# Manipulation of successive crystalline transformations to control electron transfer and switchable functions

Manipulation of successive crystalline transformations to control electron transfer and... ABSTRACT Electron transfer in solid is crucial to switchable magnetic, electrical, optical and mechanical properties. However, it is a formidable challenge to control electron-transfer behaviors via manipulation of crystalline phases, especially through dynamic crystalline transformation. Herein, three crystalline phases of an {Fe2Co2} compound were obtained via enhancement of intermolecular π···π interactions inducing successive single-crystal-to-single-crystal transformations, from solvated 1·2CH3OH·4H2O, to desolvated 1 and its polymorph 1a accompanying electron transfer. 1·2CH3OH·4H2O showed thermally induced reversible intermetallic electron transfer in mother liquor. No electron-transfer behavior was observed in 1. 1a showed reversible intermetallic electron transfer upon thermal treatment or alternative irradiation with 808- and 532-nm lasers at cryogenic temperatures. The electron-transfer behaviors significantly change the magnetic and optical properties, providing a strategy to realize different electron-transfer behaviors and switchable functions via π···π interactions manipulated dynamic crystalline transformation. electron-transfer, dynamic structural transformation, successive crystalline transformations, reversible, polymorphs INTRODUCTION Electron transfer is a common phenomenon in nature and plays important roles in biology, energy, materials, catalysis and other fields [1–4]. Intermetallic electron transfer not only changes the valence states and electron configurations of the participant metal ions, but also switches the coupling interactions between them [2,5–7]. Therefore, the control of electron transfer is an efficient way to tune the magnetic, electric and optical properties of materials [8–22]. Thermally and/or photo-induced electron transfers have been utilized to induce paramagnetic and diamagnetic transformations, presenting photo-switchable magnet behavior [11–13]. Moreover, the polarity and dielectric properties can be switched by utilizing electron-transfer-induced changes in charge distribution [23,24]. Typical examples showing external stimuli-tuned intermetallic electron-transfer behaviors are cyanide-bridged complexes, wherein the electron-transfer behaviors depend on the metallocyanate building blocks and ancillary ligands [11]. The modulation of electron-transfer behavior requires chemical modification, such as ligand substitution, anion and solvent exchange in solution reaction [11,17,20]. It is a formidable challenge to realize different electron-transfer behaviors by manipulating dynamic structural transformations in the solid state, especially through physical stimuli-induced single-crystal-to-single-crystal (SCSC) transformations. Such transformation processes usually involve the movement of atoms in the crystal and rearrangement of chemical bonds, which result in drastic changes in not only the molecular structure, but also the physical/chemical properties [25–28]. On the other hand, SCSC transformations can provide access to compounds that are difficult or impossible to be directly obtained by solution reactions [28]. More importantly, the procedure of SCSC transformations can directly and accurately provide a molecular-level understanding of the mechanism of the transformation, and help to gain more insights into the correlation between the structures and properties [29,30]. It has been reported that two crystalline phases with different electron-transfer behaviors could be obtained upon solvation and desolvation, because hydrogen-bonding interactions between coordinated solvents and the framework can tune the redox potentials of metal ions [31–36]. However, the formation and breakage of hydrogen bonds could only induce one-step SCSC transformation and generate two crystalline phases with different electron-transfer behaviors [32–36]. Moreover, the crystallinity is often disrupted owing to breakage of the hydrogen-bonding interactions that are important to maintain the integrity of the crystalline framework [25,31], restricting further investigation of the electron-transfer mechanism. How to induce successive crystalline transformation to obtain more than two crystalline phases with different electron-transfer behaviors is interesting but still remains a challenge, especially for polymorphs with different electron-transfer behaviors. To induce successive SCSC transformations, introducing flexible π···π interactions may provide a rational strategy [9,37], as the distances of π···π interactions can be modulated in a continuous range (3.3–3.8 Å) [38]. Moreover, the variations in π···π interactions between ligands around the metal ions can induce different distortions of coordination spheres and the strength of the ligand field, which can tune the redox potential of metal centers and result in different electron-transfer behaviors. Herein, we were intrigued by the possibility of introducing intermolecular cooperative π···π interactions to manipulate successive crystalline transformations to obtain different crystalline phases featuring different electron-transfer behaviors. Inspired by this, we adopt an ancillary ligand prazino[2, 3-f][1, 10]phenanthroline (dpq), with an extended π-conjugation system, to prepare a tetranuclear {Fe2Co2} compound [FeII(PzTp)(CN)3]2CoIII2(dpq)4 · 2ClO4 ·2CH3OH ·4H2O (1·2CH3OH ·4H2O, PzTp = tetrakis (pyrazolyl)borate). 1 ·2CH3OH ·4H2O undergoes successive SCSC transformations as a result of enhancement of intermolecular π···π interactions in the process of desolvation and structural rearrangement, forming a pair of polymorphs [FeIII(PzTp)(CN)3]2CoII2(dpq)4·2ClO4 (1) and [FeII(PzTp)(CN)3]2CoIII2(dpq)4·2ClO4 (1a). The three crystalline phases present different electron-transfer behaviors upon thermal treatment and light irradiation. Especially, it is the first time that crystalline transformation between a pair of polymorphs in intermetallic electron-transfer-related compounds has been observed. RESULTS Crystal structure of 1·2CH3OH·4H2O and its successive crystalline transformations Single-crystal X-ray diffraction analysis revealed that 1·2CH3OH·4H2O crystallizes in the triclinic space group P$$\overline{1}$$ (see Supplementary Table 1). The crystal structure consists of cationic tetranuclear {Fe2Co2}2+ square units, ClO4– anions, uncoordinated methanol and water molecules (Fig. 1). At 298 K, the Fe–C and Co–N bond lengths are 1.877(7)–1.889(7) and 1.879(5)–1.946(5) Å, respectively (see Supplementary Table 2), which are consistent with those observed for {FeIILS(μ-CN)CoIIILS} (LS = low spin) linkages [17,33]. When the green crystals are slowly heated to 360 K in the mother liquor, a dramatic color change from green to red is observed, indicating a possible transformation to {FeIIILS(μ-CN)CoIIHS} (HS = high spin) linkages. On cooling to 298 K, the crystals return to the initial green color. This phenomenon indicates that 1·2CH3OH·4H2O undergoes a thermally induced reversible electron transfer in the mother liquor (see Supplementary Movie 1). Figure 1. View largeDownload slide Crystal structures of 1·2CH3OH·4H2O, 1 and 1a. The hydrogen atoms and ClO4− anions are omitted for clarity. Fe, dark yellow; Co, turquoise; C, gray; N, blue; B, orange; O, red. Figure 1. View largeDownload slide Crystal structures of 1·2CH3OH·4H2O, 1 and 1a. The hydrogen atoms and ClO4− anions are omitted for clarity. Fe, dark yellow; Co, turquoise; C, gray; N, blue; B, orange; O, red. There are two important structural characteristics in 1·2CH3OH·4H2O. The first is that uncoordinated water and methanol molecules are located between the {Fe2Co2}2+ square units, with hydrogen-bonding interactions between uncoordinated water molecules and terminal cyanide nitrogen atoms (see Supplementary Fig. 1 and Table 4). The second is that the {Fe2Co2}2+ square units are linked via π···π interactions (average distance = 3.753(2) Å) between the dpq ligands and C–H···π interactions (average distance = 3.148(1) Å) between the C–H moieties of the dpq/PzTp ligands and the pyrazol/pyrazine rings of the PzTp/dpq ligands (see Supplementary Fig. 2 and Table 5). These intermolecular interactions are very important to stabilize the crystalline framework and make it possible to undergo SCSC transformations upon desolvation. Furthermore, the formation and destruction of hydrogen bonding can significantly affect the redox potential of the iron centers [31,33], providing the possibility of desolvation-induced electron transfer. Thus, two crystalline phases with different electron-transfer behaviors are reasonably expected. The TGA (see Supplementary Fig. 3) of 1·2CH3OH·4H2O was measured to explore the possibility of desolvation-induced SCSC transformation. The plot shows a weight loss of 6.0% from 300 to 360 K, corresponding well to the loss of two methanol and four water molecules (calcd: 6.2%). After this weight loss, a long plateau is observed until 540 K, suggesting the formation of a new stable phase. When the green crystals are slowly heated to ∼350 K in air, the color of the crystals changes from green to red, indicating the formation of a new redox state. Moreover, the crystallinity is well retained (Fig. 2 and see Supplementary Movie 2). The crystallographic data demonstrate that the red crystals retain the {Fe2Co2} tetranuclear structure and have the formula of [Fe(PzTp)(CN)3]2Co2(dpq)4·2ClO4 (1, Fig. 1). At 298 K, the Fe–C and Co–N bond lengths are 1.925(8)–1.944(9) and 2.097(7)–2.169(7) Å, respectively (see Supplementary Table 2), corresponding with those observed for {FeIIILS(μ-CN)CoIIHS} linkages [17,39]. These results indicate that electron transfer occurs in the desolvation process with a transformation from {FeIILS(μ-CN)CoIIILS} linkages in 1·2CH3OH·4H2O to {FeIIILS(μ-CN)CoIIHS} linkages in 1. Furthermore, an endothermic peak is seen in the DSC curve of 1·2CH3OH·4H2O, further confirming the first-order phase transition from 1·2CH3OH·4H2O to 1 (see Supplementary Fig. 4). With the transformation from 1·2CH3OH·4H2O to 1, the average π···π interaction distance between the dpq ligands decreases from 3.753(2) to 3.664(1) Å, and the average C–H···π interaction distance between the C–H moieties of the dpq/PzTp ligands and the pyrazol rings of the PzTp ligands decreases from 3.148(1) to 3.102(1) Å. Such results suggest that the intermolecular interactions are enhanced in the process of crystalline transformation (see Supplementary Fig. 5 and Table 6). Figure 2. View largeDownload slide Photographic images showing the SCSC transformation. Images show the conversions among these three species through subsequent desolvation and vapor induction. Figure 2. View largeDownload slide Photographic images showing the SCSC transformation. Images show the conversions among these three species through subsequent desolvation and vapor induction. Interestingly, when the red crystals of 1 were placed in water vapor and heated at 100°C for 24 h, the color of the crystals changed from red to green (Fig. 2, see Supplementary Fig. 6), suggesting the formation of a new phase, as confirmed by single-crystal X-ray diffraction (Fig. 1) and powder XRD analyses (see Supplementary Fig. 7). The new phase exhibits a composition consistent with the formula [Fe(PzTp)(CN)3]2Co2(dpq)4·2ClO4 (1a). Although 1 and 1a are a pair of polymorphs, they exhibit different electron-transfer behaviors. For 1a at 298 K, the Fe–C and Co–N bond lengths are 1.852(4)–1.914(4) and 1.877(3)–1.949(3) Å, respectively (see Supplementary Table 2), indicating the existence of {FeIILS(μ-CN)CoIIILS} linkages. When the crystals of 1a are slowly heated to 350 K, a dramatic color change from green to red is observed. The corresponding Co–N bond lengths are 2.077(4)–2.145(3) Å (see Supplementary Table 2), indicating the formation of {FeIIILS(μ-CN)CoIIHS} linkages [17,39]. Moreover, the crystals return to the initial green color on cooling to 298 K, indicating the thermally induced reversible intermetallic electron transfer. With the transformation from 1 to 1a, the average π···π interaction distance between the dpq ligands decreases from 3.664(1) to 3.457(1) Å at room temperature, and the average C–H···π interaction distance between the C–H moieties of the dpq/PzTp ligands and the pyrazol/pyridine rings of the PzTp/dpq ligands decreases from 3.102(1) to 3.002(1) Å, leading to the enhanced intermolecular interactions (see Supplementary Fig. 8 and Table 7). When heated to 350 K, both the average π···π interaction and C–H···π interaction distances of 1a increase slightly (see Supplementary Fig. 9 and Table 8). Magnetic characterization The magnetic properties of the three crystalline phases were subsequently investigated (Fig. 3). The χT versus T curve for 1·2CH3OH·4H2O in mother liquor shows that χT values remain essentially constant at 0.46 cm3 mol−1 K below 355 K, confirming the existence of {FeIILS(μ-CN)CoIIILS} linkages. Upon heating, χT values abruptly increase to 6.44 cm3 mol−1 K at 390 K, which are in agreement with the theoretical value of 6.67 cm3 mol−1 K expected for two LS FeIII and two HS CoII ions [16]. When the temperature is lowered from 400 K, χT values decrease rapidly to its initial value at 355 K with a small thermal hysteresis loop of ∼8 K, indicating that the system regains the {FeIILS(μ-CN)CoIIILS} linkages. Thus, 1·2CH3OH·4H2O shows thermally induced reversible intermetallic electron transfer upon thermal treatment in the mother liquor. Figure 3. View largeDownload slide Magnetic characteristics for 1·2CH3OH·4H2O, 1 and 1a. Plots of χT vs temperature for 1·2CH3OH·4H2O, 1 and 1a. 1·2CH3OH·4H2O was soaked in the mother liquor upon cooling, and cycling the temperature back to 400 K; 1 upon cooling; 1·2CH3OH·4H2O upon heating and concomitant desorption of methanol and water; 1a upon cooling and reheating to 400 K (temperature sweeping rate: 1 K/min for 10–400 K and 0.5 K/min for 2–10 K). Figure 3. View largeDownload slide Magnetic characteristics for 1·2CH3OH·4H2O, 1 and 1a. Plots of χT vs temperature for 1·2CH3OH·4H2O, 1 and 1a. 1·2CH3OH·4H2O was soaked in the mother liquor upon cooling, and cycling the temperature back to 400 K; 1 upon cooling; 1·2CH3OH·4H2O upon heating and concomitant desorption of methanol and water; 1a upon cooling and reheating to 400 K (temperature sweeping rate: 1 K/min for 10–400 K and 0.5 K/min for 2–10 K). However, 1 exhibits a different electron-transfer behavior. Its χT value is 6.51 cm3 mol−1 K at 400 K (Fig. 3), corresponding to paramagnetic {FeIIILS(μ-CN)CoIIHS} linkages. As the temperature is lowered, χT values gradually decrease, reaching a minimum value of 5.36 cm3 mol−1 K at 24 K. Below this temperature, the χT value increases up to 5.44 cm3 mol−1 K at 14 K and then decreases rapidly to 3.50 cm3 mol−1 K at 2 K. This represents a typical paramagnetic behavior without electron-transfer-induced spin transition. The transformation of the magnetic behavior in the desolvation process was monitored for as-synthesized crystals of 1·2CH3OH·4H2O. χT values remain essentially constant at 0.36 cm3 mol−1 K below 310 K and reach a maximum value of 6.43 cm3 mol−1 K at 360 K, which is in agreement with paramagnetic {FeIIILS(μ-CN)CoIIHS} linkages as observed for 1. Such magnetic behavior confirms that the electron transfer occurs in the desolvation process from 1·2CH3OH·4H2O to 1. This electron-transfer process seems analogous to that observed during the heating of 1·2CH3OH·4H2O in mother liquor, but with a lower transition temperature. This is due to the difference in the intermetallic electron-transfer mechanism, as the former stems from the loss of solvent molecules, while the latter arises from thermal stimuli. For 1a, χT values remain nearly constant between 2 and 315 K at 0.43 cm3 mol−1 K (Fig. 3). Heating from 315 to 360 K causes an increase in the χT value to 6.47 cm3 mol−1 K. The χT value returns to its initial value with a small thermal hysteresis ∼5 K wide, showing reversible electron-transfer behavior that involves transformation between the diamagnetic {FeIILS(μ-CN)CoIIILS} (LT phase = low temperature phase) linkages and the paramagnetic {FeIIILS(μ-CN)CoIIHS} (HT phase = high temperature phase) linkages. Consistently with magnetic data, endothermic/exothermic peaks for 1a are observed with Tmax= 331.2 and 325.3 K, indicating the occurrence of the first-order phase transition (see Supplementary Fig. 10). Optical studies The solid-state UV–vis–NIR absorption spectra of 1·2CH3OH·4H2O, 1 and 1a were measured at room temperature to study their color changes and to further support the electronic state assignments deduced from the structural and magnetic analyses (Fig. 4a). The absorption spectra of 1·2CH3OH·4H2O and 1a are similar, presenting bands at 427 and 763 nm, respectively. The absorption band at 427 nm may be assigned to ligand-to-metal charge transfer (LMCT) of the FeII chromophore [17]. The broad band at 763 nm can be assigned as the FeII → CoIII intervalence charge-transfer (IVCT) band [17,35]. In contrast, a broad absorption band at 459 nm with a small shoulder band at 534 nm is observed for 1. The higher-energy band at 459 nm is assigned to a spin- and Laporte-allowed LMCT transition, and the small shoulder band at 534 nm can be assigned to the CoII → FeIII IVCT band [39]. These results confirm that 1·2CH3OH·4H2O and 1a possess the diamagnetic {FeIILS(μ-CN)CoIIILS} linkages, whereas 1 possesses the paramagnetic {FeIIILS(μ-CN)CoIIHS} linkages at room temperature. Figure 4. View largeDownload slide Optical spectra for 1·2CH3OH·4H2O, 1 and 1a. (a) Solid-state UV–vis spectra for 1·2CH3OH·4H2O, 1 and 1a at room temperature. (b) Variable-temperature solid-state UV–vis spectra for 1a in heating (298→360 K) mode. Figure 4. View largeDownload slide Optical spectra for 1·2CH3OH·4H2O, 1 and 1a. (a) Solid-state UV–vis spectra for 1·2CH3OH·4H2O, 1 and 1a at room temperature. (b) Variable-temperature solid-state UV–vis spectra for 1a in heating (298→360 K) mode. Variable-temperature solid-state UV–vis–NIR absorption spectra of 1a were measured in the temperature range of 298–360 K (Fig. 4b). As the temperature increases, there is a gradual decrease in the broad absorption centered at 763 nm for the characteristic band of the FeII → CoIII IVCT, and the LMCT/MLCT band at 427 nm is shifted to the lower-energy region associated with the appearance of the CoII → FeIII IVCT band at 527 nm. The observed spectral change confirms the occurrence of thermally induced electron transfer with the transformation from the {FeIILS(μ-CN)CoIIILS} to {FeIIILS(μ-CN)CoIIHS} linkages. Photomagnetic characterization The photomagnetic effects of 1a were examined to determine the possibility of photo-induced electron transfer. Because an FeII → CoIII IVCT band was observed at 763 nm for the LT phase of 1a, an 808 nm laser was selected to stimulate the FeII → CoIII IVCT band and used for photomagnetic experiments. When the sample is irradiated at 20 K for 120 min, χT values rapidly increase and reach a maximum value of 5.61 cm3 mol−1 K (Fig. 5a). When the sample is heated from 2 K after irradiation, χT values first increase steeply to a sharp maximum of 6.38 cm3 mol−1 K at 8.2 K, indicating an almost complete conversion from diamagnetic {FeIILS(μ-CN)CoIIILS} to paramagnetic {FeIIILS(μ-CN)CoIIHS} linkages (Fig. 5b). The increase in χT values from 2 to 8.2 K is attributed to the presence of intermolecular antiferromagnetic interactions and/or zero field splitting [16]. Upon further heating to 73 K, the photo-induced metastable paramagnetic {FeIIILS(μ-CN)CoIIHS} linkages relax to the initial diamagnetic {FeIILS(μ-CN)CoIIILS} linkages, indicating that magnetization can be increased by light irradiation and recovered with thermal treatment. Figure 5. View largeDownload slide Photoreversibility of the magnetization for 1a. (a) Plots of χT vs time under cycles of successive irradiation at 808 and 532 nm at 20 K for 1a. (b) Plots of χT vs temperature for 1a before irradiation, after irradiation at 808 nm, the metastable-induced state irradiated at 532 nm, and after thermal annealing treatment at 100 K. Figure 5. View largeDownload slide Photoreversibility of the magnetization for 1a. (a) Plots of χT vs time under cycles of successive irradiation at 808 and 532 nm at 20 K for 1a. (b) Plots of χT vs temperature for 1a before irradiation, after irradiation at 808 nm, the metastable-induced state irradiated at 532 nm, and after thermal annealing treatment at 100 K. On the basis of the optical studies, 1a in HT phase displays a CoII → FeIII IVCT band at 527 nm. The photo-induced metastable phase after irradiation with an 808 nm laser was further irradiated with a 532 nm laser in order to investigate the photo-induced reversibility. As a result, χT values decrease from 5.61 to 2.27 cm3 mol−1 K after 120 min irradiation at 20 K (Fig. 5b), indicating 59.5% recovery of the diamagnetic state. This successive photoreversibility of the magnetization can be well repeated (Fig. 5a and see Supplementary Fig. 11). This magnetic change verifies the occurrence of photo-induced reversible electron transfer in 1a. The relaxation of the photo-induced metastable state was monitored at different temperatures in order to probe the stability of the photo-induced phases (see Supplementary Fig. 12). In the low-temperature (10–40 K) region, relaxation time τ was less dependent on temperature. This result additionally confirms that the photoreversibility changes were induced by light rather than the thermal relaxation effect. IR spectra The IR spectra analysis of 1·2CH3OH·4H2O, 1 and 1a also supports the redox state assignments deduced from the structural and magnetic analyses. For 1·2CH3OH·4H2O, the typical absorption bands of cyanide groups (2107, 2089 and 2071 cm−1, see Supplementary Fig. 13) indicate that the compound possesses the diamagnetic {FeIILS(μ-CN)CoIIILS} linkages [17,33]. Three absorption bands for cyanide groups are observed in the IR spectrum of 1 (see Supplementary Fig. 13), corresponding to the stretching vibrations for the bridging cyanide ions in the {FeIIILS(μ-CN)CoIIHS} linkages (2148 and 2143 cm−1) and the terminal cyanide ions in the [PzTpFeIII(CN)3]– anions (2122 cm−1) [20]. At 300 K, 1a shows absorption bands at 2069, 2085 and 2106 cm−1, confirming that 1a possesses the {FeIILS(μ-CN)CoIIILS} linkages in the LT phase (see Supplementary Fig. 14). When the temperature increases to 360 K, the intensity of the cyanide stretching bands for the {FeIILS(μ-CN)CoIIILS} linkages decreases, and new bands for the {FeIIILS(μ-CN)CoIIHS} linkages appear at 2150 and 2159 cm−1 as seen in 1. Furthermore, upon cooling to room temperature, the IR spectra of 1a return to their initial state, suggesting that the thermally induced intermetallic electron transfer is reversible. The irradiation-time dependence of the IR spectra was measured to further verify the occurrence of photo-induced reversible electron transfer in 1a upon successive and alternative irradiation with 808 and 532 nm lasers at 20 K (Fig. 6). Upon irradiation at 808 nm, the intensity of the cyanide stretching bands attributed to the {FeIILS(μ-CN)CoIIILS} linkages decreases and a new peak appears at 2162 cm−1, which is attributed to the {FeIIILS(μ-CN)CoIIHS} linkages. The intensity of the new peak gradually increases with irradiation time. Conversely, the photo-induced metastable phase was irradiated at 532 nm, inducing a decrease in the intensity of the cyanide stretching band attributed to the {FeIIILS(μ-CN)CoIIHS} linkages, and the intensity of the peaks attributed to the {FeIILS(μ-CN)CoIIILS} linkages increases. By normalization of the peak intensities for the {FeIIILS(μ-CN)CoIIHS} linkages vs irradiation time upon irradiation at 808 and 532 nm, the recovery of the {FeIILS(μ-CN)CoIIILS} linkages is estimated to be 60.0%, which is comparable to the value obtained from photomagnetic measurements (see Supplementary Fig. 15). Therefore, the observed spectral changes further confirm the occurrence of photo-induced reversible electron transfer with interconversion between {FeIIILS(μ-CN)CoIIHS} and {FeIILS(μ-CN)CoIIILS} linkages through successive and alternative irradiation at 808 and 532 nm. Figure 6. View largeDownload slide Photo-induced IR spectra for 1a. Irradiation-time dependence of the IR spectra for 1a irradiated at 808 nm (a) and the photoreversibility upon irradiation at 532 nm (b) at 20 K. Figure 6. View largeDownload slide Photo-induced IR spectra for 1a. Irradiation-time dependence of the IR spectra for 1a irradiated at 808 nm (a) and the photoreversibility upon irradiation at 532 nm (b) at 20 K. Structure and property discussion Air-stable 1a can be obtained by two-step-wise SCSC transformations from the air-unstable 1·2CH3OH·4H2O. This unprecedented successive single-crystalline transformations-induced electron transfer inspired us to investigate their structural correlations. First, the structure of 1·2CH3OH·4H2O indicates that hydrogen-bonding interactions are formed between the nitrogen atom of the terminal cyanide and an uncoordinated water molecule with the N···O distance of 2.713(12) Å (see Supplementary Fig. 1 and Table 4). Hydrogen-bonding strength can have a significant effect on redox potential and may have a significant effect on the intermetallic electron-transfer behavior. Hydrogen bonding as an electron-withdrawing effect can result in the positive shifting of FeII redox potential [31,33]. FeII ions are more stable in this situation. Thus, 1·2CH3OH·4H2O exhibits stable {FeIILS(μ-CN)CoIIILS} linkages in the solvated phase. When 1·2CH3OH·4H2O loses the solvent molecules and transforms into the desolvated phase 1, the hydrogen bonding is destroyed, accompanied by a negative shift in the redox potential of FeII. The {FeIILS(μ-CN)CoIIILS} linkages become unstable and tend to transform into the {FeIIILS(μ-CN)CoIIHS} linkages. Therefore, the electron transfer from 1·2CH3OH·4H2O to 1 occurs as a result of thermally induced desolvation. Second, the average distances of π···π and C−H···π interactions between the adjacent dpq/PzTp ligands are 3.753 and 3.148 Å for 1·2CH3OH·4H2O, 3.665 and 3.102 Å for 1 and 3.457 and 3.002 Å for 1a, indicating that the interactions gradually become stronger from 1·2CH3OH·4H2O to 1 and 1a. The enhancement of intermolecular interactions increases the synergistic effects between molecules and causes an energy decrease to a more stable structure, providing an important driving force for the successive two-step irreversible SCSC transformations. In comparison, only polycrystalline powder 1a can be obtained directly using solvothermal conditions at 100°C starting from the precursors, as confirmed by powder XRD analyses (see Supplementary Fig. 16). This results show the importance of successive crystalline transformations. In addition, the coordination spheres of cobalt centers bring out different distortion accompanying the changes in intermolecular interactions (C–H···π and π···π) between dpq ligands. This can be observed directly in the deviation of the Co–N≡C bond angles (see Supplementary Table 3). Compared with the bond angles [171.8(5) and 168.6(6)°] in 1·2CH3OH·4H2O, the difference value (8.9°) of the bond angles [171.1(6) and 162.2(7)°] in 1 is larger, indicating larger deviation from the ideal octahedron for the latter. For 1a, the bond angles [166.1(3) and 166.7(3)°] are nearly the same, which suggests that the distortion of the CoN6 octahedron becomes smaller. Continuous shape measurements (CShM) analysis and the parameter Σ (the sum of $$|90-\alpha|$$ for the 12 cis-N–Co–N angles around the Co atoms) were also calculated (see Supplementary Table 2). The CShM of cobalt is 0.178 (1·2CH3OH·4H2O), 1.014 (1), 0.276 (LT phase of 1a) and 1.216 (HT phase of 1a), respectively. A smaller value is generally associated with a stronger ligand field, leading to an LS state of the metal ion, whereas the larger value corresponds to a weaker ligand field and support an HS state. From 1·2CH3OH·4H2O to 1 and 1a, the values first increase and then decrease, which suggests that the strength of the ligand field changes from strong to weak and back to strong. Thus, the Co ion exhibits HS state in 1 and the LS state in 1·2CH3OH·4H2O and 1a. This unusual behavior serves to illustrate how subtle change in C–H···π and π···π interactions can actually have a profound effect on SCSC transformations and electron-transfer properties. CONCLUSION The enhancement of intermolecular π···π interactions drives successive single-crystalline transformations and achieves three crystalline phases with different electron-transfer behaviors. Especially, this is the first time that the crystalline transformation between two polymorphs in intermetallic electron-transfer-related compounds has been observed, providing an ideal platform to study the effect of intermolecular π···π interactions on crystalline-transformation-induced change in electron-transfer behaviors. The introduction of π···π interactions not only provides a strategy to manipulate the crystalline phases, but also offers access to construct switchable multifunctional materials displaying stimuli-induced dynamic-changes functions in the future. METHODS The detailed preparation and characteristic methods of materials are available as Supplementary data at NSR online. SUPPLEMENTARY DATA Supplementary data are available at NSR online. FUNDING This work was partly supported by the National Natural Science Foundation of China (91422302, 21421005 and 21322103) and the Fundamental Research Funds for the Central Universities, China. REFERENCES 1. Salvador JR , Guo F , Hogan T et al. Zero thermal expansion in YbGaGe due to an electronic valence transition . Nature 2003 ; 425 : 702 – 5 . https://doi.org/10.1038/nature02011 Google Scholar CrossRef Search ADS PubMed 2. Long Y-W , Hayashi N , Saito T et al. Temperature-induced A–B intersite charge transfer in an A-site-ordered LaCu3Fe4O12 perovskite . Nature 2009 ; 458 : 60 – 3 . https://doi.org/10.1038/nature07816 Google Scholar CrossRef Search ADS PubMed 3. Akimov AV , Neukirch AJ , Prezhdo OV . Theoretical insights into photoinduced charge transfer and catalysis at oxide interfaces . Chem Rev 2013 ; 113 : 4496 – 565 . https://doi.org/10.1021/cr3004899 Google Scholar CrossRef Search ADS PubMed 4. Bernardo B , Cheyns D , Verreet B et al. Delocalization and dielectric screening of charge transfer states in organic photovoltaic cells . Nat Commun 2014 ; 5 : 3245 . https://doi.org/10.1038/ncomms4245 Google Scholar CrossRef Search ADS PubMed 5. Horiuchi S , Okimoto Y , Kumai R et al. Quantum phase transition in organic charge-transfer complexes . Science 2003 ; 299 : 229 – 32 . https://doi.org/10.1126/science.1076129 Google Scholar CrossRef Search ADS PubMed 6. Alves H , Molinari AS , Xie H-X et al. Metallic conduction at organic charge-transfer interfaces . Nat Mater 2008 ; 7 : 574 – 80 . https://doi.org/10.1038/nmat2205 Google Scholar CrossRef Search ADS PubMed 7. Huang Z-X , Auckett JE , Blanchard PER et al. Pressure-induced intersite Bi–M (M = Ru, Ir) valence transitions in hexagonal perovskites . Angew Chem Int Ed 2014 ; 53 : 3414 – 7 . https://doi.org/10.1002/anie.201311159 Google Scholar CrossRef Search ADS 8. Sato O , Tao J , Zhang Y-Z . Control of magnetic properties through external stimuli . Angew Chem Int Ed 2007 ; 46 : 2152 – 87 . https://doi.org/10.1002/anie.200602205 Google Scholar CrossRef Search ADS 9. Sato O . Dynamic molecular crystals with switchable physical properties . Nat Chem 2016 ; 8 : 644 – 56 . https://doi.org/10.1038/nchem.2547 Google Scholar CrossRef Search ADS PubMed 10. Jeen H , Choi WS , Biegalski MD et al. Reversible redox reactions in an epitaxially stabilized SrCoOx oxygen sponge . Nat Mater 2013 ; 12 : 1057 – 63 . https://doi.org/10.1038/nmat3736 Google Scholar CrossRef Search ADS PubMed 11. Aguilà D , Prado Y , Koumousi ES et al. Switchable Fe/Co prussian blue networks and molecular analogues . Chem Soc Rev 2016 ; 45 : 203 – 24 . https://doi.org/10.1039/C5CS00321K Google Scholar CrossRef Search ADS PubMed 12. Hoshino N , Iijima F , Newton GN et al. Three-way switching in a cyanide-bridged [CoFe] chain . Nat Chem 2012 ; 4 : 921 – 6 . https://doi.org/10.1038/nchem.1455 Google Scholar CrossRef Search ADS PubMed 13. Sato O , Iyoda T , Fujishima A et al. Photoinduced magnetization of a cobalt-iron cyanide . Science 1996 ; 272 : 704 – 5 . https://doi.org/10.1126/science.272.5262.704 Google Scholar CrossRef Search ADS PubMed 14. Berlinguette CP , Dragulescu-Andrasi A , Sieber A et al. A charge-transfer-induced spin transition in the discrete cyanide-bridged complex {[Co(tmphen)2]3[Fe(CN)6]2} . J Am Chem Soc 2004 ; 126 : 6222 – 3 . https://doi.org/10.1021/ja039451k Google Scholar CrossRef Search ADS PubMed 15. Li D-F , Clérac R , Roubeau O et al. Magnetic and optical bistability driven by thermally and photoinduced intramolecular electron transfer in a molecular cobalt-iron prussian blue analogue . J Am Chem Soc 2008 ; 130 : 252 – 8 . https://doi.org/10.1021/ja0757632 Google Scholar CrossRef Search ADS PubMed 16. Zhang Y-Z , Li D-F , Clérac R et al. Reversible thermally and photoinduced electron transfer in a cyano-bridged {Fe2Co2} square complex . Angew Chem Int Ed 2010 ; 49 : 3752 – 6 . https://doi.org/10.1002/anie.201000765 Google Scholar CrossRef Search ADS 17. Nihei M , Sekine Y , Suganami N et al. Controlled intramolecular electron transfers in cyanide-bridged molecular squares by chemical modifications and external stimuli . J Am Chem Soc 2011 ; 133 : 3592 – 600 . https://doi.org/10.1021/ja109721w Google Scholar CrossRef Search ADS PubMed 18. Mondal A , Li Y-L , Seuleiman M et al. On/off photoswitching in a cyanide-bridged {Fe2Co2} magnetic molecular square . J Am Chem Soc 2013 ; 135 : 1653 – 6 . https://doi.org/10.1021/ja3087467 Google Scholar CrossRef Search ADS PubMed 19. Koumousi ES , Jeon IR , Gao Q et al. Metal-to-metal electron transfer in Co/Fe prussian blue molecular analogues: the ultimate miniaturization . J Am Chem Soc 2014 ; 136 : 15461 – 4 . https://doi.org/10.1021/ja508094h Google Scholar CrossRef Search ADS PubMed 20. Zhang Y-Z , Ferko P , Siretanu D et al. Thermochromic and photoresponsive cyanometalate Fe/Co squares: toward control of the electron transfer temperature . J Am Chem Soc 2014 ; 136 : 16854 – 64 . https://doi.org/10.1021/ja508280n Google Scholar CrossRef Search ADS PubMed 21. Nihei M , Yanai Y , Hsu IJ et al. A hydrogen-bonded cyanide-bridged [Co2Fe2] square complex exhibiting a three-step spin transition . Angew Chem Int Ed 2017 ; 56 : 591 – 4 . https://doi.org/10.1002/anie.201610268 Google Scholar CrossRef Search ADS 22. Nihei M , Okamoto Y , Sekine Y et al. A light-induced phase exhibiting slow magnetic relaxation in a cyanide-bridged [Fe4Co2] complex . Angew Chem Int Ed 2012 ; 51 : 6361 – 4 . https://doi.org/10.1002/anie.201202225 Google Scholar CrossRef Search ADS 23. Ohkoshi SI , Tokoro H , Matsuda T et al. Coexistence of ferroelectricity and ferromagnetism in a rubidium manganese hexacyanoferrate . Angew Chem Int Ed 2007 ; 46 : 3238 – 41 . https://doi.org/10.1002/anie.200604452 Google Scholar CrossRef Search ADS 24. Hu J-X , Luo L , Lv X-J et al. Light-induced bidirectional metal-to-metal charge transfer in a linear Fe2Co complex . Angew Chem Int Ed 2017 ; 56 : 7663 – 8 . https://doi.org/10.1002/anie.201703768 Google Scholar CrossRef Search ADS 25. Zhang J-P , Liao P-Q , Zhou H-L et al. Single-crystal X-ray diffraction studies on structural transformations of porous coordination polymers . Chem Soc Rev 2014 ; 43 : 5789 – 814 . https://doi.org/10.1039/C4CS00129J Google Scholar CrossRef Search ADS PubMed 26. Kole GK , Vittal JJ . Solid-state reactivity and structural transformations involving coordination polymers . Chem Soc Rev 2013 ; 42 : 1755 – 75 . https://doi.org/10.1039/C2CS35234F Google Scholar CrossRef Search ADS PubMed 27. Schneemann A , Bon V , Schwedler I et al. Flexible metal–organic frameworks . Chem Soc Rev 2014 ; 43 : 6062 – 96 . https://doi.org/10.1039/C4CS00101J Google Scholar CrossRef Search ADS PubMed 28. Li C-P , Chen J , Liu C-S et al. Dynamic structural transformations of coordination supramolecular systems upon exogenous stimulation . Chem Commun 2015 ; 51 : 2768 – 81 . https://doi.org/10.1039/C4CC06263A Google Scholar CrossRef Search ADS 29. Ito H , Muromoto M , Kurenuma S et al. Mechanical stimulation and solid seeding trigger single-crystal-to-single-crystal molecular domino transformations . Nat Commun 2013 ; 4 : 2009 . https://doi.org/10.1038/ncomms3009 Google Scholar CrossRef Search ADS PubMed 30. Liu D-H , Liu T-F , Chen Y-P et al. A reversible crystallinity-preserving phase transition in metal–organic frameworks: discovery, mechanistic studies, and potential applications . J Am Chem Soc 2015 ; 137 : 7740 – 6 . https://doi.org/10.1021/jacs.5b02999 Google Scholar CrossRef Search ADS PubMed 31. Berlinguette CP , Dragulescu-Andrasi A , Sieber A et al. A charge-transfer-induced spin transition in a discrete complex: the role of extrinsic factors in stabilizing three electronic isomeric forms of a cyanide-bridged Co/Fe cluster . J Am Chem Soc 2005 ; 127 : 6766 – 79 . https://doi.org/10.1021/ja043162u Google Scholar CrossRef Search ADS PubMed 32. Liu T , Zhang Y-J , Kanegawa S et al. Water-switching of spin transitions induced by metal-to-metal charge transfer in a microporous framework . Angew Chem Int Ed 2010 ; 49 : 8645 – 8 . https://doi.org/10.1002/anie.201002881 Google Scholar CrossRef Search ADS 33. Cao L , Tao J , Gao Q et al. Selective on/off switching at room temperature of a magnetic bistable {Fe2Co2} complex with single crystal-to-single crystal transformation via intramolecular electron transfer . Chem Commun 2014 ; 50 : 1665 – 7 . https://doi.org/10.1039/C3CC48116F Google Scholar CrossRef Search ADS 34. Wei R-J , Nakahara R , Cameron JM et al. Solvent-induced on/off switching of intramolecular electron transfer in a cyanide-bridged trigonal bipyramidal complex . Dalton Trans 2016 ; 45 : 17104 – 7 . https://doi.org/10.1039/C6DT03416K Google Scholar CrossRef Search ADS PubMed 35. De S , Jiménez JR , Li YL et al. One synthesis: two redox states. Temperature-oriented crystallization of a charge transfer {Fe2Co2} square complex in a {FeIILSCoIIILS}2 diamagnetic or {FeIIILSCoIIHS}2 paramagnetic state . RSC Adv 2016 ; 6 : 17456 – 9 . https://doi.org/10.1039/C6RA00191B Google Scholar CrossRef Search ADS 36. Zheng C-Y , Xu J-P , Yang Z-X et al. Factors impacting electron transfer in cyano-bridged {Fe2Co2} clusters . Inorg Chem 2015 ; 54 : 9687 – 9 . https://doi.org/10.1021/acs.inorgchem.5b02272 Google Scholar CrossRef Search ADS PubMed 37. Sagara Y , Yamane S , Mitani M et al. Mechanoresponsive luminescent molecular assemblies: an emerging class of materials . Adv Mater 2016 ; 28 : 1073 – 95 . https://doi.org/10.1002/adma.201502589 Google Scholar CrossRef Search ADS PubMed 38. Janiak C . A critical account on π–π stacking in metal complexes with aromatic nitrogen-containing ligands . J Chem Soc, Dalton Trans 2000 ; 3885 – 96 . https://doi.org/10.1039/b003010o 39. Siretanu D , Li D-F , Buisson L et al. Controlling thermally induced electron transfer in cyano-bridged molecular squares: from solid state to solution . Chem Eur J 2011 ; 17 : 11704 – 8 . https://doi.org/10.1002/chem.201102042 Google Scholar CrossRef Search ADS PubMed © The Author(s) 2018. Published by Oxford University Press on behalf of China Science Publishing & Media Ltd. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/about_us/legal/notices) http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png National Science Review Oxford University Press

# Manipulation of successive crystalline transformations to control electron transfer and switchable functions

, Volume 5 (4) – Jul 1, 2018
9 pages

Loading next page...

/lp/ou_press/manipulation-of-successive-crystalline-transformations-to-control-0LH5R0F0KR
Publisher
Oxford University Press
Copyright
© The Author(s) 2018. Published by Oxford University Press on behalf of China Science Publishing & Media Ltd.
ISSN
2095-5138
eISSN
2053-714X
D.O.I.
10.1093/nsr/nwy033
Publisher site
See Article on Publisher Site

### Abstract

ABSTRACT Electron transfer in solid is crucial to switchable magnetic, electrical, optical and mechanical properties. However, it is a formidable challenge to control electron-transfer behaviors via manipulation of crystalline phases, especially through dynamic crystalline transformation. Herein, three crystalline phases of an {Fe2Co2} compound were obtained via enhancement of intermolecular π···π interactions inducing successive single-crystal-to-single-crystal transformations, from solvated 1·2CH3OH·4H2O, to desolvated 1 and its polymorph 1a accompanying electron transfer. 1·2CH3OH·4H2O showed thermally induced reversible intermetallic electron transfer in mother liquor. No electron-transfer behavior was observed in 1. 1a showed reversible intermetallic electron transfer upon thermal treatment or alternative irradiation with 808- and 532-nm lasers at cryogenic temperatures. The electron-transfer behaviors significantly change the magnetic and optical properties, providing a strategy to realize different electron-transfer behaviors and switchable functions via π···π interactions manipulated dynamic crystalline transformation. electron-transfer, dynamic structural transformation, successive crystalline transformations, reversible, polymorphs INTRODUCTION Electron transfer is a common phenomenon in nature and plays important roles in biology, energy, materials, catalysis and other fields [1–4]. Intermetallic electron transfer not only changes the valence states and electron configurations of the participant metal ions, but also switches the coupling interactions between them [2,5–7]. Therefore, the control of electron transfer is an efficient way to tune the magnetic, electric and optical properties of materials [8–22]. Thermally and/or photo-induced electron transfers have been utilized to induce paramagnetic and diamagnetic transformations, presenting photo-switchable magnet behavior [11–13]. Moreover, the polarity and dielectric properties can be switched by utilizing electron-transfer-induced changes in charge distribution [23,24]. Typical examples showing external stimuli-tuned intermetallic electron-transfer behaviors are cyanide-bridged complexes, wherein the electron-transfer behaviors depend on the metallocyanate building blocks and ancillary ligands [11]. The modulation of electron-transfer behavior requires chemical modification, such as ligand substitution, anion and solvent exchange in solution reaction [11,17,20]. It is a formidable challenge to realize different electron-transfer behaviors by manipulating dynamic structural transformations in the solid state, especially through physical stimuli-induced single-crystal-to-single-crystal (SCSC) transformations. Such transformation processes usually involve the movement of atoms in the crystal and rearrangement of chemical bonds, which result in drastic changes in not only the molecular structure, but also the physical/chemical properties [25–28]. On the other hand, SCSC transformations can provide access to compounds that are difficult or impossible to be directly obtained by solution reactions [28]. More importantly, the procedure of SCSC transformations can directly and accurately provide a molecular-level understanding of the mechanism of the transformation, and help to gain more insights into the correlation between the structures and properties [29,30]. It has been reported that two crystalline phases with different electron-transfer behaviors could be obtained upon solvation and desolvation, because hydrogen-bonding interactions between coordinated solvents and the framework can tune the redox potentials of metal ions [31–36]. However, the formation and breakage of hydrogen bonds could only induce one-step SCSC transformation and generate two crystalline phases with different electron-transfer behaviors [32–36]. Moreover, the crystallinity is often disrupted owing to breakage of the hydrogen-bonding interactions that are important to maintain the integrity of the crystalline framework [25,31], restricting further investigation of the electron-transfer mechanism. How to induce successive crystalline transformation to obtain more than two crystalline phases with different electron-transfer behaviors is interesting but still remains a challenge, especially for polymorphs with different electron-transfer behaviors. To induce successive SCSC transformations, introducing flexible π···π interactions may provide a rational strategy [9,37], as the distances of π···π interactions can be modulated in a continuous range (3.3–3.8 Å) [38]. Moreover, the variations in π···π interactions between ligands around the metal ions can induce different distortions of coordination spheres and the strength of the ligand field, which can tune the redox potential of metal centers and result in different electron-transfer behaviors. Herein, we were intrigued by the possibility of introducing intermolecular cooperative π···π interactions to manipulate successive crystalline transformations to obtain different crystalline phases featuring different electron-transfer behaviors. Inspired by this, we adopt an ancillary ligand prazino[2, 3-f][1, 10]phenanthroline (dpq), with an extended π-conjugation system, to prepare a tetranuclear {Fe2Co2} compound [FeII(PzTp)(CN)3]2CoIII2(dpq)4 · 2ClO4 ·2CH3OH ·4H2O (1·2CH3OH ·4H2O, PzTp = tetrakis (pyrazolyl)borate). 1 ·2CH3OH ·4H2O undergoes successive SCSC transformations as a result of enhancement of intermolecular π···π interactions in the process of desolvation and structural rearrangement, forming a pair of polymorphs [FeIII(PzTp)(CN)3]2CoII2(dpq)4·2ClO4 (1) and [FeII(PzTp)(CN)3]2CoIII2(dpq)4·2ClO4 (1a). The three crystalline phases present different electron-transfer behaviors upon thermal treatment and light irradiation. Especially, it is the first time that crystalline transformation between a pair of polymorphs in intermetallic electron-transfer-related compounds has been observed. RESULTS Crystal structure of 1·2CH3OH·4H2O and its successive crystalline transformations Single-crystal X-ray diffraction analysis revealed that 1·2CH3OH·4H2O crystallizes in the triclinic space group P$$\overline{1}$$ (see Supplementary Table 1). The crystal structure consists of cationic tetranuclear {Fe2Co2}2+ square units, ClO4– anions, uncoordinated methanol and water molecules (Fig. 1). At 298 K, the Fe–C and Co–N bond lengths are 1.877(7)–1.889(7) and 1.879(5)–1.946(5) Å, respectively (see Supplementary Table 2), which are consistent with those observed for {FeIILS(μ-CN)CoIIILS} (LS = low spin) linkages [17,33]. When the green crystals are slowly heated to 360 K in the mother liquor, a dramatic color change from green to red is observed, indicating a possible transformation to {FeIIILS(μ-CN)CoIIHS} (HS = high spin) linkages. On cooling to 298 K, the crystals return to the initial green color. This phenomenon indicates that 1·2CH3OH·4H2O undergoes a thermally induced reversible electron transfer in the mother liquor (see Supplementary Movie 1). Figure 1. View largeDownload slide Crystal structures of 1·2CH3OH·4H2O, 1 and 1a. The hydrogen atoms and ClO4− anions are omitted for clarity. Fe, dark yellow; Co, turquoise; C, gray; N, blue; B, orange; O, red. Figure 1. View largeDownload slide Crystal structures of 1·2CH3OH·4H2O, 1 and 1a. The hydrogen atoms and ClO4− anions are omitted for clarity. Fe, dark yellow; Co, turquoise; C, gray; N, blue; B, orange; O, red. There are two important structural characteristics in 1·2CH3OH·4H2O. The first is that uncoordinated water and methanol molecules are located between the {Fe2Co2}2+ square units, with hydrogen-bonding interactions between uncoordinated water molecules and terminal cyanide nitrogen atoms (see Supplementary Fig. 1 and Table 4). The second is that the {Fe2Co2}2+ square units are linked via π···π interactions (average distance = 3.753(2) Å) between the dpq ligands and C–H···π interactions (average distance = 3.148(1) Å) between the C–H moieties of the dpq/PzTp ligands and the pyrazol/pyrazine rings of the PzTp/dpq ligands (see Supplementary Fig. 2 and Table 5). These intermolecular interactions are very important to stabilize the crystalline framework and make it possible to undergo SCSC transformations upon desolvation. Furthermore, the formation and destruction of hydrogen bonding can significantly affect the redox potential of the iron centers [31,33], providing the possibility of desolvation-induced electron transfer. Thus, two crystalline phases with different electron-transfer behaviors are reasonably expected. The TGA (see Supplementary Fig. 3) of 1·2CH3OH·4H2O was measured to explore the possibility of desolvation-induced SCSC transformation. The plot shows a weight loss of 6.0% from 300 to 360 K, corresponding well to the loss of two methanol and four water molecules (calcd: 6.2%). After this weight loss, a long plateau is observed until 540 K, suggesting the formation of a new stable phase. When the green crystals are slowly heated to ∼350 K in air, the color of the crystals changes from green to red, indicating the formation of a new redox state. Moreover, the crystallinity is well retained (Fig. 2 and see Supplementary Movie 2). The crystallographic data demonstrate that the red crystals retain the {Fe2Co2} tetranuclear structure and have the formula of [Fe(PzTp)(CN)3]2Co2(dpq)4·2ClO4 (1, Fig. 1). At 298 K, the Fe–C and Co–N bond lengths are 1.925(8)–1.944(9) and 2.097(7)–2.169(7) Å, respectively (see Supplementary Table 2), corresponding with those observed for {FeIIILS(μ-CN)CoIIHS} linkages [17,39]. These results indicate that electron transfer occurs in the desolvation process with a transformation from {FeIILS(μ-CN)CoIIILS} linkages in 1·2CH3OH·4H2O to {FeIIILS(μ-CN)CoIIHS} linkages in 1. Furthermore, an endothermic peak is seen in the DSC curve of 1·2CH3OH·4H2O, further confirming the first-order phase transition from 1·2CH3OH·4H2O to 1 (see Supplementary Fig. 4). With the transformation from 1·2CH3OH·4H2O to 1, the average π···π interaction distance between the dpq ligands decreases from 3.753(2) to 3.664(1) Å, and the average C–H···π interaction distance between the C–H moieties of the dpq/PzTp ligands and the pyrazol rings of the PzTp ligands decreases from 3.148(1) to 3.102(1) Å. Such results suggest that the intermolecular interactions are enhanced in the process of crystalline transformation (see Supplementary Fig. 5 and Table 6). Figure 2. View largeDownload slide Photographic images showing the SCSC transformation. Images show the conversions among these three species through subsequent desolvation and vapor induction. Figure 2. View largeDownload slide Photographic images showing the SCSC transformation. Images show the conversions among these three species through subsequent desolvation and vapor induction. Interestingly, when the red crystals of 1 were placed in water vapor and heated at 100°C for 24 h, the color of the crystals changed from red to green (Fig. 2, see Supplementary Fig. 6), suggesting the formation of a new phase, as confirmed by single-crystal X-ray diffraction (Fig. 1) and powder XRD analyses (see Supplementary Fig. 7). The new phase exhibits a composition consistent with the formula [Fe(PzTp)(CN)3]2Co2(dpq)4·2ClO4 (1a). Although 1 and 1a are a pair of polymorphs, they exhibit different electron-transfer behaviors. For 1a at 298 K, the Fe–C and Co–N bond lengths are 1.852(4)–1.914(4) and 1.877(3)–1.949(3) Å, respectively (see Supplementary Table 2), indicating the existence of {FeIILS(μ-CN)CoIIILS} linkages. When the crystals of 1a are slowly heated to 350 K, a dramatic color change from green to red is observed. The corresponding Co–N bond lengths are 2.077(4)–2.145(3) Å (see Supplementary Table 2), indicating the formation of {FeIIILS(μ-CN)CoIIHS} linkages [17,39]. Moreover, the crystals return to the initial green color on cooling to 298 K, indicating the thermally induced reversible intermetallic electron transfer. With the transformation from 1 to 1a, the average π···π interaction distance between the dpq ligands decreases from 3.664(1) to 3.457(1) Å at room temperature, and the average C–H···π interaction distance between the C–H moieties of the dpq/PzTp ligands and the pyrazol/pyridine rings of the PzTp/dpq ligands decreases from 3.102(1) to 3.002(1) Å, leading to the enhanced intermolecular interactions (see Supplementary Fig. 8 and Table 7). When heated to 350 K, both the average π···π interaction and C–H···π interaction distances of 1a increase slightly (see Supplementary Fig. 9 and Table 8). Magnetic characterization The magnetic properties of the three crystalline phases were subsequently investigated (Fig. 3). The χT versus T curve for 1·2CH3OH·4H2O in mother liquor shows that χT values remain essentially constant at 0.46 cm3 mol−1 K below 355 K, confirming the existence of {FeIILS(μ-CN)CoIIILS} linkages. Upon heating, χT values abruptly increase to 6.44 cm3 mol−1 K at 390 K, which are in agreement with the theoretical value of 6.67 cm3 mol−1 K expected for two LS FeIII and two HS CoII ions [16]. When the temperature is lowered from 400 K, χT values decrease rapidly to its initial value at 355 K with a small thermal hysteresis loop of ∼8 K, indicating that the system regains the {FeIILS(μ-CN)CoIIILS} linkages. Thus, 1·2CH3OH·4H2O shows thermally induced reversible intermetallic electron transfer upon thermal treatment in the mother liquor. Figure 3. View largeDownload slide Magnetic characteristics for 1·2CH3OH·4H2O, 1 and 1a. Plots of χT vs temperature for 1·2CH3OH·4H2O, 1 and 1a. 1·2CH3OH·4H2O was soaked in the mother liquor upon cooling, and cycling the temperature back to 400 K; 1 upon cooling; 1·2CH3OH·4H2O upon heating and concomitant desorption of methanol and water; 1a upon cooling and reheating to 400 K (temperature sweeping rate: 1 K/min for 10–400 K and 0.5 K/min for 2–10 K). Figure 3. View largeDownload slide Magnetic characteristics for 1·2CH3OH·4H2O, 1 and 1a. Plots of χT vs temperature for 1·2CH3OH·4H2O, 1 and 1a. 1·2CH3OH·4H2O was soaked in the mother liquor upon cooling, and cycling the temperature back to 400 K; 1 upon cooling; 1·2CH3OH·4H2O upon heating and concomitant desorption of methanol and water; 1a upon cooling and reheating to 400 K (temperature sweeping rate: 1 K/min for 10–400 K and 0.5 K/min for 2–10 K). However, 1 exhibits a different electron-transfer behavior. Its χT value is 6.51 cm3 mol−1 K at 400 K (Fig. 3), corresponding to paramagnetic {FeIIILS(μ-CN)CoIIHS} linkages. As the temperature is lowered, χT values gradually decrease, reaching a minimum value of 5.36 cm3 mol−1 K at 24 K. Below this temperature, the χT value increases up to 5.44 cm3 mol−1 K at 14 K and then decreases rapidly to 3.50 cm3 mol−1 K at 2 K. This represents a typical paramagnetic behavior without electron-transfer-induced spin transition. The transformation of the magnetic behavior in the desolvation process was monitored for as-synthesized crystals of 1·2CH3OH·4H2O. χT values remain essentially constant at 0.36 cm3 mol−1 K below 310 K and reach a maximum value of 6.43 cm3 mol−1 K at 360 K, which is in agreement with paramagnetic {FeIIILS(μ-CN)CoIIHS} linkages as observed for 1. Such magnetic behavior confirms that the electron transfer occurs in the desolvation process from 1·2CH3OH·4H2O to 1. This electron-transfer process seems analogous to that observed during the heating of 1·2CH3OH·4H2O in mother liquor, but with a lower transition temperature. This is due to the difference in the intermetallic electron-transfer mechanism, as the former stems from the loss of solvent molecules, while the latter arises from thermal stimuli. For 1a, χT values remain nearly constant between 2 and 315 K at 0.43 cm3 mol−1 K (Fig. 3). Heating from 315 to 360 K causes an increase in the χT value to 6.47 cm3 mol−1 K. The χT value returns to its initial value with a small thermal hysteresis ∼5 K wide, showing reversible electron-transfer behavior that involves transformation between the diamagnetic {FeIILS(μ-CN)CoIIILS} (LT phase = low temperature phase) linkages and the paramagnetic {FeIIILS(μ-CN)CoIIHS} (HT phase = high temperature phase) linkages. Consistently with magnetic data, endothermic/exothermic peaks for 1a are observed with Tmax= 331.2 and 325.3 K, indicating the occurrence of the first-order phase transition (see Supplementary Fig. 10). Optical studies The solid-state UV–vis–NIR absorption spectra of 1·2CH3OH·4H2O, 1 and 1a were measured at room temperature to study their color changes and to further support the electronic state assignments deduced from the structural and magnetic analyses (Fig. 4a). The absorption spectra of 1·2CH3OH·4H2O and 1a are similar, presenting bands at 427 and 763 nm, respectively. The absorption band at 427 nm may be assigned to ligand-to-metal charge transfer (LMCT) of the FeII chromophore [17]. The broad band at 763 nm can be assigned as the FeII → CoIII intervalence charge-transfer (IVCT) band [17,35]. In contrast, a broad absorption band at 459 nm with a small shoulder band at 534 nm is observed for 1. The higher-energy band at 459 nm is assigned to a spin- and Laporte-allowed LMCT transition, and the small shoulder band at 534 nm can be assigned to the CoII → FeIII IVCT band [39]. These results confirm that 1·2CH3OH·4H2O and 1a possess the diamagnetic {FeIILS(μ-CN)CoIIILS} linkages, whereas 1 possesses the paramagnetic {FeIIILS(μ-CN)CoIIHS} linkages at room temperature. Figure 4. View largeDownload slide Optical spectra for 1·2CH3OH·4H2O, 1 and 1a. (a) Solid-state UV–vis spectra for 1·2CH3OH·4H2O, 1 and 1a at room temperature. (b) Variable-temperature solid-state UV–vis spectra for 1a in heating (298→360 K) mode. Figure 4. View largeDownload slide Optical spectra for 1·2CH3OH·4H2O, 1 and 1a. (a) Solid-state UV–vis spectra for 1·2CH3OH·4H2O, 1 and 1a at room temperature. (b) Variable-temperature solid-state UV–vis spectra for 1a in heating (298→360 K) mode. Variable-temperature solid-state UV–vis–NIR absorption spectra of 1a were measured in the temperature range of 298–360 K (Fig. 4b). As the temperature increases, there is a gradual decrease in the broad absorption centered at 763 nm for the characteristic band of the FeII → CoIII IVCT, and the LMCT/MLCT band at 427 nm is shifted to the lower-energy region associated with the appearance of the CoII → FeIII IVCT band at 527 nm. The observed spectral change confirms the occurrence of thermally induced electron transfer with the transformation from the {FeIILS(μ-CN)CoIIILS} to {FeIIILS(μ-CN)CoIIHS} linkages. Photomagnetic characterization The photomagnetic effects of 1a were examined to determine the possibility of photo-induced electron transfer. Because an FeII → CoIII IVCT band was observed at 763 nm for the LT phase of 1a, an 808 nm laser was selected to stimulate the FeII → CoIII IVCT band and used for photomagnetic experiments. When the sample is irradiated at 20 K for 120 min, χT values rapidly increase and reach a maximum value of 5.61 cm3 mol−1 K (Fig. 5a). When the sample is heated from 2 K after irradiation, χT values first increase steeply to a sharp maximum of 6.38 cm3 mol−1 K at 8.2 K, indicating an almost complete conversion from diamagnetic {FeIILS(μ-CN)CoIIILS} to paramagnetic {FeIIILS(μ-CN)CoIIHS} linkages (Fig. 5b). The increase in χT values from 2 to 8.2 K is attributed to the presence of intermolecular antiferromagnetic interactions and/or zero field splitting [16]. Upon further heating to 73 K, the photo-induced metastable paramagnetic {FeIIILS(μ-CN)CoIIHS} linkages relax to the initial diamagnetic {FeIILS(μ-CN)CoIIILS} linkages, indicating that magnetization can be increased by light irradiation and recovered with thermal treatment. Figure 5. View largeDownload slide Photoreversibility of the magnetization for 1a. (a) Plots of χT vs time under cycles of successive irradiation at 808 and 532 nm at 20 K for 1a. (b) Plots of χT vs temperature for 1a before irradiation, after irradiation at 808 nm, the metastable-induced state irradiated at 532 nm, and after thermal annealing treatment at 100 K. Figure 5. View largeDownload slide Photoreversibility of the magnetization for 1a. (a) Plots of χT vs time under cycles of successive irradiation at 808 and 532 nm at 20 K for 1a. (b) Plots of χT vs temperature for 1a before irradiation, after irradiation at 808 nm, the metastable-induced state irradiated at 532 nm, and after thermal annealing treatment at 100 K. On the basis of the optical studies, 1a in HT phase displays a CoII → FeIII IVCT band at 527 nm. The photo-induced metastable phase after irradiation with an 808 nm laser was further irradiated with a 532 nm laser in order to investigate the photo-induced reversibility. As a result, χT values decrease from 5.61 to 2.27 cm3 mol−1 K after 120 min irradiation at 20 K (Fig. 5b), indicating 59.5% recovery of the diamagnetic state. This successive photoreversibility of the magnetization can be well repeated (Fig. 5a and see Supplementary Fig. 11). This magnetic change verifies the occurrence of photo-induced reversible electron transfer in 1a. The relaxation of the photo-induced metastable state was monitored at different temperatures in order to probe the stability of the photo-induced phases (see Supplementary Fig. 12). In the low-temperature (10–40 K) region, relaxation time τ was less dependent on temperature. This result additionally confirms that the photoreversibility changes were induced by light rather than the thermal relaxation effect. IR spectra The IR spectra analysis of 1·2CH3OH·4H2O, 1 and 1a also supports the redox state assignments deduced from the structural and magnetic analyses. For 1·2CH3OH·4H2O, the typical absorption bands of cyanide groups (2107, 2089 and 2071 cm−1, see Supplementary Fig. 13) indicate that the compound possesses the diamagnetic {FeIILS(μ-CN)CoIIILS} linkages [17,33]. Three absorption bands for cyanide groups are observed in the IR spectrum of 1 (see Supplementary Fig. 13), corresponding to the stretching vibrations for the bridging cyanide ions in the {FeIIILS(μ-CN)CoIIHS} linkages (2148 and 2143 cm−1) and the terminal cyanide ions in the [PzTpFeIII(CN)3]– anions (2122 cm−1) [20]. At 300 K, 1a shows absorption bands at 2069, 2085 and 2106 cm−1, confirming that 1a possesses the {FeIILS(μ-CN)CoIIILS} linkages in the LT phase (see Supplementary Fig. 14). When the temperature increases to 360 K, the intensity of the cyanide stretching bands for the {FeIILS(μ-CN)CoIIILS} linkages decreases, and new bands for the {FeIIILS(μ-CN)CoIIHS} linkages appear at 2150 and 2159 cm−1 as seen in 1. Furthermore, upon cooling to room temperature, the IR spectra of 1a return to their initial state, suggesting that the thermally induced intermetallic electron transfer is reversible. The irradiation-time dependence of the IR spectra was measured to further verify the occurrence of photo-induced reversible electron transfer in 1a upon successive and alternative irradiation with 808 and 532 nm lasers at 20 K (Fig. 6). Upon irradiation at 808 nm, the intensity of the cyanide stretching bands attributed to the {FeIILS(μ-CN)CoIIILS} linkages decreases and a new peak appears at 2162 cm−1, which is attributed to the {FeIIILS(μ-CN)CoIIHS} linkages. The intensity of the new peak gradually increases with irradiation time. Conversely, the photo-induced metastable phase was irradiated at 532 nm, inducing a decrease in the intensity of the cyanide stretching band attributed to the {FeIIILS(μ-CN)CoIIHS} linkages, and the intensity of the peaks attributed to the {FeIILS(μ-CN)CoIIILS} linkages increases. By normalization of the peak intensities for the {FeIIILS(μ-CN)CoIIHS} linkages vs irradiation time upon irradiation at 808 and 532 nm, the recovery of the {FeIILS(μ-CN)CoIIILS} linkages is estimated to be 60.0%, which is comparable to the value obtained from photomagnetic measurements (see Supplementary Fig. 15). Therefore, the observed spectral changes further confirm the occurrence of photo-induced reversible electron transfer with interconversion between {FeIIILS(μ-CN)CoIIHS} and {FeIILS(μ-CN)CoIIILS} linkages through successive and alternative irradiation at 808 and 532 nm. Figure 6. View largeDownload slide Photo-induced IR spectra for 1a. Irradiation-time dependence of the IR spectra for 1a irradiated at 808 nm (a) and the photoreversibility upon irradiation at 532 nm (b) at 20 K. Figure 6. View largeDownload slide Photo-induced IR spectra for 1a. Irradiation-time dependence of the IR spectra for 1a irradiated at 808 nm (a) and the photoreversibility upon irradiation at 532 nm (b) at 20 K. Structure and property discussion Air-stable 1a can be obtained by two-step-wise SCSC transformations from the air-unstable 1·2CH3OH·4H2O. This unprecedented successive single-crystalline transformations-induced electron transfer inspired us to investigate their structural correlations. First, the structure of 1·2CH3OH·4H2O indicates that hydrogen-bonding interactions are formed between the nitrogen atom of the terminal cyanide and an uncoordinated water molecule with the N···O distance of 2.713(12) Å (see Supplementary Fig. 1 and Table 4). Hydrogen-bonding strength can have a significant effect on redox potential and may have a significant effect on the intermetallic electron-transfer behavior. Hydrogen bonding as an electron-withdrawing effect can result in the positive shifting of FeII redox potential [31,33]. FeII ions are more stable in this situation. Thus, 1·2CH3OH·4H2O exhibits stable {FeIILS(μ-CN)CoIIILS} linkages in the solvated phase. When 1·2CH3OH·4H2O loses the solvent molecules and transforms into the desolvated phase 1, the hydrogen bonding is destroyed, accompanied by a negative shift in the redox potential of FeII. The {FeIILS(μ-CN)CoIIILS} linkages become unstable and tend to transform into the {FeIIILS(μ-CN)CoIIHS} linkages. Therefore, the electron transfer from 1·2CH3OH·4H2O to 1 occurs as a result of thermally induced desolvation. Second, the average distances of π···π and C−H···π interactions between the adjacent dpq/PzTp ligands are 3.753 and 3.148 Å for 1·2CH3OH·4H2O, 3.665 and 3.102 Å for 1 and 3.457 and 3.002 Å for 1a, indicating that the interactions gradually become stronger from 1·2CH3OH·4H2O to 1 and 1a. The enhancement of intermolecular interactions increases the synergistic effects between molecules and causes an energy decrease to a more stable structure, providing an important driving force for the successive two-step irreversible SCSC transformations. In comparison, only polycrystalline powder 1a can be obtained directly using solvothermal conditions at 100°C starting from the precursors, as confirmed by powder XRD analyses (see Supplementary Fig. 16). This results show the importance of successive crystalline transformations. In addition, the coordination spheres of cobalt centers bring out different distortion accompanying the changes in intermolecular interactions (C–H···π and π···π) between dpq ligands. This can be observed directly in the deviation of the Co–N≡C bond angles (see Supplementary Table 3). Compared with the bond angles [171.8(5) and 168.6(6)°] in 1·2CH3OH·4H2O, the difference value (8.9°) of the bond angles [171.1(6) and 162.2(7)°] in 1 is larger, indicating larger deviation from the ideal octahedron for the latter. For 1a, the bond angles [166.1(3) and 166.7(3)°] are nearly the same, which suggests that the distortion of the CoN6 octahedron becomes smaller. Continuous shape measurements (CShM) analysis and the parameter Σ (the sum of $$|90-\alpha|$$ for the 12 cis-N–Co–N angles around the Co atoms) were also calculated (see Supplementary Table 2). The CShM of cobalt is 0.178 (1·2CH3OH·4H2O), 1.014 (1), 0.276 (LT phase of 1a) and 1.216 (HT phase of 1a), respectively. A smaller value is generally associated with a stronger ligand field, leading to an LS state of the metal ion, whereas the larger value corresponds to a weaker ligand field and support an HS state. From 1·2CH3OH·4H2O to 1 and 1a, the values first increase and then decrease, which suggests that the strength of the ligand field changes from strong to weak and back to strong. Thus, the Co ion exhibits HS state in 1 and the LS state in 1·2CH3OH·4H2O and 1a. This unusual behavior serves to illustrate how subtle change in C–H···π and π···π interactions can actually have a profound effect on SCSC transformations and electron-transfer properties. CONCLUSION The enhancement of intermolecular π···π interactions drives successive single-crystalline transformations and achieves three crystalline phases with different electron-transfer behaviors. Especially, this is the first time that the crystalline transformation between two polymorphs in intermetallic electron-transfer-related compounds has been observed, providing an ideal platform to study the effect of intermolecular π···π interactions on crystalline-transformation-induced change in electron-transfer behaviors. The introduction of π···π interactions not only provides a strategy to manipulate the crystalline phases, but also offers access to construct switchable multifunctional materials displaying stimuli-induced dynamic-changes functions in the future. METHODS The detailed preparation and characteristic methods of materials are available as Supplementary data at NSR online. SUPPLEMENTARY DATA Supplementary data are available at NSR online. FUNDING This work was partly supported by the National Natural Science Foundation of China (91422302, 21421005 and 21322103) and the Fundamental Research Funds for the Central Universities, China. REFERENCES 1. Salvador JR , Guo F , Hogan T et al. Zero thermal expansion in YbGaGe due to an electronic valence transition . Nature 2003 ; 425 : 702 – 5 . https://doi.org/10.1038/nature02011 Google Scholar CrossRef Search ADS PubMed 2. Long Y-W , Hayashi N , Saito T et al. Temperature-induced A–B intersite charge transfer in an A-site-ordered LaCu3Fe4O12 perovskite . Nature 2009 ; 458 : 60 – 3 . https://doi.org/10.1038/nature07816 Google Scholar CrossRef Search ADS PubMed 3. Akimov AV , Neukirch AJ , Prezhdo OV . Theoretical insights into photoinduced charge transfer and catalysis at oxide interfaces . Chem Rev 2013 ; 113 : 4496 – 565 . https://doi.org/10.1021/cr3004899 Google Scholar CrossRef Search ADS PubMed 4. Bernardo B , Cheyns D , Verreet B et al. Delocalization and dielectric screening of charge transfer states in organic photovoltaic cells . Nat Commun 2014 ; 5 : 3245 . https://doi.org/10.1038/ncomms4245 Google Scholar CrossRef Search ADS PubMed 5. Horiuchi S , Okimoto Y , Kumai R et al. Quantum phase transition in organic charge-transfer complexes . Science 2003 ; 299 : 229 – 32 . https://doi.org/10.1126/science.1076129 Google Scholar CrossRef Search ADS PubMed 6. Alves H , Molinari AS , Xie H-X et al. Metallic conduction at organic charge-transfer interfaces . Nat Mater 2008 ; 7 : 574 – 80 . https://doi.org/10.1038/nmat2205 Google Scholar CrossRef Search ADS PubMed 7. Huang Z-X , Auckett JE , Blanchard PER et al. Pressure-induced intersite Bi–M (M = Ru, Ir) valence transitions in hexagonal perovskites . Angew Chem Int Ed 2014 ; 53 : 3414 – 7 . https://doi.org/10.1002/anie.201311159 Google Scholar CrossRef Search ADS 8. Sato O , Tao J , Zhang Y-Z . Control of magnetic properties through external stimuli . Angew Chem Int Ed 2007 ; 46 : 2152 – 87 . https://doi.org/10.1002/anie.200602205 Google Scholar CrossRef Search ADS 9. Sato O . Dynamic molecular crystals with switchable physical properties . Nat Chem 2016 ; 8 : 644 – 56 . https://doi.org/10.1038/nchem.2547 Google Scholar CrossRef Search ADS PubMed 10. Jeen H , Choi WS , Biegalski MD et al. Reversible redox reactions in an epitaxially stabilized SrCoOx oxygen sponge . Nat Mater 2013 ; 12 : 1057 – 63 . https://doi.org/10.1038/nmat3736 Google Scholar CrossRef Search ADS PubMed 11. Aguilà D , Prado Y , Koumousi ES et al. Switchable Fe/Co prussian blue networks and molecular analogues . Chem Soc Rev 2016 ; 45 : 203 – 24 . https://doi.org/10.1039/C5CS00321K Google Scholar CrossRef Search ADS PubMed 12. Hoshino N , Iijima F , Newton GN et al. Three-way switching in a cyanide-bridged [CoFe] chain . Nat Chem 2012 ; 4 : 921 – 6 . https://doi.org/10.1038/nchem.1455 Google Scholar CrossRef Search ADS PubMed 13. Sato O , Iyoda T , Fujishima A et al. Photoinduced magnetization of a cobalt-iron cyanide . Science 1996 ; 272 : 704 – 5 . https://doi.org/10.1126/science.272.5262.704 Google Scholar CrossRef Search ADS PubMed 14. Berlinguette CP , Dragulescu-Andrasi A , Sieber A et al. A charge-transfer-induced spin transition in the discrete cyanide-bridged complex {[Co(tmphen)2]3[Fe(CN)6]2} . J Am Chem Soc 2004 ; 126 : 6222 – 3 . https://doi.org/10.1021/ja039451k Google Scholar CrossRef Search ADS PubMed 15. Li D-F , Clérac R , Roubeau O et al. Magnetic and optical bistability driven by thermally and photoinduced intramolecular electron transfer in a molecular cobalt-iron prussian blue analogue . J Am Chem Soc 2008 ; 130 : 252 – 8 . https://doi.org/10.1021/ja0757632 Google Scholar CrossRef Search ADS PubMed 16. Zhang Y-Z , Li D-F , Clérac R et al. Reversible thermally and photoinduced electron transfer in a cyano-bridged {Fe2Co2} square complex . Angew Chem Int Ed 2010 ; 49 : 3752 – 6 . https://doi.org/10.1002/anie.201000765 Google Scholar CrossRef Search ADS 17. Nihei M , Sekine Y , Suganami N et al. Controlled intramolecular electron transfers in cyanide-bridged molecular squares by chemical modifications and external stimuli . J Am Chem Soc 2011 ; 133 : 3592 – 600 . https://doi.org/10.1021/ja109721w Google Scholar CrossRef Search ADS PubMed 18. Mondal A , Li Y-L , Seuleiman M et al. On/off photoswitching in a cyanide-bridged {Fe2Co2} magnetic molecular square . J Am Chem Soc 2013 ; 135 : 1653 – 6 . https://doi.org/10.1021/ja3087467 Google Scholar CrossRef Search ADS PubMed 19. Koumousi ES , Jeon IR , Gao Q et al. Metal-to-metal electron transfer in Co/Fe prussian blue molecular analogues: the ultimate miniaturization . J Am Chem Soc 2014 ; 136 : 15461 – 4 . https://doi.org/10.1021/ja508094h Google Scholar CrossRef Search ADS PubMed 20. Zhang Y-Z , Ferko P , Siretanu D et al. Thermochromic and photoresponsive cyanometalate Fe/Co squares: toward control of the electron transfer temperature . J Am Chem Soc 2014 ; 136 : 16854 – 64 . https://doi.org/10.1021/ja508280n Google Scholar CrossRef Search ADS PubMed 21. Nihei M , Yanai Y , Hsu IJ et al. A hydrogen-bonded cyanide-bridged [Co2Fe2] square complex exhibiting a three-step spin transition . Angew Chem Int Ed 2017 ; 56 : 591 – 4 . https://doi.org/10.1002/anie.201610268 Google Scholar CrossRef Search ADS 22. Nihei M , Okamoto Y , Sekine Y et al. A light-induced phase exhibiting slow magnetic relaxation in a cyanide-bridged [Fe4Co2] complex . Angew Chem Int Ed 2012 ; 51 : 6361 – 4 . https://doi.org/10.1002/anie.201202225 Google Scholar CrossRef Search ADS 23. Ohkoshi SI , Tokoro H , Matsuda T et al. Coexistence of ferroelectricity and ferromagnetism in a rubidium manganese hexacyanoferrate . Angew Chem Int Ed 2007 ; 46 : 3238 – 41 . https://doi.org/10.1002/anie.200604452 Google Scholar CrossRef Search ADS 24. Hu J-X , Luo L , Lv X-J et al. Light-induced bidirectional metal-to-metal charge transfer in a linear Fe2Co complex . Angew Chem Int Ed 2017 ; 56 : 7663 – 8 . https://doi.org/10.1002/anie.201703768 Google Scholar CrossRef Search ADS 25. Zhang J-P , Liao P-Q , Zhou H-L et al. Single-crystal X-ray diffraction studies on structural transformations of porous coordination polymers . Chem Soc Rev 2014 ; 43 : 5789 – 814 . https://doi.org/10.1039/C4CS00129J Google Scholar CrossRef Search ADS PubMed 26. Kole GK , Vittal JJ . Solid-state reactivity and structural transformations involving coordination polymers . Chem Soc Rev 2013 ; 42 : 1755 – 75 . https://doi.org/10.1039/C2CS35234F Google Scholar CrossRef Search ADS PubMed 27. Schneemann A , Bon V , Schwedler I et al. Flexible metal–organic frameworks . Chem Soc Rev 2014 ; 43 : 6062 – 96 . https://doi.org/10.1039/C4CS00101J Google Scholar CrossRef Search ADS PubMed 28. Li C-P , Chen J , Liu C-S et al. Dynamic structural transformations of coordination supramolecular systems upon exogenous stimulation . Chem Commun 2015 ; 51 : 2768 – 81 . https://doi.org/10.1039/C4CC06263A Google Scholar CrossRef Search ADS 29. Ito H , Muromoto M , Kurenuma S et al. Mechanical stimulation and solid seeding trigger single-crystal-to-single-crystal molecular domino transformations . Nat Commun 2013 ; 4 : 2009 . https://doi.org/10.1038/ncomms3009 Google Scholar CrossRef Search ADS PubMed 30. Liu D-H , Liu T-F , Chen Y-P et al. A reversible crystallinity-preserving phase transition in metal–organic frameworks: discovery, mechanistic studies, and potential applications . J Am Chem Soc 2015 ; 137 : 7740 – 6 . https://doi.org/10.1021/jacs.5b02999 Google Scholar CrossRef Search ADS PubMed 31. Berlinguette CP , Dragulescu-Andrasi A , Sieber A et al. A charge-transfer-induced spin transition in a discrete complex: the role of extrinsic factors in stabilizing three electronic isomeric forms of a cyanide-bridged Co/Fe cluster . J Am Chem Soc 2005 ; 127 : 6766 – 79 . https://doi.org/10.1021/ja043162u Google Scholar CrossRef Search ADS PubMed 32. Liu T , Zhang Y-J , Kanegawa S et al. Water-switching of spin transitions induced by metal-to-metal charge transfer in a microporous framework . Angew Chem Int Ed 2010 ; 49 : 8645 – 8 . https://doi.org/10.1002/anie.201002881 Google Scholar CrossRef Search ADS 33. Cao L , Tao J , Gao Q et al. Selective on/off switching at room temperature of a magnetic bistable {Fe2Co2} complex with single crystal-to-single crystal transformation via intramolecular electron transfer . Chem Commun 2014 ; 50 : 1665 – 7 . https://doi.org/10.1039/C3CC48116F Google Scholar CrossRef Search ADS 34. Wei R-J , Nakahara R , Cameron JM et al. Solvent-induced on/off switching of intramolecular electron transfer in a cyanide-bridged trigonal bipyramidal complex . Dalton Trans 2016 ; 45 : 17104 – 7 . https://doi.org/10.1039/C6DT03416K Google Scholar CrossRef Search ADS PubMed 35. De S , Jiménez JR , Li YL et al. One synthesis: two redox states. Temperature-oriented crystallization of a charge transfer {Fe2Co2} square complex in a {FeIILSCoIIILS}2 diamagnetic or {FeIIILSCoIIHS}2 paramagnetic state . RSC Adv 2016 ; 6 : 17456 – 9 . https://doi.org/10.1039/C6RA00191B Google Scholar CrossRef Search ADS 36. Zheng C-Y , Xu J-P , Yang Z-X et al. Factors impacting electron transfer in cyano-bridged {Fe2Co2} clusters . Inorg Chem 2015 ; 54 : 9687 – 9 . https://doi.org/10.1021/acs.inorgchem.5b02272 Google Scholar CrossRef Search ADS PubMed 37. Sagara Y , Yamane S , Mitani M et al. Mechanoresponsive luminescent molecular assemblies: an emerging class of materials . Adv Mater 2016 ; 28 : 1073 – 95 . https://doi.org/10.1002/adma.201502589 Google Scholar CrossRef Search ADS PubMed 38. Janiak C . A critical account on π–π stacking in metal complexes with aromatic nitrogen-containing ligands . J Chem Soc, Dalton Trans 2000 ; 3885 – 96 . https://doi.org/10.1039/b003010o 39. Siretanu D , Li D-F , Buisson L et al. Controlling thermally induced electron transfer in cyano-bridged molecular squares: from solid state to solution . Chem Eur J 2011 ; 17 : 11704 – 8 . https://doi.org/10.1002/chem.201102042 Google Scholar CrossRef Search ADS PubMed © The Author(s) 2018. Published by Oxford University Press on behalf of China Science Publishing & Media Ltd. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/about_us/legal/notices)

### Journal

National Science ReviewOxford University Press

Published: Jul 1, 2018

## You’re reading a free preview. Subscribe to read the entire article.

### DeepDyve is your personal research library

It’s your single place to instantly
discover and read the research
that matters to you.

Enjoy affordable access to
over 18 million articles from more than
15,000 peer-reviewed journals.

All for just $49/month ### Explore the DeepDyve Library ### Search Query the DeepDyve database, plus search all of PubMed and Google Scholar seamlessly ### Organize Save any article or search result from DeepDyve, PubMed, and Google Scholar... all in one place. ### Access Get unlimited, online access to over 18 million full-text articles from more than 15,000 scientific journals. ### Your journals are on DeepDyve Read from thousands of the leading scholarly journals from SpringerNature, Elsevier, Wiley-Blackwell, Oxford University Press and more. All the latest content is available, no embargo periods. DeepDyve ### Freelancer DeepDyve ### Pro Price FREE$49/month
\$360/year

Save searches from
Google Scholar,
PubMed

Create lists to
organize your research

Export lists, citations

Read DeepDyve articles

Abstract access only

Unlimited access to over
18 million full-text articles

Print

20 pages / month

PDF Discount

20% off