Interchain interactions raised the photo-induced [LS] → [HS*] transition temperature to 78 K in a cyanide-bridged [Fe^III₂Co^II] chain

Abstract

Bistable mixed valence compounds have thermodynamically accessible phases at certain temperatures, and the electron transfer switches the electronic configurations by applying external stimuli like heat and light. Thermally induced phase transition temperatures range widely, while the photo-induced state conversions need irradiation at very low temperatures, such as below 30 K, and the photo-induced metastable state relaxes rapidly at low temperatures. We prepared new mixed-valence compounds of [Fe(bipy)(CN)₄]₂[CoL₂] (L = 4-[(1E)-2-phenyldiazenyl]pyridine for 1-papy and 4-(2-phenylethynyl)pyridine for 1-pepy) in which cyanide-bridged squared cores form corner-shared chains with substantial interchain π–π contacts. Mössbauser spectra revealed that 1-papy and 1-pepy are in the high-spin (HS) state [(Fe^III_LS)₂Co^II_HS] at 300 K and the low-spin (LS) state [Fe^II_LSFe^III_LSCo^III_LS] at 78 K, confirming the occurrence of the electron transfer coupled spin transition (ETCST). Magnetic susceptibility measurements suggested their T_c values of 231 and 260 K, respectively. Photoirradiation (808 nm) for 1-papy and 1-pepy at 10 K induced the state conversion from the [LS] to the [HS*] state, and the metastable [HS]* state relaxed to the thermodynamically stable [LS] states at temperatures (T_relax) of 130 and 90 K, respectively. Furthermore, the [LS] states in 1-papy and 1-pepy were fully converted to the [HS*] states by light irradiation at 78 and 50 K, respectively. The X-ray structural analyses showed characteristic coordination bond lengths for the metal ions in each electronic state before and after light irradiation, but shortened intrachain π_L⋯π_L contact distances, from 3.726(4) to 3.688(4) Å, were observed for 1-papy upon the state conversion from the [LS] to the [HS*] state, despite the swollen cell volumes from 2479 to 2566 ų, respectively. Photomagneto and structural studies suggest that the intermolecular interactions increase the light-induced state conversion and relaxation temperatures.

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10th anniversary statement

No one, except for the Editor-in-Chief, could have predicted the current status of ICF when the journal started in 2014. Since its inception, I have witnessed this journal's growth as an associate editor, reader, and author. In the early days, there were fewer submissions, and their quality wasn't as high as it is now. We recognize now that many papers are submitted worldwide, and ICF has become a well-known journal that delivers high-quality chemistry. How has this happened? I believe the enthusiasm of Chinese chemists has contributed to this journal's success, and their passion has spread among us. I hope some of the ICF papers will seed new chemistry in the future.

Hiroki Oshio, Dalian University of Technology

Introduction

Photo-responsive molecular systems have the intrinsic advantages of high spatial and temporal resolutions for future intelligent molecular devices. They have attracted intense research interests from the viewpoints of molecular energy conversion,¹ photocatalysts,^2,3 and switching materials.^4–10 Precisely controlled molecular structure and function are critical for developing on-demand molecular devices. Among the photo-responsive systems, LIESST (= light-induced excited spin state trapping) in spin crossover (SCO) complexes and electron transfer coupled spin transition (ETCST) in mixed-valence compounds have been extensively focused on because of their photo-switchable properties.^11–20 This is due to their ultrafast photoresponse on a femtosecond timescale with highly efficient state conversions and excellent recyclability in solids.^21–24 Moreover, the photoswitches, accompanied by changes in the spin state, oxidation numbers, charge distribution, and structure, potentially possess fascinating magnetic and charge ordering properties, second-harmonic generation (SHG), and magnetization-induced SHG by mechanical and optical stimuli.^25–30

A family of cyanide-bridged metal complexes derived from Prussian Blue Analogues (PBAs), formulated as M^III_B[M^II_A(CN)₆], has been extensively investigated since the discovery of the first photomagnet of K_0.2Co^III_1.4[Fe^II(CN)₆]·6.9H₂0.³¹ It exhibited reversible photo-induced conversions between diamagnetic ([Fe^II_LS(μ-CN)Co^III_LS]) and ferromagnetic ([Fe^III_LS(μ-CN)Co^II_HS]) phases, which were accompanied by electron transfers (ET) between hetero-valent metal ions. In this conversion, the thermally stable high-spin [HS] ([Fe^III_LSCo^II_HS]) state turns into the low-spin [LS] ([Fe^II_LSCo^III_LS]) state with decreasing temperatures (T_c). The light-irradiation to the [LS] state converts it into the photo-induced metastable [HS*] state, usually at low temperatures (T_irrad), which is followed by thermal relaxation to the [LS] state by raising the temperature (T_relax). Among the three-dimensional PBAs, Co₃[Os(CN)₆]₂·6H₂O and Co₃[W(CN)₈]₂(pyrimidine)₄·6H₂O showed relatively high thermal stability of the [HS*] states up to T_relax = 120 and 150 K, respectively.^32,33 In contrast, discrete and low-dimensional compounds showed thermal transitions at wide temperature ranges of T_c = 220–300K, but the [HS*] states relaxed thermally to the [LS] states below T_relax = 100 K except for {[(pzTp)Fe^III(CN)₃]₄[Co^II(pz)₃-CCH₂OH]₄[ClO₄]₄} (PzTp: pyrazole derivative) at 200 K.³⁴ It should also be noted that the photoconversion of the [LS] to the [HS*] state needs irradiation at quite low temperatures (T_irrad < 30 K). This prompts us to establish rational strategies to prepare the systems with higher photoconversion ([LS] → [HS*]) and thermal relaxation ([HS*] → [LS]) temperatures.

The thermal and photo-induced ETs are accompanied by intra- and intermolecular structural changes, charge redistributions, and charge reorganization, affecting the temperatures of thermal and photo-induced state conversions.^35–42 The energy barrier for ET in a mixed-valence compound relates to the difference in free energy, which is also affected by the reorganization energy.^43,44 The thermally stable [HS] and light-induced metastable [HS]* states relax to the thermodynamically stable state ([LS]) as the temperatures are lowered and raised, respectively. It is expected that the larger ΔG* value raises the thermal relaxation temperature (T_relax) from the [HS]/[HS]* to the [LS] states and allows a higher photoconversion temperature (T_irrad) from the [LS] to the [HS*] state (Scheme 1). There are two ways to increase the ΔG* value: (1) a greater nuclear coordinate difference between the two configurations and (2) stabilized [HS] and [HS*] configurations. Each metal ion has typical coordination bond lengths for its oxidation and spin states, which do not allow variation of the characteristic nuclear coordinate in each configuration, excluding strategy (1). Therefore, we chose the second one. We introduced intermolecular π⋯π interactions in an ETCST system to improve photo-induced transition temperatures (T_irrad) and the thermal stability (T_relax) of the photo-induced [HS*] state, in which component metal complexes were pre-designed to have a suitable redox potential for electron transfer.⁴⁵

Scheme 1 Proposed potential energy curves with the light-induced state conversion process for a class II mixed-valence system with [Fe^II_LSCo^III_LS] ([LS]) and [Fe^III_LSCo^II_HS] ([HS]) configurations.

Results and discussion

The reactions of Li[Fe^III(bipy)(CN)₄] (bipy = 2,2′-bipyridine) with Co(ClO₄)₂·6H₂O and ligands L (papy = 4-[(1E)-2-phenyldiazenyl]-pyridine) or (pepy = 4-(2-phenylethynyl)pyridine) gave cyanide-bridged corner-shared chains of [Fe(bipy)^III(CN)₄]₂[Co^II(papy)₂]·1.5H₂O (1-papy) and [Fe^III(bipy)(CN)₄]₂[Co^II(pepy)₂]·0.5H₂O·CH₃OH (1-pepy).

1-papy and 1-pepy crystallized in the triclinic space group of P1̄. The crystallographic and detailed structural data are listed in Tables S1–S5.† In both complexes, the asymmetric unit contains two [Fe^III(bipy)(CN)₄]⁻ complex anions (Fe1 and Fe2) and two [Co^IIL₂]²⁺ moieties (Co1 and Co2), where the Co1 and Co2 ions locate on the inversion center (Fig. 1a and b). The Fe^III and Co^II ions adopt six coordination environments, and the two Fe^III and two Co^II ions are alternately bridged by cyanide groups and form a slightly twisted square core. The square cores are further linked to form a corner-shared chain at the Co^II ions (Fig. 1c and d). Six coordination sites of each Fe^III ion are occupied by two nitrogen atoms from the bidentate ligand bipy and four cyanide carbon atoms. The Co^II ions adopt elongated octahedral coordination geometries, and the equatorial sites are coordinated by four nitrogen atoms from the bridging cyanide groups. The axial positions are occupied by two nitrogen atoms of the pyridine derivatives of L. In the crystals, the phenyl and pyridine rings are nearly parallel in the Co1 site, while the dihedral angles of those rings in the Co2 site are 32.8° along the azo- and ethynyl-axes for 1-papy and 31.6° for 1-pepy. 1-papy and 1-pepy showed close π⋯π contacts between phenyl and pyridyl groups of ligands L coordinated to the Co ions (π_L⋯π_L) and between the bipyridine ligands coordinated to the Fe ions (π_bipy⋯π_bipy) in the adjacent chains (Fig. S1 and S2†).

Fig. 1 Crystallographic asymmetric units of (a) 1-papy and (b) 1-pepy. Hydrogen atoms and solvent molecules were omitted for clarity. Dotted lines present interchain π_L–π_L interactions between ligands L in (c) 1-papy and (d) 1-pepy.

⁵⁷Fe Mössbauer spectra of 1-papy and 1-pepy were measured to characterize the electronic states of the complexes at 300 and 78 K (Fig. S3†), where the Mössbauer parameters of the isomer shift (δ) and quadrupole splitting (ΔE_Q) were calculated relative to stainless steel. The Mössbauer spectra of 1-papy and 1-pepy at 300 K are composed of one quadrupole doublet with the parameters δ = −0.10 mm s⁻¹ and ΔE_Q = 1.54 mm s⁻¹ and δ = −0.12 mm s⁻¹ and ΔE_Q = 1.53 mm s⁻¹, respectively, assignable to an Fe^III_LS ion. Each Mössbauer spectrum of the complexes at 78 K showed two quadrupole doublets with the parameters δ = −0.037 mm s⁻¹ and ΔE_Q = 1.62 mm s⁻¹ and δ = 0.070 mm s⁻¹ and ΔE_Q = 0.81 mm s⁻¹ for 1-papy and δ = 0.032 mm s⁻¹ and ΔE_Q = 0.82 mm s⁻¹ and δ = −0.073 mm s⁻¹ and ΔE_Q = 1.63 mm s⁻¹ for 1-pepy, characteristic of Fe^III_LS and Fe^II_LS ions, respectively. The peak area ratios of Fe^II_LS/Fe^III_LS were, respectively, 0.47/0.53 and 0.48/0.52, confirming the complete conversions from the [HS] ([(Fe^III_LS)₂Co^II_HS]) to [LS] ([Fe^II_LSFe^III_LSCo^III_LS]) states by thermally induced ETCST. UV-vis-NIR spectra of the solid samples of 1-papy and 1-pepy were measured in the temperature range of 300–78 K (Fig. S4†). As the temperature was lowered from 300 K, both compounds exhibited new broad absorption bands with the peak maxima at ca. 900 nm, which can be assigned to the intervalence charge transfer (IVCT) band from the Fe^II_LS to Co^III_LS ions,^37,38 confirming the occurrence of thermal ETCST.

The temperature-dependences of magnetic susceptibilities for 1-papy and 1-pepy were measured from 2 to 300 K in the heating (↑) and cooling (↓) modes (Fig. 2). The results suggest reversible ETCST between the [HS] and [LS] states. The transition temperatures (T_c) are 231 K for 1-papy and 260 K for 1-pepy. Note that 1-papy showed a small thermal hysteresis with transition temperatures of 234 K and 228 K in the heating and cooling mode, respectively, but 1-pepy did not show the hysteresis. The χ_mT values at 300 K are 4.34 and 4.36 cm³ mol⁻¹ K for 1-papy and 1-pepy, respectively, which are close to the value expected for the magnetically isolated two Fe^III_LS (S = 1/2) and one Co^II_HS (S = 3/2) ions in the [HS] state, with significant orbital contributions. As the temperatures were lowered, the χ_mT values gradually decreased to reach plateau values of 0.93 cm³ mol⁻¹ K for 1-papy and 0.90 cm³ mol⁻¹ K for 1-pepy at ca.120 K. These χ_mT values are slightly higher than the values for one paramagnetic Fe^III_LS (S = 1/2) ion in the [LS] state, which might be due to paramagnetic impurities. As the temperatures were further decreased, gradual increases in the χ_mT values were observed below 30 K, and the results indicate the occurrence of intrachain ferromagnetic interactions, of which the exchange coupling constant is stronger for 1-papy.

Fig. 2 Temperature dependences of χ_mT values for 1-papy (●) and 1-pepy (○) under a magnetic field of 1000 Oe from 2 to 300 K in the heating (↑) and cooling (↓) modes.

Light irradiation experiments were conducted for 1-papy and 1-pepy in the [LS] state using a laser light (808 nm), which corresponds to the MMCT band, at 10 K (Fig. 3). The light irradiation resulted in sudden increases of the χ_mT values, confirming the conversions from the thermally stable [LS] to the light-induced metastable [HS*], [(Fe^III_LS)₂Co^II_HS], states. It should be noted that single crystal X-ray analyses for 1-papy and 1-pepy before and after photoirradiation supported the substantial conversion from the [LS] to the [HS*] state, respectively, at 78 and 50 K (see below). Temperature dependences of the magnetic susceptibility data for the [HS*] state were measured from 2 to 150 K for both compounds. The χ_mT values exhibited sharp increases from 2 K to 20 K, reaching the maximum values of 40.26 cm³ mol⁻¹ K at 4.8 K for 1-papy and 10.46 cm³ mol⁻¹ K at 4.0 K for 1-pepy. The smaller maximum value for 1-pepy can be understood by the weaker intrachain ferromagnetic interactions than that for 1-papy. Upon further increasing the temperature, the χ_mT values gradually decreased and merged with the ones for the unirradiated sample at T_relax = 130 and 90 K, respectively, for 1-papy and 1-pepy. Due to the remarkable thermal stabilities of the photo-induced [HS*] states for the complexes, we explored the photomagnetic performances at 78 and 50 K, respectively (Fig. 3, inset). The laser light irradiation (808 nm) of 1-papy at 78 K showed a sudden increase of the χ_mT values, reaching the saturation value (2.60 cm³ mol⁻¹ K) within ca. 3 min, which is close to the theoretical one (2.625 cm³ mol⁻¹ K, g = 2.0) for the magnetically isolated two Fe^III_LS ions and one Co^II_HS ion. Note that the irradiation to 1-papy at 80 K took ten min. to reach the saturation value. In contrast, 1-pepy showed a subtle increase of the χ_mT values upon photoirradiation at 78 K. However, the X-ray structure analyses (see below) before and after the photoirradiation suggested photoconversion from the [LS] to the [HS*] state at 50 K for 1-pepy. The result demonstrates the occurrence of photomagnetic conversion from the [LS] to the [HS*] state even at 78 K for 1-papy, and the origin of the thermal stability of the [HS*] states will be discussed below.

Fig. 3 Temperature dependence of χ_mT values for 1-papy (●) and 1-pepy (○) after photoirradiation (808 nm) at 10 K and time courses under irradiation at 78 K (inset).

Considering a large magnetic anisotropy of Co^II_HS ions and the intrachain ferromagnetic interactions in the [HS*] state, the chains in the [HS*] state would show single-chain magnetic properties. AC magnetic susceptibility measurements for the [HS*] state of 1-papy showed frequency-dependent in-phase (χ′) and out-of-phase (χ′′) signals of which the peak maxima shifted to higher temperatures as the frequency was increased (Fig. S5a†). The Mydosh parameter, φ = (ΔT_p/T_p)/Δ(logf) (T_p: peak temperature of χ′′ and f: ac frequency), was obtained to be 0.11, suggesting that the [HS*] state is a single-chain magnet and excluding possible spin-glass or long-range magnetic ordering. The relaxation times (τ) were extracted based on the Arrhenius equation, τ = τ₀ exp(Δ/k_BT) (τ = 1/2πf), and the fitted pre-exponential factor (τ₀) and the relaxation energy barrier (Δ/k_B) for 1-papy were 9.86 × 10⁻¹¹ s and 40.08 K, respectively, which are in the typical range of single-chain magnets (Fig. S5b†). The [HS*] state for 1-pepy also showed the slow relaxation of magnetization with frequency-dependent AC signals (Fig. S6†). The φ value (0.16) fell in the range for single-chain magnets. The Arrhenius plot for the out-of-phase AC signals gave the parameters τ₀ = 5.16 × 10⁻¹⁰ s and Δ/k_B = 27.61 K for 1-pepy. The correlation lengths (n)^46,47 and the number of [(Fe^III_LS)₂Co^II_HS] units in the photo-induced SCMs were estimated as 35 and 23 for 1-papy and 1-pepy, respectively (Fig. S7†).

1-papy and 1-pepy have identical {Fe₂Co} skeletons. However, they exhibited different photomagnetic behaviors: the irradiation temperatures for the complete conversion of ETCST at T_irrad = 78 and 50 K, and the thermal relaxation temperatures of T_relax = 130 and 90 K, respectively (Fig. 3 and Fig. S8†). It is, therefore, expected that the interchain interactions would be responsible for their photomagnetic behavior, and we performed single crystal X-ray analyses for 1-papy and 1-pepy before and after photoirradiation at 78 and 50 K, respectively (Tables S6–S10†). The results revealed that upon ETCST no distinct coordination bond length changes on the iron sites were observed, with the average bond lengths of Fe^II_LS–C_CN, Fe^II_LS–N, Fe^III_LS–C_CN, and Fe^III_LS–N being 1.922, 1.950, 1.934, and 1.956 Å for 1-papy and 1.914, 1.928, 1.936, and 1.928 Å for 1-pepy, respectively. In contrast, the bond distances around cobalt ions showed substantial changes of Co^III_LS–N_CN, Co^III_LS–N, Co^II_HS–N_CN, and Co^II_HS–N to 1.902, 1.978, 2.093, and 2.151 Å for 1-papy and to 1.890, 1.965, 2.096, and 2.151 Å)for 1-pepy, respectively. Distortion of the octahedron about metal ions can be determined using the parameters and , where α_i is the cis N–Co–N angle and θ_j is the unique N–Co–N angle measured on the projection of two triangular faces of the octahedron along their common pseudo-threefold axis.^48,49 Σ is a measure of the deviation from an ideal octahedral coordination geometry and Θ represents the extent of the distortion from the perfect octahedron (O_h, Θ = 60°) toward a trigonal prismatic structure (D_3h, Θ = 0°). Upon photoirradiation, the Θ values for the Co1 and Co2 ions changed, respectively, from 20.67 and 25.19 to 20.85 and 30.36 for 1-papy and from 20.85 and 33.56 to 21.71 and 49.70 for 1-pepy (Table S11†). Given the substantial changes in the coordination bond lengths and the octahedral distortions (ΔΣ_Co2 = 6.96–12.76° and ΔΘ_Co2 = 5.17–15.51°) on the Co sites before and after the photoirradiation, such significant changes in the vicinity of the Co ions would be propagated to the whole crystals through the interchain π⋯π contacts.⁵⁰

It is worth mentioning that some interchain contact distances show substantial shortening upon photo-induced ETCST, even though the cell volume was swollen upon conversion from the [LS] to the [HS*] state (Table S11†). For 1-papy, the interchain π_L⋯π_L and π_bipy⋯π_bipy contact distances were, respectively, shortened from 3.726(4) to 3.688(4) Å (Δ_dis = −0.038 Å) and from 3.838(4) to 3.771(4) Å (Δ_dis = −0.067 Å) upon the conversion from the [LS] to the [HS*] state. At the same time, the cell volume expanded from 2479.5(5) to 2566.4(4) ų (Δ_vol = +86.9 ų). The corresponding contact distances for 1-pepy did not change much: the corresponding distances are π_L⋯π_L = 3.776(1) and 3.766(1) Å (Δ_dis = −0.01 Å) and π_bipy⋯π_bipy = 3.887(1) and 3.827(1) Å (Δ_dis = −0.06 Å), and the cell volumes are 2545 and 2655 ų (Δ_vol = +110 ų) upon the state conversion. The results suggest that the shortened interchain π⋯π contact distances upon light irradiation stabilize the [HS*] state, leading to the substantially increased ΔG*_HS value, stabilizing the [HS*] configuration, and higher T_irrad and T_relax temperatures for ETCST. On the other hand, it was proposed in SCO systems that the cooperativity contributes to the LIESST effect, such as stabilizing the photo-induced [HS*] state and raising the thermal relaxation temperature.^51–56 In the ET and ETCST processes, particularly in non zero dimensional PBAs, the electron transfers cause redistributions of charges on the whole solids, in which the intermolecular, interchain, and interlayer interactions could play crucial roles in stabilizing the photo-induced [HS*] states towards high temperatures.

Conclusion

We prepared cyanide-bridged [Fe₂Co]-based chain compounds of 1-papy and 1-pepy. The complexes showed thermal and photo-induced ETCST behavior, and the photo-induced [HS*], [(Fe^III_LS)₂Co^II_HS], states were confirmed to be single-chain magnets. Photomagnetic, structural, and spectroscopic studies suggested that interchain interactions are crucial to the thermal stabilization of the photo-induced [HS*] state. Among the cyanide bridged molecular systems showing the intermetallic electron transfer, 1-papy exhibited very high photo-induced and relaxation temperatures at T_irrad = 78 and T_relax = 130 K, respectively, which is due to the unique interchain π⋯π stacks. The present study proves the importance of intermolecular interactions to stabilize photo-induced ETCST performances. In the ET and ETCST processes, the electron transfers cause redistributions of charges on the whole solids, in which the intermolecular, interchain, and interlayer interactions could play crucial roles in stabilizing the photo-induced [HS*] states towards high temperatures. Introductions of intermolecular π⋯π interaction networks and maybe hydrogen bonds into crystalline solids provide a controllable approach for increasing the thermal stabilization of the photo-induced [HS*] state. The present results would help rational designs of molecular architectures with photo-switchable and multifunctional materials for practical applications.

Author contributions

W. J. Jiang: conceptualization, data curation, investigation, formal analysis, and writing – original draft; Y. S. Meng and H. Oshio: formal analysis and writing – review and editing; Q. Liu: investigation (⁵⁷Fe Mössbauer spectra); H. H. Lu and H. L. Zhu: validation (reproducibility of experiments); C. Y. Duan: project administration; T. Liu: conceptualization, writing – review and editing, and funding acquisition.

Data availability

The data supporting this article have been included as part of the ESI.† Crystallographic data for 1-papy and 1-pepy have been deposited at the CCDC for 1-papy under 2247067 at 110 K, 2247068 at 290 K, 2247069 at 78 K, and 2247070 after irradiation at 78 K; and for 1-pepy under 2247071 at 110 K, 2247073 at 300 K, 2247074 at 50 K, and 2247075 after irradiation at 50 K.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grants, 22025101, 22222103, 22173015, 21871039, 91961114, 22071017, 22103009 and 22203013), and the Fundamental Research Funds for the Central Universities, China (DUT22LAB606).

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2247067–2247071 and 2247073–2247075. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4qi02428a

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