Synthesis and aggregates of phenylene-ethynylene substituted polycyclic aromatic compounds

Article abstract of DOI:10.1021/ma048728d

The synthesis of polycyclic aromatic compounds and the aggregation of compounds in solutions were analyzed. The polycyclic aromatics that were capable of undergoing C-C coupling reactions were synthesized in a two step route. These compounds were found to

Full text of DOI:10.1021/ma048728d

Volume 37, Number 19
September 21, 2004
© Copyright 2004 by the American Chemical Society
Communications to the Editor
keep the synthetic effort for the preparation of the
Syn th esis a n d Aggr ega tes of
defined structures feasible, small structures such as
dimers, trimers, etc., serve as key structures for the
investigation of the corresponding structure-property
relationship in oligomers (and polymers). For example,
derivatives of 1,4-bis(phenylethynyl)benzene are widely
used as models for the investigation of the electron
transport properties of phenylene-ethynylenes.4
P h en ylen e-Eth yn ylen e Su bstitu ted
P olycyclic Ar om a tic Com p ou n d s
Xia o Hon g Ch en g,†,§ Stefa n -Sven J ester ,‡ a n d
Sigu r d Ho1ger *,†
Institut fu¨r Technische Chemie und Polymerchemie,
Engesserstr. 18, 76131 Karlsruhe, Germany, and
Institut fu¨r Physikalische Chemie, Fritz Haber Weg 4,
76131 Karlsruhe, Germany
During our studies on polycyclic aromatic hydro-
carbons (PAHs) we became interested in the incorpora-
tion of these structural motifs in defined shape-
persistent linear and cyclic molecules.5 The use of PAHs
as active materials in electronic and electrooptical
devices is an area of dynamic research.6 However, most
reports about all-benzenoic PAHs deal either with
highly substituted or with unsubstituted materials;7 the
synthesis of mono- or difunctionalized PAHs has been
only sparingly investigated.8 Therefore, large PAHs as
functional parts of more complex molecular architec-
tures (e.g., in the center or as end groups) can only be
rarely found.9 In particular, we are not aware of any
studies on the inclusion of dibenzonaphthacenes in
complex molecular architectures. Here we report the
synthesis of monofunctionalized dibenzonaphthacenes
and their sulfur analogues. We show that these building
blocks can be introduced as end groups in defined
molecular architectures by the use of transition-metal-
catalyzed C-C coupling reactions. Besides the synthesis
of these new materials, we will describe the aggregation
of the compounds in solution as well as their structures
on a solid substrate.
Received J une 25, 2004
Revised Manuscript Received August 2, 2004
In tr od u ction . Conjugated polymers and oligomers
are of significant interest due to their optical, electrical,
and electrooptical properties that lead to applications
in various electronic devices as well as sensors. How-
ever, defects in the polymeric backbone have a remark-
able effect on the device efficiency and sometimes lead
to irreproducible device performance. Oligomers can be
obtained defect free and synthesized with tunable, well-
defined molecular weights. The well-defined nature of
the oligomers facilitates also the control over their
supramolecular structure, crucial for the realization of
optimal molecular devices.1 Therefore, oligomers serve
not only as models for the corresponding polymers but
can be used in many applications as active materials.
Additionally, the molecules can be end-capped with
(different) terminal groups that alter their optoelec-
tronic as well as structure forming properties. For
example, in the case of phenylene-ethynylene oligo-
mers, donor/acceptor substituted molecules have been
shown to exhibit interesting optical properties.2 Thio-
acetyl- and thiocarbamoyl end-capped phenylene-
ethynylenes were investigated by AFM methods.3 To
Resu lts a n d Discu ssion . The synthesis of the poly-
cyclic aromatic compounds is outlined in Scheme 1.
Details of the synthesis as well as the characterization
of all compounds are given in the Supporting Informa-
tion (SI).
† Institut fu¨r Technische Chemie und Polymerchemie.
‡ Institut fu¨r Physikalische Chemie.
The key step in our synthesis is the combination of
the transformation of dihalo 2,4,6-triarylpyrylium salts
(1) to dihalo 1,2,3,5-tetraarylbenzenes by condensation
with the corresponding sodium aryl acetates and their
subsequent Pd(0)-catalyzed dehydrohalogenation.11 As
§ On leave from the School of Chemistry and Material Engineer-
ing of Yunnan University, China.
* To whom correspondence should be addressed: E-mail
hoeger@chemie.uni-karlsruhe.de.
10.1021/ma048728d CCC: $27.50 © 2004 American Chemical Society
Published on Web 08/21/2004

7066 Communications to the Editor
Macromolecules, Vol. 37, No. 19, 2004
Sch em e 1. Syn th esis of Mon ofu n ction a lized P olycyclic Ar om a ticsa
a
(a) Sodium 4-methoxyphenylacetate, Ac2O, 37%; (b) sodium 3-thienylacetate, Ac2O, 46%; (c) PdCl2(PPh3)2, DBU, 42%; (d)
PdCl2(PPh3)2, DBU, 28%; (e) BBr3, 96%; (f) Tf2O, pyridine, 68%; (g) NBS, 97%.
we have shown earlier, the metal-catalyzed “double-
barrel” cyclization of appropriate precursors gives the
corresponding dibenzonaphthacenes in good to accept-
able yields when the initially formed Pd(II) species
attacks an electron-rich aryl ring. This is the anisyl ring
in 2 and the thienyl ring in 3.5 Both polycyclic aromatics
(4 and 5) can be transformed into monofunctionalized
compounds capable of undergoing transition-metal-
catalyzed C-C coupling reactions. 4 was demethylated
with BBr3 to give the phenol 6 and transformed into
the triflate 7 by treating with Tf2O. 5 has only one
reactive position at the electron-rich part of the molecule
and gives after treating with NBS cleanly the mono-
bromo compound 8.
Aromatic bromides and triflates are widely used as
substrates in transition-metal-catalyzed C-C coupling
reactions and allow the introduction of these aromatics
in more complex molecular architectures as described
in Scheme 2.
Pd-catalyzed coupling with 912 in the presence of CuI
afforded 10 and 11 which after deprotection of the
triisopropylsilyl group with tetrabutylammonium fluo-
ride in THF yielded the phenylene ethynylenes 12 and
13. Coupling with 14 gave 15 and 16 which both contain
trialkoxybenzyl ethers as solubilizing end groups. Side
products that arose from the oxidative homocoupling of
the acetylenes are insoluble in all common solvents and
could be easily removed by filtration.13 On the contrary,
all phenylene-ethynylene derivatives containing one
terminal polycyclic aromatic moiety are soluble in
aromatic and halogenated solvents as well as in THF.
In some solvents the NMR signals of the compounds are
concentration-dependent, an indication for the aggrega-
tion tendency of the condensed aromatic part of the
structures. Under the assumption that the monomer-
dimer equilibrium is the predominant process in solu-
tion, the aggregation constant for 16 in CH2Cl2 was
determined by concentration-dependent NMR spectros-
F igu r e 1. Error signal AFM images (noncontact mode) of 16
on mica cast from THF/n-octane: (top) image size 10 × 10 µm;
(middle) image size 4 × 4 µm; (bottom) topographical profile
along the black line in the image above.
was investigated by atomic force microscopy (AFM). It
is reported that the thin film morphology and the
structure of the supramolecular organization for oligo-
(phenylene-ethynylenes) are strongly solvent depen-
dent.3b This was also found for 16. From THF only a
granular morphology of the material was observed (see
SI). From THF/n-octane mixtures different superstruc-
tures could be identified on the substrate (Figure 1).
copy to be 20 M-1 14
.
AF M Ch a r a cter iza tion of Solu tion Gr ow n Su -
p er st r u ct u r es. Solutions of 16 were dip cast onto a
freshly cleaved mica surface from THF, THF/n-octane,
and THF/n-hexane mixtures. After complete evapora-
tion of the solvent, the morphology of the dry samples
The molecules self-assemble into ribbons of different
length, a width of several hundred nanometers (200-
400 nm) and a height (mostly) between 20 and 50 nm

Macromolecules, Vol. 37, No. 19, 2004
Communications to the Editor 7067
Sch em e 2. Syn th esis of 15 a n d 16a
F igu r e 2. Error signal AFM images (contact mode) of 16 on
mica cast from THF/n-hexane: (top) image size 5 × 5 µm;
(middle) topographical profile along the black line in the image
above; (bottom) molecular double layers have a height of
approximately 5 nm.
height, ranging from 15 to 30 nm. In some cases (lower
right part of the ziggurat in Figure 2), also layers with
a thickness of approximately 3 nm could be identified,
similar to the plateau height on which the ziggurats are
located. It is also worth mentioning that the plateau on
mica has fuzzy boundaries and does not show a sym-
metrical shape. As far as we know, this morphology has
not been observed before for phenylene-ethynylenes.
At present, we attribute this difference to the strong
aggregation tendency of the polycyclic aromatic part of
the nonsymmetric molecules together with their uniform
size. Under the assumption that the ends of the mol-
ecule tend to phase separate, the height of the ziggurats
can be explained with the adsorption of double layers
as shown in Figure 2, bottom. Accordingly, layer heights
of around 3 nm could be a result of the adsorption of
monolayers. Ongoing investigations deal with the influ-
ence of the molecule length on the film morphology as
well as possible applications of these materials in
molecular electronics.
Con clu sion . Polycyclic aromatics capable of under-
going CC coupling reactions are synthesized in a two-
step route starting from the corresponding pyrylium
salts. Since the latter are available with a variety of
substitution patterns, condensed aromatics with differ-
ent degrees and positions of the reactive groups are
available. These compounds can be used as building
blocks for complex molecular architectures, in this case
phenylene-ethynylenes. These compounds aggregate in
solution as determined by concentration-dependent
NMR experiments. Additionally, on a solid substrate
different superstructures could be identified. Among
these are not only ribbon-like but also ziggurate-like
structures, indicating the formation of a highly ordered
arrangement of the molecules on the mica surface.
a
(a) Pd(PPh3)4, CuI, 68%; (b) Pd(PPh3)4, CuI, 64%; (c)
Bu4NF, 92%; (d) Bu4NF, 87%; (e) Pd(PPh3)4, CuI, 60%; (f)
Pd(PPh3)4, CuI, 63%.
(Figure 1, top). This shows that the cross section of the
ribbons is not molecular, but an aggregation of several
molecules occurs. Similar thin film morphologies are
also found for other phenylene-ethynylenes. Also in
those cases the cross section of the superstructures can
be either molecular (for larger oligomers)3a or an ag-
gregation of several molecules occurs.3b In some cases
we could observe that ribbons with a width of 200-300
nm and a height of 5-15 nm aggregate into bundles
that can reach nearly micrometer dimensions in diam-
eter (Figure 1, middle). However, films cast from THF/
n-hexane show a completely different morphology. The
AFM images resemble polymer single crystals and
indicate that the molecules can form highly ordered
superstructures when deposited from solution (Figure
2).15
They form structures with a diameter of several
microns, a hexagonal base, and relatively sharp bound-
aries, resembling Babylonian ziggurats.16 The number
of the equidistant steps of about 5 nm determines their
Ack n ow led gm en t. We thank Prof. M. Kappes for
facilitating AFM measurements and the Deutsche
Forschungsgemeinschaft for support.

7068 Communications to the Editor
Macromolecules, Vol. 37, No. 19, 2004
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MA048728D