White electroluminescence generated by a blend of bis(di-p-tolylamino) dihydroindenoindene and DCJTB

Article abstract of DOI:10.1007/s11164-014-1597-6

Structural analysis and photophysical study of bis(di-p-tolylamino) dihydroindenoindene were conducted. On the basis of its bi-functional property, a simple WOLED was fabricated in the host-guest configuration. White light fluorescence obtained from a single device was generated by two components emitting simultaneously, via an energy-transfer process.

Full text of DOI:10.1007/s11164-014-1597-6

Res Chem Intermed (2014) 40:2199–2205
DOI 10.1007/s11164-014-1597-6
White electroluminescence generated by a blend
of bis(di-p-tolylamino)dihydroindenoindene
and DCJTB
•
P.-Y. Shen S.-H. Wu Y.-T. Huang Y. Wei
•
•
Received: 25 December 2013 / Accepted: 26 January 2014 / Published online: 1 April 2014
Ó Springer Science+Business Media Dordrecht 2014
Abstract Structural analysis and photophysical study of bis(di-p-tolylamino)di-
hydroindenoindene were conducted. On the basis of its bi-functional property, a
simple WOLED was fabricated in the host–guest configuration. White light fluo-
rescence obtained from a single device was generated by two components emitting
simultaneously, via an energy-transfer process.
Keywords OLED ꢀ WOLED ꢀ White OLED ꢀ Electroluminescence ꢀ
White light fluorescence
Introduction
Since pioneering work by Pope et al. [1], methods for generating electrolumines-
cence (EL) from organic compounds have been sought. In 1987, Tang and
VanSlyke [2] reported the first promising multi-layer, high-efficiency, organic light-
emitting diodes (OLED); soon after, Burroughes et al. [3] successfully developed
polymer organic light-emitting diodes (PLED). These have since been used in flat-
panel displays [4–7] and for lighting [8–10]. This innovative technology has a
variety of merits, including rapid response, high contrast, high resolution, low
operating voltage, high efficiency, color tunability, and stability. Recently, the
potential of white OLEDs as substitutes for candlelight and daylight lamps in our
everyday lives has been demonstrated. Methods of producing white emission can be
classified into two major groups: wavelength conversion and color mixing. There
P.-Y. Shen and S.-H. Wu contributed equally to this work.
P.-Y. Shen ꢀ S.-H. Wu ꢀ Y.-T. Huang ꢀ Y. Wei (&)
Department of Chemistry, Tamkang University, No.151, Yingzhuan Rd., Tamsui Dist., New Taipei
City 25137, Taiwan, ROC
e-mail: ywei@mail.tku.edu.tw
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are several color mixing techniques; the most general integrates multi-color emitters
in a single device, for example, blue and yellow and/or orange; green, red, and blue;
and blue and red and/or orange emitting materials. To enhance EL efficiency,
however, the corresponding device architecture is complicated [11–13]. We
therefore planned to resolve this problem by using multi-functional materials for
a single-component white OLED.
Bis(diarylamino)dihydroindenoindene derivatives have been proved to be good
hole-transporting and emitting materials in OLEDs. They emit sky-blue to greenish
blue fluorescence with high efficiency and maximum emission peaks (Em. kmax) in
the range 432–492 nm [14]. Because of their bi-functional property they have
potential for use in white OLEDs. Among all candidates, 2,7-bis(N,N,-di-(p-
tolyl)amino)-5,5,10,10-tetraphenyl-5,10-dihydroindeno[2,1-a]indene (3) has an
appropriate oxidation potential for hole injection. Its synthesis is illustrated in
Scheme 1.
Experimental
Materials
Compounds 1–3 were synthesized in accordance with literature methods [14]. All
reagents were purchased from Acros Organics, Sigma-Aldrich, TCI, or Alfa-Aesar,
and used directly without further purification. Solvents used for experiments were
reagent grade unless stated otherwise. Diethyl ether and toluene were dried over Na
with benzophenone-ketyl intermediate as indicator.
5,5,10,10-Tetraphenyl-5,10-dihydroindeno[2,1-a]indene (1)
A solution of (Z)-2,20-dibromostilbene (6,761 mg, 20 mmol) in diethyl ether
(100 mL) was placed in a 250-mL, three-necked, round-bottomed flask. The flask
was cooled to 0 °C, and n-BuLi (1.6 M in hexanes, 27.5 mL, 44 mmol) was added
dropwise. The reaction mixture was stirred at 0 °C for 30 min, then a solution of
benzophenone (8,018 mg, 44 mmol) in diethyl ether (20 mL) was added. The
resulting mixture was gradually warmed to ambient temperature and then quenched
with saturated NaHCO3(aq) (50 mL). After filtration, the precipitate was washed
with distilled water (100 mL) and hexanes (100 mL) then dried under reduce
pressure. The crude residue dissolved in acetic acid (50 mL), and placed in a
100-mL, two-necked, round-bottomed flask. HCl (12 M, 36 lL, 1 mmol) was then
added and the reaction mixture was heated under reflux for 2 h. After cooling to
ambient temperature, saturated NaHCO3(aq) (50 mL) was added again and the
mixture was stirred vigorously for 20 min. The precipitate was isolated by filtration
and the yellow solid was washed with distilled water (100 mL), methanol (100 mL),
and hexanes (100 mL) in turn, then dried under reduce pressure to give 1, which
was re-crystallized from CH2Cl2 to afford 2,441 mg pure 1 (24 %): Tm 376 °C
1
(DSC); M.W.: 508.65; H NMR (600 MHz, CDCl3) d 7.42 (dd, J = 6.8, 1.8, 2H),
7.30–7.23 (m, 20H), 7.18–7.11 (m, 6H); 13C NMR (150 MHz, CD2Cl2) d 157.7,
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White light electroluminescence generation
2201
Br
Br
OH
HO
(a)
(b)
1
(d)
(c)
N
Br
N
Br
3
2
Scheme 1 Synthetic strategy for preparation of 1–3. Reagents and conditions: a n-BuLi and
benzophenone, ether, 0 °C, 2 h. b cat. HCl, acetic acid, reflux, 4 h. c CuBr2–Al2O3, CCl4, reflux, 18 h.
d Pd2(dba)3, P(t-Bu)3, NaO(t-Bu), and di-p-tolylamine, toluene, reflux, 4 h
155.7, 143.0, 138.6, 128.7, 128.6, 127.4, 127.3, 126.2, 125.3, 121.1, 63.4; EIMS
(negative) calcd for C40H28 [M-]: 508.2, found: 508.3; TLC Rf 0.75 (CH2Cl2–
hexanes, 1:3).
2,7-Dibromo-5,5,10,10-tetraphenyl-5,10-dihydroindeno[2,1-a]indene (2)
A suspension of 1 (1,017 mg, 2 mmol), CuBr2 (3,350 mg, 15 mmol), and Al2O3
(10,050 mg) in CCl4 (20 mL) was placed in a 250-mL, three-necked, round-
bottomed flask. The reaction mixture was heated to reflux for 16 h. After cooling to
ambient temperature, excess CuBr2 and Al2O3 were removed by filtration through
Celite. The filtrate was evaporated under reduce pressure and the residue was re-
crystallized from toluene to afford 1,093 mg pure 2 (82 %): Tm 393 °C (DSC);
M.W.: 666.44; 1H NMR (600 MHz, CD2Cl2) d 7.54 (d, J = 1.7, 2H), 7.31–7.24 (m,
22H), 7.03 (d, J = 8.2, 2H); 13C NMR (150 MHz, CD2Cl2) d 159.6, 155.2, 141.8,
137.3, 130.7, 129.0, 128.7, 128.5, 127.7, 122.3, 120.4, 63.6; TLC Rf 0.70 (CH2Cl2–
hexanes, 1:3).
2,7-Bis(N,N,-di-(p-tolyl)-amino)-5,5,10,10-octaphenyl-5,10-dihydroindeno[2,1-
a]indene (3)
A solution, in toluene (20 mL), of 2 (666 mg, 1 mmol), Pd2(dba)3 (18 mg,
0.02 mmol), NaO(t-Bu) (288 mg, 3 mmol), P(t-Bu)3 (0.03 M in toluene, 2 mL,
0.06 mmol), and di-p-tolylamine (434 mg, 2.2 mmol) was placed in a 25-mL, two-
necked, round-bottomed flask. The reaction mixture was heated under reflux for 4 h
then cooled to ambient temperature, filtered, and the precipitate was washed with
toluene (100 mL). The crude residue was sublimed at 393 °C under 5 9 10-6 Torr
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P.-Y. Shen et al.
to afford 557 mg pure 3 (62 %): Tm 433 °C (DSC); M.W.: 899.17; 1H NMR
(600 MHz, CD2Cl2) d 7.23 (s, 20H), 7.15 (d, J = 2.0, 2H), 6.98 (d, J = 8.3, 8H),
6.91 (d, J = 8.3, 2H), 6.86 (d, J = 8.3, 8H), 6.65 (dd, J = 8.3, 2.0, 2H), 2.25 (s,
12H); 13C NMR (150 MHz, CD2Cl2) d 158.6, 153.8, 146.5, 145.6, 143.6, 132.9,
130.0, 128.8, 128.6, 127.1, 124.7, 121.4, 120.7, 120.4, 63.3, 20.8; TLC Rf 0.75
(CH2Cl2–hexanes, 1:3); ESI–MS (positive) calcd for C68H54N2 [M?]: 898.4, found:
898.4.
Steady-state photophysical study
Absorption spectra were measured on a Thermo Scientific Evolution 60S
spectrophotometer; spectrophotometric CH2Cl2 and toluene were used as solvents.
On excitation, emission spectra were measured on a Jasco FP-6500 fluorescence
spectrometer under the same solutions.
Device fabrication and measurements
Before use, all materials were subjected to gradient sublimation twice. The cathode
was indium tin oxide (ITO)-coated glass with a shear resistance of *30 X/sq. Pre-
patterned ITO substrate was cleaned sequentially by sonication in detergent
solution, doubly distilled water, and EtOH for 5 min, in turn, before blow-drying
with a stream of nitrogen. Advanced surface cleaning was accomplished by oxygen
plasma treatment for 3 min. PEDOT:PSS and the emitting layer [DCJTB (4-
(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran;
2 wt%):2] were then spin-coated at 3,000 rpm on to the substrate before loading
into a vacuum chamber. Layer by layer deposition was conducted thermally under a
pressure of 5 9 10-6 Torr. EL efficiency was determined and spectra were recorded
by use of a Keithley 2400 source meter and a Photo Research PR-655 SpectraScan
spectroradiometer, respectively. All the measurements were made under atmo-
spheric conditions. Device configuration is described as follows: ITO/PEDOT:PSS
(30 nm)/emitting layer (40 nm)/LiF (1 nm)/Al (150 nm).
Results and discussion
The structural features of 1 were determined by X-ray crystallographic analysis. As
depicted in Fig. 1, the central aromatic dihydro-indeno-indene framework is planar.
Therefore, the charges generated in individual di-p-tolylamine segments of 3 may
communicate more easily. Moreover, the pendent phenyl groups are tilted at
73.7 8.5°, resulting in a rigid structure that facilitates thermal stability and
prevents unexpected intermolecular p–p stacking.
It has been reported that compound 3 emits sky-blue fluorescence with Em. kmax
at 459 nm and full-width at half-maximum (fwhm) of 47 nm [14]. Stacking this
emission profile with the absorption bands of DCJTB [15] suggests the energy can
be successfully transferred (Fig. 2). Accordingly, the emission behavior of mixtures
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White light electroluminescence generation
2203
Fig. 1 ORTEP diagram for the X-ray crystal structure of 3 (ellipsoids are shown at 30 % probability
level)
1.0x105
8.0x104
6.0x104
4.0x104
2.0x104
1.0
3
DCJTB
UV PL
0.8
0.6
0.4
0.2
0.0
250 300 350 400 450 500 550 600 650 700 750
0.0
Wavelength (nm)
Fig. 2 Stacked plots of the absorption and emission spectra of 3 and DCJTB
of 3 and DCJTB in toluene solutions were examined (Fig. 3). On UV excitation at
410 nm, energy transfer does not occur until the molar ratio of DCJTB is [0.7;
broad emission with Em. kmax at 459 and 565 nm can then be observed (Fig. 3
inset). It is composed of dual emission from both molecules, which supports our
concept.
On the basis of this preliminary study, we further fabricated a trilayer device
to gain more insight into the EL for blending 3 and DCJTB. PEDOT:PSS [16]
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P.-Y. Shen et al.
3:DCJTB=9:1
3:DCJTB=8:2
3:DCJTB=7:3
3:DCJTB=6:4
3:DCJTB=5:5
3:DCJTB=4:6
3:DCJTB=3:7
3:DCJTB=2:8
3:DCJTB=1:9
450
500
550
600
650
700
450
500
550
600
650
700
Wavelength (nm)
Fig. 3 Stacked plots of the emission spectra of mixtures of 3 and DCJTB
1.0
0.8
0.6
0.4
0.2
0.0
450
500
550
600
650
700
Wavelength (nm)
Fig. 4 The EL spectrum of 3
and TPBI [17] were used as hole-injection and electron-transporting materials,
respectively, along with electrodes.1 As shown in Fig. 4, while light emission
with CIEx,y at (0.33, 0.40) was obtained, as expected. This phenomenon can be
explained by the energy-level diagram (Fig. 5). Because the HOMO and LUMO
energies of 3 are higher than those of DCJTB by 0.7 0.29 eV, the excitons
generated in the host cannot completely transfer their energies to the guest.
Therefore, a broad fluorescence spectrum is observed when 3 and DCJTB emit
simultaneously.
1
Details of the device are given in the ‘‘Experimental’’ section.
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White light electroluminescence generation
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0
1
2
3
4
3
LUMO
HOMO
2.09
DCJTB
3.10
TPBI
2.70
LiF/Al
3.70
PEDOT:PSS
ITO
4.7
5
6
7
4.79
5.20
5.00
6.20
Fig. 5 Illustration of relative energy levels
Conclusions
In summary, we have fabricated a single-component white OLED on the basis of the
bi-functional property of compound 3. When DCJTB is used as guest material,
white EL is generated, because of incomplete energy transfer. The configuration of
the device will be optimized in due course.
Acknowledgments This research was sponsored by the Ministry of Science and Technology of Taiwan
(NSC: 101-2113-M-032-008-MY2) and Tamkang University. We thank Professor Chien-Tien Chen for
generous support in enabling use of laboratory equipment.
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