Brightly Luminescent (TbxLu1−x)2bdc3·nH2O MOFs: Effect of Synthesis Conditions on Structure and Luminescent Properties

Luminescent, heterometallic terbium(III)–lutetium(III) terephthalate metal-organic frameworks (MOFs) were synthesized via direct reaction between aqueous solutions of disodium terephthalate and nitrates of corresponding lanthanides by using two methods: synthesis from diluted and concentrated solutions. For (TbxLu1−x)2bdc3·nH2O MOFs (bdc = 1,4-benzenedicarboxylate) containing more than 30 at. % of Tb3+, only one crystalline phase was formed: Ln2bdc3·4H2O. At lower Tb3+ concentrations, MOFs crystallized as the mixture of Ln2bdc3·4H2O and Ln2bdc3·10H2O (diluted solutions) or Ln2bdc3 (concentrated solutions). All synthesized samples that contained Tb3+ ions demonstrated bright green luminescence upon excitation into the 1ππ* excited state of terephthalate ions. The photoluminescence quantum yields (PLQY) of the compounds corresponding to the Ln2bdc3 crystalline phase were significantly larger than for Ln2bdc3·4H2O and Ln2bdc3·10H2O phases due to absence of quenching from water molecules possessing high-energy O-H vibrational modes. One of the synthesized materials, namely, (Tb0.1Lu0.9)2bdc3·1.4H2O, had one of the highest PLQY among Tb-based MOFs, 95%.


Introduction
Rare earth elements (REE)-based compounds are promising materials for applications in medicine [1,2], sensors [3,4], catalysis [5], anticounterfeiting [6,7], bioimaging [8,9], photovoltaic systems [10][11][12], etc. due to their unique optical and magnetic properties. The positions of the narrow emission bands of REE ions attributed to f-f transitions strongly depend only on the type of lanthanide ions. This property allows photoluminescence color tuning of the REE-containing materials [13]. Usually, purely inorganic compounds of lanthanides demonstrate relatively weak photoluminescence intensity because they possess extremely low extinction coefficients due to the forbidden nature of f-f transitions, which makes the direct excitation of ions inefficient. This issue can be overcome using the so-called "antenna effect". The antenna effect is realized in some metal-organic compounds in which the light is absorbed by the chromophore group of the organic ligand followed by the energy transfer to the lanthanide ion, which then emits the light corresponding to the characteristic f-f transitions [14][15][16]. The typical ligands used in REE antenna complexes are calixarenes [17], dipicolinic acid [18], tris-bipyridines [19], and carboxylates including terephthalates [20][21][22]. Lanthanide-based metal-organic frameworks (MOFs) combine the optical properties of REE-based materials with the topological features of MOFs, which

PXRD Results and Analysis
All of the syntheses, whatever the chosen method, yielded crystalline samples. In Figure 1a,b, the PXRD patterns of the (Tb x Lu 1−x ) 2 bdc 3 ·nH 2 O (x = 0-1) MOFs synthesized from diluted and concentrated solutions are shown. We found that all compounds with concentration of terbium (III) ions 30 at. % and more were isostructural to the Ln 2 bdc 3 ·4H 2 O crystalline phase (Ln = Ce − Yb) [40], and additional peaks were not observed. At low Tb 3+ concentrations between 0 and 5 at. %, the positions of the reflexes in the PXRD patterns were different from those of the Ln 2 bdc 3 ·4H 2 O and depended on the concentration of the initial reagents (Na 2 bdc, TbCl 3 , and LuCl 3 ). Thus, diffraction patterns corresponded to Ln 2 bdc 3 ·10H 2 O [41] and Ln 2 bdc 3 [42] for compounds synthesized from diluted and concentrated solutions, respectively (see Section 3). At intermediate Tb 3+ concentrations between 10 and 25 at. %, the binary mixtures of the aforementioned crystalline phases were precipitated, namely, Ln 2 bdc 3 ·4H 2 O + Ln 2 bdc 3 ·10H 2 O and Ln 2 bdc 3 ·4H 2 O + Ln 2 bdc 3 for the (Tb x Lu 1−x ) 2 bdc 3 ·nH 2 O MOFs obtained from diluted and concentrated solutions, respectively.
The effect of molar ratio of reagents taken for synthesis on MOFs structure was observed previously (see, for example, [43,44]), but was not explained properly. It is generally accepted that MOFs are formed stepwise from the secondary building units (SBUs), metal-ligand oligomers that replicates themselves to form MOF-like structures [45]. 10  The effect of molar ratio of reagents taken for synthesis on MOFs structure was observed previously (see, for example, [43,44]), but was not explained properly. It is generally accepted that MOFs are formed stepwise from the secondary building units (SBUs), metalligand oligomers that replicates themselves to form MOF-like structures [45]. Therefore, the final structure of coordination polymer allows us to assume the possible reasons behind the differences between compounds of two synthesized series. The crystal structures of Ln 2 bdc 3 ·4H 2 O, Ln 2 bdc 3 , and Ln 2 bdc 3 ·10H 2 O are shown in Figure 2. In Ln 2 bdc 3 ·4H 2 O lanthanide (III), ions were bound to two water molecules and six terephthalate ions through oxygen atoms, where Ln 3+ coordination number (CN) is equal to 8. In Ln 2 bdc 3 structures, Ln 3+ ions with CN = 7 coordinated solely to oxygens of terephthalate ions. In the Ln 2 bdc 3 ·10H 2 O structure, the metal center coordination number was also equal to 7, but four coordination sites were occupied by water molecules. Two water molecules per one formula unit in the Ln 2 bdc 3 ·10H 2 O structure were located in interplanar channels.  [47,48]. Therefore, we expected that terbium ions will reveal larger coordination numbers than lutetium ion in our MOFs. Indeed, the analysis of the aforementioned structures ( Figure 2) revealed that lanthanide ions have coordination numbers of seven in Ln 2 bdc 3 and Ln 2 bdc 3 ·10H 2 O, which are formed in pure lutetium terephthalate, and in (Tb x Lu 1−x ) 2 bdc 3 ·nH 2 O at high Lu 3+ content levels. In Ln 2 bdc 3 ·4H 2 O, which is formed in pure terbium terephthalate and in mixed Tb-Lu terephthalates at high Tb 3+ content levels, the coordination number of Ln 3+ ions is equal to eight. The reasons that can explain the difference between structures of lutetium terephthalates synthesized from diluted (Lu 2 bdc 3 ·10H 2 O) and concentrated (Lu 2 bdc 3 ) solutions are unclear. We assume the key factor that affects the structure of precipitated MOF is the fractional distribution of initially formed metastable complexes [Ln(H 2 O) x (bdc) y ] 3−2y [49]. These complexes then aggregate into SBUs, which further form MOFs. Apparently, in concentrated solutions, complexes have higher Lu 3+ :bdc 2− ratios (1:2 or 1:3) than in diluted solution (1:1). Therefore, further formed SBUs and MOFs of Lu 2 bdc 3 had larger number of coordinated oxygens of terephthalate ligands than Lu 2 bdc 3 ·10H 2 O.

Thermogravimetric Analysis (TGA)
The thermal behavior of the selected compounds (TbxLu1-x)2bdc3‧nH2O (x = 0-1) was studied by using the thermogravimetric method (TGA). The TGA curves of the MOFs obtained from diluted and concentrated solutions were recorded in the temperature range of 35-200 °C (Figure 3). When heated, the lanthanide terephthalates decomposed in two common steps: (i) dehydration of the compounds, resulting in formation of Ln2bdc3 at about 100-200 °C , and (ii) the structural decomposition of coordination polymers [42]. The In our previous work, we reported the similar behavior of the (Eu x Lu 1−x ) 2 bdc 3 ·nH 2 O MOFs obtained from concentrated solutions [39]. We found that phase transition occurred at significantly lower Eu 3+ concentrations ( [50]. The structure with CN = 7 (Ln 2 bdc 3 ) is more advantageous for Tb 3+ than for Eu 3+ , which forms a structure Ln 2 bdc 3 ·4H 2 O with larger CN = 8 beginning at 6 at. % of Eu 3+ ions.

Thermogravimetric Analysis (TGA)
The thermal behavior of the selected compounds (Tb x Lu 1−x ) 2 bdc 3 ·nH 2 O (x = 0-1) was studied by using the thermogravimetric method (TGA). The TGA curves of the MOFs obtained from diluted and concentrated solutions were recorded in the temperature range of 35-200 • C ( Figure 3). When heated, the lanthanide terephthalates decomposed in two common steps: (i) dehydration of the compounds, resulting in formation of Ln 2 bdc 3 at about 100-200 • C, and (ii) the structural decomposition of coordination polymers [42]. The observed weight loss at 100-190 • C for all measured samples corresponded to the dehydration step; therefore, the analysis of the TGA curves allowed us to calculate the average numbers of water molecules in the coordination polymers (Tb x Lu 1−x ) 2 bdc 3 ·nH 2 O. , and Lu2bdc3‧10H2O (c) generated from the single-crystal diffraction data [40][41][42].

Thermogravimetric Analysis (TGA)
The thermal behavior of the selected compounds (TbxLu1-x)2bdc3‧nH2O (x = 0-1) was studied by using the thermogravimetric method (TGA). The TGA curves of the MOFs obtained from diluted and concentrated solutions were recorded in the temperature range of 35-200 °C ( Figure 3). When heated, the lanthanide terephthalates decomposed in two common steps: (i) dehydration of the compounds, resulting in formation of Ln2bdc3 at about 100-200 °C , and (ii) the structural decomposition of coordination polymers [42]. The observed weight loss at 100-190 °C for all measured samples corresponded to the dehydration step; therefore, the analysis of the TGA curves allowed us to calculate the average numbers of water molecules in the coordination polymers (TbxLu1-x)2bdc3‧nH2O. The number of water molecules per one formula unit N(H2O) for all selected compounds as function of Tb 3+ concentration is shown in Figure 4a,b for samples synthesized from diluted and concentrated solutions, respectively. The number of water molecules per one formula unit is equal to four for pure terbium terephthalate (100 at. % Tb 3+ ) in both series. The N(H2O) value increases from 4 to 10 upon the substitution of Tb 3+ by Lu 3+ ions (decreasing of Tb 3+ content) in (TbxLu1-x)2bdc3‧nH2O MOFs obtained from the diluted solutions ( Figure 4a)  The number of water molecules per one formula unit N(H 2 O) for all selected compounds as function of Tb 3+ concentration is shown in Figure 4a,b for samples synthesized from diluted and concentrated solutions, respectively. The number of water molecules per one formula unit is equal to four for pure terbium terephthalate (100 at. % Tb 3+ ) in both series. The N(H 2 O) value increases from 4 to 10 upon the substitution of Tb 3+ by Lu 3+ ions (decreasing of Tb 3+ content) in (Tb x Lu 1−x ) 2 bdc 3 ·nH 2 O MOFs obtained from the diluted solutions ( Figure 4a). However, for MOFs synthesized from the diluted solutions, the number of water molecules decreases from four to zero upon Tb 3+ concentration decrease ( Figure 4b). These facts are in agreement with the XRD data, in which we observed phase transitions from Ln 2 bdc 3 ·4H 2 O either to Ln 2 bdc 3 ·10H 2 O or Ln 2 bdc 3 upon the decrease of Tb 3+ content. Summarizing the TGA and XRD data, we estimated the molar fraction of each coexisting crystalline phase (Figure 4c,d). The molar fraction of Ln 2 bdc 3 ·4H 2 O increased from 0 to 30 at. % Tb 3+ for two synthesized series of MOFs (Tb x Lu 1−x ) 2 bdc 3 ·nH 2 O, and simultaneously, the molar fraction of the second coexisting phase decreased. In the Tb 3+ concentration range of 30-100 at. %, only Ln 2 bdc 3 ·4H 2 O was present in both series.

Luminescent Properties
Aromatic carboxylate ions, especially benzene dicarboxylates, are typical linkers for the luminescent antenna MOF design [15,20] due to the efficient sensitization of lanthanide luminescence. The sensitization mechanism consists of several steps. Upon UV-photon absorption, the linker is promoted into the S n ( 1 ππ*) exited electronic state, which is followed by the fast internal conversion to S 1 ( 1 ππ*). Due to the heavy atom effect, the S 1 state of the linker efficiently undergoes intersystem crossing to the T 1 ( 3 ππ*) triplet electronic excited state [32]. If the T 1 state of organic linker lies slightly higher in energy than one of the levels of activator lanthanide ion, then the energy is efficiently transferred to the lanthanide ion and followed by the photon emission corresponding to the f-f transition. Thus, terbium terephthalate, Tb 2 bdc 3 ·4H 2 O, demonstrates a relatively high Tb 3+ photoluminescence quantum yield (43-55% [32,42,51]) upon UV-excitation into terephthalate ions due to the fact that the T 1 state of the terephthalate ion (E(T 1 ) ≈ 20,400-20,650 cm −1 [32] for bdc 2-) lies only 50-300 cm −1 above the 5 D 4 level of the Tb 3+ ion (E( 5 D 4 ) ≈ 20,350 cm −1 [52]).
Molecules 2023, 28, x FOR PEER REVIEW 5 of 13 water molecules decreases from four to zero upon Tb 3+ concentration decrease (Figure 4b).
These facts are in agreement with the XRD data, in which we observed phase transitions from Ln2bdc3‧4H2O either to Ln2bdc3‧10H2O or Ln2bdc3 upon the decrease of Tb 3+ content. Summarizing the TGA and XRD data, we estimated the molar fraction of each coexisting crystalline phase (Figure 4c,d). The molar fraction of Ln2bdc3‧4H2O increased from 0 to 30 at. % Tb 3+ for two synthesized series of MOFs (TbxLu1-x)2bdc3‧nH2O, and simultaneously, the molar fraction of the second coexisting phase decreased. In the Tb 3+ concentration range of 30-100 at. %, only Ln2bdc3‧4H2O was present in both series.

Luminescent Properties
Aromatic carboxylate ions, especially benzene dicarboxylates, are typical linkers for the luminescent antenna MOF design [15,20] due to the efficient sensitization of lanthanide luminescence. The sensitization mechanism consists of several steps. Upon UV-photon absorption, the linker is promoted into the Sn( 1 ππ*) exited electronic state, which is followed by the fast internal conversion to S1( 1 ππ*). Due to the heavy atom effect, the S1 state of the linker efficiently undergoes intersystem crossing to the T1( 3 ππ*) triplet electronic excited state [32]. If the T1 state of organic linker lies slightly higher in energy than one of the levels of activator lanthanide ion, then the energy is efficiently transferred to the lanthanide ion and followed by the photon emission corresponding to the f-f transition. Thus, terbium terephthalate, Tb2bdc3‧4H2O, demonstrates a relatively high Tb 3+ photoluminescence quantum yield (43-55% [32,42,51]) upon UV-excitation into terephthalate ions due to the fact that the T1 state of the terephthalate ion (E(T1) ≈ 20,400-20,650 cm −1 [32]  The emission spectra of the synthesized compounds, which were measured upon 280-nm excitation into the S n ( 1 ππ*) excited electronic state of terephthalate ions, are shown in Figure 5. The observed emission spectra are typical for compounds containing Tb 3+ ions [53] and consist of narrow bands corresponding to 5 D 4 → 7 F J (J = 3-6) transitions of Tb 3+ : 5 D 4 → 7 F 6 (≈491 nm), 5 D 4 → 7 F 5 (≈543 nm), 5 D 4 → 7 F 4 (≈585 nm), and 5 D 4 → 7 F 3 (≈620 nm). One can observe that the fine structure of Tb 3+ emission spectra of (Tb x Lu 1−x ) 2 bdc 3 ·nH 2 O significantly changes at Tb 3+ concentration of about 20 at. % in both studied series. It is wellknown, that the fine structure of lanthanide (III) ions strictly depends on the local symmetry of emitting lanthanide ions [54][55][56][57]. Indeed, one can notice three different types of fine structure of the spectra: (i) compounds with terbium (III) content of 25 at. % and more in both series (corresponding to the (Tb x Lu 1−x ) 2 bdc 3 ·4H 2 O structure that dominates in this range of concentrations); (ii) MOFs with Tb 3+ concentrations less than 25 at. % in series obtained from diluted solutions ((Tb x Lu 1−x ) 2 bdc 3 ·10H 2 O as the dominating structure); (iii) compounds with terbium (III) concentrations less than 25 at. % in series obtained from concentrated solutions ((Tb x Lu 1−x ) 2 bdc 3 as the dominating structure). The difference is that the fine structure of the emission bands is attributed to the different symmetry of the first coordination sphere of the Tb 3+ ion in these three types of crystalline structures. known, that the fine structure of lanthanide (III) ions strictly depends on the local symmetry of emitting lanthanide ions [54][55][56][57]. Indeed, one can notice three different types of fine structure of the spectra: (i) compounds with terbium (III) content of 25 at. % and more in both series (corresponding to the (TbxLu1-x)2bdc3‧4H2O structure that dominates in this range of concentrations); (ii) MOFs with Tb 3+ concentrations less than 25 at. % in series obtained from diluted solutions ((TbxLu1-x)2bdc3‧10H2O as the dominating structure); (iii) compounds with terbium (III) concentrations less than 25 at. % in series obtained from concentrated solutions ((TbxLu1-x)2bdc3 as the dominating structure). The difference is that the fine structure of the emission bands is attributed to the different symmetry of the first coordination sphere of the Tb 3+ ion in these three types of crystalline structures.

Figure 5.
Emission spectra of (TbxLu1-x)2bdc3‧nH2O materials synthesized from the diluted (a) and concentrated (b) solutions upon 280-nm excitation at the selected Tb 3+ concentrations. Figure 6 displays the photoluminescence decay curves measured upon UV-excitation of (TbxLu1-x)2bdc3‧nH2O MOFs synthesized via the two methods mentioned as monitored at 543 nm ( 5 D4→ 7 F5 transition). At terbium (III) ion concentrations of 60 and 100 at. %, photoluminescence decay curves were well-fitted with the single exponential functions (Equation (1)) with time constants τ of about 0.7-1.1 ms. At low levels of Tb 3+ content (1, 5, and 10 at. %), the photoluminescence decay curves of the compounds obtained from concentrated solutions fit the double exponential functions (Equation (2)). The biexponential behavior of the photoluminescence decay indicates the presence of different relaxation pathways of Tb 3+ ions corresponding to two terbium ions with different coordination environments. We believe that the larger time constant τ2, which is about 2.6-3 ms (Table 1), corresponds to lifetime of 5 D4 state Tb 3+ ions in the (TbxLu1-x)2bdc3 structure. The smaller time constant τ1 value (1.0-1.5 ms) can be assigned to the lifetime of the 5 D4 state of terbium (III) ions in the (TbxLu1-x)2bdc3‧4H2O structure. The photoluminescence decay curves of the (TbxLu1-x)2bdc3‧nH2O compounds with x = 0.01, 0.05, and 0.10, which were obtained from diluted solutions, fit the single exponential functions (eq. 1) with time constants of about 1.1 ms. As the XRD and TGA data shows the coexistence of Ln2bdc3‧4H2O and Ln2bdc3‧10H2O phases in these compounds, one would expect the presence of two different exponential components of photoluminescence decay curves. Most Figure 6 displays the photoluminescence decay curves measured upon UV-excitation of (Tb x Lu 1−x ) 2 bdc 3 ·nH 2 O MOFs synthesized via the two methods mentioned as monitored at 543 nm ( 5 D 4 → 7 F 5 transition). At terbium (III) ion concentrations of 60 and 100 at. %, photoluminescence decay curves were well-fitted with the single exponential functions (Equation (1)) with time constants τ of about 0.7-1.1 ms. At low levels of Tb 3+ content (1,5, and 10 at. %), the photoluminescence decay curves of the compounds obtained from concentrated solutions fit the double exponential functions (Equation (2)). The biexponential behavior of the photoluminescence decay indicates the presence of different relaxation pathways of Tb 3+ ions corresponding to two terbium ions with different coordination environments. We believe that the larger time constant τ 2 , which is about 2.6-3 ms (Table 1), corresponds to lifetime of 5 D 4 state Tb 3+ ions in the (Tb x Lu 1−x ) 2 bdc 3 structure. The smaller time constant τ 1 value (1.0-1.5 ms) can be assigned to the lifetime of the 5 D 4 state of terbium (III) ions in the (Tb x Lu 1−x ) 2 bdc 3 ·4H 2 O structure. The photoluminescence decay curves of the (Tb x Lu 1−x ) 2 bdc 3 ·nH 2 O compounds with x = 0.01, 0.05, and 0.10, which were obtained from diluted solutions, fit the single exponential functions (eq. 1) with time constants of about 1.1 ms. As the XRD and TGA data shows the coexistence of Ln 2 bdc 3 ·4H 2 O and Ln 2 bdc 3 ·10H 2 O phases in these compounds, one would expect the presence of two different exponential components of photoluminescence decay curves. Most likely, the values of the 5 D 4 energy level lifetime of Tb 3+ ions in Ln 2 bdc 3 ·4H 2 O and Ln 2 bdc 3 ·10H 2 O structures are close to each other, as pseudo-single-exponential decay was observed.
We have found that the 5 D 4 excited state lifetimes in the (Tb x Lu 1−x ) 2 bdc 3 ·nH 2 O MOFs obtained from diluted solutions decreased from 1.122 to 0.696 ms with the increase of terbium concentration due to the increased probability of energy transfer between neighboring Tb 3+ ions with subsequent quenching of impurities. At the same time, the photoluminescent quantum yields (PLQY) of these compounds had maxima at about 60 at. % of Tb 3+ , where PLQY is equal to 60% (Table 1). Typically, emission intensity and PLQY nonlinearly depend on the concentration of Tb 3+ ions [58,59]. This type of concentration dependence can be explained by the two competitive effects in REE-containing phosphors [60,61]. Thus, the rise of the numbers of luminescent sites results in radiative emission probability increased and, as a result, the emission intensity and PLQY increased. At the same time, upon the Tb 3+ concentration's rise, the distance between Tb 3+ ions decreased, resulting in the nonradiative processes probability increase that led to the emission quenching [62], resulting in lower PLQY values of pure terbium terephthalate (100 at.% of Tb 3+ ) relative to the MOFs containing 60 at.% of Tb 3+ . The PLQY of the (Tb x Lu 1−x ) 2 bdc 3 ·nH 2 O MOFs obtained from concentrated solutions are equal to the ones obtained from the diluted solutions at the Tb 3+ concentration of 60 and 100 at. %, where the MOFs formed in the same crystalline phase, namely, Ln 2 bdc 3 ·4H 2 O. A further decrease of Tb 3+ content in the MOFs obtained from the diluted solutions resulted in a significant PLQY rise, reaching maxima of 95% for the (Tb 0.1 Lu 0.9 ) 2 bdc 3 ·1.4H 2 O sample. The higher values of PLQY and excited state lifetimes of these materials are attributed to the formation of the anhydrous Ln 2 bdc 3 crystalline phase. The PLQY of the Ln 2 bdc 3 MOFs were significantly higher than that of the Ln 2 bdc 3 ·4H 2 O and Ln 2 bdc 3 ·10H 2 O MOFs due to the absence of water molecules coordinated to Tb 3+ ions, which efficiently quenches luminescence due to energy transfer from the 5 D 4 excited state of Tb 3+ ions to the high-energy O-H stretching vibrational modes of H 2 O molecules [63].
the photoluminescent quantum yields (PLQY) of these compounds had maxima at about 60 at. % of Tb 3+ , where PLQY is equal to 60% (Table 1). Typically, emission intensity and PLQY nonlinearly depend on the concentration of Tb 3+ ions [58,59]. This type of concentration dependence can be explained by the two competitive effects in REE-containing phosphors [60,61]. Thus, the rise of the numbers of luminescent sites results in radiative emission probability increased and, as a result, the emission intensity and PLQY increased. At the same time, upon the Tb 3+ concentration's rise, the distance between Tb 3+ ions decreased, resulting in the nonradiative processes probability increase that led to the emission quenching [62], resulting in lower PLQY values of pure terbium terephthalate (100 at.% of Tb 3+ ) relative to the MOFs containing 60 at.% of Tb 3+ . The PLQY of the (TbxLu1x)2bdc3‧nH2O MOFs obtained from concentrated solutions are equal to the ones obtained from the diluted solutions at the Tb 3+ concentration of 60 and 100 at. %, where the MOFs formed in the same crystalline phase, namely, Ln2bdc3‧4H2O. A further decrease of Tb 3+ content in the MOFs obtained from the diluted solutions resulted in a significant PLQY rise, reaching maxima of 95% for the (Tb0.1Lu0.9)2bdc3‧1.4H2O sample. The higher values of PLQY and excited state lifetimes of these materials are attributed to the formation of the anhydrous Ln2bdc3 crystalline phase. The PLQY of the Ln2bdc3 MOFs were significantly higher than that of the Ln2bdc3‧4H2O and Ln2bdc3‧10H2O MOFs due to the absence of water molecules coordinated to Tb 3+ ions, which efficiently quenches luminescence due to energy transfer from the 5 D4 excited state of Tb 3+ ions to the high-energy O-H stretching vibrational modes of H2O molecules [63].  Table 1. Lifetimes (τ) and photoluminescence quantum yields (ΦPL) of (TbxLu1-x)2bdc3‧nH2O materials at the selected Tb 3+ concentrations synthesized from the diluted (Series 1) and concentrated (Series 2) solutions.
White powders of the (Tb x Lu 1−x ) 2 bdc 3 ·nH 2 O MOFs were synthesized by the direct mixing of two aqueous solutions: (1) sodium terephthalate and (2) terbium and lutetium nitrates taken in various ratios, as shown in Table 2. In order to reveal the effect of the concentrations of the initial solutions on the properties of the obtained materials, we synthesized two series of (Tb x Lu 1−x ) 2 bdc 3 ·nH 2 O MOFs. Series 1 was obtained from the Na 2 bdc and LnCl 3 diluted solutions, where 8 mL of 0.1 M Na 2 bdc solution was added dropwise under vigorous stirring to a solution containing 5 mL of distilled water and 2 mL of 0.2M TbCl 3 and LuCl 3 solutions taken in certain ratios ( Table 2). Series 2 was obtained from the Na 2 bdc and LnCl 3 concentrated solutions, where 3 mL 0.3 M Na 2 bdc solution was rapidly added to the 2 mL of TbCl 3 and LuCl 3 solutions taken in various ratios, as shown in Table 2. Obtained suspensions were kept for one 1 h at room temperature, and then, solid precipitates of the (Tb x Lu 1−x ) 2 bdc 3 ·nH 2 O MOFs were separated from the reaction mixture via centrifugation (2300 g) and washed with deionized water 5 times. The resulting white powders of terbium-lutetium terephthalates were dried in an air atmosphere at 60 • C for 24 h. The Tb 3+ /Lu 3+ ratios in the synthesized (Tb x Lu 1−x ) 2 bdc 3 ·nH 2 O compounds were confirmed with energy-dispersive X-ray spectroscopy (EDX) (EDX spectrometer EDX-800P, Shimadzu, Japan) ( Table 3). We found that the amounts of the elements are consistent with experimental EDX data. The X-ray powder diffraction (XRD) data of obtained (Tb x Lu 1−x ) 2 bdc 3 ·nH 2 O samples were taken with a D2 Phaser (Bruker, Billerica, MA, USA) X-ray diffractometer using Cu K α radiation (λ = 1.54056 Å). The thermal behavior of the compounds was studied via thermogravimetry using a Thermo-microbalance TG 209 F1 Libra (Netzsch, Selb, Germany) with a heat-up rate of 10 • C/min. To carry out photoluminescence studies, the synthesized samples (20 mg) and potassium bromide (300 mg) were pressed into pellets (diameter 13 mm). Solid-state luminescence emission spectra were recorded with a Fluoromax-4 fluorescence spectrometer (Horiba Jobin-Yvon, Kyoto, Japan). Lifetime measurements were performed with the same spectrometer using a pulsed Xe lamp (pulse duration 3 µs). The quantum yield measurements were performed by using the Fluorolog 3 Quanta-phi device (Horiba Jobin-Yvon, Kyoto, Japan).

Conclusions
In this work, we reported the phase composition and the optical properties of luminescent antenna MOFs: heterometallic terbium(III)-lutetium(III) terephthalates. The series of (Tb x Lu 1−x ) 2 bdc 3 ·nH 2 O (x = 0-1) were synthesized via direct reaction between aqueous solutions of disodium terephthalate and nitrates of corresponding lanthanides with two methods: using diluted and concentrated solutions. At Tb 3+ concentrations more than 25 at. %, synthesized compounds existed in the Ln 2 bdc 3 ·4H 2 O crystal structure with the coordination number (CN) of the lanthanide ion equal to eight. Lu 3+ ions typically have lower coordination numbers than Tb 3+ ions; hence, at high lutetium (III) content, structures with CN(Ln 3+ ) < 8 crystallized. Therefore, compounds containing small amounts of terbium (III) ions formed in crystalline phases different from Ln 2 bdc 3 ·4H 2 O. (Tb x Lu 1−x ) 2 bdc 3 ·nH 2 O (x = 0-0.01) compounds, synthesized from concentrated solutions, dominantly existed in the Ln 2 bdc 3 crystal structure with CN(Ln 3+ ) = 7. (Tb x Lu 1−x ) 2 bdc 3 ·nH 2 O (x = 0-0.01) MOFs obtained from diluted solutions formed as Ln 2 bdc 3 ·10H 2 O crystalline phases with CN(Ln 3+ ) = 7. At 2-25 at. %, Tb 3+ ion binary mixtures of the aforementioned crystalline phases were observed. All of the synthesized samples containing Tb 3+ ions demonstrated admirable green luminescence upon 280nm excitation due to the 5 D 4 → 7 F J (J = 3-6) transitions of the Tb 3+ ions. Upon UV-photon absorption, terephthalate ion was promoted into the S n ( 1 ππ*) excited electronic state, which was followed by the fast internal conversion to S 1 ( 1 ππ*) and then to the T 1 ( 3 ππ*) triplet electronic excited state via efficient intersystem crossing due to the presence of the heavy lanthanide ion. The T 1 state of the terephthalate ion lies slightly higher in energy than the 5 D 4 level of the Tb 3+ ion, resulting in the efficient energy transfer to this level that was followed by radiative 5 D 4 → 7 F J (J = 3-6) transitions. The Tb 3+ ions in Ln 2 bdc 3 ·4H 2 O, Ln 2 bdc 3 ·10H 2 O, and Ln 2 bdc 3 ·10H 2 O crystal structures demonstrated different fine structures in their emission bands due to the different local symmetry of the Tb 3+ ions in these three types of crystalline structures. The 5 D 4 excited state lifetimes and photoluminescence quantum yields of (Tb x Lu 1−x ) 2 bdc 3 (x = 0.01, 0.5, 0.1) compounds were significantly larger than for samples of (Tb x Lu 1−x ) 2 bdc 3 ·4H 2 O (x = 0.6, 1) and (Tb x Lu 1−x ) 2 bdc 3 ·10H 2 O (x = 0.01, 0.5, 0.1) due to the absence of the luminescence quenching of the Tb 3+ by coordinated water molecules. Meanwhile, we cannot rule out effect of the crystalline structure on the relative energies of the T 1 ( 3 ππ*) triplet's electronic excited state and the 5 D 4 level of Tb 3+ ions, which affect the efficiency of the T 1 -to-5 D 4 en-