Mixtures of disc-shaped and rod-shaped mesogens with chiral components

Article abstract of DOI:10.1039/b314342b

A novel mesogen has been synthesised, that not only combines two one disc-shaped and rod-shaped mesogens, but also includes a chiral group in the centre of the molecule. A protection and selective deprotection procedure allows the introduction of the chir

Full text of DOI:10.1039/b314342b

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A R T I C L E
Mixtures of disc-shaped and rod-shaped mesogens with chiral
components
Paul H. J. Kouwer, Chris J. Welch, Graeme McRobbie, Ben J. Dodds, Lee Priest and
Georg H. Mehl*
University of Hull, Department of Chemistry, Cottingham Road, Hull, UK HU6 7RX.
E-mail: g.h.mehl@hull.ac.uk; Fax: 144 1482 466411
Received 10th November 2003, Accepted 9th March 2004
First published as an Advance Article on the web 15th April 2004
A novel mesogen has been synthesised, that not only combines two one disc-shaped and rod-shaped mesogens,
but also includes a chiral group in the centre of the molecule. A protection and selective deprotection
procedure allows the introduction of the chiral group in the central linking group of the mesogen. However,
using this versatile procedure, a variety of other groups can be introduced selectively into the mesogen. The
chiral group induces the formation of a cholesteric phase. Interestingly, the N* mesophase is miscible with the
nematic phases of the precursor discs and rods. This study provides further evidence for the miscibility of discs
and rods in a (chiral) nematic phase.
rod-shaped mesogens. This allowed us to construct the first
experimental continuous phase diagram between rods and
discs, where a nematic phase was observed.4a
Chirality plays an important role in the phase formation and
also in the applications of liquid crystals. Therefore, the
introduction of a chiral centre in this new class of covalently
linked disc–rod mesogens is a logical extension of current
research.
Usually in liquid crystals, both for calamitic and columnar
systems the chiral centre is placed in one of the tails close
to the mesogenic core.5 Hence, one of the three hydroxyl
groups of gallic acid was selectively substituted with a chiral
2-methylbutyl group, leaving the other two positions free
for the calamitic cyanobiphenyl groups, see Fig. 2. This con-
tribution will discuss the convergent and versatile synthesis
of 1. The investigation of the mesomorphic properties of this
system, which is built up in a modular way will be reported.
Furthermore the role of the chiral group is investigated by
comparing the phase behaviour of 1 with that of corresponding
disc–rod mesogens with only two or three cyanobiphenyl
moieties (8 and 9 in Fig. 3).
Introduction
In the last few decades, the field of disc-shaped (discotic)
mesogens has developed enormously.1 After their discovery in
1977,1d a wide variety of columnar mesophases has been
reported and also a number of nematic phases has been
observed, though the structural variety has been found to be
smaller than that found for rod-shaped (calamitic) liquid
crystals. Despite the similarities, the fields of the calamitic and
discotic mesogens have not merged, mainly because of shape-
incompatibility of the two mesogens, which results in macro-
scopic phase separation. To avoid separation, the two entities
have been linked covalently2 and monotropic nematic and
(soft) crystal phases have been found in such systems.
Recently, we have reported on the phase behaviour of novel
amphiphilic mesogens that contained two or three rod-shaped
mesogens and a disc-shaped mesogen connected to a central
linking group via flexible spacers. When the discotic and
calamitic moieties are incompatible, the molecular topology
induces the formation of a smectic phase, with alternating
layers of discs and rods,3 see Fig. 1. A compatible combination
of discotic and calamitic mesogens resulted in the formation
of a nematic phase,4 see Fig. 1. Interestingly, these mesogens
are miscible with both classical disc-shaped mesogens and
Experimental section
General
Discotic mesogen
7 and 4’-(10-bromodecyloxy)biphenyl-
4-carbonitrile were prepared according to literature pro-
cedures.4b,6 Other materials were used as purchased. THF
was distilled from sodium prior to use.
Fig. 1 Covalently linked disc–rod mesogens.3,4 The mesogens are
connected via flexible spacers, which allows the formation of a variety
of structures. Here are shown the (mixed) nematic phase for a
compatible system and an SmA2 phase from a phase segregating
incompatible system.
Fig. 2 Investigated chiral disc–rod mesogen.
1 7 9 8
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 1 7 9 8 – 1 8 0 3
T h i s j o u r n a l i s ß T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

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7.0 g, mmol) and THF (100 mL) was stirred at room tem-
perature. Triphenylphosphine (9.0 g, 34 mmol) was added
dropwise over 30 min and the mixture was stirred overnight at
room temperature. The solvent was removed under reduced
pressure and the crude oil was purified by column chromato-
graphy (SiO2, eluent CH2Cl2) to yield a yellowish oil (7.4 g,
23 mmol, 74%).
The orthoformate protective group was removed by stirring
the product with para-toluenesufonic acid (p-TSA, 1 g) in
methanol (150 mL) and water (20 mL) at 40 uC for 16 h. The
reaction mixture was diluted with an NaHCO3 solution, the
methanol was evaporated and the aqueous mixture was washed
with CH2Cl2 (3 times). The combined organic layers were
washed with NaHCO3, water and brine, dried over MgSO4 and
the solvent was evaporated. Pure 4 was obtained after column
chromatography [SiO2, eluent CH2Cl2 to CH2Cl2/methanol
(98:2)] as a viscous colourless oil (5.2 g, 19 mmol, 84%).
1H NMR (CHCl3): d 7.34, 7.17 [2 6 d, J(H,H) ~ 1.8 Hz,
4
2 6 CH aromatic, 2H]; 5.93 (br s, OH, 2H); 4.32 [q, 3J(H,H) ~
7.1 Hz, CH2OCO, 2H]; 3.90, 3.86 [2 6 dd, 2J(Ha,Hb) ~ 32 Hz,
Fig. 3 Structures of previously corresponding rod-containing dimer
and trimer 8 and corresponding covalently linked disc–rod mesogens 9.
3
3J(Ha,H) ~ 6.5 Hz, J(Hb,H) ~ 5.9 Hz, OCH2C*, 2H]; 1.87
Instrumental
(m, C*H, 1H); 1.53, 1.26 (2 6 m, CH2 aliphatic, 2H); 1.35 [t,
3J(H,H) ~ 7.1 Hz, CH3CH2O, 3H]; 1.00 [d, 3J(H,H) ~ 6.8 Hz,
Nuclear magnetic resonance (NMR) spectra were taken on a
Jeol JNM-ECP 400 MHz FT-NMR spectrometer. Chemical
shifts are reported in ppm relative to TMS. In some 13C NMR
spectra, multiple peaks are reported because of the chemical
dissimilarity of seemingly identical carbon atoms. A detailed
peak assignment has been published in refs. 4a and 4b. The
thermal properties were investigated using a Perkin Elmer DSC
7 differential calorimeter (DSC) in nitrogen against an indium
standard. Transition temperatures were determined as the
onset of the maximum in the endotherm or exotherm. The
mesophases were studied on an Olympus BH-2 optical
polarising microscope, equipped with a Mettler FP82 HT
hot stage and a Mettler FP90 central processor. Pictures of the
mesophases were taken using a JVC digital video camera
connected to a PC. Software Studio Capture, supplied by
Studio86Designs was used for image capturing. X-Ray diffrac-
tion measurements were performed on an MAR345 diffract-
ometer with a 2D image plate detector (CuKa radiation,
3
CH3C*, 3H], 0.92 [t, J(H,H) ~ 7.4 Hz, CH3CH2C*, 3H].
13C NMR (CHCl3): d 166.64 (CLO); 145.98, 143.34, 136.99,
121.87, 110.72, 105.58 (6 6 C aromatic); 74.05 (PhOCH2);
60.95 (CH2OCO); 34.53, 26.00, 16.42, 14.27, 11.19 (CH, CH2
and CH3 aliphatic).
3,4-Bis-[10-(4’-cyanobiphenyl-4-yloxy)-decyloxy]-5-(2-methyl-
butoxy)benzoic acid ethyl ester (5). A mixture of 4 (0.4 g,
1.2 mmol), 4’-(10-bromodecyloxy)biphenyl-4-carbonitrile4b (1.3g,
3.3 mmol), K2CO3 (0.7 g, 5.1 mmol), KI (0.1 g, 0.2 mmol) and
butanone (50 mL) was refluxed for 16 h. The mixture was
cooled, the solids were filtered and washed thoroughly with
acetone. The solvent was evaporated and the crude product
was subjected to column chromatography [SiO2, eluent
CH2Cl2/hexane (50:50) to CH2Cl2]. Pure 5 was obtained
after crystallisation from methanol/CH2Cl2 as white crystals
(0.83 g, 0.95 mmol, 77%).
1H NMR (CHCl3): d 7.67, 7.62, 7.51, 6.98 (4 6 dd, 4 6 CH
biphenyl, 16H); 7.24 (s, CH aromatic, 2H); 4.32 [q, 3J(H,H) ~
˚
graphite monochromator, l ~ 1.54 A). The samples were
heated in the presence of a magnetic field using a home-built
capillary furnace.
3
7.3 Hz, CH2OCO, 2H]; 4.01, 3.98 [2 6 t, J(H,H) ~ 6.5 Hz,
2
CH2CH2OPh, 8H]; 3.84, 3.81 [2 6 dd, J(Ha,Hb) ~ 32 Hz,
3
3J(Ha,H) ~ 6.5 Hz, J(Hb,H) ~ 5.9 Hz, OCH2C*, 2H]; 1.9–
Synthesis
3
1.2 (m, CH, CH2 aliphatic, 35H); 1.38 [t, J(H,H) ~ 7.2 Hz,
CH3CH2O, 3H]; 1.03 [d, 3J(H,H) ~ 6.8 Hz, CH3C*, 3H]; 0.94
2-Ethoxy-7-hydroxy-benzo[1,3]dioxole-5-carboxylic acid ethyl
ester (3). A mixture of ethyl gallate (10.0 g, 71.4 mmol),
amberlite IR1 20 (0.5 g), triethylorthoformate (15.8 g,
3
[t, J(H,H) ~ 7.3 Hz, CH3CH2C*, 3H].
13C NMR (CHCl3): d 166.44 (CLO); 159.75, 152.92, 152.75,
145.21, 142.14, 132.53, 131.19, 128.28, 127.01, 125.05, 119.09,
115.02, 109.98, 107.82, 107.77 (C, CH aromatic); 73.84, 73.41,
69.06, 68.10, 60.97 (CH2O); 34.79, 30.52, 29.6–29.1, 26.09,
26.02, 16.62, 14.39, 11.31 (CH2, CH3 aliphatic).
˚
107 mmol), freshly ground 4 A molecular sieves (4 g) and
toluene (200 mL) was heated at 100 uC for 1 h. Ethanol was
removed by means of a Dean–Stark trap. The reaction mixture
was cooled to room temperature, the solids were filtered and
the toluene and excess of triethylorthoformate was evaporated
under reduced pressure. The crude product (14.5 g, 57.1 mol,
80%) crystallised upon standing and was used without further
purification for the next step.
3,4-Bis-[10-(4’-cyanobiphenyl-4-yloxy)-decyloxy]-5-(2-methyl-
butoxy)benzoic acid (6). A mixture of 5 (0.83 g, 95 mmol),
ethanol (50 mL) and a 1 M NaOH solution (5 mL) was refluxed
for 2 h. The warm solution was neutralised with concentrated
HCl. The reaction mixture was cooled to 220 uC and the pro-
duct was obtained by filtration and dried. The free carboxylic
acid (0.77 g, 0.95 mmol, 98%) was used without further
purification in the esterification reaction.
4
1H NMR (CHCl3): d 7.45, 7.17 [2 6 d, J(H,H) ~ 1.4 Hz,
2 6 CH aromatic, 2H]; 6.94 (s, OH, 1H); 6.65 (s, CHO3, 1H);
4.34 [q, 3J(H,H) ~ 7.2 Hz, CH2OCO, 2H]; 4.34 [q, 3J(H,H) ~
3
7.2 Hz, CH2OCH, 2H]; 1.36, 1.25 [2 6 t, J(H,H) ~ 7.2 Hz,
2 6 CH3, 6H].
13C NMR (CHCl3): d 166.72 (CLO); 147.11, 138.88, 137.09,
124.10, 119.90, 113.97, 102.44 (6 6 C aromatic); (CH); 61.41,
59.53 (2 6 CH2); 14.66, 14.14 (2 6 CH3).
1H NMR (CHCl3): d 7.67, 7.62, 7.51, 6.98 (4 6 dd, 4 6 CH
biphenyl, 16H); 7.26 (s, CH aromatic, 2H); 4.01, 3.98 [2 6 t,
3J(H,H) ~ 6.5 Hz, CH2CH2OPh, 8H]; 3.84, 3.81 [2 6 dd,
3
3
2J(Ha,Hb) ~ 32 Hz, J(Ha,H) ~ 6.5 Hz, J(Hb,H) ~ 5.9 Hz,
3,4-Dihydroxy-5-(2-methylbutoxy)-benzoic acid ethyl ester (4).
A mixture of 3 (8.0 g, 31 mmol), S-(2)-2-methylbutan-1-ol
(2.5 g, 36 mmol) and diisopropyl azodicarboxylate (DIAD,
OCH2C*, 2H]; 1.9–1.2 (m, CH, CH2 aliphatic, 35H); 1.03 [d,
3
3J(H,H) ~ 6.8 Hz, CH3C*, 3H]; 0.95 [t, J(H,H) ~ 7.3 Hz,
CH3CH2C*, 3H].
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 1 7 9 8 – 1 8 0 3
1 7 9 9

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13C NMR (CHCl3): d 170.74 (CLO); 159.76, 153.00, 152.83,
145.23, 142.99, 132.55, 131.22, 128.29, 127.03, 123.55, 119.10,
115.04, 110.00, 108.42, 108.39 (C, CH aromatic); 73.89, 73.50,
69.11, 68.12 (CH2O); 34.77, 31.34, 29.6–29.1, 26.16, 26.10,
16.60, 11.32 (CH2, CH3 aliphatic).
3,4-Bis-[10-(4’-cyanobiphenyl-4-yloxy)-decyloxy]-5-(2-methyl-
butoxy)benzoic acid 11-[pentakis(4-methoxyphenylethynyl)phe-
noxy]undecyl ester (1). A mixture of 6 (136 mg, 150 mmol),7
(91 mg, 100 mmol), para-toluenesulfonic acid (p-TSA 9.5 mg,
50 mmol), 4-(dimethylamino)pyridine (DMAP 6.1 mg, 50 mmol),
N,N-dicyclohexylcarbodiimide (DCC, 206 mg, 1 mmol) and
CH2Cl2 (6 mL) was stirred for 3 d at room temperature. TLC
indicated complete consumption of 7. The reaction mixture was
transferred to a column for purification [SiO2, eluent gradient
of CH2Cl2/hexane (2:1) to CH2Cl2]. Yield: 83 mg (46 mmol,
46%) of a pale yellow solid.
Scheme 2 Reagents and conditions: (i) DCC, p-TSA, DMAP, CH2Cl2,
5 d at room temperature.
1H NMR (CHCl3): d 7.68–7.46 (m, CH aromatic, 22H); 7.25
(s, CH aromatic, 2H); 6.99–6.86 (m, CH aromatic 14H); 4.32 [t,
3J(H,H) ~ 6.3 Hz, CH2OCO, 2H]; 4.27 [t, 3J(H,H) ~ 6.7 Hz,
CH2OPhdisc, 2H]; 4.03–3.94 (m, CH2CH2OPh, 8H); 3.84–3.83
(3 6 s, OCH3, 5H, 10H, 10H); 3.84, 3.81 [2 6 dd, 2J(Ha,Hb) ~
32 Hz, 3J(Ha,H) ~ 6.5 Hz, 3J(Hb,H) ~ 5.9 Hz, OCH2C*, 2H];
1.9–1.2 (m, CH, CH2 aliphatic, 57H); 1.02 [d, 3J(H,H) ~ 6.8 Hz,
CH3C*, 3H]; 0.94 [t, 3J(H,H) ~ 7.4 Hz, CH3CH2C*, 3H].
13C NMR (CHCl3): d 166.49 (CLO); 160.02, 159.91, 159.88,
159.83, 159.76, 152.93, 152.75, 145.22, 142.19, 133.26, 133.13,
133.10, 132.53, 132.51, 131.20, 131.16, 128.40, 128.28, 127.02,
125.10, 123.84, 119.85, 119.10, 115.71, 115.56, 115.47, 115.02,
114.11, 114.07, 109.99, 109.94, 107.86 (C, CH aromatic); 99.17,
98.99, 96.94, 86.62, 86.09, 83.56 (CMC); 74.62, 73.86, 73.43,
69.09, 68.11, 65.18 (OCH2); 55.33, 55.31 (OCH3); 34.78, 32–26,
22.63, 22.58, 16.61, 14.10, 11.32 (CH2, CH3 aliphatic).
DCC-mediated esterification, catalysed by a mixture of p-TSA
and DMAP. The orthoformate protecting group proved a
successful route towards specific functionalisation of the gallic
acid core. It is anticipated that a wide variety of functional
groups can be incorporated at the central position of the
molecule. This opens up routes to novel mesophases with
complex self-organising structures, as has been observed
already for laterally substituted mesogens.8
For comparison, molecules analogous to 5 with two and
three cyanobiphenyl mesogens have been synthesised (8a and
8b in Fig. 3). Coupling with the discotic moiety under similar
conditions afforded linked disc–rod mesogens 9a and 9b. The
synthesis and some of the thermal properties of these mesogens
have been reported recently.4
Mesomorphic properties
Results and discussion
The mesomorphic properties of 1, 9 and the precursors were
studied using differential scanning calorimetry (DSC), optical
polarising microscopy (OPM) and X-ray diffraction (XRD).
The results are summarised in Table 1. Mesogen 5 shows a
narrow cholesteric phase, characterised by the typical finger-
print and/or less clear Granjean textures (see Fig. 4a,b). At
lower temperatures, 5 shows a transition to the SmA phase.
The optical texture shows large homeotropic and small grainy
areas (Fig. 4c). No focal conics were observed. Shearing gives
rise to a full homeotropic alignment of the material. The non-
chiral dimer (8a) and trimer (8b) show a (monotropic) nematic
phase only, typically characterised by a Schlieren texture, see,
for example, Fig. 4d.
Synthesis
The synthesis of the chiral disc–rod mesogens is outlined in
Scheme 1 and 2. The regioselective introduction of the chiral
group to the central aromatic core required that two hydroxyl
groups of ethyl gallate were protected with triethylorthofor-
mate using a modified literature procedure.7 After etherifica-
tion under Mitsonobu conditions, the orthoformate group was
cleaved under mild conditions, leaving the carboxylic ester
group intact. Two cyanobiphenyl mesogens at the end of a
flexible spacer were attached using a Williamson etherification
reaction and the carboxylic ester was hydrolysed under basic
conditions. The discotic mesogen 7 was attached using a
The optical textures of the linked disc–rod mesogens are less
˚
Scheme 1 Reagents and conditions: (i) HC(OEt)3, amberlite IR1 20, 4 A molecular sieves, toluene, 1 h at 100 uC, 80%; (ii) S-(2)-2-methyl-1-butanol,
DIAD, PPh3, THF, 16 h at room temperature, 74%; (iii) cat. p-TSA, methanol/H2O, 16 h at 40 uC, 84%; (iv) 1-(10-bromodecyl) 4-(4’-cyanobiphenyl)
ether, K2CO3, KI, butanone, 16 h at 80 uC, 77%; (v) aq. EtOH, 2 h at 80 uC, quantitative.
1 8 0 0
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 1 7 9 8 – 1 8 0 3

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Table 1 Phase behaviour of investigated mesogens
Mesogen Phase behavioura,b
7
5
8a
8b
1
Cr
137 (37)
N
21 SmA 63 (4.0) N*
246 (0.2)
66 (0.5)
I
I
I
I
I
I
I
GSmA
Cr
Cr
GN*
GN
GN
118 (104)
119 (125)
[N 106 (5.1)]d
[69 (72)
N
105 (7.3)]c
50 (0.8)
77 (1.0)
52 (1.1)
23
31
8
N*
N
N
9a
9b
a Transition temperatures (second heats) in uC and latent heat values
(between the brackets) in kJ mol21 b Abbreviations: Cr ~ crystal-
line, GM ~ glass with mesophase M frozen in, SmA ~ smectic A,
nematic, N* cholesteric,
isotropic. c Monotropic
nematic phase. d See ref. 4(a).
.
N
~
~
I ~
clear if viewed in terms of characteristic textures. They
generally show grainy textures upon formation from the
isotropic phase and as domains grow, homogeneously aligned
areas appear upon extended annealing (6-24 h), as found often
for nematic systems,4b see Fig. 4e. Thin areas in the sample
show Schlieren textures (Fig. 4f). Despite the presence of the
chiral group in 1, no characteristic features of a cholesteric
phase are observed in the pure mesogen. It is interesting to
note the effect of the chiral group and its position on
liquid crystalline properties of mesogens 1 and 5. In 5, the
2-methylbutyl group is positioned far away from the mesogenic
cores, decoupled by an alkyl chain and the aromatic ring.
Despite this, the influence of the group is very clear as can be
seen form the optical textures characteristic for cholesteric
phases. The fingerprint texture indicates a pitch length of ca.
9.5 mm at 64 uC. In sharp contrast, the influence of the chiral
group in 1 is not evident from the optical textures, suggesting
that the helical pitch of this cholesteric phase is outside the
length of the visible light, either very small (UV range) or very
large (IR range). The latter option is more likely, as the helical
twisting power of the selected chiral groups is known to be
relatively weak.5 The concentration of chiral functionality in
the system is very low compared to most low mass liquid
Fig. 5 Normalised DSC traces for mesogens (a) 5; (b) 8a; (c) 1 and
(d) 9a. The arrows in (c) and (d) indicate the nematic–isotropic
transition. Traces for 8b and 9b have been published elsewhere.4b
crystals, additionally the concentration of chiral centres per
volume fraction is further diluted in going from 5 to 1.
Furthermore, the position of the chiral group is not ideal; it is
not very close to the rod–shaped mesogens, normally required
for a strong influence on the phase behaviour and quite far
away from the large disc-shaped mesogen.5,9
DSC traces of 1, 5, 8a and 9a are shown in Fig. 5. Sharp and
reversible first-order nematic–isotropic phase transitions are
observed for all materials. At low temperatures, the materials
show glass transitions, where the mesophases are frozen in.
X-Ray diffraction
Selected mesogens in this study have been investigated by
X-ray diffraction studies. The results are shown in Fig. 6 and
the reflections are summarised in Table 2. Mesogens 5 and 8b
show in the nematic phase two diffuse reflections. The small
˚
angle reflection (d ~ 13.7 and 14.3 A, respectively) corresponds
to the rigid part (the cyanobiphenyl groups) of the mesogens.
˚
The wide angle reflection (d ~ 4.4 A for both) is assigned to the
average spacing between mesogens and/or alkyl groups. In the
SmA phase, 5 shows an additional sharp small angle reflection
that relates to the spacing of the smectic layering. At 30 uC a
˚
layer spacing of 50.3 A is observed. Considering that the
˚
molecular length of 5 is approximately 33 A (as calculated with
simple molecular modelling), a partly interdigitated bilayer is
formed. On one side of the molecule, the relatively small gallic
ester unit and the attached C11-spacer can overlap with
neighbouring layers. Also from the cyanobiphenyl groups, it
is known that they form partly interdigitated bilayers, due to
their polar structures.10
Mesogens 1 and 9b show characteristic patterns of low-
ordered nematic phases. The weak and diffuse reflections in the
small angle area (mixture of average spacings between rods
and discs) show that no higher organisation is present in the
nematic phase. In other words, the nematic phase is made up of
the mixture of components rather than the entire mesogen as
the building block.
Mixing studies
Fig. 4 Optical textures of (a,b) 5 at 64 uC (N* phase), fingerprint
texture and Granjean texture; obtained after shearing the sample; (c) 5
at 62 uC (SmA phase); (d) 8b at 100 uC (N phase); and (e,f) 1 at 49 uC
(annealed homogeneous and Schlieren texture). All pictures taken with
crossed polarisers; magnification is indicated by the scale bar.
Our previous studies included the construction of phase diagrams
of 8a with 9a and of 9a with 7.11 The results showed that the
rod–disc mesogen 9a was almost linearly miscible with the
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 1 7 9 8 – 1 8 0 3
1 8 0 1

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Fig. 7 Optical textures of contact samples of (a) 5 and 1 at 55 uC:
grainy/homeotropic SmA phase of 5 (right), and cholesteric phase
mixture (left). (b) 5 and 1 at 55 uC (zoom of interface area between
SmA and N*): fingerprint texture. (c) 1 and 7 at 49 uC: N* at high
concentration of 1 (with some fingerprint features) and isotropic (right)
at more equimolar rations. (d) 1 and 7 at different temperatures (42 uC
top to 30 uC bottom). Both 1 and 7 show a nematic (grainy) texture.
The isotropic area in between disappears on cooling. All pictures taken
with crossed polarisers; the magnification is indicated by the scale bar.
high fraction of 1), small signatures of a cholesteric phase are
found (see Fig. 7c), indicating that even mixtures of 1 with
considerable amounts of 7 still show a chiral phase.
Fig. 6 Radially integrated XRD patterns of (a) 5 at 30 uC (SmA); (b) 5
at 65 uC (N*); (c) 8b at 80 uC (N); (d) 1 at 30 uC (N*) and (e) 9b at 30 uC
(N). The patterns have been shifted vertically for convenience.
Based on the results obtained by optical microscopy studies,
phase diagrams of 5/1 and 1/7 can be sketched, see Fig. 8. It is
stressed that these phase diagrams should be interpreted
qualitatively only and that for the construction of a detailed
phase diagram a number of mixtures should be prepared and
investigated individually. The phase diagrams have very similar
shapes to the phase diagram of 8/9 and 9/1 and the materials
seem to behave analogously.4 In the left diagram, the SmA
phase of 5 is strongly destabilised by the introduction disc-
shaped moieties. The stability range of the cholesteric phase,
however, is less affected and the clearing temperature decreases
nearly linearly with increasing addition 1. In phase diagram
shown on the right-hand side of Fig. 8 a distinct minimum in
the clearing temperature is observed, similar to that found for
the detailed investigation of mixtures of 9a/1. However, at the
minimum or very close to it, the glass transition temperature is
found, which hampers further evaluation of the phase diagram.
cyanobiphenyl-containing mesogen 8a. Qualitatively similar
results are observed for contact samples of 8b with 9b 4b and 5
with 1. In addition, from the contact sample it becomes clear
that upon addition of small amounts of 1–5, the SmA phase is
destabilised and only a cholesteric phase is observed instead.
Fig. 7a shows the contact sample of 1 (left, N*) and 5 (right,
SmA). The SmA phase near the interface is oriented home-
otropically (the mesogens are oriented perpendicularly to the
glass surface). The cholesteric phase at the interface, however,
shows a clear fingerprint texture (see magnification in Fig. 7b).
Mixtures of 1 with 7 did not show linear mixing behaviour,
but rather showed a minimum in the clearing temperature at
intermediate concentrations, clearly observed in Fig. 7d. At
40 uC an isotropic area is seen in the middle of the photograph.
From either side (pure 1 is on the left-hand side of the slide), the
nematic–isotropic interfaces grow nearer with decreasing
temperature, until at approximately 30 uC the interfaces meet
and slowly mix (i.e. no demixing could be detected).
Conclusions
The use of gallic acid derivatives functionalised with disc- and
rod-shaped mesogens and with a chiral hydrocarbon group
allowed the convergent synthesis of a chiral nematic molecular
This behaviour is similar to that observed in previous studies
with 9a/b and 7. However, in the currently presented materials,
the minimum clearing temperature is rather low (ca. 30 uC),
close to the glass transition temperature, which hampers
further investigation of this interesting phenomenon. Interest-
ingly, at the nematic–isotropic interface of the mixture (with a
Table 2 XRD data of mesogens 1, 5, 8b and 9b in the mesophases
˚
Spacings/A
Mesogen
T/uC
Mesophase
d001
dA
dB
5
5
8b
1
30
65
80
30
30
SmA
N*
N
N*
N
50.3a
—
—
—
14.0
13.7
14.3
14.5
14.7
4.3
4.4
4.4
4.3
4.3
Fig. 8 Sketched phase diagram of 5 and 1 (left) and 1 and 7 (right),
based on contact samples investigated by optical polarising micro-
scopy. The solid lines represent first-order phase transitions, the dashed
line the glass transition temperature. Note that the phase diagram is
purely qualitative in the x-axis, and that the y-axis is not drawn to scale.
9b
—
a A small second-order layer reflection can be observed at d002
˚
25.1 A.
~
1 8 0 2
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 1 7 9 8 – 1 8 0 3

View Article Online
ed., Wiley VCH, New York, 1998, vol. 2B, p. 781;
(d) S. Chandrasekhar, B. K. Sadashiva and K. A. Suresh,
Pramana, 1977, 9, 471.
system. The modular construction is highly versatile and allows
the insertion of other functional groups at this position in the
mesogen. However, the helical twisting power of the selected
chiral group is relatively weak, nevertheless chiral nematic
phase behaviour was observed at room temperature. Noteworthy
is that the rod–disc multipode system does not crystallise,
though some of the precursors exhibit high melting points.
Furthermore, miscibility of this system with both rod- and disc-
shaped mesogens in the nematic phase is very unusual and
has not yet been reported for a chiral nematic system.4a In
summary, the selected modular approach to chiral rod–disc
mesogens allows for facile access to systems which span the
interface between the discotic and calamitic phase regimes,
an area of investigations currently of interest for both tech-
nological application and fundamental research.
2
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Acknowledgements
We acknowledge the University of Hull (C. J. W., G. McR.,
B. J. D., L. P.), the Ramsay Memorial Fund and the EU
(project HPRN-CT2000-00016) (P. H. J. K.) for financial
support.
6
7
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J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 1 7 9 8 – 1 8 0 3
1 8 0 3