Heteroheptacenes with fused thiophene and pyrrole rings

Article abstract of DOI:10.1002/chem.200903562

The preparation of conjugated heteroheptacenes using an electrophilic coupling reaction induced by a super acid is reported. The new molecules containing thiophene and/or pyrrole rings are bisbenzo[b,b']thienodithieno[3,2- b:2',3'-d]pyrrole, bisbenzo[b,b']thienocyclopenta[2, l -b :3,4-b']dithiophene, and bisthieno[3,2no]thieno[2,3-f:5,4-f]carbazole. Dithieno[3,2-b:2',3'-d] pyrrole, cyclopenta[2, lb:3,4-b']dithiophene, and carbazole are used as the aromatic cores. This versatility provides access to molecules with systematically controllable physicochemical properties. Single-crystal X-ray analyses demonstrate that the type and position of the alkyl substituents significantly changes the packing properties of the new molecules. The optical and optoelectronic properties of the heteroheptacenes vary considerably depending on the number and position of the sulfur or nitrogen linkages and reveal the improved environmental stability over their hydrocarbon counterparts. The analysis of the experimental results from UV/Vis absorption/ photoluminescence (PL) spectroscopy and cyclic voltammetry were combined with DFT quantum-chemical calculations and compared with other model heteroheptacenes. The results suggest that among the acenes with the same number of fused rings, the thiophene ring fusion inside the skeleton stabilizes both HOMO and LUMO levels more effectively than pyrrole and benzene rings. The present study also shows that the new heteroheptacenes are promising candidates for the construction of electronic materials.

Full text of DOI:10.1002/chem.200903562

FULL PAPER
DOI: 10.1002/chem.200903562
Heteroheptacenes with Fused Thiophene and Pyrrole Rings
Peng Gao, Don Cho, Xiaoyin Yang, Volker Enkelmann, Martin Baumgarten, and
Klaus Mꢀllen*[a]
Abstract: The preparation of conjugat-
ed heteroheptacenes using an electro-
philic coupling reaction induced by a
super acid is reported. The new mole-
cules containing thiophene and/or pyr-
ray analyses demonstrate that the type
and position of the alkyl substituents
significantly changes the packing prop-
erties of the new molecules. The optical
and optoelectronic properties of the
heteroheptacenes vary considerably de-
pending on the number and position of
the sulfur or nitrogen linkages and
reveal the improved environmental sta-
bility over their hydrocarbon counter-
parts. The analysis of the experimental
results from UV/Vis absorption/photo-
luminescence (PL) spectroscopy and
cyclic voltammetry were combined
with DFT quantum-chemical calcula-
tions and compared with other model
heteroheptacenes. The results suggest
that among the acenes with the same
number of fused rings, the thiophene
ring fusion inside the skeleton stabiliz-
es both HOMO and LUMO levels
more effectively than pyrrole and ben-
zene rings. The present study also
shows that the new heteroheptacenes
are promising candidates for the con-
struction of electronic materials.
role rings are bisbenzo
dithieno[3,2-b:2’,3’-d]pyrrole, bisbenzo-
[b,b’]thienocyclopenta[2,1-b:3,4-b’]di-
thiophene, and bisthieno[3,2-
ACHUTGTNREN[UNG b,b’]CAHTUNGTRENNtUGN hieno-
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
b]thieno[2,3-f:5,4-f’]carbazole. Dithie-
no[3,2-b:2’,3’-d]pyrrole, cyclopenta[2,1-
b:3,4-b’]dithiophene, and carbazole are
used as the aromatic cores. This versa-
tility provides access to molecules with
systematically controllable physico-
chemical properties. Single-crystal X-
Keywords: conjugation · fused-ring
systems · heteroacenes · semicon-
ductors · solid-state structures
Introduction
meric semiconducting materials.[6,7] DTP, a structural ana-
logue of carbazole (Scheme 1), has received much interest
as a new functional unit in conjugated polymers due to its
Investigations in the field of organic electronics have spur-
red the advance of new molecular building blocks for appli-
cations in organic field-effect transistors (OFETs),[1] organic
light-emitting diodes (OLEDs),[2] and photovoltaic cells.[3] In
this context, materials derived from linear organic p-conju-
gated oligomers are particularly appealing.[4] These materi-
als, when suitably modified, enable low-cost fabrication pro-
cesses of stable, large-area, and lightweight electronic devi-
ces.[5]
À
planarity and N H bond, which can be easily substituted by
functional groups.[8] On the other hand, CPDT, which is re-
garded as a fused-ring analogue of 3-alkylthiophene and a
structural analogue of fluorene (Scheme 1), shows an out-of-
plane arrangement of the substituents due to the sp3-hybrid-
ized carbon atom in the bridge position. It has been report-
ed that homopolymers based on CPDT can not self-assem-
ble into ordered structures.[9] However, much less research
has been devoted to the structurally defined oligomeric mol-
ecules based on DTP[10] and CPDT as models of their poly-
meric counterparts.
In the context of organic electronics, dithieno[3,2-b:2’,3’-
d]pyrrole (DTP) and cyclopenta[2,1-b:3,4-b’]dithiophene
(CPDT) have emerged as popular building blocks for poly-
In an effort to broaden the family of heteroheptacenes,[11]
we reported the synthesis of dibenzoACHTUNTRGNEUNG[b,b’]thieno[2,3-f:5,4-
[a] Dr. P. Gao, D. Cho, Dr. X. Yang, Dr. V. Enkelmann,
Dr. M. Baumgarten, Prof. Dr. K. Mꢀllen
Max Planck Institute for Polymer Research
Ackermannweg 10
f’]carbazoles with the inclusion of both thiophene and pyr-
role ring units (Scheme 2).[12] In spite of the extended p sy-
stem, the spectroscopic and electrochemical characterization
55128, Mainz (Germany)
Fax : (+49)6131-379-350
of dibenzoACTHNUTRGNE[UNG b,b’]thieno[2,3-f:5,4-f’]carbazoles indicates that it
possesses lower HOMO energy levels and larger band gaps
than pentacene.[13] Meanwhile, subsequent single-crystal X-
ray diffraction (SCXRD) studies of this heteroheptacene
E-mail: muellen@mpip-mainz.mpg.de
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200903562.
Chem. Eur. J. 2010, 16, 5119 – 5128
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5119

DFT calculations provided val-
uable insight into the molecular
electronic structure.
Results and Discussion
Scheme 1. Carbazole versus dithienopyrrole versus cyclopentadithiophene versus fluorene as fused p systems.
Synthesis and characterization
of 1, 2, and 4: Oxidative cou-
pling of lithiated 2,3-dibromo thiophene with copper(II)
chloride in diethyl ether at À788C gave 3,3’-dibromo-2,2’-bi-
thiophene (5; Scheme 4). According to the procedure re-
ported by Koeckelberghs et al.,[15] the 4-hexyl-4H-dithieno-
pyrrole core (6) was successfully synthesized through a
Buchwald–Hartwig reaction of the 1-hexylamine with 5. Di-
(trimethyltin)-substituted dithieno[3,2-b:2’,3’-d]pyrrole 7 was
prepared to perform a Stille coupling. This reaction was ac-
complished by treating dithienopyrrole 6 with 2.1 equiva-
lents of tBuLi in THF at room temperature. The reaction
could be driven to completion by working at sufficiently low
dilution. Trimethyltin chloride was the added to quench the
dilithiated DTP. Because distannylated dithienopyrrole de-
Scheme 2. Structure of dibenzoACTHNUGRTENUNG[b,b’]thieno[2,3-f:5,4-f’]-carbazoles
have revealed the existence of p–p stacking between cofa-
cially arranged molecules and a short interplanar separation
of 3.33 ꢂ, which is smaller than the layer separation of
graphite (3.35 ꢂ) and points toward the presence of p–p in-
teractions in the crystal lattice. These results motivated us to
study similar heteroheptacenes with DTP or CPDT as the
core structure.
graded during purification by column chromatography,[16]
7
was used without purification in the next step. The Stille
coupling of 7 and 1-bromo-2-(methylsulfinyl)benzene (8)
was carried out in the presence of [PdACHTNURTGNEUNG(PPh3)4] with DMF as
Herein, we present the facile synthesis and properties of
the solvent and afforded 9 in an overall yield of 88%. Sub-
sequent intramolecular ring closure was performed with an
excess of pure triflic acid: the as-formed clear solution was
poured into water to give a yellowish precipitate, followed
by filtering, drying, and heating to reflux in pyridine. The
new heteroheptacene 1 was obtained as an off-white solid in
an overall yield of approximately 85% (relative to 9) after
purification by flash column chromatography.
Cyclopenta[2,1-b:3,4-b’]dithiophene (10) was prepared by
following a reported method[17] and was dialkylated with
hexyl bromide in the presence of KOH as the base and KI
as the catalyst in dimethyl sulfoxide (DMSO), thus affording
11 in excellent yield (Scheme 5). The distannylated com-
pound 12 was made by dilithia-
new heteroheptacenes bisbenzoACHTUGNTREN[NUNG b,b’]thienodithieno[3,2-
b:2’,3’-d]pyrrole (1) and bisbenzo[b,b’]thienocyclopenta[2,1-
AHCTUNGTRENNUNG
b:3,4-b’]dithiophene (2) with five-membered subunits (e.g.,
thiophene, pyrrole, and cyclopenta-1,3-diene). The synthesis
of 3 has been described previously.[12] For comparison, an-
other heteroheptacene, carbazole-based bisthienoACTHNUGTRNEUNG[3,2-b]
thieno[2,3-f:5,4-f’]carbazole (4), which possesses four fused
thiophene rings outside the carbazole skeleton, was also syn-
thesized (Scheme 3). All the new compounds were charac-
terized by a combination of FT-NMR and optical (UV/Vis
and photoluminescence) spectroscopic, SCXRD, and elec-
trochemical (cyclic voltammetry) techniques. In addition,
tion of 11 with tBuLi, followed
by quenching with Me3SnCl.
The tin compound was an oil
that could not be purified by
column chromatography (due
to
protiodestannylation).[18]
However, the compound was
sufficiently pure to be used in
the next step. Precursor 13 was
synthesized by a Stille coupling
of 12 and 1-bromo-2-(methyl-
sulfinyl)benzene (8) in an over-
all yield of 85%. Finally, by fol-
lowing the same procedure as
for precursor 9, product 2 was
achieved as
powder in 88% yield.
a
light-yellow
Scheme 3. Ladder-type p-conjugated heteroheptacenes.[14]
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Physicochemical Properties of Conjugated Heteroheptacenes
FULL PAPER
corresponding
species. The
3,6-dilithiated
latter was
quenched with dimethyl disul-
fide to afford 18 in 50% yield
of the isolated product. Oxida-
tion of 18 with hydrogen perox-
ide in acetic acid gave precursor
19 in 72% yield. By following
the same ring-closing procedure
as for precursor 9, 4 was ob-
tained in 95% yield as a light-
yellow solid. The structures of
the products were verified by
1
their H NMR spectra (see Fig-
ure S1 in the Supporting Infor-
mation).
Solid-state structures of the het-
eroheptacenes: Elucidation of
the solid-state packing of conju-
gated materials in single crys-
tals is essential to understand
how the molecular assembly is
influenced by substituents.[19]
Single crystals of the hetero-
heptacenes suitable for SCXRD
studies were obtained from
CH2Cl2/hexane (for 1 and 3) or
CH2Cl2/MeOH (for 2) at room
temperature. Crystals of 4 were
too thin for structural determi-
nation.
Scheme 4. Synthesis of bisbenzo
BINAP=2,2’-bis(diphenylphosphino)-1,1’-binaphthyl.
ACHTUNGTNER[NUNG b,b’]thienodithieno[3,2-b:2’,3’-d]pyrrole (1). dba=dibenzylideneacetone,
The skeleton of 1 is planar
and two molecules form a pair
in
a head-to-head structure.
The pairs further stack into a
column with the aromatic skele-
ton that forms a herringbone
structure, in which the tilt angle
between two mean planes of
the framework of compound 1
is 45.68, relative to 60 and 528
in DBTCz[12] and pentacene,[20]
respectively. Two intermolecu-
Scheme 5. Synthesis of bisbenzoACTHNUTRGNE[NUG b,b’]thienocyclopenta[2,1-b:3,4-b’]dithiophene (2). DMSO=dimethyl sulfox-
ide. NBS=N-bromosuccinimide.
À
lar S C close contacts are ob-
served between the two mole-
cules in the pair (Figure 1a,
The synthesis of 4 is outlined in Scheme 6. The Stille cou-
pling reaction between 2,7-dibromo-9-methyl-9H-carbazole
(14) and (5-hexylthiophen-2-yl)trimethylstannane (15) was
marked in dashed line); for example, the distances between
the sulfur atoms in molecules of 1 (i.e., atoms S1 and S3;
see the Supporting Information) and carbon atoms (i.e.,
atoms C14 and C2) from the adjacent molecules are 3.44
and 3.34 ꢂ, which are shorter than the sum of their van der
Waals radii (3.55 ꢂ). The same distances could be observed
between the two carbon atoms of each molecule and the
sulfur atoms of one additional molecule as well. In addition,
these columns form lamellae with aliphatic chains that lie
between every two aromatic columns (Figure 1b, marked in
carried out in DMF with [PdACHTNUGTRENUNG(PPh3)4] as the catalyst to give
16 in 80% yield. Afterwards, regioselective bromination at
the 3- and 6-positions of the carbazole core of 16 was ach-
ieved by using two equivalents of N-bromosuccinimide
(NBS) in a 1:1 mixture of CHCl3 and acetic acid. Compound
17 was obtained after a flash column chromatography and
further treated with two equivalents of nBuLi to form the
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K. Mꢀllen et al.
ed and almost perpendicular
from each other. To fill the
space, one molecule from
second pair embeds in and has
cofacial overlap, to small
a
a
extent, with one molecule in
the first pair. The loose packing
nature of the material could be
seen from the interplanar dis-
tance (3.83 ꢂ) of the cofacial
part, and no short contact was
detected in the solid. Obviously,
the perpendicularly oriented
alkyl chain causes
a distinct
bulge, which is essential to pre-
vent the molecule of 2 from
close packing.
Compound 3 crystallizes in
the monoclinic P21/c space
group with four molecules in
the unit cell. The conjugated
Scheme 6. Synthesis of bisthienoACHTNUTRGNEUNG[3,2-b]thieno[2,3-f:5,4-f’]-carbazole (4).
Figure 2. Packing diagram of 2. a) View down the b axis of the unit cell
(dashed rectangle illustrates the head-to-head pairs). b) View along the
long molecular axis that shows the perpendicularly oriented alkyl chain
(dashed lines illustrate cofacial distance; the hydrogen atoms are omitted
for clarity).
Figure 1. Packing diagram of 1. a) View down the stacking axis of head-
to-head column pairs (dashed rectangle illustrates the head-to-head
pairs; dashed lines illustrate intermolecular close contacts within and be-
tween the columns). b) View along the long molecular axis that shows a
“lamella herringbone” structure formed by antiparallel column pairs
(dashed rectangle illustrates the antiparallel lamella structure; the hydro-
gen atoms are omitted for clarity).
molecules have the same aromatic backbone as DBTCz,[12]
but with two n-butyl chains in the long molecular axis in-
stead of the short molecular axis. As expected, the packing
characteristics of 3 are dramatically changed by altering the
position of the substituents (Figure 3). The crystal structure
of 3 is based on a sandwiched herringbone structure in
which its dimers are packed tightly in the crystal (Figure 3b,
marked in the dashed rectangle). These dimers are formed
by two inversely packed molecules in an antiparallel fashion
to the direction of the short molecular axis. The plane-to-
plane distance is approximately 3.45 ꢂ, thus suggesting p–p
a dashed rectangle). Interestingly, intermolecular close con-
tacts were also found between the columns. The distance be-
tween S2 of one molecule and C8 of the molecule in another
column was 3.48 ꢂ (as marked in Figure 1a), which suggests
that a single crystal of 1 may have a two-dimensional elec-
tronic structure. A similar packing was seen in DBTCz, but
with only a one-dimensional electronic structure.[12]
À
By changing the N-alkyl substituent in 1 to a dialkyl-
methylene unit in 2, the single crystal of 2 exhibits dramatic
differences from 1 (Figure 2). The fully ladderized skeleton
is planar and with the 4,4’-substituted alkyl chains lying
above and below the plane of the backbone, respectively, in
an almost vertical geometry. In one unit cell, two adjacent
molecules are packed so that they form a pair with a head-
to-head structure as in 1. However, due to the extended
extra alkyl chains, the two molecules in the pair are separat-
overlapping, and short C S contacts (3.49 ꢂ) also exist in a
face-to-edge manner between the dimers, thus indicating the
two-dimensional electronic structure of the crystal (marked
in dashed line).
These different solid-state structures of the three hetero-
heptacenes nicely illustrate how the effect of the alkyl sub-
stituents and different heteroatoms could efficiently be uti-
lized to tune the organization in the solid state of organic
semiconductors. The more sulfur atoms that are present, the
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Physicochemical Properties of Conjugated Heteroheptacenes
FULL PAPER
assess the effect of heteroatom and alkyl substitution on the
absorption/emission maxima of the heteroheptacenes and
the optical energy gap. Table 1 collects the UV/Vis and PL
Table 1. UV/Vis absorption and PL data for 1–4.
Compound
Solution
Film (solid state)
absorption PL
absorption[a,b] PL[a]
lmax [nm]
quantum
lf [nm] yield[a,c] Ff lmax [nm]
lf [nm]
Figure 3. Packing diagrams of 3. The hydrogen atoms are omitted for
clarity, and thermal ellipsoids are drawn at 50% probability. a) View
down the stacking axis of dimers (dashed rectangle illustrates the dimers
of the molecules). b) View along the long molecular axis that shows a
sandwiched herringbone structure (dashed rectangle illustrates the
dimers of the molecules; dashed lines illustrate intermolecular close con-
tacts within and between the dimers).
1
2
3
4
387
413
423
413
407
457
439
426
0.22
>0.75
0.03
414
420
438
432
465
468
450
444
0.12
[a] Solvent: THF (10À6 m); h=1.4070. [b] A p–p* HOMO–LUMO transi-
tion as lmax. [c] Quantum yield relative to diphenylanthracene (10À7 m, cy-
clohexane solution); excitation at l=365 nm.
more intermolecular short contacts exist, which can facilitate
charge-carrier transport in devices. It is known that tight
packing of the chromophores in the solid state will lead to
self-quenching of fluorescence and excimer or exciplex for-
mation.[21] To prevent this phenomenon, bulky or branched
substituents are always employed to intrinsically disperse
the packing arrangement of the chromophores. In this study,
a branched alkyl substitution in 2 instead of a linear one in
1 effectively loosens the solid packing, and therefore may
prevent self-quenching and suppress the formation of aggre-
gates/excimers. On the other hand, changing the alkyl sub-
stitution from the short molecular axis to the long molecular
axis changed the packing motif of the compound from a
highly slipped herringbone structure to a densely packed p–
p stacking structure.
data for all the compounds in THF. The UV/Vis absorption
spectra of 1 and 2 display structured absorption bands in the
1
UV range that are attributed to the b (Platt Bb) band and
p band of the p–p* transitions. Although in the case of 3
and 4, in addition to the b band, one can also see two weak
1
a (Platt Lb) bands and/or p bands in the visible region (Fig-
ure 4a).[22] Similar UV absorption bands have been observed
in the case of diindolocarbazoles.[11c] Relative to the DTP-
based heteroheptacene 1, the absorption maximum of
CPDT-based 2 is red-shifted by Dl=26 nm from l=387 nm
when the central pyrrole ring is replaced by a 1,3-cyclopen-
tadiene unit. This result for 1 is surprising because the intro-
duction of electron-rich heterocycles into conjugated sys-
tems has been an established strategy for achieving low-
band-gap chromophores.[23] In fact, a similar case has been
observed in the homopolymers of DTP and CPDT before.[24]
Optical properties of heteroheptacenes 1–4: UV/Vis absorp-
tion and photoluminescence (PL) spectra of 1–4 were mea-
sured in solution and as drop-cast thin films (Figure 4) to
In a comprehensive study of polyACHTGNUTERNNU(G DTP)s, Ogawa and Ras-
mussen[6b] attributed this phenomenon to the difference in
the rigidity of the two monomers. With CPDT- and DTP-
based oligomers, however, they claim that by changing the
bridgehead from a CHR to NR group (R=alkyl groups) re-
sults in no change in the absorbance spectra.[16] When two
more sulfur atoms are incorporated into the skeleton, 4
shows less-resolved absorption spectra than 3 and the
a band of 4 is also blue-shifted by Dl=10 nm relative to 3.
op
Solution optical band gaps Eg of 1–4, defined by the 0–0
transition energies, were estimated based on the lmax absorp-
tion edge (Table 3). All the compounds have significantly
larger band gaps than hydrocarbon heptacene,[25,26] thus indi-
cating that in planarized aromatic systems, the increased
thiophene units widen the HOMO–LUMO gaps.[27]
The PL spectra of 1–4 were measured in THF by exciting
10À6 m solutions at the corresponding lmax values. Interesting-
ly, 2 shows an unstructured strong emission maximum (lf =
457 nm), which is bathochromically shifted by Dl=50 nm
relative to the emission wavelength of 1 and shows a Stokes
shift of Dl=44 nm. In marked contrast, 1, 3, and 4 exhibit
weak emissions with the maxima at l=407, 439, and
426 nm, respectively, and much smaller Stokes shifts due to
their rigid coplanar structures. Relative PL quantum yields
Ff of these heteroheptacenes were determined by using di-
Figure 4. a) Normalized UV/Vis absorption and PL spectra of 1–4 in
THF (10À6 m). b) Normalized UV/Vis absorption and PL spectra of 1–4
as thin film.
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5123

K. Mꢀllen et al.
phenylanthracene in cyclohexane as the standard,[28] (see
Table 1 for the PL data). The quantum yield Ff can be de-
scribed by the relative rates of the radiative and nonradia-
tive pathways, which deactivate the excited state. In this
study, 2 (with the CPDT core) shows the highest Ff value
(Ff >0.75), thus indicating that nonradiative decay pathways
are nearly suppressed in such a structure. Obviously, the two
alkyl chains on the central tetrahedral carbon atom of 2 ef-
fectively hinder the molecular association, as shown in the
single-crystal packing (Figure 2).
The solid-state optical absorption/PL data for 1–4 are col-
lected in Table 1. In general, the film absorption spectra ex-
hibit characteristic transitions that are bathochromically
shifted relative to their solution values, which is indicative
of the presence of p–p stacking in the solid state. Com-
pounds 1, 3, and 4 show a large red-shift of Dl=9–27 nm,
whereas in contrast 2 possesses a red-shift of Dl=5 nm (Fig-
ure 4b). This behavior indicates a weaker intermolecular in-
teraction of 2 relative to 1, 3, and 4. Thin-film PL spectra
were obtained by excitation at lmax (Table 1). The spectral
shapes and maxima strongly depend on the molecular struc-
ture and the packing characteristics, with most of the plots
exhibiting red-shifts relative to the emission maxima in solu-
tion. It can be seen that the intermolecular interactions
dominate the optical properties in the solid state and sur-
mount the effects of the heteroatom substitution on the pho-
tophysics of the compounds.
tively, versus E=0.98 V for 4. The reduction potentials,
however, were out of the range of our experimental setup
(< À2 eV) and could not be determined. The oxidation of 1
(see Figure S2 in the Supporting Information) occurs in two
reversible steps, the maxima of which appear at Ea1 and Ea2
(0.87 and 1.02 V, respectively), whereas 2 (with a CPDT
core) shows only one quasireversible oxidation at Ea1 =
1.04 V. According to
E
HOMO =À(Eoxonset+4.34) eV,[29] the
HOMO levels of 1–4 could be estimated (Table 3). By
taking into account their optical band gaps, which were de-
rived from the absorption onset in the UV/Vis spectrum, the
LUMO values are calculated.
Table 3. Comparison of electrochemical, optical, and calculated HOMO–
LUMO energy gaps Eg and absolute HOMO and LUMO energies for 1–
4 and known heteroheptacenes.
Compound
E [eV]
Eg [eV]
experimental[a]
HOMO LUMO
theoretical[b]
op[c]
th[d]
HOMO
LUMO
Eg
Eg
1
2
3
4
À5.09
À5.28
À5.19
À5.20
À1.99
À2.38
À2.36
À2.30
À4.98
À5.03
À5.11
À5.18
À5.50
À4.54
À5.07
À4.98
À1.42
À1.72
À1.51
À1.54
À1.71
À1.15
À1.22
À1.43
3.10
2.90
2.83
2.90
2.83
2.59
3.20
–
3.56
3.31
3.60
3.64
3.79
3.39
3.85
3.55
DBTBT
DIoCz
DIeCz
LTP
–
–
À5.30
À2.10
–
–
[a] HOMO energy was estimated from the relationship: EHOMO
=
À(Eoxonset+4.34) eV; tLUMO energy was estimated from the empirical re-
lationship: LUMO (eV)=HOMO (eV)+Egop. [b] Gaussian 03, B3LYP,
6-31G*. [c] From absorption onset of the UV/Vis absorption data.
[d] From DFT calculations.
Electrochemical properties of new heteroheptacenes 1–4:
As already indicated by the absorption and PL spectra of
the heteroheptacenes, variation in the substitution has a sig-
nificant impact on optical properties. These results led us to
investigate the electrochemical behavior of heterohepta-
cenes 1–4 further. Cyclic voltammetry (CV) measurements
were performed under N2 in 0.1m CH2Cl2/tetrabutylammoni-
um hexafluorophosphate (TBAPF6) with a scanning rate of
50 mVsÀ1. All the systems exhibited one or two reversible
and/or quasireversible one-electron oxidation waves within
the solvent/electrolyte window range. Figure S2 (see the
Supporting Information) illustrates voltammograms of 1–4
(the electrochemical data are summarized in Table 2). It is
possible to extract the formal halfwave potentials E1/2 as the
midpoints between the peak potentials for the forward and
reverse scans when the voltammograms are (quasi)reversi-
ble. Single-electron oxidation processes (versus SCE) were
obtained at E=0.84/0.99, 0.99, and 1.04 V for 1–3, respec-
Molecular-orbital calculation: To gain deeper insight into
the electronic structures of the new heteroheptacenes and
related analogues, molecular-orbital (MO) calculations of
the HOMO levels, LUMO levels, and band gaps were per-
formed by using DFT methods (B3LYP, 6-31G*). Table 3
summarizes the electrochemical data, optical data, and cal-
culated energy gaps for 1–4. Reported heptacenes (dibenzo-
thienobisbenzothiophene (DBTBT),[11a] diindolocarbazole
(DIoCz),[11c] diindenocarbazoles (DIeCz),[11d] and ladder-
type tetraphenylene (LTP)[30]) were also included and com-
pared in terms of orbital levels from both experiment and
calculation. An excellent agreement between the experi-
mental HOMO energy and the theoretical results is seen,
with differences of only approximately 0.1–0.2 eV for almost
all molecules. However, the theoretical gaps predicated
from vertical transitions are significantly larger than the op-
Table 2. Electrochemical data for 1–4 in dry CH2Cl2 under nitrogen.
Oxidation[a]
cathodic
op
Compound
tical band gaps Eg (Table 3). A systematic study of the ac-
anodic
Ea1
half
curacy of the DFT calculations has been previously shown
1/2
1/2
Eonset
Ea2
Ec1
Ec2
E1
E2
by Zhang and Musgrave.[31]
[V]
[V]
[V]
[V]
[V]
[V]
[V]
The HOMOs and LUMOs are delocalized practically
along the entire p-conjugated backbones with generally two
different types of HOMO delocalization states and identical
LUMO states (Figure 5). The HOMOs of 1 and 2 have anti-
bonding character (or intra-ring bonding) between the adja-
1
2
3
4
0.71
0.90
0.97
0.82
0.87
1.04
1.09
1.04
1.02
0.81
0.94
0.99
0.91
0.96
0.84
0.99
1.04
0.98
0.99
–
–
–
–
–
–
–
–
–
[a] Referenced to the Ag/Ag+ couple.
5124
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Chem. Eur. J. 2010, 16, 5119 – 5128

Physicochemical Properties of Conjugated Heteroheptacenes
FULL PAPER
DieCz and LTP lie close to the
EHOMO values of 1 and 2 (i.e.,
DEHOMO <0.09 eV).
The
LUMOs of DleCz and LTP
reveal similar inter-ring bond-
ing interactions to 1 and 2 and
move slightly upward. Among
all the heteroheptacenes in this
study, the thiophene-ring-con-
taining heteroheptacenes 1–4
and DBTBT have average
HOMO and LUMO energies of
À5.20 and À1.60 eV, respective-
ly, which are lower than the
other three analogues (average
E
HOMO =À4.86 and ELUMO
=
À1.27 eV). These findings sug-
gest that the thienyl ring fusion
inside the heteroacenes is an ef-
fective method to stabilize both
the HOMO and LUMO levels.
Figure 5. Schematic representation of the HOMOs and LUMOs of 1–4, DBTBT, DIoCz, DIeCz, and LTP for
comparison (DFT//B3LYP/6-31G*).
cent hetero rings and are delocalized over the molecular
backbone, whereas the LUMO represents inter-ring bonding
interactions, thus exhibiting significant quinoid character. In
contrast, the HOMO orbital plots of 3 and 4 are different
from 1 and 2, with obvious inter-ring bonding interactions,
and are mainly localized around the central carbazole
system. On the other side, the LUMOs of 3 and 4 represent
the same fashion of delocalization as 1 and 2.
It is surprising that the LUMO energy of 2 is lower by
0.3 eV than 1 when the carbon atom is replaced by a more
electronegative nitrogen atom (Figure 5). Nguyen et al.
found from a time-dependent DFT calculation that the
LUMO energy of an organic molecule is dramatically influ-
enced by its aromacity, and the LUMO level of nonaromatic
cyclopentadiene is 1.7 eV lower than that of pyrrole. The
HOMOs, however, are not significantly influenced by the ar-
omaticity of the building blocks.[32] Obviously, heterohepta-
cenes 1 and 2 in this study follow the same trend. From
DFT calculations, the small difference between the HOMO
and LUMO energy levels of 3 and 4 is in line with the CV
experiments.
To compare the electronic effects of heteroatoms on the
frontier orbitals of heteroheptacenes, we also carried out
calculations for DBTBT, DIoCz, DIeCz, and LTP by using
the same computational method. The HOMO and LUMO
energies of DBTBT and DIoCz resemble those of 3 and 4 in
shape (Figure 5), in which the orbital shows inter-ring bond-
ing interactions and is mainly localized around the central
p system. In contrast, the EHOMO and ELUMO values of DIoCz
differ markedly from those of 3, 4, and DBTBT (i.e.,
DEHOMO =0.96 eV, DELUMO =0.6 eV). It seems that more
pyrrole ring fusion into the heteroacenes tends to destabilize
both the HOMO and LUMO energy levels. The calculation
of DleCz and LTP indicates that their HOMOs resemble
those of 1 and 2 in shape, in which the bridging (hetero)a-
toms are on the node of the orbitals. The EHOMO value of
Conclusion
Three new heteroheptacenes (1, 2, and 4) with the inclusion
of both thiophene and/or pyrrole ring units have been syn-
thesized by an intramolecular electrophilic coupling reaction
induced with triflic acid. The NMR spectroscopic measure-
ments disclosed the symmetric structure of the new hetero-
heptacenes. The SCXRD characterization demonstrated
that the solid structure of the heteroheptacenes can be sig-
nificantly influenced by the mode (position and type) of
alkyl-chain substitution. The experimental results (absorp-
tion/emission spectroscopy and CV) confirmed DFT calcula-
tions that both hetero ring fusion and the substituents have
significant impact on the optical and electrochemical proper-
ties of the heteroheptacenes. Visualization of the MOs
showed that carbazole-based compounds 3 and 4 represent a
unique inter-ring bonding character between the adjacent
hetero rings and less delocalized HOMO relative to 1 and 2.
The appropriate substitution of thiophene units inside the
ladder-type heteroacenes can effectively stabilize their fron-
tier orbitals. The present study suggests that the new hetero-
heptacenes are promising candidates for optoelectronic ap-
plications. Further studies on the structure/properties rela-
tionship of more complex heteroheptacenes and the poten-
tial of using these compounds as semiconductors in devices
are currently underway.
Experimental Section
4-Hexyl-2,6-bis[2-(methylsulfinyl)phenyl]-4H-dithieno[3,2-b:2’,3’-d]pyr-
role (9): 2-Bromo(methylsulfinyl)benzene (8; 736 mg, 3.36 mmol) was
added to a solution of 7 (0.9 g, 1.53 mmol) in anhydrous DMF (25 mL).
The resulting mixture was purged with Ar for 30 min, and [PdACHTUNGTRENNUNG(PPh3)4]
Chem. Eur. J. 2010, 16, 5119 – 5128
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K. Mꢀllen et al.
(87 mg, 0.075 mmol) was added. The reaction mixture was heated over-
night to 808C. Excess DMF was removed under high vacuum, and the
residue was dissolved in ethyl acetate and treated with 10% aqueous KF.
The mixture was filtered through a pad of celite. The filtrate was dried
over Mg2SO4, filtered, and the solvent removed in vacuo. The crude
product was purified by flash chromatography on silica gel (eluent:
hexane/THF 3:1) to afford 9 as an off-white solid (0.95 g, 88% relative to
6). 1H NMR (250 MHz, CD2Cl2): d=7.03 (s, 2H), 7.40 (ddd, J=14.20,
9.58, 4.15 Hz, 6H), 7.88 (d, J=7.53 Hz, 2H), 4.06 (t, J=6.88, 6.88 Hz,
2H), 2.30 (s, 6H), 1.77–1.61 (m, 2H), 1.18–0.99 (m, 6H), 0.66 ppm (t, J=
6.68, 6.68 Hz, 3H); 13C NMR (62.5 MHz, CD2Cl2): d=145.553, 145.009,
136.753, 133.024, 131.181, 131.045, 129.456, 124.098, 115.986, 112.256,
47.98, 42.34, 31.82, 30.69, 27.11, 22.94, 14.14 ppm; FDMS: m/z (%):
539.80 (100.0) [M+]; elemental analysis (%) calcd for C28H29NO2S4: C
62.30, H 5.42; found: C 62.35, H 5.39.
THF/hexane 1:1), thus affording the pure product as a white solid
(101 mg, 95% yield). 1H NMR (250 MHz, CD2Cl2): d=8.93 (d, J=
3.97 Hz, 2H), 7.55 (d, J=1.76 Hz, 2H), 7.13 (dd, J=3.51, 1.71 Hz, 2H),
6.88 (d, J=3.51 Hz, 2H), 3.95 (d, J=0.84 Hz, 3H), 2.92 (t, J=7.59,
7.59 Hz, 4H), 2.58 (d, J=4.99 Hz, 6H), 1.85–1.71 (m, 4H), 1.48–1.33 (m,
12H), 0.96 ppm (t, J=8.77, 5.17 Hz, 6H); 13C NMR (62.5 MHz, CD2Cl2):
d=148.68, 143.20, 137.13, 136.88, 130.93, 128.19, 125.24, 122.72, 117.56,
111.43, 43.25, 32.12, 31.99, 30.51, 30.17, 29.20, 23.03, 14.29 ppm; FDMS:
m/z (%): 637.22 (100) [M+]; elemental analysis (%) calcd for-
C35H43NO2S4: C 65.89, H 6.79; found: C 65.85, H 6.88.
BisthienoACTHNUTRGNEU[GN 3,2-b]thieno[2,3-f:5,4-f’]-carbazoles (4): The same procedure
was followed as for the synthesis of 1 starting from 19. The product 4 was
isolated by chromatography on silica gel with (eluent: CH2Cl2/hexane
1:9) as a light-yellow oil, which solidified as yellow crystals on cooling
(55 mg, 80%). 1H NMR (250 MHz, CD2Cl2): d=8.55 (s, 2H), 7.74 (s,
2H), 7.10 (s, 2H), 3.98 (s, 3H), 3.02 (t, J=7.55, 7.55 Hz, 4H), 1.89–1.77
(m, 4H), 1.53–1.43 (m, 4H), 1.39 (ddd, J=7.39, 4.59, 2.45 Hz, 8H),
0.95 ppm (t, J=9.34, 4.82 Hz, 6H); 13C NMR (62.5 MHz, CD2Cl2): d=
150.10, 141.65, 138.34, 133.65, 132.48, 132.12, 121.33, 118.02, 115.13, 99.18,
32.04, 31.68, 30.08, 29.86, 29.15, 23.00, 14.25 ppm; FDMS: m/z (%):
573.90 (100) [M+]; elemental analysis (%) calcd for C33H35NS4: C 69.06,
H 6.15; found: C 69.12, H 6.23.
BisbenzoACHTUNGTRENNUNG[b,b’]thienodithieno[3,2-b:2’,3’-d]pyrrole (1): A 10-mL round-
bottomed flask was filled with 9 (200 mg, 0.37 mmol), phosphorus pent-
oxide (28 mg, 0.2 mmol), and trifluoromethanesulfonic acid (6 mL). The
mixture was stirred for 72 h at room temperature to give a dark-blue so-
lution, which was then poured into ice-water (100 mL). The yellow pre-
cipitate was collected by suction filtration and dried under vacuum. The
structure of this compound, which was insoluble in apolar organic sol-
vents, was assumed to be the sulfonium salt. Demethylation of the solid
was achieved by heating to reflux in pyridine (30 mL) for 12 h. When the
suspension was cooled to room temperature, a large volume of CH2Cl2
was added to extract the product. Compound 1 was thus obtained as
light-yellow powder after chromatography on silica gel with CH2Cl2/
hexane (1:9) as the eluent (145 mg, 85%). 1H NMR (300 MHz, CD2Cl2):
d=7.91 (d, J=7.83 Hz, 2H), 7.86 (d, J=7.87 Hz, 2H), 7.52–7.44 (m, 2H),
7.41–7.33 (m, 2H), 4.48 (t, J=7.13, 7.13 Hz, 2H), 2.13–2.00 (m, 2H), 1.32
(td, J=13.44, 5.09, 5.09 Hz, 6H), 0.93–0.84 ppm (m, 3H); 13C NMR
(62.5 MHz, CD2Cl2): d=141.14, 136.53, 134.09, 133.65, 125.53, 124.41,
122.32, 120.42, 118.34, 32.36, 31.81, 30.07, 26.95, 22.84, 14.09 ppm;
FDMS: m/z (%): 475.71 (100) [M+]; elemental analysis (%) calcd for
C26H21NS4: C 65.64, H 4.45; found: C 64.70, H 4.45.
X-ray crystallographic analysis: Single crystals of 1–3 were grown from
cold CH2Cl2 or a hexane/methanol mixture. Crystals were mounted on
glass fibers and the data collected at 173(1) K on a diffractometer with
graphite-monochromated MoKa radiation. Data were collected in a series
of f and w scans in 0.508 oscillations with 10–30-s exposure and collected
and integrated with the SAINT software package. The data were correct-
ed for absorption effects by using the multiscan technique (SADABS)
and corrected for Lorentz and polarization effects. The structures were
solved by using direct methods and refined by using the SHELXTL crys-
tallographic software package. For each structure, all the non-hydrogen
atoms were refined anisotropically and all the hydrogen atoms were in-
cluded in calculated positions but were not refined. Compounds 1 and 3
crystallize with one-half molecule that resides on an inversion center. Un-
fortunately, the crystals of 4 were too thin for structural determination
with a standard diffractometer. CCDC-769583 (1), CCDC-769584 (2),
and CCDC-769585 (4) contain the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The Cam-
bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
4,4-Dihexyl-2,6-bis2-(methylsulfinyl)phenylcyclopenta[2,1-b:3,4-b’]dithio-
phene (13): The same procedure was followed as for the synthesis of 9
starting from 12. The crude product was purified by flash chromatogra-
phy on silica gel (eluent: hexane/THF 3:1) to afford 13 (0.56 g, 88% rela-
tive to11). 1H NMR (250 MHz, CD2Cl2): d=8.06 (dd, J=6.84, 1.20 Hz,
2H), 7.63–7.47 (m, 6H), 7.11 (s, 2H), 3.92–3.83 (m, 4H), 2.49 (s, 6H),
1.18 (td, J=10.06, 9.33, 9.33 Hz, 12H), 1.04–0.89 (m, 4H), 0.81 ppm (td,
J=13.44, 6.74, 6.74 Hz, 6H); 13C NMR (62.5 MHz, CD2Cl2): d=145.424,
139.72, 133.0, 131.54, 131.31, 129.81, 124.6, 123.27, 68.36, 38.57, 30.41,
29.94, 25.44, 23.36, 14.60 ppm; FDMS: m/z (%): 622.97 (100) [M+]; ele-
mental analysis (%) calcd for C35H42O2S4: C 67.48, H 6.80; found: 67.40,
H 6.75.
Optical spectroscopic measurements: UV/Vis absorption spectra were re-
corded at room temperature on a Perkin–Elmer Lambda 9 spectropho-
tometer. The PL spectra were recorded on a SPEX-Fluorolog II (212)
spectrometer. The solutions were prepared with an absorbance between
0 and 0.1 at the wavelength region of experimental interest. The PL
quantum yields were determined by comparison with 10À7 m diphenylan-
thracenein cyclohexane as a reference and corrected for the refractive
index of different solvents.
BisbenzoACHTUNGTRENNUNG[b,b’]thienocyclopenta[2,1-b:3,4-b’]dithiophene (2): The same
procedure was followed as for the synthesis of 1 starting from 13. The
product was isolated by chromatography on silica gel (eluent: CH2Cl2/
hexane 1:9) as a light-yellow oil, which solidified as yellow crystals on
Electrochemical measurements: CV experiments were performed in 0.1m
TBAPF6 solutions in dry, oxygen-free CH2Cl2. Glassy carbon was used as
the working electrode, a platinum wire as the auxiliary electrode, and
Ag/AgCl as the reference electrode, which was checked against the ferro-
cene/ferrocenium (Fc/Fc+) couple after each measurement. These meas-
urements were carried out on a computer-controlled PGSTAT12 at room
temperature.
1
cooling (165 mg, 85%). H NMR (250 MHz, CD2Cl2): d=7.79 (t, J=7.23,
7.23 Hz, 4H), 7.39–7.31 (m, 2H), 7.30–7.21 (m, 2H), 2.14–2.01 (m, 4H),
1.10–0.91 (m, 12H), 0.89–0.74 (m, 4H), 0.63 ppm (t, J=6.73, 6.73 Hz,
6H); 13C NMR (62.5 MHz, CD2Cl2): d=148.84, 142.13, 139.97, 135.72,
133.69, 132.72, 125.37, 124.47, 124.31, 120.50, 37.04, 31.82, 29.83, 24.86,
22.86, 14.06 ppm; FDMS: m/z (%): 558.15 (100) [M+]; elemental analysis
(%) calcd for C33H34S4: C 70.92, H 6.13; found: 70.98, H 6.11.
Calculation methods: DFT calculations were carried with the Gaussian03
program package.[33] The Becke three-parameter exchange functional
combined with the LYP correlation functional (B3LYP) was used.[34] Op-
timized molecular geometries were determined on isolated entities. The
6–31G* basis was chosen for all molecules.
2,7-Bis(5-hexylthiophen-2-yl)-9-methyl-3,6-bis(methylsulfinyl)-9H-carba-
zole (19): 2,7-Bis(5-hexylthiophen-2-yl)-9-methyl-3,6-bis(methylthio)-9H-
carbazole (18; 100 mg, 0.17 mmol) was dissolved in a 1:1 mixture of gla-
cial acetic acid and CHCl3 and cooled with an ice-bath until the solvent
was about to freeze. Hydrogen peroxide (35%, 35 mg, 0.36 mmol) was
added slowly. The cooling bath was removed and the mixture was stirred
at room temperature for 12 h. Acetic acid was removed by vacuum evap-
oration and CH2Cl2 was added to the residue. The organic fraction was
washed with a saturated solution of NaHCO3 and dried over MgSO4. The
product was purified by column chromatography on silica gel (eluent:
Acknowledgements
We thank Mr. Michael Steiert for XRD analysis. Financial support from
the Max Planck Society through the program ENERCHEM, European
5126
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Chem. Eur. J. 2010, 16, 5119 – 5128

Physicochemical Properties of Conjugated Heteroheptacenes
FULL PAPER
Commission Project ONE-P, and the German Science Foundation
(Korean–German IRTG) is gratefully acknowledged.
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