Electronic alteration of end-on phenyl groups of bis-triazolyl-silanes: Electron-transport materials for blue phosphorescent OLEDs

Article abstract of DOI:10.1039/c6tc00017g

A series of modified bis(4-(4,5-diphenyl-4H-1,2,4-triazol-3-yl)phenyl)dimethylsilane (ST), ST-CF₃, ST-Me, ST-^tBu, and ST-OMe, were designed and prepared by introducing electron-withdrawing groups or electron-donating groups at the 4-

Full text of DOI:10.1039/c6tc00017g

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DOI: 10.1039/C6TC00017G
Journal Name
ARTICLE
Electronic Alteration on End-on Phenyl Groups of bis-Triazolyl-
silanes: Electron-transport Materials for Blue Phosphorescent
OLEDs
Received 00th January 20xx,
Accepted 00th January 20xx
Seungjun Yi,a Woo-Ri Bae,a Jin-Hyoung Kim,a Ah-Rang Lee,b Woo-Young Kim,c Won-Sik Han,b,* Ho-
Jin Son,a,* and Sang Ook Kanga,*
DOI: 10.1039/x0xx00000x
A series of modified bis(4-(4,5-diphenyl-4H-1,2,4-triazol-3-yl)phenyl)dimethylsilane (ST), ST-CF3, ST-Me, ST- tBu, and ST-OMe,
were designed and prepared by introducing the electron-withdrawing groups or the electron-donating groups at the 4-
position of the end-on phenyl groups on the triazole ring. Systematic investigations of their thermal-, photophysical-, and
electron-transporting (ET) properties were successfully carried out. Depending on the electronic characteristics of the
substituents at the end-phenyl group on the triazole, their frontier orbital energy levels were controlled while maintaining
energy band gaps. Low- temperature photoluminiscenec spectra indicate that all the prepared ST moieties maintained high
triplet energy states up to 2.85 eV owing to the suppression of electron delocalization by silicon centre. The electron
mobilities of all the compounds were investigated by fabricating electron only device (EOD). The EOD of ST-tBu showed
significantly higher current density than the other analogs. Finally, the EL device utilizing ST-tBu as an electron transporting
material in phosphorescent organic light-emitting diodes with bis[(4,6-di-fluorophenly)-pyridinate-N,C2]picolinate (FIrpic) as
a dopant showed an external quantum efficiency of 15.9%.
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from the relatively high triplet energy (ET = 2.65 eV),27 finding
suitable high-triplet energy materials for host and common
layers is indispensable.30,31 Namely, to achieve the high
Introduction
For both displays and solid-state lighting, efficiency is still the
crucial factor in the applications of organic light-emitting diodes
(OLEDs). The use of phosphorescent emitters is one of the most
quantum efficiency of blue phosphorescent OLEDs, host in the
emissive layer (EML) and common layer materials in hole-
transporting layer (HTL) and electron-transporting layer (ETL)
must possess higher triplet energy levels than those of dopant
materials to confine the triplet energy in dopant material.
In general, the mobility of holes is higher than that of electrons
in most organic semiconducting materials due to electron
trapping.32,33 Therefore, developing efficient electron
transporting material (ETM) is essential to improve charge
balance in OLED device. Several candidate materials have been
used in PHOLEDs. For example, oxadiazole based ETM, 2-(4-
–
important strategies to improve the efficiency of OLEDs.1
14
Despite significant achievements in red and green
phosphorescent materials, improving efficiency for blue
emission seems to be the final obstacle for the
commercialization of full colour displays and white light
application.15–19 Obtaining highly efficient blue phosphorescent
OLEDs has been difficult, because blue emission is generated
from wide-gap excited states.20 Several blue dopant have been
26
developed,21
for example, iridium (III) bis[(4,6-di-
(FIrpic).27–29
The
biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole
metal chelated ETM, bis(2-methyl-8-quinolinato)-4-
(PBD),34,35
fluorophenly)-pyridinate-N,C2]picolinate
greenish-blue emission using Firpic exhibited CIE coordinates of
(0.16, 0.29). Even though Firpic showed good performance as a
blue dopant, another obstacle must be resolved to reach the
desired high quality with long lived full-colour display. Because
blue phosphorescent emission from Firpic dopant is generated
phenylphenolate (BAlq),29 azole based ETM, 3-(biphenyl-4-yl)-5-
(4-tert-butylphenyl)-4-phenyl-4H-1,2,4-triazole (t-Bu-TAZ),36–38
and benzimidazole based ETM, 1,3,5-tris(N-phenylbenzimidizol-
2-yl)benzene (TPBi)39–41 have been developed. However, these
materials have some drawbacks. For example, PBD and TAZ
have relatively low glass-transition temperatures (Tg ≈ 60–
70 °
C)42,43 which results in crystallization over time during
a.Department of Advanced Materials Chemistry, Korea University, Sejong 30019,
South Korea.
device operation. In the case of BAlq, even though this material
is an efficient ETM for green PHOLEDs, it cannot be used in blue
PHOLEDs because the triplet energy is only 2.2 eV44 which is
lower than blue phosphorescent dopant materials.
Recently, our group45–47 and others48–51 developed silicon-
based materials as high triplet materials, because silicon atom
can block the conjugation, leading to wide band-gap and high
b.Department of Chemistry, Seoul Women’s University, Seoul 01797, South Korea.
c. Division of Green Energy and Semiconductor Engineering Hoseo University, Asan-
si 31499, South Korea.
E-mail: wshan@swu.ac.kr, hjson@korea.ac.kr, and sangok@korea.ac.kr
Electronic Supplementary Information (ESI) available: Crystal data and collection
parameters for ST-tBu, TGA, cyclic voltammograms, device diagram, and EL spectra
of the device. CCDC 1445019. For ESI and crystallographic data in CIF or other
electronic format See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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Journal of Materials Chemistry C
Page 2 of 10
ARTICLE
Journal Name
O
O
O
O
O
O
View Article Online
NH2
H2N
Me
Me
Br
HO
OH
MeO
OMe
N
H
N
H
DOI: 10.1039/C6TC00017G
NH2NH2
EtOH
H2SO4
MeOH
Mg
KMnO4
py, H2O
Me2SiCl2
2
+
THF
Si
Si
Si
Si
Me
Me
Me
Me
Me
Me
Me
Me
Me
1
2
3
4
R
R
N
N
R
R
N
N
H
N
H
N
O
O
N
N
O
N
N
PCl3
o-dichlorobenzene
TEA
NMP
+
2
O
O
R
H
H
NH2
4
+
2
Cl
Si
Si
Me
Me
R = CF3, Me, tBu, OMe
Me
Me
R = CF3 (5a), Me (5b), tBu (5c), OMe (5d)
R = CF3 (ST-CF3), Me (ST-Me), tBu (ST-tBu), OMe (ST-OMe)
Scheme 1. Synthesis for a series of ST moieties, ST-CF3, ST-Me, ST-tBu, and ST-OMe.
thermal stability. For example, phenyl carbazole units were analyzer. High-Resolution Tandem Mass Spectrometry (Jeol LTD
introduced into silicon atom and the resulting materials were JMS-HX 110/110A) was performed by the Korean Basic Science
examined as a blue host material.45 The triplet energy of these Institute. Cyclic voltammetry (CV) was performed in an
silicon-based host materials reached up to 3.0 eV, and electrolyte containing 1 mM of the electro-active compounds
fabricated device using silicon based host material showed a and 0.1 M tetrabutylammonium perchlorate (TBAP) at room
better performance than that of the device using mCP (1,3- temperature under N2 atmosphere using
a BAS 100B
bis(N-carbazolyl)benzene). Another example of silicon based electrochemical analyzer. Platinum, platinum wire, and
host material is containing 1,3,4-oxadiazole groups and it Ag/AgNO3 (0.1 M) were used as the working, counter, and
achieved current efficiency up to 39 cd/A.48 By the same token, reference electrodes, respectively. All the potentials were
electron-transporting material based on silicon atom was calibrated to the ferrocene/ferrocenium (Fc/Fc+) redox couple.
developed for green PHOLEDs by introducing a silicon atom Benzoyl
chloride,
aniline,
PCl3,
n-BuLi,
and
between two 1,8-naphthyridin units.49 By using this material dimethyldichlorosilane were purchased and used as received.
with Ir(ppy)3, 15.4% of maximum external quantum efficiency Compounds
(EQE) corresponding to 56.2 cd A−1 of maximum current procedure.52
1
–3
were prepared following literature
efficiency was obtained with a maximum power efficiency of Synthesis of 4,4'-(dimethylsilanediyl)dibenzohydrazide (4)
A
58.9 lm W−1. Moreover, triazole units were also introduced into 500 mL two-neck round bottomed flask equipped with a
the silicon atom as the electron transporting material.46 In this magnetic stir bar and a reflux condenser was charged with 4,4'-
case, its triplet energy level was as high as 2.85 eV, and its glass- (dimethylsilanediyl)dibenzoate52 (12.3 g, 37.5 mmol), hydrazine
transition temperature reached up to 115 °C.
monohydrate (26.3 mL, 750 mmol), and ethanol (100 mL). The
In this study, we are able to control the frontier orbital reaction mixture was heated to reflux for 24 h and then allowed
energies by introducing trifluoromethyl unit as the electron- to cool to room temperature. The reaction mixture was poured
withdrawing group and methyl, tert-butyl, and methoxy units as into cold water to give a white precipitate. The precipitate was
the electron-donating groups at the 4-para-position of the end- collected by filtration and washed with hexane and small
on phenyl group on triazole, as shown in Scheme 1. Central amounts of water to remove the unreacted starting materials.
silicon atom effectively breaks the conjugation between two The resulting white solid was dried in a vacuum oven for 24 h to
1
TAZ
(3-(biphenyl-4-yl)-5-(4-tert-butylphenyl)-4-phenyl-4H- afford the pure product in quantitative yield. H NMR (300.1
1,2,4-triazole) units and thus renders high-triplet energies up to MHz, DMSO-d6): (δ, ppm) 9.78 (NH, s, 2H), 7.78 (Ar-H, d, J = 6.9
2.85 eV. Among the series, ST-tBu was incorporated into the Hz, 4H), 7.58 (Ar-H, d, J = 6.9 Hz, 4H), 4.49 (NH2, s, 4H), 0.57 (-
Firpic-based blue phosphorescent OLEDs, exhibiting and EQE of CH3, s, 6H); HRMS (FAB) calcd. for C16H20N4O2Si: [M]+ 328.1356,
15.9% with reduced efficiency roll-off at a high luminance.
Found: 328.1353; Anal. calcd for C16H20N4O2Si: C, 58.51; H, 6.14;
N, 17.06; O, 9.74; Si, 8.55. Found: C, 58.50; H, 6.13; N, 17.05.
General procedure for the synthesis of ST compound
precursors (5a–5d) A 500 mL two-neck round bottomed flask
equipped with a magnetic stir bar was charged with 4,4'-
(dimethylsilanediyl)dibenzohydrazide, triethylamine (TEA), and
N-methyl-2-pyrrolidone (NMP). Corresponding 4-benzoyl
chloride was slowly added at 10 °C, and then the reaction
mixture was stirred overnight at room temperature. The
reaction mixture was poured into water to give a precipitate.
The precipitate was collected by vacuum filtration and washed
with water. The resulting white solid was dried in a vacuum
oven for 24 h affording the pure product.
Experimental
General Procedures All the reactions were formed under a dry
nitrogen or argon atmosphere using standard Schlenk
techniques. THF was freshly distilled over sodium
benzophenone. 1H and 13C NMR spectra were recorded using a
Varian Mercury 300 spectrometer operating at 300.1 and 75.4
MHz, respectively. All proton and carbon chemical shifts were
measured relative to the internal residual benzene from the
lock solvent (99.5% CDCl3). The elemental analyses were
performed using a Carlo Erba Instruments CHNSO EA 1108
2 | J. Name., 2012, 00, 1-3
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Journal of Materials Chemistry C
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Journal Name
Synthesis
ARTICLE
of
4,4'-(dimethylsilanediyl)bis(N'-(4- reflux condenser was charged with aniline and o-
View Article Online
DOI: 10.1039/C6TC00017G
(trifluoromethyl)benzoyl)benzohydrazide) (5a) Compound 5a dichlorobenzene. Flask B was cooled down to 0 °C, and PCl3 was
was obtained by the above mentioned procedure by using 4- added dropwise. Then, the solution in flask
A was slowly
(trifluoromethyl)benzoyl chloride (5 g, 24.0 mmol), 4,4'- decanted to flask under N2 atmosphere. The reaction mixture
B
(dimethylsilanediyl)dibenzohydrazide (1.97 g, 6.0 mmol), and was heated to 180 °C for 18 h, which allowed colour change to
1
TEA (1.67 mL, 12 mmol) in NMP (40 mL). Yield: 3.8 g, 94%. H orange. The reaction mixture was cooled to room temperature
NMR (300.1 MHz, DMSO-d6): (δ, ppm) 10.79 (NH, s, 2H), 10.63 and poured into water and then neutralized with 1 N NaOH
(NH, s, 2H), 8.12 (Ar-H, d, J = 7.5 Hz, 4H), 7.84−7.93 (Ar-H, m, aqueous solution, resulting in the formation of the orange
8H), 7.70 (Ar-H, d, J = 6.9 Hz, 4H), 2.69 (-CH2-, s, 4H), 0.63 (-CH3, precipitate. The precipitate was collected by filtration and dried
s, 6H); HRMS (FAB) calcd. for C32H27F6N4O4Si: [M+H]+ 673.1706, in a vacuum oven for 24 h. The crude product was purified by
Found: 673.1702; Anal. calcd for C32H26F6N4O4Si: C, 57.14; H, silica gel flash column chromatography to afford the final
3.90; F, 16.95; N, 8.33; O, 9.51; Si, 4.18. Found: C, 57.15; H, 3.89; product as white solids.
N, 8.32.
Bis(4-(4-phenyl-5-(4-(trifluoromethyl)phenyl)-4H-1,2,4-
Synthesis
of
4,4'-(dimethylsilanediyl)bis(N'-(4- triazol-3-yl)phenyl)dimethylsilane, ST-CF3 This compound was
methylbenzoyl)benzohydrazide) (5b) Compound 5b was prepared by the above mentioned procedure by using the
obtained by the above mentioned procedure by using 4- following materials: flask : 4,4'-(dimethylsilanediyl)bis(N'-(4-
methylbenzoyl chloride (9.4 g, 60.9 mmol), 4,4'- (trifluoromethyl)benzoyl)benzohydrazide) (7.7 g, 10.0 mmol)
A
(dimethylsilanediyl)dibenzohydrazide (5 g, 15.2 mmol), and TEA and o-dichlorobenzene (50 mL), flask
B
: aniline (10.7 mL, 117.5
1
(4.24 mL, 30.5 mmol) in NMP (100 mL). Yield: 10.0 g, 92%. H mmol), PCl3 (2.1 mL, 23.5 mmol), and o-dichlorobenzene (50
NMR (300.1 MHz, DMSO-d6): (δ, ppm) 10.48 (NH, s, 2H), 10.43 mL). The crude product was purified by silica gel flash column
(NH, s, 2H), 7.91 (Ar-H, d, J = 7.8 Hz, 4H), 7.83 (Ar-H, d, J = 8.4 chromatography using THF:dichloromethane = 1:5 (v:v) as an
Hz, 4H), 7.69 (Ar-H, d, J = 8.4 Hz, 4H), 7.29−7.34 (Ar-H, m, 4H), eluent to afford the product as a white solid. Yield: 3.3 g, 42%.
2.69 (-CH2-, s, 4H), 0.63 (-CH3, s, 6H); HRMS (FAB) calcd. for 1H NMR (300.1 MHz, CDCl3): (δ, ppm) 7.57 (Ar-H, s, 8H), 7.54 (Ar-
C32H33N4O4Si: [M+H]+ 565.2271, Found: 565.2269; Anal. calcd H, s, 2H), 7.51 (Ar-H, s, 2H), 7.49-7.48 (Ar-H, m, 1H), 7.46 (Ar-H,
for C32H32N4O4Si: C, 68.06; H, 5.71; N, 9.92; O, 11.33; Si, 4.97. s, 1H), 7.42 (Ar-H, d, J = 2.70 Hz, 6H), 7.24 (Ar-H, s, 3H), 7.21 (Ar-
Found: C, 68.04; H, 5.70; N, 9.93.
Synthesis of 4,4'-(dimethylsilanediyl)bis(N'-(4-tert- 154.92, 153.55, 140.57, 134.64, 134.40, 130.56, 129.18, 128.06,
H, s, 3H), 0.51 (-CH3, s, 6H); 13C NMR (75.4 MHz, CDCl3): (δ, ppm)
butylbenzoyl)benzohydrazide) (5c) Compound 5c was 127.85, 126.61, 125.70, 125.66, 125.60, 125.55, -2.64; HRMS
prepared following the by above mentioned procedure by using (FAB) calcd. for C44H33F6N6Si: [M+H]+ 787.2440, Found 787.2438;
4-tert-butylbenzoyl chloride (22.1 mL, 113.4 mmol), 4,4'- Anal. calcd for C44H32F6N6Si: C, 67.16; H, 4.10; F, 14.49; N, 10.68;
(dimethylsilanediyl)dibenzohydrazide (9.3 g, 28.3 mmol), and Si, 3.57. Found: C, 67.18; H, 4.11; N, 10.67.
TEA (7.90 mL, 56.7 mmol) in NMP (200 mL). Yield: 22.2 g, 93%. Bis(4-(4-Phenyl-5-p-tolyl-4H-1,2,4-triazol-3-
1H NMR (300.1 MHz, DMSO-d6): (δ, ppm) 10.48 (NH, s, 2H), yl)phenyl)dimethylsilane, ST-Me This compound was also
10.43 (NH, s, 2H), 7.85−7.95 (Ar-H, m, 6H), 7.69 (Ar-H, d, J = 8.4 prepared by the above mentioned procedure by using the
Hz, 4H), 7.47−7.55 (Ar-H, m, 6H), 2.69 (-CH2-, s, 4H), 1.31 (C(CH3), following materials: flask
s, 18H), 0.62 (-CH3, s, 6H); HRMS (FAB) calcd. for C38H45N4O4Si: methylbenzoyl)benzohydrazide) (5.5 g, 10.0 mmol) and o-
[M+H]+ 649.3210, Found [M+H]+: 649.3207; Anal. calcd for dichlorobenzene (50 mL), flask
: aniline (10.9 mL, 120 mmol),
C38H44N4O4Si: C, 70.34; H, 6.83; N, 8.63; O, 9.86; Si, 4.33. Found: PCl3 (2.1 mL, 24.0 mmol), and o-dichlorobenzene (50 mL). The
A: 4,4'-(dimethylsilanediyl)bis(N'-(4-
B
C, 70.36; H, 6.84; N, 8.62.
Synthesis of
crude product was purified by silica gel flash column
4,4'-(dimethylsilanediyl)bis(N'-(4- chromatography using THF:dichloromethane = 1:5 (v:v) as an
methoxybenzoyl)benzohydrazide) (5d) Compound 5d was eluent to afford the product as a white solid. Yield: 2.8 g, 41%.
prepared by the same procedure using 4-methoxybenzoyl 1H NMR (300.1 MHz, CDCl3): (δ, ppm) 7.50-7.49 (Ar-H, m, 1H),
chloride
(5.42
mL,
36.5
mmol),
4,4'- 7.48-7.46 (Ar-H, m, 2H), 7.45-7.44 (Ar-H, m, 2H), 7.42-7.41 (Ar-
(dimethylsilanediyl)dibenzohydrazide (3 g, 9.13 mmol), and TEA H, m, 1H), 7.38 (Ar-H, s, 8H), 7.32 (Ar-H, s, 2H), 7.29 (Ar-H, s, 2H),
(2.54 mL, 18.3 mmol) in NMP (60 mL). Yield: 4.9 g, 90%. 1H NMR 7.18 (Ar-H, t, J = 1.48 Hz, 2H), 7.16 (Ar-H, t, J = 1.83 Hz, 2H), 7.11
(300.1 MHz, DMSO-d6): (δ, ppm) 10.45 (NH, s, 2H), 10.36 (NH, s, (Ar-H, s, 2H), 7.08 (Ar-H, s, 2H), 2.32 (Ar-CH3, s, 6H), 0.49 (-CH3,
2H), 7.90 (Ar-H, d, J = 12.9 Hz, 8H), 7.68 (Ar-H, d, J = 6.9 Hz, 4H), s, 6H); 13C NMR (75.4 MHz, CDCl3): (δ, ppm) 134.27, 130.26,
7.05 (Ar-H, d, J = 7.5 Hz, 4H), 3.83 (OCH3, s, 6H), 2.69 (-CH2-, s, 129.36, 128.87, 127.97, 127.79, 21.41, -2.79; HRMS (FAB) calcd.
4H), 0.62 (-CH3, s, 6H); HRMS (FAB) calcd. for C32H33N4O6Si: for C44H39N6Si: [M+H]+ 679.3005, Found: 679.3002; Anal. calcd
[M+H]+: 597.2169, Found: 597.2167; Anal. calcd for for C44H38N6Si: C, 77.84; H, 5.64; N, 12.38; Si, 4.14. Found: C,
C32H32N4O6Si: C, 64.41; H, 5.41; N, 9.39; O, 16.09; Si, 4.71. Found: 77.83; H, 5.53; N, 12.36.
C, 64.40; H, 5.42; N, 9.40.
General procedure for the synthesis of ST compounds A 250 yl)phenyl)dimethylsilane, ST-tBu. This compound was also
mL two-neck round bottomed flask ( ) equipped with a prepared by the above mentioned procedure by using the
magnetic stir bar was charged with the ST compound precursors following materials: flask : 4,4'-(dimethylsilanediyl)bis(N'-(4-
and o-dichlorobenzene. Another 250 mL two-neck round (tert-butyl)benzoyl)benzohydrazide) (8.0 g, 12.2 mmol) and o-
Bis(4-(5-(4-tert-Butylphenyl)-4-phenyl-4H-1,2,4-triazol-3-
A
A
bottomed flask (
B
) equipped with a magnetic stir bar and a dichlorobenzene (50 mL), flask
B: aniline (13.4 mL, 147.5 mmol),
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Journal of Materials Chemistry C
Page 4 of 10
ARTICLE
Journal Name
PCl3 (2.57 mL, 29.5 mmol), and o-dichlorobenzene (50 mL). The
crude product was purified by silica gel flash column
chromatography using THF:dichloromethane = 1:10 (v:v) as an
eluent to afford the product as a white solid. Yield: 5.0 g, 54%.
1H NMR (300.1 MHz, CDCl3): (δ, ppm) 7.51-7.50 (Ar-H, m, 1H),
7.50-7.47 (Ar-H, m, 2H), 7.46-7.45 (Ar-H, m, 2H), 7.43-7.42 (Ar-
H, m, 1H), 7.38-7.37 (Ar-H, m, 8H), 7.35 (Ar-H, t, J = 1.80 Hz, 3H),
7.30 (Ar-H, t, J = 2.55 Hz, 3H), 7.29-7.28 (Ar-H, m, 2H), 7.21 (Ar-
H, t, J = 1.50 Hz, 2H), 7.18 (Ar-H, t, J = 1.86 Hz , 2H), 1.28 (-CH3,
s, 18H), 0.49 (-CH3, s, 6H); 13C NMR (75.4 MHz, CDCl3): (δ, ppm)
155.04, 154.66, 153.10, 139.83, 135.58, 134.31, 130.22, 129.94,
128.54, 128.14, 127.98, 127.83, 125.60, 124.05, 34.95, 31.33, -
2.48; HRMS (FAB) calcd. for C50H51N6Si: [M+H]+ 763.3944, Found:
763.3942; Anal. calcd for C50H50N6Si: C, 78.70; H, 6.60; N, 11.01;
Si, 3.68. Found: C, 78.75; H, 6.62; N, 11.00.
View Article Online
DOI: 10.1039/C6TC00017G
(a)
(b)
Bis(4-(5-(4-Methoxyphenyl)-4-phenyl-4H-1,2,4-triazol-3-
yl)phenyl)dimethylsilane, ST-OMe This compound was also
prepared by the above mentioned procedure by using the
following materials: flask
methoxybenzoyl)benzohydrazide) (8.1 g, 13.6 mmol) and o-
dichlorobenzene (50 mL), flask : aniline (14.86 mL, 163.0
A: 4,4'-(dimethylsilanediyl)bis(N'-(4-
B
Figure 1. (a) Single crystal structure of ST-tBu. Hydrogens were omitted for
clarity and (b) crystal packing of ST-tBu in a unit cell.
mmol), PCl3 (2.8 mL, 32.6 mmol), and o-dichlorobenzene (50
mL). The crude product was purified by silica gel flash column
chromatography using THF:dichloromethane = 1:4 (v:v) as an
eluent to afford the product as a white solid. Yield: 6.1 g, 63%.
1H NMR (300.1 MHz, CDCl3): (δ, ppm) 7.54-7.53 (Ar-H, m, 1H),
7.52-7.20 (Ar-H, m, 2H), 7.48-7.47 (Ar-H, m, 2H), 7.46-7.45 (Ar-
H, m, 1H), 7.39 (Ar-H, t, J = 4.35 Hz, 12H), 7.21 (Ar-H, t, J = 1.41
Hz, 2H), 7.19 (Ar-H, t, J = 3.30 Hz, 2H), 6.84 (Ar-H, t, J = 3.00 Hz,
2H), 6.80 (Ar-H, t, J = 2.55 Hz, 2H), 3.80 (-OCH3, s, 6H), 0.50 (-
CH3, s, 6H); 13C NMR (75.4 MHz, CDCl3): (δ, ppm) 160.81, 154.97,
154.53, 151.72, 139.80, 135.97, 135.56, 134.31, 130.43, 130.22,
129.89, 128.45, 128.10, 127.96, 127.86, 125.72, 119.40, 114.08,
55.46, 34.43, 31.79, 30.52, 22.86, 21.39, 14.32, -2.49; HRMS
(FAB) calcd. for C44H39N6O2Si: [M+H]+ 711.2904, Found:
711.2901; Anal. calcd for C44H38N6O2Si: C, 74.34; H, 5.39; N,
11.82; O, 4.50; Si, 3.95. Found: C, 74.33; H, 5.38; N, 11.84.
Crystal Structure Determination Single crystal of ST-tBu was
sealed in glass capillaries under argon and mounted on the
diffractometer. The preliminary examination and data
collection were performed using a Bruker SMART CCD detector
system single-crystal X-ray diffractometer equipped with a
sealed-tube X-ray source (50 kV × 30 mA) using graphite-
monochromated Mo Krad ( λ = 0.71073 Å). The preliminary unit
cell constants were determined using a set of 45 narrow-frame
(0.3° in ω) scans. A double-pass method of scanning was used
to exclude any noise. The collected frames were integrated
using an orientation matrix determined from the narrow-frame
scans. The SMART software package was used for data
collection, and SAINT was used for frame integration.53 The final
cell constants were determined by a global refinement of the
xyz centroids of the reflections harvested from the entire data
set. Structure solution and refinement were carried out using
the SHELXTL-PLUS software package.54
spectrophotometer and a VARIAN Cary Eclipse fluorescence
spectrophotometer, respectively. The triplet energies were
determined at the maximum of the first vibronic transition of
the phosphorescence spectra. The samples dissolved in 2-
MeTHF were frozen in liquid nitrogen at 77 K. The samples were
excited using a 1 ns pulsed nitrogen laser (GL-3300, Photon
Technology International) at a repetition rate of 10 Hz.
Luminance light was allowed to pass through a monochromator
(Oriel instruments) and Photomultiplier photomultiplier tube
(Oriel instruments). The signal was obtained using a gated
boxcar Integrator System (SR 250, Stanford research System).
The detection window was set to 15 µs, and the delay was set
to 0.2 ms from the rigging edge of the laser. For the transient
photoluminescent decay measurements, a nitrogen laser (GL-
3300, PTI) was used as the excitation light source, and the
phosphorescence light was passed through a monochromator
(Oriel 77200), to which a photomultiplier tube (Oriel 77346) was
attached at the exit slit.
Thermal Property Thermal property of the synthesized
materials was measured by differential scanning calorimetry
(Perkin-Elmer/Pyris Diamond DSC). A heating rate of 10 °C/min
was used after the first melting of the compound, followed by a
rapid cooling to room temperature at a rate of 40 °C/min.
DFT calculations The derivatives were theoretically calculated
using the Gaussian 09 package.55 The ground-state geometries
of compounds were optimized using the B3LYP density
functional theory (DFT) and 6/31G (d,p) basis set. The contours
of electron density were plotted using Chem3D ver.10.56
OLED Devices The OLED devices were fabricated on glass
substrates pre-coated with a 150 nm ITO layer having a sheet
resistance of 10
Ω/square. The ITO glass was pre-cleaned using
Absorption, Emission, and Phosphorescence The absorption
a conventional solvent cleaning method. The ITO surface was
cleaned again with a UV ozone treatment immediately before
and photoluminescence spectra were recorded using
SHIMADZU UV-3101PC UV-VIS-NIR scanning
a
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50
100
150
200
250
ST-CF UV
DOI: 10.1039/C6TC00017G
3
1.0
0.8
0.6
0.4
0.2
0.0
ST-Me UV
ST-tBu UV
ST-OMe UV
ST-CF3 PL
Tg = 123oC
ST-CF3
Tg = 97oC
ST-Me PL
ST-tBu PL
ST-OMe PL
ST-Me
ST-tBu
Tc = 188oC
Tg = 131oC
Tg = 110oC
ST-OMe
200 250
50
100
150
Temperature (oC)
250
300
350
400
450
500
550
Wavelength (nm)
Figure 2. DSC diagrams for ST-CF3, ST-Me, ST-tBu, and ST-OMe.
Figure 3. UV-Vis absorption and normalized PL emission spectra of ST-CF3,
ST-Me, ST-tBu, and ST-OMe in 10 μM THF solution.
depositing the hole injecting layer (HIL). The organic, Liq, and Al
layers were deposited sequentially onto the substrate without
breaking the vacuum. The current–voltage characteristics of
OLEDs were measured using a source measure unit (Keithley
2635B). The electroluminescence spectra, luminance, and CIE
coordinates were measured using a spectro-radiometer (Konica
Minolta CS-2000). Assuming Lambertian emission, the external
quantum efficiency (EQE) was calculated from the luminance,
current density, and electroluminescence spectrum.
space group at R1 = 0.0559. The silicon atom disposes a
tetrahedral geometry, which is defined by the three methyl
groups [1.850(3) Å (averaged)] and one ipso-carbon atom in the
phenyl ring [1.885(2) Å]. The triazole ring and the phenyl ring
substituted at 2-position are tilted by 41.09(8)°. In contrast, the
phenyl ring at 1-position (C17-C18-C19-C20-C21-C22) is twisted
further to 59.56(7)°. The crystal packing structure of ST-tBu is
shown in Figure 1b. As noted from the crystal packing diagram
shown in Figure 1b, two different intermolecular interaction
motifs are present in the middle molecule when three packed
molecules are considered. One is hydrogen bonding between
the hydrogen atom of phenyl ring and the nitrogen atom of
triazole ring with the bond distance of 2.614 Å. The other is π–
π interaction between the phenyl groups to which tert-butyl
groups are substituted. tert-Butyl groups encourage spatial
overlap of the two adjacent phenyls falling their closest plan-to-
plan distance into ca. 4.18 Å. Those two types of interactions
come into play alternately in one molecule to another sidewise.
So, driving force for the molecular packing in ST-tBu comes from
hydrogen bonding and intermolecular π–π interaction. In
particular, enhanced intermolecular π–π interactions seem to
be the prime reason for the higher mobility in the context of
facile electron-hopping process among adjacent molecules in
close proximity.
Thermal Properties The thermal properties of ST compounds
were analysed by thermogravimetric analysis (TGA) and
differential scanning calorimetry (DSC) under nitrogen
atmosphere and are summarized in Table 1. All the compounds
exhibited high thermal decomposition temperatures (Td) in the
range 352–474 °C (Figures S2–S5 in the ESI) and high glass
transition temperatures (Tg) in the range of 97–131 °C (Figure 2),
which are substantially higher than general electron
transporting materials such as 3-(4’-tert-butylphenyl)-4-phenyl-
5-(4”-biphenyl)-1,2,4-triazole (TAZ, Tg ≈ 70 °C)43,57 and as high as
1,3,5-tris(1-phenyl-1H-benzo[d]imidazol-2-yl)benzene (TPBi, Tg
= 124 °C).58 High thermal stabilities, in particular, having high Tg
are desirable in phosphorescence OLEDs, because they can
Results and discussion
Synthesis and Structural Characterization The target ST
compounds were prepared as illustrated in Scheme 1, and the
detailed procedures are given in the experimental section. The
tetrahedral silicon structure was initialized by the Grignard
reaction
dimethyldichlorosilane in a quantitative yield. Treatment of
with KMnO4 results in oxidation to afford , followed by the
condensation reaction with methanol to give in 95% yields.
Important intermediate, silylbenzohydrazide moiety ( ), was
prepared quantitatively by the reaction of with excess
between
p-tolylmagnesium
bromide
and
1
2
3
4
3
amounts of hydrazine monohydrate under reflux condition.
Then, it was treated with various substituted benzoyl chloride
in the presence of triethylamine as a base yielding the
precursors 5a
condensation reaction of
phosphorus trichloride at 180 °C afforded the final products in
–5d in quantitative yields. Finally, the
5
with aniline in the presence of
moderate yields (41–63%). Final products, ST-CF3, ST-Me, ST-
tBu, and ST-OMe were further purified by train sublimation. All
1
the compounds were fully characterized by H and 13C NMR,
high-resolution mass spectrometry, and elemental analysis.
Among them, the solid-state molecular structure of ST-tBu was
determined by single-crystal X-ray crystallography using
suitably grown crystals in dichloromethane/n-hexane. Figure 1a
shows the molecular structure of a single crystal for ST-tBu
.
Tables S1–S3 in the electronic supplementary Information (ESI)
summarizes the structural parameters, bond lengths, and
angles determined by the X-ray structural studies. ST-tBu was
crystallized in the monoclinic crystal system using the C2/c
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Journal Name
Table 1. Physical properties of ST compounds, ST-CF3, ST-Me, ST-tBu, and ST-OMe in THF solution
View Article Online
DOI: 10.1039/C6TC00017G
HOMO
(eV)c
−6.95
−6.78
−6.75
−6.67
LUMO
(eV)d
−3.03
−2.83
−2.81
−2.79
Eg
Comp.
Tg (°C)
abs (nm)a
em (nm)a
ET (eV)b
Ered (V)a
(eV)
3.92
3.95
3.94
3.88
ST-CF3
ST-Me
ST-tBu
123
97
131
110
276
275
276
279
354
362
364
372
2.83
2.85
2.85
2.84
−2.44
−2.64
−2.66
−2.68
ST-OMe
aMeasured in THF. bMeasured in 2-MeTHF. The HOMO and LUMO levels were determined using the following equations: cEHOMO(eV) =ELUMO−Eg.dELUMO(eV) = −e(Ered(V
vs Fc/Fc+) + 4.74 eV + 0.73 eV); Correction value of 0.73 V was used from TAZ (reported value of ELUMO = -2.7 eV).57
-2.40
-2.45
-2.50
-2.55
-2.60
-2.65
-2.70
1.0
0.5
ST-CF3
ST-Me
ST-tBu
Eonset vs 
Linear Fit
0.0
1.0
0.5
0.0
1.0
0.5
0.0
1.0
ST-OMe
TAZ
0.5
0.0
1.0
0.5
0.0
-0.4
-0.2
0.0
0.2
0.4
0.6
400
450
500
550
600
650
Hammett Parameter ()
Wavelength (nm)
Figure 5. Reduction potential (Eonset, V vs Fc/Fc+) vs Hammett parameter (σ)
for ST-CF3, ST-Me, ST-tBu, and ST-OMe.
Figure 4. Phosphorescence spectra of ST-moieties and TAZ measured in 2-
MeTHF at 77 K.
Electrochemical Properties To investigate the electrochemical
properties of ST-CF3, ST-Me,
ST-tBu, and ST-OMe, cyclic
prevent morphological changes and suppress aggregation upon
heating.
voltammetry (CV) was performed by utilizing a three-electrode
cell system: platinum disk electrode was used as the working
electrode, whereas a platinum wire and Ag/AgNO3 were used as
the counter and reference electrodes, respectively. As shown in
Figure S6 in the ESI, irreversible reduction waves appeared at
Epc = −2.44, −2.64, −2.66, and −2.68 V (versus Fc/Fc+) for
Photophysical Properties Figure 3 shows the electronic
absorption and fluorescence spectra of the ST compounds in
THF solution. All the compounds showed maximum absorptions
in the range of 275–279 nm, which are assigned to the π–π*
electronic transition of the triazole heterocycle ring. The
Hammett parameters of substituents do not affect their
absorption properties. In contrast, spectral shifts in emission
correspond well to the electronic perturbation of the
substituents, as summarized in Table 1. ST-CF3 showed
hypsochromic shifted emission at 354 nm, whereas ST-OMe
showed bathochromic shifted emission at 372 nm because of
the lower energy gap. Figure 4 shows the phosphorescence
spectra of ST-moieties measured in 2-MeTHF at 77 K. The
phosphorescence peaks of the ST-moieties are in the similar
range 434−438 nm. The energy gap of the triplet levels,
compounds ST-CF3
linear plot of reduction potential (Eonset) versus Hammett
parameter (σ), for ST-CF3 ST-Me
ST-tBu, and ST-OMe with the
, ST-Me,
ST-tBu, and ST-OMe, respectively. A
,
,
known Hammett parameters59 indicates that there is indeed a
strong dependency of the reduction potential on the nature of
the electron-withdrawing or the electron-donating ability at the
4-position of the end-on phenyl groups on the triazole, as
shown in Figure 5. Using redox potentials, the energy levels of
the highest occupied molecular orbital (HOMO) and the lowest
unoccupied molecular orbital (LUMO) in compounds ST-CF3
, ST-
Me
,
ST-tBu, and ST-OMe were estimated as −6.95/−3.03,
estimated from the phosphorescence peaks of ST-CF3, ST-Me,
ST-tBu, and ST-OMe were obtained at ET1 = 2.83, 2.85, 2.85, and
2.84 eV, respectively, as listed in Table 1. They showed much
higher triplet levels compared to that of TAZ (ET1 = 2.70 eV)43
because of the suppression of electron delocalization by the
silicon atom, whereas TAZ contains only organic biphenyl group
at the end-on position. Because the energy gap of the triplet
levels of the ST-moieties are higher than that of deep blue
dopant, for example, FIrpic (ET1 = 2.65 eV),27 the ST series are
expected to preferentially confine the triplet excitons of Firpic.
−6.78/−2.83, −6.75/−2.81, and −6.67/−2.79 eV, respectively, as
listed in Table 1. With increasing electron withdrawing
capability of the triazole units, the energy levels of LUMO were
lowered, indicating that the electron injection from a metallic
cathode could be improved for high-performance blue OLEDs.
Theoretical Calculations To gain insight into the electronic
states of ST compounds, DFT calculations were carried out using
the TD-DFT/B3LYP/6-31G(d,p) method with the Gaussian 09
package.55 All the ST compounds showed a very similar HOMO
and LUMO distributions. The calculated electron contour plots
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(a)
View Article Online
60
DOI: 10.1039/C6TC00017G
ST-Me
ST-OMe
ST-CF3
ST-tBu
50
40
30
20
10
0
0
2
4
6
8
10
Voltage (V)
Figure 6. Spatial distributions for the frontier orbitals of compounds ST-CF3,
ST-Me, ST-tBu, and ST-OMe.
(b)
1E-10
1E-11
1E-12
1E-13
of the molecular orbitals are shown in Figure 6. The HOMOs
and LUMOs of the ST compounds are distributed mainly over
the triazole and two phenyl groups at 3- and 5-positions of the
triazole ring. Because of the symmetrical molecular structure of
the ST compounds, the HOMO and HOMO–1 orbitals are
energetically degenerated, and their distributions are same but
positioned at different branches. The LUMO and LUMO+1
orbitals also show similar phenomenon, but there were very
small distributions on the phenyl group at 4-position of the
triazole ring. Since the major electronic distributions of the
LUMO levels are on the triazole unit for all the compounds,
efficient electron transfer properties of ST compounds are
expected.
Electron-only devices (EODs) To compare the electron-
transport properties of ST compounds, electron-only devices
(EODs) containing each compound were fabricated. Figure 7
shows the current density–voltage (J–V) characteristics of EODs
with the structures of ITO (150 nm)/Al (50 nm)/ST (100 nm)/Liq
(1 nm)/Al (150 nm). Under the same voltage, the EOD
containing ST-tBu showed significantly higher current densities
ST-Me
ST-OMe
ST-CF3
ST-tBu
1000
700
800
900
E1/2(V1/2/cm1/2
)
Figure 7. (a) J V characteristics of electron-only devices (EODs) for ST-CF3,
ST-Me, ST-tBu, and ST-OMe. Devices were fabricated with the following
structures: ITO (150 nm)/Al (50 nm)/ST (100 nm)/Liq (1 nm)/Al (150 nm)
and (b) the J-V characteristics were fitted to SCLC with field dependent
mobility. J = (9/8)ε0εr(E2/L)μ0exp(0.89β 퐸), where J is the current density;
√
ε0εr is the dielectric constant; E is the electric field; L is the film thickness;
μ0 is the zero-field carrier mobility; and β is the Poole Frenkel factor. The
values of μ0 and β were calculated by assuming εr is equal to 3. The field
dependent mobility was calculated using Poole Frenkel equation: μ(E) =
μ0exp(β 퐸).
√
compared to other compounds, ST-CF3, ST-Me, and ST-OMe.
Murgatroyd et al. presented the space charge limited current
(SCLC) model, which is characterized by field dependent
nm)/mCBP:FIrpic 8% (30 nm)/ST-tBu (5 nm)/ ST-tBu:Liq 50% (25
nm) /Liq (1 nm)/Al (150 nm), where HAT-CN is dipyrazino[2,3-
f:2’,3’-h]quinoxaline-2,3,6,7,10,11-hexacarbonitrile, TAPC is
4,4’-cyclohexylidene-bis[N,N-bis(4-
methylphenyl)benzenamine], mCBP is 3,3-di(9H-carbazol-9-
yl)biphenyl. The chemical structures and energy level diagram
of the used materials are shown in Figure S7. The current
density–voltage–luminance (J–V–L) characteristics, current
efficiency vs. current density, power efficiency vs. current
density, and EQE vs. current density are shown in Figure 8. The
performances of the devices are summarized in Table 2. Overall,
the PHOLED device using ST-tBu as an electron-transporting
layer showed peak external electroluminescent (EL) quantum
and power efficiencies of ηext = 15.9% and ηp = 17.9 lm/W.
mobility;60,61 the electron mobilities (μ) of ST-CF3 ST-tBu
, ST-Me, ,
and ST-OMe, which were estimated by this model, were found
to be 9.52 X 10–5, 1.62 X 10–5, 2.65 X 10–4, and 3.93 X 10–5 cm2/Vs
at 0.5 MV/cm, respectively. The ST-tBu compound showed the
highest electron mobility among ST compounds. We believe
there are two factors related to the enhanced electron mobility
on ST-tBu: one is related to electronic effect stemming from the
tert-Bu group and the other is associated with the molecular
aggregation in the solid states. As can be seen in the Hammett
plot of Figure 5, ST-tBu lies in the electron donating side second
to the ST-OMe. Also, as noted in the crystal packing structure,
ST-tBu exhibits hydrogen bonding as well as π–π interactions
between adjacent molecules among repeating molecules in the
crystal packing. We believe higher mobility of ST-tBu is due to
synergistic effect of those two factors.
PHOLEDs performances To examine ST-tBu as an electron-
transporting material, PHOLEDs was fabricated with the
structure of ITO (150 nm)/HATCN (10 nm)/TAPC (85
Conclusions
In this study, a series of modified bis(4-(4,5-diphenyl-4H-1,2,4-
ST-tBu
triazol-3-yl)phenyl)dimethylsilane (ST), ST-CF3, ST-Me, ,
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Journal Name
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DOI: 10.1039/C6TC00017G
(a)
(b)
35
30
25
20
15
10
5
100
7000
6000
5000
4000
3000
2000
1000
0
10
1
0.1
0.01
1E-3
1E-4
1E-5
1E-6
1E-7
1E-8
0
0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
0
1
2
3
4
5
Current density (mA/cm2)
Voltage (V)
(c)
(d)
20
20
18
16
14
12
10
8
18
16
14
12
10
8
6
6
4
4
2
2
0
0
0
1
2
3
4
5
0
1
2
3
4
5
Current density (mA/cm2)
Current density (mA/cm2)
Figure 8. (a) J-V-L characteristics, (b) current efficiency-current density curves, (c) power efficiency-current density curves, and (d) EQE-current density curves of device.
Table 2. EL performances of the device with ST-tBu as a ETM
Power efficiency
(lm/W)
Current efficiency
(cd/A)
EQE
(%)
Turn on
(V)
CIE
at 400 nits
Device
max
400 nits
max
400 nits
max
400 nits
14.05
ST-tBu
4.29
17.87
10.97
30.05
26.82
15.92
(0.14, 0.31)
Device configuration: ITO (150 nm)/HATCN (10 nm)/TAPC (85 nm)/mCBP:FIrpic 8% (30 nm)/ST-tBu (5 nm)/ ST-tBu:Liq 50% (250 nm) /Liq (1 nm)/Al (150 nm).
and ST-OMe, were designed and prepared as electron-
transporting materials. Their energy band gaps were
successfully tuned depending on the Hammett parameters. All
Acknowledgements
the compounds showed the triplet levels of > 2.83 eV, which are
much higher compared to that conventional electron-
transporting material, TAZ (ET1 = 2.70 eV), because of the
suppression of electron delocalization by silicon centre. Among
the series, ST-tBu exhibited highest thermal stability with Tg at
131 °C. Furthermore, the fabricated EOD of ST-tBu showed most
efficient electron-transporting capability among the series.
Finally, in the PHOLED devices using ST-tBu as an electron-
transporting material, a maximum EQE and maximum power
efficiency of 15.9% and 17.9 lm/W, respectively, were achieved.
These results suggest that the materials in the ST series can be
used as potential blue phosphorescence electron-transporting
materials.
This research was supported by the Industrial Core Technology
Development Program (10044876) funded by the Ministry of
Trade, Industry & Energy (MOTIE, Korea), Basic Science
Research Program through the National Research Foundation of
Korea (NRF) funded by the Ministry of Education (NRF-
2014R1A6A1030732), the MOTIE (Ministry of Trade, Industry &
Energy (10051379) and KDRC (Korea Display Research
Corporation) support program for the development of future
devices technology for display industry, the Functional Districts
of the Science Belt support program, Ministry of Science, ICT
and Future Planning (2015K000287), and Basic Science
Research Program through the National Research Foundation of
Korea (NRF) funded by the Ministry of Science, ICT & Future
Planning (2014R1A1A1005982 and 2014R1A1A1007625).
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32 H. Antoniadis, M. A. Abkowitz and B. R. Hsieh, Appl. Phys. Lett.,
Notes and references
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DOI: 10.1039/C6TC00017G
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