Octahedral Yb(III) complexes embedded in [CoIII(CN)6]-bridged coordination chains: combining sensitized near-infrared fluorescence with slow magnetic relaxation

Szymon Chorazy *ab, Michał Rams c, Junhao Wang b, Barbara Sieklucka a and Shin-ichi Ohkoshi *b
aFaculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Kraków, Poland. E-mail: simon.chorazy@uj.edu.pl
bDepartment 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
cInstitute of Physics, Jagiellonian University, Łojasiewicza 11, 30-348 Kraków, Poland

Received 20th June 2017 , Accepted 12th July 2017

First published on 18th July 2017

Ytterbium (3+) ions combined with 3-pyridone and hexacyanido-cobaltate(III) anions in a concentrated aqueous solution produce cyanido-bridged {[YbIII(3-pyridone)2(H2O)2][CoIII(CN)6]} (1) chains. The resulting YbIII complexes of an elongated octahedral geometry reveal two coexisting functionalities: a field-induced slow magnetic relaxation with an energy barrier of ΔE/kB = 45(1) K at Hdc = 1 kOe, and an YbIII-centered near-infrared fluorescence in the 950–1100 nm range sensitized by 3-pyridone and [CoIII(CN)6]3−.

Complexes of lanthanide (3+) ions are the subject of enormous interest due to the special magnetic properties.1 They can reveal strong magnetic anisotropy exhibiting the effect of slow relaxation of magnetization, related to the energy barrier (ΔE) of spin inversion.2 Due to the resulting long relaxation time (τ), lanthanide-based magnetic molecules exhibit the magnetic hysteresis of molecular origin opening the applications in information storage and spintronics.3 Such molecules are called Single-Molecule Magnets (SMMs), or Single-Ion Magnets (SIMs) when their magnetism comes from the well isolated single complexes. They are typically built of DyIII[thin space (1/6-em)]4 and TbIII centers,5 but magnetic anisotropy of other 4f-metal ions, including ErIII,6 NdIII,7 or YbIII,8 was also explored.

Coordination systems with rare earth metal ions also draw great attention due to attractive photoluminescence.9 They exhibit diverse light-emitting properties, such as white light,10 or multicolored tunable emission,11 near-infrared (NIR) phosphorescence,12 and up-conversion luminescence.13 These functionalities were utilized in various applications including optical communication, light emitting devices, light conversion, chemical sensing, photovoltaics, and bioimaging.14

Magnetic anisotropy and luminescence were fruitfully combined for the selected lanthanide (3+) complexes serving as emissive SMMs and SIMs that realize the idea of multifunctionality.15 Such luminescent SMMs/SIMs are based on 4f metal ions combined with blocking organic ligands that induce magnetic anisotropy by ensuring the appropriate geometry, and enhance the 4f-centered emission by the energy transfer.16–18 As a result, the emissive SMMs of DyIII,16 ErIII,17 or YbIII[thin space (1/6-em)]18 were prepared. Alternatively, magnetic anisotropy,19 or photoluminescence of 4f-metal ions can be tuned by inserting them into coordination polymers,20 but this approach was rarely explored in the pursuit of emissive SMMs.21

We lately showed that the conjunction of slow magnetic relaxation and luminescence can be achieved in the heterometallic cyanido-bridged networks.22 Polycyanidometallates are attractive in the synthesis of molecule-based magnets combining magnetic phenomena with microporosity, chirality, second-harmonic generation, ferroelectricity, or photoinduced phase transitions.23 In contrast, their application in luminescent materials was limited as most of the [M(CN)x]n anions quench the emission due to numerous electronic states exhibiting the strong absorption in the UV-Vis-NIR range. However, [MV(CN)8]3− (M = Mo, W) ions are transparent in the Vis-NIR region giving chance to observe lanthanide (3+) emission.24 We also found that diamagnetic and red emissive [CoIII(CN)6]3− can be applied in the construction of Dy–Co and Tb–Co networks combining magnetic anisotropy and tunable visible photoluminescence.22 Following this discovery, we aimed at the preparation of novel [Co(CN)6]-bridged d–f coordination polymers serving as emissive SMM-based materials. We focused on ytterbium(III) complexes which can be strongly anisotropic in the axially deformed coordination polyhedra,8,18 and offer the near-infrared emission with potential applications in NIR light-emitting diodes, telecommunication, and night vision.25 While NIR-emissive YbIII SMMs were presented,18 there is no report on the related magneto-luminescent coordination polymers. Some YbIII-based cyanido-bridged assemblies were prepared, and NIR emission sensitized by [RuII(polyimine)(CN)4]2− or [CoIII/CrIII(CN)6]3− anions was shown, but magnetic anisotropy of inserted YbIII was not explored.26 Therefore, we decided to combine YbIII with [Co(CN)6]3− and 3-hydroxypyridine (3-OHpy), existing in water in equilibrium with the tautomeric 3-pyridone,27 as these building blocks were able to induce the axial geometry, and sensitize white light emission of DyIII in Dy(3-OHpy)–Co chains.22a Here, we report {[YbIII(3-pyridone)2(H2O)2][CoIII(CN)6]} (1) chains incorporating YbIII complexes of an unusual octahedral geometry, showing both the slow magnetic relaxation and the sensitized NIR emission.

The colourless block crystals of 1 were obtained by the crystallization from the concentrated aqueous solution of Yb3+, 3-pyridone and [CoIII(CN)6]3− (ESI). The crystalline sample was studied by IR spectroscopy and TGA (Fig. S1 and S2, ESI), and later precisely by single-crystal X-ray diffraction analysis (Fig. 1, Fig. S3–S5 and Tables S1–S3, ESI). 1 is built of nearly linear cyanido-bridged chains consisting of [YbIII(3-pyridone)2(H2O)2(μ-NC)2]+ moieties alternately arranged with [CoIII(CN)6]3− (Fig. 1a). Both the Yb and Co complexes reveal an octahedral geometry, but the noticeable deformation, that is the elongation along the N1–Yb1–N1 axis, is detected only for Yb (Fig. 1b and Fig. S4, ESI). Such six-coordinated Yb units differ 1 from the analogous DyIII(3-OHpy)–CoIII chains with the eight-coordinated [DyIII(3-pyridone)2(H2O)4(μ-NC)2]+ obtained under similar synthetic conditions,22a which is due to the decreased ionic radius of YbIII. In addition, the organic ligand in 1 is in the form of 3-pyridone as depicted by structural analysis while 3-OHpy was found in the related Dy(3-OHpy)–Co chains. This is caused by the hydrogen bonding network of 1 stabilizing the H atoms in a vicinity of pyridine N atoms (Fig. S5, ESI). These hydrogen bonds involve two 3-pyridone ligands of neighbouring chains, and together with the analogous interactions between water and cyanides induce a parallel arrangement and tight packing of chains (Fig. 1c). The validity of the structural model for the bulk sample was confirmed by PXRD analysis (Fig. S6, ESI).

image file: c7dt02239e-f1.tif
Fig. 1 Crystal structure of 1: (a) the representative fragment of coordination polymer, (b) detailed insight into YbIII complex (left) and the tautomeric forms of 3-pyridone (right), and (c) the views of the supramolecular arrangement of chains within bc (left) and ab (right) crystallographic planes.

The direct-current (dc) magnetic properties of 1 are shown in Fig. 2. The room temperature χMT value is 2.30 cm3 mol−1 K which is slightly lower than 2.57 cm3 mol−1 K expected for free YbIII of the 2F7/2 ground multiplet. On cooling, χMT continuously decreases to 1.49 cm3 mol−1 K at 1.8 K. It can be ascribed to the crystal field effect, exhibiting the thermal depopulation of the excited Kramers doublets of the ground multiplet, while Yb–Yb magnetic interactions are negligible due to the separation of Yb3+ ions by diamagnetic [CoIII(CN)6]3− linkers. The saturation magnetization MS = 1.8 μB at T = 1.8 K suggests that the ground state doublet of a nearly pure mJ = ±7/2 state, as the related Seff = 1/2 and geff = 8 with Ising anisotropy should produce MS of 2μB for a powder sample. No sign of magnetic ordering effect was found in magnetic data down to 1.8 K (Fig. S7, ESI).

image file: c7dt02239e-f2.tif
Fig. 2 Direct-current (dc) magnetic properties of 1: temperature dependence of χMT at Hdc = 1 kOe, and the field dependence of magnetization, M, at T = 1.8 K (the inset).

To investigate slow magnetic relaxation in 1, the alternate-current (ac) magnetic studies were performed (Fig. 3 and Fig. S8–S11, ESI). In zero dc field, the imaginary part of susceptibility, χ′′M, is negligibly small, even at 1.8 K, but becomes significant for a dc field above 200 Oe (Fig. S8 and S9, ESI). An optimal dc field of 1 kOe induces a slow relaxation process as visible in the frequency (ν) dependent χM(T) and χ′′M(T) curves below 10 K (Fig. S10, ESI). The χM(ν), χ′′M(ν), and the related Argand χ′′M(χM) plots at various temperatures in the 1.8–10 K range were successfully analysed using the generalized Debye model for a single relaxation (Fig. 3a, b, Fig. S10, and Table S4, ESI). The relaxation times, τ, obey the Arrhenius law above 5 K, giving the thermal energy barrier, ΔE/kB = 45(1) K with τ0 = 2.6(5) × 10−7 s (Fig. 3c). These parameters prove that magnetically isolated and strongly anisotropic YbIII complexes embedded in 1 can be considered as single-ion magnets with the energy barrier relatively large among the reported YbIII-based SIMs.8,18 The deviation of the ln(τ) versus T−1 plot from linearity below 5 K, and the disappearance of the χ′′M signal at zero dc field indicate the additional low temperature relaxation processes, including quantum tunnelling of magnetization (Fig. S11, ESI).22 No hysteresis loop was detected for a field sweep rate of 10 Oe s−1 even at the lowest T of 1.8 K.

image file: c7dt02239e-f3.tif
Fig. 3 AC magnetic susceptibility of 1 at Hac = 3 Oe, Hdc = 1000 Oe and its analysis: (a) frequency dependences at different temperatures, (b) the Argand plots, and (c) the temperature dependence of the determined relaxation time, τ. Solid curves were fitted using the generalized Debye model, while the straight line in (c) is the fit of the Arrhenius law.

The optical properties of 1, including solid state absorption, emission, and excitation spectra, are shown in Fig. 4, S12, S13, and Table S5 (ESI). 1 exhibits strong UV absorption thanks to the d–d bands of CoIII and π–π* transitions of 3-pyridone, the tiny absorption tail in the visible range, and the significant NIR absorption in the 900–1000 nm range due to the f–f transitions of YbIII (Fig. S12 and Table S5, ESI). Exploring such absorption properties, we excited 1 by the UV light of 320 nm, and a distinct emission signal in the NIR range was detected (Fig. 4a). This emission consists of one complex band in the 950–1100 nm range assignable to the YbIII-centered f–f transitions from the emissive 2F5/2 to the ground 2F7/2 multiplet.9 This results in a few distinguishable components with the strongest peaks at 976 and 1023 nm. The excitation spectrum is dominated by the broad UV band at 320 nm with the shoulder at 370 nm, which is ascribed to the combined contributions of 3-pyridone and [Co(CN)6]3− (Fig. 4b and Fig. S13, ESI). This indicates the energy transfer mechanism of the NIR emission in 1, where the UV light absorbed by CoIII and the organic ligand is transferred to the 2F5/2 multiplet of YbIII, leading to the detected fluorescence (Fig. 4c). Such interpretation is supported by the lack of both ligand- and Co-centered emission bands expected in the green and red regions of the spectrum, respectively (Fig. S13, ESI).

image file: c7dt02239e-f4.tif
Fig. 4 Room temperature solid-state emission of 1 in the near-infrared range, detected under the UV light irradiation of the indicated wavelength (a), together with the excitation spectrum (b), and the related schematic energy level diagram (c). Abbreviations: A – absorption, ET – energy transfer, L – luminescence.

In conclusion, we report the unique cyanido-bridged YbIII(3-pyridone)–CoIII chains exhibiting slow magnetic relaxation and sensitized NIR fluorescence. Due to the ionic radius of Yb3+, these chains are built of six-coordinated Yb complexes of a distorted octahedral geometry, different from the previously reported eight-coordinated DyIII in the analogous Dy–Co chains.22a The octahedral geometry of YbIII was found in some inorganic solids, but is very rare for coordination polymers, especially cyanido-bridged assemblies including usually seven- or eight-coordinated Yb (Table S6, ESI). This was achieved by the synthesis in a concentrated aqueous solution and the fast crystallization limiting the growth of crystals with complexes of the increased number of ligands. The Yb complexes are elongated along cyanide bridges occupying the trans positions which leads to the significant magnetic anisotropy giving a field-induced relaxation process with the thermal energy barrier of 45(1) K, valuable among the Yb-based SIMs.8,18 The low coordination number of Yb resulted in the dense packing of the chains which enhance the thermal stability, and improve the NIR emission, that is weakened by water molecules in the structure. Thus, 1 reveals a distinct NIR Yb-based fluorescence controlled by energy transfer from 3-pyridone and [CoIII(CN)6]3− both closely attached to the 4f metal ion. We showed, for the first time, that the implementation of Yb3+ ions into coordination polymers with cyanidometallates is an efficient route towards NIR-emissive and magnetically anisotropic materials. Even more dense packing of the metal centers and organic ligands should be achieved when the dehydration process will be performed for 1. It can lead to a solvent-free hybrid material which will enhance the NIR emission, opening also the chance to induce up-conversion luminescence by inserting Er3+ into such a Yb–Co matrix.13 We are now checking these possibilities.

This work was financed by the National Science Centre, Poland within the SONATA-11 project, no. 2016/21/D/ST5/01634, and by the Japan Society for the Promotion of Sci. (JSPS), Grant-in-Aid for Specially Promoted Research, grant no. 15H05697. The research was partially carried out with equipment purchased with the financial support of the European Regional Development Fund, within the Polish Innovation Economy Operational Program (Contract No. POIG.02.01.00-12-023/08).

Notes and references

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Electronic supplementary information (ESI) available: Experimental details, infrared spectra, TGA curve, additional structural data, powder XRD pattern, detailed magnetic characteristics, UV-Vis-NIR absorption spectra, additional figures of photoluminescence properties, summary of structural features and properties of reported cyanido-bridged assemblies with Yb(III). CCDC 1550695. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c7dt02239e

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