Pyroelectric Organic Crystal of a Bisbiphenylurea with Chiral Side Chains Polarizable under Low Electric Fields Exhibiting a Nonvolatile Polarization Memory even after the Melting–Recrystallization Process

A urea derivative possessing two biphenyl moieties and four chiral side chains was prepared, and its pyroelectric properties were investigated. Investigation of the molecular packing structure and several spectroscopic measurements of the crystal revealed that the hydrogen bonds were weaker than those in the previously reported N,N′-diphenylurea compounds; however, the intercolumnar interactions were significantly stronger. During the isotropic liquid (Iso)-to-crystal (Cr) phase transition of this derivative upon cooling, the generation of polar crystal domains was observed. Recrystallization of the compound from its Iso phase under a low electric field (4 V μm–1) produced a crystal with a pyroelectric coefficient of approximately 20–70 μC m–2 K–1, which was similar to that of polyvinylidene fluoride. Furthermore, the polarization direction of the polarized crystal was not forgotten, even when the material was heated to the Iso phase (at 30 K above the Cr–Iso transition temperature). Even without an electric field, once-polarized crystals repeatedly reproduced the same polarization direction after the melting–recrystallization process. In this study, we propose mechanisms for the generation of pyroelectricity under a low electric field and the reproduction of the polarization direction after melting–recrystallization.