Improving optoelectronic properties of the 2,7-polyfluorene derivatives with carbazole and oxadiazole pendants by incorporating the blue-emitting iridium complex pendants in C-9 position of fluorine unit

Article abstract of DOI:10.1002/pola.25017

To study the influence of a blue-emitting iridium complex pendant on the optoelectronic properties of its 2,7-polyfluorene (PF) derivatives with the carbazole and oxadiazole pendants, a class of 2,7-PF derivatives containing carbazole, oxadiazole, and/wit

Full text of DOI:10.1002/pola.25017

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ARTICLE
Improving Optoelectronic Properties of the 2,7-Polyfluorene
Derivatives with Carbazole and Oxadiazole Pendants by Incorporating
the Blue-Emitting Iridium Complex Pendants in C-9 Position of
Fluorine Unit
Hua Tan, Junting Yu, Yafei Wang, Jianmin Li, Jiaojiao Cui, Jian Luo, Danyan Shi, Kai Chen,
Yu Liu, Kaixuan Nie, Weiguo Zhu
Key Laboratory of Environment-Friendly Chemistry and Application in Ministry of Education, College of Chemistry,
Xiangtan University, Xiangtan 411105, China
Correspondence to: W. Zhu (E-mail: zhuwg18@126.com) or Y. Liu (E-mail: liuyu03b@126.com)
Received 11 July 2011; accepted 12 September 2011; published online 11 October 2011
DOI: 10.1002/pola.25017
ABSTRACT: To study the influence of a blue-emitting iridium
complex pendant on the optoelectronic properties of its
2,7-polyfluorene (PF) derivatives with the carbazole and oxadia-
zole pendants, a class of 2,7-PF derivatives containing carba-
zole, oxadiazole, and/without the cyclometalated iridium
complex pendants in the C-9 positions of fluorene unit were
synthesized. Their thermal, photophysical, electrochemical, and
electroluminescent (EL) properties were investigated. Among
these 2,7-PF derivatives (P1–P4), P2 and P3 exhibited higher
photoluminescence efficiency in dichloromethane and better
EL properties in the single-emissive-layer polymer light-emit-
ting devices. The highest brightness of 3888 cd/m2 and the
maximum current efficiency of 2.9 cd/A were obtained in the
P2- and P3-based devices, respectively. The maximum bright-
ness and efficiency levels were 1.7 and 2.1 times, respectively,
higher than the corresponding levels from the parent 2,7-PF
derivative (P1)-based devices. Our work indicated that EL prop-
erties of 2,7-PF derivatives can be improved by introducing the
blue-emitting iridium complex into the alkyl side chain of fluo-
C
V
rine unit as pendant. 2011 Wiley Periodicals, Inc. J Polym Sci
Part A: Polym Chem 50: 149–155, 2012
KEYWORDS: conjugated polymers; iridium complex; 2,7-poly-
fluorene; light-emitting diodes (LED); metal-polymer complexes
INTRODUCTION 2,7-Polyfluorenes (PFs) and their derivatives
as a class of the promising blue-emitting materials have
attracted much interests, because they have high photolumi-
nescence (PL) efficiencies both in solution and in solid films
and can be substituted easily by various groups in C-9 posi-
tion to tune the solubility and optoelectronic properties. To
achieve high-efficiency polymer light-emitting diodes
(PLEDs), various 2,7-PFs and their derivatives have been
developed.1–4
reported a class of the blue-emitting PF derivatives containing
dendritic carbazole and oxadiazole pendants and presented a
PLED with an improved emission color quality and an ascend-
ant external quantum efficiency of 1.03%.
Recently, there was an increasing research activity in develop-
ing the fluorene-based copolymers as electrophosphorescent
materials, because 2,7-PF derivatives have more advantages,
and phosphorescent units have 100% internal quantum effi-
ciency. In these developed phosphorescent fluorine-based
copolymers, many red-emitting iridium (III) complex-based
PF derivatives were studied and exhibited various electrolu-
minescent (EL) properties. For example, Wang and
coworkers17 synthesized a 2,7-PF derivative containing a
pendent orange-emitting unit of iridium (III) bis(2-(40-fluoro-
In general, 2,7-PF derivatives have high ionization potential,
and there is a high-energy barrier between these 2,7-PF
derivatives and electron-blocking layer of poly-(3,4-ethylene-
dioxythiophene) (PEDOT) that results in higher driving voltages
and an unbalance in charge injection and transportation.
Accordingly, various strategies have also been developed to
overcome these deficiencies for PF derivatives.5–14 For example,
Lin15 designed a series of the fluorene-based copolymers con-
taining dendritic 1,3,4-oxadiazole pendants and obtained a
PLED with a bright luminescence of 2446 cd/m2 at 12 V and a
current efficiency of 0.24 cd/A at 100 mA/cm2. Peng et al.16
2
0
phenyl)-4-phenylquinoline-N,C ) (tetradecane-dionate-11,13)
in the C-9 position and obtained a white-emitting device
with a maximum current efficiency of 4.49 cd/A and a Com-
mission Internationale de l’Eclairage (CIE) coordinate of
(0.46, 0.33) at 6.0 V. Cao reported a type of the fluorene-
based copolymers, in which the iridium complex units with a
Additional Supporting Information may be found in the online version of this article.
C
V
2011 Wiley Periodicals, Inc.
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cyclometalated ligand of 2-p-tolyl-benzothiazole were incor-
porated into the copolymers by either embedding or end-
capping into the backbone. Their PLEDs exhibited a red
emission peaked at 599 nm with a maximum external quan-
tum efficiency of 2.19% at a current density of 0.3 mA/cm2
and a maximum luminance of 2347 cd/m2 at 17 V.18 Yang
presented a series of 2,7-PF derivatives containing red-emit-
ting iridium complexes and carrier-transporting units as the
substitutes of the C-9 position of fluorene. A maximum cur-
rent efficiency of 9.3 cd/A and a maximum power efficiency
of 10.5 lm/W were obtained.19
derivates were dissolved in the above solution (1 mg/cm3).
A platinum rod and wire were used as a working electrode
and a counter electrode, respectively. A calomel electrode
was used as a reference electrode. Devices with a structure
of indium tin oxide (ITO)/PEDOT (40 nm)/PVK (40 nm)/PF
derivatives (80 nm)/CsF (4 nm)/Al (120 nm) were fabri-
cated in a controlled dry-box (Vacuum Atmosphere) under
N2 circulation, where a poly(vinyl carbazole) (PVK) layer
was used as a hole-transporting layer and a 4-nm thin layer
of CsF with a 120-nm Al capping layer was used as cathode.
The devices were fabricated by following a standard proce-
dure. A 40-nm thick layer of PEDOT:poly(styrene sulfonic
acid) was spun onto the precleaned ITO-glass substrates.
Then, a 40-nm-thick layer of PVK was spun on the top of
PEDOT. An 80-nm-thick emissive layer of the PF derivatives
was spun on the top of PVK. A profilometer (Tencor
Alfa-Step 500) was used to determine the thickness of the
films. The current density (J)–voltage (V) data were collected
using a Keithley 236 source measurement unit. Luminance
was measured by a Si photodiode and calibrated by a PR-
705 spectrascan spectrophotometer (Photo Research). EL
spectra were recorded using a charged coupling device
spectrophotometer (Instaspec 4, Oriel).
However, there is few report on the PF derivatives attached
the blue-emitting phosphorescent units onto the alkyl side
chains of the PF. As PF has a triplet energy in the range of
2.1–2.3 eV which is lower than that of the blue-emitting iri-
2
0
dium(III) bis[(4,6-difluorophenyl)pyridinato-N,C ] picolinate
[FIr(pic)] (2.6 eV), the efficient energy transfer from FIr(pic)
to PF is expected to occur.20 Therefore, introducing a FIr(pic)
unit onto the alkyl side chains of the PF derivatives should
be a feasible strategy to improve the optoelectronic proper-
ties of its resulting PF derivatives due to the efficient energy
transfer. To study the effect of the blue-emitting iridium com-
plex unit on the optoelectronic properties of its PF deriva-
tives, the FIr(pic) unit was introduced into PF derivatives
containing the carbazole and oxadiazole pendants by an
unconjugated linkage. The resulting 2,7-PF derivatives were
synthesized, and the synthetic route is shown in Scheme 1.
As expected, the PL and devices efficiency of these FIr(pic)-
modified 2,7-PF derivatives were greatly improved when
compared to the PF derivative without the FIr(pic) unit.
Synthesis of 2-(4-(6-Bromohexyloxy)phenyl)-5-
(4-tert-butylphenyl)-1,3,4-oxadiazole (1)
A mixture of 4-(5-(4-tert-butylphenyl)-1,3,4-oxadiazol-2-yl)-
phenol (2.94 g, 10 mmol), 1,6-dibromohexane (14.7 g,
60.3 mmol), THF (50 mL), and K2CO3 (4.0 g, 40 mmol) were
ꢀ
stirred vigorously under nitrogen protection at 70 C for 24
h. The mixture was cooled to RT, poured into brine, and then
extracted with dichloromethane (DCM). The combined
organic layer was washed with brine for three times and
dried over magnesium sulfate. After removal of the solvent,
the excess 1,6-dibromohexane was separated from the mix-
EXPERIMENTAL
Materials and Measurement
All manipulations were performed under dry nitrogen flow.
All reagents were purchased from Aldrich and were directly
used without further purification. The contents of the pendent
FIr(pic) unit in PF derivatives are 2.5, 7.5, and 12.5 mol % for
P2, P3, and P4, respectively. For comparison, P1 without the
FIr(pic) pendant was synthesized at the same time. The inter-
mediates of M1, M2, and 4-(5-(4-tert-butyl-phenyl)-1,3,4-oxa-
diazol-2-yl)phenol were synthesized according to the published
procedures.4,21 M4 was synthesized based on our previous
work.22
ꢀ
ture by vacuum distillation (130 C/0.1 mmHg). The residue
was purified by flash chromatography using hexane/DCM
(V/V, 1:0–1:1) as eluent to give compound 1 (3.27 g, 71.6%)
as white solid. 1H NMR (CDCl3, 400 MHz), d (ppm): 1.33
(s, 9H), 1.53–1.58 (m, 8H), 1.93–1.95 (m, 4H), 3.45
(t, J ¼ 6.8 Hz, 2H), 4.06 (t, J ¼ 7.0 Hz, 2H), 7.02 (d, J ¼ 8.2 Hz,
2H), 7.55 (d, J ¼ 8.0 Hz, 2H), 8.03 (d, J ¼ 8.0 Hz, 4H). 13C
NMR (CDCl3, 100 MHz), d (ppm): 163.4, 163.24, 160.8, 154.1,
127.6, 125.8, 124.7, 120.4, 115.5, 114.0, 67.0, 34.0, 32.6, 31.7,
30.1, 28.0, 26.9, 24.1. TOF-MS (m/z): Calcd for C24H29BrN2O2,
456.1; found, 456.8.
1H and 13C NMR spectra were recorded on a Bruker DRX
400 spectrometer by tetramethylsilane as a reference in deu-
terated chloroform solution at 298 K. MALDI-TOF mass spec-
trometric measurements were performed on Bruker Bifiex
III MALDI-TOF. Molecular weights for these PF derivatives
were determined using a Waters GPC 2410 in tetrahydrofu-
ran via a calibration curve of polystyrene as standard. UV
absorption spectra were recorded with a HP-8453 UV–visible
system. PL spectra were recorded on a HITACHI-850 fluores-
cence spectrophotometer. Cyclic voltammetry (CV) was
performed on a CHI660A electrochemical work station in a
solution of 0.1 M tetrabutylammonium hexafluorophosphate
(Bu4NPF6) in acetonitrile at a scan rate of 100 mV/s and at
room temperature (RT) under nitrogen protection. The PF
Synthesis of M2
Compound M2 was synthesized according to a synthetic pro-
cedure of compound 1. Yield: 79%, mp: 80.0–80.4 ꢀC. 1H
NMR (CDCl3, 400 MHz),
d (ppm): 0.51–0.56 (m, 4H),
1.06–1.25 (m, 8H), 1.67–1.70 (m, 4H), 1.79–1.82 (m, 4H),
4.16 (t, J ¼ 7.2 Hz, 4H), 7.18 (t, J ¼ 7.6 Hz, 4H), 7.30–7.35
(m, 6H), 7.41–7.49 (m, 8H), 8.06 (d, J ¼ 7.8 Hz, 4H).
13C NMR (CDCl3, 100 MHz), d (ppm):152.2, 140.4, 139.0,
130.3, 126.1, 125.6, 122.8, 121.6, 121.2, 120.3, 118.7, 108.6,
55.5, 42.9, 40.0, 29.5, 28.7, 26.8, 23.5. TOF-MS (m/z): Calcd
for C49H46Br2N2, 822.7; found, 822.1.
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SCHEME 1 Synthetic route of the PF derivatives containing carbazole, oxadiazole, and iridium complex pendants.
Synthesis of M3
as eluent to give compound M3 as white solid (8.37 g, 77.8
1
ꢀ
A mixture of 2,7-dibromo-9H-fluorene (3.24 g, 10 mmol),
compound 1 (9.14 g, 20 mmol), aqueous sodium hydroxide
(33%, 20 mL), toluene(50 mL), and tetrabutyl ammonium
bromide (0.1 g) were stirred vigorously under nitrogen pro-
%), mp: 120.8–121.1 C. H NMR (CDCl3, 400 MHz), d (ppm):
0.64–0.67 (br, 4H), 1.14–1.37 (m, 30H), 1.60–1.66 (m, 4H),
1.94–1.98 (m, 4H), 3.91 (t, J ¼ 4.0 Hz, 4H), 6.96 (d, J ¼ 6.0
Hz, 4H), 7.44–7.47 (m, 4H), 7.48 (t, J ¼ 8.0 Hz, 2H),
7.52–7.55 (m, 4H), 8.00 (d, J ¼ 7.0 Hz, 8H). 13C NMR (CDCl3,
100 MHz), d (ppm):164.4, 164.2, 161.9, 155.2, 152.3, 139.1,
130.4, 128.6, 126.7, 126.1, 126.0, 121.6, 121.2, 116.2, 115.0,
68.1, 55.6, 40.1, 35.1, 31.1, 29.5, 28.9, 25.6, 23.6. TOF-MS
(m/z): Calcd for C61H64Br2N4O4, 1076.3; found, 1077.3.
ꢀ
tection for 24 h at 80 C. The mixture was cooled to RT and
extracted with DCM (3 ꢁ 50 mL). The combined organic layer
was dried over magnesium sulfate and evaporated under
reduced pressure to remove toluene. The residue was purified
by flash chromatography using ethyl acetate/DCM (V/V, 1:10)
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TABLE 1 Polymerization Results and Thermal Properties of the
PF Derivatives
b
c
Polydispersity
indexa
Td
Tg
a
a
Polymer
Mn
Mw
(ꢀC)
(ꢀC)
1
2
3
4
a
P1
P2
P3
P4
20,365
12,136
9,285
33,892
20,025
17,019
54,704
1.66
1.65
1.83
2.47
416
417
367
397
93
94
79
22,092
105
b
Mn, Mw
,
and polydispersity
Td were measured by TGA at a
index of the polymers were deter-
mined by gel permeation chro-
matography using polystyrene
standards.
temperature of 5% weight loss.
Tg were determined by DSC.
c
General Procedures of Suzuki Polycondensation, Taking
P2 For Example
FIGURE 1 Normalized UV–vis absorption spectra of the PF
derivatives in DCM at RT.
Tetrakis(triphenylphosphine)palladium (10 mg) was added
under a nitrogen atmosphere into a degassed mixture con-
taining the monomers M1 (128.5 mg, 0.2 mmol), M2
(78.16 mg, 0.095 mmol), M3 (102.2 mg, 0.095 mmol), and
M4 (12.29 mg, 0.01 mmol), Na2CO3 aqueous solution (2.0 M,
2.0 mL), and Aliquat 336 (two drops) in toluene (4 mL). The
P1, 131 mg, yellowish solid. Yield: 42%. 1H NMR (CDCl3,
400 MHz), d (ppm): 0.59–1.45 (m, 68 H), 1.46–1.86 (m, 8H),
1.93–2.45 (br, 16H), 3.79–4.00 (m, 4H), 4.06–4.25 (br, 4H),
6.85–7.00 (m, 4H), 7.16–7.21 (m, 4H), 7.21–7.92 (m, 36H),
7.93–8.15 (m, 12H). 13C NMR (CDCl3, 100 MHz), d (ppm):
164.4, 164.2, 161.9, 155.1, 151.8, 140.4, 128.6, 126.7, 126.0,
125.5, 122.8, 121.4, 120.3, 118.7, 115.0, 114.9, 108.6, 68.1,
55.4, 42.9, 40.4, 35.1, 31.8, 31.1, 30.0, 29.7, 29.1, 25.6, 23.9,
22.6, 14.1.
ꢀ
mixture was stirred vigorously and heated at 90 C for 48 h.
The end groups were capped by heating the resulting mix-
ture under reflux for 12 h with 2,4-difluorobenzeneboronic
acid (31.4 mg, 0.200 mmol) and for another 12 h with bro-
mobenzene (34 mg, 0.220 mmol). The mixture was then
cooled to RT and dropped into 200 mL MeOH to form pre-
cipitate. The precipitate was collected and dissolved in DCM.
The resulting DCM solution was dropped into 200 mL MeOH
again to obtain precipitate. This new precipitate was washed
with acetone under refluxing for 48 h in a Soxhlet apparatus
and dried under vacuum to give P2 (105 mg, 28 %) as a yel-
lowish solid. 1H NMR (CDCl3, 400 MHz), d (ppm): 0.56–1.45
(m, 68 H), 1.47–1.82 (br, 8H), 1.88–2.31 (br, 16H), 3.83–3.95
(m, 4H), 4.06–4.23 (br, 4H), 6.83–7.04 (m, 4H), 7.09–
7.23 (m, 4H), 7.26–7.92 (m, 36H), 7.94–8.16 (m, 12H). 13C
NMR (CDCl3, 100 MHz), d (ppm): 164.4, 164.2, 161.9, 155.1,
151.8, 151.4, 140.4,140.1, 132.2, 128.6, 126.7, 126.0, 125.5,
122.8, 121.8, 120.3,118.7, 116.4, 114.9, 108.6, 68.1, 55.4,
42.9, 40.4, 35.1, 32.0, 31.8,31.2, 30.0, 29.8, 29.7, 29.2, 29.0,
28.8, 26.9, 26.3, 25.9,25.6, 23.9, 22.7, 22.6, 14.1.
P3, 85 mg, yellowish solid. Yield: 26%. 1H NMR (CDCl3, 400
MHz), d (ppm): 0.63–1.46 (m, 68 H), 1.48–1.77 (m, 8H),
1.91–2.33 (br, 16H), 3.84–3.96 (m, 4H), 4.07–4.24 (br, 4H),
6.84–6.99 (m, 4H), 7.10–7.25 (m, 4H), 7.27–7.91 (m, 36H),
7.96–8.14 (m, 12H). 13C NMR (CDCl3, 100 MHz), d (ppm):
164.4, 161.9, 155.1,151.9, 140.4, 140.1, 128.8, 127.2, 126.7,
126.0, 125.5, 122.8, 121.4, 120.3, 120, 118.7, 116.4, 114.9,
108.6, 68.1, 55.4, 42.9, 40.4, 35.1, 31.8, 31.2, 30, 29.7, 29.2,
29, 28.8, 26.9, 25.7, 23.9, 22.6, 14.1.
P4, 65 mg, yellowish solid. Yield: 20%. 1H NMR (CDCl3, 400
MHz), d (ppm): 0.64–1.42 (m, 68 H), 1.42–1.79 (m, 8H),
1.93–2.23 (br, 16H), 3.81–3.96 (m, 4H), 4.07–4.27 (br, 4H),
6.89–6.98 (m, 4H), 7.11–7.25 (m, 4H), 7.27–7.91 (m, 36H),
7.94–8.12 (m, 12H). 13C NMR (CDCl3, 100 MHz), d (ppm):
164.4, 164.2, 161.9, 155.1, 151.8, 140.4, 140.1, 128.6, 126.7,
TABLE 2 Absorption, Emission Properties of the PF Derivatives
Absorptionak/nm
Emission (measured in
Emission (measured in the
Polymer
(emax/dm3 molꢂ1 cmꢂ1
)
CH2Cl2 at 298 K) kmax (nm)
neat film at 298 K) kmax (nm)
Ufb
P1
P2
P3
P4
a
294(6.8 ꢁ 105), 390(8.8 ꢁ 105)
296(8.1 ꢁ 105), 383(5.7 ꢁ 105)
296(6.3 ꢁ 105), 381(7.9 ꢁ 105)
296(5.6 ꢁ 105), 381(5.6 ꢁ 105)
417, 440
417, 439
417, 438
417, 438
449,526
447,527
450,536
449,536
0.41
0.68
0.77
0.62
Measured in CH2Cl2 at a concentration of 10ꢂ6 mol/L.
b
Measured in CH2Cl2 at 298 K. Excitation wavelength was 380 nm, and 9,10-diphenylanthrancene (Uf ¼ 0.9) was used as the reference.
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(DSC) and thermogravimetry analysis (TGA). Their glass
transition temperatures (Tg) and thermal degradation tem-
peratures (Td) for 5% weight loss are listed in Table 1. The
corresponding TGA and DSC curves are shown in Figures S1
and S2, Supporting Information. Obviously, these PF deriva-
tivesꢀexhibit high Tg and Td, in which Tg varies from 79 to
ꢀ
105 C and Td is over 367 C. Compared to poly(9,9-dihexyl-
fluorene) (Tg ¼ 55 ꢀC), these PF derivatives containing car-
bazole, oxadiazole, and iridium complex pendants present
higher Tg. Therefore, incorporating the pendants of carbazole,
oxadiazole and iridium complex into the alkyl side chain
of PF can improve the thermal stabilities of their 2,7-PF
derivatives.
Optical Properties
The UV–vis absorption spectra of these PF derivatives in
dilute DCM are shown in Figure 1, and their data are sum-
marized in Table 2. Almost similar UV–vis absorption spectra
are displayed in these PF derivatives of P1–P4. Two typical
and intense absorption bands are observed at about 300 nm
and 381 nm, respectively. The first high-lying one is attrib-
uted to the p–p* transitions from the pendants of carbazole,
oxadiazole, and iridium complex. The second low-lying one is
assigned to the p–p* transition associated with the PF back-
bone. However, the metal–ligand charge transfer absorption
from the iridium complex unit is very hard to observe
owing to the influence of much strong p–p* transition
absorption of the pendants and PF backbone. This phenom-
enon is also formed in the red-emitting phosphorescent PF
derivatives.23
The PL spectra of these PF derivatives in dilute DCM and in
their neat thin films are shown in Figure 2, as well as their
data are also summarized in Table 2. Almost identical PL
profiles with stable and strong purple-blue emission are also
exhibited for all of these PF derivatives in dilute DCM. The
maximum emissive peak at 417 nm and a shoulder at
438 nm are observed, which is a characteristic emission of
poly(9,9-dioctylfluorene) (PFO) backbone. Emissions from
the pendants of the carbazole, oxadiazole, and iridium com-
plex are not significantly displayed. This implies that efficient
energy transfer from these pendants to the polymer back-
bone has occurred. We note that the PL quantum yields of
the P1, P2, P3, and P4 in DCM were measured to be 0.41,
0.68, 0.77, and 0.62, respectively, using 9,10-diphenylan-
thrancene (Uf ¼ 0.9) as reference. Therefore, attaching the
FIGURE 2 Normalized PL spectra of the PF derivatives in DCM
(A) and their neat films (B) at RT.
126.1, 126.0, 125.5, 122.8, 121.3, 120.3, 120, 118.6, 116.4,
114.9, 108.6, 68.2, 55.4, 42.9, 40.4, 35.1, 31.8, 31.2, 30, 29.7,
29.2, 29, 28.8, 26.9, 25.7, 23.9, 22.6, 14.0.
RESULTS AND DISCUSSION
Thermal Property
The thermal property of these PF derivatives of P1, P2, P3,
and P4 were determined by differential scanning calorimeter
TABLE 3 Electrochemical Properties of the Polyfluorene Derivatives
a
b
c
c
d
ox
1=2
red
1=2
Polymer
E
(V)
E
(V)
EHOMO (eV)
ELUMO (eV)
Eg (eV)
Eopt (eV)
P1
P2
P3
P4
a
1.62
1.60
1.60
1.56
ꢂ1.65
ꢂ1.79
ꢂ1.71
ꢂ1.72
ꢂ5.96
ꢂ5.94
ꢂ5.94
ꢂ5.90
c
ꢂ2.69
ꢂ2.55
ꢂ2.63
ꢂ2.62
3.27
3.39
3.31
3.28
2.95
2.95
2.95
2.94
Potential values are reported by cyclic voltammetry versus
Fe/Feþ.
ELUMO ¼ ꢂ(4.34 þ Er1e=d2) eV, Eg ¼ ELUMOꢂEHOMO
.
d
Estimated according to the UV–vis absorption spectrum.
b
EHOMO ¼ ꢂ(4.34 þ Eo1=x2) eV.
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TABLE 4 Device Performances of the Polyfluorene Derivative-
Based PLEDs
Polymer Vturn-on (V) Lmax (cd/m2) LE (cd/A) kmax (nm) CIE
P1
P2
P3
P4
5.6
6.0
7.2
8.7
2,334
3,888
2,153
3,068
1.35
1.97
2.90
1.19
556
561
560
558
0.41, 0.55
0.40, 0.54
0.42, 0.54
0.44, 0.52
the solid state and result in enhanced excimer emission of
its PF derivatives under photoexcitation.
Electrochemical Property
The redox behaviors of these PF derivatives in the neat films
were studied by CV using an anhydrous electrolyte of 0.1 M
tetrabutylammonium hexafluorophosphate (Bu4NPF6) in
CH3CN solution. All of these polymers presented a reversible
oxidation potential (Eox) at about 1.56–1.61 V and a reversi-
ble reduction potential (Ered) at about ꢂ1.65 to ꢂ1.79 V. The
Ered and Eox data are listed in Table 3. It is obvious that the
FIr(pic)-modified polyfluorene derivatives (P2–P4) show
lower oxidation and reduction potentials than the P1 copoly-
mer. To further study the influence of the blue-emitting
FIr(pic) pendant on the energy levels of its PF derivatives,
the highest occupied molecular orbital (HOMO) and the low-
est unoccupied molecular orbital (LUMO) energy levels
(EHOMO and ELUMO) were calculated based on the following
FIGURE 3 EL spectra for the PF derivative-based PLEDs.
blue-emitting FIr(pic) pendant onto the alkyl side-chain of
the PF derivatives is available to improve the emission quan-
tum efficiency of its resulting PF derivatives.
However, these PF derivatives in the neat thin films exhibited
significantly different PL spectra instead of in dilute DCM.
Two typical emission bands were observed at around
449 and 530 nm, in which the high-lying emission band
comes from the fluorene-based backbone, and the low-lying
emission band is attributed to excimer emission of these PF
derivatives.4 As this kind of PF derivatives contains the oxa-
diazole unit with an accepting electron effect and carbazole
unit with a donating electron effect, there is a donor–
acceptor effect in them, the PF derivatives are available to
exhibit intense excimer emission.24,25 In addition, the P2, P3,
and P4 presented much stronger excimer emission than the
P1 under photoexcitation. This indicates that the incorpo-
rated blue-emitting FIr(pic) unit with an accepting electron
effect can facilitate the molecular donor–acceptor effect in
empirical formula: EHOMO ¼ ꢂ(Eox þ 4.34) eV and ELUMO
¼
ꢂ(Ered þ 4.34) eV. The results show that the HOMO and
LUMO energy levels of these PF derivatives are around ꢂ5.93
eV and ꢂ2.62 eV, respectively. Although the optical band gaps
(Eopt) are identical for these copolymers, the FIr(pic)-modi-
fied PF derivatives (P2–P4) present higher EHOMO and ELUMO
than P1. This means that incorporating the blue-emitting
FIr(pic) unit into alkyl side chain of PF is in favor of promot-
ing the HOMO and LUMO energy levels; thus, the energy bar-
rier of charge injection from anode to emitters is decreased.
Electroluminescence Properties
The EL spectra from these PF derivative-based PLEDs are
shown in Figure 3. Almost identical EL spectra with a maxi-
mum emission peak around 560 nm are observed in these
devices. No emission from the PF backbone and the pendants
of carbazole, oxadiazole, and iridium complex was observed.
Compared to the PL spectra in the neat films, the EL may be
attributed to the excimer emission of these PF derivatives.20 It
is suggested that the donor–acceptor effect in these PF deriva-
tives plays a much important role in the excimer formation and
emission under electric field.24 Unlike the significantly changed
PL spectra in the neat films, the EL spectra have little change
with increasing FIr(pic) units for these PF derivatives. This
indicates that incorporating blue-emitting FIr(pic) onto alkyl
side chain of PF derivatives has little influence on excimer
emission of its PF derivatives under electric filed.
The current density–voltage–luminance curves of these PF
derivative-based PLEDs are shown in Figure 4, and the devi-
ces performances are summarized in Table 4. The turn-on
FIGURE 4 Current density–luminance characteristics of the PF
derivative-based PLEDs.
154
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ARTICLE
3 Wu, F. I.; Shih, P. I.; Shu, C. F. Macromolecules 2005, 38,
9028–9036.
voltage is observed to increase with increasing FIr(pic) units
which is considered to be related to the carrier-trapped effect
of these PF derivatives. On the other hand, the blue-emitting
FIr(pic)-modified PF derivatives (P2,P3) exhibit higher current
efficiency than the P1 in the devices. With increasing FIr(pic)
units from 0 to 7.5 mol %, the PF derivatives exhibited an
increased current efficiency in the devices. However, with fur-
ther increasing FIr(pic) units from 7.5 to 12.5 mol %, the PF
derivatives displayed a decreased current efficiency. Among
these PF derivatives, P3 displays a highest current efficiency of
2.90 cd/A at 232 mA/cm2, and P2 exhibits a highest brightness
of 3888 cd/m2 at 237 mA/cm2 in the devices. The brightness
and current efficiency levels are 1.7 and 2.1 times higher than
the corresponding levels of the P1-based devices, respectively.
This indicates that incorporating the blue-emitting FIr(pic)
units with proper ratios into alkyl side chain of PF is available
to improve the EL performance of its PF derivatives containing
carbazole and oxadiazole units in the PLEDs.
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CONCLUSIONS
We obtained a series of the PF derivatives grafted with oxadia-
zole, carbazole, and/without the blue-emitting cyclometalated
iridium complex in the alkyl side chain units Among these
copolymers, the iridium complex modified PF derivatives
exhibit higher PL efficiencies in DCM and better device per-
formance in the PLEDs. A maximum brightness of 3888 cd/m2
and a maximum current efficiency of 2.90 cd/A were achieved
in the P2- and P3-based devices, respectively. The EL perform-
ance of the PF derivatives was significantly improved by incor-
porating a blue-emitting iridium complex into alkyl side chain
of fluorine as pendant.
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The authors thank the financial support from the National Nat-
ural Science Foundation of China (50973093, 20872124, and
20772101), the Specialized Research Fund and the New Teach-
ers Fund for the Doctoral Program of Higher Education (2009
4301110004 and 200805301013), the Scientific Research
Fund of Hunan Provincial Education Department (10A119,
11CY023, and 10B112), the Science Foundation of Hunan Prov-
ince (2009FJ2002), and the Postgraduate Science Foundation
for Innovation in Hunan Province (CX2011B263).
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