Field-induced inversion of chirality in SmCP_A phases of new achiral bent-core mesogens

Abstract

A new series of achiral bent-core mesogens is presented. The mesophase behaviour has been investigated using differential scanning calorimetry, polarizing microscopy, X-ray diffraction, and dielectric and electro-optical measurements. Depending on the length of the terminal chains the homologues form nematic phases, conventional smectic phases (SmA, SmC) and polar smectic phases (SmCP_A). It was found that the switching mechanism of the SmCP_A phase depends on the temperature. At lower temperature the switching takes place in the usual way by rotation of the director around the tilt cone. At higher temperatures the switching is based on a collective rotation around the long axes which corresponds to a field-induced inversion of the layer chirality. Surprisingly the SmC phase also shows an unusual electro-optical response.

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Introduction

Since the initial investigation of Niori et al.¹ bent-core liquid crystals have been studied extensively. The reason for the particular interest in this field is not only the occurrence of new smectic mesophases which are different from analogous smectic phases of calamitic compounds. These phases possess a polar structure which is provided by a dense packing of their cores giving rise to a restricted molecular rotation around the long axes and to ferroelectric or antiferroelectric properties. Another reason is that some of these mesophases can develop spontaneous chiral order of the smectic layers. As first shown by Link et al.² the combination of director tilt and polar order (e.g. in the polar SmCP phase) gives the layer a chiral structure. In the so-called “racemic” state the chirality alternates from layer to layer, whereas in the “homochiral” state all layers within a domain have the same chirality (see Fig. S1 in the electronic supplementary information (ESI)†). In the majority of cases the SmCP phase has an antiferroelectric ground state which can be switched into ferroelectric states. The field-induced reorientation takes place through rotation of the director around the tilt cone similar to ferroelectric or antiferroelectric phases of calamitic compounds (SmC*, SmC_A*), see Fig. 1a. In contrast, in the polar SmAP phase, where the polar packed bent molecules are aligned orthogonal to the layer planes, the field-induced reorientation is based on the collective rotation of the molecules around their long axes.³ In principle, such switching should also be possible in the tilted SmCP phase. There are experimental hints that in SmCP-like phases^4–7and in columnar phases (B_1Rev, B_1RevTilted)⁸ the polar switching can be based on the rotation about the long molecular axes (Fig. 1b).

Fig. 1 Schematic representation of the mechanisms of polar switching in SmCP phases. The smectic layers are perpendicular to the drawing plane. The different molecule symbols (⊙ and ⊗) indicate opposite bent directions perpendicular to the drawing plane (that means, opposite polar axes). Filled and open molecule symbols correspond to layers of opposite chirality:² (a) Switching by a collective rotation of the molecules around the tilt cone, and (b) switching by a collective rotation of the molecules around their long molecular axes.

In this paper we present a series of new achiral bent-core five-ring compounds which exhibit nematic and non-polar and polar smectic phases. It will be shown that the switching mechanism in the tilted polar smectic phases depends on the temperature. At lower temperature the switching is based on the rotation of the director about the layer normal on the tilt cone. However, at higher temperatures the molecules adjust their dipole moments by rotation along their long axes which corresponds to a field-induced change of chirality.

Materials

The compounds under discussion are achiral 4,6-dichlororesorcinol derivatives bearing chlorine substituents at the terminal rings (Scheme 1). The compounds were prepared by esterification of 4,6-dichlororesorcinol with 4-(4-n-alkyloxy-3-chlorophenyliminomethyl)benzoic acids by means of dicyclohexylcarbodiimide (DCC). The transition temperatures and the transition enthalpies are summarized in Table 1

Scheme 1

Table 1 Transition temperatures in °C and transition enthalpies in kJ mol⁻¹ (in square brackets)

No.	n	Cr		SmCP^b		SmC^b		SmA		N^b		I
a This transition is not indicated by a calorimetric signal. b Parentheses designate monotropic phases.
5	5	•	128	—		—		—		(•	114)	•
			[29.7]								[0.8]
6	6	•	114	—		—		—		(•	113)	•
			[39.6]								[0.7]
7	7	•	115	—		(•	68)	—		•	111)	•
			[25.9]				[1.0]				[0.9]
8 ^9	8	•	117	(•	71	•	91	•	104	•	109)	•
			[65.9]		[0.5]		[—]^a		[0.2]		[1.3]
9	9	•	101	(•	75)	•	105	•	112	—		•
			[35]		[0.5]		[—]^a		[3.2]
10	10	•	103	(•	74)	•	112	•	118	—		•
			[35.1]		[0.4]		[—]^a		[4.7]
12	12	•	106	(•	73)	•	121	•	122	—		•
			[35.8]		[0.4]		[—]^a		[5.1]

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Results

Textural observations, X-ray investigations and electro-optical investigations

It is seen from Table 1 that the homologues with short terminal chains (compounds 5, 6) form a monotropic nematic phase only, which can be identified by its characteristic textures. Compound 7 shows, in addition, a monotropic SmC phase.

The octyloxy homologue 8 which was already described in ref. 9 exhibits, with the nematic phase, three polymorphic smectic phases. The high-temperature smectic phases could be assigned as SmA and SmC phases, respectively, on the base of their textures (smooth fan-shaped texture and homeotropic texture for the SmA phase; broken fan-shaped texture and schlieren texture for the SmC phase). At the transition into the low-temperature smectic phase the textures do not markedly change. As shown in ref. 9 the polar character of the low-temperature phase was proved by current response measurements which indicate an antiferroelectric ground state. It was found by dielectric measurements that already in the SmC phase a positive dipole correlation exists which increases with decreasing temperature, but obviously the polar clusters still have short-range order character.⁹

It follows from texture observations, X-ray diffraction and electro-optical measurements that the long-chain members of the series (compounds 9, 10, 12) show smectic trimorphism SmA–SmC–SmCP analogous to compound 8 (see Table 1). The smectic high-temperature phase (which exists for compound 12 in a very small temperature range) can be easily identified as a SmA phase by the characteristic fan-shaped texture (Fig. 2a) and by the homeotropic texture. The transition of the SmA phase into the SmC phase is not indicated by a calorimetric signal which points to a phase transition of second order. But this phase transition can be recognized by the clear change of the optical textures. At this transition the fan-shaped texture is transformed into a broken fan-shaped texture (Fig. 2b) whereas the homeotropic texture forms a schlieren texture.

Fig. 2 (a) Fan-shaped texture of the SmA phase, and (b) broken fan-shaped texture of the SmC phase of compound 10.

The transition from the SmC phase into the low-temperature phase can be detected by DSC while the textural changes are very small. The occurrence of the schlieren texture and the broken fan-shaped texture is a clear indication for a tilted biaxial phase.

X-Ray patterns of powder-like samples for all smectic phases show strong Bragg reflections in the small angle region and a diffuse scattering maximum in the wide-angle region (≈10°), which means these are smectic phases without in-plane order. In the SmA phase the layer spacing d is clearly smaller than the molecular length L_str (Table 2). The molecular length is related to the stretched conformation because, according to the NMR measurements, the bending angle is 170° in the SmA phase. Furthermore, the layer spacing d decreases only slightly on cooling into the biaxial smectic phases. From the layer spacings in the SmA phase (d_SmA) and SmC phase (d_SmC) the tilt angle ϑ could be estimated according to cosϑ = d_SmA/d_SmC (see Table 2).

Table 2 Layer spacing d, molecular length of the molecules in the stretched form L_str, length of the terminal aliphatic chain L_chain, and tilt angle ϑ in the SmC phase

n	d SmA/Å	d SmC/Å	L str/Å	L chain/Å	ϑ/° (SmC)
8	45.5	44.6	52	11.2	11
9	45.5	44.2	56.6	12.6	14
10	46	45.5	58	14.1	8
12	—	45.3	62.8	16.8	—

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The differences between the values of d and L_str are obviously in the order of magnitude of the length of the corresponding aliphatic chain (L_chain). This finding suggests an intercalated layer structure which was also found for the SmA, SmC, and SmCP phases of other bent 4,6-dichlororesorcinol derivatives.¹¹ We assume an interdigitation of the aliphatic chains in all smectic phases. Probably the bulky lateral chlorine substituents cause a kind of disturbance which makes the interdigitation possible.

From compound 8, filled in a glass capillary, we could obtain a well oriented sample by slow cooling from the isotropic in the nematic phase in presence of a permanent magnetic field. The X-ray pattern of the nematic phase shows the typical crescent-like diffuse scattering at the meridian and an outer diffuse scattering which is located around the equator (Fig. 3a). During the transition in the SmA phase the small-angle scattering becomes sharper and spot-like layer reflections and their second order reflections at the meridian of the pattern arise. The outer diffuse scattering remains unchanged (Fig. 3b). On cooling down the sample from the SmA phase into the low-temperature phases the third order of the layer reflections can also be seen. (Fig. 3c).

Fig. 3 X-Ray patterns of monodomains of (a) the nematic phase (107 °C), (b) SmA phase (103 °C) and (c) SmC phase (80 °C) of compound 8.

The analysis of the profile of the outer diffuse scattering (described in refs. 10 and 11) leads to two maxima shifted out by an angle of about 30°. That proves the existence of a small tilt angle ϑ ≈ 15° at 80 °C which agrees with the results of the electro-optical measurements. Unfortunately, X-ray studies on the low-temperature phase were not possible because of its early crystallization. On the other hand, in the electro-optical measurements the low temperature phase could be supercooled up to about 40–50 °C (depending on the homologue) so that this phase could be assigned by the electro-optical response. We obtained two current peaks per half period of the applied triangular voltage (Fig. 4) which are absent in the SmC phase. This experiment gives evidence that the low-temperature phase is a SmCP phase with an antiferroelectric groundstate (SmCP_A). The switching polarization of the SmCP_A phase of the compounds under discussion reaches values of P_S = 350–450 nC cm⁻².

Fig. 4 Switching current response of the SmCP_A phase of compound 10 using the triangular voltage method (cell thickness: 6 µm; 200 Vpp; 25 Hz; 72 °C, P_S = 345 nC cm⁻²).

Dielectric measurements

Dielectric investigations were carried out in the isotropic, SmA and SmC phases of compound 10. It should be noted that the SmA and SmC phase could not be oriented by magnetic fields; furthermore, measurements were impossible in the SmCP phase because of the early crystallization. Details of the experimental method and data processing are given elsewhere.⁹ Two dielectric absorption ranges were detected. The limits of the dielectric function and the corresponding relaxation frequencies are presented in the Figs. 5 and 6.

Fig. 5 Limits of the dielectric constants in the isotropic, SmA and SmC phases of compound 10.

Fig. 6 Relaxations times in the isotropic, SmA and SmC phases of compound 10.

The low frequency mechanism observed in all fluid phases is connected with the reorientation of the molecules about the long axes. Analogous to the homologue 8⁹ the dielectric increment Δ₁ = ε₀ − ε₁ increases starting from the SmA phase by cooling. This points to the increasing lateral cooperativity of the dipoles (ferroelectric short range order). The step of the relaxation time at the phase transition I–SmA reflects the stronger hindrance of dynamics which appears in the SmA phase and is also connected with the lateral interaction. Obviously, the second relaxation range is connected with the reorientation of the substituted phenylene units.

NMR measurements

NMR investigations were carried out on the isotropic, SmA and SmC phases of compound 10. The numeration of the positions in the molecule is given in Fig. 7. The angle ε is between the molecular long axis and the para-axis of the corresponding rings (A, B). The ¹³C-NMR spectra of the isotropic and SmA phase of compound 10 are shown in Fig. 8. The assignment of the lines rests on the comparison with structurally similar banana-shaped molecules (4,6-dichloro-substituted at the centre, but non-substituted at the outer rings)¹² for ring B and C and upon the increment system.

Fig. 7 Molecular structure and numeration of the atoms of compound 10. The angle ε is the angle between the molecular long axis ζ and the para-axis of the corresponding outer rings (A, B, A′, B′) whereas φ is the torsion angle. Bending angle α = 180° − 2ε.

Fig. 8 Isotropic ¹³C spectrum of compound 10 at 400 K (above) and the spectrum in the SmA phase 2 K below the clearing point (below). Both spectra are obtained with single pulse excitation. Simple high power decoupling with 120 scans is used for the SmA spectrum. The assignment uses the numeration of Fig. 7.

The transition to the liquid crystalline phases occurs within 1 K. Fig. 8 shows a ¹³C spectrum in the SmA phase observed by continuous decoupling at the ¹H resonance of the aromatic protons. The important C9 resonance sadly overlaps with the –CHN line and limits the resolution. C10 gives small lines in the CP spectra. The temperature dependence in the SmA and SmC phase is small.

As shown earlier the simplest approximation for the ¹³C chemical shift and the macroscopic order parameter S

δⁱ_obs(T) = δⁱ_iso + Sδⁱ_ζζ

can be used for the interpretation of the data. δⁱ_ζζ are the components of the shift tensor in the molecular system ξ, η, ζ, with ζ as molecular long axis (Fig. 7). In this frame the geometry of the central ring C is fixed and can be used to define the order parameter for the whole molecule. The evaluation of the order parameter S from the shifts of C9 and C10 requires the knowledge of δ⁹_ζζ and δ¹⁰_ζζ. In accordance with the literature¹² we used δ⁹_ζζ = 72 ppm (δ⁹₁₁= 91 ppm, δ⁹₂₂ = 15 ppm, angle 30° between δ⁹₁₁ and long axis) and δ¹⁰_ζζ = 66.8 ppm. The value δ¹⁰_ζζ results from the ratio of the anisotropic shifts of C9 and C10. The obtained S(T) for compound 10 is plotted in Fig. 9. The order parameter adopts a value of 0.55 at the clearing point and increases only weakly in the SmA phase. The transition into the SmC phase is connected with a strong increase in S with decreasing temperature.

Fig. 9 Order parameter S in the SmA and SmC phase of compound 10 calculated from the anisotropic shifts of C9 and C10 in the central ring C.

The evaluated order parameter describes the ordering of the long molecular axis. Using S(T) we can extract the δⁱ_ζζ(T) for the Cⁱ belonging to the rings of the branches and the linkage groups. The values agree with earlier results of bisubstituted resorcinol derivatives for the ring B and C and the –CHN– and –COO– linkage groups (Table 3).¹² At the moment we cannot do the same procedure for ring A since the Cl substitution changes the isotropic shifts and the normalized tensor components, the smallest influence should be at meta- positions of the Cl (2 and 4). These tensor values agree with the reference molecule of earlier investigations¹² as shown in Table 3. The geometrical relation (angle ε and torsion angle φ, Table 4) between the main frame and the molecular frame must therefore be the same for both molecules. The overall bending angle α of the molecule is related to ε according to α = 180° − 2ε. In the SmA phase the bending angle was found to be α ≈ 170° which corresponds to a nearly linear shape of the molecule. In the SmC phase the bending angle slightly decreases up to α ≈ 160°; in the SmCP phase NMR measurements are impossible for the same reason as for the X-ray and dielectric measurements. The geometry of the linkage group COO at ring C is important for the relatively stretched shape of the molecules. Its tensor component in the reference compound¹² and the compound under discussion is noticeably smaller than in the rodlike and bent molecules with a strong bending.^10,13 The strong deformation of the ester linkage group at the dichloro-substituted ring C generates similar to the reference molecule¹² a nearly linear molecule with the typical liquid crystalline phases of calamitic compounds. Since peaks of symmetry-related equivalent carbons coincide exactly for all temperatures the observed averaged conformation of the two lateral branches is identical beginning with the COO-linkage group. The plane of the ester group aligns nearly perpendicular to the plane of ring C. The large neighbouring chlorine substituent favours this torsion and also a second rotation that creates the extended molecular shape.

Table 3 The tensor components δⁱ_ζζ of dichloro-substituted resorcinol derivative¹² (a) and compound 10 (b)

Position	δ ^i ζζ /ppm (a)	δ ^i ζζ /ppm (b)
1	81.6	71.5
2	35.4	34.2
2′	—	21.9
3	32.4	37.1
3′	—	19.5
4	83.7	82.9
5	91.7	92.3
6	36.3	38.3
7	38.5	40.7
8	89.5	90.4
9	72	72.0
10	66.4	66.6
COO	26.6	29.5
Z	54.0	52.2

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Table 4 Estimated angles ε_A, ε_B between the long axis and the para-axes of rings A and B respectively and the torsion angle φ_seg between the plane of ring C and the corresponding molecular segments

ε B	φ B	ε A	φ A	φ COO	φ CHN
5–10°	0–30°	5–10°	40°	110°	65°

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Investigation of the switching mechanism

It was shown in the previous section that according to the electro-optical studies the polar SmC phase is an antiferroelectric one. But in the case of compounds 9, 10 and 12 the switching mechanism of the SmCP_A phase is quite unusual which is discussed in the example of compound 10. If an electric field is applied in the SmCP_A phase the texture of the switched state is identical with that of the off-state; that means neither the birefringence nor the extinction direction is changed during the switching process. This effect is identical for an opposite sign of the applied field. During the switching in most cases an intermediate state is visible, the occurrence of which depends on the rate of the change of the field strength and on the temperature. The slower the rate of change (increase or decrease) of the voltage the shorter is the appearance of the intermediate state. For example, if in the switched state the field is removed very fast, the relaxation into the off-state needs some time which increases with decreasing temperature; for example, the relaxation time is less than 1 s at 80 °C and about 5 s at 70 °C. Below 65 °C the electro-optical behaviour is completely changed. Now the textures are clearly different for an opposite sign of the applied field and also the field-free state is different from that observed above 65 °C. During the switching the extinction direction rotates by ±20° depending on the polarity of the field which corresponds to a tilt angle of about 20°. This can be understood by an anticlinic antiferroelectric ground state which is switched about the tilt cone into a synclinic ferroelectric state and vice versa (SmC_AP_A ↔ SmC_SP_F). It is interesting that within a limited temperature interval the field-induced texture change is very complicated pointing to the coexistence of two mechanisms.

How we can explain that above 65 °C the optical textures remain unchanged during the switching process although the macroscopic polarization is inverted by the applied electric field? Such switching is not only observed for a very slow frequency of the applied voltage as reported in ref. 4 but also for a fast switching. In order to get a deeper insight into this unusual electro-optical response we carried out the following experiment which is illustrated in Fig. 10.

Fig. 10 Electro-optical response in the SmCP_A phase of compound 10: The field-induced textures were observed between crossed polarizers. In the dark domains the director is parallel to one polarizer, in the bright domains the director is shifted by about 2ϑ (ϑ: tilt angle). Book-shelf geometry is provided where the smectic layers are perpendicular to the drawing plane. (a) Texture of the switched ferroelectric state at 60 °C which exhibits domains of opposite chirality (electric field: −15 V µm⁻¹). In the dark and bright domains the tilt of the director is opposite whereas the polar axes are unidirectional (perpendicular to the drawing plane). (b) Texture of the switched state at 60 °C (+15 V µm⁻¹): The inversion of the field polarity leads to a rotation of the director around the tilt cone giving rise to a shift of the director by 2ϑ. The chirality of the domains does not change during the switching process which is indicated by the same colour of the molecule symbols. (c) Texture of the switched state at 72 °C (+15 V µm⁻¹): Since the polarity of the applied field is the same as in (b) the polar axes, the tilt direction and therefore also the texture remain unchanged. (d) Texture of the switched state at 72 °C (−15 V µm⁻¹): The reversing of the field direction leads now to an inversion of the polar axes in comparison to (c). Since above 65 °C the polar switching takes place through a collective rotation around their long molecular axes, the tilt direction and therefore the texture does not change. However, the inversion of the polar axes is accompanied by an inversion of the chirality. This fact is assigned by a different colour of the molecule symbols in comparison with (a). On cooling in the presence of a field of −15 V µm⁻¹ below 65 °C the texture is complementary to that at the beginning.

1. On applying an electric field of −15 V µm⁻¹ at 60 °C a texture with bright and dark domains arises which correspond to domains of opposite handedness (Fig. 10a).

2. On applying an electric field of +15 V µm⁻¹ at 60 °C a texture with complementary chiral domains arises (Fig. 10b).

3. In the presence of the field of +15 V µm⁻¹ the sample is heated to 72 °C. When the polarity of the field is opposite (−15 V µm⁻¹) the texture remains unchanged (Fig. 10c and Fig. 10d)

4. After that the sample is again cooled down to 60 °C in the presence of −15 V µm⁻¹. Now the texture is the same as that observed at the beginning for an opposite sign of the applied electric field (that means, it corresponds to that of Fig. 10b).

That means that at 72 °C the inversion of the field polarity from +15 V µm⁻¹ to −15 V µm⁻¹ has led to the rotation of the molecules around the long axes and therefore to a rotation of the polar axis by 180°. This experiment gives evidence that the polar switching above 65 °C is based on a collective rotation of the molecules about their long axes. Therefore the texture of the off-state and the on-state is identical and also the inversion of the applied field does not change the texture. But if the polarity of the field is changed in the last step of our experiment it is plausible that now, with the same polarity of the field as to the beginning, a texture with complementary domains occurs, which means that the bright domains are now dark and vice versa (see schematic representation of the field-induced orientation of the molecules in Fig. 10).

Also the SmC phase shows in some respects an unusual electro-optical response. Starting from a planar oriented SmA phase with a uniform director alignment parallel to the substrates, the SmC phase adopts a texture with small domains which have a uniform colour when one of the crossed polarizers is parallel to the original direction of the director. But if one polarizer (or the heating stage) is rotated by a small angle (clockwise or anti-clockwise) two kinds of domains (bright and dark) can be distinguished. A close examination of the extinction direction gives evidence that in these domains the director is shifted by a definite angle (±ϑ) with respect to the direction of the director in the SmA phase; this angle corresponds to the tilt angle (provided that we have book-shelf geometry) which is 5° at 110 °C and 20° at 75 °C for compound 10.

By application of a sufficiently high electric field (E > 1.5–3 V µm⁻¹) depending on the temperature the small domains of the SmC phase coalesce to larger ones but the direction of the director in the complementary domains does not change. It is remarkable that by application of a d.c. or a.c. field a clear switching can be observed but this switching is not accompanied by a noticeable texture change (no change of the domain walls or the birefringence). The lack of optical changes upon applying the electric field can be understood on the base of the dielectric properties which were thoroughly studied on compound 8.⁹ It was found that the SmC phase possesses a strong negative dielectric anisotropy Δε = ε₃ − ε₁ (ε₃, ε₁: dielectric permittivities along the director and perpendicular to the director in the tilt plane, respectively): Δε = −5 (at 91 °C) and Δε = −11 (at 71 °C). Considering the molecular shape and the positions of the polar groups in the compounds under discussion we can also assume a pronounced positive dielectric biaxiality δε = ε₂ − ε₁ where ε₂ is the dielectric permittivity along the C₂ symmetry axis. The application of an electric field perpendicular to the director induces a Freedericksz transition characterized by a rotation of the director around the tilt cone. In the case ε₂ > ε₁ one can expect that the reorientation takes place for all regions where the direction of ε₂ does not coincide with the field direction. But this field-induced rotation of the director would lead to a clear change of the birefringence and the extinction direction in spite of the relatively small tilt angle. This expectation is in contrast to our experimental results. A plausible explanation could be that the dielectric reorientation, which is based on the coupling of the field with the effective dielectric anisotropy (ε₂ − ε₁), can also take place through a collective rotation of the molecules around the long molecular axes. In this context it should be noted that at the transition into the SmCP_A phase the texture with complementary domains and the optical appearance of the switching remain unchanged. The only difference is that now the switching is accompanied by a current response.

Discussion

There are homologous series of bent-core mesogens the members of which form only typical “banana phases” such as SmCP, B₁, B₆ or B₇.^13–19 But there are other homologous series where the short-chain members exhibit nematic or conventional smectic phases (SmA, SmC) whereas the long-chain members are able to form nematic and conventional smectic phases as well as “banana phases”. For example, bent-shaped compounds with the following phase sequences are reported in the literature: N–SmCP_A,^20–22 SmA–SmCP_A,^10,11,20,23 SmA–SmC–SmCP_A,^10,11,23 N–SmA–SmC–SmCP_A,^10,23 SmA–SmAP_A.^3,24,25 It was found by NMR measurements that in such compounds the opening angle (the angle between the two wings) of the bent molecules is clearly higher than the expected 120°; it amounts to between 140–160°.^10,11,20 Such an enhanced bending angle and its temperature dependence is obviously an important precondition for polymorphism variants with nematic and conventional smectic phases as well as “banana phases”. For five-ring bent mesogens a significant enhancement of the opening angle was observed when the central core is substituted by a cyano group in the 4-position¹⁰ or by chlorine in 4,6-position.¹¹ This could explain that the short-chain members exhibit a nematic phase only, whereas the long-chain homologues form both polar smectic phases (SmCP) and non-polar smectic phases (SmC, SmA). We can imagine that at higher temperatures the bent molecules can freely rotate around the molecular long axes so that SmA, SmC or nematic phases can be formed. With decreasing temperature the opening angle decreases, but also the free rotation is increasingly hindered giving rise to a polar packing which is characteristic for a SmCP phase. From dielectric measurements it follows that already in the SmC phase a positive dipole correlation arises which increases with decreasing temperature, but this dipole correlation still has short range order character.

In the SmCP_A phase of compounds 9, 10, and 12 we found an unusual switching behaviour. The usual switching mechanism based on the field-induced rotation of the director around the tilt cone was only observed at lower temperatures (below 65–60 °C). Above this temperature the polar switching takes place by a collective rotation around the molecular long axes. This mechanism of polar switching is not unknown in “banana phases”. For SmAP phases³ and B_1rev phases with SmAP-like layer fragments⁸ it is the only possibility for the polar switching. Some few examples were reported in the literature where this switching mechanism also occurs in SmCP phases^4–7 or B_1rev phases with SmCP-like layer fragments.⁸ In most cases this mechanism was found under special conditions. Nakata et al.⁵ and Bedel et al.⁶ detected such switching above a critical electric field. Schröder et al.⁴ and Reddy et al.⁷ observed this switching mechanism at very low frequency. In the compounds 9, 10, and 12 presented in this paper the occurrence of this switching mechanism does not markedly depend on the frequency or on a critical field strength, but it clearly depends on the temperature (which is also indicated in ref. 7). The reason for this temperature dependence is not yet known. It can be assumed that the rotational viscosity γ related to the rotation around the tilt cone (γ_φ) and the viscosity related to the rotation around the long molecular axes (γ_α) have a different temperature dependence. That would mean that for a given electric field E and a given polarization P at higher temperature γ_α is lower than γ_φ, whereas at lower temperature the situation is opposite. It should be emphasized that the collective rotation of the molecules around the long axes leads to a change of the layer chirality; that means, in these cases the layer chirality can be switched by the electric field.

The identical optical appearance of the switching process in the high-temperature range of the SmCP_A phase and in the SmC phase leads us to assume that the mechanisms are quite similar although the driving force must be different. If this assumption is correct the question arises whether a large dielectric biaxiality (which is probably due to the existence of short-range ferroelectric clusters) is a sufficient precondition for such a switching mechanism.

Experimental

Measurements

The phase transition temperatures were determined using a differential scanning calorimeter (DSC Pyris 1, Perkin-Elmer). The optical textures were observed through a polarizing microscope (Leitz Laborlux) equipped with a Linkam hot stage (THM 600/S). X-Ray diffraction measurements on non-oriented samples were performed using Guinier methods and a small-angle equipment with a linear detector. The investigation of oriented samples was done using a 2D detector (HI-Star, Siemens AG). Electro-optical properties were studied using commercial glass cells (E.H.C. Japan). The switching polarization was measured by means of the triangular wave voltage method.²⁶ Dielectric measurements were carried out in a frequency range of 1 Hz to 10 MHz using a Solartron Schlumberger Analyzer SI 1260 and a Chelsea interface. A brass cell coated with gold (spacing: 0.05 mm) was used as capacitor; the calibration was done using cyclohexane. ¹³C and ¹H-NMR measurements were performed using a Bruker MSL 500 spectrometer at a field of 11.7 T. The applied technique is described in earlier papers.^10,12,27 All measurements were carried out at decreasing temperature.

Synthesis of compound 10

A mixture of 0.35 g (2 mmol) 4,6-dichlororesorcinol, 1.66 g (4 mmol) 4-(3-chloro-4-decyloxyphenylimino)benzoic acid, 1.0 g (4.8 mmol) dicyclohexylcarbodiimide (DCC) and a small amount of 4-dimethylaminopyridine as catalyst in 100 ml dichloromethane was stirred for 24 h at room temperature. The precipitate of dicyclohexylurea was separated. The solvent was evaporated and the crude material was recrystallized from a DMF–ethanol mixture several times. Yield: 0.63 g (37.6%).

¹H-NMR (400 MHz, CDCl₃, δ/ppm): 0.87 (m, 6H, 2 × CH₃), 1.27–1.53 (m, 28H, 14 × CH₂), 1.84 (m, 4H, 2 × OCH₂CH₂), 4.04 (t, ³J = 6.4 Hz, 4H, OCH₂), 6.94 (d, ³J = 8.9 Hz, 2H, Ar–H), 7.19 (dd, ³J = 8.7 Hz, ⁴J = 2.5 Hz, 2H, Ar–H), 7.38 (d, ⁴J = 2.5 Hz, 2H, Ar–H), 7.41 (s, 1H, Ar–H), 7.64 (s, 1H, Ar–H), 8.01 (d, ³J = 8.5 Hz, 4H, Ar–H), 8.28 (d, ³J = 8.3 Hz, 4H, Ar–H), 8.53 (s, 2H, CH=N).

Elemental analysis for C₅₄H₆₀O₆N₂Cl₄, MW = 974.89, Calc.: C 66.53, H 6.20, N 2.87, Cl 14.55; Found: C 66.87, H 6.04, N 2.93, Cl 14.19. MS (m/z): 974 (2), 398 (100), 364 (16), 258 (26), 229 (25). IR (CCl₄, ν/cm⁻¹): 1754.9 (C=O), 2928 (C–H), 1097 (C–Cl)

References

1. T. Niori, T. Sekine, J. Watanabe, T. Furukawa and H. Takezoe, J. Mater. Chem., 1996, 6, 1213.
2. D. A. Link, G. Natale, R. Shao, J. M. Maclennan, N. A. Clark, E. Körblova and D. M. Walba, Science, 1997, 278, 1924.
3. A. Eremin, S. Diele, G. Pelzl, H. Nadasi, W. Weissflog, J. Salfetnikova and H. Kresse, Phys. Rev. E: Stat. Phys., Plasmas, Fluids, Relat. Interdiscip. Top., 2001, 64, 51707.
4. M. W. Schröder, S. Diele, G. Pelzl and W. Weissflog, ChemPhysChem, 2004, 5, 99.
5. M. Nakata, R.-F. Shao, W. Weissflog, H. Takezoe and N. A. Clark, Ferroelectric Liquid Crystal Conference, Dublin, 2003, poster.
6. J. P. Bedel, J. C. Rouillon, J. P. Marcerou, H. T. Nguyen and M. F. Achard, Phys. Rev. E: Stat. Phys., Plasmas, Fluids, Relat. Interdiscip. Top., 2004, 69, 61702.
7. R. Amaranatha Reddy, M. W. Schröder, M. Bodyagin, H. Kresse, S. Diele, G. Pelzl and W. Weissflog, Angew. Chem., 2005, 117, 784.
8. J. Szydlowska, J. Mieczkowski, J. Maraszek, D. W. Bruce, E. Gorecka, D. Pociecha and D. Guillon, Phys. Rev. E: Stat. Phys., Plasmas, Fluids, Relat. Interdiscip. Top., 2003, 67, 31702.
9. H. Kresse, H. Schlacken, U. Dunemann, M. W. Schröder, G. Pelzl and W. Weissflog, Liq. Cryst., 2002, 29, 1509.
10. I. Wirth, S. Diele, A. Eremin, G. Pelzl, S. Grande, L. Kovalenko, N. Pancenko and W. Weissflog, J. Mater. Chem., 2001, 11, 1642.
11. A. Eremin, H. Nadasi, G. Pelzl, S. Diele, H. Kresse, W. Weissflog and S. Grande, Phys. Chem. Chem. Phys., 2004, 6, 1290.
12. W. Weissflog, C. Lischka, S. Diele, G. Pelzl, I. Wirth, S. Grande, H. Kresse, H. Hartung and A. Stettler, Mol. Cryst. Liq. Cryst., 1999, 333, 203.
13. G. Pelzl, S. Diele and W. Weissflog, Adv. Mater., 1999, 11, 707.
14. T. Sekine, T. Niori, M. Sone, J. Watanabe, S.-W. Choi, Y. Takaniski and H. Takezoe, Jpn. J. Appl. Phys., 1997, 36, 6455.
15. G. Pelzl, S. Diele, S. Grande, A. Jákli, C. Lischka, H. Kresse, H. Schmalfuss, I. Wirth and W. Weissflog, Liq. Cryst., 1999, 26, 401.
16. J. P. Bedel, J. C. Rouillon, J. P. Marcerou, M. Laguerre, M. F. Achard and H. T. Nguyen, Liq. Cryst., 2000, 27, 103.
17. H. N. Shreenivasa Murthy and B. K. Sadashiva, Liq. Cryst., 2002, 29, 1223; H. N. Shreenivasa Murthy and B. K. Sadashiva, J. Mater. Chem., 2003, 13, 2863.
18. R. A. Reddy and B. K. Sadashiva, Liq. Cryst., 2003, 30, 1031.
19. G. Pelzl, S. Diele, A. Jákli, C. Lischka, I. Wirth and W. Weissflog, Liq. Cryst., 1999, 26, 135.
20. W. Weissflog, H. Nadasi, U. Dunemann, G. Pelzl, S. Diele, A. Eremin and H. Kresse, J. Mater. Chem., 2001, 11, 2748.
21. R. A. Reddy, B. K. Sadashiva and S. Dhara, Chem. Commun., 2001, 1972.
22. M. W. Schröder, S. Diele, G. Pelzl, U. Dunemann, H. Kresse and W. Weissflog, J. Mater. Chem., 2003, 13, 1877.
23. M. W. Schröder, S. Diele, G. Pelzl, N. Pancenko and W. Weissflog, Liq. Cryst., 2002, 29, 1039.
24. M. W. Schröder, S. Diele, N. Pancenko, W. Weissflog and G. Pelzl, J. Mater. Chem., 2002, 12, 1331.
25. H. N. Shreenivasa Murthy and B. K. Sadashiva, Liq. Cryst., 2004, 31, 567.
26. K. Miyasato, S. Abe, H. Takezoe, A. Fukuda and E. Kuze, Jpn. J. Appl. Phys., 1983, 22, L661.
27. S. Diele, S. Grande, H. Kruth, C. Lischka, G. Pelzl, W. Weissflog and I. Wirth, Ferroelectrics, 1998, 212, 169.

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Footnote

† Electronic supplementary information (ESI) available: Schematic representation of the organisation of bent-core molecules in adjacent layers of tilted polar smectic phases. See http://www.rsc.org/suppdata/jm/b4/b415513k/

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