Highly efficient deep-blue organic electroluminescent devices (CIE y ≈ 0.08) doped with fluorinated 9,9′-bianthracene derivatives (fluorophores)

Article abstract of DOI:10.1039/c3tc31743a

A series of new fluorinated 9,9′-bianthracene derivatives (BAnFs) have been designed and synthesized to serve as deep-blue dopants in organic electroluminescent (EL) devices. With the different substitution patterns of the electron-withdrawing groups, such as F and CF₃, the photophysical properties, the energy levels and thermal stability of these BAnFs are tuned, which are supported by a density functional study of their geometry and electronic structure. In the thin film state, the fluorescent emissions of the BAnFs are fine-tuned from 448 to 439 nm, with varied fluorinated phenyl rings attached to the 9,9′-bianthracene core. All the BAnFs show a considerable thermal stability, which have high T_g values, above 150 °C. A pure blue emission at the Commission Internationale de l'Eclairage (CIE) coordinates (0.156, 0.083), has been achieved using the host 4,4′-bis(N-carbazolyl)biphenyl (CBP) doped with 10,10′-bis(3,5- bis(trifluoromethyl)phenyl)-9,9′-bianthracene (BAn-(3,5)-CF₃). The maximum current efficiency and power efficiency of the BAn-(3,5)-CF ₃-doped device are 3.05 cd A^-1 and 2.62 lm W^-1, corresponding to 5.02% of the maximum external quantum efficiency. The synthesized new fluorinated 9,9′-bianthracene derivatives show potential applications as highly efficient pure blue emitters for organic light emitting devices.

Full text of DOI:10.1039/c3tc31743a

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Materials Chemistry C
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PAPER
Highly efficient deep-blue organic electroluminescent
devices (CIEy z 0.08) doped with fluorinated
9,90-bianthracene derivatives (fluorophores)
Cite this: DOI: 10.1039/c3tc31743a
a
a
Yue Yu, Zhaoxin Wu, Zhanfeng Li,ab Bo Jiao,a Lu Li,a Lin Ma,a Dongdong Wang,c
*
Guijiang Zhou and Xun Houa
c
*
A series of new fluorinated 9,90-bianthracene derivatives (BAnFs) have been designed and synthesized to
serve as deep-blue dopants in organic electroluminescent (EL) devices. With the different substitution
patterns of the electron-withdrawing groups, such as F and CF3, the photophysical properties, the
energy levels and thermal stability of these BAnFs are tuned, which are supported by a density
functional study of their geometry and electronic structure. In the thin film state, the fluorescent
emissions of the BAnFs are fine-tuned from 448 to 439 nm, with varied fluorinated phenyl rings
attached to the 9,90-bianthracene core. All the BAnFs show a considerable thermal stability, which have
ꢀ
´
high Tg values, above 150 C. A pure blue emission at the Commission Internationale de l’Eclairage (CIE)
coordinates (0.156, 0.083), has been achieved using the host 4,40-bis(N-carbazolyl)biphenyl (CBP) doped
with 10,100-bis(3,5-bis(trifluoromethyl)phenyl)-9,90-bianthracene (BAn-(3,5)-CF3). The maximum current
Received 5th September 2013
Accepted 10th October 2013
efficiency and power efficiency of the BAn-(3,5)-CF3-doped device are 3.05 cd Aꢁ1 and 2.62 lm Wꢁ1
,
corresponding to 5.02% of the maximum external quantum efficiency. The synthesized new fluorinated
9,90-bianthracene derivatives show potential applications as highly efficient pure blue emitters for
organic light emitting devices.
DOI: 10.1039/c3tc31743a
www.rsc.org/MaterialsC
realizing efficient OLEDs and has become an intensive
studied subject at present.
1. Introduction
Organic light-emitting diodes (OLEDs) have shown great
potential as a technology of the next generation for at-panel
displays and solid-state lighting, since the rst report by Tang
and Van Slyke.1–4 For a full-color display and white lighting, it
is essential to have the three primary colors, red (R), green
(G), and blue (B). However, progress in highly efficient blue
OLEDs is far behind the counterparts of green/red OLEDs,
due to the difficulty in developing blue-emitting materials
with a high efficiency, color purity, and long operation time.
It is well known that a dopant–host doped emitter system
can signicantly avoid the concentration quenching of uo-
rescence and improve the device performance in terms of the
electroluminescent (EL) efficiency and emissive color, as well as
the operational lifetime.6 Various blue host materials and their
non-doped EL devices have been reported to date, which
include anthracene, uorene, styrylarylene, pyrene, quinoline
and triphenylene derivatives.7–12 Among these, the anthracene-
cored uorescent emitters with an intrinsically wide-energy
band gap, high uorescence quantum yield, and high thermal
stability as well as non-dispersive ambipolar carrier trans-
porting properties, have attracted considerable interest as deep-
blue OLEDs.7 Although many kinds of blue host materials have
been developed, with some of them providing highly efficient
blue EL with CIE coordinates near the National Television
System Committee (NTSC) standard blue coordinates (0.14,
0.08),13 suitable dopants that emit light in the deep blue region
are still rare. Only a little research has been reported on the
utilization of anthracene derivatives as blue dopants in OLED
applications.14
Deep-blue emitters with
a high quantum efficiency are
needed to effectively reduce power consumption and to
increase the color gamut of full-color OLEDs. A deep-blue
emitter can also be utilized to generate light of other colors
by an energy cascade to a suitably emissive dopant.5 There-
fore, the development of saturated deep-blue (CIEy coordinate
< 0.10) emitters with a high efficiency is highly crucial for
aKey Laboratory of Photonics Technology for Information, School of Electronic and
Information Engineering, Xi’an Jiaotong University, Xi’an 710049, People’s Republic
of China. E-mail: zhaoxinwu@mail.xjtu.edu.cn; Fax: +86 029-82664867
bCollege of Physics and Optoelectronics, Taiyuan University of Technology, Taiyuan
030024, People’s Republic of China
9,90-Bianthracene, made up of two anthracene units linked
by a single bond at the 9- and 90-position, is an anthracene
derivative. Crystallographic information reveals that the two
anthracene rings are almost perpendicular to each other, with a
cDepartment of Chemistry, School of Science, Xi’an Jiaotong University, Xi’an 710049,
People’s Republic of China. E-mail: zhougj@mail.xjtu.edu.cn
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dihedral angle of 89.4ꢀ due to the strongly repulsive interaction mainly attributed to the p–p* transition of the 9,90-bian-
of the hydrogen atoms at the 1,10- and 8,80-positions.15 Over the thracene moiety. Thus, the excellent luminescence efficiency of
past four decades, many studies have focussed on its photo- the 9,90-bianthracene moiety can be maintained. The calculated
physical properties in solution.16 The two anthracene groups are HOMOs and LUMOs of all the BAnFs are listed in Table 1. In
electronically decoupled because of the orthogonal structure in each BAnF, the two adjacent planar anthracene units are nearly
the ground state, while they exhibit a strong electronic inter- perpendicular to each other because of a steric repulsion of the
action with the relaxation of the geometric structure in the anthracene peri-hydrogen atoms (1,10- and 8,80-positions). For
excited state.17 The 9,90-bianthracene-cored uorescence emit- BAn-(2)-F and BAn-(2,4)-F, the dihedral angles between the
ters with particular twisted intramolecular charge transfer functionalized phenyl rings and the adjacent phenyl rings of the
(TICT) characteristics realize the electron–hole recombination 9,90-bianthracene core, are 85ꢀ and 76ꢀ, respectively, which are
via an intramolecular conversion from charge-transfer excitons slightly smaller than those of other BAnFs (90ꢀ). This discrep-
(immediate precursor) to a radiative singlet exciton (nal ancy can be explained by the intramolecular F/H interactions
state).18 Fluorination has been used in the past decade as a of the F atoms in the ortho-positions of the peripheral
route to induce stability and electron transport or ambipolar substituted phenyl, with the H atoms of the anthracene unit in
transport in organics by lowering the energy levels in the small BAn-(2)-F and BAn-(2,4)-F. The calculation results indicate that
molecules or polymers, especially for OLEDs. It also has been the BAnFs have extremely twisted geometry congurations,
used to adjust the emission spectrum and uorescence which can prevent intramolecular extending of the p-electrons
quantum yields (Ff).19–24 With different substitution patterns of and suppress intermolecular interactions, conjugation, and
electron-withdrawing groups, such as F and CF3, the photo- molecular recrystallization.
physical properties, the energy levels and thermal stability are
tuned.24
2.2 Thermal properties
In the paper, we reported the synthesis, characterization,
Their thermal properties were investigated by differential
and EL properties of new blue materials based on a 9,90-bian-
scanning calorimetry (DSC), and the resultant data, including
thracene core and various uorinated end-capping groups.
melting temperature (Tm) and glass-transition temperature (Tg),
These uorinated 9,90-bianthracene derivatives (BAnFs) exhibit
are shown in Table 1. The Tgs of compounds BAn-(2)-F, BAn-(3)-
satisfactory EL characteristics, and their emission wavelengths
F, BAn-(4)-F, and BAn-(2,4)-F were determined to be 168, 167,
can be nely tuned in the deep-blue region. The BAn-(3,5)-CF3-
167, and 156 ꢀC, respectively, by DSC in the second heating
doped device attains an EQE of 5.02% with excellent CIE coor-
scans (for THE details, see Fig. 2). BAn-(3,4,5)-F did not exhibit a
dinates (0.156, 0.083), that meet the NTSC standard blue
glass transition up to the Tm, and in the DSC curves of BAn-(3,5)-
coordinates.
CF3, no Tg and Tm were observed, although the compound was
ꢀ
heated to 420 C. The Tms of compounds BAn-(2)-F, BAn-(3)-F,
BAn-(4)-F, BAn-(2,4)-F and BAn-(3,4,5)-F were determined to be
328, 362, 347, 322, and 387 ꢀC, respectively. The high stability of
2. Results and discussion
2.1 Synthesis, structure characterization and theoretical
the amorphous glass state of these compounds is attributed to
the non-planarity of their molecular structures, demonstrating
that the perpendicular-type 9,90-bianthracene core improves the
thermal stability efficiently. Therefore, thermal analyses indi-
cate that these chromophores are thermally stable and suitable
for vapor deposition in OLEDs fabrication.
computation
A series of new BAnFs were readily obtained with a one-step
Suzuki coupling between brominated 9,90-bianthracene (BAn-
2Br) and the respective uorinated phenylboronic in the pres-
ence of a palladium catalyst, with yields ranging from 83% to
96%. The chemical structures and the synthetic routes of the
BAnFs in this study are shown in Scheme 1. We systematically
synthesized six compounds by changing the number and posi-
2.3 Photophysical properties
tion of the varied F or CF3 substituents on the two phenyl rings Fig. 3 shows the normalized UV-vis absorption and photo-
attached at the 10,100-positions of the 9,90-bianthracene core. luminescence (PL) spectra of the BAnFs in dichloromethane
Aer purication by column chromatography and recrystalli- (CH2Cl2) dilute solutions and lms (ca. 50 nm), obtained by
zation, these newly synthesized BAnFs were puried further by thermal evaporation on a pre-cleaned quartz substrate. Their
train sublimation at a reduced pressure below 10ꢁ3 Pa and fully photophysical data are summarized in Table 1. All the
1
characterized with H NMR spectroscopy and elemental anal- compounds show very similar absorption peaks in CH2Cl2 dilute
ysis. High-pressure liquid chromatography (HPLC) analysis was solutions. The UV-vis absorption spectra (Fig. 3a) in CH2Cl2
carried out to check the purity of the materials, which was, at solutions show that all of the BAnFs exhibit the common isolated
least, above 99.5%.
characteristic vibronic structure at approximately 400, 379, 358,
To understand the optimized geometry and electronic 340 nm, due to the 9,90-bianthracene core. Additionally, the
structures of the BAnFs, we carried out density functional theory absorption bands with the most prominent peak appearing at
(DFT) calculations at the B3LYP/6-31G(d,p) level in the Gaussian 260 nm are attributed to the substituted aryl groups. The
09 program. As shown in Fig. 1, the electron densities of the absorption spectra of these F-substituted BAnFs (i.e., BAn-(2)-F,
HOMO and LUMO are mostly localized on the two anthracene BAn-(3)-F, BAn-(4)-F, BAn-(2,4)-F and BAn-(3,4,5)-F) in lms
units, implying that the absorption and emission processes are are slightly red-shied by 3–5 nm relative to those in
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Scheme 1 Synthesis and structures of the BAnF compounds.
meta-position on the phenyl ring in BAn-(3)-F with respect to the
uorine atoms is the most electron-decient, because it is
affected only by the inductive electron-withdrawing character of
the uorine atom. Contrary to this, in the case of BAn-(2)-F,
BAn-(4)-F, and BAn-(2,4)-F, the coordinating carbon of the
phenyl ring is less electron-decient, because the electron-
withdrawing effect of the uorine atom is compensated for by
its electron-donating mesomeric character. The emission
maximum for BAn-(3,4,5)-F in the solution state was observed at
ca. 449 nm. From the spectra in Fig. 3b, one clearly observes a
signicant blue shi in the emission maximum for BAn-(3,5)-
CF3 (lmax ¼ 440 nm) relative to that for the F-substituted BAnFs,
by 5–9 nm, due to the shortened p-conjugation of the distorted
structure in the end-capping groups. When transferring from
the solution to the solid state, the emission maxima of the
BAnFs compounds show almost no red-shi, indicating the
aggregation effect does not occur in the lms of the synthesized
compounds. It also indicates a very weak intermolecular inter-
action in the solid state, arising from the non-coplanar struc-
ture. The PL spectra of these BAnFs were also studied in
different organic solvents. As shown in Fig. 4, the PL spectra of
the BAnFs exhibit red-shis of 37–42 nm when the solvent
polarity is raised from 2.4 (toluene) to 5.8 (acetonitrile). This
Fig. 1 The optimized geometries and the molecular orbital surfaces of the
HOMOs and LUMOs for the BAnFs obtained at the B3LYP/6-31G level.
solutions. However, the absorption spectra of the BAn-(3,5)-CF3 phenomenon is consistent with a variety of the excited states
show no spectral red shi going from CH2Cl2 solution to the from the locally excited state (LE) to an excited state with the
thin-lm state. The similarity between the absorption spectra of strong charge transfer (CT) character involving the TICT
the dilute solutions and thin lms suggests that the conforma- mechanism of the 9,90-bianthracene core.25 The full width at
tion of the solid state of the BAnFs is similar to that of the half maximum (FWHM) values in lms of BAn-(2)-F, BAn-(3)-F,
solution state of the BAnFs, which is attributed to its non- BAn-(4)-F, BAn-(2,4)-F, BAn-(3,4,5)-F and BAn-(3,5)-CF3 were 53,
coplanar structure, reducing the degree of intermolecular 55, 52, 52, 48, and 48 nm, respectively.
aggregation and p-delocalization.
We measured the uorescence quantum yields (Ff) of the
The PL maximum in the CH2Cl2 dilute solutions varies BAnFs in a dilute CH2Cl2 solution using quinine sulfate as a
depending on the number and position of the F or CF3 standard (Ff ¼ 0.56 in 1.0 M H2SO4 solution) at room temper-
substituents in the BAnFs. The PL spectra of the meta-position ature. As shown in Table 1, the location of the uorine
on the phenyl ring with respect to the uorine atom in the substituents on the phenyl group seems to be of importance.
mono-substituted molecule, BAn-(3)-F, appear to be blue-shif- The comparison of experimental values reveals an increase of Ff
ted with the most prominent peak appearing at 445 nm when by insertion of F or CF3 substituents, and the Ff increases with
compared with those of the ortho-position in BAn-(2)-F (lmax
¼
the number of F atoms. For example, the Ffs of BAn-(2)-F and
449 nm), the para-position in BAn-(4)-F (lmax ¼ 449 nm), and the BAn-(2,4)-F with mono- and di-F substituents were 0.55 and
ortho- and para-positions in BAn-(2,4)-F (lmax ¼ 448 nm). This is 0.69, respectively. BAn-(3,5)-CF3, with the CF3 substituents, had
attributed to the double nature of the electronic effect of the the highest quantum yield (0.99) in a CH2Cl2 solution, indi-
uorine atom, which is inductive and mesomeric.23 In fact, the cating that the CF3 substituent plays a greater role in increasing
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Table 1 Physical properties of the BAnFs
PL a,b
l
max
HOMO/LUMOexp
HOMO/LUMOcal
(DEHOMO–LUMO) (eV)
Compound
BAn-(2)-F
lAmbasxa,b (nm)
(nm)
Ffc
Tg/Tm (ꢀC)
Eoxe (V)
0.90
(Eogpt)f (eV)
d
g
259, 340, 358, 378, 400/270,
342, 361, 382, 404
449/448
445/447
449/447
448/443
449/445
440/438
0.55
0.48
0.41
0.69
0.93
0.99
168/299, 328
167/362
ꢁ5.70/–2.66 (3.04)
ꢁ5.73/–2.67 (3.06)
ꢁ5.68/–2.63 (3.05)
ꢁ5.77/ꢁ2.69 (3.08)
ꢁ5.74/ꢁ2.67 (3.07)
ꢁ5.86/ꢁ2.78 (3.08)
ꢁ5.26/–1.77 (3.49)
ꢁ5.29/–1.81(3.48)
ꢁ5.25/ꢁ1.77(3.48)
ꢁ5.28/ꢁ1.82 (3.46)
ꢁ5.47/ꢁ1.99 (3.48)
ꢁ5.67/ꢁ2.18 (3.49)
BAn-(3)-F
260, 340, 359, 379, 400/269,
343, 362, 383, 405
0.93
BAn-(4)-F
260, 340, 359, 379, 401/266,
343, 362, 383, 405
167/347
0.88
BAn-(2,4)-F
BAn-(3,4,5)-F
BAn-(3,5)-CF3
259, 338, 358, 378, 400/270,
342, 362, 382, 405
259, 339, 358, 378, 400/264,
342, 360, 380, 403
259, 340, 358, 379, 400/257,
341, 359, 378, 400
156/248, 322
NA/387
0.97
0.94
NA/NA
1.06
a
b
Measured in CH2Cl2. Measured in solid thin lm on quartz plates. c Determined in CH2Cl2 using quinine sulfate (FPL ¼ 0.56 in 1.0 M H2SO4
d
e
solution) as standard. Tg: glass-transition temperature; Tm: melting point; NA: not available. Determined from the onset of oxidation
potentials; measured in CH3CN; all of the potentials are reported relative to ferrocene, which was used as the internal standard in each
f
experiment. The ferrocene oxidation potential was located at 0.16 V, relative to the Pt-wire reference electrode. The HOMO and LUMO energies
g
were determined from cyclic voltammetry and the absorption onset. EHOMO ¼ ꢁ(qEox + 4.8) eV; ELUMO ¼ EHOMO + Egopt
.
Values from DFT calculation.
Fig. 3 (a) Absorption spectra of BAnFs in CH2Cl2. (b) PL spectra of BAnFs in
CH2Cl2. (c) Absorption of BAnFs in films. (d) PL spectra of BAnFs in films.
0.1 M supporting electrolyte ([Bu4N]ClO4) and 1 mM substrate
in dry acetonitrile under an nitrogen atmosphere using ferro-
cene as an internal standard. The voltammograms for the
BAnFs are shown in Fig. 5. All the compounds exhibit oxidation
waves at potentials higher than that observed for ferrocene. The
electrochemical properties, as well as the energy level parame-
ters of the BAnFs, are listed in Table 1. From the rst oxidation
onset potential, all the HOMO energy levels of the BAnFs were
estimated to be ꢁ5.68 to ꢁ5.86 eV. The LUMO energy levels of
the BAnFs were calculated to be in the range ꢁ2.63 to ꢁ2.78 eV,
from the absorption edge of the optical absorption spectra.
BAn-(3)-F, with a mono-F substituent at the meta-position, has a
slightly lower HOMO level (ꢁ5.73 eV) than that at the ortho-
position (ꢁ5.70 eV) or para-position (ꢁ5.68 eV). The HOMO
energy level decreases with the number of F atoms. By
Fig. 2 DSC scans of the BAnFs recorded under nitrogen during the second
heating cycle at a scan rate of 10 ꢀC minꢁ1
.
the Ff. The extremely high quantum yield and narrow FWHM of
BAn-(3,5)-CF3 makes it an excellent candidate for use as an
efficient blue-light-emitting material in OLEDs.
2.4 Electrochemical properties
Cyclic voltammetric (CV) studies were performed to calculate increasing the number of the highly electronegative F substit-
the HOMO and LUMO values for the BAnFs. The oxidation and uents on the phenyl groups, the electron-withdrawing capability
reduction CV experiments were carried out in solutions of a of the F substituted phenyl groups is enhanced. For example,
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Fig. 4 PL spectra of the BAnFs in various solvents.
the HOMO levels of BAn-(2)-F and BAn-(2,4)-F with mono- and meta-position (BAn-(3,5)-CF3) resulted in the lowest HOMO level
di-F substituents were ꢁ5.70 and ꢁ5.77 eV, respectively. A (ꢁ5.86 eV) for the stronger electron-withdrawing capability of
similar situation was observed by our group for 9,90-spirobi- CF3 substituent. As seen from Table 1, the experimental
uorene,24 Chen et al. for perylene bisimides and Gade et al. for HOMOs and LUMOs are in excellent agreement with the
2,9-bisaryl-tetraazaperopyrene.26 The CF3 substituent at the calculated values. The BAnFs had wide optical energy gaps (Eg)
Fig. 5 Cyclic voltammograms of the BAnFs in CH3CN.
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Fig. 7 (a) PL spectra of CBP and absorption spectra of BAnFs in CH2Cl2. (b) EL
spectra of BAnF-doped devices at 8 V and emission photograph of device 1B.
Fig. 6 (a) Structures of device 1-6A and energy levels of the BAnFs. (b) Structure
of device 1B.
doping concentrations of 3, 5, 7, 9 wt%). Since the spectral
overlap efficiency between the PL spectra of CBP and absorption
¨
spectra of the dopants is very high (Fig. 7a), Forster energy
of 3.04 to 3.08 eV. Accordingly, the BAnFs could be expected to
be suitable candidates for deep blue emitters.
transfer was efficient from the CBP host to the dopants. Fig. 7b
shows the normalized EL spectra for the BAnF-doped devices.
All of the blue EL spectra of the BAnF-doped devices are very
similar to the corresponding PL spectra in the thin-lm state,
2.5 Electroluminescent properties
In order to achieve a deep-blue EL, we chose BAnFs as the which indicates that the EL emissions are mainly contributed
dopants for device fabrication. In these devices, the well known by the uorescence of the BAnFs. The CIE chromaticity coor-
ambipolar conductive 4,40-bis(N-carbazolyl)biphenyl (CBP) was dinates of the BAnF-doped devices are in the range (0.154–
used as host material.27 As shown in Fig. 6a, the BAnF- 0.156, 0.073–0.087) at 8 V, which show the extremely pure deep-
doped devices (1-6A) were fabricated with the following blue emissions with peaks at 435–440 nm and meet the NTSC
conguration: indium tin oxide (ITO)/poly(3,4-ethylenedioxy- blue standard. The BAnFs-doped devices show narrow FWHMs
thiophene) : poly(styrenesulfonate) (PEDOT : PSS) (30 nm)/4,40- of 63–69 nm without excimer or exciplex emission. Moreover,
bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl (NPB) (30 nm)/ Fig. 8 shows the CIE values of all the devices in response to the
4,40,400-tris(carbazol-9-yl)-triphenylamine (TcTa) (10 nm)/blue driving voltage; a negligible EL color shi was observed when
emitting layer (EML) (20 nm)/1,3,5-tris(1-phenyl-1H-benzimi- the driving voltage was increased from 4 to 10 V. It also suggests
dazol-2-yl)benzene (TPBi) (40 nm)/CsCO3 (3 nm)/Al (100 nm). that the electron–hole pairs for recombination are well conned
PEDOT : PSS was used as a hole injection layer (HIL), NPB was in the blue emitter region. Fig. 9 exhibits the current density–
used as a hole transporting layer (HTL), TcTa was used as an voltage–luminance–efficiency ( J–V–L–h) characteristics of the
exciton blocking layer, TPBi was used as an electron trans- BAnF-doped devices. The key device performance parameters
porting layer (ETL) and exciton blocking layer, and CsCO3 was and EL emission characteristics are summarized in Table 2. The
used as an electron injection layer (EIL). Each dopant was co- BAnF-doped devices display low turn-on voltages (at a lumi-
evaporated with CBP to give the optimal doping concentration nance of 1 cd mꢁ2) no greater than 4.5 V. The current efficiency
of 5 wt% in the EML (we fabricated several devices with various and power efficiency of these devices are in the range 1.28–2.40
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cd Aꢁ1 and 0.87–1.96 lm Wꢁ1, respectively. These EL behaviors
are in excellent agreement with the Ffs. Notably, the device 6A
with BAn-(3,5)-CF3 as the dopant, achieves the best EL perfor-
mance, with a maximum external quantum efficiency (EQE) as
high as 4.65% (Fig. 10), a maximum current efficiency of 2.40 cd
Aꢁ1, a maximum power efficiency of 1.96 lm Wꢁ1 and a pure
blue emission at CIE (0.154, 0.073). The excellent EL efficiency
of the BAn-(3,5)-CF3-doped device can be attributed mainly to its
high Ff (0.99) and the efficient energy transfer from CBP to the
BAn-(3,5)-CF3 dopant to form LEs (the electron–hole pair
localizes at one anthracene plane). What is more, the electron
trapping followed by the direct recombination with the hole at
the dopant sites could additionally contribute to the enhanced
EL efficiency. Charge carriers direct recombination by trapping
at the dopant sites will produce both LEs and perpendicular
TICT-states (the electron and hole of a bound electron–hole pair
may reside on two anthracene rings of one molecule respec-
tively). Due to the lack of a signicant coupling between the
ground and excited state, a perpendicular TICT emission is
forbidden. While the TICT-state is a spin-orbit, charge-transfer
intersystem crossing mechanism, it is possible that the transi-
tion of 1CT 4 3CT is allowed, due to the small energy splitting
between them, especially in the perpendicular orientation
between the donor and accepter.28 The 9,90-bianthracene-cored
uorescence emitters with particular TICT characteristics
realize the electron–hole recombination via an intramolecular
conversion from the TICT-state (precursor) to a radiative singlet
1
1
3
1
1
exciton (nal state), e.g., CT / LE and CT / CT / LE.18
For the BAn-(3,5)–CF3–doped EL device, the singlet generation
fraction is more than 25%. In addition, the LUMO level of
Fig. 8 CIE coordinates of EL spectra in response to the driving voltage of BAnF-
doped devices from 4 V to 10 V.
Fig. 9 (a) Current density–voltage curves. (b) Brightness–voltage curves. (c) Current efficiency–current density curves. (d) Power efficiency–current density curves for
BAnFs-doped devices.
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Paper
Table 2 EL performance of deep-blue devices with 5 wt% BAnFs doped in the EML
EL
b
c
d
d
d
l
Von
(V)
Lmax
hext
hc
hp
(lm Wꢁ1
FWHM
(nm)
max
Device
EML
(nm)
(cd mꢁ2
)
(%)
(cd Aꢁ1
)
)
CIE (x, y)a
1A
2A
3A
4A
5A
6A
1B
CBP:BAn-(2)-F
CBP:BAn-(3)-F
CBP:BAn-(4)-F
CBP:BAn-(2,4)-F
CBP:BAn-(3,4,5)-F
CBP:BAn-(3,5)-CF3
CBP:BAn-(3,5)-CF3
438
440
437
435
437
434
435
4.2
4.1
4.4
4.0
4.5
3.5
3.7
1647
1669
2766
2016
838
2.67
1.99
1.86
2.75
3.42
4.65
5.02
1.66
1.28
1.22
1.47
2.11
2.40
3.05
1.02
0.89
0.87
1.10
1.45
1.96
2.62
67
69
68
65
67
63
66
(0.156, 0.085)
(0.154, 0.087)
(0.154, 0.087)
(0.154, 0.074)
(0.155, 0.085)
(0.154, 0.073)
(0.156, 0.083)
1471
3588
a
b
c
d
Values collected at 8 V. Turn-on voltage at 1 cd mꢁ2
. Maximum luminance. Values collected at a peak efficiency.
maximum power efficiency of device 1B are accordingly
increased to 3.05 cd Aꢁ1 and 2.62 lm Wꢁ1, corresponding to
5.02% of the maximum EQE. Additionally, a pure blue emission
at CIE (0.156, 0.083) has been achieved with the emission
peaking at ca. 435 nm. The performance of the BAn-(3,5)-CF3-
doped device was outstanding compared to previously reported
results for deep-blue organic electroluminescent devices.13,14
3. Conclusions
In summary, a series of deep-blue materials of uorinated 9,90-
bianthracene derivatives (BAnFs) have been successfully
prepared by Suzuki coupling reactions, in high yields. We have
demonstrated that the absorption, emission, electrochemical
properties, and OLED performances are signicantly affected by
the introduction of electron-withdrawing substituents such as F
and CF3 into the 9,90-bianthracene core, which were supported
by theoretical calculations employing the B3LYP functional.
The doped EL device with BAn-(3,5)-CF3 as the dopant achieved
an maximum external quantum efficiency (EQE) of 5.02% (3.05
cd Aꢁ1) in the deep-blue visible region with excellent CIE
chromaticity coordinates (0.156, 0.083) that meet the NTSC
standard blue coordinates. The BAn-(3,5)-CF3-doped device
exhibits a high EQE, which shows a potential application as an
emitter for pure blue devices.
Fig. 10 External quantum efficiency (EQE) at different current densities for
BAnFs-doped devices.
BAn-(3,5)-CF3 (ꢁ2.78 eV) is the lowest of the BAnFs; a low-lying
LUMO level implies a facilitated electron injection and an
improved hole–electron balance, where an improved hole–
electron balance in charge injection enhances the efficiency of
the devices.
We believe that further improvements in the device perfor-
mance can be achieved through an energy level adjustment
between neighboring layers, charge mobility matching, and so
on. So, we fabricated device 1B, as shown in Fig. 6b, the
conguration of BAn-(3,5)-CF3-doped device 1B is as below: ITO/
4. Experimental
4.1 General information
molybdenum trioxide (MoO3) (3 nm)/4,40-cyclohexylidene- The manipulation involving air-sensitive reagents was per-
bis[N,N-bis(4-methylphenyl)aniline] (TAPC) (40 nm)/EML formed under an inert atmosphere of dry nitrogen. Com-
(20 nm)/2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) mercially available reagents were used without further
(10 nm)/bis[2-(2-hydroxyphenyl)-pyridine]beryllium (Be(pp)2) purication unless otherwise stated. 9,10-Anthracenedione,
(30 nm)/CsCO3 (3 nm)/Al (100 nm), in which MoO3 and CsCO3 2-uorophenylboronic acid, 3-uorophenylboronic acid, 4-u-
were the HIL and the EIL, respectively, TAPC was the HTL and orophenylboronic acid, 2,4-diuorophenylboronic acid, 3,4,
the exciton blocking layer, Be(pp)2 was the ETL, BCP was the 5-triuorophenylboronic acid, 3,5-bis(triuoromethyl)phenyl-
exciton blocking layer, BAn-(3,5)-CF3 was co-evaporated with boronic acid, and tetrakis(triphenylphosphine)palladium were
CBP to give a 5 wt% dopant content in the EML. Due to the high purchased and used as received. The photoluminescence (PL)
electron mobility of Be(pp)2, carrier transmission has been and absorption spectra were recorded on a Horiba Jobin Yvon
further balanced. Moreover, TAPC and BCP have a better exciton Fluoromax-4 spectrophotometer and a Hitachi UV 3010 spec-
blocking ability to conne the excitons in the EML. Compared trophotometer, respectively. To measure the uorescence
to a maximum current efficiency of 2.40 cd Aꢁ1, a maximum quantum yields (Ff), degassed solutions of the compounds in
power efficiency of 1.96 lm Wꢁ1, and a maximum EQE of 4.65% CH2Cl2 were prepared. The concentration was adjusted so that
for device 6A, the maximum current efficiency and the the absorbance of the solution would be lower than 0.1. The
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excitation was performed at 340 nm, and quinine sulfate in 1.0 (CDCl3, 400 MHz): d 7.17–7.25 (m, 10H), 7.29–7.32 (d, J ¼ 8.4 Hz,
M H2SO4 solution, which has a Ff ¼ 0.56, was used as a stan- 2H), 7.33–7.39 (t, 4H), 7.41–7.43 (d, J ¼ 7.6 Hz, 2H), 7.61–7.68 (q,
dard. The difference between the refractive index of the solvent 4H), 7.78–7.81 (d, J ¼ 8.8 Hz, 4H). Anal. calcd for C40H24F2: C,
and that of the standard should also be counted. The glass 88.54; H, 4.46. Found: C, 88.48; H, 4.38%.
transition temperatures (Tg) were determined with a differential
10,100-Bis(4-uorophenyl)-9,90-bianthracene
(BAn-(4)-F).
1
ꢀ
scanning calorimeter (DSC, TA instruments DSC200PC) at a 1.15 g, white solid, yield: 91%. Tm ¼ 347 C. H NMR (CDCl3,
heating rate of 10 ꢀC minꢁ1 under a nitrogen atmosphere. Cyclic 400 MHz): d 7.16–7.19 (m, 4H), 7.22–7.26 (t, 4H), 7.33–7.40 (q,
voltammetry (CV) was performed using a Princeton Applied 8H), 7.57–7.62 (q, 4H), 7.77–7.81 (d, J ¼ 8.8 Hz, 4H). Anal. calcd
Research model 273 A potentiostat at a scan rate of 100 mV sꢁ1
.
for C40H24F2: C, 88.54; H, 4.46. Found: C, 88.50; H, 4.55%.
All the experiments were carried out in a three electrode
10,100-Bis(2,4-biuorophenyl)-9,90-bianthracene (BAn-(2,4)-
1
ꢀ
compartment cell with a Pt-sheet counter electrode, a glassy F). 1.22 g, pale yellow solid, yield: 90%. Tm ¼ 322 C. H NMR
carbon working electrode and a Pt-wire reference electrode. The (CDCl3, 400 MHz): d 7.17–7.23 (m, 12H), 7.38–7.41 (m, 4H),
supporting electrolyte used was a 0.1 M tetrabutylammonium 7.53–7.61 (q, 4H), 7.74–7.61 (d, J ¼ 8.0 Hz, 4H). Anal. calcd for
perchlorate ([Bu4N]ClO4) solution in dry acetonitrile. The cell
containing the solution of the sample (1 mM) and the sup-
C
40H22F4: C, 83.03; H, 3.83. Found: C, 83.13; H, 3.75%.
10,100-Bis(3,4,5-triuorophenyl)-9,90-bianthracene (BAn-(3,4,
porting electrolyte was purged with nitrogen gas thoroughly 5)-F). 1.38 g, white solid, yield: 96%. Tm ¼ 387 ꢀC. 1H NMR
before scanning for its oxidation and reduction properties. (CDCl3, 400 MHz): d 7.18–7.20 (m, 8H), 7.27–7.31 (t, 4H), 7.38–
Ferrocene was used for the potential calibration in each 7.43 (m, 4H), 7.73–7.78 (d, J ¼ 8.8 Hz, 4H). Anal. calcd for
measurement. All the potentials were reported relative to the
ferrocene-ferrocenium (Fc/Fc+) couple, whose oxidation poten-
C
40H20F6: C, 78.17; H, 3.28. Found: C, 78.20; H, 3.17%.
10,100-Bis(3,5-bis(triuoromethyl)phenyl)-9,90-bianthracene
tial was +0.16 V relative to the reference electrode. The oxidation (BAn-(3,5)-CF3). 1.39 g, white solid, yield: 83%. 1H NMR
and reduction potentials were determined by taking the average (CDCl3, 400 MHz): d 7.22–7.24 (d, J ¼ 6.4 Hz, 8H), 7.42–7.45
of the anodic and cathodic peak potentials. The HOMO and (m, 4H), 7.62–7.64 (d, J ¼ 8.8 Hz, 4H), 8.14–8.16 (d, J ¼ 6.8 Hz,
LUMO values were estimated by using the following general 4H). Anal. calcd for C44H22F12: C, 67.87; H, 2.85. Found: C,
equation: EHOMO ¼ ꢁ(qEox + 4.8) eV; ELUMO ¼ EHOMO ꢁ Egopt
,
67.75; H, 2.91%.
which were calculated using the internal standard ferrocene
value of ꢁ4.8 eV with respect to the vacuum level.
4.3 Device fabrication and testing
The OLEDs were fabricated by thermal evaporation onto a
cleaned glass substrate, pre-coated with transparent and
conductive indium tin oxide (ITO). Prior to the organic layer
deposition, the ITO substrates were exposed to a UV-ozone
ux for 10 min, following degreasing in acetone and iso-
propyl alcohol (IPA). All of the organic materials were puri-
ed by temperature-gradient sublimation in a vacuum. The
devices were fabricated by the conventional vacuum deposi-
tion of the organic layers, CsCO3 and an Al cathode onto an
ITO-coated glass substrate under a base pressure lower than
1 ꢂ 10ꢁ3 Pa. The thickness of each layer was determined by
a quartz thickness monitor. The voltage–current density (V–J)
and voltage–brightness (V–L) as well as the current density–
current efficiency (J–hc) and current density–power efficiency
( J–hp) curve characteristics of the devices were measured with
a Keithley 2602 and Source Meter. All the measurements
were carried out at room temperature under ambient
conditions.
4.2 Preparation of blue emitters BAnFs
9,90-Bianthracene and 10,100-dibromo-9,90-bianthracene were
synthesized according to the literature.29 A series of novel
uorinated 9,90-bianthracene derivatives were synthesized by
the Suzuki coupling reaction between brominated 9,90-bian-
thracene and the respective uorinated phenylboronic in the
presence of a palladium catalyst, with yields ranging from 83 to
96% (Scheme 1). THF (30 mL) and an aqueous solution of
K2CO3 (2.0 M, 10 mL) were added to a ask containing 10,100-
dibromo-9,90-bianthracene (2.34 mmol), the uorinated phe-
nylboronic (8 mmol) and Pd(PPh3)4 (0.35 mmol) under
nitrogen. The reaction mixture was heated to reux and main-
tained at this temperature for 24 h. When the reaction was
completed (judging from thin-layer chromatography), water was
added to quench the reaction. Then, the products were extrac-
ted with CH2Cl2. The organic portion was washed with brine,
dried over anhydrous MgSO4, and concentrated by evaporating
off the solvent. The solid was absorbed on silica gel and puried
by column chromatography using light petrol ether–ethyl
acetate as the eluent to give the product.
Acknowledgements
10,100-Bis(2-uorophenyl)-9,90-bianthracene
(BAn-(2)-F). This work was nancially supported by the Basic Research
1.13 g, pale yellow solid, yield: 89%. Tm ¼ 328 ꢀC. 1H NMR Program of China (2013CB328705), the National Natural
(CDCl3, 400 MHz): d 7.18–7.26 (m, 10H), 7.27–7.30 (m, 4H), Science Foundation of China (Grant nos 61275034, 61106123
7.37–7.51 (m, 4H), 7.60–7.68 (m, 4H), 7.78–7.82 (d, J ¼ 8.8 Hz, and 61308093), the Fundamental Research Funds for the
4H). Anal. calcd for C40H24F2: C, 88.54; H, 4.46. Found: C, Central Universities (Grant no. xjj2012087) and the China
88.60; H, 4.35%.
Postdoctoral Science Foundation (Grant no. 20110491653).
(BAn-(3)-F). The supporting information is available online from the
10,100-Bis(3-uorophenyl)-9,90-bianthracene
1.18 g, pale yellow solid, yield: 93%. Tm ¼ 362 ꢀC. 1H NMR author.
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Y. S. Gal, S. Kang, J. Y. Lww, J. W. Kang, S. H. Lww and
H. D. Park, Adv. Funct. Mater., 2008, 18, 3922.
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