Fluorescigenic Magnetofluids Based on Gadolinium, Terbium, and

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Fluorescigenic Magnetofluids Based on Gadolinium, Terbium, and Dysprosium-Containing Imidazolium Salts Kun-Lun Cheng, Wen-Li Yuan, Ling He,* Ning Tang, Hong-Mei Jian, Ying Zhao, Song Qin, and Guo-Hong Tao* College of Chemistry, Sichuan University, Chengdu 610064, China S Supporting Information *

ABSTRACT: Multistimuli responsive soft materials are urgently needed in many different fields, such as anticounterfeiting technology and microdroplet manipulation. Herein, the straightforward preparation of fluorescigenic magnetofluids by the introduction of the paramagnetic metal ions Gd3+, Tb3+, and Dy3+ into alkylimidazolium-based ionic liquids (ILs) is reported. Bright visible fluorescence was observed under UV irradiation for Tb- and Dy-containing ILs. Either pure samples or papers coated with these ILs exhibited pronounced magnetic responses. Consistent and stable structures of these salts were confirmed by systematical characterizations. Because of the competition of nitrate ligands, structural water in the precursors was eliminated easily under a vacuum. For Tb- and Dy-containing ILs, featured electronic transitions were observed and were assigned in the fluorescence spectra. The long lifetimes of these transitions were also confirmed. The field-cooling experiments showed that all of these ILs display paramagnetism at room temperature. At low temperature, small deviations from the Curie Law indicate the occurrence of antiferromagnetic coupling and spin canting in these ILs. Temperature-induced differences in magnetic properties were further verified by field-dependent magnetic susceptibility measurements carried out at 5 and 300 K.

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considerably higher magnetic moments.25 Furthermore, it is important to note that the apparent effective magnetic moment of a single anion can reach 7.86 μB ([PR4]3[GdCl6])11 and 10.60 μB ([C6mim]4[Dy(SCN)7(H2O)]),13 which are very close to the theoretical values of free ions and are much higher than those of the widely studied MILs based on [FeX4]− (X = Cl, Br) anions. Additionally, the 4f−4f transition of Ln3+ ions leads to efficient fluorescence. The shielding effects of the external electronic shell of the 4f electrons, together with the lifting of parity forbidden by the effect of odd terms in the crystal field effect in condensed matters, contribute to high color purity, high strength, and a long lifetime. Therefore, the introduction of rare earth ions not only provides a new approach for solving the problem of low saturated magnetization of MILs but also provides a good modification scheme for the design of fluorescigenic magnetofluids. The presence of water, particularly in coordinating spheres, results in the fluorescence quenching of luminescence centers. Thus, water molecules acting as coligands are necessary to maintain integral and stable structures in some lanthanide ILs, including [Ln(TiW11O39)2]13−·xH2O (x = 27−44),26 [C4mim]x−3[Dy(NCS) x (H 2 O) y ] (x = 0, 1, 2, x + y < 10), 27 and [C6mim]4[Dy(SCN)7(H2O)].13 Therefore, irreversible hydrolyzation of luminescent ILs should also be avoided. More

INTRODUCTION Since 1956,1 the field of ferrofluids has seen a large amount of activity in a wide variety of multidisciplinary studies involving physics, chemistry, and material science research. Wide applications of magnetic fluids in sensors,2 medicine,3 spacecraft propulsion,4 and mechanical engineering5 have inspired scientific interest in the research on related smart liquid materials. Generally, to be suitable for use in applications, these materials should be nonvolatile, chemically and thermally stable, and capable of maintaining a liquid state over a wide temperature range. These properties are the exact basic characteristics of ionic liquids (ILs),6 which are multifunctional soft materials composed entirely of anions and cations.7 Magnetic ionic liquids (MILs), based on paramagnetic anions, such as [Bmim][FeCl4],8 [C4mim2][AOT][FeCl3Br],9 [R(C8H17)3N][FeCl3Br],10 Dimim[FeBr4], [Cation][MX4]11 (M = Fe, Co, Mn, Gd; X = Cl, Br), [P6,6,6,14]3[GdCl6],12 [PR4]3[GdCl6],11 [C6mim]5−x[Dy(SCN)8−x(H2O)x]13 (x = 0−2), [C 1 2 mim] 3 [DyBr 6 ], 1 4 and [P 6 6 6 1 4 + ][Dy(III)(hfacac)4−],15 have been reported. Numerous application areas, such as microextraction,16 desulfurization,17 PAH removal,18 nanosynthesis,19 density measurements,20 catalysis,21 gas absorption,22 and electrochemical and sensing applications,23 as well as biomedical applications,24 have been explored. All of these MILs exhibit a certain response toward external magnetic fields. Lanthanide ions have been incorporated into MILs, resulting in fabulous luminescence as well as © XXXX American Chemical Society

Received: February 21, 2018

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DOI: 10.1021/acs.inorgchem.8b00435 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 1. Synthesis route of complexes 1a−3e.

broadly, comprehensive research on magnetic fluids with fluorescence is still lacking. Herein, we present novel anhydrous fluorescigenic magnetofluids based on the lanthanide ILs [RCnmim]2[Ln(NO3)5] (R = H or methyl; Cnmim = 1-alkyl-3-methylimidazolium; Ln = Gd (1), Tb (2), Dy (3); n = 1 (a), 2 (b), 4 (c), 6 (d), 8 (e)). The synthesis, material applications, precise structures, stabilities, and luminescent and magnetic properties of these complexes were studied. The highly coordinated environments in which the rare earth ions are located in these ionic liquids are shown to be beneficial for their fluorescence properties.

signs were observed on the paper after drying for a few minutes in a vacuum oven. As shown in Figure 2, the magnetic response

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RESULTS AND DISCUSSION It is well-known that O ligands are more conducive than N ligands for the construction of Ln complexes.28 Nitrate ligands are one of the favorable O ligands for the construction of highly coordinated lanthanide ILs with good liquid and luminescence properties29 because luminescence quenchers, such as O−H, N−H, and C−H, are absent from the coordinating spheres of Ln3+. Furthermore, smaller bidentate nitrate ligands save space for coordinating spheres and concurrently avoid the formation of anions with too high of a valence. Thus, based on these considerations, pentanitratogadolinate, pentanitratoterbinate, and pentanitratodysprosinate ILs were prepared by the addition of corresponding lanthanide nitrate pentahydrates to 1-alkyl-3methyl-imidazolium nitrates [RCnmim][NO3] (1:2 molar ratio) in CH3CN (Figure 1). The subsequent drying process is performed to remove byproduct water and organic solvents. Crystals of the solid compounds 1a, 2a, and 3a were cultivated and characterized by single-crystal diffraction. Meanwhile, the structures of all the compounds were fully characterized by NMR, FT-IR, and elemental analysis. Due to the introduction of versatile lanthanide ions, the magnetic and optical responses of these ILs are expected. These fluorescigenic magnetofluids with dual responses can be made into magnetic fluorescent inks which have good prospects for application in special anticounterfeiting technology (banknote, check, and credentials). Compounds 1a, 2a, and 3a were obtained as white powders (dispersed polycrystalline). The other obtained compounds are transparent and colorless or are primrose yellow liquids. To verify their usability, we chose compound [C4mim]2[Tb(NO3)5] (2c) for the coating imaging experiments. The fabrication processes were conducted carefully as follows: the proper amount of ethanol solution of 2c was sprayed on a 5 × 5 cm rectangular commercially available paper through a hollow mold with the label “Tb SCU.” No obvious

Figure 2. Paper coated with magnetic ink magnetized by a quadrate NdFeB magnet and displayed visible green writings under 365 nm UV radiation.

derived from the ferrofluid was so pronounced that the coated paper could be gripped with two NdFeB magnets without falling. Meanwhile, the uncoated papers did not respond to any magnets. Bright fluorescence on the paper was clearly observed under the irradiation of a 365 nm lamp. To observe the magnetic response of these magnetic fluids directly, we conducted U-tube experiments. As seen in Figure 3, the pure samples of 2c and 3c filled in the U-tubes were attracted by a NdFeB magnet, resulting in significant height differences between the sides of the U-tubes. Bright fluorescence (green and light yellow) was observed under the excitation of 365 nm UV light. The distinct dual responses of these transparent fluorescigenic magnetofluids revealed their superior magnetic and luminescent properties for anticounterfeiting technology, in situ imaging, motion sensing, and energy harvesting. 1 H NMR and 13C NMR spectra of all the compounds were clearly consistent with the structures of imidazolium cations (see Supporting Information Figures S1−S30). 1H NMR spectra of 1a, 2a, and 3a showed three singlet resonances at B


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Inorganic Chemistry

Figure 3. Images of U-tube filled with MILs 2c (upper) and 3c (lower) with and without a NdFeB magnet on the side under the radiation of bright light and 365 nm UV light.

7.60, 3.75, and 2.55 ppm, attributed to hydrogens in imidazolium rings, N-methyl groups, and 2-position methyl substitutes. The signals of the C atoms of the imidazolium cations were located at 144.80, 122.00, 34.70, and 9.12 ppm, according to the 13C NMR spectra of 1a, 2a, and 3a. Analogous 1 H NMR signals were also observed for the rest of the compounds. In the aromatic region, there were three singlets resonances at 9.05−9.19 and 7.69−7.79 ppm, corresponding to the imidazolium ring fragment. The singlet at 3.85−4.20 ppm was assigned to the 1,3-position methyl substitutes. The spectral peaks were shifted to lower fields with the extension of the 3-position C-chain. In the 13C NMR spectra of the rest of the compounds, three chemical shifts assigned to the imidazolium ring appeared at approximately 136.22, 123.49, and 121.90 ppm. The infrared absorption of imidazolium cations and nitrate anions of all the materials were distinguished and confirmed (Figure 4, Figures S31 and S32). For the cations, the peaks located at 2980−2800 cm−1 are originated from the methyl and methylene attached to the imidazolium rings. Because of the nitrate anions coordinating with lanthanide ions in these waterfree ILs, the decrease in the symmetry of nitrate anions led to the splitting of the dissociative nitrate ions peak at approximately 1380 cm−1. Thus, the asymmetry stretching modes of nitrates were located at 1700−1250 cm −1 .

Figure 4. Infrared spectra of 1a−1e.

Furthermore, the dissociative nitrate ions were nonexistent in all the complexes, according to their crystal structure and spectral data. Since it is impossible to remove all the moisture in the structures, the coordinating structures of the nitrates were stable, with the presence of a trace amount of moisture. C


Article

Inorganic Chemistry

Figure 5. Crystal structures and coordination polyhedrons of 1a, 2a, and 3a.

16.6228(10) Å, b = 9.7264(7) Å, c = 15.4994(9) Å) for 1a, 2a, and 3a, respectively. Similarly, the asymmetric unit consisted of half of the Ln3+ ion, two and a half NO3− ions, and one imidazolium cation. No disordered solvent or water was observed in the crystal lattices. The crystal structures revealed that [MC1mim]2[Ln(NO3)5] has similar 10 coordinating structures, and each Ln3+ ion was coordinated with five bidentate nitrate ligands in the hexadecahedron geometry. Due to the respective values for the ionic radii of Ln and O, the average Ln−O distance was 2.473(14) Å for 1a, 2.460(8) Å for 2a, and 2.436(8) Å for 3a. The differences between the maximum value and the minimum value were 0.0379, 0.049, and 0.042 Å. This result can be attributed to the self-adjusting of the Ln−O bond to fit in with the ion stacking and the different environments caused by the ionic radius difference of the central metals. The average N−O bond length of the nitrate ligands was 1.249(8) Å for 1a, 1.246(6) Å for 2a, and 1.240(8) Å for 3a. Consistently, the coordinating N−O distances were 0.052−0.064 Å longer than the distance of the uncoordinated N−O. It is anticipated that the differences increased from 1a to 3a due to the general enhancement of the interaction between Ln3+ and the nitrates, which can be attributed to the gradual increase in the ionic potential from Gd3+ to Dy3+. While it is actually true in this case, the exceptional behavior that the N−O distance of the coordinating O atoms was smaller than that of the uncoordinated one is quite unusual. However, this observation is reasonable when we consider the effects of the imidazolium

Moreover, the wavenumber difference of the asymmetric stretching mode of the nitrate was approximately 120 cm−1 for 1a−1e, indicating that all the nitrate anions were bidentate, and [Ln(NO3)5]2− was the predominant metal-containing species in these MILs. As shown in Figure S33, recoverability of 3c was confirmed by a reversible process between the noncoordinated nitrate anion in the aqueous phase and the coordinated nitrate anion in the dried complex. Therefore, water-free 10-coordinated pentanitratogadolinate, pentanitratoterbinate, and pentanitratodysprosinate ILs were obtained and handled. The crystal structures of 1a, 2a, and 3a were determined by single-crystal X-ray diffraction. The colorless plate crystals were obtained by slow recrystallization from acetonitrile/ethyl acetate at room temperature. The molecular structures of 1a, 2a, and 3a are shown in Figure 5, and the crystallographic data are summarized in Table 1. The selected bond lengths and angles are given in Table S1. All non-hydrogen atoms were refined anisotropically. Compounds 1a, 2a, and 3a crystallized in the monoclinic space groups C2/c and I2/a, with four molecular moieties in each unit cell. Every unit cell was either straight four-prism or straight parallelepiped, in which the ac plane is viewed as the base plane of the prism and any of the other two pairs of parallel faces are rectangles. The calculated densities were 1.813, 1.827, and 1.842 g cm−3, with unit cell volumes of 2446.5(4) Å3 (a = 22.1000(13) Å, b = 9.6630(11) Å, c = 15.3316(12) Å), 2513.6(2) Å3 (a = 22.4301(10) Å, b = 9.7468(4) Å, c = 15.5097(6) Å), and 2505.0(3) Å3 (a = D


Article

Inorganic Chemistry Table 1. Crystal Data and Structures Refinement for 1a, 2a, and 3a formula Mw size [mm3] cryst syst space group a [Å] b [Å] c [Å] α [deg] β [deg] γ [deg] V [Å3] Z ρ [g cm−3] T [K] μ [mm−1] F(000) ΛMo/Cu−Kα [Å] reflns Rint params S on F2 R1 [I ≥ 2σ (I)]a wR2 [I ≥ 2σ (I)]b R1 [all data]a wR2 [all data]b Δρmin/max [e Å−3] a

1a

2a

3a

C12H22N9O15Gd 689.64 0.3 × 0.3 × 0.2 monoclinic C2/c 22.1000(13) 9.6630(11) 15.3316(12) 90.00 131.650(7) 90.00 2446.5(4) 4 1.813 293(2) 2.796 1277.0 0.71073

C12H22N9O15Tb 691.30 0.3 × 0.2 × 0.2 monoclinic C2/c 22.4301(10) 9.7468(4) 15.5097(6) 90.00 132.157(3) 90.00 2513.6(2) 4 1.827 293(2) 2.899 1368 0.71073

C12H22N9O15Dy 694.86 0.6 × 0.3 × 0.1 monoclinic I2/a 16.6228(10) 9.7264(7) 15.4994(9) 90.00 91.581(6) 90.00 2505.0(3) 4 1.8423 293(2) 16.753 1338.2 1.54184

4513 0.0471 122 1.042 0.0681

5079 0.0349 172 1.12 0.0415

8196 0.1316 171 1.000 0.0883

0.1584

0.0986

0.2064

0.0729 0.1629 2.25/-2.97

0.0481 0.1032 0.67/-1.23

0.0904 0.2123 1.69/-4.01

R1 = Σ∥Fo| − |Fc∥/|Fo|. bwR2 = [Σw(Fo2 − Fc2)2/Σw(Fo2)2]1/2.

cation and the complex environments around the nitrate ligands. The distribution of Ln3+ in the 2 × 2 × 2 cells is illustrated in Figure 6. Every unit cell was a straight parallelepiped and contained 4 equiv of magnetic Ln3+ with spin quantum numbers of J = 7/2, 3, and 5/2. All the Ln3+ ions were described as points of integrated or half parallelepipeds, and the shortest path connecting the two equivalent Ln3+ ions is represented by red dotted lines. The numbers of the adjacent ion pairs connected by the red dotted lines in each unit cell are 1.25 for 1a and 2a and 2 for 3a. These nearest-neighbor magnetic centers were considered first when considering the Ln−Ln magnetic interaction. The Ln−Ln separations were in the range of 7.8 Å to 12.2 Å, and the minimum values were 7.7904, 7.8685, and 7.8661 Å for Gd3+, Tb3+, and Dy3+, respectively. These values were slightly larger than the corresponding values of the inorganic crystals, such as nitrates and oxyhydrates,30 but were smaller than the corresponding values of the crystals of the organic lanthanide complexes. The relatively small distances between Ln3+ resulted in efficient energy transfer (ET), which is quite beneficial in the systems (lanthanide-based single molecule magnets and upconversion materials) for which Ln−Ln interactions are necessary. The average value of the O−Ln−O bond angle was 51.79° for 1a, 51.50° for 2a, and 51.88° for 3a, and the max−min difference values were 0.59°, 0.69°, and 0.30°, respectively. The

Figure 6. Distributions of Ln3+ ions in 2 × 2 × 2 cells. Nearest ions are linked through red dotted lines. The lengths are 7.7904 Å for Gd3+ unit lattices (8.9520 × 9.6630 × 12.0601 Å3), 7.8685 Å for Tb3+ unit lattices (9.0392 × 9.7468 × 12.2281 Å3), and 7.8661 Å for Dy3+ unit lattices (7.8661 × 9.7264 × 12.2161 Å3). All triangular prisms are equivalent and exactly half of each four prism.

coordinating O−N−O bond angle values were 117.39° (1a), 115.59° (2a), 115.01° (3a), and the max−min difference values were 3.23° (1a), 0.62° (2a), and 0.59° (3a). The results indicate the symmetry of the coordination polyhedrons of Ln3+ (Figure 5), which can be attributed to the ionic potential and ionic radius change in the central Ln3+ ions. However, providing an explanation of why such changes might occur within physical and mathematical precision is quite a complex problem. The distorted geometry of the [Ln(NO3)5]2− anion was further confirmed by the torsion angles. Evidence for multiple bond characters is provided. The [MC1mim]+ cations were delocalized, with the positive charge dispersed over the aromatic system. These ion pairs constructed the ordered infinite three-dimensional system of 1a, 2a, and 3a (Figure 7). E


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Inorganic Chemistry

Table 2. Thermal Properties of All Products (Tg, Glass Transition Temperature; Tm, Melting Point; Td, Decomposition Temperature) 1a Tg [°C] Tm [°C] Td [°C] Tg [°C] Tm [°C] Td [°C] Tg [°C] Tm [°C] Td [°C]

103 272 2a

1b

1c

1d

1e

−44

−46

−47

−47

258 2b

247 2c

250 2d

287 2e

−47

−52

−52

−50

101 276 3a

249 3b

260 3c

267 3d

283 3e

−53 20 260

−55 20 259

−55

−55

109 281

272

284

3b and 3c, similarly, significant crystallization signals were observed at approximately 0 °C; then, sharp signals of a melting point were distinguished at approximately 20 °C. The lowtemperature phase transition, as well as the ease of forming the undercooling state, make these ionic liquids potentially useful as heat storage materials. The thermal stabilities of the salts were determined by thermogravimetric analysis (TGA) experiments in a N 2 atmosphere. Similar decomposition curves were obtained with a single step decomposition at 250−287 °C. Similar to the reported ionic liquids, based on imidazolium cations, the decomposition temperatures decreased slightly with the extension of the substitute alkyl group. Therefore, these lanthanide ionic liquids are sufficiently thermally stable materials, with a large liquidus range of over 300 °C illustrated. Due to their appropriate transparency, conductivity, and fluidity, MILs are promising tools in the field of droplet manipulation. Contact angles of 5 μL of 3c on polytetrafluoroethylene (PTFE) films were studied (Figure 8). A contact angle of 128° of 3c on PTFE film was obtained under zero external magnetic field. Lyophobic behavior and poor wettability indicated that there is a large difference between the surface energies of 3c and the PTFE film. The contact angle showed a slight decrease from 128° to 121° under the influence of the magnet. The influences of external magnetic fields on the shapes of these droplets were observed by moving a magnet from the top to the right of the droplets. The same droplets of 3c were observed rolling to the right under the influence of a magnet at the top right and the right. The advancing contact angle can be as much as 61° larger than the receding contact angle before the further motion of the droplet. Magnetic forces between the magnet and the droplets can be divided into two components. The vertical components act against gravity together with the supporting forces, while the horizontal components are responsible for the twist and distortion of the droplet surfaces. The excitation and emission spectra of compounds 2a−2e and 3a−3e were recorded at room temperature (Figure 9). All the excitation and emission spectra of the spectral peak correspond to the electron transitions in Figure 10. Unlike for the salts with antenna ligands, no excitation bands caused by nitrate ligands or imidazolium cations were observed. As expected, both terbium- and dysprosium-containing salts were highly luminescent and displayed their respective characteristic transitions in the corresponding regions, despite the absence of fluorescent antennas.

Figure 7. Unit cells viewed along the (113) direction. H atoms are omitted for clarity.

No π-stacking interaction among the imidazolium cations was found. Neither C−H···O intermolecular H-bond interactions nor C−H···N intermolecular H-bond interactions existed between the imidazolium cation and the nitrate anions. Compared with the lanthanide complexes with hydrogen bonding, the packing structure of the [Ln(NO3)5]2− complex without hydrogen bonding produced major changes in the values of the melting point as well as the viscosity, surface tension, conductivity, and other liquid properties. The melting points of all the materials were determined by differential scanning calorimetry (DSC). As illustrated in Table 2, 1a, 2a, and 3a showed distinct melting points at 103, 101, and 109 °C, respectively. The glass transition points were observed in the rest of the complexes in the range of −44 °C to −55 °C. A decreasing trend from Gd to Dy indicates that the change in the central Ln3+ ions affects the motions and structures of the cations and anions. With the exception of 3b and 3c, no clear melting points were observed for the rest of the ionic liquids, which remained liquids at room temperature. For F


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Inorganic Chemistry

Figure 8. (a) A droplet of 5 μL of 3c on polytetrafluoroethylene films. (b) 3c with a magnet at the top. (c) 3c with a magnet at the top right. (d) 3c with a magnet at the right.

Figure 9. Excitation and emission spectra of Tb- and Dy-based ionic liquids.

Figure 9a illustrates the photoluminescence (PL) excitation spectra of Tb salts obtained by monitoring at the emission wavelength of 544 nm. No distinct differences were observed, with the exception of complex 2a, showing a slight splitting that was so inconspicuous that we cannot attribute it entirely to the influence of a phase state or a change in the chemical environment around the coordinating anions due to an alteration of imidazolium cation, even if the environmental factors and measurement error are eliminated. Weak bands

observed at 381, 370, 360, 354, 352, 341, 326, and 319 nm are assigned to the 5D4 → 7F6 electronic transitions (Figure 10a), in which the electrons are first excited to energy states approximately 5D3, 5D2, or higher energy states and then jump to 5D4 by nonradiative transitions. The isolated band observed at 490 nm was assigned to the 5D4 → 7F6 electronic transition. The flat region of 90 nm between 480 and 390 nm was attributed to the energy gap of 5700 cm−1 between the energies of the 5D4 and 5D3 states. No obvious bands were observed at G


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Inorganic Chemistry

Figure 10. Energy-level diagram in PL excitation (a) and PL emission (b) of Tb3+and Dy3+ in the Tb- and Dy-based ionic liquids (polyline down arrows represent nonradiative transitions).

the shorter wavelength region, which can be attributed to the energy transition between the Tb(III) ions; when the wavelength of exciting light reaches approximately 200 nm, the absorption due to the π−π transition of imidazolium cations becomes dominant. The PL emission spectra of 2a−2e (Figure 9b) were observed under UV excitation at 382 nm. The emission bands at 490 and 496 nm were assigned to the 5D4 → 7F6 electronic transitions. The strongest peaks at 544 and 548 nm were assigned to the 5D4 → 7F5 electronic transitions. Weak bands at 587, 596, 624, and 652 nm were unambiguously attributed to the transitions from the higher energy 5D4 state to the ground state 7Fj (j = 4, 3, 2, and 1) levels (Figure 10b). The PL excitation spectra of 3a−3e, as shown in Figure 9c, were obtained by monitoring the emission at 579 nm. For 3a− 3d, the excitation at 365 nm yielded the strongest emission at 579 nm. As illustrated in Figure 9, the excitation from 325 to 477 nm can be attributed to 6H15/2 → 4F9/2, 4I15/2, 4G11/2, 4F7/2, 4 I13/2, 6P5/2, 6P7/2, 4I9/2, 4G9/2, and 6P3/2 (Figure 10a). The PL emission spectra of 3a−3e (Figure 9d) were observed under UV excitation at 365 nm. Because no signal was observed at a wavelength smaller than 450 nm, we can exclude the possibility that emissions from [RCnmim]+ cations excited by light sources are absorbed by the Dy3+ ions. As illustrated in the energy-level diagram (Figure 10b), the 4F9/2 → 6H15/2, 4F9/2 → 6H13/2, and 4 F9/2 → 6H11/2 electronic transitions were observed at 478/484, 579, and 666 nm, respectively. All the f−f transitions of Tb3+ and Dy3+ observed in 2 and 3 are illustrated in Figure 9. The coordinating environment constructed by nitrate ligands weakened the Ln3+−Ln3+ energy transition, avoiding concentration quenching effects. The absence of a significant energy exchange between imidazolium cations and Ln3+ indicates that, in all the cases, the luminescence of these ILs is derived from the lanthanide ions without the assistance of any antennae. Furthermore, the long luminescence lifetimes of 2a−2e and 3a−3e again confirm that the ILs are a suitable and favorable environment for the construction of crystal fields that can protect luminous Ln3+ from any interference, as long as the structures are integrated. As shown in Figure 11, the decay curves of 2a−2e and 3a− 3e all show monotonically decreasing trends, proving that all luminescent centers in each compound are equivalent. The identical coordination structures guarantee similar luminescent

Figure 11. Logarithmic plots of the emission decay profile of 2a−2e (a, excitation wavelength, 390 nm; monitoring wavelength, 541 nm) and 3a−3e (b, excitation wavelength, 355 nm; monitoring wavelength, 575 nm).

lifetimes. The lifetime value of complexes 2a−2e at 541 nm varied from 1.262 to 1.294 ms, and the lifetimes of complexes 3a−3e observed at 575 nm varied from 54.75 to 59.27 μs (Table S2). We attribute the fluctuation of the lifetime values of these salts with different imidazolium cations to the interplay of the influences of the different spacing of the anions and the numbers of C−H quenchers. Because the luminescent lifetime H


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Inorganic Chemistry

Figure 12. Effective magnetic moments at a field of 10 000 Oe for 1a, 2a, and 3a.

of Ln3+ is directly sensitive to the composition of the inner and a part of the outer coordination spheres,30 the long lifetimes of 2a−2e and 3a−3e can be attributed to the absence of luminescence quenchers in the coordinating spheres. The strong magnetic response to the NdFeB magnets indicates that these MILs contain high concentrations of magnetic centers. The magnetic property studies of 1a, 2a, and 3a showed that these lanthanide ILs display paramagnetism, with only small deviations from the Curie Law at low temperatures (Figure S36). A temperature dependence of the magnetic susceptibility was also observed. The configurations of 4f7, 4f8, and 4f9 of Gd3+, Tb3+, and Dy3+ ions contributed to the extraordinary electron-spin magnetic moments (Figure 12). Field-dependent magnetization curves at 5 and 300 K showed different trends, and no magnetic hysteresis was observed with magnetic field scanning. At room temperature, these paramagnetic salts showed a linearly increasing magnetization with increasing magnetic fields and reached 1.822 emu/g for 1a, 2.836 emu/g for 2a, and 3.272 emu/g for 3a at 300 K and 50 000 Oe (Figure 13a). At 5 K, the magnetizations increased logarithmically and reached 49.58 emu/g for 1a, 49.06 emu/g for 2a, and 40.64 emu/g for 3a at 50 000 Oe. The slopes of the magnetization curves decreased with the gradually increasing external magnetic field but showed no tendencies to be saturated at 50 kOe, and the magnetization intensities reached values higher than 500 G (623 G for 1a, 617 G for 2a, 511 G for 3a), indicating that at a low temperature, electronic spins of Ln3+ are gradually redirected with the enhanced magnetic field from chaotic and nondirectional states to directional states. However, the spin orientation driven by the magnetic field became increasingly difficult due to the existence of weak thermal motion and indirect interactions between Ln3+. At a high temperature (300 K), the magnetization of 3a was larger than those of 1a and 2a. However, at a low temperature (5 K), the opposite effect was observed. In similar lattices, at 5 K, the antiferromagnetic interactions of the Dy3+ ions had more obvious effects than those of the Gd3+ and Tb3+ ions, which can

Figure 13. Field dependence diagrams of the magnetization (M-H, a) and the static molar susceptibility (χ-H, b) at 300 and 5 K of 1a, 2a, and 3a. (The scanning field signals of M-H diagrams are monitored with the magnetic field rising from 0 Oe to 50 kOe, then decrease to 0 Oe, and after rising to −50 kOe, finally fall to 0 Oe. Data of χ-H diagrams are calculated from M-H values.)

I


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Inorganic Chemistry

Figure 14. Temperature dependence of the static molar susceptibility (χ-H) of 1a, 2a, and 3a from 2 to 300 K (data for this diagram are calculated from a temperature-dependent long moment of which signals are monitored from 2 to 300 K).

stronger, the spin increasingly tended to be parallel to the magnetic field direction, but the thermal disturbance still existed, leading to a slow linear increase in the ionic liquid magnetization with the magnetic field strength. At 5 K, heat disturbances decreased, and spins tended to be oriented in the antiparallel manner because of the antiferromagnetic interactions between the Ln3+ ions. Upon an increase in the vertical magnetic field strength, the spins increasingly tended to align parallel to the magnetic field direction until magnetization saturation. The temperature-dependent inverse susceptibilities are shown in Figure 16. The Curie−Weiss temperatures θ for 1a, 2a, and 3a (Table 3, Table S3), extrapolated from the temperature region of 2−300 K, were 0.32 K for 1a, −1.13 K for 2a, and −7.97 K for 3a. The results indicate that these lanthanide ions in the crystal lattice show weak dipole, ferromagnetic, or antiferromagnetic interaction at a low temperature. The room temperature χmT values were 7.54 emu K mol−1 for 1a, 11.76 emu K mol−1 for 2a, and 13.65 emu K mol−1 for 3a, which are close to that of 7.87 emu K mol−1 calculated for the free Gd3+ ion (J = 7/2, g = 2), 11.82 emu K mol−1 calculated for the free Tb3+ ion (J = 6, g = 3/2), and 14.17 emu K mol−1 calculated for the free Dy3+ ion (J = 15/2, g = 4/3; Table 4). In addition, it is worth noting that as the temperature decreases, χmT values remained essentially constant and then decreased exponentially. The interactions between Ln3+ ions at room temperature were negligible compared to thermal motion. The rapid decline at lower temperatures suggests the occurrence of antiferromagnetic coupling.32 Similar to the conventional ferrofluids, the overall magnetic properties can be described as follows: the electron spins in Ln3+ of these ionic ferrofluids are constantly attracted in the direction of the applied field gradient, and their tendency to drift in the gradient is counteracted by the diffusive motion due to thermal agitation. The changes in the spin alignments in the MILs in a constant magnetic field during the slow cooling processes are shown in Figure 17. At room temperature, the spins of unpaired electrons in the MILs were disorganized. After the magnetic field was applied, the spins fluctuated toward

be attributed to the comparatively larger free-oriented spin and ion potential of the Dy3+ ions. Susceptibility of 1a, 2a, and 3a at 5 K reached more than 1 emu/mol under a 10 kOe magnetic field (Figure 13b). The field-dependent susceptibility curves of 1a, 2a, and 3a were similar to the normal distribution curve (except that there are upturning or sagging trends, indicating a surge or a sharp drop in the susceptibility due to the flipping of the magnetic field), indicating the existence of dipole or antiferromagnetic interactions among the Ln3+ ions. At 300 K, the susceptibility was a constant. The field-independence of susceptibility was consistent with the Curie Law. The χm values were 2.52 × 10−2 emu/mol for 1a, 3.92 × 10−2 emu/mol for 2a, and 4.55 × 10−2 emu/mol for 3a. The temperature-dependent susceptibility values were close to the inverse proportional curves (Figure 14), indicating the wide temperature range of Langevin paramagnetism of 1a, 2a, and 3a. In general, these MILs had considerable magnetic susceptibility compared to all the previously reported MILs. The changes in the spin alignments in the MILs at a constant temperature under generally increasing vertical magnetic fields are shown in Figure 15. The spins of the unpaired electrons in the MILs were disorganized at 300 K. As the vertical magnetic field became

Figure 15. Changes in spin (little arrows) arrangements of MILs at constant temperature under generally increasing vertical magnetic fields. J


Article

Inorganic Chemistry

Figure 16. Temperature dependence of χmT and 1/χm for 1a, 2a, and 3a measured under 10 000 Oe.

parallel to the magnetic field. When the temperature dropped below 20 K, the thermal ion motion tended to disappear, and the antiferromagnetic coupling between the spins started to become important.

Table 3. Magnetic Properties of 1a, 2a, and 3a ILs

Mwa

χm [emu· mol−1]

χmT [emu·K· mol−1]

μeff [μB]

θe [K]

1a 2a 3a

689.61 691.28 694.86

0.252 0.039 0.046

7.54 11.76 13.65

7.76 9.69 10.44

0.32 −1.13 −7.97

b

a

c

d

■

Molecular weight. Molar magnetic susceptibility at 300 K. Measured at 300 K and H = 10 000 Oe. dEffective magnetic moment. eCurie−Weiss temperature. c

Table 4. Relevant Magnetic Information for Free Ln3+ Ions31 ion Gd3+ Tb3+ Dy3+

ground multiplet 8

S7/2 7 F6 6 H15/2

S

L

J

g

χT(cald)

χT(exptl)

3 5/2 2

0 3 5

7/2 6 15/2

2 3/2 4/3

7.87 11.82 14.17

7.54 11.76 13.65

EXPERIMENTAL SECTION

Synthetic Procedures, Materials, and Methods. All chemicals of analytical grade were obtained commercially. Solvents were dried by standard procedures. 1,2,3-Trimethylimidazolium iodide ([MC1mim]I), 1-alkyl-3-methylimidazolium bromides ([Cnmim]Br, n = 2, 4, 6 and 8), 1,2,3-trimethylimidazolium nitrate ([MC1mim]NO3), and 1-alkyl3-methylimidazolium nitrates ([Cnmim]NO3, n = 2, 4, 6 and 8) were synthesized according to the literature procedures.33 Infrared spectra (IR) were recorded on a Bruker ALPHA infrared spectrometer using KBr pellets. 1H and 13C NMR spectra were recorded on a Bruker 400 MHz nuclear magnetic resonance spectrometer operating at 400 (1H) and 100 (13C) MHz, respectively, with d6-DMSO as the locking solvent unless otherwise stated. 1H and 13C chemical shifts are reported in parts per million relative to tetramethylsilane, Si(CH3)4. Differential scanning calorimetry (DSC) measurements were performed on a TA Q20 calorimeter equipped with a cooling accessory and calibrated using standard pure indium, which was gently flooded with N2 at a flow rate of 20 mL min−1. Measurements were carried out at a heating rate of 10 °C min−1 from −100 to 100 °C. The reference sample was an Al container with nitrogen. Thermogravimetric analysis (TGA) measurements were accomplished on a NETZSCH TG 209F1 thermogravimetric analyzer at a heating rate of 10 °C min−1 from 25 to 500 °C in a dynamic nitrogen atmosphere at a flow rate of 70 mL min−1. Elemental analyses (H, C, N) were performed on an Elementar Vario MICRO CUBE elemental analyzer. UV−vis absorption spectra were recorded on a Rayleigh UV-1601 UV−vis spectrophotometer. Luminescence measurements were recorded on an Edinburgh FLS920 fluorescence spectrophotometer with a xenon lamp as the excitation source and a photomultiplier tube for detection. Excitation and emission spectra were collected at 2.5 nm band-pass at 293 K and corrected for detector response and lamp spectrum. Photoluminescent lifetimes were measured at 293 K on a Fluorolog-3 fluorometer (Horiba JobinYvon) with a SpectralLED (355 or 390 nm, S-355 and S-390, Horiba Scientific) as the excitation source and a picosecond photon detection module (PPD-850, Horiba Scientific). Magnetism measurements were recorded on Quantum Design PPMS-9T magnetometer. In order to study the magnetic properties, about 0.1 mmol of [MC1mim]2[Ln(NO3)5] (89.12 mg for

b

Figure 17. Changes in spin (little arrows) arrangements of MILs cooled down in magnetic fields.

the magnetic field. As the temperature decreased, thermal fluctuations decreased, and the spins tended to be oriented K


Article

Inorganic Chemistry

2H), 3.84 (s, 3H), 1.77 (s, 2H), 1.26 (s, 6H), 0.85 (s, 3H) ppm. 13C NMR (100 MHz, [D6]DMSO, 25 °C, Si(CH3)4): δ 136.62, 123.71, 48.85, 35.84, 29.42, 25.22, 21.96, 18.71, 15.78 ppm. IR (25 °C): ν 3155 3116, 2959, 2932, 2862, 1571, 1465, 1295, 1163, 1025, 843, 815, 740, 651, 622 cm−1. Anal. Calcd for C20H38GdN9O15, (801.82): C, 29.96; H, 4.78; N, 17.72. Found: C, 30.12; H, 5.07; N, 17.39. Di(1-methyl-3-octylimidazolium) Pentanitratogadolinate, [C8mim]2[Gd(NO3)5] (1e). A similar procedure was followed to that described above for the preparation of 1a. 1-Methyl-3-octylimidazolium nitrate (515 mg, 2 mmol) and gadolium(III)nitrate pentahydrate (434 mg, 1 mmol) were reacted in acetonitrile to give a colorless transparent liquid (841 mg, 98%). 1H NMR (400 MHz, [D6]DMSO, 25 °C, Si(CH3)4): δ 9.20 (s, 1H), 7.82/7.76 (d, 2H), 4.20 (s, 2H), 3.90 (s, 3H), 1.82 (m, 2H), 1.29 (m, 10H), 0.90 (s, 3H) ppm. 13C NMR (100 MHz, [D6]DMSO, 25 °C, Si(CH3)4): δ 136.74, 123.81, 122.47, 48.97, 35.91, 31.35, 29.57, 28.66, 28.52, 25.68, 22.25, 14.13 ppm. IR (25 °C): ν 3152, 3115, 2955, 2929, 2857, 1571, 1466, 1295, 1162, 1025, 845, 815, 740, 651, 622 cm−1. Anal. Calcd for C24H46GdN9O15 (857.92): C, 33.60; H, 5.40; N, 14.69. Found: C, 33.75; H, 5.67; N, 14.35. Di(1,2,3-Trimethylimidazolium) Pentanitratoterbate, [MC1mim]2[Tb(NO3)5] (2a). A similar procedure was followed to that described above for the preparation of 1a. 1,2,3-Trimethylimidazolium nitrate (346 mg, 2 mmol) was dissolved in acetonitrile (30 mL), and then terbium(III) nitrate pentahydrate (Tb(NO3)3·5H2O, 435 mg, 1 mmol) was added. The resulting mixture was stirred at 40 °C for 24 h. Then, the mixture was dried to yield 2a as a colorless solid (684 mg, 99%). Clear colorless plate crystals of 2a suitable for X-ray structure determination were obtained after recrystallization from acetonitrile/ethyl acetate. 1H NMR (400 MHz, [D6]DMSO, 25 °C, Si(CH3)4): δ 7.63 (t, 2H), 3.78 (s, 6H), 2.58 (s, 3H) ppm. 13C NMR (100 MHz, [D6]DMSO, 25 °C, Si(CH3)4): δ 144.93, 122.00, 34.92, 9.15 ppm. IR (KBr, 25 °C): ν 3176, 3148, 3094, 3027, 2970, 2943, 2762, 1591, 1486, 1314, 1243, 1127, 1092, 1030, 814, 762, 740, 657, 477 cm−1. Anal. Calcd for C12H22TbN9O15 (691.28): C, 20.85; H, 3.21; N, 18.24. Found: C, 20.67; H, 3.25; N, 18.08. Di(1-ethyl-3-methylimidazolium) Pentanitratoterbate, [C2mim]2[Tb(NO3)5] (2b). A similar procedure was followed to that described above for the preparation of 1a. 1-Ethyl-3-methylimidazolium nitrate (346 mg, 2 mmol) and terbium(III) nitrate pentahydrate (435 mg, 1 mmol) were reacted in acetonitrile to obtain a colorless transparent liquid (677 mg, 98%). 1H NMR (400 MHz, [D6]DMSO, 25 °C, Si(CH3)4): δ = 9.20 (s, 1H), 7.83 (s, 1H), 7.75 (s, 1H), 4.24 (q, J = 7.2 Hz, 2H), 3.88 (s, 3H), 1.44 (t, J = 7.2 Hz, 3H) ppm). 13C NMR (100 MHz, [D6]DMSO, 25 °C, Si(CH3)4): δ 136.41, 123.68, 122.09, 44.22, 35.76, 15.18 ppm. IR (KBr, 25 °C): ν 3159, 3120, 2988, 1571, 1495, 1310, 1167, 1029, 842, 816, 744, 702, 646 cm−1. Anal. Calcd for C12H22TbN9O15 (691.28): C, 20.85; H, 3.21; N, 18.24. Found: C, 20.51; H, 3.17; N, 18.27. Di(1-butyl-3-methylimidazolium) Pentanitratoterbate, [C4mim]2[Tb(NO3)5] (2c). A similar procedure was followed to that described above for the preparation of 1a. 1-Butyl-3-methylimidazolium nitrate (402 mg, 2 mmol) and terbium(III) nitrate pentahydrate (435 mg, 1 mmol) were reacted in acetonitrile to obtain a colorless transparent liquid (741 mg, 99%). 1H NMR (400 MHz, [D6]DMSO, 25 °C, Si(CH3)4): δ 9.21 (s, 1H), 7.83 (s, 1H), 7.76 (s, 1H), 4.21 (q, J = 6.8 Hz, 2H), 3.88 (s, 3H), 1.80 (t, J = 7.2 Hz, 2H) ppm), 1.28 (s, 2H), 0.91 (s, 3H). 13C NMR (100 MHz, [D6]DMSO, 25 °C, Si(CH3)4): δ 136.73, 123.74, 122.41, 48.62, 35.81, 31.47, 18.88, 13.35 ppm. IR (KBr, 25 °C): ν 3156, 3118, 2965, 2938, 2875, 1570, 1495, 1310, 1166, 1028, 843, 816, 744, 651, 622 cm−1. Anal. Calcd for C16H30TbN9O15 (747.39): C, 25.71; H, 4.05; N, 16.87. Found: C, 25.66; H, 4.16; N, 16.96. Di(1-hexyl-3-methylimidazolium) Pentanitratoterbate, [C6mim]2[Tb(NO3)5] (2d). An analogous route was employed for 1a. 1-Hexyl-3-methylimidazolium nitrate (459 mg, 2 mmol) and terbium(III) nitrate pentahydrate (435 mg, 1 mmol) were reacted in acetonitrile to give a colorless transparent liquid (780 mg, 97%). 1H NMR (400 MHz, [D6]DMSO, 25 °C, Si(CH3)4): δ 9.21 (s, 1H), 7.83 (s, 1H), 7.75 (s, 1H), 4.20 (t, J = 7.2 Hz, 2H), 3.87 (s, 3H), 1.81 (m,

1a, 65.03 mg for 2a, 83.49 mg for 3a) was contained in sealed Teflon capsules. Magnetization of 1a, 2a, and 3a were measured at 5 and 300 K in the magnetic field range −50 kOe to 50 kOe with the help of instruments that programatically control temperature. Additionally, the temperature dependence of the magnetic susceptibilities of 1a, 2a, and 3a were tested in the temperature range of 2−300 K and an applied field of 10 kOe using randomly oriented polycrystalline samples. X-ray Crystallography. Single crystals of 1a, 2a, and 3a were removed from the test tube. A suitable crystal was selected and attached to a glass fiber, and the data were collected at 293 K using an Oxford Xcalibur diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) and Cu Kα radiation (λ = 1.54184 Å). The structures were solved by direct methods and refined by the full-matrix least-squares method on F2 with the SHELXTL program package. The structures were solved in the space groups C2/c and I2/a by analysis of systematic absences. All nonhydrogen atoms were refined anisotropically, and hydrogen atoms were located and refined. No decomposition was observed during data collection. More details concerning the crystallographic data can be requested from the Cambridge Crystallographic Data Center [www.ccdc.cam.ac.uk/data_request/cif] with the deposition numbers CCDC-1819252 (1a), CCDC-1819253 (2a), and CCDC-1819254 (3a). Di(1,2,3-trimethylimidazolium) Pentanitratogadolinate, [MC1mim]2[Gd(NO3)5] (1a). 1,2,3-a nitrate (346 mg, 2 mmol) was dissolved in acetonitrile (30 mL), and then gadolium(III) nitrate pentahydrate (Gd(NO3)3·5H2O, 434 mg, 1 mmol) was added. The resulting mixture was stirred at 40 °C for 24 h. Then, the mixture was dried to yield 1a as a colorless solid (668 mg, 97%). Clear colorless plate crystals of 1a suitable for X-ray structure determination were obtained after recrystallization from acetonitrile/ethyl acetate. 1H NMR (400 MHz, [D6]DMSO, 25 °C, Si(CH3)4): δ 7.62 (s, 2H), 3.78 (s, 6H), 2.58 (s, 3H) ppm. 13C NMR (100 MHz, [D6]DMSO, 25 °C, Si(CH3)4): δ 142.61, 122.41, 35.13, 9.57 ppm. IR (KBr, 25 °C): ν 3178, 3146, 3105, 2970, 2946, 1589, 1547, 1459, 1293, 1242, 1126, 1093, 1029, 816, 811, 760, 740, 658, 477 cm−1. Anal. Calcd for C12H22GdN9O15 (689.61): C, 20.90; H, 3.22; N, 18.28. Found: C, 20.91; H, 3.35; N, 18.05. Di(1-ethyl-3-methylimidazolium) Pentanitratogadolinate, [C2mim]2[Gd(NO3)5] (1b). A similar procedure was followed to that described above for the preparation of 1a. 1-Ethyl-3-methylimidazolium nitrate (346 mg, 2 mmol) and gadolium(III) nitrate pentahydrate (434 mg, 1 mmol) were reacted in acetonitrile to obtain a colorless transparent liquid (663 mg, 96%). 1H NMR (400 MHz, [D6]DMSO, 25 °C, Si(CH3)4): δ 9.18 (s, 1H), 7.79 (s, 2H), 4.20 (s, 2H), 3.85 (s, 3H), 1.40 (s, 3H) ppm. 13C NMR (100 MHz, [D6]DMSO, 25 °C, Si(CH3)4): δ 136.68, 123.93, 122.29, 48.45, 35.99, 15.43 ppm. IR (KBr, 25 °C): ν 3157, 3120, 2993, 2945, 1572, 1462, 1288, 1164, 1025, 841, 815, 740, 621, 597 cm−1. Anal. Calcd for C12H22GdN9O15 (689.61): C, 20.90; H, 3.22; N, 18.28. Found: C, 21.13; H, 3.52; N, 18.39. Di(1-butyl-3-methylimidazolium) Pentanitratogadolinate, [C4mim]2[Gd(NO3)5] (1c). A similar procedure was followed to that described above for the preparation of 1a. 1-Butyl-3-methylimidazolium nitrate (402 mg, 2 mmol) and gadolium(III) nitrate pentahydrate (434 mg, 1 mmol) were reacted in acetonitrile to obtain a colorless transparent liquid (716 mg, 96%). 1H NMR (400 MHz, [D6]DMSO, 25 °C, Si(CH3)4): δ 9.24 (s, 1H), 7.84 (s, 2H), 4.23 (s, 2H), 3.92 (s, 3H), 1.83 (s, 2H), 1.32 (s, 2H), 0.96 (s, 3H) ppm. 13C NMR (100 MHz, [D6]DMSO, 25 °C, Si(CH3)4): δ 136.84, 123.88, 122.55, 48.75, 35.98, 31.61, 19.02, 13.53 ppm. IR (KBr, 25 °C): ν 3156, 3118, 2965, 2937, 2874, 1572, 1459, 1294, 1163, 1025, 842, 815, 740, 651, 622 cm−1. Anal. Calcd for C16H30GdN9O15 (745.72): C, 25.77; H, 4.06; N, 16.91. Found: C, 25.79; H, 4.31; N, 17.03. Di(1-hexyl-3-methylimidazolium) Pentanitratogadolinate, [C6mim]2[Gd(NO3)5] (1d). A similar procedure was followed to that described above for the preparation of 1a. 1-Hexyl-3-methylimidazolium nitrate (459 mg, 2 mmol) and gadolium(III) nitrate pentahydrate (434 mg, 1 mmol) were reacted in acetonitrile to give a pale yellow transparent liquid (795 mg, 99%). 1H NMR (400 MHz, [D6]DMSO, 25 °C, Si(CH3)4): δ 9.12 (s, 1H), 7.77 (s, 1H), 7.70 (s, 1H), 4.15 (s, L


Article

Inorganic Chemistry 2H), 1.28 (m, 6H), 0.84 (t, J = 6.8 Hz, 3H) ppm. 13C NMR (100 MHz, [D6]DMSO, 25 °C, Si(CH3)4): δ 136.73, 123.75, 122.42, 48.91, 35.82, 30.65, 29.47, 25.26, 21.97, 13.92 ppm. IR (25 °C): ν 3155 3118, 2958, 2933, 2864, 1570, 1487, 1320, 1165, 1028, 843, 816, 743, 651, 623 cm−1. Anal. Calcd for C20H38TbN9O15 (803.50): C, 29.90; H, 4.77; N, 15.69. Found: C, 29.61; H, 4.81; N, 15.90. Di(1-methyl-3-octylimidazolium) Pentanitratoterbate, [C8mim]2[Tb(NO3)5] (2e). An analogous route was employed for 1a. 1-Methyl-3-octylimidazolium nitrate (515 mg, 2 mmol) and terbium(III) nitrate pentahydrate (435 mg, 1 mmol) were reacted in acetonitrile to give a colorless transparent liquid (833 mg, 97%). 1H NMR (400 MHz, [D6]DMSO, 25 °C, Si(CH3)4): δ 9.22 (s, 1H), 7.84 (s, 1H), 7.76 (s, 1H), 4.21 (t, J = 7.2 Hz, 2H), 3.89 (s, 3H), 1.82 (m, 2H), 1.26 (m, 10H), 0.85 (t, J = 6.8 Hz, 3H) ppm. 13C NMR (100 MHz, [D6]DMSO, 25 °C, Si(CH3)4): δ 136.59, 123.59, 122.27, 48.75, 35.65, 31.11, 29.37, 28.43, 28.31, 25.47, 22.99, 13.85 ppm. IR (25 °C): ν 3155, 3117, 2929, 2858, 1569, 1499, 1309, 1165, 1028, 844, 817, 743, 652, 623 cm−1. Anal. Calcd for C24H46TbN9O15 (859.61): C, 33.53; H, 5.39; N, 14.67. Found: C, 33.65; H, 5.23; N, 14.97. Di(1,2,3-trimethylimidazolium) Pentanitratodysprosate, [MC1mim]2[Dy(NO3)5] (3a). A similar procedure was followed to that described above for the preparation of 1a. 1,2,3-Trimethylimidazolium nitrate (346 mg, 2 mmol) was dissolved in acetonitrile (30 mL), and then dysprosium(III) nitrate pentahydrate (Dy(NO3)3· 5H2O, 439 mg, 1 mmol) was added. The resulting mixture was stirred at 40 °C for 24 h. Then, the mixture was dried to yield 3a as a white solid (681 mg, 98%). Clear colorless plate crystals of 3a suitable for Xray structure determination were obtained after recrystallization from acetonitrile/ethyl acetate. 1H NMR (400 MHz, [D6]DMSO, 25 °C, Si(CH3)4): δ 7.57 (t, 2H), 3.73 (s, 6H), 2.53 (s, 3H) ppm. 13C NMR (100 MHz, [D6]DMSO, 25 °C, Si(CH3)4): δ 144.79, 121.99, 34.68, 9.11 ppm. IR (KBr, 25 °C): ν 3178, 3146, 3093, 3025, 2971, 2939, 2761, 1591, 1489, 1315, 1244, 1128, 1093, 1033, 816, 762, 744, 659, 478 cm−1. Anal. Calcd for C12H22DyN9O15 (694.85): C, 20.74; H, 3.19; N, 18.14. Found: C, 20.77; H, 3.29; N, 18.21. Di(1-ethyl-3-methylimidazolium) Pentanitratodysprosate, [C2mim]2[Dy(NO3)5] (3b). A similar procedure was followed to that described above for the preparation of 1a. 1-Ethyl-3-methylimidazolium nitrate (346 mg, 2 mmol) and dysprosium(III) nitrate pentahydrate (439 mg, 1 mmol) were reacted in acetonitrile to obtain a primrose yellow transparent liquid (674 mg, 97%). 1H NMR (400 MHz, [D6]DMSO, 25 °C, Si(CH3)4): δ 9.12 (s, 1H), 7.79 (s, 1H), 7.70 (s, 1H), 4.20 (q, J = 7.2 Hz, 2H), 3.85 (s, 3H), 1.41 (t, J = 7.2 Hz, 3H) ppm. 13C NMR (100 MHz, [D6]DMSO, 25 °C, Si(CH3)4): δ 136.36, 123.61, 122.05, 44.14, 35.63, 15.09 ppm. IR (KBr, 25 °C): ν 3158, 3119, 2992, 2945, 1573, 1466, 1297, 1164, 1025, 841, 814, 742, 621, 597 cm−1. Anal. Calcd for C12H22DyN9O15 (694.85): C, 20.74; H, 3.19; N, 18.14. Found: C, 21.04; H, 3.03; N, 18.10. Di(1-butyl-3-methylimidazolium) Pentanitratodysprosate, [C4mim]2[Dy(NO3)5], (3c). A similar procedure was followed to that described above for the preparation of 1a. 1-Butyl-3-methylimidazolium nitrate (402 mg, 2 mmol) and dysprosium(III) nitrate pentahydrate (439 mg, 1 mmol) were reacted in acetonitrile to obtain a primrose yellow transparent liquid (735 mg, 98%). 1H NMR (400 MHz, [D6]DMSO, 25 °C, Si(CH3)4): δ 9.12 (s, 1H), 7.77 (s, 1H), 7.70 (s, 1H), 4.16 (t, J = 6.8 Hz, 2H), 3.85 (s, 3H), 1.74 (m, 2H), 1.25 (m, 2H), 0.90 (t, J = 7.2 Hz, 3H) ppm. 13C NMR (100 MHz, [D6]DMSO, 25 °C, Si(CH3)4): δ 136.64, 123.73, 122.38, 48.60, 35.84, 31.46, 18.88, 13.39 ppm. IR (KBr, 25 °C): ν 3157, 3119, 2964, 2938, 2875, 1572, 1466, 1297, 1163, 1025, 842, 814, 742, 650, 622 cm−1. Anal. Calcd for C16H30DyN9O15 (750.97): C, 25.59; H, 4.03; N, 16.79. Found: C, 25.24; H, 3.99; N, 16.94. Di(1-hexyl-3-methylimidazolium) Pentanitratodysprosate, [C6mim]2[Dy(NO3)5], (3d). A similar procedure was followed to that described above for the preparation of 1a. 1-Hexyl-3-methylimidazolium nitrate (459 mg, 2 mmol) and dysprosium(III) nitrate pentahydrate (439 mg, 1 mmol) were reacted in acetonitrile to give a primrose yellow transparent liquid (799 mg, 99%). 1H NMR (400 MHz, [D6]DMSO, 25 °C, Si(CH3)4): δ 9.12 (s, 1H), 7.78 (s, 1H), 7.71 (s, 1H), 4.15 (t, J = 7.2 Hz, 2H), 3.85 (s, 3H), 1.79 (m, 2H), 1.27

(m, 6H), 0.86 (t, J = 6.8 Hz, 3H) ppm. 13C NMR (100 MHz, [D6]DMSO, 25 °C, Si(CH3)4): δ 136.61, 123.69, 122.34, 48.83, 35.79, 30.61, 29.41, 25.21, 21.94, 13.91 ppm. IR (25 °C): ν 3153, 3118, 2957, 2930, 2860, 1571, 1477, 1297, 1162, 1026, 843, 815, 742, 651, 622 cm−1. Anal. Calcd for C20H38DyN9O15 (807.07): C, 29.76; H, 4.75; N, 15.62. Found: C, 29.81; H, 4.84; N, 15.89. Di(1-methyl-3-octylimidazolium) Pentanitratodysprosate, [C8mim]2[Dy(NO3)5], (3e). A similar procedure was followed to that described above for the preparation of 1a. 1-Methyl-3-octylimidazolium nitrate (515 mg, 2 mmol) and dysprosium(III)nitrate pentahydrate (439 mg, 1 mmol) were reacted in acetonitrile to give a yellow transparent liquid (837 mg, 97%). 1H NMR (400 MHz, [D6]DMSO, 25 °C, Si(CH3)4): δ 9.12 (s, 1H), 7.78 (s, 1H), 7.71 (s, 1H), 4.15 (t, J = 7.2 Hz, 2H), 3.85 (s, 3H), 1.78 (m, 2H), 1.25 (m, 10H), 0.85 (t, J = 6.8 Hz, 3H) ppm. 13C NMR (100 MHz, [D6]DMSO, 25 °C, Si(CH3)4): δ 137.05, 124.17, 122.77, 49.30, 36.31, 31.72, 29.93, 29.03, 28.92, 26.02, 22.59, 14.50 ppm. IR (25 °C): ν 3152, 3119, 2953, 2928, 2857, 1571, 1479, 1297, 1162, 1026, 843, 815, 742, 651, 622 cm−1. Anal. Calcd for C24H46DyN9O15 (863.17): C, 33.40; H, 5.37; N, 14.60. Found: C, 33.27; H, 5.65; N, 14.60.

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CONCLUSIONS Novel stable fluorescigenic magnetofluids were obtained using a rapid and simple synthesis procedure for room temperature ILs. These fluorescigenic magnetofluids with discrete magnetic entities showed intense responses to NdFeB magnets, and displayed temperature-dependent paramagnetic susceptibility, with small deviations from the Curie law at low temperatures. It can be seen from the magnetic properties that, as for the inorganic lanthanide salts, these lanthanide ILs exhibit a variable magnetization at different temperatures, enabling magnetic applications such as magnetic heat storage materials. In addition, we also observed that Tb- and Dy-containing ILs can emit strong and long-lifetime green and yellow fluorescence under excitation by ultraviolet light. Studies on these fluorescigenic magnetofluids are promising for potential applications, such as in new materials for sensing, in situ imaging, anticounterfeiting technology, motion sensing, energy harvesting, heat storage materials, biomacromolecule extraction, advanced luminescent coatings, and screen display technologies.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00435. NMR, IR, and UV spectra; selected bond lengths and bond angles of crystal data; luminescent lifetime data; and magnetic data (PDF) Accession Codes

CCDC 1819252−1819254 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.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Guo-Hong Tao: 0000-0002-1152-7460 M


Article

Inorganic Chemistry Author Contributions

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The manuscript was written through contributions of K.-L.C., L.H., G.-H.T., and S.Q. L.H. and G.-H.T. developed the concept; K.-L.C., W.-L.Y., Y.Z., and H.-M.J. synthesized the samples and performed NMR, IR, UV, EA, PL, lifetime, susceptibility measurements, and data analysis. N.T. performed single crystal measurements and data analysis. All authors have given approval to the final version of the manuscript. Funding

The National Natural Science Foundation of China (No. 21303108, J1210004). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the Comprehensive training platform of specialized laboratory, College of Chemistry, Sichuan University for instrumental measurements. We would also thank the Analytical and Testing Center of Sichuan University for crystal and fluorescence measurements.

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Fluorescigenic Magnetofluids Based on Gadolinium, Terbium, and

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