Synthesis and Luminescence Properties of New Metal-Organic Frameworks Based on Zinc(II) Ions and 2,5-Thiophendicarboxylate Ligands

Six new metal-organic frameworks based on 2,5-thiophendicarboxylate (tdc2–) and zinc(II) ions were prepared in different reaction conditions, and their crystal structures were determined by XRD analysis. The compound [Zn(tdc)(dabco)(H2O)]·DMF (1) is based on mononuclear Zn(II) ions connected by tdc2– and dabco linkers into square-grid layered nets. The compound [Zn3(tdc)3(dabco)2] (2) is a rare example of monocoordinated dabco ligands in the metal-organic framework chemistry. Its crystal structure contains trinuclear linear carboxylate building units, connected into a distorted primitive cubic net. Similar trinuclear units were also found in [Zn5(tdc)4(Htdc)2(dabco)2]·4DMF·14H2O (3), although as a part of more complicated pentanuclear motives. The compound [Na2Zn(tdc)2(DMF)2] (4), quantitatively isolated by the addition of NaOH to the mixture of Zn(NO3)2 and H2tdc, is based on 1D chain motives, interconnected by tdc2– linkers into a three-dimensional framework. The compounds [Zn3(tdc)3(DMF)2]·0.8DMF·1.1H2O (5) and [Zn3(tdc)3(DMF)3]·0.8DMF·1.3H2O (6) were prepared in very similar reaction conditions, but with different times of heating, indirectly indicating higher thermodynamic stability of the three-dimensional metal-organic framework 6, compared to the two-dimensional metal-organic framework 5. The crystal structures of both 5 and 6 are based on the same trinuclear linear units as in 2. Luminescence properties of the compounds 4–6 were studied and compared with those for Na2tdc salt. In particular, the luminescence spectra of 4 practically coincide with those for the reference Na2tdc, while 5 and 6 exhibit coherent shifts of peaks to higher energies. Such hypsochromic shifts are likely associated with a different effective charge on the tdc2– anions in Na2tdc and sodium-containing 4, compared to zinc-based 5 and 6.


Introduction
Porous coordination polymers (metal-organic frameworks (MOFs)) represent periodic one-(1D), two-(2D) or three-dimensional (3D) structures consisting of metal ions or polynuclear fragments connected by bridging organic ligands. For the last two decades, such compounds have been attracting a great deal of attention due to their fascinating properties and potential applications, such as luminescence sensing [1][2][3][4][5], storage and/or separation of a gases [6][7][8], heterogeneous catalysts [9][10][11], drug delivery [12][13][14], etc. To a large extent, such a variety of potential applications for MOFs is conditioned by the design of porous structures with a precise distribution of functional groups along the internal surface [15][16][17][18]. However, the rational synthesis of a target metal-organic framework is

Single-Crystal X-ray Diffraction
The diffraction data for the compounds 1-6 were collected on an automated Agilent Xcalibur diffractometer (Agilent Technologies, Santa Clara, CA, USA) equipped with a two-dimensional AtlasS2 detector (graphite monochromator, λ(MoKα) = 0.71073 Å, ω-scans). Integration, absorption correction and determination of unit cell parameters were performed using the CrysAlisPro program package [20]. The structures were solved by a dual space algorithm (SHELXT [21]) and refined by the full-matrix least squares technique (SHELXL [22]) in the anisotropic approximation (except hydrogen atoms). Positions of hydrogen atoms of organic ligands were calculated geometrically and refined in the riding model. In all structures, solvate guest molecules are highly disordered and could not be modeled as a set of discrete atomic sites. The final formula of the compound 3 was calculated from the data of the PLATON/SQUEEZE procedure [23] (1191ē in 4359 Å 3 ). The crystallographic data and details of the structure refinements are summarized in Table 1. Selected interatomic distances and valence angles are given in Tables S1-S6. CCDC 1586559-1586564 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Center at http://www.ccdc.cam.ac.uk/data_request/cif.   (1) Zero-point-one-six grams (0.54 mmol) Zn(NO 3 ) 2 ·6H 2 O, 0.046 g (0.27 mmol) H 2 tdc, 0.015 g (0.13 mmol) dabco and 0.038 g (0.27 mmol) ammonium oxalate monohydrate in 3.2 mL DMF were stirred with a magnetic stirrer for 1 h. An opaque solution was filtered, and NMe 3 (9.3 µL, 0.067 mmol) was added to the supernatant. The mixture was homogenized in an ultrasonic bath and filtered again. The vial with the resulting supernatant solution was heated at 130 • C for 2 days. The product 1 was isolated as colorless plate single-crystals, whose crystal structure and chemical composition were established by a single-crystal X-ray diffraction method.

Synthesis of [Zn 3 (tdc) 3 (dabco) 2 ] (2)
Zero-point-zero-eight grams (0.27 mmol) Zn(NO 3 ) 2 ·6H 2 O, 0.046 g (0.27 mmol) H 2 tdc and 0.015 g (0.13 mmol) dabco in 2.0 mL DMF and 0.5 mL MeCN were stirred with a magnetic stirrer for 1 h. Then, 0.7 mL of ethylene glycol were added; the vial was closed and kept at 130 • C for 2 days. Some single crystals in the form of plates were isolated, and their crystal structure and chemical composition were established by a single-crystal X-ray diffraction method.
The needle-like crystals of 4, suitable for single-crystal XRD, could be obtained in a slightly modified procedure. The above-mentioned homogeneous suspension was filtered, and a supernatant solution was heated in a sealed vial at 130 • C for 1 day. The needle-like crystals of 4 were isolated (0.02 g, 13%).
The compound 4 can be also obtained, although with notably lower yields not exceeding 60%, when other sodium(I) salts (NaNO 3 , NaClO 4 , NaCl, NaBr) are added to the reaction mixture instead of NaOH.

The colorless plane single crystals [Zn(tdc)(dabco)(H
were manually isolated out of the reaction mixture of Zn(NO 3 ) 2 , 2,5-thiophenedicarboxylic acid (H 2 tdc), 1,4-diazobicyclo[2,2,2]octane (dabco) and (NH 4 ) 2 (C 2 O 4 ) in DMF solvent. The crystal structure and chemical composition of 1 were established by the single-crystal X-ray diffraction method. The asymmetric unit of 1 contains a zinc(II) cation, the coordination environment of which consists of two N atoms of two dabco ligands and four O atoms of two carboxylate groups of two tdc 2− anions and an aqua-ligand ( Figure S4). Zn-O distances are in the range 2.016(3)-2.328(4) Å, and the Zn-N distance is 2.202(2) Å. One of the COO-groups is coordinated to zinc bidentately with one Zn-O distance being longer than another one. Each tdc 2− anion connects with two Zn(II) cations to form a polymeric carboxylate chain. Polymeric chains connect with each other via bridging dabco ligands with the formation of metal-organic layers with a square-grid topology (sql), parallel to the ab plain ( Figure 1). Such grids are packed one atop another (AAAA packing mode), forming open channels in the c crystallographic direction with a rectangular cross-section, which are filled with guest DMF molecules.
The colorless plate single crystals [Zn 3 (tdc) 3 (dabco) 2 ] (2) were manually isolated out of the reaction mixture of Zn(NO 3 ) 2 , H 2 tdc and dabco in the DMF/acetonitrile/ethylene glycol mixture. The crystal structure and chemical composition of 2 were established by the single-crystal X-ray diffraction method. The asymmetric unit in 2 contains two zinc(II) cations. Zn(1) is located in a distorted octahedral coordination environment of six O atoms of six tdc 2− anions. Zn(2) has a distorted tetrahedral coordination environment, which consists of three O atoms of three tdc 2− anions and one N atom of a terminal dabco ligand ( Figure S5 with two Zn(II) cations to form a polymeric carboxylate chain. Polymeric chains connect with each other via bridging dabco ligands with the formation of metal-organic layers with a square-grid topology (sql), parallel to the ab plain ( Figure 1). Such grids are packed one atop another (AAAA packing mode), forming open channels in the c crystallographic direction with a rectangular crosssection, which are filled with guest DMF molecules.   The compound [Na2Zn(tdc)2(DMF)2] (4) was obtained with almost quantitative yield by heating of Zn(NO3)2, H2tdc and NaOH in DMF solvent. Its crystal structure and chemical composition were established by a single-crystal X-ray diffraction method and confirmed by elemental, thermogravimetric analyses and IR data. An asymmetric unit of the structure 4 contains one Zn(II) cation and one Na(I) cation ( Figure S8). The Na + cation has a distorted octahedral coordination environment consisting six O atoms of one DMF molecule and five coordinated COO-groups. The Na-O distances are in the range 2.243(3)-2.647(2) Å. The Zn(II) cation also has a distorted octahedral coordination environment, which consists of six O atoms of five COO-groups. One of the COOgroups is coordinated to zinc bidentately with one Zn-O distance being longer than the other one. The Zn-O distances are in the range 1.981(4)-2.422(5) Å. Zn(II) and Na(I) cations are interconnected via bridging COO-groups to form polymeric carboxylate chains running along the c axis (Figure 4a). The chains are interconnected via bridging tdc 2− anions along the a and b directions to form a 3D metal-organic framework (Figure 4b). The compound [Na 2 Zn(tdc) 2 (DMF) 2 ] (4) was obtained with almost quantitative yield by heating of Zn(NO 3 ) 2 , H 2 tdc and NaOH in DMF solvent. Its crystal structure and chemical composition were established by a single-crystal X-ray diffraction method and confirmed by elemental, thermogravimetric analyses and IR data. An asymmetric unit of the structure 4 contains one Zn(II) cation and one Na(I) cation ( Figure S8). The Na + cation has a distorted octahedral coordination environment consisting six O atoms of one DMF molecule and five coordinated COO-groups.  The powder X-ray diffraction data clearly support the phase purity of the sample ( Figure 5). Microelemental (Carbon-Hydrogen-Nitrogen-Sulfur, CHNS) analyses, thermogravimetric analysis and FT-IR spectroscopy confirm the chemical formula of 4 and support the nature of the compound. The TGA of 4 shows a three-step decomposition curve ( Figure S9). The first weight loss (6.0 mass. %) at 70 °C corresponds to two H2O molecules. The second step (23%) at 280 °C corresponds to the evaporation of two coordinated DMF molecules. The irreversible decomposition of the framework 4 takes place above 300 °C. The IR spectrum of 4 shows typical stretchings for carboxylate groups (1592 and 1364 cm −1 ), C-H valence vibrations (3101-2863 cm −1 ) and the characteristic peak for the C=O group of the DMF molecules (1672 cm -1 ) ( Figure S1). Photoluminescence measurements of 4 reveal a broad excitation peak at λ = 375 nm (detection at 425 nm) and a broad emission peak at λ = 434 nm (excitation at 350 nm). The powder X-ray diffraction data clearly support the phase purity of the sample ( Figure 5). Microelemental (Carbon-Hydrogen-Nitrogen-Sulfur, CHNS) analyses, thermogravimetric analysis and FT-IR spectroscopy confirm the chemical formula of 4 and support the nature of the compound. The TGA of 4 shows a three-step decomposition curve ( Figure S9). The first weight loss (6.0 mass. %) at 70 • C corresponds to two H 2 O molecules. The second step (23%) at 280 • C corresponds to the evaporation of two coordinated DMF molecules. The irreversible decomposition of the framework 4 takes place above 300 • C. The IR spectrum of 4 shows typical stretchings for carboxylate groups (1592 and 1364 cm −1 ), C-H valence vibrations (3101-2863 cm −1 ) and the characteristic peak for the C=O group of the DMF molecules (1672 cm -1 ) ( Figure S1). Photoluminescence measurements of 4 reveal a broad excitation peak at λ = 375 nm (detection at 425 nm) and a broad emission peak at λ = 434 nm (excitation at 350 nm).  The powder X-ray diffraction data clearly support the phase purity of the sample ( Figure 5). Microelemental (Carbon-Hydrogen-Nitrogen-Sulfur, CHNS) analyses, thermogravimetric analysis and FT-IR spectroscopy confirm the chemical formula of 4 and support the nature of the compound. The TGA of 4 shows a three-step decomposition curve ( Figure S9). The first weight loss (6.0 mass. %) at 70 °C corresponds to two H2O molecules. The second step (23%) at 280 °C corresponds to the evaporation of two coordinated DMF molecules. The irreversible decomposition of the framework 4 takes place above 300 °C. The IR spectrum of 4 shows typical stretchings for carboxylate groups (1592 and 1364 cm −1 ), C-H valence vibrations (3101-2863 cm −1 ) and the characteristic peak for the C=O group of the DMF molecules (1672 cm -1 ) ( Figure S1). Photoluminescence measurements of 4 reveal a broad excitation peak at λ = 375 nm (detection at 425 nm) and a broad emission peak at λ = 434 nm (excitation at 350 nm).  Figure 5. Powder X-ray diffraction data for compounds 4 (black), 5 (red) and 6 (blue). The experimental data (normal graphs) were collected at room temperature. The theoretical plots (reverse graphs) are simulated from the corresponding single-crystal X-ray diffraction data, collected at 130 K. The compound [Zn 3 (tdc) 3 (DMF) 2 ]·0.8DMF·1.1H 2 O (5) was prepared by solvothermal reaction of Zn(NO 3 ) 2 and H 2 tdc in a DMF/acetonitrile mixture at 90 • C. After one day, the product was isolated as colorless rectangular plate single crystals ( Figure S10a), the structure and chemical composition of which were established by a single-crystal X-ray diffraction method and confirmed by elemental, thermogravimetric and IR analysis. The asymmetric unit of 5 contains three Zn(II) cations. Zn (1)  . Powder X-ray diffraction data for compounds 4 (black), 5 (red) and 6 (blue). The experimental data (normal graphs) were collected at room temperature. The theoretical plots (reverse graphs) are simulated from the corresponding single-crystal X-ray diffraction data, collected at 130 K.
The compound [Zn3(tdc)3(DMF)2]•0.8DMF•1.1H2O (5) was prepared by solvothermal reaction of Zn(NO3)2 and H2tdc in a DMF/acetonitrile mixture at 90 °C. After one day, the product was isolated as colorless rectangular plate single crystals (Figure S10a), the structure and chemical composition of which were established by a single-crystal X-ray diffraction method and confirmed by elemental, thermogravimetric and IR analysis. The asymmetric unit of 5 contains three Zn(II) cations. Zn (1)   The powder X-ray diffraction data support the phase purity of the bulk sample ( Figure 5). Microelemental (CHNS) analyses, thermogravimetric analysis and FT-IR spectroscopy confirm the chemical formula of 5 and support the nature of the compound. The TGA of 5 shows a three-step decomposition curve ( Figure S11). The first weight loss (6.0 mass. %) at 165 °C corresponds to 0.8 DMF molecules. The second step (16%) at 265 °C corresponds to the evaporation of two coordinated DMF molecules. The irreversible decomposition of the framework 5 takes place above 320 °C. The IR spectrum of 5 shows typical stretchings for carboxylate groups (1559 and 1383 cm −1 ), C-H valence vibrations (3111 and 2932 cm −1 ), O-H vibrations (3398 cm −1 ) and the characteristic peak for the C=O group of the DMF molecules (1670 cm -1 ) ( Figure S2). Photoluminescence measurements of 5 reveal a broad excitation peak at λ = 347 nm (detection at 425 nm) and a broad emission peak at λ = 416 nm (excitation at 350 nm).
The compound [Zn3(tdc)3(DMF)3]•0.8DMF•1.3H2O (6) was prepared by further heating of the crystals of 5 (vide infra) at 90 °C for one day. Overall, after two days of heating, the shape of the crystalline precipitate was changed to the 3D polyhedrons ( Figure S10b). The product was isolated with moderate yield, and its crystal structure and chemical composition were established by a singlecrystal X-ray diffraction method and confirmed by elemental, thermogravimetric and IR analysis. The powder X-ray diffraction data support the phase purity of the bulk sample ( Figure 5). Microelemental (CHNS) analyses, thermogravimetric analysis and FT-IR spectroscopy confirm the chemical formula of 5 and support the nature of the compound. The TGA of 5 shows a three-step decomposition curve ( Figure S11). The first weight loss (6.0 mass. %) at 165 • C corresponds to 0.8 DMF molecules. The second step (16%) at 265 • C corresponds to the evaporation of two coordinated DMF molecules. The irreversible decomposition of the framework 5 takes place above 320 • C. The IR spectrum of 5 shows typical stretchings for carboxylate groups (1559 and 1383 cm −1 ), C-H valence vibrations (3111 and 2932 cm −1 ), O-H vibrations (3398 cm −1 ) and the characteristic peak for the C=O group of the DMF molecules (1670 cm -1 ) ( Figure S2). Photoluminescence measurements of 5 reveal a broad excitation peak at λ = 347 nm (detection at 425 nm) and a broad emission peak at λ = 416 nm (excitation at 350 nm).
The compound [Zn 3 (tdc) 3 (DMF) 3 ]·0.8DMF·1.3H 2 O (6) was prepared by further heating of the crystals of 5 (vide infra) at 90 • C for one day. Overall, after two days of heating, the shape of the crystalline precipitate was changed to the 3D polyhedrons ( Figure S10b). The product was isolated with moderate yield, and its crystal structure and chemical composition were established by a single-crystal X-ray diffraction method and confirmed by elemental, thermogravimetric and IR analysis. The asymmetric unit of the structure 6 contains three zinc(II) cations ( Figure S12). Zn(1) has a distorted octahedral coordination environment consisting of six O atoms of four tdc 2− anions and one DMF ligand. One of the COO-groups is coordinated to Zn(1) bidentately with one Zn-O distance being longer than the other one. Zn(2) has a distorted pentagonal pyramidal coordination environment consisting of five O atoms of three tdc 2− anions and two DMF ligands. Zn(3) has a distorted octahedral coordination environment of five tdc 2− anions, as one of the COO-groups is coordinated to Zn(3) in a bidentate manner. Zn-O distances are in the range 1.9746(14)-2.2615(15) Å. Zn(II) cations are interconnected via bridging carboxylate groups to form trinuclear linear unit {Zn 3 (µ 2 -RCOO) 5 (dmf) 3 (RCOO) 2 }. These units are further extended through a bridging carboxylate group into polymeric chains in a head-to-tail mode (i.e., Zn(1) of one trinuclear unit is always connected to Zn(2) of another one), running along the c axis (Figure 7a). Finally, these chains are interlinked by thiophenedicarboxylate bridges to form a 3D metal-organic framework with narrow triangular channels along the c axis (Figure 7b). The asymmetric unit of the structure 6 contains three zinc(II) cations ( Figure S12). Zn(1) has a distorted octahedral coordination environment consisting of six O atoms of four tdc 2− anions and one DMF ligand. One of the COO-groups is coordinated to Zn(1) bidentately with one Zn-O distance being longer than the other one. Zn(2) has a distorted pentagonal pyramidal coordination environment consisting of five O atoms of three tdc 2− anions and two DMF ligands. Zn(3) has a distorted octahedral coordination environment of five tdc 2− anions, as one of the COO-groups is coordinated to Zn(3) in a bidentate manner. Zn-O distances are in the range 1.9746(14)-2.2615(15) Å. Zn(II) cations are interconnected via bridging carboxylate groups to form trinuclear linear unit {Zn3(μ2-RCOO)5(dmf)3(RCOO)2}. These units are further extended through a bridging carboxylate group into polymeric chains in a head-to-tail mode (i.e., Zn(1) of one trinuclear unit is always connected to Zn(2) of another one), running along the c axis (Figure 7a). Finally, these chains are interlinked by thiophenedicarboxylate bridges to form a 3D metal-organic framework with narrow triangular channels along the c axis ( Figure 7b). The powder X-ray diffraction data support the phase purity of the sample ( Figure 5). Microelemental (CHNS) analyses, thermogravimetric analysis and FT-IR spectroscopy confirm the chemical formula of 6 and support the nature of the compound. The TGA of 6 shows a two-step decomposition curve ( Figure S13). The first weight loss (22.0 mass. %) at 260 °C corresponds to the elimination of three coordinated DMF molecules. The irreversible thermolysis of the framework 6 starts above 320 °C. The IR spectrum of 6 shows typical stretchings for carboxylate groups (1576 and 1372 cm −1 ), C-H valence vibrations (3113 and 2932 cm −1 ), O-H vibrations (3420 cm −1 ) and the characteristic peak for the C=O group of the DMF molecules (1668 cm -1 ) ( Figure S3). The powder X-ray diffraction data support the phase purity of the sample ( Figure 5). Microelemental (CHNS) analyses, thermogravimetric analysis and FT-IR spectroscopy confirm the chemical formula of 6 and support the nature of the compound. The TGA of 6 shows a two-step decomposition curve ( Figure S13). The first weight loss (22.0 mass. %) at 260 • C corresponds to the elimination of three coordinated DMF molecules. The irreversible thermolysis of the framework 6 starts above 320 • C. The IR spectrum of 6 shows typical stretchings for carboxylate groups (1576 and 1372 cm −1 ), C-H valence vibrations (3113 and 2932 cm −1 ), O-H vibrations (3420 cm −1 ) and the characteristic peak for the C=O group of the DMF molecules (1668 cm -1 ) ( Figure S3). Photoluminescence measurements of 6 reveal a broad excitation peak at λ = 350 nm (detection at 425 nm) and broad emission peak at λ = 422 nm (excitation at 350 nm).

Discussion
Despite our numerous efforts, the crystalline phases of the coordination polymers 1 and 2 were always contaminated by another crystalline phase of zinc(II) thiophenedicarboxylate, whose structure and properties will be reported elsewhere. The compound 3 was obtained with marginally low yield, rendering impossible a full characterization of the compounds aside from the single-crystal X-ray diffraction and elemental (CHN) analyses. Among all conditions, only the reaction mixture of 4 was modified with the addition of various Na(I) compounds, which expectedly results in the incorporation of Na(I) cations into the coordination structure. Furthermore, the addition of base (NaOH), compared to the neutral salts (e.g., NaBr or NaNO 3 ), facilitates a deprotonation of the 2,5-thiophenedicarboxylic acid, thus greatly increasing the yield of 4 to a virtually quantitative one. Furthermore, the compounds 5 and 6 were obtained in very similar reaction conditions with the reaction time being the only factor. It indicates that the compound 5, which crystallizes after one day of heating, is a less thermodynamically stable product than the compound 6, which is formed after re-crystallization of 5 after another day of heating. The apparently greater stability of the phase 6 could result from an increased density and dimensionality of the metal-organic framework due to "polycondensation" of the isolated trinuclear building units {Zn 3 (RCOO) 6 }, found in 5, into infinite chains, featured in 6. Such an important interplay between the thermodynamics and kinetics of the formation of the crystals of MOFs is rarely reported in such chemistry [24,25].
Three reported compounds (2, 5, 6) are based solely on trinuclear carboxylate {Zn 3 (RCOO) 6 } linear building units, sometime referred to as "pin-wheel" complexes. Furthermore, the compound 3 contains these complexes as a part of more complicated pentanuclear motives. The trinuclear linear carboxylate blocks are rather common in zinc-organic frameworks, as well as in other MOFs [26] and often result in layered structures with a trigonal pattern of the 2D grids (hxl), as found in the compound 5. However, other topologies with higher (3D) dimensionalities may also be observed in such MOFs, distorted primitive-cubic (pcu), featured in 2, being a notable example [27]. The formation of terminal dabco molecules in 2 is surprising as this ligand is a very strong donor and tends to coordinate from both nitrogen atoms [28]. The most recent Crystal Structure Database (CSD ver. 5.38, updates May 2017) search reveals only 55 hits (12% out of the total 469 dabco-based coordination complexes) where mono-coordinating dabco ligands have ever been elucidated. The zinc(II)-containing MOF structures 1, 3 and 4 are rather unique as we fail to find any similar type of networks in the CSD.
The solid-state photoluminescence properties were studied for compounds 4-6 since these were obtainable in bulk amounts. As a reference, the luminescence spectrum of a sodium thiophenedicarboxylate salt Na 2 tdc was also recorded. Due to its ionic nature, the corresponding excitation/emission may only result from intraligand π↔π* electron transitions of the tdc 2anion. Both the excitation and emission spectra of the coordination polymers 4-6, as well as Na 2 tdc were found to be alike (Figure 8), indicating a similar nature of the luminescence. This is not to be unexpected since zinc(II) cations usually do not interfere with the nature of the ligand-centered electron transitions, not to mention spectroscopically innocent Na(I) ions. In particular, the maxima of the excitation (λ = 375 nm) and emission (λ = 434 nm) peaks for 4 coincide with those for the reference compound Na 2 tdc, which agrees with the markedly anionic nature of the tdc 2species in the sodium-containing 4. The peaks of the luminescence spectra for the zinc-based coordination polymers 5 and 6 exhibit a minor shift to higher energies. The recorded maxima for the compound 5 were observed at 347 and 416 nm for the excitation and emission lines, respectively, while for 6, these were found at 350 and 422 nm. Such hypsochromic shift of the peaks is plausibly related to a lower actual charge on the thiophenedicarboxylate anions in 5 and 6 as a result of a more covalent nature of the Zn-O coordination bonds, compared to mainly ionic Na-O bonds in 4 and Na 2 tdc. The measured intensities of the photoluminescence for all compounds were relatively low; however, we were able to determine the quantum yield ϕ = 2% for 6. measured intensities of the photoluminescence for all compounds were relatively low; however, we were able to determine the quantum yield ϕ = 2% for 6.

Conclusions
In summary, six new metal-organic frameworks based on 2,5-thiophendicarboxylic acid and zinc(II) ions were obtained in a rather narrow range of synthetic conditions. Moreover, a rarely reported kinetic vs. thermodynamic interplay was observed for two particular compounds as a longer reaction time results in a complete re-crystallization of a less stable structure into a new product. Four out of six compounds feature trinuclear linear carboxylate building units {Zn3(RCOO)6}, which are rather common in MOF chemistry, interconnected into different types of topologies. Interestingly, in one compound, such units were found to be decorated by monocoordinated dabco ligands. The solid state photoluminescence measurements reveal the influence of the chemical composition on the luminescence properties of the investigated materials as lower energies (longer wavelengths) of the peaks were detected for sodium(I)-containing compounds.

Conclusions
In summary, six new metal-organic frameworks based on 2,5-thiophendicarboxylic acid and zinc(II) ions were obtained in a rather narrow range of synthetic conditions. Moreover, a rarely reported kinetic vs. thermodynamic interplay was observed for two particular compounds as a longer reaction time results in a complete re-crystallization of a less stable structure into a new product. Four out of six compounds feature trinuclear linear carboxylate building units {Zn 3 (RCOO) 6 }, which are rather common in MOF chemistry, interconnected into different types of topologies. Interestingly, in one compound, such units were found to be decorated by mono-coordinated dabco ligands. The solid state photoluminescence measurements reveal the influence of the chemical composition on the luminescence properties of the investigated materials as lower energies (longer wavelengths) of the peaks were detected for sodium(I)-containing compounds.