Heterometallic Europium(III)–Lutetium(III) Terephthalates as Bright Luminescent Antenna MOFs

Abstract

A new series of luminescent heterometallic europium(III)–lutetium(III) terephthalate metal–organic frameworks, namely (Eu_xLu_1−x)₂bdc₃·nH₂O, was synthesized using a direct reaction in a water solution. At the Eu³⁺ concentration of 1–40 at %, the MOFs were formed as a binary mixture of the (Eu_xLu_1−x)₂bdc₃ and (Eu_xLu_1−x)₂bdc₃·4H₂O crystalline phases, where the Ln₂bdc₃·4H₂O crystalline phase was enriched by europium(III) ions. At an Eu³⁺ concentration of more than 40 at %, only one crystalline phase was formed: (Eu_xLu_1−x)₂bdc₃·4H₂O. All MOFs containing Eu³⁺ exhibited sensitization of bright Eu³⁺-centered luminescence upon the 280 nm excitation into a ¹ππ* excited state of the terephthalate ion. The fine structure of the emission spectra of Eu^{3+ 5}D₀-⁷F_J (J = 0–4) significantly depended on the Eu³⁺ concentration. The luminescence quantum yield of Eu³⁺ was significantly larger for Eu-Lu terephthalates containing a low concentration of Eu³⁺ due to the absence of Eu-Eu energy migration and the presence of the Ln₂bdc₃ crystalline phase with a significantly smaller nonradiative decay rate compared to the Ln₂bdc₃·4H₂O.

1. Introduction

In recent decades, rare-earth-element metal–organic frameworks (REE-MOFs) were actively designed and synthetized due to their unique luminescence properties. They are unique platforms for fabricating advanced luminescent materials, which are widely used in various fields of science and technology [1,2,3,4]. The position of the lanthanide ionic luminescence bands strongly depends only on the lanthanide ion, which allows the construction of REE-MOFs with the desired optical properties [5]. Taking this fact into account and considering the high stability, low solubility and toxicity, and highly effective charge transport of Ln-MOFs, they are prospective materials for OLEDs [6,7], luminescent thermometers [8,9], and imaging [10,11,12,13]. Variations in organic linkers in MOFs allow synthetic chemists to form structures with different porosities large surface areas, and high structural flexibility [14,15,16], which allows the use of REE-MOFs as highly selective sensors on organic and inorganic materials [17,18,19,20,21,22,23,24,25]. Typical linkers in REE-MOFs are organic carboxylates due to the simple synthesis of REE-MOFs in undemanding conditions and the unlimited possibilities in MOF design [26,27].

Lanthanide ions possess characteristic luminescence; however, direct UV excitation of them is inefficient because they have very small light absorption coefficients: 4f-4f transitions are forbidden by selection rules. This issue can be resolved using the energy transfer from the excited linker to the lanthanide ion (antenna effect) [28,29]. Aromatic organic molecules such as 1,4-benzenedicarboxylate (bdc) are widely used as antenna linkers due to their effective UV absorbance and pronounced antenna effect [30,31]. Usually, the energy transfer takes place from the lower-level triplet electronic state (T₁) of the linker molecule but not from the lowest excited singlet state (S₁). In some cases, the heavy lanthanides increase the rate of S₁-T₁ intersystem crossing [32,33,34,35]. The high concentration of luminescent lanthanide ion in homometallic REE-MOFs could result in concentration quenching through Ln-Ln energy migration and, therefore, the drop in luminescence quantum yield (PLQY) [36]. Utochnikova et al. proposed a solution to this problem by doping of a luminescent europium(III) terephthalate with nonluminescent Gd³⁺ ions, which not only diluted the luminescent Tb³⁺ ions, but also increased the probability of intersystem crossing, which increased the PLQY. It was found that Eu-Gd and Eu-Y heterometallic terephthalates were formed in the same crystalline phases. In the current work, we studied a series of luminescent heterometallic europium(III)–lutetium(III) terephthalates MOFs and observed that the substitution of a large amount of Eu(III) for Lu(III) resulted in a crystalline phase change as well as a significant rise in the PLQY.

2. Results and Discussion

2.1. XRD Results and Analysis

The X-ray powder diffraction (XRD) patterns (Figure 1a) were measured for the range of heterometallic europium(III)–lutetium(III) terephthalates (Eu_xLu_1−x)₂bdc₃·nH₂O; bdc = 1,4-benzenedicarboxylate) with a Eu³⁺ concentration from 0 to 100 at %. An analysis of the XRD patterns demonstrated that in a range of Eu³⁺ concentration of 6 to 100 at %, the samples were isostructural to the Ln₂bdc₃·4H₂O (Ln = Ce-Yb) [37]. This structure, which is common for the rare-earth terephthalates from Ce to Yb [38], was a three-dimensional metal–organic framework (MOF) in which octacoordinated lanthanide ions were bound to the two water molecules and six terephthalate ions through the oxygen atoms (Figure 1b). The analysis of XRD patterns of Eu-Lu terephthalates with 0–2 at % of Eu³⁺ showed that the samples were isostructural to Er₂bdc₃ [38], which is a 3D MOF in which heptacoordinated lanthanide ions are bound to the seven oxygen atoms from the terephthalate ions (Figure 1c). At a Eu³⁺ concentration in the range of 3–5 at %, both the Ln₂bdc₃·4H₂O and Ln₂bdc₃ crystalline phases were observed. The XRD peaks for heterometallic europium(III)–lutetium(III) terephthalates in Eu³⁺ concentration ranged from 6 to 100 at % and were slightly shifted relative to the XRD peaks measured for Ln₂bdc₃·4H₂O reported previously [37]. To compare these Ln₂bdc₃·4H₂O structures, the refinement of unit cell parameters was performed for some samples with a Eu³⁺ concentration between 6 and 100 at % (Table 1) using UnitCell software [39], which retrieved unit cell parameters from diffraction data using a method of least squares from the 2Θ data of the XRD patterns. Calculation errors also are shown in Table 1. We observed that in the range, the unit cell parameters increased. The observed growth of the unit cell parameters of heterometallic europium(III)–lutetium(III) terephthalates was explained by the smaller ionic radius of the octacoordinated Lu³⁺ (0.977 Å) compared with the ionic radius of the Eu³⁺ ion (1.066 Å) [40].

Figure 1.

(a) The XRD patterns of (Eu_xLu_1−x)₂bdc₃·nH₂O in heterometallic europium(III)–lutetium(III) terephthalate powders from 0% Eu³⁺ to 100% Eu³⁺) and the simulated XRD pattern of Er₂bdc₃ and Eu₂bdc₃·4H₂O single-crystals structure taken from refs. [37,38]. (b,c) The generated crystal structures of Eu₂bdc₃·4H₂O and Er₂bdc₃, respectively.

Table 1.

Unit cell parameters with calculation errors for (Eu_xLu_1−x)₂bdc₃·nH₂O refined for Eu₂bdc₃·4H₂O crystalline phase.

χEu (%)	a, Å	b, Å	c, Å	α	β	γ	V, Å3
100	6.1904 ±0.0019	9.856 ±0.003	10.251 ±0.003	101.673 ±0.027	90.273 ±0.028	104.796 ±0.025	591.13 ±0.22
90	6.1862 ±0.0019	9.845 ±0.003	10.236 ±0.003	101.583 ±0.027	90.300 ±0.028	104.727 ±0.025	589.63 ±0.22
60	6.1727 ±0.0018	9.815 ±0.003	10.206 ±0.003	101.553 ±0.027	90.418 ±0.028	104.657 ±0.025	585.00 ±0.22
40	6.1635 ±0.0018	9.783 ±0.003	10.179 ±0.003	101.497 ±0.027	90.472 ±0.028	104.651 ±0.025	580.75 ±0.22
20	6.1405 ±0.0018	9.738 ±0.003	10.145 ±0.003	101.570 ±0.027	90.562 ±0.028	104.617 ±0.025	573.90 ±0.21
10	6.1411 ±0.0018	9.7178 ±0.003	10.1334 ±0.003	101.626 ±0.026	90.461 ±0.028	104.590 ±0.025	572.13 ±0.21
6	6.1313 ±0.0018	9.714 ±0.003	10.130 ±0.003	101.582 ±0.026	90.474 ±0.027	104.608 ±0.025	570.784 ±0.21

2.2. Thermogravimetric Analysis (TGA)

The thermogravimetric analysis (TGA) was carried out for the selected heterometallic europium(III)–lutetium(III) terephthalates in a temperature range of 25–300 °C (Figure 2a). The mass loss was observed at 120–180 °C for all measured samples. As previously reported [38], the mass loss in this temperature range can be assigned to the dehydration of the compounds resulting in the formation of Ln₂bdc₃. An analysis of the TGA curves allowed us to calculate the average numbers of water molecules in the heterometallic europium(III)–lutetium(III) terephthalates ((Eu_xLu_1−x)₂bdc₃·nH₂O). We observed that this number increased with the increase in the Eu³⁺ concentration (Figure 2b). An analysis of the XRD patterns demonstrated the presence of two crystalline phases: Ln₂bdc₃·4H₂O and Ln₂bdc₃. Therefore, we could estimate the molar fraction of each coexisting crystalline phase (Figure 2c). The molar fraction of Ln₂bdc₃·4H₂O increased along with the Eu³⁺ concentration in a range between 0 and 40 at %. In the Eu³⁺ concentration range of 40–100%, only Ln₂bdc₃·4H₂O was present.

Figure 2.

(a) Thermogravimetric analysis (TGA) curves showing the mass loss profile of (Eu_xLu_1−x)₂bdc₃·xH₂O during thermal decomposition; (b) the number of water molecules per one formula unit in (Eu_xLu_1−x)₂bdc₃·xH₂O; (c) the molar fraction of Ln₂bdc₃ and Ln₂bdc₃·4H₂O in heterometallic europium(III)–lutetium(III) terephthalates as a function of Eu concentration.

2.3. Luminescent Properties

The terephthalate ion is a typical linker used in luminescent antenna MOFs [41] due to its intensive UV absorbance [42] followed by efficient energy transfer to the luminescent lanthanide ion. The excitation of (Eu_xLu_1−x)₂bdc₃·nH₂O (λ_ex. = 280 nm) resulted in emission in the visible range corresponding to ⁵D₀-⁷F_J (J = 0–5) transitions of the Eu³⁺ ion [5] (Figure 3 and Figure 4). Upon UV excitation, the terephthalate ion was promoted into the S_n(¹ππ*) state followed by the fast internal conversion to S₁(¹ππ*). Due to the heavy atom effect, the S₁ state efficiently moved to the T₁(³ππ*) triplet electronic excited state [34] via intersystem crossing. The T₁ state of the terephthalate ion [34] (≈20,000 cm⁻¹) had a higher energy than the ⁵D₁ energy level of the Eu³⁺ ion [5] (≈19,000 cm⁻¹) and a significantly lower energy than that of the lower excited state of the Lu³⁺ ion [43] (80,000 cm⁻¹). Therefore, an efficient energy transfer from the T₁ triplet electronic excited state of the terephthalate ion to the ⁵D₁ energy level of the Eu³⁺ ion occurred. The ⁵D₁ level of the Eu³⁺ ion then underwent an internal conversion into the ⁵D₀ energy level followed by the emission into the ⁷F_J (J = 0–4) lower-lying levels.

Figure 3.

The normalized emission spectra of (Eu_xLu_1−x)₂bdc₃·nH₂O at selected Eu³⁺ concentrations (given in legend) upon 280 nm excitation.

Figure 4.

Fine structure of lines in emission spectra of heterometallic europium(III)–lutetium(III) terephthalates normalized at maximum point for (a) ⁵D₀-⁷F₀, (b) ⁵D₀-⁷F₁, and (c) ⁵D₀-⁷F₂ transitions.

We observed that the fine structure of the Eu³⁺ emission spectra significantly depended on the Eu³⁺ concentration in the (Eu_xLu_1−x)₂bdc₃·nH₂O (Figure 4). At Eu³⁺ ion concentrations of more than 6 at %, in which the Ln₂bdc₃·4H₂O phase dominated, the emission spectra were similar to that of Eu₂bdc₃·4H₂O [31] and consisted of narrow bands corresponding to ⁵D₀-⁷F_J (J = 0–4) transitions of Eu³⁺: ⁵D₀-⁷F₀ (577.6 nm), ⁵D₀-⁷F₁ (587.9 and 591.5 nm), ⁵D₀-⁷F₂ (614.0 nm), ⁵D₀-⁷F₃ (649.0 nm), and ⁵D₀-⁷F₄ (697.0 nm) (Figure 3). At low Eu³⁺ concentrations (2 and 4 at % Eu³⁺), in which the Ln₂bdc₃ phase dominated, the fine structure of the emission spectra was significantly different. The emission spectra contained ⁵D₀-⁷F₀ (577.2 nm and 577.6 nm), ⁵D₀-⁷F₁ (585.9, 588.4, and 595.6 nm), ⁵D₀-⁷F₂ (606.6, 610.2, 616.6, 619.4 (shoulder), and 621.8 nm), ⁵D₀-⁷F₃ (649.0 nm), and ⁵D₀-⁷F₄ (700.0 nm) Eu³⁺ narrow emission bands.

⁵D₀-⁷F₀ transition is strictly forbidden by the Judd–Ofelt theory; one can observe this transition only for europium(III) ions in coordination sites with C_n, C_nv, and C_s symmetry. For all measured samples, ⁵D₀-⁷F₀ transitions were observed. The analysis of the fine structure of this transition allowed us to determine the number of Eu³⁺ coordination sites because the ⁷F₀ level was not degenerate and did not split in the crystal field, hence ⁵D₀-⁷F₀ could present in the emission spectrum as a single line for one type of Eu³⁺ coordination. The fine structure of the (Eu_xLu_1−x)₂bdc₃·nH₂O emission bands ⁵D₀-⁷F₀ is shown in Figure 4a. In the emission spectra of Eu-Lu terephthalates with a Eu³⁺ concentration of 6–100 at %, a single line was observed in the 570–585 nm range (⁵D₀-⁷F₀) with a maximum at 577.6 nm, which indicated that Eu³⁺ ions existed in the single-crystal-phase isostructural Ln₂bdc₃·4H₂O. Meanwhile, by using a TGA, we estimated the molar fractions of Ln₂bdc₃·4H₂O and Ln₂bdc₃ as equal to 60 and 40%, respectively. Therefore, the single ⁵D₀-⁷F₀ emission band (6–100 at % Eu³⁺) can be explained by uneven ion distribution between the two phases: the Ln₂bdc₃·4H₂O crystalline phase was enriched by Eu³⁺ ions. In the Eu-Lu terephthalates with 2–4 at % Eu³⁺, two emission bands corresponding to the ⁵D₀-⁷F₀ transition were observed to peak at 577.2 and 577.6 nm, indicating the two different coordination sites of the Eu³⁺ ion. Therefore, in the terephthalates containing 2–4 at % Eu³⁺, europium(III) ions were distributed between two phases, namely Ln₂bdc₃·4H₂O and Ln₂bdc₃, which was consistent with the TGA and XRD data.

The fine structure of the ⁵D₀-⁷F_J emission bands and their relative intensities were very sensitive to the Eu³⁺ ions’ local symmetry. The degeneracy of each spin–orbit level was 2J+1 [5]. Hence, the maximum amount of crystal-field transitions of the ⁵D₀-⁷F₁ and ⁵D₀-⁷F₂ transitions were 3 and 5, respectively. According to previous studies, lanthanide(III) ions had pseudo-C₄ symmetry in Ln₂bdc₃·4H₂O (Ln = Tb, Eu) [38]. For Eu₂bdc₃·4H₂O (100 at % Eu³⁺), the ⁵D₀-⁷F₁ transition split into two crystal field transitions (587.9 and 591.6 nm), and the ⁵D₀-⁷F₂ transition was presented in the emission spectrum as a single line (614.0 nm). Interestingly, for the Eu-Lu terephthalates containing 6–60 at % Eu³⁺, different splitting patterns of the ⁵D₀-⁷F_J transitions in the crystal field were observed. The ⁵D₀-⁷F₁ and ⁵D₀-⁷F₂ transition split into three (587.6, 591.0, and 592.8 nm) and two components (608.5 and 614.0 nm), respectively (Figure 4b,c). The difference between the abovementioned emission spectra can be explained by the distortion of the coordination polyhedron due to the appearance of structural defects caused by the addition of Lu³⁺ ions, which have a lower ionic radius than europium(III) ions (lanthanide contraction). This resulted in the lowering of the local symmetry of the Eu³⁺ ion and the larger number of crystal-field transitions of Eu-Lu terephthalates compared to the Eu₂bdc₃·4H₂O. The number of ⁵D₀-⁷F_J crystal-field transitions indicated that the Eu³⁺ had symmetry of C₂ or lower [44].

In the emission spectra of Ln₂bdc₃ (2–4 at % Eu³⁺), the ⁵D₀-⁷F₁ and ⁵D₀-⁷F₂ transitions split into three (585.9, 588.4, and 595.6 nm) and five components (606.6, 610.2, 616.6, 619.4 (shoulder), and 621.8 nm), respectively. In the emission spectra of the Lu-Eu terephthalates containing 2–4 at % Eu³⁺, we observed the presence of (Eu_xLu_1−x)₂bdc₃·4H₂O emission bands (weak 608.5 and 614.0 nm signals) because the Eu³⁺ ion was distributed between the Ln₂bdc₃ and Ln₂bdc₃·4H₂O crystalline phases. A careful analysis of the Ln₂bdc₃ crystalline structure (Figure 1c) allowed us to conclude that the Ln³⁺ ion had C₁ local symmetry, which was consistent with the number of crystal-field components of the ⁵D₀-⁷F₀, ⁵D₀-⁷F₁, and ⁵D₀-⁷F₂ transitions (1, 3, and 5, respectively) [44].

The luminescence decay curves of the (Eu_xLu_1−x)₂bdc₃·nH₂O phosphors monitored at 615 nm (⁵D₀-⁷F₂ transition) are presented in Figure 5 (λ_ex. = 280 nm). At a low Eu³⁺ concentration (2 and 4 at %), the decay curves were fitted by a double exponential function (1), whereas the decay curves of the Eu-Lu terephthalates containing 6–100 at % Eu³⁺ were fitted by a single exponential function (2):

$${\mathrm{I}=I}_1·e^{-\frac{t}{{\tau }_1}}{+I}_2·e^{-\frac{t}{{\tau }_2}}$$ (1)

$${\mathrm{I}=I}_1·e^{-\frac{t}{{\tau }_1}}$$ (2)

where τ₁ and τ₂ are the observed ⁵D₀ lifetimes (Table 2).

Figure 5.

The 615 nm luminescence decay curves of heterometallic europium(III)–lutetium(III) terephthalates at 2, 4, 6, 10, 60, and 100 at % Eu.

Table 2.

The lifetimes of excitation state 5D0 of Eu³⁺ in heterometallic europium(III)–lutetium(III) terephthalates at 2, 4, 6, 10, 60, and 100 at % Eu.

χEu (%)	τ1, ms	τ2, ms	PLQY, %
100	0.390		10 ± 1
60	0.435		11 ± 1
10	0.449		12 ± 1
6	0.459		16 ± 1
4	0.392	1.602	22 ± 1
2	0.367	1.878	22 ± 1

The Eu-Lu terephthalates containing 6–100 at % Eu³⁺ had ⁵D₀ lifetimes of 0.390–0.459 ms and luminescence quantum yields of 10–16%. The measured PLQY of the Eu₂bdc₃·4H₂O was comparable with the literature data [31,38,45]. The ⁵D₀ lifetime values and the luminescence quantum yields decreased with an increase in the Eu³⁺ concentration due to the energy migration between the Eu³⁺ ions and subsequent quenching by impurities and defects. We demonstrated in this work that Eu³⁺ ions predominantly existed in the Ln₂bdc₃·4H₂O phase in the europium(III)–lutetium(III) terephthalates containing 6–100 at % Eu³⁺, in which the presence of a single Eu³⁺ coordination site resulted in a single ⁵D₀ lifetime. Interestingly, the europium(III)–lutetium(III) terephthalates containing 2–4 at % Eu³⁺ were characterized by two ⁵D₀ lifetimes. One emission decay component (τ₁ = 0.392–0.367 ms) was close to the value observed for the europium(III)–lutetium(III) terephthalates containing 6–100 at % Eu³⁺, while another component (τ₂ = 1.602–1.878 ms) was 4–4.8 times larger. In the Eu-Lu terephthalates containing 2–4 at % Eu³⁺, the Eu³⁺ ions were distributed between the Ln₂bdc₃·4H₂O and Ln₂bdc₃ crystalline phases. Therefore, τ₁ and τ₂ could be assigned to the Eu³⁺ ions located in the Ln₂bdc₃·4H₂O and Ln₂bdc₃, respectively. The water molecules in the Ln₂bdc₃·4H₂O structure were coordinated with the Eu³⁺ ion and quenched the Eu³⁺ luminescence due to efficient energy transfer to high-energy O-H stretching vibrational modes of coordinated water molecules [46,47]. In the Ln₂bdc₃ crystalline phase, the Eu³⁺ ion was coordinated only with oxygen atoms of carboxylic groups of terephthalate ions. The efficient quenching of Eu³⁺ ions by water molecules in the Ln₂bdc₃·4H₂O structure resulted in a significant decrease in the Eu³⁺ ion ⁵D₀ lifetime compared to anhydrous Ln₂bdc₃.The emission quantum yield of the Eu³⁺ was significantly larger for the Eu-Lu terephthalates doped with a low Eu³⁺ concentration. This observation can be explained by two reasons: the absence of efficient Eu-Eu energy migration and the presence of the Ln₂bdc₃ crystalline phase with a significantly smaller nonradiative decay rate compared to the Ln₂bdc₃·4H₂O.

3. Materials and Methods

Lutetium (III) chloride hexahydrate and europium (III) chloride hexahydrate were purchased from Chemcraft (Kaliningrad, Russia). Benzene-1,4-dicarboxylic (terephthalic, H₂bdc) acid (>98%), sodium hydroxide (>99%), nickel(II) chloride hexahydrate (>99%), and EDTA disodium salt (0.1 M aqueous solution) were purchased from Sigma-Aldrich Pty Ltd. (Germany) and used without additional purification. The 0.2M solutions of EuCl₃ and LuCl₃ were prepared and standardized using complexometric titration with EDTA. A total of 0.6 mole of sodium hydroxide and 0.3 mole of terephthalic acid were dissolved in distilled water to obtain a 1 L solution of a 0.3 M solution of the disodium terephthalate (Na₂bdc).

The heterometallic europium(III)–lutetium(III) terephthalates were obtained by mixing 1 mL of 0.2 M EuCl₃ and LuCl₃ aqueous solutions taken in stoichiometric ratios with 2 mL of 0.3 M Na₂bdc water solution (Table 3). White precipitates of heterometallic europium(III)–lutetium(III) terephthalates were separated from the reaction mixture using centrifugation (4000× g) and washed using deionized water 5 times. All samples were driedat 60 °C.

Table 3.

The heterometallic europium(III)–lutetium(III) terephthalates’ synthesis conditions.

χEu, %	V(0.2M EuCl3), mL	V(0.2M LuCl3), mL
0	0	1.00
1	0.01	0.99
2	0.02	0.98
3	0.03	0.97
4	0.04	0.96
5	0.05	0.95
6	0.06	0.94
7	0.07	0.93
8	0.08	0.92
9	0.09	0.91
10	0.10	0.90
20	0.20	0.80
30	0.30	0.70
40	0.40	0.60
50	0.50	0.50
60	0.60	0.40
70	0.70	0.30
80	0.80	0.20
90	0.90	0.10
100	1.00	0

The Eu³⁺/Lu³⁺ ratios in the heterometallic europium(III)–lutetium(III) terephthalates were confirmed using energy-dispersive X-ray spectroscopy (EDX) (EDX spectrometer EDX-800P, Shimadzu, Japan) (Table 4). The Eu/Lu ratios measured via EDX were consistent with the ratios of Eu³⁺/Lu³⁺ taken for the synthesis in the form of EuCl₃ and LuCl₃ aqueous solutions (Table 3). The X-ray powder diffraction (XRD) measurements were performed on a D2 Phaser (Bruker, USA) X-ray diffractometer using Cu K_α radiation (λ = 1.54056 Å). The thermogravimetry curves were obtained using a TG 209 F1 Libra thermo-microbalance (Netzsch, Germany). The luminescence spectra were recorded with a Fluoromax-4 fluorescence spectrometer (Horiba Jobin Yvon, Japan). Lifetime measurements were performed with the same spectrometer using a pulsed Xe lamp (pulse duration: 3 µs). The absolute values of the photoluminescence quantum yields were recorded using a Fluorolog 3 Quanta-phi device. All measurements were performed at 25 °C.

Table 4.

Eu³⁺ atomic fraction (relative to the total amount of Eu³⁺ and Lu³⁺) in heterometallic europium(III)–lutetium(III) terephthalates taken during synthesis and obtained from EDX data.

χEu (%), Taken	χEu (%), EDX
0	0
2	2.07 ± 0.21
4	3.9 ± 0.4
5	4.7 ± 0.5
6	6.3 ± 0.6
10	10.2 ± 1.0
20	19.3 ± 1.9
40	37 ± 4
80	80 ± 8
100	100

4. Conclusions

In this work, we reported on the photoluminescence properties of luminescent antenna MOF—heterometallic europium(III)–lutetium(III) terephthalates. The series of (Eu_xLu_1−x)₂bdc₃·nH₂O (x = 0–1) was synthesized in the aqueous solution. At Eu³⁺ concentrations of 1–40 at %, the heterometallic europium(III)–lutetium(III) terephthalates were formed as a mixture of the (Eu_xLu_1−x)₂bdc₃ and (Eu_xLu_1−x)₂bdc₃·4H₂O crystalline phases. At higher Eu³⁺ concentrations, a single crystalline phase was formed: (Eu_xLu_1−x)₂bdc₃·4H₂O. All the synthesized samples containing Eu³⁺ demonstrated a bright red emission corresponding to the ⁵D₀-⁷F_J (J = 0–4) transitions of Eu³⁺ ions upon 280 nm excitation into the singlet electronic excited state of terephthalate ions. An analysis of the fine structure of the emission spectra allowed us to conclude that the Eu³⁺ ions were unevenly distributed between the Ln₂bdc₃ and Ln₂bdc₃·4H₂O phases: the Ln₂bdc₃·4H₂O crystalline phase was enriched by Eu³⁺ ions. The local symmetry of the Eu³⁺ ions in the heterometallic Eu-Lu terephthalates was proposed based on a careful analysis of the fine structure of the emission spectra and the structural data. We demonstrated that the ⁵D₀ excited state lifetimes were 4–4.8 times larger for Eu³⁺ in the Ln₂bdc₃ crystalline phase than in Ln₂bdc₃·4H₂O due to the absence of luminescence quenching of the Eu³⁺ by coordinated water molecules. The luminescence quantum yields of terephthalate ions decreased with an increase in the europium concentration from 2 to 100 at % Eu³⁺ (λ_ex. = 280 nm).

Author Contributions

Conceptualization, A.S.M. and V.G.N.; methodology, A.S.M., A.A.V. and V.G.N.; software, A.S.M., M.Y.S. and N.A.B.; validation, M.N.R., A.S.K. and I.E.K.; formal analysis, A.S.M. and V.G.N.; investigation, A.S.M., I.I.T., I.E.K. and V.G.N.; resources, A.S.M., M.Y.S. and N.A.B.; data curation, A.S.M., V.G.N. and A.S.K.; writing—original draft preparation, A.S.M. and V.G.N.; writing—review and editing, A.S.K., M.Y.S., I.E.K., N.A.B., V.G.N. and A.S.M.; visualization, A.S.M., V.G.N. and S.S.K.; supervision, A.S.M.; project administration, A.S.M.; funding acquisition, A.S.M. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available in the article.

Conflicts of Interest

The authors declare no conflict of interest.

Sample Availability

Samples of the reported compounds are available from the authors.

Funding Statement

This work was supported by the Russian Science Foundation under grant no. 22-73-10040 (https://rscf.ru/en/project/22-73-10040/, accessed on 1 September 2022).