Chiral nematic liquid crystals are one-dimensional photonic bandgap materials whose reflection wavelength can be tuned by temperature, but only limited and irreversible tuning can be achieved by electric fields. Oblique heliconical chiral nematic materials blueshift under electric fields applied along the helix axis, whereas chiral ferroelectric nematic ( liquid crystals can be redshifted by fields applied perpendicular to the helix axis. Here we demonstrate that in liquid crystals, the reflection color can be reversibly tuned by electric fields applied along the helix axis. In sandwich cells assembled with bare conducting indium tin oxide (ITO) substrates, the reflectivity peak wavelength increases by up to 200 nm under fields up to 0.4 V/& micro;m. When the ITO substrates are treated with an electrically insulating polymer layer, the reflectivity shift is suppressed. We propose a theoretical model assuming helical deformation of the helix axis under an electric field. This model accounts for all experimental observations and yields an estimate of the splay elastic constant, which is challenging to determine by other methods. Our findings expand understanding of ferroelectric nematic liquid crystals and suggest potential applications in both tunable reflectors and energy-efficient smart windows.

Chiral nematic liquid crystals are one-dimensional photonic band-gap materials whose reflection wavelength can be well tuned by temperature, but only limited and irreversible tuning can be achieved by electric fields. In contrast, oblique heliconical chiral nematic materials blueshift with <1kV/mm fields applied along the helix axis, whereas chiral ferroelectric nematic liquid crystals can be redshifted by <0.1kV/mm fields applied perpendicular to the helix axis. Here we demonstrate that in ferroelectric nematic liquid crystals, the reflection color can be reversibly tuned also by electric fields applied along the helix axis. In sandwich cells assembled with bare conducting indium tin oxide (ITO) substrates, the reflectivity peak wavelength increases by up to 200 nm under fields up to 0.4 kV/mm. When the ITO substrates are treated with an electrically insulating polymer layer, the reflectivity shift is suppressed. We propose a theoretical model assuming helical deformation of the helix axis under electric field. This model accounts for all observations and also yields an estimate of the splay elastic constant which is challenging to determine by other methods. Our findings expand understanding of ferroelectric nematic liquid crystals and suggest potential applications in both tunable reflectors and energy-efficient smart windows.

The polarization and density modulation associated with antiferroelectric ordering is studied experimentally as a function of temperature in two ferroelectric nematic liquid crystals, the prototypical single compound (DIO) and a commercial mixture (FNLC919). The modulation wavenumber qA is determined by small angle X-ray diffraction from the weak smectic-like density wave (wavenumber qS = 2qA) that accompanies the polarization modulation. Results for qS and the saturated value of the polarization are analyzed in terms of Landau theory previously developed to describe the para-/antiferro-/feroelectric sequence of phase transitions in solid ferroelectrics. The analysis indicates that the polarization modulation is reasonably well approximated by a simple sinusoid in the antiferroelectric phase of DIO, whereas in FNLC919 the modulation develops a strongly soliton-like profile (with sharply decreasing wavenumber) close to the antiferro- to ferrolectric transition.

Among the recently developed ferroelectric nematic liquid crystals, FNLC-919, synthesized by Merck Electronics KGaA, stands out for its stable, room-temperature, ferroelectric nematic (NF) phase. This renders it a promising candidate for both fundamental research and device-level applications. In this study, we present a comprehensive experimental investigation of FNLC-919, focusing on its structural, optical, dielectric, and elastic properties in the paraelectric nematic (N) and the intermediate antiferroelectric phase (dubbed NX) that occur in a temperature range between the N and NF phases. Key material parameters such as ferroelectric polarization, viscosity, and nanostructure are characterized as functions of temperature in all mesophases, while the orientational elastic constants are determined only in the N and NX phases. Our findings are compared with prior results concerning the benchmark compound DIO that also exhibits the phase sequence N-NX-NF and reveals a smectic-like mass density wave coinciding with antiferroelectric ordering in the NX phase.

A dynamic light scattering study of director-layer fluctuations in the antiferroelectric smectic-ZA phase of the ferroelectric nematic liquid crystal DIO is reported. The dynamics are consistent with the distinctive feature of the ZA phase that the smectic layers form parallel to the axis of molecular orientational order (director). A model is developed to describe quantitatively the dispersion of the fluctuation relaxation rates. The model is based on a specialization of the elastic free energy density of the smectic-C phase to the case of 90 degree director tilt, a "first-order" approximation of the viscous stresses by their form for an incompressible uniaxial fluid, and a treatment of the effect of chevron layer structure that develops in planar sample cells due to temperature-dependent layer shrinkage, as documented in previous studies on DIO. From the modeling, the layer compression elastic constant is estimated to be 100 times lower in the smectic-ZA phase than in an ordinary smectic-A liquid crystal. Possible effects of the antiferroelectric layer polarization on the director splay elasticity and viscosity are described. The temperature dependencies of the splay, twist, and bend elastic constants and associated viscosities in the higher temperature nematic phase are also presented.

Nematic liquid crystals are anisotropic fluids which have long-range one-dimensional orientational order and short-range spatial correlations corresponding to the molecular length L. In X-ray studies this is manifested as diffuse peaks along the average direction of the molecular long axis at Q = 2π/L and weaker harmonics at 2Q and 3Q wave numbers. This is the case for the recently discovered ferroelectric nematic (NF) liquid crystals as well. Here we synthesized highly polar three ring rod-shaped compounds with a terminal thiophene ring which on cooling from the isotropic fluid directly transition to the NF phase that shows the strongest spatial correlations corresponding to 1/3 of the molecular length (L/3). The set of thiophene compounds reported here have ferroelectric polarizations about 20% larger than that of usual ferroelectric nematic liquid crystal materials. This is the result of the tighter molecular packing and larger mass density, due to the lack of flexible terminal chains of these thiophene compounds compared to most of the NF materials. Below the NF phase, compounds with a single nitro or two cyano polar groups on the terminal benzene ring exhibit a so far never observed smectic phase with periodicity ∼1/3 the molecular length. Based on our experimental results, we propose a model of this phase featuring antipolar packing of the molecules within the layers.

Low power consumption is critical for smart windows for temperature control and privacy. The recently discovered ferroelectric nematic (NF) liquid crystals exhibit strong coupling of the ferroelectric polarization with electric fields, making them promising candidates for energy-efficient electrochromic devices. Here we investigate the electrochromic properties of a room temperature chiral ferroelectric nematic () liquid crystal in films with in-plane electrodes, where the electric field is perpendicular to the helical axis. We demonstrate that smart windows based on this material can regulate interior temperatures within a 10 degrees C range using only 50 mW/m2 specific power, achieving 50% larger temperature modulation and 50-100 times lower power consumption than polymer dispersed and polymer stabilized liquid crystal windows. These findings suggest that chiral NF liquid crystals offer a highly efficient approach for smart window applications, potentially surpassing existing electrochromic technologies in energy efficiency and thermal regulation.

In-phase adenine-tracts (A-tracts) introduce intrinsic bending to DNA resulting in banana-shaped macromolecules. We investigate how such sequence-dependent bending influences DNA-based liquid crystal (LC) phases formed by gapped DNA (GDNA) constructs. By incorporating three in-phase A-tracts into duplex arms, we created GDNA constructs with bent duplexes and examined their LC phases using temperature-resolved synchrotron small-angle X-ray scattering and polarizing optical microscopy. The bent constructs exhibit a transition from a bilayer smectic-B phase to a monolayer smectic-A phase, although at ∼30 °C lower temperatures compared to GDNA with straight duplexes. At 25 °C, we estimate a bending angle of ∼11° per A-tract, which decreases with increasing DNA concentration and temperature. Moreover, we demonstrate that the stability of the LC phases increases up to ∼30 mM Mg2+ but decreases at higher concentrations. The lower stability of the LC phases of bent duplexes implies a reduced propensity for DNA condensation and heterochromatin formation.

We investigate the impact of poly adenine (poly-A) sequences on the type and stability of liquid crystalline (LC) phases formed by concentrated solutions of gapped DNA (two duplex arms bridged by a flexible single strand) using synchrotron small-angle X-ray scattering and polarizing optical microscopy. While samples with mixed sequence form layered (smectic) phases, poly-A samples demonstrate a columnar phase at lower temperatures (5-35 °C), not previously observed in GDNA samples, and a smectic-B phase of exceptional stability at higher temperatures (35-65 °C). We present a model that connects the formation of these LC phases with the unique characteristics of poly-A sequences, which manifest in various biological contexts, including DNA condensation and nucleosome formation.

A number of highly polar three ring rod-shaped compounds with a terminal thiophene ring have been synthesized and the physical properties of a subset are reported in detail. On cooling from the isotropic fluid, they directly transition to a ferroelectric nematic liquid crystal (NF) phase that shows the strongest spatial correlations corresponding to 1/3 of the molecular length (L/3). The set of thiophene compounds reported here have ferroelectric polarizations about 20 crystal materials. Such large polarization values are due to the 20 mass densities of these thiophene compounds compared to most of the NF materials with short terminal chains. These unusual properties are consequences of tighter molecular packing due to the lack of flexible terminal chains. Below the NF phase, compounds with a single nitro or two cyano polar groups on the terminal benzene ring exhibit a so far never observed smectic phase with periodicity 1/3 the molecular length. Based on our experimental results, we propose a model of this phase featuring antipolar packing of the molecules within the layers.

A dynamic light scattering study of director fluctuations in the antiferroelectric (AF) phase of the ferroelectric nematic liquid crystal DIO is reported. The AF phase occurs in a temperature range between nonpolar (paraelectric) and ferroelectric nematic states. The behavior of the observed fluctuation modes is consistent with the smectic-ZA model of the AF phase - namely, a smectic layer structure defined by fluid layers of alternating electric polarization, with the polarization field and nematic director both oriented parallel to the layer planes. This model has been advanced to explain a large body of previous experimental results on DIO. We discuss the wave vector dependence of the modes in terms of a dynamical theory based on the elastic free energy density of a smectic-C phase in the limit of 90° director tilt and an approximation of the viscous stresses by their form for an incompressible uniaxial fluid. We also present the pretransitional temperature dependencies of the splay, twist, and bend elastic constants and the corresponding viscosities in the paraelectric nematic phase.

Liquid crystalline (LC) phases formed by gapped DNA (GDNA) constructs, where two rigid duplexes are connected with a flexible single stranded linker, offer a versatile platform to investigate interactions between DNA molecules. Base pairs containing a locked nucleic acid (LNA-DNA or LNA-LNA pairs) are generally more stable compared to DNA-DNA pairs due to enhanced hydrogen bonding and/or attractive base stacking interactions. In concentrated solutions of GDNA constructs, the stability of terminal base pairs and the base stacking interactions between neighboring duplexes are critical for the formation of a bilayer smectic phase. By using temperature-resolved synchrotron small-angle X-ray scattering (SAXS) measurements, we quantified the impact of single LNA modification of terminal base pairs on the thermal stability of smectic LC phases. We observe that LNA-DNA terminal AT base pairing (A+T) increases the stability of the bilayer smectic phase by ∼9-18 °C relative to DNA-DNA pairing at the same duplex concentrations. While relatively large, this increase is still significantly less than the up to ∼30 °C increase observed when AT DNA-DNA pairs are replaced by GC pairs, suggesting the stacking interactions between A+T LNA-DNA base pairs are significantly weaker than those between unmodified GC base pairs. Our study illustrates the sensitivity of LC ordering in dense DNA solutions to a single nucleotide modification and demonstrates that LNA modifications can provide a new mechanism for tuning the stability of nucleic acid-based materials.

Manipulating light is an important area of optical research and development. To that end, tunable dichroic devices in which the reflectivity at differing wavelengths can be adjusted, are particularly valuable. This work is motivated by recent studies of the optical properties of chiral ferroelectric nematic liquid crystals (FNLCs). Here electro-optical studies are presented on two room temperature, FNLC materials that demonstrate electrically tunable reflectivity when subject to a field below 0.2 V mu m-1. Moreover, under appropriate conditions, the reflectivity can also be electrically (and reversibly) tuned (without change of color) from 0% to 40%. Reversible, low voltage tunable mirrors, having miniscule power consumption and operable around ambient temperature are expected to be useful in diverse applications ranging from energy-saving, smart windows to virtual reality interfaces. Electrically tunable reflection color is demonstrated on two room-temperature chiral ferroelectric nematic liquid crystals under in-plane field below 0.2 V mu m-1. The magnitude of the reflectivity can also be tuned without a change of color. Reversible, low voltage tunable mirrors are expected to be useful in diverse applications ranging from energy-saving, smart windows to virtual reality interfaces. image

The recently discovered ferroelectric nematic (N_{F}) liquid crystals (LC) have been reported to show an extraordinarily large value of the real part of the dielectric constant (ϵ^{'}>10^{3}) at low frequencies. However, it was argued by Clark et al. in Phys. Rev. Res. 6, 013195 (2024)PPRHAI2643-156410.1103/PhysRevResearch.6.013195 that what was measured was the capacitance of the insulating layer at LC or electrode surface and not that of the liquid crystal. Here we describe the results of dielectric spectroscopy measurements of an N_{F} material in cells with variable thickness of the insulating layers. Our measurements quantitatively verify the model by Clark et al. Additionally, our measurements in cells with bare conducting indium tin oxide surface provide a crude estimate of ϵ_{⊥}∼10^{2} in the N_{F} phase.

The first demonstration of converse piezoelectricity in 3D fluids is presented by measuring a linear electromechanical effect in ferroelectric nematic liquid crystals. The observed piezoelectric coupling constant below 6 kHz electric field is larger than 1 nC/N, comparable to, or better than, values for the strongest solid piezoelectric materials. Symmetry considerations indicate that the alignment of the ferroelectric nematic liquid crystal in the experimental study is not optimized, so the observed signal is likely only a fraction of the theoretically achievable signal. Understanding the electromechanical response of ferroelectric nematics will enable mechanical energy harvesting and open up a new avenue for developing fluid actuators, micro positioners, and electrically tunable optical lenses. The first observations of piezoelectricity is reported in a 3D fluid. For which, linear electromechanical effects, corresponding to conserve piezoelectricity, are observed and analyzed, in two room-temperature liquid ferroelectric nematic liquid crystals. The observed piezoelectric coupling constant is found to be comparable to or exceed that of the strongest solid piezoelectric materials.image

Most of the current highly polar rod-shaped molecules that form ferroelectric nematic (NF) phase do so only at elevated temperatures and multicomponent mixtures are generally needed to obtain a broad and room temperature range NF phase. In this work, we describe the synthesis, phase characterisation and measurement of various physical properties of a new ferroelectric nematic compound 4-[(4-nitrophenoxy)carbonyl]phenyl 2-isopropoxy-4-methoxybenzoate (RT11165). The molecular structure of RT11165 with a 2-isopropoxy group differs only by a substitution of the 2-methoxy group found in the prototype ferroelectric nematic material 4-[(4-nitrophenoxy)carbonyl]phenyl 2,4-dimethoxybenzoate (RM734). This small structure change produces a rather dramatic change in phase behaviour leading to an NF phase from 63 degrees C down to room temperature. Below about 45 degrees C the rotational viscosity of RT11165 increases critically and the temperature dependence indicates a glass transition at similar to 19 degrees C. The transparent and polar glassy state of RT11165, which should be also piezoelectric, is a good candidate for energy storage, piezoecatalysis, data storage and other applications. [GRAPHICS] .

The layered liquid crystalline phases formed by DNA molecules, which include rigid and flexible segments ("gapped DNA"), enable the study of both end-to-end stacking and side-to-side (helix-to-helix) lateral interactions, forming a model system to study such interactions at physiologically relevant DNA and ion concentrations. The observed layer structure exhibits long-range interlayer and in-layer positional correlations. In particular, the in-layer order has implications for DNA condensation, as it reflects whether these normally repulsive interactions become attractive under certain ionic conditions. Using synchrotron small-angle X-ray scattering measurements, we investigate the impact of divalent Mg2+ cations (in addition to a constant 150 mM Na+) on the stability of the inter- and in-layer DNA ordering as a function of temperature between 5 and 65 degrees C. DNA constructs with different terminal base pairings were created to mediate the strength of the attractive end-to-end stacking interactions between the blunt ends of the gapped DNA constructs. We demonstrate that the stabilities at a fixed DNA concentration of both interlayer and in-layer order are significantly enhanced even at a few mM Mg2+ concentration. The stabilities are even higher at 30 mM Mg2+; however, a marked decrease is observed at 100 mM Mg2+, suggesting a change in the nature of side-by-side interactions within this Mg2+ concentration range. We discuss the implications of these results in terms of counterion-mediated DNA-DNA attraction and DNA condensation.

Positionally ordered bilayer liquid crystalline nanostructures formed by gapped DNA (GDNA) constructs provide a practical window into DNA-DNA interactions at physiologically relevant DNA concentrations; concentrations several orders of magnitude greater than those in commonly used biophysical assays. The bilayer structure of these states of matter is stabilized by end-to-end base stacking interactions; moreover, such interactions also promote in-plane positional ordering of duplexes that are separated from each other by less than twice the duplex diameter. The end-to-end stacked, as well as in plane ordered duplexes exhibit distinct signatures when studied via small angle x-ray scattering (SAXS). This enables analysis of the thermal stability of both the end-to-end and side-by-side interactions. We performed synchrotron SAXS experiments over a temperature range of 5-65 °C on GDNA constructs that differ only by the terminal base-pairs at the blunt duplex ends, resulting in identical side-by-side interactions while end-to-end base stacking interactions are varied. Our key finding is that bilayers formed by constructs with GC termination transition into the monolayer state at temperatures as much as 30 °C higher than for those with AT termination, while mixed (AT/GC) terminations have intermediate stability. By modeling the bilayer melting in terms of a temperature-dependent reduction in the average fraction of end-to-end paired duplexes, we estimate the stacking free energies in DNA solutions of physiologically relevant concentrations. The free-energies thereby determined are generally smaller than those reported in single molecule studies, which might reflect the elevated DNA concentrations in our studies.

The layered liquid crystalline (LC) phases formed by DNA molecules which include rigid and flexible segments (‘gapped DNA’) enable the study of both end-to-end stacking and side-to-side lateral interactions that drive the condensation of DNA molecules. The resulting layer structure exhibits long-range inter-layer and intra-layer positional correlations. Using synchrotron small-angle x-ray scattering (SAXS) measurements, we investigate the impact of divalent Mg 2+ cations on the stability of the inter- and intra-layer DNA ordering as a function of temperature between 5-65 °C and for different terminal base pairings at the blunt ends of the gapped DNA constructs, which mediate the strength of the attractive end-to-end interaction. We demonstrate that the stabilities at a fixed DNA concentration of both inter-layer and intra-layer order are significantly enhanced even at a few mM Mg 2+ concentration. The stability continues to increase up to ∼30 mM Mg 2+ concentration, but at higher (∼100 mM) Mg 2+ content repulsion between positive ions counteracts and reverses the increase. On the other hand, sufficiently strong base-stacking interactions promote intra-layer order even in the absence of multivalent cations, which demonstrates the impact of liquid crystal layering on the DNA condensation process. We discuss the implications of these results in terms cation-mediated DNA-DNA attraction.

The occurrence of a smectic-B (Sm-B) phase is demonstrated in concentrated aqueous solutions of "gapped" DNA constructs consisting of fully paired duplexes bridged by a flexible, unpaired strand of nucleotides. The Sm-B phase, identified by small and wide angle x-ray scattering measurements and optical microscopy, develops from a smectic-A (Sm-A) phase with increasing DNA concentration at room temperature. It transitions (reversibly) to the Sm-A when the temperature is raised above similar to 50 degrees C.