Synthesis and molecular properties of methoxy-substituted diindolo[3,2-: B:2′,3′- h] carbazoles for organic electronics obtained by a consecutive twofold Suzuki and twofold Cadogan reaction

Article abstract of DOI:10.1039/c6tc02009g

A set of methoxy-substituted diindolo[3,2-b:2′,3′-h]carbazoles has been synthesized by twofold Suzuki-Miyaura, Cadogan and N-alkylation reactions starting from N-hexyl-2,7-dibromo-3,6-dinitro carbazole. Microwave accelerated reactions ensure a rapid strai

Full text of DOI:10.1039/c6tc02009g

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ARTICLE
Synthesis and Molecular Properties of Methoxy-Substituted Di-
indolo[3,2-b:2’,3’-h]carbazoles for Organic Electronics Obtained
by a Consecutive Twofold Suzuki and Twofold Cadogan Reaction
†
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
a
b
a
DOI: 10.1039/x0xx00000x
Hassan Srour, Thu-Hong Doan, Elisabeth Da Silva, Richard J. Whitby * and Bernhard Witulski *
www.rsc.org/
A set of methoxy-substituted diindolo[3,2-b:2’,3’-h]carbazoles has been synthesized by twofold Suzuki-Miyaura, Cadogan
and N-alkylation reactions starting from N-hexyl-2,7-dibromo-3,6-dinitro carbazole. Microwave accelerated reactions
ensure a rapid straightforward and step-ecomomic assemby of the diindolo[3,2-b:2’,3’-h]carbazole framework. Besides the
parent diindolo[3,2-b:2’,3’-h]carbazole four new methoxy-group bearing derivatives were obtained and their molecular
properties investigated by thermal, photophysical and electrochemical means. Dichloromethane solutions of the
diindolocarbazoles display with respect to the methoxy-group substitution pattern bright photoluminescences covering
the blue to green spectra with quantum yields of φ
F
= 0.20-0.27. Notably, the N,N,N,-tri-n-hexyl-1,4,9,12-tetramethoxy
em
diindolo[3,2-b:2’,3’-h]carbazole exhibits a remarkable aggregate induced excimer-type luminescence (
λmax = 562 nm,
solid
F
φ = 0.14) in the solid state resulting in bright yellow light emission, whereas the parent non-substituted compound and
the other methoxy-substituted derivatives show poor photoluminescence from the solid state. HOMO/LUMO energy levels
were determined by means of electrochemical and photopphysical methods and were compared to those obtained by
computational methods using B3LYP/6-31G(d) DFT calculations with a Polarisable Continuum Model (CPCM) for the
2 2
solvent CH Cl . Good linear correlation was observed between calculated and experimentally measured values, and the
correlations were used to estimate experimental HOMO/LUMO energy values of still non-synthesized heteroatom
extended heteroacenes.
2
mobilities of 2-5 cm /V s in vacuum deposited OFETs.
3
Introduction
Organic semiconductors gain remarkable interest due to their
potential use in large-area, low-cost, light-weight electronic
N
H
1
and optoelectronic materials, that are greatly needed for next
anthracene
pentacene
hexacene
carbazole
H
N
generation smart cards, solar cells, and flexible lightning
devices and displays. Many organic semiconductors are
currently discussed as active layers in organic light emitting
diodes (OLEDs), organic field-effective transistors (OFETs), or
N
H
indolo[3,2-b]carbazole
H
N
2
photovoltaic cells. Either polymers or small molecules are
addressed in this field – where the latter benefit from the fact
of being homogeneous and well defined by means of
molecular structure. Furthermore, small molecule semi-
conductors can be easily modified in their molecular structure
and adjusted to the desired properties - and equally important
N
H
carbazolo[3,2-b]carbazole
H
N
N
H
–
can be obtained in high purity. In this context acenes and
their -extended homologues are attractive materials (Figure
). For example, pentacene has set benchmarks for high hole
heptacene
bis-[(2,3)(6,7)]indoloanthracene
π
H
N
H
N
9
7
6
4
1
10
1
3
1
12
13
N
H
15
1
2
diindolo[3,2 -b :2',3'-h]carbazole
Figure 1. Linear fused acenes and nitrogen containing linear fused heteroacenes.
But the fact that pentacene is insoluble in organic solvents
limits its use to vapour phase deposition techniques.
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18
Moreover, pentacene as well as other π-extended linear fused regioselective Cadogan reaction. The parent compound as
acenes have both high energy levels of the highest occupied well as the dichloro derivative became available through a
molecular orbital (HOMO), and narrow band gaps. This makes more straightforward synthetic approach utilizing a copper
1
them photodegradable, sensitive to air oxidation and results in mediated Ullman coupling. Moreover, Yamamoto polymeri-
9
4
performance decrease. These drawbacks render pentacene - zation of 3,10-dichloro- diindolo[3,2-b;2’3’-h]carbazole gave
as well as larger fused linear acenes – as less practical for polymers with promising conductive as well as thermoelectric
2
0
further applications.
properties.
Appealing structural variants of linear fused acenes are ladder- With only three described and characterized examples the
type heteroacenes bearing thiophene, furan, pyrrole or phos- chemistry of diindolo[3,2-b:2’3’-h]carbazoles remains an open
5
phole units. In those heteroacenes, the heteroatom not only field – particularly, because these heteroacenes provide an im-
modifies the molecular structure but further alters electronic, portant class of materials for applications in organic
photophysical, thermal, material and device properties. How- electronics. The interest in extending linear fused indolocarba-
ever, compared to the remarkable work devoted to the zoles to the more conjugated π-extended diindolo[3,2-b:2’3’-
synthesis of pentacenes, examples focusing on synthesis, h]carbazoles lies hereby not only in the thus reduced band
molecular and materials properties of linear fused het- gaps but also in the enhancement of charge transport prop-
eroacenes are less well explored - particularly, with regard to erties due to efficient intermolecular π-π overlap and
the longer conjugated specimens approaching seven and more electronic coupling. Furthermore, a well-adjusted substitution
6
linear fused rings.
pattern on the diindolocarbazole scaffold might induce
Carbazole derivatives, specifically indolo[3,2-b]carbazoles and molecular self-assembly in the solid state and allow electronic
diindolo[3,2-b:2’,3’-h]carbazoles, that are isoelectronic struc- fine tuning of HOMO/LUMO energy levels. Notably, the
tural variants of pentacene and heptacene respectively, have substitution pattern also influences the reorganization energy
gained considerable interest as a promising class of hole-trans- after hole-injection compared to the non-substituted
porting semiconductors (Figure 1). Their relatively low-lying compound and thus will alter intrinsic charge-carrier mobilities
2
1
HOMO levels and large band gaps make them interesting for of materials.
applications in organic electronics. In this paper we provide an experimental and computational
Indolo[3,2-b]carbazoles have a planar molecular structure and study on a set of methoxy-substituted diindolo[3,2-b:2’3’-
attachment of alkyl chains to the two nitrogen atoms allows h]carbazoles, their thermal, electrochemical, as well as
improvement of solubility, alters molecular self-assembly in photophysical properties including solution and solid state.
7
,8a
films and solids, and defines materials properties.
dolo[3,2-b]carbazoles showed great potential as active layers
In-
8
9
Results and Discussion
in OFETs,
OLEDs,
and in solution-processed bulk
1
0
heterojunction solar cells. π
1
indolocarbazoles, i.e. bis-indolonaphthalenes like the linear
-Extended versions of
Synthesis
1
1
2,13
For the construction of the diindolo[3,2-b:2’3’-h]carbazole
fused carbazolo[3,2-b]carbazoles
and their angular fused
2
2
scaffold a building block strategy based on Suzuki-Miyaura
reactions with the readily available carbazole and a set of
aryl and heteroaryl boronic acids ( ) was chosen (Scheme 1).
Our synthetic strategy proceeds with regioselective twofold
core isomers - the carbazolo[1,2-a]carbazoles and carba-
1
3,14
1
zolo[4,3-c]carbazoles
- show promising features in OFETs,
2-9
as well as hole transporting and light emitting material in
1
4,15
phosphorescent OLEDs.
The synthesis of π-enlarged
versions of indolo[3,2-b]carbazoles, the linear fused syn- and
2
3
Cadogan reactions, and with N-alkylations that provide the
1
6
necessary solubility and processability of the final products.
anti-bis-[(2,3)(6,7)]indoloanthracenes was reported. More
recently, we described synthesis, molecular and photophysical
properties of the angular fused 7-membered ring heteroacene
Notably, all follow-up transformations of carbazole
1 (Suzuki
coupling, Cadogan reaction, and N-alkylation) are twofold
2
4
1
7a
securing a rapid and step-economic synthesis.
The Suzuki-Miyaura cross-coupling reaction of di-bromo-dini-
trocarbazole with various aryl and heteroaryl boronic acids
) was either performed by conventional heating (Schlenk
tube in oil bath) or by microwave heating. For cross-couplings
under conventional heating 5 mol% Pd(PPh as catalyst in dry
bis-[(1,2)(5,6)]indoloanthracene as well as the 9-ring angular
1
7b
fused biscarbazolo[3,4-a:3’,4’-h]anthracene and their use as
the active layer of respectively highly sky-blue and white light
emissive OLEDs.
1
(2-9
Diindolo[3,2-b:2’3’-h]carbazoles having three carbazole units
embedded in their molecular skeleton are the largest
members of solely nitrogen containing linear fused
heteroacenes being accessed so far. Chemistry and materials
application of the diindolo[3,2-b:2’3’-h]carbazole have been
greatly pioneered by Leclerc who also disclosed potential
3 4
)
THF and cesium fluoride as base were used. However, under
these conditions the coupling reactions needed 2 days for
completion. The reaction time for the twofold Suzuki-Miyaura
couplings was significantly reduced to 10 minutes, when the
reactions were performed under microwave heating (300 W,
1
8-20
applications in the fields of organic electronics.
However,
1
40 °C). Here, best reactions conditions involved the use of 5
mol% Pd(PPh in DMF/water and Na CO . The microwave
accelerated Suzuki reactions gave the cross-coupling products
to the best of our knowledge only the parent system, the 1,15-
dimethyl substituted and the 3,10-dichloro derivative have
been described. The 1,15-dimethyl-diindolo[3,2-b:2’3’-
h]carbazole was first obtained through a low-yielding non-
3
)
4
2
3
1
0-
17 respectively in yields of 72-96% (Scheme 1 and Table 1).
2
| J. Name., 2012, 00, 1-3
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The coupling reactions were equally effective by using either aryl
ARTICLE
2
-6
or
heteroaryl
7
-9
boronic
acids.
Table 1. Products and respective yields of isolated compounds obtained by the sequence of twofold Suzuki, Cadogan and N-alkylation reactions starting from carbazole 1.
Entry
1
Boronic acid
Suzuki-reaction, Yield (%)
Cadogan reaction, Yield (%)
N-Alkylation, Yield (%)
c
2
3
10 (83)
18 (66)
26 (95)
2
3
a
11 (85)
19 (50)
20 (52)
27 (91)
O2N
NO2
MeO
OMe
N
OMe
MeO
C6H13
b
4
12 (96)
28 (52)
4
5
6
13 (90)
21 (50)
22 (48)
29 (49)
30 (84)
5
6
O2N
NO2
MeO
N
OMe
OMe
MeO
C6H13
14 (78)
*
d)
7
8
9
15 (91)
16 (72)
17 (88)
23 (0)
24 (0)
31
7
d)
32*
33*
8
d)
25 (0)
a
b
c
8
7% isolated yield with conventional heating (70 °C, 2 days). synthesized by conventional heating. a yield of 25% was obtained by conventional heating (230 °C, 24
d
h). Molecule investigated by computational methods only, see text.
Next, the twofold Cadogan reaction for the assembly of the di- heated to 230 °C in triethylphosphite/1,2-dichlorobenzene the
indolo[3,2-b:2’3’-h]carbazole scaffold was investigated. reaction proceeded very slowly over a period of 24 h to give
Although the Cadogan reaction is a versatile reaction for the product 18 in 25% yield. Other variants of the Cadogan
2
3,25
synthesis of carbazoles,
its application for the synthesis of reaction, including the use of P(OEt)3 as solvent,
26
diindolocarbazoles has been reported to be non-regioselective PPh /dichlorobenzene, or related oxygen transfer reactions
3
1
and low-yielding. Indeed, when the dinitro compound 10 was mediated through a molybdenum catalyst, were even less
8
27
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effective. Gratifyingly, when the twofold Cadogan reaction was 39 in high yields of respectively 63%, 86% and 90%. The prod-
performed under microwave heating (300 W, 230 °C) in ucts where thereafter converted to the N-alkylated derivatives
2
8
2
P(OEt) /1,2-dichlorobenzene, the diindolocarbazole 18 was 40-42 facilitating purification and characterisation by analytical
obtained within 1.5 h in 66% yield as a yellow powder. The means.
microwave accelerated Cadogan reaction was also effective to Finally, the here developed synthesis of diindolo[3,2-b:2’3’-
give the methoxy-substituted diindolocarbazoles 19
spectively via a twofold ring-closure reaction in yields of 48- Cadogan and N-alkylation reactions is remarkably short and
2%. Unexpectedly, neither a reaction under conventional efficient. With only 3 steps from readily available carbazole
heating nor the microwave accelerated version delivered the and with an overall yield of 52% for the parent di-
heteroacenes 23 25. For these cases, a complete conversion of indolocarbazole 26, this synthesis is - regarding step-count and
the corresponding dinitro-compounds 15 17 was observed (as overall yield - more efficient than previously reported
indicated by TLC), but we were unable to isolate any syntheses of this important class of organic semiconductors.
-22 re- h]carbazoles using a consecutive sequence of twofold Suzuki,
5
1,
-
-
diindolocarbazole products 23-25 – most probably due to their
Thermal Properties
insolubility.
Finally, a double N-alkylation of the Cadogan products 18
-
22 Thermal properties of the diindolocarbazoles 26-30 were
with n-hexylbromide in NaH/DMF made the targeted diindolo- investigated by differential scanning calorimetry (DSC) and
carbazoles 26 30 available in high yield (49-95%). The thus ob- thermogravimetric analysis (TGA). DSC measurements
tained diindolo[3,2-b:2’3’-h]carbazoles, the parent non-substi- revealed relatively sharp melting peaks for the heteroacenes
tuted compound 26 and the four methoxy-substituted variants 26, 27 28, and 30 with maxima at 168 °C, 193 °C, 163 °C, and
7-30, were soluble in organic solvents such as chloroform, di- 166 °C respectively, as well as matching crystallisation peaks
-
,
2
chloromethane, chlorobenzene or hot toluene. Furthermore, for both the first and the second heating-cooling cycle.
they could be purified by simple flash chromatography on silica Notably, the DSC of compound 26 and 27 also show minor
gel. The molecular structures of all new diindolo[3,2-b:2’3’- phase transitions that are probably based on “chain-melting”
1
13
h]carbazoles 26-30 were confirmed by H and C NMR processes. Additionally, TGA demonstrated excellent thermal
spectroscopy and analytical methods. They are described in stabilities up to 400-450 °C (For DSC and TGA, see ESI).
detail in the electronic supplementary material (ESI).
Photophysical Properties
The unexpected complications encountered with reactions
leading to the heteroacenes 23
-
25 were investigated in more UV-vis absorption and photoluminescence spectra of 26-30 in
Cl ) as well as solids were measured, and their op-
detail. Further support, that the lack of efficiency of solution (CH
2
2
microwave accelerated Cadogan reactions to give the tical properties are summarized in Table 2.
heteroacenes 23-25 was based on difficulties encountered by
2 2
Table 2. Photophysical data of diindolocarbazoles 26-30 in CH Cl and crystals.
product insolubility – rather than by intrinsic reactivity with
2
9
abs
λ
a
max
em
λ
b
max
em
λ
c
F
d
E
g
heteroaromatic moieties - was gained with the nitro Comp.
max
Φ
Φ
F
CH
2
Cl
2
CH
2
Cl
2
solid
CH
2
Cl
2
solid
(eV)
2.59
2.68
2.62
2.53
2.76
compounds 34-36 (Scheme 1).
(nm)
(nm)
474
509
460
490
467
498
487
520
447
476
(nm)
e
--
e
2
6
465
439
449
0.22
0.26
0.22
0.20
0.27
--
1
1
) P(OEt)3
NO2
X
R
N
,2-dichlorobenzene,
W (300W, 230 °C), 1.5h
27
506
562
0.02
0.14
0.02
4
25
458
33
476
47
X
2
) C6H13Br, or MeI
NaH, DMF
2
2
3
8
9
0
X = S, thiophene: 34
37-42
f
4
606
X= S, benzothiophene: 35
X= O, benzofurane: 36
528
f
4
575
e
e
437
415
--
--
R
N
R
N
R
N
a
b
-6
c
S
S
O
in CH
2
Cl
2
, 10 M in CH
2
Cl
2
, absolute quantum yield measured with integration
Cl was purged with argon prior
to measurement, optical band gap energy determined from the λonset value of
-
6
sphere, a 10 M solution of the compound in CH
2
2
R = H: 37
R = H: 38
R = H: 39
(90%)
d
2
)
(
63%)
R = Me: 40
98%)
2)
(86%)
R = n-C6H13: 41
(99%)
2)
e
f
MeI
the long-wavelength absorption band. non-emmissive. shoulder.
R = n-C6H13: 42
(93%)
(
2 2
The UV-vis absorption spectra measured in CH Cl solutions
Scheme 1. Microwave accelerated Cadogan reaction for the synthesis of mixed display characteristic low-energy transitions in the 437-476 nm
heteroatom containing heteroacenes.
region that are individually accompanied by vibronic satellites
and reflect transitions into the S
The microwave accelerated Cadogan reaction with the nitro- the much stronger absorption band in the 378-384 nm region
compounds 34 36 having either a thiophene, benzothiophene is assigned to the So→S transition. Wavelength and intensities
1
excited state. Accordingly,
-
2
or benzofurane unit gave under otherwise identical reaction of the absorption spectra are in good agreement with previous
conditions the mixed heteroatom containing heteroacenes 37- theoretical investigations of the non-substituted diindolo[3,2-
4
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ARTICLE
3
0
b:2’3’-h]carbazole.
UV-vis absorption and photolu- argon purged dichloromethane solutions as well as solids
minescence spectra of the parent diindolocarbazole 26 and the (Table 2). Quantum yields for this set of heteroacenes in
three tetra-methoxy-substituted derivatives 28 29, and 30 solution are equally with values between = 0.2-0.3 and no
,
φ
having each a different substitution pattern are shown in trend with respect to the number and substitution pattern of
F
Figure 2 (The complete set of absorption, emission and methoxy groups was revealed. Quantum yields of solid state
excitation spectra for 26-30 can be found in the ESI).
photoluminescence from 26-30 are rather low compared to
those in solution. This is most likely due to an aggregate
caused quenching (ACQ) that is often accompanied by close
1
0
0
,0
,5
,0
face-to-face or
aromatic or heteroaromatic compounds in the solid state.
However, such a molecular packing motif can lead to the
π-π stacking interactions of π-extended
3
1
3
0
28
26
29
π-π
3
2
formation of excimer-type emission, that is recognized by a
significantly red-shifted photoluminescence emission with
2
2
2
3
6
8
9
0
1
respect to the one in solution.
em
Notably, bright yellow fluorescence (
λmax = 562 nm) with a
quantum yield of φ = 0.14 was observed for the solid state
F
photoluminescence of the N,N,N,-tri-n-hexyl-1,4,9,12-
tetramethoxy diindolo[3,2-b:2’,3’-h]carbazole 28 (Table 2 and
Figure 3). The corresponding photoluminescence spectra of
solid 28 is distinctly red-shifted (about 95 nm compared to the
emission spectra in solution) and broadened with respect to
the one in solution. This points to a specific aggregate for-
mation of 28 in the solid state that favours excimer-type light
emission.
0
550
3
50
400
450
500
Wavelength (nm)
Figure 2. Normalized UV-vis absorption (solid lines) and PL spectra (dashed lines) of the
-
6
diindolocarbazoles 26 (black), 28 (green), 29 (red) and 30 (blue) in CH
2 2
Cl (10 M). Inlet:
Solution of diindolocarbazoles 30, 28, 26 and 29 in CH Cl under UV light.
2
2
1
2000
0000
Notably, UV-vis absorption as well as photoluminescence
properties are significantly influenced by the number of
methoxy substituents and particularly by their specific
substitution pattern on the diindolo[3,2-b:2’3’-h]carbazole
1
0
,0
,8
1
8
6
4
2
000
000
000
000
0
framework. The absorption band corresponding to the S →S
0
1
abs
absλmax
0,6
transition is blue-shifted for the 3,10-dimethoxy-27
(
=
4
49 nm), as well as for the 1,4,9,12-tetramethoxy-28
abs
(
λ
max
=
0
,4
4
58 nm) and the 1,3,10,12-tetramethoxy-30
( λmax = 437 nm)
derivative. Whereas the 2,4,9,11-tetramethoxy isomer 29
abs
(
λ
the parent non-substituted compound 26
max = 476 nm) is significantly red-shifted, with respect to
abs
0,2
( λmax = 439 nm,
Table 2 and Figure 2). The photoluminescence spectra of this
0,0
set of diindolo[3,2-b:2’3’-h]carbazoles follow the same trend
400
450
500
550
600
650
700
750
800
em
with emission bands at
λ
max = 460, 467, and 447 nm for the
28, and
0 respectively, as well as for the bathochromic shifted
Wavelength (nm)
hypsochromic shifted spectra of diindolocarbazoles 27
,
Figure 3 UV-vis absorption [black line] and PL spectra of diindolocarbazole 28 in
3
-
6
solution (10 M in CH
2
Cl
2
) [dashed red line] and PL of solid [blue dashed-dotted line].
em
photoluminescence spectra of diindolocarbazole 29
(
λ
87 nm) with respect to the photoluminescence spectra of the
max
=
Insert: Solution of 28 in CH
2
Cl and solid under UV-light.
2
4
em
non-substituted 26 ( λmax = 474 nm). Such a spectroscopic
series underlines that the methoxy-substituents might allow a
The bright photoluminescence of solid 28 and its high
efficiency is remarkable, as the other diindolocarbazoles in this
series exhibit only poor solid state photoluminescence
properties (Table 2). N-alkyl side chains and methoxy-group
substitution pattern of the diindolocarbazole 28 seem to be
critical in order to control supramolecular aggregation in such
a way that segregation between heteroaromatic scaffolds and
fine tuning of optoelectronic properties. The emission spectra
of the diindolo[3,2-b:2’3’-h]carbazoles 26-30 show all very
-1
small Stokes shifts of 9-11 nm (404-474 cm ) reflecting the
expected rigidity of these planar 7-ring linear fused
heteroacenes. Furthermore, the shape of the low-energy
absorption bands and the corresponding fluorescence
emission bands mirror each other. Such low Stokes shifts and
mirror-image profiles are due to similar geometries of ground
and first excited state, as well as a similar nature of the
absorbing and emitting electronic levels.
alkyl chains favour
π-π interaction that induce effective
excimer-type formation in the solid without dominating ACQ
effects.
A red-shifted solid state photoluminescences is also found for
the methoxy-substituted diindolocarbazoles 27 and 29, albeit
for these examples the red-shift is less pronounced (about 40
Photoluminescence quantum yields of diindolocarbazoles 26
-
30
were measured with the help of an integration sphere in
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Journal Name
3
4
nm with respect to the photoluminescences in solution) and Bu
4
NPF
6
as the supporting electrolyte.
The half-wave
+
accompanied with a much less intense photoluminescence potential for the Fc/Fc redox-couple was set to E1/2 = 0.46 for
33
F
efficiency (φ ~ 0.02 for 27 and 29, Table 2). Furthermore, the calibration purpose. We chose here -5.10 eV as the formal
+
parent compound 26 and the 1,3,10,12-tetramethoxy- potential of the Fc/Fc redox couple in the Fermi scale and not
+
substituted derivative 30 are non-emissive in the solid state the quite often used value of -4.8 eV for 0.0 V versus Fc/Fc
3
For the full set of solid state photoluminescence spectra, see based on a recent discussion of Bazan et al.. The former
4
(
ESI).
calibration scale of -5.10 eV is based on several empirically ob-
Optical band gap energies (E
g
) of the parent diindolo[3,2- tained electrochemical data and reflects best the stability of
b:2’3’-h]carbazole 26 and the four methoxy-substituted the redox species involved. The HOMO energy level for the
variants 27-30 were estimated from the onset values (
λ
onset) of parent non-substituted diindolo[3,2-b:2’3’-h]carbazole 26 was
thus estimated to EHOMO = -5.06 eV. For the 3,10-dimethoxy-27
derivative the HOMO energy level was found to be EHOMO = -
the solution absorption spectra and are listed in Table 2.
Electrochemical properties
5.04 eV, and the three tetramethoxy derivatives 28, 29, and 30
Electrochemical properties of diindolo[3,2-b:2’3’-h]carbazoles revealed energy levels of EHOMO = -4.96, -4.92, and -5.00 eV
30 were investigated by cyclic voltammetry and the results respectively. These results furthermore underline the strong
are displayed in Table 3. Cyclic voltammetry experiments were electron donating properties of methoxy substituents in the di-
carried out with a scan rate of 100 mv/s, with Bu NPF (0.1 M indolo[3,2-b:2’3’-h]carbazole framework.
26-
4
6
*
*+
2 2
in CH Cl ) as the electrolyte solution, and with Fc /Fc (-0.13 V
3
3
*
vs SCE) as the internal standard (Fc : decamethyl ferrocene).
Within the solvent/electrolyte window the parent non-substi-
tuted compound 26 showed two oxidation potentials, whereas
500
2
0
400
10
0
the methoxy group bearing variants 27-30 respectively com-
piled more than three oxidation levels each.
300
-10
a
-20
0,0
Table 3. Electrochemical data of compounds 26-30.
0,1
0,2
0,3
0,4
0,5
0,6
0,7
2
00
00
0
Vf (V)
b
ox
E
Comp.
E
1/2 (V)
onset (V)
E
HOMO (eV)
-5.06
E
LUMO (eV)
-2.47
1
2
2
2
2
3
6
7
8
9
0
0.47, 1.05
0.42
0.40
0.32
0.28
0.36
0.45, 0.99, 1.12
0.39, 0.73, 0.84
0.33, 0.66, 0.86
0.43, 0.85, 1.00
-5.04
-2.36
-4.96
-2.34
-
-
100
200
-4.92
-2.39
-5.00
-2.24
0,0
0,2
0,4
0,6
0,8
1,0
1,2
1,4
1,6
a
b
Recorded in CH
2
Cl
+
2
/0.1 M n-Bu
4
PF
6
at room temperature vs SCE. Values cali-
V (V)
f
brated against Fc* /Fc* (-0.13 V).
The cyclic voltamogram of the 1,4,9,12-tetramethoxy di-
indolo[3,2-b:2’3’-h]carbazole 28 is displayed in Figure 4 as a 28 (10 M in CH
Figure 4. Cyclic Voltammetry of 1,4,9,12-tetramethoxy diindolo[3,2-b:2’3’-h]carbazole
-
3
2
Cl
2 4 6
/ 0.1 M n-Bu NPF ) at room temperature. Scan rate 100 mV/s,
recorded vs SCE. Insert: First oxidation potential.
representative example (For the full set of cyclic
voltamograms, see ESI). The first oxidation is for all
investigated diindolocarbazoles fully reversible, with quasi- The HOMO energy of the parent non-substituted diindolo[3,2-
reversible oxidations for the second and third potential. In b:2’3’-h]carbazole 26 (EHOMO = -5.06 eV) is slightly raised in
comparison with the parent non-substituted diindolocarbazole comparison to the previously reported HOMO levels of N,N-
26
, the first oxidation potential decreased with the number of dimethylindolo[3,2-b]carbazole [EHOMO = -5.12 eV] and N-
§
methoxy-group substituents increased. For example for the methylcarbazole [EHOMO = -5.64 eV]. The increase of extension
series 26
27, and 28 having none, two and four methoxy of -conjugation from N-methylcarbazole to N,N-dimethylin-
,
π
substituents respectively, the half-wave potential E1/2 dolo[3,2-b]carbazole and to N,N,N-trihexyl-diindolo[3,2-b:2’3’-
decreases in the order 0.47, 0.45, and 0.39 V. In these cases h]carbazole 26 correlates well with the observed moderate
the methoxy groups introduced onto the diindolocarbazole increase of HOMO energies.
framework simply act as strong electron-donating groups. Notably, the low-energy HOMO levels of -5.06 eV to -4.92 eV
However, distinguished contributions are caused by the found for the diindolo[3,2-b:2’3’-h]carbazoles 26-30 makes
substitution pattern as revealed by a more detailed analysis of them potentially suitable for hole injection and hole transport
the corresponding HOMO/LUMO energy levels; especially for in a thin-film based electronic device – and should guarantee a
the set 28
different positions.
From the onset of the first oxidation half-wave ( Eonset) the (LUMO) was calculated by adding the value of the optical band
,
29, and 30 having all four methoxy groups in reasonable long-term stability.
The energy level of the lowest unoccupied molecular orbital
ox
opt
HOMO energy level for the diindolocarbazoles 26-
30 was esti- gap (
ox
mated with: EHOMO = -( E
g
E in Table 2) taken from the onset of the long-wave-
+
vs Fc/Fc + 5.10) (eV) for length absorption band of the UV-vis absorption spectra to the
onset,
opt
measurements in dichloromethane solution with 0.1 M HOMO energy level with ELUMO = EHOMO
+
g
E . The experimen-
6
| J. Name., 2012, 00, 1-3
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Journal Name
tally established HOMO/LUMO energy levels of the here
ARTICLE
0
1
2
3
synthesized and investigated diindolocarbazoles 26-30 are
listed in Table 3.
-
1.172
-
-
-
-1.278
-2.34
-
1.227
-1.060 -1.270
-
1.351
Theoretical Calculations
-1.462 -1.436
-2.538 -2.516
With the aim to gain further insight how substitution degree
and pattern influence the HOMO energy level and the HOMO-
LUMO gap of diindolo[3,2-b:2’3’-h]carbazoles, and to establish
a method to predict these parameters for compounds not
synthesised, theoretical calculations were carried out.
Calculations were performed with methyl- instead of n-hexyl
groups on the nitrogen atoms for reasons of simplicity (the
computational generated diindolo[3,2-b:2’3’-h]carbazoles are
-2.24
-
2.36
-2.39
-2.394
-2.47
-4
4.593 -4.535 -4.497
-4.96
-4.92
-
-
4.690 -4.663
-4.706 -4.727
-5.074
-
4.8
-4.768
-5.117
-
5.06
-5.04
-5.00
-5.088
-5
ITO
labelled as 26*, 27*, 28*, 29*, and 30* to distinguish them
from the synthesised corresponding counterparts bearing N-n-
hexyl groups). We applied density function theory (DFT) using
the hybrid-generalized gradient approximation functional
26/26* 27/27* 30/30* 28/28* 29/29*
31*
32*
33*
Figure 5. Experimentally obtained HOMO/LUMO energies (solid lines) for 26-29 and
estimated experimental values for 31*-33*, as well as calculated (dashed lines)
3
5
B3LYP with the split-valence polarized basis set size 6- HOMO/LUMO energy levels of the heteroacenes 26*-33*. Calculation level DFT
3
6
3
2 2
1G(d) and a Conductor – like Polarisable Continuum Model B3LYP/6-31G(d) in CH Cl .
3
7
(
CPCM)
2 2
to allow for the solvent (CH Cl ) used in
electrochemical and UV measurements, as implemented in
3
8
Gaussian-09 (rev. D.01). Several studies have shown that
calculated HOMO-LUMO band gaps are linearly correlated to
observed band gaps, at least within series of related
3
9
compounds, and B3LYP/6-31G(d) performs well. The values
we obtain for HOMO and LUMO levels of 26*-30*, and
consequent energy gaps are given in Table 4. Figure 5 shows
the comparison of empirically determined HOMO/LUMO
energy levels via combination of electrochemical and optical
measurements, and calculated ones obtained from the
B3LYP/6-31G(d) DFT calculations. We observe a reasonable
linear correlation (Figure 6) between the observed optical
band gap (Table 2) and HOMO-LUMO separation (Table 4), and
(
a) 3.55$
(b)
!4.45%
st
3.5$
between the 1 oxidation potential (Table 3) and the HOMO
!4.5%
3
.45$
.4$
.35$
.3$
energy level (Table 4).
3
!4.55%
a
Table 4. Calculated HOMO, LUMO and band-gap for compounds 26*-30*.
3
!4.6%
b
b
b
3
Comp.
HOMO (eV)
LUMO (eV)
Band gap (eV)
!4.65%
2
2
2
2
3
6*
7*
8*
9*
0*
-4.690
-4.663
-4.535
-4.497
-4.593
-1.351
-1.227
-1.172
-1.278
-1.060
3.339
3.25$
3.2$
3.437
!4.7%
!5.1%
2
.5$
2.6$
2.7$
2.8$
!5.05%
!5%
!4.95%
!4.9%
3.363
Observed)op7cal)gap)
Observed)Oxida3on)Poten3al)
3.220
Figure 6. (a) Correlation between calculated and observed band gaps. (b) Correlation
5.533
between calculated and observed HOMO levels. Energies in eV.
a
All energies were calculated using the Gaussian 09 rev D.01 program using the
B3LYP functional with 6-31G(d) basis set and a CPCM solvent model for CH Cl
2
2
.
The molecular orbitals for the parent diindolocarbazole 26*
the 3,10-dimethoxy-27* and the three tetra-methoxy
derivatives 28* 30* are displayed in Figure 7. The geometry of
,
-
both the HOMO and LUMO of the methoxy substituted
compounds are very similar to those of the parent
3
.
0a
diindolo[3,2-b:2’3’-h]carbazole 26*
The coefficients of the
HOMOs are mainly localized on the middle carbazole moiety,
with smaller contributions on the two peripheral carbazole
units. The changes in HOMO and LUMO levels are roughly
consistent with methoxy substitution on carbons with a high
orbital coefficient for that orbital raising their energy. For the
HOMO there are 0 methoxy substitutions on high coefficient
carbons for 26/26*
,
27/27* and 30/30*; 2 for 28/28* and 4 for
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2
9/29*, and for LUMO 0 for 26
29*, 4 for 30 30*). Notably, HOMO/LUMO energy level lead to molecular surface extended but also to more stable
changes do not change in the same direction, for example semiconductors making these heteroatom extended molecular
from 29 29* to 30 30* the HOMO increases and the LUMO versions of heteroacenes challenging synthetic targets.
/
26*, 2 for 27
/
27*, 28
/
28*, and
π-perimeter of an diindolocarbazole framework will not only
29
/
/
/
/
decreases in energy (Figure 5).
Table 5. Calculated HOMO and HOMO-LUMO gap for compounds 31*-32* and
The good correlation observed between experimental and
calculated HOMO and band-gap values (Figure 6) were used to
estimate the experimental values for HOMO, LUMO and band-
gap we would expect to observe for the molecules 31*-33* for
which the current synthetic method failed (Table 5 and Figure
a
estimates for experimental values using correlations from Figure 6.
a
Calculated Values
HOMO (eV)
Predicted Experimental
values
Comp.
Band gap (eV)
HOMO
eV)
Band gap
(eV)
(
5
).
The expected experimental HOMO energies for the
heteroacenes 31* 33* are significantly lower than those for
the experimentally investigated diindolocarbazoles 26 30
indicating that the formal placement of heteroatoms into the
3
3
3
1*
2*
3*
-4.706
-4.727
-4.768
3.436
3.265
3.332
-5.074
-5.088
-5.117
2.680
2.550
2.601
-
-
a
b
B3LYP/6-31G(d) in CH
2
Cl
2
. using the appropriate correlations from Figure 6.
Figure 7. Calculated HOMOs and LUMOs of diindolocarbazole 26*, dimethoxy-diindolocarbazole 27* and the tetramethoxy-diindolocarbazoles 28*-30*.
calculated and experimentally measured energies of the
HOMO and HOMO-LUMO gaps.
Conclusions
Photoluminescence properties of the diindolocarbazoles 26-30
in solution are characterized by light emission in the blue to
em
Consecutive twofold Suzuki, Cadogan, and
N-alkylation reac-
blue-green region (
φF = 0.2-0.3.
λ
= 430-530 nm) with quantum yields of
tions serve as step-economic and efficient sequence for the
rapid construct of the diindolo[3,2- :2’3’- ]carbazoles frame-
work. Besides the parent non-substituted diindolocarbazole
, four new diindolo[3,2- :2’3’- ]carbazoles bearing two or
b
h
The 1,4,9,12-teramethoxy-diindolo[3,2-
b
:2’3’-h]carbazole 28 is
the first diindolocarbazole reported with noticeable
photoluminescence from the solid state. In comparison to the
blue light emission in solution the photoluminescence of solid
26
b
h
four methoxy groups at different positions have been
selectively synthesized. The developed synthetic sequence for
2
8 is significantly red-shifted resulting in a bright yellow
em
diindolo[3,2-b:2’3’-h]carbazoles is appealing by both, the
fluorescence ( λ = 530-650 nm) and a photoluminescence
peak maxima at λmax = 562 nm. This excimer-type emission
number of synthetic steps and the overall yield. It advances
the chemistry of 7-ring linear fused diindolocarbazoles as
potential materials for organic electronics. Physicochemical
properties and MO calculations of methoxy-substituted di-
em
from solid 28 is distinguishable from the poor solid state
photoluminescence
of
the
other
investigated
diindolocarbazoles (26
,
27, 29 and 30) in this series and
indolo[3,2-b:2’3’-h]carbazoles reveal that number and
underlines that
N-alkyl and methoxy-group substitution
substitution pattern of methoxy groups significantly affect
their electronic properties. Methoxy groups introduced at the
pattern are critical to balance excimer-type emission and
aggregate caused quenching effects by favouring the former.
The highest HOMO energy level for this set of compounds was
2,11- and 4,9-position have a direct influence on the electronic
structures and raise the HOMO-energy levels in comparison to
the parent non-substituted system. The observed changes of
HOMO-LUMO energies caused by the methoxy substituents
can serve as fine-tuning of optical end electronic properties.
found for the 2,4,9,11-tetramethoxy-diindolo[3,2-
b:2’3’-
h
]carbazole (29) both by empirical and computational
methods. By chemical comprehension this result can be under-
stood such as that methoxy-substituents in para- and ortho-
position to a carbazole nitrogen (i.e. the positions of
favourable electrophilic aromatic substitution in carbazoles)
Even using a very cost effective B3LYP/6-31G(d) (CH
2 2
Cl ) DFT
calculation a good linear correlation was observed between
8
| J. Name., 2012, 00, 1-3
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Journal Name
ARTICLE
contribute the most to the increase of electron density of the
aromatic system.
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6
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Acknowledgements
,
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§
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0 | J. Name., 2012, 00, 1-3
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Suggestion for Graphical abstract:
Synthesis and Molecular Properties of Methoxy-Substituted Diindolo[3,2-b:2’,3’-h]carbazoles for
Organic Electronics Obtained by a Consecutive Twofold Suzuki and Twofold Cadogan Reaction
Hassan Srour, Thu-Hong Doan, Elisabeth Da Silva, Richard J. Whitby and Bernhard Witulski
J. Mater. Chem. C, 2016, Advance Article
DOI:
Received
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