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Long-lived luminescent soft materials of hexanitratosamarate(III) complexes with orange visible emission† Ning Tang, Ying Zhao, Ling He,* Wen-Li Yuan and Guo-Hong Tao* Sm(III)-based ionic liquids incorporating hexanitratosamarate(III) anions were obtained and fully characterized as novel Sm(III)-containing organic complexes. The structure of the ionic liquids was determined by single-crystal X-ray diffraction (1: monoclinic system C2/c space group with cell parameters: a = 19.5624(4) Å, b = 10.11895(18) Å, c = 33.2256(6) Å, β = 101.2912(18)°, Z = 8). The central Sm(III) ion is 12-coordinated by six bidentate nitrate ligands with twelve oxygen donors to form a [Sm(NO3)6]3− anion. The low melting
Received 15th January 2015, Accepted 23rd March 2015 DOI: 10.1039/c5dt00191a www.rsc.org/dalton
point, high thermostability and wide liquid range of these ionic liquids were determined in detail. All the complexes 1–5 display orange luminescence, rather than red luminescence as in most Sm(III)-containing organic complexes. Three characteristic monochromatic bands and an intense emission, derived from G5/2→6HJ ( J = 5/2, 7/2, and 9/2) intraconfigurational f–f transitions, were revealed. All these complexes exhibit long luminescence lifetimes. 4
Introduction Luminescent materials are widely used for organic light emitting diodes (OLEDs), biosensing and bioimaging, etc.1 Functional luminescent soft materials, including ionic liquids (ILs), liquid crystals, and ionogels, have drawn much attention as a growing trend.2 ILs, regarded as green solvents and functional materials, have many excellent inherent properties, such as negligible vapor pressures, wide liquid ranges, good thermal stabilities, considerable electrical conductivities, and wide electrochemical windows.3 Metal-containing ILs can combine the properties of ILs with the additional intrinsic spectroscopic, catalytic or magnetic properties of the metal, and be of more interest as modifiable soft materials.4 Because of their unique luminescent and magnetic properties, lanthanides are considered ideal metals for being incorporated into ILs.5 However, the low solubility of lanthanide complexes in ILs limits their applications.6 In recent years, to increase the concentrations of lanthanides in ILs, many lanthanide-containing ILs have been synthesized and studied.7,8 The main research on luminescent lanthanidedoped ILs has been focused on Eu(III).8 Although little research has focused on the luminescent property of Sm(III)-doped ILs,
College of Chemistry, Sichuan University, Chengdu, Sichuan 610064, China. E-mail: [email protected], [email protected] † Electronic supplementary information (ESI) available: 1H NMR, 13C NMR and IR spectra of 1–5, PXRD spectra of 1 and 2. CCDC 1040595. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5dt00191a
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the emission of Sm(III) is very interesting. Unlike Eu(III) (red color) or Tb(III) (green color), Sm(III) displays visible luminescence (red/orange color) at different wavelengths. The Sm(III) ion, with its 4f 5 orbital configuration, has plenty of energy levels, which lead to emission bands covering the visible and near-infrared (NIR) spectral range. In the visible region, Sm(III) has three characteristic emission bands, which are assigned to 4 G5/2→6H5/2 (ΔJ = 0, magnetic dipole transition), 4G5/2→6H7/2 (ΔJ = 1, magnetic dipole transition), and 4G5/2→6H9/2 (ΔJ = 2, electric dipole transition). However, the emission band of most Sm(III) complexes at around 565 nm is relatively weak. In these situations, Sm(III) complexes can only emit red or orange-red luminescence in the visible region.9 Common ligands in Sm(III)-doped ILs include β-diketone, dipicolinate,10 dicyanonitrosomethanide,11 and thiocyanide,12 etc. However, the disadvantage of β-diketone and dipicolinate is the low photostability of the corresponding complexes in organic solvents.10 Recent studies on nitratolanthanate ILs with [Ln(NO3)3+x]x− anions have shown that the nitrate anion is a stable ligand for constructing novel soft materials including propellants and liquid crystals. The structures of [Ln(NO3)6]3− anions have been determined for La- and Nd-containing ILs.13 Also, for Eu-containing ILs, the [Ln(NO3)5]2− anion has been obtained.8d However, the crystal structure of [Ln(NO3)4]− anions in ILs has still not been confirmed.8e In a nitrate-coordinated system, one or two water molecules may be coordinated to the metal ion and would cause a non-radiative transition derived from the water, and then quench any luminescent emission from the lanthanides.14 Complexes containing
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water-free [Ln(NO3)6]3− anions have already been reported for many lanthanide systems including Ln = La, Ce, Pr, Nd, Eu, Gd, Tb, and Dy, but not Sm.15 Moreover, only a little research has focused on the fluorescence of [Ln(NO3)6]3−, limited to Eu and Nd.13c,15a,c [Eu(NO3)6]3− anions exhibit strong red luminescence and can reduce non-radiative transition effectively. Therefore, the structure and visible fluorescence of the [Sm(NO3)6]3− anion are of interest. Here, we first synthesized water-free hexanitratosamarate ILs. Their unusual orange photoluminescence is illustrated, and a systematic study of their luminescence properties is undertaken.
Results and discussion Synthesis Synthesis routes of complexes 1–5 are shown in Scheme 1. The nitrate ILs [MC1mim][NO3] (MC1mim = 1,2,3-trimethylimidazolium) and [Cnmim][NO3] (Cnmim = 1-alkyl-3-methylimidazolium, n = 2, 4, 6 and 8) were prepared according to the literature method.13 Reaction of samarium(III) nitrate hexahydrate and the corresponding nitrate precursors with a molar ratio of 3 : 1 in acetonitrile gave Sm(III)-containing organic complexes 1–5. The compositions of 1–5 are [MC1mim]3[Sm(NO3)6] (1) and [Cnmim]3[Sm(NO3)6] (2–5) (n = 2, 4, 6 and 8). Slow recrystallization from an acetonitrile–ethyl acetate solution afforded colourless plate crystals of 1 suitable for X-ray diffraction determination. The structures of complexes 1–5 were fully characterized by IR, NMR and elemental analysis. IR spectra The presence of water-free hexanitratosamarate(III) with bidentate nitrates is confirmed from the evidence of infrared spectra. The infrared spectra of complexes 1–5 contain the main vibration bands of the nitrato ligand: ν4 = 1444–1461 cm−1 and ν1 = 1319–1329 cm−1, assigned to N–O asymmetrical stretching vibrations, ν2 = 1036–1040 cm−1, assigned to N–O symmetrical stretching vibrations, along with the out-of-plane bending vibration ν6 = 820 cm−1. No absorbance band corresponding to the ν3(E′) mode of the free symmetric NO3− anion with D3h point group symmetry at about 1385 cm−1 was observed. This indicates that the central Sm(III) is completely surrounded by six chelating nitrate ligands, and
Scheme 1
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Synthesis routes of hexanitratosamarate(III) complexes 1–5.
no free NO3− exists. The difference between ν4 and ν1 is larger than 120 cm−1, indicating strong interactions between the bidentate coordinating ligands and Sm(III). The expected absorbance bands of the imidazolium ring are evident in the infrared spectra. C–H deformation modes can be found in the range 1300–1570 cm−1. C–H stretching modes in the range 2800–3200 cm−1, as well as C–H rocking modes around 740 cm−1, are also observed. Structural characterization The crystal structure of 1 was determined by single-crystal X-ray diffraction. Colourless crystals of 1 of dimensions 0.3 × 0.3 × 0.25 mm were obtained by slow recrystallization from acetonitrile–ethyl acetate at room temperature. The molecular structure of 1 is shown in Fig. 1 and crystallographic data are given in Table 1. Selected bond lengths and angles are summarized in Tables 2 and 3. All non-hydrogen atoms were refined anisotropically. Compound 1 belongs to the monoclinic space group C2/c (no. 15), with three imidazolium cations and two [Sm(NO3)6]3− half-anions contained in each asymmetric unit, which is different from those in [C1mim]3[Ln(NO3)6] (Ln = La, Nd), [SEt3]3[SmI6] and (N4444)3[Sm(dcnm)6].11,13,16 The unique Sm(1) and Sm(2) ions are both located in a special position. Each Sm(III) ion is coordinated by twelve oxygen atoms in a bidentate manner through six nitrato ligands to form two [Sm(NO3)6]3− anions. The polyhedra of these [Sm(NO3)6]3− anions can be described as icosahedra, but the two anions exhibit different symmetry (Fig. 1). The anion based on Sm(2) is on an inversion centre with highly idealised D2h symmetry. For this [Sm(NO3)6]3− anion, all the oxygens are distorted from icosahedral symmetry by the shorter O⋯O distances within the chelating ligands, which is indeed typical for hexanitrato lanthanide complexes.13 However, the anion based on Sm(1) is not the same as usual, with Sm(1) on a twofold crystal axis rather than an inversion centre. This anion clearly exhibits symmetry-lowering distorted icosahedral symmetry. In the unit cell, the different kinds of Sm atoms have the same relative location,
Fig. 1
Crystal structure of complex 1.
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Table 1
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Crystal data and structure refinement for 1
Formula Mw/g mol−1 Size/mm3 Crystal system Space group a/Å b/Å c/Å α/° β/° γ/° V/Å3 Z ρ/g cm−3 T/K μ/mm−1 F(000) λMoKα/Å Reflections Rint Parameters 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
C18H33N12O18Sm 855.91 0.3 × 0.3 × 0.25 Monoclinic C2/c 19.5624(4) 10.11895(18) 33.2256(6) 90 101.2912(18) 90 6449.7(2) 8 1.763 143 1.916 3448 0.71073 5895 0.0273 452 1.074 0.0277 0.0547 0.0339 0.0570 0.49/−0.61
Bond angles in the crystal structure of 1a
O–Sm–O bond angles/° O2–Sm1–O1 49.42(6) 49.42(6) O2i–Sm1–O1i O4–Sm1–O5 49.36(6) 49.36(6) O4i–Sm1–O5i O7–Sm1–O8 49.47(6) 49.47(6) O7i–Sm1–O8i O–N–O bond angles/° O2–N1–O1 117.3(2) O3–N1–O1 121.6(3) O3–N1–O2 121.2(3) O5–N2–O4 117.0(2) O6–N2–O4 121.4(2) O6–N2–O5 121.6(3) O8–N3–O7 116.7(3) O9–N3–O7 121.6(3) O9–N3–O8 121.7(3) O–N–Sm bond angles/° O7–N3–Sm1 57.48(15) O8–N3–Sm1 59.72(14) O9–N3–Sm1 173.0(2) a
O11–Sm2–O10 O11ii–Sm2–O10ii O13–Sm2–O14 O13ii–Sm2–O14ii O16–Sm2–O18ii O16ii–Sm2–O18
49.13(7) 49.13(7) 49.13(7) 49.13(7) 50.04(6) 50.04(6)
O11–N4–O10 O12–N4–O10 O12–N4–O11 O14–N5–O13 O15–N5–O13 O15–N5–O14 O17–N6–O16 O17–N6–O18ii O18ii–N6–O16
116.9(2) 121.5(3) 121.6(3) 117.4(2) 121.8(3) 120.8(3) 122.0(3) 121.8(3) 116.2(2)
O16–N6–Sm2 O17–N6–Sm2 O18ii–N6–Sm2
59.31(14) 177.2(2) 56.96(14)
Symmetry operation: i1 − X, +Y, 1/2 − Z; ii1/2 − X, 1/2 − Y, 1 − Z.
R1 = ∑||Fo| − |Fc||/∑|Fo|. b wR2 = [∑w(Fo2 − Fc2)2/∑w(Fo2)2]1/2.
Table 2
Selected bond lengths in the crystal structure of 1a
Sm–O bond lengths/Å Sm1–O1 2.599(2) 2.599(2) Sm1–O1i Sm1–O2 2.574(2) i 2.574(2) Sm1–O2 Sm1–O4 2.5497(18) 2.5497(18) Sm1–O4i Sm1–O5 2.613(2) 2.613(2) Sm1–O5i Sm1–O7 2.547(2) 2.547(2) Sm1–O7i Sm1–O8 2.595(2) 2.595(2) Sm1–O8i O–N bond lengths/Å O1–N1 1.270(3) O2–N1 1.262(3) O3–N1 1.231(3) O4–N2 1.268(3) O5–N2 1.262(3) O6–N2 1.233(3) O7–N3 1.259(3) O8–N3 1.269(3) O9–N3 1.220(3) a
Table 3
Sm2–O10 Sm2–O10ii Sm2–O11 Sm2–O11ii Sm2–O13 Sm2–O13ii Sm2–O14 Sm2–O14ii Sm2–O16 Sm2–O16ii Sm2–O18 Sm2–O18ii
2.590(2) 2.590(2) 2.608(2) 2.608(2) 2.5839(19) 2.5839(19) 2.603(2) 2.603(2) 2.575(2) 2.575(2) 2.524(2) 2.524(2)
O10–N4 O11–N4 O12–N4 O13–N5 O14–N5 O15–N5 O16–N6 O17–N6 O18–N6ii
1.266(3) 1.271(3) 1.232(3) 1.258(3) 1.266(3) 1.240(3) 1.274(3) 1.220(3) 1.267(3)
Symmetry operation: i1 − X, +Y, 1/2 − Z; ii1/2 − X, 1/2 − Y, 1 − Z.
with each type of Sm atoms located in a plane and two types of planes formed by Sm atoms arranged parallel alternately (Fig. 2). The range of Sm–O bond lengths in 1 is similar to data for the [Sm(NO3)5·H2O]2− anion.14b The same values for the angles around Sm atoms are found in both [Sm(NO3)6]3− anions.
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Fig. 2 cell.
Relative location of Sm1 (bright green) and Sm2 (tan) in the unit
However, the difference between the maximum and minimum O–Sm–O angle is interesting. The difference in the O–Sm2–O angle is 0.91°, which is much bigger than in the other [Ln(NO3)6]3− (Ln = La, Nd) anions with a similar symmetric structure.13b,c However, the difference in the O–Sm1–O angle is only 0.11°, which is similar to that in the [Eu(NO3)5]2− anion but lower than in the [Ln(NO3)6]3− (Ln = La, Nd) anions.8d,13b,c The data indicate that with a decrease in ionic radius, the [Sm(NO3)6]3− anion has a tendency to form another type of structure with lower symmetry. This distorted geometry of the [Sm(NO3)6]3− anion is further confirmed by the torsion angles. Evidence of some multiple bond characters is provided. Delocalization of nitrate anions is recorded. The data of N–O bond distances and O–N–O angles also indicate that the [MC1mim]+ cations are delocalized, with the positive charge dispersed over the aromatic system, analogous to the case found in other imidazolium-based complexes.17
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Fig. 3
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Packing diagram of 1 viewed down the b-axis.
result in weaker electrostatic interactions between the cation and the anion and even lower melting points for complexes 3–5. The thermal stability performance of complexes 1–5 was tested by thermogravimetric analysis (TGA). Measurements indicated that complexes 1–5 exhibit high decomposition temperatures in the range 287 (2) to 313 °C (5), with no neutral water coordinated to the Sm(III) ion. Similar TGA curves were recorded for complexes 1–5, with single-step decomposition. With rising temperature, exothermic decomposition occurs, leaving samarium oxide estimated as the final residue. The considerable thermal stabilities of complexes 1–5 are comparable to those of the La(III) complexes, and close to those of their precursors [Cnmim][NO3].13 Therefore, these Sm(III)based ILs are sufficiently thermally stable materials, with a large liquid range demonstrated. Spectroscopic measurements
The powder X-ray diffraction (PXRD) patterns of complexes 1 and 2 demonstrate that the two complexes have a similar structure and fewer diffraction peaks compared to simulated PXRD from single-crystal structure data of 1 (Fig. S16†). The successful introduction of a Sm(III)-doped anion to produce water-free ILs is confirmed by the single-crystal structure of complex 1. The crystal data implies non-classical hydrogen bonds exist between hydrogen atoms of water and nitrogen atoms in the imidazolium cations (Fig. 3). Furthermore, no hydrogen bonding between the cation and the anion is found. This feature is very different from the packing structure of [C6mim][Sm(tta)4], [C6mim][Sm(nta)4] or [C6mim][Sm(hfa)4].9 These simple ion pairs can be arranged in order, to construct the infinite three-dimensional system of 1. No π-stacking interaction among the imidazolium cations was found. The large volume of the hexanitratosamarate anion may hinder π-stacking of cations. Thus, packing may have a major impact on formation of a [Sm(NO3)6]3− complex with a low melting point.
All complexes 1–5 emit orange photoluminescence under ultraviolet (UV) lamp irradiation (Fig. 4). Complexes 1–5 give rise to similar excitation and emission spectra. Electronic transitions were assigned according to the energy level diagrams of trivalent rare-earth ions. The excitation spectra of complexes 1–5 are complicated (Fig. 5a and 6a). Most of the transitions were assigned to one or two multiplets, or even three multiplets.18 In addition, the typical characteristic transitions, consisting of a broad band between 250 and 330 nm, attributed to n–π* transitions of nitrate ligands, are also found, with a maximum peak at 290 nm. The intensity of this peak is weaker than those of the peaks at 404 nm, 375 nm, 363 nm, and 345 nm, but similar to that of the peak recorded at 480 nm. Therefore, direct f–f transitions, mainly excited by the effect of high symmetry of the inner coordination sphere, are indicated, rather than a contribution of energy transfer derived from n–π* transitions. The emission spectra were recorded at room temperature upon excitation at 404 nm (Fig. 5b and 6b). The emission
Thermal properties The melting points of complexes 1–5 were determined by differential scanning calorimetry (DSC). Measurements of their thermal properties indicated that complexes 2–5 have melting points below 100 °C and therefore can be classified as ILs. Alkyl-substituted imidazolium cations were utilized to produce Sm(III) complexes as ILs with low melting points, associated with delocalization of positive charge on the imidazolium ring. Complex 1, being crystallized for structural study, melts at 142 °C. Complex 2 possesses a melting point of 82 °C, along with a glass transition temperature around −49 °C. The methyl and ethyl groups attached to the imidazolium ring give rise to a major trend to relatively regular packing for the crystallization of 1 and 2. Complexes 3–5 were found to be liquids or supercooled liquids at room temperature. No melting point was recorded. The glass transition temperatures of 3–5 range from −38 °C to −45 °C. The bigger size, lower charge density and lower point symmetry of butyl, hexyl and octyl groups
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Fig. 4 Images of hexanitratosamarate complexes 1-5 under daylight (top) and under ultraviolet irradiation (bottom).
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Fig. 5 (a) Excitation spectrum of complex 2 monitored at 645 nm; (b) Emission spectrum and decay curve (upper right) of complex 2 excited at 404 nm.
Fig. 6
Excitation (a) and emission spectra (b) of complexes 1–5.
peaks correspond to transitions from the 4G5/2 level to the different J levels of the 6H term (6HJ, J = 5/2 − 15/2). Three sharp emission peaks derived from the Sm(III) ion, assigned to 4 G5/2→6H5/2, 4G5/2→6H7/2 and 4G5/2→6H9/2 transitions, are observed at 564, 599 and 645 nm, respectively. These are responsible for the orange luminescence colour. A very weak emission peak at 706 nm is assigned to 4G5/2→6H11/2. The transition of 4G5/2→6H13/2 is so weak as to be almost invisible. The other weak peak at 532 nm corresponds to a transition from a higher emissive state 4F3/2 to the ground state 6H5/2. The three Sm3+ excited states 4G7/2 (∼20 050 cm−1), 4F3/2 (∼18 700 cm−1) and 4G5/2 (∼17 700 cm−1) can be excited efficiently. However, only emission from the 4G5/2 level can be observed intensively.
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A possible reason is that the energy differences between the two higher excited states 4G7/2 and 4F3/2 and 4G5/2 are very small, which can lead electrons to relax to the lower level (4G5/2) quickly. Non-radiative transitions will occur in this process.19 The most intense emission band, assigned to the 4 G5/2→6H7/2 transition of 1–5, is found at 599 nm. Moreover, the typical transitions of 4G5/2→6H5/2 and 4G5/2→6H9/2 are also intense, located at 564 nm and 645 nm. In the Sm(III) system, the transition of 4G5/2→6H5/2 located at 564 nm, exhibits a predominant magnetic dipole character. The 4G5/2→6H9/2 transition at 645 nm is hypersensitive.10 Generally, the symmetry of the coordination structure can influence the emission spectra of lanthanide ions.8b,9,10 The structures of usual Sm-containing complexes with lower symmetry give rise to red fluorescence, whereas those with higher symmetry may emit orange-red light.9,10 In our system, the highly symmetrical [Sm(NO3)6]3− anion probably results in a decrease in the 4G5/2→6H9/2 emission. The intensity ratio I(4G5/2→6H9/2)/I(4G5/2→6H5/2) can be calculated to have a relatively low value. The orange colour of the photoluminescence from 1–5 results from the enhanced intensity of the 4 G5/2→6H5/2 emission, which is stronger than usual and contributes yellow to the overall colour. The decay curves of all complexes are found to be monoexponential, indicating that only one Sm(III) species is present. The luminescence lifetimes of complexes 1–5 vary from 114.4 to 130.3 μs in acetonitrile, which is longer than those of most Sm(III) complexes.9,20 The reason may be that, compared with Sm(III) complexes with organic ligands, there are no C–H, O–H or N–H bonds in [Sm(NO3)6]3−, which can increase non-radiative transitions.21 Furthermore, the lifetime for [MC1mim]3[Sm(NO3)6] in acetonitrile (114.4 μs) is longer than in its solid state (98.5 μs). The shorter lifetime in the solid state can be explained by the C–H vibration in [MC1mim]+. However, if a small amount of water was added to these complexes, their lifetime decreased significantly. For example, when 50 μL water was added to 5 mL 4 ([C6mim]3[Sm(NO3)6]), the lifetime of 4 sharply decreased from 114.8 μs to 3.75 μs. Moreover, the lifetime of Sm(NO3)3·6H2O (6.91 μs) is much shorter than those of complexes 1–5. This can be explained by the presence of water molecules in the first coordination sphere. The resulting data show that water is a quencher for these complexes and water-free Sm(III)-containing ILs can effectively increase their luminescent lifetime. The quantum yields of complexes 1 and 4 are 2.12% and 2.73%, respectively. These values are greater than those of Sm-containing ILs.10,11
Experimental Materials All chemical reagents and solvents of analytical grade were purchased from standard commercial sources. The syntheses of 1,2,3-trimethylimidazolium iodide ([MC1mim]I), 1,2,3-trimethylimidazolium nitrate ([MC1mim]NO3), 1-alkyl-3-methylimidazolium bromides ([Cnmim]Br, n = 2, 4, 6 and 8), and
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1-alkyl-3-methylimidazolium nitrates ([Cnmim]NO3, n = 2, 4, 6 and 8) were carried out by standard procedures.13,17
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General methods Infrared spectra (IR) were recorded on a NEXUS 670 FT-IR spectrometer using KBr pellets. 1H and 13C NMR spectra were recorded on a Bruker 400 MHz nuclear magnetic resonance spectrometer operating at 400 and 100 MHz, respectively, with [D6]DMSO as locking solvent unless otherwise stated. 1H and 13 C chemical shifts were reported in ppm relative to TMS. Melting points were obtained on a TA Q20 calorimeter with a N2 flow rate of 20 mL min−1. Measurements were performed from −85 °C to 150 °C at a scan rate of 10 °C min−1. Thermogravimetric analysis (TGA) measurements were performed on a NETZSCH TG 209 F1 thermogravimetric analyzer from 25 to 600 °C at 10 °C min−1. Elemental analyses (H, C, N) were performed on an Elementar Vario MICRO CUBE elemental analyzer. Fluorescence spectra were recorded using a Hitachi F-7000 fluorescence spectrophotometer with a xenon lamp as excitation source and a photomultiplier tube for detection. Excitation and emission spectra were collected at a 10 nm band pass at 293 K. Luminescent lifetimes were recorded on a HORIBA TEMPRO-01 transient state fluorescence spectrometer at 293 K. Quantum yields were measured by a HORIBA FluoroMax-4 fluorescence spectrophotometer with an integrating sphere. Powder X-ray diffraction (PXRD) data were obtained using a HAO YUAN DX-1000 X-ray diffractometer.
Tri(1,2,3-trimethylimidazolium) hexanitratosamarate (1). 1,2,3-Trimethylimidazolium nitrate (519 mg, 3 mmol), samarium(III) nitrate hexahydrate (444 mg, 1 mmol) and acetonitrile (20 mL) were combined in a 50 mL flask in a molar ratio of 3 : 1 to give a solution. The reaction vessel was heated statically at 50 °C for 36 h. The clear solution was dried to yield a white solid in quantitative yield. Transparent colourless plate crystals of complex 1 suitable for X-ray diffraction determination were recrystallized from acetonitrile–ethyl acetate. 1H NMR (400 MHz, [D6]DMSO, 25 °C, TMS): δ = 7.58 (s, 2H), 3.75 (s, 6H), 2.55 (s, 3H) ppm; 13C NMR (100 MHz, [D6]DMSO, 25 °C, TMS): δ = 144.74, 121.94, 34.65, 9.08 ppm; IR (KBr, 25 °C): ν = 3148, 2962, 2472, 2347, 1781, 1748, 1594, 1446, 1319, 1128, 1093, 1040, 819, 738, 656 cm−1; elemental analysis: calcd(%) for 1 (C18H33SmN12O18, 855.88): C 25.26, H 3.89, N 19.64; found: C 25.43, H 3.89, N 19.73. Tri(1-ethyl-3-methylimidazolium) hexanitratosamarate (2). An analogous route was employed for 2. 1-Ethyl-3-methylimidazolium nitrate (519 mg, 3 mmol) and samarium(III) nitrate hexahydrate (444 mg, 1 mmol) were reacted in acetonitrile to give a pale yellow solid in quantitative yield. 1H NMR (400 MHz, [D6]DMSO, 25 °C, TMS): δ = 9.13 (s, 1H), 7.78 (s, 1H), 7.69 (s, 1H), 4.19 (q, 3J (H,H) = 7.2 Hz, 2H; CH2), 3.84 (s, 3H), 1.41 (t, 3J (H,H) = 7.2 Hz, 3H; CH3) ppm; 13C NMR (100 MHz, [D6]DMSO, 25 °C, TMS): δ = 136.30, 123.59, 122.00,
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44.14, 35.69, 15.13 ppm; IR (KBr, 25 °C): ν = 3154, 3117, 2988, 2478, 1747, 1572, 1456, 1325, 1167, 1039, 847, 819, 739, 648, 622 cm−1; elemental analysis: calcd(%) for 2 (C18H33SmN12O18, 855.88): C 25.26, H 3.89, N 19.64; found: C 25.22, H 3.94, N 19.65. Tri(1-butyl-3-methylimidazolium) hexanitratosamarate (3). An analogous route was employed for 3. 1-Butyl-3-methylimidazolium nitrate (604 mg, 3 mmol) and samarium(III) nitrate hexahydrate (444 mg, 1 mmol) were reacted in acetonitrile to give a pale yellow liquid in quantitative yield. 1H NMR (400 MHz, [D6]DMSO, 25 °C, TMS): δ = 9.16 (s, 1H), 7.78 (s, 1H), 7.70 (s, 1H), 4.15 (t, 3J (H,H) = 7.2 Hz, 2H; CH2), 3.84 (s, 3H), 1.75 (m, 2H; CH2), 1.24 (m, 2H; CH2), 0.88 (t, 3J (H,H) = 7.4 Hz, 3H; CH3) ppm; 13C NMR (100 MHz, [D6]DMSO, 25 °C, TMS): δ = 136.65, 123.67, 122.34, 48.55, 35.76, 31.42, 18.83, 13.32 ppm; IR (KBr, 25 °C): ν = 3150, 3113, 2963, 2875, 1568, 1444, 1325, 1166, 1039, 851, 820, 739, 653, 624 cm−1; elemental analysis: calcd(%) for 3 (C24H45SmN12O18, 940.04): C 30.66, H 4.83, N 17.88; found: C 30.26, H 5.08, N 18.03. Tri(1-hexyl-3-methylimidazolium) hexanitratosamarate (4). An analogous route was employed for 4. 1-Hexyl-3-methylimidazolium nitrate (688 mg, 3 mmol) and samarium(III) nitrate hexahydrate (444 mg, 1 mmol) were reacted in acetonitrile to give a pale yellow liquid in quantitative yield. 1H NMR (400 MHz, [D6]DMSO, 25 °C, TMS): δ = 9.16 (s, 1H), 7.78 (s, 1H), 7.70 (s, 1H), 4.15 (t, 3J (H,H) = 7.2 Hz, 2H; CH2), 3.84 (s, 3H), 1.76 (m, 2H; CH2), 1.24 (m, 6H; CH2), 0.84 (t, 3J (H,H) = 6.9 Hz, 3H; CH3) ppm; 13C NMR (100 MHz, [D6]DMSO, 25 °C, TMS): δ = 136.66, 123.68, 122.34, 48.83, 35.76, 30.61, 29.41, 25.21, 21.94, 13.90 ppm; IR (KBr, 25 °C): ν = 3150, 3113, 2957, 2932, 2863, 1570, 1461, 1329, 1165, 1036, 851, 821, 740, 655, 624 cm−1; elemental analysis: calcd(%) for 4 (C30H57SmN12O18, 1024.20): C 35.18, H 5.61, N 16.41; found: C 34.81, H 5.97, N 16.55. Tri(1-octyl-3-methylimidazolium) hexanitratosamarate (5). An analogous route was employed for 5. 1-Octyl-3-methylimidazolium nitrate (772 mg, 3 mmol) and samarium(III) nitrate hexahydrate (444 mg, 1 mmol) were reacted in acetonitrile to give a yellow transparent liquid in quantitative yield. 1 H NMR (400 MHz, [D6]DMSO, 25 °C, TMS): δ = 9.14 (s, 1H), 7.77 (s, 1H), 7.70 (s, 1H), 4.14 (t, 3J (H,H) = 7.2 Hz, 2H; CH2), 3.84 (s, 3H), 1.77 (m, 2H; CH2), 1.25 (m, 10H; CH2), 0.85 (t, 3J (H,H) = 6.8 Hz, 3H; CH3) ppm; 13C NMR (100 MHz, [D6]DMSO, 25 °C, TMS): δ = 136.61, 123.66, 122.32, 48.83, 35.76, 31.21, 29.44, 28.52, 28.38, 25.55, 22.11, 13.99 ppm; IR (KBr, 25 °C): ν = 3149, 3112, 2928, 2857, 1571, 1458, 1326, 1165, 1039, 853, 819, 738, 652, 624 cm−1; elemental analysis: calcd(%) for 5 (C36H69SmN12O18, 1108.36): C 39.01, H 6.27, N 15.16; found: C 38.98, H 6.59, N 14.83. X-ray crystallography Details of the X-ray diffraction analysis of compound 1 are presented. Single crystals of 1 were removed from a test tube, a suitable crystal was selected and attached to a glass fiber, and the data were collected at 143 K on an Xcalibur Eos diffractometer. Using Olex2,22 the structure was solved with the olex2.
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solve23 structure solution program using charge flipping and refined with the ShelXL-201224 refinement package using least squares minimisation. The structure was solved in the space group C2/c by analysis of systematic absences. All non-hydrogen atoms were refined anisotropically, and hydrogen atoms were located and refined. No decomposition was observed during data collection. CCDC-1040595 contains the supplementary crystallographic data for this paper.
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Conclusions Sm(III)-based ILs 1–5, with formulas of [MC1mim]3[Sm(NO3)6] (1) and [Cnmim]3[Sm(NO3)6] (2–5; n = 2, 4, 6 and 8), were first prepared and confirmed by NMR, IR, and elemental analysis. The successful incorporation of a water-free hexanitratosamarate(III) anion [Sm(NO3)6]3−, with twelve oxygen atoms bonded through six nitrate ligands in a bidentate manner, was confirmed by single-crystal X-ray diffraction of complex 1. Complexes 1 and 2 exhibit melting points of 142 and 82 °C, respectively, while complexes 3–5 are typical room-temperature ILs, with glass transition temperatures around −40 °C. All complexes exhibit high stability to heat and decompose in the range of 287–313 °C. Compared to organic ligands like β-diketonates, the nitrate anion is an appropriate ligand to construct ILs with a highly symmetrical coordination environment for a Sm(III) ion. Complexes 1–5 exhibit orange luminescence when irradiated with a UV lamp. Their excitation and emission spectra were examined at room temperature. Strong and sharp emissions of 4 G5/2→6H5/2, 4G5/2→6H7/2, and 4G5/2→6H9/2 transitions can be distinctly observed. The absence of C–H, O–H and N–H bonds in ligands can help prevent the complete quenching of any emission and increase the luminescence lifetime of these Sm(III)-based ILs.
Acknowledgements
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The finance support of NSFC (no. 21103116, 21303108, J1210004), SRF for ROCS, SEM (no. 2012170774), FRFCU (no. 2013SCU04A12), and SSTIC (no. ZDSY20130331145131323) are gratefully acknowledged. We thank the Comprehensive training platform of specialized laboratory, College of chemistry, Sichuan University for instrumental measurements.
Notes and references 1 (a) S. V. Eliseeva and J. C. G. Bünzli, Chem. Soc. Rev., 2010, 39, 189; (b) J. C. G. Bünzli, Chem. Rev., 2010, 110, 2729. 2 K. Binnemans, Chem. Rev., 2009, 109, 4283. 3 (a) N. V. Plechkova and K. R. Seddon, Chem. Soc. Rev., 2008, 37, 123; (b) R. Giernoth, Angew. Chem., Int. Ed., 2010, 49, 2834; (c) G. H. Tao, L. He, W. S. Liu, L. Xu, W. Xiong, T. Wang and Y. Kou, Green Chem., 2006, 8, 639; (d) L. He, G. H. Tao, D. A. Parrish and J. M. Shreeve, J. Phys. Chem. B,
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Long-lived luminescent soft materials of hexanitratosamarate(III) complexes with orange visible emission† Ning Tang, Ying Zhao, Ling He,* Wen-Li Yuan and Guo-Hong Tao* Sm(III)-based ionic liquids incorporating hexanitratosamarate(III) anions were obtained and fully characterized as novel Sm(III)-containing organic complexes. The structure of the ionic liquids was determined by single-crystal X-ray diffraction (1: monoclinic system C2/c space group with cell parameters: a = 19.5624(4) Å, b = 10.11895(18) Å, c = 33.2256(6) Å, β = 101.2912(18)°, Z = 8). The central Sm(III) ion is 12-coordinated by six bidentate nitrate ligands with twelve oxygen donors to form a [Sm(NO3)6]3− anion. The low melting
Received 15th January 2015, Accepted 23rd March 2015 DOI: 10.1039/c5dt00191a www.rsc.org/dalton
point, high thermostability and wide liquid range of these ionic liquids were determined in detail. All the complexes 1–5 display orange luminescence, rather than red luminescence as in most Sm(III)-containing organic complexes. Three characteristic monochromatic bands and an intense emission, derived from G5/2→6HJ ( J = 5/2, 7/2, and 9/2) intraconfigurational f–f transitions, were revealed. All these complexes exhibit long luminescence lifetimes. 4
Introduction Luminescent materials are widely used for organic light emitting diodes (OLEDs), biosensing and bioimaging, etc.1 Functional luminescent soft materials, including ionic liquids (ILs), liquid crystals, and ionogels, have drawn much attention as a growing trend.2 ILs, regarded as green solvents and functional materials, have many excellent inherent properties, such as negligible vapor pressures, wide liquid ranges, good thermal stabilities, considerable electrical conductivities, and wide electrochemical windows.3 Metal-containing ILs can combine the properties of ILs with the additional intrinsic spectroscopic, catalytic or magnetic properties of the metal, and be of more interest as modifiable soft materials.4 Because of their unique luminescent and magnetic properties, lanthanides are considered ideal metals for being incorporated into ILs.5 However, the low solubility of lanthanide complexes in ILs limits their applications.6 In recent years, to increase the concentrations of lanthanides in ILs, many lanthanide-containing ILs have been synthesized and studied.7,8 The main research on luminescent lanthanidedoped ILs has been focused on Eu(III).8 Although little research has focused on the luminescent property of Sm(III)-doped ILs,
College of Chemistry, Sichuan University, Chengdu, Sichuan 610064, China. E-mail: [email protected], [email protected] † Electronic supplementary information (ESI) available: 1H NMR, 13C NMR and IR spectra of 1–5, PXRD spectra of 1 and 2. CCDC 1040595. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5dt00191a
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the emission of Sm(III) is very interesting. Unlike Eu(III) (red color) or Tb(III) (green color), Sm(III) displays visible luminescence (red/orange color) at different wavelengths. The Sm(III) ion, with its 4f 5 orbital configuration, has plenty of energy levels, which lead to emission bands covering the visible and near-infrared (NIR) spectral range. In the visible region, Sm(III) has three characteristic emission bands, which are assigned to 4 G5/2→6H5/2 (ΔJ = 0, magnetic dipole transition), 4G5/2→6H7/2 (ΔJ = 1, magnetic dipole transition), and 4G5/2→6H9/2 (ΔJ = 2, electric dipole transition). However, the emission band of most Sm(III) complexes at around 565 nm is relatively weak. In these situations, Sm(III) complexes can only emit red or orange-red luminescence in the visible region.9 Common ligands in Sm(III)-doped ILs include β-diketone, dipicolinate,10 dicyanonitrosomethanide,11 and thiocyanide,12 etc. However, the disadvantage of β-diketone and dipicolinate is the low photostability of the corresponding complexes in organic solvents.10 Recent studies on nitratolanthanate ILs with [Ln(NO3)3+x]x− anions have shown that the nitrate anion is a stable ligand for constructing novel soft materials including propellants and liquid crystals. The structures of [Ln(NO3)6]3− anions have been determined for La- and Nd-containing ILs.13 Also, for Eu-containing ILs, the [Ln(NO3)5]2− anion has been obtained.8d However, the crystal structure of [Ln(NO3)4]− anions in ILs has still not been confirmed.8e In a nitrate-coordinated system, one or two water molecules may be coordinated to the metal ion and would cause a non-radiative transition derived from the water, and then quench any luminescent emission from the lanthanides.14 Complexes containing
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water-free [Ln(NO3)6]3− anions have already been reported for many lanthanide systems including Ln = La, Ce, Pr, Nd, Eu, Gd, Tb, and Dy, but not Sm.15 Moreover, only a little research has focused on the fluorescence of [Ln(NO3)6]3−, limited to Eu and Nd.13c,15a,c [Eu(NO3)6]3− anions exhibit strong red luminescence and can reduce non-radiative transition effectively. Therefore, the structure and visible fluorescence of the [Sm(NO3)6]3− anion are of interest. Here, we first synthesized water-free hexanitratosamarate ILs. Their unusual orange photoluminescence is illustrated, and a systematic study of their luminescence properties is undertaken.
Results and discussion Synthesis Synthesis routes of complexes 1–5 are shown in Scheme 1. The nitrate ILs [MC1mim][NO3] (MC1mim = 1,2,3-trimethylimidazolium) and [Cnmim][NO3] (Cnmim = 1-alkyl-3-methylimidazolium, n = 2, 4, 6 and 8) were prepared according to the literature method.13 Reaction of samarium(III) nitrate hexahydrate and the corresponding nitrate precursors with a molar ratio of 3 : 1 in acetonitrile gave Sm(III)-containing organic complexes 1–5. The compositions of 1–5 are [MC1mim]3[Sm(NO3)6] (1) and [Cnmim]3[Sm(NO3)6] (2–5) (n = 2, 4, 6 and 8). Slow recrystallization from an acetonitrile–ethyl acetate solution afforded colourless plate crystals of 1 suitable for X-ray diffraction determination. The structures of complexes 1–5 were fully characterized by IR, NMR and elemental analysis. IR spectra The presence of water-free hexanitratosamarate(III) with bidentate nitrates is confirmed from the evidence of infrared spectra. The infrared spectra of complexes 1–5 contain the main vibration bands of the nitrato ligand: ν4 = 1444–1461 cm−1 and ν1 = 1319–1329 cm−1, assigned to N–O asymmetrical stretching vibrations, ν2 = 1036–1040 cm−1, assigned to N–O symmetrical stretching vibrations, along with the out-of-plane bending vibration ν6 = 820 cm−1. No absorbance band corresponding to the ν3(E′) mode of the free symmetric NO3− anion with D3h point group symmetry at about 1385 cm−1 was observed. This indicates that the central Sm(III) is completely surrounded by six chelating nitrate ligands, and
Scheme 1
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Synthesis routes of hexanitratosamarate(III) complexes 1–5.
no free NO3− exists. The difference between ν4 and ν1 is larger than 120 cm−1, indicating strong interactions between the bidentate coordinating ligands and Sm(III). The expected absorbance bands of the imidazolium ring are evident in the infrared spectra. C–H deformation modes can be found in the range 1300–1570 cm−1. C–H stretching modes in the range 2800–3200 cm−1, as well as C–H rocking modes around 740 cm−1, are also observed. Structural characterization The crystal structure of 1 was determined by single-crystal X-ray diffraction. Colourless crystals of 1 of dimensions 0.3 × 0.3 × 0.25 mm were obtained by slow recrystallization from acetonitrile–ethyl acetate at room temperature. The molecular structure of 1 is shown in Fig. 1 and crystallographic data are given in Table 1. Selected bond lengths and angles are summarized in Tables 2 and 3. All non-hydrogen atoms were refined anisotropically. Compound 1 belongs to the monoclinic space group C2/c (no. 15), with three imidazolium cations and two [Sm(NO3)6]3− half-anions contained in each asymmetric unit, which is different from those in [C1mim]3[Ln(NO3)6] (Ln = La, Nd), [SEt3]3[SmI6] and (N4444)3[Sm(dcnm)6].11,13,16 The unique Sm(1) and Sm(2) ions are both located in a special position. Each Sm(III) ion is coordinated by twelve oxygen atoms in a bidentate manner through six nitrato ligands to form two [Sm(NO3)6]3− anions. The polyhedra of these [Sm(NO3)6]3− anions can be described as icosahedra, but the two anions exhibit different symmetry (Fig. 1). The anion based on Sm(2) is on an inversion centre with highly idealised D2h symmetry. For this [Sm(NO3)6]3− anion, all the oxygens are distorted from icosahedral symmetry by the shorter O⋯O distances within the chelating ligands, which is indeed typical for hexanitrato lanthanide complexes.13 However, the anion based on Sm(1) is not the same as usual, with Sm(1) on a twofold crystal axis rather than an inversion centre. This anion clearly exhibits symmetry-lowering distorted icosahedral symmetry. In the unit cell, the different kinds of Sm atoms have the same relative location,
Fig. 1
Crystal structure of complex 1.
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Table 1
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Crystal data and structure refinement for 1
Formula Mw/g mol−1 Size/mm3 Crystal system Space group a/Å b/Å c/Å α/° β/° γ/° V/Å3 Z ρ/g cm−3 T/K μ/mm−1 F(000) λMoKα/Å Reflections Rint Parameters 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
C18H33N12O18Sm 855.91 0.3 × 0.3 × 0.25 Monoclinic C2/c 19.5624(4) 10.11895(18) 33.2256(6) 90 101.2912(18) 90 6449.7(2) 8 1.763 143 1.916 3448 0.71073 5895 0.0273 452 1.074 0.0277 0.0547 0.0339 0.0570 0.49/−0.61
Bond angles in the crystal structure of 1a
O–Sm–O bond angles/° O2–Sm1–O1 49.42(6) 49.42(6) O2i–Sm1–O1i O4–Sm1–O5 49.36(6) 49.36(6) O4i–Sm1–O5i O7–Sm1–O8 49.47(6) 49.47(6) O7i–Sm1–O8i O–N–O bond angles/° O2–N1–O1 117.3(2) O3–N1–O1 121.6(3) O3–N1–O2 121.2(3) O5–N2–O4 117.0(2) O6–N2–O4 121.4(2) O6–N2–O5 121.6(3) O8–N3–O7 116.7(3) O9–N3–O7 121.6(3) O9–N3–O8 121.7(3) O–N–Sm bond angles/° O7–N3–Sm1 57.48(15) O8–N3–Sm1 59.72(14) O9–N3–Sm1 173.0(2) a
O11–Sm2–O10 O11ii–Sm2–O10ii O13–Sm2–O14 O13ii–Sm2–O14ii O16–Sm2–O18ii O16ii–Sm2–O18
49.13(7) 49.13(7) 49.13(7) 49.13(7) 50.04(6) 50.04(6)
O11–N4–O10 O12–N4–O10 O12–N4–O11 O14–N5–O13 O15–N5–O13 O15–N5–O14 O17–N6–O16 O17–N6–O18ii O18ii–N6–O16
116.9(2) 121.5(3) 121.6(3) 117.4(2) 121.8(3) 120.8(3) 122.0(3) 121.8(3) 116.2(2)
O16–N6–Sm2 O17–N6–Sm2 O18ii–N6–Sm2
59.31(14) 177.2(2) 56.96(14)
Symmetry operation: i1 − X, +Y, 1/2 − Z; ii1/2 − X, 1/2 − Y, 1 − Z.
R1 = ∑||Fo| − |Fc||/∑|Fo|. b wR2 = [∑w(Fo2 − Fc2)2/∑w(Fo2)2]1/2.
Table 2
Selected bond lengths in the crystal structure of 1a
Sm–O bond lengths/Å Sm1–O1 2.599(2) 2.599(2) Sm1–O1i Sm1–O2 2.574(2) i 2.574(2) Sm1–O2 Sm1–O4 2.5497(18) 2.5497(18) Sm1–O4i Sm1–O5 2.613(2) 2.613(2) Sm1–O5i Sm1–O7 2.547(2) 2.547(2) Sm1–O7i Sm1–O8 2.595(2) 2.595(2) Sm1–O8i O–N bond lengths/Å O1–N1 1.270(3) O2–N1 1.262(3) O3–N1 1.231(3) O4–N2 1.268(3) O5–N2 1.262(3) O6–N2 1.233(3) O7–N3 1.259(3) O8–N3 1.269(3) O9–N3 1.220(3) a
Table 3
Sm2–O10 Sm2–O10ii Sm2–O11 Sm2–O11ii Sm2–O13 Sm2–O13ii Sm2–O14 Sm2–O14ii Sm2–O16 Sm2–O16ii Sm2–O18 Sm2–O18ii
2.590(2) 2.590(2) 2.608(2) 2.608(2) 2.5839(19) 2.5839(19) 2.603(2) 2.603(2) 2.575(2) 2.575(2) 2.524(2) 2.524(2)
O10–N4 O11–N4 O12–N4 O13–N5 O14–N5 O15–N5 O16–N6 O17–N6 O18–N6ii
1.266(3) 1.271(3) 1.232(3) 1.258(3) 1.266(3) 1.240(3) 1.274(3) 1.220(3) 1.267(3)
Symmetry operation: i1 − X, +Y, 1/2 − Z; ii1/2 − X, 1/2 − Y, 1 − Z.
with each type of Sm atoms located in a plane and two types of planes formed by Sm atoms arranged parallel alternately (Fig. 2). The range of Sm–O bond lengths in 1 is similar to data for the [Sm(NO3)5·H2O]2− anion.14b The same values for the angles around Sm atoms are found in both [Sm(NO3)6]3− anions.
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Fig. 2 cell.
Relative location of Sm1 (bright green) and Sm2 (tan) in the unit
However, the difference between the maximum and minimum O–Sm–O angle is interesting. The difference in the O–Sm2–O angle is 0.91°, which is much bigger than in the other [Ln(NO3)6]3− (Ln = La, Nd) anions with a similar symmetric structure.13b,c However, the difference in the O–Sm1–O angle is only 0.11°, which is similar to that in the [Eu(NO3)5]2− anion but lower than in the [Ln(NO3)6]3− (Ln = La, Nd) anions.8d,13b,c The data indicate that with a decrease in ionic radius, the [Sm(NO3)6]3− anion has a tendency to form another type of structure with lower symmetry. This distorted geometry of the [Sm(NO3)6]3− anion is further confirmed by the torsion angles. Evidence of some multiple bond characters is provided. Delocalization of nitrate anions is recorded. The data of N–O bond distances and O–N–O angles also indicate that the [MC1mim]+ cations are delocalized, with the positive charge dispersed over the aromatic system, analogous to the case found in other imidazolium-based complexes.17
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Fig. 3
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Packing diagram of 1 viewed down the b-axis.
result in weaker electrostatic interactions between the cation and the anion and even lower melting points for complexes 3–5. The thermal stability performance of complexes 1–5 was tested by thermogravimetric analysis (TGA). Measurements indicated that complexes 1–5 exhibit high decomposition temperatures in the range 287 (2) to 313 °C (5), with no neutral water coordinated to the Sm(III) ion. Similar TGA curves were recorded for complexes 1–5, with single-step decomposition. With rising temperature, exothermic decomposition occurs, leaving samarium oxide estimated as the final residue. The considerable thermal stabilities of complexes 1–5 are comparable to those of the La(III) complexes, and close to those of their precursors [Cnmim][NO3].13 Therefore, these Sm(III)based ILs are sufficiently thermally stable materials, with a large liquid range demonstrated. Spectroscopic measurements
The powder X-ray diffraction (PXRD) patterns of complexes 1 and 2 demonstrate that the two complexes have a similar structure and fewer diffraction peaks compared to simulated PXRD from single-crystal structure data of 1 (Fig. S16†). The successful introduction of a Sm(III)-doped anion to produce water-free ILs is confirmed by the single-crystal structure of complex 1. The crystal data implies non-classical hydrogen bonds exist between hydrogen atoms of water and nitrogen atoms in the imidazolium cations (Fig. 3). Furthermore, no hydrogen bonding between the cation and the anion is found. This feature is very different from the packing structure of [C6mim][Sm(tta)4], [C6mim][Sm(nta)4] or [C6mim][Sm(hfa)4].9 These simple ion pairs can be arranged in order, to construct the infinite three-dimensional system of 1. No π-stacking interaction among the imidazolium cations was found. The large volume of the hexanitratosamarate anion may hinder π-stacking of cations. Thus, packing may have a major impact on formation of a [Sm(NO3)6]3− complex with a low melting point.
All complexes 1–5 emit orange photoluminescence under ultraviolet (UV) lamp irradiation (Fig. 4). Complexes 1–5 give rise to similar excitation and emission spectra. Electronic transitions were assigned according to the energy level diagrams of trivalent rare-earth ions. The excitation spectra of complexes 1–5 are complicated (Fig. 5a and 6a). Most of the transitions were assigned to one or two multiplets, or even three multiplets.18 In addition, the typical characteristic transitions, consisting of a broad band between 250 and 330 nm, attributed to n–π* transitions of nitrate ligands, are also found, with a maximum peak at 290 nm. The intensity of this peak is weaker than those of the peaks at 404 nm, 375 nm, 363 nm, and 345 nm, but similar to that of the peak recorded at 480 nm. Therefore, direct f–f transitions, mainly excited by the effect of high symmetry of the inner coordination sphere, are indicated, rather than a contribution of energy transfer derived from n–π* transitions. The emission spectra were recorded at room temperature upon excitation at 404 nm (Fig. 5b and 6b). The emission
Thermal properties The melting points of complexes 1–5 were determined by differential scanning calorimetry (DSC). Measurements of their thermal properties indicated that complexes 2–5 have melting points below 100 °C and therefore can be classified as ILs. Alkyl-substituted imidazolium cations were utilized to produce Sm(III) complexes as ILs with low melting points, associated with delocalization of positive charge on the imidazolium ring. Complex 1, being crystallized for structural study, melts at 142 °C. Complex 2 possesses a melting point of 82 °C, along with a glass transition temperature around −49 °C. The methyl and ethyl groups attached to the imidazolium ring give rise to a major trend to relatively regular packing for the crystallization of 1 and 2. Complexes 3–5 were found to be liquids or supercooled liquids at room temperature. No melting point was recorded. The glass transition temperatures of 3–5 range from −38 °C to −45 °C. The bigger size, lower charge density and lower point symmetry of butyl, hexyl and octyl groups
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Fig. 4 Images of hexanitratosamarate complexes 1-5 under daylight (top) and under ultraviolet irradiation (bottom).
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Fig. 5 (a) Excitation spectrum of complex 2 monitored at 645 nm; (b) Emission spectrum and decay curve (upper right) of complex 2 excited at 404 nm.
Fig. 6
Excitation (a) and emission spectra (b) of complexes 1–5.
peaks correspond to transitions from the 4G5/2 level to the different J levels of the 6H term (6HJ, J = 5/2 − 15/2). Three sharp emission peaks derived from the Sm(III) ion, assigned to 4 G5/2→6H5/2, 4G5/2→6H7/2 and 4G5/2→6H9/2 transitions, are observed at 564, 599 and 645 nm, respectively. These are responsible for the orange luminescence colour. A very weak emission peak at 706 nm is assigned to 4G5/2→6H11/2. The transition of 4G5/2→6H13/2 is so weak as to be almost invisible. The other weak peak at 532 nm corresponds to a transition from a higher emissive state 4F3/2 to the ground state 6H5/2. The three Sm3+ excited states 4G7/2 (∼20 050 cm−1), 4F3/2 (∼18 700 cm−1) and 4G5/2 (∼17 700 cm−1) can be excited efficiently. However, only emission from the 4G5/2 level can be observed intensively.
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A possible reason is that the energy differences between the two higher excited states 4G7/2 and 4F3/2 and 4G5/2 are very small, which can lead electrons to relax to the lower level (4G5/2) quickly. Non-radiative transitions will occur in this process.19 The most intense emission band, assigned to the 4 G5/2→6H7/2 transition of 1–5, is found at 599 nm. Moreover, the typical transitions of 4G5/2→6H5/2 and 4G5/2→6H9/2 are also intense, located at 564 nm and 645 nm. In the Sm(III) system, the transition of 4G5/2→6H5/2 located at 564 nm, exhibits a predominant magnetic dipole character. The 4G5/2→6H9/2 transition at 645 nm is hypersensitive.10 Generally, the symmetry of the coordination structure can influence the emission spectra of lanthanide ions.8b,9,10 The structures of usual Sm-containing complexes with lower symmetry give rise to red fluorescence, whereas those with higher symmetry may emit orange-red light.9,10 In our system, the highly symmetrical [Sm(NO3)6]3− anion probably results in a decrease in the 4G5/2→6H9/2 emission. The intensity ratio I(4G5/2→6H9/2)/I(4G5/2→6H5/2) can be calculated to have a relatively low value. The orange colour of the photoluminescence from 1–5 results from the enhanced intensity of the 4 G5/2→6H5/2 emission, which is stronger than usual and contributes yellow to the overall colour. The decay curves of all complexes are found to be monoexponential, indicating that only one Sm(III) species is present. The luminescence lifetimes of complexes 1–5 vary from 114.4 to 130.3 μs in acetonitrile, which is longer than those of most Sm(III) complexes.9,20 The reason may be that, compared with Sm(III) complexes with organic ligands, there are no C–H, O–H or N–H bonds in [Sm(NO3)6]3−, which can increase non-radiative transitions.21 Furthermore, the lifetime for [MC1mim]3[Sm(NO3)6] in acetonitrile (114.4 μs) is longer than in its solid state (98.5 μs). The shorter lifetime in the solid state can be explained by the C–H vibration in [MC1mim]+. However, if a small amount of water was added to these complexes, their lifetime decreased significantly. For example, when 50 μL water was added to 5 mL 4 ([C6mim]3[Sm(NO3)6]), the lifetime of 4 sharply decreased from 114.8 μs to 3.75 μs. Moreover, the lifetime of Sm(NO3)3·6H2O (6.91 μs) is much shorter than those of complexes 1–5. This can be explained by the presence of water molecules in the first coordination sphere. The resulting data show that water is a quencher for these complexes and water-free Sm(III)-containing ILs can effectively increase their luminescent lifetime. The quantum yields of complexes 1 and 4 are 2.12% and 2.73%, respectively. These values are greater than those of Sm-containing ILs.10,11
Experimental Materials All chemical reagents and solvents of analytical grade were purchased from standard commercial sources. The syntheses of 1,2,3-trimethylimidazolium iodide ([MC1mim]I), 1,2,3-trimethylimidazolium nitrate ([MC1mim]NO3), 1-alkyl-3-methylimidazolium bromides ([Cnmim]Br, n = 2, 4, 6 and 8), and
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1-alkyl-3-methylimidazolium nitrates ([Cnmim]NO3, n = 2, 4, 6 and 8) were carried out by standard procedures.13,17
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General methods Infrared spectra (IR) were recorded on a NEXUS 670 FT-IR spectrometer using KBr pellets. 1H and 13C NMR spectra were recorded on a Bruker 400 MHz nuclear magnetic resonance spectrometer operating at 400 and 100 MHz, respectively, with [D6]DMSO as locking solvent unless otherwise stated. 1H and 13 C chemical shifts were reported in ppm relative to TMS. Melting points were obtained on a TA Q20 calorimeter with a N2 flow rate of 20 mL min−1. Measurements were performed from −85 °C to 150 °C at a scan rate of 10 °C min−1. Thermogravimetric analysis (TGA) measurements were performed on a NETZSCH TG 209 F1 thermogravimetric analyzer from 25 to 600 °C at 10 °C min−1. Elemental analyses (H, C, N) were performed on an Elementar Vario MICRO CUBE elemental analyzer. Fluorescence spectra were recorded using a Hitachi F-7000 fluorescence spectrophotometer with a xenon lamp as excitation source and a photomultiplier tube for detection. Excitation and emission spectra were collected at a 10 nm band pass at 293 K. Luminescent lifetimes were recorded on a HORIBA TEMPRO-01 transient state fluorescence spectrometer at 293 K. Quantum yields were measured by a HORIBA FluoroMax-4 fluorescence spectrophotometer with an integrating sphere. Powder X-ray diffraction (PXRD) data were obtained using a HAO YUAN DX-1000 X-ray diffractometer.
Tri(1,2,3-trimethylimidazolium) hexanitratosamarate (1). 1,2,3-Trimethylimidazolium nitrate (519 mg, 3 mmol), samarium(III) nitrate hexahydrate (444 mg, 1 mmol) and acetonitrile (20 mL) were combined in a 50 mL flask in a molar ratio of 3 : 1 to give a solution. The reaction vessel was heated statically at 50 °C for 36 h. The clear solution was dried to yield a white solid in quantitative yield. Transparent colourless plate crystals of complex 1 suitable for X-ray diffraction determination were recrystallized from acetonitrile–ethyl acetate. 1H NMR (400 MHz, [D6]DMSO, 25 °C, TMS): δ = 7.58 (s, 2H), 3.75 (s, 6H), 2.55 (s, 3H) ppm; 13C NMR (100 MHz, [D6]DMSO, 25 °C, TMS): δ = 144.74, 121.94, 34.65, 9.08 ppm; IR (KBr, 25 °C): ν = 3148, 2962, 2472, 2347, 1781, 1748, 1594, 1446, 1319, 1128, 1093, 1040, 819, 738, 656 cm−1; elemental analysis: calcd(%) for 1 (C18H33SmN12O18, 855.88): C 25.26, H 3.89, N 19.64; found: C 25.43, H 3.89, N 19.73. Tri(1-ethyl-3-methylimidazolium) hexanitratosamarate (2). An analogous route was employed for 2. 1-Ethyl-3-methylimidazolium nitrate (519 mg, 3 mmol) and samarium(III) nitrate hexahydrate (444 mg, 1 mmol) were reacted in acetonitrile to give a pale yellow solid in quantitative yield. 1H NMR (400 MHz, [D6]DMSO, 25 °C, TMS): δ = 9.13 (s, 1H), 7.78 (s, 1H), 7.69 (s, 1H), 4.19 (q, 3J (H,H) = 7.2 Hz, 2H; CH2), 3.84 (s, 3H), 1.41 (t, 3J (H,H) = 7.2 Hz, 3H; CH3) ppm; 13C NMR (100 MHz, [D6]DMSO, 25 °C, TMS): δ = 136.30, 123.59, 122.00,
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44.14, 35.69, 15.13 ppm; IR (KBr, 25 °C): ν = 3154, 3117, 2988, 2478, 1747, 1572, 1456, 1325, 1167, 1039, 847, 819, 739, 648, 622 cm−1; elemental analysis: calcd(%) for 2 (C18H33SmN12O18, 855.88): C 25.26, H 3.89, N 19.64; found: C 25.22, H 3.94, N 19.65. Tri(1-butyl-3-methylimidazolium) hexanitratosamarate (3). An analogous route was employed for 3. 1-Butyl-3-methylimidazolium nitrate (604 mg, 3 mmol) and samarium(III) nitrate hexahydrate (444 mg, 1 mmol) were reacted in acetonitrile to give a pale yellow liquid in quantitative yield. 1H NMR (400 MHz, [D6]DMSO, 25 °C, TMS): δ = 9.16 (s, 1H), 7.78 (s, 1H), 7.70 (s, 1H), 4.15 (t, 3J (H,H) = 7.2 Hz, 2H; CH2), 3.84 (s, 3H), 1.75 (m, 2H; CH2), 1.24 (m, 2H; CH2), 0.88 (t, 3J (H,H) = 7.4 Hz, 3H; CH3) ppm; 13C NMR (100 MHz, [D6]DMSO, 25 °C, TMS): δ = 136.65, 123.67, 122.34, 48.55, 35.76, 31.42, 18.83, 13.32 ppm; IR (KBr, 25 °C): ν = 3150, 3113, 2963, 2875, 1568, 1444, 1325, 1166, 1039, 851, 820, 739, 653, 624 cm−1; elemental analysis: calcd(%) for 3 (C24H45SmN12O18, 940.04): C 30.66, H 4.83, N 17.88; found: C 30.26, H 5.08, N 18.03. Tri(1-hexyl-3-methylimidazolium) hexanitratosamarate (4). An analogous route was employed for 4. 1-Hexyl-3-methylimidazolium nitrate (688 mg, 3 mmol) and samarium(III) nitrate hexahydrate (444 mg, 1 mmol) were reacted in acetonitrile to give a pale yellow liquid in quantitative yield. 1H NMR (400 MHz, [D6]DMSO, 25 °C, TMS): δ = 9.16 (s, 1H), 7.78 (s, 1H), 7.70 (s, 1H), 4.15 (t, 3J (H,H) = 7.2 Hz, 2H; CH2), 3.84 (s, 3H), 1.76 (m, 2H; CH2), 1.24 (m, 6H; CH2), 0.84 (t, 3J (H,H) = 6.9 Hz, 3H; CH3) ppm; 13C NMR (100 MHz, [D6]DMSO, 25 °C, TMS): δ = 136.66, 123.68, 122.34, 48.83, 35.76, 30.61, 29.41, 25.21, 21.94, 13.90 ppm; IR (KBr, 25 °C): ν = 3150, 3113, 2957, 2932, 2863, 1570, 1461, 1329, 1165, 1036, 851, 821, 740, 655, 624 cm−1; elemental analysis: calcd(%) for 4 (C30H57SmN12O18, 1024.20): C 35.18, H 5.61, N 16.41; found: C 34.81, H 5.97, N 16.55. Tri(1-octyl-3-methylimidazolium) hexanitratosamarate (5). An analogous route was employed for 5. 1-Octyl-3-methylimidazolium nitrate (772 mg, 3 mmol) and samarium(III) nitrate hexahydrate (444 mg, 1 mmol) were reacted in acetonitrile to give a yellow transparent liquid in quantitative yield. 1 H NMR (400 MHz, [D6]DMSO, 25 °C, TMS): δ = 9.14 (s, 1H), 7.77 (s, 1H), 7.70 (s, 1H), 4.14 (t, 3J (H,H) = 7.2 Hz, 2H; CH2), 3.84 (s, 3H), 1.77 (m, 2H; CH2), 1.25 (m, 10H; CH2), 0.85 (t, 3J (H,H) = 6.8 Hz, 3H; CH3) ppm; 13C NMR (100 MHz, [D6]DMSO, 25 °C, TMS): δ = 136.61, 123.66, 122.32, 48.83, 35.76, 31.21, 29.44, 28.52, 28.38, 25.55, 22.11, 13.99 ppm; IR (KBr, 25 °C): ν = 3149, 3112, 2928, 2857, 1571, 1458, 1326, 1165, 1039, 853, 819, 738, 652, 624 cm−1; elemental analysis: calcd(%) for 5 (C36H69SmN12O18, 1108.36): C 39.01, H 6.27, N 15.16; found: C 38.98, H 6.59, N 14.83. X-ray crystallography Details of the X-ray diffraction analysis of compound 1 are presented. Single crystals of 1 were removed from a test tube, a suitable crystal was selected and attached to a glass fiber, and the data were collected at 143 K on an Xcalibur Eos diffractometer. Using Olex2,22 the structure was solved with the olex2.
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solve23 structure solution program using charge flipping and refined with the ShelXL-201224 refinement package using least squares minimisation. The structure was solved in the space group C2/c by analysis of systematic absences. All non-hydrogen atoms were refined anisotropically, and hydrogen atoms were located and refined. No decomposition was observed during data collection. CCDC-1040595 contains the supplementary crystallographic data for this paper.
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Conclusions Sm(III)-based ILs 1–5, with formulas of [MC1mim]3[Sm(NO3)6] (1) and [Cnmim]3[Sm(NO3)6] (2–5; n = 2, 4, 6 and 8), were first prepared and confirmed by NMR, IR, and elemental analysis. The successful incorporation of a water-free hexanitratosamarate(III) anion [Sm(NO3)6]3−, with twelve oxygen atoms bonded through six nitrate ligands in a bidentate manner, was confirmed by single-crystal X-ray diffraction of complex 1. Complexes 1 and 2 exhibit melting points of 142 and 82 °C, respectively, while complexes 3–5 are typical room-temperature ILs, with glass transition temperatures around −40 °C. All complexes exhibit high stability to heat and decompose in the range of 287–313 °C. Compared to organic ligands like β-diketonates, the nitrate anion is an appropriate ligand to construct ILs with a highly symmetrical coordination environment for a Sm(III) ion. Complexes 1–5 exhibit orange luminescence when irradiated with a UV lamp. Their excitation and emission spectra were examined at room temperature. Strong and sharp emissions of 4 G5/2→6H5/2, 4G5/2→6H7/2, and 4G5/2→6H9/2 transitions can be distinctly observed. The absence of C–H, O–H and N–H bonds in ligands can help prevent the complete quenching of any emission and increase the luminescence lifetime of these Sm(III)-based ILs.
Acknowledgements
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The finance support of NSFC (no. 21103116, 21303108, J1210004), SRF for ROCS, SEM (no. 2012170774), FRFCU (no. 2013SCU04A12), and SSTIC (no. ZDSY20130331145131323) are gratefully acknowledged. We thank the Comprehensive training platform of specialized laboratory, College of chemistry, Sichuan University for instrumental measurements.
Notes and references 1 (a) S. V. Eliseeva and J. C. G. Bünzli, Chem. Soc. Rev., 2010, 39, 189; (b) J. C. G. Bünzli, Chem. Rev., 2010, 110, 2729. 2 K. Binnemans, Chem. Rev., 2009, 109, 4283. 3 (a) N. V. Plechkova and K. R. Seddon, Chem. Soc. Rev., 2008, 37, 123; (b) R. Giernoth, Angew. Chem., Int. Ed., 2010, 49, 2834; (c) G. H. Tao, L. He, W. S. Liu, L. Xu, W. Xiong, T. Wang and Y. Kou, Green Chem., 2006, 8, 639; (d) L. He, G. H. Tao, D. A. Parrish and J. M. Shreeve, J. Phys. Chem. B,
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