Strong magnetic response in all-organic ferroelectric liquid crystals: synergistic regulation of polar order and electron spin

Professors Mingjun Huang and Xiaochen Xie's team from the School of Frontier Soft Matter at South China University of Technology published a research paper, successfully constructing an all-organic ferroelectric nematic liquid crystal FNT3 by covalently combining stable nitroxide radicals TEMPO with polar liquid crystal mesocrystalline units. The study found that the system exhibits significantly enhanced nonlinear magnetic response and short-range spin correlation with the formation of the ferroelectric nematic phase, providing a new molecular design approach for exploring the synergistic electro-magnetic functions in organic soft matter. The article was published as an open access Research Article in CCS Chemistry, the flagship journal of the Chinese Chemical Society.

Background information:

Integrating ferroelectricity and magnetism into a single homogeneous material system is an important direction in the research of multiferroic materials and magnetoelectric functional materials. Traditional research has mainly focused on inorganic crystalline materials, but achieving similar functions in organic soft matter with fluidity and structural reconfigurability remains a challenge. Liquid crystals combine the fluidity of liquids with molecular order, making them an ideal platform for constructing flexible multifunctional materials. However, traditional organic liquid crystals typically exhibit weak diamagnetic responses. Existing studies have enhanced the magnetism of liquid crystals by introducing magnetic nanoparticles, metal coordination centers, or organic free radicals, but these are still limited by factors such as particle aggregation, restricted molecular motion, or weak spin correlation between free radicals. In recent years, the discovery of ferroelectric nematic liquid crystals (FNLCs) has provided a new opportunity to address this issue. Their unique macroscopic spontaneous polarization and ordered molecular orientation structure hold promise as a key to regulating the spatial correlation and spin interaction between free radicals. Based on this, this study proposes a design strategy that utilizes the ordered ferroelectric nematic phase to control the molecular spin response by covalently connecting polar mesocrystalline units to stable organic free radicals.

Highlights of this article:

1.  Molecular design: Achieving the integration of ferroelectric mesopelastic crystals and stable free radicals.
The research team designed and synthesized a series of TEMPO-functionalized liquid crystal molecules, namely FNT1, FNT2, and FNT3. Addressing the significant steric hindrance of the TEMPO groups, they improved molecular shape anisotropy by extending the mesocrystalline framework, successfully enabling FNT3 to form both a nonpolar nematic phase (N) and a ferroelectric nematic phase (N_F). DFT calculations showed that FNT3 possesses a molecular dipole moment of approximately 10.9 D, with its orientation largely along the long axis of the molecule, providing favorable conditions for the formation of ferroelectric order.

2.  Ferroelectric properties: significant spontaneous polarization.
Through experiments including polarizing microscopy, polarization reversal current, and second harmonic generation (SHG), the research team confirmed the ferroelectric properties of FNT3. As the temperature decreased to below approximately 120 °C, the system exhibited a significant SHG signal and ferroelectric polarization response, with its saturation polarization reaching approximately 2.0 μC/cm². This result demonstrates that, through rational molecular design, even with the introduction of TEMPO radicals with significant steric hindrance , significant ferroelectric nematic phase ordering can still be achieved in a single-component, all-organic system.

3.  Magnetic properties: Enhanced nonlinear magnetic response appears in the ferroelectric nematic phase.
SQUID magnetic tests revealed that FNT3 exhibits a magnetic response significantly different from that of conventional liquid crystals with diamagnetism. In the nonpolar N phase, its magnetization curve begins to deviate from ideal linear paramagnetic behavior; as the temperature further decreases, the system enters the ferroelectric nematic phase (N_F), and the nonlinear magnetic response is significantly enhanced. At 87 °C, the magnetization of FNT3 reaches approximately 0.03 emu/g, exhibiting a narrow hysteresis loop with a coercive field of approximately 200 Oe, displaying near-superparamagnetic magnetic response characteristics. In contrast, the physical blend of RM734 and TEMPO mainly exhibits an approximately linear paramagnetic response, indicating that the covalent integration of free radicals and polar mesocrystalline units is an important factor in generating the unique collective magnetic response.

4.  Mechanism of action: Revealing the connection between ferroelectric order and short-range spin correlation.
Variable-temperature electron paramagnetic resonance (EPR) results showed that as the system transitioned from a conventional nematic phase to a ferroelectric nematic phase, the triplet hyperfine line of the TEMPO radical gradually broadened and underwent significant changes, with the apparent g-value increasing from approximately 2.0088 to 2.0100, indicating that the local spin environment evolved with the ordered liquid crystal structure. Further analysis using WAXS results revealed that the enhanced magnetic response was not accompanied by a significant abrupt change in molecular packing density. Therefore, this study suggests that polar ordering in the ferroelectric nematic phase may regulate effective spin interactions between molecules by altering the orientation of radicals and local spatial correlations. This result provides new experimental evidence for understanding the relationship between polar ordering and electronic spin behavior in soft matter systems.

Summary and Outlook:

In summary, this study constructed a single-component, all-organic liquid crystal material exhibiting both spontaneous polarization and significant nonlinear magnetic response by introducing stable TEMPO radicals into ferroelectric nematic mesocrystalline molecules. This work breaks through the traditional design approach of magnetic liquid crystals, which mainly relies on inorganic magnetic components, and demonstrates the possibility of regulating spin response through molecular structure and polarity order. Although its microscopic spin coupling mechanism and direct macroscopic magnetoelectric coupling still require further verification, this research provides a new research platform for the development of flexible magnetoelectric functional materials, molecular spintronics, and intelligent responsive soft matter.

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About the journal: CCS Chemistry is the Chinese Chemical Society’s flagship publication, established to serve as the preeminent international chemistry journal published in China. It is an English language journal that covers all areas of chemistry and the chemical sciences, including groundbreaking concepts, mechanisms, methods, materials, reactions, and applications. All articles are diamond open access, with no fees for authors or readers. More information can be found at https://www.chinesechemsoc.org/journal/ccschem.

About the Chinese Chemical Society: The Chinese Chemical Society (CCS) is an academic organization formed by Chinese chemists of their own accord with the purpose of uniting Chinese chemists at home and abroad to promote the development of chemistry in China. The CCS was founded during a meeting of preeminent chemists in Nanjing on August 4, 1932. It currently has more than 120,000 individual members and 184 organizational members. There are 7 Divisions covering the major areas of chemistry: physical, inorganic, organic, polymer, analytical, applied and chemical education, as well as 31 Commissions, including catalysis, computational chemistry, photochemistry, electrochemistry, organic solid chemistry, environmental chemistry, and many other sub-fields of the chemical sciences. The CCS also has 10 committees, including the Woman’s Chemists Committee and Young Chemists Committee. More information can be found at https://www.chinesechemsoc.org/.