Efficient blue emitters based on 1,3,5-triazine for nondoped organic light emitting diode applications

Article abstract of DOI:10.1016/j.orgel.2012.06.013

Two novel efficient blue emitters (TTT-1, TTT-2) containing 1,3,5-triazine, thiophene and triphenylamine have been designed and synthesized. Organic light emitting diodes (OLEDs) using these new triazine derivatives as emissive layers, ITO/TAPC (60 nm)/TTT-1 (Device A) or TTT-2 (Device B) (40 nm)/TPBi (60 nm)/LiF (1 nm)/Al (100 nm), were fabricated and tested. The OLEDs exhibited good performances with low turn-on voltage of 3 V, maximum luminance of ca. 8990 cd/m² for TTT-1 and 15,980 cd/m² for TTT-2, and maximum luminance efficiency of 4.7 cd/A for TTT-1 and 4.0 cd/A for TTT-2, respectively.

Full text of DOI:10.1016/j.orgel.2012.06.013

Organic Electronics 13 (2012) 2177–2184
Contents lists available at SciVerse ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Efficient blue emitters based on 1,3,5-triazine for nondoped organic
light emitting diode applications
Jian Liu a, Ming-Yu Teng a, Xiao-Peng Zhang a, Kai Wang a, Cheng-Hui Li a,b,
,
⇑
a,b,
You-Xuan Zheng a, , Xiao-Zeng You
⇑
⇑
a State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing National Laboratory of Microstructures,
Nanjing University, Nanjing 210093, PR China
b Academician Work Station of Changzhou Trina Solar Energy Co. Ltd., Changzhou 213022, Jiangsu Province, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Two novel efficient blue emitters (TTT-1, TTT-2) containing 1,3,5-triazine, thiophene and
triphenylamine have been designed and synthesized. Organic light emitting diodes
(OLEDs) using these new triazine derivatives as emissive layers, ITO/TAPC (60 nm)/TTT-1
(Device A) or TTT-2 (Device B) (40 nm)/TPBi (60 nm)/LiF (1 nm)/Al (100 nm), were fabri-
cated and tested. The OLEDs exhibited good performances with low turn-on voltage of
3 V, maximum luminance of ca. 8990 cd/m2 for TTT-1 and 15,980 cd/m2 for TTT-2, and
maximum luminance efficiency of 4.7 cd/A for TTT-1 and 4.0 cd/A for TTT-2, respectively.
Ó 2012 Elsevier B.V. All rights reserved.
Received 25 February 2012
Received in revised form 21 May 2012
Accepted 2 June 2012
Available online 25 June 2012
Keywords:
OLED
Nondoped
Triazine
Triphenylamine
Thiophene
1. Introduction
ing to low quantum efficiency of the device [13,14].
Therefore, new materials with nonplanar structures (to
Organic light-emitting diodes (OLEDs) have attracted
much attention due to their promising applications in
full-color display panels, flexible displays, and lighting
sources [1–7]. Over the past decades, numerous families
of fluorescent organic compounds have been studied as
the emitting materials in OLEDs, with the aim to optimize
the device performance through modifying their conjuga-
tion, morphology, and electron/hole mobility [8–12]. How-
ever, many fluorescent organic compounds used in OLEDs
suffer from ‘‘concentration quenching’’ effect in solid state
and unbalanced carrier mobility (the hole mobility is gen-
erally several orders higher than electron mobility), lead-
prevent orderly molecular packing and consequently alle-
viate the fluorescence quenching) and high electron trans-
porting capability (to balance the carrier mobility) are
crucial in creating high efficient OLEDs [15].
In this study, we designed and synthesized new fluores-
cent organic compounds (TTT-1, TTT-2) containing tri-
azine, triphenylamine and thiophene building blocks. The
1,3,5-triazine unit is a typical electron-accepting unit and
have been frequently incorporated into the backbone of
conjugated compounds to improve their electron-injection
and electron-transportation abilities [16–27]. Introduction
of the bulky and nonplanar triphenylamine moiety has
been shown to be effective in preventing orderly molecular
packing, improving the glass-transition temperature (Tg)
and increasing the hole-transporting properties [28]. The
thiophene unit is popularly used in optoelectronic materi-
als due to its remarkable optic, electronic and redox prop-
⇑
Corresponding authors. Address: State Key Laboratory of Coordina-
tion Chemistry, School of Chemistry and Chemical Engineering, Nanjing
National Laboratory of Microstructures, Nanjing University, Nanjing
210093, PR China.
erties [29].
A combination of these three functional
E-mail addresses: chli@nju.edu.edu (C.-H. Li), yxzheng@nju.edu.cn
(Y.-X. Zheng), youxz@nju.edu.cn (X.-Z. You).
moieties is rarely reported in the literature and would be
1566-1199/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.orgel.2012.06.013

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J. Liu et al. / Organic Electronics 13 (2012) 2177–2184
effective to construct new fluorescence emitters with good
carrier-transport property and high solid-state emission
efficiency. As expected, non-doped devices employing our
new fluorescent organic compounds as emitters display
very competitive performances at low operating voltage.
Particularly, to the best of our knowledge, the perfor-
mances of OLED device based on TTT-2 are among the best
ever reported for blue OLEDs based on triazine derivatives
either with or without a dopant.
nard solution was added dropwise to a solution of cyanuric
chloride (0.61 g, 3.3 mmol) in anhydrous THF (20 mL) at 0–
10 °C. The mixture was stirred for 10 h at 50 °C and then
poured into ice water (50 mL) and extracted twice with
CH2Cl2 (100 mL). The organic phases were combined,
washed with water and brine, dried over anhydrous sodium
sulfate, and then concentrated. The residue was purified by
flash chromatography on silica gel (hexane/EA 20:1) to
afford 1 (0.6 g, 65%) as a white solid. mp 149–151 °C; IR
(KBr)
m ;
732, 810, 1257, 1421, 1473, 1521 cmꢀ1 1H NMR
(CDCl3, 500 MHz): d 8.28 (d, J1 = 4.0 Hz, J2 = 1.0 Hz, 2H),
7.71 (d, J1 = 5.0 Hz, J2 = 1.0 Hz, 2H), 7.24 (d, J1 = 5.0 Hz,
J2 = 4.0 Hz, 2H); 13C NMR (CDCl3, 500 MHz) d 169.06,
160.33, 139.65, 134.06, 133.28, 128.81. FAB (m/z): 279
(M+). Anal. Calcd for C11H6ClN3S2: C, 47.22; H, 2.16; N,
15.02. Found: C, 47.12; H, 2.21; N, 14.98.
2. Experimental section
2.1. Materials
All reactions were performed under argon and were
magnetically stirred. THF was distilled from sodium and
benzophenone prior to use. Commercially available re-
agents were used without further purification unless
otherwise stated. Column chromatography was carried
out using flash silica gel (300–400 mesh). The chemical
structures and synthetic routes of TTT-1 and TTT-2 are
shown in Scheme 1.
2.2.2. Synthesis of 2,4-dichloro-6-(thiophen-2-yl)-1,3,5-
triazine (2)
This compound was prepared similarly to 1 except that
the ratio of 2-bromothiophene to cyanuric chloride is 1.5
while the Grignard reaction temperature is 25 °C. Com-
pound 2 was obtained as a white solid in 68% yield. mp
152–153 °C; IR (KBr)
m
787, 847, 1030, 1248, 1335, 1420,
8.30 (d,
2.2. Synthetic procedure
1508 cmꢀ1 1H NMR (CDCl3, 500 MHz):
;
d
J1 = 4.0 Hz, J2 = 1.0 Hz, 1H), 7.80 (d, J1 = 5.0 Hz, J2 = 1.0 Hz,
2.2.1. Synthesis of 2-chloro-4,6-di(thiophen-2-yl)-1,3,5-
triazine (1)
1H), 7.25 (d, J1 = 5.0 Hz, J2 = 4.0 Hz, 1H); 13C NMR (CDCl3,
500 MHz)
d 171.63, 170.19, 137.98, 136.17, 135.09,
A solution of 2-bromothiophene (1.63 g, 10 mmol) in
anhydrous THF (20 mL) was added dropwise to a suspen-
sion of iodine-activated magnesium (0.29 g, 12 mmol) in
anhydrous THF (10 mL) over 30 min. After complete addi-
tion, the reaction mixture was stirred and refluxed for 3 h
and then cooled to room temperature. The resulting Grig-
129.30. FAB (m/z): 230.9 (M+). Anal. Calcd for C7H3Cl2N3S:
C, 36.23; H, 1.30; N, 18.11. Found: C, 36.17; H, 1.25; N,
18.15.
2.2.3. Synthesis of TTT-1
To a stirred solution of compound 1 (0.558 g, 2 mmol) in
1,4-dioxane (60 mL) was added 4-diphenylaminophenylbo-
ronic acid (0.867 g, 3 mmol), potassium carbonate (1.24 g,
9 mmol), H2O (15 mL), and Pd(dppf)Cl2 (0.080 g, 0.1 mmol).
The resulting mixture was stirred at 90 °C under argon
atmosphere for 12 h. The solvent was evaporated under re-
duced pressure and the residue was treated with water
(80 mL), extracted twice with CH2Cl2 (50 mL ꢁ 3). The or-
ganic phases were combined and washed twice with water
and once with brine, dried over anhydrous sodium sulfate.
After removing the solvent under reduced pressure, the
residue was loaded onto silica gel column with PE/EA (20/
1, v/v) as eluent to give a yellow solid (0.615 g, 63%). mp
182–183 °C; IR (KBr)
m 711, 813, 1172, 1278, 1373, 1506,
1589 cmꢀ1 1H NMR (DMSO-d6, 500 MHz): d 8.42 (d,
;
J = 9.0 Hz, 2H), 8.28 (dd, J1 = 3.5 Hz, J2 = 1.5 Hz, 2H), 8.00
(dd, J1 = 5.0 Hz, J2 = 1.5 Hz, 2H), 7.42–7.44 (m, 4H), 7.33
(dd, J1 = 5.0 Hz, J2 = 3.0 Hz, 2H), 7.18–7.23 (m, 6H), 7.03 (d,
J = 9.0 Hz, 2H); 13C NMR (DMSO-d6, 500 MHz) d 170.58,
167.53, 152.24, 146.50, 141.35, 134.19, 132.30, 130.59,
130.39, 129.48, 127.10, 126.27, 125.34, 120.07. FAB (m/z):
488.1 (M+). Anal. Calcd for C29H20N4S2: C, 71.28; H, 4.13;
N, 11.40. Found: C, 71.19; H, 4.23; N, 11.32.
Scheme 1. (i) 3 equiv. 2-thienylmagnesium bromide, 50 °C, 12 h; (ii)
1.5 equiv. 2-thienylmagnesium bromide, 25 °C, 12 h; (iii) 1.5 equiv. 4-
diphenylaminophenylboronic acid, K2CO3, Pd(dppf)Cl2, dioxane, H2O,
90 °C, 12 h; (iv) 2.4 equiv. 4-diphenylaminophenylboronic acid, K2CO3,
Pd(dppf)Cl2, dioxane, H2O, 90 °C, 12 h.
2.2.4. Synthesis of TTT-2
TTT-2 was prepared similarly to TTT-1 except that 1 was
replaced by 2 while the ratio of 4-diphenylaminophenylbo-
ronic acid to 2 is 3. TTT-2 was obtained as a yellow solid in

J. Liu et al. / Organic Electronics 13 (2012) 2177–2184
2179
Fig. 1. The molecular packing diagram of TTT-1 (a) and TTT-2 (b).
59% yield. mp 233–235 °C; IR (KBr)
m
696, 816, 1174, 1369,
KCl (aq.) and a platinum wire were used as working elec-
trode, reference electrode and counter electrode, respec-
tively. Differential scanning calorimetry (DSC) analyses
were performed on a METTLER TOLEDO modulating-DSC-
823 Low-Temperature Difference Scanning Calorimeter.
The samples were first heated to melt and then cooled to
25 °C before the DSC measurement at 5 °C/min. Thermo-
gravimetric analyses (TGA) were collected on a Perkin-El-
mer Pyris 1 TGA analyzer with a heating rate of 10 °C/
min under nitrogen.
1489, 1589 cmꢀ1; 1H NMR (DMSO-d6, 500 MHz): d 8.44 (d,
J = 9.0 Hz, 2H), 8.28 (dd, J1 = 3.5 Hz, J2 = 1.0 Hz, 2H), 7.96
(dd, J1 = 5.0 Hz, J2 = 1.0 Hz, 2H), 7.39–7.42 (m, 8H), 7.31
(dd, J1 = 5.0 Hz, J2 = 3.5 Hz, 2H), 7.17–7.21 (m, 12H), 7.01
(d, J = 9.0 Hz, 2H); 13C NMR (CDCl3, 500 MHz) d 170.33,
167.50, 151.79, 146.97, 131.61, 130.95, 130.13, 129.51,
128.71, 128.33, 125.64, 124.14, 121.07. FAB (m/z): 649.2
(M+). Anal. Calcd for C43H31N5S: C, 79.48; H, 4.81; N,
10.78. Found: C, 79.39; H, 4.85; N, 10.66.
2.3. Measurements
2.4. EL device fabrication and electro-optical characterization
Melting points were measured on a Melt-Temp appara-
tus and are uncorrected. Infrared spectra were recorded in
KBr pellet on a vector 22 Bruker spectrophotometer in the
All OLEDs with the emission area of 0.1 cm2 were fabri-
cated on the pre-patterned ITO-coated glass substrate hav-
ing an ITO sheet resistance of 15
X/sq. Substrates were
range of 4000–400 cmꢀ1
.
1H NMR and 13C NMR were re-
cleaned by sequentially ultrasonication in detergents and
in deionized water, followed by UV ozone treatments. The
stack of organic layers consists of a thin layer of TAPC
(60 nm) as the hole-transport layer, TTT-1 or TTT-2
(40 nm) as emitting layer and TPBi (60 nm) as the elec-
tron-transport layer. The Al cathode (100 nm) was then
prepared by evaporation of Al after LiF of (1 nm) was depos-
ited. All chemicals used for EL devices were sublimed in
vacuum prior to use. The vacuum was less than 1 ꢁ 10ꢀ5
Pa during all materials deposition. For the hole-only de-
vices, a thin layer of appointed compound (120 nm) was
deposited onto ITO substrate pretreated with UV ozone, fol-
lowed by Al (100 nm) deposition. For the electron-only de-
vices, a thin layer of BCP (10 nm) was evaporated as the
corded using a Bruker spectrometer at 500 MHz using
TMS as an internal standard. Low and high resolution mass
spectra were recorded using an EIT-OF-MS spectrometer in
FAB mode. Elemental analyses of the compounds were per-
formed on a Perkin-Elmer 240 analyzer. Absorption spectra
were recorded on a Shimazu UV-3100 spectrometer. Emis-
sion spectra were recorded on a Hitachi F-4600 lumines-
cence spectrometer upon excitation at 313 nm. Cyclic
voltammetry (CV) was performed on an Im6eX electro-
chemistry working station. All CV measurements were car-
ried out in anhydrous CH2Cl2 containing 0.1 M TBAP as a
supporting electrolyte, purging with argon prior to conduct
the experiment. Platinum electrode, Ag/AgCl in saturated

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J. Liu et al. / Organic Electronics 13 (2012) 2177–2184
hole-blocking layer on ITO (no pretreatment with UV
ozone). Layers of appointed compound (120 nm), LiF
(1 nm) and Al (100 nm) were then sequentially deposited.
The current density–voltage–luminance characteristics, cur-
rent efficiency and power efficiency versus current density
of the devices were measured with a computer controlled
KEITHLEY 2400 source meter with a calibrated silicon diode
in air without device encapsulation. On the basis of the
uncorrected EL spectra, the Commission Internationale de
l’Eclairage (CIE) coordinates were calculated using a test pro-
gram of the Spectra scan PR650 spectrophotometer.
(a)
3. Results and discussion
3.1. Synthesis and characterization
Grignard reaction between 2,4,6-trichloro-1,3,5-
triazine and 2-thienylmagnesium bromide affords the
intermediates (1, 2), which were coupled with 4-diphenyl-
aminophenylboronic acid via Suzuki reaction to yield the
desired compound (TTT-1, TTT-2), respectively. Both tri-
azine derivatives were prepared by simple reactions,
which is an important factor for commercialization. These
compounds were fully characterized by 1H NMR, 13C NMR
(see Supplementary Data), mass spectrometry, and ele-
mental analysis.
Crystals of TTT-1 and TTT-2 suitable for X-ray crystallo-
graphic analysis were obtained by slow evaporation over a
period of 3 weeks of their hexane/EA solution. TTT-1 and
TTT-2 crystallize in monoclinic space group P21/c and tri-
clinic space group P-1, respectively. The molecular struc-
ture and crystal packing diagram of them are shown in
Fig. S1 and Fig. 1. The triazine moieties are almost coplanar
to three aryl rings on all sides in these two compounds.
(b)
There is no
p–p stacking interaction between the neigh-
Fig. 2. DSC curves of TTT-1 (a) and TTT-2 (b) at a heating (cooling) rate of
5 °C/min. Three heating–cooling cycles are shown, with the first, second,
and third cycle represented by red, blue, and green line, respectively. (For
interpretation of the references to colour in this figure legend, the reader
is referred to the web version of this article.)
boring molecules due to the large steric hindrance of the
nonplanar triphenylamine groups.
3.2. Thermal properties
The thermal property and the morphological stability of
TTT-1 and TTT-2 were investigated by thermogravimetric
analysis (TGA) and differential scanning calorimetry
(DSC), respectively. The decomposition temperatures (Td)
corresponding to 5% weight loss are 330 °C for TTT-1 and
393 °C for TTT-2, respectively (Fig. S2). The higher thermal
stability of TTT-2 in comparison with TTT-1 is due to that
organic compounds with higher content of thiophene units
are more susceptible to thermal degradation. In the first
heating-cooling cycle of DSC study, melting endotherm
peaks were observed at 185 °C for TTT-1 and 240 °C for
TTT-2, respectively (Fig. 2). However, no endotherm peaks
corresponding to glass transition temperatures (Tg) was
observed. The cooling curves are flat and show no crystal-
lization. The glass transition temperatures (80 °C for TTT-1
and 115 °C for TTT-2) and recrystallization temperatures
(170 °C for TTT-1 and 210 °C for TTT-2) were detected in
the second and subsequent heating–cooling cycles. Inter-
estingly, a new melting point at 192 °C was observed for
TTT-1 in the second and subsequent heating–cooling
Fig. 3. Absorption and photoluminescence spectra of TTT-1 and TTT-2 in
dioxane solutions.
cycles, probably due to the formation of new crystalline
phase after heating.

J. Liu et al. / Organic Electronics 13 (2012) 2177–2184
2181
Table 1
Physical Properties of TTT-1 and TTT-2.
Eg (eV)c
HOMO (eV)d
LUMO (eV)e
Tg (°C)
Tm (°C)
Td (°C)
a
a
b
Compound
kabs,max (nm)
kem,max (nm)
kem,max (nm)
TTT-1
TTT-2
308, 356,390
296, 395
468
462
472
467
2.88
2.77
ꢀ5.81
ꢀ5.78
ꢀ2.93
ꢀ3.01
80
115
185
240
330
393
a
b
c
1 ꢁ 10ꢀ5 M in dioxane.
Vacuum deposited thin film.
Estimated from the onset wavelengths of the optical absorption.
Calculated from the oxidation potentials.
Calculated from the HOMO energy levels and Eg.
d
e
Fig. 4. Cyclic voltammogram of TTT-1 and TTT-2 in CH2Cl2 solutions
(1 ꢁ 10ꢀ3 M).
Fig. 5. The calculated HOMO and LUMO levels of TTT-1 and TTT-2.
TTT-1 and TTT-2 show blue emissions both in dilute
1,4-dioxiane solutions and in the solid films when excited
at 313 nm (Fig. 3 and Fig. S3). The stoke’s shift for TTT-1 is
larger than that for TTT-2, indicating a smaller structural
distortion in the excited state of TTT-2 due to the rigidity
and steric hindrance of two triphenylamine groups. This
is evidenced by the low temperature emission spectra
(Fig. S4) where the structural distortion are frozen out at
77 K, resulting in blue-shifted emission energies and simi-
lar stoke’s shift for TTT-1 and TTT-2. The quantum yields
are 0.47 and 0.48 for TTT-1 and TTT-2, respectively, using
3.3. Photophysical properties
The photophysical properties of TTT-1 and TTT-2 in
both 1,4-dioxiane solution (Fig. 3) and thin solid film
(Fig. S3) were characterized by UV-vis and photolumines-
cence (PL) spectra, and the data are summarized in Table
1. As shown in Fig. 3, the UV–Vis spectrum of TTT-1 shows
two peaks at 308 and 356 nm, and a shoulder at 390 nm.
The peak at 308 nm originates from the triazine-thiophene
units, while the peak at 356 nm comes from the triphenyl-
amine group [30–32]. According to the frontier orbital cal-
culations (see Section 3.5), the low energy shoulder can be
assigned to the intra-molecular charge transfer (ICT) tran-
sitions from the electron-donating triphenylamine moiety
to the electron-accepting triazine-thiophene moiety. For
TTT-2, the absorption maximum (296 nm) is blue-shifted
quinine sulfate monohydrate
(U313nm = 0.54) in 0.1 M
H2SO4 as a standard [33]. It is worth noting that the PL
spectra in the solid films of these two compounds exhibit
no distinct bathochromic shift in comparison with those
in solutions, suggesting that the excited states of TTT-1
and TTT-2 could have a similar conjugation length in both
states [9].
while the absorption intensity is reduced for
p–
p⁄ transi-
tions within the triazine-thiophene units, in accordance
with the less thiophene unit in TTT-2. Similarly, the oppo-
site changes (red-shift in absorption maximum and in-
3.4. Electrochemical properties
crease in absorption intensity) are observed for
transitions within the triphenylamine units. Therefore,
the
p⁄ transitions within the triphenylamine units over-
p–
p⁄
Cyclic voltammetry experiments were conducted on
TTT-1 and TTT-2 at room temperature to investigate their
electrochemical properties (Fig. 4). Both quasi-reversible
oxidation and reduction process were observed in CH2Cl2
solutions with 0.1 M n-Bu4NClO4 (TBAP) as a supporting
electrolyte. These quasi-reversible reductive and oxidative
behaviors indicate that TTT-1 and TTT-2 possess both
hole- and electron-transporting characteristics. As shown
p–
laps with the ICT transitions in TTT-2 and appears as an in-
tense single band at 395 nm. The absorption spectra of thin
films (Fig. S3) of TTT-1 and TTT-2 are almost the same to
those in 1,4-dioxiane solution, indicating that there is no
significant intermolecular interaction in the films.

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J. Liu et al. / Organic Electronics 13 (2012) 2177–2184
Fig. 6. Energy diagrams of the devices.
vacuum level) as the standard [34]. The band gaps were
estimated from the absorption spectra (absorption edge)
of the compounds. The LUMO energy level was calculated
from the values of band gap and HOMO energy. The data
are summarized in Table 1.
3.5. Density functional theory calculations
All calculations were carried out with Gaussian 03 pro-
grams [35]. The geometries of the organic dyes were fully
optimized by B3LYP method without any symmetry con-
straint. The calculations were carried out using the 6–
31 g(d,p) basis set for all the atoms. DFT calculations man-
ifest that the HOMO orbital of TTT-1 and TTT-2 are located
on triphenylamine while LUMO orbitals are located on tri-
azine-thiophene moieties (Fig. 5).
Fig. 7. EL spectra of devices A and B.
3.6. EL properties of devices based on TTT-1 and TTT-2
in Fig. 4, the oxidation potential of TTT-1 is higher than
that of the TTT-2. This is due to that TTT-2 has more elec-
tron-donating triphenylamine groups which reduce the
electron affinity of triazine moiety thus lower the oxida-
tion potential. The HOMO energy levels relative to the vac-
uum level were calculated by using the onset oxidation
potential (after being converted to the potential relative
to the standard hydrogen electrode potential, NHE) and
the Ag/AgCl energy level of ꢀ4.44 eV (relative to the
To investigate the potential applications of these
new blue emitters, two non-doped OLEDs, ITO/TAPC
(60 nm)/TTT-1 (Device A) or TTT-2 (Device B) (40 nm)/TPBi
(60 nm)/LiF (1 nm)/Al (100 nm), have been fabricated
(Fig. 6) [36]. The device A shows one sky-blue electrolumi-
nescence (EL) band centered at 473 nm while the device B
exhibits blue EL band centered at 465 nm (Fig. 7), with CIE
coordinates of (0.14, 0.24) and (0.14, 0.16), respectively.
The emission energies are very close to those of PL

J. Liu et al. / Organic Electronics 13 (2012) 2177–2184
2183
(a)
(b)
(c)
Fig. 8. Characteristics of devices A and B: (a) L–V–J characteristics. (b) Current efficiency and power efficiency versus current density of the devices. (c) L–J
characteristics.
observed for each blue fluorophore in the thin film state
(Table 1 and Fig. S3), indicating that there is no exciplex
formation at the interface of hole-transporting and light
emitting layers. Similar results have been reported previ-
ously and can be elucidated by the small difference in en-
ergy between the HOMO levels [37].
standards to compare the carrier-transport behaviors of
our compounds. Fig. S5 depicts the current vs. voltage char-
acteristics of the hole-only and electron-only devices. The
results show that TTT-1 and TTT-2 have both hole- and elec-
tron-transport abilities as we expected, although their car-
rier-transport abilities are lower than those of NPB, TAPC
and Alq3. Because of the introduction of two strong elec-
tron-donating triphenylamine moieties, the hole-transport
ability of TTT-2 is higher than that of TTT-1. Contrarily,
the electron-transport ability of TTT-1 is better than that
of TTT-2 due to its higher electron affinity as manifested
in the cyclic voltammetry study [41].
Current density–voltage–luminance (J–V–L) character-
istics of each device are presented in Fig. 8. The maximum
luminance of the TTT-1 based device reaches ca. 8990 cd/
m2 with maximum current efficiency (
gc) and power effi-
ciency (gp) of 4.7 cd/A and 2.4 lm/W, respectively. For
OLED based on TTT-2, the maximum luminance (ca.
15,980 cd/m2) is higher while the maximum current effi-
ciency and power efficiency (4.0 cd/A and 1.9 lm/W,
respectively) are slightly lower than the TTT-1 based de-
vice. These differences are due to the additional triphenyl-
amine moiety in TTT-2, which can enhance the hole
mobility. Both devices have a low turn-on voltage around
3 V. All these features are very competitive to other excel-
lent non-doped OLEDs based on fluorescent organic com-
pounds reported previously [38,39].
In order to investigate the carrier-transport behaviors of
the TTT-1 and TTT-2, the hole-only devices (ITO/Compound
(NPB, TAPC, TTT-1 or TTT-2) (120 nm)/Al (100 nm)) and
electron-only devices (ITO/BCP (10 nm)/Alq3, TTT-1 or
TTT-2 (120 nm)/LiF (1 nm)/Al (100 nm)) have been also
fabricated (Fig. 6) [40]. We chose the popular hole- or
electron-transport materials of NPB, TAPC and Alq3 as the
4. Conclusion
In summary, we have designed and synthesized two
new fluorophores containing triazine, triphenylamine and
thiophene building blocks. These two triazine derivates
exhibited efficient photophysical and electronic properties.
OLEDs based on these compounds exhibited good device
performances. Because they have both hole- and elec-
tron-transport abilities, they are also potential bipolar host
materials for the applications in OLEDs.
Acknowledgments
This work was supported by the National Basic Research
Program of China(2011CB933303, and 2011CB808704), the

2184
J. Liu et al. / Organic Electronics 13 (2012) 2177–2184
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