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1 This article was downloaded by: [North Bengal University ] On: 28 October 2011, At: 00:36 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: Registered office: Mortimer House, Mortimer Street, London W1T 3JH, UK Liquid Crystals Publication details, including instructions for authors and subscription information: New hockey stick compounds with a lateral methyl group showing nematic, synclinic and anticlinic smectic C phases A. Chakraborty a, B. Das b, M.K. Das a, S. Findeisen-Tandel c, M.-G. Tamba c, U. Baumeister c, H. Kresse c & W. Weissflog c a Department of Physics, University of North Bengal, Siliguri, Darjeeling b Department of Physics, Siliguri Institute of Technology, Siliguri, Darjeeling c Martin-Luther-Universität Halle-Wittenberg, Institut für Chemie, Physikalische Chemie, Halle, Germany Available online: 09 Sep 2011 To cite this article: A. Chakraborty, B. Das, M.K. Das, S. Findeisen-Tandel, M.-G. Tamba, U. Baumeister, H. Kresse & W. Weissflog (2011): New hockey stick compounds with a lateral methyl group showing nematic, synclinic and anticlinic smectic C phases, Liquid Crystals, 38:9, To link to this article: PLEASE SCROLL DOWN FOR ARTICLE Full terms and conditions of use: This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae, and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand, or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material.

2 Liquid Crystals, Vol. 38, No. 9, September 2011, New hockey stick compounds with a lateral methyl group showing nematic, synclinic and anticlinic smectic C phases A. Chakraborty a,b.das b, M.K. Das a, S. Findeisen-Tandel c, M.-G. Tamba c, U. Baumeister c, H. Kresse c and W. Weissflog c a Department of Physics, University of North Bengal, Siliguri, Darjeeling; b Department of Physics, Siliguri Institute of Technology, Siliguri, Darjeeling; c Martin-Luther-Universität Halle-Wittenberg, Institut für Chemie, Physikalische Chemie, Halle, Germany (Received 15 May 2011; final version received 6 June 2011) Downloaded by [North Bengal University ] at 00:36 28 October 2011 New hockey stick-shaped mesogens with a lateral methyl group inserted between the m-alkyloxy-chain and the azomethine connecting group, 4-(3-n-alkyloxy-2-methyl-phenyliminomethyl) phenyl-4-n-alkyloxycinnamates have been synthesised. The interesting feature of these hockey stick-shaped mesogens is the appearance of the nematic phase in addition to two polymorphic tilted smectic phases the synclinic smectic C (SmC s ) and the anticlinic phase. Physical characterisation of these mesogens has been carried out using polarising optical microscopy, differential scanning calorimetry, optical birefringence, X-ray diffraction (XRD), electro optical and dielectric spectroscopy measurements. The SmC s phase transition is accompanied by only a small calorimetric signal but causes a pronounced change in the optical textures. Orientational order parameters determined from the birefringence measurements have been compared with those estimated from XRD patterns. Dielectric measurements point to the formation of small soft ferroelectric clusters responsible for a low frequency absorption range. Keywords: hockey-shaped molecule; X-ray diffraction; optical birefringence; dielectric constant; orientational order parameter 1. Introduction Bent-core or banana-shaped mesogens have emerged as a field of considerable interest in liquid crystal (LC) research ever since their discovery by Niori et al. in 1996 [1]. Such a strong interest results from the unusual physical properties of the so-called banana phases. Due to their bent shape, the molecules are preferably packed in layers. The lateral correlation of the molecular dipoles yields a polar order within the layers, which can be switched on applying an electric field. Moreover, the discovery of polar switching in such compounds and the detailed analysis of the different switching processes are also driven by their potential use for application in display technology. If the molecules are tilted within the layers, a combination of tilt direction and polar vector can give rise to layer chirality although the molecules are not optically active themselves [2 7]. A pertinent question that arises is to what extent either the bend in the molecules, or structural modifications in the molecular core, can influence the mesogenic behaviour of such molecules. As observed in recent years, an interesting class of compounds with a shape intermediate between the classical rodlike molecules and the conventional banana-shaped mesogens is the hockey stick compounds. In hockey stick molecules having a bent aromatic core, the angle of the molecular long axis results from a 1,3- phenylene fragment or 2,5-disubstituted heterocyclic five-membered ring in strongly non-symmetric four or five-ring aromatic systems [8 13]. In another type of hockey stick mesogens, which is related to the type presented in this paper, the bend is introduced by an alkyl or alkyloxy chain attached in the meta-position of a terminal phenyl ring [14 19]. Some of these compounds are of considerable interest due to the occurrence of two polymorphic tilted smectic phases the synclinic smectic C (SmC s ) as well as the anticlinic ( ) phase. In the SmC s phase, the direction of the tilt is the same in adjacent smectic layers, whereas it alternates between the layers of the phase. This dimorphism is only rarely observed for nonchiral compounds. In our earlier publications [20 23] the synthesis and detailed structural and conformational investigations from X-ray diffraction (XRD) and nuclear magnetic resonance (NMR) measurements of 4-(3-n-decyloxyphenylimino-methyl) phenyl 4-n-alkyloxybenzoates, comprising a meta-alkyloxysubstituted three-ring core with a COO and a CH=N linking group between the phenyl rings have been reported. Depending upon the length of the terminal chains these compounds form a SmA phase, SmC s and phases. From NMR investigations on these compounds it was confirmed that Corresponding author. mkdnbu@yahoo.com ISSN print/issn online c 2011 Taylor & Francis DOI: /

3 1086 A. Chakraborty et al. the SmC s phase transition is accompanied by a conformational change in the molecular part containing the 3-alkyloxyphenyl fragment [20]. This conformational change is associated with a change in the shape of the molecules from a more rod-like one in the SmC s phase to a hockey stick shape in the phase and leads to a different packing of the molecules within the layers of the phase. The enlargement of the mesogenic aromatic core of the three-ring compounds by insertion of a carbon carbon double bond results in esters of the cinnamic acid. From physical characterisation [24, 25] of these hockey stick-shaped mesogens by XRD, birefringence, density and static dielectric permittivity measurements, it was found that these compounds again exhibited SmA SmC s mesomorphism, however at higher temperatures. Interestingly, the dielectric anisotropy of these compounds changes from positive to negative values in the phase, which could be related to the bent hockey stick shape of the molecules in the phase [20, 24]. Thus, it is of great interest to see whether any other modifications in the chemical structure of these hockey stick-shaped mesogens lead to new materials exhibiting interesting mesogenic properties or phase sequences at not too high temperatures. In this paper, we report on laterally methylsubstituted hockey stick-shaped compounds and their study by polarising microscopy, XRD, optical birefringence, dielectric and electro-optical methods. The lateral methyl group is introduced in the obtuse angle between the meta-alkyloxy chain attached to the terminal phenyl ring and the azomethine connecting group. The compounds were prepared by condensation of 4-formylphenyl 4-n-alkyloxycinnamates with 3-alkyloxy-2-methylanilines in ethanol as analogously reported in [25]. The phase transition behaviour of compounds 1 5 is listed in Table 1. The 4-(3-n-alkyloxy-2-methylphenyliminomethyl) phenyl 4-n-alkyloxycinnamates 1 5 exhibit a nematic (N) phase in combination with an phase or a Iso N SmC s (Iso = isotropic) phase sequence. It should be mentioned again that the phase is relatively seldom, and the sequence SmC s is also rare in case of mesogens having no asymmetric carbon atom except the case wherein the terminal chain is a branched chain (swallow tailed) [26, 27]. The occurrence of a N phase above smectic phases is pleasant because there is a better chance to orient the smectic phases. The phase transition temperatures were determined using optical polarising microscopy, differential scanning calorimetry (DSC) and optical transmission measurements. This combination is helpful because the SmC s transition exhibits only very low enthalpy values. Furthermore, this transition is clearly characterised by strong turbulences in the Schlieren texture, which can persist over a temperature range of some degrees before absolutely coming to an end. As discussed later in more detail, this phase sequence N SmC s allows physical measurements yielding more specific information on the SmC s phase sequence. 2. Experimental Calorimetric investigations were carried out using a Perkin Elmer Pyris 1 DSC. Texture observations of planar and homeotropically aligned samples were carried out with a Nikon Labophot 2 polarising microscope, the sample temperature being regulated by a Linkam hot stage. XRD measurements were carried out on surfacealigned samples obtained by slowly cooling a drop of the LC placed on a glass plate on a temperature controlled heating stage. The X-ray patterns were recorded using an area detector (HI-STAR, Siemens AG/Bruker) with Ni-filtered Cu-Kα radiation. The birefringence, n = n e n o,wheren e and n o are the extraordinary and ordinary refractive indices, respectively, of the LC medium, were measured by a procedure reported in an earlier publication [28]. AHe Ne laser (λ = nm) beam was directed onto a homogeneously aligned (planar) indium tin oxide (ITO) coated LC cell of thickness 5.1 µm (procured from AWAT Co. Ltd., Warsaw, Poland), which was placed between two crossed linear polarisers. The temperature of the cell was regulated and measured with a temperature controller (Eurotherm PID 2404) with an accuracy of ±0.1 C. During the experiment the temperature was varied at a rate of 0.5 C min 1 and the transmitted light intensity was measured by a photo diode at an interval of 12 s. The birefringence in the N phase was calculated from the measured transmitted intensity. This method was extended to the SmC phases although we know that the SmC s modification is a two-axis system and the orientation is an open question. Dielectric spectroscopy measurements were performed in the frequency range from 0.1 Hz to 10 MHz in a gold coated double plate condenser (d = 0.1 mm) with an impedance analyser Solartron SI 1260 (Schlumberger). The sample was oriented in the N phase by an external magnetic field parallel (p) and perpendicular (s) to the director. In the SmC phases these symbols should only point to the starting orientation because we do not know the actual orientation of the molecules with respect to the measuring electrical field. To reduce the conductivity, compound 5 was additionally recrystallised from heptane. The

4 Liquid Crystals 1087 Downloaded by [North Bengal University ] at 00:36 28 October 2011 electro-optical measurements were carried out using commercially available polyimide coated ITO test cells (EHC Co. Ltd., Tokyo, Japan) with a cell thickness of 2, 5 and 6 µm. To find a switching polarisation the triangular-wave voltage method was employed [29]. 3. Results and discussion The mesophase behaviour, the transition temperatures and transition enthalpies of compounds 1 5 are summarised in Table 1. To allow a comparison between the two measuring methods, calorimetric data are given in the first line together with the enthalpy values in brackets. On the next line, the temperatures obtained from optical transmission measurements are listed in which the SmC s transition is prominent. 3.1 Texture observation Compounds 1 5 exhibit a N phase that preferably shows homeotropic orientation. The clearing temperatures are near to 110 C, and surprisingly, they are about 10 K higher than those of corresponding compounds without a lateral methyl group [25]. This effect is remarkable because small lateral alkyl groups attached to rod-like molecules generally decrease the mesophase stability. The finding is in agreement with results for other hockey stick-shaped compounds reported by Wu et al. [30, 31]. Upon cooling the N phase of compounds 1 and 2, a Schlieren texture of a SmC phase appears. It shows disclination points with two brushes in addition to those with four brushes. This is evidence for an anticlinic arrangement of adjacent layers. It is in agreement with the symmetric diffraction pattern found by XRD measurements (see section 3.3) and allows us to classify the phase as. A Schlieren texture or a broken fan-shaped texture typical for SmC s phases appears on cooling the N phases of compounds 3 5 [Figures 1(a) and 1(b)]. The transition into the low temperature phase is accompanied by a clear change in the optical textures. The Schlieren texture of the SmC s phase differs strongly from that of the phase [Figure 1(c)]. Moreover, near the SmC s transition the Schlieren texture strongly fluctuates, which is a characteristic behaviour at synclinic anticlinic transitions. The fluctuations appear in a certain temperature range, and in some cases it is difficult to decide whether the transition is really finished. The Table 1. Transition temperatures ( C) and associated transition enthalpies (kj mol 1 ) for the 4-(3-n-alkyloxy- 2-methyl-phenyliminomethyl)phenyl 4-n-alkyloxycinnamates. a H (2m+1) C m O O O H N H 3 C OC n H (2n+1) No. m n Cr SmC s N I a [47.4] [1.8] [0.49] [28.9] [1.5] [0.48] b [31.3] [2.0] [0.23] [38.2] [0.03] [2.6] [0.98] [68.7] [0.15] c [5.8] c Notes: a Transition temperatures: calorimetrically determined values (onset, on heating) (first line) with their transition enthalpies in brackets (second line) and those from the optical transmission measurements (third line). b Phase transition not observed by DSC. c Phase transitions SmC s N I not sufficiently resolved in the DSC curves. : Represents the corresponding phases are present; : represents the absence of the phase.

5 1088 A. Chakraborty et al. (a) (c) Figure 1. Compound 5: (a) Schlieren texture of the SmC s phase, (b) broken fan-shaped texture of the SmC s phase, (c) Schlieren texture of the phase, (d) fan-shaped texture of the -phase with irregular stripes: (a) and (b) 108 C, (c) and (d) 100 C (colour version online). reason for this is that the reorientation of the molecular tilt can proceed step by step in every second layer [21]. The broken fan-shaped texture of the SmC s phase transforms by cooling into the modification with a fan-shaped texture with irregular stripes across the fans [Figure 1(d)]. The birefringence decreases as indicated by the change in the interference colours. 3.2 Optical transmission method An optical transmission method was used for the identification of the different LC phases in these compounds along with the texture study, XRD and (b) (d) DSC measurements. It was found that this method is an excellent tool for the determination of the phase transition temperatures, particularly for hockey stick compounds [22, 24]. We were able to clearly identify the three phase transitions, Iso N, N and SmC s, of these compounds from the sharp change of the transmitted intensity values or change in slope of the temperature dependent intensity curve. Because the SmC s phase transition temperatures were not, or only hardly detectable by DSC measurements in some of these compounds due to their very small transition enthalpies, the optical transmission method proved to be very helpful, clearly pinpointing this transition. Figure 2 shows the temperature dependence of the optical intensity for compounds 1 and 4. From the transmitted intensity curve it can be concluded that compounds 1 and 2 have a N and a phase only. Compounds 3 5 exhibit a N SmC s phase sequence. In compounds 4 and 5 the N, SmC s and phases exist in a sufficiently broad range to make physical measurements possible. It can be noted that the intensity of the transmitted laser light depends upon the phase difference, ϕ = 2π nd, where n = birefringence λ of the sample and d = cell thickness. Due to the oscillatory nature of φ, for a large change in the birefringence of the medium within the N phase, one minimum is observed for each compound except X-Ray investigations XRD measurements on surface-aligned samples of compounds 1 [Figures 3(a) and 3(b)] and 2 clearly show the patterns of the high-temperature N phase (diffuse halos in the wide and in the small angle region, the high intensity of the inner halo indicating the existence of cybotactic groups of the smectic type) and that of the smectic phase at lower temperatures (very strong first and very weak second order (a) (b) 0.40 N Iso 0.40 SmC s N Iso Intensity (a.u.) Sm C a Temperature ( C) Intensity (a.u.) Sm C a Temperature ( C) Figure 2. Transmitted intensity as a function of temperature during cooling of compounds (a) 1 and (b) 4.

6 Liquid Crystals 1089 Downloaded by [North Bengal University ] at 00:36 28 October 2011 (a) (c) (e) Figure 3. 2D XRD patterns of surface-aligned samples of compounds 1 and 4 upon cooling, meridian of the pattern normal to the aligning surface; compound 1: (a) N phase at 102 C, N director parallel to the meridian, (b) phase at 95 C, layers parallel to the aligning surface; compound 4: (c) N phase at 108 C, N director parallel to the aligning surface, (d) SmC s phase at 106 C with a majority of domains showing the same tilt direction, layers parallel to the aligning surface, (e) phase at 80 C, layers parallel to the aligning surface. layer reflections on the meridian of the pattern along with diffuse halos in the wide angle region). The layer spacing of compound 1 (2) decreases from 34.2 (37.4) Å at 99 (97) C to 32.8 (36.5) Å at 70 (70) C, respectively, while the tilt angle estimated from the positions of the maxima for the outer diffuse scattering increases from 22 (20) to 29 (29) in the same temperature interval. The azimuthal (χ-) distribution of the wide-angle scattering around the trace of the primary beam shows four maxima for both compounds. The two maxima above the equator have (b) (d) equal intensities within experimental error indicating the co-existence of equal numbers of molecules with opposite tilt. (The part of the pattern below the equator is shadowed by the heating stage.) This may arise from equally distributed domains of opposite tilt in the SmC S phase or from an anticlinic tilt of molecules in adjacent layers, as found in the phase. Analogous X-ray investigations on compound 4 confirm the presence of three LC phases. The pattern at 108 C [Figure 3(c)] shows diffuse scattering in both the inner and outer diffraction range with comparatively strong maxima on the equator (inner scattering) and on the meridian of the pattern (outer scattering), respectively, which is the typical pattern of a N phase showing cybotactic groups as in case of compounds 1 and 2, but this time the N director is oriented on the average parallel to the aligning surface. At about 107 C, the pattern changes, the smallangle and wide-angle diffraction maxima move to completely different positions and indicate that now the smectic layers are parallel to the glass plate. The maxima of the two outer diffraction arcs are rotated out off the equator by about 17 in the same sense of rotation indicating a structure with synclinically tilted molecules the SmC s phase [Figure 3(d)]. At about 104 C, the XRD pattern changes once more, now exhibiting broader halos for the outer diffuse scattering centered about the equator [Figure 3(e)] closely resembling those in the patterns of the SmC phases of compounds 1 and 2. Theχ-distribution of their intensity may be fitted by four Gaussian curves, the positions of the four maxima yielding a tilt angle of about 23. The tilt slightly increases with decreasing temperature [Figure 4(a)]. The statistical uncertainty for our determination of the tilt angles in one experimental setting is in the range of ±1 and the experimental error for the absolute value of the tilt due to the shadowing of the pattern is expected to be somewhat larger. As stated previously, the XRD measurements cannot distinguish between a rotationally disordered synclinic structure and anticlinic molecular packing. However, considering the results of the other experiments, we can safely conclude that the low-temperature phase is. In accordance with the temperature dependence of the molecular tilt, the layer spacing decreases sharply from 37.6 Å at the N SmC s transition to 36.6 Å at the transition to and then only slightly decreases to 36.0 Å at 80 C [Figure 4(b)], which means it remains more or less constant throughout the phase. 3.4 Birefringence measurements Birefringence measurements were carried out on compounds 1 5, the temperature dependence of

7 1090 A. Chakraborty et al. (a) (b) SmC s N Iso 38 SmC s N Iso Tilt angle ( ) d (Å) T ( C) T ( C) Figure 4. Temperature dependence of (a) average tilt of the molecules with respect to the layer normal and (b) layer spacing on cooling as derived from the 2D XRD patterns of compound 4. Downloaded by [North Bengal University ] at 00:36 28 October 2011 which is shown in Figure 5. Upon cooling, a significant increase of birefringence is observed within the N phase, which is due to the increase in N order. For all the samples studied, the birefringence values decrease at the SmC s phase transition and remain more or less constant throughout the phase. Such a lowering of birefringence has also been observed from the colour change of the optical texture during the phase transition. 3.5 Orientational order parameter The optical birefringence, n, obtained from the measured transmitted intensity data of the homogeneously aligned cell [28, 32] of thickness 5.1 µm was utilised to determine the temperature variation of the orientational order parameter in the LC phases of these compounds. The order parameter is defined as <P 2 > = n/ n 0, where n 0 is the birefringence in the completely ordered state and was obtained from the temperature dependence of n, which can be approximated well for LC by [33] n = n 0 (1 T T ) β (1) where n 0, T and β are adjustable parameters. T is about 1 2 K above the clearing temperature. The exponent β depends on the molecular structure and its value is close to 0.2. This procedure is only correct for oriented phases of a single axis system. Therefore, equation (1) has been fitted for compounds 1 4 by taking the values of n only for the N phase. As an attempt we did apply equation (1) for the SmC s phase of compound 5. In Table 2, the n values at a temperature 5 K below the SmC s phase transition for all the five compounds along with the extrapolated birefringence at the absolute zero temperature ( n 0 )is shown. It is evident that for a fixed value of the chain (a) N Iso (b) SmC s N Iso Δn Δn Temperature ( C) Temperature ( C) Figure 5. Temperature dependence of the birefringence for the compounds (a) 1 and (b) 4.

8 Table 2. Optical birefringence ( n) in the phase at a temperature 5 K below the SmC s phase transition for all the five compounds and calculated values of n 0 and β (m, n: chain lengths as stated in Table 1). Compound (m, n) n n 0 β 1 (8, 8) (8, 10) (10, 8) (10, 10) (12, 10) length (n = 10), n decreases with increasing chain length m. Again, if we keep m constant (m =10), n decreases with increasing chain length (n = 8 10). A similar behaviour is also observed if we consider the extrapolated birefringence at the absolute zero temperature, n 0. The data presented in Table 2 was used to calculate <P 2 >. This procedure was extended to the SmC phases in order to see special orientation effects of the molecules during the formation of the different SmC modifications. The temperature dependence of <P 2 > is shown in Figures 6(a) (e). The experimental <P 2 > in the N and SmC s phases increases with decreasing temperature. For compounds 3, 4 and 5 we observe a drop in <P 2 > at the SmC s phase boundary. This may be due to the fact that the molecular long axes in the SmC s phase of the surfacealigned bulk sample are oriented parallel to the aligning surface and the layers are tilted with respect to the surface. Therefore, we observe a normal temperature dependence of <P 2 > in the SmC s phase. However, since the transition to the phase in these compounds leads to an effective reorientation of the molecules to form an anticlinic arrangement, the molecules in this phase are now tilted with respect to the surface. This tilt of the molecules in the phase causes a drop in the parallel component of the polarisability i.e. along the molecular long axis, thereby reducing the polarisability anisotropy and hence the birefringence of the material. Within the phase the birefringence and hence <P 2 > remain constant. The orientational distribution function f (β) and hence the orientational <P 2 > of compound 4 have been determined from X-ray intensity data using the method described previously by Bhattacharya et al. [34]. For this purpose, a χ-scan (i.e. X-ray intensity I(χ) vs. azimuthal angle (χ) curve)ofthe outer diffraction arc for χ = (omitting the trace of the beam stop) was first performed for the patterns of the SmC phases and for that of the Iso liquid. Then, the intensity data were Liquid Crystals 1091 calibrated by those of the Iso liquid to correct them for influences of the experimental setting using I rel = I(80 C)/I(115 C, Iso liquid) assuming an Iso χ distribution of the outer diffuse scattering for the sample in the Iso liquid [Figure 7(a)]. This intensity distribution I rel (χ) was then utilised to determine the orientational distribution function f (β) and <P 2 >. From the values of I(χ), the f (β) and<p 2 > values have been calculated following Leadbetter s expression [35]. For determining the experimental f (β) values and hence <P 2 >, the azimuthal distribution of the X-ray intensities I(χ) in one quadrant (e.g. from χ = 0 90 ) is sufficient. In the present case, due to the shadowing of the pattern by the heating stage the average values of I(χ) to calculate f (β) and hence <P 2 >, have been determined from the nonshadowed upper quadrants from χ = only. The temperature variation of <P 2 > for compound 4 from the XRD measurements is shown in Figure 6(d). It is observed that <P 2 > varies quite strongly in the SmC s phase. As shown in Figure 6(d) such a pronounced temperature dependence of the <P 2 > values in the SmC s phase is also observed in the birefringence measurements. Similarly to the procedure for deriving the birefringence from measurements of the transmitted intensity, the data that was used for calculating <P 2 > from XRD measurements was originally developed for a well-aligned N phase of calamitic molecules [35]. Here, it is applied to the surface-aligned smectic phases of more or less calamitic (SmC s ) and hockey stick-shaped ( ) molecules to get a qualitative impression about the changes in the order with temperature. The resulting <P 2 > values in the phase are very large in comparison to those in the SmC s or N phase. Again, similar to the birefringence measurements, the orientational <P 2 > values determined from XRD measurements also show only weak temperature dependence in the phase. Figure 6(d) shows that in the phase, the <P 2 > values obtained from XRD measurements are considerably larger than those obtained from optical birefringence studies. This apparent contradiction between the two sets of values may be due to the different uncertainties in the two approaches used. From the XRD measurements we have determined the <P 2 > values after extracting the azimuthal intensity distribution from the χ-scan (Figure 7). This distribution is certainly influenced by the non-perfect alignment of the layers in different regions of the irradiated portion of the sample. In other words, a mosaic spread of the layers has to be taken into account [35]. Qualitatively, the alignment in the phase is a little better than that in the SmC s phase as can be seen by comparing the χ-distribution

9 1092 A. Chakraborty et al. Figure 6. Temperature variation of <P 2 > for compounds (a) 1, (b)2, (c)3, (d)4 and (e) 5 (open circles show <P 2 > from birefringence measurement, closed ones in (d) show <P 2 > calculated from XRD patterns). of the intensity of their layer reflections, which is related to the distribution of the normal directions of the layer [Figure 7(b)]. Generally speaking, the uncertainties are rather large in case of a surfacealigned smectic sample. If we consider that parts of the sample can gradually reorient during cooling within one phase, even the trend in the change of the orientational order parameters becomes a little uncertain. On the other hand, as mentioned earlier, the tilting of the molecules with respect to the layer normal lowers the value of the orientational order parameter <P 2 > from the birefringence measurements. Moreover, a certain degree of arbitrariness involved in the estimation of n 0 [Equation (1)] also imparts a relative shift to the values of <P 2 >,as determined from the birefringence measurements. 3.6 Dielectric and electro-optical investigations The dielectric spectra of compound 5 were taken in the frequency range between 0.1 Hz and 10 MHz. Figure 8 shows the real and imaginary part of the

10 Liquid Crystals 1093 Downloaded by [North Bengal University ] at 00:36 28 October 2011 Figure 7. χ-scans of the 2D XRD patterns for sample 4: (a) outer diffuse scattering at 80 C with Gaussian fit for four maxima [intensity calibrated by that of the Iso liquid using I rel = I(80 C)/I(115 C, Iso liquid)], the distribution function calculated from the two intense peaks which are situated well within the non-shadowed part of the diffraction pattern was used to calculate the order parameter; (b) layer reflections at 80 and 106 C with Gaussian fits; the full width on half maximum for both peaks are 9.9 at 80 C and 11.6 at 106 C. Figure 8. The complex dielectric function of compound 5 for the starting orientation parallel to the N director, measured in (a) the N (110 C) and (b) the SmC s phase (107.5 C). complex dielectric function ε (f ) = ε jε (j 2 = 1) parallel to the director. The measured values perpendicular to the director are illustrated in Figure 9. Evaluation of the measured values was carried out using equation (2), taking into account two Cole Cole mechanisms (terms 2 and 3) and the contribution of conductivity in terms 4 and 5 to describe the double layer at low frequencies: ε ε 0 ε 1 = ε (jωτ 1 ) 1 α 1 ε 1 ε (jωτ 2 ) 1 α 2 ja f M + B f N (2) with ε i as low, middle and high frequency limiting values of the dielectric constant, ω = 2πf (f = frequency), τ i = 1/2πf (τ i = relaxation time), α i = Cole Cole distribution parameter (phenomenological description of the distribution of relaxation times Figure 9. The complex dielectric function of compound 5 at C. The director was oriented in the N phase perpendicular to the electric field. It is difficult to separate the low frequency absorption at about 200 Hz due to the influence of the double layer. or the superposition of discrete relaxation processes), the conductivity term A as well as M, B and N as

11 1094 A. Chakraborty et al. further parameters. The conductivity κ, calculated according to κ = 2Aπε 0 in parallel direction, is showninfigure10. The conductivity clearly reflects all the phase transitions. The phase transition SmC s is clearly identifiable in the specific conductivity, certainly as a result of the tilt change. To evaluate the limiting values of the dielectric permittivity with the starting orientation of the measuring field parallel to the director by Equation (2), two relaxation mechanisms with relaxation frequencies at approximately 200Hz and 5 MHz were assumed for the SmC phases. In the perpendicular direction only the mechanism at 5 MHz could be detected clearly. Figure 11 shows the limiting values of the dielectric function. At the transition from the Iso to the N state, ε P1 increases, indicating a small positive dielectric anisotropy of about The lowfrequency absorption could neither be observed in the N [Figure 8(a)] nor in the Iso phase. In both phases only the high-frequency absorption region is visible. The evaluation of the data becomes difficult in the N phase because of the influence of standing waves in the measuring cell. For this reason all values for ε P2 given in Figure 11 as closed triangles and the related relaxation times in Figure 12 as closed squares show a systematic error in the N phase. Nevertheless, we have to point out that this relaxation range exists in the N phase too. Relaxation times calculated by Equation (2) are illustrated in Figure 12. Activation energies (54 kj mol 1 for the low frequency mechanism) and (92 kj mol 1 for the high frequency mechanism) have also been calculated. The relaxation times of the high frequency process agree well with those measured in the parallel direction indicating that it arises from the same Conductivity (ns/m) 10 2 Iso N SmC s /T (K 1 ) Figure 10. Conductivity of compound 5 measured parallel to the director. ε p ε p0 ε p1 ε p2 SmC s T ( C) N Iso Figure 11. Limiting values of the dielectric constants of compound 5 parallel to the director. τ p (s) SmC s 10 6 τ p1 τ p /T (K 1 ) Figure 12. Relaxation times of compound 5 measured in the parallel direction. mechanism [see Figures 8(b) and 9]. The low frequency process could be separated in the perpendicular direction only at high temperatures. This less intensive mechanism is strongly disturbed by the conductivity and the double layer. The relaxation frequencies agree approximately with those found in the parallel direction. For the N phase, the low frequency process is not observable. The high frequency relaxation process can be well separated. The dielectric permittivity values ε P2 and ε S2 are larger than 3.5 in the SmC phase. Thus, there must be at least one further high frequency relaxation process to decrease the dielectric permittivity values to about 2.8. From the facts that the relaxation times of the low frequency mechanism are of about 500 Hz and that the related activation energy is much lower than that of the high frequency mechanism we conclude that this process can be compared with the slow dynamics of banana-shaped molecules [36]. It should be noted that in classical SmC phases only

12 one reorientation process in the MHz range with activation energy of about 120 kj mol 1 is observed [37]. This process was related to the reorientation of the molecules about the short axes. Also, in this case, the high frequency limit of the dielectric permittivity is more than 3. Assuming that the hockey sticklike molecules form small soft ferroelectric clusters we can interpret the low frequency mechanism as a collective motion of the clusters like that in the phases of banana-shaped molecules [38]. Then the high frequency mechanism should be related to the reorientation of the molecules about the short axes whereas the reorientation around the long molecular axes appearing at higher frequencies was not visible in the range investigated by us. Electro-optical measurements were carried out on compound 4 using commercial polyimide coated EHC cells of different thickness. Figure 13(a) shows the fan-shaped texture of the phase. On applying an electric field the texture nearly remains the same, but the colours clearly change depending on the strength of the field. The textures of the switched states are independent of the polarity of the applied field. The electro-optical behaviour was found to be similar in 2 µm EHC cells. It should be noted that only in such very thin cells unusually nice pictures of the SmC s phase transition could be observed (Figure 14, without electric field). The texture of the SmC s phase of compound 4 at 104 C is shown in Figure 14(a). On slow cooling, zig-zag patterns grow as shown in Figure 14(b), until the entire area consists of the phase, as shown in Figure 14(c). Decrossing the polariser by ± 15 causes dark and bright domains to appear proving that the tilt of the molecular long axes is opposite in the different domains [see Figures 14(d) (f)]. Hockey-stick mesogens have a molecular shape that is neither rod-like nor banana-like. Therefore, efforts to measure the spontaneous polarisation are of great interest. However, the results are not really conclusive. Using a triangular wave voltage (a) Figure 13. (a) Fan-shaped texture of the phase without electric field (compound 4, cell thickness 6 µm, 100 C); (b) texture after applying a dc field of ±40 V (colour version online). (b) (a) (c) (e) Liquid Crystals 1095 Figure 14. Compound 4, 2 µm EHC cell polyimide coated, without electric field. (a) SmC s phase at 104 C; (b) growing of the phase from the SmC s phase, 103 C; (c) phase at 100 C. (e) phase, crossed polariser at 93 C; (d) and (f) decrossing the polariser by +15 or 15 (colour version online). (e.g. EHC cell, 6 µm, 10Hz, 320 Vpp) a current response with two peaks can be observed in both SmC phases, which however do not completely disappear on heating into the Iso liquid phase. This means we are not able to separate a possible polar answer from the conductivity and therefore, we have to say that the situation is not clear at present. It can be assumed that the field-induced switching shown in Figure 13 corresponds to a Freedericksz transition [39] driven by the dielectric anisotropy, because it did not depend on the polarity of the voltage applied. However, it is remarkable, as Yu et al. have reported the exact same behaviour and similar problems for other hockey stick-shaped compounds [15, 16]. From the electro-optical measurements using mixtures consisting of a hockey stick-shaped compound and a chiral dopant the authors have extrapolated to a low spontaneous polarisation for the pure non-chiral hockey stick-shaped compound if the concentration of the chiral dopant goes to zero. Furthermore, there is a controversial finding concerning the properties of freestanding films formed (b) (d) (f)

13 1096 A. Chakraborty et al. by hockey stick-shaped compounds. Stannarius et al. reported a low, but clearly measurable spontaneous polarisation for the high-temperature phase of hockey stick mesogens [21]. The research group of C.C. Huang prepared thin films of definite number between two and ten layers using the same materials. However, null transmission ellipsometry and depolarised reflected light microscopy have not given evidence for polar or chiral properties of both smectic phases [23, 40]. Probably, the smectic phases formed by hockey stick mesogens could exhibit a latently polar behaviour, which cannot be clearly proved or rejected by the measuring methods used up to date. 4. Conclusions The physical properties of a series of new hockey stick-shaped LCs with a lateral methyl group introduced in the obtuse angle of the molecule, between the meta-alkyloxy chain and azomethine connection group have been investigated. It is significant that such a lateral substitution into the molecular core of these mesogens results in the suppression of the SmA phase and in the introduction of the N phase. Steric interactions between the methyl group and the neighbouring alkoxy chain in the meta-position perhaps induce a more rod-like shape thereby facilitating the formation of the N phase at higher temperatures. The N phase sequence, as observed in 1 and 2, is also quite remarkable. In addition to the N phase, compounds 3 5 form two tilted smectic phases SmC s and molecular arrangement. Together with the other measuring methods used, the optical transmission method conclusively proved the existence of the N, SmC s and phases in these compounds. The 2D XRD pattern in the SmC s phase clearly proves the synclinic structure; the pattern in the phase is caused by an anticlinic arrangement of the molecules in adjacent layers. As observed previously from NMR measurements on closely related hockey stick-shaped compounds, this arrangement of alternating tilt in adjacent layers is accompanied by a pronounced change in the overall shape of the molecules from a more or less rodlike shape in the high temperature SmC s phase to a bent hockey stick-like shape in the lower temperature phase. The orientational <P 2 > values determined from the birefringence measurements decrease at the SmC s transition. The <P 2 > values determined from XRD measurements on compound 4 however increase in going from a SmC s to a phase. This apparent contradiction may be referred to the uncertainties in both methods suffering mainly (but not only) from a general non-perfect alignment of the surface-aligned samples in the smectic phases and a differing change in this alignment with temperature for the different experimental settings. The dielectric measurements show a low frequency absorption, which we interpret as a signal from ferroelectric clusters in analogy to the polar ferroelectric smectic phases of banana-shaped molecules. It is likely that the clusters formed by hockey stick molecules show stronger fluctuations. Therefore, it may be difficult to detect ferroelectricity by the reversal current methods. Acknowledgements We gratefully acknowledge financial support from the Department of Science and Technology, New Delhi (Project no.: SR/S2/CMP-20/2005 and SR/S2/CMP- 29/2007) and the University Grants Commission (New Delhi) for the award of a Junior Research Fellowship (AC). References [1] Niori, T.; Sekine, F.; Watanabe, J.; Furukawa, T.; Takezoe, H. J. Mater. Chem. 1996, 6, [2] Diele, S.; Grande, S.; Kruth, H.; Lischka, C.H.; Pelzl, G.; Weissflog, W.; Wirth, I. Ferroelectrics 1998, 212, [3] Pelzl, G.; Diele, S.; Weissflog, W. Adv. Mater. 1999, 11, [4] Link, D.R.; Natale, G.; Shao, R.; Maclennan, J.E.; Clark, N.A.; Korblova, E.; Walba, D.M. Science 1997, 278, [5] Reddy, R.A.; Tschierske, C. J. Mater. Chem. 2006, 16, [6] Takezoe, H.; Takanishi, Y. Jpn. J. Appl. Phys. 2006, 45, [7] Walba, D.M.; Dyer, D.J.; Sierra, T.; Cobben, P.L.; Shao, R.; Clark, N.A. J. Am. Chem. Soc. 1999, 118, [8] Dingemans, T.J.; Murthy, N.S.; Samulski, E.T. J. Phys. Chem. B 2001, 105, [9] Novotna, V.; Zurek, J.; Kozmik, V.; Svoboda, J.; Glogarova, M.; Kroupa, J.; Pociecha, D. Liq. Cryst. 2008, 35, [10] Kang, S.; Saito, Y.; Watanabe, N.; Tokita, M.; Takanishi, Y.; Takezoe, H.; Watanabe, J. J. Phys. Chem. B 2006, 110, [11] Cristiano, R.; Vieira, A.A.; Ely, F.; Gallardo, H. Liq. Cryst. 2006, 33, [12] Tavares, A.; Ritter, O.M.S.; Vasconcelos, U.B.; Arruda, B.C.; Schrader, A.; Schneider, P.H.; Merlo, A.A. Liq. Cryst. 2010, 37, [13] Fergusson, K.M.; Hird, M. J. Mater. Chem. 2010, 20, [14] Hird, M.; Goodby, J.W.; Gough, N.; Toyne, K.J. J. Mater Chem. 2001, 11, [15] Yu, F.C.; Yu, L.J. Chem. Mater. 2006, 18, [16] Yu, F.C.; Yu, L.J. Liq. Cryst. 2008, 35, [17] Okamoto, H.; Morita, Y.; Segawa, Y.; Takenaka, S. Mol. Cryst. Liq. Cryst. 2005, 439,

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