Chameleonic layered metal–organic frameworks with variable charge-ordered states triggered by temperature and guest molecules

Abstract

Molecular materials whose electronic states are multiply varied depending on external stimuli are among the most promising targets for the development of multiply accessible molecular switches. Here, we report a honeycomb layer composed of tetraoxolene-bridged iron (Fe) subunits whose charge-ordered states are multiply variable via thermal treatments and solvation/desolvation with the crystallinity intact. The compound is (NPr₄)₂[Fe₂(Cl₂An)₃] (1-d; NPr₄⁺ = tetra-n-propylammonium; Cl₂An²⁻ = 2,5-dichloro-3,6-dihydroxo-1,4-benzoquinonate), which possesses three charge-ordered states: a low-temperature (LT) phase [(Fe³⁺)₂(Cl₂An²⁻)(Cl₂An˙³⁻)₂]²⁻; an intermediate (IM) phase [(Fe^2.5+)₂(Cl₂An²⁻)(Cl₂An^2.5−)₂]²⁻; and a high-temperature (HT) phase [(Fe²⁺)₂(Cl₂An²⁻)₃]²⁻ that varies according to temperature. In addition, the LT phase of 1-d is reversibly changeable to another IM phase in its solvated compound 1via a solvation/desolvation process at room temperature. This example demonstrates a new multiple-switching system based on electron transfer and host–guest chemistry in a charge-flexible metal–organic framework.

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Introduction

Stimuli-responsive molecular materials (SRMMs), which change their phases according to applied external stimuli, have good potential for use as near-future molecular scale devices such as high-density molecular memories, molecular switches, or sensors.^1–3 One of the most intriguing targets for SRMMs is a class of thermally driven electron transfer (TDET) systems,^1–31 which can also be a strong candidate for SRMMs triggered by other external stimuli such as light,^{12,14,16,19,20,23,32–35} pressure,^6,36 and chemical guests.^26,37–39 Therefore, to date, various types of TDETs, such as neutral–ionic transition (N–I transition),^{5,9,18,25,27,38} valence tautomerism (VT),^{4,6,8,10,11,21,22,24,28,30,31} and inter-valence electron transfer (IVET),^7,13 have been investigated using organic charge-transfer systems and metal complexes. Most of these systems have been studied using a one-step TDET that occurs at one transition temperature T_1/2(1), and only a few examples of two- or multi-step TDET with T_1/2(1), T_1/2(2), and T_1/2(x) (T_1/2(1) < T_1/2(2) < T_1/2(x)) have been reported thus far: (i) discrete molecules, where two VT subunits interact electronically with each other, i.e., owing to electronic disproportionation,^10,21,22 (ii) discrete molecules with intermediate states composed of several alternately arranged species possessing different charge-ordered states produced by stepwise IVET, which may be affected by Coulomb interactions and/or elastic interactions,^15,16,29 and (iii) N–I transition chains with inter-chain Coulomb interactions.^18,25,27 In addition, there is another case (iv), where two crystallographically independent VT subunits, each of which undergoes VT at a different temperature, are present in one system.^17,28

Considering such TDET systems that possess electronic disproportionation states, two-dimensional (2D) layered systems with a formula of D_xA_y (x ≠ y) may be good candidates (Fig. 1). Recently, we have reported one-step TDET in a D₂A₃-type honeycomb-layered compound, (NPr₄)₂[Fe₂(Cl₂An)₃]·2(acetone)·H₂O (1), which is composed of 2,5-dichloro-3,6-dihydroxo-1,4-benzoquinonate (Cl₂An^m−) and Feⁿ⁺ ions with tetrapropylammonium cations (NPr₄⁺) as well as some crystallization solvents (acetone and water) located at hexagonal pores of layers and between layers.³¹ This layered compound revealed two ground states of the fully electron transferred state (as the low-temperature (LT) phase) of [(Fe³⁺)₂(Cl₂An²⁻)(Cl₂An˙³⁻)₂]²⁻ and a charge-disproportionate ordered state (defined as the charge-ordered intermediate (IM_o) phase) of [(Fe²⁺)(Fe³⁺)(Cl₂An²⁻)₂(Cl₂An˙³⁻)]²⁻ with a boundary at T_1/2a↑ = 237 K (T_1/2a↓ = 236 K, where the signs ↑ and ↓ refer to heating and cooling processes, respectively. Moreover, T_1/2a is T_1/2(1) for 1). The Cl₂An^m− bridging ligand can take three redox-active states of and and vice versa, whereas Feⁿ⁺ takes Fe²⁺ → Fe³⁺ + e⁻ and vice versa in this type of material (Fig. 1). Thus, the LT phase is a layer that is constructed from D⁺A⁻ chains bridged by Cl₂An²⁻ (L_B), where D⁺ and A⁻ are high-spin Fe³⁺ and Cl₂An˙³⁻ (L_A), respectively, while the IM_o phase is a layer where DA chains and D⁺A⁻ chains are alternately aligned with the Cl₂An²⁻ (L_B) bridges (Scheme 1). In other words, this D₂A₃ layer system can be assumed to be a layered DA chain as a sort of one-dimensional (1D) VT system.³⁰

Fig. 1 Structure of D₂A₃-type honeycomb-layered systems constructed from an Fe²⁺ center (D in red) and Cl₂An²⁻ (A in blue), where the Fe²⁺ center acts as the electron donor (D) to be Fe³⁺ (D⁺), whereas Cl₂An²⁻ acts as the electron donor (A) and may be reduced to be Cl₂An˙³⁻ (A⁻).

Scheme 1 Schematic illustrations of the variations of charge-ordered states in 1 and 1-d, where the fully electron transferred state, charge-disproportionate ordered state, charge-disproportionate delocalized state, and neutral state are represented as LT, IM_o, IM_d, and HT phases, respectively.

This feature of charge-ordered states made us imagine the existence of a neutral state of the high-temperature (HT) phase in 1, which comprises only DA chains at higher temperatures; practically, 1 converted itself to a solvent-free form, (NPr₄)₂[Fe₂(Cl₂An)₃] (1-d), with an elimination of its lattice solvents, and no longer changes to the HT phase within 1 upon heating. Nevertheless, 1-d afforded three TDET phases (LT → IM → HT) described in Scheme 1 with two-step TDET at T_1/2b and T_1/2c appearing continuously from the IM_o phase of 1via desolvation (T_1/2b and T_1/2c are T_1/2(1) and T_1/2(2) for 1-d, respectively) (Scheme 2). In addition, the conversion between 1-d and 1 is reversible via a solvation/desolvation treatment.

Scheme 2 Temperature-scale representation of the charge-ordered states in 1/1′ and 1-d, where the top and bottom scales are for 1-d and 1/1′, respectively.

Here, we report the first example of multi-step TDET in a layered D₂A₃ compound (1-d), which is continuously linked to another TDET-form (1) involving a guest-induced ET in a single crystal. These TDETs consequently revealed five material states with three unique charge-ordered states (LT, IM_o, and HT) that are formable in a D₂A₃ honeycomb-layered system. This type of TDET material has good potential for use as a stimuli-controllable molecular multi-switch.

Results and discussion

Preparation of 1-d and its structural overview

The pristine solvated compound, (NPr₄)₂[Fe₂(Cl₂An)₃]·2(acetone)·H₂O (1), which is prepared in accordance with a previously reported method,³¹ has a flat honeycomb anionic layer [Fe₂(Cl₂An)₃]²⁻ that is separated by two kinds of structurally distinguished NPr₄⁺ cations (C₁⁺ and C₂⁺), forming an alternating stacking form [⋯(l)(C₁⁺)(l)(C₂⁺)⋯]_∞ (l means the [Fe₂(Cl₂An)₃]²⁻ layer) with position-disordered water molecules located only at one side of cation layers and two acetone molecules located inside the hexagonal pores of the [Fe₂(Cl₂An)₃]²⁻ layer.³¹

The lattice solvent molecules in 1 can be easily removed by evacuating at room temperature for several hours to produce the desolvated compound, (NPr₄)₂[Fe₂(Cl₂An)₃] (1-d), with its crystallinity remaining intact (here, it took 12 h to complete the elimination). Thermal gravimetric analysis (TGA) shows that the elimination of lattice solvents is completed by heating up to ∼340 K under N₂ atmosphere conditions, and the compound of 1-d is very stable up to ca. 500 K (Fig. S1†). The crystallinity of 1-d was confirmed by performing single-crystal and powder X-ray diffraction (SC- and P-XRD) studies (Fig. 2 and S2;†vide infra). However, with the exposure of the crystalline sample of 1-d to a vapor of a 1 : 2 v/v mixture of water/acetone at room temperature for 2–3 h, the solvated form, namely 1, was recovered (as 1′) with the pristine structure (Fig. S3, S4 and Table S1†). TGA of 1′ proves the contamination of the same amount of lattice solvents, i.e., 2(acetone)·H₂O, and follows the same profile as that found for 1 (Fig. S1†).

Fig. 2 Crystal structure of 1-d at 103 K. (a) Structure of the formula unit (50% probability ellipsoids; symmetry operation (*) 1/2 + x, 3/2 − y, 1/2 + z; (**) −x, 1 − y, 1 − z; (***) −1/2 − x, −1/2 + y, 1/2 − z; (#) 1/2 − x, −1/2 + y, 1/2 − z; (##) −1 + x, y, z). (b) Projection of hexagonal honeycomb networks along the 〈10−1〉 direction, where Fe, C, N, O, and Cl atoms are colored in orange octahedra, gray, blue, red, and green, respectively, and hydrogen atoms are omitted for clarity. (c) Projection of layer structures along the b-axis, where the colors of atoms are the same as those in (b) and hydrogen atoms are omitted for clarity.

SC-XRD analysis of 1-d was first performed at 103 K, which revealed a 2D honeycomb-layered structure that is similar to that of 1. While 1 was crystallized in the monoclinic space group P2₁/c (Z = 4),³¹1-d crystalized in the space group of P2₁/n with Z = 2, which determined a half of the formula unit to be an asymmetric unit (1 × Feⁿ⁺, 1.5 × Cl₂An^m− (L_A × 1, L_B × 0.5), and 1 × NPr₄⁺; Fig. 2a and Table S2†) with two-fold axes through the Feⁿ⁺ and the midpoint of the L_B Cl₂An^m− ligand. The Feⁿ⁺ node has a distorted octahedral Δ or Λ coordination geometry with three bidentate O atoms from two L_A ligands and one L_B bridging ligand, forming an anionic hexagonal layer [Fe₂(Cl₂An)₃]²⁻ spreading parallel to the (10−1) plane (Fig. 2b), where the Δ and Λ geometric coordinations on the Feⁿ⁺ nodes are present alternately. The NPr₄⁺ cations are located between the [Fe₂(Cl₂An)₃]²⁻ layers, separating the layers with an interlayer distance of 8.817 Å (Fig. 2c). Thus, remarkable structural differences between the solvated (1 and 1′) and the desolvated (1-d) compounds are found in the structural symmetry of the formula unit and in the interlayer space shortened in 1-d (Fig. 2). It should be noted that the NPr₄⁺ cations are evenly located between the [Fe₂(Cl₂An)₃]²⁻ layers in 1-d, influencing equal electrostatic effects for all Feⁿ⁺ nodes and the L_A Cl₂An^m− ligands, whereas the presence of two structurally characterized NPr₄⁺ cations in 1 distinguishes the electrostatic effect for the Feⁿ⁺ sites and the L_A Cl₂An^m− ligands.

This electrostatic situation could significantly affect the stabilization of the electronic states of 1-d and 1 (vide infra).

To determine the oxidation state of the Feⁿ⁺ node and the Cl₂An^m− ligand, it is convenient to verify the local bond lengths of Fe–O and C–O in the Cl₂An^m− ligands because of their sensitivity to the oxidation states; a comparison of the mean bond lengths d_av(Fe–O) for each octahedral Feⁿ⁺ node and d_av(C–O) for each Cl₂An^m− ligand is made. In general, d_av(C–O) is found to be within the range of 1.244–1.270 Å and 1.289–1.312 Å for Cl₂An²⁻ and Cl₂An˙³⁻, respectively.^30,31 The d_av(C–O) value at 103 K for 1-d was 1.293(3) Å and 1.257(3) Å for L_A and L_B, respectively, which are assigned the oxidation states of Cl₂An˙³⁻ and Cl₂An²⁻, respectively. On the other hand, the d_av(Fe–O) value tends to be within the range of 2.091–2.140 Å and 2.000–2.050 Å for high-spin Fe²⁺ and Fe³⁺, respectively.^30,31 The d_av(Fe–O) value at 103 K for 1-d was 2.019(2) Å, which indicates the presence of the high-spin Fe³⁺ state. These situations imply that the charge-ordered state of 1-d at 103 K is in the LT state with the formula of [(Fe³⁺)₂(Cl₂An²⁻)(Cl₂An˙³⁻)₂]²⁻, where the Cl₂An˙³⁻ bridging ligands are L_A, which makes a quasi-chain with Fe³⁺ along the [101] direction, and the Cl₂An²⁻ L_B bridging ligand links these chains alternately (along the [010] direction) to form the hexagonal net. Although the LT state in 1 underwent a transition at T_1/2a = 237 K,³¹ the LT state of 1-d is stabilized even at 300 K without there being significant changes in bond lengths (Fig. S5 and Table S3†). Thus, the desolvation treatment of 1 at room temperature (300 K), changing to 1-d, successfully demonstrated a change in the charge-ordered state involving an electron transfer: the IM_o phase [(Fe²⁺)(Fe³⁺)(Cl₂An²⁻)₂(Cl₂An˙³⁻)]²⁻ in 1 → the LT phase [(Fe³⁺)₂(Cl₂An²⁻)(Cl₂An˙³⁻)₂]²⁻ in 1-d. It should be noted that this sponge behavior is reversible via the solvation procedure to form 1 (=1′) (Fig. S6†).

Magnetic characterization for the LT phase in 1-d

The magnetization (M_m) measurements for 1-d were first performed over the temperature range of 1.8–300 K under an applied magnetic field of H_dc = 1 kOe. The temperature dependence of the magnetic susceptibility (χ_m = M_m/H_dc) of 1-d in this temperature region is similar to that observed for the LT phase in 1 (Fig. 3),³¹ in that it can be described as a ferrimagnetic chain of S = 5/2 for Fe³⁺ and S = 1/2 for the L_A Cl₂An˙³⁻ ligand, which are weakly perturbed by interchain antiferromagnetic interactions through the L_B Cl₂An²⁻ bridge and interlayer through space.

Fig. 3 Field-cooled χ_m (χ_m = M_m/H_dc) for 1-d measured with a dc field (H_dc) of 1 kOe for a temperature range of 1.8 K to 300 K, where the black solid line represents the best-fit line with the Seiden model using alternating classical S = 5/2 and quantum S = 1/2 spins for the data between 35 and 300 K (see the text). Inset: H/T phase diagram for 1-d, where AF and P represent the antiferromagnetic and paramagnetic phases, respectively, where T_N (23 K) was determined from the χ′ versus T data at H_dc = 0, and the black dashed line is a guide for enhanced visibility.

The χ_mT value at 300 K is 13.3 cm³ K mol⁻¹, which is slightly greater than the theoretical value (9.5 cm³ K mol⁻¹) for a sum of magnetically isolated species of Cl₂An˙³⁻ (S = 1/2, g = 2.0) and high-spin Fe³⁺ (S = 5/2, g = 2.0). This is indicative of strongly coupled spins, even at 300 K (vide infra).^40–42 Upon decreasing the temperature, χ_m gradually increased and reached a maximum of 2.08 cm³ mol⁻¹ at 23 K, followed by an abrupt decrease to 1.45 cm³ mol⁻¹ at 1.8 K. The χ_m–T plot in the temperature range of 35–300 K was simulated by using the Seiden model with alternating S = 5/2 and S = 1/2 spins, including a mean-field approximation (zJ′) to consider interchain interactions.⁴³ The fitting of the data for 1-d provided an adequate parameter set of g_Fe = g_L = 2.0 (fixed), J = −125.2 cm⁻¹, and zJ′ = −0.27 cm⁻¹, which are comparable to those of 1 (J = −130.6 cm⁻¹ and zJ′ = −0.27 cm⁻¹) (Fig. 3).³¹

Field-cooled magnetization (FCM) curves obtained with several dc fields showed an elimination of the χ_m peak at T_ex with increasing applied external dc field, which is indicative of spin flipping behavior from an antiferromagnetic (AF) ground state (Fig. S7a†). In addition, the field dependence of the magnetization (M–H) measured at low temperatures revealed sigmoidal features with spin-flipping fields (H_ex, which was determined from a peak in dM/dH data at each temperature measured) as well as a small field hysteresis with H_c = 0.14 T at 1.8 K (Fig. S7b†). This is a typical case of metamagnetic behavior in low-dimensional magnetic systems; the inset of Fig. 3 displays an H/T phase diagram made from H_ex and T_ex, which separates the regions of paramagnetic (P) and AF phases.

To investigate in more detail the magnetic behavior, the temperature dependence of the ac susceptibilities was measured at several frequencies below 60 K by applying an oscillating magnetic field (H_ac = 3 Oe) and a dc field of H_dc = 0 Oe. The in-phase susceptibility (χ′) showed three kinds of peaks at around 23 K (broad), 20 K, and 5 K (Fig. S8a†), where the frequency-independent peaks at 23 K do not involve any anomaly of out-of-phase susceptibility (χ′′), which is indicative of the occurrence of an AF phase transition with T_N = 23 K following the phase diagram (inset of Fig. 3). The other peaks at around 20 K and 5 K are slightly frequency dependent and involve an anomaly of χ′′, albeit weak. Although a similar ac susceptibility feature was observed in 1, which revealed single-chain magnet (SCM) behavior,³¹ this behavior in 1-d ruled out the possibility of SCM, and this may be due to a strong contribution of interchain antiferromagnetic interactions that possibly form some magnetic random domains in the AF phase. Actually, a fitting of the χ′′–T curves using the Arrhenius law τ(T) = τ₀ exp(Δ_τ/k_BT) provided a much faster τ₀ value of 1.3 × 10⁻¹⁹ s with Δ_τ/k_B = 148.3 K; this is similar to a glassy behavior (Fig. S8b†).^44,45 Thus, the significant magnetic change from 1 to 1-d could be attributed primarily to the structural contraction associated with the shorter interlayer distance in 1-d (Fig. 2).

Thermally driven electron transfers in 1-d: from magnetic investigation

The temperature dependence of M_m of 1-d was investigated for the HT region of 300–400 K. The M_m–T curve clearly shows two steps as the temperature is varied; Fig. 4 shows χ_mT variations as a function of temperature (χ_mvs. T in Fig. S9†). Upon heating from 300 K, the χ_mT value of 13.3 cm³ K mol⁻¹ at 300 K gradually decreases, but it decreases steeply at around T_1/2b↑ = 317 K and T_1/2c↑ = 354 K to 6.7 cm³ K mol⁻¹ at 400 K, indicating the occurrence of TDET from the LT phase to the HT phase via an IM phase. The χ_mT value at 400 K (HT phase) is close to the theoretical value of 7.3 cm³ K mol⁻¹ for the magnetically isolated species of Fe²⁺_HS (S = 2 as g = 2.2),³¹ and the χ_mT value of 9.6 cm³ K mol⁻¹ at 335 K for the IM phase is almost half the value (10.0 cm³ K mol⁻¹) between those of the HT and LT phases. The cooling process almost follows the feature of the heating process with little thermal hysteresis (ΔT_1/2 = 1 K), where T_1/2 was determined from peaks of dχ_mT/dT plots (Fig. 4). The stepwise TDET behavior of 1-d was also confirmed by performing differential scanning calorimetry (DSC) measurements (Fig. 5).

Fig. 4 Temperature dependence of χ_mT and −d(χ_mT)/dT for 1-d measured at H_dc = 1 kOe continuously between 300 K and 400 K in initial heating (red) and post-cooling (blue) processes, where the LT, IM, and HT phases are displayed as red, green, and blue areas, respectively.

Fig. 5 Temperature dependence of the heat flow obtained from differential scanning calorimetry (DSC) values for 1-d with a sweep rate of 5 K min⁻¹.

SC-XRD study for the phases in 1-d

Because the crystals of 1-d are stable at high temperatures, SC-XRD analyses at 335 K and 380 K, which correspond to the IM phase and the HT phase, respectively, were conducted as well as at 300 K for the LT phase in order to obtain a comparison under the same measurement conditions using the same apparatus (Fig. 6a, c, e and Table S4†). At all temperatures, they crystalized in monoclinic P2₁/n with Z = 2 as well as at 103 K, and the crystal lattices are also similar to each other. Nevertheless, the local distances within these structures were characteristically varied.

Fig. 6 Charge variations of the core unit and ⁵⁷Fe Mössbauer spectra of 1-d in LT at 300 K (a and b), IM_o at 335 K (c and d), and HT at 380 K (e and f). (a, c, and e) Display of charge variations, where L²⁻, L^2−/˙³⁻, and L˙³⁻ are displayed as gray-, pink-, and red-filled C6 rings, respectively, and Fe²⁺, Fe²⁺/Fe³⁺, and Fe³⁺ are displayed in blue, green, and orange, respectively, with C, O, and Cl atoms in gray, red, and green, respectively. (b, d, and f) ⁵⁷Fe Mössbauer spectra for the respective phases. The solid lines in the Mössbauer spectra are Lorentzian curves obtained using the parameters in Table S6,† where orange and blue curves correspond to high-spin Fe³⁺ and high-spin Fe²⁺ species, respectively. The purple curves observed at 330 K and 380 K, with similar parameters of δ and ΔE_Q may correspond to the impurity of decomposition species due to the higher temperature for longer measurement times, even in a nitrogen atmosphere.

The d_av(C–O) values for L_A and L_B and the d_av(Fe–O) value at 300 K are 1.286(6), 1.260(7), and 2.027(4) Å, respectively (Fig. 6a and Table S5†), which are consistent with the LT charge-ordered state of [(Fe³⁺)₂(Cl₂An²⁻)(Cl₂An˙³⁻)₂]²⁻, as found at 103 K (vide supra). ⁵⁷Fe Mössbauer spectra recorded at 300 K show only a typical spectrum of Fe³⁺_HS (Fig. 6b and Table S6†).^30,31 However, at 335 K, only the d_av(C–O) value of L_A changes to be lower as 1.271(7) Å, which is just an intermediate between the cases of Cl₂An²⁻ and Cl₂An˙³⁻, while d_av(C–O) = 1.254(9) Å for L_B is still within the range of Cl₂An²⁻ (Fig. 6c and Table S5†). In addition, the d_av(Fe–O) value of 2.058(4) Å is also an intermediate between those of Fe²⁺–O and Fe³⁺–O (Table S5†). This bonding formation indicates that the phase at 335 K is in an IM phase, although a half of the structure was determined by this lab-level SC-XRD measurement. This IM state for 1-d at 335 K could be assigned to be a positional disorder of the [(Fe²⁺)(Fe³⁺)(Cl₂An²⁻)₂(Cl₂An˙³⁻)]²⁻ state made in each layer that is similarly found in the IM phase in 1.³¹ This conclusion was also supported by SC-XRD measurements using a synchrotron (SPring-8, Hyogo) (ESI†). Meanwhile, ⁵⁷Fe Mössbauer spectroscopy performed at 335 K revealed that the Fe²⁺_HS and Fe³⁺_HS species were evenly located in the mode of IM_o (like in 1), not in the mode of the charge-disproportionate delocalized state (IM_d) in Scheme 1, on the time scale of the Mössbauer effect (t ≈ 10⁻⁷ to 10⁻¹⁰ s) (Fig. 6d and Table S6†).^30,31,41 In the related system, fast electron exchange between mixed-valent Fe²⁺/Fe³⁺ species has been proposed using Mössbauer studies.^42,46

As the temperature was finally increased to 380 K, the d_av(C–O) value of L_A is further decreased to 1.252(8) Å, corresponding to a case of Cl₂An²⁻, as well as L_B (d_av(C–O) = 1.26(1) Å) (Fig. 6e and Table S5†). Correspondingly, d_av(Fe–O) = 2.096(5) Å is within the range for Fe²⁺–O (Table S5†). This feature satisfies the fact that the homovalent HT phase of [(Fe²⁺_HS)₂(Cl₂An²⁻)₃]²⁻ was formed. Actually, the ⁵⁷Fe Mössbauer spectra recorded at 380 K agreed well with the existence of the high-spin Fe²⁺ state,^30,31 although some impurities that may have been produced by compound decomposition were isolated in HT measurements (Fig. 6f and Table S6†).⁴⁷ Thus, the structures measured at 335 K and 380 K prove what was observed in the temperature dependence of M_m, i.e., two-step TDET behavior (vide supra).

The difference of T_1/2(1) between 1 and 1-d, where T_1/2(1)↑ = 317 K in 1-d, is much higher than T_1/2(1)↑ = 237 K in 1, and this could be associated with the effect of Coulomb interactions between the layers and NPr₄⁺ cations, in particular, between the L_A Cl₂An˙³⁻ subunit and NPr₄⁺. There are two kinds of L_A⋯NPr₄⁺ distances in 1, ca. 4.9 Å and 6.2 Å,³¹ where the shorter one could have a gain to be a higher valency of the (Fe³⁺–Cl₂An˙³⁻)_∞ chain owing to electrostatic attraction; meanwhile, it may result in electrostatic repulsion to adjacent chains through the L_B Cl₂An²⁻ ligand. In addition to the fact that the other L_A⋯NPr₄⁺ distance (6.2 Å) is much longer than that, this situation in 1 could induce TDET between the LT and IM_o phases at a lower T_1/2(1), at least relative to that in 1-d (vide infra). Moreover, only one type of NPr₄⁺ was assigned with the L_A⋯NPr₄⁺ distance of ca. 5.0 Å in 1-d, which evenly load electrostatic interaction to the L_A ligand. This situation in 1-d could stabilize the LT phase much more than that in 1, so T_1/2(1) in 1-d > T_1/2(1) in 1.

In situ continuous modifications using desolvation/solvation treatments

These transition processes are accomplished in a single crystal even though it undergoes desolvation/solvation processes to convert between 1/1′ and 1-d, which demonstrated the reversible magnetic variations in ex situ magnetic measurements (Fig. S10 and S11†). This means that we can address five states (LT and IM_o in 1; LT, IM_o, and HT in 1-d) comprising three identified phases of LT, IM_o, and HT phases (Scheme 2 and Fig. 7). To demonstrate the effects of these continuous variations of the phases on temperature control and guest molecule accommodation, in situ magnetic measurements for continuous changes between 1 and 1-d were conducted using a SQUID apparatus (MPMS-XL, Quantum Design Co. Ltd.). The use of a home-made closed cell enables the insertion of gaseous solvent molecules as well as evacuation at a turbomolecular pump (TMP) level.

Fig. 7 In situ magnetic measurements (χ_mT–T plots) performed at 1 kOe throughout the processes for 1 (1′) and 1-d in a closed cell with a valve connectable to either a vapor of the solvent or TMP (solvation/desolvation was performed at 300 K). (a) Overview of χ_mT–T plots in the temperature range of 1.8 K and 400 K, where the initial sweep of field-cooled magnetization was started from 1-d prepared under ex situ conditions at 300 K followed by a coherent sweep of 300 K → 1.8 K → 300 K → 400 K → 300 K → exposed to a vapor of acetone–water mixed solution at 300 K → 1.8 K → 300 K → evacuating at 300 K → 1.8 K → 300 K → 400 K → 300 K. The inset shows a switching demonstration at 300 K by repeating the solvent vapor-insertion and the evacuation. (b) Close-up view of the part at 200–400 K.

First, a polycrystalline sample of 1-d (the solvent-free compound) was put into the SQUID apparatus at 300 K. Then, a temperature sweep of 300 K → 1.8 K → 300 K was applied for 1-d (Fig. 7a). During this process, the absence of TDET behavior was confirmed in this temperature region, corresponding to the magnetic data for the LT phase of 1-d. As the temperature increased to 400 K, stepwise charge variations from LT (1-d) to IM_o (1-d) at T_1/2b↑ and from IM_o (1-d) to HT (1-d) at T_1/2c↑ were observed (Fig. 7b). During the cooling process from 400 K to 300 K, the charge variation followed the heating process for 1-d as HT (1-d) → IM_o (1-d) → LT (1-d) at T_1/2c↓ and T_1/2b↓, respectively (Fig. 7b). At 300 K, the sample was exposed to solution vapor of a 1 : 2 v/v mixture of water/acetone, for which the χ_mT value was monitored in a time course, showing a rapid decrease from 13.3 cm³ K mol⁻¹ for LT (1-d) into a saturation with 10.1 cm³ K mol⁻¹ for the IM phase of 1, i.e., 1′ within a few minutes (Fig. 7b and S12a†). Upon cooling to 1.8 K, the TDET for the transformation from IM_o into LT in 1′ occurred at T_1/2a↓, which was followed by TDET from LT to IM_o at T_1/2a↑ during the heating process up to 300 K (Fig. 7b). After this process, the sample of 1′ was kept standing at 300 K for 12 h with an adequate evacuation, for which the χ_mT value was monitored in a time course, showing a gradual increase from 10.2 cm³ K mol⁻¹ for the IM_o phase of 1′ followed by a saturation with 13.1 cm³ K mol⁻¹ for the LT phase of 1-d owing to the desolvation process (Fig. 7b and S12b†). This reversible charge modulation of 1/1-d was confirmed by performing the solvation/desolvation process at 300 K over several cycles (inset of Fig. 7a and S12†). Consequently, this course of processes proves that the respective TDET processes of 1′ and 1-d can be combined using the desolvation/solvation processes at around room temperature for the same sample of compounds (Fig. 7 and S12†).

Furthermore, in situ magnetic measurements from 1 to 1-d were conducted in the heating process from 300 to 400 K at a rate of 0.5 K min⁻¹ in a sweep mode (Fig. S13†). The desolvation of crystallization solvent from 1 spontaneously occurred as the temperature was increased up to ∼320 K, and resulted in the gradual variations of χ_mT values associated with the material state changes from IM_o (1) to IM_o (1-d).

Electronic conductivity of 1-d

The charge variations in 1-d associated with TDET behavior resulted in characteristic temperature-dependent electronic conductivity. The electronic conductivity measurements were performed on a hexagonal-shaped single crystal of 1-d using two-probe dc current–voltage techniques in heating/cooling processes with a sweep rate of 5 K min⁻¹ (Fig. 8a). Gold wire probes were attached (i) parallel (‖) and (ii) perpendicular (⊥_c) to the Fe–L_A chain direction, and (iii) perpendicular (⊥_l) to the layer direction (Fig. S14†), respectively. Note that the (i) arrangement corresponds to the direction experiencing TDET behavior. As expected, the room temperature conductivity parallel to the layers σ_‖ and σ_⊥c (ca. 2.6 × 10⁻⁴ S cm⁻¹ and ca. 1.0 × 10⁻⁴ S cm⁻¹, Fig. 8b and c) is much higher than that perpendicular to the hexagonal layers (σ_⊥l, ca. 4.1 × 10⁻⁸ S cm⁻¹; Fig. 8d), resulting in a significant anisotropic electronic conductivity (σ_‖/σ_⊥l = ca. 6.3 × 10³).^42,48 The observed electronic conductivity in 1-d at room temperature was comparable with that in previously reported honeycomb layers.^41,49

Fig. 8 Thermal variation of single-crystal electronic conductivity for 1-d using a typical size of hexagonal crystals (a), in which gold wire probes were attached parallel (‖; b) and perpendicular (⊥_c; c) to the Fe–L_A chains in the layer, and perpendicular to the layer-stacked direction (⊥_l; d).

During the heating process, the thermal variations of σ_‖ profiles showed an unusual incoherent feature with two-step inflections at around T_1/2b and T_1/2c (Fig. 8b). The cooling process also follows the features of the heating process. The absolute value of σ is still within the range for common semiconductors, but interestingly, the HT phase region (350–380 K) of σ_‖ showed metallic behavior, and even in the IM_o phase region (310–350 K), such a metallic feature was observed at higher temperatures of T_1/2b and T_1/2c. This behavior may be due to electronic fluctuations that are closely associated with TDET at T_1/2b and T_1/2c. A similar feature was previously reported in DA systems involving TDET (i.e., N–I transition systems).^18,50 The activation energy (E_a) for the semiconductor behavior in the LT phase and the IM_o phase during the heating process is 438.4 meV (285–310 K) and 189.9 meV (328–347 K), respectively (Fig. S15†). The decreasing tendency of E_a values associated with the phase transition from LT to HT via IM_o phases indicates that the electronic band structure was modified in each phase based on the occurrence of TDET. The inflections at around T_1/2s were significant for σ_‖; however, the thermal variation of the σ_⊥c and σ_⊥l values exhibited fewer features owing to the anisotropic environment of TDET behavior (Fig. 8c and d).

Conclusions

There are few stimuli responsive molecular materials that can multiply change their charge states. In addition, there are no cases in which two types of phase-switchable materials, each of which has its own TDET process, were combined with each other using a guest-induced ET process. The present material is the first case of such materials. The transformation between these accessible five states composed of three distinct charge-ordered states (LT, IM_o, and HT) was successfully realized using two kinds of stimuli, namely temperature control and guest molecule desorption/adsorption. These stepwise TDETs in 1-d resulted in electronic fluctuations, which had anisotropic transient electronic conductivity; the Fe–L_A chain direction, i.e., corresponding to the TDET direction, is more conductive than the other directions.

Even though the basic layer frameworks of 1 and 1-d are structurally very similar to each other, T_1/2a and T_1/2b for the LT ⇌ IM_o TDET in 1 and 1-d, respectively, are largely different, which could be attributed to the differences in the electrostatic effects in 1 and 1-d, which resulted from their packing arrangement of NPr₄⁺ cations. Thus, the desolvation/solvation process between 1 and 1-d caused a structural change. In particular, the packing arrangement of the anionic [Fe₂(Cl₂An)₃]²⁻ layer and the cations NPr₄⁺ contributed significantly to modulating the electrostatic effect in the TDET of this series. This conclusion may answer the question regarding the reason why other series of (C)₂[Fe₂(Cl₂An)₃] with different C⁺ cations reported so far did not exhibit such TDETs within the common temperature range. The bulk effect brought about by electrostatic stabilization, i.e., Madelung stabilization, is very important to the tuning of TDET in ionic D/A systems.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors thank Prof. S. Hayami (Kumamoto University) for the helpful discussions on ⁵⁷Fe Mössbauer spectra and Dr K. Sugimoto (JASRI) for the SC-XRD measurements for the IM phase using a synchrotron beam at Spring-8. This work was supported by a Grant-in-Aid for Scientific Research (No. 16H02269, 18K19050, 18K05055, and 18H05208) from MEXT, Japan, and on Innovative Areas (“π-System Figuration” Area 2601, No. 17H05137) from the Japan Society for the Promotion of Science (JSPS), Japan, and the E-IMR project. Y. S. is thankful for financial support received from the TOBE MAKI Scholarship Foundation and the Shorai Foundation for Science and Technology. J. C. gratefully acknowledges the financial support received from the Chinese Scholarship Council (CSC) and the Institute for Materials Research, Tohoku University.

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Footnote

† Electronic supplementary information (ESI) available. CCDC 1971740–1971743. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d0sc00684j

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