Red non-doped electroluminescent dyes based on arylamino fumaronitrile derivatives

Article abstract of DOI:10.1016/j.dyepig.2009.10.008

A diarylamine, 2-(phenylamino)-9,9-diethylfluorene and three red fluorescent dyes based on arylamino fumaronitrile derivatives, bis(4-(N-(1-naphthyl)phenylamino)-phenyl)fumaronitrile (1-NPAFN, 5a), bis(4-(N-(2-naphthyl)phenylamino)phenyl)-fumaronitrile (2-NPAFN, 5b) and bis(4-(N-(9,9-diethyl-2-fluorenyl)phenylamino)-phenyl)fumaronitrile (EFPAFN, 5c), were prepared. The red dyes showed strong red photoluminescence upon excitation which centered at around 635, 650, 658?nm for 1-NPAFN, 2-NPAFN and EFPAFN in solid film respectively. Fluorescence concentration quenching of red dyes was suppressed. Thermal stability and energy levels were also measured. Multilayer non-doped electroluminescent devices were fabricated using the red dyes (5b, 5c) as red emitters. Device performance kept relatively constant at a wide current density level in the range of 20-150?mA/cm².

Full text of DOI:10.1016/j.dyepig.2009.10.008

Dyes and Pigments 85 (2010) 86e92
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Red non-doped electroluminescent dyes based on arylamino
fumaronitrile derivatives
Wenguan Zhang a b, Zhiqun He a , Linping Mu , Ye Zou , Yongsheng Wang ,*, Shengmin Zhao b
,
,
a
a
a
*
a
Key Laboratory of Luminescence and Optical Information, Ministry of Education, Institute of Optoelectronic Technology, Beijing Jiaotong University, Beijing 100044, PR China
Lab of Printing & Packaging Material and Technology-Beijing Area Major Laboratory, Beijing Institute of Graphic Communication, Beijing 102600, PR China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 25 June 2009
Received in revised form
A diarylamine, 2-(phenylamino)-9,9-diethylfluorene and three red fluorescent dyes based on arylamino
fumaronitrile derivatives, bis(4-(N-(1-naphthyl)phenylamino)-phenyl)fumaronitrile (1-NPAFN, 5a), bis
(
4-(N-(2-naphthyl)phenylamino)phenyl)-fumaronitrile (2-NPAFN, 5b) and bis(4-(N-(9,9-diethyl-2-fluo-
14 October 2009
renyl)phenylamino)-phenyl)fumaronitrile (EFPAFN, 5c), were prepared. The red dyes showed strong red
photoluminescence upon excitation which centered at around 635, 650, 658 nm for 1-NPAFN, 2-NPAFN
and EFPAFN in solid film respectively. Fluorescence concentration quenching of red dyes was suppressed.
Thermal stability and energy levels were also measured. Multilayer non-doped electroluminescent
devices were fabricated using the red dyes (5b, 5c) as red emitters. Device performance kept relatively
Accepted 17 October 2009
Available online 28 October 2009
Keywords:
Red dye
Red fluorescence
Diarylamine
2
constant at a wide current density level in the range of 20e150 mA/cm .
Ó 2009 Elsevier Ltd. All rights reserved.
Arylamino fumaronitrile
Non-doped electroluminescence
Concentration quenching
1. Introduction
The first type of nondoping red materials is built with triarylamine
units, such as anthracen-9-yl-(8-{4-[anthracen-9-yl-(4-ethylphenyl)
Red color is an indispensable component in many applications
such as full color organic light-emitting display and anti-counterfeit
technology. Usually red fluorescent molecules are either polar, such
as electron-donor pyran-containing compounds [1,2], or non-polar
amino]-phenyl}-benzo[a]-aceant-hrylen-3-yl)-(4-ethylphenyl)amine
(ACEN1) [12] and 3-(N-phenyl-N-p-tolylamino)-9-(N-p-styrylphenyl-
N-p-tolylamino)perylene [(PPA)(PSA)Pe] [9], which enhances the
amorphous property and then fluorescent intensity in solid. However,
this type of materials normally loses balance on transportation of
hole and electron. The second type refers to the fluorophores with
donor-acceptor groups having a pair of antiparallel dipoles, such as
2-(4-{1-cyano-2-[4-(4-methoxyphenyl-phenylamino)phenyl]-vinyl}
phenyl)-3-[4-(4-methoxyphenyl-phenylamino)phenyl]acrylonitrile
(D-CN) [13], bis(4-(N-(1-naphthyl)-phenylamino)phenyl)fumar-
onitrile (NPA FN) [14], 4,7-bis(5-(3,5-bis-(diphenylamino)phenyl)
thien-2-yl)benzo[1,2,5]thiadiazole (btza1) [15], N-methyl-3,4-bis
(4-(N-(1-naphthy)-phenylamino)phenyl)maleimide (NPAMLMe)
[16], 2,3-bis(N, N-1-naphthyl-phenylamino)-N-methylmaleimide
(NPAMLI) [17], isophorone dye D-3 with indoline unit as electron
donor [18]. Fluorescence concentration quenching in solid can be
suppressed due to dipoleedipole interaction. NPAFN [19] is rare
material with the bulky and nonplanar arylamino substituents
which reduce molecular interactions and PL quenching. Moreover, it
shows the property of aggregation-induced emission (AIE) [20], PL
emission in solid is much brighter than in solution.
but extensively
p-conjugated, such as porphyrin-type macrocyclic
compounds [3,4]. Many red emitters are highly emissive in solution,
e.g. N, N-bis{4-[2-(4-dicyano-methylene-6-methyl-4H-pyran-2-yl)
ethylene]phenyl}aniline (BDCM) [5] and 4-(di-cyanomethylene)-8-
methyl-2-[2-(2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)
ethenyl]-5,6,7,8-tetrahydro-4H-1-benzopyran (DCMTHBP3) [6].
However, due to the dipoleedipole interaction and extensively
delocalized
p-conjugation, these red fluorescent materials are
prone to aggregation and crystallization in their solid state which
leads to concentration quenching [7]. These limit their usage in
light-emitting devices only as a dopant. Their optimum dopant
concentration is often low and the effective doping range is very
narrow [8]. OLEDs based on dopant are more difficult to adapt for
mass production in practice than those based on non-doped host
emitter. Red fluorescent materials as non-doped host emitters are
growingly attractive [9e11].
In this work, three red fluorescent dyes based on arylamino
fumaronitrile derivatives, bis(4-(N-(1-naphthyl)-phenylamino)phe
nyl)fumaronitrile(1-NPAFN,5a),bis(4-(N-(2-naphthyl)phenylamino)
*
Corresponding authors. Tel.: þ86 10 51688018; fax: þ86 10 51683933.
E-mail addresses: zhqhe@bjtu.edu.cn (Z. He), yshwang@bjtu.edu.cn (Y. Wang).
0143-7208/$ e see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2009.10.008

W. Zhang et al. / Dyes and Pigments 85 (2010) 86e92
87
phenyl)fumaronitrile (2-NPAFN, 5b) and bis(4-(N-(9,9-diethyl-2-flu-
orenyl)phenylamino)phenyl)fumaronitrile (EFPAFN, 5c), were pre-
pared. The steric hindrance and donor-electron behavior from the
naphthyl and fluorenyl groups contributed to restricting intermo-
lecular interaction in 2-NPAFN (5b) or EFPAFN (5c) and emitting
strongluminescence inaggregatesorsolid states. Photoluminescence
The crudeproductwas purifiedbyflash chromatographyonsilica gel
eluting with 1:2 hexanes/dichloro-methane, the solvent was
removed in vacuum. A pale white solid was obtained (11.1 g, 78.7%).
FT-IR (KBr pellet, cm ): 3411, 3045, 2961, 2926, 2872, 2856,
1599, 1506, 1454, 1414, 1374, 1352, 1312, 1263, 1242, 1181, 1137, 1074,
ꢁ1
1022, 864, 824, 757, 735, 691, 577, 505, 488, 462, 414.
1
(
PL), thermal stability and energy levels of the materials were inves-
tigated. The multilayer electroluminescent devices with 2-NPAFN
5b) orEFPAFN (5c) asnon-doped emitting layerswere fabricated and
characterized.
3
H NMR (400 MHz, CDCl ), d (ppm): 0.363e0.399 (6H),
13
1.937e2.047 (4H), 7.105e7.61 (12H). C NMR (CDCl
149.2, 129.3, 126.7, 125.8, 122.6, 120.5, 120.3, 118.6, 117.2, 117.1, 112.7,
55.8, 32.7, 8.48. MS: 314.3 (M þ 1) (calcd. for C23
3
),
d
¼ 151.4,
(
þ
H23N, 313.4).
2
. Experimental
2.2.4. Bis(4-bromophenyl)fumaronitrile (4) [25]
A mixture of 4-bromophenylacetonitrile (4.86 g, 24.8 mmol) and
2
.1. Materials
iodine (6.35 g, 25 mmol) was purged with N
2
and dry diethyl ether
ꢀ
(
100 ml) was injected via syringe. A solution was cooled to ꢁ78 C.
Fluorene, Pd(OAc)
2
, tris(tert-butyl)phosphine (P(t-Bu)
3
), bis(2-
-naph-
3
, sodium tert-butoxide
Sodium methoxide (2.84 g, 52.6 mmol) and methanol (40 ml) was
added slowly over a period of 30 min and stirred for 40 min. Then
the reaction solution was put to ice-water bath with mechanical
(diphenylphosphino)-phenyl)ether (DPEphos), N-phenyl-
a
thylamine, N-phenyl- -naphthylamine, Cs CO
b
2
ꢀ
were purchased from Acros Organics. Chemicals and solvents were
used as received in the highest commercially available grade unless
described otherwise. Diethyl ether, toluene, tetrahydrofuran (THF) was
refluxed with sodium and benzophenone, and purified with distilla-
tion. Dimethylsulfoxide (DMSO) was purified through distillation at
a reduced pressure.
stirring and kept at 0 C for a further 4 h. 3e6% hydrochloric acid
was added dropwise and the solution was filtered to isolate the
precipitate, which was rinsed with cold methanolewater solution.
Filtrate was concentrated further and a second crop of product was
obtained. A pale yellow solid was given in a yield of more than 90%.
ꢁ
1
FT-IR (KBr pellet, cm ): 3096, 2220, 1585, 1488, 1396, 1245,
074, 1007, 845, 816, 710, 665, 627, 573, 514.
1
1
2
2
.2. Synthetic procedures
3
H NMR (400 MHz, CDCl ), d (ppm):7.67e7.72 (m, 8H).
.2.1. 9,9-Diethylfluorene (1) [21]
2.2.5. Bis(4-(N-(substituted)phenylamino)phenyl)fumaronitrile
(5a, 5b, 5c) [15,26]
A synthetic procedure to 9,9-diethylfluorene (1) was similar to
that reported in reference [21], but with a diluted DMSO solution.
To a solution of fluorene (50 g, 0.30 mol) in DMSO (500 ml),
powdered potassium hydroxide (84 g, 1.5 mol) and potassium
iodide (2.4 g, 0.014 mol) were added with mechanical stirring in
cold water bath. Bromoethane (64 ml, 0.84 mol) was added over
a period of 50 min. The mixture was stirred at 10 C overnight. It
was then diluted with water (800 ml) and extracted with
dichloromethane (300 ml, twice). The organic layer was dried over
A mixture of bis(4-bromophenyl)fumaronitrile (0.787 g, 2.03
mmol), N-phenyl-substituted amine (4.47 mmol), Cs
5.98 mmol), toluene (13 ml) was degassed and purged with N
(t-Bu) catalyst had been prepared previously from Pd(OAc)
(0.0135 g, 0.06 mmol), P(t-Bu) (0.36 g, 0.178 mmol) and toluene
CO
2 3
(1.95 g,
2
. Pd/P
3
2
3
ꢀ
(4 ml) and added in glove box. The reaction mixture was degassed,
ꢀ
purged with nitrogen again and heated at 110 C for 24 h. The mixture
was cooled to room temperature, water (30 ml) and dichloromethane
(50 ml) were added accordingly. An organic layer was separated and
washed with brine, dried over anhydrous MgSO and evaporated to
4
MgSO
4
. Volatiles were removed and distilled under reduced pres-
ꢀ
sure to afford pale yellow oil. Yield 57.7 g (86.4%). mp 29e30 C. MS:
þ
2
23.03 (M þ 1) (calcd. for C17
H18, 222.14).
dryness under reduced pressure. The crude product was purified by
column chromatography on silica gel using 1:1 light petroleum/
dichloromethane as eluant, affording solid (5a, 5b, 5c).
Bis(4-(N-(1-naphthyl)phenylamino)phenyl)fumaronitrile (1-NP
AFN, 5a), an orange red solid, 41%.
2.2.2. 2-Bromo-9,9-diethylflurene (2) [22]
To a solution of 9,9-diethylfluorene (20 g, 0.09 mol) in dichloro-
methane (120 ml), a solution of bromine (5.6 ml) in dichloro-
methane (30 ml) was added under stirring at room temperature. The
mixture kept stirring for 4 h and was washed with water (300 ml),
dried and concentrated. The crude product was distilled at
FT-IR (KBr pellet, cmꢁ1): 3058, 2954, 2921, 2216, 1604, 1587,
1506, 1490, 1393, 1313, 1296, 1271, 1186, 1074, 1010, 827, 800, 775,
754, 696,
ꢀ
ꢀ
1H NMR (400 MHz, CDCl
1
56e163 C/2 mm (17.2 g, 63.4%). m.p. 52e53 C.
3
), d (ppm): 6.89e6.92 (d, 4H), 7.05e7.12
ꢁ1
FT-IR (KBr pellet, cm ): 2960, 2912, 2873, 2846, 1601, 1570,
440, 1406, 1375, 1257, 1110, 1060, 1002, 873, 819, 781, 763.
H NMR (400 MHz, CDCl
.98e2.03 (m, 4H), 7.32e7.34 (s, 3H), 7.44e7.46 (m, 2H), 7.55e7.57
d, 1H), 7.66e7.68(d, 1H).
(t, 2H), 7.19e7.24 (m, 4H), 7.27e7.34 (m, 4H), 7.36e7.44 (m, 4H),
7.45e7.54 (m, 4H), 7.58e7.65 (d, 4H), 7.80e7.85 (d, 2H), 7.86e7.94
1
1
3
),
d
(ppm):0.30e0.34 (m, 6H),
32 4
(m, 4H). Elemental analysis: calcd. for C48H N : C, 86.72; H, 4.85;
1
(
N, 8.43, found C, 86.41; H, 5.14; N, 8.50.
Bis(4-(N-(2-naphthyl)phenylamino)phenyl)fumaronitrile (2-NP
AFN, 5b), a red solid, 78.3%.
2.2.3. 2-(Phenylamino)-9,9-diethylfluorene (3) [23,24]
FT-IR (KBr pellet, cmꢁ1): 3057, 2958, 2923, 2216, 1627, 1588,
1506, 1489, 1465, 1357, 1296, 1265, 1253, 1188, 1025, 937, 854, 815,
To a three-necked flask aniline (54 mmol), Palladium acetate
(
0.22 mmol), DPEphos (0.337 mmol), 13.5 g 2-bromo-9,9-diethyl-
754, 696, 684, 665.
1
fluorene (45 mmol) and 18 ml of toluene were added. The mixture
was deoxygenated. After stirring for 20 min sodium tert-butoxide
(
nitrogen, and the mixture was heated with stirring to 85 C for 24 h.
The mixture was cooled to room temperature, quenched by the
addition of water (45 ml), extracted with dichloromethane (45 ml),
and washed with brine. The organic layer was dried over anhydrous
3
H NMR (400 MHz, CDCl ), d (ppm): 7.09e7.13 (d, 4H), 7.13e7.18
(m, 4H), 7.19e7.23 (d, 4H), 7.26e7.37 (m, 8H), 7.39e7.46 (m, 4H),
7.65e7.73 (m, 4H), 7.78e7.83 (d, 4H). Elemental analysis: calcd. for
6.1 g, 63 mmol) was added in oneportion. The flaskwaspurgedwith
ꢀ
C
48
H
32
N
4
: C, 86.72; H, 4.85; N, 8.43, found C, 86.51; H, 5.11; N, 8.52.
Bis(4-(N-(9,9-diethyl-2-fluorenyl)phenylamino)phenyl)fumaro-
nitrile (EFPAFN, 5c), a red solid, 82.8%.
FT-IR (KBr pellet, cmꢁ1): 3037, 2963, 2921, 2215, 1600, 1589,
1508, 1489, 1451, 1328, 1304, 1265, 1188, 1025, 827, 755, 737, 702.
4
MgSO , filtered. The solvent was removed under reduced pressure.

88
W. Zhang et al. / Dyes and Pigments 85 (2010) 86e92
1H NMR (400 MHz, CDCl
.91e1.99 (m, 4H), 7.07e7.17 (m, 10H), 7.18e7.24 (d, 4H), 7.27e7.36
m, 10H), 7.62e7.67 (m, 4H), 7.68e7.73 (d, 4H).
Elemental analysis: calcd. for C62 : C, 87.29; H, 6.14; N, 6.57,
found C, 87.16; H, 6.28; N, 6.68.
),
d
(ppm): 0.339e0.375 (t, 12H),
measured using a Keithley 2410 source meter. Electroluminescence
3
1
(
(EL) spectra were measured by using SPEX Fluolog-3 luminescence
spectrometer or Acton CCD spectrometry. Device performances
were measured at room temperature under ambient atmosphere.
52 4
H N
3. Results and discussions
2.3. Measurement
3
.1. Synthesis and characterization
Compounds synthesized were characterized using elemental
1
analysis (Carlo Erba 1106), FT-IR (Shimadzu FT-IR 8400) and
H
Synthetic routes to the three arylamino fumaronitrile deriva-
NMR spectroscopy (Bruker DMX-300), respectively. Mass Spec-
trometry analysis was measured on a Shimadzu LC-MS2010A.
Cyclic voltammetry (CV) analyses were carried out by using Zahner
Zennium. Thermogravimetric analysis (TGA) was performed on
a Netzsch TG-209 under nitrogen flow. UVevis absorption spec-
trum was obtained by using GBC Cintra 303 Spectrometer. Photo-
luminescence (PL) and excitation spectrum were recorded with
Perkin Elmer LS-55 luminescence spectrometer.
tives based red dyes and the chemical structures were illustrated in
Fig. 1. The diarylamine 2-(phenylamino)-9,9-diethylfluorene was
synthesized with an efficient catalyst combination of Pd(OAc) /
2
DPEphos in presence of sodium tert-butoxide as base. For the ary-
lation of primaryanilines byaryl bromides, DPEphos was as effective
a ligand as rac-2,2 -bis(diphenylphosphino)-1,1 -binaphthyl (rac-
BINAP) and considerably more effective than 1,1 -bis(diphenyl-
phosphino)ferrocene (DPPF) [23]. Three diarylamines, where were
substituted with N-1-naphthyl, or N-2-naphthyl or N-9,9-diethyl-2-
fluorenyl groups, reacted with bis(4-bromophenyl)fumaronitrile in
0
0
0
2
.4. Device fabrications and testing
2 3
presence of Pd(OAc) /P(t-Bu) and cesium carbonate as base to
prepare corresponding dyes based on triarylamino fumaronitrile
derivatives 1-NPAFN (5a), 2-NPAFN (5b), and EFPAFN (5c), respec-
3
tively. The bulky and electron-rich P(t-Bu) , which accelerated
elimination reactions, was widely used in preparation of triaryl-
amines via palladium-catalyzed couplings with high yields [26].
0
Compounds 4,4 -bis(N-(1-naphthyl-N-phenylamino)biphenyl)
(
NPB), 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP), 1,3,5-
tris(N-phenyl-benzimidazol-2-yl)benzene (TPBi), LiF and Al were
purchased from Acros Organics. NPB was used as hole-transporting
layer, BCP as electron-transporting and hole-blocking layer, TPBi as
electron-transporting layer, LiF/Al as cathode. 2-NPAFN and EFPAFN
were used as the non-doped host emitter in the fabrication of red
OLEDs. Indium tin oxide (ITO) glass substrate with a sheet resistance
3.2. Thermal stability
25 U/, was (1) cleaned with detergents, deionized water, ethanol
and isopropanol in an ultrasonic bath, (2) dried under nitrogen flow,
and (3) treated with oxygen plasma for 5 min before use. The three-
layer devices with configurations of device A ITO/2-NPAFN or
EFPAFN (30 nm)/BCP (10 nm)/TPBi (30 nm)/LiF (0.5 nm)/Al and four-
layer devices with configurations of device B ITO/NPB (40 nm)/
Thermal stabilities of 1-NPAFN (5a), 2-NPAFN (5b), EFPAFN (5c)
were determined using thermogravimetric analysis (TGA) in
nitrogen atmosphere (Fig. 2). Samples were heating up at a rate of
ꢀ
ꢀ
10 C/min in a temperature range of 25e600 C with a nitrogen
flow. The starts of decompositions of 5a, 5b and 5c were observed
ꢀ
2
-NPAFN or EFPAFN (30 nm)/BCP (10 nm)/TPBi (30 nm)/LiF (0.5 nm)/
at around 409, 428 and 422 C. The rates of weights remained for
ꢀ
ꢀ
Al were fabricated by sequential vacuum deposition of organic
materials on pre-treated ITO. Currentevoltage characteristic was
the samples were 0% at 485 C for 5a, 0% at 491 C for 5b and 21% at
ꢀ
473 C for 5c. Stabilities were improved for 2-NPAFN (5b) and
H
N
Br
iii
i
ii
1
2
3
Br
CN
iv
CN
Br
v
R
N
Br
R
N
NC
NC
R=
NC
5a, 5b, 5c
4
,
,
5a
5b
5c
Fig. 1. A schematic diagram for chemical structures of intermediates and three arylamino fumaronitrile derivatives. Reagents and conditions are: (i) CH
3
CH
2
Br, KOH, DMSO, cold
ꢀ
water bath, over 2.5 h; (ii) Br
2
, CH
, CH
CO
2
Cl
2
, rt; (iii) Aniline, Pd(OAc)
2
, DPEphos(bis(2-(diphenylphosphino)phenyl)ether), toluene, sodium tert-butoxide, purged with nitrogen, 85 C, 24 h.
ꢀ
ꢀ
(
(
iv) I
OAc)
2
, diethyl ether, NaOCH
, P(t-Bu) , toluene, Cs
3
2
3
OH; 30 min at ꢁ78 C; 5 h at 0 C; (v) N-phenyl-
a-naphthylamine or N-phenyl-b-naphthylamine or N-phenyl-9,9-diethyl-2-fluorenylamine), Pd
ꢀ
2
3
3
, 110 C, 18 h.

W. Zhang et al. / Dyes and Pigments 85 (2010) 86e92
89
100
1.0
T(5b) 428
5a(THF)
5a(solid)
5b(THF)
5b(solid)
5c(THF)
5c(solid)
T(5a) 409
T(5c) 422
Temperatures of obviously
endothermic character (T)
5a
5b
5c
80
60
40
20
0
0.8
0
0
0
0
.6
.4
.2
.0
remaining weight
rate 21%(473
)
0
%(485
)
0%(491
)
100
200
300
400
500 600
550
600
650
700
750
800
Temperature(
)
Wavelength (nm)
Fig. 2. TGA curves of 1-NPAFN (5a), 2-NPAFN (5b), EFPAFN (5c).
Fig. 4. Emission spectra of 5a, 5b, 5c in solid film and dilute THF solution.
EFPAFN (5c). It was expected that these dyes might perform better
in their device applications.
EFPAFN (5c) resulted in red shifts in both absorption and emission
peaks in solution compared to N-1-naphthyl substitute in 1-NPAFN
(
5a). Bulky groups in both 5b and 5c contributed to reducing the
3
.3. Optoelectronic properties
molecular interaction, which minimized aggregation and hence the
shifts in emissions in solid state.
UVevis absorption spectra of 1-NPAFN (5a), 2-NPAFN (5b) and
EFPAFN (5c) in solution were investigated as shown Fig. 3.
Absorption from 5a, 5b and 5c peaked at 288, 305 and 326 nm in
high energy range and at 471, 487 and 497 nm in low energy range
respectively. The replacement of N-2-naphthyl and N-9,9-diethyl-
3
.4. Electrochemical properties
Energy levels (e.g. HOMO, LUMO) of 1-NPAFN (5a), 2-NPAFN (5b),
EFPAFN (5c) (ferrocene shown in the inset) were investigated via
redox reactions using cyclic voltammetry (CV) [27] (Fig. 5).
Dichloromethane was used as a solvent in presence of tetra-n-buty-
lammonium hexafluorophosphate (TBAPF
electrolyte. A Hg/Hg Cl electrode was used as the reference elec-
trode. A glass carbon rod was used as the working electrode, a plat-
inum wire was used as the counter electrode. Ferrocene was used as
a reference material. The first onset oxidation potentials (Eonset) were
2
-fluorenyl groups to the fumaronitriles extended the p-conjuga-
tion and resulted in bathochromic shifts in main absorption
wavelength compared to N-1-naphthyl in 1-NPAFN (5a).
Photoluminescence (PL) of 1-NPAFN (5a), 2-NPAFN (5b) and
EFPAFN (5c) in solid and solution were measured and shown in
Fig. 4. Emission spectra of 1-NPAFN (5a), 2-NPAFN (5b) and EFPAFN
4
) (0.1 mol/L) as supporting
2
2
(5c) in THF solution were generally broad and centered at 616, 650
and 657 nm respectively. PL spectra from 5a, 5b and 5c in solid state
peaked at 635, 650 and 658 nm respectively, in which emission of
1
.03 eV for 1-NPAFN (5a), 0.95 eV for 2-NPAFN (5b) and 0.87 eV for
EFPAFN (5c). As compared to NPAEF (2,7-bis(N- -naphthyl-N-
phenyl-amino)-9,9-diethylfluorine) in our previous work [28], the
onset of NPAEF was 0.55 V, which indicated that 5a, 5b and 5c were
more difficult to be oxidized. Eonset of the red dyes decreased
1-NPAFN (5a) > 2-NPAFN (5b) > EFPAFN (5c)) in accordance with
the increasing of electron-donating ability of the substituent from
a
5
a showed a 20 nm bathochromic shift from solution to solid while
emissions of 5b and 5c were almost unchanged in different media.
The incorporation of N-2-naphthyl and N-9,9-diethyl-2-fluo-
renyl, bulky and electron-donating groups, in 2-NPAFN (5b) and
E
(
1-naphthyl, 2-naphthyl to fluorenyl groups at the nitrogen atoms.
5
5
5
a
b
c
HOMO levels (EHOMO) were therefore calculated to be ꢁ5.47 eV
1.0
0.8
0.6
0.4
0.2
0.0
for 1-NPAFN (5a), ꢁ5.39 eV for 2-NPAFN (5b) and ꢁ5.31 eV for
EFPAFN (5c) respectively. LUMO levels (ELUMO) of the dyes were
estimated in combination with optical band gap, in which the latter
g
was taken from absorption edge (E ), to be 2.17 eV (571 nm) for 1-
NPAFN (5a), 2.12 eV (584 nm) for 2-NPAFN (5b) and 2.05 eV
(606 nm) for EFPAFN (5c). ELUMO was calculated according to:
E
LUMO ¼ EHOMO þ E , i.e. ꢁ3.30 eV for 1-NPAFN (5a), ꢁ3.27 eV for 2-
g
NPAFN (5b) and ꢁ3.26 eV for EFPAFN (5c). It can also be seen from
above that incorporation of the group affect more strongly to
HOMO level and less to LUMO level. The EHOMO and ELUMO of NPAEF
were calculated to be ꢁ5.15 and ꢁ2.36 eV respectively [28], shifted
upwards compared to 5a, 5b and 5c. The changes in energy levels
may be related to the electron-withdrawing butenedinitrile group.
3.5. Performance in electroluminescent devices
2
00
300
400
500
600
Wavelength (nm)
Electroluminescence of 2-NPAFN (5b) and EFPAFN (5c) were
Fig. 3. UVevis absorption spectra of 5a, 5b, 5c in a dilute THF solution.
investigated. They were used as non-doped emitter in multilayer

90
W. Zhang et al. / Dyes and Pigments 85 (2010) 86e92
-
6
6
6
6
8
6
4
2
.0x10
-
6
6
6
6
ferrocene
8
6
.0x10
-
6
-
4.0x10
.0x10
-
-
4.0x10
.0x10
-
2
.0x10
-6
3
2
.0x10
0
.0
-
6
-
-2.0x10
.0x10
-6
-
4.0x10
-6
.0x10
-
6
-
6.0x10
-
0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
-
.0x10
Potential/V
-
6
1
0
.0x10
0
.0
.0
-
6
-
6
-
-
2.0x10
-
1.0x10
1
-NPAFN(5a)
2-NPAFN(5b)
-
6
-6
4.0x10
-2.0x10
EFPAFN(5c)
0
.0
0.4
0.8
1.2
1.6
Potential/V
Fig. 5. The cyclic voltammograms of 1-NPAFN (5a), 2-NPAFN (5b), EFPAFN (5c) (ferrocene shown in the inset) at the concentration 1 ꢂ10ꢁ3 M in CH
2 2
Cl . The scan rate was 50 mV/s.
OLEDs. Four devices were prepared with structures as follows: Device
A1: ITO/2-NPAFN (30 nm)/BCP (10 nm)/TPBi (30 nm)/LiF (0.5 nm)/Al;
Device B1: ITO/NPB (40 nm)/2-NPAFN (30 nm)/BCP (10 nm)/TPBi
a barrier (>1.1 eV) for holes and blocked the holes to migrate from
2-NPAFN or EFPAFN to BCP. In contrast, the LUMO energy levels of
2-NPAFN (ꢁ3.27 eV) and EFPAFN (ꢁ3.26 eV) were below those of
NPB (ꢁ2.3 eV) and BCP (ꢁ3.2 eV). It is feasible for electron to
transport from BCP to the 2-NPAFN or EFPAFN. Hence the recom-
bination took place in the 2-NPAFN or EFPAFN emitting layer.
EL spectra of these devices were measured as shown in Fig. 6.
The devices were stable with respect to driving voltage. Emissions
(30 nm)/LiF (0.5 nm)/Al; Device A2: ITO/EFPAFN (30 nm)/BCP
(10 nm)/TPBi (30 nm)/LiF (0.5 nm)/Al; and Device B2: ITO/NPB
(40 nm)/EFPAFN (30 nm)/BCP (10 nm)/TPBi (30 nm)/LiF (0.5 nm)/Al.
The HOMO energy levels of 2-NPAFN (ꢁ5.39 eV) and EFPAFN
ꢁ5.31 eV) lay much more above that of BCP (ꢁ6.7 eV). It formed
(
1
0
0
0
0
0
.0
.8
.6
.4
.2
.0
b 1.0
a
1
1
1V
3V
13V
15V
17V
15V
7V
0.8
0.6
0.4
0.2
0.0
1
(2-NPAFN) with NPB
Devide B1
(
2-NPAFN) without NPB
Devide A1
3
00
400
500
600
700
800
300
400
500
600
700
800
Wavelength (nm)
Wavelength (nm)
1
.0
1.0
0.8
0.6
0.4
0.2
0.0
c
d
1
1
17V
3V
5V
11V
13V
15V
17V
0
0
0
0
0
.8
.6
.4
.2
.0
(
EFPAFN) without NPB
Devide A2
(EFPAFN) with NPB
Devide B2
3
00
400
500
600
700
800
300
400
500
600
700
800
Wavelength (nm)
Wavelength (nm)
Fig. 6. EL spectra of Device A1, B1, A2 and B2.

W. Zhang et al. / Dyes and Pigments 85 (2010) 86e92
91
from Device A1 and Device B1 had their maxima at around 664 nm
and 666 nm, red-shifted w15 nm compared to its PL emission
injection layer performed better. Their luminance maxima were
higher than those from Device A1 and A2 at same operating
voltage.
(
650 nm) in the solid state. Emissions from Device A2 and Device B2
peaked at around 680 nm and 677 nm, red-shifted w 20 nm
compared to its PL emission (658 nm) in the solid film. Both three-
and four-layer devices showed pure red EL from 2-NPAFN or
EFPAFN, indicating that recombination was located in 2-NPAFN or
EFPAFN layer. The difference between PL and EL revealed that there
was a certain degree of aggregation formation of 2-NPAFN or
EFPAFN providing charge-trapped sites with lower energy. Upon
increasing driving voltages, EL spectra were almost unchanged
with a slight blue-shift when voltage bias increases from 11 V to
Current efficiency or luminance efficiency (LE) and power effi-
ciency (PE) vs current density characteristics of Device A1, B1, A2
and B2 were given in Fig. 8. Maxima of current efficiency and power
efficiency were 0.18 cd/A and 0.057 lm/W for Device A1, 1.34 cd/A
and 0.42 lm/W for Device B1, 0.51 cd/A and 0.11 lm/W for Device
A2, 2.27 cd/A and 0.72 lm/W for Device B2 respectively. Device B1
and B2 improved both brightness and efficiency, indicating a better
balance in charge injection with NPB hole injection layer, which
ensured the recombination occurred within the emitting layer. A
red EL was achieved with high intensity. This demonstrated the
material property played an important role in determining the
performance of the device. The steric hindrance in the naphthyl and
fluorenyl groups restricted intermolecular interaction, where steric
hindrance and donor-electron behavior were more effective for
EFPAFN than that of 2-NPAFN. This suppressed fluorescence
concentration quenching in solid state, which contributed to the
high brightness and efficiency.
17 V. This is important to provide red non-doped OLEDs with
improved performance. Red OLEDs with dopants reported to show
color changed with increasing driving voltage [29].
Current densityevoltage and luminanceevoltage characteristics
of the devices were given in Fig. 7. At a fix current density of 20 mA/
2
cm , corresponding driving voltages for Device A1, B1, A2 and B2
were 10.7, 14.0, 12.2 and 13.2 V respectively. At a given voltage, the
current density of the four-layer device (Device B1, Device B2) was
lower than that of the three-layer device (Device A1, Device A2).
Maximum luminances of Device A1, B1, A2 and B2 were
measured to be 650, 1420, 1290 and 3370 cd/m2 respectively at
operating voltages between 20 and 21 V. Turn-on voltages of the
devices were defined as the voltage needed to deliver a brightness
More interestingly devices A1 and A2 appeared to have constant
current or power efficiencies when current density was varying in
2
the rage of 20e150 mA/cm . The current efficiency and power
efficiency of four devices exhibited a small decline even when
2
current density increased to 150 or 300 mA/cm . These character-
2
of 1 cd/m , which were 8V, 6.1V, 8.4V and 5.9V for Devices A1, B1,
istics primarily indicated that the dyes were promising candidates
A2 and B2 respectively. Device B1 and B2 containing NPB as hole
for efficient red non-doped light-emitting materials.
600
500
400
300
200
100
0
a
1.4
1.2
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
a
2
2
-NPAFN in Device A1
-NPAFN in Device B1
EFPAFN in Devide A2
EFPAFN in Devide B2
1
0
.0
.8
2-NPAFN in Device A1 (LE)
2
2
2
-NPAFN in Device A1 (PE)
-NPAFN in Device B1 (LE)
-NPAFN in Device B1 (PE)
0.6
0
0
0
.4
.2
.0
0
20
40
60
80
100
120
140
0
2
4
6
8
10 12 14 16 18 20
2
Current Density (mA/cm)
Voltage (V)
2
2
1
1
.5
.0
.5
.0
2.5
2.0
1.5
1.0
0.5
0.0
b
b
2
2
-NPAFN in Device A1
-NPAFN in Device B1
3
500
000
500
000
500
000
EFPAFN in Devide A2 (LE)
EFPAFN in Devide A2 (PE)
EFPAFN in Devide B2 (LE)
EFPAFN in Devide B2 (PE)
EFPAFN in Devide A2
EFPAFN in Devide B2
3
2
2
1
1
0.5
5
00
0
0
.0
0
50
100
150
200
250
300
0
2
4
6
8
10 12 14 16 18 20
2
Current Density (mA/cm)
Voltage (V)
Fig. 7. Current density and luminance vs voltage characteristics.
Fig. 8. Current efficiency and power efficiency vs current density characteristics.

9
2
W. Zhang et al. / Dyes and Pigments 85 (2010) 86e92
[7] Chen BJ, Sun XW, Lin XQ, Lee CS, Lee ST. Improved luminescent efficiency of
4
. Conclusions
a red organic dye with modified molecular structure. Materials Science and
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electroluminescent materials including perylene moiety. Synthetic Metals
Diarylamine 2-(phenylamino)-9,9-diethylfluorene were synthe-
[
[
2
sized by Pd(OAc) /DPEphos. Three novel red dyes based on aryla-
mino fumaronitrile derivatives, 1-NPAFN, 2-NPAFN and EFPAFN,
as red organic non-doped electroluminescent materials in the
2000;111e112:57e61.
presence of Pd(OAc)
tion temperatures of 1-NPAFN, 2-NPAFN, EFPAFN were determined
at about 409, 428, 422 C respectively. Photoluminescent emissions
2
/P(t-Bu)
3
were prepared. Thermal decomposi-
[10] Thomas KRJ, Lin JT, Tao YT, Chuen CH. Star-shaped thione-[3,4-b]-pyrazines:
a new class of red-emitting electroluminescent materials. Advanced Materials
2002;14:822e6.
ꢀ
[
[
11] Palayangoda SS, Cai X, Adhikari RM, Neckers DC. Carbazole-based donor-
from 1-NPAFN, 2-NPAFN, EFPAFN centered at 616, 650, 657 nm
in THF solution and at 635, 650, 658 nm in solid state. HOMO
and LUMO energy levels were ꢁ5.47, ꢁ3.30 eV for 1-NPAFN, ꢁ5.39,
acceptor compounds: highly fluorescent organic nanoparticles. Organic Letters
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12] Huang TH, Lin JT, Tao YT, Chuen CH. Benzo[a]aceanthrylene derivatives for
red-emitting electroluminescent materials. Chemistry of Materials
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ꢁ
3.27 eV for 2-NPAFN and ꢁ5.31, ꢁ3.26 eV for EFPAFN. Three-
[
13] Kim DU, Paik SH, Kim SH, Tsutsui T. Design and synthesis of a novel red
electroluminescent dye. Synthetic Metals 2001;123:43e6.
14] Yeh HC, Yeh SJ, Chen CT, Chen CT. Readily synthesized arylamino fumaronitrile
for non-doped red organic light-emitting diodes. Chemical Communications
2003:2632e3.
layered (A1, A2) and four-layered (B1, B2), with additional NPB layer,
electroluminescent devices were fabricated using 2-NPAFN and
EFPAFN as non-doped emitting layers. All devices emitted red with
[
2
peak luminance 650e1420 cd/m for 2-NPAFN and 1290e3370 cd/
2
[15] Thomas KRJ, Lin JT, Velusamy M, Tao YT, Chuen CH. Color tuning in benzo[1,2,5]
thiadiazole-based small molecules by amino conjugation/ deconjugation: bright
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m for EFPAFN. EFPAFN material and four-layered devices improved
device performance. Additionally, devices with 2-NPAFN as emitting
layer showed a relatively constant efficiency within the current
density range of 20e150 mA/cm and with almost no color changes.
The results were exciting towards red organic non-doped light-
emitting devices.
2
[
17] Wu WC, Yeh HC, Chan LH, Chen CT. Red organic light-emitting diodes with
a non-doping amorphous red emitter. Advanced Materials 2002;14:1072e5.
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isophorone dyes for high efficiency dye-sensitized solar cells: tuning photo-
[
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acceptor system. Chemical Communications 2009:1766e8.
p-
Acknowledgements
[19] Chen CT. Evolution of red organic light-emitting diodes: materials and devices.
Chemistry of Materials 2004;16:4389e400.
This work was funded by the NSFC Grants (60825407, 20674004,
0776039), ISTCP (2008DFA61420), the 111 project (B08002), the
Beijing Commission of Education Grants (KM200710015009,
KM2010), and also grant from Laboratory of Printing and Packaging
Material & Technology-Beijing Area Major Laboratory Project
[
20] Yeh SJ, Chen HY, Wu MF, Chan LH, Chiang CL, Yeh HC, et al. All non-dopant
red-green-blue composing white organic light-emitting diodes. Organic
Electronics 2006;7:137e43.
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p-electron acceptors. Chemistry of
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methanes. Part _. Electronic absorption spectra of 9,9-dimethylfluorene
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