Control of electronic polarization via charge ordering and electron transfer: electronic ferroelectrics and electronic pyroelectrics

Abstract

Ferroelectric, pyroelectric, and piezoelectric compounds whose electric polarization properties can be controlled by external stimuli such as electric field, temperature, and pressure have various applications, including ferroelectric memory materials, sensors, and thermal energy-conversion devices. Numerous polarization switching compounds, particularly molecular ferroelectrics and pyroelectrics, have been developed. In these materials, the polarization switching usually proceeds via ion displacement and reorientation of polar molecules, which are responsible for the change in ionic polarization and orientational polarization, respectively. Recently, the development of electronic ferroelectrics, in which the mechanism of polarization change is charge ordering and electron transfer, has attracted great attention. In this article, representative examples of electronic ferroelectrics are summarized, including (TMTTF)₂X (TMTTF = tetramethyl-tetrathiafulvalene, X = anion), α-(BEDT-TTF)₂I₃ (BEDT-TTF = bis(ethylenedithio)-tetrathiafulvalene), TTF–CA (TTF = tetrathiafulvalene, CA = p-chloranil), and [(n-C₃H₇)₄N][Fe^IIIFe^II(dto)₃] (dto = 1,2-dithiooxalate = C₂O₂S₂). Furthermore, polarization switching materials using directional electron transfer in nonferroelectrics, the so-called electronic pyroelectrics, such as [(Cr(SS-cth))(Co(RR-cth))(μ-dhbq)](PF₆)₃ (dhbq = deprotonated 2,5-dihydroxy-1,4-benzoquinone, cth = 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraaza-cyclotetradecane), are introduced. Future prospects are also discussed, particularly the development of new properties in polarization switching through the manipulation of electronic polarization in electronic ferroelectrics and electronic pyroelectrics by taking advantage of the inherent properties of electrons.

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1. Introduction

Molecular assemblies whose physical properties can be switched in response to external stimuli have been extensively studied, leading to the development of numerous dynamic molecular crystals with switchable magnetic, electric, optical, and mechanical properties.^1–7 The change in the physical properties of such materials results from the induction of dynamic motions of electrons, ions, and molecules, that is, electron transfer, ion displacement, and molecular reorientation, among others.^8–15

Polarization is among the most basic and important properties of organic and inorganic compounds. Therefore, the development of polarization switching materials such as ferroelectrics, pyroelectrics, and piezoelectrics and their characterization are important subjects in materials science.^14,16–19 Polarization changes in polar compounds are widely employed for diverse applications, including IR light detectors, pyroelectric sensors, and thermal energy-conversion devices. In particular, polarization switching induced by an electric field is the primary mechanism of ferroelectric memory devices. Polarization switching usually occurs as a result of the displacement of ions or the reorientation of polar molecules. Conversely, materials in which charge ordering and electron transfer are the source of polarization switching, the so-called electronic ferroelectrics, have recently attracted significant attention.^18,20–23 In these materials, electrons are expected to move faster than ions and molecules, and the polarization can be modulated via light-induced excitation of the charge-transfer band. The polarization of compounds with electron spin can potentially be manipulated by applying a magnetic field. The durability of the switching behavior in electronic ferroelectric compounds is expected to be better than that in normal ferroelectric compounds because the ion displacement and molecular reorientation are usually accompanied by a relatively large lattice distortion.

Furthermore, more recently, the control of electronic polarization in nonferroelectrics has been reported. The electronic polarization switching is realized through a precise molecular design to ensure that electron transfer in molecules occurs in the same direction throughout the crystal. This approach to develop polarization switchable compounds is different from the typical approach that is based on the development of ferroelectrics. Meanwhile, polarization-switchable polar compounds (nonferroelectrics) via vectorial electron transfer are called electronic pyroelectrics.

Considering the increasing research attention that is recently being paid to the development of electronic ferroelectrics and electronic pyroelectrics,^24,25 we summarize in this article representative examples of molecular electronic ferroelectrics (Fig. 1), which include (TMTTF)₂X (TMTTF = tetramethyl-tetrathiafulvalene, X = anion), α-(BEDT-TTF)₂I₃ (BEDT-TTF = bis(ethylenedithio)-tetrathiafulvalene), TTF–CA (TTF = tetrathiafulvalene, CA = p-chloranil), and [(n-C₃H₇)₄N][Fe^IIIFe^II(dto)₃] (dto = 1,2-dithiooxalate = C₂O₂S₂), and electronic pyroelectrics such as [(Cr(SS-cth))(Co(RR-cth))(μ-dhbq)](PF₆)₃ (dhbq = deprotonated 2,5-dihydroxy-1,4-benzoquinone, cth = 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraaza-cyclotetradecane).

Fig. 1 Chemical structures of representative electronic ferroelectrics and electronic pyroelectrics: (TMTTF)₂AsF₆, α-(BEDT-TTF)₂I₃, TTF–CA, (n-C₃H₇)₄N[Fe^IIFe^III(dto)₃], and [(Cr(SS-cth))(Co(RR-cth))(μ-dhbq)](PF₆)₃.

2. Electric polarization

2.1. Polarization

The application of an electric field (E) to materials induce a dipole moment (p), which can be expressed by the formula

p = αE

where α is the polarizability. The polarizability comprises three major components, i.e., ionic polarizability (α_ion), orientational polarizability (α_or), and electronic polarizability (α_el) (Fig. 2a). Therefore, α can be expressed as α = α_ion + α_or + α_el. When an external electric field is applied to a material, ions undergo displacement along the electric field direction, which is the origin of α_ion. When materials consist of polar molecules, the orientation of the molecules changes upon exposure to an electric field, resulting in α_or. Meanwhile, electron clouds also undergo displacement in response to an electric field, which is the origin of α_el. Notably, in some compounds, another component originating from the migration of charge carriers that form space charges, the so-called space charge polarizability (α_sp), contributes to the polarization.

Fig. 2 (a) Classification of electric polarization: ionic polarization, orientational polarization, and electronic polarization. (b) Crystallographic point groups. Compounds belonging to noncentrosymmetric point groups, except those classified as 432 according to the Hermann–Mauguin notation, exhibit piezoelectric properties. Compounds belonging to 10 polar point groups exhibit pyroelectric properties.

Compounds having polar crystal structures exhibit electric polarization in the absence of an external electric field, which is called spontaneous polarization. As shown in Fig. 2b, there are 32 crystallographic point groups,²⁶ out of which 11 have a centrosymmetric structure and the remaining 21 point groups are noncentrosymmetric. Of the noncentrosymmetric 21 point groups, 10 have a polar structure with spontaneous polarization. Compounds with noncentrosymmetric point groups, except those classified as 432 according to the Hermann–Mauguin notation, have piezoelectric properties. Furthermore, polar compounds exhibit pyroelectric properties. When the polarization direction in polar compounds can be inverted by an external electric field, the compounds are ferroelectrics.

2.2. Polarization switching via electron transfer

As shown in Fig. 3, typical polarization switching mechanisms in ferroelectrics are ion displacement and reorientation of polar molecules. Another mechanisms are electron transfer and charge ordering (Fig. 3).^20,21,27,28 As already described, ferroelectrics whose polarization switching mechanism is electron transfer are called electronic ferroelectrics. Several electronic ferroelectrics that exhibit a change in electronic polarization rather than in ionic and orientational polarization have been developed.

Fig. 3 Mechanisms of polarization switching in ferroelectrics: ion displacement, molecular reorientation, and electron transfer.

A schematic illustration of the polarization change induced via charge ordering accompanied by dimerization of two molecules is shown in Fig. 4a. Molecules with charge +0.5 are aligned at the same spacing, and no macroscopic polarization is observed. In contrast, when charge ordering with dimerization is induced, macroscopic polarization, in which molecules with charge +1 and charge 0 are aligned alternately, is generated (Fig. 4a). Another example of polarization change is also shown in Fig. 4a. Again, no macroscopic polarization exists when donor molecules with charge 0 (D⁰) and acceptor molecules with charge 0 (A⁰) are aligned at the same spacing. Upon electron transfer from D⁰ to A⁰ with dimerization, macroscopic polarization is generated; in this case, molecules with charge +1 (D⁺) and charge −1 (A⁻) are aligned alternately (Fig. 4a).

Fig. 4 (a) Electronic polarization switching via charge ordering and electron transfer in ferroelectrics. (b) Electronic polarization switching via electron transfer in pyroelectrics.

Recently, nonferroelectric compounds undergoing polarization switching via electron transfer are being studied. Several polar compounds that exhibit switchable electronic polarization, i.e., electronic pyroelectrics, have been recently developed.²⁵ A schematic illustration of the polarization change in nonferroelectric polar compounds is shown in Fig. 4b. Donor (D) and acceptor (A) molecules are aligned alternately, forming pairs with a polar structure. The electron transfer between D and A induces a change in the electronic polarization between P = P₁ and P = P₂.

Although any polar compounds have pyroelectric properties, their pyroelectric effects are usually small. However, when the constituent molecules of a polar compound exhibit electron transfer along the same direction in the crystal, the resulting pyroelectric effects can be comparable to those of typical ferroelectrics. Furthermore, various polarization behaviors are expected to be realized in nonferroelectrics, because it is not restricted to the typical behavior of ferroelectrics, in which the low temperature phase is normally a polar phase with larger polarization value than the high temperature phase. The development of polarization-switchable compounds using electron transfer in nonferroelectrics is a promising subject in the study of dielectrics.

In the following sections, some examples of electronic ferroelectrics and electronic pyroelectrics are discussed.

3. Electronic ferroelectrics

3.1. (TMTTF)₂X

The generation of polarization originating from charge disproportionation, which is also called charge ordering, has been reported in quasi-one-dimensional organic charge-transfer salts, (TMTTF)₂X (X = anion).^29,30 The crystal structure of (TMTTF)₂AsF₆ is displayed in Fig. 5a, which shows that the planar TMTTF molecules stack along the a-axis direction. (TMTTF)₂X exhibits a variety of electronic and magnetic properties depending on temperature and pressure, such as spin-density-wave antiferromagnetism and metallic and superconductor features.³¹ NMR measurements revealed that (TMTTF)₂X salts exhibit charge disproportionation^27,32 with charge ordering temperatures of approximately 67 K, 102 K, and 157 K for X = PF₆, AsF₆, and SbF₆, respectively.^33,34 According to dielectric measurements, their dielectric constant (ε′) exhibits a Curie-like sharp peak of ε′ ≈ 10⁵–10⁶ at the charge ordering temperature,^29,30 indicating the occurrence of a ferroelectric transition.^18,19 The charge difference between charge-rich and charge-poor sites, i.e., 2δ (=ρ_rich − ρ_poor), of (TMTTF)₂X can be determined using various techniques.^34,35 For instance, by means of optical spectroscopy measurements, the 2δ value of (TMTTF)₂AsF₆ at 40 K was estimated to be about 0.2e (Fig. 5b),³⁴ which indicates that charge-rich TMTTF^+0.6 and charge-poor TMTTF^+0.4 are alternately arranged along the a-axis direction. A schematic illustration of the (TMTTF)₂X crystal with charge ordering and the electric dipole expected for electronic ferroelectrics is shown in Fig. 6a.³⁶ Furthermore, (TMTTF)₂SbF₆ has been reported to exhibit antiferromagnetic ordering below 8 K,³⁷ which renders it a novel multiferroic compound with electronic ferroelectricity and antiferromagnetic ordering.³⁸ Recently, polarization properties in response to light were investigated in electronic ferroelectric (TMTTF)₂X compounds (Fig. 6).³⁶ THz absorption spectra, which allow detecting intradimer charge disproportionation, showed that the short-range charge correlation is enhanced by light (1.55 eV femtosecond pulses) close to the charge-order phase boundary for X = PF₆ and AsF₆ (Fig. 6b). It should be noted that, besides the TMTTF salts discussed above (X = PF₆, AsF₆, and SbF₆), several other compounds also exhibit charge ordering behavior, which include those with X = BF₄, and ReO₄.³⁹

Fig. 5 (a) Crystal structure of (TMTTF)₂AsF₆ (TMTTF = tetramethyl-tetrathiafulvalene).²¹ (b) Temperature dependence of the charge disproportionation (2δ) in (TMTTF)₂X, where X = PF₆, AsF₆, and SbF₆.³⁴ Reproduced from ref. 34.

Fig. 6 (a) Schematic of the (TMTTF)₂X crystal with charge ordering (TMTTF = tetramethyl-tetrathiafulvalene, X = anion). Gray arrows represent the expected electric dipole.³⁶ Reproduced from ref. 36. (b) Temperature dependence of the photoinduced changes in the short-range peak intensity (left axis) and THz emission (right axis). Long-range charge-order melts upon photoexcitation, whereas short-range charge fluctuations are enhanced close to the charge-order phase boundary in (TMTTF)₂X (X = AsF₆ and PF₆).³⁶ Reproduced from ref. 36.

3.2. (BEDT-TTF)₂X

In the two-dimensional compound α-(BEDT-TTF)₂I₃, polarization is also generated due to charge disproportionation (Fig. 7). The structure and properties of this compound have been long investigated.^40–42 The conducting layers formed by BEDT-TTF molecules are separated by insulating anion sheets of I₃⁻. The BEDT-TTF molecules are arranged in a herringbone structure, with the long axis of BEDT-TTF aligned along the crystallographic c-direction. α-(BEDT-TTF)₂I₃ exhibits charge ordering below 135 K. A single-crystal structural analysis revealed that the triclinic space group P at the high-temperature phase transforms to P₁ at the low-temperature phase.⁴³ The high-temperature phase is in the metallic state, whereas the low-temperature phase is in the insulating state.⁴⁰α-(BEDT-TTF)₂I₃ consists of two independent alternating stacks along the a-axis, i.e., Stack I and Stack II. In Fig. 7b, BEDT-TTF molecules in Stack I are denoted by A and A′ and those in Stack II are denoted by B and C.⁴⁴Fig. 7b also shows the temperature dependence of the molecular charges of BEDT-TTF at A, A′, B, and C, which are estimated from the bond lengths in the molecule.⁴³ A weak charge disproportionation is observed even at the high-temperature phase, and it is more pronounced below the charge ordering temperature. The charges at A, A′, B, and C are estimated to be ρ_A = 0.49(3), ρ_A′ = 0.49(3), ρ_B = 0.57(4), and ρ_C = 0.41(3), respectively, at room temperature (high-temperature phase) and ρ_A = 0.82(9), ρ_A′ = 0.29(9), ρ_B = 0.73(9), and ρ_C = 0.26(9), respectively, at 20 K (low-temperature phase).⁴³ The temperature dependence of the second-harmonic generation (SHG) was measured upon excitation by 1400 nm light to characterize the polarization properties of the compound. As shown in Fig. 8a, an SHG signal was detected below the charge ordering temperature and its intensity increased with decreasing the temperature. This result revealed that the centrosymmetric structure at the high-temperature phase changed to a noncentrosymmetric structure at the charge ordering temperature, which is consistent with the formation of a polar structure at the low-temperature phase. Optical SHG interferometry measurements revealed that polar domain structures in the crystal varied when the crystal was annealed above the transition temperature.⁴⁵ This behavior is indicative of the emergence of spontaneous polarization at 135 K, below which α-(BEDT-TTF)₂I₃ behaves as an electronic ferroelectric compound.⁴⁴ The polarization properties were further investigated by performing polarization vs. electric field (P–E) measurements, finding that the P–E curve at 5 K showed polarization hysteresis with a saturation polarization of approximately 2 nC cm⁻² (Fig. 8b).⁴⁶ This small saturation polarization was suggested to result from the fact that only a fraction of polar domains were switched by the electric field because the measurement was performed far below the freezing temperature.⁴⁶ Electronic ferroelectricity was also suggested in κ-(BEDT-TTF)₂Cu[N(CN)₂]Cl,⁴⁷ although this is still under debate.⁴⁸ In this compound, a THz field-induced macroscopically polarized charge-order state was observed.⁴⁹

Fig. 7 (a) Crystal structure of α-(BEDT-TTF)₂I₃ (BEDT-TTF = bis(ethylenedithio)-tetrathiafulvalene). (b) Temperature dependence of the molecular charge.⁴³ Reproduced with permission from ref. 43, The Physical Society of Japan.

Fig. 8 (a) Temperature dependence of the second-harmonic generation (SHG) signal.⁴⁴ Reproduced with permission from ref. 44, The Physical Society of Japan. (b) Time-dependent current obtained from positive-up–negative-down measurements performed at 36 K. Left inset: excitation signal. Right inset: polarization (P) vs. electric field (E) hysteresis curve at 5 K.⁴⁶ Reproduced with permission from ref. 46, American Physical Society.

3.3. TTF–CA

Electronic ferroelectricity was also reported in the charge-transfer complex TTF–CA,^50–52 whose properties have been extensively investigated.^53–58 In TTF–CA, the TTF and CA units are alternately aligned along the crystallographic a-direction, forming a one-dimensional structure. TTF-CA exhibits a neutral-to-ionic (N–I) transition at approximately 81 K via charge-transfer between TTF and CA, where the high-temperature phase is the neutral (N) phase and the low-temperature phase is the ionic (I) phase (Fig. 9a).⁵³ The degree of charge-transfer (ρ_N) of the N phase in TTF^+ρ_N–CA^−ρ_N is about 0.3, whereas that (ρ_I) of the I phase in TTF^+ρ_I–CA^−ρ_I is about 0.6. The driving force of the N–I transition from N phase to I phase is an increase in the Madelung energy induced by thermal contraction upon cooling. When the gain in the Madelung energy overcomes the ionization energy of TTF–CA, the N–I transition is induced. TTF–CA exhibits ferroelectric behavior below 81 K. TTF and CA in the I phase form a donor–acceptor pair via displacement at the N–I transition temperature. The displacement of TTF and CA induces a polarization change (ΔP_ion) according to the ion displacement mechanism. Furthermore, electron transfer between TTF and CA induces another change in polarization (ΔP_el), which represents the electron transfer mechanism. Both ion displacement and electron transfer mechanisms contribute to the total polarization change (ΔP), which can be expressed as ΔP = ΔP_ion + ΔP_el, where ΔP_el is greater than ΔP_ion, with their sign in polarization change being opposite. Since the electron transfer mechanism dominates the polarization change, TTF–CA is an electronic ferroelectric material. As shown in Fig. 9b, the P–E curves of TTF-CA exhibit a hysteresis loop. The P–E curve at 59 K shows that the remanent polarization is 6.3 μC cm⁻² and the coercive field is 5.4 kV cm⁻¹. The spontaneous polarization was calculated to be 10.0 μC cm⁻² according to the Berry phase approach.^59,60 It should be noted that the analogous charge-transfer complex TTF–BA (BA = p-bromanil), which is ionic in the whole temperature range of study,⁶¹ also exhibits ferroelectric property.⁶² Spontaneous polarization is observed below 53 K. However, the ferroelectric behavior follows a typical ion displacement mechanism rather than electron transfer.

Fig. 9 (a) Schematic illustration of the neutral-to-ionic transition in TTF–CA (TTF = tetrathiafulvalene, CA = p-chloranil). P is the ferroelectric polarization.⁵⁷ Reproduced from ref. 57. (b) Hysteresis loops of TTF–CA with various frequencies at 51 K.^50,52 Reproduced with permission from ref. 50, American Physical Society.

3.4. [(n-C₃H₇)₄N][Fe^IIIFe^II(dto)₃]

Electronic ferroelectricity has also been reported in a coordination compound, [(n-C₃H₇)₄N][Fe^IIIFe^II(dto)₃],⁶³ whose magnetic behavior and charge-transfer phase transition behavior have been investigated.^64–68 [(n-C₃H₇)₄N][Fe^IIIFe^II(dto)₃] crystallizes in the P6₃ space group (Fig. 10a and b)^64,65 and exhibits a two-dimensional honeycomb network structure (Fig. 10a and c). One of the Fe ions, which is in the low-spin (LS) state, is coordinated by six S from the dto ligand, while another Fe ion in the high-spin (HS) state is coordinated by six O from dto. The HS Fe and LS Fe ions are alternately arranged through the dto ligand in the honeycomb structure, which is separated by an [(n-C₃H₇)₄N]⁺ layer. The [(n-C₃H₇)₄N][Fe^IIIFe^II(dto)₃] compound exhibits thermally induced electron transfer between Fe^III and Fe^II at approximately 120 K (Fig. 10d and 11a), adopting an [Fe^II_LS-dto-Fe^III_HS] structure and an [Fe^III_LS-dto-Fe^II_HS] structure at the low-temperature phase and the high-temperature phase, respectively, as follows:

[Fe^II_LS-dto-Fe^III_HS] ⇄ [Fe^III_LS-dto-Fe^II_HS]

Fig. 10 (a) Two-dimensional honeycomb network of [(n-C₃H₇)₄N][Fe^IIIFe^II(dto)₃] (dto = 1,2-dithiooxalate = C₂O₂S₂).⁶³ (b) Projection view of the a–c plane: C, gray; O, red; S, yellow; N, blue; Fe^II, green; Fe^III, orange.⁶³ (c) Chemical structure of the honeycomb network of (n-C₃H₇)₄N][Fe^IIIFe^II(dto)₃].⁶⁴ (d) Schematic illustration of the charge-transfer phase transition at 120 K.⁶⁴

Fig. 11 (a) Temperature dependence of the magnetic susceptibility for [(n-C₃H₇)₄N[Fe^IIFe^III(dto)₃].^64,65 Reproduced with permission from ref. 65, Wiley. (b) Temperature dependence of the field cooled magnetization, remnant magnetization, and zero-field cooled magnetization.^64,65 Reproduced with permission from ref. 65, Wiley. (c) Hysteresis loops of electric polarization of an [(n-C₃H₇)₄N[Fe^IIFe^III(dto)₃] film at 300 K in the in-plane direction.⁶³ Reproduced with permission from ref. 63, American Chemical Society. (d) Polarization (P) vs. electric field (E) plot along the out-of-plane direction.⁶³ Reproduced with permission from ref. 63, American Chemical Society.

The electronic states of the Fe ions are expressed as Fe^II_LS (t_2g⁶e_g⁰, S = 0) and Fe^III_HS (t_2g³e_g², S = 5/2) in the low-temperature phase and as Fe^III_LS (t_2g⁵e_g⁰, S = 1/2) and Fe^II_HS (t_2g⁴e_g², S = 2) in the high-temperature phase. The thermal-induced electron transfer is thought to be driven by spin entropy.⁶⁹ The spin entropy, R ln(2S + 1), in the high-temperature phase is R ln 10, i.e., (R ln 2 + R ln 5), which is greater than that in the low-temperature phase (R ln 6). The contribution of the spin entropy (4.25 J K⁻¹ mol⁻¹) is approximately half of the total entropy gain estimated from heat capacity measurements (9.20 J K⁻¹ mol⁻¹). Because the energy of the entropically favorable [Fe^III_LS-dto-Fe^II_HS] state is slightly higher than that of the [Fe^II_LS-dto-Fe^III_HS] state, the transition to the [Fe^III_LS-dto-Fe^II_HS] state is realized upon heating. Furthermore, this compound exhibits ferromagnetic ordering below 10 K (Fig. 11b). Thus, [(n-C₃H₇)₄N][Fe^IIIFe^II(dto)₃] has received attention owing to its molecular magnet and switchable magnetic properties. Moreover, the compounds [(n-C_nH_2n+1)₄N][Fe^IIIFe^II(dto)₃] (n = 4, 5, 6) have also been synthesized. Among them, only [(n-C₄H₉)₄N][Fe^IIIFe^II(dto)₃] exhibits charge transfer phase transition, whereas the other two compounds with n = 5 and 6 don't show such phase transitions.⁷⁰

Recently, the electric polarization properties of [(n-C₃H₇)₄N][Fe^IIIFe^II(dto)₃] have been investigated in depth. The P–E curve of a film of [(n-C₃H₇)₄N][Fe^IIIFe^II(dto)₃] measured at 300 K shows a clear polarization hysteresis in the in-plane direction (Fig. 11c), whereas polarization hysteresis is not observed in the out-of-plane direction (Fig. 11d). Polarization hysteresis is also observed at 50 K, 120 K, 150 K, and 200 K. The saturation polarization at 300 K and 50 K in the in-plane direction was determined to be approximately 6 and 10.0 μC cm⁻², respectively.⁶³ The in-plane ferroelectric polarization calculated using the point electric charge model is ∼11.7 μC cm⁻², which is consistent with the experimental results. The ferroelectric polarization is most likely induced by electron hopping between the Fe^II and Fe^III sites. In fact, the analogous compound [(n-C₃H₇)₄N][Fe^IIIZn^II(dto)₃], in which electron hopping between Fe^III and Zn^II is prohibited, exhibits no ferroelectric signals in the in-plane direction, supporting the electron hopping mechanism in [(n-C₃H₇)₄N][Fe^IIIFe^II(dto)₃].

4. Electronic pyroelectrics

Polarization switching compounds can be developed using the strategy shown in Fig. 12a; the strategy is the combination of synthesis of molecules that exhibit electron transfer, and control of the molecular orientation so that electron transfer occurs in the same direction in the crystal.

Fig. 12 (a) Bottom-up approach to develop polarization switching compounds using electron transfer: the strategy is the combination of (i) synthesis of molecules that exhibit electron transfer (here is shown a [CrCo] dinuclear valence tautomeric complex), and (ii) control of the molecular orientation so that electron transfer occurs in the same direction in the crystal.⁸⁷ (b) Temperature dependence of the magnetic susceptibility and pyroelectric current for [(Cr(SS-cth))(Co(RR-cth))(μ-dhbq)](PF₆)₃.^87,91 (c) Schematic illustration of electronic polarization change via electron transfer between Co and the ligand.^87,92

4.1. [(Cr(SS-cth))(Co(RR-cth))(μ-dhbq)](PF₆)₃

Various compounds that exhibit electron transfer in response to external stimuli have been reported, including valence tautomeric compounds and cyanide-bridged metal complexes.^6,10,71–84 Electron transfer in molecules is accompanied by a change in the dipole moment. However, most compounds have a centrosymmetric structure and, therefore, the change in the dipole moment induced via electron transfer is usually canceled out in the crystal. An example is [{Co(tpa)}₂(μ-dhbq)](PF₆)₃·2H₂O {tpa = tris(2-pyridylmethyl)amine and dhbq = deprotonated 2,5-dihydroxy-1,4-benzoquinone}, which exhibits electron transfer between Co and dhbq.⁸⁵ The electron transfer process can be expressed as [Co³⁺_LS-dhsq³⁻-Co³⁺_LS] ⇄ [Co²⁺_HS-dhbq²⁻-Co³⁺_LS] (dhsq = deprotonated dihydroxysemiquinone),^85,86 meaning that the dipole moment changes at a molecular level via electron transfer. However, owing to its centrosymmetric crystal structure, no macroscopic polarization is observed in this compound. Polar molecules with a [Co²⁺_HS-dhbq²⁻-Co³⁺_LS] structure align in an antiferroelectric way.

Meanwhile, macroscopic polarization changes stemming from intramolecular electron transfer between Co and dhbq are observed in the heterometallic [CrCo] dinuclear complex with chiral cth ligand [(Cr(SS-cth))(Co(RR-cth))(μ-dhbq)](PF₆)₃ (Fig. 1 and 12, SS-cth = SS isomer of cth, RR-cth = RR isomer of cth), which possesses a polar crystal structure (Fig. 12c).^87,88 Note that homometallic dinuclear complexes such as [CoCo] complex have been reported before.^89,90 The selective synthesis of the heterometallic [CrCo] complex was achieved by mixing a Cr complex with an enantiopure SS-cth ligand and a Co complex with an enantiopure RR-cth ligand. Furthermore, the left-handed chiral ligands preferentially interact with their right-handed counterparts, affording a polar crystal structure where the [CrCo] complex is aligned along the crystallographic b-direction as shown in Fig. 12c. The [CrCo] complex exhibits electron transfer at approximately 360 K (Fig. 12b), and the dipole moments of [Co³⁺_LS-dhsq³⁻-Cr³⁺] at the low-temperature phase and [Co²⁺_HS-dhbq²⁻-Cr³⁺] at the high-temperature phase are 0.06 and 9.07 Debye, respectively:

[Co³⁺_LS-dhsq³⁻-Cr³⁺] ⇄ [Co²⁺_HS-dhbq²⁻-Cr³⁺]

The electronic states of the Co and Cr ions in the low-temperature phase are expressed as Co³⁺_LS (t_2g⁶e_g⁰, S = 0) and Cr³⁺ (t_2g³e_g⁰, S = 3/2), where Cr³⁺ (S = 3/2) interacts antiferromagnetically with the dhsq³⁻ radical (S = 1/2), representing the S = 1 ground state. The electronic states in the high-temperature phase are expressed as Co²⁺_HS (t_2g⁵e_g², S = 3/2), Cr³⁺ (t_2g³e_g⁰, S = 3/2), and dhbq²⁻ (S = 0). Since the [CrCo] molecules are aligned in the same direction, intramolecular electron transfer leads to macroscopic polarization changes (Fig. 12c). Pyroelectric current is detected at around 360 K (Fig. 12b).⁹¹ The [CrCo] complex is not a ferroelectric material; however, it exhibits electron transfer-induced polarization change, representing the concept of electronic pyroelectrics. Since the dipole moment of the [CrCo] complexes at the low-temperature phase is approximately 0 Debye, polarization switching between on (P ≠ 0) and off (P = ∼0) states is realized as in ferroelectrics. The advantage of the electron transfer mechanism over the typical ion displacement and molecular reorientation mechanisms is that electron transfer can be directly manipulated via electronic excitation with light, resulting in an ultrafast polarization change. Femtosecond time-resolved absorption spectroscopy measurements revealed that the polarization change occurs at about 280 fs after photoexcitation.⁹² Furthermore, the advantage of pyroelectrics (nonferroelectrics) over ferroelectrics is that a domain with different polarization directions is not formed, meaning that all molecules undergo directional electron transfer (vectorial electron transfer) even in the absence of an electric field. The photo-induced polarization switching would be used as the mechanism for ultrafast photo-control of even-order nonlinear optical effects such as second harmonic generation.

The analogous complex [(Co(RR-cth))(Ga(SS-cth))(dhbq)](PF₆)₃ also exhibits polarization change via electron transfer; [Co³⁺_LS-dhsq³⁻-Ga³⁺] ⇄ [Co²⁺_HS-dhbq²⁻-Ga³⁺].⁹¹ Pyroelectric measurements reveal that large pyroelectric effect is detected at approximately 222 K, at which vectorial electron transfer occurs in the crystal. The polarization change (ΔP) via the electron transfer is about 2.9 μC cm⁻², which is comparable to that of typical ferroelectrics such as triglycine sulfate (TGS, ΔP = 3.8 μC cm⁻²) being used as IR detector. Furthermore, the [CoGa] compounds exhibit photoinduced electron transfer and resultant electron transfer state is trapped as a metastable state at low temperature, where the lifetime of the metastable state is approximately 3.5 × 10⁴ s at 10 K. This means that the [CoGa] compounds have the optical memory property of electric polarization. Electric current is generated during the relaxation process from the metastable state [Co²⁺_HS-dhbq²⁻-Ca³⁺] to ground state [Co³⁺_LS-dhsq³⁻-Ca³⁺]. This means photoenergy is converted to electrical energy via photoinduced polarization change, which is distinct from typical pyroelectric compounds that exhibit the conversion of thermal energy into electricity.

The complex [(Fe(RR-cth))(Co(SS-cth))(dhbq)](X)₃ (X = PF₆ or AsF₆) also exhibits polarization change via successive spin transition and electron transfer processes. The polarization change during spin transition and electron transfer is detected as pyroelectric current.^25,93,94 The [FeCo] complex exhibits magnetoelectric effect.⁹⁵ Electric polarization switching via spin transition induced by pulsed magnetic field is observed. Polarization change via electron transfer in response to temperature and light has been also observed in a mononuclear Co valence tautomeric complex, [Co(phendiox)(rac-cth)](ClO₄)·0.5EtOH (H₂ phendiox = 9,10-dihydroxyphenanthrene, rac-cth = racemic cth).^25,96

5. Summary and outlook

In this article, representative examples of electronic ferroelectrics and electronic pyroelectrics have been introduced. Electronic polarization switching via charge ordering, neutral-to-ionic transition, and metal-to-metal electron transfer is observed in electronic ferroelectrics. However, only a limited number of electronic ferroelectrics, including oxides, have been reported so far. Therefore, electronic ferroelectrics with superior polarization properties should be synthesized. In particular, the development of room temperature electronic ferroelectrics with enhanced durability is essential to realize new applications. To this aim, a design strategy to develop electronic ferroelectrics, especially of coordination compounds, should be firmly established. As a result, electronic ferroelectrics with comparable or better properties than those of typical ferroelectrics functioning via ion displacement or molecular reorientation mechanisms could be synthesized.

Furthermore, electronic pyroelectrics are promising materials in the development of novel polarization switching compounds. The combination of the synthesis of molecules that exhibit electron transfer and the control of the electron transfer direction in crystals has enabled the development of pyroelectrics showing polarization switching via vectorial electron transfer. Since this is a bottom-up approach, it could lead to a variety of polarization-switchable materials based on the molecular design. Larger changes in macroscopic polarization than those reported to date can be expected using molecules that exhibit substantial changes in the molecular dipole moment. Recently, numerous photoswitchable compounds in electric polarization,^97–102 including molecular systems,^103–106 have been reported for application in memory devices. When molecular units exhibit photoinduced redox isomerization via electron transfer at room temperature, the electronic pyroelectrics exhibit optical memory property in electronic polarization. Realizing long-life time of the photoinduced metastable state via electron transfer at room temperature is a challenge. Although several promising approaches have been made along this line in the field of molecular photomagnetism and artificial photosynthetic systems,^107,108 much further improvements are required for practical application. Meanwhile, when the molecular units have magnetic field effects, the electronic pyroelectrics exhibit magnetoelectric properties. Although polarization change via spin transition induced by a magnetic field has been reported,^109–113 to our knowledge, the major contribution to the polarization change is a molecular structural change. Coupling the electron transfer (electron redistribution) with spin transition would lead to a large change in electronic polarization, which would be the major contributor to the polarization change, rather than ionic polarization or conformational polarization. Furthermore, various temperature-dependent polarization properties can be realized, including thermally induced polarization inversion. The development of compounds exhibiting photoinduced and magnetic field-induced polarization inversion is also expected. The concept of electronic ferroelectrics/pyroelectrics can be extended to electronic piezoelectrics, in which the mechanism of the piezoelectric effect and the inverse piezoelectric effect is electron transfer instead of ion displacement and molecular reorientation. New functionalities can be expected to emerge from electronic polarization based on the electron degrees of freedom.

Author contributions

All authors equally contributed to the manuscript preparation and writing.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by JSPS KAKENHI (Grant Numbers JP20H00385, JP21K05085, JP22K14694, JP21K05086) and Iketani Fund (Grant Number: 0351039-A).

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