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Discotic Liquid Crystals with an Inverted Structure.

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Angewandte
Chemie
Liquid Crystals
Discotic Liquid Crystals with an Inverted
Structure**
Sigurd Hger,* Xiao Hong Cheng,
Anne-Dsire Ramminger, Volker Enkelmann,
Almut Rapp, Mihail Mondeshki, and Ingo Schnell
Discotic liquid crystals, first described by Chandrasekhar and
co-workers 25 years ago, can be used for a variety of optical
and electronic applications.[1] For example, discotics in the
columnar phase are interesting materials for photovoltaic
applications, and discotic nematic liquid crystals can be used
as compensation layers in display technology.[2, 3] Furthermore, compounds based on macrocyclic mesogens are valuable candidates for the formation of tubelike superstructures.[4] The common design principle of all discotic liquid
crystals is a more or less rigid core (disklike or macrocyclic)
with peripheral flexible side groups that point outward. We
recently reported the first indication that macrocycles composed of a rigid periphery and flexible side groups that point
to the inside (inverted liquid crystals) also exhibit thermotropic mesophases.[5] This new design was derived from the Xray single-crystal structure analysis of 1, which adopts a solidstate conformation in which the long alkyl chains point
inward. However, as these chains are located at the adaptable
positions of the ring, which can be easily switched between
different conformations, it is not clear whether the same
conformation is retained in the liquid-crystal phase.[6] Herein
we prove unambiguously that discotic liquid crystals with an
inverted structure actually exist.
The key compounds for our investigations are nanometersized shape-persistent macrocycles 2–5 with flexible alkyl
chains attached to the rigid backbone. The investigation of the
thermal behavior of compound 2 a by differential scanning
calorimetry (DSC) and optical microscopy (crossed polarizers) shows that it melts at 134 8C to form a mesophase that
becomes isotropic (I) at 159 8C. The observed Schlieren
texture (Figure 1, left) and the observation of only a broad
[*] Prof. S. Hger, Dr. X. H. Cheng+
Institute for Chemical Technology and Polymer Chemistry
University of Karlsruhe
Engesserstrasse 18, 76131 Karlsruhe (Germany)
Fax: (+ 49) 721-608-3151
E-mail: hoeger@chemie.uni-karlsruhe.de
A.-D. Ramminger, Dr. V. Enkelmann, Dr. A. Rapp,++ M. Mondeshki,
Dr. I. Schnell
Max Planck Institute for Polymer Research
Ackermannweg 10, 55128 Mainz (Germany)
[+] On leave from the School of Chemistry and Materials Engineering of
Yunnan University, China.
[++] Present address:
Department of Chemical Engineering
Materials Research Laboratory
University of California, Santa Barbara, CA 93106 (USA)
[**] Financial Support by the Deutsche Forschungsgemeinschaft is
gratefully acknowledged.
Angew. Chem. Int. Ed. 2005, 44, 2801 –2805
DOI: 10.1002/anie.200462319
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2801
Communications
The intraannular orientation of the alkyl chains in 2 a was
also confirmed by X-ray single-crystal structure analysis
(Figure 2).[7] As in the solid-state structure of 1, the cyclic
backbone of 2 a is not completely planar. As a result, the alkyl
Figure 2. Top: Structure of 2 a in the crystal. Bottom: The packing
behavior of the alkyl chains of 2 a in the crystal show that the flexible
side chains form a dense alkyl packing (different shading is used for
clarity).
reflection at approximately 4.5 indicates the formation of a
nematic phase (N), as also found for 1. In contrast to 1,
however, the octadecyloxy side groups of 2 a have a fixed
intraannular position and cannot flip to the outside. This
means that 2 a is the first definite example of a liquid crystal
with this new design: a rigid periphery acts as a frame for
flexible side chains that point to the inside (inverted liquid
crystal, Figure 1, right).
Figure 1. Left: Nematic Schlieren texture of 2 a at 150 8C. Middle:
Design principle of conventional cyclic molecules capable of forming
thermotropic mesophases. Right: Schematic presentation of the alkyl
chain arrangement in 2 a.
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2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
chains are located alternately above and below the ring plane
and form a dense alkyl packing with side chains of adjacent
rings. Although short intermolecular contacts are present
between the rigid portions of the macrocycle in 2 a, the alkyl
chains melt upon heating and the molecules become mobile
to form the liquid-crystalline phase by parallel arrangement.
In the liquid-crystalline phase (at 145 8C) the dynamic order
parameters for the macrocycles are on the order of S = 0.1–
0.15 (as determined by solid-state NMR spectroscopy), and
the mobility of the alkyl chains increases from the anchoring
OCH2 groups (S 0.1) toward the chain ends (S!0).[8, 9] This
mobility increase is a well-known phenomenon, but in this
case it does not occur in the usual gradual manner along the
C18 chain. Instead, most of the CH2 segments appear to
exhibit similar dynamic behavior, which is reflected in a
common order parameter of S 0.04 that only drops to zero
at the chain end.
One of the objectives of this investigation was to vary
thermal parameters (Tm, Ti, DT) through changes in molecular structure. If the macrocycle is not to be changed, there
are, in principle, two ways this can be done: 1) variation of the
length and shape of the intraannular side chains and
2) introduction of bulky side chains at the periphery to
control the distances (interactions) between rings. Transition
temperatures and enthalpies for the macrocycles investigated
herein are summarized in Table 1.[10] With all other molecular
parameters kept constant, a side-chain length of approximately 14 carbon atoms is necessary to obtain mesophases
with 2 in a temperature range in which no thermal decomposition is observed. Furthermore, the introduction of
branched (dimethyloctyl) side groups does not influence
this behavior significantly.[11, 12]
Attempts to broaden the mesophase temperature range of
these new discotics through the attachment of bulky groups at
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Angew. Chem. Int. Ed. 2005, 44, 2801 –2805
Angewandte
Chemie
Table 1: Phase-transition behavior of the macrocycles 1–5.
Compound
1
2a
2b
2c
2d
3a
3b
3c
4
5a
5b
Phase-transition temperature [8C]
(Enthalpy [kJ mol1])
185
134
148
247
266
157
266
267
185
226
218
(152.7)
(71.5)
(94.7)
(106.3)
(87.7)
(126.4)
(115.5)
(39.3)
(44.5)
(65.1)
(21.0)
N
N
N
N
N
I
I
I
I
N
N
207 (0.73)
159 (1.82)
222 (3.52)
[a]
[b]
[c]
[b]
[b]
I
I
I
[a]
[a]
[a] Isotropization temperature lies above the decomposition temperature. [b] Melting is accompanied by decomposition. [c] Solidifies upon
cooling as a glass.
the ring peripheries were ineffective. We expected the tertbutyl-substituted macrocycles 3 to have a lower melting point
owing to an increased separation between the rigid parts of
the macrocycles, but in all cases we observed slightly higher
melting points than those of the methyl analogue 2.[11a]
Unexpectedly, none of the macrocycles 3 were found to be
liquid crystalline. However, the structures of 3 a and 3 c could
be determined crystallographically (Figure 3).
Although a direct correlation between the crystal structures and organization of molecules in the mesophase is
questionable, there are some significant differences between
the crystal structures of the mesophase-forming macrocycles 1
and 2 a, and those of 3 a and 3 c, which do not form
mesophases. We assumed that the alkyl chains of 3 a would
also undergo dense alkyl packing as was observed for 2 a and
1. On the contrary, however, the side chains in 3 a show no
tendency at all to organize in one of the commonly observed
alkyl-chain-packing motifs. They deviate from the expected
all-trans conformation and instead orient themselves perpendicular to the ring plane such that one of the chains encloses
the side of an adjacent ring (Figure 3, middle left).[13]
Accordingly, in the crystalline state, the side chains in 3 a
exhibit a gradual increase in segment mobility from the
anchoring point at the macrocycle (S 0.6 at T = 54 8C)
toward the chain ends (S 0.3), as determined by solid-state
NMR spectroscopy. Such an increase of alkyl-chain flexibility
is not observed in 2 a for which the chain segments show
mobility that is substantially more uniform with a common
(and higher) dynamic order parameter of S 0.65 at T =
54 8C.
Furthermore, the macrocycles in the crystal lattice are
locked by the large tert-butyl groups (Figure 3, bottom left).
This implies that even if the alkyl chains become mobile upon
heating, melting of the side chains does not interrupt the
Figure 3. Top left: Structure of 3 a in the crystal. Middle left: Top view of three molecules of 3 a which shows the interlocking of the macrocycles.
One C18 chain encloses the rigid side of an adjacent ring. Bottom left: The side view shows that the rigid parts of the rings are locked by the bulky
tert-butyl groups at the corners of the ring. Notably, two of the four C18 side chains are disordered: the terminal 11 carbon atoms are found in two
orientations, each of which is present with 50 % probability (only one orientation is displayed for clarity). Top right: Structure of 3 c in the crystal.
Middle right: Side view of two molecules of 3 c which shows that the tert-butyl group of the upper macrocycle dips deep inside the flexible alkyl
interior of the other ring. Bottom right: Again, the side view shows that the rigid parts of the rings are locked by the bulky tert-butyl groups at the
corners of the ring (different shading is used for clarity).
Angew. Chem. Int. Ed. 2005, 44, 2801 –2805
www.angewandte.org
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2803
Communications
extensive ring–ring interactions, and the mechanical coupling
of the rings in 3 a does not break down before it is completely
(isotropically) molten.
An ordered side-chain packing cannot be expected for 3 c.
In this case the macrocyclic frame strongly deviates from
planarity and two of the side chains are bent above and below
the macrocycle (Figure 3, right). This leaves cavities in which
the bulky tert-butyl groups located above and below the
macrocycles can interdigitate. As also found for 3 a, tert-butyl
groups lock adjacent molecules together so that an interruption of the ring–ring interaction is only possible when the
material is completely molten. Additionally, the size of the
bulky side groups leads to a more spherical shape of the
molecule which can destabilize liquid-crystalline phases.[11a, 14]
To investigate the influence of ring substituents on the
thermal behavior of the compounds, macrocycle 4 was
synthesized which lacks adaptable substituents on the rigid
ring. The formation of a mesophase with 4 was also not
observed. It is highly probable that removal of the adaptable
side groups generates empty space within the macrocycles,
and the resulting conflict between molecular anisotropy and
empty space inside the rings destabilizes the formation of a
thermotropic mesophase.[15] Alternatively, it is possible that
adaptable, flexible side groups or small extraannular substituents are principally required to obtain liquid-crystal
phases with these compounds. To address this issue, macrocycles that do not contain such groups, 5 a and 5 b, were
investigated.[16] For both compounds a nematic mesophase
could be observed.[17] Under the assumptions made above, it
can be speculated that the polycyclic aromatic backbone
decreases the size of the open cavity inside the rings, which
prevents the molecules from interlocking with each other.
Macrocycles 5 a could be crystallized as a solvate from
tetrachloroethane (TCE) to give four TCE molecules per
ring (Figure 4).
The relatively flat internal void of the macrocycle is
already filled with the first few carbon atoms of the intraannular side groups. The remaining portions of the alkyl
chains are located above and below the macrocyclic framework. Two of the C18 chains are oriented parallel to the ring
and crystallize in pairs with alkyl chains from adjacent rings.
These alkyl-chain pairs are sandwiched between two polycyclic aromatic units of the rigid macrocyclic backbone,
separating them from each other. The remaining two alkyl
chains deviate further from the ring plane and fill the lateral
space between the rings. Interlocking of the molecules
through entanglement of rigid and flexible parts between
adjacent molecules cannot be observed, although close p–p
contacts could be identified in this case. Furthermore, interlocking of the molecules through the extraannular substituents does not occur. Although 5 a could be crystallized only as
solvate, the structure clearly shows that the internal void of
each macrocycle is filled by its own alkyl chains. This means
that the alkyl chains melt upon heating and the molecules can
move sufficiently to form a mesophase before complete
(isotropic) melting.[18] The mesophase formed by macrocycles
5 a and 5 b shows that the design principle described herein is
not restricted to the phenylene–ethynylene backbone of
macrocycles 1 or 2. Moreover, it shows that substituents at
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2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Figure 4. Top: Structure of 5 a in the crystal (solvent not shown).
Bottom: Side view of four molecules of 5 a in the crystal which show
that the polycyclic aromatic portions of the ring are separated by alkyl
chains. Although a strong deviation of some of the alkyl chains from
the ring plane is observed, interlocking of the molecules cannot be
found. Furthermore, no interlocking of the rigid ring parts by the
extraannular methyl or phenyl substituents can be observed (different
shading is used for clarity).
the adaptable positions of macrocycles are not a prerequisite
for the observation of a mesophase.
In conclusion, shape-persistent rings with intraannular
flexible side chains have been demonstrated as a hitherto
unknown design principle for discotic liquid crystals. From
investigations of their thermal behavior and the X-ray singlecrystal structure analysis of these compounds, some preliminary guidelines for the synthesis of liquid crystals with an
inverted structure can be deduced: 1) The formation of a
mesophase is correlated to the possibility of the system to fill
the interior space more or less with its own alkyl chains.
2) The molecules must not contain extraannular bulky substituents that cause the molecules to interlock and that also
decrease molecular asymmetry. 3) The actual conformation of
the alkyl chains in the solid state gives no apparent
information about the thermal behavior of the macrocycles.
Instead, it is important that no interlocking of the molecules
occurs. Our investigations indicate that this design principle is
rather general and that the presence of adaptable substituents
is not a structural prerequisite. Therefore, a variety of
different macrocycles with other aspect ratios are expected
to exhibit liquid crystallinity as well.[19]
Received: October 15, 2004
Revised: January 14, 2005
Published online: March 30, 2005
.
Keywords: liquid crystals · macrocycles · mesophases ·
substituent effects · supramolecular chemistry
www.angewandte.org
Angew. Chem. Int. Ed. 2005, 44, 2801 –2805
Angewandte
Chemie
orientations q0 to yield the dynamic order parameter [Eq. (2)]:
[1] a) S. Chandrasekhar, B. K. Sadashiva, K. A. Suresh, Pramana
1977, 9, 471 – 480; b) S. Chandrasekhar in Handbook of Liquid
Crystals, Vol. 2B (Eds.: D. Demus, J. Goodby, G. W. Gray, H.-W.
Spiess, V. Vill), Wiley-VCH, Weinheim, 1998, pp. 749 – 780;
c) R. J. Bushby, O. R. Lozman, Curr. Opin. Colloid Interface Sci.
2002, 7, 343 – 354.
[2] a) D. Adam, P. Schuhmacher, J. Simmerer, L. Haeussling, K.
Siemensmeyer, K. H. Etzbach, H. Ringsdorf, D. Haarer, Nature
1994, 371, 141 – 143; b) N. Boden, R. J. Bushby, J. Clements, B.
Movaghar, J. Mater. Chem. 1999, 9, 2081 – 2086; c) L. SchmidtMende, A. Fechtenktter, K. Mllen, E. Moons, R. H. Friend,
J. D. MacKenzie, Science 2001, 293, 1119 – 1122.
[3] a) H. Mori, Y. Itoh, Y. Nishiura, T. Nakamura, Y. Shinagawa,
Jpn. J. Appl. Phys. Part 1 1997, 36, 143 – 147; b) M. Okazaki, K.
Kawata, H. Nishikawa, M. Negoro, Polym. Adv. Technol. 2000,
11, 398 – 403; c) S. Kumar, S. K. Varshney, Angew. Chem. 2000,
112, 3270 – 3272; Angew. Chem. Int. Ed. 2000, 39, 3140 – 3142.
[4] a) J. P. Behr, J.-M. Lehn, A.-C. Dock, D. Moras, Nature 1982,
295, 526 – 527; b) J.-M. Lehn, J. Malthete, A.-M. Levelut, J.
Chem. Soc. Chem. Commun. 1985, 1794 – 1796; c) J. Zhang, J. S.
Moore, J. Am. Chem. Soc. 1994, 116, 2655 – 2656; d) O. Mindyuk,
M. R. Stetzer, P. A. Heiney, J. C. Nelson, J. S. Moore, Adv. Mater.
1998, 10, 1363 – 1366.
[5] S. Hger, V. Enkelmann, K. Bonrad, C. Tschierske, Angew.
Chem. 2000, 112, 3256 – 3258; Angew. Chem. Int. Ed. 2000, 39,
2268 – 2270.
[6] a) S. Hger, V. Enkelmann, Angew. Chem. 1995, 107, 2917 –
2920; Angew. Chem. Int. Ed. Engl. 1995, 34, 2713 – 2716;
b) D. L. Morrison, S. Hger, Chem. Commun. 1996, 2313 –
2314; c) S. Hger, D. L. Morrison, V. Enkelmann, J. Am.
Chem. Soc. 2002, 124, 6734 – 6736.
[7] CCDC-252531–252534 contain the supplementary crystallographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif. Selected crystal data: 2 a
(T = 250 K): triclinic, P1̄, a = 14.888(1), b = 17.448(1), c =
18.262(1) , a = 71.501(1), b = 75.386(1), g = 70.278(1)8, V =
4178.7(3) 3, Z = 2, 1x = 1.056 g cm3, 31 555 independent reflections, 6336 reflections observed, R = 0.0863, Rw = 0.1043. 3 a (T =
120 K): triclinic, P1̄, a = 16.519(1), b = 16.652(1), c =
18.742(1) , a = 108.824(2), b = 102.745(2), g = 103.448(2)8,
V = 4494.5(3) 3, Z = 2, 1x = 1.058 g cm3, 17 332 independent
reflections, 6702 reflections observed, R = 0.0730, Rw = 0.0761.
3 c (T = 120 K): triclinic, P1̄, a = 13.707(2), b = 17.783(2), c =
17.952(2) , a = 62.568(2), b = 87.979(2), g = 87.306(1)8, V =
3879.3(2) 3, Z = 2, 1x = 1.060 g cm3, 9887 independent reflections, 2856 reflections observed, R = 0.0684, Rw = 0.0710.
5 a·4 TCE (T = 220 K): triclinic, P1̄; a = 15.9580(6), b =
16.0040(7), c = 17.7550(7) , a = 96.7810(12), b = 103.2770(14),
g = 102.9350(14)8, V = 4232.1(3) 3, Z = 2, 1x = 1.213 g cm3,
17 818 independent reflections, 6577 reflections observed, R =
0.0882, Rw = 0.0901.
[8] The dynamic order parameters (S) discussed herein represent
the residual anisotropy of a given molecular segment, as
determined by NMR spectroscopy from the residual dipole–
dipole coupling between two nuclei in the segment (1H1H in
CH2, or 13C1H in CH or CH3). Formally, S can be expressed in
the form of Equation (1) in which P(q,q0) is the distribution
function of orientations q adopted by a segment in the course of
its motion starting from the initial orientation q0.
S(q0) =
R1
1
2
(3 cos2q1) P(q,q0) d cos q
1
S = hS(q0)/2 (3 cos2q01)iq0 (2)
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
These dynamic order parameters S should not be confused with
the orientational order parameters that are often used for
describing the degree of orientational order in liquid crystals.
The two parameters are not identical, yet they are related: the
dynamic order parameters are affected by both the local motions
of molecular segments and the overall motions of the molecules.
In contrast, only the overall molecular motions are relevant to
the orientational order parameters, such that dynamic order
parameters can be (and are, in our case) decreased by local
segment motions relative to the orientational order of the
molecules.
a) I. Schnell, Prog. Nucl. Magn. Reson. Spectrosc. 2004, 45, 145 –
207; b) A. Rapp, I. Schnell, D. Sebastiani, S. P. Brown, V. Percec,
H. W. Spiess, J. Am. Chem. Soc. 2003, 125, 13 284 – 13 297.
Unless otherwise stated, mesophases were assigned on the basis
of their textures.
a) D. M. Collard, C. P. Lillya, J. Am. Chem. Soc. 1991, 113, 8577 –
8583; b) S. Kumar, D. S. S. Rao, S. K. Prasad, J. Mater. Chem.
1999, 9, 2751 – 2754.
Thermogravimetric analysis (TGA) of 2 d shows no weight loss
up to 300 8C. However, partial decomposition of the materials
cannot be fully excluded above 230–250 8C.
This unusual packing behavior may result from the increased
ring width, which does not allow optimal space filling with all
alkyl chains in the trans configuration.
Assuming a similar packing behavior for 2 d, the smaller
terminal methyl groups do not cause an interlocking of the
molecules and allow mesophase formation.
a) S. Hger, K. Bonrad, A. Mourran, U. Beginn, M. Mller, J.
Am. Chem. Soc. 2001, 123, 5651 – 5659; b) M. Fischer, G. Lieser,
A. Rapp, I. Schnell, W. Mamdouh, S. De Feyter, F. C.
De Schryver, S. Hger, J. Am. Chem. Soc. 2004, 126, 214 – 222.
X. H. Cheng, S. Hger, D. Fenske, Org. Lett. 2003, 5, 2587 – 2589.
A transition for 5 a was observed at 194 8C (DH = 27.3 kJ mol1)
to another LC phase that is not yet assigned.
Additionally, it can be assumed that the large polycyclic
aromatic functions of the compound stabilize an orientation
correlation of molecules in the mesophase: P. J. Collings, M.
Hird, Introduction to Liquid Crystals, Taylor & Francis, London,
1997, p. 83. Moreover, the flexibility of the macrocyclic backbone of 5 is likely to be less than that of the other macrocycles.
K. Praefke, D. Blunk, D. Singer, J. W. Goodby, K. J. Toyne, M.
Hird, P. Styring, W. D. J. A. Norbert, Mol. Cryst. Liq. Cryst. 1998,
323, 231 – 259.
(1)
0
If the samples are not macroscopically oriented (as in our case),
the parameters S(q0) must be averaged over all possible initial
Angew. Chem. Int. Ed. 2005, 44, 2801 –2805
www.angewandte.org
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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