Controlling the formation of heliconical smectic phases by molecular design of achiral bent-core molecules

Marco Poppe a, Mohamed Alaasar ab, Anne Lehmann a, Silvio Poppe a, Maria-Gabriela Tamba c, Marharyta Kurachkina c, Alexey Eremin c, Mamatha Nagaraj d, Jagdish K. Vij d, Xiaoqian Cai e, Feng Liu e and Carsten Tschierske *a
aDepartment of Chemistry, Martin-Luther University Halle-Wittenberg, Kurt Mothes Str. 2, 06120 Halle (Saale), Germany. E-mail: carsten.tschierske@chemie.uni-halle.de
bDepartment of Chemistry, Cairo University, Giza, Egypt
cDepartment of Nonlinear Phenomena, Institute for Physics Otto von Guericke University Magdeburg, Magdeburg, Germany
dDepartment of Electronic and Electrical Engineering, Trinity College, Dublin, The University of Dublin, Dublin 2, Ireland
eState Key Laboratory for Mechanical Behaviour of Materials, Shaanxi International Research Center for Soft Matter, Xi’an Jiaotong University, Xi’an 710049, P. R. China

Received 25th November 2019 , Accepted 12th January 2020

First published on 13th January 2020


Fluids with spontaneous helical structures formed by achiral low molecular mass molecules is a newly emerging field with great application potential. Here, we explore the chemical mechanisms of the helix formation by systematically modifying the structure of a bent 4-cyanoresorcinol unit functionalized with two different phenyl benzoate based aromatic rods and terminated with two alkyl chains of variable length. The majority of these achiral compounds self-assemble, forming a short-pitch heliconical liquid crystalline phase in broad temperature ranges. In some cases, it occurs without any competing low-temperature phase. We demonstrate that the mirror symmetry broken mesophase occurs at the paraelectric–(anti)ferroelectric transition if the tilt angle of the molecules in the smectic layers is around 18–20° and if this transition coincides with a change of the tilt correlation between the layers. In the close vicinity of this transition, a field-induced heliconical phase develops as well as a new heliconical phase with polarization-randomized structure. These investigations provide a blueprint for the future design of achiral molecules capable of spontaneous mirror symmetry breaking by the formation of heliconical liquid crystalline phases.


1. Introduction

Formation of chiral superstructures from achiral molecules by spontaneous mirror symmetry breaking is a contemporary research field1–3 with great impact. It provides technologically important chiral superstructures and materials for application in chiral photonics, for circular polarized emission, optoelectronic devices and many other fields,4 thus circumventing the often expensive chiral-pool approach in materials synthesis. It also contributes to the understanding of the spontaneous chiral self-assembly, being one of the prerequisites for the emergence of life.5 Especially the investigation of spontaneous mirror symmetry breaking phenomena in fluids and soft matter6–10 and particularly in achiral isotropic liquids7,10 is a newly emerging field of scientific interest. The spontaneous formation of helical superstructures in bicontinuous cubic phases of polycatenar molecules,8,10,11 the twist-bend nematic phases (NTB) of dimesogens and oligomesogens,12–14 as well as the so-called “dark conglomerate” phases of bent-core mesogens15–20 are just a few examples.

An important feature of bent-core liquid crystals with a bending angle of around 120° (BCLCs) is the formation of lamellar (smectic) phases with long range polar order due to the restriction of the molecular rotation around their long axes (Fig. 1a), leading to ferroelectric and antiferroelectric polar smectic phases.15 Typically, the molecules in the polar smectic phases of BCLCs are tilted (SmC phases) whereas orthogonal smectic phases (SmA phases) are rare.15,21,22 Four different sub-structures result in these polar SmC phases, which can be divided based on the polar direction in adjacent layers into synpolar (“ferroelectric”, PF) and antipolar (“antiferroelectric”, PA) and based on the tilt correlation into synclinic (Cs) and anticlinic (Ca, see Fig. 1a).9 The combination of polar direction and tilt direction reduces the symmetry of the layers to C2v which is chiral (see Scheme S3 for explanation, ESI). Accordingly, the four structures can be divided into racemic, with alternating chirality of the layers, and chiral with identical layer chirality (Fig. 1b).9 A special application-relevant feature of these liquid crystalline (LC) systems is the possibility to switch the polar direction of the layers, the optical axis of the phase and the superstructural layer chirality by external electric fields. Two fundamental switching mechanisms can be distinguished, the bistable (ferroelectric) switching between two synpolar states and the tristable antiferroelectric switching with intermediate relaxation to an antipolar ground state. These switching processes can take place either by precession of the molecules on a cone which changes the orientation of the optical axis and retains the layer chirality, or by rotation around the molecular long axis which retains the orientation of the optical axis and reverses the layer chirality (Fig. 1e).


image file: c9tc06456g-f1.tif
Fig. 1 Structures, chirality, helix formation and switching in the LC phases of bent-core molecules. (a and b) The four structures of polar and tilted smectic phases. (a) Side views, showing synpolar (“ferroelectric” = PF) and antipolar (PA) configurations in adjacent layers, and (b) front views showing the tilt correlation of the bent-core molecules in adjacent layers (polar direction is indicated by dots and crosses) and the superstructural chirality of the layers (blue/red color indicates opposite chirality sense; for an explanation of the origin of layer chirality, see Scheme S3).9 Ca and Cs refer to the anticlinic and synclinic tilt in adjacent layers; reproduced from ref. 83 with permission from The Royal Society of Chemistry; (c and d) the heliconical phase. (c) Model (d = layer distance and P = helical pitch) of the incommensurate Sm(CP)hel phase; (d) like combinations of layer chirality and helix sense of the heliconical structure, tentatively assumed to be energetically favoured; double-headed arrows are used to indicate that the real existing helically twisted structure with incommensurate 3-layer pitch is in between the shown SmCsPF and SmCaPA states. (e) The two switching modes, the precession on a cone, flipping the optical axis (rotation of the dark extinction crosses in planar textures) and retaining the chirality of the layer, and the rotation around the molecular long axis, inverting the chirality sense (blue ↔ red) and retaining the optical axis; shown for the example of the ferroelectric switching between SmCsPF state.

Another source of chirality arises from the tendency of the BCLCs to escape from the developing macroscopic polarization by a splay or twist of the polar directions in the layers. If the helical twist develops preferentially parallel to the layer planes it leads to a layer deformation. The examples are helical nano-filament phases (HNF, previously known as B4 phases)23–27 and sponge-like phases (dark conglomerate phases, DC).6,15,16,28–31 However, there is also the possibility of a helical twist developing parallel to the layer normal, not requiring steric layer frustration and leading to heliconical LC phases (Fig. 1c and d),32 which are the subject of this contribution. Heliconical phases were previously known for smectic phases of permanently chiral molecules,33–36 but here we report on related phases formed by non-chiral molecules.

The investigated compounds, the 4-cyanoresorcinol bisbenzoates B–E, shown in Scheme 1, having a bending angle α of about 140°,37–39 are less bent than ordinary BCLCs with more acute bending angles around 120°. Therefore, the rotation around the molecular long axis is easier and this reduces the coherence length of polar order, leading to paraelectric and superparaelectric phases with only short to medium-range polar order in ferroelectric grains instead of polar layers.40,41 These polarization randomized smectic phases can be divided into tilted (SmCsPR, SmCsPAR),42–44 and non-tilted (SmAPR,40,41 SmAPAR41,45) subtypes showing either ferroelectric-like (PR) or antiferroelectric like (PAR) switching. As the packing density increases with lowering temperature a transition from these paraelectric to the long range polar smectic phases can occasionally be observed for this kind of molecules.40,46 4-Cyanoresorcinol-based bent-core molecules,21,37,39,42–50 especially compounds A with two identical phenyl terephthalate wings (see Scheme 1)51–54 are known to provide a variety of these intermediate phases. Recently, a new short pitch heliconical LC phase was discovered in the series An.32,55–59 This heliconical phase with incommensurate three layer pitch in the 15 nm range (Fig. 1c) was identified and confirmed by atomic force microscopy (AFM)32 and by the observation of field induced helix deformation (HDF effect) for compounds A16 and A18 (n = 16, 18),32,59 and by soft resonant X-ray scattering (RSoXS) at the carbon K-edge for compound A14 (n = 14).60 It was designated as SmCPα51,61 or Sm(CP)α,60 due to its similarity with the heliconical SmCα* phases formed by chiral rod-like molecules34 and as SmCsPFhel, because it represents a helical stacking of polar SmCsPF layers.32 Herein we use Sm(CP)hel as a general descriptor for this kind of short pitch heliconical phases, irrespectively of their precise structure (commensurate or incommensurate; pitch length).62 It was also shown that this heliconical phase and its properties are affected by diastereomeric relations between layer chirality and helix sense, thus allowing only the chiral SmCaPA and SmCsPF phases to become heliconical.62 In this heliconical phase the twist of the projections of the tilt direction on the layer planes is around 120° between adjacent layers, meaning that this heliconical phase with almost three layer pitch represents an intermediate structure between SmCaPA (180° twist) and SmCsPF (0° twist), as shown in Fig. 1d. The heliconical phases can be formed either spontaneously from the SmCaPA ground state structure or from a field-induced SmCsPF structure.32,62 To date polar heliconical phases were exclusively observed in the series of the symmetrical alkyl substituted bisterephthalates of 4-cyanoresorcinol A (Scheme 1)32,53–62 where they occur for a chain length range of n = 12–18 in small temperature ranges. It is noted that also apolar versions of heliconical phase (designated as SmCTB) have recently been reported for mesogenic dimers with odd-numbered spacers.63


image file: c9tc06456g-s1.tif
Scheme 1 Chemical structures of the 4-cyanoresorcinol bisterephthalates A62 and the compounds under investigation herein; compounds B and C differ from A in the direction of one peripheral COO group at the side pointing away from the CN group; the compounds of series D are related to B and C, but have one or both alkyl chains replaced by alkoxy chains; in the series E the rod-like wings are exchanged compared to B–D; α is the bending angle, red indicates the more and blue the less electron deficit benzene ring.

The main goal of this contribution is to generalize this concept of emerging chirality and to establish general rules for the design of compounds forming broad ranges of heliconical LC phases. A general understanding of the formation of heliconical phases depending on molecular structure, tilt-correlation and polar order is established, which provides a blueprint for future molecular design. In addition to the polar Sm(CP)hel phase, new polarization randomized versions of the heliconical phases were observed. The polar heliconical phases allow ultrafast switching in different device configurations, being of significant interest for new generations of optoelectronic devices.32,55,58,59,64–66 For this reason, the reported compounds with broad Sm(CP)hel phase ranges at low temperature and without additional low temperature phases are of significant practical importance.

2. Experimental

The synthesis of all compounds was conducted according to Scheme 2 by stepwise acylation of 4-cyanoresorcinol with the properly substituted benzoylchlorides. The synthetic procedures and analytical data are collated in the supporting information together with the description of the investigation methods (polarizing microscopy, DSC, XRD, electro-optical methods, dielectric investigations) and second harmonic generation (SHG) studies.
image file: c9tc06456g-s2.tif
Scheme 2 Synthesis of the investigated compounds B–D. Reagents and conditions: i: 1. 4-subst. benzoic acid + SOCl2, 80 °C, 2 h, 2. 4-subst. phenol, DCM, Py, 50 °C, 2 h; ii: NaClO2, KH2PO4, resorcinol, t-BuOH, 20 °C, 1 h; iii: H2, Pd/C (10%), 1,4-dioxane, 20 °C, 1.5 bar, 24 h; the synthesis of compounds E is conducted in an analogous way (see Scheme S2, ESI).

3. Results and discussion

3.1 Phase sequence of compounds Bm/6 depending on chain length

Compounds B12/6–B22/6. At first the discussion will be focussed on compounds Bm/6 with one short hexyl chain and a second much longer chain whose transition temperatures and associated enthalpy values are collated in Table 1 and graphically shown in Fig. 2. The phase sequence of all compounds Bm/6 with m = 12–22 is very similar and is described in the following for compound B12/6 as representative example. In the DSC traces of this compound there is only one phase transition at 82–83 °C with small enthalpy in the whole LC temperature range between the melting point at 69 °C and the transition to the isotropic liquid (Iso) at 134 °C. For all compounds B10/6–B22/6 this reversible transition takes place between 80 and 86 °C (Fig. 3) with a transition enthalpy decreasing upon chain elongation from 0.6 kJ mol−1 for B10/6 to only 0.1 kJ mol−1 for B22/6 (Table 1). Upon cooling a planar aligned sample of B12/6 (layers are perpendicular to the substrate surfaces) between crossed polarizers from the isotropic liquid the typical fan texture with smooth fans and dark extinction brushes aligned parallel to the polarizer and analyzer is observed (Fig. 4e), being typical for non-tilted smectic phases (SmA). This texture is retained across the phase transition at 82–83 °C down to 54 °C where a stripe pattern develops perpendicular to the fans (Fig. 4f and g). In a homeotropic aligned cell with the layer organized parallel to the surfaces, the appearance of the SmA phase between crossed polarizers is completely dark (Fig. 4a). This means, that the LC phase is optical uniaxial and uniaxiality is retained on cooling down to the onset of the phase transition at 82 °C, when a low birefringent schlieren texture appears (Fig. 4b), indicating the transition to a biaxial phase. The birefringence is retained for only about 2 K and then fades at the transition to a second optically uniaxial phase (Fig. 4c), which is discussed below in more detail.
Table 1 Phase transitions of compounds Bm/6 on heating (H→) and cooling (C←)a

image file: c9tc06456g-u1.tif

Comp. m Phase transitions T/°C [ΔH/kJ mol−1] m + n
a Abbreviations: Cr = crystalline solid, Iso = isotropic liquid, SmAPR/SmA = de Vries-like uniaxial lamellar paraelectric LC phase involving a high permittivity (SmAPR) range at lower temperature, SmCaPA = anticlinic tilted and antiferroelectric switching lamellar phase, Sm(CP)hel = heliconical smectic phase; for DSC traces, see Fig. 3 and Fig. S2, S4, S12, S14, S17, S21, S23 (ESI).
B8/6 8 H: Cr 101 [31.3] SmAPR/SmA 123 [3.6] Iso→ 14
C: Cr 48 [−18.0] SmCaPA 69 [−2.3] SmAPR/SmA 122 [−3.7] Iso←
B10/6 10 H: Cr 90 [26.9] SmAPR/SmA 130 [4.8] Iso→ 16
C: Cr 34 [−6.8] SmCaPA ∼60 [—] SmCaPA/Sm(CP)hel 78 [—] SmCaPA 81 [−0.6] SmAPR/SmA 129 [−4.9] Iso←
B12/6 12 H: Cr 69 [18.1] Sm(CP)hel 81 [—] SmCaPA 83 [0.5] SmAPR/SmA 134 [5.3] Iso→ 18
C: Cr <20 SmCaPA 54 [—] Sm(CP)hel 80 [—] SmCaPA 82 [−0.4] SmAPR/SmA 131 [−5.4] Iso←
B14/6 14 H: Cr 83 [24.0] SmAPR/SmA 137 [6.4] Iso→ 20
C: Cr <20 SmCaPA 63 [—] Sm(CP)hel 82 [—] SmCaPA 84 [−0.3] SmAPR/SmA 135 [−6.5] Iso←
B16/6 16 H: Cr 85 [17.8] SmAPR/SmA 139 [6.8] Iso→ 22
C: Cr <20 SmCaPA 65 [—] Sm(CP)hel 84 [—] SmCaPA 86 [−0.2] SmAPR/SmA 137 [−6.9] Iso←
B18/6 18 H: Cr 86 [33.7] SmAPR/SmA 140 [7.1] Iso→ 24
C: Cr 34 [−7.8] SmCaPA 69 [—] Sm(CP)hel 82 [—] SmCaPA 84 [−0.3] SmAPR/SmA 138 [−7.3] Iso←
B20/6 20 H: Cr 87 [49.9] SmAPR/SmA 140 [7.3] Iso→ 26
C: Cr 35 [−14.3] SmCaPA 72 [—] Sm(CP)hel 84 [—] SmCaPA 85 [−0.1] SmAPR/SmA 138 [−7.4] Iso←
B22/6 22 H: Cr 91 [60.6] SmAPR/SmA 139 [7.5] Iso→ 28
C: Cr 44 [−28.4] SmCaPA 76 [—] Sm(CP)hel 84 [—] SmCaPA 85 [−0.1] SmAPR/SmA 138 [−7.6] Iso←



image file: c9tc06456g-f2.tif
Fig. 2 LC phases and phase transition temperatures of compounds Bm/6 as observed on cooling; the hashed area of compound B10/6 indicates the coexistence range of SmCaPA and Sm(CP)hel; for more details, see Fig. S2–S25 (ESI).

image file: c9tc06456g-f3.tif
Fig. 3 DSC heating and cooling traces of compound B12/6 recorded at rates of 10 K min−1; the transition between SmA and SmAPR, both representing paraelectric ranges of the same SmA phase with different correlation length of the polar clusters, is continuous and therefore no transition temperature can be given.41,71

image file: c9tc06456g-f4.tif
Fig. 4 (a–g) Textures and (h–k) polarization current curves of compound B12/6 depending on temperature; the left column shows the homeotropic textures (the bright spots in (a) and (c) are defects with planar alignment) and the middle column the planar fan-texture at the given temperatures in the indicated LC phases; the directions of polarizer and analyzer are indicated by arrows in (a) and are the same for all textures; the width of the images is 0.6 mm; the right column shows the polarization current curves (blue lines) as observed under a triangular wave voltage (black lines); (h) at 200Vpp, (i–k) at 150Vpp in a 6 μm PI-coated ITO cell; see also Fig. S8 and S10 (ESI).

In XRD investigations only a layer reflection with its 2nd order (Fig. S9a, ESI) is observed in the whole LC temperature range of compound B12/6. The layer distance is slightly smaller than the molecular length (Lmol = 5.2 nm67) and increases from d = 4.25 nm at 130 °C to 4.45 nm at 60 °C. The ratio d/Lmol = 0.83–0.88 is typical for lamellar LC phases with monolayer structure (Fig. 5) and in line with a weakly tilted organization of the molecules. A step in the development of the d-spacing at 80–82 °C (see Fig. 5) supports the onset of a tilted organization of the molecules in the biaxial range around this temperature. However, the d-value does not decrease at this transition and for all following homologues of the series Bm/6 there is even a continuous increase of d with decreasing temperature (see Fig. S15b, ESI), indicating the presence of a randomized tilt already in the SmAPR/SmA phases and suggesting a de Vries-like structure of these SmA phases.68 The average birefringence of the planar sample does not significantly change at the SmA to SmCaPA transition (Fig. 4e–g), though an increase of birefringence is usually observed at the de Vries SmA to SmCs transition of rod-like molecules.69 In contrast, the de Vries SmA to anticlinic SmCa transition is associated with a decrease of birefringence.70 In our case this decrease could be compensated by the increase of birefringence due to the developing polar order.


image file: c9tc06456g-f5.tif
Fig. 5 Layer spacing (black) and position of the diffuse wide angle scattering maximum (blue) of compound B12/6 depending on temperature; see Fig. S15 (ESI) for related data of B16/6.

The diffuse wide angle scattering (Fig. S9d, e and S15c, d, ESI) confirms the LC state of all mesophases and the shift of its maximum to smaller values at lower temperature (Fig. 5) corroborates a growing packing density. Hence, optical investigations of B12/6 combined with XRD indicate a uniaxial smectic high temperature phase (SmAPR/SmA) between 132 and 82 °C transforming into a biaxial smectic phase below 82 °C.

The optical biaxiality is observed only over a small temperature range and then the homeotropic sample becomes uniaxial again at the end-set of the transition peak at 80 °C. This uniaxial smectic phase in the temperature range between 80 and 54 °C shows the typical features of the Sm(CP)hel phase, known from the series A.32,60,62 The uniaxiality results from the short pitch heliconical structure and the fan texture in planar alignment is smooth without any stripes across the fans (Fig. 4f), because the helical pitch is far below the optical wave length range. A stripe pattern across the fans, faintly occurring in the biaxial range between 80 and 82 °C, is missing in the uniaxial range between 80 and 54 °C and clearly developing in the biaxial phase below 54 °C (Fig. 4g) as a typical feature of anticlinic tilted biaxial SmCaPA phases (see also Fig. S8, ESI and discussion further below).62 This is explained by distinct modes of anchoring of the molecules, either with the bow planes parallel or perpendicular to the surfaces, which is suppressed in the presence of a helix.

In order to distinguish non-polar, paraelectric, ferroelectric and antiferroelectric phases investigations under a triangular wave field were conducted with B12/6 (Fig. 4h–k) and all other compounds B (Fig. S3, S6, S10, S13a, S16, S19, S22 and S24, ESI). In the uniaxial high temperature phase a weak and relatively broad feature can be observed in each half period of an applied electric AC field, (Fig. 4h and Fig. S10, ESI) which grows with decreasing temperature and increasing applied voltage. This is a first indication of ferroelectric-like Langevin switching as typical for the high permittivity paraelectric SmAPR phase. Hence, the SmAPR/SmA phases of the investigated compounds are considered as composed of ferroelectric SmCsPF grains, being polarization- and tilt-randomized.40,41,72 Under an electric field these grains grow in size and polarization and then can be switched. On further cooling a second broad polarization peak develops, which splits into two, while the previously existing single peak fades upon approaching the short biaxial range at 81–83 °C (Fig. 4h and i and Fig. S10, ESI). In this short biaxial smectic phase range two well-developed polarization current peaks can be observed (Fig. 4i) indicating an antiferroelectric switching. From the step in the layer distance d at this transition we conclude that it is a tilted phase. However, the extinctions in planar alignment remain parallel to the polarizers, thus indicating an anticlinic tilted SmCaPA phase (see Fig. 5). On further cooling, below T = 80 °C one of the peaks of the double peak splits into two close peaks (Fig. 4j and Fig. S10, S13 and S27b, ESI). This three peak pattern of the polarization current response is typical for a ferrielectric switching as proposed previously for the Sm(CP)hel phase of A14,60 thus confirming the heliconical three-layer structure of this mesophase.73 The biaxial smectic phase below 54 °C shows two polarization current peaks as typical for antiferroelectric switching SmCaPA phases (Fig. 4k), being in line with the evolving phase biaxiality and associated with the development of a stripe pattern across the fans in the planar texture (Fig. 4g and Fig. S8e, f, ESI).

The development of the polarization current values of compound B12/6 upon cooling is shown in Fig. 6a (see Fig. S3d, S6d and S19f for additional examples, ESI). An increase of the polarization current starts in the SmA range around 100 °C and the slope increases upon approaching the phase transition at 82 °C; the increase of Ps continues in the Sm(CP)hel and SmCaPA ranges, reaching about 850 nC cm−2 at 50 °C, which is a typical value for polar smectic phases of bent-core molecules. Dielectric investigations (Fig. 6b) additionally support the phase assignments. Two relaxation processes, P1 and P2 were observed in the measured frequency range (Fig. S11, ESI). The low frequency process P1 is attributed to conductivity. The high frequency relaxation process P2 was observed in all of the LC phases. The dielectric strength δε and the relaxation frequency fR of P2 are obtained by fitting the relaxation spectra to the Havriliak–Negami equation. The temperature dependence of these parameters is given in Fig. 6b. On cooling, δε starts growing already at the Iso-SmA transition and rises steeply upon approaching the paraelectric–(anti)ferroelectric transition at 82 °C. This is in line with a growth of polar grains. There is a steep decrease of δε in the biaxial range between 82 and 80 °C, indicating a further growth of the polar cluster with formation of polar layers. The values of δε are still high in the Sm(CP)hel phase and decrease smoothly on further cooling. The transition to the antiferroelectric SmCaPA phase at 54 °C is indicated by a stronger decrease of δε.


image file: c9tc06456g-f6.tif
Fig. 6 (a) Polarization values (Ps) of B12/6 depending on temperature (see also Fig. S3d, S6d and S19f, ESI for additional Ps = f(T) curves) and (b) temperature dependence of the δε and relaxation frequency; (c) shows the temperature dependence of the SHG response I of compound B18/6; the measurements were made on cooling in an AC electric field (Epp = 25 V μm−1) in a 10 μm thick IPS cell with ITO electrodes. (d) The field dependence of the SHG signal in the range of the Sm(CP)hel phase of B18/6 (for more data see Fig. S17–S20, ESI).

Second harmonic generation (SHG, see Fig. 6c and d) was investigated for compound B18/6 which shows essentially the same phase sequence as B12/6. No SHG signal was observed in the absence of an applied field, indicating the absence of a polar (ferroelectric) ground state of all LC phases. A weak field-induced SHG signal occurs in the temperature range of the SmAPR/SmA phase at 115 °C and increases upon approaching the transition to the first biaxial range. This confirms the polar domain structure of the uniaxial SmAPR/SmA phase. The field-dependence of I(T) is continuously increasing in the whole range of accessible field strength. This suggest a Langevin-like behaviour of the polarisation in the random phase. There is a steep increase of the SHG signal around the paraelectric–(anti)ferroelectric transition around 82–83 °C. This suggests increasing polar correlations upon approaching the antiferroelectric SmCaPA phase. The character of the field dependence changes to a more pronounced step-like form, which cannot be fit by the Langevin model (SHG, see Fig. 6d). There is a switching threshold characteristic for this phase. However, the material remains easily polarisable and the further polarisation occurs in a continuous fashion in an in-plane switching (IPS) cell. The continuous growth of the SHG response slows down in the uniaxial Sm(CP)hel range. This can be interpreted by assuming that the helical structure suppresses the polarisation alignment in the bulk of the phase. The transition to the biaxial SmCaPA phase is marked by a slight jump of the I(T), meaning that the removal of the helical constraint improves the field-induced alignment of the layer polarisation.

Hence, below 82 °C a long range polar order develops in the layers of the LC phases of B18/6 and B12/6, and the antipolar correlations between the layers as well as the long range anticlinic tilt correlation strengthen with decreasing temperature. In a certain temperature range a developing twist between the layers compensates the growing polar order and simultaneously satisfies the desire of the transiently helical molecules to assume a helical packing.6,29,74–76 This (partial) synchronization of the helix sense of molecular helical conformers provides a denser packing and thus an energetic advantage over the non-synchronized organization. There is some hysteresis of helix formation leading to a short biaxial range at the paraelectric–(anti)ferroelectric transition due to some delay of the development of the long range helical order. Therefore, the width of this short biaxial range depends on the conditions, like heating and cooling rates and surface alignment effects and it is in some cases only found on cooling. In the biaxial SmCaPA phase below the heliconical phase the antipolar and anticlinic organization becomes energetically favourable and removes the heliconical structure.

Electro-optical investigation. As shown for compound B14/6 in Fig. 7a and b there is no effect of the applied electric field on the texture of the SmAPR/SmA phase, only the birefringence is marginally growing (green to yellowish green). In the complete temperature range of the heliconical Sm(CP)hel phase of compound B14/6 and all compounds Bm/6 the birefringence of planar aligned samples is significantly increased under the applied E-field and relaxes back to the initial value after switching off the field (Fig. 7c–f). This indicates an unwinding of the helix and relaxation of the polar field induced synclinic states back to an apolar helical ground state (Fig. 7i–k). In addition, there is a field strength and temperature dependence of the textural changes. At low E-field or high temperature there is no effect of the field on the texture, whereas with increasing field strength and decreasing temperature at first a periodic stripe pattern develops perpendicular to the fans (“tiger stripes”,60 see Fig. 7d and Fig. S25, ESI), which changes into a tilt domain texture upon further increasing the field strength or lowering the temperature (Fig. 7f). This shows that the short pitch helix is unwounded and becomes a long pitch helix (tiger stripes, Fig. 7i→j) which is finally removed, thus leading to the field-induced tilt domains with opposite polarity and chirality (Fig. 7k); the threshold voltage for tilt domain formation decreases with growing chain length. In the temperature range of the SmCaPA phase below Sm(CP)hel the optical axis is parallel to the layer normal as in the heliconical phase, but the fans adopt a speckled appearance (Fig. 7g), indicating the change to a SmCaPA phase at 0 V. No change of the position of the extinction crosses is observed under the field, while the fans become smooth and the birefringence is increased under the applied field (Fig. 7g and h). This indicates a tristable (antiferroelectric) switching around the molecular long axis (SmCaPA ↔ SmCaPF).
image file: c9tc06456g-f7.tif
Fig. 7 Textures of the distinct mesophases of compound B14/6 at the given temperatures in a planar cell as observed between crossed polarizers in the ground state without applied E-field (left) and as observed under an applied E-field at the indicated voltages (right) with corresponding models of molecular organization; the width of the images is 0.2 mm; see Fig. S7 and S20 (ESI) for the field-induced textures of the related compounds B10/6 and B18/6, respectively.

So, overall, it appears that the fundamental mode of switching in the polar smectic phases (SmCaPA) of this series of compounds is the rotation around the long axis, which is caused by the relatively small bent (and tilt) of these molecules. However, as soon as a heliconical structure is involved, the switching process changes and takes place by precession on a tilt cone, because a rotation around the long axis, leading to an inversion of layer chirality, would lead to a racemic structure which is incompatible with the helix formation.62,72,77

Compounds B8/6 and B10/6 and the effect of alkyl chain length. The longer compounds B14/6 to B22/6 all behave as described for B12/6 (see Table 1 and Fig. 2, Fig. S12–S25, ESI), whereas for the shorter homologues the capability of formation of heliconical phases is reduced. Compound B10/6 (Fig. S4–S7, ESI) shows only a decrease of the birefringence of the homeotropic sample in the SmCaPA phase with a minimum around 71 °C, but without becoming optically uniaxial, indicating the coexistence of the optically uniaxial Sm(CP)hel phase with the (probably surface stabilized) biaxial SmCaPA phase (Fig. S5, ESI). On further cooling the birefringence continuously increases due to a growing contribution of the non-helical SmCaPA structure. A direct SmAPR–SmCaPA transition without indication of any intermediate Sm(CP)hel phase is observed for compound B8/6 with the shortest chains (Fig. S2 and S3, ESI).

3.2 Compounds C with two long alkyl chains – increasing contribution of synclinic tilt

The compounds C with two alkyl chains with almost identical chain length are collated in Table 2 and shown graphically in Fig. 8. The mesomorphic range is not significantly affected by the more equal distribution of the chain lengths, but the paraelectric–(anti)ferroelectric transition is shifted to slightly higher values around 92 ± 4 °C and an increasing contribution of synclinic tilted phases is recognized.
Table 2 Phase transition of compounds Cm/n on heating (H→) and cooling (C←)

image file: c9tc06456g-u2.tif

Comp. m n Phase transitions T/°C [ΔH/kJ mol−1] m + n
Abbreviations: SmCsPR[*] = high permittivity paraelectric and synclinic tilted SmC phase showing ferroelectric-like switching and chiral domains ([*]) in homeotropic alignment; SmCsPAR = high permittivity paraelectric SmCs phase showing antiferroelectric-like switching; SmCsPA = antiferroelectric switching synclinic tilted SmC phase; {Sm(CP)hel} indicates a field-induced Sm(CP)hel phase; for the other abbreviations, see Table 1. For DSCs see Fig. 9a and Fig. S26, S30, S32, S40, S43, S47 and S50 (ESI).
C12/12 12 12 H: Cr 89 [23.8] SmAPR/SmA 142 [6.8] Iso→ 24
C: Cr 62 [−43.7] Sm(CP)hel 89 [−0.6] SmAPR/SmA 140 [−7.1] Iso←
C12/14 12 14 H: Cr 91 [23.2] SmAPR/SmA 142 [6.9] Iso→ 26
C: Cr 67 [−43.9] Sm(CP)hel 88 [−0.5] SmAPR/SmA 140 [−7.0] Iso←
C14/14 14 14 H: Cr 84 [57.1] Sm(CP)hel 98 [0.8] SmCsPAR ∼ 102 [—] SmCsPR[*]118 [—] SmAPR 136 [7.3] Iso→ 28
C: Cr 63 [−55.6] Sm(CP)hel 94 [−0.8] SmCsPAR ∼ 100 [—] SmCsPR[*] 118 [—] SmAPR 134 [−7.1] Iso←
C16/14 16 14 H: Cr 86 [6.1] Sm(CP)hel 96 [0.6] SmCsPAR ∼ 103 [—] SmCsPR[*]127 [—] SmAPR 144 [7.5] Iso→ 30
C: Cr 67 [−59.3] Sm(CP)hel 95 [−0.7] SmCsPAR ∼ 102 [—] SmCsPR[*] 127 [—] SmAPR 142 [−7.4] Iso←
C18/14 18 14 H: Cr 88 [61.1] SmCsPA{Sm(CP)hel} 97 [0.5] SmCsPAR ∼ 105 [—] SmCsPR[*] 131 [—] SmAPR 143 [7.7] Iso→ 32
C: Cr 70 [−67.3] SmCsPA{Sm(CP)hel} 95 [−0.7] SmCsPAR ∼ 103 [—] SmCsPR[*] 131 [—] SmAPR 142 [−7.7] Iso←
C22/12 22 12 H: Cr 83 [63.2] SmCsPA{Sm(CP)hel} 95 [0.5] SmCsPAR ∼ 103 [—] SmCsPR[*] 133 [—] SmAPR 142 [7.5] Iso→ 34
C: Cr 63 [−15.5] SmCsPA{Sm(CP)hel} 92 [−0.4] SmCsPAR ∼ 102 [—] SmCsPR[*] 132 [—] SmAPR 140 [−7.8] Iso←
C18/18 18 18 H: Cr 102 [26.7] SmCsPR[*] 132 [—] SmAPR 142 [7.0] Iso→ 36
C: Cr 81 [−69.2] SmCsPA{Sm(CP)hel}94 [−0.8] SmCsPAR ∼ 102 [—] SmCsPR[*] 131 [—] SmAPR 140 [−7.1] Iso←
C22/18 22 18 H: Cr 95 [77.3] SmCsPAR ∼ 103 [—] SmCsPR[*] 130 [—] SmAPR 140 [6.9] Iso→ 40
C: Cr 82 [−79.6] SmCsPA{Sm(CP)hel} 95 [−1.0] SmCsPAR ∼ 102 [—] SmCsPR[*] 130 [—] SmAPR 138 [−7.0] Iso←



image file: c9tc06456g-f8.tif
Fig. 8 Chain length dependence of the LC phases and transitions of compounds C as observed on cooling in the absence of an E-field. After application of a short AC sequence all SmCsPA phases change to field induced Sm(CP)hel phases, for details, see Fig. S26–S52 (ESI).
Compounds C12/12 and C12/14 with dominating Sm(CP)hel phases. The LC phases of compounds C12/12 (Fig. S26–S29, ESI) and C12/14 (Fig. S30 and S31, ESI) are optically uniaxial (completely dark in homeotropic alignment) and show a smooth SmA-like fan texture of the planar samples in the whole mesomorphic temperature range. Only the small DSC peaks at 88 and 89 °C, respectively, indicate the paraelectric–(anti)ferroelectric transition (Table 2), where in the polarization current curves the broad single peak in the SmAPR range changes to the typical three-peak pattern of the Sm(CP)hel phase (see Fig. S27, S29 and S31, ESI). Thus, for these compounds the phase sequence on cooling is SmAPR → Sm(CP)hel, i.e. the SmCaPA phase ranges above and also below the Sm(CP)hel phase of compounds Bm/6 are completely removed, and for C12/12 and C14/12 exclusively the heliconical phase is observed in a broad temperature range below the tilt-randomized paraelectric SmAPR phase. The stabilizing effect of symmetric chain distribution on the heliconical phase is especially evident if the isomeric compounds with the same total number carbon atoms in the terminal chains i.e.B18/6 and C12/12 or B20/6 and C12/14 are compared (Fig. 2 and 8).
Compounds C14/14 and C16/14 – emergence of synclinic tilt. The next longer homologue C14/14, shows an additional synclinic tilted SmCsPR[*] phase, replacing a part of the SmAPR/SmA phase (compare Fig. 2 and 8). Similar to the shorter homologues C12/12 and C12/14 the SmCaPA phase is completely removed and replaced by the Sm(CP)hel phase. This leads to a transition from a uniaxial via a biaxial to another uniaxial phase with lowering temperature (Fig. 10a, d, g and j), but in this case the biaxial range appears above the paraelectric–(anti)ferroelectric transition and not below it and is much broader than observed for the isomeric compound B22/6 (see Fig. 9a and 10a, d, g, j). That the tilt sets in already in the paraelectric range indicates a greater tendency for tilt correlation by adjusting the alkyl chain length at both ends. Typical features of this polarization randomized SmCsPR[*] phase are a tilt of the extinctions being inclined with the directions of the polarizers in planar alignment (Fig. 10e), a broad polarization current peak (Fig. 10f) and the formation of chiral domains in homeotropically aligned samples (therefore the additional [*]; see Fig. 12 and Fig. S33 and S35, ESI).78 Just before the transition to the heliconical phase there is an additional small ca. 7–8 K wide temperature range of a SmCsPAR phase, indicated by the vanishing of the chiral domains and the emergence of two broad polarization current peaks instead of only one (Fig. 10g–i). The Langevin switching in both polarization randomized ranges of the paraelectric SmC phase takes place between two identical synclinic SmCsPF states by reorganization around the long axis (Fig. S38 and S39, ESI).
image file: c9tc06456g-f9.tif
Fig. 9 Investigation of compound C16/14. (a) DSC traces recorded with scanning rates of 10 K min−1 and (b) plot of d-values of the layer spacing (black) and maxima of the diffuse wide angle scattering (blue) depending on temperature as observed by XRD.

image file: c9tc06456g-f10.tif
Fig. 10 Investigation of compound C16/14. Homeotropic (left) and planar textures (middle) before application af an E-field, and corresponding polarization current curves (right) at the given temperatures in the indicated LC phases; (c, f and i) Vpp = 240 V, (l) Vpp = 80 V in a 6 μm PI-coated ITO cell; the width of the images is 0.6 mm; for more detail, see Fig. S35–S39 (ESI).

image file: c9tc06456g-f11.tif
Fig. 11 Investigation of compound C18/18. Homeotropic (left) and planar textures (middle) before application af an E-field, and corresponding polarization current curves (right) at the given temperatures in the indicated LC phases at (c) 320Vpp (f) 260Vpp, (i) 80Vpp in a 6 μm PI-coated ITO cell; the width of the images is 0.6 mm.

image file: c9tc06456g-f12.tif
Fig. 12 Chiral domains in the SmCsPR[*] phase of C18/18 at T = 110 °C as observed in a homeotropic sample (b) between crossed and (a and c) between slightly uncrossed polarizers (white arrows) with the analyzer twisted by ±10° out of the perpendicular direction. The inverted brightness in (a and c) indicates chiral domains; that these domains do not represent simple tilt domains is indicated by rotation of the sample between crossed polarizers which does not give an inversion of the domain brightness; the width of the images is 0.6 mm.

Whereas the Sm(CP)hel phase of compounds Bm/6 and C12/n, occurring below a tilt- and polarization-randomized SmAPR phase, have three polarization peaks as typical for the ferrielectric three-layer heliconical phases (see for example see Fig. S27b, S29 and S31, ESI), there are only two peaks in the Sm(CP)hel phases of all compounds C with a longer total chain length (C14/14, C14/16) and having an onset of tilt before helix formation (see for example Fig. 10l, Fig. S34 and S37, ESI). This might indicate a shorter or longer helix being closer to the two- or four-layer periodicity, for which antiferroelectric switching could be expected.34

Surprisingly, for all compounds C there is an almost linear increase of the d-spacing with lowering temperature and without any jump either at the SmA-SmC phase transition or at the paraelectric–(anti)ferroelectric transition temperature (Fig. 9b, Fig. S44a and S48a, ESI), even if a synclinic tilt develops. This supports the hypothesis that the tilt is present in all phases (also in the de Vries like SmAPR/SmA phases) and only the coherence length of tilt changes with temperature, either from tilt randomized to heliconical for compounds Cm/n with n + m < 28, from tilt randomized via synclinic tilted to heliconical for n + m = 28, 30 or exclusively to synclinic (SmCsPA) for compounds with n + m > 30. Moreover, the change of the tilt angle with temperature is small and the increasing layer spacing results mainly from the stretching of the alkyl chains due to the increasing packing density at lower temperature (Fig. 9b).

Compounds C18/14 to C22/18: dominating synclinic tilt and the field-induced Sm(CP)hel phase. Further increasing the total alkyl chain length to m + n ≥ 32 completely removes the heliconical Sm(CP)hel phase which is replaced by a synclinic tilted and antiferroelectric switching SmCsPA phase. The data of compound C18/18 are shown as representative example for this case in Fig. 11–13 and Fig. S49 (ESI). The typical feature of these compounds is a SmAPR–SmCsPR[*]–SmCsPAR–SmCsPA phase sequence (Fig. 11), i.e. the synclinic tilt, once formed at the SmAPR–SmCsPR transition is retained in the whole LC range and no heliconical phase is found. In this case the transition from the paraelectric SmCsPAR phase to the antiferroelectric SmCsPA phase is accompanied by an inversion of the birefringence (Fig. 13), which is assumed to be the result of the competition between the contribution of the biaxiality due to the polar order and the biaxiality due to the tilt, being perpendicular to each other, i.e. the secondary optical axis can change from being along the tilt direction to perpendicular with growing polar order.47,62 The inversion point is close to the paraelectric–(anti)ferroelectric (SmCsPAR–SmCsPA) transition, but in most cases does not coincide with this transition and usually takes place a few degrees lower.
image file: c9tc06456g-f13.tif
Fig. 13 Inversion of birefringence in the SmCs phase range of C18/18 as observed between crossed polarizers (white arrows in b) with additional λ-retarder plate (the direction of the indicatrix is indicated by the blue arrow) in a homeotropic aligned sample; the position of the bluish and yellowish/red areas exchanges with an inversion of birefringence with Δn ∼ 0 around 90 °C; in the insets (yellow) the arrow indicate the relative orientations of the polar direction (P) to the projection of the tilt direction on the layer plane (τ, dotted line); the width of the images is 0.6 mm.

Application of a short sequence of an AC field to the SmCsPA phase leads to a change of the orientation of the dark extinction in the planar samples from being inclined to parallel to the polarizers (Fig. 11e and 14a–d), indicating the transition to a state with the main optical axis parallel to the layer normal. Under an applied DC field the extinctions become inclined again, indicating the induction of a tilted state (Fig. 14d–f). The direction of inclination is inverted by changing the field direction, thus showing that the switching takes place by precession on a cone. All these observations confirm the field induced formation of a Sm(CP)hel phase, as previously observed and confirmed by the HDF effect for compound A18.32,62 The absence of any stripes or speckles in the fan texture of the ground state structure (Fig. 14e) confirms the heliconical structure and excludes the possibility of an alternative SmCsPF ↔ SmCaPA switching. Once formed the heliconical phase is stable after removing the field for all investigated compounds Cm/n in the whole SmCsPA temperature range at least for several hours, as long as crystallization is prevented.


image file: c9tc06456g-f14.tif
Fig. 14 Switching of C18/14 at 80 °C under a DC field in a 6 μm ITO cell, with the field applied between the substrate surfaces. (a–c) in the SmCsPA phase and (d–f) in the field-induced Sm(CP)hel phase; the first application of a DC voltage switches around the long axis from the racemic SmCsPA state to the chiral SmCsPF state (a and b) and the following switching cycles (b↔c and d→f) take place by precession on a cone, between the SmCsPF states and heliconical Sm(CP)hel phase; the width of the images is 0.2 mm; see also Fig. S42 for the other LC phases and Fig. S38 (ESI) for C16/14.

Overall, the formation of the heliconical Sm(CP)hel phase requires a distinct chain length range and is associated with the cross-over of the mode of tilt correlation from anticlinic to synclinic, achieved by chain elongation. The heliconical phase replaces a part of the SmCaPA phase range (compounds Bm/6), which has uniform superstructural layer chirality. Symmetric chain distribution widens the heliconical phase range which eventually replaces the SmCaPA phase completely (compounds Cm/n with m + n = 24–30). Further strengthening of the synclinic tilt correlation with respect to the anticlinic one leads to the transition to the SmCsPA phase (compounds Cm/n with m + n ≥ 30), which is achiral and thus disfavours helix formation. In the SmCsPA phase the switching initially takes place by chirality flipping around the long axis and this leads to the field induced SmCsPF state which is chiral (Fig. 14a and b). Once the SmCsPF states is formed the helix can develop (Fig. 14c and d) and the SmCsPA phase is replaced by the Sm(CP)hel phase. In the field induced Sm(CP)hel phase the switching takes place by precession on a cone which retains the layer chirality and the heliconical phase is retained also after removing the field (see Fig. 14d–f and Fig. S38, S39, S46 and S52, ESI).32,62

3.3 Effect of ether oxygens on the formation of the heliconical smectic phase and the polarization randomized heliconical phase

The effect of replacing alkyl chains by alkoxy chains was investigated for compound B18/6 (see Table 4 and Fig. 15). As expected, the introduction of ether oxygens between alkyl chain and aromatic core leads to a general stabilization of LC phases79 as indicated by the increase of the LC-Iso transition temperature. Also the melting points are enhanced with increasing number of alkoxy chains and there is a strong effect of the position of the alkoxy chain on the formation of heliconical LC phases. Replacing the shorter alkyl chain at the CN substituted phenyl terephthalate side by an alkoxy chain (compound D18/O6) removes the heliconical phase completely, only the SmCaPA phase is observed below the SmAPR phase (see Table 3 and Fig. S53–S55, ESI). In contrast, replacing the longer C18 chain at the 4-benzoyloxybenzoate substituted side by an alkoxy chain (compound DO18/6) retains and even expands the heliconical phase range (see Table 3 and Fig. 15; and Fig. S57–S59, ESI).
image file: c9tc06456g-f15.tif
Fig. 15 Effect of replacing one or both alkyl chains of B18/6 by alkoxy chain(s) on the phase transitions on cooling.
Table 3 Phase sequence and transition temperatures of compounds D with different chain length at both ends, measured on heating (H→) and cooling (C←)a

image file: c9tc06456g-u3.tif

Compd R1 R2 Phase transitions T/°C [ΔH/kJ mol−1] m + n
a For abbreviations, see Tables 1 and 2; SmChelPR and SmCxPR represent polarization randomized heliconical smectic phases; for DSCs, see Fig. 16, Fig. S53, S60 and S62 and for additional details, see Fig. S53–S67 (ESI).
D18/O6 –C18H37 –OC6H13 H: Cr 91 [27.2] SmCaPA 102 [0.1] SmAPR/SmA 145 [7.3] Iso→ 24
C: Cr 72 [−23.2] SmCaPA 100 [−0.1] SmAPR/SmA 143 [−7.4] Iso←
DO12/6 –OC12H25 –C6H13 H: Cr 104 [48.0] SmAPR/SmA 153 [6.9] Iso→ 18
C: Cr <20 SmCaPA 57 [—] Sm(CP)hel 82 [—] SmCaPA 87 [−0.4] SmAPR/SmA 153 [−6.8] Iso←
DO18/6 –OC18H37 –C6H13 H: Cr 102 [44.3] Sm(CP)helR 125 [—] SmAPR/SmA 157 [7.6] Iso→ 24
C: Cr 37 [−6.8] SmCaPA 64 [—] Sm(CP)hel 86 [−0.3] SmChelPR 108 [—] SmCxPR 125 [—] SmAPR/SmA 155 [−7.8] Iso←
DO18/O6 –OC18H37 –OC6H13 H: Cr 117 [36.9] SmAPR/SmA 161 [7.9] Iso→ 24
C: Cr 103 [−32.9] SmAPR/SmA 160 [−8.1] Iso←
DO22/O6 –OC22H45 –OC6H13 H: Cr 116 [36.7] Sm(CP)hel 125 [—] SmCsPR[*] 140 [—] SmAPR/SmA 160 [8.9] Iso→ 28
C: Cr 107 [−34.6] Sm(CP)hel 123 [—] SmCsPR[*] 139 [—] SmAPR/SmA 158 [−8.7] Iso←


An especially interesting case is compound DO18/6 which is described here in more detail. The typical fan texture of the SmA phase with extinctions parallel to the polarizers is for this compound retained in planar samples in the whole mesomorphic range down to the crystallization at T = 37 °C (see DSC traces in Fig. 16a and corresponding textures in Fig. 17). As observed for the majority of compounds A–D there is a continuous increase of the d-value of the layer reflection with lowering temperature (Fig. 16b). The paraelectric–(anti)ferroelectric transition takes place at 86 °C, indicated by a small DSC peak (Fig. 16a) which is not associated with any major textural change, neither in planar nor in homeotropic samples (Fig. 17g–i and k). In homeotropic alignment a very low birefringent and almost invisible schlieren texture evolves at 125 °C (Fig. 17e) and is retained down to 108 °C where the homeotropic sample of DO18/6 becomes completely optically isotropic (black) again (Fig. 17h). In the switching current curves only one broad polarization current peak is observed in the whole temperature range between 125 and 108 °C (Fig. 17f).


image file: c9tc06456g-f16.tif
Fig. 16 DSC traces of compound DO18/6 measured with scanning rates of 10 K min−1 and (b) d-values of the layer spacing and of the maxima of the diffuse wide angle scattering in the XRD patterns of this compound depending on temperature.

image file: c9tc06456g-f17.tif
Fig. 17 Investigation of compound DO18/6. Hometropic (left) and planar textures (middle) before application af an E-field, and corresponding polarization current curves (right) at the given temperatures in the indicated LC phases; (c, f and i) 200Vpp (l and o) 100Vpp, in a 6 μm PI-coated ITO cell; the width of the images is 0.6 mm; for additional data, see Fig. S57–S59 (ESI).

These observations indicate a polarization randomized smectic phase with ferroelectric-like Langevin switching. The main optical axis is parallel to the layer normal (Fig. 17d) which could be in line either with an anticlinic tilted SmCaPR phase or a polarization randomized SmCsPR phase with an additional heliconical superstructure. These two possibilities require RSoXS to be distinguished, and therefore the present phase assignment is SmCxPR. The almost isotropic appearance in homeotropic alignment (Fig. 17e) is in favour of an optical uniaxial heliconical structure of this polarization randomized SmC phase, where the residual birefringence is attributed to polar surface layers. If there would be a SmCaPR phase a stronger birefringence would be expected for the homeotropic sample. Below 108 °C the homeotropic texture becomes completely dark (Fig. 17h), though a single broad polarization current peak is retained and shifted almost to the zero voltage crossing (Fig. 17i).

This broad feature is retained down to 86 °C when the typical three peak pattern (the two peaks at the left are overlapping) of the ferrielectric Sm(CP)hel phase develops (Fig. 17l). This uniaxial smectic phase occurring between 108–68 °C on cooling, i.e. between the SmCxPR and Sm(CP)hel phases, must have either a randomized tilt direction (re-entrance of a SmAPR phase), or more likely, there is a helical superstructure representing a kind of polarization randomized heliconical SmC phase (SmChelPR). This would represent a new kind of heliconical smectic phase, expanding the family of polarization randomized high permittivity smectic phases (SmAPR, SmAPAR, SmCsPR, SmCsPAR) by a new heliconical mode. It is assumed to represent a polarization randomized phase with short pitch helical twist of the tilt direction between the layers. The main difference to the SmCxPR range is a larger coherence length of polar order, but also the helical pitch length and mode of correlation between pitch and layer spacing could be different.

On further cooling of DO18/6 the Sm(CP)hel phase forms at 86 °C and at 64 °C transforms into the SmCaPA phase (Fig. 17j–o). Thus, compound DO18/6 appears to form even three different heliconical LC phases (Sm(CP)hel, SmChelPR and SmCxPR). On heating only a uniaxial phase with extinction parallel to the layer normal is found in the whole mesomorphic temperature range above the melting point at 102 °C, suggesting that the SmChelPR phase once formed replaces the SmCxPR phase on heating (Table 3).

For all compounds D, combining one alkoxy and one alkyl chain with similar length at both ends (Table 4), the Sm(CP)hel phase occurs directly below a tilt- and polarization-randomized SmAPA or SmAPAR phase without any tilted or heliconical paraelectric intermediate phase and can be observed down to crystallization. As shown in Fig. 18 the ether linkage stabilizes the polar heliconical phase with respect to the paraelectric phases, though it simultaneously tightens their phase ranges due to the rising crystallization temperatures. However, for the compounds with two alkoxy chains (Tables 3, 4 and Fig. 18) the Sm(CP)hel phase cannot be observed, due to the increase of the crystallization temperature. Only for DO22/O6 (Table 3) with very different chain length at both ends the crystallization temperature is sufficiently low that the Sm(CP)hel phase is observed. In this case the Sm(CP)hel phase is stabilized by ∼40 K compared to B22/6 (Table 1) and thus becomes an enantiotropic phase.

Table 4 Phase sequence and transition temperatures of compounds D with similar chain length at both ends, measured on heating (H→) and cooling (C←)a

image file: c9tc06456g-u4.tif

Compd R1 R2 Phase transitions T/°C [ΔH/kJ mol−1] m + n
a Abbreviations: SmCsPR = high permittivity paraelectric SmCs phase without chiral domain structure in homeotropic alignment;78 for other abbreviations, see Tables 1 and 2; for DSCs, see Fig. S65, S68, S70, S72, S73 and S75, and for more details, see Fig. S65–S77 (ESI).
DO12/12 –OC12H25 –C12H25 H: Cr 98 [40.0] SmAPAR 110 SmAPR/SmA 158 [7.5] Iso→ 24
C: Cr 75 [−56.1] Sm(CP)hel 91 [−0.5] SmAPAR 110 [—] SmAPR/SmA 156 [−7.5] Iso←
DO12/14 –OC12H25 –C14H29 H: Cr 100 [57.8] SmAPAR 110 [—] SmAPR/SmA 158 [7.7] Iso→ 26
C: Cr 79 [−62.5] Sm(CP)hel 92 [−0.4] SmAPAR 110 [—] SmAPR/SmA 158 [−7.5] Iso←
D12/O12 –C12H25 –OC12H25 H: Cr 103 [36.6] SmAPR/SmA 147 [6.9] Iso→ 24
C: Cr 92 [-36.7] Sm(CP)hel 101 [−0.6] SmAPR/SmA 145 [−6.9] Iso←
DO12/O12 –OC12H25 –OC12H25 Cr 119 [39.5] SmAPR/SmA 162 [8.4] Iso→ 24
Cr 103 [−41.2] SmAPR/SmA 163 [−8.3] Iso←
DO14/O14 –OC14H29 –OC14H29 Cr 120 [39.1] SmCsPR[*] 145 [—] SmAPR/SmA 165 [7.8] Iso→ 28
Cr 108 [−39.5] SmCsPR[*] 144 [—] SmAPR/SmA 162 [−7.9] Iso←



image file: c9tc06456g-f18.tif
Fig. 18 Effect of replacing one or both alkyl chains of C12/12 by alkoxy chain(s) in compounds D; phase transitions on cooling are shown.

3.4 Compounds E and the effect of the direction of the cyano group

The effect of exchange of the phenyl terephthalate and 4-benzoyloxybenzoate wing in compounds B–D is shown for selected compounds E in Table 5. By comparison of the alkyl substituted compounds E12/12 and C12/12 (Table 2) an increasing tendency to synclinic tilt correlation is found for the compound E12/12, as indicated by the formation of tilted paraelectric SmCs, SmCsPR and SmCsPAR phase ranges replacing the majority of the SmA/SmAPR phase of C12/12 (see Fig. S78 and S79, ESI).80 As well, the heliconical Sm(CP)hel phase of C12/12 is completely removed and replaced by an antiferroelectric switching SmCsPA phase for E12/12. In this phase a heliconical Sm(CP)hel phase is induced by application of an electric field, associated with a reorganization by precession of the molecules on a cone (see Fig. S80, ESI). The same phase sequence is observed for the two other compounds E with one or two alkoxy chain, but for these compounds no helix is formed in the SmCsPA phases under the applied AC field and the switching takes place by rotation around the long axis (SmCsPA ↔ SmCsPF) confirming the absence of a field induced heliconical phase (see EO12/12 in Fig. S83, ESI). The larger tilt of compounds E (∼22° for E12/12 compared to 18–20° in the series An) and the stronger tilt correlation between the layers obviously disfavours helix formation (Table 5). The significantly smaller d-values of the layer spacing observed for compound EO12/O12 (d = 4.34 to 4.40 nm, Fig. S85, ESI) compared to compound DO18/6 with a similar total chain length (d = 4.84–5.35 nm, Fig. 16b) confirms the stronger tilt of compounds E.
Table 5 Phase sequence and transition temperatures of compounds E with exchanged position of phenyl terephthalate and 4-benzoyloxybenzoate wings with respect to the CN group, as measured on heating (H→) and cooling (C←)a

image file: c9tc06456g-u5.tif

Compd R1 R2 Phase transitions T/°C [ΔH/kJ mol−1] m + n
a Values in parentheses refer to monotropic transitions recorded in the second heating cycle, phases in braces indicate field induced phases; abbreviations: SmCs = paraelectric and synclinic tilted SmC phase without visible polarization peak; for other abbreviations, see Tables 1, 2 and 4, for DSCs see Fig. S78, S81 and S84, and for more details see Fig. S79–S83 and S85 (ESI).
E12/12 –C12H25 –C12H25 H: Cr 87 [24.9] SmCsPA{Sm(CP)hel} 90 [0.7] SmCsPAR 98 [—] SmCsPR 130 [—] SmA 134 [6.1] Iso→ 24
C: Cr 48 [−18] SmCsPA{Sm(CP)hel} 88 [−0.9] SmCsPAR 96 [—] SmCsPR 129 [—] SmA 132 [−6.7] Iso←
EO12/12 –OC12H25 –C12H25 H: Cr 76 [19.8] SmCsPA 102 [1.2] SmCsPAR 105 [—] SmCs 137 [5.5] Iso→ 24
C: Cr <20 SmCsPA 99 [−1.1] SmCsPAR 103 [—] SmCs 135 [−5.6] Iso←
EO12/O12 –OC12H25 –OC12H25 Cr 114 [37.5] (SmCsPA 106 [1.0]) SmCsPAR 116 [—] SmCsPR[*] 148 [6.3] Iso→ 24
Cr <20 SmCsPA 104 [−1.3] SmCsPAR 115 [—] SmCsPR[*] 147 [−6.6] Iso←


Moreover, compound EO12/O12 is the only investigated compound which shows a decrease of the layer spacing d with lowering temperature and a clearly visible step to larger d-values at the paraelectric–(anti)ferroelectric transition due to the increased packing density (Fig. 19), whereas for all other compounds A–D the development of d is characterized by a continuously increasing d-value with lowering temperature (see for example Fig. 16b). Overall, the inversion of the direction of the CN group increases the tilt, thus leading to a stronger layer coupling which suppresses the heliconical phases.


image file: c9tc06456g-f19.tif
Fig. 19 d-Values of the layer spacing of EO12/O12 depending on temperature.

3.5 Effects of the direction of the COO groups

A comparison of A14, C14/14 and F14 in Table 6 shows the effect of inversion of the direction of the two peripheral COO groups in the rod-like wings. Compound F14 with two 4-benzoyloxybenzoate wings forms exclusively non-polar and synclinc tilted SmCs phases with high tilt angles (∼35°) besides the skewed cybotactic nematic phase (NCybC).47 Inversion of both outer COO groups in A14 reduces the tilt, removes all synclinic tilted phases and the transition from the paraelectric SmA/SmAPR to a polar SmCaPA phase takes place via an intermediate heliconical phase. For C14/14, combining both directions of the COO group, there is a slightly larger tendency to synclinic tilt correlation which leads to the insertion of an intermediate SmCsPR[*] phase between SmAPR and Sm(CP)hel. Simultaneously with the decreasing tilt in the order F14 > C14 > A14 the mesophases stability rises. Both effects are attributed to a growing core–core interaction and growing packing density due to the increasing electron deficit character of the polyaromatic cores in this order. The surprising similarity of the LC phases of all three series A, B and C compared to the series of compounds F might be attributed to the strength of the core–core interaction. The two electron deficit phenyl terepthalate wings provide strong attractive core–core interactions81 for compounds A. Replacement of only one phenyl terephthalate wings by an electron rich phenyl benzoate wing leads to attractive donor–acceptor interactions between the electron rich and electron deficit wings in an antiparallel packing of the compounds B and C in the layers. The electrostatic interaction become weaker for compounds F having exclusively electron rich phenyl benzoate wings.81 The comparison of C12/12 with E12/12 (Tables 2 and 5) shows that the position of the electron deficit phenyl terephthalate wing at the CN substituted side is important for retaining a small tilt.
Table 6 Effect of inverting one or both peripheral COO group(s) on the phase sequence and phase transitionsa

image file: c9tc06456g-u6.tif

Compd X Y m = n Phase transitions T/°C [ΔH/kJ mol−1]
a Phase transitions on heating; values in parentheses indicate monotropic phases, observed on cooling; SmCI, SmCII and CybC are different types of non-polar synclinic SmCs phase, M is an unknown mesophase;47 for the other abbreviations, see Tables 1 and 2.
A14 60,62 OOC COO 14 Cr 114 [47.2] (SmCaPA 91 [—] Sm(CP)hel 110 [0.9]) SmAPR/SmA 165 [8.0] Iso
C14/14 COO COO 14 Cr 84 [57.1] Sm(CP)hel 98 [0.8] SmCsPR[*] 118 [—] SmA 136 [7.3] Iso
F14 47 COO OOC 14 Cr 91 (M 36 SmCI 73 SmCII 77) CybC 114 NCybC 115 Iso


The series B and C provide access to broad heliconical smectic phase regions for compounds with a total number of C-atoms in the terminal alkyl chains of 16–30, which is shorter than the 22–36 required for the related compounds A. This is in line with the reduced intrinsic tilt in series A which requires longer chains to reach the anticlinic–synclinic cross-over point. Moreover, the heliconical phase of C14/14 is stable until crystallization and no other phase can be found below it, whereas an additional anticlinic tilted phase range is formed below the Sm(CP)hel phase of A14.

Overall the major effects come from the core structure, determining the bending angle α (Scheme 1) and the electrostatic intermolecular interactions between the polyaromatic cores, whereas alkyl chain engineering is an important tool for fine-tuning of the strength and mode of layer coupling and inter-layer tilt-correlation, growing in the order random → helical → anticlinic → synclinic.

4. Conclusions

This work provides new series of 4-cyanoresorcinol bisbenzoate derived bent-core mesogens with relatively weak bend and shows that the heliconical smectic phase, designated as Sm(CP)hel is not restricted to compounds with two phenyl terephthalate wings (compounds An in Scheme 1), but represents a new kind of mirror symmetry broken LC phase generally occurring if certain conditions are fulfilled. It can be expected for the smectic phases of bent-core mesogens below the paraelectric–(anti)ferroelectric transition if the tilt angle is around 18–20° and if this transition coincides with the transition from randomized to uniform tilt and with the anticlinic-to-synclinic cross-over point.

As graphically shown in Fig. 20, the Sm(CP)hel phase appears below a polarization randomized high permittivity SmAPR, SmAPAR, SmCsPAR or SmCsPR[*] phase32,58 and replaces the emerging homogeneously chiral polar and anticlinic SmCaPA phase. As the synclinic tilt correlation becomes stronger by chain elongation, by introduction of ether oxygens between chains and core, or by flipping the direction of the CN group (= exchange of the positions of 4-benzoyloxy benzoate and phenyl terephthalate wings), the anticlinic SmCaPA phase is replaced by the synclinic SmCsPA phase which is achiral and thus incompatible with helix formation. Even if the Sm(CP)hel phase is removed completely (see Fig. 8), it still can be induced in some cases under a pulsed E-field which switches the achiral SmCsPA phase to the chiral SmCsPF state. The helix, once formed, can be stable after switching off the field, because it provides an alternative escape from the macroscopic polarization.32 The unique feature of the investigated compounds is, that the escape from the developing macroscopic polarization takes place by helix formation along the layer normal, which retains flat layers. In previous cases the layers of the bent-core mesogens (with larger tilt angle and stronger layer coupling due to stronger tilt correlation) become distorted and polarization modulated columnar phases (B1, B1rev and B7 phases) or isotropic sponge and dark-conglomerate (DC) phases were formed instead.51,82–84 It is proposed that the weak layer coupling at the anticlinic–synclinic cross-over makes a helical twist between the layers easier than the twist deformation of the layers themselves. In some cases the Sm(CP)hel phase is stable over broad temperature ranges until crystallization without transition to any other LC phase. This is important for application of these materials in electro-optical devices, either using the very fast in-plane switching of the secondary optical axis in homeotropic alignment64,65 or the V-shaped grey scale switching by helix deformation in planar alignment.32,55,56,58,59,66


image file: c9tc06456g-f20.tif
Fig. 20 The heliconical phases as structures at the coincidence of the paraelectic–(anti)ferroelectric with the anticlinic–synclinic transition.

We also provide the first evidence that such a helical structure could even persist in the polarization randomized smectic phases at higher temperature, thus leading to a new types of the polarization randomized high-permittivity paraelectric smectic phases (SmChelPR, SmCxPR). These are extremely rare phases and limited to only one of the even numbered homologues at the transition from the uniaxial SmAPR and SmAPAR phases to the synclinic tilted biaxial SmCsPR and SmCsPAR phases at the paraelectric side of the Curie line (Fig. 20). It appears that they replace the anticlinic tilted and polarization randomized SmCaPR and SmCaPAR phases. This shows that complexity of self assembly77 emerges in a very small molecular structural space range at the edge of the transition from randomized to synclinic tilt and it can be expected that additional new phases and phenomena can be found there.

Because the heliconical phases are difficult to distinguish from non-helical uniaxial smectic phases, it is possible that heliconical phases can also be observed in other series of compounds, and might have been overlooked in some cases in previous work.51 Also previously reported observations of complex behaviour of bent-core LC phases depending on surfaces and applied fields85 might be interpreted with spontaneous helix formation, chirality effects and diastereomeric coupling. Moreover, among the heliconical phase types numerous additional incommensurate types with larger or shorter pitch-length (SmCα*-like phases)34 and also including commensurate 3- and 4-layer modes (similar to the SmCFI* phases)34 are possible which requires further investigation by RSoXS to be confirmed.

Overall, this work leads to an improved understanding and generalization of the formation of heliconical smectic phases depending on details of the molecular structure. As well it shows that this is a general phenomenon occurring at the cross-over between paraelectric and (anti)ferroelectric phases of achiral bent core mesogens if the molecular bent and the tendency to form tilted smectic phases, and hence, the layer coupling are weak. The gained knowledge contributes to the development of new materials and a common understanding of the different types of heliconical phases, involving the polar heliconical smectic phases of the bent-core mesogens, considered herein,59 the apolar heliconical smectic (SmCTB)63 and the related twist bent nematic phases (NTB),12–14 both formed by bent dimesogens. There is even a similarity with the heliconical SmCα*, SmCFI1* and SmCFI2* and other intermediate phases previously reported to occur at the SmA*–SmCs*–SmCa* transitions of highly chiral rod-like molecules.34 The main difference is that for the rod-like molecules the chirality is provided by the permanent molecular chirality, whereas for the bent-core mesogens chirality develops spontaneously by mirror symmetry breaking in lamellar phases of achiral molecules having transiently chiral helical conformations. Overall, chirality synchronization between molecules and their chiral superstructures plays an important role for spontaneous mirror symmetry breaking and the development of order and complexity in LC systems,77 as also demonstrated for the cubic and non-cubic phases of polycatenar mesogens.6,8,10

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Deutsche Forschungsgemeinschaft (Grants TS 39/24-2 and ER 467/8-2). We thank SSRF (Shanghai Synchrotron Radiation Facility P. R. China) for providing beam time.

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Footnotes

Electronic supplementary information (ESI) available. See DOI: 10.1039/c9tc06456g
Present address: School of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, UK.

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