Ionic versus Electronic Ferroelectricity in Donor-Acceptor Molecular Sequences

Molecular displacement in an alternating chain of donors and acceptors triggers ferroelectric switching of large electric polarization with a low electric field in some charge-transfer complexes. While the displacement of a point charge (i.e., static charge) provides a good picture of the polarization for spin-Peierls-type transition in an ionic complex of tetrathiafulvalene (TTF) and p-bromanil (BA), it completely fails to explain both the magnitude and direction of the observed spontaneous polarization in the neutral-ionic transition of TTF-p-chloranil (CA). The most spectacular behavior is that the TTF cations are displaced toward the anode and the CA anions toward the cathode. Recent experimental and theoretical research has highlighted the strong effects of dynamic electrons traveling in the intermolecular space of TTF-CA. The new "electronic ferroelectricity" mechanism can also be envisioned as a powerful guide for guaranteeing high-performance dielectric and related electronic functionalities, especially for organic molecular systems.