Chemical Science Research Frontiers 2021 78 Control of spin transition by interface strain in heterostructured metal-organic framework thin film Metal-organic frameworks (MOFs) are formed by the self-assembly of metal ions and ligands, and thus have a tailor-made framework structure. MOFs show various physical properties (gas separation, switching, catalysis, and so on). For future practical applications of MOFs such as high performance catalysts or chemical sensors, crystalline oriented MOF thin films have been intensively pursued in recent years. Among the proposed thin film fabrication techniques, layer- by-layer method is one of the most well-established bottom-up approaches. In this method, substrates with self-assembled monolayers (SAMs) are fabricated by alternately immersing them in a solution of building blocks. However, during the fabrication process, step- by-step growth at the surface sometimes result in unconventional structures that are not seen in the bulk. For example, heterostructured MOF thin films, in which one MOF is epitaxially grown on top of another MOF, are known to exhibit unique gas separation ability. It should be noted that the heterostructured MOF thin films have tunable interfaces between different MOFs. Thus, the heterostructured MOF thin films have the ability to realize the rational design of MOF interfaces and provide a new research platform. As a research platform for the heterostructured M O F t h i n f i l m s , H o ff m a n n - t y p e M O F { F e ( p z ) [M(CN) 4 ]} [pz = pyrazine; M = Ni (Nipz), M = Pt (Ptpz)] is a very attractive candidate because it is an iron(II) coordination compound with a 3 d 6 electron configuration and exhibits spin transitions induced by temperature, light, and so on [1]. These electronic configurations can move between paramagnetic high- spin (HS) and diamagnetic low-spin (LS) states, with accompanying structural changes. In this study, we show the first example of strain- controlled spin transition in a heterostructured MOF thin film [2]. As shown in Fig. 1, we fabricated a heterostructured MOF thin film Nipz5L-Ptpz30L , composed of 5 layers of Nipz (bottom buffer layer) and 30 layers of Ptpz (upper layer), using the layer- by-layer method to control the spin transition behavior. Nipz5L-Ptpz30L showed a remarkable increase in the spin transition temperature. In order to confirm the structure and orientation of Nipz5L-Ptpz30L , synchrotron XRD measurements were carried out using SPring-8 BL13XU ( λ = 1.550 Å, room temperature). Figure 2 shows the synchrotron XRD profiles of different scattering geometries of Nipz5L-Ptpz30L in horizontal (in-plane) and vertical (out-of-plane) directions with respect to the substrate. The in-plane XRD pattern shows peaks indexed as hk 0, showing periodic ordering of cyano-bridged 2D layers. However, the out-of-plane pattern shows two peaks, 001 and 002, showing interlayer ordering through pyrazine. hk 0 and 00 l diffraction peaks are observed separately in the in-plane and out-of-plane patterns. The data clearly show that Nipz5L-Ptpz30L has high crystallinity and well-controlled orientations of crystal growth in both in-plane and out-of-plane directions. Furthermore, the lattice constants of Nipz5L-Ptpz30L (tetragonal, P4/mmm, a = 7.28 Å, c = 7.08 Å) were determined from the XRD results. These lattice constants are significantly smaller than the bulk Ptpz in the HS state ( a = 7.53 Å, c = 7.32 Å, 293 K), but similar to the bulk Ptpz in the LS state ( a = 7.33 Å, c = 6.94 Å, 293 K) [1], which indicates that Nipz5L-Ptpz 30L is in LS state at room temperature. The Raman spectrum of MOF shows a strong pyrazine internal vibration mode between 600 and 1600 cm –1 . As a probe of the change in spin state, it is well known that the strong intensity enhancement of the pyrazine stretching mode n (ring) at 1030 cm –1 is a useful marker for the transition from the LS state to the HS state. When n (ring) is normalized to the intensity of the CH bending mode d (CH) at 1230 cm –1 , the n (ring) mode becomes a quantitative measure of the spin state. Therefore, VT Raman spectra were measured in order to investigate the spin transition behavior. As shown in Fig. 3(a), the intensity ratio between the n (ring) and d (CH) modes clearly explains the spin transition behavior. During the heating process, Nipz5L-Ptpz30L shows a spin transition from 370 K to 400 K, and during the cooling process, it shows Fig. 1. Fabrication process of heterostructured thin film ( Nipz5L-Ptpz30L ). × 5 cycles for Nipz × 30 cycles for Ptpz Fe 2+ [Ni(CN) 4 ] 2– OR [Pt(CN) 4 ] 2– pyrazine Nipz5L-Ptpz30L Research Frontiers 2021 79 hysteresis at ~380K, indicating that a first-order spin transition occurs. The spin transition temperatures ( T c ) of Nipz5L-Ptpz30L during the cooling process ( T c ↓ ) and heating ( T c ↑ ) were 362K and 388K, respectively, where T c is defined as the intermediate temperature at which the normalized intensity ratio reaches 0.65. Ptpz30L showed little change compared to bulk Ptpz . On the other hand, Nipz5L-Ptpz30L showed an increase in T c of ~80K. In the case of spin-transition compounds such as Hoffmann-type MOFs, a strong ligand field stabilizes the LS state: compressive strain on the Ptpz layer brings the CN ligand closer to Fe 2+ (Fig. 3(b)), the level of d x 2 − y 2 orbitals increases due to s -donation from CN ligand to Fe 2+ , and the level of t 2g orbitals decreases due to p -back donation from Fe 2+ to CN ligand. As a result, the ligand field around Fe 2+ becomes stronger and the LS state becomes more stable. Therefore, we have demonstrated for the first time that the spin transition behavior of MOFs can be controlled by fabricating nanometer-sized heterostructured thin films. These results provide useful insights into basic science for the future practical application of MOFs with various physical properties by rationally verifying the interfacial strain of heterostructured thin films. Tomoyuki Haraguchi Department of Chemistry, Tokyo University of Science Email: haraguchi@rs.tus.ac.jp References [1] M. Ohba et al .: Angew. Chem. Int. Ed. 48 (2009) 4767. [2] T. Haraguchi, K. Otsubo, O. Sakata, A. Fujiwara, H. Kitagawa: J. Am. Chem. Soc. 143 (2021) 16128. Fig. 2. Synchrotron XRD profiles of Nipz5L-Ptpz30L . (a) In-plane XRD patterns. (b) Out-of-plane XRD patterns. Fig. 3. (a) Temperature dependence of the normalized Raman intensity ratio. (b) Schematic representations of increase in spin transition temperature induced by compressive strain. (a) (b) 2 (degree) θ θ Intensity (arb. units) Intensity (arb. units) 10 20 30 40 2 (degree) θ θ 10 15 20 25 30 X-ray X-ray θ 2 θ 2 θ Substrate Substrate Nipz5L-Ptpz30L Nipz5L-Ptpz30L Simulation pattern for Ptpz (HS) Simulation pattern for Ptpz (HS) 100 110 200 210 220 300 310 001 002 Detector Detector Pt N C a a = 7.278 Å Fe Pt N C Fe c = 7.075 Å (a) (b) Temperature (K) Normalized Intensity Ratio 250 0 0.2 0.4 0.6 0.8 1 300 350 400 Ptpz (Bulk) Ptpz30L Nipz5L-Ptpz30L Fe dx 2 – y 2 t 2 g σ -donation π -back donation * N≡C Fe N≡C π σ
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