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Synthesis and Characterization of an Isomorphous Lanthanide-Thiophenemonocarboxylate Series (Ln = La-Lu, except Pm) Amenable to Color Tuning Rami J Batrice, Alyssa K Adcock, Paula M. Cantos, Jeffery A. Bertke, and Karah E Knope Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b00400 • Publication Date (Web): 31 Jul 2017 Downloaded from http://pubs.acs.org on August 2, 2017

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Crystal Growth & Design

Synthesis and Characterization of an Isomorphous Lanthanide-Thiophenemonocarboxylate Series (Ln = La-Lu, except Pm) Amenable to Color Tuning Rami J. Batrice, Alyssa K. Adcock, Paula M. Cantos, Jeffery A. Bertke, Karah E. Knope* Department of Chemistry, Georgetown University, 37th and O Streets, NW, Washington, D.C. 20057 Lanthanide Thiophenecarboxylic Acid Isomorphous Series Luminescence Color Tuning

ABSTRACT: A series of fourteen lanthanide compounds bearing 2-thiophenecarboxylate (TC) have been prepared under aqueous conditions and structurally characterized. Single crystal X-ray diffraction studies reveal a full isomorphous series across the 4f block of the general formula [(Ln(TC)3(H2O)2)·(HPy·TC)]n, (Ln = La (La-1) - Lu (Lu-1), excluding Pm), with observable contraction of the cell volume and average metal-oxygen bond distances occurring when moving from lanthanum to lutetium. The 8-coordinate Ln metal centers are bound to two water molecules as well as six oxygen atoms from one bidentate and four bridging TC units that link LnO8 structural units into 1D chains. One lattice stabilizing pyridinium and one outer coordination sphere TC anion are present per formula unit. Reaction optimization provided moderate to high yields of the title products; a lanthanum byproduct [La(TC)3(HTC)2]n (La-2) was formed by performing the reaction at reduced temperature, or alternatively, successive

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grinding of La-1 in ethanol resulted in partial conversion to La-2. Solid-state fluorescence studies show ligand sensitized metal-based emission for Eu(III), Tb(III), and Dy(III) in the visible region, and Nd(III) and Er(III) in the NIR. Color tuning was easily achieved by varying europium:terbium metal ratios in the reaction mixtures.

INTRODUCTION Coordination compounds of trivalent rare earth ions remain an area of intense investigation, greatly owing to the characteristic luminescence and magnetic properties of the 4f metals.1-19 While excitation of the valence electrons is parity forbidden,4 organic ligands are commonly utilized to serve as antennae for ligand sensitized metal-based emission.20 These properties have given rise to a breadth of applications for luminescent lanthanide materials including optical signal amplifiers,21-23 biochemical sensors,24-26 and other electroluminescent devices.27 Various organic moieties have been used to achieve such sensitization, with organic carboxylates constituting one of the most widely investigated classes of ligands.28-31 In addition to harnessing ligand sensitized lanthanide-based emission to develop functional materials, other studies have built upon research precedent broadly utilizing carboxylate ligands to achieve color-tuning of lanthanide luminescence through the addition of other transition or lanthanide metal ions.32-42 The diverse binding modes of carboxylates to metal ions is understood to be advantageous towards the formation of novel molecular and extended structures,43,44 and the use of thiophene ring substituents in particular has been found to impart attractive photophysical properties to the resulting metal-carboxylate species.

45-46

These features arise from two

dominant features of this type of ligand. First, the presence of the sulfur atom within the fivemembered heterocycle aids in maintaining the robust aromaticity of the ring.47,48 Second, and


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perhaps more importantly, the triplet state of 2-thiophenecarboxylate (TC) at approximately 22,624 cm-1 is uniquely situated to sensitize several of the lanthanide ions.45 When complexed to a lanthanide ion, thiophenecarboxylate and similar derivatives often impose enhanced luminescent properties with many potential applications in optical devices.45,49-55 While this literature illustrates the utility of thiophenecarboxylate as a suitable sensitizing ligand for rare earth ions, the majority of these studies focus only on a select few of the 4f series. Toward our efforts of elucidating structure-property relationships in lanthanide based materials, we have sought to prepare and characterize novel rare earth materials with promising luminescent

behavior.

The current

work

reports

the

aqueous

synthesis,

structural

characterization, and photoluminescent properties of a full isomorphous trivalent lanthanide series bearing 2-thiophenecarboxylate (TC) and incorporating outer coordination sphere pyridinium. Single-crystal X-ray diffraction studies of each of the homometallic lanthanidethiophenecarboxylates have been performed and revealed novel extended 1-D chains of LnO8 polyhedra that bridge infinitely through carboxylate linkages, and the thermal stability of the compounds was examined via thermogravimetric analysis. An additional lanthanum(III) product generated from low temperature reaction conditions was found using single crystal X-ray diffraction. Luminescence studies show ligand sensitized metal-based emission in the visible and NIR region for several of the isolated materials. Altering the color of emission in heterobimetallic europium/terbium systems has been achieved by varying lanthanide nitrate ratios in the reaction mixture, and the observed emission correlates to the relative ratios of Eu(III) and Tb(III) in the isolated products that were precisely determined using ICP-MS analysis. Lifetime data are reported for the europium(III), terbium(III), and dysprosium(III) compounds prepared in this study.


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EXPERIMENTAL SECTION Materials. La(NO3)36H2O (Strem Chemicals, 99.99%), Ce(NO3)36H2O (Fisher Scientific, 99.9%), Eu(NO3)36H2O (BeanTown Chemical, 99.9%), Gd(NO3)36H2O (Rare Earth Products, 99.99%), Tm(NO3)35H2O (Sigma Aldrich, 99.9%), Lu(NO3)36H2O (Accela, 99.9%), Ln(NO3)3xH2O (Ln = Pr, Nd, Sm, Tb, Dy, Ho, Er, or Yb, x = 5 or 6, Strem Chemicals, 99.9%), 2-thiophenecarboxylic acid (Acros Organics, 99%), and pyridine (Alfa Aesar, 99%) were used as received. Nanopure water (≤0.05 µS; Millipore, USA) was used in all experiments. General Synthetic Procedure for La-1 – Lu-1. All reactions were carried out in 10 mL glass vials in common atmosphere. To each vial was added approximately 0.90 mmol of 2-thiophenecarboxylic acid (HTC) and 4 mL of nanopure water. Stock solutions containing the appropriate lanthanide nitrate hydrate in water were used to add 0.23 mmol of the metal precursor to the ligand suspension, followed by the addition of 200 µL (2.48 mmol) pyridine. Reactions were then capped and placed in a heating block set to 70 °C for three days. Reaction mixtures were filtered while hot and the filtrate allowed to evaporate in open vials at room temperature. Single crystals deposited over 3-10 days. The crystals were harvested and wicked free of residual solvent using Kimwipes, then left open to air to dry completely and used for all subsequent analyses. Full synthetic details in addition to elemental analysis are provided as Supporting Information. [La(TC)3(HTC)2]n (La-2). The reagents were combined in an analogous manner to that of La1, however, the reaction was cooled to 4 °C for one week in lieu of heating. The reaction mixture was largely unreacted but contained a few colorless block crystals which deposited. The crystals were harvested and used in subsequent single crystal X-ray diffraction studies.


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[(EuxTb1-x(TC)3(H2O)2)·(HPy·TC)]n

(EuxTb1-x).

The

heterobimetallic

products

were

synthesized using the general procedure described above. To each vial containing 2-thiophenecarboxylic acid, water, and pyridine was added precise quantities of Eu(NO3)36H2O and Tb(NO3)36H2O stock solutions. The sum of x and y in all cases was equal to one according to the product formula; in the compounds where x ≥ 0.1, the europium and terbium stock solution concentrations were 219 and 224 mM, respectively. When x < 0.1, the Eu(III) and Tb(III) stock solution concentrations were 41 and 300 mM, respectively. Reactions were worked up in the manner described previously, and the colorless rectangular block crystals that formed by slow evaporation were collected and used in further analysis. Structure Determination. Single crystals were isolated from the bulk sample and mounted in oil on a MiTeGen micromount. Single crystal diffraction studies were performed on either a Bruker D8 Quest equipped with a Photon 100 detector or a Bruker Apex DUO equipped with an APEXII CCD detector using Mo-Kα radiation (λ=0.71073 Å) at 100-102 K. The data were collected and processed using the APEX2/SHELX-201456,57 suite of crystallographic software. Crystallographic data and structure refinement details are provided in Table 1. The thiophene ring portion of all three coordinated TC ligands were disordered over two orientations. The like S–C and C–C bonds were restrained to be similar. The free TC anion is likewise disordered over two orientations. The like S–C, C–C, and C–O distances have been restrained to be similar. The two half occupancy pyridinium cations are each disordered about a symmetry site, thus they were refined with negative PART commands. Additionally, each cation is disordered over two positions and all of the pyridinium rings have thus been constrained to be ideal hexagons. Similar displacement amplitudes were imposed on disordered sites overlapping by less than the sum of van der Waals radii. Rigid-bond restraints were also imposed on displacement parameters


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for some disordered sites. The water H atoms were located in the difference map and the O–H distance was restrained to be 0.95Å. These H atoms refine to good hydrogen bonding positions. The pyridinium N–H hydrogen atoms could not be located in the difference map but were assumed to reside on the aromatic nitrogen based on pKa differences, (3.49 and 5.23 in HTC and HPy, respectively),58 and thus were placed in calculated positions.


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Table 1. Crystallographic data for compounds La-1-Lu-1 and La-2. Chemical Formula Formula Weight Crystal System Space Group a (Å) b (Å) c (Å) α (°) β (°) γ (°) V (Å3) Z T (K) λ (Mo-Kα) Dcalc (g cm-3) µ (mm-1) Rint R1 [I > 2σ(I)] wR2 [I > 2σ(I)] CCDC Number Chemical Formula Formula Weight Crystal System Space Group a (Å) b (Å) c (Å) α (°) β (°) γ (°) V (Å3) Z T (K) λ (Mo-Kα) Dcalc (g cm-3) µ (mm-1) Rint R1 [I > 2σ(I)] wR2 [I > 2σ(I)] CCDC Number

La-1 C25H22NO10S4La 763.58 Triclinic

Ce-1 C25H22NO10S4Ce 764.79 Triclinic

P1

P1

9.7924(10) 11.9174(12) 12.6012(13) 98.671(3) 98.293(3) 95.602(3) 1427.8(3) 2 102 0.71073 1.776 1.845 0.1661 0.0616 0.0929 1535030

9.8189(5) 11.8992(6) 12.6236(6) 98.655(2) 98.340(2) 95.547(2) 1432.06(12) 2 100 0.71073 1.774 1.938 0.0456 0.0210 0.0509 1535036

Eu-1 C25H22NO10S4Eu 776.63 Triclinic

Pr-1 C25H22NO10S4Pr 765.58 Triclinic

Nd-1 C25H22NO10S4Nd 768.91 Triclinic

Sm-1 C25H22NO10S4Sm 75.02 Triclinic

P1

P1

9.8233(7) 11.8728(8) 12.6154(9) 98.5804(18) 98.2167(18) 95.5700(18) 1429.36(17) 2 100 0.71073 1.779 2.053 0.0164 0.0136 0.0350 1535038

9.8071(4) 11.8553(5) 12.6141(5) 98.5240(10) 98.1790(10) 95.6370(10) 1424.94(10) 2 100 0.71073 1.792 2.172 0.0451 0.0351 0.0599 1535029

9.7746(15) 11.8380(19) 12.612(2) 98.509(2) 97.930(2) 95.739(2) 1418.7(4) 2 100 0.71073 1.814 2.421 0.0378 0.0280 0.0583 1535035

Gd-1 C25H22NO10S4Gd 781.92 Triclinic

Tb-1 C25H22NO10S4Tb 783.59 Triclinic

Dy-1 C25H22NO10S4Dy 787.17 Triclinic

Ho-1 C25H22NO10S4Ho 789.60 Triclinic

P1

P1

P1

P1

P1

9.7787(16) 11.8247(19) 12.616(2) 98.472(3) 97.903(3) 95.749(3) 1418.3(4) 2 100 0.71073 1.819 2.563 0.0438 0.0305 0.0574 1535033

9.765(3) 11.825(3) 12.632(4) 98.502(4) 97.874(4) 95.689(4) 1418.2(7) 2 100 0.71073 1.831 2.690 0.0175 0.0142 0.0369 1535032

9.7458(8) 11.8283(9) 12.6333(10) 98.4308(11) 97.7821(11) 95.8272(11) 1416.33(19) 2 100 0.71073 1.837 2.849 0.0157 0.0140 0.0355 1535031

9.7427(14) 11.8187(17) 12.6232(18) 98.453(2) 97.672(2) 95.821(2) 1413.9(4) 2 100 0.71073 1.849 2.995 0.0172 0.0141 0.0367 1535034

9.7338(18) 11.812(2) 12.631(2) 98.418(2) 97.625(3) 95.812(3) 1413.2(5) 2 100 0.71073 1.856 3.152 0.0192 0.0147 0.0384 1535037

P1


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Chemical Formula Formula Weight Crystal System Space Group a (Å) b (Å) c (Å) α (°) β (°) γ (°) V (Å3) Z T (K) λ (Mo-Kα) Dcalc (g cm-3) µ (mm-1) Rint R1 [I > 2σ(I)] wR2 [I > 2σ(I)] CCDC Number

Er-1 C25H22NO10S4Er 791.93 Triclinic

Tm-1 C25H22NO10S4Tm 793.60 Triclinic

Yb-1 C25H22NO10S4Yb 797.71 Triclinic

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Lu-1 C25H22NO10S4Lu 799.64 Triclinic

P1

P1

P1

P1

9.7315(7) 11.8012(9) 12.6296(10) 98.3956(11) 97.6278(12) 95.8790(12) 1411.17(19) 2 100 0.71073 1.864 3.327 0.0219 0.0175 0.0423 1535039

9.6944(5) 11.7918(6) 12.6282(6) 98.3974(13) 97.5648(13) 95.8587(13) 1404.81(12) 2 100 0.71073 1.876 3.513 0.0242 0.0218 0.0456 1535041

9.7024(9) 11.7917(11) 12.6242(11) 98.4204(11) 97.4978(12) 95.8671(12) 1405.6(2) 2 100 0.71073 1.885 3.681 0.0160 0.0145 0.0373 1535042

9.6572(7) 11.7539(8) 12.6047(9) 98.3444(18) 97.4927(18) 95.9249(18) 1392.60(17) 2 101 0.71073 1.907 3.903 0.0482 0.0482 0.0716 1535040

La-2 C25H17O10S5La 776.59 Monoclinic Cc 20.4554(17) 14.1116(11) 9.9375(8) 90 92.7172(19) 90 2865.3(4) 4 100 0.71073 1.800 1.910 0.0300 0.0179 0.0445 1535043

Methods. Powder X-ray diffraction data were collected on bulk reaction products using Cu-Kα radiation (λ=1.542 Å) on a Rigaku Ultima IV X-ray diffractometer from 3-60° in 2θ with a step speed of 1 degree min-1, or on a Bruker Apex DUO with a rotation speed of 0.5 degrees sec-1. Emission and excitation data were acquired on a Horiba PTI QM-400 system (Horiba PTI) on solid samples at room temperature. Luminescent spectra were additionally collected from single crystals of the EuxTb1-x-1 phases using a Raman microscope equipped with a 355 nm excitation source to confirm the monophasic character of the bulk sample and the random distribution of the lanthanide ions over the metal sites in the crystal structure. Metal ion ratios in the heterobimetallic products were determined by inductively coupled plasma-mass spectrometry with an Agilent 7700 ICP-MS in 5% HNO3 solutions. Thermogravimetric analyses were acquired on a TA Instruments Q50 Thermogravimetric Analyzer. The samples were prepared


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first by drying under dynamic vacuum at 70 °C, then analyzed by TGA under flowing nitrogen (10 mL min-1) over 25-500 °C, with a scan rate of 5 °C min-1. Fourier Tranform Infrared (FTIR) Spectroscopy was performed on Nd-1 using a Varian 3100 Spectrometer with an attenuated total reflectance (ATR) stage. The spectrum was obtained in the range of 400-4000 cm-1 as the average of 16 scans. Combustion elemental analysis was performed on Perkin Elmer Model 2400 Elemental Analyzer. RESULTS AND DISCUSSION Synthesis of La-1 – Lu-1. The title lanthanide(III) thiophenecarboxylate compounds were prepared under aerobic conditions in aqueous media. Pyridine was added to the reaction mixtures to facilitate deprotonation and dissolution of TC, thereby promoting reaction with the lanthanide nitrate precursors. Reactions were run at elevated temperature (70 °C), and after 3 days, filtered while hot. Evaporation of the solution over the course of 3 to 10 days resulted in the formation of single crystals. We note that efforts to prepare La-1 – Lu-1 at room temperature yielded the target products in addition to an impurity observed in the bulk phase that was later confirmed to be La-2 when using the lanthanum adduct as a model reaction. PXRD collected for the bulk material from the high temperature reactions indexed well to calculated powder patterns from single crystal data, however, a minor impurity in the diffraction patterns for some of the compounds was seen which corresponded to a phase isomorphous to La-2 (Figure S30). Using the synthetic condition described above for the preparation of Ln-1, mixed metal analogues of these products were produced using europium(III) and terbium(III). The reactions were run with varying ratios of europium and terbium. After isolation of the crystalline products, PXRD was performed to confirm the structural analogy to homometallic compounds (Ln-1).


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Inductively coupled plasma mass spectrometry (ICP-MS) was utilized on the bulk crystals dissolved in 5% nitric acid, and the ratios of europium to terbium calculated for the prepared solutions (Table 2). Table 2. Reaction and product ratios of Eu(III) and Tb(III) in the synthesis of [(EuxTb1-x(TC)3(H2O)2)·(HPy·TC)]n. [(EuxTb1-x(TC)3(H2O)2)·(HPy·TC)]n Reaction Ratioa Product Ratiob Entry x 1-x x 1-x 0.02 0.98 0.029 0.971 Eu0.029Tb0.971 0.04 0.96 0.047 0.953 Eu0.047Tb0.953 0.05 0.95 0.051 0.949 Eu0.051Tb0.949 0.06 0.94 0.060 0.940 Eu0.060Tb0.940 0.07 0.93 0.079 0.921 Eu0.079Tb0.921 0.09 0.91 0.094 0.906 Eu0.094Tb0.906 0.20 0.80 0.203 0.797 Eu0.203Tb0.797 0.30 0.70 0.285 0.715 Eu0.285Tb0.715 0.40 0.60 0.335 0.665 Eu0.335Tb0.665 0.50 0.50 0.519 0.481 Eu0.519Tb0.481 0.60 0.40 0.557 0.443 Eu0.557Tb0.443 0.70 0.30 0.707 0.294 Eu0.707Tb0.294 0.80 0.20 0.733 0.267 Eu0.733Tb0.267 0.90 0.10 0.885 0.116 Eu0.885Tb0.116 a Reactions performed according to general procedure described above. determined by ICP-MS.

b

Metal ratios

While the observed ratios of europium to terbium in the products deviate slightly from statistical distributions, they remain in relatively good agreement with the ratios of the metal precursor used in the reactions. Structural Characterization. Single crystal X-ray diffraction analyses revealed an isomorphous series of the general formula [(Ln(TC)3(H2O)2)·(HPy·TC)]n (La-1 – Lu-1, excluding Pm) that crystallizes in the triclinic space group, P-1. The structure of an impurity, La(TC)3(HTC) La-2, that forms upon ethanol washing and crystallizes in a monoclinic crystal


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system was also determined. As representative examples, the praseodymium(III) structure (Pr-1) and lanthanum(III) byproduct (La-2) will be discussed in detail. [(Pr(TC)3(H2O)2)·(HPy·TC)]n

(Pr-1).

The

structure

of

Pr-1

is

built

from

one

crystallographically unique praseodymium(III) ion, two water molecules, one distinct bidentate thiophenecarboxylate,

two

unique

bridging

thiophenecarboxylates,

and

a

disordered

2-thiophenecarboxylate in close association with a pyridinium cation. Overall the metal center adopts a square antiprismatic coordination geometry with Pr1 bound to eight oxygen atoms from four monodentate, bridging TC units, one bidentate TC, and two water molecules as shown in. Pr–O bond distances range from 2.382(3)-2.589(2) Å. The four bridging TC units connect mononuclear PrO8 polyhedra into neutral one-dimensional chains that extend infinitely along [100] (Figure 2a) with an average Pr---Pr separation of 4.94(5) Å. As illustrated in Figure 2b, an unbound TC and a pyridinium cation exist between the (Pr(TC)3(H2O)2)n chains, with the two metal bound water molecules forming hydrogen bonding interactions with the carbonyl of the unbound TC moiety with O7–H···O10 and O8–H···O10 distances of 2.797(3) and 2.704(2) Å, respectively. The unbound TC also forms hydrogen bonding interactions with the outer sphere pyridinium with an N–H···O distance of 2.64(1) Å. The structure is further stabilized by weak ππ stacking interactions that exist between the thiophene rings of TC units from adjacent chains as well as those from the thiophene rings of metal bound TC with the outer coordination sphere Hpy cations.


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Figure 1. Illustration of the local structure about the Pr(III) metal center in Pr-1. Symmetry codes: (i) –x+1, -y+1, -z+1; (ii) –x, -y+1, -z+1.

Figure 2. (a) Polyhedral representation Pr-1. PrO8 polyhedra are bridging through TC units to form 1D chains that propagate along the [100] direction. (b) Packing diagram of Pr-1 viewed along [100] direction. Green polyhedra represent 8-coordinate Pr(III) metal centers. Disorder in


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the thiophene rings has been removed for clarity. Red, yellow, black, and blue spheres are oxygen, sulfur, carbon, and nitrogen atoms, respectively.

[La(TC)3(HTC)2]n (La-2). The structure of La-2 is isomorphous with a previously reported Eu(III) compound,46 yet as the La-2 analog has not been reported, and for comparison to the major phase La-1, it is briefly described. The structure is built from eight-coordinate La(III) metal centers that adopt a square antiprism coordination geometry. The inner coordination sphere of the metal center is occupied entirely by oxygen atoms of the carboxylate moieties from eight monodentate TC units, with La–O bond lengths ranging from 2.412(2) to 2.575(2) Å (Figure 3). The TC moieties bridged La(III) metal centers along the [001] direction to two other metal centers via eight monoanionic thiophenecarboxylate ligands to result in extended 1D chains as shown in Figure 4.

Figure 3. Ball and stick representation of the inner coordination sphere of La in La-2. Turquoise, red, black, and yellow spheres are La(III), oxygen, carbon, and sulfur, respectively. Symmetry codes: (i) x, -y+1, z–½; (ii) x, -y+1, z+½.


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Figure 4. Polyhedral representation of La-2 highlighting (a) the LaO8 polyhedra that are bridged via TC units into 1D chains that propagate along the [001] direction and (b) the packing diagram viewed along [001] direction. Disorder in the thiophene rings has been removed for clarity. Turquoise polyhedra represent 8-coordinate La(III) metal centers. Red, yellow, and black spheres are oxygen, sulfur, and carbon atoms, respectively. Structural Comparison of La-1 – Lu-1. Due to lanthanide contraction and the resulting changes imposed on the coordination sphere of the lanthanide, the isolation of a full isomorphous series of the 4f block is relatively uncommon. Rather, it is more common to see breaks within the structural chemistry, resulting in the isolation of different compounds for early and late lanthanides. Several experimental and theoretical studies have been performed investigating this periodicity of the lanthanides, with a few studies having focused on the various levels of hydration for the full series of rare earth metals. For example, computational studies have shown that the early 4f ions are bound to nine water molecules, however, with decreasing ionic radius the metal center is seen to accommodate fewer


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bound water molecules beginning at gadolinium(III), wherein a partial occupancy of coordinated water, between eight and nine, is seen. In the smallest of the trivalent lanthanide ions, lutetium, only eight water molecules are found in the inner coordination sphere.59-61 This break in coordination behavior, arising from the lanthanide contraction, is further manifested in a recent study by Ridenour et al. wherein p-halobenzoic acids and terpyridine were reacted with lanthanide nitrates to yield a series of products with varying coordination environments. Using pbromobenzoic acid, a series of five structural types are formed with the lanthanide ions, and the evolution of these products is clearly seen to follow systematic breaks throughout the 4f block. 62 Interestingly, this break was not observed in the lanthanide series presented within this study, and the solid-state structural data revealed that compounds La-1 – Lu-1 form an isomorphous series of extended networks. An investigation of the Shannon crystal radii for eightcoordinate lanthanide(III) ions shows a contraction of 0.183 Å when moving from lanthanum to lutetium;63 by following this pattern, it is projected that a similar contraction should be present when comparing the isomorphous series reported herein. A noticeable cell contraction is observed in the lattice parameters of the La-1 and Lu-1 members of the series. Moreover, upon comparing the average Ln–O bonds between compounds La-1 – Lu-1, this contraction is observed in the solid-state structure, evident by a 7.9% (0.198 Å) shortening of the average oxygen bond lengths between La3+ and Lu3+ (Figure 5).


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Figure 5. Line graph displaying the average Ln–O bond lengths in La-1 – Lu-1. Experimental values from this study are denoted as (♦), and values calculated as sum of Shannon crystal radii are denoted as (♦).63 Data labels display average Ln–O bond lengths in La-1 – Lu-1, and error bars indicate standard deviation of the mean (σmean). Representative bond lengths and angles in addition to full solid-state structures for all compounds are provided in Figures S1 – S15 and Table S1. In order to further elucidate the nature of the bonding observed in the solid state, the FTIR spectrum of Nd-1 was obtained (Figure 6). While several features of the spectrum could not be resolved, and thus, attributed to particular features of the obtained structure, several peaks were consistent with the solid state crystal structures.


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ν COO- sym.

ν COO- asym.

ν N-H stretch

ν C=O stretch

(aromatic)

ν C-H stretch

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ν O-H stretch

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Figure 6. ATR-FTIR spectrum of Nd-1. The broad signal centered at 3350 cm-1 is attributed to the lanthanide-bound water molecules,64 and emerging at the lower energy frequency from within this resonance (3098 and 3066 cm-1) are weak, sharp signals arising from aromatic C-H stretches in TC and pyridinium ions. As the TC units within the crystal structure exist only in their ionic form, the carboxylate moieties are largely delocalized, however, the inherent carbonyl character of the free TC gives rise to the weak, sharp absorption centered at 1684 cm-1.65 The bidentate and bridging TC units bound to the neodymium center give rise to the dominant features of the FTIR spectrum, revealing signals at 1539 and 1517 cm-1 arising from asymmetric stretching modes, and 1419 and 1394 cm-1 from symmetric stretches.45,66 At a slightly lower energy, the pyridinium N-H stretch is evident at 1234 cm-1,67 yet beyond this, the lower energy frequencies remain difficult to assign but likely arise from a combination of aromatic C-C and C-H, as well as Ln-O bond stretches. The culmination of the signals seen in this analysis aid in further supporting the proposed bonding modes established from the solid-state structures.


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Optical Emission Measurements. Solid-state luminescence measurements were obtained at room temperature for the isomorphous trivalent lanthanide compounds prepared in this study (La-1 – Lu-1 and EuxTb1-x-1), and are presented within the text of the manuscript and Figures S31 – S38. In order to elucidate the nature of the observed emission for the extended lanthanide structures, the luminescence spectrum of the supporting ligand (HTC) was first investigated, and showed a single broad band luminescence centered at 421 nm upon excitation at 361 nm. Using the previously reported energy level diagrams for trivalent lanthanide ions and comparing to the reported triplet state of the ligand, a cursory prediction of the metals suited for TC sensitization was possible.4,45 Luminescence studies were immediately precluded for La(III) and Lu(III) owing to the 4f orbital electron configurations which do not generate metal-based emission. Additionally, the emissive level of Gd3+ is much greater than the TC excited state, while the emissive manifold of Ce3+ is much lower than the ligand triplet state, informing that these two metal ions will not be sensitized by TC. The remaining lanthanide ions, however, are predicted to have emissive states that are suitable to ligand sensitization in these systems. Excitation at approximately 360 nm revealed metal-based emission in the visible region for compounds Eu-1, Tb-1, and Dy-1, and in the near-infrared for Nd-1 and Er-1. The emission spectrum for the europium analogue (Eu-1) was measured at an excitation wavelength of 360 nm and displayed the characteristic Eu3+ luminescence (Figure 7).68


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Figure 7. Room temperature excitation and emission spectra for Eu compound Eu-1. Excitation is represented by the dashed line, and the solid line shows the emission spectrum. A low intensity emission at 591 nm corresponds to the Eu3+ 5D0 → 7F1 transition. The hypersensitive 5D0 → 7F2 transition of europium is seen at 614 nm and gives rise to the characteristic red color of emission, and two weaker 5D0 → 7F3 and 5D0 → 7F4 transitions are evident at 652 and 699 nm. Due to the asymmetry of the lanthanide network created by the TC ligands and bound water molecules, and in agreement with the Judd-Ofelt theory,69 the 5

D0 → 7F2 transition clearly dominates the emission spectrum. Presence of the weak peak

corresponding to the 5D0 → 7F0 transition at 579 nm is consistent with a Eu(III) metal center with Cnv, Cn, or Cs site symmetry. Further, the significantly greater intensity of the 5D0 → 7F2 transition relative to the 5D0 → 7F1 as well as the splitting of these peaks is consistent with low site symmetry, as observed in the crystal structure. 4,62,68,70,71 The excitation spectrum acquired at


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614 nm additionally confirms efficient ligand sensitization and illustrates the utility of the TC moiety as a stabilizing ligand. Time-resolved measurements centered at the 614 nm emission showed a 431.23(9) µs fluorescence lifetime. The terbium analogue (Tb-1) similarly displayed strong metal-based emission upon excitation of the ligand (Figure 8). The highest energy emission at 490 nm arises from the 5

D4 → 7F6 after excitation of the TC-based excitation band, however, the green color of

fluorescence arises from the high intensity emission at 545 nm (5D4 → 7F5). The splitting seen in this high intensity emission informs of low site symmetry of the Tb(III) metal center in solid sample, in good agreement with the crystal structure and previously reported data.72,73 Lower energy emission peaks emerge at 581 and 622 nm (5D0 → 7F4 and 5D0 → 7F3, respectively), and the three remaining low intensity emissions corresponding to relaxation to the 7FJ energy level (J = 2, 1, and 0, respectively). The most persistent radiative decay process was seen for the terbium compound as supported by the long-lived fluorescence lifetime (873.8(2) µs).


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Figure 8. Room temperature excitation and emission spectra for Tb compound Tb-1. Excitation is represented by the dashed line, and the solid line shows the emission spectrum. For compound Dy-1, three prominent bands are observed at 422, 479, and 574 (Figure 9). The first of these peaks originates from relaxation of the triplet state of the TC ligand, however, the latter two are attributed to the 4F9/2 → 6H15/2 and 4F9/2 → 6H13/2 transitions, respectively. The weak red emissions are significantly outweighed by the intense yellow emission band, yet remain evident in the acquired spectrum at 663 and 752 nm (4F9/2 → 6H11/2,6H9/2). A short lifetime of emission was measured for compound Dy-1 (1.18(17) µs).


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Figure 9. Room temperature excitation and emission spectra for Dy compound Dy-1. Excitation is represented by the dashed line, and the solid line shows the emission spectrum.

Upon investigation of the NIR emitters, the neodymium compound (Nd-1) displayed the most intense emission spectrum, with each of the three characteristic emission features being distinctly present (Figure 10). Excitation of the supporting ligand at 355 nm results in relaxation from the 4F3/2 excited state of the metal center to a 4I9/2, 4I11/2, and 4I13/2 state, resulting in the NIR emission bands at 875, 1057, and 1326 nm, respectively.


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Figure 10. Room temperature emission spectrum for Nd compound Nd-1. Of the remaining NIR emitters (Pr-1, Sm-1, Er-1, and Yb-1) only Er-1 exhibited discernible emission. The erbium compound (Er-1) showed a single broad NIR band in the emission spectrum at 1534 nm arising from the 4I13/2 → 4I15/2 transition. The remaining lanthanide networks which have hitherto been absent from the discussion of emissive properties were found to only show ligand-based emission owing to numerous factors. The La(III) and Lu(III) compounds La-1 and Lu-1, containing a vacant and filled 4f shell, respectively, are well understood to exhibit no characteristic emission properties due to their electronic configuration.3 Gadolinium compounds generally undergo metal-based emission from a 6P7/2 → 8S7/2 transition


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in the ultraviolet region, typically at 312 nm,4 however, as the triplet state of the ligand is lower in energy than the emissive state of Gd, only ligand based emission is observed for Gd-1. The remaining lanthanide ions investigated in this study (Ce-1, Ho-1, and Tm-1) did not exhibit efficient ligand-sensitization, and as such, only presented ligand fluorescence in the emission spectra. Given the formation of an isomorphous series, color tuning of the emission was attempted by preparing heterobimetallic lanthanide analogues of the target compounds. As the europium and terbium compounds Eu-1 and Tb-1 provided the most intense luminescence, mixed metal complexes of the formula [(EuxTb1-x(TC)3(H2O)2)·(HPy·TC)]n were prepared. Using an analogous synthetic protocol to that used for the preparation of La-1 – Lu-1, various ratios of europium and terbium nitrate were introduced to solutions of HTC and pyridine in water. The crystals isolated were analyzed by ICP-MS and produced the europium/terbium ratios provided in Error! Reference source not found.. The solid samples were then studied by fluorescence spectroscopy, and the effects of altering the lanthanide ion ratios in EuxTb1-x were clearly manifest in the variations of the emission spectra (Figure 11).


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Figure 11. Emission spectra of [(EuxTb1-x(TC)3(H2O)2)·(HPy·TC)]n upon excitation at 360 nm. Given that the compounds are isomorphous, and assuming random distribution of the metal ions, it is expected that the relative intensities of the characteristic Eu(III) and Tb(III) emission bands bear strong resemblance to the homometallic compounds Eu-1 and Tb-1, respectively.74,75 Increasing the europium concentration in moving from Eu0.029Tb0.971 to Eu0.885Tb0.116 results in a significant red shift of the highest intensity signal; while the general shift in color can be safely assumed given the trend of the spectra, chromaticity coordinates were calculated and plotted in the 1931 CIE coordinate diagram to more clearly present the colors of emission (Figure 12).


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Figure 12. CIE chromaticity coordinates of [(Tb(TC)3(H2O)2)·(HPy·TC)]n (Tb-1), [(Eu(TC)3(H2O)2)·(HPy·TC)]n (Eu-1), and [(EuxTb1-x(TC)3(H2O)2)·(HPy·TC)]n (EuxTb1-x), displaying a red shift in the color of emission. As previously discussed, the pure terbium phase (Tb-1) emits at 545 nm (green) upon excitation at 360 nm; increasing the relative ratio of europium in the reaction mixture systematically increased the Eu:Tb ratio in the resulting product, and the typical color of europium emission is not evident until a europium:terbium ratio of approximately 2:8 in the final product (Eu0.2034Tb0.7966). The early onset of red luminescence informs of the greater intensity of the


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europium emission as compared to terbium, and rationalizes the rather low concentrations of Eu(III) ions needed to move the color of the heterobimetallic system out of the green regime. These results provide a facile methodology to readily generate tunable colors from green, to green-yellow, yellow, orange, red-orange, and red. Thermal Stability. While the synthesis of the lanthanide frameworks presented within this study proved robust, the thermal stability of these products remained unknown. In order to investigate the thermal properties of these compounds, thermogravimetric analysis (TGA) was performed on the praseodymium analogue (Pr-1). Drying compound Pr-1 in the vacuum oven was seen to remove coordinated water molecules, and the resulting compound of the formula [(Pr(TC)3)·(HPy·TC)]n was used in subsequent thermal analysis (Figure S42). In the range of 130 to 195 °C, a mass loss of 28.41% occurs, corresponding to the loss of the pyridinium thiophenecarboxylate (28.41%). Upon heating further and beyond 300 °C, ligand-based decomposition occurs, with the observed mass loss of 39.06%, suggesting thermal decomposition to a Pr-sulfate containing phase. Further investigation into previous literature showed considerable precedent for similar decomposition in numerous sulfur heterocycles and justifies the proposed decomposition pathway.76 CONCLUSIONS We have reported herein the preparation and luminescent properties of a family of isomorphous lanthanide(III) extended networks. Using 2-thiophenecarboxylate as a sensitizing ligand proved efficient to achieve ligand-sensitized metal-based emission upon excitation at 360 nm UV irradiation. Moreover, the presence of the carboxylate motif in the organic chromophore provides a robust scaffold for the formation of thermally stable lanthanide chains. The resulting


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compounds displayed a series of visible light (Eu, Tb, and Dy) and NIR (Nd and Er) emitters, with the europium and terbium analogues achieving long-lived fluorescence lifetimes. The culmination of these properties exposed the amenability to achieve color tuning by mixing lanthanide ions in the reaction mixture, ultimately yielding the heterobimetallic europium/terbium compounds reported. Investigation of the fluorescence properties of the mixed-metal phases revealed the ability to selectively form luminescent materials capable of emitting green to red light. The syntheses were optimized by increasing reaction temperature and crystallization by slow evaporation; conversely, low reaction temperatures or workup using ethanol was found to form the byproduct of the general formula [Ln(TC)3(HTC)2]n. Further studies are currently underway in our group to investigate the ability to utilize other combinations of lanthanide ions in these structures to achieve color tuning toward color regimes not readily accessible in homometallic lanthanide systems. These results provide a viable pathway toward the development of luminescent materials with potential applications in phosphorescent optical devices.

ASSOCIATED CONTENT Supporting Information. Synthetic details, combustion elemental analyses, powder X-ray diffraction patterns, CIF files, crystallographic refinement parameters, and excitation/emission spectra. This material is available free of charge via the internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (K.E.K.)


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ORCID Alyssa K. Adcock: 0000-0001-9977-3756 Rami J. Batrice: 0000-0001-5368-3106 Karah E. Knope: 0000-0002-5690-715X

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT The authors gratefully acknowledge the Clare Boothe Luce Foundation for their support The authors also acknowledge the National Science Foundation for acquisition of the single crystal X-ray diffractometer and the Raman spectrometer under grants NSF CHE-1337975 and NSF CHE-1429079, respectively. REFERENCES 1.

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FOR TABLE OF CONTENTS USE ONLY Synthesis and Characterization of an Isomorphous Lanthanide-Thiophenemonocarboxylate Series (Ln = La-Lu, except Pm) Amenable to Color Tuning Rami J. Batrice, Alyssa K. Adcock, Paula M. Cantos, Jeffery A. Bertke, Karah E. Knope*

An isomorphous series of lanthanide-thiophenemonocarboxylate compounds (Ln = La-Lu, except Pm) has been prepared under mild aqueous conditions. Efficient ligand-sensitized emission was observed in the visible region for Eu, Tb, and Dy, and in the NIR for Nd and Er. Mixed metal compounds containing Eu and Tb showed the ability to color tune emission from green through red.


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Synthesis and Characterization of an Isomorphous ... - ACS Publications

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