Lanthanide Heterometallic MOFs with 5-Iodosophthalate Linkers: Tuning of Luminescent Properties by Varying of Metal Composition

Abstract

Coordination polymers [Gd₂(5-iip)₃(DMF)₄]·0.4DMF (1) and [Dy₂(5-iip)₃DMF₂]·0.33DMF (2) (5-iip²⁻—5-iodoisophthalate) feature two different structural types, as follows from X-ray diffractometry data. We prepared and characterized an extended series of corresponding heterometallic complexes of the composition [Gd_xDy_2−x(5-iip)₃DMF₂]·0.33DMF and [Eu_xDy_2−x(5-iip)₃DMF₂]·0.33DMF, where x = 0.2, 0.4…1.8, and examined the changes of their luminescent properties induced by variations of metal composition, finding the promising white emission sources with high light purity.

1. Introduction

For a long time, metal–organic frameworks (MOFs) remain one of the major topics in modern coordination chemistry and related materials science, attracting numerous research teams from all over the world [1,2,3,4,5,6,7]. This interest is related to numerous useful properties demonstrated by this class of hybrid porous materials. They can be used in separation of complex mixtures of gases [8,9,10,11,12,13,14] and/or liquid organic substrates [15,16,17,18], catalysis [19,20], design of sensors [21,22,23], etc. Using combinations of organic linkers of different topology, it is possible to prepare coordination polymers of highly diverse structural types, thereby providing virtually unlimited opportunities for molecular design of desired 3D MOFs.

From the point of view of supramolecular chemistry, it is quite obvious that sorption properties of porous MOF towards certain organic or inorganic substances depend not only on the geometry of pores but also on the ability of MOF linker ligands to form a certain system of non-covalent interactions with the abovementioned substrates which can be responsible for the appearance of sorption and/or sensing selectivity. Usually, hydrogen bonds (HBs) play the most important role here. However, in recent years it has been shown that so-called halogen bond (XB), a specific type of non-covalent interactions involving halogen atoms which is being extensively studied currently [24,25,26,27,28,29], can be an efficient additional tool for “fine tuning” of MOF properties [30,31,32]. This option can be implemented by using halogen (especially iodine)-substituted linkers.

Probably the most widespread linker ligands in MOF chemistry are polycarboxylates [33,34,35,36,37,38,39], especially those which are not only commercially available but also affordable, such as terephthalate or isophthalate. From this point of view, the use of their iodinated derivatives can be a promising strategy in terms of appearance of XB-related effects in resulting MOFs. Earlier, we demonstrated [40] that this approach indeed works well for 2-iodoterephthalate-based heteroleptic polymeric complexes.

Ln³⁺ coordination polymers are being studied mostly due to their luminescent behavior [41,42,43]. These properties can be tuned by isomorphous substitution of Ln³⁺ (preparation of heterometallic complexes) [44].

In this work, we prepared 5-iodoisophthalate (5-iip) Ln³⁺-MOFs (Ln = Gd (1), Dy (2)), as well as a series of heterometallic Gd_xDy_2−x-, Eu_xDy_2−x-derivatives. Compounds 1 and 2 are not isostructural; they are characterized using X-ray diffraction. Mixed-metal frameworks are isostructural to 2, regardless of the element ratio, according to X-ray powder diffraction data. The exact ratio of lanthanides was determined using ICP OES, after which the luminescent properties were studied.

2. Results and Discussion

2.1. Crystal Structure of the Complexes 1 and 2

According to Cambridge Crystallographic Data Centre (CCDC), 1 and 2 are the first examples of homometallic coordination polymers of lanthanides with 5-iodoisophtalic acid. Only two samarium(III)-gallium(III) metallacrowns were reported in 2020 [45]. For d-metals, 5-iip was considered as an XB donor in MOFs [46,47,48,49,50].

The complexes crystallize in monoclinic (1, space group P2₁/c) and triclinic (2, space group P-1) crystal systems, respectively. Both feature the same formula and 3D motif but significantly differ from the structural point of view. Building block of 1 is given in Figure 1. Both Gd atoms are interconnected into one dimeric unit via four bridging 5-iip anions. As can be seen in Figure 2, along b axis there are layers of dimeric units which are combined into layers in ABAB manner. 5-iiP molecules interconnect these layers into 3D framework. Solvent accessible volume is 560.9 ų/cell (11.5%); it is partially filled with DMF molecules with occupancy 0.4. DMF; Gd2 were refined as disordered with occupancies of 0.75:0.50:0.75 to represent two molecules. All coordinated DMF molecules are oriented into voids thus diminishing voids accessible volume.

For non-disordered I moieties, I···O contacts can be found only in 1 (Figure 1)—3.229 (I3-O3) and 3.435 Å (I3-O10), which is below the Bondi’s van der Waals radii sum (3.50 Å [51]). The disordered I atoms are oriented toward {COO} groups or coordinated DMF molecules (in 2), and there is not much space for interaction with any possible guest molecules.

For structure 2, all I atoms were refined with total occupancy 0.95, since it resulted in a much better refinement quality. However, other techniques suggest that 5-iodoisophtalic acid used in experiments was pure. In 2, there are two slightly different dimeric building blocks around two crystallographically independent Dy atoms (Figure 3). Atom Dy1 is surrounded by 4 bridging, two chelating 5-iip units and two disordered DMF molecules. In contrast, DMF molecules coordinated to Dy2 are not disordered. In addition, one of the 5-iisoPh molecules connected only through one oxygen atom—Dy2-O3 distance is 2.235 Å, which is the shortest for 1 and 2. Interestingly, “free” O4 atom is not involved in any kind of non-covalent interactions while facing into the accessible void (Figure 4). The size of these voids is even smaller than in 1 (3.5% for 2 80.6 ų/cell); they are filled with DMF (1/3 occupancy) molecules.

2.2. Photoluminescence Properties

2.2.1. 1, 2 and Series of [Gd_xDy_2−x(5-iip)₃DMF₂]·0.33DMF

The solid-state luminescence spectra of 1 demonstrate multiple emissions (Figure 5) with the band maxima at ca. 500 nm and the band maxima at ca. 400 nm. The spectra of the complex 1 is practically identical to the spectra of the ligand [52]. In both cases, the excited state lifetime in the nanosecond domain points to intraligand transitions.

The solid-state luminescence properties of complexes Gd_xDy_2−x were investigated at room temperature.

The emission spectra of complexes, as shown in Figure 6a,b, can be ascribed to the change from excited energy level to a lower energy level. The PL spectra excited at 325 nm are very similar to those excited at 386 nm. The emission characteristics of the Dy³⁺ consisted of multiple sharp peaks with multiple wavelengths due to the unique electronic transition in the system. The compact manifolds of energy levels in the upper parts of the activator cause electrons that are excited to decay only through the ⁴F_9/2 level to the lower ⁶H_j/2 (j = 15, 13, 11) levels at 478, 575, and 661 nm, respectively. The large energy difference between each lower level causes the emission spectra to be separated at a distance.

Figure 7 depicts the photoluminescence excitation (PLE) spectra of complexes. There is no difference between the excitation complexes, except the excitation intensity (λ_em = 574 nm). It can be seen that the excitation spectra consist of some narrow peaks. In the longer wavelength region from 310 to 500 nm (Figure 7), some sharp lines due to the f–f transitions of Dy³⁺ are observed, which are assigned to the electronic transitions ⁶H_15/2-⁶P_3/2 at 325 nm, ⁶H_15/2-⁶P_7/2 at 351 nm, ⁶H_15/2-⁶P_5/2 at 365 nm, ⁶H_15/2/(⁴I_13/2 + ⁴F_7/2) at 386 nm, ⁶H_15/2-⁴G_11/2 at 427 nm, ⁶H_15/2-⁴I_15/2 at 452 nm, and ⁶H_15/2-⁴F_9/2 at 475 nm.

By comparing the emission spectra of the complexes collected under two different excitation wavelengths, we conclude that the emission peaks do not change with λ_ex. Furthermore, the ratio Y/B of emission at 574 nm (yellow) and 478 nm (blue) is almost the same for both cases (~2.8). This indicates that in this synthesized series, the predominant color is yellow. That ratio is known to be highly dependent on the site occupied by Dy³⁺. The low symmetry of the site occupied by Dy³⁺ increases the 574 nm emission in comparison to the 478 nm emission. Depending on the symmetry of the local environment and the degree of its distortion, the intensity ratio R = ⁴F_9/2 → ⁶H_13/2/⁴F_9/2 → ⁶H_15/2 is presented in the Table S4. Faint broad bands are also visible in the blue (λ_ex = 325 nm) and green (λ_ex = 386 nm) regions of the spectra.

Figure 8 shows the energy transfer processes of Dy³⁺ ions. Under the 325 nm and 386 nm excitation, Dy³⁺ ions absorb the photon energy and transfer it from the ground state ⁶H_15/2 to the metastable excited states. During this process, the excited populations are rapidly released at the ⁴F_9/2 energy level by a non-radiative (NR) process that occurs between the excited states. The transitions ⁴F_9/2-⁶H_15/2, ⁴F_9/2-⁶H_13/2, ⁴F_9/2-⁶H_11/2 are radiative and produce the emission peaks at 478, 574, and 661 nm, respectively. The concentration of Dy³⁺ ions does not clearly influence the PL intensities of complexes, but the broadband emission in the spectra affected the colorimetric parameters.

In order to determine the color parameters, the CIE coordinates (x,y) and the correlated color temperature parameter (CCT) were obtained using the standard formula. The chromaticity coordinates can be calculated by using OSRAM color calculator software (version 7.77). The CCT was determined by using McCamy empirical formula [53]:

CCT = 437n3 + 3601n2 6861n + 5514.31

(1)

where n = (x − x_e)/(y − y_e) and the value x_e = 0.3320, y_e = 0.1858.

The calculated chromaticity for different concentration levels are displayed in Figure 9 and their numerical values are shown in Table S4. The shifting in the (x,y) coordinates was observed when the concentration of the Dy³⁺ changed. It can be seen that all the coordinates are located in the greenish-yellow color region of the CIE diagram, demonstrating that the complexes have potential for white LED applications. CIE values of complex [Gd_1.31Dy_0.69(5-iip)₃DMF₂]·0.33DMF show a color close to white at λ_ex = 325 and λ_ex = 386 nm. The results exhibited the characteristics of low color-correlated temperature.

The color purity is also a key factor in producing a better light source for LED applications. To determine this factor, the following formula is used:

$$\mathrm{C}\mathrm{o}\mathrm{l}\mathrm{o}\mathrm{r} \mathrm{p}\mathrm{u}\mathrm{r}\mathrm{i}\mathrm{t}\mathrm{y}={\displaystyle \frac{\sqrt{{(x-x_{ee})}^2+{(y-y_{ee})}^2}}{{(x_d-x_{ee})}^2+{(y_d-y_{ee})}^2}}\times 100\mathrm{\%}$$

(2)

where (x, y), (x_e, y_e), and (x_d, y_d) are the chromaticity coordinates of the sample point, the equal energy point (0.33, 0.33), and the dominant wavelength point, respectively. The dominant wavelength point can be calculated based on the intersection point of the connection line between the sample point and the equal energy point. Color purity for the standard white light is 0% [54]. The color purity of complex [Gd_1.31Dy_0.69(5-iip)₃DMF₂]·0.33DMF at excitation λ_ex = 325 nm is found to be 11%. The color purity should be very low to achieve white light emission from the emitting sources [55], and the color purity for the standard white light is 0% [56]. The color purity of the [Gd_1.31Dy_0.69(5-iip)₃DMF₂]·0.33DMF complex is close enough to zero to be consistently well-suited for the generation of white light, further suggesting its utility and suitability for W-LED applications.

2.2.2. Series of [Eu_xDy_2−x(5-iip)₃DMF₂]·0.33DMF

The PLE and PL spectra complexes at 300 K were analyzed. The ions Eu³⁺ form a group of lanthanides whose complexes exhibit ionic luminescence in the visible region. The PLE spectra (Figure 10) show standard transitions for Eu³⁺ cation. Region from 240 nm to 330 nm corresponds to the area with charge transfer band (CTB) and corresponds to electron transitions from oxygen to 4f levels of cations of rare elements (from O^2– to Eu³⁺). In region from 350 nm to 470 nm, the series of sharp lines is observed. These lines correspond to intraconfigurational 4f⁶–4f⁶ transitions of Eu³⁺ cation: ⁷F₀→⁵D₄ at 362 nm, ⁷F₀→⁵L₇ at 383 nm, ⁷F₀→⁵L₆ at 395 nm, ⁷F₀→⁵D₃ at 415 nm, ⁷F₀→⁵D₂ at 465 nm.

The PL spectrum (Figure 11) consists of the standard in electronic rearrangements within the f–f transitions of europium, corresponding to the transitions from ⁵D₀→⁷F_J transition’s bands. The transition ⁵D₀→⁷F₂ (electric-dipole transition) at around 618 nm is the most prominent; rest ⁷F_J (J = 0, 1, 3, 4) bands are not very intense. It is well known that the symmetry position of the Eu³⁺ ion in the host can influence the characteristic emission. If the Eu³⁺ ions are located at the inversion symmetric site in the host, the magnetic dipole transition (⁵D₀ → ⁷F₁) dominates. When Eu³⁺ ions are located at the antisymmetric site (⁵D₀ → ⁷F₂), it promotes magnetic dipole transition. It can be seen from Figure 11 that the strongest peak located at 618 nm (⁵D₀ → ⁷F₂) peaks corresponds to red emission. So, Eu³⁺ ions are located at anti-symmetry sites of the host lattice. The intensity of the ⁵D₀ → ⁷F₂ band of the electronic transition increases with decreasing symmetry of the environment of Eu³⁺ ions. Depending on the symmetry of the local environment and the degree of its distortion, the intensity ratio R = ⁵D₀ → ⁷F₂/⁵D₀ → ⁷F₁ is presented in the Table S5.

The Figure S1 shows the dependence of integral intensity and decay time for Eu³⁺. According to obtained data, linear behavior is observed with increasing Eu³⁺ concentration.

From spectra, it can be seen (Figure S2) that the introduction of Dy³⁺ ions has affected the characteristic emission peaks only in a few complexes.

One can observe changes in the chromaticity coordinates for these complexes from the shift from white area in the CIE diagram. The calculation of the corresponding CIE coordinates and CCT (correlated color temperature) values are indicated (Figure 12, Table S5).

Figure 13 illustrates the partial energy level diagram of Eu³⁺ in complexes. Under the excitation of 395 nm wavelength, the electron absorbs energy and gets excited to the higher excited states of Eu³⁺. Finally, the excited electrons fall to the ground state by emitting red emissions through 590 nm (⁵D₀ → ⁷F₁), 618 nm (⁵D₀ → ⁷F₂), 650 nm (⁵D₀ → ⁷F₃), 700 nm (⁵D₀ → ⁷F₄) transitions of Eu³⁺ ions.

3. Materials and Methods

All reagents were obtained from commercially available sources and used without further processing. 5-Iodoisophthalic acid was prepared according to the procedure in [57] with quantitative yield. ¹H and ¹³C NMR were recorded on a Bruker Avance 500 (AV500; Bruker Elemental GmbH, Berlin, Germany) in DMSO-d₆. Elemental analysis was performed on a vario MICRO cube CHNS analyzer (Elementar, Langenselbold, Germany).

3.1. Physical Measurements

X-ray powder diffraction analysis of polycrystals was performed on a Shimadzu XRD-7000 diffractometer (CuKα radiation, Ni filter, range 5–60° 2θ, step 0.03° 2θ, accumulation 1 s.). Indexing of diffraction patterns was performed based on single-crystal data.

Thermogravimetric analysis was performed on a Netzsch TG 209 F1 Iris^® thermal analyzer (Netzsch, Hanau, Germany). Samples (10 mg) in open Al₂O₃ crucibles were heated at a rate of 10 °C/min in a helium atmosphere at a flow rate of 60 mL/min in a temperature range of 30–550 °C.

Gd, Dy, and Eu concentration was determined by inductively coupled plasma optical emission spectrometry. Quantitative measurements were carried out on ICP OES spectrometer Grand-ICP (“VMK-Optoelectronic”, Novosibirsk, Russia), spectral range from 190 to 780 nm. ICP OES analysis was performed with an axial view of the plasma. Solutions were introduced into the plasma via a OneNeb pneumatic nebulizer OneNeb (Agilent Technologies, Santa Clara, CA, USA) and a cyclone-type spray chamber (Precision Glassblowing, Centennial, CO, USA). For registration emission spectra, the following instrumental parameters were used: plasma argon flow 13 L/min, nebulizer flow 0.35 L/min, auxiliary gas flow 0.5 L/min, plasma power 1300 W.

Calibration solutions were prepared by diluting a multi-element standard solution of rare earth elements (Skat, Volgograd, Russia). A quantity of 0.5 mol/L HNO₃ was used for dilution. High pure nitric acid was additionally purified using the DuoPur sub boiling distillation system (Milestone, Sorisole, Italy). Ultra-pure water with a resistivity 18.2 MΩ cm was obtained using Direct-Q3 water purification system (Milipore, Billerica, MA, USA). Argon 99.996 purity grade was used during ICP OES measurements.

The solution for analysis was prepared from 5 mg of powdered sample in 1.5 mL of DMF with heating, after which 0.5 mL of HNO₃ (conc.) was added with stirring until complete dissolved.

An amount of 1 mL concentrated purified nitric acid was added to 0.3 mL sample solution in DMF and heated for 20 min at 100 °C. After cooling to room temperature, the solutions were diluted with 0.5 mol/L HNO₃ and ICP OES analysis was performed.

The samples were recorded using a scanning electron microscope SEM 5000 manufactured by SIQTEC (Hefei, China) with an attachment for energy-dispersive spectroscopy Oxford Xplore 30 Aztec Live Lite manufactured by Oxford Instruments (Oxford, UK). The accelerating voltage was 15 keV.

Photoluminescence and PL excitation spectra as well as the PL kinetics were recorded on a Fluorolog 3 spectrophotometer (Horiba Jobin Yvon, Paris, France) with a 450 W ozone-free xenon lamp as an excitation source. All measurements were made at room temperature. The absolute quantum yield was measured using a G8 (GMP SA, Renens, Switzerland) spectralon-coated integrating sphere, which was connected to a Fluorolog 3 spectrofluorimeter.

3.2. Synthesis and Characterization of MOFs

[Gd₂(5-iip)₃(DMF)₄]·0.4DMF (1). In 1 mL of DMF, 12.7 mg (0.034 mmol) of GdCl₃·6H₂O and 15 mg (0.051 mmol) of 5-iipH₂ were dissolved; the solution was transferred to a Pasteur pipette, sealed, and kept for 2 days at T = 120 °C with slow cooling. As a result, several orange crystals were obtained. Calc., %: C 29.7, H 2.7, N 4.1; found, %: C 29.6, H 2.7, N 4.1.

[Dy₂(5-iip)₃DMF₂]·0.33DMF (2). The synthesis is completely analogous to compound (1), only instead of GdCl₃·6H₂O, a quantity of 15.0 mg (0.034 mmol) Dy(NO₃)₃·5H₂O was used. Calc., %: C 27.3, H 1.9, N 2.4; found, %: C 27.3, H 1.9, N 2.4.

Series of [Gd_xDy_2−x(5-iip)₃DMF₂]·0.33DMF. In a glass vial, 80 mg (0.27 mmol) of 5-iipH₂ were added, the corresponding amount of GdCl₃·6H₂O and Dy(NO₃)₃·5H₂O salts (

Table 1. Reagent weights for the [Gd_xDy_2−x(5-iip)₃DMF₂]·0.33DMF series.

Ratio of GdCl3·6H2O:Dy(NO3)3·5H2O	Amount of GdCl3·6H2O, mg (mmol)	Amount of Dy(NO3)3·5H2O, mg (mmol)
0.2:1.8	6.8 (0.018)	72.1 (0.164)
0.4:1.6	13.6 (0.037)	64.1 (0.146)
0.6:1.4	20.4 (0.054)	56.1 (0.128)
0.8:1.2	27.2 (0.073)	48.1 (0.109)
1.0:1.0	33.9 (0.091)	40.0 (0.091)
1.2:0.8	40.7 (0.109)	32.0 (0.073)
1.4:0.6	47.5 (0.128)	24.0 (0.055)
1.6:0.4	54.3 (0.146)	16.0 (0.036)
1.8:0.2	61.1 (0.164)	8.0 (0.018)

) were mixed, 0.8 mL of DMF was added, and the mixture was placed in an ultrasonic bath for 5 min until the reagents were completely dissolved. After that, it was transferred to Pasteur pipettes, sealed, and kept for 2 days at T = 120 °C with slow cooling, resulting in a light-yellow powder.

Series of [Eu_xDy_2−x(5-iip)₃DMF₂]·0.33DMF. Synthesis was carried out similarly to Gd/Dy-MOFs using the corresponding salts (

Table 2. Reagent weights for the [Eu_xDy_2−x(5-iip)₃DMF₂]·0.33DMF series.

Ratio of EuCl3·6H2O:Dy(NO3)3·5H2O	Amount of EuCl3·6H2O, mg (mmol)	Amount of Dy(NO3)3·5H2O, mg (mmol)
0.2:1.8	6.7 (0.018)	72.1 (0.164)
0.4:1.6	13.4 (0.037)	64.1 (0.146)
0.6:1.4	20.1 (0.055)	56.1 (0.128)
0.8:1.2	26.8 (0.073)	48.1 (0.109)
1.0:1.0	33.5 (0.091)	40.0 (0.091)
1.2:0.8	40.2 (0.110)	32.0 (0.073)
1.4:0.6	46.8 (0.128)	24.0 (0.055)
1.6:0.4	53.5 (0.146)	16.0 (0.036)
1.8:0.2	60.2 (0.164)	8.0 (0.018)

).

The heterometallic framework series [Gd_xDy_2−x(5-iip)₃DMF₂]·0.33DMF and [Eu_xDy_2−x(5-iip)₃DMF₂]·0.33DMF are isostructural to compound 2, which was confirmed by X-ray diffraction (Figures S4 and S5, respectively). Energy dispersive X-ray elemental mapping analysis showed that each compound is an individual MOF rather than a mixture of individual coordination polymers (Figures S5–S8). ICP-OES analysis showed that the amounts of Gd³⁺, Dy³⁺ and Eu³⁺ ions in the frameworks coincide with the ratios of the reagents used.

The results of the thermogravimetric analysis for complexes 1 and 2 as well as their powder diffraction patterns are presented in the Supplementary Materials (Figures S9–S12, respectively).

3.3. Crystallographic Analysis

The diffraction data for crystals of 1 and 2 were collected on a Bruker D8 Venture diffractometer (0.5° ω- and φ-scans, fixed-χ three circle goniometer, CMOS PHOTON III detector, Mo-IμS 3.0 microfocus source, focusing Montel mirrors, λ = 0.71073 Å MoK_α radiation, N₂-flow thermostat) at 150 K. Absorption correction was applied by SADABS-2016/2 [58] and SAINT, version 2018.7-2; Bruker AXS Inc.: Madison, WI, USA, 2017). Structures were solved by SHELXT [59] and refined by full-matrix least-squares treatment against |F|² in anisotropic approximation with SHELX 2014/5 [60] in ShelXle program [61] assisted with OLEX2 1.5 GUI [62]. H-atoms were constrained and refined in the geometrically calculated positions. Crystal and structure refinement data are given in SI (Table S1). M-O bond distances for 1 and 2 are given in Table S2. In 1 iodine atoms were refined with full occupancy; however, refinement of 2 resulted in better R factors with occupancies 0.95 for each I atom. The structures of 1–2 were deposited to the Cambridge Crystallographic Data Centre as a supplementary publication, No 2441447–2441448.

4. Conclusions

As a result of the work, we obtained structural data for 2 new Ln-MOFs, and synthesized 2 series of heterometallic frameworks Gd/Dy and Eu/Dy based on 5-iodisophthalic acid. Despite their fairly similar composition, their crystal structures differ greatly. For compound 1, the formation of a network of non-covalent I···O interactions was shown.

The study of luminescent properties showed that the [Gd_xDy_2−x(5-iip)₃DMF₂]·0.33DMF series is a promising source of white emission with high light purity (11%), and the introduction of even a small amount of Eu³⁺ ions, as in the [Eu_xDy_2−x(5-iip)₃DMF₂]·0.33DMF series, leads to the production of red emission.