Star-shaped oligo(p-phenylenevinylene) substituted hexaarylbenzene: Purity, stability, and chiral self-assembly

Article abstract of DOI:10.1021/ja0765417

An oligo(p-phenylenevinylene) (OPV)-substituted hexaarylbenzene has been synthesized and fully characterized. Recycling gel permeation chromatography appeared to be a powerful technique to obtain the OPV molecules in a very pure form. X-ray analysis and p

Full text of DOI:10.1021/ja0765417

Published on Web 11/29/2007
Star-Shaped Oligo(p-phenylenevinylene) Substituted
Hexaarylbenzene: Purity, Stability, and Chiral Self-assembly†
Zˇeljko Tomovic´,‡, Joost van Dongen,‡ Subi J. George,‡ Hong Xu,§
Wojciech Pisula,|,# Philippe Lecle`re,‡,@ Maarten M. J. Smulders,‡
Steven De Feyter,*,§ E. W. Meijer,*,‡ and Albertus P. H. J. Schenning*,‡
Contribution from the Laboratory of Macromolecular and Organic Chemistry, EindhoVen
UniVersity of Technology, P.O. Box 513, 5600 MB EindhoVen, The Netherlands, DiVision of
Molecular and Nanomaterials, Department of Chemistry and INPAC - Institute of Nanoscale
Physics and Chemistry, Katholieke UniVersiteit LeuVen (K.U. LeuVen), Celestijnenlaan 200 F,
B-3001 LeuVen, Belgium, and Max Planck Institute for Polymer Research,
Ackermannweg 10, D-55128, Mainz, Germany
Received August 30, 2007
Abstract: An oligo(p-phenylenevinylene) (OPV)-substituted hexaarylbenzene has been synthesized and
fully characterized. Recycling gel permeation chromatography appeared to be a powerful technique to
obtain the OPV molecules in a very pure form. X-ray analysis and polarization optical microscopy revealed
that the OPV molecule is plastic crystalline at room temperature with an ordered columnar superstructure.
In apolar solvents, the molecules self-assemble via a highly cooperative fashion into right-handed chiral
superstructures, which are stable even at high temperatures and low concentration. Atomic force microscopy
revealed right-handed fibers with a diameter of 6 nm, indicating π-stacked aggregates; on a silicon oxide
substrate, supercoiled chiral structures were observed. STM studies on a liquid-solid interface showed
that the star-shaped OPV molecule forms an organized monolayer having a chiral hexagonal lattice.
wires.5 Two important issues concern the purity of the organic
building blocks and the stability of the noncovalent interactions.
Introduction
The construction of supramolecular assemblies of π-conju-
gated systems in the 5-100 nm length scale offers an attractive
bottom-up strategy to construct semiconducting wires in the
nanometer range.1 By using supramolecular chemistry, such
nanowires can be created from almost any polymeric and
oligomeric π-conjugated system. In some cases, these nanowires
could be placed between electrodes and a current was measured
for doped single wires and bundles of wires based on polymers
and oligomers.2,3 However, there are a large number of targets
that should be reached before a single nanowire can serve as
an attractive alternative for carbon nanotubes4 and inorganic
Impurities will become important in single wires because they
can act as traps for holes and electrons and thereby hamper the
charge-carrier mobility properties. Noncovalent interactions are
often temperature- and solvent-sensitive, and one of the chal-
lenges is to construct stable, robust self-assembled nano-objects.
In the past, we studied a variety of π-conjugated oligo(p-
phenylenevinylene) (OPV) derivatives, including H-bonded
hexamers, which self-assemble into fibers by π-π interactions.6
In order to enhance the stability of these assemblies, we
(2) For examples on oligomers, see: (a) Yamamoto, Y.; Fukushima, T.; Suna,
Y.; Ishii, N.; Saeki, A.; Seki, S.; Tagawa, S.; Taniguchi, M.; Kawai, T.;
Aida, T. Science 2006, 314, 1761-1764. (b) Yamamoto, Y.; Fukushima,
T.; Jin, W.; Kosaka, A.; Hara, T.; Nakamura, T.; Saeki, A.; Seki, S.;
Tagawa, S.; Aida, T. AdV. Mater. 2006, 18, 1297-1300. (c) Xiao, S.; Tang,
J.; Beetz, T.; Guo, X.; Tremblay, N.; Siegrist, T.; Zhu, Y.; Steigerwald,
M.; Nuckolls, C. J. Am. Chem. Soc. 2006, 128, 10700-10701. (d)
Shklyarevskiy, I. O.; Jonkheijm, P.; Stutzmann, N.; Wasserberg, D.;
Wondergem, H. J.; Christianen, P. C. M.; Schenning, A. P. H. J.; de Leeuw,
D. M.; Tomovic´, Zˇ.; Wu, J.; Mu¨llen, K.; Maan, J. C. J. Am. Chem. Soc.
2005, 127, 16233-16237. (e) Jonkheijm, P.; Stutzmann, N.; Chen, Z.; de
Leeuw, D. M.; Meijer, E. W.; Schenning, A. P. H. J.; Wu¨rthner, F. J. Am.
Chem. Soc. 2006, 128, 9535-9540.
(3) For examples on polymers, see: (a) Merlo, J. A.; Frisbie, C. D. J. Poly.
Sci. B 2003, 41, 2674-2680. (b) Mas-Torrent, M.; den Boer, D.; Durkut,
M.; Hadley, P.; Schenning, A. P. H. J. Nanotechnology 2004, 15, S265-
S269. (c) Akagi, K.; Piao, G.; Kaneko, S.; Sakamaki, K.; Sirakawa, H.;
Kyotani, M. Science 1998, 282, 1683-1686.
(4) For a special issue on carbon nanotubes, see: Acc. Chem. Res. 2002, 36,
Issue 12.
* To whom correspondence should be addressed. E-mail: Steven.
DeFeyter@chem.kuleuven.be; e.w.meijer@tue.nl; a.p.h.j.schenning@tue.nl.
† Dedicated to Professor Klaus Mu¨llen on the occasion of his 60th
birthday.
‡ Eindhoven University of Technology.
§ Katholieke Universiteit Leuven.
| Max Planck Institute for Polymer Research.
Present address: Elastogran, BASF group, Global PU Specialties
Research, E-KUE/KFH - A10, Elastogranstraâe 60, 49448 Lemfo¨rde,
Germany.
# Present address: Degussa GmbH, Process Technology & Engineering,
Process Technology - New Processes, Rodenbacher Chaussee 4, D-63457
Hanau-Wolfgang, Germany.
@ Also at Service de Chimie des Mate´riaux Nouveaux, Universite´ de
Mons-Hainaut, Place du Parc, 20, B-7000 Mons, Belgium.
(1) For reviews: (a) Watson, M. D.; Fechtenko¨tter, A.; Mu¨llen, K. Chem. ReV.
2001, 101, 1267-1300. (b) Grimsdale, A. C.; Mu¨llen, K. Angew. Chem.,
Int. Ed. 2005, 44, 5592-5629. (c) Schenning, A. P. H. J.; Meijer, E. W.
Chem. Comm. 2005, 3245-3258. (d) Hoeben, F. J. M.; Jonkheijm, P.;
Meijer, E. W.; Schenning, A. P. H. J. Chem. ReV. 2005, 105, 1491-1546.
(e) Wu, J.; Pisula, W.; Mu¨llen, K. Chem. ReV. 2007, 107, 718-747.
(5) For a review on inorganic wires, see: Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.;
Mayers, B.; Gates, B.; Yin, Y., Kim, F.; Yan. H. AdV. Mater. 2003, 15,
353-389.
(6) (a) Ajayaghosh, A.; George, S, J.; Schenning, A. P. H. J. Top. Curr. Chem.
2005, 258, 83-118. (b) Schenning, A. P. H. J., et al. Synth. Met. 2004,
147, 43-48.
9
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J. AM. CHEM. SOC. 2007, 129, 16190-16196
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Star-Shaped OPV-Substituted Hexaarylbenzene
A R T I C L E S
synthesized covalently linked OPV segments via amide linkages
to yield C3-symmetrical discs7 and showed that for the self-
assembly of such disks, special demands must be taken into
account to balance the topology, directionality and strength of
multiple hydrogen bond and π-π interactions. We now report
on a star-shaped OPV-substituted hexaarylbenzene in which the
number of OPV segments is increased to six, the hydrogen bond
interactions are omitted, and the propeller-like structure results
in more stable stacks.8 Recycling gel permeation chromatog-
raphy appeared to be an appropriate technique to obtain the
π-conjugated OPV hexamer in a very pure form. Hexa-
substituted chromophoric benzenes have been reported in
literature;9 however, the self-assembly properties have been
scarcely addressed.
Results and Discussions
Synthesis. Star-shaped OPV-substituted hexaarylbenzene 3
was synthesized from bromo-substituted OPV derivative 1,
which was prepared according to a literature procedure (Figure
1).10 Disubstituted acetylene 2 was isolated in 54% yield, after
Stille-type coupling of 1 with commercially available bis-
(tributylstannyl)acetylene in the presence of catalytic Pd(PPh3)4.
Subsequent cyclotrimerization of 2 was carried out using Co2-
(CO)8 as catalyst in refluxing dioxane for 7 h, according to a
literature procedure.9a The crude product was subjected to
extensive column chromatography and gel permeation chroma-
tography (GPC). After these purification steps, thin layer
chromatography (TLC) and 1H NMR did not show any
impurities.11 Mass spectrometry, however, apart from the
molecular mass of 3 (m/z ) 7757), showed two other mass peaks
positioned at m/z ) 5203 and m/z ) 2619, which could originate
from impurities or due to fragmentation during the mass analysis
(Figure 2a). In order to analyze the purity of 3 in more detail,
we decided to use recycling gel permeation chromatography
(rGPC) on a preparative size-exclusion column. In the first cycle,
Figure 1. Synthesis of star-shaped OPV (3): (a) bis-(tributylstannyl)-
acetylene, Pd(PPh3)4, toluene, 120 °C, 24 h, 54%; (b) Co2(CO)8, dioxane,
125 °C, 7 h, 57%.
(7) van Herrikhuyzen, J.; Jonkheijm, P.; Schenning A. P. H. J.; Meijer, E. W.
Org. Biomol. Chem. 2006, 4, 1539-1545.
(8) For recent reviews on two and three-dimensional organic semiconductors,
see: (a) Roncali, J.; Leriche, P.; Cravino, A. AdV. Mater. 2007, 19, 2045-
2060. (b) Opsitnick, E.; Lee, D. Chem. Eur. J. 2007, 13, 7040-7049.
(9) For example, see: (a) Geng, Y.; Fechenkotter, A.; Mu¨llen, K. J. Mater.
Chem. 2001, 11, 1634-1641. (b) Wu, J.; Watson, M. D.; Zhang, L.; Wang,
Z. Mu¨llen, K. J. Am. Chem. Soc. 2004, 126, 177-186. (c) Takase, M.;
Ismael, R.; Murakami, R.; Ikeda, M.; Kim, D.; Shinmori, H.; Furata, H.;
Osuka, A. Tetrahedron Lett. 2002, 43, 5157-5159. (d) Cho, H. S.; Rhee,
H.; Song, J. K., Min, C.-K.; Takase, M. Aratani, N.; Cho, S.; Osuka, A.;
Joo, T.; Kim, D. J. Am. Chem. Soc. 2003, 125, 5849-5860. (e) Lidell, P.
A.; Kodis, G.; de la Garza, L.; Moore, A. L.; Moore, T. A.; Gust, D. J.
Phys. Chem. B 2004, 108, 10256-10265. (f) Biemans, H. A. M.; Rowan,
A. E.; Verhoeven, A., Vanoppen, P.; Latterini, L.; Foekema, J.; Schenning,
A. P. H. J.; Meijer, E. W.; de Schrijver, F. C.; Nolte, R. J. M. J. Am.
Chem. Soc. 1998, 120, 11054-11060. (g) Fiqueira-Duarte, T. M.; Clifford,
J.; Amandalo, V.; Ge´gout, A.; Olivier, J.; Cardinali, F.; Meneghetti, M.;
Armaroli, N.; Nierengarten, J.-F. Chem. Comm. 2006, 2054-2056. (h)
Rausch, D.; Lambert, C. Org. Lett. 2006, 8, 5037-5040. (i) Chebny, V.
J.; Dhar, D.; Lindeman, S. V.; Rathore, R. Org. Lett. 2006, 8, 5041-5044.
(j) Keegstra, M. A.; De Feyter, S.; De Schryver, F. C.; Mu¨llen, K. Angew.
Chem., Int. Ed. 1996, 35, 774-776. (k) Hasegawa, M.; Enozawa, H.;
Kawabata, Y.; Iyoda, M. J. Am. Chem. Soc. 2007, 129, 3072-3073. (l)
Hahn, U.; Maisonhaute, E.; Amatore, C.; Nierengarten, J.-F. Angew. Chem.,
Int. Ed. 2007, 46, 951-954. (m) Zhi, L.; Wu, J.; Mu¨llen, K. Org. Lett.
2005, 7, 5761-5764. (n) Ito, S.; Inabe, H.; Morita, N.; Ohta, K.; Kitamura,
T.; Imafuku, K. J. Am. Chem. Soc. 2003, 125, 1669-1680. (o) Ito, S.;
Ando, M.; Nomura, A.; Morita, N.; Kabuto, C.; Mukai, H.; Ohta, K.;
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3949. (p) Bottari G.; Torres T. Chem. Comm. 2004, 2668-2669. (q) Takase,
M.; Nakajima, A.; Takeuchi, T. Tetrahedron Lett. 2005, 46, 1739-1742.
(r) Maley, K. E.; Gagnon, E.; Maris, T.; Wuest, J. D. J. Am. Chem. Soc.
2007, 129, 4306-4322.
only one peak was observed (Figure 2b); however, after three
cycles, another peak appeared, indicating the presence of an
impurity. After rGPC, star-shaped OPV 3 could be isolated in
57% yield, as a pure, soft, waxy material, soluble in common
organic solvents (such as chloroform, dichloromethane, toluene,
THF, hexane), and was characterized by MALDI-TOF mass
spectrometry, IR spectroscopy, and 1H and 13C NMR spectros-
copy.11 During the purification, the main impurity could be
isolated as well. The MALDI-TOF spectrum showed a mass
peak at m/z ) 5203, whereas infrared spectroscopy revealed a
carbonyl vibration at 1707 cm-1, pointing to a cyclopentadi-
enone structure having four OPV units. This assignment is
supported by scanning tunneling microscope (STM) images.11
Cyclopentadienone has been reported as a side product in the
trimerization of bulky alkynes in which insertion of carbon
monoxide occurs instead of the insertion of the third alkyne.12
Thermotropic Properties. The bulk properties of star shaped
OPV molecule 3 were investigated by thermogravimetric
analysis (TGA), differential scanning calorimetry (DSC), polar-
ized optical microscopy (POM), and wide-angle X-ray scattering
(10) Schenning, A. P. H. J.; Fransen, M.; van Duren, J. K. J.; van Hal, P. A.;
Janssen, R. A. J.; Meijer, E. W. Macromol. Rapid Commun. 2002, 23, 271-
275.
(12) (a) Vollhardt, K. P. C. Angew. Chem., Int. Ed. 1984, 23, 539-556. (b)
Shibata, T.; Yamashita, K.; Takagi, K.; Ohta, T.; Soai, K. Tetrahedron
2000, 56, 9259-9267.
(11) See supporting information.
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Tomovic´ et al.
Figure 3. Images from polarized optical microscopy of the star-shaped
OPV (3) obtained by cooling from the isotropic phase at rates of (a)
10 °C/min, (b) 2 °C/min, and (c,d) 0.2 °C/min.
crystalline phase, indicating a high degree of order, as later
specified by X-ray scattering results. Alternatively, due to the
waxy and soft consistency of the compound, the phase was
assigned as plastic crystalline.
For the 2D-WAXS experiments, mechanically aligned samples
were prepared by filament extrusion.14 In agreement with the
DSC analysis, no change in the 2D-WAXS pattern was observed
up to the isotropic phase, and a characteristic 2D-WAXS pattern
of 3 is shown in Figure 4a. The distinct and sharp reflections
in the equatorial plane of the pattern indicated the formation of
columnar superstructures, which were well-oriented along the
shearing direction, and thus along the extruded filament.
Furthermore, the large number of higher-order reflections was
typical for a pronounced degree of columnar order,11 whereby
the analysis of the positions of the scattering intensities revealed
a two-dimensional lateral hexagonal unit cell describing the
intercolumnar arrangement with a packing parameter of 5.47
nm, which was in agreement with the molecular architecture.
The meridional reflections were assigned to the molecular pack-
ing within the columnar stacks. The off-meridional scattering
intensities were characteristic for a tilting of the single OPV
building blocks toward the columnar axis.15 As schematically
illustrated in Figure 4b,c, the star-shaped OPV stack on top of
each other, whereby each of the single OPVs is rotated out of
the molecular plane, giving rise to the off-meridional reflec-
tions, correlated to a relatively long π-stacking distance of 0.45
nm and a tilting angle of ∼35°. Due to this tilting, an intra-
columnar period of 0.55 nm was determined. The 2D-WAXS
results indicate a self-organization of the star-shaped OPV
molecules into highly ordered columnar superstructures, which
are oriented along the alignment direction during extrusion.
Self-Assembly in Solution. The absorption spectrum of 3 in
chloroform shows an absorption maximum at λmax ) 436 nm,
which is typical for molecular dissolved OPVs (Figure 5).10,11
Fluorescence measurements reveal an emission maximum at λem
) 501 nm with a shoulder at λem ) 537 nm.
Figure 2. (a) MALDI-TOF mass spectrum of 3 before purification by
rGPC. (b) rGPC trace of impure 3 (9 cycles). (c) MALDI-TOF mass
spectrum of 3 after purification (calculated average mass of 3: 7758).
(WAXS). TGA experiments showed a stability of 3 up to
300 °C. DSC revealed an exothermic peak at 260 °C (∆H )
76.8 kJ/mol), which we ascribe to the isotropization temperature,
while no other thermal transitions were observed between -100
and 260 °C.11 POM micrographs of 3 by cooling the sample
from the melt exhibited fan-shaped domains of which the size
increased with decreasing the cooling rate (Figure 3). The
domain sizes exceeded several hundred micrometers, consisting
of fibers oriented along the solidification direction, especially
at low cooling rates. This morphology exemplifies the pro-
nounced tendency of the star-OPV molecules to self-assemble
in well-ordered macroscopic structures (vide infra).13 The fan-
shaped highly birefringent textures are characteristic for a
(13) Pisula, W.; Dierschke, F.; Mu¨llen, K. J. Mater. Chem. 2006, 16, 4058-
4064.
(14) Pisula, W.; Tomovic´, Zˇ.; Simpson, C.; Kastler, M.; Pakula, T.; Mu¨llen, K.
Chem. Mater. 2005, 17, 4296-4303.
(15) Pisula, W.; Kastler, M.; Wasserfallen, D.; Mondeshki, M.; Piris, J.; Schnell,
I.; Mu¨llen, K. Chem. Mater. 2006, 18, 3634-3640.
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Star-Shaped OPV-Substituted Hexaarylbenzene
A R T I C L E S
Figure 4. (a) 2D-WAXS pattern recorded at 30 °C of 3 prepared as an extruded filament, (b) schematic representation of 3, and (c) schematic illustration
of the helical columnar packing based on X-ray, CD and AFM data.
The self-assembly of star-shaped OPV 3 was first studied in
heptane solution. The UV-vis absorption spectrum of 3 (1 ×
10-6 M) shows a blue-shifted (relative to chloroform) absorption
band at 422 nm with a red-shifted onset, whereas the fluores-
cence spectrum reveals a quenched red-shifted maximum at λem
) 550 nm with two shoulders at λem ) 514 nm and λem ) 590
nm, indicating aggregated OPVs (Figure 5a). Circular dichroism
measurements show a bisignated CD effect with a positive sign
at long wavelengths and a negative sign at short wavelengths
with a zero-crossing at 421 nm, indicating that the chiral packing
of the S-2-methylbutoxy side chains causes a helical packing
of the OPV segment, suggesting right-handed helical aggregates
(Figure 5b). Temperature-dependent CD measurements showed
no transitions, indicating extremely stable chiral aggregates.
Even at a concentration of 2 × 10-7 M and 90 °C, the chiral
aggregates could not be broken. This behavior is a remarkable
increase in stability with respect to our previously reported
hydrogen-bonded hexamers having similar OPV segments that
form stacks in heptane solution that can be melted at 25 °C at
a concentration of 5 × 10-5 M.16
the CD-effect disappeared, whereas the fluorescence shifted
toward a spectrum similar to the one recorded in chloroform
with a maximum at λem ) 491 nm and shoulder at λem 522 nm,
typically for molecularly dissolved 3. Temperature-dependent
CD measurements, made by monitoring the appearance of the
Cotton effect at 390 nm, reveal a highly cooperative self-
assembly process; the melting curve is not sigmoidal, and a
steep rise in the CD signal occurs at the elongation temperature
at which the self-assembly starts (Figure 5f).17 Temperature-
dependent UV spectroscopic measurements by following the
absorption intensity at 500 nm yield the same elongation
temperature as that obtained by CD, suggesting that helical
aggregates are formed without an intermediate non-helical
state.11 The CD melting curve was fitted with the nucleation-
growth model to characterize the cooperative self-assembly in
more detail.17 An enthalpy of release of he ) -184 kJ/mol was
determined while the elongation temperature (Te) ) 331 K
(Figure 5, parts g and h). The enthalpy value is higher than
that found previously for stacks composed of hydrogen-bonded
OPV dimers, which could be due to the higher number of OPV
segments in compound 3. The high degree of cooperativity is
reflected in the small value of the equilibrium constant Ka of
the nucleation step, which was determined to be 3 × 10-5. The
stacks at Te, which are an indication of the size of the nucleus,
contain 32 molecules, on average, whereas stacks at room-
temperature enclose more than 10 000 molecules. The latter
value would imply that, by assuming an intracolumnar distance
of 0.55 nm (2D WAXS data), the stacks are micrometers long.
Self-Assembly at Surfaces. The solutions, in which the star-
shaped OPV (3) is self-assembled, were further studied in the
solid state by scanning force microscopy operated in Tapping
mode. A heptane solution of 3 (1.1 × 10-5 M) was deposited
on a silicon wafer, and the sample was allowed to dry for 24 h
in a saturated atmosphere of heptane. Typical Tapping mode
images show long fibrils (few micrometers, Figure 6, parts a
We also studied the self-assembly of 3 in methylcyclohexane
(MCH). The UV-vis absorption spectrum of 3 (2.5 × 10-6
M) shows a structureless band at 424 nm, with a shoulder at
higher wavelengths (Figure 5c), whereas in CD, a bisignated
CD effect was observed having a positive sign at long
wavelengths and a negative sign at short wavelengths (Figure
5e), indicating helical aggregated OPVs. The fluorescence
spectrum reveals a similar behavior as found in heptane, a
quenched red-shifted maximum at λem ) 550 nm with two
shoulders at λem ) 514 nm and λem ) 590 nm. Concentration-
dependent PL measurements show that 3 is still aggregated at
room temperature at a concentration of 10-9 M. Temperature-
dependent UV-vis, CD, and PL measurements showed a
transition from aggregated to molecularly dissolved species upon
heating (Figure 5c-e). The vibronic shoulder in UV-vis and
(16) Jonkheijm, P.; Miura, A.; Zdanowska, M.; Hoeben, F. J. M.; De Feyter,
S.; Schenning, A. P. H. J.; De Schryver, F. C.; Meijer, E. W. Angew. Chem.,
Int. Ed. 2004, 43, 74-78.
(17) (a) Jonkheijm, P.; Schoot, V. D. P.; Schenning, A. P. H. J.; Meijer, E. W.
Science 2006, 313, 80-83. (b) Smulders, M. M. J.; Schenning, A. P. H.
J.; Meijer, E. W. J. Am. Chem. Soc., accepted.
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Tomovic´ et al.
Figure 5. (a) Normalized absorption and fluorescence spectra of 3 in chloroform and heptane (1.1 × 10-6 M), (b) CD spectra of 3 in chloroform (5.0 ×
10-5 M) and heptane (5.4 × 10-5 M), (c-e) Temperature-dependent UV-vis (c) PL (d) and CD (e) spectra of 3 in MCH (2.5 × 10-6 M) (from 90 to 0 °C
with intervals of 10 °C, arrow indicates the changes upon cooling), (f) cooling curve of 3 in MCH (2.5 × 10-6 M) obtained by monitoring CD signal at 390
nm (dT/dt ) -12 °C/hr), (g) fit of the normalized melting curve using the nucleation-growth model, (h) calculation of the number of molecules in the stacks
using the nucleation-growth model.
and b) with a constant diameter of 6 nm. Remarkably, at places
where the fibrils are densely packed, they leave the surface and
form a supercoiled organization with a right-handed helix. This
phenomena was previously observed for chiral oligo(th-
iophene)s,18 but in the present case, there is less side-by-side
interactions leading to the supramolecular chiral nanoribbons.
When a more diluted sample is used (1.1 × 10-6 M), the fibrils
are shorter (about 100 nm, Figure 6c) and have right-handed
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Star-Shaped OPV-Substituted Hexaarylbenzene
A R T I C L E S
range. The supramolecular order in this kind of assembly
systems is probably too weak to contribute to long-range
correlations of ∼10 nm between single molecules along the
stacking direction.
When the heptane solution of 3 (1.1 × 10-5 M) is deposited
on a highly oriented pyrolytic graphite (HOPG) substrate, the
observed morphology is once again fibrillar (Figure 6d). The
diameter is about 6 nm showing that the molecules are
π-stacked. Compared to the silicon surface, the fibrils are shorter
and tend to orient along a 3-fold symmetry of HOPG. The
chirality of individual fibrils is difficult to observe by AFM
even when the sample solution is diluted.
Self-Assembly at the Liquid-Solid Interface. The self-
assembly properties of 3 were also probed at the liquid (liquid
1-phenyloctane-solid HOPG substrate interface by scanning
tunneling microscopy (STM).19,20 Typical STM images (Figure
7) show six-armed stars: the tunneling efficiency through the
OPV units is higher compared to the alkyl chains,21 and
therefore, the former ones appear bright. In some images,
submolecular features appear along the rods that we tentatively
assign to the phenyl rings of the OPV arms. The alkyl chains
can under favorable measuring conditions be discriminated as
faint gray lines (Figure 7a), which run parallel to one of the
main symmetry axes of graphite, a phenomenon which is typical
for alkyl chains.19,22 In addition, the STM images show that 3
assembles into a hexagonal lattice. The unit cell vectors a and
b are identical in length (5.56 ( 0.07 nm) and the angle between
both unit cell vectors measures 61 ( 2° (Figure 7c). The STM
images are two-dimensionally chiral,23-26 and the molecules
appear as stars, which belong to the crystallographic plane point
group 6mm (the combination of a mirror with a hexad).
Neglecting the intrinsic chirality27 of the molecules, these stars
can only order into an achiral pattern if their “arms” are oriented
parallel or perpendicular to the unit cell vectors. This is,
however, not the case, and this has an effect on the relative
distance between parallel oriented OPV-units of adjacent
molecules. Consider, for instance, the orientation of the longer
dashed and shorter solid white lines that connect the terminal
Figure 6. Tapping mode phase images of 3 deposited from heptane
solutions on silicon wafer (a) 1.1 × 10-5 M (scan size ) 500 nm); (b) 1.1
× 10-5 M (scan size ) 200 nm); (c) 1.1 × 10-6 M (scan size ) 250 nm),
inset: enlargement of one of the fibers (image size ) 30 nm); (d) Tapping
mode phase images of 3 deposited from heptane solutions on HOPG (1.1
× 10-5 M, scan size ) 1.0 µm).
helical structure. The pitch of these chiral fibrils is about 10
nm (Figure 6c) and the handedness is in agreement with the
that found in solution (vide supra). Taking into account an
intracolumnar period of 0.55 nm between individual molecules
along the columnar structures, as determined by 2D-WAXS,
this indicates that ∼18 building blocks within one helical pitch.
This leads to a lateral rotation angle of ∼3.3° between molecules
which is rather small (Figure 4c). However, it was not possible
to confirm this helical arrangement by 2D small-angle X-ray
scattering measurements (SAXS) in the corresponding scattering
Figure 7. (a and b) Small scale STM images of 3 at the 1-phenyloctane-HOPG interface. The bright rods are the OPV-units. Alkyl chains can clearly be
identified as the gray stripes between the OPV-rods of adjacent molecules. Insert: the orientation of the main symmetry axes of graphite (<11 -2 0>). c)
Large scale image. The unit cell is indicated. The longer dashed and shorter solid white lines connect the terminal phenyl groups of similarly oriented OPV
units along unit cell vector b.
9
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A R T I C L E S
Tomovic´ et al.
phenyl groups of similarly oriented OPV units along unit cell
vector b in Figure 7c. Note that these dashed and solid marker
lines are not in line. Their relative orientation can be described
as dashed-up and solid-down: 3 self-assembles into a chiral
pattern in accordance with the plane group p6. Mirror plane
images are never observed for this molecule,11 as expected for
an enantiopure molecule.28
In contrast to other systems, the chirality at the level of the
monolayer is not or only very weakly expressed in terms of the
monolayer orientation with respect to the substrate. The alkyl
chains run parallel to the main symmetry axes of graphite (inset
in Figure 7a), and also the unit cell vectors run within error
parallel to the main symmetry axes of graphite. It is interesting
to note that the surface pattern formed by this covalent star
shows many similarities with our previous reported hydrogen-
bonded hexamers having the similar OPV segments that further
organize into a hexagonal lattice (the plane group is p6, Figure
S15 of the Supporting Information).11,16,29 In addition, the same
chirality aspects also apply to the surface organization of this
OPV-oligomer which carries the same (S)-2-methylbutoxy
substituents.
purified by recycling GPC. This separation method can serve
as a very helpful technique to purify π-conjugated molecules
to a high extent. The star-shaped molecule is a plastic crystalline
at room temperature, in which highly ordered columnar
superstructures are formed. These properties make these mol-
ecules attractive to apply in electronic devices in which the
mesoscopic order can possibly be aligned.30 Interestingly, in
apolar solution, chiral fibers are formed which still do not
disassemble at 90 °C at a very low concentration. This shows
that the stability of π-stacked fibers can be greatly enhanced
by using a hexaarylbenzene scaffold in which the number of
π-conjugated segments is increased without using other non-
covalent interactions such as hydrogen bonding. The star-shaped
OPV molecule also forms organized monolayers at the liquid-
solid interface having a chiral hexagonal lattice. All of these
properties make the star-shaped oligo(p-phenylenevinylene)
(OPV)-substituted hexaarylbenzene potentially useful in the field
of supramolecular electronics.1
Acknowledgment. This work was supported by the Council
for Chemical Sciences of The Netherlands Organization for
Scientific Research (CW-NWO). We would like to thank to
Dr. Xianwen Lou for matrix-assisted laser desorption ionization
time-of-flight (MALDI-TOF) measurements and Maarten J.
Pouderoijen for the synthesis of the starting compound. S.D.F.
would like to thank the Fund for Scientific ResearchsFlanders
and the Belgian Federal Science Policy Office via IAP-6/27.
H.X. thanks the Marie Curie Research Training Network
CHEXTAN (MRTN-CT-2004-512161). Ph.L. is Chercheur
Qualifie´ of the Fonds National de la Recherche Scientifique
(FNRS-Belgium).
Conclusions
We have synthesized a star-shaped oligo(p-phenylenevi-
nylene) (OPV) substituted hexaarylbenzene which could be
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Schenning, A. P. H. J. J. Mater. Chem. 2004, 14, 1959-1963.
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Supporting Information Available: Experimental details,
synthetic procedures, characterization data and complete refer-
ence 6b. This material is available free of charge via the Internet
at http:// pubs.acs.org.
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(just a hexad) because of the chiral substituents.
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(28) De Feyer, S.; De Schryver, F. In Scanning Probe Microscopy beyond
Imaging; Samor´ı, P., Ed.; Wiley-VCH: New York, 2006, 3-36.
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