Donor Interlocked Molecular Design for Fluorescence-like Narrow Emission in Deep Blue Thermally Activated Delayed Fluorescent Emitters

Article abstract of DOI:10.1021/acs.chemmater.6b01484

Deep blue thermally activated delayed fluorescent (TADF) emitters with a narrow emission spectrum were developed by managing the molecular structure of the TADF emitters. The deep blue TADF emitters were designed to show large steric hindrance at the central core of the molecule and small singlet-triplet energy gap. The molecular engineering of the deep blue TADF emitters enabled the fabrication of the deep blue TADF device with a full width at half-maximum of only 48 nm and a quantum efficiency of 14.0%. The full width at half-maximum of the deep blue TADF device was similar to that of conventional fluorescent devices, while the quantum efficiency was more than tripled.

Full text of DOI:10.1021/acs.chemmater.6b01484

Article
pubs.acs.org/cm
Donor Interlocked Molecular Design for Fluorescence-like Narrow
Emission in Deep Blue Thermally Activated Delayed Fluorescent
Emitters
Yong Joo Cho,†,∥ Sang Kyu Jeon,‡ Sang-Shin Lee,§ Eunsun Yu,§ and Jun Yeob Lee*,‡
†Department of Polymer Science and Engineering, Dankook University, 152, Jukjeon-ro, Suji-gu, Yongin-si, Gyeonggi 448-701, Korea
‡School of Chemical Engineering, Sungkyunkwan University, 2066, Seobu-ro, Jangan-gu, Gyeonggi 440-746, Korea
§Central R&D Center, Samsung SDI, 56, Gosan-ro, Uiwang-si, Gyeonggi 16073, Korea
S
* Supporting Information
ABSTRACT: Deep blue thermally activated delayed fluorescent
(TADF) emitters with a narrow emission spectrum were developed
by managing the molecular structure of the TADF emitters. The
deep blue TADF emitters were designed to show large steric
hindrance at the central core of the molecule and small singlet−
triplet energy gap. The molecular engineering of the deep blue
TADF emitters enabled the fabrication of the deep blue TADF
device with a full width at half-maximum of only 48 nm and a
quantum efficiency of 14.0%. The full width at half-maximum of the
deep blue TADF device was similar to that of conventional
fluorescent devices, while the quantum efficiency was more than
tripled.
emission.14,15 In general, full width at half-maximum (fwhm)
of common blue TADF emitters is in the range from 70 to 80
INTRODUCTION
■
Blue thermally activated delayed fluorescent (TADF) devices
have been intensively studied for the last couple of years
because they have a potential as a substitute for blue
nm compared to below 50 nm of conventional blue fluorescent
emitters.1−16 The broad emission spectrum makes it difficult to
phosphorescent devices.1−13 Since the paper reporting 20%
apply the blue TADF emitters in a display application because a
sharp emission spectrum is essential in the display panel. The
other issue is color purity of the blue TADF devices. Most
TADF devices developed so far show sky blue emission rather
than deep blue emission, but deep blue TADF devices are
preferred in real applications in the organic light-emitting diode
(OLED) panel. In fact, these two issues are related to each
other, and material development to overcome the hurdles of
current blue TADF emitters is necessary.
In this work, we report a new molecular design approach to
improve the color purity and broad emission spectrum of
common TADF emitters. We designed a target molecule with
interlocked donor moieties at the central core and electron
donor−acceptor moieties in the backbone structure for small
singlet−triplet energy gap (ΔEST). Comparison of the target
molecule having the interlocked donor based molecular design
with a reference molecule revealed that the interlocked donor
structure is crucial to reduce the fwhm and to obtain deep blue
emission color as well as high EQE. A deep blue TADF device
with a fwhm of 48 nm, high EQE of 14.0%, and deep blue color
external quantum efficiency (EQE) in the green TADF
devices,1 more interest is being focused on developing high
efficiency blue TADF devices, and high EQE above 20% has
been already demonstrated.10−13
There have been several different design approaches for the
development of the blue TADF emitters, but the main
approach was to control the electron donating and accepting
character of the donor and acceptor moieties comprising the
TADF backbone structure. For example, a strong electron
accepting dicyanobenzene moiety was combined with a weak
electron donating carbazole moiety,1,7 while a weak electron
accepting diphenylsulfone moiety was merged with a strong
electron donating acridine moiety.2,5,6,9,11 In another approach,
a diphenyltriazine or phenyltriazine acceptor was linked with
multiple carbazole units.8,10,13 These approaches were
successful to control the electron donating and accepting
power and corresponding singlet and triplet energy of the
TADF emitters. Therefore, several blue TADF emitters could
reach high EQE close to 20% in the blue TADF devices.
Although high efficiency blue TADF devices have been
reported, there are two main issues related with the emission
spectrum to overcome in the development of the blue TADF
devices. One issue is broad emission spectra of the current
TADF emitters due to strong charge transfer (CT)
Received: April 13, 2016
Revised: July 18, 2016
© XXXX American Chemical Society
A
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a
Scheme 1. Synthetic Routes of CNBPCz and CzBPCN
a
NaH = sodium hydride, DMF = dimethylformamide, CuCN = copper(I) cyanide, Pd(dppf)Cl2 = [1,1′-bis(diphenylphosphino)ferrocene]-
dichloropalladium(II), DMSO = dimethyl sulfoxide, CsF = cesium fluoride.
10 h. The solution of the reaction mixture was washed with cold
distilled water in order to remove DMF solvent and then extracted by
ethyl ether three times. The mixture was purified by column
chromatography on silica gel with n-hexane and dichloromethane.
The white needle shape product was obtained in 1.50 g (19%) yield.
MS (FI+) m/z 216.96 [(M)].
coordinate of (0.14, 0.12) was demonstrated using the newly
designed deep blue TADF emitter.
EXPERIMENTAL SECTION
■
General Information. 9H-Carbazole, [1, 1′-bis-
(diphenylphosphino)ferrocene]dichloropalladium(II) (Pd(dppf)Cl2),
and copper(I) cyanide were purchased from Aldrich Chemical Co.,
and dimethyl sulfoxide (DMSO), magnesium sulfate and dimethyl-
formamide (DMF) were purchased from Duksan Sci. Co. Sodium
hydride, 1,5-dibromo-2,4-difluorobenzene, and 1,2-dibromo-4,5-di-
fluorobenzene were purchased from Fluorochem Ltd. 9,9-Dimethyl-
9,10-dihydroacridine was purchased from P&H tech. These chemicals
were used without further purification. THF was distilled over sodium
and calcium hydride. Nuclear magnetic resonance (NMR) spectra
were recorded on a Jeol 400 ECX (1H NMR: 400 MHZ, 13C NMR:
4,4′,6,6′-Tetrafluoro-[1,1′-biphenyl]-3,3′-dicarbonitrile. 5-
Bromo-2,4-difluorobenzonitrile (3.00 g, 13.76 mmol), cesium fluoride
(15.6 g, 103.21 mmol), and Pd(dppf)Cl2 (1.12 g, 1.38 mmol) was
dissolved in DMSO solvent at 100 °C. The reaction mixture was
stirred during 3 h, and then distilled water was poured into the mixture
flask in order to quench the reaction. The mixture was extracted with
dichloromethane and then purified by column chromatography on
silica gel with n-hexane/dichloromethane. The white powdery product
was obtained in 0.30 g (16%) yield.
1H NMR (200 MHZ, CDCl3): δ 8.29 (t, J = 5.2 HZ, 2H), 7.90 (t, J =
6.6 HZ, 2H). MS (FI+) m/z 276.13 [(M)].
1
100 MHZ) at room temperature. Chemical shifts of the H and 13C
NMR signals were quoted relative to tetramethylsilane (δ = 0.00).
Ultraviolet−visible (UV−vis) absorption spectra were recorded on a
PerkinElmer Lamda 950, and fluorescence spectra were recorded on
Hitachi F-7000. The highest occupied molecular orbital energy level
was determined using an IVIUM STAT in three electrodes with a
carbon working electrode, a platinum wire counter electrode, and a
Ag/AgCl reference electrode. The lowest unoccupied molecular orbital
energy level was determined using an IVIUM STAT in three
electrodes with a carbon working electrode, a platinum wire counter
electrode, and a Ag/AgCl reference electrode after nitrogen bubbling
to remove oxygen from the sample. Ferrocene was used as the
standard material for the cyclic voltammetry measurement. The
elemental analyses were carried out with a flash 2000 model. The mass
spectra were collected on a Jeol JMS-AX505WA spectrometer in fast
atom bombardment mode and AccuTOF-GCv 4G spectrometer in
field ionization mode. Transient photoluminescence measurement was
performed using a pulsed Nd:YAG laser (355 nm) as the excitation
source and a photomultiplier tube as an optical detector system.
Synthesis. The synthesis of two emitters is schematically described
in Scheme 1.
4,4′,6,6′-Tetra(9H-carbazol-9-yl)-[1,1′-biphenyl]-3,3′-dicar-
bonitrile (CzBPCN). Sodium hydride (60% in paraffin, 0.13 g, 5.43
mmol) was washed with n-hexane three times. A solution of 9H-
carbazole (0.91 g, 5.43 mmol) in anhydrous DMF was poured into the
dispersed solution of purified sodium hydride in dry DMF at room
temperature. After stirring for 30 min, 4,4′,6,6′-tetrafluoro-[1,1′-
biphenyl]-3,3′-dicarbonitrile (0.30 g, 1.09 mmol) was dissolved in
anhydrous DMF (20 mL) and then added to the stirred reaction
solution under nitrogen atmosphere. The reaction solution was
allowed to stir at room temperature overnight. The reaction mixture
was extracted with dichloromethane and distilled water and then dried
with anhydrous magnesium sulfate. The reaction product of CzBPCN
was purified by silica gel chromatography with a mixture of chloroform
and n-hexane. A light yellow powdery product was obtained in 0.60 g
(64%) yield.
1H NMR (200 MHZ, CDCl3): δ 8.70 (s, 4H), 8.15 (d, J = 7.8 HZ,
8H), 7.53−7.29 (m, 24H). 13C NMR (50 MHZ, CDCl3): δ 141.85,
141.51, 140.43, 139.71, 135.11, 129.42, 126.59, 124.26, 121.45, 121.31,
120.90, 115.37, 111.99, 109. 68. MS (FAB) m/z 865 [(M + H)+].
Anal. Calcd for C62H36N6: C, 86.09; H, 4.19; N, 9.72. Found: C, 85.90;
H, 4.03; N, 9.32.
5-Bromo-2,4-difluorobenzonitrile. 1,5-Dibromo-2,4-difluoro-
benzene (10.00 g, 36.78 mmol) and copper(I) cyanide (4.94 g.
55.17 mmol) were dissolved with anhydrous DMF solvent under 100
°C. The reaction mixture was stirred with ambient atmosphere during
2-Bromo-4,5-difluorobenzonitrile. 2-Bromo-4,5-difluorobenzo-
nitrile was synthesized by the same route as for 5-bromo-2,4-
B
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Figure 1. Gaussian simulation images of CNBPCz and CzBPCN. Electron density distribution of the HOMO and LUMO was calculated by TD-
DFT using Gaussian 09 software.
difluorobenzonitrile but using 1,2-dibromo-4,5-difluorobenzene in-
stead of 1,5-dibromo-2,4-difluorobenzene.
MS (FI+) m/z 216.96 [(M)].
RESULTS AND DISCUSSION
■
The new TADF emitter, 4,4′,6,6′-tetra(9H-carbazol-9-yl)-[1,1′-
biphenyl]-3,3′-dicarbonitrile (CzBPCN), was designed to have
interlocked donor moieties at the central core to restrict
molecular motion of the TADF emitters for a sharp light-
emission spectrum. To prove the concept of the interlocked
donor based molecular design, an isomer molecule without any
interlocked donor moieties at the central core, 4,4′,5,5′-
tetra(9H-carbazol-9-yl)-[1,1′-biphenyl]-2,2′-dicarbonitrile
(CNBPCz), was also synthesized as a reference material. The
CNBPCz and CzBPCN emitters were constructed based on a
biphenyl core structure. Four carbazole units were attached to
the biphenyl core as donor moieties and two CN units were
attached as acceptor moieties. Both CNBPCz and CzBPCN
have two carbazole moieties at 4- and 4′- position of the
biphenyl core, but the position of the two carbazole and two
CN units were different. In the case of CNBPCz, two carbazole
units were substituted at 5,5′-positions and two CN units were
attached at 2,2′-positions. The positions of the carbazole and
CN units were reversed in the CzBPCN emitter with two
carbazole units at 2,2′-positions and two CN units at 5,5′-
positions. The CzBPCN emitter was intended to have large
steric hindrance for the rotation of the central biphenyl ring by
interlocking the bulky carbazole units, while the CNBPCz
emitter was designed to have little steric hindrance.
Starting materials of CNBPCz and CzBPCN were 1,2-
dibromo-4,5-difluorobenzene and 1,3-dibromo-4,6-difluoroben-
zene. Only one Br unit of each material was substituted with a
CN unit using CuCN, and Pd catalyzed halogen coupling of the
CN modified intermediates produced two intermediates with
two CN units and four F units for carbazole modification. The
carbazole modification was conducted using NaH mediated
substitution of the carbazole units. The CNBPCz and CzBPCN
were synthesized at synthetic yields of 60% and 85%,
respectively, after vacuum train sublimation.
4,4′,5,5′-Tetrafluoro-[1,1′-biphenyl]-2,2′-dicarbonitrile.
4,4′,5,5′-Tetrafluoro-[1,1′-biphenyl]-2,2′-dicarbonitrile was synthe-
sized by the same route as for 4,4′,6,6′-tetrafluoro-[1,1′-biphenyl]-
3,3′-dicarbonitrile but using 2-bromo-4,5-difluorobenzonitrile instead
of 5-bromo-2,4-difluorobenzonitrile.
1H NMR (200 MHZ, CDCl3): δ 8.46 (t, J = 5.0 HZ, 2H), 8.03 (t, J =
5.2 HZ, 2H), MS (FI+) m/z 276.13 [(M)].
4,4′,5,5′-Tetra(9H-carbazol-9-yl)-[1,1′-biphenyl]-2,2′-dicar-
bonitrile (CNBPCz). CNBPCz was synthesized by the same route as
for CzBPCN but using 4,4′,5,5′-tetrafluoro-[1,1′-biphenyl]-2,2′-
dicarbonitrile instead of 4,4′,6,6′-tetrafluoro-[1,1′-biphenyl]-3,3′-dicar-
bonitrile.
1
Yield: 62%. H NMR (400 MHZ, CDCl3): δ 8.43 (s, 2H), 8.33 (s,
2H), 7.84−7.80 (m, 8H), 7.26−7.12 (m, 24H). 13C NMR (100 MHZ,
CDCl3): δ 139.60, 139.00, 138.75, 135.88, 135.63, 133.34, 126.14,
124.27, 124.17, 121.31, 121.20, 120.39, 120.27, 117.11, 111.84, 109.86,
109.44. MS (FAB) m/z 865 [(M + H)+]. Anal. Calcd for C62H36N6: C,
86.09; H, 4.19; N, 9.72. Found: C, 85.94; H, 4.15; N, 9.36.
Device Fabrication and Measurements. Vacuum thermal
evaporation was a basic process to grow the CNBPCz and CzBPCN
devices with organic layers sandwiched between an indium tin oxide
and LiF/Al double layer cathode. Spin coating of poly(3,4-ethyl-
enedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS) formed a
60 nm thick hole injection layer, and consecutive vacuum evaporation
of 20 nm thick 4,4′-cyclohexylidenebis[N,N-bis(4-methylphenyl)-
aniline] (TAPC) hole transport layer, 10 nm thick 1,3-bis(N-
carbazolyl)benzene (mCP) exciton blocking layer, 25 nm thick
DPEPO:CNBPCz or DPEPO:CzBPCN emitting layer with 5%
doping concentration, 5 nm thick diphenylphosphine oxide-4-
(triphenylsilyl)phenyl (TSPO1) exciton blocking layer, and 30 nm
thick 1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene (TPBI) electron
transport layer followed. Encapsulation of the evaporated devices was
conducted after cathode deposition for device characterization in
ambient conditions. Current density−voltage testing and device
performances of the narrow emission blue TADF devices were
measured by Keithley 2400 source measurement and CS-1000
spectroradiometer. Lambertian distribution of light emission was
confirmed in all quantum efficiency measurements.
The sublimed CNBPCz and CzBPCN emitters with a high
purity above 99% were used for identification of chemical
structure and analysis of photophysical parameters. Detailed
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chemical analysis methods and analysis results of the CNBPCz
and CzBPCN emitters were added in the Experimental Section.
The analysis results include 1H and 13C nuclear magnetic
resonance (NMR) data, mass data, and thermal analysis data.
The purity level of the sublimed compounds was confirmed by
high performance liquid chromatography (HPLC). The
CNBPCz and CzBPCN compounds could be differentiated
by different retention times in the HPLC analysis and NMR
spectra in Supporting Information (Figures S1 and S2).
Singlet energy and triplet energy of the two emitters were
estimated by measuring fluorescence and phosphorescence
emission spectra in Figure 2. Ultraviolet−visible spectra of the
The molecular design concept of interlocking two carbazole
units was theoretically proven by calculating the molecular
orbital and geometrical structure of the synthesized materials
using the Gaussian 09 software program which employed Lee−
Yang−Parr functional (B3LYP) with 6-31G* basis sets for the
density functional of Becke’s 3-parameters.19 The molecular
orbital and geometrical structure of the two materials are shown
in Figure 1. At the central biphenyl core, two carbazole units of
CzBPCN are interlocked because rotation of the central
biphenyl core is prohibited by large steric hindrance of the
carbazole unit. In the case of CNBPCz, the steric hindarance is
not significant because two CN units are substituted at the
ortho-position of the biphenyl core. The small size CN unit has
no effect on the rotation of two phenyl units of the biphenyl
core. As a result, CzBPCN can be considered as a rigid
molecule with a restricted molecular motion, and CNBPCz can
be treated as a flexible molecule. The highest occupied
molecular orbital (HOMO) and the lowest unoccupied
molecular orbital (LUMO) of the CNBPCz and CzBPCN
were also estimated by the molecular calculation, and the
HOMO and LUMO were extensively isolated with weak
overlap at the central biphenyl core. Both CNBPCz and
CzBPCN exhibited the molecular orbital distribution required
for TADF emitters. Small ΔEST for intersystem crossing from
triplet excited state to singlet excited state is expected from the
separated HOMO and LUMO, and high photoluminescence
quantum yield is anticipated from the overlapped HOMO and
LUMO. The HOMO−LUMO overlap was rather extensive in
the CzBPCN emitter, which was reflected in the large oscillator
strength of CzBPCN (0.230) relative to that of CNBPCz
(0.193). This implies that the CzBPCN emitter may possess
better light absorption and accompanying light emission
characteristics than the CNBPCz emitter. Calculated physical
parameters of CNBPCz and CzBPCN are presented in Table 1.
Figure 2. Photoluminescence (PL) spectra of CNBPCz and CzBPCN.
PL spectra were obtained from toluene solution (concentrations 1.0 ×
10−5 M). All data were collected at room temperature except for the
low temperature PL data at 77 K.
emitters are in the Supporting Information (Figure S3). The
singlet energy and triplet energy were calculated from the onset
of fluorescence and phosphorescence spectra measured in
toluene solvent at a concentration of 1.0 × 10−5 M,
respectively. The singlet energy/triplet energy of CNBPCz
and CzBPCN were 3.10/2.83 eV and 3.14/2.87 eV,
respectively. The singlet/triplet energy gap was 0.27 eV in
the two emitters. The difference of geometrical structure did
not affect the singlet−triplet energy gap, although increase of
singlet and triplet energy was observed in the CzBPCN by the
distortion of the central biphenyl ring due to interlocked
carbazole units. The dihedral angles between two phenyl units
of the biphenyl core were 60° and 47° in the CNBPCz and
CzBPCN, while those between the phenyl unit of the biphenyl
core and carbazole were 60° and 90° in the CNBPCz and
CzBPCN, respectively. The large dihedral angles of the
CzBPCN emitter increased the emission energy.17 The
fluorescence spectra of the CNBPCz and CzBPCN also clearly
showed the effect of the interlocked carbazole units. The
fluorescent spectrum of CzBPCN was sharper than that of
CNBPCz. The full width at half-maximum (fwhm) of CzBPCN
was only 55 nm, which was much smaller than that of other
TADF emitters and even smaller than that of common blue
phosphorescent emitters. The large fwhm of 71 nm of
CNBPCz certifies that the CzBPCN emitter shows a very
narrow emission spectrum as a TADF emitter.
Delayed fluorescence behavior of CNBPCz and CzBPCN
was analyzed by transient photoluminescence (PL) measure-
ment and absolute PL measurement with and without nitrogen.
In the transient PL measurement results in Figure 3, the
delayed fluorescence component was very weak in the
CNBPCz both at low and room temperatures, but it was
gradually increased according to temperature in the CzBPCN
TADF emitter. This indicates that the CzBPCN TADF emitter
has large contribution of the delayed fluorescence in the light
emission process, while the CNBPCz emitter has small
contribution of the delayed fluorescence for light emission.
This can also be confirmed by the absolute PL quantum yield of
the two emitters. The PL quantum yields with/without
nitrogen were 0.45/0.46 and 0.20/0.76 in the CNBPCz and
CzBPCN emitters. The PL quantum yield of CNBPCz was not
affected by the presence of nitrogen, but that of CzBPCN was
largely increased under nitrogen, proving a large contribution of
triplet excitons to the PL emission by a delayed emission
Table 1. Summary of the Photophysical Properties of
CNBPCz and CzBPCN
b
ΦPL
PL peak (prompt/
ES/ET
d
ΔEST fwhm
e f
a
c
emitter
(nm)
total)
τ (μs)
(eV)
(eV)
(nm)
CNBPCz
CzBPCN
458
453
0.45/0.46
0.20/0.76
24.34
48.22
3.10/2.83
3.14/2.87
0.27
0.27
71
55
a
b
Measured in toluene solution. Absolute photoluminance quantum
efficiency yield evaluated using an integrating sphere in toluene
solution (concentrations in the 10 μM range) without degassing
(prompt) and after bubbling nitrogen gas in a few minutes (total).
c
Exciton lifetime at 297 K; transient PL measured from 1 wt % doped
d
film in polystryrene on quartz substrate. Singlet energy (ES)
estimated from onset of wavelength in toluene solution. Triplet
energy (ET) estimated from onset of wavelength in tetrahydrofuran
solution (concenctrations in the 10 μM range) emission spectra at 77
e
f
K. ΔEST = ES − ET. Calculated by PL spectra in toluene solution.
D
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Figure 3. Transient PL decay curves of solution coated DPEPO:CNBPCz and DPEPO:CzBPCN films according to different temperatures (50 K,
100 K, 150 K, 200 K, 250 K, and 300 K) with vacuum state measured using a pulse Nd:YAG laser (340 nm) as the excitation source and a
photomultiplier tube as an optical detector system.
process. The PL quantum yields of the vacuum deposited
CNBPCz and CzBPCN were 0.37 and 0.94, respectively. The
high PL quantum yield of the CzBPCN is in agreement with
the oscillator strength of the emitters obtained by molecular
calculation.
As the objective of designing the CzBPCN was to sharpen
the emission spectrum of the TADF devices, electrolumines-
cence (EL) spectra of the CzBPCN devices were investigated.
Figure 4 shows EL spectra of the CNBPCz and CzBPCN
Figure 5. Quantum efficiency versus current density curves of
CNBPCz and CzBPCN devices.
CNBPCz device was only 4.8%. This work reported high EQE
in the deep blue device with a very narrow emission spectrum
with a fwhm of 48 nm. The EQE was tripled while keeping the
color performances of common blue fluorescent device having a
common emitting layer of 2-methyl-9,10-di(2-naphthyl)-
anthracene:2,5,8,11-tetra-tert-butylperylene(MADN:TBPe).11
Figure 4. EL spectra of CNBPCz and CzBPCN devices. The EL
spectra were measured at 1000 cd/m2.
CONCLUSIONS
■
In conclusion, a novel TADF emitter having a very narrow
emission spectrum with a fwhm of 48 nm was developed by a
new molecular design approach interlocking donor moieties for
restricted molecular motion. A high efficiency of 14.0%, a deep
blue color coordinate of (0.14, 0.12), and a fwhm of 48 nm
were achieved in the deep blue TADF device using a new
molecular design approach.
devices fabricated by doping of CNBPCz and CzBPCN in a
common bis[2-(diphenylphosphino)phenyl] ether oxide
(DPEPO) host. The CNBPCz and CzBPCN emitters showed
similar deep blue emission spectra peaked at 456 and 460 nm,
respectively, but the fwhm of the two emitters was quite
different. The fwhm values of CNBPCz and CzBPCN were 76
and 48 nm, respectively. As expected in the molecular design,
the CzBPCN emitter with the interlocked carbazole units
showed a very sharp emission spectrum comparable to that of a
common fluorescent emitter. The fwhm of the CzBPCN
emitter was even smaller than that of common phosphorescent
emitters. The narrow emission spectrum of the CzBPCN
device realized a deep blue color coordinate of (0.14, 0.12)
even though the peak wavelength of the CzBPCN device was in
the long wavelength range compared to that of CNBPCz or
other deep blue TADF emitters.18
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.chemma-
ter.6b01484.
■
S
NMR spectra and UV−vis absorption spectra of
synthesized materials (PDF)
The external quantum efficiency (EQE) of the two TADF
devices is presented in Figure 5. The EQE of the CzBPCN
device was much higher than that of the CNBPCz device
because of triplet exciton harvesting for fluorescence emission
by delayed fluorescence. As confirmed by the PL analysis data,
the delayed fluorescence was activated in the CzBPCN emitter,
which contributed to the high EQE of the CzBPCN device.
The EQE of the CzBPCN device was 14.0%, while that of the
AUTHOR INFORMATION
Corresponding Author
*J. Y. Lee. E-mail: leej17@skku.edu.
■
Present Address
∥Y. J. Cho. Department of Electrical and Computer Engineer-
ing, University of Waterloo, Waterloo, Ontario N2L 3G1,
Canada.
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(16) Nakanotani, H.; Higuchi, T.; Furukawa, T.; Masui, K.;
Morimoto, K.; Numata, M.; Tanaka, H.; Sagara, Y.; Yasuda, T.;
Adachi, C. High-efficiency organic light-emitting diodes with
fluorescent emitters. Nat. Commun. 2014, 5, 4016.
(17) Ward, J. S.; Nobuyasu, R. S.; Batsanov, A. S.; Data, P.;
Monkman, A. P.; Dias, F. B.; Bryce, M. R. The interplay of thermally
activated delayed fluorescence (TADF) and room temperature organic
phosphorescence in sterically-constrained donor-acceptor charge-
transfer molecules. Chem. Commun. 2016, 52, 2612−2615.
(18) Lee, I.; Lee, J. Y. Molecular design of deep blue florescent
emitters with 20% external quantum efficiency and narrow emission
spectrum. Org. Electron. 2016, 29, 160−164.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by Samsung SDI and Basic Science
Research Program through the National Research Foundation
of Korea (NRF) funded by the Ministry of Science, ICT, and
f u t u r e P l a n n i n g ( 2 0 1 3 R 1 A 2 A 2 A 0 1 0 6 7 4 4 7 ,
2016R1A2B3008845).
(19) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.;
Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.;
Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.;
Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.;
Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao,
O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J.
B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R.
E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.;
Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J.
J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.;
Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman,
J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.;
Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.;
Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.;
Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen,
W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 09; Gaussian,
Inc.: Wallingford, CT, 2004.
REFERENCES
■
(1) Uoyama, H.; Goushi, K.; Shizu, K.; Nomura, H.; Adachi, C.
Highly efficient organic light-emitting diodes from delayed fluo-
rescence. Nature 2012, 492, 234−238.
(2) Zhang, Q.; Li, J.; Shizu, K.; Huang, S.; Hirata, S.; Miyazaki, H.;
Adachi, C. Design of efficient thermally activated delayed fluorescence
materials for pure blue organic light emitting diodes. J. Am. Chem. Soc.
2012, 134, 14706−14709.
(3) Lee, J.; Shizu, K.; Tanaka, H.; Nomura, H.; Yasuda, T.; Adachi, C.
Oxadiazole- and triazole- based highly-efficient thermally activated
delayed fluorescence emitters for organic light-emitting diodes. J.
Mater. Chem. C 2013, 1, 4599−4604.
(4) Lee, S. Y.; Yasuda, T.; Yang, Y. S.; Zhang, Q.; Adachi, C.
Luminous Butterflies: Efficient Exciton Harvesting by Benzophenone
Derivatives for Full-Color Delayed Fluorescence OLEDs. Angew.
Chem., Int. Ed. 2014, 53, 6402−6406.
(5) Wu, S.; Aonuma, M.; Zhang, Q.; Huang, S.; Nakagawa, T.;
Kuwabara, K.; Adachi, C. High-efficiency deep-blue organic light-
emitting diodes based on a thermally activated delayed fluorescence
emitter. J. Mater. Chem. C 2014, 2, 421−424.
(6) Zhang, Q.; Li, B.; Huang, S.; Nomura, H.; Tanaka, H.; Adachi, C.
Efficiency blue organic light-emitting diodes employing thermally
activated delayed fluorescence. Nat. Photonics 2014, 8, 326−332.
(7) Cho, Y. J.; Yook, K. S.; Lee, J. Y. Cool and warm hybrid white
organic light-emitting diode with blue delayed fluorescent emitter both
as blue emitter and triplet host. Sci. Rep. 2015, 5, 7859−7866.
(8) Kim, M.; Jeon, S. K.; Hwang, S. H.; Lee, J. Y. Stable Blue
Thermally Activated Delayed Fluorescent Organic Light-Emitting
Diodes with Three Times Longer Lifetime than Phosphorescent
Organic Light-emitting Diodes. Adv. Mater. 2015, 27, 2515−2520.
(9) Zhang, Q.; Tsang, D.; Kuwabara, H.; Hatae, Y.; Li, B.; Takahashi,
T.; Lee, S. Y.; Yasuda, T.; Adachi, C. Nearly 100% Internal Quantum
Efficiency in Undoped Electroluminescent Device Employing Pure
Organic Emitters. Adv. Mater. 2015, 27, 2096−2100.
(10) Lee, D. R.; Kim, M.; Jeon, S. K.; Hwang, S. H.; Lee, C. W.; Lee,
J. Y. Design Strategy for 25% External Quantum Efficiency in Green
and Blue Thermally Activated Delayed Fluorescent Devices. Adv.
Mater. 2015, 27, 5861−5867.
(11) Lee, I. H.; Song, W.; Lee, J. Y.; Hwang, S. H. High efficiency
blue fluorescent organic light-emitting diodes using a conventional
blue fluorescent emitter. J. Mater. Chem. C 2015, 3, 8834−8838.
(12) Numata, M.; Yasuda, T.; Adachi, C. High efficiency pure blue
thermally activated delayed fluorescence molecules having 10H-
phenoxaborin and acridan units. Chem. Commun. 2015, 51, 9443−
9446.
(13) Hirata, S.; Sakai, Y.; Masui, K.; Tanaka, H.; Lee, S. Y.; Nomura,
H.; Nakamura, N.; Yasumatsu, M.; Nakanotani, H.; Zhang, Q.; Shizu,
K.; Miyazaki, H.; Adachi, C. Highly efficient blue electroluminescence
based on thermally activated delayed fluorescence. Nat. Mater. 2015,
14, 330−336.
(14) Dias, F. B.; Bourdakos, K. N.; Jankus, V.; Moss, K. C.; Kamtekar,
K. T.; Bhalla, V.; Santos, J.; Bryce, M. R.; Monkman, A. P. Triplet
Harvesting with 100% Efficiency by Way of Thermally Activated
Delayed Fluorescence in Charge Transfer OLED Emitters. Adv. Mater.
2013, 25, 3707−3714.
(15) Reineke, S. Organic light-emitting diodes: Phosphorescence
meets its match. Nat. Photonics 2014, 8, 269−270.
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DOI: 10.1021/acs.chemmater.6b01484
Chem. Mater. XXXX, XXX, XXX−XXX