Ladder-type Heteroarenes: Up to 15 Rings with Five Imide Groups

Article abstract of DOI:10.1002/anie.201702225

A series of novel imide-functionalized ladder-type heteroarenes with well-defined structure and controllable conjugation lengths were synthesized and characterized. The synthetic route shows remarkable efficacy for constructing the electron-deficient ladder backbones. π-Conjugation extension leads to narrowed band gaps with enhanced electron affinities. The ladder arenes are incorporated into organic thin-film transistors, and show encouraging electron mobilities of 0.013–0.045 cm² V^?1 s^?1. The heteroarenes reported here provide a remarkable platform for fundamental physicochemical studies and materials innovation in organic electronics.

Full text of DOI:10.1002/anie.201702225

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201702225
German Edition: DOI: 10.1002/ange.201702225
Polymers
Ladder-type Heteroarenes: Up to 15 Rings with Five Imide Groups
+
+
Yingfeng Wang , Han Guo , Shaohua Ling, Iratxe Arrechea-Marcos, Yuxi Wang,
Juan Teodomiro Lꢀpez Navarrete, Rocio Ponce Ortiz,* and Xugang Guo*
Dedicated to Professor Tobin J. Marks
[
23]
Abstract: A series of novel imide-functionalized ladder-type
heteroarenes with well-defined structure and controllable
conjugation lengths were synthesized and characterized. The
synthetic route shows remarkable efficacy for constructing the
electron-deficient ladder backbones. p-Conjugation extension
leads to narrowed band gaps with enhanced electron affinities.
The ladder arenes are incorporated into organic thin-film
transistors, and show encouraging electron mobilities of 0.013–
interesting properties.
However, most ladder-type mole-
[6]
cules typically exhibit electron-rich characteristics and the
synthesis of electron-deficient analogues remains a great
challenge because of the reduced chemical reactivity and
unfavored steric hindrance created by typical electron-with-
[
24–26]
drawing substituents.
Hence it is highly imperative to
develop electron-deficient ladder-type p-conjugated systems,
which are highly desired for constructing donor-acceptor
and developing organic semiconductors using the
donor-acceptor strategy.
2
À1 À1
[27]
0
.045 cm V s . The heteroarenes reported here provide
dyads
[
28]
a remarkable platform for fundamental physicochemical
studies and materials innovation in organic electronics.
Imide-functionalized arenes are highly promising p-con-
[
1,29]
jugated materials,
naphthalene diimide,
c]pyrrole-4,6-dione
and the semiconductors derived from
[
30,31]
[32]
O
rganic p-conjugated materials are emerging semiconduc-
tors for next-generation optoelectronic devices, such as
organic thin-film transistors (OTFTs) and organic solar cells
perylene diimide, and thieno[3,4-
[
33]
are the representative materials in
OTFT and OSC fields, thus showing highly encouraging
device performance. Among various imide-functionalized
arenes, the bithiophene imide (BTI, Figure 1) was first
synthesized as an electron-deficient unit by Marks et al.,
and its incorporation into polymers affords a series of
semiconductors with remarkable performance in OTFTs
(
OSCs), owing to the advantages of low-cost, light-weight,
[
1,2]
and mechanical flexibility.
The device performance of
[34]
organic semiconductors is highly related to their backbone
[
3,4]
structure.
In addition, intermolecular packing and film
[
5]
morphology on larger scale also play critical roles. Ladder-
type molecules with highly planar backbones and well-
delocalized p-conjugation have attracted intensive attention
as small-molecule semiconductors and building blocks for
[
35,36]
and OSCs.
Inspired by the interesting physicochemical properties
and encouraging device performance of ladder-type materials
and the great success of imide-functionalized arenes, we
herein report a series of novel imide-functionalized ladder-
type heteroarenes BTI-BTI5 (Figure 1) with controllable
[6–19]
polymers, molecular wires, and ladder polymers.
Among
[
4]
them, pentacenes show substantial hole mobility, and
benzothienobenzothiophene and dinaphthothienothiophene
exhibit remarkable mobility with excellent OTFT stabil-
ity. Recently, Bꢀuerle and co-workers reported the
synthesis of a fused thiophene-pyrrole heterodecacene by
[
20,21]
[
22]
multiple aminations. A series of ladder-type thienoacenes
based on benzodithiophene are synthesized, thus showing
[
+]
[+]
[
*] Dr. Y. Wang, Dr. H. Guo, S. Ling, Y. Wang, Prof. X. Guo
Department of Materials Science and Engineering and The Shenzhen
Key Laboratory for Printed Organic Electronics
South University of Science and Technology of China
No. 1088, Xueyuan Road, Shenzhen, Guangdong 518055 (China)
E-mail: guoxg@sustc.edu.cn
I. Arrechea-Marcos, Prof. J. T. Lꢀpez Navarrete, Dr. R. P. Ortiz
Department of Physical Chemistry, University of Mꢁlaga
Campus de Teatinos s/n, 29071 Mꢁlaga (Spain)
E-mail: rocioponce@uma.es
Y. Wang
Department of Chemistry, Wuhan University
Wuhan, Hubei 430072 (China)
+
[
] These authors contributed equally to this work.
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.201702225.
Figure 1. Chemical structures of the imide-functionalized ladder-type
heteroarenes BTI–BTI5 with varied conjugation lengths.
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Scheme 1. Synthetic route to the ladder-type heteroarene BTI5 with 15 Rings and 5 imide groups.
conjugation lengths and varied BTI-fused heterocycles. The
ladder-type arenes show excellent solubility, highly planar
backbone, substantial crystallinity, and tunable optoelectrical
properties. When tested in OTFTs, the BTI derivatives
exhibit encouraging electron mobility.
(90%), and it is used for the following imidization without
further purification because of its limited solubility. The
thermodynamic nature of the ring-closing reaction favors the
[38,39]
formation of ladder-type anhydrides.
Upon the synthesis of 11, its full imidization becomes the
Scheme 1 depicts the synthetic route to BTI5, the longest
arene with 15 rings and 5 imide groups in a row, and it covers
most reactions needed for the synthesis of the shorter BTI
derivatives BTI2–BTI4 (see the Supporting Information).
Palladium-catalyzed Stille coupling and/or nickel-mediated
Yamamoto coupling are employed to construct the extended
backbones. The construction of multiple imide groups is
similar to the one used in the monoimide synthesis for BTI.
key step. Fortunately, the imidization shows great success
[
36]
using a two-step protocol
and toluene is chosen as the
solvent in the alkylcarbamoyl-carboxylic acid formation step
to increase the solubility and reaction temperature. The
formed alkylcarbamoyl-carboxylic acid cannot be readily
converted into the undesired dialkylcarbamoyl (or diamide)
under these reaction conditions, which is critical for the imide
synthesis. The multiple reaction sites on the penta-anhydride
11 result in a variety of carbamoyl-carboxylic acid isomers,
which are difficult to purify and characterize. Without further
[37]
Compared to the method developed for the BTI2 synthesis,
the methyl ester is replaced with an ethyl ester to increase
solubility and reaction yield, which are critical for the
synthesis of the more extended BTI3–BTI5. One of the key
steps is the monobromination of the compounds 3 and 5,
a step which is challenging because of the comparable
reactivities at the a-positions on both ends of the molecules.
purification, the isomers are subjected to SOCl -assisted
2
cyclization to afford the penta-imide 12 in 20% overall yield.
The moderate yield is likely due to the unpurified anhydride
11. In contrast, it was found that the iminochloride byprod-
[
40]
uct is formed and the byproduct yield gets higher as the
anhydride number increases. As a result, imidization results in
greatly improved yields for the shorter BTI derivatives, which
are 37%, 49%, 76%, and 90% for BTI4, BTI3, BTI2, and
BTI, respectively (see the Supporting Information). In
addition, BTI–BTI5 can be readily brominated, and thus
enables materials development by incorporating these novel
electron-deficient units into small molecules and polymer
semiconductors.
It was found that the Br addition conditions are critical, and
2
Br2 (1.0 equiv totally) addition in five different portions
affords the monobrominated 4 and 6 in fair yield (40–50%)
with about 20% unreacted starting material and about 20%
of the dibrominated byproducts, which can be used for the
synthesis of BTI2 and BTI3 (see the Supporting Information).
By using the monobrominated 4, both copper-mediated
Ullmann coupling and palladium-catalyzed Stille coupling
were attempted, but gave 5 in low yields (20–30%). To our
delight, under nickel-mediated Yamamoto coupling, 4 and 6
can be successfully dimerized to afford 5 and 7, respectively,
in excellent yields (ca. 90%). The compound 7 is readily
In spite of the high efficacy for the synthesis of these
ladder-type BTI derivatives, we must admit that the synthetic
route to the longest BTI5 is tedious. In contrast, the nickel-
mediated Yamamoto coupling is remarkably powerful for
backbone extension, and has also been used for the synthesis
dibrominated using Br to afford 8 in nearly quantitative
2
[
35]
yield, and it is then coupled with 2-(trimethylstannyl)thio-
phene-3-carboxylate to yield 9. Hydrolysis in NaOH solution
can readily convert the deca-ester 9 into the deca-acid 10 with
remarkable yield (97%) and the subsequent dehydration
affords the purple penta-anhydride 11 in excellent yield
of BTI-based homopolymers. The synthesis of BTI-based
ladder-type polymers is ongoing in our lab and we expect that
high-quality ladder polymers are achievable if quantitative
postpolymerization imidization can be realized.
2
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All ladder-type heteroarenes show ready solubility at
room temperature, even for the longer BTI4 and BTI5 with
À1
solubilities of 14 and 13 mgmL , respectively, in chloroform.
The chemical structures of BTI–BTI5 were confirmed by
NMR spectra, MALDI-TOF mass spectra (Figure 2), high-
resolution mass spectrometry (HRMS), and elemental anal-
ysis (see the Supporting Information). Among the series, BTI,
Figure 3. Absorption spectra of a) BTI–BTI5 solutions in chloroben-
zene at 298 K; b) BTI–BTI5 solutions in chlorobenzene at 373 K;
c) BTI–BTI5 films. d) Optical gap of BTI–BTI5 in correlation with the
reciprocal number of double bonds. The optical gap is derived from
BTI–BTI5 absorption in solution at 373 K.
structured absorption shoulder and a sharp absorption edge
[7–9]
as expected for ladder-type oligomers.
However on the
basis of the absorption spectra in solution and film state, BTI5
shows distinctive aggregation in solution at room temper-
ature. As the temperature is increased to 373 K, BTI5 then
de-aggregates. For more consistent comparison, the absorp-
tion onsets of BTI–BTI5 solutions at 373 K (Figure 3b) are
used for the calculation of optical gaps since all chromophores
are in isolated form. The optical gaps decrease from 3.19 eV
for BTI to 2.04 eV for BTI5 and correlate linearly with the
reciprocal number of double bonds with a correlation coef-
1
Figure 2. a) MALDI-TOF mass spectra and b) H NMR spectra (in
aromatic region) of BTI–BTI5. CDCl is used for NMR measurements
of BT1–BT3 at room temperature and C Cl D is used for BTI4 and
3
2
4
2
BTI5 at 1208C.
[
22]
ficient of 0.995 (Figure 3d). Such a linear correlation is in
agreement with theoretical calculations (see Figure S5). The
limit of the extrapolated optical gap is about 1.7 eV, thus
showing good tunability from wide to medium optical gaps by
backbone extension. The thermal properties of BTI–BTI5 are
studied by thermogravimetric analysis (TGA) and the TGA
curves exhibit excellent stability with the decomposition
temperatures typically larger than 4508C, except for BTI (see
Figure S2). Hence as the conjugation length increases, the
thermal stability is enhanced and then saturates for these
ladder-type arenes.
BTI2, and BTI3 show well-resolved NMR chemical shifts,
however the longer BTI4 and BTI5 cannot achieve high-
quality H NMR signals at room temperature because of their
1
strong aggregation induced by the extended ladder p-systems.
At 1208C, the NMR spectra (Figure 2b) of BTI4 and BTI5
show improved resolution. It is interesting to note that on the
basis of the HMRS spectra of BTI4 and BTI5 (see Figures S64
and S65 in the Supporting Information), the isotope peaks
indicate that z should be equal to + 2, therefore m is double
the molecular weights of BTI4 and BTI5. Although the exact
reason for such HMRS spectra is unknown at this stage, we
speculate that the doubled m value is likely due to the
formation of BTI4 and BTI5 dimers induced by their strong
intermolecular interactions, and is in good accordance with
their UV-vis absorption and NMR measurement. Cultivation
of single crystals of BTI2–BTI5 failed because of the long and
branched side chains and the analogues with straight chains
show limited solubility.
The optical properties of these heteroarenes are inves-
tigated by measuring their absorption spectra (Figure 3). The
absorption onsets in chlorobenzene show a gradual bath-
ochromic shift from l = 390 nm for BTI to l = 637 nm for
BTI5 (Figure 3a), and the films exhibit similar transition with
the decreased optical gaps (Figure 3c). At room temperature
BTI2–BTI4 show absorption spectra with a vibrationally
Electrochemical properties of BTI–BTI5 were investi-
gated by cyclic voltammetry (CV) with referencing to
+
a ferrocene/ferrocenium (Fc/Fc ) internal standard (see Fig-
ure S1). The onsets of reduction potentials are À2.07, À1.46,
À1.21, À1.10, and À0.81 V for BTI, BTI2, BTI3, BTI4, and
BTI5, respectively. Hence with the p-conjugation extension,
the derived LUMOs are gradually decreased from À2.29 eV
for BTI to À3.55 eV for BTI5, thus indicating increased
electron affinity. The gradual decrement trend of the LUMOs
is in good accord with the results from theoretical calcula-
tions, where a stabilization of about 1 eV is recorded on going
from BTI to BTI5. On the contrary, the experimentally
derived HOMOs partially differ from the trend extracted
from DFT calculations, and is likely due to its tendency to
overestimate conjugation. The CV data render similar
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[
41]
HOMOs for BTI–BTI5, while a HOMO destabilization of
about 0.6 eV is found theoretically from BTI to BTI5. Hence,
HOMOs seems to be less affected than LUMOs with chain
lengthening (Figure 4a). The results indicate that the band-
gap lowering is mainly attributed to the decreased LUMOs
phenes. Focusing on the second most intense band, whose
theoretical eigenvector indicates that it arises from an in-
phase antisymmetric n(C=C) mostly localized on the outer
À1
rings, a notable downshift of 30 cm is recorded on going
À1
À1
from BTI (1553 cm ) to BTI3 (1523 cm ). However, it stays
unaltered for longer chains (see Figures S7–10). This Raman
mode can be related to the well-known line A of linear
oligothiophenes, which is remarkably sensitive to conjugation
[
41]
length. Note however that the most intense band is still
slightly downshifted for BTI5. Therefore, the results obtained
here indicate that although the band gaps are still diminishing
and some conjugation changes are still evidenced by Raman
spectra, we are reaching saturation at the longer analogues
BTI3–BTI5, and it is in good accord with the trend in
Figure 3d and the localization of the LUMO orbital top-
ologies in the central part of the conjugated skeleton, with
modest contribution of the external rings for BTI4 and BTI5
(see Figure S6). All the above evidence indicates that electron
delocalization, a key parameter in charge transport, may be
quite similar for BTI3–BTI5. Thus, synthesizing longer fused
systems may not be necessary, in terms of p-conjugation.
Top-gate/bottom-contact OTFTs were fabricated to inves-
tigate the potential applications of these building blocks in
organic electronics. The OTFTs of all these ladder-type BTI
derivatives show encouraging electron-transporting charac-
teristics with average electron mobilities of 0.013–
Figure 4. a) Experimentally measured and B3LYP/6-31G** calculated
FMO energy levels of BTI-BTI5. b) B3LYP/6-31G** optimized BTI5
geometry.
2
À1 À1
0
.045 cm V
s
in the saturated regime (see Tables S3 and
S4) except BTI. The inactivity of BTI OTFTs is attributed to
no quality film formation and substantial charge carrier
injection barrier. The representative transfer curve of BTI5
OTFTs (Figure 5b) shows a “kink-free” feature, and is
indicative of the reliability of these mobility values. From
with extended conjugation, and contrasts from previous
reports, wherein the main contribution to band-gap lowering
was from the HOMO or both HOMO and LUMO in these
[22,23]
kinds of ladder-type arenes.
The DFT optimized energies
show highly planar backbones for all these ladder-type arenes,
with dihedral angles of less than 18 (Figure 4b; see Figure S6),
and in good accord with the BTI single-crystal structure. In
addition, the DFT calculations reveal a straight BTI5 back-
bone (Figure 4b). The planar and straight backbones are
beneficial to materials packing and film ordering.
BTI2 to BTI4, the threshold voltage (V ) gradually decreases,
t
and is in good accordance with the LUMO evolvement. As
the backbone extends, in addition to n-channel performance,
the longest BTI5 exhibits p-channel performance with a hole
À3
2
À1 À1
mobility of more than 10 cm V s . It should be pointed
Internal reorganization energies were computed for all
these ladder-type arenes. As expected, for both hole and
electron transport the energies decrease substantially in the
longest derivatives (see the Supporting Information), because
of delocalization of the injected charge over a larger number
of conjugated CÀC/C=C bonds. Please note that the decrease
is significant for electron transport on going from BTI
(
0.40 eV) to BTI2 (0.26 eV) and to BTI3 (0.22 eV). However
it stays quite balanced for the longer systems, at 0.20 and
.18 eV for BTI4 and BTI5, respectively. These data are in
0
good agreement with the DFT-derived LUMO topologies
which show an electron density confinement in the central
parts of BTI4 and BTI5, thus rendering a similar extension
(
see Figure S6). To further analyze the conjugation extension,
FT-Raman spectra were recorded for BTI–BTI5 (see Fig-
ure S3). The most intense Raman mode, which is ascribed to
a symmetric n(C=C/CÀC) vibration, and can be related to line
B in linear oligothiophenes, gives valuable information on the
À1
conjugated extension. This band downshifts from 1470 cm
À1
for BTI to 1451 cm for BTI5. This modest shift with chain
Figure 5. a) Output and b) transfer characteristics of BTI5 OTFTs. The
elongation is similar to that observed for linear oligothio-
active layer is deposited from solution.
4
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out that all these BTI derivatives contain very bulky
-octyldodecyl side chains, which are detrimental to molecule
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To conclude, we have successfully synthesized a series of
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to 5 imide groups and 15 rings in a row. The synthetic route
shows remarkable efficacy for constructing ladder-type back-
bones with multiple imide functionalities. These fused arenes
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Manuscript received: March 1, 2017
Revised manuscript received: May 30, 2017
Version of record online: && &&, &&&&
6
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ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2017, 56, 1 – 7
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Polymers
Climbing the ladder: A series of novel
ladder-type heteroarenes with well-
defined structures and precisely control-
lable conjugation lengths were synthe-
sized with remarkable efficacy and char-
acterized. Compared to most ladder-type
arenes, these imide-functionalized build-
ing blocks are highly electron-deficient,
and thus provide a remarkable platform
for materials innovation in organic elec-
tronics.
Y. Wang, H. Guo, S. Ling,
I. Arrechea-Marcos, Y. Wang,
J. T. Lꢁpez Navarrete, R. P. Ortiz,*
X. Guo*
&&&& — &&&&
Ladder-type Heteroarenes: Up to 15
Rings with Five Imide Groups
Angew. Chem. Int. Ed. 2017, 56, 1 – 7
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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These are not the final page numbers!