New blue and bluish green electroluminescent properties of fully substituted ethylene moieties

Article abstract of DOI:10.1080/15421400600932371

We synthesized 1,1,2,2-tetrakis(4'-tert-butylbiphenyl)ethene[TBBPE] and 1,2-di(4'-tert-butyl-phenyl)-1,2-bis(4'-tert-butyl-biphenyl) ethene [BPBBPE]. TBBPE and BPBBPE film showed PL maximum value of 489nm and 511nm due to longer conjugation length of TBBP

Full text of DOI:10.1080/15421400600932371

Mol. Cryst. Liq. Cryst., Vol. 458, pp. 209–216, 2006
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400600932371
Synthesis and Electroluminescent Properties of Fully
Substituted Ethylene Moieties
Soo Kang Kim
Young Il Park
Jong Wook Park
Department of Chemistry=Display Research Center, The Catholic
University of Korea, Pucheon, Korea
We synthesized a new bluish-green emitting material using fully substituted ethyl-
ene moieties, 1,1,2,2-tetrakis(40-tert-butyl-phenyl)ethene [TBPE] and 1,1,2,2-tetra-
kis(40-tert-butyl-biphenyl)ethene [TBBPE] by McMurry reaction. These products
were identified by NMR, IR, Fab-Mass analysis. The maximum absorbance of
TBPE and TBBPE appeared at around 318 nm and 358 nm and two compounds
also showed different PL maximum value as 431 nm (TBPE) and 511 nm
(TBBPE). ITO=2-TNATA=TBBPE=Alq3=LiF=Al device showed bluish-green EL
color and 4cd=A efficiency.
Keywords: blue organic light-emitting diode; electroluminescent; McMurry reaction;
tetraphenylethylene
INTRODUCTION
Organic light-emitting diodes (OLEDs) based on organic molecules are
currently the subject of an intense research effort due to their promise
as devices for full-color large display applications [1–3], therefore,
numerous conjugated organic molecules have been synthesized and
reported to exhibit electroluminescence (EL), from red, to green, to
blue [4–8]. Although considerable research efforts have been carried
out to enhance the performance suitable for practical use, however,
a lot of problems to solve still remain, for example the blue emitting
material, the device lifetime and the luminous efficiency, etc.
This work was supported by the Catholic University of Korea, Research Fund, 2006.
Address correspondence to Jong Wook Park, Department of Chemistry=Display
Research Center, The Catholic University of Korea, Pucheon 420-743, Korea. E-mail:
hahapark@catholic.ac.kr
209

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S. K. Kim et al.
In this work we synthesized a new bluish-green emitting material
using fully substituted ethylene moieties. Double-layered and
triple-layered EL devices were fabricated with synthesized
material and evaluated in terms of emission color and luminescence
efficiency.
EXPERIMENTAL
Synthesis
Synthesis of 1,1,2,2-tetrakis(40-tert-butyl-phenyl)ethene [TBPE],
1,1,2,2-tetrakis(40-tert-butyl-biphenyl)ethene [TBBPE]. TBPE and
TBBPE in Schemes 1, 2 were obtained by McMurry reaction and the
reactants containing ketone moiety were synthesized by Friedel Crafts
acylation and Suzuki coupling reaction. Bis(4-tert-butylphenyl)metha-
none (2 g, 6.8 mmol), Zn-powder (1.06 g, 17 mmol) were added to dried
THF solvent (150 ml), then TiCl4 (1.5 ml, 13.6 mmol) were added into
the reaction mixture. The mixture was heated to 60ꢀC for 1 hr under
nitrogen. Reactant mixture was cooling down after 1 hr and then the
product was extracted by diethyl-ether. The organic layer was dried
by anhydrous MgSO4 and filtered. The solution was evaporated. The
residue was dissolved in CHCl3 and added to methanol. The precipi-
tate was filtered and washed with methanol.
Yellowish powder was purified by column chromatography with
CHCl3: n-hexane (1:1) eluent to afford white solid (TBPE) (1.5 g, 79%).
1H NMR (d, ppm): 6.93(d, 8H), 7.06(d, 8H), 1.25 (s, 36H).
SCHEME 1 Synthetic route of TBPE.

Synthesis and Electroluminescent Properties
211
SCHEME 2 Synthetic route of TBBPE.
TBBPE was obtained by using bis(40-tert-butyl-biphenyl)methanone-
(2g, 4.5 mmol) for the McMurry reaction (1.2 g, 63%).
1H NMR (d, ppm): 7.16(d, 8H), 7.37(d, 8H), 7.42(d, 8H), 7.51(d, 8H),
1.34 (s, 36H).
Fabrication of OLED
For EL device, TBPE and TBBPE were vacuum-ꢁd6eposited on top of
˚
˚
2-TNATA (500 A)=ITO (1200 A=30 ohm) under 10 torr, the rate of
2
˚
deposition being 1 A=sec to give an emitting area of 4 mm and other
organic layer and aluminum layer were continuously deposited with
same vacuum condition. Current-voltage (I-V) characteristics of the
fabricated OLEDs were measured using Keithley 2400 electrometer.
Light intensity was obtained by Minolta CS-1000.
RESULTS AND DISCUSSION
These products were finally purified with recrystallization and silica
column method to have highly pure powder and were identified by
NMR, IR, Fab-Mass analysis. As the last step reaction proceeded, car-
bonyl peak at around 1645 cmꢁ1 in IR was completely disappeared in
compounds (Fig. 1).

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FIGURE 1 IR spectrum of bis(4-tert-butylbiphenyl)methanone (a) and
TBBPE (b).
Figure 2 shows representative NMR data of TBBPE. At NMR data,
we identified proton peaks of TBBPE at around 7.0 ppm and 8.0 ppm.
We confirmed synthesis of TBBPE through proton a ꢂ e peaks. Each
proton peak a, b, c, d, e was located at 7.16, 7.37, 7.42, 7.51, 1.34 ppm,
respectively. Proton a and b were more shielded compared to c and d,
because a pair of proton a and b is far from ethylene moiety and close
to tert-butyl group. As a resulting proton (a–b) and (c–d) pairs showed
FIGURE 2 NMR spectrum of TBBPE at 500 MHz.

Synthesis and Electroluminescent Properties
213
FIGURE 3 UV-visible (solid) and PL spectra (open) of TBPE (a, circle) and
TBBPE (b, square) film on glass.
multiplet skewing interacting each other. Fab Mass data of TBBPE was
860. The resulting products are soluble in common organic solvents,
such as chloroform and THF.
Figure 3 shows UV-visble and photoluminescence (PL) spectra of
TBPE and TBBPE film on glass. The maximum absorbance of TBPE
and TBBPE appeared at around 318 nm and 358 nm, which are origi-
nated from the p ! pꢃ transition of conjugated double bond. Two com-
pounds also showed different PL maximum value as 431 nm (TBPE)
and 511 nm (TBBPE) due to longer conjugation length of TBBPE.
The energy band gaps and HOMO, LUMO level of TBPE and
TBBPE were estimated by cyclic voltammograms (CV) and the analy-
sis of absorption edge with a plot of (hv) vs. (ahv)2, where a, h, and v
are absorbance, Planck’s constant, and the frequency of light, respect-
ively. Based on these CV results, electro-stability of compounds was
achieved. The result accounts that we could get HOMO level value.
Electronic levels were summarized in Table 1.
TABLE 1 Electronic Levels of TBPE and TBBPE
HOMO (eV)
LUMO (eV)
Eg (eV)
TBPE
TBBPE
5.25
5.24
1.84
2.24
3.41
3.00

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S. K. Kim et al.
FIGURE 4 EL spectrum of ITO=2-TNATA=TBBPE=Alq3=LiF=Al device at
10 mA=cm2.
EL devices were fabricated in ITO=2-TNATA=TBPE or TBBPE=
Alq3=LiF=Al and ITO=2-TNATA=TBPE or TBBPE=LiF=Al configura-
tions.
Figure 4 showed EL maximum value at 496 nm in ITO=2-TNATA-
(70 nm)=TBBPE (30 nm)=Alq3 (30 nm)=LiF (1 nm)=Al (200 nm) device,
whereas there is no EL light in TBPE device. It corresponds that PL
intensity of TBPE is also about 20 times weaker than TBBPE’s. It
might be explained by large energy barrier between high LUMO level
(1.84 eV) of TBPE and Alq3 LUMO level (3.1 eV). In order to find out
original EL maximum wavelength of TBPE and TBBPE, we made EL
device without using Alq3 layer. TBPE showed no EL light and TBBPE
showed blue EL maximum value of 485 nm. We believe that real EL
TABLE 2 Performance Characteristics of Double Layer Device (ITO=2-TNATA
(70 nm)=TBBPE (50 nm)=LiF (1 nm)=Al (200nm))
Current
density
(mA=cm2)
Current
efficiency
(cd=A)
Power
efficiency
(Im=w)
Voltage
(V)
Brightness
(cd=m2)
C.I.E
TBBPE
10
25
16.6
18.4
126
244
(0.24, 0.38)
(0.24, 0.37)
1.26
0.98
0.24
0.17

Synthesis and Electroluminescent Properties
215
TABLE 3 Performance Characteristics of Triple Layer Device (ITO=2-TNATA
(70 nm)=TBBPE (30 nm)=Alq3 (30 nm)=LiF (1nm)=Al (200nm))
Current
density
(mA=cm2)
Current
efficiency
(cd=A)
Power
efficiency
(Im=w)
Voltage
(V)
Brightness
(cd=m2)
C.I.E
TBBPE
10
25
10.0
11.8
444
852
(0.24, 0.43)
(0.24, 0.43)
4.44
3.41
1.39
0.91
maximum value of TBBPE is 485 nm. We also suppose that recombi-
nation center is near emitter and Alq3 interface in case of the device
containing an Alq3 layer. It is not clear that there is 26 nm difference
between EL and PL maximum values.
We summarized EL efficiency and CIE values in Tables 2 and 3.
Table 2 showed bluish-green CIE values, (0.23, 0.38) and 1.26cd=A
at 10 mA=cm2 in double layer device (ITO=2-TNATA=TBBPE=LiF=Al).
In Table 3, we observed that the triple layer device (ITO=2-TNATA=
TBBPE=Alq3=LiF=Al) shows relatively higher EL efficiency of 4cd=A,
because the carrier charge was normally balanced by Alq3 ETL layer.
Further studies on these kinds of material derivatives and devices
are underway [9].
CONCLUSIONS
We synthesized a new bluish-green emitting material using fully sub-
stituted ethylene moieties, 1,1,2,2-tetrakis(40-tert-butyl-phenyl)ethene
[TBPE] and 1,1,2,2-tetrakis(40-tert-butyl-biphenyl)ethene [TBBPE] by
McMurry reaction. The maximum absorbance of TBPE and TBBPE
appeared at around 318 nm and 358 nm, which are originated from
the p ! pꢃ transition of conjugated double bond. Two compounds also
showed different PL maximum value as 431 nm (TBPE) and 511 nm
(TBBPE) due to longer conjugation length of TBBPE.
EL devices were fabricated in ITO=2-TNATA=TBPE or
TBBPE=Alq3=LiF=Al and ITO=2-TNATA=TBPE or TBBPE=LiF=Al
configurations. TBPE showed no EL light and TBBPE showed blue
EL maximum value of 485 nm. We observed that the triple layer device
(ITO=2-TNATA=TBBPE=Alq3=LiF=Al) shows relatively higher EL
efficiency of 4cd=A.
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