Dimesogenic compounds consisting of cholesterol and fluorinated azobenzene moieties: Dependence of liquid crystal properties on spacer length and fluorination of the terminal tail

Article abstract of DOI:10.1039/b516141j

The liquid crystalline (LC) properties of two series of non-symmetric dimesogenic compounds consisting of cholesterol and ring-fluorinated azobenzene mesogenic moieties interconnected by ω-oxyalkanoyl spacers of varying length are compared; one series (F-AOC-n) has the octyloxy tail attached to the fluorinated azobenzene mesogen unit while the other (F-AOCF-n) has the perfluoroheptylmethyloxy tail. In general, compounds with the fluorinated alkoxy tail exhibited a mesophase over a much wider temperature range than those with the alkoxy tail. On the other hand, the F-AOC-n series exhibited a greater variety of mesophases: TGB (twisted grain boundary), chiral smectic C (S_C*), antiphase smectic A (S_A), cholesteric (N*) and solid state smectic (S_X) phases were observed depending on the length of the central spacer, whereas the F-AOCF-n series favored the formation of only the S_A phase with the N* phase completely suppressed. Both series showed an odd-even dependence of the isotropization temperature on the spacer length. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b516141j

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www.rsc.org/materials | Journal of Materials Chemistry
Dimesogenic compounds consisting of cholesterol and fluorinated
azobenzene moieties: dependence of liquid crystal properties on spacer
length and fluorination of the terminal tail{
Woo-Keun Lee,a Kyu-Nam Kim,a Marie France Achardb and Jung-Il Jin*a
Received 14th November 2005, Accepted 20th March 2006
First published as an Advance Article on the web 25th April 2006
DOI: 10.1039/b516141j
The liquid crystalline (LC) properties of two series of non-symmetric dimesogenic compounds
consisting of cholesterol and ring-fluorinated azobenzene mesogenic moieties interconnected by
v-oxyalkanoyl spacers of varying length are compared; one series (F-AOC-n) has the octyloxy tail
attached to the fluorinated azobenzene mesogen unit while the other (F-AOCF-n) has the
perfluoroheptylmethyloxy tail. In general, compounds with the fluorinated alkoxy tail exhibited a
mesophase over a much wider temperature range than those with the alkoxy tail. On the other hand,
the F-AOC-n series exhibited a greater variety of mesophases: TGB (twisted grain boundary), chiral
smectic C (SC*), antiphase smectic A (SA˜ ), cholesteric (N*) and solid state smectic (SX) phases were
observed depending on the length of the central spacer, whereas the F-AOCF-n series favored the
formation of only the SA phase with the N* phase completely suppressed. Both series showed an
odd-even dependence of the isotropization temperature on the spacer length.
probably the Dh phase. De Givenchy et al.28 recently reported
Introduction
that inclusion of perfluorinated tails in a series of 4,49-biphenyl
Dimesogenic compounds consisting of two different mesogenic
derivatives bearing the allylic group formed smectic phases
units interlinked through a central spacer are a relatively new
of high order, the SE and SF phases. More recently, it was
demonstrated by Bilgin-Eran et al.29 that they could prepare a
class of liquid crystalline (LC) compounds that still require
more research in order to establish the relationship between
series of LC chloro-bridged dinuclear Pd organyls carrying
their structure and LC properties.1–22 The possibility of the
fluorinated alkyl chains. The compounds with six semifluori-
formation by those compounds of uncommon phases such as
nated alkyl chains and two alkyl chains revealed hexagonal
incommensurate phases,4–6,8–10 the blue phase8,15,23,24 and the
columnar LC phases with enhanced mesophase stabilities. The
twisted grain boundary (TGB) phases,4,10,22,24 is rather
possible technological applications of these compounds are
intriguing. Most of the non-symmetric dimesogens described
also considered to be very important.
earlier by ourselves4–10,12,13,25 contained a cholesterol moiety as
Recently, we12,25 reported the LC properties of a series of
an integral structural component, carrying several chiral centers.
non-symmetric dimesogenic compounds in which the choles-
Yelamaggad et al.18 also reported the LC properties of dimeso-
terol moiety is interlinked through the v-oxyalkanoyl spacer to
genic compounds in which one of the mesogenic units was the
the second mesogen having either an octyloxy or perfluoro-
cholesterol moiety. Recently, Mallia and Tamaoki19,20 reported
heptylmethyloxy terminal tail. In particular, distinctively
the photochemically driven phase transition in dimesogenic
different mesomorphic behavior was observed for two series
compounds consisting of azobenzene and cholesterol.
of non-symmetric dimesogenic compounds consisting of
The effect of fluorinating aromatic mesogenic cores and of
cholesterol and azobenzene-based mesogens8,12,25 or Schiff
alkyl or alkoxy tails on the LC properties of monomeric
base mesogens8,10,12,13 having an octyloxy or perfluoroheptyl-
compounds has also been the subject of many studies. Wide
methyloxy tail with varying length of the central spacer linking
mesomorphic temperature ranges, low melt viscosities and
the two mesogenic units. In general, fluorination of the tail on
the ferroelectricity of fluorinated compounds (with or without
the azobenzene or Schiff base mesogen12,25 tends to reduce the
chiral centers) appear to be the major findings by these
diversity of mesophases and exhibits a high favor for the forma-
studies.12 There also was an effort to increase optical aniso-
tion of the SA phase. This is ascribed to the strong interac-
tropy of LC compounds by ring fluorination.26 Boden et al.27
tions among the fluorinated segments, whose incompatibility
showed that monofluorination of hexahexyloxy triphenylene
with hydrogenated parts is believed to be strong.12,25
resulted in the formation of the columnar LC phase, most
As part of our continuing effort to understand the structure–
property relationships for the cholesterol-based dimesogens in
aDepartment of Chemistry and Center for Electro- and
this investigation, we have partially fluorinated one of the two
phenylene rings of the azobenzene moiety and also the alkyl
tail. These compounds have different numbers of methylene
units (n), in the central linking group, as shown in the structure
below:
Photo-Responsive Molecules, Korea University, Seoul 136-701, Korea.
E-mail: jijin@korea.ac.kr; Fax: +82-2-921-6901; Tel: +82-2-3290-3123
bCentre de Recherche Paul Pascal, Universite´ Bordeaux I, Av. A.
Schweitzer, 33600 Pessec, France.
{ Electronic supplementary information (ESI) available: spectroscopic
information and elemental analysis. See DOI: 10.1039/b516141j
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7.0 mL of water. And separately 2,3-difluorophenol (5.88 g;
45.2 mmol), NaOH (1.81 g; 45.2 mmol), and Na2CO3 (4.79 g;
45.2 mmol) were dissolved in 15.0 mL of water. The whole
mixture was then stirred for 12 h at room temperature. The
product was extracted with methylene chloride followed by
recrystallization in hexane. A yellowish brown solid product
was obtained, 10.2 g (62.0%), mp 89 uC. dH (300 MHz; CDCl3;
Me4Si): 0.89 (t, CH3), ca. 1.29–1.47 (m, CH2), 1.82 (q, O–CH2–
CH2), 4.00 (t, O–CH3), 5.45 (s, Ar–OH), 6.84 (t, ArH), 6.99 (d,
ArH), 7.53 (t, ArH), 7.90 (d, ArH). FT-IR (KBr, cm21): 3100
(ArC–OH stretching), 2941 and 2869 (C–H stretching), 1602
and 1468 (ArCLC stretching), 1248 and 1169 (C–O stretching).
Elemental analysis: calcd. for C20H24F2N2O2: C 66.28, H 6.67,
N 7.73; found C 66.30, H 6.68, N 7.74%
Fluorination of the two hydrogens on the same side of the
phenylene ring is expected to strongly increase lateral
interactions. The main purpose of the present investigation is
to learn how this structural modification would change the
mesomorphic behavior of the resulting compounds.
Experimental
Synthesis
4-{8-(Cholesteryloxycarbonyl)heptyloxy}49-octyloxy-29,39-
Since the synthetic method of all of the present compounds is
the same (see Scheme 1), representative procedures are given
below in detail only for the preparation of compounds 1 and 2.
For others, only spectroscopic information and elemental
analysis results can be found in the Electronic Supplementary
Information.{
difluoroazobenzene (2) (F-AOC-7)
Cholesteryl-8-bromooctanoate (0.45 g; 0.76 mmol), 4-(4-
octyloxy-2,3-difluorophenylazo)phenol (1) (0.25 g; 0.69 mmol),
and sodium carbonate (0.11 g; 1.00 mmol) were dissolved in
40 mL of DMF. The mixture was stirred for 12 h at 110 uC.
The product was extracted with methylene chloride followed
by recrystallization from a mixture of methylene chloride and
methanol (1 : 20 by vol.). A yellow solid product was obtained,
0.45 g (74.6%), mp 96.4 uC. dH (300 MHz; CDCl3; Me4Si): ca.
0.67–2.04 (m, CH, CH2, CH3), 2.29 (q, O–CH–CH2), 4.03 (t,
CH2–O), 4.10 (t, CH2–O), 5.37 (d, CLCH), 6.84 (d, ArH), 6.99
(d, ArH), 7.53 (t, ArH), 7.90 (d, ArH). FT-IR (KBr, cm21):
4-(4-Octyloxy-2,3-difluorophenylazo)phenol (1)
4-Octyloxyphenylamine (10.0 g; 45.2 mmol) was mixed with
19.0 mL of water, to which concentrated hydrochloric acid
(14.0 mL) was added dropwise. To the mixture was added a
solution of sodium nitrite (3.12 g; 45.2 mmol) dissolved in
Scheme 1 Synthetic route to F-AOC-n and F-AOCF-n.
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2941 and 2869 (C–H stretching), 1730 (CLO stretching), 1602
and 1468 (ArCLC stretching), 1248 and 1169 (C–O stretching).
Elemental analysis: calcd. for C55H82F2N2O4: C 75.65, H 9.46,
N 3.21; found C 75.68, H 9.43, N 3.22%.
(FP-82HT, Mettler, Germany), which was controlled by an
automatic controller (FP-90, Mettler, Germany).
Results and discussion
Phase transitions and thermal stability of mesophases
Experimental techniques
1
The IR and H NMR spectra of the intermediates and final
Tables 1 and 2 summarize the thermal behavior of the two
series of compounds, F-AOC-n and F-AOCF-n, that were
collected from DSC thermograms; the nature of the meso-
phases is also included in the tables.
compounds were recorded on a Bomem MB FTIR spectro-
photometer and a Bruker AM 300 spectrometer, respectively.
Thermal transitions of the LC compounds were studied under
a nitrogen atmosphere using a Mettler Toledo DSC 821e
(Germany) differential scanning calorimeter. The heating and
cooling rates were kept at 2 uC min21. Indium was employed
as a reference material for the calibration of temperature and
enthalpy. The peak maximum or minimum point was taken as
the transition temperature. Thermodynamic parameters for
the transitions were obtained from the DSC thermograms.
The optical textures of the mesophases formed by the com-
pounds obtained were observed using a polarizing microscope
(Olympus BH-2, Japan) equipped with a hot stage (FP-82HT,
Germany) and an automatic temperature controller (Mettler
FP-90, Germany). When the sample formed a homeotropic
texture on a regular glass slide, a rubbed polyimide substrate
was used to observe the optical texture.
The data given in Table 1 tell us that most of the phase
transitions of the F-AOC-n compounds are reversible or
enantiotropic with a notable peculiarity of occurrence for some
of the members (F-AOC-1 and F-AOC-9) of the twisted grain
boundary C (TGBC), and SX phases, with most probably the
SX phases being solid-like smectic phases, monotropically
formed in most of the cases during the cooling cycle.
F-AOC-5 is unique in that it shows most abundant
mesophases including a two-dimensional anti-SmA (SA˜ ) phase
and the solid-like SX phase. It is rather remarkable that
F-AOC-7, -9, and -11 show the TGBC phase monotropically.
A further discussion on the structural analysis of this series will
be made later in detail. In contrast, all the compounds of the
F-AOCF-n series formed only the SA phase enantiotropically.
In spite of the presence of the cholesterol moiety, none of the
members formed the cholesteric phase. Lateral substitution of
the two phenylene protons on the same side of the azobenzene
mesogen with fluorine and the fluorinated tail together
suppressed the formation of cholesteric and other LC phases.
Increased lateral dipolar interaction between the molecules,
and strong affinity among the fluorinated parts, must be
responsible for this contrasting LC behavior of the F-AOCF-
n’s. We25 earlier observed that the same series of compounds
without the lateral fluorine substituents form only the SA
phase when the tail was fluorinated.
X-Ray diffractograms of the compounds were obtained at
˚
varying temperatures using the synchrotron radiation (1.542 A)
of the 3C2 beam line at the Pohang Synchrotron Laboratory,
Korea.
Polarization switching30 of the dimesogenic compounds was
studied using indium tin oxide (ITO) electro-optic cells 1.5 mm
thick coated with a rubbed polyimide. The measurement
instrument consisted of
a function and arbitrary wave
generator (DS345, Stanford Research Inc., USA), a high
voltage amplifier (FA20A, FLC Electronics, Sweden), a
digital storage oscilloscope (54600B, Agilent, USA),
a
The two series exhibit different melting behavior (Fig. 1) as
far as the melting temperature (Tm) dependence on n is
concerned. First, the Tm values (80.3–107 uC) of the F-AOC-n
transimpedance amplifier with an operation amplifier chip
(OPA-627), and a 10 kV detection resistor. During the
measurements the sample cells were placed on a hot stage
Table 1 Thermodynamic data for the phase transitions of the F-AOC-n seriesa
n
Transition temperature (uC) and enthalpy change (kJ mol21
)
DT/„C
(26.1)
Cr 88.4
(0.210)
SC* 139
(36.8)
Cr 105
(0.262)
SC* 131
(0.106)
SX 115
(0.108)
SC* 142
(41.0)
Cr 96.1
(0.398)
SC* 140
(51.9)
Cr 80.3
(1.20)
N* 149
(3.55)
N* 184
(0.730)
N* 143
(5.96)
N* 170
(1.62)
N* 140
(4.34)
N* 156
(2.20)
N* 140
(6.10)
N* 150
(3.09)
N* 136
(2.10)
I 144
(4.65)
I 183
(1.58)
I 142
(5.07)
I 169
(1.68)
I 140
(5.78)
I 152
(2.32)
I 140
(6.41)
I 148
(2.60)
I 135
(0.0910)
N* 75.2
(0.421)
N* 138
(0.223)
N* 72.8
(0.166)
N* 131
(0.204)
N* 63.8
(0.144)
N* 126
(12.0)
N* 38.5
(0.488)
N* 130
(2.39)
N* 48.1
(0.210)
SA 33.1
(0.0511)
SC* 126
(0.516)
SX 49.3
(0.0130)
SC* 96.6
(11.8)
SX 45.2
(0.0205)
SC* 87.9
(3.48)
SX 6.81
(0.0306)
SC* 86.3
(1.73)
SX 8.51
F-AOC-4
(28.5)
F-AOC-5 Cr 89.5 SA
Recr
60.6
(20.4)
SX 48.1 Recr 94.5
(0.0410)
127
(0.705)
60.2
˜
S
˜
A
F-AOC-6
F-AOC-7
F-AOC-8
Recr
38
(41.7)
(32.8)
Cr 100
(42.6)
Cr 107
(0.198)
TGBC 68.6
Recr
S2
70
33
Recr
(36.2)
(0.117)
(3.95)
33.7 Recr 56.4
F-AOC-9 Cr 99.6 S1 126
TGBC 51.2
F-AOC-10
Recr
43.9
(10.8)
SX 31.4 Recr 50.4
(44.5)
Cr 99.6
(0.0295)
F-AOC-11
TGBC 74.1
F-AOC-12
a
Recr
55.7
Estimated from DSC thermogram obtained at a heating and cooling rate of 2 uC min21
.
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Table 2 Thermodynamic data for the phase transitions of the F-AOCF-n seriesa
n
Transition temperature (uC) and enthalpy change (kJ mol21
)
DT/uC
(14.9)
73.6
(26.2)
111
(20.9)
70.6
(24.0)
85.6
(24.3)
69.8
(26.7)
92.4
(24.0)
75.3
(37.0)
97.9
(9.40)
211
(13.7)
224
(9.16)
189
(13.9)
207
(8.99)
178
(14.6)
187
(9.43)
169
(11.4)
175
(11.4)
204
(12.0)
222
(8.82)
184
(14.6)
203
(9.22)
172
(14.4)
183
(9.41)
161
(11.4)
171
(15.6)
40.6
(22.9)
90.1
(21.2)
54.3
(22.3)
59.3
(23.3)
55.8
(28.2)
66.5
(26.1)
59.8
(34.3)
79.2
F-AOCF-4
F-AOCF-5
F-AOCF-6
F-AOCF-7
F-AOCF-8
F-AOCF-9
F-AOCF-10
F-AOCF-11
Cr
Cr
Cr
Cr
Cr
Cr
Cr
Cr
Cr
SA
SA
SA
SA
SA
SA
SA
SA
SA
I
I
I
I
I
I
I
I
I
SA
SA
SA
SA
SA
SA
SA
SA
SA
Recr
Recr
Recr
Recr
Recr
Recr
Recr
Recr
Recr
137.4
113
118.4
121.6
108.2
94.6
93.7
77.1
(28.3)
75.5
(10.0)
160
(10.7)
154
(28.1)
59.4
F-AOCF-12
a
84.5
Estimated from DSC thermogram obtained at a heating and cooling rate of 2 uC min21
.
series are consistently higher, with the sole exception (111 uC)
of F-AOCF-5, than those (70.6–97.9 uC) of the F-AOCF-n
series. Second, the F-AOCF-n series shows the so-called
regular odd-even dependence of their Tm values on n, whereas
a much reduced dependence is observed for the F-AOC-n
series. Although information on the three-dimensional crystal
structures of the present compounds is lacking, the heat of
melting (DHm) data given in Tables 1 and 2 provide at least a
partial answer as to why the Tm values of the F-AOC-n series
are higher than those of F-AOCF-n compounds. The DHm
values of the former range from 26.1 to 51.9 kJ mol21, whereas
the values of the latter are only 14.9–37.0 kJ mol21. In short,
partial fluorination of the phenylene rings of the azobenzene
mesogen and the presence of the fluorinated tail lower the
DHm values and, thus, the Tm values too.
On the other hand, both series commonly reveal a clear cut
odd-even dependence for their isotropization temperature, i.e.
Ti, with those with odd n’s showing higher Ti values than those
with even n’s. And the amplitude of alternation decreases
with increasing n. It should be noted that the apparently odd-
membered spacers actually contain an even number of linking
atoms in the spacer and vice versa, because the ester and ether
groups between the spacers and the two linked mesogens bring
in three additional (one carbon and two oxygen) atoms.
Although there are various molecular theories1–3,17,31 to
explain the odd-even alternation of transition temperatures
with the number of the CH2 groups in the central spacer, it is
evident, in the present cases, that the greater heat of
isotropization (DHi) for the compounds with odd n’s than
those with even n’s results in the higher Ti’s for the former than
the latter. In fact, the DHi values for both series also exhibit the
odd-even alternation, see Tables 1 and 2. Although the values
of the entropy changes, i.e. DSi, are not given in the tables, one
can easily estimate the values from the DSi = DHi/Ti relation
and find that a similar odd-even effect exists in DSi. This
implies that the mesophases of compounds having even
numbers of atoms (i.e. carbon and oxygen atoms in the
present case) in the spacer are of a higher degree of order than
those with odd numbers. The same trend has been repeatedly
observed in monomesogenic compounds1–3 and also in main-
chain liquid crystalline polymers.1,2
This trend also becomes very clear if we compare the Tm and
DHm values of the present two series with those of similar
series containing the unsubstituted azobenzene mesogen,
whose LC properties were previously reported by us.25
A scrutiny of the Tm and Ti values in Tables 1 and 2 and
Fig. 1 tells us that the mesophase temperature range, i.e. DT or
Tm 2 Ti, and Ti of the F-AOCF-n series are much greater than
those of the F-AOC-n series. In other words, the thermal
stability of the mesophase of the F-AOCF-n members is
enhanced by the fluorination of the alkyl tail, if we disregard
the difference in the nature of the mesophases formed by the
two series. The exact same phenomenon was observed earlier
by us25 for AOC-n and AOCF-n series which did not have
fluorine atoms in the azobenzene moiety.
The comparison of the thermal and LC properties of the
F-AOC-n series with those of the unfluorinated AOC-n series
reported earlier by us25 tells us that fluorination in the one of
Fig. 1 The dependence of Tm and Ti on the number of methylene
units (n) in F-AOC-n and F-AOCF-n. The data were taken from the
heating DSC curves.
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the phenylene rings of the azobenzene moiety lowers the Tm
and Ti values of the compounds. This can be ascribed to the
destruction of the symmetry of the chemical structure of the
azobenzene moiety, which is expected to reduce the molecular
packing efficiency.
Fig. 3 Photomicrographs of (a) F-AOCF-8 taken at 153.9 uC on
heating, (b) F-AOCF-12 taken at 84.2 uC on cooling (magnification
6400).
Fluorination, however, increases the diversity of mesophases
formed by the compounds, especially in those related to
smectic orders. In addition, the mesophase temperature range
(i.e. DT or Ti 2 Tm) of the present F-AOC-n series is
consistently greater than that of the unfluorinated AOC-n
series, due to greater suppression in Tm than in Ti by
fluorination. The Tm and Ti values of the fluorinated series,
F-AOCF-n, again are lower than those of the AOCF-n series.25
Both series commonly form only the SA phase. The only
difference observed is that AOCF-10 formed additionally the
SC* phase monotropically, whereas F-AOCF-10 formed only
the SA phase. It appears that fluorination of the tail part
dominates the LC properties of the compounds.
Mesophase structure of the F-AOC-n series
We studied the nature of the mesophases formed by the
present compounds relying on DSC, polarizing microscopy,
and small- (SAXD) and wide-angle X-ray diffractometry
(WAXD). As described above, Tables 1 and 2 list the phase
transition temperatures and nature of the mesophases of the
present two series based on the information gathered by the
three experimental methods just mentioned. Figs. 2 and 3 show
optical textures of broken focal conics (Fig. 2), focal conics
(Fig. 3), and oily streaks (Fig. 2) typical to the SC*, SA, and
cholesteric phases, respectively, and textures that we assign to
be the TGBC and SX phases. F-AOC-5 is peculiar in that its
optical texture in the solid-like SX phase exhibits strings
(Fig. 4). This can probably be related to the so-called banana
phases described by Lee et al.32 Table 3 shows the dependence
of the layer spacings of the compounds in the SA phase on the
length of the central spacers, which were estimated from the
SAXDs. The interesting point to be noted for the F-AOC-n
series is that we observe two long spacings: one close to, but
less than, the estimated lengths of molecules, assuming that the
central spacers and the tails on the azobenzene mesogen are of
anti–trans conformation around the connecting carbon atoms,
and the other is close to the length of the cholesterol moiety
˚
(20 A), but increases steadily with the spacer length or n.
Since the phase transition behavior of the odd n series of
F-AOC-n’s is much more complicated than the even-numbered
series, we will first devote ourselves in a deeper discussion of
the X-ray investigation results of the former series.
Fig. 2 Photomicrographs of (a) F-AOC-7 taken at 103 uC on cooling,
(b) F-AOC-10 taken at 25 uC on cooling, (c) F-AOC-7 taken at 84 uC
on cooling, (d) F-AOC-9 taken at 84 uC on cooling, (e) F-AOC-12
taken at 117 uC on cooling, (f) F-AOC-6 taken at 131 uC on heating
(magnification 6400).
Fig. 4 Photomicrographs of F-AOC-5 taken at 56.0 uC on cooling
(magnification 6400).
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Table 3 Layer spacing in the smectic phases for the F-AOC-n and
F-AOCF-n series
Estimated Estimated
molecular Layer spacing of molecular
Layer spacing of
˚
F-AOC-n/A
˚
length/A F-AOCF-n/A
˚
˚
length/A
n
4
5
6
7
8
9
43.3 and 21.4 (72 uC) 47.0
37.1 and 22.4 (135 uC) 48.1
44.2 and 24.0 (70 uC) 49.1
41.3 and 23.4 (127 uC) 50.3
45.3 and 24.3 (60 uC) 51.4
42.7 and 23.6 (125 uC) 52.6
46.5
48.6
47.5
50.5
50.2
52.6
52.6
54.6
54.6
47.8
48.3
49.2
51.2
52.2
53.2
54.3
55.7
57.6
10 48.8 and 25.0 (35 uC) 53.9
11 50.2 and 25.7 (125 uC) 55.3
12 50.5 and 25.4 (45 uC) 56.7
The F-AOC-5 compound is unique in that it forms a two-
dimensional antiphase, SA˜ . Fig. 5 compares the X-ray patterns
of unoriented and oriented F-AOC-5 samples. In order to
orient the LC molecules, the sample was sandwiched between
two ITO glass plates coated with rubbed polyimide layers. It is
clear from Fig. 5 that the molecular ordering of F-AOC-5 at
125 and 135 uC is not the same: Figs. 5(a) and 5(c) show an
X-ray diffraction typical of the SC* phase. In contrast, the low
temperature diffractions [Figs. 5(b) and 5(d)] show spots at q1
(q = 4p/dsinh = 2p/d where d is the layer spacing; 0.171 A21 or
Fig. 6 X-Ray diffraction intensity profiles of the two phases of
F-AOC-5 at 125 and 135 uC.
Other evidence for the SC*ASA˜ transition was obtained
from the changes in optical textures observed for aligned
samples on a rubbed polyimide-coated surface. As can be seen
from Fig. 7, the transition from the lower temperature
phase (125 uC) to the higher temperature (135 uC) one is
characterized by multiplication of the focal conics, which is
˚
d = 36.7 A) condensed out of the z-axis, whereas those at q3
(0.281 A21 or d = 22.3 A) remain on the z-axis. Moreover, the
˚
q3 spots and the projection of q1 spots on the z-axis reveal the
relation 2q1z = q3, suggesting that they are commensurate. In
other words, the phase at 125 uC is, most probably, a two-
˚
accompanied by a significant change (from 36.74 A at 125 uC
˚
to 36.96 A at 135 uC) in the layer spacing. In short, we can
clearly define all of the mesophases formed by the F-AOC-5
compound: the N*, SC*, SA˜ and SX phases, with the SX phase
being the banana phase.
˚
dimensional antiphase, SA˜ . The long spacing (37.1 A) is
˚
smaller than the molecular length (48.1 A) indicating that in
this phase the molecules are tilted and most probably also
intercalated. Fig. 6 shows the X-ray diffraction intensity
profiles for the corresponding patterns shown in Fig. 5.
F-AOC-7 also shows rather complex phase transitions. Fig. 8
shows the X-ray diffraction pattern of F-AOC-7 obtained at
100 uC (SC* phase) on cooling the isotropic liquid, which
˚
exhibits a sharp outer ring of d # 22–23 A (q3 # 0.273–
0.286 A21) and a diffused inner ring of d # 40 A (q1
=
˚
0.157 A21). The larger (q1 vector) spacing is much less than the
molecular length while the shorter spacing is close to the length
˚
(20 A) of the cholesteric moiety. This X-ray analysis and
optical texture [Fig. 2(b)] together with spontaneous polariza-
tion experiments (Fig. 9) strongly tell us that this mesophase is
the SC* phase. The fact that we observe a very sharp outer
diffraction ring while the inner ring is much diffused [Fig. 5(a)]
implies that we have an intercalated fluidic SC* phase. This
compound exhibited a fairly large spontaneous polarization
(Fig. 9) of 41.8 nC cm22. F-AOC-5, -9, and -11 also exhibited a
Fig. 5 X-Ray patterns of unorientated and orientated F-AOC-5
samples on cooling: (a) and (c) in the SC* at 135 uC; (b) and (d) in the
2D antiphase SA˜ at 125 uC. The q3 spots and the projection of the q1
spots along the z-axis in (d) are commensurate, i.e. 2q1z = q3.
Fig. 7 Change of optical textures between the SA˜ (125 uC) and SC*
phases (135 uC) of F-AOC-5.
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Fig. 8 X-Ray diffraction pattern of F-AOC-7 obtained at 100 uC (the
SC* phase) on cooling the isotropic liquid.
spontaneous polarization of 19.7, 65.0, and 5.2 nC cm22
,
respectively. However we noted that F-AOC-11 contains the
longer spacer and could not be well aligned before the
measurement of spontaneous polarization.
Fig. 10 Dependence of layer spacing on temperature of F-AOC-7.
spacing value continuously decreases as the temperature
Fig. 10 represents the revolution of the layer spacings.
Surprisingly, the two spacings show very different temperature
˚
dependence: the smaller spacing (ca. 23–24 A) continuously
is lowered from 130 to 96 uC where an upturn is observed.
˚
The extent of the change (41–38.5 A) is significantly larger
˚
than the case (23.4–23.7 A) of the smaller spacing. This
increases as we lower the temperature from 130 to 80 uC with a
slight change in slope at about 95 uC. On crystallization its
˚
value increases abruptly to 25.6 A. We also observe a sudden
contrasting temperature dependence is ascribed to a combined
change in the intercalated structure and the tilt angle in the
SC* phase.
decrease in the magnitude of this spacing at 130 uC. All the
breaking points coincide with the transition temperatures of
this compound (F-AOC-7) determined by DSC analysis and
polarizing microscopy: N* 131 SC* 96.6 TGBC 68.6 Recr. On
the other hand, the temperature dependence of the larger
spacing related to the molecular length is very contrasting, but
the same transition temperatures reveal changes in the slope.
Since the long spacings have been estimated from much-
diffused diffractograms (Fig. 11), one should understand that
there are many fluctuations in the values of this spacing.
Nevertheless, the most striking phenomenon is that this
Since it is well known that the tilt angle of the SC* phase
increases as the temperature is lowered,33 the temperature
dependence of the larger spacing appears to parallel the
general observations. The smaller spacing, however, changes in
the opposite direction, which we believe implies that the degree
of intercalation decreases as the temperature is lowered. In
short, the temperature dependence of the tilt angle seems to
overwhelm the change in the intercalated structure. One
intriguing observation is that a new mesophase appears on
cooling between the SC* and crystalline phases. This phase
Fig. 9 Switching current response in the SC* phase of F-AOC-7
obtained by applying a triangular voltage wave (30 V mm21, 100 Hz)
at 110 uC.
Fig. 11 X-Ray diffractogram obtained for F-AOC-7 at 125, 100, and
80 uC (q = 2p/d).
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appears to be the TGBC phase whose X-ray diffraction
˚
crystallization, as we can see from the appearance of sharp
low-angle diffraction peaks (Fig. 12, diffractogram at 45 uC).
This phase (S2), however, shows a different diffraction pattern
as compared with the pattern observed for the S1 phase on
(Fig. 11) shows two peaks at 2h = 2.14u (d = 41.3 A) and 3.73u
˚
(d = 23.7 A). When compared with the diffraction of the SC*
phase, the latter peak is much broadened. However, it requires
a further study to confirm this assignment of the phase,
because the TGBC phase usually is observed between the N*
and smectic phases.
˚
heating. In the former, the peak at 2h = 2.07u (d = 42.7 A) is
relatively sharp [see the diffraction peak (Fig. 12) taken at
45 uC on cooling], whereas the latter shows a sharp peak at
˚
The X-ray analysis results of the F-AOC-9 compound are
basically the same as those above described for F-AOC-7
with a remarkable exception that it most probably forms a
solid-like smectic phase on cooling, the SC* phase, before
2h = 3.63u (d = 24.4 A). Also, again it appears that F-AOC-9
forms the TGBC phase on cooling between the SC* and S2
phases. F-AOC-11 shows a very similar evolution of X-ray
intensity profiles as F-AOC-7 and F-AOC-9.
Fig. 12 X-Ray diffractogram obtained for F-AOC-9 (q = 2p/d).
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4 F. Hardouin, M. F. Achard, J.-I. J. J. W. Shin and Y.-K. Yun,
J. Phys. II, 1994, 4, 627–643.
5 F. Hardouin, M. F. Achard, J.-I. Jin and Y.-K. Yun, J. Phys. II,
1995, 5, 927–935.
As far as the nature of the mesophases formed by the even n
series, X-ray analysis results coincide with the phase assign-
ments given in Table 1.
6 F. Hardouin, M. F. Achard, J.-I. Jin, Y.-K. Yun and S.-J. Chung,
Eur. Phys. J. B, 1998, 1, 47–56.
7 F. Hardouin, M. F. Achard, M. Laguerre, J.-I. Jin and D.-H. Ko,
Liq. Cryst., 1999, 26, 589–599.
8 S. W. Cha, J.-I. Jin, M. Laguerre, M. F. Achard and F. Hardouin,
Liq. Cryst., 1999, 26, 1325–1337.
9 D. W. Lee, J.-I. Jin, M. Laguerre, M. F. Achard and F. Hardouin,
Liq. Cryst., 2000, 27, 145–152.
X-Ray analysis of the F-AOCF-n series
The situation is much simpler for the structural analysis of the
F-AOCF-n series. As shown in Table 2, this series forms only
the SA phase, and the long spacings for their SA phase
determined by SAXD are included in Table 3. Moreover, we
˚
do not observe the second or shorter spacing of about 23 A for
10 S. W. Cha, J.-I. Jin, M. F. Achard and F. Hardouin, Liq. Cryst.,
2002, 29, 755–763.
this series. The long spacing values are slightly shorter, with
the single exception of F-AOCF-5, than the estimated
molecular lengths. There are two possibilities for this
observation: the presence of the gauche conformation around
some carbons in the central spacer, and the alkoxy tails or
slight intercalation between the molecules. It is our conjecture
that both irregularities cause the slightly reduced spacings for
the SA phase when compared with the molecular lengths
estimated for fully extended conformations.
11 V. Prasad, K. H. Lee, Y. Park, J.-W. Lee, D. K. Oh, D. Y. Han
and J.-I. Jin, Liq. Cryst., 2002, 29, 1113–1119.
12 J.-W. Lee, Y. Park, J.-I. Jin, M. F. Achard and F. Hardouin,
J. Mater. Chem., 2003, 13, 1367–1372.
13 Y. Park, K. H. Lee, J.-W. Lee and J.-I. Jin, Liq. Cryst., 2003, 30,
173–179.
14 V. Faye, A. Babeau, F. Placin, H. T. Nguyen, P. Barois, V. Laux
and N. Isaert, Liq. Cryst., 1996, 21, 485–503.
15 A. E. Blatch, I. D. Fletcher and G. R. Luckhurst, J. Mater. Chem.,
1997, 7, 9–17.
16 M. J. Wallage and C. T. Imrie, J. Mater. Chem., 1997, 7,
1163–1167.
17 C. J. Dunn, P. J. L. Masurier and G. R. Luckhurst, Phys. Chem.
Chem. Phys., 1999, 1, 3757–3764.
Conclusions
We observe a very interesting mesophase transition behavior
for a series of dimesogenic compounds consisting of choles-
terol and laterally substituted difluoroazobenzene moieties
interconnected through oligomethylene spacers of varying
length. Especially, the compounds of odd numbered spacers
bearing octyloxy tail exhibit much more complicated phase
transition behavior than those of the even series, for which
molecular interpretation is lacking at the present moment. The
appearance of the TGBC phase monotropically between SC*
and SX or the crystalline phase is extremely intriguing, which
we believe deserves a further detailed study. F-AOC-5 is
unique in that it forms a solid-like mesophase that is very
similar to those reported for so-called banana shaped
mesocompounds.
18 C. V. Yelamaggad, A. Srikrishna, D. S. S. Rao and S. K. Prasad,
Liq. Cryst., 1999, 26, 1547–1554.
19 V. A. Mallia and N. Tamaoki, J. Mater. Chem., 2003, 13, 219–224.
20 V. A. Mallia and N. Tamaoki, Chem. Commun., 2004, 2538–2539.
21 D. Pociecha, D. Kardas, E. Gorecka, J. Szydlowska,
J. Mieczkowski and D. Guillon, J. Mater. Chem., 2003, 13, 34–37.
22 H. T. Nguyen, M. Ismaili, N. Isaert and M. F. Achard, J. Mater.
Chem., 2004, 14, 1560–1566.
23 J. Rokunohe and A. Yoshizawa, J. Mater. Chem., 2005, 15,
275–279.
24 A. Yoshizawa, M. Sato and J. Rokunohe, J. Mater. Chem., 2005,
15, 3285–3290.
25 K.-N. Kim, E.-D. Do, Y.-W. Kwon and J.-I. Jin, Liq. Cryst., 2005,
32, 229–237.
26 M. Hird, K. J. Toyne, J. W. Goodby, G. W. Gray, V. Minter,
R. P. Tuffin and D. G. McDonnell, J. Mater. Chem., 2004, 14,
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27 N. Boden, R. J. Bushby, A. N. Cammidge, S. Duckworth and
G. Headdock, J. Mater. Chem., 1997, 7, 601.
When the octyloxy tail was replaced with the corresponding
perfluoroalkoxy group, all the compounds formed only the SA
phase indicating that intermolecular attraction among the
fluorinated part overwhelms any other interactions, most
probably leading to a phase separation on the micro scale
between the fluorinated and unfluorinated parts.
28 E. T. de Givenchy, F. Guittard, L. Caillier, J. Munuera and
S. Geribaldi, Mol. Cryst. Liq. Cryst., 2005, 436, 237.
29 B. Bilgin-Eran, C. Tschierske, S. Diele and U. Baumeister,
J. Mater. Chem., 2006, 16, DOI: 10.1039/b511758e.
30 J. W. Goodby, R. Blinc, N. A. Clark, S. T. Lagerwall,
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Ferroelectric Liquid Crystals: Principles, Properties and Applica-
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31 C. T. Imrie, D. Stewart, C. Remy, D. W. Christie, I. W. Hamley
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32 C. K. Lee, A. Primak, A. Jakli, E. J. Choi, W. C. Zin and
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