New five-ring bent-core mesogens that possess only ester connecting groups between the aromatic rings and different lateral substituents at the central phenyl ring are presented. The mesophases have been assigned by polarizing microscopy, differential scanning calorimetry, X-ray diffraction and electro-optical measurements. It is shown that the mesophase behaviour depends strongly on the position of the lateral substituents. Compounds, which are derived from 4-cyano-, 4-chloro- and 4,6-dichloro-resorcinol, show polymorphism variants where polar phases (SmAP, SmCP) occur together with nematic and conventional smectic phases, e.g. SmA–SmAP, SmA–SmCSPA–Colob–SmCSPA, N–SmA–SmCPA, SmA–SmC–SmCPA and SmC–SmCPA. On the basis of the behaviour of two series of materials, the occurrence of different polar-switching mechanisms could be demonstrated. Apart from the usual mechanism by director rotation around the tilt cone, the polar switching can also take place through collective rotation of the molecules around their long axes, which corresponds to a field-induced switching of the layer chirality. A remarkable finding is the polar switching in the crystalline modification of long-chain, bent-core compounds with a methyl group in 2-position, which is accompanied by a clear change of the optical texture and by a relatively high switching polarization (approx. 600 nC cm−2). It was found for selected bent-core compounds that, above the transition temperature of a polar to a non-polar phase, the non-polar phase can be transformed to the polar phase by application of an electric field, which was proved for the transitions isotropic–SmCPF, SmA–SmCPF and isotropic–CrII polar.
Niori et al. (1996) found that liquid crystalline compounds with bent molecular shape are able to form unusual mesophases with polar order. Since then, bent-core mesogens have developed into a fascinating field of liquid crystal (LC) research. Owing to their bent shape, the molecules can be packed tightly within smectic layers giving rise to a restricted rotation around the long molecular axes. In this way, a polar axis parallel to the layers results, provided that the molecules possess a lateral dipole moment. In the polar SmA phase (designated as SmAP where P stands for polar), the molecules are arranged, on an average, perpendicular to the layer planes, whereas in the most frequently observed polar SmC phase (designated as SmCP), the molecules are tilted with respect to the layer normal. If the polar axes are the same in adjacent layers, the SmAP or SmCP phase behaves to be ferroelectric (FE). If the polar axes alternate from layer to layer, the groundstate is antiferroelectric (AFE). In the case of the SmCP phase, four distinct structural variants can be distinguished depending on the stacking of the molecules in adjacent layers (Link et al. 1997). A synclinic or an anticlinic arrangement of the molecules can occur which is indicated in the code letter by the suffixes ‘S’ or ‘A’ after C. On the other hand, FE structures (polar axes parallel) or AFE structures (polar axes antiparallel) are designated by the suffixes ‘F’ and ‘A’, respectively, after P. In figure 1, these structural variants are schematically presented: SmCSPA, SmCAPF, SmCAPA and SmCSPF. It should be emphasized that in most cases, the groundstate is AFE in order to escape from a bulk polarization. In addition to the polar structures, another aspect is of fundamental interest, which was first discussed by Link et al. (1997). The combination of director tilt and polar order leads to a chirality of the smectic layers, even though the constituent molecules are achiral; this can be regarded as a ‘superstructural’ chirality (Amaranatha Reddy & Tschierske 2006). It is seen from figure 1 that there are two equivalent layer structures with antiparallel polar axes, which are mirror images of each other. These enantiomeric structures are indicated by filled (black) and open (white) molecule symbols, respectively. In the so-called ‘racemic’ states, the chirality alternates from layer to layer (SmCSPA and SmCAPF), whereas in the so-called ‘homochiral’ states, the chirality is the same in adjacent layers (SmCAPA and SmCSPF; figure 1).
It is also indicated in figure 1 that by the application of an electric field, the AFE groundstate can be switched to the corresponding FE states: SmCSPA to SmCAPF and SmCAPA to SmCSPF. If the groundstate is FE, the switching takes place between opposite FE states. In general, this switching is based on the collective rotation of the molecules around the tilt cone—like in FE SmC* and AFE SmCA* phases of calamitic compounds.
The materials presented here are five-ring bent-core compounds containing only carboxylic connecting groups between the aromatic rings. These materials are suitable to demonstrate the strong influence of lateral substituents attached at the central phenyl ring on the mesophase behaviour of banana-shaped molecules, which can result in new phases and unusual phase sequences. Not only is the polymorphism of mesophases of interest, but also the physical properties, which can be different in phases having the same phase symbol. It will be shown that this concept of molecular design is a successful way to prepare LCs exhibiting polar mesophases typical of bent-core mesogens, as well as nematic and conventional smectic phases, characteristic for calamitic mesogens. To date, such mesophase behaviour has been observed in only a few classes of bent-core compounds (Weissflog et al. 2001; Dunemann et al. 2005). Furthermore, it is remarkable that the isotropic, nematic, smectic as well as crystalline phases formed by bent-core mesogens can exhibit unusual physical properties as proved by electro-optical studies.
2. Materials and their liquid crystalline behaviour
The new compounds under study have been prepared by the esterification of substituted resorcinols with the corresponding 4-(4′-alkyloxyphenoxycarbonyl)benzoic acids using N,N′-dicyclohexylcarbodimide in the presence of 4-(N,N-dimethylamino)pyridine as catalyst in dichloromethane as described for the parent series (Amaranatha Reddy et al. 2005). The transition temperatures of the substituted 1,3-phenylene bis[4′-(4″-alkyloxyphenoxycarbonyl)benzoates] are listed in tables 1–7. Below the transition temperatures, the transition enthalpies are given (kJ mol−1). The abbreviation of the compounds is obtained from the length of the terminal chains connected by a hyphen to the position and type of the lateral substituents. The isomeric compounds listed in table 8 have been synthesized using the same procedure; however, substituted resorcinols were reacted with 4-(4′-dodecyloxybenzoyloxy)benzoic acid. The abbreviations of the isomeric compounds are I-n- (table 8).
(a) Resorcinol derivatives n-H (R2–R6H)
In table 1, three members of the non-substituted parent series that do not bear lateral substituents are listed. The mesophase behaviour and the physical properties of the complete homologous series have been reported recently (Amaranatha Reddy et al. 2005). All the homologues exhibit a polar SmCP phase with an undulated layer structure (). It is of interest to note that the electro-optical behaviour changes in the homologous series. The phase of the short-chain members (n=5–9) show AFE behaviour, whereas the phase of the longer chain homologues (n>9) exhibits a FE switching. This series represents the first example having such a drastic change of polar properties with increasing length of the terminal chains. It is remarkable that in the FE phase of the longer chain members, two different mechanisms of switching can be distinguished, which depend on the temperature as well as on the frequency of the electric field, discussed in §4.
(b) 5-Fluororesorcinol derivatives n-5F (R5F)
The compounds, which have fluorine at the apex position, are discussed here. For all the three homologues, only one type of mesophase has been observed (table 2). On cooling the sample slowly from the isotropic phase, a schlieren texture or a fan-shaped texture with irregular stripes appears, which is an indication for an SmCPA phase.
The X-ray diffraction (XRD) measurements gave evidence that this phase has a layer structure without in-plane order and a tilted arrangement of the molecules in the smectic layers. The layer thickness for compound 16-5F amounts to 51.5 Å. On applying a triangular wave electric field, two peaks per half period were recorded indicating an AFE groundstate. The polarity of the applied field has no effect on the optical texture of the mesophases, which is typical for the racemic groundstate, where the chirality alternates from layer to layer, i.e. the switching corresponds to the transition from SmCSPA to SmCAPF (figure 1). The value of the spontaneous polarization was found to be 685, 660 and 675 nC cm−2 for compounds 8-5F, 12-5F and 16-5F, respectively.
(c) 2-Nitroresorcinol derivatives n-2NO2 (R2NO2)
These homologous compounds (see table 3) possess the strong electron-withdrawing nitro group in 2-position, i.e. in the obtuse angle of the bent molecules. On cooling the isotropic liquid, beautiful spiral filaments were obtained (a photomicrograph is shown in figure 2).
Such a texture is an indication that the phase might be a B7 phase. The powder XRD pattern of compound 12-2NO2 shows several incommensurate reflections in the small-angle region and a diffuse scattering maximum in the wide-angle region. This rules out a simple layer structure and is in agreement with the pattern of some B7 phases described to date in the literature (Pelzl et al. 1999, 2004; Bedel et al. 2000; Shankar Rao et al. 2001; Amaranatha Reddy & Sadashiva 2002; Coleman et al. 2003; Folcia et al. 2005; Novotna et al. 2005a). No electro-optical response could be detected even at rather high electric fields.
(d) 2-Methylresorcinol derivatives n-2CH3 (R2CH3)
To study the effect of electron-donating groups in the same position, 2-methylresorcinol derivatives have been prepared. As in table 4, compounds form only crystalline phases. The crystalline state is proved by the X-ray pattern of partially oriented samples (obtained by slowly cooling a drop of the isotropic liquid on a glass plate), which show several strong reflections in the small-angle region as well as in the wide-angle region (Pelzl et al. 2006). Surprisingly, for the homologues with the longer terminal chains, 14-2CH3, and 18-2CH3, polar switching could be detected, which will be discussed in §4 in more detail.
(e) 4-Cyanoresorcinol derivatives n-4CN (R4CN)
The homologues 8-4CN and 12-4CN show smectic dimorphism (see table 5), as reported recently by Kovalenko et al. (2005). The high-temperature phase is a smectic A phase, which appears as a homeotropic or fan-shaped texture. On cooling down the homeotropic texture of the SmA phase, the low-temperature phase forms a fluctuating schlieren texture, whereas the fan-shaped texture remains nearly unchanged. Only irregular stripes grow on further cooling (figure 3), which disappear on applying an electric field. In addition, the birefringence of the fan-shaped texture is clearly changed by the application of an external field; however, this field-induced texture is identical for the opposite polarity of the applied field. The current response under a triangular voltage shows two peaks per half period, which indicates an AFE groundstate. The switching polarization was found to be 640 and 320 nC cm−2 for compounds 8-4CN and 12-4CN, respectively (Kovalenko et al. 2005). The X-ray pattern obtained from an oriented sample indicates that the molecular long axes are, on an average, perpendicular to the layer planes. Since both the phases exhibit a liquid-like order in the layers, the high-temperature phase is a uniaxial SmA phase, whereas the low-temperature phase can be assigned as an AFE polar biaxial SmA phase (SmAPA). Both the phases are different only in the existence and hindrance, respectively, of a free rotation of the molecules around their long axes. Such a phase sequence was also reported by Eremin et al. (2001), Schröder et al. (2002) and Shreenivasa Murthy & Sadashiva (2004).
(f) 4-Chlororesorcinol derivatives n-4Cl (R4Cl)
It can be seen from table 6 that the homologues with shorter chains (8-4Cl; 12-4Cl) form an SmCPA phase, whereas the hexadecyloxy homologue 16-4Cl exhibits liquid crystalline tetramorphism SmA–SmCSPA–Colob–SmCSPA. Two aspects should be stressed here. In the last-mentioned compound, there is a combination of a conventional non-polar smectic A phase, together with polar smectic phases and a columnar phase. The electro-optical studies give a lot of interesting information about all the existing phases. For example, although the phase structure of both SmCSPA phases is identical, they differ in the mechanism of polar switching, which is explained in §4. Furthermore, the oblique columnar mesophase can be irreversibly transformed into the SmCPA by application of a sufficiently high electric field. On the other hand, we observed for the first time a reversible, field-induced transition from the non-polar SmA phase to the polar SmCPF phase above the transition temperature of SmCPA–SmA. Detailed information about this series is reported elsewhere (Shreenivasa Murthy et al. 2006).
(g) 4,6-Dichlororesorcinol derivatives n-4,6Cl (R4, R6Cl)
Depending on the length of the terminal chains, three different phase sequences have been observed for the homologues listed in table 7. It is remarkable that these homologues form the polar SmCP phase as well as nematic or conventional smectic phases (SmA and SmC) typical for calamitic mesogens. The octyloxy homologue 8-4,6Cl forms an SmCPA phase in addition to a nematic and an SmA phase. The longer chain homologues, 12-4,6Cl and 16-4,6Cl, exhibit the phase sequences SmA–SmC–SmCPA and SmC–SmCPA, respectively. For the transition SmA–SmC, no transition enthalpy is found by calorimetry, which is characteristic for a phase transition of second order. In contrast, the transition from the SmC into the SmCPA phase is accompanied by a small but clear calorimetric signal (0.9–2.5 kJ mol−1). The assignment of the mesophase is demonstrated for compound 8-4,6Cl. At first, the occurrence of the mesophases can clearly be distinguished by their characteristic optical texture. As shown in figure 4, the nematic phase shows a birefringent texture with characteristic defects. On cooling the sample, the smooth fan-shaped texture of the SmA phase and then the broken fan-shaped texture of the SmCPA phase appear (figure 4). Having a homeotropic alignment, the transition into the SmCPA phase is indicated by the appearance of a schlieren texture.
The phase sequence given for compound 8-4,6Cl is confirmed by the XRD measurements, for which well-oriented samples could be obtained. The small-angle region of the X-ray pattern displays the typical, diffuse, crescent-like scattering in the nematic phase (figure 5).
The scattering develops into quasi Bragg spots at the transition into to the smectic phases. The outer diffuse scattering is located around the equator of the pattern, where no significant change is observed at transition from the SmA phase into the SmCP phase (figure 6).
According to this, only a very small tilt angle can be expected; this is supported by the temperature dependence of the d-values (figure 7). Only a small decrease can be seen at the transition into the tilted SmCP phase. Using the ratio dSmCP/dSmA, a tilt angle of ca 8° could be estimated. Such small tilt angle could hardly be resolved by an analysis of the outer diffuse scattering.
An additional hint for a tilted alignment in the SmCP phase results from a more detailed check of the small-angle reflections (figure 5). The reflections are surrounded by diffuse lines (streaks) and the origin of such streaks is a translational movement of the molecules parallel to each other around an equilibrium position. Information about the direction of the movement, which should be parallel to the long axis of the molecules, can be obtained. In the SmA phase, the direction of the streaks is perpendicular to that of the layer reflections, i.e. the long axis of the molecules are aligned parallel to layer normal—as known from conventional SmA phases. In the SmCP phase, however, the streaks are inclined with respect to the layer reflections, which indicates that the molecules are now slightly tilted.
In the case of compound 12-4,6Cl, the layer spacing slightly decreases on cooling (SmA 49.3 Å; SmC 48.7 Å; SmCP 48.8 Å), whereas for compound 16-4,6Cl a slight increase is observed (SmC 51.5 Å and SmCP 53 Å).
The X-ray investigations gave evidence of the smectic layer structure, but cannot distinguish between polar and non-polar smectic phases. The polar character could be detected by current response measurements. As shown in figure 8, in the SmCP phase, two current peaks could be recorded per half period of the applied field, which is characteristic for an AFE groundstate. The switching polarization was found to be 460, 400 and 315 nC cm−2 for compounds 8-4,6Cl, 12-4,6Cl and 16-4,6Cl, respectively.
On cooling the isotropic liquid in the presence of a DC field of 3.5 V μm−1, circular domains appear in which the smectic layers are arranged cylindrically around the centre of the domains. In the field-off state, the extinction cross is parallel to the crossed polarizers, which indicates an anticlinic stacking of the layers. On applying an electric field, the extinction crosses rotate clockwise or anticlockwise to the corresponding FE state depending on the polarity of the applied field (figure 9), i.e. the switching corresponds to a transition from SmCAPA to SmCSPF.
3. Relationships between chemical structure and mesophase behaviour
The compounds under study are suitable materials to discuss some aspects of the relationships between chemical structure and liquid crystalline properties. Using a symmetric, parent mesogen bearing alkyloxy chains with 8, 12 and 16 methylene groups in both the terminal positions, the polymorphism can be changed by the attachment of small lateral substituents on different positions of the central phenyl ring. As listed in the tables 1–7, seven mesophases in different sequences could be found by such a variation in the series under study.
In some cases, the relationships agree with results reported in the original series of bent-core mesogens that contain azomethine-linking groups between the outer rings (Pelzl et al. 1999a,b; Weissflog et al. 2001), but there are also new aspects. The introduction of a nitro group between the two legs on the central ring leads to a B7 phase as expected. In contrast, the existence of a methyl group in the same position suppresses any mesophase in the new series. Only small substituents like fluoro atoms, attached at position 5—that means at the top of the molecules—are accepted without loss of the liquid crystalline behaviour. Attachment of chlorine atoms in the neighbourhood of the connecting groups (positions 4 and 6) increases the probability of finding nematic and conventional smectic phases. The reason for this tendency is an increase of the bending angle between the two legs of the molecules caused by such lateral substituents (Pelzl et al. 1999; Eremin et al. 2004). Having this in mind, it is understandable that the dichloro-substituted compounds show nematic, SmA and SmC phases together with the polar SmCP phase. A similar effect could be seen for a cyano group in 4-position of the central core. In this case, a transition from the non-polar uniaxial SmA phase to the polar biaxial SmAPA phase occurs, which was found only for a few bent compounds (Eremin et al. 2001; Schröder et al. 2002; Shreenivasa Murthy & Sadashiva 2004).
Another point is to compare the phase behaviour of the new compounds with isomeric compounds that differ only in the direction of the outer carboxylic groups. In table 8, the transition behaviour of the corresponding dodecyloxy-substituted homologues is listed.
A general tendency is remarkable, namely the clearing temperatures of all new compounds under study (tables 1–7) are higher by between 31 and 56 K than those of the analogous derivatives listed in table 8. Comparable phases exist for the laterally non-substituted compounds as well as for the esters derived from 2-nitro-and 5-fluororesorcinol. The substitution in 2-position by a methyl group suppresses mesophases in both the series. Furthermore, the tendency to form conventional smectic and nematic phases caused by substitution near to the connecting groups, which means in 4- or 6-position of the central ring, is conspicuous. For both the types of isomers derived from 4-chloro-, 4-cyano- and 4,6-dichlororesorcinol, the phase sequences are clearly different and also the isomeric compounds listed in tables 5–7 exhibit unusual phase sequences. For example, compound I-12-4Cl forms, apart from a nematic phase, an optically isotropic mesophase. This phase shows spontaneously randomly distributed domains of opposite handedness and a FE current response (Weissflog et al. 2004; Liao et al. 2005). Furthermore, compound I-12-4CN exhibits additionally to the nematic and the SmC phase, three polar SmCPA subphases (Kovalenko et al. 2005).
The new materials open the possibility not only to investigate the polar phases, but also to study conventional nematic, smectic and crystalline phases formed by bent-core mesogens.
4. Electro-optical studies
(a) Polar switching in SmAP and SmCP phases and field-induced reversal of layer chirality
In §1, it was shown that the field-induced transitions in SmCP phases are generally based on the rotation of the director around the tilt cone. In the case of the n-5F homologues, the switching corresponds to the transition from the racemic SmCSPA phase to the racemic SmCAPF phase, whereas in the case of the 4,6-dichlororesorcinol derivatives n-4,6Cl, the switching corresponds to the transition from the homochiral SmCAPA into the homochiral SmCSPF phase. It is seen from figure 1 that in both cases, the chirality of the smectic layers does not change (see also figure 10a). Recently, it was found that for a few bent-core compounds depending on the experimental conditions, the polar switching of the SmCP phase can take place by another mechanism, by the collective rotation of the molecules around their long axes (Szydlowska et al. 2003; Bedel et al. 2004; Schröder et al. 2004; Amaranatha Reddy et al. 2005; Gorecka et al. 2005; Novotna et al. 2005a,b; Weissflog et al. 2005a,b; Nakata et al. 2006). It should be emphasized that during this switching process, the chirality of the smectic layers is reversed by the applied field (figure 10b).
In the case of the undulated FE, phase of the non-substituted homologues n-H (n>9; table 1), the switching mechanism depends on the experimental conditions. At lower temperatures and higher frequencies of the applied field, polar switching occurs by the rotation of the director around the tilt cone. However, at higher temperatures and low frequencies, polar switching takes place through collective rotation of the molecules around the long axes.
In the undulated phase of compound 12-H, we observed an interesting electro-optical response that demonstrates impressively the field-induced reversal of chirality (Amaranatha Reddy et al. 2005). On cooling the isotropic liquid in the presence of a strong triangular electric field (±40 V μm−1; 35 Hz), a texture with many well-developed circular domains appears (figure 11a). In circular domains, the smectic layers are arranged cylindrically around the centre of the domains. The extinction crosses of the circular domains are tilted with respect to the crossed polarizers, which points to a tilt of the molecules in the layers. If an alternating field (±40 V μm−1; 35 Hz) is applied over some time (see snapshots in figure 11b,c), a new circular domain nucleates in the centre of the original domain. This new domain possesses opposite handedness indicated by opposite field-induced rotation of the extinction crosses (clockwise or anticlockwise). After a certain time (approx. 2 min) the original circular domain is completely transformed into a domain of opposite handedness. This process can be repeated several times. In this way, it is remarkable that the field-induced inversion of chirality can be directly visualized. The possible mechanism of this transition is discussed by Amaranatha Reddy & Tschierske (2006).
A similar temperature dependence of the switching mechanism, as reported for compound 12-H, was also observed for the SmCSPA phase of the longer chained homologues of the n-4Cl series, i.e. for compound 16-4Cl. In this case, the high-temperature SmCP phase possesses a synclinic, AFE groundstate (SmCSPA), which is switched to the synclinic FE SmCSPF phase by the rotation of the molecules around their long axes. In this case, the textures of the field-on and field-off states are identical and independent of the polarity of the applied field. Furthermore, this switching leads to an inversion of the layer chirality. In contrast, the re-entrant SmCSPA phase (table 6), which exists below the Colob phase, switches to the SmCAPF phase by the rotation of the molecules around the tilt cone (without change of the layer chirality). It should be noted that in the SmAP phase, the rotation of the molecules around the long axes is the only possibility for polar switching. This is true for the SmAPA phase of compounds 8-4CN and 12-4CN (table 5). Additionally, in this case, the texture of the switched state is independent of the polarity of the field. It should be stressed that no chiral layers exist in the SmAPA phase since the director is non-tilted with respect to the layer normal.
(b) A crystalline modification exhibiting polar switching
The homologues derived from 2-methylresorcinol (n-2CH3; table 4) are the only bent-core compounds under discussion that do not form a mesophase. On the other hand, the high-temperature solid modification of compounds 14-2CH3 and 18-2CH3 (designated as CrII) exhibits an unusual electro-optical response. On slowly cooling the isotropic liquid, this solid modification shows a colourful texture, which can be slightly deformed by a strong mechanical stress. By the application of a sufficiently high electric field (12–15 V μm−1), this texture can be switched, where the texture of the switched state is clearly different for an opposite sign of the electric field (figure 12 for compound 18-2CH3).
This is an indication of polar switching, which is further confirmed by current response measurements. As seen from figure 13, one current peak could be recorded per half period of an applied triangular voltage. The switching polarization determined from the current response was found to be 610 and 630 nC cm−2 for compounds 14-2CH3 and 18-2CH3, respectively, which is of the same order of magnitude as in switchable SmCP phases. At first sight, the results of the electro-optical studies point to a FE groundstate; but the switching is not really bistable (as expected for FEs) because the texture of the switched state slowly changes after the removal of the field. This finding suggests that the groundstate is probably AFE, but the relaxation to the AFE groundstate needs some time (2 min). It should be mentioned that the crystalline nature of the CrII phase was proved by XRD measurements, which yielded a pattern with several strong reflections in the small-angle as well as in the wide-angle region. The relatively high transition enthalpy is also a hint that the CrII phase is solid. The mechanism of the polar switching is not yet clear. It seems that the CrII phase has a structure with sufficient free volume to allow collective motions of molecules or parts of molecules. Such motions—also rotations of whole molecules—are not unknown in crystalline phases of organic compounds (Thompson & Pintar 1976; Reichert et al. 2000; Stumber et al. 2001).
(c) Field-induced transitions of non-polar into polar phases
It is interesting to note that non-polar phases formed by bent molecules (isotropic and SmA) can show an unusual electro-optical behaviour. This is demonstrated by the example of compound 12-5F, which exhibits an SmCPA–isotropic transition. If a sufficiently high electric field is applied above the clearing temperature (i.e. in the isotropic liquid), nuclei of the SmCPA phase appear (figure 14).
With increasing field strength, the whole field of view of the microscope is covered by a texture comparable to that known for SmCPA phases. If the field is switched off, the isotropic liquid state reappears immediately. We found that the threshold field necessary for the nucleation process increases with increasing temperature T, i.e. with increasing distance T−Tcl from the clearing temperature Tcl. In the case of compound 12-5F, this electro-optical effect could be proved up to 3 K above the clearing temperature by application of an electric field of ca 25 V μm−1. This field-induced nucleation effect was first reported by Bourny et al. (2002) and Weissflog et al. (2003). In the latter paper, a material was investigated consisting of molecules isomeric to compound 12-5F, i.e. I-12-5F (table 8). For this compound, the effect is much stronger and the field-induced polar phase can be observed up to 9 K above the clearing temperature, which is the highest value proved to date. Plotting T−Tcl versus the field intensity shows that the slope of the curve is nearly linear (Weissflog et al. 2003).
The field-induced transition of the isotropic liquid into the SmCPA phase can be understood if we assume that polar clusters already exist in the isotropic liquid in the short-range order regions. These clusters are aligned by the external electric field. This would explain why the field-induced phase disappears immediately on terminating the field. The formation of FE clusters already in the isotropic liquid was proved by dielectric measurements. On approximation to the transition temperature from isotropic to SmCPA, the static dielectric constant strongly increases indicating a positive dipole correlation (Kresse et al. 2001). There are also other examples reported in the literature that above the clearing temperature, clusters of the preceding mesophase (blue phase, twist grain boundary (TGB) phase, cubic phase) can exist (Goodby et al. 1995).
Another example for a field-induced transition of a non-polar to a polar phase is compound 16-4Cl, which exhibits the polymorphism variant SmA–SmCSPA–Colob–SmCSPA. Above the transition temperature, SmCSPA–SmA (i.e. in the existence range of the SmA phase), the SmA fan-shaped texture can be transformed to the texture of the SmCPF phase. During this transition, a current response typical for the AFE SmCPA phase could be detected. If the field is switched off, the smooth fan-shaped texture of the SmA phase reappears. This effect corresponds to a field-induced enhancement of the transition temperature, the maximum enhancement was found to be ca 2 K at a field of 35 V μm−1.
Surprisingly, for the compounds 14-2CH3 and 18-2CH3, an analogous behaviour was observed for the transition CrII to isotropic. By an electric field of ca 25 V μm−1, the nucleation of the CrII phase could be induced up to 2 K above the melting temperature. In this case, circular, oval, lancet-like and ribbon-like nuclei grow within the isotropic liquid (figure 15), which coalesce to a beautiful optical texture with further increasing field.
We believe that the field-induced formation of the polar SmCPA phase from the non-polar SmA phase, as well as the field-induced formation of the CrII phase from the isotropic phase, can also be explained by the alignment of polar clusters, which already exist in the non-polar phases.
In bent-core mesogens, the influence of lateral substituents on the mesophase behaviour is much more pronounced than in calamitic mesogens. In our paper, it is shown that the attachment of small substituents at the central core of a five-ring bent-core mesogen can lead to a diversity of mesophases and unusual polymorphism variants. Of particular interest are polymorphism with polar SmAP or SmCP phases as well as nematic or conventional smectic phases in different phase sequences. Depending on the structural features, some polar phases formed by bent-core molecules show unusual properties. For example, in SmCPA and SmCPF phases, an unusual mechanism of polar switching is observed which is based on the collective rotation of the molecules around their long axes. During this switching, the sign of the layer chirality is reversed by a reversal of the polarity of the applied field.
For the selected laterally substituted bent-core compounds, it was found that above the transition temperature from a polar to a non-polar phase by application of a sufficiently high electric field, the non-polar phase can be transformed to the polar phase (i.e. isotropic–SmCPF or SmA–SmCPF). Another unexpected finding is the proof of a polar switching in a solid modification of a bent-core compound, which is indicated by a clear change of the optical texture and a polar current response.
V. Percec (Department of Chemistry, University of Pennsylvania, USA). Your bent-core mesogens exhibited smectic mesophases in most cases. However, they all had a single alkoxy group on the periphery. What about 2 or 3? What phases do they form?
W. Weissflog. With increasing number of aliphatic chains at the terminal phenyl rings the tendency to form columnar phases is growing like polycatenar compounds of calamitic mesogens. Corresponding compounds having seven phenyl rings in the aromatic core have been reported by Ewa Gorecka.
V. Percec. Did you check the purity of the molecules found in the Vorländer collection that were made about one century ago? Just a curiosity.
W. Weissflog. The Vorländer collection is a real treasure in our institute. The final products of the different co-workers have been sorted in splendid cigar-boxes. The purity of most materials is surprisingly high. Using the original sample, for example, we could prove that one of these compounds indeed forms a ‘banana phase’ (B6), which is characteristic for bent-core mesogens.
M.-H. Li (Institut Curie, France). If we add an asymmetric carbon (C*) centre in banana-shaped molecules, what will happen?
W. Weissflog. In principle, the same types of mesophases can be formed as described for achiral banana-shaped LCs. It should be emphasized that in mesophases of chiral banana-shaped mesogens two sources of chirality exist: the molecular chirality caused by the asymmetric carbon atom and the layer chirality resulting from the combination of polar axis and tilt of the molecular long axis. In the case of bent-core compounds with chiral groups, there is a competition between these two kinds of chirality which gives rise to particular properties, e.g. the ratio between chiral domains of opposite handedness will not be balanced, but is shifted by the excess of one enantiomer. Such materials are of interest to be used as dopants.
M.-H. Li. Is it possible to find TGB phases or blue phases (frustrated phases) in bent-core molecules?
W. Weissflog. A solid-like phase depicted as B4 phase is described in the literature for which a TGB structure is assumed. On the other hand, some bent-core compounds form optically isotropic mesophases, the structure of which is still under investigation.
A. S. Matharu (Department of Chemistry, University of York, UK). Can you comment on the mechanism of field-induced crystallization? Is this a special form of electro-crystallization and can uniform single crystals be grown?
W. Weissflog. In my talk I reported on ferro- or antiferroelectric behaviour in the solid-state of a material formed by bent-core molecules. Liquid crystalline behaviour could not be observed. The effect depends on the type of the crystalline modification and, furthermore, on the frequency of the electric field we used. We assume that parts of the molecules can be switched by the electric field in the solid-state, which is proved by a clear change of the birefringence but also by the current response.
It seems that the field-induced growing of the polar crystalline phase is a kind of electro-crystallization because the growing process and habit of the crystalline nuclei are influenced by an applied electric field.
We would like to thank the Deutsche Forschungsgemeinschaft GRK 894, for the financial support.
One contribution of 18 to a Discussion Meeting Issue ‘New directions in liquid crystals’.
- © 2006 The Royal Society