An efficient red organic light-emitting diode using DCJTB type emitters based on silicone-containing julolidine derivatives

Article abstract of DOI:10.1080/15421406.2011.599776

This paper reports the synthesis and electroluminescent properties of new red fluorescent materials, 4-(dicyanomethylene)-2-(1,1-dimethyl-7- trimethylsiliyl methyl-julolidyl-9-enyl)-6-(4-pentylbicyclo[2.2.2]octan-1-yl)- 4H-pyran (DCJTMS) and 4-(dicyanomethylene)-2-(1,7-bistrimethylsiliylmethyl- julolidyl-9-enyl)-6-(4- pentylbicyclo[2.2.2]octan-1-yl)-4H-pyran (DCJDTMS). In these red emitters, the bulky groups such as trimethylsilane and n-pentyl-bicyclo[2,2,2]octane were introduced into and 4-(dicyanomethylene)-2- methyl-6-[p-(dimethylamino)-styryl]-4Hpyran (DCM) skeleton to increase the steric hindrance between the red emitters and prevent concentration quenching. An OLED device using 4-(dicyanomethylene)-2-(1,7- bis(trimethylsiliylmethyl)- julolidyl-9-enyl)-6-(4-pentylbicyclo[2.2.2]octan-1-yl)-4Hpyran (DCJDTMS) as a dopant exhibited the efficient red emission with a Luminous efficiency, Power efficiency and Commision Internationale De L'Eclairage (CIEx,y) coordinates of 3.66 cd/A, 2.13 lm/W at 20 mA/cm² and (0.62, 0.38) at 7.0 V, respectively. Compared to device using 4-(dicyanomethylene)-2-(1,1,7,7-tetra- methyljulolidyl- 9-enyl)-6-tert-butyl-4H-pyran (DCJTB), the red device using DCJDTMS showed the greatly improved EL efficiency due to the more bulky substituents of compound DCJDTMS. Copyright Taylor & Francis Group, LLC.

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An Efficient Red Organic Light-Emitting
Diode Using DCJTB Type Emitters
Based on Silicone-Containing Julolidine
Derivatives
b
a
a
a
Kum Hee Lee , Su Hyun Oh , Min Hye Park , Young Kwan Kim
&
a
Seung Soo Yoon
a
Department of Chemistry , Sungkyunkwan University , Suwon ,
4
40-746 , Korea
b
Department of Information Display , Hongik University , Seoul ,
21-791 , Korea
1
Published online: 18 Oct 2011.
To cite this article: Kum Hee Lee , Su Hyun Oh , Min Hye Park , Young Kwan Kim & Seung Soo Yoon
(
2011) An Efficient Red Organic Light-Emitting Diode Using DCJTB Type Emitters Based on Silicone-
Containing Julolidine Derivatives, Molecular Crystals and Liquid Crystals, 550:1, 270-277, DOI:
0.1080/15421406.2011.599776
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Mol. Cryst. Liq. Cryst., Vol. 550: pp. 270–277, 2011
Copyright © Sungkyunkwan University
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2011.599776
An Efficient Red Organic Light-Emitting Diode
Using DCJTB Type Emitters Based on
Silicone-Containing Julolidine Derivatives
1
1
1
KUM HEE LEE, SU HYUN OH, MIN HYE PARK, YOUNG
2
,∗
1,∗
KWAN KIM, AND SEUNG SOO YOON
1
Department of Chemistry, Sungkyunkwan University, Suwon 440-746, Korea
Department of Information Display, Hongik University, Seoul 121-791, Korea
2
This paper reports the synthesis and electroluminescent properties of new
red fluorescent materials, 4-(dicyanomethylene)-2-(1,1-dimethyl-7-trimethylsiliyl
methyl-julolidyl-9-enyl)-6-(4-pentylbicyclo[2.2.2]octan-1-yl)-4H-pyran (DCJTMS)
and
4-(dicyanomethylene)-2-(1,7-bistrimethylsiliylmethyl-julolidyl-9-enyl)-6-(4-
pentylbicyclo[2.2.2]octan-1-yl)-4H-pyran (DCJDTMS). In these red emitters,
the bulky groups such as trimethylsilane and n-pentyl-bicyclo[2,2,2]octane were
introduced into and 4-(dicyanomethylene)-2-methyl-6-[p-(dimethylamino)-styryl]-4H-
pyran (DCM) skeleton to increase the steric hindrance between the red emitters and
prevent concentration quenching. An OLED device using 4-(dicyanomethylene)-2-(1,7-
bis(trimethylsiliylmethyl)-julolidyl-9-enyl)-6-(4-pentylbicyclo[2.2.2]octan-1-yl)-4H-
pyran (DCJDTMS) as a dopant exhibited the efficient red emission with a Luminous
efficiency, Power efficiency and Commision Internationale De L’Eclairage (CIEx,y
coordinates of 3.66 cd/A, 2.13 lm/W at 20 mA/cm and (0.62, 0.38) at 7.0 V, respectively.
Compared to device using 4-(dicyanomethylene)-2-(1,1,7,7-tetra-methyljulolidyl-
)
2
9
-enyl)-6-tert-butyl-4H-pyran (DCJTB), the red device using DCJDTMS showed
the greatly improved EL efficiency due to the more bulky substituents of compound
DCJDTMS.
Keywords
OLED; Red fluorescence; Trimethylsiliyl group; n-Pentyl-
bicyclo[2,2,2]octane; DCJTB derivatives
Introduction
Since the first application of a multilayer structure in organic light-emitting diodes (OLEDs)
by Tang and Vanslyke, OLEDs have attracted considerable attention owing to their easy
processability, flexibility, thinness, low operating voltages, high resolution, wide viewing
angles, and faster response time [1]. Therefore, OLEDs have become a technology for the
next generation full color flat-panel displays. Many researchers have examined fluorescent
and phosphorescent emitters for high device performance in OLEDs [2]. However, fluores-
cent red emitters are a limitation in realizing a full color display with low efficiency and
satisfactory color purity [3].
∗
Corresponding author. E-mail: ssyoon@skku.edu (S. S. Yoon), kimyk@hongik.ac.kr
(Y.K. Kim)
270

An Efficient Red OLED Using DCJTB Type Emitters
271
Up to now, a variety of red fluorescent emitters have been developed, including the
push-pull red emitters based on arylamine-substituted polyaromatic hydrocarbon deriva-
tives [4], polyacenes [5], metal chelates [6] and DCM derivatives [7]. Generally, red fluo-
rescent emitters have electron donor-acceptor structures with a narrow band gap. In most
cases, the concentration quenching due to excimer and exciplex formation reduce the EL
performance dramatically. Therefore, there has been considerable effort in developing ef-
ficient red fluorescent materials by preventing excimer and exciplexes formation through
the incorporation of sterically bulky moieties in the emitters. In particular, DCJTB was
reported to show improved EL performance due to the introduction of a bulky tert-butyl
group, which prevented concentration quenching [8]. However, their EL performance still
requires improvement.
This paper describes the synthesis and electroluminescent properties of new red fluores-
cent materials, 4-(dicyanomethylene)-2-(1,1-dimethyl-7-trimethylsiliylmethyl-julolidyl-9-
enyl)-6-(4-pentylbicyclo[2.2.2]octan-1-yl)-4H-pyran (DCJTMS) and 4-(dicyanomethy
lene)- 2-(1,7-bis(trimethylsiliylmethyl)-julolidyl-9-enyl)-6-(4-pentylbicyclo[2.2.2]octan-
1
-yl)-4H-pyran (DCJDTMS). In these red emitters, bulky groups, such as trimethylsilane
and n-pentyl-bicyclo[2,2,2]octane, are introduced into the DCM skeleton to increase steric
hindrance between red emitters and prevent concentration quenching. As will be demon-
strated in below, a fluorescent red OLED using DCJDTMS exhibites efficient red emission
with improved EL efficiency, compared to a device using DCJTB.
Experimental
Materials and Measurement
1H- and 13C- nuclear magnetic resonance (NMR) were recorded on a Varian (Unity Inova
300Nb) spectrometer. The Fourier transform infrared (FT-IR) spectra were recorded using a
Thermo Nicolet Avatar 320 FT-IR spectrometer. The low- and high-resolution mass spectra
were measured using a Jeol JMS-AX505WA spectrometer in FAB mode, a Jeol JMS-600W
spectrometer in EI mode and a JMS-T100TD (AccuTOF-TLC) in positive ion mode. The
UV-Vis absorption and photoluminescence spectra of the newly designed red dopants were
−
5
measured in a 10 M solution in 1,2-dichloroethane. The fluorescent quantum yields were
12
determined in 1.2-dichloroethane at 293 K against DCJTB (Ф = 0.78). The HOMO
energy levels were measured using a low energy photo-electron spectrometer (Riken-Keiki
AC-2). The LUMO energy levels were estimated by subtracting the energy gap from the
HOMO energy levels.
4
-(dicyanomethylene)-2-(1,1-dimethyl-7-trimethylsiliylmethyl-julolidyl-9-enyl)-6-
(4-pentylbicyclo[2.2.2]octan-1-yl)-4H-pyran (DCJTMS). Piperidine (1.17 mmol) was
added to a solution of compounds 1a (0.26 mmol) and 2 (0.26 mmol) in anhydrous ethyl
◦
alcohol (10 ml) and heated to 140 C for 5 h with a dean-stark trap. After cooling in a
refrigerator, the mixture was filtered with hexane and extracted with ethyl acetate and brine.
After evaporating the solvent, the mixture was purified by silica column chromatography
with ethyl acetate and hexane and re-crystallized with hexane. The obtained compound
1
was a reddish solid (Yield : 70 %). H-NMR (300 MHz, CDCl3): δ ppm 7.25 (d, J = 15.7
Hz, 1H), 7.18 (d, J = 1.9 Hz, 1H), 7.06 (d, J = 1.9 Hz, 1H), 6.56 (d, J = 2.1 Hz, 1H),
6
3
1
.41 (d, J = 2.1 Hz, 1H), 6.40 (d, J = 15.7 Hz, 1H), 3.45-3.36 (m, 1H), 3.32-3.18 (m,
H), 2.96-2.91 (m, 1H), 1.88-1.73 (m, 10H), 1.54-1.49 (m, 6H), 1.33 (s, 3H), 1.30 (s, 3H),
13
.26-1.17 (m, 8H), 0.96-0.93 (m, 2H), 0.89 (t, J = 7.0 Hz, 3H), 0.09 (s, 9H); C-NMR (75

272
K. H. Lee et al.
MHz, CDCl3): δ ppm 171.8, 161.0, 156.9, 143.1, 139.4, 130.2, 128.7, 126.0, 124.9, 121.3,
1
3
1
16.4, 116.3, 111.5, 105.2, 103.2, 56.6, 46.7, 41.5, 37.2, 36.1, 33.3, 32.9, 32.3, 30.8, 30.7,
0.6, 30.2, 30.0, 27.9, 25.0, 23.6, 22.9, 14.3, -0.30; FT-IR (KBr): ν = 2926, 2861, 2204,
−1
+
640, 1597, 1540, 1419, 1319, 1304, 1176, 843 cm ; Mass (EI) m/z = 633 (M ); HRMS
◦
(EI) calcd for C41H55N3OSi, 633.4114; found,633.4107; mp: 234 C.
4
-(dicyanomethylene)-2-(1,7-bis(trimethylsiliylmethyl)-julolidyl-9-enyl)-6-(4-
pentylbicyclo[2.2.2]octan-1-yl)-4H-pyran (DCJDTMS). DCJDTMS was prepared using
the method for synthesizing DCJTMS using compound 1b instead of compound 1a. The
1
obtained compound was a reddish solid (Yield : 80 %). H-NMR (300 MHz, CDCl3): δ
ppm 7.25 (d, J = 15.6 Hz, 1H), 7.02 (d, J = 4.7 Hz, 2H), 6.54 (d, J = 2.1 Hz, 1H), 6.40
(
d, J = 2.1 Hz, 1H), 6.37 (d, J = 15.6 Hz, 1H), 3.43-3.35 (m, 2H), 3.26-3.17 (m, 2H),
2
6
3
1
3
.96-2.88 (m, 2H), 2.04-1.93 (m, 2H), 1.87-1.82 (m, 6H), 1.81-1.71 (m, 2H), 1.54-1.49 (m,
H), 1.35-1.16 (m, 8H), 0.98-0.86 (m, 4H), 0.89 (t, J = 7.0 Hz, 3H), 0.09 (s, 9H), 0.08 (s,
13
H); C-NMR (75 MHz, CDCl3): δ ppm 171.8, 161.0, 156.9, 143.5, 143.0, 139.4, 128.4,
28.3, 126.5, 121.2, 121.0, 116.5, 116.4, 111.4, 111.3, 105.1, 103.2, 56.5, 46.6, 46.1, 41.5,
7.2, 33.2, 33.1, 33.0, 30.8, 30.6, 30.0, 28.1, 27.9, 25.2, 23.6, 22.9, 14.3, −0.25, −0.30;
FT-IR (KBr): ν = 2927, 2858, 2204, 1640, 1594, 1542, 1492, 1423, 1312, 1161, 837
−
1
+
cm ; Mass (EI) m/z = 691 (M ); HRMS (EI) calcd for C43H61N3OSi2, 691.4353; found,
◦
6
91.4351; mp: 236 C.
Results and Discussion
Scheme 1 shows the synthetic scheme of DCJTMS, DCJDTMS and DCJTB. The aldehyde
intermediates were prepared using the known procedure [9]. Knoevenagel condensation of
the corresponding aldehyde intermediates with the pyran intermediate 2 [9] provided red
emitters DCJTMS, DCJDTMS and DCJTB in moderate yields. These compounds were
1
13
fully characterized with H- and C-NMR, FTIR, and low- and high-resolution mass
spectrometry.
Figure 1(a) presents the ultraviolet-visible (UV-Vis) absorption spectra of compounds
DCJTMS, DCJDTMS and DCJTB. The maximum absorption peaks of compounds
DCJTMS, DCJDTMS and DCJTB were 507, 504 and 513 nm, respectively. Figure 1(a)
shows good overlap between the emission spectra of Alq3 and the absorption spectra of
compounds DCJTMS, DCJDTMS and DCJTB. The results suggest that F o¨ rster singlet
energy transfer from Alq3 to red emitters DCJTMS, DCJDTMS and DCJTB is efficient,
and Alq3 served well as a host in the OLEDs using these compounds as red dopant ma-
terials. Red emitters DCJTMS, DCJDTMS and DCJTB exhibited efficient red emission
with maximum emission peaks of 621, 618 and 621 nm, respectively, as shown in Figure
1
0
(b). The quantum yields of compounds DCJTMS, DCJDTMS and DCJTB were 0.49,
.10 and 0.78, respectively [10]. Table 1 lists the physical properties.
To examine the electroluminescent properties of compound DCJTMS and
DCJDTMS, multilayer OLED devices were fabricated with the following structure: Indium
Tin Oxide (ITO)/ N,N’-diphenyl-N,N’-(2-napthyl)-(1,1’-phenyl)-4,4’-diamine(NPB) (400
Å)/Alq3 (DCJDTMS 1 or 2 %, device A or B), (DCJTMS 1 %, device C), (DCJTB 1 %,
device D) or (DCJDTMS 1 % with 30 % rubrene, device E) (200 Å) /tris-(8-quinolinolate)
aluminium (Alq3) (200 Å)/Liq (20 Å)/Al (1000 Å), respectively, as shown in Figure 2. NPB
Alq3 and Liq were used as the hole transporting layer, red host in the emitting layer as well
as electron transporting layer (ETL), and electron injection layer, respectively. DCJTMS,

An Efficient Red OLED Using DCJTB Type Emitters
273
Scheme 1. Synthesis and structures of DCJDTMS, DCJTMS, and DCJTB.
DCJDTMS and DCJTB were used as red dopants in the emitting layer, respectively. Par-
ticularly, in device E, rubrene was used as cohost to improve the EL performances. Table
2
summarizes the electroluminescent data.
Figure 3 shows the EL efficiencies of devices A-E. The luminous and power efficiency
2
of device A using DCJDTMS was 3.54 cd/A and 1.48 lm/W at 20 mA/cm , respectively.
Compared to a device C and D using DCJTMS and DCJTB at the same doping con-
centration, a device A showed the improved EL efficiencies. For examples, compared to
devices C and D, the luminous efficiency of device A increased by 134% and 49% and
Table 1. Physical properties of red emitters DCJTMS, DCJDTMS and DCJTB
compound UVmax(nm)[a] PLmax(nm)[a] FWHM[b] Eg[c] Q.Y[d] HOMO/LUMO[e]
DCJDTMS
DCJTMS
DCJTB
504
507
513
618
621
621
83
73
72
2.20 0.10
2.17 0.49
2.16 0.78
−5.71/−3.51
−5.44/−3.27
−5.26/−3.10
−
5
[
a] Maximum absorption or emission wavelength in 1,2-dichloroethane (ca. 1 × 10 M). [b] The
Full width at Half-Maximum peaks of Emission Spectra. [c] Energy band-gap [d] Using DCJTB as
a standard; λex = 550 nm (Ф = 0.78 in 1,2-dichloroethene). [e] Obtained from AC-2 and absorption
measurement.

274
K. H. Lee et al.
Figure 1. (a) UV-Vis absorption spectra and (b) PL spectra of DCJDTMS, DCJTMS and DCJTB.
2
the power efficiency of device A increased by 179% and 37% at 20 mA/cm , respectively.
The bulky two trimethylsiliyl groups in compound DCJDTMS could partially contribute
to the improved EL efficiency of device A. Presumably, the bulky groups of compound
DCJDTMS in device A would increase the steric hindrance between red emitters in the
emitting layer of the device A and lead to improve EL performance by preventing con-
centration quenching. In devices A and B using DCJDTMS as a dopant at the different
dopping concentrations, with the increase of doping concentration from 1 % to 2 %, the
luminous and power efficiencies decreased due to the concentration quenching effect.

An Efficient Red OLED Using DCJTB Type Emitters
275
Figure 2. Structure and Energy-level diagram of devices.
Figure 4 shows the EL emission spectra of the red devices. The EL wavelengths of
devices A–E were 608, 620, 616, 616, and 616 nm, respectively. The EL peaks of devices A
and B showed increases in the intensities of the shoulder peaks due to the enhanced excimer
formation under higher-doping conditions. The EL wavelengths of devices A, C, and D
using different dopants at the same doping level were 608, 616, and 616 nm, respectively.
The CIE coordinates of devices A, C, and D were (0.58, 0.42), (0.60, 0.39), and (0.60,
0.39) at 7.0 V, respectively. Device A showed smaller excimer peaks in the EL spectra than
devices C and D, suggesting less excimer formation due to the bulky two trimethylsiliyl
groups in the julolidine moiety of DCJTMS and DCJTB. Interestingly, there were shoulder
peaks at approximately 510 nm in the EL spectra of devices A, C, and D, which might have
originated from the green emission peak of Alq3 host. This indicates incomplete energy
transfer between host and dopant in the Alq3 single-host emitting system.
The CIE coordinates of device A is (0.58, 0.42) in the orange-red region. Interestingly,
compared to device A, device E using rubrene as a cohost had the improved CIE coordinates
Table 2. EL performance characteristic of devices A-E
Dopant
(doping %)
L [a]
LE-J [b] PE-J[b] EL[c]
CIE[c]
(x,y)
V on[d]
(V)
2
Device
(cd/m ) (cd/A) (lm/W) (nm)
A
B
C
D
E
DCJDTMS (1%)
DCJDTMS (2%)
DCJTMS (1%)
DCJTB (1 %)
DCJDTMS (1%)
Rubrene (30%)
4880
496
1410
6299
2380
3.54
1.90
1.51
2.38
3.66
1.48
0.52
0.53
1.08
2.13
608 (0.58, 0.42)
620 (0.62, 0.38)
616 (0.60, 0.39)
616 (0.60, 0.39)
616 (0.62, 0.38)
4
4
4
3
4
2
[
a] Value at 12V. [b] The obtained value at 20 mA/cm , [c] The obtained value at 7.0 V. [d] Turn-on
2
voltage at 1cd/m .

276
K. H. Lee et al.
Figure 3. (a) J-V, (b) L-V, (c) LE versus current density, and (d) PE versus current density charac-
teristics of devices A-E.
of (0.62, 0.38) at 7.0 V. Compared to EL spectra of device A, device E was not exhibited
a Alq3 emission, which shows that energy transfer between the host and dopant is more
effective in the rubrene-Alq3 co-host emitting system than the Alq3 single-host emitting
system [11]. These observations are imply that a cohost such as rubrene increase energy
Figure 4. EL spectra of device A-E.

An Efficient Red OLED Using DCJTB Type Emitters
277
transfer efficiency from Alq3 to DCJDTMS and thus improve the color purity. Furthermore,
compared to device A, the luminous and power efficiencies of device E increased by 3.4%
2
and 44% at 20 mA/cm , respectively. These phenomena give rise to the higher luminance
efficiency and improved color purity. This study demonstrates that DCJDTMS with a
modified julolidine moiety have excellent properties for use as red-emitting materials for
OLEDs.
Conclusions
In conclusion, a red organic light-emitting diode using a new fluorescent red emitter
DCJDTMS exhibited efficient red emission with a LE and PE of 3.66 cd/A and 2.13 lm/W at
2
2
0 mA/cm , respectively. The device showed the CIEx,y coordinates of (0.62, 0.38) at 7.0 V.
Compared to the control device using DCJTB, the red device using DCJDTMS showed
greatly improved EL efficiency due to the more bulky substituents in dopant DCJDTMS
than those of DCJTB, Thus DCJDTMS is an excellent fluorescent red material for efficient
OLEDs.
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