Fluorinated liquid crystals and their mixtures giving polar phases with enhanced low-temperature stability

ABSTRACT The fluid ferroelectrics, called ferroelectric nematics (NF), have recently become available by incorporating strong polarity into rod-shaped liquid crystal molecules. Its unprecedented electro-optic properties have created significant excitement in soft matter research. The further progression from the NF phase to the antiferroelectric smectic Z (SmZA) phase, and ultimately to the ferroelectric Smectic A (SmAF) phase, represents a remarkable journey in emerging polar liquid crystal states. Nevertheless, the limitation of NF liquid crystal materials remains one of the prominent obstacles to physical property optimization and optoelectronic device development. In this work, we synthesized a series of fluorinated liquid crystal molecules with large dipole moments and systematically investigated their phase behavior. We designed them with a similar fluorinated aromatic skeleton and varied the structures of the terminal group and bridging bond. We found that the dipole moment density and shape anisotropy significantly affect the phase behavior. Notably, diverse polar liquid crystal phases, including NF, SmZA, and SmAF were observed. Through a multi-component mixing strategy, we successfully achieved a much-expanded temperature window and improved low-temperature stability not only in the NF phase but also in the SmZA and SmAF phases. GRAPHICAL ABSTRACT


Introduction
Liquid crystals (LCs) have extensive applications in modern displays [1][2][3][4], smart windows [5][6][7], and numerous other tunable devices, such as beam steering devices [8,9], tunable waveguides [10], optical fibers [11], and spatial light modulators [12,13].The LC state comprises diverse phases, each exhibiting distinct physical properties.The nematic phase, with the long-range orientational order but no translational order, is the least ordered yet most widely exploited in industry.On the other hand, chiral molecules are known to form the cholesteric phase and blue phases (BP) characterized by a lattice of line defects.It has not been until the last decade that new nematic phases were added to this exclusive list.In 2011, the twist-bend nematic phase (NTB) was discovered in achiral flexible dimer molecules [14].Most recently, in 2017, Nishikawa et al. [15] and Mandle et al. [16] independently reported rodshaped LC molecules with high polarity, where an CONTACT Mingjun Huang huangmj25@scut.edu.cnSupplemental data for this article can be accessed online at https://doi.org/10.1080/02678292.2024.2306309.
unknown nematic phase appeared.This unusual nematic phase was later proved to be the ferroelectric nematic (N F ) by Chen et al. [17].The equal distribution of two permissible director orientations, i.e. n and -n, is broken, producing spontaneous polarization.The symmetry-breaking and ferroelectricity are attributed to the strong dipolar interaction among LC mesogens [18,19].
There already exist some categories of LC materials possessing ferroelectricity.The first ferroelectric LC was discovered in the chiral smectic C (SmC*) phase by Meyer et al. [20].SmC* phase has been extensively studied and even integrated into LC display applications [20][21][22][23].However, the unavoidable defect generation in the devices originating from the layering has been one of the main technical difficulties.Ferroelectricity also exists in some of the phases of bent-core mesogens [24,25], but the relatively high phase transition temperature and uniform alignment on the substrate of the LC cell have limited their application in optoelectronic devices.The N F phase with characteristic nematic feature provides a new opportunity for LC technology innovation [26].The spontaneous polarization in the N F state is comparable to that of some solid ferroelectric materials (e.g.BaTiO 3 ) [19,27].Following the discovery and validation of the N F phase, other polar LC states were subsequently reported.The helielectric nematic (HN*) phase carries a helically rotating polar vector and exhibits the combined optical properties of the chiral nematic phase and dielectric properties of the N F phase [28].The smectic Z A (SmZ A ) phase, identified by Chen et al., resides as an intermediate phase between the N and N F phases [29].It is antiferroelectric, with the nematic director and polarization oriented parallel to the smectic layer planes and the polarization alternating in sign from layer to layer.More recently, a new ferroelectric smectic A (SmA F ) was discovered, where the polarization is longitudinally coupled to the smectic quasilayer order [30].SmA F exhibits a lamellar structure with a higher molecular packing density, thus offering exceptional capability for polarization field switching and memory [31,32].
Previously, the LC species exhibiting the N F phase were primarily RM734 [16], DIO [15,33,34] and UUQU [35] molecules and their structural variants [36].Li et al. have conducted a systematic investigation into the general molecular design principles and phase transitions of N F LC [18,19].An increasing number of LC molecular structures have been developed recently exhibiting the N F phase [37][38][39][40][41][42][43][44].Nevertheless, the limitations of N F LC materials in chemical structure diversity and lowtemperature stability remain prominent obstacles to fundamental physics research and optoelectronic device development.On the one hand, further specific investigation for clarifying the relationship between molecular structures and the behavior of polar LC phases is needed [45,46].This would benefit the LC industry by expanding the diversity of N F materials and optimizing their physical properties.On the other hand, increasing the low-temperature stability is a critical challenge for the polar LC materials.Despite some recent works made through multi-component mixing to improve the lowtemperature stability and expand the temperature range [31,47,48], particularly for the N F LC, the newly discovered SmZ A and SmA F phases have received less exploration.These two phases usually exist at relatively high temperatures or within narrow temperature windows, hindering in-depth studies of their physical properties and material performance.
In this study, we synthesized a series of fluorinated LC molecules (Figure 1) with large molecular dipole moments and systematically investigated their phase behavior.These molecules possess a similar molecular skeleton consisting of fluorinated aromatic rings and a nitrile head group.The molecular structures vary in two aspects: the terminal groups (n-butyl, 4-propylcyclohexan, or 5-propyl-1,3-dioxan groups); the bridging group (ester or difluoromethylether) and its position on LC mesogens.We found that dipole moment density and shape anisotropy significantly affect the phase behavior, such as phase type and phase transition temperatures.Notably, the recently reported polar LC phases including N F , SmZ A , and SmA F were all observed.Furthermore, we employed the multi-component mixing strategy among these synthesized molecules to fine-tune the phase stability and temperature range of various polar LC phases.As a result, we have not only successfully expanded the temperature window of the N F phase at some optimized mixing formulations, but also improved the low-temperature stability of the SmZ A and SmA F phases.

Results and discussion
Based on a similar molecular skeleton composed of fluorinated aromatic rings and a nitrile head group, we synthesized 12 compounds and categorized them into three series according to their terminal group types (Figure 1(a)): nBu series (n-butyl in samples nBu1-nBu4), DIO series (5-propyl-1,3-dioxan in samples DIO1-DIO4) and CyH3 series (4-propyl-cyclohexan in samples CyH1-CyH4).The nBu4 and DIO1 compounds have been studied and reported in other references [32,35].Further details regarding the synthesis can be found in supporting information (SI).
The phase transition behavior of the synthesized compounds was comprehensively determined upon cooling by utilizing polarized light microscopy (PLM) (   upon crystallization.The ultrahigh dielectric constant was interpreted using polarization-capacitance Goldstone (PCG) mode by Clark et al. [49].It is proposed that polarization reorientation induced by the external field causes the N F layer to function as a lowvalue resistor that enables the charging of the interfacial capacitors, lowering the restoring force of the PCG mode and resulting in a robust reactive dielectric behavior.Furthermore, the hysteresis loop (P-E) measurements were performed to study the polarization switching of DIO3 material by using a triangular wave voltage.Figure 3(c) depicts the P-E curves at different temperatures at 390 Hz.In the Iso and N phases, approximately linear paraelectric behavior was observed, whereas the SmZ A and N F phases exhibit nonlinear hysteresis loops.For the SmZ A phase, double hysteresis loops were observed (Figure 3(d)), indicating the antiferroelectric response as reported by Chen et al. [29].In the N F phase, parallelogram-shaped loops were observed between 60-110°C, indicating its ferroelectric nature (Figure 3(e)).
The phase behavior of these samples is summarized in Figure 1(b) and Table S1.Despite sharing a similar molecular skeleton, they exhibit distinct phase transition behavior.To gain deeper insights into the underlying factors governing the phase behavior and the emergence of various polar LC phases, we calculated the essential molecular parameters using density functional theory (DFT) with B3LYP/6-311+G(d,p) basic function, including dihedral angle (φ) between the benzylic hydrogen atom or the homobenzylic carbon atom and the adjacent benzene ring, the dipole moments of the whole molecule (μ), the angle between the dipole and long axis of the molecule (β), the length along the long axis of the molecule (a), dipole density (μ 2 /m, where m represents molecular mass), and the parameter of shape anisotropy (a 3/2 /m 1/2 ).For detailed information on these parameters, please refer to Table 1 and Figure S17.
In our previous work [18], we demonstrated that the molecular shape anisotropy (represented by a 3/2 /m 1/2 ) and the molecular dipole density (represented by μ 2 /m) play crucial roles in dictating the formation of nonpolar N phase and polar N F phase.By evaluating these molecular parameters, the phase behavior exhibited by the synthesized molecules in this work closely aligns with the reported quadrant diagram, which is governed by the dimensions of dipolar strength and shape anisotropy.The molecules within Series DIO, characterized by the highest dipole density (μ 2 /m > 0.33 D 2 mol/g) and suitable shape anisotropy (4.99-5.5 Å 3/2 mol 1/2 g −1/2 ), follow the same phase transition sequence (Iso-N-SmZ A -N F ) with nBu1 and nBu2 in Series nBu.The direct crystallization of nBu3 without any LC phase appearance can be attributed to its small shape anisotropy and dipole density.Despite having a similarly small shape anisotropy, nBu4 exhibits a N F phase transition from the isotropic phase at a relatively low temperature (below 10°C) due to its higher dipole density.Notably, DIO1 and DIO2 molecules undergo a phase transition from N F to SmA F phase.Our previous study has proved that the SmA F phase exhibits a lamellar structure with higher molecular packing density and enhanced orientational order compared to the N F phase [32].We assume that the stronger dipolar interaction strength (μ 2 /m ~0.40 D 2 mol/g) and high shape anisotropy (a 3/ 2 /m 1/2 ~ 5.5 Å 3/2 mol 1/2 g −1/2 ) in DIO1 and DIO2 facilitate the denser packing of rigid rod-shaped LC mesogens in the SmA F phase.In strong contrast, molecules in the CyH series favor the dominance of the N phase.This is certainly related to their relatively low dipole density (μ 2 /m < 0.32 D 2 mol/g) coupled with high shape anisotropy (a 3/2 /m 1/2 >5.47 Å 3/2 mol 1/2 g −1/2 ).After checking the DFT calculation results (Table 1 and Figure S17), we found that molecular conformations in the CyH series may also hinder the formation of the N F phase.The dihedral angles φ between the benzylic hydrogen atom (acetal position in DIO series and benzyl position in CyH series) or the homebenzylic carbon atom (homebenzylic position in nBu series) and the adjacent benzene ring were distinct [37], being ~ 88º in DIO series and ~ 180º in CyH series.This means that the dioxane unit is nearly in the same plane with the adjacent benzene ring; however, the cyclohexane unit is perpendicular to the adjacent benzene ring (Figure S17).This perpendicular orientation of the bulky cyclohexane group in the CyH series disfavors dense packing of the LC mesogens and suppresses the N F phase formation.
The presence of strong dipolar interaction among LC molecules is crucial for stabilizing the N F phase [16,18,19,37,50,51].Consequently, the molecules in the DIO series, which have a higher dipole density, present a stable N F phase at higher temperatures and over broader temperature ranges compared to the nBu series.In addition, the transition from ester to difluoromethylether as the bridging group type (e.g.DIO1 to DIO3 and DIO2 to DIO4) yields a significant reduction in molecular phase transition temperature from isotropic to LC phases.This shift is believed to be closely related to the decrease of dipole density and shape anisotropy.Intriguingly, when the position of the bridging groups is changed, molecules within the nBu and CyH series connected via bridging groups located behind the biphenyl experience a decrease in phase transition temperature correlated with the rise in dipole density.However, molecules in the DIO group exhibit an opposite trend in terms of change of phase transition temperature.It is important to emphasize that the introduction of an ester bond directly connected to the aromatic ring with strong electron-withdrawing substitutions, namely two fluorine atoms and one nitrile group, would weaken the thermal stability of the LC molecules.For instance, our observations of DIO2 and CyH2 molecules through PLM unveiled the appearance of a dark texture at high temperatures (Figure S18).Further analysis through nuclear magnetic resonance (NMR) on heated samples revealed new peaks, indicating the thermal decomposition of the ester bonds at high temperatures.
Previous research conducted by Chen et al. [48] showcased significant progress in achieving a stable wideinterval N F phase by blending DIO and RM734.Later on, they successfully realized the stabilization of SmZ A and SmA F phases in a relatively low-temperature range through binary blending of DIO with AUUQU2N and AUUQU7N [31].Thus, the use of a multicomponent mixing strategy has proven effective in tailoring the phase stability and temperature range of polar LC phases.In this study, the inherent structural similarity and interplay of substantial dipole moments lay the foundation for the ideal blending approach.Specifically, compounds featuring ester bridging groups were chosen for binary blending.The phase diagrams depicting the binary blends of different proportions upon slow cooling, as determined using PLM, are shown in Figure 4.
Figure 4(a) presents the behavior of the binary mixture of nBu1/nBu2, which illustrates that the Iso, N, SmZ A , and N F phases appear in a continuous fashion throughout the phase diagram, with the phase transition behavior remaining essentially unchanged.This is usual that the same phases mix additively in mixtures as observed in both Figure 4(a,b).This remarkable degree of ideal mixing is attributed to the closely aligned chemical structures, dipole densities, and shape anisotropy of the molecules.
Figure 4(c) presents the behavior of the binary mixture of DIO1/CyH1, with their partial PLM textures shown in Figure S19.When the doping content of CyH1 remains below 50%, the mixture exhibits a phase sequence following pathway I upon cooling: N SmZ A -N F -SmA F -Cry.The LC phase window (N F and SmA F ) gradually narrows as the doping ratio of molecule CyH1 increases, resulting in the replacement of the N F and SmA F phases by the N phase and SmZ A phase.At a balanced DIO1/CyH1 ratio of 50/50, both the N F and SmA F phases vanish, giving rise to a widewindow (~ 100°C) SmZ A phase that remains stable even at room temperature.As the amount of CyH1 is further increased, the SmZ A -Cry transition progressively shifts towards higher temperatures while the N-SmZ A transition shifts towards lower temperatures, leading to a continuous narrowing of the SmZ A interval.Notably, a phase sequence at a fixed temperature following pathway II was identified with increased doping of CyH1, i.e.SmA F -N F -SmZ A -N.As shown in Table 1, DIO1 possesses high dipole density and good shape anisotropy, while CyH1 possesses low dipole density but rather a high shape anisotropy.The doping of CyH1 into DIO1 gradually reduced the dipolar interaction strength of the system while retaining the high shape anisotropy.The phase evolution along pathway II reveals that SmA F prefers the stronger intermolecular interaction and  The symbols φ, µ, β, a, µ 2 /m and a 3/2 /m 1/2 represent the dihedral angle between the benzylic hydrogen atom or the homobenzylic carbon atom and the adjacent benzene ring, the vector sum of the dipole moments of the whole molecule, the angle between µ and the long axis of the molecule, the length along the long axis of the molecule, the volume density of dipole interaction strength and a form of the parameter of shape anisotropy, respectively.Among them, m refers to molecular weight.respectively.This could be explained by that CyH1 and CyH2 are suitable candidates for weakening the dipolar interaction strength and increasing the shape anisotropy of molecules in Series nBu and DIO.We could also conclude that an intermediate dipolar interaction strength combined with high shape anisotropy facilitates the SmZ A phase formation.Molecules nBu1 and nBu2 display diverse LC phase transitions (Iso-N-SmZ A -N F ) within a relatively narrow temperature range (~ 30°C).Their inherently low phase transition temperatures render them favorable candidates for blending.In the nBu1/DIO1 mixture (Figure 4(e)), all LC phase transition temperatures in the system decrease simultaneously as the proportion of nBu1 doping increases.This occurrence is accompanied by the maintenance of a stable phase transition window across a wide region when the nBu1 doping is less than 60%. Figure 4(f) shows the results of DIO2/nBu2 mixture, demonstrating a broad range of N F phase across all ratios.Particularly noteworthy is the occurrence of low crystallization temperatures (23.4°C), with the temperature window of N F phase ~ 120°C.
The SmA F phase boasts notable attributes, including fluidity, elevated structure order, and memory capability, rendering it highly promising for optoelectronic device innovations.Through straightforward binary mixing, such as the blending of DIO1 and DIO2 molecules (Figure 4(a)), the SmA F phase with a wider window is obtained.In addition, nBu2 is believed to significantly reduce the phase transition temperature based on the DIO2/nBu2 twocomponent mixture.To further enhance the stability of the SmA F phase at room temperature and even lower temperatures, a strategic blending approach involving nBu2, DIO1 and DIO2 was conducted (Figures 5 and S28-30).Comprehensive insights into the phase transition behavior are documented in Table S10.Importantly, all samples derived from these mixtures exhibit a fluid SmA F state that proves remarkably stable, with no crystallization observed over extended periods.Upon examining these ternary mixtures, a clear trend emerges: the transition of N F -SmA F shifts towards lower temperatures as the DIO1 content decreases.For a ternary mixture with a ratio of nBu2/DIO1/DIO2 = 30/30/40, the SmA F phase was observed by PLM at around 70°C upon cooling and remained stable at 27°C (Figure 5(a-f)).The corresponding P-E hysteresis loop was recorded at 35°C (Figure 5(g)), consistent with the polarization shape recorded in our previous work [40].As revealed by the DSC profile in Figure S24, a SmA F possessing stability at low temperatures and a wide temperature window (>50°C) was obtained using ternary mixing.The span of the temperature window can be adjusted by changing the blend ratio.

Conclusion
To enrich the N F LC materials and elucidate their phase behavior dependence on chemical structures, we synthesized three series of LC molecules using a similar fluorinated aromatic molecular skeleton.The key molecular parameters were finely tuned via changing the structures of the terminal group and bridging bond.The dipole density and shape anisotropy parameters were identified to be the crucial factors dictating the phase behavior.Besides, the introduction of difluoromethylether bonds and alkyl chain terminal groups emerges as an effective strategy for lowering the phase transition temperatures.These synthesized LC molecules with similar chemical structures provide fertile ground for blending to adjust the phase behavior.Eight groups of binary mixtures and three proportions of ternary mixtures were explored.Particularly, with the continuous weakening of the dipolar interaction strength but retaining high shape anisotropy, SmA F , N F , SmZ A , and N phases would become thermodynamically stable sequentially.The SmA F phase prefers the stronger intermolecular interaction and thus denser packing density among LC molecules, while an intermediate dipolar interaction strength combined with high shape anisotropy facilitates the SmZ A phase formation.Moreover, we obtained optimized mixing formulations for all the N F , SmZ A , and SmA F polar phases which are stable down to 30°C, and realized the controllable customization of LC phase transformation.This work would benefit LC material research by expanding the diversity of polar LC materials and optimizing their physical properties.

Disclosure statement
No potential conflict of interest was reported by the author(s).
Figure 2 illustrates a representative texture evolution of the compound DIO3, demonstrating a phase transition pathway of Iso-N-SmZ A -N F -Cry.This phase transition process was also captured by DSC, showing similar transition temperatures during the cooling scan (Figure 3(a)).In the N F state, a characteristic band texture was observed, where the ferroelectric domains with reversed polarity are separated by disclination lines (Figure 2(e)).The dynamic polarity in the N F phase was probed by the dielectric measurement.DIO3 exhibits a sudden increase of dielectric constant when entering the N F state (Figure 3(b)), followed by a dramatic decrease

Figure 1 .
Figure 1.(Colour online) (a) Chemical structures of 12 synthesized molecules, which are classified into three series based on the characteristics of the terminal groups.(b) Phase behavior of the synthesized samples.The phase transition temperatures were obtained by DSC and PLM.The Iso phases in DIO2 and CyH2 were not reached due to severe decomposition upon heating beyond 200°C.

Figure 3 .
Figure 3. (Colour online) (a) DSC profiles (top: first cooling; bottom: second heating) of samples DIO3.The scanning rate is 10 K/min.(b) Temperature dependence of real part of ε´ (effective) at different frequencies.Voltage: 50 mV.Indium tin oxide (ITO) cell thickness: 5 μm.(c) P-E hysteresis loops of DIO3 at different temperatures.The applied voltage: V P = 100 V, f = 390 Hz. (d), (e) P−E hysteresis loop of DIO3 at 110°C for SmZ A and 90°C for N F phase corresponding to (c).
thus denser packing density among LC molecules.With the weakening of the dipolar interaction strength, N F , SmZ A , and N phases would become thermodynamically stable, consistent with the phase behavior along pathway I.In summary, N F and SmA F phases that are formed only by one component of binary mixture are destabilized in a mixture in DIO1/CyH1, while the antiferroelectric SmZ A expands first and then shrinks.We can see that all four diagrams (Figure4(c,d,g,h)) look similar, where the ferroelectric phases are destabilized and the N phases expand with the addition of CyH series molecules, since pure CyH series form only non-polar N phase.Interestingly, although the SmZ A phase is not observed in CyH1 or CyH2 molecules, a stable SmZ A phase can be easily obtained when they are mixed with other series of molecules (shown in Figs.4(c,d,g,h)).The phase transition temperatures from the SmZ A phase to the crystal phase are recorded as 26.7°C and 26.4°C for blends nBu1/CyH1 = 50/50 and nBu2/CyH2 = 50/50,

Figure 5 .
Figure 5. (Colour online) (a-f) The representative PLM texture evolution of ternary mixture nBu2/DIO2/DIO1 (the corresponding mass fraction ratio is 30:30:40) in the cooling process.The textures were observed in a syn-parallel rubbed cell under the crossed polarizers with a cell thickness of 5 μm.(g) P-E hysteresis loop at 35°C.V p = 100 V, f = 150 Hz.Scale bar: 100 μm.
This work is supported by National Key Research and Development Program of China [No.2022YFA1405000], the National Natural Science Foundation of China [NSFC No. 52273292], the Recruitment Program of Guangdong [No.2016ZT06C322], the International Science and Technology Cooperation Program of Guangdong province [No.2022A0505050006] and the Fundamental Research Funds for the Central University [No.2022ZYGXZR001].

Table 1 .
Molecular parameters of synthesized compounds calculated by DFT.