Azaindolo[3,2,1-jk]carbazoles: New Building Blocks for Functional Organic Materials

Article abstract of DOI:10.1002/chem.201805578

The preparation and characterization of 12 azaindolo[3,2,1-jk]carbazoles is presented. Ring-closing C?H activation allowed for the convenient preparation of six singly and six doubly nitrogen-substituted indolo[3,2,1-jk]carbazole derivatives in which ten

Full text of DOI:10.1002/chem.201805578

A Journal of
Accepted Article
Title: Azaindolo[3,2,1-jk]carbazoles: New Building Blocks for Functional
Organic Materials
Authors: Thomas Kader, Berthold Stöger, Johannes Fröhlich, and Paul
Kautny
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Azaindolo[3,2,1-jk]carbazoles: New Building Blocks for
Functional Organic Materials.
Thomas Kader,[a] Berthold Stöger,[b] Johannes Fröhlich,[a] and Paul Kautny*[a]
Abstract: The preparation and characterization of twelve
azaindolo[3,2,1-jk]carbazoles is being presented. Ring closing C-H
activation allowed for the convenient preparation of six mono and six
twofold nitrogen substituted indolo[3,2,1-jk]carbazole derivatives
whereupon ten of the materials have not been described in literature
before. The detailed photophysical and electrochemical
characterization of the developed materials revealed a significant
impact of the incorporation of pyridine like nitrogen into the fully
planar indolo[3,2,1-jk]carbazole backbone. Furthermore, the nitrogen
position decisively impacts intermolecular hydrogen bonding and
thus the solid state alignment. Ultimately, the versatility of the
azaindolo[3,2,1-jk]carbazoles scaffold suggests this class of
materials as an attractive new building block for the design of
functional organic materials.
new synthetic strategy based on modern C-H activation renders
the wider application of this building block possible.23,32
Compared to TPA and phenylcarbazole (PCz) ICz is a weaker
electron donor (Chart 1). Owing to the increasing planarization,
the electron donating power of the triarylamines significantly
decreases from TPA to PCz and ICz. This gradual decrease of
donor strength is due to the contribution of the lone pair of the
nitrogen to the aromaticity of the pyrroles formed by
planarization. Therefore, the lone pair is tighter bound to the
arylamine core and less prone to be delocalized in a donor-
acceptor molecule.24 Effectively, the ICz moiety can exhibit light
acceptor properties itself as evidenced in bipolar host
materials.23,33 Furthermore, the ICz building block has been
utilized as weak electron acceptor in thermally activated delayed
fluorescent (TADF) emitters.34 Increasing the electron accepting
strength by substitution of the ICz scaffold with electron
withdrawing cyano groups enabled the preparation of pure blue
TADF emitters.35
Introduction
Based on these results and the growing interest regarding the
application of the indolo[3,2,1-jk]carbazole as building block for
optoelectronic materials, we set out to further expand the
concept of planarization to intrinsically modulate the acceptor
strength by incorporation of electron withdrawing pyridine like
nitrogen atoms into the ICz moiety (Chart 1; henceforth the
atoms in the nitrogen substituted ICzs (NICzs) will be numbered
according to the original numbering of ICz, whereby the
numbers specify the positions of nitrogen atoms). The concept
of heteroatom doping is widely employed in silicon based
semiconductor technology. Analogously, the incorporation of
heteroatoms into polycyclic aromatic scaffolds has been
demonstrated to effectively modify the photophysical and
electrochemical properties of the parent material.36-37 In
particular nitrogen incorporation into acenes has been
successfully realized to induce n-type charge transport
properties in materials for OFET applications.38-40 Moreover,
carbolines, which contain an electron deficient pyridine unit,
were successfully employed as electron transporting units in
host materials for efficient phosphorescent OLEDs.41-42 Notably,
the exact position of the nitrogen atom within the carboline
scaffold significantly impacted the molecular properties of the
materials and the device efficiency.42-43
The development of π-conjugated small molecules with tailored
molecular properties has been one of the major driving forces for
the rapid development of the field of organic electronics over the
last decades.1-2 In particular small to medium sized polycyclic
(hetero)aromatic molecules with a defined molecular structure
are of tremendous importance.2-6 Accordingly, many materials
based on these building blocks have been developed for
applications in Organic Light Emitting Diodes (OLEDs),2,7-12
Organic Field Effect Transistors (OFETs),1-2,6-7,13-15 Organic
Photovoltaics (OPVs),2,16-19 and sensing technology11,20-22 to
name a few. As a consequence, there is an ongoing quest for
novel fused aromatic moieties to fulfill the particular
requirements of the respective technological application.2-3,12
Recently we have introduced indolo[3,2,1-jk]carbazole (ICz) as
new building block for optoelectronic materials.23-26 ICz can be
considered a fully planarized derivative of triphenylamine (TPA).
Although arylamines are widely employed as electron donating
moieties, reports on the application of ICz have been scarce for
a long time,27-29 owing to the elaborate preparation of the
planarized scaffold (e.g. vacuum flash pyrolysis).30-31 However, a
[a]
[b]
T. Kader, Prof. Dr. J. Fröhlich, Dr. P. Kautny
Institute of Applied Synthetic Chemistry
TU Wien
Getreidemarkt 9/163, A-1060, Vienna, Austria
E-mail: paul.kautny@tuwien.ac.at
Dr. B. Stöger
Besides the electronic effects, also the structure directing
potential of nitrogen atoms by means of weak CH···N hydrogen
bonds can be employed to induce intra-44-45 or intermolecular46-47
interactions in order to control optical or electronic properties of
the organic materials.48
Consequently, the NICzs are an interesting class of materials. A
reliable synthetic access to this scaffold would allow for the
intriguing possibility to fine-tune the molecular properties of
optoelectronic materials by the subtle variation of nitrogen
X-Ray Centre
TU Wien
Getreidemarkt 9, A-1060, Vienna, Austria
Supporting information for this article is given via a link at the end of
the document.
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Planarization
N-Content
Donor-Strength
Acceptor-Strength
Chart 1. Schematic representation of the concept of planarization of the triarylamine framework and nitrogen incorporation to control donor and acceptor strength.
position and content of the NICz building block. Notably, three
NICz derivatives have already been prepared, namely 5NICz,
7NICz, and 7,9NICz, which are accessible as single
regioisomers by vacuum flash pyrolysis.49 However, all other
NICzs are unknown to literature.
In this work we present a comprehensive synthetic approach
towards all possible mono-, as well as six twofold substituted
NICzs employing a convenient C-H activation reaction. This
strategy allows control of the substitution pattern of the NICz by
choice of substrate as well as the regioselectivity of the ring
closing reaction by electronic manipulation of the nitrogen atom
by oxidation. Ultimately, our approach enables the preparation of
a full set of NICz building blocks with tunable photophysical and
electrochemical properties.
desymmetrization of the ICz scaffold by nitrogen incorporation.
Therefore, we opted to investigate different approaches for the
synthesis of these precursors (Scheme 1). These different
strategies allow not only for the convenient synthesis of the
required precursors but also pave the way for the future
preparation of substituted NICzs as well as for the incorporation
of the NICz moiety into larger π-conjugated systems. The
pursued approaches can be divided into two groups and the
results of the respective reactions towards NICz precursors are
summarized in Table 1.
The first strategy relies on the establishment of a carbazole unit
attached to a halogen substituted pyridine. Notably, the outcome
of the cyclization reaction for these precursors is solely
determined by the substitution pattern of the pyridine.
Accordingly, this strategy allows for the preparation of NICzs
with the nitrogen atom incorporated in the peripheral benzene
unit of the ICz scaffold (position 4-7). The most straightforward
preparation of the respective precursors is a nucleophilic
aromatic substitution of a properly substituted dihalogenated
pyridine (Scheme 1). Following this approach the bromo-
precursors bromo-5PCz and bromo-7PCz were obtained in 69
and 37% yield by heating the according bromochloropyridines
and carbazole in the presence of a base (Table 1). The two
remaining carbazole precursors bromo-4PCz and bromo-6PCz,
Results and Discussion
Precursor synthesis
Compared to plain ICz, the preparation of the substrates for C-H
activation towards NICz proved to be more challenging, owing to
altered reactivity, stability, and availability of the required
substituted pyridines. Furthermore, the issue of regioselectivity
during the ring closing step arises as
a result of the
a X and Y are either Br and Cl or Cl and F
Scheme 1. Synthetic approaches towards precursors for C-H-activation; carbazole and carboline strategya.
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dimethoxytetrahydrofuran50 with a yield of 52%. In the case of
bromo-6PCz the carbazole moiety was formed by a Pd
catalyzed Nozaki type Buchwald-Hartwig amination.51 Notably,
the condensation reaction did not yield bromo-6PCz and the
Nozaki approach failed in the preparation of bromo-4PCz. To
fully explore the accessibility of the carbazole precursors we
employed these two methods also for the preparation of bromo-
5PCz, which was obtained in good yields from both reactions, as
well as bromo-7PCz, which could not be successfully prepared
following these approaches.
Table 1. Synthesis of the precursors for C-H activation towards mono
substituted NICzs
precursor
method[a]
halogen
X=Br
X=Br
X=Br
X=Cl
X=Br
X=Br
X=Br
X=Cl
X=Br
X=Br
X=Br
X=Cl
X=Br
X=Br
X=Br
X=Cl
yield
52%
-
condensation
Buchwald-Hartwig
substitution
[b]
-
substitution
69%
32%
81%
69%
95%
-
condensation
Buchwald-Hartwig
substitution
The second strategy employs the four different carboline
derivatives as starting materials (Scheme 1). The corresponding
four carboline-based NICz precursors (PCbs) could be prepared
substitution
by
nucleophilic
aromatic
substitution
of
1-bromo-2-
condensation
Buchwald-Hartwig
substitution
fluorobenzene and were obtained in excellent yields (82-92%)
with the exception of bromo-7PCb (34%; Table 1). In contrast to
the PCz precursors, the unsymmetrical nature of the carbolines
entails two possible sites at which ring closure may occur in the
case of 4PCb, 5PCb and 6PCb. Whereas reactions at the
benzene unit of the carbolines would yield the same products,
which are available through the carbazole strategy, ring closure
at the pyridine unit would give the two remaining NICz
derivatives. Notably, the site selectivity and ratio of product is
determined by the electronic demand of the ring closing reaction
and thus may be significantly influenced by the nitrogen position.
Furthermore, we aimed to explore the substrate scope of the C-
H activation. Accordingly, the chloro-PCz precursors were
prepared starting from carbazole and the corresponding
chlorofluoropyridines. In contrast to the bromo-precursors all of
the chloro-derivatives could be obtained by nucleophilic aromatic
substitution in acceptable to excellent yield (57-95%; Table 1).
67%
[c]
-
substitution
57%
39%[d]
4%
condensation
Buchwald-Hartwig
substitution
37%
94%
substitution
substitution
substitution
substitution
substitution
X=Br
X=Br
X=Br
X=Br
86%
82%
92%
34%
C-H activation
During initial experiments regarding the ring closing reaction by
C-H activation, we encountered difficulties with reproducibility of
reaction time and yields of the reactions. Thus, we decided to
revisit the reaction conditions of this Pd-catalyzed key step on
the example of the reaction towards 4NICz starting from bromo-
4PCz (Scheme 2, Table 2).
[a] Reaction conditions: condensation: 2,5-dimethoxytetrahydrofuran (4
eq.), AcOH, reflux; Buchwald-Hartwig amination: 2,2’-diiodo-1,1’-
biphenyl (1 eq.), NaOtBu (6 eq.), Pd2(dba)3 (2 mol%), dppf (4 mol%),
toluene, reflux; nucleophilic aromatic substitution towards PCzs:
dihalogenated pyridine (1 eq.), Cs2CO3 (1.1 eq.), DMF, 130 °C, and
nucleophilic substitution towards PCbs: 1-bromo-2-fluorobenzene (2
eq.), Cs2CO3 (2 eq.), DMF, 130 °C. [b] Chloro-7PCz was isolated with a
yield of 35%. [c]Chloro-5PCz was isolated with a yield of 37%. [d]
Containing inseparable byproducts.
Scheme 2. Model system for the optimization of the reaction conditions;
molecular structure of (NHC)Pd(allyl)Cl.
however, could not be prepared using this method. Instead,
nucleophilic substitution occurred in the more activated 2 and 4
positions of the pyridines, yielding chloro-7PCz and chloro-5PCz
in low yields. Therefore, we employed brominated
aminopyridines as alternative substrates (Scheme 1) and
bromo-4PCz could be obtained by condensation of 3-amino 2-
First, various catalytic systems (10 mol%) were investigated
applying a catalyst based on an N-heterocyclic carbene ligand
((NHC)Pd(allyl)Cl)52-53 (Scheme 2) as well as different phosphine
ligands, which have been previously applied in the preparation
of ICz23,32 or other intramolecular arylation reactions.54-56 Yields
were determined using GC (FID). As depicted in Figure 1,
bromopyridine
and
three
molecules
of
2,5-
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after 120 min. Variations of base (Figure S64, support
information) and solvent decreased the reaction rate. In addition,
the reactivity of (NHC)Pd(allyl)Cl towards the chlorine precursor
chloro-4PCz was investigated to explore the scope of the
reaction. Notably, 4NICz was obtained in 68% yield after 180
min. In contrast, starting from chloro-4PCz PdCl2(PPh3)2 gave
4NICz with a yield of only 10% after 180 min (Figure S65,
supporting information), highlighting the importance of the NHC
ligand. Accordingly, the combination of (NHC)Pd(allyl)Cl and
K2CO3 in DMA represents the optimum reaction conditions.
Hence, we decreased the catalyst loading to establish the final
reaction conditions with a catalyst loading of 5 mol% (Figure S66,
supporting information).
First experiments on the 1 mmol scale were performed using the
carbazole precursors (Table 3). To our delight, the conversion of
the bromo-substrates smoothly delivered all four NICz
derivatives (4NICz, 5NICz, 6NICz, 7NICz) in excellent yields of
80-96% after reasonable reaction times (4-8 h). Inspired by this
initial success we investigated the reactivity of the respective
chloro precursors. Surprisingly, the results obtained for the
chloro substrates in these quantitative experiments exceeded
those of the bromo precursors. This finding is in opposition to
previous results, which suggest a lower reactivity of chloro
substrates in the C-H activation step towards ICz.23
Nevertheless, 4NICz, 5NICz, 6NICz and 7NICz were obtained
from the corresponding chloro-PCzs in excellent yields between
92 and 98% after a short reaction time of 4 h (Table 3). In a next
step we investigated the reactivity of the carboline precursors
(PCbs, Table 4). In contrast to the carbazole strategy, two.
Figure 1. C-H activation of bromo-4PCz towards 4NICz applying different
catalysts. Reactions conditions: bromo-4PCz (0.025 mmol), K2CO3 (2 eq.),
catalyst (10 mol%) and ligand (12 mol%: NHC, dppf; 22 mol%: PPh3,
PCy3*HBF4, JohnPhos), DMA, 130 °C.
(NHC)Pd(allyl)Cl provides by far the best results with over 90%
yield within 90 minutes. In comparison, the use of phosphine
based catalysts resulted in longer reaction times and lower
yields (47-68%) due to increased formation of dehalogenated
byproduct (Figure S63, supporting information). Notably, also
the precursor salt of the ligand, (1,3-bis(2,6-diisopropylphenyl)-
1H-imidazol-3-ium chloride), could directly be applied in
26
combination with Pd(OAc)2 to give the product in 76% yield
Table 2. Screening of the reaction conditions for the ring closure towards 4NICz employing bromo-4PCz (entry 1-15) or chloro-4PCz (entry 16-17).
entry
catalyst
ligand
base[a]
solvent[b]
temperature [°C]
time [min][c]
yield [%][d]
[e]
1
2
3
4
Pd(OAc)2
NHC•HCl[f]
K2CO3
K2CO3
K2CO3
K2CO3
DMA
DMA
DMA
DMA
130
130
130
130
120 (91%)
76
68
59
50
[e]
PdCl2(PPh3)2
-
60
60
35
[e]
[g]
Pd(OAc)2
PPh3
[e]
dppf[f]
Pd(OAc)2
[e]
[g]
5
Pd(OAc)2
PCy3•BF4
K2CO3
K2CO3
K2CO3
K2CO3
Na2CO3
K3PO4
Et3N
DMA
DMA
DMA
DMA
DMA
DMA
DMA
toluene
dioxane
DMA
DMA
DMA
DMA
130
130
130
130
130
130
130
110
100
130
130
130
130
120 (65%)
120
47
47
36
93
94
84
10
3
[e]
6
Pd(OAc)2
JohnPhos[g]
[e]
7
Pd(OAc)2
-
-
-
-
-
-
-
-
-
-
-
120 (71%)
60
8
(NHC)Pd(allyl)Cl[e]
(NHC)Pd(allyl)Cl[e]
(NHC)Pd(allyl)Cl[e]
(NHC)Pd(allyl)Cl[e]
(NHC)Pd(allyl)Cl[e]
(NHC)Pd(allyl)Cl[e]
(NHC)Pd(allyl)Cl[h]
(NHC)Pd(allyl)Cl[i]
(NHC)Pd(allyl)Cl[e]
9
180
10
11
12
13
14
15
16[j]
17[j]
60
1200 (76%)
150 (6%)
150 (4%)
360
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
1
73
52
68
10
1200 (93%)
180 (72%)
180 (47%)
[e]
PdCl2(PPh3)2
[a] 2 eq.; [b] the water content of DMA was adjusted to 3000 ppm; [c] reaction time until ≥97% conversion unless noted otherwise in bracket; [d] determined by
GC-FID analysis; [e] 10 mol%; [f] 12 mol%; [g] 22 mol%; [h] 5 mol%; [i] 2 mol%; [j] starting from chloro-4PCz.
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Table 3. C-H activation towards mono substituted NICzs starting from
Table 4. C-H activation towards mono substituted NICzs starting from
carbazole precursors.
carboline precursors.
precursor
product
halogen
X=Br
time
6 h
yield
93%
precursor
time
yield[a]
X=Cl
X=Br
X=Cl
4 h
4 h
4 h
98%
96%
97%
6 h
n.d.[b]/(15%)
n.d.[b]/(45%)
X=Br
X=Cl
X=Br
X=Cl
4 h
4 h
8 h
4 h
80%
92%
84%
96%
8 h
7%/(3%)
61%/(58%)
possible products can be formed in the C-H activation step from
the PCbs. While the formation of a product mixture is a clear
disadvantage, 1NICz and 2NICz are only accessible via this
strategy. Precursors 4PCb, 5PCb and 6PCb were all converted
towards ring-closed NICzs. In contrast, hardly any conversion of
7PCb was observed and starting material along with small
amounts of dehalogenated byproduct was recovered even after
prolonged reaction times of 96 h. We assume that the ortho
nitrogen of 7PCb can stabilize the initially formed Pd-complex
and thus prevents productive ring-closure. Analyzing the
products formed in the C-H activation, we determined that ring
closure regioselectively occurred at the 4 and 3 position of the
pyridine ring in the case of 4PCb and 5PCb, respectively.
Accordingly, 1NICz and 4NICz were obtained from 4PCb in a
ratio of 3:1 after separation, whereas 5PCb yielded 2NICz and
5NICz in a ratio of 6.9:1. Notably, the separation of the
regioisomers was achieved by simple column chromatography.
Thus, the two missing NICz regioisomers could be indeed
prepared using the carboline strategy. In the case of 6PCb, ring
closure on the benzene ring of the carboline was preferred,
presumably owing to the highly electron poor alternative 2
position of the pyridine. Therefore, 1NICz and 6NICz were
formed in a ratio of 1:3.9.
4 h
11%/(0%)
14%/(4%)
76%/(46%)
62%/(44%)
4 h
6 h
74%/(61%)
16%/(12%)
19%/(16%)
71%/(57%)
4 h
Inspired by these results, we wondered if we could control the
regioselectivity of the C-H activation by manipulation the
electron density of the pyridine unit. Thus, we oxidized the
nitrogen atom of the pyridine to increase the electron density of
the aromatic ring applying mCPBA yielding the corresponding N-
oxides in good to excellent yields (61-90%). Employing these N-
[a] Overall yield of ring-closed products (ratio determined by 1H NMR) and
isolated yields in bracket. For reactions starting from N-oxides yields over
two steps (C-H activation and reduction) are given; [b] not determined owing
to overlapping signals from small amounts of dehalogenated byproduct.
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oxides we indeed observed an increased tendency for the ring-
closure to occur on the now more electron rich pyridine rings
remarkable result underlines the impact of the electronic layout
of the carboline systems on the outcome of the ring closing step.
(Table 4), while high yields (between 89 and 92%) were retained. Notably, this oxidation-reduction sequence was established as a
Reduction of the N- oxides was smoothly accomplished using Fe
powder in AcOH with yields ranging from 76 to 97%. In the case
of 4PCb the ratio of isolated 1NICz to 4NICz could be
significantly shifted towards 1NICz upon starting form 4PCb-Ox
(3:1 vs. 19.3:1), whereat the ratio of the products remained
roughly the same for 5PCb and 5PCb-Ox. Most strikingly,
however, the regioselectivity could be inverted for 6PCb. While
6NICz is the favored product starting from 6PCb, predominately
1NCz is formed from 6PCb-Ox (1NICz:6NICz=4.4:1). This
powerful tool to control the regioselectivity of the C-H activation
reaction, which is of particular interest for the synthesis of more
complex and/or extended annulated systems (see section 2.1.3).
Diazaindolo[3,2,1-jk]carbazoles
Inspired by the successful preparation of all mono substituted
NICz regioisomers, we explored the potential to further increase
the electron deficiency of the ICz scaffold by twofold pyridine
incorporation. Owing to the numerous possibilities to position
two nitrogen atoms in the annulated system we limited these
investigations to substrates that can yield symmetrically
substituted products 4,12NICz, 5,11NICz and 6,10NICz (Table
5). The according precursors 4,12PyCb, 5,11PyCb and
6,10PyCb were prepared by nucleophilic substitution of suitably
substituted dihalogenated pyridines with the respective
carbolines. To our delight also these precursors underwent
smooth ring-closure under the optimized conditions, giving the
twofold substituted NICzs in high overall yields. In analogy to the
mono substituted compounds, ring-closure occurred on the
pyridine ring in the case of 4,12PyCb and 5,11PyCB,
preferentially yielding unymmetric 1,4NICz and 2,5NICz,
respectively (Table 5). Separation of the regioisomers was
accomplished by HPLC on the preparative scale with an
acceptable product loss. In contrast 6,10PyCb yielded
symmetric 6,10NICz as the major product. Compared to the
mono substituted precursor 6PCb the electron deficient 2-
position of the pyridine ring was disfavored more pronouncedly
in 6,10PyCb as symmetric 6,10NICz and unymmetric 1,10NICz
were obtained in a ratio of 15.2:1. To control the regioselectivity
of the C-H activation we again employed the oxidation-reduction
strategy. Indeed, the selectivity could be inverted and 1,10NICz
was the preferred product starting from 6,10PyCb-Ox
(6,10NICz:1,10NICz=1:12.7). The complete inversion of the
regioselectivity decisively underlines the potential of the N-oxide
methodology to control the outcome of the ring-closing step.
Table 5. C-H activation towards twofold substituted NICzs.
precursor
time
yield[a]
4 h
28%/(21%)
65%/(40%)
4 h
19%/(15%)
72%/(53%)
Characterization
With all six possible mono substituted NICzs and three
symmetric twofold substituted NICzs as well as their respective
unsymmetric congeners isolated we investigated the molecular
properties of the new building blocks.
Photophysical properties
4 h
4 h
76%/(76%)
3%/(3%)
n.d. [b]/(5%)
38%/(38%)
To investigate the effects of the nitrogen incorporation on the
photophysical properties of the synthesized building blocks UV-
Vis absorption and fluorescence spectra in dichloromethane
(Figure 2), as well as low temperature phosphorescence spectra
(Figure 3) were recorded (Table 6). Molar attenuation
coefficients are given in Table S1. Plain ICz shows a distinct
absorption peak at 285 nm which can be attributed to a π-π*
transition of the conjugated molecular scaffold.25-26 This peak
disappears almost completely with the incorporation of nitrogen
in the twofold annulated central benzene ring in 1NICz and
[a] Overall yield of ring-closed products (ratio determined by 1H NMR) and
isolated yields in bracket. For reactions starting from N-oxides yields over
two steps (C-H activation and reduction) are given; [b] not determined owing
to overlapping signals from small amounts of dehalogenated byproduct.
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2NICz. The lowest energy transition of ICz can be observed as
clear peak at 363 nm with a shoulder at shorter wavelength. In
contrast, 1NICz and 2NICz show rather broad and weak
absorption at longer wavelengths. While ICz and 2NICz have
nearly the same absorption onset at approximately 375 nm, the
absorption onset of 1NICz is clearly redshifted to 400 nm.
The unsymmetric twofold substituted isomers 1,4NICz, 2,5NICz
and 1,10NICz all show one broad emission peak. The emission
maxima of 1,4NICz and 1,10NICz are similar to 1NICz rather
redshifted at 410 nm and 416 nm, respectively. In contrast,
2,5NICz exhibits an emission maximum at 361 nm, which is
blueshifted to that of 2NICz and closer to the emission of
peripheral substituted 5NICz. Analogously to the absorption
properties, 5,11NICz and 6,10NICz feature similar emission
properties as their respective mono substituted derivatives.
Notably, 5,11NICz exhibits the highest energy emission of the
investigated materials with an emission maxima at 345 nm.
Unlike the other derivatives, the emission characteristic of
4,12NICz clearly differs from that of 4NICz. While the emission
maximum of 4,12NICz is redshifted to 392 nm compared to
4NICz an additional high energy band at 372 nm emerges..
The isomers with a nitrogen in one peripheral benzene ring
show distinct peaks between 280–300 nm. Analogously to ICz,
5NICz and 6NICz exhibit a sharp absorption peak at about 285
nm. 7NICz and 4NICz feature peaks at 286 nm and 290 nm,
respectively, but those two compounds also feature additional
further redshifted absorption peaks at 297 nm. At longer
wavelengths the absorption of 5NICz and 6NICz resemble that
of ICz with a distinct peak with a shoulder towards shorter
wavelengths and absorption onsets at 359 and 386 nm,
respectively. In contrast, 7NICz and 4NICz exhibit broader and
weaker absorption with onsets at 366 nm and 375 nm. Notably,
nitrogen incorporation and the altered substitution position
significantly impacts the electronic layout and thus absorption
behavior of the NICz building blocks. Accordingly, the optical
bandgap of the mono substituted NICz, which is an important
characteristic for practical applications, can be tuned over a
range of 0.35 eV from 359 to 400 nm solely by choice of
nitrogen position.
The absorption characteristics of unsymmetrically twofold
derivatives 1,4NICz and 1,10NICz as well as 2,5NICz clearly
resemble those of 1NICz and 2NICz, respectively. This result is
in line with the finding that compounds 4-7NICz more closely
resemble plain ICz. Thus, nitrogen substitution in the central
benzene ring is the decisive factor determining the absorption
properties of the twofold substituted derivatives. Notably, the
distinct absorption band of 4NICz at 295 nm is also observed for
1,4NICz. The onset of absorption of unsymmetric 1,4NICz is
identical to that of 1NICz at 400 nm, while the broad lowest
energy transition of 1,10NICz is shifted to longer wavelengths
with an absorption onset of 410 nm. In contrast, the absorption
onset of unsymmetric isomer 2,5NICz at 353 nm is shifted
towards higher energy compared to 2NICz (373 nm).
Accordingly, the optical bandgaps of the unsymmetrically twofold
substituted derivatives span a larger range of 0.49 eV compared
to the mono substituted NICzs.
Symmetric building blocks 4,12NICz, 5,11NICz and 6,10NICz
exhibit absorption properties similar to their respective mono
substituted congeners. The absorption onset of 4,12NICz and
5,11NICz are slightly blueshifted compared to 4NICz and 5NICz,
while the onset of 6,10NICz is shifted to somewhat longer
wavelength compared to 6NICz.
1NICz and 2NICz exhibit similar fluorescence characteristics as
ICz. Analogously to the absorption onset, the emission
maximum of 2NICz displays the same emission maximum at 375
nm as ICz, while 1NICz is distinctly redshifted to 407 nm. In
contrast, all isomers with nitrogen substitution in the peripheral
benzene ring exhibit a sharper peak maximum as well as a
redshifted shoulder. The nitrogen position influences the
emission maximum which follow the same order as the
absorption onset from 5NICz (354 nm) to 7NICz (364 nm),
4NICz (375 nm) and 6NICz (384 nm).
Figure 2. UV-Vis absorption (full lines) and normalized fluorescence spectra
(dotted lines) of ICz and the synthesized NICz building blocks. All spectra
recorded as 5 µM solutions in DCM.
The triplet energy, corresponding to the highest energy
phosphorescent transition, of building blocks for organic
electronics is a decisive factor in particular for applications in
OLEDs.10 Therefore, phosphorescence spectra of the developed
building blocks were recorded. All compounds exhibit
vibronically resolved phosphorescence. The phosphorescence
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spectra of the NICz isomers with the nitrogen in the peripheral
ring are similar to the spectrum of ICz. While the highest energy
maximum of 5NICz at 436 nm is the same compared to ICz, this
transition is slightly redshifted for 7NICz (439 nm), 4NICz (443
nm) and 6NICz (446 nm). Notably, this redshift occurs in the
same order as for the fluorescence. In contrast, 1NICz and
2NICz show clearly different phosphorescence. The first
maximum is blueshifted to 428 nm in the case of 2NICz, but
redshifted to 445 nm for 1NICz. The spectra of the unsymmetric
twofold NICz isomers are clearly impacted by the nitrogens in
the central benzene ring, as they show great similarity to the
spectra of 1NICz and 2NICz. While 1,4NICz and 1,10NICz have
highest energy maxima at 456 nm and 455 nm, 2,5NICz is
blueshifted to 429 nm. The spectra of the symmetric twofold
isomers are similar to the peripheral mono substituted
derivatives. Accordingly, the highest energy vibronic transitions
of 5,11NICz and 4,12NICz are located at about the same energy
as those of 5NICz and 4NICz at 432 nm respectively 442 nm. In
contrast, in the case of 6,10NICz this transition (436 nm) is
slightly blueshifted compared to 6NICz. Notably, the triplet
energies of the NICz derivatives are located between 2.90 and
2.72 eV and therefore suitable for the development of functional
organic materials for applications in blue and even deep blue
phosphorescent and TADF based OLEDs.57
Summarizing, clear trends for the impact of the different nitrogen
substitution positions are found. Substitution of one of the
peripheral benzene rings (position 4-7) results in photophysical
properties similar to those of plain ICz, whereby the exact
energetic localization (blueshift, redshift) of absorption and
emission is dependent on the substitution position. Derivatives
with symmetric, twofold substitution of the peripheral rings follow
these tendencies, albeit the resulting shifts are generally more
pronounced. In contrast, substitution of the central benzene ring
(position 1 or 2) significantly alters the photophysical behavior,
whereby nitrogen incorporation at position 1 leads to a shift
towards lower and incorporation at position 2 towards higher
energies. In the case of the unsymmetric twofold substituted
derivatives with one pyridine like nitrogen in the central and one
in a peripheral benzene ring, the effect of the nitrogen in the
central benzene ring clearly dominates and those derivatives
feature similar properties as 1NICz and 2NICz, respectively.
Table 6. Electrochemical and photophysical data of the developed
materials.
[b]
[c]
opt. BG[a]
[eV]
λmax
ET
HOMO[d]
[eV]
LUMO[d]
[eV]
[nm]
375
407
375
375
354
384
364
410
361
416
392
345
387
[eV]
2.84
2.79
2.90
2.80
2.84
2.78
2.82
2.72
2.89
2.72
2.81
2.87
2.84
ICz
3.30
3.10
3.33
3.30
3.45
3.21
3.38
3.10
3.51
3.03
3.37
3.56
3.18
-5.78
-6.22
-6.28
-6.22
-6.32
-6.23
-6.00
-6.28
-6.48
-6.31
-6.35
-6.42
-6.44
-2.27
-2.68
-2.43
-2.57
-2.42
-2.62
-2.46
-2.84
-2.58
-2.85
-2.70
-2.54
-2.75
1NICz
2NICz
4NICz
5NICz
6NICz
7NICz
1,4NICz
2,5NICz
1,10NICz
4,12NICz
5,11NICz
6,10NICz
[a] Optical bandgap determined from the absorption onset measured in
DCM solutions (5 µM) at room temperature; [b] emission maximum
measured in DCM solutions (5 µM) at room temperature; [c] determined
from the highest vibronic transition in solid solutions of toluene/EtOH (9/1;
0.5 mg/ml) at 77 K; [d] calculated from the onset of the oxidation and
reduction peak observed during cyclic voltammetry.
Electrochemical properties
The exact energetic alignment of the highest occupied molecular
orbitals (HOMOs) and lowest unoccupied molecular orbitals
(LUMOs) of organic materials is of tremendous importance
regarding charge transport in and charge injection into electronic
devices. Therefore, cyclic voltammetric measurements were
conducted to explore the effects of nitrogen incorporation and
the impact of nitrogen position on the frontier molecular orbitals
(Figure S67-S73, supporting information). All NICz derivatives
exhibited irreversible oxidation, as typically observed for 9H-
Figure 3. Normalized low temperature phosphorescence spectra (77 K) of ICz
and the synthesized NICz building blocks.
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carbazole and ICz derivatives, owing to the instability of the
formed oxidation products.25,58
range of -6.22 to -6.23 eV between those of the ortho and para
substituted materials.
The determined energy values of the HOMOs and LUMOs of the
developed NICzs are summarized in Table 6 and selected
energy levels are depicted in Figure 4. In general nitrogen
incorporation decreased the energy of both orbitals of all
compounds compared to ICz (HOMO: -5.78 eV; LUMO: -2.27),
whereat the effect is slightly more pronounced for the HOMO
levels. The HOMO levels of the mono substituted NICzs were
decreased by 0.22-0.54 eV, while the LUMO energy levels are
lowered by 0.15-0.41 eV. In the case of the twofold substituted
NICzs, HOMO and LUMO levels are decreased by 0.50-0.70
and 0.27-0.58 eV, respectively. Notably, the magnitude of this
effect is dependent on the exact position of the pyridine nitrogen
in the ICz scaffold.
For the mono substituted NICz the biggest impact on the HOMO
levels is observed for compounds 2NICz (-6.28 eV) and 5NICz (-
6.32 eV) with the pyridine nitrogen in para position to the central
nitrogen atom. Notably, the effect on the LUMO levels of these
two compounds is distinctly weaker and thus 2NICz (-2.43 eV)
and 5NICz (-2.42 eV) feature the highest LUMO levels of the
mono substituted series. In contrast, the overall effect of
nitrogen incorporation is least pronounced for ortho substituted
7NICz. Therefore, within this series 7NICz exhibits the highest
HOMO energy of -6.00 eV corresponding to a 0.22 eV decrease
compared to ICz and a LUMO level at -2.46 eV. Compounds
1NICz, 4NICz and 6NICz with a meta substitution pattern display
similar electrochemical behavior. The LUMO levels of these
three compounds are impacted the most by nitrogen
incorporation and are decreased to -2.68, -2.57 and -2.62 eV,
respectively, while their HOMO levels are located in a narrow
The influence of the nitrogen position on the electrochemical
properties can be rationalized by the exploration of the spatial
distribution of the HOMO and the LUMO of the investigated
derivatives (Figure
5
and Figure S74-S75, supporting
information). HOMO and LUMO levels of the materials were
calculated employing density functional theory. As can be seen
in Figure 5 significant electron density of the HOMO level of ICz
is located on the central nitrogen, as well as C2, C5 and C11
which are located para to the central nitrogen. Analogously,
hardly any electron density is located on C1, C3, C6 and C10
which are all located meta to the central nitrogen atom. The
distribution of the electron density explains why para nitrogen
substitution influences the energetic location of the HOMO levels
the most. Indeed, the HOMO level of 2NICz is strongly distorted
compared to ICz (Figure 5). The concentration of the electron
density on the two benzene rings and the reduction of the spatial
extension of the molecular orbital decrease the orbital energy of
the HOMO of 2NICz. In contrast, the shape of the HOMO of
1NICz is highly similar to that of ICz and in the case of the meta
substituted NICz the decreased orbital energy can be explained
by a general inductive effect of the pyridine nitrogen. The
electron density of the LUMO of ICz is located virtually
exclusively on carbon atoms located meta to the central nitrogen
and the annulation positions. This finding explains the
significantly decreased energy of the LUMOs of 1NICz, 4NICz
and 6NICz. Analogously to the HOMO of 2NICz, a similar
distortion of the shape of the LUMO of 1NICz is observed.
However, the restriction of the LUMO is less pronounced which
is in agreement with the finding that the LUMO energy levels are
generally influenced to a slightly lower degree by the nitrogen
Figure 4. Schematic representation of the energy levels of HOMOs and LUMOs of selected materials.
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Chart 1. In the case of more than one crystallographically unique
molecules (Z’=2), prime characters are added to the atom
names.
The molecular packing in the solid state is generally determined
by non-classical C—H···N hydrogen bonding.61-62 In these kind
of non-classical hydrogen bonds electrostatic interactions are
more prominent than covalent bonding.61 Their influence on the
structure is not as pronounced as in the case of strong hydrogen
bonds. Notably, some of the interactions reported herein feature
H...N distances longer than the sum of the van-der-Waals radii
[2.75 Å]. If possible, the lone pairs of the N atoms connect to the
Hs in 7 and 9 position (Table S2). In a few exceptions, the H
atom in 2 position acts as donor. From a topological point of
view, the hydrogen-contacts form one-dimensional chains, which
usually are connected by π-π interactions to layers. The layers
in turn are stacked to give the final crystal structure. A more
detailed description of the molecular arrangements will be given
first for the molecules with N-substitution para to the central N8
atom (2NICz, 5NICz, 2,5NICz), then meta (1NICz, 1,10NICz,
6NICz, 6,10NICz) and finally ortho (7NICz).
Figure 5. Spatial distribution of the HOMO and LUMO levels of ICz, 1NICz
and 2NICz.
para-N: The hydrogen bonding of 2NICz and 2,5NICz molecules
forms straight chains as shown in Figure 6. In both cases, the
molecules are located on a (pseudo-)twofold rotation axis. This
(pseudo-)symmetry is the crucial feature of the phase transitions
described elsewhere.60 The mutual inclination of adjacent
molecules with respect to this axis is determined by the crystal
packing: In 2NICz, the least squares (LS) planes defined by the
C and N atoms of the individual aromatic planes of two adjacent
molecules are inclined by 63.4°. 2,5NICz, on the other hand,
exists as two polymorphs. The first is isostructural with 2NICz,
whereas in the second, adjacent molecules are perfectly
coplanar (adjacent molecules related by a b+c lattice translation).
In both 2,5NICz polymorphs, the molecule is 1:1 disordered with
respect to the N5 atom which is not involved in hydrogen
bonding.
incorporation. As expected, the energy levels of the HOMOs and
LUMOs are further lowered by the introduction of a second
pyridine ring. Compared to ICz the HOMO levels of the twofold
substituted NICzs were decreased by 0.50-0.70 eV and the
LUMO energy levels are lowered by 0.27-0.58 eV. The same
trends as for the mono substituted derivatives were observed.
Accordingly, para substituted 2,5NICz exhibits the lowest HOMO
energy level among the developed compounds at -6.48 eV while
the HOMO level of 5,11NICz is located slightly higher at -6.42
eV. Analogously to 2NICz and 5NICz, the LUMO levels of the
twofold substituted derivatives are located at -2.58 and -2.54 eV
and thus significantly higher compared to the LUMOs of the
meta substituted compounds. The meta substituted derivatives
can be divided into two groups. Unsymmetric 1,4NICz and
1,10NICz feature a smaller electrochemical bandgap compared
to symmetric 4,12NICz and 6,10NICz. Accordingly, 1,4NICz and
1,10NICz exhibit the lowest LUMO energy levels of -2.84 and -
2.85 eV among the developed materials.
Crystal packing
The arrangement and π-π interactions of individual molecules in
the solid state can significantly impact the macroscopic
photophysical and electronic properties of thin films of organic
materials.59 In particular, CH···N hydrogen bonds can be
employed to induce or control specific interactions between
molecules and influence their alignment.46-47 Hence, we were
interested in the packing of the newly developed NICz molecules
as the incorporation of the pyridine like nitrogen atom into the
molecular scaffold entails the potential to induce CH···N
hydrogen bonding. Therefore, single crystals suitable for X-ray
structure determination were grown by recrystallization of 2NICz,
5NICz, 6NICz, 7NICz, 2,5NICz and 6,10NICz. Crystals of 1NICz
were only obtained as the acetonitrile solvate. 2NICz, 5NICz and
2,5NICz featured temperature-dependent polymorphism and
twinning, which is beyond the scope of this contribution and will
be detailed elsewhere.60 Atoms were labeled as outlined in
Figure 6. Chains of (a) 2NICz and (b) 2,5NICz molecules, second polymorph,
connected by non-classical C—H···N hydrogen bonding. C (grey) and N (blue)
atoms are represented by ellipsoids drawn at the 50% probability levels; H
atoms by white spheres of arbitrary radius. The hydrogen bonds are indicated
by dotted lines.
The chains are in all cases connected to layers by π-π
interactions. Adjacent chains within these layers are related by
an a (2NICz, 2,5NICz first polymorph) or a b (2,5NICz, second
polymorph) lattice translation. The distances between the least
squares planes of individual molecules of neighboring chains are
3.45 Å in all cases. Neighboring chains of two adjacent layers
feature different propagation directions for the two structures
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(Figure 7). In the crystals of 2NICz and the first polymorph of
hydrogen bonding in 2,5NICz, indicating that this interaction is
2,5NICz the chains are parallel and extend in opposite directions. preferred.
In contrast, in the second polymorph of 2,5NICz neighboring
interlayer chains are inclined by 42.71° but propagate in the
same direction. No notable inter-layer C—H··· π interactions are
observed in either of the structures.
meta-N: Concerning the meta substituted 1NICz, only
acetonitrile solvate crystals were formed, whereby the N-
electron lone pair is directed towards the solvent filled voids of
the structure (Figure S76, supporting information). This is a first
indication that forming a hydrogen-bond network is more difficult
in these cases. The 1NICz molecules are connected via π-π
interactions to rods extending along the [100] direction [Adjacent
molecules related by an a lattice translation; distance of LP
planes: 3.41 Å]. These rods are located around the solvent filled
voids.
Crystals of 1,10NICz, on the other hand, consist of two
crystallographically independent molecules. These molecules
are connected by H2···N10’ and H9’···N1 contacts, each
molecule of the pair acting as donor and acceptor (Figure 9a).
Adjacent molecules are related by a pseudo-b[001] glide reflection
[the P21 structure has pseudo-Pc21b symmetry] forming chains
extending in the [010] direction. The two crystallographically
independent molecules are nearly coplanar [angle between LS
planes: 3.65°]. Analogously to 2NICz and 2,5NICz, the chains
are connected to layers by π-π
Figure 7. Packing of chains of 2NICz (a) and 2,5NICz second polymorph (b).
Color codes as in Figure 6.
For 5NICz, analogous chains form in the solid state (Figure 8a).
Here, the hydrogen bond acceptor N5 is located off the
molecular axis (C2-N8). Accordingly, the H7···N5 and H9···N5
contacts and the molecular axis form angles of approximately
90° and the formed molecular chains adopt a zig-zag pattern.
Adjacent molecules in these chains are related by a 21 screw
rotation and are nearly coplanar [angle between LS planes:
3.55°].
Figure 9. Packing of chains of 1,10NICz (a) and side view of chains of
1,10NICz (b) and 2,5NICz (c). Color codes as in Figure 6.
interactions [adjacent chains related by an a lattice translation;
distance of LS planes 3.47 Å in both cases]. Notably, the
arrangement of the 1,10NICz chains closely resembles the
structure formed by 2,5NICz (second polymorph). Chains in
neighboring layers propagate in the same direction and are
inclined by 56.41° (Figure 9b and 9c). The hydrogen bonding
networks in meta substituted 6NICz is complex. Crystals of
6NICz contain two crystallographically independent 6NICz
molecules (Z’=2). In contrast to 1,10NICz these molecules are
also not related by pseudo-symmetry. Molecules of the first kind
are connected by C—H···N hydrogen bonding, whereby each
molecule connects to two others, acting as hydrogen-bond
donor and acceptor, respectively. Since the donor (H7, H9) and
acceptor (N6) atoms are in close vicinity, this is only possible
owing to a strong twisting of the two connecting molecules
[angle between LS planes: 77.8°; Figure 10a]. The two
molecules connecting as donor and acceptor to one twisted
Figure 8. a) Chain of 5NICz molecules in the solid state. b) Packing of 5NICz.
Color codes as in Figure 6.
Again, the chains are connected by π-π interactions, whereby
adjacent chains are related by inversion symmetry (Figure 8b).
The distance between connected molecules is approximately
3.55 Å (a precise value cannot be given because the connected
molecules are not perfectly coplanar). The thus formed layers
are stacked along [100]. In principle, nitrogen atoms in 2 and 5
position are capable of forming similar C—H···N hydrogen
bonds with the Hs in 7 and 9 position. Yet, only N2 is involved in
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molecule are coplanar (related by a b lattice translation) and
their LS planes are spaced by only 3.26 Å, indicating a strong
interaction of the π systems (Figure 10b). These fragments form
chains extending along the [010] direction. Notably, π-π stacking
does not occur between adjacent chains but between two
molecules of the same chain, which are connected by a third
6NICz molecule, which acts as a hydrogen donor and acceptor
for the stacked molecules, respectively (Figure 10a). Therefore,
in contrast to the previous cases, the π-π interaction does not
connect adjacent chains to two-dimensional layers but forms
one-dimensional rods extending along [010] by intra-chain π-π
interaction. For the second kind of 6NICz molecule, a rather
short C—H···N contact to a H2 atom is observed. The observed
C—H···N angle of 135° indicates a rather weak interaction,
which is probably not structure-directing. These weak
interactions form chains extending in the [001] direction (Figure
10c). Analogously to 2NICz, these chains are connected by π-π
interactions to layers by a b lattice translation with a distance of
the LS planes of 3.31 Å.
are related to one of the 6NICz chains, the actual geometry of
the chains is distinctly influenced by packing effects. The chains
are packed in a checkerboard pattern connected only by Van-
der-Waals interactions (Figure 11).
Figure 11. Packing of 6,10NICz. Each 6,10NICz molecule connects as
hydrogen bond donor and hydrogen bond acceptor to two different adjacent
6,10NICz molecules. Color codes as in Figure 6.
ortho-N: Finally, in 7NICz, no notable intermolecular hydrogen-
bonding interactions are observed, owing to steric shielding of
the N in 7-position. Instead, molecules are connected by π-π
interactions to rods [adjacent molecules related by an a lattice
translation; distance of LS planes: 3.30 Å]. The rods are packed
by van-der-Waals interactions (Figure S79, supporting
information).
In summary, the investigated NICzs exhibited
a
rich
Figure 10. a) Three 6NICz molecules (molecule 1) connected via C—H···N
contacts to a chain fragment and (b) resulting chain. c) Chain of 6NICz
molecules (molecule 2) connected via non-structure directing C—H···N
contacts. Color codes as in Figure 6.
crystallization behavior whereat π-π stacking and non-classical
C—H···N hydrogen bonds proofed to be structure determining
factors. While all compounds featured π-π stacking of the planar
aromatic scaffolds, hydrogen bonds were not formed between
molecules of 1NICz and 7NICz. Notably, the sole pyridine like
nitrogen of these two compounds is located next to the
annulation position and thus not effectively available for
hydrogen bonding due to steric reasons. Although the
interactions between isolated molecules of the developed
compounds are similar, the position of the nitrogen has a
decisive impact on the exact packing of the crystals. Thus,
depending on the nitrogen position one- or two-dimensional
supramolecular arrangements are formed. Compounds lacking
hydrogen bonds are organized in rods by π-π stacking, which
are packed by van-der Waals interactions (1NICz, 7NICz). If
hydrogen bonding is possible, chains will be formed which are
connected to neighboring chains by π-π stacking to form layers
(2NICz, 5NICz, 2,5NICz, 1,10NICz). Notably, the propagation
direction of chains in adjacent layers is dependent on the
nitrogen position. If a linear propagation of the chains in direction
of the molecular axis (C2-N8) is not possible owing to the
geometrical requirements of the hydrogen bonds, twisted chains
will be formed and π-π stacking occurs between molecules of
Thus, in the case of 6NICz, the π-π interaction between the
planar molecules results in different packing features for the two
crystallographically independent molecules. While intrachain
stacking yields one-dimensional rods, interchain stacking leads
to two-dimensional layers.
Finally, the whole 6NICz crystal is built of a stacking of an
alternation of two layers of which the first corresponds to the
layer formed by the π-π stacking of chains of molecule 2 and the
second is made of neighboring rods of molecule 1 which are
packed by van-der-Waals interactions (Figure S77, supporting
information).
In 6,10NICz, only one of the N atoms is involved in hydrogen
bonding. As with 6NICz, the connected molecules [related by a
21 screw rotation] are distinctly non-coplanar [angle between LS
planes: 53.0°]. In the resulting chains (Figure S78, supporting
information), pairs of molecules are again connected by
intrachain π-π interactions [distance between LS planes: 3.41 Å]
forming rods along [100]. Thus, even though topologically
(considering hydrogen bonding and π-π interactions) the chains
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the same chain (6NICz, 6,10NICz). In such a way the chains are
arranged to one-dimensional rods.
delay of 1 ms. Cyclic voltammetry was measured using a three
electrode configuration consisting of a Pt working electrode, a Pt
counter electrode, and an Ag/AgCl reference electrode and a
PGSTAT128N potentiostat provided by Metrohm Autolab B.V.
The measurements were carried out in a 0.5 mM solution in
HPLC-grade ACN employing Bu4NBF4 (0.1 M) as supporting
electrolyte. Prior to the measurements, the solutions were
purged with nitrogen for approximately 15 minutes. The HOMO
and LUMO energy levels were calculated from the onset of the
oxidation and reduction peaks, respectively. The onset potential
was determined by the intersection of two tangents drawn at the
background and the rising of the oxidation and reduction peaks.
Synthetic details are described in the Supporting Information.
General procedure for C-H activation: A glass vial was
charged with the corresponding halogenated precursor (1 eq.),
K2CO3 (2 eq.) and (NHC)Pd(allyl)Cl (5 mol%) and flushed with
argon. After addition of 10 ml/mmol degassed DMA, the reaction
was stirred under argon atmosphere until full conversion at
130 °C (4 h – 8 h). After cooling, the reaction mixture was
poured into water and repeatedly extracted with CH2Cl2. The
organic phases were dried over anhydrous Na2SO4 and
concentrated under reduced pressure. The crude product was
purified by column chromatography.
Conclusions
We have described the convenient synthesis of all six possible
mono substituted azaindolo[3,2,1-jk]carbazoles as well as three
symmetric twofold substituted derivatives and their unsymmetric
regioisomers. Notably, the presented C-H activation approach
not only allows for the preparation of the azaindolo[3,2,1-
jk]carbazole congeners described in this work but the developed
oxidation-reduction
strategy
enables
to
control
the
regioselectivity of the ring closing C-H activation step. Therefore,
numerous further twofold substituted regioisomers are
accessible employing this method and additional nitrogen
incorporation could be realized. We investigated the
photophysical and electrochemical properties as well as the
solid state interaction of the developed materials and revealed
the impact of nitrogen incorporation in different positions of the
indolo[3,2,1-jk]carbazole scaffold. Ultimately, we provide
synthetic chemists with a toolbox full of NICz building blocks.
The established structure properties relationship and the
possibility to tune their molecular properties and to control the
supramolecular arrangement of individual molecules predestines
the class of azaindolo[3,2,1-jk]carbazoles as a novel molecular
platform and will guide material scientists in the design of
functional organic materials with tailored properties.
Computational details: DFT calculations were performed using
the Gaussian 09 package63 applying the Becke three-parameter
hybrid functional with Lee–Yang–Perdew correlation (B3LYP)64-
65 in combination with Pople basis sets 6-311G(d,p).66 Geometry
optimizations were performed in the gas phase and without
symmetry constraints. Orbital plots were generated using
GaussView.67
Single crystal diffraction: X-ray diffraction data of
1NICz·xCH3CN, 2NICz, 5NICz, 6NICz, 7NICz, 1,10NICz,
2,5NICz and 6,10NICz (CCDC 1864321-1864328) were
collected at T = 100–270 K in a dry stream of nitrogen on a
Bruker Kappa APEX II diffractometer system using graphite-
monochromatized Mo-Kα radiation (λ = 0.71073 Å) and fine
sliced φ- and ω-scans. Data were reduced to intensity values
with SAINT and an absorption correction was applied with the
multi-scan approach implemented in SADABS or TWINABS.68
The structures were solved by the dual-space approach
implemented in SHELXT69 and refined against F2 with
JANA2006.70 Non-hydrogen atoms were refined anisotropically.
The H atoms connected to C atoms were placed in calculated
positions and thereafter refined as riding on the parent atoms.
Contributions of disordered solvent molecules to the intensity
data were removed for 1NICz using the SQUEEZE routine of the
PLATON71 software suite. 2NICz, 5NICz and 1,10NICz
crystallize as twins by pseudo-merohedry. 7NICz forms twins
with non-overlapping reflections and was refined against HKLF5
data with information on reflection overlap. The absolute
structure of non-centrosymmetric crystals (1NICz·xCH3CN,
1,10NICz, 2,5NICz and 6,10NICz) was not determined owing to
Experimental Section
All reagents and solvents were obtained commercially and used
without further purification. Anhydrous solvents were prepared
by filtration through drying columns. The water content of
purchased DMA was determined by Karl Fischer titration using a
Mitsubishi CA-21 Moisture Meter and corrected to 3000 ppm for
C-H activation screening reactions. As further experiments
indicated no significant impact of the water content on the
reaction outcome large scale experiments were conducted using
unmodified DMA with roughly 100 ppm H2O. Screening
reactions were performed in glass vials under argon atmosphere
in a controlled heating block using argon degassed substrate
solutions including 1-methylnaphthalene as internal standard as
well as a catalyst solution. All screening experiments were
performed three times independently. The depicted results
represent the mean of these experiments. Yields of the
screening reactions were determined by GC using a Thermo
Scientific TRACE 1310 gas chromatograph with dual
configuration consisting of two AS 1310 autosamplers, Thermo
Scientific TR-5MS columns and FID detectors. Absorption and
photoluminescence measurements were conducted using a
Perkin Elmer Lambda 750 spectrometer and a PerkinElmer LS
55 fluorescence spectrometer, respectively. DCM solutions (5
µM) were employed for solution measurements while
phosphorescence spectra were recorded at 77 K using solid
solutions of the materials in toluene/EtOH (9/1; 0.5 mg/ml) with a
a
lack of resonant scatterers. Molecular graphics were
generated with the program MERCURY.72
Acknowledgements
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10.1002/chem.201805578
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Property tuning by nitrogen
incorporation: a reliable synthetic
protocol for the preparation of novel
azaindolo[3,2,1-jk]carbazole
Thomas Kader, Berthold Stöger,
Johannes Fröhlich, and Paul Kautny*
Page No. – Page No.
derivatives was established. Nitrogen
position and amount decisively impact
photo-physical and electrochemical
properties as well as supramolecular
arrangement.
Azaindolo[3,2,1-jk]carbazoles: New
Building Blocks for Functional
Organic Materials.
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Author(s), Corresponding Author(s)*
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Title
Text for Table of Contents
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