Entry to coronene chemistry - Making large electron donors and acceptors

Article abstract of DOI:10.1002/chem.200800832

A study was conducted to demonstrate the synthesis of methoxy-substituted coronenes by paracyclophanes into coronenes using ultra-violet (UV) irradiation process. The study also performed a double Z-selective Witting reaction in dichloromethance or dimethylformamide (DMF) with the addition lithium ethanolate. The study observed that the photochemical conversion of cyclophanes to coronenes can be used to prepare new coronene-based nanographenes with different functionality and symmetry. The method can be used to obtain new donor and acceptor molecules with different electronic properties and the symmetry. The study found that the interplane distance increases with the increasing number of methoxy groups. The study also observed a decrease in the π-π interaction and an increase in solubility.

Full text of DOI:10.1002/chem.200800832

DOI: 10.1002/chem.200800832
Entry to Coronene Chemistry—Making Large Electron Donors and
Acceptors
[a]
Ralph Rieger,[a] Marcel Kastler,[b] Volker Enkelmann,[a] andKlaus Müllen*
Single layers of graphite, commonly called graphenes,
have recently attracted much attention.[1] It could be shown
that graphenes are accessible by exfoliation from highly or-
dered pyrolitic graphite (HOPG), which led to a rapid de-
velopment of this exciting research field.[2] These microme-
ter-sized layers show highly interesting properties, such as
ballistic charge transport and the quantum Hall effect.[3,4]
Nevertheless, the exfoliated graphenes are structurally not
well defined. Small model systems of graphene have been
synthesized with perfect structural control and hence defect-
free.[5] One example of such nanographenes is hexa-peri-
hexabenzocoronene (HBC), which serves as a semiconduc-
tor and even as a single-molecule transistor.[6,7] Much effort
has been spent to increase the size of these nanographenes;
however, the increase in size of these systems has always led
to dramatic solubility problems.[8,9] Going to smaller but still
neering, for example, in the field of single-crystal semicon-
ductors or charge transfer salts.[14,15]
Herein, we have synthesized a series of methoxy-substi-
tuted coronenes by converting paracyclophanes into coro-
nenes by UV irradiation (Scheme 1).[16,17] In analogy to the
method employed by Otsubo et al.,[18] a doubly Z-selective
Wittig reaction was performed by reacting 1 and 2 at À408C
in dichloromethane or dimethylformamide (DMF) by addi-
tion of lithium ethanolate. The dialdehyde 1 (R1 =OMe) is
available by double lithiiation of veratrol and subsequent
relatively large nanoACHTREgUNG raphenes offers the possibilities of sub-
limation and crystallization. Hence, versatile ways of study-
ing molecular properties on surfaces and crystals become
available with highly developed techniques such as X-ray
diffractommetry and scanning tunneling microscopy.[10,11]
Coronene is in that sense a very promising D6h-nanogra-
phene with a zigzag periphery and just the right size for
processing techniques. Functionalized coronenes are, howev-
er, very rare.[12,13] To tune the electronic properties and in-
termolecular order, a versatile synthetic method is required
to introduce substituents and control the symmetry of coro-
nenes. This helps to match up the energetic levels of coro-
nene with those of electrode materials, as needed for organ-
ic electronics. The symmetry plays a key role in crystal engi-
[a] R. Rieger, Dr. V. Enkelmann, Prof. Dr. K. Müllen
Max-Planck-Institute for Polymer Research
Ackermannweg 10, 55128 Mainz (Germany)
Fax : (+49)6131-379-100
E-mail: muellen@mpip-mainz.mpg.de
[b] Dr. M. Kastler
BASF Aktiengesellschaft
GKS/A - B1, 67056 Ludwigshafen (Germany)
Scheme 1. Synthesis of three methoxy-substituted coronenes; conditions:
a) LiOEt, À408C, CH2Cl2 or DMF, 27–46%; b) TiCl4, Zn, THF, reflux,
34–48%; c) 300 nm 40W, cyclohexane, I2, 60–80%; d) 1. BBr3, CH2Cl2, 2.
HNO3, 80–90%.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200800832.
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COMMUNICATION
treatment with DMF.[19] Compound 1 can be converted into
compound 2 (R2 =OMe) by reduction, followed by bromi-
nation, and reaction with triphenylphosphine. A high Z-se-
lectivity of the Wittig reaction is needed to be able to close
the molecule in a McMurry reaction to get to the [2.2.2]par-
acyclophanes 4 under pseudo-dilution conditions. A major
side reaction is the formation of reduced aldehydes.
Irradiation of 4 in the presence of an oxidant such as
iodine led to the corresponding coronenes 5 in high yield. In
comparable stilbene systems the elimination of methoxy
groups has been observed.[20] In the present case, only a few
percent of side products that have lost methoxy groups are
obtained. They can be separated easily by preparative chro-
matography. In contrast to the previous work of Otsubo
et al.,[18] who only mention the formation of unsubstituted
coronene as a side product in their experimental section, it
is now possible to introduce functional groups and different
symmetries, a fundamental requirement for further chemis-
try.
material, possibly a pronounced network of partially hydro-
lyzed intermediates. In contrast to 6a and 6b, which are
formed directly under theses conditions, 6c is only obtained
after dissolution in dimethyl sulfoxide followed by treatment
in air to quantitatively convert the hydroxy groups to ke-
tones. Coronene-quinone (6a) has been reported previously
by direct oxidation of coronene with chromium(VI)
oxide.[22] Compounds 6b and 6c are reddish powders that
are sparingly soluble in DMF and DMSO (1 mgmLÀ1), but
otherwise practically insoluble.
The reaction from 5c to 6c is remarkable as it converts
the donor 5c into a strong acceptor in just a single step—a
very useful process with regard to the manifold of applica-
tions of donor–acceptor systems.[23,24] DFT calculations
(Figure 2) reveal that the HOMO energy decreased by
When the irradiation of 4c is terminated after two hours,
two intermediates are obtained, one of which could be iden-
tified as 4c’ by mass spectrometry and NMR analysis. The
second compound is unstable and leads to the formation of
coronene 5c after a short time. It can be assumed to be 4c’’.
This highly strained ring can thermally planarize and is oxi-
dized by air. Therefore, it seems very likely that the reaction
occurs stepwise as depicted in Figure 1.
In contrast to unsubstituted coronene and the analogous
hexamethoxytriphenylene,[21] the methoxy-substituted coro-
nenes are very soluble (>50 mgmLÀ1 hexamethoxycoronene
(5c) in dichloromethane, more than 10 times the solubility
of hexamethoxytriphenylene). The methoxy groups are ex-
pected to increase the p–p distance and therefore lead to a
decrease in the intermolecular forces.
Figure 2. DFT calculations of the electronic states of 5c and 6c (B3LYP
method, 6-31G** basis set).
2.2 eV and that the LUMO value decreased by 2.9 eV. These
values are supported by cyclovoltammetry data. Hexame-
thoxycoronene (5c) is oxidized at 0.74 V against ferrocene
(standard), which corresponds to a value for the HOMO of
À5.5 eV.[25] Coronene-hexaone (6c) is reduced at À0.38 V
against ferrocene (standard), which corresponds to a value
for the LUMO of À4.4 eV. This is close to that for 7,7,8,8-
tetracyanoquinodimethane (TCNQ), which is outstanding
for discs of such a size.
Treatment of 5a–c with boron tribromide followed by oxi-
dative workup gave the corresponding coronene quinones
6a–c. The initially formed boron tribromide adducts of all of
the coronenes are practically insoluble compounds that are
hydrolyzed successfully only with concentrated nitric acid.
All other attempted reactions ended with a black insoluble
Coronene-hexaone (6c) reacts with o-phenylenediamine
as expected for a-diketones by forming quinoxaline units
Figure 1. Planarization process of 4c (calculated by Merck molecular force field (MMFF)).
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K. Müllen et al.
(Scheme 2), which leads to the larger p-system 7. Many
more such extension reactions are conceivable, so 6c offers
a new platform for further chemistry and many more inter-
esting molecules.
Figure 3. UV/Vis spectra of the methoxy-substituted coronenes 5a–c; all
recorded in cyclohexane (10À5 m), fluorescence excited at 280 nm.
Scheme 2. Quinoxaline condensation of 6c.
Figure 3 displays the electronic spectra of the methoxy-
substituted coronenes. The main absorption peaks as well as
the fluorescence peaks are almost identical with that of un-
substituted coronene. The methoxy groups thus influence
the electronic structure only slightly, which is further sup-
ported by cyclovoltammetry (HOMO level at À5.5 eV for
all molecules). The lower symmetry of 5a and 5b in contrast
to coronene (D3h instead of C2v) leads to peak broadening
particularly in the fluorescence spectra as more forbidden
transitions become allowed when the symmetry is reduced.
A dramatic change in the absorption behavior can be ob-
served when going from the methoxy-coronenes to the cor-
responding ketones 6a–c (Figure 4). The main absorption is
shifted to shorter wavelength, which is attributed to the fact
that the p system of the disc has decreased. The interaction
of the carbonyl bonds with the rest of the molecule creates
more allowed transitions, a p–
Figure 4. UV/Vis absorption spectra of the coronene ketones 6a–c; all re-
corded in THF.
n transition can be observed at
a wavelength above 400 nm,
which causes the reddish color
of the substances. With an in-
creasing number of carbonyl
bonds, the bands become
broader, which is another
effect of the coupling between
the aromatic core and the car-
bonyl groups.
Figure 5 depicts the single-
crystal structures of 5a–c to-
gether with that of unsubstitut-
ed coronene for comparison.
Coronenes 5a and 5b form a
herringbone-like, 5c a sand-
wich-herringbone-like
struc-
ture. The unsubstituted aro-
matic edge points towards the
p
system of the next disc,
which is typical behavior for
polyaromatic
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hydrocabrons
Figure 5. Single crystal structures of coronene and 5a–c.[26]
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Coronene Chemistry
COMMUNICATION
(PAHs).[27] On the other hand, the methoxy groups maxi-
mize their mutual contact, presumably through dipolar inter-
actions. Coronene 5a even forms planes containing all the
methoxy groups.
The interplane distance increases with the increasing
number of methoxy groups. In the structure of 5a, the discs
are 3.4 Š apart, which is comparable to coronene, in 5b the
distance is 3.5 Š, and for 5c it is 3.6 Š. This causes a de-
crease in the p–p interaction and thus the observed increase
in solubility as expected above.
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[12] S. Alibert-Fouet, I. Seguy, J. F. Bobo, P. Destruel, H. Bock, Chem.
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Herein we have shown that the photochemical conversion
of cyclophanes to coronenes is a very versatile method for
the preparation of new coronene-based nanographenes with
different functionality and symmetry. Coronenes with differ-
ent degrees of methoxy substitution have been synthesized.
Their absorption and emission as well as their packing in
the crystal have been studied with regard to symmetry and
number of substituents. These molecules can switch in just a
single step from donors to strong acceptors by cleaving the
ether group and oxidizing the alcohols to a-diketones.
Therefore, a set of new donor and acceptor molecules with
a systematic variation of the electronic properties and the
symmetry are available to investigate their properties as
crystalline semiconductors, charge transfer systems, or
metal–organic frameworks. Manifold known condensation
reactions for a-diketones can now be used to access a whole
range of new molecules, one example has already been
shown here. In the future, it should be possible to introduce
further functionalities to coronene by making different cy-
clophanes. This will enable one to fully exploit the potential
of coronene as a model system for graphene as well as to
grow much larger systems, for example, by fusing appropri-
ate precursors on surfaces.
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[25] A. J. Bard, L. R. Faulkner, Electrochemical Methods. Fundamentals
and Applications, 2 ed., Wiley, 2001.
[26] Data collections for the crystal-structure analysis were performed
on a Nonius KCCD diffractometer equipped with a Cryostream
cooler with graphite-monochromated MoKa radiation (l=0.7107 Š).
The samples were measured at 120 K. The structures were solved by
direct methods (Shelxs) and refined on F with anisotropic tempera-
ture factors for the non-hydrogen atoms. The Hatoms were refined
with fixed isotropic temperature factors in the riding mode. 5a:
C26H16O2, Mr =360.41 gmolÀ1, monoclinic, space group P21/a, a=
16.0660(6), b=4.5830(4), c=23.2180(7) Š, b=99.842(1)8, V=
1684.39(17) Š3, Z=4, 1calcd =1.421 gcmÀ3, m=0.089, 2qmax =27.4968,
Acknowledgements
This work was financed by Deutsche Forschungsgemeinschaft (Korean
German IRTG). R. R. thanks the “Fond der Chemischen Industrie”for fi-
nancial support. We thank Dr. M. Wagner for NMR spectroscopy and
many fruitful discussions.
22586 reflections measured, 3817 unique, 1854 observed, Rint
0.087, R=0.0519, Rw =0.0432; 5b: C28H20O4, Mr =420.46 gmolÀ1
monoclinic, space group P21/c, a=12.7901(6), b=11.7359(5), c=
13.5211(6) Š, b=107.344(1)8,
V=1937.29(15) Š3, Z=4, 1calcd
=
,
,
=
1.442 gcmÀ3, m=0.096, 2qmax =30.0008, 21479 reflections measured,
5540 unique, 2224 observed, Rint =0.077, R=0.0656, Rw =0.0299; 5c:
C30H24O6, Mr =480.52 gmolÀ1, monoclinic, space group P21/c, a=
10.2336(5), b=13.6684(5), c=16.1664(6) Š, b=101.541(1)8, V=
2215.58(16) Š3, Z=4, 1calcd =1.440 gcmÀ3, m=0.100, 2qmax =29.4998,
Keywords: coronene
acceptor systems · photosynthesis
· crystals · cyclophanes · donor–
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0.076, R=0.0491, Rw =0.0536; CCDC-678259, CCDC-678260, and
CCDC-678261 contain the supplementary crystallographic data for
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Received: May 2, 2008
Published online: June 3, 2008
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