Article pubs.acs.org/IC
Fine Tuning of Multicolored Photoluminescence in Crystalline Magnetic Materials Constructed of Trimetallic EuxTb1−x[Co(CN)6] Cyanido-Bridged Chains Szymon Chorazy,*,†,‡ Kunal Kumar,† Koji Nakabayashi,† Barbara Sieklucka,‡ and Shin-ichi Ohkoshi*,† †
Department of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Kraków, Poland
‡
S Supporting Information *
ABSTRACT: Coordination compounds built of trivalent lanthanide ions have been demonstrated as promising solidstate materials for diverse photoluminescent applications and as attractive magnetic objects with the prospective application in information storage and spintronics. We present a synthetic methodology in which both luminescent and magnetic functionalities are induced within lanthanide-based coordination polymers by the application of hexacyanocobaltate(III) anions and 3-hydroxypyridine (3-OHpy), both coordinated to 4f-metal ions modulating the lanthanide-centered properties. We report a series of trimetallic cyanido-bridged chains {[EuIIIxTbIII1−x(3-OHpy)2(H2O)4][CoIII(CN)6]}·H2O (x = 1, 0.8, 0.5, 0.4, 0.3, 0.2, 0.1, 0; compounds 1, 2, ..., 7, 8). They reveal tunable visible photoluminescence ranging from green, through yellow and orange, to red color depending on the composition of material and the wavelength of UV excitation light. Such multicolored emission is realized by the adjusted ratio between red emissive Eu3+ and green emissive Tb3+ and by the selection of wavelengths of UV light controlling the intensities of Eu- and Tb-based components of visible luminescence. The photoluminescence is enhanced by the energy-transfer (ET) process from [CoIII(CN)6]3− and 3-OHpy to lanthanides, and the efficiencies of ET to Eu and Tb play an important role in the switchable emission. The whole family, 1−8, exhibits temperature-dependent paramagnetism due to the intrinsic property of lanthanide(3+) ions. Tb-containing 2−8 reveal the field-induced slow relaxation of magnetization due to the magnetic anisotropy of Tb3+. Moreover, the compounds built of large amounts of Tb reveal the double relaxation, the faster of a typical TbIII singleion origin, and the slower originating from the magnetic dipole−magnetic dipole interactions between neighboring TbIII centers. Compound 2, which can be considered as a magnetically diluted sample, exhibits almost single relaxation process with the thermal energy barrier ΔE/kB of 35.8(6) K and τ0 = 1.1(2) × 10−8 s at Hdc = 1500 Oe, indicating a single-molecule magnet behavior.
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INTRODUCTION
magnetization which is related to the energy barrier (ΔE) of spin inversion.23 Below blocking temperature, magnetically anisotropic lanthanide-based coordination molecules and chains, named single-molecule magnets (SMMs) and singlechain magnets (SCMs), respectively, show magnetic hysteresis of the molecular origin which opens the applications in high density information storage and molecular spintronics.24,25 Lanthanide-based SMMs are constructed of coordination clusters of f-metal ions,26,27 and mixed d/f-metal ions,21,28,29 or even mononuclear 4f-metal complexes, often called singleion magnets (SIMs).30,31 Among them, the highest energy barriers were found for DyIII and TbIII,32,33 however magnetic anisotropy of other 4f-metals, e.g., ErIII or YbIII,34,35 was also explored.
Trivalent lanthanide ions embedded in coordination systems arouse a considerable scientific interest due to the attractive photoluminescent properties.1−5 Lanthanide-based materials found diverse applications as tunable laser systems, lightemitting devices, amplifiers for optical communication, lowenergy scintillators, optical storage materials, devices for light conversion, chemical sensors, molecular thermometers, photovoltaic light concentrators, and bioimaging tools.3−10 Thus, there is a strong need to search for novel coordination compounds built of 4f-metal ions revealing effective luminescence, in particular such light-emitting functionalities as white light,11−13 or multicolored tunable emission,14−16 near-infrared phosphorescence,17,18 and up-conversion luminescence.19,20 Complexes of lanthanides are also broadly investigated due to their remarkable magnetic properties.21−23 They can exhibit strong magnetic anisotropy leading to slow relaxation of © 2017 American Chemical Society
Received: February 9, 2017 Published: April 18, 2017 5239
DOI: 10.1021/acs.inorgchem.7b00369 Inorg. Chem. 2017, 56, 5239−5252
Article
Inorganic Chemistry
recent results on DyIII−[CoIII(CN)6]3− coordination systems, we focused our attention of lanthanide complexes with pyridine-based ligands that are able to induce diverse coordination topologies amending the magnetic and optical functionalities of cyanido-bridged materials.52,62−64 Here, we present the synthesis, crystal structure, and detailed magnetic and optical properties of a unique family of crystalline magnetoluminescent materials constructed of a series of trimetallic cyanido-bridged {[Eu I I I x Tb I I I 1 − x (3-OHpy) 2 (H 2 O) 4 ] [CoIII(CN)6]}·H2O (x = 1, 0.8, 0.5, 0.4, 0.3, 0.2, 0.1, 0; compounds 1, 2, ..., 7, 8; 3-OHpy =3-hydroxypyridine; Scheme 1) chains combining the field-induced slow magnetic relaxation
In pursuit of multifunctional materials, that can give a new quality by combining two or more physical properties,36 luminescent molecule-based magnets based on lanthanides(3+) ions appear as a promising idea.37−50 Such bifunctionality was realized for magnetically ordered systems built of emissive 4fmetal ions and polycyanidometallates. 37−40 However, lanthanides(3+) ions can give only weak magnetic coupling, limiting their application in design of luminescent magnets of high critical temperatures.38 On the contrary, they offer strong magnetic anisotropy which,22 together with a diversity of luminescent effects,1 results in a great potential toward emissive SMMs. In fact, a considerable number of photoluminescent SMMs constructed of DyIII, ErIII, and YbIII complexes were reported.41−50 The reports on lanthanide-based SMMs focused the main attention on the correlation between magnetic and optical properties, that is the optical estimation of anisotropic energy barrier, ΔE, from the high-resolution low-temperature emission spectra, while the possibly attractive luminescent effects of lanthanide(3+) complexes were not explored.41−50 In this regard, lately, we decided to take fresh and much more comprehensive approach toward emissive SMMs that truly combines adjustable magnetic characteristics with tunable lightemitting functionalities such as multicolored, white, or nearinfrared emission. To achieve this, we explored the rare materials based on bimetallic coordination polymers constructed of emissive lanthanide(III) complexes combined with red emissive hexacyanocobaltate(III) anion. Such a synthetic approach is part of a more general methodology of combining 4f metal ions with polycyanidometallates which was realized in our group, and by others, resulting in several types of functional materials with interesting magnetic, optical, and thermal expansion properties.51−58 We have recently prepared DyIII(3-hydroxypyridine)− [CoIII(CN)6]3− material based on bimetallic cyanido-bridged chains which reveals DyIII-centered white light emission coexisting with SMM behavior of strongly anisotropic DyIII complexes with a high thermal energy barrier of 385 K.59 In this report, we proved that a diamagnetic and red emissive [CoIII(CN)6]3− linker can modulate both the magnetic and optical properties of DyIII, being a good sensitizer for DyIII emission, and extorting the axially elongated geometry of DyIII complexes. Recently, we presented also the layered DyIII(4hydroxypyridine)−[CoIII(CN)6]3− material in which another attractive luminescent property, that is the excitation switchable emission, originating from the equilibrium between properties of DyIII and organic ligand, can be induced within the cyanidobridged skeleton, and such a property coexists with a fieldinduced slow magnetic relaxation.60 These reports indicated that new bifunctional magneto-luminescent materials can be obtained by rational design from the self-assembly of hexacyanocobaltate(III) and complexes of lanthanides(III) with selected organic ligands. Prompted by this discovery, we aimed at the preparation of functional materials combining tunable photoluminescence and SMM behavior that are constructed of EuIII and TbIII complexes which are known to reveal the strongest visible emission among lanthanides, in the red and green range, respectively.1 While Eu(3+) ions do not offer interesting magnetic properties due to the diamagnetic 7F0 ground state,54 Tb(3+) ions of the 7F6 ground state are often strongly magnetically anisotropic, leading to SMM characteristics with large energy barriers,22,32 but their magnetoluminescent potential is not well recognized.61 Following our
Scheme 1. Investigated Molecular Building Blocks Together with the Synthetic Eu/Tb Ratio and the Formulae of the Resulting Heterometallic Coordination Polymers, 1−8
of TbIII complexes with multicolored photoluminescence tunable both by the Eu/Tb ratio and by wavelength of UV excitation light.
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EXPERIMENTAL SECTION
Materials. Europium(III) chloride hexahydrate (EuIIICl3·6H2O, CAS: 13759-92-7), terbium(III) chloride hexahydrate (TbIIICl3·6H2O, CAS: 13798-24-8), potassium he xacyanocobaltate(III) (K3[CoIII(CN)6], CAS: 13963-58-1), and 3-hydroxypyridine (3OHpy, 3-pyridinol, CAS: 109-00-2) were reagent grade and purchased from commercial sources (Wako Pure Chemical Industries, Ltd. and Sigma-Aldrich) and used without additional purification. Synthetic Procedure. 1 was synthesized using the following procedure: The 300 mg (0.9 mmol) portion of K3[CoIII(CN)6] was dissolved in 2 mL of water, and the resulting light yellow solution A was stirred for a few minutes. The 220 mg (0.6 mmol) portion of EuIIICl3·6H2O and the 115 mg (1.2 mmol) portion of 3-OHpy were combined together with 2.5 mL of water. The resulting mixture was stirred with continuous heating up to the boiling point to obtain the colorless solution B. Hot solution B was quickly added to the cold solution A giving a small amount of white precipitate which was separated by filtration. The resulting light yellow solution was closed and left for crystallization for 3 h. Then, the block shaped crystals of 1 were collected by filtration, washed by a small amount of cold water, and later heavily with ethanol which was followed by drying the crystals in the air. The crystals of 1 were air-stable, and their composition was found by the single-crystal X-ray diffraction analysis, supported by the CHN and metals (ICP-MS) elemental analyses. Yield was calculated based on lanthanide as the excess of K3[CoIII(CN)6] was used. We found that the simple 1:1 molar ratio between CoIII and EuIII resulted in the crystals of the moderate quality which was improved by the addition of extra K3[CoIII(CN)6] precursor. All other compounds, 2−8, were prepared following the analogous procedure as described for 1, using the appropriate mixture of EuIIICl3· 6H2O and TbIIICl3·6H2O, keeping the total 0.6 mmol portion of lanthanide(III) salts. The observations and the treatment of the product during the synthesis and basic characterization of 2−8 were 5240
DOI: 10.1021/acs.inorgchem.7b00369 Inorg. Chem. 2017, 56, 5239−5252
Article
Inorganic Chemistry Table 1. Crystal Data and Structure Refinement for 1, 3, and 8 compound formula formula weight T [K] λ [Å] crystal system space group unit cell
1
a [Å] b [Å] c [Å] β [deg]
V [Å3] Z calcd density abs. coefficient F(000) crystal size [mm3] crystal type Θ range [deg] limiting indices
collected refls unique refls Rint completeness data/restraints/parameters GOF on F2 final R indices largest diff peak/hole
3
Eu1Co1C16H20N8O7 647.29 g·mol−1
15.8502(7) 9.2263(4) 17.6466(8) 116.564(8) 2308.2(2) 4 1.863 g·cm−3 3.464 cm−1 1272 0.18 × 0.08 × 0.07 colorless blocks 3.204−25.242 −20 < h < 20 −11 < k < 11 −22 < l < 22 10866 2629 0.0282 99.8% 2629/6/186 1.311 R1= 0.0269 [I > 2σ(I)] wR2 = 0.0554 (all) 1.086/−2.004 e·A−3
8
Eu0.5Tb0.5 Co1C16H20N8O7 650.77 g·mol−1 100(2) 0.71075 (Mo Kα) monoclinic C 2/c 15.8179(5) 9.2054(3) 17.6221(5) 116.600(8) 2294.36(19) 4 1.884 g·cm−3 3.660 cm−1 1276 0.13 × 0.13 × 0.12
Tb1Co1C16H20N8O7 654.25 g·mol−1
15.8279(6) 9.1964(4) 17.6223(7) 116.614(8) 2293.3(2) 4 1.895 g·cm−3 3.836 cm−1 1280 0.13 × 0.05 × 0.04
3.211−27.478 −20 < h < 20 −11 < k < 10 −22 < l < 21 10908 2628 0.0179 99.8% 2628/6/186 1.213 R1= 0.0192 [I > 2σ(I)] wR2 = 0.0390 (all) 0.877/−1.384 e·A−3
3.212−27.456 −18 < h < 20 −11 < k < 11 −22 < l < 22 10859 2618 0.0687 99.8% 2618/13/186 1.164 R1= 0.0480 [I > 2σ(I)] wR2 = 0.0789 (all) 1.911/−4.171 e·A−3
Eu, 7.0%; Tb, 17.0%; Co, 9.0%; C, 29.5%; H, 3.1%; N, 17.2%. Found: Eu, 7.4%; Tb, 16.6%; Co, 8.6%; C, 29.2%; H, 3.2%; N, 17.3%. IR spectrum (KBr, cm−1). CN− stretching vibrations: 2162w, 2143vs, 2135vs. {[EuIII0.2TbIII0.8(3-OHpy)2(H2O)4][CoIII(CN)6]·H2O}n (6). Yield: 289 mg, 74%. Anal. calcd for Eu0.2Tb0.8Co1C16H20N8O7 (MW = 652.9 g·mol−1): Eu, 4.7%; Tb, 19.5%; Co, 9.0%; C, 29.4%; H, 3.1%; N, 17.2%. Found: Eu, 4.9%; Tb, 19.6%; Co, 8.8%; C, 29.0%; H, 3.1%; N, 17.3%. IR spectrum (KBr, cm−1). CN− stretching vibrations: 2162w, 2144vs, 2136vs. {[EuIII0.1TbIII0.9(3-OHpy)2(H2O)4][CoIII(CN)6]·H2O}n (7). Yield: 234 mg, 60%. Anal. calcd for Eu0.1Tb0.9Co1C16H20N8O7 (MW = 653.5 g·mol−1): Eu, 2.3%; Tb, 21.9%; Co, 9.0%; C, 29.4%; H, 3.1%; N, 17.2%. Found: Eu, 2.5%; Tb, 22.1%; Co, 8.8%; C, 29.1%; H, 3.2%; N, 17.2%. IR spectrum (KBr, cm−1). CN− stretching vibrations: 2162w, 2144vs, 2135vs. {[TbIII(3-OHpy)2(H2O)4][CoIII(CN)6]·H2O}n (8). Yield: 256 mg, 65%. Anal. calcd for Tb1Co1C16H20N8O7 (MW = 654.2 g·mol−1): Tb, 24.3%; Co, 9.0%; C, 29.4%; H, 3.1%; N, 17.1%. Found: Tb, 24.7%; Co, 8.6%; C, 29.0%; H, 3.5%; N, 17.0%. IR spectrum (KBr, cm−1). CN− stretching vibrations: 2163w, 2144vs, 2135vs X-ray Crystallography. Single-crystal diffraction measurements were performed for 1, 3, and 8 using a Rigaku R-AXIS RAPID imaging plate area detector with graphite monochromated Mo Kα radiation. The single crystals were taken from the mother solution, dispersed in a paratone N-oil, mounted on Micro Mounts holder, and measured at the temperature of 100(2) K. The crystal structures were solved by a direct method using SHELXS-97 and refined by a full-matrix leastsquares technique using a SHELXL-2014/7.65 Calculations were performed using partially Crystal Structure crystallographic software and partially WinGX (ver. 1.80.05) integrated system.66 All nonhydrogen atoms were refined anisotropically. The hydrogen atoms in
identical to that presented for 1. As was proved by the elemental analysis of CHN and metals (ICP-MS), the compositions of 1−8 correspond well to the formulas expected from the molar ratios between metal ions used in the synthesis. Taking into account the experimental limitations of the elemental analyses, the deviations from the presented amounts of Eu and Tb metal ions per one Co center are estimated to be
