Novel violet emitting material synthesized by stepwise chemical reactions

Article abstract of DOI:10.1039/c4tc00456f

In this paper, we report the design and synthesis of a novel bipolar violet emitting molecule CzPySiSF. The carbazole and pyridine moieties are employed to facilitate charge injection and balance carrier transport, whereas the spirofluorene is used as the violet emitter. The three functional groups, i.e., carbazole, pyridine and spirofluorene, are connected to tetraphenylsilane in a stepwise fashion using classical coupling reactions such as Suzuki and Ullmann. The resultant bipolar molecule CzPySiSF exhibits very stable thermal properties and a uniform amorphous morphology. The introduction of spirofluorene greatly enhances the fluorescence quantum efficiency of the molecule; moreover the PL spectrum of CzPySiSF in THF is mainly located in the violet region. The EL spectrum of CzPySiSF matches well with the PL spectrum with a maximum at 408 nm. The violet OLED of CzPySiSF exhibits the maximum external quantum efficiency of 0.59%. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4tc00456f

Journal of
Materials Chemistry C
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PAPER
Novel violet emitting material synthesized by
stepwise chemical reactions†
Cite this: J. Mater. Chem. C, 2014, 2,
5019
Xiangyang Tang, Liang Yao, He Liu, Fangzhong Shen, Shitong Zhang, Yunan Zhang,
*
Huanhuan Zhang, Ping Lu and Yuguang Ma
In this paper, we report the design and synthesis of a novel bipolar violet emitting molecule CzPySiSF. The
carbazole and pyridine moieties are employed to facilitate charge injection and balance carrier transport,
whereas the spirofluorene is used as the violet emitter. The three functional groups, i.e., carbazole,
pyridine and spirofluorene, are connected to tetraphenylsilane in a stepwise fashion using classical
coupling reactions such as Suzuki and Ullmann. The resultant bipolar molecule CzPySiSF exhibits very
stable thermal properties and a uniform amorphous morphology. The introduction of spirofluorene
greatly enhances the fluorescence quantum efficiency of the molecule; moreover the PL spectrum of
CzPySiSF in THF is mainly located in the violet region. The EL spectrum of CzPySiSF matches well with
the PL spectrum with a maximum at 408 nm. The violet OLED of CzPySiSF exhibits the maximum
external quantum efficiency of 0.59%.
Received 7th March 2014
Accepted 16th April 2014
DOI: 10.1039/c4tc00456f
www.rsc.org/MaterialsC
caused by the difficulty of charge injection and/or lopsided
carrier transport that produces interface emission or energy
transfer from the emitter to the adjacent layers.20 Therefore,
judicious molecular design that can achieve a compromise
between the above requirements is imperative.
Introduction
Efficient violet emitting materials play an important role in the
eld of organic light-emitting devices (OLEDs) as efficient violet
emitters can be utilized to generate light over the entire visible
region and to emit white light either by irradiation of proper
luminescent dyes or by an energy transfer process from host
materials to uorescent and phosphorescent dyes.1–9 However,
in contrast with the rapid advancement of visible light
OLEDs,10–13 the development of violet OLEDs has not made any
signicant progress. To date, the majority of violet emitting
small molecules are based on uorene, carbazole or purine
derivatives.14–20 Among these reported molecules, only a handful
can simultaneously achieve emission below 400 nm and high
external quantum efficiency (EQE).14 The scarcity of efficient
violet small molecules is due to conicting demands on the
molecular design. On the one hand, to attain violet emission,
the effective conjugation length of the molecule must be
restricted, which would usually result in constraining the
molecular size to a certain scale. Unfortunately, the resultant
limited molecular size would lead to unstable thermal proper-
ties and inability to form stable and uniform amorphous thin
lms, all of which is detrimental for device operation. On the
other hand, the wide band gap of violet molecules will make it
onerous for device fabrication and oen causes undesirable EL
emission in the long-wavelength region. This effect may be
Accordingly, some intriguing properties observed in tetra-
phenylsilane may shed light on the molecular design of violet
emitting material.21 The band gap of tetraphenylsilane is as wide
as 4.0 eV and its derivatives usually exhibit high thermal stability
and the tendency to form stable amorphous lms.22 Hence, by
introducing a violet uorophore such as uorene to tetraphe-
nylsilane, it is possible to obtain thermally stable and amor-
phous violet emitters. However, this alone is not adequate for
applications of violet OLEDs. The extremely wide band gap of
tetraphenylsilane makes it difficult to achieve efficient charge
injection and carrier transport. To resolve this problem, a
general method is to adopt bipolar design tactics, i.e., to incor-
porate electron rich and electron decient moieties into one
molecule simultaneously. Generally, it is inevitable to prolong
the molecular conjugation and to cause a possible CT effect, both
of which are adverse for the realization of violet emission.23,24
Luckily, these problems can be prevented if the electron rich and
electron decient moieties are attached to different phenyl
groups of tetraphenylsilane as a result of d-Si interaction and the
relatively large size of tetraphenylsilane. As such, carbazole and
pyridine were chosen as the hole/electron transport groups
because they have suitable energy levels that are favorable for
charge injection and carrier transport.25–27 Moreover, spiro-
uorene (SF) is chosen as the violet emitter due to its highly
efficient violet emission.28 Note that all three functional groups
are connected to different phenyl groups of tetraphenylsilane.
State Key Laboratory of Supramolecular Structure and Materials, Jilin University,
Changchun, 130012, P.R. China. E-mail: lup@jlu.edu.cn; Fax: +86 431 85193421;
Tel: +86 431 85167057
† Electronic supplementary information (ESI) available: 1H NMR attribution,
detailed thermal, photophysical and EL properties. See DOI: 10.1039/c4tc00456f
This journal is © The Royal Society of Chemistry 2014
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We anticipate that the three functional groups (carbazole, pyri-
dine and spirouorene) can work independently without inter-
fering with each other; i.e., the molecule can retain the functions
of each of the three groups: facilitating charge injection,
balancing carrier transport and achieving efficient violet emis-
sion. Moreover, we hope that as an adduct of carbazole, pyridine,
spirouorene and tetraphenylsilane, the molecule will exhibit
some integrated properties such as good thermal stability and
stable amorphous morphology. Tetraphenylsilanes with three
different kindsofsubstituentshave not been reported thusfar, to
the best of our knowledge. Although the synthesis of such a
molecule is quite challenging, it is still possible to fulll this goal
considering the multiple reaction sites of tetraphenylsilane. In a
chemical reaction between 1,4-dibromobenzene and
Ph(4ꢀx)SiClx (x ¼ 1, 2, 3, 4), the phenyl group of tetraphenylsilane
can react with a Br atom, which acts as the reaction site. From the
perspective of synthetic chemistry, it is possible to add three
different functional groups to the tetraphenylsilane core by
means of classical coupling reactions such as Suzuki and Ull-
mann.29,30 Accordingly, we have succeeded in synthesizing a
bipolar violet molecule, CzPySiSF, by the sequential attachment
of carbazole, spirouorene and pyridine moieties to the tetra-
phenylsilane core. For a more comprehensive study, the hole-
type molecule CzSiSF and electron-type molecule PySiSF were
also synthesized.
Experimental section
Materials
Fig. 1 The TGA (a) and DSC (b) of CzSiSF, PySiSF and CzPySiSF.
All of the compounds in this work contain a tetraphenylsilane
core. Tetrahydrofuran (THF) and diethyl ether were dried and
puried by fractional distillation over sodium in the presence of
benzophenone. All other chemicals were purchased from Acros
and used without further purication.
N,N-dimethylformamide (DMF) or CH2Cl2 as the electrolyte.
The devices were fabricated by vacuum evaporation. The ITO-
coated glass substrates were sequentially cleaned in an ultra-
sonic bath with toluene, acetone, ethanol, and deionized water.
They were then dried with nitrogen and nally irradiated in a
UV-ozone chamber. The organic materials were sequentially
deposited onto cleaned ITO glass substrates by thermal evapo-
ration. The layer thickness of the deposited material was
monitored in situ using an oscillating-quartz thickness monitor.
Finally, a LiF buffer layer and an Al cathode were deposited onto
the organic lms. The background pressure of the chamber was
less than 10ꢀ6 Torr during the deposition process. The lumi-
nance–current characteristics of devices were measured using a
PR650 spectroscan spectrometer. The current–voltage charac-
teristics were studied using a Keithley 2400 sourcemeter. All of
the device measurements were carried out at room temperature
under ambient conditions.
General characterization
The 1H and 13C NMR spectra were recorded using a Bruker
AVANCE 500 spectrometer at 500 MHz and 125 MHz, respec-
tively, at 298 K using CDCl3 as the solvent and tetramethylsilane
(TMS) as the internal standard. The elemental analysis was
performed on a Flash EA 1112, CHNS–O elemental analysis
instrument. The MALDI-TOF-MS mass spectra were recorded
using an AXIMA-CFR™ plus instrument. The optical rotation a
was measured using a Shen Guang SGW-2 automatic polarim-
eter. Thermogravimetric analysis (TGA) was performed on a
Perkin-Elmer thermal analysis system at a heating rate of 10 ꢁC
minꢀ1 at a nitrogen ow rate of 80 mL minꢀ1 (Fig. 1). DSC
analysis was carried out using a NETZSCH (DSC-204) instru-
ment at 10 K minꢀ1 under a nitrogen ow. UV-vis and uores-
Device fabrications
cence spectra were recorded by
a Shimadzu UV-3100
spectrophotometer and a Shimadzu RF-5301PC spectropho- Devices consisting of ITO/PEDOT:PSS (40 nm)/NPB (80 nm)/
tometer, respectively, using 1 cm path-length quartz cells. TCTA (10 nm)/emitting-layer (30 nm)/TPBi (30 nm)/LiF (0.5 nm)/
Electrochemical measurements were performed using a BAS Al (100 nm) were fabricated as follows: PEDOT:PSS was spin-
100W Bioanalytical System: a glass-carbon disc electrode was coated onto the cleaned ITO-coated glass electronic substrate
used as the working electrode, a Pt wire as the counter elec- from its aqueous solution and then heated to 120 ꢁC for
trode, Ag/Ag+ as the reference electrode and Bu4NPF6 (0.1 M) in 20 minutes to remove the residual water solvent. Then, the
5020 | J. Mater. Chem. C, 2014, 2, 5019–5027
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organic layers, 0.5 nm thick LiF, and 100 nm thick Al were in (6H, d, J ¼ 8.20 Hz, Ar-H), 7.49–7.45 (3H, m, Ar-H), 7.41–7.39
deposited in sequence by thermal evaporation at a pressure of (2H, d, J ¼ 7.41 Hz, Ar-H), 7.37–7.35 (6H, d, J ¼ 7.72 Hz, Ar-H).
4.0 ꢂ 10ꢀ6 mbar.
13C NMR (125 MHz, CDCl3) d (ppm): 137.76, 136.18, 132.46,
132.09, 131.40, 130.27, 128.27, 125.26. MALDI-TOF (m/z): [M+]
calcd: 569.86; found: 570.00.
Synthesis
SFB. Suzuki reaction: in a 100 mL round-bottom ask, a
(4-Bromophenyl)triphenylsilane (1). In a 250 mL round- mixture of 2-bromo-9,90-spirobi[9H-uorene] (1.18 g, 3.0 mmol),
bottom ask, 1,4-dibromobenzene (7.02 g, 30 mmol) was dis- bis(pinacolato)diboron (1.15 g, 4.5 mmol, 1.5 equiv.),
solved in dry diethyl ether. The solution was frozen into solid [1,10-bis(diphenylphosphino)ferrocene]dichloropalladium(II)
form followed by expelling air from the system for about 5 (70 mg, 0.09 mmol, 0.03 equiv.), KOAc (885 mg, 9.0 mmol, 3.0
minutes, then thawing diethyl ether completely under nitrogen equiv.), and 50 mL 1,4-dioxane was degassed, then stirred and
atmosphere, and then freezing the solution into a solid form reuxed at 90 ꢁC for 48 h under nitrogen atmosphere. Water was
again to expel air. Aer that, the solution was cooled to 0 ꢁC in added to quench the reaction, and the mixture was then
an ice/water bath. Subsequently, n-BuLi (2.4 M, 13.75 mL, extracted with dichloromethane. Aer evaporating the solvent,
33 mmol, 1.1 equiv.) was added into the solution dropwise via a the crude product was puried by column chromatography
syringe. Aer stirring for 3 h, chlorotriphenylsilane (8.82 g, (petroleum ether : dichloromethane ¼ 3 : 1) to give the product
30 mmol) was dissolved in dry diethyl ether and then added as a white solid. Yield: (1.32 g, 95%). 1H NMR (500 MHz, CDCl3)
dropwise using a syringe. The reaction system was then stirred d (ppm): 7.88–7.82 (5H, m, Ar-H), 7.35 (3H, td, J ¼ 7.50, 3.70 Hz,
overnight at room temperature. Upon completion, the reaction Ar-H), 7.19 (1H, s, Ar-H), 7.13–7.06 (3H, m, Ar-H), 6.69 (3H, dd, J
mixture was poured into 100 mL water and extracted with ¼ 14.70, 7.60 Hz, Ar-H), 1.25 (12H, s, Me). 13C NMR (125 MHz,
diethyl ether. The combined organic solution was washed with CDCl3) d (ppm): 149.73, 148.61, 147.70, 144.92, 141.93, 141.34,
water, and dried over anhydrous magnesium sulphate. The 134.80, 130.27, 128.33, 127.77, 127.65, 124.16, 123.97, 120.39,
crude product was puried by recrystallization in ethanol as a 120.02, 119.34, 83.70, 24.82. MALDI-TOF (m/z): [M+] calcd:
1
white solid. Yield: (9.82 g, 79%). H NMR (500 MHz, CDCl3) d 442.21; found: 442.10.
(ppm): 7.55–7.53 (6H, d, J ¼ 7.02 Hz, Ar-H), 7.51–7.50 (2H, d, J ¼
CzSiBr. Ullmann reaction: in a 100 mL round-bottom ask, a
8.55 Hz, Ar-H), 7.45–7.41 (5H, m, Ar-H), 7.39–7.36 (6H, t, J ¼ 7.32 mixture of bis(4-bromophenyl)diphenylsilane (2.21 g, 4.5 mmol,
Hz, Ar-H). 13C NMR (125 MHz, CDCl3) d (ppm): 138.36, 136.74, 1.5 equiv.), carbazole (501 mg, 3 mmol), CuI (57 mg, 0.3 mmol,
134.04, 133.72, 131.52, 130.23, 128.41, 125.17. MALDI-TOF 0.1 equiv.), K3PO4 (1.4 g, 6.6 mmol, 2.2 equiv.), (ꢃ)-trans-1,2-
(m/z): [M+] calcd: 414.04; found: 414.30.
diaminocyclohexane (34.2 mg, 0.3 mmol, 0.1 equiv.), 15 mL
Bis(4-bromophenyl)diphenylsilane (2). In a 250 mL round- toluene was degassed, then stirred and reuxed at 110 ꢁC for 24
bottom ask, 1,4-dibromobenzene (11.7 g, 50 mmol) was dis- h under a nitrogen atmosphere. Aer cooling to room temper-
solved in dry diethyl ether. The solution was frozen into solid ature, the mixture was poured into a (NH4)2CO3 solution and
form followed by expelling air from the system for about extracted with dichloromethane. The combined organic
5 minutes, then thawing diethyl ether completely under solution was then washed with water and dried over
nitrogen atmosphere, and then freezing the solution into a solid anhydrous magnesium sulphate. Aer evaporating the solvent,
foꢁrm again to expel air. Aer that, the solution was cooled to the residue was puried by column chromatography (petroleum
0 C in an ice/water bath. Subsequently, n-BuLi (2.4 M, 23 mL, ether : dichloromethane ¼ 8 : 1)to give the product as a white
55 mmol, 1.1 equiv.) was added into the solution dropwise via a solid. Yield: (960 mg, 55%). 1H NMR (500 MHz, CDCl3) d (ppm):
syringe. Aer stirring for 3 h, dichlorodiphenylsilane (5.27 mL, 8.14 (2H, d, J ¼ 7.80 Hz, Ar-H), 7.78 (2H, d, J ¼ 8.10 Hz, Ar-H),
25 mmol) was added dropwise in one portion. The reaction 7.62 (6H, t, J ¼ 6.90 Hz, Ar-H), 7.57 (2H, d, J ¼ 8.20 Hz, Ar-H),
system was then stirred overnight at room temperature. Upon 7.49 (6H, dd, J ¼ 9.80, 4.70 Hz, Ar-H), 7.46–7.39 (6H, m, Ar-H),
completion, the reaction mixture was poured into 100 mL water 7.32–7.27 (2H, m, Ar-H). 13C NMR (125 MHz, CDCl3) d (ppm):
and extracted with diethyl ether. The combined organic solu- 140.47, 139.15, 137.88, 137.74, 136.28, 133.22, 132.86, 131.21,
tion was washed with water, and dried over anhydrous 129.97, 128.10, 126.16, 125.90, 124.93, 123.48, 120.27, 120.07,
magnesium sulphate. The crude product was puried by 109.81. MALDI-TOF (m/z): [M+] calcd: 579.10; found: 579.00.
recrystallization in ethanol as a white solid. Yield: (9.88 g, 80%).
CzSiBr2. Ullmann reaction: compound tris(4-bromophenyl)-
1H NMR (500 MHz, CDCl3) d (ppm): 7.46–7.44 (8H, m, Ar-H), phenylsilane (2.57 g, 4.5 mmol, 1.5 equiv.) was reacted with
7.40–7.37 (2H, t, J ¼ 7.32 Hz, Ar-H), 7.33–7.30 (8H, m, Ar-H). 13
C
carbazole (501 mg, 3 mmol) by the same Ullmann reaction
NMR (125 MHz, CDCl3) d (ppm): 140.38, 138.77, 135.56, 135.21, procedure as in the synthesis of CzSiBr, and the product was
133.78, 132.56, 130.66, 127.53. MALDI-TOF (m/z): [M+] calcd: puried by column chromatography and eluted with a mixture
491.95; found: 492.10.
of petroleum ether : dichloromethane ¼ 8 : 1 to give the
1
Tris(4-bromophenyl)phenylsilane (3). Tris(4-bromophenyl)- product as a white solid. Yield: (605 mg, 30%). H NMR (500
phenylsilane was prepared in the same manner as 1,4-dibro- MHz, CDCl3) d (ppm): 8.15 (2H, d, J ¼ 7.70 Hz, Ar-H), 7.75 (2H,
mobenzene; however, 1,4-dibromobenzene (11.7 g, 50 mmol) d, J ¼ 8.30 Hz, Ar-H), 7.64–7.56 (8H, m, Ar-H), 7.51–7.39 (11H,
and trichlorophenylsilane (2.67 mL, 16.67 mmol) were used as m, Ar-H), 7.32–7.28 (2H, m, Ar-H). 13C NMR (125 MHz, CDCl3) d
the starting materials. The product was a white solid. Yield: (ppm): 140.55, 137.95, 137.83, 136.45, 136.36, 132.78, 132.42,
(7.81 g, 82%). 1H NMR (500 MHz, CDCl3) d (ppm): 7.53–7.52 132.32, 131.51, 130.35, 128.39, 126.40, 126.09, 125.34, 123.68,
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120.47, 120.31, 109.93. MALDI-TOF (m/z): [M+] calcd: 657.01; 126.83, 126.43, 124.14, 124.10, 122.79, 120.33, 120.11, 120.04.
found: 657.00.
MALDI-TOF (m/z): [M+] calcd: 650.24; found: 650.50. Anal. calcd
SFSiBr. Suzuki reaction: in a 100 mL round-bottom ask, a for C49H34Si: C, 90.42; H, 5.27%. Found: C, 90.58; H, 5.24%.
mixture of bis(4-bromophenyl)diphenylsilane (2.21 g, 4.5 mmol, PySiSF. Suzuki reaction: in a 50 mL round-bottom ask,
1.5 equiv.), SFB (1.33 g, 3 mmol), Na2CO3 (2 M, 2.1 g, 20 mmol), compound SFSiBr (728 mg, 1 mmol) was reacted with 4-pyr-
Pd(PPh3)4 (65 mg, 0.06 mmol, 0.02 equiv.), distilled water (10 idinylboronic acid (185 mg, 1.5 mmol, 1.5 equiv.) for 2 days at
mL), toluene (20 mL) was degassed, then stirred and reuxed at 95 ꢁC. The rest of the procedure was the same as for the
85 ꢁC for 24 h under a nitrogen atmosphere. Water was added to synthesis of SFSiBr, and the product was puried by column
quench the reaction, and the mixture was then extracted with chromatography (dichloromethane : ethyl acetate ¼ 8 : 1) to
dichloromethane. The organic solution was then washed with give the product as a white solid. Yield: (627 mg, 86%). 1H NMR
water and dried over anhydrous magnesium sulphate. Aer (500 MHz, CDCl3) d (ppm): 8.66 (2H, d, J ¼ 4.60 Hz, Ar-H), 7.91
evaporating the solvent, the residue was puried by column (1H, d, J ¼ 8.00 Hz, Ar-H), 7.85 (3H, dd, J ¼ 12.70, 7.60 Hz, Ar-H),
chromatography (petroleum ether : dichloromethane ¼ 8 : 1) to 7.63 (5H, dt, J ¼ 15.40, 8.20 Hz, Ar-H), 7.54 (8H, ddd, J ¼ 16.60,
1
give the product as a white solid. Yield: (1.18 g, 55%). H NMR 10.70, 5.50 Hz, Ar-H), 7.48–7.33 (11H, m, Ar-H), 7.11 (3H, qd, J ¼
(500 MHz, CDCl3) d (ppm): 7.94 (1H, d, J ¼ 8.00 Hz, Ar-H), 7.89 7.50, 0.90 Hz, Ar-H), 6.95 (1H, d, J ¼ 1.30 Hz, Ar-H), 6.75 (3H, dd,
(3H, dd, J ¼ 12.00, 7.70 Hz, Ar-H), 7.67 (1H, dd, J ¼ 8.00, 1.50 Hz, J ¼ 13.10, 7.60 Hz, Ar-H). 13C NMR (125 MHz, CDCl3) d (ppm):
Ar-H), 7.55–7.49 (8H, m, Ar-H), 7.48–7.35 (13H, m, Ar-H), 7.14 150.19, 149.67, 149.13, 148.67, 148.24, 142.18, 141.81, 141.37,
(3H, q, J ¼ 7.40 Hz, Ar-H), 6.98 (1H, d, J ¼ 1.20 Hz, Ar-H), 6.79 140.47, 139.06, 137.11, 136.72, 136.33, 135.72, 133.74, 132.37,
(3H, dd, J ¼ 12.10, 7.60 Hz, Ar-H). 13C NMR (125 MHz, CDCl3) d 129.78, 128.00, 127.88, 127.83, 127.79, 126.82, 126.54, 126.36,
(ppm): 149.64, 149.12, 148.64, 142.20, 141.78, 141.35, 140.43, 124.13, 122.78, 121.67, 120.36, 120.12, 120.04. MALDI-TOF (m/
137.86, 136.63, 136.33, 136.24, 133.55, 133.24, 132.17, 131.07, z): [M+] calcd: 727.27; found: 727.20. Anal. calcd for C54H37NSi:
129.78, 127.96, 127.93, 127.84, 127.79, 127.76, 126.80, 126.52, C, 89.09; H, 5.12; N, 1.92%. Found: C, 88.97; H, 5.08; N, 1.97%.
124.72, 124.10, 124.08, 122.76, 120.32, 120.09, 120.01. MALDI-
CzSiSF. Suzuki reaction: in a 50 mL round-bottom ask,
CzSiBr (580 mg, 1 mmol) was reacted with SFB (486 mg, 1.1
TOF (m/z): [M+] calcd: 728.15; found: 728.10.
CzSFSiBr. Suzuki reaction: compound CzSiBr2 (1.97 g, 3 mmol, 1.1 equiv.) for 2 days at 95 ꢁC. The rest of the procedure
mmol, 1.5 equiv.) was reacted with SFB (884 mg, 2 mmol) using was the same as for the synthesis of SFSiBr, and the product was
the same Suzuki reaction procedure as for the synthesis of puried by column chromatography (petroleum ether-
SFSiBr. The product was puried by column chromatography
: dichloromethane ¼ 8 : 1) to give the product as a white solid.
(petroleum ether : dichloromethane ¼ 8 : 1) to give the product Yield: (694 mg, 85%). 1H NMR (500 MHz, CDCl3) d (ppm): 8.13
as a white solid. Yield: (928 mg, 52%). 1H NMR (500 MHz, (2H, d, J ¼ 7.70 Hz, Ar-H), 7.92 (1H, d, J ¼ 8.00 Hz, Ar-H), 7.85
CDCl3) d (ppm): 8.16 (2H, d, J ¼ 7.70 Hz, Ar-H), 7.95 (1H, d, J ¼ (3H, dd, J ¼ 13.40, 7.60 Hz, Ar-H), 7.76 (2H, d, J ¼ 8.20 Hz, Ar-H),
8.00 Hz, Ar-H), 7.88 (3H, dd, J ¼ 13.30, 7.70 Hz, Ar-H), 7.76 (2H, 7.66 (1H, dd, J ¼ 8.00, 1.60 Hz, Ar-H), 7.63–7.59 (4H, m, Ar-H),
d, J ¼ 7.70 Hz, Ar-H), 7.59 (8H, dt, J ¼ 14.00, 5.80 Hz, Ar-H), 7.51 7.56 (4H, d, J ¼ 8.00 Hz, Ar-H), 7.50–7.43 (6H, m, Ar-H), 7.38 (9H,
(7H, dd, J ¼ 10.60, 7.80 Hz, Ar-H), 7.41 (8H, dt, J ¼ 16.80, 7.20 dt, J ¼ 17.40, 7.60 Hz, Ar-H), 7.30–7.27 (2H, m, Ar-H), 7.14–7.08
Hz, Ar-H), 7.32 (2H, d, J ¼ 7.30 Hz, Ar-H), 7.14 (3H, q, J ¼ 7.20 (3H, m, Ar-H), 6.97 (1H, d, J ¼ 1.20 Hz, Ar-H), 6.75 (3H, dd, J ¼
Hz, Ar-H), 7.00 (1H, s, Ar-H), 6.79 (3H, dd, J ¼ 14.10,7.60 Hz, Ar- 14.60, 7.60 Hz, Ar-H). 13C NMR (125 MHz, CDCl3) d (ppm):
H). 13C NMR (125 MHz, CDCl3) d (ppm): 149.68, 149.12, 148.63, 149.67, 149.14, 148.67, 142.22, 141.81, 141.38, 140.59, 140.50,
142.43, 141.79, 141.43, 141.32, 140.52, 140.35, 139.19, 137.88, 138.99, 137.84, 136.77, 136.39, 133.83, 133.55, 132.48, 129.83,
137.74, 136.65, 136.28, 133.21, 132.88, 131.79, 131.25, 130.02, 128.04, 127.95, 127.88, 127.79, 126.89, 126.59, 126.14, 125.95,
128.14, 128.01, 127.97, 127.69, 127.82, 127.78, 126.82, 126.69, 124.13, 123.52, 122.80, 120.31, 120.06. MALDI-TOF (m/z): [M+]
126.56, 126.20, 126.11, 125.95, 124.97, 124.11, 123.53, 122.77, calcd: 815.30; found: 815.10. Anal. calcd for C61H41NSi: C, 89.78;
120.37, 120.31, 120.11, 120.03, 109.87. MALDI-TOF (m/z): [M+] H, 5.06; N, 1.72%. Found: C, 90.02; H, 5.11; N, 1.75%.
calcd: 893.21; found: 893.30.
CzPySiSF. Suzuki reaction: in a 50 mL round-bottom ask,
SiSF. Suzuki reaction: in a 50 mL round-bottom ask, 4- CzSFSiBr (893 mg, 1 mmol) was reacted with 4-pyridinylboronic
(bromophenyl)triphenylsilane (82 mg, 0.2 mmol) was reacted acid (185 mg, 1.5 mmol, 1.5 equiv.) for 2 days at 95 ꢁC. The rest
with SFB (88 mg, 0.2 mmol) for 2 days at 95 ꢁC. The rest of the of the procedure was the same as for the synthesis of SFSiBr; the
procedure was the same as for the synthesis of SFSiBr, and the product was puried by column chromatography (dichlor-
product was puried by column chromatography (petroleum omethane : ethyl acetate ¼ 8 : 1) to give the product as a white
ether : dichloromethane ¼ 8 : 1) to give the product as a white solid. Yield: (778 mg, 87%). 1H NMR (500 MHz, CDCl3) d (ppm):
solid. Yield: (92 mg, 71%). 1H NMR (500 MHz, CDCl3) d (ppm): 8.72 (2H, d, J ¼ 3.50 Hz, Ar-H), 8.17 (2H, d, J ¼ 7.70 Hz, Ar-H),
7.94 (1H, d, J ¼ 8.00 Hz, Ar-H), 7.88 (3H, dd, J ¼ 11.80, 7.60 Hz, 7.96 (1H, d, J ¼ 8.00 Hz, Ar-H), 7.89 (3H, dd, J ¼ 14.30, 7.60 Hz,
Ar-H), 7.67 (1H, dd, J ¼ 8.00, 1.60 Hz, Ar-H), 7.59–7.51 (8H, m, Ar-H), 7.80 (4H, dd, J ¼ 20.30, 8.20 Hz, Ar-H), 7.72–7.58 (11H, m,
Ar-H), 7.48–7.34 (14H, m, Ar-H), 7.13 (3H, dt, J ¼ 7.40, 3.70 Hz, Ar-H), 7.55–7.49 (5H, m, Ar-H), 7.42 (7H, ddd, J ¼ 18.30, 14.70,
Ar-H), 6.99 (1H, d, J ¼ 1.30 Hz, Ar-H), 6.79 (3H, dd, J ¼ 12.80, 7.50 Hz, Ar-H), 7.32 (2H, t, J ¼ 7.40 Hz, Ar-H), 7.15 (3H, q, J ¼
7.60 Hz, Ar-H). 13C NMR (125 MHz, CDCl3) d (ppm): 149.64, 7.50, Ar-H), 7.01 (1H, s, Ar-H), 6.79 (3H, dd, J ¼ 14.90, 7.60 Hz,
149.14, 148.68, 141.99, 141.81, 141.42, 141.28, 140.59, 136.76, Ar-H). 13C NMR (125 MHz, CDCl3) d (ppm): 150.35, 149.72,
136.37, 134.18, 132.86, 129.60, 127.92, 127.87, 127.81, 127.78, 149.14, 148.67, 148.11, 142.43, 141.82, 141.46, 141.35, 140.56,
5022 | J. Mater. Chem. C, 2014, 2, 5019–5027
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140.39, 139.34, 139.20, 137.83, 137.15, 136.75, 136.37, 135.31, synthesis, we successfully obtained the four tetraphenylsilane
133.42, 133.08, 132.02, 130.04, 128.17, 128.01, 127.89, 127.86, derivatives in reasonable yields. The synthesis began with a
127.81, 126.84, 126.71, 126.53, 126.24, 125.98, 124.14, 123.56, reactions between 1,4-dibromobenzene and Ph(4ꢀx)SiClx (x ¼ 1,
122.81, 121.68, 120.41, 120.36, 120.15, 120.06, 109.90. MALDI- 2, 3) to afford Ph4SiBrx (x ¼ 1, 2, 3, corresponding to compound
TOF (m/z): [M+] calcd: 892.33; found: 892.30. Anal. calcd for 1, 2 and 3, respectively). SiSF was synthesized in a single step
C
66H44N2Si: C, 88.75; H, 4.97; N, 3.14%. Found: C, 88.87; H, using a Suzuki coupling reaction between SFB and compound 1,
5.01; N, 3.17%.
whereas CzSiSF was obtained in two steps. Firstly, one carbazole
unit was introduced on the tetraphenylsilane core using an
Ullmann coupling reaction, which was realized by controlling
the amount of carbazole added to afford CzSiBr. Secondly,
CzSiBr was reacted with SFB via a Suzuki coupling reaction to
achieve CzSiSF. PySiSF was also synthesized in a similar manner
as CzSiSF by sequentially introducing spirouorene and
Results and discussion
Synthesis
The synthetic routes employed to afford SiSF, CzSiSF, PySiSF
and CzPySiSF are shown in Scheme 1. Using a stepwise
Scheme 1 Synthetic routes to SiSF, CzSiSF, PySiSF and CzPySiSF. (a) Suzuki coupling reaction: PdCl2(dppf), KOAc and 1,4-dioxane, 90 ꢁC, 48 h.
(b) Suzuki coupling reaction: Pd(PPh3)4, Na2CO3 and toluene, 85 ꢁC, 48 h. (c) Ullmann coupling reaction: CuI, K3PO4, (ꢃ)-trans-1,2-dia-
minocyclohexane and toluene, 110 ꢁC, 24 h.
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pyridine to the tetraphenylsilane core in two steps. CzPySiSF results suggest that the spirouorene contributed to the emis-
was obtained in three steps, in which carbazole, spirouorene sion of CzSiSF and CzPySiSF.
and pyridine were sequentially attached to the tetraphenylsi-
The thin lm emission spectra of the four compounds
lane using Ullmann and Suzuki coupling reactions to obtain the exhibited red-shis of about 40 nm in comparison with the
target compound. Column chromatography was used to purify emission in THF (Fig. 3), which was still located deep in the blue
these molecules. All of the products were characterized spec- region. This observation might be due to the aggregation effect
troscopically, and the results were in agreement with their as the intermolecular interactions in the thin lms were much
respective structures (ESI†).
stronger than those of the dilute solution (10ꢀ5 M).33,34 Espe-
cially for CzPySiSF, the emission peak at 402 nm became even
stronger and a new shoulder was observed at 424 nm in the
solid state.
Thermal properties and morphology
Thermogravimetric analyses (TGA) and differential canning
We attempted to measure the uorescence quantum effi-
calorimetry (DSC) were used to investigate the thermal proper- ciency to investigate whether the introduction of spirouorene
ties. All the three compounds exhibited excellent thermal can improve the efficiency of the molecules. However, a typical
stability with a very high Td (corresponding to 5 wt% loss) at reference, such as sulphate quinone, could not be used as the
487 ꢁC, 425 ꢁC and 502 ꢁC for CzSiSF, PySiSF and CzPySiSF, reference. Therefore, we synthesized CzSiCz,32 which was used
respectively. The DSC results revealed that CzSiSF, PySiSF and as a relative reference to measure the uorescence quantum
CzPySiSF exhibited a very high Tg at 156 ꢁC, 141 ꢁC and 179 ꢁC, efficiency of CzSiSF and CzPySiSF. The method is described as
respectively. Specically, no crystallization or phase transitions follows:
ꢀ
ꢁꢀ
ꢁꢀ ꢁꢀ
ꢁ
were observed for any molecule upon heating above Tg. The
results indicate that the three compounds are stable amor-
phous materials. Such good thermal stability is desirable for
violet OLEDs applications.
ArðlrÞ
IðlrÞ n2x
Dx
Dr
Qx ¼ Qr
(1)
AxðlxÞ IðlxÞ n2r
AFM was used to investigate the surface of CzSiSF, PySiSF
and CzPySiSF. Films were prepared by evaporation on quartz
substrates and the thickness of the lms was measured to be
around 30 nm. As shown in Fig. S2,† CzSiSF, PySiSF and CzPy-
SiSF exhibited a fairly smooth and uniform morphology with a
roughness of only 3.27 nm, 1.52 nm and 2.18 nm, respectively.
Such a uniform amorphous morphology is oen observed in
silane-based compounds31,32 and is favorable for OLEDs
applications.
Photophysical properties
In the absorption spectra recorded in THF, the characteristic
absorption peaks of each functional group were observed in
these compounds. The absorption peak at around 308 nm could
be ascribed to the p–p* transition of the spirouorene moiety.
The absorption peak at around 293 nm was caused by the
characteristic p–p* transition of the carbazole unit. The
absorption peak at around 265 nm was attributed to the pyri-
dine unit. The lm absorption spectra of the four compounds
were very similar to their solution absorption spectra (Fig. 3).
More details about the absorption spectra are provided in
Table 1.
Fig. 2 The UV and PL spectra of the four compounds in THF,
concentration: 10ꢀ5 M.
The emission maxima of the four compounds in THF were
the same at 344 nm and 361 nm (Fig. 2). Notably, all four
compounds in THF exhibited almost completely violet emission
with negligible emission in the visible region (>400 nm). It can
be seen that the conjugation length was effectively restricted
due to the d-Si interaction of tetraphenylsilane. The emission
spectra of CzSiSF and CzPySiSF in THF were almost the same,
and both were very similar to the emission of CzPh in THF
(Fig. S3†). The uorescence quantum efficiencies of CzSiSF and
CzPySiSF in THF were both about 1.5-fold higher than that of
CzSiCz, which will be discussed later in the manuscript. These Fig. 3 The UV and PL spectra of the four compounds in thin films.
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Table 1 The thermal, photophysical and electrochemistry data of CzSiSF, PySiSF and CzPySiSF
a
b
c
d
c
d
e
e
f
Td
Tg
lmax,ab
lmax,ab
PL lmax
PL lmax
EHOMO
(eV)
ELUMO
(eV)
Eg
Compound
(ꢁC)
(ꢁC)
(nm) (THF)
(nm) (lm)
(nm) (THF)
(nm) (lm)
(eV)
CzSiSF
PySiSF
CzPySiSF
487
425
502
156
141
179
293, 308
265, 308
293, 308
297, 310
266, 310
297, 310
344, 361
344, 361
344, 361
385, 405
390, 408
373, 402,
424
ꢀ5.62
ꢀ5.81
ꢀ5.63
ꢀ2.39
ꢀ2.41
ꢀ2.43
3.50
3.50
3.50
a
b
c
d
Obtained from TGA measurements. Obtained from DSC measurements. Measured in THF (10ꢀ5 M). Measured in thin solid lm by spin
e
f
coating using THF as the solvent. Determined from the onset of the oxidation/reduction voltages. Calculated from optical absorption band
edge in THF.
Table 2 EL performance of CzPySiSFa
Compound
Von (V)
Lmax (cd mꢀ2
)
LEmax (cd Aꢀ1
)
EQEmax/100 (%)
EL lmax (nm)
CzSiSF
PySiSF
CzPySiSF
4.8
5.0
5.8
615
421
627
0.32
0.52
0.35
0.48/0.32
0.31/0.24
0.59/0.41
410
412, 517
408
a
Device conguration: ITO/PEDOT:PSS (40 nm)/NPB (80 nm)/TCTA (10 nm)/emitting-layer (CzPySiSF) (30 nm)/TPBi (30 nm)/LiF (0.5 nm)/Al
(100 nm); abbreviations: Von ¼ turn on voltage, Lmax ¼ maximum luminance, LEmax ¼ maximum luminous efficiency, EQEmax/100 ¼ maximum
external quantum efficiency/external quantum efficiency at 100 cd mꢀ2
.
where Q is the uorescence quantum efficiency,35 A is the value CzSiSF and CzPySiSF, exhibited very similar oxidation onset
of the absorbance, I is the intensity of the excitation source, n is potentials and their HOMO energy levels were calculated to be
the refractive index of the solvent, D is the area of the emission ꢀ5.62 and ꢀ5.63 eV, respectively, which resembled that of CzSi
spectra, and l is the corresponding wavelength. The excitation (ꢀ5.61 eV). The pyridine-containing molecules, i.e., PySiSF and
wavelength was 291 nm as all the three compounds have strong CzPySiSF, exhibited similar initial reduction potentials and
2
absorption at this wavelength. The value of nx /nr2 and I(lr)/I(lx) their LUMO energy levels were calculated to be ꢀ2.41 and ꢀ2.43
were both equal to 1 due to the use of the same THF solvent and eV, respectively, which is similar to that of PySi (ꢀ2.43 eV).
excitation source. The uorescence quantum efficiency of Compared with SiSF, the injection of holes and electrons
CzSiCz was set to q, according to the formula. Consequently, the became easier in CzPySiSF due to the modication of silane
uorescence quantum efficiency of CzSiSF and CzPySiSF were with carbazole and pyridine.
calculated to be 1.55q and 1.51q, respectively (ESI, Fig. S5 and
To further understand the electronic properties, density
Table S1†). Hence, the uorescence quantum efficiency of functional theory (DFT) calculations were carried out with the
CzSiSF and CzPySiSF were improved by 50% compared with that geometry optimized at the B3LYP/6-31G* level (Fig. 5).
of CzSiCz. Thus, it is reasonable to conclude that the intro-
duction of the spirouorene contributes signicantly to the
emission of CzSiSF and CzPySiSF and makes them promising
materials for violet OLEDs applications.
Moreover, we measured the uorescence of CzPySiSF in
different solvents with various polarities (ESI, Fig. S6†), and they
all exhibited similar emission spectra, which indicates that no
intramolecular charge transfer exists in CzPySiSF. Incorpo-
rating the electron rich and the electron decient moieties into
tetraphenylsilane simultaneously did not cause a CT effect,
which was consistent with our molecular design tactics.
Electrochemical properties and theoretical calculations
Cyclic voltammetry (CV) measurements were employed to
investigate the electrochemical behavior. The model
compounds CzSi and PySi (Fig. 4) were also synthesized to
explore the effect of carbazole and pyridine on the energy
levels.21 It was found that carbazole-containing molecules, DMF solution, respectively.
Fig. 4 Cyclic voltammogram and the HOMO–LUMO levels. The
oxidation and reduction processes were performed in CH2Cl2 and
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Fig. 6 The current efficiency and external efficiency of CzPySiSF.
Inset: EL and PL spectra of CzPySiSF, the EL spectrum was recorded at
7 V.
Fig. 5 The relevant molecular orbitals density maps calculated at the
B3LYP/6-31G* level.
According to the theoretical calculations, the HOMOs and
LUMOs of CzSiSF, PySiSF and CzPySiSF were all distributed
separately on different moieties. For example, in CzPySiSF, the
HOMO energy level was located on the carbazole moiety,
whereas the LUMO was located on the pyridine moiety. The
separated HOMO and LUMO could be ascribed to the tetrahe-
dral structure and the d-Si interaction of tetraphenylsilane. The
results of CV measurements and theoretical calculations veri-
ed our initial hypothesis: i.e., adopting tetraphenylsilane as
the linker, the hole/electron transport groups and emitting unit
could work independently. In the case of the bipolar molecule,
CzPySiSF, the HOMO and LUMO were distributed on the
carbazole and pyridine separately. The introduction of both
carbazole and pyridine can elevate the HOMO level and lower
the LUMO level to a certain extent in comparison with SiSF.
Such modications of the HOMO/LUMO levels will facilitate
charge injection from adjacent organic layers to CzPySiSF in an
EL device.
in other uorene-based small molecules and polymers
devices.14,36–38 The maximum external quantum efficiency (EQE)
of CzPySiSF was calculated to be 0.59% and should be higher if
shorter wavelengths could be detected. For a more compre-
hensive study, the hole-type molecule CzSiSF and the electron-
type molecule PySiSF were also employed as the emitting layer
using the same device structure. However, both CzSiSF and
PySiSF exhibited long wavelength emission as demonstrated in
Fig. S6,† which might be due to the interface emission between
the emitting layer and the adjacent organic layer. This result
also conrmed that the bipolar injection of charge is very
important for EL devices. The EQE of the bipolar molecule
CzPySiSF (0.59%) was a little bit higher than that of CzSiSF (EQE
¼ 0.48%) and PySiSF (EQE ¼ 0.31%). Moreover, the EL spectra
of CzPySiSF were very stable even as the voltage was increased to
10.0 eV (Fig. S7†).
Conclusions
Electroluminescence properties
In summary, we have successfully synthesized a bipolar violet
To investigate the EL properties of the bipolar compound emissive molecule, CzPySiSF, using stepwise chemical synthesis
CzPySiSF as a violet or UV emitter, OLEDs were fabricated using techniques. CzPySiSF exhibits very good thermal stability and a
vacuum evaporation with the conguration of ITO/PEDOT:PSS uniform amorphous morphology. The introduction of spiro-
(40 nm)/NPB (80 nm)/TCTA (10 nm)/emitting-layer (CzPySiSF) uorene enhances the uorescence quantum efficiency by 50%
(30 nm)/TPBi (30 nm)/LiF (0.5 nm)/Al (100 nm), in which NPB in comparison with CzSiCz. The tetraphenylsilane-bridged
(N,N0-di-(naphthalen-1-yl)-N,N0-diphenyl benzidine) was utilized linkage mode effectively interrupts communication between
as the hole transport layer, TCTA (4,40,400-tris(carbazol-9-yl)-tri- each group and leads to separated distributions of HOMO and
phenylamine) as the buffer layer, and TPBi (1,3,5-tris(N-phe- LUMO levels. The EL spectra of CzPySiSF is in good agreement
nylbenzimidazol-2-yl)benzene) as the electron transport layer. with the PL spectra without long-wavelength emission, which
As shown in Fig. 6, the EL spectrum of CzPySiSF was similar to illustrates that the bipolar design strategy employing an insu-
that of the PL spectrum. The maximum peak in the EL spectrum lator tetraphenylsilane as the linkage can retain violet emission
was observed at 408 nm (Table 2), which was very close to the and facilitate device operation as a result of a lower charge
maximum peak in the PL spectrum (402 nm). The EL spectrum injection barrier. The maximum EQE for CzPySiSF reaches
could not be completely measured as wavelengths shorter than 0.59% and can be further improved if shorter emission (<380
380 nm cannot be detected by our PR650 spectroscan spec- nm) can be detected. This inspiring result gives us a new
trometer. The vibronic structure observed in the PL spectrum template for the violet material design. By rationally choosing
disappeared in the EL spectrum due to a different luminescence bipolar groups and efficient violet emitters, it is possible to
mechanism; moreover, such a phenomenon was also reported design and synthesize highly efficient violet emitting materials
5026 | J. Mater. Chem. C, 2014, 2, 5019–5027
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with balanced carrier transport characteristics as well as 16 H. Zhang, B. Yang, Y. Zheng, G. Yang, L. Ye, Y. Ma and
thermal stability and morphological properties using a tetra-
phenylsilane-bridged linkage.
X. Chen, J. Phys. Chem. B, 2004, 108, 9571.
17 K. T. Wong, Y. L. Liao, Y. T. Lin, H. C. Su and C. C. Wu, Org.
Lett., 2005, 7, 5131.
18 F. Niu, H. Niu, Y. Liu, J. Lian and P. Zeng, RSC Adv., 2011, 1,
415.
Acknowledgements
19 Y. Yang, P. Cohn, A. L. Dyer, S. H. Eom, J. R. Reynolds,
R. K. Castellano and J. Xue, Chem. Mater., 2010, 22, 3580.
20 Y. Yang, P. Cohn, S. H. Eom, K. A. Abboud, R. K. Castellano
and J. Xue, J. Mater. Chem. C, 2013, 1, 2867.
21 D. Hu, F. Shen, H. Liu, P. Lu, Y. Lv, D. Liu and Y. Ma, Chem.
Commun., 2012, 48, 3015.
We are grateful for support from the National Basic Research
Program
of
China
(973
Program,
2013CB834701,
2013CB834800), National Science Foundation of China (Grant
no. 21174050, 21374038) and PCSIRT.
22 S. Gong, Y. Chen, J. Luo, C. Yang, C. Zhong, J. Qin and D. Ma,
Adv. Funct. Mater., 2011, 21, 1168.
23 C. T. Chen, Y. Wei, J. S. Lin, M. V. R. K. Moturu, W. S. Chao,
Y. T. Tao and C. H. Chien, J. Am. Chem. Soc., 2006, 128,
10992.
24 K. R. Wee, H. C. Ahn, H. J. Son, W. S. Han, J. E. Kim,
D. W. Cho and S. O. Kang, J. Org. Chem., 2009, 74, 8472.
25 L. Xiao, S. J. Su, Y. Agata, H. Lan and J. Kido, Adv. Mater.,
2009, 21, 1271.
Notes and references
1 Y. Sun, N. C. Giebink, H. Kanno, B. Ma, M. E. Thompson and
S. R. Forrest, Nature, 2006, 440, 908.
2 M. A. Baldo, D. F. O'Brien, Y. You, A. Shoustikov, S. Sibley,
M. E. Thompson and S. R. Forrest, Nature, 1998, 395, 151.
3 Y. Mo, R. Tian, W. Shi and Y. Cao, Chem. Commun., 2005,
4925.
4 Y. Tao, C. Yang and J. Qin, Chem. Soc. Rev., 2011, 40, 2943.
5 R. J. Holmes, S. R. Forrest, Y. J. Tung, R. C. Kwong,
J. J. Brown, S. Garon and M. E. Thompson, Appl. Phys. Lett.,
2003, 82, 2422.
¨
26 K. Brunner, A. V. Dijken, H. Borner, J. J. A. M. Bastiaansen,
N. M. M. Kiggen and B. M. W. Langeveld, J. Am. Chem.
Soc., 2004, 126, 6035.
6 K. L. Chan, S. E. Watkins, C. S. K. Mak, M. J. Mckiernan,
C. R. Towns, S. I. Pascu and A. B. Holmes, Chem. Commun.,
2005, 5766.
7 H. Sasabe and J. Kido, Chem. Mater., 2011, 23, 621.
8 A. V. Dijken, J. J. A. M. Bastiaansen, N. M. M. Kiggen,
B. M. W. Langeveld, C. Rothe, A. Monkman, I. Bach,
´
27 D. Kim, V. Coropceanu and J. L. Bredas, J. Am. Chem. Soc.,
2011, 133, 17895.
28 T. P. I. Saragi, T. Spehr, A. Siebert, T. F. Lieker and J. Salbeck,
Chem. Rev., 2007, 107, 1011.
29 N. Miyaura and A. Suzuki, Chem. Rev., 1995, 95, 2457.
30 F. Ullmann and J. Bielecki, Chem. Ber., 1901, 34, 2174.
31 D. Hu, P. Lu, C. Wang, H. Liu, H. Wang, Z. Wang, T. Fei,
X. Gu and Y. Ma, J. Mater. Chem., 2009, 19, 6143.
32 D. Hu, G. Cheng, H. Liu, Y. Lv, P. Lu and Y. Ma, Org.
Electron., 2012, 13, 2825.
33 S. Tang, M. Liu, P. Lu, G. Cheng, M. Zeng, Z. Xie, H. Xu,
H. Wang, B. Yang, Y. Ma and D. Yan, Org. Electron., 2008,
9, 241.
¨
P. Stossel and K. Brunner, J. Am. Chem. Soc., 2004, 126, 7718.
9 K. R. Wee, Y. J. Cho, S. Jeong, S. Kwon, J. D. Lee, I. H. Suh and
S. O. Kang, J. Am. Chem. Soc., 2012, 134, 17982.
10 H. Uoyama, K. Goushi, K. Shizu, H. Nomura and C. Adachi,
Nature, 2012, 492, 234.
11 S. Lamansky, P. Djurovich, D. Murphy, F. A. Razzaq,
H. E. Lee, C. Adachi, P. E. Burrows, S. R. Forrest and
M. E. Thompson, J. Am. Chem. Soc., 2001, 123, 4304.
12 A. J. Sandee, C. K. Williams, N. R. Evans, J. E. Davies,
34 Y. Mo, L. Du, L. Liu, J. Huang, Y. Pan, B. Yang, Z. Xie and
Y. Ma, J. Phys. Chem. C, 2013, 117, 27081.
¨
C. E. Boothby, A. Kohler, R. H. Friend and A. B. Holmes, J.
35 J. N. Demas and G. A. Crosby, J. Phys. Chem., 1971, 75, 991.
36 P. Lu, H. Zhang, F. Shen, B. Yang, D. Li, Y. Ma, X. Chen, J. Li
and N. Tamai, Macromol. Chem. Phys., 2003, 204, 2274.
37 D. Yu, F. Zhao, Z. Zhang, C. Han, H. Xu, J. Li, D. Ma and
P. Yan, Chem. Commun., 2012, 48, 6157.
Am. Chem. Soc., 2004, 126, 7041.
13 M. T. Lee, C. H. Liao, C. H. Tsai and C. H. Chen, Adv. Mater.,
2005, 17, 2493.
14 T. C. Chao, Y. T. Lin, C. Y. Yang, T. S. Hung, H. C. Chou,
C. C. Wu and K. T. Wong, Adv. Mater., 2005, 17, 992.
15 H. Etori, X. L. Jin, T. Yasuda, S. Mataka and T. Tsutsui, Synth.
Met., 2006, 156, 1090.
38 Z. R. Grabowski, K. Rotkiewicz and W. Rettig, Chem. Rev.,
2003, 103, 3899.
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