FTIR Study of Thermally Induced Magnetostructural Transitions in

Mar 11, 2015 - The pronounced difference in the VT-FTIR spectra of two states in breathing crystals is a fingerprint of magnetostructural transition, ...
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FTIR Study of Thermally Induced Magnetostructural Transitions in Breathing Crystals Sergey L. Veber,*,†,‡ Elizaveta A. Suturina,‡,§ Matvey V. Fedin,†,‡ Kirill N. Boldyrev,∥ Kseniya Y. Maryunina,† Renad Z. Sagdeev,†,⊥ Victor I. Ovcharenko,† Nina P. Gritsan,‡,§ and Elena G. Bagryanskaya†,‡,# †

International Tomography Center, §Institute of Chemical Kinetics and Combustion, #N.N. Vorozhtsov Novosibirsk Institute of Organic Chemistry, SB RAS, 630090 Novosibirsk, Russia ‡ Novosibirsk State University, 630090 Novosibirsk, Russia ∥ Institute of Spectroscopy, RAS, 142190 Troitsk, Russia ⊥ Kazan (Volga Region) Federal University, Kremlevskaya St. 18, 420008 Kazan, Russia S Supporting Information *

ABSTRACT: “Breathing crystals” based on copper(II) hexafluoroacetylacetonates and pyrazolyl-substituted nitronyl nitroxides comprise the exchange-coupled clusters within the polymeric chains. Owing to an interplay of exchange interaction between copper(II) and nitroxide spins and Jahn−Teller nature of copper(II) complex, the breathing crystals demonstrate thermally and light-induced magnetostructural transitions in many aspects similar to the classical spin crossover. Herewith, we report the first application of variable temperature (VT) far/mid Fourier transform infrared (FTIR) spectroscopy and mid FTIR microscopy to breathing crystals. This VT-FTIR study was aimed toward clarification of the transitions mechanism previously debated on the basis of superconducting quantum interference device, X-ray diffraction, and electron paramagnetic resonance data. VT-FTIR showed the onset of new vibrational bands during phase transitions occurring at the expense of several existing ones, whose intensity was significantly reduced. The most pronounced spectral changes were assigned to corresponding vibrational modes using quantum chemical calculations. A clear-cut correlation was found between temperature-dependent effective magnetic moment of studied compounds and the observed VT-FTIR spectra. Importantly, VT-FTIR confirmed coexistence of two types of copper(II)−nitroxide clusters during gradual magnetostructural transition. Such clusters correspond to weakly coupled and strongly coupled spin states, whose relative contribution depends on temperature. The pronounced difference in the VT-FTIR spectra of two states in breathing crystals is a fingerprint of magnetostructural transition, and understanding of these characteristics achieved by us will be useful for future studies of breathing crystals as well as their diamagnetic analogues.

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INTRODUCTION An ability of magnetic materials for thermo- and photoswitching is intensively pursued for potential applications in spintronics and data storage.1−7 Along with the studies of classical spin-crossover (SCO) materials, a significant progress in this field has been achieved recently for copper(II) complexes with stable nitroxide radicals. Although the majority of known copper(II) compounds is resistant to polymorphic transformations,8 a large family of complexes with nitroxides experiences thermally induced structural rearrangements in heterospin coordination units that lead to various SCO-like magnetic anomalies.9−19 Unlike the classic SCO, the individual spin states of copper(II) and nitroxide do not change; however, the spin state of the cluster as a whole and its magnetic moment may change significantly. In most cases these structural rearrangements refer to the conversion of spin-carrying NO group of nitroxide from © XXXX American Chemical Society

equatorial to axial coordination of the metal ion. As a result, the intracluster exchange interaction between copper(II) and nitroxide becomes strongly dependent on temperature. This effect for copper(II) complexes was first discovered and characterized by P. Rey and coauthors in 1995.10 However, single crystals of molecular compound discussed in ref 10 cracked at the temperature of phase transition, complicating Xray diffraction (XRD) study of the effect and limiting the use of such compounds for practical applications. Molecular magnets Cu(hfac)2LR based on copper(II) hexafluoroacetylacetonates (Cu(hfac)2) and pyrazolyl-substituted nitronylnitroxides (LR) have polymer-chain structure and demonstrate similar magnetic effects but occur without crystal destruction.20−23 Single crystals of these compounds possess Received: December 29, 2014

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states,50 determination of spin transition temperatures,51,52 studying the hysteresis loops,53 and even LIESST.54−57 In this work we report the first application of far/mid range VT-FTIR to breathing crystals, with the primary focus on molecular-level mechanism of magnetostructural transitions. Two representative compounds Cu(hfac)2LMe and Cu(hfac)2LPr showing abrupt and gradual magnetostructural transition,20,21 respectively, were selected and investigated. On the basis of the obtained experimental results, quantum chemical calculations, and comparison with VT-XRD data, the origin of gradual magnetostructural transitions was clarified.

unique mechanical plasticity, which allows them to repeatedly undergo thermally induced phase transitions keeping original crystal quality. Because of that, the structure of heterospin complex at certain temperature can be unambiguously correlated with its magnetic characteristics. Variable-temperature (VT) XRD study showed that reversible change of the unit cell volume during phase transition reaches up to 13% for some of Cu(hfac)2LR crystals; because of such a remarkable property, these heterospin complexes were grouped together under the name “breathing crystals”.24 Systematic analysis of relationships between thermally induced structural rearrangements and corresponding magnetic anomalies in breathing crystals motivated a series of quantum chemical studies, which addressed exchange interactions and their temperature dependence,25−27 as well as developed theoretical approaches for description of spin transitions.28−31 Along with above-mentioned techniques, electron paramagnetic resonance (EPR) is a method of choice for studying magnetic behavior of breathing crystals. It is applicable for both polycrystalline powders and single crystals in a wide temperature range (4−300 K). EPR spectra of breathing crystals are very informative, since their temperature dependence reflects the occurring magnetic anomalies.32,33 EPR was found sensitive for both intra- and intercluster exchange interactions in breathing crystals. In former case EPR provides useful information complementary to magnetometry data,34 whereas in the latter case it reveals the topology of interchain exchange interactions that can hardly be unveiled and interpreted by other techniques.35,36 Moreover, the remarkable ability of breathing crystals to undergo light-induced spin state switching and light-induced excited spin state trapping (LIESST) was first discovered and investigated by EPR,37−40 and only recently complemented by femtosecond optical spectroscopy.41 Although VT-XRD crystallography, superconducting quantum interference device (SQUID) magnetometry and EPR have extensively characterized manifestations of magnetic anomalies in breathing crystals, they were not suitable to investigate phase transitions on the level of one molecule (one cluster). Because of the principal technical limitations (SQUID, XRD) or the presence of exchange interaction networks (EPR), all these methods measured “bulk” properties, whereas addressing the “individual” heterospin cluster and its thermally induced behavior was not feasible.42 For instance, the molecular mechanism of gradual change of magnetic moment in breathing crystals remained unclear. It could have been explained by gradually changing properties of individual cluster or, alternatively, by a presence of magnetically nonequivalent species in an ensemble of clusters, whose relative ratio is temperature-dependent. Although quantum chemical studies suppose the latter mechanism,25 so far there was no experimental evidence to support or disprove it. Similar questions arise also for many other magnetoactive complexes, for example, for classical iron-based SCO compounds.3,4 In this case 57Fe Mössbauer spectroscopy is perhaps most suitable and allows determination of the spin and oxidation states for individual paramagnetic center.43−46 At the same time, the overwhelming majority of magnetoactive compounds can also be studied by vibrational spectroscopies, which are highly sensitive to the metal−ligand bond lengths and hence to the spin states of SCO centers. Variabletemperature Fourier transform infrared spectroscopy (VTFTIR) was successfully applied to SCO phenomena47−49 and proved suitable for distinguishing between different spin

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EXPERIMENTAL AND COMPUTATIONAL DETAILS

Studied Compounds. Two studied compounds of breathing crystals family Cu(hfac)2LPr and Cu(hfac)2LMe were synthesized according to the previously developed procedures.20,21 Their structure, magnetic susceptibility,20,21 and magnetic resonance behavior35,42 were also investigated previously. Cu(hfac)2LMe has the “head-to-tail” polymer chain motif leading to formation of exchange-coupled twospin copper(II)−nitroxide clusters (Figure 1a). Cu(hfac)2LPr has the

Figure 1. Structure of Cu(hfac)2LMe head-to-tail (a) and Cu(hfac)2LPr head-to-head (b) polymer chains. The color code: Cu−orange, F− yellow, O−red, N−blue, C−gray; hydrogen are hidden for clarity. Tinctured areas show the elementary units of the Cu(hfac)2LPr chain taken into account in quantum chemical calculations: (I) Cu(hfac)2 and (II) LPrCu(hfac)2LPr. “head-to-head” polymer chain motif (Figure 1b), implying the alternation of exchange-coupled three-spin nitroxide−copper(II)− nitroxide clusters and one-spin magnetically isolated copper(II) units along the chain. VT-XRD crystallography shows that, for both compounds, the most pronounced temperature-induced structural rearrangements take place in exchange-coupled clusters. Sample Preparation and Infrared Measurements. FTIR spectra of polycrystalline powders of studied compounds were measured in the mid- and far-IR ranges at T = 20−295 K. In the mid-IR range the temperature dependence of the IR spectra was obtained with a step of 10−15 K; in far-IR range the spectra were obtained with a step of 40−60 K. To prepare the sample pellet for mid-IR range, 1 mg of the studied compound was ground in agate mortar with 200 mg of potassium bromide powder; the resulting B

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Figure 2. Mid-range VT-FTIR spectra (a,b) and temperature dependence of the effective magnetic moment (c) of Cu(hfac)2LMe compound. (a) Mid-range VT-FTIR spectra measured for single crystal (SC) and pellet (Plt) of Cu(hfac)2LMe. Temperatures are shown on the right. (b) Zoomed area of VT-FTIR spectra, demonstrating absorption bands characteristic for magnetostructural transition. (c) Temperature dependence of the effective magnetic moment of Cu(hfac)2LMe compound. Cu(hfac)2 complex were taken for the calculations (Figure 1b). The geometry optimization and IR spectra calculations were performed at D359,60-BP8661,62/def2-TZVP63,64 level of theory using ORCA65 program package.

mixture was placed into the pellet die (13 mm diameter) and compressed at room temperature under the load of six tons applied for 1 min. During the whole compression procedure the pellet die with the sample was evacuated using a membrane pump (ultimate pressure of 7−10 mbar). For the far-IR range the procedure of sample preparation was similar, but the ultrafine polyethylene powder58 was used instead of potassium bromide, and the produced pellet consisted of 50 mg of polyethylene and 5 mg of studied compound. Spectra were measured in the ranges of 4000−550 cm−1 (mid-IR) and 650−50 cm−1 (far-IR) using FTIR spectrometer Bruker Vertex 80v (Bruker Optics, Germany) equipped with continuous flow liquid He cryostat Oxford OptistatCF and with KBr/DLaTGS D301 and PE/DLaTGS D201 detectors. Spectral resolution was 1 cm−1 in both spectral ranges. FTIR spectra of thin single crystals were measured in the mid-IR in the temperature range of 80−295 K. The temperature dependences of the IR spectra were obtained with a step of 5 K. The thickness of crystals was ∼5−10 μm (according to IR transparency), which corresponds to the IR absorption of the same level as that of the pellet samples. The probing area of the crystals was ∼0.2 × 0.2 mm2, that is, a bit smaller than the crystals size. Spectra were measured in the range of 4000−700 cm−1 using IR microscope HYPERION 2000 (Bruker Optics, Germany) equipped with MCT detector D316 and coupled to FTIR spectrometer Bruker Vertex 80v. Spectral resolution was 1 cm−1. Sample stage Linkam FTIR600 (Linkam Scientific Instruments, U.K.) equipped with BaF2 windows was used to control the temperature of single crystals. FTIR spectra of molecular complex Cu(hfac)2 were measured in both mid- and far-IR ranges at room temperature. The pellets of the sample were prepared using the procedure described above. Before the measurements in mid-IR range, the KBr pellet was kept at 100 °C for 30 min to minimize the effect of hydration of Cu(hfac)2 in the atmosphere. Quantum Chemical Calculations. To simulate IR spectra of the Cu(hfac)2LPr chain, the LPr-Cu(hfac)2-LPr fragment and separate

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RESULTS AND DISCUSSION Spectroscopic Characterization. Manifestation of Abrupt Magnetostructural Transition in Mid-Infrared Range (Cu(hfac)2LMe Molecular Magnet). It is well-known that sample preparation can significantly influence physical properties of SCO compounds.66−68 An enormous influence of high pressure on phase transitions in breathing crystals was recently found by SQUID; for example, 0.03 GPa applied to Cu(hfac)2LMe shifts its phase transition temperature from ∼140 to ∼200 K, and 0.1 GPa shifts this value even to ∼250 K.69 Similar (yet smaller) effects might be expected during preparation of the sample for IR spectra measurements. Therefore, we used two types of samples for VT-FTIR studies of molecular magnets Cu(hfac)2LR: (i) pellets of IR-transparent material (potassium bromide or polyethylene powder, depending on IR range) with ground crystals, and (ii) thin single crystals of studied compounds. In the first case, grinding of crystals and their subsequent compression into IR-transparent matrix could significantly change the characteristics of phase transition. In the second case, no treatment was applied; therefore, the compound was expected to demonstrate the same behavior as in SQUID and XRD studies. To clarify this, we first investigated the VT mid-IR spectra of Cu(hfac)2LMe undergoing an abrupt magnetostructural transition at ∼141 K on cooling (and 146 K on warming, Figure 2c), and compared C

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Inorganic Chemistry them for the samples of (i) KBr pellet (Plt) and (ii) thin single crystal (SC). Figure 2a shows the mid-IR spectra (1800−700 cm−1) of Cu(hfac)2LMe (Plt and SC) at temperatures above and below 141 K (magnetostructural transition). Spectra of Plt and SC samples measured at 190 K (far above magnetostructural transition) are almost identical; therefore, the temperature behavior of SC spectra can be used as a reference for tracking thermal changes in the Plt. The SC spectrum at 150 K, a few kelvin above magnetostructural transition (Figure 2c), shows no differences from the SC spectrum at 190 K. Thus, the structure of complex is the same at both temperatures. However, at 140 K (1 K below magnetostructural transition), the SC spectrum is strikingly different from that at 150 K due to the onset of new vibrational bands. In most cases these absorption bands considerably overlap with the previously existing ones; therefore, such changes are difficult to quantify. Much more promising and straightforward for interpretation are the changes manifested in (i) the splitting of well-resolved vibrational peak into two peaks (e.g., A(1356 cm−1) → A(1363 cm−1) + A(1334 cm−1)) and (ii) onset of the new bands with simultaneous disappearance of some of previously existing ones. Figure 2b shows zoomed-in spectral region of 1125−800 cm−1 with such changes being best visible. Comparison of the SC spectra at 150 and 140 K shows that the magnetostructural transition leads to the onset of new vibrational bands at 1098, 1086, and 1034 cm−1, shift of the band at 884 to 890 cm−1, and nearly complete disappearance of vibrational band at 815 cm−1. Note also that subsequent lowering of the temperature to 80 K does not induce any further spectral changes except for a narrowing of some bands at 80 K (Figure 2a,b). Thus, pronounced changes in the single-crystal IR spectrum of Cu(hfac)2LMe occur only due to the magnetostructural transition, in complete agreement with the trends observed previously by VT-XRD and SQUID magnetometry.20,21 As mentioned above, the spectra of SC and Plt measured at high temperature (190 K) are almost identical. However, this is not the case for the corresponding spectra measured at low temperature (80 K). Although the magnetostructural transition leads to appearance of the same set of new absorption peaks in the Plt spectrum, their intensity is remarkably weaker (Figure 2b); in addition, the peaks that have disappeared in SC still contribute to the spectrum of Plt at 80 K. This indicates that the magnetostructural transition in Cu(hfac)2LMe compressed into a KBr pellet becomes more gradual and partially suppressed: it is still incomplete even at as low temperature as 20 K. Since the low-temperature (LT) and high-temperature (HT) phases of Cu(hfac)2LMe manifest characteristic and wellresolved vibrational bands, VT-FTIR allows measuring the ratio of these phases versus temperature. For this, we selected two bands with maxima at 884 and 890 cm−1, being characteristic of the HT and LT phases, respectively. Presumably, these bands belong to the same vibrational mode, whose frequency is slightly different in two phases (similar effect for ∼1650 cm−1 and other bands will be demonstrated below for Cu(hfac)2LPr by quantum chemical calculations). Figure 3a shows the temperature dependence of these two bands for the SC sample. At high temperatures only one of these bands centered at 884 cm−1 is present in the spectra. As the temperature is lowered, this band retains its shape and intensity until the temperature of magnetostructural transition

Figure 3. Temperature dependence of the absorption areas for the bands centered at 890 and 884 cm−1, which are characteristic for LT and HT phases in Cu(hfac)2LMe, respectively. (a) Fragment of VTFTIR spectra of single crystal (SC) measured in a temperature range of 190−80 K with a step of 5 K. For the spectra at 190 and 80 K the peak areas centered at 980 cm−1 are colored in yellow. (b) Fragment of VT-FTIR spectra of a pellet (Plt) measured in a temperature range of 190−20 K with a step of 10−15 K. For the spectra at 190 and 20 K the peak areas centered at 980 cm−1 are colored in green. (c) Temperature dependence of the absorption area centered at 890 cm−1 normalized to the integral absorption at 890 and 884 cm−1. Yellow ■ and green ● correspond to the SC and Plt samples, respectively. In the case of SC sample, the asymptotic values of the 980 cm−1 absorption area at high (T > 150 K) and low (T < 140 K) temperatures are assigned to 0% and 100% of LT phase. On the basis of these values, HT → LT phase conversion for the Plt sample is estimated.

is reached, where it suddenly jumps from 884 to 890 cm−1. Further temperature lowering to 80 K does not induce any significant changes in the spectra. These data clearly show that switching between HT and LT phases in the SC sample occurs at the temperature of magnetostructural transition. Contrary to SC spectra, the Plt spectra demonstrate gradual buildup of the low-temperature phase (Figure 2b). At the transition temperature the band centered at 890 cm−1 does appear; however, it is much weaker compared to the SC sample. In turn, the intensity of the band centered at 884 cm−1 remains almost unchanged. Further lowering of the temperature leads to the gradual increase of the 890 cm−1 band intensity, whereas 884 cm−1 band becomes weaker. Nevertheless, even at 20 K the conversion of HT phase to LT phase is incomplete: the band centered at 884 cm−1 still contributes to the spectra with the intensity comparable to that of the 890 cm−1 band. Figure 3c shows temperature dependence of the intensity of 890 cm−1 absorption area normalized to the integral intensity of the 884 and 890 cm−1 absorption areas. For the SC spectra this dependence consists of two plateaus, because the HT↔LT switching occurs completely at the magnetostructural transition temperature. The plateau values correspond to the pure HT and LT phases. The deviation of these values from 0 and 1 at high and low temperatures, respectively, is caused by partial overlap of the 884 and 890 cm−1 lines (e.g., the absorption area at 890 cm−1 is not equal to zero at 190 K for both SC and Plt samples). In the case of Plt sample, only a few percent of the clusters undergo structural rearrangements to LT phase at the transition temperature (141 K). The subsequent temperature decrease to 20 K leads to the gradual growth of this ratio to maximum of 60−70%. Thus, sample preparation does influence both the abruptness and completeness of the magnetostructural transition, and in relevant cases this must be taken into account. D

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Figure 4. Mid-range VT-FTIR spectra (a, b), temperature dependence of the effective magnetic moment (c), and temperature dependence of absorption peak area of Cu(hfac)2LPr compound (d). (a) Mid-range VT-FTIR spectra measured for single crystal (SC) and pellet (Plt) Cu(hfac)2LPr samples. Temperatures are shown on the right. (b) Zoomed area of VT-FTIR spectra of Plt demonstrating the dynamics of new absorption bands appearing at temperature lowering. Tinctured areas show frequency ranges used for peak intensity calculation. (c) Temperature dependence of the effective magnetic moment of Cu(hfac)2LPr compound. (d) Temperature dependence of the absorption peak areas for Plt sample shown in (b). All the intensities are normalized to unity at the lowest temperature (20 K).

the ratio between these phases depends on the temperature. Taking into account temporal resolution of the FTIR and its ability to distinguish between the different structures, it is a suitable technique to clarify the mechanism of magnetostructural transition in the Cu(hfac)2LPr compound. Similar to the case of Cu(hfac)2LMe, two types of Cu(hfac)2LPr samples were used, namely, thin single crystals and KBr pellets. In the former case, the mid-IR spectra provided clear signatures of vibrational bands characteristic for the low and high temperature phases. In the latter case, we investigated the temperature dependence of mid-IR spectra in a wide temperature range (20−295 K) to elucidate the mechanism of magnetostructural transition on “molecular level”. Figure 4a demonstrates the mid-IR spectra (1800−700 cm−1) of Cu(hfac)2LPr measured in a wide temperature range (20−295 K) for both the KBr pellet (Plt) and thin single crystal (SC). In the case of SC, the spectra recorded at 295−80 K clearly demonstrate changes due to the magnetostructural transition. Similar to previously studied Cu(hfac)2LMe compound, among numerous changes of the band intensities and shapes, a few well-defined ones can be selected. Upon the temperature decrease of the SC sample, the band with the maximum at 1650 cm−1 splits into two bands, one with nearly the same maximum and another one shifted to the higher energy (1670 cm−1). In addition, a set of new bands appears in the mid-IR: one of them at 1504 cm−1 noticeably overlaps with

It also should be mentioned that the grinding of the Cu(hfac)2LMe crystals and subsequent compression of them into KBr matrix makes HT phase favorable compared to LT phase, the trend being opposite to the influence of high external pressure.69 One assumes that grinding of microcrystals changes the surface/volume ratio and introduces new defects in polymer chains, altogether leading to a stabilization of HT phase. Summarizing the experimental VT-FTIR results on Cu(hfac)2LMe, we conclude that (i) different structural phases have characteristic spectrally resolved vibrational bands; (ii) the intensities of these characteristic bands can be used to estimate the ratio between two structural phases involved in the magnetostructural transition; (iii) grinding of the crystals and subsequent compression with KBr matrix remarkably influences the abruptness and completeness of the phase transition. Manifestation of Gradual Magnetostructural Transition in Mid-Infrared Range (Cu(hfac)2LPr Molecular Magnet). Cu(hfac)2LPr represents an example of breathing crystal having gradual temperature dependence of the effective magnetic moment (Figure 4c). Such magnetic behavior supposes different interpretations, for example, (i) all three-spin clusters have identical geometry and effective magnetic moment, which are gradually changed at the temperature lowering, or (ii) the three-spin cluster can be found in one of two geometries (LT or HT phase) characterized by different magnetic moment, and E

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Inorganic Chemistry the adjacent bands; two bands in the range of 1000−1100 cm−1 are intense and well-resolved; two bands at 903 and 893 cm−1 have relatively weak intensity; and the band at 792 cm−1 appears on the top of existing band at 800 cm−1. Contrary to Cu(hfac)2LMe, SC of Cu(hfac)2LPr shows no mid-IR bands that completely disappear (or shift) upon the temperature decrease. Figure 4a demonstrates also that IR spectra of the Plt sample measured at 270 and 80 K have the same set of lines as the corresponding spectra of SC. Therefore, the temperature dependence of the Plt spectra measured in a broad temperature range (20−295 K) can be used to clarify the mechanism of structural conversion. All new vibrational bands shown in Figure 4a demonstrate the same temperature behavior, which is most clearly shown for the new bands in the 1100−1000 cm−1 spectral region (Figure 4b). Note that frequencies of these bands remain constant, but the intensities do depend on the temperature. Thus, we conclude that similar to Cu(hfac)2LMe the temperature decrease leads to formation of a new LT structural phase in Cu(hfac)2LPr. However, the amount of LT phase depends on temperature implying the coexistence of the HT and LT phases. To study the latter in more detail, below we first clarify the influence of the sample preparation on the properties of magnetostructural transition in Cu(hfac)2LPr by comparison of SC and Plt samples. Then we analyze the temperature dependence of the absorption lines using the Plt sample (Figure 4b), because SC samples can only be measured at T ≥ 80 K, whereas for Plt samples the temperature range extends to 20 K. To compare the HT → LT phase conversion in the Plt and SC samples we selected two vibrational bands: the first one is permanently present in the spectra (1105 cm−1), and the second one appears upon magnetostructural transition (1092 cm−1). Figure 5a shows these bands for the SC and Plt samples measured at 295 and 80 K. At low temperature (80 K) both bands have almost the same intensity for the SC sample,

whereas in the case of Plt sample the band at 1092 cm−1 is noticeably weaker compared to 1105 cm−1. This indicates that the ratio of HT → LT conversion is remarkably higher in the SC compared to Plt. Figure 5b demonstrates the temperature dependence of 1092 cm−1 band intensity normalized to the overall intensity of 1105 and 1092 cm−1 bands. Since the 1092 cm−1 band is assigned to LT phase, the fraction of LT phase appears to be smaller in the Plt sample compared to SC sample in the whole temperature range. Temperature dependence for the SC resembles the curve of the effective magnetic moment within its two plateaus at 1.8 and 2.6 β (Figure 4c) and even has similar pronounced curvature at ∼230−250 K. Temperature dependence for the Plt is smoothed, and the curvature is not pronounced anymore, implying that the magnetostructural transition in Plt sample is partially suppressed. On the basis of the relative intensity of 1092 cm−1 band for SC and Plt samples, the suppression degree can be roughly estimated as follows: at 80 K we find only 60−70% of LT phase in Plt compared to SC. The fraction of LT phase (60−70%) might be slightly underestimated, because, as follows from Figure 5b, the transition in SC used as a reference is not fully complete at T = 80 K (lowest available for our FTIR microscope). However, exceeding the 80% level for maximum LT fraction is very unlikely, since SQUID data (Figure 4c) show negligible changes of μ eff at T = 20−80 K. Thus, despite the magnetostructural transition being smoothed and partially suppressed in the Plt sample, the vibrational bands appearing at low temperatures can be used to monitor the coexistence of the LT and HT phases and the transfer between them. The vibrational bands shown in Figure 4b demonstrate various temperature behaviors. The frequencies of these bands remain almost the same in a whole temperature range, but their intensities strongly depend on the temperature (Figure 4d). Two lines centered at 1092 and 1039 cm−1 are absent at high temperature (270 K) but show dramatic increase at T = 250− 20 K. The third line (1105 cm−1) follows the opposite trend: it is most intense at high temperature, weakening to a certain extent as the temperature is decreased, but does not completely disappear at low temperatures. The first two bands show the same dynamics of the temperature growth, and thus both belong to the LT phase. The behavior of the third band (1105 cm−1) can be rationalized assuming that the two bands with the same frequency are superimposed at high temperature: the band corresponding to the “magnetically isolated” copper cluster and the band of the three-spin cluster in HT geometry (Figure 1b). As the temperature is lowered, the band of the three-spin cluster in HT geometry decreases due to the HT → LT phase conversion (with simultaneous increase of the band centered at 1092 cm−1). Finally, at T < 80 K the band at 1105 cm−1 is predominantly contributed by magnetically isolated copper(II) cluster, whereas the contribution of the three-spin cluster gets negligible. Note that the integral intensity of bands at 1105 and 1092 cm−1 is almost temperature-independent in a whole temperature range (changes fall within 10%, Figure 4d). It implies that these bands belong to the same vibrational mode, as will be confirmed below by quantum chemical calculations. Summarizing the experimental results observed for Cu(hfac)2LPr, (i) different structural phases have characteristic vibrational bands, some of which are spectrally well-separated to be used for monitoring the progress of magnetostructural transition; (ii) the intensities of these bands versus temperature follow temperature dependence of the effective magnetic

Figure 5. Comparison of the HT → LT conversion rate for the single crystal (SC) and pellet (Plt) samples of the Cu(hfac)2LPr compound depending on temperature. (a) Vibrational bands used to evaluate the progress of magnetostructural transition in Cu(hfac)2LPr. (b) Temperature dependence of 1092 cm−1 absorption area normalized to the overall intensity of 1092 and 1105 cm−1 absorption bands. F

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tion and IR spectra simulation for the three-spin cluster (Cu(hfac)2 coordinated by two LPr ligands) and magnetically isolated Cu(hfac)2 complex (Figure 1b). Structure optimization of the three-spin cluster LPr-Cu(hfac)2-LPr was performed for both the high-spin (S = 3/2) and low-spin (S = 1/2) states. In both cases, the minimum on the potential energy surface was localized. Optimized structures of high-spin and low-spin clusters were found to be similar to those of HT and LT XRD structures, respectively. For instance, in the HT phase, the Cu−O distances are equal to 1.955 and 1.977 Å for coordination by the hfac ligand and 2.318 Å for both nitronyl nitroxide radicals NN. For high-spin (S = 3/2) complex, calculations predict these distances as 1.985 and 1.987 Å for hfac ligand and 2.350 for NN, that is, in good agreement with XRD data. In turn, the LT XRD data give Cu−O distances equal to 1.949 and 2.274 Å for the hfac ligand and 1.994 Å for NN coordination. In fairly good agreement with experiment, calculations predict the following Cu−O distances for low-spin (S = 1/2) state: 1.987 and 2.313 Å for coordination of hfac ligand and 2.09 Å for NN coordination. An agreement of experimental and calculated structures enables one to expect reasonable agreement of experimental and calculated IR spectra. Figure 7 shows the experimental and calculated IR spectra of individual Cu(hfac)2 complex. The calculations reproduce the

moment; (iii) analysis of the intensities of characteristic vibrational bands versus temperature yields the ratio of structural phases; (iv) grinding of the studied compound and subsequent compression of obtained microcrystals into KBr matrix remarkably suppresses the magnetostructural transition observed. Manifestations of the Magnetostructural Transitions in Far-Infrared Range. Far-range VT-FTIR spectra of studied compounds are also temperature-dependent (Figure 6);

Figure 6. Far-range VT-FTIR spectra of the pellet samples of Cu(hfac)2LMe (a) and Cu(hfac)2LPr (b). Lines with numbers indicate the vibrational bands characteristic for LT phase.

however, their analysis is more complicated compared to mid-IR because (i) the technical limitations do not allow working with single crystals but only with pellet samples, (ii) a lot of vibrational bands overlap, and (iii) the width of these bands strongly depends on temperature. Nevertheless, a set of lines can be assigned to the LT phase by monitoring the temperature at which they start growing. Thus, in the case of Cu(hfac)2LMe compound (Figure 6a) the spectra measured above magnetostructural transition (280, 220, and 160 K) are described by the same set of vibrational bands. New bands (marked by lines) appear only at T = 100 K and grow in intensity at 60 and 20 K, in a way similar to that observed for Plt sample in the mid-IR range. The same tendency was observed for Cu(hfac)2LPr compound (Figure 6b). Special attention should be drawn to the far-IR region at ∼200 cm−1, where the temperature-induced spectral changes are most pronounced. In both compounds the LT phase manifests the onset of three new bands at 225−213 cm−1, which are absent in HT phase. In the next section we discuss this effect in more detail. Quantum Chemical Calculations. Assignment of the FTIR bands (Figures 2 and 4) to particular vibration modes is not straightforward. Therefore, we performed a series of quantum chemical calculations, including geometry optimiza-

Figure 7. Experimental spectra of Cu(hfac)2 recorded in mid- and farIR ranges at room temperature. Black sticks correspond to positions and relative intensities of IR transitions calculated using D3-BP86/ def2-TZVP level of theory; black spectrum was simulated using results of calculations and Gaussian shape of individual bands with 20 cm−1 width.

experimental spectrum very well; only calculated bands in the 1400−1600 cm−1 range are slightly shifted to the low energy. Although the efforts were made to achieve anhydrous Cu(hfac)2 in the KBr sample pellet (see Experimental Section), partial hydration of the Cu(hfac)2 complex can not be excluded. Note that water in the complex affects some of its vibrational frequencies. Calculations revealed that the most intense absorption bands of Cu(hfac)2 in the 1100−1200 cm−1 region are associated with the deformational vibration of CO groups and stretching CF vibrations of the CF3 groups of the hfac ligands. Table S1 (Supporting Information) represents calculated frequencies, intensities, and displacement vectors of these and other modes corresponding to the most intense IR bands of Cu(hfac)2 complex. G

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Cu(hfac)2 unit in LPrCu(hfac)2LPr, the frequency difference in HT and LT phase equals 7 cm−1 (1608 cm−1 for HT (S = 3/2) and 1615 cm−1 for LT (S = 1/2), see Table S2, Supporting Information). This value coincides well with the experimental difference of 8 cm−1 for the bands at ∼1650 cm−1 recorded at LT. The vibrational bands centered at 1105 and 1091 cm−1 (Figure 4b) can not be assigned unambiguously to the certain mode, because a lot of lines are present in this area of the calculated spectra (see Table S2, Supporting Information). Nevertheless, taking into account the frequency change observed experimentally (14 cm−1), the most probable mode is CO and CF stretching centered at 1068 cm−1 at high temperature (HT → LT change is 14 cm−1). However, it can also be the set of CF stretching modes (most intense at 1083 cm−1 at high temperature), which have HT → LT frequency change of 6−15 cm−1. The calculated spectral changes in the far-IR range are also in good agreement with experimentally observed ones. Thus, the most pronounced spectral changes are caused by the appearance of new intense bands with three maxima at 224, 220, and 218 cm−1 in LT phase. The calculated spectral changes are almost the same: very intense band at 194 cm−1 was predicted for HT state, three bands of medium intensity at 204, 213, and 219 cm−1 were predicted for LT state, and one band of medium intensity at 199 cm−1 for isolated Cu(hfac)2 complex (Table S3, Supporting Information). The observed spectral changes due to the magnetostructural transition are caused by the spectral shifts and noticeable intensity redistribution among similar modes of HT and LT states. Mechanism of the Magnetostructural Transition. General characteristics of phase transitions in complexes Cu(hfac)2LMe and Cu(hfac)2LPr obtained by VT-FTIR are in good agreement with the results of other techniques (XRD, SQUID, and EPR). However, in addition VT-FTIR data provides novel experimental conclusions on the mechanism of magnetostructural transitions. In case of Cu(hfac)2LMe, most pronounced spectral changes detected by FTIR on single crystal occur at the phase transition temperature, coinciding well with those measured by XRD, SQUID, and EPR. In case of Cu(hfac)2LPr, the gradual magnetostructural transition leads to the gradual spectral changes in VT-FTIR manifested by the onset or disappearance/decrease of certain vibrational bands. This behavior indicates the coexistence of two structural phases in the studied compound (LT and HT phase), whose structures are largely temperature-independent, but the ratio does depend on temperature. Such information requires reconsideration or adjustment of previously proposed models for gradual magnetostructural transitions in breathing crystals.42 SQUID magnetometry measures the integral magnetization of the sample and thus is not able to distinguish between the two superimposed phases. If transitions of clusters between HT and LT states are spatially disordered, XRD data can also give only the average structure. EPR has an advantage of selective monitoring of spin triads, whose spectra are drastically different in HT and LT phases. However, different complications arise due to the pronounced intercluster exchange interactions effectively averaging signals of HT and LT states.36 In this regard, obviously FTIR results provide vital complementary information that excludes the gradual structural changes during gradual magnetostructural transitions. It still remains unclear whether the HT↔LT switching is static or dynamic on molecular level. Static model implies consecutive switching of clusters from one state to another as the temperature changes;

The structural changes associated with the magnetic transitions in the complexes Cu(hfac)2LMe and Cu(hfac)2LPr affect mainly the Cu-based octahedral units (CuO5N and CuO6, respectively). Because of the Jahn−Teller effect, the Cu−O (or Cu−N) bonds along one of three coordination axes of the Cu2+ ion are elongated: ∼ 2.2 Å for atoms in axial position versus ∼2.0 Å for atoms in equatorial position. At the temperature of magnetostructural transition the previously elongated bonds shorten, whereas two others become longer, and thus the Jahn−Teller axis of the Cu2+ octahedron changes its direction. Therefore, one would expect the most pronounced temperature changes to be observed for the IR bands associated with the modes involving the displacement of the coordinating oxygen atoms. Table S1 demonstrates that the corresponding modes of Cu(hfac)2 have the following calculated frequencies: 1589, 1466, 1080, 935, 793, 661, 581, 572, 501, 342, 312, 262, 242, and 199 cm−1. However, most of these transitions have very low intensities (Table S1, Supporting Information). As expected, the experimental IR spectrum of Cu(hfac)2LPr has more lines compared to that of Cu(hfac)2. Although all calculations were performed for the isolated complexes, the calculated IR spectra (Figure 8, vertical lines) reproduce the experimental HT/LT spectra of the polymer-chain complex Cu(hfac)2LPr fairly well (Figure 8, curves). Moreover, calculations also satisfactorily predict the values and directions of the frequency change of vibrational bands caused by HT → LT phase conversion (Figures 4 and 8). For example, according to the calculations for the CO stretching mode of the

Figure 8. Experimental mid-IR spectra recoded for the KBr pellet of Cu(hfac)2LPr at 290 K (red) and at 20 K (blue) and for the Cu(hfac)2 at room temperature (black). Vertial lines indicate positions and relative intensities of the IR transitions calculated using D3-BP86/ def2-TZVP level of theory for the LPrCu(hfac)2LPr spin triad at S = 3/2 state (red) and at broken-symmetry S = 1/2 state (blue); calculated spectrum of Cu(hfac)2 is shown by black. H

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temperature-dependent absorption lines to corresponding vibrational modes and successful description of experimental mid- and far-IR spectra.

in this case, similar to many SCO compounds, magnetostructural transitions are expected to occur in domains,70 that is, sufficiently large volumes of one phase within the other one. This model might be difficult to reconcile with the EPR data, since intercluster exchange interactions are not strong enough to average the signals of large domains.36,42 Dynamic model implies that clusters stochastically switch between HT and LT states during gradual magnetostructural transition, and the ratio of phases at a certain temperature is a measure of time spent by a particular cluster in one or another state. This model is in good agreement with concepts developed for dynamic Jahn− Teller effect in copper(II) ions.71−73 A detailed analysis of VT FTIR, EPR, and XRD data obtained on other breathing crystals might provide further insight into the mechanism of gradual magnetostructural transitions in this family of compounds.

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ASSOCIATED CONTENT

S Supporting Information *

Wavenumbers of band maxima in calculated IR spectra, their relative intensities, and figures representing the main displacement vectors characteristic for these vibrations. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

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Notes

CONCLUSIONS This work demonstrates that VT-FTIR spectroscopy combined with quantum chemical calculations provides a valuable approach to study magnetostructural transitions in molecular magnets of the breathing crystals family. Two representative compounds demonstrating abrupt (Cu(hfac)2LMe) and gradual (Cu(hfac)2LPr) magnetostructural transitions were studied by VT-FTIR in mid- and far-IR ranges at T = 20−295 K. First, the samples of studied compounds were prepared following typical procedure (grinding of the crystals, mixing them with IRtransparent powder, and subsequent compression to the pellet) and investigated in both IR ranges. Additionally, in the mid-IR range thin single crystals (10−15 μm thickness) of both studied compounds were investigated by means of IR-microscopy (T = 80−295 K). It was shown that grinding of Cu(hfac)2LMe and compression of microcrystals into KBr pellet lead to significant suppression of magnetostructural transition. VT-FTIR of single crystal demonstrates abrupt phase transition at ∼141 K (in good agreement with previous XRD, SQUID, and EPR studies), whereas the transition in the KBr pellet becomes smoothed and lagged, and even at 20 K it is completed only by 50−60%. Except for the influence of grinding and compression, the manifestations of magnetostructural transitions in VT-FTIR were found similar in both single crystals and pellets, and the same holds for the second complex Cu(hfac)2LPr. Magnetostructural transitions manifest themselves in the onset of new vibrational bands and disappearance of some of previously existing ones. Many of these bands overlap with those permanently present in the spectra, but some of them are spectrally well-resolved in the mid-IR range. The intensities of these bands depend on temperature in good agreement with the shape of magnetostructural transition obtained by SQUID magnetometry. The manifestations of magnetostructural transitions in far-IR range are similar to those in mid-IR; however, the interpretation of spectral changes is complicated by pronounced temperature-dependent broadening and overlap of individual vibrational bands. Remarkably, in all cases we have not observed gradual frequency shift of the vibrational bands caused by magnetostructural transition but only the rise or decrease of some particular bands. This confirms that gradual magnetostructural transitions found in many breathing crystals occur via the formation of new structural phase and not via the gradual structural changes in the original one. When magnetostructural transition occurs within a broad temperature range (e.g., in Cu(hfac)2LPr), one structural phase gradually replaces another one, and they coexist at intermediate temperature. Quantum chemical calculations allowed the assignment of

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. S. Fokin for providing us with anhydrous Cu(hfac)2 complex. This work was supported by the Russian Foundation for Basic Research (Nos. 14-03-00224, 15-0303242, and 15-03-07640), the RF President’s Grant (MK3241.2014.3, MD-276.2014.3).

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REFERENCES

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