Synthesis, characterization, physical properties, and applications of highly fluorescent pyrene-functionalized 9,9-bis(4-diarylaminophenyl)fluorene in organic light-emitting diodes

Article abstract of DOI:10.1016/j.tetlet.2012.07.112

A new, highly fluorescent pyrene-functionalized 9,9-bis(4- diarylaminophenyl)fluorene, namely PTF, was synthesized and characterized. This material is an amorphous molecular glass with notably high T_g, is electrochemically stable, and exhibits strong blue emission both in solution and solid state. It shows promising ability as a solution processed blue emitter and hole-transporter for OLEDs. High-efficiency sky-blue and Alq3-based green devices with luminance efficiencies of 1.13 and 4.08 cd/A are achieved, respectively. 2012 Elsevier Ltd. All rights reserved.

Full text of DOI:10.1016/j.tetlet.2012.07.112

Tetrahedron Letters 53 (2012) 5492–5496
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis, characterization, physical properties, and applications of highly
fluorescent pyrene-functionalized 9,9-bis(4-diarylaminophenyl)fluorene
in organic light-emitting diodes
Narid Prachumrak, A-monrat Thangthong, Ruangchai Tarsang, Tinnagon Keawin, Siriporn Jungsuttiwong,
⇑
Taweesak Sudyoadsuk, Vinich Promarak
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
a r t i c l e i n f o
a b s t r a c t
Article history:
A new, highly fluorescent pyrene-functionalized 9,9-bis(4-diarylaminophenyl)fluorene, namely PTF, was
synthesized and characterized. This material is an amorphous molecular glass with notably high Tg, is
electrochemically stable, and exhibits strong blue emission both in solution and solid state. It shows
promising ability as a solution processed blue emitter and hole-transporter for OLEDs. High-efficiency
sky-blue and Alq3-based green devices with luminance efficiencies of 1.13 and 4.08 cd/A are achieved,
respectively.
Received 11 May 2012
Revised 13 July 2012
Accepted 27 July 2012
Available online 2 August 2012
Keywords:
Pyrene
Ó 2012 Elsevier Ltd. All rights reserved.
Fluorene
Blue-emitting material
Hole-transporting material
Organic light-emitting diode
Organic light-emitting diodes (OLEDs) have attracted significant
attention due to their applications as next-generation flat-screen
displays and in general illumination.1 In the past decade, we have
seen great progress in both material development and device fabri-
cation techniques.2 One area of on-going research is the pursuit of a
stable blue-emitting material.3 Although, many types of blue light-
emitting materials have been investigated, such as derivatives of
anthracene, fluorene, and polycyclic aromatic hydrocarbons,3c,4 fur-
ther improvement for blue OLEDs compared to red and green
devices is still required. Pyrene has also been widely used as a build-
ing block to form many emissive materials5 due to its high photolu-
minescence efficiency in the blue spectrum, high carrier mobility,
and improved hole-injection ability compared with other blue chro-
mophores.6 Though many pyrene-functionalized materials were
proven to be promising blue emitters for OLEDs,7 solution-
processed analogues remain rare and largely unexplored in OLEDs.8
To enhance the color purity of pyrene-based materials, it is essential
to integrate either large bulky groups9 or multi-substituted rigid
moieties10 to suppress aggregates and excimers. 9,9-Bis(4-diphe-
blue-emitting molecules and polymers with improved hole-injec-
tion, thermal stability, and suppressed aggregation.11
Therefore, we herein implemented all the required features in
the synthesized molecule (Scheme 1). Incorporation of pyrene units
into a cruciform of 9,9-bis(4-diphenylaminophenyl)fluorene would
offer an effective way to control the p–p stacking interactions and
the associated red shift in emission, and maintain the high blue
emissive ability of pyrene in the solid state. Moreover, triarylamine
units also improve hole injection and transport ability with high
thermal stability. Owing to its supramolecular steric hindrance,
the material would possess good solubility and hence a thermally
stable amorphous thin film could be deposited by a cheap solution
process. Accordingly, this would result in a new solution-process-
able molecular material with combined blue-emitting and hole-
transporting properties for OLEDs.
In this work, we present the synthesis and characterization of
such a material, namely 9,9-bis{4-di[4-(pyren-2-yl)phenyl]amino-
phenyl}-2,7-di(pyren-2-yl)fluorene (PTF). An investigation of its
physical and photophysical properties, and its application as active
layers in OLEDs are also reported. Scheme 1 outlines the synthesis
of the designed molecule, PTF. We began with the aerial oxidation
of 2,7-dibromofluorene (1) with KOH/air in DMSO to give 2,7-dib-
romofluorenone (2) as a yellow solid in a low yield of 31% and
2-bromofluorenone was also isolated in 23% as a by-product. An
improved 92% yield of fluorenone 2 was obtained by oxidation of 1
nylaminophenyl)fluorene possesses
a highly rigid framework
and has been successfully used as a building block to form many
⇑
Corresponding author.
E-mail addresses: pvinich@ubu.ac.th, pvinich@gmail.com (V. Promarak).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.07.112

N. Prachumrak et al. / Tetrahedron Letters 53 (2012) 5492–5496
5493
Ph3N, MeSO3H
190oC
61%
CrO3, Ac2O
92%
Br
Br
Br
Br
Br
Br
1
2
O
N
N
3
NBS, THF, rt
95%
1-pyreneboronic acid
Pd(PPh3)4, THF
2 M Na2CO3, Δ
N
N
Br
Br
N
75%
Br
Br
N
PTF
4
Br
Br
Scheme 1. Synthesis of PTF.
with CrO3 in acetic anhydride along with a trace amountof 2-bromo-
fluorenone. The 9,9-bis(4-diphenylaminophenyl)fluorene (3) inter-
mediate was then synthesized via a tandem protocol involving the
reaction of fluorenone 2 with excess triphenylamine catalyzed by
MeSO3H at 190 °C. Product 3 was obtained as a white solid in 61%
yield. Selective bromination of compound 3 with NBS in THF gave
9,9-bis[4-di(4-bromophenyl)aminophenyl]-2,7-dibromofluorene
(4) as a white solid in excellent yield. Coupling of the resultant hexa-
bromide 4 withreadilyavailable 1-pyrene boronic acidunder Suzuki
cross-coupling conditions catalyzed by Pd(PPh3)4/Na2CO3 (aq) in
THF afforded PTF as a light-yellow solid in a good yield of 75%. The
structure was characterized unambiguously by NMR spectroscopy
and mass spectrometry.12 PTF showed good solubility in organic
solvents, opening the door to solution processing techniques.
Quantum chemical calculations on PTF performed using the
TDDFT/B3LYP/6-31G(d,p) method13 revealed that the two triaryl-
amine units at the C-9 position of the fluorene ring generated high
steric hindrance, resulting in a bulky molecular structure with high
steric repulsion between the aromatic rings and thereby prevent-
effectively the intermolecular
p
–
p
interactions of the pyrene units
in the solid state. This material exhibited strong blue fluorescence
in both solution and solid state with a high solution fluorescence
quantum yield (UF) of 0.85. A cyclic voltammetry (CV) study on
PTF revealed a quasi-reversible oxidation wave at 0.86 V with no
distinct reduction process being detected (Fig. 2a). The oxidation
assigned to removal of electrons from the triphenylamine-pyrene
resulting radical cations occurred at a lower potential value than
that of other triarylamine derivatives having p-unsubstituted phe-
nyl rings (E1/2 = 0.93 V),14 supporting the existence of
p-electron
interactions between the triphenylamine and substituted pyrene
as revealed by the calculations. Multiple CV scans displayed identi-
cal CV curves indicating an electrochemically stable molecule. The
HOMO and LUMO levels of PTF were calculated to be 5.21 and
2.24 eV, respectively.
Thermogravimetric analysis (TGA) revealed that PTF was ther-
mally stable with a 5% weight loss (T5d) at a temperature well over
475 °C (Fig. 2b). In repeated differential scanning calorimetry (DSC),
all the thermograms displayed only endothermic baseline shifts
owing to glass transition (Tg) at 237 °C with no crystallization and
melting being observed at higher temperatures, indicating a highly
stable amorphous material. The Tg value of this material is among
the highest values in amorphous hole-transporting materials
(AHTMs) reported so far, and is significantly higher than those of
the most widely used HTMs, N,N0-diphenyl-N,N0-bis(1-naphthyl)-
(1,10-biphenyl)-4,40-diamine (NPB) (Tg = 100 °C) and N,N0-bis(3-
methylphenyl)-N,N0-bis(phenyl)benzidine (TPD) (Tg = 63 °C).15 The
amorphous features of PTF were further studied by powder X-ray
diffraction (XRD) using silicon wafer as a substrate. Its XRD pattern
showed broad diffraction peaks between 10.5 and 34.6o with char-
ing close
p–p stacking interactions and imparting good ability to
dissolve in a solvent (Supplementary data). In the HOMO, electrons
are delocalized over the pyrene-triphenylamine moiety resulting
in less
p-electron interactions between the fluorene and pyrene
moieties, while in the LUMO, the excited electrons are delocalized
over the quinoid-like pyrene-fluorene plane. The HOMO-LUMO
energy gap (Eg calcd) was calculated to be 3.01 eV which slightly
deviated from that estimated from the optical absorption edge
(2.97 eV). There are factors responsible for errors in the Eg calcd
values because the orbital energy difference between the HOMO
and LUMO is still an approximate estimation of the transition en-
ergy which 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.
In a CH2Cl2 solution, the absorption spectrum of PTF showed
bands at 279 and 364 nm, while the photoluminescence spectrum
displayed a featureless emission peak at 478 nm (Fig. 1). In the solid
state, the emission spectrum of the thin film obtained by spin-coat-
ing on a quartz substrate was identical to its solution spectrum.
This suggested that the bulky molecular structure of PTF reduced
acteristic peaks ascribed to the p–p stacking of pyrene units being
observed (Supplementary data). The morphology of PTF was char-
acterized by atomic force microscopy. The film spin-coated from
THF:toluene solution showed quite a smooth surface indicating
its good film-forming ability (Supplementary data). These results
proved that the use of 9,9-bis(4-diarylaminophenyl)fluorene as a
molecular platform could reduce the crystallization of pyrene and
improve the amorphous stability of the material, which in turn
could increase the service time in device operation and enhance
the morphological stability of the thin film. Moreover, the ability

5494
N. Prachumrak et al. / Tetrahedron Letters 53 (2012) 5492–5496
1.0
0.5
0.0
1.0
solution solid state
0.5
0.0
solution
thin film
ΦF = 0.85
300 350 400 450 500 550 600
Wavelength (nm)
Figure 1. UV–Vis absorption and PL spectra in solution (CH2Cl2) and as a thin film.
15
10
5
a
b
1st scan
2nd scan
100
95
90
85
80
75
3rd scan
4th scan
T5d 475 oC
0
-5
Tg 237 oC
-10
100 200 300 400 500 600
0.5
0.6
0.7
0.8
0.9
1.0
1.1
Temperature (oC)
Potential vs Ag/Ag+ (V)
Figure 2. (a) Multiple CV curves measured in CH2Cl2 at a scan rate of 50 mV/s. (b) DSC (2nd scan), and TGA curves measured at 10 °C/min under N2.
of PTF to form a good thin film by solution casting techniques is
highly desirable for fabrication of cheap OLED devices.
The properties of PTF as a blue emissive layer (EML) and hole-
transporting layer (HTL) in OLEDs were investigated. Devices with
layers, which often occurs in devices fabricated from EMLs with
planar molecular structure,16 was detected. This shoulder emission
(at 463 nm) was observed in the NPB-based blue OLED (device II).
In our case, the formation of these species might be prevented by
the bulky nature of 9,9-bis(4-diarylaminophenyl)fluorene used as
the molecular platform of PTF. Moreover, stable emission was ob-
tained from device I and the EL spectra did not change over the en-
tire driven voltages.
structures:
ITO/PEDOT:PSS/EML
(40 nm)/BCP
(40 nm)/LiF
(0.5 nm):Al (150 nm) and ITO/PEDOT:PSS/HTL (40 nm)/Alq3
(50 nm)/LiF (0.5 nm):Al (150 nm) were fabricated. The PTF layer
in both devices was spin-coated from THF:toluene (1:1) solution
with controlled thickness. To compare the bifunctional ability of
PTF, commercially available NPB was employed as a reference
EML and HTL material, and the reference devices II, IV, and V were
fabricated using the same structure and device fabrication pro-
cesses. The electroluminescent (EL) spectra and current density-
voltage-luminance (I-V-L) characteristics of the devices are shown
in Figure 3, and their electrical parameters are summarized in Ta-
ble 1. As a blue emitter, the PTF-based device I exhibited a high
maximum brightness of 5997 cd mꢀ2 (blue OLED) at 10.2 V, a low
turn-on voltage of 3.2 V, and a maximum luminance efficiency of
As a HTL, the PTF-based device III exhibited good performance
with a high maximum brightness of 19560 cd mꢀ2 as a green OLED
at 10.8 V, a low turn-on voltage of 3.0 V, and a maximum lumi-
nance efficiency of 4.08 cd Aꢀ1 (Fig. 3 and Table 1). The operating
voltage at 100 cd mꢀ2 was 4.2 V indicating that good performance
was achieved. By comparison with the reference device V, it was
found that the incorporation of PTF as a HTL not only increased
the maximum luminance from 4961 cd mꢀ2
(g
of 0.91 cd Aꢀ1) in
device V to 19560 cd mꢀ2 of 4.08 cd Aꢀ1) in device III, but also
(g
significantly decreased the turn-on voltage from 4.2 V to 3.0 V. Be-
sides, their EL spectra were nearly identical. Moreover, the device
characteristics in terms of luminous efficiency clearly verified that
the solution-processed HTM ability of PTF was greater than that of
the NPB-based device IV. Device III emitted a bright green lumi-
nescence with EL spectra matching with the PL spectrum of Alq3,
and the EL of the reference devices and other reported devices.17
No emission at a longer wavelength due to exciplex species formed
at the interface of the HTL and ETL materials, which often occurs in
the devices fabricated from HTLs with planar molecular structures,
was detected.18 More importantly, a stable emission was obtained
from this diode with the EL spectra and CIE coordinates not chang-
1.13 cd Aꢀ1 (Fig.
3 and Table 1). The operating voltage at
100 cd mꢀ2 was 4.8 V indicating that good performance was
achieved. The device characteristics in terms of maximum bright-
ness, turn-on voltage, and maximum luminous efficiency clearly
demonstrated that the solution-processed blue-emitting ability of
PTF was greater than the NPB-based device II. Under applied volt-
age, device I emitted a bright sky-blue luminescence with peaks
centered at 474 nm and CIE coordinates of x = 0.15 and y = 0.24.
The EL spectrum of the diode matched with the PL spectrum of
PTF. No emission shoulder at a longer wavelength due to excimer
and exciplex species formed at the interface of the EML and BCP

N. Prachumrak et al. / Tetrahedron Letters 53 (2012) 5492–5496
5495
a
b
PL of PTF
PL of NPB
EL I
1500
1000
500
0
device I
I
104
103
102
101
100
1.0
0.5
0.0
II
III
IV
V
VI
EL II
EL III
EL IV
device III
EL V
EL VI
300
400
500
600
700
0
2
4
6
8
10
12
Applied voltage (V)
Wavelength (nm)
c
5
4
3
2
1
0
d
1.0
0.8
0.6
0.4
0.2
0.0
I
5 V
II
6 V
7 V
8 V
9 V
III
IV
V
VI
400
500
600
700
800
0
400
800
1200
1600
Current density (mA/cm2)
Wavelength (nm)
Figure 3. (a) EL spectra, (b) I-V-L characteristics, (c) variation of g-I of the fabricated OLEDs, and (d) normalized EL spectra of device I under different applied voltages.
Table 1
Device characteristics of OLEDs fabricated with PTF (II and III) and NPB (II and IV) as EMLs or HTLs
Device
Structure
Turn-on voltage (V)a
Emission maxima (nm)
Maximum luminance
Luminance efficiency
, cd Aꢀ1
CIE coordinates
(x, y)
(L, cd mꢀ2
)
(
g
)
I
ITO/PEDOT:PSS/PTF/BCP/LiF:Al
ITO/PEDOT:PSS/NPB/BCP/LiF:Al
ITO/PEDOT:PSS/PTF/Alq3/LiF:Al
ITO/PEDOT:PSS/NPB/Alq3/LiF:Al
ITO/PEDOT:PSS/Alq3/LiF:Al
3.2
3.7
3.0
3.0
4.2
474
438
518
518
518
5997
558
19560
20373
4961
1.13
0.31
4.08
3.22
0.91
0.15, 0.24
0.16, 0.10
0.29, 0.53
0.29, 0.53
0.30, 0.54
II
III
IV
V
a
At a luminance of 1 cd mꢀ2
.
ing over the entire applied voltages (Supplementary data).
Although, many blue emitting and hole-transporting materials
have been reported, in terms of their amorphous morphology, sig-
nificantly high Tg value solution processability and device effi-
ciency, PTF is among the good bifunctional materials reported to
date.
In summary, we have designed and synthesized PTF as a
highly fluorescent blue light-emitting small molecule for OLEDs.
Using 9,9-bis(4-diarylaminophenyl)fluorene as a molecular plat-
form, we were able to reduce the crystallization and maintained
the high blue emissive ability of pyrene in the solid state, and
improved the amorphous stability of the material. PTF is an
amorphous molecular glass with a very high Tg, is electrochem-
ically stable, and exhibits strong blue emission both in solution
and solid state. Its ability as both a blue light-emitter for blue
OLEDs and hole-transporter for green OLEDs, in terms of device
performance and thermal properties, was superior than com-
monly used NPB. Non-doped sky-blue and green OLEDs with
luminance efficiencies of 1.13 and 4.08 cd/A were achieved,
respectively.
Acknowledgements
This work was supported financially by the Thailand Research
Fund (RMU5080052), Center for Excellence in Innovation in Chem-
istry, Ubon Ratchathani University, and the Strategic Scholarships
for Frontier Research Network for Research Groups (CHE-RES-
RG50) from the Office of the Higher Education Council, Thailand.
We acknowledge the scholarship support for N. Prachumrak from
the Science Achievement Scholarship of Thailand (SAST).
Supplementary data
Supplementary data associated with this article can be found, in
theonlineversion, at http://dx.doi.org/10.1016/j.tetlet.2012.07.112.
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calcd for C49H34Br2N2, 808.1089; found, 809.1169 [MH+].Compound 4: white
solid; mp. >250 °C; 1H NMR (300 MHz, CDCl3) d 6.93 (12H, t, J = 9.01 Hz), 7.02
(4H, d, J = 8.40 Hz), 7.36 (8H, d, J = 8.70 Hz), 7.51 (4H, d, J = 6.90 Hz) and 7.61 (2
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12. Characterization data for 3: white solid; 1H NMR (300 MHz, CDCl3) d 6.99 (4H, d,
J = 9.0 Hz), 7.00 (8H, t, J = 9.0 Hz), 7.09 (8H, d, J = 9.0 Hz), 7.26 (8H, t, J = 9.0 Hz),
7.55 (4H, t, J = 9.0 Hz) and 7.58 (2H, d, J = 9.0 Hz) ppm; 13C NMR (75 MHz,
CDCl3) d 64.65, 121.55, 121.76, 122.77, 123.06, 124.5, 124.68, 128.69, 129.27,
129.38, 130.82, 137.66, 137.98, 146.74, 147.52 and 153.47 ppm; HRMS m/z
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