Light-Induced Spin State Switching and Relaxation in Spin Pairs of

Article pubs.acs.org/IC

Light-Induced Spin State Switching and Relaxation in Spin Pairs of Copper(II)−Nitroxide Based Molecular Magnets Sergey V. Tumanov,†,‡ Sergey L. Veber,†,‡ Svyatoslav E. Tolstikov,† Natalia A. Artiukhova,† Galina V. Romanenko,† Victor I. Ovcharenko,† and Matvey V. Fedin*,†,‡ †

International Tomography Center SB RAS, Institutskaya Str. 3a, 630090 Novosibirsk, Russia Novosibirsk State University, Pirogova Str. 2, 630090 Novosibirsk, Russia

‡

S Supporting Information *

ABSTRACT: Similar to spin-crossover (SCO) compounds, spin states of copper(II)−nitroxide based molecular magnets can be switched by various external stimuli including temperature and light. Although photoswitching and reverse relaxation of nitroxide−copper(II)−nitroxide triads were investigated in some detail, similar study for copper(II)−nitroxide spin pairs was still missing. In this work we address photoswitching and relaxation phenomena in exchangecoupled spin pairs of this family of molecular magnets. Using electron paramagnetic resonance (EPR) spectroscopy with photoexcitation, we demonstrate that compared to triad-containing compounds the photoinduced weakly coupled spin (WS) states of copper(II)−nitroxide pairs are remarkably more stable at cryogenic temperatures and relax to the ground strongly coupled spin (SS) states on the scale of days. The structural changes between SS and WS states, e.g., differences in Cu−Onitroxide distances, are much more pronounced for spin pairs than for spin triads in most of the studied copper(II)− nitroxide based molecular magnets. This results in higher energy barrier between WS and SS states of spin pairs and governs higher stability of their photoinduced WS states. Therefore, the longer-lived photoinduced states in copper(II)−nitroxide molecular magnets should be searched within the compounds experiencing largest structural changes upon thermal spin transition. This advancement in understanding of LIESST-like phenomena in copper(II)−nitroxide molecular magnets allows us to propose them as interesting playgrounds for benchmarking the basic factors governing the stability of photoinduced states in other SCO and SCO-like photoswitchable systems.

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INTRODUCTION Molecule-based magnetic materials are among the most promising candidates for applications in nanotechnology, data processing and storage, quantum computing, etc.1,2 Singlemolecule magnets (SMMs) are able to store information in electron spin states of the molecules in the absence of the magnetic field, making them potential building blocks for realization of ultradense magnetic memory.3−6 Very recently, reading and writing information was demonstrated even on the scale of single atoms.7 Spins of SMMs can also be used for quantum operations by means of microwave pulses and electron paramagnetic resonance (EPR) detection.8,9 This field was recently advanced by developing new approaches for prolongation of the electron decoherence times of molecular magnets.10−13 Another type of promising moleculebased magnetic materials is switchable compounds, mostly represented by spin-crossover (SCO) complexes, whose magnetostructural states can be switched by temperature, light, pressure, and other external stimuli.14−17 Thermochromism of SCO compounds and the ability to manipulate their states by light, including the room-temperature switching back and forward for some complexes,18−20 are good prerequisites for their potential applications. Recently, special attention was also attracted to the light-induced processes in SCO compounds occurring on the short femtosecond-to-nano© 2017 American Chemical Society

second time scales, with the ultimate goal of creating ultrafast-responsive magnetic materials.21−26 A number of close SCO-like phenomena involving electron transfer and ligands influence on spin states were elaborated.27−29 SCO-like phenomena were also found in exchangecoupled clusters of copper(II) with stable nitroxide radicals.30−40 These copper−nitroxide based molecular magnets exhibit thermally induced magnetostructural transitions, manifesting themselves very similar to a classical spin crossover. The observed anomalies of the magnetic moment are owed to the structural rearrangements in coordination environment of copper(II) ion, where the elongated Jahn−Teller axis flips and nitroxide ligands change coordination between axial and equatorial. In most cases, high-temperature configuration corresponds to the nitroxides in axial coordination positions of copper. This layout favors weak ferromagnetic exchange interaction between copper(II) and nitroxide spins (so-called weakly coupled spin (WS) state). In contrast, low-temperature configuration typically features nitroxides in equatorial coordination positions of copper, and this layout favors strong antiferromagnetic exchange effectively coupling two of three spins (strongly coupled spin (SS) state). The temperature and Received: July 3, 2017 Published: September 21, 2017 11729

DOI: 10.1021/acs.inorgchem.7b01689 Inorg. Chem. 2017, 56, 11729−11737

Inorganic Chemistry

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RESULTS AND DISCUSSION Representative “Breathing Crystals” under Study. In this work we aimed at establishing general trends of photoswitching and relaxation from photoinduced to the ground state in breathing crystals containing spin pairs. Such breathing crystals are formed when copper(II) hexafluoroacetylacetonates Cu(hfac)2 coordinate nitroxide radicals in headto-tail manner, so that the two-spin units CuO5N are formed (Figure 1a). Breathing crystals of this type are rare, and not all

the character of spin transitions between WS and SS states strongly depend on the structure of the crystal. Despite the unit volume changes upon thermal WS ↔ SS transitions by up to ∼13%, in the overwhelming majority of these compounds transitions are fully reversible; therefore they are often called “breathing crystals”. Over the past decade, the family of breathing crystals was strongly expanded and extensively studied.41−58 Although there are principal differences between SCO compounds and SCO-like breathing crystals, a lot of similarities in manifestations of spin state switching and physics behind were found between them. In particular, photoswitching and light-induced excited spin state trapping (LIESST) phenomena were found for breathing crystals similar to their occurrence in SCO compounds. For the present, some trends and characteristics of photoswitching in breathing crystals are already investigated, including its discovery,43 excited state relaxation,49,51 nano- and femtosecond light-induced spin dynamics,48,53 and magnetic and structural specifics of photoinduced states;52,58 yet many crucial aspects still remain unexplored. One of the issues that have not been addressed to date is the detailed comparison of the photoswitching and reverse relaxation between breathing crystals containing spin triads nitroxide−copper(II)−nitroxide and spin pairs copper(II)− nitroxide. These two types of compounds differ in structure of copper(II) coordination octahedra being either CuO6 (triads) or CuO5N (pairs). Most of the breathing crystals are represented by triad-containing compounds, therefore to date most photoswitching studies were performed for spin triads. However, photoswitching in spin pairs was also demonstrated recently.58 This motivated us to perform for the first time a detailed comparison of photoswitching and relaxation in triads and pairs. We have shown previously that the relaxation from the photoinduced WS state to the ground SS state in spin triads has an unusual self-decelerating character and occurs on the scale of hours at cryogenic temperatures.43,49,51,52 Such behavior was assigned to the broad distribution of the energy barrier between the potential wells of WS and SS states. In the present work we investigate photoswitching and relaxation in spin pairs of breathing crystals, compare the obtained trends with spin triads, and draw general conclusions on the factors governing the reverse relaxation in these and similar systems.

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Article

EXPERIMENTAL SECTION

The breathing crystals Cu(hfac)2LMe, [[Cu(hfac)2]2L2PyMe][Cu(hfac)2], and Cu(hfac)2LPr were synthesized using the procedures described previously, and their structural and magnetic properties were studied in detail.31,55 The compounds were dispersed in KBr, and then optically transparent pellets were prepared by applying the pressure of 3 tons, cut to appropriate size, and placed into quartz EPR tubes (OD 3.8 mm, i.d. 2.8 mm) for measurements. The EPR measurements were performed using a Bruker Elexsys E580 spectrometer equipped with an Oxford Instruments temperature control system (4−300 K) at X-band (∼9.7 GHz). A standard ER 4118 X-MD5 resonator was used in all experiments. Illumination with light was done using the optical window of cryostat and light-emitting diode with the wavelength of ∼720 nm (see Supporting Information) and the outcoming light power up to 1 W; however, only ∼1−3% of this power reaches the sample due to the losses at the light guides and optical windows. The absolute values of power given below are rough estimates, however their ratios are accurately controlled at the LED output.

Figure 1. (a−c) Structure of the polymer chains of studied compounds I−III. Spin pairs are highlighted with green, spin triads with red. (d) Temperature dependence of the effective magnetic moment μeff(T) for the compounds I−III.31,55 Sketch of the structural differences between WS and SS states.

of them exhibit clear thermal and/or photoinduced switching. For this study we have selected the first and the most studied representative of this type, Cu(hfac)2LMe (I), whose polymerchain structure and magnetic behavior are shown in Figure 1a,d.31 Recently, we have demonstrated that photoswitching 11730


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Inorganic Chemistry does occur for this compound, being the first example in spin pairs copper(II)−nitroxide.58 As the second representative of breathing crystals with spin pairs, we selected the complex [[Cu(hfac)2]2L2PyMe][Cu(hfac)2] (II), which has another structure of the polymer chains with alternating four-spin loops [[Cu(hfac)2]2L2PyMe] and one-spin copper units (Figure 1b). Each loop contains two spin pairs copper(II)−nitroxide in CuO5N units, and both the magnetic moment behavior and XRD data indicate the beginning of the thermal spin state switching around room temperature.55 In contrast to I, the character of spin transition in II is gradual (Figure 1d), and the photoswitching in this compound is reported below for the first time. Finally, in order to compare the trends for spin pairs with those in spin triads, we mainly refer to our previous studies on photoswitching/relaxation in triads;43,48,49,51,52,58 however, some new experiments related to the photoswitching cooperativity are considered here for the first time, therefore we perform them for the well-studied representative of spin triads Cu(hfac)2LPr (III) (Figure 1c,d) as well. This compound has the polymer chains with the head-to-head coordination motif, where one-spin CuO4N2 units alternate with three-spin CuO6 units, and the spin transition gradually develops between ∼100 and 250 K.31 Light-Induced Spin State Switching. Figure 2 shows the effect of light on the studied compounds I−III at cryogenic temperatures. In each case, the illumination with light (∼720 nm, corresponding mainly to the d−d band of copper) was accomplished for ∼20 min at high power of light, until further changes in the EPR spectrum became negligible. The shapes of the EPR spectra in WS and SS states are noticeably different for each studied compound (Figure 2), which simplifies the observation of SS → WS photoswitching. At the same time, the spectra of WS/SS states differ between the compounds I−III due to the different structure of copper−nitroxide units and polymer chains in each case; therefore below we briefly describe various spectral shapes observed. EPR spectroscopy of I was studied in detail previously.44 The WS state is characterized by a broad single line with g ≈ 2.1 (Figure 2a); no manifestations of the dimeric structures are observed due to the exchange narrowing along the structural polymer chain. In contrast, the SS state features the transition in half magnetic field characteristic for triplet states of dimers, and the signal in the central field noticeably narrows (Figure 2a). This occurs because one half of spin pairs converts to diamagnetic state at temperatures below spin transition (∼141 K), so that effective diamagnetic dilution suppresses the exchange narrowing between individual spin pairs. Clearly, the illumination with light reduces the amplitude of the halffield signal and simultaneously increases the line width and intensity of the central-field signal. This transformation corresponds to the switching of some fraction of pairs from the SS to the WS state (Figure 2a). The EPR spectrum of II in the WS state cannot be obtained experimentally, because at room temperature this compound is found in the mixed SS/WS state and the spin transition is incomplete (Figure 1d). Further warming leads to decomposition of the compound and does not allow reaching the pure WS state. In this regard, the possibility to photoinduce this WS state at cryogenic temperatures provides a unique and elegant way for its study, as was recently demonstrated for another thermally unswitchable compound.52 The SS state of II corresponds to all spin pairs being in diamagnetic state (Figure

Figure 2. X-band (∼9.7 GHz) CW EPR spectra of I−III in thermal SS and WS states, and the corresponding spectra of photoinduced WS states. For II the mixed WS/SS state at 300 K is shown. The temperatures are indicated in the plot. The spectra are normalized; however, the dark/light spectra of each compound at 5 K are shown on the same scale.

2b); therefore its EPR spectrum is close to the typical one for magnetically isolated copper(II) ions and originates from CuO4N2 units between the loops (see Figure 1b). The illumination with light results in drastic change in the shape and integral intensity of the observed EPR spectrum. The narrow line at the g⊥ region (∼2.064) of the copper(II) ion disappears, and the broad weakly structured line appears instead. The increase of the integral intensity clearly indicates the conversion of spin pairs inside the loops into paramagnetic state, and disappearance of the narrow SS spectrum implies that the one-spin copper(II) ion linking neighboring loops gets involved in the exchange-coupled network, so that the exchange narrowing along the polymer chain becomes efficient. As was mentioned above, the EPR spectrum of pure WS state is not available for comparison; however the trend in the spectral transformation upon illumination clearly corresponds to the SS 11731


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Inorganic Chemistry → WS photoswitching. Thus, EPR detects the photoswitching of II from the SS to the WS state. Finally, both EPR spectroscopy in the dark and the effect of light on compound III were studied previously in detail.43,49,54 The spectrum of triads in the SS state shows intense signals at the g < 2 region, whereas in the WS state a broad single line with g ∼ 2 is observed, making the discrimination between two states very easy (Figure 2c). In addition, typical copper(II) signals with g⊥ ∼ 2.075 and g|| ∼ 2.371 are observed in both states, originating from CuO4N2 units between spin triads (see Figure 1c). Figure 2c shows the spectra in the WS and SS states, as well as the effect of photoswitching observed in III. Relaxation of Light-Induced State. For the study of structural relaxation from photoinduced WS state to the ground SS state, first we illuminated each sample sufficiently long to achieve maximum photoswitching for the particular experimental conditions (sample, temperature, and light power). Then the light was switched off, and a series of EPR spectra vs time spaced with 2 min was accumulated. To process these data and obtain the dependence γWS(t), where γWS is the fraction of photoinduced WS states, we used the approach developed previously for spin triads.49 Briefly, the subtraction of dark spectrum (measured before illumination) from the whole array results in spectra whose shape is time-independent, but intensity is proportional to the value of γWS.49 Figures 3a and 3b show that virtually no WS → SS relaxation occurs for I and II on the scale of 2 h. This is drastically different from previous observations at similar temperatures for spin triads, in particular in III, where unusual self-decelerating relaxation curves were obtained (Figure 3c).49 One would anticipate that elevation of temperature will gradually increase the relaxation rate and transform near plateaus in Figures 3a and 3b into self-decelerating shapes in Figure 3c; however this does not happen. Instead, the observed relaxation does not indicate any noticeable acceleration with temperature between ∼5 and 20 K, whereas the amplitude of photoswitching (the maximum γWS at t = 0) decreases with temperature, so that the experiments at T > 20 K become not feasible. In order to gain more insight into this controversial trend, we investigated the photoswitching kinetics vs light intensity and the cooperativity issues. Photoswitching Kinetics and Cooperativity Issues. Self-decelerating WS → SS relaxation in spin triads was previously rationalized by a distribution of relaxation parameters in photoinduced WS states, so that some clusters relax faster and contribute at short times, whereas the others relax slower and sustain the relaxation curve at longer time. In this sense, the dependence of maximum γWS on temperature can be explained by a contribution of fast-relaxing clusters, whose relaxation to the SS state occurs within a few seconds, i.e., beyond temporal resolution of continuous wave EPR, and therefore is not detectable. The same explanation can, in principle, be applied to spin pairs. But then, in the case of spin pairs, one should assume the essentially bimodal distribution of relaxation rates, i.e., the coexistence of fast-relaxing clusters and extremely slow-relaxing clusters (we call them below fastrelaxing fraction and slow-relaxing fraction). The former would explain the dependence of maximum γWS on temperature, whereas the latter would be responsible for the plateaus shown in Figure 3a,b. Since γWS changes with temperature, we must assume that the ratio of two fractions also depends on temperature, similar to the logic implemented previously to spin triads.49

Figure 3. Structural relaxation from photoinduced WS state to the ground SS state in I−III (a−c) at different temperatures (indicated). In all cases the kinetic curves are normalized to the γWS(t = 0) at 5 K. Panel c is adapted from ref 49 and used with permission. Copyright 2012 American Chemical Society.

One of the possibilities to verify the above hypothesis experimentally is to measure the photoswitching vs time as a function of light power P (under continuous illumination). The rates of SS → WS photoswitching (kp) and WS → SS relaxation (kr) must compete if fast relaxation processes are present. Therefore, assuming that the relaxation rate kr does not depend on the intensity of light, one should be able to vary the ratio kp/kr by changing the power of the incident light. Figure 4 shows the photoswitching kinetics for I−III recorded at 4−5 different powers of light keeping all other parameters of the experiment fixed. Most clearly for II and III, but also indicative of I, the function γWS(t) tends to different plateaus at long times for different powers of light. The kinetics of photoswitching γWS(t) develops on the time scale of tens of minutes, but the observed relaxation (slow-relaxing fraction) almost does not occur during this time (Figure 3a,b). Therefore, different plateaus (Figure 4) and different maximum γWS(T) values (Figure 3) must be assigned to the competition 11732


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Inorganic Chemistry

equation considers kr as the average relaxation rate over all clusters (fast- and slow-relaxing fractions), and the amount of fast-relaxing clusters at 5 K is small. Therefore, rather small absolute kr values obtained from simulations are very reasonable. At the moment the origin of the fast-relaxing fraction of clusters is not clear. Possibly, a photoswitching of all clusters to the WS state having larger volume induces stronger elastic strain in the polymer chain than the crystal can endure; therefore some fraction of clusters rapidly relaxes to the SS state due to elastic interactions. The other view of the same cause is the occurrence of the specific structural distributions of photoinduced WS states, leading to the corresponding distributions of the WS → SS relaxation rates, and being grounded on the elastic interactions as well. The deviations of γWS(t) curves from monoexponential law occur at ∼ γWS > 0.6 (Figure 4). Previously, we have observed that the self-decelerating WS → SS relaxation in III depends on the initial degree of photoconversion, which was justified by a dependence of relaxation rate on the location of SCO center with respect to the surface of microcrystal.49 Gradual growth of γWS(t) at long times of illumination (∼γWS > 0.6, Figure 4) is fully coherent with this explanation. Since optical density of WS states is smaller than that of SS states, light penetrates deeper into the crystal as γWS grows and reaches slower relaxing clusters, ultimately resulting in the increase of quasistationary value γWS,q = kp/(kp + kr). Even though we did not take into account this process rigorously in the master equation above, the presented analysis clearly describes the main tendencies of photoswitching and relaxation which are important for this work. In addition, simulations of γWS(t) dependences shown in Figure 4 provide means to assess influence of cooperativity on the photoswitching. In some SCO compounds it has been shown that noticeable amplification of photoswitching might occur via elastic interactions, so that formal quantum yield of photoswitching can exceed 100% by 4−5 times.22 Although in the present study we cannot discriminate between fs-scale photoswtiching and ns-scale elastic-driven switching,22 still it is possible to characterize the “bulk” photoswitching rate kp vs the number of incident photons, i.e., power of light (P). Figure 4 (insets) shows that in all cases the dependence kp(P) is roughly linear, implying the proportionality of absorbed photons and switched clusters. The linearity on P clearly indicates that the character of photoswitching is local in these conditions. If any elastic-driven amplification of photoswitching would occur, the elastic contribution should be proportional to the pure photoswitching amplitude, and this can be further investigated only by means of femto/nanosecond optical spectroscopy. Fundamental Reasons for Higher Stability of Photoinduced States in Spin Pairs Compared to Spin Triads. The difference between the character of structural relaxation from photoinduced WS to ground SS state in spin triads and spin pairs is striking. Although the statistics for spin pairs are not that convincing at the moment (being limited by the number of available breathing crystals of this type), we emphasize that the observable WS → SS relaxation for I and II is nearly absent at T = 5−20 K, whereas the same relaxation for III and four more studied compounds with spin triads43,49,51,52 is much faster at similar temperatures and has a self-decelerating character.

Figure 4. Photoswitching kinetics of I−III (a−c) as a function of power of incident light (indicated on the plot). The measurements are done at 5 K. Insets show the dependence of photoswitching rate (kp) on the power of incident light.

of photoswitching with relaxation occurring in the fast-relaxing fraction. The plateaus in experimentally measured dependences are not reached in most cases; therefore theoretical simulations are needed. All curves in Figure 4 can be approximated by monoexponential function at small degree of photoconversion, and then they continue a gradual ascent (vide infra). In the range of satisfactory monoexponentiality (RMSD < 5%), the photoconversion curves can be modeled by the master equation dγWS/dt = kp(1 − γWS) − krγWS.59 This equation results in quasistationary value of γWS at t → ∞ being γWS,q = kp/(kp + kr). For each compound I−III we simultaneously fitted all γWS(P) curves with both kp and kr being variables, but with kr values being independent of P. The obtained kp(P) dependences are given in the insets of Figure 4, whereas the obtained kr values equal 0.0211, 0.0057, and 0.1115 min−1 for I, II, and III, respectively. Remarkably, the values of kr and kp are comparable for each studied compound at small P, clearly meaning that there is a competition of photoswitching and relaxation in fastrelaxing fraction of clusters. Note that the above master 11733


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Inorganic Chemistry Similar to SCO compounds, relaxation from photoinduced to the ground state in breathing crystals is governed by interplay of quantum tunneling and thermally activated processes. Both vertical and horizontal displacements of the potential wells of SS and WS states are therefore important for the relaxation (sketched in Figure 5). Vertical displacement can be roughly

Table 1. Structural Data for Selected Breathing Crystals Containing Spin Pairs and Spin Triadsa spin pairs Cu(hfac)2LCD3 Cu(hfac)2LMe (I) Cu(hfac)2LEtCP [Cu(hfac)2L5/Me]2 [(Cu(hfac)2]4LPyCP2 spin triads Cu(hfac)2LPr (III) Cu(hfac)2LBu·0.5C7H16 Cu(hfac)2LMe/Et Cu(hfac)2LAll Cu(hfac)2LBu·0.5C6H5−C2H5 Cu(hfac)2LBu·0.5 C6H5−C2H3 Cu(hfac)2LBu·0.5 C6H5−C3H7 Cu(hfac)2LBu·0.5m-C6H4(CH3)2 Cu(hfac)2LBu·0.5p-C6H4(CH3)2 Cu(hfac)2LBu·0.5p-C6H4(C2H3,CH3)

Figure 5. Schematic representation of the potential energy surface in breathing crystals as a function of the Cu−Onitroxide distance rCu−O considered as a reaction coordinate. The differences in shapes and vibronic level displacements of SS/WS potential wells are exaggerated for clarity.

r(WS)/Å

r(SS)/Å

Δr/Å

ref

2.477 2.484 2.451 2.354 2.403

1.958 1.992 2.002 2.034 2.002

0.519 0.492 0.449 0.320 0.401

31 31 56 74 75

2.315 2.320 2.353 2.307 2.332 2.367 2.343 2.285

2.016 2.007 1.990 1.994 1.997 2.020 1.978 2.002

0.299 0.313 0.363 0.313 0.335 0.347 0.365 0.283

31 31 76 77 78 78 78 78

2.329

1.998

0.331

78

2.338

1.996

0.342

78

a The parameter r corresponds to the distance between the copper atom of Cu(hfac)2 and the oxygen atom of nitroxide, in the SS and WS states (as indicated). Δr = r(WS) − r(SS).

estimated from the temperature of thermal spin transition T1/2.14 For I and III the values T1/2 are not very different (∼141 and ∼175 K, respectively), thus the differences in vertical displacement are not likely to account for most important differences found for relaxation in I and III. In addition, previous study on spin triads having noticeably different T1/2 and hence vertical displacements did not reveal any profound differences in the relaxation rates.49 At the same time, remarkably, the available structural data indicates that the horizontal displacement, i.e., the difference in Cu−Onitroxide distances, is much larger (up to a factor of ∼1.5) for spin pairs compared to spin triads, see Table 1. The Cu−Onitroxide distances in WS state are systematically larger for spin pairs, possibly due to the peculiarities of crystal packing, which results in higher Δr values in pairs compared to triads. Obviously, larger horizontal displacement for spin pairs leads to the larger potential barrier for thermally activated processes, as well as thermally assisted tunneling, Epair > Etriad (Figure 5). We believe that this is the major factor governing much higher stability of photoinduced WS states in spin pairs compared to those in spin triads. In order to compare this trend found for breathing crystals with SCO compounds, we attempted to analyze the available literature data on lifetimes of the photoinduced high-spin states in iron-based SCO compounds. Among available relevant data, most often the values of TLIESST are reported, which characterize the temperature where the photoswitched (γHS) fraction vanishes.60 We assume that this value correlates with the magnitude of energy barrier between the potential wells of HS and LS states, at least to some extent. Table S1 reports some available data for the changes in bond lengths in HS and LS states and corresponding TLIESST values for Fe(II) and Fe(III) SCO compounds.61−71 Although the considered list of SCO examples is by far not complete, it is evident that establishing correlations between Δr = r(HS) − r(LS) and TLIESST values is problematic. We assume that the reason for that is relatively small values of Δr found for SCO compounds

compared to breathing crystals (∼0.2 Å vs ∼0.52 Å, respectively). In exceptional cases the Δr for SCO compounds can reach ∼0.3 Å,72 but still, to the best of our knowledge, such high values as Δr ∼ 0.5 Å were never found. It is therefore plausible that the other processes and structural peculiarities (e.g., distortions of FeN6 octahedra60) contribute to the stability of photoinduced states in SCO compounds with weights comparable to those originating from the simple energetic argument sketched in Figure 5. In some cases, the single configurational coordinate model can even become insufficient, and SCO-induced torsion might critically affect the stability of photoinduced states.73 In this point of view, the present study does not only report advancements in understanding of LIESST-like phenomena in breathing crystals; it also presents these copper−nitroxide compounds as interesting playgrounds for benchmarking the basic factors governing the stability of photoinduced states in photoswitchable systems of similar types.

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CONCLUSIONS In this work we have studied general trends of photoswitching and excited state relaxation in molecular magnets Cu(hfac)2LR (breathing crystals) containing spin pairs copper(II)−nitroxide and compared them with those obtained for more common breathing crystals containing spin triads. We have found that the photoswitching from the ground SS state to the excited WS state occurs similar in two types of compounds; however, the following WS → SS relaxation is strikingly different in the two systems. While in spin triads it occurs on the time scale of hours at T ∼ 5−20 K, in spin pairs the corresponding relaxation is negligible during a couple of hours, providing the estimation of the WS state lifetime as at least several days. The comparison of typical structures of CuO6 and CuO5N units for triads and pairs, correspondingly, indicates that the lengthening of the Cu−Onitroxide bond upon conversion to the WS state (ΔrCu−O) is much larger in studied pairs compared to studied triads, 11734


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Inorganic Chemistry

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being the most plausible reason for the higher stability of WS states in spin pairs. Although some variations of ΔrCu−O are found for different triad-containing breathing crystals, no correlations between structure and relaxation properties could be derived solely from triads to date. The opportunity to investigate WS → SS relaxation in spin pairs allowed us to reveal structural factors correlating with the stability of photoinduced states in breathing crystals. More investigations of WS → SS relaxation in spin pairs should be done in the future; however, it is already evident that the search for the most stable WS states should be done within the compounds exhibiting the most pronounced structural changes upon thermal WS ↔ SS switching. This strategy will be used in further research on photoswitchable copper(II)−nitroxide based molecular magnets, and can also be used as a “rule of thumb” for chemical design of other photoswitchable SCO and SCO-like materials, toward their potential applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01689. Correlations between structural data and stability of the corresponding photoinduced high-spin states for ironbased SCO compounds, spectrum of light-emitting diode used for photoexcitation, data processing details, and reversibility of light-induced switching (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Sergey L. Veber: 0000-0002-5445-3713 Victor I. Ovcharenko: 0000-0002-8280-8112 Matvey V. Fedin: 0000-0002-0537-5755 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been supported by FASO Russia (Project 03332016-0001), RFBR (No. 15-03-07640, 17-33-80025, and 15-0300488), and the RF President’s Grants (MK-3597.2017.3 and MK-6040.2016.3). S.V.T. thanks Russian Science Foundation (No. 17-13-01412).

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REFERENCES

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Light-Induced Spin State Switching and Relaxation in Spin Pairs of

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