Reversible On/Off Switching of Ferroelectricity in a Molecular FeCo Prussian Blue Analogue with Multiple Control

Domain: chemistry
Published: 2026-03-07
Editorial outcome: minor_revision

Abstract
Polar molecule-based magnetic materials capable of multistate switching have garnered significant interest for their potential applications in next-generation memory devices, sensors, and energy conversion. Among such materials, Prussian blue analogues that exhibit electron transfer–coupled spin transition (ETCST) behavior stand out due to their unique switching properties. In this study, we report a trinuclear cyanide-bridged FeCo compound (1) that exhibits photo- and thermo-induced ferroelectric phase transition via ETCST mechanism. Notably, this FeCo complex also demonstrates a quenching effect, whereby a non-polar state is trapped as a metastable state at low temperature, allowing the polarization control via the cooling rate variations. In addition, reversible single-crystal-to-single-crystal transformation via the desorption and absorption of solvent molecules are observed. The EtOH-removed FeCo compound (1’) loses its ferroelectricity, revealing that the ferroelectric phase transition can be modulated by dynamic desorption/adsorption of the guest molecule. These findings advance the design of functional ferroelectric materials with multistate switching capabilities, offering potential for future applications in contactless memory devices and beyond.

Introduction
Ferroelectric memory devices are distinguished by their inherent ability to retain data in the absence of a continuous power supply, thus offering substantial energy-saving advantages.1-3 Conventional ferroelectric materials typically modulate macroscopic polarization through mechanisms such as order–disorder transitions, ion displacements, and intermolecular charge transfer.4-8 However, these systems often encounter integration challenges stemming from the unavoidable physical contact between electrodes and the ferroelectric layer, underscoring the need for novel strategies to control ferroelectricity.9,10 In addition, the remnant polarization of ferroelectric materials tends to diminish with repeated switching cycles (fatigue), necessitating effective anti-fatigue solutions.11-14 One promising approach is to exploit guest molecules to toggle ferroelectricity on and off, enabling the resets of ferroelectric states without resorting to conventional thermal cycling.

Dynamic molecular materials capable of adopting multiple states have attracted growing interest for their ability to switch various physical properties—such as magnetism and electric polarization—under external stimuli.15-23 To date, most of these systems have been primarily studied for magnetic switching, driven by mechanisms such as spin crossover, electron transfer, and structural transformations.20,22,24-29 Although many of these studies have focused on magnetic switching mechanisms (e.g., spin crossover, electron transfer, and structural transformations), recent findings in molecular Prussian blue analogues demonstrate that thermal- and visible-light-induced polarization switching can be realized through electron transfer-coupled spin transition (ETCST).30,31 Here, directional intramolecular electron transfer induces pronounced changes in molecular dipole moments, thereby enabling macroscopic polarization switching and paving the way for contactless, light-driven memory devices. Despite this progress, there still lacks the direct evidence of electric-field control of polarization in these systems, and the full range of strategies for manipulating polarization remains far from exhausted.

The particularly compelling feature of molecular Prussian blue analogues lies in their long-lived electron-transferred metastable states, which can be accessed with high efficiency via non-thermal channels such as light irradiation or rapid cooling.32 This capability has led to the observation of multistep phase transitions and hidden phases with distinct properties.33-40 Furthermore, solvent molecules could provide an additional freedom of tunability in cyanide-bridged compounds, exemplified by molecular magnetic sponges during guest absorption–desorption processes.41-43 Given the significant dipole changes associated with ETCST, integrating it with ferroelectricity facilitates access to a rich variety of polarization states (vanishing, positive and negative), each responsive to certain stimuli. As a result, multistate polarization manipulation can be achieved by application of electric fields, photoinduced, thermally induced, and guest molecule–dependent processes (Figure 1). In particular, the reversible generation and erasure of ferroelectricity via metastable states created through light exposure or thermal quenching opens up new opportunities for circumventing fatigue, as guest molecule–driven on/off switching of ferroelectricity can be used to reset the material through dynamic desorption–absorption cycles.

In this study, we developed a compound exhibiting directional ETCST by incorporating distinct solvent molecules based on our previous work30, i.e., [Fe(Tp)(CN)3]2[Co(dpa)2]·2H2O·1.5EtOH (1) (Tp = trispyrazolylborate, dpa = 2,2'-dipyridylamine, EtOH = ethanol) to achieve multistate polarization switching (Figure S1a). As anticipated, 1 undergoes a nonpolar-to-polar phase transition during the ETCST process, a distinguishing feature among Prussian blue analogues. Its ferroelectric nature is confirmed by the reversal of spontaneous polarization under an applied electric field. Notably, the metastable FeIII LS(μ-CN)CoII HS state can be trapped as an MS* (quenched) state at low temperature via thermal quenching (rapid cooling), which closely resembles its photoinduced metastable phase (MS) and high-temperature (HT) phase. Moreover, a controlled thermal treatment yields the ethanol-removed compound [Fe(Tp)(CN)3]₂[Co(dpa)₂]·2H₂O (1’) via a single-crystal-to-single-crystal transition, which undergoes a distinct ETCST process without polarization switching—highlighting the crucial role of solvent molecules in modulating ferroelectricity. These trapping effects, in combination with tunable vapor absorption–desorption, enable the formation of multiple states, including the HT, low-temperature (LT), quenched, and photoinduced MS in 1, as well as the HT and LT states in 1’. These states were characterized by single-crystal X-ray diffraction (XRD), infrared (IR) spectroscopy, and magnetic susceptibility measurements, and polarization switching behavior could be directly tracked by pyroelectric measurements. Therefore, in addition to conventional light- and thermal-driven ferroelectric phase transitions, we demonstrate polarization control through both the cooling rate and the dynamic desorption–adsorption of guest molecules.

Results
The temperature-dependent direct-current magnetic susceptibility of 1 was measured with an external magnetic field of 2000 Oe at a temperature range of 5–200 K using freshly filtered microcrystalline samples (Figures 2 and S1b). Below 140 K, the product of magnetic susceptibility and temperature (χmT) was maintained at ca. 0.78 cm3 mol−1 K, which was consistent with a FeIII LS ion with S = 1/2, suggesting that the diamagnetic pair [FeII LS–CoIII LS] was formed in the LT phase. Further heating caused an abrupt increase in the χmT to ca. 4.05 cm3 mol−1 K above 184 K, which was higher than the spin-only value obtained with an uncoupled CoII HS ion (S = 3/2) and two FeIII LS ions (S = 1/2) in the HT phase. This is caused by the significant unquenched orbital angular momentum for both ions in octahedral ligand fields. During the cooling process, the same transition of magnetic susceptibility occurred at a different temperature. By analyzing the χmT with respect to temperature, phase transition temperatures of T1/2(↓) = 148 K and T1/2(↑) = 170 K with a hysteresis of 22 K were obtained. Thus, the reversible bistability from the thermal-induced ETCST between the LT phase [FeII LS–CoIII LS–FeIII LS] and HT phase [FeIII LS–CoII HS–FeIII LS] were confirmed. This process was strongly supported by recording variable-temperature IR spectra from 140 to 200 K (Figure S2). In the LT phase, vibration bands observed at 2089 and 2187 cm−1 were attributable to the bridging νCN stretching mode of the FeII LS(μ-CN)CoIII LS and FeIII LS(μ-CN)CoIII LS linkages, respectively. Two absorptions at 2053 and 2062 cm−1 were attributable to the terminal cyanides of [FeII(Tp)(CN)3]2− , whereas only one peak at 2131 cm−1 was ascribable to the νCN mode of the uncoordinated cyanide of [FeIII(Tp)(CN)3] −. As the temperature increased above 165 K, the peak at 2187 cm−1 gradually declined and new bands were observed at 2127 and 2164 cm−1, assigned to the terminal νCN absorption of [FeIII(Tp)(CN)3]− and the bridging νCN absorption of FeIII LS(μ-CN)CoII HS, respectively.44-47 In addition, the ultraviolet–visible–near IR (UV–VIS–NIR) absorption spectra of 1 was recorded in the HT and LT phases (Figure S3). The UV–VIS-NIR spectrum of the LT phase exhibited a broad absorption peak centered at 880 nm, which was assignable to the FeII → CoIII intervalence charge transfer band.44-47 With an increase in the temperature, the absorption of this peak disappeared in the HT phase.

Single-crystal structures of freshly filtered 1 were collected to characterize the structural rearrangements during thermal-induced ETCST processes (Figure 3). Notably, in the LT phase, 1 was observed to adopt a polar crystal structure in the Pn space group (Table S1). The asymmetric unit comprised two V-shaped neutral [Fe(Tp)(CN)3]2[Co(dpa)2] motifs, four water molecules, and three EtOH molecules. The Co–N bond lengths ranged from 1.893(7) to 1.954(7) Å, suggesting the typical coordination environment of a CoIII LS ion. The Fe ions were coordinated by three N atoms of a Tp ligand and three C atoms from cyanides, resulting in two pair chemically nonequivalent Fe ions (Fe1, Fe2 and Fe3, Fe4), which were characterized by the first coordination sphere as follows. The bond lengths of Fe1–N and Fe1–C were in ranges of 1.998(6)–2.007(6) Å and 1.850(7)–1.911(8) Å, respectively. The Fe2–N and Fe2–C bond lengths were in ranges of 1.956(6)–1.993(7) Å and 1.905(8)–1.937(8) Å, respectively. The Fe3–N and Fe3–C bond lengths ranged from 1.961(6) to 1.980(6) Å and 1.908(7) to 1.941(7) Å, respectively. Finally, the Fe4–N and Fe4–C bond lengths ranged from 2.003(6) to 2.009(6) Å and 1.869(7) to 1.911(8) Å, respectively (Table S2). The differences in the bond lengths of Fe–N and Fe–C between both pairs of Fe ions, which exceeded the experimental error, represented the difference between FeIII LS and FeII LS, compared with the reported structures constructed with Tp building blocks. This indicated that Fe2, Fe3 corresponded to FeIII LS, and Fe1, Fe4 corresponded to FeII LS, respectively.30,48-51 The bond length data indicated that the formation of [FeII LS–CoIII LS–FeIII LS], FeII, and FeIII were distinguishable. In the crystal lattice, one ordered ethanol molecule forms a hydrogen bond with a non-bridging cyanide group, while each of the other two ethanol molecules simultaneously engages in hydrogen bonding with both a water molecule and another non-bridging cyanide group. All water molecules participate in hydrogen bonding with the nitrogen atoms of the dpa ligands. Through these interactions, the trinuclear [Fe₂Co] units are further extended into a three-dimensional network stabilized by hydrogen bonds involving both water and ethanol molecules.

As the temperature increased to the HT phase, the space group changed to P2/n with a nonpolar structure. The asymmetric unit comprised two half [Fe(Tp)(CN)3]2[Co(dpa)2] trinuclear motifs, two water molecules, one ordered EtOH molecule, and a half disordered EtOH molecule. The Co–N bond lengths of both Co ions ranged from 2.119(2) to 2.167(2) Å, suggesting the typical coordination of CoII HS. The Fe–N and Fe–C bond lengths of both Fe ions ranged from 1.968(2) to 1.986(2) Å and 1.909(2) to 1.935(2) Å, respectively, which were common values for FeIII LS ions. The bond length data indicated the formation of [FeIII LS–CoII HS–FeIII LS]. Interestingly, one of the EtOH molecules, which form hydrogen bonds with the uncoordinated cyanide group of FeII in the LT phase, was disordered in the HT phase because of the oxidation of FeII. These features of the HT and LT phases were consistent with that of a thermal-induced directional ETCST between the [FeII LS–CoIII LS–FeIII LS] and [FeIII LS–CoII HS–FeIII LS] phases, implying a thermal-induced ferroelectric phase transition process. The remaining ethanol and water molecules exhibit only minor changes in orientation. From the perspective of crystal packing, the directional electron transfer process is accompanied by an order–disorder transition of the ethanol molecules. This observation highlights the crucial role of ethanol in facilitating the ferroelectric phase transition. These findings underscore the importance of guest molecular dynamics in governing the switchable ferroelectric properties of the system.

Moreover, the optical second harmonic generation (SHG) measurements on crushed single crystal samples were also performed to investigate the symmetry change during the phase transition process (Figure 4). At temperatures above the phase transition, the SHG intensities were almost vanishing, consistent with a centrosymmetric structure. Upon cooling across the transition temperatures, the SHG intensities increase abruptly, indicating a change from centrosymmetric to noncentrosymmetric space groups. The SHG response displayed clear thermal hysteresis, with transition temperatures in excellent agreement with those obtained from magnetometry. Therefore, the symmetry-breaking nature of this phase transition process was further confirmed.

Metastable states often exhibit distinct physical properties compared to those of the ground state and can be achieved through light irradiation and rapid cooling. Given that the low-temperature phase is polar, we propose that the metastable state could facilitate access to the paraelectric phase at low temperatures.

When the sample were directly placed at a temperature below 60 K from HT phase, the χmT would be maintained at ca. 4.05 cm3 mol−1 K, indicating the presence of a quenching effect (Figure 2). This value was consistent with that of the HT phase, which suggests that the metastable state induced by quenching resembles the high-temperature phase. Further heating caused an abrupt decrease in the χmT to approximately 0.78 cm3 mol−1 K above 120 K, through a relaxation process from a quenching (MS*) state back to the LT state at T1/2 = 107 K. To further explore the structural rearrangements, we collected single-crystal structures of freshly filtered 1 at 30 K under quenching (MS*) conditions. Notably, the quenching effect stabilized the P2/n space group, indicating that the MS* structure closely resembles the HT phase (Table S3). The asymmetric unit comprised two half trinuclear motifs, two water molecules, half a disordered EtOH molecule, and an ordered EtOH molecule. The Co–N bond lengths of both Co ions ranged from 2.066(2) to 2.149(2) Å, suggesting the typical coordination of CoII HS. The Fe–N and Fe–C bond lengths of both Fe ions ranged from 1.945(2) to 1.980(4) Å and 1.903(3) to 1.923(3) Å, attributable to two FeIII LS ions (Table S4). The disordered EtOH molecule formed hydrogen bonds with the cyanide groups in the two trinuclear units that were adjacent to it (Figure 2). The main structural difference between the HT phase and the MS* originated from the different orientations of the ordered EtOH molecules.

Rapid cooling of the sample preserves the paraelectric phase at low temperatures, rather than transitioning to the ferroelectric phase, indicating that ferroelectricity can be controlled through the quenching effect (Figure 3 and S4). During relaxation to the low-temperature phase, a ferroelectric phase transition occurs from the P2/n (MS* phase) to the Pn (LT phase), implying that the paraelectric phase can be the thermodynamically metastable state.

In addition, photoinduced ETCTST behavior of 1 was also detected. The MS phase was obtained by irradiating a fine freshly filtered microcrystalline sample with a red light (785 nm, 10 mW) at 5 K (Figure 1). Upon irradiation, the χmT rapidly increased with a saturation of ca. 4.05 cm3 mol−1 K around 15 K after 30 min. This suggested that the MS formed with a practically quantitative light-excitation effect. A sudden decrease from ca. 4.05 to ca. 0.78 cm3 mol−1 K was observed at T1/2 = 84 K, suggesting a relaxation process from the MS back to the LT state.

Light-induced ferroelectric phase transition has also been confirmed by single-crystal structural analysis (Figure S4). After irradiation with 785 nm laser (10 mW) for 30 min, the compound underwent a transition from the ferroelectric phase (Pn) to the paraelectric phase (P2/n). The data showed that the structure in MS was similar to that in the HT phase and MS* state (Table S3), with the primary distinction among the three phases being the subtle differences in the orientations of the crystallization solvents, namely water and ethanol. The asymmetric unit comprised two half trinuclear motifs, two water molecules, half a disordered EtOH molecule, and an ordered EtOH molecule. The Co–N bond lengths of both Co ions ranged from 2.074(4) to 2.159(3) Å, suggesting the typical coordination of CoII HS. The Fe–N and Fe–C bond lengths of both Fe ions ranged from 1.953(4) to 1.980(4) Å and 1.902(4) to 1.932(4) Å, attributable to two FeIII LS ions (Table S4).

Through the quenching effect and light radiation, we successfully stabilized the paraelectric phase at low temperatures in this ferroelectric material by utilizing the metastable state. This achievement also demonstrates effective control over the ferroelectric phase transition through multistate switching.

The occurrence of a thermal-induced ferroelectric type phase transition indicates that the pristine sample is in the ferroelectricity “on” state. As shown in Figure 3, in this state, there are two kinds of [Fe(Tp)(CN)3]2[Co(dpa)2]–EtOH structural units labeled with red (A) and blue (B) circles in the HT and LT phases, respectively. In the HT phase, the ordered EtOH molecule formed a hydrogen bond with the neighboring cyanide, and the disordered EtOH molecule formed a hydrogen bond with two neighboring cyanides. After the transition from LT to HT phase, the dipole moment of the trinuclear molecular unit changed via ETCST, where the orientation of EtOH in the A-unit changed from the ordered state in the LT phase to a disordered state in the HT phase. The EtOH in the A-unit was rearranged to form a hydrogen bond with the noncoordinating cyanide group of FeII. For the B-unit, the EtOH molecule is always in an ordered state with only slight orientation changes during the thermal-induced ETCST process. The structural analysis showed that the compound underwent a directional ETCST during the structural transition in response to the temperature, where the EtOH molecules play an important role.

Given that ethanol molecules play a key role in the ferroelectric phase transition, we speculate that the compound will lose its ferroelectricity after removal of ethanol. The process of ethanol removal was characterized by powder XRD (Figure S1b) and thermogravimetric analysis (TGA, Figure S5a). The TGA of 1 revealed a weight loss of 8.45% from 65°C to 200°C, corresponding to the loss of two water and one and a half EtOH molecules (8.75%), thus confirming the chemical composition. After the weight loss, a plateau was observed up to the point where decomposition occurred at ca. 200°C, suggesting the formation of [FeIII(Tp)(CN)3]2[CoII(dpa)2]. When the pristine sample was heated to 150°C and then cooled, a gradual increase in weight is observed during the natural cooling process in the air, eventually stabilizing at a total weight loss of approximately 6% (Figure S5b), which corresponds to a new dihydrate phase with the release of 1.5 EtOH molecules, 1’. To confirm the stability of this 1’ phase, we performed TGA on 1’ single crystals that had been stored at room temperature for one month (Figure S5c). Upon heating, a weight loss of 3.18% was observed, consistent with the release of two water molecules. Notably, this weight loss was fully recovered during the subsequent natural cooling process, indicating that 1’ does not absorb more water molecules from ambient air under these conditions and remains structurally stable.

Interestingly, a solvent-dependent change in χmT was observed during the direct-current magnetometry (Figure 5b). After removal of ethanol, the temperature-dependent magnetic susceptibility showed that 1’ underwent an ETCST process (the 1’ was obtained by exposing the fresh 1 sample to the air at room temperature for one month, which was confirmed by the powder XRD patterns). Below 230 K, the magnetic susceptibility and temperature (χmT) value was maintained at ca. 0.85 cm3 mol−1 K, which correlated with a FeIII LS ion with S = 1/2, suggesting that the diamagnetic pair [FeII LS–CoIII LS] was formed in the new low-temperature phase (LT*). Further heating led to an abrupt increase in the χmT to ca. 4.05 cm3 mol−1 K above 262 K, which was consistent with the value obtained with an uncoupled CoII HS ion (S = 3/2) and two FeIII LS ions (S = 1/2) in the new high-temperature phase (HT*). In the cooling process, the same transition of magnetic susceptibility occurred at a different temperature with the pristine sample. By analyzing the χmT with respect to temperature, phase transition temperatures of T1/2(↓) = 245 K and T1/2(↑) = 258 K with a hysteresis of 13 K was obtained. Thus, another reversible bistability from the thermally induced ETCST between the LT* phase [FeII LS–CoIII LS–FeIII LS] and HT* phase [FeIII LS–CoII HS–FeIII LS] states was confirmed. This process was supported by variable-temperature IR spectra recorded from 250 to 300 K (Figure S6).

After removal of ethanol, single-crystal structures of 1’ were collected at 120 K (LT* phase) and 300 K (HT* phase) to characterize the structural rearrangements during both thermally induced ETCST processes (Figure 5a, Table S5). Notably, during the transformation from 1 into 1’, the space group changed from nonpolar P2/n to nonpolar C2/c in the HT phase. The asymmetric unit comprised a half [Fe(Tp)(CN)3]2[Co(dpa)2] trinuclear motif and two water molecules. The Co–N bond lengths of both Co ions ranged from 2.084(3) to 2.152(3) Å, suggesting the typical coordination of CoII HS. The Fe–N and Fe–C bond lengths of both Fe ions ranged from 1.986(3) to 1.992(6) Å and 1.909(3) to 1.935(4) Å, respectively, which were common values for FeIII LS ions. The unit cell volume shrank because the distance of two trinuclear motifs significantly reduced after the EtOH desorption.

With a decrease in temperature, the crystals remain in the nonpolar C2/c space group at the LT* phase. The asymmetric unit comprised a half [Fe(Tp)(CN)3]2[Co(dpa)2] trinuclear motif and two water molecules, similar to the HT* phase. The Co–N bond lengths of both Co ions ranged from 1.887(2) to 1.945(2) Å, suggesting the typical coordination of CoIII LS. The Fe–N and Fe–C bond lengths of both Fe ions ranged from 1.992(2) to 1.999(2) Å and 1.875(3) to 1.925(3) Å, respectively (Table S6). These structural characteristics suggested the existence of [FeIII LS–CoIII LS–FeII LS]. However, FeIII and FeII could not be distinguished because of random occupation at different lattice sites.49,52,53 This non-directional ETCST behavior prevents polarization switching at the crystal level, resulting in the ferroelectricity “off” state upon EtOH desorption (Figure S7).

This ethanol removal process was observed to be reversible. After exposure to the vapor atmosphere of ethanol, the sample was recovered and exhibited the same thermal stability shown in the TGA curves. Consequently, this ethanol removal and absorption process can occur in the same crystal, and the reversibility of this process suggests that the sample underwent dynamic vapor sorption. Upon absorption process, 1’ transforms to 1, thereby restoring the ferroelectric properties. This transition enables on/off switching of ferroelectricity through the reversible absorption and desorption of guest molecules.

A ferroelectric phase transition was observed during the thermally induced ETCST.30 The structural analysis showed that during the thermally induced ETCST process, the orientation of EtOH molecules underwent directional changes with the emergence of distinguishable FeII, which implied the existence of a directional ETCST process at the single-crystal level. The molecular packing revealed a directional electron transfer between Fe and Co ions. This electron transfer process was accompanied by a transformation between the disordered and ordered states of the EtOH molecule (Figures 2 and S8). This is tentatively ascribed to the change in the dipole of the trinuclear molecule after the directional electron transfer, leading to a directional rotation of the EtOH molecule.

To provide more direct evidence of polarization change, a single-crystal sample was sandwiched with silver pastes on the (001) and (00–1) surfaces for pyroelectric measurement. For the ferroelectric phase transition process from HT to LT, during the cooling process, the peak of pyroelectric coefficient (p) was observed at T(↓) = 148 K, which is consistent with the magnetic properties and structural changes during the phase transition from HT to LT, indicating a polarization change (Figure S9). The polarization change (ΔP) obtained by integrating p is about 140 nC/cm2 during the temperature decreased from 170 to 130 K, which is almost same with the polarization change during the relaxation process from MS* to LT, indicating that the ferroelectric phase transition process is reliable. Noting that crystals tend to crack during the ETCST process, it is difficult to observe reversible polarization changes of single crystal sample during the heating and cooling processes in the experiments. To further demonstrate the ferroelectric transition, the temperature-dependent polarization change was obtained by integrating the pyroelectric current under a reverse electric field based on powder pellet of 1 (Figure S10). The results of reversed spontaneous polarization under a direct-current electric field confirmed the ferroelectricity of 1. Thus, the thermally induced polarization switching is well characterized, indicating a thermo-induced ferroelectric phase transition.

Noting the quenching effect of 1, we placed the crystal sample directly at 30 K from room temperature (300 K) and recorded the current signal during the heating process to verify the occurrence of the ferroelectric phase transition. During the thermal relaxation process, the peak of pyroelectric coefficient (p) was observed at T(↑) = 118 K, which is consistent with the magnetic properties and structure change during the phase transition from MS* to LT, indicating a polarization change (Figure 6). The polarization change(ΔP) obtained by integrating p is about 120 nC/cm2 during the temperature increased from 50 to 150 K, which is consistent with the thermally induced ferroelectric phase transition process. Additionally, the polarization change during the photoinduced ferroelectric phase transition was detected at the single-crystal level. Upon irradiation with a 785 nm laser at 5 K, a sharp current peak was immediately observed, indicating a transition from the polar phase to a nonpolar phase. The light exposure was maintained for 500 seconds, during which the current gradually decreased to a negligible level (Figure S11). Subsequently, the temperature was increased. As the photoinduced nonpolar MS state relaxed back to the polar LT state, a reverse peak in the pyroelectric current was observed at approximately 110 K, confirming the recovery of the polar phase.

To theoretically investigate the origin of polarization change during the structural transition, point charge and density functional theory (DFT) calculations were conducted (Figure 7a and 7b). The point charge calculations suggested that the polarization change caused by the pure electron transfer behavior along the c-axis was 2.45 μC cm−2 (supporting information). The DFT calculations were performed on the asymmetric unit that comprised two trinuclear units and three EtOH molecules, which were extracted from the LT X-ray structure with optimized positions of hydrogen atoms. Given the symmetry constraints, spontaneous polarization was expected to occur in the ac plane. To distinguish the contributions of electron transfer and EtOH reorientation to the polarization change, polarization was calculated for the models containing only the two trinuclear units or the three EtOH molecules, each held at fixed positions. The results showed that electron transfer contributed 3.22 µC cm−2, whereas the reorientation of EtOH molecules contributed 0.43 µC cm−2, with an angle of 160.4°. Thus, we concluded that directional electron transfer played a major role in the observed polarization change.

To summarize the above results, we observed that, in this crystal, polarization can be controlled through desorption/absorption of guest molecules and metastable state via quenching effect and light irradiation. During rapid cooling, MS* enables retention of the high temperature paraelectric phase at low temperatures due to the quenching effect. In contrast, during slow cooling, the spontaneous polarization from the ferroelectric phase transition can be recorded via the current signal, thereby achieving polarization control via the cooling rate. After removal of ethanol, a single-crystal to single-crystal transformation occurred; however, no ferroelectric phase transition was observed in the new phase, 1’. Thus, polarization control through desorption/adsorption of guest molecules was also achieved, which demonstrates that the orientation-directed change of the lattice solvent facilitates directional ETCST at the crystal level.

The results of this study demonstrate that six stable states—HT, LT, HT*, LT*, MS, and MS*—can be achieved in 1 complex via guest molecule-dependent, thermally induced, and photo-induced ETCST processes. These states and their stability have been demonstrated via single-crystal structural analyses, magnetometry, and IR spectroscopy. During the ETCST process, the directional electron transfer between Fe and Co, accompanied by the rearrangement of the EtOH molecules within the crystal lattice, induces polarization switching, representing an example of polarization control via ETCST. The observed ferroelectric behavior, driven by intramolecular electron transfer, highlights the promise of cyanide-bridged compounds as candidates for next-generation memory devices. Moreover, the ability to control the polarization through both cooling rate and desorption/absorption of the guest molecules introduces new strategies for manipulating the spontaneous polarization of ferroelectrics. The reversible on/off switching of ferroelectricity via the dynamic desorption/adsorption of guest molecules offers a promising new approach to mitigate ferroelectric fatigue. This study builds upon our previous findings, further validating the feasibility of directional ETCST at the single-crystal level, providing valuable insights to guide the design and synthesis of additional electron transfer compounds with polarization switching capabilities. 54-59 Our light-induced polar to non-polar conversion, combined with guest-controlled on/off ferroelectricity, complements prior light-assisted polar-state control and establishes a distinct axis for photo-programmable ferroics. We anticipate that coupling ETCST to lattice symmetry will enable new device concepts for non-volatile, optically addressable states.60,61

Methods
Materials and Synthesis Methods. All chemicals used were of analytical grade, and solvents were kept anhydrous using molecular sieves. The dpa was commercially available (dpa = 2,2'-dipyridylamine).

Synthesis of [Fe(II)(Tp)2]. FeCl2·6H2O (59 g 0.25 mol) was added to a solution of KTp (126 g, 0.5 mol) under vigorous stirring. A purple precipitation of Fe(II)Tp2 was formed, which was collected by filtration and dried in the air. Yield: 99% (119 g).

Synthesis of Bu4N[Fe(Tp)(CN)3]. [Fe(II)(Tp)2] (48 g, 0.1 mol) and NaCN (25 g, 0.5 mol) were mixed in 2-propanol (300 mL), and the resulting suspension was heated at 80 °C for five hours under continuous stirring. The initial purple solid turned yellowish brown and was poured off while leaving unreacted NaCN at the bottom. The solid was washed with hot water, air dried, dissolved in 500 mL water, and filtrated again to remove undissolved materials. A solution of Bu4NBr (32.2 g, 0.1 mol) was added to the filtrate under stirring, with formation of a brownish solid. Dropwise addition of 30% H2O2 to this suspension caused precipitation of the crude product as an orange-red crystalline solid. Yield: 85% (48 g).

Synthesis of [Fe(Tp)(CN)3]2(Co(dpa)2)·2H2O·1.5EtOH (1). An aqueous solution of Co(II)(ClO4)2·6H2O (36.5 mg, 0.1 mmol, 2.5 mL) was placed at the bottom of a long test tube followed by a buffer layer of 0.6 ml EtOH and 0.4 ml water, and a solution of Bu4N[Fe(Tp)(CN)3] (112 mg, 0.2 mmol) and dpa (34 mg, 0.2 mmol) in 2.5 mL EtOH was layered over it. This was left undisturbed at 40 °C for one month, and light red prism crystals were separated by filtration and washed with water (3 mL × 3). The yield was about 36% based on Co(II)(ClO4)2·6H2O. (Elemental analysis calcd (%) for C47H51N24O3.5B2Fe2Co: C 47.03, H 4.28, N 28.01; Found: C 47.11, H 4.31, N 28.11)

Synthesis of [Fe(Tp)(CN)3]2(Co(dpa)2)·2H2O (1’). The ethanol-removed sample 1’ was obtained by exposing the fresh 1 sample to the air at room temperature for one month.

X-ray crystallography. All single crystals were coated with an oil-based cryoprotectant and mounted on nylon loops. Single-crystal X-ray structures were obtained for all phases. The Crystallographic Information Files of the crystal structures can be obtained free of charge from the Cambridge Crystallographic Data Centre for 1 at 100 K (LT) and 300 K (HT), for 1’ at 120 K (LT*) and 300 K (HT*) (CCDC Rigaku FR-E+ diffractometer equipped with a HyPix-6000 area detector, using a multilayer mirror monochromated Mo-Kα radiation (λ = 0.71073 Å), for 1 after irradiation (MS) and quenching (MS*) at 30 K (CCDC Rigaku varimax diffractometer equipped with a HyPix-6000 area detector, using a multilayer mirror monochromated Mo-Kα radiation (λ = 0.71073 Å)). The structures were solved by a direct method and refined via full-matrix least-squares on F2 using the SHELX program62,63 implemented in the OLEX2 program64 with anisotropic thermal parameters for all nonhydrogen atoms. The hydrogen atoms were geometrically added and refined by the riding model. Powder X-ray diffraction patterns were recorded on a Rigaku-TTR diffractometer to examine purity and difference between 1 and 1’.

Magnetism measurements. Magnetic susceptibility measurements were conducted on a Quantum Design MPMS-XL superconducting quantum interference device magnetometer under a 2000 Oe field, with a sweeping rate of 2 K·min−1 in a 5–300 K temperature range. The samples were prepared by encapsulating polycrystalline samples (ca. ~15mg) into a gelatin capsule. Photomagnetic measurements were performed on powdered samples attached to transparent tape. A 785 nm laser (10 mW) was used as the excitation source. The sample was irradiated for 1 h with the cooling valve open to maintain a low-temperature environment. Relaxation measurements were performed from 5 to 130 K.

IR spectroscopy. Temperature-dependent IR spectra were recorded on a JASCO FT/IR-600 Plus spectrometer equipped with temperature control system. The powder samples were held between grained and flat CaF2 plates. The experiment of 1 was performed with the temperature ranging from 140 to 200 K in heating process. The experiment of 1’was performed with the temperature ranging from 100 to 300 K in heating process.

Optical second-harmonic generation. Optical second-harmonic generation (SHG) was measured using a femtosecond Er³⁺-doped fiber laser (1550 nm, 100 fs, 20 MHz; Calmar FPL-03C). Thin flakes from a crushed single crystal were mounted in a heat-exchange-gas optical cryostat for continuous temperature control. A chopped excitation beam was normally incident, and the transmitted SHG signal was isolated by multiple optical filters, detected with a cooled photomultiplier, and analyzed by lock-in detection.

Theoretical Calculations. DFT calculations were conducted using the ORCA 5.0.1 quantum chemistry package.65,66 An asymmetric pair consisting of [Fe2Co] and ethanol molecules was extracted from the single-crystal structure recorded at 100 K, with the Cartesian x- and y-axes aligned to the crystalline a- and b-axes, respectively. The z-axis was defined as the cross product of the x-vector and y-vector, and this orientation remained fixed throughout the process. The coordinates of all non-hydrogen atoms were kept fixed, while only the hydrogen atoms' coordinates were optimized geometrically. To prevent self-interaction errors that could lead to a mixing of the two Fe sites' valence, the range-separation functional cam-B3LYP67-71 was employed for all calculations, using the Karlsruhe triple-zeta basis def2-TZVP72-74. Atom-pairwise dispersion correction was applied along with Becke–Johnson damping.75,76 The molecular dipole moments were then calculated based on structural fragments extracted from the optimized model. The macroscopic polarization was determined as the volume average of the sum of the molecular dipole moments in a unit cell.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "In this work, the authors report an interesting electron transfer–coupled spin transition (ETCST) mechanism for the implementation of photo- and thermo-induced ferroelectric phase transition, based on the use of a trinuclear cyanide-bridged [Fe2Co]·1.5EtOH compound. ETCST behaviors with unique switching performances are in the spotlight of devices for next-generation memory devices, sensors, and energy conversion. In this sense, the present work, by introducing a new way to ensure multistate switching capabilities, may give a significant contribution to the field. The manuscript is well-organised, and several experimental results have been provided to validate the proposed design, although some further explanations are needed to improve the scientific soundness of the paper.\n1. To confirm the existence of reversible structural phase transition and to further elucidate its origin, DSC and temperature-dependent dielectric measurements should be performed. Such experiments would provide critical evidence to validate the proposed mechanisms and enhance the understanding of the material's dielectric behavior.\n2. It appears clear from Figure S11 that the single-crystal device is sensitive to irradiation. However, no significant reverse pyroelectric current signal was observed after the irradiation was canceled. Does this affect the reliability and the reproducibility of the pyroelectric current signal? I suggest the authors to elaborate a little bit more in this and supplement the discussion on irradiation intensity dependence.\n3. Authors need to supplement the existing representative literature to compare with the light-induced ferroelectric phase transition performance of [Fe2Co]·1.5EtOH. You can refer to \"Angew. Chem. Int. Ed. 2025, 64, 1, e202413898 and Angew. Chem. Int. Ed. 2025, 64, 21, e202501238\".\n4. The manuscript contains several minor writing errors that should be addressed in revision. For example, \"SH Intensity\" should be substitute with \"SHG Intensity\" in Figure 3; in the section \"Optical second-harmonic generation\", \"SH signal\" should be substitute with \"SHG signal\".", "reviewer_2": "The manuscript submitted by Huang et al. follows a recently developped research line of professor Sato group devoted to the study of thermally-induced or photo-induced ferroelectric switching in Fe–Co charge transfer complexes that are able to show reversible iron-cobalt electron transfer coupled to a spin transition on the cobalt ions (so called ETCST). The authors have previously shown on a very similar cyanide-bridged Fe2Co trimetallic complex as the one investigated in this work (a solvatomorph) a light-induced change of electric polarity (see cited article, Sato et coll, J. Am. Chem. Soc. 2024). In the present submitted work, they have extended these ideas showing that a reversible and controllable adsorption/desorption of ethanol solvent molecules (associated to single-crystal to single-crystal phase transition) can also be used to control the ferroelectric state in this Fe2Co complex. Additionally, they also show that other stimuli that are already known to control the iron-cobalt reversible electron transfer are also efficient in the present case. Overall, the experimental work is well carried out, and the article easy to read, the conclusion are supported by sound experimental results. However, I am afraid the present contribution lacks a bit of novelty in comparison to the recently published JACS article. It appears rather incremental for a publication in Nature communication. Once again, the compound is almost the same as the one in JACS, the key mechanism, ETCST driven polarization switching, is the same. The newly introduced solvent-mediated control (which is known to control ETCST in various FeCo systems) represents an incremental extension rather than a breakthrough. Moreover, the ferroelectric control via solvent adsorption/desorption might be conceptually interesting but it has very limited technological relevance, given the slow kinetics of the phenomenon and the very high sample sensitivity (loss of crystallinity upon cycling etc). I would rather recommand publication in a high factor journal as a complementary full work of the JACS article , for exemple in ACS Chem Mater.", "reviewer_3": "Su, Wu, Sato and coworkers explore a trinuclear FeCo Prussian blue analogue which shows On/Off switching of ferroelectricity. The current compound is a new EtOH solvate of a known compound (Ref. 30) and as such represents another example in the series of trinuclear systems explored by these authors that all show a polar/non-polar state accessible thermally and by light. Moreover, recent references beyond the authors’ own work reveal that solvent effects and SC-SC transformations are not new in PBA chemistry: https://doi.org/10.1021/jacsau.5c00681 (an example of solvent effects in a 1D PBA) and Chem. Sci., 2025, 16, 130-138; a recent example of SC-SC transformation in a ETCST system.\nHowever, these systems simply show ETCST and do not have the additional ferroelectric component. Hence, to the best of my knowledge this is the first time that anyone has demonstrated multiple switching in a molecular PBA analogue with on/off switching of ferroelectricity. The authors also make every effort to characterise the different phases upon thermal, light and EtOH cycling, clearly demonstrating the level of novelty and, for the most part, the experimental completeness I would expect for Nat. Commun.\nDespite this there are areas of improvement in the paper. The so-called desolvated system, is in fact a dihydrate only system. I understand the desire to keep the nomenclature simple, but this description of the compound as being desolvated is inaccurate. I suggest another term is used perhaps dihydrate for [Fe(Tp)(CN)3]2[Co(dpa)2]·2H₂O ([Fe₂Co]) and alcohol-dihydrate for [Fe(Tp)(CN)3]2[Co(dpa)2]·2H2O·1.5EtOH ([Fe2Co]·1.5EtOH).\nI note that neither of the samples have elemental analysis, why is this? This is essential to ensure there are no amorphous impurities in the samples.\nThe crystallographic data looks OK, but I am a little concerned about the use of Mo in a system that crystallizes in a polar space group. Even though the Flack parameter is decent for the Pn structure, this can happen when Mo is used. Why did the authors not use Cu, where Flack parameters are more reliable?\nThe structure of [Fe2Co]·1.5EtOH lacks hydrogen atoms on the EtOH molecules. I did a quick refinement, and I was able to add these and get the structure to settle, so the authors should do this. Similar issues seem to be present for the 30 K structures, where H atoms are missing for some of the solvent molecules. These can be added and refine well (put Hs on the water first then add to the EtOH to get it to settle), so there’s no need to omit them. In both structures there are some clearly disagreeable reflections that should be omitted.\nThe structure of the dihydrate (i.e. -EtOH) is good, but there’s a piece of analysis that I feel is missing. The hysteresis drops from 22 K to 13 K, but we are not provided with a structural rationalisation of this. What changes occur beyond the change to a C2/c group? What interactions replace those mediated by the EtOH? Do the chains of trinuclear units get closer together?\nThe authors use 785 nm light to access the MS state and I wondered if there’s any particular rationale for this? In their latest 2024 Angew Chem paper (Ref. 31) the authors show that the IPA solvate shows different MS states depending on the wavelength of light used. It seems like a missed opportunity not to explore the same wavelength dependence here.\nFinally, the polarization change is a key highlight of this work, but it’s lacking context. The value is found to 140 nC/cm2, but how does this compare with other molecular ferroelectric materials? I note that the IPA solvate (Ref. 30) seems to be a little higher than the EtOH solvate. I think it would be a good idea for the authors to provide a table in the ESI that compares this system with the others that have been reported.\nOverall, if the authors can update and improve the work, I can see it being a good paper for Nat Commun.\nMinor points\n1. Line 73, the phrase ‘..based on our previous work,’ is unreferenced, please correct.\n2. Figure 1 is missing the x-axis.\n3. In Figure 2, the use of blue for C and grey for N, is very unusual and could lead to widespread misunderstanding as these are typically the other way round. Previous work by these authors has used grey for C and blue for N, I see no reason to change this.\n4. Figure 5 is not as clear as it could be, can the authors not plot the data as they did in their 2024 JACS paper?\n5. In Figure 6 caption, it would be a good idea to put in the net polarization value of 2.45 μC/cm2.\n6. Line 365, the formula should Bu4N[Fe(Tp)(CN)3], with the 3 next to the CN ligands.\n7. What are the CCDC nos. of the structures and what program was used to solve the structures, SHELXS or SHELXT?\n8. Ref. 44 is incorrect and should J Am Chem Soc.\n9. Figure S2 is a little fuzzy, please correct.\n10. There is substantial noise in the UV-Vis in Figure S3 around 800-900 nm, why is this?\n11. Figure S5, should be: thermogravimetric analysis."}, "author_response": "Response: Thank you for your helpful comment. Following your suggestion, we performed DSC measurements to further support the reversible ETCST behavior in the 1 complex (In consideration of Reviewer 3’s comments, we have revised the nomenclature of the compound from [Fe Co]·1.5EtOH to 1 and from [Fe Co] to 1’). As shown in Fig. R1, clear endothermic and exothermic transitions were observed at 174 and 147 K, respectively, in good agreement with the ETCST temperatures (170 and 148 K) determined by magnetometry. Notably, the exothermic peak is much weaker. This is likely because the cooling transition occurs very close to the lower temperature limit of our DSC apparatus, where unavoidable thermal leakage reduces the signal intensity.\nIn addition, we also conducted DSC measurements on the desolvated 1’ complex, which also exhibited consistent transition temperatures (239 and 257 K) with the ones from magnetometry (245 and 258 K, Fig. R2).\nThese measurements support the occurrence of ETCST in 1 and 1’, which is also consistent with the experimental results obtained through variable-temperature single-crystal X-ray diffraction, and pyroelectric current measurements.\n1’\nFig. R2. DSC measurement of the EtOH-removed complex 1’.\nOwing to the small capacitance of single crystals of complex 1, we attempted dielectric testing using pressed pellets made from polycrystalline samples. Indeed, we did detect a dielectric peak signal within the thermally induced phase transition range (Fig. R3), consistent with other characterizations. However, we also noted that the peak shape was rather diffuse. This may be related to the polycrystalline nature of the sample or the influence of strain induced by pressure on the ETCST in pellet samples (as discussed, for instance, in the recent publications, Chem. Commun. 10.1039/D5CC06296A). Consequently, we regret that we were unable to extract sufficiently precise information for discussion. Therefore, this data was not included in the article or the Supplementary Information. We hope that you find our explanations address you concerns.\nFig. R3. Temperature-dependent dielectric property based on pellet sample of 1.\n2. It appears clear from Figure S11 that the single-crystal device is sensitive to irradiation. However, no significant reverse pyroelectric current signal was observed after the irradiation was canceled. Does this affect the reliability and the reproducibility of the pyroelectric current signal? I suggest the authors to elaborate a little bit more in this and supplement the discussion on irradiation intensity dependence.\nResponse: Thank you for raising this important point. We agree this point deserves clarification because this clearly distinguishes the conventional photopyroelectricity and the mechanism behind our phenomena. Our system at low temperatures undergoes a photo-induced phase transition from a polar phase to a non-polar phase. This differs fundamentally from photopyroelectricity, which is a purely thermal modulation of polarization within a single polar phase. The current therefore contains only a minor photothermal term and a dominant phase-transition term associated with the disappearance of spontaneous polarization. Once converted, the metastable state becomes centrosymmetric and the spontaneous polarization vanishes; turning the light off cools the crystal but does not restore polarization, hence no reverse light-off peak is expected. Therefore, the apparent lack of a reverse current when the light is turned off (Fig. S11) is expected for our mechanism and should not be interpreted as a lack of reproducibility.\n3. Authors need to supplement the existing representative literature to compare with the light-induced ferroelectric phase transition performance of [Fe2Co]·1.5EtOH. You can refer to \"Angew. Chem. Int. Ed. 2025, 64, 1, e202413898 and Angew. Chem. Int. Ed. 2025, 64, 21, e202501238\".\nResponse: Thank you for pointing out these important papers to us. The light-induced ferroelectric phase transition and domain manipulation are indeed intriguing phenomena. In the I −-intercalated 3 perovskite, visible light assists the antiferro-to-ferroelectric transition (Angew. Chem. Int. Ed. 2025, 64, e202413898), while in the diarylethene crystals, light reversibly switches the system between two polar phases via covalent bond switching (Phys. Rev. Lett. 2023, 130, 176802). In addition, optical manipulation of domain structure in the ferroelectric phase of a perovskite ferroelectric material, (C H NH ) (CH NH ) Pb Br (Angew. Chem. Int. Ed. 2025, 64, e202501238). 7 15 3 2 3 3 2 3 10\nComplementary to these advances, our work realizes a light-induced erasure of ferroelectricity (converting a polar phase into a metastable non-polar phase) and on/off ferroelectricity via guest sorption. Together, these approaches broaden the operational palette of photo-programmable ferroics by spanning (i) light-assisted polar-state control and (ii) polar to non-polar conversion.\nWe have added a comparison with existing literature at the end of the conclusion section as follows.\n“Our light-induced polar to non-polar conversion, combined with guest-controlled on/off ferroelectricity, complements prior light-assisted polar-state control and establishes a distinct axis for photo-programmable ferroics. We anticipate that coupling ETCST to lattice symmetry will enable new device concepts for non-volatile, optically addressable states.60,61”\nhttps://doi.org:10.1002/anie.202413898\nhttps://doi.org:10.1002/anie.202501238\n4. The manuscript contains several minor writing errors that should be addressed in revision. For example, \"SH Intensity\" should be substitute with \"SHG Intensity\" in Figure 3; in the section \"Optical second-harmonic generation\", \"SH signal\" should be substitute with \"SHG signal\".\nResponse: Thank you for your comment. We have changed SH to SHG: “Optical second-harmonic generation (SHG) was measured using a femtosecond Er³⁺-doped fiber laser (1550 nm, 100 fs, 20 MHz; Calmar FPL-03C). Thin flakes from a crushed single crystal were mounted in a heat-exchange-gas optical cryostat for continuous temperature control. A chopped excitation beam was normally incident, and the transmitted SHG signal was isolated by multiple optical filters, detected with a cooled photomultiplier, and analyzed by lock-in detection.”.\nReviewer #2 (Remarks to the Author):\nThe manuscript submitted by Huang et al. follows a recently developped research line of professor Sato group devoted to the study of thermally-induced or photo-induced ferroelectric switching in Fe–Co charge transfer complexes that are able to show reversible iron-cobalt electron transfer coupled to a spin transition on the cobalt ions (so called ETCST). The authors have previously shown on a very similar cyanide-bridged Fe2Co trimetallic complex as the one investigated in this work (a solvatomorph) a light-induced change of electric polarity (see cited article, Sato et all, J. Am. Chem. Soc. 2024). In the present submitted work, they have extended these ideas showing that a reversible and controllable adsorption/desorption of ethanol solvent molecules (associated to single-crystal to single-crystal phase transition) can also be used to control the ferroelectric state in this Fe2Co complex. Additionally, they also show that other stimuli that are already known to control the iron-cobalt reversible electron transfer are also efficient in the present case. Overall, the experimental work is well carried out, and the article easy to read, the conclusion are supported by sound experimental results. However, I am afraid the present contribution lacks a bit of novelty in comparison to the recently published JACS article. It appears rather incremental for a publication in Nature communication. Once again, the compound is almost the same as the one in JACS, the key mechanism, ETCST driven polarization switching, is the same. The newly introduced solvent-mediated control (which is known to control ETCST in various FeCo systems) represents an incremental extension rather than a breakthrough. Moreover, the ferroelectric control via solvent adsorption/desorption might be conceptually interesting but it has very limited technological relevance, given the slow kinetics of the phenomenon and the very high sample sensitivity (loss of crystallinity upon cycling etc). I would rather recommand publication in a high factor journal as a complementary full work of the JACS article , for exemple in ACS Chem Mater.\nResponse: Thank you for your detailed and thoughtful evaluation. We sincerely appreciate your recognition of the high experimental quality and clarity of our work. However, we respectfully disagree with the assessment that our contribution is incremental relative to our prior JACS paper. Although the molecular system is related, the questions asked, measurements performed, and conclusions reached here are different in scope and depth.\nOur prior JACS work reported photo-induced electric polarization changes in an IPA solvate, it did not establish electronic ferroelectricity under electric field nor demonstrate guest molecule-regulated on/off switching of ferroelectricity. In fact, only a fully desolvated high-spin phase could be obtained by thermal treatment of the IPA solvate, precluding the reversible, single-crystal-to-single-crystal (SCSC) on/off control we demonstrate here. This highlights the fact that, albeit very similar molecules, this is standard and productive in molecular materials: changing the guest landscape can alter space-group symmetry, elastic coupling, and the ETCST balance, thereby revealing new emergent behavior. Academically, in the present study, we reveal previously unexplored key physical properties of this [Fe Co] system:\n2\n(1) We demonstrate electric-field-induced polarization switching in single crystals that are independently verified by X-ray crystallography, SHG, and pyroelectric measurements. This closes a key conceptual loop left open in the JACS paper: we now show that ETCST in this lattice produces a ferroelectric phase (not merely a ferroelectric-type phase transition).\n(2) We realize reversible absorption/desorption of EtOH to switch between a ferroelectric phase and a non-ferroelectric phase in bulk single crystals, which can be isothermally obtained in principle and without destroying crystallinity. This stimulus is orthogonal to temperature, light, or magnetic field and demonstrates a chemical gate for electronic ferroelectricity. The ability to write/erase polar order by guest molecules is a qualitatively different methodology from the JACS paper.\n(3) We determine how light, temperature, and guest molecules access distinct regions of the free-energy landscape: light at low T writes a metastable non-polar state (polar to non-polar), while EtOH sorption controls ferroelectricity on/off through SCSC. This symmetry control (polar to non-polar) is complementary to the literature where light typically assists switching within the polar manifold (AFE to FE or polar to polar).\nRegarding technological relevance, while solvent-responsive ferroelectricity may not immediately translate to fast device operation, fundamental breakthroughs in dynamic ferroelectric control and electronic polarization mechanisms often precede practical applications. Here we introduce guest-regulated electronic ferroelectricity as a design principle and provide a proof-of-concept material that can be translated to vapor-phase gate and porous hosts. These provides the possibility of chemical sensors with memory, opto-chemical logic, non-volatile states with thermally gated erase.\nFinally, Nature Communications is a multidisciplinary forum bridging chemistry, condensed-matter physics, and functional materials. Given that ferroelectricity was not previously established in this [Fe Co] PBA system, and that our work significantly advances understanding of molecular ferroelectrics, we believe this platform will maximize constructive feedback from the ferroelectric research community and support the development of next-generation molecular ferroelectric devices.\nBased on this point, we maintain that this work advances molecular ferroelectric science beyond our previous reports and deserves consideration at the present journal. We hope the reviewer will reconsider the broader conceptual contribution and multidisciplinary relevance of this study. We believe the results presented here represent a meaningful advance beyond our previous work and warrant publication in Nature Communications.\nReviewer #3 (Remarks to the Author):\nSu, Wu, Sato and coworkers explore a trinuclear FeCo Prussian blue analogue which shows On/Off switching of ferroelectricity. The current compound is a new EtOH solvate of a known compound (Ref. 30) and as such represents another example in the series of trinuclear systems explored by these authors that all show a polar/non-polar state accessible thermally and by light. Moreover, recent references beyond the authors’ own work reveal that solvent effects and SC-SC transformations are not new in PBA chemistry: https://doi.org/10.1021/jacsau.5c00681 (an example of solvent effects in a 1D PBA) and Chem. Sci., 2025, 16, 130-138; a recent example of SC-SC transformation in a ETCST system.\nHowever, these systems simply show ETCST and do not have the additional ferroelectric component. Hence, to the best of my knowledge this is the first time that anyone has demonstrated multiple switching in a molecular PBA analogue with on/off switching of ferroelectricity. The authors also make every effort to characterise the different phases upon thermal, light and EtOH cycling, clearly demonstrating the level of novelty and, for the most part, the experimental completeness I would expect for Nat. Commun.\nDespite this there are areas of improvement in the paper. The so-called desolvated system, is in fact a dihydrate only system. I understand the desire to keep the nomenclature simple, but this description of the compound as being desolvated is inaccurate. I suggest another term is used perhaps dihydrate for [Fe(Tp)(CN)3]2[Co(dpa)2]·2H₂O ([Fe₂Co]) and alcohol-dihydrate for [Fe(Tp)(CN)3]2[Co(dpa)2]·2H2O·1.5EtOH ([Fe2Co]·1.5EtOH).\nResponse: Thank you for this thoughtful suggestion. To avoid the ambiguity of the term “desolvated,” we now use the following explicit nomenclature throughout: complex 1 = [Fe(Tp)(CN) ] [Co(dpa) ]·2H O·1.5EtOH (alcohol-dihydrate); complex 1’ = 3 2 2 2 [Fe(Tp)(CN) ] [Co(dpa) ]·2H O (dihydrate). We state on first use that loss of co-crystallized 3 2 2 2 ethanol from 1 gives 1’ via a single-crystal-to-single-crystal transformation. All instances of “desolvated” have been replaced with “EtOH-removed”. We believe this resolves the terminology issue while keeping the text concise.\nI note that neither of the samples have elemental analysis, why is this? This is essential to ensure there are no amorphous impurities in the samples.\nResponse: Thank you for your comment. Since most of our measurements were performed on single-crystal samples, we overlooked including elemental analysis data. We fully agree that elemental analysis is important to exclude amorphous impurities. We have now performed elemental analysis for the pristine complex 1 (Elemental analysis calcd (%) for C H N O B Fe Co: C 47 51 24 3.5 2 2 47.03, H 4.28, N 28.01; Found: C 47.11, H 4.31, N 28.11) and added the results to the revised manuscript.\nThe crystallographic data looks OK, but I am a little concerned about the use of Mo in a system that crystallizes in a polar space group. Even though the Flack parameter is decent for the Pn structure, this can happen when Mo is used. Why did the authors not use Cu, where Flack parameters are more reliable?\nResponse: Thank you for this insightful comment. While Cu Kα can be advantageous for very light-atom structures (like organic crystals) due to its stronger intensity and longer wavelength, it also increases the absorption coefficients for the 3d transition metals markedly. The reliability of the Flack parameter is governed by the precision of Bijvoet differences and control of systematics rather than wavelength. Our crystal contains Fe and Co atoms, which already provide sufficient resonant scattering at Mo Kα. The present Mo dataset has excellent Friedel coverage (0.894; fraction_full 0.996), high redundancy (122,617/27,004), and low merging R (0.0472). Using the Flack quotient int method on 8,580 Friedel quotients gives a Flack parameter of 0.063(5), indicating a small inversion-twin fraction consistent with ferroelectric domain formation. Moreover, independent SHG and pyroelectric measurements further confirm the polar space group. Given these metrics, Mo Kα provides a reliable absolute-structure determination for this composition, while Cu Kα would substantially increase absorption and the risk of systematic error owing to its larger absorption coefficients. We hope these explanations satisfactorily address your concerns.\nThe structure of [Fe2Co]·1.5EtOH lacks hydrogen atoms on the EtOH molecules. I did a quick refinement, and I was able to add these and get the structure to settle, so the authors should do this. Similar issues seem to be present for the 30 K structures, where H atoms are missing for some of the solvent molecules. These can be added and refine well (put Hs on the water first then add to the EtOH to get it to settle), so there’s no need to omit them. In both structures there are some clearly disagreeable reflections that should be omitted.\nResponse: Thank you for your helpful suggestion. We have revised the single-crystal data and updated the datasets deposited in the CCDC system according to your suggestion.\nThe structure of the dihydrate (i.e. -EtOH) is good, but there’s a piece of analysis that I feel is missing. The hysteresis drops from 22 K to 13 K, but we are not provided with a structural rationalisation of this. What changes occur beyond the change to a C2/c group? What interactions replace those mediated by the EtOH? Do the chains of trinuclear units get closer together?\nResponse: Thank you for this insightful comment. Upon removal of EtOH, analysis of the unit‐cell parameters shows a contraction along the a and c axes, while the b axis remains nearly unchanged. Simultaneously, the hydrogen-bonding network changes from an H O⋯EtOH⋯NC framework in 1 2 to a water-mediated network in 1’ (EtOH H-bond donors/acceptors are absent). This indicates that the trinuclear units move closer to each other in the absence of ethanol molecules.\nWith these quantitative observations in hand, we offer a cautious interpretation. In ETCST lattices, the thermal hysteresis width reflects the overall cooperativity including elastic, dipolar, and host-guest interactions that couple local ET/spin events across the 3D lattice. The increased packing density and loss of EtOH-mediated links in 1’ plausibly reduce the effective interchain coupling pathway relative to 1, which is consistent with the observed narrowing of hysteresis from 22 K to 13 K. However, because cooperativity depends on the full network (including anisotropic elastic constants and domain-wall energetics), we refrain from asserting a direct conclusion on this point.\nThe authors use 785 nm light to access the MS state and I wondered if there’s any particular rationale for this? In their latest 2024 Angew Chem paper (Ref. 31) the authors show that the IPA solvate shows different MS states depending on the wavelength of light used. It seems like a missed opportunity not to explore the same wavelength dependence here.\nResponse: Thank you very much for your thoughtful comment and for recognizing our previous work. In contrast to the IPA solvate, the present EtOH co-crystal (1) is mechanically fragile during the photo-induced phase transition: photo-cycling and especially multi-wavelength protocols systematically induce cracking, preventing us from reliable electric measurements. Therefore, we only used the excitation of 785 nm that reliably triggers a complete photoconversion in the photomagnetic measurements.\nOn the other hand, the scopes of these two studies are also different. In the present study, our goals are to establish macroscopic ferroelectricity, to demonstrate guest-regulated on/off ferroelectricity at the bulk-single-crystal level, and to clarify symmetry-changing pathways. A comprehensive wavelength survey would be valuable, however, this direction lies beyond the primary scope of the present manuscript and would not significantly reinforce the core conclusions demonstrated here. We consider such wavelength‐dependent studies on a more robust sample for the future work.\nFinally, the polarization change is a key highlight of this work, but it’s lacking context. The value is found to 140 nC/cm2, but how does this compare with other molecular ferroelectric materials? I note that the IPA solvate (Ref. 30) seems to be a little higher than the EtOH solvate. I think it would be a good idea for the authors to provide a table in the ESI that compares this system with the others that have been reported.\nResponse: Thank you for this insightful suggestion. In this work, our primary goal is to provide further evidence that this system represents a new electronic ferroelectric, and the reported polarization value is obtained on a pellet sample made from polycrystalline sample. Therefore, the current value reflects a partial polarization change owing to the crystalline/domain cancelation rather than the fully saturated polarization. At this stage, a direct quantitative comparison with other molecular ferroelectrics would not be appropriate, as it would not represent the intrinsic maximum polarization of this system.\nWe fully agree that such a comparison would be valuable once saturated P–E loop measurements on the single crystals are obtained. We are actively working toward performing such measurements to determine the saturated polarization and will include a comparative analysis in future work.\nWe appreciate the reviewer’s thoughtful insight and believe that the suggested comparison will greatly benefit the field once complete data become available.\nOverall, if the authors can update and improve the work, I can see it being a good paper for Nat Commun.\nMinor points\n1. Line 73, the phrase ‘..based on our previous work,’ is unreferenced, please correct.\nResponse: Thank you for your comment. We have revised this sentence to: “In this study, we developed a compound exhibiting directional ETCST by incorporating distinct solvent molecules based on our previous work30”.\n2. Figure 1 is missing the x-axis.\nResponse: Thank you for your comment. We have revised it.\n3. In Figure 2, the use of blue for C and grey for N, is very unusual and could lead to widespread misunderstanding as these are typically the other way round. Previous work by these authors has used grey for C and blue for N, I see no reason to change this.\nResponse: Thank you for your comment. We fully acknowledge the conventional colour scheme where carbon is depicted in grey and nitrogen in blue. In the present work, however, we intentionally adopted a different colour scheme to clearly distinguish these figures from those in our previous publications on related systems, thereby helping readers easily differentiate between separate studies.\nWhile the colour choice is unconventional, we believe it does not affect the interpretation of the structures, as all atom types are clearly labelled in the figures. For clarity, we have also added a note in the caption to explicitly state the atom colour assignments.\nWe hope this explanation clarifies our intention and appreciate your understanding.\n4. Figure 5 is not as clear as it could be, can the authors not plot the data as they did in their 2024 JACS paper?\nResponse: Thank you for your helpful comment. We have further improved Figure 5 in the revised manuscript to enhance the clarity of the plotted information while maintaining the format that best highlights the characteristics of the current data set.\n5. In Figure 6 caption, it would be a good idea to put in the net polarization value of 2.45 μC/cm2.\nResponse: Thank you for the helpful suggestion. We have now added the net polarization value of 2.45 μC cm⁻² to the caption of Figure 6 in the revised manuscript: “Black arrow, net polarization change (2.45 μC cm⁻²)”.\n6. Line 365, the formula should Bu4N[Fe(Tp)(CN)3], with the 3 next to the CN ligands.\nResponse: Thank you for your comment. We have revised it to Bu N[Fe(Tp)(CN) ]: “and a solution 4 3 of Bu N[Fe(Tp)(CN) ] (112 mg, 0.2 mmol)”. 4 3\n7. What are the CCDC nos. of the structures and what program was used to solve the structures, SHELXS or SHELXT?\nResponse: Thank you for your comment. We have now added the CCDC deposition numbers for all reported crystal structures in the revised manuscript: “CCDC 2352581-2352583, 2352585, 2352586 and 2377474 containing the supplementary crystallographic data can be obtained free of charge from the Cambridge Crystallographic Data Center via www.ccdc.cam.ac.uk/data_request/cif.”\nThe structures were solved using SHELXT.\n8. Ref. 44 is incorrect and should J Am Chem Soc.\nResponse: Thank you for pointing this out. We have carefully re-checked Ref. 44, and confirm that the cited work was indeed published in Inorganic Chemistry, not J. Am. Chem. Soc. Therefore, the reference is correct as listed.\n9. Figure S2 is a little fuzzy, please correct.\nResponse: Thank you for your comment. We've improved the resolution of this figure.\n10. There is substantial noise in the UV-Vis in Figure S3 around 800-900 nm, why is this?\nResponse: Thank you for your comment. The noise observed in the UV-Vis spectrum around 800–900 nm in Figure S3 originates from the instrument light-source switching region. In this wavelength range, our spectrometer switches between different light sources, which inevitably introduces signal fluctuations.\n11. Figure S5, should be: thermogravimetric analysis.\nResponse: Thank you for your comment. We have now revised the caption of Figure S5: “Thermogravimetric analyses (TGA) of 1 and 1’ under air flow.”."}, {"round": 2, "reviewer_comments": {"reviewer_1": "In the 2nd-round revision, the authors have made the detailed responses to all the reviews' concerns, and I think the manuscript has bee well improved. So, I recommend it for publication in the current form now.", "reviewer_3": "The revision by Su, Wu, Sato and coworkers adequately addresses most of the comments but there are still a few points that I think need work. Provided that these changes are made I would be happy to see the work published in nature Communications.\nSpecific points\n1. Spin crossover is sometimes spelt as spin-crossover. I don’t mind which the authors use as long as they are consistent.\n2. Lines 188-189, ‘When the sample were directly placed at a temperature below 60 K from room temperature, the χmT would be maintained at ca. 4.05 cm3 mol−1 K, indicating the presence of a quenching effect.’ is a little confusing as it suggests 60 K below RT which would be 238 K. What the authors really mean is that quench cooling results in trapping of the MS* phase, provided that the sample is kept below 60 K.\n3. Having conducted DSC measurements, why are these not presented in the SI?\n4. The suggestion that in Figure 2, the authors cannot use a sensible colouring system for the crystal structures because readers will mistake the work for previous papers is nonsensical. The aim of figures is to communicate information quickly and accurately. Using blue for C almost guarantees that readers will misunderstand what the figures are trying to show and I must insist that this change is made."}, "author_response": "Response: Thank you for your recommendation. We have revised the manuscript and supporting information to incorporate feedback from the other reviewers, and we hope these revisions satisfactorily address all of your concerns.\nResponse: Thank you for the suggestion. We have unified the terminology throughout the manuscript and now consistently use “spin crossover”.\nResponse: Thank you for pointing out the ambiguity in the original wording. The sentence has been revised to clarify the meaning: “When the sample were directly placed at a temperature below 60 K from HT phase, the χ T would be maintained at ca. 4.05 cm3 mol−1 K, indicating the presence of a m quenching effect.”.\nResponse: Thank you for the suggestion. Although DSC measurements were presented in the response letter, we chose not to include them in the Supporting Information because the data quality does not allow reliable interpretation beyond a qualitative confirmation of the transition temperatures. As explained to reviewer 1, endothermic/exothermic features were observed at ~174 K (heating) and ~147 K (cooling), consistent with the ETCST temperatures determined by magnetometry (~170 K and ~148 K). However, the cooling transition occurs very close to the lower temperature limit of our DSC apparatus, where unavoidable thermal leakage significantly suppresses the exothermic signal. As a result, the peak area is not sufficiently reproducible for extracting thermodynamic parameters (ΔH, ΔS), and presenting the data could invite over-interpretation. Given that magnetometry already establishes the transition temperatures and hysteresis robustly, we believe including these DSC data would not meaningfully strengthen our conclusions.\nResponse: Thank you for this comment. The coloring system in Figure 2 has been modified according to the reviewer’s suggestion. A conventional and chemically intuitive color scheme is now used, ensuring that the structural information is clearly and accurately conveyed.\nFigure 3. Molecular structure and space group switching of 1. The LT phase was in the polar space group Pn, whereas the HT, and MS* phases were in the nonpolar space group P2/n. The MS* phase was obtained by placing the single-crystal sample directly at 30 K, and no ETCST behavior occurred. With a temperature increase, the MS* phase returned to the LT phase via relaxation. During the thermal-induced ETCST process from the HT to LT phases, the orientation of part of the EtOH molecules changed from disorder to order. The gray dashed lines represent hydrogen bonds. The faintly drawn part is the adjacent structure with a disordered EtOH molecule connected by hydrogen bonds. Ethanol molecules are indicated with an ellipse (type A, red; type B, blue). Orange, FeIII; green, FeII; purple, CoII; pink, CoIII; yellow, B; gray, C; red, O; blue, N; light gray, H.\nSimilarly, Figures 5 and 7 have been revised as follows:\na\nFeII + T FeIII + EtOH CoII + T CoIII\nCoIII CoII\n-T -EtOH -T\nFeIII FeIII FeIII FeIII Fe Fe\nLT: Pn HT: P2/n HT*: C2/c LT*: C2/c\nb\nFigure 5. Molecular structure, space group, and spin and valence states of the metal centers between 1 and 1’ during dynamic vapor sorption. a. Schematic representation of the on/off switching of ferroelectricity via desorption/adsorption of the guest molecule. The HT phase of 1 was in the nonpolar space group P2/n, the LT phase of 1 was in the polar space group Pn, while the LT* and HT* phases of 1’ were in the nonpolar space group C2/c. In the LT* phase, the two irons were in the average state of FeII LS and FeIII LS. The transition between the LT* and HT* phases was a temperature-dependent reversible process. Orange, FeIII; green, FeII; purple, CoII; pink, CoIII; yellow, B; gray, C; red, O; blue, N; light gray, H.\nb. Temperature-dependent χmT product of 1 (blue) and 1’ (red). Open circle, heating process; filled circle, cooling process.\na b\ne- e-\na\nFeIILS\nFeIIILS\nFeIIILS\nFeIIILS\nCoIIILS CoIIHS\ne- e- e- e- e- e-\nFeII\nFeIII\nCoIII + T CoII\nFeIII FeIII\ne- e- e- e- e- e-\n-T\na a\no c\nc c\nLT: Pn HT: P2/n\nFigure 7. Directional ETCST induced ferroelectric phase transition and polarization switching mechanism. a. Schematic representation of 1 in the ferroelectricity “on” state via directional ETCST. The LT phase was in the polar space group Pn, whereas the HT phase was in the nonpolar space group P2/n. During the phase transition, the compound underwent the directional ETCST. Orange, FeIII; green, FeII; purple, CoII; pink, CoIII; yellow, B; gray, C; red, O; blue, N. Red arrow, direction of electron transfer; yellow arrow, polarization direction within the crystal lattice. b. Calculated polarization change from the directional charge transfer and EtOH reorientation. Green arrow, contribution from EtOH reorientation (0.43 μC/cm2); Red arrow, contribution from directional electron transfer (3.22 μC/cm2); Black arrow, net polarization change (2.45 μC/cm2).\nFigures S4, S7, and S8 in the Supporting Information have also been revised.\nFigure S4. Molecular structure, spin and valence states of metal centers, and space group switching of 1. The LT phase was in the polar space group Pn, whereas the HT, MS and MS* phases were in the nonpolar space group P2/n. The MS* phase was obtained by placing the high temperature phase directly at the low temperature, and no ETCST behavior occurred. With a temperature increase, the MS* phase returned to the LT phase via relaxation. The MS phase was obtained by irradiating LT phase by 785 nm light. During the thermal-induced ETCST process from the HT to LT phases, the orientation of part of the EtOH molecules changed from disorder to order. The gray dashed lines represent hydrogen bonds. The electron transfer process is indicated with a curved arrow. The blurred part is the adjacent structure with a disordered EtOH molecule connected by hydrogen bonds. Orange, FeIII; green, FeII; lavender, CoIII; plum, CoII; yellow, B; gray, C; red, O; blue, N; light gray,\nFigure S7. Molecular structure, space group, and spin and valence states of the metal centers between 1 and 1’ during dynamic vapor sorption. Top: The HT phase of 1 was in the nonpolar space group P2/n, and the LT* and HT* phases of [Fe Co] were in the nonpolar space group C2/c. In the 2 LT* phase, the two irons were in the average state of FeII and FeIII . The transition between the LS LS LT* and HT* phases was a temperature-dependent reversible process. Bottom: Schematic representation of [Fe Co] in the ferroelectricity “off” state due to nondirectional ETCST. Both the 2 LT* and HT* phases were in the nonpolar space group C2/c. Orange, FeIII; green, FeII; lime, Fe; gold, CoII; plum, CoIII; yellow, B; gray, C; red, O; blue, N; light gray, H.\nFigure S8. Molecular stacking and space group of 1 at LT, MS* and HT phase. The 1 crystalizes in a polar space group Pn in the LT phase, while the 1 crystalizes in the nonpolar space group P2/n in the HT phase and MS* phase. During thermo-induced ETCST process, the disordered part of ethanol molecules in the HT phase become ordered in the LT phase. Orange, FeIII; green, FeII; lavender, CoII; plum, CoIII; yellow, B; gray, C; red, O; blue, N."}]

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