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. 2026 Apr 29;59(10):1632–1643. doi: 10.1021/acs.accounts.6c00023

Guest-Triggered Charge Transfer for Magnetic Change in Redox-Active MOF Magnets

Jun Zhang †,∥,⊥,#,*, Yang Cao ‡, Wataru Kosaka §, Hitoshi Miyasaka §,*
PMCID: PMC13192255  PMID: 42054543

Conspectus

The synergistic control of lattice properties and porosity with mass transport is a defining feature of molecular lattices known as metal–organic frameworks (MOFs), a capability largely absent in conventional rigid inorganic materials. These open frameworks provide adaptive chemical environments in which guest inclusion can directly reorganize electronic structure and magnetic order. In redox-active MOFs, guest insertion, ejection, and transport, collectively referred to as guest dynamics, can reversibly modulate charge distribution and spin states, enabling electronic and magnetic phase switching. Such dynamic coupling between framework, space, and closely spaced electronic states establishes porous magnets as a versatile platform for stimulus-responsive molecular materials with potential applications in information storage and chemical sensing. This Account summarizes our efforts to develop redox-active MOF magnets based on donor–acceptor (D/A) architectures. These systems integrate redox-active paddlewheel-type diruthenium (II,II) complexes ([Ru2II,II]; donors, D) with π-acidic TCNQ derivatives (acceptors, A), forming layered D2A frameworks that support multiple, closely spaced electronic states. Because these states lie in delicate energetic balance, subtle structural perturbations such as guest adsorption can trigger charge transfer and reorganize magnetic ground states. To enable guest-induced magnetic switching, we have developed two key mechanisms: (1) on-host charge transfer (CT), where neutral guests modulate the electronic state of the host framework, and (2) host–guest CT, where redox-active guests directly exchange electrons with the framework. Whereas host–guest CT is limited to strongly redox-active guests, on-host CT exploits the intrinsic energetic competition between donor and acceptor units, amplified by lattice electrostatics. The central question addressed in this Account is how to rationally design D/A-MOFs poised at electronic instability, such that minor external stimuli can tip the balance between competing charge states. We outline two guiding strategies: positioning donor–acceptor pairs at the boundary of multiple electronic states and targeting systems that display emergent electronic configurations beyond initial predictions. Guided by these principles, five representative systems showing guest-induced on-host CT have been discovered. We hope this Account will encourage continued investigation into these multifunctional materials at the interface of magnetism, electronic regulation, and host–guest chemistry.

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KEY REFERENCES

  • Zhang, J. ; Kosaka, W. ; Liu, Q. ; Amamizu, N. ; Kitagawa, Y. ; Miyasaka, H. . CO2-Sensitive Porous Magnet: Antiferromagnet Creation from a Paramagnetic Charge-Transfer Layered Metal–Organic Framework. J. Am. Chem. Soc. 2023, 145 (48), 26179–26189. DOI: 10.1021/jacs.3c08583 . This study reports the first example of gas-induced magnet creation, where a paramagnetic layered MOF becomes an antiferromagnet upon CO 2 adsorption.

  • Zhang, J. ; Kosaka, W. ; Kitagawa, Y. ; Miyasaka, H. . A Metal–Organic Framework That Exhibits CO2-Induced Transitions between Paramagnetism and Ferrimagnetism. Nat. Chem. 2021, 13 (2), 191–199. DOI: 10.1038/s41557-020-00577-y . This study reports a first CO 2 -responsive porous magnet that undergoes a reversible magnetic phase transition from a ferrimagnetic state to a paramagnetic state upon CO 2 adsorption.

  • Zhang, J. ; Kosaka, W. ; Sato, H. ; Miyasaka, H. . Magnet Creation by Guest Insertion into a Paramagnetic Charge-Flexible Layered Metal–Organic Framework. J. Am. Chem. Soc. 2021, 143 (18), 7021–7031. DOI: 10.1021/jacs.1c01537 . This study reports the guest-induced creation of magnetism in a charge-flexible layered MOF, where insertion of different neutral molecules (e.g., benzene, DCM, CS 2 ) transforms the paramagnet into either ferrimagnetic or antiferromagnetic states.

  • Zhang, J. ; Kosaka, W. ; Sugimoto, K. ; Miyasaka, H. . Magnetic Sponge Behavior via Electronic State Modulations. J. Am. Chem. Soc. 2018, 140 (16), 5644–5652. DOI: 10.1021/jacs.8b02428 This study reports a large, reversible shift in the magnetic phase transition temperature in a layered MOF from a high-temperature ferrimagnet (T C = 101 K) to a low-temperature ferrimagnet (T C = 34 K) upon desolvation.

1. Introduction

The 2025 Nobel Prize in Chemistry was awarded to three individuals who pioneered Metal–Organic Frameworks (MOFs). − This recognition suggests the transformative impact of reticular chemistry in enabling functional control over matter at the molecular level. The most crucial aspect of this chemistry is that it is the chemistry of a nanosized “space” surrounded by designed frameworks. Moreover, MOFs introduce a paradigm in which porosity is not passive but dynamic, possessing selectivity, flexibility, and functional diversity; these qualities are absent in rigid, conventional porous inorganic materials. The true essence of MOF chemistry lies in its ability to simultaneously and synergistically control two distinct elements, “framework” and “space” and to orchestrate their interplay.

Within this context, porous magnets combining magnetic functionality with porosity, − denoted as MOF magnets, represent a robust platform for the investigation of molecular magnets that can functionally respond to the transport and inclusion of guest molecules. − An effective approach to trigger magnetic phase transformations/transitions via guest accommodation is to manipulate electronic states within strongly correlated electronic systems through charge transfer (CT) between distinct components, such as metal ions and organic ligands, thereby varying charge distribution and spin arrangement. Thus, our concept is to address the guest-induced magnetic responsiveness by constructing redox-active MOF magnets, − in which the inherent electronic flexibility allows magnetic ordering to be reversibly tuned by molecular guests. Such frameworks offer nearly degenerate electronic states that can be shifted by subtle perturbations, directly coupling porosity with controllable magnetism. In this regard, electron-conjugated MOFs consisting of an electron donor (D) and an electron acceptor (A), denoted as D/A-MOFs (Figure a), represent an attractive platform. ,

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(a) Construction of layered D/A-MOF from electron-rich paddlewheel diruthenium­(II,II) complexes ([Ru2(RCO2)4], donor units) and electron-deficient TCNQ derivatives (TCNQ­(R2)2, acceptor units). (b) Construction of different electronic states (neutral, one-electron transfer, and two-electron transfer states) in D/A-MOFs. (c) Chemical strategies for manipulating charge transfer in D/A-MOFs: On-host charge transfer (left), mediated by nonredox-active solvents or gaseous guests through structural or weak interactions; Host–guest charge transfer (right), involving direct electron transfer between the host framework and redox-active guest molecules.

Our groups have long focused on frameworks with distinct electronic states constructed from carboxylate-bridged paddlewheel-type diruthenium­(II,II) complexes ([Ru2 II,II(RCO2)4] or [Ru2 II,II]) and polycyano organic compounds such as 7,7,8,8-tetracyano-p-quinodimethane (TCNQ) and N,N′-dicyanoquinodiimine (DCNQI), where [Ru2 II,II] complexes act as one-electron donors ([Ru2 II,II] ⇄ [Ru2 II,III]+ + e–) and TCNQ and DCNQI derivatives act as stepwise two-electron acceptors (TCNQR + e– ⇄ TCNQR•– and TCNQR•– + e– ⇄ TCNQR2–). , In addition, [Ru2 II,II] and [Ru2 II,III]+ are paramagnetic with multiple spin states of S = 1 and 3/2, respectively, − and TCNQR•–/DCNQIR•– has a radical spin with S = 1/2, acting as good π-type magnetic modules, while TCNQR0/DCNQIR0 and TCNQR2–/DCNQIR2– are diamagnetic. In particular, the frameworks of the series of D:A = 2:1 assembly (i.e., D2A) can exhibit three principal electronic configurations as shown in Figure b: a neutral (N) state, − a one-electron-transferred (1e-I) state, ,− and a two-electron-transferred (2e-I) state, , each associated with unique spin arrangements and magnetic ground states. In the N state, spins remain localized on donor units with weak interactions, while the 1e-I state stabilizes ferrimagnetic ordering within the framework. The 2e-I state contains diamagnetic TCNQR2–, so spins are localized on [Ru2 II,III]+ unit to be paramagnetic, as well as the N state. An intermediate state between the 1e-I and 2e-I states, i.e., 1.5e-I state, was also assigned, where a charge-disproportionated electronic state composed of 1e-I and 2e-I basic moieties in a 1:1 ratio was confirmed, which suppressed strong magnetic exchanges because of the partial presence of TCNQR2– moieties.

Layered D2A systems can incorporate interstitial molecules between layers, subtly modifying interlayer distance and registry. Although these structural variations may appear minor, they significantly influence long-range magnetic exchange by altering electrostatic stabilization and π-overlap. Chemical manipulation of CT in such systems generally follows two mechanisms (Figure c): (1) on-host CT, where neutral guests modulate the charge distribution within the D/A-MOFs, and (2) host–guest CT, where redox-active guests directly exchange electrons with the host. For on-host CT, guest molecules do not directly participate in redox chemistry but indirectly tune the energetic balance between donor and acceptor units through structural distortion, Coulombic Madelung stabilization (M a), and weak host–guest interactions owing to direct host–guest van der Waals contacts or through hydrogen bonding in a relationship of ΔE CT ≈ (I D – E A) – M a, where ΔE CT is charge transfer stabilization energy, I D is ionization potential of D, and E A is electron affinity of A. The (I D – E A) term relates to the HOMO–LUMO energy difference. When this E CT takes a positive value, no electron transfer occurs between D and A, and the neutral state (D0A0) is maintained. Conversely, when E CT is negative, electron transfer occurs, forming an ionic state (D+A–). Three-dimensional electrostatic interactions due to packing are thought to relate to M a, and weak host–guest interactions are thought to relate to the adjustment of the (I D – E A) term.

The mechanism whereby intercalated molecules perturb the host layers and induce intraframework CT offers greater structural and electronic diversity than host–guest CT pathways. To date, host–guest CT has been realized primarily through the introduction of redox-active guests such as iodine. Consequently, our strategy for on-host CT systems relies on modular synthetic chemistry: (i) tuning donor strength and HOMO level energies of [Ru2(RCO2)4] units via carboxylate substitution, (ii) engineering LUMO levels and planarity of TCNQ derivatives to modulate acceptor properties, and (iii) regulating interlayer geometry and pore topology through steric design of linkers. Guided by these principles, we envisioned two strategies for constructing layered D2A systems capable of guest-induced on-host CT:

  • (I)

    Constructing a D/A-MOF using a D–A set positioned precisely at the boundary of three electronic states (N, 1e-I, and 2e-I).

  • (II)

    Targeting D/A-MOFs that exhibit unexpected electronic states, significantly different from the predicted ones, when constructed.

The evolution of this field reflects a conceptual shift from constructing electronically versatile D/A frameworks to understanding how guest inclusion can reversibly reorganize charge, spin, and magnetic order. Early studies established the accessibility of multiple electronic states within D/A-MOFs. Subsequent investigations demonstrated solvent-induced transitions of critical ordering temperature (T C) or magnetic phases, followed by gas-triggered transitions such as CO2-induced stabilization of distinct charge states. Host–guest CT processes, such as iodine-induced spin quenching, demonstrated how redox-active guests can switch magnetic order via electron exchange.

In this Account, we systematically review the progress on chemical manipulation of magnetic regulation in redox-active MOF magnets: on-host CT and host–guest CT. We detail how these mechanisms enable controllable magnetic switching in D/A-MOFs and provide design principles for achieving specific magnetic responses. We reveal how guest inclusion can precisely regulate electronic distribution and critical temperatures, and even switch the magnetic ground state.

2. How Can We Design D/A-MOF Enabling Guest-Induced ON-Host CT

In line with strategies (I) and (II), we began selecting D and A units by utilizing an ionicity diagram (Figure a). ,, This diagram was based on the energy gap between the HOMO level of the D unit and the LUMO level of the A unit (ΔE H–L), as well as the on-site Coulomb repulsion U, which was derived from the gap between the second and first redox potentials of TCNQR. This ionicity diagram semiempirically distinguishes the N, 1e-I, and 2e-I states based on the electronic states of D2A compounds consisting of [Ru2] and TCNQR reported to date. First, following strategy (I), we selected sets of D and A species with ΔE H‑L values near the boundaries of each electronic-state phase and synthesized D2A aggregates. This D–A choice led to the discovery of five layered D2A compounds (denoted as solvent-free forms): [{Ru2(2,3,5-Cl3ArCO2)4}2(TCNQMe2)] (1), [{Ru2(m-FArCO2)4}2{TCNQ­(MeO)2}] (2), [{Ru2(2,4-F2ArCO2)4}2{TCNQ­(EtO)2}] (3), , [{Ru2(o-ClArCO2)4}2{TCNQ­(MeO)2}] (4), , [{Ru2(2,4,6-F3ArCO2)4}2{TCNQ­(EtO)2}] (5), where 2,3,5-Cl3ArCO2 – = 2,3,5-trichlorobenzoate; m-FArCO2 – = m-fluorobenzoate; 2,4-F2ArCO2 – = 2,4-difluorobenzoate; o-ClArCO2 – = o-chlorobenzoate; 2,4,6-F3ArCO2 – = 2,4,6-trifluorobenzoate; TCNQMe2 = 2,5-dimethyl-7,7,8,8-tetracyanoquinodimethane; TCNQ­(MeO)2 = 2,5-dimethoxy-7,7,8,8-tetracyanoquinodimethane; TCNQ­(EtO)2 = 2,5-diethoxy-7,7,8,8-tetracyanoquinodimethane.

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Ionicity diagrams of ΔE H‑L vs |2 E 1/2(A) – 1 E 1/2(A)| for [Ru2 II,II]/TCNQR D2A frameworks. The symbols denote the electronic states: N (red circles), 1e–I (blue squares), 1.5e–I (blue triangles), and 2e–I (green diamonds). The dashed horizontal line corresponds to ΔE H‑L = 0 eV, which approximately separates the N and I regimes, while the solid sloped line separating the 1e–I (blue) and 2e–I (green) regions is defined based on the TCNQR LUMO energy (y = −0.006451x + 2.911). Ionicity diagrams for D2A compounds 1–5 in the (a) guest-accommodated and (b) guest-free phases. Guest accommodation expands the interlayer spacing, reduces Coulombic stabilization, and shifts the framework away from the theoretical charge-separated state. The representative compounds used in this design are [{Ru2(2,3,5-Cl3ArCO2)4}2(TCNQMe2)] (1), [{Ru2(m-FArCO2)4}2{TCNQ­(MeO)2}] (2), [{Ru2(2,4-F2ArCO2)4}2{TCNQ­(EtO)2}] (3), , [{Ru2(o-ClArCO2)4}2{TCNQ­(MeO)2}] (4), , and [{Ru2(2,4,6-F3ArCO2)4}2{TCNQ­(EtO)2}] (5).

Compound 1 displayed an electronic state of 1.5e-I, contrary to its predicted state of N with ΔE H–L = 0.010 eV, suggesting that it adopted a potential aligned with strategy (II). The D–A set of 2 is precisely positioned at the boundary between the 1e-I and 2e-I electronic states (ΔE H–L = – 0.204 eV), aligning with strategy (I). In fact, solvent-free compound 2 assumes a 1.5e-I electronic state. Compound 3 exhibited the 2e-I state, even though its D–A combination was predicted to be in the 1e-I state (ΔE H–L = −0.059 eV), thus conforming to strategy (II). Compound 4 was previously assigned as the 1e-I state, but its charge partially fluctuated to form 2e-I moieties. In fact, the D–A combination is situated at the boundary between the 1e-I and 2e-I states (ΔE H–L = −0.459 eV). Although the D–A combination in 5 is situated at the boundary between the N and 1e-I states (ΔE H–L = 0.196 eV), its actual state was 1e-I in both cases of (I) and (II). Thus, these five layered D2A-MOFs achieve electronic Coulombic Madelung energy stabilization in a solvent-free state, that is, with dense packing without significant pores or closed pores between layers, and adopt a high potential charge-separated state (Figure b). This state holds the potential to converge toward a lower charge-separated state when guests are inserted between layers during the gate-open process, causing the interlayer pores to open with increasing interlayer distances. Indeed, all five materials induced CT within their layered lattices upon insertion of crystalline solvents or gaseous guests, transitioning to states with lower charge separation.

Thus, the difference in three-dimensional charge stability due to Coulomb interactions, namely, the Madelung potential resulting from interlayer charge separation, is thought to influence intralayer CT significantly. Selecting D–A units and designing lattices with this charge stability in mind are key to designing guest-induced CT systems. On the other hand, it is well-known that accurately predicting three-dimensional charge stabilization, such as the Madelung potential, in MOF design is difficult. Therefore, it becomes important to select D–A combinations where a certain degree of overpotential can be anticipated based on the difference between the HOMO of the D unit and the LUMO of the A unit using ionic phase diagrams. ,,

3. ON–Host Charge Transfer for Magnetic Regulation in D/A-MOF

3.1. Solvent-Induced Magnetic Regulation in D/A-MOF

Solvent-induced structural transformations provide a finely tunable approach for regulating the electronic and magnetic behavior of D/A-MOFs. Because the layered D2A frameworks possess diverse electronic states (N, 1e-I, 1.5e-I, and 2e-I) that are stabilized by a delicate balance between intralayer donor–acceptor CT and three-dimensional Madelung interactions, even subtle variations in the interlayer environment can shift the electronic ground state. The inclusion or removal of solvent molecules perturbs key structural parameters, such as interlayer distance, lateral layer slippage, pore occupation, and weak host–guest interactions (e.g., van der Waals contacts, π–π stacking, and hydrogen bonding), thereby modulating the Coulombic stabilization of charge-separated states.

A preliminary solvent-mediated example was observed in a simple DA chain compound, [{Ru2(3,4-Cl2ArCO2)4}­{TCNQ­(EtO)2}]·DCE (DCE = 1,2-dichloroethane), which exhibited a thermally induced neutral–ionic (N–I) phase transition. Reversible insertion and removal of DCE produced an N–I state switch below 230 K, accompanied by the emergence and disappearance of ferrimagnetic short-range ordering through the chain. In the solvated state, enhanced π–π stacking and a more linear chain geometry promote electron transfer from [Ru2 II,II] to TCNQ­(EtO)2 at 230 K (= critical temperature, T c), generating [Ru2 II,III]+–TCNQ•– pairs and inducing a ferrimagnetic chain with an alternating spin set of S = 3/2 and 1/2. Complete desolvation suppresses this CT process, maintaining a neutral paramagnetic state at temperatures below 300 K. Although this system demonstrates the feasibility of solvent-driven valence modulation, its one-dimensional connectivity limits the diversity of accessible electronic states and the dimensionality of magnetic ordering.

Layered D2A frameworks, owing to their extended two-dimensional donor–acceptor connectivity, exhibit diverse electronic states and support ferrimagnetic ordering over the layer. Simultaneously, the denser arrangement of D–A–D units renders their electronic configurations highly sensitive to modest lattice perturbations, making guest-molecule modulation correspondingly more delicate. Guided by these considerations, we selected a D–A pair of [Ru2(2,3,5-Cl3ArCO2)4] and TCNQMe2, positioned near the boundary between two competing electronic states of N and 1e–I with a ΔE H–L of only 0.010 eV. Nevertheless, [{Ru2(2,3,5-Cl3ArCO2)4}2(TCNQMe2)] (1) possessed the 1.5e-I state, being in an overpotential state corresponding to the strategy (II).

The solvated compound, [{Ru2(2,3,5-Cl3ArCO2)4}2(TCNQMe2)]·4DCM (1-DCM), incorporates four dichloromethane (DCM) molecules per formula unit and exhibits highly cooperative, solvent-driven electronic-state transformations (Figure ). This compound underwent an electronic transformation from the 1.5e-I state to the 1e-I state in the desolvated form, modulated by the inclusion of DCM guest molecules (Figure a). The guest molecules occupy the interlayer channels (Figure b), and their removal leads to subtle structural relaxations, accompanied by a contraction of the interlayer spacing. These changes in the electronic states consequently induce large variations in the magnetic ordering temperatures. Specifically, the solvated compound displayed ferromagnetic long-range ordering at a high Curie temperature of T C = 101 K, whereas upon solvent removal, a drastic reduction to T C = 34 K (Figure c) occurred due to the partial formation of diamagnetic TCNQMe2 2– species that interrupted significant spin exchange. Notably, the readsorption of DCM restored both the electronic configuration and the high T C FM state, confirming the complete reversibility of this transformation. This system illustrates how layered D2A frameworks can access competing electronic states that respond sensitively to solvent-induced structural modulation, enabling reversible control over their magnetic properties.

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(a) Schematic depiction of reversible electronic state transitions between the 1.5e-I and 1e-I states induced by solvent inclusion. (b) Crystal structure of the solvated phase [{Ru2(2,3,5-Cl3ArCO2)4}2(TCNQMe2)]·4DCM (1-DCM) along the a- and b-axes, with dichloromethane (DCM) guests highlighted in cyan. (c) Time-dependent AC susceptibility (χ’ and χ’’) during in situ desolvation. Reprinted with permission from Reference . Copyright 2018, American Chemical Society.

To further evaluate whether the charge state of the solvated phase is governed not only by Madelung stabilization associated with lattice contraction but also by direct host–guest interactions, we selected a structurally rigid framework positioned at the boundary region of the ionicity diagram: the D–A set in [{Ru2(m-FArCO2)4}2{TCNQ­(MeO)2}] (2) is precisely positioned at the boundary between the 1e–I and 2e–I electronic states (ΔE H–L = −0.204 eV). The framework undergoes reversible transitions between two distinct electronic states, 1e-I and 1.5e-I states, depending on the presence or absence of one molar equivalent of DCE in its isostructural analogues. Interestingly, this system revealed the critical role of host−guest hydrogen-bonding interactions in regulating its charge-state behavior and magnetic properties (Figure ). In the DCE-free form (Figure a, top), the absence of hydrogen bonding stabilized the 1.5e-I state. In contrast, the solvated phase (Figure a, bottom) introduces a distinct C–H···O hydrogen bond (3.32 Å) between DCE guest molecules and carboxylate oxygens of the [Ru2] units, which stabilizes the 1e-I state. Complementary computational studies further confirmed that host−guest hydrogen-bonding interactions significantly perturbed the orbital energies of diruthenium centers, thereby ttuning the electron-donating ability of the [Ru2 II,II] moiety. The magnetic properties of these compounds reflect their electronic states: the DCE-free form exhibits FM long-range ordering at T C = 30 K, while the solvated hydrogen-bonded phase elevates T C to 88 K (Figure b).

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(a) Crystal structure of the [{Ru2(m-FArCO2)4}2{TCNQ­(MeO)2}] (2) framework, illustrating the hydrogen bonding interactions between DCE molecules and oxygen atoms of the carboxylate ligands, which stabilize the intermediate 1e-I state. The atoms are denoted as follows: H (white), N (blue), O (red), C (gray), F (green), Cl (cyan), Ru (purple). (b) Field-cooled magnetization curves at 100 Oe for solvated and desolvated forms. Reprinted with permission from Reference . Copyright 2019, Wiley-VCH GmbH.

The above cases show that solvent molecules can modulate the charge-transfer equilibrium in layered D2A frameworks through a global electrostatic effect via a change in Madelung stabilization and an intrinsic HOMO–LUMO energy modulation affected by local host–guest hydrogen bonding interactions. These systems, however, mainly represent cases in which the charge state is tuned between the 1e−I and 1.5e−I states. As a result, the magnetic response is mainly reflected in shifts of T C, while the magnetic ground state remains the same. From a broader perspective, it is particularly appealing to realize guest-induced switching between distinct electronic states of 1e-I and N, or 2e-I via one-electron transfer, since such changes directly alter oxidation states of magnetic centers and reconfigure the spin topology and magnetic ground state.

A representative example of this CT-level control is provided by the charge-flexible layered framework [{Ru2(2,4-F2ArCO2)4}2{TCNQ­(EtO)2}] (3) (Figure ). Notably, the desolvated form 3 exhibits a paramagnetic 2e–I ground state even though its D–A combination is predicted, based on ΔE H–L (−0.059 eV), to fall within the 1e–I region, underscoring the crucial role of lattice electrostatics and packing effects beyond molecular-level energy considerations, i.e., the desolvated form 3 is in an overpotential state. Upon inclusion of volatile organic compounds (VOCs), such as benzene (PhH), p-xylene (PX), DCE, DCM, and carbon disulfide (CS2), the system undergoes a guest-triggered intralattice electron transfer from TCNQ­(EtO)2 2– back to the [Ru2] sites, converting the material to a 1e–I state. This discrete electronic state change regenerates a spin configuration composed entirely of paramagnetic units, leading to the formation of ferrimagnetic layers (Figure a). Magnetic susceptibility measurements reveal a clear transformation from a PM ground state (2e–I) to FM (1e–I), with T C in the range of 70–92 K depending on the specific VOC guest molecule. Moreover, the nature of the magnetic ordering itself is guest-dependent. While PhH, PX, DCE, and DCM stabilize FM ground states, CS2 uniquely induces AFM ordering at approximately 78 K, reflecting the interlayer environment (Figure c). These guest-driven electronic and magnetic phase transitions are fully reversible upon guest removal, indicating that the host framework functions as a reusable, magnetically responsive platform. Structural analyses suggest that CT switching and the associated magnetic responses originate from subtle, guest-specific adjustments in interlayer packing and interactions, which tune both donor–acceptor CT and the resulting exchange pathways. Viewed alongside the previous examples, 3 highlights an important conceptual progression: guest molecules can be used not only to fine-tune ordering temperatures within a fixed electronic-state manifold, but also to switch between distinct electronic states and generate qualitatively different magnetic phases. This distinction is central to the design of charge-flexible D2A frameworks as truly multistate, guest-controllable magnetic materials.

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(a) Electronic configuration transitions from the 2e-I state to the 1e-I state induced by guest molecule insertion. (b) Crystal packing structures of the [{Ru2(2,4-F2ArCO2)4}2{TCNQ­(EtO)2}] (3) framework in the desolvated (top) and benzene-solvated (bottom) phases, viewed along the a-axis. (c) Field-cooled magnetization curves at 100 Oe for the desolvated phase (solid black circles), solvated phase with different pure solvents: benzene (PhH, orange), p-xylene (PX, red), 1,2-dichloroethane (DCE, green), dichloromethane (DCM, azure), and carbon disulfide (CS2, blue), as well as the desolvated phase after solvent removal (open black circles). Reprinted with permission from Reference . Copyright 2021, American Chemical Society.

3.2. Gas-Induced Magnetic Regulation in D/A-MOF

In addition to liquid solvents, gaseous guests, including ubiquitous gases such as carbon dioxide (CO2), oxygen (O2), and nitrogen (N2), represent an important class of chemical external stimuli for D/A-MOFs. Gas molecules offer intrinsically fast adsorption–desorption transport kinetics and excellent reversibility under mild pressure and temperature changes, making them, in principle, highly suitable for dynamic control of framework properties. Furthermore, since typical gas molecules have lower boiling points than room temperature, they can be used over a wide temperature range from their low boiling points. Nevertheless, gas-induced modulation of magnetic behavior in D/A-MOFs has remained comparatively underdeveloped. Conceptually, gas molecules are often treated as weak, nonperturbative adsorbates that primarily probe porosity rather than substantially reshape the electronic landscape, so significant changes in magnetic response have been considered difficult to achieve. Experimentally, rigorous elucidation of gas-driven transformations typically requires in situ crystallographic and magnetic measurements under well-defined gas atmospheres, in combination with sorption and structural analyses, which impose stringent instrumental and methodological requirements and have thus constrained systematic progress in this area. Addressing these challenges requires the integration of controlled gas environments with synchronized structural and magnetic measurements. To this end, we combined controlled gas dosing with in situ physical characterization. In this approach, the sample is sealed in a holder connected to a calibrated gas manifold, enabling precise regulation of gas pressure prior to and during magnetic measurements. For crystallographic tracking, single crystals are loaded into gastight capillaries or cells under fixed atmospheres to maintain constant pressure throughout temperature variation.

A previously reported layered framework, [{Ru2(o-ClArCO2)4}2{TCNQ­(MeO)2}] (4), has been utilized to demonstrate gas-adsorption-induced electronic and magnetic transformation. The electronic state of this guest-free compound 4 has been assigned to be 1e-I state with a ferromagnetic ground state at T C = 65 K. However, the framework adopts a partially charge-fluctuated ordered configuration, in which local disorder leads to the presence of diamagnetic TCNQ­(MeO)2 2– species and additional [Ru2 II,III]+ units (Figure a). These charge inhomogeneities hinder a regular spin alignment of 1e-I state, in a charge-fluctuated state, thereby limiting T C to 65 K. In fact, the D–A set of 4 lies precisely at the boundary between 1e-I and 2e-I states in the ionic diagram (ΔE H–L = −0.459 eV), suggesting that even slight external perturbations can readily induce charge transfer or fluctuations. Upon CO2 adsorption, however, the framework transforms into a charge-ordered state of 1e-I (Figure a), as CO2 molecules densely populate the interlayer voids, suppressing charge fluctuations without significantly altering the crystal structure (Figure b). This gas-induced charge stabilization enables a uniform spin distribution across the layers, raising T C to 100 K. Moreover, the magnetic hysteresis loops at 1.8 K reveal that the CO2-loaded framework slightly enhances the coercivity and remanence relative to the guest-free phase, confirming a robust and reversible magnetic enhancement via CO2 accommodation (Figure c,d).

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(a) Scheme showing the transition from the partially charge-fluctuated ordered configuration in [{Ru2(o-ClArCO2)4}2{TCNQ­(MeO)2}] (4) to the charge-ordered 1e-I state of 1e-I state induced by CO2 adsorption. (b) Crystal structures of the guest-free phase and the CO2-loaded phase viewed along the a-axis. The adsorbed CO2 molecules are shown in CPK models. (c) FCM curves at H = 100 Oe and (d) magnetic hysteresis loops at 1.8 K, showing reversible magnetic switching between guest-free and CO2-loaded phases upon successive He and CO2 dosing cycles. Reprinted with permission from Reference . Copyright 2023, Wiley-VCH GmbH.

Given that [{Ru2(2,4-F2ArCO2)4}2{TCNQ­(EtO)2}] (3) already displays VOC-induced electronic state switching and guest-dependent magnetic responses, its behavior toward gas adsorption was also investigated. The framework underwent a CO2-triggered transition from a 2e–I PM state to a 1e–I AFM state, enabled by charge redistribution between [Ru2] units and TCNQ­(EtO)2 acceptors (Figure a). In the CO2-loaded phase, three equivalents of CO2 per formula unit were incorporated into the interlayer voids (Figure b). Whereas the guest-free phase shows typical paramagnetic behavior (Figure c,d), the CO2-loaded phase exhibits Néel-type AFM ordering at T N = 62 K. The reversible switching between PM 2e–I state and AFM ordered 1e–I state through CO2 adsorption and desorption thus provides a clear example of gas-induced electronic state conversion directly coupled to a magnetic phase transformation within a single D2A framework.

7.

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(a) Scheme showing the transition from 2e-I state in [{Ru2(2,4-F2ArCO2)4}2{TCNQ­(EtO)2}] (3) to 1e-I state induced by CO2 adsorption. (b) Crystal structure of CO2-loaded phase viewed along the a-axis. The right side shows the portion of the structure of the CO2-loaded phase with selected host–guest distances. The atoms are colored as follows: N (blue), O (red), C (gray), F (green), and Ru (pink). Hydrogen atoms are omitted for clarity. The adsorbed CO2 molecules are shown in the CPK models. (c) FCM curves at H = 100 Oe and (d) magnetic hysteresis loops at 1.8 K between guest-free phase (black) and CO2-loaded phase (blue) upon dosing of CO2 and evacuation for the first (square), second (circle), third (triangle), fourth (star), and fifth (diamond) cycle. Reprinted with permission from Reference . Copyright 2023, American Chemical Society.

To further examine gas-induced electronic state changes in D2A frameworks, we synthesized a compound of [{Ru2(2,4,6-F3ArCO2)4}2{TCNQ­(EtO)2}] (5), in which the D–A combination is situated at the boundary between N and 1e–I states (ΔE H–L = 0.196 eV). This layered metal–organic framework 5 exhibits a reversible magnetic phase transition governed by CO2 adsorption (Figure ). The guest-free compound is ferrimagnetic in a 1e−I state, and it undergoes a stepwise transformation upon CO2 uptake into two successive guest-included phases, denoted CO2–I and CO2–II. The adsorption of four CO2 molecules per formula unit into the in-plane pores affords CO2–I, which retains the ferrimagnetic [D+–A––D0] configuration with 1e-I state (Figure a,b, left). With further CO2 uptake into the newly generated interlayer voids, the system converts to CO2–II, in which a complete reversal of electron transfer occurs to produce a neutral [D0–A0–D0] state with PM behavior (Figure a,b, right). These transitions are accompanied by distinct structural reorganizations, including the planarization of TCNQ­(EtO)2 acceptors and expansion of the interlayer spacing. The evolution of structural and electronic states is clearly reflected in the temperature-dependent magnetization curves. At low CO2 pressures (≤5 kPa), the framework remained FM with T C = 110 K. Upon increasing CO2 pressure (≥10 kPa), magnetization dropped abruptly, indicating a transition to PM neutral phase CO2–II (Figure c). The transformation exhibited thermal hysteresis, consistent with a first-order guest-induced magnetic switching process, as shown in the inset of Figure c. Together, these results demonstrate that placing the D–A pair near the N/1e–I boundary enables CO2 adsorption to drive a genuine electronic state reversal and a corresponding FM-to-PM phase transition within a single-layered D2A framework.

8.

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(a) Schematic illustration of electronic configuration transition from 1e-I state in [{Ru2(2,4,6-F3ArCO2)4}2{TCNQ­(EtO)2}] (5) to N state triggered by CO2 adsorption. (b) Crystal structures of CO2 accommodated phases corresponding to the first and second gate-opening steps, viewed along the a-axis and a′-axis, respectively. (c) Temperature-dependent magnetization (M–T) curves measured at H dc = 1 kOe under various CO2 pressures during cooling (filled symbols) and heating (open symbols) processes. The inset highlights the region where the transition between the 1e-I and N states occurred. Reprinted with permission from Reference . Copyright 2021, Springer Nature.

4. Conclusion and Outlook

In this Account, we summarize recent progress in guest-responsive magnetic regulation in redox-active D/A-MOFs, with a particular focus on layered D2A systems assembled from [Ru2(RCO2)4] and TCNQ derivatives. These frameworks are uniquely suited for guest control because their electronic states (N, 1e–I, 1.5e–I, and 2e–I) are diverse and can be shifted by small perturbations in lattice electrostatics and local host–guest interactions. Within this platform, we distinguished two complementary mechanisms: (i) on-host CT, in which neutral solvents or gases modulate charge distribution and spin topology not only indirectly through structural changes associated with Madelung stabilization, but also directly through weak host–guest interactions, and (ii) host–guest CT, in which redox-active guests directly exchange electrons with the framework. Together, these mechanisms enable the reversible tuning of magnetic ground states, including PM–FM and PM–AFM transformations, under mild conditions.

A central outcome of these studies is a set of design principles for constructing MOF magnets with flexible charge states. First, the ionicity diagram approach, based on ΔE H–L and an effective on-site Coulomb term U, provides a practical guide for placing D–A combinations near electronic phase boundaries, where modest structural or electrostatic changes are sufficient to switch between electronic states. Second, the layered D2A topology, with its interlayer voids and gate-opening behavior, offers a natural arena in which guest accommodation modifies interlayer distance, lateral slippage, and local interactions (van der Waals contacts, π–π stacking, C–H···O hydrogen bonding), thereby tuning the Madelung stabilization and CT equilibria. Third, the introduction of redox-active guests, such as iodine, demonstrates that host–guest CT can complement on-host CT, allowing the direct manipulation of radical densities and thus magnetic order, albeit with stricter requirements for framework stability and reversibility.

Future efforts may focus on three research directions to advance this field. The first is the development of multistimuli-responsive frameworks that couple molecular guest adsorption with physical inputs, such as light, pressure, or electric fields, as well as investigation of the framework dynamics. Such synergistic control may enable more adaptive regulation of spin states and lead to the design of magnetic frameworks with stimuli-tunable functionalities. The second key focus is multistate magnetic phase transitions, in which stepwise guest inclusion or redox tuning enables continuous or discrete modulation across multiple magnetic ground states generated by cooperative domains rather than binary switching. Furthermore, more research interest may focus on the diffusion dynamics of guests in D/A-MOFs. Elucidating the diffusion kinetics of gaseous guests and their interactions with spin centers is essential for real-time sensing and memory applications. The third direction involves constructing information transmission functions using hierarchical structures. This holds the potential to create the kind of coordination between mass transport and electrical/magnetic signals seen in biological systems.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (Nos. 22305180, 22471202), Guang Dong Basic and Applied Basic Research Foundation (2025A1515010686), Basic Research Program of Jiangsu (BK20241811), Open Fund of the State Key Laboratory of Coordination Chemistry of Nanjing University (SKLCC2403), and Global Institute for Materials Research Tohoku (GIMRT) of Tohoku University (No. 202405-RDKGE-0519). H.M. and W.K. appreciate the financial support from MEXT/JSPS KAKENHI (JP20H00381, JP21H01900, JP22H00324, JP23K17899, JP25K22264, JP23K21104) as well as support from the GIMRT and E-IMR projects at the Institute for Materials Research, Tohoku University.

Biographies

Jun Zhang received her Ph.D. degree from Tohoku University, Japan, in 2019. She then continued her research as a postdoctoral fellow from 2019 to 2020. In 2021, she was promoted as an assistant professor at the Frontier Research Institute for Interdisciplinary Sciences, Tohoku University. In 2023, she joined Wuhan University as a full professor. Her research interests include the molecular magnetism and chiral molecular framework.

Yang Cao received his Ph.D. degree from Tohoku University, Japan, in 2016. He then worked as a postdoctoral researcher at Tohoku University from 2017 to 2019, followed by a position as an assistant professor at Tohoku University from 2019 to 2023. In 2023, he joined Hubei University as a full professor in the School of Microelectronics. His research interests include nanomagnetism, molecular magnetism, and magnetic sensing.

Wataru Kosaka received his Ph.D. from the University of Tokyo in 2009. After conducting postdoctoral research at Kyoto University, he began his independent academic career in 2011 as an assistant professor at Kanazawa University. In 2013, he joined the Institute for Materials Research, Tohoku University, as an assistant professor and was promoted to associate professor in 2022. His research focuses on molecular magnetism and porous materials.

Hitoshi Miyasaka received his Ph.D. in Chemistry from Kyushu University in 1998. After conducting postdoctoral research at Kyoto University and Texas A&M University, he began his independent career in 2000 as an assistant professor at Tokyo Metropolitan University. He was also a research member of the PRESTO project of JST in 2002–2004. In 2006, he became an associate professor at Tohoku University. Then, he became a full professor at Kanazawa University in 2011. Since 2013, he has been a full professor at the Institute for Materials Research, Tohoku University. His research focuses on solid state chemistry, molecular magnetism, and porous materials.

All authors contributed equally to this work.

The authors declare no competing financial interest.

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