Synthesis and self-assembly of a triphenylene-containing amphiphilic conjugated macrocycle

Article abstract of DOI:10.1016/j.tetlet.2009.02.177

A new amphiphilic π-conjugated system containing a shape-persistent conjugated macrocycle with two triphenylene chromophores as the anchoring units has been successfully synthesized. The amphiphilic macrocycle has been found to self-assemble into robust a

Full text of DOI:10.1016/j.tetlet.2009.02.177

Tetrahedron Letters 50 (2009) 2147–2149
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis and self-assembly of a triphenylene-containing amphiphilic
conjugated macrocycle
Degang Wang a, Jeffrey Fu Hsu a, Mahuya Bagui a, Vladimir Dusevich b, Yong Wang b, Yi Liu a,
Andrew J. Holder a, Zhonghua Peng a,
*
a Department of Chemistry, University of Missouri-Kansas City, Kansas City, MO 64110, USA
b Department of Oral Biology, University of Missouri-Kansas City, Kansas City, MO 64110, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
A new amphiphilic p-conjugated system containing a shape-persistent conjugated macrocycle with two
triphenylene chromophores as the anchoring units has been successfully synthesized. The amphiphilic
macrocycle has been found to self-assemble into robust and uniform microspheres, presumably due to
Received 10 February 2009
Revised 18 February 2009
Accepted 23 February 2009
Available online 26 February 2009
the strong
pÀ
p
stacking interaction between triphenylene chromophores.
Ó 2009 Elsevier Ltd. All rights reserved.
Keywords:
Triphenylene
Macrocycle
Amphiphilic
1. Introduction
hydrophobic component9 or have both hydrophilic and hydropho-
bic chains laterally attached to rotatable phenyl rings in the frame
Amphiphilic or in a broader term dual-character molecules
(including block copolymers) self-organize into a variety of struc-
tures such as spheres, vesicles, cylinders, and lamellars.1 These
nano-objects may be rendered electrically and optically active if
a p-conjugated component is built into the self-assembling mole-
cule.2,3 Indeed, amphiphilic conjugated systems have drawn tre-
mendous attention in recent years.4 Such systems can be realized
by having a rigid
p-conjugated segment as the core, asymmetri-
cally flanked with hydrophilic chains on one end and hydrophobic
tails on the other, as shown schematically in Figure 1. Indeed, a
number of such systems have been reported, where the
p-conju-
gated component is either a rigid rod (linear
p-conjugated oligo-
mers, model A)2,3,5 or a planar disk (polycyclic aromatic systems,
model B).6 One elegant representative example for each type is
shown in Figure 1. Interestingly, a
p-conjugated amphiphile with
a rigid -conjugated cyclic core has not yet been realized (model
p
C in Fig. 1). Such amphiphilic rigid macrocycles with their unique
molecular geometry may self-assemble to new interesting nano-
objects.
Shape-persistent macrocycles based on phenylene ethynylenes
have been actively studied in recent years.7 There are also reports
of amphiphilic rigid macrocycles.8,9 However, the reported amphi-
philic macrocycles either have the cyclic component itself as the
* Corresponding author. Tel.: +1 816 235 2288; fax: 1 816 235 5502.
E-mail address: pengz@umkc.edu (Z. Peng).
Figure 1. Schematic models of amphiphilic p-conjugated systems.
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.02.177

2148
D. Wang et al. / Tetrahedron Letters 50 (2009) 2147–2149
of the macrocycle.8 Such macrocycles were designed to possess
switchable interior environments. The new amphiphilic rigid
macrocycles as shown in model C of Figure 1 have a hydrophilic
head and a hydrophobic tail and thus represent a new class of con-
jugated amphiphiles. In this Letter, we report the synthesis of the
first such shape-persistent amphiphiles.
Scheme 1 shows the structure of the macrocyclic amphiphile.
Two planar polycyclic aromatic hydrocarbon structures (triphenyl-
ene), one with hydrophilic chains and the other with hydrophobic
tails, are used as corner units, which not only enhance inter-ring
corresponding larger macrocycles. The 1H NMR spectra of 5 in
CDCl3 show an extremely broad hump from 6.6 to 8.2 ppm (see
Supplementary data for the spectra) in the aromatic region,
indicating strong inter-ring p-stacking. Sitting on this broad hump,
one can identify five discernible peaks. There is also one well-iso-
lated aromatic signal appearing at 8.7 ppm. The complete disap-
pearance of signals corresponding to proton a in 4 and proton a
in 2 (see Scheme 1 for proton labeling and Fig. S3 of Supplementary
data for the 1H NMR spectra of 2, 4, and 5)), coupled with the
observation of the expected 6 aromatic signals in the 1H NMR
spectrum of 5 supports, albeit not conclusively, the macrocycle
structure of 5. MALDI-TOF measurements show a sharp dominant
peak at m/z 1514.12 (100%), corresponding to the molecular ion
(m/z = 1513.81) and a less intense peak at 1622.00 (30%) which is
attributed to (M+Ag+). The purity of 5 is also confirmed by the
elemental analysis.
p p interactions, but also act as anchors to direct the positioning
À
of the approaching rings during self-assembly.
The synthesis of the new amphiphilic system is shown in
Scheme 1 as well. Diiodo-functionalized triphenylenes 1 and 2
were synthesized according to an approach reported previously.10
Sonogashira coupling of 1 with 3, followed by desilylation, gave
compound 4 in 78% overall yield. Subsequent coupling of 4 with
compound 2 resulted in amphiphilic macrocycle 5. To improve
the yield of the cyclized product, the reaction system was kept at
high-dilution condition by dropping the two reactants (2 and 4)
into the catalyst solution over days (see Supplementary data for
detailed experimental procedures). Even so, a yield of only 20% is
obtained, presumably due to the formation of oligomers and their
Figure 2 shows the UV/Vis absorption and fluorescence emis-
sion spectra of compound 4 and macrocycle 5 in chloroform. As
expected by the meta linkage of the phenyl ring which breaks
p-conjugation, macrocycle 5 shows absorption and emission spec-
tra similar to those of compound 4. For example, both compounds
show absorption bands at around 300 and 340 nm and emission
peaks at 400 and 422 nm. It is noted however that macrocyle 5
has a slightly red-shifted (by 4 nm) band edge in its absorption
spectrum and a clear emission tail extending to much longer wave-
lengths. These observations may indicate the formation of some
inter-ring aggregates in the solution of 5.
C12H25
O
OC12H25
RO
OR
When methanol vapor was diffused into a chloroform solution
of 5, the clear solution became opaque and eventually precipitates
were formed. Scanning electron microscopy (SEM) indicated that
the precipitates are composed of microspheres with remarkable
size uniformity (Fig. 3a). The average size of the microspheres
2
O
O
tips
3
a
O
O
Pd(Ph3P)Cl2,
CuI, (i-Pr)2NH,
THF, 60-85 °C
20%
a) Pd(Ph3P)4, CuI
Et3N, Toluene
b) TBAF
a
I
I
1. R = C12H25
2.
78%
R = (CH2CH2O)3CH3
4
O
O
O
O
O
O
O
O
O
O
O
O
O
O
5
Scheme 1. Synthesis of triphenylene-based shape-persistent macrocycles.
Figure 3. SEM images of microspheres prepared by vapor diffusion of methanol to a
chloroform solution of 5: (a) sample on tape with Pt-coating (b) sample on tape
without Pt-coating; (c) and (d) sample on mica, after dispersion in methanol; (e)
and (f) sample on mica after heating at 60 °C for 1 h.
Figure 2. UV–vis absorption and fluorescence emission spectra of compound 4 and
macrocycle 5 in chloroform solutions.

D. Wang et al. / Tetrahedron Letters 50 (2009) 2147–2149
2149
In conclusion, a new type of shape-persistent macrocyclic
amphiphile has been successfully prepared. Such a new amphi-
phile self-assembles into robust and uniformly sized microspheres.
Acknowledgments
We thank J. Andrew Keightley for MALDI-TOF measurements.
This research was financially supported by the National Science
Foundation (DMR-0804158) of USA.
Supplementary data
Figure 4. TEM images of microspheres prepared by vapor diffusion of methanol to a
chloroform solution of 5.
Synthetic details, 1H NMR spectra, MALDI-TOF mass spectra,
additional SEM and TEM micrographs, and diameter histogram
are available. Supplementary data associated with this Letter can
be found, in the online version, at doi:10.1016/j.tetlet.2009.02.177.
was found to be 1.1 0.2 lm. The contact edge between adjacent
balls is clearly visible (Fig. 3b). The microsphere aggregates can
be dispersed by diluting with methanol (Fig. 3c and d). The micro-
spheres exhibit excellent stability as no change was observed in
regard to their size and shape after they were heated at 60 °C
for an hour and subjected to high-vacuum environments (Fig. 3e
and f).
References and notes
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To figure out whether the microspheres are solid balls or ves-
icles, methanol-dispersed microspheres were subjected to trans-
mission electron microscopy (TEM). As shown in Figure 4a,
microspheres without staining appeared as a dark disk. When
images of some of the smallest spheres were visualized, darker
cores and lighter edges were noticed (Fig. 4b), indicating that
they are solid balls. The preference for microsphere formation
may indicate that the rigid macrocyclic core of 5 does not have
a truly planar geometry. Molecular modeling indeed showed that
the ring system in 5, when interacting with methanol, exhibits a
certain curvature with the two triphenylene rings both bending
upward (see Supplementary data). One intriguing question has
to do with how the cyclic cores stack to form the microspheres:
do they stack concentrically or radially with the macrocyclic
plane either perpendicular or parallel to the spherical surface.
Preliminary studies using polarized microscopy on those micro-
spheres were not successful due to limited instrument resolution.
To realize other self-assembled nano- and micro-objects, such as
coiled nanotubes, macrocyclic amphiphiles with more planar
p-
conjugated cyclic cores may be needed. The amphiphilicity of
the system may also need to be enhanced. In particular, the
hydrophilicity of the system needs to be improved so that a bal-
anced amphiphilicity can be achieved. We are currently synthe-
sizing such systems.
9. Seo, S. H.; Chang, J. Y.; Tew, G. N. Angew. Chem., Int. Ed. 2006, 45, 7526.
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