OPTO−ELECTRONICS REVIEW 17(1), 40–44
DOI: 10.2478/s11772−008−0044−x
Effect of fluorination of molecular rigid core in liquid crystal biphenyl
benzoate based homologous series
P. MORAWIAK1, W. PIECEK*1, M. ŻUROWSKA2, P. PERKOWSKI1, Z. RASZEWSKI1,
R. DĄBROWSKI2, K. CZUPRYŃSKI2, and X.W. SUN3
1Institute
of Applied Physics, Military University of Technology, 2 Kaliskiego Str., 00−908 Warsaw, Poland
of Chemistry, Military University of Technology, 2 Kaliskiego Str., 00−908 Warsaw, Poland
3School of Electrical and Electronic Engineering, Nanyang Technological University,
Nanyang Avenue, Singapore 639798, Singapore
2Institute
Liquid crystalline perfluorinated biphenyl benzoates were synthesized and investigated. A highly tilted, neat orthoconic
smectic antiferroelectric phase was observed. A homologous series of compounds without and with a single fluorine atom
substituted at different positions of a molecular rigid core was investigated by standard methods. Influence of fluorine substi−
tution on physical properties of antiferroelectric smectic phase was discussed as well as influence of fluorination on molecu−
lar dipole moment orientation and its value were presented. Decrease in rotational viscosity as a result of fluorine substitu−
tion within a molecular rigid core was ascribed to changes of molecular packing.
Keywords: liquid crystals, antiferroelectric smectic phase, orthoconic smectic phase, perfluorination, fluorine substitution.
1. Introduction
Among antiferroelectric smectic liquid crystalline materials
(AFLC) being utilised for electrooptical applications those
having high optical tilt are the most promising. When the tilt
angle in AFLC reaches finally 45°, the surface stabilised
AFLC, in its anticlinic state, produces optically negative
uniaxial slab with the optic axis perpendicular to the cell
surfaces [1,2]. When such a cell is placed between two
crossed polarizers and the direction of the incident light
beam is normal to the bounding plates, perfect dark state is
produced (without any light leakage). What is more, the
quality, of this black state completely does not depend on
uniformity of the direction of the smectic layer normal to
polarizer direction, while the optical axis of the AFLC struc−
ture is parallel to the light beam. The electrooptical switch−
ing of such materials between two states, driven by opposite
polarisation of the external electric field, causes rotation of
the director by 90° and simultaneously orientation of the
slow axis of the optical indicatrix in parallel with AFLC slab
plane. Such materials may find very wide spectrum of appli−
cations because the alignment problems causing degrada−
tion of the optical uniformity of the structure are here almost
eliminated. However, wide application of those materials is
still hampered mainly by a very short molecular pitch and
relatively high rotational viscosity. Moreover, according to
active matrix driving requirements, the value of spontane−
*e−mail:
40
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ous polarisation is too high. The contemporary engineering
of orthoconic materials is aimed at elimination of these ob−
stacles. A few attempts were undertaken to figure out what
molecular parameters and properties govern the main
physical properties important for induction of a uniform
bookshelf structure of orthoconic material and its perfect
electrooptical performance.
Among others, those compounds having a molecular rigid
core and build up from biphenyl benzoates exhibit low temper−
ature, wide range, near orthoconic antiferroelectric smectic
phase even they are having very different aliphatic chains at−
tached at terminal positions [3]. A homologous series with
protonated and perfluorinated aliphatic chain have been syn−
thesized and investigated [4]. The observation of the meso−
morphic behaviour of homologous series of compounds with
biphenyl benzoates molecular rigid core proved that the per−
fluorination of the aliphatic chain affects greatly mesomorphic
behaviour of the compound. Extensive investigations show
that the induction of anticlinic properties of those materials is
ascribed mainly to the influence of interactions of the per−
fluorinated aliphatic chains which induce a molecular micro
segregation within molecular layers [5]. During perfluori−
nation of the aliphatic chain, physical properties like spontane−
ous polarisation and tilt are affected to some extend, too. Un−
fortunately, the helical pitch and spontaneous polarisation ob−
served for the obtained compounds within homologous series
utilising the mentioned molecular rigid core with different
terminal chains as well as with side polar groups are still out of
the range needed for applications.
Opto−Electron. Rev., 17, no. 1, 2009
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The limited success of the up to now material engineer−
ing of compounds indicated the aim of this work, which is to
observe the influence of fluorination of the molecular rigid
core on the physical properties of the phase and correlation
of the molecular rigid core polarity with such a phase prop−
erties like tilt angle, spontaneous polarisation, and tilt.
2. Experiment
The mesogenic behaviour of the parent compound and com−
pounds with fluorine substituted molecular core was inves−
tigated by DSC method and routine polarising microscopy
phase transition observations (see Fig. 1). The phase tran−
sition temperatures and enthalpies were determined by differen−
tial scanning calorimetry (DSC) using a SETARAM 141 in−
strument. Some melting temperatures, not detectable upon
cooling, are taken from the DSC measurements which were
done upon heating. Liquid crystal transition temperatures
and phase textures were observed using polarizing optical
microscope BIOLAR PI equipped with the LINKAM 660
hot stage controlled by the TMS 93 unit.
A number of structural and physical parameters were
studied by standard methods [5–7]. The tilt angles q, the
spontaneous polarizations Ps, the switching times t and the
rotational viscosity g were investigated using 1.6−µm thick
cells with ITO electrodes, specially prepared in our labora−
tory. Both cell surfaces were spin−coated by RN 1199 (Nip−
pon Chem.) polyimide. For the uniform orientation, anti−
parallel rubbing was applied. The cells were filled with the
investigated materials by capillary actions at the isotropic
phase. The uniform quasi−bookshelf structures were finally
obtained upon several slow melting−cooling cycles (~0.1
K/min) in the presence of the electric field (E »14 V/µm).
All measurements were done upon cooling from the isotro−
pic (Iso) phases at the cooling rate of about 0.1 K/min. The
spontaneous polarization Ps was evaluated from the integra−
tion of the polarization reversal current peaks in the same
cell as used for the tilt angle measurements under a triangle
electric pulse. The results of the Ps measurements are pre−
sented in Fig. 2.
The tilt angles q were studied by means of optical swi−
tching angle measurements and using uniformly oriented
samples with the planar orientation of the smectic layer nor−
mal (see Fig. 3). The transmitted light intensity vs. the angle
between the direction of the layer normal and the orientation
of the polarizer was obtained for both electric field polariza−
tions. The tilt angle was obtained as a half of the angle be−
tween transmission minima for both polarizations of the
electric field.
The dynamics of the materials under study was investi−
gated under a pulse of square wave of the electric field at
the frequency of 50 Hz. The switching time t10–90 was eval−
Table 1. Phase transition temperatures TD (°C) and enthalpies DH (kJ/mol) for Cf compounds obtained at cooling rate of 5 K/min.
Molecule
Cr
Cf 0
l
SmC*A
60.51
l
23.95
Cf 1
l
57.25
l
42.19
l
l
39.48
19.30
l
74.03
l
69.64
SmA*
124.94
l
108.95
l
79.52
0.02
Iso
126.37
l
l
112.96
l
l
110.98
0.97
l
TD (°C)
DH (kJ/mol)
4.98
l
TD (°C)
DH (kJ/mol)
4.09
104.95
TD (°C)
DH (kJ/mol)
3.52
1.19
0.04
l
l
1.52
0.02
34.38
Cf 3
89.01
0.03
24.32
Cf 2
SmC*
113.07
3.82
l
TD (°C)
DH (kJ/mol)
Fig. 1. Structure of materials under study (a) and graph displaying mesogenic behaviour of investigated compounds (b).
Opto−Electron. Rev., 17, no. 1, 2009
41
P. Morawiak
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Effect of fluorination of molecular rigid core in liquid crystal biphenyl benzoate based homologous series
Fig. 2. Spontaneous polarisation of compounds under study
measured by switching current integration method.
Fig. 3. Tilt angle of materials under study measured by optical
method in 1.56−µm cell with polyimide RN 1199 planar orienting
layers.
uated from the curve of the light intensity change from 10%
to 90% of the maximum transmission upon switching. The
rotational viscosity has been calculated using a practical Eq.
(1) [8]
gj =
1
.
P Et
. s 10 - 90
18
Table 2. Activation energy of parent compound Cf0 and its fluori−
nated analogues.
Compound
Activation energy F (eV)
(2)
where A is the constant, TR is the reduced temperature, and
F is the stand for activation energy. The results are pre−
sented in Table 2.
42
Fig. 5. Arhenius plot for rotational viscosity calculated from
electrooptical measurements.
Cf 0
Cf 1
Cf 2
Cf 3
2.10
2.39
2.30
2.68
(1)
The Arhenius plot has been presented in Fig. 5. The acti−
vation energy was calculated using linear approximation ac−
cording to Eq. (2)
F
,
ln g = ln A +
TR
Fig. 4. Switching time (time on) of materials under study measured
for a square driving pulse of electric field with amplitude of
20 V/µm.
The SAXS investigation was done by using Bruker ap−
paratus with Cu lamp and monochromator. Thin−wall glass
capillary, filled with the material under study, was placed
in a thermo stabilised chamber. The diffraction maxima
were detected upon slow sample cooling (0.1 K/min) from
the isotropic state. The layer spacing d (in ) (see Fig. 6)
was calculated using commercial Bruker software.
Opto−Electron. Rev., 17, no. 1, 2009
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molecular core on the phase properties mainly to the
change of the molecular polarity. At this particular exam−
ple of the homologous series, the fluorination within the
molecular core improves mainly the dynamic properties of
the materials what should be concerned a as a method for
future engineering of effective working materials for LCD
technology.
Acknowledgements
Fig. 6. Layer spacing vs. temperature measured by SAXS method.
References
3. Conclusions
The fluorination of the molecular rigid core apparently af−
fects molecular as well as physical phase parameters. The
fluorine substitution within the molecular rigid core de−
presses the clearing temperature as it was observed earlier
for smectogenic compounds with perfluorinated aliphatic
chains [9]. The depressing ratio depends on both, the num−
ber of fluorine atoms as well as on the place of the substitu−
tion. The isotropic−smectic phase transition enthalpy is
changed significantly due to the fluorination of the molecu−
lar rigid core what suggests the change in the intermolecular
interactions (see Table 1).
As far as the tilt angle presented in Fig. 3 is concerned,
the magnitude of the tilt is depressed when the fluorine atom
is double substituted within the molecular rigid core. The tilt
depressing is not accompanied by the spontaneous polaris−
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In opposite, spontaneous polarisation is significantly lo−
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All effects discussed above are accompanied with the
very small change in molecular packing (see Fig. 6), so
one can ascribe influence of the fluorination within the
Opto−Electron. Rev., 17, no. 1, 2009
This work was done under the Ministry of Science and
Higher Education grant for Polish−Singapore cooperation
no.: Singapore/13/2006 (Grant WAT SPG 29041/WAT/
2006). We would like to express our gratitude to Prof. E.
Górecka and Dr Damian Pociecha (Department of Chemis−
try, Warsaw University, Poland) for the SAXS measure−
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