Yb to Tb Cooperative Upconversion in Supramolecularly Assembled Complexes in a Solution

: The podand-type ligand L, based on a tertiary amine substituted by three pyridyl-6-phosphonic acid functions, forms hydrated complexes with Ln 3+ cations. The luminescence properties of the YbL complex were studied in D 2 O as a function of the pD and temperature. In basic conditions, increases in the luminescence quantum yield and the excited state lifetime of the Yb centered emission associated with the 2 F 5/2 → 2 F 7/2 transition were observed and attributed to a change in the hydration number from two water molecules in the ﬁrst coordination sphere of Yb at acidic pH to a single one in basic conditions. Upon the addition of TbCl 3 salts to a solution containing the YbL complex in D 2 O, heteropolynuclear Yb/Tb species formed, and excitation of the Yb at 980 nm resulted in the observation of the typical visible emission of Tb as a result of a cooperative upconversion (UC) photosensitization process. The UC was further evidenced by the quadratic dependence of the UC emission as a function of the laser power density.


Introduction
The upconversion (UC) phenomenon is an anti-Stokes process in which low-energy photons are converted into higher energy ones. This generally results in the observation of visible-to-NIR emission upon excitation in the NIR domain [1,2]. This process is particularly useful for applications in biology, as the excitation and emission wavelengths match the biological window where the background is low. It thus provides a low autofluorescence background and a high penetration depth [3][4][5].
Although it is mostly observed in the solid state [6][7][8], examples at the molecular scale in solutions [9][10][11][12] are flourishing thanks to the wide use of lanthanides, with their unique luminescence properties such as long luminescent lifetimes and line-like emission spectra [13,14]. The molecular strategy opens the door to new perspectives in biological applications, considering that it might resolve the drawbacks due to the biotoxicity and hazardous reproducibility of nanomaterials [15].
However, in contrast to solid state devices and nanoparticles [7], the design of the molecular devices in solution is still challenging as a result of important non-radiative deactivations due to energy transfer to OH, NH and CH oscillators [16,17]. These oscillators favor the decay of the intermediate excited state and inhibit the absorption of a second photon or of an energy quantum that is mandatory to reach and emit at a higher energy. One can limit these deactivations by (1) using deuterated oscillators as in D 2 O which have less impact on quenching of the intermediate state [18]; (2) protecting the cation with a polydentate ligand, thereby limiting the presence of the solvent in the first coordination polydentate ligand, thereby limiting the presence of the solvent in the first coordination sphere [19]; and (3) introducing bulky substituents which will keep the solvent molecules away from the NIR emitters [20]. Thanks to these strategies, the number of reported molecular upconverting devices has increased in the last decade.
Among the different mechanisms used to develop upconverting devices [1,21], ETU is reported to be the most efficient for the observation of UC at the molecular scale [22]. Its efficiency results from the possibility of having at least one sensitizer transferring its energy to an emitting acceptor center. Thanks to its unique 2 F5/2 → 2 F7/2 electronic transition at 980 nm, Yb(III) is the most common sensitizer used, and it allows for reaching the excited state of numerous other lanthanides such as Eu, Tb, Ho, Er or Tm, all of which can emit in the visible spectrum [11,[23][24][25]. Between those different pairs, the Yb/Tb one introduces a particular mechanism called cooperative sensitization [11,12,26,27]. This mechanism helps to overstep the absence of resonant electronic transition between Yb and Tb, but the efficiency is expected to be weak. In that case, the UC process is possible by exciting 2 Yb centers, which then transfer their energy to the excited state of the Tb ( 5 D4), allowing its emission in the visible spectrum (Scheme 1).
Considering our previous findings on the cooperative sensitization of Tb by Yb complexes [11,12], we anticipated that a Yb complex formed by complexation with the heptadentate ligand L (Scheme 1) [28] might offer opportunities to observe UC in D2O solutions. The spectroscopic properties of the YbL complex were studied in detail together with the interactions of the complex with Tb cations and were shown to allow the formation of heteropolynuclear Yb/Tb complexes that exhibit UC in D2O. Scheme 1. (left) Principle of the cooperative sensitization between Yb and Tb [27]. (right) Representation of ligand L and its dihydrated Yb complex (H atoms are omitted for clarity).

Materials and Methods
The (YbL) complex was prepared by following a previously described procedure [28]. Spectroscopic measurements were performed with 10 × 10 mm² quartz suprasil certified cells (Helma Analytics). The steady state emission spectra were recorded on an Edinburgh Instrument FLP920 spectrometer working with a continuous 450-W Xe Lamp and a red-sensitive R928 photomultiplier from Hamamatsu in a Peltier housing for visible detection (230-900 nm) or a Hamamatsu R5 509-72 photomultiplier for the Vis-NIR part. 330nm and 850-nm high-pass filters were used to eliminate the second-order artifacts when recording in the visible and NIR spectra, respectively. The steady state upconversion emission spectra were recorded on the FLP920 spectrometer using an MDL-3 III-980-nm Scheme 1. (left) Principle of the cooperative sensitization between Yb and Tb [27]. (right) Representation of ligand L and its dihydrated Yb complex (H atoms are omitted for clarity).
Considering our previous findings on the cooperative sensitization of Tb by Yb complexes [11,12], we anticipated that a Yb complex formed by complexation with the heptadentate ligand L (Scheme 1) [28] might offer opportunities to observe UC in D 2 O solutions. The spectroscopic properties of the YbL complex were studied in detail together with the interactions of the complex with Tb cations and were shown to allow the formation of heteropolynuclear Yb/Tb complexes that exhibit UC in D 2 O.

Materials and Methods
The (YbL) complex was prepared by following a previously described procedure [28]. Spectroscopic measurements were performed with 10 × 10 mm 2 quartz suprasil certified cells (Helma Analytics). The steady state emission spectra were recorded on an Edinburgh Instrument FLP920 spectrometer working with a continuous 450-W Xe Lamp and a redsensitive R928 photomultiplier from Hamamatsu in a Peltier housing for visible detection (230-900 nm) or a Hamamatsu R5 509-72 photomultiplier for the Vis-NIR part. 330-nm and 850-nm high-pass filters were used to eliminate the second-order artifacts when recording in the visible and NIR spectra, respectively. The steady state upconversion emission spectra were recorded on the FLP920 spectrometer using an MDL-3 III-980-nm 2-W laser (CNI Laser) with a 850-nm high-pass filter between the source and the sample to remove any potential visible excitation. The spectra were recorded using the Peltier-cooled R928 photomultiplier from Hamamatsu, using a fully opened emission slit width (20 nm) by accumulation of 10 spectra at 1 point/nm, with a dwell time of 0.1 s per nm. The pD value was adjusted with a solution of NaOD and a solution of DCl and was calculated with use of the equation of Mikkelsen (pD = pH + 0.4) [29].
The phosphorescence lifetimes were measured with the same instrument working in the multi-channel spectroscopy mode and using a Xenon flash lamp as the excitation source. Errors for the luminescence lifetimes were estimated to be ±10%.
The upconversion quantum yields were measured by following the published procedures [6,11] with a diluted solution (optical density < 0.05) using [TbGlu(H 2 O)]Na as a reference [30].
Low-temperature measurements were performed with a Mercury iTC thermocryostat equipped with a HiCUBE turbopump. The solution was prepared with 10% glycerol in a Tris buffer (10 mM, pH 7).
Power-dependent steady state and time-resolved UC luminescence measurements were performed in quartz cuvettes (Hellma QS Semi-Micro Cells). The continuous-wave laser beam (λ = 975 nm) of a single-mode fiber-coupled laser diode was collimated (Thorlabs, TC12APC-980, Ottawa, Canada) and focused in the cuvette by a +100-mm focal length lens. From the measured beam waist radius (w = 30.6 µm), we estimated the average excitation intensity in the beam waist to be 8.1 kW/cm 2 at the maximum laser power. The emission of the sample was collected by a 10 × 0.25 numerical aperture objective and focused on an avalanche photodiode (Excelitas, SPCM-AQRH-16, Wiesbaden, germany). The scattered laser light was removed by a low-pass filter (Chroma, ET750-SP-2P, Olching, Germany) and a band-pass filter (Semrock, FF01-550/88-25, Vanves, France). For the steady state measurements (emitted intensity as a function of the excitation intensity), the output power of the laser was adjusted electronically (by varying the current intensity sent to the diode) and optically by using neutral density filters (Thorlabs) and then systematically measured by a power meter (Newport, 1917R and Ophir, 30A-BB-18). The signal of the photodiode was collected by a computer equipped with a multifunction input/output card (National Instruments multifunction board, PCIe 6711). During the time-resolved measurements, the laser output was modulated electronically at 32 Hz with a 12% duty cycle square waveform by a multifunction input/output card (National Instruments multifunction board, PCIe 6361). The single-photon events detected by the photodiode and the synchronization signal of the laser were recorded by a time-correlated single-photon counting board (Becker & Hickl, SPC-830, Berlin, Germany). SPCM 9.75 software (Becker & Hickl) was used to both record the data and build the photon distribution over time. Experimental luminescence decays were fitted by using the coupled differential rate equations model discussed in the main text in Matlab 2017b.

Spectroscopic Properties of the YbL Complex
The YbL complex was prepared as previously described [28]. Considering the peculiar hydration change observed for the parent Gd complex [31], the spectroscopic properties of the Yb analog were first studied as a function of the pD in the D 2 O solution. Figure 1 shows the evolution of the emission spectra and of the excited state lifetime of the YbL complex at various pD values. When moving from acidic conditions to basic conditions, an increase in the Yb intensity and lifetime was observed. This phenomenon can be explained by a change in the hydration state of the complex as a function of the acidity of the medium. At pD values below six, the complex contained two water molecules in the first coordination sphere, which was completed by the coordination of the heptadentate ligand, forming the [(YbL)(H 2 O) 2 ] species. Above a pD of six, deprotonation of one of the phosphonate functions resulted in the hydrogen bridging of two phosphonate functions with a concomitant overall contraction of the ligand around the Ln atom, resulting in the repulsion of one of the two water molecules and the formation of the [(YbL)(H 2 O)] species.
Due to the strong quenching effect of water [16,17], the displacement of one water molecule led to a large increase in the luminescence intensity and lifetime. In addition to the main transition observed at 975 nm at a low pD, a hypsochromic shift of the main band toward 970 nm was observed, as well as new bands appearing at ca. 1000 nm and 1040 nm when increasing the pD, together with a shoulder on the high energy tail at ca. 960 nm. While the former bands were ascribed to the 2 F 5/2 → 2 F 7/2 transitions of Yb with M J splitting of the ground and excited states [32], the latter was ascribed to hot bands of Yb, as evidenced by the variable temperature measurements (see below) [33]. ry 2021, 3, FOR PEER REVIEW 4 with a concomitant overall contraction of the ligand around the Ln atom, resulting in the repulsion of one of the two water molecules and the formation of the [(YbL)(H2O)] species. Due to the strong quenching effect of water [16,17], the displacement of one water molecule led to a large increase in the luminescence intensity and lifetime. In addition to the main transition observed at 975 nm at a low pD, a hypsochromic shift of the main band toward 970 nm was observed, as well as new bands appearing at ca. 1000 nm and 1040 nm when increasing the pD, together with a shoulder on the high energy tail at ca. 960 nm. While the former bands were ascribed to the 2 F5/2 → 2 F7/2 transitions of Yb with MJ splitting of the ground and excited states [32], the latter was ascribed to hot bands of Yb, as evidenced by the variable temperature measurements (see below) [33].

Determination of the Yb-Centered Quantum Yield
In order to obtain more insights into the photophysical properties of the Yb complex, the intrinsic luminescence quantum yield of YbL in D2O was determined using Equation (1). τobs corresponds to the observed excited state lifetime measured upon excitation of the ligand, and it amounts to 16.7 μs. τrad is the radiative lifetime of Yb in the complex, which could be determined using the modified Einstein Equation (2) [34][35][36]. In this second equation, NA is Avogadro's number, c is the speed of light in a vacuum (in cm.s −1 ), ῦ is the barycenter of the transition in cm −1 , n is the refractive index of the medium ( = 1.33), ε(ῦ) is the absorption spectrum of the transition (in M −1 .cm −1 ) and gl and gu are related to the degeneracies of the ground and excited states, respectively, and are equal to 2J + 1, where J = 7/2 for gl and J = 5/2 for gu. A value of 1.13 ms was calculated for τrad, which is generally between 0.5 and 1.3 ms in the solution [21,37]. Thus, an intrinsic Yb quantum yield of 1.5% could be calculated at a pD of 7.5: The efficiency of sensitization ηsens was also determined using Equation (3) [36]. is the luminescent quantum yield upon ligand excitation, and it was equal to 1.2% [28]. Compared with other complexes [38,39], the value of 0.8 obtained for ηsens in YbL was higher and pointed to a remarkable transfer from the ligand triplet state to the Yb center:

Determination of the Yb-Centered Quantum Yield
In order to obtain more insights into the photophysical properties of the Yb complex, the intrinsic luminescence quantum yield of YbL in D 2 O was determined using Equation (1). τ obs corresponds to the observed excited state lifetime measured upon excitation of the ligand, and it amounts to 16.7 µs. τ rad is the radiative lifetime of Yb in the complex, which could be determined using the modified Einstein Equation (2) [34][35][36]. In this second equation, N A is Avogadro's number, c is the speed of light in a vacuum (in cm s −1 ),υ is the barycenter of the transition in cm −1 , n is the refractive index of the medium (n D 2 O = 1.33), ε(υ) is the absorption spectrum of the transition (in M −1 cm −1 ) and g l and g u are related to the degeneracies of the ground and excited states, respectively, and are equal to 2J + 1, where J = 7/2 for g l and J = 5/2 for g u . A value of 1.13 ms was calculated for τ rad , which is generally between 0.5 and 1.3 ms in the solution [21,37]. Thus, an intrinsic Yb quantum yield of 1.5% could be calculated at a pD of 7.5: The efficiency of sensitization η sens was also determined using Equation (3) [36]. φ Yb L is the luminescent quantum yield upon ligand excitation, and it was equal to 1.2% [28]. Compared with other complexes [38,39], the value of 0.8 obtained for η sens in YbL was higher and pointed to a remarkable transfer from the ligand triplet state to the Yb center:

Low Temperature Measurements
To further characterize this complex, the Yb emission was monitored as a function of the temperature by excitation of the ligand to observe the evolution of the Yb signal in the 77-300 K range in D 2 O containing 12% glycerol. As shown in Figure 2, three sharp bands were observed, which could be ascribed to the 2 F 5/2 → 2 F 7/2 transitions of Yb with M J splitting of the ground and excited states [32]. The lifetime was also measured (Table 1), showing a gradual decrease upon heating. The higher lifetime values at a low temperature can be explained by the minimization of the non-radiative transitions due to the solvent. = (3)

Low Temperature Measurements
To further characterize this complex, the Yb emission was monitored as a function of the temperature by excitation of the ligand to observe the evolution of the Yb signal in the 77-300 K range in D2O containing 12% glycerol. As shown in Figure 2, three sharp bands were observed, which could be ascribed to the 2 F5/2 → 2 F7/2 transitions of Yb with MJ splitting of the ground and excited states [32]. The lifetime was also measured (Table 1), showing a gradual decrease upon heating. The higher lifetime values at a low temperature can be explained by the minimization of the non-radiative transitions due to the solvent.

Upconversion Experiments
In order to check the possibility of a Yb-to-Tb UC process, titration was performed in D2O in which the Yb complex was titrated by additions of aliquots of a solution of TbCl3 salts. The emission spectrum was recorded in the visible domain upon excitation at 980 nm in the 2 F5/2 ← 2 F7/2 absorption band of Yb ( Figure 3). After the first additions, characteristic Tb-centered emission bands emerged at 485, 545, 580 and 620 nm, corresponding to the 5 D4 → 7 FJ (with J = 6, 5, 4 and 3, respectively) transitions of Tb. The signal then increased up to 1.5 eq and then formed a plateau right after. To confirm the UC process, a Log I vs. Log P plot was performed by monitoring the 5 D4 → 7 F5 transition as a function of the power of the laser at 980 nm ( Figure 4) for a mixture containing a 1.4 equivalent of Tb added to the Yb complex. A value close to 2 (1.92 found) indicated the involvement of two photons in the process and confirmed the UC phenomenon. The UC quantum yield was determined using a protocol described in the literature [6,11]. A value of 2.6 × 10 −8 was obtained (P = 10.8 W.cm −2 ), which was of the same order of magnitude as those reported for similar compounds [11].

Upconversion Experiments
In order to check the possibility of a Yb-to-Tb UC process, titration was performed in D 2 O in which the Yb complex was titrated by additions of aliquots of a solution of TbCl 3 salts. The emission spectrum was recorded in the visible domain upon excitation at 980 nm in the 2 F 5/2 ← 2 F 7/2 absorption band of Yb ( Figure 3). After the first additions, characteristic Tb-centered emission bands emerged at 485, 545, 580 and 620 nm, corresponding to the 5 D 4 → 7 F J (with J = 6, 5, 4 and 3, respectively) transitions of Tb. The signal then increased up to 1.5 eq and then formed a plateau right after. To confirm the UC process, a Log I vs. Log P plot was performed by monitoring the 5 D 4 → 7 F 5 transition as a function of the power of the laser at 980 nm ( Figure 4) for a mixture containing a 1.4 equivalent of Tb added to the Yb complex. A value close to 2 (1.92 found) indicated the involvement of two photons in the process and confirmed the UC phenomenon. The UC quantum yield was determined using a protocol described in the literature [6,11]. A value of 2.6 × 10 −8 was obtained (P = 10.8 W cm −2 ), which was of the same order of magnitude as those reported for similar compounds [11]. Chemistry 2021, 3, FOR PEER REVIEW 6  The data of the titration were analyzed by a nonlinear least-squares method using Specfit software [40,41]. The analysis pointed to the formation of three emitting species, which were ascribed to [(YbL)2Tbx] complexes with x = 1, 2 or 3. Their calculated distributions are presented in the inset of Figure 3. These compositions were consistent with previous spectroscopy titrations of the ligand by the addition of Ln(III) (Ln = Eu or Tb) [28]. All three species were able to generate UC, but the relative efficiencies, which were obtained thanks to the fitting procedure, pointed to the UC efficiencies of [(YbL)2Tb2] and [(YbL)2Tb3] being 1.84 and 5.6 times higher than that of [(YbL)2Tb], respectively.
To get deeper insight into the UC mechanism, time-resolved measurements were performed with a modulated 975-nm laser diode generating excitation pulses with a square temporal profile. As shown in Figure 5, 3-4 milliseconds were required for the rise  The data of the titration were analyzed by a nonlinear least-squares method using Specfit software [40,41]. The analysis pointed to the formation of three emitting species, which were ascribed to [(YbL)2Tbx] complexes with x = 1, 2 or 3. Their calculated distributions are presented in the inset of Figure 3. These compositions were consistent with previous spectroscopy titrations of the ligand by the addition of Ln(III) (Ln = Eu or Tb) [28]. All three species were able to generate UC, but the relative efficiencies, which were obtained thanks to the fitting procedure, pointed to the UC efficiencies of [(YbL)2Tb2] and [(YbL)2Tb3] being 1.84 and 5.6 times higher than that of [(YbL)2Tb], respectively.
To get deeper insight into the UC mechanism, time-resolved measurements were performed with a modulated 975-nm laser diode generating excitation pulses with a square temporal profile. As shown in Figure 5, 3-4 milliseconds were required for the rise The data of the titration were analyzed by a nonlinear least-squares method using Specfit software [40,41]. The analysis pointed to the formation of three emitting species, which were ascribed to [(YbL) 2 Tb x ] complexes with x = 1, 2 or 3. Their calculated distributions are presented in the inset of Figure 3. These compositions were consistent with previous spectroscopy titrations of the ligand by the addition of Ln(III) (Ln = Eu or Tb) [28]. All three species were able to generate UC, but the relative efficiencies, which were obtained thanks to the fitting procedure, pointed to the UC efficiencies of [(YbL) 2 Tb 2 ] and [(YbL) 2 Tb 3 ] being 1.84 and 5.6 times higher than that of [(YbL) 2 Tb], respectively.
To get deeper insight into the UC mechanism, time-resolved measurements were performed with a modulated 975-nm laser diode generating excitation pulses with a square temporal profile. As shown in Figure 5, 3-4 milliseconds were required for the rise of the UC emission before the steady state was reached. This result indicated a slow kinetic step in the UC process which was related to the energy transfer occurring throughout the cooperative energy transfer UC process [27] (Scheme 2). The fit of the rising and decaying signals was made by using the model described by Nonat et al. [11] by updating the fitting data with the initial parameters (τ Yb and ε Yb ). In this model, the presence of two species was assumed as a result of the presence of the different complexes in the solution. In our case, the addition of the 1.4 equivalent of Tb per Yb complex pointed to the presence of [(YbL) 2 Tb 2 ] and [(YbL) 2 Tb 3 ] as the dominant species in the solution. The upconversion lifetimes (i.e., the Tb luminescence lifetimes upon excitation at 980 nm) were determined to be 0.38 ± 0.04 ms and 1.86 ± 0.2 ms. The shorter Tb lifetime may be attributed to a Tb cation more exposed to the solvent molecules (with a higher hydration number). The associated UC rate constants k UC were determined to be of 755 and 355 s −1 . In comparison with the literature, the UC rate constants obtained were somehow smaller but consistent with those reported in the literature [11,27,42].
Chemistry 2021, 3, FOR PEER REVIEW 7 of the UC emission before the steady state was reached. This result indicated a slow kinetic step in the UC process which was related to the energy transfer occurring throughout the cooperative energy transfer UC process [27] (Scheme 2). The fit of the rising and decaying signals was made by using the model described by Nonat et al. [11] by updating the fitting data with the initial parameters (τYb and εYb). In this model, the presence of two species was assumed as a result of the presence of the different complexes in the solution. In our case, the addition of the 1.4 equivalent of Tb per Yb complex pointed to the presence of [(YbL)2Tb2] and [(YbL)2Tb3] as the dominant species in the solution. The upconversion lifetimes (i.e., the Tb luminescence lifetimes upon excitation at 980 nm) were determined to be 0.38 ± 0.04 ms and 1.86 ± 0.2 ms. The shorter Tb lifetime may be attributed to a Tb cation more exposed to the solvent molecules (with a higher hydration number). The associated UC rate constants kUC were determined to be of 755 and 355 s −1 . In comparison with the literature, the UC rate constants obtained were somehow smaller but consistent with those reported in the literature [11,27,42]. The energy transfer efficiency of the UC process ηUC was then calculated using Equation (4) [11]: where kUC represents the UC energy transfer rate. The radiative rate constant kr of the Yb complex is the inverse of τr (1.13 ms) and is equal to 885 s −1 . The non-radiative rate constant knr is obtained from the intrinsic quantum yield by = + Using = 1.5%, a value of 58 100 s −1 was obtained for knr. The energy transfer efficiency of the UC process ηUC was then calculated to be equal to 1.8%.

Conclusions
Our in-depth study of the YbL complex in D2O showed a dependence on the hydration state as a function of the pD, resulting in the presence of two deuterated water mole-Scheme 2. Energy level diagram of the cooperative UC sensitization process in the [(YbL) 2 Tb x ] complex (x = 1-3). Symbols * and ** denote singly and doubly excited states respectively.
The energy transfer efficiency of the UC process η UC was then calculated using Equation (4) [11]: η UC = ∑ k UC ∑ k UC + k r + k nr (4) where k UC represents the UC energy transfer rate. The radiative rate constant k r of the Yb complex is the inverse of τ r (1.13 ms) and is equal to 885 s −1 . The non-radiative rate constant k nr is obtained from the intrinsic quantum yield Φ Yb Yb by Φ Yb Yb = k r k r + k nr (5) Using Φ Yb Yb = 1.5%, a value of 58,100 s −1 was obtained for k nr . The energy transfer efficiency of the UC process η UC was then calculated to be equal to 1.8%.

Conclusions
Our in-depth study of the YbL complex in D 2 O showed a dependence on the hydration state as a function of the pD, resulting in the presence of two deuterated water molecules in the first coordination sphere of the complex at low pD values and a single one around the physiological pD. The dehydration was accompanied by a large increase in both the luminescence intensity and the excited state lifetime of the Yb emission. An increase in Yb emission was also observed at low temperatures as a result of decreased non-radiative transitions due to water molecules.
When the Yb complex was titrated by Tb 3+ salts in a D 2 O solution, heteropolynuclear [(YbL) 2 Tb x ] (x = 1-3) complexes formed, and direct excitation of the Yb atom at 980 nm in the heteronuclear complexes resulted in emission of the Tb atoms in the visible domain as a result of a cooperative sensitization UC mechanism. The energy transfer efficiency of the UC process was determined to be 1.8%, while the overall UC quantum yield value of 2.6 × 10 −8 in D 2 O was obtained for an excitation power density of 10.8 W cm −2 . Although weak, this value was in line with previous results in the literature [11], despite the presence of water molecules in the first coordination sphere of Yb. The improvement of the protection of Yb toward water de-excitation paves the way to even better UC devices for luminescence bioanalytical applications.