Single-component organic molecular ferroelectrics based on disk- or wheel-like rotation

Abstract

Large polarizations and their reversible switching, which are indispensable for many functional ferroelectric applications, are achieved in new single-component organic molecular ferroelectrics with rotating strongly dipolar molecules or substituents. In a pre-screening process using a crystal structural database, both local-molecular and whole-crystal symmetries are inspected for the existence of pseudosymmetry required for rotational ferroelectricity, revealing several candidates: disk-type rotation in 1,2,3,4,5-pentamethyl-6-nitrobenzene (PMNB) molecules and wheel (or rotary knob)-type rotations of the substituents of 2-(methylsulfonyl)malonamide (MSMA), tris(4-acetylphenyl)amine (TAPA), and 4-methylsulfonyl-2-nitrotoluene (MSNT) crystals. The theoretically computed polarizations successfully reproduce experimental values, demonstrating that the rotational processes reverse most of the observed macroscopic polarizations. The theoretically large polarizations (7–8 μC cm⁻²) being comparable to those of polymer ferroelectrics are attributed to the strongly electron-withdrawing nitro group in PMNB as well as the methylsulfonyl group of MSMA and MSNT being highly polarized in the radial direction.