Hydrogen-bond stabilized columnar discotic benzenetrisamides with pendant triphenylene groups

Article abstract of DOI:10.1039/b805283b

A series of 1,3,5-benzenetrisamide derivatives with three hexaalkoxytriphenylene pendant groups were prepared, in which the triphenylene groups are connected to the central 1,3,5-benzenetrisamide core through a flexible spacer. The length of this spacer, as well as the size of the ortho-substituent at the triphenylene core, influences the columnar packing of these molecules. All compounds show liquid crystalline behavior with high isotropization temperatures. Different columnar hexagonal phases (Col _h, Col_hp, Col_obl and Col_r) have been identified using optical polarization microscopy, differential scanning calorimetry and X-ray diffraction. A chiral 1,3,5-benzenetrisamide derivative forms columnar stacks with a single helical sense, both in an apolar solvent and in a film, as observed by circular dichroism studies. Pulse radiolysis time-resolved microwave conductivity (PR-TRMC) studies show that the high ordering in a Col_hp phase results in high charge carrier mobilities. On the other hand, the highest mobility was observed for the chiral compound, which has a Col_r ordering.

Full text of DOI:10.1039/b805283b

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Hydrogen-bond stabilized columnar discotic benzenetrisamides
with pendant triphenylene groups†
Ioan Paraschiv,‡a Kim de Lange,a Marcel Giesbers,a Barend van Lagen,a Ferdinand C. Grozema,b
b
b
a
a
*
¨
Ruben D. Abellon, Laurens D. A. Siebbeles, Ernst J. R. Sudholter,x Han Zuilhof
a
*
and Antonius T. M. Marcelis
Received 31st March 2008, Accepted 10th September 2008
First published as an Advance Article on the web 16th October 2008
DOI: 10.1039/b805283b
A series of 1,3,5-benzenetrisamide derivatives with three hexaalkoxytriphenylene pendant groups were
prepared, in which the triphenylene groups are connected to the central 1,3,5-benzenetrisamide core
through a flexible spacer. The length of this spacer, as well as the size of the ortho-substituent at the
triphenylene core, influences the columnar packing of these molecules. All compounds show liquid
crystalline behavior with high isotropization temperatures. Different columnar hexagonal phases
(Colh, Colhp, Colobl and Colr) have been identified using optical polarization microscopy, differential
scanning calorimetry and X-ray diffraction. A chiral 1,3,5-benzenetrisamide derivative forms columnar
stacks with a single helical sense, both in an apolar solvent and in a film, as observed by circular
dichroism studies. Pulse radiolysis time-resolved microwave conductivity (PR-TRMC) studies show
that the high ordering in a Colhp phase results in high charge carrier mobilities. On the other hand,
the highest mobility was observed for the chiral compound, which has a Colr ordering.
To obtain discotic materials with optimal processability and
high charge carrier mobilities, a controlled balance is required
between the p–p stacking of the aromatic cores and alkyl–alkyl
interactions between the liquid-like chains around the disk-like
core of the molecule.11 To stabilize the columnar ordering and
obtain better performing discotics, useful for building up
organic-based electronic devices,12 complex molecular architec-
tures have been constructed recently. One of the methods to
stabilize the columns is by making use of intermolecular
hydrogen bonds, since they constitute a relatively strong and
tunable interaction.13
Introduction
Discotic liquid crystals are a particularly interesting class of
self-organizing soft materials, generally consisting of rigid
polyaromatic cores surrounded by alkyl chains that provide the
flexibility required for the liquid-crystalline properties.1 Several
classes of discotics have been described in the literature; two of
the most studied ones are triphenylene2 and hexabenzocoronene3
derivatives. These molecules often form columnar aggregates
through p–p stacking of the aromatic cores, and as a direct
consequence of this organization, charge carrier migration along
the columns is possible.4 Significant advantages of organic
conducting materials are their flexible synthesis, easy process-
ability and low production costs. Therefore, these organic
optoelectronic materials are of interest for a number of appli-
cations,5 such as photovoltaic solar cells,6 electroluminescent
displays,7 and field-effect transistors.8 Because charge transport
across the organic layer(s), quantified by the charge carrier
mobility,9 plays a key role in the performance of such devices,10
the stability of the organization of discotic molecules in the
columnar stacking is very important.
In this paper, a series of 1,3,5-benzenetrisamide derivatives
with three pendant triphenylene moieties is investigated as a new
class of H-bond-stabilized self-assembled disk-like molecules
(Fig. 1a). Analogous C3-symmetrical trisamide molecules that
associate into supramolecular stacks through H-bonding have
been previously described for smaller disk-like molecules.14,15 In
a previously reported example of one such molecule (T-4-Hex),
it was found that the intermolecular H-bonding by the 1,3,5-
benzenetrisamide groups considerably slows the dynamics of the
attached triphenylene groups, resulting in a highly ordered
columnar hexagonal plastic phase (Colhp).16 In this columnar
phase, the intermolecular hydrogen bonding connects the central
benzene cores in such a way that the adjacent benzene cores are
rotated over 60ꢀ, therefore giving rise to a helical orientation of
the resulting columnar stacks (see Fig. 1b).15,16 Furthermore, for
that particular compound, molecular modeling indicates that
adjacent triphenylene disks are rotated 15ꢀ with respect to each
other. This would correspond to ꢁ85% of the maximum of the
charge transfer integral.17 As a consequence, the resulting highly
stable mesophase is characterized by a charge carrier mobility of
aLaboratory of Organic Chemistry, Wageningen University, Dreijenplein 8,
6703 HB Wageningen, The Netherlands. E-mail: Han.Zuilhof@wur.nl;
Ton.Marcelis@wur.nl
bOptoelectronic Materials Section, Delft University of Technology,
Mekelweg 15, 2629 JB Delft, The Netherlands
† Electronic supplementary information (ESI) available: Experimental
details of measurements and synthesis. See DOI: 10.1039/b805283b
‡ Present address: Avantium Technologies BV, Zekeringstraat 29, 1014
BV Amsterdam, The Netherlands.
x Present address: Department of Chemical Technology DelftChemTech,
Delft University of Technology, Julianalaan 136, 2628 BL Delft, The
Netherlands.
0.12 cm2 Vꢂ1 sꢂ1 at 180 C, the second highest ever reported for
ꢀ
triphenylene-based liquid crystalline materials.16
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Fig. 1 a) Structures of the C3-symmetrical benzenetrisamide T-n-R compounds investigated in this study. b) Molecular modeling representation of four
stacked trimethyl 1,3,5-benzenetrisamide molecules showing the helical hydrogen bonding network.
Our modeling studies suggested that the conformation of the
butyl spacer dictates the p–p overlap of neighbouring triphe-
nylene units.16 A better overlap of the p-orbitals would result in
a larger splitting of the HOMO (LUMO) levels, and ultimately in
a higher charge transfer integral.4,11,17 Therefore, it can be
expected that variation of the spacer length, as well as the size of
the ortho-substituent of the triphenylene core, will influence the
degree of ordering and charge carrier mobility in the columnar
phases of these molecules.
Therefore, a series of new trisamide compounds with various
spacer lengths (Fig. 1a) were synthesized and investigated with
respect to their liquid-crystalline organization. A chiral 1,3,5-
benzenetrisamide triphenylene derivative was also synthesized,
by introducing a chiral center in one of the ortho-substituents of
the triphenylene core. For this compound, the effect of the
Scheme 1 Reagents and conditions for the synthesis of compounds T-n-
chirality on the molecular packing was investigated with circular
R: (i) FeCl3, CH2Cl2; methanol; (ii) a: Br(CH2)nBr, K2CO3, 2-butanone;
dichroism.
b: NaN3, ethanol; c: LiAlH4, THF; d: NaOH, H2O, Na2SO4$10H2O; (iii)
for n ¼ 3: a: N-(3-bromopropyl)phthalimide, K2CO3, 2-butanone; b:
NH2–NH2$H2O, ethanol; (iv) N(CH2CH3)3, CH2Cl2, RT.
Results and discussion
Synthesis
T-n-R were investigated using optical polarization microscopy
The synthesis of compounds T-n-R is analogous to that previ-
(OPM), differential scanning calorimetry (DSC), temperature-
ously reported for T-4-Hex16 and is depicted in Scheme 1. Details
dependent Fourier transform infrared spectroscopy (FTIR) and
of the synthesis and other experimental procedures can be found
X-ray diffraction (XRD) techniques. Additionally, circular
in the ESI.†
dichroism (CD) studies were performed on the chiral derivative
The route was not successful for the compound with the propyl
T-4-MeBu.
spacer (T-3-Hex), due to the fact that dibromopropane did not
All compounds T-n-R display liquid crystalline behavior with
give the desired product upon reaction with 3-R. Therefore,
relatively high isotropization temperatures (Table 1). The
an alternative route was developed for this compound. N-(3-
compounds were obtained as white (glassy) powders, for which
Bromopropyl)phthalimide reacts nicely with 3-R to give the
no crystal melting was observed upon heating. Also, upon
triphenylene derivative with a phthalimido-protected amine
cooling to room temperature no crystallization occurred. Even
group. Deprotection with hydrazine hydrate gives amine 4-n-R,
which was further reacted with 1,3,5-benzenetricarbonyl
when the samples were kept at room temperature for weeks, no
crystallization was apparent, neither by optical microscopy nor
trichloride in a similar manner as the other amine compounds to
by DSC. Observation by polarized optical microscopy shows
yield trisamide T-3-Hex.
that compound T-3-Hex exhibits a dendritic texture,18 as seen
in Fig. 2a. The textures of the optical micrographs are indicative
of a columnar mesophase.19 Upon cooling from the isotropic
Characterization
The structures of each of the intermediary compounds and
final products were confirmed by 1H-NMR, 13C-NMR and exact
mass measurements. The mesogenic properties of compounds
state, compound T-4-Bu exhibits needle- or ribbon-like textures,
as seen in Fig. 2b. As reported before,16 T-4-Hex also shows
growth of ribbon-like structures upon cooling from the isotropic
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Table 1 Type of mesophase,a phase transition temperatures (ꢀC) and enthalpies (kJ molꢂ1, in parentheses) of compounds T-n-R as determined by DSC;
scanning rate 10 ꢀC minꢂ1
Compound
Heating
Cooling
T-3-Hex
T-4-Bu
T-4-Hex
T-4-MeBu
T-5-Hex
T-6-Hex
Colh
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
172 [29]
199 [102]
208 [88]
182 [42]
190 [41]
183 [39]
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
I
I
I
I
I
I
I
I
I
I
I
I
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
160 [29]
186 [99]
196 [88]
167 [37]
181 [35]
170 [35]
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
Colh
Colhp
Colhp
Colx
Colx
Colobl
Colhp
Colhp
Colr
Colobl
Colobl
ꢃ
ꢃ
135 [11]
162 [7]
ꢃ
ꢃ
Colr
Colobl
ꢃ
ꢃ
129 [10]
146 [3]
ꢃ
ꢃ
Colx
Colx
a
Colx ¼ unidentified columnar phase; Colh ¼ columnar hexagonal; Colhp ¼ columnar hexagonal plastic; Colr ¼ columnar rectangular; Colobl
¼
columnar oblique.
Fig. 2 Polarized optical micrographs of the compounds T-3-Hex at 169 ꢀC (a), T-4-Bu at 193 ꢀC (b), T-4-Hex at 196 ꢀC (c) T-4-MeBu at 172 ꢀC (d),
T-5-Hex at 187 ꢀC (e) and T-6-Hex at 177 ꢀC (f).
state (Fig. 2c). Compounds T-4-MeBu, T-5-Hex and T-6-Hex
show fan-like textures, which are often found for columnar
phases.
about 40 kJ molꢂ1 for the other compounds. These observations,
together with the observations by polarization microscopy,
suggest that at least three types of columnar ordering are present
in this series of compounds.
The DSC data of the trisamides are also given in Table 1.
A small hysteresis of about 10–15 ꢀC was observed for the
isotropization temperatures. This effect was smaller when lower
heating and cooling rates were used. Most compounds only show
one transition upon both heating and cooling, namely at their
isotropization temperature. Compounds T-4-MeBu and T-5-Hex
show an additional lower temperature transition to an uniden-
tified columnar phase.
Besides having the highest isotropization temperatures,
compounds T-4-Bu and T-4-Hex also show the highest iso-
tropization enthalpies within the series. Together with the rather
characteristic X-ray diffractograms (vide infra) this leads us to
assign the phase of these two compounds to the highly ordered
columnar hexagonal plastic (Colhp) phase. Compound T-4-Hex
was previously assigned the Colhp phase.16
As is also seen in Table 1, the isotropization temperatures are
all rather high, ranging from about 170 to 210 ꢀC. The high
isotropization temperatures are the result of p–p stacking in the
columns of mainly the triphenylenes and of hydrogen bond
interactions of the amide groups of the benzenetrisamide groups,
which reinforce the columnar organization.16 The corresponding
heat effects (DH) fall into three groups, a low one of about 30 kJ
molꢂ1 for T-3-Hex, two very high ones for T-4-Bu and T-4-Hex
of 102 and 88 kJ molꢂ1, respectively, and intermediate ones of
Compound T-4-Bu shows an even higher isotropization
enthalpy than T-4-Hex. The difference of 14 kJ molꢂ1 between
these two relatively large heat effects suggests an even better
packing for T-4-Bu molecules in the liquid-crystalline phase.
Since this compound bears a shorter, butyloxy ortho-substituent,
instead of a hexyloxy substituent, a more compact organization
of the molecules within the liquid-crystalline phase is proposed as
the reason for this higher isotropization enthalpy. The lowest
isotropization temperature and enthalpy was found for T-3-Hex.
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Table 2 X-ray diffraction data for the liquid crystalline phases of
compounds T-n-R
This indicates that this compound has the least ordered columnar
phase of this series. From these data it seems that a C-4 spacer is
optimal for obtaining the highly ordered Colhp phase. However,
when the ortho-substituent on the triphenylene is branched, like
in T-4-MeBu, the Colhp ordering is lost.
˚
d-Spacing/A
Lattice Miller
spacing/A observed (calcd) indices
˚
Compound Mesophase
T-3-Hex
Colh at 170 ꢀ
C
a ¼ 21.03
36.4 (36.4)
18.28 (18.21)
10.50 (10.51)
9.12 (9.10)
31.9 (36.8)
18.41 (18.41)
10.60 (10.63)
6.98 (6.96)
9.21 (9.21)
4.98 (4.97)
4.15 (4.02)
3.56 (3.56)
3.50 (3.49)
35.7 (37.8)
18.82 (18.88)
10.93 (10.90)
9.43 (9.44)
7.12 (7.14)
5.09 (5.08)
4.25 (4.12)
3.62 (3.61)
3.54 (3.55)
32.5 (32.7)
18.8 (18.5)
16.30 (16.35)
12.34 (12.35)
10.89 (10.90)
9.03 (9.05)
8.18 (8.16)
36.5 (36.2)
30.8 (31.1)
18.10 (18.11)
15.37 (15.41)
12.05 (12.07)
9.15 (9.20)
34.9 (35.0)
20.7 (20.5)
19.21 (19.03)
17.47 (17.48)
16.25 (16.59)
13.46 (13.50)
12.45 (12.46)
11.59 (11.65)
½00
100
110
200
½00
100
110
210
200
211
410
002
102
½00
100
110
200
210
211
410
002
102
100
020
200
220
300
140
240
010
100
020
200
030
230
100
020
120
200
ꢂ120
130
ꢂ220
300
XRD analysis was performed on all compounds T-n-R to
further investigate the possible ordering in their mesophases.
From the X-ray patterns (Fig. 3) it is clear that compound
T-3-Hex has the most simple ordering. It can be explained by
a Colh type of ordering of the triphenylene columns with an
additional superlattice caused by the benzenetrisamide columns,
which occupy one out of every six void spaces between the
triphenylene columns. These additional columns give rise to an
additional peak in the X-ray diffractograms corresponding to
a Miller index of {½, 0, 0}. This is also indicated in Table 2. This
type of indexation has often been used to indicate the presence
and influence of a superstructure.16,20
T-4-Bu
Colhp at 140 ꢀ
C
a ¼ 21.26
c ¼ 7.12
T-4-Hex
Colhp at 170 ꢀ
C
a ¼ 21.80
c ¼ 7.22
Compounds T-4-Hex and T-4-Bu show very similar diffrac-
tion patterns. The phase of T-4-Hex was described previously
as being columnar hexagonal plastic (Colhp).16 The d-spacings of
T-4-Bu can be explained similarly by a Colhp phase, although
with smaller lattice constants as can be expected for the smaller
side chains. Especially characteristic for this Colhp phase are the
sharp {0, 0, 2} and {1, 0, 2} reflections. The broad peak at about
20ꢀ confirms the liquid behavior of the alkyl tails.
T-4-MeBu
Colr at 150 ꢀ
C
a ¼ 32.7
b ¼ 37.7
g ¼ 90ꢀ
The remaining compounds show more peaks in the small-angle
region. Therefore, it is assumed that the lattice spacings a and
b are not equal any more. In Table 2 the observed d-spacings are
given together with the calculated d-spacings obtained from
a fitting of the peaks and the assignment to Miller indices.
T-5-Hex
T-6-Hex
Colobl at 170 ꢀ
C
C
a ¼ 30.9
b ¼ 36.3
g ¼ 86ꢀ
Colobl at 170 ꢀ
a ¼ 35.4
b ¼ 41.5
g ¼ 99ꢀ
For the fits of the last 3 compounds the superlattice was taken
as a basis, which means twice the normal value of the lattice. For
T-4-MeBu the fit corresponds quite nicely with a rectangular
columnar phase with different lengths of a and b. For T-5-Hex
and T-6-Hex nice fits were only possible for an angle g that was
not exactly 90ꢀ. It should be pointed out that the exact lattice
parameters as given in Table 2 depend to a certain degree on the
correct attribution of the Miller indices. To index the peaks with
more certainty, XRD studies on oriented samples are desirable.
For the compounds with the same mesophase assignments it
is seen that the cell parameters increase with increasing spacer
(T-5-Hex and T-6-Hex) or tail (T-4-Bu and T-4-Hex) length, as is
expected.
As can be seen from the diffractograms, the low-angle peak
from the superlattice is often broad, which means that determi-
nation of its position is not very accurate. As can be seen in
Fig. 4, at least two regular packing models for the superlattice
can be drawn. As a result, a more or less random position of the
Fig. 3 XRD-diffractograms of the compounds T-n-R in their liquid
crystalline phases at 170 ꢀC.
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the frequencies of the characteristic C]O (amide I) and N–H
stretching vibration bands. These changes clearly demonstrate
a strengthening of the H-bonding interactions in the liquid-
crystalline phase.22
Compound T-4-MeBu contains three chiral S-2-methylbutyl
side chains. As deduced from model studies, it is expected that
the hydrogen-bond network around the benzenetrisamide
column follows a helical path (see Fig. 1b).16 With chiral building
blocks it can be expected that a preferred helical orientation of
the hydrogen-bonded network is present, which should in prin-
ciple be observable by circular dichroism (CD). Therefore, CD
measurements23 of T-4-MeBu were performed, both in hexane
and as a thin neat film on a quartz substrate. As can be seen in
Fig. 6, a strong CD effect was found for the compound in hexane,
which points to one sense of helical aggregates in this solvent.
The strong CD effect can only be caused by a single helical sense
of the columns in the aggregates, which is effected by the chiral
center of the S-2-methylbutyl ortho-substituent. Achiral aggre-
gates of compound T-4-Hex in hexane do not give a CD signal.
By adding dichloromethane to the hexane solution, the CD effect
decreases as expected for an H-bonded network, since the
aggregates that are initially present in hexane are solubilized by
the dichloromethane.
Fig. 4 Possibilities for the superlattices of the trisamides in the columnar
phases: antiparallel (left) and parallel (right).
benzenetrisamide columns between the triphenylene columns can
probably be expected. This would mean a short correlation
length for reflections involving the benzenetrisamide columns,
and would yield broad low-angle peaks, as indeed observed. All
compounds show a broad halo in the wide-angle region caused
by the liquid-like behaviour of the alkyl tails in the liquid-
crystalline phase. For the two compounds with a Colhp ordering
two sharp peaks in this region contain information about the
˚
disk–disk distance. For T-4-Hex this distance is 3.61 A and for
˚
T-4-Bu it is 3.56 A. For the other compounds an average distance
between the disks is similar, but not easily determined exactly,
because of the broad nature of this peak.
A sample of T-4-MeBu measured as a neat film on a quartz
plate at room temperature, after cooling from the isotropic
phase, gave a very similar CD effect, indicating that in the liquid-
crystalline phase the aggregated columnar stacks also exhibit
a single helical sense. These results show that the chirality from
the alkyl side chains is transferred to the columnar organization.
The compounds described in this report are especially inter-
esting for their possible high conductivity along the columns.
Therefore, the one-dimensional charge carrier mobility in
compounds T-4-R was measured in their liquid-crystalline state
using pulse radiolysis time-resolved microwave conductivity
(PR-TRMC) measurements.24 In this technique, pulse radiolysis
yields the charge carriers, and from the time-resolved microwave
conductivity, the mobility of these charge carriers can be derived.
As can be seen from Table 3, the charge carrier mobilities of
T-4-Bu and T-4-Hex in the Colhp phase are much higher than
those of T-3-Hex in the Colh phase and of the achiral compounds
T-5-Hex and T-6-Hex in the Colobl phase. This can be attributed
to the high ordering in the Colhp phase and the good favourable
overlap of the triphenylene disks as explained previously for
compound T-4-Hex.16
Above the isotropization point the N–H stretching vibration is
between that of strong hydrogen bonding and very weak to
absent hydrogen bonding (as found in CHCl3 solution). This
could indicate that even in the (macroscopically) isotropic phase
some hydrogen bonding is still present. The other compounds of
the series show very similar behaviour, with a corresponding
decrease of the N–H stretching frequency of 100–150 cmꢂ1, and
a decrease of the C]O stretching vibration frequency of
about 30–40 cmꢂ1 upon going from the isotropic to the liquid-
crystalline phase.
The presence of H-bonding in the liquid-crystalline phases of
these discotic molecules is demonstrated by temperature-
dependent FT-IR studies.21 The infrared spectra were recorded
for all compounds upon cooling from the isotropic phase to
room temperature. As an example Fig. 5 displays for compound
T-4-Bu the frequencies of the N–H and C]O stretching vibra-
tions as a function of temperature. Upon going from the
isotropic to the columnar mesophase a strong decrease is seen of
Fig. 5 FTIR stretching vibrations of the N–H (–C–) and C]O (–B–)
bands of compound T-4-Bu as a function of temperature upon cooling
from the isotropic phase.
Fig. 6 CD spectra of T-4-MeBu and T-4-Hex in hexane at 5 ꢄ 10ꢂ5 M.
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Table
compounds T-n-R in their liquid crystalline columnar phase at 150 ꢀC
3 ) of
PR-TRMC charge carrier mobilities (cm2 Vꢂ1 sꢂ1
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