Magneto-Luminescence Correlation in the Textbook Dysprosium(III) Nitrate Single-Ion Magnet Ekaterina Mamontova, Jérôme Long, Rute A

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Magneto-Luminescence Correlation in the Textbook Dysprosium(III) Nitrate Single-Ion Magnet Ekaterina Mamontova, Jérôme Long, Rute A. S. Ferreira, Alexandre Botas, Dominique Luneau, Yannick Guari, Luis D. Carlos, Joulia Larionova

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Ekaterina Mamontova, Jérôme Long, Rute A. S. Ferreira, Alexandre Botas, Dominique Luneau, et al.. Magneto-Luminescence Correlation in the Textbook Dysprosium(III) Nitrate Single-Ion Magnet. Magnetochemistry, MDPI, 2016, 2 (4), pp.41. 10.3390/magnetochemistry2040041. hal-01399122

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Article Magneto-Luminescence Correlation in the Textbook Dysprosium(III) Nitrate Single-Ion Magnet

Ekaterina Mamontova 1, Jérôme Long 1,*, Rute A. S. Ferreira 2, Alexandre M. P. Botas 2,3, Dominique Luneau 4, Yannick Guari 1, Luis D. Carlos 2 and Joulia Larionova 1 1 Institut Charles Gerhardt Montpellier, UMR 5253, Ingénierie Moléculaire et Nano-Objects, Université de Montpellier, ENSCM, CNRS, Place E. Bataillon, 34095 Montpellier CEDEX 5, France; [email protected] (E.M.); [email protected] (Y.G.); [email protected] (J.L.) 2 Physics Department and CICECO—Aveiro Institute of Materials, University of Aveiro, 3810-193 Aveiro, Portugal; [email protected] (R.A.S.F.); [email protected] (A.M.P.B.); [email protected] (L.D.C.) 3 I3N, University of Aveiro, 3810-193 Aveiro, Portugal 4 Laboratoire des Multimatériaux et Interfaces (UMR 5616), Université Claude Bernard Lyon 1, Campus de la Doua, 69622 CEDEX Villeurbanne, France; [email protected] * Correspondence: [email protected]; Tel.: +33-4-67-14-38-33

Academic Editor: Kevin Bernot Received: 11 October 2016; Accepted: 12 November 2016; Published: 18 November 2016

Abstract: Multifunctional Single-Molecule Magnets (SMMs) or Single-Ion Magnets (SIMs) are intriguing molecule-based materials presenting an association of the slow magnetic relaxation with other physical properties. In this article, we present an example of a very simple molecule based on Dy3+ ion exhibiting a field induced SIM property and a characteristic Dy3+ based emission. The [Dy(NO3)3(H2O)4]·2H2O(1) complex is characterized by the means of single crystal X-Ray diffraction and their magnetic and photo-luminescent properties are investigated. We demonstrate here that it is possible to correlate the magnetic and luminescent properties and to obtain the Orbach barrier from the low temperature emission spectra, which is often difficult to properly extract from the magnetic measurements, especially in the case of field induced SIMs.

Keywords: lanthanides; single-ion magnets; single-molecule magnet; luminescence; magnetic relaxation

1. Introduction Single-Ion Magnets (SIMs) are intriguing mononuclear coordination complexes which represent one of the smallest magnetically bistable units since they are composed of a single paramagnetic transition metal or lanthanide ion [1–3]. Consequently, such molecular objects appear as promising candidates for high-density data storage or quantum computing [1,4]. In the case of lanthanide ions, the magnitude of the anisotropic barrier responsible for the reversal of the magnetization arises from the intrinsic electronic structure and in particular from the sub-levels structure of the Stark components. Such energetic considerations rely on numerous parameters including the nature of the lanthanide ion (Kramers/non-Kramers character, oblate/prolate electronic density), its geometry as well as the overall coordination environment often governed by the donor strength of the ligands [5]. In addition, the relaxation of the magnetization may be governed by various mechanisms that are either spin-lattice processes (Orbach, Raman and direct relaxations) or the Quantum Tunneling of the Magnetization (QTM). The latter is known to depend also on numerous parameters such as the occurrence of magnetic exchange interactions, hyperﬁne coupling or dipolar interactions [6]. Understanding the mechanisms that govern the relaxation of the magnetization and the parameters that affect these relaxation processes is particularly important to ultimately optimize the SIM features.

Magnetochemistry 2016, 2, 41; doi:10.3390/magnetochemistry2040041 www.mdpi.com/journal/magnetochemistry MagnetochemistryMagnetochemistry 20162016,,2 2,, 41 41 22 ofof 1111

In this line of scope, luminescent SIMs are particularly appropriate for these investigations since In this line of scope, luminescent SIMs are particularly appropriate for these investigations the photo‐luminescence measurements permit determining spectroscopically the Stark sub‐level since the photo-luminescence measurements permit determining spectroscopically the Stark sub-level structure of the lanthanides ions and comparing it with the magnetic data. However, examples of structure of the lanthanides ions and comparing it with the magnetic data. However, examples of such such magneto‐optical correlations are rather scarce [7–18] in comparison with the large number of magneto-optical correlations are rather scarce [7–18] in comparison with the large number of reported reported lanthanides‐based SIMs. This fact is often due to the weak or completely quenched emission lanthanides-based SIMs. This fact is often due to the weak or completely quenched emission in these in these complexes. In order to overcome this problem, antenna ligands are used to enhance the complexes. In order to overcome this problem, antenna ligands are used to enhance the lanthanide lanthanide emission. Thus, in all cases of magneto‐luminescent SIM, the observed lanthanide emission. Thus, in all cases of magneto-luminescent SIM, the observed lanthanide luminescence relies luminescence relies on the use of coordinated ligands or complexes as sensitizers to enhance the on the use of coordinated ligands or complexes as sensitizers to enhance the lanthanide luminescence lanthanide luminescence via an energy transfer mechanism [19]. We report in this article that such via an energy transfer mechanism [19]. We report in this article that such magneto-optical correlations magneto‐optical correlations can be extended to a textbook dysprosium(III) nitrate coordination can be extended to a textbook dysprosium(III) nitrate coordination complex by direct excitation in complex by direct excitation in the f–f transitions of the lanthanide ion. Surprisingly, this complex the f–f transitions of the lanthanide ion. Surprisingly, this complex behaves as a ﬁeld induced SIM behaves as a field induced SIM allowing to perform a magneto‐optical correlation. allowing to perform a magneto-optical correlation. 2. Results 2. Results

2.1.2.1. StructureStructure

X‐Ray Single Crystal diffraction indicates that [Dy(NO3)3(H2O)4]∙2H2O (1) crystallizes in the X-Ray Single Crystal diffraction indicates that [Dy(NO3)3(H2O)4]·2H2O(1) crystallizes in the triclinictriclinic PP-1‐1 spacespace groupgroup (Table(Table S1) S1) withwith aa uniqueunique crystallographiccrystallographic molecule.molecule. TheThe coordinationcoordination spheresphere 3+ ofof the Dy Dy3+ ionion is isconstituted constituted by bythree three bidentate bidentate nitrate nitrate and four and water four water molecules molecules leading leading to an overall to an overallcoordination coordination number number of 10 of(Figure 10 (Figure 1). The1). The Dy–O Dy–O distances distances are are comprised comprised between between 2.357(6)2.357(6) andand 2.792(5)2.792(5) Å.Å. The analysis of the geometry using using the the SHAPE SHAPE software software [20] [20 ]indicates indicates that that it it is is close close to to a aspherocorona spherocorona (Table (Table S2). S2). Study Study of of the the crystal crystal packing packing reveals a complex hydrogen bondsbonds networknetwork involvinginvolving oxygenoxygen nitratesnitrates andand bothboth coordinatedcoordinated andand uncoordinateduncoordinated waterwater moleculesmolecules givinggiving riserise toto aa three-dimensionalthree‐dimensional supramolecular supramolecular network. network. The The shortest shortest Dy–Dy Dy–Dy distance distance in in the the crystal crystal is foundis found to beto 6.3282(7)be 6.3282(7) Å. Å.

(a) (b)

FigureFigure 1.1. ((a)) molecular structurestructure of of1 1.. ColorColor code:code: orangeorange,, Dy;Dy; redred,, O;O; blueblue,, N; lightlight grey,, C. Hydrogen atomsatoms are are omitted omitted for for clarity; clarity; and and (b )(b view) view of theof the packing packing arrangement arrangement along along the a thecrystallographic a crystallographic axis. axis. 2.2. Magnetic Properties 2.2. Magnetic Properties The magnetic properties of 1 were investigated using a Superconducting QUantum Interference The magnetic properties of 1 were investigated using a Superconducting QUantum Interference Device (SQUID) magnetometer (Quantum Design, San Diego, CA, USA) working in the temperature Device (SQUID) magnetometer (Quantum Design, San Diego, CA, USA) working in the temperature range 1.8–350 K up to 7 T in direct current (DC) and alternating currents (AC) modes. range 1.8–350 K up to 7 T in direct current (DC) and alternating currents (AC) modes.

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2.2.1. DC Magnetic Properties The room temperature valuevalue ofof χχTT isis equalequal to to 14.04 14.04 cm cm33·∙KK·∙molmol−−11, ,which which is is in in a a good agreement with the the value value of of 14.17 14.17 cm cm3∙K3·∙Kmol·mol−1 expected−1 expected for a for Dy a3+ Dyion3+ usingion usingthe free the‐ion free-ion approximation. approximation. Upon cooling,Upon cooling, χT remainsχT constantremains down constant to 50 down K before to decreasing 50 K before to reach decreasing the value to reachof 11.87 the cm value3∙K∙mol of−1 11.87at 1.8 cm K 3(Figure·K·mol −2).1 at Such 1.8 K a (Figure phenomenon2). Such can a phenomenon be ascribed can to bethe ascribed thermal to depopulation the thermal depopulation of the Stark sublevels.of the Stark The sublevels. field dependence The ﬁeld dependence of the magnetization of the magnetization measured at measured 1.8 K reaches at 1.8 the K reaches value of the 5.85 value μB underof 5.85 70µB kOeunder without 70 kOe evidence without of evidence a clear saturation of a clear saturation (inset of Figure (inset 2). of Figure2).

6 12 5 -1 10 B 4  8 / 3

.K.mol M 3 6 2

/ cm 4 T  0 2 0 20000 40000 60000 H / Oe 0 0 50 100 150 200 250 300 T / K

Figure 2. TemperatureTemperature dependence of χχT measured under a 1000 Oe DC field. ﬁeld. Inset: field ﬁeld dependence dependence of the magnetization measured at 1.8 K.

2.2.2. AC Magnetic Properties AC magneticmagnetic properties properties were were performed performed in order in order to investigate to investigate the occurrence the occurrence of a slow of relaxation a slow relaxationof the magnetization. of the magnetization. Under a zeroUnder DC a zero field, DC no field, significant no significant out-of-phase out‐of susceptibility,‐phase susceptibility,χ00, can χ′′ be, canobserved. be observed. This directly This directly reflects reflects a strong a QTMstrong which QTM maywhich arise may from arise external from external factors suchfactors as such dipolar as dipolarinteractions, interactions, intrinsic effectsintrinsic due effects to hyperfine due to interactionshyperfine interactions or symmetry or deviations symmetry [6 ].deviations Applying DC[6]. Applyingfields induce DC afields short-cut induce of a this short QTM‐cut andof this allow QTM the and observation allow the ofobservation an out-of-phase of an out component.‐of‐phase component.The highest relaxationThe highest time, relaxationτ, is found time, at τ 4.0, is K found around at 4.0 1500–2000 K around Oe 1500–2000 DC fields (FigureOe DC 3fields). For (Figure higher 3).field For values, higher a field decrease values, of aτ decreaseis observed of τ is and observed can be and imputed can be to imputed the direct to the process direct that process becomes that becomespredominant. predominant. Thermally Thermally activated activated and Raman and Raman processes processes are not are field not field dependent dependent but but cannot cannot be beneglected neglected even even at at 4.0 4.0 K, K, and and thethe fieldfield dependence of of the the relaxation relaxation time time can can be befitted fitted by using by using the followingthe following equation equation [21]: [ 21]:

τ−1 = DH4T + B1/(1 + B2H2) + τ0−1exp(−∆/kT) + CTm. (1) −1 4 2 −1 m τ = DH T + B1/(1 + B2H ) + τ0 exp(−∆/kT) + CT . (1) The first term accounts for the direct process (for Kramers‐ion), the second one stands for the QTMThe while first the term third accounts and fourth for the are direct relative process to (forthermally Kramers-ion), activated the and second Raman one standsprocesses, for respectively.the QTM while The thebest thirdfit leads and to fourththe following are relative values: to D thermally = 1.37 × 10 activated−12 s−1∙K−1∙Oe and−4; B Raman1 = 293 s processes,−1, B2 = 2.6 −12 −1 −1 −4 −1 ×respectively. 10−6 Oe−2; C The= 0.0005 best s fit−1∙ leadsK−9; ∆ to= 39 the cm following−1; τ0 = 8 × values: 10−10 s.D The= 1.37 magnitudes× 10 sof the·K B1· Oeand B; 2B parameters1 = 293 s , −6 −2 −1 −9 −1 −10 directlyB2 = 2.6 ×reflect10 theOe degree; C = 0.0005 of mixing s ·K between; ∆ = 39 the cm ±mJ; τlevels0 = 8 ×and10 consequentlys. The magnitudes are a sign of of the theB1 magnitudeand B2 parameters of the QTM directly process. reflect the degree of mixing between the ±mJ levels and consequently are a sign of the magnitude of the QTM process.

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1.0

Magnetochemistry 2016, 2, 41 4 of 11

0.9 1.0

 / ms  0.8 0.9

 / ms 0.7 0.8

0 500 1000 1500 2000 2500 3000 0.7 H / Oe Figure 3. Field dependence of the 0 relaxation 500 1000 time measured 1500 2000 at 4.0 2500 K. The 3000 red solid line represents the H / Oe fitfit using Equation (1). Figure 3. Field dependence of the relaxation time measured at 4.0 K. The red solid line represents the The frequency fit using Equation dependence (1). of of χχʺ00 forfor various various temperatures temperatures under under a a2000 2000 Oe Oe DC DC field field reveals reveals a afrequency frequency dependent dependent asymmetric asymmetric peak peak (Figure (Figure 4a).4a). The The maximum maximum of of the peakpeak shiftsshifts toto higherhigher temperaturesThe uponfrequency increasing dependence frequency, of χʺ for variouswhich temperaturesindicates a fieldunder‐induced a 2000 Oe slow DC field relaxation reveals aof the temperaturesfrequency upon dependent increasing asymmetric frequency, peak (Figure which 4a). indicates The maximum a field-induced of the peak slow shifts relaxation to higher of the magnetization. At low temperature, a plateau at low frequencies can be observed demonstrating the magnetization.temperatures At upon low temperature,increasing frequency, a plateau which at indicates low frequencies a field‐induced can be slow observed relaxation demonstrating of the occurrence of a second relaxation process. We note that this process is essentially frequency the occurrencemagnetization. of a At second low temperature, relaxation a process.plateau at low We frequencies note that thiscan be process observed is demonstrating essentially frequency the independent,occurrence suggesting of a second that relaxation it results process. from a collectiveWecollective note that behaviorbehavior this process mediated is essentially by dipolardipolar frequency interactionsinteractions and/orindependent, hydrogen hydrogen bond bondsuggesting network. network. that itAs As results regards regards from the the a collective frequency frequency behavior dependent mediated process, by dipolar it can interactions be noticed that the magnitudeand/or hydrogen of the out-of-phaseoutbond‐of network.‐phase signals As regards increases the frequency with frequency, dependent process, which isit usuallycan be noticed ascribed that to the the magnitude of the out‐of‐phase signals increases with frequency, which is usually ascribed to the presence of intermolecularintermolecular interactions or spin-glassspin‐glass behavior [[22].22]. The same feature can also be presence of intermolecular interactions or spin‐glass behavior [22]. The same feature can also be observed on the temperature dependence of the out-of-phaseout‐of‐phase susceptibility with various frequencies (Figureobserved 4b). on the temperature dependence of the out‐of‐phase susceptibility with various frequencies (Figure(Figure4b). 4b).

2.5 2.5 2.02.0 -1 -1 -1 2.0 -1 2.0 1.51.5

.mol 1.5 .mol 1.5 . mol 3 . mol 3 3

3 10 Hz 1.01.0 10 Hz 1.0 100 Hz 1.0 100 Hz

' / cm 500 Hz ' / cm 

' / cm 0.5 500 Hz 0.5  1000 Hz ' / cm  0.5 0.5  1488 Hz1000 Hz 0.0 0.0 1488 Hz 1.8 K 0.0 1.0 0.80.0 1.0 1.8 K 5.0 K 0.8 -1 0.8 -1 0.6 5.0 K -1 0.8 -1 .mol 0.6 0.6 3 . mol

3 0.4

.mol 0.6 3 0.4 . mol 3 0.4 '' / cm 0.2 " / cm  0.4 0.2 

'' / cm 0.2 0.0" / cm  0.0 0.2 1 10 100 1000  234567

0.0  / Hz 0.0 T / K 1 10 100 1000 234567 (a) (b)  / Hz T / K Figure 4. (a) frequency dependence of the in ‐phase (χ′) and out‐of‐phase (χ′′) components of the AC Figure 4. (a) frequency dependence of the in-phase (χ0) and out-of-phase (χ00) components of the AC susceptibility under(a) an optimal magnetic DC field of 2000 Oe for 1; and (b) temperature(b) dependence susceptibility under an optimal magnetic DC ﬁeld of 2000 Oe for 1; and (b) temperature dependence of of the in‐phase (χ′) and out‐of‐phase (χ′′) components of the AC susceptibility under a 2000 Oe DC χ0 χ00 theFigure in-phasefield 4. ( afor) (frequency 1. )Solid and lines out-of-phase dependence are guides ( to of the) the components eye. in ‐phase of(χ′ the) and AC out susceptibility‐of‐phase (χ′′ under) components a 2000 Oe of DC the ﬁeld AC forsusceptibility1. Solid lines under are guidesan optimal to the magnetic eye. DC field of 2000 Oe for 1; and (b) temperature dependence of the in‐phase (χ′) and out‐of‐phase (χ′′) components of the AC susceptibility under a 2000 Oe DC field for 1. Solid lines are guides to the eye.

Magnetochemistry 2016, 2, 41 5 of 11

Magnetochemistry 2016, 2, 41 5 of 11 The presence of a second relaxation process at a low temperature becomes evident when looking at theThe Cole–Cole presence plots of a (Figure second5 a).relaxation Although process a second at a semi-circlelow temperature is not observed,becomes evident a plateau when is present. looking Therefore,at the Cole–Cole the Cole–Cole plots (Figure plots 5a). were Although fitted by a second taking semi into‐ accountcircle is not only observed, the data a of plateau the semi-circle is present. correspondingTherefore, the to Cole–Cole the high-temperature plots were fitted process by (Table taking S3). into The account values only of the theα parameterdata of the are semi ranging‐circle betweencorresponding 0.114 and to the 0.227, high indicating‐temperature a narrow process distribution (Table S3). of The the relaxationvalues of the processes. α parameter Further are insights ranging intobetween the mechanisms 0.114 and 0.227, of the indicating slow relaxation a narrow of distribution the magnetization of the relaxation can be obtained processes. by Further studying insights the temperatureinto the mechanisms dependence of the of τ slow(Figure relaxation5b). It turns of the out magnetization that a deviation can from be obtained the thermally by studying activated the behaviortemperature can be dependence observed at of low τ (Figure temperatures 5b). It turns and originates out that a from deviation the occurrence from the thermally of other relaxation activated processes,behavior can such be as observed Raman and/orat low temperatures direct. Fitting and the originates temperature from dependence the occurrence of the of relaxationother relaxation time wasprocesses, performed such using as Raman the following and/or direct. model: Fitting [23] the temperature dependence of the relaxation time was performed using the following model: [23] −1 −1 m n τ = τ0 exp(−∆/kT) + CT + AT , (2) τ−1 = τ0−1exp(−∆/kT) + CTm + ATn, (2) forfor which which the the first first term term accounts accounts for afor thermally a thermally activated activated process process such as such Orbach as Orbach or direct or spin–phonon direct spin– transitionsphonon transitions involving involving higher excited higher states excited [24], whilestates the[24], second while and the thirdsecond ones and stand third for ones two-phonon stand for Ramantwo‐phonon and direct Raman relaxation and processes, direct relaxation respectively. processes, To avoid the respectively. over-parameterization, To avoid the the values over of ‐ mparameterization,= 9 and n = 1 were the fixed values to theof m values = 9 and found n = 1 forwere two-phonon fixed to the Raman values (forfound Kramers for two ions)‐phonon and Raman direct processes(for Kramers [25,26 ions)]. and direct processes. [25,26].

1.8 K -5 0.8 -6

-1 0.6 -7 .mol / s / 3 5.0 K  0.4 ln -8 / cm "  0.2 -9

0.20 0.22 0.24 0.26 0.28 0.30 0.5 1.0 1.5 2.0 2.5 3 -1 -1 -1 ' / cm .mol T / K (a) (b)

FigureFigure 5. 5.(a ()a Cole–Cole) Cole–Cole plots plots using using the the AC AC data data performed performed under under a 2000 a 2000 Oe DC Oe field. DC field. The red Thesolid red lines solid correspondlines correspond to the fitto with the afit generalized with a generalized Debye model; Debye (b )model; temperature (b) temperature dependence dependence of the relaxation of the timerelaxation using thetime AC using data the under AC data a 2000 under Oe a DC 2000 field. Oe DC The field.blue Thesolid blue line solid corresponds line corresponds to the fitto withthe fit with Equation (2), the purple solid line represents the fit with Equation (2) by fixing the ∆Orbach Equation (2), the purple solid line represents the fit with Equation (2) by fixing the ∆Orbach parameter parameter from photoluminescence, while the red dashed line represents the Orbach process and from photoluminescence, while the red dashed line represents the Orbach process and (∆Orbach fixed (∆Orbach fixed from photoluminescence and τ0 obtained from Equation (2)). from photoluminescence and τ0 obtained from Equation (2)).

The best fit parameters give ∆ = 40 ± 4 cm−1; τ0 = (1.38 ± 0.09) × 10−9 s and C = 0.003 ± 0.001 s−1∙K−9 The best ﬁt parameters give ∆ = 40 ± 4 cm−1; τ = (1.38 ± 0.09) × 10−9 s and while the value for A is found negligible. Consequently, the0 deviation from linearity observed in C = 0.003 ± 0.001 s−1·K−9 while the value for A is found negligible. Consequently, the deviation Figure 5b can be explained by the overlap between thermally activated and Raman processes at low from linearity observed in Figure5b can be explained by the overlap between thermally activated temperature. Fitting the temperature dependence of the relaxation time with only a Raman process and Raman processes at low temperature. Fitting the temperature dependence of the relaxation time (m being free) did not provide meaningful results with an m exponent value larger (10.7) than the with only a Raman process (m being free) did not provide meaningful results with an m exponent expected value for the Kramers ion. Consequently, the thermally activated relaxation process seems value larger (10.7) than the expected value for the Kramers ion. Consequently, the thermally activated to dominate in the high‐temperature region. relaxation process seems to dominate in the high-temperature region. 2.3. Luminescence 2.3. Luminescence The photoluminescence properties of 1 were investigated at 14 K and at 300 K, Figure 6a,b. The photoluminescence properties of 1 were investigated at 14 K and at 300 K, Figure6a,b. Independently of the temperature, the emission spectra reveal the characteristic luminescence of the Independently of the temperature, the emission spectra reveal the characteristic luminescence of the 3+ 4 6 Dy3+ ions ascribed to the4 F9/2→ 6H15/2–11/2 transitions. By decreasing the temperature from 300 K to 14 Dy ions ascribed to the F9/2→ H15/2–11/2 transitions. By decreasing the temperature from 300 K to K, a decrease in the full‐width‐at‐half‐maximum (fwhm) is observed, as well as the Stark splitting of the intra‐4f9 lines. The excitation spectra monitored within the more intense 4F9/2→6H13/2 transition reveal a series of intra‐4f9 straight lines attributed to transitions between the 4K15/2,17/2, 4I9/2–13/2, 4G9/2–11/2,

Magnetochemistry 2016, 2, 41 6 of 11

14 K, a decrease in the full-width-at-half-maximum (fwhm) is observed, as well as the Stark splitting of 9 4 6 Magnetochemistrythe intra-4f lines. 2016, The2, 41 excitation spectra monitored within the more intense F9/2→ H13/2 transition6 of 11 9 4 4 reveal a series of intra-4f straight lines attributed to transitions between the K15/2,17/2, I9/2–13/2, 4 6 4 4 6 4 4 4 6 6 MG9/2–11/215/2–21/2, ,P3/2–7/2M15/2–21/2, F7/2–9/2, , andP3/2–7/2 H15/2, Fexcited7/2–9/2 ,levels and andH15/2 theexcited H11/2 levelsground and multiplet. the H11/2 Similarlyground to multiplet. what is foundSimilarly for tothe what emission is found spectra, for the apart emission from a spectra, decrease apart of the from fwhm, a decrease the intra of‐ the4f9 lines, fwhm, the the excitation intra-4f9 4 4 spectrumlines, the excitationobserved spectrumat 14 K resembles observed that at 14 measured K resembles at 300 that K. measured The F9/2 emission at 300 K. ThedecayF curves9/2 emission were measureddecay curves under were UV measured excitation under at 355 UV nm excitation (Figure S1) at 355for nm1. Both (Figure decay S1) curves for 1. Bothare well decay‐described curves areby well-describeda single‐exponential by a single-exponentialfunction yielding lifetime function values yielding of τ = lifetime (7.7 ± 0.2) values × 10− of9 s τ(14= K) (7.7 and± τ 0.2)= (5.9× 10 ± −0.3)9 s ×(14 10 K)−9 s and (300τ K).= (5.9 ± 0.3) × 10−9 s (300 K).

4 6 F  H (a) 9/2 13/2 7/2 (b) P 6 , 7/2 F 4 , 15/2 5/2,3/2 M 17/2 P 21/2 4 6 15/2 K M 4 K 4 4 , 4 6 9/2 G 15/2 13/2 F  H 4 I 9/2 15/2 , 19/2 4 H 4 9/2 M I 11/2 4 6 4 F  H 4 9/2 11/2 G 4 9/2

14 K F 4

14 K Normalized Intensity Normalized 300 K

300 K 450 495 540 585 630 675 310 355 400 445 490 Wavelength / nm Figure 6. (a) emission; and (b) excitation spectra (14 K and 300 K) excited at 385 nm and monitored at Figure 6. (a) emission; and (b) excitation spectra (14 K and 300 K) excited at 385 nm and monitored at 573 nm, respectively. In (b), the 66H11/2 ground state is omitted and only the excited state is represented 573 nm, respectively. In (b), the H11/2 ground state is omitted and only the excited state is represented for clarity.

In order to gain additional insight into the correlation between SIM behaviour and luminescence In order to gain additional insight into the correlation between SIM behaviour and luminescence properties, the crystal field splitting of the ground state of the Dy(III) ion in 1 was studied. The low properties, the crystal field splitting of the ground state of the Dy(III) ion in 1 was studied. The low temperature (14 K) high‐resolution emission spectrum in the spectral region showing the 4F9/2→6H15/2 temperature (14 K) high-resolution emission spectrum in the spectral region showing the 4F →6H transitions was acquired for several crystals (Figure 7 and Figure S2). Apart from changes9/2 in 15/2the transitions was acquired for several crystals (Figure7 and Figure S2). Apart from changes in the relative relative intensity of the Stark components, all the spectra reveal the presence of 11 components, as intensity of the Stark components, all the spectra reveal the presence of 11 components, as illustrated illustrated in Figure 7 for a selected ensemble of crystals. in Figure7 for a selected ensemble of crystals. Attending to the fact that the Dy(III) ions occupy a low‐symmetry group, the splitting of the Attending to the fact that the Dy(III) ions occupy a low-symmetry group, the splitting of the electronic levels (4F9/2 and 6H15/2) into the maximum number of allowed components to (2J + 1)/2 4 6 J electronic levels ( F9/2 and H15/2) into the maximum number of allowed4 components6 to (2 + 1)/2 Kramers doublets is expected, leading to five and eight components for4 F9/2 and 6 H15/2, respectively. Kramers doublets is expected, leading to five and eight components for F9/2 and H15/2, respectively. Thus, at least eight Stark components are expected for the 4F9/2→6H15/2 transitions assuming that only 4 →6 Thus, at least eight Stark components are expected4 for the F9/2 H15/2 transitions assuming that only the lower‐energy Stark component of the4 F9/2 excited state is populated and all the transitions end at the lower-energy Stark component of the F9/2 excited6 state is populated and all the transitions end at the Stark components of the ground multiplet 6 H15/2. Since 11 components are clearly expressed, it the Stark components of the ground multiplet H15/2. Since 11 components are clearly expressed, it readily points out that the second Stark component of the 4F9/2 excited state must also be populated. readily points out that the second Stark component of the 4F excited state must also be populated. This is a feasible situation due to the low‐energetic difference9/2 between the first Stark component and This is a feasible situation due to the low-energetic difference between the first Stark component and the the second one (35 ± 5 cm−1) which enables the population of the second Stark component of the 4F9/2 second one (35 ± 5 cm−1) which enables the population of the second Stark component of the 4F at at 14 K. The high‐resolution emission spectrum was fitted with 11 components using Gaussian9/2 14 K. The high-resolution emission spectrum was fitted with 11 components using Gaussian functions, functions, whose energy was constrained to the peak position analysis based on the spectrum whose energy was constrained to the peak position analysis based on the spectrum observation taking observation taking into account the experimental uncertainty (±5 cm−1); the fwhm and the relative into account the experimental uncertainty (±5 cm−1); the fwhm and the relative intensity were free intensity were free to vary. From the emission spectra best fit, an energy diagram of the Stark‐sub‐ levels is proposed in Figure 7d. From the data acquired for the three distinct ensemble of crystals (Figure S2), an average energy barrier, ∆Orbach = 34 ± 5 cm−1 is estimated, which is in excellent agreement with that obtained from the AC magnetic susceptibility data.

Magnetochemistry 2016, 2, 41 7 of 11 to vary. From the emission spectra best ﬁt, an energy diagram of the Stark-sub-levels is proposed in Figure7d. From the data acquired for the three distinct ensemble of crystals (Figure S2), an average −1 energy barrier, ∆Orbach = 34 ± 5 cm is estimated, which is in excellent agreement with that obtained fromMagnetochemistry the AC magnetic 2016, 2, 41 susceptibility data. 7 of 11

4 4 6 6 FigureFigure 7. 7.( (aa,b,b)) magnificationmagnification of of the the F9/2F9/2→ H→15/2H transition15/2 transition at 14 K at and 14 excited K and at excited 385 nm. at Multi 385 nm.‐ Multi-GaussianGaussian functions functions envelope envelope fit fit (circles) andand the the components components arising arising from from the ( orangethe (orangeshadow) shadow) first and (purple shadow)4 second 4F9/2 Stark sublevels6 to the 6H15/2 multiplet; (c) fit first and (purple shadow) second F9/2 Stark sublevels to the H15/2 multiplet; (c) fit regular residual regular residual plot; and (d) schematic diagram of the radiative transitions between the Stark4 plot; and (d) schematic diagram of the radiative transitions between the Stark sublevels of the F9/2 sublevels6 of the 4F9/2 and 6H15/2 multiplets of the Dy(III) ion. and H15/2 multiplets of the Dy(III) ion.

3. Discussion 3. Discussion Compound 1 behaves as a field induced SIM showing two relaxation processes. Multiple Compound 1 behaves as a field induced SIM showing two relaxation processes. relaxation processes have been widely observed in lanthanide zero‐field or field induced SIMs Multiple[3,27,28]. relaxation For 1, the processes independent have frequency been widely process observed observed in in lanthanide the low frequency zero-field range or field most induced likely SIMsarises [3,27 from,28]. a For collective1, the independent behavior resulting frequency from process strong observed dipolar interactions in the low frequency or hydrogen range bond most likelynetwork arises involving from a collective water molecules. behavior Such resulting intermolecular from strong interactions dipolar interactionsmay also affect or hydrogenthe frequency bond networkdependent involving process water associated molecules. with a Such slow intermolecularrelaxation of the interactionsmagnetization. may Indeed, also affect a clear the increase frequency of dependentthe out‐of process‐phase signal associated are observed with a slow with relaxation increasing of frequencies. the magnetization. This fact Indeed, is frequently a clear viewed increase as of thethe out-of-phase signature for signal either are spin observed‐glass withor superparamagnetic increasing frequencies. systems This with fact strong is frequently intermolecular viewed asinteractions the signature [29]. for Dilution either into spin-glass a diamagnetic or superparamagnetic matrix would have systems been the with ultimate strong proof intermolecular to confirm interactionsthe origin [of29 the]. Dilution frequency into independent a diamagnetic relaxation matrix process. would However, have been attempts the ultimate to grow proof crystals to confirm of a thesolid origin solution of the of frequency Y3+/Dy3+ ions independent were unsuccessful. relaxation process. However, attempts to grow crystals of a solid solutionThe field of‐ Yinduced3+/Dy3+ SIMions character were unsuccessful. of 1 appears at first glance rather surprising regarding the coordinationThe field-induced environment SIM of character the dysprosium of 1 appears site. Based at first on the glance electrostatic rather surprising proposed by regarding Rinehardt the coordinationand Long [5] environment and later extended of the dysprosiumby Chilton et site.al. [30], Based the stabilization on the electrostatic of the oblate proposed electronic by Rinehardt density andof Long the Dy [5]3+ and ion laterrequires extended an axial by crystal Chilton field. et al. Although [30], the stabilization this model only of the relies oblate on electronic the charge density of the of theatoms Dy3+ ionbelonging requires to an the axial ligands crystal (it field.does Althoughnot take into this account model only the reliesdonor on strength), the charge it allows of the atoms for belongingestimating to thethe ligandsorientation (it does of the not anisotropic take into accountaxis by theusing donor the MAGELLAN strength), it allows software for [30]. estimating The orientation of the latter is highly dependent on the magnitude of the interaction between the the orientation of the anisotropic axis by using the MAGELLAN software [30]. The orientation of the negatively charged atom of the ligand and the Dy3+ ion. It turns out that the anisotropic axis (Figure latter is highly dependent on the magnitude of the interaction between the negatively charged atom of 8) is found nearly collinear (deviation of 23.62° with respect to the central nitrogen atom) to the nitrate the ligand and the Dy3+ ion. It turns out that the anisotropic axis (Figure8) is found nearly collinear anion presenting the shortest Dy–O distance (Dy–O8 = 2.463 Å). The ideal stabilization of oblate (deviation of 23.62◦ with respect to the central nitrogen atom) to the nitrate anion presenting the density would have been to place two nitrate in trans fashion. However, the three nitrate are located shortestin cis fashion Dy–O distance to each others, (Dy–O8 forming = 2.463 a pseudo Å). The‐triangular ideal stabilization arrangement. of oblate We note density also wouldthat the have second been to placeshortest two Dy–O4 nitrate = in2.491trans Å fashion.distance However,forms an angle the three of 32.48° nitrate with are the located anisotropic in cis fashion axis. The to eachdeviation others, formingof the aanisotropic pseudo-triangular axis with arrangement.respect to the WeO4–Dy–O8 note also bond that most the secondlikely relies shortest on the Dy–O4 effect = of 2.491 the Å ◦ distancepositively forms charged an angle nitrogen of 32.48 that withattracts the the anisotropic electron density axis. The of the deviation lanthanide of theions. anisotropic Significant axis longer with respectDy–O to bonds the O4–Dy–O8 (Dy–O1 2.529 bond Å and most Dy–O5 likely = 2.568 relies Å) on are the found effect almost of the perpendicular positively charged to the nitrogenanisotropic that axis in order to limit the repulsion with the radial plane of the oblate electron density from the Dy3+ ion.

Magnetochemistry 2016, 2, 41 8 of 11 attracts the electron density of the lanthanide ions. Signiﬁcant longer Dy–O bonds (Dy–O1 2.529 Å and Dy–O5 = 2.568 Å) are found almost perpendicular to the anisotropic axis in order to limit the repulsion Magnetochemistrywith the radial 2016 plane, 2, 41 of the oblate electron density from the Dy3+ ion. 8 of 11

FigureFigure 8. 8. ArrangementArrangement of of the the anisotropic anisotropic axis axis (purple (purple) )obtained obtained with with the the MAGELLAN MAGELLAN software. software.

3+ PhotoluminescencePhotoluminescence studies studies show show that that the characteristic the characteristic emission emission of Dy of ion Dy in3+ 1 ioncan be in obtained1 can be withoutobtained any without sensitizer any sensitizer ligands by ligands direct by excitation direct excitation into the into f–f transitions. the f–f transitions. To our To knowledge, our knowledge, this constitutesthis constitutes the first the example first example of a luminescent of a luminescent SIM showing SIM such showing an excitation such process. an excitation The magneto process.‐ −1 opticalThe magneto-optical correlation shows correlation that ∆ obtained shows thatby magnetism∆ obtained (i.e., by 40 magnetism ± 4 cm ) is (i.e., very 40 close± 4 to cm the−1) energy is very gapclose between to the energythe ground gap betweenand first the‐excited ground Kramers and first-excited doublet extracted Kramers from doublet photoluminescence extracted from −1 measurementsphotoluminescence (i.e., measurements34 ± 5 cm ), (i.e.,indicating 34 ± 5 cmthat−1 ),the indicating dominant that relaxation the dominant pathways relaxation in the pathways high temperaturein the high region temperature is of the region Orbach is of type. the The Orbach observed type. slight The observeddeviation slightmost likely deviation arises most from likely the overlaparises from between the overlapOrbach and between Raman Orbach processes and due Raman to the processes limited frequency due to the of limited the oscillating frequency field of by the standardoscillating SQUID field bymagnetometry standard SQUID (i.e., magnetometry1488 Hz), even (i.e.,in the 1488 highest Hz), eventemperature in the highest range. temperatureThe energy gaprange. between The energythe ground gap betweenand first the‐excited ground Kramers and first-excited doublet obtained Kramers from doublet photoluminescence obtained from correspondsphotoluminescence definitely corresponds to the correct definitely value. to Consequently, the correct value. fitting Consequently, of the temperature fitting of dependence the temperature of the relaxation time with Equation (2) can be performed by fixing ∆Orbach to the one obtained by dependence of the relaxation time with Equation (2) can be performed by fixing ∆Orbach to the one luminescence (~34 cm−1). The best fit− parameters1 obtained are τ0 = (5.7 ± 0.4) × 10−9 s and C = 0.0009− 9± obtained by luminescence (~34 cm ). The best fit parameters obtained are τ0 = (5.7 ± 0.4) × 10 s −1 −9 0.0003and C s=∙K 0.0009, while± 0.0003the value s−1 for·K− A9, is while still negligible. the value forAttemptsA is still to let negligible. the m parameter Attempts free to does let the notm improveparameter significantly free does notthe improvequality of significantly the fit, and the the quality m parameter of the fit,is found and the slightlym parameter higher than is found its theoreticalslightly higher value than of nine its theoretical (fit m = 10.7) value [31]. of From nine these (fit m results,= 10.7) [ the31]. C From parameter these results, is found the loweredC parameter with respect to the fit without fixing ∆Orbach. This confirms that the main source of relaxation is clearly an is found lowered with respect to the fit without fixing ∆Orbach. This confirms that the main source of Orbachrelaxation process is clearly (red andashed Orbach line, process Figure (red 5b). dashed This constitutes line, Figure a5 b).rare This example constitutes of a luminescent a rare example SIM of withouta luminescent an underestimation SIM without anof ∆ underestimation obtained by magnetism of ∆ obtained as frequently by magnetism observed as frequently in luminescent observed SIMs in [7–18].luminescent SIMs [7–18].

4.4. Materials Materials and and Methods Methods

4.1.4.1. Synthesis Synthesis and and Crystal Crystal Structure Structure TheThe compound compound was was purchased purchased from from Alfa Alfa Aesar Aesar and and used used as received. as received. Single Single crystals crystals of 1 were of 1 selectedwere selected and measured and measured on an onXcalibur, an Xcalibur, Onyx Onyxdiffractometer diffractometer (Oxford (Oxford Diffraction, Diffraction, Oxford, Oxford, UK). The UK). crystalThe crystal was kept was kept at 300 at K 300 during K during data data collection. collection. Using Using Olex2 Olex2 [32], [32 the], the structure structure was was solved solved with with the the SuperflipSuperflip [33] [33] structure structure solutionsolution programprogram using using Charge Charge Flipping Flipping and and refined refined with with the the olex2.refine olex2.refine [34 ] [34]refinement refinement package package using using Gauss–Newton Gauss–Newton minimisation. minimisation.

4.2. Magnetic Measurements Magnetic susceptibility data were collected with a Quantum Design (San Diego, CA, USA) MPMS‐XL SQUID magnetometer working between 1.8 and 350 K with the magnetic field up to 7 Tesla. The data were corrected for the sample holder and the diamagnetic contributions calculated

Magnetochemistry 2016, 2, 41 9 of 11

4.2. Magnetic Measurements Magnetic susceptibility data were collected with a Quantum Design (San Diego, CA, USA) MPMS-XL SQUID magnetometer working between 1.8 and 350 K with the magnetic field up to 7 Tesla. The data were corrected for the sample holder and the diamagnetic contributions calculated from Pascal's constants. The AC magnetic susceptibility measurements were carried out in the presence of a 3 Oe oscillating field in zero or applied external DC field.

4.3. Photoluminescence Measurements The photoluminescence spectra were recorded at 14 K and at 300 K with a modular double grating excitation spectrofluorimeter with a TRIAX 320 emission monochromator (Fluorolog-3, Horiba Scientific (Kyoto, Japan)) coupled to a R928 photomultiplier (Hamamatsu) using a front face acquisition mode. The excitation source was a 450 W Xe arc lamp. All the emission spectra were measured under vaccum conditions (10−7 bar) and corrected for detection and optical spectral response of the spectrofluorimeter, and the excitation spectra were corrected for the spectral distribution of the lamp intensity using a photodiode reference detector. The nominal spectral dispersion response of the spectrofluorimeter is 2.64 nm·cm−1, yielding a spectral resolution of 0.1 nm (±5 cm−1) for the high-resolution emission spectra (slits width of 5 × 10−2 mm). The emission decay curves were measured at 14 K and at 300 K using a pulsed LED peaking at 355 nm (SpectraLED-355, Horiba Scientific (Kyoto, Japan)) coupled to a TBX-04 photomultiplier tube module. The emission spectrum of the SpectraLED-355 displays a Gaussian profile with a fwmh of ca. 15 nm. The room temperature emission quantum yield was measured using the C9920-02 (Hamamatsu) setup with a 150 W Xe lamp coupled to a monochromator for wavelength discrimination, an integration sphere as sample chamber and a multichannel analyzer for signal detection. The accuracy is within 10% according to the manufacturer. Three measurements were made for each sample, revealing values below the detection limits of our experimental apparatus (0.01).

5. Conclusions Although known for decades and thoroughly used for the synthesis of SIMs and SMMs, we have demonstrated that the dysprosium(III) nitrate salt, [Dy(NO3)3(H2O)4]·2H2O(1), behaves as a ﬁeld-induced SIM. This can be rationalized by the presence of the negatively charged oxygen of the nitrate ions, which helps to stabilize the oblate electronic density of the Dy3+ ion. In addition, the excitation in the intra 4f transitions induces the characteristic luminescence of the Dy3+ ion and allows to perform a magneto-structural correlation, giving additional insights into the relaxation processes. In the high-temperature region, it proceeds via an Orbach mechanism, which overlaps with the Raman process upon lowering of the temperature.

Supplementary Materials: The following are available online at www.mdpi.com/2312-7481/2/4/41/s1, Table S1: Crystal and structure refinement data for compound 1; Table S2: SHAPE analysis for compound 1; Table S3: Fitting of the Cole–Cole plots with a generalized Debye model for temperature ranging from 1.8 to 4.1 K under a 2000 Oe DC field for 1; Figure S1: Emission decay curves; Figure S2: High-resolution emission spectra in the 4 6 F9/2→ H15/2 transition acquired for distinct ensemble of crystals. Acknowledgments: The authors thank the University of Montpellier, Centre Nationale de la Recherche Scientifique (CNRS) and Plateforme d'Analyse et de Caractérisation (PAC) Balard Institut Charles Gerhardt Montpellier (ICGM). E.M. acknowledges the financial support from the LABoratoire d'EXcellence (LABEX) CheMISyst Agence Nationale de la Recherche ANR-10-LABX-05-01. This work was developed within the scope of the project CICECO-Aveiro Institute of Materials, POCI-01-0145-FEDER-007679 (FCT Ref. UID /CTM /50011/2013) and I3N (UID/CTM/50025/2013) financed by national funds through the FCT/MEC and co-financed by Fonds Européen de Développement Régional (FEDER) under the PT2020 Partnership. A.M.P.B. thanks FCT for a PhD fellowship (SFRH/BD/104789/2014). The Portugal–France bilateral action, PESSOA Program (Hubert Curien) “Multifunctional magneto-luminescent molecular architectures” is also acknowledged. Magnetochemistry 2016, 2, 41 10 of 11

Author Contributions: All authors contributed equally to this work. Rute A. S. Ferreira, Alexandre M. P. Botas and Luis D. Carlos performed, interpreted and wrote the luminescence part; Dominique Luneau resolved the crystal structure; Ekaterina Mamontova and Jérôme Long performed the magnetic characterization; Jérome Long wrote the draft of the article; Yannick Guari and Joulia Larionova discussed the results and implications and commented on the manuscript at all stages. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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