Soluble bipolar star-shaped molecule as highly stable and efficient blue light emitter

Article abstract of DOI:10.1039/c4ra14934c

A star-shaped blue small-molecule 1,3,5-tris(5-(4-(3,6-di-tert-butylcarbazol-9-yl)phenyl)-1,3,4-oxadiazol-2-yl)benzene (S-Cz-OXD) with donor-π-acceptor (D-π-A) structure was designed and synthesized by incorporating three (3,6-di-tert-butyl-9H-carbazol-9-

Full text of DOI:10.1039/c4ra14934c

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ARTICLE
Soluble Bipolar Star-Shaped Molecule as Highly
Stable and Efficient Blue Light Emitter
Cite this: DOI: 10.1039/x0xx00000x
Xuehan He,a Lei Chen,b Yongbiao Zhao,c Siu Choon Ng,d Xizu Wang,e Xiaowei
Sun,c and Xiao ‘Matthew’ Hu *a
Received 00th January 2012,
Accepted 00th January 2012
A starꢀshaped blue smallꢀmolecule 1,3,5ꢀtris(5ꢀ(4ꢀ(3,6ꢀdiꢀtertꢀbutylcarbazolꢀ9ꢀyl)phenyl)ꢀ1,3,4ꢀoxadiazolꢀ
2ꢀyl)benzene (SꢀCzꢀOXD) with donorꢀπꢀacceptor (DꢀπꢀA) structure was designed and synthesized by
DOI: 10.1039/x0xx00000x
incorporating
three
(3,6ꢀdiꢀtertꢀbutylꢀ9Hꢀcarbazolꢀ9ꢀyl)phenyl
arms
into
an
tris(1,3,4ꢀ
oxadiazole)phenylene core. It gives pure blue photoluminescence emission at 424 nm with high quantum
efficiency (Φf) (93%) in dilute toluene and has good solubility in common solvents. This starꢀshaped
molecule also demonstrates enhanced Φf (72%) in film state. Moreover, the spectral stability of SꢀCzꢀ
OXD is also excellent due to its low lying HOMO level and absence of fluorine moiety. The PL spectrum
www.rsc.org/
o
of SꢀCzꢀOXD remains virtually unchanged even after annealing at 150 C for 220 hours in air. Solutionꢀ
processed nonꢀdoped electroluminescent device shows a low turnꢀon voltage of 3.7 V although there
exists big energy barrier due to the bipolar design. The device emits stable pure blue light, CIE
(Commission Internationale de L’Eclairage) coordinates of (0.16, 0.16) with high current efficiency (CE)
and external quantum efficiency (EQE) of 4.22 cd Aꢀ1and 3.37% respectively. Mechanistic discussion is
also carried out in relation to the molecular structure.
KEYWORDS: Organic semiconductor, Starꢀshaped bipolar molecule, Light emitting diode, Blue emission
that of linear polyfluorene (55%).11 Using
a
planar
1. Introduction
triphenylaminecore, Yang et al designed a series of starburst
fluorene oligomers with a maximum CE of 3.83 cd Aꢀ1 and a
maximum EQE of 4.9%.12 Zou et al also reported a series of
starburst oligofluorenes using phenyl as the core with a
maximum EQE of 6.8% and CE of 5.4 cd Aꢀ1.7 Subsequently,
they fineꢀtuned the turnꢀon voltage at 3.6 V by introducing
electron donor diphenylamine endꢀcaps to raise the HOMO
level although this may reduce the spectral stability.13Improved
device performance was also reported in starꢀlike branched
polyfluorene,14,15 even though polymers tend to have more
batch to batch difference in average molecular weight and
polydispersity, structural defects and more tedious procedures
in removing residue catalysts.
Although numerous starꢀlike blue oligomers/polymers were
reported with good device efficiency, some issues remain
hindering their practical application. Firstly, most of the
reported materials were fluoreneꢀbased molecules showing
good solubility and high PL quantum efficiency. However,
fluorine has an intrinsic drawback of unstable EL spectrum. An
additional green emission could be observed during long time
operation or thermal treatment in air due to the generation of
ketonic defects.16–19 Besides, more balanced carrier flow can be
better achieved in a bipolar molecular design than that in
Solutionꢀprocessed organic lightꢀemitting diodes (OLEDs)is the
most promising technology for low cost, flexible, large area
fullꢀcolor displays and solid state illumiantions.1,2 As one of the
threeꢀprimary colors, blue emitters could also act as the host
material for green and red emitters to realize fullꢀvisibleꢀspectra
emission. Therefore, it is important to develop highly efficient,
solution processable, stable and colorꢀpure blue emitting
materials.3
Due to their highly branched and globular features, starꢀ
shaped molecules could effectively suppress the intermolecular
interaction in film state, leading to unique properties, such as
enhanced solidꢀstate luminescence, good film forming ability
and excellent thermal stability.4–7 Therefore, starꢀshaped
molecular design strategy is an effective way to improve the EL
performance of OLEDs. For instance, Luo et al first reported a
series of blue oligmers and polymers using truxene as the core
and fluorene derivatives as the repeat unit of arms. They show
improved EL efficiency with an EQE value of 2.9%,8 which is
higher than that of linear polyfluorene (around 1.0%).9 Lai et al
developed
a
set of fluoreneꢀbased sixꢀarmed nanostar
with greatly improved solidꢀstate
macromolecules10
fluorescence quantum efficiency (around 90%) compared to
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DOI: 10.1039/C4RA14934C
fluorine based materials which are strong pꢀtype hole starburst structure while ensuring blue light emission. The
conductors. While, donorꢀπꢀAcceptor (DꢀπꢀA)pushꢀpull design target molecule 1,3,5ꢀtris(5ꢀ(4ꢀ(3,6ꢀdiꢀtertꢀbutylcarbazolꢀ9ꢀ
is usually used to improve the balance of charge transportation, yl)phenyl)ꢀ1,3,4ꢀoxadiazolꢀ2ꢀyl)benzene
(SꢀCzꢀOXD)
few reported blue light emitting starꢀshaped molecules with Dꢀ comprising of tris(1,3,4ꢀoxadiazole) phenylene (OXD) electron
πꢀA structures because the pushꢀpull structure usually leads to acceptor and 3,6ꢀdiꢀtertꢀbutyl carbazole (tꢀBCz) electron donor
lower bandꢀgap and redꢀshift of the spectra.20,21 Jose et al (See Scheme 1). This molecule is readily synthesized and
synthesized two interesting blue light emitting DꢀπꢀA bipolar purified, and exhibits very high PL quantum efficiency.
starburst molecules, but no device data were reported.22
Furthermore, the cast film of SꢀCzꢀOXD shows outstanding
Based on the above consideration, we designed a solution oxidation resistance and spectra stability. Nonꢀdoped device
processable starꢀshaped smallꢀmolecule with high PL based on SꢀCzꢀOXD show pure blue emission with a high
efficiency, excellent spectra stability and bipolar characteristics efficiencies of 4.22 cd Aꢀ1, an EQE of 3.37% and CIE
by (1) utilizing carefully selected nonꢀfluorene based building coordinates of (0.16, 0.16).
blocks with high stability; (2) introducing DꢀπꢀA pairs into
Br
N
N
N
N
N
O
O
COCl
H
(
O
O
N
(
N
N
Br
Br
N
N
N N
N N
ClOC
COCl
N
N
N
N
O
O
1 (core)
S-Cz-OXD
Br
N
O
O
C
O
(
(
(
O
Br
COCl
Br
N
N
Br
Br
C NHNH
Br
N N
N N
2
3
L-Cz-OXD
Scheme 1 Synthesis route to S-Cz-OXD and L-Cz-OXD.i) anhydrous pyridine; reflux for 24 hours. ii) 3,6-Di-tert-butyl-9H-carbazole, CuI, K2CO3, 1,3-dimethyl-3,4,5,6-
tetrahydro-2(1H)-pyrimidinone (DMPU), and 18-crown-6; 170 oC for 48 hours. iii) H2NNH2·H2O, Et3N, CH2Cl2; 40 oC for 2 hours. iv) SOCl2, toluene; reflux for 12 hours.
Bulky tꢀbutyl group on carbazole unit was introduced to
2. Results and discussion
improve the solubility and further suppress the molecular
aggregation in solid state. For comparison, a linear analogue
2,5ꢀbis(3,6ꢀdiꢀtertꢀbutylꢀcarbazolꢀ9ꢀyl)phenylꢀ1,3,4ꢀoxadiazol
2.1. Molecular design and synthesis
The chemical structure of SꢀCzꢀOXD is shown in Scheme 1
.
(
LꢀCzꢀOXD) (Scheme 1) was also synthesized and studied.
The synthesis route is also shown in Scheme 1. Intermediate
The electronꢀrich carbazole and electronꢀdeficient oxadiazole
units were selected because of their excellent charge transport
properties and good chemical stability. Such a donorꢀπꢀacceptor
structure renders the molecule bipolar characteristics which are
favorable for more balanced charge injection and transport. For
instance, adding an electron acceptor onto an oligofluorene was
reported to substantially increase the device current
efficiency.23 On the other hand, DꢀπꢀA structures will lead to
hole and electron traps on a same molecular and thus enhance
the carrier recombination efficiency.24 The three sterically
hindered arms are designed to prevent aggregation and suppress
fluorescence quenching, leading to high quantum efficiency.
1 (the core) was synthesized by a cyclization reaction between
5ꢀ(4ꢀBromophenyl)ꢀ1Hꢀtetrazole
and
1,3,5ꢀ
Benzenetricarboxylic acid chloride in dry pyridine.25 Bis(4ꢀ
bromobenzoyl)hydrazine (intermediate 2) and 2,5ꢀBis(4’ꢀ
bromophenyl)ꢀ1,3,4ꢀoxadiazole
(intermediate
3)
were
synthesized according to literature.26 SꢀCzꢀOXD and LꢀCzꢀ
OXD were obtained through an Ullmann reaction in the
presence of CuI, K2CO3, 1,3ꢀdimethylꢀ3,4,5,6ꢀtetrahydroꢀ
2(1H)ꢀpyrimidinone (DMPU), and 18ꢀcrownꢀ6.27 Their identity
and purity were confirmed by H NMR, 13C NMR spectra, MS
(MALDIꢀTOF), and elemental analysis, as detailed in the
1
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DOI: 10.1039/C4RA14934C
experimental part. SꢀCzꢀOXD was checked to have good Figure 1a and
b
show the UVꢀvisible absorption and PL
solubility. It could be dissolved no less than 30 mg mLꢀ1 in spectra of SꢀCzꢀOXD and LꢀCzꢀOXD in both diluted toluene
CHCl3 and 15 mg mLꢀ1 in toluene or chlorobenzene. While, Lꢀ solution and film state, their corresponding peak emission
CzꢀOXD just can be dissolved around 20mg mLꢀ1 in CHCl3, wavelengths and Фf are listed in Table 1. The main absorption
and 5 mg mLꢀ1 in toluene or chlorobenzene under heating peaks of intramolecular charge transfer of SꢀCzꢀOXD and Lꢀ
(60oC). The superior solubility of starꢀshaped molecules could CzꢀOXD in diluted toluene are 369 nm and 360 nm,
be ascribed to the bulky steric structure impedes the respectively. The peaks of PL emission wavelengths in
aggregation of molecules in solution.
solutionare located at 422 nm for SꢀCzꢀOXD and 397 nm for Lꢀ
CzꢀOXD. Both of them show very high PL quantum efficiency
in dilute toluene solution, 93% for SꢀCzꢀOXD, and 95% for Lꢀ
CzꢀOXD with 9,10ꢀdiphenylanthracene as reference.
2.2. Properties and characterizations of the blue emitters
2.2.1.
Photophysical properties
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
Absorbance in film state
Absorbance in toluene
PL in toluene
Absorbance in film state
Absorbance in toluene
PL in toluene
PL in film state
PL in film state
b
a
300
400
500
600
700
800
300
400
500
600
700
800
Wavelength (nm)
Wavelength (nm)
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
Toluene:C-Hexane
Toluene
Chloroform
THF
Toluene:C-Hexane
Toluene
Chloroform
THF
THF:Ethanol
THF:Ethanol
d
c
350
400
450
500
550
600
650
700
400
500
600
700
800
Wavelength (nm)
Wavelength (nm)
Figure 1. Absorption and PL spectra of (a) S-Cz-OXD and (b) L-Cz-OXD in dilute toluene solution and film state, and PL spectra of (c) S-Cz-OXD and (d) L-Cz-OXD in
various solvents.
To study the condensed state optical properties, thin films of indicating that SꢀCzꢀOXD can effectively suppress the
the molecules were prepared on quartz substrates by spinꢀ concentration quenching in film due to its highly branched
o
coating from solutions followed by annealing at 150 C for 30 structure.
minutes. The absorption peak of SꢀCzꢀOXD and LꢀCzꢀOXD
Most molecules with donorꢀacceptor structure usually show
show slightly redꢀshift to 372 nm and 362 nm in film state. The bathochromic shift in polar solvent because the exciton could
emission peaks of SꢀCzꢀOXD and LꢀCzꢀOXD exhibit obviously be more stabilized under polar surrounding. Therefore, we also
redꢀshifts to 455 nm and 425 nm, respectively. The solidꢀstate studied the solvatochromism effect of our compounds by
PL quantum efficiency of these films were recorded by employing solvents with different polarities, including mixed
integrated sphere. SꢀCzꢀOXD exhibits a quite high Φf value of solvent of toluene:cyclohexane (1:1, v/v), toluene, chloroform,
72% in solidꢀstate, this value of LꢀCzꢀOXD is only 47%, THF, and mixed solvent of THF:ethanol (1:1, v/v). The reason
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for using mixed solvent is that the solubility of LꢀCzꢀOXD is the maximum emission peaks show bathochromic shift from
too low to make PL measurement in individual cyclohexane 411 nm to 511 nm for SꢀCzꢀOXD and from 391 nm to 458 nm
and ethanol solvent. The resultant spectra are shown in Figure for LꢀCzꢀOXD. Their emission peaks redꢀshift about 100 nm
1, c and d. It can be seen that with increased solvent polarity, and 67 nm, respectively.
Table 1.Photophysical, electrochemical, and thermal properties of SꢀCzꢀOXD and LꢀCzꢀOXD
Abs
max
PL
max
oxd
onset
red
onset
Compound
λ
(nm) [a]
λ
(nm) [a]
Φf (%)[a]
E
(V) [b]
E
(V) [b]
EHOMO (eV)
ELUMO (eV)
Eg (eV)
Tg (oC)
Td (oC)
SꢀCzꢀOXD
LꢀCzꢀOXD
369/372
360/362
422/455
397/425
93/72
95/47
1.26
1.27
ꢀ1.51
ꢀ1.58
ꢀ5.66
ꢀ5.67
ꢀ2.89
ꢀ2.82
2.77
2.85
137
141
447
423
[a]Measured in around 10ꢀ6 M toluene solution (former) and film (later) state; [b] Using 0.1 M (C4H9)4NClO4 in CH2Cl2as supporting electrolyte, Ag/AgCl as
reference electrode.
o
o
temperature (Td, 5% weight loss) is 447 C and 423 C, and the
2.2.2.
Electrochemical and thermal properties
distinct glass transition temperature (Tg) is 137 C and 141oC
o
Cyclic voltammetry measurement has been demonstrated to
investigate the electrochemical properties of the blue emitters.
The results are shown in Figure 2. SꢀCzꢀOXD and LꢀCzꢀOXD
exhibited reversible redox behavior under both positive and
for SꢀCzꢀOXD and LꢀCzꢀOXD, respectively. The high Td and
Tg temperatures further guarantee the thermal stability during
practical application.
negative drive bias, which can be attributed to the tꢀBCz donor
units and the OXD acceptor core, respectively, indicating their
distinct bipolar properties. Their highest occupied molecular
orbital (HOMO) and lowest unoccupied molecular orbital
(LUMO) energy levels are calculated from the onset oxidation
and reduction potential according to the formula EHOMO = ꢀe(E
oxd
onset
red
onset
+ 4.4)(eV) and ELUMO = ꢀe(E + 4.4)(eV). All these data
are listed in Table 1. The calculated HOMO energy levels are
quite similar for SꢀCzꢀOXD (ꢀ5.66 eV) and LꢀCzꢀOXD (ꢀ5.67
eV), suggesting the electronꢀdonating ability of two compounds
are almost the same. The calculated LUMO energy level of Sꢀ
CzꢀOXD is ꢀ2.89 eV, slightly deeper than that of LꢀCzꢀOXD (ꢀ
2.82eV), declaring stronger electronꢀwithdrawing ability for
tris(1,3,4ꢀoxadiazole)phenylene compared to the single 1,3,4ꢀ
oxadiazole unit. The calculated energy bandgap for SꢀCzꢀOXD
is 2.77 eV, slightly lower than 2.85 eV for LꢀCzꢀOXD. These
findings are consistent with the PL measurement results. SꢀCzꢀ
OXD displays longer emission wavelength in both solution and
solid state as well as stronger solvatochromism effect. It should
possess stronger intramolecular charge transfer effect.
Considering the same HOMO level of the two compounds, it
can be attributed to the deeper LUMO energy level of the
tris(1,3,4ꢀoxadiazole)phenylene core.
S-Cz-OXD
L-Cz-OXD
-2.0 -1.5 -1.0 -0.5
0.5 1.0 1.5 2.0
Voltage(V) vsAg/AgCl
Figure 2. Cyclic voltammograms of S-Cz-OXD and L-Cz-OXD
2.3. Electroluminescent properties of non-doped devices
1.0
0.8
0.6
0.4
0.2
0.0
-0.2
-0.4
-0.6
-0.8
-1.0
-1.2
-1.4
S-Cz-OXD
L-Cz-OXD
a
b
S-Cz-OXD
L-Cz-OXD
50
100
150
200
250
300
0
100
200
300
400
500
600
700
Temperature (oC)
Temperature (oC)
Scheme 2.Energy level diagram of the OLED devices and molecular structure of
3TPYMB
Figure 3.(a) DSC traces and (b) TGA thermogram of S-Cz-OXD and L-Cz-OXD
The thermal properties of SꢀCzꢀOXD and LꢀCzꢀOXD were
explored by thermogravimetric analysis (TGA) and differential
scanning calorimetry (DSC) measurement as shown in Figure
To investigate the EL properties of SꢀCzꢀOXD, nonꢀdoped
devices
withconfigurations
of
ITO/PEDOT:PSS/SꢀCzꢀ
OXD/3TPYMB/Cs2CO3/Al was fabricated. Here, PEDOT:PSS
3
. The results are also listed in Table 1. The decomposition
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is Poly(3,4ꢀethylenedioxythiophene) Polystyrene sulfonate. layer were also fabricated for comparison. The energy diagram
3TPYMB is tris(2,4,6ꢀtrimethylꢀ3ꢀ(pyridinꢀ3ꢀyl)phenyl)borane, of the devices and the chemical structure of 3TPYMB were
serving as both electron transporting and hole blocking shown in Scheme 2
.
materials. Control device using LꢀCzꢀOXD as the emitting
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
7V
8V
9V
6V
8V
10V
b
a
400
500
600
700
800
400
500
600
700
800
Wavelength (nm)
Wavelength (nm)
1.0
0.8
0.6
0.4
0.2
0.0
1.0
d
1 hour
5 hour
c
1 hour
5 hour
0.8
0.6
0.4
0.2
0.0
10 hours
24 hours
40 hours
80 hours
120 hours
160 hours
180 hours
220 hours
10 hours
24 hours
40 hours
80 hours
120 hours
160 hours
180 hours
220 hours
400
500
600
700
800
400
500
600
700
800
Wavelength (nm)
Wavelength (nm)
Figure 4.Normalized EL spectra of devices based on (a) S-Cz-OXD and (b) L-Cz-OXD at different applied voltages, and PL spectra of (c) S-Cz-OXD and (d) L-Cz-OXD
during heating process at 150 oC in ambient air
The electroluminescent (EL) spectra of the devices at from the additional green emission band in PL spectrum after
different applied voltages could be observed in Figure 4, a and thermal treatment under high temperature in air.28 In order to
b. The common way to reduce the turnꢀon voltage is to increase investigate the emissive stability of SꢀCzꢀOXD and LꢀCzꢀOXD,
the HOMO level of the material to get a narrow energy barrier the PL spectra of their solutionꢀcasted films were recorded after
(usually PEDOT:PSS for solution processing OLED device).13 different durations of heat treatment at 150 oC in ambient
In our case, although there exists large hole injection barrier condition (Figure 4, c and d). It could be observed that there is
from PEDOT:PSS layer (ꢀ5.2 eV) to the HOMO level of almost no change of the PL spectra even after 220 hours
emitting layer (ꢀ5.66 eV and ꢀ5.67 eV), both devices show low thermal treatment. The excellent spectra stability of SꢀCzꢀOXD
turnꢀon voltages especially the one based on SꢀCzꢀOXD has a and LꢀCzꢀOXD is a greatly improvement compared to those
turnꢀon voltage at 3.7 V. This can be ascribed to its bipolar fluorineꢀbased organic light emitting materials. We believe that
structure with good hole/electron transporting properties. Pure this is due to the high stability of buildingꢀblocks such as
and deep blue emission peaking at 459nm and 429nm were carbazole and oxadiazole as well as very low lying HOMO
obtained for device based on SꢀCzꢀOXD and LꢀCzꢀOXD. They energy level at ꢀ5.66 eV resulting in excellent the oxidative
are nearly identical with the corresponding PL spectra of the stability.
films with only a little redꢀshift. The CIE coordinates of the
Figure 5 illustrates the detailed performances of the nonꢀ
devices are demonstrated to be (0.16, 0.16), (0.16, 0.07), as doped device based on the blue emitters. The
shown in Table 2. Moreover, no significant change in the EL electroluminescent performances were outlined in Table 2. The
spectra was observed at various driving voltages, indicating an devices of SꢀCzꢀOXD show high EL performances. It can reach
excellent voltageꢀindependent and stable EL spectrum.
a maximum luminance of 3601 cd mꢀ2, a maximum CE of 4.22
As mentioned above, one of the serious problems for cd Aꢀ1, an EQE of 3.37%, and a PE of 2.60 lm Wꢀ1. It still
fluoreneꢀbased materials is the unstable EL spectrum arising remains the efficiency as high as of 3.44 cd Aꢀ1 and an EQE of
from the generation of fluorenone defects. It can be verified 2.48% at the brightness of 1000 cd mꢀ2. However, the control
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device of LꢀCzꢀOXD exhibits much poorer EL performance, performance could be ascribed to the low solidꢀstate PL
with a maximum brightness of 295 cd mꢀ2, a CE of 0.62 cd Aꢀ1, efficiency, poor solubility and inferior film formation ability.
an EQE of 1.68% and a PE of 0.38 lm Wꢀ1. The inferior
4
3
2
1
0
10000
1000
100
10
900
800
700
600
500
400
300
200
100
0
S-Cz-OXD
L-Cz-OXD
S-Cz-OXD
L-Cz-OXD
1
b
a
12
0,1
0
2
4
6
8
10
0
50
100
Current Density (mA cm-2)
Voltage (V)
5
4
3
2
1
0
3
2
1
0
S-Cz-OXD
L-Cz-OXD
S-Cz-OXD
L-Cz-OXD
c
d
0
50
100
0
50
100
Current Density (mA cm-2)
Current Density (mA cm-2)
Figure 5.(a) Current density−voltage and brightness−voltage spectra of devices; (b) Current efficiency versus current density of devices; (c) Power efficiency versus
current density of devices; (d) External quantum efficiency versus current density of devices.
Table 2.Electroluminescence characteristics of the devices based on SꢀCzꢀOXD and LꢀCzꢀOXD
Device
Von [a]
(V)
Max Brightness
EQE [b]
(%)
CE [b]
PE [b]
λmax
(nm)
CIE
(x, y)
(cd mꢀ2)
(cd Aꢀ1)
(lm Wꢀ1)
SꢀCzꢀOXD
LꢀCzꢀOXD
3.7
4.2
3601
295
3.37 / 3.26 / 2.48
1.68 / 0.36 / n.a.
4.22 / 4.21 / 3.44
0.62 / 0.39 / n.a.
2.60 / 2.50 / 1.74
0.38 / 0.18 / n.a.
459
429
(0.16, 0.16)
(0.16, 0.07)
[a] Turnꢀon voltage at a brightness of 1 cd mꢀ2; [b] Maximum value and estimated values at 100 cd mꢀ2 and 1000 cd mꢀ2
processed nonꢀdoped blue device based on fluoreneꢀfree
materials, indicating its potential for practical applications.
3. Conclusion
In summary, we report a highly soluble starꢀshaped blue
molecule SꢀCzꢀOXD with bipolar feature. The unique branched
molecular structure endows it superior filmꢀforming ability,
high PL efficiency in film state as well as good thermal
stability. Furthermore, SꢀCzꢀOXD demonstrates outstanding
spectral stability under longꢀterm heat treatment in air. As a
result, efficient and stable blue electroluminescence based on Sꢀ
CzꢀOXD was realized with a current efficiency of 4.22 cd Aꢀ1
and an EQE of 3.37%. It is one of the best results for solutionꢀ
Experimental Section
3.1. Materials and synthesis
Materials:
Chemical reagents were purchased from Aldrich, Acros or
Scientific Resources Pte Ltd, and used as received without
further purified. Anhydrous toluene and THF were obtained
6 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C4RA14934C
through a solvent purification system. All manipulations Found: 774.4. Anal.Calcd. For C54H56N4O: C, 83.47; H, 7.26;
involving air and water sensitive reagents were performed N, 7.21; O, 2.06. Found: C, 83.44; H, 7.29; N, 7.15; O, 2.08.
under an atmosphere of dry nitrogen. All glassware was dried
in 120 oC oven for more than 2 hours before usage.
3.2. Measurement and instruments
1H and 13C nuclear magnetic resonance (NMR) spectra were
collected on a Bruker DRX spectrometer operating at 400 MHz
using deuterated chloroform as solvent with tetramethylsilane
(TMS) as reference. Elemental analyses were carried out on
Synthesis:
Synthesis of the core (intermediate 1): 1, 3, 5-Tris(4-
bromophenyl)-1,3,4-oxadiazol-2-yl)benzene
3.98g of 5ꢀ(4ꢀBromophenyl)ꢀ1Hꢀtetrazole (15 mmol) and 13.5g
ElementarVario EL. Molecular weight was determined using
of 1,3,5ꢀBenzenetricarboxylic acid chloride (60 mmol) were
Matrix assisted laser desorption ionization/Timeꢀofꢀflight
mixed with 250 ml anhydrous pyridine in a 500 ml round
(MALDIꢀTOF, AXIMA Performance). Thermal analyses were
bottom flask. The resulting mixture was refluxed in a nitrogen
atmosphere for 24 hours. After cooled the reaction to ambient
temperature, the solid product, collected by filtration, was
washed repeatedly with methanol and water each three times to
afford a gray powder with a yield of 75%. MS (MALDIꢀTOF,
m/z): [M]+Calcd. For C30H15Br3N6O3: 747.19. Found: 749.2.
No NMR datawas recorded forthis compound because it is
insoluble in organic solvents such as CHCl3, THF, and DMSO.
carried out using differential scanning calorimetry (DSC, Q10
TA Instrument) and thermogravimetric analyses (TGA 2950,
TA Instrument) at a heating rate of 10oCminꢀ1.UVꢀvisible
absorption spectra and emission spectra were recorded on UVꢀ
Vis Spectrophotometer (Shimadzu UVꢀ2501 PC) and Shimadzu
RF 5301 Spectrofuorophotometer, respectively. Fluorescence
quantum yields (Φf) of the samples both in THF solutions and
as films were measured by using 9,10ꢀdiphenylanthracene (Φf
=
0.92 in toluene and Φf = 0.83 dispersed in PMMA films with a
Synthesis of S-Cz-OXD
concentration lower than 1
×
10ꢀ3 M) as standards. The
5.0 g of 3,6ꢀDiꢀtertꢀbutylꢀ9Hꢀcarbazole (18 mmol), 2.3 gof
intermediate 1 (3 mmol), 2.76 g of K2CO3 (20 mmol), 0.19 g of
CuI (1mmol), 1 ml of DMPU, and 0.09 g of 18ꢀcrownꢀ6
(0.3mmol)were added into a150 ml round bottom flask
followed by evacuated and backfilled with N2. The mixture was
concentrations of their dilute toluene solution were around 10ꢀ6
M with an absorbance lower than 0.05, while the thickness of
films was around 70 nm. Quantum yield was calculated
according to equation Φn
= ΦsInAsηn /
2 AnIsηs2 where Φn is the
fluorescence quantum yield of the sample, Φs is the
fluorescence quantum yield of the standard reference, In and Is
are the integrated emission intensities of the sample and the
standard, respectively, An and As are the absorbance of the
sample and the standard at the excitation wavelength,
respectively, and ηn and ηs are the refractive indexes of the
corresponding solutions. Cyclic voltammetry was carried out on
a CHI660A electrochemical workstation at a scan rate of 100
o
stirred at 170 C for 48 h. After cooling to room temperature,
the mixture was dissolved with dichloromethane and washed
with brine for several times. Then the solution was dried with
anhydrous sodium sulfate. After filtration and removal of the
solvent, the residue was purified by silica column
chromatography followed by two times of recrystallization to
1
give the SꢀCzꢀOXD as a white solid (yield: 37%). H NMR
(CDCl3, 500 MHz,
(d, = 8.5 Hz, 6H), 8.17 (d,
Hz, 6H), 9.15 (s, 3H). 13C NMR (CDCl3, 125 MHz,
δ
): 1.49 (s, 54H), 7.49ꢀ7.53 (m, 12H), 7.86
= 0.9 Hz, 6H), 8.48 (d, = 8.5
): 32.0,
mV sꢀ1 in
a
nitrogenꢀsaturated solution of 0.1
M
J
J
J
Tetrabutylammonium perchlorate ((C4H9)4NClO4) in anhydrous
methylene chloride (CH2Cl2), using a glassy carbon electrode
as working electrode, a Pt wire as counter electrode, and an
Ag/AgCl electrode as reference electrode. The energy band gap
was calculated from Eg = ELUMOꢀ EHOMO (eV).
δ
34.8, 109.2, 116.5, 121.1, 123.9, 124.0, 126.3, 126.7, 127.3,
128.9, 138.5, 142.1, 143.8, 162.8, 165.1. MS (MALDIꢀTOF,
m/z): [M]+Calcd. For C90H87N9O3: 1341.7. Found: 1339.19.
Anal.Calcd. For C90H87N9O3: C, 80.51; H, 6.53; N, 9.39; O,
3.57. Found: C, 80.47; H, 6.56; N, 9.41; O, 3.53.
3.3. Device fabrication and characterizations
Patterned ITOꢀcoated glass (sheet resistance 10 ꢁ per square)
was cleaned sequentially with detergent, deꢀionized water,
acetone and isopropanol, each for 20 min in an ultrasonic bath.
Then the ITO substrates were blowꢀdried with nitrogen gas and
undergo O2 plasma cleaning for 10 min, PEDOT:PSS (4083)
(30 nm) was spinꢀcoated on the ITO substrate at 4000 rpm for
Synthesis of L-Cz-OXD
Similar procedures were used to synthesize LꢀCzꢀOXD.5.6 gof
3,6ꢀDiꢀtertꢀbutylꢀ9Hꢀcarbazole (20mmol), 1.9 g of intermediate
3 (5mmol), 2.76 g of K2CO3 (20 mmol), 0.19 g of CuI (2
mmol), 1 ml of DMPU, and 0.09 g of 18ꢀcrownꢀ6 (0.3mmol)
were mixed and stirred at 170 oC for 48 h under a N2
atmosphere. After purification and recrystallization, LꢀCzꢀOXD
o
60sec and then annealed for 15 min at 120 C. Then emitting
layer was spin coated on PEDOT:PSS layer from a 12 mg mLꢀ1
1
white solid was obtained (yield: 60%). H NMR (CDCl3, 400
solution
in
a
mixed
solvent
(1:1
v/v)
of
MHz,
4H), 8.16 (d,
NMR (CDCl3, 400 MHz,
δ
): 1.48 (s, 36H), 7.46ꢀ7.53 (m, 8H), 7.81 (d,
= 1.3 Hz, 4H), 8.40 (d,
= 8.5 Hz, 4H). 13C
): 29.9, 32.1, 35.0, 109.4, 116.6,
J = 8.5 Hz,
chloroform:chlorobenzene for 60sec at 2500rpm, and annealed
at 120 oC for 30 min (70 nm). Then 3TPYMB (40 nm), Cs2CO3
(1 nm) and Al (120 nm) were successively evaporated on the
emitting layer in a multiꢀsource vacuum evaporator at a base
pressure of 4×10−4 Pa. The evaporation rates and thickness
J
J
δ
122.0, 124.1, 126.9, 128.7, 138.8, 141.8, 143.9, 164.4. MS
(MALDIꢀTOF, m/z): [M]+Calcd. For C54H56N4O: 776.5.
This journal is © The Royal Society of Chemistry 2012
J. Name., 2012, 00, 1-3 | 7

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VJieowuArrtnicalelONnlainme e
ARTICLE
DOI: 10.1039/C4RA14934C
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