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

Four-step electron transfer coupled spin transition in a cyano-bridged [Fe2Co2] square complex

The design of molecular functional materials with multi-step magnetic transitions has attracted considerable attention. However, the development of such materials is still infrequent and challenging. Here, a cyano-bridged square Prussian blue complex that exhibits a thermally induced four-step electron transfer coupled spin transition (ETCST) is reported. The magnetic and spectroscopic analyses confirm this multi-step transition. Variable-temperature infrared spectrum suggested the electronic structures in each phase and a four-step transition model is proposed.

Polar Crystals Using Molecular Chirality: Pseudosymmetric Crystallization toward Polarization Switching Materials

Polar compounds with switchable polarization properties are applicable in various devices such as ferroelectric memory and pyroelectric sensors. However, a strategy to prepare polar compounds has not been established. We report a rational synthesis of a polar CoGa crystal using chiral cth ligands (SS-cth and RR-cth, cth = 5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane). Both the original homo metal Co crystal and Ga crystal exhibit a centrosymmetric isostructure, where the dipole moment of metal complexes with the SS-cth ligand and those with the RR-cth ligand are canceled out. To obtain a polar compound, the Co valence tautomeric complex with SS-cth in the homo metal Co crystal is replaced with the Ga complex with SS-cth by mixing Co valence tautomeric complexes with RR-cth and Ga complexes with SS-cth. The CoGa crystal exhibits polarization switching between the pseudononpolar state at a low temperature and the polar state at a high temperature because only Co complexes exhibit changes in electric dipole moment due to metal-to-ligand charge transfer. Following the same strategy, the polarization-switchable CoZn complex was synthesized. The CoZn crystal exhibits polarization switching between the polar state at a low temperature and the pseudononpolar state at a high temperature, which is the opposite temperature dependence to that of the CoGa crystal. These results revealed that the polar crystal can be synthesized by design, using a chiral ligand. Moreover, our method allows for the control of temperature-dependent polarization changes, which contrasts with typical ferroelectric compounds, in which the polar ferroelectric phase typically occurs at low temperatures.

Photo-induced valence tautomerism and polarization switching in mononuclear cobalt complexes with an enantiopure chiral ligand

Light-induced polarization switchable molecular materials have attracted attention for decades owing to their potential remote manipulation and ultrafast responsiveness. Here we report a valence tautomeric (VT) complex with an enantiopure chiral ligand. By a suitable choice of counter anions, a significant improvement in photoconversion has been demonstrated, leading to novel photo-responsive polarization switching materials.

Electrically Detectable Photoinduced Polarization Switching in a Molecular Prussian Blue Analogue

Light, a nondestructive and remotely controllable external stimulus, effectively triggers a variety of electron-transfer phenomena in metal complexes. One prime example includes using light in molecular cyanide-bridged [FeCo] bimetallic Prussian blue analogues, where it switches the system between the electron-transferred metastable state and the system's ground state. If this process is coupled to a ferroelectric-type phase transition, the generation and disappearance of macroscopic polarization, entirely under light control, become possible. In this research, we successfully executed a nonpolar-to-polar phase transition in a trinuclear cyanide-bridged [Fe2Co] complex crystal via directional electron transfer. Intriguingly, by exposing the crystal to the wavelength of light─785 nm─without any electric field─we can drive this ferroelectric phase transition to completely depolarize the crystal, during which a measurable electric current response can be detected. These discoveries signify an important step toward the realization of fully light-controlled ferroelectric memory devices.