric stretching of the C-N bond of tetramethyl ammonium (TMA) groups, which are in interaction with O − atoms of the inorganic cage [62]. After complexation of the POSS units with Tb 3+ (Figure 2, curve c) or both Tb 3+ and Eu 3+ ions in different stoichiometric ratios (Figure 2, curves d and e), a coalescence of the bands at 1000 and 1100 cm −1 is observed, leading to a wide multi-component absorption in the 1200-1000 cm −1 range of the spectrum. This feature can be ascribed to different inorganic architectures of the Si-O-Si groups of POSS cages after the polycondensation mechanism [24,32]. The same characteristics are found in the self-condensed reference PSQ material (Figure 2, curve b). Moreover, the signals of the TMA groups at 1490 and 950 cm −1 are almost completely eroded in the mono and co-doped samples, while some residuals are still visible in PSQ; this behavior is due to replacement of the organic groups in the presence of Tb 3+ or both Tb 3+ /Eu 3+ metal ions in the final polycondensed inorganic network, as previously observed in the literature for parent Eu-PSQ materials [24].  Further information on the local structure of the final samples was obtained through FTIR spectroscopy with the powders dispersed in KBr matrix (0.5 wt.%). FTIR spectra of Tb-PSQ, TbEu-PSQA and TbEu-PSQB are reported in Figure 2 and compared to the FTIR spectrum of OctaTMA-POSS. The FTIR spectrum of OctaTMA POSS (Figure 2, curve a) presents two main absorptions at ca. 1100 and 1000 cm −1 of the asymmetric stretching of Si-O-Si and Si-O − groups of the POSS cage [61], and two narrow peaks at 1490 and 950 cm −1 due to bending modes of -CH3 and symmetric stretching of the C-N bond of tetramethyl ammonium (TMA) groups, which are in interaction with O − atoms of the inorganic cage [62]. After complexation of the POSS units with Tb 3+ (Figure 2, curve c) or both Tb 3+ and Eu 3+ ions in different stoichiometric ratios (Figure 2, curves d and e), a coalescence of the bands at 1000 and 1100 cm −1 is observed, leading to a wide multi-component absorption in the 1200-1000 cm −1 range of the spectrum. This feature can be ascribed to different inorganic architectures of the Si-O-Si groups of POSS cages after the polycondensation mechanism [24,32]. The same characteristics are found in the self-condensed reference PSQ material ( Figure 2, curve b). Moreover, the signals of the TMA groups at 1490 and 950 cm −1 are almost completely eroded in the mono and co-doped samples, while some residuals are still visible in PSQ; this behavior is due to replacement of the organic groups in the presence of Tb 3+ or both Tb 3+ /Eu 3+ metal ions in the final polycondensed inorganic network, as previously observed in the literature for parent Eu-PSQ materials [24].  The aggregation state of co-doped TbEu-PSQA and TbEu-PSQB solids in water solution was analyzed by the dynamic light scattering technique. The suspensions of the materials were prepared by mixing 1 mg of each sample with 1 mL of pure water. DLS and surface ζ-potential analysis were carried out at neutral pH and 25 • C. The solid suspensions appear qualitatively homogeneous in the water medium. They are mainly composed by some aggregates of particles with distribution of hydrodynamic diameters centered at 220 and 345 nm for TbEu-PSQA and TbEu-PSQB, respectively, with a polydispersity index (PDI) of 0.74 and 0.51 ( Figure S3). The high PDI value registered in both cases suggests the presence of particles with varying sizes, as testified by the broadness of DLS distributions. The DLS measurements were found to be highly reproducible, and the data meet the analysis quality criteria. Furthermore, both co-doped lanthanide-PSQs present a negative surface charge density due to the presence of several free Si-O − groups after synthesis, as  Table S1, thus corroborating the FTIR data.
sions appear qualitatively homogeneous in the water medium. They are mainly composed by some aggregates of particles with distribution of hydrodynamic diameters centered at 220 and 345 nm for TbEu-PSQA and TbEu-PSQB, respectively, with a polydispersity index (PDI) of 0.74 and 0.51 ( Figure S3). The high PDI value registered in both cases suggests the presence of particles with varying sizes, as testified by the broadness of DLS distributions. The DLS measurements were found to be highly reproducible, and the data meet the analysis quality criteria. Furthermore, both co-doped lanthanide-PSQs present a negative surface charge density due to the presence of several free Si-O − groups after synthesis, as demonstrated by the values of surface ζ-potential reported in Table S1, thus corroborating the FTIR data.
tions [25,[63][64][65][66], respectively, with the most intense line centered at 615 nm, as typical of Eu 3+ -doped materials [41,57]. Interestingly, upon excitation at 270 nm, where only Tb 3+ is expected to be excited, in addition to the typical emission bands of Tb 3+ in the 480-550 nm range ( 5 D4 → 7 F6 at 488-495 nm and 5 D4 → 7 F5 at 542-548 nm) both the co-doped samples exhibit the emission features of Eu 3+ as previously discussed. This is a further strong confirmation of the metal-to-metal energy transfer occurring from Tb 3+ to Eu 3+ ions [25,[67][68][69]. The photoemission stability of the co-doped systems was assessed by monitoring the photoemission intensity at 615 nm for 1 h, upon continuous illumination under 270 nm light. Both samples exhibited good photostability, as the photoemission intensity after 1 h of illumination was found to be 94% of the initial value in the case of TbEu-PSQA and 90% of the initial value in the case of TbEu-PSQB ( Figure S5).
In order to gain insight into the effect of the Tb 3+ /Eu 3+ ratio on the overall color of the co-doped sample emission, the photometric characteristics of TbEu-PSQA and TbEu-PSQB were derived from their emission spectra, calculating the xy chromaticity coordinates as well as related RGB, Hex and color purity parameters according to the CIE 1931 color space [70], as reported in Figure 4B and Table S2. Both samples, when excited at 395 nm, are characterized by red emission (dots c and d in Figure 4B), with high color purity. Excitation at 270 nm generates emissions with lower color purity and characterized by more orange-red color (dots a and b in Figure 4B).
As a first general insight into the local environment experienced by Eu 3+ ions, the local geometry of Eu 3+ sites can be estimated by the asymmetry factor (R21), defined as the ratio between the integrated intensity of 5 D0-7 F2 and 5 D0-7 F1 electronic transitions [71,72]. The asymmetry factor was calculated for TbEu-PSQA and TbEu-PSQB on the emission spectra obtained upon excitation at 395 nm (in order to have the contribution by Eu 3+ emission only), considering the following spectral ranges for the integration: 602-638 nm ( 5 D0-7 F2 transition) and 581-602 nm ( 5 D0-7 F1 transition). The obtained R21 values were 2.13 for TbEu-PSQA and 2.36 in the case of TbEu-PSQB, indicating the heterogeneous nature of the coordination chemistry of Eu 3+ in both co-doped materials.
The number of water molecules present in the first coordination sphere of the Tb 3+ and Eu 3+ centers (q Tb and q Eu , respectively) in TbEu-PSQA and TbEu-PSQB were determined by measuring the experimental lifetime values (τ) of the transitions at 545 (λem Tb) The photoemission stability of the co-doped systems was assessed by monitoring the photoemission intensity at 615 nm for 1 h, upon continuous illumination under 270 nm light. Both samples exhibited good photostability, as the photoemission intensity after 1 h of illumination was found to be 94% of the initial value in the case of TbEu-PSQA and 90% of the initial value in the case of TbEu-PSQB ( Figure S5).
In order to gain insight into the effect of the Tb 3+ /Eu 3+ ratio on the overall color of the co-doped sample emission, the photometric characteristics of TbEu-PSQA and TbEu-PSQB were derived from their emission spectra, calculating the xy chromaticity coordinates as well as related RGB, Hex and color purity parameters according to the CIE 1931 color space [70], as reported in Figure 4B and Table S2. Both samples, when excited at 395 nm, are characterized by red emission (dots c and d in Figure 4B), with high color purity. Excitation at 270 nm generates emissions with lower color purity and characterized by more orange-red color (dots a and b in Figure 4B).
As a first general insight into the local environment experienced by Eu 3+ ions, the local geometry of Eu 3+ sites can be estimated by the asymmetry factor (R 21 ), defined as the ratio between the integrated intensity of 5 D 0 -7 F 2 and 5 D 0 -7 F 1 electronic transitions [71,72]. The asymmetry factor was calculated for TbEu-PSQA and TbEu-PSQB on the emission spectra obtained upon excitation at 395 nm (in order to have the contribution by Eu 3+ emission only), considering the following spectral ranges for the integration: 602-638 nm ( 5 D 0 -7 F 2 transition) and 581-602 nm ( 5 D 0 -7 F 1 transition). The obtained R 21 values were 2.13 for TbEu-PSQA and 2.36 in the case of TbEu-PSQB, indicating the heterogeneous nature of the coordination chemistry of Eu 3+ in both co-doped materials.
The number of water molecules present in the first coordination sphere of the Tb 3+ and Eu 3+ centers (q Tb and q Eu , respectively) in TbEu-PSQA and TbEu-PSQB were determined by measuring the experimental lifetime values (τ) of the transitions at 545 (λ em Tb) and 615 nm (λ em Eu) ( Table S3). The analyses were performed for the H 2 O and D 2 O dispersions of each solid under excitation at 370 nm with a SpectraLED source, taking advantage of the time-resolved fluorescence spectroscopy technique. The q Ln values were then derived from Equations (1) and (2) [73][74][75][76].
The photoluminescence decay curves of 5 D 0 and 5 D 4 excited states of Eu 3+ and Tb 3+ ( Figure S6), respectively, were fitted with a double-exponential function, suggesting the pres-ence of (at least) two types of different Tb 3+ /Eu 3+ sites in the disordered polysilsesquioxane networks [77]. On average, from the above equations, a value of ca. 1-2 water molecules in coordination with Eu 3+ or Tb 3+ ions were found (Table 2); the low hydration state of both ions corroborates the previous structural data on the formation of a complex and unordered metal-containing polysilsesquioxane framework. Lastly, the metal-to-metal energy transfer (MMET) process from Tb 3+ to Eu 3+ centers was investigated in more detail though measurements of τ values at the solid state of the donor Tb-PSQ (D) and donor-acceptor TbEu-PSQA and TbEu-PSQB (DA) systems (Table S3). The intensity decay curves of 5 D 4 excited state of the donor (or sensitizer) ion Tb 3+ were collected at a λ em of 545 nm under irradiation at 370 nm, in the presence (τ DA , Figure S7A) or absence (τ D , Figure S7B,C) of the acceptor (or activator) lanthanide (Eu 3+ ). A bi-exponential fitting was applied to the experimental curves, extrapolating average τ values for each measurement. The energy transfer efficiency (E EnT ) and rate (k EnT ) parameters were then calculated from Equations (3) and (4) [67,68,78].
The data analysis, reported in Table 2, showed an almost double energy transfer efficiency for the TbEu-PSQB sample, equal to ca. 60% versus a ca. 32% of the TbEu-PSQA solid. Moreover, the MMET rate appears to be faster for TbEu-PSQB, with a value equal to 1.24 × 10 3 s −1 in comparison to 4.05 × 10 2 s −1 for the TbEu-PSQA solid. This is in line with what is usually observed in energy transfer pairs, where to enhance the efficiency of the phenomenon the sensitizer concentration should be kept lower than the activator concentration [79]. The calculated parameters are comparable to other inorganic materials containing Tb 3+ and Eu 3+ ions in their structure as presented in the literature [67,69,78], such as the recent double-decker Tb 3+ /Eu 3+ -functionalized TetraSilanolPhenyl POSS materials proposed in Ref. [25].

Conclusions
In summary, novel luminescent amorphous PSQs co-doped with Tb 3+ and Eu 3+ ions directly integrated in the polysilsesquioxane matrix ( Figure S8) were synthesized with a simple and sustainable approach in aqueous conditions and at room temperature, followed by water evaporation. Two co-doped materials were prepared by tuning the Tb 3+ /Eu 3+ molar ratio, obtaining TbEu-PSQ solids characterized by high photostability and colors of the emitted light ranging from orange to deep red as a function of both the Tb 3+ /Eu 3+ molar ratio and the chosen excitation wavelength. Despite the disordered nature of the obtained solids, confirmed by structural and spectroscopic characterization, a good energy transfer from Tb 3+ to Eu 3+ was observed, with higher efficiency displayed when sensitizer concentration was lower than activator concentration, as expected. Due to the easiness of preparation and their tunable photoluminescence emission properties, these materials are promising candidates for several applications, ranging from bioimaging to technological applications such as sensors, ratiometric luminescence-based thermometers and lightemitting components in inorganic or hybrid optical devices.