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Article

Structures and Luminescent Properties of Rare-Earth Metal–Organic Framework Series with Thieno[3,2b]thiophene-2,5-dicarboxylate

by
Yuliya A. Yudina
1,2,
Pavel A. Demakov
1,
Alexey A. Ryadun
1,
Vladimir P. Fedin
1 and
Danil N. Dybtsev
1,*
1
Nikolaev Institute of Inorganic Chemistry, Siberian Branch of the Russian Academy of Sciences, 3 Akad. Lavrentieva Ave., Novosibirsk 630090, Russia
2
Department of Natural Sciences, Novosibirsk State University, 2 Pirogova St., Novosibirsk 630090, Russia
*
Author to whom correspondence should be addressed.
Crystals 2022, 12(10), 1374; https://doi.org/10.3390/cryst12101374
Submission received: 31 August 2022 / Revised: 23 September 2022 / Accepted: 25 September 2022 / Published: 28 September 2022
(This article belongs to the Special Issue Structure and Properties of Metal-Organic Frameworks and MOF-Derived Materials)
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Abstract

:
Four new rare-earth metal–organic frameworks containing thieno[3,2b]thiophene-2,5-dicarboxylate (ttdc2−) with general formulae [M2(DMF)4(ttdc)3] (M3+ = Y3+ for 1, La3+ for 2, Tb3+ for 3) and [M2(H2O)2(ttdc)3] (M3+ = Lu3+ for 4) were synthesized. Their crystal structures were determined by performing a single-crystal X-ray diffraction analysis. Coordination polymers 1–3 are based on the binuclear metal-carboxylate building units with the formulae {M2(DMF)4(OOCR)6}. The six-connected blocks in 1–3 form a three-dimensional network with the primitive cubic (pcu) topology. Coordination framework 4 is based on chains comprised by stretched pseudo-binuclear metal-carboxylate building units. The chains are interconnected in four directions with ttdc2− linkers forming the 3D framework. The luminescent properties were studied for the synthesized frameworks in the solid state. All the coordination frameworks show a broad blue emission band (λex = 380 nm) typical for intra-ligand electron transitions. The sensing properties of 3 dispersions in solutions were investigated in detail and the luminescent response (quenching) was discovered in the presence of cinnamaldehyde and quinoline in diluted solutions at concentrations of as low as 4 × 10−1 vol.% and 4 × 10−2 vol.% (~3 × 10−3 M), respectively.

1. Introduction

Metal–organic frameworks (MOFs) represent a prominent class of coordination compounds. The vast variety of available organic linkers and metal nodes creates opportunity to design materials with highly tunable characteristics such as porosity [1,2,3], chemical affinity [4,5], catalytic activity [6,7,8,9], and optical properties [10,11,12] etc.
Lanthanide(III) MOFs attract special interest due to the high versatility of their coordination modes and presence of the partially occupied f-sublevel in metal centers which is responsible for magnetism [13,14,15,16], luminescence [17,18,19,20,21,22,23,24,25] and other properties. However, since f-f electron transitions are forbidden by the Laporte selection rule, lanthanide cations demonstrate low molar absorption coefficients and low quantum yields. The incorporation of a ligand with an extended π-system to effectively absorb light energy and to transfer it to the Ln3+ cation is a promising approach to overcome the Laporte prohibition. This photosensitization phenomenon is also known as the «antenna effect» [25,26].
Thieno[3,2b]thiophene-2,5-dicarboxylic acid (trans-H2ttdc, Scheme 1) is based on two conjugated thiophene heterocycles, containing the 10 e− aromatic π-system. It is a linear, structurally robust molecule with two carboxylate functional groups. Even though its structure closely resembles naphthalene-2,6-dicarboxylic acid—one of the extensively used linkers in MOF chemistry, sulfur heteroatoms predetermine a number of unique features. The polarizable sulfur atoms were shown to enhance host–guest interactions and thus increase the gas adsorption capacity and selectivity of the corresponding MOFs [27,28,29,30]. Additionally, the electron-rich aromatic thiophene heterocycle is an effective photon trap [31,32,33] and may serve as a proper photosensitizer for luminescence centers provided that HOMO/LUMO energy levels are appropriately arranged. Although the sulfur atom in five-membered aromatic rings might be considered as a potential ligand, S coordination is not usually observed in cases of the presence of stronger donor atoms in the heterocycle [34,35] or strong electron acceptor substituents, such as carboxylic groups [36,37,38]. Therefore, combining lanthanide cations with thiophene-containing ligands is a potential approach for obtaining materials with high adsorption uptakes, effective luminescence and, as a consequence, prospective sensing applications. Of all the range of organic compounds, acetone, amides and several strong-donor or strong-acceptor aromatic compounds [18,25,26,39,40,41,42,43,44,45,46] are most often described in the literature.
The present work reports the synthesis and characterization of four new rare-earth-based MOFs containing thieno[3,2b]thiophene-2,5-dicarboxylate with the formulae [M2(DMF)4(ttdc)3]·xDMF, where M3+ = Y3+ (1), La3+ (2), Tb3+ (3), and [Lu2(H2O)2(ttdc)3]·4DMF (4). A luminescence study revealed a guest-dependent quenching which allows for the semi-quantitative determination of quinoline or cinnamaldehyde at low concentrations in solutions.

2. Materials and Methods

2.1. Materials

Thieno[3,2b]thiophene-2,5-dicarboxylic acid (H2ttdc, >97.0%) was synthesized according to the previously published procedure [37]. Y(NO3)3∙6H2O (99.9% REO) and La(NO3)3∙6H2O (99.9% REO) were received from Strem Chemicals. Lu(NO3)3∙7H2O (reagent grade) was received from Reakhim. Tb(NO3)3∙5H2O (reagent grade) and N,N-dimethylformamide (DMF, reagent grade) were supplied by Vekton. Trans-Cinnamaldehyde (99%) was supplied by Sigma Aldrich. Quinoline (>97.0%) was received from TCI. All the reagents were used as received without further purification.

2.2. Instruments

Infrared (IR) spectra were obtained in the 4000−400 cm−1 range using a Bruker Scimitar FTS 2000 spectrometer in KBr pellets. Elemental CHNS analyses were carried out using a VarioMICROcube device. Powder X-ray diffraction (PXRD) data were acquired with a Shimadzu XRD-7000 diffractometer (Cu-Kα radiation, λ = 1.54178 Å) at room temperature. Thermogravimetric analyses (TGA) were performed on a Netzsch TG 209 F1 Iris instrument at a 10 K∙min−1 heating rate under an inert atmosphere. Photoluminescence excitation and emission spectra were recorded with a spectrofluorometer Horiba Jobin Yvon Fluorolog 3 equipped with 450W power ozone-free Xe-lamp, cooled R928/1860 PFR technologies photon detector with a PC177CE-010 refrigerated chamber and double grating monochromators. Standard correction curves were used for the spectra correction for source intensity and detector response. Diffraction data for single crystals of 1–4 were collected with an automated Agilent Xcalibur diffractometer equipped with AtlasS2 area detector and graphite monochromator (λ(MoKα) = 0.71073 Å). The CrysAlisPro program package [47] was used for the integration, absorption correction and determination of unit cell parameters. Dual space algorithm (SHELXT [48]) was used for structure solution and the full-matrix least squares technique (SHELXL [49]) was used for structure refinement. Anisotropic approximation was applied for all atoms, except hydrogens. Positions of hydrogen atoms of organic ligands were calculated geometrically and refined in the riding model. Details for single crystal structure determination experiments and structure refinements are summarized in Appendix A, Table A1. CCDC 2191621–2191624 entries contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Center at https://www.ccdc.cam.ac.uk/structures/ (accessed at 27 September 2022).

2.3. Synthetic Methods

Synthesis of [Y2(DMF)4(ttdc)3]·5DMF (1) First, 23.0 mg (0.06 mmol) of Y(NO3)3∙6H2O and 20.6 mg (0.09 mmol) of H2ttdc were mixed and dissolved in 2.0 mL of DMF in a 5 mL screw-capped glass vial using an ultrasonic bath. The obtained solution was thermostated at 80 °C for 48 h. After cooling in air to room temperature, the resultant colorless crystals were filtered, washed twice with DMF to remove unreacted metal salt and H2ttdc, and dried in air. Yield: 23.3 mg (54%). IR spectrum (KBr, cm−1) main bands: 3084 (w., νCsp2–H); 2928 (m., νCsp3–H); 1660 (s., νCODMF); 1485 (s., νCOOas); 1387 (s., νCOOs). Elemental CHNS analysis data, calculated for [Y2(DMF)4(ttdc)3]·3.5DMF (%): C, 39.5; H, 4.6; N, 7.4; S, 13.6. Found (%): C, 39.8; H, 4.3; N, 7.7; S, 14.2. PXRD of the filtered sample: Figure S1.
Synthesis of [La2(DMF)4(ttdc)3]·3.7DMF (2) First, 26.0 mg (0.06 mmol) of La(NO3)3∙6H2O and 20.6 mg (0.09 mmol) of H2ttdc were mixed and dissolved in 2.0 mL of DMF in a 5 mL screw-capped glass vial via ultrasonic bath assistance. The obtained solution was thermostated at 80 °C for 48 h. After cooling in air to room temperature, the resultant colorless crystals were filtered, washed twice with DMF to remove unreacted metal salt and H2ttdc, and dried in air. Yield: 26.4 mg (57%). IR spectrum (KBr, cm−1) main bands: 3090 and 3082 (w., νCsp2–H); 2926 (m., νCsp3–H); 1653 (m., νCODMF); 1485 (s., νCOOas); 1387 (s., νCOOs). Elemental CHNS analysis data, calculated for [La2(DMF)4(ttdc)3]·4DMF (%): C, 37.4; H, 4.1; N, 7.3; S, 12.5. Found (%): C, 37.8; H, 4.3; N, 7.1; S, 13.0. PXRD of the filtered sample: Figure S2.
Synthesis of [Tb2(DMF)4(ttdc)3]·5.3DMF (3) First, 26.1 mg (0.06 mmol) of Tb(NO3)3∙5H2O and 20.6 mg (0.09 mmol) of H2ttdc were mixed and dissolved in 2.0 mL of DMF in a 5 mL screw-capped glass vial with ultrasonic bath assistance. The obtained solution was thermostated at 100 °C for 48 h. After cooling in air to room temperature, the resultant colorless crystals were filtered off, washed twice with DMF to remove unreacted metal salt and H2ttdc, and dried in air. Yield: 22.3 mg (47%). IR spectrum (KBr, cm−1) main bands: 3078 and 3090 (w., νCsp2–H); 2920 (m., νCsp3–H); 1659 (s., νCODMF); 1485 (s., νCOOas); 1387 (s., νCOOs). Elemental CHNS analysis data, calculated for [Tb2(DMF)4(ttdc)3]·3DMF (%): C, 35.8; H, 3.7; N, 6.5; S, 12.8. Found (%): C, 35.9; H, 3.7; N, 6.8; S, 12.9. PXRD of the filtered sample: Figure S3.
Synthesis of [Lu2(H2O)2(ttdc)3]·4DMF (4) A total of 29.2 mg (0.06 mmol) of Lu(NO3)3∙7H2O and 20.6 mg (0.09 mmol) of H2ttdc were mixed and dissolved in 2.0 mL of DMF in a 5 mL screw-capped glass vial with ultrasonic bath assistance. The obtained solution was thermostated at 80 °C for 48 h. After cooling in air to room temperature, the resultant colorless crystals were filtered, washed twice with DMF to remove unreacted metal salt and H2ttdc, and dried in air. Yield: 25.6 mg (63%). IR spectrum (KBr, cm−1) main bands: 3427 (s., br., νO–H); 3099 and 3082 (w., νCsp2–H); 2928 (m., νCsp3–H); 1655 (s., νCODMF); 1484 (s., νCOOas); 1417 (s., νCOOs). Elemental CHNS analysis data, calculated for [Lu2(H2O)2(ttdc)3]·3.5DMF (%): C, 31.4; H, 2.6; N, 3.7; S, 14.6. Found (%): C, 31.6; H, 2.6; N, 4.1; S, 15.5. PXRD of the filtered sample: Figure S4.
Synthesis of [Tb2(DMF)4(ttdc)3]·2.5DMF·1.3cinnamal (3⊃cinnamal). A sample of [Tb2(DMF)4(ttdc)3]·3DMF was placed in a glass vial and soaked in cinnamaldehyde for 48 h at room temperature. The obtained yellow paste was filtered, washed with a minimal amount of DMF to remove cinnamaldehyde from the surface of the crystals and dried in air. IR spectrum (KBr, cm−1) main bands: 3086 (w., νCsp2–H); 2932 (m., νCsp3–H); 2810 (w., νC(O)-H); 1672 (s., νCOcinnamal); 1647 (s., νCODMF); 1485 (s., νCOOas); 1391 (s., νCOOs). Elemental CHNS analysis data, calculated for [Tb2(DMF)4(ttdc)3]·2.5DMF·1.3cinnamal (%): C, 40.3; H, 3.8; N, 5.5; S, 11.7. Found (%): C, 40.1; H, 4.0; N, 5.2; S, 11.7.
Synthesis of [Tb2(DMF)4(ttdc)3]·DMF·2.5quinoline (3⊃quinoline). A sample of [Tb2(DMF)4(ttdc)3]·3DMF was placed in a glass vial and soaked in quinoline for 48 h at room temperature. The obtained brown paste was filtered, washed with minimal amount of DMF to remove quinoline from the surface of the crystals and dried in air dried in air. IR spectrum (KBr, cm−1) main bands: 3090 (w., νCsp2–H); 2928 (m., νCsp3–H); 1618 (s., νCODMF); 1489 (s., νCOOas); 1389 (s., νCOOs). Elemental CHNS analysis data, calculated for [Tb2(DMF)4(ttdc)3]·DMF·2.5quinoline (%): C, 43.8; H, 3.5; N, 6.2; S, 11.4. Found (%): C, 43.5; H, 3.7; N, 5.9; S, 11.4.
Preparation of suspensions for luminescence measurements. Powder sample of 3 suspended in DMF was prepared according to the scaled synthetic method described above. A total of 130.5 mg (0.30 mmol) of Tb(NO3)3∙5H2O and 103.0 mg (0.45 mmol) of H2ttdc were mixed and dissolved in 10.0 mL of DMF in a 20 mL screw-capped glass vial with ultrasonic bath assistance. Then, the solution was thermostated at 100 °C for 48 h at continuous intensive stirring. After cooling in air to room temperature, the resultant thin powder dispersion was decanted, washed with 10 mL of DMF and decanted again. The washing procedure was repeated 5 times to remove unreacted Tb(III) salt and H2ttdc. A 1.0 mL of suspension of 3, prepared as described above, was sampled in a volumetric flask with 1.0 mL of the corresponding analyte solutions and diluted with DMF to a 25.0 mL total volume. No sedimentation of 3 to the bottom of the flask occurred for at least 24 h. The resultant diluted suspensions were transferred into cuvettes and analyzed with luminescent spectroscopy.

3. Results and Discussion

3.1. Synthesis and Crystal Structure Description

Compounds 1–4 were synthesized in similar solvothermal conditions, using N,N-dimethylformamide (DMF) as a solvent. Compound [Y2(DMF)4(ttdc)3]·5DMF (1) crystallizes in the P–1 space group and is similar to the previously reported MOF [Eu2(DMF)4(ttdc)3]·4DMF [44]. Unlike that Eu3+-based compound, frameworks containing Y3+, La3+ and Lu3+ appeared to form poor quality single crystals under similar synthetic conditions. Therefore, optimization experiments were carried out and the optimal temperature was found to be 80 °C, as opposed to the 100 °C for [Eu2(DMF)4(ttdc)3]·4DMF. An asymmetric unit of structure 1 is shown in Figure S5. Y(III) adopts a capped square-antiprismatic environment, which consists of two oxygen atoms of DMF, four oxygen atoms of chelate carboxylate groups and three oxygen atoms of bridging carboxylate groups. The selected coordination bond lengths are listed in Table 1. Two symmetry-equivalent Y(III) ions form binuclear metal-carboxylate blocks with the formula {Y2(DMF)4(RCOO-κ2)2(μ-RCOO-κ1,κ1)2(μ-RCOO-κ1,κ2)2} (Figure 1a). Such blocks represent six connected nodes which are interconnected by thieno[3,2b]thiophene-2,5-dicarboxylate bridges and form a three-dimensional coordination lattice bearing a pcu topology (Figure S6). The polymeric framework in 1 contains channels ca. 3 Å × 6 Å in size (Figure 2a) with a 38% total solvent accessible volume, as calculated with the PLATON [50] program. Only two DMF molecules per formula unit in 1 were located directly. The residual electron density in the voids was analyzed using the SQUEEZE [51] procedure in PLATON and the obtained electron count (121 e− in 404 Å3 void volume per formula unit) was assigned to three additional DMF molecules per f.u., resulting in [Y2(DMF)4(ttdc)3]⋅5DMF as the final composition of the crystal.
The asymmetric unit of compound [La2(DMF)4(ttdc)3]·3.7DMF (2) is shown in Figure S7. 2 has a similar form to 1 binuclear {La2(DMF)4(RCOO-κ2)2(μ-RCOO-κ1,κ1)2(μ-RCOO-κ1,κ2)2} building units and possesses the same pcu topology. However, coordinated DMF molecules, κ2- and κ1,κ1-carboxylic groups appear to be disordered between two positions (Figure 1b), which is apparently related to a larger ionic radius of La3+ (103 pm) compared to that of Y3+ (90 pm). Moreover, the metal node deformation in 2 results in the pronounced bending of the (κ1,κ1;κ1,κ1)-ttdc linker and a subsequent decrease in the total solvent accessible volume in 2 to 29%. Similar features were previously observed by our group in a cerium(III) thieno[3,2b]thiophene-2,5-dicarboxylate [52]. No guest molecules were located directly in the voids of 2. The residual electron density in these voids was analyzed using PLATON/SQUEEZE and the obtained electron count (148 e− in 453 Å3 void volume per formula unit) was assigned as 3.7 DMF molecules per f.u., leading to [La2(DMF)4(ttdc)3]⋅3.7DMF as the final composition of the crystal.
The compound [Tb2(DMF)4(ttdc)3]·5.3DMF (3) is isostructural to the yttrium-based 1, that can apparently be explained by the similarity of Tb3+ and Y3+ in the ionic radii (92 pm and 90 pm, respectively). Therefore, the lanthanide contraction phenomenon appears to be a significant factor manipulating the local structural features of the metal cation in otherwise chemically and topologically similar coordination frameworks [M2(DMF)4(ttdc)3] (M = Y3+, La3+, Tb3+). Bond lengths in the metal coordination environment for 1–3 are summarized in Table 1 and fit the typical values for Ln(III)-carboxylate complexes containing similar binuclear building blocks [53,54,55,56]. Similarly to 1, only two DMF molecules were located directly in the voids of 3. The residual electron density in these voids was analyzed by PLATON/SQUEEZE and the obtained electron count (131 e− in 422 Å3 void volume per formula unit) was assigned to 5.3 DMF molecules per f.u., giving [Tb2(DMF)4(ttdc)3]⋅5.3DMF as the final composition of the crystal.
Compound [Lu2(H2O)2(ttdc)3]·3.64DMF (4) crystallizes in the C2/c space group. The asymmetric unit of 4 is shown in Figure S8. The coordination environment of Lu(III) consists of six oxygen atoms of six carboxylate groups and one oxygen atom of a water molecule. The Lu3+ features the smallest ionic radius (86 pm) among the heavy rare-earth elements, which results in relatively short Lu–O bond lengths (2.211(3)–2.291(3) Å for Lu–O(COO), 2.349(3) Å for Lu–O(H2O)). Nevertheless, such metal–oxygen distances are typical for lutetium(III)-carboxylate MOFs or complexes containing Lu atom with a similar coordination environment [57,58,59]. The apparent coordination number (CN) of Lu3+ is reduced to 7, in comparison with CN = 9 for the other metal cations in 1–3, which again manifests the lanthanide contraction effect. Two symmetry-equivalent Lu(III) cations are linked by four (κ1,κ1)-carboxylate bridges to form a pseudo-binuclear metal-carboxylate block with the shortest Lu…Lu distance of 3.99 Å. Due to an insufficient space around the small Lu3+ cation, its coordination environment includes a H2O molecule in contrast to bulkier DMF ligands in 1–3. Neighboring pseudo-binuclear blocks are interlinked by two additional (κ1,κ1)-carboxylate groups creating an infinite chain (Figure 1c) with a 5.39 Å Lu…Lu distance between those blocks. The chains are interconnected in four directions by ttdc2− linkers forming 3D framework containing one-dimensional channels ca. 5 Å × 5 Å (Figure 2b) in size and a 46% calculated total solvent-accessible volume. Only two DMF molecules per formula unit were located directly, apparently due to the presence of hydrogen bonding between the coordinated water and DMF guests with 1.83 Å H(H2O)…O(DMF) contacts (see Figure S8). A non-ordered electron density present in the residual voids (320 e− in 1212 Å3 void volume per unit cell or 80 e− in 303 Å3 void volume per formula unit, as calculated by SQUEEZE) was attributed to two additional DMF molecules per f.u. resulting in [Lu2(H2O)2(ttdc)3]·4DMF as a general crystal composition for 4.

3.2. Thermal Properties and IR Spectra

The chemical identity and composition of compounds 1–4 were confirmed by performing elemental CHN analyses and IR spectroscopy. All the IR (Figure 3a) spectra contain typical bands corresponding to C(sp2)–H vibrations of an aromatic ligand core, C(sp3)–H vibrations of DMF molecule methyl groups, C=O vibrations in the DMF molecule, and antisymmetric and symmetric COO vibrations. The major difference between the spectra is a presence of strong O–H vibrations (broad band at ~3450 cm−1) in Lu-based compound 4, which corresponds to the coordinated water molecules, perfectly matching the crystal structure data.

3.3. Luminescence Spectrocopy

Solid-state luminescence measurements were performed for the synthesized compounds. All four coordination frameworks show a broad blue emission band (λex = 380 nm) typical for intra-ligand π-π* electron transitions. The corresponding emission spectra are presented in Figure 4. The spectral maxima for 1 (Y) and 3 (Tb) appear in a close range of λmax = 457–460 nm. The maximum spectra for 2 (La) were slightly red-shifted (λmax = 472 nm) in comparison to 1 and 3; this is apparently caused by the presence of three structurally different ttdc2− ligands in 1–3 and a subsequent variation in their vibrational modes which affects intra-ligand luminescence. As previously reported, [Eu2(DMF)4(ttdc)3]·4DMF [44] demonstrates the characteristic red emission, indicating an efficient energy transfer from the ttdc2− ligand to the Eu3+ center. On the contrary, no apparent energy transfer occurs from ttdc2− to Tb3+ likely because the LUMO π* orbitals of the ligand lie between the LUMO levels of green-emitting Tb3+ and red-emitting Eu3+.
Since the luminescence properties of the title MOFs are generally comparable, only Tb(III)-based compound 3 was chosen for further guest-dependent luminescence experiments. Several typical donors (o-, m- and p-cresoles) or typical acceptors (cinnamaldehyde, quinoline and pyridine), representing important industrial analytes, were selected. At first, the substrates were added to the suspension of 3 in DMF (see the Experimental section for details) at up to 4 vol.%. Compared to the crystalline sample, the emission spectrum of suspension 3 (Figure S9; λex = 320 and 360 nm) is more complex and contains additional bands in the UV range (λem = 360, 380, 410 nm). Such a difference may be explained again by the different vibrational modes of structurally independent ttdc2− ligands, as the lower excitation wavelengths (λex = 320 or 360 nm vs. λex = 380 nm) make possible the emergence of the more energy-demanding transitions in the emission spectra. More interestingly, the addition of cresoles and pyridine did not change luminescence properties of 3 in the visible range while an addition of cinnamaldehyde prevents luminescence completely. An addition of quinoline also quenches the luminescence but only at λex = 320 nm.
So long as both cinnamal and quinoline feature strong absorption in the near-UV region of their own, additional experiments are to be carried out to prove the luminescent detection of those substrates with a porous MOF instead of just competitive absorption. First, an incorporation of the guests into 3 was confirmed as the solid adducts 3⊃cinnamal and 3⊃quinoline were obtained. Elemental analysis data unambiguously show a partial substitution of DMF by cinnamal or quinoline resulting in the chemically reasonable formula [Tb2(DMF)4(ttdc)3]·2.5DMF·1.3cinnamal (3⊃cinnamal) or [Tb2(DMF)4(ttdc)3]·DMF·2.5quinoline (3⊃quinoline). IR spectra are also consistent with the presence of cinnamal and quinoline in the pores of 3. Absorbtion bands in the 3⊃cinnamal spectrum (Figure S10) at 2810 cm−1 and 1672 cm−1 match the oscillations of the aldehyde group conjugated with C=C double bond as in cinnamaldehyde. The 3⊃quinoline spectrum (Figure S10) shows a characteristic band at 1200 cm−1 indicating a presence of a moiety containing a pyridine core, i.e., quinoline. Next, DMF solutions of cinnamal and quinoline with lower concentrations were prepared to elucidate the minimal concentration at which the quenching response occurs. The excitation spectra for λem = 440 and 470 nm were recorded and shown in Figure S11 and Figure S12. For the spectra of pure suspension of 3 in DMF, a broad multimodal excitation peak appears in the range ca. 270–420 nm. Upon addition of cinnamal or quinoline, the lower-than-320 nm components of the spectra completely vanished, which might be attributed to the competitive absorption of an excess of cinnamal and quinoline, dissolved in solution. Such a decrease in the higher-energy excitation of 3 in quinoline solutions corresponds well with the UV/vis absorption spectrum of quinoline in the DMF solution represented in Figure S13. Moreover, a similar UV/vis spectrum for cinnamal shows very weak absorption in the λ > 330 nm region, so the quenching of 3 in cinnamal solutions at λex = 360 nm (Figure 5) indicates intensive energy transfer between the host and the adsorbed guest. At the same time, the excitation spectra at higher wavelengths increase substantially, most notably in the case of quinoline. All these data point to an efficient interplay between the luminescent MOF host and the guest substrate molecules through some short-range intermolecular interactions, additionally confirming the adsorption detection mechanism.
The emission spectra of the DMF suspension of 3 in the presence of the analytes are shown on Figure 5. As it can be clearly seen, the substantial decrease in luminescence intensity was already observed for 4 × 10−1% v/v of cinnamal (λex = 360 nm). A comparable level of cinnamal detection was reported previously for other Tb-based MOF [Tb2(phen)2(NO3)2(chdc)2]·2DMF (phen = 1,10-phenantroline, chdc2− = thans-1,4cyclohexanedicarboxylate) [60]. Remarkably, the reliable detection of quinoline through luminescence quenching is secured at as low as 4 × 10−2 % v/v (ca. 3 × 10−3 M) concentrations (λex = 320 nm). Interestingly, completely different luminescence behaviors were obtained upon sample excitation by λex = 360 nm (Figure S14) as the intensity of the emission spectra increased by up to three times upon an addition of the quinoline to the DMF suspension of 3. Such complex luminescence behavior is rather unusual and requires more experimental/theoretical efforts for its accurate explanation. Nevertheless, on the basis of the experimental data reported here, it is clear that compound 3 has a promising application potential in the detection or semi-quantitative determination of quinoline, cinnamal or, possibly, other important chemicals.

4. Conclusions

In summary, four new rare-earth metal–organic frameworks containing thieno[3,2b]thiophene-2,5-dicarboxylate (ttdc2−) were synthesized and characterized. Their crystal structures were established by performing an X-ray diffraction analysis of single crystals. A lanthanide contraction phenomenon was found to be a considerable factor driving the structural features of the frameworks along the rare earth metals row. All the compounds possess three-dimensional porous coordination networks with specific solvent-accessible volumes ranging from 30% to 46%. Luminescent properties measurements revealed the intra-ligand blue emission for all the synthesized frameworks. The luminescence properties of the terbium-based framework [Tb2(DMF)4(ttdc)3] in the presence of different organic guest molecules were investigated. The obtained experimental data show that a reliable detection of quinoline and cinnamal with luminescence quenching is possible at milimolar concentrations, suggesting promising sensing applications for the studied MOFs.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/cryst12101374/s1. Figure S1: Experimental PXRD pattern for 1 in comparison to the theoretical pattern; Figure S2: Experimental PXRD pattern for 2 in comparison to the theoretical pattern; Figure S3: Experimental PXRD pattern for 3 in comparison to the theoretical pattern; Figure S4: Experimental PXRD pattern for 4 in comparison to the theoretical pattern; Figure S5: The asymmetric unit of Y-containing structure 1; Figure S6: Topological representation of coordination framework in 1; Figure S7: The asymmetric unit of La-containing structure 2; Figure S8: The asymmetric unit of Lu-containing structure 4; Figure S9: Emission spectra for 3 suspension with selected substrates at λex = 360 nm (left) and 320 nm (right); Figure S10: IR spectra for 3 soaked in quinoline and cinnamal; Figure S11: Excitation spectra for 3 suspensions in cinnamal solutions at λem = 440 nm (a) and λem = 470 nm (b); Figure S12: Excitation spectra for 3 suspensions in quinoline solutions at λem = 440 nm (a) and λem = 470 nm (b); Figure S13: UV/vis absorption spectra for DMF solutions of cinnamal (3·10−5 M) and quinoline (6·10−5 M); Figure S14: Emission spectra for 3 suspensions in quinoline solutions at λex = 360 nm; Figure S15: Solid-state luminescence excitation spectrum of 3 at λem = 450 nm.

Author Contributions

Y.A.Y.—writing (original draft preparation), synthesis, characterization, graphing. P.A.D.—writing (original draft preparation), graphing, single-crystal XRD. A.A.R.—solid-state luminescence measurements. V.P.F.—writing (manuscript review and editing), resources. D.N.D.—writing (manuscript review and editing), conceptualization, project administration, funding acquisition. All authors have read and agreed to the published version of the manuscript.

Funding

The research was supported by Russian Science Foundation, project № 18-13-00203, https://rscf.ru/project/18-13-00203/ (accessed at 27 September 2022). The analytical services were supported by the Ministry of Science and Higher Education of the Russian Federation, projects № 121031700321-3 and 121031700313-8.

Informed Consent Statement

Not applicable.

Data Availability Statement

Samples of compounds 1–4 are available from authors.

Conflicts of Interest

The authors declare no conflict of interest.

Appendix A. The Crystallographic Data for 1–4

Table
Table A1. Single crystal X-ray diffraction analysis and structure refinement details.
Table A1. Single crystal X-ray diffraction analysis and structure refinement details.
1234
Chemical formulaC51H69N9O21S6Y2C47.1H59.9La2N7.7O19.7S6C51.9H71.1N9.3O21.3S6Tb2C36H38Lu2N4O18S6
Mr, g/mol1514.331519.301676.281357.00
Crystal systemTriclinicTriclinicTriclinicMonoclinic
Space groupP¯1P¯1P¯1C2/c
Temperature, K150150150170
a, Å11.9443(6)10.8057(4)11.9884(4)20.8816(5)
b, Å12.1749(7)12.0790(4)12.1592(5)12.7885(3)
c, Å12.9163(7)13.1351(4)13.0067(3)18.4579(6)
α, °101.820(4)105.625(3)101.739(3)90
b, °101.592(4)98.423(3)101.689(3)108.470(3)
γ, °108.524(5)100.939(3)108.713(3)90
V, Å31670.07(17)1585.27(10)1683.24(10)4675.2(2)
Z1114
F(000)7807648442656
D(calc.), g·cm−31.5061.5911.6541.928
μ, mm−11.991.602.354.54
Crystal size, mm0.37 × 0.35 × 0.120.22 × 0.11 × 0.070.42 × 0.23 × 0.120.43 × 0.37 × 0.20
θ range for data collection, °2.1 ≤ θ ≤ 25.42.0 ≤ θ ≤ 25.42.1 ≤ θ ≤ 25.42.1 ≤ θ ≤ 25.4
No. of reflections: measured/independent/observed [I > 2σ(I)]10061/
6002/
5090		11546/
5795/
4987		18265/
5984/
5470		8366/
4277/
3785
Rint0.04970.02130.03550.0250
Index ranges–11 ≤ h ≤ 14
–14 ≤ k ≤ 14
–15 ≤ l ≤ 12		–13 ≤ h ≤ 10
–14 ≤ k ≤ 14
–13 ≤ l ≤ 15		–14 ≤ h ≤ 14
–14 ≤ k ≤ 14
–15 ≤ l ≤ 15		–16 ≤ h ≤ 25
–15 ≤ k ≤ 13
–22 ≤ l ≤ 14
Final R indices [I > 2σ(I)]R1 = 0.0660
wR2 = 0.1743		R1 = 0.0316
wR2 = 0.0746		R1 = 0.0460
wR2 = 0.1227		R1 = 0.0259
wR2 = 0.0652
Final R indices
(all data)		R1 = 0.0769
wR2 = 0.1821		R1 = 0.0406
wR2 = 0.0784		R1 = 0.0506
wR2 = 0.1252		R1 = 0.0304
wR2 = 0.0667
Goodness-of-fit on F21.0541.0431.0331.060
Largest diff. peak, hole, e/Å31.86/–1.040.59/–1.322.77/–1.291.60/–1.00

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Crystals 12 01374 sch001 550
Scheme 1. Structural formula of thieno[3,2b]thiophene-2,5-dicarboxylic acid (trans-H2ttdc).
Scheme 1. Structural formula of thieno[3,2b]thiophene-2,5-dicarboxylic acid (trans-H2ttdc).
Crystals 12 01374 sch001
Crystals 12 01374 g001 550
Figure 1. Binuclear carboxylate secondary building units in 1 (a), 2 (b) and one-dimensional lutetium(III)-carboxylate chains in 4 (c). Metal atoms are turquoise, N atoms are blue, O atoms are red. H atoms (grey) are shown only in H2O ligands.
Figure 1. Binuclear carboxylate secondary building units in 1 (a), 2 (b) and one-dimensional lutetium(III)-carboxylate chains in 4 (c). Metal atoms are turquoise, N atoms are blue, O atoms are red. H atoms (grey) are shown only in H2O ligands.
Crystals 12 01374 g001
Crystals 12 01374 g002 550
Figure 2. Projections of three-dimensional coordination frameworks in 1 along b axis (a) and in 4 along c axis (b). Ln atoms are turquoise, N atoms are blue, O atoms are red, S atoms are yellow. H atoms (grey) are shown only in water molecules.
Figure 2. Projections of three-dimensional coordination frameworks in 1 along b axis (a) and in 4 along c axis (b). Ln atoms are turquoise, N atoms are blue, O atoms are red, S atoms are yellow. H atoms (grey) are shown only in water molecules.
Crystals 12 01374 g002
Crystals 12 01374 g003 550
Figure 3. IR spectra for compounds 1–4 (a). TG plots for compounds 1–4 (b). According to thermogravimetric analyses (TGA, Figure 3b), all the compounds slowly lose their guest and coordinated solvent molecules at heating temperatures of up to 340 °C. The stepwise curve for 1 shows the loss of 3.5 guest DMF molecules at ca. 100 °C (22% weight loss; calculated for 3.5DMF–18%) and then four coordinated DMF molecules at ca. 190 °C (18% weight loss; calculated for 4DMF–21%). The total weight losses for 2 and 3 at 400 °C correspond to four and seven DMF molecules, respectively. The stepwise TGA curve for 4 demonstrates the 20% weight loss in the 100–140 °C interval, corresponding to ~3.6 guest DMF molecules (calculated for 3.6 guest DMF–20%) followed by the removal of two coordinated H2O molecules at ca. 310 °C (4% weight loss; calculated for 2H2O–3%). Therefore, the thermogravimetric analysis agrees well with the elemental analysis data, except for in the case of 2, where some level of underestimation of the guest content based on TGA data might be attributed to the weathering of two DMF guest molecules during sample handling. An irreversible framework decomposition in 1–4 starts at ca. 440 °C. Such thermal stabilities of the coordination frameworks are comparable to those for other reported Ln(III)-based MOFs based on ttdc2− ligand and its derivatives [39,41,44,52].
Figure 3. IR spectra for compounds 1–4 (a). TG plots for compounds 1–4 (b). According to thermogravimetric analyses (TGA, Figure 3b), all the compounds slowly lose their guest and coordinated solvent molecules at heating temperatures of up to 340 °C. The stepwise curve for 1 shows the loss of 3.5 guest DMF molecules at ca. 100 °C (22% weight loss; calculated for 3.5DMF–18%) and then four coordinated DMF molecules at ca. 190 °C (18% weight loss; calculated for 4DMF–21%). The total weight losses for 2 and 3 at 400 °C correspond to four and seven DMF molecules, respectively. The stepwise TGA curve for 4 demonstrates the 20% weight loss in the 100–140 °C interval, corresponding to ~3.6 guest DMF molecules (calculated for 3.6 guest DMF–20%) followed by the removal of two coordinated H2O molecules at ca. 310 °C (4% weight loss; calculated for 2H2O–3%). Therefore, the thermogravimetric analysis agrees well with the elemental analysis data, except for in the case of 2, where some level of underestimation of the guest content based on TGA data might be attributed to the weathering of two DMF guest molecules during sample handling. An irreversible framework decomposition in 1–4 starts at ca. 440 °C. Such thermal stabilities of the coordination frameworks are comparable to those for other reported Ln(III)-based MOFs based on ttdc2− ligand and its derivatives [39,41,44,52].
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Figure 4. The normalized emission spectra and CIE 1931 chromaticity diagram for 1–4 at λex = 380 nm.
Figure 4. The normalized emission spectra and CIE 1931 chromaticity diagram for 1–4 at λex = 380 nm.
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Figure 5. Emission spectra for DMF suspension of 3 with cinnamaldehyde at λex = 360 nm (a) and with quinoline at λex = 320 nm (b) at different concentrations.
Figure 5. Emission spectra for DMF suspension of 3 with cinnamaldehyde at λex = 360 nm (a) and with quinoline at λex = 320 nm (b) at different concentrations.
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Table
Table 1. Selected bond lengths in the structures 1–3.
Table 1. Selected bond lengths in the structures 1–3.
Bond1 (M = Y)2 (M = La)3 (M = Tb)
M–O(DMF), Å2.334(4); 2.363(4)2.365(5)–
2.667(5)		2.362(5); 2.395(5)
M–O(COO-κ1,κ1), Å2.415(3); 2.454(4)2.444(6)–
2.698(5)		2.448(4);
2.483(4)
M–O(COObridge), Å2.284(4); 2.343(3);
2.373(3); 2.385(4)		2.351(8)–
2.651(8)		2.325(4); 2.364(4);
2.409(4); 2.433(4)
M–Ocap, Å2.767(4)2.706(2)2.719(4)
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Yudina, Y.A.; Demakov, P.A.; Ryadun, A.A.; Fedin, V.P.; Dybtsev, D.N. Structures and Luminescent Properties of Rare-Earth Metal–Organic Framework Series with Thieno[3,2b]thiophene-2,5-dicarboxylate. Crystals 2022, 12, 1374. https://doi.org/10.3390/cryst12101374

AMA Style

Yudina YA, Demakov PA, Ryadun AA, Fedin VP, Dybtsev DN. Structures and Luminescent Properties of Rare-Earth Metal–Organic Framework Series with Thieno[3,2b]thiophene-2,5-dicarboxylate. Crystals. 2022; 12(10):1374. https://doi.org/10.3390/cryst12101374

Chicago/Turabian Style

Yudina, Yuliya A., Pavel A. Demakov, Alexey A. Ryadun, Vladimir P. Fedin, and Danil N. Dybtsev. 2022. "Structures and Luminescent Properties of Rare-Earth Metal–Organic Framework Series with Thieno[3,2b]thiophene-2,5-dicarboxylate" Crystals 12, no. 10: 1374. https://doi.org/10.3390/cryst12101374

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