Highly Luminescent Ionic Liquids Based on Complex Lanthanide

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Highly Luminescent Ionic Liquids Based on Complex Lanthanide Saccharinates Si-Fu Tang† and Anja-Verena Mudring*,‡ †

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College of Chemistry and Pharmaceutical Sciences, Qingdao Agricultural University, Changcheng Road 700, Chengyang District, Qingdao 266109, China ‡ Department of Materials and Environmental Chemistry, Stockholm University, Svante Arrhenius väg 16 C, 106 91 Stockholm, Sweden S Supporting Information *

ABSTRACT: Four strongly luminescent ionic liquids with complex lanthanide saccharinate anions, [C 4 mim] 3 [Eu(Sac) 6 (H 2 O) 2 ] (1), {C 4 mpy} 5 {[Ln(Sac) 6 (H 2 O) 2 ][Ln(Sac)5(H2O)3]}{(H2O)2(CH3CN)2} (Ln = Sm for 2a; Ln = Eu for 2b), and [C4mpy]3[Eu(Sac)6][CH3CN] (3) (C4mim = 1butyl-3-methylimidazolium; C4mpy = N-butyl-4-methylpyridinium; Sac = saccharinate), have been obtained by reacting the ionic liquids 1-butyl-3-methylimidazolium saccharinate, [C4mim][Sac], and N-butyl-4-methylpyridinium saccharinate, [C4mpy][Sac], with the respective lanthanide saccharinates. Single-crystal X-ray diffraction analyses reveal the respective lanthanide center to be six- or eight-coordinated by five or six saccharinate anions and two or three aqua ligands in the cases of 1 and 2 when lanthanide saccharinated hydrate was employed as the starting material. Coordination of water to the lanthanide can be avoided when using the anhydrous lanthanide saccharinate as shown by the structure of 3. Using a co-solvent, acetonitrile, to facilitate the reaction led to incorporation of solvent molecules into the crystal structure of the final materials (2 and 3). Differential scanning calorimetry analyses reveal that 1 is an ionic liquid whereas 2 and 3 are low-temperature molten salts. The three europium(III)-containing compounds (1, 2b, and 3) all show characteristic intense red emissions of Eu(III) upon excitation into levels of the saccharinate ligands (321 nm) or Eu(III) (393 nm). At room temperature, the decay times of 1 and 2b are all ∼0.5 ms, whereas the decay time of 3 amounts to 3.85 ms due to removal of aqua ligands in the first coordination sphere; accordingly, the quantum efficiencies of 1, 2b, and 3 were determined to be 15.9%, 22.95%, and 57.75%, respectively. The CIE chromaticity coordinates for all Eu compounds are in the red region and approach the NTSC standard CIE values when the temperature is increased. Sm(III)-containing compound 2a shows characteristic Sm(III) emission peaks. As expected, the CIE coordinates of samarium compound 2a fall in the orangered region.

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them excellent optical solvents.18,19 Optically active components such as lanthanide ions can be introduced into an IL by simply dissolving a lanthanide salt in the IL. However, this is often hampered by low solubility. Higher lanthanide ion concentrations can be reached when the lanthanide ions become part of the ionic liquids in the form of a complex anion. Lanthanide-containing ionic liquids are very promising materials because, albeit liquid, ILs provide a comparatively low phonon environment to the optical active center leading to an appreciable excited state lifetime.20,21 In the past several years, a few lanthanide-containing ionic liquids have been synthesized and studied. The first examples of strongly luminescent ionic liquids contained the Eu(III)bis(trifluoromethanesulfonyl)amide complex anions. They show excellent photophysical properties such as long luminescence lifetimes and high color purities.22 The three dysprosium-based ionic liquids [C 6 mim] 5 − x [Dy(SCN)8−x(H2O)x] (x = 0−2; C6 mim = 1-hexyl-3-methyl-

INTRODUCTION Ionic liquids (ILs) have been attracting more and more attention in the past few years because of their unique nature and wide variety of potential or practical applications in organic synthesis, catalysis electrochemistry, and many other fields or chemistry and materials science.1−6 Ionic liquids are also often labeled as “green”, mainly for two reasons.7 One is that many ionic liquids have a negligible vapor pressure, low flammability, and low toxicity and therefore can be used as stand-ins for conventional organic solvents. On the other hand, they also have been extensively studied for making chemical processes “greener”, as in the BASIL process or in the separation of lanthanides and nuclear waste disposal.8−12 ILs are known for their wide “tunability”, and with the right combination of a cation and an anion that can be endowed with various functional groups, it is possible to engineer a liquid for a certain application. Indeed, today the most important field of IL research might be the development of ILs for specialized applications such as optics and soft luminescent materials.13−17 Many ILs are transparent through almost the whole visible and near-infrared spectral regions, which makes © XXXX American Chemical Society

Received: May 14, 2019

A

DOI: 10.1021/acs.inorgchem.9b01411 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

[C4mim][Sac]. Slightly yellow liquid. Elemental analysis (%) calcd for [C4mim][Sac] [C15H19N3O3S, formula weight (fw) of 321.39]: C, 56.06; H, 5.96; N, 13.07; S, 9.98. Found: C, 55.97; H, 5.53; N, 13.06; S, 9.89. [C4mpy][Sac]. White solid. Elemental analysis (%) calcd for [C4mpy][Sac] (C17H20N2O3S, fw of 332.42): C, 61.42; H, 6.06; N, 8.43; S, 9.65. Found: C, 61.23; H, 6.02; N, 8.40; S, 9.59. Eu(Sac)3(H2O)4. The title compound was obtained by reacting a slight excess of Eu2O3 with an aqueous solution of saccharin [H(Sac)], followed by filtering of the unreacted Eu2O3 and removal of water by evaporation. The obtained salt was dried under dynamic vacuum at room temperature for 1 day. The formula Eu(Sac)3(H2O)4 was verified by thermogravimetric (TGA) and element analysis (EA). Elemental analysis (%) calcd for Eu(Sac)3(H2O)4 (C21H20N3O13S3Eu, fw of 770.56): C, 32.73; H, 2.62; N, 5.45; S, 12.48. Found: C, 32.50; H, 2.77; N, 5.48; S, 12.54. Sm(Sac)3(H2O)4. The title compound was synthesized from HSac and Sm2O3 using the same method that was used for Eu(Sac)3(H2O)4. Elemental analysis (%) calcd for Sm(Sac)3(H2O)4 (C21H20N3O13S3Sm, fw of 768.96): C, 32.80; H, 2.62; N, 5.46; S, 12.51. Found: C, 32.67; H, 2.70; N, 5.41; S, 12.60. Eu(Sac)3. The title compound was obtained by drying Eu(Sac)3(H2O)4 at an elevated temperature (120 °C) under high vacuum (10−3 mbar) for 2 days. Elemental analysis (%) calcd for Eu(Sac)3 (C21H12N3O9S3Eu, fw of 698.50): C, 36.11; H, 1.73; N, 6.02; S, 13.77. Found: C, 36.13; H, 1.82; N, 5.98; S, 13.90. [C4mim]3[Eu(Sac)6(H2O)2] (1). The title compound was synthesized from [C4mim][Sac] and Eu(Sac)3(H2O)4. 0.2410 g of [C4mim][Sac] (0.7498 mmol) was mixed with 0.1926 g of Eu(Sac)3(H2O)4 (0.2499 mmol) in a small Schlenk tube and stirred for 1 h at ∼90 °C until a homogeneous solution was obtained. When the mixture was cooled to room temperature slowly, block-shaped colorless crystals formed. Elemental analysis (%) calcd for [C4mim]3[Eu(Sac)6(H2O)2] (C66H73EuN12O20S6, fw of 1698.68): C, 46.67; H, 4.33; N, 9.89; S, 11.33. Found: C, 46.60; H, 4.12; N, 9.80; S, 11.27. {C4mpy}5{[Ln(Sac)6(H2O)2][Ln(Sac)5(H2O)3]}{[(H2O)2(CH3CN)2]} (Ln = Sm for 2a; Ln = Eu for 2b). In a 20 mL Schlenk tube, [C4mpy][Sac] (1.0 mmol, 0.3324 g) and Ln(Sac)3(H2O)4 (1/3 mmol, 0.2563 g of Sm, 0.2569 g of Eu) were dissolved in 2 mL of dry acetonitrile. The obtained solution was stirred at 60 °C for 2 h and then allowed to cool slowly to room temperature. Colorless brick crystals appeared within a couple of days. Elemental analysis (%) calcd for 2a (C131H144N18O40S11Sm2, fw of 3264.00): C, 48.20; H, 4.45; N, 7.72; S, 10.81. Found: C, 48.27; H, 4.50; N, 7.88; S, 10.92. Elemental analysis (%) calcd for 2b (C131H144N18O40S11Eu2, fw of 3267.3270): C, 48.16; H, 4.44; N, 7.72; S, 10.80. Found: C, 48.27; H, 4.44; N, 7.62; S, 10.82. [C4mpy]3[Eu(Sac)6](CH3CN) (3). The title compound was obtained in a manner similar to that used for 2b except that anhydrous Eu(Sac)3 was used instead of Eu(Sac)3(H2O)4. Elemental analysis (%) calcd for 3 (C74H75EuN10O18S6, fw of 1736.76): C, 51.17; H, 4.35; N, 8.06; S, 11.08. Found: C, 51.24; H, 4.27; N, 8.16; S, 11.12. Elemental analyses were performed on a Vario EL III elemental analyzer. Infrared (IR) spectra were recorded on a Bruker Alpha-P FT-IR spectrometer in the range of 400−4000 cm−1 (see Figures S1 and S2 and the corresponding IR data in the Supporting Information). Raman spectra were recorded at room temperature on a Bruker IFS-FRA-106/s instrument at 150 mW with the powder samples sealed in glass capillaries under an argon atmosphere (see Figures S3−S5 and the corresponding Raman data in the Supporting Information). Powder X-ray diffraction (PXRD) measurements (see Figures S6−S9) were performed on powder samples sealed in Lindemann capillaries (diameter of 0.3 mm) and collected on a G670 Guinier camera diffractometer (Huber, Rimsting, Germany) equipped with an image plate detector using Mo Kα radiation. Phase transition behavior was investigated with a differential scanning calorimeter (NETZSCH DSC 240 F1). The measurements were performed on samples sealed in aluminum pans at a heating rate

imidazolium) not only are strongly luminescent but also respond strongly to external magnetic fields. 23 With tetraalkylphosphonium as the countercation, an intrinsic photoluminescent ionic liquid based on the europium(III) tetrakis(β-diketonate) complex can also be synthesized, and these kinds of ILs show characteristic Eu(III) emissions and interesting temperature effects above its melting point.24 These successes verified lanthanide-containing ILs as outstanding and promising optical materials. However, this field of research is still in its infancy when it is compared with other fields of ionic liquid chemistry. Saccharinate ionic liquids have been promoted in recent years as the next generation of sweeteners.25,26 A low toxicity is asserted,27−29 and interesting combination of properties can be achieved with respect to biological systems.30 The saccharinate anion has variable electronic structures upon complexation with hard or soft metal ions31 and therefore is very interesting for anticrystal engineering ionic liquids (see Scheme 1). Scheme 1. Ionic Components of the Starting Ionic Liquids

Additionally, it is aromatic in nature, which is very helpful for harvesting light and transferring the energy to the lanthanide ion (antenna effect), thus overcoming the poor absorption efficiencies of the lanthanide ion itself. In view of these findings and on the basis of our previous studies, we continued our work by investigating saccharin as a coordination ligand to form the complex anion in the hope that it may be able to improve the luminescence efficiency of the obtained ionic liquids. Here, we report on the syntheses, structures, thermal behaviors, and spectroscopic studies of four saccharinate-based lanthanide-containing ionic liquids, namely, [C4mim]3[Eu(Sac) 6 (H 2 O) 2 ] (1), {C 4 mpy} 5 {[Ln(Sac) 6 (H 2 O) 2 ][Ln(Sac)5(H2O)3]}{(H2O)2(CH3CN)2} (Ln = Sm for 2a; Ln = Eu for 2b), and [C4mpy]3[Eu(Sac)6][CH3CN] (3).

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EXPERIMENTAL SECTION

Materials and Methods. Saccharin (1,1-dioxo-1,2-benzothiazol3-one, HSac, Acros), Eu2O3 and Sm2O3 (Smart Elements), 1-butyl-3methylimidazolium chloride (C4mim)Cl (iolitec), N-butyl-4-methylpyridinium (C4mpy)Cl (iolitec), and acetonitrile (Acros, AcroSeal, ≤10 ppm H2O) were obtained from commercial sources and used as received. AgSac was synthesized from saccharin according to the literature.32,33 First, saccharin was dissolved in water at 80 °C and neutralized with a NaOH solution. After the solution was allowed to cool to room temperature, 1 N AgNO3 was dropped slowly into the system while it was being vigorously stirred. A white precipitate formed immediately, which was isolated by filtration after the mixture had been stirred for 30 min. [C4mim][Sac] and [C4mpy][Sac]. At room temperature, an aqueous solution of (C4mim)Cl or (C4mpy)Cl was reacted with an aqueous slurry of a slight excess of AgSac by being stirred overnight in a flask covered with aluminum foil. After completion of the reaction, silver chloride was removed from the solutions by filtration and the remainder was washed with a small amount of water. The collected filtrates were dried using a rotating evaporator. To ensure complete removal of silver salt from the product, the obtained viscous liquids were redissolved in dry acetone and cooled in a freezer overnight followed by additional filtration. The final products were dried under dynamic vacuum for 2 days at 60 °C. B

DOI: 10.1021/acs.inorgchem.9b01411 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry of 5 °C/min and an argon flow rate of 20 mL/min. TGAs were carried out on a Shimadzu TGA-50 thermogravimetric analyzer at a heating rate of 10 °C min−1 using dried N2 as the purging gas (10 mL min−1). Optical analyses were performed via polarized optical microscopy using an Axio Imager A1 microscope (Carl Zeiss MicroImaging GmbH, Göttingen, Germany) equipped with a hot stage (THMS600, Linkam Scientific Instruments Ltd., Surrey, U.K.) and a temperature controller (Linkam TMS 94, Linkam Scientific Instruments Ltd.). Excitation and emission spectra were recorded at three different temperatures (77, 298, and 323 K) on a Fluorolog 3 fluorescence spectrophotometer (Jobin Yvon Gmbh, Munich, Germany) equipped with a continuous xenon lamp for steady state spectra and a pulsed xenon lamp for time-dependent spectra as the excitation source and a photomultiplier tube for detection. Powdered samples were sealed in quartz tubes for the measurements. The metastable liquid samples were carried out by heating the tube to 90 °C and then cooling it to 50 °C and measured at this temperature. The electronic transitions of trivalent rare earth ions were assigned according to the energy level diagrams.34,35 Determination of the Crystal Structures of 1, 2a, and 3. Single crystals of 1, 2a, and 3 with suitable quality were sealed in glass capillaries and measured on a single-crystal X-ray diffractometer (IPDS II, Stoe, Darmstadt, Germany). The diffraction data sets were collected at room temperature for 1 and 100 K for 2a and 3. X-red36 was used for the data reduction, and X-Shape was used for the numerical absorption.37 Direct methods was employed for the determination of crystal structures using the program package SIR9238 and yielded the heavy atom positions. Subsequent difference Fourier analyses and least-squares refinement with SHELXL-201339 allowed for the location of the remaining atomic positions. The hydrogen atoms were added theoretically with the riding atom mode. The hydrogen atoms on the water molecules and acetonitrile are located from the difference Fourier maps with their isotropic displacement factors set at 1.2 times the preceding oxygen atoms. All non-hydrogen atoms were refined anisotropically. For the sake of high thermal parameters, O3, C5, C7, C13, C25, C27, and C110 in compound 3 were refined istropically. The data have been deposited in the Cambridge Crystallographic Data Centre (CCDC), deposition numbers CCDC 938855−938857 for compounds 1−3, respectively. These data can be obtained free of charge via www.ccdc.cam.ac.uk/ data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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Scheme 2. Synthesis Route for Compounds 1−3

material the lanthanide center was coordinated not only by saccharinate anions but also by aqua ligands (cf. 1 and 2). It was possible to avoid this by using the anhydrous salt as the starting material (see 3). Structure Analysis. Compound 1, [C 4 mim] 3 [Eu(Sac)6(H2O)2], crystallizes in orthorhombic space group Pna21 with four formula units in the unit cell (see Table 1). The asymmetric unit contains one europium(III) ion, three [C4mim]+ cations, six (Sac)− anions, and two aqua ligands. The europium(III) ion is coordinated by six carbonyl oxgen atoms from six (Sac)− ligands and two water molecules to give a complex anion of the composition [Eu(Sac)6(H2O)2]3−. The coordination geometry around Eu(III) in the complex anion can be best viewed as a bicapped triangular prism with the two aqua ligands as the two caps (see Figure 1). The Eu−O bond lengths range from 2.326(5) to 2.516(5) Å with an average of ∼2.392 Å (see Table 2). This value is comparable to those of europium salts with similar ligands.22,24 A substantial number of supramolecular O−H···N and C− H···O hydrogen bonds are found in the structure (see Figure 2 and Table S1). The two crystallographically independent water molecules form two intramolecular O−H···N hydrogen bonds with two adjacent N atoms of (Sac)− ligands. The O···N distances fall in the range of 2.710(4)−2.778(5) Å, whereas the ∠(O−H···N) angles range from 121° to 157°. In addition, both intra- and intermolecular C−H···O interactions are observed. Compared to the O−H···N interactions, the C− H···O hydrogen bonds are weaker and less directional, which becomes obvious from the C···O distances and C−H···O angles. C−H···O hydrogen bond lengths are found to be between 3.170(6) and 3.430(1) Å with ∠(C−H···O) angles of 134−158°. These intra- and intermolecular hydrogen bonds link the [C4mim]+ cations and [Eu(Sac)6(H2O)2]3− anions into a supramolecular structure (see Figure 2). Compound 2a, {C 4 mpy} 5 {[Sm(Sac) 6 (H 2 O) 2 ][Sm(Sac)5(H2O)3]}{(H2O)2(CH3CN)2}, crystallizes in monoclinic space group P21/c. Its asymmetric unit is comprised of five [C4mpy]+ cations, one [Sm(Sac)5(H2O)3]2− anion, one [Sm(Sac)6(H2O)2]3− anion, two lattice waters, and two acetonitrile molecules (see Figure 3). In the two complex anions, both of the two samarium(III) ions are 8-fold coordinated; however, they contain different numbers of aqua ligands. Sm(1) is surrounded by five Sac− ligands and three water molecules, whereas Sm(2) is coordinated by six Sac− anions and two aqua ligands (see Figure 4). The Sm(1)−

RESULTS AND DISCUSSION

Synthesis. The saccharinate ionic liquids, 1-butyl-3methylimidazolium saccharinate, [C4mim][Sac], and N-butyl4-methylpyridinium saccharinate, [C4mpy][Sac], were obtained by reacting metathesis of silver saccharinate with 1butyl-3-methylimidazolium chloride, [C4mim]Cl, or N-butyl-4methylpyridinium chloride, [C4mpy]Cl. Europium(III) saccharinate tetrahydrate and samarium(III) saccharinate tetrahydrate were obtained by reacting saccharin with the respective lanthanide oxide. The anhydrous Eu(Sac)3 could be obtained by heating the hydrate to 120 °C under vacuum. Compounds 1−3 were synthesized from two precursor ionic liquids and the respective lanthanide saccharinates (M) with a M:IL molar ratio of 1:3 (see Scheme 2). [C4mim][Sac] is liquid at room temperature, and Eu(sac)3·4H2O easily dissolves in [C4mim][Sac] to give compound 1 in a direct reaction. On the contrary, [C4mpy][Sac] is solid at room temperature and it turned out to be difficult to obtain homogeneous solutions of Eu(sac)3· 4H2O in [C4mpy][Sac] even at elevated temperatures. To overcome this problem, acetonitrile was employed as a cosolvent to facilitate the reaction. It was observed that whenever a hydrous lanthanide saccharinate was employed as the starting C

DOI: 10.1021/acs.inorgchem.9b01411 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Crystallographic Details for Compounds 1, 2a, and 3 formula fw space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z Dcalcd (g cm−3) absorption coefficient (mm−1) no. of reflections collected no. of independent reflections/Rint F(000) GOF on F2 final R indices [I > 2σ(I)]: R1,a wR2b R indices (all data): R1,a wR2b

1

2a

3

C66H73EuN12O20S6 1698.68 Pna21 25.574(5) 19.104(4) 15.648(3) 90 90 90 7645(3) 4 1.476 1.061 87226 13365/0.0608 3488 0.888 0.0337, 0.0541 0.0511, 0.0570

C131H144N18O40S11Sm2 3263.99 P21/c 26.628(5) 20.823(4) 26.952(5) 90 108.54(3) 90 14168(5) 4 1.530 1.070 52726 18805/0.0781 6704 0.838 0.0455, 0.0817 0.0951, 0.0930

C74H75EuN10O18S6 1736.76 P1̅ 11.584(2) 14.089(3) 25.660(5) 77.44(3) 79.25(3) 73.12(3) 3877.3(15) 2 1.488 1.046 17119 9553/0.0653 1784 1.080 0.0576, 0.1726 0.0712, 0.1818

R1 = ∑||Fo| − |Fc||/∑|Fo|. bwR2 = {∑[w(Fo2 − Fc2)2]/∑w(Fo2)2}1/2.

a

Figure 1. Coordination environment of Eu(III) in 1. Eu, O, H, S, N, and C are drawn as green, red, white, yellow, blue, and black spheres, respectively, and the [EuO8] polyhedron is shaded in violet. The [C4mim]+ cations have been omitted for the sake of clarity.

Figure 2. View of the crystal structure of 1 down the c-axis. Hydrogen bonds are shown by dotted lines. Hydrogen atoms have been omitted for the sake of clarity.

Table 2. Selected Bond Lengths (angstroms) in 1, 2a, and 3 Eu(1)−O(1) Eu(1)−O(13) Eu(1)−O(7) Eu(1)−O(4) Sm(1)−O(1) Sm(1)−O(2) Sm(1)−O(3) Sm(1)−O(5) Sm(1)−O(6) Sm(1)−O(4) Sm(1)−O(1W) Sm(1)−O(2W) Eu(1)−O(12) Eu(1)−O(9) Eu(1)−O(3)

Compound 1 2.326(5) Eu(1)−O(10) 2.331(5) Eu(1)−O(16) 2.338(5) Eu(1)−O(1W) 2.377(6) Eu(1)−O(2W) Compound 2a 2.372(3) Sm(2)−O(20) 2.376(3) Sm(2)−O(23) 2.396(3) Sm(2)−O(21) 2.393(3) Sm(2)−O(22) 2.408(4) Sm(2)−O(19) 2.422(3) Sm(2)−O(5W) 2.477(3) Sm(2)−O(3W) 2.473(3) Sm(2)−O(4W) Compound 3 2.283(4) Eu(1)−O(15) 2.276(4) Eu(1)−O(18) 2.296(5) Eu(1)−O(6)

2.390(5) 2.425(5) 2.451(5) 2.516(5) 2.316(4) 2.370(4) 2.390(3) 2.413(4) 2.423(3) 2.449(4) 2.461(3) 2.466(3)

Figure 3. Coordination environment of the two crystallographically independent Sm(III) cations [Sm(Sac)6(H2O)2]3− (left) and [Sm(Sac)5(H2O)3]2− (right) in 2a.

O distances range from 2.372(3) to 2.477(3) Å. Sm(2)−O bond lengths vary from 2.316(4) to 2.466(3) Å (see Table 2). All are in the normal ranges.40 Although for these two complex anions the average Sm−O bond lengths are all ∼2.42 Å, more uniform values are found in the [Sm(Sac)6(H2O)2]3− anion. Plenty of O−H···N, C−H···O, and O−H···O interactions (see Table S2) assemble the complex anions and cations into a

2.294(4) 2.303(4) 2.305(5)

D

DOI: 10.1021/acs.inorgchem.9b01411 Inorg. Chem. XXXX, XXX, XXX−XXX

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liquids (C4mim)(Sac) and (C4mpy)(Sac). (C4mim)(Sac) exhibited glass transitions during both heating (−37 °C) and cooling (−44 °C) processes. In contrast, crystalline (C4mpy)(Sac) could be obtained by crystallization from acetonitrile. The crystalline material melted at ∼78 °C. However, in the following heating and cooling cycles, only glass transitions like that for (C4mim)(Sac) were observed. They occurred at approximately −22.1 °C upon heating and −30.0 °C upon cooling. This is not an uncommon thermal behavior for ionic liquids.41 The lanthanide-containing compound 1 showed a thermal behavior similar to that of (C4mpy)(Sac). When the crystalline material was carefully cooled, a melting point of 85 °C could be obtained. However, upon being moderately to quickly cooled like in the DSC experiments with thermal ramps of 5− 10 °C/min, the compound solidified as a glass. Glass transitions were observed at −10.1 °C on cooling and 18.1 °C on heating. In contrast, even at higher thermal ramps the thermobehaviors of compounds 2a, 2b, and 3 are reversible and the compounds crystallize. However, as found for many ionic liquids, supercooling was substantial. It may be as high as ∼30 °C as observed for 2a and 2b, which melted around 107 °C and crystallized around 76−79 °C from the melt. For compound 3, two endothermic and two exothermic phase transitions could be observed on the heating and cooling traces, respectively. The thermal events at higher temperatures could be associated with the melting and crystallization of 3, the one at lower temperatures with a solid−solid phase transition. Judging from polarized optical microscope measurements, compound 3 did not form a liquid crystal but a second solid polymorph; the solid−solid phase transitions were most probably caused by a rotation of the pyridinium cation. The melting points of pyridinium compounds 2a, 2b, and 3 were also found at temperatures higher than that of imidazolium compound 1, which was expected. Thermal Stability. TGAs were carried out to examine the thermal stabilities of Eu(Sac)3(H2O)4, Eu(Sac)3, and compounds 1−3 in the temperature ranges of 25−600 and ≤1000 °C (see Figures S14 and S15). The TGA curve for Eu(Sac)3(H2O)4 shows a first weight loss of ∼9.6% between 50 and 178 °C, which corresponds to the loss of the four water molecules (calcd 9.4%). A second weight loss involving three badly resolved steps starts at ∼380 °C and does not conclude at 600 °C. The observed weight loss amounts to 44.9%. For Eu(Sac)3 no obvious weight loss occurs before 420 °C. For compound 1, a first small weight loss from 75 to 180 °C (2.2%) corresponding to the loss of two aqua ligands (calcd 2.1%) was observed. A second drastic weight loss sets in at ∼285 °C, earlier than that observed for Eu(Sac)3(H2O)4. At 600 °C, the total weight loss is ∼73.9%. As expected, the TGA curves of isotypic compounds 2a and 2b are very similar. Different from those of Eu(Sac)3(H2O)4 and 1, three distinct steps of weight losses could be observed. In the range of 70−130 °C, one weight loss corresponding to the loss of solvent molecules (acetonitrile and lattice water) could be found (3.3% for 2a, 3.6% for 2b, the theoretical value is 3.6%); then, their weights were steady until 260 °C, but dramatic decomposition becomes evident with a further increase in temperature. From 450 to 520 °C, the weight loss slows but becomes fast again until 685 °C; at this temperature, the weight losses were 86.76% and 85.87%, respectively. Thus, the residue should be mainly Sm2O3 and Eu2O3.

Figure 4. Structure of complex anion [Eu(Sac)6]3− in compound 3. Three [C4mpy]+ cations and one acetonitrile have been omitted for the sake of clarity.

three-dimensional (3D) supramolecular structure (see Figure S10). Due to poor crystal quality, the intensity data of the europium analogue compound 2b were not collected, but PXRD analysis showed it to crystallize isotypic with 2a. The use of the anhydrous europium(III) salt as the lanthanide source resulted in the formation of compound 3, [C4mpy]3[Eu(Sac)6](CH3CN), which crystallizes in triclinic space group P1̅. As opposed to compound 2a, there is only one crystallographically independent europium(III) ion in the asymmetric unit. It is coordinated in an octahedral-like fashion by six Sac ligands to give the [Eu(Sac)6]3− complex anion. The Eu−O distances fall in the range of 2.276(4)−2.305(5) Å (average of 2.293 Å) (see Table 2), a little shorter than that found in 1, which features a higher coordination number. Three [C4mpy]+ cations balance the charge, and one acetonitrile molecule completes the asymmetric unit (see Figure 4). These cations and anions pack into the 3D supramolecular structure through π−π interactions between the pyridine moieties and aromatic saccharin rings (see Figure S11). Thermal Investigations. The DSC thermograms of ionic liquids (C4mim)(Sac) and (C4mpy)(Sac) and lanthanide compounds 1−3 are shown in Figures S12 and S13, respectively. Their phase transition properties are summarized in Table 3. Different thermobehaviors were found for ionic Table 3. Phase Transition Properties of [C4mim][Sac], [C4mpy][Sac], and Compounds 1−3a compound [C4mim] [Sac] [C4mpy] [Sac] 1 2a 2b 3

Tm (°C)

Tc (°C)

Tpt‑h (°C)

Tpt‑c (°C)

74.5 85.2 107.4 106.5

Tg‑h (°C)

Tg‑c (°C)

−37.1

−44.4

−22.1

−30.0

18.1

−10.1

78.5 75.9 107.8, 142.8

62.4, 127.4

a Tm, melting temperature; Tc, crystallization temperature; Tpt‑h, phase transition temperature during heating; Tpt‑c, phase transition temperature during cooling; Tg‑h, glass transition temperature during heating; Tg‑h, glass transition temperature during cooling.

E

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Inorganic Chemistry For compound 3, the first weight loss of ∼2.5% is found in the temperature range of 115−195 °C, a little higher than those observed for compounds 2a and 2b. It can be ascribed to the loss of one acetonitrile (calcd 2.4%). The second weight loss also starts at ∼260 °C similar to those of 2a and 2b. Until 600 °C, a weight loss of ∼66.6% was observed, but obviously, the decomposition of 3 was not complete at that temperature. Photophysical Properties. The excitation and emission spectra were recorded at 77, 298, and 353 K for compound 1 and at 77 and 298 K for compounds 2 and 3. The respective spectra of compounds 1 and 2b show only small differences, while that for compound 3 is notably different. All spectra for compounds 1, 2b, and 3 are dominated by the characteristic Eu(III) f−f transitions (see Figures 5−8). The emission spectra of compound 2a are typical for a Sm(III) compound (see Figure 9).

Figure 7. Enlarged portion of the 5D0 → 7F0 transition in compound 2b.

Figure 8. (a) Excitation and (b) emission spectra of compound 3 recorded at 77 (LT) and 298 (RT) K, respectively.

Figure 5. (a) Excitation and (b and c) emission spectra of compound 1 recorded at 77 (LT), 298 (RT), and 353 (HT) K, respectively.

Figure 9. (a) Excitation and (b) emission spectra of compound 2a recorded at 77 (LT) and 298 (RT) K, respectively.

D4), 375 (7F0/1 → 5GJ), 379.5, 382.5 (7F0/1 → 5L7, 5GJ), 393, 396 (7F0 → 5L6), 414.5 (7F0 → 5D3), 464 (7F0 → 5D2), 524.5 (7F0 → 5D1), and 533.5 nm (7F1 → 5D1). The emission spectra of 1 recorded at three different temperatures (77, 298, and 353 K) show a strong red light emission (λex = 321 and 393 nm) (see Figure 5). When excited into the ligand, the transitions from the lowest excited state 5 D0 to the different J levels of the lower 7FJ states of the Eu(III) ion were observed (J = 0−4): 5D0 → 7F0 at 578.9 nm, 5 D0 → 7F1 at 590.9 nm, 5D0 → 7F2 at 615.7 and 619.7 nm, 5D0 → 7F3 at 653.1 nm, and 5D0 → 7F4 at 708 nm. No ligand-based emission is observed, again confirming an efficient ligand-tometal energy transfer process. The spectrum is dominated by a band with a maximum around 615.7 nm that is sensitive to Eu(III) site symmetry. The high relative intensity ratio of the 5 D0 → 7F2 to 5D0 → 7F1 transition (the so-called asymmetry ratio, which here is 3.33) reveals the lack of inversion symmetry for the Eu(III) ion, as anticipated and confirmed by the single-crystal X-ray structure. Further evidence of a single Eu(III) local environment is given by the presence of a single line (17274.1 cm−1, full width at half-maximum of 14.9 cm−1) 5

Figure 6. (a) Excitation and (b) emission spectra of compound 2b recorded at 77 (LT) and 298 (RT) K, respectively.

At room temperature, the excitation spectrum of 1, when monitoring the emission of the 5D0 → 7F2 transition at λem = 612 nm, shows a broad band between 250 and 350 nm and some narrow lines (see Figure 5). The broad band (due to the saccharinate ligand) proves that the saccharinate ligands harvest photons and transfer them efficiently to the Eu(III) ions. The narrow and weak lines in the spectra correspond to discrete f−f transitions that are located at 361, 365 (7F0 → F

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Inorganic Chemistry for the nondegenerate 5D0 → 7F0 transition.42 Upon variation of the excitation wavelength from 321 to 393 nm and the temperature, the components of the respective transitions remain the same but their relative intensities differ slightly. The lifetimes of the excited states could be derived from luminescence intensity decay curves. For compound 1, the decay times of the 5D0 states were monitored within the most intense 5D0 → 7F2 line upon excitation at 321 (or 393) nm. The decay curves all reveal typical single-exponential behavior. This yielded lifetimes of 0.62 (0.66) ms at 77 K, 0.53 (0.54) ms at 298 K, and 0.48 (0.52) ms at 353 K. Thus, the emission lifetimes show the expected trend of a decrease with an increase in temperature but on the whole are not greatly dependent on temperature. In the emission spectra of 2b, the hypersensitive 5D0 → 7F2 transitions also dominate the overall emission spectra (see Figure 6), but some differences are notable. Upon closer examination of the wavelength region near 579 nm, the single weak peak in compound 1 that belongs to the 5D0 → 7F0 transition here is split into two peaks of similar intensity at 578.6 and 578.8 nm (see Figure 7). The presence of two 5D0 → 7F0 lines indicates the existence of at least two Eu3+ local environments, which is consistent with the crystallographic results. We also found that the 5D0 → 7F1 transition splits into two components, with maxima at 588.25 and 593 nm, whereas in the 5D0 → 7F2 transition region, four peaks are clearly located (611.9, 613.9, 618.5, and 620.2 nm). The decay curves (λem = 612 nm) under 393 nm excitation can be fitted with a double-exponential function, revealing fast and slow components around 0.43 and 0.03 ms at 298 K and 0.40 and 0.05 ms at 77 K, respectively. This is not the case for the decay curves obtained by exciting into the saccharinate ligand (λex = 321 nm), where the decay curves are monoexponential, yielding decay times of 0.45 ms at 298 K and 0.50 ms at 77 K. This difference could be due to the fact that upon 321 nm excitation, the 612 nm emission originates from energy transfer of the saccharinate ligands. A similar phenomenon was also reported for Eu(III)-doped aluminum oxide films.43 The excitation spectra of compound 3 are also dominated by the ligand band (see Figure 8), but the relative intensity of the f−f transitions is even larger than in 1 and 2b, indicating better energy transfer between the saccharin ligand and the europium(III) ion. It is noteworthy that the emission spectra of 3 are dominated by the 5D0 → 7F1 magnetic dipole transition (MD) at 593 nm, which is different from that observed in compounds 1 and 2b. As a result, the value of asymmetry ratio I(5D0 → 7F2)/I(5D0 → 7F1) is very close to 1 (1.04 at 298 K), indicating a higher coordination symmetry around the europium center that is normal for an octahedrally coordinated europium(III) compound. When the temperature is decreased to 77 K, the ratio is 0.58, indicating an increasingly symmetrical octahedral coordination sphere. As expected, the 5 D0 decay times (5.31 ms at 77 K and 3.85 ms at 298 K) of compound 3 are far longer than those of compounds 1 and 2b as no aqua ligands are in the first coordination sphere of the emitting lanthanide center. Water molecules are known to be efficient quenchers of the excited state in trivalent lanthanide ions, owing to the O−H vibrations, which couple to the lanthanide’s energy levels (energy gap law).44,45 By assuming that only nonradiative (knr) and radiative (kr) processes lead to the depopulation of the 5D0 state, we can estimate the 5D0 quantum efficiency (q) from the emission

spectrum and lifetime of the efficiency q can be defined as

5

D0 state.46−49 Quantum

q = k r /(k r + k nr)

(1)

Because of the negligible contribution to the depopulation of the 5D0 state, the influence of the 5D0 → 7F5 and 5D0 → 7F6 transitions is ignored. The radiative contribution thus can be calculated from the relative intensities of the 5D0 → 7F0, 5D0 → 7 F1, 5D0 → 7F2, 5D0 → 7F3, and 5D0 → 7F4 transitions as kr = A 0→1

S0 − J ℏω0 → 1 4 ∑J=0 S0 → 1 ℏω0 − J

(2)

where A0−1 is the Einstein coefficient of spontaneous emission between the 5D0 and 7F1 Stark levels, usually considered to be equal to 50 s−1.46−49 The ℏω0−J and S0−J are the energy and the integrated intensity of the 5D0 → 7FJ transitions, respectively. The calculated values are listed in Table 4. Table 4. Experimental Lifetimes, Calculated Radiative (kr) and Nonradiative (knr) Decay Rates, and Quantum Efficiencies (q) of the 5D0 State for Europium(III)Containing Compounds 1, 2b, and 3 at Room Temperature compound

τ (ms)

kr

knr

q (%)

1 2b 3

0.53 0.45 3.85

0.30 0.45 0.15

1.59 1.77 0.11

15.90 20.39 57.75

Upon closer examination of the radiative and nonradiative parameters of these compounds, we found that anhydrous compound 3 has the smallest nonradiative rate, from which we can understand that this compound also has the longest decay time. Appropriate analyses of the emission spectra of compounds 1, 2b, and 3 yield quantum efficiencies of 15.9%, 22.95%, and 57.75%, respectively. Unsurprisingly, the quantum efficiency of compound 3, where no water is coordinating to Eu3+, is the largest. The room-temperature (RT) and liquid nitrogen (LT)temperature excitation and emission spectra of samarium compound 2a are shown in Figure 9. The excitation spectrum was obtained by monitoring the 646 nm line of the 4G5/2 → 6 H9/2 emission. It contains a very intense broadband that corresponds to the excitation of the organic chromophore (S0 → S1) and a series of weak lines corresponding to intraconfigurational f−f transitions from the ground state of Sm3+ (see Figure 9a).50 The emission spectra of compound 2a were measured with the excitation wavelength set at 321 nm (ligand-localized transition) and 401 nm (6H5/2 → 6P3/2). The respective spectra obtained at room temperature and liquid nitrogen temperature with different excitation wavelengths all show four characteristic Sm3+ emission peaks that are characteristic of the Sm(III) compound: 4G5/2 → 6H5/2 (560.75 nm), 4G5/2 → 6 H7/2 (595.5 nm), 4G5/2 → 6H9/2 (641.75 nm), and 4G5/2 → 6 H11/2 (701.25 nm). The bright luminescence observed for the Sm(III) complex is reflective of a good match between the ligand-centered triplet state and the Sm3+ emissive states. Similar to the 5D0 → 7F2 electric dipole transition in the europium compound, the 4G5/2 → 6H9/2 transition in the samarium-containing compound is also hypersensitive. Intensity ratio I(4G5/2 → 6H9/2)/I(4G5/2 → 6H5/2) can be calculated G

DOI: 10.1021/acs.inorgchem.9b01411 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

into saccharin ligand (321 nm) or directly into Eu(III) (393 nm) levels. Compounds 1 and 2b have comparatively short 5 D0 lifetimes due to aqua ligands in the first coordination sphere of Eu3+ that can be vibrationally excited. This problem is overcome in compound 3, which exhibits a decay time that is higher by an order of magnitude (3.85 ms). This is also reflected in the quantum yield. Compound 3 has a quantum yield of 58% that is roughly 3.5 times higher than that of the water-containing compounds. The optical properties of both the europium and the samarium ionic liquids benefit from the efficient energy transfer from the saccharinate ligand to the emitting lanthanide centers, rendering the ionic liquids efficient emitters. As these novel saccharinate-based ionic liquids have been synthesized, we hope to pave the way for the development of potentially useful optical soft materials.

(here it is 7.68) and used as a measure of the polarizability of the chemical environment of the samarium(III) ion.40,51 Further investigation was carried out by decreasing the temperature to liquid nitrogen temperature (77 K) and varying the excitation wavelength from 321 to 401 nm. It was found that changing the excitation wavelength had a very small effect on the energy. When spectra were recorded at 77 K, better resolution of the crystal-field fine structure can be obtained (see Figure 9) and I(4G5/2 → 6H9/2)/I(4G5/2 → 6H5/2) increased to 9.84 (under 401 nm excitation, the values are 4.95 at room temperature and 6.19 at low temperature). The luminescence lifetimes of the 4G5/2 state can be derived from the luminescence decay curves. For both measurements (λex = 401 nm, and λem = 596 nm), the decay curves were found to be monoexponential with values of 16.4 μs at 298 K and 18.5 μs at 77 K. The CIE chromaticity coordinates for all compounds are calculated on the basis of their emission spectra (see Table 5

■

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b01411. Hydrogen bonds, IR and Raman spectra, XRD patterns, DSC traces, TGA curves, 3D packing diagrams of compounds 2a and 3, and a CIE chromaticity diagram (PDF)

Table 5. CIE Chromaticity Coordinatesa of Compounds 1− 3 compound 1 2a 2b 3

LT 0.654, 0.522, 0.658, 0.633,

0.345 0.322 0.324 0.366

RT 0.656, 0.569, 0.662, 0.638,

0.342 0.371 0.333 0.362

ASSOCIATED CONTENT

S Supporting Information *

HT 0.666, 0.331 not measured not measured not measured

Accession Codes

a

CCDC 938855−938857 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

The coordinates are calculated from the emission spectra under 321 nm excitation.

and Figure S16). For compound 1, x = 0.654 and y = 0.345 at 77 K, x = 0.656 and y = 0.342 at 298 K, and x = 0.666 and y = 0.331 at 353 K. For compound 2b, x = 0.658 and y = 0.324 at 77 K, x = 0.662 and y = 0.333 at 298 K. Obviously, with an increase in temperature, the coordinates are coming closer to the NTSC standard CIE values for red (x = 0.67, and y = 0.33).52 For compound 3, x = 0.638 and y = 0.362 at room temperature and x = 0.633 and y = 0.366 at low temperature (77 K). As expected, the coordinates of samarium compound 2a fall in the orange-red region (x = 0.522 and y = 0.322 at 77 K, and x = 0.569 and y = 0.371 at 298 K).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Si-Fu Tang: 0000-0002-7151-9876 Anja-Verena Mudring: 0000-0002-2800-1684

■

Notes

The authors declare no competing financial interest.

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CONCLUSIONS Four luminescent lanthanide-containing soft materials were synthesized from the respective lanthanide saccharinates and saccharinate-based ionic liquids. Structural analysis reveals that in all cases the saccharinate ligand does not bridge lanthanide centers but serves as a terminal ligand yielding isolated complex anions of various compositions. When hydrous lanthanide saccharinates were used as the starting material, the formed complex anions contained water that coordinates to the lanthanide ion, which occurs because of the strong oxophilic character of trivalent lanthanides and strong donor properties of water. However, loss of water was observed upon heating. When using anhydrous lanthanide saccharinate as the starting material, it is possible to obtain the homoleptic complex anion [Eu(Sac)6]3−. Thermal analysis indicates that compound 1 is an ionic liquid with a melting point of ∼85 °C. When the cation is changed from imidazolium to pyridinium, compounds that have higher melting points are obtained and it might be more suitable to call compounds 2a, 2b, and 3 lowmelting salts. The three europium(III)-containing compounds (1, 2b, and 3) all show intense red emissions upon excitation

ACKNOWLEDGMENTS A.-V.M. thanks the Kungl. Vetenskapsakademien for support through the Göran Gustafsson prize in Chemistry. Energymyndigheten is acknowledged for support through Project 46676-1. S.-F.T. acknowledges support from the National Natural Science Foundation of China (21171173), Qingdao Science and Technology Program (No. 19-6-1-43-nsh) and the Advanced Talents Foundation of Qingdao Agricultural University.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.9b01411 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.9b01411 Inorg. Chem. XXXX, XXX, XXX−XXX