Synthesis and properties of novel spirobifluorene-cored dendrimers

Article abstract of DOI:10.1016/j.dyepig.2011.12.005

Two novel spirobifluorene-cored dendrimers containing polyphenylene dendrons with the carbazole (spiro-Cz) and cyano surface groups (spiro-CN) were synthesized and characterized. Both the dendrimers show good solubility in common organic solvents. Spiro-Cz is amorphous even when it is obtained directly from organic solvents and an extremely high glass transition temperature of 332 °C is detected. These dendrimers can be reversibly oxidized and reduced in electrochemical measurements. They are blue fluorescent with small red-shift in solid film absorption and fluorescence spectra. These advantageous merits are suggested to benefit from the combined contribution from the spirobifluorene core and the bulky polyphenylene dendrons. They were used as emitting layer to fabricate organic light-emitting diodes (OLEDs). Deep-blue electroluminescence was obtained for both dendrimers devices.

Full text of DOI:10.1016/j.dyepig.2011.12.005

Dyes and Pigments 94 (2012) 136e142
Contents lists available at SciVerse ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Synthesis and properties of novel spirobifluorene-cored dendrimers
Huicai Ren, Qian Tao, Zhanxian Gao, Di Liu*
School of Chemistry, Dalian University of Technology, 2 Linggong Road, Dalian 116024, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 24 October 2011
Received in revised form
Two novel spirobifluorene-cored dendrimers containing polyphenylene dendrons with the carbazole
(
spiro-Cz) and cyano surface groups (spiro-CN) were synthesized and characterized. Both the den-
drimers show good solubility in common organic solvents. Spiro-Cz is amorphous even when it is
12 December 2011
ꢀ
obtained directly from organic solvents and an extremely high glass transition temperature of 332 C is
Accepted 15 December 2011
Available online 21 December 2011
detected. These dendrimers can be reversibly oxidized and reduced in electrochemical measurements.
They are blue fluorescent with small red-shift in solid film absorption and fluorescence spectra. These
advantageous merits are suggested to benefit from the combined contribution from the spirobifluorene
core and the bulky polyphenylene dendrons. They were used as emitting layer to fabricate organic
light-emitting diodes (OLEDs). Deep-blue electroluminescence was obtained for both dendrimers
devices.
Keywords:
Dendrimer
Polyphenylene dendron
Spirobifluorene
Amorphous
High glass transition temperature
Ó 2011 Elsevier Ltd. All rights reserved.
ꢀ
1
. Introduction
to 205 C [12]. Furthermore, the chemical modification could be
0
0
easily achieved at the 2,2 ,7,7 -positions of spirobifluorene.
On the other hand, a new class of functional organic material,
dendrimer, has been extensively applied as active materials for
optoelectronic devices such as organic light-emitting diodes
(OLEDs) [13e17] and solar cells [13,18]. The well-defined three-
dimensional dendritic structures can greatly improve the solubility
and film forming ability and make these materials show high
thermal and chemical stabilities. In addition, the emissive core,
dendrons and surface groups of dendrimers can be individually
modified to optimize the overall properties including light-emitting
wavelength and efficiency, the charge-transporting ability and the
solubility. This is the advantageous merit of dendrimers over the
traditional small molecules and polymers based optoelectronic
materials.
For the past decades, polyfluorene and its derivatives have been
known as the most promising blue light-emitting materials due to
their high fluorescent quantum yields and excellent chemical and
thermal stability [1,2]. However, the rigid and planar biphenyl
structure of these compounds always leads to the reduction of color
purity and stability of the devices due to the tendency to aggregate
and crystallize in the solid state or excimer formation [3e5].
Because of these drawbacks, there has been a considerable interest
0
in 9,9 -spirobifluorene that consists of two more or less extended
p
-systems with identical or different functions (emission, charge
3
transport) via a sp -hybridized carbon atom at the spiro center
6e9]. The rings of the connected bifluorene entities are arranged
[
orthogonally in the spiro segment. This high steric and rigid
structure not only efficiently suppresses the excimer formation and
Our interest is to take advantages of spirobifluorene skeleton to
develop novel spirobifluorene-cored dendrimers for optoelectronic
application. The bulky polyphenylene groups (Müllen type den-
molecular interactions between the
a high stability of the amorphous state, that is, a high glass tran-
sition temperature (T ) which is an important factor to improve the
durability of organic light-emitting diodes (OLEDs). Recently,
several spirobifluorene-based compounds with high T have been
p-systems, but also results in
0
0
g
drons) [19] are selected and incorporated at the 2,2 ,7,7 -positions
of the spirobifluorene core in order to further enhance the non-
planar conformation of the resultant molecules and consequently
tune the stability, solubility and so on. 3,6-Di-tert-butylcarbazole or
cyano groups are grafted to the periphery of dendrons with the aim
to tune the electrochemical redox behavior and thus the charge
injecting or transporting ability of the target dendrimers, respec-
tively. The dendrimers designed in such a way are expected to have
three-dimensional non-planar conformation and to show remark-
able thermal and morphological stabilities. Herein, we report the
g
successfully synthesized: fully spiro-configured terfluorene with
ꢀ
a T
a T
g
up to 296 C [10], spirobifluorene-linked bisanthracene with
ꢀ
g
up to 223 C [11], and spiro-linked oligothiophene with a T
g
up
*
Corresponding author. Fax: þ86 411 84986233.
E-mail address: liudi@dlut.edu.cn (D. Liu).
0143-7208/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2011.12.005

H. Ren et al. / Dyes and Pigments 94 (2012) 136e142
137
synthesis and characterization of these novel spirobifluorene-cored
dendrimers, spiro-Cz and spiro-CN (Scheme 1). The optical,
thermal and electrochemical properties of these dendrimers are
investigated as well. These dendrimers were also used as emitting
layer to fabricate OLEDs and their electroluminescent properties
are studied.
unoccupied molecular orbital (LUMO) energy levels of the studied
dendrimers were determined from the onset potential of the first
ox
red
oxidation ðE
Þ and first reduction ðE
Þ wave, based on the
onset
onset
þ
reference energy level of Fc/Fc (4.8 eV below the vacuum level)
[20].
2
2
2
.3. Synthesis
2
. Experimental
0
.3.1. Synthesis of 4, 4 -Dibromobenzoin (1)
A 250 ml round-bottom flask was charged with VB1 (5 g,
0 mmol) dissolved in distilled water (5 ml) and ethanol (100 ml).
2.1. Materials and instruments
All the reagents are of analytical grade and used as received
The mixture was cooled by an ice-salt bath. When the temperature
was below zero, a 10% NaOH solution in distilled water was added
dropwisely until the pH reached to 9. Then 4-bromobenzaldehyde
1
from commercial sources without further purification. H NMR
spectra were recorded on a Varian INOVA spectrometer (400 MHz).
Chemical shifts were referenced to TMS. Mass spectra were recor-
ded on a Micromass Q-Tof (Micromass, Wythenshawe, UK) mass
spectrometer for ESI-MS and a Micro MX (Micromass) Mass Spec-
trometer for MALDI-TOF-MS. Thermogravimetric analyses (TGA)
were carried out using a PerkineElmer thermogravimeter (Model
TGA 7) under a dry nitrogen gas flow by heating samples from room
ꢀ
(20 g, 108 mmol) was added, and the mixture was kept at 65 C and
stirred for 12 h. The solution was cooled and the precipitate was
filtered off, washed with water and ethanol. The crude product was
a mixture of benzoin and benzil. Crystallization from ethanol gave
ꢀ
pure compound 1 (11 g, 55%) as white crystals. mp: 92e93 C (lit.
ꢀ
[21]: 94e96.5 C).
ꢀ
ꢀ
ꢁ1
temperature to 600 C at a rate of 10 C min . The differential
scanning calorimetry (DSC) was performed on a NETZSCH DSC 204
0
2.3.2. Synthesis of 4, 4 -Dibromobenzil (2)
ꢀ
ꢁ1
at a scan rate of 10 C min . The fluorescence and absorption
spectra were measured on a PerkineElmer LS55 fluorescence
spectrometer and a PerkineElmer Lambda 35 UVeVis spec-
traphotometer, respectively. The relative fluorescent quantum
A solution of compound 1 (500 mg, 1.35 mmol), NH NO
4
3
(135 mg, 1.69 mmol), and Cu(OAc) $H O (31.4 mg, 0.17 mmol) in
2
2
acetic acid (5.5 ml) was stirred and refluxed for 2 h. Upon cooling to
room temperature, a precipitate was formed. Filtration followed by
washing with water and ethanol gave pure 8 (409 mg, 84.5%) as
yields (
hexane solutions using 9,10-diphenylanthracene as the standard
¼ 90% in cyclohexane).
F
F ) of these dendrimers were measured in dilute cyclo-
ꢀ
ꢁ1
1
yellow crystal. mp: 230e232 C. IR (KBr, cm ): 3090 (¼CeH
(
F
F
stretch), 1664 (C=O stretch), 1586 (C¼C stretch). H NMR (400 MHz,
CDCl
3
)
d
(ppm): 7.83 (d, J ¼ 8.4 Hz, 4H, ArH), 7.67 (d, J ¼ 8.4 Hz, 4H,
2
.2. Electrochemistry
ArH). ESI-MS (m/z): calcd. for C14
H
8
2
Br O
2
368.02; found 368.0
þ
(
[M] ).
Cyclic voltammetry (CV) measurements were performed using
0
a conventional three-electrode cell with a glassy carbon working
electrode, a platinum wire counter electrode, and an Ag/AgCl
reference electrode on a computer-controlled BAS 100W electro-
chemical analyzer at room temperature. Positive values are typi-
2.3.3. Synthesis of 4, 4 -Di(3,6-di-tert-butylcarbazol-9-yl) benzil (3)
A solution of compound 2 (2 g, 5.4 mmol), 3,6-Di-tert-carbazole
(3.34 g, 12 mmol), CuI (207 mg, 1.1 mmol), anhydrous K CO (2.25 g,
2 3
16 mmol) and 18-crown-6 (537 mg, 2 mmol) in nitrobenzene
(35 ml) was stirred and refluxed for 5 h under nitrogen. Upon
cooling, the precipitate was filtered. The solvent was removed
under reduced pressure, and the crude product was purified by
column chromatography on silica gel to afford 2.2 g (55% in yield) 3
as pale yellow solid.
cally measured in CH
2 2 4 6
Cl and negative values in THF with Bu NPF
(
0.1 M) as the supporting electrolyte. The redox potentials were
obtained at the scan rate of 100 mV/s, and with ferrocene/ferro-
þ
cenium (Fc/Fc ) as an internal standard of the redox system. The
highest occupied molecular orbital (HOMO) and the lowest
Scheme 1. Chemical structures of the dendrimers in present study.

138
H. Ren et al. / Dyes and Pigments 94 (2012) 136e142
1H NMR (400 MHz, CDCl
)
d
(ppm): 8.28 (d, J ¼ 8.4 Hz, 4H), 8.14
d
(ppm): 6.95 (d, J ¼ 8.6 Hz, 4H, ArH), 7.12e7.05 (m, 4H, ArH),
3
(
s, 4H), 7.80 (d, J ¼ 8.4 Hz, 4H), 7.50 (s, 8H), 1.47 (s, 36H). ESI-MS (m/
7.28e7.20 (m, 6H, ArH), 7.45 (d, J ¼ 8.6 Hz, 4H, ArH). ESI-MS (m/z):
þ
þ
z): calcd. for C54
H
56
N
2
O
2
, 764.4; found, 764.5 ([M] )
calcd. for C31
18
H N
2
O 434.1419; found 434.1423 ([M] ).
0
0
0
0
2
.3.4. Synthesis of 3,4-Bis[4,4 -Di(3,6-di-tert-butylcarbazol-9-yl)
2.3.7. Synthesis of 2,2 ,7,7 -tetra((trimethylsily) ethyny)l-9,9 -
phenyl]2,5-diphenyl-cyclopenta-2,4-di-enone (Cp-Cz)
spirobifluorene (4)
0
0
0
A 25 ml round-bottom flask was charged with compound 3
2,2 ,7,7 -Tetrabromo-9,9 -spirobifluorene (2 g, 3.16 mmol),
Pd(PPh Cl (266 mg, 0.379 mmol), CuI (301 mg, 1.58 mmol), and
PPh (414 mg, 1.58 mmol) were dissoved in 20 ml diethylamine.
(
800 mg, 1.05 mmol), 1, 3-diphenylacetone (219.6 mg, 1.05 mmol),
anhydrous KOH (58.8 mg, 1.16 mmol) and absolute ethyl alcohol
10 ml). The mixture was stirring for 30 min under reflux. Removing
3
)
2
2
3
(
Then trimethylsilylacetylene (2.7 mL, 19 mmol) was added via
a syringe. The reaction mixture was brought to reflux overnight and
then cooled down to room temperature. The solvent was removed
in vacuum, and the crude product was purified by chromatography
on silica gel to obtain white solid product (1.87 g, 84% yield). H
3 3
NMR (400 MHz, CDCl ) d (ppm): 0.16 (s, 36H, CH ), 6.77 (s, 4H, ArH),
of the solvent under reduced pressure gave the crude product,
purification of which by column chromatography on silica gel
yielded pure Cp-Cz (550 mg, 56%) as a brown solid.
1
1
H NMR (400 MHz, CDCl
3
)
d
(ppm): 8.13 (s, 4H), 7.49 (d, J ¼ 8 Hz,
4
(
9
H), 7.42 (d, J ¼ 8.8 Hz, 4H), 7.36 (m, 14H), 7.23 (d, J ¼ 8 Hz, 4H), 1.47
s, 36H). ESI-MS (m/z): calcd. for C69 O, 939.2751; found,
H
66
N
2
7.49 (dd, J ¼ 1.2, 7.6 Hz, 4H, ArH), 7.74 (d, J ¼ 8 Hz, 4H, ArH). ESI-MS
þ
þ
39.4039 ([M] ).
(m/z): calcd. for C45
H48Si
4
700.2833; found 700.2850 ([M] ).
0
0
0
2.3.5. Synthesis of 3,4-bis-(4-bromophenyl)-2,5-diphenyl-
2.3.8. Synthesis of 2,2 ,7,7 -tetraethynyl-9,9 -spirobifluorene (5)
cyclopentadienone (Cp-Br)
NaOH (220 mg, 20 mmol) was dissolved in 5 mL of CH
added to a solution of compound 4 (1.4 g, 2 mmol) in 20 mL of
CH Cl , and then stirred over night at room temperature. The
reaction mixture was washed with water, and the aqueous phase
was extracted with CH Cl and then dried over anhydrous MgSO
3
OH, then
A 25 ml round-bottom flask was charged with compound 2
(
1.2 g, 3.26 mmol), 1, 3-diphenylacetone (600 mg, 3.26 mmol),
anhydrous KOH (200 mg, 3.59 mmol) and absolute ethyl alcohol
15 ml). The mixture was stirring for 30 min under reflux. Removing
2
2
(
2
2
4
.
of the solvent under reduced pressure gave the crude product,
The solvent was removed under reduced pressure. The crude
purification of which by column chromatography on silica gel
yielded pure compound 3 (1.5 g, 85%) as a dark brown solid. H
product was purified by chromatography on silica gel to obtain
1
1
yellow solid product (0.66 g, 80% yield). H NMR (400 MHz, CDCl
3
)
NMR (400 MHz, CDCl
3
)
d
(ppm): 6.77 (dd, J ¼ 2.1, 8.7 Hz, 4H, ArH),
d
(ppm): 3.01(s, 4H, C^CH), 6.83(s, 4H, ArH), 7.54(dd, J ¼ 1.2, 7.6 Hz,
7.1e7.3 (m, 10H, ArH), 7.34 (dd, J ¼ 2.1, 8.7 Hz, 4H, ArH). ESI-MS (m/
4H, ArH), 7.79(d, J ¼ 8 Hz, 4H, ArH). ESI-MS (m/z): calcd. for C33
H
16
þ
þ
z): calcd. for C29H18Br
2
O 542.2606; found 542.2610 ([M] ).
412.1252; found 412.1263 ([M] ).
2.3.6. Synthesis of 3,4-bis-(4-cyanophenyl)-2,5-diphenylcyclopenta-
2.3.9. General procedure for synthesis of the target dendrimers
dienone (Cp-CN)
A mixture of compound 5 (0.15 mmol) and the corresponding
cyclopentadienone (6 equiv) in o-xylene (15 ml) was stirred at
140 C for 12 h under nitrogen. The solvent was removed under
A 25 ml round-bottom flask was charged with compound Cp-Br
ꢀ
(
(
(
500 mg, 0.92 mmol), K
2 mg, 0.01 mmol), P(t-Bu)
4
Fe(CN)
6
(156 mg, 0.37 mmol), Pd(OAc)
CO
2
3
(0.08 ml, 0.03 mmol) and Na
2
3
reduced pressure. The crude product was purified by column
chromatography on silica gel and then recrystallized from chloro-
form and methanol to afford the target dendrimers.
39 mg, 0.37 mmol) and NMP (10 ml). The mixture was stirring for
ꢀ
7
h at 140 C under nitrogen. Removing of the solvent under
reduced pressure gave the crude product, purification of which by
spiro-Cz: yield 75% as a white powder. 1H NMR (400 MHz,
column chromatography on silica gel yielded pure compound
CDCl
3
)
d
(ppm): 8.08 (d, J ¼ 2 Hz,16H, ArH), 7.62 (s, 4H, ArH), 7.46 (d,
1
Cp-CN (200 mg, 50%) as a dark solid. H NMR (400 MHz, CDCl
3
)
J ¼ 7.6 Hz, 4H, ArH), 7.31e6.99(m, 88 H, ArH), 6.82e6.68(m, 24H,
O
O
HO
O
O
O
CHO
Br
A
B
D
E
Br
Br
Br
Br
1
2
Br
Br
NC
CN
Cp-Br
Cp-CN
C
O
O
D
N
N
O
N
N
3
Cp-Cz
(
A): VB1, NaOH, H O, EtOH; (B): NH NO Cu(OAc) ; (C): CuI, 18-crown-6, K CO , reflux,
2 4 3, 2 2 3
nitrobenzene; (D): KOH, EtOH, 1,3-diphenylacetone, reflux; (E): K Fe(CN) , Pd(OAc) , P(t-Bu) ,
4
6
2
3
Na CO , NMP.
2
3
Scheme 2. Synthetic routes of the important intermediate Cp-Cz and Cp-CN.

H. Ren et al. / Dyes and Pigments 94 (2012) 136e142
139
ArH), 1.40 (s, 144H, CH
3
). MALDI-TOF-MS: (m/z): calcd for
2
.8
.6
þ
C
305
H
280
N
8
4054.2156; found 4056.1604 ([M] ).
spiro-CN: yield 80% as a white powder. 1H NMR (400 MHz,
spiro-CZ
spiro-CN
CDCl
3
)
d
(ppm): 7.45 (s, 4H, ArH), 7.36 (d, J ¼ 8 Hz, 4H, ArH), 7.24 (d,
2
J ¼ 7.6 Hz, 8H, ArH), 7.19e7.15 (m, 20H, ArH), 7.04e7.02 (m, 8H,
ArH), 6.93e6.90 (dd, J ¼ 1.2, 8 Hz, 4H, ArH), 6.88 (d, J ¼ 8 Hz, 8H,
ArH), 6.78 (d, J ¼ 8.4 Hz, 8H, ArH), 6.61e6.48 (m, 24H, ArH). MALDI-
2.4
.2
2.0
.8
TOF-MS: (m/z): calcd for C153
H
88
N
8
2036.7132; found 2036.7667
þ
(
[M] ).
2
2.4. OLED fabrication and measurements
The light-emitting devices have a configuration of ITO/
PEDOT:PSS (40 nm)/dendrimer (40 nm)/TPBI (30 nm)/LiF (1 nm)/Al
100 nm). The patterned ITO substrates were cleaned by successive
(
1
ultrasonications in detergent, deionized water, ethanol, and chlo-
roform, followed by treatment with UVeozone. PEDOT:PSS (Bayer
AG) was spin-coated on pretreated ITO substrate from aqueous
1
00
200
300
400
500
600
700
ꢀ
dispersion and baked at 120 C for 1 h. Subsequently the dendrimer
Fig. 1. TGA thermograms for dendrimers spiro-Cz and spiro-CN.
layer was deposited on PEDOT:PSS film by spin-coating the den-
drimer solutions in chlorobenzene, the thickness of which was
controlled as 40 nm by tuning the solution concentration and spin
rate. Then a thin layer of TPBI was vacuum deposited on top of
dendrimer layer, followed by vacuum deposition of LiF (1 nm) and
Al (100 nm) as cathode. The emitting area of each pixel is deter-
1
MALDI-TOF mass spectrometry and H NMR spectroscopy confirmed
their chemical structures.
3.2. Thermal properties
2
mined by overlapping of the two electrodes as 9 mm . The EL
spectra, CIE coordinates, and current-voltage-luminance charac-
teristics were measured with computer-controlled Spectrascan PR
The thermal properties of the spirobifluorene-cored dendrimers
were investigated by thermogravimetric analysis (TGA) and
differential scanning calorimetry (DSC). Both TGA and DSC
measurements were carried out in nitrogen atmosphere at a heat-
7
05 photometer and a Keithley 236 source-measure-unit. All the
measurements were carried out at room temperature under
ambient conditions.
ꢀ
ꢁ1
ing rate of 10 C min . As shown in Fig. 1, both spiro-Cz and spiro-
ꢀ
CN start to decompose at a high temperature up to 480 C, indi-
3
. Results and discussion
cating that such dendron functionalized materials have excellent
thermal stability, which is an essential merit for fabricating stable
optoelectronic devices. It should be noted that spiro-CN started to
show a tiny weight loss at lower temperature than spiro-Cz, and
then exhibited an obvious weight loss at a higher temperature,
indicating spiro-Cz may possess higher thermal stability than
spiro-CN. This should benefit from the extra contribution from
carbazole groups that is characterized by excellent thermal and
chemical stabilities. The phase transition behaviors of these den-
drimers were investigated by means of DSC measurements. The
DSC trace for spiro-Cz is shown in Fig. 2. When a dry powder of
spiro-Cz obtained directly from organic solvents was heated at
3.1. Synthesis
The target demdrimers spiro-Cz and spiro-CN were synthesized
through the typical DielseAlder cycloaddition of the functionalized
tetraphenyl-cyclopentedieneone (Cp-Cz and Cp-CN) to a spirobi-
0
0
fluorene core containing four terminal ethynyl groups at the 2,2 ,7,7 -
positions (compound 5). The important building blocks, Cp-Cz and
Cp-CN, were obtained conveniently from 4-bromobenzaldehyde as
the starting material through the route shown in Scheme 2. Both
dendrimers were prepared in the divergent way (Scheme 3) from
compound 5, which was involved 2,2 ,7,7 -tetrabromo-9,9 -spirobi-
fluorene by treatment with trimethylsilylacetylene, followed by
deprotection. The dendrimers are readily soluble in common organic
solvents such as dichloromethane, chloroform, or toluene and were
easily purified by column chromatograph. Characterization by
0
0
0
ꢀ
ꢁ1
a rate of 10 C min , a typical endothermic process was detected at
ꢀ
332 C, which is ascribed to the glass transition temperature (T
g
) of
this dendrimer. With further increasing temperature, an
ꢀ
exothermic transition was observed at 372 C, which corresponds
to the crystallization of the dendrimer. No further change was
Scheme 3. Synthetic routes of the dendrimers.

140
H. Ren et al. / Dyes and Pigments 94 (2012) 136e142
ꢀ
detected before 450 C. The detection of T
g
in the first heating run
a
indicates that the dendrimer spiro-Cz is naturally amorphous even
1
0
0
0
.0
.8
.6
.4
Sol.
Film
1.0
0.8
0.6
0.4
0.2
0.0
when it was obtained from organic solvents. It should be noted that
ꢀ
such a T
a high T
g
of 332 C is remarkably high for organic molecules. Such
of 332 C indicates that dendrimer spiro-Cz has extra
ꢀ
g
stable amorphous state. For dendrimer spiro-CN, no glass transi-
tion or other phase change was observed when it was heated to
ꢀ
4
50 C. We attribute the excellent thermal and amorphous stabil-
ities of dendrimer spiro-Cz to the contribution from both the bulk
dendritic polyphenylenes and the tetrahedral structure of spirobi-
fluorene. Such a high T
g
and high decomposition temperature
0.2
0.0
determine that the spirobifluorene-based dendrimers would be
desirable especially for high-temperature device applications.
3.3. Optical properties
200 250 300 350 400 450 500 550
Wavelength (nm)
The absorption and fluorescence spectra of these dendrimers
were measured in dichloromethane solution as well as in solid film
on quartz substrates. The spectra are shown in Fig. 3. In CH Cl
solutions, the absorption bands between 200 and 300 nm are from
b
1
.0
Sol.
Film
1.0
2
2
0.8
0.8
0.6
0.4
0.2
0.0
the benzene units while the weak absorption between 300 and
*
3
40 nm is ascribed to the
p-
p
transition of the molecular skeleton.
One additional absorption peak at 345 nm was only observed for
spiro-Cz, which should be assigned to the transition absorption of
carbazole units. For both dendrimers, the absorption spectra in
films are nearly identical with the spectra in solutions, suggesting
that there is minimal intermolecular interaction in thin films at the
ground state. Upon photoexcitation at 330 nm, both dendrimers
emit deep-blue fluorescence in dilute solutions with the emission
maxima at 396 and 391 nm for spiro-Cz and spiro-CN, respectively.
In solid state, only small red-shift of 5 nm were observed for the
two dendrimers. These observations suggest that the orthogonal
structure of spirobifluorene and the bulky polyphenylene dendrons
efficiently suppress molecular interactions and excimer formation,
which are frequently observed in solid state. The fluorescent
0.6
0
0
0
.4
.2
.0
2
00 250 300 350 400 450 500 550
Wavelength (nm)
Fig. 3. Absorption and fluorescence spectra of a) spiro-Cz and b) spiro-CN in CH
2
Cl
2
solutions (1ꢂ 10ꢁ mol L ) and solid films (40 nm thick).
5
ꢁ1
F
quantum yields (F ) of spiro-Cz and spiro-CN are determined as ca.
0
.50 and 0.46 in dilute cyclohexane solutions, respectively, relative
to 9, 10-diphenylanthracene in cyclohexane (
a standard.
F
F
¼ 0.90) [22] as
2
atmosphere. As shown in Fig. 4, during the anodic scan in CH Cl
2
solutions, reversible oxidation waves were observed with the onset
þ
potentials at 0.73 V and 1.1 V (vs. Fc/Fc ) for spiro-Cz and spiro-CN,
3
.4. Electrochemical properties
respectively. Upon the cathodic sweep in THF, a reversible reduc-
þ
tion was detected with the onset potential at ꢁ2.3 V (vs. Fc/Fc )
The electrochemical behavior of spiro-Cz and spiro-CN was
only for spiro-CN, while no reduction signal was observed for
spiro-Cz within the detected potential range. Distinctively, the
investigated by cyclic voltammetry method in
a
nitrogen
spiro-Cz
spiro-CN
100
200
300
400
500
-3000
-2000
-1000
0
1000
+
Potential mV Vs. Fc/Fc
Fig. 2. DSC thermogram of spiro-Cz in the first heating cycle.
Fig. 4. Cyclic voltammograms of the studied dendrimers.

H. Ren et al. / Dyes and Pigments 94 (2012) 136e142
141
Tabel 1
EL performance data for dendrimers spiro-Cz and spiro-CN based OLEDs.
300
250
200
150
300
250
200
150
0.40
.35
0.30
.25
.20
0
Dendrimer
Von (V)
J
max (mA/cm2)a
L
max (cd/m2)a
hmax (cd/A)
0
Spiro-Cz
Spiro-CN
6
6
302 (14)
277 (11)
287 (13)
197 (11)
0.35
0.47
0
0.15
0.10
a
Values in parentheses are the voltages at which they were obtained.
0
.05
.00
0.05
0
-
0
50 100 150 200 250 300 350
oxidation of spiro-Cz occurs at less positive potential if compared
with spiro-CN. This should be because of the peripheral carbazole
groups is electron-donating and make the corresponding den-
drimer molecules much easier to be oxidized. The reversible
reductive or oxidative behavior indicates that the dendrimers could
be applied as hole- or electron- transporter in optoelectronic
100
50
0
1
00
50
0
0
2
4
6
8
10
12
14
devices. The HOMO and LUMO energy levels were estimated from
Voltage (V)
ox
the onset potential of the first oxidation ðE
Þ and reduction
onset
red
ðE
Þ waves, respectively, by using the following equations:
onset
Fig. 6. The current density-voltage-brightness (JeVeB) characteristics of spiro-Cz
OLED. Insert: the plot of current efficiency versus current density of the same device.
ox
red
HOMO (eV) ¼ ꢁe(E
þ 4.8 V), LUMO (eV) ¼ ꢁe(E
þ 4.8 V),
onset
onset
based on the energy level of the ferrocene reference (4.8 eV below
the vacuum level) [20]. The HOMO energy levels for spiro-Cz and
spiro-CN are determined as ꢁ5.53 and ꢁ5.9 eV, respectively. The
LUMO energy level for spiro-CN is ꢁ2.5 eV and the electrochemical
energy band gap is 3.4 eV. Since no reduction process was detected
for spiro-Cz, its LUMO value was estimated as ꢁ2.11 eV by using the
optical band gap of 3.42 eV that is determined by absorption edge
technique [23]. The wide band gaps and proper HOMO and LUMO
levels as well as their good solubility make these dendrimers
promising materials to act as host in solution-processed optoelec-
tronic devices such as OLEDs.
chlorobenzene. The performance data are collected in Table 1.
As shown by the EL spectra in Fig. 5, the spiro-CN based
OLED exhibited blue electroluminescence with emission peaks at
4
22 and 448 nm, which is slightly red-shifted in comparison with
the PL emission peak. The bathochromic effect in EL relative to
the PL, which is frequently observed in OLEDs, should be ascribed
to the additional influence of the electrical field on the excited
state in OLEDs. The EL spectra show little change in position and
profile with increasing driving voltage, indicating a relatively
stable blue emission. The current density-voltage-brightness
(
JeVeB) characteristics and the EL efficiency curve of the spiro-
3
.5. Electroluminescence properties
Cz OLED are shown in Fig. 6. This device exhibited a maximum
brightness of 287 cd/m at 13 V and a maximum luminance
efficiency of 0.35 cd/A. Similarly, the spiro-CN device emitted
deep-blue electroluminescence at 412 nm with two shoulders at
2
In order to evaluate the electroluminescence properties of
these spirobifluorene-cored dendrimers, they were used as the
non-doped emitting layer to fabricate OLEDs. The OLEDs have the
configuration of ITO/PEDOT:PSS (40 nm)/dendrimer (40 nm)/TPBI
4
48 and 480 nm (Fig. 7). This OLED exhibited a maximum
2
brightness of 197 cd/m at 11 V and a peak luminance efficiency
of 0.47 cd/A. Although these EL performance data are not satis-
fied, the deep-blue electroluminescence and the extremely
high glass transition temperature still indicate that these
spirobifluorene-cored dendrimers are potential light-emitting
materials for OLEDs application, especially for applications
under high temperature.
(
30 nm)/LiF (1 nm)/Al (100 nm), in which ITO (indium tin oxide)
and LiF/Al are the anode and cathode, respectively. PEDOT:PSS
poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate)) acts
as the hole-injecting and -transporting layer, TPBI (1,3,5-tris[N-
phenyl)benzimidazole]-benzene) as the electron-transporting
(
(
and hole-blocking layer. Based on the good solubility and film-
forming ability of these dendrimers, the emitting layers of the
OLEDs were obtained by spin-coating the dendrimer solutions in
3
00
50
00
50
1.0
0.8
0.6
0.4
0.2
0.0
2
00
2
1
.0
8
9
1
1
V
V
0 V
1 V
2
150
100
50
1
0.8
100
50
0
0.6
0
0
2
4
6
8
10
12
Voltage (V)
0.4
0.2
0.0
300
400
500
600
700
800
3
00
400
500
600
700
800
Wavelength (nm)
Wavelength (nm)
Fig. 7. The EL spectrum of spiro-CN OLED at 10 V. Insert: The current density-voltage-
brightness (JeVeB) characteristics of the same device.
Fig. 5. The EL spectra of the spiro-Cz device under different voltages.

1
42
H. Ren et al. / Dyes and Pigments 94 (2012) 136e142
[7] Johansson N, Santos DA, Guo S, Cornil J, Fahlman M, Salbeck J, et al. Electronic
4
. Conclusions
structure and optical properties of electroluminescent spiro-type molecules.
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organic light-emitting devices using spirobifluorene-cored conjugated
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stereoselective alkoxy-substituted spirobifluorene derivatives for blue light
emitting materials. Tetrahedron 2003;59:2773e9.
In summary, we have synthesized two novel spirobifluorene-
[
[
cored dendrimers, spiro-Cz and spiro-CN, with polyphenylene
dendrons and carbazole or cyano surface groups. Spiro-Cz is char-
acterized by the amorphous nature evenwhen it is obtained directly
from organic solvents. An extremely high glass transition tempera-
ꢀ
[10] Geng YH, Katsis D, Culligan SW, Ou JJ, Chen SW, Rothberg LJ. Fully spiro-
configured terfluorenes as novel amorphous materials emitting blue light.
Chem Mater 2002;14:463e70.
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bisanthracene: an efficient blue emitter with pronounced thermal stability.
Chem Mater 2004;16:930e4.
ture of 332 C was detected for Spiro-Cz, indicating a remarkable
amorphous stability. Both dendrimers are also thermally stable and
have good solubility in organic solvents. The absorption and fluo-
rescence spectra of these dendrimers are almost identical to those in
dilute solutions. All these good properties are suggested to benefit
from the molecular design strategy by combining the spirobi-
fluorene core and the bulky polyphenylene dendrons. The prelimi-
nary EL results of the devices made with these dendrimers
demonstrate that they could be applied as active emitters in OLEDs.
[
12] Pei J, Ni J, Zhou XH, Cao XY, Lai YH. Regioregular head-to-tail oligothiophene-
functioalized 9,9
-spirobiflurene derivatives 2. NMR characterization,
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104e13.
0
8
[
[
[
13] Lo SC, Burn PL. Development of dendrimers: macromolecules for use in
organic light-emitting diodes and solar cells. Chem Rev 2007;107:1097e116.
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Polymer 2005;46:7658e69.
Acknowledgments
We thank the National Natural Science Foundation of China
[16] Liu D, Ren H, Li J, Tao Q, Gao Z. Novel perylene bisimide derivative with
fluorinated shell: a multifunctional material for use in optoelectronic devices.
Chem Phys Lett 2009;482:72e6.
(
20704002) and the Fundamental Research Funds for the Central
Universities (DUT10LK16) for financial support of this work.
[
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complex as a single dopant for white-light electroluminescent devices.
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