The interplay of bent-shape, lateral dipole and chirality in thiophene based di-, tri-, and tetracatenar liquid crystals

Article abstract of DOI:10.1021/ja0268027

A range of mesogenic molecules varying in both bend angle and strength of lateral dipole were synthesized, and their phase behavior was characterized by polarizing microscopy, thermal analysis, X-ray diffraction, and electrooptical measurements. We find t

Full text of DOI:10.1021/ja0268027

Published on Web 10/05/2002
The Interplay of Bent-Shape, Lateral Dipole and Chirality in
Thiophene Based Di-, Tri-, and Tetracatenar Liquid Crystals
S. Holger Eichhorn, Alexander J. Paraskos, Keiki Kishikawa, and
Timothy M. Swager*
Contribution from the Department of Chemistry and Center for Materials Science and
Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
Received May 6, 2002
Abstract: A range of mesogenic molecules varying in both bend angle and strength of lateral dipole were
synthesized, and their phase behavior was characterized by polarizing microscopy, thermal analysis, X-ray
diffraction, and electrooptical measurements. We find the general destabilization of the liquid crystallinity
caused by strong lateral dipolar groups and the bent molecular shape are off-set in mesomorphic
tetracatenars, which display stable nematic, smectic, columnar, and cubic mesophases. The broad
mesomorphism of the tetracatenars containing lateral dipoles and their incompatibility with chiral induction
are explained by considering that loosely correlated dimers exist within the mesophases. Chiral mesophases
of derivatives with strong lateral dipoles were achieved by attaching fewer or different side chains to each
end of the mesogen.
Introduction
New types of liquid crystalline phases displayed by certain
bent-rod molecules (banana phases) have received much atten-
Dipole-dipole interactions and shape anisotropy have been
recognized as fundamental elements in the design of liquid
crystals since the first systematic synthetic studies in the early
decades of the 20th century.1,2 More recently, microphase
segregation3 and noncovalent interactions such as H-bonding4-6
have been added to the list of design concepts. By applying
these principles, it is possible to incorporate almost any chemical
structure into a liquid crystal as has been shown most notably
for C60.7
One of the present goals of synthetic liquid crystal research
is to understand the interplay between different elements of
design.8 For example, molecules with unfavorable shape can
be made mesogenic if favorable dipole-dipole interactions and/
or microphase segregation are used to compensate for the
deficiencies in shape. Exciting and surprising examples include
liquid crystalline dendrimers,9 columnar liquid crystals that lack
side chains,10 and unusually shaped amphiphilic molecules.11
tion in recent years.12-16 Novel ferroelectric17,18 and antiferro-
electric19-24 mesophases were obtained from initially achiral
molecules and macrophase separation of racemic mixtures16 into
homo-chiral domains was observed. However, the design
principles appear to be subtle, as the majority of bent-rod
(banana) mesogens did not display these new mesophases, if
any mesophase at all, despite possessing similar molecular
structures.25-27 The ultimate goal of being able to accurately
predict the liquid crystalline properties of many molecular
structures remains elusive, despite the numerous mesomorphic
(12) Niori, T.; Sekine, T.; Watanabe, J.; Furukawa, T.; Takezoe, H. J. Mater.
Chem. 1996, 6, 1231-1233.
(13) Sekine, T.; Niori, T.; Sone, M.; Watanabe, J.; Choi, S. W.; Takanishi, Y.;
Takezoe, H. Jpn. J. Appl. Phys., Part 1 1997, 36, 6455-6463.
(14) Heppke, G.; Moro, D. Science 1998, 279, 1872-1873.
(15) Watanabe, J.; Niori, T.; Sekine, T.; Takezoe, H. Jpn. J. Appl. Phys., Part
2 1998, 37, L139-L142.
(16) Link, D. R.; Natale, G.; Shao, R.; Maclennan, J. E.; Clark, N. A.; Koerblova,
E.; Walba, D. M. Science 1997, 278, 1924-1927.
(17) Sekine, T.; Takanishi, Y.; Niori, T.; Watanabe, J.; Takezoe, H. Jpn. J. Appl.
Phys., Part 2 1997, 36, L1201-L1203.
* Address correspondence to this author. e-mail: tswager@mit.edu.
(1) Demus, D.; Goodby, J.; Gray, G. W.; Spiess, H.-W.; Vill, V. In Handbook
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12742
J. AM. CHEM. SOC. 2002, 124, 12742-12751
10.1021/ja0268027 CCC: $22.00 © 2002 American Chemical Society

Tetracatenar Liquid Crystals
A R T I C L E S
Scheme 1. Synthesis of Thiophene Based Tetracatenarsa
a (a) Pd (PPh3)4, CuI, iPr2NH, toluene, 25 °C, >90%. (b) and (c) CuCN, CuI, DMI, 120 °C, 3.5 h, >60%. (d) 2.2 equiv LDA, I2, THF, -78 °C, 77%.
(e) Pd (PPh3)4, CuI, iPr2NH, toluene, 25 °C, >90%. (f) TsOH, MeOH, 25 °C, >90%. (g) BrB-catechol, DCM, 0-25 °C, >90%. (h) (iPr)NdCdN(iPr),
DMAP, DCM, 0-25 °C, 70-90%.
molecular structures and liquid crystalline phases that have been
studied over the last hundred years.
crystalline properties of thiophene-based polycatenar liquid
crystals. The tetracatenar (four side chains) and tricatenar (three
side chains) structures allow us to access a wide variety of
mesophases simply by changing the length, number, and position
of side chains. Nematic (N), smectic (Sm), columnar (Col), and
cubic mesophases of polycatenars were obtained as well as
chiral-nematic (N*) and smectic C (SmC*) phases. Their
potential as new building blocks for polar materials is discussed.
We have been investigating thiophene-based, bent-rod meso-
gens which contain lateral dipolar groups attached to the
thiophene in the center of the molecule.28,29 In a preliminary
study on the mesomorphism of bent-rod 2,5-bis(4-alkoxyphenyl-
acetylene)thiophene derivatives,29 we found that molecules with
local lateral dipoles of up to 6.3 D at the central thiophene ring
displayed nematic mesophases, and single-crystal structures
revealed a dimer formation of neighboring molecules with an
antiparallel orientation of their dipoles. Antiparallel packing was
also found in the hexagonal columnar mesophase of the
hexacatenar (six side chains) 2,5-bis[4-(3,4,5-tridodecyloxy-
phenylcarbonyloxy)phenylethynyl]-3,4-dicyanothiophene.28
We now present an expanded study on the effects of bend
angle, lateral dipole, chirality, and symmetry on the liquid
Results and Discussion
Synthesis. To guide the reader through the many structures
reported herein, we have developed the following nomenclature
system. The mesogenic cores are described as catenars with DC
standing for dicatenar, TrC standing for tricatenar, and TC
standing for tetracatenar. The very central portion of the core
is either a 2,5-thienyl (XXT), para-phenyl (pXXP), or meta-
phenyl (mXXP) where the XX represents the type of catenar.
The central rings of the core can be substituted with hydrogens
(H), bromides (Br) or nitriles (NC) as shown in Schemes 1 and
2. Hence, NC-TCT stands for a nitrile containing tetracatenar
(28) Levitsky, I. A.; Kishikawa, K.; Eichhorn, S. H.; Swager, T. M. J. Am.
Chem. Soc. 2000, 122, 2474-2479.
(29) Kishikawa, K.; Harris, M. C.; Swager, T. M. Chem. Mater. 1999, 11, 867-
871.
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A R T I C L E S
Eichhorn et al.
Scheme 2. Synthesis of Unsymmetric and Benzene Based Polycatenarsa
a (a) (iPr)NdCdN(iPr), DMAP, DCM, 0-25 °C, 70-85%.
Table 1. Mesomorphism of the Symmetric Thiophene and 3,4-Dibromothiophene- Based Tetracatenars Determined by Polarizing
Microscopy, DSC (5 °C/min) and X-ray Diffraction
T/deg C,
T/deg C,
T/deg C,
T/deg C,
heating
cooling
heating
cooling
(∆H/kJ mol-1
)
compound
transition
(∆H/kJ mol-1
)
(∆H/kJ mol-1
)
compound
transition
(∆H/kJ mol-1
)
H-TCT-mp4
Br--TCT-mp4
Cr-(N)a
(N)-I
135.2 (57.03) 87.1 (-28.31)
133.9 (-1.10)
H-TCT-mp6
H-TCT-mp8
H-TCT-mp10
Cr-N
137.6 (69.88) 122.9 (-67.77)
144.1 (1.11)
138.1 (74.57) 117.6 (-73.14)
Br-TCT-mp6
Br-TCT-mp8
Br-TCT-mp10
Cr-(N)a
(N)-I
189.6 (95.28) 127.9 (-55.14)
161.1 (-1.22)
N-I
142.9 (-1.14)
Cr-(N)a
(N)-I
Cr-(N)a
(N)-I
119.5 (52.93) 84.8 (-51.14)
107.6(-1.01)
133.2 (-1.22)
Cr-(SmC)a 127.3 (71.58) 112.0 (-62.11)
Cr-(SmC)a 110.8 (47.86) 82.7 (-98.56)
(SmC)-(N)
(N)-I
114.8 (-6.92)
125.9 (-1.09)
(SmC)-I
97.3 (-7.51)
H-TCT-mp12
H-TCT-mp14
Cr-(N)a
(N)-I
133.6 (89.77) 116.9 (-87.66)
120.5 (-1.26)
Br-TCT-mp12
Cr-(SmC)a 117.3 (67.74) 89.4 (-53.95)
(SmC)-I
Br-TCT-mp10*(S) Cr-I
98.3 (-3.16)
103.3 (42.74) 60.9 (-47.01)
Cr-I
125.4 (91.41) 115.3 (-98.49)
H--TCT-mp10*(S) Cr-I
99.2 (66.39)
63.2 (-44.49)
a Monotropic mesophases in brackets.
based on the 2,5-thienyl group. At the ends of the mesogens
are benzoates containing alkyl-oxy side chains that are para (p)
or meta (m) with respect to the ester, and for example, a mpm12
designation indicates that 12 carbon alkyl-oxy side chains are
attached to both of the meta positions in addition to the para
position of the benzoate. Chiral compounds are labeled (*), and
the stereochemistry is given as R, S, or rac. The connectivity
of the chiral side chains is shown in Schemes 1 and 2.
We chose a 2,5-bis(4-hydroxyphenylacetylene)thiophene 6
and its derivatives 7 and 8, all fluorescent compounds, as central
building blocks (Scheme 1). They were obtained in three steps
from 2,5-dibromothiophene or tetrabromothiophene in analogy
to previously published procedures.28,29 Palladium-catalyzed
cross-coupling of 4-(benzyloxy)phenylacetylene 2 with tetra-
bromothiophene results in selective substitution of the bromine
atoms in the 2 and 5 positions to give 4, which was subsequently
treated with CuCN/CuI in 2,5-dimethylimidazlidinone at 120
°C to give the 3,4-dicyanothiophene derivative 5a. Monitoring
the cyanation by thin-layer chromatography suggests rapid
substitution of the first bromide, while the substitution of the
second bromide was much slower and was accompanied by
several side reactions. Polymerization and exchange of the
second bromine by hydrogen were the most prominent side
reactions, and were responsible for the only moderate yields of
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Tetracatenar Liquid Crystals
A R T I C L E S
Table 2. Mesomorphism of the Symmetric 3,4-Dicyanothiophene-Based Tetracatenars Determined by Polarizing Microscopy, DSC (5
°C/min) and X-ray Diffraction (2D-waxs)
T/deg C,
heating
(∆H/kJ mol-1
T/deg C,
cooling
(∆H/kJ mol-1
)
d-values/Å
(sample temp
degC)a
tilt angle/deg
SmC phaseb
compound
transition
Cr-N
N-I
Cr-N
)
NC-TCT-mp4
148.4 (44.40)c
162.5 (1.14)
133.5 (40.33)c
147.6 (0.94)
113.9 (32.37)
121.6 (7.08)
144.4 (0.96)
127.1 (46.46)
129 (shoulder)
131.2 (39.89)
135.3 (6.92)
119.3 (35.27)
130.9 (5.50)
100.5 (50.88)
123.4 (2.95)
131.4 (3.95)
129.2 (-44.04)
161.4 (-1.03)
116.8 (-40.87)
146.4 (-1.67)
98.9 (-21.83)
121.6 (-7.42)
142.8 (-0.96)
91.8 (-19.85)
125.2 (-5.02)
109.9 (-38.48)
132.4 (-7.72)
98.9 (-30.22)
129.4 (-5.92)
86.8 (-49.88)
111.7 (-4.38)
129.7 (-3.66)
NC-TCT-mp5
NC-TCT-mp6
29.3, 3.9 (145)
N-I
Cr-SmC
SmC-N
N-I
28.9, 14.6, 6.3, 4.5
(118);
30.4, 4.0 (134)
31.5, 15.5, 6.3, 4.6
(120)
30.9, 15.5, 6.4, 4.7
(133)
32.6, 16.4, 6.5, 4.7
(130)
34.5, 17.6, 6.8, 4.7
(110);
39.0, 22.5, 19.7, 6.5,
4.6 (130)
39.9, 23.2, 20.2, 6.8,
4.7 (120)
41.1, 23.9, 20.8,
6.3(small),4.7 (115)
38
NC-TCT-mp7
NC-TCT-mp8
NC-TCT-mp9
NC-TCT-mp10
Cr-SmC
SmC-I
Cr-SmC
SmC-I
Cr-SmC
SmC-I
Cr-SmC
SmC-Colh
Colh-I
53
54
56
NC-TCT-mp11
NC-TCT-mp12
Cr-Colh
Colh-I
Cr-Colh
Colh-I
Cr-I
Cr-(N)
(N)-I
Cr-I
106.2 (59.99)d
137.5 (5.29)
102.8 (89.29)
138.0 (2.78)
161.0 (31.38)
101.6 (62.89)
94.7 (-61.61)
136.1 (-4.19)
87.8 (-81.35)
130.3 (-6.79)
107.2 (-34.33)
69.4 (-37.88)
74.7 (-2.14)
NC-TCT-mp4*(S)
NC-TCT-mp10*(rac)
NC-TCT-mp10*(R),(S)
104.1 (56.57)
66.9 (-15.62)e
a Reflections at 6-7 Å and 4-5 Å are broad halos. b Angle between layer and side-chain reflections of magnetically aligned samples. c Nematic phases
were not observed at the first heating runs since the Cr-I transitions were at 189.0 (79.63) for mp-TC-4 and at 154.8 (56.67) for mp-TC-5. d A
Cr-SmC transition was observed on the first heating run, 110.6 (2.94) e Cold crystallization on heating run.
about 60%. Cleavage of the benzyl-protecting groups in 4 and
5a with B-bromocatecholborane afforded the diphenolic building
blocks 7 and 8, respectively, in good yields. The same synthetic
approach was initially applied to obtain 6, but the cleavage of
the benzyl groups with boroncatecholbromide or BBr3 was low-
yielding due to the occurrence of side-reactions. Compound 6
was therefore synthesized via the tetrahydropyran (THP)-
protected precursor 3.
Mesomorphism. The phase diagrams of all compounds were
determined by temperature controlled polarizing microscopy,
differential scanning calorimetry (DSC), and X-ray diffraction.
All diffraction data agreed with our phase assignments, and
details are given for higher ordered enantiotropic mesophases.
The results for the thiophene based symmetric tetracatenars are
summarized in Tables 1 and 2. Tables 3 and 4 contain the data
for the symmetric dicatenars, the unsymmetric tri and tetracat-
enars, as well as the bistolane-based straight-rod tetracatenars.
An alternative synthetic approach to 8 involves cyanation of
3,4-dibromothiophene followed by the lithiation and subsequent
iodination of the 2 and 5 positions (Scheme 1). The resulting
2,5-diiodo-3,4-dibromothiophene was then converted to 5b via
palladium-catalyzed cross-coupling. The 42% overall yield of
this pathway is similar to the yield obtained for the approach
described above, but the lowest-yielding cyanation step is carried
out at the beginning of the synthetic scheme, with the result
that purification of the following intermediates is much easier.
Incorporation of a bent core within a molecular design
typically destabilizes the resulting liquid crystalline phases as
the bent-rod shape of the molecules interferes with their
rotational freedom.30-33 Both the H-TCT-mp as well as Br-
TCT-mp tetracatenar derivatives follow this trend (Table 1).
Despite possessing moderate bend angles (154°) these com-
pounds display exclusively monotropic mesophases (with the
exception of the nematic phase of H-TCT-mp6). This stands
in direct contrast to the corresponding straight-rod tetracatenars
H-CPpTCP-mp6 and -mp10, which exhibit SmC and N phases
over wide temperature ranges, suggesting that the introduction
of even small bend angles has dramatic consequences upon the
stability of the resulting liquid crystal phases of these compounds
(Table 4). As one would expect, further increasing the bend to
120° as in the H-mTCP-mp and Br-mTCP-mp derivatives
exaggerates the effect and in fact results in the complete
suppression of mesomorphism (Table 4).
Esterification of the lateral hydroxy groups of 6-8 with the
corresponding benzoic acid derivatives in the presence of
diisopropylcarbodiimide and 4-(dimethylamino)pyridine gave
the desired symmetric di- and tetracatenars (Scheme 1) in good
yields. Polycatenars possessing two different benzoic ester
groups were obtained by statistical mono-esterification and
subsequent chromatographic separation of the two symmetric
side products (Scheme 2). 1,3-Bis(4-hydroxyphenylacetylene)-
benzene (9a) and 1,4-bis(4-hydroxyphenylacetylene)benzene
(10) derivatives were synthesized for comparison purposes
(Scheme 2). The reaction pathways were identical to the
preparation of 6 and H-TCT-mp4-14 except that 1,3-dibro-
mobenzene and 1,2-dibromobenzene were used as central cores
instead of 2,5-dibromothiophene. Experimental details are given
as Supporting Information.
(30) Friedman, M. R.; Toyne, K. J.; Goodby, J. W.; Hird, M. J. Mater. Chem.
2001, 11, 2759-2772.
(31) Fazio, D.; Mongin, C.; Donnio, B.; Galerne, Y.; Guillon, D.; Bruce, D. W.
J. Mater. Chem. 2001, 11, 2852-2863.
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105, 8845-8860.
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A R T I C L E S
Eichhorn et al.
steric demand of the side chains causes the phase transition from
SmC to Col.34,35 A cubic mesophase is displayed by NC-TCT-
mpm12,p8 which is also a reported behavior of classical
tetracatenars with three terminal side chains at one end and only
one side chain at the other end.36
The strong lateral dipole (calculated to be 6.3 D) and steric
hindrance introduced by the two cyano groups increases the
rotational barrier about the long axis of the mesogens and results
in strong intermolecular dipole-dipole interactions. Both of
these effects would be expected to promote higher melting points
and decreased mesomorphism. Following this line of reasoning,
tetracatenars H-TCT-mp (with the smallest lateral dipoles and
least steric hindrance to rotation) would have been predicted to
display mesophases that were significantly more stable than
those of the NC-TCT-mp analogues, which is in direct contrast
to our observations.
We believe that the unexpectedly broad mesomorphism of
the NC-TCT-mp derivatives can be reasoned by considering
two factors. First, the individual NC-TCT-mp molecules form
loosely associated dimers with antiparallel dipole orientations
that function as the mesomorphic unit in the liquid crystalline
phase. While strict antiparallel association of nearest-neighbor
molecules has not been observed for calamitic liquid crystals,
several classical nematic and smectic liquid crystals with strong
dipoles along their molecular long axis, such as cyanobiphenyl
type systems,37 have been shown to statistically adopt antipar-
allel orientations. The formation of these dimers results in a
mesomorphic unit in which the deleterious effects of both the
bent-shape and large lateral dipoles are largely negated, as such
a dimer sweeps out an approximately cylindrical volume upon
rotation around its long axis. Experimental support of our
hypothesis was provided by antiparallel dimer formation in
single crystals of related compounds29 and by two-dimensional
X-ray diffraction data of magnetically aligned Colhex and SmC
mesophases. A weak and diffuse peak centered at 0.75 nm, the
expected distance between the sulfur atoms of every other
thiophene unit in stacks of alternating antiparallel packing, was
found in all of the diffraction patterns (Figure 2). A similar but
sharper peak was found in the X-ray diffraction pattern of a
related hexacatenar derivative, for which antiparallel columnar
packing was independently verified by photophysical methods.28
Second, the lateral dipole introduced by the two cyano groups
is likely to influence the conformation of the tetracatenars driven
by the intramolecular interactions between the dipoles of the
cyano groups, the ester groups, and the alkoxyphenyl ether
groups. Semiempirical calculations on individual molecules in
the gas phase reveals four low-energy conformers (Figure 3).
Their overall shape (bend) and lateral dipole vary considerably,
which emphasizes the wide range of conformations these
molecules are able to adapt despite their rigid bent core. The
terminal benzoic ester groups can either oppose or enhance the
bend of the central 2,5-bis(phenylacetylene)thiophene entity and
can impart an overall more bent, more elongated, or zigzag
shape to the molecules.
Figure 1. Microphotographs of the nematic Schlieren texture of mp-TC-4
at 162 °C (×160) [top], the SmC Schlieren texture of mp-TC-8 at 115 °C
(×200) [middle], and the pseudofocal conic type texture of the Colh
mesophase of mp-TC-10 at 124 °C (×320) [bottom]. All samples were
sandwiched between untreated glass slides and viewed through crossed
polarizers
We found that the introduction of a strong lateral dipole
through the incorporation of cyano groups at the 3 and 4
positions of the central thiophene unit changes the mesomorphic
properties of the tetracatenars. The NC-TCT-mp derivatives
are liquid crystalline over wide temperature ranges and display
a polymesomorphism typical of tetracatenars (Table 2). The
types of mesophases progress from N, to SmC, to Col with
increasing side chain length (Figure 1). X-ray tilt angle
measurements on magnetically aligned samples reveal a constant
increase of tilt angle of the SmC phases from NC-TCT-mp6
to NC-TCT-mp9 with the SmC phase of NC-TCT-mp10
eventually converting into a high-temperature hexagonal co-
lumnar mesophase (Colhex). This behavior is typical of tetra-
catenars and is consistent with a model in which the increasing
The two conformers found in the single-crystal structures of
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(37) Cook, M. J.; Wilson, M. R. Liq. Cryst. 2000, 27, 1573-1583.
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A R T I C L E S
Figure 2. (a) 1D X-ray diffraction pattern (wide-angle region) of sample mp-TCT-7 at 120 °C and illustrated antiparallel packing of molecules. (b) 2D
X-ray diffraction pattern of a previously magnetically aligned sample of mp-TCT-7 at 120 °C, which shows a 53° tilt angle in the SmC phase.
Figure 3. (a) Four lowest-energy conformers of a NC-TCT-mp derivative with methoxy groups. The conformer minima were first determined by molecular
mechanics (MMFF94) and subsequent energy minimization by AM1 calculations in Spartan. (b) A conformation in which the dipoles of the carbonyl groups
and cyano goups point toward the same direction was found in the single-crystal structure of NC-TCT-mp4 shown for comparison.
NC-TCT-mp4 and NC-DCT-p4* were very different (Figures
3 and 4); this is possibly due to molecular packing forces. NC-
TCT-mp4 showed an interdigitated structure with antiparallel
dipoles, while NC-DCT-p4* displayed layers with parallel
dipole orientation within a layer. While the large hysteresis (>10
°C) in the melting transition temperatures of the NC-TCT-mp
compounds suggests major structural changes are occurring at
the crystal-mesophase transitions and that the single-crystal
structure is therefore unlikely to reflect the order in the liquid
crystalline phase, the crystal structure does help to elucidate
generally favored and disfavored intermolecular interactions as
well as conformations. NC-mTCP-mp bistolane derivatives
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A R T I C L E S
Eichhorn et al.
Figure 4. Molecular packing in the single-crystal structures of (a) NC-DCT-p4* with polar layer structure and (b) nonchiral NC-TCT-mp4 with interdigitated
antiparallel dipole orientation.
Table 3. Mesomorphism of 3,4-Dicyanothiophene-Based Symmetric Dicatenars as Well as Unsymmetric Tricatenars and Tetracatenars
Determined by Polarizing Microscopy, DSC (5 °C/min) and X-ray Diffraction
T/deg C, heating
(∆H/kJ mol-1
T/deg C, cooling
(∆H/kJ mol-1
d-values/Å
(sample temp/degC)a
compound
transition
)
)
NC-DCT-p10
Cr-SmC
SmC-N
N-I
143.5 (12.74)
196.1 (0.64)
281.8 (0.12)
131.4 (29.68)
221.9 (0.48)
206 (24.76)
279 (0.14)
158.8 (39.95)
171.9 (2.002)
247.6 (0.94)
157.2 (55.59)
213.2 (1.22)
119.4 (-10.65)
192.0 (-0.57)
279.9 (-0.12)
111.1 (-11.40)
220.1 (-0.72)
198 (-22.03)
278 (-0.11)
NC-DCT-p12
Cr-N
N-I
Cr-N*
N*-I
NC-DCT-
p4*(S)
NC-DCT-
p10*(S)
Cr-SmC*
SmC*-N*
N*-I
125.9 (-33.84)
171.8 (-1.47)
233.8 (-0.12)
122.2 (-40.85)
127.5 (-2.54)
210.8 (-1.16)
104.7 (-30.76)
145.5 (-4.26)
193.3 (-1.09)
54.0 (-16.11)
98.6 (-5.44)
29.6, 14.9, 4.5
(165 °C)
NC-TrCT-
mp4,p10*(S)
Cr-(SmC*)b
(SmC*)-N*
N*-I
NC-TrCT-
mp6,p10*(S)
Cr-SmC*
SmC*-N*
N*-I
Cr-SmC*
SmC*-I
Cr-cubic
cubic-I
138.7 (39.81)
146.2 (4.23)
194.1 (1.09)
92.8 (32.33)
100.7 (5.18)
65.6 (82.61)
115.1 (1.80)
30.3, 15.2, 4.5
(130 degC)
NC-TCT-
mp6, mp10*(R)
NC-TCT-
pm12,p8
30.0, 15.1, 4.5
(80 deg C)
58(shoulder), 44,
39(shoulder), 12.5,
10.5, 6.2, 5.3, 4.5, 4.2
(68 deg C)
41.7 (-24.37)
109.5 (-0.94)
a Reflections at 6-7 and 4-5 Å are broad halos. b Monotropic mesophase.
(with a bend angle of 120°) cannot adapt rodlike elongated
conformations and are not mesomorphic, despite their surpris-
ingly low melting and crystallization temperatures between 116
°C and 93 °C (Scheme 2 and Table 4).
The NC-TCT-mp derivatives were considered candidates to
display biaxial nematic phases because of the sterically and
electronically hindered rotation about their molecular long axis.
A tendency of mesogens to display antiparallel dimeric cor-
relations results in mesomorphic units that may not contain
lateral dipoles and a more rodlike shape. Apparently, dimers
can rotate freely, and no evidence for the existence of biaxiality
was found by optical microscopy.38 However, more elaborate
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A R T I C L E S
Table 4. Mesomorphism of Bistolane-Based Symmetric
Tetracatenars Determined by Polarizing Microscopy, DSC (5
°C/min) and X-ray Diffraction (2D-WAXS)
T/deg C,
T/deg C,
d-values/Å
(sample
heating
cooling
compound
transition (∆H/kJ mol-1
)
(∆H/kJ mol-1
)
temp/deg C)a
H-mTCP-mp6
Cr-I
122 (73.79)
98.0 (-68.56)
89.6 (-71.76)
H-mTCP-mp10 Cr1-Cr2 95.9 (71.79)
Cr2-I
Br-mTCP-mp6 Cr-I
Br-mTCP-mp8 Cr-I
101.2 (13.60)
127.9 (56.91)b 82.0 (-50.81)
108.9 (43.22) 88.8 (-46.43)
Br-mTCP-mp10 Cr1-Cr2 71.5 (17.52)
Cr2-I 109.4 (36.79)
Br-mTCP-mp12 Cr1-Cr2 83.5 (11.69)c 76.1 (-4.08)
Cr2-I 108.4 (49.16) 82.8 (71.18)
NC-mTCP-mp8 Cr1-Cr2 66.31 (11.43) 66.2 (-7.03)
Cr2-I 116.5 (39.04) 95.5 (-39.03)
NC-mTCP-mp12 Cr1-Cr2 84.2 (42.38) 83.1 (-39.31)
Cr2-I 110.2 (122.56) 93.7 (123.78)
89.8 (-44.89)
H-pTCP-mp6
Cr-SmC 147.5 (28.61) 141.0 (-33.17) 27.9, 13.8, 4.5
SmC-N 164.9 (9.57)
N-I 217.8 (1.47)
167.5 (-8.31) (155 deg C)
217.7 (-1.67)
H-pTCP-mp10 Cr-SmC 114.6 (58.15) 112.6 (-59.66) 31.6, 15.8, 4.5
SmC-N 159.7 (5.10)
N-I 175.7 (0.67)
163.3 (-5.12) (140 deg C)
174.6 (-1.18)
a Reflections at 6-7 and 4-5 Å are broad halos. b Cold crystallization
on heating at 109 °C. Cold crystallization at 97 °C.
NMR measurements would be required to provide conclusive
evidence of the formation of dimers and the presence or absence
of biaxiality within the phases.39
Control over the macroscopic alignment of liquid crystals is
essential to many optical investigations and applications.
Homogeneous (parallel) alignment was easily achieved for all
of the symmetric tetracatenars by either the use of surface
treatments such as rubbed polyimide or the application of shear
forces. Homeotropic (vertical) alignment in glass cells treated
with cetyl trimethylammonium bromide or octadecyltrimethoxy-
silane was not successful for the materials reported herein, and
small, slightly tilted domains were obtained. The inability to
achieve vertical alignment seems to be an attribute of the
tetracatenar substitution rather than of the bent-rod structure.
Bent-rod dicatenars NC-DCT-p10, 12 showed a better homeo-
tropic alignment under the same conditions, while the straight-
rod tetracatenars H-pTCP-mp6, 10 did not. We considered that
the high clearing temperatures of some of the derivatives could
be problematic, but reference measurement with known liquid
crystals did not suggest any thermal damage to the alignment
layers when heated to temperatures up to 220 °C.
Untreated hydrophilic surfaces resulted in a preferred homo-
geneous organization of NC-TCT derivatives that can be
explained due to surface interactions with the lateral polar cyano
groups. All NC-TCT derivatives were further investigated as
free-standing films, and the fact that NC-TCT-mp4, -5, and -6
display stable freely suspended films is noteworthy, because
few nematic liquid crystals are capable of this behavior.38,40
These compounds show easily interpretable defect textures as
free-standing films, whereas on solid surfaces unusual and small
domain defect textures are often observed due to the strong
surface interactions of their bent-rod shapes and large lateral
dipoles. Figure 5 shows a sequence of textures monitoring the
Colh to SmC transition of NC-TCT-mp10 in a free-standing
Figure 5. Phase transition from a Colh to a SmC phase in a free-standing
film of NC-TCT-mp8 (on cooling, ×200).
film of 1 mm diameter. In these photographs, the texture changes
from fanlike to Schlieren-like, which indicates a transition from
a homogeneously aligned columnar to a homeotropically aligned
smectic phase.
As none of the aforementioned liquid crystals display
ferroelectric or antiferroelectric mesophases, we decided to
introduce chiral side chains to induce polar phases in NC-TCT-
mp derivatives. This effort was encouraged by the polar sheet
structure found in the single-crystal structure of NC-DCTp4*-
(S). A parallel orientation of the cyano-groups of neighboring
molecules within each sheet was revealed, but the overall
material was not polar since the orientation of the macroscopic
dipole alternates from sheet to sheet (Figure 4).
Surprisingly, the introduction of four chiral branched side
chains fully suppressed liquid crystallinity of the derivatives
X-TCT-mp10* and NC-TCT-mp4*. Only the 8-stereoisomer
mixture NC-TCT-mp10*(rac), derived from the attachment of
racemic side chains, displayed a monotropic N-phase. Similarly,
a narrow enantiotropic nematic mesophase (4 °C) was exhibited
by the 1:1 mixture of the (R) and (S) enantiomers.
An explanation for the suppression of the liquid crystallinity
in systems with chiral side chains is that disturbances caused
by the methyl-group branches of the chiral side chains push
the molecules apart. Models show that this steric hindrance is
even stronger when we assume that antiparallel packing is
preferred, since this organization results in the methyl groups
being directed toward each other rather than all pointing in a
similar direction such as is the case in a SmC*. A 1:1 mixture
of the (R) and (S) enantiomers results in a decreased disturbance,
and consistent with this model, the 1:1 mixture was the only
(38) Chandrasekhar, S.; Nair, G. G.; Shankar Rao, D. S.; Krishna Prasad, S.;
Praefcke, K.; Blunk, D. Liq. Cryst. 1998, 24, 67-70.
(39) Hunt, J. J.; Date, R. W.; Timimi, B. A.; Luckhurst, G. R.; Bruce, D. W. J.
Am. Chem. Soc. 2001, 123, 10115-10116.
(40) Faetti, S.; Palleschi, V. Phys. ReV. A: At., Mol., Opt. Phys. 1984, 30, 3241-
3251.
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Figure 6. Polarizing microscope pictures (crossed polarizers, ×100) of NC-DCT-p10*(S) at different temperatures. Grandjean textures of the N* phase
indicate a spontaneous orthogonal surface alignment of the helices (planar boundary conditions of the molecules), and the change of the iridescent color, a
decrease in pitch with increasing temperature. On approaching the isotropic phase the alignment of the helices was lost, and a focal-conic texture appeared
at 243 °C.
material to display an enantiotropic nematic mesophase. How-
ever, the suppression of liquid crystalline behavior is very strong,
since doping of the nonchiral NC-TCT-mp derivatives with
chiral molecules, chiral-liquid crystals, or the chiral NC-TCT-
mp derivatives discussed previously all result in phase separation
into an isotropic liquid phase and the nonchiral mesophase of
the NC-TCT-mp derivative. Thus, mesophases of the NC-TCT-
mp tetracatenars are not compatible with chiral induction, most
likely due to unfavorable conformational changes.
and tricatenars as well as possibly antiferroelectric behavior of
the tetracatenar. Triangular wave measurements show evidence
of switching, and we also observe a commensurate optical
switching of the focal conic domains. However, large resistive
currents, possibly due to ionic impurities that could not be
removed, precluded detailed characterization of the ferroelectric
and antiferroelectric behavior at this time. Future efforts will
seek to resolve this issue.
In consideration of our hypothesis that steric repulsion
originating from the chiral centers destabilized the mesophase,
we investigated lowering the number of side chains and
attaching chiral side chains to only one end of the molecule.
Derivatives NC-DCT-p10*(S), NC-TrCT-mp4,p10*(S), and
NC-TrCT-mp6,p10*(S) display SmC*- and N*-phases between
138° and 248 °C, while NC-DCT-p4*(S) displays a N*-phase
and NC-TCT-mp6,mp10*(R) a SmC*-phase only (Schemes 1
and 2, Table 3). The N*-phases of NC-DCT-p10*(S); NC-
TrCT-mp4,p10*(S); and NC-TrCT-mp6,p10*(S) displayed a
pitch in the wavelength range of visible light as can be
concluded from their iridescent Grandjean textures (Figure 6).
The (complementary) color change of NC-DCT-p10*(S) from
purple to green indicates a decrease in pitch with increasing
temperature. Heating of the sample to temperatures above 240
°C, close to the clearing transition, resulted in a loss of the
orthogonal surface alignment of the helices and appearance of
a focal-conic texture.41,42
Conclusions
Though bent cores and strong lateral dipoles typically disturb
calamitic mesophases, we were able to demonstrate that
combining both of these structural features can result in
enantiotropic mesomorphism over wide temperature ranges. In
fact, the phase diagram of the bent tetracatenars containing two
lateral cyano groups at the central thiophene ring (NC-TCT) is
very similar to what has been found for common straight-rod
tetracatenars. We obtained nematic, smectic, columnar, and
cubic mesophases by changing the length and substitution
pattern of the four aliphatic side chains. At a molecular level,
however, we propose distinct differences and explain the unusual
behavior of the NC-TCT derivatives with an antiparallel dimer
formation. The antiparallel dimer formation is incompatible with
the formation of chiral mesophases. Only tetracatenars of lower
symmetry and analogues with fewer side chains tolerated chiral
induction, and chiral-nematic and SmC phases were obtained.
Investigations of electrooptical properties of the chiral phases
are the focus of our ongoing efforts.
Preliminary electrooptical measurements on the SmC* of NC-
DCT-p10*(S); NC-TrCT-mp6,p10*(S); and NC-TCT-mp6,-
mp10*(R) are suggestive of ferroelectric behavior of the di-
(41) Coles, H. in Handbook of Liquid Crystals Demus, D., Goodby, J., Gray,
G. W., Spiess, H.-W., Vill, V., Eds.; Wiley-VCH: Weinheim, 1998; Vol.
1, p 343, 350, 365.
(42) Demus D.; Richter, L. In Textures of Liquid Crystals; 1st ed.; VEB
Deutscher Verlag, fuer Grundstoffindustrie: Leipzig, 1978; pp 55-57.
Acknowledgment. We are grateful for the generous financial
support from NEDO and the National Science Foundation. This
work made use of equipment provided by the MRSEC Program
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12750 J. AM. CHEM. SOC. VOL. 124, NO. 43, 2002

Tetracatenar Liquid Crystals
A R T I C L E S
of the National Science Foundation under award number DMR
98-08941
This material is available free of charge via the Internet at
http://pubs.acs.org.
Supporting Information Available: Experimental details,
characterization data and crystallographic information (PDF).
JA0268027
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