Synthesis and characterization of unsymmetric Lndolodithienopyrrole and extended diindolodithienopyrrole

Article abstract of DOI:10.1021/ol902528b

(Figure presented) Indole-fused dithieno[3,2-b:2′,3′-d]pyrrole- based unsymmetrical and extended heteroacenes were synthesized and characterized. Solid-state structures were examined using single-crystal X-ray diffraction to understand their packing behav

Full text of DOI:10.1021/ol902528b

ORGANIC
LETTERS
2010
Vol. 12, No. 2
232-235
Synthesis and Characterization of
Unsymmetric Indolodithienopyrrole and
Extended Diindolodithienopyrrole
Ganapathy Balaji, Dazril Izrar Phua, Wong Low Shim, and Suresh Valiyaveettil*
Department of Chemistry, National UniVersity of Singapore,
3 Science DriVe 3, Singapore 117 543
chmsV@nus.edu.sg
Received November 2, 2009
ABSTRACT
Indole-fused dithieno[3,2-b:2′,3′-d]pyrrole-based unsymmetrical and extended heteroacenes were synthesized and characterized. Solid-state
structures were examined using single-crystal X-ray diffraction to understand their packing behavior. The optical and electrochemical properties
of these new heteroacenes are also described in detail.
Ladder-type π-conjugated systems with fused-rigid back-
bones are actively explored in organic electronic applica-
tions.1 Pentacene and its derivatives have shown a hole
mobility of ∼3 cm2·V-1·s-1.2 Unfortunately, pentacene and
higher acenes are unstable and undergo photo-oxidation due
to their high-lying HOMO and narrow band gap.3 Such
higher [n]acenes also exhibit poor solubility in common
solvents, which limits their processability.4 The incorporation
of heteroatoms and solublizing groups has been part of the
ongoing efforts to prepare new [n]acene analogues with
improved device performances.5 These heteroacenes with low
HOMO level offer stability to the fused ring systems.6
Furthermore, easy functionalization on the heteroatoms (e.g.,
N, P, Si) allows for facile derivatization7 and incorporation
of substituents designed to tune the molecular organization
and solubility.8 Heteroacenes are expected to possess better
organic field effect transistor (OFET) performance9 due to
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10.1021/ol902528b  2010 American Chemical Society
Published on Web 12/09/2009

Scheme 1. Synthesis of Unsymmetrical and Extended Heteroacene
secondary interactions such as hydrogen bonding, π-stacking,
and sulfur-sulfur (S· · ·S) interactions originating from the
high polarizability of sulfur electrons in thiophene rings.10
Thiophene-based dithieno[3,2-b:2′,3′-d]pyrrole (DTP) rep-
resents a promising class of semiconducting materials
because of their fused-rigid backbone and good molecular
planarity.11 Thiophene- and pyrrole-based symmetrical het-
eroacenes were also successfully tested for OFET applica-
tions.12 Most of the reported heteroacenes (five rings fused)
are symmetrical in nature.13 Only a few unsymmetrical
heteroacenes14 and extended heteroacenes have been re-
ported.15 In this regard, a series of unsymmetrical in-
dolodithienopyrroles (7, 9) and the extended (seven-fused
ring) diindolodithienopyrroles (8, 10) were synthesized. Their
photophysical and electrochemical properties along with their
molecular organization in the crystal lattice are presented in
this study.
The synthetic route leading to the unsymmetrical and
extended heteroacenes is presented in Scheme 1. The
palladium-catalyzed (Pd2(dba)3/PtBu3) amination of 3,3′-
dibromo-2,2′-bithiophene 1 with excess (1.4 equiv) aniline/
4-hexyl aniline afforded DTP in 60% yield. Only aryl
substituents were incorporated as the alkyl-substituted bromo-
DTPs were unstable.16 It was postulated that the presence
of the CH group R to N in the N-alkyl derivatives plays a
role in the decomposition of the brominated product. The
dibromination of DTP was carried out by following a
reported procedure.16 Monobromination of DTP was achieved
by the addition of 1.2 equiv of N-bromosuccinimide (NBS)
under nitrogen atmosphere. Reactions under normal condi-
tions led to decomposed products as indicated by the presence
of black insoluble particles. By applying an inert nitrogen
atmosphere throughout the reaction, decomposition was
avoided, and compound 3 was obtained in a reasonable yield
(45%). The dibrominated product 4 was also isolated (20%)
during the reaction along with the unreacted starting material
2. Suzuki coupling of 3a, 3b, and 4a was carried out with
2-nitrophenyl boronic acid (NBA) to obtain compounds 5a,
5b, and 6a, respectively. Suzuki coupling of 2,6-dibromo-
N-phenyldithieno[3,2-b:2′,3′-d]pyrrole resulted in an in-
soluble product. Hence the synthesis of 8b and 10b becomes
unfeasible. The thienopyrrole-based heteroacenes were pre-
pared by reductive Cadogan cyclization.17 Refluxing the
compounds 5 and 6 with excess of triethyl phosphite and
1,2-dichlorobenzene for 24 h resulted in the target com-
pounds 7 and 8, respectively, in around 35% yield. N-Methyl
products, 9 and 10, were obtained by reacting 7 and 8 with
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J. Am. Chem. Soc. 1998, 120, 13453. (c) Ogawa, K.; Rasmussen, S. C. J.
Org. Chem. 2003, 68, 2921.
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K.; Du, C.; Yu, G.; Zhu, D. Chem. Commun. 2008, 6227.
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Mu¨llen, K. J. Org. Chem. 2008, 73, 9207. (b) Ebata, H.; Izawa, T.; Miyazaki,
E.; Takimiya, K.; Ikeda, M.; Kuwabara, H.; Yui, T. J. Am. Chem. Soc.
2007, 129, 15732. (c) Pietrangelo, A.; MacLachlan, M. J.; Wolf, M. O.;
Patrick, B. O. Org. Lett. 2007, 9, 3571.
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Mu¨llen, K. J. Org. Chem. 2008, 73, 9207. (b) Sonnlag, M.; Strohriegl, P.
Tetrahedron 2006, 62, 8103. (c) Sirringhaus, H.; Friend, R. H.; Wang, C.;
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Org. Lett., Vol. 12, No. 2, 2010
233

iodomethane. Structures of the synthesized compounds were
(Figure S2, Supporting Information). In the crystal lattice,
10a form π-stacked dimers which are rotated 180° to each
other. The distance between these dimers was found to be
around 3.34 Å, which equals the sum of van der Waals radii
of carbon atoms (Figure 1E). The existence of C-H· · ·π
interactions between the hydrogen atoms of the methyl group
with a π cloud of the adjacent molecule also accounts for
the close interactions in these dimers (2.84 Å, Figure S3,
Supporting Information). These dimers interact with other
dimers in the lattice through C-H· · ·S interactions between
the pendant phenyl group of one dimer with the π-backbone
of another dimer (2.89 Å). As a consequence of this
interaction, the fused π-backbones (dimer) are well separated
from other dimers by a distance of 8.45 Å. Though there
exists short S· · ·S contacts (3.34 Å) between the molecules
of adjacent columns, a less densely packed structure of 10a
was obtained.
1
confirmed by H NMR, 13C NMR spectroscopy, and high-
resolution mass spectrometry. These heteroacenes are highly
soluble in organic solvents which is a useful property for
thin film processing. Single crystals of 9b and 10a suitable
for single-crystal X-ray analysis were obtained by slow
evaporation from a mixture of hexane and dichloromethane
at room temperature. The detailed crystallographic informa-
tion of 9b and 10a are given in the Supporting Information.
The unsymmetrical heteroacene 9b is not planar, as it
deviates 12° from planarity (Figure 1A and 1B). The phenyl
Absorption and emission spectra of heteroacenes are
shown in Figure 2, and the spectral characteristics are
Figure 2. Normalized UV-vis absorption and emission spectra of
unsymmetric indolodithienopyrrole (A) and extended diin-
dolodithienopyrrole (B) recorded in THF.
Figure 1. Thermal ellipsoid plot of 9b (A), lateral view of 9b (B),
crystal packing of 9b (C), thermal ellipsoid plots of 10a (D), crystal
packing of 10a (E). Hydrogen atoms are removed for clarity.
summarized in Table 1. All unsymmetrical heteroacenes
showed similar absorption properties with λmax around 340
nm and absorption onset at 370 nm (Figure 2A). Absorption
spectra of extended heteroacenes 8a and 10a show similar
spectral characteristics (Figure 2B) illustrating the insignifi-
cant influence of the alkyl substituent on the electronic states
of these compounds. It is noteworthy that the absorption
spectra of the compounds are featured with several vibronic
peaks, characteristic of fused-ring systems with well-defined
electronic states. The HOMO-LUMO gap estimated from
the onset of UV-vis absorption is identical for both 8a and
10a (i.e., ca. 3.02 eV). Interestingly, 8a and 10a possess a
larger HOMO-LUMO gap compared to diindolocarbazole
(2.73 eV).16 An identical energy level gap of 3.35 eV was
obtained for all unsymmetrical heteroacenes. The fluores-
cence spectra are virtually the mirror images of absorption
spectra, indicating the rigid nature of the system.18 The
fluorescence quantum yield (ΦF) of all compounds was
measured in THF using quinine sulfate in 0.1 M sulfuric
acid as standard. The solution state quantum yields of around
5% and 18% are observed for unsymmetric and extended
heteroacenes, respectively.
group attached to the pyrrole ring is twisted by an angle of
64° with respect to the acene core. In 9b, molecules pack in
an edge-to-face manner leading to a sandwiched herringbone
packing with a herringbone angle of 60° along the a-axis.
Such a packing mode favors two-dimensional electronic
interactions in the solid which is advantageous for better
charge carrier mobility.7a The molecule 9b forms dimers with
an antiparallel, slipped cofacial arrangement, presumably to
minimize the dipolar interactions and the steric effect
imposed by the pendant phenyl ring. Within each dimer, the
molecules are held together by short C-H· · ·π contacts of
2.76 Å between the pendant phenyl ring and the acene core
(Figure S1, Supporting Information). The molecules in these
dimers are 3.6 Å apart at the shortest intermolecular distance.
The S· · ·S interactions of 3.5 Å between the molecules from
adjacent columns are also dominant (shown in Figure 1C).
Similar to 9b, compound 10a was also not strictly planar,
with a deviaton of 13.4° from planarity (Figure 1D). The
phenyl group attached to the pyrrole ring is twisted by an
angle of 84.6° with respect to the acene core. This pendant
phenyl group is tilted from the plane of the molecule due to
the steric hindrance imposed by the adjacent methyl groups
(18) Belletete, M.; Wakim, S.; Leclerc, M.; Durocher, G. J. Mol. Struct.:
THEOCHEM 2006, 760, 147–152.
234
Org. Lett., Vol. 12, No. 2, 2010

Table 1. Photophysical and Electrochemical Properties of Indolo- and Diindolodithienopyrrole
a
b
d
compd
λAbs (nm)
λEm (nm)
ΦF (%)
HOMO-LUMO gapc (eV)
Eoxid (V)
HOMOe
LUMOf
7a
7b
9a
9b
8a
357, 341, 264
357, 340, 264
386
386
387
388
425, 403
423, 400
325
6
4
6
3.35
3.35
3.35
3.35
3.02
3.02
-
0.81
0.78
0.79
0.79
0.59, 1.23
0.56, 1.14
0.65, 1.1
-5.04
-5.01
-5.01
-5.01
-4.84
-4.82
-
-1.69
-1.66
-1.66
-1.66
-1.82
-1.8
-
358, 342, 291, 265
357, 342, 290, 265
391, 370, 343, 329, 273
391, 370, 345, 330, 270
310, 300, 293, 261h
4
18
18
0.28
10a
2ag
a The samples were excited at the absorption maximum. b Solution state quantum yield in THF calculated with quinine sulfate as standard. c Estimated
from the onset of absorption. d Potentials (vs Ag/AgCl) calculated from cyclic voltammetry: 0.1 M Bu4NPF6 in acetonitrile, Pt as working electrode under
the scan rate of 100 mV·s-1. The potentials are calibrated with ferrocene as the internal standard. e Calculated using the relationship EHOMO ) -(Eox
4.38). f Derived from HOMO-LUMO gap and HOMO. g From ref 11c. h In CH2Cl2.
+
onset
Electrochemical properties of the compounds were inves-
tigated with cyclic voltammetry performed using Ag/Ag+
as the reference electrode (Figure S4, Supporting Informa-
tion). Electrochemical data and energy levels are summarized
in Table 1. Unsymmetrical heteroacenes 7a, 7b, 9a, and 9b
showed an irreversible oxidation peak around 0.8 V. Oxida-
tion potential gradually shifted to lower potential with an
increase in peak current during repeated anodic scans (Figure
S5, Supporting Information). This can be attributed to the
presence of an unblocked electroactive unit (thiophene end)
which can undergo oxidative coupling under electrochemical
conditions. Unlike the unsymmetrical heteroacenes, the
oxidation process in an extended compound was found to
be quasi-reversible. The cyclic voltammograms of 8a and
10a showed two oxidation peaks around 0.6 and 1.2 V. The
with the extension of π-conjugation. The HOMO energy
level of these compounds is significantly lower than that
reported for pentacene (ca. -4.4 eV).20 The low-lying
HOMO and relatively large band gap afford environmental
stability to these molecules.
In summary, we report a simple synthesis of new het-
eroacenes (unsymmetrical and extended) containing thiophene
and pyrrole units. These acenes are stable at ambient
conditions and soluble in common organic solvents. Unsym-
metrical heteroacene packs in sandwich herringbone manner
leading to densely packed structures. Incorporation of these
compounds in OFET application is currently in progress in
our laboratory.
Acknowledgment. The authors would like to thank
Agency for Science, Technology and Research (A*STAR),
National University of Singapore, for financial support, and
Miss Tan Geok Kheng of Department of Chemistry, National
University of Singapore, for the X-ray crystal diffraction
experiment and data refinement.
HOMO energy level was calculated from the oxidation onset
onset
potential using the empirical formula EHOMO ) -(Eox
+
4.38) eV.19 As we were unable to observe any reduction
within our scan range, LUMO energy level was empirically
calculated from the HOMO energy level and optical data
(absorption edges in UV-vis spectra). All unsymmetrical
compounds showed an oxidation onset potential around 0.7
V, which corresponds to HOMO energy levels of -5.01 eV.
The smaller band gaps for extended ring compounds (8a and
10a) compared to the unsymmetrical systems are consistent
Supporting Information Available: Details on experi-
1
mental procedures, spectroscopic data, H and 13C NMR
spectra, cyclic voltammograms of all heteroacenes, and
crystal data in CIF format of 10a and 9b. This material is
available free of charge via the Internet at http://pubs.acs.org.
OL902528B
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Einerhand, R. E. F. Synth. Met. 1997, 87, 53. (c) Cui, Y.; Zhang, X.; Jenekhe,
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