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Photoluminescent Lanthanide-Organic Framework based on a Tetraphosphonic Acid Linker Ricardo F. Mendes, Duarte Ananias, Luis D. Carlos, Joao Rocha, and Filipe A. Almeida Paz Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b00667 • Publication Date (Web): 25 Aug 2017 Downloaded from http://pubs.acs.org on August 26, 2017

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Crystal Growth & Design

Photoluminescent Lanthanide-Organic Framework based on a Tetraphosphonic Acid Linker Ricardo F. Mendes,a Duarte Ananias,a,b Luís D. Carlos,b João Rocha,a Filipe A. Almeida Paza,* A contribution from a

Department of Chemistry, CICECO – Aveiro Institute of Materials, University of Aveiro, 3810-193 Aveiro, Portugal. E-mail: [email protected] b

Department of Physics, CICECO – Aveiro Institute of Materials, University of Aveiro, 3810-193 Aveiro, Portugal

* To whom correspondence should be addressed: Filipe A. Almeida Paz Department of Chemistry, CICECO – Aveiro Institute of Materials University of Aveiro 3810-193 Aveiro Portugal E-mail: [email protected] FAX: (+351) 234 401470 Telephone: (+351) 234 401418 ACS Paragon Plus Environment


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Mendes et al. - Submitted to Crystal Growth and Design 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract A new Metal-Organic Framework based on the highly flexible tetraphosphonic acid linker hexamethylenediamine-N,N,N',N'-tetrakis(methylphosphonic 3+

3+

[Ln2(SO4)2(H6htp)(H2O)4]·10H2O [Ln = Eu

(1), Sm

3+

acid)

(H8htp) 3+

(2) and Gd

is

reported.

(3)] was readily obtained by

microwave heating at moderate temperatures (80 ºC) and low reaction time (15 minutes). The reaction was carried out in aqueous medium and, because of the high flexibility of the organic linker, sulfuric acid was added in small quantities. This acid delays the coordination process and blocks the access of the phosphonic acid groups by coordinating the sulfate anion to the metal center, leading to the formation of a compact 3D network. Sulfuric acid further proved to be crucial for the formation of the materials because the use of different acids led to either no precipitation or amorphous compounds. When compared to the only known and reported material based on the same building blocks, this approach allowed us to significantly reduce the reaction time to just 15 min with an immediate crystal formation (compared to the 2 months reported). Crystals were obtained with sizes suitable for single-crystal X-ray diffraction analysis for 1. Materials consist in a 3D network with the metal centers forming a close packed layer, being interconnected by the organic linker, forming cavities which are filled with solvent water molecules. Topologically, 1-3 are binodal networks with a 4,8-connectivity and a Schäfli point symbol of {412·612·84}{46}2. This topology is unusual for MOFs, especially for phosphonic acid based linkers, resembling the known mineral fluorite. The photoluminescence properties of 1 were studied showing an emission lifetime of 0.43±0.01 ms and 0.57±0.01 at 297 and 13 K, respectively.

2

ACS Paragon Plus Environment

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1. Introduction Metal-Organic Frameworks (MOF), or Coordination Polymers (CP), have been raising much interest over the past two decades mainly because these materials are amenable to be tailored for specific applications, having specific physicochemical functionalities.1, 2 This ability to fine tune the properties of MOF is evident in various areas, both academic and industrial, with applications ranging from catalysis3-5 to photoluminescence,6, 7 contrast agents8 or in the fabrication of membranes or thin films.9, 10 Despite the fact that the majority of MOFs are based on the self-assembly of d-block metal centers with carboxylatebased organic linkers, exhibiting nonetheless interesting results,11, 12 the synthesis and characterization of metal phosphonate frameworks have gained ground in the last decade, prompted by the high versatility, structural diversity and high flexibility of phosphonic acids. These molecules have a more stable covalent bond between the carbon and the phosphorous atoms,13 form more robust networks due to their chelating effect14, 15 and can also be obtained by simple and rapid methodologies.16 We have been focusing our research efforts on the preparation of new phosphonate hybrid materials having simultaneously a large concentrations of acidic protons and optically active centers, with the networks being based on flexible ligands and lanthanide cations. Materials exhibit interesting properties and can be used as heterogeneous catalysts and in photoluminescence devices. The main drawback of using highly flexible organic linkers is their usual rapid coordination to metal centers, especially lanthanides yielding, in most cases, amorphous materials. However, if one overcomes this experimental challenge, new structures with larger pores may be obtained with interesting properties (e.g. breathing effect). In previous reports we used different organic linkers bearing from two to four phosphonic acid groups (Figure 1).15,

17-21

According the Cambridge Structural Database the distance

between metal centers (M···M) increases with the increase of linker length, even for highly flexible ones. Thus, in the latter case, to design MOFs with increased pore sizes the flexible linker should, as expected, be in their extended form, which can only achieved by a precise control of the experimental conditions. To overcome the limitation in using highly flexible organic linkers we have recently introduced hydrochloric acid in the reaction, which acts as a nucleation retardant by slowing the organic linker deprotonation and, as a result, favor crystal growth. In our recent work we have showed the importance of using hydrochloric acid in the formation of a 2D material exhibiting positively charged layers, counterbalanced by chloride anions.20 Hydrochloric acid was not only responsible for the high crystallinity of the material but also crucial for the formation of a new MOF. Sulfuric acid has also been shown to be of interest for the preparation of MOFs with highly flexible linkers. Constantino et al.22 were able to hydrothermally synthesize three different MOFs based on Ce3+ and two different highly flexible organic 3



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linkers (1,4-N,N,N’,N’-diaminobutyl and 1,4-N,N,N’,N’-diaminocyclohexyl tetraphosphonic acids) by only adding sulfuric acid at 100 ºC. This acid was responsible not only to slow down the deprotonation of the organic linker (in a similar way as for the hydrochloric acid) but it coordinates to the metal center, diminishing the number of metal coordination sites for the phosphonic acid groups. In this report we describe the preparation of three isotypical three-dimensional MOFs, [Ln2(SO4)2(H6htp)(H2O)4]·10H2O [Ln3+= Eu3+ (1), Sm3+ (2) and Gd3+ (3)], assembled from lanthanides and the highly flexible tetraphosphonic acid, hexamethylenediamine-N,N,N',N'-tetrakis(methylphosphonic acid) (H8htp) using a simple, cost-effective and fast microwave-assisted methodology (80 ºC for 15 min), using water as the solvent. Small quantities of sulfuric acid were used in the reaction, which proved to be crucial in the preparation of 1, since no other acid enabled the formation of a crystalline material. Because of the presence in 1 of the optically active Eu3+ the photoluminescence properties of the system were studied.

---Insert Figure 1---

2. Experimental Section 2.1 - General Instrumentation SEM (Scanning Electron Microscopy) images were acquired using a high-resolution Hitachi SU70 working at 4 kV. Samples were prepared by deposition on aluminum sample holders followed by carbon coating using an Emitech K950X carbon evaporator. EDS (Energy Dispersive X-ray Spectroscopy) data and SEM mapping images were recorded using the same microscope working at 15 kV while employing either a Bruker Quantax 400 or a Sprit 1.9 EDS microanalysis system. Thermogravimetric analyses (TGA) were carried out using a Shimadzu TGA 50, from ambient temperature to ca. 800 ºC (heating rate of 5 ºC/min) and from ambient temperature to ca. 200 ºC (heating rate of 1 ºC/min) under a continuous stream of air at a flow rate of 20 mL min-1. Fourier Transform Infrared (FT-IR) spectra in the spectra range of 4000-350 cm-1 were recorded as KBr pellets (2 mg of sample were mixed in a mortar with 200 mg of KBr) using a Bruker Tensor 27 spectrometer by averaging 256 scans at a maximum resolution of 2 cm-1. Elemental analyses for C, N and H were performed with a Truspec Micro CHNS 630-200-200 elemental analyzer at the Department of Chemistry, University of Aveiro. Analysis parameters: sample amount between 1 and 2 mg; combustion furnace temperature = 1075 ºC; after burner temperature = 850 ºC. Detection method: carbon - infrared absorption; hydrogen - infrared absorption; nitrogen – infrared 4


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absorption. Analysis time = 4 minutes. Gasses required: carrier – helium; combustion – oxygen; pneumatic – compressed air. Routine Powder X-Ray Diffraction (PXRD) data for all prepared materials were collected at ambient temperature on a Empyrean PANalytical diffractometer (Cu Kα1,2 X-radiation, λ1 = 1.540598 Å; λ2 = 1.544426 Å), equipped with an PIXcel 1D detector and a flat-plate sample holder in a BraggBrentano para-focusing optics configuration (45 kV, 40 mA). Intensity data were collected by the stepcounting method (step 0.01º), in continuous mode, in the ca. 3.5 ≤ 2θ ≤ 50º range.

2.2 - Reagents Chemicals were readily available from commercial sources and were used as received without further purification: europium(III), samarium(III) and gadolinium(III) oxide (at least 99.99%, Jinan Henghua Sci. & Tec. Co. Ltd); hexamethylenediamine-N,N,N',N'-tetrakis(methylphosphonic acid) solution (~25% (T) in water, Fluka); sulfuric acid (98%, José Manuel Gomes dos Santos); potassium bromide (KBr for infra-red spectroscopy, > 99%, BDH SpectrosoL).

2.3 - Preparation of [Ln2(SO4)2(H6htp)(H2O)4]·10H2O A reactive mixture containing Ln2O3 (Ln3+= Eu3+, Sm3+ and Gd3+, 0.0426 g, 0.124 mmol), 125 µL of concentrated sulfuric acid in ca. 4 mL of distilled water and 125 µL of the hexamethylenediamineN,N,N',N'-tetrakis(methylphosphonic acid) solution (H8htp) was prepared at ambient temperature inside a 10 mL IntelliVent microwave reactor under vigorous stirring. The resulting white solution was placed inside a CEM Focused Microwave Synthesis System Discover S-Class equipment, under constant magnetic stirring (controlled by the microwave equipment) for 15 minutes at 80 ºC using an irradiation power of 50 W. A constant flow of air (ca. 20-30 psi of pressure) ensured a close control of the temperature inside the reactor. The resulting product, [Ln2(SO4)2(H6htp)(H2O)4]·10H2O [Ln3+= Eu3+ (1), Sm3+ (2) and Gd3+ (3)] was isolated as white crystals, recovered by vacuum filtration and washed with copious amounts of water.

[Eu2(SO4)2(H6htp)(H2O)4]·10H2O (1) Elemental composition. Calculated (in %) (MW = 618.24 g/mol): C 9.69, H 4.39, N 2.26. Found: C 10.94, H 4.25, N 2.35.

5



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Selected FT-IR data (in cm-1): ν(H2O) = 3640-3150w; ν(N–H) andνsym+asym(C–H) = 3000-2800w; ν(POH) = 2825-2570w; δ(H2O) = 1645w; δ(P−CH2) = 1520-1380m; ν(P=O) = 1280-1140m-vs; ν[P−O] = 1040-990vs; ν(P−C) = 800-690m. Thermogravimetric analysis (TGA) data (weight losses in %) and derivative thermogravimetric peaks (DTG; in italics inside the parentheses): 17-65 ºC -9.5% (45 ºC), 65-120 ºC -6.8% (90 ºC), 112-160 ºC -4.8% (138 ºC), 160-800ºC -24.6% (281 ºC).

[Sm2(SO4)2(H6htp)(H2O)4]·10H2O (2) Elemental composition. Calculated (in %) (MW = 649.63 g/mol): C 12.01, H 4.35, N 2.95. Found: C 11.59, H 4.04, N 3.24. Selected FT-IR data (in cm-1): ν(H2O) = 3640-3060w; ν(N–H) andνsym+asym(C–H) = 3060-2833w; ν(POH) = 2833-2472w; δ(H2O) = 1644w; δ(P−CH2) = 1545-1371m; ν(P=O) = 1371-1139m-vs; ν[P−O] = 1039-978vs; ν(P−C) = 866-698m. Thermogravimetric analysis (TGA) data (weight losses in %) and derivative thermogravimetric peaks (DTG; in italics inside the parentheses): 25-75 ºC -7.6% (65 ºC), 75-125 ºC -5.9% (105 ºC), 125245 ºC -6.2% (154 ºC), 245-800ºC -25.7% (352 ºC).

[Gd2(SO4)2(H6htp)(H2O)4]·10H2O (3) Elemental composition. Calculated (in %) (MW = 618.24 g/mol): C 10.03, H 4.32, N 2.93. Found: C 11.13, H 3.89, N 3.16. Selected FT-IR data (in cm-1): ν(H2O) = 3640-3114w; ν(N–H) and νsym+asym(C–H) = 3070-2843w; ν(POH) = 2843-2513w; δ(H2O) = 1651w; δ(P−CH2) = 1540-1357m; ν(P=O) = 1357-1146m-vs; ν[P−O] = 1046-982vs; ν(P−C) = 865-689m. Thermogravimetric analysis (TGA) data (weight losses in %) and derivative thermogravimetric peaks (DTG; in italics inside the parentheses): 25-75 ºC -7.6% (60 ºC), 75-125 ºC -5.9% (105 ºC), 125245 ºC -5.8% (151 ºC), 245-800ºC -25.7% (353 ºC).

2.4 - Photoluminescence spectroscopy The emission and excitation spectra were recorded at 13 and 296 K using a Fluorolog-2® Horiba Scientific (Model FL3-2T) spectroscope, with a modular double grating excitation spectrometer (fitted with a 1200 grooves/mm grating blazed at 330 nm) and a TRIAX 320 single emission monochromator (fitted with a 1200 grooves/mm grating blazed at 500 nm, reciprocal linear density of 2.6 nm mm-1), 6


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coupled to a R928 Hamamatsu photomultiplier, using the front face acquisition mode. The excitation source was a 450 W Xe arc lamp. The emission spectra were corrected for detection and optical spectral response of the spectrofluorimeter and the excitation spectra were corrected for the spectral distribution of the lamp intensity using a photodiode reference detector. Time-resolved measurements have been carried in the Fluorolog-3® and a Xe-Hg flash lamp (6 µs/pulse half width and 20-30 µs tail) was used as the excitation source. The low temperature measurements (13 K) were performed using a helium-closed cycle cryostat with vacuum system measuring ca. 5×10-6 mbar and a Lakeshore 330 auto-tuning temperature controller with a resistance heater.

2.5 - Structural Determination Using Single Crystal X-ray Diffraction Inspection of batches of the microcrystalline powder isolated from the microwave synthesis approach was performed using a Stemi 2000 stereomicroscope equipped with Carl Zeiss lenses. This allowed the identification of small colorless plates (dimensions of about 0.03 x 0.08 x 0.08 mm) which were investigated using single-crystal X-ray diffraction. One colorless plate was manually selected and harvested from the batch powder and immersed in highly viscous FOMBLIN Y perfluoropolyether vacuum oil (LVAC 140/13, Sigma-Aldrich).23 The crystal was then mounted on a Hampton Research CryoLoop and X-ray diffraction data were collected at 150(2) K on a Bruker D8 QUEST equipped with Mo Kα sealed tube (λ = 0.71073 Å), a multilayer TRIUMPH X-ray mirror, a PHOTON 100 CMOS detector, and a Oxford Instruments Cryostrem 700+ Series low temperature device. The instrument was controlled with the APEX2 software package.24 Diffraction images were processed using the software package SAINT+,25 and data were corrected for absorption by the multiscan semi-empirical method implemented in SADABS.26 The crystal structure of [Eu2(SO4)2(H6htp)(H2O)4]·10H2O (1) was solved using the direct space algorithm implemented in SHELXT-2014,27 which allowed the immediate location of almost all of the heaviest atoms composing the molecular unit. The remaining missing non-hydrogen atoms were located from difference Fourier maps calculated from successive full-matrix least-squares refinement cycles on F2 using SHELXL from the 2014 release.28 All structural refinements were performed using the graphical interface ShelXle.29 Hydrogen atoms bound to carbon, nitrogen and to the P–OH groups were placed at their idealized positions using appropriate HFIX instructions in SHELXL-2014: 147 for the hydroxyl groups (identified using a combination of the chemical environment and the P–O distances) and 23 for the –CH2– moieties. All hydrogen atoms were included in subsequent refinement cycles with isotropic displacement parameters (Uiso) fixed at 1.5 (for the P–OH and N–H groups) or 1.2×Ueq (for the –CH2– groups) of the 7



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parent atoms. Hydrogen atoms associated with the two coordinated and one crystallization water molecules, were directly located from difference Fourier maps and included in the structural model. These hydrogen atoms were further included in the final structural model with the Uiso values fixed at 1.5×Ueq of the parent oxygen atoms. The O–H and N–H distances were restrained to 0.95(1) Å and the H···H distances in the water molecules were restrained to 1.55(1) Å. This procedure ensures that these moieties refine using chemically reasonable environments. Four water molecules were disordered and were refined over nine positions with different occupancy rates (from O4W to O12W), ensuring a total of four crystallization water molecules. The hydrogen atoms of these remaining molecules were not directly located from difference Fourier maps and no attempt was made to place them in calculated positions. These hydrogen atoms were, however, included in the empirical formula of the compound (Table 1). The last difference Fourier map synthesis showed the highest peak (1.342 eÅ-3) and the deepest hole (-0.639 eÅ-3) located both at 0.83 and 0.75 Å of Eu1, respectively. All structural refinements were performed using the graphical interface ShelXle.29 Structural drawings have been created using the software package Crystal Impact Diamond.30 Information concerning crystallographic data collection and structure refinement details is summarized in Table 1. Tables 2 and S1 (in the ESI) gather the most significant geometrical parameters of the crystallographically independent Eu3+ coordination sphere and supramolecular interactions, respectively. CCDC 1496999 contains the supplementary crystallographic data for this paper. The data can be obtained

free

of

charge

from

The

Cambridge

Crystallographic

Data

Centre

via

www.ccdc.cam.ac.uk/getstructures.

---Insert Table 1-----Insert Table 2---

3. Results and Discussion 3.1. MOF synthesis strategy: Three new isotypical MOFs (as evidenced in Figure S1 in the ESI),, [Ln2(SO4)2(H6htp)(H2O)4]·10H2O [Ln3+= Eu3+ (1), Sm3+ (2) and Gd3+ (3)], based on different lanthanides and hexamethylenediamine-N,N,N',N'-tetrakis(methylphosphonic acid) (H8htp) have been prepared by employing the microwave-assisted synthesis methodology. The H8htp organic linker is highly flexible, posing a challenge in the preparation of these types of materials. Most reports with this linker use alkaline earth or transition metals, such as calcium, barium or copper.31-36 Due to the usually higher coordination 8


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numbers of lanthanides, the preparation of MOFs using these two building blocks is, thus, more demanding. To the best of our knowledge, there is only one reported MOF based on the H8htp organic linker and lanthanides, reported by Colodrero and co-workers:37 [La(H5htp)]·7H2O. This material was obtained by fixing the reaction to a low pH and by slow crystallizing over a period of 2 months. Comparatively to this, we were able to decrease considerably the reaction time to just 15 minutes by employing a microwave approach at a rather moderate temperature (80 ºC). The inclusion of sulfuric acid in the reaction medium proved to be a crucial step in the preparation of 1-3, because the addition of other acids could not lead to the same results.

3.2. Structural elucidation: Compound 1 was formulated as [Eu2(SO4)2(H6htp)(H2O)4]·10H2O by singlecrystal X-ray diffraction and crystallizes in the centrosymmetric space group P21/c. The 3D structure has large but compact pores occupied with disordered water molecules. The asymmetric unit is composed of one europium center, half H6htp2- residue and seven water molecules (two coordinated and five of crystallization) (Figure S2 in ESI). The Eu3+ is octacoordinated to one sulfate anion in a typical bidentate coordination fashion, two coordination water molecules and four different phosphonate residues, with the {EuO8} coordination polyhedron resembling a distorted dodecahedron (Figure 2). The Eu–O bond lengths were found in the 2.519(3)-2.545(3) Å range for the Eu–O4S, between 2.427(3) and 2.457(3) Å for the Eu–O(1,2)W bonds to the solvent molecules, and in the 2.292(2)-2.381(3) Å range for the Eu–O3P connectivitires. These values are comparable to those reported for other Ln3+-based phosphonate compounds15, 19 and with the data present in the CSD database (mean value of 2.64 Å for the Ln–O4S and 2.37 Å for the remaining Ln–O). The internal O–Eu–O polyhedral angles were found as 55.70(8)º for O1Eu-O2 (belonging to the sulfonate group), 70.22(9)º for O1W–Eu–O2W and between 70.52(9)º and 157.68(9)º for the remaining O–Eu–O angles (Table 2). The H6htp2- linker acts as an octodentate organic linker connecting eight symmetry-related metal centers. All phosphonate groups have a κ1-O mode of coordination, with all of them connecting to a different metal center. Each polyhedron has the two water molecules in opposite sides of the sulfate ion, both directed towards the pores of 1. The remaining four phosphonates are in the same “plane” connecting to the adjacent metals, forming 2D metallic layers (Figure S3 in the ESI) with Eu···Eu intermetallic distances of 6.2789(9) Å along the b axis and 12.4885(17) Å between each layer parallel to the ab plane. These 2D layers are connected by the outstretched organic linker, originating a compact 3D structure (Figure 3). The disordered water molecules of crystallization occupy the pores of the structure. These molecules are engaged in several hydrogen bonding interactions between each other and with the phosphonate and sulfate groups (Figure 3 – bottom). We note that the removal of these solvent molecules 9



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leads to successive structural transformations. Moreover, the rehydration of the dried material leads to an amorphous material. Although we were unable to unveil the structure of the dried materials, we present a possible explanation: the increase in temperature of 1 should lead to the removal of the crystallization water molecules, which is accompanied by a shift in the [002] reflection to higher 2θ (see Figure 4a). This is indicative of a decrease in the c axis length, resulting in an approximation of the 2D metallic layers (Figure 4b). The rehydration of 1 introduces some water molecules but not in an orderly fashion, originating an amorphous material. This explanation is further supported by FTIR studies (see section 3.3 for further details). In terms of topological analysis, and based on the recommendations of Alexandrov et al.,38 who suggested that any moiety (ligand, atom or clusters of atoms) connecting more than two metallic centers (µn) should be considered as a network node, 1 is a binodal network with a 4,8-connectivity (four from the metal center and eight from the organic linker) and a Schäfli point symbol of {412·612·84}{46}2. 1 shares the topology with the well-known mineral fluorite. This type of topology is very interesting not only for the fact that most known MOF compounds with this topology have large cavities but also to the fact that, to the best of our knowledge, no self-interpenetrating structures are reported.39 This is clearly a structural feature largely desired in the design of MOFs with large cavities.

---Insert Figure 1-----Insert Figure 2---

1 has a number of structural similarities to another lanthanide MOF, also based on H8btp, reported by Colodrero and co-workers: [La(H5htp)]·7H2O.37 The two frameworks are 3D with cavities defined by a 30-membered ring (Figure 5). On the other hand, while in 1 the Eu3+ centers form a 2D lamthanide phosphonate layer, in [La(H5htp)]·7H2O the metal centers are solely distributed along a 1D chain. The mode of coordination is also strikingly distinct. The slow crystallization of [La(H5htp)]·7H2O allowed a peculiar connectivity: the La3+ centers are hexacoordinated to six different phosphonate residues forming a highly disordered octahedron, with with the network having 6,6-connected nodes. In 1 the connectivity is different mainly because of the use of sulfuric acid: it not only increases the crystallite size by slowing down the deprotonation of the organic linker (which favors crystal growth over nucleation) but it also coordinates to the metal center. This difference in connectivity is also related to the difference in thermal stability of the two materials. While [La(H5htp)]·7H2O is stable up to 190 ºC (the crystallization water molecules are removed promoting a structural transformation into [La(H5htp)]), 1 is only stable up to 120 ºC, temperature at which a structural transformation is evident. 10


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3.2. Thermogravimetry Studies: The thermal stability of the bulk [Eu2(SO4)2(H6htp)(H2O)4]·10H2O (1) material was investigated between ambient temperature and ca. 800 ºC (heating rate of 5 ºC/min), to provide further information on the hydration level and stability of the material. As observed in Figure S4 (in the ESI) the weight losses of 1 are continuous, being difficult to assign the temperature ranges to the expected residue release. For this reason a second analysis was performed from ambient temperature to 200 ºC at a much slower heating rate (1 ºC/min), which allowed a division of the thermogram into five well defined main regions. The first three weight loss are due to the release of water molecules, between ambient temperature and ca. 160 ºC, attributed to 5 crystallization and 2 coordination water molecules corresponding to a total weight loss of 21.1% (calculated 20.3%).

---Insert Figure 4--The following weight loss, located between 213 and 433 ºC is attributed to the release of the SO42ion in the form of one molecule of sulfuric acid, corresponding to a weight loss of 16.8% (calculated 15.8%). The last final weight loss is attributed to the complete calcinations of the material. This overall thermal behavior was also observed for the two other isotypical materials reported in this manuscript (Figure S5 in the ESI).

3.3. FTIR spectroscopy: The vibrational FT-IR spectroscopy studies clearly support the structural features unveiled by the X-ray diffraction studies. Figure S6 (in the ESI) depicts the FT-IR spectrum of [Eu2(SO4)2(H6htp)(H2O)4]·10H2O (1) in the 4000-350 cm-1 region, including assignments for each main observed band. 1 contains a series of bands between 3640 and 3150 cm-1 attributed to the ν(O–H) stretching vibrational modes from both coordination and crystallization water molecules. The typical symmetric and asymmetric ν(C–H) and ν(N–H) stretching vibrational modes appear in the 3100-2900 cm-1 spectral region. The latter mode is found at about 3025 cm-1, while the signals peaking at about 3000, 2960, 2950 and 2870 cm-1 are attributed to the νsym+asym(C–H) modes.

11



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In the central spectral region, between 1650 and 1440 cm-1, a number of very weak bands can be attributed to ν(C–H) modes characteristic of P–CH2 groups. The typical P–OH stretching modes were also observed between 2825 and 2570 cm-1 as faint and broad bands. The ν(P–C) stretching vibrational modes are also observed, in particular between ca. 800-690 cm-1. Also in this region, the stretching vibrational modes of ν(P=O) are clearly noticed between ca. 1280 and 1140 cm-1, plus those of ν(P–O) between ca. 1040 and 990 cm-1. The stretching vibrational modes of ν(SO42-) are also present but superimposed by the vibrational modes of ν(P=O) in the range between 1200-1140 cm-1. Similar vibrational features were further registered for the two other isotypical materials reported in this manuscript (Figure S7 in the ESI).

3.4.

Photoluminescence

studies:

The

optically

active

Eu3+

centers

present

in

[Eu2(SO4)2(H6htp)(H2O)4]·10H2O (1) allowed the study of this material as a converter of UV radiation into visible light at ambient temperature. The excitation spectra of 1 were recorded at ambient temperature (296 K) and at 13 K while monitoring within the Eu3+ 5D0→7F2 transition (Figure 6). Spectra are dominated by a series of sharp lines assigned to the 7F0,1→5D0-4, 5L6, 5G2-6, 5H3-7 and 5F1-5 Eu3+ intra4f6 transitions. An additional UV broad band was visible after vacuum exposure between ca. 260 to 375 nm. At 13 K this UV band is more prominent, peaking at ca. 280nm, strongly suggesting a temperature dependence. A similar temperature dependence was previously reported for the rare-earth silicate Na3[(Y,Eu)Si3O9]·3H2O, attributed to a O2--to-Eu3+ charge transfer (CT) excitation.40 The absence of ligand excitation bands indicates that the H8htp ligand is not a suitable sensitize for the Eu3+ emission via energy transfer process.


The emission spectra of 1 at ambient temperature (Figure 7) exhibits, mainly, the characteristic narrow lines ascribed to the Eu3+ 5D0→7F0-4 transitions. The 5D1→7F0-3 transitions are also experimentally detected in the high energy region from 525 to 590 nm, although with residual intensities. The predominance of the 5D0→7F2 emission, the presence of a single 5D0→7F0 transition and the local-field splitting of the 7F1,2 levels in three and five Stark components, respectively, suggests the presence of a single Eu3+ local site without inversion centre. Moreover, the integrated intensity ratio between the 5

D0→7F2 and 5D0→7F1 transitions for 1 is 3.0, significantly higher than the 0.68 value reported for a

centrosymmetric Eu3+ emitting center.41 As described from the crystallographic studies the eight12


Page 13 of 28



coordinated environment of Eu3+ resembles a highly distorted {EuO8} dodecahedron. Thus, the point group of Eu3+ agrees well with the information derived from the Eu3+ emission spectroscopy. The exposure of 1 to the vacuum results in slight broadening and shifts of the Stark components (Figure 7) presumably due to a minor structural rearrangement promoted by the release of water molecules.

---Insert Figure 6--At ambient temperature, with and without vacuum of 5×10-3 mbar, 5D0 decay curves have been measured for 1 under direct 4f excitation at 393 nm, by monitoring the strongest 7F2 Stark component. Experimental data are well described by single-exponential functions in both cases (Figure 7, inset), yielding lifetimes of 0.48±0.01 and 0.43±0.01 ms with and without vacuum, respectively, in agreement with the presence of a unique Eu3+ crystallographic site, as ascertained by the crystallographic studies. The slight improvement of the emission lifetime with application of vacuum presumably results from the reduction of the non-radiative deactivation pathways associated with the water O–H oscillators. The emission spectra of 1 were also recorded at 13 K under direct intra-4f (393 nm) and O2--toEu3+ CT (280 nm) excitations (Figure 8). Spectra are different: under CT excitation the emission lines are broader and their Stark components are slight shifted, relatively to that under intra-4f excitation. In addition, the 5D0 decay curves recorded at low temperature (Figure 7, inset) also shows considerable changes in its profile when the excitation changes from 464.5 to 393 and 280 nm. Whereas under intra-4f excitation at 464.5 nm the experimental data is well fitted by a single exponential function (lifetime of 0.57±0.01 ms), the data obtained under excitation at 280 and at 393 nm are only properly fitted by biexponential decay curves yielding lifetimes of 0.57±0.02 (78%) and 1.75±0.05 ms (22%), and of 0.57±0.01 (97%) and 1.75±0.21 ms (3%), respectively. In this later case, the bi-exponential behavior is explained by the superposition of the CT band with the 7F0→5L6 intra-4f line (Figure 5). The detection of a second lifetime and the dependence of the emission spectra on the excitation wavelength, particularly with the CT excitation at 13 K, indicate the presence of a second Eu3+ local site whose emission is strongly temperature dependent and characterized by a larger lifetime. Considering the structural characterization performed, which demonstrates a phase transformation with dehydration at temperatures above 100 ºC (ambient pressure), it is plausible that the application of a vacuum of 5×10-3 mbar, even at ambient temperature, promotes a partial phase transformation due to sample dehydration, inducing the appearance of a second Eu3+ local site.

---Insert Figure 6--13



Page 14 of 28


4 – Conclusions Three novel isotypical MOFs based on a highly flexible organic linker (H8htp), formulated as [Ln2(SO4)2(H6htp)(H2O)4]·10H2O (1-3) by single-crystal X-Ray diffraction, were prepared by the selfassembly of the linker with different lanthanide metal cations [Eu3+ (1), Sm3+ (2) and Gd3+ (3)]. The fast and simple adopted microwave methodology (80 ºC for 15 min), using solely water as the solvent medium, allowed the preparation of a novel network, whose structure contains several cavities defined by a 30-membered ring filled with disordered crystallization water molecules. The removal of these solvent molecules at 120 ºC is accompanied by an irreversible structural change. To the best of our knowledge this is the second MOF structure in which this highly flexible organic linker is self-assembled with lanthanide cations. We were, however, able to improve greatly the reaction approach with a considerable decrease in reaction time (from 2 months to 15 minutes). This was achieved by the inclusion of sulfuric acid in the reaction medium. Photoluminescence spectroscopy of1, bearing Eu3+, yields lifetimes of 0.43±0.01 ms and 0.57±0.01 at 297 and 13 K, respectively. The low lifetimes can be explained by the presence of several water molecules with the associated O-H luminescence quenchers. The presence of only an aliphatic component in the organic linker is further evident by the absence of ligand excitation bands which indicates that the H8htp ligand is not a suitable sensitize for the Eu3+ emission via energy transfer process. The lifetime of 1 can, however, be improved using a different excitation: under intra-4f excitation the lifetime is fitted to 0.57±0.01 ms, while the data obtained under charge-transfer excitation leads to a larger averaged lifetime of 1.10±0.01 ms. We are currently exploring in our labs different approaches to expand this family of lanthanidebased MOFs based on highly flexible organic linkers. The current observation of the removal of coordination sites on the rare-earth has further prompted us to explore this structural feature so to try to reduce the number of coordinating solvent molecules which also constitute a way to quench the photoluminescence of the materials.

14


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ASSOCIATED CONTENT Electronic Supporting Information Structural characterization details for compound 1, including additional crystallographic representations and tables, crystal structure data in CIF format, crystal structure validation, Electron Microscopy (EDS and SEM), and vibrational spectroscopy.

ACKNOWLEDGEMENTS Funding agencies and projects We wish to thank Fundação para a Ciência e a Tecnologia (FCT, Portugal), the European Union, QREN, FEDER through Programa Operacional Factores de Competitividade (COMPETE), and CICECO Aveiro Institute of Materials, POCI-01-0145-FEDER-007679 (FCT Ref. UID/CTM/50011/2013), financed by national funds through the FCT/MEC and when appropriate co-financed by FEDER under the PT2020 Partnership Agreement. Individual grants and scholarships FCT is also gratefully acknowledged for the Ph.D. grant No. SFRH/BD/84231/2012 (to RFM), and the post-doctoral research grant No. SFRH/BPD/95032/2013 (to DA).

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Figure 1. Schematic representation of the organic linkers used in our research group in recent years with emphasis on the average metal-to-metal distances [d(M···M)].

18


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Figure 2. Polyhedral representation of the distorted {EuO8} dodecahedron coordination environment present in the crystal structure of [Eu2(SO4)2(H6htp)(H2O)4]·10H2O (1). Symmetry transformations used to generate equivalent atoms: (i) –x+1, y-1/2,- z+1/2; (ii) x+1, y, z; (iii) -x+1, y+1/2,- z+1/2. See Table 2 for geometrical details on the bond distances and angles.

19



Page 20 of 28


Figure 3. 3D packing of [Eu2(SO4)2(H6htp)(H2O)4]·10H2O (1) along the [100] direction, (inset) close up of the pore of 1 emphasizing the hydrogen interactions between water molecules and the phosphonate and sulfonate groups. 20


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Figure 4. (a) Powder X-ray diffraction patterns of [Eu2(SO4)2(H6htp)(H2O)4]·10H2O (1) under different conditions. A structural transformation occurs when the material is dried at different temperatures and leads to an amorphous compound after rehydration. (b) Schematic representation of the possible transformation that pccurs in 1. The numbers direct the readers to the corresponding PXRD patterns.

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Figure 5. Structural comparison between the crystal features, topologies and lanthanide coordination spheres of compounds [Eu2(SO4)2(H6htp)(H2O)4]·10H2O (1) and La(H5htp)·7H2O with emphasis of the 30-membered rings comprising the pores. The orange node in the topological representations corresponds to the centre of gravity of the connecting organic ligand.

22


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Figure 6. Excitation spectra of [Eu2(SO4)2(H6htp)(H2O)4]·10H2O (1) acquired at ambient conditions (black line – 296 K, 1 bar), under vacuum (blue line – 296 K, 5×10-3 mbar) and at low temperature (red line – 13 K, 5×10-6 mbar), while monitoring the emission at 612.5 nm.

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Page 24 of 28

Mendes et al. - Submitted to Crystal Growth and Design 4

10

5

τRT= 0.43 ± 0.01 ms τRTV= 0.48 ± 0.01 ms

7

D0→ F2

3

10

2

10

Intensity (a. u.)

1

10

5 5

7

520

540

1

2

3

4

5

6

7

Time (ms)

5

7

7

D0 → F 0

D0→ F4

5

5

7

D1→ F2

0

7

D0→ F1

7

D0→ F3

5

7

D1→ F1

5

7

D1→ F0

D1→ F3

5

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

560

580

600

620

640

660

680

700

720

Wavelength(nm)

Figure 7. Emission spectra of [Eu2(SO4)2(H6htp)(H2O)4]·10H2O (1) acquired at ambient conditions (black line – 296 K, 1 bar), under vacuum (blue line – 296 K, 5×10-3 mbar), fixing the excitation at 393 nm. (Inset) Corresponding 5D0 decay curves monitored at 612.5 nm (black squares, 296 K, 1 bar and blue triangles, 296 K, 5×10-3 mbar) and fitted with the single exponential function ( y = y0 + A1 exp − x /τ1 ), r2 > 0.999.

24


Page 25 of 28



Figure 8. Emission spectra of [Eu2(SO4)2(H6htp)(H2O)4]·10H2O (1) acquired at 13 K with the excitation fixed at 393 (black line) and 280 nm (red line). The inset shows the corresponding 5 D0 decay curves monitored at 612.5 nm and excited at 464.5 nm (blue circles), 393 nm (black squares) and 280 nm (red triangles). The curves were fitted with single ( y = y0 + A1 exp − x /τ1 ), 464.5 nm, and bi-exponential ( y = y0 + A1 exp − x /τ1 + A2 exp − x /τ 2 ) decay functions, 280 and 393 nm, always with r2 > 0.999.

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Page 26 of 28


Table 1. Crystal data collection and [Eu2(SO4)2(H6htp)(H2O)4]·10H2O (1). Formula Formula weight Temperature / K Crystal system Space group a/Å b/Å c/Å β/º Volume / Å3 Z ρcalc / g cm-3 µ(Mo Ka) / mm-1 Crystal type Crystal size / mm θ range (°) Index ranges

Collected Reflections Independent Reflections Completeness to θ=25.242º Final R indices [I > 2σ(I)] Final R indices (all data) Largest diff. peak and hole /eÅ-3

structure

refinement

R1 = ∑ Fo − Fc / ∑ Fo

b

wR 2 =

c

w = 1/ σ 2 ( Fo2 ) + ( mP ) + nP  where P = ( Fo2 + 2 Fc2 ) / 3  

2 o

for

C5H27EuNO17P2S 619.23 150(2) Monoclinic P21/c 8.3245(11) 10.0478(13) 24.972(3) 95.151(4) 2080.3(5) 4 1.977 3.345 Colourless plate 0.03×0.08×0.08 3.68–25.35 −10 ≤ h ≤ 10 −12 ≤ k ≤ 12 −30 ≤ l ≤ 30 57946 3746 (Rint = 0.0575) 98.4% R1 = 0.0259 wR2 = 0.0519 R1 = 0.0331 wR2 = 0.0540 1.342 and -0.639

a

∑ w ( F

details

2 2 − Fc2 )  / ∑  w ( Fo2 )    

2

26


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Table 2. Selected bond lengths (in Å) and angles (in degrees) for the {EuO8} coordination environment present in [Eu2(SO4)2(H6htp)(H2O)4]·10H2O (1).a Eu1–O1

2.545(3)

Eu1–O8ii

2.306(2)

Eu1–O2

2.519(3)

Eu1–O9iii

2.381(3)

Eu1–O5

2.329(3)

Eu1–O1W

2.427(3)

Eu1–O6i

2.292(2)

Eu1–O2W

2.457(3)

O2–Eu1–O1

55.70(8)

O8ii–Eu1–O2

77.21(9)

O5–Eu1–O1

72.55(9)

O8ii–Eu1–O5

146.90(9)

O5–Eu1–O2

80.17(9)

O8ii–Eu1–O9iii

89.69(9)

O5–Eu1–O9iii

105.37(9)

O8ii–Eu1–O1W

140.24(9)

O5–Eu1–O1W

72.69(9)

O8ii–Eu1–O2W

70.52(9)

O5–Eu1–O2W

140.50(9)

O9iii–Eu1–O1

126.32(9)

O6i–Eu1–O1

75.16(9)

O9iii–Eu1–O2

70.89(9)

O6i–Eu1–O2

130.86(9)

O9iii–Eu1–O1W

77.03(9)

O6i–Eu1–O5

85.84(9)

O9iii–Eu1–O2W

79.12(10)

O6i–Eu1–O8ii

90.95(9)

O1W–Eu1–O1

142.36(9)

O6i–Eu1–O9iii

157.68(9)

O1W–Eu1–O2

130.14(9)

O6i–Eu1–O1W

88.40(9)

O1W–Eu1–O2W

70.22(9)

O6i–Eu1–O2W

80.08(9)

O2W–Eu1–O1

136.58(9)

O8ii–Eu1–O1

74.78(9)

O2W–Eu1–O2

135.59(9)

a

Symmetry transformations used to generate equivalent atoms:

(i) –x+1, y-1/2,- z+1/2; (ii) x+1, y, z; (iii) -x+1, y+1/2,- z+1/2.

27



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For Table of Contents Only Photoluminescent Lanthanide-Organic Framework based on a Tetraphosphonic Acid Linker Ricardo F. Mendes,a Duarte Ananias,a,b Luís D. Carlos,b João Rocha,a Filipe A. Almeida Paza,* a

Department of Chemistry, CICECO – Aveiro Institute of Materials, University of Aveiro, 3810-193 Aveiro, Portugal. E-mail: [email protected] b

Department of Physics, CICECO – Aveiro Institute of Materials, University of Aveiro, 3810-193 Aveiro, Portugal

The Metal-Organic Framework family [Ln2(SO4)2(H6htp)(H2O)4]·10H2O [Ln3+= Eu3+, Sm3+ and Gd3+; hexamethylenediamine-N,N,N',N'-tetrakis(methylphosphonic acid = H8htp] was obtained by microwave heating at moderate temperatures (80 ºC) and low reaction time (15 minutes). The reaction was carried out in aqueous medium in the presence of sulfuric acid. Materials consist in a 3D network with cavities filled with solvent water molecules.

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Photoluminescent Lanthanide-Organic Framework ... - ACS Publications

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