Gold Catalysis Meets Materials Science – A New Approach to π-Extended Indolocarbazoles

Article abstract of DOI:10.1002/adsc.202001123

Herein we describe a modular, convergent synthesis of substituted benzo[a]benzo[6,7]-indolo[2,3-h]carbazoles (BBICZs) using a bidirectional gold-catalyzed cyclization reaction as a key step. A building block strategy enabled the easy variation of substituents at different positions of the core structure and a general analysis of substitution effects on the materials properties of the target compounds. All BBICZs were fully characterized and their optical and electronic properties were studied experimentally as well as by computational methods. Organic thin-film transistors based on eight selected derivatives were fabricated by vacuum deposition and charge-carrier mobilities up to 1 cm²/Vs were measured. (Figure presented.).

Full text of DOI:10.1002/adsc.202001123

Title: Gold Catalysis Meets Materials Science – A New Approach to pi-
Extended Indolocarbazoles
Authors: Christoph Hendrich, Lukas M. Bongartz, Marvin Hoffmann,
Ute Zschieschang, James W. Borchert, Désirée Sauter, Petra
Krämer, Frank Rominger, Florian F. Mulks, Matthias Rudolph,
Andreas Dreuw, Hagen Klauk, and A. Stephen K. Hashmi
This manuscript has been accepted after peer review and appears as an
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to this Accepted Article as a result of editing. Readers should obtain
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content of this Accepted Article.
To be cited as: Adv. Synth. Catal. 10.1002/adsc.202001123
Link to VoR: https://doi.org/10.1002/adsc.202001123

10.1002/adsc.202001123
Advanced Synthesis & Catalysis
FULL PAPER
DOI: 10.1002/adsc.202001123
Gold Catalysis Meets Materials Science – A New Approach to
-Extended Indolocarbazoles
Christoph M. Hendrich,a Lukas M. Bongartz,a Marvin T. Hoffmann,b Ute
Zschieschang,c James W. Borchert,c Désirée Sauter†,a Petra Krämer,a Frank
Rominger+,a Florian F. Mulks‡,a Matthias Rudolph,a Andreas Dreuw,b Hagen Klauk,c A.
Stephen K. Hashmia*
a
Organisch-Chemisches Institut, Heidelberg University, Im Neuenheimer Feld 270, 69120 Heidelberg (Germany).
Fax: (+49)-6221-54-4205; e-mail: hashmi@hashmi.de (homepage: http://www.hashmi.de)
Interdisciplinary Center for Scientific Computing (IWR), Heidelberg University, Im Neuenheimer Feld 205A, 69120
b
Heidelberg (Germany).
Max Planck Institute for Solid State Research, Heisenbergstr. 1, 70569 Stuttgart (Germany).
Present address: Department for Cellular Biophysics, Max Planck Institute for Medical Research, Jahnstraße 29, 69120
c
†
Heidelberg, Germany; Institute for Physical Chemistry, Department for Biophysical Chemistry, University of
Heidelberg, Im Neuenheimer Feld 253, 69120 Heidelberg, Germany.
Present address: Center for Catalytic Hydrocarbon Functionalizations, Institute for Basic Science (IBS), Daejeon 34141,
‡
Republic of Korea; Department of Chemistry, Korea Advanced Institute of Science and Technology (KAIST), Daejeon
34141, Republic of Korea.
Crystallographic investigation.
+
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
Abstract. Herein we describe a modular, convergent
synthesis of substituted benzo[a]benzo[6,7]-indolo[2,3-
h]carbazoles (BBICZs) using a bidirectional gold-catalyzed
cyclization reaction as a key step. A building block strategy
enabled the easy variation of substituents at different
positions of the core structure and a general analysis of
substitution effects on the materials properties of the target
compounds. All BBICZs were fully characterized and their
optical and electronic properties were studied experimentally
as well as by computational methods. Organic thin-film
transistors based on eight selected derivatives were
fabricated by vacuum deposition and charge-carrier
mobilities up to 1 cm2/Vs were measured.
Keywords: Materials science; gold catalysis;
indolocarbazoles; organic semiconductors; modular
synthesis; thin-film transistors
substitutions at the various positions of the molecular
core. For example, Ong et al. introduced a 4-
octylphenyl substituent and reported a TFT mobility
of 0.14 cm2/Vs.[5] Bao and Leclerc synthesized 3,9-
di(p-octylbenzene)-5,11-dihydroindolocarbazole and
measured a carrier mobility of 0.22 cm2/Vs.[6]
Introduction
Small-molecule organic semiconductors are of
interest for a variety of electronic devices, including
organic thin-film transistors (TFTs).[1] Among the
large number of molecular motifs that have been
explored for the fabrication of organic p-channel
The fact that even small modifications to the
molecular structure can have significant effects on the
electronic properties was demonstrated by the
introduction of chloro-substituents at the 2- and 8-
positions of N-alkylated ICZs, which led to a carrier
mobility of 0.5 cm2/Vs in single-crystal transistors.[7]
In 2013, Sung et al. reported on the synthesis of 7-
ring -extended indolocarbazoles (Scheme 1), so
TFTs
are
the
indolocarbazoles
(ICZs).[2,3]
Indolocarbazoles are typically characterized by good
solubility in common organic solvents and good air
stability, which can be attributed to their low-lying
HOMO-energy level. While the performance of the
first ICZ-based TFTs was quite disappointing, with a
carrier mobility of 0.001 cm2/Vs,[4] the performance
of ICZ-based TFTs has been continuously and
significantly improved, mainly through strategic
called
benzo[a]benzo[6,7]indolo[2,3-h]carbazoles
(BBICZs), with alkyl substituents ranging in chain
length from C5 to C16.[8] The -extension led to a
1
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10.1002/adsc.202001123
Advanced Synthesis & Catalysis
carrier mobility of 1.5 cm2/Vs for transistors based on
a C12-BBICZ single-crystal. C12-BBICZ and its
non-alkylated derivative have also been used as a
hole-transporting material in perovskite solar cells[9]
and as a donor in solution-processed organic
semiconductors.[10]
especially effective in which polyynes serve as
starting materials.[14] As an example, the well-known
gold-catalyzed indole synthesis from Utimoto et al.
served as a first step of a cascade reaction, in which
this nucleophilic unit was further transferred to a
larger -system by one or more additional alkyne
moieties that react in a nucleophilic cascade towards
the extended -systems.[15] Additionally, bidirectional
approaches and cascade cyclizations can be combined,
which has already been demonstrated by Yamamoto
et al. and our group.[16] In the present study, we
present a new access to -extended indolocarbazoles
by a gold-catalyzed bidirectional tandem cyclization
as crucial step. Tetraynes 1a-g, which serve as
starting materials for the subsequent gold-catalyzed
step, were made accessible by a modular building
block principle which opens up a highly variable,
convergent synthetic approach towards differently
substituted derivatives of the target molecules
(Scheme 3).
Scheme 1. -Extension of indolocarbazoles.
What is currently lacking is a systematic analysis
of the effects of the substitution patterns on the
material properties of -extended indolocarbazoles.
This is in part due to the fact that the traditional
synthesis involves a double Fischer indole reaction of
3,4-dihydronaphthalen-1(2H)-one and Cbz-protected
1,4-dihydrazineylbenzene that is characterized by
rather harsh conditions and requires precursors with
limited commercial availability (Scheme 2). In order
to conduct a deeper study on the effect of substitution
manipulations on the electronic properties of BBICZs,
as a missing link to the already mentioned current
research on ICZs, we envisioned that a modular
approach using a gold-catalyzed key step might
contribute to a further evolution of this promising
substrate class.[11]
Scheme 3. Building block principle for the BBICZ-
synthesis.
The diamine building block A was synthesized
through
a modified literature procedure from
Lepoittevin et al. in three steps starting from p-
diaminobenzene in excellent yield (scheme 4).[17]
This building block acts as the inner core and is
therefore part of all BBICZs.
Scheme 2. Previous approach towards BBICZs and our
strategy.
Results and Discussion
Scheme 4. Synthesis of building blocks A and B.
In contrast to the established approaches to
indolocarbazoles, we reasoned an entirely new
approach for the synthesis of BBICZs.[12] Numerous
publications during the past decade impressively
demonstrated gold catalysis as an effective tool for
the formation of extended -systems.[13] In this
context, cascade cyclizations turned out to be
The diyne-building block B was derived from
different
commercially
available
2-
bromobenzaldehydes (Ba) and terminal alkynes (Bb).
A Sonogashira cross-coupling, followed by a
Seyferth-Gilbert homologation with the Bestmann-
2
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10.1002/adsc.202001123
Advanced Synthesis & Catalysis
Ohira reagent or a Corey-Fuchs strategy (for
R1 = TMS) delivered this subunit (overall
7
examples). By using the chlorinated 2-bromo-4-
chloro-benzaldehyde, it was also possible to
introduce a hexyl substituent as an example for an
alkyl chain at the R2-position through a Suzuki
reaction with hexylboronic acid. Based on the
availability of many different alkynes and 2-
bromobenzaldehydes in combination with mild
reaction conditions, building block B turned out to be
the most variable one in our strategy. In accordance
with already reported procedures, the R3 substituent
from building block C was introduced by a late stage
modification of the N-unprotected molecules. Alkyl
groups with different chain lengths turned out to be
the most suitable units.
By combining the two building blocks A and B via
a bidirectional Sonogashira reaction, tetraynes 1a-g
were achieved in reasonable to excellent yields from
51-88% (scheme 5). Only in the case of TMS-1f
(R1 = TMS, R2 = Cl), the yield dropped down to 27%.
Owing to their amino-functionality and their
moderate solubility, purification of these -extended
alkyne systems was challenging at first, but simple
filtration over a short pad of silica gel and subsequent
recrystallisation/precipitation from EA or an EA/PE-
mixture turned out to be a simple way to furnish the
compounds in high purity
Scheme 6. Gold-catalyzed conversion of tetraynes 1a-g.
For 1a and 1b 10 mol% IPrAuNTf2 were used. For TMS-
1c, 29% of deprotected 2c was obtained.
It is noteworthy, that it was also possible to
synthesize BBICZs with R1 = H (2c) via the
cyclization of the corresponding terminal alkynes.
These are of particularly interest, because cross-
couplings like Sonogashira reaction are once more
feasible at this stage of the sequence. To demonstrate
this approach, 1a was synthesized by a Sonogashira
reaction from 1c with 2 eq of iodobenzene in 74%
yield (scheme 7).
Scheme 7. Gold-catalyzed conversion of tetrayne 1c and
further Sonogashira-modification.
Interestingly, the TMS-protected tetrayne TMS-1c
furnished the desilylated BBICZ 2c in 29% yield
after the gold-catalyzed step. The reason for this fact
can be explained by the isolobal-analogy of LAu+ and
a proton combined by the use of non-dry solvents for
the gold catalysis.[19,20] To cleave the Boc-protection
group, which was introduced for synthetic reasons at
the beginning of the synthesis, we applied a modified
procedure of Cava and Bergman.[21] This thermal
deprotection worked quantitatively under vacuum at
temperatures above 200 °C without any solvent.
Usually no further purification was needed (scheme
8). Other attempts using more classic deprotection
methods like acidic cleavage with TFA or HCl in
dioxane resulted in difficult workup or low
conversion.
The work by Sung et al. already demonstrated a
dependence of the product solubility from the length
of the alkylchain.[8] In this study, the dodecyl chain
gave the best results, hence all synthesized BBICZs
were alkylated analogously following Sung’s
protocol with aqueous NaOH and dodecylbromide in
anhydrous DMSO. For 4a, 4b and 4e, THF was used
as solvent due to higher solubility of the
corresponding starting materials. 3d, 3f and 3g were
alkylated using KOtBu instead of a NaOH solution as
Scheme 5. Combination of building blocks A and B via a
bidirectional Sonogashira reaction. For 1a Pd(PPh3)4 was
used instead of PdCl2(PPh3)2.
The tetraynes themselves might appear as
promising materials as well.[17] Analyzing the optical
properties (absorption and emission spectroscopy) of
the respective compounds revealed fluorescence
quantum yields of up to 68%, indicating potential
applicability in organic devices (for absorption and
emission spectra see SI). Scheme 6 shows the crucial
gold-catalyzed cyclization step of tetraynes 1a-g to
Boc-BBICZs 2a-g. In general, 5 mol% of the
commercially available IPrAuNTf2 served as catalyst
in 1,2-dichloroethane at 60 °C over night. Overall, 6
new bonds are formed during the reaction in a single
step. Products 2a-g were either isolated via common
flash column chromatography or, for compounds 2c-f
which precipitate due to the formation of the large
planar -system causing low solubility, by simple
filtration of the product from the reaction mixture.
3
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10.1002/adsc.202001123
Advanced Synthesis & Catalysis
base. Following the same strategy it was also possible
to introduce a methyl-group (4h). Remarkably, R1
seems to be crucial for the solubility of 3a-g as well.
For R1 = Ph and iPr (3a, 3d), the products are soluble
in common organic solvents such as chloroform or
dichloromethane, whereas all other non-alkylated
BBICZs were only soluble in THF or DMSO. In
contrast, all alkylated BBICZs showed a decent
solubility in chloroform and dichloromethane.
Overall 15 different BBICZs were synthesized in
good to excellent yields.
Figure 1. X-ray structure with --stacking of the
naphthyl ends of 3a. Left: top view, right: side view.
X-Ray structures for BBICZs 3a, 3b, 3d and the
alkylated 4a and 4f were obtained.[22] The solid state
molecular structures are in excellent agreement with
expected bond lengths and angles.[8] It should be
noted that for the non alkylated 3a-g in most cases a
solvent molecule is part of the determined solid state
structure (except for 3d). For 3a the molecules show
--stacking of the naphthyl ends between each other,
which leads to one-dimensional chains in direction of
the molecule plane (Figure 1). In between there is one
solvent molecule (DCM). For 3b and 3d no -
stacking was observed in the solid state structure. 3b
forms parallel columns, but one solvent molecule
(DMSO) and the alkylchains prevent decent -
stacking. 3d also forms columns; however, molecules
are strongly laterally shifted against each other so that
there is hardly any -contact. The alkylated 4a forms
one-dimensional columns perpendicular to the
molecular plane with significant -stacking. 4f shows
a similar behaviour.
Figure 2. Absorption and emission spectra of 3a-g (left, in
THF) and 4a-h (right, in DCM). All emission spectra were
normalized to the maximum emission signal.
Scheme 8. Boc-deprotection to the final BBICZs 3a-g and alkylation methods to get 4a-h. Method A: aqueous NaOH,
benzyltriethylammonium chloride (25 mol%), DMSO; method B: KOtBu (4 eq), DMSO.
Table 1. Yields of gold cyclization and final alkylation.
Starting material
R1
R2
Yield of the gold Method of
Yield of the
alkylation
catalysis
alkylation
Ph
H
H
H
H
54%
82%
29%a)
74%
A
A
/
92%
68%
/
77%
60%b)
88%
58%
75%
85%
1a
1b
TMS-1c
1c
Hexyl
TMS
H
A
Bb)
B
iPr
H
H
Cl
Hexyl
78%
53%
82%
73%
1d
1e
1f
Cyclopentyl
H
H
Ac)
B
B
1g
a)
b)
c)
29% of 2c was obtained. methyl (=4h) by using MeI instead of dodecylbromide. THF was used due to better
solubility.
Our next efforts concentrated on the evaluation of
the optical and redox properties of the obtained
BBICZs 3a-g, the alkylated BBICZs 4a-h and the
Boc-protected derivatives (2, see Supporting
Information). Among themselves, 3a-g and 4a-h
show quite similar absorption spectra with maxima of
4
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10.1002/adsc.202001123
Advanced Synthesis & Catalysis
395-406 nm and 417-425 nm, respectively (Figure 2).
It is striking that while a hexyl substitution at the
3,11-position of derivatives 3g/4g leads to the most
hypsochromice shift, the same substituents attached
to the 6,14-position only have a minor influence on
the absorption signal maximum. A similar effect was
observed for the fluorescence quantum yields (QY).
All 6,14-substituted BBICZs show similar quantum
yields varying between 33-37% for 3a-g and 24-36%
for 4a-h. However, 3,11-substitution has a notable
greater influence. 3f/4f with the electron-withdrawing
chloro substituent show the lowest QY (15% for 3f,
14% for 4f), while the hexyl-substituent led to the
highest yield with 58% (3g) and 44%, respectively
(4g).
Cyclic voltammograms were recorded for all
dichloromethane soluble compounds, namely 3a, 3d
and all alkylated 4a-h (see Supporting Information
for more details). All compounds show two reversible
oxidation potentials. For 3a and 3d, the second
oxidation potential is almost the same (774 mV vs.
739 mV), but a big difference in the first oxidation
potential can be detected (125 mV vs. 317 mV). The
alkylated compounds 4a-h show greater differences
between both potentials. 4f and 4a with an electron-
withdrawing chloro, respectively an aryl substituent,
have similar and high values for both oxidation states
(321-333 mV; 919-934 mV). In contrast the values of
the alkyl-substituted (4b, 4d, 4e, 4g) vary much
stronger (11-240 mV; 646-981 mV).
details see SI) of selected compounds were conducted
next, followed by device fabrications of the most
notable compounds. Calculation of the optical
properties using time dependent density functional
theory (TDDFT) illuminated the character of the
emitting excited state being mainly unchanged and
derived from the parent molecule 3c. Thus, in the
case of 3c, a bright -* transition (oscillator strength
of 0.23) constitutes the first excited singlet state,
classified as the typical Ls state of N-
heteropolycyles,[23] exhibiting a strongly correlated
electron-hole pair (exciton) with a pronounced
correlation coefficient of 0.56. The excitation energy
(optical gap) for 3c of 3.93 eV is in the UV-A region.
The effect of alkylation at the carbon skeleton (e.g.
for 3b) on the photophysical properties is only
marginal, except a change in oscillator strength with
3g exhibiting an almost twice as large oscillator
strength of the first excited singlet state (0.28)
compared to 3b (0.14). The difference in the coupling
of the S1 and S0 states is in line with the
experimental findings of an increased quantum yield
for the 3,11-substitution pattern in contrast to the
6,14-position. The addition of additional -systems as
substituents, i.e. in 3a, results mainly in the reduction
of the exciton binding energy (difference between
fundamental and optical gap) from 2.15 eV (3c) to
1.99 eV. An alkyl substituent (3d) shows the same
trend, but less pronounced, with an exciton binding
energy of 2.04 eV. Generally, the alkylation of the
amino groups (4a and 4d) leads to a decreased
exciton binding energy (1.95 eV and 1.98 eV) as well
as a smaller optical gap (3.78 eV and 3.80 eV).
In order to get a better understanding of
substitution effects on optical and electronic
properties, theoretical calculations (computational
Table 2. Overview of quantum yield (QY), photophysical and electrochemical properties of 3a-g and 4a-h, including
theoretical and device properties of selected BBICZs. abs, max = maximum of the longest absorption wavelength; em
=
maximum of the shortest emission wavelength, Eg,cal = calculated optical Gap, IPcal = calculated ionization potential, EAcal
= calculated electron affinity, cal = reorganization energy, FGcal = fundamental gap, µh = hole transport mobility.
IPcal/
EAcal
[eV]
6.32/0.43 100.94 5.88
FGcal
[eV]
Compound abs,max em
Eg,opt
QY
Ox-pot. Eg,cal
[mV]b)
[eV]
cal
[meV]
µh [cm2/Vs]
[nm]
404
[nm]
[eV]a)
Silicon
PEN
substrate substrate
414
2.99
37%
317/774 3.89
3×10-5
No field
effect
0.01
3a
3bc)
3c
3d
405
397
406
406
404
395
425
423
420
423
413
403
413
414
413
400
434
430
420
425
2.95
2.99
2.98
2.98
2.99
3.06
2.82
2.83
2.88
2.83
36%
15%
34%
33%
24%
58%
36%
24%
36%
30%
3.93
3.93
125/739 3.91
6.36/0.34 93.42
6.44/0.37 100.52 6.07
6.33/0.38 96.48 5.95
6.02
0.1
1
0.5
0.003
0.008
3e
3f
3gc)
4ac)
4b
3.94
333/934 3.78
11/646
6.32/0.29 116.23 6.03
6.11/0.38 84.17
0.1
0.2
0.05
0.02
4cc)
4dc)
0.02
0.002
240/650 3.80
6.16/0.38 96.30
No field 0.001
effect
424
422
417
418
431
429
425
425
2.82
2.86
2.91
2.87
33%
14%
44%
41%
195/875
321/919
130/981
4e
4f
4g
4h
a) from onset of the absorption spectra. b) from cyclic voltammographic measurements. alkyl chains were simplified
c)
using a methyl group.
5
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10.1002/adsc.202001123
Advanced Synthesis & Catalysis
The calculations provide an additional puzzle piece
channel transistors. The best TFT performance was
obtained for the parent compound BBICZ 3c, which
showed effective charge-carrier mobilities of
1 cm2/Vs in TFTs fabricated on a silicon substrate
and 0.5 cm2/Vs in TFTs fabricated on flexible PEN.
These mobilities are larger compared to the literature
value of 0.21 cm2/Vs reported previously for this
compound.[10e] In contrast to the theoretical
calculations, which predict carrier mobilities larger
than that of 3c for some of the derivatives, our
measurements of TFTs based on 3a, 3b, 3d, 3g, 4a,
4c and 4d all yielded mobilities smaller than that of
3c. For example, hexyl substitution at the carbon
skeleton led to a mobility of 0.1 cm2/Vs, regardless of
the position of the substituents (3b and 3g).
Especially for 3g we expected a higher value, based
on previous results on similar substituted DNTTs
(dinaphtho[2,3-b:2,3‘-f]thieno[3,2-b]thiophenes).[26]
However, the morphology of the formed layer and so
the obtained value are dependent on the fabrication
process, which might be improved further.
Phenyl (3a) or isopropyl (3d) substitution at the
6,14-positions of the N-unsubstituted compounds
produced mobilities below 0.01 cm2/Vs. Interestingly,
a phenyl substitution at the 6,14-positions in
combination with a dodecyl substitution at the
nitrogen atoms (4a) led to carrier mobilities of
0.2 cm2/Vs on silicon and 0.02 cm2/Vs on flexible
PEN substrates. The fact that these mobilities are
larger than those of the N-unalkylated counterparts
(3a and 3d) is consistent with the theoretical
predictions which indicate a beneficial effect of N-
alkylation. In contrast, this effect was not observed
for the isopropyl derivate (4d), which completely
falls out of the series with a carrier mobility of just
0.001 cm2/Vs on PEN and without yielding a field
effect on a silicon substrate.
to understand structure-property relationships and
therefore assist to uncover the potential of deliberate
tuning of desired electronic and optical properties.
The fundamental gap (single-molecule band gap),
being the difference of non-adiabatic (vertical)
ionization potential and electron affinity, gives an
indication of the energetic barrier of charge transport
(creating and eliminating a charge carrier). Therefore,
a smaller fundamental gap should have a positive
effect on the charge carrier mobility. Furthermore, a
decreasing reorganization energy, as defined within
Marcus theory (see SI), leads to an acceleration of an
electron transfer process. While the fundamental gap
turned out to be invariant from the substitution
pattern of compounds 3b and 3g (3,11- vs. 6,14-hexyl
substitution, Table 3), a significant effect on the
reorganization energy was calculated. While the 3,11-
derivative showed a higher reorganization energy
(116.23 meV) compared to 3c (100.52 meV), the
6,14-derivative showed a lower one (93.42 meV).
The type of substituent on the 6,14-position with
either being an alkyl 3b or an aryl type substituent 3a,
influences mainly the electron affinity (0.43 eV for
3a and 0.34 eV for 3b) leading to a decrease in the
fundamental gap in the case of 3a (5.88 eV). The
difference in reorganization energy is relatively
marginal in comparison with 3b exhibiting slightly
smaller reorganization energy (93.42 meV) compared
to 3a (100.94 meV). Varying the type of alkyl
substituent (3b compared to 3d) has insignificant
influence on electronic properties indicating the steric
load being responsible for varying measured charge
carrier mobilities. The most notable change happens
in the case of alkylation (methylation) of the amino
group, with the 4a derivative showing a significantly
lower reorganization energy (84.17 meV) compared
to its non-alkylated counterpart 3a (100.64 meV). In
addition, the ionization potential decreases from
6.32 eV to 6.11 eV which consequently leads to a
smaller fundamental gap of 5.73 eV. The same trend
can be observed for the alkylated substituents (3d
compared to 4d). However, the steric hindrance due
to the bulkiness of the iPr group (3d/4d) should again
be the predominant factor in resulting thin film
morphologies and subsequently measured mobilities.
Consequently, the substitution pattern of 4a should
have the most positive influence, as both, the
fundamental gap as well as the reorganization energy
are the lowest calculated and the ability for -
stacking in aggregation should remedy the increased
sterics due the aryl substituent.
To investigate the charge-transport properties of
BBICZ 3c and its alkylated and non-alkylated
derivatives 3a, 3b, 3d, 3g, 4a, 4c and 4d, we
fabricated TFTs in the inverted staggered (bottom-
gate, top-contact) device architecture on heavily
doped silicon substrates[24] and on flexible
polyethylene naphthalate (PEN) substrates[25] (Figure
3). The organic semiconductors were deposited by
thermal sublimation in vacuum (for more details see
the Supporting Information). All TFTs operate as p-
Figure 3. Above: Schematic cross-section of organic TFTs
fabricated on silicon substrates. Below: Schematic cross-
section of organic TFTs fabricated on polyethylene
naphthalate (PEN) substrates.
For the previously reported 4c we measured a
mobility of 0.02 cm2/Vs, which is substantially
6
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10.1002/adsc.202001123
Advanced Synthesis & Catalysis
smaller compared to the value of 1.5 cm2/Vs reported
by Sung et al. for a transistor based on solution-
grown single-crystals of this compound.[8] These
results serve as a reminder that the specific crystal
packing for which theory may predict a large carrier
mobility is not always identical to the crystal packing
obtained in thin films, especially when these are
prepared by vacuum deposition, which typically
provides less control over the crystallization process
than some solution-based deposition methods. In
other words, while vacuum deposition can yield high
mobilities for molecules with a simple rod-like shape
(such as 3c), it is often less suitable for molecules
with sterically demanding substituents, which means
that some of the BBICZ derivatives presented here
might perform better if processed using an optimized
solution-based protocol. Nevertheless, the fact that
we measured carrier mobilities between 0.1 and
1 cm2/Vs for four different compounds (3b, 3c, 3g,
4a) confirms that BBICZ is indeed an interesting
molecular motif for future device-fabrication efforts.
Scheme 9. Overview of BBICZs used for fabrication of TFTs.
deposition. Such compounds may provide better
performance when processed into single-crystals or
from solution, much like TIPS pentacene for which
the best mobilities obtained by solution processing
are significantly larger than the best mobilities
obtained by vacuum deposition. More importantly,
the modular synthetic approach presented here will
greatly accelerate the rapid screening of this class of
compounds for materials science. Further screenings
of this class of compounds is ongoing in our
laboratories.
Conclusion
We present a new highly modular, bidirectional
synthetic strategy to -extended indolocarbazoles via
a variable building block principle. The starting
materials for this route are easily accessible by well-
known procedures. Bidirectional Sonogashira cross-
coupling, followed by gold catalysis as a key step and
a final quantitative thermal deprotection, provides
access to BBICZs in very good yields. In total, 13
new BBICZ derivatives were synthesized, and their
optical, electronic and redox properties were studied.
The solid-state structures of some of the compounds
were also obtained. Based on these BBICZs, we
analyzed a number of potentially useful structure-
property relationships. The effects of different
substitution patterns on the electronic properties of
the molecules were studied theoretically and by
fabricating TFTs, in which we measured carrier
mobilities up to 1 cm2/Vs. The measurements
confirmed some of the trends predicted by the
theoretical calculations regarding the effect of certain
substitution patterns on the charge-carrier mobility,
such as the beneficial effect of N-alkylation. In
general, however, we measured smaller carrier
mobilities in the substituted BBICZs compared to the
unsubstituted parent compound 3c, which is partly in
contradiction to the results of the theoretical analysis.
This might be due to the fact that certain substitution
patterns make it difficult, if not impossible, to obtain
a favorable thin-film morphology by vacuum
Experimental Section
General Procedure for the Gold Catalysis
1.00 eq of the corresponding alkyne was dissolved in DCE.
5 mol% IPrAuNTf2 were added and the mixture was stirred
at given temperature for the given time until TLC showed
full conversion. The solvent was removed under reduced
pressure and the crude product was purified by flash
column chromatography.
General Procedure for the thermal Boc deprotection
In a flask with vacuum-connection the Boc-protected
BBICZ was heated up to 200-220 °C on a vacuum of 5*10-
3 mbar for 3-5 h. The progress of the reaction could be
determined by colour change and meantime decrease of the
applied vacuum.
Di-tert-butyl
6,14-diphenylbenzo[a]benzo[6,7]-
indolo[2,3-h]carbazole-8,16-dicarboxylate (2a):
7
This article is protected by copyright. All rights reserved.

10.1002/adsc.202001123
Advanced Synthesis & Catalysis
According to the general procedure, 1.00 eq of the
corresponding tetrayne (100 mg, 141 µmol) was dissolved
in 20 mL DCE and 10 mol% IPrAuNTf2 (12.2 mg,
14.1 µmol) were added to the solution. After stirring over
3 d at 60 °C, the mixture was treated according to general
procedure (silica gel, PE:EA = 50:1). A beige solid was
obtained (54.0 mg, 76.2 µmol, 54%).
14, 868-874; b) T. Janosik, N. Wahlström, J. Bergman,
Tetrahedron 2008, 64, 9159-9180; c) M. Zhao, B.
Zhang, Q. Miao, Angew. Chem. Int. Ed. 2020, 59,
9678-9683; Angew. Chem. 2020, 132, 9765-9770.
[4] S. Wakim, J. Bouchard, M. Simard, N. Drolet, Y. Tao,
M. Leclerc, Chem. Mater. 2004, 16, 4386-4388.
Mp.: decomp. >192 °C; Rf: 0.27 (silica gel, PE:EA = 10:1);
IR (ATR): 푣̃ [cm-1] = 3056, 2979, 2932, 1731, 1495, 1438,
1392, 1368, 1339, 1286, 1254, 1144, 1122, 1065, 878, 852,
832, 784, 864, 742, 698, 643, 617; 1H NMR (CD2Cl2,
600.2 MHz): δ[ppm] = 1.49 (s, 18H), 7.52-7.57 (m, 4H),
7.62-7.64 (m, 6H), 7.67 (s, 2H), 7.72-7.74 (m, 4H), 7.97 (d,
3JH-H = 7.6 Hz, 2H), 8.18-8.19 (m, 4H); 13C NMR (CD2Cl2,
150.9 MHz): δ[ppm] = 28.0 (q, 6C), 84.8 (s, 2C), 107.8 (d,
2C), 122.1 (s, 2C), 123.0 (s, 2C), 125.2 (d, 2C), 125.4 (d,
2C), 125.5 (s, 2C), 126.1 (d, 2C), 126.5 (d, 2C), 128.5 (d,
2C), 129.0 (d, 2C), 129.2 (d, 4C), 129.7 (d, 4C), 133.3 (s,
2C), 135.8 (s, 2C), 136.9 (s, 2C), 137.7 (s, 2C), 140.8 (s,
[5] a) Y. Li, Y. Wu, S. Gardner, B. S. Ong, Adv. Mater.
2005, 17, 849-853; b) Y. Wu, Y. Li, S. Gardner, B. S.
Ong, J. Am. Chem. Soc. 2005, 127, 614-618.
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Bao, M. Leclerc, J. Mater. Chem. 2009, 19, 2921-2928.
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Q. Meng, W. Hu, J. Mater. Chem. 2012, 22, 4409-4417
+
[8] K. S. Park, S. M. Salunkhe, I. Lim, C. G. Cho, S. H.
2C), 152.0 (s, 2C); HRMS (MALDI+): C48H40N2O4 ,
calculated: 708.2983 [M+], observed: 708.2983 [M+];
UV/VIS (DCM, 2.20 µg/mL): λ[nm] (logε) = 238 (4.87),
272 (4.79), 298 (4.71), 335 (4.74), 348 (4.79), 365 (4.62),
Han, M. M. Sung, Adv. Mater. 2013, 25, 3351-3356.
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RSC Adv. 2015, 5, 55321–55327.
380 (4.43); Fluorescence (DCM): λAnr = 380 nm, λmax
394 nm; Quantum yield: Ф = 52.5%.
=
6,14-Diphenyl-8,16-dihydrobenzo[a]benzo[6,7]-
[10] a) P. J. Jeon, K. Lee, E. Y. Park, S. Im, H. Bae, Org.
Electron. 2016, 32, 208-212; b) H. S. Lee , J. M. Shin ,
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ACS Appl. Mater. Interfaces 2016, 8, 8637-8643; d) J.
Lee, S. R. A. Raza, P. J. Jeon, J. S. Kim, S. Im, NPG
Asia Mater. 2016, 8, e278; e) J. H. Park, H. S. Lee, S.
Park, S.-W. Min, Y. Yi, C.-G. Cho, J. Han, T. W. Kin,
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indolo[2,3-h]carbazole 3a: According to the general
procedure, the Boc-protected BBICZ (137 mg, 193 µmol)
was heated up to 200 °C for 4 h. The deprotected BBICZ
was obtained as yellow solid in quantitave yields (98.2 mg,
193 µmol).
Mp.: >300 °C; IR (ATR): 푣̃ [cm-1] = 3429, 3052, 3027,
1566, 1520, 1493, 1452, 1398, 1370, 1343, 1309, 1280,
1225, 1152, 1103, 1072, 1027, 935, 911, 838, 782, 766,
740, 701, 670, 615; 1H NMR (CD2Cl2, 600.2 MHz):
δ[ppm] = 7.49 (s, 2H), 7.55-7.57 (m, 4H), 7.60-7.63 (m,
4H), 7.65-7.67 (m, 4H), 7.77-7.79 (m, 4H), 8.01 (d, 3JH-H
=
3
8.0 Hz, 2H), 8.16 (d, JH-H = 8.0 Hz, 2H), 8.88 (s, 2H); 13C
NMR (CD2Cl2, 150.9 MHz): δ[ppm] = 102.7 (d, 2C),
116.4 (s, 2C), 120.5 (s, 2C), 120.6 (d, 2C), 120.9 (d, 2C),
123.2 (s, 2C), 125.8 (d, 2C), 126.1 (d, 2C), 128.0 (d, 2C),
128.9 (d, 4C), 129.1 (d, 2C), 129.8 (d, 4C), 132.5 (s, 2C),
135.1 (s, 2C), 136.9 (s, 2C), 137.2 (s, 2C), 141.8 (s, 2C);
[11] H. Jiang, P. Hu, J. Ye, A. Chaturvedi, K. K. Zhang, Y.
Li, Y. Long, D. Fichou, C. Kloc, W. Hu, Angew. Chem.
Int. Ed. 2018, 57, 8875-8880; Angew. Chem. 2018, 130,
9013-9018.
+
HRMS (DART+): C38H25N2 , calculated: 509.2012
[12] J. Bintinger, S. Yang, P. Fruhmann, B. Holzer, B.
Stöger, A. Svirkova, M. Marchetti-Deschmann, E.
Horkel, C. Hametner, J. Fröhlich, I. Kymissis, H.
Mikula, Synth. Met. 2017, 228, 9-17.
[M++H], observed: 509.2005 [M++H]; UV/VIS (THF,
6.10 µg/mL): λ[nm] (logε) = 238 (4.69), 274 (4.72), 320
(4.57), 342 (4.74), 355 (4.77), 391 (4.16), 410 (4.10);
Fluorescence (DCM): λAnr = 400 nm, λmax = 414 nm,
439 nm, 464 nm; Quantum yield: Ф = 36.8%.
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Acknowledgements
The authors are grateful to funding by the DFG (SFB
1249‐ N‐ Heteropolyzyklen als Funktionsmaterialien).
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9
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Advanced Synthesis & Catalysis
FULL PAPER
Gold Catalysis Meets Materials Science – A New
Approach to -Extended Indolocarbazoles
Adv. Synth. Catal. Year, Volume, Page – Page
Christoph M. Hendrich, Lukas M. Bongartz,
Marvin T. Hoffmann, Ute Zschieschang, James W.
Borchert, Désirée Sauter, Petra Krämer, Frank
Rominger, Florian F. Mulks, Matthias Rudolph,
Andreas Dreuw, Hagen Klauk, A. Stephen K.
Hashmi*
10
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