Ligand Modified and Light Switched On/Off Single-Chain Magnets of {Fe 2 Co} Coordination Polymers via Metal-to-Metal Charge Transfer
2022; Chinese Chemical Society; Volume: 5; Issue: 4 Linguagem: Inglês
10.31635/ccschem.022.202202023
ISSN2096-5745
AutoresJi‐Xiang Hu, Hai‐Lang Zhu, Yin‐Shan Meng, Jiandong Pang, Na Li, Tao Liu, Xian‐He Bu,
Tópico(s)Metal complexes synthesis and properties
ResumoOpen AccessCCS ChemistryRESEARCH ARTICLE2 Jun 2022Ligand Modified and Light Switched On/Off Single-Chain Magnets of {Fe2Co} Coordination Polymers via Metal-to-Metal Charge Transfer Ji-Xiang Hu†, Hai-Lang Zhu†, Yin-Shan Meng, Jiandong Pang, Na Li, Tao Liu and Xian-He Bu Ji-Xiang Hu† School of Materials Science and Engineering, Nankai University, Tianjin 300350 College of Chemistry and Chemical Engineering, Qingdao University, Qingdao 266071 , Hai-Lang Zhu† State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024 , Yin-Shan Meng State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024 , Jiandong Pang School of Materials Science and Engineering, Nankai University, Tianjin 300350 , Na Li School of Materials Science and Engineering, Nankai University, Tianjin 300350 , Tao Liu *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024 and Xian-He Bu *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] School of Materials Science and Engineering, Nankai University, Tianjin 300350 https://doi.org/10.31635/ccschem.022.202202023 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail It is still a formidable challenge to simultaneously switch single-chain magnet (SCM) behavior via ligand modification and light irradiation in the field of molecular spintronics. Herein, we present a ligand-bridged layer {[pzTpFe(CN)3]4Co2(Bib)4}·3H2O ( 1; pzTp, tetra-kis(1-pyrazolyl)borate; Bib, 1,4-bis-(1H-imidazol-1-yl)benzene) and a well-isolated double chain {[pzTpFe(CN)3]2Co(Bpi)2}·CH3CN·4H2O ( 2; Bpi, 1-Biphenyl-4-yl-1H-imidazole) that display reversible metal-to-metal charge transfer (MMCT) between FeIIILS(μ-CN)CoIIHS(μ-NC)FeIIILS (LS, low spin; HS, high spin) and FeIIILS(μ-CN)CoIIILS(μ-NC)FeIILS linkages under alternating irradiation with 808 and 532 nm lasers. The bidirectional light irradiations induces significant changes in anisotropy and intrachain magnetic interactions, resulting in the on/off switching of SCM behavior with observable hysteresis loops by 808 and 532 nm light irradiations for both compounds. Because of the ligand modification, the SCM property of 2 with the monodentate ligand is greatly improved with a correlation length increased to 83, which is the largest correlation length among all reported light actuated SCMs. Furthermore, the influence of ligand modification on their thermally induced MMCT is also discussed. This study provides a rational approach for the swift and reversible control of SCM behavior via ligand modified and light induced MMCT, which is crucial to the future technological demand for high-density data storage and processing. Download figure Download PowerPoint Introduction Single-chain magnets (SCMs), which can exhibit slow dynamics for the relaxation of their magnetization and magnetic hysteresis, have been attracting much interest due to the potential applications in spintronic, quantum computing, and intelligent storage of information.1–7 Various SCMs have been developed since the first experimental observation of Glauber dynamics in a one-dimensional system in 2001.8–14 A recent focus is the design of switchable SCMs, which could exhibit sensitive responses to external stimuli, such as heat, pressure, guest molecules, and especially light.15–18 Due to the matched energy of the interconversion of different electronic states and picosecond response to functional signals, light-induced SCMs actuated by metal-to-metal charge transfer (MMCT) have been specially preferred in constructing these functional materials.19–24 By utilizing metallocyanate building blocks to incorporate the MMCT units into the chain structures, the spin states, anisotropy, and magnetic couplings can be photoswitched, so as to efficiently tune the SCM behavior. Until now, several light-driven SCMs have been synthesized, wherein SCM behavior can be triggered by light and erased upon heating.25–27 Recently, Liu et al. utilized the cyano-bridged FeIII–CoII assembly to achieve the photo- switchable SCM behavior actuated by bidirectionally light-induced MMCT, where the production and elimination of the SCM property was driven by different laser irradiations at a specific temperature.28,29 Such controllable magnetic states with the switching of 0→1→0 sequences may contribute to the currently used molecular model device-systems. So far, only two Fe–Co compounds are reported to show the bidirectionally light-induced regulation of SCM behavior through MMCT.28,29 Although it is still a formidable challenge to design photoswitchable SCMs, incorporating the charge transfer Fe-CN-Co units into the magnetically isolated chains is considered to be an efficient strategy for constructing light switchable SCMs, wherein the diamagnetic and isotropic FeIILS(μ-CN)CoIIILS (LS, low spin) linkages could be transformed to the paramagnetic and anisotropic FeIIILS(μ-CN)CoIIHS (HS, high spin) analogues, and photodriven SCMs could be achieved under the specific light irradiation. Moreover, the HS CoII ion can possess a large negative anisotropy constant D value in a suitable coordination environment, and the cyanide ligands are prone to induce effective magnetic couplings when coordinating with metal ions. As a result, these metallocyanate species could generate strong intrachain magnetic interactions and be helpful for constructing photoswitchable SCMs when combining these Fe-CN-Co units into the chain structures.30–33 Moreover, interchain interaction is another important factor that may affect the SCM performance.34–37 Interchain interaction can be tuned through the modulation of ligands by cutting the interactions between different chains through changing the coordinated ligands from bridging to separating modes. Herein, we synthesized a ligand-bridged layer compound {[pzTpFe(CN)3]4Co2(Bib)4}·3H2O ( 1; pzTp, tetra-kis(1-pyrazolyl)borate; Bib, 1,4-bis-(1H-imidazol-1-yl)benzene) and an isolated double zigzag chain compound {[pzTpFe(CN)3]2Co(Bpi)2}·CH3CN·4H2O ( 2; Bpi, 1-biphenyl-4-yl-1H-imidazole) under the reaction of [pzTpFe(CN)3]− building blocks, CoII ions, and corresponding auxiliary ligands with different lengths and coordination modes (Scheme 1). The cyanide bridges link the metal ions in a one-dimensional (1D) alignment and serve as an ideal channel for magnetic interactions. The auxiliary ligands in the chains are adopted to adjust the ligand field on the CoII sites and tune the intra- and interchain magnetic interactions. It is expected that the longer and monodentate Bpi ligand could effectively cut off the chain connections and thereby weaken the interchain interactions and enhance the SCM property. Photomagnetic study revealed that both compounds exhibit SCM behaviors with hysteresis loops and slow magnetic relaxations after 808 nm light irradiation. Attributed to the cancellation of interchain interactions, 2 exhibits impressive SCM property, accompanied by a correlation length increase to 83 (30 for 1), the highest value among the reported light-induced SCMs. Furthermore, the interconversion between the paramagnetic FeIILS(μ-CN)CoIIILS(μ-CN)FeIIILS units and ferromagnetic FeIIILS(μ-CN)CoIIHS(μ-CN)FeIIILS linkages were reversibly manipulated in both compounds under cryogenic conditions by applying the alternating illuminations with 808 and 532 nm lasers, accompanied by the successful on/off switching of SCM behavior. Scheme 1 | The pzTp building block, Bib, and Bpi ligands. Download figure Download PowerPoint Experimental Methods Materials and syntheses All reagents were obtained from commercial suppliers and used without further purification. The ligands Bu4N[pzTpFe(CN)3], Bib, and Bpi ligands were synthesized according to the literature methods.38,39 Synthesis of 1 A mixture of Bu4N[pzTpFe(CN)3] (0.10 mmol, 65.5 mg), Bib (0.20 mmol, 42.0 mg), and Co(NO3)2·6H2O (0.10 mmol, 29.1 mg) was dissolved in a solution containing 10.0 mL methanol and 10.0 mL water. The resulting suspension was stirred for 0.5 h and then slowly filtered to remove the precipitate. Blackish green crystals were obtained by slow evaporation of the filtrate in air for about 2 weeks. Elemental analysis calculated for C84H74B2Fe4Co2N52O3: C, 45.39; H, 3.36; N, 32.77. Found: C, 45.47; H, 3.51; N, 32.68. Synthesis of 2 Blackish green crystals of 2 were synthesized using the same method as 1 by replacing Bib with Bpi ligand (0.20 mmol, 44.0 mg). Elemental analysis calculated for C62H59B2Fe2CoN27O4: C, 51.76; H, 4.13; N, 26.29. Found: C, 51.57; H, 4.28; N, 26.21. Physical measurements Elemental, thermogravimetric, and optical analyses The polycrystalline samples of 1 and 2 for all measurements were freshly prepared by removing the crystals from the mother liquid and soaking up the solvent from crystal surfaces. Elemental analyses were performed on a PerkinElmer 240C analyzer (PerkinElmer Scientific, United States). Thermogravimetric analysis was performed under a N2 atmosphere at 10 K min−1 using a TG/DTA STD-Q600 system (TA Instruments, United States). UV–vis–near-infrared (NIR) absorption spectra were recorded on a Hitachi U-4100 UV–vis–NIR spectrophotometer (HITACHI Company, Japan) at room temperature. Infrared (IR) spectra were collected on a KBr plate using a Nicolet iS10 Fourier transform infrared (FT-IR) spectrometer (Thermo Nicolet, United States) with a liquid helium type cryostat (OptistatCF2). Mössbauer experiments were carried out by a WissEl MSS-10 Mössbauer spectrometer (Topologic Systems Company, Japan) with a 57Co/Rh source in the transmission mode at room temperature. The spectrometer was calibrated by a standard α-Fe foil. Magnetic and photomagnetic studies Magnetic measurements of the polycrystalline samples for both compounds were carried out on a Quantum Design SQUID (MPMS-XL-7) magnetometer (Quantum Design, United States). Both compounds were restrained in a frozen eicosane matrix with polycarbonate capsules to prevent the loss of lattice solvent molecules on pumping at room temperature. Photomagnetic measurements were performed by utilizing a sample holder equipped with an optical fiber. The photoirradiation experiments were measured at 10 K with a laser diode pumped Nd:YAG laser (λ = 808 nm, 15 mW/cm2, and λ = 532 nm, 10 mW/cm2). The samples were ground into powders, spread on commercial transparent adhesive tape, placed on the optical fiber, and directly inserted into the sample chamber at 110 K. Structure determination and refinement The single-crystal X-ray diffraction (SCXRD) experiments were carried out on a Bruker D8 Venture CMOS-based single-crystal X-ray diffractometer (Bruker AXS Company, Germany) equipped with a mirror-monochromated Mo-Kα radiation (λ = 0.71073 Å). Both structures were solved by direct method and refined on F2 by the full-matrix least-squares methods using SHELXTL-14.40 All non-hydrogen atoms were anisotropically refined and all H atoms were localized in their calculated positions. The supplementary crystallographic data for this paper is available: CCDC-2145338 (1 at 296 K) and CCDC-2145339 (2 at 100 K). These data can be obtained free of charge from the Cambridge Crystallographic Data Center via www.ccdc.cam.ac.uk/data_request/cif Powder X-ray diffraction (PXRD) patterns of both compounds were obtained on a Rigaku 2400 diffractometer at 298 K with Cu-Kα radiation (λ = 1.5418 Å) with a step size of 0.03° and a scanning rate of 8° per min. Results and Discussion The SCXRD revealed that compounds 1 and 2 crystallized in the monoclinic P21/m and triclinic Pī space groups ( Supporting Information Table S1), respectively. Both crystal structures were comprised of double zigzag chains {[pzTpFe(CN)3]2Co(L)2} (L = Bib for 1 and Bpi for 2) with lattice solvent molecules occupying solvent-accessible voids (Figures 1a and 1b and Supporting Information Figure S1). The solvent molecules and thermal stabilities for both compounds were confirmed by thermogravimetric analyses ( Supporting Information Figure S2). Within the repeating chain, each [pzTpFe(CN)3]− unit was cis-bridged with two CoII ions through two of its three cyanide groups, and the coordination sphere of CoII ion was occupied by four nitrogen atoms from four [pzTpFe(CN)3]− units. In 1, these {Fe2Co}-based chains were further linked by ditopic Bib ligands through the CoII centers, affording a layered structure. For 2, the rest sites of CoII centers were coordinated by monodentate Bpi ligands, forming a well-isolated chain structure. Furthermore, hydrogen bonding interactions were found between lattice water molecules and terminal cyanide nitrogen atoms ( Supporting Information Figure S3). Such interactions would affect the redox potential of iron ions and lead to different MMCT behavior for both compounds. Figure 1 | Crystal structures for both compounds. Side view of the 1D chain in 1 (a) and 2 (b). The dihedral angle between the planes crossing the triangular {Fe2Co} units for 1 (c) and for 2 (d), respectively, which were arranged in an interlaced pattern. Atomic scheme: Fe, black; Co, pink; C, gray; N, blue; B, orange. The hydrogen atoms and solvent molecules are omitted for clarity. Download figure Download PowerPoint The asymmetric unit of 1 consisted of two unique iron centers and one unique cobalt center. The Co1–N bond lengths were measured as 1.876(5)–1.947(5) Å, the Fe1–C and N bond lengths were 1.851(6)–1.905(11) and 1.998(5)–2.032(7) Å, respectively, while the Fe2–C and N bond lengths were 1.891(11)–1.921(7) and 1.951(8)–1.975(6) Å ( Supporting Information Table S2), respectively. According to the structural parameters and charge compensation, the Co centers were presented as LS CoIII states, while one Fe center was in the FeII LS and the other was in FeIII LS, which are consistent with other compounds containing {FeIILS(μ-CN)CoIIILS(μ-NC)FeIIILS} linkages.41 The dihedral angle was 147.71(8)° between the planes of the interlaced triangular {Fe2Co} units (Figure 1c). The nearest intrachain and interchain Co…Co distances were 6.883(3) and 13.312(6) Å, respectively. The metal centers in 2 had a similar coordination environment with 1, with two unique iron centers (site occupancy factor of 1) and two unique cobalt centers (site occupancy factor of 0.5). The Co1–N and Co2–N bond lengths were 1.875(5)–1.938(5) and 1.871(5)–1.932(5) Å, respectively; the Fe1–C and N bond lengths were 1.863(6)–1.902(7) and 1.988(4)–2.014(5) Å, respectively; and the Fe2–C and N bond lengths were 1.912(6)–1.916(6) and 1.950(5)–1.973(5) Å ( Supporting Information Table S3), respectively. Similiarly, compound 2 contained the same {FeIILS(μ-CN)CoIIILS(μ-NC)FeIIILS} linkages. The dihedral angle between the interlaced triangular {Fe2Co} planes was larger (139.71(6)°) because of the relatively larger steric hindrance of the Bpi ligands (Figure 1d), accompanied by a shortened distance of 6.716(1) Å and an elongated distance of 17.657(1) Å for the nearest intrachain and interchain Co…Co distances, respectively. Moreover, continuous shape measurement analyses of the metal centers for both compounds were calculated ( Supporting Information Table S4). Compared with 1, the relatively smaller shape values for all the metal centers suggested that 2 had more regular coordination spheres. According to the above results, such increased dihedral angle and decreased intrachain Co…Co distances may favor the enhanced intrachain ferromagnetic interactions, and the elongated interchain distance may diminish the interchain interactions. Both of the two aspects will benefit the occurrence of SCM properties. Before conducting the magnetic properties measurements, PXRD experiments were performed ( Supporting Information Figure S4) and revealed the phase purities of the bulk materials for both compounds. Temperature-dependent magnetic susceptibility (χT) measurements were then collected on the polycrystalline samples upon heating from 2 to 300 K under a direct current (DC) field of 1000 Oe. As shown in Figure 2a and Supporting Information Figure S5, the χT value for compound 1 was 1.15 cm3 mol−1 K at 300 K and remained this value in the full temperature region measured, corresponding to the expected value for two LS FeIII ions (S = 1/2) in two FeIIILS(μ-CN)CoIIILS(μ-NC)FeIILS linkages. For 2 in Figure 2b, a similar behavior occurred with nearly unchanged χT values of 0.57 cm3 mol−1 K between 300 and 2 K, indicating one LS FeIII ion (S = 1/2) in the FeIIILS(μ-CN)CoIIILS(μ-NC)FeIILS linkage. The isothermal magnetization curve at 1.8 K increased slowly to 2.12 and 1.12 Nβ for 1 and 2 at 50 kOe (Figure 2, inset), respectively, suggesting paramagnetic behavior of the isolated FeIIILS ion in the {FeIILS(μ-CN)CoIIILS(μ-NC)FeIIILS} unit. 57Fe Mössbauer spectra were collected to illustrate the spin states of the Fe centers for both compounds ( Supporting Information Figure S6). Two quadrupole doublets were observed at 297 K: one doublet of δ (isomer shift) equal to −0.08 and −0.06, and ΔEQ (quadrupole splitting) equal to 0.85 and 0.87 mm s−1 for 1 and 2, respectively, typical of the LS FeIII species. The other with δ equal to 0.16 and 0.15, and ΔEQ equal to 0.56 and 0.58 mm s−1 for 1 and 2, respectively, corresponding to the LS FeII species. The peak area ratio of FeIII/FeII was 0.50/0.50 for both compounds, confirming the FeIIILS(μ-CN)CoIIILS(μ-NC)FeIILS states. Moreover, the IR spectra at 10 K confirmed the paramagnetic states of both compounds. As shown in Figures 3a and 3b, the cyanide stretching bands (νCN) attributed to the free νCN mode of [FeII(Tp)(CN)3]2− (2069 and 2092 cm–1 for 1, 2070 and 2098 cm–1 for 2) and the bridging νCN modes of the FeIILS(μ-CN)CoIIILS (2131 cm–1 for 1, 2126 cm–1 for 2) and FeIIILS(μ-CN)CoIIILS (2204 and 2222 cm–1 for 1, 2203 cm–1 for 2) were observed at 10 K, demonstrating the FeIIILS(μ-CN)CoIIILS(μ-NC)FeIILS state. Figure 2 | Magnetic susceptibility measurements under a DC field of 1000 Oe. Plots of χT vs T for 1 (a) and 2 (b) before and after irradiation at 808 and 532 nm, respectively. χT, temperature-dependent magnetic susceptibility. Inset: Isothermal magnetization (Nβ) of 1 (a) and 2 (b) before and after light irradiation at 1.8 K. Download figure Download PowerPoint Before we further explored the photomagnetic behaviors, solid-state UV–vis–NIR absorption spectroscopy ( Supporting Information Figure S7) was carried out to help choose suitable light irradiation. Similar absorption bands were observed at 296 K for 1 and 2, respectively. The ones at 711 nm for 1 and 784 nm for 2 were ascribed to the FeII→CoIII intervalence charge transfer (IVCT) followed by intersystem crossing of the excited state to the metastable state; the others at 430 nm for 1 and 451 nm for 2 was attributed to ligand-to-metal charge transfer in the Fe and Co chromophores.42,43 According to the UV–vis spectroscopic results, we recorded photomagnetic susceptibility measurements at 10 K with the light irradiation of 808 and 532 nm, respectively. When the compounds were irradiated with an 808 nm laser, the χT values rapidly increased to the maximum value of 155.9 cm3 mol−1 K at 5.7 K for 1 and 41.9 cm3 mol−1 K at 4.9 K for 2, indicating a highly photosensitive MMCT from the FeIIILS(μ-CN)CoIIILS(μ-NC)FeIILS to the light-induced metastable FeIIILS(μ-CN)CoIIHS(μ-NC)FeIIILS phases (HS* state). Interestingly, the photoinduced χT values exhibited extremely large increases in the current photomagnetic systems, showing about 135 and 73 times enhancement for 1 and 2, respectively. The increases below ca. 5 K may be due to weak interchain antiferromagnetic interactions or zero-field splitting of metal centers. Above 5 K, the χT values sharply decreased upon heating because of strong intrachain ferromagnetic interactions between light-induced LS FeIII and HS CoII magnetic centers in the HS* state. At last, owing to fast thermal relaxation, both curves slowly returned to the thermodynamic FeIIILS(μ-CN)CoIIILS(μ-NC)FeIILS species at 89 K for 1 and 101 K for 2, respectively. The above results showed that MMCT behavior from the FeIIILS(μ-CN)CoIIILS(μ-NC)FeIILS linkage to the FeIIILS(μ-CN)CoIIHS(μ-NC)FeIIILS configuration could be activated by light irradiation and deactivated upon heat treatment. After irradiation, isothermal magnetization curves (Figure 2, inset) increased to 8.78 Nβ for 1 and 3.25 Nβ for 2 at 50 kOe, respectively, demonstrating that the spin topology at the low temperature phase changed from the isolated FeIIILS ions to the FeIIILS(μ-CN)CoIIHS(μ-NC)FeIIILS 1D ordered structure. Furthermore, hysteresis loops were observed at 1.8 K ( Supporting Information Figure S8), with a coercive field of 122 and 55 Oe for 1 and 2, respectively. Zero-field-cooling (ZFC) and field-cooling (FC) curves showed an irreversibility ( Supporting Information Figure S9) at 5.6 K for 1 and 4.9 K for 2 after 808 nm light irradiation. The maximum of the ZFC/FC curve in 1 was observed after irradiation because the antiferromagnetic order originated from nearer chains by the bridged Bib ligands. Figure 3 | IR spectra for compounds 1 (a) and 2 (b) before and after light irradiation at 10 K. Download figure Download PowerPoint Temperature- and frequency-dependent alternating current (AC) susceptibilities were measured to probe the dynamic magnetic behaviors after 808 nm light irradiation. As shown in Figures 4a and 4b, both in-phase (χ′) and out-of-phase (χ″) components exhibited strong frequency dependence at low temperatures, suggesting slow relaxation of the magnetization. The shifts of the peak temperature (Tp) in the χ″ component of both compounds were given by the parameter Φ = (ΔTp/Tp)/Δ(logν) = 0.13 and 0.23 for 1 and 2, respectively, which were in the expected range for an SCM (0.1 ≤ Φ ≤ 0.3) and eliminated the possibility of a spin-glass behavior. The Arrhenius law of τ = τo exp(ΔE/kBT) (τ = 1/2πν) was applied to fit the plot of lnτ versus 1/T using the χ″ component of temperature-dependent AC susceptibility data, where τ, τo, ν, and ΔE/kB represent relaxation time, pre-exponential factor, frequency, and relaxation energy barrier, respectively. The fitting results gave τo = 6.2 × 10−11 s and ΔE/kB = 56.8 K for 1 and τo = 2.5 × 10−11 s and ΔE/kB = 50.2 K for 2, ( Supporting Information Figure S10), in accordance with those reported values for typical SCMs.8–14 In addition, Cole–Cole diagrams for 1 at 3.0 K and for 2 at 3.5 K showed the semicircular characteristic, giving α-values of 0.65 and 0.36 by applying the generalized Debye model ( Supporting Information Figure S11). The SCM behavior in the HS* state after 808 nm irradiation was further demonstrated by AC data at 1 Hz in a zero DC field, following the correlation length that was proportional to the χ′T product in the chain system. As shown in Supporting Information Figure S12, the ln(χ′T) vs T−1 plot showed a clear linear region between 6 and 19 K for 1 and 4 and 15 K for 2, indicating the Ising-like 1D chain behavior.44,45 The linear region was then fitted by the equation χT = Ceff exp(ξ/kBT), giving ξ/kB = 21.9 K and Ceff = 5.64 cm3 mol−1 K for 1 and Δξ/kB = 17.0 K and Ceff = 1.75 cm3 mol−1 K for 2, where Ceff was the effective Curie constant and Δξ was the energy for creating a domain wall along the chain. It is noted that the ln(χ′T) value for 1 reached saturation due to the finite-size effects with [χ′T]max = 166.48 cm3 mol−1 K. As such, the correlation length n that described the number of FeIIILS(μ-CN)CoIIHS(μ-NC)FeIIILS units in the chain was estimated to be 30 according to the relation of ln(χ′T) = nCeff, which is comparable with the reported SCMs.2,46–48 Impressively, the correlation length of 2 is 2.2 times higher than that of 1, showing the record n value of 83 among all reported photo-induced SCMs. Therefore, all analyses together demonstrated that 808 nm light irradiation produced remarkable SCM behavior for both compounds. By tuning the ligand from bridging to isolating, the antiferromagnetic order phase finally disappeared and a real photoswitchable SCM with high performance was constructed. Figure 4 | Dynamic magnetic behaviors (χ′, in-phase; χ″, out-of-phase) after 808 nm light irradiation. Temperature dependence of the in-phase and out-of-phase components of AC magnetic susceptibility for 1 (a) and 2 (b) after 808 nm light irradiation in a zero DC field and 3.5 Oe AC field at different frequencies. Download figure Download PowerPoint Furthermore, the temperature-dependent magnetization decays of the light-induced HS* state ( Supporting Information Figures S13 and S14) were also monitored to check the thermal stability. The decay of magnetization was normalized as photoinduced fraction γ and fitted to a stretched exponential law ( Supporting Information Eq. (1)). The obtained relaxation time t was fitted to an Arrhenius relationship (t(T) = t0exp(ΔE/(kBT)). Two distinct dependencies of relaxation time t on temperature were observed for both compounds ( Supporting Information Figures S13b and S14b). In the high-temperature region (40–65 K), the relaxation time was strongly dependent on temperature, and the best fitting results gave ΔE/kB equal to 330.6 (±24.1) and 431.7 (±28.1) cm−1, and t0 equal to 350.7 and 99.5 s for 1 and 2, respectively. In the low temperature (10–30 K) region, the relaxation time showed less dependency on temperature, giving an energy barrier ΔE/kB equal to 27.0 (±2.6) and 9.5 (±1.1) cm−1, and t0 equal to 3.0 × 106 and 1.3 × 108 s for 1 and 2, respectively. The magnetization decay measurements suggested that the switching from the metastable FeIIILSCoIIHSFeIIILS state to the ground FeIILSCoIIILSFeIIILS state was dominated by a quantum tunneling mechanism in the low temperature region. When a 532 nm laser was applied, a photo-demagnetization effect was observed for both compounds (Figure 2). This phenomenon suggested that the 532 nm light irradiation induced the transition from the FeIIILS(μ-CN)CoIIHS(μ-NC)FeIIILS phase to the FeIIILS(μ-CN)CoIIILS(μ-NC)FeIILS analogue. The maximum χT value decreased from 155.9 to 39.8 cm3 mol–1 K for 1 and from 41.9 to 8.5 cm3 mol−1 K for 2 (Figure 2) after the 532 nm irradiation, with a decreased proportion of more than 75% for both compounds. The incomplete phase transition was mainly attributed to the partial overlap between green light and the FeII→CoIII IVCT band. Both χ′ and χ″ components exhibited poor frequency-dependence at low temperatures (Figures 5a and 5b and Supporting Information Figure S15). The Arrhenius law was also performed to give τo = 7.6 × 10−10 s and Δ/kB = 40.2 K for 1 and τo = 1.4 × 10−11 s and Δ/kB = 39.9 K for 2 ( Supporting Information Figure S16). The α-values in the semicircular Cole–Cole diagrams were fitted as 0.72 and 0.60, indicating wide distribution of relaxation times ( Supporting Information Figure S17). Noting that the coupling channel within the chain had been destroyed, the dynamic spin reversal along the chain was therefore attenuated to a large extent. Furthermore, no linear region was found in the ln(χ′T) versus T−1 plots for the two compounds ( Supporting Information Figure S18), demonstrating that the 532 nm laser broke the intrachain magnetic interactions and overshadowed the SCM behavior. The remnant frequency dependence was probably caused by the segments of the coupled FeIIILS(μ-CN)CoIIHS(μ-NC)FeIIILS phase. Therefore, the photodriven SCM behavior of both compounds can be switched on by the 808 nm laser and off by the 532 nm light. Furthermore, the successive changing of the magnetization was well reproduced by alternating irradiations between 808 and 532 nm light at 10 K (Figures 5c and 5d), thus elucidating the bidirectionally light-induced on/off SCM behavior for both compounds. Figure 5 | The photoreversibility of the dynamic magnetic behaviors for compounds 1 and 2. Temperature dependence of the in-phase (χ′) components of AC magnetic susceptibility for 1 (a) and 2 (b) after light irradiation at 808 and 532 nm in a zero DC and 3.5 Oe AC field at various AC frequencies. Plots of χT vs time under cycles of successive irradiation at 808 and 532 nm at 10 K for 1 (c) and 2 (d), respectively. χT, temperature-dependent magnetic susceptibility. Download figure Download PowerPoint The bidirectionally light-induced MMCT were also revealed by IR spectroscopy at 10 K, as the variations of the CN stretching bands could
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