Thieno[2,3-b]indole-Based Small Push-Pull Chromophores: Synthesis, Structure, and Electronic Properties

Article abstract of DOI:10.1021/acs.orglett.6b00438

Small push-pull molecules were synthesized in high yields by connecting a N-methyl or N-phenyl substituted thieno[2,3-b]indole electron-donating block directly to a 2,2-dicyanovinyl or (1-(dicyanomethylene)-3-oxo-1-inden-2-ylidene)methyl electron-withdraw

Full text of DOI:10.1021/acs.orglett.6b00438

Letter
pubs.acs.org/OrgLett
Thieno[2,3‑b]indole-Based Small Push−Pull Chromophores:
Synthesis, Structure, and Electronic Properties
Franco
and Philippe Blanchard*
̧
is Baert, Clement Cabanetos, Magali Allain, Virginie Silvestre,† Philippe Leriche,*
́
MOLTECH-Anjou, CNRS UMR 6200, University of Angers, Linear Conjugated Systems Group, 2 Bd Lavoisier, 49045 Angers,
France
S
* Supporting Information
ABSTRACT: Small push−pull molecules were synthesized in
high yields by connecting a N-methyl or N-phenyl substituted
thieno[2,3-b]indole electron-donating block directly to a 2,2-
dicyanovinyl or (1-(dicyanomethylene)-3-oxo-1-inden-2-
ylidene)methyl electron-withdrawing group. The effects of
the N-substitution on thieno[2,3-b]indole and the nature of
the electron-accepting group on the electrochemical, optical,
and charge-transport properties were investigated by cyclic
voltammetry, UV−vis spectroscopy, and the space-charge-
limited current method, respectively. These results, together
with the 1% power conversion efficiency of a bilayer solar cell
prepared with the smallest compound of the series, show the
potential of thieno[2,3-b]indole for organic electronics.
properties of fused thieno[2,3-b]indole benzothiadiazole
hile arylamine derivatives have been used as hole-
Wtransporting materials for organic-light emitting diodes derivatives have been recently described.22 Few organic
(OLEDs),1,2 organic (OSCs),3 dye-sensitized (DSSCs),4 and
electroluminescent compounds derived from N-arylthieno-
[2,3-b]indole23 and condensed N-aroylthieno[2,3-b]indole24
have been reported in recent patents. Two dyes derived from
thieno[2,3-b]indole have been recently evaluated in DSSCs.25
We present herein the synthesis and the electronic properties
of small push−pull D-π−A molecules where N-methyl or N-
phenyl substituted thieno[2,3-b]indole 3a (MTI) or 3b (PTI)
as D building-block is directly connected to 2,2-dicyanovinyl
(DCV) or (1-(dicyanomethylene)-3-oxo-1-inden-2-ylidene)-
methyl (DCI) as electron-accepting group (A) (Scheme 1).
Preliminary results on the hole-transporting properties of these
compounds and their potential in OSCs are also reported.
The synthetic strategy is depicted in Scheme 1. Phenyl-
substituted thieno[2,3-b]indole 3a was prepared in one step via
a pallado catalyzed domino N−H/C−H bond activation
reaction involving diphenylamine and 2,3-dibromothiophene.26
Interestingly, the use of the N-methylaniline instead of the
diarylamine does not afford the methyl analogue 3b despite
multiple attempts and conditions changes. Consequently, the
latter was prepared according to a three-step procedure starting
with a microwave assisted Suzuki-Miyaura cross-coupling
reaction between 1-iodo-2-nitrobenzene and 3-thienylboronic
acid under atmospheric pressure. The resulting 3-(2-
nitrophenyl)thiophene 1 was subsequently involved in a
Cadogan reductive cyclization yielding 8H-thieno[2,3-b]indole
perovskite-sensitized solar cells,5 their strong electron-donating
properties make them ideal donor building-blocks (D) for the
synthesis of D-π-A push−pull molecules, which are efficient
electron-donor materials in OSCs3,6 or metal-free sensitizers for
DSSCs.7
Triarylamine units have been widely used in linear6g,8−10 or
star-shaped6g,11−13 push−pull molecules for OSCs, whereas
alkyldiarylamine14 and dialkylarylamine15 have also led to D-π−
A systems with interesting photovoltaic performance. To
restrict the well-known propeller shape associated with
triarylamine derivatives and thus favoring π−π intermolecular
interactions, covalent bridging by methylene groups or
heteroatoms has been pursued leading to planarized triphenyl-
amine,16 indoline,17 and phenoxa(thia)zine18 derivatives.
Similarly, merocyanine dyes with constrained arylamino
electron-donating groups have been successfully reported for
OSCs.19 On the other hand, fused aromatic N-heterocycles,
such as carbazole and its derivatives, represent also a well-
known class of planarized D building-blocks for the synthesis of
push−pull molecules for photovoltaic applications.3,6,7,10c,20
In this context, thieno[2,3-b]indole (TI) which constitutes
one isoelectronic structure of carbazole, could be of interest in
the field of organic electronics. While TI itself or derivatives
constitute important targets due to their biological activities
leading to potential applications in treating diseases or plant-
growth regulation,21 their use as functional material in organic
electronics is scarce. Thus, the structure and the electronic
Received: February 15, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b00438
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Organic Letters
Letter
Scheme 1. Synthesis of Thieno[2,3-b]indole-Based Push−
Pull Molecules
Figure 1. Cyclic voltammograms of PTI-DCV (orange), PTI-DCI
(green), MTI-DCV (red), and MTI-DCI (dark blue): 1 mM in 0.1 M
Bu4NPF6/CH2Cl2, Pt electrode, scan rate 100 mV s−1.
methyl by a phenyl substituent on the thienoindole unit leads
to a ca. 0.2 V positive shift of Epc resulting from the
concomitant suppression of the +I inductive effect of the
alkyl chain and occurrence of the −M mesomeric effect of the
phenyl ring.
On the other hand, one oxidation wave with an anodic peak
potential Epa associated with the formation of a radical cation is
also observed on each CV. While this oxidation process is
irreversible for PTI-DCV (Epa = +1.15 V) and PTI-DCI (Epa =
+1.04 V), the N-methyl substituted thienoindole derivatives
MTI-DCV (Epa = +0.97 V) and MTI-DCI (Epa = +0.87 V)
exhibit more reversible oxidation waves at more negative Epa
values (ca. − 0.2 V shift) owing to the +I effect of methyl
groups. It is worth noting that the use of the stronger electron
accepting group DCI leads to ca. −0.1 V shifts of Epa which is
counterintuitive but may be related to the extension of the π-
conjugated system and hence to a better stabilization of the
radical-cation.
The UV−vis absorption spectrum of titled compounds in
CH2Cl2 shows a main band in the visible region attributed to an
intramolecular charge transfer (Figure 2). Comparison of the
λmax of DCV (424 and 452 nm, respectively, for PTI-DCV and
MTI-DCV) and DCI derivatives (548 and 566 nm,
respectively, for PTI-DCI and MTI-DCI) shows that the
2, as already reported.27 For scale-up purpose, the microwave
irradiation was replaced by a conventional oil bath and P(OEt)3
by a mixture of 10 equiv of PPh3 in 1,2-dichlorobenzene since
P(OEt)3, used as a solvent and reactant, systematically afforded
undesired byproducts. In these optimized conditions, several
grams of compound 2 were prepared in 62% yield (Scheme 1).
Since dioxomolybdenum(VI) complexes are efficient catalysts
for oxo-transfer reactions, the Cadogan reaction was finally
performed in the presence of a catalytic amount of
MoCl2O2(dmf)2.28 While improving the yield to 88%, it
allowed us to reduce the quantities of PPh3 and to switch
over toluene (Scheme 1). Thieno[2,3-b]indole 2, thus prepared
in good yields and grams scale quantity, was then easily
alkylated with methyl iodide affording compound 3b. Note that
an alternative route to compound 2 starting from 1 has been
recently described.29 Finally, an aldehyde group was introduced
at the 2-position of both N-aryl and N-alkyl thienoindoles 3a
and 3b via a regioselective Vilsmeier−Haack formylation. The
resulting formyl derivatives 4a and 4b were subjected to a
Knœvenagel condensation with either malononitrile or 2-(3-
oxo-2,3-dihydroinden-1-ylidene)malononitrile leading to the
four target push−pull molecules in high overall yields.
Figure 1 presents the cyclic voltammograms (CVs) of the
push−pull molecules recorded in CH2Cl2 in the presence of
Bu4NPF6 as supporting electrolyte. Each CV shows an
irreversible reduction wave with a cathodic peak potential
value Epc which depends on both the nature of the electron-
accepting group and the N-substitution of the thienoindole
moiety. The Epc values of the DCI derivatives PTI-DCI (Epc
=
−1.16 V) and MTI-DCI (Epc = −1.29 V) are significantly
shifted toward positive potentials compared to DCV com-
Figure 2. Absorption spectra of 10−5 M solutions of PTI-DCV
(orange), PTI-DCI (green), MTI-DCV (red), and MTI-DCI (dark
blue) in CH2Cl2.
pounds PTI-DCV (Epc = −1.55 V) and MTI-DCV (Epc
=
−1.72 V) due to the stronger electron-accepting ability of DCI
group. Similarly, albeit to a lesser extent, the replacement of the
B
DOI: 10.1021/acs.orglett.6b00438
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Organic Letters
Letter
stronger electron-withdrawing DCI group leads to a significant
bathochromic shift of ca. 120 nm. Absorption maxima of MTI
derivatives are subject to 20−30 nm bathochromic shifts
compared to the corresponding PTI derivatives. In addition, it
is interesting to note that MTI derivatives present higher
extinction molar coefficients ε than those of PTI ones with high
values for MTI-DCV (ε = 62 000 M−1·cm−1) and MTI-DCI (ε
= 71 000 M−1·cm−1).
× 10−5 cm2 V−1 s−1), and MTI-DCI (μh = 1.3 × 10−6 cm2 V−1
s−1) show close values for the DCV derivatives at least 1 to 2
orders of magnitude higher than those of DCI derivatives.
Single crystals of MTI-DCV grown by slow evaporation of a
solution of DMF were analyzed by X-ray diffraction (SI). Two
independent molecules are present per asymmetric unit. With
the exception of methyl groups, the molecules adopt a quasi-
planar conformation. As expected, the thieno[2,3-b]indole core
is fully planar, whereas a slight deviation from planarity results
from small dihedral angles of 4° or 6.8° between the thiophene
and DCV units, the latter adopting a cis conformation relative
to the sulfur atom. Figure 4 shows that molecules are stacked
The thermal properties of titled compounds have been
analyzed by differential scanning calorimetry (DSC) and
thermogravimetric analysis (TGA). Except for MTI-DCV that
decomposes at 298 °C before melting, the first DSC traces
show a sharp endothermic peak with an onset corresponding to
the melting point (Table S1, Figure S20). TGA indicates
decomposition temperatures Td, defined at 5% weight loss, of
ca. 230 and 310 °C for DCV and DCI derivatives, respectively.
Thermal evaporation of these compounds under vacuum
readily affords homogeneous thin-films without any sign of
decomposition as confirmed by MALDI-TOF and UV−vis
analysis of redissolved films. The absorption spectrum of films
of 15 nm thickness of these compounds evaporated on
PEDOT−PSS coated ITO on glass shows a red shift of λmax
and a broadening of the absorption band compared to the
solution spectrum together with the apparition of well-defined
shoulders attributed to intermolecular π-interactions in the
solid state (Figure S22). The thin-films absorption onsets at
low energy give optical band gaps Eg of 2.39 eV (PTI-DCV),
1.93 eV (PTI-DCI), 2.38 eV (MTI-DCV), and 1.86 eV (MTI-
DCI). Note that the absorption coefficient of the film derived
from MTI-DCV exhibits the highest value of 2.22 × 105 cm−1
(Supporting Information (SI)).
Figure 4. Molecular packing of MTI-DCV along the b axis.
along the b axis with a parallel orientation and significant
overlaps (SI). The short mean intermolecular distances inside
the column d1 (3.402 Å) and d2 (3.414 Å) are consistent with a
compact packing associated with π−π stacking interactions of
high interest for charge transport.
The hole mobility (μh) of titled compounds has been
estimated on hole only devices using the space-charge-limited
current (SCLC) method (SI). The devices consist of a 115 nm
thick layer of each push−pull molecule vacuum-deposited on a
PEDOT−PSS/ITO/glass substrate on which a gold cathode
(100 nm) is thermally evaporated. X-ray diffraction analysis of
these organic thin films did not show any diffraction peak
suggesting their amorphous character. Figure 3 displays the
current density vs voltage plots of hole-only diodes based on
the different compounds and clearly shows higher current
density for DCV derivatives. The hole-mobility extracted from
these curves for PTI-DCV (μh = 2.68 × 10−5 cm2 V−1 s−1),
PTI-DCI (μh = 2.3 × 10−7 cm2 V−1 s−1), MTI-DCV (μh = 2.27
The relatively good hole transport properties of thin-films of
MTI-DCV combined with their high absorption coefficient and
thermal stability incite us to evaluate the photovoltaic
performance of this small push−pull molecule as donor
material. As a preliminary test, simple bilayer planar
heterojunction solar cells of 27 mm2 active area were fabricated
by successive thermal evaporations of 15 nm of MTI-DCV, 20
nm of C60, as acceptor material, and 150 nm of aluminum on
ITO substrates precoated with a 40 nm layer of spin-cast
PEDOT−PSS. After a thermal treatment of 15 min at 100 °C, 8
devices under AM 1.5 simulated solar illumination (80 mW·
cm−2) led to average values of short-circuit current density Jsc of
2.92 mA·cm−2, open-circuit voltage Voc of 0.69 V, fill-factor FF
of 0.40, and power conversion efficiency PCE of 1.00% with a
maximum of 1.06% for the best device (SI).
In conclusion, we have developed a straightforward, scalable,
and high yield synthetic strategy to N-alkyl and N-aryl
thieno[2,3-b]indole heterocycles and used them as electron-
donating blocks for the preparation of new push−pull
chromophores. The latter combine good absorption properties
in the visible spectrum and hole transport properties; as such,
the photovoltaic performance of one of these reduced-size
molecules has been evaluated showing interesting potential. To
the best of our knowledge, MTI-DCV represents the smallest
push−pull molecule ever reported with a PCE higher than
1%.6f Further investigations of the photovoltaic properties of
the other thieno[2,3-b]indole derivatives and the synthesis of
more extended push−pull systems are underway. More
generally, these results show that thieno[2,3-b]indole can be
a useful electron-donating building block for various
applications in organic electronics or in nonlinear optics, for
example.
Figure 3. Current density−voltage characteristics of hole-only diodes
based on PTI-DCV (orange), PTI-DCI (green), MTI-DCV (red),
and MTI-DCI (dark blue).
C
DOI: 10.1021/acs.orglett.6b00438
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Organic Letters
Letter
M.; Grosu, I.; Roncali, J. Chem. - Eur. J. 2015, 21, 1598. (e) Mohamed,
S.; Demeter, D.; Laffitte, J.-A.; Blanchard, P.; Roncali, J. Sci. Rep. 2015,
5, 09031.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.6b00438.
■
S
́ ̂ ̀
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̈
̈
̈
AUTHOR INFORMATION
Corresponding Authors
■
M.; Meerholz, K.; Wurthner, F. J. Mater. Chem. 2010, 20, 240.
̈
(b) Burckstummer, H.; Tulyakova, E. V.; Deppisch, M.; Lenze, M. R.;
̈
̈
Kronenberg, N. M.; Gsanger, M.; Stolte, M.; Meerholz, K.; Wurthner,
F. Angew. Chem., Int. Ed. 2011, 50, 11628.
̈
̈
* E-mail: philippe.leriche@univ-angers.fr.
* E-mail: philippe.blanchard@univ-angers.fr.
Present Address
†CEISAM, CNRS UMR 6230, 2 rue de la Houssinier
̀
e, 44322
Nantes cedex 3, France.
(16) Do, K.; Kim, C.; Song, K.; Yun, S.; Lee, J. K.; Ko, J. Sol. Energy
Mater. Sol. Cells 2013, 115, 52.
(17) Montcada, N. F.; Cabau, L.; Kumar, C. V.; Cambarau, W.;
Palomares, E. Org. Electron. 2015, 20, 15.
(18) (a) Cheng, M.; Yang, X.; Chen, C.; Tan, Q.; Sun, L. J. Mater.
Chem. A 2014, 2, 10465. (b) Tan, Q.; Yang, X.; Cheng, M.; Wang, H.;
Wang, X.; Sun, L. J. Phys. Chem. C 2014, 118, 16851.
(19) Steinmann, V.; Kronenberg, N. M.; Lenze, M. R.; Graf, S. M.;
Notes
The authors declare no competing financial interest.
Hertel, D.; Meerholz, K.; Burckstummer, H.; Tulyakova, E. V.;
̈
̈
ACKNOWLEDGMENTS
F. Baert acknowledges The Minister
Ph-D grant. We thank the PIAM of the University of Angers for
characterization of organic compounds. Jonhson Mattey is
acknowledged for the gift of PdCl2 used for the preparation of
Pd(PPh3)4 catalyst.
■
Wurthner, F. Adv. Energy Mater. 2011, 1, 888.
̈
̀
e de la Recherche for his
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DOI: 10.1021/acs.orglett.6b00438
Org. Lett. XXXX, XXX, XXX−XXX