Blue light-emitting and hole-transporting materials based on 9,9-bis(4-diphenylaminophenyl)fluorenes for efficient electroluminescent devices

Article abstract of DOI:10.1039/c2jm15480c

New bifunctional materials namely BTTF, TPTF and BPTF having 9,9-bis(4-diphenylaminophenyl)fluorene as a molecular platform were synthesized and characterized. These molecules showed strong blue emission in both solution and solid state with a solution fl

Full text of DOI:10.1039/c2jm15480c

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PAPER
Blue light-emitting and hole-transporting materials based on 9,9-bis(4-
diphenylaminophenyl)fluorenes for efficient electroluminescent devices†
A-monrat Thangthong, Narid Prachumrak, Ruangchai Tarsang, Tinnagon Keawin, Siriporn Jungsuttiwong,
*
Taweesak Sudyoadsuk and Vinich Promarak
Received 27th October 2011, Accepted 22nd January 2012
DOI: 10.1039/c2jm15480c
New bifunctional materials namely BTTF, TPTF and BPTF having 9,9-bis(4-diphenylaminophenyl)
fluorene as a molecular platform were synthesized and characterized. These molecules showed strong
blue emission in both solution and solid state with a solution fluorescence quantum efficiency of up
to 74% and were thermally stable amorphous materials with glass transition temperature well above
170 ꢀC. The abilities of these materials as blue light-emitting materials for blue OLEDs and hole-
transporting materials for green OLEDs in terms of device performance and thermal property were
superior to a commonly used N,N0-diphenyl-N,N0-bis(1-naphthyl)-(1,10-biphenyl)-4,40-diamine (NPB).
Efficient non-doped blue and Alq3-based green OLEDs with maximum luminance efficiencies and CIE
coordinates of 2.06 cd Aꢁ1 and (0.15, 0.13), and 4.94 cd Aꢁ1 and (0.29, 0.52) were achieved, respectively,
with BPTF having two pyrene substituents as the emitting layer and the hole-transporting layer,
respectively.
with triarylamines.10 The latter approach was proven to be more
effective and less complex synthesis is required. For example, 9,9-
bis(4-diphenylaminophenyl)fluorene (9,9-bis(triphenylamine)
Introduction
Organic light-emitting diodes (OLEDs) have attracted a great
deal of attention due to their applications, such as in full-color or
large-area flat panel displays, backlights, and general illumina-
tion.1 In the past decade, we have seen great progress in both
materials development and device fabrication techniques.2 One
area of continuing research is the pursuit of a stable-blue emit-
ting material. Poly(p-phenylene) (PPP)3 and related polymers,
such as ladder-type PPP and polyfluorenes (PFs), have received
strong interest in recent years.4 Although PFs in particular have
been used to make efficient blue light-emitting devices,5 they still
have a number of deficiencies as potential candidates for blue
OLEDs including low colour purity due to side emission from the
aggregates and defect sites, and requiring the use of hole-trans-
porting layers to obtain a balance in charge carriers.6 The hole-
injection ability of these PFs has been successfully improved
by many approaches such as blending with hole-transporting
triarylamines,7 end-capping the polymer chain with triaryl-
amines,8 copolymerization with triarylamine-containing mono-
mers9 and substitutions at the C-9 position of the fluorene block
fluorene) has been successfully used as a building block for the
synthesis of many blue emitting PFs and copolymers with
improving hole-injection and suppressing aggregation.11 It has
been shown that the formation of molecular fluorenes or oligo-
fluorenes efficiently improved the colour purity of these poly-
mers.12 Hence, using this fluorene building block to construct
new molecular materials might be a simple solution to develop
efficient pure blue emitters.
Owing to its high photoluminescence efficiency in the blue
region, high carrier mobility, and improved hole-injection ability
than other blue chromophores such as oligofluorenes, PFs or
PPP, pyrene has been used as a building block to form many
emissive materials.13 Recently, many kinds of pyrene-function-
alized materials have been synthesized and considered for several
applications,14 and some of them are proven to be promising blue
emitters for OLEDs.15 Pyrene-based molecules as bifunctional
materials, blue emitters and hole-transporters remain rare and
largely unexplored in OLEDs.16 Therefore, we herein imple-
mented all required aspects in the synthesized molecules (Fig. 1).
The use of 9,9-bis(4-diphenylaminophenyl)fluorene as a molec-
ular framework offers a perfect bulky molecule with high thermal
stability and an improved hole injection and transport ability
from the substituted triphenylamine units. Incorporation of
a pyrene unit into the cruciform of this platform is an effective
way to control the p–p stacking interactions and to maintain the
high blue emissive ability of pyrene in the solid state, which
Center for Organic Electronic and Alternative Energy, Department of
Chemistry and Center for Innovation in Chemistry, Faculty of Science,
Ubon Ratchathani University, Warinchumrap, Ubon Ratchathani, 34190,
Thailand. E-mail: pvinich@ubu.ac.th; pvinich@gmail.com; Fax: +66
4528 8379; Tel: +66 4535 3400 ext. 4510
† Electronic supplementary information (ESI) available: Synthesis of 2,
CV curves, energy diagrams and EL spectra of the devices, and 1H-,
13C-NMR and mass spectra. See DOI: 10.1039/c2jm15480c.
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with a three electrode system (platinum counter electrode, glassy
carbon working electrode and Ag/Ag+ reference electrode) at
a scan rate of 50 mV sꢁ1 in the presence of tetrabutyl ammonium
hexafluorophosphate (n-Bu4NPF6) as a supporting electrolyte in
dichloromethane under argon atmosphere. Melting points were
measured using an Electrothermal IA 9100 series of digital
melting point instruments and are uncorrected. High resolution
€
mass spectrometry (HRMS) analysis was performed on Bruker
micrOTOF (Q-ToF II) mass spectrometer. Elemental analysis
was performed on a PerkinElmer 2400 Series II CHNS/O
Elemental Analyzer at Chulalongkorn University.
Synthesis
2,7,9,9-Tatrakis(4-diphenylaminophenyl)fluorene (BTTF).
A
mixture of 2 (1.00 g, 0.55 mmol), triphenylamine-4-boronic acid
(0.39 g, 1.35 mmol), Pd(PPh3)4 (21 mg, 0.13 mmol) and an
aqueous Na2CO3 solution (2 M, 10 ml) in THF (15 ml) was
degassed with N2 for 5 min. The mixture was heated at reflux
under N2 atmosphere for 20 h. After the mixture was cooled to
room temperature water (50 ml) was added. The mixture was
extracted with CH2Cl2 (50 ml ꢂ 2). The combined organic phase
was washed with water (50 ml) and brine solution (50 ml), dried
over anhydrous Na2SO4, filtered, and the solvents were removed
to dryness. Purification by column chromatography using silica
gel eluting with a mixture of CH2Cl2 and hexane followed by
recrystallisation in a mixture of CH2Cl2 and methanol afforded
white solids (1.09 g, 78%): mp 200–201 ꢀC; 1H NMR (300 MHz,
CDCl3) d 6.92–7.09 (20H, m), 7.16–7.31 (32H, m), 7.47 (4H, d,
J ¼ 8.40 Hz), 7.50 (2H, d, J ¼ 9.01 Hz), 7.61 (2H, s), 7.8 (2H, d,
J ¼ 7.8 Hz); 13C NMR (75 MHz, CDCl3) d 64.68, 120.35, 122.77,
122.99, 123.09, 123.81, 124.45, 126.15, 127.79, 129.00, 129.17,
129.29, 135.24, 138.62, 139.62, 140.01, 146.27, 147.19, 147.65,
147.71, 152.52; IR (KBr disc) 3415, 3031, 1600, 1505, 1250, 1177,
813; found: C, 89.71; H, 5.34; N, 5.65%; HRMS (m/z) M+,
1139.5376. C85H62N4 requires C, 89.60; H, 5.48; N, 4.92%; M+,
1138.4977.
Fig. 1 Molecular structures of BTTF, TPTF and BPTF.
significantly impact the emission behaviour of devices. The high
steric hindrance in the molecule also offers good solubility and
thereby thermally stable amorphous thin film could be deposited
by a cheap solution process. Fluorene also has a number of
advantages, including its capability to emit in the blue part of the
visible spectrum, chemical and photochemical stability.4,5,12
Accordingly, this would result in new bifunctional materials with
combined blue-emitting and hole-transporting properties. An
investigation of their physical and photophysical properties, and
blue OLED fabrication and characterization is also reported.
Experimental section
2,7-Bis(pyren-1-yl)-9,9-bis(4-diphenylaminophenyl)fluorene (BP-
TF). BPTF was synthesized in the same manner as BTTF from 2
and pyrene-1-boronic acid, and obtained as yellow solids (0.56 g,
68%); mp > 250 ꢀC; 1H NMR (300 MHz, CDCl3) d 6.98 ppm (8H,
t, J ¼ 9.01 Hz), 7.07 (8H, d, J ¼ 7.69 Hz), 7.15–7.29 (20H, m), 7.51
(2H, d, J ¼ 8.18 Hz), 7.67 (2H, d, J ¼ 7.85 Hz), 7.77 (2H, s), 7.89–
8.00 (2H, m), 8.03–8.06 (2H, m), 8.10 (4H, t,
J ¼ 7.23 Hz), 8.22 (2H, t, J ¼ 8.10 Hz); 13C NMR (75 MHz,
CDCl3) d 50.50, 120.44, 122.73, 123.45, 124.30, 124.72, 124.80,
124.94, 125.12, 125.18, 125.26, 126.07, 127.44, 127.50, 127.69,
128.48, 128.83, 129.05, 129.21, 130.05, 130.64, 130.97, 131.53,
137.77, 138.96, 140.51, 146.39, 147.73; IR (KBr disc) 3415, 3031,
1589, 1510, 1260, 813; found: C, 92.32; H, 5.28; N, 2.59%; HRMS
(m/z) M+, 1052.4506. C81H52N2 requires C, 92.36; H, 4.98; N,
2.66%; M+, 1052.4130.
General procedure
1H and 13C nuclear magnetic resonance (NMR) spectra were
€
recorded on a Bruker AVANCE 300 MHz spectrometer with
tetramethylsilane as the internal reference using CDCl3 as the
solvent in all cases. Infrared (IR) spectra were measured on
a PerkinElmer FTIR spectroscopy spectrum RXI spectrometer
as potassium bromide (KBr) disc. Ultraviolet-visible (UV-Vis)
spectra were recorded for a dilute solution in dichloromethane
on
a PerkinElmer UV Lambda 25 spectrometer. Photo-
luminescence spectra and the fluorescence quantum yields (FF)
were recorded with a PerkinElmer LS 50B Luminescence
Spectrometer for a dilute solution in dichloromethane and a thin
film obtained by thermal evaporation. A quinine sulfate solution
in 0.01 M H2SO4 (FF ¼ 0.54) was used as the reference stan-
dard.17 Differential scanning calorimetry (DSC) analysis and
thermogravimetry analysis (TGA) were performed on a MET-
TLER DSC823e thermal analyzer and a Rigaku TG-DTA 8120
thermal analyzer, respectively, with a heating rate of 10 ꢀC minꢁ1
under nitrogen atmosphere. Cyclic voltammetry (CV) measure-
ments were carried out on an Autolab potentiostat PGSTAT 12
7-Bromo-2,9,9-tris(4-diphenylaminophenyl)fluorene (3). 3 was
synthesized in the same manner as BTTF from 2 and triphenyl-
amine-4-boronic acid, and obtained as white solids (1.02 g, 72%);
mp 167–170 ꢀC; 1H NMR (300 MHz, CDCl3) d 6.93 ppm (4H, d,
J ¼ 9.01), 7.00 (5H, t, J ¼ 8.10), 7.15–7.07 (19H, m), 7.31–7.21
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(12H, m), 7.46 (2H, d, J ¼ 7.8.40), 7.51 (1H, s) 7.58 (1H, s), 7.62
(2H, d, J ¼ 5.10), 7.64 (1H, s), 7.77 (1H, d, J ¼ 7.8 Hz); 13C NMR
(75 MHz, CDCl3) d 64.66, 120.45, 120.45, 121.16, 121.42, 122.92,
123.07, 123.70, 124.40, 124.56, 126.25, 127.77, 128.84, 129.22,
129.30, 129.37, 130.66, 137.68, 138.65, 138.87, 140.58, 146.48,
147.60, 152.09, 153.88; IR (KBr disc) 3415, 3030, 1600, 1520,
1256, 1170, 813; found: C, 82.44; H, 5.31; N, 4.37%; HRMS (m/z)
M+, 973.3099. C67H48BrN3 requires C, 82.53; H, 4.96; N, 4.31%;
M+, 973.3032.
7-(Pyren-1-yl)-2,9,9-tris(4-diphenylaminophenyl)fluorene (TP-
TF). TPTF was synthesized in the same manner as BTTF from 3
and pyrene-1-boronic acid, and obtained as light yellow solids
1
(0.48 g, 75%); mp 189–199 ꢀC; H NMR (300 MHz, CDCl3)
d 6.99 ppm (8H, t, J ¼ 6.90 Hz), 7.12 (10H, d, J ¼ 7.00 Hz), 7.29
(24H, m), 7.48 (2H, d, J ¼ 12 Hz), 7.62 (3H, t, J ¼ 12 Hz), 7.68
(1H, s), 7.87–7.95 (3H, m), 7.98–8.03 (2H, m), 8.08 (4H, t, J ¼ 12
Hz); 13C NMR (75 MHz, CDCl3) d 64.74, 120.24, 120.63, 122.76,
123.30, 124.38, 124.70, 124.77, 124.93, 125.09, 125.15, 125.26,
126.05, 127.45, 127.67, 128.45, 128.70, 129.02, 129.19, 129.91,
130.59, 130.95, 131.51, 137.79, 138.89, 139.68, 140.25, 146.30,
147.69, 152.38; IR (KBr disc) 3415, 3031, 1602, 1504, 1260, 813;
found: C, 91.13; H, 5.36; N, 3.89%; HRMS (m/z) M+, 1096.4537.
C83H59N3 requires C, 90.93; H, 5.24; N, 3.83%; M+, 1097.4709.
Scheme 1 Synthesis of BTTF, TPTF and BPTF: (a) triphenylamine,
MeSO3H, 190 ꢀC; (b) pyrene-1-boronic acid; (c) triphenylamine-4-
boronic acid, Pd(PPh3)4, 2 M Na2CO3, THF, reflux.
the cathode materials and pumped back; a 0.5 nm thick LiF and
a 150 nm thick aluminium layers were subsequently deposited
through a shadow mask on the top of EML/HTL film without
braking vacuum to from an active diode area of 4 mm2. The
measurement of the device efficiency was performed according to
M.E. Thomson’s protocol and the device external quantum
efficiencies were calculated using a procedure reported previ-
ously.19 Current density–voltage–luminescence (J–V–L) charac-
teristics were measured simultaneously by the use of a Keithley
2400 source meter and a Newport 1835C power meter equipped
with a Newport 818-UV/CM calibrated silicon photodiode. The
EL spectra were acquired by an Ocean Optics USB4000 multi-
channel spectrometer. All the measurements were performed
under ambient atmosphere at room temperature.
Quantum calculation
The ground state geometries of all molecules were fully optimized
using density functional theory (DFT) at the B3LYP/6-31G (d,p)
level, as implemented in Gaussian 03.18 TDDFT/B3LYP calcu-
lation of lowest excitation energies was performed at the opti-
mized geometries of the ground states.
Fabrication and characterisation of OLEDs
OLED devices using BTTF, TPTF and BPTF as EML with
a configuration of ITO/PEDOT:PSS/EML(50 nm)/BCP(40 nm)/
LiF(0.5 nm):Al(150 nm) and double-layer green OLED devices
using BTTF, TPTF, BPTF and NPB as HTL with a configura-
tion of ITO/PEDOT:PSS/HTL(40 nm)/Alq3(50 nm)/
LiF(0.5 nm):Al(150 nm) were fabricated and characterized as
followed. The patterned indium tin oxide (ITO) glass substrate
with a sheet resistance 14 U ,ꢁ1 (purchased from Kintec
Company) was thoroughly cleaned by successive ultrasonic
treatment in detergent, deionised water, isopropanol, and
acetone, and then dried at 60 ꢀC in a vacuum oven. A 50 nm thick
PEDOT:PSS hole injection layer was spin-coated on top of ITO
from a 0.75 wt% dispersion in water at a spin speed of 3000 rpm
for 20 s and dried at 200 ꢀC for 15 min under vacuum. Thin films
of each organic EML or HTL were deposited on top of the
PEDOT:PSS layer by evaporation from resistively heated
alumina crucibles at the evaporation rate of 0.5–1.0 nm sꢁ1 in
vacuum evaporator deposition (ES280, ANS Technology) under
a base pressure of ꢃ10ꢁ5 mbar. The film thickness was monitored
and recorded by a quartz oscillator thickness meter (TM-350,
MAXTEK). A 40 nm thick hole-blocking layer of BCP or
a 50 nm thick green-emitting layer of Alq3 was then deposited on
the organic EML or HTL, respectively, without breaking the
vacuum chamber. The chamber was vented with dry air to load
Fig. 2 The HOMO (right) and LUMO (left) orbitals of BTTF, TPTF
and BPTF.
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obtained from the experimental results (ꢃ0.03 to 0.11 eV). There
are factors responsible for the errors because the orbital energy
difference between HOMO and LUMO is still an approximate
estimation of the transition energy since the transition energy
also contains significant contributions from some two-electron
integrals. The real situation is that an accurate description of the
lowest singlet excited state requires a linear combination of
a number of excited configurations.
Results and discussion
Synthesis
The target molecules were synthesized as outlined in Scheme 1.
Firstly, an intermediate 2 was synthesized via a tandem protocol
of readily obtained 2,7-dibrofluorenone (1) with an excess of
triphenylamine catalysed by methanesulfonic acid at 190 ꢀC.
Coupling of 2 with either pyrene-1-boronic acid or triphenyl-
amine-4-boronic acid by the Suzuki cross-coupling reaction
catalysed by Pd(PPh3)4 afforded BPTF and BTTF in good yields
as yellow and white solids, respectively. Under the same
Pd-catalysed coupling conditions, a stepwise coupling of 2 with
triphenylamine-4-boronic acid followed by 1-pyreneboronic acid
gave TPTF as a light yellow solid in 54% yield over two steps.
Their chemical structures were characterized unambiguously
The optical properties of BTTF, TPTF and BPTF were
investigated in dilute CH2Cl2 solution and thin film obtained by
thermal evaporation on a quartz substrate. The pertinent data
are presented in Fig. 3a and summarized in Table 1. Their
solution absorption spectra showed two absorption bands:
a strong absorption band at 303 nm corresponding to the p–p*
local electron transition of the pendant triphenylamine
moieties and a less intense absorption band at a longer wave-
length (350–400 nm) assigned to the p–p* electron transition of
the p-conjugated backbone. In solid state, similar absorption
features with a slight red shift compared to their corresponding
solution spectra were observed. These materials exhibited
a strong blue fluorescence in both solution and solid state. The
fluorescence quantum yields (FF) of BTTF, TPTF and BPTF
measured in dilute solution using quinine sulfate solution in 0.01
M H2SO4 (FF ¼ 0.54) as a standard were 0.43, 0.64 and 0.74,
respectively. The results indicated that the direct attachment of
the pyrene ring to the molecule boosted the FF of these materials.
Their solution photoluminescence (PL) spectra showed
a featureless emission peak in the blue region (430–470 nm). The
PL spectrum of TPTF red-shifted by 28 nm with respect to those
of BTTF and BPTF, resulting from its donor–acceptor character
as indicated by quantum calculation. These materials showed
small Stokes shifts (66–89 nm) suggesting less energy loss during
the relaxation process and thereby ensuing efficient fluorescence.
The thin film PL emission spectra of BTTF and BPTF were
similar to their solution PL spectra, hence indicating no or less if
any solid state packing occurred in this case due to their bulky
molecular structures.
1
with H-NMR, 13C-NMR spectroscopy and mass spectrometry.
These materials showed good solubility in organic solvents,
opening the door to solution processing techniques.
Quantum calculation and optical properties
To understand the electronic properties and the geometries of the
synthesized molecules, quantum chemical calculations were
performed using the TDDFT/B3LYP/6-31G (d,p) method.18 The
optimized structures of these compounds revealed that both
triphenylamine moieties at the C-9 position of the fluorene ring
generated high steric hindrance, resulting in a bulky molecular
structure and thereby preventing a closed p–p stacking interac-
tion of the molecule. The molecules (TPTF and BTTF) became
more steric when the C-2 or -7 positions of the fluorene ring being
attached by extra triphenylamine units. In the highest occupied
molecular orbitals (HOMO) of BTTF and TPTF, p-electrons
delocalized over the p-conjugated 2-(4-diphenylaminophenyl-
fluorene) backbone and also located on triphenylamine pendant
groups, while in the HOMO orbital of BPTF, p-electrons
localized only on the triphenylamine (Fig. 2). This suggested that
there is no p-electron interaction between the fluorene and pyr-
ene moieties. In the lowest unoccupied molecular orbitals
(LUMO) of all compounds, the excited electrons delocalized
over the quinoid-like pyrene–fluorene plane, created in the case
of TPTF a donor–acceptor molecule with triphenylamine acting
as the donor and pyrene acting as the acceptor. The HOMO–
LUMO energy gaps (Eg cal.) were calculated and are presented in
Table 1. These calculated values slightly deviated from those
Thermal and electrochemical properties
For OLED applications, thermal stability of organic materials is
crucial for device stability and lifetime. The thermal instability or
low glass transition temperature (Tg) of the amorphous organic
layer may result in the degradation of organic devices due to
Table 1 Physical data of the synthesized molecules
Solutiona/nm
Thin filmc/nm
lem Stokes shiftb labs
lem FF
(Tg/T5d)e/ꢀC E1/2 (Eonset)f/V
Egg/eV Eg calh/eV HOMO/LUMOi/eV
d
Comp. labs
BTTF
TPTF
BPTF
303, 363 437 71
303, 373 462 89
303, 369 436 66
310, 375 436 0.43 171/390
310, 380 447 0.64 174/405
310, 375 441 0.74 179/472
0.96 (0.85)
0.98, 1.27 (0.87) 3.01
0.97, 1.13 (0.87) 3.02
3.06
3.11
3.04
3.13
ꢁ5.29/ꢁ2.23
ꢁ5.31/ꢁ2.30
ꢁ5.31/ꢁ2.28
a
Measured in CH2Cl2 solution. b Stokes shift calculated from the difference between lmax of the absorption and emission spectra. c Measured on the
thermal evaporated film on a quartz substrate. d Determined in CH2Cl2 solutions (A < 0.1) at room temperature using quinine sulfate solution in 0.01 M
H2SO4 (FF ¼ 0.54) as a standard. e Obtained from DSC (2nd heating scan) and TGA at a heating rate of 10 ꢀC minꢁ1
.
Measured by CV (Pt counter,
glassy carbon working and SCE reference electrodes) in CH2Cl2 containing n-Bu4NPF6 as a supporting electrolyte. Estimated from the optical
f
g
h
i
absorption edge. Obtained from the quantum chemical calculation using TDDFT/B3LYP/6-31G (d,p). Calculated by HOMO ¼ ꢁ(4.44 + Eonset),
and LUMO ¼ HOMO ꢁ Eg, where Eonset is the onset potential of the oxidation peak.
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materials with high Tg which was higher thanꢀ those of the
commonly used HTMs such as NPB (Tg ¼ 100 C) and N,N0-
bis(3-methylphenyl)-N,N0-bis(phenyl)benzidine (TPD) (Tg
¼
63 ꢀC), and other triphenylamine derivatives.20 The results
proved that the use of 9,9-bis(4-diphenylaminophenyl)fluorene
as a molecular platform could reduce the crystallization of
pyrene and improve the amorphous stability of the materials,
which in turn could increase the service time in device operation
and enhance the morphological stability of the thin film.21
Moreover, the abilities of these materials to form molecular glass
and dissolve in organic solvents offer the possibility to prepare
good thin films by both thermal evaporation and solution casting
techniques which are highly desirable for fabrication of an
OLED device.
Electrochemical behaviours of all compounds were investi-
gated by cyclic voltammetry (CV), and the resulting data are
shown in Fig. 4 and summarized in Table 1. The experiment was
carried out in degassed CH2Cl2 containing 0.1 M n-Bu4NPF6 as
a supporting electrolyte under argon atmosphere. BTTF was
found to exhibit one quasi-reversible oxidation process at 0.96 V,
while both TPTF and BPTF showed two quasi-reversible
oxidation processes with an additional peak at a lower potential
(0.74 V) on the cathodic scan (Epc) being observed. The first
oxidation potentials of all compounds were nearly identical
(0.96–0.98 V) and were assigned to the removal of electrons from
the triphenylamines resulting in radical cations as demonstrated
in Fig. 5. The multiple CV scans of BTTF revealed identical CV
curves, indicating no oxidative coupling or a weak oxidative
coupling if any at p-phenyl rings of the peripheral triphenylamine
led to electro-polymerization and BTTF is electrochemically
stable molecule. However, this type of electrochemical reaction
was detected in both TPTF and BPTF as indicated by the pres-
ence of the cathodic peak at 0.74 V in their first CV scan. The
repeated CV scans of these compounds also revealed an
increasing change in their CV curves as illustrated in Fig. S1
(ESI†). Usually, this type of electrochemical coupling reaction is
detected in most triphenylamine derivatives with unsubstituted
p-position of the phenyl ring such as in the case of 2,7-bis-
[2-(4-diphenylaminophenyl)-1,3,4-oxadiazol-5-yl]-9,9-bis-n-hexyl-
fluorene.22 A proposed oxidation and electrochemical coupling
reaction of these materials is outlined in Fig. 5. During the first
oxidation, the electrons are removed from the N-atom of all
available triphenylamine moieties to give a radical cation form A
which undergoes electron delocalization to get resonance forms
B, C and D, respectively. These radical cations are highly reactive
Fig. 3 (a) UV-vis absorption and PL spectra measured in CH2Cl2
solution and as thin film obtained by thermal deposition on a quartz
substrate. (b) DSC (2nd heating scan) and TGA thermograms measured
under N2 at a heating rate of 10 ꢀC minꢁ1
.
morphological changes. The thermal properties of BTTF, TPTF
and BPTF were investigated by the thermogravimetric analysis
(TGA) and differential scanning calorimetry (DSC), and the
results are shown in Fig. 3b and summarized in Table 1. Those
results suggested that all compounds were thermally stable
materials with 5% weight loss temperatures (T5d) well over
390 ꢀC. In the 1st and 2nd heating scans, DSC thermograms of all
compounds displayed only a baseline shift due to glass transition
ꢀ
temperatures (Tg) ranging from 171 to 179 C with no crystalli-
zation and melting peaks observed at higher temperatures. These
results indicated that these materials were stable amorphous
Fig. 4 (a) CV curves of BTTF, TPTF and BPTF and (b) multiple CV
scans of TPTF measured in CH2Cl2 at a scan rate of 50 mV sꢁ1
.
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Fig. 5 A proposed oxidation and electrochemical reaction of the triphenylamine moiety.
species and readily undergo dimerization coupling to form
a stable neutral molecule. Resonance forms A, B and D having
a radical on the N atom and ortho-C atoms, respectively, are less
reactive to a dimerization coupling reaction due to high steric
hindrance and thereby this electrochemical reaction would take
place through the resonance form C. However, triphenylamine
units X and Y are unlikely to undergo such a coupling reaction
because the radical cation formed is stabilized by electron delo-
calization through the attached fluorene ring20a and is prevented
by the steric hindrance of the adjacent triphenylamine unit,
respectively. Hence, the oxidative coupling of the triphenylamine
occurs only with the unstable and less steric triphenylamine unit
Z. Steric hindrance and stability of triphenylamine moieties
therefore played an important role in preventing such an elec-
trochemical reaction in BTTF. While in cases of TPTF and
BPTF having one or both of C-2 and -7 positions of the fluorene
ring being substituted by a pyrene ring which is less bulky than
the triphenylamine unit (in BTTF), the coupling reaction took
place through less steric triphenylamine pendant groups (X).
However, this type of electrochemical reaction will become
inactive in the non-diffusion system or solid state. Moreover,
under these CV experiment conditions, no distinct reduction
process was observed in all cases. The HOMO and LUMO
energy levels of BTTF, TPTF and BPTF were calculated from
the oxidation onset potentials (Eonset) and energy gaps (Eg) and
the results are summarized in Table 1. The HOMO levels of these
materials ranged from 5.29 to 5.31 eV matching well with the
work functions of the gold (Au) or indium tin oxide (ITO)
electrodes and hence favouring the injection and transport of
holes.
Electroluminescent properties
Owing to their high blue fluorescence (FF ¼ 0.43–0.74) and their
HOMO levels (5.30 eV) lying between those of PEDOT:PSS
(5.00 eV) and Alq3 (5.80 eV), the new synthesized materials
BTTF, TPTF and BPTF could be used as bifunctional materials
namely blue light-emitting and hole-transporting materials. This
encouraged us to investigate the use of these compounds as an
emissive layer (EML) for blue OLED and hole-transporting
layer (HTL) for Alq3-based green OLED. The blue light-emit-
ting diodes with the device structure of indium tin oxide (ITO)/
PEDOT:PSS/EML(50 nm)/BCP(40 nm)/LiF(0.5 nm):Al(150 nm)
and green light-emitting diodes with the device structure of ITO/
PEDOT:PSS/HTL(40 nm)/Alq3(50 nm)/LiF(0.5 nm):Al(150 nm)
were fabricated. Tris-(8-hydroxyquinoline) aluminium (Alq3)
acts as the green light-emitting (EML) and electron-transporting
layers
(ETL).
Conductive
polymer,
poly(3,4-ethyl-
enedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS), as
the hole injection layer and 2,9-dimethyl-4,7-diphenyl-1,10-phe-
nanthroline (BCP) as the hole blocking layer (HBL) were inte-
grated to enable high-performance devices. From our study and
Table 2 Device characteristics of the fabricated OLED devices
hd
EQEe
CIEf
a
b
c
Device
Structure
Von
lmax
Lmax
I
II
ITO/PEDOT:PSS/BTTF/BCP/LiF:Al
ITO/PEDOT:PSS/TPTF/BCP/LiF:Al
ITO/PEDOT:PSS/BPTF/BCP/LiF:Al
ITO/PEDOT:PSS/NPB/BCP/LiF:Al
ITO/PEDOT:PSS/BTTF/Alq3/LiF:Al
ITO/PEDOT:PSS/TPTF/Alq3/LiF:Al
ITO/PEDOT:PSS/BPTF/Alq3/LiF:Al
ITO/PEDOT:PSS/NPB/Alq3/LiF:Al
ITO/BPTF/Alq3/LiF:Al
3.3
3.3
3.4
3.4
3.1
3.2
3.0
3.1
5.9
4.2
427
437
438
436
519
518
519
516
520
518
2340
3639
6245
0.83
0.88
2.06
0.73
4.78
4.88
4.94
4.45
1.24
0.91
0.22
0.29
0.56
0.24
0.23
0.24
0.24
0.22
0.06
0.05
0.16, 0.08
0.15, 0.12
0.15, 0.13
0.15, 0.07
0.29, 0.53
0.28, 0.53
0.29, 0.52
0.30, 0.54
0.27, 0.53
0.30, 0.54
III
IV
V
VI
VII
VIII
IX
X
343
28 895
26 375
30 760
30 044
1236
ITO/PEDOT:PSS/Alq3/LiF:Al
4961
a
b
c
d
Turn-on voltage (V) at a luminance of 1 cd mꢁ2
.
Emission maximum. Maximum luminance (cd mꢁ2) at the applied voltage (V). Luminance
efficiency (cd Aꢁ1). External quantum efficiency (%). f CIE coordinates (x, y).
e
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other reports,23 it was found that the incorporation of
PEDOT:PSS in the device as the hole injection layer not only
increased the maximum luminance from 1236 cd mꢁ2 (h of
1.24 cd Aꢁ1) in device IX to 30 760 cd mꢁ2 (h of 4.94 cd Aꢁ1) in
device VII, but also significantly decreased the turn-on voltage
from 5.9 V to 3.0 V (Table 2). Moreover, their EL spectra were
near identical. It has been pointed out that the lower operating
voltage of the PEDOT:PSS-based device can be attributed to the
rough and porous surface of the spin-coated PEDOT:PSS
polymer layer, which increases the contact area to enhance hole
injection and lowers the barrier at the organic–organic interface
by relocating the barrier to the more conductive PEDOT:PSS
layer.24 To enable high-performance devices therefore
PEDOT:PSS as the hole injection layer was integrated into all
devices. To compare the blue light-emitting and hole-trans-
porting abilities of the synthesized materials, N,N0-diphenyl-
platform. Additionally, a stable emission was obtained from all
devices and the EL spectra did not change much over the entire
drive voltages (Fig. S2, ESI†). Device I emitted a deep blue
(427 nm) light with CIE coordinates of (0.16, 0.08), which is close
to the National Television System Committee (NTSC) deep blue
(0.14, 0.10) standard. The voltage–luminance and voltage–
current density (J–V–L) characteristics of the devices are shown
in Fig. 6b–d, and their electrical parameters are summarized in
Table 2. The light turn-on voltage at 1 cd mꢁ2 for all devices was
in the range of 3.3–3.4 V and the operating voltage at 100 cd mꢁ2
was in the range of 4.0–5.0 V, indicating good performance is
achieved for all the devices. The device characteristics in terms of
maximum brightness, turn-on voltage and maximum luminous
efficiency clearly verified that the blue-emitting abilities of these
newly synthesized materials were greater than the NPB-based
device (device IV). The results also revealed that BPTF bearing
two pyrene rings attached to the fluorene ring of the 9,9-bis(4-
diphenylaminophenyl)fluorene had the best EML properties
among these three materials. The device (III) showed a high
maximum brightness of 6245 cd mꢁ2 at 8.8 V, a low turn-on
voltage of 3.4 V, a maximum luminous efficiency of 2.06 cd Aꢁ1
and a maximum external quantum efficiency of 0.56%. A
reasonably lower device performance was observed in devices I
(BTTF as EML) and II (TPTF as EML) displaying a maximum
luminous efficiency of 0.83 and 0.88 cd Aꢁ1, respectively. The
trend in device luminous efficiencies matched very well with the
observed decrease in PL quantum efficiencies in going from
BPTF to TPTF to BTTF (Table 1). The fluorescence quantum
yield (FF) of BPTF was significantly higher than those of TPTF
and BTTF. It has been demonstrated that the efficiency of an
OLED depends both on the balance of electrons and holes and
the FF of the emitter.25 Analysis of the band energy diagrams of
N,N0-bis(1-naphthyl)-(1,10-biphenyl)-4,40-diamine
(NPB),
a commonly used commercial HTM, was employed as reference
EML and HTL materials and the reference devices (IV and VIII)
of the same structure were fabricated.
As non-doped blue emitters, under an applied voltage, devices
I–III emitted a bright blue light with peaks centered at 427, 437
and 438, respectively (Fig. 6a). The electroluminescence (EL)
spectra of all diodes matched with their corresponding PL
spectra with the EL of device I being slightly blue shifted (9 nm)
from the PL of BTTF. Moreover, no emission shoulder at the
longer wavelength owing to excimer and exciplex species formed
at the interface of EML and HBL materials, which often occurs
in the devices fabricated from EML with a planar molecular
structure, was detected. In our case, the formation of these
species could be prevented by the bulky nature of the 9,9-bis(4-
diphenylaminophenyl)fluorene implemented as the molecular
Fig. 6 (a) EL spectra, (b) V–L characteristics, (c) I–V characteristics, and (d) variation of luminance efficiency with current density of the fabricated
blue OLED devices using the synthesized materials as EML.
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Fig. 7 (a) EL spectra, (b) V–L characteristics, (c) I–V characteristics, and (d) variation of luminance efficiency with current density of the fabricated
green OLED devices using the synthesized materials as HTL.
these diodes also revealed that the HOMO levels of BTTF, TPTF
and BPTF (5.28–5.31 eV) sat perfectly between those of the hole
injection layer (PEDOT:PSS, 5.00 eV) and HBL (BCP, 6.50 eV),
resulting in the charge efficiently recombining in the emitting
layer and better device performance (Fig. S3, ESI†).
longer wavelength owing to exciplex species formed at the
interface of HTL and ETL materials, which often occurs in the
devices fabricated from HTL with a planar molecular structure,
was detected.27 In our case, the formation of exciplex species
could be prevented by the bulky nature of the 9,9-bis(4-diphe-
nylaminophenyl)fluorene. From device performance results and
the fact that the barrier for electron-migration at the Alq3/HTL
interface (ꢃ0.7 eV) is higher than that for hole-migration at the
HTL/Alq3 interface (ꢃ0.5 eV), under the present device config-
uration of ITO/PEDOT:PSS/HTL(50 nm)/Alq3(50 nm)/
LiF(0.5 nm):Al(200 nm), BTTF, TPTF and BPTF would act only
as HTLs, and Alq3 would act preferably as an electron blocker
more than as a hole blocker, and charge recombination was thus
confined to the Alq3 layer. More importantly, a stable emission
was obtained from all diodes (V–VII) and the EL spectra and
CIE coordinates did not change over the entire applied voltages
(Fig. S4, ESI†).
As HTLs, devices (V–VII) exhibited the light turn-on voltage
at 1 cd mꢁ2 in the range of 3.0–3.1 V and the operating voltage at
100 cd mꢁ2 in the range of 4.2–4.6 V, indicating good perfor-
mance is achieved for all the devices (Fig. 7 and Table 2). By
comparison with the reference device X, it was found that the
incorporation of BTTF, TPTF and BPTF in the device as HTL
not only increased the maximum luminance from 4961 cd mꢁ2
(h of 0.91 cd Aꢁ1) to 26 375–30 760 cd mꢁ2 (h of 4.78–4.94 cd Aꢁ1
)
in device III, but also significantly decreased the turn-on voltage
from 4.2 V to 3.0–3.2 V. Besides, their EL spectra were nearly
identical. Moreover, the device characteristics in terms of lumi-
nous efficiency clearly demonstrated that the HTM abilities of
these materials were greater than the NPB-based device (device
VIII). Device VII having compound BPTF as HTL exhibited the
best performance with a high maximum brightness of 30 760 cd
mꢁ2 for green OLED at 10.6 V, a low turn-on voltage of 3.0 V,
a maximum luminous efficiency of 4.94 cd Aꢁ1 and a maximum
external quantum efficiency of 0.24%. A comparable device
performance was observed from devices V (BTTF as HTL) and
VI (TPTF as HTL) (Table 1). Under an applied voltage, all
devices (V–VII) exhibited a bright green emission with peaks
centered at 519 nm and CIE coordinates of (0.29, 0.53) (Fig. 7a).
The electroluminescence (EL) spectra of these diodes were
identical, and matched with the PL spectrum of Alq3, the EL
spectra of the reference devices (VII and X) and also other
reported EL spectra of Alq3-based devices.26 No emission at the
Although, many blue emitting and hole-transporting materials
have been reported, in terms of the amorphous morphology, high
Tg and device efficiency, BPTF is among the good bifunctional
materials reported.
Conclusions
In conclusion, we successfully designed and synthesized three
bifunctional materials namely BTTF, TPTF and BPTF. By the
use of 9,9-bis(4-diphenylaminophenyl)fluorene as a molecular
platform, we were able to reduce the crystallization and main-
tained high blue emissive ability of pyrene in the solid state, and
improved the amorphous stability of these materials. Strong blue
emission in both solution and solid state with solution
6876 | J. Mater. Chem., 2012, 22, 6869–6877
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Acknowledgements
This work was financially supported by the Thailand Research
Fund (RMU5080052). We acknowledge the scholarship support
from Center of Excellence for Innovation in Chemistry
(PERCH-CIC), Ubon Ratchathani University, and the Office of
the Higher Education Commission, Thailand.
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