Synthesis and mesogenic properties of novel board-like liquid crystals

Article abstract of DOI:10.1039/b105351p

The syntheses of novel octa- and tetra-substituted systems based on tetrabenz[a,c,h,j]anthracene are reported. The disc-shaped octa-substituted material exhibits a columnar mesophase, whereas the tetra-substituted material exhibits a lamellar phase. Binar

Full text of DOI:10.1039/b105351p

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Synthesis and mesogenic properties of novel board-like liquid
crystals{
M. Carmen Artal,a Kenneth J. Toyne,a John W. Goodby,a Joaqu´ın Barbera´b and Demetri
J. Photinosc
aThe Department of Chemistry, The University of Hull, Hull, UK HU6 7RX
bDepartamento de Qu´ımica Orga´nica, Facultad de Ciencias, Universidad de Zaragoza, 50009
Zaragoza, Spain
cDepartment of Physics, University of Patras, Patras 26110, Greece
Received 19th June 2001, Accepted 15th August 2001
First published as an Advance Article on the web 9th October 2001
The syntheses of novel octa- and tetra-substituted systems based on tetrabenz[a,c,h,j]anthracene are reported.
The disc-shaped octa-substituted material exhibits a columnar mesophase, whereas the tetra-substituted
material exhibits a lamellar phase. Binary mixtures of the two materials were examined in order to explore the
change over from discotic to calamitic behaviour as a function of concentration. At the crossover point, the
possibility of the formation of a biaxial phase was investigated.
give columnar mesophases12 and the tetrabenz[a,c,h,j]anthra-
1. Introduction
cene unit is formally related to a triphenylene structure but is
more elongated and board-like because of the fusion of a
phenanthrene unit at its 9,10-bond with the 2,3-bond of
triphenylene. This new architecture therefore gave us the
opportunity to study both rod-like (via tetra-substituted) and
disc-like (via octa-substituted) systems.
Molecular shape determines to a large extent the type of
mesophase observed for thermotropic liquid crystals. For
example, in general, rod-like molecules give rise to smectic and/
or nematic mesophases, whereas disc-like molecules self-
organise to give columnar mesophases. Columnar phases are
also observed when a flat core is replaced by a pyramidal or
conical shaped unit1 or even when the central core is absent.2
Since the theoretical prediction in 1970 of the possibility of
the existence of a biaxial nematic phase for thermotropic liquid
crystals,3 much effort has been directed towards designing
novel molecules, which in some way combine the features of
rods and discs. Several different approaches have been utilised
thus far in attempting to obtain low-molecular-weight thermo-
tropic biaxial nematic materials. In the course of these
investigations, board-like molecules (sanidic),4 polycatenar
compounds derived from calamitic compounds modified with
several terminal chains,5,6 disc-like molecules where the
number of substituents was reduced,7 rod–disc dimers,8 disc–
disc dimers,9 different combinations of disc–rod–disc and
rod–half-disc dimers10 were prepared. Theoreticians have
suggested an alternative route to producing a thermotropic
biaxial nematic phase, which involves mixing rod-like with
disc-like molecules rather than specifically linking rods and
discs. However, the main drawback of this approach with
respect to biaxial nematics is the tendency of both components
to phase separate, because of their very different structural
features. The design of the above mentioned novel dimeric
structures by covalently linking geometrically different units9–10
was used with the intention of overcoming issues of phase-
separation.
2. Experimental
2.1. Materials and characterisation techniques
Starting materials were purchased from Aldrich or Lancaster
and used without further purification; 2-bromooctane was of
technical grade. Column chromatography was performed over
silica gel (35–75 mm particle size, 200 to 400 mesh, Merck).
Analytical TLC was performed on Kieselgel F-254 precoated
silica gel plates (Merck).
The structures of the compounds prepared were confirmed
by spectroscopic methods. 1H NMR spectra were recorded
using either a JEOL JNM-GX (270 MHz) or a JEOL JNM-LA
(400 MHz) spectrometer with tetramethylsilane (TMS) as an
internal standard; J values are given in Hz. IR spectra were
recorded on a Perkin–Elmer 882 infrared spectrophotometer.
Mass spectra were measured with a Finnigan-MAT 1020 GC/
MS. Elemental analyses were performed on a Fisons Instru-
ments Carlo Erba EA 1108 CHN analyser using acetanilide as
the reference standard. A Perkin–Elmer DSC 7 Differential
Scanning Calorimeter was used for determining the transition
temperatures and transition enthalpies; heating and cooling
rates were 10 uC min21 and the transition temperatures
reported are the onset values. The calorimeter was calibrated
against pure indium metal (mp~156.6 uC, enthalpy change~
28.5 J g21). An Olympus BH-2 polarizing microscope equipped
with a Mettler FP82 HT hot-stage and a Mettler FP90 central
processor was used to determine melting points, examine the
thermal transitions and observe the defect textures of the liquid
crystal phases. XRD measurements were performed with a
Pinhole camera (Anton-Paar) operating with a point-focused
Ni-filtered Cu-Ka beam. The sample was held in Lindemann
glass capillaries (1 mm diameter) and heated, when necessary,
with a variable-temperature attachment. The patterns were
collected on flat photographic film.
The aim of our work was to explore the borderline, as
described by theory,11 between calamitic and discotic systems
by studying novel board-like molecules in the hope of
observing a nematic biaxial phase either for individual
materials or in binary mixtures between calamitic and discotic
systems. The targeted compounds (I) were based on the
tetrabenz[a,c,h,j]anthracene structure. Triphenylene is an
excellent core unit for generating disc-like molecules that
{Basis of a presentation given at Materials Discussion No. 4, 11–14
September 2001, Grasmere, UK.
DOI: 10.1039/b105351p
J. Mater. Chem., 2001, 11, 2801–2807
This journal is # The Royal Society of Chemistry 2001
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absolute zero, in an ideal motionless state, and the force fields
used are those described in CHARMm V22.2. In addition to
Quanta and CHARMm, molecular simulations were generated
using an Apple MacIntosh G3 computer and ChemDraw3DTM
as part of a ChemDraw Ultra 6.0 programme.
2.2. Synthesis of materials
The preparation of tetrabenz[a,c,h,j]anthracene and some
related derivatives have been achieved using extreme reaction
conditions involving pyrolysis at temperatures up to 400 uC.13
Furthermore, none of the compounds reported so far have
lateral alkoxy substituents. We have developed an unambig-
uous and efficient route to these substituted derivatives, which
have more flexible regions and are therefore conducive to the
formation of liquid-crystalline materials. The synthetic path-
ways used in the preparation of the targeted compounds are
shown in Schemes 1 and 2.
Alkylation of 4-bromophenol and 4-bromo-1,2-dihydroxy-
benzene with the appropriate bromoalkane, following standard
procedures yielded compounds 1a–1c, 4a and 4c. Boronic acids
2a–2c, 5a and 5c were prepared by the reaction of the alkoxy-
bromo derivatives with n-butyllithium and subsequent treat-
ment with trimethyl borate followed by in situ hydrolysis
using hydrochloric acid.14 Demethylation of 4-bromo-1,2-
dimethoxybenzene using boron tribromide afforded the
required starting material 4-bromo-1,2-dihydroxybenzene 3.15
Compounds 6a–c, 8a and 8c were prepared by the
tetrakis(triphenylphosphine)palladium(0)-catalysed coupling of
commercially available 1,2,4,5-tetrabromobenzene with the
boronic acids, according to the procedure described by Hird
et al.16 Nearly quantitative conversions were achieved in spite
of the steric hindrance arising from the ortho-substituents and
the presence of four reactive sites in the molecule. The key step
in the synthesis was the oxidative cyclodehydrogenation17 of the
1,2,4,5-tetrasubstituted intermediates using FeCl3 as the Lewis
acid, to afford the final compounds. In the case of compound
8a, with disubstituted peripheral aromatic rings, the cyclisation
reaction proceeded regioselectively affording only the desired
compound 9a. The presence of a single reaction product was
confirmed by the 1H NMR spectra, which showed only singlets
for the aromatic protons. It has been reported previously that
Molecular modelling studies were performed on a Silicon
Graphics workstation (Indigo XS24, 4000) using the programs
Quanta and CHARMm. Within CHARMm, the Adopted
Basis Newton Raphson (ABNR) algorithm was used to locate
the molecular conformation with the lowest potential energy.
The minimisation calculations were performed until the root
21, which
˚
A
mean square (RMS) force reached 4.184 kJ mol21
is close to the resolution limit. The RMS force is a direct
measure of the tolerance applied to the energy gradient (i.e., the
rate of change of potential energy with step number) during
each cycle of minimisation. The calculation was terminated in
cases where the average energy gradient was less than the
specified value. The results of the molecular mechanics
calculations were generated using the programs QUANTA
V4.0 and CHARMm V22.2. The programs were developed and
integrated by Molecular Simulations Inc. The modelling
packages assume the molecules to be a collection of hard
particles held together by elastic forces, in the gas phase, at
Scheme 1 Reagents: 1.1. RBr, K2CO3, butanone; 1.2. (i) n-BuLi, THF,
278 uC (ii) (MeO)3B (iii) 10% HCl; 1.3. BBr3, CH2Cl2. a: R~C8H17, b:
R~C12H25, c: R~CH(CH3)C6H13
.
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t), 6.77 (2H, d), 7.35 (2H, d). IR (KBr) nmax/cm21 2933, 1492,
1288, 1246, 822, 641.
1-Bromo-4-dodecyloxybenzene 1b was prepared as described
for compound 9 in ref. 14. The excess of 1-bromododecane
was removed by distillation and the residual oil crystallised at
room temperature. The compound was used without further
1
purification in the next step. Yield 90%, mp 34 uC. H-NMR
(CDCl3) d 0.88 (3H, t), 1.25–1.50 (18H, m), 1.76 (2H, quint),
3.91 (2H, t), 6.77 (2H, d), 7.36 (2H, d). IR (KBr) nmax/cm21
2931, 1492, 1287, 1245, 822, 640.
1-Bromo-4-(1-methylheptyloxy)benzene 1c was prepared as
described for compound 9 in ref. 14. Yield 77%, bp 130 uC at
0.05 mmHg. 1H-NMR (CDCl3) d 0.87 (3H, t), 1.25–1.80 (13H,
m), 4.29 (1H, sext), 6.76 (2H, d), 7.35 (2H, d). IR (KBr) nmax
/
cm21 2937, 1483, 1244, 823, 641.
4-Octyloxyphenylboronic acid 2a was prepared as described
1
for compound 53 in ref. 14. Yield 91%. H-NMR (CDCl3) d
0.89 (3H, t), 1.25–1.55 (10H, m), 1.72–1.88 (2H, m), 4.05 (2H,
t), 7.00 (2H, d), 8.14 (2H, d).
4-Dodecyloxyphenylboronic acid 2b was prepared as
described for compound 53 in ref. 14. Yield 65%. 1H-NMR
(CDCl3) d 0.88 (3H, t), 1.20–1.50 (18H, m), 1.75–1.90 (2H, m),
4.40 (2H, t), 7.00 (2H, d), 8.15 (2H, d).
4-(1-Methylheptyloxy)phenylboronic acid 2c was prepared as
described for compound 53 in ref. 14. Yield 95%. 1H-NMR
(CDCl3) d 0.89 (3H, t), 1.25–1.80 (13H, m), 4.48 (1H, sext), 6.98
(2H, d), 8.14 (2H, d).
4-Bromo-1,2-dihydroxybenzene 3 was prepared as described
in ref. 15. Yield 98%, mp 85 uC (lit. 85–86 uC).15 1H-NMR
(CDCl3) d 5.25 (s, 1H), 5.40 (s, 1H), 6.74 (d, 1H), 6.93 (dd, 1H),
7.02 (d, 1H). IR (KBr) nmax/cm21 3600–3000, 1616, 1502, 1277,
1181, 1111, 887, 803, 777.
4-Bromo-1,2-bis(octyloxy)benzene 4a was prepared as des-
cribed for compound 9 in ref. 14. Yield 95% (liquid; crystallises
at room temperature but melts below 30 uC). 1H-NMR (CDCl3)
d 0.86–0.90 (6H, m), 1.25–1.50 (20H, m), 1.75–1.85 (4H, m), 3.95
(2H, t), 3.96 (2H, t), 6.73 (1H, d), 6.97–7.00 (2H, m). IR (KBr)
n
max/cm21 2932, 2861, 1503, 1471, 1255, 1222, 1024, 797.
4-Bromo-1,2-bis(1-methylheptyloxy)benzene 4c was prepared
as described for compound 9 in ref. 14. Yield 79%, bp 180 uC at
0.02 mmHg, 1H-NMR (CDCl3) d 0.88 (6H, t), 1.20–1.80 (26H,
m), 4.20–4.30 (2H, m), 6.75 (1H, dd), 6.95–7.02 (2H, m). IR
(KBr) nmax/cm21 2935, 2862, 1491, 1379, 1256, 1213, 1134,
1059, 942.
3,4-Bis(octyloxyphenyl)boronic acid 5a was prepared as des-
cribed for compound 53 in ref. 14. Yield 80%. 1H-NMR (CDCl3)
d 0.40–0.70 (6H, m), 1.25–1.55 (20H, m), 1.82–1.90 (4H, m), 4.08
(2H, t), 4.13 (2H, t), 7.00 (1H, d), 7.68 (1H, d), 7.81 (1H, dd).
3,4-Bis(1-methylheptyloxy)phenylboronic acid 5c was pre-
pared as described for compound 53 in ref. 14. Yield 75%.
1H-NMR (CDCl3) d 0.85–0.90 (6H, m), 1.25–1.88 (26H, m),
4.40–4.50 (2H, m), 7.00 (1H, d), 7.72 (1H, s), 7.80 (1H, d).
1,2,4,5-Tetrakis(4-octyloxyphenyl)benzene 6a: Column chro-
matography: eluent, hexane–dichloromethane (4 : 1). Recrys-
tallised from ethanol. Yield 65%, mp 103.2 uC. 1H-NMR
(CDCl3) d 0.89 (12H, t, J 7.0), 1.28–1.48 (40H, m), 1.73–1.80
(8H, m), 3.92 (8H, t, J 6.6), 6.77 (8H, d, J 8.8), 7.12 (8H, d, J
8.8), 7.42 (2H, s). IR (KBr) nmax/cm21 2933, 2860, 1612, 1519,
1478, 1246, 1176, 831. MS m/z 895 (Mz), 354, 186, 120, 94, 66
(100%). Calc. for C62H86O4: C, 83.17; H, 9.68%; found: C,
83.28; H, 9.89%.
1,2,4,5-Tetrakis(4-dodecyloxyphenyl)benzene 6b: Column
chromatography: eluent, hexane–dichloromethane (4 : 1). Re-
crystallised from ethanol with drops of toluene. Yield 60%, mp
82.7 uC. 1H-NMR (CDCl3) d 0.88 (12H, t, J 6.8), 1.25–1.45
(72H, m), 1.72–1.80 (8H, m), 3.92 (8H, t, J 6.6), 6.76 (8H, d,
J 8.8), 7.12 (8H, d, J 8.8), 7.42 (2H, s). IR (KBr) nmax/cm21
2927, 2858, 1612, 1521, 1476, 1245, 1177, 1032, 831. Calc. for
C78H118O4: C, 83.66; H, 10.62%; found: C, 83.70; H, 10.71%.
1,2,4,5-Tetrakis[4-(1-methylheptyloxy)phenyl]benzene 6c:
Scheme 2 Reagents: 2.1. Pd(PPh3)4, DME, 20% Na2CO3; 2.2. FeCl3,
dichloromethane, CH3NO2. a: R~C8H17, b: R~C12H25, c: R~
CH(CH3)C6H13
.
compounds with alkoxy substituents undergo oxidative de-
alkylation in the presence of Lewis acids, which involves ether
cleavage and oxidation to quinonoid aromatic structures.18 It is
remarkable that in the work reported here for compounds with
n-alkoxy substituents, only the cyclised compounds 7a, 7b and
9a were obtained and in 80% yield, and there was no evidence
of quinone derivatives being formed. However, when the
substituents in the aromatic ring were branched alkoxy chains
(compound 6c), only decomposition of the starting material
was observed.
General synthetic procedures
1-Bromo-4-octyloxybenzene 1a was prepared as described in
ref. 14. Yield 95%, bp 76 uC at 0.05 mmHg. 1H-NMR (CDCl3)
d 0.89 (3H, t), 1.25–1.50 (10H, m), 1.76 (2H, quint), 3.91 (2H,
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Column chromatography: eluent, hexane–dichloromethane
(4 : 1). Recrystallised from ethanol. Yield 50%, mp 66.5 uC.
1H-NMR (CDCl3) d 0.88 (12H, t, J 6.8), 1.25–1.75 (52H, m),
4.28–4.35 (4H, m), 6.75 (8H, d, J 8.2), 7.11 (8H, d, J 8.2), 7.44
(2H, s). IR (KBr) nmax/cm21 2953, 2862, 1611, 1516, 1482, 1246,
1177, 833. MS m/z 897, 446, 427, 352, 267, 71 (100%). Calc. for
C62H86O4: C, 83.17; H, 9.68%; found: C, 83.29; H, 9.75%.
3,6,12,15-Tetrakis(octyloxy)tetrabenz[a,c,h,j]anthracene 7a:
Recrystallised from ethanol–toluene (2 : 1). Yield 80%, mp
Table 1 Phase transitions and transition enthalpy changes for 7a, 7b,
8a and 9a
Transition temperatures (uC) and enthalpy changes
Compound (kJ mol21 in [ ])
7a
7b
8a
9a
Iso 179.1 [36.91] Cryst B-Cr
Iso 145.2 [39.53] Cryst B-Cr
Iso 74.4 [24.28] Colh
Iso 251.8 [7.05] Colh
229.9 Glass
154.6 [57.83] Cr
1
132.3 uC. H-NMR (CDCl3) d 0.92 (12H, t, J 6.8), 1.34–1.46
(32H, m), 1.53–1.61 (8H, m), 1.88–1.95 (8H, m), 4.19 (8H, m),
7.28 (4H, dd, J 2.3 and 9.0), 7.91 (4H, d, J 2.3), 8.71 (4H, d,
J 9.0), 9.48 (2H, s). IR (KBr) nmax/cm21 2927, 2859, 1618, 1472,
1425, 1232, 1203, 1038, 834, 804. MS m/z 286, 174, 143, 110, 93,
76, 65. Calc. for C62H82O4: C, 83.55; H, 9.27%; found: C, 83.46;
H, 9.33%.
compound 9a. Mixtures were therefore prepared by dissolving
specific amounts of two materials in toluene (Aldrich, 99.8%,
HPLC grade). The solvent was evaporated by heating the
samples at atmospheric pressure and traces of remaining
solvent were removed by keeping the samples under reduced
pressure (0.05 mmHg) at 60 uC for several hours. Toluene was
used because of the insolubility of compound 9a in the other
solvents that were tested.
3,6,12,15-Tetrakis(dodecyloxy)tetrabenz[a,c,h,j]anthracene
7b: Recrystallised from ethanol–toluene (1 : 1). Yield 80%, mp
1
112.0 uC. H-NMR (CDCl3) d 0.88 (12H, t, J 8.8), 1.25–1.50
(64H, m), 1.52–1.62 (8H, m), 1.87–1.95 (8H, m), 4.19 (8H, t,
J 6.0), 7.29 (4H, dd, J 1.9 and 8.5), 7.93 (4H, d, J 1.9), 8.73 (4H,
d, J 8.5), 9.50 (2H, s). IR (KBr) nmax/cm21 2927, 2856, 1617,
1425, 1233, 1202, 1038, 861, 806. Calc. for C78H114O4: C, 83.97;
H, 10.30%; found: C, 83.80; H, 10.64%.
3. Results and discussion
The liquid crystal properties of the synthesised compounds
were investigated by polarized light optical microscopy and
differential scanning calorimetry (DSC). The results obtained
are summarised in Table 1.
1,2,4,5-Tetrakis[3,4-bis(octyloxy)phenyl]benzene 8a: Column
chromatography: eluent, hexane–dichloromethane (3 : 1). Re-
crystallised from ethanol with drops of toluene. Yield 91%, mp
64.9 uC. 1H-NMR (CD2Cl2) d 0.87–0.91 (24H, m), 1.29–1.49
(80H, m), 1.59–1.66 (8H, m), 1.74–1.81 (8H, m), 3.69 (8H, t, J
6.6), 3.94 (8H, t, J 6.7), 6.67 (4H, s), 6.79 (8H, s), 7.47 (2H, s).
IR (KBr) nmax/cm21 2931, 2860, 1523, 1472, 1251, 1140, 1020,
817, 727. MS m/z 1408 (Mz, 100%), 1297, 1185, 1073. Calc. for
C94H150O8: C, 80.17; H, 10.74%; found: C, 80.15; H, 10.69%.
1,2,4,5-Tetrakis[3,4-bis(1-methylheptyloxy)phenyl]benzene
8c: Column chromatography: eluent, hexane–dichloromethane
(4 : 1). Yield 40% (liquid). 1H-NMR (CDCl3) d 0.85–0.90 (24H,
m), 1.25–1.80 (104H, m), 3.90–4.00 (4H, m), 4.17–4.27 (4H, m),
For compounds 7a and 7b, the DSC first heating scan
showed two major peaks, but further heating and cooling scans
showed only several low enthalpy peaks after the isotropic
liquid to mesophase transition (Fig. 1). By X-ray diffraction
experiments these materials only showed a crystalline structure
over the whole temperature range studied (from room
temperature to isotropization). In particular, for compound
7a this was revealed by the complexity of the patterns, which
contained numerous sharp rings in all the angular regions.
However, upon annealing at the clearing point and further
cooling a defect texture typical of a crystal B texture was
observed by optical microscopy (see Fig. 2). When cooling at
10 uC min21, a rapid transition from the isotropic liquid
occurred, that gave rise to a defect texture which was typical
of a crystal. The formation of the crystal state was nucleated
via an intermediary crystal B phase which was present at the
growing edges of the crystal. This is not inconsistent with the
X-ray measurements, that need exposure times of, at least,
several minutes to obtain sufficiently exposed photographs.
Therefore, the X-ray patterns taken at high temperatures, even
very close to the transition to/from the isotropic liquid, contain
the reflections from the crystal phase and not totally from
the thermodynamically unstable crystal B. Furthermore, this
hypothesis is consistent with the results obtained by DSC where
the exotherm associated with the process of nucleation exhibits
a high temperature shoulder which would correspond to the
6.69 (4H, s), 6.75–6.88 (8H, m), 7.47 (2H, s). IR (KBr) nmax
cm21 2935, 2863, 1516, 1480, 1255, 1136.
/
2,3,6,7,11,12,15,16-Octakis(octyloxy)tetrabenz[a,c,h,j]-
anthracene 9a: Recrystallised from toluene. Yield 80%, mp
164.6 uC. 1H-NMR (C6D5CD3, 70 uC) d 0.90–1.00 (24H, m),
1.30–1.50 (80H, m), 1.56–1.70 (8H, m), 1.87–2.02 (8H, m), 4.20
(8H, t, J 6.4), 4.26 (8H, t, J 6.5), 8.09 (4H, s), 8.44 (4H, s), 9.69
(2H, s). IR (KBr) nmax/cm21 2929, 2858, 1524, 1443, 1262, 1173,
1070, 838. MS m/z 1404 (Mz, 100%). Calc. for C94H146O8: C,
80.40; H, 10.48%; found: C, 80.00; H 10.61%.
Synthesis of 1,2,4,5-tetraphenylbenzene derivatives (com-
pounds 6a–c, 8a and 8c): A mixture of 1,2,4,5-tetrabromoben-
zene (1 mmol) in DME (1,2-dimethoxyethane) (8 ml) and 20%
aq. Na2CO3 (10 ml) was stirred under a positive pressure of
nitrogen. Tetrakis(triphenylphosphine)palladium(0) (0.10 mmol)
was added followed by the boronic acid (6 mmol) dissolved in
DME (5 ml). The resulting mixture was heated under reflux for
24 h and allowed to cool to room temperature. The organic
phase was separated and the aqueous phase was washed with
diethyl ether (62). The combined organic layers were washed
with water, brine and dried (MgSO4). The solvent was removed
in vacuo and the crude product was purified by column
chromatography and recrystallisation.
Synthesis of final compounds (compounds 7a, 7b and 9a): The
1,2,4,5-tetraphenylbenzene derivative (1 mmol) was stirred
in dry dichloromethane (100 ml) under nitrogen. A solution
of FeCl3 (10 mmol) in nitromethane (2 ml) was then added
dropwise via a syringe. After 1 h the mixture was quenched
with methanol and the precipitate was filtered off and purified
by recrystallisation.
Fig. 1 DSC traces of compound 7a (rate 10 uC min21). The solid line
corresponds to the first scan; the dashed line corresponds to the second
scan.
Preparation of binary mixtures of 7a and 9a: The preparation
of binary mixtures of compounds 7a and 9a in the isotropic
phase was not possible due to the high clearing temperature of
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Fig. 4 DSC traces of compound 9a (rate 10 uC min21), showing both
the heating and cooling cycles.
Fig. 2 Optical texture of the crystal B phase exhibited by compound 7a
under crossed polarizers at 189.4 uC (on heating).
isotropic liquid to crystal B transition, which occurs just before
recrystallisation.
An increase in the number of peripheral alkoxy substituents
attached to the aromatic core from four to eight induced a
change from calamitic to discotic nature. Compound 9a
exhibited a columnar hexagonal phase for which the texture
observed under crossed polarizers (Fig. 3) was a combination
of mosaics with linear birefringent defects and homeotropic
digitated stars; both are typical features for columnar hexa-
gonal phases.19 X-Ray diffraction studies confirmed the
mesophase assignment made by optical microscopy and DSC
(Fig. 4). The patterns taken above 155 uC show a strong
maximum in the low-angle region, that corresponds to the (1 0)
reflection from a two-dimensional hexagonal lattice with a
˚
lattice constant a~26 A. Although, in principle, other low-
angle reflections should be observed to unambiguously assign
the mesophase symmetry, the optical textures are unequi-
vocally characteristic of a hexagonal columnar mesophase.
Moreover, the absence of other reflections at low angles apart
from the (1 0) reflection is frequent in diffraction patterns of
hexagonal columnar mesophases, and it is due to a minimum in
the form factor, which precludes the observation of peaks in
this angle region.20 Molecular modelling gives the approximate
˚
diameter of the molecular disc as 31 A which corresponds to an
˚
inter-columnar distance of 26.8 A.
At high angles, two diffuse scattering maxima are observed
˚
corresponding to distances of about 4.6 and 3.8 A. The first of
them is common in all kinds of mesomorphic phases and arises
from short-range correlations between the conformationally-
Fig. 5 Molecular models to show the shape of (a) compound 7a and (b)
compound 9a.
˚
disordered hydrocarbon chains. The 3.8 A maximum is due to
Fig. 3 Optical texture of the Colh phase exhibited by compound 9a
under crossed polarizers at 224.0 uC (on cooling).
Fig. 6 Optical texture of the Colh phase exhibited by compound 8a
under crossed polarizers at 67.0 uC (on cooling).
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the intra-columnar order and corresponds to the average
stacking distance of the aromatic cores.
mesogenic properties.23 However, trialkoxy-substituted ben-
zene rings linked to a central 1,3,5-trisubstituted benzene via an
ester group exhibit discotic liquid-crystalline behaviour (VI).24
Since compound 9a displays discotic behaviour, it appears that
the number of substituents providing an efficient filling of the
space around the central ring has a stronger influence than the
nature of the linking group between the peripheral ring and
the central ones.21
Molecular modelling of compound 8a shows that the
peripheral aromatic rings are twisted out of the plane defined
by the central aromatic ring by between 40 and 50u (54.3u has
been calculated to be the value for the interannular twist in a
molecule of o-terphenyl).25 This is in agreement with previous
results showing that structural planarity is not a prerequisite to
observe a columnar arrangement21 (cf. structure VI, cyclohex-
ane derivatives,26 sugar-based liquid crystals27).
The main effect induced by the planarity of the structure
induced by cyclisation is to promote mesophase formation.
Compounds of type 7 derived from compounds 6a and 6b
exhibit mesomorphic behaviour, whereas the parent com-
pounds are non-mesomorphic. In comparison with the struc-
tures bearing eight alkoxy chains (compounds 8a and 9a), the
stronger interaction between the molecules for compound 9a
causes a change from a hexagonal disordered phase exhibited
by the parent compound 8a to a hexagonal ordered phase, as
well as increasing the transition temperatures.
None of the compounds prepared exhibited a nematic phase,
and so, to study in more detail the border-line between discotic
and calamitic behaviour, we prepared binary mixtures of
compound 9a (with a hexagonal columnar phase) and com-
pound 7a (with a crystal B phase). The usual problem of poor
miscibility and ready phase separation in mixtures of calamitic
and discotics28 was expected to be minimised for these two
components since they have identical core structures which
would be expected to enhance the interactions between the
rod-like and disc-like molecules.
Molecular models of compounds 7a and 9a (Fig. 5) show
planar extended aromatic cores. The eight alkoxy chains in
compound 9a can be arranged to provide a substantial covering
of the peripheral region of the molecule, a condition which is
required in molecular design in order to form a columnar
phase.21 However, for compounds of type 7, incomplete filling
of the space around the core results, thereby giving rise to an
overall calamitic or lath-like molecular shape.
Surprising results were found when studying compound 8a.
In the cooling process this compound exhibited a dendritic
growth from the isotropic liquid, which is typical for the
formation of a hexagonal columnar phase. Upon annealing,
the mesophase defects changed to give a fan-like texture typical
of a discotic hexagonal disordered phase (Colh) (Fig. 6). The
material did not crystallise on further cooling, and only showed
a transition to the isotropic liquid upon subsequent heating. By
DSC, the first heating scan showed two major transitions;
the first peak corresponds to the transition from the crystal to
the columnar mesophase and the second one corresponds to the
transition to the isotropic liquid. On cooling, crystallisation did
not occur but a glass transition at 229.9 uC was detected and
only the peak related to the isotropization was observed on
further heating scans. However, when the DSC scan was
repeated after one week, the peak corresponding to the melting
of the material was again observed, showing that crystallisation
of the compound takes place when it is left for a long period
of time. The slowness in its crystallisation allowed the X-ray
diffraction study of the mesophase at room temperature on a
sample of 8a cooled from the isotropic liquid. The low-angle
region of the pattern contained a set of four rings in a
reciprocal spacing ratio 1 : d3 : d4 : d9. These maxima are
indexed, respectively, as the (1 0), (1 1), (2 0) and (3 0)
reflections from the two-dimensional hexagonal lattice and give
˚
a lattice constant a~27.5 A. The occurrence of a higher num-
ber of low-angle reflections for this compound compared to
compound 9a is probably related to the different temperature
of the experiments. At higher temperatures, the mobility of the
molecules increases, thus reducing the degree of order. In fact,
in the X-ray patterns of 8a taken at temperatures close to the
clearing point, the intensity of the low-angle maxima (excepting
the (1 0) reflection) diminishes and the (1 1) and (3 0) reflections
practically vanish. However, the hexagonal lattice constant a
does not change at high temperature and its value remains
Binary mixtures of compounds 7a and 9a were studied by
DSC and polarized light optical microscopy and the resulting
binary phase diagram is shown in Fig. 7.
The first point to note about these results is that the mixtures
of the compounds were homogeneous across the whole phase
diagram and no immiscibility or phase separation was observed
in any of the individual binary mixtures that was tested. No
additional mesophases were induced for any of the mixtures
˚
equal to 27.5 A. In the high-angle region only a diffuse halo at
˚
4.4 A characteristic of the molten chains is observed. The
absence of scattering related to the molecular stacking distance
along the column axes, even at room temperature, means that
the intra-columnar order is poorer for 8a than for 9a.
Structures of type 6 and 8 are related to compounds
previously reported where the linkage between the central
1,2,4,5-tetrasubstituted benzene ring and the outer aromatic
rings was an alkyne4b or ester group.22 In these cases the greater
length of the alkyne (II) or ester linking group (III and IV)
allowed intramolecular interactions between adjacent periph-
eral rings which resulted in the generation of calamitic
behaviour. For the shorter direct C–C bond connection, it
appears that intermolecular interactions dominate leading to a
columnar arrangement of the molecules. In these cases the
number of substituents, and the small free volume between
the core and the peripheral chains, play a decisive role in the
mesogenic character of the systems, since no liquid-crystalline
behaviour was detected when only four chains were present
(compounds 6a–c).
Fig. 7 Phase diagram of binary mixtures (mol %) between 7a and 9a
taken on cooling. The crystal B region exists over a small temperature
range to the right of the figure. Key: & the isotropic to columnar phase
transition, $ the isotropic liquid to crystal B transition, the columnar
to solid phase transition is shown as a continuous line, the crystal B to
solid phase transition could not be detected by microscopy because of
either rapid crystallisation or the formation of a glass.
Furthermore, the mesogenic behaviour of 9a can be
compared to some reported 1,3,5-trisubstituted benzene
derivatives. The hexaalkoxy-substituted structure V, where
the linkage between the central and the peripheral rings is a
direct C–C bond, have been reported not to show any
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2
J. Maltheˆte, N. T. Nguyen and A. M. Levelut, J. Chem. Soc.,
Chem. Commun., 1985, 1794.
and it is remarkable that the columnar phase exists even with a
high concentration of the calamitic compound 7a present. It
seems that molecules of 7a can be accommodated within the
columnar structure without disturbing the packing of com-
pound 9a. In a complementary fashion, adding the discotic
compound 9a to calamitic compound 7a simply gives a crystal
B phase that has an almost constant thermal stability with
respect to change in concentration. However, there is a sudden
change in the nature of the mesophase shown by the mixtures
of about 30% molar of the discotic component. The discotic
mesophase for mixtures at 32% content of component 9a then
change to a crystal B mesophase at 30% of component 9a; i.e.
over a 2% difference in concentration the nature of the liquid
crystal morphology is totally changed. Thus, our attempts to
generate a biaxial smectic phase were unsuccessful, as with
nematic phases there appears to be an immiscibility gap
between the two uniaxial systems which extends only over a
percent in change in concentration.
3
4
M. J. Freiser, Phys. Rev. Lett., 1970, 24, 1041.
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K. J. Toyne, Liq. Cryst., 1997, 22, 631; (c) S. Chandrasekar, Mol.
Cryst. Liq. Cryst., 1994, 243, 1.
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K. Praefcke and D. Blunk, Liq. Cryst., 1998, 24, 67.
K. Ohta, R. Higashi, M. Ikejima, I. Yamamoto and N. Kobayashi,
J. Mater. Chem., 1998, 8, 1979.
5
6
7
8
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11 A. G. Vamakaras, A. F. Terzis and D. J. Photinos, Mol. Cryst.
Liq. Cryst., 2001, 362, 67.
12 A. N. Cammidge and R. J. Bushby, in Handbook of Liquid
Crystals, Vol. 2B, Eds. D. Demus, J. Goodby, G. W. Gray,
H.-W. Spiess and V. Vill, Wiley-VCH, 1998, Chapter VII, pp. 702–
712.
4. Conclusions
13 (a) P. Lambert and R. H. Martin, Bull. Soc. Chim. Belg., 1952,
61, 124; (b) E. Clar, in Polycyclic Hydrocarbons, Academic
Press, New York, 1964; (c) R. A. Pascal, Jr., N. D. McMillan,
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14 G. W. Gray, M. Hird, D. Lacey and K. J. Toyne, J. Chem. Soc.,
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15 D. Stewart, G. S. McHattie and C. T. Imrie, J. Mater. Chem.,
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16 M. Hird, G. W. Gray and K. J. Toyne, Mol. Cryst. Liq. Cryst.,
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17 A. J. Berresheim, M. Muller and K. Mullen, Chem. Rev., 1999, 99,
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18 K. Weiss, G. Beernik, F. Do¨tz, A. Birkner, K. Mu¨llen and
C. H. Wo¨ll, Angew. Chem., Int. Ed., 1999, 38, 3748.
19 (a) C. Destrade, P. Foucher, H. Gasparoux, H. T. Nguyen,
A. M. Levelut and J. Maltheˆte, Mol. Cryst. Liq. Cryst., 1984, 106,
21; (b) C. Desdrade, H. T. Nguyen, J. Maltheˆte and A. M. Levelut,
J. Phys. (Paris), 1983, 44, 597.
Novel board-like liquid-crystalline materials based on tetra-
benz[a,c,h,j]anthracene have been synthesised and charac-
terised. A change from lamellar to discotic mesomorphic
behaviour was observed when increasing the number of
pheripheral alkoxy substituents from four (compounds 7a
and 7b showed a crystal B phase) to eight (the material 9a
exhibited a Colh phase). The non-planar tetra-substituted
benzene intermediate 8a bearing eight alkoxy chains also
showed columnar liquid-crystalline behaviour (Colh phase).
Binary mixtures of the structurally related compounds 7a
(calamitic) and 9a (discotic) were mesogenic and did not show
any evidence of phase-separation across the whole phase
diagram, although no additional mesophases were induced.
Theory suggests11 that the packing restrictions which could
promote biaxiality in the nematic binary mixture of rod-like
and plate-like molecules may at some time cause demixing of
the phase. Calculations show11 that the thermodynamic
stabilisation against demixing can be achieved by introducing
selective rod–plate interactions. Attempts to chemically
incorporate such interactions by hydrogen bonding lead,
however, to strong distortions of the board-like structures.
20 J. Barbera´, R. Gime´nez and J. L. Serrano, Chem. Mater., 2000, 12,
481.
21 B. Khone and K. Praefcke, Chem.-Ztg., 1985, 109, 121–127.
22 W. D. J. A. Norbert, J. W. Goodby, M. Hird, K. J. Toyne,
J. C. Jones and J. S. Patel, Mol. Cryst. Liq. Cryst., 1995, 260,
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23 E. Frackowiak and G. Scherowsky, Z. Naturforsch., 1997, 52b,
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24 G. Latterman, Liq. Cryst., 1982, 2, 723.
Acknowledgements
25 W. R. Busing, J. Am. Chem. Soc., 1982, 104, 4829.
26 A. N. Cammidge and R. J. Bushby, in Handbook of Liquid
Crystals, Vol. 2B, Eds. D. Demus, J. Goodby, G. W. Gray,
H.-W. Spiess and V. Vill, Wiley-VCH, 1998, Chapter VII, pp. 739–
741.
We would like to thank the European Union for financial
support through the Molecular Design of Functional Liquid
Crystals TMR Network.
27 A. N. Cammidge and R. J. Bushby, in Handbook of Liquid
Crystals, Vol. 2B, Eds. D. Demus, J. Goodby, G. W. Gray,
H.-W. Spiess and V. Vill, Wiley-VCH, 1998, Chapter VII, pp. 741–
742.
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