Triaryl boron derivatives of pyridine as electron transporting materials for blue phosphorescent organic light-emitting diodes

Article abstract of DOI:10.1016/j.orgel.2018.07.014

Two duryl borane derivatives, tris(2,3,5,6-tetramethyl-4-(pyridin-3-yl)phenyl)borane (TDPB) and tris(2,3,5,6-tetramethyl-4'-(pyridin-3-yl)biphenyl-4-yl)borane (TDPPB), which contain heteroaromatic pyridine rings at peripheral positions were synthesized to

Full text of DOI:10.1016/j.orgel.2018.07.014

Accepted Manuscript
Triaryl boron derivatives of pyridine as electron transporting materials for blue
phosphorescent organic light-emitting diodes
Jiewon Lee, Sunhee Lee, Jin-Hyoung Kim, Sang Ook Kang, Won-Sik Han
PII:
S1566-1199(18)30371-9
10.1016/j.orgel.2018.07.014
ORGELE 4794
DOI:
Reference:
To appear in: Organic Electronics
Received Date: 31 May 2018
Revised Date: 6 July 2018
Accepted Date: 16 July 2018
Please cite this article as: J. Lee, S. Lee, J.-H. Kim, S.O. Kang, W.-S. Han, Triaryl boron derivatives
of pyridine as electron transporting materials for blue phosphorescent organic light-emitting diodes,
Organic Electronics (2018), doi: 10.1016/j.orgel.2018.07.014.
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ACCEPTED MANUSCRIPT

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Triaryl Boron Derivatives of Pyridine as
Electron Transporting Materials for Blue
Phosphorescent Organic Light-emitting
Diodes
Jiewon Lee,a,† Sunhee Lee,a,† Jin-Hyoung Kim,b Sang Ook Kang,b and Won-Sik Han*,a
aDepartment of Chemistry, Seoul Women’s University, Seoul 01797, South Korea.
bDepartment of Advanced Materials Chemistry, Korea University, Sejong 30019, South
Korea.
†These authors contributed equally to this work.
E-mail: wshan@swu.ac.kr
ABSTRACT
Two duryl borane derivatives, tris(2,3,5,6-tetramethyl-4-(pyridin-3-yl)phenyl)borane (TDPB)
and tris(2,3,5,6-tetramethyl-4'-(pyridin-3-yl)biphenyl-4-yl)borane (TDPPB), which contain
heteroaromatic pyridine rings at peripheral positions were synthesized to investigate the
effects of bridging phenyl group on their thermal, photophysical, and electrochemical
properties. Upon adding a phenyl unit between duryl and pyridine units, TDPPB showed
slightly higher thermal stability (Td = 446 °C and Tg = 166 °C) than TDPB (Td = 439 °C and
Tg = 142 °C). Both TDPB and TDPPB have deep highest occupied molecular orbital levels
to block the holes/excitons and appropriate lowest unoccupied molecular orbital levels for
smooth electron injection. Notably, TDPB and TDPPB exhibited relatively high triplet
energies of 2.84 eV and 2.69 eV, respectively. Thus, the triplet energy level of TDPB is
sufficient for use in bis[2-(4,6-difluorophenyl)pyridinato-C2,N](picolinato)iridium(III)
(FIrpic) based blue phosphorescent organic light-emitting diodes (PHOLEDs) as an electron

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transport material. Accordingly, TDPB was used as an electron transport material in FIrpic-
based blue PHOLEDs and the optimized device showed high external quantum efficiency of
up to 18.8 %.
1. Introduction
Triaryl boron based compounds have been widely investigated owing to their unique
structural and electronic properties such as a triangular planar structure, existence of a vacant
p orbital allowing effective orbital interaction with π-conjugated compounds, relatively high
Lewis acidity, and a π electron-accepting property in the excited state. Thus, when
manipulating molecules with borane, it is possible to synthesize elaborate π-functional
materials with effective photophysical and electronic properties using the properties of boron-
based compounds.
In particular, trivalent boron compounds can act as an electron-accepting group in donor–
acceptor dyad systems owing to their electron-deficient nature.1,2 Thus, by attaching an
electron-donating group such as pyridine to a boryl compound, the latter can exhibit
intramolecular charge transfer (CT) transitions. Therefore, it is possible to apply a substance
having a specific photophysical characteristic to the photochemical field, e.g., non-linear
optical materials,3,4 two-photon absorption materials,5,6 and in particular, electron transport
materials (ETMs),7−9 or emitting materials10-12 in organic light-emitting diodes (OLEDs).
The essential materials in OLED devices can be roughly classified into three types: hole
transport material, emitting material, and ETM. To achieve high efficiency in OLED devices,
it is necessary to develop materials with suitable properties for each layer. For example,
materials for charge transport layers, i.e., hole transport layer (HTL) and electron transport
layer (ETL), must have suitable energy levels and high charge mobility to effectively transfer
holes and electrons, respectively, to generate excitons with high quantum yield.13 In this

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regard, various strategies have been attempted to develop highly efficient HTL14 and ETL15-17
materials. Accordingly, triarylborane-based materials are utilized as HTL18 and ETL7-9 in
OLEDs. In particular, Kido et al. report tris[3-(3-pyridyl)mesityl]borane as an electron
transporting material which has a wide HOMO–LUMO energy gap (3.45 eV) and a high
triplet excited energy level (2.95 eV).7,19
In this work, a series of duryl borane derivatives, tris(2,3,5,6-tetramethyl-4-(pyridin-3-
yl)phenyl)borane (TDPB) and tris(2,3,5,6-tetramethyl-4'-(pyridin-3-yl)biphenyl-4-yl)borane
(TDPPB), is designed and synthesized to study structure-property relationship. The chemical
structures of TDPB and TDPPB are illustrated in Chart 1. In this molecular design, we
expect that they can be used as ETMs in OLEDs because, owing to the presence of a vacant p
orbital at the boron center, effective orbital interactions with π-conjugated pyridine-based
peripheral groups are possible. In this regard, the effects of phenyl bridge on the thermal,
photophysical, and electrochemical properties of TDPB and TDPPB were systematically
investigated. Furthermore, the nature of their excited states was investigated using density
functional theory (DFT) calculations and the results were compared with the experimental
results. Finally, a bis[2-(4,6-difluorophenyl)pyridinato-C2,N](picolinato)iridium(III) (FIrpic)
based blue phosphorescent device was fabricated with TDPB as an ETM and the optimized
device showed a relatively high external quantum efficiency (EQE) of up to 18.8 %.
N
N
B
B
N
N
N
N
TDPB
TDPPB

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Chart 1. Chemical structures of TDPB and TDPPB.
2. Experimental
2.1. General information
All the experimental procedures were performed under a dry nitrogen or argon atmosphere
using standard Schlenk techniques. All solvents were distilled freshly and used under dry
1
nitrogen or argon purging. H and 13C nuclear magnetic resonance (NMR) spectra were
recorded using a Bruker Fourier 300 MHz spectrometer operated at 300.1 and 75.4 MHz,
1
respectively. H and 13C NMR chemical shifts were measured in CDCl3 and CD2Cl2.
1
Referenced to the relative peaks of CDCl3 and CD2Cl2 are 7.26 and 5.32 ppm for H NMR
13
and 77.16 and 54.00 ppm for C NMR, respectively. Elemental analyses were performed
using a Carlo Erba Instruments CHNS-O EA 1108 analyzer. A high-resolution mass
spectrometry (HR-MS) analysis was performed using a highly sensitive liquid
chromatography–multistage MS (LC/MS/MSn) (n = 10) spectrometer (Thermo Fisher
Scientific, LCQ Fleet Hyperbolic Ion Trap MS/MSn Spectrometer). Cyclic voltammetry (CV)
was performed in an electrolytic solution prepared using 0.1 M tetrabutyl ammonium
hexafluorophosphate (NBu4PF6) at room temperature under an atmosphere of argon. For this
purpose, CHI600E was used. Glassy carbon, platinum wire, and Ag/AgNO3 (0.1 M) were
used as the working, counter, and reference electrodes, respectively. All the potentials were
calibrated to the ferrocene/ferrocenium (Fc/Fc+) redox couple. 1,4-Dibromobenzene, boron
trifluoride diethyl etherate, trimethyl borate, 1,2,4,5-tetramethylbenzene, 3-bromopyridine,
potassium
carbonate,
bis(pinacolato)diboron,
potassium
acetate,
tetrakis(triphenylphosphine)palladium(0), and n-BuLi (2.5 M solution in n-hexane) were
purchased from Aldrich or TCI and used without further purification. The starting materials,
1,4-dibromo-2,3,5,6-tetramethylbenzene
(1),20
tris(bromoduryl)borane
(2),21
3-

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pyridinylboronic acid (3),22 and 3-(4-bromophenyl)pyridine23 were prepared according to
previously reported procedures.
2.2. Synthesis
3-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)pyridine (4) A mixture of 3-(4-
bromophenyl)pyridine (1.4 g,
6
mmol), 4,4,4',4',5,5,5',5'-octamethyl-2,2'-bi(1,3,2-
dioxaborolane) (2.28 g, 9 mmol), Pd(PPh3)2Cl2 (0.25 g, 0.3 mmol), and KOAc (1.76 g, 18
mmol) in 1,4-dioxane (35 mL) was stirred at 100 °C for 5 h under argon atmosphere. After
cooling to room temperature, the mixture was poured into distilled water and extracted with
dichloromethane. The combined organic layers were dried over MgSO4 and evaporated under
reduced pressure. The crude product was purified via silica-gel column chromatography
using ethyl acetate/n-hexane = 1/1(v:v) as an eluent. The resulting oily product was dissolved
1
in dichloromethane and evaporated. Yield: 77%; H NMR (300 MHz, CDCl3, δ): 8.84–8.83
(d, J = 3 Hz, 1H), 8.57–8.55 (m, 1H), 7.91–7.88 (m, 2H), 7.87–7.84 (m, 1H), 7.58–7.55 (m,
13
2H), 7.35–7.30 (m, 1H), 1.33 (s, 12H), C NMR (75 MHz, CDCl3, δ): 150.5, 149.4, 147.8,
139.3, 138.8, 137.2, 136.1, 136.0, 135.9, 131.8, 131.7, 131.7, 131.6, 123.3, 31.0, 20.4.
tris(2,3,5,6-Tetramethyl-4-(pyridin-3-yl)phenyl)borane (TDPB) A mixture of tris(4-
bromo-2,3,5,6-tetramethylphenyl)borane (3) (1.06 g, 1.63 mmol), 3-pyridinylboronic acid (1)
(0.8 g, 6.52 mmol), tetrakis(triphenylphosphine)palladium (0.04 g, 0.03 mmol), and 2 M
K2CO3 (4.9 mL, 9.78 mmol) in toluene/ethanol (v:v = 12 mL/4 mL) was stirred at 80 °C for
48 h under argon atmosphere. After cooling to room temperature, the mixture was poured into
distilled water and extracted with dichloromethane. The combined organic layers were dried
over MgSO4 and evaporated under reduced pressure. The crude product was purified via
silica-gel column chromatography using ethyl acetate as an eluent. The resulting white solid

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1
was dissolved in dichloromethane and evaporated. Yield: 81%; H NMR (300 MHz, CDCl3,
δ): 8.60–8.59 (d, J = 3.6 Hz, 3H), 8.41 (s, 3H), 7.50–7.48 (d, 3H), 7.39–7.34 (m, 3H), 2.07 (b,
13
s, 18H), 1.85 (b, s, 18H), C NMR (75 MHz, CDCl3, δ): 150.5, 149.4, 147.8, 139.3, 138.8,
137.2, 136.1, 136.0, 135.9, 131.8, 131.7, 131.7, 131.6, 123.3, 31.0, 20.4. HRMS (FAB) calcd
for C45H48BN3: 641.3941, found 641.3939; Elem. anal. calcd for C45H48BN3: C, 84.23; H,
7.54; B, 1.68; N, 6.55; Si, 4.72. Found: C, 84.22; H, 7.52; N, 6.56.
tris(2,3,5,6-Tetramethyl-4'-(pyridin-3-yl)biphenyl-4-yl)borane (TDPPB) A mixture of
tris(4-bromo-2,3,5,6-tetramethylphenyl)borane (3) (0.74 g, 1.15 mmol), 3-(4-(4,4,5,5-
tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)pyridine
(5)
(1.3
g,
4.6
mmol),
tetrakis(triphenylphosphine)palladium (0.07 g, 0.06 mmol), and K2CO3 (0.95 g, 6.9 mmol) in
THF/H2O (v:v = 12 mL/4 mL) was stirred at 80 °C for 48 h under argon atmosphere. After
cooling to room temperature, the mixture was poured into distilled water and extracted with
dichloromethane. The combined organic layers were dried over MgSO4 and evaporated under
reduced pressure. The crude product was purified via silica-gel column chromatography
using ethyl acetate as an eluent. The resulting white solid was dissolved in dichloromethane
1
and evaporated. Yield: 80%; H NMR (300 MHz, CD2Cl2, δ): 8.93–8.92 (m, 3H), 8.58–8.56
(m, 3H), 8.00–7.96 (m, 3H), 7.71–7.67 (m, 6H), 7.42–7.38 (m, 3H), 7.28–7.25 (m, 6H), 2.12
13
(b, s, 18H), 1.91 (b, s, 18H), C NMR (75 MHz, CD2Cl2, δ): 148.7, 148.0, 147.8, 142.8,
142.2, 135.9, 135.4, 135.3, 133.7, 131.0, 129.9, 126.5, 123.1, 19.6, 19.3. HRMS (FAB) calcd
for C63H60BN3: 869.4880, found 869.4882; Elem. anal. calcd for C63H60BN3: C, 86.98; H,
6.95; B, 1.24; N, 4.83. Found: C, 86.99; H, 6.94; N, 4.84.
2.3. Absorption and emission spectra

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The absorption and photoluminescence spectra were recorded using a SHIMADZU UV-2450
UV–vis–NIR scanning spectrophotometer and a VARIAN Cary Eclipse fluorescence
spectrophotometer, respectively.
2.4. Phosphorescence emission spectra
At 77 K, the third harmonic (355 nm) pulse of a Q-switched Nd:YAG laser (neodymium-
doped yttrium aluminum garnet; Continuum, Surelite II; pulse width of 4.5 ns) was used to
excite the molecules. The phosphorescence spectra were recorded using an ICCD detector
(Andor, iStar) equipped with a monochromator (DongWoo Optron, Monora 500i). The
temporal profiles were measured using a monochromator equipped with a photomultiplier
(Zolix Instruments Co., CR 131) and a digital oscilloscope (Tektronix, TDS-784D). Oxygen
in the sample solutions was removed via the argon purging method.
2.5. Thermal properties
Thermogravimetric analysis (TGA) was performed using a TGA/DSC 1 (Mettler-Toledo Inc.)
instrument. TGA was conducted several times, and the data were confirmed. The sample was
loaded into a weight-tared alumina pan, and heated at a rate of 5 °C/min from 25 to 800 °C
under nitrogen flow (50 mL/min). The thermal properties were measured using differential
scanning calorimetry (DSC; Pyris Diamond DSC; Perkin Elmer). The compound was first
melted at a heating rate of 10 °C/min and subsequently cooled at a rapid rate of 40 °C/min
until room temperature.
2.6. DFT calculations
The derivatives were theoretically calculated using the Gaussian 09 package.24 The ground-
state geometries of the compounds were optimized using the B3LYP DFT and the 6/31G(d,p)

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basis set. The contours of electron density were plotted using Chem3D ver.10.25
2.7. Device fabrication and characterization
The OLED devices were fabricated on glass substrates pre-coated with a 150-nm-thick
indium tin oxide (ITO) layer having a sheet resistance of 10 Ω square−1. The ITO glass was
pre-cleaned using a conventional solvent cleaning method. The ITO surface was cleaned
again with UV ozone treatment immediately before depositing the hole injection layer. The
organic, Liq, and Al layers were deposited sequentially onto the substrate without breaking
the vacuum. The current–voltage characteristics of the OLEDs were measured using a source
measure unit (Keithley 2635B). The electroluminescence spectra, luminance, and CIE
coordinates were measured using a spectroradiometer (Konica Minolta CS-2000). Assuming
Lambertian emission, the EQE was calculated using the luminance, current density, and
electroluminescence spectrum.
3. Results and Discussion
Br
N
Pd(PPh3)4, K2CO3
tol/EtOH, reflux
(HO)2B
+
+
TDPB
i) n-BuLi,
3
ii) BF3-OEt2
Br
Br
B
Et2O, -78 o
C
Br
Br
O
O
N
Pd(PPh3)4, K2CO3
THF/H2O, reflux
B
TDPPB
2
1
4
Scheme 1. Synthesis of TDPB and TDPPB.
3.1. Synthesis
Scheme 1 illustrates the synthetic procedures used for the preparation of TDPB and TDPPB.
Initially, the starting compounds were prepared according to the previously reported
procedures. Briefly, 1,4-dibromo-2,3,5,6-tetramethylbenzene (1) was obtained via

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bromination reaction at 1 and 4 positions of 2,3,5,6-tetramethylbenzene with bromine
solution in dichloromethane with a yield of 64%. tris(Bromoduryl)borane (2) was synthesized
via mono-lithiation of (1) followed by the addition of boron trifluoride diethyl etherate. 3-
Pyridinylboronic acid (3) was synthesized by conducting a direct substitution reaction
between 3-bromopyridine and trimethyl borate to obtain a monosubstituted product. TDPB
was prepared via palladium-catalyzed Suzuki–Miyaura coupling reaction in the presence of
excess amounts of base as a white solid with a yield of 81%. Similarly, TDPPB was prepared
via Suzuki–Miyaura coupling reaction of (2) with 3-(4-(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)phenyl)pyridine (4) in the presence of Pd(PPh3)4 as a catalyst and excess
amounts of base as a white solid with a yield of 80%. The structures and chemical purities of
the final products were adequately verified using 1H NMR and 13C NMR, mass spectrometry,
and elemental analysis (see Experimental Section).
3.2. Thermal properties
The thermal properties of TDPB and TDPPB were investigated using TGA and DSC under a
nitrogen environment at a heating rate of 10 °C min−1. Upon adding a phenyl unit between
duryl and pyridine units, TDPPB showed higher thermal stability with the thermal
decomposition temperature (Td) of 446 °C as compared with that of TDPB at 439 °C, as
shown in Electronic Supplementary Information, Figure S1. In addition, high thermal
stability with a high glass transition temperature is desirable in OLED device fabrication
process because it can prevent morphological changes and suppress the formation of
aggregation upon heating. In this regard, the glass transition temperatures of TDPB and
TDPPB were investigated using DSC. The results are illustrated in Figure 1 and summarize
d in Table 1. The glass transition temperatures (Tg) of TDPB and TDPPB were measured to

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be 142 and 166 °C, respectively. The improvement in Td and Tg of TDPPB compared with
the corresponding values of TDPB might be attributed to the higher molecular weight.
TDPB
TDPPB
142 oC
166 oC
50
100
150
200
250
300
Temperature (oC)
Figure 1. Glass transition temperatures (Tg) of TDPB and TDPPB analyzed using DSC scan
rate at 10 °C min−1.
Table 1. Thermal, photophysical, and electrochemical properties of TDPB and TDPPB
λem (nm)
Td
Tg
HOMOc
(eV)
−6.47
−6.41
LUMOd
(eV)
−2.98
−2.95
λabsa (nm)
ET (ev)
Ereda (V)
Eg (eV)
RTa
403
404
77 Kb
436
(°C) (°C)
439
446
142
243, 330
2.84
2.69
−1.12
−1.11
3.49
3.46
TDPB
TDPPB
166 256, 279, 331
461
aMeasured in dichloromethane at room temperature, bmeasured in 2-MeTHF at 77 K, cEHOMO (eV) = ELUMO−Eg, and dELUMO
(eV) = −e(Ered(V vs. Fc/Fc+) + 4.8 eV).
3.3. Photophysical properties
Steady-state absorption and emission spectra of TDPB and TDPPB were measured in
dichloromethane solution at room temperature and the resulting spectral data are summarized
in Table 1. As shown in Figure 2, the two compounds exhibited similar absorption spectra but
different intensities depending on the phenyl bridge units. TDPPB showed a hyperchromic
shift compared with TDPB without significant changes in wavelength. The absorption peaks
at approximately 250 nm are mainly attributed to the ligand-centered π-π* transitions,

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whereas the absorption peaks at approximately 330 nm are assigned to the CT band from the
duryl units to the central boron atom in the molecule18 which supported by DFT calculations
(vide infra).
3.5x105
TDPB abs.
TDPPB abs.
TDPB em.
TDPPB em.
1.0
3.0x105
0.8
2.5x105
2.0x105
1.5x105
0.6
0.4
1.0x105
0.2
5.0x104
0.0
0.0
250 300 350 400 450 500 550 600
Wavelength (nm)
Figure 2. UV–vis absorption and photoluminescence spectra of TDPB and TDPPB in 5 µM
dichloromethane solution. Photoluminescence spectra were obtained via excitation at 330 nm.
The photoluminescence spectra of TDPB and TDPPB in dichloromethane solution were
also similar to each other, as shown in Figure 2. Notably, TDPPB shows an identical
emission spectrum to that of TDPPB even though it has a lengthened conjugation in the
molecular structure. We obtained the solvent-dependent emission spectra to determine the
origin of emission. As TDPB and TDPPB are donor–acceptor type dyad compounds, the CT
state in the excited states would be formed. Therefore, the sensitivity of the solvent to
polarity can be analyzed in terms of the difference between the dipole moments in the ground
and excited states. As shown in Figure S2, no significant solvatochromic shifts were observed
in both compounds in the absorption spectra, whereas the emission spectra were red-shifted
with the increase in solvent polarity, which indicates the presence of CT state in the excited
state. Thus, the emissions of TDPB and TDPPB originated from the CT state and had a
similar wavelength even though the two compounds have different conjugation lengths. The

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CT phenomenon was analyzed using a Lippert–Mataga plot (Figure 3), a plot of the Stokes
shift of the emission versus the solvent polarity.26 The difference (Stokes shift) between the
maximum absorption and emission wavelengths, in wavenumbers (∆ν), is expressed using
equation (1):
where µe − µg (∆µ) is the difference between the dipole moments of the excited and ground
states, c is the speed of light, h is Planck’s constant, and a0 is the radius of the Onsager cavity
around the fluorophore. The dielectric constant (ε) and the refractive index (n) of the solvents
are included in the term ∆f, known as orientation polarizability. The Onsager radius was
considered as half the average of 6.7 and 10.9 Å for compounds TDPB and TDPPB,
respectively, which were calculated from the optimized structures obtained using DFT
calculations (B3LYP method, 6-31G(d,p) basis using Gaussian program24). As shown in
Figure 3, the Stokes shift for the CT emissions in various solvents changed linearly with ∆f.
Using equation (1), the dipole moment differences between the excited state and ground state
(∆µ) for TDPB and TDPPB were estimated to be 4.53 and 7.68 Debye, respectively, as listed
in Table 1. As expected, ∆µ of TDPPB was larger than that of TDPB because TDPPB has a
longer distance between the electron donor and electron acceptor groups. This solvent-
dependent emission is consistent with the presence of a highly polarized excited state, thus
confirming that the lowest unoccupied molecular orbital (LUMO) level is dominated by the
empty p orbital of the boron atom and the lowest energy electronic transition is a CT from the
peripheral aryl groups to the boron center.

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6400
6200
6000
5800
5600
5400
5200
5000
4800
4600
4400
TDPB
TDPPB
(4)
(3)
(2)
(1)
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35
∆f
Figure 3. Lippert–Mataga plots for TDPB and TDPPB (1: in n-hexane, 2: THF, 3:
dichloromethane, and 4: acetonitrile).
The triplet energy levels of host, HTL, and ETL materials are important factors for the
application in phosphorescent OLEDs (PHOLEDS) because the back energy transfer from
the triplet of the dopant to the triplet of the host or adjacent layers will reduce the emission
efficiency. In this regard, the triplet energy levels of TDPB and TDPPB were measured in 2-
methyltetrahydrofuran (2-MeTHF) at 77 K. As shown in Figure 4, TDPB showed a higher
triplet energy level (437 nm, 2.84 eV) than TDPPB (461 nm, 2.69 eV), which were estimated
by considering the highest peak energy of phosphorescence as the transition energy of T1 →
S0, which corresponded to the vibronic 0-0 transition between these two electronic states. The
higher triplet energy level of TDPB compared with that of TDPPB might be due to its
shorter conjugation length. Notably, the triplet energy level of TDPB is sufficiently higher
than that of FIrpic (T1 = 2.6 eV),27 a phosphorescent dopant that emits blue light. Therefore,
we expect that the utilization of TDPB as an electron transporting layer in FIrpic based blue
devices would effectively confine the triplet excitons on the dopant and prevent the back
energy transfer process.

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1.0
0.8
0.6
0.4
0.2
0.0
2.84 eV
TDPB at 77 K
350
400
400
450
500
500
550
550
600
650
1.0
0.8
0.6
0.4
0.2
0.0
2.69 eV
TDPPB at 77 K
350
450
600
650
Wavelength (nm)
Figure 4. Phosphorescence spectra of TDPB (top) and TDPPB (bottom) measured in 2-
MeTHF at 77 K.
3.4. Electrochemical properties
To investigate the electrochemical properties of TDPB and TDPPB, CV was performed by
employing a three-electrode cell system; a glassy carbon 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 5, TDPB and TDPPB showed reduction waves
at −1.30 (Epc) and −1.12 V (Epa), and −1.33 (Epc) and −1.11 V (Epa) (versus Fc/Fc+),
respectively. These peaks can be ascribed to the duryl borane unit in its reduced form. Using
redox potentials and the optical band gap calculated from the edge of the absorption spectra,
the energy levels of the highest occupied molecular orbital (HOMO) in TDPB and TDPPB
were estimated to be −6.47 and −6.41 eV, respectively, and those of the LUMO in TDPB and
TDPPB were estimated to be −2.98 and −2.95 eV, respectively. Notably, the relatively low
LUMO energy levels of TDPB and TDPPB might form a low electron-injection barrier and
thus result in a low driving voltage when they are applied to OLED devices. Furthermore, the
deep HOMO levels of TDPB and TDPPB can be an additional advantage when they are

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applied to OLED devices because they can act as hole blocking materials and prevent exciton
quenching process occurring owing to hole leakage into the ETL.28
TDPB
TDPPB
5 µA
-0.4
-0.6
-0.8
-1.0
-1.2
-1.4
-1.6
-1.8
V vs Fc | Fc+
Figure 5. CV curves of TDPB and TDPPB in dichloromethane solution containing 0.1
M NBu4PF6 as electrolyte, at a scan rate of 0.1 V s−1.
3.5. DFT calculations
To gain insight into the electronic properties of the compounds, the DFT calculations of the
frontier molecular orbitals were performed by using the B3LYP/6-31G(d,p) method with the
Gaussian 09 package.24 In the optimized structure of TDPB, the inner duryl groups and
peripheral pyridine groups are significantly twisted together with averaged dihedral angles of
99.7° owing to the steric hindrance of tetramethyl groups. Similarly, the inner duryl groups
and bridged phenyl groups in TDPPB are also in perpendicular positions with averaged
dihedral angles of 99.7°. In contrast, the bridged phenyl groups and peripheral pyridine
groups in TDPPB are in horizontal positions with averaged dihedral angles of 161.6°. Figure
6 shows the orbital distributions for the HOMO and LUMO of TDPB and TDPPB and
Figure S3 shows the orbital distributions for selected orbitals. The HOMOs for TDPB and
TDPPB were mainly located on the duryl groups, and the LUMOs were mainly located on

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the electron-deficient central boron atom. Time-dependent DFT (TD-DFT) with the same
functional and basis set confirms the origin of the main transitions of TDPB and TDPPB at
approximately 330 and 250 nm, as shown in Figure S4 and Table S1. The lowest energy
absorption λmax at approximately 330 nm is mainly associated with the CT transitions from
the duryl units to the central boron atom in the molecule, whereas the absorption λmax at
approximately 250 nm is mainly attributed to the ligand-centered π-π* transitions in duryl–
pyridine groups. The additional strong transition for TDPPB at approximately 275 nm is
attributed to the π-π* transitions in phenyl-pyridine groups, as shown in Figure S4. According
to the results of TD-DFT calculation, we ascertained that the hyperchromic shift in TDPPB is
due to the presence of additional bridged phenyl groups. These results are consistent with the
experimental results.
Figure 6. Optimized structures of TDPB and TDPPB with their HOMO and LUMO orbital
distributions calculated using DFT calculations.
3.6. Electron Only Device (EOD)
EODs for TDPB and TPBi (2,2′,2"-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole))
were fabricated to verify the electron transport property of TDPB. TPBi was chosen because

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it is well-known and widely used ETM. A similar device was not prepared with TDPPB
because this compound was decomposed during the train-sublimation process. Figure 7
shows the current density–voltage (J–V) characteristics of EOD with the structure of ITO
(150 nm)/Al (100 nm)/TDPB or TPBi (50 nm)/Liq (1 nm)/Al (150 nm). According to the
space-charge-limited current (SCLC) model, which is characterized by field-dependent
mobility,29,30 the electron mobility (µ) of TDPB was estimated to be 1.24 × 10−3 cm2 V−1 s−1
at 1 MV cm−1, which is significantly higher than that of TPBi (2.46 × 10−5 cm2 V−1 s−1 at 1
MV cm−1). The high electron mobility of TDPB at low driving voltage may result from such
a coordination effect of the nitrogen atom in peripheral heteroaromatic rings of TDPB with
lithium cations, which induces an effective electron injection.31
(a)
(b)
1E-8
1E-9
1000
100
10
1E-10
1E-11
1E-12
1E-13
1E-14
1E-15
1E-16
1
0.1
0.01
1E-3
1E-4
1E-5
1E-6
1E-7
1E-8
TDPB
TPBi
TDPB
TPBi
700
800
900
1000
1100
1200
0
1
2
3
4
5
6
7
8
9
10
E1/2 (V/cm)1/2
Voltage (V)
Figure 7. (a) J–V characteristics of EOD with TDPB and TPBi. The devices were fabricated
with the following structure: ITO (150 nm)/Al (100 nm)/ TDPB or TPBi (50 nm)/Liq (1
nm)/Al (150 nm) and (b) the J–V characteristics were fitted to SCLC with field-dependent
mobility. J = (9/8)ꢀ ꢀ (E2/L) exp(0.89)ꢄ ꢅ, where J is the current density; ꢆ ꢆ is the
√
ꢃ
ꢁ
ꢇ ꢈ
ꢁ
ꢂ
dielectric constant; E is the electric field; L is the film thickness; ꢉꢇ is the zero-field carrier
mobility; β is the Poole–Frenkel factor. The values of ꢉꢇ and β were calculated by assuming
that ꢆꢈ is equal to 3. The field-dependent mobility was calculated using the Poole–Frenkel
equation: µ(E) = µ exp(β ꢅ).
√
0
3.7. Device Performance

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To examine TDPB as an ETM, PHOLEDs were fabricated with the structure of ITO (150
nm)/HATCN (10 nm)/TAPC (x nm)/mCBP:FIrpic 8% (30 nm)/TDPB(y nm)/Liq (1 nm)/Al
(150 nm), where HATCN 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],
and mCBP is 3,3-di(9H-carbazol-9-yl)-biphenyl. The chemical structures and the energy
level diagram of the used materials are shown in Figure S5. To optimize the efficiency, five
different devices were fabricated with variations of the thickness of HTL and ETL; device 1:
HTL 85 nm, ETL 30 nm, device 2: HTL 100 nm, ETL 30 nm, device 3: HTL 70 nm, ETL 30
nm, device 4: HTL 70 nm, ETL 35 nm, and device 5: HTL 85 nm, ETL 35 nm. The
performances of the five devices are summarized in Figure 8 and Table 2. Among the five
devices, device 4 showed the best performance with the peak external electroluminescence
quantum and power efficiencies of ηext = 18.8% and ηp = 31.3 lm W−1, respectively.
(c)
(b)
(a)
20
15
10
5
1000
100
10
10000
8000
6000
4000
2000
0
1.0
0.8
0.6
0.4
0.2
0.0
device 1
device 2
device 3
device 4
device 5
device 1
device 2
device 3
device 4
device 5
device 1
device 2
device 3
device 4
device 5
1
0.1
0.01
1E-3
1E-4
1E-5
1E-6
1E-7
0
0
2
4
6
8
10
12
14
0
2
4
6
8
10
350
400
450
500
550
600
650
700
Current density (mA/cm2)
Voltage (V)
Wavelength (nm)
Figure 8. (a) Current density–voltage–luminance curves, (b) quantum efficiency–luminance
curves, and (c) electroluminescence spectra of devices 1–5.
Table 2. Summary of the PHOLED devices 1–5
Current Efficiency
(cd/A)
Power Efficiency
(lm/W)
Turn-On
Voltage (V)
(at 1 nits)
EQE (%)
CIE (x, y)
(at 400 nits)
100
nits
1000
nits
100
nits
1000
nits
7.79
5.20
9.30
4.86
7.15
100
nits
17.44 16.51
17.12 15.96
17.91 16.26 10.06
18.84 17.23
17.42 16.96
1000
nits
8.74
6.14
Max
Max
Max
34.83 32.77 17.40 30.38 22.39
36.11 33.42 13.00 30.34 21.94
33.44 31.25 19.30 31.17 21.13
36.76 33.36 12.33 31.13 20.72
34.76 33.64 16.00 27.46 22.96
3.44
3.53
3.39
3.53
3.58
0.14, 0.33
0.15. 0.36
0.15, 0.31
0.15, 0.32
0.15, 0.33
Device 1
Device 2
Device 3
Device 4
Device 5
6.34
8.04
Device 1: ITO(150 nm)/HATCN(10 nm)/TAPC(85 nm)/mCBP:FIrpic 8%(30 nm)/TDPB(30 nm)/Liq(1 nm)/Al(150 nm).
Device 2: ITO(150 nm)/HATCN(10 nm)/TAPC(100 nm)/mCBP:FIrpic 8%(30 nm)/TDPB(30 nm)/Liq(1 nm)/Al(150 nm).
Device 3: ITO(150 nm)/HATCN(10 nm)/TAPC (70 nm)/mCBP:FIrpic 8%(30 nm)/TDPB(30 nm)/Liq(1 nm)/Al(150 nm).
Device 4: ITO(150 nm)/HATCN(10 nm)/TAPC (70 nm)/mCBP:FIrpic 8%(30 nm)/TDPB(35 nm)/Liq(1 nm)/Al(150 nm).

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Device 5: ITO(150 nm)/HATCN(10 nm)/TAPC(85 nm)/mCBP:FIrpic 8%(30 nm)/TDPB(35 nm)/Liq(1 nm)/Al(150 nm).
4. Conclusions
In this study, two duryl borane derivatives, TDPB and TDPPB, which contain
heteroaromatic pyridine rings at peripheral positions, were prepared and their thermal,
photophysical, and electrochemical properties were systematically investigated. TDPB and
TDPPB showed high thermal stabilities with Tg values at 142 and 166 °C, respectively. Low
LUMO energy levels (≈2.96 eV) and deep HOMO energy levels (≈6.45 eV) of both
compounds would be beneficial when they are applied in OLED devices because they would
form a low electron-injection barrier and may act as a hole-blocking material. The triplet
energies of these compounds can be tuned from 2.69 to 2.84 eV by shortening the
conjugation length, and TDPB shows the higher value of 2.84 eV. The electron mobility (µ)
of TDPB was estimated to be 1.24 × 10−3 cm2 V−1 s−1 and an optimized FIrpic-based blue
phosphorescence device with TDPB achieved a maximum EQE of 18.84% with the CIE
coordinates of (0.15, 0.32). These results suggest that these duryl borane derivatives can be
used as high-efficient blue phosphorescent ETMs.
5. Acknowledgements
This work was supported by the National Research Foundation of Korea (NRF-
2016R1D1A1B03935823).
6. References
(1) C. H. Zhao, A. Wakamiya, Y. Inukai, S. Yamaguchi, J. Am. Chem. Soc., 2006, 128,
15934−15935.
(2) Z. Zhang, R. M. Edkins, J. Nitsch, K. Fucke, A. Steffen, L. E. Longobardi, D. W. Stephan,
C. Lambert, T. B. Marder, Chem. Sci., 2015, 6, 308−321.

ACCEPTED MANUSCRIPT
(3) Z. Yuan, N. J. Taylor, T. B. Marder, I. D. Williams, S. K. Kurtz, L. T. Cheng, J. Chem.
Soc., Chem. Commun., 1990, 1489−1492.
(4) N. S. Makarov, S. Mukhopadhyay, K. Yesudas, J. L. Brédas, J. W. Perry, A. Pron, M.
Kivala, K. Müllen, J. Phys. Chem. A, 2012, 116, 3781−3793.
(5) A. Hayek, J. F. Nicoud, F. Bolze, C. Bourgogne, P. L. Baldeck, Angew. Chem. Int. Ed.
2006, 45, 6466−6469.
(6) Z. Q. Liu, Q. Fang, D. Wang, D. X. Cao, G. Xue, W. T. Yu, H. Lei, Chem. Eur. J. 2003, 9,
5074−5084.
(7) D. Tanaka, T. Takeda, T. Chiba, S. Watanabe, J. Kido, Chem. Lett., 2007, 36, 262−263.
(8) K. Shizu, T. Sato, K. Tanaka, H. Kaji, Appl. Phys. Lett., 2010, 97, 142111.
(9) T. Noda, Y. Shirota, J. Am. Chem. Soc., 1998, 120, 9714−9715.
(10) F. Li, W. Jia, S. Wang, Y. Zhao, Z. H. Lu, J. Appl. Phys., 2008, 103, 034509.
(11) H. W. Mo, Y. Tsuchiya, Y. Geng, T. Sagawa, C. Kikuchi, H. Nakanotani, F. Ito, C.
Adachi, Adv. Funct. Mater. 2016, 26, 6703−6710.
(12) H. Zhang, C. Huo, J. Zhang, P. Zhang, W. Tian, Y. Wang, Chem. Commun., 2006,
281−283.
(13) Y. Shirota, H. Kageyama, Chem. Rev., 2007, 107, 953−1010.
(14) M. Aonuma, T. Oyamada, H. Sasabe, T. Miki, C. Adachi, Appl. Phys. Lett. 2007, 90,
183503.
(15) H. Sasabe, J. Kido, Chem. Mater. 2011, 23, 621–630.
(16) L. Xiao, Z. Chen, B. Qu, J. Luo, S. Kong, Q. Gong, J. Kido, Adv. Mater. 2011, 23, 926–
952.
(17) D. Chen, S. J. Su, Y. Cao, J. Mater. Chem. C, 2014, 2, 9565–9578.
(18) W. L. Jia, M. J. Moran, Y. Y. Yuan, Z. H. Lu, S. Wang, J. Mater. Chem., 2005, 15,
3326−3333.
(19) D. Tanaka, Y. Agata, T. Takeda, S. Watanabe, J. Kido, Jpn. J. Appl. Phys., 2007, 46,
L117.

ACCEPTED MANUSCRIPT
(20) C. Gu, N. Huang, Y. Chen, H. Zhang, S. Zhang, F. Li, Y. Ma, D. Jiang, Angew. Chem. Int.
Ed., 2016, 55, 3049−3053.
(21) S. Yamaguchi, T. Shirasaka, K. Tamao, Org. Lett., 2000, 26, 4129−4132.
(22) C. Chakraborty, M. K. Bera, S. Malik, New J. Chem., 2014, 38, 3522−3528.
(23) S. Basceken, M. Balci, J. Org. Chem. 2015, 80, 3806−3814.
(24) M. J. Frisch, et al., Gaussian 09, Revision D.01, Gaussian Inc., Wallingford, CT, 2013.
(25) Cambridge Soft Corporation, Cambridge, MA, USA, http://www.cambridgesoft.com/.
(26) Principles of Fluorescence Spectroscopy, ed. J. R. Lakowicz, Corrected 3rd, Springer,
Singapore, 2010, ch. 6.
(27) J. W. Kim, S. I. You, N. H. Kim, J. A. Yoon, K. W. Cheah, F. R. Zhu, W. Y. Kim, Sci.
Rep., 2014, 4, 7009.
(28) M. A. Baldo, S. Lamansky, P. E. Burrows, M. E. Thompson, S. R. Forrest, Appl. Phys.
Lett., 1999, 75, 4−6.
(29) T. Y. Asuda, Y. Yamaguchi, D. C. Zou, T. Tsutsui, Jpn. J. Appl. Phys., 2002, 41,
5626−5629.
(30) P. N. Murgatroyd, J. Phys. D: Appl. Phys., 1970, 3, 151−156.
(31) S. J. Su, T. Chiba, T. Takeda, J. Kido, Adv. Mater. 2008, 20, 2125−2130.

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Highlights
 Two boron derivatives of pyridine were synthesized and systematically investigated their
thermal, photophysical, and electrochemical properties.
 Among the series, TDPB compound has proper properties as ETM; high thermal
stability with Tg value of 142 °C, high triplet energy of 2.84 eV, proper LUMO energy
level of −2.98 eV, and deep HOMO energy level of −6.47 eV.
 EOD using TDPB showed relatively high electron mobility with 1.24 × 10−3 cm2 V−1 s−1
at 1 MV cm−1 and optimized FIrpic based blue PhOLEDs device achieved a maximum
EQE of 18.84% with the CIE coordinates of (0.15, 0.32).