A novel amorphous oligo(phenylenevinylene) dimer with a biphenyl linkage center and fluorene end groups for electroluminescent devices

Article abstract of DOI:10.1039/b407262f

A new type of oligo(phenylenevinylene) dimer, 2,5,2,5- tetra(9,9-dihexylfluorenyl)biphenyl (TFB), with a biphenyl linkage center and four fluorene end groups, has been synthesized by the Wittig reaction. The full characterization of its structure and optical properties, as well as the performance of its electroluminescent devices are presented. TFB shows strong blue fluorescence both in solution and as a solid film. High-quality films of TFB for light-emitting devices (LEDs) can be fabricated both by vacuum evaporation and the spin-coating technique, which is very special and interesting. Single-layer and multi-layer light-emitting devices using TFB as the active layer all show efficient blue emission.

Full text of DOI:10.1039/b407262f

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A R T I C L E
A novel amorphous oligo(phenylenevinylene) dimer with a
biphenyl linkage center and fluorene end groups for
electroluminescent devices
a
Feng He, Hong Xia, Shi Tang, Yu Duan, Ming Zeng, Linlin Liu, Mao Li,
a
a
b
a
a
a
a
Haiquan Zhang, Bing Yang, Yuguang Ma,* Shiyong Liu and Jiacong Shen
a
a
b
a
a
Key Laboratory for Supramolecular Structure and Materials of Ministry of Education,
Jilin University, Changchun 130012, P. R. China. E-mail: ygma@jlu.edu.cn;
Fax: 186-431-5193421; Tel: 186-431-5618480
b
National Laboratory of Integrated Optoelectronics, Jilin University, Changchun,
P. R. China
Received 13th May 2004, Accepted 9th June 2004
First published as an Advance Article on the web 5th August 2004
A new type of oligo(phenylenevinylene) dimer, 2,5,2’,5’-tetra(9,9’-dihexylfluorenyl)biphenyl (TFB), with a
biphenyl linkage center and four fluorene end groups, has been synthesized by the Wittig reaction. The full
characterization of its structure and optical properties, as well as the performance of its electroluminescent
devices are presented. TFB shows strong blue fluorescence both in solution and as a solid film. High-quality
films of TFB for light-emitting devices (LEDs) can be fabricated both by vacuum evaporation and the spin-
coating technique, which is very special and interesting. Single-layer and multi-layer light-emitting devices using
TFB as the active layer all show efficient blue emission.
high-performance blue electroluminescent (EL) devices using
Introduction
the new oligomers as the active layer have been demonstrated.
In this paper, we report the synthesis of a new oligo(pheny-
lenevinylene) with a biphenyl linkage center and fluorene end
groups, TFB, and its optical and EL properties. The four
fluorene end groups in TFB strongly improve its solubility in
common solvents and high quality films can be fabricated by
solution spin-coating and also by vacuum evaporation.
Generally, LEDs can be fabricated from small organic
Conjugated organic materials such as poly(phenylenevinylene)s
(PPVs) have attracted considerable attention in recent years
due to their applications in organic light-emitting devices
1
–5
(
OLEDs).
As model compounds for the corresponding
polymers and active materials themselves, oligomers have
induced great interest because their conjugation length can be
defined and adjusted, and consequently the color of the
emission can be easily controlled. Trimeric phenylenevinylene
1
5,16
molecules by a vacuum-deposition technique
and by a
6
–8
(
TPV) and its derivatives have been widely investigated,
spin-coating technique for conjugated polymers, which has the
advantage of relative simplicity and suitability for large-area
which has resulted in the deep understanding of the important
conjugated polymer poly(phenylenevinylene) (PPV), while TPV
itself is also a bright pure blue emitter. Unfortunately, however,
the excellent luminescent properties of TPV, e.g. a photo-
luminescence (PL) efficiency of over 90%, are only obtained in
dilute solution; in the solid state it emits very weakly. This solid
state fluorescent quenching behavior has been associated with the
formation of intermolecular aggregates, especially those
1
7,18
flat displays.
Therefore, TFB combines the advantages of
small organic molecules, e.g. well-defined molecular structure
and easy purification, with the advantages of conjugated
polymers, e.g. easy device fabrication.
Experimental
9
arranged in a side-by-side fashion (H-aggregates). This means
Materials
that the stacking of TPV molecules plays a determining role in the
physical properties of these materials. Much work has been
done to prevent the aggregation of TPV, which aims to achieve
Fluorene, N-bromosuccinimide (NBS), 1-bromohexane,
n-butyllithium (1.6 M hexane solution), and sodium tert-
butoxide were purchased from Aldrich and used without further
purification unless otherwise noted. Tetra(phosphonate) 5 was
1
0,11
high-efficiency emission in the solid state and in films.
involves either the introduction of a chemical substitutent to
This
12
16
TPV, forming a tetrahedral linkage of four TPVs, i.e.
13
prepared according to our previous report. Tetrahydrofuran
t
tetrakis(4-tert-butylstyrylstilbenyl)methane (C( BuSSB)
4
)
or
forming covalent assemblies of four TPVs around a calix[4]arene
(THF) was dried and purified by fractional distillation over
sodium/benzophenone and N,N-dimethylformamide (DMF)
was vacuum-distilled over P O . Column chromatography was
1
core. Some of them e.g. C( BuSSB) , have been used in the
4
t
4
2
5
1
5
fabrication of devices, however, the device performance in this
case is not as good as expected from its high PL efficiency in the
solid state (42%), likely to be due to low carrier mobility induced
performed using silica gel (200–300 mesh).
3
Synthesis of 2-bromofluorene (2)
by a central sp -hybridized carbon atom.
Recently, we developed a versatile synthesis of a new class of
dimeric TPVs in which two TPVs are linked through the
phenyl–phenyl bond of the central phenylene ring to form
a crossing dimeric structure with a twist angle of y70u
between the planes of the two TPVs. High photolumine-
scence efficiences for these new solid TPV oligomers and
A mixture of fluorene (5 g, 60 mmol) and FeCl3 (0.26 g,
4.4 mmol) was dissolved in anhydrous DMF (50 mL) in a
100 mL round-bottomed flask, which was equipped with a
magnetic stirring bar and capped with a rubber septum. A
solution of N-bromosuccinimide (0.090 g, 0.8 mmol) in DMF
(50 mL) was added dropwise to the reaction flask at room
1
6
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 , 2 7 3 5 – 2 7 4 0
2 7 3 5

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temperature in the dark. The reaction mixture was stirred at
room temperature for 24 h, the resulting solution treated with
HCl (0.1 M, 10 mL), neutralized with NaHCO , and then
extracted with chloroform several times. After filtration and
solvent evaporation, the residual solid was recrystallized from
1
hexane, to give a white crystalline solid (yield 56%). H NMR
Characterization methods
1
The H NMR spectra were recorded on a AVANCE 500
spectrometer at 298 K with deuterated chloroform or dimethyl
sulfoxide (DMSO) as the solvent and tetramethylsilane (TMS)
as the standard. The compounds were characterized on a Flash
EA 1112 CHNS–O elemental analysis instrument. IR spectra
3
(
1
500 MHz, CDCl
H, ArH), 7.650–7.634 (d, 1H, ArH), 7.546–7.531 (d, 1H,
ArH), 7.510–7.491 (d, 2H, ArH), 7.398–7.368 (t, 1H, ArH),
.342–7.310 (t, 1H, ArH), 3.890 (s, 2H, ArCH Ar). Anal.: calcd
for C13 Br: C, 63.70; H, 3.70; found: C, 63.59; H, 3.76.
3
): d ~ 7.766–7.751 (d, 1H, ArH), 7.682 (s,
were recorded on a Perkin–Elmer spectrophotometer in the
4
21
00–4000 cm region using a powdered sample on a KBr
plate. The LC–MS and MS–TOF mass spectra were recorded
using a Applied Biosystems QSTAR instrument. UV–Vis
absorption spectra were recorded on a UV-3100 spectro-
photometer. Fluorescence measurements were carried out with
a RF-5301PC instrument. Differential scanning calorimetry
7
2
H
9
Synthesis of 2-bromo-9,9’-dihexylfluorene (3)
Compound 2 (1 g, 4.08 mmol), 1-bromohexane (1.7 mL,
2 mmol), tert-butylammonium bromide (0.129 g, 0.4 mmol)
and NaOH (0.48 mL, 50% w/w) were mixed in a 100 mL round-
bottomed flask under a nitrogen atmosphere. The mixture was
heated at 90 uC for 24 h. After the mixture was cooled to room
temperature, water was added to the resulting solution and it
was extracted with chloroform several times. The organic phase
was dried over anhydrous magnesium sulfate. After filtration
and solvent evaporation, the yellow liquid was purified by
(
DSC) analysis was carried out using a NETZSCH (DSC-204)
2
1
instrument at 10 uC min
whilst flushing with nitrogen.
Electrochemical measurements were performed with a Bioa-
nalytical Systems BAS 100 W instrument, using a glassy carbon
working electrode, a platinum wire auxiliary electrode with a
porous ceramic wick, and a Ag/Ag reference electrode,
standardized for the redox couple ferricinium/ferrocene. Cyclic
voltammetric studies of TFB were carried out in MeCN
containing 0.1 M NBu PF (tetra-n-butylammonium hexa-
4 6
fluorophosphate) as the supporting electrolyte. The electro-
chemical investigations on the oligomer were carried out on a
1
1
column chromatography (silica gel, cyclohexane) to afford a
1
colorless oil (yield 41%). H NMR (500 MHz, CDCl ): d ~
thin layer of material deposited on glassy carbon from a CHCl
solution.
3
3
7
1
1
6
.670–7.651 (m, 1H, ArH), 7.560–7.542 (d, 1H, ArH), 7.469 (s,
H, ArH), 7.336–7.275 (m, 4H, ArH), 1.95–1.92 (m, 4H, CH2),
.12–1.09 (m, 4H, CH ), 1.04–1.03 (m, 8H, CH ), 0.78–0.75 (m,
Devices fabricated by spin-coating
2
2
H, CH
3 2
), 0.59–0.57 (m, 4H, CH ) ppm.
The typical device configuration fabricated by spin-coating was
ITO/PEDOT/TFB/Ba/Al. Indium–tin oxide (ITO)-coated glass
2
1
with a sheet resistance of v50 V % was used as the substrate.
Pre-treatment of ITO included a routine chemical cleaning
using detergent and alcohol in sequence, followed by oxygen
plasma cleaning. The PEDOT (3,4-polyethylenedioxythiophene–
polystyrenesulfonate) layer was deposited onto the ITO-coated
substrate as the buffer layer. The TFB film was formed on the
PEDOT layer by spin-coating a toluene solution with a
Synthesis of 9,9’-dihexylfluorene-2-carbaldehyde (4)
Compound 3 (0.413 g, 1 mmol) was dissolved in THF (15 mL)
and the flask purged with nitrogen. A 15% solution of n-BuLi
in hexane (0.7 mL, 1.4 mmol) was added dropwise at
278 uC. After 3 h at this temperature, dry degassed DMF
(
0.4 mL) was added, and the reaction was allowed to continue
2
1
concentration of 5 mg mL to give a film thickness of 50–
0 nm. The Ba cathode was deposited by thermo-evaporation
and was followed by a thick Al capping layer.
for an additional 12 h at 0 uC. The resulting solution was
evaporated under reduced pressure, and solid residue was
7
2
1
stirred with 10 mL CH
temperature for 20 min. The organic layer was separated,
washed with saturated NaHCO , dried with Na SO , and the
2 2
Cl and 10 mL HCl (3 mol L ) at room
Devices fabricated by vacuum evaporation
3
2
4
solvent removed under reduced pressure. The crude product
was purified by column chromatography using dichloro-
The typical device configuration fabricated by vacuum
evaporation was ITO/NPB/TFB/BCP/LiF/Al. The organic
1
2
6
methane as solvent to give a colorless oil (yield 66%).
H
layer was deposited by high vacuum (10
Torr) thermal
NMR (500 MHz, DMSO): d ~ 10.05 (s, 1H, CHO), 8.05–8.04
d, 1H, ArH), 7.97 (s, 1H, ArH), 7.96–7.94 (d, 1H, ArH), 7.93–
.91 (d, 4H, ArH), ), 7.52–7.51 (d, 1H, ArH), 7.44–7.38 (m, 2H,
evaporation onto a cleaned ITO glass substrate. The layer
thickness of the deposited material was monitored in situ using
an oscillating quartz thickness monitor. Finally a LiF buffer
layer and Al cathode were vapor-deposited onto the organic
films at a background pressure of 10 Torr. The EL spectra
´
and Commission Internationale de l’Eclairage (CIE) coordi-
(
7
ArH), 2.07–2.02 (m, 4H, CH ), 1.03–0.95 (m, 12H, CH ), 0.72–
2
2
2
6
0
.69 (m, 6H, CH ), 0.47–0.44 (m, 4H, CH ) ppm.
3
2
nates of these devices were measured by a PR650 Spectroscan
spectrometer. The luminance–current density–voltage charac-
teristics were recorded simultaneously with the EL spectra by
combining the spectrometer with a Keithley model 2400
programmable voltage–current source. All measurements
were carried out at room temperature under ambient
conditions.
Synthesis of 2,5,2’,5’-tetra(9,9’-dihexylfluorenyl)biphenyl (TFB)
Compound 4 (0.217 g, 0.6 mmol) and tetra(phosphonate) 5
(
0.158 g, 0.1 mmol) were dissolved in a mixture of anhydrous
ethanol (10 mL) and THF (10 mL) in a 100 mL oven-dried
round-bottomed flask, equipped with a magnetic stirring bar
and capped with a rubber septum. A freshly prepared 5%
solution of sodium ethoxide in ethanol (0.090 g, 0.8 mmol) was
added dropwise to the reaction flask via a syringe at room
temperature. The reaction mixture was stirred at room
temperature under an argon atmosphere. The resulting
solution was twice precipitated from methanol to give a light
Results and discussion
Synthesis and characterization of TFB
As shown in Scheme 1, the oligo(phenylenevinylene) dimer
TFB containing a biphenyl linkage center and fluorene end
segments was prepared by a Wittig reaction. The structure of
1
yellow solid (yield 60%). H NMR (500 MHz, DMSO): d ~
7
.78–7.28 (m, 42H), 2.09–1.93 (m, 16H, CH
8H, CH ), 0.71–0.62 (m, 24H, CH ), 0.60–0.49 (m, 16H, CH )
2
), 0.98–0.85 (m,
1
4
ppm.
2
3
2
TFB was identified by FT-IR, H NMR and HPLC mass
spectrometry. The FT-IR spectrum of TFB is shown in Fig. 1.
2
7 3 6
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 2 7 3 5 – 2 7 4 0

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Scheme 1 Synthesis of TFB.
1
Fig. 2 The H NMR spectrum of TFB.
getting a high emission efficiency. Fig. 3 shows the mass
spectrum corresponding to the ‘‘molecular ion’’ of the TFB
molecule (C120H146). We observed the characteristic peak
1
[M 1 H] ) at 1588.19. The experimentally measured isotopic
(
distribution showed excellent agreement with that calculated
Fig. 1 The FT-IR spectrum of TFB.
(
1588.14).
As depicted in Fig. 4, the wide-angle X-ray diffraction
In particular, TFB showed characteristic absorption bands at
3
010, 2927, 2848 (nC–H); 1466, 1452, 1376 (dCH2, dCH3); 962
trans CH~CH, cC–H); and 825, 739 (nArC–H). From Fig. 1, we
could easily find the characteristic absorption bands of the
pattern for powdered specimens of TFB showed a broad band
in the region around 20u, which revealed its entirely amorphous
character. Because of this amorphous nature, molecules of TFB
might have a weak interaction in the solid state and therefore
have promise as blue-emitting materials with a weak (or no)
concentration-quenching effect.
The DSC thermograms of TFB are shown in Fig. 5. A
sample of TFB was heated to 150 uC with subsequent cooling to
2
heating scan at a rate of 10 uC min
100 uC. The glass phase transition Tg w2a2s found at 43 uC;
similar to other fluorene based oligomers.
We also investigated the electrochemical behavior of TFB
by cyclic voltammetry (CV) to obtain information about the
(
2
1
trans double bonds at 962 cm . This suggested that the
vinylene group formed in this reaction was in the trans
1
9
configuration.
1
The H NMR spectrum of TFB in DMSO-d
Fig. 2) showed that the ratio of aliphatic/aromatic protons
6
solution
(
2
1
observed for TFB is 2.53, which is in good agreement with the
calculated value (2.48). In addition, there weren’t any peaks
characteristic of a cis-double bond around 6.5 ppm in the
20 uC at a rate of 10 uC min before gathering the second
21
from 220 uC to
1
H NMR spectrum of TFB, which also confirmed that the
2
0,21
vinylene group was in trans configuration. This trans
configuration of the vinylene group in TFB was propitious to
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 2 7 3 5 – 2 7 4 0
2 7 3 7

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Fig. 3 The mass spectrum corresponding to the ‘‘molecular ion’’ of
TFB (C120
H
146).
Fig. 6 UV–Vis and PL spectra of TFB in THF solution (y2.0 6
2
4
1
3
0
75 nm.
M) and as a film at room temperature; the samples were excited at
band gap. The result showed a reversible redox peak in the
reductive potential range with the reductive onset at 22.20 V
and an obvious single oxidation peak in the oxidative potential
range with the oxidative onset at 0.86 V. The LUMO level of
TFB was estimated to be 22.48 eV and the HOMO level was
about 25.54 eV. Therefore, the band gap of TFB is 3.06 eV,
which is close to the value obtained from the absorption spectra
(
2.82 eV).
Optical properties of TFB
The absorption and emission spectra of the oligomer both in
dilute THF solution and as a solid film are shown in Fig. 6;
detailed data are tabulated in Table 1. The optical absorption
spectrum of TFB in THF solution showed lmax at 374 nm,
which is almost the same as that of its analog 2,5,2’,5’-tetra(4’-
1
6
biphenylenevinyl)biphenyl (TBVB), indicating that the bulky
and non-coplanar aliphatic side groups have little effect on the
conjugation degree of the oligomer. The PL spectra of TFB in
THF exhibited a maximum (lem) around 465 nm (the blue light
region). The PL efficiency of TFB solution was about 50%,
which was close to that for the homogeneous PPV oligomers
Fig. 4 Wide-angle X-ray diffraction pattern of TFB recorded at room
temperature.
1
6
that we had reported previously,
such as 2,5,2’,5’-
tetra(styryl)biphenyl (TSB) and TBVB. As shown in Fig. 6,
there are only slight differences in the absorption spectra
between the solution and the film of TFB. The PL spectrum of
the solid film of TFB displayed a maximum of about 474 nm
and a shoulder at about 503 nm, which was only a y10 nm red
shift compared to that of the solution, which could be the result
of self-absorption. Both in solution and in the film state, TFB
showed a very bright blue emission. This was different from
9
TPV, which emitted very weakly in the solid state. Conse-
quently, with its central phenylene–phenylene bond and the
bulky side chain, TFB can efficiently avoid intermolecular
aggregation.
Electroluminescence properties of TFB
TFB-based LEDs can be fabricated both by vacuum evapora-
tion and the spin-coating technique, and two types of devices
show high performance.
Fig. 5 DSC thermograms of TFB with sequential heating and cooling.
Table 1 Optical properties of TFB; the samples were excited at 375 nm
2
1
21
a
Oligomer
la, max/nm (THF solution)
e / mol L cm
lf, max/nm (THF solution)
lf, max/nm (film)
QPL (solution)
3
TFB
a
374
54.47 6 10
465
474
0.50
In dilute THF, using quinine sulfate in 0.1 M sulfuric acid as standard.
2
7 3 8
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 2 7 3 5 – 2 7 4 0

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Characteristics of LEDs fabricated by vacuum evaporation
Characteristics of LEDs fabricated by spin-coating
The multi-layer organic light-emitting devices (OLEDs)
with the structure ITO/NPB (30 nm)/TFB (30–40 nm)/BCP
The single-layer organic light-emitting devices (OLEDs) with
the configuration of ITO/PEDOT/TFB/Ba/Al were fabricated
to investigate the electroluminescent properties of the spin-
coating oligomer devices. As shown in Fig. 9, the EL spectra of
TFB (insert) showed a peak wavelength around 468 nm with
CIE coordinates of (0.23, 0.29), which was similar to both the
PL spectra and that of devices fabricated by vacuum
evaporation. The spectra showed a small change in difference
voltage, probably produced by Joule heat when the devices
operated. The device had a turn-on voltage of 5 V with a
(
30 nm)/LiF/Al were fabricated by thermal deposition in a
vacuum chamber, in which NPB (N,N’-di-1-naphthyl-N,N’-
diphenylbenzidine) acted as the hole transporting layer and
BCP (bathocuprine) as the hole and exciton block layer. Fig. 7
shows the electroluminescence spectra of TFB. Fig. 8 shows the
current density (I)–voltage (V)–luminance (L) characteristics;
the insert shows the LE–V (luminance efficiency vs. voltage)
characteristics of the device. As for other homogeneous PPV
oligomers, TFB exhibits blue light emission with CIE
coordinates of (0.26, 0.27). The EL spectrum of TFB devices
with a peak at 472 nm was the same as its PL spectrum. The
devices based on TFB exhibited a turn-on voltage of 5 V with a
2
2
maximum luminance of 314 cd m , but the luminance
efficiency of the single-layer organic light-emitting devices is
2
1
only about 0.023 cd A . In TFB LEDs, since the p-type
conjugated molecules are much better hole-transporters than
electron-transporters, the resulting charge imbalance may limit
EL efficiency. An improvement in device efficiency could be
obtained by inserting the electron injecting layer between the
spin-coated TFB layer and the electrode metals. Fig. 10 shows
the power efficiency (PE)–voltage (V)–luminance efficiency
(LE) characteristics of the multi-layered ITO/PEDOT/TFB/
2
1
21
)
maximum luminance efficiency of 0.66 cd A (0.21 lm W
2
2
and a luminance of 954 cd m . Although the devices
fabricated by vacuum evaporation showed acceptable char-
acter, some key operations should be noticed in the process; in
particular, the evaporative temperature must be kept under
3
50 uC during device fabrication, because TFB might begin to
decompose if the operating temperature is too high. In
addition, TFB evaporated very slowly, which made the
whole process too long. Therefore, we investigated devices
formed by spin-coating in our subsequent studies.
PBD/Alq
coating, PBD (2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazole)
acted as the hole and exciton block layer, and Alq
(aluminium(III tris(8-hydroxyquinolate)) as the electron
3
/LiF/Al devices, where TFB was fabricated by spin-
3
)
Fig. 9 The current density (I)–voltage (V)–luminance (L) character-
istics of the single-layer device ITO/PEDOT/TFB/Ba/Al fabricated by
spin-coating; the insert shows its electroluminescence spectra.
Fig. 7 Electroluminescence spectrum of the ITO/NPB/TFB/BCP/LiF/
Al device.
Fig. 8 The current density (I)–voltage (V)–luminance (L) character-
istics of the ITO/NPB/TFB/BCP/LiF/Al device; the insert shows its LE–
V (luminance efficiency vs. voltage) characteristics.
Fig. 10 The power efficiency (P)–voltage (V)–luminance efficiency
(LE) characteristics of the multi-layer device ITO/PEDOT/TFB/PBD/
Alq /LiF/Al fabricated by spin-coating.
3
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 2 7 3 5 – 2 7 4 0
2 7 3 9

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transporting layer. The luminance efficiency of the devices was
enhanced greatly compared with that of the single-layer
devices, which had a maximum luminance efficiency of
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stabilization of the amorphous state of the oligomers is
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4
5
6
7
8
9
g
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Acknowledgements
We are grateful for financial support from the National Science
Foundation of China (grant numbers 20125421, 90101026,
2
1
600777014, 50303007), and by the Ministry of Science and
Technology of China (grant number 2002CB6134003).
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2
7 4 0
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 2 7 3 5 – 2 7 4 0