Revisiting Indolo[3,2-b]carbazole: Synthesis, Structures, Properties, and Applications

Article abstract of DOI:10.1002/anie.202001803

Indolo[3,2-b]carbazole presents a π-skeleton with a remarkable electronic structure and interesting potential applications. It is, however, also associated with ambiguity and controversy. Herein, new derivatives of indolo[3,2-b]carbazole are reported and they have enabled a comprehensive study on the electronic structure of indolo[3,2-b]carbazole and the development of a new n-type organic semiconductor. Experimental and computational studies show that indolo[3,2-b]carbazole has a largely localized p-benzoquinonediimine moiety and significant antiaromaticity. When substituted with (4-silylethynyl)phenyl groups, the indolo[3,2-b]carbazole exhibits one-dimensional π–π stacking and functions as an n-type organic semiconductor in solution-processed field effect transistors.

Full text of DOI:10.1002/anie.202001803

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Accepted Article
Title: Revisiting Indolo[3,2-b]carbazole: Synthesis, Structures,
Properties and Applications
Authors: Mengna Zhao, Binghao Zhang, and Qian Miao
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10.1002/anie.202001803
Angewandte Chemie International Edition
RESEARCH ARTICLE
Revisiting Indolo[3,2-b]carbazole: Synthesis, Structures,
Properties and Applications
Mengna Zhao#, Binghao Zhang# and Qian Miao*
[*]
M. Zhao, B. Zhang, Prof. Q. Miao
Department of Chemistry
The Chinese University of Hong Kong
Shatin, New Territories, Hong Kong, China
E-mail: miaoqian@cuhk.edu.hk
Equal contribution
#
Supporting information for this article is given via a link at the end of the document.
Abstract: Indolo[3,2-b]carbazole presents
a
π-skeleton with
remarkable electronic structure and interesting potential applications,
and, on the other hand, is associated with ambiguity and controversy.
Herein, we report new derivatives of indolo[3,2-b]carbazole, which
have enabled a comprehensive study on the electronic structure of
indolo[3,2-b]carbazole and development of a new n-type organic
semiconductor. It is found from experimental and computational
studies that indolo[3,2-b]carbazole has
a largely localized p-
benzoquinonediimine moiety and significant antiaromaticity. When
substituted with (4-silylethynyl)phenyl groups, indolo[3,2-b]carbazole
exhibits one-dimensional π-π stacking and functions as an n-type
organic semiconductor in solution-processed field effect transistors.
Introduction
Indolo[3,2-b]carbazole is a molecule of ambiguity and
controversy. First, “indolo[3,2-b]carbazole” in literature refers to
two different structures, 1 and 2, as shown in Figure 1a.
According to the IUPAC recommendations on nomenclature of
fused and bridged fused ring systems since 1998, cyclic parent
hydrides are either fully saturated or fully unsaturated.[
1 , 2 ]
Therefore the preferred IUPAC name of
1 is indolo[3,2-
b]carbazole, while the preferred IUPAC name of 2 is 5,11-
dihydroindolo[3,2-b]carbazole. However, indolo[3,2-b]carbazole
still widely appears in recent literature as the out-of-date
conventional name of 2, and dehydroindolo[3,2-b]carbazole
appears in old literature as the out-of-date conventional name of
1. A very recent search of the structure of 1 within the SciFinder®
database resulted in 101 references and 18 commercial
resources. However, the vast majority of these references, in
fact, are about 5,11-dihydroindolo[3,2-b]carbazole (2) or its
derivatives, and none of these commercial resources could
provide indolo[3,2-b]carbazole (1) without ambiguity upon our
request.[3] Second, the synthesis of indolo[3,2-b]carbazole (1) is
in question. To the best of our knowledge, Hünig and
Steinmetzer in 1976 first synthesized 1 by oxidation of 2 with
PbO2, but only provided characterization data (multiplets in the
region of 6.8 to 8.0 ppm in 1H NMR) that are not detailed enough
to identify the structure unambiguously.[4] The next synthesis of
1 was reported in 2000 by Yudina et al., who obtained 1 by
oxidation of 2 with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone
(DDQ) and provided convincing 1H and 13C NMR of 1.[ 5 ]
However, Dehaen and coworkers reported in 2008 that, under
the same reaction conditions, they failed to observe formation of
1.[6]
Figure 1. (a) Indolo[3,2-b]carbazole (1), 5,11-dihydroindolo[3,2-b]carbazole (2)
and indeno[1,2-b]fluorene (3); (b) resonance structures of indolo[3,2-
b]carbazole (1) with Clar’s aromatic sextets highlighted in light blue; (c)
substituted indolo[3,2-b]carbazoles.
On the other hand, indolo[3,2-b]carbazole (1) presents a
π-skeleton with remarkable electronic structure and interesting
potential applications.[ 7 ] Similar to its hydrocarbon analogue,
indeno[1,2-b]fluorene (3 in Figure 1a),[8,9] 1 can be described
with three resonance structures as shown in Figure 1b. Structure
1 features a localized p-benzoquinonediimine moiety (shown
with bold lines) and two Clar’s aromatic sextets (highlighted in
light blue) in the terminal benzenoid rings; structure 1’ features a
1
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Scheme 1. Synthesis of indolo[3,2-b]carbazoles
formally antiaromatic diaza[20]annulene structure (shown with
bold lines) with 20 π-electrons; and structure 1” features a
diradical structure with three Clar’s aromatic sextets (highlighted
[10 , 11 , 12 ]
in light blue). N-alkylated derivatives of 2
and 6,12-
diarylated derivatives of 3 were successfully applied as p-type
and ambipolar organic semiconductors in organic field effect
transistors (OFETs). Having a nitrogen-rich electron-deficient π-
skeleton[13] with essentially the same shape as those of 2 and 3,
indolo[3,2-b]carbazole (1) promises a new design of n-type
organic semiconductors. So far, the only known substituted
indolo[3,2-b]carbazoles are the t-butylated derivatives, 4a and
4b (Figure 1c).[6] However, their electronic structure was not
studied to understand the antiaromatic and diradical characters
of indolo[3,2-b]carbazole, and the t-butyl groups block π-π
interactions necessary for organic semiconductors as found from
the crystal structure of 4a.
Therefore, we carried out an investigation to revisit
indolo[3,2-b]carbazole, with emphasis on their electronic
structure and application as an n-type organic semiconductor in
OFETs. Herein, we report new derivatives of indolo[3,2-
b]carbazole, 5a–c (Figure 1c), where the substituents are
introduced not only to increase solubility and stability. Because
the chemical shift of the hydrogen atoms on the central
benzenoid ring is a good probe for studying aromaticity and
antiaromaticity with NMR spectroscopy, 5a is designed to have
the central benzenoid ring unsubstituted. In order to develop n-
type organic semiconductors based on indolo[3,2-b]carbazole,
triethylsilylethynyl in 5b and (4-triethylsilylethynyl)phenyl in 5c
are introduced to tune π-stacking in the solid state.
Results and Discussion
To synthesize new derivatives of indolo[3,2-b]carbazole,
we first tested the reported synthesis of 1 by oxidation of 2. It is
[14 , 15 ]
found that refluxing a solution of 2
and DDQ in ethyl
[5]
acetate following Yudina’s report did not afford 1 but resulted
in recovery of 2, in agreement with the report by Dehaen et al.[6]
In contrast, following Hünig and Steinmetzer’s method
The electrochemical and optical properties of 5a–c were
studied with cyclic voltammetry (CV) and UV‐vis absorption
spectroscopy, respectively. In the test window of CV, 5a–c all
exhibited one quasi-reversible and one irreversible reduction
waves but did not show any oxidation waves (Figure S1 in the
Supporting Information). The first half wave potential of 5a is
−0.82 V versus ferrocenium/ferrocene, from which the LUMO
level of 5a is estimated as −4.28 eV.[20] As estimated using the
same method, 5b and 5c have lower LUMO energy levels
(−4.47 eV and −4.32 eV, respectively), as a result of the ethynyl
and phenyl substituting groups at the 6 and 12 positions.
Indolo[3,2-b]carbazoles 5a–c all form red solutions, which are
nonfluorescent when excited with UV light similar to their
hydrocarbon analogues, indeno[1,2-b]fluorene [8] and indeno[2,1-
a]fluorene.[21] As shown in Figure 2, 5a exhibits a low-energy
absorption band from 450 nm to 600 nm and a high-energy
absorption band centered at 302 nm. The absorption of 5b in the
visible region is very similar to that of 5a, but is red shifted
relative to that of 5c as shown in Figure S2 in the Supporting
Information. In contrast, dihydroindolo[3,2-b]carbazole 6a is
yellow in solution with an absorption edge at 435 nm, and its
solution exhibits strong purple fluorescence when excited with
UV light.
[4]
with
minor modification, oxidation of 2 with PbO2 at room temperature
in dark gave 1 in a yield of 42%, and 1 was unambiguously
characterized by 1H NMR spectroscopy and high resolution
mass spectroscopy. The UV-vis absorption spectrum of 1 in
DMF (Figure S2 in the Supporting Information) is similar but not
the same as that reported by Hünig and Steinmetzer.[4] To
synthesize 5a, its precursor (6a) was prepared following the
reported procedure.[16] To synthesize 5b–c, the corresponding
substituted
dihydroindolo[3,2-b]carbazoles
(6b–c)
were
synthesized from the known brominated dihydroindolo[3,2-
[ 17 ]
[ 18 , 19 ]
b]carbazoles 7
and 8
by Pd-catalyzed coupling
reactions as shown in Scheme 1. Finally, oxidation of 6a–c with
PbO2 resulted in 5a–c, respectively, in fair yields.
2
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Single crystals of 5b and 5c suitable for X-ray diffraction
analysis were grown by slow diffusion of CH3CN vapor into the
solution in CH2Cl2. The crystal structures of 5b and 5c [22] exhibit
essentially the same pentacyclic framework, which is in good
agreement with that in the reported crystal structure of 4a.[6]
Figure 3a shows the pentacyclic framework of 5b, where each of
the two terminal six-membered rings have averaged carbon-
carbon bond length in the range of 1.37 to 1.39Å. This indicates
aromatic character of these two rings. In contrast, the central
six-membered ring has four C–C single bonds (C5a–C6, C11a–
C12, C6a–C11a, C5a–C12a) with bond lengths of 1.46−1.47 Å
and two C–C double bonds (C6–C6a, C12–C12a) with bond
lengths of 1.35 Å.[23] The central six-membered ring is bonded to
two nitrogen atoms, N5 and N11, with short C–N bond length of
1.30 Å, which is even shorter than the typical bond length of C–
N double bonds in imines (1.35 Å).[23] These bond lengths are
Figure 2. UV-vis absorption spectra of 5a and 6a in CH2Cl2 (3×10−5 mol/L)
with the photograph of the same solutions upon irradiation with UV light at 365
nm.
similar to the corresponding bond lengths in indeno[1,2-
[8]
b]fluorene
and indicate a localized p-benzoquinonediimine
moiety. As shown in Figure 3b, the crystal structure of 5b
exhibits herringbone packing with π-π overlap between adjacent
molecules. As a result, molecules of 5b stack along two
directions that form an angle of 65°, and two adjacent π-planes
within each stack are separated by a distance of 3.40 Å. As
shown in Figure 3c, the crystal structure of 5c exhibits one-
dimensional π-π stacking with two slightly different π-π
distances, 3.60 and 3.66 Å. Within each stack, two adjacent π-
planes overlap with about three rings, and as a result, 5c
exhibits a larger degree of π-overlap than that of 5b as
displayed in Figure S3 in the Supporting Information despite of
the larger π-π distances.
The bond length analysis of the crystal structures of 5b
and 5c indicates that the major resonance contributor of
indolo[3,2-b]carbazole has a localized p-benzoquinonediimine
moiety. In order to better understand the electronic structure of
indolo[3,2-b]carbazole, particularly, the contribution of the other
two resonance forms (1’ and 1’’ in Figure 1b), the ring currents
were examined computationally with the nucleus-independent
chemical shift (NICS)-XY scan[ 24 ] and the anisotropy of the
induced current density (ACID) method,[25] and experimentally
with 1H NMR spectroscopy. Figure 4a shows the NICS-XY scans
of indolo[3,2-b]carbazole and two related molecules, pyrrolo[2,3-
f]indole and indeno[1,2-b]fluorene,[26] as calculated at the GIAO-
B3LYP/6-311+G(d) level of density functional theory (DFT) using
the π-only method. It is found that all rings in pyrrolo[2,3-f]indole
have positive NICS values, which suggest a global paratropic
ring current, and the largest NICS value (17.7 ppm) occurs at
the center of ring A. The central pyrrolo[2,3-f]indole moiety in
indolo[3,2-b]carbazole is less paratropic with its largest NICS
value (13.1 ppm) also occurring at the center of ring A, while the
central indacene moiety in indeno[1,2-b]fluorene is weakly
paratropic with its largest NICS value (7.4 ppm) occurring at the
center of ring B. The terminal benzenoid rings in both indolo[3,2-
b]carbazole and indeno[1,2-b]fluorene exhibit negative NICS
values, and those in indolo[3,2-b]carbazole are less diatropic.
The ACID plot of indolo[3,2-b]carbazole (Figure 4b) as
calculated at the CSGT-B3LYP/6-311+G(d) level of DFT shows
a counter-clockwise ring current in the central pyrrolo[2,3-
f]indole moiety and two clockwise ring currents in the terminal
benzenoid rings. This is in good agreement with the calculated
NICS values. In contrast, no directional ring current is observed
in the central indacene core of indeno[1,2-b]fluorene.[26] The
Figure 3. (a) The pentacyclic backbone of 5b in the crystal structure with
some bond lengths; (b) molecular packing of 5b in the single crystal as viewed
along the a-axis of the unit cell; (c) π-stacking between molecules of 5c. (C
and N atoms in the polycyclic backbone are shown as ellipsoids at the 50%
probability level; the triethylsilylethynyl and (4-triethylsilylethynyl)phenyl
substituents are shown as capped sticks; and H atoms are removed for clarity).
3
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NICS-XY scans together with the ACID plots indicate that
indolo[3,2-b]carbazole has
a more significant antiaromatic
character in its central pyrrolo[2,3-f]indole moiety than
indeno[1,2-b]fluorene in its central indacene moiety. The
calculated antiaromaticity in indolo[3,2-b]carbazole is in
agreement with the 1H NMR spectrum of 5a. Figure 5 compares
1H NMR spectra of 5a and 6a. It is found that the peaks of 5a in
the aromatic region are significantly upfield shifted relative to
those of 6a. In particular, the protons in the central six-
membered ring in 6a exhibit a singlet at 8.13 ppm, while those in
5a exhibit a singlet at 6.80 ppm, which is attributable to the
paratropic ring current with a shielding effect on the surrounding
protons. In agreement with the calculated lower NICS value (5.2
ppm) at the center of ring A of indeno[1,2-b]fluorene, the central
proton in a series of indeno[1,2-b]fluorene derivatives exhibits
chemical shifts in the range of 7.18 to 7.39 ppm,[9] which is
downfield shifted relative to that in 5a by 0.38 to 0.59 ppm.
Moreover, the diradical characters (y0) of 1, 5b and 5c are all 0
as calculated at the UCAM-B3LYP/6-31G(d,p) level of DFT. This
indicates that the diradical resonance form (1” in Figure 1b)
makes little contribution to the structure of indolo[3,2-b]carbazole.
Figure 5. 1H NMR spectra (400 MHz, CDCl3, 25 ºC) of 6a (top) and 5a
(bottom) in the aromatic region.
In relation to the antiaromaticity of indolo[3,2-b]carbazole,
the stability of 5a–c was monitored with UV‐vis absorption and
1H NMR spectroscopy. As shown in Figure S7 in the Supporting
Information, when solutions of 5a–c in CH2Cl2 (3×10−5 mol/L) in
capped volumetric flasks were exposed to the ambient light over
7 days, their UV-vis absorption spectra only showed slight
decrease in intensity. However, presence of acid or water was
found to dramatically decrease the stability of 5a in solutions. A
solution of 5a in THF saturated with water lost its red color in a
few seconds. In a more quantitative experiment, the absorption
spectrum of 5a in CH2Cl2 containing acetic acid (CH2Cl2/acetic
acid, 50/1, v/v) showed a large decrease in the absorption peaks
at 524 and 302 nm in accompany with an increase in the
absorption peaks at 262, 337 and 378 nm within one hour
(Figure S8 in the Supporting Information). These changes in
absorption spectrum suggest conversion of 5a to 6a. However,
after the solution of 5a in CH2Cl2 containing acetic acid was
concentrated, the resulting product exhibited very complicated
1H NMR spectrum, from which 6a was not unambiguously
identified. In the experiments to monitor the stability with 1H
NMR spectroscopy, the solution of 5a–c (~ 10-3 mol/L) was kept
in an NMR tube, which was wrapped in alumina foil to avoid light.
The deep red solution of 5a in CD2Cl2 gradually faded within two
days, and all the signals corresponding to 5a in 1H NMR became
weaker apparently as shown in Figure S9 (the Supporting
Information). From this solution, dimer, trimer and tetramer of 5a
were found by high resolution mass spectroscopy. This suggests
1
oligomerization of 5a, which occurs at the concentration for H
NMR spectroscopy but not at the concentration for UV-vis
1
spectroscopy. In contrast, the H NMR spectra of 5b and 5c did
not exhibit apparent change over 7 days under the same
conditions as shown in Figure S11 and S12 (Supporting
Information). The fact that 5b and 5c are more stable than 5a
can be attributed to the substituents at 6 and 12 positions, which
can kinetically block reactions of the central pyrrolo[2,3-f]indole
moiety. In the solid state, 5a–c all exhibit good stability under
ambient conditions at room temperature without obvious change
within at least 4 months.
Figure 4. (a) NICS-XY scans of indolo[3,2-b]carbazole, pyrrolo[2,3-f]indole
and indeno[1,2-b]fluorene. Capital letters (top) refer to the rings in the
structures and the dashed red lines describe the scanning path. (b) ACID plot
of indolo[3,2-b]carbazole at an isosurface value of 0.05. The magnetic field is
chosen to be orthogonal to the ring planes and pointing towards the viewer.
The low-lying LUMO energy levels and π-π interactions
found in the single crystals suggest that 5b and 5c can in
principle function as n-type organic semiconductors in the solid
4
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state. To fabricate organic field effect transistors, thin films of 5b
and 5c were deposited by solution-based methods onto a silicon
substrate, which had successive layers of thermally grown silica,
alumina, and 12-cyclohexyldodecylphosphonic acid (CDPA) as a
composite dielectric material.[ 27 ] Then, a layer of gold was
vacuum deposited on the organic films through a shadow mask
to form top-contact source and drain electrodes. 5b formed
crystalline films by dropping its solution in CH2Cl2 and acetone
(1/1, v/v, 0.5 mg/mL) onto a substrate. But the film changed its
appearance after vacuum deposition of gold (Figure S13 in the
Supporting Information) likely because 5b decomposed at
elevated temperature. High quality films of 5c were formed by
immersing a silicon substrate in a solution of 5c in CH2Cl2 and
acetone (1/1, v/v, 1.5 mg/mL) and then pulling it up with a
constant speed of 5 μm/s. The film was composed of aligned
crystalline fibers, as observed from the polarized-light
micrographs and atomic force micrographs shown in Figure S14
and S15 in the Supporting Information. X-ray diffraction from the
film exhibited four strong peaks, corresponding to the (001),
(002), (003) and (004) crystallographic planes of the single
crystal. These diffractions indicate that 5c favors an edge-on
orientation on the surface and forms lamellar structures in the
films as shown in Figure S17 in the Supporting Information. The
thin films of 5c didn’t decompose during vacuum deposition of
gold, in agreement with the fact that the decomposition
temperature of 5c (243 °C) is higher than that of 5b (153 °C) as
found from differential scanning calorimetry (DSC). When
measured under vacuum, the films of 5c functioned as n-type
semiconductors with an average electron mobility of 0.25 ± 0.09
cm2/Vs as extracted from the transfer I-V curves in the saturation
region. Figure 6 shows the output I-V curves of the best
performing device, and Figure S18 in the Supporting Information
shows the transfer I-V curves of the same device.
moiety is the major contributor; and the NICS-XY scans and
ACID plots also reveal significant antiaromaticity in the central
pyrrolo[2,3-f]indole moiety, in agreement with the 1H NMR
spectrum of 5a and the calculated diradical character of zero. It
is found that the stability of indolo[3,2-b]carbazole can be
increased by substitution at the 6 and 12 positions with aryl or
ethynyl groups, but dramatically reduced by acid in solution.
With two (4-silylethynyl)phenyl groups, 1c exhibits one-
dimensional π-π stacking and functions as an n-type organic
semiconductor in solution-processed OFETs with a field-effect
mobility of 0.25 ± 0.09 cm2/Vs for electrons.
Acknowledgements
We thank Prof. Chunyan Chi (National University of Singapore)
for calculation of diradical character (y0) and Ms. Hoi Shan Chan
(the Chinese University of Hong Kong) for the single crystal
crystallography. This work was supported by the Research
Grants Council of Hong Kong (GRF 14300217) and the
Croucher Senior Research Fellowship.
Keywords: fused-ring systems • polycycles • heterocycles •
antiaromaticity • organic semiconductors
[1]
[2]
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[3]
We contacted the eighteen commercial resources by email in Feb.
2020 for purchase of indolo[3,2-b]carbazole, and got ten replies. Seven
of them replied that it was out of stock; and three of them provided
quotation on the basis of customized synthesis. However, upon our
further request, these three commercial resources replied that they
were not able to provide any information about the appearance and
characterization data of indolo[3,2-b]carbazole because they had not
synthesized it before.
[4]
[5]
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Conclusion
[13] For examples of n-type or ambipolar organic semiconductors based on
a nitrogen-rich electron-deficient π-skeleton, see: a) Y.-Y. Liu, C.-L.
Song, W.-J. Zeng, K.-G. Zhou, Z.-F. Shi, C.-B. Ma, F. Yang, H.-L. Zhang,
X. Gong, J. Am. Chem. Soc. 2010, 132, 16349–16351; b) Z. Liang, Q.
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Tang, R. Mao, D. Liu, J. Xu, Q. Miao, Adv. Mater. 2011, 23, 5514–5518;
In summary, the above study puts forth three new
derivatives of indolo[3,2-b]carbazole, which have enabled a
comprehensive study on the electronic structure of indolo[3,2-
b]carbazole and development of
a new n-type organic
semiconductor. The crystal structures of 5b and 5c indicate that
the resonance structure with a localized p-benzoquinonediimine
5
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10.1002/anie.202001803
Angewandte Chemie International Edition
RESEARCH ARTICLE
Entry for the Table of Contents
Synthesis of new derivatives of indolo[3,2-b]carbazole has enabled a comprehensive study on the remarkable electronic structure of
indolo[3,2-b]carbazole and development of a new n-type organic semiconductor for solution-processed organic field effect transistors.
Institute and/or researcher Twitter usernames: The Miao Group
7
This article is protected by copyright. All rights reserved.