Perylene bisimide macrocycles and their self-assembly on HOPG surfaces

Article abstract of DOI:10.1039/c0cc03004j

Acetylene-linked macrocycles incorporating multiple perylene tetracarboxylic acid bisimide (PBI) chromophores have been synthesised and separated by recycling GPC. The very first example of such macrocycles, i.e., cyclic trimer 5, containing three PBI dye

Full text of DOI:10.1039/c0cc03004j

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Perylene bisimide macrocycles and their self-assembly on HOPG
surfacesw
Felix Schlosser, Vladimir Stepanenko and Frank Wurthner*
¨
Received 3rd August 2010, Accepted 14th September 2010
DOI: 10.1039/c0cc03004j
Acetylene-linked macrocycles incorporating multiple perylene
tetracarboxylic acid bisimide (PBI) chromophores have been
synthesised and separated by recycling GPC. The very first
example of such macrocycles, i.e., cyclic trimer 5, containing
three PBI dyes self-assembles into highly ordered donut-shaped
unique hexagonal nanopatterns on HOPG surfaces as revealed
by atomic force microscopy (AFM).
Macrocyclic compounds have always been attractive targets
due to the challenges involved in their synthesis and the
prospects arising from their distinct molecular shape and
geometry. In the field of p-conjugated systems, acetylene-
based macrocycles occupy a prominent standing.1 The concept
of acetylene and acetylenephenylene linkage has indeed pro-
vided large-sized macrocyclic scaffolds with highly defined
molecular shapes and properties that are quite distinctive from
conformationally flexible open-chain analogues.2 In extension
of fundamental work on the synthesis of acetylene-based
macrocycles, the successful incorporation of different chromo-
phores and electrophores such as oligothiophenes,3 tetrathia-
fulvalenes,4 carbazoles,5 salphens,6 or porphyrins7–9 in such
macrocyclic scaffolds has been demonstrated. Many of these
macrocycles are rigid, planar architectures that facilitated
fascinating investigations of their self-organisation on surfaces
by scanning tunneling microscopy (STM).10,11
Scheme 1 Synthesis of PBI macrocycle 5. (a) Imidazole, 120 1C,
30 min, 99%; (b) Pd(PPh3)2Cl2, CuI, THF, HNiPr2, 50 1C, 1 h, 99%;
(c) nBu4NF, THF, rt, 2 min, 90%; (d) Pd(PPh3)2Cl2, CuI, CH2Cl2,
HNiPr2, ꢀ30 1C, 3 d, 46% yield of crude macrocyclic products;
TIPS = triisopropylsilyl.
could be synthesised in remarkably high yield, and show that
this cyclic trimer self-assembles into highly ordered two-
dimensional (2D) nanopatterns on highly ordered pyrolytic
graphite (HOPG) surface.
Embedding of chromophores into rigid macrocyclic
scaffolds in a precise arrangement is of great significance to
ensure defined electronic and excitonic coupling patterns that
are otherwise difficult to attain in synthetic molecular
scaffolds, but such photophysical functions are common in
natural light-harvesting complexes, where protein matrices
ensure the desired fixation of circular dye arrays.12 Inspired
by the cyclic organisation of chlorophyll dyes in natural light-
harvesting systems, appreciable number of acetylene- and
phenylene-linked macrocycles containing up to 32 porphyrin
units have been synthesised by the groups of Sanders,7
Lindsey,8 and Osuka.9 It is however surprising, keeping in
mind the modest fluorescence and exciton transport properties
of porphyrins, that related macrocyclic systems based on
outstanding fluorophores such as perylene bisimides13–16 are
still unprecedented.
The synthesis of macrocycle 5 was accomplished in four
straightforward steps as depicted in Scheme 1, starting with
the condensation of perylene tetracarboxylic acid bisanhydride
(PBA) 1 with 3-iodoaniline in imidazole at 120 1C to give the
corresponding PBI chromophore 2. The latter was then
reacted with triisopropylsilylacetylene in a Sonogashira coupling
reaction to obtain PBI 3. Removal of the protecting groups
with tetrabutylammoniumfluoride afforded the building block
PBI 4 containing terminal acetylene groups at the imide
positions. The subsequent macrocyclisation of
4 was
performed by palladium-catalysed oxidative homocoupling
under high dilution conditions. For this purpose, the precursor
and the catalysts were dissolved in CH2Cl2/HNiPr2
4
(9 : 1 vol%, ca. 0.2 mM) under ambient atmosphere and the
reaction mixture was stored in a freezer at ꢀ30 1C for three
days. Afterwards, the solvent was removed under reduced
pressure and the crude mixture was filtered over a silica gel
pad with CH2Cl2 to remove residual reagents and polymeric
side products. The product mixture obtained after filtration
does not contain any terminal acetylene groups as revealed by
1H NMR, indicating that the product mixture consists of only
macrocycles. MALDI-TOF analysis reveals that cyclic trimer
(n = 3) is the major product, although higher cyclic oligomers
up to the octamer could be detected (Fig. 1). It is remarkable
Herein, we report the first example of a rigid acetylene-
linked perylene bisimide (PBI) macrocycle 5 (Scheme 1) that
Universitat Wurzburg, Institut fur Organische Chemie, Am Hubland,
¨
¨
97074 Wurzburg, Germany.
¨
¨
E-mail: wuerthner@chemie.uni-wuerzburg.de;
Fax: +49 (0)931 31 84756; Tel: +49 (0)931 31 85340
w Electronic supplementary information (ESI) available: Synthetic
details, spectroscopic characterisation, NMR and mass spectra. See
DOI: 10.1039/c0cc03004j
c
8350 Chem. Commun., 2010, 46, 8350–8352
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observed in CH2Cl2 with an absorption coefficient of
49 200 Mꢀ1 cmꢀ1 and 139 500 Mꢀ1 cmꢀ1, respectively. The
absorption coefficient of trimer 5 is nearly three times higher
than that for the monomer 4 but otherwise both spectra are of
same shape,z indicating that the electronic interaction between
the chromophores in macrocycle 5 is negligible in the ground
state. The emission spectra for 4 and 5 are both almost the
mirror image of the corresponding absorption spectra with
maxima at 611 nm and 614 nm, respectively. The fluorescence
quantum yield of monomer 4 is 95% and for the cyclic trimer 5
with 91% is surprisingly high, which is to our knowledge
unprecedented among multichromophoric macrocycles.
The electrochemical properties of 4 and 5 have been
investigated by CV, revealing two reversible reductions and
one reversible oxidation for each of them that resemble the
electrochemical characteristics of similar PBI chromophores.13
The redox potentials of monomer 4 and cyclic trimer 5 are very
similar, indicating that neither the circular arrangement of the
chromophores nor the dialkyne bridge has an influence on the
redox properties of 5 (for CV data and cyclovoltammograms,
see ESIw).
Fig. 1 MALDI-TOF mass spectrum of macrocyclic product mixture.
that the overall yield for the three steps from PBA 1 to
precursor molecule 4 is almost 90% and that the macro-
cyclisation step affords 46% of macrocyclic products,17 hence
an easy access to this new class of PBI-based macrocycles is
worked out.
The capability of the cyclic trimer 5 to self-assemble into 2D
nanopatterns on HOPG was investigated by AFM. A dilute
solution of 5 in a THF/MCH mixture (7 : 93 vol%) was spin-
coated onto HOPG under 4000 rpm. As shown in Fig. 3,
macrocycle 5 self-assembles into 2D donut-like structures in a
unique hexagonal arrangement.
The mixture of macrocycles was then further purified by
recycling GPC (JAIGEL 2H + 2.5H columns; CHCl3,
3.5 mL minꢀ1). The separation is illustrated in Fig. 2 wherein
the peak of the cyclic trimer (n = 3: 5) is marked with an
asterisk. Macrocycles larger than the hexamer were cut off
during the purification procedure to avoid overlapping of the
peaks. After six cycles, the pure trimeric macrocycle 5 was
obtained in a yield of 18%. The molecular structure of
macrocycle 5 was confirmed by NMR spectroscopy and mass
spectrometry. The optical and electrochemical properties of 5
have been studied by UV-vis and fluorescence spectroscopy,
and cyclic voltammetry (CV) (see ESIw).
The optical properties of the monomeric building block PBI
4 and macrocycle 5 have been characterised by UV-vis absorption
and fluorescence emission measurements. For both monomer
4 and cyclic trimer 5, an absorption maximum at 582 nm is
Fig. 3 AFM height images of a monolayer of 5 (5.25 ꢁ 10ꢀ6 M)
obtained by spin-coating from THF/MCH solution onto HOPG. (a–c)
Topographic images with z-scale of 0.8 nm (a) and 0.6 nm (b, c); inset
in (b) is cross-section analysis along the yellow line 1-10; (c) zoomed
unit of the hexagonal domains and proposed model (Hyperchem) for
the molecular arrangement of cyclic trimer 5 in donut-like structures
(the distance between the centres of the donuts and the width of a
donut ring are indicated by yellow arrows, and the average values are
8.2–8.6 nm and 1.6–1.8 nm, respectively); (d) an expanded section of
the model shown in (c).
Fig. 2 Recycling GPC traces of the mixture of macrocycles (JAIGEL
2H + 2.5H columns; CHCl3, 3.5 mL minꢀ1; UV-vis detection at
500 nm). The peak of the trimer 5 is marked with an asterisk; to the left
of this peak follow the peaks for tetramer, pentamer, and so on.
c
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From the size and the shape of the hexagonal nanopattern,
it is most likely that six cyclic trimers constitute one hexagon
(molecular donut). In Fig. 3c, we show the proposed arrangement
of the chromophores modelled with Hyperchem. In this
model, the dialkyne edges are laying next to each other and
the bulky substituents of the perylene core are pointing
outside, building a donut-like structure out of six trimers in
the sterically most favourable arrangement. These molecular
donuts are hierarchically organised in even bigger patterned
domains. The distances proposed in this model are in excellent
agreement with those determined for the 2D structure by
AFM. The distance between two donut holes is estimated to
be 8.2–8.6 nm by AFM and 8.6–8.8 nm in the model. The
width of a donut ring that corresponds to the diameter of the
macrocycle is determined to be 1.6–1.8 nm in average, which is
quite close to that in the model (1.7–1.9 nm). The height was
estimated from the cross-section analysis to be 0.20 ꢂ 0.03 nm
(see inset in Fig. 3b), indicating a monolayer.
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To conclude, we have reported here the first synthesis of
acetylene-linked macrocycles incorporating multiple perylene
bisimide chromophores. The obtained cyclic trimer 5 exhibits
a fluorescence quantum yield of 91%, which is to the best of
our knowledge unprecedented for macrocyclic multichromo-
phoric arrays. Macrocycle 5 self-assembles on HOPG surfaces
into a unique well-ordered hexagonal nanopattern with
donut-like structure. In our future work, the organisational
and functional properties of macrocyle 5 and its higher
homologues will be approached. This new class of macrocycles
is highly promising for future exploration of exciton and
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8352 Chem. Commun., 2010, 46, 8350–8352
This journal is The Royal Society of Chemistry 2010