South China University of Technology makes all-organic ferroelectric liquid crystal with magnetic response
Chemists covalently attached stable TEMPO radicals to a polar liquid crystal molecule. In the ferroelectric nematic phase, its nonlinear magnetic response grew markedly.
Researchers led by Mingjun Huang and Xiaochen Xie at the School of Frontier Soft Matter, South China University of Technology, synthesized an all-organic ferroelectric nematic liquid crystal, FNT3, according to EurekAlert. The material was made by covalently joining stable TEMPO nitroxide radicals to polar mesogenic units eurekalert.org
EurekAlerteurekalert.orgOpen source ↗. The work was published as an open access Research Article in CCS Chemistry. The authors report that the nonlinear magnetic response and short-range spin correlation increase markedly as the ferroelectric nematic phase forms
eurekalert.org
EurekAlerteurekalert.orgOpen source ↗.
Molecular design and ferroelectric order
The team designed three TEMPO-functionalized molecules: FNT1, FNT2 and FNT3. The TEMPO groups cause significant steric hindrance, so the authors extended the mesogenic framework to improve molecular shape anisotropy. As a result, FNT3 forms both a nonpolar nematic phase (N) and a ferroelectric nematic phase (NF) eurekalert.org
EurekAlerteurekalert.orgOpen source ↗. DFT calculations gave FNT3 a molecular dipole moment of about 10.9 D, directed largely along the long axis of the molecule.
Polarizing microscopy, polarization reversal current and second harmonic generation (SHG) measurements confirmed the ferroelectric properties. Below about 120 °C the system showed a significant SHG signal and a ferroelectric polarization response, with saturation polarization of about 2.0 μC/cm² eurekalert.org
EurekAlerteurekalert.orgOpen source ↗. Ferroelectric nematic ordering was thus achieved in a single-component, all-organic system despite the bulky radical groups.
Magnetic response and spin environment
SQUID measurements showed that FNT3 differs from conventional diamagnetic liquid crystals. In the N phase its magnetization curve begins to deviate from ideal linear paramagnetic behavior. In the NF phase the nonlinear response is significantly enhanced eurekalert.org
EurekAlerteurekalert.orgOpen source ↗. At 87 °C the magnetization of FNT3 is about 0.03 emu/g, with a narrow hysteresis loop and a coercive field of about 200 Oe, close to superparamagnetic behavior. A physical blend of RM734 and TEMPO mainly showed an approximately linear paramagnetic response, which points to the importance of covalent bonding between radical and mesogenic unit.
Variable-temperature EPR showed that on the transition from the nematic to the ferroelectric nematic phase the triplet hyperfine line of TEMPO broadened and changed, and the apparent g-value rose from about 2.0088 to 2.0100. WAXS showed no significant abrupt change in molecular packing density alongside the stronger magnetic response. The authors therefore suggest that polar order may regulate effective spin interactions between molecules by changing radical orientation and local spatial correlations eurekalert.org
EurekAlerteurekalert.orgOpen source ↗.
The authors note that the microscopic spin coupling mechanism and direct macroscopic magnetoelectric coupling still require further verification. The article is titled "All-Organic Strong-Paramagnetic Ferroelectric Nematic Fluids" (DOI 10.31635/ccschem.026.202608172).
