Brightly Luminescent (Tb_xLu_1−x)₂bdc₃·nH₂O MOFs: Effect of Synthesis Conditions on Structure and Luminescent Properties

Abstract

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 (Tb_xLu_1−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, (Tb_0.1Lu_0.9)₂bdc₃·1.4H₂O, had one of the highest PLQY among Tb-based MOFs, 95%.

1. 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 makes them exceptional materials for chemical sensors [23], optical thermometers [24,25], and OLED components [26,27]. Eu³⁺ and Tb³⁺ ions are often used as activators in such materials because of their strong red and green emissions, respectively [21,28,29,30]. The simultaneous presence of several lanthanide ions in one compound provides the possibility to gain the properties of multimodal imaging agents and allows one to discover the energy transfer mechanisms in such compounds [31,32,33]. Moreover, some studies showed the enhancement of luminescence of Eu³⁺, Tb³⁺, and Sm³⁺ containing antenna MOFs upon dilution with paramagnetic Gd³⁺ ions, whereas the substitution of luminescent REE ions by diamagnetic La³⁺, Y³⁺, and Lu³⁺ ions does not lead to the luminescence intensity increase [20]. It is important to note that the doping by the aforementioned ions does not result in the crystalline phase change in the majority of studies of heterometallic REE MOFs. At the same time, the number of studies of luminescent antenna MOFs containing both luminescent and non-luminescent REE ions is still insignificant in contrast with those of solid solutions of purely inorganic compounds [34,35,36,37,38]. Recently, we studied the optical properties of heterometallic europium–lutetium terephthalates [39]. The luminescence quantum yields of the terephthalate ions were found to be increased with a decrease in the europium concentrations in these compounds. We also observed that the substitution of a large amount of Eu³⁺ for Lu³⁺ resulted in a crystalline phase change from Ln₂bdc₃·4H₂O to Ln₂bdc₃ (bdc = 1,4-benzenedicarboxylate). The lifetimes of europium (III)’s ⁵D₀ excited state were found to be larger by 4–4.8 times in an anhydrous phase with low Eu³⁺ content.

In order to further reveal the doping effect of Lu³⁺ ions on the structural and optical properties of antenna luminescent MOFs, in the current work, we studied bimetallic terbium(III)–lutetium(III) terephthalates as obtained by using two methods.

2. Results and Discussion

2.1. 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_xLu_1−x)₂bdc₃·nH₂O (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 Ln₂bdc₃·4H₂O crystalline phase (Ln = Ce − Yb) [40], and additional peaks were not observed. At low Tb³⁺ concentrations between 0 and 5 at. %, the positions of the reflexes in the PXRD patterns were different from those of the Ln₂bdc₃·4H₂O and depended on the concentration of the initial reagents (Na₂bdc, TbCl₃, and LuCl₃). Thus, diffraction patterns corresponded to Ln₂bdc₃·10H₂O [41] and Ln₂bdc₃ [42] for compounds synthesized from diluted and concentrated solutions, respectively (see Section 3). At intermediate Tb³⁺ concentrations between 10 and 25 at. %, the binary mixtures of the aforementioned crystalline phases were precipitated, namely, Ln₂bdc₃·4H₂O + Ln₂bdc₃·10H₂O and Ln₂bdc₃·4H₂O + Ln₂bdc₃ for the (Tb_xLu_1−x)₂bdc₃·nH₂O 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]. 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 Ln₂bdc₃·4H₂O, Ln₂bdc₃, and Ln₂bdc₃·10H₂O are shown in Figure 2. In Ln₂bdc₃·4H₂O lanthanide (III), ions were bound to two water molecules and six terephthalate ions through oxygen atoms, where Ln³⁺ coordination number (CN) is equal to 8. In Ln₂bdc₃ structures, Ln³⁺ ions with CN = 7 coordinated solely to oxygens of terephthalate ions. In the Ln₂bdc₃·10H₂O 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 Ln₂bdc₃·10H₂O structure were located in interplanar channels. Commonly, Tb³⁺ ions have relatively larger coordination numbers than Lu³⁺ ions. For example, in aqueous solutions, Tb³⁺ dominantly exists in a nonacoordinated form as the [Tb(H₂O)₉]³⁺ complex [46], but smaller Yb³⁺ and Lu³⁺ ions possess lower coordination numbers and exist as [Ln(H₂O)₈]³⁺ [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 Ln₂bdc₃ and Ln₂bdc₃·10H₂O, which are formed in pure lutetium terephthalate, and in (Tb_xLu_1−x)₂bdc₃·nH₂O at high Lu³⁺ content levels. In Ln₂bdc₃·4H₂O, which is formed in pure terbium terephthalate and in mixed Tb–Lu terephthalates at high Tb³⁺ content levels, the coordination number of Ln³⁺ ions is equal to eight. The reasons that can explain the difference between structures of lutetium terephthalates synthesized from diluted (Lu₂bdc₃·10H₂O) and concentrated (Lu₂bdc₃) 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(H₂O)_x(bdc)_y]^3−2y [49]. These complexes then aggregate into SBUs, which further form MOFs. Apparently, in concentrated solutions, complexes have higher Lu³⁺:bdc²⁻ ratios (1:2 or 1:3) than in diluted solution (1:1). Therefore, further formed SBUs and MOFs of Lu₂bdc₃ had larger number of coordinated oxygens of terephthalate ligands than Lu₂bdc₃·10H₂O.

In our previous work, we reported the similar behavior of the (Eu_xLu_1−x)₂bdc₃·nH₂O MOFs obtained from concentrated solutions [39]. We found that phase transition occurred at significantly lower Eu³⁺ concentrations (6 at. % of Eu³⁺ vs. 30 at. % of Tb³⁺). This observation can be explained by the lower ionic radius of Tb³⁺ (1.040Å) than that of Eu³⁺ (1.066Å) [50]. The structure with CN = 7 (Ln₂bdc₃) is more advantageous for Tb³⁺ than for Eu³⁺, which forms a structure Ln₂bdc₃·4H₂O with larger CN = 8 beginning at 6 at. % of Eu³⁺ ions.

2.2. Thermogravimetric Analysis (TGA)

The thermal behavior of the selected compounds (Tb_xLu_1−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 (Tb_xLu_1−x)₂bdc₃·nH₂O.

The number of water molecules per one formula unit N(H₂O) for all selected compounds as function of Tb³⁺ 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. % Tb³⁺) in both series. The N(H₂O) value increases from 4 to 10 upon the substitution of Tb³⁺ by Lu³⁺ ions (decreasing of Tb³⁺ content) in (Tb_xLu_1−x)₂bdc₃·nH₂O 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 Tb³⁺ 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 Tb³⁺ 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. % Tb³⁺ for two synthesized series of MOFs (Tb_xLu_1−x)₂bdc₃·nH₂O, and simultaneously, the molar fraction of the second coexisting phase decreased. In the Tb³⁺ concentration range of 30–100 at. %, only Ln₂bdc₃·4H₂O was present in both series.

2.3. 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(¹ππ*) exited electronic state, which is followed by the fast internal conversion to S₁(¹ππ*). Due to the heavy atom effect, the S₁ state of the linker efficiently undergoes intersystem crossing to the T₁(³ππ*) 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 Tb³⁺ 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] for bdc^2-) lies only 50—300 cm⁻¹ above the ⁵D₄ level of the Tb³⁺ ion (E(⁵D₄) ≈ 20,350 cm⁻¹ [52]).

The emission spectra of the synthesized compounds, which were measured upon 280-nm excitation into the S_n(¹ππ*) excited electronic state of terephthalate ions, are shown in Figure 5. The observed emission spectra are typical for compounds containing Tb³⁺ ions [53] and consist of narrow bands corresponding to ⁵D₄→⁷F_J (J = 3–6) transitions of Tb³⁺: ⁵D₄→⁷F₆ (≈491 nm), ⁵D₄→⁷F₅ (≈543 nm), ⁵D₄→⁷F₄ (≈585 nm), and ⁵D₄→⁷F₃ (≈620 nm). One can observe that the fine structure of Tb³⁺ emission spectra of (Tb_xLu_1−x)₂bdc₃·nH₂O significantly changes at Tb³⁺ concentration of about 20 at. % in both studied series. It is well-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 (Tb_xLu_1−x)₂bdc₃·4H₂O structure that dominates in this range of concentrations); (ii) MOFs with Tb³⁺ concentrations less than 25 at. % in series obtained from diluted solutions ((Tb_xLu_1−x)₂bdc₃·10H₂O as the dominating structure); (iii) compounds with terbium (III) concentrations less than 25 at. % in series obtained from concentrated solutions ((Tb_xLu_1−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 Tb³⁺ ion in these three types of crystalline structures.

Figure 6 displays the photoluminescence decay curves measured upon UV-excitation of (Tb_xLu_1−x)₂bdc₃·nH₂O MOFs synthesized via the two methods mentioned as monitored at 543 nm (⁵D₄→⁷F₅ 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 Tb³⁺ 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 Tb³⁺ 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. Lifetimes (τ) and photoluminescence quantum yields (Φ_PL) of (Tb_xLu_1−x)₂bdc₃·nH₂O materials at the selected Tb³⁺ concentrations synthesized from the diluted (Series 1) and concentrated (Series 2) solutions.

Series 1 (From Diluted Solutions)			Series 2 (From Concentrated Solutions)
ΧTb (at. %)	τ, ms	ΦPL, %	ΧTb (at. %)	τ1, ms	τ2, ms	ΦPL, %
1	1.12 ± 0.02	38	1	1.17 ± 0.04	2.63 ± 0.10	77
5	1.12 ± 0.02	56	5	1.53 ± 0.05	3.00 ± 0.24	88
10	1.08 ± 0.01	58	10	1.02 ± 0.03	2.61 ± 0.08	95
60	0.92 ± 0.02	60	60	0.94 ± 0.02		60
100	0.70 ± 0.01	49	100	0.69 ± 0.01		49

), corresponds to lifetime of ⁵D₄ state Tb³⁺ ions in the (Tb_xLu_1−x)₂bdc₃ structure. The smaller time constant τ₁ value (1.0–1.5 ms) can be assigned to the lifetime of the ⁵D₄ state of terbium (III) ions in the (Tb_xLu_1−x)₂bdc₃·4H₂O structure. The photoluminescence decay curves of the (Tb_xLu_1−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 likely, the values of the ⁵D₄ energy level lifetime of Tb³⁺ ions in Ln₂bdc₃·4H₂O and Ln₂bdc₃·10H₂O structures are close to each other, as pseudo-single-exponential decay was observed.

$$I=I_1·e^{-\frac{t}{\tau }}$$

(1)

$$I=I_1·e^{-\frac{t}{{\tau }_1}}+I_2·e^{-\frac{t}{{\tau }_2}}$$

(2)

We have found that the ⁵D₄ excited state lifetimes in the (Tb_xLu_1−x)₂bdc₃·nH₂O 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 Tb³⁺ 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 Tb³⁺, where PLQY is equal to 60% (Table 1). Typically, emission intensity and PLQY nonlinearly depend on the concentration of Tb³⁺ 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 Tb³⁺ concentration’s rise, the distance between Tb³⁺ 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 Tb³⁺) relative to the MOFs containing 60 at.% of Tb³⁺. The PLQY of the (Tb_xLu_1−x)₂bdc₃·nH₂O MOFs obtained from concentrated solutions are equal to the ones obtained from the diluted solutions at the Tb³⁺ concentration of 60 and 100 at. %, where the MOFs formed in the same crystalline phase, namely, Ln₂bdc₃·4H₂O. A further decrease of Tb³⁺ content in the MOFs obtained from the diluted solutions resulted in a significant PLQY rise, reaching maxima of 95% for the (Tb_0.1Lu_0.9)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 Tb³⁺ ions, which efficiently quenches luminescence due to energy transfer from the ⁵D₄ excited state of Tb³⁺ ions to the high-energy O-H stretching vibrational modes of H₂O molecules [63].

3. Materials and Methods

Benzene-1,4-dicarboxylic (terephthalic, H₂bdc) acid (>98%), sodium hydroxide (>99%), nickel(II) chloride hexahydrate (>99%), EDTA disodium salt (0.05M aqueous solution), and murexide were purchased from Sigma-Aldrich Chemie GmbH (Taufkirchen, Germany) and used without additional purification. Lutetium (III) nitrate pentahydrate and terbium (III) nitrate pentahydrate were purchased from Chemcraft (Kaliningrad, Russia). The 0.3 M solution of disodium terephthalate (Na₂bdc) was prepared by dissolving 0.6 moles of sodium hydroxide and 0.3 moles of terephthalic acid in 1 L of distilled water. Volumes of 0.2 M of TbCl₃ and LuCl₃ solutions were prepared and standardized using back complexometric titration. Thus, 1 mL of LnCl₃ (Ln = Tb, Lu) solution with a concentration of about 0.3 M, 20 mL of 0.05 M EDTA, 10 mL of ammonium buffer solution (pH = 9), and a pinch of murexide indicator were taken in a conical flask. The obtained solution was titrated with 0.05 M NiCl₂ [64]. Then, standardized LnCl₃ solutions were diluted to 0.2 M.

White powders of the (Tb_xLu_1−x)₂bdc₃·nH₂O 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. The volumes of the initial TbCl₃ and LuCl₃ solutions used for the synthesis of (Tb_xLu_1−x)₂bdc₃·nH₂O MOFs.

ΧTb (at. %)	V(0.2M TbCl3), mL	V(0.2M LuCl3), mL
0	0.00	2.00
1	0.02	1.98
5	0.10	1.90
10	0.20	1.80
15	0.30	1.70
20	0.40	1.60
25	0.50	1.50
30	0.60	1.40
60	1.20	0.80
100	2.00	0.00

. 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_xLu_1−x)₂bdc₃·nH₂O MOFs. Series 1 was obtained from the Na₂bdc and LnCl₃ diluted solutions, where 8 mL of 0.1 M Na₂bdc solution was added dropwise under vigorous stirring to a solution containing 5 mL of distilled water and 2 mL of 0.2M TbCl₃ and LuCl₃ solutions taken in certain ratios (Table 2). Series 2 was obtained from the Na₂bdc and LnCl₃ concentrated solutions, where 3 mL 0.3 M Na₂bdc solution was rapidly added to the 2 mL of TbCl₃ and LuCl₃ 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_xLu_1−x)₂bdc₃·nH₂O 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 Tb³⁺/Lu³⁺ ratios in the synthesized (Tb_xLu_1−x)₂bdc₃·nH₂O compounds were confirmed with energy-dispersive X-ray spectroscopy (EDX) (EDX spectrometer EDX-800P, Shimadzu, Japan) (

Table 3. Tb³⁺ atomic fractions (relative to the total amount of Tb³⁺ and Lu³⁺) in (Tb_xLu_1−x)₂bdc₃·nH₂O compounds synthesized from the diluted (Series 1) and concentrated (Series 2) solutions. Measurements were taken during synthesis and obtained from EDX data.

Series 1 (from Diluted Solutions)		Series 2 (from Concentrated Solutions)
Χtb (At. %), Taken	ΧTb (%), EDX	ΧTb (at. %), Taken	ΧTb (%), EDX
0	0	0	0
1	0.74 ± 0.07	1	0.70 ± 0.07
5	4.6 ± 0.5	5	4.6 ± 0.5
10	9 ± 1	10	10 ± 1
15	15 ± 3	15	14 ± 1
20	19 ± 2	20	20 ± 2
25	26 ± 3	25	23 ± 2
30	29 ± 3	30	27 ± 3
60	57 ± 5	60	57 ± 5
100	100	100	100

). We found that the amounts of the elements are consistent with experimental EDX data. The X-ray powder diffraction (XRD) data of obtained (Tb_xLu_1−x)₂bdc₃·nH₂O 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).

4. 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_xLu_1−x)₂bdc₃·nH₂O (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 Tb³⁺ concentrations more than 25 at. %, synthesized compounds existed in the Ln₂bdc₃·4H₂O crystal structure with the coordination number (CN) of the lanthanide ion equal to eight. Lu³⁺ ions typically have lower coordination numbers than Tb³⁺ ions; hence, at high lutetium (III) content, structures with CN(Ln³⁺) < 8 crystallized. Therefore, compounds containing small amounts of terbium (III) ions formed in crystalline phases different from Ln₂bdc₃·4H₂O. (Tb_xLu_1−x)₂bdc₃·nH₂O (x = 0–0.01) compounds, synthesized from concentrated solutions, dominantly existed in the Ln₂bdc₃ crystal structure with CN(Ln³⁺) = 7. (Tb_xLu_1−x)₂bdc₃·nH₂O (x = 0–0.01) MOFs obtained from diluted solutions formed as Ln₂bdc₃·10H₂O crystalline phases with CN(Ln³⁺) = 7. At 2–25 at. %, Tb³⁺ ion binary mixtures of the aforementioned crystalline phases were observed. All of the synthesized samples containing Tb³⁺ ions demonstrated admirable green luminescence upon 280nm excitation due to the ⁵D₄→⁷F_J (J = 3–6) transitions of the Tb³⁺ ions. Upon UV-photon absorption, terephthalate ion was promoted into the S_n(¹ππ*) excited electronic state, which was followed by the fast internal conversion to S₁(¹ππ*) and then to the T₁(³ππ*) triplet electronic excited state via efficient intersystem crossing due to the presence of the heavy lanthanide ion. The T₁ state of the terephthalate ion lies slightly higher in energy than the ⁵D₄ level of the Tb³⁺ ion, resulting in the efficient energy transfer to this level that was followed by radiative ⁵D₄→⁷F_J (J = 3–6) transitions. The Tb³⁺ ions in Ln₂bdc₃·4H₂O, Ln₂bdc₃·10H₂O, and Ln₂bdc₃·10H₂O crystal structures demonstrated different fine structures in their emission bands due to the different local symmetry of the Tb³⁺ ions in these three types of crystalline structures. The ⁵D₄ excited state lifetimes and photoluminescence quantum yields of (Tb_xLu_1−x)₂bdc₃ (x = 0.01, 0.5, 0.1) compounds were significantly larger than for samples of (Tb_xLu_1−x)₂bdc₃·4H₂O (x = 0.6, 1) and (Tb_xLu_1−x)₂bdc₃·10H₂O (x = 0.01, 0.5, 0.1) due to the absence of the luminescence quenching of the Tb³⁺ by coordinated water molecules. Meanwhile, we cannot rule out effect of the crystalline structure on the relative energies of the T₁(³ππ*) triplet’s electronic excited state and the ⁵D₄ level of Tb³⁺ ions, which affect the efficiency of the T₁-to-⁵D₄ energy transfer efficiency, resulting in PLQY changes. As a result of our study, we synthesized the material, namely (Tb_0.1Lu_0.9)₂bdc₃·1.4H₂O, which has one of the highest PLQY among Tb-based MOFs, 95%.