A twist grain boundary (TGB A*) or twisted smectic A* state was observed in two compounds of a homologous series of diarylethane &#945;-chloroester ferroelectric liquid crystals. The phases have been characterized by optical polarizing microscopy and differential scanning calorimetry. Textures of the TGB A* state obtained by preparation on a glass slide and in thin homeotropically orienting liquid crystal cells or as free-standing films clearly show the helical structure, whereas preparation in homogeneously orienting LC cells suggests that the helical structure is suppressed by the cell geometry, in a similar way to that observed for S*C phases in the surface stabilized geometry (SSFLCs)

Dierking, Ingo

Gießelmann, Frank

Zugenmaier, Peter

Online Publikationen der Universität Stuttgart

LIQumCRYSTALS, 1994, VOL.l? , No. 1, 17- 22 
TGB A * state in a homologous series of 
diarylethane IX-chloroester ferroelectric liquid crystals 
by I. DIERKING. F. GlEBELMANN and P. ZUGENMAIER* 
Institut filr Physikalischc Chemic, TU Clausthal, 
Arnold-Sommerfeld-Str. 4, 0 -38678 Clausthal-Zellerfeld, Germany 
(Received 23 August 1993; accepted 8 November /993) 
A twist grain boundary (TO B A"') or twisted smectic A '" state was observed in 
twO compounds of a homologous series of diarylethane a-chloroester ferroelectric 
liquid crystals. The phases have been characterized by optical polarizing 
microscopy and differential scanning calorimetry. Textures of the TOB A* state 
obtained by preparation on aglass slide and in thin homeotropically orienting liquid 
crystal cells or as frcc-standing films clearly show the helical structure, whereas 
preparati on in homogeneously orienti ng LC cell s suggests that the helical structure 
is suppressed by the ce ll geometry, in a similar way to that observed for St phases 
in the surface stabilized geometry (SSFLCs). 
1. Introduction 
Recently the di scovery of a novel smectic A>I< vari ant, the twisted smectic A * or twist 
grain boundary (TGB A *) phase [I] , was reported by Goodby el al. [2,3]. In thi s paper 
we report on two compounds of a homologous series of diarylethane a-chloroesters (4] 
(n = 7, 8) deri ved from D-valine, which will be referred to as 07 and 08, having the 
general formula 
(n = 7: D7; fl = 8: 0 8) which exhibit the phase sequences 
D7: I 133·2 N* 12 1·1 TGB A* 120·8 SI' 92-4 p~ 86·1 S;; 
D8: 134·5 N* 129-4 TGB A* 129·2 S! 125·6 51' 92·9 st 83·7 Sr 
The mesophases were identified by texmre observation and comparison with results in 
[4]. The temperature control of the hot stage was within 0· 1 °C and the accuracy of 
absolute temperature values was about IOC. Since the distinctive invariant property of 
chiral phases, as compared to their non-chiral analogues, is the local symmetry, meaning 
the chirality itself and not the macroscopic helical structure (5], the notation SA is 
used to describe the chiral smecti c A phase, that is the commonly observed smectic 
A phase, but one which is built up of chiral molecules, in contrast to the TGB A* 
state, which is built up of chiral molecules and exhibits a twisted laycr configu ration 
[2.3]. 
* Author for correspondence. 
02fl7-8292194 SI 0-00 (I 1994 Taylor & Francis Ltd. 
18 I. Dierking et ai. 
Figure 1. Polarizing optical micrographs of some mesophases of compound D8 contained 
in thin liquid crystal cells with homeotropic alignment layers. (a) Transition from the 
homeotropically aligned smectic A* phase (black texture) to the twisted p~ state 
(TGB A" state) (yellow line texture, T = t25 ·9°C. (b) The TGB A* state at T = 126·0°C. 
(c) Transition from the TGB A* state (yellow texture) to the cholesteric phase (grey 
texture) at T = 126· 15°C. (d) Texture oflhe cholesteric phase al T = 116S'C (10 units of 
scale A 60 }lru), 
Figure 2. Polarizing optical micrographs of some mesophases of compound D8 mounted 
between two untreated glass slides. (a) smectic C* phase with typical disclination lines. 
T = 125·6°C. (b) fan-shaped smcctic A'" texture. T = 128·9°C, (c) line tyxture of the TGB 
A* state, T = 129·2°C and (d ) oily streak texture of the cholesteric phase at T = 129·5°C 
(10 units of scale l:! 60tlm). 
19 
Figure 3. Polarizing optical micrographs of some mesophases of compound D7 mounted as 
a small droplet between two untreated glass slides. (a) Smectic C* phase with disclination 
lines arranged in a concentric fashion, T= 121·0°C, (b) TGB A" slate, T= 121 ·5°C; the 
disclination lines are arranged in a spherulite-like manner, (c) fingerprint texture of the 
cholesteric phase at T= 122·3°C (10 units of scales A 60 ~mF. 
20 I. Dierking et aL. 
A similar homologous series was first synthesized from L-valine by Nguyen et al. 
[4J, but they did not report any observations on TGB A * states. This is most likely due 
to differences in optical purities, since the optical purity has a great influence on the 
stability and temperature range of the TGB A* phase (6,7]. 
2. Experimental 
Polarizing optical microscopic studies were carried out with an Olympus BH-2 
microscope equipped with a Mettler FP 52 hot stage and a Mettler FP 5 temperature 
controller. The textures of phases possessing a helical st structure were investigated 
by preparing the samples on slides without surface alignment layers and in 
commercially available liquid crystal cells (E.H.C. Co. Ltd., cell gap 4,urn) with 
homeotropic aligning surface layers and as free-standing films. The surface induced 
unwinding of the helical structure of the TGB A * state was studied by preparing samples 
in commercially available liquid crystal cells (E.H.C. Co. Ltd. , cell gap 4 Jim) with 
homogeneous aligning surface layers. 
Calorimetric investigations were carried out with a differential scanning calorimeter 
(Perkin Elmer OSC 7) at variable heating and cooling rates of ::t I, ::t 2 and 
::t 5°C min - 1 and different sample masses rrom 2-8 mg. 
3. Experimental results and discussion 
Compounds D7 and 08 exhibit the phase sequences given above; these are slightly 
different from those reported in the literature [4], since a TGB A * state over a small 
temperature interval of about O·2°C was detected by polarizing optical microscopy for 
both substances. The TGB A "+ state mediates the smectic A* to cholesteric transition 
in 08 and the smectic C* to cholesteric transition in 07. 
Figure I (a)-{d) depicts some of the liquid crystalline phases of compound D8, 
observed by polarizing optical microscopy on heating samples prepared in liquid crystal 
cells with homeotropic alignment layers. Figure 1 (a) shows the transition from the 
non-helical, homeotropically aligned smectic A * phase (black texture, T = I 25·9°C) to 
the helical TOB A* state (yellow line texture, see also f3, 6, 81). The transition is 
accompanied by the formation of a characteristic filament texture, which is completely 
developed at T = 126°C (see figure 1 (b)). The transition from the helical TGB A * state 
(yellow texture) to the cholesteric phase (grey texture) is shown in figure 1 (c) at 
T = 126·15°C and clearly detected by the change in the interference colour due to the 
variation of the birefringence. Figure I (d) depicts the N* phase at T = 126·5°C. 
Preparation of compound D8 on slides without surface alignment layers produces 
textures which also reveal the existence of an intermediary slate between the commonly 
observed non-helical, chiral SA phase and the cholesteric phase, as depicted in figure 
2 (a)-(d). Figure 2 (a) shows a typical fan-shaped texture of the ferroelectric St phase 
with lines due to the helical director configuration [9, 10] (see plate 164 in [11)) at 
temperature T= I 25·6°C. On passing through the transition temperature into the 
non-helical, chiral smectic A * phase. the lines disappear and a fan-shaped texture of 
the S): phase (see plates 99 and 105 in [II]) is observed (see figure 2 (b), T= 128·9°C). 
Mediating the chiral smectic A * phase and the cholesteric phase, the fannation of a lined 
texture, due to the layer twist of the TOB A * state can be seen (see figure 2 (c), 
T= 129·2°C), before, on further heating, the typical oily-streak texture (see plates 21 
and 22 in f II]) of the cholestcric phase (sec figure 2 (d), T = 129·5°C) appears. The 
transition temperatures are somewhat different from the ones observed by preparation 
of the sample in thin liquid crystal cell s, which is most likely due LO sLrong surface 
TGB A'" state 21 
interactions in thin LC cells [10]. Observation of free-standing films between crossed 
polarizers revealed the same characteristic filament textures of the TGB A* state as 
those observed by Goodby et al. [3]. 
Figure 3 (a)-(c) depicts some of the mesophases of compound D7 mounted as a 
small droplet between two untreated glass slides. Figure 3 (a) shows the smectic C* 
phase at temperature T = I21·0°e. The disclination lines due to the helical structure 
of the phase are arranged as concentric rings, since the helical axis of the St phase 
lies perpendicular to the smectic layers. Figure 3 (b) depicts the same droplet in the 
TOB A'" state at T = 121 ·SoC. The photograph exhibits a line texture with lines arranged 
in a spherulite-like, radial fashion, as the helical axis of the configuration has turned 
90°, compared to the helical axis of the p~ phase. Passing through the transition into 
the cholesteric phase, a finger-print texture appears (see figure 3(c), T= I22·3°C). 
A non-helical, chiral smectic A'* phase (st phase), in addition to the twisted TGB A* 
state as observed for D8, was not detected for this compound by preparation of the 
sample on slides without alignment layers. 
Calorimetric studies at variable heating and cooling rates showed that for compound 
08, the p~~pA and the TGB A'*--4-N* transitions are clearly detected, whereas the 
transition from SA 10 TGB A * cannot be observed within the experimental resolution. 
For compound D7, the lransitions p~ ~ TGB A* and TOB A* ~ N* could be detected. 
This suggests, that the transitional enthalpy change between the SA and the TGB A* 
phase is very small, like the energy barrier between the helical s~ phase and the 
corresponding surface stabilized geometry [12]. An analogous behaviour can also be 
observed for cholesteric systems with temperature induced helix inversion, where the 
twist inversion cannot be detected by DSC [13]. 
Further evidence that the energy barrier for the change from SA to TOB A * is very 
small can be found by preparing the samples in thin liquid crystal cells with 
homogeneous surface alignment layers. Very well-oriented, defect-free alignment is 
obtained for compound 08 by slowly cooling the sample from the isotropic phase. The 
transition from st to TGB A'* is no longer observed, as no change in the appearance 
of the texture is detected. The sample can be brought into an optically extinct position 
by rotating the cell between crossed polarizers, just as is the case for a normal , 
non-helical SA phase. For compound D7, a defect-free, very well-oriented sampled 
appears, without the fingerprint-like line textures which were observed by preparing the 
sample on slides without alignment layers orin cells with homeotropic alignment layers. 
The helical structure of the TOB A'" phase seems to be suppressed by the cell geometry, 
in an analogous way to that in a SSFLC cell. 
4. Conclusions 
In contrast to reports on diarylethane C(-chloroesters in the literature [4], we have 
detected a TOB A * state, which mediates the p~ and N'" phases in compound D7 and 
the st and N'" phases in compound D8. CaJorimetric studies and texture observation 
of samples in thin liquid crystal cells suggest that the TOB A'" state can be suppressed 
by an appropriate cell geometry, as it is found in SSFLC cells. It should be noted, that 
the formation of the TGB state of the s1 phase is very sensitive to conditions of 
preparation of the sample. 
This work was supported by a grant from the Deutsche Forschungsgemeinschaft. 
The compounds were kindly supplied by Dr J. KuBerow, Institut fUr Organische Chemie 
der Technischen UniversiUi.t Clausthal. 
22 TGB A* state 
References 
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[4] NGUYEN. H. T ., BABEAU, A.. LOON, c., MARCEROU, I .-P ., DnsTRADE, C, SOLDERA, A ., 
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[9] MARTINOT-LAGARDE, PH ., 1976, J. Phys .• Paris, 37, C3-129. 
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TGB A* state in a homologous series of diarylethane &#945;-chloroester ferroelectric liquid crystals

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TGB A* state in a homologous series of diarylethane α-chloroester ferroelectric liquid crystals

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TGB A* state in a homologous series of diarylethane &#945;-chloroester ferroelectric liquid crystals

Abstract

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Full text

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TGB A* state in a homologous series of diarylethane α-chloroester ferroelectric liquid crystals