Suppressing aggregation in a large polycyclic aromatic hydrocarbon

Article abstract of DOI:10.1021/ja056782j

With the approach presented herein, a large aromatic π-system is synthesized, which shows extraordinarily high solubility and an effective suppression of aggregation. This was due to a distortion of the aromatic core by bulky tert-butyl groups and the sol

Full text of DOI:10.1021/ja056782j

Published on Web 01/05/2006
Suppressing Aggregation in a Large Polycyclic Aromatic
Hydrocarbon
†
†
†
‡
Daniel Wasserfallen, Marcel Kastler, Wojciech Pisula, Werner A. Hofer,
†
†
,†
Yulia Fogel, Zhaohui Wang, and Klaus M u¨ llen*
Contribution from the Max-Planck-Institute for Polymer Research, Ackermannweg 10, 55128
Mainz, Germany, and Hofer, Surface Science Research Centre, Department of Chemistry,
UniVersity of LiVerpool, LiVerpool L69 3BX
Received October 4, 2005
Abstract: With the approach presented herein, a large aromatic π-system is synthesized, which shows
extraordinarily high solubility and an effective suppression of aggregation. This was due to a distortion of
the aromatic core by bulky tert-butyl groups and the solubilizing effects of alkyl chains in the corona of the
aromatic core. Therefore not only the processing and cleaning of the materials with standard laboratory
1
techniques became possible, but moreover the first structure-rich UV/vis and a resolved H NMR spectra
for an aromatic system two times larger than hexa-peri-hexabenzocoronene were recorded. The bulk
properties in an extruded fiber as well as on the surface showed a columnar self-assembly including a
phase in which a homeotropic alignment on a substrate was observed, which turns the material into an
interesting candidate for future applications in electronic devices.
Introduction
to exhibit an improved self-ordering due to the extended π-area,
which is another key feature to yield high performances in
electronic devices.
Polycyclic aromatic hydrocarbons (PAHs) have proven their
1
potential as active components in field-effect transistors,
The main problem with increasing the aromatic core size,
however, is the much stronger π-stacking interaction, which
reduces solubility and limits analytical methods such as NMR
spectroscopy. Additionally, it has been shown for HBCs that a
reduced solubility also hampers the processing of such materials
2
3
photovoltaic devices, and light-emitting diodes. Alkyl substi-
tuted hexa-peri-hexabenzocoronenes (HBC) are known for their
4
unique self-organization into columnar superstructures and
exhibit one of the highest charge-carrier mobilities for mesogens.
As the charge-carrier mobility is in close relation to the size of
the aromatic core, larger π-areas are expected to lead to
improved mobilities.5 For photovoltaic applications a high
extinction coefficient over a broad range of the spectrum is a
prerequisite, which can also be achieved by enlarging the
6
into highly oriented films. Until now large PAHs were
synthesized by preparing oligophenylene precursors, which were
directly converted to the planarized products by an oxidative
cyclodehydrogenation step. By this method it is possible to
establish a remarkable number of intramolecular aryl-aryl
a
5
aromatic core component. Moreover such disks are assumed
7
bonds.
The solubilities of these compounds are based on the alkyl
chains in the corona of the molecule. Even for HBCs with
strongly solubilizing alkyl chains, concentrations of 10 mol/L
*
To whom correspondence should be addressed. Fax: (+49) (0)6131-
3
79100. E-mail: muellen@mpip-mainz.mpg.de.
†
Max-Planck-Institute for Polymer Research.
University of Liverpool.
-4
‡
(
1) (a) van de Craats, A. M.; Stutzmann, N.; Bunk, O.; Nielsen, M. M.; Watson,
M.; M u¨ llen, K.; Chanzy, H. D.; Sirringhaus, H.; Friend, R. H. AdV. Mater.
(5) (a) Van de Craats, A. M.; Warman, J. M. AdV. Mater. 2001, 13, 130-133.
(b) Lemaur, V.; da Silva Filho, D. A.; Coropceanu, V.; Lehmann, M.;
Geerts, Y.; Piris, J.; Debije, M. G.; van de Craats, A. M.; Senthilkumar,
L.; Siebbeles, L. D. A.; Warman, J. M.; Br e´ das, J. L.; Cornil, J. J. Am.
Chem. Soc. 2004, 126, 3271-3279. (c) Debije, M. G.; Piris, J.; de Haas,
M. P.; Warman, J. M.; Tomovic, H.; Simpson, C. D.; Watson, M. D.;
M u¨ llen, K. J. Am. Chem. Soc. 2004, 126, 4641-4645.
2
003, 15, 495-499. (b) Dimitrakopoulos, C. D.; Brown, A. R.; Pomp, A.
J. Appl. Phys. 1996, 80, 2501-2508. (c) Dimitrakopoulos, C. D.; Malenfant,
P. R. L. AdV. Mater. 2002, 14, 99-116. (d) Horowitz, G. AdV. Mater.
1
998, 10, 365-377. (e) Roberson, L. B.; Kowalik, J.; Tolbert, L. M.; Kloc,
C.; Zeis, R.; Chi, X.; Fleming, R.; Wilkins, C. J. Am Chem. Soc. 2005,
27, 3069-3075.
1
(
2) (a) Schmidt-Mende, L.; Fechtenk o¨ tter, A.; M u¨ llen, K.; Moons, E.; Friend,
R. H.; MacKenzie, J. D. Science, 2001, 293, 1119-1122. (b) Adam, D.;
Schuhmacher, P.; Simmerer, J.; H a¨ ussling, L.; Siemensmeyer, K.; Etzbach,
K. H.; Ringsdorf, H.; Haarer, D. Nature 1994, 371, 141-143. (c) Brabec,
C. J.; Sariciftci, N. S.; Hummelen, J. C. AdV. Funct. Mater. 2001, 11, 15-
(6) (a) Liu, C. Y.; Bard, A. J. Chem. Mater. 2000, 12, 2353-2362. (b) Perahia,
D.; Traiphol, R.; Bunz, U. H. F. Macromolecules 2001, 34, 151-155. (c)
Kastler, M.; Pisula, W.; Wasserfallen, D.; Pakula, T.; M u¨ llen, K. J. Am.
Chem. Soc. 2005, 127, 4286-4296. (d) Tracz, A.; Jeszka, J. K.; Watson,
M. D.; Pisula, W.; M u¨ llen, K.; Pakula, T. J. Am. Chem. Soc. 2003, 125,
1682-1683. (e) Piris, J.; Debije, M. G.; Stutzmann, N.; Laursen, B. W.;
Pisula, W.; Watson, M. D.; Bjornholm, T.; M u¨ llen, K.; Warman, J. M.
AdV. Funct. Mater. 2004, 14, 1053-1061. (f) Pisula, W.; Tomovic, Z.;
Stepputat, M.; Kolb, U.; Pakula, T.; M u¨ llen, K. Chem. Mater. 2005, 17,
2641-2647. (g) Breiby, D. W.; Bunk, O.; Pisula, W.; Sølling, T. I.; Tracz,
A.; Pakula, T.; M u¨ llen, K.; Nielsen, M. M. J. Am. Chem. Soc. 2005, 127,
11288-11293.
2
6. (d) Boden, N.; Bushby, R. J.; Clements, J.; Movaghar, B.; Donovan,
K. J.; Kreouzis, T. Phys. ReV. B 1995, 52, 13274-13280.
(
3) De Halleux, V.; Calbert, J. P.; Brocorens, P.; Cornil, J.; Declercq, J. P.;
Br e´ das, J. L.; Geerts, Y. AdV. Funct. Mater. 2004, 14, 649-659.
4) (a) Herwig, P.; Kayser, C. W.; M u¨ llen, K.; Spiess, H. W. AdV. Mater. 1996,
(
6
, 510-513. (b) Fechtenk o¨ tter, A.; Saalw a¨ chter, K.; Harbison, M. A.;
M u¨ llen, K.; Spiess, H. W. Angew. Chem., Int. Ed. Engl. 1999, 38, 3039-
042. (c) Pisula, W.; Tomovic, Z.; Simpson, C.; Kastler, M.; Pakula, T.;
M u¨ llen, K. Chem. Mater. 2005, 17, 4296-4303.
3
(7) Simpson, C. D.; Brand, J. D.; Berresheim, A. J.; Przybilla, L.; R a¨ der, H.
J.; M u¨ llen, K. Chem.-Eur. J. 2002, 8, 1424-1429.
1
334 J. AM. CHEM. SOC. 2006, 128, 1334-1339
9
10.1021/ja056782j CCC: $33.50 © 2006 American Chemical Society

Suppressing Aggregation in a Large PAH
A R T I C L E S
9
Figure 1. (A) Published synthesis toward “supernaphthalene” (2) where i ) FeCl3, CH3NO2, CH2Cl2; (B) (a) simulated mass distribution of 2, MALDI-
TOF spectra of same material with different matrixes; (b) dithranol; (c) TCNQ; (C) PL excitation and UV/vis spectrum of 2.
6
are needed until they appear as monomers in solution, and for
introduce effective solubilizing features into this type of PAH
to extend the range of possible analytical methods and to
additionally enhance the material properties such as columnar
self-assembly, film formation, and thermal phase behavior.
Since it is known that a distortion of the aromatic skeletal
structure will induce a higher solubility due to suppression of
8
larger PAHs, the aggregation is exceedingly stronger. Only by
a sufficient suppression of the π-stacking it can be expected to
acquire resolved NMR spectra and a good processability of
PAHs of that size. These issues and additional difficulties have
been encountered while revisiting the synthesis of a PAH
consisting of 72 aromatic carbon atoms with an enlarged
11
association, two bulky tert-butyl group were introduced in the
indentation (Scheme 1, red area) leading to the required
distortion. They are also known to hinder the aromatic cores
9
naphthalene-like shape, called “supernaphthalene”.
Results and Discussion
6
from approaching each other. Second, dodecyl chains were
The cyclodehydrogenation reaction of 1 toward 2 (Figure 1A)
showed a poor reproducibility, and the prepared batches differed
severely in their macroscopic behavior (phase transitions,
solubility, UV/vis spectra). This can be explained by the
presence of different amounts of side products, such as partially
fused species, where not all of the bonds have been established.
Furthermore, a methodological problem was determined for
matrix-assisted laser desorption-ionization time-of-flight (MAL-
DI-TOF), which has so far been used as the main characteriza-
tion technique for such PAHs.10 In the case of 2, the choice of
matrix proved to be crucial for the resulting spectra (Figure 1B).
While for 7,7,8,8-tetracyanoquinodimethane (TCNQ) only the
mass peak of the product was observed, dithranol provided
solely the signals of the side products. Another indication for
the presence of these partially fused byproducts was the strong
discrepancy between the photoluminescence (PL) excitation and
the UV/vis absorption spectra (Figure 1C).
introduced in the corona of the molecule together with four
2-decyl-tetradecyl chains to further increase the solubility due
to the suppressed chain crystallization.
1
2
The synthesis of 8 started with a 4-fold Knoevennagel-
condensation from 2,7-di-tert-butyl-pyrene-4,5,9,10-tetraone
13
(3) and 1,3-bis(4-dodecylphenyl)-propan-2-one (4) to yield the
thermally unstable biscyclopentadienone-derivative 5 (Scheme
1), which was directly converted in a microwave-assisted Diels-
Alder cycloaddition with the alkylated diphenylacetylene (6)
to yield the precursor 7. Compound 7 was submitted to a highly
reproducible oxidative cyclodehydrogenation using FeCl to
3
afford the strained PAH 8 in 85% yield. After removal of the
tert-butyl groups using oleum (65%) and benzene, one obtained
compound 9. Interestingly the tert-butyl groups could not be
removed from the precursor 7 although the same harsh condi-
tions were applied. For the cyclodehydrogenated compound 8,
the relaxation of the twisted aromatic core into the planar
product 9 seems to be a key driving force for the cleavage of
the substituents. The resulting material 9 showed very limited
All these arguments led on one hand to the necessity to
develop a more reliable synthetic route and on the other to
(
8) (a) Hunter, C. A.; Sanders, J. K. M. J. Am. Chem. Soc. 1990, 112, 5525-
(11) (a) Seybold, G.; Wagenblast, G. Dyes Pigm. 1989, 11, 303-317. (b) Scott,
L. T.; Cheng, P. C.; Hashemi, M. M.; Bratcher, M. S.; Meyer, D. T.;
Warren, H. B. J. Am. Chem. Soc. 1997, 119, 10963-10968. (c) Hagen, S.;
Bratcher, M. S.; Erickson, M. S.; Zimmermann, G.; Scott, L. T. Angew.
Chem., Int. Ed. Engl. 1997, 36, 406-408.
(12) Wegner, G. Macromol. Chem. Phys. 2003, 204, 347-357.
(13) Yamato, T.; Fujimoto, M.; Miyazawa, A.; Matsuo, K. J. Chem. Soc., Perkin
Trans. 1 1997, 1201-1207 (modified procedure).
5
534. (b) Van Nostrum, C. F.; Nolte, R. J. M. Chem. Commun. 1996, 2385-
2
392. (c) Brunsveld, L.; Folmer, B. J. B.; Meijer, E. W.; Sijbesma, R. P.
Chem. ReV. 2001, 101, 4071-4097.
(9) M u¨ ller, M.; K u¨ bel, C.; Morgenroth, F.; Iyer, V. S.; M u¨ llen, K. Carbon
1
998, 36, 827-831.
(
10) Przybilla, L.; Brand, J. D.; Yoshimura, K.; R a¨ der, H. J.; M u¨ llen, K. Anal.
Chem. 2000, 72, 4591-4597.
J. AM. CHEM. SOC.
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A R T I C L E S
Wasserfallen et al.
Scheme 1. Novel Synthesis of “Supernaphthalene”a
a
Reagents and conditions: (i) KOH, MeOH, EtOH; 48%; (ii) Ph2O, 21%; (iii) FeCl3, CH2Cl2, CH3NO2, 85%; (iv) benzene, oleum (65%), 95%.
1
Figure 2. (A) MALDI-TOF of 8; (B) H NMR of 8 in CD2Cl2/CS2. Protons were assigned using HH correlation spectroscopy and nuclear Overhauser
enhancement spectroscopy.
solubility and therefore contradicted the observations made for
the good soluble compound 2, supporting the possible existence
of side products, which interfere with the π-interactions and
induce the observed solubility for 2.
In the MALDI-TOF spectrum (Figure 2A), the product was
clearly identified, and the comparison with the simulation
confirmed the successful cyclodehydrogenation. Depending on
the laser attenuation, a fragmentation of the molecule could be
material that allowed the recording of a resolved NMR spectrum,
indicating the enhanced purity of the new material 8.
1
4
On the basis of theoretical calculations, it was shown that
the cis and the trans conformation exhibit almost the same heat
of formation and therefore both conformers can be formed
during the cyclodehydrogenation (Figure 3A). However, the
1
observed four aromatic signals in the H NMR implied that
either only one conformer is present or a flipping of the tert-
butyl groups occurs.
observed, leading to the observed small peaks in the spectrum
1
(
Figure 2A). The H NMR spectrum of 8 (Figure 2B) (CD2-
The possibility for the conformational change from cis to trans
was investigated by density functional calculations of a section
Cl2/CS2) showed the resolved resonances of the four aromatic
protons, which could be assigned by HH COSY and NOESY.
Contrary to other PAHs, such as alkylated hexa-peri-hexaben-
zocoronenes, the chemical shifts of the aromatic resonances for
15
of the molecule (parts B and C of Figure 3). Within this section
the space coordinates of four carbon atoms were fixed in the
simulations (Figure 3B, orange), whereby all other atoms were
8
were independent of concentration and temperature, clearly
6
(14) AM1. HyperChem 6.0 for Windows; Hypercube, Inc.
indicating minimum or no aggregation in solution. In contrast
to the novel synthesis, the published route did not yield a
(
15) (a) Kresse, G.; Hafer, J. Phys. ReV. B 1993, 47, 558-561. (b) Kresse, G.;
9
Furthm u¨ ller, J. Phys. ReV. B 1996, 54, 11169-11186.
1336 J. AM. CHEM. SOC.
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Suppressing Aggregation in a Large PAH
A R T I C L E S
Figure 3. (A) Cis-trans conformation of 8 (alkyl chains are ommitted for clarity); (B) structure of trans conformation, selected section (red dotted line) for
the density functional simulation and carbon atoms fixed in the simulations (orange), the tert-butyl group was colored blue for a better visualization; (C)
ground-state conformation of cis and trans functional groups.
-
6
Figure 4. (A) UV/vis of 8 (black) compared to the fully planarized derivative 9 (grey); (B) PL excitation (black) and emission (gray) of 8 (4.52 × 10
mol/L) in THF.
fully relaxed. This fragment showed a similar flexibility with
respect to the elasticity of the carbon-carbon bonds of the whole
molecule, as the rigid entities were placed at the very end of
the substructure. It also has to be considered that this part is
usually connected to the rest of the molecule by seven covalent
bonds, exerting further resistance to a conformational change.
The energy barrier for transitions was calculated with the nudged
elastic band method16 and was found to exceed 1 eV/atom. This
indicates that every bond of the segment would have to stabilize
at least a strain energy of 450 kJ/mol; hence the molecule is
more likely to fragment than changing its conformation. As a
consequence the efficient aggregation suppression observed in
tert-butyl groups and was not partially enhanced by any
disturbing effect of a dynamic conformational change.
The UV/vis spectrum of 8, recorded in THF, showed a large
number of transition bands (Figure 4A) typical for PAHs and a
more than three times higher molar extinction coefficient relative
6
to the smaller HBC. As already reported for [n]paracyclo-
1
7
phanes, the bathochromic shift seen for 8 with respect to 9
resulted from the distortion of the aromatic core caused by the
tert-butyl groups, which induced a severe bending from planarity
of the aromatic entities attached to them (Figure SI 1). The
barely soluble material 9 could not be subjected to any solution-
1
(17) (a) Allinger, N. L.; Sprague, J. T.; Liljefors, T. J. Am. Chem. Soc. 1974,
the H NMR was due to the bulky side chains and the fixed
9
6, 5100-5104. (b) Jenneskens, L. W.; de Kanter, F. J. J.; Kraakman, P.
A.; Turkenburg, L. A. M.; Koolhaas, W. E.; de Wolf, W. H.; Bickelhaupt,
(16) Mills, G.; Jonsson, H.; Schenter, G. K. Surf. Sci. 1995, 324, 305-337.
F. J. Am. Chem. Soc. 1985, 107, 3716-3717.
J. AM. CHEM. SOC.
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A R T I C L E S
Wasserfallen et al.
Figure 5. (A) 2D-WAXS of an extruded fiber of 8 at room temperature (arrows indicate tilted arrangement); (B) optical microsopy image of 8 at 370 °C;
C) cross-polarized optical microscopy image of 8 at 370 °C; (D) self-assembled column of 8 (top view); (E) hexagonal columnar arrangement of 8 in the
bulk (alkyl chains are ommitted for clarity); (F) calculated dimensions of 8 with respect to hexagonal packing parameter (a ) 3.3 nm).
(
based analytical techniques except UV/vis and PL. For the UV/
vis measurements the compound had to be refluxed overnight
in THF to dissolve sufficient material. The very pronounced
association in even strongly diluted solutions led to a broadening
of the absorption spectrum (Figure 4A) with a maximum
absorption peak at 433 nm.
fits the calculated dimensions by taking into account an
20
interdigitation of the alkyl chains (Figure 5F). The meridional
reflections combined with the weak offset signals (Figure 5A,
arrows) implied that the disks adopted a tilted arrangement with
2
1
respect to the columnar axis. This stands in contrast to the
9
old pathway, which only yielded materials that could not be
The PL emission spectrum (Figure 4B) of 8, appeared
mechanically oriented.
18
simplified, which can be attributed to the low symmetry, while
the high aggregation of 9 led to a complete quenching of the
fluorescence. For 8 quantum yields of 3% were determined,
Although the effective suppression of the π-stacking was
expected to lead to a strong reduction of the supramolecular
organization of the material in the bulk, 8 featured a high
22
6
which correspond roughly to those of HBCs. In contrast to
degree of order in the 2D-WAXS experiment. Therefore the
remaining π-interaction was sufficient for the self-assembly into
the already known columnar superstructures. The found hex-
agonal arrangement is nevertheless contradictory to the D2h
symmetry of the molecule, which is explained by the intermo-
lecular arrangement induced by the bulky tert-butyl groups. This
forces the molecules to adopt an offset angle for gaining a higher
π-contact area, leading to a round columnar structure (top view)
and a hexagonal arrangement as seen for HBCs (parts D and E
compound 2 (parts A and C of Figure 1), the band structure of
the PL excitation spectrum of 8 perfectly matched the one
obtained in the UV/vis measurements (parts A and B of Figure
), indicating the high purity of the material.
In the differential scanning calorimetry (DSC) of 8, an
4
endothermic transition was observed at 60 °C. As in the 2D-
wide-angle X-ray scattering (2D-WAXS) diffractogram of an
extruded fiber no change in the reflections appeared at this
temperature, the transition was attributed to a reorganization of
the alkyl chains. As expected, the fully planarized compound 9
did not show any thermal transition until degradation. In an
(
20) Ito, S.; Wehmeier, M.; Brand, J. D.; K u¨ bel, C.; Epsch, R.; Rabe, J. P.;
M u¨ llen, K. Chem.-Eur. J. 2000, 6, 4327-4342.
(
21) (a) Fechtenk o¨ tter, A.; Chebotareva, N.; Watson, M. D.; M u¨ llen, K.
Tetrahedron 2001, 57, 3769-3783. (b) Fischbach, I.; Pakula, T.; Minkin,
P.; Fechtenk o¨ tter, A.; Saalw a¨ chter, K.; M u¨ llen, K.; Spiess, H. W. J. Phys.
Chem. B 2002, 106, 6408-6418. (c) Brown, S. P.; Schnell, I.; Brand, J.
D.; M u¨ llen, K.; Spiess, H. W. J. Am. Chem. Soc. 1999, 121, 6712-6718.
1
9
extruded fiber of 8, the disks self-organized in columnar
structures along the extrusion direction (Figure 5A). From the
equatorial reflections, an intercolumnar hexagonal unit cell could
be assigned with a ) 3.3 nm as the packing parameter, which
(
d) Brown, S. P.; Schnell, I.; Brand, J. D.; M u¨ llen, K.; Spiess, H. W. J.
Mol. Struct., 2000, 521, 179-195.
(
22) (a) Aoki, K.; Nakagawa, M.; Seki, T.; Ichimura, K. Bull. Chem. Soc. Jpn.
2002, 75, 2533-2539. (b) Aoki, K.; Nakagawa, M.; Ichimura, K. Chem.
Lett. 1999, 1205-1206. (c) Aoki, K.; Nakagawa, M.; Seki, T.; Ichimura,
K. J. Am. Chem. Soc. 2000, 122, 10997-11004.
(
(
18) Beier, J. PhD Thesis, University Bayreuth, 2000.
19) Fischbach, I.; Pakula, T.; Minkin, P.; Fechtenk o¨ tter, A.; Saalw a¨ chter, K.;
M u¨ llen, K.; Spiess, H. W. J. Phys. Chem. B 2002, 106, 6408-6418.
1338 J. AM. CHEM. SOC.
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Suppressing Aggregation in a Large PAH
A R T I C L E S
of Figure 5). The high transition temperature to the isotropic
melt confirms the significant degree of intracolumnar π-stacking
in the bulk, which is indispensable for the intrinsic charge-carrier
migration in the bulk.
highly reproducible and no discrepancies between the batches,
as for 2, could be observed for 8 and 9. It was verified that one
can record well-resolved NMR and UV/vis data from large
PAHs, as compound 1 is the largest reported PAH with a
1
24
Polarization microscopy (POM) allowed the application of
high heating rates to suppress thermal decomposition of the
material, which was not possible with DSC, and was used to
investigate the crystallization behavior from the isotropic phase.
Compound 8 was sandwiched between two glass slides and
heated under nitrogen flow at maximum heating rate. After
reaching the isotropic phase, the sample was immediately
cooled. At approximately 370 °C, a dendritic texture appeared,
which did not reveal birefringence in polarized light, except
for some defects. This indicated a homeotropic order (parts B
and C of Figure 5), which is the preferred orientation on
substrates for photovoltaic applications.23
resolved H NMR spectrum. To achieve this goal, two main
solubilizing features had to be introduced, whereby the induced
distortion and the steric requirements of the bulky tert-butyl
groups proved to be the main argument for the solubility, the
effective aggregation suppression and the thermal properties of
8. The compound kept, however, the necessary columnar self-
assembly in the bulk, which is a prerequisite for a successful
charge-carrier migration along the columnar axis. Also from
drop casting the material already formed homogeneous films
with low crystallinity, indicating good film forming properties.
With the more than three times higher molar extinction
coefficient of 8 and the bathochromically shifted absorption
profile relative to HBCs the material becomes an interesting
As for 9, no transitions were observed; any further thermal
processing of this material is futile. Nevertheless it was possible
to gain drop-casted films from THF (parts A and B of Figure
SI 2), where small crystallinelike objects formed. It has been
2
a
candidate for future photovoltaic applications.
Acknowledgment. Financial support from the Deutsche
Forschungsgemeinschaft (Schwerpunktprogramm: Organische
Feldeffekttransistoren), the EU projects NAIMO (Integrated
Project Number NMP4-CT-2004-500355), and RADSAS (In-
tegrated Project Number NMP3-CT-2004-001561) is gratefully
acknowledged. M. Kastler thanks the “Fonds der Chemischen
Industrie” for a graduate fellowship.
6
shown for HBCs that a close correlation exists between the
association constants of the disks in solution and their film-
forming properties. Only compounds with already high associa-
tion constants of the disks in solution provided films with a
high degree of crystallinity. Therefore it becomes obvious that
9
exhibits a high association constant in solution pointing toward
a pronounced degree of aggregation and cannot be used for any
kind of application in electronic devices, as for these purposes
a far better solubility and a thermal phase behavior are
prerequisites.
Supporting Information Available: Experimental Section, PL
excitation and emission spectra of 8, POM images of 9 and
schematic arrangement of 8 in the bulk. This material is available
free of charge via the Internet at http://pubs.acs.org.
In conclusion, the novel synthetic approach proved to be
JA056782J
(
23) (a) Pisula, W.; Tomovic, Z.; El Hamaoui, B.; Watson, M. D.; Pakula, T.;
M u¨ llen, K. AdV. Funct. Mater. 2005, 15, 893-904. (b) Hatsusaka, K.; Ohta,
K.; Yamamoto, I.; Shirai, H. J. Mater. Chem. 2001, 11, 423-433. (c)
Terasawa, N.; Monobe, H.; Kiyohara, K.; Shimizu, Y. Chem. Commun.
(24) (a) Tomovic, Z.; Watson, M. D.; M u¨ llen, K. Angew. Chem., Int. Ed. 2004,
43, 755-758. (b) Iyer, V. S.; Yoshimura, K.; Enkelmann, V.; Epsch, R.;
Rabe, J. P.; M u¨ llen, K. Angew. Chem., Int. Ed. Engl. 1998, 37, 2696-
2699.
2
003, 1678-1679.
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