Reinforced self-assembly of hexa-peri-hexabenzocoronenes by hydrogen bonds: From microscopic aggregates to macroscopic fluorescent organogels

Article abstract of DOI:10.1002/chem.200700921

Hexa-peri-hexabenzocoronene derivatives (HBCs) that have hydrogen-bonding functionalities (either amido or ureido groups) adjacent to the aromatic cores have been synthesized to study the effects of intracolumnar hydrogen bonds on the self-assembly behavior of HBCs. The hydrogen bonds effectively increased the aggregation tendency of these compounds in solution. In the bulk state, the typical columnar supramolecular arrangement of HBCs was either stabilized substantially (1a, 1b, 2a, and 2b), or suppressed by dominant hydrogen-bonding interactions (3). For some of the compounds (1a, 2a, and 2b), the supramolecular arrangement adopted in the liquid-crystalline state was even retained after annealing, presumably owing to the reinforcement of the π-stacking interactions by the hydrogen bonds. Additionally, the combined effect of the hydrogen bonds and π-stacking of the aromatic moieties led to the formation of fluorescent organogels, whereby some derivatives were further investigated as novel low molecular-mass organic gelators (LMOGs).

Full text of DOI:10.1002/chem.200700921

DOI: 10.1002/chem.200700921
Reinforced Self-Assembly of Hexa-peri-hexabenzocoronenes by Hydrogen
Bonds: From Microscopic Aggregates to Macroscopic Fluorescent
Organogels
Xi Dou,[a] Wojciech Pisula,[a] Jishan Wu,[a] Graham J. Bodwell,[b] and Klaus Müllen*[a]
Abstract:
Hexa-peri-hexabenzocoro-
of HBCs was either stabilized substan-
tially (1a, 1b, 2a, and 2b), or sup-
pressed by dominant hydrogen-bonding
interactions (3). For some of the com-
pounds (1a, 2a, and 2b), the supra-
molecular arrangement adopted in the
liquid-crystalline state was even re-
tained after annealing, presumably
owing to the reinforcement of the p-
stacking interactions by the hydrogen
bonds. Additionally, the combined
effect of the hydrogen bonds and p-
stacking of the aromatic moieties led
to the formation of fluorescent organo-
gels, whereby some derivatives were
further investigated as novel low mo-
nene derivatives (HBCs) that have hy-
drogen-bonding functionalities (either
amido or ureido groups) adjacent to
the aromatic cores have been synthe-
sized to study the effects of intracolum-
nar hydrogen bonds on the self-assem-
bly behavior of HBCs. The hydrogen
bonds effectively increased the aggre-
gation tendency of these compounds in
solution. In the bulkstate, the typical
columnar supramolecular arrangement
Keywords: gels · hydrogen bonds ·
liquid crystals · organic gelators ·
pi interactions
lecular-mass
(LMOGs).
organic
gelators
Introduction
p Interactions have generated both theoretical[1] and experi-
mental[2] interest in recent decades owing to their unique
importance in supramolecular self-assembly behavior. Like
hydrogen bonds, the strength of which ranges from 4 to
120 kJmolÀ1,[3] p interactions (0 to 50 kJmolÀ1) are impor-
tant secondary interactions for the construction of supra-
molecular architectures.[3] Because of its relatively strong
and highly directional nature, hydrogen bonding has been
described as the “master key interaction in supramolecular
chemistry”.[3] In particular, hydrogen bonds are responsible
for the overall shape of many proteins, the recognition of
substrates by numerous enzymes and for the double helix
structure of DNA.[3] On the other hand, the comparatively
weak p interactions are responsible for the slippery feel of
graphite and its useful lubricant properties.[3] By combining
hydrogen bonding and p interactions, it has been possible to
obtain, for example, nanotubes,[4] organogels,[5] and a diversi-
ty of helical filaments[6,7] through self-assembly.
[a] X. Dou, Dr. W. Pisula,+ Dr. J. Wu,++ Prof. Dr. K. Müllen
Max-Planck-Institute for Polymer Research
Ackermannweg 10, 55128 Mainz (Germany)
Fax : (+49)6131-379-350
E-mail: muellen@mpip-mainz.mpg.de
[b] Prof. Dr. G. J. Bodwell
Department of Chemistry, Memorial University
St. Johnꢀs, NL, A1B 3X7 (Canada)
[+] Present address: Degussa AG
Process Technology & Engineering
Process Technology—New Processes
Rodenbacher Chaussee 4
63457 Hanau-Wolfgang (Germany)
[
++] Present address: California NanoSystems Institute
and Department of Chemistry and Biochemistry
California University at Los Angeles
607 Charles E. Young Dr. East
CA 90095–1569 Los Angeles (USA)
In the case of hexa-peri-hexabenzocoronenes (HBCs), p
interactions induce columnar self-assembly and provide the
basis for their distinct, intrinsic charge-carrier ability.[8]
Therefore, considerable effort has been invested in the suc-
cessful implementation of these and similar materials in
electronic devices, such as field-effect transistors (FETs),[9]
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author. It contains
1
MALDI-TOF and H NMR spectra for the described compounds, ad-
ditional UV-visible spectra, FTIR spectra, additional 2D WAXS pat-
terns, additional SEM images, and LSCM pictures for the organogels.
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Chem. Eur. J. 2008, 14, 240 – 249

FULL PAPER
light-emitting diodes (LEDs),[10] and photovoltaic cells.[11]
The performance of the device depends, to a large extent,
upon the degree of intracolumnar order of these discotic
molecules in thin films, which is an important factor for effi-
cient charge-carrier mobility.[12] The processing of such ma-
terials is a key issue in the production of well-ordered films,
which can be obtained by either a thermal[13] or a solution-
dependent treatment.[12,14] It has already been shown for
HBCs that have carboxylic acid terminated alkyl chains that
intercolumnar hydrogen bonds can strongly influence the p-
amined by differential scanning calorimetry (DSC) and two-
dimensional wide-angle X-ray scattering (2D WAXS) ex-
periments.
Finally, the expectation of gel formation was realized for
some of the compounds and the gelation ability of these
compounds was tested in different solvent systems. Thus, a
novel potential application for HBC derivatives as low mo-
lecular-mass organic gelators (LMOGs)[5] has been demon-
strated.
[15]
mediated columnar self-assembly in the bulkstate.
To enhance the degree of order of discotic molecules
within a single column, relatively strong and directional hy-
drogen bonds were brought into play through the attach-
ment of ureido or amido groups to the HBC moieties, either
directly or through rigid linkers. In so doing, it was expected
that intracolumnar hydrogen bonds would form instead of
the aforementioned intercolumnar hydrogen-bond net-
works.[15] In this context, some novel HBC derivatives that
contain either amido or ureido units were synthesized
(Figure 1). These functional groups are commonly used in
hydrogen-bond-assisted supramolecular chemistry.[3,16] Gen-
erally, ureido groups (2a and 2b) supply stronger and more
Results
Synthesis: Monofunctionalized compounds 1a, 1b, 2a, and
2b were synthesized from 4[18] as shown in Scheme 1. Com-
pound 4 was treated with benzophenonimine in a Buch-
wald–Hartwig coupling reaction to give 5 in a yield of 95%.
The hydrolysis of 5 in 2.5m aqueous hydrochloric acid quan-
titatively afforded amino-substituted HBC 6 as its hydro-
chloride salt. Compound 6 was then treated with the corre-
sponding acetyl chlorides or isocyanates to give compounds
1a, 1b, 2a, and 2b. Compound 3 was generated from a Ha-
gihara–Sonogashira coupling reaction between 7 and 8[19] in
a yield of 13%. The low yield originated from the difficult
workup, which was presumably a consequence of strong in-
termolecular hydrogen bonds.
rigid interactions than amido groups (1a and 1b).[17]
A
second ureido group (3) was added to further enhance the
interactions between the discotic molecules. The intermolec-
ular interactions between compounds with the same kind of
hydrogen-bond providers were further adapted by adjusting
the size of the substituents. The self-assembly properties of
these compounds in solution were investigated by electronic
spectroscopy, whereas the behavior in the bulkstate was ex-
Self-assembly behavior in solution: The way in which HBCs
self-associate in solution serves as a valuable indicator for
how these materials will align in films.[20] The electronic
spectra of p-conjugated chromophores in solution are highly
Figure 1. A) HBC derivatives that have been synthesized with amido (1a and 1b) and ureido groups (2a, 2b, and 3). B) Schematic representation of the
assumed intracolumnar hydrogen bonding and p-stacking interactions of monosubstituted HBC moieties.
Chem. Eur. J. 2008, 14, 240 – 249
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241

K. Müllen et al.
trations for some of the absorp-
tion
bands
(l=301
and
373 nm).
In the temperature-depen-
dent absorption spectra of the
monofunctionalized
HBCs,
compound 2b exhibited the
most pronounced effects. Com-
pounds 1a (Figure 3A) and 2a
(Figure S7B in the Supporting
Information) showed similar,
but less-pronounced effects,
whereas 1b did not display any
recognizable change (Figure
S7A in the Supporting Informa-
tion). Upon increasing the tem-
perature of solutions of 1a (Fig-
ure 3A) and 2b (Figure 3B),
the resolution of the absorption
bands at shorter wavelengths
improved and some broad
bands at longer wavelengths
(approximately
410–420 nm)
became less intense with the
appearance of an isosbestic
point at around 400 nm. More-
over, the a bands (named ac-
cording to Clarꢀs nomencla-
ture[23]), which are generally
very weakfor HBC deriva-
tives,[24] were also much better
resolved at higher tempera-
tures.
Significant changes were also
observed in the temperature-
dependent UV-visible absorp-
tion spectrum of 3 (Figure 4), in
which two isosbestic points
were apparent (at 349 and
389 nm). For the bands at about
Scheme 1. Synthesis of compounds 1a, 1b, 2a, 2b, and 3: i) [Pd2
(diphenylphosphino)-1,1’-binaphthyl (BINAP), NaOC(CH3)3, toluene, 808C, 16 h, 95%; ii) 2.5m HCl/THF, 5 h,
87%; iii) appropriate acetyl chlorides or isocyanates, Et3N, THF/toluene (1:1), 808C, overnight, 1a=88%;
1b=81%; 2a=60%; 2b=54%; iv) [Pd(PPh3)4], CuI, Et3N/THF (2:1), 508C, 4 d, 13%.
(dba)3] (dba=dibenzylideneacetone), 2,2’-bis-
AHCTREUNG
AHCTREUNG
300 nm and those at longer
wavelengths, the intensity de-
creased with increasing temper-
ature, whereas the band be-
sensitive to aggregational effects. For example, a variation in
concentration and/or temperature can result in significant
changes in the intensities, shapes, positions, and/or number
of the bands.[21,22] Therefore, such investigations were con-
ducted for all of the compounds described herein.
In contrast to the other compounds, only 2b (Figure S6 in
the Supporting Information) and 3 (Figure 2) showed dis-
tinct changes in their respective concentration-dependent
UV-visible absorption spectra. With decreasing concentra-
tion, not only did the extinction coefficients of compound 3
increase substantially, but the resolution of some of the
bands (l=297, 349, and 397 nm) also improved. Additional-
ly, a slight blueshift of 4 nm was recorded at higher concen-
tween the isosbestic points changed in the opposite sense.
The fluorescence spectra of 2a, 2b, and 3 varied with in-
creasing concentration (Figure 5A). In particular, the bands
at longer wavelengths became more intense and broader.
Compounds 1b and 2b each have one chiral center in the
functional substituent. Circular dichroism (CD) spectrosco-
py revealed weakCotton effects for 1b, but not 2b, presum-
ably owing to the size of the molecule in relation to the
number of chiral centers.
Hydrogen-bonding interactions and self-assembly behavior
in the bulk state: Infrared spectroscopy of the bulkstate
samples supported the formation of hydrogen bonds in
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Self-Assembly of Hexa-peri-hexabenzocoronenes
FULL PAPER
Figure 4. Temperature-dependent UV-visible absorption spectra of 3,
measured in solution in toluene (1.56”10À6 m); arrows indicate the direc-
tion of change with increasing temperature.
Figure 2. Concentration dependent UV-visible absorption spectra of 3 in
solution in toluene for 1.56”10À5 (c), 1.56”10À6 (a), and 1.56”
10À7 m (g).
Figure 5. Concentration-dependent fluorescence spectra of 1a, 1b, 2a, 2b
(A) and 3 (B) in toluene at 10À5 (c), 10À6 (a), and 10À7 m (g); ex-
cited at 360 nm and normalized at 471 nm for 1a, 1b, 2a, and 2b; excited
at 369 nm and normalized at 482 nm for 3.
Table 1. Stretching frequencies [cmÀ1
(n˜C O) groups of compounds 1a, 1b, 2a, 2b and 3 in KBr pellet.
=
]
of N H (n˜NÀ
)
and carbonyl
À
H
Compound
n˜NÀ [cmÀ1
]
n˜C O [cmÀ1
]
=
H
1a
3430 (medium, broad)
3310 (shoulder)
1660 (medium)
1b
2a
3400 (medium, broad)
3430 (medium, broad)
3310 (shoulder)
1700 (medium)
1710 (medium)
1650 (weak)
2b
3
3390 (medium, broad)
3310 (medium, broad)
3310 (medium, broad)
1640 (medium)
Figure 3. Temperature-dependent UV-visible absorption spectra of 1a
(A) and 2b (B) measured in solution in toluene (1.0 ”10À5 m); arrows in-
dicate the direction of change with increasing temperature.
1650 (medium)
À
bonds, the characteristic N H stretching vibrations usually
give multiple bands in the range of n˜NÀH =3400–3060 cmÀ1
and the carbonyl stretching vibrations, so-called amide I
these compounds according to the distinct changes induced
À
in the vibrational frequency of the N H and carbonyl
groups.[25]
bands, appear at lower frequencies (n˜C O =1700–1640 cmÀ1)
=
À
The characteristic stretching vibrations of the N H bond
in contrast to a saturated aliphatic ketone (n˜ =1715 cmÀ1)
owing to the resonance effect.[25a] For compounds 1b, which
has a sterically hindered amido functionality, and 3, which
and the carbonyl group are listed in Table 1 (see Figure S8
in the Supporting Information for full spectra). For secon-
dary amide and urea compounds, which involve hydrogen
À
has two ureido groups, only one kind of N H and carbonyl
Chem. Eur. J. 2008, 14, 240 – 249
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243

K. Müllen et al.
À
stretching bands were recorded. Specifically, both the N H
and the carbonyl stretching bands for 1b were located at rel-
atively higher wavenumbers than those for 3. The other
ganization did not return to the crystalline phase previously
recorded (as observed for other alkylated disk-type
PAHs),[28] but rather retained the nontilted, hexagonal 2D
packing observed in the mesophase.
À
compounds all had two kinds of N H stretching bands and
2a even had two carbonyl stretching signals.
Compound 3 showed significantly less order over the
entire temperature range, which rendered the assignment of
a unit cell impossible. At room temperature, no p-stacking
signal was observed. At 1208C, meridional reflections were
apparent, which correspond to a cofacial stacking distance
of 0.38 nm. The p–p distance decreased to a typical aromatic
stacking value of 0.36 nm at 1808C. In contrast to the behav-
ior observed for 1a, 2a, and 2b, the LC organization at
higher temperatures was not retained after annealing (Fig-
ure S10 in the Supporting Information). Unlike the above-
mentioned derivatives, 1b showed nontilted, hexagonal col-
umnar stacking (ahex =2.58 nm) with a cofacial distance of
0.36 nm over the whole temperature range investigated (Fig-
ure S11 in the Supporting Information).
DSC is a method commonly used for investigating ther-
motropic phase behavior and gives information about
phase-transition temperatures and the corresponding enthal-
pies.[26] For 1a, 1b, and 3, there was no visible phase-transi-
tion peakrecorded in the temperature range from À100 to
2508C. In the case of 2b, the DSC trace showed only an ir-
reversible endothermic transition at 2208C during both the
first and second heating cycles.
2D WAXS experiments,[27] which provide information
about the supramolecular arrangement in the solid state,
were conducted for all of the compounds at both ambient
and elevated temperatures. Compounds 1a, 2a, and 2b ex-
hibited similar phase behavior. Figure 6 presents the temper-
Gelation ability test: For initial gelation testing, attempts to
dissolve each of the compounds in a small amount of an
apolar solvent (e.g., hexane, heptane, or toluene) at reflux
temperatures were made. THF was then progressively
added as a polar solvent if a particular compound did not
dissolve under the initial conditions. The solution was then
slowly cooled (by placing it in a 608C oil bath and allowing
the oil bath to cool to ambient temperatures) and the sub-
stance was judged to be a gel if the vessel could be inverted
without apparent movement of the content.[17] The minimum
gelation concentration, which describes the gelation ability
of the compounds, was determined by continuously diluting
a gel (with the same solvent mixture as the one used to
form the gel) with heating and cooling cycles until the gel
did not regenerate. Another important parameter of gels,
the so-called sol-to-gel phase-transition temperature (Tg),
which describes their thermodynamic stability, was mea-
sured by using the falling-drop method.[29]
Figure 6. Temperature-dependent 2D WAXS experiments of A) 1a and
B) 2b; 2D monoclinic unit cell parameters (at 308C): a=2.46, b=
1.93 nm; g=1158 for 1a and a=2.50, b=1.86 nm; g=958 for 2b; hexag-
onal unit cell parameters (at 1708C and after annealing): ahex =2.77 nm
for 1a and ahex =2.80 nm for 2b.
The efficient combination of hydrogen bonding and p in-
teractions led to distinct gelation properties for two of the
compounds (2b and 3). Only these two examples developed
stable fluorescent organogels (Figure 7A) in the solvents
tested. The minimum gelation concentrations are shown
with the corresponding Tg values in Table 2. Laser scanning
ature-dependent 2D WAXS patterns of 1a and 2b (for 2a
see Figure S9 in the Supporting Information). At 308C, the
characteristic distribution of the reflections indicated identi-
cal monoclinic columnar stacking for both compounds.[27]
From the meridional and off-meridional scattering intensi-
ties, it was determined that the discotic molecules were
tilted by approximately 408, the columnar axis had a period-
ic distance of 0.47 nm, and the p-stacking distance was
0.37 nm.[27a]
Upon heating to 1708C, the 2D patterns of both com-
pounds (Figure 6) displayed a typical liquid-crystalline (LC)
arrangement of the molecules,[27] in which the discs self-ar-
ranged perpendicular to the columnar axis with a stacking
distance of 0.37 nm in a hexagonal columnar lattice. Inter-
estingly, upon cooling the samples to room temperature
after about one hour at 1708C, the highly ordered LC or-
Figure 7. A) Photograph of fluorescent gel from 3 in toluene. B) SEM
image of xerogel prepared from 2b in hexane/THF.
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Self-Assembly of Hexa-peri-hexabenzocoronenes
FULL PAPER
Table 2. Gelation test for 2b and 3 in n-hexane/THF, n-heptane/THF,
and toluene/THF.
fore, the different self-assembling abilities can be attributed
to the differences in the degree of hydrogen bonding,[16,17a]
which is strongly influenced by the temperature,[32] the steric
demands of the side chains attached to the hydrogen bond-
ing groups, and the number of hydrogen-bonding functional-
ities. Concerning the influence of temperature, regardless of
the nature of the hydrogen-bonding group, the effects were
effectively suppressed upon moderate heating, as indicated
by the close similarity of the absorption spectra at 828C and
their marked differences at 258C (Figure 3 and Figure S7 in
the Supporting Information). Superimposed upon the tem-
perature effect was a steric effect. Specifically, the spectrum
of 1b, which carries the bulkiest side chain (the 2-(R)-(À)-a-
[b]
Apolar solvent[a]
[mL]
THF
[mL]
G.C.min
[wt%]
Phase/
Tg [8C]
A
hexane
2b[c]
0.55
0.60
1.30
1.10
0.068
0.081
G[d]/43
G/31
heptane
2b[c]
toluene
2b[c]
0.50
1.90
0.35
0
0.15
0.035
G/A[e]
G/42
3[f]
[a] Hexane, heptane, or toluene. [b] G.C.min =minimum gelation concen-
tration. [c] 1 mgvolÀ1. [d] G=gel at room temperature; [e] A=destroyed
by mechanical agitation. [f] 0.6 mgvolÀ1
methoxy-a-(trifluoromethyl)phenylethylamido
group),
showed very little changes with respect to temperature var-
iation (Figure S7A in the Supporting Information). In con-
trast, the spectrum of 1a, which has a less bulky appendage,
displayed more pronounced changes (Figure 3A). The domi-
nant effect is the nature of the hydrogen-bonding group.
The spectra of the ureido-containing HBCs (2a and 2b; Fig-
ures S7B and 3B in the Supporting Information) exhibit
much greater temperature dependence than those of the
amido-containing HBCs (1a and 1b). The number of hydro-
gen-bonding groups present also played a significant role, as
demonstrated by the strong temperature dependence ob-
served in the spectrum of 3 (two ureido groups). In this
case, the spectrum was still changing at the upper tempera-
ture limit (858C), which indicated that hydrogen-bonding in-
teractions had not yet been suppressed.
confocal microscopy (LSCM) indicated that the fluorescence
comes from the fibrillar structure itself and not from amor-
phous material (Figure S19 in the Supporting Information).
To visualize how the nanoscale hydrogen-bonding interac-
tions manifest themselves in the macroscopic superstruc-
tures, a scanning electron microscopy (SEM) image of a xe-
rogel of 2b was recorded (Figure 7B).[30] The molecules ag-
gregated into long fibrillar structures (up to several microns
in length) with a diameter of approximately 100 nm. These
fibers further interwound with each other to establish three-
dimensional networks, which are the only known examples
for HBC derivatives (for more SEM pictures, see Figures
S20A and B for 2b and C and D for 3 in the Supporting In-
formation).
In the CD spectrum of 1b (Figure S21 in the Supporting
Information), the Cotton effects are quite weak. However,
the pattern observed is consistent with a clockwise helical
aggregation of molecular dipole moments in solution.[33] The
SEM image of the xerogel prepared from 2b, which showed
no CD signal, appeared to show discernable left-handed hel-
ical structures in the gel fibers of 2b (Figure S20A in the
Supporting Information).
Discussion
As indicated by the results, the presence of hydrogen bonds
strongly influenced not only the solution behavior, but also
the bulkproperties of the HBC derivatives. In solution, the
non-Lambert–Beer law behavior and the concentration de-
pendence of the absorption spectra (sharp, well-resolved
band shape with larger extinction coefficients at lower con-
centrations) indicated the existence of larger aggregates at
higher concentrations.[13c] The slight blueshift of one of the
absorption bands (from 373 to 369 nm) in the spectrum of 3
was also consistent with cofacial stacking of the aromatic p-
systems in the aggregates.[13a,31b] This phenomenon has been
studied both theoretically and experimentally.[31] The exis-
tence of increasing aggregation with increasing concentra-
tion for 2a, 2b, and 3 was further supported by concentra-
tion-dependent fluorescence experiments, which exhibited
broad, less-resolved concentration-dependent bands above
500 nm. The less-pronounced photophysical changes in 1a
and 1b indicated lower self-association propensities, which
are likely to be as a result of the weaker hydrogen-bonding
interactions between amido functionalities than ureido
groups.[17]
In the bulkstate, the differences in the degree of hydro-
gen-bonding interactions also became obvious. The observa-
À
tion of multiple N H stretching vibrations for 1a, 2a, and
2b in their FTIR spectra (Table 1, Figure S8 in the Support-
ing Information) suggested that the amido/ureido groups
might bind to produce dimers with an s-cis conformation or
polymers with an s-trans conformation.[25a] When comparing
the two amido-containing compounds (1a and 1b), the car-
bonyl stretching of 1a was observed at a frequency that was
40 cmÀ1 lower than that of 1b, which suggested the existence
of stronger hydrogen bonds in 1a. Among the ureido-con-
taining compounds, compounds 2b and 3 gave only one car-
bonyl stretching signal (n˜ =1640 (2b) and 1650 cmÀ1 (3)),
whereas two carbonyl stretching bands were observed for
2a (n˜ =1710 and 1650 cmÀ1). This result may be a reflection
of different hydrogen-bonding interactions in these three
compounds.[34] These observations are in agreement with the
results obtained from solution studies that 1b, which has a
bulky amido substituent, formed the weakest hydrogen
bonds and that hydrogen bonding was significantly strength-
As the aromatic moieties of compounds 1a, 1b, 2a, and
2b consist of the same basic structure, very little difference
in the strength of the p interactions can be expected. There-
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245

K. Müllen et al.
ened both by reducing the size of the substituent and by
using a more effective hydrogen-bonding ureido group.
The supramolecular columnar arrangements of the com-
pounds at different temperatures were investigated by 2D
WAXS, whereby it was found that the hydrogen-bonding in-
teractions strongly influenced the stacking and especially
the thermal properties of these materials. The most interest-
ing observation was the irreversibility of mesophase forma-
tion after annealing for compounds 1a, 2a, and 2b, which is
a unique property for HBC derivatives. The columnar stack-
ing at room temperature before annealing resembled that of
most other HBC molecules in the crystalline phase.[27a] The
periodic distance along columns was determined to be
0.47 nm for 1a and 2b, and 0.46 nm for 2a, which is in good
agreement with the length of the hydrogen-bonding units
between the urea and amide groups (0.44–0.48 nm).[16,17a,35]
This provides the opportunity for a near-optimum arrange-
ment of hydrogen-bonding functional groups. As the tem-
perature was increased to 1708C, the effect of the hydrogen
bonds was weakened and p interactions became the domi-
nant influence on the supramolecular arrangement. This led
to a typical LC organization with nontilted molecular discs,
a typical intermolecular p-stacking distance (0.36 nm), and
hexagonal 2D columnar packing. Thereby, stacked arenes
separated by 0.36 nm and hydrogen bonds of 0.47 or
0.46 nm can be simultaneously accommodated in the struc-
ture shown in Figure 8, which has a helical arrangement.
The unexpected preservation of the LC supramolecular or-
ganization after prolonged annealing is consistent with the
observation of only an irreversible endothermic transition
during the heating cycle on the DSC diagram (2b). This be-
havior can be explained in terms of the relative importance
of various weakinteractions at different temperatures. At
low temperatures, the hydrogen bonding and the crystallini-
ty of the alkyl substituents dictate the supramolecular ar-
rangement to give monoclinic columnar packing with tilted
molecular discs. At high temperatures, the relative impor-
tance of hydrogen bonding lessens[32] and p interactions
become dominant. As such, a hexagonal columnar arrange-
ment forms. Upon cooling, the order imposed by p stacking
remains in place and intermolecular hydrogen bonding be-
tween side chains cements the structure. Thus, the coopera-
tive effects of the weakinteractions ( p stacking, hydrogen
bonding, and side chain crystallinity) are responsible for the
preservation of the LC phase. Again, it is noteworthy that
the functional groups are attached directly to the aromatic
cores and the intracolumnar hydrogen bonds are thus estab-
lished between discs within the same supramolecular
column. This stands in starkcontrast to the previously re-
ported HBC derivatives, which had hydrogen-bonding func-
tionalities that were separated from the HBC cores by long
alkyl spacers and resulted in the formation of intercolumnar
hydrogen bonds.[15]
The observation that 1b maintained the same columnar
arrangement over the whole temperature range suggested a
less pronounced influence of hydrogen bonds on the supra-
molecular arrangement at low temperatures.
Upon inclusion of a second ureido group (i.e., 3), the 2D
WAXS pattern revealed an increased disorder at low tem-
peratures, which implied an even stronger influence of the
hydrogen bonds. The decrease in the cofacial stacking dis-
tance (from 0.38 to 0.36 nm) upon increasing the tempera-
ture (from 1208C to 1808C) showed that the supramolecular
arrangement induced by the p interactions could only be ob-
tained after the hydrogen bonds were significantly weak-
ened at higher temperatures. The loss of long range order
upon cooling not only indicates that hydrogen bonding is
more important than p interactions at room temperature,
but also that the two effects do not operate cooperatively in
this system.
All of the results discussed above clearly demonstrate
that for successful cooperative implementation of two secon-
dary forces to improve the self-assembling propensity, the
spatial demands and geometrical requirements need to coin-
cide. By selecting a stronger hydrogen-bond donor, that is,
the ureido unit, and reducing the size of the side chain at-
tached to the functional group, for example, as in 2b, the
HBC derivative exhibited excellent self-assembling ability
both in solution and in the solid state. The introduction of a
second strong hydrogen-bond provider at the para position
(in the case of 3) was, however, counterproductive. A bulky
side chain and relatively feeble hydrogen-bonding group, in
the case of 1b, resulted in an almost undetectable influence
on the self-assembly behavior of the HBCs.
It has been observed previously that amphiphilic HBCs
are capable of gelling some organic solvents under certain
conditions.[36] For compounds 2b and 3, the combination of
stronger and more rigid hydrogen bonds and their intrinsic
p interactions caused the HBCs to behave like LMOGs.
Both materials exhibited excellent gelation abilities with
very low minimum gelation concentrations compared with
Figure 8. Schematic representation of the coincidence of hydrogen-bond-
ing interactions and p interactions after annealing for 2b.
246
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Chem. Eur. J. 2008, 14, 240 – 249

Self-Assembly of Hexa-peri-hexabenzocoronenes
FULL PAPER
perylene-[37] and porphyrin-based[17c,d,35g] LMOGs, which also
combine p interactions with hydrogen bonds. The gelling
process could be assumed to be as follows: The hydrogen
bonds were initially significantly weakened in solution by
heating at reflux and the p interactions dominated the mo-
lecular aggregation. As in the bulkstate, the hydrogen-
bonding interactions stabilized these aggregates upon cool-
ing and further accumulated these microscopic aggregates
into fibrillar supramolecular structures. An SEM image of
the xerogel (Figure 7B) showed that the resulting three-di-
mensional networktrapped small solvent molecules and sta-
bilized the substance in a metastate between liquid and
solid, which led to the formation of fluorescent organogels.
one-dimensional aligned chromophores,[17d,38a] enhanced
charge transport based on organic semi-conducting
layers,[38b] light-scattering electrooptical materials through
the cooperation of LC moieties,[38c] and sensor devices[6,38d]
(following the introduction of additional functionalities).
Experimental Section
General methods: Chemicals were obtained from Fluka, Aldrich, and
ABCR and used as received. Column chromatography and TLC were
performed with Mercksilica gel 60 (70–230 mesh) and Mercksilica gel
60 F254, respectively. 1H NMR and 13C NMR spectra were recorded by
using a Bruker DPX 250, Bruker 300 AMX, Bruker DRX 500, or Bruker
DRX 700 spectrometer with the solvent proton or carbon signal as an in-
ternal standard. MALDI-TOF mass spectra were measured by using a
Bruker Reflex II-TOF spectrometer with a 337 nm nitrogen laser and
7,7,8,8-tetracyanoquinodimethane (TCNQ) as the matrix. The optical ab-
sorption measurements were performed at ambient temperatures by
using a UV/Vis/NIR Perkin–Elmer Lambda 900 spectrometer. The pho-
toluminescence in solution was recorded by using a SPEX Fluorolog 2
type 212 steady-state fluorometer. Infrared spectroscopy was measured
by using Nicolet FTIR 320 spectrometer. DSC measurements were con-
ducted by using a mettler DSC 30 instrument with a heating and cooling
rate of 108CminÀ1. The 2D-WAXS experiments were performed by
means of a rotating anode (Rigaku 18 kW) X-ray beam with a pinhole
collimation and a 2D Siemens detector. A double graphite monochroma-
tor for the CuKa radiation (l=0.154 nm) was used. Electron microscopy
was performed by using a Zeiss 1530 electron microscope. LSCM was
conducted by using a LSM 510 laser scanning module coupled to a Zeiss
Axiovert 200m inverted microscope.
Conclusion
Through the introduction of hydrogen bonds along the HBC
columnar axis, the self-association propensities of HBC de-
rivatives have been improved considerably and were also
strongly influenced by varying both the size of the substitu-
ents and the nature of the hydrogen-bonding groups at the
same time. The self-assembling abilities of these compounds
increased substantially as the hydrogen-bonding interactions
became stronger and the steric demands of the side chains
diminished. Accordingly, the compound with the weaker hy-
drogen-bonding group and the bulkier substituent (1b) ex-
hibited the lowest aggregation tendency. By the same token,
either reducing the steric hindrance of the substituent (1a)
or incorporating a stronger hydrogen-bonding unit (2a, 2b)
endowed the HBCs with a stronger self-assembling ability.
When a second hydrogen-bond donating group was connect-
ed to the aromatic core at the para position (3), the mole-
cule displayed the strongest aggregation tendency in solu-
tion.
Nevertheless, in the bulkstate, depending upon the
strength of the intermolecular interactions and the geometry
of functional groups, hydrogen bonds either cooperated with
the p interactions to stabilize the HBC columnar superstruc-
tures, which even resulted in the preservation of a highly or-
dered LC phase after annealing (1a, 2a and 2b), or compet-
ed against the p interactions, which led to a short-range or-
dered supramolecular arrangement (3).
Finally, the relatively stronger hydrogen bonds of 2b and
3 rendered these HBC derivatives as LMOGs, which has
never been characterized previously for HBC derivatives.
HBC molecules 2b and 3 assembled into stable, three-di-
mensional networks under the collaboration of hydrogen
bonding and p interactions, which trap small solvent mole-
cules to form fluorescent organogels. The hydrogen bonds
significantly enhanced the self-assembling abilities of the
HBCs and thus endowed them with excellent gelation abili-
ties. In so doing, the optoelectronic properties of HBCs
have been made available for exploitation in the area of
functional organogels. Owing to the intrinsic semiconducting
properties of stacked HBCs, such gels are promising candi-
dates for applications in areas such as energy transfer along
2-Benzophenonimino-5,8,11,14,17-penta(3,7-dimethyloctanyl)hexa-peri-
hexabenzocoronene (5): 2-Bromo-5,8,11,14,17-penta(3,7-dimethylocta-
nyl)hexa-peri-hexabenzocoronene (4) (261.3 mg, 201.0 mmol), [Pd2(dba)3]
A
(4.9 mg, 5.0 mmol), BINAP (10.0 mg, 16.0 mmol), and sodium tert-butox-
ide (461.0 mg, 4.8 mmol) were dissolved in dry toluene (10 mL) and
heated to 808C in a 25 mL Schlenkflaskunder argon. Then benzopheno-
nimene (870.0 mg, 4.8 mmol) was added dropwise. The mixture was
stirred at 808C for 16 h. After cooling, the mixture was poured into meth-
anol (100 mL). The yellow solid that precipitated was filtrated, redis-
solved in a minimum amount of THF and precipitated by adding metha-
nol twice. Compound 5 was obtained as yellow solid (265 mg, 95%).
1
À
H NMR (250 MHz, CDCl3, 258C): d=8.30–8.26 (m, 6H; Ar H), 8.16–
À
À
À
8.12 (m, 6H; Ar H), 7.60–7.59 (m, 4H; Ar H), 7.40–7.37 (m, 2H; Ar
H), 7.20–7.11 (m, 4H; Ar H), 3.04–2.90 (m, 10H; a-CH2), 1.98–1.14 (m,
À
65H; CH, CH2, CH3), 0.92–0.90 ppm (m, 30H; CH3); 13C NMR
(62.5 MHz, [D8]THF, 258C): d=168.1, 148.9, 141.2, 140.8, 140.7, 140.6,
138.0, 131.6, 130.9, 130.6, 130.5, 130.4, 130.3, 129.4, 129.1, 129.0, 123.8,
123.7, 123.6, 122.3, 121.9, 121.7, 120.0, 119.9, 119.8, 116.2, 41.1, 41.0, 40.5,
38.6, 38.5, 35.8, 34.4, 29.1, 23.3, 23.2, 20.4 ppm; MS (MALDI-TOF,
TCNQ): m/z (%): 1404 (100%) [M+H]+.
À
À
À
À
2-Amino-5,8,11,14,17-penta(3,7-dimethyloctanyl)hexa-peri-hexabenzocor-
onene hydrochloride (6): Compound 5 (297.3 mg, 0.2 mmol) was dis-
solved in THF (10 mL) and 2.5m HCl (70 mL) was added under an argon
atmosphere. The solution immediately, but temporarily, turned blackfol-
lowed by the formation of a suspension of yellow powder in the aqueous
solution. The mixture was stirred vigorously for 5 h and filtered. The
solid was washed with water until the pH value of the filtrate was close
to 7, then it was washed with acetone to remove excess diphenylketone,
as monitored by TLC. Ammonium salt 6 was collected as a yellow
powder after being dried under high vacuum (236 mg, 87%). MS
(MALDI-TOF, TCNQ): m/z (%): 1239 (100%) [MÀH]+. This compound
was used directly in the next step, owing to the instability of the free
base. NMR spectroscopy was not used for structural characterization, but
rather the spectra of derivatives were measured.
Chem. Eur. J. 2008, 14, 240 – 249
ꢁ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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247

K. Müllen et al.
À
À
À
General procedure for the synthesis of compounds 1a and 1b: Com-
pound (0.06 mmol, 74.3 mg), triethylamine (0.5 mL) and LiBr
CH2), 1.23–2.05 (m, 65H; CH, CH2), 0.93–0.97 ppm (m, 33H; CH3);
13C NMR (125 MHz, [D8]THF/LiBr, 508C): d=157.1, 147.2, 140.6, 140.1,
139.9, 139.7, 130.9, 130.4, 130.3, 130.2, 129.9, 129.1, 127.3, 127.1, 123.7,
123.6, 123.5, 122.5, 121.6, 121.5, 121.3, 120.9, 119.9, 119.5, 119.2, 119.1,
113.8, 50.5, 41.1, 40.9, 40.8, 40.5, 38.5, 35.9, 35.7, 35.6, 34.4, 34.34, 30.5,
29.0, 23.2, 23.1, 20.4 ppm; MS (MALDI-TOF, TCNQ): m/z (%): 1386
(100%) [M]+.
6
(110.0 mg) were added to a mixture of toluene/THF (1:1, 5 mL) and
heated to 808C under an argon atmosphere. The appropriate acetyl chlo-
ride (0.6 mmol) was added dropwise. The mixture was stirred at 808C
overnight. After cooling to room temperature, the mixture was poured
into methanol (100 mL). The precipitate was collected and further puri-
fied by column chromatography.
2,11-Bis-{N’-[3,4,5-tris-(dodecyloxyphenyl)ureido]-4-phenyl-1-ethylnyl}-
5,8,14,17-tetradodecanylhexa-peri-hexabenzocoronene (3): Compound 7
2-Nonylamido-5,8,11,14,17-penta(3,7-dimethyloctanyl)hexa-peri-hexaben-
zocoronene (1a): Compound 1a was prepared according to the general
procedure. The product was purified by using a silica gel column and a
mixture of low-boiling petroleum ether/CH2Cl2 (2:1) as the eluent (Rf =
(109 mg, 0,08 mmol),
8
(152 mg, 0,19 mmol), [Pd(PPh3)4] (18 mg,
A
0,02 mmol), CuI (6 mg, 0,03 mmol) and LiBr (100 mg) were dissolved in
a mixture of Et3N/THF (2:1, 15 mL). The mixture was carefully degassed
twice by freeze-thaw method and the reaction lasted for 4 days at 508C
under argon atmosphere. After it was cooled to room temperature, the
reaction was quenched with water (30 mL). The organic phase was ex-
tracted with CH2Cl2 (3”50 mL) and dried over MgSO4. The solvent was
removed in vacuo and the product was purified by column chromatogra-
phy by using THF/CH2Cl2 (3:1) with LiBr as the eluent to afford 3 as a
1
0.36) to afford 1a as a yellow powder (79 mg, 88%). H NMR (250 MHz,
À
À
[D8]THF/LiBr, 508C): d=10.91 (s, 1H; NH), 9.32 (s, 2H; Ar H), 8.47
À
À
À
(s, 2H; Ar H), 8.36 (s, 2H; Ar H), 8.27–8.19 (m, 6H; Ar H), 3.05–2.96
À
(m, 12H; a-CH2), 2.01 (br s, 12H; b-CH2), 1.57–1.15 (m, 65H; CH,
CH2), 0.96–0.88 ppm (m, 33H; CH3); 13C NMR (62.5 MHz, [D8]THF/
LiBr, 508C): d=173.3, 146.3, 140.8, 140.3, 140.2, 138.9, 135.7, 130.8,
130.6, 130.4, 130.2, 130.0, 129.9, 125.9, 123.7, 123.5, 122.4, 121.7, 121.6,
121.4, 119.9, 119.6, 119.5, 119.4, 114.6, 41.0, 40.9, 40.5, 38.5, 34.4, 34.3,
32.8, 31.8, 31.0, 30.9, 30.5, 29.0, 23.2, 23.1, 20.3, 14.4 ppm; MS (MALDI-
TOF, TCNQ): m/z (%): 1379 (100%) [M]+.
À
À
1
yellow powder (81 mg, 13%). H NMR (500 MHz, [D8]THF/LiBr, 508C):
d=8.53 (s, 4H; Ar H), 8.22–8.01 (m, 12H; Ar H), 7.72 (d, 4H; 3J-
À
À
3
À
À
A
ACHTREUNG
2
À
À
(s, 4H; Ar H), 3.91 (t, J
(H,H)=6 Hz, 12H; OCH2), 2.89 (br s, 8H; a-
À
À
CH2), 1.56–1.30 (m, 180H; CH2), 0.92–0.87 ppm (m, 50H; CH2,
2-(R)-(À)-a-methoxy-a-(trifluoromethyl)phenylethylamido-5,8,11,14,17-
penta(3,7-dimethyloctanyl)hexa-peri-hexabenzocoronene (1b): Compound
1b was prepared according to the general procedure. The product was
purified by using a silica gel column and a mixture of low-boiling petrole-
um ether/THF (10:1) as the eluent (Rf =0.56) to afford 1b as a yellow
powder (72 mg, 81%). 1H NMR (250 MHz, [D8]THF, 258C): d=10.01 (s,
CH3); 13C NMR (125 MHz, [D8]THF/LiBr, 508C)): d=166.0, 159.5,
À
151.7, 140.8, 138.6, 135.8, 133.9, 133.7, 133.1, 123.2, 118.3, 116.51, 116.47,
114.6, 114.5, 114.2, 113.8, 113.0, 112.8, 112.6, 109.3, 107.3, 106.2, 60.1,
33.0, 32.7, 30.6, 27.9, 26.4, 25.0, 22.7, 21.2, 18.1, 18.0, 15.3, 15.2, 12.5 ppm;
MS (MALDI-TOF, TCNQ): m/z (%): 2770 (100%) [M]+.
À
À
À
À
1H; NH), 9.57 (s, 2H; Ar H), 8.77 (br s, 4H; Ar H), 8.69 (s, 2H; Ar
H), 8.63–8.61 (m, 4H; Ar H), 7.96 (d, 3J
A
À
À
À
À
7.57–7.53 (m, 3H; Ar H), 3.90 (s, 3H; OCH3), 3.29–3.19 (m, 10H; a-
À
À
À
CH2), 2.01–1.21 (m, 68H; CH, CH2), 0.93–0.91 ppm (m, 33H; CH3);
13C NMR (175 MHz, [D8]THF, 258C): d=168.8, 144.5, 144.3, 144.2, 140.4,
137.8, 134.7, 133.9, 133.7, 133.6, 127.2, 126.9, 126.1, 123.3, 123.1, 116.9,
44.1, 43.9, 43.5, 41.5, 38.8, 38.7, 37.3, 37.2, 33.6, 32.0, 26.2, 26.1, 23.3 ppm;
MS (MALDI-TOF, TCNQ) m/z (%): 1455 (100%) [M]+.
Acknowledgements
This workwas supported by the EU Integrated Project NAIMO (No.
NMP4-CT-2004-500355). We are grateful to Dr. Daniel Wasserfallen and
Dr. Xiaoyin Yang for their valuable help and discussions in the prepara-
tion of the manuscript.
General procedure for the synthesis of compounds 2a and 2b: Com-
pound 6 (123.9 mg, 0.1 mmol), LiBr (55 mg), and triethylamine (0.5 mL)
were added to a mixture of toluene/THF (1:1, 5 mL) and heated to 808C
under argon. The appropriate isocyanate (1.3 mmol) was added dropwise,
and the reaction stirred at 808C for 45 h. After the mixture was cooled,
the crude product was precipitated in methanol (250 mL) and dried
under vacuum after filtration. The crude product was further purified by
column chromatography.
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2-(N’-Octanylureido)-5,8,11,14,17-penta(3,7-dimethyloctanyl)hexa-peri-
hexabenzocoronene (2a): Compound 2a was prepared according to the
general procedure. The product was purified by using a mixture of low-
boiling petroleum ether/CH2Cl2 (3:1) with LiBr and then LiBr/THF as
the eluents (Rf =0.4) to afford 2a as a brown solid (75 mg, 60%).
1
À
H NMR (250 MHz, [D8]THF/LiBr, 508C): d=9.81 (s, 1H; NH), 8.97
À
À
À
(br s, 2H; Ar H), 8.64 (s, 2H; Ar H), 8.41–8.17 (m, 8H; Ar H), 7.59 (s,
1H; NH), 3.41 (br s, 2H; a-CH2), 3.13–3.05 (m, 10H; a-CH2), 2.17–1.83
À
À
À
(m 12H; b-CH2), 1.65–1.14 (m, 65H; CH, CH2), 0.95–0.82 ppm (m,
33H; CH3); 13C NMR (175 MHz, [D8]THF/LiBr, 508C): d=160.8,
À
144.5, 144.6, 144.3, 144.2, 135.8, 133.6, 126.9, 126.6, 125.7, 125.3, 125.1,
123.6, 123.3, 122.6, 44.7, 44.3, 44.2, 43.8, 43.6, 41.6, 38.8, 38.6, 37.5, 37.4,
37.2, 36.1, 35.9, 33.9, 33.8, 33.7, 33.5, 33,4, 33,3 32.1, 31.2, 23.4, 17.5 ppm;
MS (MALDI-TOF, TCNQ): m/z (%): 1394 (100%) [M]+.
2-(N’-(S)-(À)-1-phenylethylureido)-5,8,11,14,17-penta(3,7-dimethylocta-
nyl)hexa-peri-hexabenzocoronene (2b): Compound 2b was prepared ac-
cording to the general procedure. The residue was purified with a mix-
ture of low-boiling petroleum ether/CH2Cl2 (3:2) with LiBr and then
LiBr/THF as the eluents (Rf =0.15) to afford 2b as a brown solid (81 mg,
54%). 1H NMR (250 MHz, [D8]THF/LiBr; 508C): d=10.12 (s, 1H;
À
À
À
À
NH), 9.01 (s, 2H; Ar H), 8.51 (br s, 4H; Ar H), 8.42 (br s, 2H; Ar
H), 8.33 (br s, 4H; Ar H), 8.03 (br s, 1H; NH), 7.78 (d, 3J
ACHTREUNG
À
À
7.2 Hz, 2H; Ar H), 7.41 (t, 3J
AHCTREUNG
À
À
À
A
248
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Chem. Eur. J. 2008, 14, 240 – 249

Self-Assembly of Hexa-peri-hexabenzocoronenes
FULL PAPER
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Received: June 17, 2007
Published online: October 8, 2007
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.
Chem. Eur. J. 2008, 14, 240 – 249
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