Phase behavior and mesophase structures of 1,3,5-benzene- and 1,3,5-cyclohexanetricarboxamides: Towards an understanding of the losing order at the transition into the isotropic phase

Article abstract of DOI:10.1002/chem.201103216

One of the simplest and most-versatile motifs in supramolecular chemistry is based on 1,3,5-benzenetricarboxamides. Variation of the core structure and subtle changes in the structures of the lateral substituents govern the self-assembly and determine the

Full text of DOI:10.1002/chem.201103216

FULL PAPER
DOI: 10.1002/chem.201103216
Phase Behavior and Mesophase Structures of 1,3,5-Benzene- and 1,3,5-
Cyclohexanetricarboxamides: Towards an Understanding of the Losing Order
at the Transition into the Isotropic Phase
Andreas Timme,[a] Roman Kress,[a] Rodrigo Q. Albuquerque,[b] and
Hans-Werner Schmidt*[a]
Abstract: One of the simplest and
most-versatile motifs in supramolecular
chemistry is based on 1,3,5-benzenetri-
carboxamides. Variation of the core
structure and subtle changes in the
structures of the lateral substituents
govern the self-assembly and deter-
mine the phase behavior. Herein, we
provide a comprehensive comparison
between the phase behavior and meso-
phase structure of a series of 1,3,5-ben-
zene- and 1,3,5-cyclohexanetricarbox-
plastic crystalline, and liquid crystalline
phases were formed. The relatively
rare columnar nematic (NC) phase was
only observed in cyclohexane-based
the order decreases. Temperature-de-
pendent IR spectroscopy and XRD
measurements revealed that columnar
H-bonded aggregates were still present
in the isotropic phase. At the clearing
transition, mainly the lateral order was
lost, whilst shorter columnar aggregates
still remained. A thorough understand-
ing of the phase behavior and the
trisACTHNUTRGENUGaN mides that contained linear alkyl
substituents. Of fundamental interest in
liquid crystalline supramolecular sys-
tems is the transition from the meso-
morphic state into the isotropic state
and, in particular, the question of how
mesoACTHNUTRGENUGpN hase structure is relevant for se-
lecting processing conditions that use
supramolecular structures in devices or
as fibrillar nanomaterials.
Keywords: liquid crystals · self-as-
sembly · structure elucidation · ther-
mal analysis · X-ray diffraction
ACHTUNGTRENNUNGamides that contain linear and
branched alkyl substituents. Depending
on the substituent, different crystalline,
Introduction
disc-shaped molecules. To a large extent, it was stabilized by
the presence of three amide groups per molecule, thereby
yielding strong hydrogen bonds, which led to the formation
of stable columnar aggregates.[4–9] Owing to the formation of
supramolecular structures, these compounds were capable of
forming physical gels of a variety of organic liquids[10–13] as
well as aqueous solutions.[14,15] These compounds were also
applied as functional additives to nucleation and clarifica-
tion agents for isotactic polypropylene and polyvinyliden-
fluoride[16–18] and as additives to improve the electret perfor-
mance of polypropylene.[19,20] Bushey et al. described hexa-
substituted aromatic cores that consisted of three amide sub-
stituents in the 1,3,5-positions that were flanked by substitu-
ents other than hydrogen at each of the remaining
positions.[21]
A variety of compounds that are composed of disk-like mol-
ecules are known to form liquid crystalline (LC) mesophas-
es. The first series of such compounds, which consisted of
hexakis(alkanoyloxy)benzenes, was reported by Chandrase-
khar et al. in 1977.[1] Another series of compounds, which
consisted of 1,3,5-benzenetricarboxamides with certain alkyl
substituents, was reported by Matsunaga et al. to exhibit
thermotropic liquid-crystalline behavior over a broad tem-
perature range.[2,3] Mesophase formation with columnar hex-
agonal order resulted not only from the anisometry of the
[a] A. Timme, R. Kress, Prof. Dr. H.-W. Schmidt
Macromolecular Chemistry I
Bayreuther Institut fꢀr Makromolekꢀlforschung (BIMF)
and Bayreuther Zentrum fꢀr Kolloide und
Grenzflꢁchen (BZKG)
University Bayreuth, 95440 Bayreuth (Germany)
Fax : (+49)921-55-3206
A structurally similar series to 1,3,5-benzenetricarbox-
AHCTUNGTREGaNNUN mides are 1,3,5-cyclohexanetricarboxamides, first reported
by Tomatsu et al.[22] The different core strongly affected the
stacking behavior and thermal properties compared with
benzenetricarboxamides.[23,24] Some of these cyclohexane de-
rivatives were also excellent gel-forming molecules for non-
polar organic solvents[25] and aqueous solutions.[26–28]
E-mail: hans-werner.schmidt@uni-bayreuth.de
[b] Prof.Dr. R. Q. Albuquerque
Theoretical Physics IV
University Bayreuth, 95440 Bayreuth (Germany)
Current Address: Institute of Chemistry of S¼o Carlos
University of S¼o Paulo, Av. Trab. S¼ocarlense 400
13560-970 S¼o Carlos (Brazil)
Herein, two series of 1,3,5-tricarboxamides, one with
a benzene core and one with a cyclohexane core, with linear
and branched alkyl substituents (Table 1) were compared
with respect to their phase behavior and mesophase struc-
ture to obtain a deeper understanding of the structure–prop-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201103216.
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Table 1. Chemical structures of 1,3,5-tricarboxamides, based on either ben-
zene (1) or cyclohexane cores (2).
molecular order: First, the stacking along the column axis,
caused by strong threefold hydrogen bonding, and, second,
the intercolumnar interactions. At the transition tempera-
ture of these ordered columnar phases into the nematic or
isotropic phase, different possibilities of losing order may
occur depending on the strength of interactions inside
a column: In the case of strong hydrogen bonding, the in-
tercolumnar order is lost, whilst the columns themselves
persist. A columnar nematic (NC) phase is formed. If the
hydrogen bonds are less strong, the columns break into
smaller pieces. Below a certain column lengths, the sample
appears to be optical isotropic, although columnar aggre-
gates are still present. If the order inside a column is lost
completely, an isotropic phase is obtained, which is disor-
dered on a molecular level.
R
1a
1b
1c
2a
2b
2c
1d
1e
1 f
2d
2e
2 f
1g
1h
1i
2g
2h
2i
Results and Discussion
Phase-transitions and liquid-crystalline behavior: Although
the thermal data for some of these compounds have been
published previously in part,[2–6,22] we re-measured them to
have comparable data that were obtained under identical
experimental conditions. The phase transitions, as mea-
sured by differential scanning calorimetry (DSC), polariz-
ing optical microscopy (POM), and X-ray diffraction
(XRD), are compared in Figure 1. For the transition tem-
peratures, including the enthalpies, see the Supporting In-
formation, section I. Complex polymorphisms, such as
solid–solid transitions and plastic crystalline and different
LC phases, were observed for many compounds.
1j
2j
1k
2k
1l
2l
2m
2n
2o
1m
1n
1o
The derivatives with a benzene core and linear side-
chains of at least six C atoms (1c–1 f; Figure 1a) exhibited
an ordered, columnar, hexagonal, LC phase (Colho phase).
Compounds 1a and 1b did not possess LC phases, but
rather possessed plastic crystalline mesophases instead. Plas-
tic phases are characterized by a 3D crystal-like order,
whilst the molecules within the columns are able to rotate.
Therefore, these phases appear to be plastic.[30–32] Compound
1c actually showed two hexagonal phases: a plastic Colhp
phase followed by a Colho phase. The temperature range, in
which the LC phase was present, became broader with in-
creasing chain length. Whereas the clearing temperatures
(Tc) of the LC compounds were located between 207 and
2188C, independent of the chain length, the melting temper-
ature (Tm) decreased with increasing chain length from
1728C (1c) to 588C (1 f). The DSC heating traces are shown
in Figure 2. The temperature range, over which the LC
phases were thermodynamically stable, is indicated by hori-
zontal arrows. The textures of the LC phases were recorded
upon cooling at the marked temperatures.
erty relationships in these compounds. Although a lot of
1,3,5-benzenetricarboxamides have been synthesized and
characterized by Matsunaga et al., not all of their mesophas-
es could be classified at the time.[2,3] The supramolecular
structures of stacked molecules of 1,3,5-benzene- and 1,3,5-
cyclohexanetricarboxamides have already been compared
by Lightfoot et al., but only one compound of each series
that contained short polar substituents was described.[8]
Special attention was given to the stepwise losing of order
during heating at the transition from the mesophase into the
isotropic phase. The structure of the nematic phase, which is
formed by some 1,3,5-cyclohexanetricarboxamides, was of
fundamental interest.[22] As this phase is an intermediate
state in between a higher-ordered columnar phase and the
isotropic liquid, it may help to understand what happens at
the transition into the isotropic state. Laschat et al. investi-
gated discotic liquid crystals and found that the hexagonal
arrangement of the columns was lost in the isotropic melt,
but they did not describe what happened to the columns
themselves.[29] Coming from 3D order in the crystalline
state, hexagonal or rectangular columnar mesophases were
formed at higher temperatures, and possessed two types of
Branched benzene tricarboxamides (Figure 1b) were
transformed into isotropic liquids at markedly higher tem-
peratures, except for compound 1o. Their corresponding
phase-transition enthalpies were also higher. Branched side-
chains stabilized aggregation in the mesophases. Just like
their linear analogues, the compounds with four C atoms in
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1,3,5-Benzene- and 1,3,5-Cyclohexanetricarboxamides
FULL PAPER
compounds with linear alkyl
substituents, compounds 2c–2 f
formed columnar rectangular
plastic (Colrp) mesophases fol-
lowed by nematic LC phases.
XRD of compound 2 f revealed
that its nematic phase was clas-
sified as NC, which is very un-
common (see below).
Cyclohexane derivatives with
branched alkyl substituents
(Figure 1d) showed numerous
DSC transitions in most cases.
Nearly all of these compounds
exhibited more than one meso-
phase and, in some cases, addi-
tional crystalline phases were
present. The temperatures of
transition into the isotropic
phase were between 338 and
3828C, which pointed to stron-
ger interactions than in the ben-
zene derivatives. Compound 2o
was the only one that definitely
formed a Colho phase, as known
from the aromatic analogues.
With a stability range of more
than 200 K (121–3388C), com-
pound 2o shows the broadest
LC phase of all of the com-
pounds investigated herein.
As examples of the influence
of linear versus branched sub-
stituents and benzene versus cy-
clohexane tricarboxamides, the
thermal behavior of compounds
1 f, 1o, 2 f, and 2o were com-
pared (Figure 3). Higher tem-
peratures of transition into the
isotropic phase were observed
Figure 1. Phase-transition temperature as a function of the substituent and the core (DSC, 1st heating): a) ben-
zene core with linear alkyl substituents; b) benzene core with branched alkyl substituents; c) cyclohexane core
with linear alkyl substituents; d) cyclohexane core with branched alkyl substituents. Cr=crystalline, Col=col-
umnar mesophase, h=hexagonal, r=rectangular, p=plastic crystalline, o=ordered liquid crystalline, NC =col-
umnar nematic, M=unidentified mesophase, X=unknown solid phase (not ordered enough for a crystalline
!
~
phase). Transitions: crystalline–mesophase ( ), crystalline–isotropic ( ), mesophase–isotropic ( ), Colhp–Colho
J
*
^
(
), Colrp–NC (&), mesophase–mesophase ( ).
for the branched compounds.
the longest side-chain (1g–1j) did not show LC behavior
but rather showed a plastic crystalline mesophase, which
transformed directly into the isotropic phase. Similar to
compound 1c, compounds with longer side-chains (1k–1o)
showed two Colh phases: a Colhp phase followed by a Colho-
phase. Whilst compound 1o showed an exceptionally broad
Colho phase between 57 and 2448C, compounds 1k–1n were
liquid crystalline over a smaller range (around 250–3008C).
Regarding the temperature of transition into the isotropic
state, it was not only important whether the side-chain was
branched or linear, but also the position of branching was
important: The transition temperatures increased if the
branching was closer to the core.
The transition temperature of compound 1o was 26 K
higher than that of compound 1 f and that of compound 2o
was 21 K higher than that of compound 2 f. For the cyclo-
hexane tricarboxamides, the clearing temperatures were
higher (317 and 3388C for compounds 2 f and 2o, respec-
tively) than those of the analogous benzene derivatives (281
and 2448C for compounds 1 f and 1o, respectively). Whilst
compounds 1 f, 1o, and 2o possessed a broad Colho phase,
compound 2 f showed two crystalline phases followed by
a plastic crystalline Colrp phase and a liquid crystalline NC
phase.
Investigation of the mesomorphic state: To use these supra-
molecular structures in devices or as supramolecular nano-
fibers, it is of fundamental importance to gain an insight
into their molecular arrangement within the different meso-
Cyclohexane tricarboxamides with linear alkyl substitu-
ents (Figure 1c) showed considerably higher transition tem-
peratures into the isotropic state. In contrast to the aromatic
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H.-W. Schmidt et al.
phases. Therefore, these compounds were studied by XRD
to investigate the interdisc distance and the lateral order
and to identify the type of mesophases present. For exam-
ple, the diffraction patterns of compounds 1o and 2o, which
both formed a Colho phase, are shown in Figure 4. The dis-
Figure 4. XRD patterns of a benzene tricarboxamide (1o) and its analo-
gous cyclohexane derivative (2o) in the LC state (Colho) at 1508C: The
cyclohexane derivative showed a smaller column distance but a larger in-
terdisc distance. The halo in the wide-angle range was caused by interac-
tions between the amorphous parts without long-range order. Such halos
are common in the LC phase, whereas the occurrence of distinct signals
in this range pointed to 3D crystalline order.
Figure 2. DSC–temperature plots of the first heating scan of linear-substi-
tuted benzene tricarboxamides, which indicated the formation of a broad-
er LC phase ($) and a lower Tm with increasing chain length (1c–1 f);
mesophase textures were recorded under cooling at the indicated temper-
ature, scale bar: 50 mm.
tance of interdisc spacing [001] rose from 0.35 nm for the
benzene core to 0.47 nm for the cyclohexane core, independ-
ent of the alkyl substitution pattern (see below). This rise of
about one third was due to packing reasons. A hexagonal
lattice was characterized by the presence of at least three
signals with the ratio of their corresponding lattice distances
p
of 1:1/ 3:1/2, which corresponded to the [100], [110], and
[200] spacing, respectively. The column distance was slightly
smaller for the alicyclic compounds. The columns packed
closer together than the aromatic compounds.
For all of the liquid crystalline benzene tricarboxamides,
the LC phase was assigned to be a Colho phase. The hexago-
nal arrangement of compound 1o is shown in Figure 5. The
same hexagonal order was also present in the plastic Colhp
phase that was formed by several compounds. The core–
core distances (a) were calculated with ChemSketch.[33] The
distances between the molecules were drawn such that a=
1.99 nm, as obtained from the XRD investigations. Assum-
ing that the side-chains were elongated, they nearly touched
the core of the adjacent molecule, which meant that the
side-chains interlaced with each other.
The column distance in the hexagonal arrangement was
obtained by XRD from the lattice distances that were calcu-
lated by using Braggꢃs law (Figure 6 and Figure 7). The
column distances, as well as the interdisc spacing, are listed
in the Supporting Information, Table S1. The aromatic com-
pounds that contained linear side-chains (1b–1 f) showed
a linear increase in the column distance of 0.1 nm per addi-
Figure 3. DSC–temperature plots of the first heating scan of linear (f)
and branched (o) benzene (1) and cyclohexane tricarboxamides (2)
showed higher transition temperatures for branched compounds and for
the cyclohexane compounds. *: Micrographs of the textures were record-
ed under cooling at the indicated temperature, scale bar: 50 mm.
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1,3,5-Benzene- and 1,3,5-Cyclohexanetricarboxamides
FULL PAPER
phase (Colhp for compounds 1g and 1k–1n, Colho for com-
pound 1o) showed a larger spacing than the unbranched
compounds owing to hindered intercalation (Figure 6). In
these compounds, the space consumption increased from
1.66 nm (1c) to 1.79 nm (1k, ethyl substitution) and 1.80 nm
(1l, dimethyl substitution). For compounds 1m and 1n,
which were both methyl-substituted, less additional space
was needed. The column distance rose from 1.76 nm (1d) to
1.81 nm (1m) and 1.80 nm (1n). Also, compound 1g showed
a larger distance than the extrapolated value of compound
1a. The column distance also depended on the position of
the substituents. The closer the branching was to the core,
the larger the space consumption, which hindered the inter-
calation of the columns. Comparing the total number of
C atoms in the side-chain (including branches), the branched
compounds needed less space than their elongated ana-
logues, because they were more compact (Figure 7). For ex-
ample, compounds 1k–1n, which had side-chains that were
constructed from eight C atoms, showed smaller column dis-
tances than unbranched 1e.
Figure 5. Hexagonal arrangement in a columnar hexagonal phase of com-
pound 1o; the core–core distances are drawn with the calculated distance
of a=1.99 nm, as obtained from the XRD investigations.
Temperature-dependent XRD patterns of compound 1 f
were collected to gain an insight into the morphologies of
different phases (Figure 8). At 120, 150, and 1808C, three
Figure 6. Column distance a (Figure 5) of benzene tricarboxamides in the
*
hexagonal phase at 1508C: Compounds without branches ( ) showed
a linear increase with chain length (line); larger spacing was determined
*
for branched compounds ( ) with the same main chain length (›).
Figure 8. XRD patterns of compound 1 f at different temperatures, which
revealed the morphologies in the crystalline, liquid crystalline, and iso-
tropic phases: The pattern at room temperature was measured before
heating the sample. A large number of signals were evident for a crystal-
line phase.
sharp signals in the small-angle range, which corresponded
to the [100], [110], and [200] lattice distances, were evidence
for a hexagonal phase. The distances increased with temper-
ature owing to thermal expansion of about 0.9 pmKꢀ1. In
the wide-angle range, a distinct columnar stacking was
shown in the LC phase, with a constant spacing of 347 pm.
This signal became weaker at 1808C, which meant that the
columnar order decreased at high temperature. At 2308C,
two diffuse signals pointed to an isotropic liquid with re-
maining columnar aggregates. If the molecules were totally
disordered, no signal would have been observed. The broad
diffuse signal at small angles described an average column
Figure 7. Column distance a (Figure 5) of benzene tricarboxamides in the
hexagonal phase at 1508C versus the total number of C atoms in the
*
side-chain, including branches; the branched compounds ( ) needed less
*
space than their elongated analogues ( ).
tional C atom, which was independent of the type of Colh
mesophase (Colhp for compounds 1b and 1c, Colho for com-
pounds 1d–1 f) at a fixed temperature of 1508C for compari-
son. Comparing compounds with the same main chain
length, the branched compounds that formed a hexagonal
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H.-W. Schmidt et al.
distance of about 2.1 nm, which was larger than in the or-
dered LC phase (21% larger than at 1508C). However, the
second signal in the wide-angle region, which came from the
regular stacking of discs along the column axis, did not
appear in the isotropic melt. This result meant that the exist-
ing columns were less ordered and/or shorter. The presence
of columnar aggregates will be proven later with additional
data (see below).
For comparison, a diffraction pattern was collected at
room temperature, at which the crystal structure was com-
pletely different. No hexagonal order was present, but in-
stead a monoclinic lattice was found with calculated values
of a=2.48 nm, b=0.70 nm, c=1.23 nm, and b=1018.
The geometries of stacked molecules were simulated for
both cores with the same substituent (compounds 1o and
2o; Figure 9). The simulated interdisc distances were in very
had a noncircular cross-section, which favored a rectangular
lattice (Colr) instead of a hexagonal lattice.[22]
Investigation of the nematic phase: 1,3,5-Cyclohexanetricar-
boxamides 2d–2 f formed a nematic phase, which was an in-
termediate state between the higher-ordered columnar
phase and the isotropic liquid. The molecular structure of
this nematic phase is of special interest, as it may help to un-
derstand what happens at the transition to the isotropic
state. Although the existence of this nematic phase has al-
ready been described by Tomatsu et al.[22] for compounds
2d–2 f, the type of nematic phase was not specified. Two dif-
ferent types of nematic phases needed to be considered,
namely the discotic nematic (ND) and columnar nematic
(NC) mesophases.[29] XRD of compound 2 f in the nematic
phase revealed a broad but distinct signal that corresponded
to the [001] interdisc distance of 0.47 nm, as also obtained
from the higher-ordered columnar phases (Figure 10). In
Figure 10. XRD patterns of compound 2 f in the columnar nematic phase
(top) and in the columnar rectangular plastic phase (bottom); the signal
at around 9.58 showed that intracolumnar stacking was also in the nemat-
ic phase.
contrast, in the small-angle range, a broad diffuse signal re-
vealed a liquid-like arrangement of the columns. According
to the absence of intercolumnar order and the still-existing
intracolumnar spacing, the nematic phase of compound 2 f
was classified as NC. NC phases are very rare and have only
been reported in a few examples. A NC phase was known to
be induced by charge-transfer interactions between a binary
mixture of a disk-shaped electron donor and an electron ac-
ceptor.[29,34–36] Such a phase was also found for ionic liquid
crystals,[37] oligomesogenes,[38] bent-core mesogenes,[39] and
phthalocyanine complexes.[40] In contrast to the binary sys-
tems and the structurally complex compounds, we observed
for the first time, a nematic NC phase for a single-compo-
nent system based on a simple molecule. Analogous to com-
pound 2 f, the nematic phases of compounds 2d and 2e
were also most-likely NC phases. However, owing to their
higher transition temperatures, they were not experimentally
investigated by XRD.
Figure 9. Orthogonal views of the simulated stacking geometries of con-
secutive molecules within a column for compounds 1o (benzene core)
and 2o (cyclohexane core); different stacking, depending on the core, led
to different interdisc distances for both compounds, which was larger in
the case of the cyclohexane core.
good agreement with the results obtained by XRD. The
side-chains of consecutive molecules adopted staggered ori-
entations for aromatic compounds and eclipsed orientations
for alicyclic compounds. The simulations confirmed the ar-
rangement inside a column, as previously described by
Lightfoot et al.[8] This remarkable difference in packing
within a column led to a larger interdisc distance for the cy-
clohexane derivatives. Because of the eclipsed orientation of
their side-chains, the columns of the cyclohexane derivatives
Transition from the mesomorphic state into the isotropic
state: Herein, we gave special focus on the stepwise losing
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1,3,5-Benzene- and 1,3,5-Cyclohexanetricarboxamides
FULL PAPER
ꢀ
of order during heating. Laschat et al. stated that the hexag-
onal arrangement of columns in discotic liquid crystals was
lost in the isotropic melt, although they did not report what
happened to the columns themselves.[29] Therefore, we inves-
tigated the transition from the mesomorphic state into the
isotropic phase in more detail to obtain an insight into how
the order was lost during heating. Coming from 3D order in
the crystalline state, columnar hexagonal or rectangular
mesophases were formed at higher temperatures, which pos-
sessed two types of molecular order: the stacking along the
column axis caused by strong threefold hydrogen bonding
and weaker 2D lateral order. In this case, the interdisc inter-
actions were several orders of magnitude stronger.[7] At the
upper transition temperature of these ordered columnar
phases, there were different possibilities of losing order de-
pending on the strength of interactions inside a column
(Figure 11). If the weaker lateral interactions were lost first,
bonds. In the hydrogen-bonded state, the N H and C=O
bonds were weaker and the absorption bands of their vibra-
tions appeared at lower wavenumbers.[5,41,42] As an example,
ꢀ
the n
(N H) and n
(C=O) vibrations of compound 1 f were
investigated in the solid state, in solution, and in the optical
isotropic melt. The different vibrations were indicated as n1
for strongly hydrogen-bonded species, n2 for weakly hydro-
gen-bonded species, and n3 for molecularly dissolved species.
For compound 1 f, these types of N H species at different
conditions are shown in Figure 12. Vertical lines mark the
ꢀ
Figure 12. IR spectra of compound 1 f showed strong hydrogen bonds in
the solid and partially in 10 mmolLꢀ1 o-dichlorobenzene solution (n1),
whereas compound 1 f was molecularly dissolved in dilute o-dichloroben-
zene as well as in CHCl3 (n3). In the melt, weak hydrogen-bonded aggre-
gates were present (n2).
Figure 11. Losing of order during heating: Depending on the strength of
the interactions inside a column, an anisotropic columnar nematic phase,
an optical isotropic phase that consisted of short columns, or a molecular
isotropic phase with disordered molecules was formed.
ꢀ
wavenumbers of the absorption maxima of the n3
(N H) ab-
ꢀ
ꢀ
whilst the hydrogen bonds persisted completely, a columnar
nematic (NC) phase was formed, which appeared optical an-
sorption and the n1
G
N
tion at 3240 cmꢀ1, which is typical for strong hydrogen-bond-
A
ing interactions, was present in the solid state and partially
umns would break into smaller pieces. Below a certain
column lengths, the sample appeared to be optical isotropic.
This optical isotropic phase, in which short columns were
still present, must be distinguished from the molecular iso-
tropic phase, in which the columnar order was lost entirely.
The optical isotropic phase is an intermediate state in be-
tween the anisotropic nematic and the molecular isotropic
phases.
present in 10 mmolLꢀ1 o-dichlorobenzene. The n3
G
ꢀ
sorption at about 3450 cmꢀ1, which indicated the presence of
free amide groups, was detected in solution, as would be ex-
pected in a molecularly dissolved state. Similar compounds
have been reported to be molecularly dissolved in
CHCl3.[6,10] In the optical isotropic melt of compound 1 f, the
ꢀ
n3ACHTUNGTREN(NUNG N H) absorption only occurred very weakly. Instead, an-
ꢀ
other broad absorption band n2ACTHNUTRGNE(NUG N H) appeared at
Infrared (IR) spectroscopy allows the study of changes in
H-bonded aggregates by different absorptions of amide
3375 cmꢀ1, which belonged to a weakly hydrogen-bonded
species. This absorption was located between the molecular-
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H.-W. Schmidt et al.
ly dissolved and the strongly associated species. Although
these hydrogen bonds were weaker than at room tempera-
ture, the columnar aggregates were still present in the opti-
cal isotropic melt.
melting transition temperature, the monoclinic lattice was
transformed into a hexagonal lattice. The corresponding
high transition enthalpy only resulted from intercolumnar
interactions. Because the stacking inside a column was not
affected, no change in H-bonding should be observed. Ap-
proximately 15 K below the transition temperature (DSC)
from the mesophase into the optical isotropic phase, the ab-
ꢀ
The IR spectra of compound 1 f as a function of tempera-
ture were collected from 30–2908C (for the spectra, see the
Supporting Information, section II). The absorbance of the
ꢀ
strongly bonded amide groups (n1
G
N
sorbance of the n1ACTHUNGTRENNU(G N H) and n1CAHTUNGTRNE(NUNG C=O) vibrations that corre-
sponded to the strongly bonded species decreased, whereas
ꢀ
creased, whilst the absorbance of the weakly bonded (n2
G
ꢀ
ꢀ
H) and n2
C
G
the absorbance of the two n2
N
N
creased as expected during heating. Figure 13 shows the
that corresponded to the weakly bonded species increased.
In the optical isotropic phase, these changes continued,
albeit with a smaller slope. This result confirmed the exis-
tence of aggregates that broke up slowly with an increase in
temperature.
Similar to compound 1 f, alicyclic compound 2 f also did
not show a significant change in the absorbance at the tran-
sition from the crystalline phase into the columnar meso-
phase (Figure 14). At higher temperatures in the range 220–
Figure 13. IR absorbance of compound 1 f as a function of the tempera-
ꢀ
ture; squares: n
G
G
gen-bonded species (n1), open symbols: weakly hydrogen-bonded species
(n2); vertical lines: transition temperatures obtained by DSC (first heat-
ing, 1 Kminꢀ1).
transition from the strongly associated to the weakly associ-
ated species. The intensity at the absorption maxima of the
Figure 14. IR absorbance of compound 2 f as function of the tempera-
ꢀ
vibrations n
N
H
ꢀ
ture; squares: n
N
N
vestigated as a function of temperature. The absorbance (A)
was calculated from the IR spectra by using the baseline
method.[43] A is defined as in Equation (1):
gen-bonded species (n1), open symbols: weakly hydrogen-bonded species
(n2); vertical lines: transition temperatures obtained by DSC (first heat-
ing, 1 Kminꢀ1).
A ¼ lg ðI0=IÞ
ð1Þ
2908C, the absorbance of both n1 vibrations decreased,
whereas the absorbance of the two n2 vibrations increased.
This result correlated with the transition from the Colrp
phase into the NC phase at 2498C (DSC, 1 Kminꢀ1). At this
transition, the intercolumnar order was lost and the columns
broke into shorter pieces. In contrast to compound 1 f, these
pieces were still long enough to form a columnar nematic
phase. On further heating, the columns became shorter,
thereby resulting in an optical isotropic phase above the
clearing temperature at 3168C (DSC, 1 Kminꢀ1). However,
the changes in the absorbance continued, with a significantly
smaller slope. This result confirmed the slow break-up of ag-
gregates and them becoming shorter with increasing temper-
I0 and I are the transmission intensities of the beam
before and after its transmission through the sample. These
relative absorbance values were normalized to the absorb-
ꢀ
ance of the aliphatic nACTHUNGTRENNG(U C H) vibration, which was not in-
volved in the phase-transition and remained constant.
For compound 1 f, no significant change in the absorbance
was observed for the transition from the crystalline phase
into the columnar hexagonal mesophase. XRD measure-
ments showed that the morphology in the crystalline phase
was different to that in the mesophase (Figure 8) with re-
spect to the arrangement of the columns with each other but
not with respect to the structure within the columns. At the
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1,3,5-Benzene- and 1,3,5-Cyclohexanetricarboxamides
FULL PAPER
ature. The absorption of the free amide groups (n3) in the
melt was not detected for the alicyclic compound (2 f) in
contrast to compound 1 f.
The intercolumnar and intracolumnar interactions cannot
be regarded individually, as they influence each other. Obvi-
ously, the long columnar aggregates were stabilized by inter-
columnar interactions in the mesophases. Within a column,
the molecules were stacked and all of the amide groups
pointed in the same direction, thereby generating a macrodi-
pole that governed the intermolecular interactions. This phe-
nomenon was described in the literature for tricarbox-
ACHTUNGTRENNUNG
amides.[8,12,13,23,24,44] Through a partly antiparallel arrange-
ment of adjacent columns, the dipole moments were com-
pensated and the columnar structure was stabilized.[9] If the
lateral order was lost at the clearing temperature, the col-
umns would become unstable and break into shorter pieces.
Only in those cases with exceptionally strong H-bonds
would the possibility of the formation of NC phases exist. At
higher temperatures in the optical isotropic melt, there were
still columnar aggregates present. The molecules were defi-
nitely not disordered on a molecular level.
Brunsveld et al.[6] reported that the unusually high clear-
ing enthalpy of compound 1o resulted from the melting of
the three intermolecular hydrogen bonds. This result did not
agree with our findings that, at Tc, mainly the lateral interac-
tions were lost. Our interpretation was supported by the fol-
lowing example: The amide groups of compounds 1 f and 1o
absorbed at the same wavenumbers in the LC phase as well
as in the isotropic phase. Although the energy difference
necessary to weaken the hydrogen bonds was the same in
both cases, compounds 1 f and 1o differed from each other
in terms of the clearing enthalpy (19 and 31 kJmolꢀ1 for
compounds 1 f and 1o, respectively). This result meant that
the enthalpy difference between compounds 1 f and 1o
could not be explained by the break-up of hydrogen bonds,
but by different lateral interactions. In conclusion, the inter-
molecular H-bonds were only broken up to a small extent
into shorter columns.
The presence of columnar aggregates in the optical iso-
tropic phase was also confirmed by XRD measurements.
The diffraction pattern of compound 1 f showed a distinct
signal at small angles, which corresponded to the columnar
distance (Figure 15). The observation of a columnar distance
indicated the presence of columnar aggregates in the optical
isotropic phase. The diffuse halo in the wide-angle region
came from short-range order, which was also present in iso-
tropic phases of small molecules (c.f. the toluene curve,
Figure 15). Compound 3 was investigated as a reference
compound. Here, the hydrogen atoms of the amide groups
were substituted by methyl groups; therefore, compound 3
was not able to build up stable columns through hydrogen
bonds. The XRD pattern of compound 3 in the isotropic
phase only showed a very weak signal at small angles, thus
confirming the absence of columnar aggregates.
Figure 15. Diffraction patterns from the optical isotropic melts of com-
pound 1 f (2308C), compound 3 (1508C), and toluene (RT); the signal of
compound 1 f in the small-angle region indicated columnar aggregation.
ance of nematic liquid crystalline phases is an axial ratio
(length/diameter) above which rod-like molecules orient
themselves along a director. Flory characterized rod-like
liquid crystals by their excluded volume. He found that
nematic order in a thermotropic system was only stable if
the axial ratio was above 6.42.[45] This theory did not consid-
er macrodipole interactions, which were present in our case.
Therefore, the minimum axial ratio might differ for the dis-
cussed compounds. The columnar aggregates could be con-
sidered as single rod-like molecules; these aggregates were
able to form a NC phase above a certain length. The diame-
ter of a column was estimated by assuming the presence of
elongated side-chains and a virtual circle around the mole-
cule. For example, compound 1 f had a diameter of 3.5 nm.
To obtain a suitable axial ratio to form a stable nematic
phase, a length of the aggregates of more than 22.5 nm cor-
responded to at least 65 molecules, by taking the measured
disc spacing of 0.35 nm into account. Shorter aggregates ap-
peared to be optical isotropic and the interdisc spacing was
not present in the XRD pattern. A signal could only be ob-
served for regular spacing above a certain number of repeat-
ing units. This optical isotropic phase where short columns
Although in the case of compound 1 f, columnar aggre-
gates were present in the optical isotropic melt, no columnar
nematic phase was observed. One condition for the appear-
Chem. Eur. J. 2012, 18, 8329 – 8339
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8337

H.-W. Schmidt et al.
were still present (1 f) must be distinguished from the mo-
lecular isotropic phase (3), in which the columnar order was
entirely lost.
Experimental Section
Methods: The phase-transition temperatures and their corresponding en-
thalpies were determined by using a Perkin–Elmer Diamond DSC with
a heating rate of 10 Kminꢀ1. For comparison with the IR data, a heating
rate of 1 Kminꢀ1 was used. The XRD measurements in the range q=0.5–
158 were carried out on a Huber Guinier diffractometer 600 that was
equipped with a Huber germanium monochromator 611 to get CuKa1 ra-
diation (l=154.051 pm). An extra slit diaphragm reduced the broadening
of the primary beam owing to scattering in air. A custom-made furnace
was integrated into the diffractometer, which allowed investigation at
temperatures up to 2508C. The samples were prepared in soda-glass ca-
pillaries (1.5–2 mm diameter). IR spectra were collected on a Bio-Rad
Digilab FTS-40 that was equipped with a heating device by using KBr
pellets. Solution samples were prepared between NaCl plates (layer
thickness: 0.5 mm). The geometries of the stacked molecules were opti-
mized by using the PM6 semi-empirical method that was implemented in
Conclusion
Two series of 1,3,5-tricarboxamides, one with a benzene
core and one with a cyclohexane core, with linear and
branched alkyl substituents were compared with respect to
their phase behavior and mesophase structure to obtain
a deeper understanding of the structure–property relation-
ships in these compounds. Depending on the substituent and
the core, a large variety of columnar mesophases were ob-
tained, including columnar hexagonal plastic, columnar rec-
tangular plastic, and columnar hexagonal liquid crystalline
phases, as well as a columnar nematic (NC) phase. NC phases
are very rare and have only been reported in a few exam-
ples. For compounds 2d–2 f, we observed for the first time
a NC phase for a single-component system that was based on
a simple molecule. Linear-substituted benzene tricarbox-
the MOPAC2009 program,[46] which was able to describe not only hydro-
[47]
ꢀ
gen bonds, but also p p interactions. The reliability of the optimized
geometries was checked by calculating the vibrational frequencies, which
only gave rise to positive frequencies. For compound 2o, the theoretical
core–core separation was about 498 pm, whilst the H-bond distances
were about 214 pm.
Synthesis of tricarboxamides: 1,3,5-tricarboxamides were prepared based
on both a benzene and a cyclohexane core. The 1,3,5-benzenetricarbox-
AHCTUNGERTGaNNUN mides (1a–1o) were synthesized from the reaction of 1,3,5-benzenetri-
ACHTUNGTRENNUNGamides showed a broader LC-phase with increasing chain
carboxylic acid chloride with the corresponding amine in N-methylpyrro-
lidone or THF as the solvent and pyridine or triethylamine as the base.
Lithium chloride was added to weaken the hydrogen-bonding interac-
tions during the reaction to improve solubility. The cis,cis-1,3,5-cyclohex-
anetricarboxamides (2a–2o) were prepared in an analogous manner, but
starting from commercially available cis,cis-1,3,5-cyclohexanetricarboxylic
acid, which was allowed to react with oxalyl chloride to yield the corre-
sponding trichlorides. For further details regarding the synthesis and
characterization of these compounds, see the Supporting Information,
sections IV and V.
length. Branched compounds showed higher transition tem-
peratures into the isotropic phase than their linear ana-
logues. The transition temperatures for cyclohexane tricar-
boxamides were higher than those for benzene tricarbox-
ACHTUNGTRENNUNG
pounds were smaller than their linear analogues in relation
to the total amount of C atoms. In the case of a columnar
hexagonal mesophase, a smaller column distance was found
for the alicyclic compounds compared to the aromatic ones.
The interdisc distance was larger for the alicyclic com-
pounds. The stacking geometries in the columnar phases
were simulated for both core types with the MOPAC2009
program. The simulated interdisc distances were in very
good agreement with the results obtained by XRD.
The columnar hexagonal or rectangular mesophases,
which were formed in most cases, possessed two types of
molecular order: First, they were stacked along the column
axis as a result of strong hydrogen bonding and, second,
they showed 2D lateral order between the columns. At the
temperature for the transition of ordered columnar phases
into phases with lower order, three types were observed de-
pending on the internal strength of the interactions inside
the column. In the case of strong hydrogen bonding, the in-
tercolumnar order was lost, whilst the columns themselves
persisted and a NC phase was formed. The NC phase was
only observed within the series that was based on the cyclo-
hexane core. The aromatic derivatives were transformed
from a Colh phase directly into the isotropic phase. In this
case, isotropic means optical isotropic, but the molecules
were not molecularly disordered. Temperature-dependent
XRD and IR spectroscopy measurements revealed that the
columnar aggregates were still present above the clearing
temperature.
Acknowledgements
We gratefully acknowledge Sandra Ganzleben and Jutta Failner (Macro-
molecular Chemistry I, University Bayreuth) for the synthesis of most of
the tricarboxamides. We thank Dr. Wolfgang Milius (Inorganic Chemistry
I, University Bayreuth) for calculating the unit cell of compound 1 f in
the crystalline phase. We would like to acknowledge the financial support
by the Deutsche Forschungsgemeinschaft (SFB 840-B8).
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Received: October 12, 2011
Revised: January 16, 2012
Published online: May 31, 2012
Chem. Eur. J. 2012, 18, 8329 – 8339
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www.chemeurj.org
8339