Synthesis, molecular and photovoltaic properties of an indolo[3,2-b]indole- based acceptor-donor-acceptor small molecule

Article abstract of DOI:10.1002/ejoc.201300443

Indolo[3,2-b]indole, containing two fused indole units, is an unexplored but promising electron-rich molecule for constructing donor-acceptor materials due to its planar, symmetric, and extended conjugated structure. We have successfully developed a new synthetic pathway to prepare 2,7-diboronic ester-indolo[3,2-b]indole, which was then reacted with dithienodiketopyrrolo- pyrrole acceptor to afford a new acceptor-donor-acceptor (A-D-A) conjugated molecule, 2,7-bis(dithienodiketopyrrolo-pyrrole)indolo[3,2-b]indole (2,7-DPPIIDPP). II is used to stand for indolo[3,2-b]indole in order to emphasize that this compound is constructed from two indole units. The A-D-A linkage through the 2,7-positions of II not only preserves the phenylene units in the para-conjugation but also renders stronger electron-donating strength. This material exhibited good thermal stability, high crystallinity, and broad UV/Vis absorption. The solution-processed bulk heterojunction device using the configuration of ITO/PEDOT:PSS/2,7-DPPIIDPP:PC₇₁BM/Ca/Al exhibited a V_oc of 0.72 V, a J_sc of 6.88 mA/cm², and an FF of 49.6 %, leading to a power conversion efficiency (PCE) of 2.45 %. A new acceptor-donor-acceptor conjugated molecule, 2,7-bis(dithienodiketopyrrolo- pyrrole)indolo[3,2-b]indole (2,7-DPPIIDPP) was designed and synthesized. This material exhibited good thermal stability, high crystallinity, and broad UV/Vis absorption. The solution-processed bulk heterojunction device using the configuration of ITO/PEDOT:PSS/ 2,7-DPPIIDPP:PC₇₁BM/Ca/Al was characterized. Copyright

Full text of DOI:10.1002/ejoc.201300443

FULL PAPER
DOI: 10.1002/ejoc.201300443
Synthesis, Molecular and Photovoltaic Properties of an Indolo[3,2-b]indole-
Based Acceptor–Donor–Acceptor Small Molecule
Yu-Ying Lai,[a] Jyun-Ming Yeh,[a] Che-En Tsai,[a] and Yen-Ju Cheng*[a]
Keywords: Nitrogen heterocycles / Donor–acceptor systems / Conjugation / Semiconductors / Photophysics
Indolo[3,2-b]indole, containing two fused indole units, is an
unexplored but promising electron-rich molecule for con-
structing donor–acceptor materials due to its planar, symmet-
ric, and extended conjugated structure. We have successfully
developed a new synthetic pathway to prepare 2,7-diboronic
ester-indolo[3,2-b]indole, which was then reacted with dithi-
enodiketopyrrolo-pyrrole acceptor to afford a new acceptor–
donor–acceptor (A–D–A) conjugated molecule, 2,7-bis(di-
thienodiketopyrrolo-pyrrole)indolo[3,2-b]indole (2,7-DPPI-
IDPP). II is used to stand for indolo[3,2-b]indole in order to
emphasize that this compound is constructed from two indole
units. The A–D–A linkage through the 2,7-positions of II not
only preserves the phenylene units in the para-conjugation
but also renders stronger electron-donating strength. This
material exhibited good thermal stability, high crystallinity,
and broad UV/Vis absorption. The solution-processed bulk
heterojunction device using the configuration of ITO/PE-
DOT:PSS/2,7-DPPIIDPP:PC71BM/Ca/Al exhibited a Voc of
0.72 V, a Jsc of 6.88 mA/cm2, and an FF of 49.6%, leading to
a power conversion efficiency (PCE) of 2.45%.
citation.[4] It was therefore considered promising to further
develop a new class of elongated and coplanar conjugated
systems comprising multifused benzene and pyrrole compo-
nents. One molecule belonging to this category is the penta-
cyclic indolo[3,2-b]carbazole, which has been introduced to
D–A-conjugated copolymers showing decent photovoltaic
performance.[5] On the other hand, indolo[3,2-b]indole (II),
in which the central pyrrolopyrrole is fused with two exter-
nal benzene rings, emerges as an appealing synthetic target
due to its many superior molecular properties (Scheme 1).
With one more embedded pyrrole moiety than carbazole,
indolo[3,2-b]indole has more extended conjugation and
stronger donating strength, which should result in efficient
ICT transition and thereby better light-harvesting. Further-
more, unlike carbazole, with the C2v symmetry, II is a C2h
symmetrical molecule. Pei and co-workers have demon-
strated that a C2h centro-symmetric donor unit can facili-
tate intermolecular π–π stacking, whereas a C2v axis-sym-
metric donor unit has a weaker effect on intermolecular
interactions.[6] Therefore, centro-symmetric II-based D–A
molecules would align in higher order in the solid state,
leading to improvements in the hole-transporting properties
of the corresponding materials. Furthermore, planar conju-
gated molecules usually suffer from poor solubility, which
hinders solution processability. Two aliphatic chains can be
introduced to the nitrogen atoms in II to enhance the solu-
bility of the highly planar D–A conjugated molecule. We
anticipate that the nitrogen-containing small molecule II
may not only inherit the intrinsic advantages of carbazole
but should also exhibit very interesting optical, electronic,
and morphological properties. Herein, we report a new syn-
thetic pathway leading to the preparation of 2,7-diboronic
Introduction
Research on organic p-type (donor) and n-type (ac-
ceptor) semiconductors in the field of bulk-heterojuction
(BHJ) solar cells has developed rapidly in recent years.[1]
Fullerene derivatives, such as phenyl-C61-butyric acid
methyl ester (PC61BM), have been regarded as irreplaceable
n-type materials for high-performance BHJ solar cells. Con-
siderable research effort is thus been directed toward the
development of p-type conjugated molecules, polymers es-
pecially. In this context, studies of small molecules are rela-
tively scarce.[2] However, in comparison to polymeric coun-
terparts, small molecules in principle have many advan-
tages, including monodispersity, well-defined structures, no
end group contamination, simple purification, more effec-
tive intermolecular π–π stacking, and better batch-to-batch
reproducibility. Consequently, BHJ solar cells utilizing
small molecule p-type materials continue to receive growing
attention.[2] The strategy of integrating an electron-rich mo-
lecule (donor) with an electron-deficient molecule (ac-
ceptor) has been demonstrated to be effective in producing
low band-gap conjugated molecules.[3] Carbazole, in which
a central pyrrole ring is embedded between two benzene
rings, is one of the most classical electron donors for the
construction of donor-acceptor conjugated polymers due to
its good hole-transportation properties and the ability to
induce intramolecular charge transfer (ICT) upon photoex-
[a] Department of Applied Chemistry, National Chiao Tung
University,
1001 Ta Hseuh Road, Hsin-Chu 30010, Taiwan
E-mail: yjcheng@mail.nctu.edu.tw
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201300443.
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Indolo[3,2-b]indole-Based Acceptor–Donor–Acceptors
Scheme 1. Structures of carbazole, indolo[3,2-b]indole (II), and 2,7-DPPIIDPP. Numerical locants of indolo[3,2-b]indole are listed for
clarity in nomenclature.
ester indolo[3,2-b]indole. Electron-rich monomer II was
Results and Discussion
coupled with two electron-deficient dithienodiketopyrrolo-
pyrrole (DPP)[7] moieties to yield 2,7-bis(dithienodiketopyr-
Synthesis
rolo-pyrrole)indolo[3,2-b]indole (2,7-DPPIIDPP) with
an acceptor–donor–acceptor (A–D–A) arrangement
Attachment of II with two dithienodiketopyrrolo-pyrrole
(Scheme 1). Its thermal, optical, and electrochemical prop- (DPP) units can be carried out either through the 3,8-posi-
erties and X-ray diffraction have been characterized and tions or 2,7-positions of II to produce DPPIIDPP conju-
theoretical calculations were performed. Photovoltaic de- gated molecules. The position of the linkage may play an
vices based on this solution-processed 2,7-DPPIIDPP small important role in determining the electronic and steric
molecule have delivered good performance.
properties of the resulting DPPIIDPP molecules. 2,7-Exten-
Scheme 2. Synthesis of the 2,7-diboronic ester of the indolo[3,2-b]indole unit II .
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sion can maintain para-linkage of the phenylene, resulting clarify this controversy, we re-examined this reaction
1
in a more effective conjugation length. On the other hand, (Scheme 3). By comparing the H NMR spectra of com-
3,8-extension leads to the nitrogen atoms para to the ac- pound 8 and the product of the direct bromination, it can
ceptor moieties, which may induce stronger D–A interac- be concluded that bromination of indolo[3,2-b]indole oc-
tion. The disparity between the electronic and steric proper- curs at the 2,7-positions rather than the 3,8-positions. This
ties derived from the two different connecting modes (2,7- result also implies that the 2,7-positions of indolo[3,2-b]-
vs. 3,8-positions of II) can therefore be investigated. The indole actually have stronger nucleophilicity than the 3,8-
synthetic route via 2,7-dibromoindolo[3,2-b]indole is de- positions, which can be rationalized by analyzing the reso-
picted in Scheme 2. Regioselective iodination of 3-nitroanil- nance structures of II. As illustrated in Scheme 4, the lone
ine with N-iodosuccinimide (NIS) in dimethyl sulfoxide pair electrons on the nitrogen can delocalize into the phenyl
(DMSO) yielded 1,[8] which then underwent Sonogashira ring through the central olefin in the pyrrolopyrrole ring,
reaction with (trimethylsilyl)acetylene to afford 2. Desil- pushing the π-electrons on the 2- or 7-positions. Conse-
ylation of 2 by treatment with potassium carbonate fur- quently, electrophilic aromatic substitution of II with
nished 3. Sonogashira reaction of 1 with 3 led to the forma- bromine selectively occurs at the 2,7-positions. Considering
tion of 4. Treatment of the latter with BF3·OEt2/tBuNO2 to that 2,7-linkage can preserve the phenylene units in the
form the diazonium salt followed by reaction with CuBr2 para-conjugation, and 2,7-positions are actually the
yielded the brominated compound 5. Oxidation of the stronger electron-donating sites, we thus focus on the syn-
acetylene group of 5 by KMnO4/AcOH gave 6. Reduction
of the nitro groups of 6 by SnCl2, followed by acid-
prompted intramolecular cyclization furnished 7. N-Alkyl-
ation of 7 by using 2-ethylhexyl bromide in the presence
of sodium hydride in tetrahydrofuran (THF) afforded 2,7-
dibromo-5,10-diethylhexylindolo[3,2-b]indole (8), which
was treated with bis(pinacolato)diboron in the presence of
[PdCl2(dppf)] [dppf = 1,1Ј-bis(diphenylphosphino)ferro-
cene] in N,N-dimethylformamide (DMF) to give 2,7-di-
boronic ester-indolo[3,2-b]indole (9).
Scheme 3. Bromination of N-alkyl-substituted indolo[3,2-b]indole
by Br2 and pyridine in CCl4.
Bromination of indolo[3,2-b]indole is an intriguing
chemistry. Bromination at the 3,8-positions of indolo[3,2-b]-
indole seems to be reasonable according to the para-direct-
ing effect of the nitrogen atom. However, the regioselec-
tivity of direct bromination of II is ambiguous because both
3,8-dibromo- and 2,7-dibromo-indolo[3,2-b]indole have
been reported to take place under similar conditions.[9] To Scheme 4. Resonance structures of indolo[3,2-b]indole.
Scheme 5. Synthesis of the DPP unit and 2,7-DPPIIDPP.
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Indolo[3,2-b]indole-Based Acceptor–Donor–Acceptors
thesis of 2,7-substituted II monomer and its corresponding
acceptor-donor-acceptor small molecule, 2,7-DPPIIDPP.
The synthesis of DPP is described in Scheme 5. Tandem
aldol reactions between dimethyl succinate and thiophene-
2-carbonitrile accompanied by intramolecular cyclization
led to the formation of 10, which was then alkylated by
using 2-ethylhexyl bromide in the presence of potassium
carbonate to give 11. Monobromination of 11 was achieved
by using N-bromosuccinimide (NBS; 1 equiv.) in chloro-
form to furnish 12. The desired product, 2,7-DPPIIDPP,
was then successfully synthesized from 9 and 12 through the
palladium-catalyzed Suzuki coupling reaction (Scheme 5).
Physical Properties
The thermal properties of 2,7-DPPIIDPP were analyzed
by thermogravimetric analysis (TGA). The decomposition
temperature (Td) obtained by TGA was 386 °C, suggesting
its thermal stability is sufficient for organic-solar-cell (OSC)
applications (Figure 1).
X-ray diffraction (XRD) analysis was employed to exam-
ine the morphology of the thin film that was prepared by
spin-coating a CHCl3 solution of 2,7-DPPIIDPP on a glass
substrate. The XRD patterns are illustrated in Figure 2 and
the key d-spacing values are summarized in Table 1. The
Figure 1. Thermogravimetric analysis (TGA) of 2,7-DPPIIDPP.
Table 1. XRD data of 2,7-DPPIIDPP.
2θ [rad]
d-Spacing [Å]
5.46
7.60
11.37
18.38
16.20
11.71
7.78
4.83
2,7-DPPIIDPP film exhibited a strong diffraction peak at respectively, with the corresponding band gap being
2θ = 5.46°, corresponding to a d-spacing value of 16.20 Å 1.67 eV.
and diffraction peaks at 2θ = 7.60°, 11.37°, and 18.38° were
The absorption spectra of 2,7-DPPIIDPP are shown in
also observed, suggesting an organized assembly and crys- Figure 3 and its λmax values are listed in Table 2. It exhibits
tallinity of this π-conjugated molecule in the solid state. The two distinct bands in the absorption spectrum in the CHCl3
crystallinity of this material is highly associated with its co- solution. One band at the shorter wavelength region results
planar structure (the optimized geometry of 2,7-DPPIIDPP from localized π–π* transitions and the second band at
is depicted in Figure 5 below).
longer wavelength is attributed to intramolecular charge
Cyclic voltammetry (CV) was employed to study the elec- transfer (see below for a detailed assignment of the absorp-
trochemical properties. The HOMO and LUMO levels of tion bands). Compared with the solution state, the thin-film
2,7-DPPIIDPP were estimated to be –5.14 and –3.47 eV, spectrum exhibited bathochromic shifts and broadening of
Figure 2. 1-Dimension X-ray diffraction patterns of 2,7-DPPIIDPP.
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Table 2. Calculated[a] HOMO/LUMO energy, excitation energy, oscillator strength, and configurations (with large CI coefficients) of the
excited state of 2,7-DPPIIDPP.
HOMO [eV][e] LUMO [eV][e]
Excitation energy [nm]
Oscillator strength Symmetry
Configuration[d]
[b]
[c]
λmax,exp
λmax,exp
λcalc
–4.98 (–5.14) –2.86 (–3.47)
615
415
625
401
673
424
2.20826
0.3767
Singlet-A
Singlet-AЈЈ
HǞL
HǞL+2
H–3ǞL
H–1ǞL+3
[a] TD-B3LYP/6-311G(d,p), PCM = CHCl3. [b] Experimental values were measured for non-simplified 2,7-DPPIIDPP in the thin film.
[c] Experimental values were measured for non-simplified 2,7-DPPIIDPP in CHCl3 solution. [d] Configurations with largest coefficients
in the CI expansion of each state are highlighted in boldface. [e] Experimental values obtained by cyclic voltammetry are given in
parentheses.
the π–π* and ICT bands with a shoulder, suggesting that
2,7-DPPIIDPP has stronger intermolecular aggregation in
the solid state. Moreover, 2,7-DPPIIDPP possesses a broad
absorption coverage ranging from 300 to 800 nm in the so-
lid state, indicating that the electron-donating unit II has
strong electronic interaction with the electron-deficient
DPP units through its 2,7-linkage.
Figure 4. Plots (isovalue = 0.04 au) of the HOMO of II calculated
at the B3LYP/6-311G (d,P) level.
Furthermore, to gain more insight into the molecular or-
bital properties of 2,7-DPPIIDPP, quantum-chemical cal-
culations were performed with the Gaussian09 suite, em-
ploying the B3LYP and TD-B3LYP density functionals in
combination with the 6-311G (d,p) basis set. Again, all the
side-chain substituents were replaced by methyl groups for
simplicity. The computational details are provided in the
Supporting Information. The optimized geometry of 2,7-
DPPIIDPP is depicted in Figure 5. As can be seen from the
top-view graph in Figure 5, this molecule has a fairly planar
structure, which is believed to be the primary factor that
renders this material crystalline.
Figure 3. Normalized absorption spectra of 2,7-DPPIIDPP in
chloroform solution (squares) and the solid state (circles).
Theoretical Calculations
DFT calculations were carried out with the Gaussian 09
suite at the B3LYP/6-311G (d,p) level of theory to investi-
gate the chemical reactivity of II; the calculations were sim-
plified by replacing the hexyl substituents with methyl
groups. Chemical reactivity of a molecule is highly associ-
ated to its frontier orbitals, i.e., HOMO and LUMO,[10]
which can provide useful information on the regioselectivity
of II for electrophilic aromatic substitution. Figure 4 illus-
trates the calculated HOMO of II, in which the carbon
atoms at the 2,7-positions possess much higher electron
gray: hydrogen.
density than those at the 3,8-positions, implying that the
Figure 5. (a) Front view and (b) top view of the optimized geome-
try of 2,7-DPPIIDPP at the B3LYP/6-311G(d,p) level, with sol-
vation in CHCl3. Yellow: sulfur; blue: nitrogen; gray: carbon; light-
2,7-positions of indolo[3,2-b]indole have stronger nucleo-
The calculated HOMO/LUMO energy, excitation energy,
philicity than the 3,8-positions. The result is consistent with oscillator strength, and configuration of the excited state
the experimental observation that bromination of II occurs are summarized in Table 2. The most probable vertical exci-
at the 2,7-positions selectively.
tations are calculated to be 673 and 424 nm, which are both
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Indolo[3,2-b]indole-Based Acceptor–Donor–Acceptors
more redshifted than their experimental λmax (625 and units and the π–π* transition within the DPP unit has only
401 nm). Although there are variations in the absolute val- a minor contribution. Another electronic transition at λmax
ues, the calculated estimates should still be representative of = 424 nm shows localized π–π* transitions independently
the experimental data. The frontier orbitals of the model on the DPP and II groups, respectively.
compound 2,7-DPPIIDPP are illustrated in Figure 6. The
electron density of the HOMO is homogeneously distrib-
Photovoltaic Characteristics
uted along the molecular backbone, except that the outmost
thiophenes have lower electron density. However, the elec-
tron density of the LUMO is mainly localized in the DPP
acceptor. Such an electronic redistribution would normally
result in a pronounced intramolecular charge separation be-
tween donor and acceptor after excitation. To gain further
understanding of these electronic transitions, electron den-
sity difference maps (EDDMs) were constructed.[11] The
electronic transitions can therefore be visualized through
EDDMs. Red indicates a decrease in charge density, while
green indicates an increase. As shown in Figure 7, the low-
est energy singlet electronic transition (λmax = 673 nm) of
2,7-DPPIIDPP is mainly a result of ICT from II to DPP
Bulk heterojunction photovoltaic cells were fabricated on
the basis of indium tin oxide (ITO)/poly-3,4-ethylenedioxy-
thiophene (PEDOT):poly-styrenesulfonate (PSS)/2,7-DPPI-
IDPP:PC71BM/Ca/Al device configuration and their per-
formances were measured under a simulated AM1.5G illu-
mination of 100 mWcm–2. The current density-voltage
characteristics of the devices are illustrated in Figure 8. The
optimized conditions for the devices are summarized in
Table 3. The device based on a 2,7-DPPIIDPP/PC71BM
blend (optimal ratio, 1:1 wt.-%) showed a Voc of 0.78 V, a
Jsc of 4.54 mA/cm2, a FF of 38.0%, and a power conversion
efficiency (PCE) of 1.35%. More importantly, the device
efficiency could be enhanced significantly by thermal an-
nealing of the active layers. After thermal annealing at
150 °C for 7 min, the device exhibited a Voc of 0.72 V, a Jsc
of 6.88 mA/cm2, a FF of 49.6%, and a higher PCE of
2.45%. The Jsc value calculated from the integral of the
external quantum efficiency (EQE) curve with an AM1.5G
reference spectrum was 6.85 mA/cm2, which is consistent
with the value obtained from the J-V measurement (Fig-
ure 8). It should be noted that there was almost no response
between 800 to 900 nm in the EQE spectrum, although the
absorption of 2,7-DPPIIDPP actually spans from 300 to
900 nm. It can be reasonably deduced that the presence of
PC71BM may reduce the degree of aggregation of 2,7-
DPPIIDPP in the solid state. Furthermore, the 2,7-DPPI-
Figure 6. Plots (isovalue = 0.03 au) of frontier orbitals of 2,7-
DPPIIDPP calculated at the B3LYP/6-311G (d,P) level in chloro-
form.
Figure 7. Electron density difference maps (EDDMs) of selected
singlet electronic transitions of 2,7-DPPIIDPP at 673 and 424 nm.
Red indicates a decrease in charge density, while green indicates an
increase. All EDDMS were plotted with isovalue 0.0012 au.
Figure 8. Current density-voltage characteristics (top) and EQE
spectrum (bottom) of ITO/PEDOT:PSS/2,7-DPPIIDPP:PC71BM/
Ca/Al devices under illumination by AM 1.5G at 100 mWcm–2.
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IDPP content was reduced when PC71BM was included in the 2,7-linkage, this material exhibited crystalline nature
a 1:1 ratio. Therefore, diminution of absorption intensity in and broad UV/Vis absorption. The molecular orbital prop-
the longer wavelengths for 2,7-DPPIIDPP in the blending erties and electronic transitions of 2,7-DPPIIDPP have
systems is rational and even expected. Subsequently, in- been elucidated in detail by TD-DFT calculations. Solu-
creasing the thermal-annealing time to 10 or 15 min does tion-processed small-molecule bulk heterojunction photo-
not further change the efficiency (Table 3). To investigate voltaic
cells
based
on
ITO/PEDOT:PSS/2,7-
the influence of thermal annealing on the active layer, tem- DPPIIDPP:PC71BM (1:1, w/w)/Ca/Al were fabricated and
perature-dependent UV/Vis absorption measurements were characterized. The device thermally annealed at 150 °C ex-
conducted. After thermal annealing at 150 °C, the 2,7- hibited a Voc of 0.72 V, a Jsc of 6.88 mA/cm2, and a FF of
DPPIIDPP:PC71BM blend film exhibited a substantial en- 49.6%, leading to a PCE of 2.45%. This value is promising
hancement of the absorption intensity due to the thermal- for small molecule-based solution processed BHJ solar cells.
driven molecular assembly leading to stronger interchain We anticipate that the 2,7-diboronic ester-indolo[3,2-b]-
interactions (Figure 9). This improved device performance indole monomer presented in this research will emerge as
may be attributed to better charge transporting properties an attractive building block for the construction of a series
associated with higher molecular ordering of 2,7-DPPI- of new conjugated polymers and small molecules based on
IDPP.
2,7-substituted II. Follow-up research is underway in our
laboratory.
Table 3. Device characteristics with 2,7-DPPIIDPP/PC71BM (1:1
w/w).
Conditions
No TA[a]
TA 150 °C 7 min
TA 150 °C 10 min
TA 150 °C 15 min
Voc [V] Jsc [mA/cm2] FF [%] PCE [%]
Experimental Section
0.78
0.72
0.70
0.72
–4.54
–6.88
–6.70
–6.72
38.0
49.6
50.3
49.4
1.35
2.45
2.36
2.39
General Measurement and Characterization: All chemicals were ob-
tained from commercial sources and used as received unless other-
wise specified. Tetrahydrofuran (THF) was dried by distillation
from sodium benzophenone ketyl. Anhydrous toluene and di-
chloromethane were obtained from a SD-300 solvent purification
system (AsiaWong Enterprise). Fourier transform infrared (FTIR)
spectra were recorded with a Perkin–Elmer One instrument. NMR
measurements were recorded with a Varian Unity-300 spectrometer
(1H, 300 MHz; 13C, 75 MHz). Chemical shifts (δ) are reported in
ppm with respect to Me4Si (δ = 0 ppm) for 13C and 1H NMR spec-
troscopy. Coupling constants (J) are given in Hz. 13C NMR was
proton broadband-decoupled. Multiplicities of peaks are denoted
by the following abbreviations: s, singlet; d, doublet; t, triplet; m,
multiplet; br, broad. Thermogravimetric analysis (TGA) was re-
corded with a PerkinElmer Pyris analyzer under nitrogen atmo-
sphere at a heating rate of 10 °C/min. Absorption spectra were col-
lected with a HP8453 UV/Vis spectrophotometer. The molecular
weight of polymers was determined by gel permeation chromatog-
raphy with a Viscotek VE2001GPC instrument, polystyrene was
used as the internal standard, and THF as the eluent. Electrochem-
ical cyclic voltammetry (CV) was conducted with a CH Instru-
ments Model 611D analyzer. A carbon glass coated with a thin
film was used as the working electrode and a Ag/Ag+ electrode as
the reference electrode; 0.1 m tetrabutylammonium hexafluoro-
phosphate in acetonitrile was used as the electrolyte. CV curves
were calibrated by using ferrocene as the standard, the oxidation
potential of which was set at –4.8 eV with respect to zero vacuum
level. The HOMO energy levels were obtained from the equation
[a] TA: thermal annealing.
Figure 9. Absorption spectra of 2,7-DPPIIDPP/PC71BM (1:1 wt.-
%) before (squares) and after (circles) thermal annealing at 150 °C.
onset
onset
HOMO = –(Eox
– E(ferrocene)
+ 4.8) eV. The LUMO levels
onset
Conclusions
were obtained from the equation LUMO
=
–(Ered
–
onset
E(ferrocene)
+ 4.8) eV.
We have developed a new synthetic route leading to
indolo[3,2-b]indole derivatives. We discovered that the 2,7-
positions of these fused ring systems have stronger electron-
density and thus nucleophilicity than the 3,8-positions.
Functionalization at the 2,7-positions of II not only pre-
serve the phenylene units in the para-conjugation but also
render stronger electron-donating strength. A conjugated
A–D–A small molecule, 2,7-DPPIIDPP, was therefore ob-
tained by the Suzuki coupling reaction. Due to its high co-
4,4Ј-(Ethyne-1,2-diyl)bis(3-nitroaniline) (4): To a mixture of 3
(7.44 g, 45.88 mmol), 4-iodo-3-nitroaniline (10.81 g, 40.94 mmol),
degassed triethylamine (10 mL), and degassed THF (100 mL), was
added [PdCl2(PPh3)2] (0.72 g, 1.03 mmol) and copper iodide
(0.39 g, 2.05 mmol). The reaction mixture was stirred for 2 h at
room temperature under a nitrogen atmosphere. The solvent was
removed and the residue was then purified by column chromatog-
raphy on silica gel (hexane/THF, 5:1 v/v) to furnish 4 (10.8 g, 88%)
1
as a purple solid. H NMR ([D6]DMSO, 300 MHz): δ = 7.39 (d, J
planarity and strong donor-acceptor interaction through = 8.4 Hz, 2 H), 7.27 (d, J = 2.1 Hz, 2 H), 6.91 (dd, J = 2.3, 8.6 Hz,
5082
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Eur. J. Org. Chem. 2013, 5076–5084

Indolo[3,2-b]indole-Based Acceptor–Donor–Acceptors
2 H), 6.28 (br., 4 H) ppm. 13C NMR ([D6]DMSO, 75 MHz): δ =
150.0, 135.5, 132.2, 118.5, 108.8, 104.9, 89.5 ppm. HRMS (FAB):
calcd. for C14H10N4O4 [M]+· 298.0702; found 298.0708.
14.2, 11.2 ppm. HRMS (EI): calcd. for C30H40Br2N2 [M]+·
586.1558; found 586.1550.
5,10-Bis(2-ethylhexyl)-2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxabor-
olan-2-yl)indolo[3,2-b]indole (9):
A
mixture of
8
(500 mg,
1,2-Bis(4-bromo-2-nitrophenyl)acetylene (5): To a solution of 4
(3.30 g, 11.06 mmol) in CH3CN (50 mL) and sulfolane (25 mL) was
added boron trifluoride–diethyl ether (d = 1.12 g/mL, 98+%,
5.6 mL, 43.31 mmol) at –30 °C and stirring was continued at this
temperature for 10 min. tert-Butyl nitrite (d = 0.86 g/mol, 90%,
7.4 mL, 55.54 mmol) was then introduced and the mixture was
gradually warmed to room temperature, stirred for 30 min, and
charged with diethyl ether (100 mL). The formed salt was collected
and mixed with copper(II) bromide and DMSO (40 mL). The mix-
ture was stirred at room temperature for 2 h (Caution: this reaction
is exothermic and generates a significant amount of nitrogen gas)
and then poured into water. The resultant precipitate was collected
and purified by column chromatography on silica gel (hexane/
CH2Cl2, 5:1 v/v) to afford 5 (1.88 g, 40%) as a yellow solid. IR
0.85 mmol), bis(pinacolato)diborane (518 mg, 2.04 mmol),
[PdCl2(dppf)] (16 mg, 0.02 mmol), potassium acetate (500 mg,
5.09 mmol), and DMF (20 mL; deoxygenated by a freeze-pump-
thaw process) was heated at 140 °C for 2 d. After cooling to room
temperature, the reaction mixture was poured into water and ex-
tracted with CH2Cl2. The collected organic layer was dried under
vacuum. The residue was purified by column chromatography on
silica gel (hexane/CH2Cl2, 5:1 v/v) and then recrystallized from eth-
anol to give 9 (240 mg, 41%) as light-olivine crystals. 1H NMR
(CDCl3, 300 MHz): δ = 7.93 (s, 2 H), 7.84 (d, J = 8.1 Hz, 2 H),
7.59–7.62 (m, 2 H), 4.41 (br., 4 H), 2.17 (br., 2 H), 1.19–1.57 (m,
40 H), 0.81–0.89 (m, 12 H) ppm. 13C NMR (CDCl3, 75 MHz): δ =
141.0, 127.4, 123.9, 117.4, 116.7, 116.2, 83.6, 49.4, 40.3, 30.6, 28.7,
24.9, 24.2, 23.0, 14.1, 11.0 ppm. HRMS (EI): calcd. for
C42H64B2N2O4 [M]+· 682.5052; found 682.5054.
(KBr): ν = 3089, 2919, 2850, 1549, 1342, 1273, 1093, 890, 835, 760,
˜
1
536 cm–1. H NMR (CDCl3, 300 MHz): δ = 8.31 (d, J = 2.0 Hz, 2
H), 7.79 (dd, J = 2.0, 8.3 Hz, 2 H), 7.68 (d, J = 8.3 Hz, 2 H) ppm.
13C NMR (CDCl3, 75 MHz): δ = 136.4, 136.1, 128.3, 128.1, 123.5,
118.5, 116.7 ppm. HRMS (FAB): calcd. for C14H6Br2N2O4 [M]+·
423.8694; found 423.8699.
2,7-DPPIIDPP: A mixture of 12 (60 mg, 0.09 mmol), 9 (106.1 mg,
0.18 mmol), potassium carbonate (91.7 mg, 0.66 mmol), Aliquat
336 (15.2 mg, 0.04 mmol), [Pd(PPh3)4] (2.0 mg, 0.002 mmol), tolu-
ene (5 mL), and water (1 mL) was added to a sealed tube filled with
nitrogen. The mixture was heated at 110 °C for 2 d, then cooled to
room temperature, poured into water, and extracted with CH2Cl2.
The organic layer was mixed with Pd-thio gel and Pd-TAAcOH
(Silicycle Inc.) to remove any residual metal. After filtration and
removal of the solvent, the residue was recrystallized from ethanol/
THF to give 2,7-DPPIIDPP (30 mg, 23%). 1H NMR (CDCl3,
300 MHz): δ = 9.10 (d, J = 3.9 Hz, 2 H), 8.88 (d, J = 2.7 Hz, 2 H),
7.84 (d, J = 8.1 Hz, 2 H), 7.71 (s, 2 H), 7.60–7.62 (m, 2 H), 7.51–
7.56 (m, 4 H), 7.29 (s, 2 H), 4.39–4.42 (m, 4 H), 4.05–4.13 (m, 8
H), 2.19 (br., 2 H), 1.99 (br., 2 H), 1.89 (br., 2 H), 1.26–1.49 (m,
48 H), 0.83–0.98 (m, 36 H) ppm. 13C NMR (CDCl3, 75 MHz): δ =
162.1, 161.8, 152.2, 141.8, 140.8, 139.5, 137.9, 135.1, 130.4, 130.3,
128.6, 128.1, 128.0, 127.2, 123.9, 118.7, 117.2, 114.5, 108.4, 107.7,
107.7, 49.8, 46.2, 40.7, 39.5, 39.3, 31.1, 30.6, 30.5, 29.0, 28.7, 28.6,
24.5, 23.9, 23.8, 23.4, 23.3, 23.2, 14.3, 14.3, 14.2, 11.2, 10.9,
10.8 ppm. HRMS (FAB): calcd. for C90H118N6O4S4 [M]+·
1474.8097; found 1474.8088.
1,2-Bis(4-bromo-2-nitrophenyl)ethane-1,2-dione (6): To a solution of
5 (1.06 g, 2.49 mmol) in acetone (100 mL) and acetic acid (1 mL)
was added KMnO4 (0.87 g, 5.51 mmol). The mixture was heated
to reflux for 2 h, poured into acetone (500 mL), and sonicated for
10 min. After removal of the MnO2 precipitate by filtration, the
filtrate was evaporated and the residue was purified by chromatog-
raphy on silica gel (hexane/CH2Cl2, 5:1 v/v) to afford 6 (1.13 g,
99%) as a yellow solid. IR (KBr): ν = 3426, 3087, 2923, 2853,
˜
1721, 1602, 1535, 1464, 1341, 1262, 1201, 1080, 968, 865, 823, 732,
1
548 cm–1. H NMR (CDCl3, 300 MHz): δ = 8.41 (d, J = 1.5 Hz, 2
H), 8.01 (dd, J = 1.8, 8.1 Hz, 2 H), 7.53 (d, J = 8.1 Hz, 2 H) ppm.
HRMS (FAB): calcd. for C14H6Br2N2O6 [M]+· 455.8593; found
455.8585.
2,7-Dibromo-5,10-dihydroindolo[3,2-b]indole (7): A mixture of 6
(0.72 g, 1.75 mmol) and acetic acid (25 mL) was heated to 80 °C
and stirred vigorously for 5 min. Tin(II) chloride dihydrate (3.55 g,
15.73 mmol) and HCl(aq) (1N, 3 mL) were introduced and the mix-
ture was stirred for 5 h at 80 °C. The reaction proceeded through
stages characterized by a yellow suspension, a red solution, and
finally a yellow suspension. The resulting precipitate was collected
and washed with acetic acid and methanol to give 7 (0.21 g, 37%)
Device Fabrication: Indium tin oxide (ITO)-coated glass substrate
was ultrasonically washed with detergent, deionized water, acetone,
and 2-propanol sequentially (15 min each) and then cleaned under
UV/ozone for another 15 min. PEDOT:PSS (Clevios PVP AI-4083)
was filtered and spin-coated on the cleaned ITO-coated glass at
2000 rpm for 40 s to produce a 30 nm interlayer, followed by baking
at 170 °C under a nitrogen atmosphere for 15 min. 2,7-DPPIIDPP
was then mixed with PC71BM in 1:1 weight ratio in chloroform
(the concentration of 2,7-DPPIIDPP was kept at 6 mg/mL). The
prepared solution was heated to 60 °C, stirred for 6 h, and filtered
with a 0.45 μm Teflon® syringe filter. It was then spin-coated on
top of the PEDOT:PSS interlayer at 1200 rpm. The resultant film
was covered with a Petri dish to allow the solvent to slowly evapo-
rate (solvent annealing) until the film dried and then baked at
150 °C for 7–15 min (thermal annealing). The top electrode was
then prepared by sequential thermal evaporation of Ca (35 nm) and
Al (100 nm) at reduced pressure (below 10–6 Torr) to furnish the
BHJ solar-cell devices. All devices contained an active area of
0.04 cm2 and the photovoltaic parameters were measured at room
temperature under air atmosphere with a Xenon lamp coupled to
as a yellow solid. IR (KBr): ν = 3647, 3414, 2956, 2921, 2852, 1646,
˜
1442, 1402, 1363, 1313, 1252, 1161, 1049, 928, 849, 798 cm–1. 1H
NMR ([D6]DMSO, 300 MHz): δ = 10.61 (br., 2 H), 7.77 (d, J =
8.3 Hz, 2 H), 7.77 (d, J = 1.8 Hz, 2 H), 7.27 (dd, J = 1.7, 8.5 Hz, 2
H) ppm. HRMS (EI): calcd. for C14H8Br2N2 [M]+· 361.9054; found
361.9059.
2,7-Dibromo-5,10-bis(2-ethylhexyl)indolo[3,2-b]indole (8): A mixture
of NaH (60%, 273 mg, 6.83 mmol), 7 (620 mg, 1.70 mmol), and
THF (50 mL) was heated to 70 °C for 10 min and 2-ethylhexyl
bromide (95%, d = 1.08 g/mL, 0.96 mL, 5.10 mmol) was then intro-
duced. The reaction was continued at this temperature for 24 h.
After cooling to room temperature, the mixture was poured into
water, extracted with diethyl ether, evaporated, and purified by col-
umn chromatography to furnish 8 (632 mg, 63%) as a white solid.
1H NMR (CDCl3, 300 MHz): δ = 7.61 (d, J = 8.4 Hz, 2 H), 7.54
(s, 2 H), 7.25 (br., 2 H), 4.20 (br., 4 H), 2.07 (br., 2 H), 0.81–1.38 an AM 1.5G solar filter (SAN-EIXES-301S solar simulator). J-V
(m, 28 H) ppm. 13C NMR (CDCl3, 75 MHz): δ = 141.8, 126.3, characteristics were recorded with a Keitheley 2400 Source Mea-
121.5, 119.0, 115.5, 113.3, 113.2, 49.9, 40.4, 31.0, 28.8, 24.4, 23.2,
surement Unit.
Eur. J. Org. Chem. 2013, 5076–5084
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
5083

Y.-Y. Lai, J.-M. Yeh, C.-E. Tsai, Y.-J. Cheng
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Acknowledgments
The authors thank the National Science Council of Taiwan and
the “ATU Program” of the Ministry of Education, Taiwan, the
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Received: March 26, 2013
Published Online: July 1, 2013
5084
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Eur. J. Org. Chem. 2013, 5076–5084