Active Polarization Engineering between Symmetry Inequivalent Polar States Using Electron Transfer: A Nonferroelectric Approach

Conspectus Compounds with polarization switching properties have a wide range of applications including ferroelectric memories, pyroelectric sensors, and piezoelectric actuators. Ferroelectric compounds are primarily focused owing to their ability to switch between two or more symmetry-equivalent polarization states. Besides ferroelectrics, numerous compounds with polar structures exhibit polarization changes in response to external stimuli such as temperature and pressure; however, such effects are normally too small to be considered in practical applications. Recently, we proposed a strategy to achieve polarization switching via electron transfer in polar crystals. The strategy consists of the synthesis of molecules exhibiting intramolecular electron transfer, combined with crystal engineering to align these molecules so that the molecular dipole moments are not canceled at the lattice level. Consequently, vectorial (or directional) electron transfer results in a significant polarization change comparable to what is found for conventional ferroelectrics. Chemically, since the functional motifs are molecules, operational parameters such as the working temperatures and polarization change can be fine-tuned by adjusting the energy levels of the electron donor and acceptor sites and their separation, which enables a more active control of polarization than ferroelectrics. From a physical perspective, the key difference between these systems and ferroelectrics is that the polarization switching occurs between symmetry inequivalent and nondegenerate polar states. As a direct result, they can be switched by various external stimuli other than electric fields, including temperature, magnetic fields, pressure, and light, owing to their different physical properties such as entropy, magnetization, volume, absorption, etc. Moreover, although thermally- and photoinduced ferroelectrics have been reported, they typically form domain structures with different polarization direction due to a symmetry-related degenerate ground state, causing macroscopic polarization to be largely canceled. In contrast, our compounds, which lack accessible symmetry equivalent states, can achieve a perfect polarization alignment without polarization domains upon temperature changes, photoirradiation, or magnetic field. In this Account, we discuss the synthesis of polarization switching compounds, i.e., dinuclear [CoGa], [FeCo], and [CrCo] complexes, via a chirality-assisted method. The thermally induced polarization switching behavior, or pyroelectric effect, is then explained, highlighting the large polarization change (2.9 μC cm^-2) in the [CoGa] complex, which is comparable to the widely used infrared (IR) detector material, triglycine sulfate (TGS). We then discuss the optical polarization memory effect and photoenergy conversion properties, which are a consequence of the photoinduced valence tautomeric behavior with a long-lived photoinduced metastable state. Furthermore, a magnetoelectric effect in the [FeCo] complex is described. The change in polarization is, to the best of our knowledge, the largest one induced by a magnetic field in molecular compounds to date. Notably, the polarization changes induced by temperature variation, photoirradiation, and magnetic field were detected as an electric current without the need of an electric field because polarization domains are not formed, unlike ferroelectric materials.