Columnar propeller-like 1,3,5-triphenylbenzenes: The missing link of shape-persistent hekates

Article abstract of DOI:10.1039/c5sm02489g

Triphenylbenzenes with different substitution patterns at the outer phenyl rings have been successfully synthesised. Sixfold n-alkoxy substitution was insufficient for mesomorphism, but already increasing the number of side chains by three methoxy groups

Full text of DOI:10.1039/c5sm02489g

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DOI: 10.1039/C5SM02489G
Columnar propeller-like 1,3,5-triphenylbenzenes: the missing link of shape-
persistent hekates†
Tobias Wöhrle,a Stuart James Beardsworth,a Christopher Schilling,a Angelika Baro,a Frank
Giesselmannb and Sabine Laschat*a
Abstract
Triphenylbenzenes with different substitution patterns at the outer phenyl rings have been
successfully synthesised. Sixfold nꢀalkoxy substitution was insufficient for mesomorphism,
but already increasing this number of side chains by three methoxy groups led to liquid
crystalline behaviour and mesophase formation. Symmetrical triphenylbenzenes with nine
nꢀalkoxy side chains (≥ C9) formed broad enantiotropic mesophases. The symmetry of the
liquid crystalline phases was unambiguously determined by Xꢀray diffraction measurements
to be Colh and Colho for symmetryꢀreduced methoxyꢀalkoxy derivatives and symmetrical
nonaalkoxyꢀtriphenylbenzenes, respectively. Based on Xꢀray diffraction data a stacking model
was proposed in which the single molecules aggregate to helical columns forming the
mesophase.
Keywords discotic, helical selfꢀassembly, liquid crystals, triphenylbenzenes
a Institut für Organische Chemie, Universität Stuttgart, Pfaffenwaldring 55, Dꢀ70569
Stuttgart, Germany. Eꢀmail: sabine.laschat@oc.uniꢀstuttgart.de
b Institut für Physikalische Chemie, Universität Stuttgart, Pfaffenwaldring 55, Dꢀ70569
Stuttgart, Germany.
† Electronic supplementary information (ESI) available. See DOI
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Introduction
Discotic liquid crystals, which selfꢀassemble into columnar mesophases have been intensively
studied over the last decades, because their anisotropic properties, high 1D charge carrier
mobility and selfꢀhealing of defects makes them attractive for molecular electronics.1,2
Among the various discotic liquid crystals are starꢀshaped mesogens with three or more arms
pointing into the vortices of 2D polygons or 3D polyhedra. The star topology provides access
to oligomers and dendrimers with wellꢀdefined size and allows further functionalization of the
arms.3 A particularly interesting subfamily are the shapeꢀpersistent hekates, i.e. starꢀshaped
mesogens with three arms such as phenyl, oligophenylene, oligo(het)arylene, or
oligo(phenylenevinylene) groups, because the space between the arms cannot be compensated
by folding. Thus, simple columnar stacking of the central aromatic cores does not provide
sufficient nanosegregation required for the formation of stable columnar mesophases. In such
systems space filling and nanosegregation is often achieved by rotating neighboring
molecules by a certain angle along the intracolumnar axis. This selfꢀassembly can be further
enforced by hydrogen bonding. In some cases helical columnar stacking is possible, even in
the absence of chiral substituents.4–11 Therefore, these compounds allow a detailed view into
the interplay between molecular structure and modes of selfꢀassembly. While starꢀshaped trisꢀ
styrylbenzenes 112–20 and the corresponding trisꢀstyryltriazines 221–23 as well as 1,3,5ꢀtriꢀ
phenyltriazines 3 have been extensively studied,24–34 the parent 1,3,5ꢀtriphenylbenzenes 5, 7
are much less explored24,35–37 (Scheme 1).
In particular, Scherowsky38 and Kotka24 reported that bisalkoxyꢀsubstituted 1,3,5ꢀtriphenylꢀ
benzenes do not show any mesomorphic properties. Only for the corresponding donor–
acceptor complexes with trinitrofluorenone (TNF) and C8 chains in the triphenylbenzene
columnar mesophases were observed. In light of these results24,38 triphenylbenzenes 5 with
only six short alkoxy substituents (< C9) were deemed unsuitable as mesogens and thus only
hexaꢀ and nonaꢀsubstituted derivatives bearing longer side chains (C9–C18) were considered
further. We were curious whether the missing link in star shaped discotics, i.e. the simple bisꢀ
or trisalkoxyꢀsubstituted triphenylbenzenes with columnar mesomorphism in the neat form,
could be obtained. In addition, due to the fact that the corresponding planar
tris(trialkoxyphenyl)triazines 331 and the tris(trialkoxyphenyl)boroxines 439 display stable
columnar mesophases over a broad temperature range, it was of interest how the propellerꢀ
shape of triphenylbenzenes 5–7 affects the selfꢀassembly behaviour in comparison with 3, 4.
The results are reported below.
2

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Scheme 1 Known shapeꢀpersistent hekates 1–5a and related novel compounds 5bꢀf, 6 and 7.
Synthesis of triphenylbenzenes
The hexaalkoxyꢀ1,3,5ꢀtriphenylbenzenes 5 were prepared by Suzuki cross coupling of
borolanes 840 with 1,3,5ꢀtribromobenzene in moderate to good yields (33–75%) (Scheme 2).
Derivative 5f with C18 alkoxy side chains, however, was obtained in only 7% due to
solubility problems during purification.
Scheme 2 Preparation of nonꢀsymmetrical triphenylbenzene 5.
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Triphenylbenzenes 6 with decreased symmetry in the substitution pattern of the aryl ring were
synthesized from commercially available 1,2,3ꢀtrimethoxyꢀ5ꢀbromobenzene 9 (Scheme 3).
Selective demethylation of 9,41 Williamson etherification and borylation gave methoxyꢀ
bisalkoxyꢀsubstituted borolanes 11, which were submitted to Suzuki cross coupling providing
the desired triphenylbenzenes 6 in 10–25% total yield.
The symmetrically nonaalkoxyꢀsubstituted triphenylbenzenes 7 were accessible in 16–17%
total yield from 9 via borolane 1240 followed by Suzuki cross coupling, demethylation and
final Williamson etherification (Scheme 3). This route is advantageous due to less amount of
expensive reagent BBr3 for the demethylation of 13 to 1438 and fewer purification steps.
Scheme 3 Preparation of triphenylbenzene derivatives 6 and 7.
DSC and POM investigations
The liquid crystalline properties of compounds 5–7 were first studied by differential scanning
calorimetry (DSC). As the known sixfold octyloxyꢀsubstituted triphenylbenzene 5a,38 the
higher homologues 5b–f did not show any mesomorphic behaviour. In contrast, both the nonꢀ
symmetrical derivatives 6 and symmetrical compounds 7 with nine peripheral alkoxy side
chains form stable enantiotropic mesophases. The results of the DSC investigations are
summarised in Table 1 (for original DSC traces see ESI†).
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Table 1 Thermotropic data of the mesogenic triphenylbenzenes 6 and 7 obtained by DSC
(heating/cooling rate 5 K min–1, onset values)a
Compd
Phase Tm (°C)
(∆H / kJꢁmol–1)
53 (Tg)
Phase Tc (°C)
(∆H / kJꢁmol–1)
Colho 126 (30.0)
Colho 123 (30.4)
Colho 121 (35.4)
Colho 119 (34.3)
Phase Cycles
2nd heating
G
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
6a
G
90 (Tg)
2nd cooling
2nd heating
2nd cooling
2nd heating
2nd cooling
2nd heating
2nd cooling
2nd heating
2nd cooling
2nd heating
2nd cooling
2nd heating
2nd cooling
2nd heating
2nd cooling
G
51 (Tg)
6b
6c
6d
7a
7b
7c
7d
a
G
86 (Tg)
Cr
Cr
Cr
Cr
Cr
Cr
Cr
Cr
Cr
Cr
Cr
Cr
35 (33.2)
5 (20.1)
52 (55.1)
23 (41.7)
29 (41.3)
28 (42.5)
41 (40.9)
38 (40.8)
49 (74.1)
47 (52.3)
59 (98.3)
52 (82.0)
Colh
Colh
Colh
Colh
113 (27.7)
113 (30.5)
110 (25.3)
110 (24.5)
Colho 143 (63.3)
Colho 139 (68.8)
Colho 140 (67.0)
Colho 135 (69.7)
Colho 136 (66.9)
Colho 131 (67.1)
Colho 130 (63.1)
Colho 127 (58.9)
Melting point (Tm), clearing point (Tc), glass transition temperature (Tg), crystalline
phase (Cr), glassy state (G). Colh: columnar hexagonal phase, Colho: ordered columnar
hexagonal phase, I: isotropic liquid.
In contrast to the sixfold substituted derivatives 5 tris(dialkoxyꢀmethoxyphenyl)benzenes 6
displayed different phase behaviour depending on their chain lengths. For 6a,b with shorter
alkyl chains (C9, C10) no melting points were visible, neither in the heating nor in the cooling
scans. Both derivatives showed distinctive glass transitions at 53 °C and 51 °C, respectively,
in the second heating cycle. Upon further heating clearing from the mesophase to the isotropic
liquid took place at 126 °C for 6a and 121 °C for 6b. In the cooling cycle formation of the
mesophase took place at 123 °C and 119 °C, respectively. The glass transitions were shifted
to higher temperatures (90 °C for 6a and 86 °C for 6b) reducing the mesophase width by half
to 33 K for both compounds.
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Triphenylbenzenes 6c and 6d with longer alkoxy side chains (C11, C12) displayed
reproducible melting, at 35 °C and 52 °C, and clearing peaks at 113 °C and 110 °C,
respectively, upon heating. Whilst the temperatures of the clearing points are maintained in
the cooling cycle with respect to those measured upon heating supercooling was observed for
the mesophaseꢀtoꢀcrystalline transitions. Crystallisation took place at 5 °C and 23 °C,
respectively, giving mesophase widths of 108 K for 6c and 87 K for 6d.
All tris(trialkoxyphenyl)benzenes 7a–d formed stable mesophases, displaying increasing
melting points from 29 °C (7a) to 59 °C (7d) with increasing chain lengths. Conversely, the
clearing points decreased with increasing chain length from 143 °C (7a) to 130 °C (7d)
whereby the mesophase widths decrease linearly through the series with increasing alkoxy
chain length. This behaviour is in good agreement with the structurally related triphenylꢀ
boroxines 4.39
Under the polarizing optical microscope (POM) all compounds gave textures typical of those
for columnar mesophases (Fig. 1). 6a and 7a displayed broken fan textures, 6c a pseudo focal
conic texture, 6b, 6d and 7b–d fanꢀshaped textures.42 All textures also exhibited small
homeotropic areas which is common for uniaxial columnar mesophases.
Fig. 1 POM images of 6a–d and 7a–d upon cooling from the isotropic liquid (cooling rate
1 K min–1, magnification ×100).
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X-ray diffraction (XRD) investigations
Further insight into the mesophase geometries of 6a–d and 7a–d came from Xꢀray diffraction
measurements. Two derivatives of each type of mesogens with different substitution patterns,
namely 6a, 6d, 7b and 7d, were representatively investigated (SAXS and WAXS, for further
details see Table S2, ESI†). The SAXS profile and the corresponding diffraction pattern of 6a
showed two reflexes which were assigned as (10) and (20) as they are in a relative ratio of
1:1/2 (Fig. 3a).43 This set of signals might be assigned to a discotic lamellar phase geometry44
due to the missing (11) peak which would be necessary for an unambiguous assignment of
Colh phase geometry. However, the partially oriented SAXS pattern (Fig. 2a, inset) clearly
indicates a hexagonal lattice. In the SAXS diffractogramm of 6d four reflexes in a ratio of
1:1/√3:1/√7:1/√9 were visible (Fig. 2b), which have been unambiguously assigned as (10),
(11), (21) and (30). Despite the missing (20) peak, which might occur due to superimposing
of areas with different orientation in the sample, the observed reflexes were in good
agreement with a Colh lattice.
Fig. 2 SAXS profile and diffraction pattern (inset) during cooling of a) 6a at 105 °C, b) 6d
85 °C.
The smallꢀangle Xꢀray scattering profile and the diffraction pattern of 7b are depicted in
Fig. 3a. The SAXS pattern (Fig. 3a inset) is highly ordered and exhibits an oriented hexagon
of the innermost reflection together with four additional sets of oriented reflections.
Integration of the inner hexagon over 2 Θ is shown in Fig. 3b. The relative angle of 60(±2)°
between the peaks indicates a hexagonal columnar mesophase. In the smallꢀangle region five
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reflections were visible in a ratio of 1:1/√3:1/2:1/√7:1/√9 which were indicated as (10), (11),
(20), (21) and (30), confirming a hexagonal lattice (Fig. 3a).43
The SAXS investigations of 7d (Fig. 3c) confirmed the results of the previous XRD
experiments. The SAXS profile displayed five reflections in the ratio of
1:1/√3:1/2:1/√7:1/√12 which fit to a hexagonal lattice and were therefore indicated as (10),
(11), (21), (30), (22) (Fig. 3c).
c)
Fig. 3 a) SAXS profile and diffraction pattern (inset) of 7b during cooling at 88 °C, b)
azimuthal intensity profile of the innermost hexagon in the SAXS pattern, c) SAXS profile
and diffraction pattern (inset) of 7d during cooling at 90 °C.
In the wideꢀangle region of 6d and 7b a broad halo at d = 4.5 Å and d = 4.3 Å was visible
related to the liquidꢀlike alkoxy chains (Fig 4a, b). In addition, the wideꢀangle Xꢀray
scattering profile of 7b showed a multitude of discernable reflections labelled as (10), (11),
(20), (21), (30), (22) and (40) besides the halo at d = 4.6 Å and a peak at d = 4.3 Å, which
originate from π–π interactions between the aromatic cores and corresponds to their mean
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distance within the columns. The latter reflection suggested the formation of an ordered
columnar hexagonal mesophase.
a)
b)
Fig. 4 a) WAXS profile and diffraction pattern (inset) of 6d during cooling at 85 °C,
b) WAXS profile and diffraction pattern (inset) of 7b during cooling at 88 °C.
For compounds 6a and 7d the WAXS samples were prepared by fibre extrusion. Due to the
better alignment of the sample in the WAXS of 7d as compared to 6a the former was analysed
first. The WAXS pattern of 7d showed two sets of diffraction peaks, one in the equatorial and
one in the meridional region (Fig. 5a on the left side). The reflexes in the equatorial region,
perpendicular to the fibre axis, correspond to the lattice geometry of the mesophase which
was determined as columnar hexagonal (Fig. 3c). The meridional region displayed four pairs
of scattering peaks on the left and right hand side of the meridian and two distinct reflexes on
the meridian axis, typical for helical superstructures. The obtained WAXS pattern is in good
agreement with the simulated one shown in Figure 5a on the right.45 For this simulation it was
assumed that, because of their C3 symmetry, the molecules stack in a manner where they are
rotated by φ = 60 °. Thus the inner benzene rings are piled up on top of each other while the
outer phenyl groups describe a triple helix around the inner column. Owing to the C3ꢀ
symmetry of the triphenylbenzenes only the reflexes on the layer lines L = 3 and L = 6 are
visible. The reflex on the meridional axis on the layer line L = 6 corresponds to the stacking
inside of the column giving a stacking distance of d = 4.3 Å. The layer line L =3 corresponds
to the pitch p of the helix, which is three molecules or p = 8.6 Å. Altogether the WAXS
pattern of 7d indicates that a triple 61 helix with a azimuthal rotation angle of φ = 60 °, a pitch
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p = 8.6 Å and a repeat C of three pitches, giving C = 25.8 Å, is formed (Fig. 5b). Although
this resembles an antiparallel “zigꢀzag” like packing, the high orientation in the WAXS
pattern indicated a periodical setup where the peripheral phenyl rings are twisted in the same
direction respective to each other. In an antiparallel stacking a random orientation would be
expected.
The WAXS diffractogram of 6a (Fig. 5c on the left side) displayed a similar structure than
that of 7d, although the scattering peaks on the equatorial axis were not visible. In the
meridional region four peaks on the right and left hand side of the axis were visible as well as
two distinct reflexes on the meridional axis. With the assumption of the same rotational angle
as for 7d (φ = 60 °) a diffraction pattern was simulated which is in good agreement with the
measurement.45 The stacking distance inside the column at L = 6 is d = 4.0 Å which leads to
slightly different helical coefficients than for 7d.
Fig. 5 a) WAXS pattern of 7d during heating at 90 °C (left) and a simulated WAXS pattern
(right, HELIX program, φ = 60 °, d = 4.3 Å), b) schematic structure of a 61 helix with a
highlighted (red) rotation of one phenyl group throughout the repeat C, c) WAXS pattern of
6a during heating at 105 °C (left) and a simulated WAXS pattern (right, HELIX program,
φ = 60 °, d = 4.0 Å).
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The analysis of the WAXS pattern indicates the formation of a triple 61 helix with a rotational
angle of φ = 60 °, a pitch of p = 8.0 Å and a repeat of C = 24.0 Å. In both cases, indication of
the handedness could not be obtained. The helix formation seems to be a statistical process
due the absence of chiral information in the triphenylbenzenes, in contrast to resent work of
Casado et al.46 who studied C3 tricarboxamides bearing chiral side chains by ECD, VPC and
ECL techniques.
Proposed Packing Model
The peripheral phenyl rings of 1,3,5ꢀtriphenylbenzene derivatives are well known to be
twisted by up to 48° in the solid state.47–49 According to Scherowsky this nonꢀplanar structure
surpresses mesomorphism in hexaoctyloxyꢀ and various esterꢀfunctionalized triphenylꢀ
benzenes 5.38 As discussed above, side chain elongation in the prepared novel sixfold
substituted triphenylbenzenes 5b–f did not promote liquid crystalline behaviour. However,
introducing a third alkoxy group in each peripheral phenyl ring led to the formation of
enantiotropic mesophases in derivatives 6a–d and 7a–d. For this purpose, even the
minimalistic OCH3 group is sufficient, as was demonstrated for 6a–d. To explain this finding
we considered two aspects, the influence of the nonꢀplanar triphenylbenzene core system and
the varying substitution patterns.
Fig. 6 a) Rotational latitude of triphenylbenzenes (red arrows) and the voids resulting from
the rigid framework, b) proposed helical stacking model of triphenylbenzenes, c) hexagonal
lattice consisting of helical columns (lattice spacing a).
The Xꢀray data of 6a and 7d indicate a helical structure of the formed columns. In a helical
setup parts of the voids resulted by the rigid star shape (Fig. 6a) can be compensated by the
twisted outer phenyl rings above and beneath each disk in the column, respectively (Fig. 6b).
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The remaining empty spaces then have to be filled by the peripheral alkoxy chains. This can
be achieved either by backfolding of the side chains or by their interdigitation.11,50 Both
effects should lead to a reduced diameter of the discs (determined by the lattice spacings a of
the Colh phase geometry) as can be seen in Table 2. For example, the estimated diameters for
6d and 7d are the same yet the measured matrix parameter of 6d is considerably smaller than
that of 7d, indicating that the compensation of the minimalistic methoxy groups requires more
backfolding or interdigitation of the remaining alkoxy chains. Taking in account that the rigid
core cannot take part in these effects, the resulting reduction amounts 47% for 6d and 37% for
7d, respectively.
Table 2 Calculated (acalcd, for the maximum stretched out state), observed matrix parameter
(aobsd) from Xꢀray diffraction measurements and reduction of the alkyl chains due to
backfolding and interdigitation (ꢀdalk).
Compd acalcd / Å aobsd / Å
ꢀ
a / Å
ꢀdalk
36.4
43.4
37.8
43.4
25.3
28.4
28.3
31.8
11.1
44%
47%
38%
37%
6a
6d
7b
7d
15
9.5
11.6
The absence of mesomorphism in the series of hexaꢀsubstituted triphenylbenzenes 5b-f is not
uncommon among shape persistent hekates. For structurally related hexaꢀ and nonaꢀalkoxy
substituted triphenylboroxines39 as well as for hexaꢀ30 and nonaꢀalkoxy31 substituted
triphenyltriazines similar behaviour has been found. This indicates that the formation of a
liquid crystalline phase in these small star shaped mesogens strongly depends on the number
of alkoxy substituents and not so much on their length. Apparently nine side chains are
necessary for successful nanosegregation. This might originate from two effects: The
additional group at each peripheral aryl unit increases the shielding of the aromatic core, thus
improving segregation of the cores and stabilising columnar mesophases. Alternatively, the
additional methoxy group deplanarises the remaining alkoxy chains, forcing them to fill the
void between the rings.51 Considering this, both possibilities lead to the fact that any
substituent in the 5ꢀposition of the outer phenyl groups should promote mesomorphism. To
elucidate this interesting topic further investigations are currently in progress.
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Conclusions
In conclusion, we have synthesised triphenylbenzenes with 3,4ꢀbisalkoxyꢀ and 3,4,5ꢀ
trisalkoxyphenyl moieties, which were not liquid crystalline (5) if only two alkoxy
substituents were present at each peripheral phenyl ring. However, derivatives 7 with three
alkoxy substituents on each phenyl ring showed enantiotropic hexagonal columnar
mesophases with phase widths of 75–114 K depending on the chain lengths. Surprisingly,
even methoxy served as a third minimalistic alkoxy unit at the peripheral phenyl rings,
resulting in enantiotropic hexagonal columnar mesophases with phase widths of 58–80 K for
compounds 6. Extensive XRD studies indicate that a packing model with helical selfꢀ
assembly seems very likely. The mesomorphic properties indicate that compounds 6, 7 are
indeed the missing links in the series of shapeꢀpersistent hekates and might be useful for
applications such as lightꢀemitting diodes or photovoltaic cells.
Acknowledgements
Generous financial support by the Deutsche Forschungsgemeinschaft, the Fonds der
Chemischen Industrie (Ph.D. felloship for S. J. Beardsworth), the Ministerium für
Wissenschaft und Kunst des Landes BadenꢀWürttemberg and the Bundesministerium für
Bildung und Forschung (shared instrumentation grant 01 RI05177) is gratefully
acknowledged.
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Table of contents
Triphenylbenzenes with nine peripheral nꢀalkoxy side chains form enantiotropic mesophases
near room temperature. Surprisingly, one alkoxy group per phenyl ring can be mimicked by
methoxy. Xꢀray investigations show highly ordered columnar hexagonal mesophases. Wideꢀ
angle Xꢀray data indicates a helical stacking in the columns.
16