Synthesis of an aromatic amine derivative with novel double spirobifluorene core and its application as a hole transport material

Article abstract of DOI:10.1016/j.orgel.2011.11.028

A synthetic method to prepare a novel double spirobifluorene core structure was developed and a hole transport type exciton blocking material with the double spirobifluorene core was synthesized. A two step ring closing method was used to synthesize the double spirobifluorene core. The double spirobifluorene core based hole transport material showed high glass transition temperature due to rigid structure, and high quantum efficiency in green phosphorescent organic light emitting diodes because of efficient hole injection and triplet exciton blocking properties.

Full text of DOI:10.1016/j.orgel.2011.11.028

Organic Electronics 13 (2012) 351–355
Contents lists available at SciVerse ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Synthesis of an aromatic amine derivative with novel double
spirobifluorene core and its application as a hole transport material
⇑
Yong Joo Cho, Oh Young Kim, Jun Yeob Lee
Department of Polymer Science and Engineering, Dankook University, Jukjeon-dong, Suji-gu, Yongin-si, Gyeonggi-do 448-701, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 18 August 2011
Received in revised form 25 October 2011
Accepted 22 November 2011
Available online 14 December 2011
A synthetic method to prepare a novel double spirobifluorene core structure was devel-
oped and a hole transport type exciton blocking material with the double spirobifluorene
core was synthesized. A two step ring closing method was used to synthesize the double
spirobifluorene core. The double spirobifluorene core based hole transport material
showed high glass transition temperature due to rigid structure, and high quantum effi-
ciency in green phosphorescent organic light emitting diodes because of efficient hole
injection and triplet exciton blocking properties.
Keywords:
Double spirobifluorene core
Hole transport material
High efficiency
Ó 2011 Elsevier B.V. All rights reserved.
High glass transition temperature
1. Introduction
However, further development of new double spirobifluo-
rene based compounds is required.
Organic light-emitting diodes (OLEDs) have been devel-
oped for more than 20 years and are being used in display
and lighting applications. Various organic emitters and
charge transport materials were synthesized for OLED
applications and one of the well-known core structures is
spirobifluorene. The spirobifluorene core has been widely
used due to the merits of thermal stability, morphological
stability, chemical stability, chemical versatility and high
triplet energy [1–10].
In this work, a new core structure with a double spiro-
bifluorene moiety was developed as a thermally stable core
and the synthetic procedure to prepare the double spirobi-
fluorene structure was studied. It was demonstrated that
two step ring closing procedure can be used to synthesize
the double spirobifluorene core. In addition, the applica-
tion of the new double spirobifluorene core based com-
pound as the hole transport material for phosphorescent
organic light emitting diodes (PHOLEDs) was investigated.
As the spirobifluorene moiety was effective as the core
structure of hole transport materials and emitters, many
derivatives of the spirobifluorene were synthesized
[11–17]. Among the various spirobifluorene compounds,
dispirobifluorene based compounds were better than spiro-
fluorene in terms of light-emitting efficiency, thermal and
morphological stability [18–22]. Dispirofluorene–indeno-
fluorene based compounds were synthesized as the double
spiro type compounds [18–22]. Other than these, several
dispiro-fused fluorene compounds were reported [23,24].
2. Experimental
2.1. General information
1,2-Dibromobenzene, n-butyllithium (n-BuLi), sodium-
t-butoxide, palladium(II)acetate, diphenylamine and
chlorodiphenylphosphine (Aldrich Chem. Co.) were used
without further purification. Hydrogen peroxide (Duksan
Sci. Co.) and 2,7-dibromofluorenone (P&H Co.) were used
as received. Tetrahydrofuran (THF) was distilled over so-
dium and calcium hydride.
⇑
Corresponding author. Tel./fax: +82 31 8005 3585.
E-mail address: leej17@dankook.ac.kr (J.Y. Lee).
1566-1199/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.orgel.2011.11.028

352
Y.J. Cho et al. / Organic Electronics 13 (2012) 351–355
The 1H and 13C nuclear magnetic resonance (NMR)
spectra were recorded on Varian 200 (200 MHz)
sodium-t-butoxide (0.37 g, 39.20 mmol) dropwise slowly.
a
The reaction mixture was stirred for 12 h at 100 °C. The
mixture were diluted with dichloromethane and washed
with distilled water (50 mL) three times. The organic layer
was dried over anhydrous magnesium sulfate and evapo-
rated in vacuo to give the crude product, which was puri-
fied by column chromatography using n-hexane. The final
yellowish powdery product was obtained in 75% yield.
1H NMR (200 MHz, CDCl3): 8.05 (d, J = 8.0 Hz, 2H), 7.81
(d, J = 7.2 Hz, 4H), 7.61 (d, J = 7.4 Hz, 4H), 7.38–6.67 (m,
20H), 6.50 (d, J = 7.6 Hz, 8H), 6.06 (s, 2H).
spectrometer. The photoluminescence (PL) spectra were
recorded on a fluorescence spectrophotometer (HITACHI,
F-7000) and the ultraviolet–visible (UV–Vis) spectra were
obtained using UV–Vis spectrophotometer (Shimadzu,
UV-2501PC). Samples were dissolved in THF at a concen-
tration of 1.0 ꢀ 10ꢁ4 M. The differential scanning calorime-
try (DSC) measurements were performed on a Mettler DSC
822 under nitrogen at a heating rate of 10 °C/min. The
mass spectrometry (MS) was performed using a JEOL,
JMS-AX505WA spectrometer in fast atom bombardment
mode. Cyclic voltametry (CV) measurement of organic
material was carried out in acetonitrile solution with tetra-
butylammonium perchlorate at 0.1 M concentration. Fer-
rocene was used as the internal standard material.
Elemental analysis of the materials was carried out using
EA1110 (CE instrument).
13C NMR (50 MHz, CDCl3): 149.6, 149.2, 147.8, 146.7,
142.0, 141.9, 140.1, 136.9, 136.4, 135.8, 129.8, 128.8,
128.5, 127.5, 127.11, 124.9, 123.9, 123.3, 122.3, 121.3,
120.7, 119.5, 65.7, 50.9. MS (FAB) m/z 814 [(M+H)+]. Anal.
Calcd for C62H40N2: C, 91.60; H, 4.96; N, 3.45. Found: C,
91.67; H, 4.90; N, 3.37.
2.3. Device fabrication
2.2. Synthesis
The basic device structure used to evaluate DSPN was
indium tin oxide (ITO, 50 nm)/N,N0-diphenyl-N,N0-bis-[4-
(phenyl-m-tolyl-amino)-phenyl]-biphenyl-4,40-diamine
(DNTPD, 60 nm)/N,N0-di(1-naphthyl)-N,N0-diphenylbenzi-
dine (NPB) or DSPN (30 nm)/bis-9,90-spirobi[fluoren-2-
yl]-methanone (BSFM):tris(2-phenylpyridine) iridium
(Ir(ppy)3)(30 nm, 10%)/diphenylphosphine oxide-4-(tri-
phenylsilyl)phenyl(TSPO1, 25 nm)/LiF (1 nm)/Al (100 nm).
All organic materials were deposited by vacuum thermal
evaporation. Devices were encapsulated with a glass lid
and a CaO getter in glove box after device fabrication. Hole
only device with a device structure of ITO/DNTPD/NPB or
DSPN/Al was also fabricated to compare hole injection
and transport properties of DSPN and NPB. Current den-
sity–voltage–luminance characteristics of the devices were
measured with Keithley 2400 source measurement unit
and CS1000 spectroradiometer.
2.2.1. 20,70-Dibromo-spiro(cyclopenta[def]fluorene-1,5,90900-
bifluorene)
4-Bromo-9,90-spirobi[fluorene] (5.70 g, 14.40 mmol)
was dissolved in anhydrous THF (76 mL) under ambient
atmosphere. The reaction flask was cooled to ꢁ78 °C and
n-BuLi (2.5 M in hexane, 7.49 mL) was added dropwise
slowly. Stirring was continued for 2 h at ꢁ78 °C, followed
by addition of
a solution of 2,7-dibromo-fluorenone
(6.33 g, 18.70 mmol) in anhydrous THF (123 mL) under
nitrogen atmosphere. The resulting mixture was gradually
warmed to ambient temperature and quenched by adding
saturated, aqueous sodium bicarbonate (200 mL). The mix-
ture was extracted with dichloromethane. The combined
organic layers were dried over magnesium sulfate, filtered,
and evaporated under reduced pressure. A yellow powdery
product was obtained. The crude residue was placed in an-
other two-neck flask (250 mL) and dissolved in acetic acid
(150 mL). A catalytic amount of sulfuric acid (15 mL) was
then added and the whole solution was refluxed for 12 h.
After cooling to ambient temperature, purification by silica
gel chromatography using dichloromethane/n-hexane
gave a white powder.
3. Results and discussion
The double spirobifluorene moiety was designed as the
core structure to obtain good thermal stability due to the
twisted structure and rigidity of the double spirobifluorene
core. A two step ring closing method was used to synthe-
size the double spirobifluorene core structure with two
substituents at the spirobifluorene core. One spirobifluo-
rene unit can be formed by lithiating only one bromine
unit of 2,20-dibromobiphenyl followed by reaction with
fluorenone and ring closing reaction. The other bromine
unit can be lithiated and reacted with 2,7-dibromofluore-
none followed by ring closing reaction, yielding the double
spirobifluorene structure. The two step synthetic process
can be used to functionalize only one spirobifluorene of
the double spirobifluorene core. The functionalized double
spirobifluorene core can be used to synthesize various
materials and a hole transport material, DSPN, was synthe-
sized as the hole transport material for green PHOLEDs in
this work. Synthetic scheme of the DSPN is shown in
Scheme 1. The DSPN was designed as the high triplet en-
ergy hole transport material for green PHOLEDs. The
1H NMR (200 MHz, CDCl3): 8.26 (d, J = 8.0 Hz, 2H), 7.87
(d, J = 7.2 Hz, 2H), 7.71ꢂ7.54 (m, 4H), 7.39 (t, J = 7.2 Hz,
2H), 7.17 (t, J = 7.1 Hz, 2H), 6.85 (d, J = 7.4 Hz, 2H), 6.62
(d, J = 7.2 Hz, 2H), 6.50 (d, J = 7.6 Hz, 2H), 6.25 (s, 2H).
13C NMR (50 MHz, CDCl3): 150.1, 149.0, 141.9, 140.0,
139.5, 135.0, 131.6, 130.6, 129.4, 128.6, 127.6, 124.7,
123.8, 122.2, 121.7, 121.3, 120.8, 119.8, 65.9, 50.0. MS
(FAB) m/z 637 [(M+H)+]. Anal. Calcd for C38H20Br2: C,
71.72; H, 3.17 Found: C, 69.94; H, 3.31.
2.2.2. N,N,N0,N0-Tetraphenyl-spiro(cyclopenta[def]fluorene-
1,5,90,900-bifluorene)-20,70-diamine (DSPN)
20,70-Dibromo-spiro(cyclopenta[def]fluorene-1,5,90,900-bi
fluorene) (1.00 g, 15.71 mmol) diphenylamine (0.66 g,
39.20 mmol) and palladium acetate (0.02 g, 0.94 mmol)
were dissolved in anhydrous toluene under a nitrogen
atmosphere. To the reaction mixture was added a solution
of tri-t-butylphosphine (1 M, 0.31 g, 15.71 mmol) and

Y.J. Cho et al. / Organic Electronics 13 (2012) 351–355
353
O
Br
Br
Br
O
Br
Br
Br
Br
HO
H SO
2
4
OH
n-BuLi
CH COOH
3
n-BuLi
Br
H
N
Br
N
Br
N
H SO
2
4
CH COOH
Pd(2)
3
DSPN
Scheme 1. Synthetic scheme of DSPN.
diphenylamine unit was introduced as the hole transport
unit and was substituted at the double spirobifluorene
core.
UV-Vis
Solution PL
Low T PL
Density functional theory calculation of DSPN was car-
ried out using a suite of Gaussian 03 program to study
the orbital distribution of the DSPN. The nonlocal density
functional of Becke’s 3-parameters employing Lee–Yang–
Parr functional with 6-31G⁄ basis sets was used for the cal-
culation. Fig. 1 shows the molecular simulation result of
the hole transport material. The highest occupied molecu-
lar orbital (HOMO) and the lowest unoccupied molecular
orbital (LUMO) of the DSPN were localized on the spirobi-
fluorene with two diphenylamine groups. The other spiro-
bifluorene without the attached aromatic amine group had
little effect on the HOMO and LUMO. The simulated HOMO
and LUMO levels of the DSPN were 4.62 and 0.90 eV,
respectively.
300
350
400
450
500
550
600
Wavelength (nm)
Fig. 2. UV–Vis absorption, solution PL and low temperature PL spectra of
DSPN.
Photophysical properties of the DSPN were analyzed
using UV–Vis and PL spectrometer. UV–Vis, solution PL
and low temperature PL spectra of the DSPN are shown
in Fig. 2. UV–Vis absorption peaks of DSPN were observed
at 309 and 373 nm. The peak at 309 nm is assigned to the
of the extended double spirobifluorene core through two
diphenylamine units [11]. The absorption peak appeared
at long wavelength due to extended conjugation. As shown
in the molecular orbital simulation results, the conjugation
of the DSPN is extended between two diphenylamine units,
leading to the long wavelength absorption. The bandgap
could be calculated from the edge of the UV–Vis absorption
and was 3.12 eV. The PL emission of the DSPN was ob-
served at 404 nm. Low temperature PL measurement was
also carried out at 77 K to obtain the triplet energy of the
DSPN. As the diphenylamine group extends the conjuga-
tion of the double spirobifluorene core through amine
units, the triplet energy of the DSPN was 2.44 eV. The trip-
let energy of the DSPN was suitable for exciton blocking in
green PHOLEDs.
p–
p⁄ transition of the spirobifluorene unit of the double
spirobifluorene core without the diphenylamine units,
while the peak at 373 nm is assigned to the
p
–
p⁄ transition
LUMO
HOMO
DSPN
The HOMO and LUMO of the DSPN were measured
using CV and optical bandgap from UV–Vis absorption
data. CV data of the DSPN is shown in Fig. 3. The HOMO
4.62 eV
0.90 eV
Fig. 1. HOMO and LUMO distribution of DSPN.

354
Y.J. Cho et al. / Organic Electronics 13 (2012) 351–355
ence of HOMO level between DSPN and NPB, the similar
hole current density implies similar hole transport
properties.
800
700
600
500
400
300
200
100
0
The DSPN has a triplet energy for exciton blocking in
green PHOLEDs and was tested as the hole transport mate-
rial in green PHOLEDs. Device structure and energy level
diagram of the green PHOLED are shown in Fig. 5. Fig. 6
shows the current density–voltage–luminance curves of
the green PHOLEDs with the DSPN as the hole transport
material. The DSPN was compared with common NPB hole
transport material. The current density was similar in the
NPB and DSPN devices, while the luminance was high in
the DSPN device. The NPB and DSPN showed similar cur-
rent density due to similar energy barrier for hole injec-
tion. Although the energy barrier for hole injection from
DNTPD to DSPN (0.46 eV) was slightly higher than that
from DNTPD to NPB (0.40 eV), the energy barrier for hole
injection from DSPN to BSFM (0.34 eV) was slightly lower
than that from NPB to BSFM (0.40 eV). Therefore, the NPB
and DSPN were similar in terms of energy barrier for hole
injection. As shown in Fig. 4, the similar hole transport
properties induced the similar current density. The lumi-
nance was rather high in the DSPN device, which is due
to high recombination efficiency of DSPN device.
Quantum efficiency–luminance curves of the green
PHOLEDs are shown in Fig. 7. The quantum efficiency of
DSPN device was much higher than that of NPB device.
Maximum quantum efficiency of NPB device was 10.4%,
while that of DSPN device was 16.5%. In particular, the
DSPN showed high quantum efficiency of 16.6% even at
1000 cd/m2. The improved quantum efficiency of DSPN de-
vice is mainly due to efficient hole injection into the emit-
ting layer and triplet exciton blocking effect. The HOMO
level of the BSFM host is 5.90 eV and there is an energy
barrier of 0.40 eV for hole injection from NPB to BSFM host.
The high energy barrier for hole injection leads to hole
accumulation at the interface between NPB and BSFM
emitting layer as the BSFM host is an electron transport
type host material [29,30]. Therefore, hole injection from
NPB layer to BSFM emitting layer is limited by the hole
accumulation. This can be confirmed in the electrolumi-
nescence (EL) spectra of the NPB device (Fig. 8). NPB emis-
sion at 450 nm was observed in the NPB device [29].
However, the hole transport layer emission disappeared
in the DSPN device. The energy barrier between DSPN
and BSFM was 0.34 eV, which was lower than that between
NPB and BSFM. The reduced energy barrier for hole injec-
-100
Vo l t a g e ( V )
Fig. 3. CV curve of the DSPN.
and LUMO levels of the DSPN were 5.56 and 2.44 eV,
respectively. The HOMO level of the DSPN was 0.06 eV dee-
per than that of common hole transport material like NPB
(5.50 eV). The HOMO level of the DSPN is suitable for hole
injection from common hole injection layer to the emitting
layer and the LUMO was high enough to block electron
injection from the emitting layer to the DSPN hole trans-
port layer. In addition to the hole transport and electron
blocking, the DSPN can suppress the triplet exciton
quenching due to the triplet energy of 2.44 eV of DSPN
compared with 2.40 eV of green phosphorescent emitter
[25].
The double spirobifluorene core structure may improve
the thermal properties because it has serially connected ri-
gid spirobifluorene structure. Thermal analysis of the DSPN
was carried out and high glass transition temperatures (Tg)
of 152 °C was observed. Compared with common hole
transport materials based on spirobiflurorene [26–28],
the DSPN showed high Tg due to the rigidity of the back-
bone structure.
As DSPN was synthesized as the hole transport material,
the hole transport properties of the DSPN were compared
with those of NPB. Hole only devices DSPN and NPB were
fabricated to compare the hole transport properties.
Fig. 4 shows the hole only device data of DSPN and NPB.
Similar hole current density was observed in the hole only
devices of DSPN and NPB, indicating similar hole transport
properties of DSPN and NPB. As there was 0.06 eV differ-
1000
DSPN
2.10
10
NPB
2.40
2.44
DSPN
5.56
2.52
TSPO1
6.79
0.1
0.001
DNTPD
LiF/Al
TSPO1
2.80
NPB
BSFM:Ir(ppy)3
NPB or DSPN
DNTPD
BSFM
5.10
0.00001
5.50
ITO
0.0000001
5.90
0
1
2
3
4
5
Voltage(V)
Fig. 4. Hole current density–voltage curves of DSPN and NPB hole only
devices.
Fig. 5. Device structure and energy level diagram of DSPN device.

Y.J. Cho et al. / Organic Electronics 13 (2012) 351–355
355
Ir(ppy)3 (2.40 eV). Therefore, triplet exciton quenching of
Ir(ppy)3 by DSPN is suppressed in DSPN devices. However,
the triplet energy of NPB (2.30 eV) is lower than that of
Ir(ppy)3, resulting in triplet exciton quenching by NPB
[30]. Therefore, the appropriate HOMO level for hole injec-
tion and high triplet energy for triplet exciton blocking im-
proved the quantum efficiency of DSPN device.
1000
10
10000
8000
6000
4000
2000
0
NPB
DSPN
0.1
0.001
0.00001
0.0000001
4. Conclusions
In conclusion, a novel double spirobifluorene based hole
transport material, DSPN, was effectively synthesized by
two step ring closing reaction. The DSPN were efficient as
thermally stable hole transport material for green PHOL-
EDs. Our results indicate that the double spiro type core
structure is useful as the core structure for hole transport
layer.
0
2
4
6
8
Voltage (V)
Fig. 6. Current density–voltage–luminance curves of DSPN device com-
pared with NPB device.
20
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