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Lanthanide metal-organic frameworks based on a thiophenedicarboxylate linker: Characterization and luminescence
Solid State Sciences 15 (2013) 36e41
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Lanthanide metal-organic frameworks based on a thiophenedicarboxylate linker: Characterization and luminescence Paul J. Calderone a, *, Anna M. Plonka b, Debasis Banerjee a, Quddus A. Nizami a, John B. Parise a a b
Department of Chemistry, Stony Brook University, Stony Brook, NY 11794-3400, United States Department of Geosciences, Earth and Space Science Building, Stony Brook University, Stony Brook, NY 11794-2100, United States
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 July 2012 Received in revised form 13 September 2012 Accepted 14 September 2012 Available online 25 September 2012
Three topologically-related lanthanide thiophenedicarboxylate (TDC) metal-organic frameworks were synthesized using an identical metal:linker:solvet ratio. Nd(TDC)3(EtOH)3(H2O)$H2O (1; space group Cc, a ¼ 24.035(2) A, b ¼ 10.063(1) A, c ¼ 18.998(1) A, b ¼ 132.41(1) ) contains the same metal-TDC coordination modes as two other compounds which have the isostructural formula Ln(TDC)3(EtOH)3(H2O)$ A, b ¼ 14.557(1) A, c ¼ 19.128(1) A, a ¼ 106.66(2) , H2O; Ln ¼ Tb (2; space group P 1, a ¼ 12.807(9) b ¼ 105.62(2) , g ¼ 93.691(2) ), Dy (3; space group P 1, a ¼ 12.793(8) A, b ¼ 14.682(1) A, c ¼ 19.077(1) A, a ¼ 107.12(1) , b ¼ 105.54(1) , g ¼ 93.518(2) ). An equimolar solvent mixture of water and ethanol causes both types of solvent molecules coordinating to metal centers. The fluorescence spectra of compounds 2 and 3 show characteristic bands related to their respective metal ions, but Dy-based 3 is very weak compared to Tb-based 2, indicating coordinating solvent molecules may be quenching Dy fluorescence. Ó 2012 Elsevier Masson SAS. All rights reserved.
Keywords: Lanthanides Metal organic frameworks Coordination networks Photoluminescence Sensing
1. Introduction Metal-organic frameworks (MOFs) have been shown in recent years to be one of the most versatile classes of materials [1e4]. Despite a relatively short history, a wide range of structures have been discovered along with many potential applications in areas such as gas storage [5e7], separations [8], catalysis [9], magnetism [10,11], and luminescence/sensing [12e17]. This variety of topologies and uses is possible because of the seemingly infinite combinations of linkers and metal centers available [18,19], meaning MOFs can be tailored for specific purposes based on their structures and component parts. For instance, lightweight, open-framework materials are desired for their gas storage potential [4], while dense transition-metal MOFs often have interesting magnetic properties [10]. Luminescence is yet another possible application from MOFs and the photoluminescence properties can be tuned since they are dependent on the combination and interaction of the metal and linker [13,14]. Lanthanide luminescence occurs with the excitation and subsequent relaxation of f electrons [14], resulting in very specific color emissions unique to each metal ion. However, the low molar
* Corresponding author. 255 ESS, Room 343, Stony Brook University, NY 117942100, United States. Tel.: þ1 631 632 8196; fax: þ1 631 632 8240. E-mail address: [email protected] (P.J. Calderone). 1293-2558/$ e see front matter Ó 2012 Elsevier Masson SAS. All rights reserved. http://dx.doi.org/10.1016/j.solidstatesciences.2012.09.005
absorbtivity of the lanthanide ions leads to weak emission from direct metal excitation. One way around this is the use of coordinated or guest organic antenna ligands [12,14,20,21]. Through the “antenna effect”, the ligands, often with high degrees of conjugation, further sensitize the metal excitation and enhance the luminescence intensity of the complex. When constructing lanthanide MOFs, the linkers serving as connective building blocks for the structure can also function as antennae for the metal [22e26]. Networks based on lanthanide metals offer the possibility of new topologies not normally accessible when using other metals [27e30]. Lanthanide metal ions have variable coordination environments depending on the ligand used and can adopt larger coordination spheres than more common metal centers, such as the 3d series. Additionally, the lanthanide contraction effectda decrease in atomic radius with increasing atomic numberdcan influence how linkers coordinate and allows for additional degrees of freedom across the lanthanide series [31,32]. This makes the coordination geometry and preferences of the ligand important factors in lanthanide MOF design. Carboxylate-based linkers are a good choice when constructing lanthanide networks because the Ln3þ ions are hard acceptors, making coordination with the hard carboxylate groups more favorable. To this end, the 2,5thiophenedicarboxylate (TDC) has been used previously as a linker in lanthanide MOFs because its multiple coordination modes lead to a number of different topologies [32e37].
P.J. Calderone et al. / Solid State Sciences 15 (2013) 36e41
Our synthetic approach builds on previous reports of lanthanide-TDC MOFs by using the alternative solvents and synthetic conditions. It has been shown that varying the solvent or temperature in a given synthetic system can cause changes in the topology and properties of the product [38e40]. Solvent activity can dictate how the molecules will coordinate to metal centers or occupy pore space, thus directing the assembly of the overall structure [41]. Our work sought to apply this knowledge using an equimolar water/ethanol mixture as a solvent and explore its effects on structure and properties in the resulting lanthanide networks. Herein, we report the synthesis and structural characterization, of a series of three related lanthanide-TDC MOFs, Nd(TDC)3(EtOH)3(H2O), and Ln(TDC)3(EtOH)3(H2O)$2H2O (Ln ¼ Tb, Dy). Additionally, the luminescence properties and structural origin of these compounds are examined and discussed. 2. Experimental details 2.1. Synthesis Compounds 1e3 were synthesized under solvothermal conditions using Teflon e lined Parr stainless steel autoclaves. Starting materials, including neodymium nitrate pentahydrate (Nd(NO3)3.5H20, 99.9%, Alfa Products), terbium nitrate pentahydrate (Tb(NO3)3.5H20, 99.9%, Aldrich), dysprosium nitrate pentahydrate (Dy(NO3)3.5H20, 99.99%, Reaction), 2,5-thophenedicarboxylic acid (2,5-TDC, C6H4O4S, 97%, SigmaeAldrich), ethanol (C2H5OH, 99%, SigmaeAldrich) and distilled water (H2O), were used without any further purification.
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A) and 0.5 4 scans. The synchrotron X-ray radiation (l ¼ 0.41328 raw intensity data were analyzed using the APEXII suite of software [42]. Raw intensity data were integrated with the software packages SMART [43] and SAINT [43]. Empirical absorption corrections for 1e3 were applied using SADABS [43]. Structures were solved using direct methods and refined using SHELXS [43]. Mg atoms were located first, followed by the determination of all other atom positions (O, C, N) from the Fourier difference map. Non-hydrogen atoms comprising the main frame were refined with anisotropic displacement parameters while solvent atoms (EtOH, H2O) were refined with isotropic parameters. Hydrogen atoms were placed in idealized positions using geometric constraints. Bulk sample identification and phase purity were determined using powder Xray diffraction. The data were collected using a Scintag Pad-X A) radiation diffractometer equipped with Cu Ka (l ¼ 1.5405 within a range of 5 2q 40 (step size: 0.02 , counting time: 1 s/ step).
2.6. Thermal data Combined TGAeDSC data of 1e3 were collected using an STA 449 C Jupiter Netzch instrument. Powder samples were placed in an Al2O3 crucible for TGAeDSC and analyzed using a range of 30e 650 C and 5 per minute temperature under N2 atmosphere. The thermogravimetric data of 1 shows two weight losses with the first beginning at approximately 235 C and accounting for
2.2. Synthesis of 1 Nd (TDC)3(EtOH)3(H2O)$H2O In a 23 ml Teflon liner, 1 mmol (0.435 g) of Nd(NO3)3$5H2O and 1 mmol (0.172 g) of 2,5-TDC were dissolved in 8.52 g of EtOH and 3.33 g of distilled H2O and stirred for 3 h to achieve homogeneity (molar ratio of metal salt:ligand:ethanol:water ¼ 1:1:185:185). The Teflon liner was transferred to stainless steel autoclave and the heated for a period of three days at 100 C. After cooling to room temperature, the product was recovered by filtration and subsequently washed with ethanol and water with a yield of 52.0% (based on Nd). Elemental analysis for C, H, and N found 29.70% C, 1.96% H, <0.5% N (calc. 30.21% C, 2.53% H, 0.00% N). 2.3. Synthesis of 2 Tb(TDC)3(EtOH)3(H2O)$H2O The synthesis was the same as 1, except that the metal source used was Tb(NO3)3$5H2O. The yield was 67.6% (based on Tb). Elemental analysis for C, H, and N found 27.80% C, 2.39% H, <0.5% N (calc. 28.57% C, 2.53% H, 0.00% N). 2.4. Synthesis of 3 Dy(TDC)3(EtOH)3(H2O)$H2O The synthesis was the same as 1, except that the metal source used was Dy(NO3)3$5H2O. The yield was 59.5% (based on Dy). Elemental analysis for C, H, and N found 27.72% C, 2.19% H, <0.5% N (calc. 28.56 C, 2.50% H, 0.00% N). 2.5. X-ray crystallography Suitable crystal of compounds 1e3 were selected from the bulk samples and were mounted on a glass fiber using epoxy. In order to address low resolution from standard laboratory diffractometers, reflections were collected at ChemMatCars (Sector 15) at the Advanced Photon Source using a three-circle Bruker D8 diffractometer equipped with an APEXII detector at 100 K using
Fig. 1. The network of 1 as viewed in the 0 1 0 (top) and 0 0 1 (bottom) directions. A similar network connectivity is observed in the networks of 2 and 3.
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2.7. Photoluminescence The preparation of samples for fluorescence analysis of 1e3 and free TDC followed the procedure as described below. Solid samples were ground into powder. Quartz slides of dimension 16 mm 60 mm were first rinsed with de-ionized water and ethanol, dried, and then scotch (black) tape was applied to the lower half of the dried slide. The tape was then removed to leave behind a small amount of adhesive, onto which a continuous thin layer of sample was sprinkled. Excess sample was removed by gently tapping the slide face down. The effect of glass slide and adhesive was evaluated and found to be negligible. 3. Results 3.1. Structure of 1, Nd(TDC)3(EtOH)3(H2O)$H2O
Fig. 2. Coordination modes of (k2) (k1 k1) m3 (a) and (k1 k1) (k1 k1) m4 (b) for TDC (left) and the metal carboxylate chain (right) observed in 1e3.
a loss of two ethanol molecules. After a plateau, the second weight loss begins at 415 C and accounts for the removal of remaining coordinating solvent and pyrolysis of the ligand. Thermogravimetric data of 2 and 3 show no well defined weight losses before the first plateaus, beginning at approximately 210 C and 200 C, respectively and account for the loss of two ethanol molecules. The second major weight losses for 2 and 3 occur at 420 C and 445 C, respectively and account for the loss of remaining coordinated solvent and pyrolysis of the ligands.
The structure of 1 consists of chains of individual 8-coordinate Nd polyhedra linked together with TDC carboxylate groups. The chains are connected via a TDC linker into a diamond-like threedimensional network (Fig. 1). Both EtOH molecules and H2O molecules coordinate to the metal centers. The asymmetric unit of 1 consists of two unique Nd atoms, three TDC linkers, three metalcoordinated EtOH molecules, one metal-coordinated H2O molecule, and one guest H2O molecule. Two of the TDC linkers adopt the (k2) e (k1 e k1) e m3 coordination mode, while the other TDC linker coordinates in the (k1 e k1) e (k1 e k1) e m4 mode (Fig. 2). The coordination environment of Nd1 consists of eight oxygen atoms at an average NdeO distance of 2.444 A (Table 2); two oxygen atoms come from a bidentate TDC carboxylate group (O3, O4), four from monodentate carboxylate groups (O2, O8, O10, O12), and two from coordinated ethanol molecules (O2E, O3E). Eight oxygen atoms also make up the coordination environment of Nd2 coordinating at an average distance of 2.455 A; two oxygen atoms come from a bidentate TDC carboxylate group (O5, O6), four from monodentate carboxylate groups (O1, O7, O7, O11), one from a coordinated ethanol molecule (O1E) and one from a coordinated water molecule (O1W). A guest H2O molecule (O2W) is also present. To determine the topology of the network, an Nd3 cluster was defined
Table 1 Crystallographic data for 1e3.
Empirical formula Formula weight Collection temperature (K) Wavelength ( A) Space group a (A) b ( A) c ( A) a ( ) b ( ) g ( ) Volume ( A3) Z Calculated Density (g/cm-3) Absorption Coefficient (mm-1) F(000) Crystal size (mm) q range of data collection Index range
Data/restraints/parameters R1 (on Fo2,I > 2s(I)) wR2 (on Fo2,I > 2s(I))
1
2
3
C24H25Nd2O17S3 970.13 100 0.41328 Cc 24.0351(2) 10.0634(1) 18.9982(1) 90 132.414(1) 90 3392.58(7) 4 1.899 0.771 1892 0.08 0.04 0.01 2.23e14.41 28 h 28 12 k 12 22 l 22 5868/341/353 0.0542 0.1386
C48H50Tb4O34S6 1994.99 100 0.41328 P1 12.807(9) 14.557(1) 19.128(1) 106.66(2) 105.62(2) 93.691(2) 3251.2(4) 2 2.038 1.083 1924 0.10 0.05 0.02 2.21e15.99 17 h 16 19 k 19 21 l 25 15333/0/812 0.0423 0.1011
C48H50Dy4O34S6 2013.24 100 0.41328 P1 12.793(8) 14.682(1) 19.077(1) 107.12(1) 105.54(1) 93.518(2) 3261.1(4) 2 2.047 0.170 1936.8 0.08 0.05 0.02 2.20e14.97 15 h 15 18 k 18 22 l 23 12779/0/778 0.0430 0.1005
P.J. Calderone et al. / Solid State Sciences 15 (2013) 36e41
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as an 11-connected node and the three TDC linkers were defined as 3-, 4-, and 4-connected nodes respectively. A calculation using TOPOS software [44] was done, revealing a tetranodal net with a point symbol of (42$6)(424$630$8)(43$63)(44$62). Crystallographic details are shown in Table 1. 3.2. Structures of 2 and 3, Ln(TDC)3(EtOH)3(H2O)$H2O, Ln ¼ Tb, Dy The structures of 2 and 3 are isostructural and topologically similar to the overall structure and net described in 1, with the main difference being the packing arrangement of connecting TDC ligands (Fig. 3). The asymmetric units of 2 and 3 consists of four unique Ln atoms, six TDC linkers, six metal-coordinated EtOH molecules, two metal-coordinated H2O molecules, and two guest H2O molecules. The coordination modes of TDC observed in 1 are also found in 2 and 3 as four TDC linkers coordinate in a (k2) (k1 k1) m3 mode and two TDC linkers adopt the (k1 k1) (k1 k1) m4 coordination mode. Each Ln metal center in 2 and 3 is 8-coordinate, consisting of oxygen atoms from TDC carboxylate groups, ethanol molecules and water molecules. In 2, the environment of Tb1 is a bidentate TDC carboxylate group (O5, O6), four monodentate carboxylates (O1, O12, O14, O22) and two coordinated ethanol molecules (O1E, O2E). Tb2 also has a bidentate TDC carboxylate group (O15, O16) and four monodentate carboxylates (O2, O8, O11, O20), but only one coordinated EtOH molecule (O3E) is observed as the final position is occupied by a water molecule (O3W). Tb3 is similar to Tb1 with a bidentate TDC carboxylate group (O23, O24), four monodentate carboxylates (O3, O7, O9, O19) and two coordinated EtOH molecules (O4E, O5E). Likewise, Tb4 is similar to Tb2 with a bidentate TDC carboxylate group (O17, O18), four monodentate carboxylates (O4, O13, O10, O21), one coordinated EtOH (O6E), and one coordinated H2O molecule (O4W). Two guest H2O molecules (O1W, O2W) are also present. Selected bond lengths and angles for 2 are shown in Table 3. In 3, the environment of Dy1 consists of a bidentate TDC carboxylate group (O23, O24), four monodentate carboxylates (O4, O6, O10, O20) and two coordinated ethanol molecules (O1E, O2E). Dy2 has a bidentate TDC carboxylate group (O13, O14), four monodentate carboxylates (O3, O5, O11, O21), one coordinated EtOH molecule (O3E), and one H2O molecule (O1W). Dy3 has a bidentate TDC carboxylate group (O1, O2), four monodentate carboxylates (O9, O15, O19, O22), one coordinated EtOH molecule (O4E), and one H2O molecule (O2W). Tb4 has a bidentate TDC carboxylate group Table 2 Selected bond lengths ( A) and angles ( ) for 1. Nd(1)eO(2)b Nd(1)eO(3) Nd(1)eO(2E) Nd(1)eO(3E) Nd(1)eO(4) Nd(1)eO(8) Nd(1)-O(10)a Nd(1)eO(12) Nd(2)eO(1)d Nd(2)eO(1E) Nd(2)eO(1W) Nd(2)eO(5)e Nd(2)eO(6)e Nd(2)eO(12)c Nd(2)eO(9)a Nd(2)eO(11)
2.387(18) 2.453(17) 2.488(17) 2.464(15) 2.595(16) 2.431(15) 2.320(2) 2.325(19) 2.459(16) 2.489(17) 2.488(16) 2.547(16) 2.522(14) 2.368(17) 2.396(13) 2.365(12)
O(2)beNd(1)eO(3) O(10)aeNd(1)e(12) O(3)eNd(1)eO(4) O(2)beNd(1)eO(8) O(10)aeNd(1)eO(2)b O(8)eNd(1)eO(4) O(12)eNd(1)eO(2)b O(10)aeNd(1)eO(8) O(12)eNd(1)eO(8) O(10)aeNd(1)eO(4) O(11)eNd(2)eO(9)a O(6)eeNd(2)eO(5)e O(11)eNd(2)eO(1)d O(9)aeNd(2)eO(1)d O(7)ceNd(2)eO(9)a O(11)eNd(2)eO(6)e O(7)ceNd(2)eO(6)e O(9)aeNd(2)eO(5)e O(11)eNd(2)eO(7)c O(7)ceNd(2)eO(1)d
74.5(7) 89.8(7) 53.3(5) 111.5(5) 92.2(7) 74.1(6) 157.8(5) 140.2(6) 79.9(5) 66.3(7) 115.7(5) 49.6(5) 73.2(5) 142.3(5) 79.4(5) 127.9(5) 78.5(6) 72.1(6) 150.8(5) 111.0(5)
Symmetry transformations used to generate equivalent atoms: ax,y 1,z; bx þ 1/ 2,y 1/2,z þ 1/2; cx,y,z 1/2; dx þ 1/2,y þ 1/2,z; ex þ 1/2,y 1/2,z.
Fig. 3. Difference in ligand packing between the network of 1 (a) and the networks of 2 and 3 (b).
(O7, O8), four monodentate carboxylates (O12, O16, O17, O21), and two coordinated EtOH molecules (O5E, O6E). The network also includes two guest H2O molecules (O3W, O4W). Selected bond lengths and angles for 3 are shown in Table 4.
Table 3 Selected bond lengths ( A) and angles ( ) for 2. Tb(1)eO1 Tb(1)eO(5)a Tb(1)eO(6)a Tb(1)eO(12)g Tb(1)eO(14)f Tb(1)eO(22)f Tb(2)eO(2) Tb(2)eO(8)d Tb(2)eO(11)g Tb(2)eO(15)c Tb(2)eO(16)c Tb(2)eO(20)h Tb(3)eO(3) Tb(3)eO(9) Tb(3)eO(7) Tb(3)eeO(19) Tb(3)aeO(23) Tb(3)aeO(24) Tb(4)eO(4) Tb(4)eO(10) Tb(4)eO(13) Tb(4)eO(17) Tb(4)eO(18) Tb(4)eO(21)
2.273(4) 2.432(4) 2.564(5) 2.292(5) 2.350(4) 2.292(5) 2.332(5) 2.272(4) 2.236(4) 2.511(5) 2.420(4) 2.372(4) 2.312(4) 2.269(5) 2.381(4) 2.262(4) 2.424(4) 2.536(5) 2.234(5) 2.340(4) 2.278(4) 2.530(5) 2.418(4) 2.377(4)
O(5)aeTb(1)eO(6)a O(1)eTb(1)eO(12)g O(22)feTb(1)eO(6)a O(12)geTb(1)eO(5)a O(14)feTb(1)eO(5)a O(13)eTb(4)eO(18) O(16)ceTb(2)eO(15)c O(8)deTb(2)eO(20)h O(11)geTb(2)eO(8)d O(2)eTb(2)eO(15)c O(8)deTb(2)eO(2) O(8)deTb(2)eO(16)c O(23)beTb(3)eO(24)b O(9)eTb(3)eO(3) O(9)eTb(3)eO(24)b O(7)eTb(3)eO(24)b O(7)eTb(3)eO(23)b O(3)eTb(3)eO(24)b O(4)eTb(4)eO(21) O(13)eTb(4)eO(10) O(13)eTb(4)eO(18) O(18)eTb(4)eO(17) O(10)eTb(4)eO(21) O(13)eTb(4)eO(17)
51.86(15) 104.48(18) 125.55(16) 128.70(18) 73.19(16) 78.70(16) 52.81(14) 109.67(15) 156.75(17) 72.06(17) 80.60(17) 77.88(15) 52.28(14) 104.44(17) 70.20(18) 71.90(16) 72.49(16) 81.39(16) 81.12(17) 79.55(16) 78.70(16) 52.56(15) 143.26(18) 126.95(16)
Symmetry transformations used to generate equivalent atoms: ax 1,y,z; bx þ 1,y,z; c x þ 2,y þ 1,z; dx þ 2,y þ 2,z; ex þ 2,y þ 2,z þ 1; fx þ 1,y þ 1,z; g x 1,y,z 1; hx,y,z 1.
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Table 4 Selected bond lengths ( A) and angles ( ) for 3. Dy(1)eO(4) Dy(1)eO(6) Dy(1)eO(10)b Dy(1)eO(20)a Dy(1)eO(23)c Dy(1)eO(24)c Dy(2)eO(3) Dy(2)eO(5) Dy(2)eO(11)f Dy(2)eO(13)g Dy(2)eO(14)g Dy(2)eO(18)f Dy(3)eO(1)e Dy(3)eO(2)e Dy(3)eO(9)c Dy(3)eO(15) Dy(3)eO(19)d Dy(3)eO(22) Dy(4)eO(7) Dy(4)eO(8) Dy(4)eO(12) Dy(4)eO(16) Dy(4)eO(17) Dy(4)eO(21)
2.248(6) 2.359(5) 2.300(6) 2.267(6) 2.509(6) 2.423(5) 2.365(5) 2.261(5) 2.325(6) 2.486(6) 2.415(5) 2.221(5) 2.418(5) 2.494(6) 2.217(6) 2.253(5) 2.332(6) 2.359(6) 2.536(6) 2.420(5) 2.255(6) 2.339(5) 2.285(6) 2.264(6)
O(20)aeDy(1)eO(10)b O(20)aeDy(1)eO(24)c O(10)beDy(1)eO(23)c O(4)eDy(1)eO(6) O(4)eDy(1)eO(23)c O(20)aeDy(1)eO(6) O(5)eDy(2)eO(3) O(18)feDy(2)eO(11)f O(18)feDy(2)eO(14)g O(5)eDy(2)eO(13)g O(14)geDy(2)eO(13)g O(18)feDy(2)eO(13)g O(15)eDy(3)eO(22) O(9)ceDy(3)eO(19)d O(15)eDy(3)eO(2)e O(9)ceDy(3)eO(22) O(9)ceDy(3)eO(2)e O(15)eDy(3)eO(1)e O(21)eDy(4)eO(16) O(12)eDy(4)eO(21) O(21)eDy(4)eO(7) O(17)eDy(4)eO(8) O(17)eDy(4)eO(7) O(12)eDy(4)eO(16)
103.4(2) 80.6(2) 80.8(2) 108.8(2) 124.6(2) 142.4(2) 108.14(19) 105.6(2) 125.5(2) 127.19(18) 53.05(18) 75.7(2) 108.7(2) 104.3(2) 127.01(19) 81.6(2) 75.1(2) 78.04(19) 110.2(2) 86.9(2) 126.0(2) 129.4(2) 80.4(2) 142.1(2)
Symmetry transformations used to generate equivalent atoms: ax,y,z þ 1; bx 1,y,z; c x þ 2,y,z þ 2; dx þ 1,y,z þ 1; ex,y 1,z 1; fx þ 2,y þ 1,z þ 2; g x þ 1,y þ 1,z þ 2; hx þ 1,y,z.
4. Discussion Compounds 1e3 were synthesized using the lanthanide metal centers Nd, Tb, and Dy, while holding constant solvent and reactant ratios. The lanthanide TDC networks that resulted are all topologically related and have the structural formula Ln(TDC)3(EtOH)3(H2O). The TDC linker in 1e3 adopts the same coordination mode in each of the three compounds, but is arranged slightly different in compounds 2 and 3, which are isostructural. The observed structural differences suggest that the lanthanide contraction effect influences the way the linkers coordinate to the A, metal. Eight-coordinate Nd3þ has an effective ionic radius of 1.11 whereas eight-coordinate Tb3þ and Dy3þ are smaller and much similar to one another, having ionic radii of 1.04 A and 1.03 A, respectively. However, the smaller ionic radii of Tb3þ and Dy3þ cannot be definitively given as the reason for the structural variance, as other effects, such as metal solubility differences can also play a role in network assembly. Previous studies in the synthesis of lanthanide TDC networks have used a variety of solvents and solvent mixtures, including DMSO/DMF [33], DMF/H2O [36], and H2O/EtOH [34]. We set out to explore the possible topological differences in this family with a 1:1 M EtOH/H2O solvent mixture. Similar work by Wang and coworkers used an H2O/EtOH mixture comprised of mostly H2O and found that EtOH is not incorporated into the structure [34]. Curiously, we found that our use of an equimolar mixture of EtOH/ H2O led to coordination of both EtOH and H2O to the metal centers in each of the compounds, with the empirical ratio being three coordinated EtOH molecules to one coordinated H2O. Previous work done in our group showed that mixtures of solvents have a unique character compared to pure solvents and lead to different topologies for a given metal/ligand combination [38]. Since EtOH coordinates preferentially, we can conclude it is a more active solvent than H2O under these synthetic conditions. The fluorescence spectra of 1e3 were analyzed based on the maximum ligand excitation wavelength, which was determined to be 300 nm. Each compound shows the characteristic p* / n and p* / p transitions of the TDC linker, with a broad peak centered at 362 nm (Fig. 4). Compound 2 shows the characteristic Tb emission
Fig. 4. Fluorescence spectra of 1e3 and free H2TDC acid showing emission of the TDC linkers.
with lines corresponding to the 5D4 / 7F6 (492 nm), 5D4 / 7F5 (539 nm and 551 nm), 5D4 / 7F4 (585 nm), and 5D4 / 7F3 (621 nm) transitions (Fig. 5). When irradiated with a UV lamp on a benchtop, 2 emits bright green light (Fig. 6). The spectrum of 3 is comparatively weaker and shows an emission line at 479 nm that is characteristic of the 4F9/2 / 6H15/2 transition and another emission line at 575 nm corresponding to the 4F9/2 / 6H13/2 transition (Fig. 5). Other reported compounds have shown strong Dy emission using the TDC linker, but the weakness of emission lines in the Dy compounds reported in our work may be related to non-emissive
Fig. 5. Photoluminescence emission spectra of compounds 2 and 3 obtained using an excitation wavelength of 300 nm.
P.J. Calderone et al. / Solid State Sciences 15 (2013) 36e41
41
Fig. 6. The response of compound 2 under visible light (left) and under UV light (right).
luminescence quenching of the coordinated solvent molecules EtOH and H2O. 5. Conclusion Three structurally-related lanthanide TDC coordination networks were synthesized using Nd, Tb, and Dy metal centers, with each reaction using the same metal:linker:solvent ratio. Each network shares the formula Ln(TDC)3(EtOH)3(H2O)$H2O and the same TDC linker coordination modes are observed. However, a difference in TDC linker packing due to the lanthanide contraction effect causes the topology of 1 to differ slightly from the isostructures 2 and 3. Both water and ethanol solvent molecules were found to coordinate to the metal center when using a 1:1 mixture, but the ratio of three coordinated EtOH to one H2O molecule suggests a preference for EtOH under these conditions. Fluorescence spectra show the characteristic Ln transitions in 2 and 3, but the emission in Dy-based 3 is considerably weaker indicating that the coordinating solvent molecules may be quenching the luminescence. Acknowledgements Synthesis and characterization work (PJC, AMP, DB, QAN) is funded by the Division of Materials Research of the National Science Foundation; grant number DMR-0800415 (JBP). We especially thank Zhichao Hu and Jing Li (Department of Chemistry & Chemical Biology, Rutgers University) for use of facilities and assistance in collecting photoluminescence data. Thanks also go to Dr. Robert LaDuca (Department of Chemistry, Michigan State University) for consultation regarding the use of TOPOS software. ChemMatCars (Sector 15) is principally supported by the National Science Foundation/Department of Energy under Grant Number CHE-0535644. Use of the Advanced Photon Source was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.solidstatesciences.2012.09.005. References [1] [2] [3] [4] [5]
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Lanthanide metal-organic frameworks based on a thiophenedicarboxylate linker: Characterization and luminescence Paul J. Calderone a, *, Anna M. Plonka b, Debasis Banerjee a, Quddus A. Nizami a, John B. Parise a a b
Department of Chemistry, Stony Brook University, Stony Brook, NY 11794-3400, United States Department of Geosciences, Earth and Space Science Building, Stony Brook University, Stony Brook, NY 11794-2100, United States
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 July 2012 Received in revised form 13 September 2012 Accepted 14 September 2012 Available online 25 September 2012
Three topologically-related lanthanide thiophenedicarboxylate (TDC) metal-organic frameworks were synthesized using an identical metal:linker:solvet ratio. Nd(TDC)3(EtOH)3(H2O)$H2O (1; space group Cc, a ¼ 24.035(2) A, b ¼ 10.063(1) A, c ¼ 18.998(1) A, b ¼ 132.41(1) ) contains the same metal-TDC coordination modes as two other compounds which have the isostructural formula Ln(TDC)3(EtOH)3(H2O)$ A, b ¼ 14.557(1) A, c ¼ 19.128(1) A, a ¼ 106.66(2) , H2O; Ln ¼ Tb (2; space group P 1, a ¼ 12.807(9) b ¼ 105.62(2) , g ¼ 93.691(2) ), Dy (3; space group P 1, a ¼ 12.793(8) A, b ¼ 14.682(1) A, c ¼ 19.077(1) A, a ¼ 107.12(1) , b ¼ 105.54(1) , g ¼ 93.518(2) ). An equimolar solvent mixture of water and ethanol causes both types of solvent molecules coordinating to metal centers. The fluorescence spectra of compounds 2 and 3 show characteristic bands related to their respective metal ions, but Dy-based 3 is very weak compared to Tb-based 2, indicating coordinating solvent molecules may be quenching Dy fluorescence. Ó 2012 Elsevier Masson SAS. All rights reserved.
Keywords: Lanthanides Metal organic frameworks Coordination networks Photoluminescence Sensing
1. Introduction Metal-organic frameworks (MOFs) have been shown in recent years to be one of the most versatile classes of materials [1e4]. Despite a relatively short history, a wide range of structures have been discovered along with many potential applications in areas such as gas storage [5e7], separations [8], catalysis [9], magnetism [10,11], and luminescence/sensing [12e17]. This variety of topologies and uses is possible because of the seemingly infinite combinations of linkers and metal centers available [18,19], meaning MOFs can be tailored for specific purposes based on their structures and component parts. For instance, lightweight, open-framework materials are desired for their gas storage potential [4], while dense transition-metal MOFs often have interesting magnetic properties [10]. Luminescence is yet another possible application from MOFs and the photoluminescence properties can be tuned since they are dependent on the combination and interaction of the metal and linker [13,14]. Lanthanide luminescence occurs with the excitation and subsequent relaxation of f electrons [14], resulting in very specific color emissions unique to each metal ion. However, the low molar
* Corresponding author. 255 ESS, Room 343, Stony Brook University, NY 117942100, United States. Tel.: þ1 631 632 8196; fax: þ1 631 632 8240. E-mail address: [email protected] (P.J. Calderone). 1293-2558/$ e see front matter Ó 2012 Elsevier Masson SAS. All rights reserved. http://dx.doi.org/10.1016/j.solidstatesciences.2012.09.005
absorbtivity of the lanthanide ions leads to weak emission from direct metal excitation. One way around this is the use of coordinated or guest organic antenna ligands [12,14,20,21]. Through the “antenna effect”, the ligands, often with high degrees of conjugation, further sensitize the metal excitation and enhance the luminescence intensity of the complex. When constructing lanthanide MOFs, the linkers serving as connective building blocks for the structure can also function as antennae for the metal [22e26]. Networks based on lanthanide metals offer the possibility of new topologies not normally accessible when using other metals [27e30]. Lanthanide metal ions have variable coordination environments depending on the ligand used and can adopt larger coordination spheres than more common metal centers, such as the 3d series. Additionally, the lanthanide contraction effectda decrease in atomic radius with increasing atomic numberdcan influence how linkers coordinate and allows for additional degrees of freedom across the lanthanide series [31,32]. This makes the coordination geometry and preferences of the ligand important factors in lanthanide MOF design. Carboxylate-based linkers are a good choice when constructing lanthanide networks because the Ln3þ ions are hard acceptors, making coordination with the hard carboxylate groups more favorable. To this end, the 2,5thiophenedicarboxylate (TDC) has been used previously as a linker in lanthanide MOFs because its multiple coordination modes lead to a number of different topologies [32e37].
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Our synthetic approach builds on previous reports of lanthanide-TDC MOFs by using the alternative solvents and synthetic conditions. It has been shown that varying the solvent or temperature in a given synthetic system can cause changes in the topology and properties of the product [38e40]. Solvent activity can dictate how the molecules will coordinate to metal centers or occupy pore space, thus directing the assembly of the overall structure [41]. Our work sought to apply this knowledge using an equimolar water/ethanol mixture as a solvent and explore its effects on structure and properties in the resulting lanthanide networks. Herein, we report the synthesis and structural characterization, of a series of three related lanthanide-TDC MOFs, Nd(TDC)3(EtOH)3(H2O), and Ln(TDC)3(EtOH)3(H2O)$2H2O (Ln ¼ Tb, Dy). Additionally, the luminescence properties and structural origin of these compounds are examined and discussed. 2. Experimental details 2.1. Synthesis Compounds 1e3 were synthesized under solvothermal conditions using Teflon e lined Parr stainless steel autoclaves. Starting materials, including neodymium nitrate pentahydrate (Nd(NO3)3.5H20, 99.9%, Alfa Products), terbium nitrate pentahydrate (Tb(NO3)3.5H20, 99.9%, Aldrich), dysprosium nitrate pentahydrate (Dy(NO3)3.5H20, 99.99%, Reaction), 2,5-thophenedicarboxylic acid (2,5-TDC, C6H4O4S, 97%, SigmaeAldrich), ethanol (C2H5OH, 99%, SigmaeAldrich) and distilled water (H2O), were used without any further purification.
37
A) and 0.5 4 scans. The synchrotron X-ray radiation (l ¼ 0.41328 raw intensity data were analyzed using the APEXII suite of software [42]. Raw intensity data were integrated with the software packages SMART [43] and SAINT [43]. Empirical absorption corrections for 1e3 were applied using SADABS [43]. Structures were solved using direct methods and refined using SHELXS [43]. Mg atoms were located first, followed by the determination of all other atom positions (O, C, N) from the Fourier difference map. Non-hydrogen atoms comprising the main frame were refined with anisotropic displacement parameters while solvent atoms (EtOH, H2O) were refined with isotropic parameters. Hydrogen atoms were placed in idealized positions using geometric constraints. Bulk sample identification and phase purity were determined using powder Xray diffraction. The data were collected using a Scintag Pad-X A) radiation diffractometer equipped with Cu Ka (l ¼ 1.5405 within a range of 5 2q 40 (step size: 0.02 , counting time: 1 s/ step).
2.6. Thermal data Combined TGAeDSC data of 1e3 were collected using an STA 449 C Jupiter Netzch instrument. Powder samples were placed in an Al2O3 crucible for TGAeDSC and analyzed using a range of 30e 650 C and 5 per minute temperature under N2 atmosphere. The thermogravimetric data of 1 shows two weight losses with the first beginning at approximately 235 C and accounting for
2.2. Synthesis of 1 Nd (TDC)3(EtOH)3(H2O)$H2O In a 23 ml Teflon liner, 1 mmol (0.435 g) of Nd(NO3)3$5H2O and 1 mmol (0.172 g) of 2,5-TDC were dissolved in 8.52 g of EtOH and 3.33 g of distilled H2O and stirred for 3 h to achieve homogeneity (molar ratio of metal salt:ligand:ethanol:water ¼ 1:1:185:185). The Teflon liner was transferred to stainless steel autoclave and the heated for a period of three days at 100 C. After cooling to room temperature, the product was recovered by filtration and subsequently washed with ethanol and water with a yield of 52.0% (based on Nd). Elemental analysis for C, H, and N found 29.70% C, 1.96% H, <0.5% N (calc. 30.21% C, 2.53% H, 0.00% N). 2.3. Synthesis of 2 Tb(TDC)3(EtOH)3(H2O)$H2O The synthesis was the same as 1, except that the metal source used was Tb(NO3)3$5H2O. The yield was 67.6% (based on Tb). Elemental analysis for C, H, and N found 27.80% C, 2.39% H, <0.5% N (calc. 28.57% C, 2.53% H, 0.00% N). 2.4. Synthesis of 3 Dy(TDC)3(EtOH)3(H2O)$H2O The synthesis was the same as 1, except that the metal source used was Dy(NO3)3$5H2O. The yield was 59.5% (based on Dy). Elemental analysis for C, H, and N found 27.72% C, 2.19% H, <0.5% N (calc. 28.56 C, 2.50% H, 0.00% N). 2.5. X-ray crystallography Suitable crystal of compounds 1e3 were selected from the bulk samples and were mounted on a glass fiber using epoxy. In order to address low resolution from standard laboratory diffractometers, reflections were collected at ChemMatCars (Sector 15) at the Advanced Photon Source using a three-circle Bruker D8 diffractometer equipped with an APEXII detector at 100 K using
Fig. 1. The network of 1 as viewed in the 0 1 0 (top) and 0 0 1 (bottom) directions. A similar network connectivity is observed in the networks of 2 and 3.
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2.7. Photoluminescence The preparation of samples for fluorescence analysis of 1e3 and free TDC followed the procedure as described below. Solid samples were ground into powder. Quartz slides of dimension 16 mm 60 mm were first rinsed with de-ionized water and ethanol, dried, and then scotch (black) tape was applied to the lower half of the dried slide. The tape was then removed to leave behind a small amount of adhesive, onto which a continuous thin layer of sample was sprinkled. Excess sample was removed by gently tapping the slide face down. The effect of glass slide and adhesive was evaluated and found to be negligible. 3. Results 3.1. Structure of 1, Nd(TDC)3(EtOH)3(H2O)$H2O
Fig. 2. Coordination modes of (k2) (k1 k1) m3 (a) and (k1 k1) (k1 k1) m4 (b) for TDC (left) and the metal carboxylate chain (right) observed in 1e3.
a loss of two ethanol molecules. After a plateau, the second weight loss begins at 415 C and accounts for the removal of remaining coordinating solvent and pyrolysis of the ligand. Thermogravimetric data of 2 and 3 show no well defined weight losses before the first plateaus, beginning at approximately 210 C and 200 C, respectively and account for the loss of two ethanol molecules. The second major weight losses for 2 and 3 occur at 420 C and 445 C, respectively and account for the loss of remaining coordinated solvent and pyrolysis of the ligands.
The structure of 1 consists of chains of individual 8-coordinate Nd polyhedra linked together with TDC carboxylate groups. The chains are connected via a TDC linker into a diamond-like threedimensional network (Fig. 1). Both EtOH molecules and H2O molecules coordinate to the metal centers. The asymmetric unit of 1 consists of two unique Nd atoms, three TDC linkers, three metalcoordinated EtOH molecules, one metal-coordinated H2O molecule, and one guest H2O molecule. Two of the TDC linkers adopt the (k2) e (k1 e k1) e m3 coordination mode, while the other TDC linker coordinates in the (k1 e k1) e (k1 e k1) e m4 mode (Fig. 2). The coordination environment of Nd1 consists of eight oxygen atoms at an average NdeO distance of 2.444 A (Table 2); two oxygen atoms come from a bidentate TDC carboxylate group (O3, O4), four from monodentate carboxylate groups (O2, O8, O10, O12), and two from coordinated ethanol molecules (O2E, O3E). Eight oxygen atoms also make up the coordination environment of Nd2 coordinating at an average distance of 2.455 A; two oxygen atoms come from a bidentate TDC carboxylate group (O5, O6), four from monodentate carboxylate groups (O1, O7, O7, O11), one from a coordinated ethanol molecule (O1E) and one from a coordinated water molecule (O1W). A guest H2O molecule (O2W) is also present. To determine the topology of the network, an Nd3 cluster was defined
Table 1 Crystallographic data for 1e3.
Empirical formula Formula weight Collection temperature (K) Wavelength ( A) Space group a (A) b ( A) c ( A) a ( ) b ( ) g ( ) Volume ( A3) Z Calculated Density (g/cm-3) Absorption Coefficient (mm-1) F(000) Crystal size (mm) q range of data collection Index range
Data/restraints/parameters R1 (on Fo2,I > 2s(I)) wR2 (on Fo2,I > 2s(I))
1
2
3
C24H25Nd2O17S3 970.13 100 0.41328 Cc 24.0351(2) 10.0634(1) 18.9982(1) 90 132.414(1) 90 3392.58(7) 4 1.899 0.771 1892 0.08 0.04 0.01 2.23e14.41 28 h 28 12 k 12 22 l 22 5868/341/353 0.0542 0.1386
C48H50Tb4O34S6 1994.99 100 0.41328 P1 12.807(9) 14.557(1) 19.128(1) 106.66(2) 105.62(2) 93.691(2) 3251.2(4) 2 2.038 1.083 1924 0.10 0.05 0.02 2.21e15.99 17 h 16 19 k 19 21 l 25 15333/0/812 0.0423 0.1011
C48H50Dy4O34S6 2013.24 100 0.41328 P1 12.793(8) 14.682(1) 19.077(1) 107.12(1) 105.54(1) 93.518(2) 3261.1(4) 2 2.047 0.170 1936.8 0.08 0.05 0.02 2.20e14.97 15 h 15 18 k 18 22 l 23 12779/0/778 0.0430 0.1005
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39
as an 11-connected node and the three TDC linkers were defined as 3-, 4-, and 4-connected nodes respectively. A calculation using TOPOS software [44] was done, revealing a tetranodal net with a point symbol of (42$6)(424$630$8)(43$63)(44$62). Crystallographic details are shown in Table 1. 3.2. Structures of 2 and 3, Ln(TDC)3(EtOH)3(H2O)$H2O, Ln ¼ Tb, Dy The structures of 2 and 3 are isostructural and topologically similar to the overall structure and net described in 1, with the main difference being the packing arrangement of connecting TDC ligands (Fig. 3). The asymmetric units of 2 and 3 consists of four unique Ln atoms, six TDC linkers, six metal-coordinated EtOH molecules, two metal-coordinated H2O molecules, and two guest H2O molecules. The coordination modes of TDC observed in 1 are also found in 2 and 3 as four TDC linkers coordinate in a (k2) (k1 k1) m3 mode and two TDC linkers adopt the (k1 k1) (k1 k1) m4 coordination mode. Each Ln metal center in 2 and 3 is 8-coordinate, consisting of oxygen atoms from TDC carboxylate groups, ethanol molecules and water molecules. In 2, the environment of Tb1 is a bidentate TDC carboxylate group (O5, O6), four monodentate carboxylates (O1, O12, O14, O22) and two coordinated ethanol molecules (O1E, O2E). Tb2 also has a bidentate TDC carboxylate group (O15, O16) and four monodentate carboxylates (O2, O8, O11, O20), but only one coordinated EtOH molecule (O3E) is observed as the final position is occupied by a water molecule (O3W). Tb3 is similar to Tb1 with a bidentate TDC carboxylate group (O23, O24), four monodentate carboxylates (O3, O7, O9, O19) and two coordinated EtOH molecules (O4E, O5E). Likewise, Tb4 is similar to Tb2 with a bidentate TDC carboxylate group (O17, O18), four monodentate carboxylates (O4, O13, O10, O21), one coordinated EtOH (O6E), and one coordinated H2O molecule (O4W). Two guest H2O molecules (O1W, O2W) are also present. Selected bond lengths and angles for 2 are shown in Table 3. In 3, the environment of Dy1 consists of a bidentate TDC carboxylate group (O23, O24), four monodentate carboxylates (O4, O6, O10, O20) and two coordinated ethanol molecules (O1E, O2E). Dy2 has a bidentate TDC carboxylate group (O13, O14), four monodentate carboxylates (O3, O5, O11, O21), one coordinated EtOH molecule (O3E), and one H2O molecule (O1W). Dy3 has a bidentate TDC carboxylate group (O1, O2), four monodentate carboxylates (O9, O15, O19, O22), one coordinated EtOH molecule (O4E), and one H2O molecule (O2W). Tb4 has a bidentate TDC carboxylate group Table 2 Selected bond lengths ( A) and angles ( ) for 1. Nd(1)eO(2)b Nd(1)eO(3) Nd(1)eO(2E) Nd(1)eO(3E) Nd(1)eO(4) Nd(1)eO(8) Nd(1)-O(10)a Nd(1)eO(12) Nd(2)eO(1)d Nd(2)eO(1E) Nd(2)eO(1W) Nd(2)eO(5)e Nd(2)eO(6)e Nd(2)eO(12)c Nd(2)eO(9)a Nd(2)eO(11)
2.387(18) 2.453(17) 2.488(17) 2.464(15) 2.595(16) 2.431(15) 2.320(2) 2.325(19) 2.459(16) 2.489(17) 2.488(16) 2.547(16) 2.522(14) 2.368(17) 2.396(13) 2.365(12)
O(2)beNd(1)eO(3) O(10)aeNd(1)e(12) O(3)eNd(1)eO(4) O(2)beNd(1)eO(8) O(10)aeNd(1)eO(2)b O(8)eNd(1)eO(4) O(12)eNd(1)eO(2)b O(10)aeNd(1)eO(8) O(12)eNd(1)eO(8) O(10)aeNd(1)eO(4) O(11)eNd(2)eO(9)a O(6)eeNd(2)eO(5)e O(11)eNd(2)eO(1)d O(9)aeNd(2)eO(1)d O(7)ceNd(2)eO(9)a O(11)eNd(2)eO(6)e O(7)ceNd(2)eO(6)e O(9)aeNd(2)eO(5)e O(11)eNd(2)eO(7)c O(7)ceNd(2)eO(1)d
74.5(7) 89.8(7) 53.3(5) 111.5(5) 92.2(7) 74.1(6) 157.8(5) 140.2(6) 79.9(5) 66.3(7) 115.7(5) 49.6(5) 73.2(5) 142.3(5) 79.4(5) 127.9(5) 78.5(6) 72.1(6) 150.8(5) 111.0(5)
Symmetry transformations used to generate equivalent atoms: ax,y 1,z; bx þ 1/ 2,y 1/2,z þ 1/2; cx,y,z 1/2; dx þ 1/2,y þ 1/2,z; ex þ 1/2,y 1/2,z.
Fig. 3. Difference in ligand packing between the network of 1 (a) and the networks of 2 and 3 (b).
(O7, O8), four monodentate carboxylates (O12, O16, O17, O21), and two coordinated EtOH molecules (O5E, O6E). The network also includes two guest H2O molecules (O3W, O4W). Selected bond lengths and angles for 3 are shown in Table 4.
Table 3 Selected bond lengths ( A) and angles ( ) for 2. Tb(1)eO1 Tb(1)eO(5)a Tb(1)eO(6)a Tb(1)eO(12)g Tb(1)eO(14)f Tb(1)eO(22)f Tb(2)eO(2) Tb(2)eO(8)d Tb(2)eO(11)g Tb(2)eO(15)c Tb(2)eO(16)c Tb(2)eO(20)h Tb(3)eO(3) Tb(3)eO(9) Tb(3)eO(7) Tb(3)eeO(19) Tb(3)aeO(23) Tb(3)aeO(24) Tb(4)eO(4) Tb(4)eO(10) Tb(4)eO(13) Tb(4)eO(17) Tb(4)eO(18) Tb(4)eO(21)
2.273(4) 2.432(4) 2.564(5) 2.292(5) 2.350(4) 2.292(5) 2.332(5) 2.272(4) 2.236(4) 2.511(5) 2.420(4) 2.372(4) 2.312(4) 2.269(5) 2.381(4) 2.262(4) 2.424(4) 2.536(5) 2.234(5) 2.340(4) 2.278(4) 2.530(5) 2.418(4) 2.377(4)
O(5)aeTb(1)eO(6)a O(1)eTb(1)eO(12)g O(22)feTb(1)eO(6)a O(12)geTb(1)eO(5)a O(14)feTb(1)eO(5)a O(13)eTb(4)eO(18) O(16)ceTb(2)eO(15)c O(8)deTb(2)eO(20)h O(11)geTb(2)eO(8)d O(2)eTb(2)eO(15)c O(8)deTb(2)eO(2) O(8)deTb(2)eO(16)c O(23)beTb(3)eO(24)b O(9)eTb(3)eO(3) O(9)eTb(3)eO(24)b O(7)eTb(3)eO(24)b O(7)eTb(3)eO(23)b O(3)eTb(3)eO(24)b O(4)eTb(4)eO(21) O(13)eTb(4)eO(10) O(13)eTb(4)eO(18) O(18)eTb(4)eO(17) O(10)eTb(4)eO(21) O(13)eTb(4)eO(17)
51.86(15) 104.48(18) 125.55(16) 128.70(18) 73.19(16) 78.70(16) 52.81(14) 109.67(15) 156.75(17) 72.06(17) 80.60(17) 77.88(15) 52.28(14) 104.44(17) 70.20(18) 71.90(16) 72.49(16) 81.39(16) 81.12(17) 79.55(16) 78.70(16) 52.56(15) 143.26(18) 126.95(16)
Symmetry transformations used to generate equivalent atoms: ax 1,y,z; bx þ 1,y,z; c x þ 2,y þ 1,z; dx þ 2,y þ 2,z; ex þ 2,y þ 2,z þ 1; fx þ 1,y þ 1,z; g x 1,y,z 1; hx,y,z 1.
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Table 4 Selected bond lengths ( A) and angles ( ) for 3. Dy(1)eO(4) Dy(1)eO(6) Dy(1)eO(10)b Dy(1)eO(20)a Dy(1)eO(23)c Dy(1)eO(24)c Dy(2)eO(3) Dy(2)eO(5) Dy(2)eO(11)f Dy(2)eO(13)g Dy(2)eO(14)g Dy(2)eO(18)f Dy(3)eO(1)e Dy(3)eO(2)e Dy(3)eO(9)c Dy(3)eO(15) Dy(3)eO(19)d Dy(3)eO(22) Dy(4)eO(7) Dy(4)eO(8) Dy(4)eO(12) Dy(4)eO(16) Dy(4)eO(17) Dy(4)eO(21)
2.248(6) 2.359(5) 2.300(6) 2.267(6) 2.509(6) 2.423(5) 2.365(5) 2.261(5) 2.325(6) 2.486(6) 2.415(5) 2.221(5) 2.418(5) 2.494(6) 2.217(6) 2.253(5) 2.332(6) 2.359(6) 2.536(6) 2.420(5) 2.255(6) 2.339(5) 2.285(6) 2.264(6)
O(20)aeDy(1)eO(10)b O(20)aeDy(1)eO(24)c O(10)beDy(1)eO(23)c O(4)eDy(1)eO(6) O(4)eDy(1)eO(23)c O(20)aeDy(1)eO(6) O(5)eDy(2)eO(3) O(18)feDy(2)eO(11)f O(18)feDy(2)eO(14)g O(5)eDy(2)eO(13)g O(14)geDy(2)eO(13)g O(18)feDy(2)eO(13)g O(15)eDy(3)eO(22) O(9)ceDy(3)eO(19)d O(15)eDy(3)eO(2)e O(9)ceDy(3)eO(22) O(9)ceDy(3)eO(2)e O(15)eDy(3)eO(1)e O(21)eDy(4)eO(16) O(12)eDy(4)eO(21) O(21)eDy(4)eO(7) O(17)eDy(4)eO(8) O(17)eDy(4)eO(7) O(12)eDy(4)eO(16)
103.4(2) 80.6(2) 80.8(2) 108.8(2) 124.6(2) 142.4(2) 108.14(19) 105.6(2) 125.5(2) 127.19(18) 53.05(18) 75.7(2) 108.7(2) 104.3(2) 127.01(19) 81.6(2) 75.1(2) 78.04(19) 110.2(2) 86.9(2) 126.0(2) 129.4(2) 80.4(2) 142.1(2)
Symmetry transformations used to generate equivalent atoms: ax,y,z þ 1; bx 1,y,z; c x þ 2,y,z þ 2; dx þ 1,y,z þ 1; ex,y 1,z 1; fx þ 2,y þ 1,z þ 2; g x þ 1,y þ 1,z þ 2; hx þ 1,y,z.
4. Discussion Compounds 1e3 were synthesized using the lanthanide metal centers Nd, Tb, and Dy, while holding constant solvent and reactant ratios. The lanthanide TDC networks that resulted are all topologically related and have the structural formula Ln(TDC)3(EtOH)3(H2O). The TDC linker in 1e3 adopts the same coordination mode in each of the three compounds, but is arranged slightly different in compounds 2 and 3, which are isostructural. The observed structural differences suggest that the lanthanide contraction effect influences the way the linkers coordinate to the A, metal. Eight-coordinate Nd3þ has an effective ionic radius of 1.11 whereas eight-coordinate Tb3þ and Dy3þ are smaller and much similar to one another, having ionic radii of 1.04 A and 1.03 A, respectively. However, the smaller ionic radii of Tb3þ and Dy3þ cannot be definitively given as the reason for the structural variance, as other effects, such as metal solubility differences can also play a role in network assembly. Previous studies in the synthesis of lanthanide TDC networks have used a variety of solvents and solvent mixtures, including DMSO/DMF [33], DMF/H2O [36], and H2O/EtOH [34]. We set out to explore the possible topological differences in this family with a 1:1 M EtOH/H2O solvent mixture. Similar work by Wang and coworkers used an H2O/EtOH mixture comprised of mostly H2O and found that EtOH is not incorporated into the structure [34]. Curiously, we found that our use of an equimolar mixture of EtOH/ H2O led to coordination of both EtOH and H2O to the metal centers in each of the compounds, with the empirical ratio being three coordinated EtOH molecules to one coordinated H2O. Previous work done in our group showed that mixtures of solvents have a unique character compared to pure solvents and lead to different topologies for a given metal/ligand combination [38]. Since EtOH coordinates preferentially, we can conclude it is a more active solvent than H2O under these synthetic conditions. The fluorescence spectra of 1e3 were analyzed based on the maximum ligand excitation wavelength, which was determined to be 300 nm. Each compound shows the characteristic p* / n and p* / p transitions of the TDC linker, with a broad peak centered at 362 nm (Fig. 4). Compound 2 shows the characteristic Tb emission
Fig. 4. Fluorescence spectra of 1e3 and free H2TDC acid showing emission of the TDC linkers.
with lines corresponding to the 5D4 / 7F6 (492 nm), 5D4 / 7F5 (539 nm and 551 nm), 5D4 / 7F4 (585 nm), and 5D4 / 7F3 (621 nm) transitions (Fig. 5). When irradiated with a UV lamp on a benchtop, 2 emits bright green light (Fig. 6). The spectrum of 3 is comparatively weaker and shows an emission line at 479 nm that is characteristic of the 4F9/2 / 6H15/2 transition and another emission line at 575 nm corresponding to the 4F9/2 / 6H13/2 transition (Fig. 5). Other reported compounds have shown strong Dy emission using the TDC linker, but the weakness of emission lines in the Dy compounds reported in our work may be related to non-emissive
Fig. 5. Photoluminescence emission spectra of compounds 2 and 3 obtained using an excitation wavelength of 300 nm.
P.J. Calderone et al. / Solid State Sciences 15 (2013) 36e41
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Fig. 6. The response of compound 2 under visible light (left) and under UV light (right).
luminescence quenching of the coordinated solvent molecules EtOH and H2O. 5. Conclusion Three structurally-related lanthanide TDC coordination networks were synthesized using Nd, Tb, and Dy metal centers, with each reaction using the same metal:linker:solvent ratio. Each network shares the formula Ln(TDC)3(EtOH)3(H2O)$H2O and the same TDC linker coordination modes are observed. However, a difference in TDC linker packing due to the lanthanide contraction effect causes the topology of 1 to differ slightly from the isostructures 2 and 3. Both water and ethanol solvent molecules were found to coordinate to the metal center when using a 1:1 mixture, but the ratio of three coordinated EtOH to one H2O molecule suggests a preference for EtOH under these conditions. Fluorescence spectra show the characteristic Ln transitions in 2 and 3, but the emission in Dy-based 3 is considerably weaker indicating that the coordinating solvent molecules may be quenching the luminescence. Acknowledgements Synthesis and characterization work (PJC, AMP, DB, QAN) is funded by the Division of Materials Research of the National Science Foundation; grant number DMR-0800415 (JBP). We especially thank Zhichao Hu and Jing Li (Department of Chemistry & Chemical Biology, Rutgers University) for use of facilities and assistance in collecting photoluminescence data. Thanks also go to Dr. Robert LaDuca (Department of Chemistry, Michigan State University) for consultation regarding the use of TOPOS software. ChemMatCars (Sector 15) is principally supported by the National Science Foundation/Department of Energy under Grant Number CHE-0535644. Use of the Advanced Photon Source was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.solidstatesciences.2012.09.005. References [1] [2] [3] [4] [5]
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