Starburst 4,4′,4′′-tris(carbazol-9-yl)-triphenylamine-based deep-blue fluorescent emitters with tunable oligophenyl length for solution-processed undoped organic light-emitting diodes

Article abstract of DOI:10.1039/c4tc02173h

On the basis of a well-known hole transporting material, namely 4,4′,4′′-tris(carbazol-9-yl)-triphenylamine (TCTA), a series of star-shaped deep-blue fluorescent emitters (2P-TCTA, 3P-TCTA, 4P-TCTA and 5P-TCTA) have been successfully developed via a simpl

Full text of DOI:10.1039/c4tc02173h

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Starburst 4,4’,4’’-Tris(carbazol-9-yl)-triphenylamine-Based
Deep-Blue Fluorescent Emitters with Tunable Oligophenyl Length
for Solution-Processed Undoped Organic Light-Emitting Diodes
Mingquan Yu,†,‡ Shumeng Wang,†,‡ Shiyang Shao,† Junqiao Ding,*,† Lixiang Wang,*,†
Xiabin Jing,† and Fosong Wang†
†State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of
Applied Chemistry, Chinese Academy of Sciences, Changchun, 130022, P. R. China
‡University of Chinese Academy of Sciences, Beijing, 100049, P. R. China
Eꢀmail: junqiaod@ciac.ac.cn; lixiang@ciac.ac.cn
1

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Abstract: On the basis of a wellꢀknown hole transporting material, namely
4,4’,4’’ꢀtris(carbazolꢀ9ꢀyl)ꢀtriphenylamine (TCTA), a series of starꢀshaped deepꢀblue
fluorescent emitters (2PꢀTCTA, 3PꢀTCTA, 4PꢀTCTA and 5PꢀTCTA) have been
successfully developed via a simple extension of oligophenyl chain between two N
atoms. When the number of phenyl ring increases, it is found that both the absorption
and emission for these TCTAꢀbased starbursts are redꢀshifted and finally become
saturated for 5PꢀTCTA consisting of a pentaphenyl bridge. Interestingly, on going
from 2PꢀTCTA to 5PꢀTCTA, the film photoluminescent quantum yield is gradually
enhanced from 11.4% to 35.5%. The same trend is also observed for their
corresponding solutionꢀprocessed undoped OLEDs. As a consequence, 5PꢀTCTA
shows the best device performance, revealing a maximum luminescence of 7300
cd/m2, and a peak luminous efficiency of 2.48 cd/A (2.15 lm/W; 2.30%) together with
CIE coordinates of (0.15, 0.09).
2

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Introduction
Since 1987, organic lightꢀemitting diodes (OLEDs) have attracted tremendous
attention due to their potential applications in flatꢀpanel displays and solidꢀstate
lightings.1ꢀ10 To achieve fullꢀcolor displays, three primary RGB luminescent materials
with excellent stability, efficiency and color purity are required. With respect to green
and red counterparts, it seems to be a big challenge to develop blue emitters,
especially deepꢀblue ones that have a Commission Internationale de l’Eclairage (CIE)
y coordinate value of < 0.10, because the intrinsic wide bandgap would inevitably
result in inefficient charge injection to an emitting layer (EML).11 Therefore, great
efforts should be paid on the design of deepꢀblue emitters to push forward the
commercialization of fullꢀcolor OLEDs.
In general, OLEDs can be fabricated through either vacuum evaporation or solution
processes, such as inkjet printing and spinꢀcoating. Compared with the former, the wet
method is superior for realizing lowꢀcost and largeꢀarea displays.12 We note that most
reported solutionꢀprocessed deepꢀblue OLEDs are based on polymers rather than
small molecules.13ꢀ18 The multidispersity of polymers as well as some defects
including monomer sequence, end groups and catalyst residues may limit their device
performance and reproducibility.19, 20 As an alternative, starburst conjugated oligomers,
in which several conjugated arms are joined together to a central core, are believed to
be capable of preventing these drawbacks given their wellꢀdefined structures.21ꢀ32 In
addition, when deposited via a solution process, they possess relatively good
filmꢀforming properties owing to the characteristic starꢀshaped orientation.
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Recently, many starburst deepꢀblue emitters with different cores, such as benzene,22
26
truxene,23 porphyrin,24 triazatruxene,25,
pyrene,27 spirofluorene,28 and
triphenylamine (TPA)29ꢀ32 have been demonstrated. Among them, TPAꢀcored
molecules represent a group of very promising deepꢀblue lightꢀemitting materials
since they have suitable highest occupied molecular orbital (HOMO) levels to
facilitate the hole injection from the anode to the EML. For example,
4,4’,4’’ꢀtris(carbazolꢀ9ꢀyl)ꢀtriphenylamine (TCTA), bearing a TPA core and three
carbazole endꢀcappers, has been widely used as the holeꢀtransporting material in
OLEDs because of its high hole mobility.33 By connecting six oligofluorene arms at
the periphery of TCTA, monodisperse starburst oligomers were reported to display
bright, deepꢀblue fluorescence both in solution and in the solid state.30 Their
corresponding undoped OLEDs revealed a luminescence up to 1025 cd/m2 and a
maximum luminous efficiency of 0.47 cd/A with CIE coordinates of (0.16, 0.07).
However, these oligofluorene functionalized oligomers may suffer from the unwanted
long wavelength emission under longꢀterm device operation, similar to
polyfluoreneꢀbased macromolecules.34ꢀ36
In this paper, we propose a novel strategy for the development of starburst
TCTAꢀbased deepꢀblue fluorescent emitters without oligofluorene. Stemming from
TCTA, as indicated in Scheme 1, 2PꢀTCTA, 3PꢀTCTA, 4PꢀTCTA and 5PꢀTCTA have
been successfully designed and synthesized by increasing the number of phenyl ring
between two N atoms. Such a simple modification is expected to have several
advantages: firstly, the ease of hole injection may be kept in the presence of a TPA
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core; secondly, the fluorene moiety is not adopted any longer in this case, thereby
avoiding the risk of spectral instability induced by fluorene; most importantly, the
extension of oligophenyl chain could not only enlarge the conjugation length and thus
redꢀshift the emission from 419 nm of 2PꢀTCTA to 448 nm of 5PꢀTCTA, but also
significantly improve the film photoluminescent quantum yield (PLQY) from 11.4%
to 35.5%. As a result, 5PꢀTCTA reveals a maximum luminescence of 7300 cd/m2, a
maximum luminous efficiency of 2.48 cd/A, and CIE coordinates of (0.15, 0.09). To
the best of our knowledge, the obtained undoped device performance is among the
highest for solution processible nonfluorene deepꢀblue emitters.37ꢀ39
Results and Discussion
Synthesis and characterization:
Scheme 1 outlines the synthetic route for TCTAꢀbased deepꢀblue emitters. Starting
from 1,4ꢀdibromobenzene and 4,4'ꢀdibromoꢀ1,1'ꢀbiphenyl, the selective CꢀN coupling
reactions were carried out to produce CzPBr and Cz2PBr, which were converted to
the corresponding boronic esters CzPBO and Cz2PBO, respectively. They
subsequently reacted with 4,4'ꢀdibromoꢀ1,1'ꢀbiphenyl to afford the monocoupling
products Cz3PBr and Cz4PBr in good yields (72ꢀ89%) under room temperature
conditions by using Pd(PPh3)4 as the catalyst, Cs2CO3 as the base and
toluene/methanol as the solvent.40 Similar to CzPBO and Cz2PBO, Cz3PBO and
Cz4PBO were prepared through Pdꢀcatalyzed Miyaura reactions of Cz3PBr and
Cz4PBr. In the case of the synthesis of Cz4PBO, a mixed solvent of
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N,Nꢀdimethylformide (DMF) and mesitylene was used to improve the system
solubility, and hence considerably increase the yield from 10% to 91%.
With CzPBO
~
Cz4PBO in hand, the Suzuki crossꢀcouplings of
tris(4ꢀbromophenyl)amine were finally performed to give the desired starbursts
2PꢀTCTA, 3PꢀTCTA, 4PꢀTCTA and 5PꢀTCTA in moderate yields of 45ꢀ55%. The
1
structures of 2PꢀTCTA ~ 5PꢀTCTA were fully characterized by H and 13C NMR
spectroscopy, MALDIꢀTOF mass spectroscopy and elemental analysis. Moreover,
they were all readily soluble in chloroform, tetrahedronfuran, chlorobenzene,
oꢀdichlorobenzene, and mesitylene, ensuring their capabilities to form high quality
films via spinꢀcoating.
Thermal properties:
The thermal properties of TCTAꢀbased deepꢀblue emitters 2PꢀTCTA ~ 5PꢀTCTA
were measured by thermogravimetric analysis (TGA) and differential scanning
calorimetry (DSC). They all exhibit close decomposition temperatures (Tds) of 520 °C,
corresponding to a 5% weight loss (Figure 1). When the number of phenyl ring grows,
nevertheless, the glass transition temperature (Tg) is found to be slightly enhanced
from 249 oC of 2PꢀTCTA to 265 oC of 4PꢀTCTA. As for 5PꢀTCTA, no glass transition
o
is observed, and a melting process appears at about 389 C, which may be tentatively
ascribed to the tendency of easy intermolecular packing and hence ready
crystalization induced by the extension of oligophenyl. Meanwhile, we note that the
Tgs of these TCTAꢀbased derivatives are much higher than that of the deepꢀblue
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emitters with oligofluorene arms (60ꢀ130 oC),30, 31 indicative of their excellent thermal
and morphological stability.
Taking 5PꢀTCTA as an example, its morphological stability was then investigated
by atomic force microscopy (AFM). Figure 2 compares the AFM images of 5PꢀTCTA
films before and after annealing under vacuum at 150 oC for 2 hours. As one can see,
both the pristine and annealed films show a fairly smooth surface morphology with a
rootꢀmeanꢀsquare roughness of 0.408 and 0.722 nm, respectively. The observed good
morphological stability is believed to be favorable for the preparation of homogenious
and stable thin films via solution process, thereby improving the reliability of the
lightꢀemitting diodes.
Electrochemical properties:
Cyclic voltammetry (CV) was used to study the electrochemical properties of
2PꢀTCTA ~ 5PꢀTCTA. Upon anodic sweeping in dichloromethane (DCM), all the
starbursts show two reversible oxidation waves at 0.4/0.7 V, respectively (Figure 3).
Consistent with the literatures,41 the first oxidation is from the TPA core, while the
second is corresponding to the carbazole terminals. With regard to the energy level of
the ferrocene reference (4.8 eV below the vacuum level), the HOMO levels of
2PꢀTCTA ~ 5PꢀTCTA are determined to be about ꢀ5.2 eV. The value matches well
with that of poly(3,4ꢀethylenedioxythiopene):poly(styrenesulfonate) (PEDOT:PSS)
(ꢀ5.1 eV), indicating that the holeꢀinjection from PEDOT:PSS to these emitters would
be very efficient. During cathodic scan in DCM, no reduction waves are detected, and
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so their lowest unoccupied molecular orbital (LUMO) levels are estimated from the
HOMO and optical bandgap (Eg). As tabulated in Table 1, the LUMO is decreased
gradually from ꢀ2.16 eV of 2PꢀTCTA to ꢀ2.29 eV of 5PꢀTCTA.This is reasonable
when considering that the electron could be more delocalized along the extended
oligophenyl chain. Furthermore, the lowꢀlying LUMO level for 5PꢀTCTA means the
reduced electron injection barrier, which will be discussed later.
Photophysical properties:
The UVꢀVis spectra of 2PꢀTCTA ~ 5PꢀTCTA in toluene are shown in Figure 4a.
The lowꢀenergy absorption bands in the range of 320ꢀ450 nm can be safely attributed
to the πꢀπ* transitions of TPAꢀcored oligophenyl, while the highꢀenergy absorption
bands located at 280ꢀ320 nm are related to the πꢀπ* transitions of periphery carbazole
42
moieties. Meanwhile, the absorption onset shows a significant redꢀshift of 15 nm
from 2PꢀTCTA to 4PꢀTCTA, but a slight redꢀshift of 1 nm from 4PꢀTCTA to
5PꢀTCTA (Table 1), implying the conjugation saturation for 5PꢀTCTA with 5 benzene
units between two N atoms. To further verify this trend, the effective conjugation
length (ECL) is then estimated. According to the theory proposed by Meier et al.,43
the dependence of the absorption onset λ on the number of benzene unit n can be
described by the exponential function (1):
(1)
λ1 belongs to the parent compound TCTA (λ1 = 360 nm)42;λ∞ is the limiting value
for n = ∞. And the maximum conjugation is virtually reached for an nECL, which
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fulfills the equation (2):
(2)
By applying the above equations (1) and (2), we find λ∞ = 412.5 nm and nECL = 5
(Inset of Figure 4a). The result correlates well with both the absorption and
photoluminescence (PL). As indicated in Figure 4b and 4c, the emission maxima in
both solutions and films display a bathochromic shift with the extended oligophenyl
chain, and become saturated for 5PꢀTCTA at 425 and 448 nm, respectively. These
observations suggest that a pentaphenyl bridge is enough to tune the conjugation of
TCTAꢀbased starbursts.
On going from solution to film, in addition, the PL spectra of 2PꢀTCTA ~ 5PꢀTCTA
are all found to be redꢀshifted about 15ꢀ23 nm, indicative of the existence of
aggregation to some degree. Noticeably, 2PꢀTCTA ~ 5PꢀTCTA posscess significant
spectral stability since no fluorene unit is used here for the composition of blue
o
emitters. For example, even after annealing at 150 C for 2 hours in air, the PL
spectrum of the 5PꢀTCTA film remains nearly unchanged without the appearance of
lowꢀenergy green emission band observed in polyfluoreneꢀbased macromolecules
(Figure 5).34ꢀ36 Furthermore, the corresponding film PLQY is measured to be 11.4%,
11.6%, 22.4% and 35.5% for 2PꢀTCTA, 3PꢀTCTA, 4PꢀTCTA and 5PꢀTCTA,
respectively. Compared with 2PꢀTCTA, the PLQY of 5PꢀTCTA is improved by about
three folds when the number of phenyl rings increases. The reason is not very clear
now, but the same phenomenon has been previously reported for 4PꢀNPD with a
tetraphenyl bridge, which has a much higher film PLQY than that of αꢀNPD with a
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biphenyl bridge.44
Electroluminescence properties:
To investigate the electroluminescence (EL) properties of 2PꢀTCTA ~ 5PꢀTCTA,
solutionꢀprocessed undoped OLEDs were fabricated with the configuration of
ITO/PEDOT:PSS/2PꢀTCTA ~ 5PꢀTCTA/TPBI/LiF/Al. Here TPBI stands for
1,3,5ꢀtris(1ꢀphenylꢀbenzimidazolꢀ2ꢀyl)benzene, and acts as the electron transporting
layer. Figure 6 portrays their EL spectra at a driving voltage of 6 V. Noticeably, the EL
spectra of 3PꢀTCTA ~ 5PꢀTCTA are similar to their PL counterparts in solid states,
whereas 2PꢀTCTA exhibits a much broader EL spectrum relative to its PL one. The
possible formation of an exciplex at the interface between the EML and TPBI may
contribute to the wideness of the EL spectrum for 2PꢀTCTA.31 Despite this difference,
all of them display bright deepꢀblue emission with CIE coordinates of (0.16, 0.05) for
2PꢀTCTA, (0.16, 0.08) for 3PꢀTCTA, (0.15, 0.09) for 4PꢀTCTA and (0.15, 0.09) for
5PꢀTCTA.
The current density―voltage―brightness characteristics and luminous efficiency
versus current density curves of 2PꢀTCTA ~ 5PꢀTCTA are depicted in Figure 7a and
7b, respectively. And the related data are also summarized in Table 2. It should be
noted that all these deepꢀblue emitters show turnꢀon voltages below 5 V. This is
consistent with their highꢀlying HOMO levels close to PEDOT:PSS, favorable to the
hole injection. Moreover, the turnꢀon voltage is obviously down from 4.4 V of
2PꢀTCTA to 3.3 V of 5PꢀTCTA. With respect to 2PꢀTCTA (ꢀ2.16 eV), the
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aboveꢀmentioned decreased LUMO level of 5PꢀTCTA (ꢀ2.29 eV) can facilitate the
electron injection and transporting, thus leading to the reduction of the turnꢀon
voltage.31 Consequently, the brightness as well as current density at the same driving
voltage seems to be enhanced monotonically from 2PꢀTCTA to 5PꢀTCTA (Figure 7a).
Most interestingly, the device performance is found to increase with the increasing
number of phenyl ring between two N atoms (Figure 7b). For instance, 2PꢀTCTA
reveals a maximum luminous efficiency (ηc,max) of 0.83 cd/A, a maximum power
efficiency (ηp,max) of 0.54 lm/W, a maximum external quantum efficiency (ηext,max) of
1.33%, and a maximum luminescence (Lmax) of 1740 cd/m2. As for 5PꢀTCTA, these
values are up to 2.48 cd/A, 2.15 lm/W, 2.30% and 7300 cd/m2 for ηc,max, ηp,max, ηext,max
and Lmax, respectively. The comprehensive performance of 5PꢀTCTA is superior to
that of the starꢀshaped oligofluorenes with TCTA as the core (0.47 cd/A, 1025
cd/m2),30 and among the highest for solutionꢀprocessable nonfluorene deepꢀblue
emitters.37
Compared with 2PꢀTCTA, the significant improvement of the device performance
for 5PꢀTCTA could be explained as follows: 1) Among 2PꢀTCTA ~ 5PꢀTCTA,
5PꢀTCTA possesses the highest PLQY in the film, rendering the generated excitons in
OLEDs with high radiative decay constant. 2) The similar highꢀlying HOMO levels
for all these emitters could ensuring the effective hole injection from the adjacent
PEDOT:PSS layer to the EML (Figure 8). On the other hand, the gradual reduce of the
LUMO level from 2PꢀTCTA to 5PꢀTCTA would lower the electron injection barrier
between TPBI and the EML. Therefore, a more balanced charge flux could be
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anticipated for 5PꢀTCTA, increasing the recombination possibility of holes and
electrons.
Conclusion
In conclusion, by simply tuning the conjugation length of oligophenyl chain
between two N atoms, we have reported a series of starꢀshaped deepꢀblue emitters
2PꢀTCTA ~ 5PꢀTCTA. They all possess good solution processibility and excellent
thermal stability. As the number of phenyl ring grows, the film PLQY is up from
11.4% of 2PꢀTCTA to 35.5% of 5PꢀTCTA, and the LUMO level is decreased by about
0.13 eV. Solutionꢀprocessed undoped OLEDs based on these starbursts exhibit bright
deepꢀblue emissions with CIE coordinates ranging from (0.16, 0.05) to (0.15, 0.09).
Owing to the highest PLQY as well as the obtained more balanced charge flux
induced by the lowered LUMO level, the best device performance is achieved for
5PꢀTCTA, which gives a stateꢀofꢀart luminous efficiency as high as 2.48 cd/A. These
results suggest that the simple extension of oligophenyl chain would be a promising
strategy to optimize the device performance of solutionꢀprocessed deepꢀblue OLEDs.
Experimental Section
General information:
1H NMR and 13C NMR spectra were measured on a Bruker Avance 300 NMR
spectrometer or Bruker Avance 400 NMR spectrometer. Elemental analyses of carbon,
hydrogen, and nitrogen were performed on a VarioEL elemental analyzer.
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MALDIꢀTOF mass spectra were recorded on an AXIMA CFR MS apparatus
(COMPACT). Thermal properties of the compounds were analyzed with a
o
PerkinꢀElmerꢀTGA 7 instrument under nitrogen at a heating rate of 10 C/min.
Differential scanning calorimetry (DSC) was performed on a PerkinꢀElmerꢀDSC 7
o
o
instrument at a heating rate of 10 C/min from room temperature to 400 C under
nitrogen. The glass transition temperature was determined from the second heating
scan. Atomic force microscopy (AFM) image was recorded on a SPA300HV with a
SPI3800N controller (Seiko Instruments, Inc., Japan).
Photophysical characterization
The UV/Vis absorption and photoluminescence spectra were measured by a
Perkin–Elmer Lambda 35 UV/Vis spectrometer and a Perkin–Elmer LS 50B
spectrofluorometer, respectively. Solution spectra were recorded in toluene for
UV/Vis absorption with a concentration of 10ꢀ6 M and toluene for photoluminescence
with a concentration of 10ꢀ5 M. Thin films on quartz for spectroscopic measurements
were prepared by dropꢀcoating. The quantum efficiency of the solidꢀfilms were
collected by measuring the total light output in all directions in an integrating sphere
(C9920ꢀ02, HAMAMATSU).
Cyclic voltammetry
All measurements were carried out in dichloromethane with a conventional
threeꢀelectrode system consisting of a platinum working electrode, a platinum counter
electrode, and an Ag/AgCl reference electrode. The supporting electrolyte was 0.1 M
tetrabutylammonium perchlorate (nꢀBu4NClO4). All potentials were calibrated against
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the ferrocene/ferrocenium couple (Fc/Fc+). The HOMO levels were calculated
according to the equation HOMO = ꢀe [Eox + 4.8 V], where Eox was the initial
oxidation peak value, and the LUMO levels were calculated according to the equation
LUMO = HOMO + ꢀEg, where ꢀEg is the optical band gap estimated from the onset
of the absorption spectrum.
Device fabrication and measurement:
Patterned glass substrates coated with indium tin oxide (ITO) (20 ꢁ per square)
were cleaned with acetone, detergent, distilled water, and then in an ultrasonic solvent
o
bath. After baking in a heating chamber at 120 C for 8 h, the ITOꢀglass substrates
were treated with O2 plasma for 25 min. Subsequently, a 40ꢀnmꢀthick
poly(3,4ꢀethylenedioxythiophene):poly(styrenesulfonate)
(PEDOT:PSS,
BatronꢀP4083, Bayer AG) film was spinꢀcoated on top of the ITO and then baked at
o
120 C for 40 min. Then, solutions of the deepꢀblue emitters in oꢀdichlorobenzene or
mesitylene were filtered through a filter (0.45 um) and spinꢀcoated on PEDOT:PSS as
o
the emissive layer (EML). After annealing at 120 C for 30 min, the thickness of the
EML was about 40 nm. Successively, the substrate was transferred to a vacuum
thermal evaporator and a 50ꢀnmꢀthick film of TPBI was evaporated on top of the
EML at a base pressure less than 10ꢀ6 Torr (1 Torr = 133.32 Pa). Finally, 1 nm LiF and
100 nm Al were deposited subsequently as the cathode through a shadow mask with
an array of 14 mm2 openings. The EL spectra and CIE coordinates were measured
using a PR650 spectra colorimeter. The current–voltage and brightness–voltage
curves of devices were measured using a Keithley 2400/2000 source meter and a
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calibrated silicon photodiode. All the measurements were carried out at room
temperature under ambient conditions.
Synthesis:
1,4ꢀdibromobenzene, 4,4'ꢀdibromoꢀ1,1'ꢀbiphenyl were bought and recrystallized in
nꢀhexane before using. All solvents for chemical synthesis were purified according to
the
standard
procedures.
Tris(4ꢀbromophenyl)amine,45
3,6ꢀdiꢀtertꢀbutylꢀ9Hꢀcarbazole46 were prepared according to the literatures.
Synthesis of 9-(4-bromophenyl)-3,6-di-tert-butyl-9H-carbazole (CzPBr):
3,6ꢀdiꢀtertꢀbutylꢀ9Hꢀcarbazole (6.00 g, 21.50 mmol), Cu powder (2.73 g, 42.95
mmol), CuI (0.41g, 2.15 mmol), K2CO3 (13.67 g, 64.40 mmol) were added to a
solution of 1,4ꢀdibromobenzene (10.13 g, 42.95 mmol) in 30 ml
o
N,NꢀDimethylethyleneurea (DMI). The mixture was stirred at 160 C for 12 h. After
o
cooling to room temperature, the solvent was removed under vacuum at 120 C. The
residue was poured into dichloromethane (DCM) and the organic layer was washed
with water and then dried over anhydrous Na2SO4. After the solvent was removed, the
residue was purified by column chromatography on silica gel with nꢀhexane as eluent
to get a white solid in 70% yield. 1H NMR (300 MHz, CDCl3): δ 8.13 (d, J = 1.5 Hz,
2H), 7.70 (d, J = 8.6 Hz, 2H), 7.46 (dd, J = 8.3, 1.9 Hz, 2H), 7.43 (d, J = 6.1 Hz, 2H),
7.31 (d, J = 8.6 Hz, 2H), 1.46 (s, 18H).
Synthesis of 9-(4'-bromo-[1,1'-biphenyl]-4-yl)-3,6-di-tert-butyl-9H-carbazole
(Cz2PBr):
3,6ꢀdiꢀtertꢀbutylꢀ9Hꢀcarbazole (6.00 g, 21.5 mmol), Cu powder (2.73 g, 42.95
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mmol), CuI (0.41 g, 2.15 mmol), K2CO3 (13.67 g, 64.40 mmol) were added to a
solution of 4,4'ꢀdibromoꢀ1,1'ꢀbiphenyl (10.10 g, 32.21 mmol) in 30 ml DMI. The
o
reaction was carried out at 140 C for 12 h. After cooling to room temperature, the
solution was washed with water and dried with anhydrous sodium sulfate. The residue
was dissolved in dichloromethane and purified by column chromatography on silica
1
gel with nꢀhexane as eluent to get a white solid in 60% yield. H NMR (400 MHz,
CDCl3): δ = 8.18 (d, J = 1.2, 2H), 7.76 (d, J = 8.4, 2H), 7.64 (dd, J = 8.3, 4.7, 4H),
7.56 (d, J = 8.4, 2H), 7.49 (dd, J = 8.6, 1.7, 2H), 7.42 (d, J = 8.6, 2H), 1.50 (s, 18H).
Synthesis
of
3,6-di-tert-butyl-9-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)-9H-ca
rbazole (CzPBO):
CzPBr (5.40 g, 12.43 mmol), Bis(pinacolato)diboron (7.89 g, 31.08 mmol),
Pd(dppf)Cl2 (0.18 g, 0.25 mmol), KOAc (3.66 g, 37.30 mmol) were add to a clean dry
threeꢀneck flask filled with argon. 30 ml N,Nꢀdimethylformide (DMF) was added into
o
the mixture. Then the solution was stirred and heated to reflux at 100 C for 12 h.
After cooling to room temperature, 100 ml water was slowly add to the solution. The
mixture was filtered and the residue was washed with water and dried with anhydrous
sodium sulfate and then purified by column chromatography on silica gel with
1
nꢀhexane/ethyl acetate (20/1 v/v) as eluent to get a white solid in 96% yield (% ). H
NMR (300 MHz, CDCl3): δ = 8.13 (d, J = 1.5, 2H), 8.02 (d, J = 8.2, 2H), 7.58 (d, J =
8.2, 2H), 7.46 (dd, J = 8.7, 1.8, 2H), 7.39 (d, J = 8.7, 2H), 1.46 (s, 18H), 1.40 (s,
12H).
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Synthesis
of
3,6-di-tert-butyl-9-(4'-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-[1,1'-bipheny
l]-4-yl)-9H-carbazole (Cz2PBO):
Cz2PBO was prepared in 95% yield according to a similar procedure to CzPBO by
1
using Cz2PBr instead of CzPBr. H NMR (400 MHz, CDCl3): δ = 8.15 (d, J = 1.7,
2H), 7.94 (d, J = 8.1, 2H), 7.82 (d, J = 8.8, 2H), 7.70 (d, J = 8.2, 2H), 7.63 (d, J = 8.4,
2H), 7.47 (dd, J = 8.7, 1.9, 2H), 7.40 (d, J = 8.6, 2H), 1.47 (s, 18H), 1.38 (s, 12H).
Synthesis
of
9-(4''-bromo-[1,1':4',1''-terphenyl]-4-yl)-3,6-di-tert-butyl-9H-carbazole
(Cz3PBr):
CzPBO (5.72 g, 11.92 mmol), 4,4'ꢀdibromoꢀ1,1'ꢀbiphenyl (7.44 g, 23.83 mmol),
Pd(PPh3)4 (0.14 g, 0.12 mmol) and Cs2CO3 (7.71 g, 23.83 mmol) were added into a
roundꢀbottom flask. After 105 ml solvent of methanol/toluene (20:1 v/v) was added,
the solution was stirred at room temperature and protected under argon for 24 h. 200
ml methanol was added into the flask to precipitate a white solid which was collected
by filtration and washed by methanol. The solid was purified by column
chromatography on silica gel with cyclohexane/dichloromethane (10/1 v/v) as eluent
1
to get a white solid in 72% yield. H NMR (300 MHz, CDCl3): δ = 8.16 (d, J = 1.4,
2H), 7.84 (d, J = 8.4, 2H), 7.78 (d, J = 8.3, 2H), 7.69 (d, J = 8.4, 2H), 7.65 (d, J = 8.4,
2H), 7.61 (d, J = 8.6, 2H), 7.54 (d, J = 8.5, 2H), 7.49 (dd, J = 8.7, 1.7, 2H), 7.43 (d, J
= 8.6, 2H), 1.48 (s, 18H).
Synthesis
of
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9-(4'''-bromo-[1,1':4',1'':4'',1'''-quaterphenyl]-4-yl)-3,6-di-tert-butyl-9H-carbazo
le (Cz4PBr):
Cz4PBr was prepared in 89% yield according to a similar procedure to Cz3PBr by
using Cz2PBO instead of CzPBO. 1H NMR (400 MHz, CDCl3): δ = 8.16 (s, 2H), 7.85
(d, J = 7.8, 2H), 7.77 (d, J = 14.2, 6H), 7.67 (t, J = 7.4, 4H), 7.60 (d, J = 8.3, 2H),
7.53 (d, J = 8.3, 2H), 7.49 (d, J = 8.5, 2H), 7.43 (d, J = 8.7, 2H), 1.48 (s, 18H).
Synthesis
of
3,6-di-tert-butyl-9-(4''-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-[1,1':4',1''-te
rphenyl]-4-yl)-9H-carbazole (Cz3PBO):
Cz3PBO was prepared in 95% yield according to a similar procedure to CzPBO by
1
using Cz3PBr instead of CzPBr. H NMR (400 MHz, CDCl3): δ = 8.16 (d, J = 1.3,
2H), 7.93 (d, J = 8.0, 2H), 7.84 (d, J = 8.4, 2H), 7.77 (s, 4H), 7.69 (d, J = 8.0, 2H),
7.65 (d, J = 8.4, 2H), 7.48 (dd, J = 8.7, 1.6, 2H), 7.43 (d, J = 8.6, 2H), 1.46 (d, J = 9.3,
18H), 1.38 (s, 12H).
Synthesis
of
3,6-di-tert-butyl-9-(4'''-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-[1,1':4',1'':4
'',1'''-quaterphenyl]-4-yl)-9H-carbazole (Cz4PBO):
Cz4PBr (9.80 g, 14.79 mmol), Bis(pinacolato)diboron (7.51 g, 29.58 mmol),
Pd(dppf)Cl2 (0.36 g, 0.44 mmol), KOAc (4.35 g, 44.3 mmol) were add to a clean dry
threeꢀneck flask filled with argon. 50 ml N,Nꢀdimethylformide (DMF)/mesitylene
(1:5) was added into the mixture. Then the solution was stirred and heated to reflux at
o
100 C for 24 h. After cooling to room temperature, 100 ml dichloromethane was
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added to the solution and washed with water. The organic layer was collected and
dried with anhydrous sodium sulfate. The solvent was removed under vacuum and
o
120 C. The residue was purified by column chromatography on silica gel with
nꢀhexane/ethyl acetate (20/1 v/v) as eluent to get a white solid in 91% yield. 1H
NMR (400 MHz, CDCl3) δ = 8.16 (d, J = 1.6, 2H), 7.92 (d, J = 8.2, 2H), 7.85 (d, J =
8.4, 2H), 7.81 – 7.77 (m, 4H), 7.77 – 7.72 (m, 4H), 7.68 (d, J = 8.2, 2H), 7.65 (d, J =
8.4, 2H), 7.49 (dd, J = 8.7, 1.9, 2H), 7.43 (d, J = 8.6, 2H), 1.48 (s, 18H), 1.38 (s,
12H).
Synthesis
of
Tris(4'-(3,6-di-tert-butyl-9H-carbazol-9-yl)-[1,1'-biphenyl]-4-yl)amine
(2P-TCTA):
Tris(4ꢀbromophenyl)amine (1.00 g, 2.07 mmol), CzPBO (3.61 g, 7.47 mmol),
Pd(OAc)2 (0.014 g, 0.062 mmol) and SPhos (0.051 g, 0.124 mmol) were dissolved in
50ml toluene. An aqueous solution of 2M K2CO3 (9.4 ml, 18.67 mmol K2CO3) and
one drop of Aliquat 336 was added to the solution and then the mixture was heated to
o
100 C for 24 h under argon. After reaction, 100 ml DCM was added to the solution
after cooled to room temperature and washed with water and dried with anhydrous
sodium sulfate Solvents were removed under vacuum and the residue was purified by
column chromatography on silica gel with nꢀhexane/dichloromethane (5/1 v/v) as
1
eluent to get a white solid in 45% yield. H NMR (400 MHz, CDCl3): δ 8.16 (d, J =
1.7 Hz, 6H), 7.82 (d, J = 8.4 Hz, 6H), 7.67 (d, J = 8.6 Hz, 6H), 7.63 (d, J = 8.5 Hz,
6H), 7.48 (dd, J = 8.7, 1.9 Hz, 6H), 7.42 (d, J = 8.6 Hz, 6H), 7.35 (d, J = 8.5 Hz, 6H),
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1.48 (s, 54H). 13C NMR (101 MHz, CDCl3): δ = 147.00, 142.90, 139.25, 139.09,
137.09, 134.94, 127.97, 127.90, 126.98, 124.63, 123.62, 123.44, 116.26, 109.29,
34.75, 32.03. MS (MALDIꢀTOF): m/z: 1304.8. Elemental analysis calcd. (%) for
C96H96N4: C, 88.30; H, 7.41; N, 4.29; found: C, 87.81; H, 7.24; N, 4.06.
Synthesis
of
Tris(4''-(3,6-di-tert-butyl-9H-carbazol-9-yl)-[1,1':4',1''-terphenyl]-4-yl)amine
(3P-TCTA):
3PꢀTCTA was prepared in 50% yield according to a similar procedure to 2PꢀTCTA
1
by using Cz2PBO instead of CzPBO. H NMR (400 MHz, CDCl3): δ = 8.16 (d, J =
1.6, 6H), 7.86 (d, J = 8.4, 6H), 7.77 (q, J = 8.5, 12H), 7.65 (d, J = 8.4, 12H), 7.49 (dd,
J = 8.7, 1.8, 6H), 7.43 (d, J = 8.6, 6H), 7.32 (d, J = 7.9, 6H), 1.48 (s, 54H). 13C NMR
(101 MHz, CDCl3): δ = 146.93, 142.92, 139.68, 139.21, 139.14, 138.80, 137.43,
135.04, 128.15, 127.85, 127.45, 127.15, 126.95, 124.56, 123.64, 123.46, 116.25,
109.30, 34.73, 32.03. MS (MALDIꢀTOF): m/z: 1532.9. Elemental analysis calcd. (%)
for C114H108N4: C, 89.25; H, 7.10; N, 3.65; found: C, 89.01; H, 7.07; N, 3.47.
Synthesis
of
Tris(4'''-(3,6-di-tert-butyl-9H-carbazol-9-yl)-[1,1':4',1'':4'',1'''-quaterphenyl]-4-y
l)amine (4P-TCTA):
4PꢀTCTA was prepared in 48% yield according to a similar procedure to 2PꢀTCTA
1
by using Cz3PBO instead of CzPBO. H NMR (400 MHz, CDCl3): δ = 8.16 (d, J =
1.5, 6H), 7.86 (d, J = 8.4, 6H), 7.80 (s, 12H), 7.76 (dd, J = 16.1, 8.5, 12H), 7.64 (t, J =
8.6, 12H), 7.49 (dd, J = 8.7, 1.8, 6H), 7.43 (d, J = 8.6, 6H), 7.31 (d, J = 8.4, 6H), 1.48
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13
(s, 54H). C NMR (101 MHz, C6Cl2D4) δ = 147.38, 143.39, 140.07, 139.83, 139.78,
139.38, 139.32, 138.13, 135.60, 132.73, 128.25, 128.00, 127.50, 127.40, 127.15,
126.90, 124.91, 124.34, 123.87, 116.45, 109.74, 34.71, 32.10. MS (MALDI TOF):
m/z: 1761.0. Elemental analysis calcd. (%) for C132H120N4: C, 89.96; H, 6.86; N, 3.18;
found: C, 89.70; H, 6.92; N, 2.96.
Synthesis
of
Tris(4''''-(3,6-di-tert-butyl-9H-carbazol-9-yl)-[1,1':4',1'':4'',1''':4''',1''''-quinquep
henyl]-4-yl)amine (5P-TCTA):
5PꢀTCTA was prepared in 55% yield according to a similar procedure to 2PꢀTCTA
by using Cz4PBO instead of CzPBO. 1H NMR (400 MHz, C6Cl2D4): δ = 8.45 (s, 6H),
8.06 (d, J = 8.2, 6H), 7.99 – 7.92 (m, 36H), 7.88 (d, J = 8.2, 6H), 7.81 (d, J = 0.81,
6H), 7.69 (d, J = 8.6, 6H), 7.62 (d, J = 8.6, 6H), 7.57 (d, J = 8.0, 6H), 1.68 (s, 54H).
13C NMR (101 MHz, C6Cl2D4): δ = 147.37, 143.39, 140.03, 139.92, 139.78, 139.64,
139.36, 138.14, 135.42, 132.73, 128.24, 127.99, 127.52, 127.39, 127.14, 126.90,
124.90, 124.34, 123.86, 116.45, 109.73, 34.70, 32.09; MS (MALDIꢀTOF): m/z:
1989.0. Elemental analysis calcd. (%) for C150H132N4: C, 90.50; H, 6.68; N, 2.81;
found: C, 90.44; H, 6.60; N, 2.66.
Acknowledgments
The authors are grateful to the 973 Project (No. 2009CB623601), and the National
Natural Science Foundation of China (Nos. 21174144, 51322308 and 91333205) for
financial support of this research.
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Scheme 1. Synthesis of TCTAꢀbased deepꢀblue emitters.
Reagents and conditions: a) Cu, CuI, K2CO3, N,NꢀDimethylethyleneurea (DMI),
140ꢀ160 oC; b) Pd(dppf)Cl2, Bis(pinacolato)diboron, KOAc, DMF, 100 oC; c)
Pd(PPh3)4, Cs2CO3, toluene/methanol, room temperature; d) Bis(pinacolato)diboron,
Pd(dppf)Cl2,
KOAc,
DMF/mesitylene,
100
oC;
e)
Pd(OAc)2,
2ꢀDicyclohexylphosphinoꢀ2’,6’ꢀdimethoxybiphenyl (SPhos), K2CO3, Aliquat 336,
toluene, reflux.
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Figure 1. TGA curves of deepꢀblue emitters 2PꢀTCTA ~ 5PꢀTCTA and their DSC
traces (inset).
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Figure 2. Comparison of AFM topographic images for 5PꢀTCTA between the pristine
film (a) and annealed film (b).
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Figure 3. CV characteristics of deepꢀblue emitters 2PꢀTCTA ~ 5PꢀTCTA.
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Figure 4. UVꢀVis spectra in toluene (a), PL spectra in toluene (b), and PL spectra in
solid states (c) for deepꢀblue emitters 2PꢀTCTA ~ 5PꢀTCTA. Inset: Correlation of the
absorption onset and number of phenyl ring.
29