Hexathienocoronenes: Synthesis and self-organization

Article abstract of DOI:10.1021/ja3082395

Here we report hexathienocoronenes (HTCs), fully thiophene-annelated coronenes in which six double bonds in the periphery are thieno-fused. The derivatives tetrasubstituted with hexyl and dodecyl chains show a phase formation that strongly depends on the chain length. HTCs are remarkably stronger donors than the known thiophene-annelated coronenes but do not readily assemble into well-ordered films when deposited from the vapor phase. Thus, thin-film transistors fabricated by vacuum deposition have only modest field-effect mobilities of 0.002 cm² V^-1 s^-1.

Full text of DOI:10.1021/ja3082395

Communication
pubs.acs.org/JACS
Hexathienocoronenes: Synthesis and Self-Organization
Long Chen,† Sreenivasa R. Puniredd,† Yuan-Zhi Tan,† Martin Baumgarten,† Ute Zschieschang,‡
Volker Enkelmann,† Wojciech Pisula,† Xinliang Feng,† Hagen Klauk,‡ and Klaus Mullen*,†
̈
†Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany
‡Max Planck Institute for Solid State Research, Heisenbergstr. 1, 70569 Stuttgart, Germany
S
* Supporting Information
The synthesis of the HTCs is shown in Scheme 1. The key
building block is 5,11-bis(dibromomethylene)anthradithio-
ABSTRACT: Here we report hexathienocoronenes
(HTCs), fully thiophene-annelated coronenes in which
six double bonds in the periphery are thieno-fused. The
derivatives tetrasubstituted with hexyl and dodecyl chains
show a phase formation that strongly depends on the chain
length. HTCs are remarkably stronger donors than the
known thiophene-annelated coronenes but do not readily
assemble into well-ordered films when deposited from the
vapor phase. Thus, thin-film transistors fabricated by
vacuum deposition have only modest field-effect mobilities
of 0.002 cm2 V−1 s−1.
Scheme 1. Synthesis of HTCs 1 and 2 Based on 3
olycyclic aromatic hydrocarbons (PAHs) can be regarded as
Ptwo-dimensional graphene segments. Because of their
optoelectronic and self-assembling properties,1 PAHs have
served as active components of organic electronic devices such
as field-effect transistors, light-emitting diodes, and solar cells.2
Compared with their all-hydrocarbon analogues, PAHs contain-
ing heteroatoms such as N,3 S,4 or O5a in the aromatic skeleton,
either neutral or positively charged,5 exhibit unprecedented
chemical and physical properties. Increasing attention has been
paid to heterocyclic PAHs, especially those containing thiophene
units.6 To date, multiple-thiophene-fused benzene,7 naphtha-
lene,8 anthracene,9,18 triphenylene,10 pyrene,4d tetracene,11a−e
pentacene,11f and coronene12 have been developed. We are
particularly interested in thiophene-annelated coronenes for
their interesting assembly behavior. Nuckolls12a described a
contorted dibenzotetrathienocoronene (c-DBTTC), a tetrathio-
phene-fused version of the previously reported contorted
hexabenzocoronene13 (c-HBC), although c-DBTTC is flatter
than its analogue c-HBC. Spurred by the beautiful structure of the
first fully heterocyclic circulene, “sulflower” (octathio[8]-
circulene),4a which has eight S atoms pointing out of the
molecular plane, here we report hexathienocoronenes (HTCs),
fully thiophene-annelated coronenes wherein six double bonds in
the periphery are thieno-fused. The HTCs were obtained as
inseparable mixtures of syn and anti isomers derived from the
regioisomers of the anthradithiophene quinone precursor. To
investigate the effect of the regioisomerism on the molecular
packing and physical properties, a syn form, HTC syn-1b, was
synthesized. The organization was studied by X-ray scattering of
thin films. For two derivatives with hexyl and dodecyl
substituents, the order and phase formation strongly depend
on the length of the alkyl side chains.
phene (4). The conventional Corey−Fuchs (CF) reaction of
anthradithiophene-5,11-dione (3) with PPh3 and CBr4 either in
dichloromethane with stirring at room temperature14 or refluxing
in toluene15 failed or gave only trace amounts of product because
of the low reactivity of 3. However, with a slightly modified
procedure using (i-PrO)3P16 instead of PPh3, the CF reaction of
3 smoothly afforded 4 in 67% yield. Subsequently, fourfold
Suzuki cross-coupling of 4 with thiophene-α-boronic esters 5
afforded the bisolefin skeletons 6 in high yields (86−89%). In
contrast to Nuckolls’ case, photocyclization of trimethylsilyl
(TMS)-substituted 6b gave only the half-closed product 7,17
while nonsubstituted 6a afforded the nonsubstituted, fully
cyclodehydrogenated HTC 2a in 88% yield. On the other
hand, 7 could be further dehydrogenated17 using ferric chloride
to obtain 2b. For comparison, a TMS-substituted tetrathieno-
[a,c,j,l]coronene (TMS-TTC) was also synthesized [Scheme S1
in the Supporting Information (SI)]. Toward a facile and
efficient synthesis of HTCs, we used Suzuki coupling of
thiophene-β-boronic ester 8 with 4 followed by photocyclization
to afford the target HTCs 1 directly in high yields (65−92%). To
investigate the effect of the regioisomerism on the molecular
packing and physical properties, we also synthesized the
Received: August 20, 2012
Published: October 15, 2012
© 2012 American Chemical Society
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isomerically pure HTC syn-1b using syn-318 as the precursor
(Scheme S2).
HTCs 1 and 2 were fully characterized by NMR spectroscopy,
MALDI-TOF mass spectrometry, UV/vis absorption spectros-
copy. MALDI-TOF mass spectra of 1 and 2 revealed single
species with isotopic distributions in accordance with calcu-
lations (Figure S1 in the SI). UV/vis spectra of 2b (Figure 1)
substituent alkyl chains. The phase was further characterized by
means of POM on thin films cooled from the isotropic phase.
Slow cooling at 0.1 °C/min resulted in a highly birefringent
fanlike texture (Figure 2a), as observed for discotic liquid-
Figure 2. POM images of 1c at cooling rates of (a) 0.1 and (b) 1 °C/
min.
crystalline (LC) phases.20 With more rapid thermal processing
(1 °C/min), the shape of the texture changed to spiral, indicating
that the columns were approximately parallel to the glass slides
and that the molecules were tilted with respect to the column axis
(Figure 2b).21 This is in agreement with the structural analysis
discussed below.
To provide a better understanding of the nature of the HTC
molecules and their intermolecular interactions, single crystals of
1b were grown from p-xylene solutions by slow evaporation at
room temperature. Although these HTC materials existed as
inseparable mixtures of syn and anti isomers, this did not
significantly interfere with the packing in the crystal,11f as shown
in Figure 3.22a In contrast to c-DBTTC, because of the relatively
Figure 1. Normalized UV/vis spectra (solid) and photoluminescence
(PL) spectra (dashed) of TMS-TTC (blue) and 2b (green) (1.0 × 10−6
M in CH2Cl2). Three distinct emission bands with maxima at 458, 488,
and 522 nm and at 480, 512, and 548 nm were observed in the PL
spectra of TMS-TTC and 2b, respectively.
showed three well-resolved absorption bands (α, β, p)
characteristic of large PAHs. The absorption maximum of 2b
(β band, 370 nm, ε = 2.48 × 105 M−1 cm−1) exhibited a significant
bathochromic shift (20 nm) relative to the corresponding band
of TMS-TTC (350 nm, ε = 1.83 × 105 M−1 cm−1). The same
bathochromic shift was observed for the α band of 2b (476 nm)
relative TMS-TTC (456 nm). However, a small bathochromic
shift (10 nm) was found for the p band of 2b (433 nm) relative to
TMS-TTC (423 nm). 1 and 2 showed almost identical
absorption bands, although the four thiophenes are annelated
at different positions (Figure S2a). The absorption spectra of
spin-coated films exhibited bathochromic shifts of 16 nm for both
1b and 1c and spectral broadening, indicating that the HTCs
have a strong tendency to aggregate in the solid state (Figure
S2b).
To elucidate the influence of the peripheral thiophenes on the
molecular energy levels of the coronene core, the HOMO
energies of the HTCs were obtained via cyclic voltammetry
(Figure S3). For example, 2b showed four quasi-reversible
oxidation waves, with the onset of the first oxidation indicating a
HOMO energy of −5.08 eV, which is in good agreement with
calculations (−4.93 eV; Figure S4) and comparable to the values
reported for [a,g,m]TTC (−5.51 eV)12c and c-DBTTC (−5.10
eV).12a The HOMO energies of 1b and 1c are −5.00 and −4.97
eV, respectively (Table S1 in the SI). Thus, the new HTCs are
stronger donors than [a,g,m]TTC and c-DBTTC and might
serve as efficient active layers in photovoltaic devices.19
Figure 3. Crystal structure of 1b obtained from p-xylene. (a) Top and
(b) side views shown as ORTEP plots. (c) Side view of columnar
packing with interlayer distances of 3.37 Å in the dimer stack and 3.5 Å
between the dimers. (d) Eclipsed interlayer packing in the distorted
hexagonal columnar arrangement. H atoms and the hexyl side chains in
(c) and (d) have been removed for clarity. C, gray; S, yellow.
The bulk thermotropic properties of HTCs 1b and 1c were
investigated by differential scanning calorimetry (DSC) and
polarized optical microscopy (POM). 1b and 1c showed only
one main phase transition during cooling, related to their
solidification from the isotropic melt. The transition temper-
atures were 232 °C for 1b and 128 °C for 1c, indicating the effect
of the alkyl side chains on the thermal properties, as observed
previously for other discotic PAHs26(Figure S5). An additional
peak at 38 °C for 1c was assigned to reorganization of the
weak steric congestion of the six outer thiophene rings, 1b adopts
an almost planar conformation (Figure 3a,b); the maximal
deviation of the C and S atoms from the molecular plane is 0.56
Å, and the angle between the plane of the outer thiophene and
inner benzenoid rings is 7.4°. X-ray diffraction (XRD) also
revealed the assembly of 1b. Columnar packing was observed,
with stacks of molecules oriented along the a axis. Interestingly,
the repeating moiety with the stack is not the monomer; instead,
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dimers are packed in a graphite-like AB fashion (Figure 3c).
Within the dimer stacks, the π−π interactions dominate, with a
distance of 3.37 Å between two molecules, while the distance
between the dimers is expanded to ca. 3.5 Å. These molecular
stacks are arranged in a distorted hexagonal arrangement of
columns in the solid state (Figure 3d). Serendipitously, another
crystal polymorph of 1b was obtained from dilute toluene
solution.22b Interestingly, this crystal polymorph of 1b forms
dimers by π−π interactions with an expanded interlayer distance
of 3.5 Å, with no obvious π−π stacking among the dimers.
Furthermore, the aromatic plane of 1b is much more twisted
(Figure S6). On the other hand, single crystals of isomerically
pure syn-1b were also obtained by slow evaporation from p-
xylene.22c However, the crystal structure of syn-1b was
remarkably similar to that of the syn/anti mixture and also
displayed disorder at two sulfur positions, probably because the
molecules were simply flipped over in the lattice and present in
equal abundance (Figure S7).27 Comparison of the 13C NMR
with the alkyl chains pointing toward the substrate, and the
aromatic cores are tilted ca. 45° toward the surface (Figure 4b).
This correlates with the calculated molecular size of 2.58 nm
[arccos(1.84/2.58) ≈ 45°]. On the basis of these findings, the
GIWAXS pattern of 1c was then analyzed (Figure 4c). The
reflections related to the intracolumnar arrangement for 1c are
significantly broader than those for 1b but are located on
identical scattering lines at identical positions. This indicates not
only a lower crystallinity for 1c because of the longer flexible alkyl
side chains but also a similar molecular organization on the
surface. This latter conclusion was drawn from the first scattering
line in the middle-angle region and the wide-angle off-meridional
reflections, all of which were located at the same scattering angles
as for 1b. The out-of-plane spacing of 3.0 nm determined from
the XRD plot (Figure 4d) is in agreement with the meridional
plane of the GIWAXS pattern and is attributed to molecules
tilted by ∼45° [arccos(3.0/4.2) ≈ 45°]. The decreased order is
related to the LC phase observed by POM.
1
spectra of syn-9b and syn/anti-9b together with the H NMR
Bearing in mind these highly ordered structures formed in the
assemblies, we further investigated the charge transport
properties in polycrystalline vacuum-deposited thin films of
HTCs. 1a and 1b were selected for the first attempt because of
their high crystallinity. Bottom-gate, top-contact thin-film
transistors (TFTs) with a 110 nm thick SiO2/AlOx gate dielectric
functionalized with a fluoroalkylphosphonic acid self-assembled
monolayer24 and a 30 nm thick vacuum-deposited semi-
conductor layer were fabricated. 1a and 1b were deposited at
substrate temperatures of 100 and 120 °C, respectively. Figure
S10 shows the output and transfer characteristics of the TFTs,
which exhibited p-type behavior with field-effect mobilities of
0.002 cm2 V−1 s−1 for 1a and 0.001 cm2 V−1 s−1 for 1b. These
mobilities are notably smaller than the best mobilities reported
for TFTs based on oligoacenes25 and thienoacenes.6c,11 The
reasons for these relatively small mobilities are the self-assembly
into one-dimensional columnar stacks and the poor crystallinity
of the vacuum-deposited thin films, as revealed by atomic force
microscopy (Figure S11a,c) and XRD (Figure S11b,d). The poor
crystallinity of the vacuum-deposited films is in stark contrast to
the excellent crystallinity of the drop-cast films (see Figure 4),
which suggests that solution processing may be a more suitable
deposition method for HTCs.
spectrum of syn-1b showed much better resolution of peaks than
for syn/anti-1b, further confirming that the observed disorder
was not a result of the presence of a mixture of syn and anti
isomers but appeared because half of the molecules were inverted
in the crystal lattice (Figure S8).
To gain further insight into the supramolecular organization of
1b and 1c on the surface, grazing-incidence wide-angle X-ray
scattering (GIWAXS) measurements on thin films were
performed. The samples were prepared by drop-casting from
toluene solution (2 mg/mL) with subsequent annealing at 90 °C.
The GIWAXS pattern of 1b indicates a crystalline phase with
molecules arranged edge-on, while the columnar stacks are
parallel to the surface (Figure 4a). An identical organization was
In summary, we have reported unprecedented sulfur-
containing heterocyclic PAHs and established a facile synthetic
method for these fully thiophene-annelated coronenes that
allows various substituents to be introduced easily. They reveal
remarkable self-assembly behavior in solution, the solid state, and
at the solution−substrate interface. HTC 1c forms a columnar
mesophase over a wide temperature range close to room
temperature, which is much lower than that for the well-
investigated hexadodecylhexabenzocoronene,26 and thus is good
for processing in electronic devices. Moreover, compared with
the reported c-DBTTCs, the tetrasubstituted HTCs leave the
two α-positions of the annelated thiophenes on the anthradi-
thiophene backbone open, allowing further functionalization and
polymerization.27 Investigations of the formation of charge
transfer complexes with acceptor counterparts (e.g., TCNQ) and
polymerization of HTCs are currently underway.
Figure 4. (a) GIWAXS pattern of a thin film of 1b and (b)
corresponding schematic illustration of the molecular organization on
the surface. (c) GIWAXS pattern and (d) XRD of a thin film of 1c.
Dashed lines are guides for the reflections positioned on scattering lines.
recently determined for columnar terrylene diimides.23 Meri-
dional reflections along qz for qx,y = 0 are related to an
intercolumnar out-of-plane spacing of 1.84 nm. Further
scattering lines parallel to qz suggest a complex intracolumnar
packing of the molecules. To clarify the surface organization, the
single-crystal data for both solvents (toluene and p-xylene) were
used to simulate the scattering patterns with Cerius2 for
comparison to the experimental results. As expected, only the
simulated pattern for the crystal from toluene was in good
agreement with the GIWAXS pattern (Figure S9). In this way, it
was possible to derive precisely the molecular packing of 1b on
the surface after thermal treatment. The molecules are arranged
ASSOCIATED CONTENT
■
S
* Supporting Information
Synthetic details and characterization data. This material is
available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
muellen@mpip-mainz.mpg.de
■
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the DFG Priority Program (SPP 1355, SPP1459) and
ERC NANOGRAPH for support. L.C. is grateful for funding by
the Alexander von Humboldt Foundation.
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