Manipulating Spin Transition To Achieve Switchable Multifunctions

Article Cite This: Acc. Chem. Res. XXXX, XXX, XXX−XXX

pubs.acs.org/accounts

Manipulating Spin Transition To Achieve Switchable Multifunctions Yin-Shan Meng and Tao Liu*

Downloaded via UNIV OF MELBOURNE on April 12, 2019 at 01:12:57 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

State Key Laboratory of Fine Chemicals, Dalian University of Technology, 2 Linggong Road, Dalian 116024, P. R. China CONSPECTUS: The spin transition of metal ions involves interconversion between electron configurations exhibiting considerably different functions and plays a substantial role in the chemical, physical, and biological fields. The photoinduced spin transition offers a promising approach to tune various physical properties with high spatial and temporal resolutions for producing smart multifunctional materials not only to explore their basic science but also to satisfy the demands of the nextgeneration photoswitchable-molecule-based devices. Therefore, it is attracting considerable interest to utilize photoinduced spin transition to simultaneously tune multifunctions. However, two issues are challenging in obtaining reversible and swift manipulation of functions: (1) the interconversion between different electron configurations of photoresponsive units should be reversibly switched via photoinduced spin transition; (2) effective coupling should be built between the photoresponsive and functional units to produce photoswitchable functions utilizing photoinduced spin transition. In this Account, we will review our recent advances in the usage of spin transition of metal ions as actuators for tuning the magnetic, dielectric, fluorescence, and mechanical properties, wherein the role of a photoswitchable spin transition is highlighted. We mainly focus on the study of two spin-transition categories, including spin-crossover (SCO) of one metal ion and metal-to-metal charge transfer (MMCT). Initially, we will describe a strategy for developing photoinduced reversible SCO and MMCT. The role of flexible intermolecular interactions, in particular, π···π interactions, is discussed with respect to a photoinduced reversible MMCT. Then, the SCO and MMCT units were assembled using metallocyanate building blocks to form a chain, wherein the spin states, anisotropy, and magnetic coupling interactions can be photoswitched to tune the singlechain magnet behavior. Besides magnetic properties, the photoinduced spin transition that is associated with the concomitant changing of charge distribution, bond lengths, and absorption spectra can be utilized to tune the multifunctions. Therefore, the transfer of an electron from a central cobalt site to one of the two iron sites in linear trinuclear Fe2Co compounds resulted in the transformation of a centrosymmetric nonpolar molecule into an asymmetric polar molecule, and the molecular electric dipole and dielectric properties can be reversibly switched. Moreover, the spin transition usually involved significant expansion or contraction of the coordination sphere of metal ions because of the population/depopulation of the antibonding eg orbitals. Therefore, colossal positive and negative thermal expansion behaviors were achieved in a layered compound by manipulating the spin-transition process and the rotation of the functional units, thereby providing a strategy for synthesizing phototunable nanomotors. Photoinduced spin transition can also be used to modulate the fluorescence properties by controlling the energy transfer between the fluorescent ligands and the metal sites showing SCO. Finally, we will provide a perspective and detail the remaining challenges that are associated with this research area. We believe that an increasing number of fascinating photoswitchable SCO and MMCT systems will emerge in the near future and that the materials exhibiting various properties and functions that can be manipulated using photoinduced spin transition will provide novel opportunities for the development of smart multifunctional materials and devices.

1. INTRODUCTION

crucial: (1) the interconversion between the two states of photoresponsive units should be reversibly switched via light irradiation for ensuring the existence of bistable states with respect to binary code (0 and 1) and can, therefore, serve as elementary binary units (bits) for application in both information storage and processing; (2) further, a method should be developed for coupling the photoresponsive and functional units to produce photoswitchable functions and to

The functional molecular materials that exhibit response to external stimuli, such as light, heat, and pressure, are attracting considerable interest owing to their promising applications in switches, sensors, and storage devices.1−4 In particular, photoswitchable functions allow convenient photoirradiation with high spatial and temporal resolutions and can be used to develop smart devices for application to molecular machines,5 molecular electronics,6 liquid crystals,7 and biosystems.8 The following two issues that are associated with the practical application of photoswitchable materials are considered to be © XXXX American Chemical Society

Received: January 27, 2019

A

DOI: 10.1021/acs.accounts.9b00049 Acc. Chem. Res. XXXX, XXX, XXX−XXX

Article

Accounts of Chemical Research

different metal ions, resulting in the generation of valence isomers with different electronic configurations, which can be referred to as the electron-transfer-coupled spin transition (ETCST). Such ETCST processes have been observed in several 3d−3d, 3d−4d, and 3d−5d metal pairs, where the redox potentials of metal ions are sensitive to any change in their coordination spheres.17−19 For example, the Fe−CN−Co system exhibits ETCST with interconversion between the FeIILS−CN−CoIIILS and FeIIILS−CN−CoIIHS linkages. The FeIIILS (S = 1/2) and CoIIHS (S = 3/2) ions are paramagnetic, whereas the FeIILS (S = 0) and CoIIILS (S = 0) ions are diamagnetic, providing switchable magnetic properties.17 ETCST not only changes the spin states of respective metal sites and the coupling interactions between them but also changes the symmetry of charge distribution, resulting in drastic changes in both the magnetic and dielectric properties. Furthermore, spin transition causes significant variations in the bond lengths and absorption spectra, resulting in unusual thermal expansion and photochromic behavior. Thus, the materials that exhibit external-stimuli-tuned spin transition are considered to be excellent candidates for achieving synergistically switchable multifunctions. Recently, the use of LIST units as actuators for tuning the physical and chemical properties has emerged as a popular research topic. Although several advancements have been reported, it still remains a challenge to reversibly and synergistically manipulate multifunctions by alternating the light irradiation. In this Account, we intend to summarize our efforts to investigate the aforementioned research topic. First, we will introduce a strategy to achieve photoswitchable spin transition to generate a bistable state as a result of significant variation in the magnetization value. Further, we will introduce a building block strategy for achieving a photoswitchable nanomagnet behavior that can not only exhibit a change in magnetization value upon light irradiation but also interchange the magnetization polarization direction upon being exposed to an alternating external magnetic field. On the basis of the photoswitchable magnetic properties that are achieved via LIST, we will describe our efforts for designing photoswitchable multifunctional materials, including dielectric, mechanical, and luminescent properties. Finally, a perspective of this area will be provided.

satisfy the requirements of various applications. The design and synthesis of photoswitchable materials have exhibited considerable progress via the combination of photoresponsive units and different functional units. A majority of photoswitchable functional materials exhibit the photoisomerism of organic photochromic groups, which undergo cis/trans or open/closed ring isomerization under alternating ultraviolet and visible light irradiations.3,9 However, such photoisomerism processes are often accompanied by the drastic movement of atoms and/or bond formation/cleavage. Thus, efficient photoisomerization reactions normally occur in solution rather than in the solid state because of steric tension and hindrance, limiting their application in photoswitchable solid-state devices. When compared with the photoisomerization of organic molecules exhibiting significant atom displacement, lightinduced spin transition (LIST) of metal ions is observed to involve an interconversion between the low-spin (LS) and high-spin (HS) states because of electron rearrangement with a small structural deformation. Thus, the LIST can be achieved with high efficiency and good reversibility in the solid state under light irradiation. Furthermore, LIST can induce significant changes in the spin state, charge distribution, second-harmonic generation, and energy transfer, which can be utilized to tune the magnetic,10 dielectric,11 magneto− optical,12 and luminescent13 properties. To this end, two types of LISTs have attracted considerable interest (Scheme 1). One is light-induced excited spin-state trapping (LIESST), Scheme 1. Electron Rearrangements and Spin Transition of the Spin-Crossover (SCO) and Metal-to-Metal Charge Transfer (MMCT) Processes

2. PHOTOINDUCED SPIN-TRANSITION-SWITCHED MULTIFUNCTIONS 2.1. Reversible Photoswitching of Spin Transition

which was initially reported by Gütlich and co-workers.14 LIESST is associated with the elongation of metal−ligand bonds because of the population of the antibonding eg orbitals accompanied with the conversion from the LS state to the HS state and the fast coherent activation and damping of the molecular breathing trap of the photogenerated HS states.14−16 Generally, the trapping of photogenerated states requires structural deformation for inhibiting relaxation from the metastable HS state to the LS state. Thus, LIESST is limited to d6 FeII compounds, with only rare reports of d5 FeIII complexes owing to the relatively large changes observed in the bond lengths of FeII (ca. 0.2 Å) and FeIII (ca. 0.15 Å), where two electrons are observed to move from the t2g to the eg orbital during the spin-crossover (SCO) process.14,15 For example, the FeII ion exhibits LIESST with interconversion between FeIILS (t2g6eg0, S = 0) and FeIIHS (t2g4eg2, S = 2). Another type of LIST involves electron transfer between

It is desirable to have an in-depth understanding of the factors that govern the photoswitchable spin-transition behavior and properties before realizing the reversible manipulation of functions that are actuated by the photoswitchable SCO and MMCT units. A large number of spin-transition compounds have been designed and summarized in previously published books and reviews.14−19 Our group mainly focused on various strategies for designing the building blocks that can be used to construct the SCO and MMCT compounds (i.e., using rigid metallocyanate building blocks to rationally connect the metal ions in the presence of appropriate ancillary ligands). The metallocyanate building blocks were selected because of the linear coordination linkage of cyanide with other metal ions.20−22 Furthermore, the metallocyanate building blocks exhibit a suitable ligand field and can serve as bridges to effectively transfer electrons and to allow magnetic interaction B


Article


The photogenerated HS FeII ions were observed to be antiferromagnetically coupled with the adjacent LS FeIII ions and were able to revert to the LS state after thermal treatment. The LIESST and photoswitchable magnetic properties are unidirectional for the three SCO complexes above. However, the reversible photoswitchable spin transition should be considered for developing smart photoresponsive-moleculebased devices. We synthesized an asymmetric dinuclear complex 4 [FeII2L2(μ-L)3(NCSe)4]·2DMF·2H2O (L = 1naphthylimino-1,2,4-triazole) that can exhibit reversible LIESST behaviors (Figure 2a).24 Variable-temperature Raman and infrared spectroscopic study revealed the remarkable temperature dependence of the vibration modes of the [NCSe]− groups, confirming the thermally induced SCO behavior (Figure 2b). Light irradiation at 532 nm can rapidly increase the magnetization values of this material because of the photoinduced spin transition from FeIILS to metastable FeIIHS. Metastable FeIIHS can be reverted to FeIILS by applying light irradiation at 808 nm (Figure 2c). The almost completely reversible LIESST behavior can be attributed to the small spectral overlap between the 5T2 → 5E band of the FeIIHS ions and the spin-forbidden 1A1 → 3T1 and 1A1 → 3T2 bands of the FeIILS ions. This reversible LIESST behavior can be successfully repeated several times, confirming the reversible on/off switching of the spin states and the magnetic coupling interactions. Besides the realization of reversible LIESST, we intended to achieve reversible photoinduced ETCST. The MMCT behavior depends not only on the coordination environments of the metal sites but also on intermolecular interactions, including hydrogen bonding and π···π interactions. Recently, we have investigated the manner in which the π···π interactions affect the crystalline transformations and the reversible ETCST behavior (Figure 3). A model square planar complex, [(TpPz)FeII(CN)3]2CoIII2(dpq)4·2ClO4·2CH3OH·4H2O (5· 2CH3OH·4H2O, TpPz = tetrakis(pyrazolyl)borate, dpq = pyrazino[2, 3-f][1, 10]phenanthroline), was prepared.25 When 5·2CH3OH·4H2O was subjected to desolvation in air, it was transformed into [(TpPz)FeIII(CN)3]2CoII2(dpq)4·

between metal ions. Ancillary ligands were selected to provide an appropriate ligand field for ensuring photoswitchable SCO and MMCT properties. Recently, we investigated the influence of steric hindrance effects on the SCO behavior with different building blocks.23 Three linear trinuclear complexes, {[(Tp*)FeIII(CN)3]2FeII(Bpi)4}·2H2O (1; Tp* = hydrotris(3,5-dimethylpyrazol-1-yl)borate), {[(TpMe)FeIII(CN)3]2FeII(Bpi)4}· 4H2O (2; TpMe = hydrotris(3-methyl-pyrazol-1-yl)borate), and {[(Tp)FeIII(CN)3]2FeII(Bpi)4}·4H2O (3; Tp = hydrotris(pyrazolyl)borate), were synthesized using diphenyl substituted imidazole (Bpi). Temperature-dependent Mössbauer and infrared spectroscopic studies revealed their SCO properties (Figure 1a). A variable-temperature susceptibility

Figure 1. (a) Infrared spectra of 1. (b) Temperature-dependent susceptibilities of 1.

study indicated the presence of a complete thermally induced spin transition between the HS and LS states (Figure 1b). Upon a decrease in the steric effects of the TpR ligands, there was an increase in the spin-transition temperature T1/2 (182 K for 1, 200 K for 2, and 263 K for 3). Detailed magnetostructural analysis indicated that the large steric hindrance of TpR led to the presence of bent C≡N−FeII angles, which elongated the Fe−N bond lengths and distorted the FeN6 octahedron. Therefore, the ligand field of the FeII site was weakened, leading to a lower spin-transition temperature. A photomagnetic study denoted that all the three complexes exhibited LIESST behavior under light irradiation at 808 nm.

Figure 2. (a) Molecular structure of 4 at 280 K. Color code: Fe, pink; C, gray; N, blue; Se, orange. (b) Plots of χT vs. temperature. (c) Plots of χT vs. time under cycles of successive irradiation at 532 nm (green dots) and 808 nm (red dots) at 10 K. C


Article


Figure 3. (a) Images showing the single-crystal-to-single-crystal transformations. The images denote the conversions among the three species through subsequent desolvation and vapor induction processes. Inset: Molecular structure of the tetranuclear cluster. Color code: Fe, dark yellow; Co, turquoise; C, gray; N, blue; B, orange. (b) Plots of χT vs. temperature.

Figure 4. (a) Molecular structure of 6. (b) LS ferromagnetic triplet state, FeIIILS(↑)−FeIILS−FeIIILS(↑). (c) HS ferromagnetic septet state, FeIIILS(↑)−FeIIHS(↑↑↑↑)−FeIIILS(↑). (d) Plots of χT vs. temperature for 6. (e) Temperature dependence of AC magnetic susceptibility after irradiation.

the strength of cooperative interactions between the molecules, providing an additional driving force for performing successive two-step single-crystal-to-single-crystal transformation. The variation in intermolecular π···π interactions also led to the distortion of cobalt sites. The distortion degree of the cobalt sites inevitably affected the ligand-field strength, redox potential, and the MMCT behavior. Therefore, the introduction of flexible intermolecular interactions not only provides an efficient strategy to manipulate crystalline transformations but also offers a methodology to modulate the photoswitchable MMCT properties.

2ClO4 (5) with an accompanying color change from green to red. Further, when the red complex 5 was exposed to water vapor and heated to 100 °C, it changed into a green phase and transformed into its polymorph 5a. A magnetic study revealed that 5·2CH3OH·4H2O exhibited reversible ETCST behavior in mother liquor with a transition temperature T1/2 of approximately 370 K. The desolvated compound 5 maintained the FeIIILS−CN−CoIIHS phase across all the measured temperatures. A detailed structural study indicated that the hydrogen bonding interactions were destroyed upon the loss of solvent molecules, which influenced the redox potential of the iron sites. However, its polymorph 5a exhibited a reversible charge transfer behavior with an interconversion being observed between FeIIILS−CN−CoIIHS and FeIILS−CN−CoIIILS with a T1/2 of approximately 330 K. The intervalence changing of 5a could be reversibly switched by alternating between 808 and 532 nm light irradiations. Structural analysis showed that the average distances of the π···π and C−H···π interactions converged upon transforming from 5·2CH3OH·4H2O to 5 to 5a. The enhancement of intermolecular interactions increased

2.2. Manipulating the Photoswitchable SCO and MMCT toward Molecular Nanomagnets

Besides switching the magnetization values of the SCO and MMCT complexes using light irradiation, the manipulation of the orientation of magnetic dipoles can also provide another type of bistable states. This type of bistable materials, known as molecular nanomagnets, can exhibit slow magnetic relaxation and magnetic hysteresis under the blocking temperature and have attracted an increasing amount of attention owing to the D


Article


Figure 5. (a) Molecular structure of 8. Color code: Fe, green; Co, pink; C, gray; N, blue; B, yellow. (b) Temperature dependence of the AC susceptibility after irradiation under a zero DC field.

promising applications in high-density information storage and quantum computing at a purely molecular level.26−30 To this, photoinduced SCO and MMCT compounds can provide an additional and independent methodology to control the magnetic bistable states. This is because electron rearrangement involves the changing of spin states as well as the magnetic anisotropies of the respective metal ions and the magnetic interactions between them, thus offering an efficient manner through which the single-chain magnet (SCM) and single-molecule magnet properties can be switched on/ off.31−34 The photoinduced SCO can induce significant changing of the spin state; magnetic anisotropy of the SCO centers, thus, can be used to construct photoswitchable nanomagnets.31,32 Inspired by this, we linked the FeII SCO units using [(Tp*)FeIII(CN)3]− building blocks.35 A long ditopic magnetically inert ancillary ligand N,N′-bis-pyridin-4-ylmethylene hydrazine (bpmh) was selected to tune the ligand field of the FeII sites and to provide a large ratio of intra and interchain magnetic interactions (Figure 4a). The variable-temperature crystal structure and Mössbauer spectroscopic analysis of compound {[(Tp*)FeIII(CN)3]2FeII(bpmh)}·2H2O (6) confirmed the SCO behavior of the FeII sites, and the variabletemperature and variable-field magnetization measurements suggested that a photoinduced ferromagnetic FeIIILS−CN− FeIIHS−CN−FeIIILS chain was formed after light irradiation (Figure 4b−d). Further, a strong frequency dependence of the in-phase and out-of-phase signals was observed (Figure 4e). The thermal relaxation behavior was observed to be in accordance with the Arrhenius law, resulting in a preexponential factor, τ0, of 5.0 × 10−10 s and a relaxation barrier of 43.0 K, which were in agreement with those that were reported for SCMs.

Compared with the photoswitchable SCO of one type of metal ions, the photoinduced MMCT involves the electron transfer between different metal sites, thereby offering more choices for manipulating their spin states and anisotropies as well as the magnetic interactions between them. Among the various MMCT materials, the cobalt−iron Prussian blue compounds have been extensively investigated, wherein the diamagnetic FeIILS−CN−CoIIILS and paramagnetic FeIIILS− CN−CoIIHS linkages can interconvert.17 The FeIIILS ions exhibit unquenched angular momentum, and the CoIIHS ions normally adopt an elongated N6 octahedral shape, thereby satisfying the magnetic anisotropy requirement of nanomagnets. Initially, we synthesized a cyanide-bridged FeIII2CoII double-zigzag {[FeIII(bpy)(CN)4]2-CoII(4,4′-bipyridine)}· 4H2O (7; bpy = 2,2′-bipyridine) chain.36 An AC susceptibility study revealed a slow magnetic relaxation behavior with a relaxation energy barrier of 29 K that stemmed from the photogenerated FeIIILS−CN−CoIIHS phase. However, because of the occurrence of interchain antiferromagnetic interactions, complex 7 formed an antiferromagnetic ordering phase below 3.8 K. To obtain a well-isolated magnetic chain, a bulky [(TpPz)FeIII(CN)3]− building block and monodentate 4styrylpyridine were utilized to construct the {[(TpPz)FeIII(CN)3]2CoII(4-styrylpyridine)}·2H2O·2CH3OH (8; Figure 5a).37 Magnetic and infrared spectroscopic study indicated the MMCT behavior. An AC susceptibility study on the 532 nm light irradiated compound 8 confirmed the SCM behavior utilizing photoinduced transformation from the FeIILS−CN− CoIIILS to the FeIIILS−CN−CoIIHS linkages (Figure 5b). A few photoinduced SCMs have been reported.33,36−38 However, the SCM properties can be activated by light irradiation but deactivated only via a time-consuming thermal relaxation process upon heating. To achieve reversible E


Article


Figure 6. (a) Molecular structure of 9. (b) Plots of χT vs. temperature for 9. (c) Plots of χT vs. time under cycles of successive irradiation at 808 and 532 nm at 10 K. (d) AC magnetic susceptibility under a zero DC field.

Figure 7. (a,b) Molecular structure of 10. Color code: FeIII, orange; FeII, green; C, gray; N, blue; B, gold. (c) Plots of χT vs. temperature under different hydrostatic pressures. (d) Plots of T1/2 vs. hydrostatic pressure. (e) Temperature dependence of the dielectric constants. (f) Structural variation of the {FeIII2FeII} unit at 10 K (blue) and 250 K (red).

= 4-phenylpyridine) chain (Figure 6a).38 It should be mentioned that the changing of flexible π···π interactions was involved during the MMCT, which would be helpful for the

manipulation of the SCM properties, the photoinduced reversible MMCT behavior was introduced into a well-isolated double-zigzag {[Fe(bpy)(CN)4]2Co(phpy)2}·2H2O (9; phpy F


Article


Figure 8. (a) Plots of χT vs. temperature for 11. Inset: Spin densities of 11 in the HS and LS phases. (b) Plots of χT vs. temperature for 12. (c) Temperature-dependent dielectric constants of 12.

FeIII(CN)3]2FeII(azp)4}·4H2O (10; azp = 4,4′-azopyridine; Figure 7b).43 The Mössbauer spectroscopic and magnetic studies confirmed the SCO behavior, and a photomagnetic study revealed that the layered compound 10 formed an antiferromagnetic ordering phase below 10 K after 532 nm light irradiation. Interestingly, the SCO process denoted a sensitive response to a small amount of hydrostatic pressure with a large changing rate of 1113 K GPa−1 (Figure 7c,d). This sensitive response to the external stimuli was also reflected in the changing of dielectric constants from 3.4 to 5.3 with thermal hysteresis (Figure 7e). Detailed structural analysis revealed that this anomaly in the dielectric constants was caused by cooperative structural changes, including the stretching of the SCO centers and the rotation of negatively charged building blocks (Figure 7f). Because the double-zigzag chain systems could be activated by light irradiation, we contend that it is possible to control the dielectric properties via LIESST. Because MMCT can induce the redistribution of charges, a drastic change in the electric dipole can be highly anticipated. Previously, we reported a linear trinuclear compound {[(Tp)FeIII(CN)3]2CoII(Meim)4}·6H2O (11; Meim = N-methylimidazole).44 This compound exhibited a thermally induced charge transfer behavior at temperatures ranging from 200 to 250 K (Figure 8a). In an individual molecule, one electron moved from the cobalt site to one of the two iron sites, which would transform the centrosymmetric nonpolar molecule into an asymmetric polar structure. To verify this, the DFT calculations were performed based on the HS and LS structures, and the calculated results indicated that the LS structure exhibited a permanent electric dipole of 18.4 D; further, the HS structure exhibited no permanent dipole, confirming the nonpolar−polar interconversion (Figure 8a, inset). Complex 11 can be photoactivated from the FeIILS− CN−CoIIILS−NC−FeIIILS phase to the FeIIILS−CN−CoIIHS− NC−FeIIILS phase, enabling photoswitching of the molecular electric dipoles. Recently, reversible photoinduced MMCT was

photoinduced reversible ETCST. Light at 808 nm can sufficiently induce the generation of a metastable ferromagnetic FeIIILS−CN−CoIIHS−NC−FeIIILS chain (Figure 6b,c). AC susceptibility measurements revealed an SCM behavior after the 808 nm light irradiation, with an energy barrier of 40.21 K (Figure 6d). When 532 nm light irradiation was applied to the metastable ferromagnetic phase, the FeIIILS−CN−CoIIHS− NC−FeIIILS linkages transformed into FeIILS−CN−CoIIILS− NC−FeIIILS linkages (Figure 6c,d), resulting in the disappearance of SCM behavior. The SCM behavior can be switched in the on−off−on sequence by alternatively applying 808- and 532 nm laser irradiation. 2.3. Synergistic Modulation of the Photoswitchable Spin Transition and Coupled Multifunctions

The spin transition of metal ions not only involves the variation of spin states and magnetic properties but also involves other physical properties. The spin transitions that resulted in electron rearrangement induced charge redistribution as well as the variation of the molecular electric dipole. Therefore, the conductivity and dielectric properties also change.39 Moreover, significant structural variations, including the contraction and/or expansion of the coordination spheres of metal ions in the SCO and MMCT processes, are sufficiently substantial to design colossal thermal expansion materials and molecular motors.40 Apart from these, the fluorescence can be modified when the spin transition tunes the energy transfer between the fluorophore and the metal sites.41,42 Furthermore, when the photoswitchable SCO and MMCT units are coupled with multifunctional signals, it is plausible that the synergistic transformations of their dielectric, mechanical, and fluorescence properties can be tuned using a convenient, rapid, and noncontact light irradiation approach. Recently, we used negatively charged [(Tp)FeIII(CN)3]− building blocks (Figure 7a, left) to connect positively charged FeII SCO units (Figure 7a, right) and obtained {[(Tp)G


Article


Figure 9. (a) Temperature-dependent susceptibilities of 13. Inset: Layered structure of 13. (b) Temperature-dependent change in the unit cell volume. (c) Thermally and light-tuned positive thermal expansion. (d) Local structural rotation-induced negative thermal expansion.

Figure 10. (a) Temperature-dependent magnetic susceptibility of 14. Inset: Molecular structure of 14. Color code: Fe, red; C, gray; N, blue; S, yellow. (b) Temperature-dependent fluorescence emission spectra of 14.

realized in {[(Tp)FeIII(CN)3]2CoII(Bpi)4}·3H2O (12; Bpi = diphenyl substituted imidazole) via introducing flexible intermolecular π···π interactions (Figure 8b).45 Further, the spin state and charge distribution could be reversibly controlled by alternating 532 and 808 nm light irradiations, providing both photoswitchable polarity and magnetization. Dielectric studies showed that the trinuclear compound exhibited a dielectric anomaly associated with the reorientation of the molecular dipole during the MMCT process (Figure 8c). Apart from ETCST and charge redistribution, the accompanying structural variation in the MMCT complexes has also caught our attention. Normally, intermetallic charge transfer involves a change in the bond lengths of 0.1−0.2 Å. Thus, colossal thermal expansion was possible when the external-stimuli-responsive MMCT units were incorporated. The two-dimensional cyanide-bridged compound {[(Tp)-

FeIII(CN)3]2CoII(Bib)2}·5H2O (13; Bib = 1,4-Bis(1H-imidazol-1-yl)benzene; Figure 9a) exhibits both colossal positive and negative thermal expansions.46 A structural study revealed a four-stage volumetric thermal expansion behavior (Figure 9b). A drastic increase in the unit cell volume was observed over a temperature ranging from 180 to 240 K with a volumetric thermal expansion coefficient of 1498 MK−1. Detailed study revealed that the colossal positive thermal expansion was mainly caused by the elongation of the coordination bonds around the cobalt sites and the interlayer π···π distances derived from MMCT (Figure 9c). Interestingly, the layered compound exhibited a negative thermal expansion behavior between 300 and 350 K with a negative volume expansion coefficient of −489 MK−1. The volumetric contraction was caused by the rotation of the [Fe2Tp(CN)3]− building blocks leading enhancement of interlayer π···π interactions (Figure 9d). Furthermore, the FeIILS−CN−CoIIILS−NC−FeIIILS phase H


Article


Figure 11. (a) Modulation of the fluorescence emission with a photoinduced spin transition. (b) Normalized maximum fluorescence emission intensity as a function of temperature. Inset: Molecular structure of 16. Color code: Fe, pink; C, gray; N, blue; S, orange. (c) Thermal variations in the fluorescence emission of 16 after irradiation.

could be transformed into the FeIIILS−CN−CoIIHS−NC− FeIIILS phase by applying a 532 nm light irradiation, suggesting that the colossal positive thermal expansion can be controlled by light irradiation. It can be seen that the aforementioned properties were mainly focused on the spin transition of metal ions. It is also plausible to integrate functional organic ligands into the spintransition systems and manipulate their functions via LIST, which can produce novel molecular materials with more than two functions. To this end, the modulation of fluorescence signals of the coordinating ligand using LIST provides a promising alternative for application in noninvasive optical read-out technologies. Initially, we designed a fluorophore ligand (naphth-1-yl)-N-(3,5-di(pyridin-2-yl)-4H-1,2,4-triazol4-yl)methanimine (L) to satisfy the ligand-field requirements of the SCO properties.47 The [Fe(L)2(NCS)2] (14) (Figure 10a, inset) and [Fe(L)2(NCSe)2] (15) compounds can exhibit SCO behavior. The fluorescence emission was preserved, exhibiting a broad peak at 412 nm. However, the fluorescence emission intensities exhibited a monotonous increase upon cooling (Figure 10b). The lack of synergy between the SCO and temperature-dependent evolution of the fluorescence emission intensity can be attributed to the fact that the fluorescence emission spectra exhibits poor overlapping with the absorption band of the SCO FeII center. To solve this problem, a more conjugated pyrene group was coupled with 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole (Figure 11a).48 The photoresponsive properties were wellmaintained in compound [Fe(L′)2(NCS)2] (L′ = (pyrene-1yl)-N-(3,5-di(pyridin-2-yl)-4H-1,2,4-triazol-4-yl)methanimine; 16). The temperature-dependent fluorescence emission analysis elucidated the correlation between the fluorescence emission and SCO, with an abnormal decrease being observed in the emission intensity from 300 to 250 K in the SCO temperature range (Figure 11b). In addition, the fluorescence emission showed a significant increase after irradiation at 10 K, which is in agreement with the increase of the magnetization

value after irradiation (Figure 11c). This is due to the excited energy of the photoirradiated ligand being transferred to the FeIILS centers via a resonant energy transfer process, resulting in the quenching of the fluorescence during the spin transition from the HS state to the LS state. This conclusion was further clarified from the results of the variable-temperature UV−vis absorption study and time-dependent DFT calculations. Therefore, these observations provide direct evidence of the photoswitchable fluorescence emission.

3. OUTLOOK In this Account, we have reviewed the recent advances in the usage of photoresponsive SCO and MMCT units for tuning various properties. On the basis of a building block strategy, target functions, including the magnetic, dielectric, fluorescence, and mechanical properties, were integrated with the spin-transition units. These interesting properties can be reversibly switched using light irradiations at different wavelengths. The newly developed photoresponsive complexes provide considerable alternatives and opportunities for the production of multifunctional materials. Despite some progress being achieved in this field, some considerable challenges still remain. (1) The development of novel photoresponsive complexes and an investigation of their underlying mechanisms are required, because the LIESST effect is still limited to the d6 FeII compounds and rare examples of d5 FeIII complexes. There have been considerable d4 CrII/MnIII and d7 CoII SCO complexes reported, although neither of them can exhibit LIESST because of the relatively small structural changing. To this goal, we envisage that by connecting the d4 and d7 SCO units with rigid linkages, the photoinduced metastable state can be stabilized with the help of the cooperative interactions throughout the coordination frameworks. In fact, the realization of LIESST with small structural changing will also facilitate the resistance to fatigue, which is very important for solid-state materials and devices. (2) In addition, the design strategy of integrating different functions should be further I


Article


Bonded Single-Molecule Junctions with Stable and Reversible Photoswitched Conductivity. Science 2016, 352, 1443−1445. (7) Bisoyi, H. K.; Li, Q. Light-Driven Liquid Crystalline Materials: from Photo-Induced Phase Transitions and Property Modulations to Applications. Chem. Rev. 2016, 116, 15089−15166. (8) Szymański, W.; Beierle, J. M.; Kistemaker, H. A.; Velema, W. A.; Feringa, B. L. Reversible Photocontrol of Biological Systems by the Incorporation of Molecular Photoswitches. Chem. Rev. 2013, 113, 6114−6178. (9) Bandara, H. D.; Burdette, S. C. Photoisomerization in Different Classes of Azobenzene. Chem. Soc. Rev. 2012, 41, 1809−1825. (10) Ohkoshi, S.; Imoto, K.; Tsunobuchi, Y.; Takano, S.; Tokoro, H. Light-Induced Spin-Crossover Magnet. Nat. Chem. 2011, 3, 564−569. (11) Bonhommeau, S.; Guillon, T.; Lawson Daku, L. M.; Demont, P.; Sanchez Costa, J.; Létard, J. F.; Molnár, G.; Bousseksou, A. Photoswitching of the Dielectric Constant of the SpinCrossover Complex [Fe(L)(CN)22O. Angew. Chem., Int. Ed. 2006, 45, 1625− 1629. (12) Ohkoshi, S.; Takano, S.; Imoto, K.; Yoshikiyo, M.; Namai, A.; Tokoro, H. 90-Degree Optical Switching of Output SecondHarmonic Light in Chiral Photomagnet. Nat. Photonics 2014, 8, 65−71. (13) Estrader, M.; Salinas Uber, J.; Barrios, L. A.; Garcia, J.; LloydWilliams, P.; Roubeau, O.; Teat, S. J.; Aromí, G. A Magneto−optical Molecular Device: Interplay of Spin Crossover, Luminescence, Photomagnetism, and Photochromism. Angew. Chem., Int. Ed. 2017, 56, 15622−15627. (14) Gütlich, P., Goodwin, H. A., Eds. Spin Crossover in Transition Metal Compounds I−III; Springer Science & Business Media, 2004. (15) Halcrow, M. A., Ed. Spin-Crossover Materials: Properties and Applications.; John Wiley & Sons, 2013. (16) Hogue, R. W.; Singh, S.; Brooker, S. Spin Crossover in Discrete Polynuclear Iron (II) Complexes. Chem. Soc. Rev. 2018, 47, 7303− 7338. (17) Aguila, D.; Prado, Y.; Koumousi, E. S.; Mathonière, C.; Clérac, R. Switchable Fe/Co Prussian Blue Networks and Molecular Analogues. Chem. Soc. Rev. 2016, 45, 203−224. (18) Mathonière, C. Metal-to-Metal Electron Transfer: A Powerful Tool for the Design of Switchable Coordination Compounds. Eur. J. Inorg. Chem. 2018, 2018, 248−258. (19) Meng, Y.-S.; Sato, O.; Liu, T. Manipulating Metal-to-Metal Charge Transfer for Materials with Switchable Functionality. Angew. Chem., Int. Ed. 2018, 57, 12216−12226. (20) Ferlay, S.; Mallah, T.; Ouahès, R.; Veillet, P.; Verdaguer, M. A Room-Temperature Organometallic Magnet Based on Prussian Blue. Nature 1995, 378, 701. (21) Beltran, L. M. C.; Long, J. R. Directed Assembly of Metal− Cyanide Cluster Magnets. Acc. Chem. Res. 2005, 38, 325−334. (22) Li, D.; Clérac, R.; Roubeau, O.; Harté, E.; Mathonière, C.; Le Bris, R.; Holmes, S. M. 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, 252−258. (23) Hu, J.-X.; Meng, Y.-S.; Zhao, L.; Zhu, H.-L.; Liu, L.; Liu, Q.; Jiao, C.-Q.; Liu, T. A Series of Linear {FeIII2FeII} Complexes with Paramagnetic Building-Block-Modified Spin Crossover Behaviors. Chem. - Eur. J. 2017, 23, 15930−15936. (24) Wang, J.-L.; Liu, Q.; Meng, Y.-S.; Zheng, H.; Zhu, H.-L.; Shi, Q.; Liu, T. Synergic on/off Photoswitching Spin State and Magnetic Coupling between Spin Crossover Centers. Inorg. Chem. 2017, 56, 10674−10680. (25) Jiao, C.-Q.; Jiang, W.-J.; Meng, Y.-S.; Wen, W.; Zhao, L.; Wang, J.-L.; Hu, J.-X.; Gurzadyan, G. G.; Duan, C.-Y.; Liu, T. Manipulation of Successive Crystalline Transformations to Control Electron Transfer and Switchable Functions. Natl. Sci. Rev. 2018, 5, 507−515. (26) Gatteschi, D.; Sessoli, R.; Villain, J. Molecular Nanomagnets; Oxford University Press, 2006. (27) Leuenberger, M. N.; Loss, D. Quantum Computing in Molecular Magnets. Nature 2001, 410, 789−793.

developed. An efficient coupling between the functions should be built to realize the synergic modulation of multifunctions. As shown in Section 2.3, we have observed the photoinduced electric dipole changing for the individual molecule, whereas how to realize the photoinduced ferroelectric transition and further multiferroics featuring strong magnetoelectric coupling still provides huge challenges. (3) The application of the aforementioned materials to design molecule-based devices and prototypes is also urgently required. We believe that the fascinating properties of photoresponsive spin-transition materials will continue to be exploited in future research even though extensive collaboration between researchers with expertise in synthetic chemistry, materials chemistry, and application techniques is required.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Tao Liu: 0000-0003-2891-603X Notes

The authors declare no competing financial interest. Biographies Yin-Shan Meng is working in the State Key Laboratory of Fine Chemicals, Dalian University of Technology. He received his B.S. degree from the College of Chemistry, Beijing Normal University. He completed his Ph.D. study in 2017 at the College of Chemistry and Molecular Engineering, Peking University, under the supervision of Prof. Song Gao and Bing-Wu Wang. His research interests are dielectric and thermal expansion properties of spin-transition materials. Tao Liu received his Ph.D. in 2008 at Peking University under the supervision of Prof. Song Gao and Zhe-Ming Wang. From 2008− 2010, he was a postdoctoral researcher with Prof. Osamu Sato at Kyushu University. He joined the faculty at Dalian University of Technology in 2010 and is currently professor of the State Key Laboratory of Fine Chemicals. His research interest focuses on manipulating spin transition toward switchable functions.

■

ACKNOWLEDGMENTS This work was supported by the NSFC (21871039, 21801037, 21421005, and 91422302) and the Fundamental Research Funds for the Central Universities, China.

■

REFERENCES

(1) Gütlich, P.; Garcia, Y.; Woike, T. Photoswitchable Coordination Compounds. Coord. Chem. Rev. 2001, 219-221, 839−879. (2) Kobatake, S.; Takami, S.; Muto, H.; Ishikawa, T.; Irie, M. Rapid and Reversible Shape Changes of Molecular Crystals on Photoirradiation. Nature 2007, 446, 778−781. (3) Irie, M.; Fukaminato, T.; Matsuda, K.; Kobatake, S. Photochromism of Diarylethene Molecules and Crystals: Memories, Switches, and Actuators. Chem. Rev. 2014, 114, 12174−12277. (4) Sato, O. Dynamic Molecular Crystals with Switchable Physical Properties. Nat. Chem. 2016, 8, 644−656. (5) Balzani, V.; Credi, A.; Venturi, M. Light Powered Molecular Machines. Chem. Soc. Rev. 2009, 38, 1542−1550. (6) Jia, C.; Migliore, A.; Xin, N.; Huang, S.; Wang, J.; Yang, Q.; Wang, S.; Chen, H.; Wang, D.; Feng, B.; Liu, Z.; Zhang, G.; Qu, D.H.; Tian, H.; Ratner, M. A.; Xu, H. Q.; Nitzan, A.; Guo, X. Covalently J


Article


(46) Hu, J.-X.; Xu, Y.; Meng, Y.-S.; Zhao, L.; Hayami, S.; Sato, O.; Liu, T. A Material Showing Colossal Positive and Negative Volumetric Thermal Expansion with Hysteretic Magnetic Transition. Angew. Chem., Int. Ed. 2017, 56, 13052−13055. (47) Wang, J.-L.; Liu, Q.; Lv, X.-J.; Wang, R.-L.; Duan, C.-Y.; Liu, T. Magnetic Fluorescent Bifunctional Spin-Crossover Complexes. Dalton Trans. 2016, 45, 18552−18558. (48) Wang, J.-L.; Liu, Q.; Meng, Y.-S.; Liu, X.; Zheng, H.; Shi, Q.; Duan, C.-Y.; Liu, T. Fluorescence Modulation via Photoinduced Spin Crossover Switched Energy Transfer from Fluorophores to FeII Ions. Chem. Sci. 2018, 9, 2892−2897.

(28) Goodwin, C. A. P.; Ortu, F.; Reta, D.; Chilton, N. F.; Mills, D. P. Molecular Magnetic Hysteresis at 60 K in Dysprosocenium. Nature 2017, 548, 439−442. (29) Guo, F.-S.; Day, B. M.; Chen, Y.-C.; Tong, M.-L.; Mansikkamäki, A.; Layfield, R. A. A Dysprosium Metallocene Single-Molecule Magnet Functioning at the Axial Limit. Angew. Chem., Int. Ed. 2017, 56, 11445−11449. (30) Guo, F.-S.; Day, B. M.; Chen, Y.-C.; Tong, M.-L.; Mansikkamäki, A.; Layfield, R. A. Magnetic Hysteresis up to 80 K in a Dysprosium Metallocene Single-Molecule Magnet. Science 2018, 362, 1400−1403. (31) Feng, X.; Mathonière, C.; Jeon, I.-R.; Rouzières, M.; Ozarowski, A.; Aubrey, M. L.; Gonzalez, M. I.; Clérac, R.; Long, J. R. Tristability in a Light-Actuated Single-Molecule Magnet. J. Am. Chem. Soc. 2013, 135, 15880−15884. (32) Mathonière, C.; Lin, H.-J.; Siretanu, D.; Clèrac, R.; Smith, J. M. Photoinduced Single-Molecule Magnet Properties in a FourCoordinate Iron(II) Spin Crossover Complex. J. Am. Chem. Soc. 2013, 135, 19083−19086. (33) Hoshino, N.; Iijima, F.; Newton, G. N.; Yoshida, N.; Shiga, T.; Nojiri, H.; Nakao, A.; Kumai, R.; Murakami, Y.; Oshio, H. Three-Way Switching in a Cyanide-Bridged [CoFe] Chain. Nat. Chem. 2012, 4, 921−926. (34) Nihei, M.; Okamoto, Y.; Sekine, Y.; Hoshino, N.; Shiga, T.; Liu, I. P.-C.; Oshio, H. A Light-Induced Phase Exhibiting Slow Magnetic Relaxation in a Cyanide-Bridged [Fe4Co2] Complex. Angew. Chem., Int. Ed. 2012, 51, 6361−6364. (35) Liu, T.; Zheng, H.; Kang, S.; Shiota, Y.; Hayami, S.; Mito, M.; Sato, O.; Yoshizawa, K.; Kanegawa, S.; Duan, C. A Light-Induced Spin Crossover Actuated Single-Chain Magnet. Nat. Commun. 2013, 4, 2826. (36) Liu, T.; Zhang, Y.-J.; Kanegawa, S.; Sato, O. Photoinduced Metal-to-Metal Charge Transfer toward Single-Chain Magnet. J. Am. Chem. Soc. 2010, 132, 8250−8251. (37) Dong, D.-P.; Liu, T.; Kanegawa, S.; Kang, S.; Sato, O.; He, C.; Duan, C.-Y. Photoswitchable Dynamic Magnetic Relaxation in a WellIsolated {Fe2Co} Double-Zigzag Chain. Angew. Chem., Int. Ed. 2012, 51, 5119−5123. (38) Jiang, W.; Jiao, C.; Meng, Y.; Zhao, L.; Liu, Q.; Liu, T. Switching Single Chain Magnet Behavior via Photoinduced Bidirectional Metal-to-Metal Charge Transfer. Chem. Sci. 2018, 9, 617−622. (39) Jornet-Mollá, V.; Duan, Y.; Giménez-Saiz, C.; Tang, Y.-Y.; Li, P.-F.; Romero, F. M.; Xiong, R.-G. A Ferroelectric Iron(II) Spin Crossover Material. Angew. Chem., Int. Ed. 2017, 56, 14052−14056. (40) Mullaney, B. R.; Goux-Capes, L.; Price, D. J.; Chastanet, G.; Létard, J.-F.; Kepert, C. J. Spin Crossover-Induced Colossal Positive and Negative Thermal Expansion in a Nanoporous Coordination Framework Material. Nat. Commun. 2017, 8, 1053. (41) Wang, C. F.; Li, R. F.; Chen, X. Y.; Wei, R. J.; Zheng, L. S.; Tao, J. Synergetic Spin Crossover and Fluorescence in One−Dimensional Hybrid Complexes. Angew. Chem., Int. Ed. 2015, 54, 1574−1577. (42) Yuan, J.; Wu, S.-Q.; Liu, M.-J.; Sato, O.; Kou, H.-Z. Rhodamine 6G-Labeled Pyridyl Aroylhydrazone Fe(II) Complex Exhibiting Synergetic Spin Crossover and Fluorescence. J. Am. Chem. Soc. 2018, 140, 9426−9433. (43) Zheng, H.; Meng, Y.-S.; Zhou, G.-L.; Duan, C.-Y.; Sato, O.; Hayami, S.; Luo, Y.; Liu, T. Simultaneous Modulation of Magnetic and Dielectric Transition via Spin-Crossover-Tuned Spin Arrangement and Charge Distribution. Angew. Chem., Int. Ed. 2018, 57, 8468−8472. (44) Liu, T.; Dong, D.-P.; Kanegawa, S.; Kang, S.; Sato, O.; Shiota, Y.; Yoshizawa, K.; Hayami, S.; Wu, S.; He, C.; Duan, C.-Y. Reversible Electron Transfer in a Linear {Fe2Co} Trinuclear Complex Induced by Thermal Treatment and Photoirraditaion. Angew. Chem., Int. Ed. 2012, 51, 4367−4370. (45) Hu, J.-X.; Luo, L.; Lv, X.-J.; Liu, L.; Liu, Q.; Yang, Y.-K.; Duan, C.-Y.; Luo, Y.; Liu, T. Light-Induced Bidirectional Metal-to-Metal Charge Transfer in a Linear Fe2Co Complex. Angew. Chem., Int. Ed. 2017, 56, 7663−7668. K


Manipulating Spin Transition To Achieve Switchable Multifunctions

Recommend Documents