Introduction

For more than half a century, mechanically interlocked molecules (MIMs) have aroused great research interest owing to their esthetic appeal and dynamic physical properties1,2,3,4,5,6. Introducing bistability into these MIM systems has proved to be of particular interest, which endows these systems with the capacity to switch between two distinct stable states when subjected to specific external stimuli, paving the way for various sophisticated molecular switches. Notable examples include rotaxanes and catenanes, which can exhibit switching behavior in response to changes in light, pH value, or redox states and are expected to be applied to molecular electronic devices and drug delivery7,8,9,10,11. In recent years, a number of discrete interlocking molecular catenanes with increasingly intricate structures have been synthesized. In contrast, the design and synthesis of the [∞] catenated metal-organic frameworks (catena-MOFs) is still in its infancy12,13,14,15,16,17,18. The concept of robust dynamics envisages that MOF materials can significantly improve orderliness and performances while preventing degradation of their components during repeated switching processes6,8, enabling the internal dynamics of MIMs to transfer from solution to solid state, thus achieving macroscopic applications. One of the most critical issues for bistable catena-MOFs is how to promote switchable motion between two well-defined states, which can respond in situ to diverse stimuli19,20.

To achieve these goals, it is essential to control the coupling between individual switchable molecules and the environment, as well as to integrate these bistable molecules into ordered components. Herein, we focus on molecular spin crossover (SCO) complexes of 3d4−3d7 octahedral transition metal ions, whose spin states can be reversibly switched between high spin (HS) and low spin (LS) states, thus allowing the observation of the characteristic bistability in magnetism21,22,23,24,25,26. During spin transition, structural changes at the molecular level lead to crystal deformation, which can be used to promote a mechanical effect27. Given these structural changes, SCO complexes can serve as effective switching units. Any external stimuli capable of manipulating spin states, such as temperature, pressure, light irradiation, magnetic fields, and guest molecules, can be employed to operate the desired switching devices. This versatility leads to a broader range of switching techniques and widens the scope of potential applications28. Therefore, SCO molecules can be ideal candidates for developing micro- and macroscopic molecular switches for various physical properties, such as magnetism29, conductivity30,31, luminescence32,33,34,35,36,37, dielectric properties38,39,40,41, and mechanical effects42,43. Particularly, the spin-dependent synergistic switching of photoluminescence (PL) and dielectric properties, combined with the stiffness brought about by mechanical interlocking, makes SCO-based MIMs44 promising for robust optoelectronic devices. This provides the opportunity for a bi-channel (i.e. optical and electrical) read-out of the magnetic states of such systems. Moreover, the influence of the dielectric background on photoluminescence, through adjusting the energy levels of excited states with different dipole moments and also the optical properties such as refractive index, presents a unique playground for understanding the interplay between these physical properties, providing different perspectives for the development of molecular switches and multichannel devices.

The rational design of organic bis-pyridinyl linkers is the key to the construction of catena-MOFs. Flexible organic ligands facilitate the coexistence of coordination networks and p-stacked. In our work, the rigid naphthalene diimide (NDI) group is opted as the p-conjugated moiety45, which contributes to favorable aromatic π-π stacking interactions and excellent luminescent properties. The sp3-hybridized carbon atoms within their flexible methylene group allow two pyridine arms to rotate freely, which in turn enables the supramolecular polymerization of the resulting catenanes via NDI-NDI dimerization46,47,48. We envisage that integrating the NDI-based ligand N, N’-bis(4-pyridylmethyl)−1,4,5,8-naphthalene diimide (BPND) with the classical bimetallic Hofmann-type FeII{MI(CN)2}2 (M = Ag or Au) building blocks will lead to multifunctional catena-SCO MOFs.

In line with this strategy, we synthesized two catena-MOFs [FeII(BPND){Ag(CN)2}2]·3CHCl3 (1Ag) and [FeII(BPND){Au(CN)2}2]·2CHCl3·2H2O (1Au), which exhibited two-dimensional (2D) → three-dimensional (3D) parallel polycatenation. Both complexes undergo thermal- and light-induced spin transition. The reversible SCO was confirmed by variable-temperature single-crystal X-ray diffraction, magnetic susceptibility measurements, and Mössbauer spectroscopy. As expected, strong coupling between SCO and luminescence was clearly observed in 1Ag and 1Au. It is noteworthy that 1Ag has two emission bands: the emission at around 465 nm is due to the monomer NDI groups, and the broad peak at around 625 nm at room temperature is attributed to intermolecular excimer from NDI groups. The intensities of both emissions are correlated with the spin state transition. Moreover, the dielectric constant of 1Ag and 1Au in the electronic configurations from LS to HS can be well observed in the temperature-dependent dielectric constant, which is coupled with the SCO synchronously, confirming the SCO-induced dielectric switching behavior.

Results

Self-assembly and crystal structures

The reaction between BPND, [M(CN)2]− (M = Ag and Au, respectively) and Fe2+ in a molar ratio of 1:2:1 gave rise to two catena-MOFs 1Ag and 1Au (Fig. 1, Supplementary Figs. 1–3). Single crystals of 1Ag and 1Au were obtained by slowly liquid-to-liquid diffusing the methanolic solution of Fe(ClO4)2·6H2O into the chloroform-methanol solution of BPND and [M(CN)2]−. The homologous diamagnetic complex [ZnII(BPND){Ag(CN)2}2]·3CHCl3 (2Ag) was similarly prepared (Supplementary Fig. 4). Variable-temperature single-crystal X-ray diffraction data were collected at 100, 170, and 250 K for 1Ag and 100 K and 220 K for 1Au to characterize the structural changes during spin transition. Supplementary Tables 1–5 summarize the crystal data, structure refinement parameters, and selected bond lengths for 1Ag, 1Au, and 2Ag at different temperatures.

Fig. 1: Synthetic strategy and crystal structure of 1Ag.
figure 1

a Self-Assembly of 1Ag by the combination of BPND ligand, [Ag(CN)2]− and FeII ions. b and c Top and side view of the 2D layered structure for 1Ag. d, e Supramolecular π-π stacking and C-Cl···O XB interactions in 1Ag at 100 K. The red dashed lines represent supramolecular π-π stacking and C-Cl···O XB interactions. Hydrogen atoms and solvent molecules are omitted for clarity.

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Single-crystal X-ray diffraction analysis revealed that 1Ag and 2Ag crystallize in the polar tetragonal space group (Z = 16), I41cd at all measured temperatures. While the Flack parameter for 1Ag is close to 0.5, the polar crystal structure can be clearly verified by the photo-pyroelectric measurements: when the single crystals undergo instantaneous heating with a 532 nm laser, a transient electric current signal can be detected as a result of the change in the macroscopic polarization. This phenomenon can be repeatedly observed in the temperature range of 100–300 K, which falls outside the temperature range associated with the photomagnetic effect (Supplementary Fig. 5). Crystal data show that all FeII ions are crystallographically equivalent. Each FeII unit contains three chloroform molecules embedded within the hole of the framework (Supplementary Figs. 6 and 7). The presence of chloroform molecules can also be confirmed by infrared (IR) spectrum and elemental mapping photographs (Supplementary Figs. 8–10). The FeII ions are bridged by four bidentate [Ag(CN)2]− linkers in the equatorial plane to form rhombic [Fe4{Ag(CN)2}4] grids with sizes of 10.550(1) Å × 10.379(1) Å at 100 K (10.486(1) Å × 10.666(1) Å at 170 K, and 10.696(1) Å × 10.525(1) Å at 250 K) (Fig. 1b). The axial sites of the [FeIIN6] octahedral are occupied by two nitrogen atoms of two BPND ligands. Two 4-pyridyl arms of the BPND ligand are in syn-conformation, connecting two FeII ions on the diagonal in the rhombic grids (Fe···Fe distances are 13.473(3) Å and 13.335(3) Å at 100 K, 13.575(3) Å and 13.468(3) Å at 170 K, 13.665(2) Å and 13.547(2) Å at 250 K) (Fig. 1c). The resulting 6-connected structural units lead to the formation of a 2D layer with hxl topology (Supplementary Figs. 11 and 12)49. It is noteworthy that due to the bent conformation of BPND ligand, the layer thickness allows for the formation of entanglement by parallel polycatenation. Therefore, the adjacent 2D layers are interlocked by the bent BPND ligands, and the polycatenation of 2D layers gives rise to an overall 3D framework. Moreover, according to reports in the TopCryst database of ToposPro, among 2D layers of hxl topology no polycatenated examples have been found16,49,50.

In each interlocked NDI unit, two NDI groups are parallel and arranged in an orthogonal array with the center-to-center and interplanar separations of 3.470(18) Å at 100 K (3.506(15) Å at 170 K and 3.504(13) Å at 250 K) (Fig. 1d), indicating the presence of strong π-π interactions. In addition, weak π-π stacking interactions exist between pyridine rings of ligands with the center-to-center distance of 3.580(3) Å and the dihedral angle between two pyridine rings is 1.1(6)° at 100 K (3.592(3) Å and 0.8(6)° at 170 K; 3.615(3) Å and 0.8(5)° at 250 K). Interestingly, the Cl···O distances between the solvent molecule chloroform and the NDI group are shorter than the sum of the van der Waals radii51, i.e., C31-Cl2···O2A (100 K: 2.974(10) Å, 154.9(6)°; 170 K: 2.995(9) Å, 155.9(6)°; 250 K: 3.024(9) Å, 154.6(7)°) and C32-Cl6···O4B (100 K: 3.041(11) Å, 150.1(12)°; 170 K: 3.201(15) Å, 143.6(16)°; 250 K: 3.165(13) Å, 146.8(13)°), which indicates that there are two groups of halogen bond (XB, where X = Cl, Br, or I and B = nucleophile) non-covalent interactions (Fig. 1e)52. The formation of these supramolecular interactions builds a firm structure showing good thermal stability. The Fe-N bond lengths at 250 K are in the range of 2.106(6)−2.242(7) Å with an average value of 2.179(7) Å, indicating that FeII ions are in the HS state. The average Fe-N bond lengths are 2.156(8) Å (170 K) and 2.082(10) Å (100 K), corresponding to an incomplete and gradual interconversion from the HS to LS states of the FeII ions and match well with the magnetic susceptibilities (vide post).

The main 3D structure of 1Au is basically isostructural to that of 1Ag, in which the bridging ligand [Ag(CN)2]− was replaced with [Au(CN)2]− for 1Au, with cell volume has been reduced by half (Supplementary Figs. 13–15). 1Au crystallizes in the orthorhombic space group Ccce at 100 K and 220 K. Each FeII unit contains two chloroform molecules and two water molecules embedded within the hole of the framework (Supplementary Figs. 16 and 17). The oxygen atoms of water molecules and the chlorine atoms of chloroform molecules are crystallographically disordered in the framework at 100 K. The presence of chloroform molecules was confirmed by IR spectrum and elemental mapping photographs (Supplementary Figs. 18–20). Since the sp3-hybridized carbon atoms of methylene allow two pyridine arms to rotate freely, the asymmetric unit in 1Ag contains two BPND ligands with different rotational angles (Supplementary Fig. 21), while 1Au has only one type of BPND ligand. Therefore, different stacking modes between 2D layers are observed, the ABCD stacking pattern of 1Ag and the ABAB stacking pattern of 1Au, respectively (Fig. 2, Supplementary Figs. 15 and 22). For 1Au, at 100 K, the π-π stacking interactions between the parallel pyridine rings have the interplanar distance of 3.298(18) Å and the center-to-center distance of 3.810(18) Å (3.397(10) Å and 3.778(10) Å, respectively, at 220 K), while 3.482(20) Å (3.505(13) Å at 220 K) between parallel NDI centers in an orthogonal array (Supplementary Fig. 23). Moreover, at 100 K, the C5–H5···Cl5C (2.916(18) Å, 118.7(10)°) hydrogen bonds and the C17-Cl3···O1D (3.070(14) Å, 150.8(12)°) halogen bonds between the chloroform molecules and the framework lead to the removal of chloroform at an abnormally high temperature. The average Fe-N bond lengths are 2.178(8) Å (220 K) and 2.081(11) Å (100 K), corresponding to an incomplete spin transition in 1Au.

Fig. 2: Packing modes and entanglement of hxl 2D layers for 1Ag and 1Au.
figure 2

a ABCD stacking pattern of 2D layers for 1Ag. Four colors (red, blue, gray and purple) are used to illustrate interlocking adjacent 2D layers. b ABAB stacking pattern of 2D layers for 1Au. c, d Illustration of parallel polycatenation in 1Ag and 1Au. Hydrogen atoms and solvent molecules are omitted for clarity.

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Magnetic properties and Mössbauer spectroscopy

The variable-temperature magnetic susceptibilities of 1Ag and 1Au on polycrystalline samples revealed that 1Ag and 1Au underwent gradual and incomplete spin transition without hysteresis (Fig. 3a, Supplementary Figs. 24 and 25). The χmT value of 1Ag (χm is the molar magnetic susceptibility) is 4.08 cm3 K mol−1 at 300 K, which is obviously larger than the spin-only value (3.0 cm3 K mol−1) of a HS FeII ion. The potential electron transfer between the redox-active HS FeII and BPND should form HS FeIII and reduced BPND ligand in 1Ag with larger χmT values. However, the variable-temperature infrared absorption spectra for 1Ag and its ZnII analog 2Ag show that the absorption features are similar and there are no bands for reduced BPND ligand, suggesting identical oxidation states for Fe and Zn ions in 1Ag and 2Ag (Supplementary Fig. 26). This result rules out the possibility of electron transfer in 1Ag. The magnetic susceptibility was then calculated by the well-established ab initio protocol using the relativistic Complete-Active-Space Self-Consistent Field/N-Electron-Valence Perturbation Theory (CASSCF/NEVPT2) methods based on the truncated model extracted from the single-crystal structure of 1Ag (250 K, HS, Supplementary Table 6). The results indicate that 1Ag exhibits unquenched orbital angular momentum with a χmT value of 3.95 cm3 K mol−1 at 300 K in this coordination geometry, consistent with the experimental values (Supplementary Fig. 27). Notably, the slight increase in the calculated χmT values upon cooling was also consistent with the experimental results (220–300 K). The χmT value remains almost constant upon cooling until 210 K and slowly decreases to 1.57 cm3 K mol−1 at 50 K. This value corresponds to ca. 38% of HS FeII centers, indicating that 62% of FeII centers changed from HS to LS state. A further decrease below 10 K is possibly due to zero-field splitting of the HS FeII ions. The diffuse reflectance spectra display the absorption bands centered at 555 nm for 1Ag (Supplementary Figs. 28 and 29). Therefore, irradiation to photo saturation was carried out using laser light of 532 nm (15 mW). When the sample was irradiated at 5 K for 4 h, the χmT value attained saturation at 2.45 cm3 K mol−1. After stopping irradiation, the χmT value increases up to the maximum value of 3.20 cm3 K mol−1 at 31 K. Complex 1Au exhibits thermal- and light-induced SCO behavior similar to that of 1Ag (Supplementary Fig. 30). At 300 and 50 K, the χmT values are 3.74 and 1.77 cm3 K mol−1, respectively, corresponding to about 53% of the FeII ions undergoing spin transition.

Fig. 3: Magnetic properties and 57Fe Mössbauer spectra of 1Ag.
figure 3

a Temperature-dependent χmT product and light-induced spin transition effect induced by laser. The measurements were performed with a sweeping rate of 2 K min−1. Temperature-dependent χmT product before (blue) and after irradiation (red) with 532 nm laser light in the heating mode. b 57Fe Mössbauer spectra at room temperature and 50 K, respectively.

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To further verify the spin state of FeII centers, the Mössbauer spectra (Fig. 3b) of 1Ag were measured at room temperature and 50 K, respectively. The corresponding hyperfine parameters are summarized in Supplementary Table 7. At room temperature, 1Ag has only one doublet with an isomer shift (δ) of 1.061 mm s−1 and a quadrupole splitting (ΔEQ) of 0.565 mm s−1, indicating that all FeII centers are in HS state (S = 2)53. At 50 K, one additional doublet corresponding to LS FeII sites appears, indicating that partial FeII centers (68.3%) have undergone the spin transition (LS FeII: δ = 0.499 mm s−1, ΔEQ = 0.457 mm s−1; HS FeII: δ = 1.147 mm s−1, ΔEQ = 0.999 mm s−1). It is worth noticing that the ΔEQ values for HS FeII are somewhat smaller than that previously reported (1.43–2.95 mm s−1)54, probably due to the less distorted local surrounding of HS FeII in 1Ag that causes a small deviation of the iron nucleus from the sphere. Unfortunately, it is difficult to observe signals for 1Au at room temperature and low temperature (50 K), which might be related to the presence of heavy AuI ions in the complex that absorb the incident γ-radiation, hindering the resonance absorption of FeII 55.

Dielectric properties

For HS octahedral FeII complexes, two electrons occupy anti-bonding 3d orbitals (eg), resulting in the increase of coordination bond lengths and structural distortion with the spin transition. Consequently, changes in local electrical dipoles caused by spin transition can be expected, leading to changes in dielectric properties during spin transition41. The variation of complex dielectric permittivity ε* (ε* = ε′ - iε″, where ε′ and ε″ are the real and imaginary parts of ε*) was measured between 15 to 300 K at different electric field frequencies for 1Ag and 1Au (Fig. 4, Supplementary Fig. 31). The frequency scans were carried out isothermally. The temperature-dependent dielectric constant ε′ at 105 Hz (in the heating mode) is compared with the magnetic susceptibility data (Fig. 4a). The dielectric constant is 2.90 at 50 K, and then gradually increases until 245 K reaches the maximum value of 3.46, which is consistent with the gradual SCO behavior of 1Ag. During the SCO process, frequency-dependent peaks in dielectric loss (tan δ, tan δ = ε″/ε′) and imaginary part of the dielectric constant (ε″) were also observed, characteristic of dielectric relaxation (Supplementary Fig. 31). Notably, this phenomenon has been observed in other reported SCO systems56,57. The corresponding temperature-dependent relaxation time could be well fitted by the Arrhenius law, τ = τ0exp(Ea/kBT) (where τ0 is defined as the pre-exponential factor, Ea is the activation energy, T is the temperature of the ε″ peak, and kB is the Boltzmann constant), giving the activation energy Ea of 18.0(6) kJ/mol and τ0 of 2.6 × 10−13 s (Supplementary Fig. 31c). This low activation energy, along with the broad temperature range of the peak shift, suggests that the SCO transition likely occurs in a non-correlated manner across a wide timescale. Such findings are in agreement with the gradual SCO behavior observed through magnetometry. These results directly demonstrate that the spin state-dependent dynamic local electrical dipoles change in 1Ag.

Fig. 4: Dielectric properties between 15 and 300 K of 1Ag.
figure 4

a Temperature-dependent dielectric constants (ε′) versus frequency plots (f = 102 Hz to 107 Hz) with a sweeping rate of 2 K min−1. b ε′ (f = 105 Hz) as comparison with the χmT products in the heating mode.

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Theoretically, the octahedral [FeN6] geometry of FeII ions is sensitive to its spin state, and the distorted octahedron stabilizes the HS state owing to the Jahn-Teller effect for its t2g4eg2 electronic configuration. To verify this, the variation has been quantified by calculating the octahedral distortion parameter Σ, which is defined by the sum of the deviations from 90° of the 12 cis-N-Fe-N angles58. The Σ value increased from 31.9° at 100 K to 36.3° at 250 K (Supplementary Table 8). The local symmetry distortion changes the local electric field significantly, and large changes in the dipole moment can be detected by the dielectric spectrum.

The dielectric constant ε′ and dielectric loss tan δ of 1Au showed a temperature-dependent variation trend similar to that of 1Ag (Supplementary Fig. 32 and Table 9). To elucidate the correlation between spin crossover and dielectric properties, the dielectric constants of the homologous diamagnetic ZnII complex 2Ag at variable temperatures were measured under the same conditions (Supplementary Fig. 33). The dielectric constant ε′ and dielectric loss tan δ of 2Ag remained almost unchanged from 15 to 300 K. Therefore, the spin transition of FeII ions should be responsible for the dielectric switching response.

Photoluminescence properties

In order to verify the correlation between SCO and photoluminescence, the temperature-dependent emission spectra for pure BPND ligand, 1Ag, 1Au and 2Ag were measured in the heating mode. The emission intensity of free BPND ligand decreases monotonically with increasing temperature due to the expected thermal quenching effect (Supplementary Fig. 34). The excitation wavelength of 355 nm was determined based on the excitation spectra of 1Ag and 1Au (Supplementary Figs. 35 and 36).

At 80 K, the emission spectrum of 1Ag shows a dual emission peak: one is due to the monomer species of NDI groups at around 465 nm (λ1), and the other is an unstructured broad peak at around 640 nm (λ2), which is assigned to the intermolecular excimer emission (Fig. 5a, Supplementary Figs. 37 and 38). As temperature increases, the intensity of monomer emission (λ1) increases slowly to 120 K, then increases rapidly, reaches the maximum at 210 K, and then decreases due to the thermal quenching effect. The emission intensity of the excimer (λ2) decreases rapidly from 80 K to 120 K, and then slowly to 170 K. Above 170 K, the emission intensity begins to increase and reaches the maximum value at 210 K (Fig. 5b). The abnormal and discontinuous change of emission intensity of 1Ag is obviously different from the monotonic decrease of the pure ligand (Supplementary Fig. 39). 1Ag exhibits gradual SCO in a relatively wide temperature range of 50–210 K. The abnormal temperature range of monomer emission falls within the range of spin transition temperature, certificating that the intensity of luminescence is mainly controlled by the spin state of FeII ions. Similarly, the inflection point of excimer emission intensity appears at 210 K, but its intensity change is not as obvious as that of the monomer. The lattice expansion during the spin transition process (ΔV/V ∼4%) may induce spatial distortion in the formation of NDI-NDI excited dimerization. Such mechanically induced interference leads to changes in the emission displacement and intensity of excimers59,60. In our previous studies, the luminescence-SCO coupling originated from the energy transfer between the luminescence donor and the spin center receptor61. Therefore, the regulation of spin state transition on excimer emission is reflected in the change of spectral overlap and interplanar NDI-NDI dimerization distance.

Fig. 5: Luminescent properties of 1Ag.
figure 5

a A 2D color map of the temperature-dependent emission spectra (λex = 355 nm). Dashed lines represent the characteristic emission peaks of monomer (λ1) and excimer (λ2). b Normalized maximum emission intensity (λ1 and λ2) as comparison with the χmT products in the heating mode.

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Unlike 1Ag, the emission spectrum of 1Au only contains an emission peak of around 460 nm, which is attributed to the monomer species of NDI (Supplementary Fig. 40). Considering that the NDI-NDI stacking in 1Au is very similar to that in 1Ag, it is unexpected that no excimer emission was observed in 1Au, which may be related to the existence of some non-radiative transition pathways. The monomer emission intensity of 1Au shows a temperature-dependent variation trend similar to that of 1Ag, while the inflection point appears at 220 K (Supplementary Figs. 41 and 42). These results clearly show that both complexes have an apparent coupling effect between luminescence and SCO properties. To further elucidate the regulation of the spin state transition of FeII ions on the emission properties, we measured the temperature-dependent emission spectra of the ZnII complex 2Ag. The emission intensity monotonically decreases with increasing temperature (Supplementary Figs. 43 and 44) owing to the thermal quenching effect.

The UV-vis diffuse reflectance spectra of 1Ag and 1Au at different temperatures (Supplementary Figs. 28 and 29) were recorded to better elucidate the coupling mechanism between SCO and luminescence. For 1Ag at 80 K, there are two well-separated absorption bands at 400 nm and 550 nm, respectively. As the temperature increases, the absorption intensity gradually decreases. Similar temperature-dependent absorption features can also be found for 1Au. Therefore, an energy transfer mechanism should be responsible for the SCO-luminescence coupling effect.

The time-dependent density functional theory (TD-DFT) calculations were performed to investigate the vertical excitation energies and the corresponding oscillation strengths for both the HS and the LS structures of 1Ag at B3LYP/def2-SVP level (Supplementary Figs. 45 and 46). Supplementary Table 10 summarizes selected excitation energies, corresponding oscillator strengths ( f) and the assignments of absorption peaks for LS 1Ag and HS 1Ag. As shown in Fig. 6, the two absorption bands of LS 1Ag centered at 466 nm and 548 nm overlap the emission band effectively. Therefore, luminescence is quenched through energy transfer. The two absorption bands of HS 1Ag are blue shifted to 420 nm and 518 nm respectively, far away from the emission band, and the oscillator strength is markedly lower than that of LS state. Consequently, it was observed that the emission intensity synchronously increases with the increase of the proportion of HS FeII ions.

Fig. 6: TD-DFT-calculated absorption spectra of 1Ag.
figure 6

Electronic absorption spectra predicted by TD-DFT for LS and HS 1Ag and the photoluminescence emission spectrum at 270 K. The full width of the half-maximum is set to be 0.3 eV.

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In conclusion, we have successfully synthesized and characterized two FeII SCO-based catenated MOFs. Both complexes exhibit thermal- and light-induced spin transition, verified by temperature-dependent Mössbauer spectra, structural analyses, and magnetic measurements. Moreover, temperature-dependent emission spectra and dielectric constant demonstrated that the dual coupling of emission and dielectric properties were regulated by the spin state, where the coupling of SCO-luminescence/dielectricity comes from the energy transfer caused by spectral overlap (LS state), or the change of local electrical dipoles caused by structural deformation, respectively. These results provide strong evidence for the practical application of SCO materials. The mechanically interlocked structure improves the stiffness of SCO materials and is conducive to enhancing the coupling effect between luminescence and SCO. Correspondingly, the introduction of SCO units brings magnetic, optical, and electrical properties to bistable MIM materials, which makes SCO-based MIMs promising for advanced sensing materials. Research to explore these possibilities is underway in our laboratory.

Methods

Synthesis

All chemicals and solvents were purchased from commercial sources and used without further purification.

N,N’-bis(4-pyridylmethyl)-1,4,5,8-naphthalene diimide (BPND)

BPND was synthesized by adapting literature methods62. 1,4,5,8-Naphthalenetetracarboxylic dianhydride (4.5 mmol) and 4-(aminomethyl)pyridine (11.3 mmol) were refluxed in anhydrous DMF (25 mL) under N2 atmosphere 12 h, an orange-brown suspension was obtained. The solid was isolated by vacuum filtration and washed with dichloromethane and acetone before vacuum drying. Yield: ∼85%. Anal. Calcd for C26H16N4O4: C, 69.64; H, 3.60; N, 12.49. Found: C, 69.56; H, 3.65; N, 12.54.

[FeII(BPND){Ag(CN)2}2]·3CHCl3 (1Ag)

A mixture of ligand (0.02 mmol) in chloroform (3 mL) and Na[Ag(CN)2] (0.04 mmol) in methanol (1 mL) was placed in the bottom of a test tube, and then a mixture of methanol and chloroform [1:1 (v/v); 3 mL] was added as a buffering solution. A methanol solution (0.5 mL) of Fe(ClO4)2·6H2O (0.02 mmol) was carefully added to the top of the test tube. The test tube was sealed and left undisturbed at room temperature. Brown crystals were obtained after 2 weeks in the second layer (yield: ∼50%). Anal. Calcd. for C33H19Ag2Cl9FeN8O4: C, 33.53; H, 1.62; N, 9.48. Found: C, 33.46; H, 1.69; N, 9.43.

[FeII(BPND){Au(CN)2}2]·2CHCl3·2H2O (1Au)

1Au was synthesized by following the method of 1Ag except that Na[Ag(CN)2] and Fe(ClO4)2·6H2O were replaced with K[Au(CN)2] and FeCl2·4H2O, respectively. (yield: ∼40%). Anal. Calcd. for C32H22Au2Cl6FeN8O6: C, 30.10; H, 1.74; N, 8.77. Found: C, 30.22; H, 1.70; N, 8.70.

[ZnII(BPND){Ag(CN)2}2]·3CHCl3 (2Ag)

2Ag was synthesized by following the method of 1Ag except that Fe(ClO4)2 was replaced with Zn(ClO4)2 (yield: ∼40%). Anal. Calcd. for C33H19Ag2Cl9ZnN8O4: C, 33.26; H, 1.61; N, 9.40. Found: C, 33.32; H, 1.55; N, 9.39.

Single-crystal X-ray diffraction

Single crystal X-ray diffraction data for 1Ag and 2Ag were collected using a Rigaku SuperNova, Dual, AtlasS2 diffractometer. Single-crystal X-ray diffraction data for 1Au on XtaLAB Synergy R diffractometer using Rigaku (Cu) X-ray Source (Rotating-anode X-ray tube). The structures were solved by direct method Olex2 1.3 and refined by full-matrix least-squares (SHELXL or Olex2 1.3) on F2. Hydrogen atoms were added geometrically and refined using a riding model. For 1Au (220 K), SQUEEZE was employed to calculate the diffraction from the solvent region and thereby produced a set of solvent-free diffraction intensities. A total of 1172 electrons per unit cell were masked, consistent with 16 chloroform molecules and 16 water molecules (two chloroform molecules and two water molecules per FeII unit).

Magnetic measurements

Variable-temperature magnetic susceptibility measurements were carried out on a Quantum Design MPMS XL7 magnetometer under a magnetic field of 5000 Oe at a temperature range of 2–300 K with a sweeping rate of 2 K min−1. All data were corrected for diamagnetic contributions63. Photomagnetic measurements were performed on the powdered sample attached to transparent tape. A green laser (532 nm) was adopted as the excitation source.

Fluorescence spectroscopy

Emission spectra of fluorescence were measured on an Edinburgh FLS 980 fluorescence spectrophotometer. The samples were placed in an electrically heatable continuous flow cryostat (Oxford Instruments). The waiting time for each temperature stabilization was 300 s.

Electrical property measurements

Temperature-dependent dielectric constants were measured using the two-probe a.c. impedance methods in the frequency range from 100 Hz to 10 MHz on a Wayne Kerr 6500B Precise Impendance Analyzer. The electric contacts were prepared by using silver paste (DAD-87) to attach 25 µm gold wires to the pressed powder samples. The sample was placed into a Janis cryogenic refrigeration system with a warming rate of 2 K min−1. Pyroelectric measurements were performed with Keithley 6517B electrometer and the Quantum Design MPMS-XL chamber as temperature controller. The single-crystal sample (0.2 × 0.18 × 0.05 mm3) was sandwiched between the silver pastes on its (001) and (00–1) surfaces.

Other physical measurements

Elemental analyses (C, H, and N) were performed on a Cario Erballo elemental analyzer. The UV-vis reflectance spectra were recorded on a Shimadzu UV 3600 UV-vis-NIR spectrophotometer under N2 atmosphere. Thermogravimetric analyses (TGA) were performed on a DTU-3A simultaneous thermal analyzer from room temperature to 800 °C under N2 atmosphere at a heating rate of 5 °C min−1. 57Fe Mössbauer spectra were recorded on a Topologic 500AV spectrometer. Powder X-ray diffraction (PXRD) data were measured by Cu Kα radiation (λ = 1.5418 Å) on a Rigaku diffractometer with a scanning rate of 5 ° min−1 at 298 K. The infrared (IR) spectra were recorded on a WQF-510A FTIR spectrometer. Field-emission scanning electron microscopy (FE-SEM) images were performed on a SU-8010 scanning electron microscope.

Density functional theory calculations

The ab initio calculations were performed with the ORCA program package version 5.0.364. The CASSCF method was used to take the static correlation effect into account, while the NEVPT2 correction was adopted to consider the dynamic correlation effect. The def2-TZVP65 basis set was used for all atoms. The magnetic susceptibility was calculated in a 5000 Oe external field.

Truncated molecular models (containing four FeII ions, each Fe center coordinated by one BPND ligand and one pyridine molecule in the axial direction, and four CN− on the equatorial plane) were constructed based on the single-crystal structure of 1Ag measured at 100 K and 250 K without further optimizations for the TD-DFT calculations (Supplementary Data 1). The TD-DFT calculations were performed in ORCA 5.0.3 program, and the Tamm Dancoff Approximation (TDA) approximation method was used to investigate the vertical excitation energies and the corresponding oscillation strengths on the models at B3LYP/def2-SVP level65,66. Simultaneously using auxiliary fitting basis sets def2-SVP/C67 and def2/J68 to support the RIJCOSX density fitting approximation method. To cover the energy range of interest, 360 excited states were included in the TD-DFT calculations for both HS and LS states. The assignments of absorption peaks were made based on the CI expansions and charge density difference calculated by Multiwfn 3.769. The molecular orbital diagrams were made by ChemCraft (Chemcraft - graphical software for visualization of quantum chemistry computations. https://www.chemcraftprog.com).