The Versatile Synthesis and Self-Assembly of Star-Type Hexabenzocoronenes

Article abstract of DOI:10.1002/anie.200352047

An insoluble disclike building block hexa(4-iodophenyl)-peri-hexabenzocoronene (see picture) was synthesized in an efficient way, and subsequent functionalization led to a series of soluble, star-type hexabenzocoronenes (HBCs), all showing remarkable self-assembly behavior: 1) highly ordered liquid-crystalline materials, 2) dendronized HBCs, and 3) triarylamine-substituted HBCs.

Full text of DOI:10.1002/anie.200352047

Angewandte
Chemie
Carbon-Rich Compounds
Herein, we address these functions by varying pendant
substituents; thus hexa(4-iodophenyl)-peri-hexabenzocoro-
The Versatile Synthesis and Self-Assembly of Star- nene (2) acts as an HBC building block with versatile reactive
Type Hexabenzocoronenes**
sites, which can be converted into a range of useful
functionalities. Such an atom economy is important when
enriching the library of HBCs, since their synthetic modifi-
cation has so far involved variation of the diphenylacetylene
Jishan Wu, Mark D. Watson, and Klaus Müllen*
[
2c]
starting compounds.
p-Stacking is a powerful motif for self-assembly that is
ubiquitous in nature and utilized extensively by synthetic
chemists to engineer complex supramolecular architectures—
particularly in recent years, having been spurred on by the
increasing technological promise of the p stacks as conducting
The synthesis of the key building block 2 is outlined in
Scheme 1. The precursor hexa(4-iodophenyl)hexaphenylben-
zene (9) was prepared in 92% yield by reaction of excess
[1]
pathways in organic-based electronics. Hexa-peri-hexaben-
[
2]
zocoronenes (HBCs) 1 are larger than the widely studied
[
3]
[4]
[5]
triphenylenes, porphyrins, and phthalocyanines. As a
result, alkyl-substituted HBCs exhibit aggregation behavior
in solution that persists to nanomolar concentrations, as well
as bulk columnar mesophases with high order parameters,
high charge-carrier mobilities, and exceptional thermal sta-
[
2]
bility. The bulk properties qualify them as active compo-
[
6]
nents in organic field-effect transistors (OFETs) and as
hole-conducting layers in photovoltaic devices or light-emit-
[
7]
ting diodes (LEDs). Important targets include the under-
standing of the self-assembly of large disc molecules and
improving device performance through 1) more specific
assembly in solution, 2) further increasing the order in the
columnar mesophases, and 3) modifying the electronic prop-
erties of the discs, in particular, for better hole stabilization.
Scheme 1. Synthesis of compound 2: a) [Pd(PPh ) ], toluene, 1258C,
3
4
7
5%; b) [Co (CO) ], dioxane, 1258C, 92%; c) neat bromine, ꢀ208C!
2
8
+
[
*] Prof. Dr. K. Müllen, J. Wu, Dr. M. D. Watson
Max-Planck Institute for Polymer Research
Ackermannweg 10, 55128 Mainz (Germany)
Fax: (+49)6131-379-350
RT, 96%; d) [Pd(PPh ) ], K CO (aq), 958C, 92%; e) ICl, chloroform,
3
4
2
3
92%; f) FeCl /nitromethane, CH Cl /CS =2:1.
3 2 2 2
E-mail: muellen@mpip-mainz.mpg.de
iodine monochloride with hexa(4-trimethylsilylphenyl)hexa-
phenylbenzene (6), which was synthesized by two different
routes. The first so-called convergent route (Route 1)
+
[
] Current address:
University of Kentucky
Department of Chemistry
Lexington, KY 40506-0055 (USA)
[8]
involves a Stille coupling and a cobalt-catalyzed cyclo-
[2c]
trimerization. To avoid the toxic organotin chemistry, as
[
**] This work was financially supported by the Zentrum für Multi-
well as to simplify the synthesis, an alternate divergent route
funktionelle Werkstoffe und Miniaturisierte Funktionseinheiten
[
9]
(
Route 2) requiring one sixfold bromination and one sixfold
(
BMBF 03N6500), EU-TMR project SISITOMAS, the Deutsche
[
10]
Suzuki coupling reaction was developed. Final cyclodehy-
drogenation of 9 with iron(iii) chloride gave the desired
compound 2 as a yellow powder in quantitative yield. Notably,
this divergent route involves four sixfold transformations, all
Forschungsgemeinschaft (Schwerpunkt Feldeffekttransistoren),
and the EU project DISCEL (G5RD-CT-2000-00321).
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2003, 42, 5329 –5333
DOI: 10.1002/anie.200352047
ꢀ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
5329

Communications
in excellent to near quantitative yields and provides com-
pound 2 on a gram scale.
temperature charge-carrier mobility for a discotic liquid-
[11]
[2d,e]
crystalline material.
Thus a functionalizable core such as 2
Despite the virtual insolubility of compound 2, palladium-
is expected to provide a new series of highly ordered liquid-
crystalline materials. As shown in Scheme 2, cross-coupling
between 2 and soluble monoacetylenes 10a–c gave a series of
extended, star-type HBC derivatives 11a–c in high yield.
Their good solubility facilitates their full structural character-
ization, despite their size. The bulk thermotropic liquid-
crystalline properties of 11a–c were investigated by differ-
[
12]
catalyzed Hagihara–Sonagashira coupling reactions
with
solubilizing acetylene groups work smoothly, thus paving the
way to versatile functionalization. Compared to n-alkyl
substituents, 4-alkylphenyl groups provide the following
advantages to HBCs: good solubility, longer range order in
the columnar mesophase, and the highest reported room-
Scheme 2. Versatile functionalization based on building block 2. a) [Pd(PPh ) ]/CuI, piperidine, 508C; b) Ph O, 2508C; c) tetrabutylammonium
3
4
2
fluoride, THF, RT.
5
330
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ential scanning calorimetry, polarized optical microscopy, and
wide-angle X-ray diffraction (WAXD). As expected, highly
ordered hexagonal columnar mesophases were observed at
ambient temperature with compounds 11a–c. Most interest-
ingly, the 2D WAXD pattern of extruded fibers of 11b,c
equilibrium in solution. The representative temperature-
1
dependent H NMR spetra of 12a in [D ]tetrachloroethane
4
are shown in Figure 2. Two independent species, one expected
[
13]
revealed a helical stacking in the HBC column. As a typical
example, clear reflection peaks positioned in a first layer (l =
1
) above/below the equator corresponding to a repeating
distance along the columns that is four times the p-p stacking
distance of HBC disks are observed in the 2D X-ray pattern of
1
4b (Figure 1). This pattern indicates the existence of super-
1
Figure 2. Temperature-dependent H NMR (500 MHz) spectra of 12a
ꢀ1
in [D ]tetrachloroethane (3 mgmL ) which reveal two independent
4
species, the ratio of which strongly depends on the temperature, con-
centration, and solvent. Resonances at about d=8.7 ppm and 9.4 ppm
are assigned to HBC core protons in the dimer and monomer, respec-
tively.
Figure 1. 2D X-ray pattern of 11b showing a typical helical column
stacking. The sample was oriented by extrusion from a mini-extruder
to give a fiber with a diameter of about 0.7 mm, then 2-D X-ray diffrac-
tograms were collected in transmission mode with the beam perpen-
dicular to the fiber(fiber/column axes vertical).
monomer (labeled by star) and another dimer (labeled by a
square), were identified. The monomer/dimer ratio increases
with increasing temperature or decreasing concentration.
Furthermore, only dimer exists in a relatively poor solvent
such as cyclohexane, while only monomer can be observed in
carbon disulfide (an excellent solvent for large aromatic
compounds). Clearly, the bulky dendrons suppress the
formation of higher aggregates and allow only a dimer
aggregate to exist in solution. The appearance of separate
lattice periodicity in the direction of stacks of discs. Further
analysis discloses that every 5th disc along the column axis is
positionally/rotationally correlated to form a helical super-
structure. As a consequence, reflection peaks appear at l = 2
and 3 in addition to the reflection peaks at l = 4, which is
correlated to the normal p-p stacking distance. This packing
could arise from additional p-p interactions between, and
space-filling requirements introduced by, the rigid-rod side
groups in the star-type molecules. A helical arrangement has
rarely been observed experimentally by X-ray diffraction for
1
H NMR resonances, instead of averaged ones which shift
with temperature/concentration/solvent, suggests that this
self-assembly is slow on the NMR time scale. This could be
ascribed to the bulky dendron structure as well as the steric
hinderance produced by the dodecyl chains at the bay
position. To further investigate the effect of the dodecyl
group in 12a, an analogous HBC molecule 12b, in which the
dodecyl groups are replaced by hydrogen atoms, was pre-
pared (Scheme 2). Sixfold Hagihara–Sonogashira coupling
between 2 and triisopropylacetylene gave the insoluble
material 13a, which was deprotected with tetrabutylammo-
nium fluoride (TBAF) to provide insoluble material 13b.
[
14]
discs without chiral substituents,
and to the best of our
knowledge, this is the first case from a disc with sixfold
symmetry. For example, such a helical Dho mesophase was
found in hexathioalkyltriphenylene and has exceptional one-
[
14a]
dimensional charge-carrier mobilities.
Disc molecules and structurally related shape-persistent
macrocycles are known to exhibit dynamic aggregation in
[
15]
solution. This process is relevant for the formation of films
from solution and also serves as a model for phase formation
from the melt. Attaching bulky substituents is expected to
inhibit or even completely suppress face-to-face aggregation
of the HBC discoid core. As shown in Scheme 2, a sixfold
Diels–Alder cycloaddition between 11a and tetraphenylcy-
clopentadienone proceeds smoothly in refluxing diphenyl
ether to provide HBC 12a, which is substituted with
[
11]
Solid-state MALDI MS
of compounds 13a,b indicated
quantitative conversion even though both the starting mate-
rials and products are insoluble(!), further proving the high
reactivity of the building block 2. Diels–Alder cycloaddition
between 13b and tetraphenylcyclopentdienone proceeded
1
smoothly to give the soluble 12b. The H NMR spectra of 12b
in [D ]tetrachloroethane reveal only one aggregated spe-
4
[16]
1
oligophenylene dendrons. H NMR spectroscopic analysis
cies, which does not show concentration dependence, which
reveals that molecule 12a exhibits a unique monomer–dimer
thus most probably pointing toward a dimeric structure.
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Triarylamines are commonly used as hole-transport
[
17]
[1] a) Handbook of Liquid Crystals, Vol. 1 (Eds.: D. Demus, J.
Goodby, G. W.Gray, H. W. Spiess, V. Vill), Wiley-VCH, Wein-
heim, 1998; b) S. Chandrasekhar, Liq. Cryst. 1993, 14, 3 – 1 4;
c) D. Adam, F. Closs, T. Frey, D. Funhoff, D. Haarer, H.
Ringsdorf, P. Schuhmacher, K. Siemensmeyer, Phys. Rev. Lett.
materials in organic LEDs. Sixfold triarylamine-substituted
HBC 15 can be prepared by similar Hagihara–Sonogashira
coupling reaction between compound 2 and N,N’-di(4-octyl-
phenyl)-N’’-(4-ethynylphenyl)amine (14) as shown in
Scheme 2. It should be pointed out that the triarylamine-
substituted hexaphenylbenzenes are usually excluded from
the standard FeCl3 oxidative cyclodehydrogenation condi-
tions as a result of preferential formation of charge-localized
1993, 70, 457 – 460.
[2] a) M. D. Watson, A. Fechtenkötter, K. Müllen, Chem. Rev. 2001,
101, 1267 – 1300; b) P. Herwig, C. Kayser, K. Müllen, H. W.
Spiess, Adv. Mater. 1996, 8, 510 – 513; c) A. Stabel, P. Herwig, K.
Mullen, Angew. Chem. 1995, 107, 335 – 339; Angew. Chem. Int.
Ed. Engl. 1995, 34, 1609 – 1611; d) A. M. van de Craats, J. M.
Warman, A. Fechenkötter, J. D. Brand, M. A. Harbison, K.
Müllen, Adv. Mater. 1999, 11, 1469 – 1472; e) A. Fechtenkötter,
K. Saalwächter, M. A. Harbison, K. Müllen, H. W. Spiess,
Angew. Chem. 1999, 111, 3224 – 3228; Angew. Chem. Int. Ed.
1999, 38, 3039 – 3042.
[3] a) J. A. Rego, S. Kumar, H. Ringsdorf, Chem. Mater. 1996, 8,
1402 – 1409; b) D. Adam, P. Schuhmacher, J. Simmerer, L.
Häussling, K Siemensmeyer, K. H. Etzbach, H. Ringsdorf, D.
Haarer, Nature 1994, 371, 141 – 143; c) E. F. Gramsbergen, H. J.
Hoving, W. H. de Jeu, K. Praefcke, B. Kohne, Liq. Cryst. 1986, 1,
[
18]
radical cations. The successful synthesis of the electroactive
materials 15 suggests that more electroactive materials could
be made from building block 2. Besides the hole-transfer
channel of the HBC column, the triarylamine moieties in
molecule 15 are expected to form an additional hole-transport
channel. By adapting the term “double-cable” used for
conjugated polymers with tethered electronic functional
units the columnar structures formed from 15 could be
described as coaxial cables. Scientific interest in this molecule
also includes the study, after formation of a radical cation, of
the electronic coupling of the electroactive triarylamines
through the rigid p system including the “superbenzene”
HBC core.
3
97 – 400.
4] a) B. A. Gregg, M. A. Fox, A. J. Bard, J. Am. Chem. Soc. 1989,
11, 3024 – 3029; b) M. Kimura, Y. Saito, K. Ohta, K. Hanabusa,
[
1
A large number of useful peripheral substituents can now
be attached to the HBC core as a last synthetic step, which
leads to three new types of materials: 1) more highly ordered
discotic columnar mesogens 11b,c which exhibit unique
helical superstructures, 2) peripherally dendronized large
p systems 12a,b which provide for programmed self-assembly
in solution, and 3) a novel electroactive material 15 contain-
ing seven positionally defined hole-transport units of two
different types. Work is under way to prepare new mesogenic
versions with six peripheral electron acceptor/transport units
that would lead to core–shell columns (coaxial cables) in
which each individual column is a self-contained photovoltaic
element. As an extension of the above synthetic concept,
another two functionalizable HBC building blocks (16 and
H. Shirai, N. Kobayashi, J. Am. Chem. Soc. 2002, 124, 5274 –
5275.
[5] a) C. Piechoki, J. Simon, A. Skoulios, D. Guillon, P. Weber, J.
Am. Chem. Soc. 1982, 104, 5245 – 5247; b) S. Chandrasekhar,
Mol. Cryst. Liq. Cryst. 1981, 63, 171 – 180.
6] a) A. Kraft, ChemPhysChem 2001, 2, 163 – 165; b) H. E. Katz,
A. J. Lovinger, J. Johnson, C. Kloc, T. Siegrist, W. Li, Y.-Y. Lin,
A. Dodabalapur, Nature 2000, 404, 478 – 481; c) A. M. van de -
Craats, N. Stutzmann, O. Bunk, M. M. Nielsen, M. D. Watson, K.
Müllen, H. D. Chanzy, H. Sirringhaus, R. H. Friend, Adv. Mater.
2003, 15, 495 – 499.
[7] a) L. Schmidt-Mende, A. Fechtenkötter, K. Müllen, E. Moons,
R. H. Friend, J. MacKenzie, Science 2001, 293, 1119 – 1122;
b) C. W. Tang, Appl. Phys. Lett. 1986, 48, 183 – 185.
8] T. N. Mitchell, Synthesis 1992, 803 – 815.
9] R. Rathore, C. L. Burns, M. I. Deselnicu, Org. Lett. 2001, 3,
[
[
[
1
7) have been prepared by shorter synthetic routes and
2887 – 2890.
[
[
10] N. Miyaura, A. Suzuki, Chem. Rev. 1995, 95, 2457 – 2483.
1
11] The very poor solubility of 2 precludes standard H NMR and
1
3
C NMR characterization, however, solid-state matrix-assisted
laser-desorption/ionization mass spectroscopy (MALDI-MS)
studies reveal a single species with an isotopic distribution in
accord with that simulated for compound 2. See Supporting
Information and: L. Przybilla, J. D. Brand, K. Yoshimura, J.
Räder, K. Müllen, Anal. Chem. 2000, 72, 4591– 4597.
12] S. Taskahashi, Y. Kuroyama, K. Sonogashira, N. Hagihara,
Synthesis 1980, 627 – 630.
13] Details of the characterization of this unique helical Dho meso-
phase and nanoscale self-assembling properties of 11a–c will be
reported elsewhere; J. Wu, M. D. Watson, L. Zhang, Z. Wang, K.
Müllen, unpublished results.
[
[
functionalized by palladium-catalyzed Hagihara–Sonogashira
[
19]
and Buchwald reactions. The fact that substituents which
may be incompatible with the normal synthetic build-up can
be introduced to all three halo derivatives dramatically
broadens the scope of HBC materials.
[14] a) D. Adam, P. Schuhmacher, J. Simmerer, L. Häussling, K.
Siemensmeyer, K. H. Etzbach, H. Ringsdorf, D. Haarer, Nature
1994, 371, 141 – 143; b) F. Charra, J. Cousty, Phys. Rev. Lett. 1998,
80, 1682 – 1685; c) E. Fontes, P. A. Heiney, W. H. de Jeu, Phys.
Rev. Lett. 1988, 61, 1202 – 1205.
[
15] a) J.-S. Zhang, J. S. Moore, J. Am. Chem. Soc. 1992, 114, 9701–
Received: June 4, 2003 [Z52047]
9702; b) A. S. Shetty, J.-S. Zhang, J. S. Moore, J. Am. Chem. Soc.
1996, 118, 1019 – 1027; c) Y. Tobe, N. Utsumi, K. Kawabata, A.
Keywords: cross-coupling · liquid crystals · polycycles ·
self-assembly · X-ray diffraction
Nagano, K. Adachi, S. Araki, M. Sonoda, K. Hirose, K.
Naemura, J. Am. Chem. Soc. 2002, 124, 5350 – 5364.
.
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Chemie
1
[
16] Only a single H NMR resonance (ca. d = 8.7 ppm) is observed
above 8 ppm. A single crystal of 12b has been obtained and
hopefully will disclose the atom positions of this supramolecular
dimer. A more detailed analysis of this unique self-assembly
process will be presented in a separate publication.
[
[
[
17] a) U. Mitschke, P. Bäuerle, J. Mater. Chem. 2000, 10, 1471 – 1507;
b) S. J. Campbell, G. G. Malliaras, Semicond. Polym. 2000, 41 1 –
461; c) M. D'lorio, Can. J. Phys. 2000, 78, 231– 241.
18] a) C. Lambert, G. Nöll, Angew. Chem. 1998, 110, 2239 – 2242;
Angew. Chem. Int. Ed. 1998, 37, 2107 – 2110; b) C. Lambert, G.
Nöll, Chem. Eur. J. 2002, 8, 3467 – 3477.
19] a) J. P. Wolfe, S. L. Buchwald, J. Org. Chem. 1997, 62, 6066 –
6068; b) M. H. Ali, S. L. Buchwald, J. Org. Chem. 2001, 66,
2560 – 2565.
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