Low turn-on voltage, high-power-efficiency, solution-processed deep-blue organic light-emitting diodes based on starburst oligofluorenes with diphenylamine end-capper to enhance the HOMO level

Article abstract of DOI:10.1021/cm4039522

Two new star-shaped oligofluorenes, HFB-diF-Dpa and HFB-terF-Dpa, with a hexakis(fluoren-2-yl)benzene core and six diphenylamine end-cappers were designed and synthesized. The peripheral diphenylamine groups enhance the HOMO energy levels of the materials, and the bulky star-shaped structures efficiently suppress the intermolecular interaction. Their thermal, photophysical, and electrochemical properties were investigated. The two compounds display strong deep-blue emission both in solution and solid state. Solution-processed devices based on these starbursts exhibit highly efficient and stable deep-blue electroluminescence. Their high-lying HOMO energy levels match very well with that of the hole-injecting material. The double-layered device featuring HFB-diF-Dpa as emitter shows a low turn-on voltage of 3.6 V, a maximum current efficiency of 6.99 cd A^-1, and a maximum external quantum efficiency of 5.45% with the CIE coordinate of (0.154, 0.136). In particular, the combination of low driving voltage and high EQE provides an outstanding maximum power efficiency of 6.10 lm W^-1, which is the highest for nondoped deep-blue OLEDs based on solution-processable materials. Moreover, these devices present small values of efficiency roll-off at high brightness up to 1000 cd m^-2.

Full text of DOI:10.1021/cm4039522

Article
pubs.acs.org/cm
Low Turn-on Voltage, High-Power-Efficiency, Solution-Processed
Deep-Blue Organic Light-Emitting Diodes Based on Starburst
Oligofluorenes with Diphenylamine End-Capper to Enhance the
HOMO Level
Cui Liu,†,§ Qiang Fu,‡,§ Yang Zou,† Chuluo Yang,*,† Dongge Ma,*,‡ and Jingui Qin†
†Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Hubei Key Lab on Organic and Polymeric
Optoelectronic Materials, Department of Chemistry, Wuhan University, Wuhan 430072, People’s Republic of China
‡State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences,
Changchun 130022, People’s Republic of China
ABSTRACT: Two new star-shaped oligofluorenes, HFB-diF-Dpa and HFB-terF-Dpa, with a hexakis(fluoren-2-yl)benzene core
and six diphenylamine end-cappers were designed and synthesized. The peripheral diphenylamine groups enhance the HOMO
energy levels of the materials, and the bulky star-shaped structures efficiently suppress the intermolecular interaction. Their
thermal, photophysical, and electrochemical properties were investigated. The two compounds display strong deep-blue emission
both in solution and solid state. Solution-processed devices based on these starbursts exhibit highly efficient and stable deep-blue
electroluminescence. Their high-lying HOMO energy levels match very well with that of the hole-injecting material. The double-
layered device featuring HFB-diF-Dpa as emitter shows a low turn-on voltage of 3.6 V, a maximum current efficiency of 6.99 cd
A−1, and a maximum external quantum efficiency of 5.45% with the CIE coordinate of (0.154, 0.136). In particular, the
combination of low driving voltage and high EQE provides an outstanding maximum power efficiency of 6.10 lm W−1, which is
the highest for nondoped deep-blue OLEDs based on solution-processable materials. Moreover, these devices present small
values of efficiency roll-off at high brightness up to 1000 cd m−2.
charge injection and balances, consequently impairing the
efficiency of blue OLEDs.
INTRODUCTION
■
Organic light-emitting diodes (OLEDs) have attracted
considerable scientific and industrial attention because of
their applications in flat-panel displays and solid-state lighting
resources.1−13 Full-color displays require primary RGB
emission of relatively equal stability, efficiency, and color
purity. The development of deep-blue emission, which is
defined as having blue electroluminescence (EL) emission with
a Commission Internationale de l’Eclairage (CIE) y-coordinate
value of less than 0.15, is of special significance because such
emitters can not only effectively reduce the power consumption
of a full-color OLED but also be utilized to generate light of
other colors by energy cascade to a lower-energy fluorescent or
phosphorescent dopant.14−16 However, the intrinsic wide band
gap essential for blue emission often results in low electron
affinities of these blue-light-emitting materials. This hampers
Polyfluorene and its derivatives are considered to be the most
promising blue emitting materials17−19 due to their high
photoluminescence (PL) efficiencies, wide band-gaps, good
thermal stabilities, interesting morphological properties, and
easy tunability of properties by substitution and/or copoly-
merization.20−25 However, the application of polyfluorenes as
blue emitters in OLEDs is hampered by their undesirable long-
wavelength emission in the process of photoirradiation, heat
treatment, or device operation, which has been attributed to
physical26−30 or chemical31,32 degradation processes. Recently,
Received: November 30, 2013
Revised: April 27, 2014
Published: April 27, 2014
© 2014 American Chemical Society
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Chemistry of Materials
Article
pseudoreference electrode with ferrocenium−ferrocene (Fc+/Fc) as
the internal standard. Cyclic voltammograms were obtained at a scan
rate of 100 mV s−1. Formal potentials are calculated as the average of
cyclic voltammetric anodic and cathodic peaks. The onset potential
was determined from half-wave potential of the oxidation.
monodisperse oligofluorenes that can be deposited via solution
process have been of great interest for use in OLEDs.33−42 In
comparison with polymers, monodisperse oligofluorenes are
characterized by well-defined and uniform molecular structures
as well as high chemical purity. It has been demonstrated that
the performance and stability of monodisperse oligofluorenes in
OLEDs are superior to those of polyfluorenes.43−45 Several
series of star-shaped oligofluorenes with various cores such as
benzene,46 triazatruxene,34 pyrene,37 4,4′,4″-tris(carbazol-9-yl)-
triphenylamine (TCTA),35 1,3,5-tri(anthracen-10-yl)benzene,39
fully bridged triphenylamine,40 truxene,47 and spirofluorene33
have been reported.
In our previous work, we have reported a series of six-arm
star-shaped oligofluorenes with a hexakis(fluoren-2-yl)benzene
core.42 These oligomers exhibit good solubility, efficient deep-
blue emission, and good film-forming ability. The devices based
on these materials achieved good performances and striking
deep-blue EL color stability. However, the highest occupied
molecular orbital (HOMO) energy levels of these materials are
very low, resulting in a very high turn-on voltage, and then low
luminance and power efficiency. To address this issue, herein
we designed and synthesized two new star-shaped oligofluor-
enes HFB-diF-Dpa and HFB-terF-Dpa by introducing a
diphenylamine group in each fluorene arm to enhance the
HOMO energy level of the materials. The thermal, photo-
physical, and electrochemical properties of HFB-diF-Dpa and
HFB-terF-Dpa as well as the characteristics of devices
incorporating these materials were investigated. All these
compounds show deep-blue emission with high fluorescence
quantum yields in film. Their high-lying HOMO energy levels
match very well with that of the hole-injecting material.
Solution-processed devices based on these starbursts exhibit
efficient deep-blue electroluminescence and achieve a max-
imum current efficiency of 6.99 cd A−1, and a maximum power
efficiency of 6.10 lm W−1, with the CIE coordinate of (0.154,
0.136), which is among the highest for nondoped deep-blue
OLEDs based on solution-processed materials.
Device Fabrication and Measurement. The two types of hole-
injection material poly(3,4-ethylenedioxythiopene):poly-
(styrenesulfonate) (PEDOT:PSS) and electron-transporting material
TPBI were commercially available. Commercial ITO coated glass with
sheet resistance of 10 Ω square−1 was used as the starting substrates.
Before device fabrication, the ITO glass substrates were precleaned
carefully and treated by oxygen plasma for 2 min. PEDOT:PSS was
spin-coated to smooth the ITO surface and promote hole injection,
and then the emissive layer was spin-coated from chlorobenzene
solution. The samples were annealed at 120 °C for 30 min to remove
residual solvent. The thickness of the EML was about 50 nm. Finally,
an electron-transporting layer of 1,3,5-tris(N-phenylbenzimidazol-2-
yl)benzene (TPBI, 30 nm) and a cathode composed of lithium
fluoride (LiF, 1 nm) and aluminum (Al, 100 nm) were sequentially
deposited onto the substrate by vacuum deposition in the vacuum of
10−6 Torr. The current density−voltage−brightness (J−V−L) of the
devices was measured with a Keithley 2400 Source meter and a
Keithley 2000 Source multimeter equipped with a calibrated silicon
photodiode. The electroluminiscence (EL) spectra were measured by
JY SPEX CCD3000 spectrometer. The EQE values were calculated
according to previously reported methods.48 All measurements were
carried out at room temperature under ambient conditions.
Synthesis. (9,9-Dihexyl-7-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-
2-yl)-9H-fluoren-2-yl)(trimethyl)silane (F1) and (7′-bromo-9,9,9′,9′-
tetrahexyl-9H,9′H-2,2′-bifluoren-7-yl)(trimethyl)silane (F2) were pre-
pared following the reported procedures.49 2,2′-Ethyne-1,2-diylbis(7-
bromo-9,9-dihexyl-9H-fluorene) (A0) was synthesized according to
our previous method.42
7-Bromo-9,9,9′,9′-tetrahexyl-7′-iodo-9H,9′H-2,2′-bifluorene (F3).
To a solution of F2 (3.11 g, 3.80 mmol) in anhydrous CH2Cl2 (40
mL), ICl (0.63 M in CH2Cl2, 6.0 mL, 3.80 mmol) was added dropwise
at 0 °C. After stirring for 30 min at 0 °C, the reaction mixture was
poured into a Na2S2O3 aqueous solution with vigorous stirring until
discoloration for extraction with CH2Cl2. The organic layer was
washed with brine for several times and dried over anhydrous Na2SO4.
The extract was evaporated to dryness, and the residue was purified by
column chromatography on silica gel using petroleum ether as the
eluent to give a white solid (3.26 g). Yield: 98%. 1H NMR (300 MHz,
CDCl3, δ): 7.76 (d, J = 8.1 Hz, 2H), 7.71−7.52 (m, 7H), 7.50−7.43
(m, 3H), 2.10−1.88 (m, 8H), 1.19−0.99 (m, 24H), 0.77 (t, J = 6.9 Hz,
12H) 0.73−0.61 (m, 8H). 13C NMR (75 MHz, CDCl3, δ): 153.68,
153.51, 151.40, 151.18, 141.23, 141.06, 140.65, 140.02, 139.64, 136.18,
132.40, 130.29, 126.53, 121.71, 121.35, 120.31, 92.78, 55.71, 55.79,
40.49, 31.69, 29.85, 23.98, 22.80, 14.25. MS (EI) m/z: 870.21. Anal.
Calcd for C50H64BrI: C 68.88, H 7.40. Found: C 68.58, H 7.13.
(7″-Bromo-9,9,9′,9′,9″,9″-hexahexyl-9H,9′H,9″H-2,2′:7′,2″-ter-
fluoren-7-yl)(trimethyl)silane (F4). To a mixture of F1 (1.86 g, 3.50
mmol), F3 (3.05 g, 3.50 mmol), Pd(PPh3)4 (0.058 g, 0.05 mmol), and
2 M K2CO3 (9.00 mL, 18.00 mmol) was added degassed toluene (40
mL). The solution was heated to 100 °C and stirred at this
temperature for 48 h under argon. After cooling to room temperature,
the solution was extracted with CH2Cl2 and the organic layer was
washed with brine and water, and then dried over anhydrous Na2SO4.
After the solvent had been removed, the residue was purified by
column chromatography on silica gel using petroleum ether as the
eluent to give a white solid (3.10 g). Yield: 77%. 1H NMR (300 MHz,
CDCl3, δ): 7.84−7.70 (m, 6H), 7.70−7.57 (m, 8H), 7.57−7.44 (m,
4H), 2.18−1.92 (m, 12H), 1.19−1.00 (m, 36H), 0.90−0.62 (m, 30H),
0.33 (s, 9H). 13C NMR (75 MHz, CDCl3, δ): 153.42, 151.89, 151.27,
150.34, 141.57, 141.19, 140.77, 140.47, 140.33, 140.03, 139.38, 139.16,
132.04, 130.19, 128.96, 127.81, 127.38, 126.44, 126.35, 126.21, 121.67,
121.27, 121.17, 120.20, 119.20, 55.71, 55.50, 55.29, 40.53, 40.33,
31.67, 31.57, 29.85, 23.92, 22.78, 22.73, 14.25, −0.62. MALDI-TOF
m/z: 1151.7. Anal. Calcd for C78H105BrSi: C 81.42, H 9.20. Found: C
81.62, H 9.49.
EXPERIMENTAL SECTION
■
General Information. 1H NMR and 13C NMR spectra were
measured on a MECUYR-VX300 spectrometer. Elemental analyses of
carbon, hydrogen, and nitrogen were performed on a Vario EL III
microanalyzer. Mass spectra were measured on a ZAB 3F-HF mass
spectrophotometer. Matrix-assisted laser-desorption/ionization time-
of-flight (MALDI-TOF) mass spectra were performed on Bruker
BIFLEX III TOF mass spectrometer. UV−vis absorption spectra were
recorded on a Shimadzu UV-2500 recording spectrophotometer.
Photoluminescence (PL) spectra were recorded on a Hitachi F-4500
fluorescence spectrophotometer. The PL quantum yields of solid state
films were measured by an absolute method using the Edinburgh
Instruments (FLS920) integrating sphere excited with Xe lamp.
Differential scanning calorimetry (DSC) was performed on a
NETZSCH DSC 200 PC unit at a heating rate of 10 °C min−1
from room temperature to 300 °C under argon. The glass transition
temperature was determined from the second heating scan.
Thermogravimetric analysis (TGA) was undertaken with a NETZSCH
STA 449C instrument. The thermal stability of the samples under a
nitrogen atmosphere was determined by measuring their weight loss
while heating at a rate of 15 °C min−1 from 25 to 800 °C. Cyclic
voltammetry was carried out in nitrogen-purged THF (reduction scan)
and dichloromethane (oxidation scan), respectively, with a CHI
voltammetric analyzer. Tetrabutylammonium hexafluorophosphate
(TBAPF6) (0.1 M) was used as the supporting electrolyte. The
conventional three-electrode configuration consists of a platinum
working electrode, a platinum wire auxiliary electrode, and an Ag wire
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Chemistry of Materials
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7-Bromo-9,9,9′,9′,9″,9″-hexahexyl-7″-iodo-9H,9′H,9″H-2,2′:7′,2″-
terfluorene (F5). F5 was prepared according to the same procedure as
F3 but using compound F4 to give a white solid. Yield: 96%. 1H NMR
(300 MHz, CDCl3, δ): 7.83 (d, J = 7.5 Hz, 2H), 7.77 (d, J = 8.1 Hz,
2H), 7.72−7.58 (m, 10H), 7.52−7.45 (m, 4H), 2.18−1.89 (m, 12H),
1.22−1.01 (m, 36H), 0.91−0.61 (m, 30H). 13C NMR (75 MHz,
CDCl3, δ): 153.62, 153.42, 151.99, 151.28, 151.08, 141.34, 141.15,
140.63, 140.52, 140.25, 140.00, 139.47, 136.07, 132.26, 130.18, 127.39,
126.44, 126.37, 121.66, 121.28, 121.20, 120.23, 92.70, 55.72, 55.65,
55.52, 40.51, 31.66, 29.84, 23.93, 22.78, 14.25. MALDI-TOF m/z:
1202.8. Anal. Calcd for C75H96BrI: C 74.79, H 8.03. Found: C 74.96,
H 8.19.
(7′-Bromo-9,9,9′,9′-tetrahexyl-9H,9′H-2,2′-bifluoren-7-yl)-
diphenylamine (F6). To a mixture of F3 (10.90 g, 12.50 mmol),
diphenylamine (1.69 g, 10.00 mmol), CuI (0.095 g, 0.50 mmol),
sodium tert-butoxide (1.44 g, 15.00 mmol), and trans-1,2-diaminocy-
clohexane (0.23 g, 2.00 mmol) was added degassed 1,4-dioxane (40
mL).The solution was heated to 110 °C and stirred at this temperature
for 18 h under argon. After cooling to room temperature, the reaction
mixture was poured into 150 mL of brine. The solution was extracted
with CH2Cl2, and the organic layer was washed with brine and water
and then dried over anhydrous Na2SO4. After the solvent had been
removed, the residue was purified by column chromatography on silica
gel using petroleum ether/dichloromethane (50:1 by vol) as the eluent
to give a white solid (7.85 g). Yield: 86%. 1H NMR (300 MHz, CDCl3,
δ): 7.76−7.53 (m, 8H), 7.53−7.44 (m, 2H), 7.31−7.21 (m, 4H),
7.21−7.10 (m, 5H), 7.10−6.98 (m, 3H), 2.03−1.92 (m, 8H), 1.22−
1.00 (m, 24H), 0.91−0.62 (m, 20H). 13C NMR (75 MHz, CDCl3, δ):
153.51, 152.69, 151.73, 151.38, 148.30, 147.45, 141.31, 140.60, 140.16,
139.78, 139.36, 136.21, 130.26, 129.47, 129.03, 127.38, 126.48, 124.09,
123.87, 122.78, 121.60, 121.32, 120.72, 120.28, 119.70, 55.79, 55.46,
40.57, 31.77, 29.90, 24.01, 22.83, 14.33. MS (EI) m/z: 911.28. Anal.
Calcd for C62H74BrN: C 81.55, H 8.17, N 1.53. Found: C 81.73, H
8.25, N 1.67.
22.32, 14.15. MS (EI) m/z: 959.32. Anal. Calcd for C68H86BNO2: C
85.06, H 9.03, N 1.46. Found: C 85.07, H 9.08, N 1.49.
(9,9,9′,9′,9″,9″-Hexahexyl-7″-(4,4,5,5-tetramethyl-1,3,2-dioxa-
borolan-2-yl)-9H,9′H,9″H-2,2′:7′,2″-terfluoren-7-yl)diphenylamine
(F9). F9 was prepared according to the same procedure as F8 but
1
using compound F7 to give a light yellow solid. Yield: 50%. H NMR
(300 MHz, CDCl3, δ): 7.87−7.72 (m, 6H), 7.72−7.56 (m, 10H),
7.29−7.20 (m, 4H), 7.17−7.09 (m, 5H), 7.07−6.98 (m, 3H), 2.18−
1.82 (m, 12H), 1.40 (s, 12H), 1.22−0.99 (m, 36H), 0.90−0.61 (m,
30H). 13C NMR (75 MHz, CDCl3, δ): 152.60, 152.27, 151.96, 151.60,
150.38, 148.19, 147.28, 144.03, 141.21, 140.70, 140.63, 140.35, 140.07,
139.91, 136.21, 134.05, 129.38, 129.10, 126.27, 123.99, 123.80, 122.68,
121.71, 121.55, 121.42, 120.61, 120.18, 119.62, 119.27, 83.91, 55.51,
55.46, 55.39, 40.57, 40.49, 31.73, 31.68, 29.88, 25.18, 24.03, 22.80,
14.30. MALDI-TOF m/z: 1292.9. Anal. Calcd for C93H118BNO2: C
86.40, H 9.20, N 1.08. Found: C 86.66, H 9.31, N 1.12.
7″,7″″′-(Ethyne-1,2-diyl)bis(9,9,9′,9′,9″,9″-hexahexyl-N,N-di-
phenyl-9H,9′H,9″H-(2,2′:7′,2″-terfluoren)-7-amine) (A1). To a mix-
ture of A0 (0.34 g, 0.40 mmol), F8 (1,23 g, 1.27 mmol), Pd(PPh3)4
(0.028 g, 0.024 mmol), and 2 M K2CO3 (2.00 mL, 4.00 mmol) was
added degassed toluene (10 mL) and ethanol (2 mL). The solution
was heated to 100 °C and stirred at this temperature for 48 h under
argon. After cooling to room temperature, the solution was extracted
with CH2Cl2, and the organic layer was washed with brine and water
and then dried over anhydrous Na2SO4. After the solvent was
removed, the residue was purified by column chromatography on silica
gel using petroleum ether/dichloromethane (5:1 by vol) as the eluent
1
to give a yellow solid (0.67 g). Yield: 71%. H NMR (300 MHz,
CDCl3, δ): 7.85−7.78 (m, 5H), 7.78−7.72 (m, 2H), 7.72−7.61 (m,
16H), 7.61−7.57 (m, 7H), 7.31−7.22 (m, 8H), 7.18−7.11 (m, 10H),
7.08−6.98 (m, 8H), 2.17−1.82 (m, 24H), 1.21−0.98 (m, 72H), 0.90−
0.62 (m, 60H). 13C NMR (75 MHz, CDCl3, δ): 152.60, 151.98,
151.60, 151.28, 148.18, 147.28, 141.26, 141.18, 140.75, 140.45, 140.35,
140.02, 139.85, 136.18, 129.36, 126.42, 126.35, 126.26, 126.15, 123.99,
123.79, 122.66, 121.71, 121.66, 121.58, 121.42, 120.61, 120.47, 120.18,
119.95, 119.64, 119.56, 90.91, 55.52, 55.38, 40.70, 40.57, 40.49, 31.74,
31.67, 29.95, 29.87, 24.03, 22.84, 22.79, 14.28. MALDI-TOF m/z:
2356.3. Anal. Calcd for C176H212N2: C 89.74, H 9.07, N 1.19. Found:
C 89.48, H 9.27, N 1.23.
(7″-Bromo-9,9,9′,9′,9″,9″-hexahexyl-9H,9′H,9″H-2,2′:7′,2″-ter-
fluoren-7-yl)diphenylamine (F7). F7 was prepared according to the
same procedure as F6 but using compound F5 to give a yellow solid.
1
Yield: 88%. H NMR (300 MHz, CDCl3, δ): 7.83−7.73 (m, 4H),
7.73−7.56 (m, 10H), 7.51−7.45 (m, 2H), 7.30−7.21 (m, 4H), 7.17−
7.11 (m, 5H), 7.08−6.98 (m, 3H), 2.09−1.82 (m, 12H), 1.22−1.01
(m, 36H), 0.90−0.62 (m, 30H). 13C NMR (75 MHz, CDCl3, δ):
153.44, 152.61, 151.99, 151.62, 151.30, 148.20, 147.31, 141.22, 140.79,
140.41, 140.22, 140.03, 139.89, 139.41, 136.21, 130.22, 129.39, 126.47,
126.37, 126.31, 124.22, 124.01, 123.81, 122.69, 121.65, 121.61, 121.42,
121.30, 121.21, 120.66, 120.23, 119.63, 55.74, 55.54, 55.39, 40.56,
31.74, 31.70, 29.88, 24.05, 22.81, 14.66, 14.32. MALDI-TOF m/z:
1246.7. Anal. Calcd for C87H106BrN: C 83.88, H 8.58, N 1.12. Found:
C 83.81, H 8.44, N 1.16.
7‴,7″″″′-(Ethyne-1,2-diyl)bis(9,9,9′,9′,9″,9″,9‴,9‴-octahexyl-N,N-
diphenyl-9H,9′H,9″H,9‴H-(2,2′:7′,2″:7″,2‴-quaterfluoren)-7-amine)
(A2). A2 was prepared according to the same procedure as A1 but
1
using compounds A0 and F9 to give a yellow solid. Yield: 56%. H
NMR (300 MHz, CDCl3, δ): 7.87−7.76 (m, 12H), 7.76−7.57 (m,
30H), 7.31−7.22 (m, 8H), 7.18−7.11 (m, 10H), 7.08−6.97 (m, 8H),
2.19−1.81 (m, 32H), 1.21−0.95 (m, 96H), 0.90−0.60 (m, 80H). 13C
NMR (75 MHz, CDCl3, δ): 152.58, 152.12, 151.97, 148.16, 141.25,
140.71, 140.58, 140.48, 140.31, 140.22, 140.13, 140.04, 139.84, 136.17,
129.36, 126.51, 126.41, 126.32, 123.96, 122.65, 121.66, 121.57, 120.60,
120.47, 120.19, 119.59, 90.90, 55.65, 55.52, 40.71, 40.59, 31.73, 31.68,
29.95, 29.88, 24.21, 24.02, 22.85, 22.80, 14.29. MALDI-TOF m/z:
3021.6. Anal. Calcd for C226H276N2: C 89.86, H 9.21, N 0.93. Found:
C 90.17, H 9.36, N 1.05.
1,2,3,4,5,6-Hexakis(7″-(diphenylamino)-9,9,9′,9′,9″,9″-hexahex-
yl-9H,9′H,9″H-2,2′:7′,2″-terfluoren-7-yl)phenyl (HFB-diF-Dpa). To a
mixture of A1 (0.60 g, 0.26 mmol) and dicobaltoctacarbonyl (10 mg,
0.03 mmol) was added 60 mL of degassed 1,4-dioxane. The solution
was heated to 120 °C and stirred at this temperature for 24 h under
argon. After cooling to room temperature, the reaction mixture was
poured into water. The solution was extracted with CH2Cl2, and the
organic layer was dried over anhydrous Na2SO4. After the solvent was
removed, the residue was purified by column chromatography on silica
gel using petroleum ether/chloroform (10:1 by vol) as the eluent to
give a yellow powder (0.40 g). Yield: 67%. 1H NMR (300 MHz,
CDCl3, δ): 7.80−7.71 (m, 18H), 7.71−7.48 (m, 48H), 7.48−7.38 (m,
18H), 7.30−7.19 (m, 18H), 7.17−7.08 (m, 36H), 7.08−6.91 (m,
30H), 2.10−1.72 (m, 72H), 1.21−0.92 (m, 208H), 0.90−0.62 (m,
165H), 0.47−0.31 (m, 23H). 13C NMR (75 MHz, CDCl3, δ): 152.75,
152.55, 151.90, 151.77, 151.54, 148.13, 147.20, 140.52, 140.35, 140.26,
140.05, 139.92, 136.16, 129.34, 126.34, 126.18, 123.93, 123.76, 122.61,
(9,9,9′,9′-Tetrahexyl-7′-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-
2-yl)-9H,9′H-2,2′-bifluoren-7-yl)diphenylamine (F8). To a mixture of
F6 (4.83 g, 5.29 mmol), bis(pinacolato)diborane (1.61 g, 6.35 mmol),
and KOAc (3.14 g, 32 mmol), degassed 1,4-dioxane (25 mL) was
added under a flow of argon. Afterward, Pd(dppf)Cl2 (0.12 g, 0.16
mmol) was added. The solution was heated to 85 °C and stirring at
this temperature for 8 h under argon. After cooling to room
temperature, the reaction mixture was poured into ice water. The
solution was extracted with CH2Cl2, and the organic layer was washed
with brine and water and then dried over anhydrous Na2SO4. After the
solvent had been removed, the residue was purified by column
chromatography on silica gel using petroleum ether/dichloromethane
(2:1 by vol) as the eluent to give a white solid (3.73 g). Yield: 73%. 1H
NMR (300 MHz, CDCl3, δ): 7.85−7.70 (m, 4H), 7.70−7.55 (m, 6H),
7.29−7.22 (m, 4H), 7.17−7.11 (m, 5H), 7.07−6.98 (m, 3H), 2.07−
1.82 (m, 8H), 1.40 (s, 12H), 1.20−1.00 (m, 24H), 0.82−0.62 (m,
20H). 13C NMR (75 MHz, CDCl3, δ): 152.47, 152.11, 151.45, 150.22,
148.04, 147.13, 143.91, 141.05, 140.24, 140.06, 139.74, 136.04, 133.85,
129.23, 128.92, 127.23, 126.14, 126.01, 123.84, 123.64, 122.52, 121.43,
121.32, 120.46, 120.40, 119.47, 119.40, 119.09, 83.79, 55.30, 55.23,
40.32, 40.14, 40.05, 31.58, 31.54, 31.35, 31.22, 29.71, 29.55, 29.46,
25.04, 24.85, 24.82, 24.71, 23.87, 23.76, 23.64, 23.49, 22.66, 22.49,
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Scheme 1. Synthetic Routes of HFB-diF-Dpa and HFB-terF-Dpa
121.48, 121.36, 120.05, 119.57, 119.51, 55.36, 55.33, 54.71, 40.46,
31.77, 31.70, 31.58, 30.16, 29.81, 23.96, 23.82, 23.06, 22.76, 14.37,
14.29. MALDI-TOF m/z: 7066.5. Anal. Calcd for C528H636N6: C
89.74, H 9.07, N 1.19. Found: C 89.79, H 9.40, N 1.23.
1,2,3,4,5,6-Hexakis(7‴-(diphenylamino)-9,9,9′,9′,9″,9″,9‴,9‴-oc-
tahexy-9H,9′H,9″H,9‴H-2,2′:7′,2″:7″,2‴-quaterfluoren-7-yl)phenyl
(HFB-terF-Dpa). HFB-terF-Dpa was prepared according to the same
procedure as HFB-diF-Dpa but using compounds A2 to give a yellow
powder. Yield: 30% .1H NMR (300 MHz, CDCl3, δ): 7.87−7.74 (m,
24H), 7.74−7.50 (m, 72H), 7.50−7.38 (m, 24H), 7.32−7.18 (m,
18H), 7.18−7.08 (m, 36H), 7.08−6.85 (m, 30H), 2.19−1.77 (m,
96H), 1.23−0.96 (m, 306H), 0.96−0.66 (m, 209H), 0.50−0.30 (m,
13H). 13C NMR (75 MHz, CDCl3, δ): 152.66, 151.91, 151.78, 148.14,
147.25, 140.61, 140.33, 140.15, 140.05, 139.89, 139.87, 136.26, 129.34,
126.24, 123.94, 122.51, 121.63, 119.43, 55.46, 55.38, 54.73, 40.51,
40.48, 31.78, 31.70, 31.64, 29.84, 23.99, 23.07, 22.77, 14.38, 14.29,
14.27. MALDI-TOF m/z: 9060.0. Anal. Calcd for C678H828N6: C
89.86, H 9.21, N 0.93. Found: C 90.09, H 9.43, N 0.96.
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RESULTS AND DISCUSSION
■
Synthesis and Characterization. The new star-shaped
oligofluorenes HFB-diF-Dpa and HFB-terF-Dpa were synthe-
Figure 1. TGA thermograms of HFB-diF-Dpa and HFB-terF-Dpa
recorded at a heating rate of 10 °C min−1. Inset: DSC traces recorded
at a heating rate of 10 °C min−1.
Figure 3. UV−vis absorption and PL spectra of HFB-diF-Dpa and
HFB-terF-Dpa in (a) toluene solution at 5 × 10−6 M and (b) film
state.
Figure 2. AFM topographic images of HFB-diF-Dpa and HFB-terF-
Dpa in thin solid films.
sized by convergent core-creating approach as depicted in
Scheme 1. The stepwise preparation of the key intermediates of
alkyne derivatives A1 and A2 commenced with the synthesis of
F3 and F5, respectively, which were then subjected to the
selective amination of the aryl iodide to generate F6 and F7 by
a modified, mild Buchwald cross-coupling reaction.50 Sub-
sequent borylation using bis(pinacolato)diboron afforded the
desired boronic acid pinacol esters F8 and F9. By using Suzuki
cross-coupling reactions of building block A0 with the
corresponding oligofluorene boronic acid pinacol esters (F8
and F9), the alkyne derivatives of A1 and A2 were obtained.
Finally, the target compounds HFB-diF-Dpa and HFB-terF-
Dpa were successfully prepared by using dicobaltoctacarbonyl-
catalyzed cyclotrimerization of A1 and A2. The well-defined
structures and chemical purities of all intermediates and the
previously reported.42 The high Tg values can attribute to the
rigid hexakis(fluoren-2-yl)benzene core and the extending
molecular system.42,52
We also recorded atomic force microscopy (AFM) images of
the compounds on ITO/PEDOT substrate to investigate their
morphological stability (Figure 2). The thin film was prepared
by spin-coating and then annealed under N2 gas condition at
120 °C for 30 min. The annealed film has a fairly smooth
surface morphology with a root-mean-square (rms) roughness
of 0.338 for HFB-diF-Dpa and 0.380 nm for HFB-terF-Dpa.
The excellent thermal and morphological stability of these
materials enable the preparation of homogeneous and stable
amorphous thin films through solution processing.
Photophysical Properties. Figure 3 shows the absorption
and fluorescence spectra of HFB-diF-Dpa and HFB-terF-Dpa
both in toluene solution and solid state films. The photo-
physical data of the compounds are summarized in Table 1.
The absorption spectrum of each compound in toluene
solution shows absorption peak around 380 nm assigned to
the π−π* transition of the oligofluorenes, which reveals no red-
shift in solid state film. The thin film of the two compounds
display emission peaks at 429 nm and shoulder emissions at
452 nm, which only bathochromically shift 1−3 nm relative to
their solution spectra, indicating the absence of intermolecular
interactions in thin films. This can be attributed to the bulky
star-shaped structures that have effectively restrained inter-
molecular aggregation. In addtion, almost the same emission
peaks of the two compounds indicate that their emission tends
to be saturated with sufficiently long arms. The peak emissions
of HFB-diF-Dpa and HFB-terF-Dpa exhibit 12 and 20 nm
1
final star-shaped compounds were adequately verified by H
and 13C NMR spectroscopy, elemental analysis, and MALDI-
TOF mass spectrometry (see experimental section).
Thermal Properties. The good thermal stability of the
compounds is indicated by their high decomposition temper-
atures (Td, corresponding to 5% weight loss) of ca. 430 °C in
the thermogravimetric analysis (Figure 1). Their glass transition
temperatures (Tg) determined through differential scanning
calorimetry are 123 °C for HFB-diF-Dpa and 142 °C for HFB-
terF-Dpa (inset of Figure 1), which are significantly higher than
those of the corresponding three-armed oligofluorenes based
on a benzene core (Tg = 76−88 °C),46 and poly(9,9-
dihexylfluorene) (Tg = 103 °C).51 These values are also higher
than those of the six-armed oligofluorenes without diphenyl-
amine end-capper (102 °C for T2 and 114 °C for T3) we
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Table 1. Thermal, Photophysical, and Electrochemical data of HFB-diF-Dpa and HFB-terF-Dpa
a
b
c
d
c
d
e
f
g
compd
Tg (°C)
Td (°C)
λabs (nm)
λabs (nm)
λem,max (nm)
λem,max (nm)
HOMO /LUMO (eV)
ΦPL
HFB-diF-Dpa
HFB-terF-Dpa
123
142
430
378
379
380
380
426, 449
428, 447
429, 452
−5.10/−2.14
−5.10/−2.15
0.77
0.83
429
b
429, 453
d
a
c
e
Obtained from DSC measurements. Obtained from TGA measurements. Measured in toluene. Measured in film. Determined from the onset of
oxidation potentials. Deduced from HOMO and Eg estimated from the red edge of the longest absorption wavelength for the solid-film sample.
Fluorescence quantum yields in solid state films, measured on the quartz plate using an integrating sphere.
f
g
Figure 4. Cyclic voltammograms of HFB-diF-Dpa and HFB-terF-Dpa
in CH2Cl2 for oxidation.
Figure 6. (a) Current efficiency and EQE versus luninance curves and
(b) current density−voltage−brightness (J−V−L) characteristics for
devices A and B.
integrating sphere are 0.77 for HFB-diF-Dpa and 0.83 for
HFB-terF-Dpa.
Electrochemical Properties. The electrochemical behav-
iors of the compounds were probed by cyclic voltammetry
(Figure 4). Both HFB-diF-Dpa and HFB-terF-Dpa exhibit the
reversible oxidation process. The oxidation wave around 0.4 V
can be attributed to the oxidation of the terminal diphenyl-
amine groups, while oxidation waves beyond 0.5 V can be
attributed to the oxidation of oligofluorene arms. HFB-terF-
Dpa shows the second oxidation peak at 0.78 V that is slightly
lower than 0.80 V of HFB-diF-Dpa, owing to the extended
conjugation of the former. In addition, the two compounds
exhibit similar oxidation peaks of oligofluorenes to their
Figure 5. Normalized EL spectra of devices (a) A and (b) B recorded
at various driving voltage.
analogues without diphenylamine end-capper.42 The highest
occupied molecular orbital (HOMO) energy levels estimated
from the onsets of the oxidation potentials are −5.10 eV for
redshifts in comparsion with their six-armed analogues without
diphenylamine end-capper (409 nm for T2 and 417 nm for
T3).42 This suggests the excitation on the diphenylamine end
groups. Fluorescence quantum yields (ΦPL) of these materials
in the solid state measured on the quartz plate using an
both the two compounds with regard to the energy level of
ferrocene (4.8 eV below vacuum), which are 0.46 and 0.41 eV
higher than those of their corresponding analogues without
diphenylamine end-capper (5.56 eV for T2 and 5.51 eV for
T3).42 Noticeably, the HOMO levels of the two compounds
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Table 2. Electroluminescence Characteristics of the Devices
b
c
e
Lmax (cd m−2), voltage
ηc,max (cd A−1),
ηp, max (lm W−1),
a
d
f
device
A
emitter
HIL
Von (V)
(V)
L (cd m−2
)
ηext,max (%)
L (cd m−2
)
CIE (x,y)
HFB-diF-
Dpa
PEDOT:PSS4083
3.6
9524, 9.2
5.80, 53.6
5.30
4.78, 8.2
0.149,
0.104
5.72 0.08
3.97, 339.3
3.91 0.06
6.99, 1.5
5.27 0.04
3.70
4.71 0.07
2.91, 3.5
B
HFB-terF-
Dpa
PEDOT:PSS4083
PEDOT:PSS8000
PEDOT:PSS8000
3.6
3.6
3.6
8596, 9.8
0.151,
0.101
3.64 0.06
5.45
2.78 0.13
6.10, 1.5
C
D
HFB-diF-
Dpa
6378, 12.0
0.154,
0.136
6.86 0.14
6.73, 1.7
5.41 0.04
4.86
6.02 0.08
5.87, 1.7
HFB-terF-
Dpa
6057, 11.8
0.154,
0.150
6.64 0.10
c
4.75 0.11
5.74 0.13
a
b
Turn-on voltage, recorded at the brightness of 1 cd m−2. Maximum luminance. Maximum current efficiency, statistics (italic) based on 10 cells of
each type. Maximum external quantum efficiency, statistics (italic) based on 10 cells of each type. Maximum power efficiency, statistics (italic)
based on 10 cells of each type. Measured at 7 V.
d
e
f
match well with that of the hole-injecting material poly(3,4-
ethylenedioxythiopene):poly(styrenesulfonate) (PEDOT:PSS)
(−5.10 eV) and would result in an efficient hole-injection from
PEDOT:PSS to the emissive layer. The band gaps (Eg) of
HFB-diF-Dpa and HFB-terF-Dpa, on the basis of the red edge
of the longest absorption wavelength for the solid-film sample,
were estimated to be 2.96 and 2.95 eV, respectively. Previous
researchers had done systematical studies of the electro-
chemical and electronic properties for oligofluorenes. They
demonstrated that the reduction behavior could not be
observed in CV for oligofluorenes because a stable anionic
state was not experimentally accessible.53 In this work, we have
scanned the reduction curves in THF for these multibranched
oligofluorenes, and we also have not observed their electro-
chemical reduction behavior. Thus, the lowest unoccupied
molecular orbital (LUMO) levels are deduced to be −2.15 eV
by the relation LUMO = HOMO − Eg (Table 1).
Deep-Blue Electroluminescence. Both HFB-diF-Dpa
and HFB-terF-Dpa exhibit deep-blue emission, high fluo-
rescence quantum yield and good solubility in common
solvents. To evaluate EL properties of these materials, simple
double-layer devices, featuring the two compounds as non-
doped blue emitters, were fabricated with the configurations of
indium tin oxide (ITO)/PEDOT:PSS4083 (40 nm)/emission
layer (EML, 50 nm)/1,3,5-tris(N-phenylbenzimidazol-2-yl)-
benzene (TPBI, 30 nm)/lithium fluoride (LiF, 1 nm)/
aluminum (Al, 100 nm) (emitter: HFB-diF-Dpa, device A;
HFB-terF-Dpa, device B), in which the emission layer was
spin-coated from chlorobenzene solution. PEDOT:PSS4083 and
LiF served as hole- and electron-injecting layers, respectively.
TPBI was inserted between the EML and LiF as the electron-
transporting layer.
The devices based on these emitters exhibit deep-blue
emission with emission maxima of 456 nm and CIE coordinates
of (0.149, 0.104) for device A and (0.151, 0.101) for device B
(Figure 5). The EL spectra of these materials are similar to their
PL spectra in the thin film state, indicating that the formation of
excimer or exciplex is effectively suppressed during the EL
process. Noticeably, these devices exhibit striking blue EL color
stability at various driving voltages. With increasing driving
voltage from 4 to 8 V, the EL spectra of all these devices remain
nearly unchanged, and the CIE coordinate values show
negligible variation, suggesting a remarkable voltage-independ-
ent EL emission. The excellent color stability can be attributed
to the bulky star-shaped structures that may suppress the close-
packing of molecules in the solid state and then improve the
morphological stability that would resist crystallization and
morphological transition-induced deterioration during the
device operation.
Figure 6 shows the current density−voltage−brightness (J−
V−L) characteristics and efficiency versus luminance curves of
the devices (Table 2). Both two devices have a low turn-on
voltage of 3.6 V, which are much lower than those of their
corresponding diphenylamine-absent analogues (7.8 V for T2-
based device and 6.9 V for T3-based device).42 This should be
attributed to the high-lying HOMO levels of these materials,
which match well with the HOMO level of adjacent
PEDOT:PSS and thereby facilitate hole injection of the device.
Device B exhibits good performance with a maximum current
efficiency of 3.97 cd A−1, a maximum power efficiency of 2.91
lm W−1, and a maximum external quantum efficiency of 3.70%.
Device A achieves excellent performance with a maximum
luminance of 9524 cd m−2 at 9.2 V, a maximum current
efficiency of 5.80 cd A−1, a maximum power efficiency of 4.78
lm W−1, and a maximum external quantum efficiency of 5.30%.
The superior efficiency of devices A to B could be attributed to
the following facts: (i) HFB-diF-Dpa possesses more smooth
surface morphology (0.338 nm) than HFB-terF-Dpa (0.380
nm); (ii) HFB-diF-Dpa exhibits lower hole mobility than
HFB-terF-Dpa, and thus, the charge flux in device A tends to
be more balanced in comparison with device B. Notably, upon
increasing to the practical brightness of 100 cd m−2, the current
and external quantum efficiency of device A are 5.73 cd A−1 and
5.25%, respectively, with a roll-off value of below 2%. At a high
brightness of 1000 cd m−2, the roll-off value of external
quantum efficiency is only 12%. Even at the brightness of 5000
cd m−2, the current and external quantum efficiency of device A
is still above 50% of the maximum value. Although device B
shows inferior performances to device A, it exhibits much lower
efficiency roll-off. At a high brightness of 1000 cd m−2, the roll-
off values of current and external quantum efficiency are 7.8%
and 6.2%, respectively.
To further improve the EL efficiency, we fabricated HFB-
diF-Dpa-based device C and HFB-terF-Dpa-based device D in
the same device configurations except using PEDOT:PSS8000
with low electrical conductivity of ∼10−5 S cm−1 to replace
PEDOT:PSS4083 (∼10−3 S cm−1). It was reported that the
device with PEDOT:PSS8000 presented more efficient attenu-
ation of hole flux and control over electrical leakage.54 The J−
V−L characteristics and efficiency versus luminance curves are
shown in Figure 7. The current densities in device C and D are
lower than those of the PEDOT:PSS4083 devices, which can be
attributed to the decreased leakage current and balanced charge
flux. Remarkably, the current/power efficiencies are elevated to
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C and D show a lighter blue color than devices A and B. In
addition, device D exhibits more significant improvement than
device C. This can be elucidated as follows: when using
PEDOT:PSS8000 with low electrical conductivity as the hole-
injecting and transporting layers, more balanced charge flux
may be achieved in device D owing to the superior hole
mobility of HFB-terF-Dpa to HFB-diF-Dpa. On the contrary,
the charge flux in device A tends to be more balanced when
using PEDOT:PSS4083 with relative high electrical conductivity
as hole-injecting and transporting layers as we discussed before.
To the best of our knowledge, these efficiencies are among
the highest for nondoped deep-blue OLEDs based on solution-
processed materials.34−42,55−65 For example, the device using
TCTA-cored oligofluorenes as emitters acquired a ηc,max of 0.47
cd A−1 and the CIE coordinates of (0.16, 0.07);35 the device
based on six-oligofluorene-arm triazatruxenes showed ηc,max of
2.07 cd A−1 and CIE coordinates of (0.15, 0.09);34 the device
based on 1,3,5-tri(anthracen-10-yl)-benzene starburst oligo-
fluorenes showed ηc,max of 1.80 cd A−1 and CIE coordinates of
(0.15, 0.10);39 the device based on the star-shaped oligo-
fluorenes with a planar triphenylamine core obtained ηc,max of
3.83 cd A−1 and CIE coordinates of (0.15, 0.09).40 It is
noteworthy that the low driving voltage and high EQE provides
an outstanding maximum power efficiency of 6.10 lm W−1,
which is the highest for nondoped deep-blue OLEDs based on
solution-processed materials.
The unencapsulated devices can be lighted for 3 weeks in the
glovebox. When the devices were exposed to the air, they
remained to be emissive in 1 week, and the brightness was
gradually decreased.
CONCLUSIONS
■
In summary, we have designed and synthesized two new star-
shaped oligofluorenes, HFB-diF-Dpa and HFB-terF-Dpa, with
a hexakis(fluoren-2-yl)benzene core and six diphenylamine
end-cappers. The two compounds show excellent thermal
stabilities and morphological stability, pronounced PL
efficiencies, and good solution-processability. Double-layer
nondoped OLEDs by solution process exhibit highly efficient
and stable deep-blue electroluminescence. The HFB-diF-Dpa-
based device achieves a maximum current efficiency of 6.99 cd
A−1, a maximum external quantum efficiency of 5.45%.
Noticeably, the low driving voltage and high EQE result in a
maximum power efficiency of 6.10 lm W−1, which is the highest
for nondoped deep-blue OLEDs based on solution-processed
materials. Moreover, these devices present small values of
efficiency roll-off at high brightness up to 1000 cd m−2. Our
findings provide a valuable strategy to the rational design and
development of efficient blue oligofluorenes and other related
materials.
Figure 7. (a) Current efficiency and EQE versus luninance curves, (b)
current density−voltage−brightness (J−V−L) characteristics, and (c)
normalized EL spectra recorded at 7 V for devices C and D.
a higher level. Device C shows a maximum current efficiency of
6.99 cd A−1, a maximum external quantum efficiency of 5.45%,
and a maximum power efficiency of 6.10 lm W−1, with the CIE
coordinate of (0.154, 0.136). The HFB-terF-Dpa-based device
D exhibits a maximum current efficiency of 6.73 cd A−1, a
maximum external quantum efficiency of 4.86%, and a
maximum power efficiency of 5.87 lm W−1, with the CIE
coordinate of (0.154, 0.150). This drastic improvement could
be rationalized from the fact that excess hole carriers are
retarded to transport into the EML. However, because of the
inferior electrical conductivity of PEDOT:PSS8000, the
recombination zones in devices C and D may shift toward
the interface of HTL/EML, resulting in the formation of some
excimers and/or exciplexes (Figure 7c). Consequently, devices
AUTHOR INFORMATION
■
Corresponding Authors
*(C.Y.) E-mail: clyang@whu.edu.cn.
*(D.M.) E-mail: mdg1014@ciac.jl.cn.
Author Contributions
§(C.L. and Q.F.) These authors contributed equally to this
work.
Notes
The authors declare no competing financial interest.
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(31) List, E. J.; Guentner, R.; Scanducci de Freitas, P.; Scherf, U. Adv.
Mater. 2002, 14, 374.
ACKNOWLEDGMENTS
■
We thank the National Basic Research Program of China (973
Program 2013CB834805), the National Science Fund for
Distinguished Young Scholars of China (No.51125013), and
the Research Fund for the Doctoral Program of Higher
Education of China (20120141110029) for financial support.
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