Tetrasubstituted-pyrene derivatives for electroluminescent application

Article abstract of DOI:10.1016/j.orgel.2014.06.003

Tetrasubstituted-pyrenes containing peripheral diarylamines (1-4) or fluorenes (5-6) have been synthesized. These compounds are highly fluorescent and possess high morphological stability and thermal stability. Compounds containing peripheral arylamines (

Full text of DOI:10.1016/j.orgel.2014.06.003

Organic Electronics 15 (2014) 2148–2157
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Tetrasubstituted-pyrene derivatives for electroluminescent
application
Rossatorn Muangpaisal a,b,c, Ming-Chi Ho b,d, Tai-Hsiang Huang b, Chih-Hsin Chen b,
e
e
Jiun-Yi Shen b, Jen-Shyang Ni b, Jiann T. Lin b, , Tung-Huei Ke , Li-Yin Chen ,
⇑
d,
Chung-Chih Wu e, , Chiitang Tsai
⇑
⇑
a Taiwan International Graduate Program, Taiwan
b Institute Chemistry, Academia Sinica, Taipei 11529, Taiwan
c Department of Chemistry, National Tsing Hua University, 101 Section 2 Kuang-Fu Road, Hsinchu 30031,Taiwan
d Department of Chemistry, Chinese Culture University, Taipei 111, Taiwan
e Department of Electrical Engineering, Graduate Institute of Electro-optical Engineering and Graduate Institute of Electronics Engineering, National
Taiwan University, Taipei 106, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history:
Tetrasubstituted-pyrenes containing peripheral diarylamines (1–4) or fluorenes (5–6) have
been synthesized. These compounds are highly fluorescent and possess high morphological
stability and thermal stability. Compounds containing peripheral arylamines (1–3) can be
used as the hole-transport and green-emitting materials for two-layered electrolumines-
cent devices. Compounds with peripheral fluorenes (5–6) are efficient blue emitters and
exhibit ambipolar carrier-transport characteristics with high electron mobilities
(10ꢀ3–10ꢀ2 cm2/V s) and high hole mobilities (>10ꢀ3 cm2/V s). Non-doped blue-emitting
devices with promising electroluminescent performance (i.e., high efficiency and narrow/
saturated emission) can be achieved using fluorene–substituted pyrenes as either the hole-
transport/emitting layer or the electron-transport/emitting layer in the two-layered devices.
Ó 2014 Elsevier B.V. All rights reserved.
Received 21 February 2014
Received in revised form 21 April 2014
Accepted 2 June 2014
Available online 21 June 2014
Keywords:
Pyrene
Star-shaped materials
Organic light-emitting diode
Ambipolar transport
1. Introduction
Moreover, longevity of EL devices is also highly demanded
for commercial applications. Therefore, the durability, e.g.,
Two decades after Kodak’s [1] and Cambridge’s [2] sem-
inal work on organic electroluminescence (EL), organic
light-emitting diodes (OLEDs) have become widely used
in practical appliances such as car stereos, mobile phones,
and digital cameras. In OLEDs, balanced electron and hole
mobilities are extremely important in order to achieve
good EL performance. Through proper choice of carrier-
transport and emitting materials, and engineering of
device structures, better confinement of charge recombi-
nation can generally be achieved in a multilayer device.
thermal and morphological stability of deposited organic
films, cannot be overlooked. Unlike polymeric materials
which can easily form films via the spin-coating technique,
films of small molecules normally are vacuum-deposited.
Consequently, the compounds must be able to tolerate
the deposition temperature, which frequently exceeds
250 °C. Amorphous compounds possessing high glass tran-
sition temperatures (Tg) are highly desired as crystalliza-
tion can be avoided and film morphology can be retained
at elevated temperatures during device operation [3–6].
Molecules possessing a non-planar structure and differ-
ent conformers have a great tendency to form amorphous
⇑
Corresponding authors. Tel.: +886 2 27898522.
E-mail addresses: jtlin@chem.sinica.edu.tw (J.T. Lin), chungwu@cc.ee.
glasses [5]. Among these, star-shaped
p-conjugated
molecules allowing incorporation of several bulky and
ntu.edu.tw (C.-C. Wu), tsaictom@faculty.pccu.edu.tw (C. Tsai).
http://dx.doi.org/10.1016/j.orgel.2014.06.003
1566-1199/Ó 2014 Elsevier B.V. All rights reserved.

R. Muangpaisal et al. / Organic Electronics 15 (2014) 2148–2157
2149
rigid substituents are of particular interest. Previously we
developed several series of star-shaped compounds which
readily formed glasses once isolated. These high Tg materi-
als were successfully used to fabricate efficient electrolumi-
nescent (EL) devices [7,8]. In our continuation of developing
high Tg materials for EL applications, we selected 1,3,6,8-
tetrabromo-pyrene as the building block for new star-
shaped compounds based on the following reasons: (1)
according to our previous studies and other literature
reports, incorporation of the pyrenyl unit was beneficial in
raising the thermal stability of the compounds [9,10], (2)
tetraphenylpyrene was reported to be strongly emissive
[11] and (3) pyrenyl unit can be readily functionalized and
its emission color can be tuned [12,13]. Compounds con-
taining pyrenyl units have found useful applications in
liquid crystals [14,15] and field-effect transistors [16,17].
Besides, the pyrenyl unit and its excimer was found useful
in sensing [18–20]. However, formation of excimers
normally quenches the fluorescence and deteriorate the
performance of EL devices [21]. Star-shaped pyrenyl deriv-
atives obtained by replacing four bromine atoms of 1,3,6,
8-tetrabromopyrene with appropriate peripheral substitu-
ents are expected to encapsulate the pyrenyl moiety and
prevent the formation of excimers. Moreover, incorporation
of carrier-transporting peripheral substituents is possible.
There have been reports on using 1,3,6,8-substituted pyre-
nyl derivatives as the light emitting materials in OLEDs
[19]. Liu et al. developed pyrene derivatives containing four
peripheral oligofluorenes of varied chain lengths. Single-
layered OLED based on these materials exhibited a lumi-
nance efficiency of 1.28 cd Aꢀ1 with CIE coordinates at
(0.19, 0.32) and 1.75 cd Aꢀ1 with CIE coordinates at (0.20,
0.32) for bifluorene and tris(fluorene) derivatives, respec-
tively [22]. Sonar et al. developed pyrene-based compounds
capable of emitting various colors by incorporation of dif-
ferent moieties. Among these, only the deep blue-emitting
1,3,6,8-tetrakis-(4-butoxyphenyl)pyrene was subjected to
OLED fabrication, and high luminance efficiency of
2.56 cd Aꢀ1 with CIE coordinates at (0.15, 0.18) was
achieved [23]. Recently, Thomas et al. reported blue to yel-
low light-emitting materials based on tetrasubstituted-pyr-
ene core with acetylene unit as the linker to the peripheries,
and these compounds exhibited red-shifted emission com-
pared to the monosubstituted derivatives. However, these
materials need to be doped in a host for OLEDs, possibly
due to lack of amorphous morphology [24].
Herein, we report light-emitting 1,3,6,8-substituted
pyrene derivatives possessing high quantum efficiency
and carrier mobility. EL devices fabricated from these
star-shaped materials will be also presented.
Chart 1. Structure of compounds 1–6.

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R. Muangpaisal et al. / Organic Electronics 15 (2014) 2148–2157
crude product was further purified by column chromatog-
2. Experimental section
2.1. Materials and methods
raphy using CH2Cl2/hexane (1:10 by v/v) as eluent to give 1
as a yellow powder (0.82 g, 92%). 1H NMR (400 MHz, ace-
tone-d6): d (ppm) 7.96 (s, 4 H, pyrene), 7.67 (s, 2 H, pyrene),
7.15 (t, J = 8.64 Hz, 16 H, meta-C6H5), 6.99 (d, J = 8.67 Hz, 16
H, ortho-C6H5), 6.89 (t, J = 8.66 Hz, 8 H, para-C6H5); MS
(FAB): m/z 870.1 (M+). Anal. Calcd for C64H46N4: C, 88.25;
H, 5.32; N, 6.43. Found: C, 87.97; H, 5.45; N, 6.41.
All reactions and manipulations were carried out under
N2 with the use of standard inert atmosphere and Schlenk
techniques. Solvents were dried by standard procedures.
All column chromatography was performed with the use
of silica gel (230–400 mesh, Macherey–Nagel GmbH &
Co.) as the stationary phase in a column 30 cm in length
and 4.0 cm in diameter. The NMR spectra were recorded
on Bruker AMX400 and AC300 spectrometers. Electronic
absorption spectra were measured in dichloromethane
using a Cary 50 Probe UV–visible spectrophotometer.
Emission spectra were recorded by a Jasco FP-6500 fluores-
2.2.2. Tetra(naphthalen-1-yl)-tetraphenylpyrene-1,3,6,8-tetra-
amine (2)
Yield: 52%. Yellow powder. 1H NMR (400 MHz, acetone-
d6): d (ppm) 7.90 (s, 4 H, pyrene), 7.84 (d, J = 8.47 Hz, 4 H,
C
10H11), 7.81 (d, 8.45 Hz, 4 H, C10H11), 7.37 (s, 2 H, pyrene),
7.60 (d, J = 8.40 Hz, 4 H, C10H11), 7.39 (t, J = 8.42 Hz, 4 H,
10H11), 7.23 (t, J = 8.39 Hz, H, 10H11), 7.18
C
4
C
cence spectrometer. Luminescence quantum yields (Uf
were calculated using Coumarin 1 as primary standard
Uf = 0.99 in ethyl acetate) [25]. Cyclic voltammetry exper-
)
(t, J = 8.40 Hz, 4 H, C10H11), 7.00–6.96 (m, J = 8.44 Hz, 8 H,
meta-C6H5), 6.81 (t, J = 8.45 Hz, 8 H, para-C6H5), 6.60 (d,
J = 8.47 Hz, 8 H, ortho-C6H5); MS (FAB): m/z 1070.3 (M+).
Anal. Calcd for C80H54N4: C, 89.69; H, 5.08; N, 5.23. Found:
C, 89.10; H, 5.64; N, 5.18.
(
iments were performed with a CHI-621B electrochemical
analyzer. All measurements were carried out at room tem-
perature with a conventional three electrode configuration
consisting of a platinum working electrode, an auxiliary
2.2.3. 1,3,6,8-Tetrakis(4-tert-butylphenyl)-1,3,6,8-tetraphenyl-
pyrene-1,3,6,8-tetraamine (3)
electrodes and
a
non-aqueous Ag/AgNO3 reference
electrode. The E1/2 values were determined as
1=2ðEap þ EcpÞ, where Epa and Ecp are the anodic and cathodic
peak potentials, respectively. The solvent in all experi-
ments was CH2Cl2 and the supporting electrolyte was
0.1 M tetrabutylammonium hexafluorophosphate. DSC
measurements were carried out using a Perkin Elmer 7 ser-
ies thermal analyzer at a heating rate of 10 °C/min. TGA
measurements were performed on a Perkin Elmer TGA7
thermal analyzer. FAB-mass spectra were collected on a
JMS-700 double focusing mass spectrometer (JEOL, Tokyo,
Japan) with a resolution of 8000 (5% valley definition). For
FAB-mass spectra, the source accelerating voltage was
operated at 10 kV with a Xe gun, using 3-nitrobenzyl alco-
hol as the matrix. Elementary analyses were performed on
a Perkin-Elmer 2400 CHN analyzer.
Yield: 52%. Yellow powder. 1H NMR (400 MHz, acetone-
d6): d (ppm) 8.00 (s, 4 H, pyrene). 7.66 (s, 2 H, pyrene), 7.24
(d, J = 7.04 Hz, 8 H, C6H4), 7.17 (t, J = 7.20 Hz, 8 H, C6H5),
6.98 (m, 16 H, meta-C6H5 and meta-C6H4), 6.86
(t, J = 7.26 Hz, 4 H, C6H5), 1.23 (s, 36 H, CH3); MS (FAB):
m/z 1095.5 (M+). Anal. Calcd for C80H78N4: C, 87.71; H,
7.18; N, 5.11. Found: C, 87.93; H, 6.70; N, 5.19.
2.2.4. 5-(5-(1,3,6-Tris(5-(3,5-bis(diphenylamino)phenyl)
thiophen-2-yl)pyren-8-yl)thio-phen-2-yl)-1,1,3,3-
tetraphenylbenzene-1,3-diamine (4)
To the flask containing a mixture of 1,3,6,8-tetrabromop-
yrene (0.52 g, 1.0 mmol), 5-(5-(tributylstannyl)thiophen-2-
yl)-1,1,3,3-tetraphenylbenzene-1,3-diamine (3.45 g, 4.4
mmol) and PdCl2(PPh3)2 (22 mg, 0.030 mmol) was added
DMF (5 mL), and the mixture was heated at 80 °C for
48 h. After cooling, MeOH was added and the precipitate
formed was collected by filtration. the crude product was
further purified by column chromatography using CH2Cl2/
hexane (2:5 by v/v) as eluent to give the 4 as a pale yellow
powder (1.76 g, 61%). 1H NMR (400 MHz, acetone-d6):
d (ppm) 8.41 (s, 4 H, pyrene), 8.07 (s, 2 H, pyrene),
7.23–7.17 (m, 40 H, meta-C6H5 and C4SH2), 7.11
(d, J = 7.47 Hz, 32 H, ortho-C6H5), 6.98–6.95 (m, 24 H,
para-C6H5, C6H3), 6.75 (s, 4 H, C6H3); MS (FAB): m/z
2173.6 (M+). Anal. Calcd for C80H78N4: C, 84.02; H, 4.92;
N, 5.16. Found: C, 83.63; H, 5.01; N, 5.05.
2.2. Synthesis and characterization
1,3,6,8-Tetrabromopyrene was synthesized as reported
in the literature [26]. Compounds octaphenylpyrene-1,3,
6,8-tetraamine (1), tetra(naphthalen-1-yl)-tetraphenylpy-
rene-1,3,6,8-tetra-amine (2), and 1,3,6,8-tetrakis(4-tert-
butylphenyl)-1,3,6,8-tetraphenyl-pyrene-1,3,6,8-tetraamine
(3) were synthesized by similar procedures, and only the
preparation of 1 will be described in detail.
2.2.1. Octaphenylpyrene-1,3,6,8-tetraamine (1)
To a flask containing a mixture of 1,3,6,8-tetrabromop-
yrene (0.52 g, 1.0 mmol), diphenylamine (0.74 g,
4.4 mmol), sodium tert-butoxide (0.51 g, 5.28 mmol) and
Pd(dba)2 (0.040 mmol) was added dry toluene (30 mL).
After 30 min. P(tBu)3 (1.2 mol%) was added and the solu-
tion mixture was heated to reflux and stirred for 24 h. After
cooling, the solution was pumped dry and the residue was
extracted with CH2Cl2/brine. The organic layer was dried
over magnesium sulfate, then filtered and dried. After
recrystallization from dichloromethane and hexane, the
2.2.5. 1,3,6,8-Tetrakis(9,9-dihexyl-9H-fluoren-2-yl)pyrene (5)
To the flask containing a mixture of 9,9-dihexyl-9H-flu-
oren-2-ylboronic acid (7.93 g, 21.0 mmol), 1,3,6,8-tetrab-
romopyrene (2.11 g, 4.04 mmol), Na2CO3 (2 M in H2O,
9.0 mL, 8.9 mmol), and Pd(PPh3)4 (0.49 g, 0.42 mmol) was
added 70 mL of dry toluene. After the reaction mixture
was refluxed for 24 h, the solvent was removed and the
residue was extracted with CH2Cl2/brine. The organic layer
was dried over magnesium sulfate, then filtered and dried.

R. Muangpaisal et al. / Organic Electronics 15 (2014) 2148–2157
2151
The crude product was further purified by column chroma-
tography using CH2Cl2/hexane (1:5 v/v) as eluent to give
the 5 as a pale yellow powder (3.5 g, 57%). 1H NMR
(400 MHz, acetone-d6): d (ppm) 8.18 (s, 4 H, pyrene),
8.14 (s, 2 H, pyrene), 7.84 (d, J = 7.45 Hz, 4 H, C6H3),
7.66–7.65 (m, 8 H, C6H3), 7.73 (d, J = 7.41 Hz, 4 H, C6H4),
7.37–7.29 (m, 12 H, C6H4), 2.00 (t, J = 7.32 Hz, 16 H,
C6H13), 0.87 (m, 48 H, C6H13), 0.78–0.63 (m, 40 H, C6H13);
MS (FAB): m/z 1532.9 (M+). Anal. Calcd for C116H138: C,
90.92; H, 9.08. Found: C, 90.90; H, 8.85.
aluminum (Alq3) as the electron-transporting layer were
fabricated (devices I and II). The devices were prepared by
vacuum deposition of 40 nm of the hole-transporting layer,
followed by 40 nm of TPBI or Alq3. Inorganic LiF of 1 nm
thick was then deposited as the buffer layer. Aluminum
was finally deposited as the cathode (150 nm). In device
III, 4,40-bis[N-(1-naphthylphenyl)amino]biphenyl (NPB)
was used as the hole-transport layer, and 5 was used as
emitting as well as electron-transport layer. In devices IV
and V, the films of PVK doped with compound 6 were
spin-coated from dichloroethane solution (concentra-
tion = 10 mg/mL) at a spin rate of 3000 rpm for 40 s. I–V
curves were measured in a Keithley 2400 Source Meter
under the ambient environment. Light intensity was mea-
sured with a Newport 1835 Optical Meter. I–V curve was
measured on a Keithley 2000 Source Meter in ambient envi-
ronment. Light intensity was measured with a Newport
1835 Optical Meter.
2.2.6. 1,3,6,8-Tetrakis(7-(9,9-dihexyl-9H-fluoren-2-yl)-9,9-
dihexyl-9H-fluoren-2-yl)pyrene (6)
Compounds 6 were prepared by similar procedures
for preparation of compound 5, except that 2-bromo-7-
(9,9-dihexyl-9H-fluoren-2-yl)-9,9-dihexyl-9H-fluorene are
used. Yield: 92%. Yellow powder. 1H NMR (400 MHz, ace-
tone-d6): d (ppm) 8.25 (s, 4 H, pyrene), 8.19 (s, 2 H, pyrene),
7.9 (d, J = 8.12 Hz, 4 H, C6H3), 7.81 (d, J = 7.92 Hz, 4 H, C6H3),
7.77 (d, J = 7.96 Hz, 4 H, C6H3), 7.73–7.62 (m, 28 H, C6H3),
7.73–7.62 (m, 12 H, C6H4), 2.15–2.00 (m, 32 H, C6H13),
1.08–1.06 (m, 96 H, C6H13), 0.76–0.68 (m, 80 H, C6H13);
2.4. Time-of-flight (TOF) mobility measurement
The samples 5 and 6 for the TOF measurement were
prepared by melt deposition using the structure of: glass/
Ag (30 nm)/5 (or 6) (11–12 lm)/Al (150 nm) with an active
MS (FAB): m/z 2863.7 (M+). Anal. Calcd for C216H266
:
C, 90.63; H, 9.37. Found: C, 90.62; H, 9.12.
area of 2 ꢁ 2 mm2 as described in previous reports [27,28].
A frequency-tripled Nd:YAG laser (355 nm) with ꢂ10 ns
pulse duration was used for pulsed illumination through
the semitransparent Ag. Under an applied DC bias, the
transient photocurrent as a function of time was recorded
with a digital storage oscilloscope. The TOF measurements
were typically performed in a 10ꢀ5-torr vacuum chamber.
Depending on the polarity of the applied bias V, selected
photogenerated carriers (holes or electrons) are swept
across the sample thickness D with a transit time tT, the
2.3. EL device fabrication and characterization of OLEDs
Pre-patterned ITO substrates with an effective individual
device area of 3.14 mm2 were cleaned by using standard
procedure. The organic materials were fabricated by
thermal evaporation technique, except compound 6.
Two-layered devices using compounds 1–5 as the hole-
transporting/emitting layer and 1,3,5-tris(N-phenylbenzi-
midizol-2-yl)benzene (TPBI) or tris(8-hydroxy-quinolinato)
Scheme 1. Synthesis of compounds 1–6, R = C6H13
.

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R. Muangpaisal et al. / Organic Electronics 15 (2014) 2148–2157
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
applied electric field E is then V/D, and the carrier mobility
is given by = D/(tTꢃE) = D2/(VꢃtT).
(a)
3
4
5
6
3. Results and discussion
3.1. Synthesis
The compounds synthesized in this study are shown in
Chart 1. Scheme 1 illustrates synthetic procedures of these
compounds. Compounds 1–3 were synthesized from
1,3,6,8-tetrabromopyrene and 4.4 equivalents of the corre-
sponding diarylamine following the literature method
[29]. Fourfold thienylation of 1,3,6,8-tetrabromopyrene
with 4.4 equivalents of stannyl compound, 5-(5-(tributyl-
stannyl)thiophen-2-yl)-1,1,3,3-tetraphenylbenzene-1,3-
diamine, using palladium(0)-catalyzed Stille coupling reac-
tion, resulted in formation of 4. Suzuki coupling reactions
of 1,3,6,8-tetrabromopyrene with 5.2 equivalents of the
corresponding fluorenylboronic acids yielded 5 and 6
[22]. The new compounds were isolated in moderate to
high yields (52–92%). All the compounds were soluble in
common solvents (e.g., acetone, CH2Cl2) and characterized
by 1H NMR, FAB-MS and elemental analysis.
300
400
500
600
Wavelength, (nm)
(b)
3
4
5
6
1.0
0.8
0.6
0.4
0.2
0.0
3.2. Thermal properties
The thermal properties of the new compounds were
determined by DSC and TGA (Table 1). These compounds
have high thermal decomposition temperature (Td = 414–
522 °C). All the compounds exhibited melting isotherms
during the first heating cycle, but rapid cooling of the melt
led to the formation of a glassy state which persisted in the
subsequent heating cycles. No glass transition was noticed
for 1 and 5, while the glass transition temperatures (Tg) of
2–4 are higher than 160 °C. Compound 6 has a much lower
Tg value, which is attributed to the presence of the flexible
alkyl chains of the fluorenyl segments.
400
500
600
700
800
Wavelength, (nm)
Fig. 1. The absorption (a) and emission (b) spectra of the compounds in
CH2Cl2.
absorption and emission spectra in solution are shown in
Fig. 1. The bands at 400–469 nm are ascribed to the
p
?
p* transition of the pyrene moiety. The longer wave-
3.3. Photophysical properties
length absorption in 1–3 than in 5–6 can be attributed
the mixing of
* transition with n ? * transition from
the nitrogen lone pair to the pyrene moiety. Compound 4
p
?
p
p
The absorption and luminescence spectral data of the
compounds are presented in Table 1. Representative
Table 1
Physical data for compounds 1–6.
Tg/Tc/Tm (°C)a
Td (°C)a
kmax (nm)b
kem (nm) solb/filmc
Uf (%) sold/solide
Eox (4Ep) (mV)f
HOMO/LUMO (eV)g
1
2
3
4
5
6
NA/287/367
172/261/342
162/214/298
167/266/372
NA/164/186
87/NA/212
414
440
426
522
409
414
257, 299, 383, 463
257, 300, 367, 466
263, 302, 388, 462
301, 351, 443
262, 326, 401
289, 351, 405
500/524
500/525
507/526
532/533
455/485
465/493
69/57
62/55
74/60
16/13
83/68
79/62
137 (163), 1187 (i)
112 (160), 1197 (i)
95 (158), 1217 (i)
617 (i)
847 (163), 1177 (174)
886 (143), 1200 (169)
ꢀ4.94/ꢀ2.50
ꢀ4.91/ꢀ2.49
ꢀ4.90/ꢀ2.43
ꢀ5.41/ꢀ3.00
ꢀ5.65/ꢀ2.91
ꢀ5.69/ꢀ3.02
a
b
c
Tg, glass transition temperature; Tc, crystallization temperature; Tm, melting point; Td, thermal decomposition temperature; NA, not detected.
kmax, absorption maximum; measured in CH2Cl2 solution; kem, emission maximum; measured in CH2Cl2 solution.
kem, emission maximum; measured in the film cast on glass substance.
d
Quantum yield (Uf) was measured relative to Coumarin 1 (99% in ethyl acetate) [25]. Corrections due to change in solvent refractive indices were
applied.
e
Quantum yield (Uf) was measured in solid state by integration sphere.
f
All Eox data are reported relative to ferrocene which has an Eox at 223 mV relative to Ag/Ag+ and the anodic peak–cathodic peak separation (4Ep) is
75 mV. The concentration of the compound was 5 ꢁ 10ꢀ3 M and the scan rate was 100 mV sꢀ1; i, irreversible.
g
LUMO levels were derived via eq. Eg = HOMO–LUMO.

R. Muangpaisal et al. / Organic Electronics 15 (2014) 2148–2157
2153
exhibits a blue shift of absorption compared with 1–3,
which may be due to the meta-substitution of the periph-
eral diphenylamines, leading to less efficient interaction
between the amines and the pyrene core. In the solution,
the emission colors range from blue (5–6, 455–465 nm)
to green (1–4, 500–532 nm) (Fig. 1b). The photolumines-
cence spectra in the film states were also measured. The
film emission spectra (Fig. S1, Supporting Information)
were broadened only slightly compared with the solution
spectra, indicating no significant dye aggregation. All the
compounds except 4 are strongly emissive in dichloro-
methane, with the PL quantum yields (Uf) ranging from
16% to 83%. Insufficient orbital overlap between HOMO
and LUMO in the S0 ? S1 transition likely causes lower
solution quantum yield of 4 as the arylamines reside at
meta-disposition relative to the conjugated spacer. The
large difference in MO population between the S0 and S1
states also leads to large Stokes shift in 4 than the others.
The full widths at half maximum (FWHM) of the emission
spectra in general are very small (<60 nm) except for 4
(90 nm), which is beneficial to saturated monochromatic
emission.
exhibits two reversible redox waves due to the oxidation
of the amine [30]. However, the oxidation of arylamines
(1–3) was observed as a quasi-reversible wave at slightly
higher potential than previous report. The oxidation poten-
tial of the compounds increases in the order of 3 < 2 < 1, in
accordance with the decreasing electron-donating ability
of the substituents. The oxidation of the arylamines in 4
is irreversible and at higher potential than that in 1–3. This
is in accordance with our previous observation: pyrenyl-
amine has a lower oxidation potential than phenylamine
[9]. Compounds 5 and 6 are oxidized at a much higher
potential than 1–4, due to the absence of the amino units.
In contrast to 1–4, two quasi-reversible oxidation pro-
cesses were detected for 5 and 6.
3.5. Theoretical calculation
Density functional calculations at B3LYP/6-31G* level
were carried out on the pyrene compounds. Fig. 2 shows
the frontier orbitals for the representative compounds 3,
4 and 5. The HOMOs of 1–3 are distributed from pyrene
to the arylamine, whereas their LUMOs mainly localized
on the pyrene entity (Fig. S3, Supporting Information).
For compound 4–6, both HOMO and LUMO for each com-
pound populate at pyrene and four aryl substituents. There
is only little MO population on the diphenylamino groups
of compound 4, showing the interruption of electronic
communication between pyrene and diphenylamine by
meta-conjugation. The calculated energy gap (Eg) increased
3.4. Electrochemical properties
The electrochemical data of all compounds are listed in
Table 1. Representative cyclic voltammograms of selected
compounds are shown in Fig. S2 (Supporting Information).
1,3,6,8-Tetrakis-(dimethylamino)pyrene was reported to
Fig. 2. Calculated molecular orbital and energy levels of model compounds of 3, 4 and 5.

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R. Muangpaisal et al. / Organic Electronics 15 (2014) 2148–2157
Table 2
Performance characteristics of the devices.
Devices
1a
2a
3a
4a
5b
6c
Von (V)
2.6; 2.4
2.5; 2.3
22,212; 23,329
1.1; 1.3
3.7; 4.0
3.8; 4.2
2.1; 2.1
8784; 11,290
2.0; 1.1
6.2; 3.6
6.5; 3.7
4.2; 3.8
3.3; 3.0; 4.2
10,035; 11,088; 17,118
2.7; 1.0; 3.2
4.3; 3.1; 5.0
2.7; 2.5; 2.1
5.6; 4.2
Lmax (cd/m2) 13,696; 14,313
6765; 18,351
0.59; 0.62
1.6; 1.8
3249; 9755
0.34; 3.2
0.42; 4.5
0.11; 1.6
gex,max (%)
gc,max (cd/A) 4.4; 5.5
gp,max (lm/
W)
1.4; 1.7
4.6; 5.8
1.3; 1.4
kmax (nm)
FWHM
(nm)
502; 504
60; 66
508; 508
52; 52
504; 504
46; 48
532; 526
126; 94
468; 516; 470
62; 104; 64
472; 468
54; 54
CIE (x, y)
0.28, 0.58; 0.28,
0.59
5.5; 6.3
2730; 2797
0.88; 0.89
2.7; 2.8
0.24, 0.63; 0.25,
0.63
4.8; 6.0
2829; 2625
0.87; 0.80
2.8; 2.6
0.19, 0.62; 0.20,
0.62
7.0; 7.0
970; 1945
0.71; 0.64
2.2; 2.0
0.39, 0.52; 0.33,
0.56
8.5; 8.2
2168; 194
0.36; 0.63
0.97; 2.0
0.36; 0.72
0.16, 0.22; 0.27; 0.49; 0.14,
0.22
6.5; 7.1; 11
3300; 2870; 4165
2.2; 0.97; 2.7
3.3; 2.9; 4.2
0.12, 0.17; 0.15,
0.19
13; 14.5
420; 2679
0.34; 2.0
0.42; 2.7
0.10; 0.58
Vd
L (cd/m2)
d
d
gex (%)
d
gc (cd/A)
gp (lm/W)
d
1.6; 1.4
1.8; 1.4
0.98; 0.90
1.6; 1.3; 1.2
a
b
c
The measured values are given in order of the devices I and II.
The measured values are given in order of the devices I–III.
The measured values are in order of the devices IV and V.
d
The measured values were taken at a current density of 100 mA. Lmax, maximum luminance;
p,max, maximum power efficiency; FWHM, full width at half maximum; V, voltage; L, luminance;
c, current efficiency; p, power efficiency.
g
ex,max, maximum external quantum efficiency; gc,max
,
maximum current efficiency;
efficiency;
g
g
ex, external quantum
g
g
from ca. 2.8 eV (1–4) to 3.14 eV (5) with the absence of the
strong amino donor. These values are in good agreement
with those estimated from the optical absorption edge
(Table 1). It is clear that the Eg of pyrene derivatives can
be adjusted via substituted aryl groups, indicating tunable
emission wavelength of the pyrene chromophores.
are mainly confined in the EML. The observation of the
broad emission band is attributed to exciplex formation
since similar spectra was also observed in the blend film
2000
(a)
1800
1, device I
2, device I
3, device I
4, device I
5, device I
1600
1400
1200
1000
800
600
400
200
0
3.6. Electroluminescent properties
The HOMO energy levels of the compounds were calcu-
lated from cyclic voltammetry reference to ferrocene
(4.8 eV) [31]. These data together with the absorption
spectra were then used to obtain the LUMO energy levels
(Table 1). Considering the energy levels (Fig. 7) and car-
rier-transporting characteristics of these compounds,
two-layered devices having three different structures were
fabricated: (I) ITO/1 (or 2–5) (40 nm)/TPBI (40 nm)/LiF
(1 nm)/Al (150 nm); (II) ITO/1 (or 2–5) (40 nm)/Alq3
(40 nm)/LiF (1 nm)/Al (150 nm); (III) ITO/NPB (40 nm)/5
(40 nm)/LiF (1 nm)/Al (150 nm). Compounds 1–5 were
used as the emitting as well as hole-transporting layer in
both devices I and II. TPBI and Alq3 were used as elec-
tron-transporting layer in the devices I and II, respectively.
In viewing that 5 exhibits good electron transport ability
(vide infra), device III consisting of NPB as the hole-trans-
porting layer, and 5 as the emitting as well as electron-
transporting layer was also fabricated. The performances
of various devices are summarized in Table 2. I–V–L char-
acteristics for the devices I–III are shown in Figs. 3 and 4,
and the EL spectra are shown in Fig. 5. For 1–3, the EL spec-
tra of the devices I and II bear resemblance to the film
spectra of EML, indicating the excitons are effectively con-
fined in the EML. For 4, the EL spectrum of the device II
resembles the film spectra of 4. In comparison, the EL spec-
trum of the device I is broader than the film spectra of 4 at
the longer wavelength portion, even though the excitons
0
2
4
6
8
10
12
14
16
Voltage, V
1800
1600
1400
1200
1000
800
600
400
200
0
(b)
1, device II
2, device II
3, device II
4, device II
5, device III
0
2
4
6
8
10
12
14
16
Voltage, V
Fig. 3. Current density–voltage characteristics of the devices. (a) Device I.
(b) Devices II and III.

R. Muangpaisal et al. / Organic Electronics 15 (2014) 2148–2157
2155
of 4 and TPBI. Efficiencies (Table 2) of both devices I and II
for 1–4 drop significantly as the applied voltage (or current
density) increases.
1.0
0.8
0.6
0.4
0.2
0.0
(a)
1, device I
2, device I
3, device I
4, device I
5, device I
Both devices I and III of 5 emit blue light characteristic of
5, confirming that 5 is capable of transporting both elec-
trons and holes (which will be further discussed in next
section). Both devices have good performance parameters
(I: L, 3300 cd/m2;
III: L, 4165 cd/m2;
g
g
ex, 2.2%;
ex, 2.7%;
g
c, 3.3 cd/A;
g
g
p, 1.6 lm/W and
p, 1.2 lm/W at a
gc, 4.2 cd/A;
current density of 100 mA/cm2). Moreover, the non-opti-
mized device III also appears to have reasonably stable
external quantum efficiency and current efficiency at high
current levels (Fig. 6). It is worth noting that the two
devices have very narrow full widths at half maximum
(FWHM ꢂ 60 nm) in EL spectra, which is required for satu-
rated monochromatic emission. In contrast to the devices I
and III, the device II of 5 emitted green light characteristic of
Alq3. Obviously the smaller energy barrier between the
HOMO of 5 and Alq3 (6.0 eV) allows passage of holes form
5 to Alq3. In comparison, TPBI acts as an effective hole
blocker due to its lower HOMO level (6.2 eV). Consequently,
excitons are effectively confined inside the layer of 5.
Due to the low volatility of 6 (and thus difficulty in
preparing films via vacuum deposition), device IV having
the structure of ITO/PEDOT:PSS (70 nm)/PVK + 6
(30 wt.%) (60 nm)/LiF (1 nm)/Al (150 nm) (PEDOT:PSS =
polyethylenedioxythiophene:polystyrene-sulfonate; PVK =
400
500
600
700
800
Wavelength, nm
1.0
0.8
0.6
0.4
0.2
0.0
(b)
1, device II
2, device II
3, device II
4, device II
5, device III
6, device V
24000
22000
(a)
400
500
600
700
800
20000
18000
16000
14000
12000
10000
Wavelength, nm
Fig. 5. EL spectra of the devices. (a) Device I. (b) Devices II, III and V.
8.0
8000
6000
4000
2000
0
1, device I
2, device I
3, device I
4, device I
5, device I
3.5
7.0
6.0
5.0
4.0
3.0
2.0
1.0
0.0
-1.0
5, device III
5, device III
3.0
2.5
2.0
1.5
1.0
0.5
0.0
-0.5
0
200 400 600 800 1000 1200 1400 1600 1800 2000
Current Density, mA/cm2
26000
24000
22000
20000
18000
16000
14000
12000
10000
8000
(b)
-100
0
100
200
300
400
500
600
700
Current Density, mA/cm2
Fig. 6. Current efficiency and external quantum efficiency versus current
density for device III of 5.
1, device II
2, device II
3, device II
4, device II
5, device III
6000
poly(vinylcarbazole)), was fabricated via the spin-coating
technique. Blue light (kem = 472 nm; (x, y) = (0.12, 0.17);
FWHM = 54 nm) characteristic of 6 was observed in this
device. However, the performance (Lmax
ex,max, 0.34%; c,max, 0.42 cd/A; gp,max 0.11 lm/W) is only
mediocre. If TPBI is deposited on the top of the PVK/6
blend layer, i.e., ITO/PEDOT:PSS (70 nm)/PVK + 6 (30 wt.%)
(60 nm)/TPBI (40 nm)/LiF (1 nm)/Al (150 nm) (device V),
4000
2000
0
,
3249 cd/m2;
0
200 400 600 800 1000 1200 1400 1600 1800
g
g
Current Density, mA/cm2
Fig. 4. Luminescence–current density characteristics of the devices. (a)
Device I. (b) Devices II and III.

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R. Muangpaisal et al. / Organic Electronics 15 (2014) 2148–2157
Fig. 7. Energy level diagram of the materials used for the fabrication of devices.
amorphous solid states [27]. The presence of oligofluorene
5
6
moiety in compound 5 and 6 may prevent of the formation
of deep electron trapping due to oxygen, which affect the
carrier transport in molecule.
Interestingly, compound 5 and 6 exhibit ambipolar
transporting properties though both do not contain
strongly electron-deficient moieties in the molecular skel-
eton [34]. The hole mobilities of 5 and 6 also appear to be
very high, (>10ꢀ3 cm2/V s), and are higher than those of the
typical hole-transport material NPB [35].
Electron
Hole
Electron
Hole
4. Conclusions
10-3
We have synthesized star-shaped molecules based on
tetra-substituted pyrene. They are highly thermally stable
and exhibit high glass transition temperatures. Com-
pounds containing peripheral arylamines can be used as
hole-transporting and green-emitting materials for elec-
troluminescent devices. The compounds with peripheral
fluorenes are blue-emitting and exhibit bipolar carrier
transport characteristics. Both electron and hole mobilities
measured by TOF method surpass 10ꢀ3 cm2/V s. Two-lay-
ered devices using the blue-emitting compound as the
hole-transporting and the emitting layer or as the elec-
tron-transporting and the emitting layer exhibited good
performances. Most devices in this study have relatively
narrow FWHMs (ꢂ60 nm).
200
250
300
350
400
450
500
E1/2 [V/cm]1/2
Fig. 8. Electron and hole mobilities versus E1/2 for 5 and 6.
the performance increases significantly: (Lmax, 9755 cd/
m2;
ex,max, 3.2%; c,max, 4.6 cd/A; gp,max 1.6 lm/W). If
g
g
TPBI was replaced with Alq3, both 6 and Alq3 made sig-
nificant contributions to the light emission.
3.7. Charge-transport properties
Since 5 can be used as an electron- as well as a hole-
transporting layer and similar structural motifs are present
in 5 and 6, carrier-transport properties of both were inves-
tigated by the TOF transient photocurrent technique at
room temperature. Both 5 and 6 reveal non-dispersive
hole-transport and dispersive electron-transport charac-
teristics. The carrier transit time tT needed for determining
carrier mobilities was evaluated from the intersection
point of two asymptotes in the double logarithmic repre-
sentation (insets of Fig. S4, Supporting Information). The
carrier mobilities of 5 and 6 thus determined are shown
in Fig. 8 as a function of the electric field E. The field depen-
dence of carrier mobilities of 5 and 6 follows the nearly
Acknowledgements
This work was supported by the TIGP program (Acade-
mia Sinica), National Science Council, Academia Sinica
Institute of Chemistry (AC), National Taiwan University,
and the Chinese Culture University, the Instrumental Cen-
ter of Institute of Chemistry (AC).
Appendix A. Supplementary material
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/
j.orgel.2014.06.003.
universal Poole–Frenkel relationship:
l
/ exp(bE1/2),
where b is the Poole–Frenkel factor [32]. Electron mobili-
ties of 5 and 6 are in the range of 10ꢀ3–10ꢀ2 cm2/V s, which
are nearly two-orders higher than those of the typical elec-
tron-transport material Alq3 (ꢂ10ꢀ5 cm2/V s) [33]. Some
ter(9,10-diarylfluorene) derivatives were also reported to
have rather high electron mobilities (>10ꢀ3 cm2/V s) in
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