Bimodal Magneto-Luminescent Dysprosium (DyIII)-Potassium (KI

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Bimodal Magneto-Luminescent Dysprosium (DyIII)Potassium (KI)-Oxalate Framework: Magnetic Switchability with High Anisotropic Barrier and Solvent Sensing Sudip Mohapatra, Bharath Rajeswaran, Anindita Chakraborty, Athinarayanan Sundaresan, and Tapas Kumar Maji Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm400116h • Publication Date (Web): 24 Mar 2013 Downloaded from http://pubs.acs.org on April 12, 2013

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Chemistry of Materials

Bimodal Magneto-Luminescent Dysprosium (DyIII)-Potassium (KI)-Oxalate Framework: Magnetic Switchability with High Anisotropic Barrier and Solvent Sensing Sudip Mohapatra, Bharath Rajeswaran, Anindita Chakraborty, A. Sundaresan, and Tapas Kumar Maji* Chemistry and Physics of Materials Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Bangalore, 560064, India *E-mail: [email protected] Keywords: Metal-organic Framework, Photoluminescence, Ferromagnetism, Antiferromagnetism, Magnetic phase transition, Slow magnetic relaxation, Sensor Abstract: We report synthesis, characterization and properties of a multifunctional oxalate framework, {KDy(C2O4)2(H2O)4}n (1) (C2O42- = oxalate dianion) composed of two absolutely different metal ions in terms of their size, charge and electronic configuration. Dehydrated framework (1) exhibits permanent porosity and interesting solvent (H2O, MeOH, CH3CN and EtOH) vapor sorption characteristics based on specific interactions with unsaturated alkali metal sites on the pore surface. Compound 1 shows solvent responsive bimodal magnetic and luminescence properties related to DyIII center. 1 exhibits reversible ferromagnetic to antiferromagnetric phase tranisition upon dehydration and rehydration, hitherto known for any lathanide based coordination polymer or metal-organic frameworks. Both the compounds 1 and 1 exhibit slow magnetic relaxation with very high anisotropic barrier (417 9 K for 1 and 418  7 K for 1) which has been ascribed to the single ion magnetic anisotropy of the DyIII centers. Nevertheless, compound 1 shows metal based luminescence property in the visible region and H2O molecules exhibit strongest quenching effect compared to other solvents MeOH, MeCN and EtOH, evoking 1 as a potential H2O sensor. Sudip Mohapatra, Bharath Rajeswaran, Anindita Chakraborty, A. Sundaresan and Tapas Kumar Maji * Chem. Mater. 2013, 0, 00 Bimodal

Magneto-Luminescent

Dysprosium(DyIII)-Potassium(KI)-Oxalate Framework: Magnetic Switch ability with

A bimetallic lanthanide- alkali metal-oxalate framework has been synthesized and characterized. The framework exhibits solvent responsive magnetic and luminescence properties. The framework shows reversible ferromagnetic to antiferromagnetic phase transition upon dehydration and rehydration, slow magnetic relaxation with very high anisotropic barrier, and water sensing properties.

High Anisotropic Barrier and Solvent Sensing

ACS Paragon Plus Environment


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1. Introduction During the last two decades, the design and synthesis of metal-organic hybrid compounds have attracted considerable interest to the chemists, physicists and material scientists due to their fascinating structures and potential application as magnetic, optoelectronic, and porous material. 1-6 Now, one of the most appealing aims is to fabricate multifunctional hybrid material which combines a set of well-defined properties (e.g porosity and magnetism, porosity and optical, magnetism and conductivity) for specific applications.712

In this context, lanthanide-based metal-organic hybrid compounds are excellent candidates

because of their interesting magnetic and luminescent properties.13 Such a synergism, where two or more different functionalities are united would lead to a novel smart material.14-16 The field of molecular magnetism based on coordination compounds has experienced great deal of attention in the last two decades with the discovery of single molecule magnet (SMM) and single chain magnet (SCM) which exhibit slow relaxation of magnetization and magnetic hysteresis below the blocking temperature (TB).17-23 The origin of the SMM behaviour is the existence of an energy barrier (ΔE) that prevents reversal of magnetization which depends on the large ground state spin multiplicity and the magnetic anisotropy of the compound (D < 0). Lanthanide based mononuclear, polynuclear or extended systems can provide an important platform to explore the SMM or SCM behavior because of their high spin and large intrinsic magnetic anisotropy.23,

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In this context, recent works suggest that

DyIII ions would be a potential candidate for a novel magnetic system by using the appropriate ligand system because of high spin, large magnetic anisotropy and reduced quantum tunneling of the magnetization (QTM).25-32 Furthermore, lanthanide compounds have high emission quantum yields under ambient condition.12,33 We envisioned that the presence of porosity in such lanthanide-organic frameworks would further provide the opportunities to study gas storage, sensing for small molecules and guest dependent magnetic and luminescent properties which are of particular interest for applications as magnetic


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devices and sensors. Some elegant examples of porous magnets based on transition metal ions with reversible solvent induced change in the magnetic ordering have been recently documented.

34-37

It has been observed that slight change around the metal coordination

environment has significant influence on the magnetic exchange coupling. Very few porous magnetic coordination frameworks are known to exhibit reversible ferromagnetic to antiferromagnetic phase transition through desolvation and resolvation,34 such phenomenon in lanthanide-organic systems yet to be accounted. In this contribution we have exploited the large intrinsic magnetic anisotropy, high ground state spin and luminescence property of DyIII to fabricate a novel multifunctional bimetallic 3D porous compound, {KDy(C2O4)2(H2O)4}n (1). Here the magnetically innocent KI ion is an integral part of the framework and acts as a potentate of the structure. The temperature dependence magnetic measurement of 1 reveals ferromagnetic interaction through oxalate linker and exhibits slow magnetic relaxation, very high anisotropic barrier (418 K), and reversible switching of magnetic property from ferromagnetic to antiferromagnetic via dehydration and rehydration. Dehydrated compound {KDy(C2O4)2} (1) exhibits permanent porosity and interesting vapour (H2O, MeOH, MeCN, EtOH) sorption properties. Compound 1 has well characterized emissions at visible region related to DyIII which has been exploited for selective sensing for solvent molecules (H2O, MeOH, MeCN and EtOH) based on quenching of the emission intensities. To the best of our knowledge this is the first report of a lanthanide MOF where three interesting properties like porosity, solvent induced modulation in magnetic and luminescent properties have been combined together (Scheme 1).



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2. Experimental Section Materials: All the reagents were commercially available and used as supplied without further purification. Dy(NO3)3∙5H2O, Y(NO3)3.6H2O and potassium oxalate (K2C2O4∙H2O) were obtained from Aldrich Chemical Co.

Synthesis of {KDy(C2O4)2(H2O)4}n (1): Dy(NO3)3·5H2O (0.219 g, 0.5 mmol), K2C2O4·2H2O (0.184 g, 1 mmol), and 10 mL distilled H2O were placed in a 25 mL beaker, and the whole reaction mixture was vigorously stirred for 1 hour. The white color reaction mixture was transferred into a 23 mL Teflon-lined stainless steel autoclave and the system was kept in the oven at 180 oC for 8 days. After 8 days the autoclave was kept out and allowed to cool to room temperature. The white colour crystalline solid was filtered and washed several times with H2O to remove unreacted starting materials and dried under vacuum. Yield 72.5 % (relative to DyIII). Anal. Calcd. for KDyH8C4O12: C, 10.68; H, 1.78. Found: C, 10.71; H, 1.75. IR (KBr cm-1), 3530 br (OH); 1623 br s(COO); 1329 s(COO). Synthesis of {KDy0.25Y0.75(C2O4)2(H2O)4}n (2): Compound 2 was prepared using the similar reaction condition as of 1 except Dy(NO3)3·5H2O and Y(NO3)3.6H2O metal source have been used in a 1 : 3 molar ratio instead of only Dy(NO3)3.5H2O. Anal. Calcd. for KDy0.25Y0.75H8C4O12: C, 12.17; H, 2.08. Found: C, 12.06; H, 2.15. IR (KBr cm-1), 3536 br (OH); 1625 br s(COO); 1324 s(COO). Physical measurements: The elemental analyses were carried out in a Perkin Elmer 1800 instrument. IR spectra were recorded on a Bruker IFS 66v/S spectrophotometer with samples prepared in KBr pellets in the region 4000-400 cm-1. Thermogravimetric analysis (TGA) was carried out using Metler Toledo instrument in the range of 28 - 600°C using the heating rate of 3°C/min under N2 atmosphere. X-ray powder diffraction (PXRD) patterns in the different state of the sample were recorded on a Bruker D8 Discover instrument using Cu-K radiation


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(step size = 0.019467, step time = 87.99). Magnetic measurements were carried out with vibrating sample magnetometer in Physical Properties Measurement Systems (PPMS, Quantum Design, USA). UV-vis and fluorescence spectra were recorded on a Perkin Elmer model Lambda 900 spectrophotometer and Perkin Elmer model LS 55 spectrophotometer, respectively. Lifetime measurements were carried out at room temperature using Edinburgh Instrument FLSP 920 spectrometer.

Adsorption measurements: The adsorption isotherm of different solvents (H2O, CH3CN, EtOH at 298 K and MeOH at 293 K) were measured in the vapour state by using BELSORPaqua volumetric adsorption instrument from BEL, Japan. In the sample chamber (~12 mL) maintained at T ± 0.03 K was placed the adsorbent sample (100-150 mg), which had been prepared at 433 K at 10-1 Pa for 18 hours prior to measurement of the isotherms. The adsorbate was charged into the sample tube, and then the change of pressure was monitored and the degree of adsorption was determined by the decrease of pressure at the equilibrium state. All operations were computer-controlled and automatic.

X-ray crystallography: A suitable single-crystal of compound 1 was mounted on a thin glass fibre using commercially available super glue. X-ray single-crystal structural data were collected on a Bruker Smart–CCD diffractometer equipped with a normal focus, 2.4 kW sealed tube X-ray source with graphite monochromated Mo-Kα radiation (λ = 0.71073 Å) operating at 50 kV and 30 mA, with ω scan mode. The programme SAINT

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was used for

integration of diffraction profiles and absorption correction was made with SADABS programme.39 The structure was solved by direct method using SIR-92

40

and followed by

successive Fourier and difference Fourier Syntheses. All the non-hydrogen atoms were refined anisotropically. All calculations were carried out using SHELXL 97, 41 SHELXS 97,



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42

PLATON 99, 43 and WinGX system, ver. 1.70.01.44 Crystal data and structure refinement

parameters for 1 are given in Table S1 and selected bond lengths and angles are provided in Table S2. [Crystallographic data (excluding structure factors) for the structure reported in this paper have been deposited with the Cambridge Crystallographic Data Centre and the corresponding CCDC no. 906708] 3. Results and discussion 3.1. Structural description of {KDy(C2O4)2(H2O)4}n (1) Compound 1 crystallizes in tetragonal space group I41/amd. Single-crystal X-ray structural determination reveals that 1 is a neutral 3D bimetallic coordination framework of DyIII and KI bridged by the oxalate linkers (ox2-) (Figure 1a). The compound 1 is isomorphous to {KHo(C2O4)2(H2O)4}n reported previously by us.45 Similarly here each octacoordinated DyIII center chelated to four different ox2- through the oxygen atoms (O1 and

O2) forming a distorted square-antiprismatic coordination geometry. It is worth

mentioning that the coordination number eight of DyIII is satisfied without any ancillary solvent molecule. Each octacoordinated KI center is connected to four oxygen atoms (µ 2-O1) of ox2- and rest of the four coordination numbers are satisfied with four H2O molecules (O1w) (Figure 1a). Oxalate linker is connecting DyIII and KI centers along crystallographic a, b and c direction to form a 3D architecture (Figure 1b). DyIII-O1/O2 bond distances are 2.390(3) and 2.351(2) Å respectively and KI-O1/O1w bond distances are 2.849(3) and 2.908(4) Å respectively (see Table S1 and S2). The HoIII-O bond distances in previously reported {KHo(C2O4)2(H2O)4}n 40 are slightly smaller than the DyIII-O bond distances, which would be due to the lanthanide contraction. After removal of the KI bound H2O molecules the 3D framework may generate bidirectional channels with the dimensions of 3.5 × 3.5 Å2 along the c-axis and 2.0 × 1.1 Å2 along perpendicular to a axis (Figures 1c, 1d and S1). The distance between two ox2- bridged DyIII centers is 6.172 Å.


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3.2. Thermal stability and structural reversibility

Powder X-ray diffraction (PXRD) pattern of the assynthesized sample obtained in bulk is similar to the simulated pattern calculated from single crystal structural data suggesting the compound is pure (Figure S2). To study the framework stability of 1, TGA and PXRD patterns at different states were performed. TGA suggests that four KI bound H2O molecules are released in the temperature range of 45 - 120 ºC and the dehydrated solid (1′) is stable up to 380 ºC without further weight loss (Figure S3). The weight loss (obs. 15.01wt %) is consistent with the four H2O molecules (calc. 14.16 wt %). The PXRD pattern of 1′ shows sharp lines with shifting of some peak positions, and appearance of some new peaks compared to the as-synthesized compound 1, suggesting structural transformation to lower symmetry after removal of KI bound H2O molecules. Indexing of the powder pattern of 1′ by using the TREOR programme

46

suggests triclinic crystal system with a = 9.079(9) Ǻ, b =

12.201(2) Ǻ, c = 13.990(7) Ǻ, α = 60.473(3) º β = 84.824(2) º, γ = 68.854(8) º and V = 1250.35 Ǻ3, which indicates the structural transformation in 1 after removal of the H2O molecules (see Table S3). When 1′ is exposed to H2O vapour for three days, assynthesized framework regenerated as revealed by the PXRD pattern indicating reversibility in structural transformation upon dehydration and rehydration. Indexing of the powder pattern of the rehydrated solid indicating the tetragonal crystal system with cell parameters a = b = 11.467(7) Å; c = 8.902(5) Å; V = 1170.75 Å3, which is similar to the assynthesized framework 1. Compound 1 was heated at 130 °C for six hours under vacuum and then TGA and IR spectroscopic measurements were performed. TGA shows negligible weight loss up to 380 °C and no peak corresponds to H2O molecules in the IR spectrum indicating dehydration was completed in the above mentioned condition (Figure S3).



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3.3. Porous property

Inspired by the squared shaped channels decorated with the unsaturated alkali metal ions, we anticipated that 1' could selectively adsorb solvent molecules on the basis of the size and polarity of the pore surface and H2O, MeOH, MeCN and EtOH adsorption experiments were carried out (Figure 2). The dehydrated compound 1' uptakes H2O (kinetic diameter 2.8 Å), MeCN (4.3 Å) and MeOH (4.0 Å) in two steps but the occlusion of EtOH (4.5 Å) occurs in one step. The H2O vapor sorption profile exhibits stepwise adsorption with rapid uptake at low pressure region and incomplete desorption suggesting strong interaction with the pore surface and it has been correlated to the presence of unsaturated KI site. The calculation using saturation sorption amount suggest that 1' uptakes 3.74 molecules of H2O (220 mL g-1), 0.58 molecule of MeOH (35 mL g-1), 0.74 molecule of MeCN (44 mL g-1) and 0.37 molecule of EtOH (22 mLg-1) per formula unit. The sorption amount of the different solvent vapors depends not only to their size but also to their polarity and interaction with the framework structure having unsaturated KI sites. The structural reversibility with dehydration and rehydration can be realized by the similar PXRD pattern and almost same amount of H2O adsorption revealed from the sorption profile.

3.4. Photoluminescent property The luminescence property of assynthesized, dehydrated and rehydrated compound (1ʹʹ) were investigated in the solid state. As shown in Figure 3A, the emission spectra of 1, 1ʹ and 1ʹʹ were recorded in the range 450 to 700 nm using the excitation wavelength (λex) of 365 nm (Figure S4). The emission spectrum of 1 exhibits two characteristics peaks of DyIII at 485 nm and 576 nm, which correspond to 4F9/2→6H15/2, 4F9/2→6H13/2 transitions respectively.13-15 To realize the effect of KI bound H2O molecules on the emission intensity, the luminescence


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measurement was carried out for the dehydrated compound (heating at 130º C for 6 hours under vacuum) under same experimental conditions. Both the characteristic emission peaks in 1ʹ were enhanced to almost double in intensity and slightly blue shifted compared to 1 (Figure 3A). This study clearly demonstrates that H2O molecules act as a quencher although they are not directly coordinated to DyIII. The slight blue shifted emission can be correlated to the structural change after removal of H2O molecules which we observed from the PXRD pattern. It is worth noting that H2O exposed dehydrated compound (1ʹʹ) shows decrease in emission intensity consistent with the quenching effect of the H2O molecules (Figure 3A). We did not observe the same emission intensity as of as-synthesized compound because of incomplete rehydration. As the dehydrated compound 1 uptakes different solvent vapors (vide infra) of different amount (Figure 2), we studied their effect on emission intensity after exposing to them. Time dependent measurements were carried out by keeping a glass slide containing thin layer of 1ʹ inside a quartz cuvoid containing different solvents. The emission intensity at 576 nm was recorded for 40 minutes with certain time interval after exciting 1ʹ at 365 nm. The extent of intensity change at different time interval with different solvent vapors has been shown in Figure 5C which suggests the quenching effect is maximum in case of H2O compared to MeCN, MeOH and EtOH. Indeed, the extent of quenching depends upon the amount of solvent included into the framework. Vapour sorption isotherms suggest the adsorption amount decreases in the order, H2O> MeCN > MeOH> EtOH and the extent of quenching by these solvents also follow the same order. The high quenching effect of H2O compared to other solvents (MeOH, EtOH, MeCN) may find application of compound 1 as a H2O sensor. Surprisingly here in 1, H2O molecules are not directly coordinated to the DyIII center, although its effective quenching effect has been observed. Significantly high enhancement of intensity after dehydration is due to the inhibition of nonradiative decay



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process. The quantum yield depends on the energy gap between emissive state and highest sublevel of the ground state multiplet. The smaller the energy gap easier will be the nonradiative decay through the vibration of O-H group from H2O or other solvent molecules which are in sufficiently close proximity to the emissive lanthanide ion. Furthermore, to support the quenching effect of H2O, we performed the lifetime measurements of the 4F9/2 state of DyIII for 1 and 1 (Figure 3B). The significant increase in the lifetime of 4F9/2 in 1 (τ1 = 10.258 μs, τ2 = 2.063 μs) from 1 (τ1 = 7.894 μs, τ2=1.558 μs) indeed suggesting H2O molecules is an effective quencher in this system. It is worth mentioning that we measured emission properties for the dehydrated and rehydrated compounds for a same sample for three consecutive cycles and observed almost similar change in emission intensities in two different states, further confirming it can acts as a potential water sensor material (Figure S5).

3.5. Magnetic properties

3.5.1. Static magnetic property

The temperature dependent zero field cooled dc magnetic susceptibility measurement of 1 was performed in the temperature range of 300 – 3 K. The plots of temperature dependence of molar magnetic susceptibility χM and χMT of 1 show a very clear signature of weak ferromagnetic interaction between the DyIII centers (Figure 4a). At room temperature χMT value is 14.25 emu K mol-1, close to the value (14.17 emu K mol-1) expected for a magnetically isolated DyIII center (g = 4/3, J = 15/2). χMT value gradually increases upon decreasing the temperature and reaches a maximum value of 16.68 emu K mol-1 at 50 K and then χMT value remain constant up to 32 K. Further cooling χMT rapidly decreases to 14.95 emu mol-1 K at 3 K due to the depopulation of the excited state Stark levels. The increase of χMT up to 50 K from 300 K suggests the presence of the ferromagnetic interaction in the


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system. The linear fitting of χM-1 vs T of 1 at 290-40 K deduce the Weiss constant (θ) = 11.3 K which supports the presence of ferromagnetic interaction operating between the DyIII centers at a distance of 6.17 Å connected by the ox2- linker (Figure S6). The ox2- is a small linker simultaneously can offer both σ and π electronic pathways for magnetic superexchange between the DyIII centers. To further prove that the ferromagnetic interaction operating between two DyIII centers, we prepared a magnetically diluted compound {KDy0.25Y0.75(C2O4)2(H2O)4}n (2) containing 3:1 ratio of YIII and DyIII and structurally characterized from PXRD pattern (Figure S7). Similarity in PXRD patterns of 1 and 2 suggests both are isostructural but the composition is different. Both EDAX and room temperature χMT value suggests YIII and DyIII are present in 3:1 ratio in 2. Temperature dependent dc susceptibility measurement of 2 suggests the compounds are magnetically different from 1 (Figure S8). Compound 2 exhibit a parallel response with respect to temperature up to 90 K and χM-1 vs T profile deduced a very small negative θ value (-2.1 K) compared to 1 suggesting ferromagnetic interaction is not operating in the YIII diluted sample due to longer DyIII …DyIII distance (Figure S9). To study the effect of dehydration on the magnetic properties, we measured the magnetic susceptibility of the dehydrated compound (1) in the temperature range of 300 – 3 K and observed a different χMT vs T profile (Figure 4b). In 1ʹ χMT value continuously decreases from 14.23 emu K mol-1 at 300 K and reaches a minimum at 3 K suggesting possible antiferromagnetic interaction operating in the system. Curie-Weiss fitting of χM-1 vs T plot for 1' over the whole temperature range deduced Weiss constant (θ) is -3.9 K in 1' (Figure S10). Furthermore to prove the weak antiferromagnetic interaction operating between the DyIII in the dehydrated phase χMT vs T profile and θ values are compared for 1ʹ and the dehydrated phase of {KDy0.25Y0.75(C2O4)2(H2O)4}n (2), i.e. 2ʹ.(Figure 4, Figures S8, S10, S11). Negligibly small θ value (- 0.5 K) in 2ʹ indicating the antiferromagnetic interaction is



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also get effected through the diamagnetic YIII dilution. To understand the effect of rehydration in magnetic properties, 1' was exposed to H2O vapor for three days and then rehydrated framework 1'' was subjected for magnetic measurement which shows similar χMT vs T profile as of compound 1. The χMT value gradually increases from 300 K upon cooling and reaches maximum at 50 K and again decreases on further cooling due to the depopulation of excited state Starks level (Figure 4c). Linear fitting of χM-1 vs T at 290 - 40 K, deduced positive θ value of 3.65 K suggesting reappearance of ferromagnetic interaction and the smaller θ value compared to the as-synthesized compound evoking rehydration was incomplete (Figure S12). This result indicates that reversible ferromagnetic to antiferromagnetic phase transition based on dehydration and rehydration in the framework which is unprecedented in the lanthanide-organic framework systems. The ferromagnetic to antiferromagnetic phase transition after dehydration is due to the structural change leading to the change in magnetic interaction. The magnetic interaction between the lanthanide ions is weak due to the core shell nature of f electrons and complicated due to unquenched orbital contribution and spin orbit coupling. The magnetic properties of the magnetically diluted compounds 2 and 2 are further confirming the presence ferromagnetic interaction in 1 and antiferromagnetic interaction in 1ʹ.

3.5.2. Dynamics of magnetization To understand the dynamics of magnetization we performed alternating current (ac) magnetic measurement of compound 1 under the dc field Hdc = 0 and Hac = 10 Oe from 5 K to 80 K temperature range.

Both the real χMT and imaginary χM part of the ac

susceptibilities shows strong frequency dependence below 53 K and peaks are more prominently observed in χM profile (Figure 5 and S13). The peaks are observed above 1000 Hz and shift to higher temperature with increasing frequencies which is a signature of slow


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magnetic relaxation. Completely dehydrated framework (1ʹ) obtained after heating at 130˚C for 6 hours also shows similar frequency dependence (Figure S14). Furthermore in both cases both the χM and χM value increases with decreasing temperature below 30 K indicating superparamagnism plus paramagnetism. Similar behavior have also been reported by Song et al. in purely lanthanide based 3D framkework, {Dy(TDA)1.5(H2O)2}n [TDA =thiophene-2,5dicarboxylic acid anion)].20 Superparamagnetic property of the compound was also confirmed from the field dependent magnetic property study at 3 K, a saturation magnetization value of 6.05 µ B was obtained without any hysteresis (Figure S16). The peak temperature Tp obtained from the plots of χM -T after Gaussian fitting deduce a linear plot of 1/Tp versus lnτ and obey the Arrhenius law τ = τ0 exp(ΔE/KBT). The best fitting yields the energy barrier, ΔE/KB = 417  9 K and relaxation time τ0 = 1.23 ×10-09 S. (Figure 6) On the other hand to obtain dynamical exponent, ᴢѵ, frequency dependence of ac χM was fitted by conventional critical scaling law of spin dynamics as described by τ = τ0 [((Tp-Tf)/Tf)]-ᴢѵ, where τ = 1/2πf providing ᴢѵ = 2.58. It is worth mentioning that the ᴢѵ value for spin glasses is 4-12 (Figure 6). This result suggests that it is not a spin glass behavior; rather it is a conventional magnetic phase transition. To confirm our results we have repeated the measurements several times and able to reproduce results. The dehydrated compound, confirmed from IR and TGA was subjected for the ac susceptibility measurements and shows the similar behaviour with ΔE/KB, τ0 and ᴢѵ values of 418 7 K, 3.0 × 10-09 S and 2.24 respectively (Figures S14 and S15). To further confirm the slow relaxation of magnetization is due to single ion anisotropy and is independent of the magnetic interaction between two DyIII centers, we have studied ac susceptibility measurements for magnetically diluted sample 2, and similar frequency dependence was observed as of 1 and 1ʹ (Figure S17). The ac magnetic measurements of 2 further support that the dynamics of magnetization is independent of the nature of magnetic interaction between two neighbouring DyIII centers.



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Therefore this slow magnetic relaxation is attributed to the single-ion magnetic anisotropy which is also observed in other DyIII based single ion, single molecule magnet or extended coordination polymer. 47-51 4. Conclusions We have successfully synthesized a novel bimetallic (DyIII-KI) 3D framework connected by oxalate linker and having 2D channels filled with the K I bound H2O molecules. By exploiting DyIII as a metal ion, we could obtain multiple properties in the same compound. The dehydrated framework is permanently porous and shows interesting vapor sorption properties (H2O, MeOH, MeCN and EtOH) and also reveals stronger metal-based emission properties compared to assynthesized compound. Based on luminescence quenching by the different solvent molecules, this compound can act as a potential H 2O sensor. The compound 1 unveils switching of magnetic properties (ferromagnetic to antiferromagnetic upon dehydration and rehydration) and shows second highest anisotropic barrier (418 K) among the other reported lanthanide compounds including single-molecule magnets (SMMs) and single chain magnets (SCMs).48 The anisotropic barrier arises due to the isolated DyIII ion and independent of nature of interaction between neighbouring DyIII center. The framework demands high impact in the field of molecular magnetism and as a luminescent sensor material.

Acknowledgements Authors are thankful to Prof. C.N.R Rao for his constant support and encouragements. S. M thankful to Council of Scientific and Industrial Research (India) for financial support.


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TKM gratefully acknowledges the Seikh Saqr career award fellowship. Authors are thankful to Dr. P. Mandal for fruitful discussion and support.

5. References

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Figure Caption

Scheme 1: Solvent induced change in structure and the corresponding magnetic and luminescent properties in 1. Figure 1. (a) View of the coordination environment of DyIII and KI in 1. (b) Extended 3D structure of 1 built by the ox2- linker bridging DyIII and KI alternatively, viewed along the crystallographic c axis. (c) View of the 1D channels after removing H2O molecules along the crystallographic c axis. (d) View of the small channels perpendicular to crystallographic b axis.

Figure 2. Solvent sorption isotherms for 1: (left) H2O at 298 K, (right) (a) MeCN at 298 K (b) MeOH at 293 K, (c) EtOH at 298 K. P0 is the saturated vapor of the respective solvent at the mentioned temperature.

Figure 3. (A) Luminescent spectra of the (a) as-synthesized compound (1), (b) dehydrated compound (1ʹ), (c) rehydrated compound (1ʹʹ) excited at 365 nm. (B) Decay profile of excited state of DyIII (4F9/2) for 1 (b) and 1ʹ (a) showing longer lifetime in 1' compared to 1. (C) Change of luminescent intensity with time ; (a) H2O (b) MeOH (c) EtOH (d) MeCN; (D) Histogram showing the same.

Figure 4. Left Side: χMT as a function of temperature plot for (a) as-synthesized (1) (b) dehydrated (1') (c) rehydrated (1'') in the temperature range 5 K to 300K. Right Side: χM as a function of temperature for 1.

Figure 5. Frequency dependence of imaginary part of the ac susceptibility for as-synthesized compound 1.

Figure 6. (a) Frequency dependence of ac χMˊˊ fitted by Arrhenius law, the solid line representing the least square fitting of the experimental data to the Arrhenius equation; (b) Frequency dependence of ac χMˊˊ for 1 fitted by conventional critical scaling law of the spin dynamics as described by  = 0 [((Tp-Tf/Tf))]-ᴢѵ .



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Scheme1


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Figure 1



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Figure 2

Figure 3


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Figure 4

Figure 5



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Figure 6


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Bimodal Magneto-Luminescent Dysprosium (DyIII)-Potassium (KI

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