Bifunctional anthracene derivatives as non-doped blue emitters and hole-transporters for electroluminescent devices

Article abstract of DOI:10.1039/c1cc11624j

New highly fluorescent bifunctional anthracenes showed high thermal and electrochemical stability, and great potential as both blue emitters and hole-transporters for OLEDs. Deep-blue and Alq3-based green devices with maximum efficiencies and CIE coordina

Full text of DOI:10.1039/c1cc11624j

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COMMUNICATION
Bifunctional anthracene derivatives as non-doped blue emitters and
hole-transporters for electroluminescent devicesw
A-monrat Thangthong, Duangratchaneekorn Meunmart, Narid Prachumrak,
Siriporn Jungsuttiwong, Tinnagon Keawin, Taweesak Sudyoadsuk and Vinich Promarak*
Received 21st March 2011, Accepted 10th May 2011
DOI: 10.1039/c1cc11624j
New highly fluorescent bifunctional anthracenes showed high
thermal and electrochemical stability, and great potential as
both blue emitters and hole-transporters for OLEDs. Deep-blue
and Alq3-based green devices with maximum efficiencies and
CIE coordinates of 1.65 and 6.25 cd Aꢀ1, and (0.15, 0.16) and
(0.26, 0.49) were achieved, respectively.
The use of p-conjugated organic compounds as electrolumines-
cent materials in organic light-emitting diodes (OLEDs) was
introduced by Tang and van Slyke over two decades ago.1
Since then, extensive academic and industrial research has
been carried out to bring OLEDs into the display markets.2
OLEDs for full colour displays are showing great promise, and
are required for red, green and blue-emitting materials.
Although many blue light-emitting materials have been
reported, such as pyrene, fluorene, and triarylamine derivatives,3
materials emitting in the deep-blue colour (CIE coordinates of
(0.15, 0.15)) with high efficiency are still rare.4 Thus, there is
Scheme 1 Synthetic route of 1 and 2: (i) Pd(PPh3)4, 2 M Na2CO3, THF.
still a clear need for further improvement for blue OLED
compared to red and green devices.
synthesis of compounds 1 and 2 (Scheme 1), physical
and photophysical properties. Investigation on OLED device
fabrication and characterization is also reported. We began
with Suzuki coupling of readily obtained 2-iodo-9,9-bis-n-
hexylfluorene 38 in excess amounts with 9-bromoanthracene-
10-boronic acid to give intermediate 4 in a fair yield of 52%.
Anthracene 1 was obtained as light yellow solids in a good
yield of 79% by Suzuki coupling of 4 with 4-(diphenylamino)-
phenyl boronic acid.9 Anthracene 2, a dimer of 1, was success-
fully synthesized by carrying out firstly Suzuki coupling of
2,7-diiodo-9,9-bis-n-hexylfluorene 58 with 9-bromoanthracene-
10-boronic acid (2.11 equiv.) followed by coupling of the
resultant 6 with 4-(diphenylamino)phenyl boronic acid, and
was obtained as deep yellow solids in 75% yield.
Anthracene is a thermally and electronically stable rigid
aromatic and its derivatives have been widely used as blue
chromophores for many applications.5 A number of anthracene
derivatives have also been developed as blue emitters for
OLEDs, but the factors of colour purity and efficiency continue
to require improvement.6 To this end, we report the new
anthracene-based blue emitters. Our design involved an intro-
duction of the bulky triphenylamine and 9,9-bis-n-hexylfluorene
as 9,10-disubstituents. The former can suppress aggregation
of the planar anthracene ring as well as increase the hole-
transporting capability and thermal stability, while the latter
also has a number of advantages including its ability to emit in
the blue region and increase solubility.7 This would result in
new bifunctional anthracenes with combined blue emitting
and hole-transporting properties. Herein, we report a detailed
Quantum chemical calculations performed using the
TDDFT/B3LYP/6-31G(d,p) method revealed that all molecules
adopt non-coplanar conformations. Particularly, the planar
anthracene unit twists nearly perpendicular to the adjacent
fluorene and triphenylamine moieties because of a steric
repulsion of anthracene peri-hydrogen atoms (1,8 and 4,5
positions) with hydrogen atoms of those aromatic rings. Such
structural characteristics can influence some of their electronic
and physical properties such as suppression of the conjugation.
Center for Organic Electronic and Alternative Energy, Department of
Chemistry and Center for Innovation in Chemistry, Faculty of
Science, Ubon Ratchathani University, Ubon Ratchathani, 34190,
Thailand. E-mail: pvinich@ubu.ac.th; Fax: +66 4528 8379;
Tel: +66 4535 3400 ext. 4510
w Electronic supplementary information (ESI) available: General
procedure, synthetic detail, characterization data, computer calculation
data and OLED device characteristics. See DOI: 10.1039/c1cc11624j
c
7122 Chem. Commun., 2011, 47, 7122–7124
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In the HOMO orbitals of 1 and 2, p-electrons delocalize over
the anthracene and electron donor triphenylamine groups, while
in their LUMO, electrons locate only on the anthracene rings.
The UV-vis spectra of 1 and 2 in solutions showed two
absorption bands: strong absorption band (250–310 nm)
agreeing with the p–p* local electron transition of the individual
aromatic units and the less intense three absorption bands
(350–400 nm) attributed to the p–p* electron transition
of the anthracene–triphenylamine conjugated backbone. The
HOMO–LUMO energy gaps (Eg) estimated from the onset of
the absorption spectra are nearly identical (2.9 eV), despite
somewhat different molecular size, indicating an equal
p-conjugation length. This can be explained by an out-of-plane
twisting of the fluorene unit from adjacent anthracene rings
limiting the delocalization of p-electrons on the anthracene and
triphenylamine units only. Both 1 and 2 showed deep blue
fluorescence with high quantum yields (FF) of 0.73 and 0.77,
respectively. Their PL spectra are identical with a featureless
pattern which is similar to what was observed for most
9,10-diphenylanthracenes. These materials showed a slight
Stokes shift (85 nm) suggesting less energy loss during the
relaxation process and efficient fluorescence. The thin film PL
emission spectra (Fig. 1) of 1 and 2, having bulky structures,
exhibited a hypsochromic shift of about 19–26 nm compared
to their solution spectra. This result may also be attributed
to the aforementioned solid state packing force which prohibits
the electron-vibration coupling between the triphenylamine
substituent and the anthracene photoactive unit.
electrochemical reactions in both molecules. The result suggests
that these materials are electrochemically stable molecules.
Moreover, under these CV conditions, no reduction process
was observed in both cases.
The thermal properties were investigated by the thermo-
gravimetric analysis (TGA) and differential scanning
calorimetry (DSC). Those results suggest that 1 and 2 were
thermally stable materials with temperature at 5% weight loss
(T5d) well over 400 1C. A DSC thermogram of 1 revealed only
an endothermic baseline shift owing to glass transition (Tg)
that was detected at 79 1C with no crystallization and melting
peaks observed at higher temperature. The thermogram of 2
displayed Tg at 155 1C and exothermic peaks due to the
crystallization (Tc) around 225 1C that gave the same crystal
as obtained by crystallization from solution, which then
melted at 338 1C. Their ability to form a molecular glass with
the possibility to prepare good thin films by both evaporation
and solution casting processes is highly desirable for application
in electroluminescent devices.
In order to investigate the electroluminescence properties
of 1 and 2, OLEDs (devices I, II) were fabricated with
the following device structure: indium tin oxide (ITO)/
PEDOT:PSS/EL(50 nm)/BCP(40 nm)/LiF(0.5 nm):Al(150 nm),
where these materials were used as the emitting layer (EL) and
poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate)
(PEDOT:PSS) and 2,9-dimethyl-4,7-diphenyl-1,10-phenan-
throline (BCP) act as the hole injection layer and the hole
blocking layer, respectively. Both diodes exhibited deep blue
emission with peaks centered at 458 and 466 nm and CIE
coordinates of (0.14, 0.13) and (0.15, 0.16), respectively. The
EL spectra are similar to their corresponding PL spectra in
film (Fig. 1) indicating that the EL emission originates from
the singlet-excited states of 1 and 2. Significantly, a stable
emission was obtained from all diodes and the EL spectra and
CIE coordinates did not change over the entire applied
voltage. Device II having 2 as the emitting layer showed the
best performance with a high maximum brightness of 4586 cd mꢀ2
at 8.8 V, a low turn-on voltage of 4.6 V, a maximum luminous
efficiency of 1.65 cd Aꢀ1 and a maximum external quantum
efficiency of 0.34%. A slightly lower device performance was
observed from device I (Fig. 2, Table 1). Although many blue
fluorescent emitters have been reported, high efficiency materials
emitting in the deep blue region are still rare. The performance
of these materials in terms of colour purity and efficiency is
Electrochemical behaviours of 1 and 2 were investigated by
cyclic voltammetry (CV). They were found to exhibit two
quasi-reversible oxidation processes (0.95–0.96 and 1.21–1.25 V)
with an identical onset potential of 0.88 V, which is very close
to the data reported for most triphenylamine derivatives (0.90 V).
The first oxidation wave was assigned to the removal of
electrons from the peripheral triphenylamines resulting in
radical cations. Their multiple CV scans revealed identical
CV curves with no additional peak at lower potential on the
cathodic scan (Epc) being observed. This indicates no oxidative
coupling or a weak oxidative coupling if any at p-phenyl rings
of the peripheral triphenylamine led to electro-polymerization.
This type of electrochemical coupling reaction can be detected
in some triphenylamine derivatives with an unsubstituted
p-position of the phenyl ring.10 Steric hindrance of an adjacent
anthracene ring might play a key role to prevent such
Fig. 1 PL spectra of 1 and 2 measured as a thin film obtained by vapor
deposition, and EL spectra of OLEDs with 1 and 2 as EL and HTL.
Fig. 2 J–V–L characteristics of OLED devices.
Chem. Commun., 2011, 47, 7122–7124 7123
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Table 1 Device characteristics of OLEDs with 1 and 2 as EL
and HTL
preferably as an electron blocker more than as a hole blocker
and charge recombination is thus confined to the Alq3 layer.
The device characteristics clearly demonstrate that the hole-
transporting ability of 1 and 2 with superior device performance
(maximum brightness and efficiency) is comparable to that of
an NPB-based device (Fig. 2, Table 1). Device IV having
compound 2 as HTL exhibited the best performance with a
high maximum brightness of 32270 cd mꢀ2 for green OLED at
11.4 V, a low turn-on voltage of 4.0 V, a maximum luminous
efficiency of 6.25 cd Aꢀ1 and a maximum external quantum
efficiency of 0.30%.
Device EL/HTL Von lem Lc
Jd
Ze
EQEf CIEg
a
b
I
1
2
1
4.5 457
4.6 465
2621 436 1.28 0.26 0.14,0.13
4586 512 1.65 0.34 0.15,0.16
II
III
IV
V
4.4 515 18 035 891 4.13 0.20 0.27,0.52
4.0 509 32 270 894 6.25 0.30 0.26,0.49
3.6 515 30 044 1362 4.42 0.22 0.25,0.48
2
NPB
a
b
Turn-on voltage (V) at a luminance of 10 cd mꢀ2
.
Emission
maximum. Maximum luminance (cd mꢀ2) at the applied voltage (V).
d
c
Current density (mA cmꢀ2). Luminance efficiency (cd
g
External quantum efficiency (%). CIE coordinates (x, y).
A
ꢀ1).
e
f
New anthracene derivatives with the combined characteristics
of deep blue light-emitting and hole-transporting materials have
been developed. These materials showed deep blue emission with
high emission quantum efficiency over 73% in the solution and
strong luminance in the solid state and were electrochemically
and thermally stable with degradation temperature well above
400 1C. Non-doped blue OLEDs with a maximum efficiency of
1.65 cd Aꢀ1 and CIE coordinate of (0.15, 0.16), and green
OLEDs with a maximum efficiency of 6.25 cd Aꢀ1 and CIE
coordinate of (0.26, 0.49), were achieved. Their ability as HTL
for green OLEDs in terms of device performance and thermal
properties was greater than the common hole-transporter NPB.
This work was financially supported by the Thailand Research
Fund (MRU5080052). We acknowledge the scholarship support
from Center for Innovation in Chemistry (PERCH-CIC), Ubon
Ratchathani University, and the Office of the Higher Education
Commission, Thailand.
among good non-doped deep blue emitters reported. The
trend in device luminous efficiencies matches very well with
the observed decrease in PL quantum efficiencies (FF) on
going from 2 to 1. The efficiency of an OLED depends both
on the balance of electrons and holes and the FF of the
emitter.11 Analysis of band energy diagrams of all devices also
revealed that there is a barrier around 0.32 eV for holes to
migrate from the PEDOT:PSS/EL interface suggesting that a
migration of hole at that interface is more effective in both
devices and the charge efficiently recombines in the EL resulting
in good device performance.
As HOMO levels of these materials (5.32 eV) match
well with the work function of the ITO (4.8 eV) electrode,
they may potentially serve as hole-transporting material
(HTM). To test this hypothesis, double-layer green OLEDs
(devices III, IV) with the structure of ITO/PEDOT:PSS/
HTL(40 nm)/Alq3(50 nm)/LiF(0.5 nm):Al(150 nm) were fabri-
cated, where these materials were used as the hole-transporting
layer (HTL) and tris-(8-hydroxyquinoline) aluminium (Alq3)
as the green light-emitting and electron-transporting layers
(ETL). The reference device (device V) with the same structure
based on commonly used commercial HTM, N,N0-diphenyl-
N,N0-bis(1-naphthyl)-(1,10-biphenyl)-4,40-diamine (NPB), as
HTL was made for comparison. On comparison of HOMO
and LUMO levels of layers for the devices, it was found that
there is a barrier around 0.32 eV for holes to migrate from the
HTL layer to the Alq3 layer, while that for electrons to
transport from the Alq3 to the HTL layer is about 0.57 eV.
According to this band diagram and device configuration,
Notes and references
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both compounds can transport
a hole injected from
ITO/PEDOT:PSS to the Alq3 emitting layer. Under applied
voltage, all devices exhibited a bright green emission with
peaks centered at 509–515 nm and CIE coordinates of
(0.25–0.27, 0.48–0.52). The EL spectra match with the PL
spectrum of Alq3 and also other reported EL spectra of Alq3
devices (Fig. 1).12 No emission at the longer wavelength,
owing to exciplex species formed at the interface of HTL
and ETL materials, which has often occurred in the devices
fabricated from HTL with a planar molecular structure, was
detected.13 In our case, the formation of exciplex species could
be prevented by the bulky nature of both the anthracene core
and triphenylamine at the periphery of the molecules. From
these results and in view of the fact that barrier for electron-
migration at the Alq3/HTL interface (0.57 eV) is nearly twice
higher than those for hole-migration at the HTL/Alq3 interface
(0.32 eV), 1 and 2 act only as HTM and Alq3 would act
c
7124 Chem. Commun., 2011, 47, 7122–7124
This journal is The Royal Society of Chemistry 2011