Achieving white light emission and increased magnetic anisotropy by transition metal substitution in functional materials based on dinuclear DyIII(4-pyridone)[MIII(CN)6]3− (M = Co, Rh) molecules

Junhao Wang a, Szymon Chorazy *b, Koji Nakabayashi a, Barbara Sieklucka b and Shin-ichi Ohkoshi *a
aDepartment of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan. E-mail: ohkoshi@chem.s.u-tokyo.ac.jp
bFaculty of Chemistry, Jagiellonian University, Gronostajowa 2, 30-387 Kraków, Poland. E-mail: simon.chorazy@uj.edu.pl

Received 30th August 2017 , Accepted 18th November 2017

First published on 18th November 2017


A building block approach has led to the construction of two unique bifunctional magneto-luminescent molecular materials, {[DyIII(4-pyridone)4(H2O)2][MIII(CN)6]}·nH2O (M = Co, n = 2, 1; M = Rh, n = 4, 2), incorporating the cyanido-bridged dinuclear {DyIIICoIII} (1) or {DyIIIRhIII} (2) molecules, that crystallize within the supramolecular network in the attractive non-centrosymmetric Cmc21 space group. Both compounds reveal dual physical properties of colour-tunable photoluminescence and slow relaxation of magnetization. While 1 exhibits multi-coloured photoluminescence ranging from yellow to blue, tuned by the excitation wavelength, 2 additionally reveals nearly white light emission under 336 nm excitation at room temperature. 1 and 2 show 4f-metal-centered strong magnetic anisotropy presenting Single-Molecule Magnet (SMM) behaviour with the large anisotropic energy barriers of 187(6) K for 1, and 214(4) K for 2. We have shown and discussed that the replacement of [CoIII(CN)6]3− by the heavier [RhIII(CN)6]3− analogue in {[DyIII(4-pyridone)4(H2O)2][MIII(CN)6]}·2H2O crystalline materials is an efficient route towards white light emissive solid state matrices composed of Single-Molecule Magnets with enhanced magnetic anisotropy. Such extraordinary photoluminescent molecule-based magnets can become good prerequisites for future application in bifunctional optical and magnetic devices.


Introduction

The design of smart stimuli-responsive molecular materials has generated much interest over the last several years.1 Considerable attention has been given to the systems that exhibit switching of optical, magnetic and magneto-optical properties. The extraordinary effects of photoinduced magnetization in spin-crossover magnets,2 magnetization-induced second harmonic generation (MSHG),3 and magneto-chiral dichroism (MChD)4 has prompted researchers to design systems with more exciting magneto-optical synergistic effects.

In this regard, a great perspective can be found for the simultaneous implementation of photoluminescence and single molecule magnet (SMM) behaviour in a single solid-state matrix.5 Photoluminescent materials find various applications by taking advantage of their white light emission (WLE),6 tunable multi-coloured luminescence,7 near-infrared phosphorescence,8 and up-conversion luminescence (UCL).9 In contrast, Single-Molecule Magnets (SMMs) exhibiting slow magnetic relaxation behaviour at the molecular scale due to the thermally activated energy barrier of spin reversal are of potential interest in applications in high-density memory storage, spintronics, as well as in quantum computing.10 Thus, the combination of the luminescence and magnetic anisotropy is expected to be a fruitful multifunctionality leading to a new class of optical molecular magnets, not achievable for classical metal- or metal oxide-based materials.

From the perspective of photoluminescent SMM-based materials, our efforts are concentrated on the application of trivalent lanthanide ions, which have been proved to be one of the most promising candidates in both luminescent materials11 and magnetically anisotropic systems.12 The variety and versatility of lanthanide-based photoluminescent materials, including nanomaterials,13 ionogels,14 metallogels,15 lanthanidomesogens,16 stimuli-responsive probes,17 and organic light emitting diodes (OLEDs),18 is ascribed to their intrinsic well-structured 4f levels. However, in many cases, sensitization by coordinated organic ligands,19 or d-metal ions,20 is also of great importance as the quantum yield of photoluminescence can be substantially enhanced by means of the energy transfer process. In addition, organic ligands can essentially influence the magnetic anisotropy of lanthanide(3+) ions by modifying the geometry of the coordination sphere, which strongly affects the resulting effective anisotropic energy barrier (Ueff).21 In this context, an attractive idea is to implement the diamagnetic transition metal complex as a linker between 4f-metal centers that not only precludes the disturbing magnetic coupling between lanthanide ions, but also contributes to the modulation of the coordination geometry of the 4f-metal moiety by restraining the coordination topology.22,23

Following these perspectives, we decided to explore the heterobimetallic d–f cyanido-bridged coordination polymers incorporating lanthanide(3+) ions bonded to both selected organic ligands and polycyanidometallates. Lately, our groups proved that the bifunctional magneto-luminescent materials incorporating emissive Single-Molecule Magnets can be achieved by combining magnetically anisotropic and white-to-yellow emissive Dy(3+) ions with diamagnetic and red-emissive [CoIII(CN)6]3− ions.23 The first exciting results prompted us to explore these systems further, and replace the red emissive [CoIII(CN)6]3− ions by their heavier, similarly diamagnetic, [RhIII(CN)6]3− revealing blue emission under UV excitation.

While the luminescent properties of the [RhIII(CN)6]3− anion are known,24 its sensitizing character for lanthanide-based photoluminescent materials is unexplored. Moreover, to the best of our knowledge, there are no reports on Rh-containing molecular systems of the SMM character. Thus, we concentrated our efforts on the construction and characterization of two novel molecular materials constructed of dinuclear species, {[DyIII(4-pyridone)4(H2O)2][MIII(CN)6]}·nH2O (M = Co, n = 2 for 1, and M = Rh, n = 4 for 2), where DyIII serves as the luminescent and magnetic center isolated by diamagnetic and emissive [MIII(CN)6]3− ions (M = Co, Rh), and additionally sensitized by the blue-emissive 4-pyridone ligand, which in total give colour-tunable photoluminescence and slow magnetic relaxation, with significant enhancement and modulation of the optical and magnetic properties resulting from the replacement of CoIII with RhIII in the hexacyanide complex.

Experimental section

Materials

Dysprosium(III) chloride hexahydrate (DyIIICl3·6H2O, CAS: 15059-52-6, Wako Pure Chemicals Industries, Ltd), yttrium(III) chloride hexahydrate (YIIICl3·6H2O, CAS: 10025-94-2, Wako Pure Chemicals Industries, Ltd), 4-pyridone (4-hydroxy-pyridine, 4-pyridinol, CAS: 626-64-2, Sigma-Aldrich), potassium hexacyanidocobaltate(III) (K3[Co(CN)6], CAS: 13963-58-1, Sigma-Aldrich), potassium cyanide (KCN, CAS: 151-50-8, Wako Pure Chemicals Industries, Ltd), and rhodium(III) chloride (RhIIICl3·xH2O, Rh: 38–40%, CAS: 20765-98-4, Sigma-Aldrich) were purchased from commercial sources, and used without further purification.

The starting material for the synthesis of 2, K3[Rh(CN)6]·2H2O, was prepared from KCN and RhCl3 following the modified literature procedure.25 In the optimized method, the 1.9 mmol (0.50 g) portion of RhCl3·xH2O was dissolved in 3.5 ml of distilled water. After 1 hour of continuous stirring at 60 °C, the 12 mmol (0.8 g) portion of KCN was slowly added to the RhCl3 solution, followed by another 1 hour of continuous stirring at 60 °C. After that, 1.0 ml of hydrochloric acid (2 mol L−1) was added to the resulting solution, which was immediately followed by 10 minutes of air bubbling to exclude the extra amount of free cyanide anion. Then, the resulting solution was concentrated to 1.2 ml to precipitate the salt of KCl. After removing the resulting precipitate by suction filtration, the filtrate containing the target product was diluted to 2 ml of volume. To extract the K3[Rh(CN)6] from the diluted filtrate, the 10 ml portion of ethanol was slowly and carefully layered on the top of the filtrate. By very careful stirring of the resulting mixture, the K3[Rh(CN)6] precipitated in the ethanol phase, and, then, the two phases were separated. The extraction processes were repeated until there was no white precipitation appearing in the ethanol layer. After collecting all the ethanol layers, the white precipitation was filtered by using a vacuum pump to yield the crude sample of K3[Rh(CN)6]·2H2O. The basic characterization of K3[Rh(CN)6]·2H2O was carried out by CHN elemental analysis and IR spectroscopy, and the crude sample was used without further purification. IR spectrum (KBr, cm−1). Peaks: 2135vs, stretching mode of CN, 3340s, br, OH hydrogen bonding. CHN elemental analysis. Anal. calcd for K3Rh1C6H4N6O2 (Mw = 412.3 g mol−1): C, 17.5%; H, 1.0%; N, 20.4%. Found: C, 17.1%; H, 0.9%; N, 20.2%.

Synthesis and characterization of 1. Two precursor solutions were prepared as follows. The 0.12 mmol (45 mg) portion of DyCl3·6H2O and the 1.2 mmol (115 mg) of 4-pyridone were dissolved together in 0.5 ml of distilled water to get solution I, and the 0.12 mmol (40 mg) portion of K3[Co(CN)6] was dissolved in 0.5 ml of distilled water to get solution II. Solution I was heated with stirring, and was immediately poured into solution II after reaching boiling point. The resulting mixture was filtered by using a vacuum pump after quick stirring for a few seconds, then the vial with the clear filtrate was left closed for crystallization. After several hours, yellow transparent block-shaped crystals of 1 appeared in the solution. After one day, the whole crystalline product was collected by suction filtration, washed with small portions of water and ethanol, and dried overnight under an air atmosphere to give 58 mg of the product (yield: 58%), with the chemical formula of {[DyIII(4-pyridone)4(H2O)2][CoIII(CN)6]}·2H2O (1), as determined by the CHN elemental analysis supported by the IR spectrum (Fig. S1, ESI) and thermogravimetric studies (Fig. S2, ESI). IR spectrum (Fig. S1, ESI, KBr, cm−1). CN stretching vibrations: 2160vs, 2144vs, 2134vs, 2130vs, 2123vs, indicating the presence of both bridging and terminal modes at the splitting of the peak with characteristic shifts observed when compared with the ν(CN) of 2129vs for K3[CoIII(CN)6].23a Other peaks are as follows: 3335s, 3250s, 3139s, br, 3095s, 2981s, N–H stretching mode, O–H stretching mode, and hydrogen bonding; 1637s, 1611m, 1528vs, 1516vs, ring stretching mode; 1576m, C[double bond, length as m-dash]O stretching mode; 1391s, 1382s, ring vibration mode; 1195m, 1183s, 999s, in-plane CH deformation; 863m, 833w, 744w, out-of-plane CH deformation. All these peaks can be found in pure 4-pyridone, but their positions are variously shifted indicating the coordination of 4-pyridone with DyIII in 1.26 CHN elemental analysis. Anal. calcd for Dy1Co1C26H28N10O8 (Mw = 830.0 g mol−1): C, 37.6%; H, 3.4%; N, 16.9%. Found: C, 37.4%; H, 3.4%; N, 16.9%. TGA (Fig. S2, ESI): loss of 1 H2O per {DyCo}, calcd 2.2%, found 2.0%; loss of the next 3 H2O per {DyCo} (one hydrogen bonded water, and two coordinated water molecules), calcd 6.5%, found, 6.6%.
Synthesis and characterization of 2. The synthesis of 2 was started with the preparation of two precursor solutions: the 0.12 mmol (45 mg) portion of DyCl3·6H2O and the 1.2 mmol (115 mg) of 4-pyridone were dissolved together in 0.5 ml of distilled water to get solution I, and the 0.12 mmol (49 mg) portion of K3[Rh(CN)6] was dissolved in 0.5 ml of distilled water to get solution II. Solution I was heated with stirring, and it was immediately poured into solution II after reaching boiling point. The mixture solution was filtered by using a vacuum pump after quick stirring for a few seconds, then the vial with the clear filtrate was left closed for crystallization. After several hours, colourless elongated crystals of 2 appeared in the solution. After one day, the whole crystalline product was collected by suction filtration, washed with small portions of water and ethanol, and dried overnight under an air atmosphere, to give 25 mg of product (yield: 23%), with the chemical formula of {[DyIII(4-pyridone)4(H2O)2][RhIII(CN)6]}·4H2O (2) as determined by CHN elemental analysis supported by the IR spectrum (Fig. S3, ESI) and thermogravimetric studies (Fig. S2, ESI). IR spectrum (KBr, cm−1). CN stretching vibrations: 2176vs, 2156vs, 2144vs, 2138vs, 2133vs, indicating the presence of both bridging and terminal modes at the splitting of the peak with characteristic shifts observed when compared with the ν(CN) of 2135vs for K3[RhIII(CN)6] (Fig. S3, ESI). The other peaks were as follows: 3342s, 3251s, br, 3138s, 3094s, 2978s, N–H stretching mode, O–H stretching mode, and hydrogen bonding; 1635s, 1611m, 1527vs, 1517vs, ring stretching mode; 1576m, C[double bond, length as m-dash]O stretching mode; 1381s, ring vibration mode; 1194m, 1184s, 999s, in-plane CH deformation; 862m, 832w, 737w, out-of-plane CH deformation. All these peaks are variously shifted compared to the pure 4-pyrdione, indicating the coordination of 4-pyridone by DyIII in 2.26 CHN elemental analysis. Anal. calcd for Dy1Rh1C26H32N10O10 (Mw = 910.0 g mol−1): C, 34.3%; H, 3.5%; N, 15.4%. Found: C, 34.6%; H, 3.4%; N, 15.7%. TGA (Fig. S2, ESI): loss of 3 H2O per {DyRh} unit (three weakly-bonded, non-coordinated water molecules), calcd 4.0%, found 4.0%; loss of the next 3 H2O (one crystallization water, and two coordinated water molecules), calcd 6.1%, found, 6.2%.

X-ray crystallography

Single-crystal X-ray diffraction analyses of 1 and 2 were executed on a Rigaku R-AXIS RAPID diffractometer equipped with an imaging plate area detector and graphite monochromated Mo Kα radiation. The single crystals were taken from the solution and dispersed in a small portion of paratone-N oil on a glass plate. The selected single crystals were then mounted on a Micro Mounts™ holder and measured at T = 90(2) K. The crystal structures of 1 and 2 were solved using a direct method of SHELXS-97, while the refinement was performed by using a full-matrix least squares technique of SHELXL-2014/7.27 Calculations were preliminarily executed by the Crystal Structure software, and later using the WinGX (ver. 1.80.05) integrated system. Only [MIII(CN)6]3− (M = Co for 1, Rh for 2) units, DyIII, and O atoms that were coordinated by DyIII were refined anisotropically. However, it was necessary to apply the ISOR and DELU restraints on their ellipsoids. Due to the disorder of the pyridone rings, the C and N atoms from the pyridone ring and the other O atoms were refined isotropically. The H atoms were found from the electron density map, but the command of DFIX restraints were performed on C–H distances to keep the correct geometries during the refinement. The disorder on 4-pyridone molecules generates the unusually reported contacts between the overlapped 4-pyridone rings. Therefore, the PART command was used. Structural diagrams were prepared by using the Mercury 3.7 program. The details of the crystal structure and structure refinement are gathered in Table S1, and the structural details are listed in Table S2 (1) and Table S3 (2) (ESI). The CCDC reference numbers for the crystal structures of 1 and 2 are 1571678 and 1571679, respectively.

Physical techniques

The IR absorption spectra were obtained by measuring the grinded and pressed polycrystalline samples with potassium bromide using a JASCO FRIT-4100 spectrometer. Elemental analysis of metals (Dy, Y, and Co) was conducted using an Agilent 7700 Inductively coupled plasma (ICP) mass spectrometer. Thermogravimetric analysis was conducted using a Rigaku Thermo Plus TG8120 in the temperature range of 20–400 K (heating rate: 1 K min−1) under an air atmosphere with aluminum oxide as the reference. UV-vis-NIR diffuse reflectance spectra were collected by using a JASCO V-670 spectrophotometer on the polycrystalline samples mixed with barium sulfate. Power X-ray diffraction patterns were obtained by using a Rigaku Ultima-IV diffractometer equipped with Cu Kα radiation (λ = 1.5418 Å). Photoluminescent emission and excitation spectra were measured by using a Horiba Jobin-Yvon Fluorolog®-3 (FL3-211) spectrofluorimeter (model TKN-7) with a Xe lamp (450 W) as an excitation source, and the R928P detector working in a photo-counting mode. The collection and analysis of the emission and excitation spectra were executed using FluorEssence® software. At room temperature, the samples were loaded in the EPR tube, which served as a sample holder, while at the low temperature of 77 K the EPR tube was cooled by liquid nitrogen in the optical cryostat. The magnetic properties were investigated using a Quantum Design MPMS XL magnetometer on the polycrystalline samples dispersed in the frozen mother solution, or submerged in a small portion of paraffin oil, to avoid the rotation of crystals under an applied magnetic field.

Calculations

Continuous Shape Measure Analysis for the determination of the geometry of the seven-coordinated DyIII complexes of 1 and 2 was performed by using SHAPE software ver. 2.1.28

Results and discussion

Structural studies

The yellow transparent single crystals of 1 and 2 that crystallized from the aqueous solution containing Dy3+, 4-pyridone, and [MIII(CN)6]3− (M = Co for 1, and M = Rh for 2) were studied by single-crystal X-ray diffraction analyses (Fig. 1, Fig. S4–S7 and Tables S1–S4, ESI). 1 and 2 were found to be isostructural, crystallizing in the non-centrosymmetric space group of Cmc21 of the orthorhombic crystal system. They are built of dinuclear cyanido-bridged molecules, {DyIIIMIII} (M = Co, 1; M = Rh, 2), which consist of the octahedral [MIII(CN)6]3− ion and the DyIII complex. One of the cyanide ligands of the [MIII(CN)6]3− ion serves as a molecular bridge to the Dy3+ ion, while the remaining five cyanides are terminal (Fig. 1a). The seven-coordinated Dy complex is formed by the coordination of four O atoms of 4-pyridone ligands, two O atoms of coordinated water molecules, and one N atom of bridging cyanide, resulting in the geometry of an elongated pentagonal bipyramid (PBPY-7) (Fig. 1b, Fig. S4, S5 and Tables S2–S4, ESI). The distortion from ideal PBPY-7 is due to the discrepancy of the O4–Dy–O5 angle from 180°. Besides, the relatively longer bond lengths of Dy–N1, and the two Dy–O1 distances result in the axial elongation of the Dy complex in the direction of the bridging cyanide (Fig. S5, ESI). It is noteworthy that the arrangement of coordinated 4-pyridone has two types, giving two four-blade propeller-shaped enantiomers of DyIII (Fig. S4, ESI). However, the flack parameters, which are close to 0.50 for both 1 and 2, indicate that they crystallize as a racemic twin.29 The dinuclear molecules interact through π–π interactions of two pairs of 4-pyridone rings, forming a supramolecular chain along the c axis (Fig. S4, ESI). The chains are interlocked to each other by hydrogen bonds between the coordinated water and the terminal cyanides in the a direction, stabilizing the robust three-dimensional supramolecular network (Fig. 1c, Fig. S4, S6 and S7). The closest distance of Dy centers between the supramolecular chains is 10.5 Å, while the closest intrachain distance between the Dy centers is 7.2 Å. Powder XRD, IR, thermogravimetric, and CHN elemental analyses confirmed the validity of the structural models with respect to the bulk samples used in other physical experiments (Fig. S8, Å).
image file: c7tc03963h-f1.tif
Fig. 1 Crystal structure of 1: (a) the structural view of the dinuclear cyanido-bridged molecules, (b) the geometry of the seven-coordinated DyIII complex, and (c) the three-dimensional supramolecular network presented within the bc crystallographic plane. The crystal structure of 2 is analogous to that presented above for 1, with RhIII replacing the CoIII metal center.

Optical properties

The optical properties of 1 and 2 including solid state absorption, emission and excitation were investigated (Fig. 2, Fig. S9–S13 and Tables S5–S8, ESI). Both compounds show strong absorption peaks in the UV range due to the π–π* transitions of 4-pyridone mixed with the d–d bands of [MIII(CN)6]3− ions, and an array of weak sharp peaks in the visible and NIR ranges corresponding to the characteristic f–f transitions of DyIII (Fig. S9, S10, Tables S5 and S6, ESI). Photoluminescence studies reveal that 1 and 2 can give visible emission with varieties of colours tuned by the excitation wavelength (Fig. 2, Fig. S11, S12, Tables S7 and S8, ESI). At room temperature, the emission of 1 covers the colours from yellow to greenish blue, while the emission of 2 is slightly blue shifted ranging from yellow to light blue including the attractive white light emission (WLE) for the excitation with 336 nm light. This prominent tunability of emission colour is ascribed to the mixing of the two emissive components in various ratios by a set of selected excitation wavelengths. Exciting 1 by the wavelength of 281 nm yields the emissive component of the Dy3+ ion with two strong emission peaks at 480 nm and 574 nm, assignable to the 4F9/26H15/2,13/2 transitions. Similarly, the pure emission of Dy3+ ion in 2 can also be obtained by the UV excitation of 322 nm. Upon the specifically selected excitation wavelengths, the other emissive component, related to the phosphorescent T1 → S0 transition of 4-pyridone, starts to appear, and becomes dominant, while the relative intensity of the DyIII component decreases. When irradiated by 410 nm, both 1 and 2 give the almost pure emission of 4-pyridone, centered at 500 nm and 485 nm, respectively. The designated excitation wavelengths were found in the excitation spectra obtained by monitoring the emission intensities at 574 nm and 510 nm, at which the intensities are solely contributed by the pure components of DyIII and 4-pyridone, respectively (Fig. S11 and S12, ESI). In the excitation spectra of the DyIII component for both 1 and 2, the sharp peaks, corresponding to the electronic transitions of DyIII from the ground 6H15/2 level to the higher f levels, are overlapped within the range of 240–340 nm with the broad peak that can be ascribed to the 1A1g1T2g transition of [CoIII(CN)6]3− and [RhIII(CN)6]3−. This indicates that the emission of 1 and 2 can be realized both by the direct f–f excitation pathway and the energy transfer process from CoIII/RhIII to the 4f metal ion. This is also supported by the lack of [M(CN)6]3−-based emission expected in the red or blue ranges for CoIII and RhIII, respectively (Fig. S13, ESI).23,24 At the low temperature of 77 K, the colour-tunable emission of 1 and 2 can also be achieved by varying the excitation wavelengths, and the multiple emission colours of both compounds are quite similar, ranging from yellow to blue (Fig. S11, S12, Tables S7 and S8, ESI). When compared with the previously reported 2D Dy(4-hydroxypyridine)–Co material,23b the dinuclear structural matrixes of 1 and 2 exhibit similar colour-tunable photoluminescent properties, resulting from the interplay between the electronic levels of DyIII, [MIII(CN)6]3−, and the organic ligand (Fig. 2c). However, the chromatic ranges in 1 and 2 offer a richer colour spectrum at room temperature (RT). Besides, 2 is specifically more prominent since the almost white light emission, that is x = 0.29, y = 0.35 of the CIE 1931 chromaticity scale, can be achieved at room temperature under excitation of 336 nm light, proving the non-innocent role of the introduced [RhIII(CN)6]3− anion.
image file: c7tc03963h-f2.tif
Fig. 2 Room temperature solid-state emission spectra of 1 (left) and 2 (right) by the indicated excitation wavelengths with the resulting emission colours (a), the estimated emission colours shown on the CIE 1931 chromaticity diagrams (b), and schematic energy level diagrams showing the mechanism of electronic interplay between the building blocks (c).

Magnetic studies

The direct current (dc) magnetic properties of 1 and 2 are illustrated in Fig. 3. The value of the product of the magnetic susceptibility and the temperature χMT of 1 and 2 reaches 14.3 and 14.2 cm3 mol−1 K, respectively, which are in good agreement with the expected value of 14.1 cm3 mol−1 K for free DyIII of the 6H15/2 ground multiplet. On cooling from 300 K to 10 K, the values of χMT for both compounds decrease gradually reaching ca. 13 cm3 mol−1 K at 5 K. At lower temperatures, the values of χMT decrease rapidly to 10.6 cm3 mol−1 K and 11.5 cm3 mol−1 K at 2 K, for 1 and 2, respectively. This magnetic behaviour can be explained by the thermal depopulation of the mJ levels of the 6H15/2 multiplet, according to the crystal field effect. This indicates the good magnetic isolation of DyIII complexes both in 1 and 2, which is ensured by the diamagnetic hexacyanidometallates, and the supporting 4-pyridone ligands. Such an interpretation is also supported by the magnetic characteristics of the magnetically diluted sample of 1, 1md of the {[Dy0.12YIII0.88(4-pyridone)4(H2O)2][CoIII(CN)6]}·2H2O formula (Fig. S14, ESI), exhibiting dc magnetic behaviour very similar to that observed in 1. The saturation values of magnetization reach 5.2 μB at 50 kOe for both 1 and 2, which are close to the value of 5.0 μB for the pure mJ = ±15/2 states of an Ising anisotropy (Seff = 1/2, geff = 20) (Fig. S15 and S16, ESI).
image file: c7tc03963h-f3.tif
Fig. 3 Direct-current (dc) magnetic properties of 1 (a) and 2 (b): temperature dependence of χMT under Hdc = 1 kOe, and the field dependence of molar magnetization at the indicated temperatures showing hysteresis loops (the insets).

The SMM behaviour of 1 and 2 was investigated by alternate current (ac) magnetic measurements (Fig. 4, Fig. S17–S20, Tables S9 and S10, ESI). Under zero dc field, the signals of in-phase and out-of-phase magnetic susceptibilities for both compounds are clearly observed in the χM′(T) and χM′′(T) plots below 18 K (Fig. 4a). The plots of frequency dependence of χM′ and χM′′, and the Cole–Cole plots of χM′′(χM′), in the temperature range of 6–15 K were fitted by the generalized Debye model (Tables S10 and S11, ESI). The α parameters are between 0.06–0.26 for 1, and 0.24–0.43 for 2, indicating a narrow distribution of the relaxation times (Fig. 4b and c). By plotting the relaxation time τ as ln(τ) versus T−1 (Fig. 4d), it was found that the magnetic relaxation process is thermally activated within the 11–15 K range. The fitting procedure following the Arrhenius law gives the effective anisotropic energy barriers Ueff = 187(6) K (ca. 130(4) cm−1) with τ0 = 1.9(8) × 10−10 s for 1, and 214(4) K (ca. 149(3) cm−1), with τ0 = 1.7(5) × 10−11 s for 2. Below 10 K, a quantum tunneling of magnetization (QTM) effect becomes dominant so that the relaxation times for both compounds are independent of temperature, resulting in the discrepancy from linear Arrhenius curves. Applying an external dc field to the samples partially suppresses the QTM effect giving a relatively weaker discrepancy of the ln(τ) versus T−1 plot from the linear Arrhenius dependence. However, the anisotropic energy barriers are not visibly enhanced, giving Ueff = 186(2) K (ca. 129(1) cm−1) with τ0 = 1.9(3) × 10−10 s for 1 (Fig. S18, ESI), and 224(4) K (ca. 156(6) cm−1), with τ0 = 7(6) × 10−12 s for 2 (Fig. S20, ESI). Thus, the additional contribution of the phonon-driven relaxation process can also be postulated, especially for 2, exhibiting the rather large scope of the α parameters. The AC magnetic measurement was also performed on the magnetically diluted sample of 1, 1md of the {[DyIII0.12YIII0.88(4-pyridone)4(H2O)2][CoIII(CN)6]}·2H2O formula. The thermal energy barrier for 1md remains at the same level as for 1, 186(1) K (ca. 130(4) cm−1) with τ0 = 1.89(17) × 10−10 s (Hdc = 0 Oe), proving that good magnetic isolation of Dy3+ ions is achieved in 1, and further magnetic dilution does not significantly change its SMM behaviour (Fig. S21, S22 and Table S11, ESI). As a result of the high Ueff, 1 and 2 exhibit magnetization hysteresis loops, which are observed below 4 K for a field sweep rate of 10 Oe s−1 (Fig. 3, the insets). The characteristic butterfly shape of the MH loops is connected with the strong QTM effect dominating at low fields and temperatures.


image file: c7tc03963h-f4.tif
Fig. 4 Alternate-current (ac) magnetic properties of 1 (left column) and 2 (right column) under zero dc field with Hac = 3 Oe: (a) temperature dependences of χM′′ at the indicated ac frequencies, (b) frequency dependence of χM′′ at the indicated temperatures with the (c) related Cole–Cole plots of χM′′(χM′), and (d) the temperature dependence of relaxation time in the form of ln(τ) versus T−1 plots. The solid lines in (b and c) are fitted according to the generalized Debye model, and the solid line in (d) is the fitted curve according to Arrhenius law.

From the comparison of the ac magnetic properties of 1 and 2, it is noticeable that the introduction of heavier RhIII can significantly boost the effective energy barrier of DyIII by improving the single-ion magnetic anisotropy. The reason is that the geometry of the Dy complex in 2 is relatively more axially elongated than that of 1, so that the related magnetic anisotropy of Dy in 1 is comparatively greater. This is concluded from the comparison of O1–Dy1–O1 angle, which is 70.5° for 2 and 71.3° for 1. This can be explained because the steric effect on the larger Rh(III) center influences the cyanido-bridged Dy complex more strongly than the relatively smaller Co(III).

A comparison of compounds 1 and 2 with the two-dimensional Dy(4-hydroxypyridine)–Co analogue23b containing essentially the same ligand existing here in the tautomeric 4-pyridone form reveals a significant improvement in the ac magnetic properties such that the inducing dc field is no longer necessary for slow magnetic relaxation, and the energy barrier is several-fold higher than that of the reported two-dimensional analogue. This is ascribed to the very different coordination geometry of Dy that is the axially elongated seven-coordinated Dy complex, which can better induce magnetic anisotropy when compared to the laterally flattened geometry of the eight-coordinated Dy complex found in the 2D analogue. In addition, the higher thermal energy barrier also results in the hysteresis loop in the M(H) plot below 4 K, which was not detectable in the 2D analogue.23b

Conclusions

Here, we report two novel functional crystalline materials incorporating the cyanido-bridged d–f dinuclear molecules {Dy(4-pyridone)-NC-MIII(CN)5} (M = Co, 1; M = Rh, 2) that are built from the DyIII complex with axially elongated pentagonal bipyramid geometry bonded to the diamagnetic [MIII(CN)6]3− ions. Both materials exhibit a dual magneto-luminescent nature, which is colour-tunable photoluminescence and slow magnetic relaxation. Chromatic ranges of photoluminescence with rich colours, including close to white light emission, were observed for 1 and 2 at both room temperature and low temperature, tuned by variation of the excitation wavelength. 1 and 2 reveal single-molecule magnet (SMM) behaviour with large thermal energy barriers of Ueff = 187(6) K for 1, and 214(4) K for 2, under a zero dc field. 2 is the first SMM containing a structurally supporting RhIII metal center. We showed that the replacement of CoIII by heavier RhIII in the analogous bimetallic Dy–Co and Dy–Rh molecules enhances both the optical and magnetic functionalities of the functional material by shifting the emission colours towards the desired white light emission, and by increasing the anisotropic thermal energy barrier due to the increasing steric effect. The reported materials are also attractive as they crystallize in the non-centrosymmetric Cmc21 space group, thus opening their possible functionalities of ferroelectricity and second harmonic generation (SHG) activity.30 This will increase their multifunctional character towards the fruitful combination of various linear and non-linear optical properties with electric and magnetic effects. Further exploration of the presented materials along these lines is in progress.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

This work was financed by the Japan Society for the Promotion of Science (JSPS) within the Grant-in-Aid for Specially Promoted Research, grant no. 15H05697, and by the National Science Centre of Poland within the SONATA-11 project, grant no. 2016/21/D/ST5/01634. K. Nakabayashi is thankful to the Grant-in-Aid for Challenging Exploratory Research, no. 15K1366, from JSPS. We are grateful to Dr Olaf Stefańczyk (the University of Tokyo) for the valuable discussion concerning the crystallographic studies.

Notes and references

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Footnote

Electronic supplementary information (ESI) available: Infrared spectra, TGA curves, powder XRD patterns, UV-vis-NIR absorption spectra, results of continuous shape measure analysis, structural details, and additional figures of crystal structures, photoluminescence, and magnetism of the reported materials. CCDC 1571678 and 1571679. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c7tc03963h

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