Effect of Noble Metals on Luminescence and Single-Molecule Magnet

2 days ago - Synopsis. Six dinuclear cyanido-bridged complexes containing noble metals and lanthanides [LnIII(terpy)(H2O)(NO3)2][MI(CN)2] (LnIII = Dy,...
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Effect of Noble Metals on Luminescence and Single-Molecule Magnet Behavior in the Cyanido-Bridged Ln−Ag and Ln−Au (Ln = Dy, Yb, Er) Complexes Kunal Kumar,† Olaf Stefańczyk,† Szymon Chorazy,†,‡ 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, Gronostajowa 2, 30-387 Kraków, Poland

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‡

S Supporting Information *

ABSTRACT: Self-assembly of lanthanide(III) complexes of 2,2′:6′,2″-terpyridine (terpy) with dicyanidoargentate(I) and dicyanidoaurate(I) anions in water results in the formation of six isostructural dinuclear systems [Ln III (terpy)(H 2 O)(NO 3 ) 2 ][MI(CN)2] (LnIII/MI = Dy/Ag, 1; Dy/Au, 2; Yb/Ag, 3; Yb/Au, 4; Er/Ag, 5; Er/Au, 6). They form three-dimensional supramolecular networks based on dinuclear molecules linked by hydrogen bonds, π−π interactions, and argentophilic (Ag···Ag) or aurophilic (Au···Au) interactions. All of the assemblies show complex solid-state strong UV and weak vis−NIR absorption due to overlapping contributions from 2,2′:6′,2″-terpyridine, dicyanidoargentate(I), dicyanidoaurate(I), and lanthanide(III) ions. Moreover, they exhibit excitation-wavelengthdependent multicolor photoluminescence ranging from bright white to blue via yellow, green, and cyan colors due to variable contributions from the dicyanidometalate and ligand. Assemblies 3−6 show NIR emission originating from YbIII and ErIII metal centers. Furthermore, compounds 1−6 and their magnetically diluted samples are magnetic-field-induced single-molecule magnets with energy barriers of up to 35 K. The effect of noble metal substitution on the magnetic properties of particular lanthanide ions is described. The influence on the thermal anisotropic energy barrier, which relates to the strength of the magnetic anisotropy, depends on the type of lanthanide used. The Ag-to-Au substitution enhances the anisotropy of the prolate YbIII ion and decreases it for the oblate DyIII ion. It also modifies the strength of dipolar interactions affecting the slow magnetic relaxation processes.

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INTRODUCTION The design, synthesis, and characterization of magnetic cyanido-bridged metal assemblies have attracted a lot of attention in materials science, especially Prussian blue analogues offering various functionalities.1−25 From the viewpoint of magnetism, the lanthanide-based metal−organic frameworks are an excellent base for the construction of multifunctional magnetic materials since most of the lanthanides reveal intrinsic magnetic and/or luminescence properties. Until now, numerous single-molecule magnets (SMMs) based on DyIII, TbIII, and ErIII and much rarer examples of compounds based on CeIII, NdIII, HoIII, TmIII, and YbIII have been reported.26−33 It is also well-established that paramagnetic DyIII and TbIII compounds can emit intense and sharp visible (vis) luminescence while complexes with NdIII, YbIII, and ErIII can generate near-infrared (NIR) light.34−42 These characteristics give a remarkable opportunity to combine magnetism with luminescence and even next to incorporate additional physical properties such as nonlinear optical activity, conductivity, or porosity.13,43,44 In some rare cases, the combination of two or more different physicochem© XXXX American Chemical Society

ical properties resulted in new cross-effects leading to unique phenomena.45 This successful approach resulted in the formation of emissive SMMs with tunable light-emitting characteristics for homo- and heterometallic materials, including polycyanidometalate-based systems.32−42,46−51 Dicyanidometalate complexes of noble metals with lanthanides have been investigated for their interesting luminescence properties.52−60 Recently, Sykora et al. reported bimetallic GdIII−[MI(CN)2]− systems showing tunable excimer and exciplex properties.59 However, the combined tunable magnetic and luminescence properties obtained by substituting AgI by AuI have barely been elucidated. The exploration of such systems is of great importance in research focused on the deposition of SMM systems on gold, silicon, or other surfaces.61,62 SMM compounds with noble metals will enable understanding of the effect of argentophilic (Ag···Ag) or aurophilic (Au···Au) interactions on the magnetic relaxation and energy barrier. These diamagnetic linkers also provide high Received: January 4, 2019

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DOI: 10.1021/acs.inorgchem.8b03634 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

acetonitrile solution (1 mL) of Ln(NO3)3·6H2O and Y(NO3)3·6H2O in a 1:19 molar ratio instead of pure Ln(NO3)3·6H2O solution. Colorless needlelike crystals of each assembly formed within 24 h. Results of infrared spectroscopy and thermogravimetric analyses with interpretation are collected in Figures S1 and S2. [Dy0.05Y0.95(terpy)(H2O)(NO3)2][Ag(CN)2] (1md). Yield: 48.7 mg (51.3% based on Ag). Anal. Calcd for Dy0.05Y0.95AgC17H13N7O7 (molar mass = 627.8 g/mol): Dy, 1.29%; Y, 13.45%; Ag, 17.18%; C, 32.52%; H, 2.09%; N, 15.62%. Found: Dy, 1.29%; Y, 13.17%; Ag, 17.12%; C, 32.41%; H, 2.25%; N, 15.54%. FT-IR (KBr, cm−1): 2184w, 2164vs [ν(CN)]. [Dy0.07Y0.93(terpy)(H2O)(NO3)2][Au(CN)2] (2md). Yield: 54.2 mg (46.35% based on Au). Anal. Calcd for Dy0.05Y0.95AuC17H13N7O7 (molar mass = 716.9 g/mol): Dy, 1.51%; Y, 11.56%; Au, 27.43%; C, 28.43%; H, 1.82%; N, 13.65%. Found: Dy, 1.54%; Y, 11.84%; Au, 27.49%; C, 28.46%; H, 1.98%; N, 13.71%. FT-IR (KBr, cm−1): 2184w, 2164vs [ν(CN)]. [Yb0.05Y0.95(terpy)(H2O)(NO3)2][Ag(CN)2] (3md). Yield: 41 mg (62.7% based on Ag). Anal. Calcd for Yb0.05Y0.95AgC17H13N7O7 (molar mass = 628.3 g/mol): Yb, 1.38%; Y, 13.44%; Ag, 17.17%; C, 32.50%; H, 2.09%; N, 15.60%. Found: Yb, 1.36%; Y, 13.56%; Ag, 17.16%; C, 32.41%; H, 2.24%; N, 15.54%. FT-IR (KBr, cm−1): 2184w, 2164vs [ν(CN)]. X-ray Crystallography. Single-crystal diffraction analyses were performed for 1−6 using a Rigaku R-AXIS RAPID diffractometer equipped with an imaging plate area detector with graphitemonochromatized Mo Kα radiation. The single-crystal diffraction measurements were performed at 90 K for the pristine crystals immersed in Paratone-N oil and mounted on a Micro Mounts holder. Single-crystal X-ray diffraction data were integrated by Rigaku RAPID AUTO software. The crystal structures were solved by the direct method using SHELXS-97 and refined using the F2 full-matrix leastsquares technique of SHELXL-2014/7 incorporated in OLEX2 1.2 crystallographic software.63−65 Anisotropic refinement was performed for all non-hydrogen atoms, while hydrogen atoms were positioned with the help of the electron density map. Crystallographic data and refinement parameters for 1−6 are listed in Table S1 in the Supporting Information. CCDC 1887723−1887728 contain the supplementary crystallographic data for 1−6 in series. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. The figures presenting the structural data were prepared with the use of the CCDC Mercury 3.10 visualization software.66 Geometries of metal centers were estimated by continuous shape measures analysis using SHAPE v2.1 software.67 Physical Techniques. Infrared absorption spectra of all compounds dispersed in KBr pellets were measured using a JASCO FTIR-4100 spectrometer. Elemental analyses of metal concentrations (Ag, Au, Dy, Er, Y, Yb) were performed using an Agilent 7700 inductively coupled plasma mass spectrometer. CHN analyses were performed on a vario MICRO cube (Elementar Analysensysteme GmbH). Powder X-ray diffraction (PXRD) patterns were recorded with a RIGAKU Ultima IV diffractometer. Thermogravimetric analyses were measured using a Rigaku Thermo Plus TG8120 apparatus in the 20−400 °C range under an air atmosphere with a 1 °C/min heating rate and Al2O3 as a reference material. UV−vis−NIR diffuse-reflectance (absorption) spectra were collected using a JASCO V-670 spectrophotometer on polycrystalline samples mixed and pressed with barium sulfate. Emission and excitation spectra were measured on a Horiba Jobin-Yvon Fluorolog-3 (FL3-211) spectrofluorometer (model TKN-7) equipped with a Xe (450 W) lamp as an excitation source with room-temperature R928P emission and liquidnitrogen-cooled InGaAs NIR linear array detectors working in photon-counting mode. The background correction and further analyses of the emission and excitation spectra were performed using FluorEssence Software from HORIBA Jobin Yvon. The luminescence measurements were performed at room temperature and at a low temperature of 77 K using an optical cryostat cooled by liquid nitrogen. Temperature-dependent emission and excitation spectra were also measured using the same spectrofluorometer and lamp as

charge density around the lanthanide centers, improving the SMM behavior.51 In this context, we selected DyIII, YbIII, and ErIII complexes with 2,2′:6′,2″-terpyridine (terpy) and [M(CN)2]− (M = Ag, Au) as building blocks to construct cyanido-bridged [LnIII(terpy)(H2O)(NO3)2][MI(CN)2] (LnIII/MI = Dy/Ag, 1; Dy/Au, 2; Yb/Ag, 3; Yb/Au, 4; Er/Ag, 5; Er/Au, 6) molecular systems showing distinct metallophilic Ag···Ag and Au···Au interactions as well as SMM behavior and multicolor excitation-wavelength-dependent visible and NIR emissions, both tunable by application of different dicyanidometalates.

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EXPERIMENTAL SECTION

Materials. Dysprosium(III) nitrate hexahydrate, ytterbium(III) nitrate hexahydrate, erbium(III) nitrate hexahydrate, yttrium(III) nitrate hexahydrate, 2,2′:6′,2″-terpyridine, potassium dicyanidoaurate(I), potassium dicyanidoargentate(I), and acetonitrile were reagent grade and were purchased from commercial sources (Wako Pure Chemical Industries, Ltd. and Sigma-Aldrich) and used without purification. General Synthetic Procedure for Pure [LnIII(terpy)(H2O)(NO3)2][MI(CN)2] (LnIII = Dy, Yb, Er; MI = Ag, Au) Compounds. The reaction of a 0.1 M acetonitrile solution of Ln(NO3)3·6H2O (1 mL) with a 0.1 M solution of K[M(CN)2] (M = Ag, Au) in a H2O/ CH3CN (1:4 v/v) mixture (1 mL) with vigorous stirring for 30 min resulted in the formation of a white suspension, from which the suspended solid was filtered and discarded. Afterward, the clear, colorless solution was layered with a 0.1 M acetonitrile solution of 2,2′:6′,2″-terpyridine (1 mL). Colorless needlelike crystals formed within 24 h. The product was collected by filtration under reduced pressure, washed with methanol, and dried under the ambient conditions. The crystalline material was used for further investigations. Results of infrared spectroscopy and thermogravimetric analyses with interpretation are collected in Figures S1 and S2 in the Supporting Information. [Dy(terpy)(H2O)(NO3)2][Ag(CN)2] (1). Yield: 51 mg (74.5% based on Dy). Anal. Calcd for DyAgC17H13N7O7 (molar mass = 697.7 g/ mol): Dy, 23.29%; Ag, 15.46%; C, 29.27%; H, 1.88%; N, 14.05%. Found: Dy, 23.28%; Ag, 15.63%; C, 29.62%; H, 2.26%; N, 13.89%. FT-IR (KBr, cm−1): 2168w, 2154vs [ν(CN)]. [Dy(terpy)(H2O)(NO3)2][Au(CN)2] (2). Yield: 57.2 mg (68.4% based on Dy). Anal. Calcd for DyAuC17H13N7O7 (molar mass = 786.8 g/ mol): Dy, 20.65%; Au, 25.03%; C, 25.09%; H, 1.98%; N, 12.05%. Found: Dy, 20.73%; Au, 25.05%; C, 24.90%; H, 1.75%; N, 12.46%. FT-IR (KBr, cm−1): 2183w, 2164vs [ν(CN)]. [Yb(terpy)(H2O)(NO3)2][Ag(CN)2] (3). Yield: 38 mg (49.1% based on Yb). Anal. Calcd for YbAgC17H13N7O7 (molar mass = 708.2 g/ mol): Yb, 24.43%; Ag, 15.23%; C, 28.11%; H, 2.08%; N, 13.50%. Found: Yb, 24.02%; Ag, 14.99%; C, 27.96%; H, 2.38%; N, 13.23%. FT-IR (KBr, cm−1): 2172w, 2153vs [ν(CN)]. [Yb(terpy)(H2O)(NO3)2][Au(CN)2] (4). Yield: 65.1 mg (77.5% based on Yb). Anal. Calcd for YbAuC17H13N7O7 (molar mass = 797.3 g/ mol): Yb, 21.70%; Au, 24.70%; C, 25.65%; H, 1.85%; N, 12.03%. Found: Yb, 21.88%; Au, 24.67%; C, 26.16%; H, 2.15%; N, 12.26%. FT-IR (KBr, cm−1): 2186w, 2164vs [ν(CN)]. [Er(terpy)(H2O)(NO3)2][Ag(CN)2] (5). Yield: 45 mg (62.6% based on Er). Anal. Calcd for ErAgC17H13N7O7 (molar mass = 702.5 g/mol): Er, 23.81%; Ag, 15.36%; C, 29.07%; H, 1.87%; N, 13.96%. Found: Er, 23.72%; Ag, 15.31%; C, 28.82%; H, 1.96%; N, 13.80%. FT-IR (KBr, cm−1): 2170w, 2154vs [ν(CN)]. [Er(terpy)(H2O)(NO3)2][Au(CN)2] (6). Yield: 67 mg (75.3% based on Er). Anal. Calcd for ErAuC17H13N7O7 (molar mass = 791.6 g/mol): Er, 20.66%; Au, 24.33%; C, 25.80%; H, 1.66%; N, 12.39%. Found: Er, 20.70%; Au, 24.38%; C, 25.95%; H, 1.90%; N, 12.34%. FT-IR (KBr, cm−1): 2184w, 2164vs [ν(CN)]. General Synthetic Procedure for Magnetically Diluted (md) [LnxIIIY1−xIII (terpy)(H2O)(NO3)2][MI(CN)2] (LnIII = Dy, Yb; MI = Ag, Au) Assemblies. The magnetically diluted complexes were synthesized in a similar way as the pure assemblies by using a 0.1 M B

DOI: 10.1021/acs.inorgchem.8b03634 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry mentioned above, but the samples were mounted between two plates of quartz glass inside the cryostat. Magnetic studies were performed with a Quantum Design MPMS XL magnetometer on polycrystalline samples dispersed in paraffin oil to avoid dislocation of the crystals under an applied magnetic field. The diamagnetic contributions from the sample, oil, and the holder were estimated and corrected.

dicyanidometalate anion have one bridging and one terminal cyanide with an almost linear structure. The lanthanides in the asymmetric units are situated on the vertices of the two parallelograms aligned along the a and b axes (Figures S3−S8). The first parallelogram arrayed along the a axis has two lanthanides separated by 7.41 and 7.65 Å, while the sides of the second parallelogram are aligned along the b axis at 9.95 and 9.78 Å. A similar type of arrangement can be observed for all of the other complexes with different side lengths for the parallelograms. The dinuclear complexes form a three-dimensional supramolecular network stabilized by (i) hydrogen bonds between oxygen atoms of nitrates and hydrogen atoms of water molecules and between nitrogen atoms of terminal cyanides and hydrogen atoms of water molecules (Figure S9), (ii) argentophilic (Ag···Ag)68 and aurophilic (Au···Au)69 interactions with shortest distances between 3.45−3.63 Å, and (iii) parallel π−π interactions between 2,2′:6′,2″terpyridine aromatic rings with average distance of 3.71 Å. The high crystallinity and phase purity of 1−6 were confirmed by comparison of their PXRD patterns with simulated ones based on the crystal structures from the single-crystal X-ray diffraction measurements (Figure S10). For the purpose of the detailed magnetic characterization (Figures S35−S38, S43 and S44), we also prepared trimetallic magnetically dilute (md) samples 1md, 2md, and 3md. The perfect isostructurality and phase purity of these compounds were also confirmed by PXRD measurements (Figure S11) as well as by the IR spectra in range of 750−4000 cm−1 (Figure S1). The IR peaks in the range of 2140−2200 cm−1 correspond to the CN stretching bands of cyanide, and the IR peaks at 1000−1700 cm−1 match the C−H and C−C stretching bands of 2,2′:6′,2″-terpyridine. More importantly, the diffraction patterns do not show any broadened or split peaks, indicating homogeneity of the crystals and a random distribution of lanthanide(III) ions in the yttrium(III) matrix in 1md, 2md, and 3md. Luminescence Properties of 1−6. The luminescence studies of 1−6 were preceded by solid-state UV−vis−NIR absorption spectroscopy at room temperature, which showed similar absorption bands regardless of the noble metal used (Figures S12−S15 and Table S4). The solid-state visible and NIR emission spectra for 1−6 and 3−6, respectively, were recorded at 300 and 50 K (Figures 2−4 and S19−S26 and Table S6). The emission spectra of 1 (Figure 2) and 2 (Figure S20) measured at room and low temperature show similar patterns with two sharp peaks around 480 and 570 nm corresponding to the 4F9/2 → 6H15/2 and 4F9/2 → 6H13/2 transitions of the Dy(III) center, respectively, and five broad emission peaks centered around 450, 460, 520, 530, and 550 nm assigned to the phosphorescence of 2,2′:6′,2″-terpyridine due to transitions from the 3T1 triplet state to the various singlet vibrational ground states 1S7−10.70,71 There are no emission peaks of [M(CN)2]− (M = Ag, 1; Au, 2) at about 440 nm (Figures S17 and S18 and Table S5), which implies that energy is directly transferred from the dicyanidometalate anion to the neighboring Dy3+ ion.72−74 The existence of two emissive centers (2,2′:6′,2″-terpyridine and lanthanide) has a strong impact on the observed color change of the emitted light with respect to the excitation light at different temperatures. The colors of the luminescence at 300 and 50 K do not change significantly for excitation wavelengths ranging from 270 to 360 nm and result in whitish-yellow emission as shown in the chromaticity diagrams in Figures 2 and S20. This indicates a dominant role of Dy(III)-centered

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RESULTS AND DISCUSSION Structural Studies. Single-crystal X-ray diffraction studies of 1−6 indicated that they crystallize in the centrosymmetric triclinic space group P1̅ (No. 2) (Figures 1 and S3−S9 and

Figure 1. Crystal structure of 1: (a) structural unit with atom labeling; (b, c) arrangement of the dinuclear complexes along the ac and ab planes, respectively.

Tables S1−S3). [Ln(terpy)(H2O)(NO3)2] units are bridged by one cyanide to the [M(CN)2]− (M = Ag, Au) anion, resulting in the formation of zero-dimensional binuclear complexes. All of the complexes exhibit a nine-coordinate distorted spherical capped square antiprism geometry around the Ln(III) ions, as revealed by continuous shape measure analysis (Table S3). The central lanthanide ions are coordinated by one oxygen atom of water, three nitrogen atoms of the terpy ligand, and the nitrogen atom of the bridging cyanide, whereas the silver(I) or gold(I) center in the C

DOI: 10.1021/acs.inorgchem.8b03634 Inorg. Chem. XXXX, XXX, XXX−XXX

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excitation spectra (Figures S19 and S20), mostly dicyanidometalate(I) moieties are excited and participate in sensitizing the DyIII ion. On the other hand, strong greenishblue and blue emission is observed for lower-energy excitation light (λexc = 365−415 nm), which is even more predominant at low temperature (Figure 2). This can be assigned to the emission spectra of 2,2′:6′,2″-terpyridine (Figure S16 and Table S5), showing the maximum at excitation wavelengths around 400 nm and corresponding emissive peaks in the blue (ca. 436 and 460 nm) and green (ca. 525 nm) regions. The change in the ratio of the mutual intensities of the blue and green components with temperature and the applied excitation wavelength is accompanied by the neutral white emission from Dy(III), resulting in different shades of blue light emission. Furthermore, substitution of silver(I) (1) with gold(I) (2) expands the palette of blue luminescence light for lower-energy excitation at 50 K because Au(I) has slightly better sensitizer character than Ag(I). On the contrary to 1 and 2, the luminescence spectra of 3 and 4 do not show lanthanide(III)-centered emission in the 400−700 nm range because of absence of visible-light emission from YbIII complexes. The room-temperature (300 K) and low-temperature (50 K) emission spectra of 3 show sharp and broad peaks attributed to the phosphorescence from [Ag(CN)2]− anions and 2,2′:6′,2″-terpyridine (Figures 3 and S21 and Table S6).70−74 The first two peaks centered around 414 and 440 nm are associated with the transitions from various triplet excited states to the 1Σg+ state of [Ag(CN)2]−, while the other peaks around 458, 471, 489, 519, and 578 nm mainly originate from 2,2′:6′,2″-terpyridine phosphorescence from the 3 T1 triplet state to the different singlet vibrational ground states

Figure 2. (a, b) Solid-state visible-light emission spectra of 1 and (c, d) corresponding emission colors presented on the CIE 1931 chromaticity diagram recorded at various excitation wavelengths at 300 and 50 K, respectively. Red lines marked on the emission spectra in (a) and (b) represent the corresponding emission peaks originating from 2,2′:6′,2″-terpyridine. In (a), the intensities are scaled with respect to the 572 nm emission peak with an intensity of 1.0, while in (b) the intensities are scaled corresponding to the relative intensities for the 482 nm excitation.

emission and almost negligible contribution of phosphorescence from 2,2′:6′,2″-terpyridine. As indicated in the

Figure 3. (a, b) Solid-state visible-light emission spectra of 3 and (d, e) corresponding emission colors presented on the CIE 1931 chromaticity diagram recorded at various excitation wavelengths at 300 and 50 K, respectively. Red lines marked on the emission spectra in (a) and (b) represent the corresponding emission peaks originating from 2,2′:6′,2″-terpyridine. In (a) and (b), the intensities are scaled with respect to the 520 nm emission peak with an intensity of 1.0. (c) Solid-state NIR emission spectra of 3 measured for 364 nm excitation and (f) excitation spectrum recorded for 982 nm at 300 and 3.75 K. D

DOI: 10.1021/acs.inorgchem.8b03634 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 4. (a, b) Solid-state visible-light emission spectra of 5 and (d, e) corresponding emission colors presented on the CIE 1931 chromaticity diagram recorded at various excitation wavelengths at 300 and 50 K, respectively. Red lines marked on the emission spectra (a) and (b) represent the corresponding emission peaks originating from 2,2′:6′,2″-terpyridine. In (a), the intensities are scaled with respect to the 520 nm emission peak with an intensity of 1.0, while in (b) the intensities are scaled corresponding to the relative intensities for the 432 nm excitation. (c) Solid-state NIR emission spectra of 5 measured for 625 nm excitation and (f) excitation spectrum recorded for 1477 nm at 300 and 77 K.

that both the AgI/AuI center as well as 2,2′:6′,2″-terpyridine transfer their energy to the YbIII ion, resulting in NIR emission. The emission spectra recorded for complexes 5 and 6 have no lanthanide-based emission, similarly to 3 and 4. The emission peaks of both complexes at room temperature (300 K) and low temperature (77 K) centered around 428, 445, 462, 511, 534, and 549 nm originate from transitions from the 3 T1 state to the various singlet vibrational ground states 1S7−10 of 2,2′:6′,2″-terpyridine (Figures 4, S24, and S25 and Table S6).70−74 Similarity in the peak patterns for both complexes indicates that phosphorescence contribution of [Ag(CN)2]− and [Au(CN)2]− are very small. This is completely different from what we have observed for 3 and 4 where both Ag(I) and Au(I) ions emit significant amount of light at room and low temperature. The weakly efficient and less broad nature of spectra is mainly due to energy transfer to neighboring Er(III) ions from Ag(I) in complex 5 and Au(I) in complex 6. Further, the transferred energy dissipates through nonradiative pathways. The Ag(I) containing complex 5 reveals visible emission with color ranging from whitish-blue to light green at 300 K and deep blue to cyan at 77 K with the hues of various CIE 1931 xy parameters depending on the excitation wavelength (Figure 4). Similarly, Au(I) containing complex 6 shows cyan and light blue emission at 300 K and deep blue to light blue emission at 77 K (Figure S25). In conclusion, dicyanidoargentate(I) and dicyanidoaurate(I) sensitize the neighboring DyIII ion, resulting in the disappearance of the corresponding emission peaks from the dicyanidometalates. On the other hand, visible emission for complexes 3 and 4 from the ligand and d-transition metal (Ag and Au) ion occurs along with intrinsic NIR emission of the YbIII ion. However, the compounds containing erbium(III)

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S7−10. As a result of the excitation-wavelength-dependent phosphorescence, the emission color can be tuned from greenish-yellow to greenish-blue via green color at 300 K and greenish-blue to blue via cyan at 50 K (Figure 3). This difference in the phosphorescence color can be rationalized in terms of the increase of the emission peak intensities around 440 and 458 nm for higher-energy-excitation wavelengths (λexc = 270 and 317 nm) as the temperature decreases with contributions from both the [AgI(CN)2]− and 2,2′:6′,2″terpyridine moieties as a result of suppression of nonradiative relaxation pathways. However, lower-energy excitation (λexc = 360−400 nm) at room and low temperature mainly populates excited 2,2′:6′,2″-terpyridine energy levels, which make a large contribution to the broad green emission peak at 520 nm, resulting in greenish-blue emission (Figure 3). The room-temperature emission spectra of 4 reveal a broad emission band with two distinguishable peaks centered at 518 and 616 nm that are the result of a combination of [Au(CN)2]− and 2,2′:6′,2″-terpyridine phosphorescences (Figure S22).70−74 However, the low-temperature spectra have relatively less broad features with visible phosphorescence from the 3T1 triplet state to the different singlet vibrational ground states 1S7−10 of 2,2′:6′,2″-terpyridine because of the blue shift in the emission spectra of the Au(I) moiety, as indicated in the phosphorescence spectra of K[Au(CN)2] (Figure S18). The effect of such shift is clearly visible in the chromaticity diagram, where the emission color is shifted toward the blue side (Figure S22). The NIR emission spectra of 3 and 4 have peaks centered at 982 nm at 300 K as well as at 3.75 K originating from transitions from the 2F5/2 excited state to the 2F7/2 ground state of Yb3+ (Figures 3 and S23 and Tables S7 and S8). The excitation spectra of 3 and 4 reveal E

DOI: 10.1021/acs.inorgchem.8b03634 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry reveal multicolor emission mostly from the 2,2′:6′,2″terpyridine ligand due to partial energy transfer from the [M(CN)2]− moiety to the central lanthanide ion. Magnetic Properties of 1−6. The direct-current (dc) magnetic properties of 1−6 are shown in Figure S27. The χMT values at room temperature for the Dy(III)-containing complexes (1 and 2) adopt values around 14.0 cm3 mol−1 K, which are slightly lower than the predicted value of 14.1 cm3 mol−1 K for isolated Dy(III) centers (6H15/2 ground multiplet, J = 15/2, gJ = 4/3) isolated by diamagnetic [M(CN)2] ions.28,29 The χMT values at 300 K are 2.5 cm3 mol−1 K for both YbIII compounds (3 and 4) and 11.5 cm3 mol−1 K for both Er(III) assemblies (5 and 6). They are close to the calculated values (2.57 cm3 mol−1 K for isolated Yb3+ ions with a 2F7/2 ground state, J = 7/2, gJ = 8/7; and 11.5 cm3 mol−1 K for isolated Er3+ ions with a 4I15/2 ground state, J = 15/2, gJ = 6/5). Upon cooling, the χMT values decrease gradually at high temperature and rapidly in the low-temperature range. This effect can be attributed to several factors, such as depopulation of mJ levels within the ground multiplet related to crystal field effects and/or weak antiferromagnetic interactions between neighboring lanthanides (Figures S27 and S28). The magnetization (M) versus external magnetic field curves at T = 1.8 K indicate a rapid increase in the magnetization at low field followed by a slow linear increase under an applied magnetic field of 70 kOe for complexes 1, 2, 5, and 6, indicating the presence of magnetic anisotropy. The saturation magnetizations of 6.0μB and 6.7μB for 1 and 2, respectively, are slightly higher than the predicted value for an isolated Dy3+ ion in the ground mJ = ±15/2 doublet state (Seff = 1/2, geff = 20, producing a saturation magnetization of 5μB). Moreover, saturation magnetizations of 1.7μB and 1.8μB for 3 and 4, respectively, suggest a ground-state doublet of a nearly pure mJ = ±7/2 state for Yb(III) ion (Seff = 1/2, geff = 8 with Ising anisotropy, producing a saturation magnetization of 2μB). Assemblies 5 and 6 reach saturation magnetizations of 5.2μB and 5.3μB, respectively, at Hdc = 70 kOe, corresponding to the ground-state doublet of a nearly pure mJ = ±15/2 state for Er(III) ion (Seff = 1/2, geff = 20, producing a saturation magnetization of 5μB). Dynamic Magnetic Properties of 1−6. The alternatingcurrent (ac) magnetic susceptibility measurements at Hdc = 0 Oe and Hac = 3 Oe indicate that none of the complexes exhibit frequency-dependent signals above 1.85 K, which can be related either to fast quantum tunneling of magnetization (QTM) or to weak magnetic anisotropy with a relatively low anisotropic thermal energy barrier. However, the application of small external dc magnetic field leads to full or partial suppression of the QTM relaxation process for all compounds and produces frequency- and temperature-dependent out-ofphase magnetic susceptibility (χ″M) (Figures 5−7 and S29− S48). The frequency dependence of the out-of-phase magnetic susceptibility χM ″ for 1 measured in dc magnetic fields of 0− 4000 Oe show a single maximum, with the highest signal for Hdc = 1000 Oe (Figure S29). Furthermore, the maximum starts to shift toward lower frequency after application of a dc magnetic field higher than 1200 Oe. Following these two observations, the dynamic magnetic behavior of assembly 1 was investigated over the 1.85−5 K temperature range at applied dc magnetic fields of 1000 and 2000 Oe (Figures 5, S30, and S31). The ac magnetic studies at Hdc = 1000 Oe show a single relaxation process described by the Arrhenius law with

Figure 5. Comparison of ac magnetic characteristics. (a, b, e, f) Frequency (ν) dependence of the out-of-phase component of the complex magnetic susceptibility (χ″M) for (a) 1, (b) 1md, (e) 2, and (f) 2md at Hdc = 1000 Oe and Hac = 3 Oe at various temperatures. The solid lines in (a), (b), and (e) were fitted using the generalized Debye model. (c, g) Temperature dependence of the relaxation time (τ), presented as a plot of ln(τ) vs T−1, for (c) 1 and 1md and (g) 2. The solid lines in (c) are fits to the Arrhenius law. (d, h) Magnetic anisotropy axes (red solid lines) for (d) 1 and (h) 2 calculated using Magellan software.

an energy barrier of ΔE/kB = 25.50(3) K and pre-exponential factor of τ0 = 1.20(4) × 10−9 s (Table 1), which indicated the Table 1. Summary of ac Magnetic Data for 1 Hdc (Oe)

τ0 (s)

ΔE/kB (K)

1000 2000

1.20(4) × 10−9 5.4(4) × 10−7

25.50(3) 12.6(5)

SMM behavior related to the magnetic anisotropy of the isolated Dy(III) centers. Nevertheless, an increase in the dc magnetic field up to 2000 Oe induced a second, slower relaxation pathway with a temperature-independent relaxation time and suppressed the thermal relaxation pathway to give a reduced energy barrier of 12.6(5) K and τ0 = 5.4(4) × 10−7 s (Table 1 and Figure S31). The plausible origin of the second relaxation process can be attributed to the induced magnetic dipole−dipole interaction with neighboring Dy(III) ions linked through Ag···Ag interactions. The χ″M(ν) plots measured in dc F

DOI: 10.1021/acs.inorgchem.8b03634 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry magnetic fields up to 4000 Oe show that substitution of Ag(I) with Au(I) in 2 retains two relaxation process even at lower dc magnetic fields (Figure S32). Furthermore, slower and faster relaxation processes reach the highest value of χ″M versus ν in applied dc magnetic fields of 1000 and 2000 Oe, respectively (Figures S33 and S34). Comparison of the out-of-phase components of the magnetic susceptibility for 1 and 2 at Hdc = 1000 Oe shows that compound 1 exhibits ac signals at a higher temperature than 2 (Figures 5 and S33). Replacement of Ag(I) by the larger and heavier Au(I) atom should lead to a more oblate geometry of the complex and hence should improve the SMM behavior from dysprosium(III) centers. However, the axes of magnetic anisotropy calculated using the Magellan software suggest otherwise.75 The magnetic anisotropy of the AuI-containing complex deviates from the oblate-type geometry by 2.05° compared with that of AgI-containing 1 (Figure 5d,h). The observation of a second, slower magnetic relaxation process in 2 unlike 1 can be further explained by the presence of Au···Au interactions which facilitate intermolecular dipolar interactions. Furthermore, to support this argument, magnetically diluted compounds [Dy0.05Y0.95(terpy)(H2O)(NO3)2][M(CN)2] (M = Ag, 1md; Au, 2md) were prepared to minimize dipole−dipole interactions between the two Dy(III) centers and to highlight the effect of the noble metal on the slow magnetic behavior of isolated Dy(III). Measurements of the ac magnetic properties for 1md and 2md in an applied dc magnetic field of 1000 Oe exhibited a single thermal relaxation process for both assemblies, and the slower relaxation process related to dipole−dipole coupling was almost completely suppressed (Figures 5 and S35−S38). Analysis of the slow magnetic relaxation in the Y(III)/Dy(III)−Ag(I) system (1md) gave a slightly higher thermal energy barrier of ΔE/ kB = 30.6(8) K and slightly shorter attempt time of relaxation of τ0 = 2.7(1) × 10−10 s than for complex 1. In the case of the Y(III)/Dy(III)−Au(I) system (2md), the maximum signal for the out-of-phase component was shifted toward higher frequency and located outside the measurement range. To summarize this part of the research, substitution of Ag(I) with Au(I) has a significant effect on shortening the slow magnetic relaxation time through a decrease of the intrinsic anisotropy of oblate Dy(III) ions. Ytterbium(III)-based complexes 3 and 4 show two competing relaxation processes under the optimal applied dc field of 2000 Oe (Figures 6 and S39−S42). The faster relaxation process shows energy barriers of 24.5(8) and 25.3(8) K and pre-exponential factors of 7.6(7) × 10−8 and 5.1(1) × 10−8 s for 3 and 4, respectively. Meanwhile, the slower relaxation process has slightly increased energy barriers of 29.5(3) and 33.9(4) K and longer τ0 values of 1.2(8) × 10−6 and 4.4(3) × 10−7 s for 3 and 4, respectively. The relaxation mechanism for both processes can be assigned to the combination of Orbach relaxation, phonon-assisted Raman relaxation and the QTM process, which was confirmed by fitting the ln(τ) versus T−1 data to eq 1 (values of the fitting parameters are shown in Table 2): τ −1 = τ0−1e−ΔE / kBT + BRaman T n + τQTM −1

Figure 6. Comparison of ac magnetic characteristics. (a, c) Frequency (ν) dependence of the out-of-phase component of the complex ″ ) for (a) 3 and (c) 4 at Hdc = 2000 Oe and magnetic susceptibility (χM Hac = 3 Oe at various temperatures. The solid lines are fits to the generalized Debye model. (b, d) Temperature dependence of the relaxation time (τ), presented as a plot of ln(τ) versus T −1, for (b) 3 and (d) 4. Solid lines are fits to eq 1.

can be related to the presence of individual Yb(III) centers in the structural unit, showing single fast magnetic relaxation, and dipole−dipole interactions76 with the closest ytterbium(III) centers connected through argentophilic (Ag···Ag) or aurophilic (Au···Au) interactions, resulting in the second, slower magnetic relaxation process. To confirm further the above hypotheses, the magnetically diluted sample [Yb0.05Y0.95(terpy)(H2O)(NO3)2][Ag(CN)2] (3md) was prepared to reduce the probability of magnetic interactions within dinuclear systems. The ac magnetic investigation at Hdc = 2000 Oe (Figure S44) indicated a single thermal relaxation pathway for 3md with an energy barrier of ΔE/kB = 16.0(1) K and preexponential factor of τ0 = 4.9(5) × 10−6 s. This implies that upon dilution the magnetic centers are well-separated to give a single relaxation pathway. The slight increase in the energy barrier for 4 suggests that replacement of silver(I) by gold(I) improves the SMM behavior, which might be due to the enhancement of the prolate magnetic anisotropy of ytterbium(III) ions by the substitution of the higher-electron-density atom.75 Additionally, the first attempt to determine experimentally the values of the energy barriers for ytterbium(III) complexes by measuring the NIR luminescence at 3.75 K was performed (Figure S49). The established energy barriers of 79 and 76 cm−1 for 3 and 4, respectively, are on the same order as the calculated ones with the difference being due to the absence of the magnetic field and the presence of QTM effects for the SMMs. Interestingly, assemblies 5 and 6 with erbium(III) ions show a single relaxation process upon application of a dc magnetic field of 800 Oe (Figures 7 and S45−S48). The ln(τ) versus T −1 plots were fitted using eq 2, which includes three terms describing the direct process, QTM, and Orbach thermal relaxation in sequence (values of the fitting parameters are shown in Table 3): B1 τ −1 = Adirect TH 4 + + τ0−1e−ΔE / kBT 2 1 + B2 H (2)

(1)

where Orbach thermal relaxation is described by τ0 and ΔE, the Raman contribution is represented by the parameters BRaman and n, and QTM is shown by τQTM−1. Above 2.25 K, the two relaxation processes are equally significant and clearer with two distinct maxima. The origin of two relaxation processes G

DOI: 10.1021/acs.inorgchem.8b03634 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 2. Summary of ac Magnetic Data for 3 and 4 complex (component) 3 3 4 4

(faster) (slower) (faster) (slower)

τ0 (s)

Hdc (Oe)

−8

2000 2000 2000 2000

7.6(7)·10 1.2(8) × 10−6 5.1(1)·10−8 4.4(3) × 10−7

ΔE/kB (K)

BRaman (s−1 K−n)

n

τQTM−1 (s−1)

24.5(8) 29.5(3) 25.3(8) 33.9(4)

1.2(6) 0.1(1) 0.9(5) 0.4(2)

5.1(1) 5.6(6) 1.8(2) 2.0(9)

421.9(3) 71.3(3) 180.2(6) 77.5(9)

well as for dipole−dipole interactions. Interestingly, assemblies with Er(III) ion (5 and 6), showing an almost spherical geometry with a minor prolate contribution,77,78 exhibit a decrease in the energy barrier similar to that for complexes 1 and 2 but without occurrence of the second relaxation process. Au(I) seems to decrease the intrinsic magnetic anisotropy of the oblate Dy(III) while enhancing that for Yb(III); this may be due to the similar steric effects in both cases, which have different influences depending on the electronic nature of the lanthanide; it strongly depends on the case, as Er(III), which is also closer to prolate, seems to behave similarly to Dy(III). The second point that Ag-to-Au substitution affects is the intermolecular interactions, which is nicely visible in Dy(III).

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CONCLUSIONS In this work, we have presented comprehensive structural, spectroscopic, and magnetic studies of six dinuclear complexes with the general formula [Ln III (terpy)(H 2 O)(NO 3 ) 2 ][MI(CN)2] (LnIII/MI = Dy/Ag, 1; Dy/Au, 2; Yb/Ag, 3; Yb/ Au, 4; Er/Ag, 5; Er/Au, 6). All of these assemblies are isostructural, and they consist of dinuclear molecules stabilized in a three-dimensional network by hydrogen bonds, π−π interactions, and argentophilic (Ag···Ag) or aurophilic (Au··· Au) interactions. Furthermore, all of the compounds exhibit visible luminescence with excitation-wavelength-induced tuning of the luminescence color from whitish-yellow to blue via green and cyan for 1 and 2, greenish-yellow to blue via green and cyan for 3, greenish-yellow to cyan via green for 4, and whitish-blue to deep blue via cyan for 5 and 6. Additionally, all of the Er(III) and Yb(III) compounds reveal NIR luminescence at room and low temperature as a result of energy transfer from both sensitizers ([M(CN)2]− and 2,2′:6′,2″-terpyridine) for 3 and 4 and from 2,2′:6′,2″terpyridine for 5 and 6. Moreover, all of the complexes exhibit external-magnetic-field-induced single-molecule magnet behavior with energy barriers of up to 35 K. It is noteworthy that substitution of Ag(I) with Au(I) seems to affect negatively the value of the energy barrier for Dy(III) ions with an oblate geometry and Er(III) ions with almost spherical geometry and a minor prolate contribution but induces a moderate positive effect to increase the energy barrier for Yb(III) ions with a prolate geometry. The steric effect of the heavier Au(I) seems to decrease the magnetic anisotropy for Dy(III) and Er(III) but augments the magnetic anisotropy for the case of the prolate charge distribution of Yb(III). Dicyanidometalates can be used for further design of novel SMMs with enhanced properties, as they affect both the intermolecular interactions by metallophilic interactions and the anisotropy by constrain-

Figure 7. Comparison of ac magnetic characteristics. Frequency (ν) dependence of the out-of-phase component of the complex magnetic ″ ) for (a) 5 and (c) 6 at Hdc = 800 Oe and Hac = 3 susceptibility (χM Oe at various temperatures. The solid lines are fits to the generalized Debye model. (b, d) Temperature dependence of the relaxation time (τ), presented as a plot of ln(τ) versus T −1, for (b) 5 and (d) 6. Solid lines are fits to eq 2.

In order to avoid overparametrization, the parameters corresponding to the direct process (Adirect) and QTM relaxation (B1 and B2) were obtained by fitting the relaxation time versus external magnetic field data at 1.85 K using the first two terms of eq 2 (Figure S50). The energy barrier for thermal relaxation in the case of 5 is 35.7(8) K with τ0 = 2.2(4) × 10−11 s, while assembly 6 exhibits an energy barrier of 34.4(8) K with τ0 = 6.7(8) × 10−11 s. The observation of single relaxation process can be attributed to the slow magnetic relaxation of single Er(III) ions and the absence of magnetic dipole−dipole interactions due to the smaller optimal dc field compared with 3 and 4. As explained above, argentophilic (Ag···Ag) and aurophilic (Au···Au) interactions and magnetic anisotropy of lanthanides play a vital role in affecting the SMM properties. Consequently, in the case of 1 and 2 with an oblate-shaped dysprosium ion,77,78 the substitution of Ag(I) by Au(I) results in a decrease in the energy barrier for relaxation of Dy(III) and amplification of dipole−dipole interactions, leading to the second relaxation process. In contrast, the substitution of silver(I) by gold(I) in the case of 3 and 4 containing the Yb(III) ion with a wellpronounced prolate geometry77,78 results in a moderate increases in the energy barrier for relaxation of lanthanide as Table 3. Summary of ac Magnetic Data for 5 and 6 complex 5 6

Hdc (Oe) 800 800

τ0 (s)

Adirect (s−1 K−1 Oe−4)

ΔE/kB (K) −11

2.2(4) × 10 6.7(8) × 10−11

−10

1.1(5) × 10 2.3(5) × 10−10

35.7(8) 34.4(8) H

B1 (s−1)

B2 (Oe−2)

1172.1(5) 1299.1(3)

6.5(0) × 10−7 9.3(3) × 10−7 DOI: 10.1021/acs.inorgchem.8b03634 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

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ing the geometry and changing the charge distribution around the metal centers.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b03634. IR spectroscopy and thermogravimetric analysis; crystal structures of 1−6 and detailed structural information; results of continuous shape measure analysis; PXRD patterns; UV−vis−NIR absorption spectra at room temperature; results of solid-state luminescence studies for 1−6, 1md−3md, 2,2′:6′,2″-terpyridine, K[Ag(CN)2], and K[Au(CN)2] at different temperatures; magnetic properties in ac and dc magnetic fields (PDF) Accession Codes

CCDC 1887723−1887728 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by e-mailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, U.K.; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Olaf Stefańczyk: 0000-0003-0955-5646 Szymon Chorazy: 0000-0002-1669-9835 Barbara Sieklucka: 0000-0003-3211-5008 Shin-ichi Ohkoshi: 0000-0001-9359-5928 Author Contributions

The manuscript was written through contributions of all authors. All of the authors approved the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The present research was partially supported by a Grant-in-Aid for Specially Promoted Research (Grant 15H05697), a Grantin-Aid for Scientific Research on Innovative Areas Soft Crystals (Area 2903, Grant 17H06367), and APSA from MEXT. The Global Science course from MEXT, the Cryogenic Research Center at The University of Tokyo and the Center for Nano Lithography & Analysis at The University of Tokyo supported by MEXT, and the Quantum Leap Flagship Program (QLEAP) by MEXT are acknowledged. The authors acknowledge partial support of this research by the Polish National Science Centre within the OPUS-15 Project (Grant 2018/29/B/ST5/ 00337).

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DOI: 10.1021/acs.inorgchem.8b03634 Inorg. Chem. XXXX, XXX, XXX−XXX