Synthesis and properties of thienopyrrole based heteroacenes - Indolodibenzothienopyrrole and dicarbazolodithienopyrrole

Article abstract of DOI:10.1039/c2ob25087j

We report the syntheses and properties of thienopyrrole based unsymmetrical and extended heteroacenes, which are isoelectronic with heptacene (30π) and nonacene (38π), respectively. Optical and electrochemical properties of these seven and nine rings fused systems are studied. The optoelectronic properties of the syn and anti-isomers of the unsymmetrical heteroacenes are also compared. The influence of the position of the heteroatoms in the fused corona, upon the optical and electrochemical properties, is rationalized based on the contributions from the benzenoid vs. quinonoid-type structures of these molecules.

Full text of DOI:10.1039/c2ob25087j

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links
Organic &
Biomolecular
Chemistry
Cite this: Org. Biomol. Chem., 2012, 10, 3455
www.rsc.org/obc
PAPER
Synthesis and properties of thienopyrrole based heteroacenes –
indolodibenzothienopyrrole and dicarbazolodithienopyrrole†
Ganapathy Balaji, Andrea M. Della Pelle, Bhooshan C. Popere, A. Chandrasekaran and S. Thayumanavan*
Received 12th January 2012, Accepted 22nd February 2012
DOI: 10.1039/c2ob25087j
We report the syntheses and properties of thienopyrrole based unsymmetrical and extended heteroacenes,
which are isoelectronic with heptacene (30π) and nonacene (38π), respectively. Optical and
electrochemical properties of these seven and nine rings fused systems are studied. The optoelectronic
properties of the syn and anti-isomers of the unsymmetrical heteroacenes are also compared. The
influence of the position of the heteroatoms in the fused corona, upon the optical and electrochemical
properties, is rationalized based on the contributions from the benzenoid vs. quinonoid-type structures of
these molecules.
introduction of functional groups that facilitate solubilization
necessary for processibility and to optimize packing in con-
Introduction
Extended π-structures have attracted significant attention from
several fields, especially in organic electronics.1 The reason for
the interest stems from the fact that intermolecular frontier
orbital overlap in molecular organizations is thought to greatly
influence charge transport in these materials.2 Then, it is reason-
able to assume that these desired MO overlaps will be enhanced
in flat aromatic systems with a large π-surface area. Thus,
extended acenes have become interesting synthetic targets. The
relatively limited access to extended acenes has led to the use of
heteroacenes.3 Heteroacenes have the advantage of structural
tunability based on the chosen heterocyclic systems.4 Heteroa-
cenes are also versatile in their structural variations due to the
differences in number, nature, and position of heteroatoms in the
fused aromatic system which enables the variation of their struc-
tures for a targeted property.5 Several families of heteroacenes
bearing heteroatoms such as boron, nitrogen, oxygen, silicon,
phosphorous, sulfur or a combination of these heteroatoms have
been reported.6 Among them, thiophene and pyrrole based het-
eroacenes gained significant attention, due to their prior success
as components of materials for organic field effect transistor
(OFET) or photovoltaics applications.7
densed phase for efficient charge transport. Similarly, thiophene
units are thought to bring about secondary interactions such as
S⋯S contacts in the condensed phase, which should also
influence packing.8 Therefore, we considered the thienopyrrole
core as a promising unit to construct the extended fused struc-
tures. In this manuscript, we report on the syntheses and proper-
ties of unsymmetrically fused (seven rings) and symmetrically
fused (nine rings), thienopyrrole based heteroacenes utilizing
Cadogan reductive cyclization. Photophysical and electrochemi-
cal properties of the target molecules are also presented. The
structures of the synthesized molecules are represented in Fig. 1.
Experimental
General considerations
All chemicals and reagents were purchased from commercial
suppliers and used without further purification, unless otherwise
mentioned. Solvents used for spectroscopic measurements were
spectral grade quality. Tetrahydrofuran (THF) was distilled over
sodium. All reactions were monitored by thin-layer chromato-
graphy (TLC) carried out on silica gel plates. Preparative separ-
ations were performed using Combiflash Rf-200 automated flash
chromatography.
Fused systems containing both thiophene and pyrrole units are
interesting, because they have the potential to retain the advan-
tages of both pyrrole and thiophene units. The trivalent nature of
the nitrogen in the pyrrole units allows for the convenient
Characterization techniques
Department of Chemistry, University of Massachusetts, Amherst,
Massachusetts 01003, United States. E-mail: thai@chem.umass.edu
†Electronic supplementary information (ESI) available: 1H and 13C
spectra, and crystallographic information file (CIF). CCDC 864076 and
864077. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c2ob25087j
All molecules were characterized using 1H NMR, 13C NMR and
high resolution mass spectrometry to confirm the molecular
1
structure. H NMR spectra were recorded on a 400 MHz Bruker
NMR spectrometer using the residual proton resonance of the
solvent as the internal standard. Chemical shifts are reported in
This journal is © The Royal Society of Chemistry 2012
Org. Biomol. Chem., 2012, 10, 3455–3462 | 3455

View Article Online
Fig. 1 Structures of synthesized heteroacenes (A) indolodibenzothienopyrrole and (B) dicarbazolodithienopyrrole.
X-ray crystallography
parts per million (ppm). When peak multiplicities are given, the
following abbreviations are used: s, singlet; d, doublet; t, triplet;
dd, doublet of doublet; m, multiplet; b, broad. 13C NMR spectra
were proton decoupled and recorded on a 100 MHz Bruker
spectrometer using the carbon signal of the deuterated solvent as
an internal standard. Cyclic voltammetry experiments were
carried out at room temperature using BASi C3 cell stand fitted
with three electrodes; a platinum disk working electrode, plati-
num auxiliary electrode and Ag/Ag+ reference electrode. The
voltammograms were recorded in dry dichloromethane (DCM)
using tetrabutylammonium hexafluorophosphate as the support-
ing electrolyte and ferrocene as the internal standard under
nitrogen atmosphere. The scan rate in all experiments was
100 mV s−1. Theoretical calculations were performed in Gaus-
sian 031 at the density functional theory (DFT) level with the
B3LYP functional and a 6-311g(d,p) basis set.9 The HOMO and
LUMO surfaces were generated from the optimized geometries.
UV-Visible spectra were obtained using a Cary 100 spectro-
photometer and fluorescence data were collected using a JASCO
FP-6500 spectrofluorimeter. Solution state photoluminescence
quantum yields were measured using quinine sulfate (0.1 M
H2SO4) as the standard.10 Spectroscopic grade solvent was used
to prepare sample solutions and checked for background fluor-
escence. The solution of each sample and standard at five differ-
ent concentrations (concentration was chosen such that the
absorption of the sample was less than 0.1 at the excitation wave-
length) was analyzed. The absorbance and integrated fluor-
escence intensity values at each concentration was obtained and
plotted to get the gradient (m) with intercept = 0. The quantum
yield value was calculated using the relation,
The X-ray crystallographic studies were performed using a
Nonius KappaCCD diffractometer and graphite monochromated
MoKα radiation (λ = 0.71073 Å). Data was collected at 293 K,
θMoKα ≤ 25°. All data was included in the refinement. The struc-
tures were solved by direct methods and difference Fourier tech-
niques and were refined by full-matrix least squares.
Refinements were based on F2 and computations were performed
using SHELXS-86 for solution and SHELXL-97 for refinement.
All non-hydrogen atoms were refined anisotropically. All hydro-
gen atoms were included in the refinement as isotropic scatterers
riding in ideal positions on the bonded atoms. The hydrogens on
disordered atoms were not included in the calculations. The final
agreement factors are based on the reflections with I ≥ 2σI.
Synthesis of compound 7
To a 250 mL round-bottom flask under argon atmosphere, cata-
lyst Pd2(dba)3 (0.1 mmol), ligand ( )-BINAP (0.1 mmol) and
toluene (20 mL) were added and allowed to stir for 5 minutes.
The base NaOtBu (3 mmol) and dibromodibenzo[b]thiophene
(6) (1 mmol) were added followed by the addition of 2-ethylhex-
ylamine (1.2 mmol) and the reaction mixture was allowed to
reflux for six hours. Reaction mixture was cooled to room temp-
erature and extracted with ethyl acetate–brine. The combined
organic layer was dried over anhydrous sodium sulfate, filtered
and concentrated in vacuo. The crude compound was purified by
column chromatography with hexane as the eluent to give the
desired compound as white solid. 7: (70% yield). Mp
1
147–148 °C. H NMR (400 MHz, CDCl3) δ: 7.91 (d, J = 8 Hz,
2H), 7.86 (d, J = 8 Hz, 2H), 7.41 (m, 2H), 7.29 (m, 2H), 4.67
(m, 2H), 2.18 (m, 1H), 1.48–1.39 (m, 5H), 1.23 (m, 3H), 0.89 (t,
J = 7.4 Hz, 3H), 0.82 (t, J = 7.12 Hz, 3H). 13C NMR (100 MHz,
CDCl3) δ: 141.9, 137.9, 127.7, 124.5, 124.2, 123.1, 119.2,
114.6, 51.8, 40.7, 30.5, 28.6, 23.8, 23.0, 13.9, 11.1. IR (solid)
2
2
Φx ¼ Φs½Gra X=Gra Sꢀ½ηx =ηs ꢀ
where S and X denotes the standard and test sample respectively.
Φs of quinine sulfate in 0.1 M H2SO4 = 54%.
3456 | Org. Biomol. Chem., 2012, 10, 3455–3462
This journal is © The Royal Society of Chemistry 2012

View Article Online
3048, 2922, 1587, 1406, 742, 724 cm−1. HRMS m/z calculated
to column chromatography with ethyl acetate–hexane as eluent
for C24H25NS2: 391.1428, found by EI: 391.1419.
to get the pure product.
9: (60% yield). Mp 91–93 °C. H NMR (400 MHz, CDCl3)
1
7.92–7.83 (m, 5H), 7.68–7.64 (m, 1H), 7.57–7.49 (m, 2H),
7.44–7.40 (m, 1H), 7.36–7.29 (m, 2H), 4.67 (m, 2H), 2.17 (m,
1H), 1.53–1.21 (m, 8H), 0.90 (t, J = 7.44 Hz, 3H), 0.83 (m, J =
7.18 Hz, 3H). 13C NMR (100 MHz, CDCl3) δ: 149.4, 142.3,
142.0, 138.4, 137.5, 136.0, 133.4, 132.3, 132.2, 132.0, 128.1,
127.6, 127.3, 124.5, 124.3, 123.7, 123.3, 119.3, 119.1, 115.6,
114.6, 51.8, 40.8, 30.5, 28.6, 23.9, 23.0, 13.9, 11.1. IR (solid)
3054, 2927, 1648, 1520, 1340, 748, 693 cm−1. HRMS m/z cal-
culated for C30H28N2O2S2: 512.1592, found by EI: 512.1567.
13: (50% yield). Mp 191–192 °C. 1H NMR (400 MHz,
CDCl3) δ: 7.95–7.90 (m, 4H), 7.85 (d, J = 1.44 Hz, 2H),
7.68–7.64 (m, 2H), 7.57–7.50 (m, 4H), 7.36 (dd, J1 = 8.32 Hz,
J2 = 1.60 Hz, 2H), 4.68 (m, 2H), 2.21 (m, 1H), 1.51–1.26 (m,
6H), 1.21–1.18 (m, 2H), 0.92 (t, J = 7.40 Hz, 3H), 0.84 (t, J =
7.16 Hz, 3H). 13C NMR (100 MHz, CDCl3) δ: 149.3, 142.5,
138.0, 135.9, 132.4, 132.3, 132.2, 128.2, 127.2, 124.3, 124.2,
123.7, 119.3, 115.6, 51.9, 40.8, 30.6, 28.6, 23.9, 23.0, 13.9,
Synthesis of compound 8
Bromine (2 mmol) was added drop wise to a solution of com-
pound 7 (2 mmol) in chloroform (50 mL) over a period of
20 min. The reaction mixture was stirred for 4 hours at room
temperature. After completion of the reaction, the reaction
mixture was extracted with dichloromethane–water twice and
finally with sodium thiosulphate solution. The combined organic
layer was dried over anhydrous sodium sulfate, filtered and con-
centrated in vacuo. The crude product was purified by column
chromatography to give the monobromo compound as a white
solid. 8: (60% yield). Mp 153–154 °C. 1H NMR (400 MHz,
CDCl3) δ: 7.96 (d, J = 1.72 Hz, 1H), 7.87 (d, J = 8.06 Hz, 1H),
7.85 (d, J = 7.96 Hz, 1H), 7.70 (m, 1H), 7.49 (dd, J1 = 8.0 Hz,
J2 = 1.8 Hz, 1H), 7.41 (m, 1H), 7.31 (m, 1H), 4.66 (m, 2H),
2.10 (m, 1H), 1.46–1.18 (m, 8H), 0.88 (t, J = 7.4 Hz, 3H), 0.82
(t, J = 7.28 Hz, 3H). 13C NMR (100 MHz, CDCl3) δ: 143.3,
142.0, 137.1, 127.5, 127.4, 126.8, 126.4, 124.5, 124.3, 123.3,
120.0, 119.9, 119.3, 116.2, 114.8, 114.5, 51.7, 40.7, 30.5, 28.6,
23.9, 23.0, 13.9, 11.0. IR (solid) 3060, 2922, 1587, 1551, 797,
724 cm−1. HRMS m/z calculated for C24H24NBrS2: 469.0534,
found by EI: 469.0545.
11.1. IR (solid) 3060, 2922, 1611, 1520, 1346, 748 cm−1
.
HRMS m/z calculated for C36H31N3O4S2: 633.1756, found by
EI: 633.1754.
General procedure for Cadogan reaction – synthesis of
compounds 10, 11, 14 and 4
Synthesis of compound 12
To a 100 mL two-neck round-bottomed flask under argon atmos-
phere, the nitro compound (9/13) (0.5 mmol) was added fol-
lowed by the addition of triethyl phosphite (5 mL) and 1,2-
dichlorobenzene (5 mL). The reaction mixture was heated under
reflux for 12 hours. After completion, the reaction mixture was
cooled to ambient temperature and concentrated in vacuo. The
crude product was purified by column chromatography (ethyl
acetate–hexane) to result in the desired fused compounds.
10: (50% yield). Mp 205–207 °C. 1H NMR (400 MHz,
acetone-d6) δ: 10.92 (b, 1H), 8.21 (d, J = 8.21 Hz, 1H), 8.16 (d,
J = 7.76 Hz, 1H), 8.11 (d, J = 8.04 Hz, 1H), 7.97–7.94 (m, 2H),
7.59 (d, J = 8.08 Hz, 1H), 7.48 (m, 1H), 7.41 (m, 1H), 7.34 (m,
1H), 7.24 (m, 1H), 4.90 (d, J = 7.92 Hz, 2H), 2.25–2.20 (m,
1H), 1.57–1.37 (m, 6H), 1.18–1.13 (m, 2H), 0.92 (t, J = 7.44
Hz, 3H), 0.76 (t, J = 7.26 Hz, 3H). 13C NMR (100 MHz,
acetone-d6) δ: 141.2, 139.6, 139.5, 137.5, 135.3, 127.5, 125.7,
125.0, 124.2, 124.1, 123.5, 122.9, 119.5, 119.3, 119.3, 118.8,
116.9, 114.0, 113.0, 111.5, 110.9, 110.8, 51.1, 40.6, 30.0, 28.2,
23.5, 22.5, 12.9, 10.2. IR (solid) 3368, 3043, 2923, 1613,
724 cm−1. HRMS m/z calculated for C30H28N2S2: 480.1694,
found by EI: 480.1694.
Bromine (5 mmol) was added dropwise to a solution of com-
pound 7 (2 mmol) in chloroform (50 mL). The reaction mixture
was stirred for 8 hours at room temperature. After completion,
the reaction mixture was extracted with dichloromethane–water
twice and finally with a sodium thiosulphate solution. The com-
bined organic layer was dried over anhydrous sodium sulfate.
The solvent was removed under pressure and the crude product
was purified by column chromatography to give the dibromo
1
compound 12 as a white solid (80% yield). Mp 175–176 °C. H
NMR (400 MHz, CDCl3) δ: 7.97 (d, J = 1.72 Hz, 2H), 7.69 (d,
J = 8.6 Hz, 2H), 7.49 (dd, J1 = 8.6 Hz, J2 = 1.8 Hz, 2H), 4.54
(m, 2H), 2.05 (m, 1H), 1.47–1.16 (m, 8H), 0.86 (t, J = 7.44 Hz,
3H), 0.81 (t, J = 7.04 Hz, 3H). 13C NMR (100 MHz, CDCl3) δ:
143.4, 137.4, 127.5, 126.9, 126.2, 120.0, 116.5, 114.8, 51.8,
40.7, 30.5, 28.6, 23.9, 22.9, 13.9, 11.0. IR (solid) 3072, 2919,
1546, 1499, 1089, 794 cm−1. HRMS m/z calculated for
C24H23NBr2S2: 546.9639, found by EI: 546.9603.
General procedure for Suzuki coupling – synthesis of
compounds 9 and 13
1
14: (40% yield). Mp > 350 °C. H NMR (400 MHz, acetone-
To a 250 mL two-neck round-bottom flask, THF (20 mL), mono-
bromo or dibromo compound (8 or 12) (2.5 mmol) and 2-nitro-
phenylboronic acid (3.0 mmol for 9/6.0 mmol for 13) were
added followed by the addition of catalyst Pd2(dba)3
(0.25 mmol), P(o-Tol)3 (0.25 mmol) and 2 M aqueous potass-
ium carbonate (15 mL). The reaction mixture was heated to
50 °C for 8 hours. The reaction mixture was then allowed to cool
to room temperature and extracted with ethyl acetate–brine. The
collected organic layer was dried over anhydrous sodium sulfate,
filtered and concentrated in vacuo. The crude solid was subjected
d6) δ: 10.91 (s, 2H), 8.23 (d, J = 8.4 Hz, 2H), 8.17 (d, J = 7.68
Hz, 2H), 8.00 (d, J = 8.4 Hz, 2H), 7.59 (d, J = 8.08 Hz, 2H),
7.41 (m, 2H), 7.25 (m, 2H), 5.03 (d, J = 8.24 Hz, 2H), 2.33 (m,
1H), 1.61–1.53 (m, 4H), 1.21–1.15 (m, 4H), 0.95 (t, J = 7.40
Hz, 3H), 0.77 (t, J = 7.30 Hz, 3H). 13C NMR (100 MHz CDCl3)
δ:139.6, 139.3, 125.9, 124.9, 123.59, 123.55, 119.5, 119.3,
118.6, 117.0, 113.1, 111.4, 110.83, 110.78, 51.1, 40.6, 30.1,
23.56, 22.5, 12.9, 10.27. IR (solid) 3397, 3052, 2926, 1609,
724 cm−1. HRMS m/z calculated for C36H31N3S2: 569.1959,
found by EI: 569.1955.
This journal is © The Royal Society of Chemistry 2012
Org. Biomol. Chem., 2012, 10, 3455–3462 | 3457

View Article Online
1
General procedure for alkylation – synthesis of compounds 1, 2
3: (85% yield). Mp 226–228 °C. H NMR (400 MHz CDCl3)
δ: 8.12 (d, J = 7.60 Hz, 2H), 8.05 (d, J = 8.36 Hz, 2H), 7.68 (d,
J = 7.4 Hz, 2H), 7.46 (m, 2H), 7.40 (d, J = 8.04 Hz, 2H), 7.29
(m, 2H), 4.60–4.56 (m, 2H), 4.51 (t, J = 7.8 Hz, 4H), 2.19 (m,
1H), 1.98 (m, 4H), 1.58 (m, 5H), 1.39 (m, 12H), 1.20 (m, 3H),
0.93 (t, J = 7.00 Hz, 6H), 0.86–0.80 (m, 6H). 13C NMR
(100 MHz CDCl3) δ: 140.2, 139.2, 136.2, 126.5, 124.9, 123.4,
122.1, 119.7, 119.3, 118.4, 117.0, 113.3, 111.3, 108.8, 51.3,
44.8, 40.7, 31.6, 31.0, 30.5, 28.6, 26.8, 23.8, 23.0, 22.7, 14.1,
14.0, 11.1. IR (solid) 3050, 2925, 1591, 721 cm−1. HRMS m/z
calculated for C48H55N3S2: 737.3837, found by EI: 737.3876.
and 3
To a 100 mL round-bottom flask under argon atmosphere, fused
compound (10/11/14) (0.1 mmol), dry THF (10 mL) and alkyl
bromide (0.3 mmol) were added. To this reaction mixture NaH
(60% in mineral oil) (0.25 mmol) was added and refluxed for
6 hours. The reaction mixture was quenched by careful addition
of water and extracted with ethyl acetate–water three times. The
combined organic layer was dried over anhydrous sodium
sulfate, filtered and concentrated in vacuo. The crude product
was purified by column chromatography to obtain the corre-
sponding alkyl derivative.
1
1: (80% yield). Mp 106–108 °C. H NMR (400 MHz CDCl3)
Results and discussion
δ: 8.12 (d, J = 7.72 Hz, 1H), 8.09 (d, J = 8.36 Hz, 1H), 7.87 (d,
J = 8.60 Hz, 2H), 7.74 (d, J = 8.36 Hz, 1H), 7.49–7.39 (m, 3H),
7.31–7.27 (m, 2H), 4.64 (m, 2H), 4.55 (t, J = 8.00 Hz, 2H),
2.24–2.20 (m, 1H), 2.01–1.93 (m, 2H), 1.56–1.21 (m, 14H),
0.93–0.81 (m, 9H). 13C NMR (100 MHz CDCl3) δ: 141.7,
140.2, 139.5, 137.9, 136.2, 127.8, 126.4, 125.0, 124.4, 124.2,
123.4, 122.8, 122.4, 119.8, 119.4, 119.2, 118.5, 117.0, 114.3,
113.7, 111.3, 108.9, 51.6, 44.9, 40.7, 31.6, 31.0, 30.5, 28.6.
26.8, 23.8, 23.0, 22.6, 14.1, 13.9, 11.1. IR (solid) 3043, 2923,
1690, 1597, 743, 724 cm−1. HRMS m/z calculated for
C36H40N2S2: 564.2633, found by EI: 564.2655.
The synthetic route towards the seven-ring fused heteroacenes is
shown in Scheme 1. Commercially available 3-bromobenzo[b]
thiophene (5) was deprotonated at the 2-position with lithium
diisopropylamide (LDA) followed by oxidative coupling with
CuCl2 to obtain 3,3′-dibromo-2,2′-dibenzo[b]thiophene (6) in
90% yield.11 Buchwald–Hartwig amination of compound 6 with
alkyl amines using Pd2(dba)3/( )-BINAP as the catalyst resulted
in dibenzodithienopyrrole 7 in a moderate yield of 70%. Con-
trolled bromination of dibenzodithienopyrrole with one equival-
ent of Br2 in CHCl3 for 2 hours resulted in the mono-bromo
product (8). The regioselectivity in bromination is influenced by
the directing effect of the pyrrole and possibly by the steric
effect of the N-alkyl chains which favors substitution at the 2-
and 8-positions. The monobromo compound (8), upon Suzuki
coupling with 2-nitrophenylboronic acid in the presence of
Pd2(dba)3 as the catalyst and P(o-Tol)3 as the ligand resulted in
the nitroarene molecule 9. The heteroacene skeleton was
achieved by a Cadogan reductive ring-closure reaction.12 The
compound 9 was refluxed with triethyl phosphite in 1,2-dichloro-
benzene for 12 hours to achieve the seven-ring fused heteroacene.
The Cadogan ring-closure reaction was non-regioselective to
result in a mixture of two regioisomeric heteroacenes 10 (anti)
1
2: (65% yield). Mp 144–146 °C. H NMR (400 MHz CDCl3)
δ: 8.47 (s, 1H), 8.10 (d, J = 7.68 Hz, 1H), 7.82 (d, J = 8.08 Hz,
2H), 7.66 (s, 1H), 7.46 (m, 1H), 7.36 (m, 2H), 7.27–7.22 (m,
2H), 4.61–4.58 (m, 2H), 4.23 (t, J = 7.28 Hz, 2H), 2.21 (m, 1H),
1.90 (m, 2H), 1.52–1.21 (m, 14H), 0.93–0.81 (m, 9H). 13C
NMR (100 MHz CDCl3) δ: 141.9, 141.5, 139.1, 137.9, 137.7,
133.4, 127.6, 126.1, 125.7, 124.4, 124.1, 123.9, 122.1, 120.8,
120.1, 119.1, 118.8, 115.6, 115.1, 114.8, 108.3, 98.2, 51.6, 43.2,
40.8, 31.7, 30.6, 28.8, 28.6, 27.1, 23.8, 23.1, 22.6, 14.0, 13.9,
11.0. IR (solid) 3047, 2920, 1598, 1471, 739, 725 cm−1. HRMS
m/z calculated for C36H40N2S2: 564.2633, found by EI:
564.2618.
Scheme 1 Synthesis of thienopyrrole based seven-ring fused unsymmetrical heteroacenes.
3458 | Org. Biomol. Chem., 2012, 10, 3455–3462
This journal is © The Royal Society of Chemistry 2012

View Article Online
Fig. 2 1H NMR spectra of anti (1) and syn (2) isomer recorded in CDCl3.
Scheme 2 Synthesis of dicarbazolodithienopyrrole.
and 11 (syn) (90 : 10). We were successful in isolating the pure
form of the major isomer 10 by column chromatography. The
minor syn-isomer was always contaminated with anti-isomer,
and owing to its smaller percentage in the mixture, our attempts
to achieve a pure form of 11 was unfruitful. Alkylation of these
fused cores with hexylbromide in the presence of NaH resulted
in the target compounds 1 and 2. Pure syn isomer 2 free of anti-
isomer was obtained upon alkylation of the mixture of anti and
syn isomers. These isomers are conveniently distinguished by 1H
NMR spectroscopy. The syn isomer showed the characteristic
singlet at 8.47 and 7.67 ppm corresponding to He and Hj
protons, whereas the anti isomer showed the characteristic
doublet at 8.09 and 7.74 ppm for He and Hf protons (Fig. 2).
All synthesised compounds were characterized by 1H NMR,
13C NMR and high resolution mass spectrometric (HRMS)
investigation.
excess bromine (2.5 equivalents) for 6 hours, afforded the
dibromo product 12. Suzuki coupling with the nitroarylboronic
acid afforded the dinitro arene compound 13 in 50% yield.
Cadogan ring-closure of 13 is expected to result in three regio-
isomers out of which two isomers were observed. Only the
major isomer 14 was isolated, and the other isomer 4 was always
contaminated with the major isomer. A similar orientation
pattern (substitution ortho- to heteroatom) was reported for
Cadogan reactions involving carbazole to synthesize diindolocar-
bazole and the orientation was attributed to the strong ortho-
directing nature of the hetero atoms.13 It is noteworthy that the
current synthetic scheme allows for the orthogonal substitution
on pyrrole nitrogens which provides an additional path to
influence their properties. All molecues 1–3 are stable under
ambient conditions and soluble in common organic solvents
such as THF, CHCl3 and toluene.
The synthetic route towards nine rings fused heteroacene is
outlined in Scheme 2. Bromination of 7, when carried out with
Among the compounds synthesized, crystals of 14 were
obtained by slow evaporation from ethyl acetate. The molecule
This journal is © The Royal Society of Chemistry 2012
Org. Biomol. Chem., 2012, 10, 3455–3462 | 3459

View Article Online
Fig. 3 Thermal ellipsoid plot of 14 (A), lateral view (B), and packing of 14 in solid state (solvent molecules and hydrogen atoms removed for
clarity) (C).
Fig. 4 Normalized absorption spectra (A) and emission spectra (B) of heteroacenes in THF.
Table 1 Optoelectronic properties of heteroacenes
Compd λmax a (nm)
log ε (L mol−1 cm−1
)
λemi (nm)
ϕF b (%) HOMO–LUMO gapc (eV) Epa d(V)
HOMOe(eV) LUMOf(eV)
10
1
2
14
3
370, 350, 273 2.08
372, 351, 274 1.98
383, 362, 282 2.17
388, 367, 283 2.39
390, 370, 286 2.36
396, 377 36
397, 378 35
421, 404 30
416, 396 31
419, 399 30
3.28
3.26
3.09
3.03
3.01
0.51, 1.27 −5.15
−1.87
−1.92
−1.99
−1.95
−1.99
0.60, 1.30 −5.18
0.42, 1.06 −5.08
0.30, 1.04 −4.98
0.37, 1.04 −5.00
a Solvent: tetrahydrofuran. b Solution state quantum yield calculated with quinine sulfate as standard. c Estimated from the onset of absorption spectra
1240/λ. d Oxidation peak potential versus Fc/Fc+. e Calculated from the empirical relationship EHOMO = −(Eoxid
from the relationship ELUMO = EHOMO − Eg(optical).
+ 4.8) eV. f Calculated LUMO
onset
co-crystallizes with two molecules of ethyl acetate which are
hydrogen bonded with the pyrrole hydrogen. The thermal ellip-
soid plot at 50% probability and crystal packing in solid state are
represented in Fig. 3. The crystal structure reveals that the conju-
gated backbone is highly planar with a deviation of less than 3°
from planarity. In solid state, these molecules form a slip-stacked
structure of alternate C–H⋯π interactions of 3.37 Å and close
contact of 3.50 Å. No significant π overlap is apparent from the
packing along these stacks. These stacked layers are insulated
from the other layers by solvent molecule and also by bulky 2-
ethylhexyl chains.
Absorption and emission spectra of the fused systems were
recorded in THF (Fig. 4) and the values are summarized in
Table 1. All fused systems show multiple absorption peaks with
the absorption maxima around 370–390 nm. The seven-rings
fused anti-isomer (10) showed absorption maxima at 370 nm.
3460 | Org. Biomol. Chem., 2012, 10, 3455–3462
This journal is © The Royal Society of Chemistry 2012

View Article Online
Fig. 5 Cyclic voltammogram of seven-ring (A) and nine-ring fused systems (B) recorded at the scan rate of 100 mV s−1. Potentials are calibrated
with respect to ferrocene–ferrocenium couple.
However, syn-isomer 2 showed a red shifted absorption with the
absorption maximum at 383 nm. This difference in absorption
maxima in anti and syn isomers can be attributed to the differ-
ence in conjugation (vide infra). Alkylation of the pyrrole unit in
compound 10 did not have any effect on the absorption spec-
trum. Similarly, the absorption spectra of the nine rings fused
compound is red-shifted compared to the seven-ring fused
systems, with absorption maxima around 390 nm due to an
extension in conjugation. The HOMO–LUMO energy gap calcu-
lated from absorption onset was estimated to be 3.26, 3.09, and
3.01 eV for compounds 1, 2 and 3 respectively.
Fig. 6 Canonical structures for syn and anti isomers.
All heteroacenes are blue emissive in solution state with com-
pounds 1, 2 and 3 showing the emission maximum at 378, 401
and 399 nm respectively with a quantum yield of around 30%.
The absorption and emission spectra of these fused systems hold
a mirror image relationship with a small Stokes shift, character-
istic of rigid conjugated systems with well-defined electronic
states.14
renders these molecules as viable candidates for components of
electronic materials.
In order to understand the difference in optoelectronic proper-
ties of syn and anti-isomers, their canonical structures were com-
pared. A number of canonical structures are possible for both syn
and anti-isomers, of which we consider the one involving the
central benzene ring (point of fusion). From Fig. 6, it can be
noted that in case of the syn-isomer either one of the heterocycles
fused to the central benzene ring [pyrrole (A) or thiophene (B)],
resides in a partial quinoidal state, whereas, in case of the anti-
isomer, canonical structure C, with all fused rings in the benze-
noid state, will be favored over the canonical structure D, with
both pyrrole and thiophene in a partial quinoid state. This
explains the better conjugation in the syn-isomer over the anti-
isomer which is reflected in their energy gap, absorption maxima
and oxidation potential (Table 1). To obtain further support for
this, we also calculated HOMO energy levels of the syn- and
anti-isomers of the seven-ring system using DFT calculations.
As shown in Fig. 7, anti and syn isomers showed subtle differ-
ences in the distribution of HOMO energy levels. The difference
in HOMO energy levels is more pronounced along the central
benzene ring, consistent with the canonical structure analysis
provided above.
The redox properties of these compounds were studied using
cyclic voltammetry (Fig. 5). All compounds showed two quasi-
reversible oxidation peaks; the anodic peak potentials, Epa vs.
Fc/Fc+, are tabulated in Table 1. They showed two oxidation
peaks attributed to the successive oxidation of the pyrrole nitro-
gens. Alkylation of 10 resulted in a 90 mV positive shift of the
first oxidation potential and a 30 mV shift in the second oxi-
dation potential, suggesting that the first oxidation peak can be
ascribed to the indole nitrogen. Peak separation between con-
secutive redox events in a molecule can be used as a tool for the
characterization of coupling strength.15 The anti-isomer 1
showed a greater peak separation (700 mV) compared to the syn-
isomer (640 mV) 2 indicating a slightly better communication
between the redox centers in the anti-isomer, which can be
understood from the canonical structures of these molecules
shown below.
The HOMO energy levels were calculated from the oxidation
onset potentials using the empirical relationship EHOMO
(Eoxid
=
+ 4.8) eV.16 The HOMO levels of −5.18, −5.08 and
In summary, we report a new family of seven and nine rings
fused heteroacenes which are isoelectronic with heptacene and
nonacene. The handiness of using thienopyrrole core to construct
highly stable and soluble fused aromatic systems is demon-
strated. The current synthetic routes also allow for the orthogonal
onset
−5.00 eV were estimated for compounds 1, 2 and 3 respectively.
It is interesting to note that all compounds have a low-lying
HOMO as compared to heptacene (−4.8 eV)17 and nonacene
(−4.69 eV).18 The low lying HOMO imparts stability and
This journal is © The Royal Society of Chemistry 2012
Org. Biomol. Chem., 2012, 10, 3455–3462 | 3461

View Article Online
2005, 7, 5301; (g) S. Yamaguchi, C. Xu and T. Okamoto, Pure Appl.
Chem., 2006, 78, 721.
7 (a) K. Takimiya, H. Ebata, K. Sakamoto, T. Izawa, T. Otsubo and
Y. Kunugi, J. Am. Chem. Soc., 2006, 128, 12604; (b) Y. Zhou, W. J. Liu,
Y. Ma, H. Wang, L. Qi, Y. Cao, J. Wang and J. Pei, J. Am. Chem. Soc.,
2007, 129, 12386; (c) B. Wex, B. R. Kaafarani, R. Schoreder, L.
A. Majewski, P. Burckel, M. Grell and D. C. Neckers, J. Mater. Chem.,
2006, 16, 1121; (d) Y. Li, Y. Wu, S. Gardner and B. S. Ong, Adv. Mater.,
2005, 17, 849; (e) T. Qi, Y. Guo, Y. Liu, H. Xi, H. Zhang, X. Gao, Y. Liu,
K. Lu, C. Ku, G. Yu and D. Zhu, Chem. Commun., 2008, 6227;
(f) P. Gao, D. Beckmann, H. N. Tsao, X. Feng, V. Enkelmann,
M. Baumgarten, W. Pisula and K. Müllen, Adv. Mater., 2009, 21, 213;
(g) S. Shinamura, I. Osaka, E. Miyazaki, A. Nakao, M. Yamagishi,
J. Takeya and K. Takimiya, J. Am. Chem. Soc., 2011, 133, 5024;
(h) G. Balaji, D. I. Phua, W. L. Shim and S. Valiyaveettil, Org. Lett.,
2010, 12, 232.
8 (a) K. Pluchackova, S. Grimme and P. Hobza, J. Phys. Chem. A, 2008,
112, 12469; (b) X.-K. Chen, L,-Y. Zou, S. Huang, C.-G. Min and A.-
M Ren, Org. Electron., 2011, 12, 1198; (c) V. Coropceanu, J. Cornil, D.
A. da Silva Filho, Y. Oliver, R. Silbey and J.-L. Brédas, Chem. Rev.,
2007, 107, 926; (d) V. Lemaur, D. A. da Silva Filho, V. Coropceanu,
M. Lehmann, Y. Geerts, J. Piris, M. G. Debije, A. M. van de Craats,
K. Senthilkumar, L. D. A. Siebbeles, J. M. Warman, J.-L. Brédas and
J. Cornil, J. Am. Chem. Soc., 2004, 126, 3271; (e) E. A. Marseglia,
F. Grepioni, E. Tedesco and D. Braga, Mol. Cryst. Liq. Cryst. Sci.
Technol., Sect. A, 2000, 348, 137; (f) G. Barbarella, M. Zambianchi,
A. Bongini and L. Antolini, Adv. Mater., 1993, 5, 834.
Fig. 7 HOMO energy calculated by density functional theory (DFT) at
the B3LYP/6-311g (d,p) level.
substitution on the pyrrole rings. Utilizing the non-regioselective
nature of Cadogan cyclization, both syn and anti isomers are
synthesized in one pot. The synthesized heteroacenes are highly
stable and are soluble in common organic solvents. The photo-
physical and electrochemical properties of the syn- and anti-
isomers were studied, both of which indicate that the positions
of heteroatoms has a significant influence on the physiochemical
properties of these molecules. Investigation of charge transport
behavior and incorporation of these fused heterocycles into a
semiconducting polymeric backbone is under progress in our
laboratory.
9 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb,
J. R. Cheeseman, J. A. Montgomery, Jr., T. Vreven, K. N. Kudin, J.
C. Burant, J. M. Millam, S. S. Iyengar, J. Tomasi, V. Barone,
B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G. A. Petersson,
H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa,
M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li,
J. E. Knox, H. P. Hratchian, J. B. Cross, V. Bakken, C. Adamo,
J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin,
R. Cammi, C. Pomelli, J. Ochterski, P. Y. Ayala, K. Morokuma, G.
A. Voth, P. Salvador, J. J. Dannenberg, V. G. Zakrzewski, S. Dapprich, A.
D. Daniels, M. C. Strain, O. Farkas, D. K. Malick, A. D. Rabuck,
K. Raghavachari, J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul,
S. Clifford, J. Cioslowski, B. B. Stefanov, G. Liu, A. Liashenko,
P. Piskorz, I. Komaromi, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-
Laham, C. Y. Peng, A. Nanayakkara, M. Challacombe, P. M. W. Gill, B.
G. Johnson, W. Chen, M. W. Wong, C. Gonzalez and J. A. Pople, GAUS-
SIAN 03 (Revision D.02), Gaussian, Inc., Wallingford, CT, 2004.
10 G. A. Crosby and J. N. Demas, J. Phys. Chem., 1971, 75, 991.
11 (a) U. Dahlmann and R. Neidlein, Helv. Chim. Acta, 1997, 80, 111;
(b) J. Gao, R. Li, L. Li, Q. Meng, H. Jiang, H. Li and W. Hu, Adv.
Mater., 2007, 19, 3008; (c) G. Balaji and S. Valiyaveettil, Org. Lett.,
2009, 11, 3358.
Acknowledgements
We thank the US Department of Energy for support through the
EFRC at UMass Amherst (DE-SC0001087) and the US Army
Research Office (54635CH) for support through the Green
Energy Center.
References
1 (a) J. Roncali, Chem. Rev., 1997, 97, 173; (b) M. Bendikov, F. Wudl and
D. Perepichka, Chem. Rev., 2004, 104, 4891; (c) J. E. Anthony, Chem.
Rev., 2006, 106, 5028; (d) J. E. Anthony, Angew. Chem., Int. Ed., 2008,
47, 452; (e) G. H. Ronald, Polycyclic Aromatic Hydrocarbons, Wiley,
VCH, New York, 1997.
2 (a) J. L. Brédas, J. P. Calbert, D. A. da, S. Filho and J. Cornil, Proc. Natl.
Acad. Sci. U. S. A., 2002, 99, 5804; (b) H. Dong, C. Wang and W. Hu,
Chem. Commun., 2010, 46, 5211.
12 (a) J. I. G. Cadogan, M. Carmeron-Wood, R. K. Makie and
R. J. G. Searle, J. Chem. Soc., 1965, 4831; (b) J. I. G. Cadagon, Organo-
phosphorus Reagents in Organic Synthesis, Academic Press Inc.,
London, 1979, p. 269.
3 (a) A. Maliakal, K. Raghavachari, H. Katz, E. Chandross and T. Siegrist,
Chem. Mater., 2004, 16, 4980; (b) U. H. F. Bunz, Pure Appl. Chem.,
2010, 82, 953.
13 J. Bouchard, S. Wakim and M. Leclerc, J. Org. Chem., 2004, 69, 5705.
14 M. Belletete, S. Wakim, M. Leclerc and G. Durocher, THEOCHEM,
2006, 760, 147.
15 (a) N. S. Hush, Prog. Inorg. Chem., 1967, 10, 247; (b) K. Lancaster, S.
A. Odom, S. C. Jones, S. Thayumanavan, S. R. Marder, J.-L. Brédas,
V. Coropceanu and S. Barlow, J. Am. Chem. Soc., 2009, 131, 1717.
16 (a) V. Gebhardt, A. Bacher, M. Thelakkat, U. Stalmach, H. Meier, H.-
W. Schmidt and D. Haarer, Adv. Mater., 1999, 11, 119; (b) J. Liu,
R. Zhang, G. Sauvé, T. Kowalewski and R. D. McCullough, J. Am.
Chem. Soc., 2008, 130, 13167.
4 S. Yamaguchi, C. Xu and T. Okamoto, Pure Appl. Chem., 2006, 78, 721.
5 (a) Q. Hemi and C. Chunyan, Curr. Org. Chem., 2010, 14, 2070;
(b) U. H. F. Bunz, Chem.–Eur. J., 2009, 15, 6780; (c) S. Miao, S.
M. Brombosz, P. v. R. Schleyer, J. I. Wu, S. Barlow, S. R. Marder, K.
I. Hardcastle and U. H. F. Bunz, J. Am. Chem. Soc., 2008, 130, 7339.
6 (a) M. Lepeltier, O. Lukoyanova, A. Jacobson, S. Jeeva and D.
F. Perepichka, Chem. Commun., 2010, 46, 7007; (b) B. Gao, M. Wang,
Y. Cheng, L. Wang, X. Jing and F. Wang, J. Am. Chem. Soc., 2008, 130,
8297; (c) A. L. Appleton, S. M. Brombosz, S. Barlow, J. S. Sears, J.-
L. Bredas, S. R. Marder and U. H. F. Bunz, Nat. Commun., 2010, 1, 91;
(d) Y. Ren and T. Baumgartner, J. Am. Chem. Soc., 2011, 133, 1328;
(e) K. Kawaguchi, K. Nakano and K. Nozaki, Org. Lett., 2008, 10, 1199;
(f) T. Okamoto, K. Kudoh, A. Wakamiya and S. Yamaguchi, Org. Lett.,
17 D. Chun, Y. Cheng and F. Wudl, Angew. Chem., Int. Ed., 2008, 47, 8380.
18 I. Kaur, M. Jazdzyk, N. N. Stein, P. Prusevich and G. P. Miller, J. Am.
Chem. Soc., 2010, 132, 1261.
3462 | Org. Biomol. Chem., 2012, 10, 3455–3462
This journal is © The Royal Society of Chemistry 2012