Rational monomer design towards 2D polymers: Synthesis of a macrocycle with three 1,8-anthn lene units

Article abstract of DOI:10.1002/chem.200900781

A study was conducted to demonstrate the synthesis of a macrocycle with three 1,8-anthrylene units. Anthracene units were selected as connection sites to conduct the investigations. The 1,8-anthrylene building blocks were prepared starting from 1-(triisop

Full text of DOI:10.1002/chem.200900781

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DOI: 10.1002/chem.200900781
Rational Monomer Design towards 2D Polymers: Synthesis of a Macrocycle
with Three 1,8-Anthrylene Units
Patrick Kissel, A. Dieter Schlꢀter,* and Junji Sakamoto*[a]
One monomer unit thick (or better: thin) covalent net-
works with a translational periodicity are referred to as 2D
polymers, and these are still on the wish list of synthetic
chemistry.[1] Such structures have been proposed repeated-
ly,[2] but have never been made and unequivocally proven.
Limited numbers of 2D polymers are available from Nature.
They are usually obtained by exfoliation of graphite (gra-
phene)[3] or inorganic lamellar crystals like montmorillon-
ite.[4] Rational synthetic avenues to 2D polymers are still
awaiting to be developed. A couple of years back we started
a research project aimed at providing organic chemical solu-
tions to this burning question.
The first attempt to create sheetlike polymers was report-
ed by Gee back in 1935.[5] This and most of the subsequent
related studies are based on covalent cross-linking of mono-
layers by radical polymerization at interfaces[5,6] or in lay-
ered assemblies.[7] Despite the visionary character of this ap-
proach, it intrinsically gives rise to irregularly networked ul-
trathin structures only. The covalent stitching process un-
avoidably has a “random-walk” nature. In some cases of in-
terfacial/surface polymerizations, translational order within
thin films was referred to, but the aimed structures were
either not realized[8] or remained unproven,[9] or were ob-
tained only within tiny domains still having molecular di-
mensions.[10] Herein, we introduce some aspects of a strategy
with which we hope to eventually arrive at 2D polymers. We
also describe the synthesis of a potential monomer for this
ultimate goal and some model reactions of relevance to this
compoundꢀs usability in the concept.
In previous studies at interfaces, most of the monomers
were connected to one another through flexible linkers.
This, however, prevents control over the growth directions
during the polymerization, so that formation of ill-defined,
irregular networks is a necessary consequence. The same is
likely to hold true even if the monomers are pre-organized
in a periodical array prior to the polymerization.[11] There-
fore, we consider rational design of monomer structures that
ensures directionality of growth a fundamental prerequisite
for a successful 2D polymer synthesis.
Structural inspection of graphene can provide hints for
monomer design. In a Gedanken experiment its sp2-hybrid-
ized carbon atoms can be regarded as the “repeating unit”.
Their three sp2 orbitals lie in the same plane separated by
1208 angles and ensure the regular lateral bond pattern en-
countered in this fascinating sheetlike polymer. These de-
fined and fixed directions in which monomers have to grow,
play a crucial role in obtaining honeycomb-like 2D net-
works. In other words, the structure of a 2D polymer is
“programmed” in the structure of the monomer it was made
from.
With this consideration in mind, a rational monomer
structure for 2D polymer synthesis can be envisaged as fol-
lows (Scheme 1): three bond-forming sites are embedded in
a shape-persistent cyclic skeleton so as to allow the bond
formation to take place laterally within the same plane and
in radial directions from the center separated by 1208. Such
monomers are referred to as M3 (Scheme 1). On the basis
of analogous considerations, one can also design other mo-
nomer types possessing four bond-forming sites with a 908
angular distance (M4), or even six with a 608 angular dis-
tance (M6), which can lead to formation of tetragonal and
hexagonal 2D networks as a result of polymerizations, re-
spectively.
[a] M. Sc. P. Kissel, Prof. Dr. A. D. Schlꢁter, Dr. J. Sakamoto
Laboratory of Polymer Chemistry, Department of Materials
ETH Zurich
Wolfgang-Pauli-Strasse 10, HCI J541
8093 Zurich (Switzerland)
Fax : (+41)446331395
In the present study, we selected anthracene units as the
connection sites. Their dimerizations upon UV irradiation
by [4+4] cycloaddition are well known[12] (Scheme 2a) and
provide advantages: First, the [4+4] cycloaddition generates
two parallel bonds simultaneously. This arrests the two
former anthracene units in a fixed relative geometry that is
E-mail: schlueter@mat.ethz.ch
sakamoto@mat.ethz.ch
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200900781.
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within preorganized layers[9d,13] as well as in bulk conditions
where transport is hindered.[14]
Scheme 2b sketches a belt-shaped potential M3 monomer
in which three anthracene units are embedded in a cyclic
skeleton at the 2,3 and 6,7 positions so as to be D3h symmet-
ric. This structure looks apparently simple and potentially
useful, but the synthesis will be rather laborious.[15] On the
other hand, Scheme 2c depicts a C3v-symmetric macrocycle
bearing three 1,8-anthrylenes. This latter design has advan-
tages: First, precursors of 1,8-anthrylene derivatives, a key
building block for the monomer, are readily available on a
multigram scale.[16] Second, related structures based on 1,8-
anthrylenes were investigated intensively by Toyota, et al. so
that their photophysical/-chemical properties are quite well
known.[17] Third, the 1,8-carbons of two anthracenes of dif-
ferent monomers stacked face-to-face at van der Waals dis-
tance prior to polymerization do not suffer a change of their
mutual distances before and after the dimerization. This
unique feature allows the 1,8-anthrylene-based monomer to
be available for a topochemical polymerization in a solid
Scheme 1. Laterally periodic 2D network formation using a M3 mono-
mer.
Scheme 3. A designed M3 monomer structure with three 1,8-anthrylenes
built-in (1).
Scheme 2. Design of M3 monomers using anthracenes as bond-forming
sites. a) Photo-induced [4+4] cycloaddition dimerization of anthracene,
b) a double-stranded macrocycle, and c) a macrocycle with three anthra-
cenes in D3h and C3v symmetry, respectively.
state as it avoids shrinkage.[1] Scheme 3 describes a concrete
molecular structure for this M3 design in which the 1,8-an-
thylene units were bridged by m-terphenylenes through
ethynylene spacers (1).[18] Note that the methyl groups are
introduced intentionally at the edges of the m-terphenylene
bridges so that the monomers are expected to align prefer-
entially in an antiparallel manner circumventing the steric
hindrance imposed by them in the parallel alignment.
Compounds 2–4 were selected as building blocks for 1
(Scheme 4). Starting from the previously reported com-
pound 5,[19] the m-terphenylene bridge 2 was prepared by
Suzuki–Miyaura cross-coupling (SMC), in which the reactiv-
ity difference between bromo and iodo aromatics[20] was
combined with a trimethylsilyl (TMS) masking strategy
(Scheme 4a).[21] Compound 5 was first subjected to SMC
with 6[20c] to give 7, followed by electrophilic replacement of
the TMS group of 7 by iodide with iodine monochloride
(ICl). The obtained 8 was then subjected to SMC with 9[20c]
not subject to conformational changes through rotations.
Second, these new bonds are oriented perpendicular to the
initially opposing anthracene faces. These two aspects will
help to achieve what is so highly needed in 2D polymer syn-
thesis, namely a lateral periodic growth event. A third ad-
vantage is that anthracene absorbs UV light with a range of
wavelengths not exciting its photodimer. This allows bond
formation to take place under certain conditions, while
under others the reverse process can be enforced, for exam-
ple, by shorter wavelength irradiation. This is an important
issue for aspects such as efficiency of polymerization, heal-
ing of polymer structure, and depolymerization within se-
lected application areas such as photo lithography. Last but
not least, the reaction proceeds without any use of reagents
or catalysts so that it can work not only in solution but also
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Synthesis of a Macrocycle with Three 1,8-Anthrylene Units
COMMUNICATION
action was carried out under the standard Sonogashira con-
ditions, using cuprous iodide in combination with a Pd0 cata-
lyst, the yield of 1 tended to be low despite of the pseudo-
dilution (syringe pump) conditions applied. GPC analysis of
the products suggested that intermolecular coupling to pro-
duce linear oligomers was the predominant reaction. On the
other hand, when the reaction was performed under cupr-
ous-free conditions, the yield of 1 surprisingly increased to
about 45%. This result is consistent with our previous obser-
vation in other macrocycle synthesis: the absence of CuI
from the Sonogashira reaction can have a positive impact on
the cyclization efficiency.[20b] Even though the reaction pro-
ceeded rather slowly and therefore required more Pd cata-
lyst for its acceleration, GPC analysis of the products clearly
indicated grossly reduced formation of the oligomeric side
products (see the Supporting Information). Figure 1 displays
Scheme 4. Synthesis of the building blocks 2 (a) as well as 3 and 4 (b).
to give 10, and the TMS group was substituted by iodide
using ICl to furnish 2, which was obtained on 4 g scale.
The 1,8-anthrylene building blocks 3 and 4 were prepared
starting from 1-(triisopropylsilyl)ethynyl-8-(trimethylsilyl)e-
thynyl-anthracene (11) and 1,8-di[(trimethylsilyl)ethynyl]an-
thracene (12), respectively, whose multigram scale syntheses
were reported previously (Scheme 4b).[16] Compound 3 was
obtained by selective deprotection of the TMS group of 11.
As for 4, both TMS groups of 12 were first deprotected to
give 13. This was followed by silylation of 13 using one
molar equivalent of BuLi and subsequent quenching with
TMSCl. This reaction gave rise to a mixture of 12, 13, and 4.
The last one was isolated from the others by gel-permeation
chromatography (GPC). Compounds 3 and 4 were thus ob-
tained on 4 g and 1.5 g scale, respectively.
Figure 1. 1H NMR spectra of the linear precursor 20 (a) and its intramo-
lecularly cyclized product 1 (b).
1
the H NMR spectra of 20 and 1, the simplicity of the latter
of which demonstrates three anthrylene units to be arranged
symmetrically, as expected after cyclization.
The set of building blocks 2–4 were iteratively assembled
into the target compound 1 on the basis of the Sonogashira
cross-coupling protocol (Scheme 5).[22] First, 2 was subjected
to the cross-coupling with one molar equivalent of 4 to
afford 14. This reaction proceeded selectively at the iodo
terminus under the present reaction conditions at 608C.[20]
The remaining bromide terminus of 14 was then used for
the second Sonogashira cross-coupling with 3 to give 15.
The TMS group of 15 was selectively deprotected with aque-
ous NaOH to give 16, which was followed by cross-coupling
with 14 to afford 17. After TMS deprotection, 18 was sub-
jected to the cross-coupling with 2 to obtain 19, followed by
TIPS deprotection with tetra-n-butylammonium fluoride
(TBAF) to furnish 20, which is the linear precursor of the
target macrocycle 1.
Compound 21 was prepared as a model to provide in-
sights into photochemical properties of monomer
1
(Scheme 6). Because of the rather complex structure of 1 as
well as its limited amount so far available (ca. 50 mg), this
simple model compound 21 was instead chosen to inspect
the reactivity of 1 towards 2D polymer synthesis.[23] A de-
gassed solution of 21 in benzene (28 mm) was UV irradiated
at 208C. After 5 h, about 80% of the compound was con-
verted to its dimers 22 (anti) and 23 (syn) in a molar ratio of
about 8:1. Note that virtually no side reaction was observed
as long as oxygen was carefully excluded.[24] In particular,
the ethynylene spacers stayed intact. The obtained anti
dimer 22 was isolated by recycling GPC and then incubated
in the dark as a degassed solution in deuterated 1,1,2,2-tet-
rachloroethylene (5 mm). Even after incubation for one
week at 808C, 22 was found to be stable; no trace of the
Several different conditions were tested to accomplish the
final cyclization to 1 as efficiently as possible. When the re-
1
back reaction to 21 was observed by H NMR spectroscopy.
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P. Kissel, A. D. Schlꢁter, and J. Sakamoto
Notably, no excimer emission
was observed for 1, suggesting
that intramolecular [4+4] cy-
cloaddition is unlikely.[17a,b] This
is an important finding in con-
nection with the planned poly-
merization for which intramo-
lecular encounters are obvious-
ly detrimental. Furthermore,
comparison of the UV absorp-
tion spectra of 1 and 22 con-
firmed that selective photo-ex-
citation of
choosing the UV wavelength in
range of l=350–450 nm.
1 is possible by
a
These results suggest that inter-
molecular [4+4] cycloaddition
can take place irreversibly lead-
ing to the polymerization of 1.
In conclusion, macrocycle 1
in which three 1,8-anthrylene
parts are embedded in C3v sym-
metry has been designed as a
monomer for rational 2D poly-
mer synthesis.[18] Feasibility of
its synthesis was successfully
demonstrated by iterative use
of palladium-catalyzed cross-
coupling protocols. A key for
the success were the copper-
free Sonogashira conditions ap-
plied to the final cyclization
step. Accordingly, multigram-
scale synthesis of this macrocy-
cle as well as a similar structure
which carries free OH groups
(R=H in Scheme 3) available
for any subsequent functionali-
zations are now in progress.
The results from photochemical
experiments using
a
model
compound suggest that macro-
cycle 1 has the potential to un-
dergo UV-induced [4+4] cyclo-
addition without undesired pos-
sible side reactions, which is en-
Scheme 5. Iterative assembly of the building block 2–4 to the latent monomer 1.
couraging for our ongoing in-
vestigations on the UV-induced
When the temperature was raised to 1208C, however, 23%
back reaction was detected after two weeks at the tempera-
ture. These results indicate a considerable thermal stability
of the [4+4] dimeric product.
polymerization of 1 including a solid-state topochemical
polymerization.
Figure 2 exhibits UV absorption and emission spectra of
model compound 21, monomer 1, and anti dimer 22 record-
ed from dilute dichloromethane solutions. The very similar
UV absorption and emission spectra of 21 and 1 suggests
that both compounds have similar photochemical properties.
Experimental Section
All experimental procedures, NMR and MS data as well as GPC elution
curves on cyclization experiments are described in the Supporting Infor-
mation.
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Synthesis of a Macrocycle with Three 1,8-Anthrylene Units
COMMUNICATION
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Scheme 6. Model compound 21 of monomer 1 and its two photodimers
22 and 23.
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Figure 2. UV absorption and emission spectra of model compound 21 and
monomer 1 together with UV spectrum of the anti dimer 22. Black, dark
gray, and light gray lines correspond to the spectra of 1, 21, and 22, re-
spectively. Broken and solid lines were used for the emission and absorp-
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Acknowledgements
The authors thank L. Federer, S. Breitler, F. Weibel, and T. Mingarcha
(ETH Zurich) for their help with the synthesis, Dr. X. Zhang and L.
Bertschi (ETH Zurich) for mass measurements, and Prof. P. Walde and
his co-workers for their help in UV absorption and emission measure-
ments. Financial support by the ETH grant (TH-05 07-1) is gratefully ac-
knowledged.
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Keywords: anthracenes
·
cross-coupling
·
photodimerization
polymerization
·
macrocycles
·
topochemical
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[18] Scheme 3 depicts a presumably stable conformation of 1 with a C3v
point group. In principle, however, a planar D3h conformation is also
possible. The solution conformation of 1 is under investigation. This
[1] J. Sakamoto, J. van Heijst, O. Lukin, A. D. Schlꢁter, Angew. Chem.
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P. Kissel, A. D. Schlꢁter, and J. Sakamoto
includes complexation studies with guest molecules such as carbor-
ane or fullerenes. Such host–guest complexes may also be of direct
interest as monomers for 2D polymer synthesis.
this matter, see: T. Ljungdahl, K. Pettersson, B. Albinsson, J. Mar-
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this comment, however, and as the former name is used much more
often in the literature we will use it here as well.
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[23] Given the literature evidence on the polymerizability of monomers
containing at least two anthracene units,[9d,12d] solution polymeri-
zations of monomer 1 were not performed. Most likely they would
only result in poorly soluble, irregular 3D networks of little value
and difficult to analyze.[9d]
[21] See, for example: a) V. Hensel, A. D. Schlꢁter, Chem. Eur. J. 1999,
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[24] The solution needed to be degassed carefully; otherwise formation
of peroxidized anthracene was observed by 1H NMR spectroscopy
as a side reaction upon the UV irradiation.
[22] For the Sonogashira reaction without cuprous iodide two different
names are being used in the literature, namely the “copper-free So-
nogashira reaction” and the “Heck alkynylation.” For a comment on
Received: March 25, 2009
Published online: July 27, 2009
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