Wrapping a single crystal spin-crossover complex with a single crystal non-spin-crossover complex to modulate the spin-transition temperature

Abstract

The formation of a heterojunction interface between different materials could lead to emergent functions that are not the simple sum of the properties of each component. Here we report the modulation of physical properties due to the lattice mismatch between two molecular crystals. Using metal complexes as a motif, we created a crystalline object that can be referred to as a core–shell crystal, in which a single crystal of one complex (Fe^II complex) is wrapped with a single crystal of a different complex (Co^II or Zn^II complex). X-ray analysis revealed that the Fe^II complex constituting the inner core exhibits spin-crossover behavior, while the Co^II (or Zn^II) complex constituting the outer shell does not. The crystal structure of the outer shell is different from that formed through spontaneous crystallization of the Co^II (or Zn^II) complex alone, but similar to that of the high-temperature phase of the spin-crossover Fe^II complex. Interestingly, the spin transition behavior of the Fe^II complex inside the core–shell crystals changes, demonstrating that in molecular materials, the formation of a heterojunction interface can modulate the properties of the entire bulk crystal. The fabrication of a ternary core–shell crystal using Fe^II, Co^II and Zn^II complexes is also presented.

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Introduction

It is well known that in epitaxially grown films of inorganic materials, the lattice mismatch between two compounds leads to modulation of the physical properties of the material deposited on the substrate.^1–3 Does this hold true for molecular crystals? This question appears to be non-trivial, since unlike inorganic materials in which atoms are infinitely connected by strong chemical bonds, these materials consist of discrete molecules assembled together by weak intermolecular forces. It is interesting to consider whether, for example, if two different molecular crystals were brought into close contact, the structural information of each component could influence the other, causing a change in the overall properties. In reality, however, it is not easy to create a material in which two types of molecular crystals are closely linked together at the molecular level. This is because the lattice mismatch between dissimilar molecular crystals is too large due to the complexity and anisotropy of their components. Hence, when growing crystals from a solution of a mixture of two compounds, fractional recrystallization usually occurs.

In the past, there have been several reports on the construction of heterostructures using metal–organic frameworks (MOFs),^4–7 π-electronic molecules^8–10 and metal complexes^11–13 as constituents. However, little attention has been paid to the effect of lattice mismatches at heterojunction interfaces on the physical properties. In the present work, to create an elaborate heterojunction interface between two molecular crystals, we used metal complexes, which allow control over the physical properties without significantly changing the molecular structure by fixing the ligand while changing the type of metal ion. To explore the changes in physical properties due to the formation of heterojunction interfaces, we focused on the spin-crossover phenomenon, in which the spin state of a metal complex changes in response to external stimuli such as heat and light.^14–17 Previous studies on nanoscale core–shell spin-crossover systems have reported that interfacial lattice strain can modulate the spin-transition temperature.^18–24 However, this phenomenon has yet to be experimentally demonstrated in macroscopic single crystals. In the present work, we address this gap by demonstrating that even a small fraction of a heterojunction interface within a bulk crystal can modulate its overall spin-crossover behavior.

The above material design concept was embodied using metal complexes ligated with 4-carboxy-2,6-di(pyrazol-1-yl)pyridine^25,26 (cdpp) with compositions of [M(cdpp)₂](BF₄)₂ [M_cdpp (M = Fe, Co, Zn), Fig. 1]. Fe_cdpp undergoes a first-order phase transition between a low-spin state [total spin quantum number (S) = 0] and a high-spin state (S = 2) as the low-temperature and high-temperature phases, respectively, accompanied by a change in the molecular structure.^25,26 On the other hand, Co_cdpp has only a high-spin state (S = 3/2), and Zn_cdpp is diamagnetic in nature (S = 0), with both of them exhibiting no first-order phase transitions. We created a crystalline object that can be referred to as a core–shell crystal, in which a single crystal of Fe_cdpp is wrapped with a single crystal of Co_cdpp or Zn_cdpp. A key to the successful formation of such core–shell crystals (Fe_cdpp@Co_cdpp and Fe_cdpp@Zn_cdpp, Fig. 1b) lies in the seeded crystal growth method, which has been used to construct block supramolecular assemblies^27–32 and block crystallizable polymer-based micelles.^33–36 Interestingly, in this seeded crystallization, the crystal structure of Co_cdpp or Zn_cdpp that forms the outer shell is different from that when it crystallizes alone. What is even more interesting is that the spin transition temperature of the Fe_cdpp core changes, where the greater the lattice mismatch between the core and shell crystals, the lower the spin transition temperature. Here we report the preparation, crystal structures and spin-crossover behavior of these core–shell crystals. The fabrication and characterization of a ternary core–shell crystal (Fe_cdpp@Co_cdpp@Zn_cdpp, Fig. 1b) is also described.

Fig. 1 (a) Synthetic scheme of M_cdpp (M = Fe, Co, Zn). cdpp: 4-carboxy-2,6-di(pyrazol-1-yl)pyridine. (b) Schematic illustration of core–shell crystals of M_cdpp using a seeded crystal growth method.

Results and discussion

Preparation and characterization of single crystals of M_cdpp

The 4-carboxy-2,6-di(pyrazol-1-yl)pyridine (cdpp) ligand was synthesized according to previously reported procedures.³⁷ Single crystals of M_cdpp (M = Fe, Co, Zn) were prepared using a liquid–liquid diffusion crystallization method (Fig. S1, SI). Typically, an acetonitrile solution of a mixture of cdpp (1.0 eq.) and Fe(BF₄)₂·6H₂O (0.5 eq.) is layered on top of a highly concentrated acetonitrile solution (6.6 M) of tetrabutylammonium tetrafluoroborate (TBABF₄) and allowed to stand at 25 °C for two days, leading to the formation of single crystals of Fe_cdpp (Fig. 2a). By following a similar procedure as for Fe_cdpp, except that Co(BF₄)₂·6H₂O (0.5 eq.) or Zn(BF₄)₂·nH₂O (0.5 eq.) are used instead of Fe(BF₄)₂·6H₂O, single crystals of Co_cdpp or Zn_cdpp can be prepared (Fig. 2b and c).

Fig. 2 POM images and X-ray crystal structures (molecular and packing structures) of (a) Fe_cdpp, (b) Co_cdpp and (c) Zn_cdpp. Color code: carbon = gray, nitrogen = blue, oxygen = red, iron = orange, cobalt = purple, zinc = sky blue, fluorine = green, boron = pink. Hydrogen atoms have been omitted for clarity.

Fig. 2 shows the X-ray crystal structures of Fe_cdpp, Co_cdpp and Zn_cdpp at 93 K, along with polarized optical micrographs (POM) of single crystal samples for each complex. The single crystal of Fe_cdpp belongs to a monoclinic system with space group C2/c (Fig. 2a). Fe_cdpp has an average Fe–N bond length (d_Fe–N) of 1.946 Å and an octahedral distortion parameter (Σ) of 85.32°. These values are typical for low-spin state Fe^II complexes.^{14–16,25,26} In terms of packing structure, the Fe_cdpp molecules assemble to form a one-dimensional (1D) hydrogen-bonded chain structure along the b-axis. When single crystalline Fe_cdpp is heated, it undergoes a phase transition involving a spin crossover with a change from a low- to a high-spin state. Thus, as determined from the X-ray crystal structure, the values of d_Fe–N (1.946 Å) and Σ (85.32°) at 93 K change at 393 K to 2.156 Å and 163.1°, respectively (Fig. S2, SI).

Co_cdpp crystallizes into an orthorhombic system with space group Pbcn, in which a 1D hydrogen-bonded chain structure of Co_cdpp is formed along the a-axis (Fig. 2b). In the crystal, the average Co–N bond length (d_Co–N) of Co_cdpp is 2.122 Å, and its Σ value is 135.4°, indicating that Co_cdpp is in a high-spin state.³⁸ Likewise, Zn_cdpp crystallizes into an orthorhombic space group (Pbcn), and the packing structure of the Zn_cdpp molecules is almost identical to that observed for single-crystalline Co_cdpp (Fig. 2c).

Preparation and characterization of core–shell crystals

Using single crystals of the spin-crossover Fe_cdpp complex as seeds, we examined crystal growth of the Co_cdpp complex from the seed surface (Fig. S1, SI). Thus, single crystals of Fe_cdpp [number-average area (A_n) = 8.07 × 10³ μm² and dispersity index (A_w/A_n) = 1.04] were immersed in an acetonitrile solution (6.6 M) of TBABF₄. On top of this, a saturated acetonitrile solution of Co_cdpp was layered slowly. After two days, core–shell crystals (denoted hereafter as Fe_cdpp@Co_cdpp) were formed (Fig. 3a). An optical micrograph of the resulting crystals illustrates that the Fe_cdpp seed crystals are covered with an outer shell of crystalline Co_cdpp (Fig. S3, SI), indicating quantitative formation of Fe_cdpp@Co_cdpp (A_n = 1.21 × 10⁴ μm² and A_w/A_n = 1.02). Scanning electron microscopy energy dispersive X-ray spectroscopy (SEM-EDX) of Fe_cdpp@Co_cdpp confirmed that the Fe content in the outer shell is below the detection limit (Fig. 3b). For analysis of the inner core, a Fe_cdpp@Co_cdpp crystal was embedded in an epoxy resin and sliced using an ultramicrotome. The SEM-EDX profile of a sliced sample of Fe_cdpp@Co_cdpp shows the presence of a distinct heterojunction interface between the Fe_cdpp core and the Co_cdpp shell (Fig. 3c). These results demonstrate the formation of core–shell crystals having well-defined heterojunction interfaces.

Fig. 3 (a) POM image of Fe_cdpp@Co_cdpp. (b and c) SEM-EDX elemental maps (top) and total area traces (bottom) of Fe_cdpp@Co_cdpp (b) before and (c) after slicing by a ultramicrotome. (d) Crystal structure of Co_cdpp in the outer shell along the a-axis. (e) POM image of Fe_cdpp@Zn_cdpp. (f and g) SEM-EDX elemental maps (top) and total area traces (bottom) of Fe_cdpp@Zn_cdpp (f) before and (g) after slicing by a ultramicrotome. (h) Crystal structure of Zn_cdpp in the outer shell along the a-axis.

When the seeded crystallization event was monitored in real time, the formation of Fe_cdpp@Co_cdpp was observed after leaving the two-layer solution to stand for about 5 h (Fig. S4, SI), which is faster than the spontaneous crystallization of Co_cdpp (about 20 h) without seeds under otherwise identical conditions (Fig. S5, SI). Thus, the surface of the seed Fe_cdpp crystal may provide kinetic perturbation in the crystallization of Co_cdpp.

The inner core and outer shell of the crystals were cut out and each segment analyzed by single-crystal X-ray analysis at 93 K. As expected, before and after the seeded crystal growth process, the crystal structure of the core Fe_cdpp remains unchanged. On the other hand, the crystal structure of the Co_cdpp shell is different from when Co_cdpp crystallizes alone in acetonitrile (Fig. 2b), and instead, it inherits the structural feature of the core Fe_cdpp (Fig. 3d and S6, SI). Thus, the Co_cdpp forming the outer shell has the space group C2/c, where the difference in lattice constant with the core Fe_cdpp is within a range of 2.5% or less (Table S1, SI). This suggests that on the surface of the Fe_cdpp seed crystal, a kinetic assembly of Co_cdpp is induced, which grows to become the outer shell. Notably, optical microscopy (OM) and scanning electron microscopy (SEM) have not shown any seams on the surfaces or edges of the core–shell crystals (Fig. S3 and S7, SI). In POM (Fig. S8, SI), the molecular orientations in the core and shell of Fe_cdpp@Co_cdpp are identical to each other. These observations indicate that Fe_cdpp@Co_cdpp is formed through a topotactic crystal growth process,^39,40 resulting in a core–shell type “pseudo-single crystal” with two distinct well-defined domains.

Despite the difference in crystal structure and space group, there is no significant difference in the molecular structure of Co_cdpp in the crystal formed in the absence or presence of the Fe_cdpp seed. For example, the values of d_Co–N (2.118 Å) and Σ (134.1°) are virtually identical to those observed in the Co_cdpp crystal alone (d_Co–N = 2.122 Å and Σ = 135.4°), indicating that the Co_cdpp shell crystal is in the high-spin state.³⁸

Similar to Fe_cdpp@Co_cdpp, using single crystals of Fe_cdpp (A_n = 3.63 × 10³ μm²) as seeds, crystals of Zn_cdpp can grow around them (Fig. 3e). The resulting core–shell crystals, Fe_cdpp@Zn_cdpp (A_n = 6.05 × 10³ μm² and A_w/A_n = 1.06), have been unambiguously characterized by SEM-EDX (Fig. 3f and g) and single-crystal X-ray analysis (Fig. 3h and S9, SI). The Zn_cdpp shell of Fe_cdpp@Zn_cdpp adopts the C2/c space group (Fig. 3h), while crystals with the Pbcn space group grow from an acetonitrile solution of Zn_cdpp alone (Fig. 2c). The difference in lattice constant with the core Fe_cdpp is within a range of 3.1% or less (Table S1, SI). As in the case of Fe_cdpp@Co_cdpp, the Zn_cdpp shell inherits the structural feature of the core Fe_cdpp (Fig. 3h and S9, SI).

Unfortunately, attempts to crystallize the Fe_cdpp complex from the seed surface of either Co_cdpp or Zn_cdpp have not been successful. Presumably, Fe_cdpp would be unable to give Pbcn space group crystals as a kinetic form on the Co_cdpp and Zn_cdpp crystal surfaces.

Spin-crossover behavior of core–shell crystals

Since spin-crossover is a first-order phase transition involving structural changes,^14–16 its occurrence can be detected using differential scanning calorimetry (DSC) measurements. Prior to DSC measurements, we confirmed by thermogravimetric analysis (TGA) that all samples are thermally stable up to approximately 450 K, with no significant weight loss within the experimental temperature range (Fig. S10, SI). Fig. 4a shows the DSC profiles of Fe_cdpp, Fe_cdpp@Co_cdpp and Fe_cdpp@Zn_cdpp. Upon heating from 223 K at a rate of 1 K min⁻¹, Fe_cdpp exhibits an endothermic peak at 348 K due to a spin-crossover phenomenon. At the same temperature, the colour of the crystals changes from red to orange (Fig. S11, SI). Similarly, Fe_cdpp@Co_cdpp and Fe_cdpp@Zn_cdpp undergo spin-crossover, with the appearance of endothermic peaks at 347 K and 346 K, respectively. While the difference in phase transition temperature is rather small, a clear difference can be observed between Fe_cdpp and the core–shell crystals. OM visualized the colour change occurring only in the cores of Fe_cdpp@Co_cdpp and Fe_cdpp@Zn_cdpp (Fig. S11, SI).^41,42 For comparison, we also measured DSC of a sample in which Fe_cdpp single crystals are wrapped in an epoxy resin and confirmed that there is no change in the phase transition temperature associated with the spin crossover of Fe_cdpp (Fig. S12, SI).

Fig. 4 (a) DSC profiles of Fe_cdpp (black), Fe_cdpp@Co_cdpp (red) and Fe_cdpp@Zn_cdpp (orange) upon heating at a rate of 1 K min⁻¹. (b) Temperature-dependent magnetic susceptibility of Fe_cdpp (black), Fe_cdpp@Co_cdpp (red) and Fe_cdpp@Zn_cdpp (orange) upon heating at a rate of 1 K min⁻¹ under a magnetic field of 1 T (see Fig. S13 for the cooling process, SI). The χ_mT values were calculated assuming a 50 : 50 molar ratio between Fe_cdpp and M_cdpp (M = Co, Zn) in the core–shell crystals. Plots of spin-transition temperatures (T_1/2) versus core/shell lattice constant mismatch at (c) b and (d) β.

Using a superconducting quantum interference device (SQUID) magnetometer, we investigated the temperature-dependence of the magnetic susceptibility of Fe_cdpp, Fe_cdpp@Co_cdpp and Fe_cdpp@Zn_cdpp, upon heating from 4 K at a rate of 1 K min⁻¹ under an external magnetic field of 1 T (Fig. 4b). Each SQUID measurement was repeated twice independently, which confirmed the reproducibility of the data. Consistent with previous reports,^25,26 we observed that the χ_mT value of Fe_cdpp abruptly increases to 3.0 emu K mol⁻¹ with a spin-transition temperature (T_1/2) of 351 K, which is due to spin-crossover from the low- (S = 0) to high-spin state (S = 2). For Fe_cdpp@Co_cdpp, the χ_mT value below 345 K was determined to be approximately 1.6 emu K mol⁻¹, reflecting both high-spin state of Co_cdpp (S = 3/2) and low-spin state of Fe_cdpp (S = 0). The χ_mT value greatly increases to 3.0 emu K mol⁻¹ due to the spin-crossover behavior of the core Fe_cdpp with T_1/2 of 349 K. Fe_cdpp@Zn_cdpp also displays spin-crossover behavior with T_1/2 of 348 K (Fig. 4b). The fact that the T_1/2 values for Fe_cdpp@Co_cdpp (349 K) and Fe_cdpp@Zn_cdpp (348 K) are similar but apparently lower than that for Fe_cdpp (T_1/2 = 351 K) shows that the wrapping of crystalline Fe_cdpp with other complex crystals perturbs the original spin-crossover behavior. It has been reported that the high-spin state of Fe^II complexes is stabilized in solid-solution systems diluted with Ni^II, Co^II, or Zn^II ions, which have ionic radii similar to that of Fe^II in the high-spin state.^43–45 However, it is notable that the phase-transition behavior of the entire bulk crystal changes when spatially distinct crystalline domains are closely linked together.

We found that the degree of lattice constant mismatch of each block segment is related to T_1/2. In Fe_cdpp@Co_cdpp, when comparing the values of b and β between the Fe_cdpp core and the Co_cdpp shell, there are differences of 2.42% and 1.91%, respectively (Fig. S14 and Table S1, SI). Fe_cdpp@Zn_cdpp has differences in b and β of 3.02% and 2.02%, respectively (Table S1, SI), which are slightly larger than the case of Fe_cdpp@Co_cdpp. Clearly, the greater the lattice mismatch between the core and shell crystals, the lower the spin transition temperature (Fig. 4c, d and S15, SI). It should be noted that the values of b and β of Co_cdpp and Zn_cdpp, which form the outer shell, are closer to those of the high temperature phase of Fe_cdpp (i.e., high-spin state), than to those of its low temperature phase (i.e., low-spin state).

Since the interface of the core–shell crystals would be in a higher energy state than the interior of the Fe_cdpp core, the phase transition is thought to be initiated from the interface. The interface is covered by the crystalline shell of Co_cdpp or Zn_cdpp with a structure similar to the high-temperature phase of Fe_cdpp. That is, for the low-temperature phase of Fe_cdpp before the transition, the shell has a structural mismatch, while for the high-temperature phase of Fe_cdpp generating after the transition, the shell has a structural similarity. This feature of the heterojunction interface is considered responsible for lowering the phase transition temperature in the core–shell crystal systems. In fact, the structure of the shell Zn_cdpp is more similar to the high-temperature phase of Fe_cdpp than that of the Co_cdpp shell, and Fe_cdpp@Zn_cdpp undergoes the phase transition at a lower temperature than Fe_cdpp@Co_cdpp. Such interfacial effects have also been discussed previously in nanoscale coordination polymer core–shell systems, where the modulation of spin-transition behavior is understood in terms of elastic effects due to a mismatch in lattice parameters at the interface.^18–24 The present results may extend this nanoscale understanding to macroscopic core–shell structures composed of molecular crystals.

Preparation and characterization of a ternary core–shell crystal

We have shown that sequential seeded crystallization allows for the preparation of an intriguing ternary core–shell crystals. As mentioned above, crystalline Co_cdpp and Zn_cdpp can be grown from the surface of a seed crystal of Fe_cdpp, and also both can form a crystal structure with the monoclinic C2/c space group. Taking advantage of these structuring properties, Fe_cdpp@Co_cdpp@Zn_cdpp can be obtained through the sequential seeded crystallization of Co_cdpp and Zn_cdpp from a single crystal of Fe_cdpp as seed (Fig. 1b). The ternary core–shell crystals have been characterized by OM and SEM-EDX (Fig. 5a, b and S16, SI). In DSC, Fe_cdpp@Co_cdpp@Zn_cdpp, upon heating, exhibits an endothermic peak at 347 K (Fig. S12, SI), which is the same as in the case of Fe_cdpp@Co_cdpp. As expected, the phase transition behavior of the core Fe_cdpp is affected only by the outer shells in direct contact with it.

Fig. 5 (a) POM image of Fe_cdpp@Co_cdpp@Zn_cdpp. (b) SEM-EDX elemental map and total area trace along one axis of Fe_cdpp@Co_cdpp@Zn_cdpp after slicing by a ultramicrotome. Color code: iron = red, cobalt = green, zinc = sky blue.

Conclusions

We have presented the preparation and characterization of core–shell crystals, in which a single crystal spin-crossover complex (Fe_cdpp) is wrapped with a single crystal of a non-spin-crossover metal complex (Co_cdpp or Zn_cdpp), using the seeded crystallization approach. Interestingly, the crystal structures of the Co_cdpp and Zn_cdpp shells are different from those formed by their spontaneous crystallization, and instead they inherit the crystal structure of the inner Fe_cdpp core. When Fe_cdpp is surrounded by an outer shell of Co_cdpp or Zn_cdpp, its phase transition shifts to a lower temperature, with the degree of variation becoming larger the greater the similarity between the lattice constants of Co_cdpp or Zn_cdpp, and the structure of the high temperature phase of Fe_cdpp. In this seeded crystallization, the Fe_cdpp seed induces the outer shell complex to crystallize into a structure different from its intrinsic one, and thus indirectly tunes the inherent phase transition behavior. The present study, which demonstrates that a heterojunction interface in different molecular crystals can have a noticeable effect on the physical properties of the bulk crystals, sparks new interest in the seeded crystallization approach to interface design of molecular crystals.

Author contributions

To. F. conceived the project; To. F. and Ta. F. designed the experiments; M. T., N. M. and To. F. carried out the experiments and analysed the data; To. F. and Ta. F. co-wrote the manuscript.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). The supplementary information includes crystal data, OM/POM images, TGA data, DSC profiles, SEM images and magnetic susceptibility data. See DOI: https://doi.org/10.1039/d5sc04311e.

CCDC 2214914 (for Fe_cdpp), 2408263 (for Fe_cdpp at 393 K), 2214916 (for Co_cdpp), 2214918 (for Zn_cdpp), 2214915 (for Co_cdpp in the shell) and 2214919 (for Zn_cdpp in the shell) contain the supplementary crystallographic data for this paper.^46a–f

Acknowledgements

This work was supported by Japan Science and Technology Agency (JST) ACT-X (JPMJAX24DH for To.F.), JSPS KAKENHI (JP23K13728 for To.F.) and Grant-in-Aid for Transformative Research Areas (A) “Supra-ceramics” (JP23H04617 for To.F.). This work was also supported by “Crossover Alliance to Create the Future with People, Intelligence and Materials” from MEXT, Japan. We thank the Materials Analysis Division, Core Facility Center, Institute of Science Tokyo, for their support with SEM-EDX and single-crystal X-ray diffraction measurements.

Notes and references

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