Synthesis and characterization of heteroatom substituted carbazole derivatives: Potential host materials for phosphorescent organic light-emitting diodes

Article abstract of DOI:10.1039/c2nj40900c

A series of carbazole derivatives based on 9-ethyl-carbazole substituted at the 3-position with diphenylamine, carbazole, phenoxazine, phenothiazine, and phenothiazine-S,S-dioxide units were successfully developed, and a comparative study of their thermal and photoelectrical properties was carried out by differential scanning calorimetric measurements, thermogravimetric analysis, UV-vis absorption spectroscopy, photoluminescence spectroscopy, cyclic voltammetry, and phosphorescent spectroscopy. All the synthesized compounds were found to have high thermal decomposition temperature in the range of 304-344 °C (except for C-SO, 145 °C) and showed a remarkable improvement when compared with 9-ethyl-carbazole (180 °C). The emission properties can be effectively tuned by systematically changing the substituent units. The highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO), and triplet energy (E_T) of the synthesized compounds were determined both theoretically by quantum chemical calculations and experimentally by examination. Their energy levels can also be tuned by the change of the moieties connected to the 3-position through coupling reaction. The experimental values of ionization potentials range from 5.37 to 5.46 eV. Some of these compounds exhibit high E_T, and C-SO achieves the highest E_T value of 2.79 eV, which exhibits a marked improvement over that of 4,4′-bis(9- carbazolyl)-biphenyl (2.6 eV). Green phosphorescent devices using tris(2-phenylpyridine)iridium as a guest and C-O as a host show excellent performances. Remarkably, a device with C-O as host exhibits the best performance with a maximum luminance of 10270 cd m^-2, a maximum current efficiency of 34.8 cd A^-1, and a maximum power efficiency of 26.0 lm W^-1. These results illustrate that the introduction of heteroatoms into carbazole can result in a significant change in their optoelectronic characteristics. The low LUMO, high HOMO, suitable E_T and thermal stability of the carbazole derivatives endow them with the potential to be green, red, and even blue host materials for phosphorescent organic light-emitting diodes.

Full text of DOI:10.1039/c2nj40900c

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New Journal of Chemistry
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ARTICLE TYPE
Synthesis and Characterization of Heteroatom Substituted Carbazole
Derivatives: Potential Host Materials for Phosphorescent Organic
Light-emitting Diodes
5
Jian Sun, Hong-ji Jiang ∗a, Jin-long Zhang, Ye Tao, and Run-feng Chen
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
A series of novel heteroatom substituted carbazole derivatives based on 9ꢀethylꢀcarbazole, diphenylamine,
carbazole, phenoxazine, phenothiazine, and phenothiazineꢀS,Sꢀdioxide units were successfully developed,
10 and a comparative study on their thermal and photoelectrical properties were investigated by differential
scanning calorimetric measurement, thermogravimetric analysis, UVꢀvis absorption, photoluminescent
spectra, cyclic voltammetry, and phosphorescent spectra. All the synthesized compounds were found to
have high thermal decomposition temperature in the range of 304ꢀ344 oC (except for CꢀSO, 145 oC) and
showed a remarkable improvement when compared with 9ꢀethylꢀcarbazole (180 oC). The emission
15 properties can be effectively tuned by systematically changing the substituent units. The highest occupied
molecular orbital (HOMO), lowest unoccupied molecular orbital(LUMO), and triplet energy (ET) of the
synthesized compounds were determined both theoretically by quantum chemical calculations and
experimentally by examination. Their energy levels can also be tuned by the change of the moieties
connected to the 3ꢀposition through CꢀN coupling reaction. The experimental values of ionization
20 potentials range from 5.37 to 5.46 eV. Some of these compounds exhibit high ET, and CꢀSO achieves the
highest ET value of 2.79 eV, which exhibits a marked improvement than that of 4,4'ꢀbis(9ꢀcarbazolyl)ꢀ
biphenyl (2.6 eV). Green phosphorescent devices using tris (2ꢀphenylpyridine) iridium as guest and CꢀO
as host show excellent performances. Remarkably, one device hosted by CꢀO exhibits the best
performance with a maximum luminance of 10270 cd mꢀ2, a maximum current efficiency of 34.8 cd Aꢀ1, a
25 maximum power efficiency of 26.0 lmWꢀ1. The results illustrate that the introduction of heteroatoms into
carbazole can result in significant change in their optoelectronic characteristics. The low LUMO, high
HOMO, suitable ET and thermal stability of the carbazole derivatives endow them with potential as green,
red, and even blue host materials for phosphorescent organic lightꢀemitting diodes.
1. Introduction
quenching, a phosphorescent emitter is dispersed in a suitable
Phosphorescent organic lightꢀemitting diodes (PHOLEDs) have
recently attracted considerable attention mainly because it is
possible to increase the internal quantum efficiency of the devices
up to 100% as a result of their harvesting of both singlet and
triplet excitons. [1] Generally, in order to suppress concentration
____________________________________________________
host material to obtain a high photoluminescent (PL) quantum
yield. Good host materials used in PHOLEDs commonly have the
following properties: (1) a triplet energy(ET) larger than that of
the phosphorescent guest, which facilitates an efficient energy
transfer from the host to the guest and prevents reverse energy
transfer from the guest back to the host; (2) appropriate values of
highest occupied molecular orbital (HOMO) energy and lowest
occupied molecular orbital (LUMO) energy to facilitate charge
injection from adjacent holeꢀtransporting and electronꢀ
transporting layers, which is also the key to balance the
transporting of holes and electrons; (3) high glass transition
temperatures (Tg) and thermal decomposition temperatures (Td)
confer better device endurance.
Key Laboratory of Organic Electronics and Information Displays,
Institute of Advanced Materials, Nanjing University of Posts and
Telecommunications Nanjing 210046, China
Eꢀmail: iamhjjiang@njupt.edu.cn
†Electronic Supplementary Information (ESI) available: NMR , the
details of quantum chemical calculations. See DOI: 10.1039/b000000x/
This journal is © The Royal Society of Chemistry [year]
[journal], [year], [vol], 00–00 | 1

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Both H NMR and 13C NMR spectra were measured on a Varian
Mercury Plus 400 spectrometer with CDCl3 as solvent and
tetramethylsilane as internal reference. Chemical shifts were
reported in ppm relative to CDCl3 as internal standard, which was
set to be 7.26 ppm, and coupling constant (J) was expressed in
hertz. The signals have been designed as follows: s (singlet), d
(doublet), t (triplet), m (multiplets). Matrix assistant laser
desorption/ionization timeꢀofꢀflight mass (MALDIꢀTOFꢀMS)
spectrometry was performed on a Buker Daltonics flex Analysis.
Elemental analysis of carbon, hydrogen and nitrogen was
obtained on Elementar Vario MICRO elemental analyzer. UVꢀ
Vis absorption spectra were recorded on an UVꢀ3600
SHIMADZU UVꢀVisꢀNIR spectrophotometer. PL spectra were
recorded on a RFꢀ5301PC spectro fluorophotometer with a xenon
lamp as a light source. The concentration of these compounds
solution (in CHCl3) was adjusted to be about 0.01 mg/mL or less.
The thin solid films were prepared by spinꢀcoating on quartz
substrates from solution in CDCl3 at a spin rate of 3500 rpm. The
phosphorescence spectra of the compounds were measured using
an Edinburgh LFS920 fluorescence spectrophotometer at 77 K,
with a 5 ms delay time between the excitation with a microsecond
flash lamp. Differential scanning calorimetry (DSC)
measurements were carried out using a PerkinꢀElmer Diamond
DSC at a heating rate of 100C min–1 from room temperature to
500 oC under nitrogen. Thermogravimetric analysis (TGA) was
undertaken using a SEIKO EXSTAR 6000 TG/DTA 6200 unit
under nitrogen atmosphere at a heating rate of 10 oC min–1. Cyclic
voltammetric measurements were carried out on the Chi660e
system in a conventional typical threeꢀelectrode cell with a Pt
work electrode (glass carbon), a platinumꢀwire counter electrode
1
Among host materials, carbazoleꢀbased materials have received
special attention for their sufficient ET (up to 3.05 eV) and
excellent holeꢀtransporting properties, wellꢀpositioned HOMO[2ꢀ
9].The most commonly used host material of carbazole derivatives
in organic lightꢀemitting diodes(OLEDs) is 4,4'ꢀbis(9ꢀcarbazolyl)ꢀ
biphenyl, which is used along with the iridium(III) bis[[4,6ꢀ
difluorophenyl] pyridinatoꢀN,C2] picolinate to fabricate blue
PHOLEDs with high quantum efficiency.[10] So far, many host
materials, such as carbazole derivatives containing diarylamine
units,[11] phenylsilane[12,13] and phosphine oxide moieties,[14,15]
heterofluorenes,[16] and heterocyclic cores,[17ꢀ22] carbazoleꢀbased
conjugated dendrimer units,[23] and noncoplanar units,[24] with
high Tg values have been reported, where the carbazoles were
substituted on the 2,7ꢀposition, 1,8ꢀposition, 3,6ꢀposition or Nꢀ
position, respectively.
Recently, phenoxazine and phenothiazine derivatives were
synthesized in an attempt to modify the HOMO/LUMO energy
level and further improve the device performance [25ꢀ26]. The
meterials containing phenothiazine and phenoxazine have been
mainly developed as pꢀtype or ambipolar materials that can
transport hole and/or electron in OLEDs. Especially, the materials,
which were incorporated with heterocyclics and heteroatoms to
break points of πꢀconjugation, have attracted more attention. The
chemical modification of the heterocyclic could induce new
desired properties to the material. These attractive features can
promise tremendous opportunities as
a host material in
PHOLEDs. However, there is still room for improvement, as
these host materials still present issues related to large band gaps,
unbalanced charge transporting, and low device power efficiency.
and
a
Ag/Ag+ reference electrode (referenced against
acetonitrile solution of
ferrocene/ferrocenium (FOC)) in
a
Bu4NPF6 (0.10 M) at a sweeping rate of 100 mV/s. The HOMO
and LUMO were measured by CV and calculated according to
the formula: HOMO=ꢀEoxi+0.0042 ꢀ 4.8, LUMO=ꢀEredꢀ0.061 ꢀ 4.8,
where the 0.0042 and ꢀ0.061 are the Eoxi and Ered of FOC
respectively. Eoxi , Ered are the values of compounds.
2.2 Synthesis
Scheme 1. The chemical structure of heteroatom substituted carbazole
derivatives EꢀC, CꢀC, CꢀN, CꢀO, CꢀS and CꢀSO
9-Ethyl-carbazole (E-C). EꢀC was synthesized through the NꢀH
alkylation reaction[27] as shown in Scheme 2. Carbazole (0.21 g,
1.25 mmol), bromoethane (0.54 g, 5.02 mmol), NaOH (aq, 12.5
mol/L, 5 ml, 62.5 mmol) and catalytic amount of tetrabutyl
ammonium bromide (0.05 g, 0.14 mmol) was charged in a flask.
The flask was heated at 70 oC continuously for 8 h. After the
completion of the reaction monitored by thinꢀlayer
chromatography, the reaction mixture was cooled to room
temperature, and extracted with dichoromethane. The organic
layer was washed with water and dried over anhydrous sodium
sulfate (Na2SO4). The solvent was removed under reduced
pressure and the crude product was chromatographed on a silica
gel column with petroleum ether/ dichloromethane (2:1) as eluent.
The recovered yield of the solid was 0.20 g (83%). 1H NMR
(400MHz, CDCl3, ppm):δ 8.15 (d, J=8.0 Hz, 2H), 7.52 (t, J=15.2
Hz, 2H), 7.44 (d, J=8 Hz, 2H) ,7.28 (t, J=14.8 Hz, 2H), 4.43ꢀ4.37
(m, 2H, ꢀCH2CH3), 1.47 (t, J=14.4 Hz, 3H, ꢀCH2CH3). 13CNMR
(100MHz, CDCl3, ppm) :δ 139.96, 125.62, 122.95, 120.44,
118.76, 108.44, 37.52, 13.82. MALDIꢀTOF, m/z cacld for
C14H13N, 196.0; Found: 196.7. Elemental analysis: calcd (%) for
Therefore, in an attempt to address the above problems, we
reported in this work the synthesis of six types of heteroatom
substituted carbazole derivatives, which were modified with
carbazole (CꢀC), biphenylamine (CꢀN), phenoxazine (CꢀO),
phenothiazine (CꢀS), and phenothiazineꢀS,Sꢀdioxide (CꢀSO)
moieties on the 3ꢀpositions of 9ꢀethylꢀcarbazole(EꢀC) (Scheme 1).
We also address the effects of different units on the energies of
the frontier molecular orbitals, thermal stability and T1 of these
carbazoleꢀbased derivatives, one phosphorescent device hosted
by CꢀO exhibits the excellent performance.
2. Experimental Section
2.1 Materials and measurements
Materials purchased from commercial suppliers were used
without further purification. The common solvents were purified
according to their standard methods.
2
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C14H13N: C 86.12, H 6.71, N 7.17; Found: C 86.04, H 6.58, N
7.12.
a silica gel column with petroleum ether/dichloromethane (6:1) as
eluent. Recrystallize several times from ethanol or hexane to
afford the pure solid compounds in a yield of 1.64 g (70 %).
9-Ethyl-9H-3,9'-bicarbazole (C-C). CꢀC was prepared
according to the general procedure. CꢀI (0.53 g, 1.64 mmol),
carbazole (0.63 g, 3.79 mmol), copper powder (0.05 g, 0.78
mmol), anhydrous potassium carbonate (0.91 g, 6.61 mmol). The
resulting solid product was purified by column chromatography
with petroleum ether/ dichloromethane (6:1) as eluent. Yield:
0.51 g (85%). 1H NMR (400MHz, CDCl3, ppm): δ 8.35ꢀ8.26 (m,
3H, Ar), 8.12 (d,J=8 Hz, 1H, Ar), 7.67ꢀ7.47 (m, 8H, Ar), 7.39ꢀ
7.30 (m, 3H, Ar), 4.51ꢀ4.46 (m, 2H, ꢀCH2CH3), 1.59ꢀ1.55 (t,
J=14.4 Hz, 3H, ꢀCH2CH3). 13CNMR (100MHz, CDCl3, ppm):
δ142.13, 140.69, 139.18, 128.92, 126.45, 125.94, 125.33, 123.96,
123.19, 122.67, 120.79, 120.36, 119.76, 119.65, 119.38, 109.98,
109.51, 108.93, 37.87, 14.01. MALDIꢀTOF, m/z cacld for
C26H20N2 361.1; found, 361.6. Elemental analysis: calcd (%) for
C26H20N2: C 86.64, H 5.59, N 7.77; found C 86.53, H 5.66, N
7.69.
3-Iodo-9ethyl-carbazole (C-I). CꢀI was synthesized through
Sandmeyer reaction[28] as shown in Scheme 2. A solution of 3ꢀ
amineꢀ9ꢀethylꢀcarbazole (10.68 g, 50.8 mmol) and concentrated
HCl (37.2 mL) in water (42.8 mL) was cooled to 0 oC. Next, 5.32
g (77.12 mmol) of NaNO2 in 26.8 mL of water was added
dropwise to the diamine solution over a period of 30 min, keeping
the temperature at 0 0C. After the addition of NaNO2 was
completed, the resulting mixture was stirred for an additional 30
min, and an aqueous solution of KI (12.48 mol/L, 100 ml, 1.25
mol) was added dropwise over 30 min at ꢀ5 oC. The reaction
mixture was then stirred for 1 h at room temperature and 3 h at 60
oC, giving a dark brown solution. The solution was then cooled to
0
25 C and the brown precipitate was collected by filtration. The
crude brown solid was then purified by column chromatography
(petroleum ether/ dichloromethane=6:1) yielding the title
1
compound as a white solid in a yield of 4.56 g (28 %). HNMR
(400MHz, CDCl3, ppm): δ8.41 (s, 1H, Ar), 8.03 (d, J=8 Hz,1H,
Ar), 7.72ꢀ7.70 (m, 1H, Ar), 7.50 (t, J=15.6 Hz, 1H, Ar), 7.39 (d,
J=4 Hz,1H, Ar), 7.26 (t, J=14.8 Hz, 1H, Ar), 7.17 (d, J=8.4 Hz,
1H, Ar), 4.34ꢀ4.29 (m, 2H, ꢀCH2CH3), 1.41 (t, J=14.4 Hz, 3H, ꢀ
CH2CH3). 13CNMR (100MHz, CDCl3, ppm): 139.93,
139.06,133.81, 129.27, 126.37, 125.63, 125.49, 121.69, 120.61,
119.33, 110.53, 108.66, 37.62, 13.76. MALDIꢀTOF, m/z cacld
for C14H12IN , 322.0; found, 323.7.
9-Ethyl-N,N-diphenyl-9H-carbazol-3-amine (C-N). CꢀN was
prepared according to the general procedure. CꢀI (0.50 g, 1.56
mmol), diphenylamine (0.31 g, 1.81 mmol), copper powder (0.02
g, 0.31 mmol), anhydrous potassium carbonate (0.44g, 3.18
mmol). The resulting solid product was purified by column
chromatography with petroleum ether/ dichloromethane (5:1) as
1
eluent in a yield of 0.45 g (80%). H NMR (400MHz, CDCl3,
ppm): δ 8.02 (d, J=8 Hz, 1H, Ar), 7.93 (d, J=1.6 Hz, 1H, Ar),
7.58 (t, J=18 Hz, 2H, Ar), 7.41 (t, J=15.6 Hz, 1H, Ar), 7.21 (t,
J=15.6 Hz, 5H, Ar), 7.10 (t, J=15.2 Hz, 1H, Ar), 6.97ꢀ6.89 (m,
6H, Ar), 4.43ꢀ4.38 (m, 2H, ꢀCH2CH3), 1.31 (t, J=14 Hz, 3H, ꢀ
CH2CH3). 13CNMR (100 MHz, CDCl3, ppm): δ148.66, 140.52,
139.07, 137.52, 129.69, 126.43, 126.16, 123.70, 122.35, 121.92,
121.12, 119.50, 119.16, 110.81, 109.63, 37.53, 14.25. MALDIꢀ
TOF, m/z cacld for C26H22N2, 363.1; found, 362.2. Elemental
analysis: calcd (%) for C26H22N2: C 86.15, H 6.12, N 7.73; found
C 86.62, H 5.59, N 7.58.
Scheme 2. Synthesis of compounds EꢀC, CꢀI. i: Tetrabutyl ammonium bromide,
NaOH(aq). ii: NaNO2, HCl, KI.
C2H5
N
N
N
I
10-(9-Ethyl-9H-carbazol-3-yl)-10H-phenoxazine (C-O). CꢀO
was prepared according to the general procedure. CꢀI (0.50 g,
1.55 mmol), phenoxazine (1.32 g, 1.8 mmol), copper powder
(0.02 g, 0.34 mmol), anhydrous potassium carbonate (0.43 g, 3.14
mmol). The resulting solid product was purified by column
chromatography with petroleum ether/ dichloromethane (6:1) as
N
C2H5
H
N
iii
C-C
O
C-N
S
O
O
+
X
N
S
N
C2H5
N
N
iv
N
N
N
C2H5
C2H5
C2H5
1
eluent in a yield of 0.41 g (70%). H NMR (400MHz, CDCl3,
C-SO
C-O
C-S
Scheme 3. Synthesis of compound CꢀC, CꢀN, CꢀO, CꢀS and CꢀSO. iii: K2CO3, Cu,
PhNO3, reflux, 190 oC, 24h. iv: CH3COOH, H2O2, 5h.
ppm):δ 8.05 (d, J=8 Hz, 2H, Ar),7.60 (d, J=8.8 Hz, 1H, Ar), 7.55ꢀ
7.47 (m, 2H, Ar), 7.41ꢀ7.38 (m, 1H, Ar), 7.29ꢀ7.25 (m, 1H, Ar),
6.72ꢀ6.54 (m, 6H, Ar), 5.95 (d, J=7.6 Hz, 2H, Ar), 4.47ꢀ4.42 (m,
2H, ꢀCH2CH3), 1.52 (t,J=8.0 Hz, 3H, ꢀCH2CH3). 13C NMR (100
MHz, CDCl3, ppm): δ 144.05, 140.47, 139.36, 135.36, 129.85,
127.77, 126.38, 124.89, 123.23, 122.79, 122.56, 121.01, 120.72,
119.39, 115.25, 113.51, 110.76, 108.86, 37.83, 13.93. MALDIꢀ
TOF, m/z cacld for C26H20N20, 377.1; found, 376.5. Elemental
analysis: calcd (%) for C26H20N2O: C 82.95, H 5.35, N 7.44;
found C 83.01, H 5.38, N 7.45.
10-(9-Ethyl-9H-carbazol-3-yl)-10H-phenothiazine (C-S). CꢀS
was prepared according to the general procedure. CꢀI (2.00 g,
6.23 mmol), phenothiazine (1.44 g, 7.22 mmol), copper powder
(0.09 g, 1.41 mmol), anhydrous potassium carbonate (1.74 g,
12.59 mmol). The resulting solid product was purified by column
General Procedure. The new carbazole derivative CꢀC, CꢀN, Cꢀ
O and CꢀS were synthesized by substituting the CꢀI on the 3ꢀ
position through Ullmann CꢀN coupling reaction[29] (Scheme 3)
with different units. Take the synthesis of CꢀO as an example: CꢀI
(2.00 g, 6.2 mmol), phenoxazine (1.32 g, 7.2 mmol), copper
powder (0.09 g, 1.41 mmol), anhydrous potassium carbonate
(1.74 g, 12.59) and nitrobenzene (44.5 mL) were placed into a
roundꢀbottom flask. The mixture was refluxed for 24 h under
argon. After evaporation of the solvent in vacuo, distilled water
was added into and the reaction mixture was extracted with
dichoromethane. The combined organic layers were collected,
dried over anhydrous Na2SO4, filtered and evaporated to remove
the solvent. The resulted crude product was chromatographed on
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chromatography with petroleum ether/ dichloromethane (7:1) as
3. Results and Discussion
eluent in a yield of 1.59 g (65%). 1HNMR (400MHz, CDCl3,
ppm): δ 8.14 (d, J=1.6 Hz, 1H, Ar), 8.07 (d, J=8 Hz, 1H, Ar),
7.62 (d, J=8.4 Hz, 1H, Ar), 7.54ꢀ7.46 (m, 3H, Ar), 7.27 (t, J=13.6
Hz, 1H, Ar), 7.03ꢀ7.01 (m, 2H, Ar), 6.80 (t, J=7.2 Hz, 4H, Ar),
6.25ꢀ6.23 (m, 2H, Ar), 4.49ꢀ4.43 (m, 2H, ꢀCH2CH3), 1.54
(t,J=14.4 Hz, 3H, ꢀCH2CH3). 13CNMR (100MHz, CDCl3, ppm):
δ145.12, 140.51, 139.26, 131.81, 128.46, 126.80, 126.52, 126.38,
124.70, 123.18, 122.65, 122.14, 120.74, 119.47, 119.39, 115.90,
110.35, 108.87, 37.86, 13.96. MALDIꢀTOF,m/z cacld for
C26H20N2S, 393.0; found, 394.1. Elemental analysis: calcd (%)
for C26H20N2S: C 79.56, H 5.14, N 7.14; found C 79.61, H 5.15,
N 7.09.
The new compound CꢀC, CꢀN, CꢀO and CꢀS, which are based on
EꢀC, were synthesized using an optimized Ullmann CꢀN crossꢀ
coupling reaction between the CꢀI and carbazole, diphenylamine,
phenoxazine, phenothiazine. The optimized Ullmann reaction
proceeded in refluxing nitrobenzene in the presence of copper
bronze as catalyst and K2CO3 as base and produced the desired
compounds in relative high yields. CꢀSO was obtained from the
oxidation of CꢀS in the presence of hydrogen peroxide.
E- C
C- C
C- N
C- O
C- S
C- SO
1.0
0.8
0.6
0.4
0.2
0.0
(a)
10-(9-Ethyl-9H-carbazol-3-yl)-10H-phenothiazine-S,S-dioxide
(C-SO). CꢀSO was obtained from the oxidation of CꢀS in the
presence of hydrogen peroxide. CꢀS (0.4 g, 1.0 mmol) was
dissolved in acetic acid (8.1 mL) and hydrogen peroxide (2.4 mL)
was carefully added and the solution was refluxed for 5 h at 130
oC. A further 1.6mL of hydrogen peroxide was added and the
solution was refluxed over night. After cooling, water was added
and the resultant solid was filtered and washed with water. The
solid was dissolved in dichloromethane and washed with brine,
dried (MgSO4), filtered and concentrated. The resulting solid
product was purified by column chromatography with petroleum
ether/ dichloromethane (2:1) as eluent to give the targeted solid
100
(b)
200
300
400
500
Temperature0C
1
compound CꢀSO in a yield of 0.37 g (86%). H NMR (400MHz,
CDCl3, ppm): δ 8.19 (d, J=8 Hz, 2H, Ar), 8.10 (d, J=2 Hz, 1H,
Ar), 8.05 (d, J=8 Hz, 1H, Ar), 7.67 (d, J= 8.41 Hz,1H, Ar), 7.59ꢀ
7.51 (m, 2H, Ar), 7.42ꢀ7.21 (m, 6H, Ar), 6.70 (d, J=8.4 Hz, 2H,
Ar), 4.53ꢀ4.47 (m, 2H, ꢀCH2CH3), 1.56 (t, J=14.4 Hz, 3H, ꢀ
CH2CH3). 13C NMR (100 MHz, CDCl3, ppm): δ141.60, 140.68,
139.74, 132.66, 129.79, 127.04, 126.90, 124.84, 123.27, 122.45,
122.33, 122.31, 121.82, 120.83, 119.78, 117.68, 110.74, 109.08,
37.96, 13.95. MALDIꢀTOF, m/z cacld for C26H20N2O2S, 425.1;
found 425.9. Elemental analysis: calcd (%) for C26H20N2O2S: C
73.56, H 4.75, N 6.60; found C 73.69, H 4.72, N 6.51.
Tm=235 0
C
Tm=230 0
C
Tm=129 0
C
E-C
C-C
C-N
C-O
C-S
Tm=70 0
C
Tm=201 0
C
C-SO
50
100
150
200
250
Temperature 0C
2.3 Device fabrication and measurement
Figure 1. TGA (a), DSC (b) traces of compounds EꢀC, CꢀC, CꢀN, CꢀO, Cꢀ
S and CꢀSO
The hole injection material MoO3, hole transporting material 1,4ꢀ
bis[(1ꢀnaphthylphenyl)amino]biphenyl(NPB), electron/excitonꢀ
blocking material 4,4’,4’’ꢀtri(Nꢀcarbazolyl) triphenylamine
All the target compounds were purified on a silica column and
recrystallized to yield pure solid compounds. The chemical
structures of the synthetic compounds were confirmed by 1H
NMR, 13C NMR (supporting information), elemental analysis and
MALDIꢀTOFꢀMS. All these compounds were readily soluble in
common organic solvents such as chloroform, dichloromethane at
room temperature. The thermal stabilities of the compounds were
studied by DSC and TGA under a nitrogen atmosphere, and the
details were summarized in Table 1 and Figure 1. All the
synthesized compounds demonstrated high thermal stability. The
DSC measurements revealed melting points (Tm) ranging from
(TCTA)
and
electronꢀtransporting
material
(TPBI)
1,3,5ꢀ
were
tris(Nphenylbenzimidazolꢀ2ꢀyl)benzene
commercially available. Commercial indium tin oxide (ITO)
coated glass with sheet resistance of 10 ꢁ per square was used as
the starting substrate. Before device fabrication, the ITO glass
substrates were preꢀcleaned carefully and treated by oxygen
plasma for 2 min. Then the sample was transferred to the
deposition system. MoO3 (8 nm) was firstly deposited to ITO
substrate, followed by NPB (50 nm), TCTA (5 nm), emissive
layer, and TPBI (25 nm). Finally, a cathode composed of lithium
fluoride (1 nm) and aluminium (100 nm) was sequentially
deposited onto the substrate in the vacuum of 10ꢀ6 Torr. The LꢀVꢀ
J of the devices was measured with a Keithley 2400 Source meter
and a Keithley 2000 Source multimeter equipped with a
calibrated silicon photodiode. The electroluminescent (EL)
spectra were measured by JY SPEX CCD3000 spectrometer. All
measurements were carried out at room temperature under
ambient conditions.
o
129 to 235 C for carbazole derivatives, which were higher than
0
o
that of EꢀC (70 C), and the Tm of CꢀO and CꢀS are 235 C, 230
oC respectively, which are the highest of all. All the values of Td
confirmed by TGA with a heating rate of 20 oC /min were higher
than 304 oC (except CꢀSO). The values of CꢀO and CꢀS are 318
oC, 344 oC respectively, and that of compound CꢀS is the highest
of all. This shows that the incorporation of carbazole,
4
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diphenylamine, phenoxazine, phenothizine can significantly
improve the Td and Tm of EꢀC.
Table 1. Absorption, PL emission spectra and thermal characteristics of
compounds EꢀC, CꢀC, CꢀN, CꢀO, CꢀS and CꢀSO.
UV
PL
C-C PL
C-N PL
E-C PL
E-C UV
C-C UV
C-N UV
(a)
1.0
0.8
0.6
0.4
0.2
0.0
CHCl3
λem
thin solid film
λem
Compound
η[c]
（%）
1.36
[a]
[a]
[b]
[b]
λabs
Eg
λabs
Eg
Td
Tm
EꢀC
CꢀC
CꢀN
CꢀO
292 358 3.47 345(295) 446
3.34 180 70
3.29 334 201
427(499) 3.00 304 129
7.92
2.2
266 392 3.39 295
300 416 3.02 302
265 370 3.40 245
412
250
300
350
400
450
500
550
600
650
Wavelength (nm)
1.25
2.16
458
460
472
3.34 318 235
3.27 344 230
CꢀS
259 371 3.41 260
298 503 3.43 258
UV
PL
C-O PL
C-S PL
(b)
1.0
5.97
C-SO PL
C-O UV
C-S UV
C-SO UV
CꢀSO
3.32 145
ꢀ
0.8
0.6
0.4
0.2
0.0
[a]Optical band gaps obtained from the absorption edge; [b] Tm and
temperatures at which 5% loss of mass (Td)；[c] absolute PL quantum
yields.
300
400
500
600
Wavelength (nm)
Table 2. Experimental and theoretical HOMO, LUMO, energy band gaps
of the compounds (eV)
UV
PL
E-C UV
(c)
[a]
[b]
[c]
Compound HOMO[a] LUMO[a] Eg
ET
HOMO[c] LUMO[c] Eg
C-C UV
C-N UV
E-C PL
C-C PL
C-N PL
1.0
EꢀC
CꢀC
CꢀN
CꢀO
CꢀS
ꢀ5.58
ꢀ5.44
ꢀ5.45
ꢀ5.46
ꢀ5.37
ꢀ2.34
ꢀ1.89
ꢀ1.99
ꢀ1.84
ꢀ1.86
3.24 2.61 ꢀ5.29
3.55 2.70 ꢀ5.04
3.46 2.49 ꢀ4.64
3.62 2.57 ꢀ4.67
3.51 2.34 ꢀ4.75
ꢀ0.60
ꢀ0.87
ꢀ0.58
ꢀ0.68
ꢀ0.68
4.70
4.17
4.06
2.76
2.79
0.8
0.6
0.4
0.2
0.0
CꢀSO
ꢀ5.38
ꢀ2.03
3.35 2.79 ꢀ5.42
ꢀ1.15
3.15
[a] Results obtained from the CV curves on spinꢀcoated films; [b] Results
obtained from phosphorescent spectra; [c] Results obtained from the
theoretical calculations at B3LYP/6ꢀ31G*.
250
300
UV
350
400
450
500
550
600
Wavelength (nm)
C-O UV
C-S UV
C-SO UV
C-O PL
C-S PL
PL
(d)
1.0
The optical properties of the derivatives in chloroform and in thin
solid film were investigated and summarized in Table 1 and
Figure 2. The first absorption band around 290 nm can be
assigned to the carbazoleꢀcentered nꢀπ* transition, and the other
long wavelength absorption around 330ꢀ343 nm is attributed to
the πꢀπ* transition of the entire conjugated backbone. An
absorption peak at around 245 nm and a shoulder at 265 nm can
be attributed to πꢀπ* and nꢀπ* transitions of arylenes respectively,
which belong to carbazole, diphenylamine, phenothizine,
phenothizine, and phenothiazineꢀS, Sꢀdioxide. The UVꢀvis
absorption maxima in the solution state were 292, 266, 300, 265,
259, 298 nm for EꢀC, CꢀC, CꢀN, CꢀO, CꢀS and CꢀSO respectively,
and 245, 295, 302, 245, 260, 258 nm for EꢀC, CꢀC, CꢀN, CꢀO, Cꢀ
S and CꢀSO respectively, in the film state. The UVꢀvis spectra of
these compounds displayed virtually slight changes on going
from the solution to the film state, as a result of the dihedral angle
between the carbazole and the hetrocyclics which can prevent the
aggregation in the films. However, the absorption maxima and
0.8
0.6
0.4
0.2
0.0
C-SO PL
250 300 350 400 450
500
550
600
Wavelength (nm)
Figure 2. Normalized UVꢀvis and PL emission spectra of compounds EꢀC,
CꢀC, CꢀN, CꢀO, CꢀS and CꢀSO in chloroform (a, b) and thin solid films (c,
d)
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the position of absorption edges changed obviously. The optical
energy band gaps (Eg) of the new compounds determined from
the onset of the UVꢀvis spectra in solid film are in a range of
3.00ꢀ3.34eV. For the PL emission spectra, the emission maxima
in the solution were 358, 392, 416, 370, 371, 503 nm for EꢀC, Cꢀ
C, CꢀN, CꢀO, CꢀS and CꢀSO respectively, and 446, 412, 427, 458,
460, 472 nm for EꢀC, CꢀC, CꢀN, CꢀO, CꢀS and CꢀSO,
respectively, in the film state. A bathochromic shift of the PL
spectra is found when heteroatoms are introduced into the
arylenes. The emission in the thin solid film was generally redꢀ
shifted about 50 nm as a result of the stronger intermolecular
interactions in the solid state. For CꢀO, CꢀS, CꢀSO in the dilute
CHCl3 solutions, more PL peaks appear and the spectra are quite
complicated, however, the three compounds have only one
emission peak in the film state. It was interesting to notice that
the absolute PL quantum efficiency of CꢀC and CꢀSO in thin
solid film was relatively higher than that of compound EꢀC, CꢀN,
CꢀO and CꢀS, and the potential mechanism is still unclear at this
stage.
all (ꢀ5.37 eV), showing that the units, especial hetetocyclics on
the 3ꢀposition, can improve the HOMO and may enhance the hole
transporting ability of these compounds. The LUMO values
range from ꢀ2.03 to ꢀ1.89 eV, which have a strong improvement
when compared with EꢀC (ꢀ2.34 eV), and target compounds CꢀO,
CꢀS have the highest LUMO value. It is obvious that the
incorporation
of carbazole, diphenylamine, especially
heterocyclic penoxazine, phenothizine can improve the
HOMO/LUMO energy levels of the derivatives and may endow
them with excellent hole transporting ability. The Eg estimated
from the corresponding HOMO and LUMO has the similar
values ranging from 3.35 to 3.62 eV. The HOMO and LUMO
orbitals are shown in Table 3. As can be seen from the molecular
orbital distribution of the compounds, the LUMO contour of the
compounds are all localized on the carbazole units of EꢀC, while
the HOMO contour of compound CꢀC, CꢀN, and CꢀS have
covered the whole molecular framework, and the HOMO contour
of compound CꢀO and CꢀSO are both localized on the
phenoxazine and phenothiazineꢀS,Sꢀdioxide units. There might
exist obvious intramolecular charge transfer in compounds CꢀO
and CꢀSO, and compounds CꢀO and CꢀSO might have potential
applications as bipolar host materials in PHOLEDs. The variation
tendency of band gaps obtained from DFT calculations for
compounds EꢀC, CꢀC, CꢀN, CꢀO, CꢀS, CꢀSO is approximately
consistent with those of obtained from experimental
characterization.
C-S
(a)
C-O
C-C
C-N
C-SO
E-C
C-O
C-S
C-SO
(a)
1.0
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Potential Ag/Ag+(v)
0.8
(b)
0.6
0.4
0.2
0.0
E- C
C- C
C- N
C- O
C- S
C- SO
450
500
550
600
650
700
750
Wavelength(nm)
-3.5
-3.0
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
Potential vs Ag/Ag+ (v)
E-C
C-C
C-N
(b)
1.0
0.8
0.6
0.4
0.2
0.0
Figure 3. Oxidative CV (a) and reductive CV (b) curves of the
compounds in the film state.
The electrochemical behaviors of synthesized compounds were
investigated by cyclic voltammetry (CV), and the HOMO/LUMO
values were determined. The electrochemical waves are showed
in Figure 3. On the basis of these results, the HOMO and LUMO
energy levels were calculated and summarized in Table 2.
Carbazole derivatives have clearly localized frontier orbitals with
HOMO predominately localized in heterocyclic moieties and
LUMO in EꢀC moieties. The HOMO of carbazole derivatives
have the close values ranging from ꢀ5.46 to ꢀ5.37 eV, which are
higher than that of EꢀC (ꢀ5.58 eV), and CꢀS have the highest of
450
500
550
600
Wavelength (nm)
Figure 4. Normalized phosphorescent spectra of compounds EꢀC, CꢀC, Cꢀ
N, CꢀO, CꢀS and CꢀSO at 77 K with 5 ms delay.
6
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As shown in Figure 4, the phosphorescent spectra of synthesized
PHOLEDs. To evaluate the utility of these compounds as host
materials for green phosphor tris (2ꢀphenylpyridine)
iridium(Ir(ppy)3), we initially fabricated green devices A–C by
using CꢀO as the host and Ir(ppy)3 as the guest with the
configuration of ITO/MoO3 (8 nm)/NPB (50 nm)/TCTA (5
nm)/host CꢀO: Ir(ppy)3 (10 nm)/TPBI (25 nm)/LiF (1 nm)/Al
(100 nm).
compounds were detected at 77K with 5 ms delay. The ET was
determined by taking the highest energy peak of phosphorescence
spectra as the transition energy of T1ꢀS0. The carbazole
derivatives showed high ET (2.34ꢀ2.79 eV), and CꢀSO exhibited
the highest value of all, which showed that the introduction of
heterocyclics,
especial
phenothiazineꢀS,Sꢀdioxide,
could
remarkably improve the ET value. When the oxygen atom in
phenoxazine group is changed into sulfur atom, the ET value
would have a clear decrease.
Device
Device
Device
A
B
C
300
250
200
150
100
50
10000
1000
100
10
Table 3．The molecular orbital contours of HOMO and LUMO energy
levels (b) of compounds EꢀC, CꢀC, CꢀN, CꢀO, CꢀS, CꢀSO from DFT
calculations.
a)
Compound
HOMO
LUMO
1
Device
Device
Device
A
B
C
0
0.1
0
2
4
6
8
10
12
14
Voltage (V)
EꢀC
b)
10
1
CꢀC
Device
Device
Device
A
B
C
0.01
0.1
1
10
CꢀN
Current density (mA/cm2)
c)
10
1
CꢀO
CꢀS
Device
Device
Device
A
B
C
0.01
0.1
1
10
100
Current density (mA/cm2)
Device
Device
Device
A
B
C
15000
12000
9000
6000
3000
0
CꢀSO
d)
However, when the phenothiazine is oxidized into phenothiazineꢀ
S,Sꢀdioxide, the ET shows an increase of about 0.45eV. This may
be attributed to different sizes and inductive effects from different
atoms. The phenomena indicate that these compounds with high
ET are promising host materials for green, red and even blue
0
50
100 150 200 250 300
Current density (mA/cm2)
Figure 5. a) Current densityꢀvoltageꢀbrightness characteristics for devices
Journal Name, [year], [vol], 00–00 |
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7

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device C. EL spectra of devices A–C are shown in Figure 6.
Devices A–C show the same main peak at 509 nm.
A–C. b) Power efficiency versus current density for devices A–C. c)
Current efficiency versus current density for devices A–C. d) luminance
versus current density for devices A–C.
4. Conclusions
1.0
Device
Device
Device
A
B
C
In conclusion, a series of derivatives obtained by substituting Eꢀ
C with different units in the 3ꢀposition were synthesized through
Ullmann CꢀN coupling reaction. Their energy levels including
HOMO, LUMO and ET can be effectively tuned by the choice of
substituent moieties. Especilly the incorporation of heterocyclics
can further optimize the HOMO/LUMO and ET of these
carbazole derivatives. The properties of carbazole derivatives
were further validated by theoretical calculations. We have
0.8
0.6
0.4
0.2
0.0
300
400
500
Wavelenth(nm)
Figure 6. EL spectra of devices A–C
600
700
800
successfully
fabricated
highly
efficient
green
electrophosphorescences by using CꢀO as host material, with a
maximum luminance of 10270 cd mꢀ2, a maximum current
efficiency (ηc.max) of 34.8 cd Aꢀ1, and a maximum power
efficiency (ηp.max) of 26.0 lm Wꢀ1. New bipolar molecules capable
of transporting both electrons and holes have been obtained while
maintaining a desired energy gap for using as the host for both
singlet and triplet emitters. These carbazole derivatives have
potential as green, red, and even blue host materials for
PHOLEDs. This can serve as an excellent exploratory example
for fine band gap and optoelectronic control principle through the
heteroatom replacement of carbazole derivatives.
Table 4. Electroluminescence characteristics of the devices.a
Ic/ mA
cmꢀ2
0.5
5.2
54.7
0.3
Lmaxd/cd
mꢀ2
ηc.maxe/cd
Aꢀ1
ηp.maxf/lm
Wꢀ1
Device Vb/V
4.6
125
26.0
20.3
7.6
17.7
9.1
2.3
26.0
16.9
8.4
A
7
1049
4157
117
10.4
4.2
6.2
8.8
10.6
4.2
6.4
8.8
10.6
34.8
33.3
23.4
15.0
31.3
33.4
23.7
15.2
3.0
998
B
22.0
68.6
0.4
3.5
21.9
68.3
5162
10270
119
1151
5195
10406
4.4
23.4
16.4
8.5
Acknowledgements
This work was financially supported by the Major Research
Program from the State Ministry of Science and Technology
(2009CB930600 and 2012CB933301), the Ministry of Education
of China (IRT1148), the Priority Academic Program
Development of Jiangsu Higher Education Institutions, the
Natural Science Foudation of the Jiangsu Higher Education
Institutions of China (10KJB150012) and Nanjing University of
Posts and Telecommunications (NY211131). Besides, this work
was strongly supported by Dongge Ma (Changchun Institute of
Applied Chemistry, Chinese Academy of Sciences) and Hongmei
Zhang (Nanjing University of Posts and Telecommunications) in
device fabrication.
C
4.5
a
Device configuration: ITO/MoO3 (8 nm)/NPB (50 nm)/TCTA
(5 nm)/host CꢀO: Ir(ppy)3 (10 nm)/TPBI (25 nm)/LiF (1 nm)/Al
(100 nm). b Voltages. Current density. Maximum luminance.
c
d
e
Maximum current efficiency.f Maximum power efficiency.
NPB and TPBI were used as the holeꢀand electron transporting
layers, respectively; TCTA was used as the electron/excitonꢀ
blocking layer; MoO3 and LiF served as holeꢀ and electronꢀ
injecting layers, respectively; Ir(ppy)3 doped in host CꢀO was
used as the emitting layer. Figure 5 shows the current densityꢀ
voltageꢀbrightness (JꢀVꢀL) characteristics, efficiency versus
current density curves and Luminance versus current density
curves for the devices. The electroluminescence (EL) data are
summarized in Table 4. Device A with 5% Ir(ppy)3 doped in host
CꢀO achieves a maximum current efficiency (ηc.max) of 26.0 cd Aꢀ
1, and a maximum power efficiency (ηp.max) of 17.7 lm Wꢀ1. In
comparison, device C with 10% Ir(ppy)3 doped in host CꢀO
exhibits higher efficiencies with ηc.max of 31.3 cd Aꢀ1, and ηp.max
of 23.4 lm Wꢀ1. Device B, with 8% Ir(ppy)3 doped in host CꢀO,
achieves the highest performance with a maximum luminance of
10270 cd mꢀ2, a maximum current efficiency (ηc.max) of 34.8 cd Aꢀ
1, and a maximum power efficiency (ηp.max) of 26.0 lm Wꢀ1. It is
worth noting that these devices show high efficiencies and low
efficiency rollꢀoff at high luminance. For example, when the
brightness reaches 1000 cd mꢀ2, ηc still remains as high as 20.3 cd
Aꢀ1 for device A, 33.3 cd Aꢀ1 for device B , and 33.4 cd Aꢀ1 for
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This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 | 9

New Journal of Chemistry
Page 10 of 10
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Synthesis and Characterization of
Heteroatom Substituted Carbazole
Derivatives: Potential Host Materials
for Phosphorescent Organic Lightꢀ
emitting Diodes.
A series of carbazole derivatives
based on 9ꢀethylꢀcarbazole substituted
on the 3ꢀposition with different units
were synthesized through Ullmann CꢀN
coupling reaction. Their energy levels
can be effectively tuned by the choice
of suitable moieties. Especilly the
incorporation of heterocyclics can
further optimize the highest occupied
molecular orbital / lowest unoccupied
molecular orbital and triple energy
levels of these carbazole derivatives.
New bipolar molecules capable of
transporting both electrons and holes
have also been obtained while
maintaining a desired energy gap for
using as the host for both singlet and
triplet emitters. One phosphorescent
device hosted by CꢀO exhibits the
excellent performance, and these
carbazole derivatives have the potential
as green, red, and even blue host
materials for phosphorescent organic
lightꢀemitting diodes.
10 | Journal Name, [year], [vol], 00–00
This journal is © The Royal Society of Chemistry [year]