Japanese Journal of Applied Physics 48 (2009) 081604
REGULAR PAPER
Fast Switching in the Vicinity of Smectic-to-Nematic Phase Transition
in High Birefringence Isothiocyanate Mixtures
Sebastian Gauza, Amanda Parish, Shin-Tson Wu, Anna Spadło1 , and Roman Da˛browski1
College of Optics and Photonics, University of Central Florida, Orlando, FL 32816, U.S.A.
1
Institute of Chemistry, Military University of Technology, 00-908 Warsaw, Poland
Received February 4, 2009; accepted June 2, 2009; published online August 20, 2009
Two series of high birefringence liquid crystal compounds, isothiocyanato phenyl-tolane and terphenyl, and mixtures were prepared and
their physical properties evaluated. Depending on the terminal alkyl chain length, different mesomorphic sequences were observed. In the
vicinity of nematic-to-smectic transition, the visco-elastic coefficient decreases, instead of increase, as the temperature decreases. We
investigated the possibility of extending the observed phenomenon from single compounds to mixtures for practical applications. This
behavior is useful for improving the response time of a liquid crystal device, if the operating temperature can be accurately controlled.
# 2009 The Japan Society of Applied Physics
DOI: 10.1143/JJAP.48.081604
1.
Introduction
Liquid crystal (LC) single compounds and mixtures are
typically designed to exhibit a wide range of single
mesomorphic phase, say nematic or smectic, intended for
an application. This desired mesomorphic phase is carried on
by single compounds, eutectic mixture, or induced within
certain formulations of the mixtures. Numerous reports are
available in the literature regarding binary or multicomponent systems exhibiting induced smectic A (specifically Ad)
phase, reentrant nematic phase and also ferroelectric and
antiferroelectric phases.1–3) Induced phases are often of
interest for phase transition boundary study. Typically close
proximity of the phase transition introduces unexpected
deviations of the major physical parameters, e.g., dielectric
permittivity, elastic constants, flow viscosity or rotational
viscosity, as the order parameter of the LC changes
drastically.4–10) The phase transition between nematic phase
(which has orientational order) and smectic phase (which
has both orientational and positional order) has long been
studied for subtle effects that arise from the intrinsic
coupling of their order parameters.11) To combine the
typical nematic behavior observed by different experimental
techniques (optical polarizing microscopy, miscibility studies, and dielectric spectroscopy) and the smectic-like
fluctuations (short-range positional order) in the systems
with smectic and nematic phase, the term cybotactic nematic
has been used. It is known from studies of X-ray diffraction
patterns that small domains with a local smectic organization
appear in the nematic phase just above the transition point,
and the domains are cybotactic clusters.12)
Recently we have developed some new high birefringence
(n) single compounds13–15) and formulated high n
mixtures for laser beam steering and optical shutter designed
for infrared applications.16–18) These applications require a
fast response time. In order to achieve 5 ms response time,
thin cell gap filled with a high n mixture and operated at
elevated temperature are the common approaches.19–21) LC
system is typically enclosed in a temperature-stabilized oven
because the operating temperature under different laser
power levels has to be maintained. Our research focuses on
elongated rigid core molecular systems like terphenyl and
E-mail address: sgauza@mail.ucf.edu
phenyl-tolane with isothiocyanato (NCS) terminal polar
group. The advantage of using NCS group over the CN
group is mainly on the lower rotational viscosity. However,
the NCS compounds tend to exhibit smectic phases.14) It is
difficult to suppress the smectic phase without sacrificing the
mixture’s electro-optical performance, therefore, we investigated this intrinsic smectic-to-nematic (S–N) phase transition phenomenon of some single LC compounds.22)
In this paper, we present some high n LC mixtures
exhibiting unusual boost of figure-of-merit (FoM) while
operated at a temperature near the S–N transition. Measured
visco-elastic coefficient was significantly reduced near the
S–N transition temperature. As a result, the FoM is
improved. Here, FoM is linked directly to the response time
of a LC device.
2.
Experimental Procedure
Several measurement techniques were used to measure the
physical properties of the single compounds and mixtures.
Differential scanning calorimetry (DSC; TA Instrument Q100) was used to determine the phase transition temperatures. Results were obtained from 3 – 6 mg samples in the
heating and cooling cycles with rate 2 C/min. The electrooptic properties of the LC compounds and mixtures were
measured using 5-mm homogenous cells with indium tin
oxide (ITO) electrodes coated in the inner sides of the
glass substrates. A thin polyimide layer was overcoated
on ITO and buffed in antiparallel directions to produce a
small pretilt angle (2 ). A linearly polarized He–Ne
laser with ¼ 633 nm was used as the light source for the
electro-optic measurements. Experimental setup and measurement technique were the similar to those reported in
ref. 23.
The physical properties of the single compounds were
measured at several temperatures within their nematic phase
and results were used to fit with theoretical models. In this
study, we focused on the birefringence (n), visco-elastic
coefficient ( 1 =K11 ) and figure-of-merit (FoM) which is
defined as24)
FoM ¼ K11 n2 = 1 ;
ð1Þ
where K11 is the splay elastic constant and 1 the rotational
viscosity. The temperature dependent birefringence of a LC
can be described as
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S. Gauza et al.
n ¼ no ð1 T=Tc Þ ;
ð2Þ
where no represents the birefringence at T ¼ 0 K, is a
material constant, and TC is the clearing temperature of the
LC. By fitting the experimental data using eq. (2), we can
obtain no and . Once these two parameters are determined, the birefringence of the LC at room temperature can
be extrapolated. Similarly, we fit FoM with following
equation:24)
T 3
E
FoM ¼ aðno Þ2 1
exp
;
ð3Þ
Tc
T
where a is a fitting parameter. The extrapolated visco-elastic
coefficient at room temperature can be calculated from
eq. (1).
3.
Results and Discussion
Compounds with phenyl-tolane (400 -alkyl-20 ,30 -difluoro-4-(3fluoro-4-isothiocyanato-phenylethynyl)biphenyl; Structure I
and 400 -alkyl-20 ,30 -difluoro-4-(3,5-difluoro-4-isothiocyanatophenylethynyl)biphenyl; Structure II) and terphenyl (4alkyl-300 ,500 -difluoro-400 -isothiocyanato-[1,10 ;40 100 ]terphenyl;
Structure III) rigid cores were chosen for mixtures under
investigation. All three are known as high optical anisotropy LCs and used for high birefringence mixtures
formulation.15)
Structure (I)
F
F
F
R
L
Structure (II)
F
F
F
R
L
F
Structure (III)
F
R
L
F
Structure (IV)
F
R
L
F
Typically for isothiocyanates, homologues with longer alkyl
chains possess smectic and nematic mesophases while
shorter chain members show only a nematic phase.
Included for comparison is a set of fluorinated phenyltolanes (compounds 1a–c and 2a–b) and a pair of fluorinated
terphenyls (compounds 3a–b). Triple and quatro fluorination
of presented phenyl-tolane cores was introduced in order to
lower melting point temperature. Phenyl ring with polar
terminal group was single or di-fluorinated at the position(s)
adjacent to the NCS terminal group. Different alkyl chain
length homologues were intentionally chosen to experience
different mesomorphic sequence within compared sets. All
homologues are listed in Table I with detailed structures
and phase transition temperatures included.22,25,26) Isothiocyanato tolane compounds (Structure IV) are included as a
dilutant component in our experiment.
In general, the trifluoro substituted 400 -alkyl-20 ,30 -difluoro4-(3-fluoro-4-isothiocyanatophenylethynyl)-biphenyl compounds 1a, 1b, and 1c show a wide nematic range with
clearing temperature exceeding 200 C. Only ethyl homologue (C2) does not show a smectic phase. Both propyl (C3)
and butyl (C5) homologues exhibit S–N transition respectively at 89.2 and 135.0 C. For the pentyl homologue, its
H value is 3.4 kcal/mole, which is exceptionally low. The
smectic phase which is present for propyl and pentyl
homologues is particularly attractive for investigating the
S–N transition phenomenon. Similarly, 400 -alkyl-20 ,30 -difluoro-4-(3,5-difluoro-4-isothiocyanato-phenylethynyl)biphenyl (compounds 2a–b) does not show smectic phase for short
alkyl chain homologue (C2; ethyl homologue). Extending an
alkyl chain to five carbons (C5; pentyl homologue) introduces a wide range of smectic phase with S–N phase
transition at 116 C.
Compound 3a, a short chain homologue of 4-alkyl-300 ,500 difluoro-400 -isothiocyanato[1,10 ;40 100 ]terphenyl series, exhibited only nematic phase while pentyl homologue (compound 3b) showed also smectic phase with transition to
nematic at 112.4 C. A lower melting point temperature of
longer alkyl chain compounds, while smectic phase appeared within the mesomorphic sequence, was revealed in
all three sets of the compounds. Compounds without smectic
phase had significantly higher melting temperatures. Therefore, their application potential is limited to mixture
formulations also because their melting point heat enthalpy
is typically significantly higher than their longer alkyl chain
counterparts, Table I.
Tolane isothiocyanates (Structure IV) show no mesomorphic phase while double lateral fluorination is exercised on
the ring with terminal polar group and propyl or pentyl
flexible chains as a second terminal moiety.
As for terphenyl isothiocyanates investigated in our
current experiment, their birefringence was lower than that
of phenyl-tolane compounds. This effect is mainly because
of a shorter, conjugated rigid core as there is no carbon–
carbon triple bond involved in terphenyl structure. Molecular modeling supported length difference of the rigid core of
2.56 Å based on 17.92 and 15.36 Å was calculated for
structures I(II) and III, respectively. HyperChem molecular
modeling software was applied with modified neglect of
diatomic differential overlap (MNDO) calculation method.
The measured temperature dependent birefringence followed the theoretical prediction by eq. (2) regardless of
compound’s rigid core structure and either smectic and
nematic or nematic only mesomorphic sequence. Results are
detailed in our earlier work.22) Figures 1(a) and 1(b) show
the measured visco-elastic coefficient data that exhibit the
opposite trend. Here, it is obvious for the compounds with
smectic phase (1b and 1c) that visco-elastic coefficient
values resulting from relaxation time measurement do not
follow theoretical prediction at the temperatures near the
S–N transition. In fact, the 1 =K11 decreases instead of
increasing as the temperature decreases, leading to 45 and
35% lower than the expected value from theoretical fitting at
the same temperature, respectively, for the phenyl-tolane
and terphenyl compound 1c and 2b. Meanwhile, compounds
without smectic phase behaved within our expectation with
their visco-elastic coefficient following closely the theoret-
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S. Gauza et al.
Table I. Single compounds structures and phase transition temperatures.
No.
R
L
Phase transition temperature ( C)
Compound
F
F
[H] Cr–N
(cal/mol)
F
R
L
1a
C2 H5 –
–NCS
C2PP(23F)TP(3F)NCS
Cr 73.7 N 223.3 Iso
5452
1b
C3 H7 –
–NCS
C3PP(23F)TP(3F)NCS
Cr 82.2 S 89.2 N 240.5 Iso
5814
1c
C5 H11 –
–NCS
C5PP(23F)TP(3F)NCS
Cr 53.5 S 135.0 N 218.0 Iso
3359
F
F
F
R
L
F
2a
C2 H5 –
–NCS
C2PP(23F)TP(35F)NCS
Cr 97.2 N 204.3 Iso
5807
2b
C5 H11 –
–NCS
C5PP(23F)TP(35F)NCS
Cr 46.0 S 116.1 N 202.2 Iso
6380
F
R
NCS
F
3a
3b
C3 H7 –
C5 H11 –
–NCS
–NCS
C3PPP(35F)NCS
C5PPP(35F)NCS
Cr 106.9 N 202.6 Iso
Cr 96.9 S 112.4 N 188.6 Iso
3359
2174
F
R
L
F
4a
C3 H7 –
–NCS
C3PTP(35F)NCS
Cr 51.6 Iso
6300
4b
C5 H11 –
–NCS
C5PTP(35F)NCS
Cr 46.5 Iso
7100
ical prediction. Consequently, the FoM calculated for the
compounds with S–N transition was also affected. As
shown in Figs. 2(a) and 2(b), the FoM of phenyl-tolane
and terphenyl compounds is 38 and 71% higher than the
predicted ones when measured at a temperature close to a
S–N boundary.
Most of single compound liquid crystals possess a
relatively high melting point and their nematic phase
temperature range is not wide enough for practical applications. Therefore formulating eutectic mixtures is a necessary
step. Based on previous results for single phenyl tolane and
terphenyl compounds we formulated several mixtures with
various concentration of the components listed in Table I.
Primary interest in this case was to closely monitor S–N
pretransitional effect on FoM, similar to single compounds
study while changing the temperature of S–N transition.
To shift the temperature of smectic-nematic transition we
balanced formulation of our ‘‘M’’ mixtures by changing
the ratio between phenyl-tolane and terphenyl compounds.
Additionally, mixtures were treated with 10 – 25 wt % of
isothiocyanatotolane compounds (Structure IV) to differentiate more the structures involved into composition and,
therefore, enforce smectic to nematic transition shift toward
lower temperatures. As a result, we obtained several
mixtures based on the same set of components but with
different S–N phase transition temperature. Three representative mixtures (designated as M1, M2, and M3) with S–N
phase transition were chosen for this report. Difference in
the mixture compositions resulted in a gradually decreasing
S–N phase transition temperature as shown in Fig. 3.
Table II shows the concentration of the components in
mixture M1, M2, and M3. The clearing and S–N transition
temperatures ( C) of M1, M2, and M3 are ½184; 115,
½149; 69, and ½120; 31, respectively. Similar to the single
compounds, the temperature dependent birefringence of
these mixtures also follows the theoretical prediction
[eq. (2)] well. Birefringence of the mixtures varies upon
different concentration of Structures I–IV. Moreover significant drop in clearing point temperature results in a lower
n at the extrapolated temperature of 25 C (Fig. 4).
Detailed analysis of a visco-elastic coefficient reveals that
mixtures M1 and M2 exhibit a similar reversed trend as
smectic single compounds.
Figure 5 shows the measured temperature dependent
visco-elastic coefficient of the three mixtures investigated.
The visco-elastic coefficient of M1 and M2 was significantly
lower than the theoretical prediction. Measured 1 =K11 of
M1 and M2 is only 56 and 54% of the theoretical value at
2 C above the respective S–N transition, which is 117 C for
M1 and 71 C for M2. However, mixture M3 shows no
reversed trend in 1 =K11 even in very close proximity to the
S–N transition temperature, as depicted in Fig. 5. Consequently, the FoM of M1 and M2 shows significant increase
(53 and 48%, respectively) while mixture M3’s FoM value
decreases with temperature until nematic to smectic transition takes place at 31 C, as shown in Fig. 6. At this point
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S. Gauza et al.
(a)
(a)
(b)
(b)
Fig. 1. Temperature dependent visco-elastic coefficient of (a)
trifluorinated phenyl-tolane isothiocyanates, and (b) difluorinated
tertphenyl isothiocyanates.
we do not fully understand the reason why the reversed trend
of visco-elastic coefficient was not observed when the S–N
transition took place at below 40 C. Here we consider that
either the amount of tolane compounds (Structure IV) is
too high to enable local smectic domain formation or
impact of overall higher viscosity at lower temperature of
the S–N transition (31 C in case of M3) may diminish
the investigated phenomenon. Increase of the rotational
viscosity, as the temperature decreases, reduces molecule’s
ability for quick relaxation. At a high operational temperature, rotational viscosity is low. As a result, the effect
of reverse change on splay elastic constant can be noticeable.
4.
Conclusions
Detailed study of flow viscosities, dielectric anisotropy and
elastic constants of the cyanobipheyl systems near smectic
to nematic transition are known for cyanobiphenyl type of
liquid crystals and are widely described and discussed by
Fig. 2. Temperature dependent figure of merit of (a) trifluorinated
phenyl-tolane isothiocyanates, and (b) difluorinated tertphenyl isothiocyanates.
Fig. 3. Mesomorphic properties of isothiocyanates based high
birefringence mixtures.
several authors.5–10) The conclusion of some of these
experiments shows that at the temperatures slightly higher
than smectic to nematic transition there is significant,
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Table II. Components concentration in M1, M2, and M3 mixtures
designed for our experiment.
No
Compound
Eut
M1
M2
M3
(wt %)
(wt %)
(wt %)
(wt %)
100
80
60
+
+
+
0
20
60
1a
C2PP(23F)TP(3F)NCS
13
1c
C5PP(23F)TP(3F)NCS
19
2a
2b
C3PP(23F)TP(35F)NCS
C5PP(23F)TP(35F)NCS
3
18
3a
C3PPP(35F)NCS
16
3b
C5PPP(35F)NCS
31
4a
C3PTP(35F)NCS
36
4b
C5PTP(35F)NCS
64
Near eutectic composition of smectogenic compounds and diluter
(4a–b).
Fig. 5. Temperature dependent visco-elastic coefficient of isothiocyanates based high birefringence mixtures.
Fig. 4. Temperature dependent birefringence of isothiocyanates
based high birefringence mixtures.
opposite to theoretical, change of the splay elastic constant
K11 . At the same time, rotational viscosity remains rather
unaffected.27) Phenomenon of opposite change of viscoelastic coefficient reported here is observed for the high
birefringence phenyl-tolane and terphenyl isothiocyanates
and first time for their mixtures. Comparison of our results
on viscoelastic coefficient from measured optical relaxation
time with the results for cyanobiphenyl compounds shows
good agreement. Investigation of isothiocyanato phenyltolanes and terphenyl compounds shows dramatic decrease
of viscoelastic coefficient at the close proximity of a nematic
to smectic phase transition. Consequently, FoM achieves the
values higher than measured at the optimum operating
temperature according to eq. (3). However, to utilize the
observed phenomenon, the device temperature should be
controlled within 1– 4 C. We extended our investigation to
high birefringence isothiocyanato phenyl-tolanes and terphenyl based mixtures. Mixtures, based entirely on laterally
fluorinated phenyltolane isothiocyanates, terphenyl, and
tolane isothiocyanates show similar behavior as reported
for single compounds. Exception within several investigated
systems was M3 mixture. There was no obvious improve-
Fig. 6. Temperature dependent FoM of isothiocyanates based high
birefringence mixtures.
ment observed in close proximity of smectic to nematic
transition. We consider two possible mechanisms of diminishing tailored phenomenon. Firstly amount of tolane
compounds (Structure IV) is too high to enable local
smectic domains formation. Secondly impact of overall
higher viscosity at lower temperature of the smectic to
nematic transition could reduces molecules ability to quickly
relax back and therefore the expected reverse change of
the splay elastic constant cannot be observed. Although,
previously reported experiments indicates adequate thermal
stability of similar materials,16,28) the temperatures at
which we operate single LCs for this experiment could
be too high for reliable long term operation. Therefore, we
will further continue to search similar systems with
reverse trend in 1 =K11 and S–N transition temperature
close to room temperature. The low operational temperature will be beneficial from LCs thermal stability standpoint.
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