Substituent effect of fluoranthene derivatives in electroluminescence

Article abstract of DOI:10.1080/15421400802615352

7,8,10-triphenylfluoranthene [TPF], 7,10-diphenyl-8-p-tolylfluoranthene [DPTF] and 7,10-diphenyl-8-p-cyanophenlyfluoranthene [DPCF] were synthesized by using the Knoevenagel condensation and Diels-Alder addition. TPF showed sky-blue CIE value of (0.192, 0

Full text of DOI:10.1080/15421400802615352

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Substituent Effect of
Fluoranthene Derivatives in
Electroluminescence
a
b
Soo-Kang Kim , Jae-Yun Jaung & Jong-Wook Park
a
a
Department of Chemistry/Display Research Center,
The Catholic University of Korea , Wonmi, Bucheon,
Korea
b
Department of Fiber and Polymer Engineering,
Hanyang University , Seongdong-gu, Seoul, Korea
Published online: 22 Jul 2010.
To cite this article: Soo-Kang Kim , Jae-Yun Jaung & Jong-Wook Park (2009)
Substituent Effect of Fluoranthene Derivatives in Electroluminescence, Molecular
Crystals and Liquid Crystals, 498:1, 140-150, DOI: 10.1080/15421400802615352
To link to this article: http://dx.doi.org/10.1080/15421400802615352
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Mol. Cryst. Liq. Cryst., Vol. 498, pp. 140–150, 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400802615352
Substituent Effect of Fluoranthene Derivatives
in Electroluminescence
1
2
1
Soo-Kang Kim , Jae-Yun Jaung , and Jong-Wook Park
1
Department of Chemistry=Display Research Center, The Catholic
University of Korea, Wonmi, Bucheon, Korea
2
Department of Fiber and Polymer Engineering, Hanyang University,
Seongdong-gu, Seoul, Korea
7
,8,10-triphenylfluoranthene [TPF], 7,10-diphenyl-8-p-tolylfluoranthene [DPTF]
and 7,10-diphenyl-8-p-cyanophenlyfluoranthene [DPCF] were synthesized by
using the Knoevenagel condensation and Diels–Alder addition. TPF showed sky-
2
blue CIE value of (0.192, 0.269) and 3.27cd=A at 10 mA=cm . DPTF also showed
2
sky-blue CIE value of (0.185, 0.252) and 2.31cd=A at 10 mA=cm .
Keywords: blue emitting material; Diels–Alder addition; electroluminescence;
fluoranthene; substituent effect
INTRODUCTION
Organic light-emitting diodes (OLEDs) based on organic molecules are
currently the intensive subject due to their promise for full-color large
display applications [1–5].
Recent studies developed materials with which both red or green
have high efficiency of 10cd=A and 100,000 hours of lifetime, but blue
emitting materials have not reached these with maximum capacity of
7
cd=A and 12,000 hours [6]. That blue material having relatively low
efficiency and short device life-time is attributed to the fact that more
than 3.0 eV of band-gap is needed to have blue light. Compared to red or
This study was supported by the Research Fund, 2008 The Catholic University of
Korea. This study was supported by a grant from the Fundamental R&D Program for
Core Technology of Materials funded by the Ministry of Commerce, Industry and
Energy, Republic of Korea.
Address correspondence to Prof. Jong Wook Park, Department of Chemistry=Display
Research Center, The Catholic University of Korea 43-1, Yeokgok, Wonmi, Bucheon
4
20–743, Korea (ROK). E-mail: hahapark@catholic.ac.kr
1
40

Effect of Fluoranthene Derivatives in Electroluminescence
141
green emitting materials, band-gap is relatively wide, increasing
energy gap between layers, which causes charge-balance to mismatch
between hole and electron. As a result, low luminance efficiency and
high operating voltage are found, which leads material degradation
by heating and further reduction of lifetime. Of the materials that have
reported as blue materials such as Diarylanthracene [7], di(styryl) ary-
0
0
lene [8] and pyrene [9], 4,4 -bis(2,2-diphenylvinyl)-1,1 -biphenyl
(
DPVBi) [8] is considered most representative. However, DPVBi has
ꢀ
the lowest value of Tg, 64 C [10] and it is well known that low Tg has
a considerable influence on life-time. Therefore, it is imperative to find
new blue emitting materials with long lifetime, and it is necessary to
carry out studies on various types of new blue emitting materials.
In this study, we synthesized 7,8,10-triphenylfluoranthene [TPF]
[
11] and TPF derivatives which are substituted by p-tolyl and
p-cyanobenzene instead of phenyl at 8-position.
The materials 7,8,10-triphenylfluoranthene [TPF], 7,10-diphenyl-
8
-p-tolylfluoranthene [DPTF] and 7,10-diphenyl-8-p-cyanophenly-
fluoranthene [DPCF] were synthesized by using the Knoevenagel
condensation and Diels–Alder addition.
The thermal and electro-optical properties of these materials were
characterized with differential scanning calorimetry (DSC), thermo-
gravimetric analysis (TGA), cyclic voltammetry (CV), and UV-visible
and photoluminescence (PL) spectroscopy. Moreover, multilayered
EL devices were fabricated using these materials as non-doped
emitting layers.
EXPERIMENTAL
Synthesis
7,9-diphenyl-8H-cyclopenta[a]acenaphthylen-8-one(1)
1,3-Diphenyl-2-propanone(10 g, 47 mmol) and acenaphthoquinone(8 g,
4
4
5
4 mmol) were added to Ethanol (300 mL), then 1 M KOH solution(47 mL,
7 mmol) was added into the reaction mixture. The mixture was heated to
ꢀ
0 C for 3 h under nitrogen. The reactant mixture was cooled after 1 h and
then the product was filtered and washed with ethanol (yield¼ 14.5 g,
9
2.5%).
1
H NMR (d, ppm): 8.07(d, 2H), 7.87(d, 2H), 7.83 (d, 4H), 7.59 (t, 2H),
þ
7
.52 (t, 4H), 7.41(t, 2H); Fab -mass: 356.
7
,8,10-triphenylfluoranthene [TPF]
,9-diphenyl-8H-cyclopenta[a]acenaphthylen-8-one (2 g, 5.62 mmol)
and ethynylbenzene (0.7 ml, 6.3 mmol) were added to xylene
7

1
42
S.-K. Kim et al.
ꢀ
200 mL). The mixture was heated to 130 C for 10 h under nitrogen.
(
After the reaction had finished, the reaction mixture was extracted
with diethyl ether and water. The organic layer was dried with anhy-
drous MgSO and filtered. The solution was evaporated. The residue
4
was dissolved in CHCl and added to methanol. The precipitate was
3
filtered and washed with methanol.
The yellow powder was purified by using column chromatography
with CHCl : n-hexane (1:3) eluent to afford a yellowish-beige solid
3
(
TPF) (yield ¼ 2.1 g, 86.9%).
1
H NMR (d, ppm): 7.72(m, 4H), 7.55(t, 2H), 7.52 (t, 1H), 7.36 (m,
6
H), 7.33 (s, 1H), 7.30(t, 2H), 7.23(d, 2H), 7.17(m, 3H), 6.68(d, 1H) ;
þ
Fab -mass: 430.
7
8
,10-diphenyl-8-p-tolylfluoranthene [DPTF] and 7,10-diphenyl-
-p-cyanophenlyfluoranthene [DPCF]
DPTF and DPCF were synthesized as for the above series by using
Diels–Alder addition.
1
[
DPTF] (yield ¼ 87%) H NMR (d, ppm): 7.71(m, 4H), 7.55(t, 2H),
7
6
.51 (t, 1H), 7.39 (m, 6H), 7.31 (s, 1H), 7.28(m, 2H), 7.11(d, 2H),
þ
.99(d, 2H), 6.65(d, 1H), 3.00(s,3H); Fab -mass: 444.
1
[
DPCF] (yield ¼ 73%) H NMR (d, ppm): 7.77(d, 1H), 7.75(d, 1H),
7
7
.69 (d, 2H), 7.55 (m, 3H), 7.46 (d, 2H), 7.40(m, 4H), 7.32(m, 5H),
þ
.29(d, 2H), 6.72(d, 1H), 3.00; Fab -mass: 455.
Characterization
1
H NMR spectra were recorded on Bruker Avance 500 spectrometers.
Fast atom bombardment (FAB) mass spectra were recorded on a
JEOL, JMS-AX505WA, HP5890 series II. The optical absorption
spectra were obtained with a HP 8453 UV-VIS-NIR spectrometer.
The melting temperatures (T ), glass-transition temperatures (T ),
m
g
crystallization temperatures (T ), and degradation temperatures (T )
c
d
of the compounds were measured by carrying out differential scanning
calorimetry (DSC) under a nitrogen atmosphere using a DSC2910 (TA
Instruments) and thermogravimetric analysis (TGA) using
a
SDP-TGA2960 (TA Instruments). A Perkin Elmer luminescence spec-
trometer LS50 (Xenon flash tube) was used for photo- and electro-
luminescence spectroscopy. The redox potentials of the compounds
were determined with cyclic voltammetry (CV) using an EG&G 362
electrochemical workstation with a scanning rate of 50ꢁ200 mV=s.
The synthesized materials were dissolved in N,N-dimethylformamide
(
DMF) with 0.1 M tetrabutylammonium tetrafluoroborate as the elec-
trolyte. We used a platinum working electrode and a saturated

Effect of Fluoranthene Derivatives in Electroluminescence
143
Ag=AgNO reference electrode. Ferrocene was used for potential cali-
3
bration and for reversibility criteria.
Density functional theoretical calculations were performed using
Spartan’04 (for Windows) [12]. The structures were drawn at the
entry-level of input and minimized. Equilibrium geometry was
obtained at the B3LYP level of DFT for each molecule at the ground
ꢂ
state from its initial geometry subject to symmetry with a 6–31G
basis set [13]. Orbitals and energies, atomic charges, vibrational
modes, and thermodynamic properties were chosen as output para-
meters. HOMO and LUMO orbital surfaces were then obtained from
the output.
ꢃ6
2
For the EL devices, all organic layers were deposited under 10
˚
torr, with a rate of deposition of 1 A=s to give an emitting area of 4 mm .
The LiF and aluminum layers were continuously deposited under the
same vacuum conditions.
The current-voltage (I-V) characteristics of the fabricated EL
devices were obtained with a Keithley 2400 electrometer. Light inten-
sity was obtained with a Minolta CS-100A.
RESULTS AND DISCUSSION
Synthetic routes of three fluoranthene derivatives are described in
Scheme 1. All compounds were obtained by using the Knoevenagel
condensation and Diels–Alder addition.
SCHEME 1 Synthetic routes of synthesized materials.

1
44
S.-K. Kim et al.
All steps showed high yield of more than 80%. Especially it was
possible to synthesize the final compound without using metal com-
plex catalyst. This is intended to exclude a possibility that even a very
small amount of metal complex could exist in the final compound. It is
not easy to remove small residue portion of metal complex in synthe-
sized compounds with various purification methods as well as subli-
mation. Therefore, it is possible to expect high purity of the
materials and the improved life-time of device in case of this synthesis
scheme.
In the last step of purification, recrystallization and silica column
were performed for all synthesized materials to increase their
purity, and structures were identified with NMR, FT-IR, and Fab-
Mass analysis.
Synthesized materials are placed on glass to make a film through
vacuum deposition, and UV-Visible and PL spectra are measured
(
Fig. 1). The data are summarized in Table 1.
The maximum absorbance of TPF, DPTF and DPCF appeared at
around 381, 380 and 384 nm, respectively. Compared to TPF or DPTF,
DPCF showed UVonset and UVmax values that are red-shifted about
4
ꢁ5 nm, which is expected to be resulted from the increase of conju-
gation length due to triple bond of –CN.
PLmax values of three compounds showed 458 nm (TPF), 461 nm
(
DPTF) and 453 nm (DPCF) wavelength, which are blue colors. From
PLmax values, it was found that DPTF with electron-donating –CH3
group was red-shifted about 3 nm more than TPF. Also, despite the
increase of conjugation length by –CN group, DPCF with electro-
withdrawing –CN group was blue-shifted about 5 nm. However, UVmax
and PLmax values of the three materials were found to be similarly
close to 381 nm and 458 nm. Based on these results, it is possible to
assume that fluoranthene provides a major optical property that
determines UVmax and PLmax
.
The ground-state structure and electron state of HOMO and LUMO
ꢂ
were calculated using by B3LYP=6-31G method of Spartan’04
program [12,13].
TABLE 1 Optical, Electrical and Thermal Properties of Synthetic Compounds
UVonset
UVmax
(nm)
PLmax
(nm)
HOMO
(eV)
LUMO
(eV)
Eg
(eV)
T
( C)
g
T
( C)
m
T
( C)
d
ꢀ
ꢀ
ꢀ
(nm)
TPF
DPTF
DPCF
410
409
415
381
380
384
458
461
453
5.84
5.85
5.89
2.82
2.82
2.90
3.02
3.03
2.99
–
–
–
199
262
236
323
337
393

Effect of Fluoranthene Derivatives in Electroluminescence
145
FIGURE 1 Normalized PL Spectra of TPF(&), DPTF(ꢄ) and DPCF(4) film on
glass.
When the ground-state structures of three molecules were exam-
ined, it was found that the distance and the dihedral angle between
fluoranthene and 7-, 8-, 10-phenyl groups were almost identical. This
indicates that in TPF structure, the addition of –CH or –CN would
3
not have a considerable impact on the ground state structure of
molecule (Table 2).
Figure 2 shows electron distribution of HOMO and LUMO in TPF,
DPTF and DPCF. According to Figure 2, only DPCF shows a slight
difference. Unlike TPF or DPTF, DPCF in the state of LUMO has
some electron density at benzene ring of the 8-position, while the case
TABLE 2 Comparison of Distances and Dihedral Angles in Fluoranthene
ꢂ
Derivatives at the Ground State (at Spartan’04=B3LYP=6–31G )
7
8
10
Distance Dihedral Distance Dihedral Distance Dihedral
˚
˚
˚
Position@fluoranthene
(A)
(deg)
(A)
(deg)
(A)
(deg)
TPF
DPTF
DPCF
1.496
1.496
1.496
71.91
72.25
70.48
1.493
1.493
1.491
55.10
54.27
54.57
1.492
1.491
1.492
58.20
57.44
61.04

1
46
S.-K. Kim et al.
FIGURE 2 HOMO and LUMO levels of (a) TPF, (b) DPTF and (c) DPCF cal-
ꢂ
ꢂ
culated at the B3LYP=6-31G level of density functional theory with a 6-31G
basis set using Spartan’04.
of HOMO did not have electron density at benzene ring of the 8-
position. On the contrary, TPF or DPTF in the state of HOMO has
some electron density at benzene ring of the 8-position, but no electron
density was found in the state of LUMO. This shows a possibility that
when electrons come from the outside to LUMO, the withdrawing
effect of –CN group may allow electron to arrive at the 8-position.
However, the electron cloud of fluoranthene cannot make much
change with the withdrawing effect of a single –CN group. HOMO
and LUMO levels, which determine band gap in all compounds,

Effect of Fluoranthene Derivatives in Electroluminescence
147
confirmed that electron density is concentrated on fluoranthene.
HOMO and LUMO results explain why UVmax and PLmax values of
three compounds are similar. CV was measured to identify HOMO
values of the synthesized materials. Even when cycles are scanned
for more than 50 times or so, CV result appeared stable.
By using band-gap identified with UVonset and HOMO values
measured through CV, LUMO was confirmed, which are summarized
in Table 1. The HOMO levels of three materials showed very similar
values in the range of 5.8ꢁ5.9 eV. That is, it is expected that oxidation
and reduction within molecules occur in fluoranthene regardless of
substituents. This result can be associated with the finding obtained
through molecular simulation that the most of electrons on HOMO
and LUMO exist in fluoranthene. The HOMO levels of both com-
pounds were lower than NPB (5.4 eV) which is a hole transporting
material; the LUMO levels were higher than Alq3 (3.1 eV) which is
electron transporting material, showing an adequate electronic level
as emitting layer.
TGA and DSC data analysis identified important thermal proper-
ties of the synthesized materials which is directly related to the life-
time property of device as mentioned above (Table 1). Through DSC,
ꢀ
it were found that TPF, DPTF and DPCF had T values of 199 C,
2
m
ꢀ ꢀ
62 C and 236 C, but a clear T was not observed. TGA data under
g
the condition of nitrogen showed Td of the materials as shown in
Table 1. Temperatures at which the weight is reduced by 5% were
ꢀ ꢀ ꢀ
23 C (TPF), 337 C (DPTF) and 393 C (DPCF). Considering that T
m
of DPVBi is 204 C , thermal properties of these materials are
assumed to be excellent.
3
ꢀ
14
Multi-layered OLED device was fabricated in order to identify elec-
troluminescent properties of synthesized materials. EL devices were
0
00
fabricated
using
4,4 ,4 -tris
{N,-(2-naphthyl)-N-phenylamino}-
0
triphenylamine (2-TNATA) as a hole injection layer, 4,4 -bis[N-(1-
naphthyl)-N-phenyl-amino]biphenyl (NPB) as a hole transporting
layer, tris(8-hydroxyquinoline) aluminum (Alq ) as electron transport-
3
ing layer, synthesized materials as a emitting layer, ITO as the anode
and LiF=Al as the cathode; ITO=2-TNATA (60 nm)=NPB (15 nm)=TPF
or DPTF or DPCF (30 nm)=Alq (30 nm)=LiF (1 nm)=Al (200 nm).
3
Figure 3 shows the I-V curves of the devices. DPCF showed I-V pro-
perty with slightly increasing turn-on voltage. This is because DPCF
has a lower level of HOMO level and a little barrier energy gap with
NPB than TPF or DPTF.
Figure 4 illustrates EL spectrum of the fabricated device. The
device that used TPF and DPTF as emitting layers shows ELmax
values of 471 nm and 470 nm, respectively, in the sky-blue area.

1
48
S.-K. Kim et al.
FIGURE 3 Energy diagram and current density-voltage characteristics of
ITO=2-TNATA(60 nm)=NPB(15 nm)=TPF(&), DPTF(ꢀ) and DPCF(4)(30 nm)=
3
Alq (30 nm)=LiF(1 nm)=Al devices.
FIGURE 4 EL Spectrum of ITO=2-TNATA=NPB=TPF(&), DPTF(ꢀ) and
2
DPCF(4)=Alq
3
=LiF=Al devices at 10 mA=cm .

Effect of Fluoranthene Derivatives in Electroluminescence
149
TABLE 3 EL Performance of Multi-Layered Devices with the Structure:
ITO=2-TNATA(60 nm)=NPB(15 nm)=Synthesized Material(30 nm)=Alq
3
2
(
30 nm)=LiF(1 nm)=Al(200 nm) at 10 mA=cm
ELmax
Voltage
(V)
Efficiency
(cd=A)
Efficieny
(1 m=W)
CIE
(x,y)
(nm)
TPF
DPTF
DPCF
471
470
513
6.3
6.1
6.9
3.27
3.62
0.28
1.63
1.85
0.13
(0.192, 0.269)
(0.185, 0.252)
(0.285, 0.465)
Whereas, it was identified that the device with DPCF as an emitting
layer has weak emission that is expected to be Alq3 wavelength
(
513 nm).
Table 3 shows efficiency of the fabricated device at 10 mA=cm of
2
current density. TPF showed sky-blue CIE value of (0.192, 0.269)
2
and 3.27cd=A at 10 mA=cm . DPTF also showed sky-blue CIE value
2
of (0.185, 0.252) and 2.31cd=A at 10 mA=cm . However, DPCF device
exhibited relatively low efficiency. This result can be explained by
the mismatched HOMO levels of DPCF and Alq3 that is automatically
injected hole carrier from DPCF to Alq3 when hole is injected from
NPB. That is why EL spectrum of DPCF device shows EL maximum
value of 513 nm of Alq3 light.
In particular, DPTF device has better thermal properties, a better
luminance efficiency, a lower operating voltage, and a higher power
efficiency than the TPF device. As a result, we conclude that the EL
efficiencies and CIE values of these synthesized compounds are affec-
ted by the presence of methyl group and their inductive effect, as was
also concluded in a previous study of 9,10-di(2-napthyl)anthracene
(
ADN) and 2-methyl-9,10-di(2-napthyl)anthracene (MADN) [15].
CONCLUSIONS
In this study, we synthesized 7,8,10-triphenylfluoranthene [TPF] and
TPF derivatives which are substituted by p-tolyl and p-cyanobenzene
instead of phenyl at 8-position.
Multilayered EL devices were fabricated using these materials as
non-doped blue emitting layers. DPCF device exhibited relatively
low efficiency. This result can be explained by the mismatched HOMO
levels of DPCF and Alq3 that is automatically injected hole carrier
from DPCF to Alq3 when hole is injected from NPB. DPTF device
including a methyl group has better thermal properties, a better

1
50
S.-K. Kim et al.
luminance efficiency, a lower operating voltage, and a higher power
efficiency than the TPF device.
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