Fluid Molecules Gain Controllable Electrical Order

Researchers are increasingly focused on fluid molecular ferroelectrics, a novel class of organic materials exhibiting both ferroelectricity and three-dimensional fluidity with spontaneous polarisation values matching solid state materials. Calum J. Gibb, Jordan Hobbs, and William C. Ogle, from the School of Chemistry and School of Physics and Astronomy at the University of Leeds, alongside Richard J. Mandle and colleagues, demonstrate how subtle hydrogen fluorine substitution within forty-five systematically varied molecules allows for the control of syn-parallel pairing motifs. Their work reveals that these motifs can be tuned to produce either geometrically constrained lamellar (smectic) order or diversified pairings stabilising nematic ordering, as validated through large-scale molecular dynamics simulations. These findings establish crucial, experimentally verified design principles for fluid molecular ferroelectrics and offer a predictive framework for developing functional polar fluids, representing a significant advance in soft physics and materials engineering.

Controlling electrical polarisation in flowing materials has long been a difficult goal for materials science. Now, a detailed investigation of molecular structure reveals how to reliably create these ‘fluid ferroelectrics’ and predict their behaviour, offering a pathway to designing advanced materials for adaptable electronics and optics. Scientists are increasingly focused on organic molecular ferroelectrics for use in technologies such as energy harvesting, medical imaging, and robotics.

These materials offer advantages over traditional inorganic counterparts, being lighter, more flexible, and easier to synthesise, allowing for greater structural diversity. Design-driven synthesis has advanced considerably, with key mechanisms including reduced molecular symmetry, introduction of homochirality, and hydrogen/fluorine substitution to tune desired properties.

Recently discovered fluid molecular ferroelectrics represent a new class of organic materials exhibiting both ferroelectricity and three-dimensional fluidity, while maintaining spontaneous polarization levels comparable to solid-state materials. This combination of properties presents distinct advantages, including ease of monodomain fabrication and structural diversity, opening avenues for novel applications in areas like quantum optics and nanoscale electrooptical devices.

Predicting whether a fluid molecular material will form a ferroelectric phase with nematic or smectic order remains a significant challenge in soft condensed matter physics. Nematic phases display long-range orientational order, while smectic phases also exhibit layered structures, influencing material behaviour. Researchers have now explored the relationship between molecular structure and these emergent phases through a systematic investigation of 45 carefully designed molecules.

Subtle alterations in hydrogen-fluorine substitution patterns enabled control of the arrangement of molecules, leading to either geometrically constrained lamellar order or diversified pairings that stabilise nematic ordering. Large-scale molecular dynamics simulations revealed how smectic ferroelectricity arises from discrete lateral pairing modes, whereas nematic phases emerge from a multitude of equivalent polar configurations.

By combining experimental synthesis with computational modelling, these investigations establish validated design principles for fluid molecular ferroelectrics. For instance, a single hydrogen/fluorine substitution can determine whether a material exhibits nematic or smectic ordering, echoing observations in solid-state ferroelectrics where such substitutions impact thermal stability.

A predictive framework for engineering functional polar fluids is becoming increasingly attainable. Currently, 316 of the 438 reported fluid molecular ferroelectrics exhibit nematic order, with only 74 displaying lamellar structures. The research highlights that favourable lateral electrostatic interactions and uniformly distributed partial dipoles are important for promoting ferroelectricity, offering a pathway towards rationally designing fluid ferroelectrics with tailored properties, potentially unlocking a new generation of advanced materials for diverse technological applications.

Synthesis and characterisation techniques employed for compound production

Chemicals required for synthesis were obtained from commercial sources including Fluorochem, Merck, and Apollo Scientific, and researchers employed them without additional purification procedures. Reactions proceeded within standard laboratory glassware under ambient conditions, with progress monitored via thin layer chromatography using appropriate eluents and visualisation under ultraviolet light at 254nm or 365nm.

Purification of chromatographed materials involved a Combiflash NextGen 300+ System (Teledyne Isco) utilising a silica gel stationary phase and a hexane/ethyl acetate gradient as the mobile phase, with detection occurring between 200-800nm. Structural confirmation of intermediates and final products relied on 1H, 13C{1H} and 19F NMR spectroscopy, recorded using either a Bruker Avance III HDNMR spectrometer operating at frequencies of 400MHz, 100.5MHz or 376.4MHz, or a Bruker AV4 NEO 11.75T spectrometer at 500MHz or 125.5MHz.

Mesophase transitions and associated enthalpy changes were determined by differential scanning calorimetry using a TA Instruments Q2000 heat flux calorimeter, equipped with liquid nitrogen cooling. Samples, weighing between 3-8mg, were sealed within T-zero aluminium DSC pans and researchers assessed them under a nitrogen atmosphere at heating and cooling rates of 10°C min-1.

Transition temperatures were measured on cooling to identify the onset of transitions, while melting points were recorded during heating to avoid crystallisation artefacts. Polarised optical microscopy, performed with a Leica DM 2700 P microscope and Linkam TMS 92 heating stage, aided in phase identification by observing samples sandwiched between untreated glass coverslips.

Detailed structural information was obtained through X-ray scattering measurements, encompassing both small angle X-ray scattering (SAXS) and wide angle X-ray scattering (WAXS), conducted using an Anton Paar SAXSpoint 5.0 system. This instrument featured a primux 100 Cu X-ray source coupled with a 2D EIGER2 R detector, utilising X-rays with a wavelength of 0.154nm.

Samples were contained within either thin-walled quartz capillaries or held between Kapton tape, with temperature regulated by an Anton Paar heated sampler ranging from 20°C to 300°C under atmospheric pressure exceeding 1 mbar. Background scattering was subtracted from sample data after appropriate scaling based on sample transmission. For current response measurements, a current reversal technique was employed, applying triangular waveform AC voltages via an Agilent 33220A signal generator.

Resulting current outflow was amplified and recorded using a RIGOL DHO4204 oscilloscope, while temperature control was maintained by an Instec HCS402 hot stage with 10 mK stability, governed by an Instec mK1000 temperature controller. Liquid crystal samples were housed in 4μm thick cells lacking alignment layers, supplied by Instec, and cooled at 1 K min-1 with a 20Hz voltage applied every 1 K to saturate the measured polarisation.

Ferroelectric phase behaviour modulated by halogen substitution in FWXYZ liquid crystals

Nine out of fifteen FWXYZ analogues exhibited ferroelectricity, with the resulting phase, either nematic or smectic, dependent on the degree of hydrogen/fluorine substitution on the molecule’s tail-group. Phase sequences were determined using polarized optical microscopy and differential scanning calorimetry, revealing distinct textures corresponding to nematic, smectic A, nematic ferroelectric, smectic antiferroelectric, and smectic chiral polar phases.

Current reversal techniques confirmed the existence of ferroelectricity, with a single peak indicating a ferroelectric phase and a small pre-voltage peak observed in SmCP phases signifying the removal of molecular tilt. Increasing hydrogen/fluorine substitution at the head-group raised the apolar-polar transition temperature by approximately 35°C per additional fluorine atom, irrespective of the observed mesophase.

This increase correlates with stronger lateral core-core interactions and a more uniform partial dipole organisation along the molecular axis, promoting favourable electrostatic charge interactions. The tail-group’s degree of substitution had a different effect; smectic phases were observed when fewer than two fluorine atoms were present, with their thermal stability decreasing as substitution increased.

Beyond two fluorine atoms on the tail-group, nematic ferroelectric phases emerged, their stability increasing with substitution. X-ray scattering measurements confirmed increased lateral spacing with increased tail-group substitution, alongside stronger π-π interactions in materials exhibiting ferroelectric smectic phases, evidenced by a significant shoulder in the wide-angle scattering peak.

Molecular engineering unlocks ferroelectric order within fluid materials

Researchers have long sought to create functional materials that combine the fluidity of liquids with the order of crystalline solids, and recent work brings this goal closer to realisation. Achieving ferroelectricity, a spontaneous electric polarisation, in fluid systems proved elusive, as the inherent disorder of liquids typically opposes the alignment needed for this property.

A detailed investigation into a series of molecularly engineered fluids demonstrates a clear link between chemical structure and the emergence of ferroelectric behaviour. By systematically altering the composition of these molecules, scientists have successfully designed materials exhibiting both nematic and smectic ferroelectric phases, offering unprecedented control over polar order in fluid systems.

Previously, the relationship between molecular structure and the type of liquid crystal phase formed was often empirical, relying on trial and error. This study establishes design principles based on the way molecules pair and align, revealing that subtle changes in chemical composition can steer the system towards either layered (smectic) or more disordered (nematic) arrangements.

These arrangements dictate the type of ferroelectricity observed, offering a pathway to engineer fluids with tailored properties. The materials currently require relatively low temperatures to exhibit these behaviours, limiting immediate practical applications. Unlike conventional solid-state ferroelectrics, which operate at room temperature, these fluid systems are not yet there.

However, the predictive framework developed here could accelerate the discovery of more stable, room-temperature polar fluids. The ability to control polar order at the molecular level opens up possibilities for novel devices, including adaptable optics, sensors, and even new forms of data storage. Further research addressing the temperature limitations may lead to incorporation of these materials into applications demanding active control over electrical polarisation.