3D monodisperse oligofluorenes with non-conjugated triphenylamine-based cores: Synthesis and optoelectronic properties

Article abstract of DOI:10.1002/ejoc.200901140

Two 3D monodisperse oligofluorenes with non-conjugated triphenylamine-based cores have been synthesized by Friedel-Crafts copolycondensation reaction. The oligomers, PF₃-TPA and PF₃-TPA₃, consist of three fluorene pentamer arms that are connected non-conjugately through a triphenylamine (TPA) and 1,3,5-tris(triphenylammo)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 PF₃-TPA₃ and 86% for PF₃-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/PBDOT-PSS/PF₃-TPA₃ and PF₃-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/m² at 7.5 V in the PF₃-TPA ₃ device and 1055 cd/m² at 8 V in the PF₃-TPA device. The EL efficiencies at this luminance were 1.63 and 1.57 cd/A, respectively.

Full text of DOI:10.1002/ejoc.200901140

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,
PF3-TPA and PF3-TPA3, consist of three fluorene pentamer
arms that are connected non-conjugately through a tri-
phenylamine (TPA) and 1,3,5-tris(triphenylamino)benzene
deep-blue fluorescence with high efficiencies in thin films
(φpl–film = 67% for PF3-TPA3 and 86% for PF3-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/PF3-TPA3 and PF3-TPA/TPBI/LiF/Al
core (TPA3), respectively, at the 9-position of the central fluor- were fabricated to investigate the electroluminescence (EL)
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
properties of the two oligomers. Both oligomers showed a low
turn-on voltage of 4 V. The luminances reached 1946 cd/m2
at 7.5 V in the PF3-TPA3 device and 1055 cd/m2 at 8 V in the
PF3-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.
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),
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,
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.
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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 (TPA3) 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 2. The structures of the 3D monodisperse oligofluorenes
PF3-TPA and PF3-TPA3 with a TPA-based core prepared by the
Friedel–Crafts copolycondensation reaction.
Results and Discussion
Synthesis and Characterization
The synthetic procedures used to prepare PF3-TPA3 and
PF3-TPA are outlined in Scheme 3 and Scheme 4 respec-
tively. PF3-TPA3 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
9,9,9Ј,9Ј-tetra-n-octyl-2,2Ј-bifluorenyl-7-boronic acid under
Suzuki coupling reaction conditions in a mixture of toluene
Scheme 1. The structure of monodisperse triphenylamine-substi-
tuted oligofluorenes prepared by the Friedel–Crafts self-condensa-
tion reaction.
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3D Monodisperse Oligofluorenes
and a 2  aqueous solution of sodium carbonate (Na2CO3). the core. The final Friedel–Crafts reaction afforded high
Tetrakis(triphenylphosphane)palladium [(PPh3)4Pd0] was yields of 73% for PF3-TPA3 and 88% for PF3-TPA after
used as the catalyst and tricaprylylmethylammonium chlo- purification by column chromatography.
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 (PF3-
TPA3) was then prepared by a copolycondensation reaction
of an excess of compound 4 with compound 2 (TPA3) 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 PF3-TPA was similar to the
procedure used for PF3-TPA3 with (4-bromophenyl)di-p-
tolylamine as the starting material and triphenylamine as
Scheme 4. Synthetic route for the preparation of the steric mono-
disperse oligofluorene with a non-conjugated TPA core (PF3-TPA).
Reagents and conditions: (a) nBuLi/THF, –78 °C 1 h, 2  HCl,
room temp. 1 h; (b) [(PPh3)4Pd0], toluene, 2  Na2CO3, Aliquat
336, reflux, 24 h; (c) p-toluenesulfonic acid, mesitylene, 80 °C, 3 h.
The structures and monodispersities of PF3-TPA3 and
PF3-TPA were identified by NMR, size-exclusion
chromatography (SEC) and MALDI-TOF mass spec-
troscopy. In the 1H NMR spectrum of PF3-TPA3, 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 CH3 groups in the tolyl moieties at the 9-position of the
central fluorene units, the CH3 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 PF3-TPA3 in which the three pentafluorene arms are
connected to the TPA3 core. Similar characteristic peaks
1
have also been assigned in the H NMR spectrum of PF3-
TPA. 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 SEC traces displayed narrow peaks at 27.04 and
26.85 min corresponding, respectively, to an Mn value of
6190 Da with a Mw/Mn value of 1.02 for PF3-TPA3 and an
Scheme 3. Synthetic route for the preparation of the 3D monodis-
Mn value of 6180 Da with a Mw/Mn value of 1.05 for PF3-
TPA. The Mn 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 PF3-TPA3 and one
perse oligofluorenes with a non-conjugated TPA3 core (PF3-TPA3).
Reagents and conditions: (a) nBuLi/THF, –78 °C, 1 h, B(OiPr)3,
–78 °C to room temp., 10 h, 2  HCl, room temp. 1 h; (b) [(PPh3)4-
Pd0], toluene, 2  Na2CO3, reflux, 24 h; (c) nBuLi/THF, –78 °C 1 h,
2  HCl, room temp. 1 h; (d) [(PPh3)4Pd0], toluene, 2  Na2CO3,
Aliquat 336, reflux, 24 h; (e) p-toluenesulfonic acid, mesitylene,
80 °C, 3 h.
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at m/z = 6215.2 for PF3-TPA, proving the monodisperse served in the solution spectrum with the same absorption
molecular weights of the final products with values consis- maximum at 368 nm for PF3-TPA3 and PF3-TPA. Both ma-
tent with the theoretical values of m/z = 6276.6 for PF3- terials exhibit very strong fluorescence in the pure-blue re-
TPA3 and m/z = 6215.5 for PF3-TPA.
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 PF3-TPA3 and PF3-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]
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. The DSC curves showed a glass
transition at 72 °C for PF3-TPA3 and 75 °C PF3-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 PF3-TPA3 and PF3-
TPA were measured and the thermal stabilities of the PL
spectra of the films were also studied by annealing the spin-
coated PF3-TPA3 and PF3-TPA films on quartz slides in a
vacuum at 100 °C for 24 h. The results are shown in Fig-
Figure 1. UV/Vis absorption and PL spectra of PF3-TPA3 and PF3-
ure 1 and Figure 2. A broad strong absorption band is ob- TPA in dichloromethane.
Table 1. Thermal and optical properties of PF3-TPA3 and PF3-TPA.
Oligomer
Tg [°C]
Td 5% [°C]
λmax [nm]
φ (film) [%]
λmax [nm]
Solution UV Solution PL Film UV
Film PL
Film PL*[a]
PF3-TPA3
PF3-TPA
72
75
420
425
368
368
418, 438
417, 439
365
370
420, 441
417, 439
67
86
417, 438
420, 440
[a] Film PL* after thermal annealing.
Figure 2. UV/Vis absorption and PL spectra of PF3-TPA3 and PF3-TPA thin films (75 nm) before and after annealing.
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3D Monodisperse Oligofluorenes
After thermal annealing only changes in the relative in- and reduction relative to the Ag quasi-reference electrode.
tensity were observed in the vibronic peaks with the emis-
The thin films of PF3-TPA3 and PF3-TPA have typical
sion peak positions remaining almost unchanged. The PL CV curves similar to those reported previously for fluorene
efficiencies of PF3-TPA3 and PF3-TPA after thermal an- polymers.[5,16] Figure 3 shows that both the p- and n-doping
nealing changed slightly to 64 and 85%, respectively. More- processes for PF3-TPA3 and PF3-TPA3 are reversible. Un-
over, no additional PL peak was observed between 500 and der successive multiple potential scans, as shown in Fig-
600 nm in the spectra of PF3-TPA3 and PF3-TPA3 films, as ure 3, the PF3-TPA film only showed a very small decrease
often appear in the PL spectra of poly(dialkylfluorene) in the current intensity in the n-doping process with the CV
films after thermal annealing. These results demonstrate curve remaining almost unchanged after 10 scans, which
that the incorporation of triphenylamine-based cores into indicates good stability towards both electrochemical oxi-
the uorene units exert a spacer function, improving thermal dation and reduction. However, the PF3-TPA3 film showed
and morphological stability and preventing chain aggrega- lower stability in the n-doping process. Both oligomer films
tion effectively. Even after the films were annealed at 130 °C displayed a 15–30% loss of current intensity after 10 suc-
in air for 24 h, the PL spectra for both compounds re- cessive p-doping scans. The decrease in the current on cy-
mained largely the same (see the Supporting Information), cling arises from the fact that PF3-TPA and PF3-TPA3 are
however, a tiny shoulder peak at around 520 nm associated not polymers but short oligomers that are somewhat soluble
with fluorenone defects was evident.
in their neutral or charged forms in the solvent used. Thus,
on cycling, partial dissolution of the material in the films
might occur. The onset oxidation (EpЈ) and reduction (EnЈ)
potentials of PF3-TPA3 and PF3-TPA are 0.98 and
Cyclic Voltammetry
Cyclic voltammetry (CV) was used to investigate the ion- –2.10 eV, and 0.90 and –2.08 eV respectively. These values
ization potentials (EHOMO), the electron affinities (ELUMO correspond to an EHOMO of –5.36 eV and an ELUMO of
)
and the electrochemical stabilities of PF3-TPA3 and PF3- –2.28 eV for PF3-TPA3 and an EHOMO of –5.28 eV and an
TPA. The thin films of oligomers PF3-TPA3 and PF3-TPA ELUMO of –2.20 eV for PF3-TPA, which are similar to those
were measured in a 0.1  Bu4NPF6/acetonitrile solution at of triphenylamine-substituted linear polyfluorenes.[8a] These
a scan rate of 50 mV/s at room temperature. The CV curves data indicate that the energy barriers for hole-injection
were referenced to an Ag quasi-reference electrode, which from the ITO anode (WITO = –4.8 eV) to these oligofluor-
was calibrated using an internal standard, the ferrocene/ enes are significantly lower than that to the fluorene homo-
ferrocenium redox couple (0.34 V vs. SCE).[5] According to polymer.[16] Traditionally, the introduction of tri-
de Leeuw et al.,[15] the ionization potential (ELUMO) and phenylamine units into conjugated polymers or organic mo-
electron affinity (EHOMO) of a material are approximately lecules was found to effectively enhance the hole-injecting
equal to the onset oxidation potential (vs. SCE) and the properties of the resulting materials. At the same time, it
onset reduction potential (vs. SCE) plus 4.4 eV (the SCE also causes an increase in the LUMO energy level, which
energy level below the vacuum level), respectively. The po- results in an increased energy barrier for electron injection
tential of the Ag quasi-reference electrode used in this ex- from the metal cathode. The existence of the triphenylamine
periment was determined to be –0.02 V vs. SCE. Therefore units in PF3-TPA3 and PF3-TPA does not apparently lead
the LUMO and HOMO energy levels of the materials can to this effect. It effectively increased the HOMO level, but
be estimated by using the equations EHOMO = –(EpЈ + maintained the LUMO energy level at about –2.20 eV, com-
4.38) eV and ELUMO = –(EnЈ + 4.38) eV, respectively, in parable to that of the fluorene homopolymer.[16a] This result
which EpЈ and EnЈ are the onset potentials for oxidation indicates that introducing a non-conjugated triphenylamine
Figure 3. CV curves of the thin films of oligomers PF3-TPA3 and PF3-TPA measured in a 0.1  Bu4NPF6/acetonitrile solution at a scan
rate of 50 mV/s at room temperature (10 successive scans vs. an Ag quasi-reference electrode).
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unit core into the oligomers promotes their hole-injection PF3-TPA3 device and 56 cd/m2 in the PF3-TPA device at
capability, but does not sacrifice their electron-injection 5 V. The EL efficiencies at this luminance are 1.44 and
capability.
1.36 cd/A, respectively. Compared with POF,[10] these re-
sults suggest that introducing
a non-conjugated tri-
phenylamine unit into a fluorene side-chain helps to im-
prove the efficiency of hole injection and transport. Both
compounds had a higher luminous efficiency of around
1.5 cd/A (at 6–8 V), which may be attributed to their higher
solid-film PL efficiencies. In addition, more current can be
injected into the PF3-TPA3-based device at the same ap-
plied voltages compared with the PF3-TPA-based device,
which leads to a higher luminance. For example, at 7.5 V
the luminance of the PF3-TPA3-based device already
reached 1946 cd/m2, whereas the PF3-TPA-based device
gave 1055 cd/m2 at 8 V. These values are comparable to the
literature data for triphenylamine-substituted linear poly-
fluorene derivatives for solution-processed light-emitting
devices.[8a,8b,8d,8f]
Electroluminescence Spectra
Two multi-layer devices with a configuration of ITO/PE-
DOT-PSS/PF3-TPA3 and PF3-TPA/TPBI/LiF/Al were fab-
ricated to investigate the electroluminescence (EL) proper-
ties of the two synthesized compounds. In these devices,
poly(3,4-ethylenedioxythiophene)
poly(styrenesulfonate)
(PEDOT-PSS) is a hole-injection layer and wide-band-gap
1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBI) func-
tions as a hole-blocking and electron-transporting layer.
The EL spectra of PF3-TPA3 and PF3-TPA are shown in
Figure 4. At low voltages, these oligofluorenes exhibit deep-
blue EL, however, the low-energy peak at around 580 nm
increased in intensity with applied biases. For PF3-TPA3,
the CIE chromaticity coordinate is (0.192, 0.101) at 5 V, but
changes to (0.226, 0.137) at 6 V and (0.257, 0.168) at 7.5 V.
PF3-TPA was more stable than PF3-TPA3: at 5 V, the CIE
coordinate of PF3-TPA is (0.178, 0.069) and at 8 V the CIE
coordinate became (0.269, 0.156). At the moment, the
reason for this instability is not very clear because this low-
energy emission peak was not observed even in the PL spec-
tra of the annealed films. Jen and co-workers also observed
a similar EL emission peak at 580 nm for polyfluorenes
with triphenylamine and oxadiazole pendant groups at the
9-position of the fluorene units.[8f] At first, we assigned this
emission band to fluorenone defects arising from electro-
oxidation of the fluorene units. However, numerous litera-
ture data indicate that the emission from fluorenone defects
should appear at 520–540 nm.[17] Therefore, the appearance
of this low-energy emission band may be due to the forma-
tion of exciplexes between the emitting layer and TPBI dur-
ing device operation.
Figure 5. L–V and LE–V characteristics of the devices prepared
from oligomers PF3-TPA3 and PF3-TPA.
Conclusions
Two 3D steric monodisperse oligofluorenes with a TPA3
and TPA core (PF3-TPA3 and PF3-TPA) have been synthe-
sized 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.
Figure 4. The EL spectra of oligomers PF3-TPA3 and PF3-TPA at
5 V.
The luminance–voltage (L–V) and luminous efficiency–
voltage (LE–V) characteristics of the fabricated devices are
outlined in Figure 5. Both compounds show a low turn-on
Experimental Section
General: The 1H NMR spectra of the materials were recorded with
voltage of 4 V. The luminances reached 160 cd/m2 in the a Bruker spectrometer (500 MHz) in CDCl3 and tetramethylsilane
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3D Monodisperse Oligofluorenes
was used as the internal standard. Differential scanning calori-
metric (DSC) measurements were performed with a DSC 2920 TA
Instrument. Thermogravimetric analyses (TGA) were performed
using a Perkin–Elmer Thermal Analysis system. The heating rate
was 10 °C/min for DSC measurements and 20 °C/min for TGA
measurements in nitrogen. Positive-ion MALDI-TOF/TOF mass
spectrometry was performed using an Autoflex II TOF/TOF time-
of-flight mass spectrometer (Bruker Daltonics, Germany). The
sample solutions were prepared by dissolving sample (0.5 mg) in
dichloromethane (0.5 mL), which was then mixed with the matrix
solution (50% acetonitrile with 0.5% trifluoroacetic acid solution
saturated with cinnamonic acid, 0.05 mL). Size-exclusion
chromatography (SEC) was performed at room temperature with a
Waters 515 instrument equipped with a Waters 2487 Ultraviolet
absorbance detector and a Wyatt Technology Optilab rEX refrac-
tive index detector. Styragel HR3, HR4 and HR5 (300ϫ7.8 mm)
columns from Waters were used. HPLC-grade THF was used as
eluent at a flow rate of 1 mL/min. The sample solutions were fil-
tered through a 0.45 µm filter (Nylon, Millex-HN 13 mm Syringes
Filters, Millipore, U.S.). The columns were calibrated by using
polystyrene standards with molecular weights in the range of 900
and 1.74ϫ106 g/mol. UV/Vis absorption spectra were collected
with a Shimadzu MPC-3100 UV/Vis/NIR spectrophotometer.
Fluorescence measurements were performed with a Spex Fluorolog
3 spectrometer using the absorption maxima of the materials as the
excitation wavelengths. The PL quantum yields of the thin films of
the oligomers spin-coated on quartz slides were measured with a
Spex Fluorolog 3 spectrometer combined with an integrating
sphere according to the literature procedure.[14]
was supplied by Zhenjiang Haitong Chemical industry Co., Ltd,
China. All other reagents were purchased from Alfa Aesar and
used without further purification. (4-Bromophenyl)(phenyl)p-tolyl-
amine,[18] 2,7-dibromofluorenone[19] and 9,9,9Ј,9Ј-tetra-n-octyl-
2,2Ј-bifluorenyl-7-boronic acid[20] were prepared according to lit-
erature procedures.
4-[Phenyl(p-tolyl)amino)]phenylboronic Acid (1): nBuLi (2.5  in n-
hexane, 6 mL, 15 mmol) was slowly added to a solution of (4-bro-
mophenyl)(phenyl)p-tolylamine (3.88 g, 11.5 mmol) in anhydrous
tetrahydrofuran (30 mL) at –78 °C under nitrogen. The reaction
mixture was stirred for 1 h before the addition of triisopropoxybor-
ane (8 mL, 6.5 g, 34.5 mmol). The solution was then allowed to
warm to room temperature, stirred overnight and then quenched
with 2.0  HCl (25 mL). The aqueous layer was extracted with di-
ethyl ether three times. The combined organic layers were washed
with brine before drying with anhydrous Na2SO4. The crude prod-
uct was collected by evaporating the solvent and then purified by
column chromatography on silica gel to afford 4-[phenyl(p-tolyl)-
amino]phenylboronic acid as a white solid (1.85 g, 53%). 1H NMR
(500 MHz, [D6]acetone): δ = 2.32 (s, 3 H, CH3), 6.94 (d, J = 8.5 Hz,
2 H, ArH), 7.00 (d, J = 8.3 Hz, 2 H, ArH), 7.05–7.08 (m, 3 H,
ArH), 7.16 (d, J = 8.2 Hz, 2 H, ArH), 7.27–7.33 (m, 2 H, ArH),
7.76 (d, J = 8.5 Hz, 2 H, ArH) ppm. C19H18BNO2 (303.16): calcd.
C 75.27, H 5.98, N 4.62; found C 75.36, H 5.89, N 4.72.
1,3,5-Tris{4-[phenyl(p-tolyl)amino]phenyl}benzene (TPA3, 2): 1,3,5-
Tribromobenzene (0.37 g, 1.18 mmol), compound
1 (1.38 g,
4.55 mmol) and Aliquat 336 (20% by weight of the monomer) were
added to a mixture of toluene (10 mL) and a 2.0  aqueous solu-
tion of Na2CO3 (10 mL) in a 100 mL flask. The mixture was evacu-
ated and filled with nitrogen three times. [Pd(PPh3)4] (0.005 equiv.)
was then added under nitrogen. The reaction mixture was stirred
under reflux for 24 h. The organic layer was then separated, dried
with anhydrous Na2SO4 and concentrated by evaporating the sol-
vent prior to further purification by column chromatography on
silica gel to afford TPA3 as a pale-yellow solid (0.83 g, 83%). 1H
NMR (500 MHz, [D6]DMSO): δ = 2.29 (s, 9 H, CH3), 6.90–7.10
(m, 21 H, ArH), 7.17, (d, J = 7.7 Hz, 6 H, ArH), 7.31 (m, 6 H,
ArH), 7.72–7.82 (m, 9 H, ArH) ppm. MS (AutoflexTOF/TOF): m/z
= 849.3. C63H51N3 (850.10): calcd. C 89.01, H 6.05, N 4.94; found
C 88.98, H 6.12, N 4.86.
Cyclic voltammetry (CV) measurements were conducted at room
temperature in a 0.1  Bu4NPF6/acetonitrile solution using a gas-
tight electrochemical cell under the protection of argon with a So-
lartron SI 1287 potentiostat at a scan rate of 50 mV/s. A silver wire
(φ = 1 mm), a platinum wire (φ = 0.5 mm) and a platinum disk (φ
= 1 mm) sealed in a soft glass rod were used as the quasi-reference
electrode, counter-electrode and working electrode, respectively.
The working electrode was coated with the polymer film using a
toluene solution and heated in an oven at 80 °C for 5 min. After
the cell had been evacuated and filled with argon three times, a
0.1  Bu4NPF6 solution in anhydrous acetonitrile was loaded.
EL Device Fabrication and Testing: ITO-coated glass substrates
(15 Ω/square) were patterned by a conventional wet-etching process
using a 1:1 mixture of HCl (6 )/HNO3 (0.6 ) as the etchant. The
active area of each device was 5.5ϫ6.5 mm2. After patterning, the
substrates were rinsed in deionized water and then sequentially sub-
jected to ultrasound in acetone (20 °C) and 2-propanol (65 °C).
Immediately prior to device fabrication, the ITO substrate was
treated with UV/ozone for 15 min. PEDOT-PSS (50 nm) was spin-
coated at 5000 rpm from its aqueous suspension onto the treated
substrate and then baked at 135 °C under nitrogen for 30 min. Sub-
sequently, a 1% PF3-TPA3 or PF3-TPA solution in CHCl3 was
spin-coated on top of PEDOT-PSS. The resulting film was about
60 nm thick, as measured by a Dektak surface profilometer. 1,3,5-
Tris(N-phenylbenzimidazol-2-yl)benzene (TPBI, 30 nm) was vac-
uum-deposited as a hole-blocking layer. Finally, LiF (1 nm) and Al
(100 nm) were deposited as the cathode. The devices were tested in
air under ambient conditions with no protective encapsulation. EL
spectra, device luminance and current–voltage characteristics were
recorded by using a combination of a Photo Research PR-650
SpectraScan and a Keithley 238 Source meter.
2,7-Dibromo-9-(p-tolyl)fluoren-9-ol (3): nBuLi (2.5  in n-hexane,
9.1 mL, 22.79 mmol) was slowly added to a solution of 1-bromo-4-
methylbenzene (3.39 g, 19.82 mmol) in anhydrous tetrahydrofuran
(15 mL) at –78 °C under nitrogen. The reaction mixture was then
stirred for 1 h before the addition of 2,7-dibromofluorenone
(6.03 g, 17.84 mmol) in anhydrous tetrahydrofuran (50 mL). The
solution was then allowed to warm to room temperature, stirred
overnight and quenched with 2.0  HCl (30 mL). The aqueous
layer was extracted with diethyl ether three times. The combined
organic layers were dried with anhydrous Na2SO4. The crude prod-
uct was collected by evaporating the solvent and then purified by
column chromatography on silica gel to afford 3 as a light-yellow
solid (5.60 g, 73%). 1H NMR (500 MHz, CDCl3): δ = 2.35 (s, 3 H,
CH3), 7.14 (d, J = 7.4 Hz, 2 H, ArH), 7.25 (d, J = 8.0 Hz, 2 H,
ArH), 7.38–7.55 (m, 6 H, ArH) ppm. MS (AutoflexTOF/TOF): m/z
= 430.2. C20H14Br2O (430.13): calcd. C 55.85, H 3.28; found C
55.94, H 3.21.
Pentafluorene (4): Compound 3 (0.33 g, 0.76 mmol), 9,9,9Ј,9Ј-tetra-
n-octyl-2,2Ј-bifluorenyl-7-boronic acid (1.50 g, 1.82 mmol) and
Aliquat 336 (20% by weight of the monomer) were added to a
mixture of toluene (15 mL) and a 2.0  aqueous solution of
Na2CO3 (10 mL) in a 100 mL flask. The mixture was evacuated
Materials: Tetrahydrofuran, diethyl ether and toluene were dried
by Na and distilled prior to use. (4-Bromophenyl)di-p-tolylamine
Eur. J. Org. Chem. 2010, 2295–2303
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
2301

X. Zhang, Y. Quan Z. Cui, Q. Chen, J. Ding, J. Lu
FULL PAPER
under vacuum and filled with nitrogen three times. [(PPh3)4Pd0]
(0.005 equiv.) was then added under nitrogen. The reaction mixture
was stirred under reflux for 24 h. The organic layer was then sepa-
rated, dried with anhydrous Na2SO4 and concentrated by evaporat-
ing the solvent prior to further purification by column chromatog-
raphy on silica gel to afford 4 as a pale-yellow solid (0.85g, 62%).
1H NMR (500 MHz, CDCl3): δ = 0.68–0.87 (m, 40 H, CH2CH3),
1.07–1.27 (m, 80 H, CH2CH2CH2CH2CH2), 2.03–2.13 (m, 16 H,
CH2), 2.36 (s, 3 H, CH3), 7.17–7.19 (d, J = 8.0 Hz, 2 H, ArH),
7.34–7.42 (m, 6 H, ArH), 7.48 (d, J = 8.0 Hz, 2 H, ArH), 7.58–
7.86 (m, 26 H, ArH) ppm. MS (AutoflexTOF/TOF): m/z = 1826.8.
2008.5. C149H187NO (2008.09): calcd. C 89.12, H 9.39, N 0.70;
found C 89.01, H 9.45, N 0.63.
Oligomer PF3-TPA: Compound 6 (0.87 g, 0.43 mmol) and tri-
phenylamine (31.8 mg, 0.13 mmol) were dissolved in mesitylene
(10 mL) in a 100 mL flask and 1 drop of p-toluenesulfonic acid was
added. The reaction mixture was stirred at 80 °C for 3 h under
nitrogen and was then concentrated by evaporating the solvent
prior to further purification by column chromatography on silica
gel to afford oligomer PF3-TPA as a pale-yellow solid (0.71 g,
88%). 1H NMR (500 MHz, CDCl3): δ = 0.75–0.85 (m, 120 H,
CH2CH3), 0.95–1.23 (m, 240 H, CH2CH2CH2CH2CH2), 1.95–2.05
(m, 48 H, CH2), 2.22 (s, 18 H, CH3), 6.82–6.97 (m, 30 H, ArH),
7.05 (d, J = 8.5 Hz, 6 H, ArH), 7.13 (d, J = 7.0 Hz, 6 H, ArH),
7.32–7.39 (m, 24 H, ArH), 7.46–7.56 (m, 18 H, ArH), 7.62–7.78
(m, 54 H, ArH), 7.85 (d, J = 8.0 Hz, 6 H, ArH) ppm. MS (Auto-
flexTOF/TOF): m/z = 6215.2. C465H570N4 (6215.53): calcd. C
89.86, H 9.24, N 0.90; found C 89.73, H 9.30, N 0.83.
C136H176O (1826.85): calcd. C 89.41, H 9.71; found C 89.28, H
9.65.
Oligomer PF3-TPA3: Compound 4 (0.505 g, 0.2765 mmol) and 2
(67.80 mg, 0.0798 mmol) were dissolved in mesitylene (7 mL) in a
50 mL flask and 1 drop of p-toluenesulfonic acid was added. The
reaction mixture was stirred at 80 °C for 3 h under nitrogen and
was then concentrated by evaporating the solvent prior to further
purification by column chromatography on silica gel to afford PF3-
Supporting Information (see also the footnote on the first page of
this article): 1H NMR spectra, SEC traces, MALDI-TOF mass
spectra, DSC curves, TGA curves and PL spectra of oligomers af-
ter annealing.
1
TPA3 as a pale-yellow solid (0.365 g, 73%). H NMR (500 MHz,
CDCl3): δ = 0.73–0.92 (m, 120 H, CH2CH3), 1.08–1.28 (m, 240 H,
CH2CH2CH2CH2CH2), 2.06–2.15 (m, 48 H, CH2), 2.30 (s, 9 H,
CH3), 2.33 (s, 9 H, CH3), 7.02 (d, J = 7.7 Hz, 6 H, ArH), 7.05–
7.20 (m, 24 H, ArH), 7.24 (d, J = 7.9 Hz, 6 H, ArH), 7.32–7.39
(m, 24 H, ArH), 7.50 (d, J = 6.5 Hz, 6 H, ArH), 7.57–7.82 (m, 75
H, ArH), 7.90 (d, J = 7.7 Hz, 6 H, ArH) ppm. MS (AutoflexTOF/
TOF): m/z = 6275.9. C471H573N3 (6276.61): calcd. C 90.13, H 9.20,
N 0.67; found C 90.07, H 9.15, N 0.63.
Acknowledgments
The authors thank Professor Yu-hua Mei of Nanjing University
for MS analysis.
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Published Online: March 5, 2010
Eur. J. Org. Chem. 2010, 2295–2303
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
2303