Thermally activated delayed fluorescence emitters with a m,m-di-tert-butyl-carbazolyl benzoylpyridine core achieving extremely high blue electroluminescence efficiencies

Article abstract of DOI:10.1039/c7tc00457e

Thermally activated delayed fluorescence (TADF) emitters are attractive for display and lighting applications. Here, a series of highly efficient blue TADF emitters including 3,5-bis((3,6-di-tert-butyl-9H-carbazol-9-yl)phenyl)(pyridin-4-yl)methanone (4BPy

Full text of DOI:10.1039/c7tc00457e

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Nguyên T. K. Thanh, Xiaodi Su et al.
Fine-tuning of gold nanorod dimensions and plasmonic properties using
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Thermally activated delayed fluorescence emitters with a m,m-di-
tert-butyl-carbazolyl benzoylpyridine core achieving extremely
high blue electroluminescence efficiencies
Received 00th January 20xx,
Accepted 00th January 20xx
Pachaiyappan Rajamalli,a Vasudevan Thangaraji,a Natarajan Senthilkumar,a Chen-Cheng Ren-Wu,b
Hao-Wu Linb and Chien-Hong Cheng*,a
DOI: 10.1039/x0xx00000x
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Thermally activated delayed fluorescence (TADF) emitters are attractive for the display and lighting applications.
Here,
a
series of highly efficient blue TADF emitters including 3,5-bis((3,6-di-tert-butyl-9H-carbazol-9-
(4BPy-mDTC), (3,5-bis(3,6-di-tert-butyl-9H-carbazol-9-yl)phenyl)(pyridin-3-
yl)phenyl)(pyridin-4-yl)methanone
yl)methanone (3BPy-mDTC), (3,5-bis(3,6-di-tert-butyl-9H-carbazol-9-yl)phenyl)(pyridin-2-yl)methanone (2BPy-
mDTC) and (3,5-bis(3,6-di-tert-butyl-9H-carbazol-9-yl)phenyl)(phenyl)methanone (BP-mDTC) were designed and
synthesized. The molecular structures feature two meta carbazole substituents attached to a benzolylpyridine (BPy)
group or to a benzophenone (BP) group. These compounds show high thermal stability (Td = 371 ~ 439 °C ), blue
emissions (458 ~ 488 nm), high photoluminescence quantum yields in thin films (PLQY) (75 ~ 96%) and very small
energy gaps between S1 and T1 (ΔEST) of 0.01 ~ 0.05 eV. In addition, they all reveal TADF properties including small
ΔEST, two components in the transit PL decays, the prompt emission and the temperature-dependent delayed
emission. The BPy series appears to give much higher photoluminescence quantum yields (PLQY >92%) than that of
BP-mDTC (75%) plausibly due to the more rigid structure caused by the interaction between pyridine nitrogen and
the aromatic C-H bond. Further, 4BPy-mDTC shows more delayed component compared to 3BPy-mDTC and 2BPy-
mDTC. The electroluminescence devices based on 4BPy-mDTC and 2BPy-mDTC as the dopant emitters exhibit sky
blue emission with maximum external quantum efficiencies (EQEs) over 28%, and current and power efficiency and
maximum luminance up to 67.0 cd A-1, 60.1 lm W-1 and 20000 cd m-2, respectively. The presence of pyridine ring
and the position of nitrogen atom in the molecules are critical for the high quantum yield and device efficiency. The
PLQY EQE and luminance are dramatically improved by changing the phenyl into the pyridine group in the dopant
in
these
devices.
1. Introduction
Intensive research has continued to focus on OLEDs due to fluorescence
(1st
generation),
phosphorescence
(2nd
many performance advantages such as access to flexible panels, generation).4ꢀ10 The radiative decay of triplet excitons (75%) is
low power consumption, wider viewing angles, a higher spinꢀforbidden in the fluorescent OLED, and only singlet
contrast ratio, faster response times, high luminous efficiency, excitons (25%) can be recruited to produce light.4 In the 2nd
facile color tuning of emitters and are lighter in weight.1ꢀ3 generation materials of late transition metal complexes as
Because of the reduced energy consumptions of OLEDs, emitters access the emissive triplet states due to singletꢀtriplet
electronic companies have invested significantly for adopting state mixing via efficient spinꢀorbit coupling, and thus they can
this forefront technology as a tool to reduce the electricity harvest both singlet and triplet excitons (100%).11ꢀ18 However,
usage across the world. In the past few decades, intensive the 2nd generation phosphorescent emitters containing precious
research towards OLEDs has successively led to development metals, such as Ir and Pt are expensive and bespoke blueꢀ
of different generations of dopant materials such as emitting complexes remains unreliable in terms of their
performance in OLED devices for practical applications. Very
recently, TADF materials (3rd generation) have been realized
for the efficient OLED display and lightings.19ꢀ26 Among these
three generation materials, TADF materials (3rd generation)
have rapidly gained great attentions owing to their high device
efficiencies by harvesting 100% of the excitons without
requiring the expensive noble metals.
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Although many TADF emitters have been developed for
Journal Name
red, green, and blue emitting OLEDs.27ꢀ32 The molecular design
for efficient blue TADF emitters is not very clear and
systematic plan for the rational molecular designs are highly
desired. Particularly, to achieve inexpensive OLEDs for
commercial application, there is an urgent need for highly
efficient blue emitting materials to overcome the weakness of
currently existing blue emitters. In principle, TADF originates
from molecules with a small energy gap between the lowest
singlet (S1) and triplet (T1) excited states that enable the
harvesting of both singlet and triplet excitons under electrical
excitation.33ꢀ40 The molecular design plays a critical role in
achieving high efficiency of the TADF based OLEDs.29ꢀ32 Here,
we present an ideal simple molecular design for blue TADF
emitters consisting of a benzoylpyridine and a benzophenone
core as the electron accepting unit and two di(tꢀbutyl)carbazole
groups as the electron donating units. Importantly, the
molecules show very small
ꢁEST and extremely high
photoluminescence quantum yield (PLQY) in the thin films
indicating that they are ideal TADF materials. By using these
new TADF materials as emitters, the blue OLEDs can achieve
an external quantum efficiency over 28%. These results will
open up a new path to achieve efficient blue TADF devices.
2. Results and discussion
2.1. Synthesis and DFT calculation
We have designed and synthesized four TADF materials,
including 4BPyꢀ
mDTC, 3BPyꢀmDTC, 2BPyꢀmDTC, and BPꢀ
mDTC. The structures of these species consist of
a
benzoylpyridine and a benzophenone core as the electron
accepting unit and two di(tꢀbutyl))carbazole groups attached to
the meta positions of the central phenyl ring as the electron
donating units. These four compounds were synthesized in high
yields via the Ullmann coupling reaction of 3,6ꢀdiꢀtertꢀbutylꢀ
9Hꢀcarbazole (1) with the corresponding meta diꢀbromobenzoyl
pyridine derivatives (Scheme 1), while the details of synthetic
procedures are given in the Experimental section.
Fig.1 Molecular orbitals of a) 4BPy-mDTC, b) 3BPy-mDTC, c) 2BPy-mDTC and d) BP-
mDTC.
In these four compounds, the reactive C3 and C6 positions of
the carbazole groups were protected by a tꢀbutyl group to
improve the chemical and electrochemical stability and to
enhance the PLQY.29 In addition, the carbonyl acceptor unit is
more stable compared to the phosphine oxide or sulfone
counterpart.41 These synthesized emitters were purified by the
temperature gradient vacuum sublimation and were
characterized by NMR spectroscopy and highꢀresolution mass
spectrometry. The purity of the materials was confirmed by
high performance liquid chromatography (HPLC) analysis
given in the Experimental section and Supporting Information.
To gain insight into the electronic states of these
compounds, timeꢀdependent density functional theory (TDꢀ
DFT) calculations were performed for all the compounds at the
B3LYP/6ꢀ31G (d,p) level. As shown in Fig.1, the LUMOs of
O
O
i) n-BuLi
1
N
Br
Br
Br
Cu, K2CO3
ii) 4-Cyanopyridine
iii) 2N HCl
N
N
DCB, 180 °C
N
Br
Br
4BPy-mDBr
4BPy-mDTC
O
O
O
N
N
N
N
N
N
N
N
4BPyꢀ
mDTC, 3BPyꢀ
mDTC, and 2BPyꢀ
mDTC are localized on
3BPy-mDTC
2BPy-mDTC
BP-mDTC
the benzoylpyridine group, and BPꢀ
benzophenone moiety due to the strong electron withdrawing
m
DTC is on the
Scheme 1. Synthetic scheme for 4BPy-mDTC and structure of 3BPy-mDTC, 2BPy-
mDTC and BP-mDTC
2 | J. Name., 2012, 00, 1-3
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ARTICLE
nature of these moieties. Although both
t
ꢀbutyl carbazole units photoexcitation, the 4BPyꢀmDTC, 3BPyꢀmDTC, 2BPyꢀmDTC
are attached meta to the pyridoyl or benzoyl unit, the HOMO is and BPꢀ
mainly on one ꢀbutylcarbazole group, and the HOMOꢀ1 are peaks centered at 495, 480, 494 and 465, respectively. All the
distributed over the other ꢀbutylcarbazole. The main transitions compounds display significant positive solvatochromic effects
mDTC solutions emit sky blue light with PL emission
t
t
of these materials along with oscillator strengths and contour of its fluorescent emission in different solvents (Fig. S2). These
plots of the occupied and unoccupied molecular orbitals are results suggest that these donorꢀacceptor molecules show strong
listed in Tables S1ꢀS4. The two carbazolyl groups are slightly charge transfer
property
from
tꢀbutylcarbazole to
different in energy because they interact differently with the benzoylpyridine or benzophenone and are in good agreement
pyridoyl unit or benzoyl unit. The separated frontier molecular with the DFT calculation (vide supra).
orbitals lead to small calculated ꢁEST values of 0.13, 0.19, 0.11
and 0.15 eV for 4BPyꢀ DTC, 3BPyꢀ DTC, 2BPyꢀ DTC and the singlet energies were calculated to be 2.82, 2.90, 2.78 and
BPꢀ DTC, respectively. Besides, in all the molecules, the 2.98 eV for 4BPyꢀ DTC, 3BPyꢀ DTC, 2BPyꢀ DTC, and BPꢀ
HOMO and HOMOꢀ1 are extended to the central phenyl linker DTC, respectively (Table 1). The phosphorescent emission
From the onset of fluorescence spectra of these compounds,
m
m
m
m
m
m
m
m
and show weakly overlap between HOMO and LUMO, which spectra of these compounds were measured at 77 K in toluene
can ensure efficient radiative decay and high PL quantum (10ꢀ5 M) and are shown in Fig. 1 and Fig. S1. The triplet
efficiency.
energies were calculated to be 2.81, 2.85, 2.76 and 2.95 eV for
4BPyꢀ DTC, 3BPyꢀ DTC, 2BPyꢀ DTC, and BPꢀ DTC,
respectively, from the onset of phosphorescence spectra. The
EST was estimated to be 0.01 eV for 4BPyꢀ DTC, 0.05 for
DTC, 0.02 for 2BPyꢀ DTC and 0.03 eV for BPꢀ
DTC, indicating that they plausibly possess TADF property
before 350 nm is probably from the π→π* transitions and weak with efficient upꢀconversion from T1 to S1.29 The ꢁEST
m
m
m
m
2.2. Photophysical properties
The UVꢀvis absorption and PL spectra of these compounds in
ꢁ
m
toluene solutions are depicted in Fig. 2 and Fig. S1 while their 3BPyꢀ
physical parameters are summarized in Table 1. The absorption
m
m
m
broadband at 392 nm is assigned to the intramolecular
calculated are very small (0.01~0.05 eV) for these four
molecules in toluene indicating that they plausibly possess
TADF property with efficient upꢀconversion from T1 to S1. In
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
a)
4BPy-mDTC(Fl.)
4BPy-mDTC(Phos.)
BP-mDTC(Fl.)
4BPy-mDTC (Abs.)
BP-mDTC (Abs.)
addition, 4BPyꢀ
compared to the 3BPyꢀ
DTC).
The electrochemical properties of these materials were
mDTC and 2BPyꢀ
mDTC show very small ꢁEST
1.0
0.8
0.6
0.4
0.2
0.0
mDTC and benzophenone unit (BPꢀ
BP-mDTC(Phos.)
m
investigated by Cyclic Voltammetry (CV) and are shown in
Fig. S3. The CV shows reversible oxidation curves for all the
compounds, indicating promising electrochemical stability. The
HOMO levels were estimated to be ꢀ5.65, ꢀ5.60, ꢀ5.61 and ꢀ5.65
eV for 4BPyꢀmDTC, 3BPyꢀmDTC 2BPyꢀmDTC, and BPꢀ
300 350 400 450 500 550 600 650
mDTC, respectively, obtained from the onset of oxidation
waves (vs Fc/Fc+). The LUMO levels were estimated from
HOMO ꢀ ES to be ꢀ2.83, ꢀ2.71, ꢀ2.82, and ꢀ2.65 eV for 4BPyꢀ
Wavelength, nm
mDTC, 3BPyꢀmDTC, 2BPyꢀmDTC, and BPꢀmDTC,
b)
1
respectively. To further study the photophysical property in thin
film, the doped films of these materials in a host matrix of 3, 3′ꢀ
4BPy-mDTC
BP-mDTC
0.1
0.01
1E-3
bis(carbazolꢀ9ꢀyl)ꢀ1,1′ꢀbiphenyl
(mCBP) were prepared by
vacuum deposition at a concentration of 7 wt%. In oxygenꢀfree
toluene solutions, the quantum yields are 7.5%, 7.0%, 7.2% and
7.3% for 4BPyꢀ
mDTC, 3BPyꢀmDTC, 2BPyꢀmDTC and BPꢀ
mDTC, respectively. In the presence of oxygen, the values
decrease to 1.7%, 1.9%, 1.9% and 2.0%. The quantum yield
decrease of these compounds in the presence of oxygen
supports that these molecules possess TADF property. It is
known that the T1 states of these molecules are readily
quenched by the triplet ground state of oxygen molecules
leading to the decrease of RISC and subsequently the
fluorescence intensity.
0
1
2
3
Time, ꢀs
Fig. 2 a) Absorption (Abs.) and fluorescence (Fl.) spectra in toluene (10-5 M) measured
at room temperature and phosphorescence (Phos.) spectra in toluene (10-5 M)
measured at 77 K; b) their transient PL characteristics in toluene (10-5 M) at room
temperature under vacuum.
Surprisingly, the PLQYs of 7 wt% 4BPyꢀ DTC, 3BPyꢀ
m
m
DTC, 2BPyꢀ DTC, and BPꢀ DTC doped in mCBP thin films
m
m
charge transfer (ICT) absorption associated with the electron
are 97, 92, 96 and 75%, respectively, measured using an
integrating sphere under N2 atmosphere. These values are much
transfer from the
benzoylpyridine or
t
ꢀbutylcarbazole groups to the
benzophenone moiety. Upon
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Table 1. Physical properties of 4BPy-mDTC, 3BPy-mDTC, 2BPy-mDTC and BP-mDTC
Dopant
λabs
(nm)a
λem
(nm)a
λem
(nm)b
Td (°C)c
HOMO
(eV)d
LUMO
(eV)e
Eg (eV)f
ET (eV)g
ΔEST
(eV)h
ΦPL (%)
Soli/film(%)j
τp (ns)/τd (ꢀs)j
28.1/18.0
4.7/8.3
4BPy-mDTC
3BPy-mDTC
2BPy-mDTC
BP-mDTC
342, 396
339, 381
495
480
478
476
488
458
408
439
411
371
˗5.65
˗5.61
˗5.60
˗5.63
˗2.83
˗2.71
˗2.82
˗2.65
2.82
2.90
2.78
2.98
2.81
2.85
2.76
2.95
0.01
0.05
0.02
0.03
7.5/97
7.0/92
7.2/96
7.3/75
5.3/6.9
342, 394
341, 391
494
465
6.1/12.4
a
Measured in toluene (1×10-5 M) at room temperature. bPhosphorescence measured in toluene (1×10-5 M) at 77 K. cObtained from TGA measurements. dMeasured
from the oxidation potential in 10-3 M DCM solution by cyclic voltammetry. eCalculated from HOMO - Eg. fEstimated from the onset of fluorescence spectrum.
gEstimated from the onset of phosphorescence spectrum. hΔEST = ES - ET. Measured in degassed toluene solution using diphenyl anthracene as a standard. Absolute
i
j
total PL quantum yield evaluated for 7 wt% dopant in mCBP film using an integrating sphere. jPL lifetimes of the prompt (τp) and delayed (τd) decay components in 7
wt%-doped
mCBP
film
measured
at
300
K
higher than those in the solution. This quantum yield
enhancement is probably due to the suppression of
intramolecular rotational and collisional quenching in the solid
state.29, 42 The emission spectra of the doped films were shown
in Fig. S4. The results reveal that the emissions are only from
the dopants. It is noteworthy that the three benzoylpyridine
(BPy) derivatives show higher quantum yields compared to the
benzophenone (BP) derivative. Further, the BPy derivatives
b)
1.0
a)
1.0
Prompt
Delay
Prompt
Delay
0.8
0.6
0.4
0.2
0.0
0.8
0.6
0.4
0.2
0.0
400
450
500
550
600
650
400
450
500
550
600
650
700
Wavelength, nm
Wavelength, nm
with the nitrogen atom at 4 (4BPyꢀmDTC) and 2 (2BPyꢀmDTC)
position of the pyridine ring show slightly higher PLQY (97
and 96%, respectively) compared to that with the nitrogen atom
Fig. 3 Prompt and delayed Pl spectra of the co-doped thin films, a) mCBP: 4BPy-mDTC
(7 wt%) and b) mCBP: BP-mDTC (7 wt%) measured at 300 K.
at 3 (3BPyꢀmDTC) position (92% for 3BPyꢀmDTC). The
To confirm the TADF property, the transient PL decay
presence of the nitrogen atom and the position in the acceptor
unit appear to plays an essential role in the fine tuning of the
structures to reach very high quantum yield.43,44
To further examine the emission properties of these
compounds, prompt and delayed spectra of the coꢀdoped thin
characteristics of these materials were measured in 10ꢀ5
M
toluene solution under vacuum and are shown in Fig. 2b. As
shown in Fig. 2b, the transient decay curves can be divided into
two components. The first one is the prompt emission decay
from S1 to ground state S0 with a lifetime (τ) of 2.0 ns and 5.7
films
wt%) were measured (Fig. 3a and 3b). The prompt and delayed
spectra coincide with each other for the 4BPyꢀ DTC coꢀdoped
mCBP: 4BPyꢀmDTC (7 wt%) and mCBP: BPꢀmDTC (7
ns for 4BPyꢀ
component is a delayed emission with a lifetime τ of 0.46 µs
and 0.49 µs for 4BPyꢀ DTC and BPꢀ DTC, respectively.
mDTC and BPꢀmDTC, respectively. The second
m
m
m
film. On the other hand, a slight redshift of the delayed
fluorescence relative to the prompt spectrum was observed for
BPꢀmDTC. The slight redshift of BPꢀmDTC coꢀdoped film is
likely from the conformation change in the thin film: the
excited state undergoes conformation adjustment during the
RISC to a slightly lower S1 state.45 We suggest that the nitrogen
atom restricts the excited state conformational changes via
hydrogen bonding in the thin film, which is important in
improving the PLQYs in the thin film. The thermal properties
of these emitters were determined by thermogravimetric
These delayed emissions can be explained as the upꢀconversion
of T1 to S1, followed by fluorescence to the ground state.
Further evidence for the TADF property of these materials is
from the temperature dependent transient PL decays of coꢀ
doped thin films of
mCBP:4BPyꢀmDTC (7 wt%) and
m
CBP:BPꢀ DTC (7 wt%) measured at 100, 200 and 300 K
m
(Fig. 4a and 4b). The results reveal that the delayed component
intensified when the temperature increases. This observation
clearly demonstrates that the delayed emission is an RISC
process activated by the thermal energy and is a characteristic
analysis (TGA) under
a nitrogen atmosphere and the
of TADF emitter.29ꢀ32 As shown in S6, 4BPyꢀ
m
DTC in the
CBP film shows more delayed component than 3BPyꢀ DTC
DTC in their transient PLs at 300 K. The lifetimes
thermograms are shown in Fig. S5 and the values are
summarized in Table 1. The results show that all four
m
m
and 2BPyꢀ
m
compounds possess very high Td with 4BPyꢀ
mDTC, 3BPyꢀ
of prompt and delayed components of these compounds in the
CBP coꢀdoped thin films are summarized in Table 1.
m
DTC, and 2BPyꢀ DTC ~40 °C higher than BPꢀm
m
DTC. The
m
high Td values are important for high morphological stability in
the film.
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2.3. Electroluminescence performance
a)
1
To investigate the electroluminescence properties of these
TADF materials, multilayer devices A, B, C and D were
4BPy-mDTC 100 K
4BPy-mDTC 200 K
4BPy-mDTC 300 K
fabricated using 4BPyꢀ
mDTC, 3BPyꢀmDTC, 2BPyꢀmDTC and
BPꢀ DTC as the dopants, respectively. The molecular
m
0.1
0.01
1E-3
structures used in the devices are shown in Fig. S7. Devices Aꢀ
D were constructed with the following device structure:
ITO/NPB (30 nm)/TAPC (20 nm)/
nm)/DPEPO (5 nm) or PPT (10 nm)/TmPyPb (60 nm)/LiF (0.8
nm)/Al (100 nm). In the devices, ′ꢀbis(1ꢀnaphthyl)ꢀ ′ꢀ
diphenylꢀ1,1′ꢀbiphenylꢀ4,4′ꢀdiamine (NPB) acts as hole
mCBP:dopant (7 wt%) (30
N
,
N
N,N
0
10
20
30
40
50
a
Time, ꢀs
injection material, 1,1ꢀbis[4ꢀ[N,N′ꢀdi(ptolyl)amino]phenyl]
cyclohexane (TAPC) as a holeꢀtransporting material, 3,3′ꢀ
b)
1
BP-mDTC 100 K
BP-mDTC 200 K
BP-mDTC 300 K
bis(
1,3,5ꢀtri(
electronꢀtransporting material. In addition, (oxybis(2,1ꢀ
N
ꢀcarbazolyl)ꢀ1,1′ꢀbiphenyl (
mCBP) as a host material,
1
0.1
mꢀpyridꢀ3ꢀylꢀphenyl)benzene (TmPyPb) is the
0.1
0.01
1E-3
phenylene))bis(diphenylphosphine
dibenzo[b,d]thiopheneꢀ2,8ꢀdiylbis(diphenylphosphine
(PPT) is used as an exciton blocker for devices A and BꢀD,
respectively. The electroluminescent properties of these devices
are displayed in Fig. 5, Fig. S8 and summarized in Table 2.
oxide)
(DPEPO)
or
0.01
1E-3
oxide)
0
1
2
Time, ꢀs
0
10
20
30
Time, ꢀs
Fig. 4 Temperature dependent transient PL decays of doped films a) mCBP: 4BPy-mDTC
(7 wt%) and b) mCBP: BP-mDTC (7 wt%) ranging from 100 K to 300 K.
a)
b)
600
c)
1.0
d)
Device A
Device A
Device D
1
Device B
Device C
Device D
10000
1000
100
10
Device B
Device A
Device C
Device D
0.8
0.6
0.4
10
Device A
Device B
Device C
Device D
400
0.1
0.2
200
1
0.0
1
10
4
100
1000
10000
16
400
500
600
100
0
1
2
0
50
Wavelength, nm
8
12
Luminance, cd/m
V
Time, ꢀs
Fig. 5 The EL characteristic plots of devices A-D: a) external quantum efficiency vs luminance, b) electroluminescent spectra, c) current density and luminance vs driving voltage, d)
transient electroluminescence characteristics of devices A and D.
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Table 2. EL performances of the devices using 4BPy-mDTC, 3BPy-mDTC, 2BPy-mDTC and BP-mDTCa,b
Device
dopant
Vd (Vb
L (cd m-2, V)
EQE (%, V)
CE (cd A-1, V)
PE (lm W-1, V)
λ
max (nm)
CIE (x,y), 8V
A
B
C
D
E
4BPy-mDTC
3BPy-mDTC
2BPy-mDTC
BP-mDTC
FIrpic
3.3
3.6
3.6
3.8
3.3
17038 (13.0)
8470 (12.5)
20000 (14.5)
4475 (12.5)
27332 (12.0)
28.1 (3.5)
24.6 (4.5)
28.0 (4.0)
18.2 (4.5)
18.7 (3.5)
67.0 (3.5)
49.1 (4.5)
65.4 (4.5)
34.6 (4.5)
34.6 (3.5)
60.1 (3.5)
34.2 (4.5)
50.7 (4.0)
25.4 (4.0)
30.9 (3.5)
490
476
490
472
462
(0.17:0.37)
(0.15:0.28)
(0.16;0.37)
(0.15;0.25)
(0.15;0.25)
aDevice configuration for A-D: ITO/NPB (30 nm)/TAPC (20 nm)/mCBP: dopant (7 wt%) (30 nm)/DPEPO (5 nm) (Device A) or PPT (10 nm) (Devices B-D)/TmPyPb (60
nm)/LiF (0.8 nm)/Al (100 nm), respectively; bVd, The operating voltage at a brightness of 1 cd m-2; L, maximum luminance; EQE, maximum external quantum efficiency;
CE, maximum current efficiency; PE, maximum power efficiency; and λmax, the wavelength where the EL spectrum has the highest intensity.
The EQE vs luminance of devices AꢀD is shown in Fig. 5a. emitters, confirming that the EL emissions are from the
Devices A and C gave sky blue electroluminescence with EQEs fluorescence of the emitters. The EL peak positions of devices
of 28.1 and 28.0%, respectively. These efficiencies are much A, B, C and D appear at 490, 476, 490 and 472 nm, respectively
higher than the earlier reported device based on a carbazole without emission from other materials. The color coordinates of
derived blue TADF emitter (EQE~18%).27 The current and the devices are listed in Table
2 along with device
power efficiency of devices A and C are 67.0 and 65.4 cd Aꢀ1 performances. To check the device emission stability at various
and 60.1 and 50.7 lm Wꢀ1, respectively. Moreover, device C operating voltages, the EL spectra were measured from 6 to 12
with 2BPyꢀmDTC as the dopant emitter showed a maximum V and no significant change of the spectra with applied voltage
luminance of 20000 cd mꢀ2 without any light outꢀcoupling was observed for all the devices (Fig. S9). To compare the
enhancement. On the other hand, the maximum EQE and device performance with the standard blue phosphorescent
luminance of devices B and D drop to 24.6 and 18.3% and 8470 emitter (FIrpic), device E was fabricated with the device
cd mꢀ2 and 4475 cd mꢀ2, respectively. Although BPꢀ
shows lower ꢁEST of 0.03 eV than that of 3BPyꢀ
eV), the EQE of 3BPyꢀ DTCꢀbased device B is much higher summarized in Table 2. The observed EQE for device E is
than that of BPꢀ DTCꢀbased device D. This observation may ~18.7% and is comparable to the reported FIrpic based
be explained by the substantially higher PLQY of 3BPyꢀ
DTC device.46 As shown in Table 2 and Fig. 5, devices A, B and C
in the CBP thin film and the presence of the pyridine group with 4BPyꢀ DTC, 3BPyꢀ DTC and 2BPyꢀ DTC as the
likely accounts for the higher PLQY. Surprisingly, 4ꢀpyridine dopants, respectively, surpass device E in terms of EQE,
(4BPyꢀ DTC) and 2ꢀpyridine (2BPyꢀ DTC) derived current efficiency, and power efficiency. These results reveal
compounds show higher luminance (17000~20000 cd mꢀ2) than that TADF emitters have the potential as the replacement for
the 3ꢀpyridine (3BPyꢀ
DTC) derivative (8470 cd mꢀ2). Also, the expensive blue phosphorescent emitters.
the 4ꢀpyridine derivative shows low turnꢀon voltage (3.3 V) Furthermore, to confirm the TADF property of the devices,
mDTC structure similar to BꢀD except that FIrpic is used as the dopant
mDTC (0.05 and the device performance is shown in Fig. S10 and
m
m
m
m
m
m
m
m
m
m
compared 2ꢀpyridine and 3ꢀpyridine (3.6 V) derivatives, due to the transient electroluminescence decays for devices AꢀD were
the enhanced electron transporting ability of 4ꢀpyridine.46 These measured at room temperature under electrical excitation. Fig.
results suggest that the position of the nitrogen atom in the 5d and Fig. S11 show that the delayed electroluminescence
molecules is vital to the luminance and the operating voltage.
The EL performance of these devices is in the order A
component lasts for several tens of microseconds with a very
high delayed component. The results indicate that the EL
≈
C
> B >> D. The EQE and luminance were dramatically efficiency is largely contributed by the delayed fluorescence,
improved by changing the phenyl to a pyridine group in the supporting the importance of TADF process in these devices.
dopant in these devices. This can be attributed to the Device A shows a stronger delayed component compared to
substantially lower PLQY of BPꢀ
corresponding values of the BPy series. Further, the lower device performance. The RISC process is more efficient under
device efficiency of 3BPyꢀ DTCꢀbased device B relative to electrical excitation for device A (4BPyꢀ DTC) than for device
4BPyꢀ DTCꢀbased A and 2BPyꢀ DTCꢀbased C is also likely D (BPꢀ DTC) (Fig. 5d).
due to the slightly lower PLQY of 3BPyꢀ DTC and the slightly
higher ꢁ ST compared to those of the other two materials.
mDTC relative to the device D; this observation is consistent with the observed
m
m
m
m
m
m
E
The electroluminescence (EL) spectra are presented in Fig.
5. They are similar to the corresponding PL spectra of the four
6 | J. Name., 2012, 00, 1-3
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250 nm; 10 % iꢀPrOH / hexane, 1.0 mL/min, 254 nm; tr (100%)
= 3.58 min (Fig. S12c).
3. Experimental section
3.1. Synthesis of the target molecules
3.1.4. Synthesis of (3,5ꢀbis(3,6ꢀdiꢀtertꢀbutylꢀ9Hꢀcarbazolꢀ9ꢀ
yl)phenyl)(phenyl)methanone (BPꢀ
to that of 4BPyꢀ DTC was followed for the synthesis of BPꢀ
DTC using (3,5ꢀdibromophenyl)(phenyl)methanone (BPꢀ
DBr) (2.5 g, 7.33 mmol), ꢀbutyl carbazole (5.11 g, 18.33
mDTC). A procedure similar
3.1.1. Synthesis of (3,5ꢀbis(3,6ꢀdiꢀtertꢀbutylꢀ9Hꢀcarbazolꢀ9ꢀ
m
yl)phenyl)(pyridinꢀ4ꢀyl)methanone (4BPyꢀ
dried seal tube (3,5ꢀdibromophenyl)(pyridineꢀ4ꢀyl)methanone
(4BPyꢀ DBr) (2.50 g, 7.33 mmol), ꢀbutyl carbazole (5.11 g,
mDTC). To an oven
m
m
t
m
t
mmol), Cu (0.93 g, 14.66 mmol), K2CO3 (5.06 g, 36.65 mmol)
18.3 mmol), Cu (0.93 g, 14.7 mmol), K2CO3 (5.06 g, 36.7
mmol) and 1,2ꢀdichlorobenzene (20 mL) was added. The
system was evacuated and was purged with nitrogen and the
mixture was heated with stirring at 180 °C for 48 h. After
completion of the reaction, the reaction mixture was filtered
through Celite and washed with ethyl acetate (50 mL). The
combined filtrate was evaporated under reduced pressure and
the residue was purified by column chromatography using 25%
1
and 1,2ꢀdichlorobenzene (20 mL) in 71% yield. H NMR (400
MHz, CDCl3): δ 8.13 (s, 4 H), 8.04 (s, 2 H), 7.93ꢀ7.95 (d,
J =
8.0 Hz, 2 H), 7.60 (t, = 7.2 Hz, 1 H), 7.49ꢀ7.53 (m, 11 H),
J
1.49 (s, 36 H); 13C NMR (100 MHz, CDCl3): δ 195.18 (ꢀCOꢀ),
143.63, 140.90, 139.89, 138.58, 136.73, 133.17, 130.10,
128.63, 127.49, 125.63, 123.92, 123.75, 116.49, 108.99, 34.76,
31.95; HRMS (FAB+) cal for C52H55N2O 736.4393, found
736.4398. HPLC (CHIRALCEL ODꢀH, 4.6 x 250 nm; 10 % iꢀ
PrOH / hexane, 1.0 mL/min, 254 nm; tr (100%) = 3.25 min (Fig.
S12d).
ethyl acetate/
yield. 1H NMR (400 MHz, CDCl3): δ 8.84 (d,
8.13 (s, 4 H), 8.09 (d, = 1.6 Hz, 1 H), 8.04 (d,
H), 7.71 (d, = 5.6 Hz, 2 H), 7.49ꢀ7.44 (m, 8 H), 1.44 (s, 36 H);
n
ꢀhexane as eluent to afford 4BPyꢀ
= 5.2 Hz, 2 H),
= 2.0 Hz, 2
mDTC in 78%
J
J
J
J
13C NMR (100 MHz, CDCl3): δ 193.78 (ꢀCOꢀ), 150.67, 143.87,
143.47, 140.42, 139.09, 138.49, 128.54, 125.53, 124.01,
123.87, 122.74, 116.59, 108.84, 34.78, 31.94; HRMS (FAB+)
cal for C52H55N3O 737.4345, found 737.4335. HPLC
4. Conclusions
In summary, we have designed and synthesized a series of the
TADF emitters, bearing a benzoylpyridine or benzophenone
core as the electronꢀaccepting unit and two meta
butylcarbazolyl groups as the electronꢀdonating units. These
molecules show small ꢁEST and very high quantum yields
(~97%) in the thin films. The EQE and luminance were
dramatically improved from 18 to 28% and from 4475 to 20000
cd mꢀ2, respectively, by changing a phenyl to a pyridine group
in the acceptor units. Among the pyridine derived emitters,
tꢀ
(CHIRALCEL ODꢀH, 4.6 x 250 nm; 10 %
mL/min, 254 nm; tr (100%) = 4.85 min (Fig. S12a).
3.1.2. Synthesis of (3,5ꢀbis(3,6ꢀdiꢀtertꢀbutylꢀ9
iꢀPrOH / hexane, 1.0
H
ꢀcarbazolꢀ9ꢀ
DTC).
DTC was followed for the
DBr (2.5 g, 7.33
ꢀbutyl carbazole (5.11 g, 18.33 mmol), Cu (0.93 g,
yl)phenyl)(pyridinꢀ3ꢀyl)methanone
procedure similar to that of 4BPyꢀ
(3BPyꢀ
m
A
m
synthesis of 3BPyꢀ
mmol),
mDTC using 3BPyꢀm
t
4BPyꢀ
mDTC and 2BPyꢀ
mDTC show high efficiency compared
14.66 mmol), K2CO3 (5.06 g, 36.65 mmol) and 1,2ꢀ
to the 3BPyꢀ
mDTC. The nitrogen atom in the acceptor unit is
1
dichlorobenzene (20 mL) in 75% yield. H NMR (400 MHz,
playing a more vital role than the low ꢁEST values for
improving the PLQY and EL performances. This work presents
a judicious design strategy for tuning TADF materials to
achieve excellent skyꢀblue OLEDs. The light blue emission and
the high EL efficiency would make these materials excellent
candidates for the practical application in twoꢀcolor white
OLEDs.
CDCl3): δ 9.17 (s, 1 H), 8.85 (d,
6.0 Hz, 1 H), 8.13 (s, 4 H), 8.09 (t,
J
= 3.6 Hz, 1 H), 8.24 (d,
= 2.0 Hz, 1 H), 8.04 (t,
J
J
=
=
J
2.0 Hz, 2 H), 7.46ꢀ7.51 (m, 9 H), 1.45 (s, 36 H); 13C NMR (100
MHz, CDCl3): δ 193.75 (ꢀCOꢀ), 153.75, 151.16, 144.12,
140.68, 140.28, 138.87, 137.45, 132.81, 128.55, 125.79,
124.35, 124.17, 123.90, 116.87, 109.23, 35.08, 32.07; HRMS
[ESI, m/z] [M+H]+ cal for C52H56N3O+H+: 738.4423, found
738.4423. HPLC (CHIRALCEL ODꢀH, 4.6 x 250 nm; 10 % iꢀ
PrOH / hexane, 1.0 mL/min, 254 nm; tr (100%) = 4.04 min (Fig.
S12b).
Acknowledgements
We thank the Ministry of Science and Technology of Republic
of China (MOST 104ꢀ2633ꢀMꢀ007ꢀ001) for support of this
research and the National Center for HighꢀPerformance
Computing (Account number: u32chc04) of Taiwan for
providing the computing time.
3.1.3. Synthesis of (3,5ꢀbis(3,6ꢀdiꢀtertꢀbutylꢀ9
H
ꢀcarbazolꢀ9ꢀ
DTC).
DTC was followed for the
DBr (2.5 g, 7.33
ꢀbutyl carbazole (5.11 g, 18.33 mmol), Cu (0.93 g,
yl)phenyl)(pyridinꢀ2ꢀyl)methanone
procedure similar to that of 4BPyꢀ
(2BPyꢀ
m
A
m
synthesis of 2BPyꢀ
mmol),
mDTC using 2BPyꢀm
t
14.66 mmol), K2CO3 (5.06 g, 36.65 mmol) and 1,2ꢀ
1
dichlorobenzene (20 mL) in 80% yield. H NMR (400 MHz,
Notes and references
CDCl3): δ 8.78 (d,
5 H), 8.04 (s, 1 H), 7.91ꢀ7.95 (t,
J
= 4.0 Hz, 1 H), 8.36 (s, 2 H), 8.13ꢀ8.19 (m,
= 6.4 Hz, 1 H), 7.47ꢀ7.57 (m,
1
Highly Efficient OLEDs with Phosphorescent Materials (Ed.: H.
Yersin), WileyꢀVCH, Weinheim, 2008
J
.
9 H), 1.46 (s, 36 H); 13C NMR (100 MHz, CDCl3): δ 192.19 (ꢀ
COꢀ), 154.14, 148.72, 143.48, 139.50, 139.23, 138.68, 137.34,
127.58, 127.03, 126.83, 124.80, 123.86, 123.72, 116.40,
109.22, 34.75, 31.97; HRMS (FAB+) cal for C52H55N3O
737.4345, found 737.4349. HPLC (CHIRALCEL ODꢀH, 4.6 x
2
Organic LightꢀEmitting Devices: Synthesis Properties and
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Journal of Materials Chemistry C
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DOI: 10.1039/C7TC00457E
Thermally activated delayed fluorescence emitters with a m,m-di-
tert-butyl-carbazolyl benzoylpyridine core achieving extremely high
blue electroluminescence efficiencies
Pachaiyappan Rajamalli,a Vasudevan Thangaraji,a Natarajan Senthilkumar,a Chen-Cheng Ren-
Wu,b Hao-Wu Linb and Chien-Hong Cheng*,a
aDepartment of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan.
E-mail: chcheng@mx.nthu.edu.tw
bDepartment of Materials Science and Engineering, National Tsing Hua University,
Hsinchu 30013, Taiwan
High performance blue OLEDs with maximum EQEs over 28% are achieved from TADF
emitters bearing a m,m-di-tert-butyl-carbazolyl benzoylpyridine unit. The heteroatom and
position of hetroatom in the molecules are essential for the very high PL quantum yield and
device efficiency.