Novel hole-transporting materials based on 1,4-bis(carbazolyl)benzene for organic light-emitting devices

Article abstract of DOI:10.1039/b309630k

Novel hole-transporting molecules containing 1,4-bis(carbazolyl)benzene as a central unit and different numbers of diphenylamine moieties as the peripheral groups have been synthesized and characterized. These compounds are thermally stable with high glass transition temperatures of 141-157°C and exhibit chemically reversible redox processes. Their amorphous state stability and hole transport properties can be significantly improved by increasing the number of diphenylamine moieties in the outer part and by controlling the symmetry of the carbazole-based molecules. These compounds can be used as good hole-transporting materials for organic electroluminescent (EL) devices. The device performance based on tri- and tetra-substituted carbazole derivatives is comparable to that of a typical 4,4′-bis[N-(1-naphthyl)-N-phenylamino] biphenyl (NPB)-based device.

Full text of DOI:10.1039/b309630k

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A R T I C L E
Novel hole-transporting materials based on 1,4-
bis(carbazolyl)benzene for organic light-emitting devices
Qian Zhang,a,b Jiangshan Chen,a Yanxiang Cheng,a Lixiang Wang,*a Dongge Ma,a
Xiabin Jinga and Fosong Wanga
aState Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of
Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China.
E-mail: lixiang@ciac.jl.cn
bDepartment of Chemistry, Northeast Normal University, Changchun 130024, P. R. China
Received 11th August 2003, Accepted 15th December 2003
First published as an Advance Article on the web 20th January 2004
Novel hole-transporting molecules containing 1,4-bis(carbazolyl)benzene as a central unit and different numbers
of diphenylamine moieties as the peripheral groups have been synthesized and characterized. These compounds
are thermally stable with high glass transition temperatures of 141–157 uC and exhibit chemically reversible
redox processes. Their amorphous state stability and hole transport properties can be significantly improved by
increasing the number of diphenylamine moieties in the outer part and by controlling the symmetry of the
carbazole-based molecules. These compounds can be used as good hole-transporting materials for organic
electroluminescent (EL) devices. The device performance based on tri- and tetra-substituted carbazole
derivatives is comparable to that of a typical 4,4’-bis[N-(1-naphthyl)-N-phenylamino] biphenyl (NPB)-based
device.
stability, morphological stability and good hole-transporting
properties.
Introduction
The development of organic light-emitting diodes (OLEDs) has
been the subject of intense academic and industrial research
directed to the fabrication of large-area multicolor and full
color flat-panel displays.1 Since the discovery of double-layer
organic light-emitting diodes (LEDs) based on Alq3 [tris-(8-
quinolinolato)aluminium] as the emitting material and tri-
phenylamine as hole-transporting material by Tang and
VanSlyke,2 intensive research has been focused on the
development of new hole-transporting molecules and tremen-
dous progress has been made in this area.3 Among them,
triphenylamine-like compounds, such as N,N’-di-m-tolyl-N,N’-
diphenyl-1,1’-biphenyl-4,4’-diamine (TPD) and 4,4’-bis[N-(1-
naphthyl)-N-phenylamino] biphenyl (NPB) have been proved
to be excellent hole-transporting materials and have shown a
wide range of practical applications. These classes of materials
offer many attractive properties such as high charge carrier
mobility and ease of sublimation. However, they possess some
disadvantages for use in long-lifetime LED devices such as their
relatively low glass transition temperature (Tg ~ 65 uC for
TPD and Tg ~ 100 uC for NPB), ease of crystallization and
unsatisfactory morphological stability.4 To solve these pro-
blems, many efforts have been devoted to the synthesis of new
hole-transporting materials with high thermal stability and
good morphological stability. For example, Shirota proposed
the starburst-shape concept to develop a series of dendrimer-
like amines with high charge carrier mobility, for suppressing
crystallization formation and improving the morphological
stability of deposited films during the device operation.5 Some
spiro-linked amines6 and oligomeric amines3c,i were reported to
exhibit good thermal and morphological stability. Recently,
The carbazole derivatives generally possess good thermal
stability and hole transport properties. The triphenylamine
derivatives such as TPD and NPB have shown promising hole
transport characteristics. The combination of carbazole
derivatives and triphenylamine derivatives is expected to
offer the improved thermal and morphological stabilities as
well as their good hole transport properties. In the present
work, we design and synthesize a series of novel hole-
transporting molecules with 1,4-bis(carbazolyl)benzene (1)
(Chart 1) as the central blocks and diphenylamine as the
outer blocks for organic light-emitting devices. Using 1,4-
bis(carbazolyl)benzene as a center building block is expected to
increase the glass transition temperature (Tg) and thermal
stability. The incorporation of diphenylamine moieties in the
periphery can provide the improved hole injection and
transport properties.
Experimental
General
The resulting compounds were characterized by 1H NMR
spectra using a Varian 400 MHz spectrometer with chloro-
form-d as solvent and tetramethylsilane as internal standard.
UV-vis and fluorescence spectra were recorded on a Varian
Cray 50 spectrophotometer and on a Perkin-Elmer LS 50 B
luminescence spectrometer, respectively; the elemental analyses
were performed using a Bio-Rad elemental analysis system.
Thermal gravimetric analysis (TGA) and differential scanning
calorimetry (DSC) measurements were carried out under a
nitrogen atmosphere at a heating rate of 20 uC min21, with a
TGA 2950 thermogravimetric analyzer and a Perkin-Elmer
TGA 7 Thermal analysis system, respectively. Cyclic voltam-
metry (CV) was performed on an EG&G model 283
potentiostat/galvanostat system with a three-electrode cell in
a solution of Bu4NClO4 (0.1 M)/dichloromethane at a scan rate
of 100 mV s21 with a platinum counter electrode and Ag/AgCl
silver reference electrode.
Tao and co-workers7 synthesized
a series of carbazole
compounds with peripheral arylamines possessing high glass
transition temperatures (Tg ~ 120–194 uC) and found the
OLED devices based on the resulting carbazole compounds
to be promising in terms of lifetime. Nevertheless, for effect-
ive molecular design aspects, well-defined conjugated mole-
cules need to be developed which present excellent thermal
T h i s j o u r n a l i s ß T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 8 9 5 – 9 0 0
8 9 5

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Chart 1 The structure of compounds 1–6.
Materials
(1.1 g, 6.6 mmol) and finely-powdered potassium iodate (1.6 g,
7.5 mmol) were added. The solution was refluxed at 80 uC for
4 h. Then the mixture was poured into water, the precipitate
was filtered and washed with water. Without further purifica-
tion, this iodinated mixture in which 2I was the main product
was reacted directly with diphenylamine (6.8 g, 40 mmol), CuI
(0.2 g, 1 mmol), 18-Crown-6 (0.08 g, 0.3 mmol), K2CO3 (11.1 g,
80 mol), and 1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidi-
none (DMPU) (1.5 mL) at 170 uC for 18 h under nitrogen.
After cooling to room temperature, the mixture was subse-
quently poured into 1 N HCl and then extracted with dichloro-
methane. The organic layer was washed with NH3H2O and
water, dried with anhydrous Na2SO4, followed by evaporation;
the resulting residue was purified with column chromatography
using hexane/methylene chloride as eluant. Yield 62%. UV-vis
absorption spectra (CH2Cl2): lmax ~ 294 (e ~ 12 400), 307 (e ~
Synthesis of 1. A mixture of CuI (0.57 g, 3 mmol), 18-Crown-6
(0.264 g, 1 mmol), K2CO3 (16.6 g, 120 mmol), 1,3-dimethyl-
3,4,5,6-tetrahydro-2(1H)-pyrimidinone (DMPU) (2 mL), dibro-
mobenzene (7.1 g, 30 mmol) and carbazole (10 g, 60 mmol) was
heated at 170 uC for 11 h under nitrogen. After cooling to room
temperature, the mixture was quenched with 1 N HCl, the
precipitate was filtered and was washed with NH3H2O and water.
The grey solid was recrystallized twice from chloroform to afford
1 as colorless crystals (19.3 g, 79%). UV-vis absorption spectra
(CH2Cl2): lmax ~ 295 (e ~ 10 900), 325, 340 nm. 1H NMR
(400 MHz, CDCl3): d/ppm 7.35 (td, 4 H, J ~ 7.3, 1.2 Hz), 7.48
(td, 4 H, J ~ 7.3, 1.2 Hz), 7.57 (d, 4 H, J ~ 8.4 Hz), 7.83 (s, 4 H),
8.19 (d, 4 H, J ~ 7.9 Hz).
Synthesis of 2. Compound 1 (4.1 g, 10 mmol) was dissolved
in boiling glacial acetic acid (100 mL), and potassium iodide
1
10 100) nm. H NMR (400 MHz, CDCl3): d/ppm 6.98 (t, 2 H,
8 9 6
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 8 9 5 – 9 0 0

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J ~ 7.6 Hz), 7.12 (d, 4 H, J ~ 7.6 Hz), 7.25 (t, 4 H, J ~ 7.6 Hz),
7.29–7.52 (m, 8 H), 7.56 (d, 3 H, J ~ 8.0 Hz), 7.83 (s, 4 H), 7.97
(s, 1 H), 8.04 (d, 1 H, J ~ 8.0 Hz), 8.19 (d, 2 H, J ~ 8.0 Hz).
MS (FAB): m/z 576 (M1, 100%). Elemental analysis: calc.: C,
87.62; H, 5.08; N, 7.30. Found: C, 87.41; H, 5.13; N, 7.26%.
dichloromethane. The organic layer was washed with NH3H2O
and water, dried with anhydrous Na2SO4, followed by evapor-
ation; the resulting residue was purified with column chro-
matography using hexane/methylene chloride as eluant.Yield
32%. UV-vis absorption spectra (CH2Cl2): lmax ~ 309 (e ~
1
10 600) nm. H NMR (400 MHz, CDCl3): d/ppm 6.97 (t, 8 H,
J ~ 7.6 Hz), 7.12 (d, 16 H, J ~ 7.6 Hz), 7.24 (t, 16 H, J ~
7.6 Hz), 7.27–7.39 (m, 8 H), 7.49 (d, 4 H, J ~ 8.0 Hz), 7.86 (s,
4 H). MS (FAB): m/z 1077 (M1, 100%). Elemental analysis:
calc.: C, 86.96; H, 5.24; N, 7.80. Found: C, 86.67; H, 5.36; N,
7.64%.
Synthesis of 4. Compound 1 (4.1 g, 10 mmol) was dissolved
in boiling glacial acetic acid (100 mL), and potassium iodide
(2.2 g, 13.2 mmol) and finely-powdered potassium iodate (3.2 g,
15 mmol) were added. The solution was refluxed at 80 uC for
4 h. Then the mixture was poured into water, the precipitate
was filtered and washed with water. Without further purifica-
tion, this iodinated mixture in which 4I was the main product
was reacted directly with diphenylamine (13.6 g, 80 mmol), CuI
(0.4 g, 2 mmol), 18-Crown-6 (0.16 g, 0.6 mmol), K2CO3 (22.2 g,
160 mol), and 1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimi-
dinone (DMPU) (2 mL) at 170 uC for 18 h under nitrogen.
After cooling to room temperature, the mixture was subse-
quently poured into 1 N HCl and then extracted with
dichloromethane. The organic layer was washed with
NH3H2O and water, dried with anhydrous Na2SO4, followed
by evaporation; the resulting residue was purified with column
chromatography using hexane/methylene chloride as eluant.
Yield 60%. UV-vis absorption spectra (CH2Cl2): lmax ~ 304
(e ~ 10 200) nm. 1H NMR (400 MHz, CDCl3): d/ppm 7.03 (t,
4 H, J ~ 7.6 Hz), 7.19 (d, 8 H, J ~ 7.6 Hz), 7.30 (t, 8 H, J ~
7.6 Hz), 7.36–7.62 (m, 8 H), 7.89 (s, 4 H), 8.02 (s, 2 H), 8.08 (d,
4 H, J ~ 8.0 Hz). MS (FAB): m/z 743 (M1, 100%). Elemental
analysis: calc.: C, 87.30; H, 5.16; N, 7.54. Found: C, 87.11; H,
5.36; N, 7.44%.
Synthesis of 3. Synthesis of 9-(4-bromophenyl)carbazole (7).
A mixture of CuI (1.14 g, 6 mmol), 18-Crown-6 (0.53 g,
2 mmol), K2CO3 (16.6 g, 120 mmol), 1,3-dimethyl-3,4,5,6-
tetrahydro-2(1H)-pyrimidinone (DMPU) (2 mL), dibromo-
benzene (14.2 g, 60 mmol) and carbazole (10 g, 60 mmol) was
heated at 170 uC for 11 h under nitrogen. After cooling to room
temperature, the mixture was quenched with 1 N HCl, the
precipitate was filtered and washed with NH3H2O and water.
The grey solid was purified with column chromatography using
hexane as eluant. Yield 62%. 1H NMR (400 MHz, CDCl3):
d/ppm 7.28–7.41 (m, 6 H), 7.45 (d, 2 H, J ~ 8.2 Hz), 7.72 (d,
2 H, J ~ 8.2 Hz), 8.13 (d, 2 H, J ~ 7.6 Hz).
Synthesis of 3,6-diiodo-9-(4-bromophenyl)carbazole (8).7
(0.65 g, 2 mmol) was dissolved in boiling glacial acetic acid
(40 mL) and potassium iodide (0.66 g, 3.96 mmol) and finely-
powdered potassium iodate (0.96 g, 4.5 mmol) were added. The
solution was refluxed at 80 uC for 4 h. Then the mixture was
poured into water, the precipitate was filtered and washed with
water. A purified 8 (1.14 g, 98%) was obtained as a white solid
by recrystalization from chloroform/ethanol.
Synthesis of 5. Compound 1 (4.1 g, 10 mmol) was dissolved
in boiling glacial acetic acid (150 mL), and potassium iodide
(4.4 g, 26.4 mmol) and finely-powdered potassium iodate (6.4 g,
30 mmol) were added. The solution was refluxed at 80 uC for
4 h. Then the mixture was poured into water, the precipitate
was filtered and washed with water. Without further purifica-
tion, this iodinated mixture in which 5I was the main product
was reacted directly with diphenylamine (27.2 g, 160 mmol),
CuI (0.8 g, 4 mmol), 18-Crown-6 (0.32 g, 1.2 mmol), K2CO3
(44.4 g, 320 mol), and 1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-
pyrimidinone (DMPU) (2.5 mL) at 170 uC for 18 h under
nitrogen. After cooling to room temperature, the mixture was
subsequently poured into 1 N HCl and then extracted with
dichloromethane. The organic layer was washed with NH3H2O
and water, dried with anhydrous Na2SO4, followed by
evaporation; the resulting residue was purified with column
chromatography using hexane/methylene chloride as eluant.
Yield 85%. UV-vis absorption spectra (CH2Cl2): lmax ~ 309
(e ~ 10 200) nm.1H NMR (400 MHz, CDCl3): d/ppm 7.00 (t,
6 H, J ~ 7.6 Hz), 7.16 (d, 12 H, J ~ 7.6 Hz), 7.25 (t, 12 H, J ~
7.6 Hz), 7.29–7.58 (m, 9 H), 7.86 (s, 6 H), 7.99 (s, 1 H), 8.04 (d,
1 H, J ~ 8.0 Hz). MS (FAB): m/z 910 (M1, 100%). Elemental
analysis: calc.: C, 87.10; H, 5.21; N, 7.70. Found: C, 86.99; H,
5.46; N, 7.34%.
Synthesis of 3,6-diphenylamine-9-(4-bromophenyl)carbazole
(9). To a 250 mL round-bottomed flask containing 70 mL of
toluene, using argon as the purge gas and a Dean–Stark trap
under a reflux condenser, were added in the following order
while maintaining good stirring: 8 (1.15 g, 2 mmol), diphenyl-
amine (1.35 g, 8 mmol), 1,10-phenanthroline (29.7 mg,
0.15 mmol), cuprous chloride (14.9 mg, 0.15 mmol), potassium
hydroxide (1.35 mg, 24 mmol) and toluene (50 mL). The
reaction mixture was refluxed at 125 uC for 20 h. The mixture
was quenched with 1 N HCl, extracted with toluene, washed
with NH3H2O and water, and dried with Na2SO4. After
evaporation of the solvent under vacuum, the residue was
purified with column chromatography using hexane/methylene
1
chloride as eluant. Yield 76%. H NMR (400 MHz, CDCl3):
d/ppm 6.93 (t, 4 H, J ~ 7.6 Hz), 7.06 (d, 8 H, J ~ 7.6 Hz), 7.20
(t, 8 H, J ~ 7.6 Hz), 7.27–7.30 (m, 4 H), 7.47 (d, 2 H, J ~
8.4 Hz), 7.72 (d, 2 H, J ~ 8.4 Hz), 7.80 (s, 2 H). MS (FAB): m/z
657 (M1, 100%).
Synthesis of 3. Compound 9 was reacted with carbazole by
the same Ullmann reaction procedure as for the synthesis of 1,
and the product was purified with column chromatography
using hexane/methylene chloride as eluant. Yield 81%. UV-vis
absorption spectra (CH2Cl2): lmax ~ 294 (e ~ 10 900), 313 (e ~
1
10300) nm. H NMR (400 MHz, CDCl3): d/ppm 6.95 (t, 4 H,
J ~ 7.6 Hz), 7.09 (d, 8 H, J ~ 7.6 Hz), 7.22 (t, 8 H, J ~ 7.6 Hz),
7.27–7.50 (m, 8 H), 7.55 (d, 2 H, J ~ 8.0 Hz), 7.80 (s, 2 H),
7.83 (s, 4 H), 8.18 (d, 2 H, J ~ 7.6 Hz). MS (FAB): m/z 743
(M1, 100%). Elemental analysis: calc.: C, 87.30; H, 5.16; N,
7.54. Found: C, 87.21; H, 5.36; N, 7.34%.
Synthesis of 6. Compound 1 (4.1 g, 10 mmol) was dissolved
in boiling glacial acetic acid (200 mL), and potassium iodide
(8.8 g, 52.8 mmol) and finely-powdered potassium iodate
(12,8 g, 60 mmol) were added. The solution was refluxed at
80 uC for 4 hours. Then the mixture was poured into water,
the precipitate was filtered and washed with water. Without
further purification, this iodinated mixture in which 6I was the
main product was reacted directly with diphenylamine (54.4 g,
320 mmol), CuI (1.6 g, 8 mmol), 18-Crown-6 (0.64 g, 2.4 mmol),
K2CO3 (88.8 g, 640 mol), and 1,3-dimethyl-3,4,5,6-tetrahydro-
2(1H)-pyrimidinone (DMPU) (3 mL) at 170 uC for 18 h under
nitrogen. After cooling to room temperature, the mixture was
subsequently poured into 1 N HCl and then extracted with
LED fabrication and measurements
Double-layer OLED devices using 1–6 as the hole-transporting
layer and Alq3 [tris-(8-quinolinolato)aluminium] as the emit-
ting as well as the electron transport layer were fabricated. For
comparison, a typical device using NPB as the hole transport
layer was also constructed. All devices were prepared by
˚
vacuum deposition of 400 A of the hole-transporting layer,
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 8 9 5 – 9 0 0
8 9 7

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Scheme 1 Synthetic routes to compounds 1, 2, 4, 5 and 6
˚
followed by 600 A of Alq3. Lithium fluoride (10 A) was
˚
deposited as the cathode, which was capped with 1000 A of
˚
in good yields, such as 3-diphenylamino-1,4-bis(carbazolyl)-
benzene (2), 3,6’-di(diphenylamino)-1,4-bis(carbazolyl)benzene
(4), 3,3’,6-tri(diphenylamino)-1,4-bis(carbazolyl)benzene (5)
and 3,3’,6,6’-tetra(diphenylamino)-1,4-bis(carbazolyl)benzene
(6). In order to further identify the structure of 4, 3,6-
di(diphenylamino)-1,4-bis(carbazolyl)benzene (3), in which
two diphenylamino substituents are located at the same side
of the 1,4-bis(carbazolyl)benzene core, is synthesized by a
different method from the syntheses of 2, 4, 5 and 6, as shown
in Scheme 2. The structure of 4 is confirmed by comparing the
1H NMR spectra of 3 and 4. All carbazole-based compounds
are obtained as yellow-green powders and are characterized by
1H-NMR, MS spectroscopy and elemental analysis.
aluminium. Hole-only devices were fabricated by sequential
vacuum deposition of 1–6 onto an indium-tin-oxide (ITO)-
coated glass substrate, followed by vacuum deposition of
aluminium onto the organic layer. Vacuum deposition was
carried out at a pressure of 1025 Torr. The electroluminescence
(EL) spectra were recorded using a Perkin-Elmer LS 50B
luminescence spectrometer while applying a direct current from
a current/voltage source. The current–voltage (I–V) and light
intensity–voltage (L–V) curves were measured using a Keithley
2400/2000 current/voltage source unit with a calibrated silicon
photodiode.
Thermal properties
Results and discussion
The thermal properties of the new compounds were determined
by DSC and TGA measurements (Table 1). As expected, the
resulting compounds possess excellent thermal stability.
Their thermal decomposition temperatures (Td) are in the
range of 377–487 uC and are increased with increasing the
number of diphenylamine moieties in the periphery. The glass
transition temperatures (Tg) are higher than those of
commercially used NPB (Tg ~ 100 uC), depending upon the
number of diphenylamine moieties and their structural
symmetry. For 4 (Tg ~ 141 uC) and 6 (Tg ~ 157 uC), their
symmetric structures may be responsible to their corresponding
Syntheses
The synthetic routes of the six carbazole-based compounds are
shown in Scheme 1 and Scheme 2. The 1,4-bis(carbazolyl)ben-
zene (1)3b is synthesized from carbazole as the starting material
by a modified Ullmann reaction. After iodination of 1, the
intermediates with different numbers of iodine substituents (by
controlling the quantity of potassium iodide and potassium
iodate) are directly reacted with diphenylamine. Purification by
column chromatography afforded pure 1,4-bis(carbazolyl)ben-
zene derivatives with different numbers of diphenylamine units
Scheme 2 Synthetic route to compound 3
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 8 9 5 – 9 0 0
8 9 8

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Table 1 Physical data for compounds 1–6 and NPB
Compound
Tg/Tma/uC
Tdb/uC
lmaxc/nm
lemc/nm
Eox/mV
HOMO/LUMOd/eV
Hole drift mobility [cm2 (Vs)21
]
NPB
100/265
—e/327
110/216
—e/316
141/248
147/—f
157/—f
479
321
377
426
424
436
487
271, 342
295, 325, 340
294, 307
294, 313
304
762, 936
993, 1320
776, 1194
783, 1031
796, 1245
763, 906, 1199
744, 939, 1088
5.12/2.12
5.40/1.94
5.13/1.58
5.14/1.68
5.15/1.64
5.12/1.72
5.10/1.74
1.89 6 1026
1
2
3
4
5
6
345
421
432
421
429
432
6.71 6 1028
3.29 6 1028
1.74 6 1026
1.93 6 1026
2.47 6 1026
309
309
a Obtained from DSC measurements. b Obtained from TGA measurements. c Measured in CH2Cl2 solution. d HOMO energy was calculated
with reference to ferrocence (4.8 eV).8 LUMO energy was derived from the relation, band gap ~ HOMO 2 LUMO (where the band gap was
derived from the observed optical edge). e Cannot form stable amorphous glass. f Obtained as stable amorphous glass.
higher Tg values. Although 3 and 4 have the same numbers of
diphenylamine units, they exhibit quite different thermal
behavior due to the different symmetry in their structures.
DSC shows no evidence of the glass transition for 3. It should
be noticed that there is no indication of crystallization and
melting in DSC curves for 5 and 6, implying that they have
excellent amorphous glass state stability.3b,9 These results
indicate that the 1,4-bis(carbazolyl)benzene unit increases the
rigidity of the molecular backbone, resulting in high Tg, where
the symmetry plays the important role in the enhancement of
Tg and the mophological stability can be improved by
increasing the number of the diphenylamine moieties in the
periphery.
Optical properties
The absorption and photoluminescence spectra of all carbazole-
based compounds in CHCl3 solution were measured and values
are listed in Table 1. It can be seen that compounds 2 and 3 show
two absorption bands centered at about 294 and 310 nm, which
can be attributed to the cabazole moiety and p–p* transition of
the molecular backbone. With increasing the number of
diphenylamine moieties, the feature bands at 294 nm, character-
periphery and their structural symmetry, and which are very
istic of the carbazole moiety, disappear. In the case of 4, 5 and 6,
only one-absorption band centered at around 306 nm is
Fig. 1 Cyclic voltammograms of compounds in CH2Cl2 containing
0.1 M tetrabutylammonium perchlorate at 25 uC.
voltammetry and are listed in Table 1. We can see from Table 1
that their HOMOs fall in the range of 5.10–5.15 eV, which is
independent of both the number of diphenylamine moieties in the
closed to the HOMO value of NPB. These are lower than that of
1, without the diphenylamino moieties, which shows
a
observed, implying that there is no unsubstituted carbazole
unit within the structure. In contrast, all these new compounds
exhibit quite different potoluminesent spectra with 1. Their
photoluminescence spectra are located in the range of 420–
430 nm, about 80–90 nm red-shifted with respect to 1, which is
caused by the electron-donating nature of the diphenylamine
moiety. Compared to the 3-substituted 1 derivatives 2 and 4, the
maximum emissions of the 3,6-disubstituted 1 derivatives 3, 5 and
6 are red-shifted by about 10 nm, indicating that 3,6-disubstituted
1 derivatives have much better conjugation than the 3-substituted
ones.
HOMO level of 5.40 eV. From these results, we can conclude
that the HOMO levels of the resulting compounds are
determined by the diphenylamino moieties. Thus, the role of the
diphenylamino moieties for significant improvement in the hole-
injection properties can be expected, as observed in the following
double-layer device results. To investigate their hole-transporting
properties, the hole drift mobility of the resulting compounds is
calculated by using the hole-only devices with the structure of
ITO/2–6/Al,11,12 as summarized in Table 1. It can be seen that the
hole drift mobility is greatly dependent upon the number and the
position of the diphenylamine moiety. For instance, the hole drift
mobility is ordered in 6 w 5 w 4 w 2, in accordance with the
increasing number of diphenylamine moieties. Compound 6
Electrochemical properties
exhibits a high hole drift mobility of 2.47 6 1026 cm2 (Vs)21
,
The electrochemical properties of all carbazole-based com-
pounds were investigated by cyclic voltammetry, as shown in
Fig. 1. The resulting data are also summarized in Table 1. In the
mono- and di-substituted derivatives (2 and 4), there are two
chemically reversible oxidation processes, which can be attri-
buted to the oxidation of the diphenylamino moiety at the
3-position of 1 and of the 1,4-bis(carbazolyl)benzene core itself,
respectively.7a In the tri- and tetra-substituted derivatives (5 and
6), three chemically reversible oxidation processes are observed,
in which the first and third oxidation waves are corresponding to
two oxidation potentials of 2 and 4, and the second oxidation
potential is consistent with that of the 3,6-disubstituted derivative
3, which can be attributed to the oxidation of the diphenylamino
moiety at the 6-position of 1. These results suggest that the
carbazole derivatives containing the 3,6-disubstituted diphenyl-
amino moieties undergo the stepwise oxidation process.3b,10 The
HOMO values of these compouds are estimated by cyclic
which is in same order of magnitude with typical NPB under the
same measurement conditions. Although 4 and 3 have the same
number of diphenylamine moieties, their hole drift mobilities are
quite different, depending upon the position of diphenylamine.
The hole drift mobility in the former one is found to be two orders
of magnitude larger than that in the latter, implying that the hole
drift mobility is strongly influenced by the symmetry of the
molecules. Based on these results, it is deduced that the symmetric
structure may be the practical way for achieving high charge
mobility.
Electroluminescent properties
In order to further study their charge-transporting properties,
double-layer LED devices with the structure of ITO/
HTL(40 nm)/Alq3(60 nm)/LiF/Al were fabricated using 1–6 as
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 8 9 5 – 9 0 0
8 9 9

View Article Online
Fig. 2 Current and luminance versus voltage characteristics of the double-layer devices based on 6 (left) and NPB (right).
materials may be promising in long-lifetime LED device
applications.3i,13 This synthetic strategy seems to be general for
developing new hole-transporting materials with large molecular
size having excellent thermal and morphological stability.
Further investigation on this aspect is currently in progress in
our laboratory.
Table 2 Performance characteristics of 1–6 and the NPB-based
devices
Turn-on
voltage/
V
Max.
brightness/
cd m22
Max. current
efficiency/
cd A21
Max. power
efficiency/
lm W21
Compound
NPB
4.5
8.2
4.2
3.4
3.5
3.3
3.2
14 628
254
4867
10 109
14 090
13 740
15 739
4.62
0.10
1.76
3.83
4.47
4.47
4.79
1.90
0.03
0.57
1.48
1.95
2.14
2.56
1
2
3
4
5
6
Acknowledgements
This work was supported by the National Natural Science
Foundation of China, No. 29725410 and 29992530, 973 Project
(2002CB613401 and 2002CB613402) and the Northeast
Normal University Science Foundation for the Youth.
the hole-transporting layer and Alq3 as the emitting layer and
electron-transporting layer. For comparison, the typical double-
layer device with the configuration of ITO/NPB/Alq3/LiF/Al was
also fabricated. Their device performance results are summarized
in Table 2 and Fig. 2. Except for 1, all of the devices exhibit a low
turn-on voltage of 3.2–4.5 V, which is consistent with their
HOMO levels. These suggest that the peripheral diphenylamine
moiety plays a critical role in determining the hole-injection
ability of the resulting molecules. The devices based on 4, 5 and 6
have much better performance in terms of brightness, current and
power efficiency than those of 2 and 3, due to the relatively low
hole-drift mobility in these later two compounds. For example, in
the case of 6, its maximum brightness is about 15739 cd m22 at a
driving voltage of 16 V. The current efficiency and power
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Conclusions
In summary, we have designed and synthesized a series of novel
hole-transporting molecules with 1,4-bis(carbazolyl)benzene as
the central unit and the diphenylamine moieties as substituents at
the 3- and/or 6-positions of the carbazole group. The resulting
carbazole-based molecules are thermally stable with high glass
transition temperatures of 141–157 uC, due to the rigid carbazole
central unit. Their amorphous stability, hole injection and
transport properties can be significantly improved by increasing
the number of peripheral diphenylamine moieties and by
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9 0 0
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 8 9 5 – 9 0 0