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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Irradiation Temperature Dependence of the Photomagnetic Mechanisms in a Cyanido-Bridged CuII2MoIV Trinuclear Molecule Olaf Stefanć zyk,†,‡,§,# Robert Pełka,∥ Anna M. Majcher,⊥ Corine Mathonier̀ e,*,†,‡ and Barbara Sieklucka*,§ †

CNRS, ICMCB, UPR 9048, 87 Avenue du Docteur Schweitzer, F−33608 Pessac, France University Bordeaux, ICMCB, UPR 9048, 87 Avenue du Docteur Schweitzer, F−33608 Pessac, France § Faculty of Chemistry, Jagiellonian University in Krakow, Gronostajowa 2, 30−387 Kraków, Poland ∥ Polish Academy of Science, H. Niewodniczański Institute of Nuclear Physics PAN, Radzikowskiego 152, 31−342 Kraków, Poland ⊥ Faculty of Physics, Astronomy and Applied Computer Science, Jagiellonian University in Krakow, Łojasiewicza 11, 30−348 Kraków, Poland

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‡

S Supporting Information *

ABSTRACT: We report a new bimetallic cyanido-bridged trinuclear complex [CuII(enpnen)]2[MoIV(CN)8]·6.75H2O (1) (enpnen = N,N′-bis(2-aminoethyl)-1,3-propanediamine) that shows reversible photomagnetic effect. The photoinduced increase of magnetization is characterized by the irradiation temperature-dependent shapes of the χMT(T) plots and different magnetization values at low temperature in high magnetic field, suggesting multiple photoexcited states. The photomagnetic effect in 1 is explained through two possible processes simultaneously: the light-induced metal-to-metal charge transfer (MMCT) in the CuII−NC−MoIV pair and the light-induced excited spin-state trapping (LIESST) effect in MoIV center. A numerical model assuming the simultaneous existence of three possible states after irradiation: the MMCT CuI−NC− MoV−CN−CuII state, the LIESST CuII−NC−MoIVHS−CN−CuII state, and the ground-state CuII−NC−MoIVLS−CN−CuII was applied to the data and resulted in Cu−Mo exchange coupling constants J1MMCT = 11 cm−1 and J2LIESST = 109 cm−1 for the MMCT and LIESST mechanisms induced states, respectively. Fractions of respective states after irradiations at different temperatures were also calculated, shedding light on the influence of irradiation temperature on the photomagnetic mechanism. The proposed model can provide the interpretative framework for testing and refinement of the mechanism of photomagnetic effect in other coordination networks with cyanido-bridged Cu−[Mo(CN)8]4− linkages.

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pairs.41,50−71 The LIESST mechanism here is based on electronic excitation within the d-orbitals of the octacyanidomolybdate(IV) anion, leading to singlet−triplet transition in closed shell MoIVLS (S = 0) to triplet-state MoIVHS (S = 1) resulting in an increase of the total number of spins for the isolated centers.44,47,48 The newly generated metastable HS molybdenum(IV) centers and the paramagnetic copper(II) centers can couple magnetically, triggering development of isolated coordination clusters with HS ground states and longrange magnetic ordering for the extended frameworks. Extensive studies of the mechanism of singlet−triplet excitation of [MoIV(CN)8]4− anions demonstrated its strong dependence on the d-orbital splitting pattern, that is, upon the coordination geometry.41,44,47,48 Consequently, the singlet− triplet transition is predominant for the [Mo(CN)8]4− ion adopting deformed bicapped trigonal prism geometry, which was confirmed experimentally for [Zn2−xCux(tren)2][Mo-

INTRODUCTION Photomagnetism of functional molecular materials offers an uncommon way of manipulating spins and magnetization by electromagnetic radiation, which is not attainable in conventional magnets.1−12 The bistable character in the photoswitching process can refer to the light-induced excited spinstate trapping (LIESST) effect (photo-induced transformation from low-spin (LS) to high-spin (HS) states) of a single metal center.13−26 The change in magnetic properties of molecular compounds can also be generated by modifying redox states of two different metal centers through the photo-induced electron transfer (metal-to-metal charge transfer (MMCT) process) of the bimetallic assembly.27−39 The mechanism of photomagnetic phenomenon in coordination frameworks based on diamagnetic 4d2/5d2 [M(CN)8]4− (M = MoIV, WIV) elementary units is generally interpreted using either of the two above processes, the LIESST on the sole MIV center in [M(CN)8]4− (M = MoIV, WIV) building block40−49 or the MMCT mechanism in the majority of investigated materials, containing CuII−NC−[MIV(CN)8]4− © XXXX American Chemical Society

Received: March 2, 2018

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DOI: 10.1021/acs.inorgchem.8b00545 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry (CN)8] (tren = tripodal N4 amine ligand) molecules.44 Recently, the existence of singlet-triplet transition was also confirmed for assemblies with bidentate 2,2′-bipyridine (bpy): {[Cd(bpy)2]4[Mo(CN)8]2}·10H2O hexanuclear cluster43 and {[MnII(bpy)2][MnII(bpy)(H2O)2][M(CN)8]·5H2O}n (M = MoIV, WIV) chains.42 According to the photo-induced electron transfer mechanism, exposition of {CuII−NC−MoIV} pair to electromagnetic radiation to visible light range can induce electron transfer, generating the metastable {CuI−NC−MoV} redox state.50−71 This process can account for magnetic coupling between the photogenerated MoV centers and the remaining paramagnetic CuII centers, leading to development of isolated coordination clusters with HS ground states and long-range magnetic ordering for the extended frameworks. Nevertheless, the total magnetization for the isolated spin centers remains unchanged after the photo-induced process. Taking into account that the mechanism and factors influencing photomagnetic phenomena of the coordination compounds incorporating CuII−NC−MoIV linkage is not yet fully explained,4,37 we constructed a novel molecular material to establish the favored pathway of the photomagnetic process and magnetostructural correlations. Learning from the previous systems,72,73 we designed new photoswitchable cyanido-bridged bimetallic trinuclear complex [CuII(enpnen)]2[MoIV(CN)8]·6.75H2O (1) (enpnen = N,N′bis(2-aminoethyl)-1,3-propanediamine) through a rational building-block approach. Moreover, the near-room-temperature relaxation of the photogenerated metastable state for molecular systems built of cyanido-bridged Cu II − [MoIV(CN)8]4− coordination networks40−71 inspired us to investigate for the first time the photomagnetic effect as a function of temperature of irradiation to clarify the photomagnetic behavior. A numerical model assuming the coinciding existence of three possible states after irradiation: light-induced MMCT state: CuI −NC−Mo V −CN−CuII; LIESST state: CuII−NC−MoIVHS−CN−CuII, and a ground state: CuII−NC−MoIVLS−CN−CuII was applied to the magnetic susceptibility and magnetization data.

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optically diluted 1 in BaSO4 (nm): 250vs, 309s, 381s [metal-to-ligand charge transfer (MLCT) and ligand field (LF) bands of [Mo(CN)8]4−]; 432m, 568vs(br) [Mo(IV)−Cu(II) MMCT bands], 803m(br) [Cu(II) d−d band]. Single-Crystal X-Ray Diffraction. Data for 1 were collected at 153 K on a Nonius KappaCCD diffractometer, operating at 50 kV and 30 mA using graphite monochromated molybdenum radiation [λ(Mo Kα) = 0.710 73 Å]. Data collection and reduction were performed utilizing the Denzo and Scalepack (KappaCCD) programs. The structure of 1 was solved by direct methods using SHELXT and refined using a F2 full-matrix least-squares technique of SHELXL2014/7 included in the OLEX-2 1.2 software package.75,76 The non-H atoms were refined anisotropically adopting weighted full-matrix leastsquares on F2. Water molecule (O14) near the b-axis was found disordered between the two unit cells with occupation factor of 1/2. Part of N,N′-bis(2-aminoethyl)-1,3-propanediamine ligand coordinated to the copper(II) center (Cu6) is disordered in two position with occupation factors of 0.539 and 0.461. Majority of hydrogen atoms was positioned with an idealized geometry and refined using a riding model. Selected hydrogen atoms bonded to O7, O8, and 013 are disordered between two positions with an occupancy factor of 1/2 to reproduce two possible orientations of water molecules in the hydrogen-bond network. The positions of hydrogen atoms bonded to O14 were not available on the electron density map. Crystal data, data collection, and refinement parameters for 1 are listed in Supporting Information, Table S1. CCDC 1822556 contains the supplementary crystallographic data for these compounds, and additional crystallographic information is available in the Supporting Information. The structural data presented as figures were prepared with the use of the CCDC Mercury 3.9 visualization software.77 Geometries of metal centers are estimated with the Continuous Shape Measures (CShM) analysis with the use of SHAPE v2.0 software.78,79 Physical Techniques. Elemental analysis (C, H, N) was performed on Elementar Analysensysteme GmbH: vario MICRO cube. Infrared spectrum was recorded in the 4000−550 cm−1 range using a Thermo Scientific Nicolet iS5 spectrometer equipped with iD5 ATR-Diamond. Solid-state UV−vis−NIR absorption spectrum of 1 diluted in BaSO4 was recorded with a PerkinElmer Lambda 35 UV/ vis spectrophotometer equipped with a PerkinElmer Labsphere RSAPE-20 Reflectance Spectroscopy Accessory and converted with the Kubelka−Munk function. The deconvolution of the solid-state UV− vis−NIR absorption spectrum was performed by means of the Origin 8.0 software.80 All magnetic properties were measured by means of a Quantum Design MPMS XL system in the range of temperatures 1.8−300 K and fields up to 70 kOe. Basic magnetic measurements were done for a large sample (ca. 30 mg) placed in a 30 μm thick polyethylene bag (30 × 5 mm). Photomagnetic studies were conducted on a smaller sample (ca. 2 mg) scattered as a thin film between two pieces of a Scotch tape, blocked tightly between two transparent polypropylene films and mounted in the probe equipped with an optical fiber entry enabling the transmission of laser light of 405 nm line (P ≈ 5 mW/cm2) into the sample space. Diamagnetism of the sample holders and of the constituent atoms (Pascal’s tables) was accounted for in all the obtained magnetic and photomagnetic data.81 The numerical calculations of magnetic and photomagnetic properties were performed in an especially designed notebook of the Mathematica8.0 environment.82 The details of the fitting procedure with the equations used are presented in the Supporting Information.

EXPERIMENTAL SECTION

Materials. Copper(II) perchlorate hexahydrate and N,N′-bis(2aminoethyl)-1,3-propanediamine were acquired from Sigma-Aldrich and used without further purification. Potassium octacyanidomolybdate(IV) salt was prepared as stated in the published procedure.74 Caution! We did not encounter any problems during our studies; nevertheless, the perchlorate salts compounds are potentially explosive and should be handled with care. Synthesis and Characterization of [CuII(enpnen)]2[MoIV(CN)8]·6.75H2O (1). Aqueous solutions of Cu(ClO4)2·6H2O (37 mg, 0.1 mmol, 5 mL) with N,N′-bis(2aminoethyl)-1,3-propanediamine (15 mg, 0.1 mmol) were mixed with a water solution of K4[Mo(CN)8]·2H2O (25 mg, 0.05 mmol, 5 mL). The resulting clear purple solution was left in an open vessel in the dark at room temperature for crystallization. Dark violet crystals of 1 were formed within a week. Yield: 31 mg (72%). Anal. Calcd (%) for C22H51.5Cu2MoN16O5.75 (1·1.25H2O, 855.3 g/mol): C, 30.89; H, 6.07; N, 26.20. Found: C, 30.88; H, 5.94; N, 26.00. Fourier transform infrared (FT-IR) (cm−1): 3327w(br), 3314m, 3283m, 3245m, 3159w [ν(O−H), ν(N−H)]; 2964vw, 2942vw, 2876vw [ν(C−H)]; 2114vs, 2096vs [ν(CN)]; 1640w, 1602w [γ(O−H)]; 1472w, 1452w, 1433w, 1324vw, 1289w [δ(H−C−H), νa(C−N), ν(C−H)]; 1144w, 1108w(sh), 1094m, 1080w, 1058m, 1043m, 1023s, 981vs, 961m, 936w(sh), 885w, 876w, 668s(br) [γ(C−H), γ(N−H out-of-plane), ν(C−N)]. Solid-state UV−vis−NIR absorption spectrum of the

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RESULTS AND DISCUSSION Structural Description. Compound 1 crystallized in the triclinic space group P1̅. The crystal structure of 1 consists of two types almost identical trinuclear molecules [((enpnen)CuII(μ-CN))2MoIV(CN)6] arranged into two-dimensional (2D) supramolecular square grids revealing 8-metallic units through long coordination bonds (ca. 2.8 Å) between terminal cyanides and copper(II) centers (Figure 1). Selected structural parameters for 1 are listed in Supporting Information, Table B

DOI: 10.1021/acs.inorgchem.8b00545 Inorg. Chem. XXXX, XXX, XXX−XXX

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(vOC-5) (Supporting Information, Table S3) with one axial CN− with average Cu−N distance of 2.316(2) Å and Cu−N C angle of 142.61(19)°, whereas the equatorial positions are coordinated by the nitrogen atoms of the enpnen ligand with average Cu−N length of 2.023(2) Å. Structure of 1 is stabilized by the hydrogen-bond network divided in two subnetworks (Figure 1 and Supporting Information, Figure S1). The first subnetwork involves water molecules (O1−O6) mainly connected through hydrogen bonds to each other (O−H··· O) and to terminal cyanides (O−H···Ncyanide), as well as from enpnen ligand to water molecules O2 and O3 (N−Henpnen··· O). The second subnetwork, based on water molecules (O7− O14), is much more extended and complex. The hydrogen bonds are formed only between water molecules (O−H···O) and terminal cyanides (O−H···Ncyanide) with exception of single hydrogen bond between enpnen and water molecule O12 (N44−H44Aenpnen···O12). Finally, two hydrogen-bond subnetworks are placed alternately in relation to the 2-D supramolecular layer build of [((enpnen)Cu I I (μCN))2MoIV(CN)6] entities. The closest distances between two metal centers of two different trinuclear molecules through hydrogen-bond network are more than 10 Å and involve two or more water molecules. Long distances between paramagnetic centers ensure almost perfect magnetic isolation between the molecules in respect to hydrogen bond interactions. Only weak interactions magnetic between molecules can be observed within 2-D supramolecular layers. Incorporation of the unexplored linear N-tetradentate enpnen ligand in the synthesis of Cu(II)−Mo(IV) systems resulted in the formation of a trinuclear [CuII(enpnen)]2[MoIV(CN)8]·6.75H2O molecule (1). Trinuclear cyanidobridged Cu2Mo molecules are commonly observed among Cu(II)−Mo(IV) systems.40,49,50,52,55,58,68,83−86 Such a structural arrangement is favored due to the charge compensation of two [Cu(L)n]2+ ions by [Mo(CN)8]4− anion. In all the assemblies, the coordination sphere of copper(II) is completed either by four nitrogen atoms of two terminal N-bidentate ligands40,49,55,68,85,86 or one terminal N-tetradentate ligand.50,52,58,83,84 Analysis of local geometries of copper(II) centers (Supporting Information, Table S3) reveals that trigonal bipyramid geometry is exceptional among trinuclear systems and can be achieved mainly for tripodal ligands tris(2aminoethyl)amine and tris(2-pyridylmethy)amine52 and bulky bidentate aromatic 2,2′-bipyridine.55,68 In other cases copper(II) adopts intermediate geometry between square pyramid and vacant octahedron with a slight predominance of the first one. There is no strict correlation between the type of ligand coordinated to Cu(II) center and the distortion of three perfect geometries of [Mo(CN)8]4− complex. However, we can deduce that assemblies with the trigonal bipyramid geometry of copper(II) entities prefer almost perfect triangular dodecahedron geometry of [Mo(CN)8]4− ion, while the square pyramid and vacant octahedron ones usually correlate to square antiprism geometry of octacyanidomolybdate(IV). These observations can be very useful in the rational design of photomagnetic materials, taking into account that the geometry of [Mo(CN)8]4− ion is considered as an important factor for observation of LIESST effect on Mo(IV) center.41,44,47,48 Compound 1 shows very similar local geometries of Cu(II) and Mo(IV) to Cu2Mo photomagnetic material with linear tetradentate poliamine 1,2-bis(3aminopropylamino)ethane coordinated to Cu(II).52 Comprehensive analysis of structural parameters of 1 and other

Figure 1. (a) ORTEP diagram of asymmetric unit of 1 with selected atoms labeling. Colors used: C−gray, Cu−orange, N−blue, Mo− green, O−red. Thermal ellipsoids of 50% probability are shown. (b) Crystal packing of 1 in [001] crystallographic direction. Colors used: trinuclear motifs with Mo1 center−magenta sticks, trinuclear motifs with Mo2 center−green sticks. The dotted orange lines represent supramolecular connections. (c) Three-dimensional packing along aaxis with two hydrogen-bond subnetworks (yellow and blue dotted sticks). In all figures hydrogen atoms are omitted for clarity.

S2. Both octacyanidomolybdate(IV) anions in asymmetric unit reveal square antiprism (SAPR-8) geometries with two bridging cyanides. This geometry is quite common among trinuclear Cu−Mo systems (Supporting Information, Table S3). Average Mo−C and CN bonds in [Cu(enpnen)]2[Mo(CN)8] are equal to 2.157(2) and 1.153(3) Å, respectively, and the average Mo−CN angle is 176.9(2)°. The Cu···Mo··· Cu angles between the two arms adopt 152.78(1) and 151.22(1)° for Mo1 and Mo2 centers, respectively. All [Cu(enpnen)(NC)] entities reveal intermediate geometries between square pyramidal (SPY-5) and vacant octahedral C

DOI: 10.1021/acs.inorgchem.8b00545 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry CuII2MoIV trinuclear systems provides the following information (Supporting Information, Table S4). The average Cu− Nligand distances are roughly the same with values ∼2.0 Å despite the fact that each trinuclear system contains different types of ligands, and they exhibit different local geometries of Cu(II) units. The largest differences among trinuclear molecules has been observed for cyanido bridges. Cu−Ncyanide bond lengths adopt values in a wide range between 1.9 and 2.6 Å. However, these distances for assemblies with the trigonal bipyramid geometry of copper(II) complexes are reduced to 2.0 Å. Furthermore, Cu−NC angle values deviate more strongly from straight angle being inversely proportional to the Cu−Ncyanide distances. The angle decreases from ∼175° to 135°, while the Cu−N distance increases from 1.9 to 2.6 Å. Finally, Cu···Mo···Cu angle values do no depend on the type of ligand. However, two groups regarding the Cu···Mo···Cu angle can be distinguished: “acute angle” (∼80°) and “obtuse angle” (∼150°). Assembly 1 with the average Cu···Mo···Cu angle of 141.86° belongs to the second group. Furthermore, the average Cu−NC angle of 1 adopts one of the smallest values among the trinuclear systems, while the average Cu−N distance for bridging cyanide is in the middle of the range. We were not able to find clear correlation between structural parameters and photomagnetic phenomenon in Cu(II)−Mo(IV) trinuclear molecules due to the different experimental conditions in each photomagnetic experiment. However, there are indications that assemblies with acute angle demand less time of photoirradiation than those with obtuse angle ones to reach significant increase of the photoinduced magnetic signal. Reflectance Spectroscopy. Fitting peak functions to the experimental solid-state UV−vis−NIR absorption spectrum of 1 (Supporting Information, Figure S2) delivered six Gaussian components with maxima at 250, 309, 381, 432, 568, and 803 nm. Three bands at 250, 309, and 381 nm are assigned to the MLCT transition and two LF transitions of the [Mo(CN)8]4− anion, respectively.4 Next two bands at 432 and 568 nm are attributed to the MMCT optical transition from the two highest valence bands of molybdenum(IV) to the two lowest conduction bands of copper(II) centers.87 The last band at 803 nm is related to the d−d transition of [CuII(enpnen)]2+ entities with vacant octahedral/square pyramidal geometry.50−56,88 The 405 nm excitation line for photomagnetic studies was selected based on the above interpretation of the reflectance spectrum. Analysis of UV−vis−NIR absorption spectra of 1 and other previously reported Cu(II)−Mo(IV) photomagnetic assemblies (Supporting Information, Table S5), along with the most recent results of the first-principle calculations by the GGA+U method for Cu2[Mo(CN)8]·2H2O,87 reveals several features. The spectra of three compounds: (H4cyclam)[Mo(CN)8]· 1.5H2O,42 {[Cd(bpy)2]4[Mo(CN)8]2}·10H2O,43 and [Mn(bpy)2][Mn(bpy)(H2O)2][Mo(CN)8]·5H2O,42 exhibiting single LIESST phenomenon on Mo(IV) center, show three strong bands around 250, 300, and 400 nm, corresponding to MLCT and LF bands of octacyanidomolybdate(IV) only. Incorporation of bare copper(II) ions in compounds with [Mo(CN)8]4− anion results in the appearance of additional medium intense bands in 400−650 nm range,65,66,71,87 corresponding to MMCT bands in CuII−NC−MoIV pairs, which was confirmed by the results of the first-principles calculations.87 Finally, incorporation of the organic ligands leads to appearance of further d−d bands of copper(II) complex above 600 nm. Absorption spectra of 1 and other

Cu(II)−Mo(IV) complexes with organic ligands are rather similar to several bands below 350 nm, single band around 430 nm, and two bands in 500−700 nm and 800−900 nm ranges. The energies and intensities of absorption bands for compounds including the same organic ligand but with different topologies are almost identical. All attempts to correlate ligand donor strength with changes in electronic spectra were unsuccessful due to similar spectrochemical character of aliphatic polyamines. Magnetic Properties of 1. The magnetic properties of 1 in the ground state (Supporting Information, Figure S3) reveal that it is an almost exclusively paramagnetic system with a steep downshift below 6 K. The experimental χMT value at 300 K reaches 0.77 cm3 K mol−1, which agrees with the contribution from two spins 1/2 of the copper(II) centers with Landé factor g = 2.04. The magnetization at 1.8 K versus applied field is shown in inset of Supporting Information, Figure S3 and is close to saturation reaching the value of 1.94 NAμB in the field of 70 kOe, which complies with the value of 2.0 NAμB calculated for two spins 1/2 with g = 2.0. To acquire a more quantitative understanding of the details of possible interactions, a molecular field theory model was fitted to the magnetic measurements data (for details see Supporting Information, Figure S3 and Model “0” calculation section). The fit yielded weak ferromagnetic intermolecular coupling zJ′ = 0.13(1) cm−1 and gCu = 2.02(2) with good agreement. Photomagnetic Studies of 1. Time dependencies of the magnetic response of the assembly during excitation with 405 nm line at 10, 50, 100, 150, and 200 K are presented in Supporting Information, Figure S4. During irradiation for 24 h the χMT value increased steadily attaining the values of 1.08 cm3 K mol−1 at 10 K, 1.00 cm3 K mol−1 at 50 K, 0.91 cm3 K mol−1 at 100 K, 0.89 cm3 K mol−1 at 150 K, and 0.78 cm3 K mol−1 at 200 K. The relative increase of the signal is reduced with the increasing temperature of irradiation and amounts to 44% at 10 K, 31% at 50 K, 18% 100 K, 17% at 150 K, and 2% at 200 K. The highest efficiency of photoconversion was observed at the lowest irradiation temperatures of 10 and 50 K. At both temperatures the magnetic signal increases rapidly at the beginning of the light excitation (within few hours), after which the growth rate systematically deteriorates and approaches a constant value. Studies at 100 and 150 K display similar trajectory of the time evolution product of magnetic susceptibility and temperature; however, the maximal value of magnetic signal is almost 2 times smaller in comparison to results obtained in the low-temperature regime. Investigation at 200 K shows a very small but non-negligible increase of magnetic susceptibility, evidently proving unprecedented photomagnetic effect in CuII−[MoIV(CN)8]4− system induced at near-room temperature. When the irradiation was stopped, the time evolution of magnetic signal was observed for at least 30 min at each temperature. The detected magnetic signals were constant, therefore proving the high stability of the photo-induced metastable state and very long time of thermal relaxation under experimental conditions employed. The samples after irradiation were cooled to low temperatures, and their magnetization was measured as a function of the applied field at 1.8 and 5 K and then in 10 kOe with increasing temperature (a sweep rate of 0.4 K/min up to 300 K). The irradiation of the sample at each temperature (ranging from 10 to 200 K) resulted in an apparent increase of the χMT values (Figure 2). The magnitude of the χMT curves obtained from D

DOI: 10.1021/acs.inorgchem.8b00545 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 3. Schematic of the ground-state model “0”, the MMCT excited-state model “1”, and the LIESST-state model “2”.

Figure 2. ΧMT(T) plots for 1 before and after excitation with 405 nm laser line performed at diverse temperatures and after heating to 300 K. Black solid lines are fits of a model representing the three assumed possible states after irradiation.

CuII2MoIVLS (SCu(II) = 1/2, gCu(II) = 2.0, SMo(IV−CS) = 0) species and 12% CuII2MoIVHS (Sall = 2, gall = 2.0) is lower than the observed value of 1.14 cm3 K mol−1. Detailed consideration of the temperature dependence of the χT signal and the growth of the low-temperature magnetizations in high fields after irradiation demonstrate clearly that the photomagnetic effect in 1 is more complex and may originate from both MMCT and LIESST processes. In this approximation, we assume that irradiation of CuII−NC− MoIVLS−CN−CuII ground state (model “0”, see Supporting Information) leads to the simultaneous occurrence of the CuI− NC−MoV−CN−CuII and the CuII−NC−MoIVHS−CN−CuII metastable excited states (described by models “1”and “2”, respectively), which relax back to the ground state upon thermal treatment above their characteristic relaxation temperatures, which are independent of each other (Figure 2) (for details, see Supporting Information). J1MMCT and J2LIESST denote superexchange constants of the coupling between MoV of S = 1/2 and the remaining CuII of S = 1/2 and the MoIVHS of S = 1 and the CuII of S = 1/2, respectively. Simple approximation of the structural identity of all trinuclear molecules in 1 allows to ascribe them a single exchange coupling constant each. The Hamiltonians describing each model are therefore Ĥ 1 = −J1Ŝ Mo·Ŝ Cu + gμB(Ŝ Cu + Ŝ Mo)·Ĥ and Ĥ 2 = −J2Ŝ Mo·(Ŝ Cu1 + Ŝ Cu2) + gμB(Ŝ Cu1 + Ŝ Cu2 + Ŝ Mo)·Ĥ for models “1” and “2”, respectively. For the sake of simplicity, a single average Landé factor g was assumed in both cases. It must be underlined that the fit was done to all the available χMT(T) data after irradiation. The calculation of the exchange couplings in both the MMCT excited state (J1MMCT = 11 cm−1) and the LIESST state (J2LIESST = 109 cm−1) confirms their ferromagnetic character, which is consistent with the general increase of the magnetic susceptibility upon irradiation. Furthermore, the determined exchange coupling constant J1MMCT for CuII−NC− MoV pair, generated as a result of photo-induced MMCT process, adopts a value of the same order as the one reported for similar trinuclear CuII2MoV molecule.52 Consequently, the second exchange coupling J2LIESST is one order larger than J1MMCT, but there are no experimental reports for CuII−NC− MoIVHS pairs to which we can refer. Parameter ρ implies that the value of the high-temperature limit of the lifetime τ0 of photo-induced MMCT metastable state is τ0 ≈ 90 ± 65 min, indicating that MMCT excited state is practically stable for long period at very low temperature. On the one hand, only at

data collected upon heating decrease with the increasing temperature of irradiation. More importantly, the shapes of the χMT curves change, depending on the temperature at which the irradiation was performed. The way in which the irradiation temperature affects the shape of the χMT(T) curves is nontrivial and was fitted using the model of the two contributing mechanisms of the photomagnetic effect (solid lines in Figure 2), which is discussed in more detail below. The relaxation temperature of 255 K remains unchanged regardless of the temperature at which the irradiation was performed. The photo-induced magnetization disappeared after the sample was heated to 300 K in each case. The isothermal M versus H plots at 1.8 and 5 K before and after irradiations done at various temperatures are presented in Supporting Information, Figure S5. After excitations, the magnetization is increased compared to the magnetization plots of non-irradiated samples. Like in our previous work, and in other similar reported cases,50−56 the increase of magnetization in 70 kOe (up to 2.24 NAμB for the sample irradiated at 10 K) suggests that the mechanism of the photomagnetic effect is more complex than just the photo-induced electron transfer mechanism, which does not produce new spins. Mechanisms of the Photomagnetic Behavior of 1. The photomagnetic phenomena involving CuII−NC−MoIVLS− CN−CuII linkage have been interpreted in terms of either light-induced MMCT in CuII−NC−MoIV pairs41,50−71 or LIESST in the MoIV centers separately40−49 (Figure 3), without any attempts to incorporate these two mechanisms in one model. The major disadvantage of the MMCT mechanism in the case of 1 is the failure to explain the augmentation of magnetizations at low temperature in high external magnetic fields after photoexcitation (Supporting Information, Table S6 and Figure S5), which ought not to be observed in systems with an invariant number of spins. Taking into account the sole LIESST effect and assuming the full conversion of Mo(IV)LS (S = 0) closed-shell singlet centers in 1 into their Mo(IV)HS (S = 1) triplet state, we would anticipate a magnetization value close to 4 Nβ at 1.8 K.44,47,48 The collation with the highest observed value of 2.24 Nβ suggests ∼12% conversion of the MoIVLS → MoIVHS in 1. However, the Curie constant value of 1.06 cm3 K mol−1 estimated for 88% E

DOI: 10.1021/acs.inorgchem.8b00545 Inorg. Chem. XXXX, XXX, XXX−XXX

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magnetic susceptibility is satisfactory. The largest discrepancies can be observed in the low-temperature limit, which may be because (for the sake of simplicity) we neglected the intermolecular interactions between the trinuclear moieties in their excited states. The introduction of two additional parameters would certainly have an adverse effect on the convergence of the optimizing procedure. The agreement of the calculated values (solid lines) with the experimental ones (symbols) of the molar magnetization is apparently better at 5 than at 1.8 K. At 1.8 K the calculated values overestimate the experimental signal especially for the intermediate field values. This again may be due to the lack of accounting for the intermolecular interactions of the trinuclear units in their excited states, which most probably are antiferromagnetic in character. However, this should rather result in a slight underestimation of the fractions and therefore lead to a further enhancement of the calculated values. For the sake of transparency of presentation, the initial abundances (fractions) of the ground and the MMCT and LIESST states are presented in Figure 4 in a form of a stack column chart. It can

higher temperatures the relaxation back to the ground state starts to be visible within measurable time scale. On the other hand, the parameter ρ, containing the information about τ0, has the largest relative error (72%) among all the parameters of the fitting procedure. The ensuing value of the relative error of τ0 is just the same, and it may well be that the true value should be measured in minutes instead of hours. In conclusion, the very value of τ0 implied by the fit must be taken and interpreted with great caution. In the irradiation experiments performed at 10, 50, and 100 K, the temperatures are relatively low with respect to the energy scale of this process quantified by Δ1 ≈ 100 K; the relaxation process is apparent with the χMT signal diminishing gradually with increasing temperature. By contrast, for the irradiation experiments performed at 150 and 200 K, the relaxation of the MMCT process is not visible. This is because the corresponding excited states managed to relax back completely within the time span of the experiment, as the temperatures of irradiation are comparable with the pertinent energy scale. The time span of the experiment is measured in hours, and in view of the above facts the characteristic relaxation time is expected to have roughly the same order of magnitude. The same relaxation effect but at higher temperature (∼200 K) is observed for LIESST state due to higher energy barrier and larger value of the high-temperature limit of the lifetime. The height of the energy barrier for the relaxation of the MMCT and LIESST excited states is around Δ1 ≈ 100 K and Δ2 ≈ 4400 K, respectively. The first value Δ1 is quite small with respect to other assemblies showing MMCT and LIESST phenomena;11,23,30 however, in the combination with unusually long lifetime of the MMCT excited state at low temperature (which can be counted in a wide intervalfrom several days to several years due to large relative errors deriving from the multiparameter nature of the fit, see Supporting Information for details), it governs to relaxation temperature slightly above 100 K (see Table 1). Besides, the Δ2 value is Table 1. Fractions of the Initial Unperturbed State ( fω00), the MMCT Excited State ( fω10), and the LIESST State (fω20) after Excitation at Different Temperatures ω

10 K

50 K

100 K

150 K

200 K

fω00 fω10 fω20

0.477 0.444 0.079

0.476 0.458 0.066

0.841 0.100 0.059

0.945 0 0.055

0.992 0 0.008

Figure 4. Initial abundances of the ground state (GS) and lightinduced MMCT and LIESST states for the irradiation at 10, 50, 100, 150, and 200 K.

be seen that the level of population of both the MMCT and LIESST metastable states diminishes with increasing temperature of irradiation. The LIESST state has a non-vanishing contribution in the whole temperature of irradiation range (10−200 K), while the contribution of the MMCT state is negligibly small for the irradiations above 100 K. Apparently, the time that elapses between the end of irradiation and the start of susceptibility measurement is sufficiently long for the MMCT metastable state to decay to the ground state for the irradiations above 100 K.

uncommonly large and implies near-room temperature relaxation of the LIESST metastable state. The calculated threshold temperature T0 of the decay of metastable state process is 243(7) K, which is in compliance with the experimentally evaluated relaxation temperature of 255 K. The value of the average Landé factor g = 2.13(2) is reasonable and indicates that the Landé factor of the Mo ion in the metastable state gMo, either MoV or MoIVHS, takes on the value in the interval [2.13, 2.24]. Finally, the fractions of the initial unperturbed-state model “0” (fω00), the MMCT excited-state model “1” (fω10), and the LIESST-state model “2” (fω20), where ω = {10, 50, 100, 150, and 200 K} are all positive and consistently attain values below 1 (Table 1). The curves corresponding to the resultant set of values are depicted with solid lines in Figure 2 (molar magnetic susceptibility) and in Supporting Information, Figure S5 (molar magnetization). The agreement of the calculated values (solid lines) with the experimental ones (symbols) of the

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CONCLUSIONS Detailed consideration of the temperature dependence of the photomagnetic behavior of trinuclear [CuII(enpnen)]2[MoIV(CN)8]·6.75H2O clearly demonstrates the complex character of the photo-induced magnetic properties. The successful analysis corroborates the assumption that the irradiation of the ground CuII−NC−MoIVLS−CN−CuII state of 1 triggers two coinciding processes: the light-induced MMCT leading to a metastable CuI−NC−MoV−CN−CuII state with the Arrhenius-type relaxation and the singlet−triplet F

DOI: 10.1021/acs.inorgchem.8b00545 Inorg. Chem. XXXX, XXX, XXX−XXX

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transition associated with the LIESST effect resulting in a metastable state CuII−NC−MoIVHS−CN−CuII, the relaxation of which displays a threshold behavior. Despite the modeling of the photo-induced curves involving as many as 17 parameters, the calculations presented in this study converged to give the best fit with physically acceptable parameters in compliance with the experimental χMT(T) data. Moreover, the simultaneous implementation of these parameters predict the isothermal magnetization, which agrees remarkably well with the experimental results. In all the cases, the level of population of both the MMCT and LIESST metastable states diminishes with increasing irradiation temperature. It is important to note that the LIESST metastable state has a non-vanishing contribution within the whole temperature range, while the contribution of the MMCT metastable state is negligibly small for the temperature of irradiation higher than 100 K, which is consistent with the height of the energy barriers and the relaxation times determined from the numerical calculations. The calculation of the exchange couplings in both the MMCT excited state (J1MMCT = 11 cm−1) and the LIESST state (J2LIESST = 109 cm−1) confirms their ferromagnetic character. The presented experimental approach and the outlined model can provide an interpretative framework for testing and refining of the mechanism of photomagnetic effect in other cyanido-bridged CuII−[Mo(CN)8]4− coordination networks.

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The University of Tokyo, Department of Chemistry, School of Science, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Centre National de la Recherche Scientifique (CNRS), the Institut Universitaire de France (IUF), and the Polish National Science Centre within the Opus 8 research project (Decision No. DEC-2014/15/B/ST5/04465) for financial support.

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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.8b00545. Crystal data, data collection, and refinement parameters; selected bond lengths and angles; listings of SHAPE parameters calculated for the CuII and MoIV centers; graphical presentation of hydrogen bonds network of 1; structural parameters and photomagnetic properties of trinuclear CuII2MoIV systems; deconvoluted solid-state UV−vis−NIR absorption spectrum; analysis of UV− vis−NIR solid-state spectra for 1 and other literaturereported CuII−[MoIV(CN)8]4− systems; χMT(T) and M(H) curves for 1 before irradiation; χMT(time) and M(H) curves after excitation at different temperatures; details of calculations of magnetic and photomagnetic properties of 1 (PDF) Accession Codes

CCDC 1822556 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (C.M.) *E-mail: [email protected]. (B.S.) ORCID

Olaf Stefańczyk: 0000-0003-0955-5646 Robert Pełka: 0000-0002-9796-250X Anna M. Majcher: 0000-0001-7921-5968 G

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DOI: 10.1021/acs.inorgchem.8b00545 Inorg. Chem. XXXX, XXX, XXX−XXX