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Journal ArticleDOI

3D Monodisperse Oligofluorenes with Non-Conjugated Triphenylamine-Based Cores: Synthesis and Optoelectronic Properties

01 Apr 2010-European Journal of Organic Chemistry (John Wiley & Sons, Ltd)-Vol. 2010, Iss: 12, pp 2295-2303

TL;DR: Two 3D monodisperse oligofluorenes with non-conjugated triphenylamine-based cores have been synthesized by Friedel-Crafts copolycondensation reaction and emit deep-blue fluorescence with high efficiencies in thin films.

AbstractTwo 3D monodisperse oligofluorenes with non-conjugated triphenylamine-based cores have been synthesized by Friedel-Crafts copolycondensation reaction. The oligomers, PF 3 -TPA and PF 3 -TPA 3 , consist of three fluorene pentamer arms that are connected non-conjugately through a triphenylamine (TPA) and 1,3,5-tris(triphenylamino)benzene core (TPA 3 ), respectively, at the 9-position of the central fluorene of the pentafluorene arms. The coplanar structures of the cores and the linkages at the centre of the pentafluorene arms produced a 3D structure of oligomers. This specific structure efficiently retarded the crystallization tendencies of the pentafluorene arms and gave the materials completely amorphous morphological structures. Both oligomers emit deep-blue fluorescence with high efficiencies in thin films (Φ pl-film = 67 % for PF 3 -TPA 3 and 86 % for PF 3 -TPA). The introduction of triphenylamine units into the core promoted the hole-injection ability while not obviously scarificing the electron-injection ability of the oligomers. The multi-layer devices ITO/PEDOT-PSS/PF 3 -TPA 3 and PF 3 -TPA/TPBI/LiF/Al were fabricated to investigate the electroluminescence (EL) properties of the two oligomers. Both oligomers showed a low turn-on voltage of 4 V. The luminances reached 1946 cd/m 2 at 7.5 V in the PF 3 -TPA 3 device and 1055 cd/m 2 at 8 V in the PF 3 -TPA device. The EL efficiencies at this luminance were 1.63 and 1.57 cd/A, respectively.

Topics: Triphenylamine (57%)

Summary (2 min read)

Introduction

  • In the past decade, fluorene-based conjugated polymers have emerged as a very promising class of blue-light-emit- ting materials for use in polymer light-emitting diodes because of their high photoluminescence (PL) and electroluminescence (EL) quantum efficiencies, thermal sta- bility, good solubility and facile functionalization at the 9- position of fluorene. [1].
  • Some researchers have demonstrated that the introduction of bulky groups at the 9-position of fluorene or introduction of cross-linkable moieties tended to suppress this emission and to improve the thermal stability of the PL spectra.[3].
  • Inspired by these results, the authors recently synthesized monodisperse triphenylamine-substituted oligofluorenes[10] (shown in Scheme 1) in which the triphenylamine cyclic core serves as a non-conjugated spacer bearing oligofluorene arms in a multi-H shaped structure of oligomers.

Synthesis and Characterization

  • The synthetic procedures used to prepare PF3-TPA3 and PF3-TPA are outlined in Scheme 3 and Scheme 4 respectively.
  • Scheme 4. Synthetic route for the preparation of the steric monodisperse oligofluorene with a non-conjugated TPA core (PF3-TPA).
  • The structures and monodispersities of PF3-TPA3 and PF3-TPA were identified by NMR, size-exclusion chromatography (SEC) and MALDI-TOF mass spectroscopy.
  • The peaks at δ = 2.29 (6 H, singlet) and 2.05 ppm (16 H, multiplet) are due to the resonance of the CH3 groups in the TPA unit and the methylene protons of the octyl groups adjacent to C-9 of the fluorene units, respectively.
  • The Mn values determined from the SEC analysis were based on polystyrene standards.

Thermal and Photophysical Properties

  • The thermal properties of PF3-TPA3 and PF3-TPA were characterized by differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA) and the results are summarized in Table 1.
  • In addition, there is a negligible shift in the absorption and PL spectra for both steric oligomers on changing from the solution to solid state, which indicates the absence of strong interchain interactions in the solid-state films.
  • The PF3-TPA3 film showed lower stability in the n-doping process.
  • This result indicates that introducing a non-conjugated triphenylamine X. Zhang, Y. Quan Z. Cui, Q. Chen, J. Ding, J. LuFULL PAPER unit core into the oligomers promotes their hole-injection capability, but does not sacrifice their electron-injection capability.

Conclusions

  • Two 3D steric monodisperse oligofluorenes with a TPA3 and TPA core (PF3-TPA3 and PF3-TPA) have been synthesized by the Friedel-Crafts copolycondensation reaction.
  • The triphenylamine-based cores serve as non-conjugated spacers bearing oligofluorene arms in a multi-H-shaped structure of oligomers such that the optoelectronic proper- ties of the individual oligofluorene arms remain relatively unperturbed.
  • Both oligomers show excellent thermal sta- bility and high photoluminescence quantum efficiency.
  • Electrochemical analysis showed that the non-conjugated triphenylamine core promotes the hole-injection capability of the oligomers, but does not sacrifice their electron-injec- tion capability.
  • LED devices based on these two oligomers showed comparable performances with respect to tri- phenylamine-substituted linear polyfluorene derivatives in solution-processed light-emitting devices.

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3D monodisperse oligofluorenes with non-conjugated triphenylamine-
based cores : Synthesis and optoelectronic properties
Zhang, Xiaomo; Quan, Yiwu; Cui, Zhe; Chen, Qingmin; Ding, Jianfu; Lu,
Jianping
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FULL PAPER
DOI: 10.1002/ejoc.200901140
3D Monodisperse Oligofluorenes with Non-Conjugated Triphenylamine-Based
Cores: Synthesis and Optoelectronic Properties
Xiaomo Zhang,
[a]
Yiwu Quan,*
[a]
Zhe Cui,
[a,b]
Qingmin Chen,
[a]
Jianfu Ding,
[b]
and
Jianping Lu*
[c]
Keywords: Optoelectronic properties / Oligomers / Amines / Fluorescence / Electrochemistry
Two 3D monodisperse oligofluorenes with non-conjugated
triphenylamine-based cores have been synthesized by
Friedel–Crafts copolycondensation reaction. The oligomers,
PF
3
-TPA and PF
3
-TPA
3
, consist of three fluorene pentamer
arms that are connected non-conjugately through a tri-
phenylamine (TPA) and 1,3,5-tris(triphenylamino)benzene
core (TPA
3
), respectively, at the 9-position of the central fluor-
ene of the pentafluorene arms. The coplanar structures of
the cores and the linkages at the centre of the pentafluorene
arms produced a 3D structure of oligomers. This specific
structure efficiently retarded the crystallization tendencies of
the pentafluorene arms and gave the materials completely
amorphous morphological structures. Both oligomers emit
Introduction
In the past decade, fluorene-based conjugated polymers
have emerged as a very promising class of blue-light-emit-
ting materials for use in polymer light-emitting diodes
(PLEDs) because of their high photoluminescence (PL) and
electroluminescence (EL) quantum efficiencies, thermal sta-
bility, good solubility and facile functionalization at the 9-
position of fluorene.
[1]
However, the application of poly-
fluorenes in PLEDs has been hampered by the formation
of an emission tail at long wavelengths during device opera-
tion due to undesired chain aggregation, excimer formation
and keto defects.
[2]
Some researchers have demonstrated
that the introduction of bulky groups at the 9-position of
fluorene or introduction of cross-linkable moieties tended
to suppress this emission and to improve the thermal sta-
bility of the PL spectra.
[3]
Another challenge is how to
[a] Department of Polymer Science & Engineering, State Key Lab-
oratory of Coordination Chemistry, School of Chemistry and
Chemical Engineering, Nanjing University,
Nanjing 210093, P. R. China
E-mail: quanyiwu@nju.edu.cn
[b] Institute for Chemical Process and Environmental Technology
(ICPET), National Research Council of Canada (NRC),
1200 Montreal Road, Ottawa, Ontario K1A 0R6, Canada
[c] Institute for Microstructural Sciences (IMS), National Research
Council of Canada (NRC),
1200 Montreal Road, Ottawa, Ontario K1A 0R6, Canada
E-mail: jianping.lu@nrc-cnrc.gc.ca
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.200901140.
Eur. J. Org. Chem. 2010, 2295–2303 © 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2295
deep-blue fluorescence with high efficiencies in thin films
(
φ
pl–film
= 67 % for PF
3
-TPA
3
and 86 % for PF
3
-TPA). The in-
troduction of triphenylamine units into the core promoted the
hole-injection ability while not obviously scarificing the elec-
tron-injection ability of the oligomers. The multi-layer de-
vices ITO/PEDOT-PSS/PF
3
-TPA
3
and PF
3
-TPA/TPBI/LiF/Al
were fabricated to investigate the electroluminescence (EL)
properties of the two oligomers. Both oligomers showed a low
turn-on voltage of 4 V. The luminances reached 1946 cd/m
2
at 7.5 V in the PF
3
-TPA
3
device and 1055 cd/m
2
at 8 V in the
PF
3
-TPA device. The EL efficiencies at this luminance were
1.63 and 1.57 cd/A, respectively.
achieve balanced charge injection and mobility in fluorene
oligomers/polymers. One of the approaches used to address
this issue is to modify the chemical structures of the poly-
fluorenes by copolymerization with dibenzothiophene S,S-
dioxide,
[4]
carbazole
[5]
or triphenylamine.
[6]
The introduc-
tion of electron-donating units into conjugated polyfluor-
ene can effectively enhance the hole-injecting properties of
the resulting materials. However, at the same time, it also
causes an increase in the LUMO energy level, which results
in an increased energy barrier for electron injection from
the metal cathode. For example, introducing triphenylamine
units into a polyfluorene main chain raises the LUMO level
by about 0.2 eV relative to the parent polymer.
[5a,7]
To pro-
mote hole-injection capability in the polyfluorene without
sacrificing its electron-injection capability, researchers re-
cently directed their attention towards triphenylamine-sub-
stituted polyfluorene derivatives
[8]
and found that the intro-
duction of triphenylamine groups at the 9-position of the
fluorene simultaneously has the advantages of 1) higher sol-
ubility, 2) reduced interchain interaction and 3) improved
hole injection. This non-conjugated connection between the
triphenylamine and fluorene is advantageous for balancing
hole and electron injection into emitting layers.
On the other hand, monodisperse oligofluorenes have re-
cently become the subject of intense study for their opto-
electronic applications
[9]
due to their well-defined conjuga-
tion lengths and molecular structures, ease of purification
and characterization, and solution processing. Moreover,

X. Zhang, Y. Quan Z. Cui, Q. Chen, J. Ding, J. Lu
FULL PAPER
recent rapid development of new synthetic methodologies
has made it possible to design a variety of monodisperse
oligomers, permitting efficient colour and energy-level tun-
ing by the control of effective conjugation length as well as
by the introduction of electron-donating and -withdrawing
moieties into the conjugated systems. For example, Cao and
co-workers synthesized extended π-conjugated dendrimers
and star-shaped molecules,
[9e,9f]
Lai et al. synthesized six-
arm triazatruxenes,
[9a]
Liu et al. reported a series of Si-
based tetrahedral luminescent materials,
[9g]
Shih et al. de-
signed and synthesized a series of carbazole/fluorene hy-
brids
[9h]
and Huang and co-workers synthesized a novel
series of monodisperse starburst macromolecular materials
based on fluorene.
[9k]
Inspired by these results, we recently synthesized mono-
disperse triphenylamine-substituted oligofluorenes
[10]
(shown
in Scheme 1) in which the triphenylamine cyclic core serves
as a non-conjugated spacer bearing oligofluorene arms in
a multi-H shaped structure of oligomers. As a result, the
optoelectronic properties of the individual oligofluorene
arms remained relatively unperturbed. These cyclic oligo-
mers were prepared by a Friedel–Crafts self-condensation
reaction
[10]
so that oligomers of different sizes are possibly
prepared. In this article we report two 3D monodisperse
oligofluorenes, one with a 1,3,5-tris(triphenylamino)-
benzene (TPA
3
) core and the other with a triphenylamine
(TPA) core (shown in Scheme 2), prepared by a Friedel–
Crafts copolycondensation reaction. This design gave the
products well-defined structures. The coplanar structures of
the TPA-based cores and the connection at the centre of
the pentafluorene arms gave the oligomers a 3D steric con-
formation. In the products, the triphenylamine units and
oligofluorenes are also non-conjugately connected at the 9-
position of the fluorenes, as reported in our previous
work.
[10]
The thermal, optical and electrochemical proper-
ties of the polymers were investigated and OLEDs from
these polymers were fabricated and characterized.
Scheme 1. The structure of monodisperse triphenylamine-substi-
tuted oligofluorenes prepared by the Friedel–Crafts self-condensa-
tion reaction.
www.eurjoc.org © 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Eur. J. Org. Chem. 2010, 2295–23032296
Scheme 2. The structures of the 3D monodisperse oligofluorenes
PF
3
-TPA and PF
3
-TPA
3
with a TPA-based core prepared by the
Friedel–Crafts copolycondensation reaction.
Results and Discussion
Synthesis and Characterization
The synthetic procedures used to prepare PF
3
-TPA
3
and
PF
3
-TPA are outlined in Scheme 3 and Scheme 4 respec-
tively. PF
3
-TPA
3
was synthesized according to the reported
procedure.
[10]
First, 1-bromo-4-methylbenzene was treated
with nBuLi in tetrahydrofuran at –78 °C followed by the
addition of 2,7-dibromofluorenone to give 2,7-dibromo-9-
(p-tolyl)fluoren-9-ol (3). This compound was treated with
9,9,9,9-tetra-n-octyl-2,2-bifluorenyl-7-boronic acid under
Suzuki coupling reaction conditions in a mixture of toluene

3D Monodisperse Oligofluorenes
and a 2 aqueous solution of sodium carbonate (Na
2
CO
3
).
Tetrakis(triphenylphosphane)palladium [(PPh
3
)
4
Pd
0
]was
used as the catalyst and tricaprylylmethylammonium chlo-
ride (Aliquat 336) was used as the phase-transfer catalyst.
The mixture was heated at reflux with stirring for 24 h to
give the fluorene pentamer 4. The 3D oligofluorene (PF
3
-
TPA
3
) was then prepared by a copolycondensation reaction
of an excess of compound 4 with compound 2 (TPA
3
)in
mesitylene at 80 °C for 3 h in the presence of p-toluenesul-
fonic acid. This polycondensation reaction follows the aro-
matic electrophilic substitution mechanism. Triphenylamine
is a very reactive aromatic compound for the electrophilic
substitution of fluorenol in the presence of a strong
acid.
[11–13]
The para position with respect to the nitrogen is
a reactive site whereas the ortho position has proven to be
non-reactive as a result of steric hindrance.
[10–12]
The syn-
thetic route used to synthesize PF
3
-TPA was similar to the
procedure used for PF
3
-TPA
3
with (4-bromophenyl)di-p-
tolylamine as the starting material and triphenylamine as
Scheme 3. Synthetic route for the preparation of the 3D monodis-
perse oligofluorenes with a non-conjugated TPA
3
core (PF
3
-TPA
3
).
Reagents and conditions: (a) nBuLi/THF, –78 °C, 1 h, B(OiPr)
3
,
–78 °Ctoroomtemp.,10h,2 HCl, room temp. 1 h; (b) [(PPh
3
)
4
-
Pd
0
], toluene, 2 Na
2
CO
3
, reflux, 24 h; (c) nBuLi/THF, –78 °C1h,
2 HCl, room temp. 1 h; (d) [(PPh
3
)
4
Pd
0
], toluene, 2 Na
2
CO
3
,
Aliquat 336, reflux, 24 h; (e) p-toluenesulfonic acid, mesitylene,
80 °C, 3 h.
Eur. J. Org. Chem. 2010, 2295–2303 © 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.eurjoc.org 2297
the core. The final Friedel–Crafts reaction afforded high
yields of 73 % for PF
3
-TPA
3
and 88 % for PF
3
-TPA after
purification by column chromatography.
Scheme 4. Synthetic route for the preparation of the steric mono-
disperse oligofluorene with a non-conjugated TPA core (PF
3
-TPA).
Reagents and conditions: (a) nBuLi/THF, –78 °C 1 h, 2 HCl,
room temp. 1 h; (b) [(PPh
3
)
4
Pd
0
], toluene, 2 Na
2
CO
3
, Aliquat
336, reflux, 24 h; (c) p-toluenesulfonic acid, mesitylene, 80 °C, 3 h.
The structures and monodispersities of PF
3
-TPA
3
and
PF
3
-TPA were identified by NMR, size-exclusion
chromatography (SEC) and MALDI-TOF mass spec-
troscopy. In the
1
H NMR spectrum of PF
3
-TPA
3
, the peaks
at δ = 2.33 (3 H, singlet), 2.29 (3 H, singlet) and 2.05 ppm
(16 H, multiplet) have been attributed to the resonance of
the CH
3
groups in the tolyl moieties at the 9-position of the
central fluorene units, the CH
3
groups in the TPA units and
the methylene units in the octyl groups adjacent to the C-9
of the fluorene units. This result is consistent with the struc-
ture of PF
3
-TPA
3
in which the three pentafluorene arms are
connected to the TPA
3
core. Similar characteristic peaks
have also been assigned in the
1
H NMR spectrum of PF
3
-
TPA. The peaks at δ = 2.29 (6 H, singlet) and 2.05 ppm (16
H, multiplet) are due to the resonance of the CH
3
groups
in the TPA unit and the methylene protons of the octyl
groups adjacent to C-9 of the fluorene units, respectively.
The SEC traces displayed narrow peaks at 27.04 and
26.85 min corresponding, respectively, to an M
n
value of
6190 Da with a M
w
/M
n
value of 1.02 for PF
3
-TPA
3
and an
M
n
value of 6180 Da with a M
w
/M
n
value of 1.05 for PF
3
-
TPA. The M
n
values determined from the SEC analysis
were based on polystyrene standards. The absolute molecu-
lar weights of the products were verified by MALDI-TOF
mass spectroscopy. The MALDI-TOF mass spectra dis-
played a single peak at m/z = 6275.9 for PF
3
-TPA
3
and one

X. Zhang, Y. Quan Z. Cui, Q. Chen, J. Ding, J. Lu
FULL PAPER
at m/z = 6215.2 for PF
3
-TPA, proving the monodisperse
molecular weights of the final products with values consis-
tent with the theoretical values of m/z = 6276.6 for PF
3
-
TPA
3
and m/z = 6215.5 for PF
3
-TPA.
Thermal and Photophysical Properties
The thermal properties of PF
3
-TPA
3
and PF
3
-TPA were
characterized by differential scanning calorimetry (DSC)
and thermal gravimetric analysis (TGA) and the results are
summarized in Table 1. The DSC curves showed a glass
transition at 72 °C for PF
3
-TPA
3
and 75 °CPF
3
-TPA. No
first-order transition related to a crystalline structure was
observed in the tested temperature range of –50 to 300 °C,
which indicates an amorphous nature of the oligomers. Ac-
cording to the literature, poly(dioctylfluorene) (POF) dis-
plays a high crystallization tendency; forming crystals at a
low temperature (113 °C).
[10]
These results indicate that the
introduction of the triphenylamine-based core into the
oligomers (as a spacer) indeed effectively prevents close-
packing of the molecules. On the other hand, a small broad
exothermal peak was observed when the temperature was
above 200 °C, which might be related to a degradation pro-
cess of the oligomers. But the TGA analysis did not show
any apparent weight loss at these temperatures; indeed, 5 %
weight loss was observed at about 420 °C.
The UV/Vis absorption and photoluminescence (PL)
spectra of solution and film samples of PF
3
-TPA
3
and PF
3
-
TPA were measured and the thermal stabilities of the PL
spectra of the films were also studied by annealing the spin-
coated PF
3
-TPA
3
and PF
3
-TPA films on quartz slides in a
vacuum at 100 °C for 24 h. The results are shown in Fig-
ure 1 and Figure 2. A broad strong absorption band is ob-
Table 1. Thermal and optical properties of PF
3
-TPA
3
and PF
3
-TPA.
Oligomer T
g
C] T
d5%
C] λ
max
[nm]
φ
(film) [%] λ
max
[nm]
Solution UV Solution PL Film UV Film PL Film PL*
[a]
PF
3
-TPA
3
72 420 368 418, 438 365 420, 441 67 417, 438
PF
3
-TPA 75 425 368 417, 439 370 417, 439 86 420, 440
[a] Film PL* after thermal annealing.
Figure 2. UV/Vis absorption and PL spectra of PF
3
-TPA
3
and PF
3
-TPA thin films (75 nm) before and after annealing.
www.eurjoc.org © 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Eur. J. Org. Chem. 2010, 2295–23032298
served in the solution spectrum with the same absorption
maximum at 368 nm for PF
3
-TPA
3
and PF
3
-TPA. Both ma-
terials exhibit very strong fluorescence in the pure-blue re-
gion. In addition, there is a negligible shift in the absorption
and PL spectra for both steric oligomers on changing from
the solution to solid state, which indicates the absence of
strong interchain interactions in the solid-state films.
Clearly, the novel steric molecular architecture retains an
isolated-molecule emission in the condensed state. This is
important for enhancing LED performance.
[9k]
The solid-
state PL quantum efficiencies of PF
3
-TPA
3
and PF
3
-TPA
were measured in an integrating sphere according to a lit-
erature procedure
[14]
and were found to be 67 and 86 %,
respectively. These values are much higher than those of
triphenylamine-substituted linear polyfluorenes and poly-
(dialkylfluorenes), which showed efficiencies of 22–50 and
around 50 %, respectively.
[8a,8d,8f]
Figure 1. UV/Vis absorption and PL spectra of PF
3
-TPA
3
and PF
3
-
TPA in dichloromethane.

Citations
More filters

01 Jan 2003
Abstract: Polyfluorene (PF)-based light-emitting diodes (LEDs) typically exhibit device degradation under operation with the emergence of a strong low-energy emission band (at ∼ 2.2–2.4 eV). This longer wavelength band converts the desired blue emission to blue–green or even yellow. We have studied both the photoluminescence (PL) and electroluminescence (EL) of PFs with different molecular structures and found that the low-energy emission band originates from fluorenone defects which are introduced by photo-oxidization, thermal oxidation, or during device fabrication. X-ray photo-emission spectroscopy (XPS) results show that the oxidation of PF is strongly catalyzed by the presence of calcium. The fluorenone defects generate a stronger contribution to the EL than to the PL. By utilization of a novel electron-transporting material as a buffer layer between the emissive PF and the Ca/Ag (Ba/Ag) cathode, the blue EL emission from the PF was stabilized.

276 citations


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Abstract: A series of starburst materials (T1−T3) bearing a 1,3,5-tri(anthracen-10-yl)benze-ne core (T0) and three oligofluorenes arms have been synthesized and characterized. Single-crystal diffraction analysis has shown that the core of these starburst materials possess a propeller twist topology, which made the starburst materials exhibit good film-forming capabilities and display deep blue emission both in solution and in the thin solid film. The compounds (T1−T3) possess high glass transition temperatures (Tg’s) at 107, 109, and 110 °C, and high decomposition temperatures (Td’s) at 438, 440, and 434 °C, respectively. In addition, the double-layered devices fabricated with the three materials as the emitter show a stable deep-blue emission and the device performance increases with arm length at some extent. The double-layered device based on T2 has a maximum brightness of over 3400 cd/m2 and a maximum current efficiency of 1.80 cd/A with CIE coordinates of (0.149, 0.098), which is among the best of the deep-blu...

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Abstract: A novel triphenylamine-fluorene oligomer with macro-spirocyclic structure was designed and prepared as a host for exciplex based white phosphorescent organic light-emitting diodes (white PhOLEDs), in which only iridium(III)bis(4,6-(difluorophenyl)pyridinato-N,C2)picolinate (FIrpic) was employed as the dopant. The device exhibited a comparatively high performance with a maximum luminance and current efficiency of 14 213 cd m−2 and 22.6 cd A−1, respectively.

14 citations


Journal Article
Abstract: The properties of alternating copolymers of 9,9-dioctylfluorene and oxadiazole (F/Ox copolymers) have been studied by cyclic voltammetry (CV), photoluminescence (PL) and electroluminescence (EL). The copolymers contained oxadiazole units symmetrically dispersed in the main chain between one, P(F 1 -alt-Ox), three, P(F 3 -alt-Ox), or four, P(F 4 -alt-Ox), fluorene units, as well as asymmetrically distributed in the chain, P(F 2 -as-Ox). CV studies revealed that all the copolymers, except P(F 1 -alt-Ox), had high electrochemical stabilities, and exhibited both reversible n- and p-doping processes. Meanwhile P(F 1 -alt-Ox) only showed a stable and reversible n-doping process. The CV data also showed that the LUMO levels for the copolymers are close to the work functions of cathode materials such as Ca and Mg, indicating a favourable electron transport property of the copolymers. The PL spectra of the copolymers show a stable blue light-emitting behaviour in the solid state upon annealing under inert gas at temperatures from 40 to 150 °C. However, a broad peak appeared at 510 nm when the samples were annealed at 120 °C in the presence of oxygen. EL studies of a bi-layer light-emitting diode (LED) device produced from P(F 3 -alt-Ox) demonstrated stable spectra with features similar to those of the PL spectra. The results of this study suggest that F/Ox copolymers are good candidates for use as electron transport layers and blue light-emitting materials in bi-layer LED devices.

13 citations


Journal ArticleDOI
Xiao-Xu Wang1, Tao Tao1, Jiao Geng1, Bin-Bin Ma1, Yu-Xin Peng1, Wei Huang1 
TL;DR: A series of T- and H-shaped donor-acceptor (D-A) types of dipyrido[3,2-a:2',3'-c]phenazine (DPPZ)-based molecules, extended by thienyl and triphenylamino chromophores at the 2,7-(bottom) and/or 10,13-positions (top), have been designed and prepared successfully.
Abstract: A series of T- and H-shaped donor-acceptor (D-A) types of dipyrido[3,2-a:2',3'-c]phenazine (DPPZ)-based molecules, extended by thienyl and triphenylamino chromophores at the 2,7-(bottom) and/or 10,13-positions (top), have been designed and prepared successfully. Synthetic, structural, thermal, spectral, and computational comparisons have been carried out for related compounds because of their adjustable intramolecular charge-transfer properties. It is noted that a pair of structural isomers (5 and 6) has been obtained, respectively, where distinguishable UV/Vis and fluorescence spectra, electrochemical activity, thermal stability, and bandgaps are observed. Furthermore, compounds 6, 8, 10, 11, 13, and 15 exhibit excellent thermal stability, and the Td10 values for them are found to range from 524 to 646 °C, which can be regarded as one of the best groups of thermally stable compounds among organic small molecules. In addition, theoretical calculations were performed, and the structure-property relationships were examined to reveal the effects of the position and number of donor arms on the DPPZ acceptor core.

12 citations


References
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Journal ArticleDOI
Abstract: Present polymeric microelectronic devices are typically unipolar devices, based on p-type semiconducting polymers. Bipolar devices stable under ambient conditions are desirable, but have not yet been reported due to a lack of stable n-type doped conducting polymers. Starting from the standard redox potentials of, especially, water and oxygen, stability requirements on electrode potentials of n-type doped conducting polymers are derived. The predictions are then compared with experimental data on stability of conducting polymers. A good agreement is obtained. An electrode potential of about 0 to + 0.5 V (SCE) is required for stable n-type doped polymers, similar to the requirement on the electrode potential for stable undoped p-type polymers. Consequences for bipolar devices are analysed. Huge overpotentials for the redox reaction with wet oxygen are required in order to realize thermodynamically stable bipolar devices from known doped p-type and n-type conducting polymers. Finally, possible solutions, accepting thermodynamic instability, are discussed.

1,267 citations




Journal ArticleDOI
Abstract: Efficient blue electroluminescence, peaked at 436 nm, is demonstrated from polymer light-emitting diodes operating at high brightness A dioctyl-substituted polyfluorene was used as the emissive layer in combination with a polymeric triphenyldiamine hole transport layer The luminance reaches 600 cd/m2 at a current density of 150 mA/cm2 for a bias voltage of 20 V, corresponding to an efficiency of 025 cd/A and a luminosity of 004 lm/W These values are optimized at a critical emissive layer thickness

606 citations