An oxidative coupling route to macrocyclic thiophenes and its application in the synthesis of a donor/acceptor hybrid molecule

Article abstract of DOI:10.1039/c1cc14664e

A route towards phenylene-bithiophene macrocycles via oxidative thiophene coupling under pseudo-high dilution conditions is reported. This method is applied to the synthesis of a shape-persistent thiophene macrocycle with extraannularly attached peryleneb

Full text of DOI:10.1039/c1cc14664e

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COMMUNICATION
An oxidative coupling route to macrocyclic thiophenes and its application
in the synthesis of a donor/acceptor hybrid moleculew
Stefan K. Maier, Stefan-S. Jester, Ute Muller, Walter M. Muller and Sigurd Hoger*
¨ ¨
¨
Received 29th July 2011, Accepted 26th August 2011
DOI: 10.1039/c1cc14664e
A route towards phenylene-bithiophene macrocycles via oxida-
tive thiophene coupling under pseudo-high dilution conditions is
reported. This method is applied to the synthesis of a shape-
persistent thiophene macrocycle with extraannularly attached
perylenebisimide moieties that forms supramolecular aggregates
at the solid/liquid interface.
discussed as inevitable for an efficient extraction of charges, which
is limited by fast recombination processes in such a solar cell.9
Despite some successful reports, the synthesis of thiophene
macrocycles is still a challenging task, as reactive functional
groups in the 2-position of the thiophene, such as metals
or unprotected ethynylenes, often do not possess sufficient
chemical stability to be coupled without considerable side
reactions. Because of these difficulties, the cyclisation reaction
is mostly not performed at the thiophene sites,1c or requires a
rather lengthy and advanced synthesis.10
Macrocycles containing thiophene segments have proven to
be fascinating compounds considering their ability to form
functionalised self-assembled monolayers at the solid/liquid
interface that allow the epitaxial coadsorption of guest molecules
such as fullerenes or metallacycles on top of the preformed
macrocycle pattern.1 Furthermore, supramolecular empty nano-
tubes have been observed for thiophene macrocycles in the
liquid crystalline mesophases.2 Besides self-assembly, the opto-
electronic properties of macrocyclic thiophenes are very inter-
esting as the syn configuration of adjacent thiophene units
gives rise to unusual properties.3 This might become of special
interest as linear oligothiophenes have proven to be prominent
electron rich organic semiconductors applied as donor materials
in organic solar cells. In the latter, one of the key steps is the
photoinduced electron transfer from an electron rich to an
electron poor organic semiconductor.4 For the conventional case
of two-component active layers this electron transfer occurs
intermolecularly. Therefore the covalent joint of electron rich
and electron poor molecular building blocks towards hybrid
molecules leads to one-compound active layers with intra-
molecular electron transfer which is discussed as a route towards
a maximised contact of donor and acceptor materials.5
In this work we will present an efficient method to directly
connect two thiophene terminated precursor molecules at the
2-positions by oxidative coupling. This allows the synthesis of
macrocycles from the respective thiophene-terminated ‘‘half-
ring’’ structures in only one synthetic step.11
The synthesis of macrocycle 5 is shown in Scheme 1. The
bridged bithiophene building block 1 involves thiophene moi-
eties constrained to planarity.1c,12 Furthermore, the bridge
between the thiophene units reduces the rotational degrees
of freedom and is thus expected to support the cyclisation
reaction.13 To overcome the general low solubility of rigid
structures,14 long branched alkoxy sidechains were attached to
the bithiophene building blocks.
Bithiophene 1 was statistically iodinated with N-iodo-
succinimide (NIS) to give the monoiodinated compound 2.
Perylenebisimide (PBI) units are promising candidates
for such electron accepting building blocks. Additionally, they
are known for their outstanding properties as highly stable
fluorescent organic dyes.6 They exhibit a pronounced tendency
to form stacked arrays which may support the aggregation of
PBI-functionalised molecules towards well-defined supra-
molecular stacks.7,8 This is of great importance for functional
donor/acceptor hybrid materials, as an ordered structure is
Universitat Bonn, Kekule´-Institut fur Organische Chemie und
¨
¨
Biochemie, Gerhard-Domagk-Str. 1, 53121 Bonn, Germany.
E-mail: hoeger@uni-bonn.de; Fax: +49 (0)228 73 5662;
Tel: +49 (0)228 73 3495
w Electronic supplementary information (ESI) available: Synthetic
procedures, additional STM images and characterisation. See DOI:
10.1039/c1cc14664e
Scheme 1 Synthesis of phenylene-bithiophene macrocycle
5 via
oxidative coupling. (a) NIS, AcOH, CHCl3, rt, 12 h, 60%; (b) PEPPSI-
IPr,15 KOH, THF, 60 1C, 40 h, 79%; (c) FeCl3, CHCl3, rt, 110 h, 27%.
c
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Chem. Commun., 2011, 47, 11023–11025 11023

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Fig. 1 GPC analysis of the precursor 4 (blue), macrocycle 5 (green)
and the crude reaction product (red) of the cyclisation reaction of 4
with FeCl3 (PS-calibration).
In a twofold Suzuki cross-coupling reaction 4 was obtained in
good yield.
The polymerisation of unfunctionalised thiophenes with
iron(III) chloride is a well-known and facile route to poly-
thiophenes.16 Following this pathway, macrocycle 5 was
synthesised by slowly adding the precursor molecule 4 to a
dispersion of iron(III) chloride in chloroform. The outcome of
the cyclisation reaction is shown in Fig. 1. Gel permeation
chromatography (GPC) of the crude product shows an elution
diagram typical for a cyclisation reaction under pseudo-high
dilution conditions.17 The major component is the cyclic dimer,
along with trimer, higher oligomers and polymers. The dimer 5
was purified by recycling GPC (rec GPC), and its cyclic structure
was confirmed by high resolution mass spectrometry (MS) as
well as high temperature NMR spectroscopy. Room temperature
NMR signals were broadened especially in the aromatic region
most probably due to strong p–p interactions between the
expanded aromatic systems of the molecules (see ESIw).
The oblate donor/acceptor hybrid molecule 10 is functionalised
with PBI substituents directly attached to the m-phenylene corner
pieces of the macrocyclic backbone. The synthetic pathway
is shown in Scheme 2. Peryleneimidemonoanhydride 618
was condensed with 3,5-diiodoaniline19 to yield the PBI 7
which was subsequently converted to the diboronic ester 8.
The latter was reacted with the monoiodinated bithiophene 2
under Suzuki coupling conditions to yield 9. If cyclised under
similar conditions as applied to the synthesis of 5, 9 shows a
larger tendency to form higher oligomers with a trimeric species
as a major product (see GPC data in the ESIw). However,
oxidative coupling under even higher diluted conditions
yielded up to 23% of cyclodimer 10. The crude product was
again purified by rec GPC and characterised by MS. While 5
could be investigated by high temperature NMR, even at
elevated temperature the NMR signals of 10 are significantly
broadened.
Scheme 2 Synthesis of the donor/acceptor hybrid macrocycle 10.
(a) m-cresol/isoquinoline, 80 - 200 1C, 9 h, 81%; (b) (Bpin)2, KOAc,
PdCl2(dppf), DMF, 60 1C, 15 h, 84%; (c) PEPPSI-IPr, KOH, THF,
60 1C, 24 h, 55%; (d) FeCl3, CHCl3, rt, 116 h, 23%.
Fig. 2 UV/Vis absorption spectra of the hybrid molecule 10, the ring
5 and the PBI 11 (solid lines, left axis, all 10À5 M in CH2Cl2) and
normalised fluorescence spectra of 5 and 11 (dotted lines, right axis).
The UV/Vis spectra of the two macrocycles 5 and 10 as well
as of the PBI 11 (cf. Fig. 3c, 11 is used as a reference system)
are displayed in Fig. 2. The spectrum of the hybrid molecule 10
can be drawn back to a superposition of the spectra of compo-
nents 5 and 11 indicating weak coupling of the chromophore
units. However, despite of 5 and 11 being strong fluorescent
dyes, 10 shows no fluorescence. This may be a result of a rapid
electron transfer from the macrocycle to the PBI that offers a
non-radiative pathway towards the ground state.20
rich compound which can be easily oxidised to the doubly
charged cation. The energy of the HOMO (À4.7 eV) was
obtained from the onset of the first oxidation step. The
LUMO energy (À2.3 eV) was obtained from the bandgap
derived from UV/Vis spectroscopy (Fig. 2). With regard to the
HOMO/LUMO potentials of the PBI 11 (À3.8 and À6.1 eV
respectively), in 10 a photoinduced electron transfer from the
macrocyclic backbone to the extraannular PBI moieties is
possible after excitation of either one of the chromophores.
This assumption is supported by cyclic voltammetry (CV)
measurements displayed in Fig. 3. Macrocycle 5 is an electron
c
11024 Chem. Commun., 2011, 47, 11023–11025
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the structure stabilising effect of the PBI–PBI interaction that
leads to robust 1D aggregates at the TCB/HOPG interface.
In conclusion, an oxidative coupling route towards thiophene
macrocycles was applied to prepare a shape-persistent electron
rich thiophene macrocycle with extraannularly attached electron
poor PBI moieties. We have shown that the organisation in
one dimension is dominated by the strong attractive forces
between the PBI units. The results prove the concept of
interaction of PBI substituents applied for supramolecular
structure formation of macrocycles.
Fig. 3 (a) Cyclic voltammogram of 5 in CH2Cl2 at a concentration of
4 Â 10À4 M and a sweep rate of 100 mV sÀ1; (b) HOMO/LUMO
potentials of 5 compared to the HOMO/LUMO energies of the PBI
dye 11 shown in (c).
We gratefully acknowledge the DFG (SFB 813 and SFB
624) for financial support.
Notes and references
We were able to gain additional structural information of 10
by directly visualising molecular patterns formed by the
macrocycles at the interface of 1,2,4-trichlorobenzene (TCB)
and highly oriented pyrolytic graphite (HOPG) using scanning
tunneling microscopy (STM; see Fig. 4). Stacking between
PBI substituents of adjacent molecules leads to the formation
of (brightly appearing)21 chain-like supramolecular one-
dimensional (1D) assemblies with a variable line distance of
w1D Z 5.5 nm (cf. Fig. 4b).22 The latter are separated by
(darker appearing) intermediate regions covered by the flexible,
branched 2-octyldodecyloxy substituents that tend to assemble
along the HOPG main axes.23 From the large distance we
assume that all alkoxy substituents are adsorbed and do not
interdigitate, which frustrates the interaction of molecules of
adjacent lines, so that only 1D order is observed. However,
some of the lines form a most dense, two-dimensional (2D)
crystalline packing (Fig. 4a, dashed box, and Fig. 4b) with
observed line distances of w2D = 3.5 Æ 0.3 nm, which are
consistent with an incomplete adsorption of the branched
alkoxy sidechains. It is of interest to note that both packing
motifs exist under the imaging conditions used here, indicating
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extraannular PBI units (bright spots) and consistent with the molecular
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represent the 1D and 2D ordered adsorbate patterns, as observed in (a).
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Chem. Commun., 2011, 47, 11023–11025 11025