New 4-ring FLC’s with different positions of lateral substituents and bridge fragments in the central core were synthesized. These compounds have a wide temperature range of the smectic C* phase and similar length of the molecules. Different ways of the preparation of the FLC mixtures with a high tilt angle and the results of electrooptical and XRD measurements of the FLC mixtures based upon the chiral compounds are presented

Bezborodov, V.

Haase, W.

Lapanik, A.

Lapanik, V.

English

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XRD Study of New FLC's: Correlation
Between the Chemical Structure of
Molecules, the Sign of the Optical
Rotation and the Tilt Angle
V. Lapanik b , V. Bezborodov b , W. Haase a & A. Lapanik a
a Institute of Physical Chemistry, Darmstadt University of
Technology, Darmstadt, 64287, Germany
b Institute of Applied Physics Problems, Belarusian State University,
Minsk, 220064, Belarus
Version of record first published: 10 Mar 2011
To cite this article: V. Lapanik, V. Bezborodov, W. Haase & A. Lapanik (2006): XRD Study of New FLC's:
Correlation Between the Chemical Structure of Molecules, the Sign of the Optical Rotation and the
Tilt Angle, Ferroelectrics, 343:1, 127-132
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Ferroelectrics, 343:127–132, 2006
Copyright © Taylor & Francis Group, LLC
ISSN: 0015-0193 print / 1563-5112 online
DOI: 10.1080/00150190600962226
XRD Study of New FLC’s: Correlation Between
the Chemical Structure of Molecules, the Sign
of the Optical Rotation and the Tilt Angle
V. LAPANIK,2 V. BEZBORODOV,2 W. HAASE,1
AND A. LAPANIK1,∗
1Institute of Physical Chemistry, Darmstadt University of Technology, 64287,
Darmstadt, Germany
2Institute of Applied Physics Problems, Belarusian State University,
Minsk 220064, Belarus
New 4-ring FLC’s with different positions of lateral substituents and bridge fragments
in the central core were synthesized. These compounds have a wide temperature range
of the smectic C* phase and similar length of the molecules. Different ways of the
preparation of the FLC mixtures with a high tilt angle and the results of electrooptical
and XRD measurements of the FLC mixtures based upon the chiral compounds are
presented.
Keywords Ferroelectric; liquid crystalline materials; tilt angle; XRD
Introduction
The development of mixtures with a high optical tilt angle and a broad range of the SmC
phase play the key role in the production and the practical application of the ferroelectric
liquid crystalline (FLC) displays. One of the main parameters of FLC cells for displays
is the contrast ratio which is strongly depended on the tilt angle of the FLC mixtures or
single compounds and is responsible for the quality of the image of a display. It is well
known, that the optical tilt angle can be directly measured by the electro-optical methods.
Unfortunately, the accuracy is unsatisfactory and sometimes it is impossible to carry out
the measurements fast and successfully [1]. In contrast to this, the X-ray diffraction (XRD)
on smectic layers allows to measure the tilt angle of the molecules with high accuracy and
does not need much time and good practical skills of the preparation of testing cells. But, it
should be noted, that the XRD data correspond to the tilt angle of the whole molecules in
the smectic layers, whereas the electro-optical measurements – to the angle of more tilted
central core of the molecules, which is always larger than the previous one [2].
Nevertheless, in spite of these differences, it is clear, that the optical tilt angle within
certain chemical classes can be easy estimated from the X-ray data, using the ratio between
the two tilt angles.
Received September 12, 2005.
∗Corresponding author. E-mail: artem.lapanik@web.de
127
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128 V. Lapanik et al.
Chiral mesomorphic 4,4’-disubstituted biphenyls as well as similar terphenyl deriva-
tives are promising candidates for the use in liquid crystal mixtures intended for FLC
displays. These compounds are characterized by low melting points, a wide temperature
range of SmC* phase and high values of the tilt angle. In order to continue our investiga-
tions of the mesomorphic and physical properties of ferroelectric derivatives of biphenyl
and terphenyl and to ascertain the regularities in the change in their tilt angle depending on
the chemical structure of the molecules, as well as the variation of the parameters of the FLC
mixtures containing these compounds, we have prepared the new four-ring FLC compounds
(SC-5, SC-48, SC-90, SC-91, SC-92, SC-94, SC-95, SC-97, SC-98) with different types
and the positions of lateral substituents and bridge fragments in the central core [3]. These
compounds have a wide temperature range of the smectic C∗ phase and similar length of
the molecules.
In this paper we describe the influence of the molecular structure of synthesized com-
pounds on the tilt angle and the layer thickness of Sm C∗ phase.
Results and Discussion
We investigated the main physical properties (tilt angle, temperature range of the SmC∗
phase and spontaneous polarization) and performed X-ray measurements (thickness of the
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XRD Study of New FLC’s 129
Table 1
Electrooptical and X-ray properties of single compounds (SC) and of mixtures
Tilt angle
SmC∗ Spontaneous Tilt angle extracted
temperature polarization [ ◦] d [ ˚A] in d [ ˚A] in from X-ray
Mixtures range [◦C] [nC/cm2] electro-optical SmC∗ SmA data [ ◦]
LBHS-37 <20–120.9 — 27.7 28.6 — —
LBHS- 38 <20–119.5 — 23.2 28.5 — —
LBHS- 39 <20–117.1 — 22.8 28.7 — —
LBHS-40 <20–118.2 — 27.1 28.6 — —
SC-5 37–139 160 44.0 31.2 37.5 33.6
SC-48 44–107 140 36.8 33.6 37.5 26.4
SC-90 <20–113.5 175 35.7 28.4 — 28.1
SC-91 (70.2) — 37.1 26.5 — 29.6
SC-92 (70.3) — 37.7 26.6 — 29.5
SC-94 <20–123.6 170 35.8 28.4 — 28.1
SC-95 52–93 38.5 29.9 — 30.5
SC-97 <20–110.3 172 37.0 25.6 — 29.0
SC-98 <20–132.4 175 38.8 27.5 — 31.0
smectic layers) of all synthesized compounds and some FLC mixtures based upon them.
The obtained results are presented in Tables 1 and 2.
As can be seen from the electro-optical measurements (Table 1) the tilt angle for SC-5
is larger as for SC-48 and for SC-98 is larger than for SC-94. The difference between the
single compounds in these two pairs is only in lateral substituents in the same positions of
the central core: (Cl) for SC-5 and SC-98 and (CH3) for SC-48 and SC-94. The temperature
dependences of the tilt angles of SC-5 and SC-58 are presented in figure 1 and show that
the substitution of the CH3 group by a Cl atom leads to the increasing of the tilt angle by
10 degrees. It was also found that the lateral CH3 derivatives, compared to the Cl derivatives,
have higher transition temperatures from the crystalline to smectic C∗ phase [4].
The XRD data confirmed the results of the electro-optical measurements of these FLC
compounds. As can be seen from Fig. 2, the thickness of the SmC∗ layers of SC-5 is smaller
than of SC-48. Since both compounds have the same molecular length we can conclude
that the tilt angle in case of SC-5 is enlarged, compared to SC-48 (see Table 1). It is clear
that the substitution of a CH3 group by a Cl atom leads to the increase of the tilt angle.
Additionally to these measurements we investigated the influence of the sign of the
optical rotation on the thickness of smectic layers (see Table 1). From the results of the in-
vestigations of four single compounds (SC-90, SC-91, SC-92 and SC-94) and four mixtures
Table 2
FLC mixtures based on left and right handed chiral compounds
Compounds LBHS-37 wt% LBHS-38 wt% LBHS-39 wt% LBHS-40 wt%
SC-90 (S+) 10 30 70 90
SC-94 (R-) 90 70 30 10
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130 V. Lapanik et al.
Figure 1. Temperature dependence of the tilt angle for SC-5 and SC-48.
(LBHS-37 and LBHS-38, LBHS-39 and LBHS-40 (see Table 2)) we have found that the
thickness of the SmC* layers for SC-90 and SC-94 (left and right handed) is in full agreement
with the thickness of SC-91 and SC-92 (right and left handed) (see Table 1).
As can be seen from XRD data there are no changes in the thickness of smectic layers
when we mix right- and left-handed components in different proportions (see Fig. 3), but
the electro-optical measurements indicated that there is a difference in the tilt angles of
these mixtures and of single compounds. In spite of that the tilt angle for each of the two
pairs of single components (SC-90 and SC-94, SC-91 and SC-92) is the same, but for the
FLC mixtures (LBHS-38 and LBHS-39, LBHS-37 and LBHS-40) the tilt angle is lower
(see Table 1).
In continuation of our investigation we have used other pair of FLC compounds (SC-91
and SC-90). Both single compounds are left-handed with the same molecular length. The
difference between them is only the position of the CH3 substituent in the central core.
Figure 2. XRD pattern for SC-5 and SC-48 at 75◦C. (See Color Plate V).
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XRD Study of New FLC’s 131
Figure 3. XRD pattern for LBHS-38 and LBHS-39 at 57◦C. (See Color Plate VI).
X-ray measurements of these compounds have shown that the thickness of the SmC*
layer for SC-91 is lower than for SC-90 (see Table 1 and Fig. 4). It may be explained by a
different packing of molecules inside the layer, leading to the different tilt of them. It should
be noted that these results are in agreement with results from electro-optical measurements
(see Table 1).
For the investigation of the influence of the length of alkyl chain on the tilt angle
we have used the (SC-94 and SC-97) single compounds. The results of the electro-optical
measurements of these compounds demonstrated that the tilt angle of a compound with
Figure 4. XRD pattern for SC 90 and SC 91 at 57◦C. (See Color Plate VII).
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132 V. Lapanik et al.
shorter alkyl chain (SC-97) is higher in comparison to SC-94 (see Table 1). The same
results we have got for other single compounds. The decrease of the length of the alkyl
chain results in a higher tilt angle.
Finally, on the example of the esters (SC-92 and SC-95) we studied the influence of the
positions and the amount of COO bridge fragments on the tilt angle and the thickness of the
smectic layers of the FLC compounds. As can be seen from electro-optical measurements
(see Table 1) the tilt angle of SC-95 is higher. It means that the introduction of an additional
COO fragment in the molecule improves the properties of FLC compounds and leads to the
increasing of the tilt angle.
Conclusion
 A correlation between the chemical structures of new four-ring FLC compounds, the
sign of the optical rotation and the tilt angle have been done.
 The prepared results allow to predict the structures of FLC compounds promising
for the practical application and to choose the right direction for the synthesis of
new FLC compounds with high tilt angle and a wide temperature range of the SmC*
phase.
Experimental
The structures of the prepared compounds were confirmed by 1H-NMR and mass spec-
troscopy. Phase transition temperatures were measured using a Linkam heating stage hav-
ing a polarizing PZO microscope and also by using a Setaram DSC 92. Electro-optical
studies were performed using cells supplied with ITO electrodes (with the resistivity about
30 Ohm/cm2). Aligning layers (nylon 6, polyimide (Al-1254) 100–120 nm) were spun at
3000 rpm rate by a Spincoater and then baked at 180◦C for 1 hour and unidirectional rubbed.
The thickness of the cells was about 2 µm measured in each case interferometrically. During
electro-optic and X-ray measurements the temperature of the cells and of capillaries was
controlled with the accuracy of ±0.3◦C.
XRD measurements were made by using STOE Stadi 4 diffractometer with standard
1, 1.5, 2 mm capillaries (Hilgenberg).
References
1. I. Dierking, J. Phys. Condens. Matter 17, 4403–4410 (2005).
2. S. J. Watson, L. S. Matkin, L. J. Baylis, N. Bowring, H. F. Gleeson, M. Hird, and J. Goodby, Phys.
Rev. E. 65, 03175 (2002).
3. V. Lapanik, V. Bezborodov, and W. Haase, New chiral compounds with high optical anisotropy
and the FLC compositions on their base, Book of Abstracts, 20th International Liquid Crystal
Conference, July 4–9, 2004, Ljubljana, Slovenia, SYN P-020.
4. V. Lapanik, V. Bezborodov, A. Minko, and W. Haase, Mol. Cryst. Liq. Cryst. 439, 1945 (2005).
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XRD Study of New FLC's: Correlation Between the Chemical Structure of Molecules, the Sign of the Optical Rotation and the Tilt Angle

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XRD Study of New FLC's: Correlation Between the Chemical Structure of Molecules, the Sign of the Optical Rotation and the Tilt Angle

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