Eaton's reagent assisted aromatic C-C coupling of carbazoles for optoelectronic applications

Article abstract of DOI:10.1039/c8nj03296c

An Eaton's reagent assisted C(sp²)-C(sp²) coupling via the Scholl reaction mechanism was developed to prepare bicarbazoles without using any solvents and catalysts. The resulting biaryl exhibits unique optoelectronic properties as a promising host material of blue phosphorescent organic light emitting diodes, achieving high current efficiency up to 31.7 cd A^-1 with ultralow roll-off.

Full text of DOI:10.1039/c8nj03296c

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PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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Received 00th January 20xx,
Accepted 00th January 20xx
Eaton's reagent Assisted Aromatic C-C Coupling of Carbazoles for
Optoelectronic Applications
†
DOI: 10.1039/x0xx00000x
a
a
a
a
a
a
a
Jie Yuan, Lu Jin, Runfeng Chen, * Xingxing Tang, Xiang Xie, Yuting Tang, Wei Huang*
www.rsc.org/
2
2
An Eaton's reagent assisted C(sp )-C(sp ) coupling in Scholl approach, which seems to be atomically economic, generally
reaction meachism was developed to prepare bicarbazoles requires oxidants of FeCl , MoCl , and K [Fe(CN) ] or corrosive
without using any solvents and catalysts. The resulting biaryl AlCl at high temperature, leading to troubles in the post
exhibits unique optoelectronic properties as promising host treatments of the reaction .
3
5
3
6
3
1
3
1
4
material of blue phosphorescent organic light emitting diodes,
achieving high current efficiency up to 31.7 cd/A with ultralow pentoxide (P
roll-off. alternative to polyphosphoric acid in chemical synthesis to
Eaton’s reagent , composed of 7.5 wt% phosphorus
2 5
O
) in methanesulfonic acid, has been used as an
15
promote acylation reactions . As a mobile and colorless liquid,
this reagent is inexpensive, readily available, and safe to
handle with easy and clean work-ups of the reaction mixture
and organic compounds can be dissolved readily in this
Dimerization of aromatic molecules is an important
strategy in designing high-performance organic optoelectronic
materials and the resulting biaryls generally show improved
thermal physical and photochemical properties for a wide
14
medium . Here, we found occasionally that the
dehydrogenative coupling of phenylcarbazole (PhCz) can be
processed facilely in Eaton’s reagent to get the dimer of 9, 9'-
diphenyl-9H, 9'H-3, 3'-bicarbazole (DPhBCz) without using any
catalysts or solvents. With the careful optimization of the
reaction conditions and mechanism research, the newly
observed aromatic C-C coupling was revealed to be a kind of
Scholl reaction involving Lewis acid mediated arenium cation.
The geometric and electronic structures of thus obtained biaryl
1
-3
range of device applications . For example, bicarbazoles,
4
which are dimers of carbazole linked directly via C-C , C-N , or
5
6
N-N chemical bond, have been extensively studied as
luminescent, hole-transport and host materials in
optoelectronics owing to the good hole transport, excellent
thermal stability and high triplet energy of the rigid
7
-9
bicarbazole core . To chemically construct the aryl-aryl C-C
linkage, two basic synthetic approaches are available (Scheme
1). The first one is metal-catalyzed cross-coupling between
were
investigated
systematically
through
X-ray
aromatic halide and organic boron, aryl-magnesium halides or
organostannanes through Yamamoto, Suzuki, Kumada or Stille
crystallography, optical spectroscopy, cyclic voltammetry (CV),
and density functional theory (DFT) calculations. We also
explored the device performance of the biaryl as host
materials of blue phosphorescent organic light emitting diodes
1
0, 11
coupling
. The second one is dehydrogenative coupling via
double C-H activation by either Lewis acid-mediated Scholl
1
2
reaction or oxidant-mediated oxidative aromatic coupling .
The first approach, although has high diversity in preparing
various biaryls, needs two different precursors and metal
complex catalysts, which would bring significant difficulties in
the mass production with high costs in materials. The second
(
PhOLEDs) and a high external quantum efficiency (EQE) up to
1
6.9% with very low efficiency roll-off was achieved.
Scheme 1. (a) Metal-catalyzed cross-coupling and (b) dehydro-
genative coupling for aromatic C-C bonding to construct biaryls.
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suggested to be performed in Eaton’s reagent with 0.1 equiv.
at 110ºC for 12 h.
To reveal the mechanism of this extraordinary aromatic C-C
coupling, varied reaction conditions were explored (Table S2).
Using only P , which is one component of Eaton’s reagent,
no reactions can be processed, suggesting that this reaction
cannot belong to oxidative coupling, since P is the only
2 5
P O
2 5
O
2 5
O
oxidant in the reagent. Using only methanesulfonic acid, which
is the other component of Eaton’s reagent, still no reactions
can be observed. Moreover, when organic solvents were
added into the reaction mixture, the coupling cannot be
conducted; but, the reaction can proceed in air with lower
yields. With these observations, we can exclude the oxidative
radical cation mechanism of the dehydrogenative C-C coupling
and propose that this Eaton’s reagent assisted aromatic C-C
coupling is through an arenium cation mechanism as in Scholl
1
3
reaction (Figures 1a and S2) . Also, the clean reaction mixture
without various intermediates and byproducts (Figure S1)
support the arenium cation mechanistic pathway, because the
first C-C bonding is the slowest. Thus, in the initiation step,
PhCz combines a proton ionized from the methanesulfonic
acid in Eaton’s reagent at C-3 position, which is far from
molecular center and possesses the highest reaction activity.
Then, C-3 is transform by
a
rapid intramolecular
Figure 1. (a) Mechanisms of Suzuki coupling and the
intermolecular Scholl reaction and (b) single-crystal structure
and packing diagrams of DPhBCz.
rearrangement to a carbocation, which subsequently attacks
another PhCz at its C-3 position for the first C-C coupling by
delivering the combined proton. After the coupling,
dehydrogenation/aromatization further takes place to
Extraordinarily, DPhBCz was facilely prepared from the regenerate the stable biaryl structure through hydrogen
commercially available PhCz in one step with a simple work-up elimination.
1
The resulting biaryl was fully characterized by H NMR, C
13
method using Eaton’s reagent (Scheme 1). Compared to the
commonly used Suzuki coupling reaction et al (Table S1), which NMR, HRMS spectrum, and X-ray crystallography (Figures S3-6
requires separately prepared boron and bromide precursors, and Table S3). An extended molecular planarity and π-
Pd(0) complex and phase transfer catalysts, and organic conjugation can be identified from the single crystal structure,
1
6
solvents, the newly developed method is much more leading to excellent thermal properties with high melting
convenient by simply heating the mixture of PhCz and Eaton’s temperatures (T ) of 208ºC and decomposition temperature
m
reagent at 110ºC for 12 h without using any catalysts and of 413 ºC (Figure S7 and Table S4). The crystals of DPhBCz
solvents. And, excessive reagent can be destroyed (CCDC 1579239) exhibit a staggered packing pattern (Figure
1
8
conveniently with water to afford the target compound and 1b) . In each of the π-stacked column, the molecules were
the remaining PhCz without any other by-products or alternatively oriented to the reverse directions, adopting a
intermediates, as confirmed by the in situ liquid slipped stacking. Such kind of packing diagram would be
chromatography measurements (Figure S1). Moreover, the helpful for the charge transport to function as organic
1
9
unreacted PhCz can be reused in the next cycle of the Eaton’s semiconductors . In addition, the highly planar structure of
reagent assisted C-C coupling.
DPhBCz is beneficial for the charge carrier delocalization,
To optimize the reaction, various amount of Eaton’s which may reduce the reactivity of the C-3 position in the C-C
reagent at different reaction temperatures for varied time coupling; this explains why only dimer was obtained without
1
7
were tested (Table S2) . Using 1.5 g starting material of PhCz the formation of any other oligomers. Further, X-ray
6.2 mmol), the yield of the reaction increases when the photoelectronic spectroscopy (XPS) shows that the binding
amount of Eaton’s reagent increases to 0.78 mL (0.1 equiv. of energy of N 1s of DPhBCz (399 eV) is significantly higher than
) for the highest isolated yield of 46%. Excessive Eaton’s that of PhCz (398 eV), suggesting the increased valent state of
reagent cannot help the reaction but will reduce the yield to N 1s and the decreased dehydrogenative coupling in DPhBCz
(
2 5
P O
2
0
3
5% when 1.56 mL Eaton’s reagent was used. Also, the for the formation of other oligomers (Figure 2a) . Excellent
reaction time shorter than 12 h and temperature lower than solubility of DPhBCz was identified and good morphology of
10ºC is insufficient, and prolonged reaction time and higher the vacuum evaporated films was also revealed by atom force
1
temperature are harmful, leading to lower yields. Therefore, microscopy (AFM) (Figure S8), showing smooth and uniform
this Eaton’s reagent assisted dehydrogenative C-C coupling is morphology with root-mean-square (RMS) surface roughness
2
| J. Name., 2012, 00, 1-3
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20
lower than 0.54 nm in either pure or guest doped (15 wt%) basis of the reference energy level of ferrocene (-4.80 eV) . A high-
films. lying HOMO of DPhBCz was found to be at -5.45 eV, which is very
UV-vis absorption spectra of PhCz and DPhBCz were close to the widely used hole-transport materials. Further, from the
opt
investigated in both dilute CH
2 2 g
Cl solution and thin solid film (Figure HOMO energy levels and the optical energy gaps ( E ), the lowest
2
b). The strong absorption peak at ~300 nm and the weak unoccupied molecular orbital (LUMO) energy level was deduced to -
23
absorption bands around 340 nm originated from the π-π* and n- 2.01 eV . Therefore, increased HOMO and slightly decreased
LUMO were resulted in DPhBCz after the dimerization of PhCz.
These changing tendency of the frontier orbital energy levels is well
confirmed by the density functional theory (DFT) calculations
2
4
(
Figure 2c) . Furthermore, both HOMO and LUMO isosurfaces are
delocalized on the whole carbazole and bicarbazole, leading to
lower bandgap of DPhBCz with larger π-conjugation. The lowest
triplet states (T ) of PhCz and DPhBCz were also found to be
1
determined by carbazole or bicarbazole from the triplet spin density
distributions (TSDD); the delocalized TSDD in DPhBCz explains well
its slightly lower E found experimentally. Also, the electrostatic
T
2
5
potential (ESP) is delocalized on the whole molecular architecture,
indicating the effective extension of π-conjugation in DPhBCz for its
high charge transport properties.
In light of the good optical and electronic properties of the
biaryl, especially its high-lying T
1
energy, we investigated its
performance as host material of PhOLEDs using the sky-blue FIrpic
as the phosphorescent guest under the following two different
2
6
device configurations : ITO/PEDOT:PSS (30 nm)/TAPC (25
nm)/DPhBCz:15 wt % FIrpic (20 nm)/DPEPO (10 nm)/TmPyPB (35
nm)/LiF (0.5 nm)/Al (100 nm) in Device A and ITO/PEDOT:PSS (30
nm)/TAPC (25 nm)/ mCP (10 nm)/ DPhBCz:15 wt % FIrpic (20
nm)/TmPyPB (35 nm)/LiF (0.5 nm)/Al (100 nm) in Device B (Figure
S11). In the devices, poly(3,4-ethylenedioxythiophene): poly
Figure 2. (a) X-ray photoelectron spectroscopy (XPS) spectra of N
s of PhCz and DPhBCz. (b) UV-vis absorption (left) and
fluorescence (right) spectra in dilute CH Cl and thin films at
room temperature (c) DFT calculated (black) and experimental
red) energy levels of HOMO, LUMO, and bandgap (E ) (in eV) , as
1
2
2
(
g
well as isosurfaces of HOMO, LUMO, triplet spin density
distribution (TSDD) and electrostatic potential (ESP) of PhCz and
DPhBCz.
(
styrenesulfonic acid) (PEDOT:PSS) and LiF serve as the hole- and
electron-injecting materials, respectively; di-[4-(N,N-ditolyl-amino)-
phenyl]cyclohexan (TAPC) and 1,3,5-tri[(3-pyridyl)-phen-3-
9
yl]benzene (TmPyPB) act as the hole and electron transport
π* transitions of the carbazole moiety respectively , are slightly red-
shifted (5 nm) in DPhBCz owing to the conjugation plane expansion.
From the onsets of the absorption spectra in solid films, the optical
opt
energy gaps (
g
E ) were deduced to be 3.60 and 3.44 eV for PhCz
and DPhBCz, respectively. Similar to the absorption spectra, the
emission spectra of DPhBCz were also red-shifted compared to
PhCz, exhibiting emission peaks at 389 and 405 nm in solution and
3
5
91 and 410 nm in thin film. A weak excimer emission band around
60 nm can be observed in the thin film of DPhBCz, suggesting its
2
1
strong intermolecular interactions in the solid states .
Phosphorescent spectra of PhCz and DPhBCz were measured in 2-
methyltetrahydrofuran at 77 K with a delay time of 10 ms (Figure
S9). From the highest energy vibronic sub-bands of the
phosphorescence spectra, the triplet energies (E
to be 3.05 eV for PhCz and 2.82 eV for DPhBCz. Although DPhBCz
has slightly lower E than its precursor, this value is still high enough
T
) were determined
T
for the application as host materials of blue phosphors, such as
bis[2-(4,6-difluorophenyl)pyridinato-C2,N](picolinato) iridium (III)
2
2
(
FIrpic) with a E
T
of 2.65 eV .
Figure 3. (a) Current density-voltage-luminance of Device A and B.
(b) Current efficiency (CE), power efficiency (PE) and external
quantum efficiency (EQE) versus luminance. (c) Current density-
voltage characteristic of the single-carrier transporting devices of
DPhBCz. Solid and open symbols are for electron-only and hole-
only devices, respectively. (c) Electroluminescence (EL) spectra
recorded at 5 V and the insets showing the photography of Device
A and B under the operating voltage of 5.0 V.
The electrochemical behaviors of PhCz and DPhBCz films were
examined by cyclic voltammetry (CV). Reversible oxidation
processes were observed in the CV profiles (Figure S10) and the
highest occupied molecular orbital (HOMO) energy levels can be
evaluated from the onset of the first oxidation potential on the
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Table 1. Electroluminescent properties of the blue PhOLEDs hosted dimerization of carbazoles for high-performance optoelectronic
by DPhBCz.
applications.
Financial support for this research was provided by the National
Natural Science Foundation of China (21304049, 21674049, and
1703077), Science Fund for Distinguished Young Scholars of
[
b]
[b]
V
on
Maximum
Efficiency
CE PE EQE
Roll-off
CE PE EQE
Device
[a]
(
V) efficiency
1.7
18.0
2
3
28.8 16.1 15.8
30.0 15.2 16.7
9.1 13.0
26.5 14.8 12.4
29.7 14.5 14.4
9.1 10.6 6.5
5.4 15.6 1.2
20.5 49.4 23.1
11.7 5.1 15.1
A
3.6
Jiangsu Province of China (BK20150041), the Six Talent Plan of
Jiangsu Province (2016XCL050), and Postgraduate Research &
Practice Innovation Program of Jiangsu Province (46030CX17750).
1
3
6.9
0.0
25.2
B
3.8
15.6
4.6
1.0
7.1
1.4
1
25.3
9.0 12.3
15.7 42.3 15.8
Conflicts of interest
[
a]
-1
-1
[b]
In the order of CE (cd·A ), PE (lm·W ), and EQE (%); In
-
2
There are no conflicts to declare.
the order of 100, 1000, and 10000 cd·m (%).
materials,
bis(diphenylphosphoryl)diphenylether
bis(carbazol-9-yl)benzene (mCP) with high E
respectively;
whereas,
and
are introduced as
(DPEPO)
1,3- Notes and references
T
1
.
S. Feuillastre, M. Pauton, L. Gao, A. Desmarchelier, A. J. Riives,
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Pieters, J. Am. Chem. Soc., 2016, 138, 3990.
exciton blocking material to confine the excitons in the emitting
layer of the devices.
2
.
L. V. Ivanova, T. S. Navale, D. Wang, S. Lindeman, M. V. Ivanov
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The DPhBCz-hosted blue PhOLEDs exhibit low turn-on voltages
of 3.6 and 3.8 V and high maximum luminance of 24300 and 24150
-2
3. Y. Inoue, D. Sakamaki, Y. Tsutsui, M. Gon, Y. Chujo and S. Seki, J.
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cd m for Device A and B, respectively (Figure 3a). High maximum
-1
current efficiencies (CE) of 31.7 and 30.0 cd A , power efficiencies
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(a) K. Okura, T. Teranishi, Y. Yoshida and E. Shirakawa, Angew.
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(
PE) of 18.0 and 15.6 lm W , and EQE of 16.9% and 14.6% were also
achieved in corresponding Device A and B (Figure 3b). The single
carrier devices reveal that the hole transport mobility of DPhBCz is
about 3 orders of magnitude higher than the electron mobility
1
996, 118, 3930.
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.
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1605444.
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Device B due to the higher hole transporting feature of DPhBCz,
which moves the charge combination zone closer to the cathode
side. Both Device A and B show typical sky-blue emission from FIrpic
with the Commission Internationale de l’Eclairage (CIE) coordinates
of (0.15, 0.34) and (0.16, 0.37) respectively, indicating that the
triplet exitons were well confined in the emitting layer for FIrpic
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-2
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16
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-2
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4
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Eaton's reagent Assisted Aromatic C-C Coupling of Carbazoles
for Optoelectronic Applications†
a
a
a
a
a
a
Jie Yuan, Lu Jin, Runfeng Chen, * Xingxing Tang, Xiang Xie, Yuting Tang, Wei
Huang*a
Bicarbazoles can be facilely synthesized by Eaton’s reagent assisted C-C coupling in
Scholl reaction mechanism without using solvents and catalysts.