Star-shaped hexa-peri-hexabenzocoronene "heptamer": Synthesis and self-assembly

Article abstract of DOI:10.1021/ol052201j

(Chemical Equation Presented) A star-shaped hexa-peri-hexabenzocoronene "heptamer" was prepared, which showed a strong tendency to aggregate in solution and in the bulk states. Higher order was found in a high-temperature, columnar liquid crystalline phase due to the higher mobility of the molecules in comparison with the low-temperature solid. In addition, physical gel formation was observed due to the presence of covalent intercolumnar interactions.

Full text of DOI:10.1021/ol052201j

ORGANIC
LETTERS
2005
Vol. 7, No. 26
5761-5764
Star-Shaped
Hexa-peri-hexabenzocoronene
“Heptamer”: Synthesis and
Self-Assembly
Linjie Zhi, Jishan Wu, and Klaus Mu1llen*
Max-Planck-Institute for Polymer Research, Ackermannweg 10,
55128 Mainz, Germany
muellen@mpip-mainz.mpg.de
Received September 13, 2005
ABSTRACT
A star-shaped hexa-peri-hexabenzocoronene “heptamer” was prepared, which showed a strong tendency to aggregate in solution and in the
bulk states. Higher order was found in a high-temperature, columnar liquid crystalline phase due to the higher mobility of the molecules in
comparison with the low-temperature solid. In addition, physical gel formation was observed due to the presence of covalent intercolumnar
interactions.
Columnar liquid crystals based on disc-like mesogens such
as triphenylene, porphyrin, phthalocyanine, and hexa-peri-
hexabenzocoronene (HBC) have attracted attention due to
their remarkable self-assembling properties and promising
applications as charge transporting materials in organic
electronic devices.1 Incorporation of these discotics into the
main chain or side chain of polymers has resulted in a new
class of liquid crystalline materials.2 The large aromatic core
of HBC derivatives leads to high order parameters in the
mesophase and to high photoconductivities.3 However, one-
dimensional topologically extended para- and ortho-con-
nected HBC oligomers and polymers showed less ordered
columnar structures presumably due to the linker stiffness
(1) (a) Watson, M. D.; Fechtenko¨tter, A.; Mu¨llen, K. Chem. ReV. 2001,
101, 1267-1300. (b) Bushby, R. J.; Lozman, O. R. Curr. Opin. Solid State
Mater. Sci. 2002, 6, 569-578. (c) Simpson, C. D.; Wu, J.; Watson, M. D.;
Mu¨llen, K. J. Mater. Chem. 2004, 14, 494-504. (d) de la Torre, G.; Nicolau,
M.; Torres, T. In Phthalocyanines: Synthesis, Supramolecular Organization
and Physical Properties (Supramolecular PhotosensitiVe and ElectroactiVe
Materials); Nalwa, H. S., Ed.; Academic Press: New York, 2001.
(2) Ringsdorf, H.; Wu¨stefeld, R.; Zerta, E.; Ebert, M.; Wendorff, J. H.
Angew. Chem., Int. Ed. Engl. 1989, 28, 914-918.
10.1021/ol052201j CCC: $30.25
© 2005 American Chemical Society
Published on Web 11/24/2005

and large intermolecular torsion angles.4 On the other hand,
an HBC dimer 1 (Figure 1) connected by a flexible, aliphatic
HBC derivatives have been conventionally synthesized by
oxidative cyclodehydrogenation of the corresponding hexa-
phenylbenzene (HPB) precursors.8 As such, the key step
toward molecule 2 was the synthesis of the star-shaped HPB
heptamer 3, which was then treated with FeCl3, dissolved
in CH3NO2, affording compound 2 in quantitative yield
(Scheme 1).
Scheme 1. Synthesis of HBC Heptamer 2
Figure 1. Chemical structures of compounds 1 and 2.
A divergent approach was used to prepare the precursor
3 (Scheme 2). Pd-catalyzed Kumada coupling of the 1-bromo-
4-trimethylsilylbenzene (4) with freshly prepared 11-mag-
nesium bromoundec-1-ene (5) afforded trimethyl-(4-undec-
10-enyl-phenyl)-silane (6) in 84% yield. Transformation of
the olefin in 6 into alkylborane by treating with 9-BBN and
subsequent Suzuki coupling9 reaction with bis(4-bromophen-
yl)acetylene 7 10 gave compound 8 in 54% yield. The hexa-
phenylbenzene derivative 9 carrying six trimethylsilyl (TMS)
groups was then synthesized by dicobalt octacarbonyl cata-
lyzed cyclotrimerization of 8 in refluxing dioxane in 81%
yield. Attempts at preparing compound 9 by 6-fold Suzuki
coupling reactions between hexakis (4-bromophenyl)ben-
zene11 and the borane compound of 6 failed due to the in-
stability of the TMS groups under basic conditions. The TMS
groups in 9 were then converted to iodides by treating with
iodine monochloride to obtain molecule 10 in high yield
(90%). Six-fold Sonogashira coupling of 10 with 4-n-
dodecylphenylacetylene afforded 11 in 66% yield. The target
compound 3 was then obtained by 6-fold [4 + 2] Diels-
Alder cycloaddition between 11 and tetrakis(4-dodecylphen-
yl)cyclopentadienone (12 ) in 70% yield.
alkyl chain displayed an ordered hexagonal columnar LC
phase similar to that of hexakis(n-dodecyl)-peri-hexabenzo-
coronene (HBC-C12).5 Thereby, the dimer 1 suppressed the
room-temperature crystalline phase found in HBC-C12 and
gelated organic solvents such as toluene, suggesting the
existence of intercolumnar physical cross-linking. Gel forma-
tion has been widely observed in hydrogen-bonded π-con-
jugated discotics and oligomers due to the H-bonding, π-π
interactions, and dipole-dipole interactions;6 however, non-
H-bonded aromatic compounds rarely formed gels.7 To
further understand the structure-property relationships, a
star-shaped HBC “heptamer” 2 (heptamer of HBC units) in
which seven HBC units are connected by flexible aliphatic
chains in a hexagonal symmetry was synthesized and self-
assembly in solution, bulk-state and gel-state was studied.
(3) (a) Herwig, P.; Kayser, C. W.; Mu¨llen, K.; Spiess, H. W. AdV. Mater.
1996, 8, 510-513. (b) Schmidt-Mende, L.; Fechtenko¨tter, A.; Mu¨llen, K.;
Moons, E.; Friend, R. H.; MacKenzie, J. D. Science 2001, 293, 1119-
1123.
(4) Wu, J.; Watson, M. D.; Tchebotareva, N.; Wang, Z.; Mu¨llen, K. J.
Org. Chem. 2004, 69, 8194-8204.
(5) Ito, S.; Herwig, P. T.; Bo¨hme, T.; Rabe, J. P.; Rettig, W.; Mu¨llen,
K. J. Am. Chem. Soc. 2000, 122, 7698-7706.
(8) (a) Mu¨ller, M.; Ku¨bel, C.; Mu¨llen, K. Chem. Eur. J. 1998, 4, 2099-
2109. (b) Stabel, A.; Herwig, P.; Mu¨llen, K.; Rabe, J. P. Angew. Chem.,
Int. Ed. Engl. 1995, 34, 1609-1611.
(9) (a) Miyaura, N.; Ishiyama, T.; Sasaki, H.; Ishikawa, M.; Satoh, M.;
Suzuki, A. J. Am. Chem. Soc. 1989, 111, 314-321. (b) Miyaura, N.; Suzuki,
A. Chem. ReV. 1995, 95, 2457-2483.
(10) Pisula, W.; Tomovic, Z.; El Hamaoui, B.; Watson, M. D.; Pakula,
T.; Mu¨llen, K. AdV. Funct. Mater. 2005, 15, 893-904.
(11) Wu, J.; Watson, M. D.; Mu¨llen, K. Angew. Chem., Int. Ed. 2003,
42, 5329-5333.
(6) (a) Tamaru, S.; Nakamura, M.; Takeuchi, M.; Shinkai, S. Org. Lett.
2001, 3, 3631-3634. (b) Ikeda, M.; Takeuchi, M.; Shinkai, S. Chem.
Commun. 2003, 1354-1355.
(7) Pozzo, J. L.; Clavier, G. M.; Colomes, M.; Bouas-Laurent, H.
Tetrahedron 1997, 53, 6377-6390. (b) Kato, T.; Kutsuna, T.; Yabuuchi,
K.; Mizoshita, N. Langmuir 2002, 18, 7086-7088. (c) Perahia, D.; Traiphol,
R.; Bunz, U. H. F. J. Chem. Phys. 2002, 117 (4), 1827-1832. (d) Desvergne,
J. P.; Brotin, T.; Meerschaut, D.; Clavier, G.; Placin, F.; Pozzo, J. L.; Bouas-
Laurent, H. New J. Chem. 2004, 28, 234-243.
5762
Org. Lett., Vol. 7, No. 26, 2005

Scheme 2. Synthesis of Compound 3a
a Reaction conditions: (a) Pd(dppf)Cl2, THF, rt, 84%; (b) 9-BBN, NaOH, Pd(dppf)Cl2, 54%; (c) Co2(CO)8, dioxane, reflux, 81%; (d)
ICl, CHCl3, 90%; (e) 4-n-dodecylphenylacetylene, Pd(PPh3)4, CuI, piperidine, 55 °C, 66%; (f) diphenyl ether, reflux, 70%
All intermediate compounds were well characterized by
mass spectroscopy, microanalysis, and NMR spectroscopy
(Supporting Information). The complete cyclodehydrogena-
tion (loss of 84 H) of 3 was confirmed by MALDI-TOF mass
spectrum of 2 for which only one sharp molecular ionic peak
at 9624 was observed (Supporting Information). Compound
2 is soluble in organic solvents such as THF and toluene.
Solution NMR measurements revealed, however, only a weak
and broad resonance at the aromatic region even at higher
temperatures (130 °C) for long time (2 days) (Supporting
Information). This can be explained by aggregation of the
HBC molecules induced by strong π-stacking and interco-
lumnar interactions (vide infra).
In comparison with a hexakis(3,7-dimethyloctyl)-peri-hexa-
benzocoronene12 (HBC-C8,2, inset in Figure 2), which has
good solubility in normal organic solvents and well-resolved
UV-vis and fluorescence spectra, the HBC heptamer 2 in
toluene showed broad absorption and emission bands (Fig-
ure 2). Hyperchromic shift (ca. 9 nm) in absorption and a
large bathochromic shift (more than 40 nm) in the fluores-
cence spectrum of 2 were observed, suggesting strong face-
to-face π-interactions between the HBC units. The long-
wavelength band is assigned to excimer emission as found
Figure 2. Normalized UV-vis absorption spectra of HBC-C8,2
(black) and 2 (red) in toluene. Fluorescence spectra of HBC-C8,2
(green) and 2 (cyan to yellow). The arrow at 594-599 nm indicates
trend with an increase on concentration.
in other π-aggregates.5 With increasing concentration, the
relative intensity of the shoulders at 594-599 nm increased,
indicating a larger size of the aggregates at higher concentra-
tions.
(12) Fechtenko¨tter, A.; Tchebotareva, N.; Watson, M. D.; Mu¨llen, K.
Tetrahedron 2001, 57, 3769-3783.
Org. Lett., Vol. 7, No. 26, 2005
5763

The thermal behavior of the HBC heptamer 2 in the bulk
state was investigated by means of differential scanning
calorimetry (DSC), polarized optical microscopy (POM), and
X-ray diffractometry (XRD). The second DSC heating scan
of 2 revealed a broad endothermal peak at 35 °C that
corresponds to the transition from the solid state into the
mesophase. After annealing 2 at 400 °C for 1 h under argon,
fanlike textures were obtained under POM (Figure 3),
example, at room temperature, only a weak and broad (001)
reflection at ca. 3.65 Å due to the intracolumnar π-stacking
distance was observed. Upon heating, this reflection became
sharper and the relative intensity increased, suggesting a
longer-range columnar order at higher temperatures. When
the sample was cooled to room tempearure, the sharp (001)
reflection peaks became weak again. In addition, the mono-
mer HBC-C12 has a highly ordered crystalline phase below
60 °C, but the star-shaped HBC heptamer only displayed a
less ordered solid phase at room temperature. This is
reasonable because the HBC units in 2 are covalently linked
by flexible aliphatic chains and an ideal “top-on-top” super
seven-HBC-column structure is entropically disfavored; thus,
statistical intramolecular stacking and intercolumnar cross-
linking by π-π interactions will each occur in the solid phase
at low temperature, leading to a disordered structure. How-
ever, at higher temperatures, the HBC units become highly
mobile, such self-healing behavior resulting in more ordered
columnar structure at higher temperatures. Similar star-
shaped triphenylene heptamers have been reported,14 where
covalent connection of triphenylene suppressed the crystal-
lization of triphenylene derivatives; however, a detailed study
of the temperature dependence was not made.
Gel formation was observed for HBC heptamer 2 in
n-heptane and toluene, respectively, with a concentration of
10 g/L. The gel structure formed in toluene was a kind of
colloidal, three-dimensional architecture with cross-linked
aggregates as the frame of the architecture (Supporting Infor-
mation), suggesting strong intra- and intermolecule π-π
interactions between HBC units. It should be noted that such
gel formation occurred mainly due to π-π interactions, but
without any additional H-bonding or dipole-dipole interac-
tions!
Figure 3. Wide-angleXRD patterns of 2 at (a) room temperature,
(b) heating to 100 °C and then cooling to room temperature, (c)
100 °C, (d) 150 °C. Inset is the POM image of 2 after annealing at
400 °C for 1 h, showing fan-shaped textures.
In summary, a novel star-shaped HBC heptamer 2 was
synthesized by a facile procedure. Compared to the monomer
HBC-C12, the heptamer 2 showed different self-assemblying
motifs, for example, suppression of the crystallization at low
temperature and formation of long-range order phase at
higher temperatures. Organic solvents could also be gelated
by heptamer 2, from which the gelation was mainly induced
by π-π interactions. Charge transport through the multiply
self-assembled HBC columns is currently under investigation.
suggesting the formation of a hexagonal, ordered, columnar
liquid crystalline phase (Dh).
Wide-angle XRD measurements on solid sample 2 were
performed at different temperatures (Figure 3). In all cases,
a sequence of reflections at small angle region were ob-
served and their reciprocal space distances disclosed a ratio
of 1:2:x3, indicating a hexagonal two-dimensional lattice
of the HBC columns. With increasing temperature, the unit
cell parameter also (i.e., the intercolumnar distance) increased
from 2.48 nm at room temperature to 2.68 nm at 100 °C
and 2.77 nm at 150 °C. This tendency is expected for discotic
LCs and can be explained by the higher mobility of both
HBC molecules and alkyl chains at higher temperatures. The
broad halos found at ca. 4.3-4.8 Å were assigned to the
alkyl chain interactions and displayed a similar temperature
dependence.
The single alkyl chain substituted HBCs typically show
long-range, ordered columnar superstructures at room tem-
perature and less ordered columnar LC phases at high
temperatures as indicated by the sharpness and intensity of
the (001) reflections in the wide-angle region.13 In contrast,
the HBC heptamer 2 exhibited the reverse behavior. For
Acknowledgment. This work was supported by EU
project NAIMO (NMP4-CT-2004-500355). We thank Dr.
C. G. Clark, Jr. for his assistance in preparation of the
manuscript.
Supporting Information Available: Synthetic procedures
and additional characterization data. This material is available
free of charge via the Internet at http://pubs.acs.org.
OL052201J
(13) Fischbach, I.; Pakula, T.; Minkin, P.; Fechtenko¨tter, A.; Mu¨llen,
K. Spiess, H. W. J. Phys. Chem. B 2002, 106, 6408-6418.
(14) Plesnivy, T.; Ringsdorf, H.; Schuhmacher, P.; Nu¨tz, U.; Diele, S.
Liq. Cryst. 1995, 18, 185-190.
5764
Org. Lett., Vol. 7, No. 26, 2005