Synthesis and characterization of diphenylamine derivative containing malononitrile for thermally activated delayed fluorescent emitter

Article abstract of DOI:10.1016/j.dyepig.2017.01.010

To increase the efficiency of blue organic light-emitting diodes (OLEDs), new blue dopants, 2-(bis(4-(diphenylamino)phenyl)methylene)malononitrile (4-DPM) and 2-(bis(3-(diphenylamino)phenyl) methylene)malononitrile (3-DPM), with thermally activated delayed fluorescence were synthesized. 4-DPM with a p-phenyl linker between the electron-donating diphenylamine unit and the electron-withdrawing malonitrile unit showed a large overlap in the phenyl linker as well as the malonitrile unit, while 3-DPM with an m-phenyl linker showed a weak overlap in the phenyl linker. The OLED with the 3-DPM dopant showed higher efficiencies of 12.62?cd/A and 8.81?lm/W compared to that with the 4-DPM dopant (1.38?cd/A and 0.70?lm/W). This result was attributed to the small overlap between the electron-donating and electron-withdrawing units, the small charge-transfer singlet-triplet state splitting below 0.1?eV, and the well-aligned structure in the horizontal direction, as determined by grazing incidence X-ray diffraction.

Full text of DOI:10.1016/j.dyepig.2017.01.010

Accepted Manuscript
Synthesis and characterization of diphenylamine derivative containing malononitrile
for thermally activated delayed fluorescent emitter
Sunyoung Sohn, Bong Hyun Koh, Jae Yeul Baek, Hyun Chan Byun, Jae Hyun Lee,
Dong-Seon Shin, Hyungju Ahn, Han-Koo Lee, Jaeyoung Hwang, Sungjune Jung,
Yun-Hi Kim
PII:
S0143-7208(16)31223-2
10.1016/j.dyepig.2017.01.010
DYPI 5709
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 18 November 2016
Revised Date: 17 December 2016
Accepted Date: 6 January 2017
Please cite this article as: Sohn S, Hyun Koh B, Baek JY, Chan Byun H, Lee JH, Shin D-S, Ahn H, Lee
H-K, Hwang J, Jung S, Kim Y-H, Synthesis and characterization of diphenylamine derivative containing
malononitrile for thermally activated delayed fluorescent emitter, Dyes and Pigments (2017), doi:
10.1016/j.dyepig.2017.01.010.
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ACCEPTED MANUSCRIPT
Graphical abstract

ACCEPTED MANUSCRIPT
Synthesis and characterization of diphenylamine derivative containing
malononitrile for thermally activated delayed fluorescent emitter
a,1
b,1
b
b
b
Sunyoung Sohn , Bong Hyun Koh , Jae Yeul Baek , Hyun Chan Byun , Jae Hyun Lee ,
b
c
c
d
a,**
Dong-Seon Shin , Hyungju Ahn , Han-Koo Lee , Jaeyoung Hwang , Sungjune Jung
,
d,*
Yun-Hi Kim
a
Department of Creative IT Engineering, Pohang University of Science and Technology,
Pohang 37673, Republic of Korea
b
Kyeong-Nam Science High School, Jinju 52620, Republic of Korea
c
Pohang Accelerator Laboratory, Pohang 37673, Republic of Korea
d
Department of Chemistry and Research Institute for Green Energy Convergence
Technology, Gyeonsang National University, Jinju 52828, Republic of Korea
To increase the efficiency of blue organic light-emitting diodes (OLEDs), new blue dopants,
2
-(bis(4-(diphenylamino)phenyl)methylene)malononitrile
(4-DPM)
and
2-(bis(3-
(diphenylamino)phenyl) methylene)malononitrile (3-DPM), with thermally activated delayed
fluorescence were synthesized. 4-DPM with a p-phenyl linker between the electron-donating
diphenylamine unit and the electron-withdrawing malonitrile unit showed a large overlap in
the phenyl linker as well as the malonitrile unit, while 3-DPM with an m-phenyl linker
showed a weak overlap in the phenyl linker. The OLED with the 3-DPM dopant showed
higher efficiencies of 12.62 cd/A and 8.81 lm/W compared to that with the 4-DPM dopant
(1.38 cd/A and 0.70 lm/W). This result was attributed to the small overlap between the
electron-donating and electron-withdrawing units, the small charge-transfer singlet-triplet
state splitting below 0.1 eV, and the well-aligned structure in the horizontal direction, as
determined by grazing incidence X-ray diffraction.
1

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Keywords: OLEDs, TADF, GIXD, Horizontal orientation, ∆E , Phenyl linker
ST
*
Corresponding author. Department of Chemistry and Research Institute for Green Energy
Convergence Technology, Gyeonsang National University, Jinju 52828, Republic of Korea.
Tel.: +82-55-772-1491 ; fax: +82-55-772-1489.
*
* Corresponding author.
E-mail addresses : sjjung@postech.ac.kr (S. Jung), ykim@gnu.ac.kr (Y.-H. Kim).
1
Both authors contributed equally to this work.
2

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1
. Introduction
Organic light-emitting diodes (OLEDs) have been used in flat-panel displays and general
light applications because of their many advantages such as self-emission, flexibility, large-
area fabrication, low cost, and low power consumption [1,2]. However, the efficiency of first-
generation fluorescence-based OLEDs is limited because spin-allowed emission is generated
only by singlet (S ) exciton. In contrast, in second-generation phosphorescent OLEDs
1
(
PHOLEDs), exciton harvesting by the triplet (T ) state via intersystem crossing leads to an
1
internal efficiency of nearly 100%. However, it remains challenging to produce cost-effective
and efficient pure-blue OLEDs with long-term stability because of the incorporation of
organometallic complexes containing rare and heavy transition metals (such as iridium and
platinum), for example, tris(2-phenylpyridinato)iridium(III) [3]. In 2009, thermally activated
delayed fluorescence (TADF) materials were proposed as a third-generation material and
have since been studied intensively by Adachi et al [4,5]. As compared to PHOLEDs
containing rare metal complexes, TADF materials have many advantages such as easy
synthesis using pure organic molecules, low driving voltage, and lower triplet and singlet
energy levels. In addition, it harvesting both single and triplet excitons because TADF
involves small charge-transfer singlet-triplet state splitting (∆E ) by thermal activation
ST
between the lowest T and lowest exited S states, resulting in fluorescence in S [6-8].
1
1
1
Currently, blue TADF devices show relatively low quantum efficiencies (e.g., 19.5% in 2014
and 22.3% in 2015 with deep blue color, and 25% in 2016 with light blue color) compared to
green TADF devices (29.6% in 2014) [7, 9-11]. However, a few studies have achieved high
efficiency and good color purity in blue TADF OLEDs. For example, for deep-blue color, the
synthesis of a new donor unit of the azasiline group, design of a carbazole donor moiety, and
triazine acceptor moiety were developed. For high efficiency, very large dihedral angles,
small ∆E (<0.1 eV), and molecular design without complex chemical modification, which
ST
3

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is only substituting into meta positions of the electron-accepting units [5,7,10,12].
Minimizing ∆E is the first priority because TADF efficiency depends on the rate of S ← T
1
ST
1
intersystem crossing as well as a small overlap of the work function of the highest occupied
molecular orbital (HOMO, ΦHOMO) and the lowest unoccupied molecular orbital (LUMO,
ΦLUMO), which results in a spatial separation between HOMO and LUMO [12].
In addition, the molecular orientation of the emitting material is known as a key to
determine OLED efficiency by considering the outcoupling enhancement caused by a
reduced coupling to surface plasmons. For example, emitters with perfectly horizontal
orientations can increase the external efficiency by up to 46%, which is higher than that
achieved by isotropically oriented molecules [13-15]. In 2014, Brutting analyzed the
molecular orientations of the transition dipole moments in sky-blue TADF molecules and
fluorescent green emitters based on the angular dependent photoluminescence (PL) spectra
and the calculated dipole moments of the TADF emitters obtained using variable-angle
spectroscopic ellipsometry [13,14]. Kim et al. have been widely studing the molecular
orientations in some iridium complexes, and a non-doped, Pt-based emitter by grazing
incidence wide-angle X-ray diffraction (GIXD), along with a mixed host consisting of a hole-
transporting material and an electron-transporting material sky-blue TADF OLEDs by angle
dependent PL spectra for sky-blue TADF OLEDs [15,16]. Although the preferred dipole
orientation may be one of the most important properties of a TADF emitter, two-dimensional
GIXD (2D-GIXD) analysis using scattered X-rays has not been used to clarify the molecular
orientations. In this study, we synthesized new TADF blue dopants, 2-(bis(4-
(
diphenylamino)phenyl)methylene) malononitrile (4-DPM) with a p-phenyl linker and 2-
bis(3-(diphenylamino)phenyl)methylene)malononitrile (3-DPM) with an m-phenyl linker
(
between an electron-donating diphenylamine unit and an electron-withdrawing malonitrile
unit. Furthermore, we focused on the effect of molecular orientations (para and meta
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positions) of the substitutent phenyl linkers on the device efficiency using 2D-GIXD analysis,
because the functional linker position leads to harvesting of both singlet and triplet excitons,
which in turn affects the device properties. In addition, we analyzed the scattered X-ray
intensities of the host and host:dopant films along with the spectra of in-plane and out-of-
plane scattering to the plane and azimuthal intensity profiles for a plot of orientation
distribution. To the best of our knowledge, this is the first study on molecular orientation
using 2D-GIXD analysis in blue TADF emitters, and the results will contribute to basic
research on TADF molecular design.
2
. Experiments
2
.1. Synthesis of marterials
O
NC
CN
H
O
N
NC CN
3
N
N
N
N
t
o
THF, Et N, 40 hr
F
F
KO Bu/ DMF/ 110 C
8hr
4
1
4-DPM
H
O
N
NC
CN
NC CN
F
F
O
N
N
N
N
t
o
KO Bu/ DMF/ 110 C
2hr
3
THF, Et N, 40 hr
7
3-DPM
2
Scheme 1. Synthetic route of compounds 4-DPM and 3-DPM.
2
.1.1 Synthesis of bis(4-(diphenylamino)phenyl)methanone
Diphenylamine (3.4 g, 20 mmol) and KOtBu (3.37 g, 30 mmol) were dissolved in 100 mL
anhydrous N,N-dimethylformamide (DMF). Bis(3-fluorophenyl)methanone (2.18 g, 10
mmol) was slowly dropped into 30 mL anhydrous DMF solution. The reaction mixture was
refluxed for 48 h at 110 °C under N atmosphere. After cooling to room temperature, water
2
was added and the mixture was extracted with CH Cl . The combined extract was dried over
2
2
5

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anhydrous MgSO and filtered. The solvent was removed by rotary evaporation, and the
4
residue was purified by column chromatography over silica gel with hexane-EtOAc (4:1) as
the eluent to yield 4-DPM as a yellow solid (2.3 g, 44.5%). 1H NMR (300 MHz, CDCl ), δ
3
(ppm): 7.51 (m, 4H), 7.35 (m, 8H), 7.20 (m, 8H), 7.16 (m, 4H), 7.05 (d, J = 8.6 Hz), 13C
NMR (75 MHz, CDCl ), δ (ppm): 192.83, 151.20, 146.37, 131.24, 130.51, 129.15, 125.47,
3
1
2
5
24.11, 119.70.
.1.2. Synthesis of 4-DPM
Bis(4-(diphenylamino)phenyl)methanone (1.8 g, 3.48 mmol) and malononitrile (0.33 mL,
.23 mmol) were dissolved in anhydrous THF (30 mL). Next, four drops of triethylamine
were added and the solution was stirred at room temperature for 40 h. The solvent was
removed by rotary evaporation, and the residue was purified by column chromatography over
silica gel with CH Cl /petroleum ether (1:1) as the eluent to yield 4-DPM as a yellow solid
2
2
(0.52 g, 21%). IR (KBr) ν (cm-1) 2225 (-CN), 1H NMR (300 MHz, CD2Cl2), δ (ppm): 7.71
(d, J = 8.0 Hz, 4H), 7.33 (m, 8H), 7.15 (m, 12H), 7.03 (d, J = 7.8 Hz, 4H), 13C NMR (75
MHz, CD2Cl2), δ (ppm): 192.82, 152.37, 146.45, 131.82, 130.37, 130.04, 126.31, 125.34,
1
19.13, MS (EI): m/z (%) 564 (100).
2
.1.3. Synthesis of bis(3-(diphenylamino)phenyl)methanone
Bis(3-(diphenylamino)phenyl) methanone was obtained as a yellow solid (2.3 g, 44.5 %)
using a method similar to that used to prepare bis(4-(diphenylamino)phenyl)methanone. 1H
NMR (300 MHz, CDCl3): 7.74 (d, J = 8.0 Hz, 4H), 7.35 (m, 8H), 7.22(m, 8H), 7.14 (m, 4H),
7
1
.02 (d, J = 8.0 Hz, 4H), 13C NMR (75 MHz, CDCl3): 119.53, 120.38, 123.50, 124.65,
28.69, 129.48, 135.93, 138.20, 147.27, 148.42, 193.23.
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2
.1.4. Synthesis of 3-DPM
-DPM was obtained by a method similar to 4-DPM and was obtained as a yellow solid
3
-1
1
(
0.52 g, 21 %). IR (KBr) ν (cm ) 2223 (-CN), H NMR (300 MHz, CD Cl ), δ (ppm): 7.71
2 2
1
3
(
d, J = 8.0 Hz, 4H), 7.33 (m, 8H), 7.15 (m, 12H), 7.03 (d, J = 7.80 Hz, 4H), C NMR (75
MHz, CD Cl ), δ (ppm): 192.82, 152.37, 146.45, 131.82, 130.37, 130.04, 126.31, 125.34,
2
2
1
19.13, MS (EI): m/z (%) 564 (100).
2
.2. Instruments
1
H NMR spectra were recorded using a Bruker Avance-300 MHz FT-NMR spectrometer,
and chemical shifts were reported in parts per million with tetramethylsilane as the internal
standard. Thermogravimetric analysis (TGA) was performed under nitrogen using a TA
instrument 2050 thermogravimetric analyzer. Differential scanning calorimetry (DSC) was
conducted under nitrogen using a TA instrument DSC Q10. Samples for both TGA and DSC
were heated at a rate of 10 °C/min. UV-visible spectra were measured using a Shimadsu UV-
1
065PC UV-visible spectrophotometer. PL spectra were measured using a Perkin Elmer
LS50B fluorescence spectrophotometer. The electrochemical properties of the materials were
measured by cyclic voltammetry (CV) using an Epsilon C3 in a 0.1 M solution of tetrabutyl
ammonium
perchlorate
in
acetonitrile.
The
topographies
of
bis[2-
(diphenylphosphino)phenyl]ether oxide (DPEPO), DPEPO:4-DPM(10 wt.%), and DPEPO:3-
DPM(10 wt.%) films were analyzed using atomic force microscopy (AFM; VEECO
Dimension 3100+Nanoscope V) in the non-contact mode. Near-edge X-ray absorption fine
structure (NEXAFS) measurements were performed at room temperature at the 4D beamline
of the Pohang Light Source (PLS-II) of the Pohang Accelerator Laboratory (PAL). We used
the partial-electron-yield detection mode for NEXAFS spectra by recording the sample
current normalized to a signal current measured simultaneously using a gold mesh in
7

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ultrahigh vacuum. We used a polarized (p-polarized) synchrotron photon beam (∼85%) with
energy in the range of 279 to 325 eV, a spectral energy resolution of ∆E = 150 meV, and a
probing depth of ~50 Å for surface-sensitive measurements. Two-dimensional-GIXD
measurements were conducted at the PLS-II 9A U-SAXS beamline of PAL. The X-rays
coming from the in-vacuum undulator were monochromated (wavelength λ = 1.120 Å) using
a double-crystal monochromator and focused both horizontally and vertically (FWHM 300
µm (H) × 30 (V) µm at sample position) using K-B-type mirrors. The vacuum GIXD system
was equipped with a seven-axis-motorized sample stage for a fine alignment of thin films.
GIXD patterns were recorded with a 2D CCD detector (Rayonix SX165, USA) under an X-
ray irradiation time of 10 s.
2
.3. Device fabrication and measurements
OLED devices were fabricated on ITO-coated glass substrates with a structure of
ITO/NPB(40 nm)/TcTa(20 nm)/host:dopant(20 nm)/DPEPO(10 nm)/TPBi(30 nm)/Liq(1
nm)/Al (120 nm) in a class-1000 cleanroom. Before use, the ITO-coated glass with a
resistance of 15 Ω/sq was sequentially cleaned with deionized water, acetone, and isopropyl
alcohol for 15 min in an ultrasonic bath, and then, treated with O plasma for 60 s. To
2
fabricate the OLEDs, N,N′-di(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl)-4,4′-diamine (NPB)
and tris(4-carbazoyl-9-ylphenyl)amine (TcTa) were used as a hole transport layer and a hole
injection layer, respectively. To achieve blue emission, we used an emitting material layer
consisting of 10 wt.% 4-DPM and 3-DPM dopants in DPEPO host. DPEPO was also used as
an exciton-blocking layer due its high current density, shallow LUMO level, and deep
HOMO level for the carrier injection. 1,3,5-tris(N-phenylbenzimiazole-2-yl)benzene (TPBi)
was used as an electron transport layer, and 8-hydroxyquinolatolithium (Liq) and aluminum
8

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(Al) were thermally evaporated as an interlayer and a cathode, respectively. Current density,
luminance, and efficiencies versus driving voltage were measured using the Keithley 236 and
CS-1000 (Konica Minolta Co.) system. Transient electroluminescence (EL) curves of devices
were measured using a pulsed ND-YAG laser; the wavelength of the excitation source was
3
55 nm.
3
. Results and discussion
3
.1. Computational calculation
To predict the optimized geometries and frontier molecular orbital energy levels for 4-
DPM and 3-DPM, density functional theory (DFT) calculations are carried out at the
B3LYP/6-31G(d) level using Gaussian 09. The HOMO and LUMO distributions of 4-DPM
and 3-DPM are shown in Fig 1. The HOMO of 4-DPM, which has a p-phenyl linker between
the electron-donating diphenylamine unit and the electron-withdrawing malonitrile unit, is
distributed throughout the molecule, while the LUMO is distributed on the malonitrile unit.
In 3-DPM, which has an m-phenyl linker, the HOMO is distributed on the diphenylamine
unit and the LUMO is distributed on the malonitrile unit. 4-DPM has a large overlap in the
phenyl linker and the malonitrile unit, whereas 3-DPM has a weak overlap in the phenyl
linker.
9

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Fig. 1. Results of density functional theory (DFT) calculations of 4-DPM and 3-DPM
performed at the B3LYP/6-31G level: (a) chemical structures and (b) HOMO and LUMO
electron density maps.
Table 1
HOMO, LUMO, E , S , T , and ∆E values of 4-DPM, 3-DPM.
g
1
1
ST
HOMO(eV) LUMO(eV) E (eV)
S1(eV)
3.7316
3.7452
T (eV)
∆EST(eV)
0.114
g
1
4
-DPM
-DPM
7.22
7.27
5.15
5.42
2.07
1.85
3.6176
3.6472
3
0.098
We also calculate the energy band gaps (E ) and the lowest singlet and triplet excited
g
states for the optimized ground-state geometries of 4-DPM and 3-DPM using time-dependent
DFT (TD-DFT) calculations with the same functional and basis set. The calculated excited
states are used to estimate ∆EST values of 0.114 and 0.098 eV for 4-DPM and 3-DPM,
respectively (Table 1). The TD-DFT calculation provides ∆E < 0.2 eV for both compounds.
ST
3
.2. Photophysics properties
The absorption and emission spectra of 4-DPM and 3-DPM in cyclohexane, toluene,
chloroform, acetone, and DMF solvent are shown in Fig. 2. In all solvents, the PL spectra of
4
-DPM and 3-DPM show solvatochromic shifts proportional to solvent-induced polarization.
3
.3. Thermal properties
The thermal properties of 4-DPM and 3-DPM are investigated by TGA and DSC. The
thermal decomposition temperature (T ), defined as the temperature at which 5% mass loss
d
occurs, is determined from the TGA results. As shown in Fig. 3, the T values of 4-DPM and
d
1
0

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3
-DPM are 376 °C and 359 °C, respectively. The glass transition temperature (T ) values of
g
both 4-DPM and 3-DPM are 189 °C.
Fig. 2. Absorption and PL spectra of (a), (b) 4-DPM and (c), (d) 3-DPM in solvents with
different polarities.
Fig. 3. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC)
curves
of
(a),
(b)
4-DPM
and
(c),
(d)
3-DPM.
1
1

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3
.4. Electrochemical analysis
The electrochemical properties of 4-DPM and 3-DPM are studied in CHCl solution using
3
CV with ferrocene as the internal standard. The HOMO and LUMO energy levels of both
compounds are estimated according to the electrochemical performance and the UV-Vis
absorption spectra. The E of 4-DPM and 3-DPM, estimated from the long-wavelength
g
absorption edges in CHCl solvent, are 2.92 and 2.91 eV, and the HOMO energy levels are
3
estimated to be -5.18 and -5.21 eV, respectively (Fig. 4). From the equation ELUMO = EHOMO
+
Eg, the LUMO energy levels of 4-DPM and 3-DPM are determined to be -2.26 and -2.30 eV,
respectively.
Fig. 4. Cyclic voltammetry (CV) measurements of (a) 4-DPM and (b) 3-DPM dissolved in
-1
acetonitrile with 0.1 M n-Bu NClO . Working electrode: glassy carbon, scan rate: 100 mVs .
4
4
3
.5. General physical properties
The topographical 2D and 3D images of DPEPO, DPEPO:4-DPM, and DPEPO:3-DPM
films with thicknesses of 50 nm are measured by AFM (scan size = 5 µm × 5 µm; see Fig. 5).
The R values corresponding to root-mean-square (rms) averages of the height deviations of
q
the DPEPO, DPEPO:4-DPM(10 wt.%), and DPEPO:3-DPM(10 wt.%) films are 0.33, 0.56,
and 0.46 nm, respectively. The surfaces of all films are sufficiently uniform to fabricate
1
2

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TADF-OLEDs with low leakage current [15]. However, the DPEPO:4-DPM film shows non-
uniformly dispersed surface with partially nanocrystals (width = 0.9 nm, height = 0.6 nm)
compared to those of the DPEPO film (width = 0.3 nm, height = 0.19 nm) and DPEPO:3-
DPM film (width = 0.6 nm, height = 0.3 nm). This may be because the DPEPO:4-DPM film
has a relatively rough surface morphology and non-uniformly stacked molecules due to
random orientation.
Fig. 5. Two-dimensional and 3D topographical AFM images of (a) DPEPO, (b) DPEPO:4-
DPM, and (c) DPEPO:3-DPM films at 5 µm scan size.
The features in the NEXAFS spectra (Supplementary Fig. S1) acquired from the
∗
DPEPO:4-DPM and DPEPO:3-DPM films are assigned as the π (C=C) orbital at 284.5 eV,
∗
the σ (C–H) orbital mixed with Rydberg orbitals around 287 to 291 eV, and several σ∗
orbitals in the energy range of 292 to 307 eV. We attempt to determine the tilt angle α
between the C=C double bond in the conjugated planes and the substrate surface with a
2
photon beam size of approximately 0.1×0.3 mm . From the best fit in the inset, we obtain an
∗
ensemble average tilt angle of approximately 55° ± 5°. The peak intensity of the π resonance
of the DPEPO, DPEPO:4-DPM, and DPEPO:3-DPM films does not change with increasing
1
3

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incidence angle (θ), and the changes in crystallinity of all films after C=C/C-H change are
insignificant. This may be because all films have broad orientation distributions.
To clarify the molecular orientations as a function of substituent positions of a phenyl
linker in TADF-OLED characteristics, we analyze the scattered X-ray intensity in the 2D-
GIXD patterns of DPEPO, DPEPO:4-DPM, and DPEPO:3-DPM films with the spectra of
scattering in-plane and out-of-plane to the plane and azimuthal angle scan (see Fig. 6). In the
2
D-GIXD patterns, qxy and qz represent the in-plane and out-of-plane components of the
scattering vector , which are normal to the plane of incidence and the sample surface plane,
q
respectively. The azimuthal intensity profiles are plots of orientation distribution. The 2D-
GIXD patterns reveal the crystallinities of DPEPO, DPEPO:4-DPM, and DPEPO:3-DPM on
the substrate. As shown in Figs. 6(a), (b), and (c), pristine DPEPO, DPEPO:4-DPM, and
-1
-
1
DPEPO:3-DPM show intensified out-of-plane reflections at qr,z= 0.706 Å and 1.406 Å ,
corresponding to the (100) and (200) reflections of DPEPO. The (100) layer distance of
DPEPO (= 2π/q*) is 8.9 Å. In case of DPEPO:4-DPM, additional reflections [highlighted by
red circles in Fig. 6(b)] are also observed along the out-of-plane direction at the surface
(αi=0.08) in Fig. 6(e). The multiple reflections observed at the surface are induced by the
aggregation and self-crystallization of the 4-DPM dopant at the surface. This result is
consistent with the surface morphology of DPEPO:4-DPM shown in Fig. 5(b). The inner
structures and out-of-plane intensity profiles for the surfaces of pristine DPEPO, DPEPO:4-
DPM, and DPEPO:3-DPM are shown in Figs. 6(d) (α =0.12°) and (e) (α =0.08°). As shown in
i
i
the intensity profiles, the (100) and (200) reflections of DPEPO are observed at the surfaces
and inner structures. However, multiple reflections for 4-DPM aggregates (red solid arrows)
are only observed at the surface, indicating that 4-DPM aggregates are mainly distributed at
the film surface. Moreover, the peak intensity and width of the (100) reflection of DPEPO:4-
DPM are significantly weakened and broadened. These results imply that the crystallinity of
1
4

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DPEPO:4-DPM is weaker than that of pristine DPEPO and DPEPO:3-DPM because the
aggregated 4-DPM interrupts the crystallization of DPEPO.
Fig. 6. Two-dimensional GIXD images of scattered X-ray intensity from surface to full depth
for (a) DPEPO, (b) DPEPO:4-DPM, and (c) DPEPO:3-DPM films on SiO /Si substrates. The
2
vertical and horizontal axes correspond to in-plane and out-of-plane scattering to the plane of
the films, whose spectra are, respectively, shown in (d) and (e). (f) Azimuthal intensity
plotting the orientation distributions of particular sets of crystallographic reciprocal lattice
planes of the films.
To elucidate the crystalline orientation of DPEPO, the azimuthal angle scans of (100)
reflection are shown in Fig. 6(f). The azimuthal intensity profiles allow the study of the
orientational distribution of the reflections as a function of all possible crystalline orientations
[
15]. The angular volume fractions of the films show that all films adopt a preferential
1
5

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horizontal orientation in the emission layer since the maximum intensities are observed at
o
about χ=0 . In particular, most of the crystallinity in the pristine DPEPO film is preferentially
horizontal plane-on orientation within the FWHM of 23°, while the crystallinity of 3-DPM is
a relatively narrow distribution within the FWHM of 25° compared with that of DPEPO:4-
DPM; that the crystallinity of DPEPO:4-DPM is more randomly distributed and exhibits low
o
crystallinity within the FWHM of 28 compare to DPEPO and DPEPO:3-DPM.
3
.6. Device characteristics
As the evidence of the contribution of TADF triplet excitions, we conduct time-resolved
EL measurements on the microsecond timescale (Supplementary Fig. S2). Two TADF
devices with DPEPO:4-DPM and DPEPO:3-DPM emitters show delayed fluorescence and
have similar values of about 10 µs, as shown in inset. It is because of the similar ∆E values
ST
of 0.114 and 0.098 eV for 4-DPM and 3-DPM, respectively. The current density of TADF-
OLEDs with the 4-DPM and 3-DPM show similar property (Supplementary Fig. S3). In
contrast, the current efficiency of TADF-OLED with the 3-DPM dopant (12.62 cd/A) is
approximately 9.1 times higher that of the device with the 4-DPM dopant (1.38 cd/A), and its
luminance efficiency is 12.5 times greater (8.81 vs. 0.70 lm/W); the CIE coordinates of the 3-
DPM and 4-DPM devices are (0.16, 0.20) and (0.17, 0.17), respectively (Fig. 7(a)). The
enhanced efficiency of the DPEPO:3-DPM device results from the following two reasons.
First is because of the increased internal quantum efficiency by up-converted triplet excitions
to the singlet state due to a small overlap between the electron-donating and electron-
withdrawing units, as well as a small charge-transfer with ∆E below 0.1 eV, as shown in
ST
Fig. 1 and Table 1. Second is because of the external quantum efficiency with the light
outcoupling effect by the horizontal orientation of the transition dipole moments. The
molecular orientation of the DPEPO:3-DPM is similar to that of DPEPO (i.e., horizontal
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plane-on molecular alignment), whereas the DPEPO:4-DPM shows weak crystallinity and
random orientation of weak horizontal and plane-on orientations (Fig. 6(f)). Thus, the charge
transport and light outcoupling of the DPEPO host and 3-DPM dopant are improved due to
the well-aligned stacking structure with horizontal orientation compared to the DPEPO:4-
DPM with random orientation [13-16].
Fig. 7. Current efficiency and luminance efficiency versus applied voltage of TADF devices
with (a) DPEPO:4-DPM and (b) DPEPO:3-DPM emitters.
4
. CONCLUSION
In summary, we synthesized new deep-blue TADF dopants by using dissolved bis(4-
(diphenylamino)phenyl)methanone and malononitrile. We observed that the phenyl linker
positions between the electron-donating diphenylamine unit and electron-withdrawing
malonitrile unit significantly affected device efficiency. 4-DPM with a p-phenyl linker
showed that the HOMO level was distributed on the whole molecule and the LUMO level
was distributed on the malonitrile unit, resulting in a large overlap in the phenyl linker as well
as the malonitrie unit. In 3-DPM with an m-phenyl linker, the HOMO was distributed on the
diphenylamine unit and the LUMO was distributed on the malonitrile unit, resulting in a
weak overlap in the phenyl linker. The calculated ∆E values for the optimized ground-state
ST
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geometries were similar (0.114 and 0.098 eV for 4-DPM and 3-DPM, respectively). In the
final analysis, TADF efficiency depended strongly on the molecular orientation as well as the
linker positions. The current efficiency of the TADF-OLED with the 3-DPM dopant (12.62
cd/A) was approximately 9.1 times higher than that of the device with the 4-DPM dopant
(1.38 cd/A), and its luminance efficiency was 12.5 times greater (8.81 vs. 0.70 lm/W).
Acknowledgments
This research was supported by Basic Science Research Program through the National
Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future
Planning(2016R1C1B1011745) and by the Ministry of Science, ICT and Planning of South
Korea under the “IT Consilience Creative Program” (IITP-2015-R0346-15-1007) supervised
by IITP (Institute for Information & Communications Technology Promotion). The work also
supported
by
the
NRF(2015R1A2A1A10055620)
and
R&D
program
of
MOTIE/KEIT(10048317).
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Highlight
•
•
•
•
•
Synthesized TADF blue dopants with diphenylamine derivative malononitrile.
High current efficiency of 12.62 cd/A by dopant with m-phenyl linker.
Low current efficiency of 1.38 cd/A by dopant with p-phenyl linker.
Small charge-transfer singlet-triplet state splitting below 0.1 eV.
Molecular orientations by substituent positions of a phenyl linker.