Highly twisted pyrene derivatives for non-doped blue OLEDs

Article abstract of DOI:10.1016/j.dyepig.2015.12.024

New highly twisted rigid blue light-emitting materials were designed, composed of pyrene with a xylene core unit and either naphthalene or phenyl end units. These blue-emitting materials were synthesized via the Suzuki cross-coupling reaction and their structures were confirmed using FT-IR, ¹H NMR, ¹³C NMR, and mass spectroscopy. The optical, electrochemical and thermal properties of the materials were investigated. The non-coplanar structure introduced by highly twisted xylene units provides steric hindrance, resulting in very deep blue emission. The fabricated devices exhibited a maximum external quantum efficiency (EQE) of 3.69% with CIE color coordinates (x, y: 0.15, 0.06).

Full text of DOI:10.1016/j.dyepig.2015.12.024

Accepted Manuscript
Highly twisted pyrene derivatives for non-doped blue OLEDs
Sang Bong Lee, Kwang Hun Park, Chul Woong Joo, Jeong- Ik Lee, Jonghee Lee, Dr.,
Yun-Hi Kim, Prof. Dr.
PII:
S0143-7208(15)00511-2
10.1016/j.dyepig.2015.12.024
DYPI 5047
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 29 September 2015
Revised Date: 14 November 2015
Accepted Date: 22 December 2015
Please cite this article as: Lee SB, Park KH, Joo CW, Lee J-I, Lee J, Kim Y-H, Highly twisted pyrene
derivatives for non-doped blue OLEDs, Dyes and Pigments (2016), doi: 10.1016/j.dyepig.2015.12.024.
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ACCEPTED MANUSCRIPT
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0
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0
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.6
.4
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at 1000 nits
(
(
A) (0.15,0.06)
B) (0.16,0.06)
(A)BDPP
(B)BDNP
3
50 400 450 500 550 600 650 700 750 800
Wavelength (nm)

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Highly twisted pyrene derivatives for non-doped blue OLEDs
a, 1
a, 1
b
b
b, **
Sang Bong Lee ; Kwang Hun Park , Chul Woong Joo , Jeong- Ik Lee , Jonghee Lee
,
a, *
and Yun-Hi Kim
a
Department of Chemistry and RIGET, Gyeongsang National Univ, 900, Gajwa-dong, Jinju,
Gyeongsangnam-do, 660-701, South Korea
b
Soft I/O Interface Research Section, Electronics and Telecommunications Research Institute
(ETRI), Daejeon 305-700, Republic of Korea
Abstract:
New highly twisted rigid blue light-emitting materials were designed, composed of pyrene
with a xylene core unit and either naphthalene or phenyl end units. These blue-emitting
materials were synthesized via the Suzuki cross-coupling reaction and their structures were
1
13
confirmed using FT-IR, H NMR, C NMR, and mass spectroscopy. The optical,
electrochemical and thermal properties of the materials were investigated. The non-coplanar
structure introduced by highly twisted xylene units provides steric hindrance, resulting in
very deep blue emission. The fabricated devices exhibited a maximum external quantum
efficiency (EQE) of 3.69% with CIE color coordinates (x, y: 0.15, 0.06).
Keywords: blue, fluorescence, OLED, pyrene, xylene
PACS code: To be determined
*
*
Author to whom correspondence should be addressed.
Prof. Dr. Yun-Hi Kim
E-mail: ykim@gnu.ac.kr, Fax: +82-55-772-1489; Tel: +82-55-772-1491
1

ACCEPTED MANUSCRIPT
*
Dr. Jonghee Lee
E-mail: jonghee.lee@etri.re.kr
1
. Introduction
Organic light-emitting diodes (OLEDs) have attracted considerable attention or their
emission properties at wide viewing angle angles and their application to flat-panel displays
1-6]. Blue, green and red emitters with have high emission efficiency and high color purity
[
are required for full color display applications. Green and red emitters are usually used in
phosphorescent systems, while blue emitters are still used in fluorescent systems, so
developing effective blue OLEDs is an important part of developing these systems. The
color purity of blue emitters can affect power consumption and is an important issue for
researchers [7-11]. It is very difficult to develop color pure and efficient blue emitters due to
the wide band gap required, regardless of the type of material.
Research has revealed that host-guest systems experience some problems such as phase
separation upon heating, complexity due to addition of dopants, and the high cost of mass
production, despite improved EL efficiency [12-13]. Non-doped emitting layer systems may
therefore have advantages over host-guest systems.
Pyrene has strong π electron delocalization energy and efficient fluorescence properties due
to its large planar conjugated aromatic characteristics [14]. Some pyrene derivatives have
been used in OLEDs to improve hole transport through their electron-rich characterization
[
15].
Recently, our group reported anthracene derivatives with a xylene group, with highly
twisted and rigid non-planar structures, which prevents the close-packing of molecules in the
solid state and increases efficiency through reduced vibronic coupling. These anthracene
derivatives exhibited high efficiency and color- pure blue emission [11, 16].
2

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In this study, we designed and developed new pyrene derivatives containing xylene units,
specially, 1,6-bis(2,5-dimethyl-4-naphthalene-2-yl)phenyl)pyrene (BDNP) and 1,6-bis(2,5-
dimethyl-4-phenyl)phenyl)pyrene (BDPP). BDNP and BDPP which have bulky, rigid non-
coplanar structures due to steric tortional hindrance of the 2,5-dimethyl phenyl (xylene) unit,
are expected to demonstrate efficient and color-pure blue emission.
2
. Experimental Section
2
.1. Materials
All starting materials were purchased from Aldrich and TCI. Pd catalyst was purchased
from Umicore. All starting materials were used without further purification. 3,4-
Dibromothiophene-1,1-dioxide and 2,3-dibromoanthraquinone were prepared according to
literature procedures [17]. All solvents were further purified prior to use.
2
.2. Instruments
1
H NMR spectra were recorded using a Bruker Avance 300 MHzFT NMR spectrometer,
and chemical shifts (ppm) were reported with tetramethylsilane (TMS) as an internal standard.
Fourier transform (FT)-IR spectra were recorded using a Shimadazu FT-IR spectrometer.
Thermogravimetric analysis (TGA) was carried out on a TA instruments 2100 TGA analyzer
o
under nitrogen atmosphere at a rate of 10 C/min. Differential scanning calorimeter (DSC)
o
was performed using a TA DSC 2010 device under nitrogen atmosphere at a rate of 10 C
/
min. Melting points was determined using an Electrothermal digital melting point IA 9000
analyzer. UV–vis absorption spectra were measured using a Shimadsu UV-1065PCUV–vis
spectrophotometer. The photoluminescence (PL) spectra were measured by Perkin-Elmer
LS50B fluorescence spectro meter. The electrochemical properties of the materials were
3

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measured by cyclic voltammetry (CV) using Epsilon C3 in 0.1 M solution of tetrabutyl
ammonium perchlorate (TBAP) in chloroform.
2
2
.3. Synthesis
.3.1 Synthesis of 2-naphthalene boronic acid
In the THF (300 mL) and 2-bromonaphthalene (30 g, 0.03 mmol) at -78 C, 2.5 M n-
butyllithium (69.54 mL, 0.17 mmol) was slowly added. After 1 h stirring, triethyl borate
o
(
31.7 g, 0.22 mmol) was added at -78 C. The mixture was stirred for 3 h, and the mixture
was extracted with ethyl acetate and water. After the crude product was dried by MgSO the
4
1
solvent was evaporated. The product was purified by hexane. Yield: 21.8 g (87%); H NMR
(
CDCl , 300MHz, ppm): δ = 8.37 (s, 1H), 8.18 (s, 2H), 7.95-7.83 (m, 4H), 7.55-7.48 (m, 2H).
3
2
.3.2 Synthesis of (1-bromo-2,5-dimethyl-4-phenyl)benzene
The mixture of 1,4-dibromo-2,5-dimethylbenzene (40 g, 151.54 mmol), phenyl boronic acid
(
14.8 g, 121. 23 mmol), 2M K CO (120 mL), and tetrahydrofuran (400 mL) was degassed.
2 3
o
After Pd(PPh ) (7.22 g, 4.55 mmol) was added in the mixture, it was stirred for 24 h at 90 C.
3
4
The mixture was extracted with methylene dichloride and water. The crude product was
1
purified by column chromatography using hexane. Yield: 23.2 g (36.7 %); H NMR (CDCl ,
3
3
00MHz, ppm): δ = 7.57 (s, 1H), 7.56-7.45(m, 4H), 7.37-7.33 (dd, 1H), 7.25 (s, 1H), 2.44 (s,
3
H), 2.27 (s, 3H).
2
.3.3 Synthesis of (1-bromo-2,5-dimethyl-4-naphthyl)benzene
4

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The reaction was similarly proceeded as synthesis of 1-bromo-2,5-dimethyl-4-phenyl
benzene.
1
Yield: 25.5 g, 72%; H NMR (CDCl , 300MHz, ppm): δ = 7.93-7.87 (m, 3H), 7.77 (s, 1H),
3
7
.58-7.51(m, 3H), 7.47-7.43 (dd, 1H), 7.22 (s, 1H), 2.44 (s, 3H), 2.27 (s, 3H).
2
.3.4 Synthesis of (2,5-dimethyl-4-phenyl)phenyl boronic acid
In the THF (300 mL) and (1-bromo-2,5-dimethyl-4-phenyl)benzene (5 g, 19.1 mmol) at -78
o
C, 2.5 M n-butyllithium (8.42 mL, 21.06 mmol) was slowly added. After 1 h stirring, triethyl
o
borate (3.07 g, 21.06 mmol) was added at -78 C. The mixture was stirred for 8 h, and the
mixture was extracted with ethyl acetate and water. After the crude product was dried by
MgSO the solvent was evaporated. The product was purified by hexane. Yield: 2.87 g
4
1
(
66.3%); H NMR (DMSO, 300MHz, ppm): δ =8.022 (s, 2H), 7.43(m, 2H), 7.36(m, 4H),
6
.95 (s, 1H), 2.37 (s, 3H), 2.17 (s, 3H).
2
.3.5 Synthesis of (2,5-dimethyl-4-(2’-naphthyl)phenyl boronic acid
The synthesis was proceeded by same synthetic method of (2,5-dimethyl-4-phenyl)phenyl
1
boronic acid. Yield: 23.2 g, 86.3%; H NMR (300 MHz, CDCl , ppm): δ=8.10 (s, 2H), 7.97
3
(m, 3H), 7.86 (s, 1H), 7.55 (m, 3H), 7.29 (s, 1H), 7.19 (s, 1H), 2.28 (s, 3H), 2.26 (s, 3H).
2
.3.6 Synthesis of 1,6-bis(2,5-dimethyl-4-phenyl)phenyl)pyrene (BDPP)
5

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1,6-Dibromopyrene (2g, 4.95 mmol), (2,5-dimethyl-4-phenyl)phenyl boronic acid (2.8 g,
1
2.38 mmol), 2M NaOH (8mL) and THF (100 mL) were mixed. Pd(PPh ) (0.20 g, 0.12
3 4
o
mmol) was added to the mixture, and it was stirred for 24 h at 90 C. The mixture was added
to ethanol, and the crude solid was filtered. The product was purified by soxhlet using
1
chloroform. Yield: 2.03 g, 72.8 %; H NMR (300 MHz, CD Cl , ppm): δ = 8.25 (2H, d), 7.93
2
2
(
2H, s), 8.08 (2H, d), 7.96 (2H, d), 7.88 (4H, d), 7.52-7.50 (8H, m), 7.33-7.32 (6H, s), 2.37
6H, s), 2.09 (6H, s). HRMS (EI+): m/z calcd for (C H )562.2661; found 562.2664.
(
4
4
34
2
.3.7 Synthesis of 1,6-bis(2,5-dimethyl-4-naphthyl)phenyl)pyrene (BDNP)
The synthesis was proceeded by same synthetic method of 1,6-bis(2,5-dimethyl-4-
1
phenyl)phenyl)pyrene (BDNP). Yield: 4.9 g, 88.7 %; H NMR (300 MHz, CDCl , ppm): δ =
3
8
.29 (2H, d), 8.12 (2H, d) 8.09 (12H, m), 7.69 (2H, dd), 7.58-7.55 (4H, m), 7.41 (2H, s), 2.42
(
6H, s), 2.13 (6H, s). HRMS (EI+): m/z calcd for (C H ) 662.2974; found 662.2976.
5
2
38
2
.4. OLEDs device fabrication and measurement
ITO was cleaned by the standard oxygen plasma treatment. The OLED grade materials
except BDPP and BDNP were purchased and used with-out further purification. All organic
–
5
layers were deposited in a high vacuum chamber below 6.67ⅹ10 Pa and thin films of LiF
and Al were deposited as a cathode electrode by a thermal evaporation method. The OLEDs
were transferred directly from vacuum into an inert environment glove-box, where they were
encapsulated using a UV-curable epoxy, and a glass cap with a moisture getter. The
electroluminescence spectrum and current density-voltage-luminescence (J-V-L) were
6

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measured using a Minolta CS-2000 and a current/voltage source/measure unit (Keithley 238)
[
18].
3
. Results and discussion
3
.1. Synthesis and characterization
Synthetic scheme for the two pyrene derivatives, BDNP and BDPP, are shown in Scheme 1.
BDNP and BDPP were synthesized using a simple twofold by two times Suzuki coupling
reaction. Phenyl boronic acid and naphthyl boronic acid were reacted with 1,4-dibromo-2,5-
dimethylbenzene to give 1-bromo-2,5-dimethyl-4-phenylbenzene and 1-bromo-2,5-dimethyl-
4
-naphthylbenzene, respectively. The Suzuki coupling reaction was employed between
monobromide and pyrene boronic ester to obtain the target compounds. The molecular
1
structures of BDNP and BDPP were fully characterized using H-NMR and high resolution
mass spectrometry
3
.2. Computational investigations
To improve EL performance, highly twisted pyrene derivatives with xylene groups were
designed. Computer simulations using density function theory (DFT) were performed to
optimize the structures, using the B3LYP/6-31G* method in Gaussian 03. The optimized
molecular structures of BDNP and BDPP are shown in Fig. 1. The electron densities of the
highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital
(
LUMO) of both BDNP and BDPP were delocalized on the pyrene core regardless of
whether they contained phenyl or naphthyl end groups; this may be due to the steric
7

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hindrance of the xylene units resulting in a highly twisted structure. This structure improves
the color purity because intermolecular interaction is disrupted and recrystallization is
suppressed.
3
.3. Thermal properties
Thermo gravimetric analysis (TGA) and differential scanning calorimetry (DSC) of
measurements for BDPP and BDNP are shown in Fig. 2. In the DSC spectra of heating (10
o
C/min), the two materials have no obvious glass transition. In addition, no crystallization
processes were observed in the DSC trace. These results imply that the bulk structures of
BDNP and BDPP are morphological stable. Both BDPP and BDNP were thermally stable up
o
to 447 C. These results show that these materials are amorphous with good thermal stability,
desirable characteristics for OLED performance.
3
.4. Optical properties
The solution and film state UV absorption and PL spectra of BDNP and BDPP are shown in
Fig. 3. Absorption maxima of BDPP in solution state were observed at 284, 338, and 352 nm,
and in the film state at 287, 343, and 358 nm. For BDNP, absorption maxima in solution were
observed at 283, 338, and 351 nm and in the film state at 286, 342, and 359 nm. Although a
small bathochromic shift occurred in the film state, both BDNP and BDPP exhibited similar
UV-absorption properties. The results suggest that both BDNP and BDPP are highly twisted
structures regardless of whether they contain phenyl or naphthyl side groups, and that
extended conjugation to side groups is inhibited because of the steric stain of the xylene
8

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group. The absorption maxima of both BDPP and BDNP were 40-50 nm blue shifted
compared to anthracene core with phenyl or naphthyl end capped xylene groups.
The fluorescence spectra in solution revealed feature bands at 386 and 403 nm for BDPP
and at 399 nm for BDNP. Fluorescence spectra for thin film revealed bands at 381, 402 and
4
26 nm for BDPP and at 404 and 425 nm for BDPP. Both BDPP and BDNP exhibited
similar deep blue emission even in the film state.
3.5. Electrochemical properties
The electrochemical characteristics of BDNP and BDPP were investigated using cyclic
voltammetry and measuring the optical band gap from the absorption edge. The HOMO,
LUMO, and band gap values for BDNP and BDPP are summarized in Table 1. BDNP and
BDPP both exhibited wide optical energy gaps (E ) of 3.06 and 3.16 eV, respectively, and are
g
expected to be suitable candidates for deep blue emitters.
3
.6. Electroluminescence properties
Figure 4 shows the structures of blue OLEDs fabricated in this study using non-doped
BDNP or BDPP. The detailed structure of Device A was as follows: indium tin oxide (ITO)/
N,N’-di(naphthalene-1-yl)-N,N’-diphenylbenzidine (NPB) (50 nm)/BDPP (40 nm)/1,3-
bis(3,5-di-pyrid-3-yl-phenyl)benzene (BmPyPB) (10 nm)/LiF/Al, where NPB is the hole
transporting layer (HTL); BDPP is the blue emitting layer (EML); BmPyPB is the electron
transporting layer (ETL), and LiF is the electron injection layer (EIL) at the Al cathode
interface [17-19]. Devices B had an identical structure, except that BDNP was used for the
EML material instead of BDPP. Figure 5 shows the current–luminance–voltage
9

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characteristics of the fabricated devices. Both devices exhibited low turn-on voltages of 4.0 V
2
at 10 cd m .
Figure 6 shows the normalized EL spectra of the fabricated devices. The spectra revealed
deep blue emissions with peaks at 436 (BDPP), and 440 nm (BDNP). Moreover, both BDPP
and BDNP exhibited a narrow full-width at half-maximum (FWHMs) of about 50 nm without
excimer or exciplex emission. This small value can be explained by the highly twisted
structure of BDPP and BDNP due to dimethyl groups containing a xylene group, which
results in steric hindrance toward the pyrene unit and the end group [20-21]. Hence, close-
packing is inhibited in the synthesized molecules. The CIE coordinates of Devices A (DBPP),
and Device B (BDNP) were (0.15, 0.06), and (0.16, 0.06), respectively, at a luminance of
2
1
000 cd/m .
The external quantum efficiency (EQE) and current efficiency (CE) vs. current density
characteristics of the fabricated blue OLEDs are plotted shown in Fig. 7. Both devices
exhibited good light-emitting performance, and in particular, Device A containing BDPP
EML material exhibited a maximum EQE of 3.69% and CE of 2.03 cd/A. Key device
performance parameters and EL emission characteristics are summarized in Table 2.
4
. Conclusions
We designed and developed new deep blue light-emitting materials composed of a pyrene
core with highly twisted xylene units. The non-coplanar structure due to steric hindrance of
the introduced substituent resulted in very deep blue emissions with a maximum EL peak of
4
36 nm and narrow FWHM of 50 nm. In particular, the device using BDPP exhibited a peak
EQE of 3.69% with CIE color coordinates (x, y: 0.15, 0.06).
Acknowledgement:
1
0

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The authors acknowledge the Industrial Strategic Technology Development Program
10045269, Development of Soluble TFT and Pixel Formation Materials/Process
(
Technologies for AMOLED TV) funded by MOTIE/KEIT.
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Scheme 1. Synthesis of 1,6-Bis(2,5-dimethyl-4-phenyl)phenyl)pyrene (BDPP) and 1,6-
Bis(2,5-dimethyl-4-naphthyl)phenyl)pyrene (BDNP)
Table Caption
Table 1 Physical properties of BDPP and BDNP.
Table 2 OLED device characteristics using BDPP and BDNP as non-doped and doped blue
emitters
Figure Captions
Fig. 1 Calculated stereostructure and frontier orbital of BDPP (left) and BDNP (right).
Fig. 2 TGA thermogram and DSC thermogram of (a) BDPP and (b) BDNP at a heating rate
o
of 10 C/min.
Fig. 3 UV-vis absorption and photoluminescence emission spectra of (a) BDPP and (b)
BDNP.
Fig. 4 Energy diagrams of devices using BDPP and BDNP as a non-doped emitter.
Fig. 5 Current density–voltage–luminance (J-V-L) characteristics of the fabricated blue
OLEDs.
Fig. 6 Normalized EL spectra of the devices using BDPP and BDNP as a non-doped emitter.
Fig. 7 (a) External quantum and (b) current efficiency vs. current density characteristics of
the fabricated blue OLEDs.
Scheme 1. Synthesis of 1,6-Bis(2,5-dimethyl-4-phenyl)phenyl)pyrene (BDPP) and 1,6-
Bis(2,5-dimethyl-4-naphthyl)phenyl)pyrene (BDNP)
1
4

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Br
Br
Br
B(OH)2
, K CO
2 3
triethyl borate
n-BuLi/ THF
triethyl borate
n-BuLi/ THF
Pd(PPh ) , THF
3
4
Br
B(OH)2
Br
NaOH, Pd(PPh3)4
THF
+
Br
BDPP
B(OH)2
1
5

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Table 1 Physical properties of BDPP and BDNP.
Band
gap
PL
film
HOMO LUMO
UV sol UV film PL sol
Td
( C)
Tg
( C)
Tm
( C)
o
o
o
(
nm)
(nm)
(nm)
(
eV)
(eV)
(
nm)
(
eV)
2
2
3
3
73,
84,
38,
52
272,
287,
343,
358
3
81,
402,
26
386,
403
BDPP
PBNP
372
446
-
-
-
-
5.85
5.59
2.69
3.16
3.06
4
283,
338,
351
286,
342,
359
392
425
2.53
1
6

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Table 2 OLED device characteristics using BDPP and BDNP as non-doped and doped blue
emitters
Device
A (BDPP)
B (BDNP)
CEmax [cd/A]
2.03
EQEmax [%]
3.69
ELmax [nm]
436
CIE [x, y]
(0.15, 0.06)
(0.16, 0.06)
1.88
3.15
440
Device structure A: ITO / NPB (50 nm) / BDPP (40 nm) / BmPyPB (10 nm) / LiF (0.5 nm) /
Al (70nm)
Device structure B: ITO / NPB (50 nm) / BDNP (40 nm) / BmPyPB (10 nm) / LiF (0.5 nm) /
Al (70nm)
1
7

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Fig.1 Calculated stereostructure and frontier orbital of BDPP (left) and BDNP (right)
1
8

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Fig. 2 TGA thermogram and DSC thermogram of (a) BDPP and (b) BDNP at a heating rate
o
of 10 C/min.
(a) BDPP
(b) BDNP
1
9

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Fig. 3 UV-vis absorption and photoluminescence emission spectra of (a) BDPP and (b)
BDNP.
(a) BDPP
1.5
1.2
0.9
0.6
0.3
0.0
UV-Sol (CHCl3)
UV-Film
PL- Sol (CHCl3)
PL- Film
320
400
480
560
640
Wavelength (nm)
(b) BDNP
UV-Sol (CHCl3)
1.0
UV-Film
PL-Sol (CHCl3)
0.8
0.4
0.0
PL-Film
0.8
0.6
0.4
0
.2
.0
0
200 240 280 320 360 400 440 480 520 560 600 640 680
Wavelength (nm)
2
0

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Fig. 4 Energy diagrams of devices using BDPP and BDNP as a non-doped emitter.
2
1

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Fig. 5 Current density–voltage–luminance (J-V-L) characteristics of the fabricated blue
OLEDs.
1
.1
.01
10000
Device A (BDPP)
Device B (BDNP)
0
0
1000
100
10
1
1
1
1
1
1
1
1
E-3
E-4
E-5
E-6
E-7
E-8
E-9
1
1
E-10
E-11
-
6
-4 -2
0
2
4
6
8
10 12
Voltage (V)
2
2

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Fig. 6 Normalized EL spectra of the devices using BDPP and BDNP as a non-doped emitter.
1
0
0
0
0
0
.0
.8
.6
.4
.2
.0
at 1000 nits
BDPP (A) (0.15,0.06)
BDNP (B) (0.16,0.06)
3
50 400 450 500 550 600 650 700 750 800
Wavelength (nm)
2
3

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Fig. 7 (a) External quantum and (b) current efficiency vs. current density characteristics of
the fabricated blue OLEDs.
(
a)
5
4
3
2
1
Device A (BDPP)
Device B (BDNP)
0
1
E-4
1E-3
0.01
0.1
1
2
Current density (A/cm )
(
b)
2
2
1
1
0
0
.5
.0
.5
.0
.5
Device A (BDPP)
Device B (BDNP)
.0
1
E-4
1E-3
0.01
02.1
1
Current density (A/cm )
2
4

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Research highlights
Deep blue emitting materials based on pyrene core ▶ Highly twisted blue emitting
▶
materials by xylene units ▶ 3-Dimensionally highly twisted non-coplanar ▶ Non-doped
device using new pyrene exhibited efficiency of 3.69%. ▶ Non-doped EL devices showed
high color purity of (0.15, 0.06).

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Keywords: Blue OLED; limb structure; anthracene derivatives; quantum efficiency; color
purity; non-coplanar structure