Shape-persistent arylenevinylene macrocycles (AVMs) prepared via acyclic diene metathesis macrocyclization (ADMAC)

Article abstract of DOI:10.1039/c0cc02941f

Shape-persistent arylenevinylene macrocycles (AVMs) were successfully prepared in one step from readily available aromatic diene monomers through olefin metathesis in good yields. ¹H NMR, UV-Vis absorption and fluorescence studies revealed the

Full text of DOI:10.1039/c0cc02941f

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Shape-persistent arylenevinylene macrocycles (AVMs) prepared
via acyclic diene metathesis macrocyclization (ADMAC)w
Yinghua Jin,a Aibo Zhang,ab Yongshun Huanga and Wei Zhang*a
Received 31st July 2010, Accepted 10th September 2010
DOI: 10.1039/c0cc02941f
Shape-persistent arylenevinylene macrocycles (AVMs) were
successfully prepared in one step from readily available aromatic
diene monomers through olefin metathesis in good yields.
1H NMR, UV-Vis absorption and fluorescence studies revealed
the aggregation behavior of the obtained macrocycles. SEM
characterization showed AVM nanofibril formation.
and the second most stable product determines whether a
unique product will be formed under reversible conditions.1a
Given the above considerations, the potential challenges in
AVM synthesis can be envisioned as follows: (1) the uncontrolled
double bond configuration (E/Z) in olefin metathesis could
lead to a variety of macrocyclic products with different angles
and geometry; (2) the higher reactivity of the alkene metathesis
catalyst on end groups than on internal double bonds6 may also
hamper selective generation of a complex, thermodynamically
most stable structure: formation of some intermediate
compounds may not be truly reversible due to the relative
inertness of the internal double bonds. Nonetheless, it is highly
desired to explore the feasiblity of utilizing olefin metathesis in
AVM preparation, and the success of the ADMAC approach
would open new possibilities for generating novel AVMs and
studying their unique properties.
Shape-persistent macrocycles have attracted great attention in
the past two decades, due to their unique structures, novel
properties, and potential applications.1 These macrocycles
usually have rigid, planar hydrocarbon backbones, which can
form ordered structures under certain conditions. There has
been significant progress in the preparation of aryleneethynylene
macrocycles (AEMs) through dynamic covalent chemistry
(DCC),2 namely alkyne-metathesis-mediated cyclooligomeri-
zation in which the thermodynamically most favored target
macrocycles can be obtained in one step and high yield from
simple diyne monomers.3 In contrast to the intensive research
efforts devoted to AEMs, the studies on their hydrogenated
analogues, arylenevinylene macrocycles (AVMs), have lagged
far behind.4 AVMs presumably have more conformational
freedom compared to AEMs, and would diminish the rigidity
of the backbone since the vinylene group could undergo
conformational isomerization. Although olefin metathesis5
has been intensively used in synthesis of flexible large cyclic
compounds, to our best knowledge, there has been no report
on its application in shape-persistent macrocycle synthesis
through one-step cyclooligomerization. In this work, we
report the first successful synthesis of AVMs through Acyclic
Diene Metathesis Macrocyclization (ADMAC). Such a dynamic
covalent approach shows a general scope. The observed self-
aggregation behavior of the obtained AVMs and their fabrication
into nanofibrils will also be discussed.
Initially, we conducted the ADMAC reaction of divinyl-
substituted monomer 1 catalyzed by Grubbs’ 2nd generation
catalyst. The reaction that was conducted in 1,2,4-trichloro-
benzene under nitrogen at 35 1C turned out to be very successful,
7
affording AVM 2 in 64% isolated yield (eqn (1)).z The
reaction progress was monitored by gel permeation chromato-
graphy (GPC, Fig. 1). The GPC traces after 1, 4, 8 and
18 h showed the initial formation of high molecular weight
polymers and/or large cyclic intermediates, which gradually
transformed into cyclohexamer 2 along the reaction pathway,
thus indicating the reversibility of such a cyclooligomeri-
zation process and the thermodynamic stability of the target
macrocycle.
In contrast to conformationally flexible cycles, shape-
persistent macrocycles generally have a regular repeating unit
with few degrees of conformational freedom. Under a certain
monomer concentration, macrocyclic strain and entropy con-
siderations determine the preferred ring size, with macrocycles
having the smallest number of monomer units being entropi-
cally favored. The bond angles and conformational rigidity of
the monomer determine if a particular macrocycle is strained
or strain-free. The energy gap between the most stable product
ð1Þ
The cyclic structure of AVM 2 precludes the cis conforma-
tion of the –CQC– bond, which would induce significant angle
strain. The H NMR spectrum of AVM 2 showed only three
1
aromatic signals, including vinyl protons, which suggests
that the conformational isomerism arising from the –CQC–
rotation around phenylene–vinylene–phenylene linkages, if
any, is occurring rapidly on the NMR time scale.
a Department of Chemistry and Biochemistry, University of Colorado,
Boulder, CO 80309, USA. E-mail: wei.zhang@colorado.edu;
Fax: (+1) 303-492-5894; Tel: (+1) 303-492-0652
b School of Natural and Applied Science, Northwestern Polytechnical
University, Xian, P. R. China
w Electronic supplementary information (ESI) available: Experimental
details, characterization data, and NMR and MALDI spectra. See
DOI: 10.1039/c0cc02941f
It has been well demonstrated that shape-persistent phenylene–
acetylene macrocycles (PAMs) tend to form aggregates
via self-association induced by aromatic p–p interactions.8
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Fig. 3 UV-Vis absorption spectra (left) and fluorescence emission
spectra (right, lex = 324 nm) of AVM 2 in various solvents (5 mM).
In cyclohexane, strong fluorescence signals (Fig. 3) were
observed at 375 nm and 394 nm with a shoulder band at
414 nm. The spectrum obtained in chloroform showed similar
emission intensity, but the emission shifted to 380 nm and
398 nm respectively, and the shoulder band is less significant.
In contrast to the strong emissions observed in cyclohexane
and chloroform, AVM 2 in acetonitrile and methanol only
showed very weak fluorescence emissions, presumably due to
the fluorescence quenching,10 which resulted from the macro-
cycle aggregation induced by the poor solvent. The signal
observed in methanol is almost featureless.
Fig. 1 ADMAC reaction progress of diene monomer 1 as monitored
by GPC (THF, 30 1C).
The electronic character and orientation of the substituents on
the PAMs strongly influence their self-associating tendency.
PAMs substituted with electron donating alkoxy or alkanoate
groups don’t favor aggregation, while PAMs with electron with-
drawing ester groups showed strong aggregation behavior.8d
Interestingly, in great contrast to the non-aggregation
character of alkoxy substituted PAMs, AVM 2, which is
substituted with six decyloxy groups,9 showed strong self-
aggregation in organic solvents. A 1H NMR study showed
the strong dependence of the aromatic chemical shifts of this
macrocycle on its concentration. At ambient temperature, the
chemical shifts in CDCl3 of the two anisochronous aromatic
protons and vinyl protons shifted from d 7.38 ppm to 7.24 ppm,
from d 7.15 ppm to 7.00 ppm, and from 6.98 ppm to 6.85 ppm
respectively as the concentration changed from 0.1 to 10 mM
(Fig. 2). The chemical shifts of the aliphatic protons, unlike
those of the aromatic and vinylic protons, remained relatively
unchanged over the same concentration range. These observa-
tions suggest that AVM 2 self-associates in the CHCl3 solution,
and the –CQC– bond does not significantly interfere with the
backbone p–p stacking interactions.
To demonstrate the general scope of the ADMAC approach
in preparation of AVMs, we also successfully synthesized a
carbazole-based AVM 4 starting from the diene monomer 3.
The reaction was also conducted in 1,2,4-trichlorobenzene
at 35 1C, and the cyclotetramer 4 was obtained in 57%
yield (eqn (2)), thus showing a general scope of such a cyclo-
oligomerization approach to AVMs.
ð2Þ
The UV-Vis spectra of AVM 2 in four different solvents are
shown in Fig. 3. The absorption shifts from 302 nm in
cyclohexane to 318 nm in methanol, and an isosbestic point
at 323 nm is observed, indicating a conversion from a free
single macrocycle to aggregated species. In acetonitrile and
methanol, the absorption band was broader along with a long
tail absorption extending up to 500 nm, which also indicates
the aggregation of AVM 2 in these solvents.
More interestingly, rapid dispersion of the concentrated
solution of AVM 2 or 4 in CHCl3 into CH3CN induced
self-assembly of these two macrocycles into 1D nanofibrils,
which were characterized by scanning electron microscopy
(SEM, Fig. 4). Most of the fibers are around tens of mm in
length, and their diameters are around a few hundred nm.
Presumably, the nanofibril formation is induced by the strong
self-aggregation of these macrocycles. The p–p interactions
between aromatic backbones and the side chain entanglement
are the two key factors contributing to the observed morpho-
logy of the aggregates.11
Investigation of the fluorescence emission of AVM 2 in
various solvents at a concentration of 5 mM gave more insight.
Fig. 2 Concentration-variable NMR spectra of AVM 2 in CDCl3 at
20 1C.
Fig. 4 SEM images of AVM 2 (left) and AVM 4 (right) fabricated by
rapid solution dispersion.
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The red solution was heated at 35 1C under nitrogen for 18 h.
All the solvent was removed and diethyl ether (100 mL) was
added. The ethereal solution was washed with water (3 ꢀ 50 mL),
dried over Na2SO4, and concentrated to give the crude product.
Purification by flash column chromatography using CH2Cl2 and
hexane (1 : 1, v/v) as the eluent afforded AVM 2 as a white solid
(109 mg, 64%).
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Fig. 5 The most favored conformer of AVM 20: top view (top left),
side view (bottom left), and AVM 40: top view (top right), side view
(bottom right). Methyl group was used in calculation instead of decyl
for simplification. Energy minimization was performed with Spartan
04 at semi-empirical PM3 level.
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The most favored conformer of methyl substituted analogue
AVM 20 has a planar carbon backbone, and the molecule is
highly symmetrical, which is fully consistent with the NMR
characterization data (Fig. 5). The favored conformer of AVM
40 is not perfectly planar, and has a saddle-like conformation
with alternating carbazole moieties tilted up and down.
However, the distorted non-planar conformation seems not
to impede the self-aggregation of AVM 4, as evidenced by
successful nanofibril fabrication, which is probably due to the
conformational flexibility provided by the vinylene linkage.
In summary, we successfully developed a one-step synthetic
approach to shape-persistent AVMs from simple aromatic
dienes. The highly efficient ADMAC method is generally
applicable, and AVMs with different shape and geometry
can be prepared. The AVMs showed a strong self-aggregation
behavior, and can be fabricated into nanofibrils. Considering
the great importance and growing interest in 1D self-assembly
of planar aromatic molecules into well-defined nanowires or
nanotubes,12 the AVMs reported herein open many new
possibilities for novel tubular nanomaterial fabrication. The
substrate scope of such an ADMAC approach, the AVM
self-association behaviors, as well as the structure–function
relationship of the aggregation-induced nanofibrils are being
investigated in our lab and will be reported in due course.
We thank Dr Kuthanapillil Jyothish for helpful discussions,
and Dr Richard K. Shoemaker for NMR assistance. W. Z.
thanks the CRCW Junior Faculty Development Award and
the University of Colorado for funding support through the
innovative seed grant program.
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strain in the macrocycles, so the unfavored cyclic products were
converted into the most stable cyclohexamer 1 under the reversible,
thermodynamically-controlled condition.
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9 Linear decyl group was installed to enhance the solubilityof AVM
2 and oligomeric reaction intermediates. For the effect of side chain
substituents on the self assembly process, see K. Balakrishnan,
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AEMs has been reported, and the fibril formation is primarily
due to the columnar stacking of the macrocycles, see
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Notes and references
z Procedure for the synthesis of AVM 2: To a Schlenk tube were added
1,3-divinyl-5-decyloxy-benzene (1, 217 mg, 0.76 mmol) and a solution
of Grubbs’ 2nd generation catalyst (48 mg, 0.076 mmol) in 1,2,4-
trichlorobenzene (14 mL). The reaction apparatus was evacuated
and refilled with nitrogen, and this process was repeated three times.
12 (a) L. Zang, Y. Che and J. S. Moore, Acc. Chem. Res., 2008, 41,
1596; (b) J. Wu, W. Pisula and K. Mullen, Chem. Rev., 2007, 107,
¨
718; A. P. H. J. Schenning and E. W. Meijer, Chem. Commun.,
2005, 3245.
c
8260 Chem. Commun., 2010, 46, 8258–8260
This journal is The Royal Society of Chemistry 2010