Highly efficient blue organic light-emitting diodes from pyrimidine-based thermally activated delayed fluorescence emitters

Article abstract of DOI:10.1039/c7tc05746f

Three highly efficient blue thermally activated delayed fluorescence (TADF) emitters, 2SPAc-HPM, 2SPAc-MPM and 2SPAc-PPM, have been synthesised based on pyrimidine (PM), 2-methylpyrimidine (MPM) and 2-phenylpyrimidine (PPM) as the acceptor and 10H-spiro[acridan-9,9′-fluorene] (2SPAc) as the donor moiety. With their appropriate molecular design, these emitters successfully achieve small singlet-triplet splitting energies for efficient reverse intersystem crossing, and exhibit high photoluminescence quantum yields. As a result, TADF OLEDs based on 2SPAc-HPM, 2SPAc-MPM and 2SPAc-PPM emit blue light peaking at 479-489 nm and generate very high external quantum efficiencies of 25.56%, 24.34% and 31.45%, respectively. The 2SPAc-PPM-based TADF OLED is also one of the very few blue TADF emitters with an external quantum efficiency of more than 30%, and it has the best OLED performance among the pyrimidine-based TADF emitters.

Full text of DOI:10.1039/c7tc05746f

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Y. Zhang, Y. Wang, Y. Yi, F. Guo and L. Zhao, J. Mater. Chem. C, 2018, DOI: 10.1039/C7TC05746F.
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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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Highly Efficient Blue Organic Light-Emitting DiodeDO
s
I: 1
f
0
r
.10
o
39/C7TC05746F
m
Pyrimidine-Based Thermally Activated Delayed
Fluorescence Emitters
1
#
2#
3
1*
2*
3
Bowen Li , Zhiyi Li , Taiping Hu , Yong Zhang , Ying Wang , Yuanping Yi , Fengyun
1
1
Guo , Liancheng Zhao
1School of Materials Science and Engineering, Harbin Institute of Technology, Harbin
1
50001, China
2Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical
Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China
3Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese
Academy of Sciences, Beijing 100190, China
E-mail: yongzhang@hit.edu.cn; wangy@mail.ipc.ac.cn
#
Both authors contributed equally to this work.
1

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DOI: 10.1039/C7TC05746F
Abstract
Three highly efficient blue thermally activated delayed fluorescence (TADF) emitters,
SPAc-HPM, 2SPAc-MPM and 2SPAc-PPM, have been synthesised based on pyrimidine
PM), 2-methylpyrimidine (MPM) and 2-phenylpyrimidine (PPM) as the acceptor and 10H-
2
(
spiro[acridan-9,9'-fluorene] (2SPAc) as the donor moiety, respectively. With the appropriate
molecular design, these emitters successfully achieve small singlet-triplet splitting energies
for efficient reverse intersystem crossing, and exhibit high photoluminescence quantum
yields. As a result, the TADF OLEDs based on 2SPAc-HPM, 2SPAc-MPM and 2SPAc-PPM
emit blue light peaking at 479-489 nm and generate the very high the external quantum
efficiency of 25.56%, 24.34% and 31.45%, respectively. The 2SPAc-PPM based TADF
OLEDs also is one of the very few blue TADF emitters with the external quantum efficiency
of more than 30% and is the best OLED performance among the pyrimidine-based TADF
emitters.
2

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DOI: 10.1039/C7TC05746F
Introduction
Organic lighting-emitting diodes (OLEDs) with the advantages of such as lightweight,
ultra-thin, large-area and flexibility, etc have been considered as one of the most promising
applications in the new generation of full-color flat panel displays and solid-state lightings
1
since the first report by Tang in 1987. In order to achieve the near-unity internal quantum
efficiency, the utilizations of both singlet (25%) and triplet excitons (75%) should be
maximized. In the past decades, noble metals contained phosphorescent OLEDs (ph-OLEDs)
has become a representative technology to realize 100% conversion of electrons to photons.
The green and red ph-OLEDs containing the phosphorescent iridium-based emitters have
been easily achieved with high efficiencies,2, 3 however the blue ph-OLEDs currently are still
suffering the inferior device performances for the practical applications compared to its green
and red counterparts, regardless of the critical high cost and stability issues.
Recently, the metal-free pure-organic thermally activated delay fluorescent (TADF)
OLEDs have received considerable attentions as the new generation and alternative
technology to reach the high efficiency, especially for the blue OLEDs.4-6 In TADF emitters,
the up-conversion of triplet states (T
1
) to emissive singlet states (S ) via a reverse intersystem
1
crossing (RISC) realizes the harvests of both singlet and triplet excitons, in which a small
splitting energy (ΔEST) between singlet and triplet excited energies is critically important in
the thermodynamical RISC process. Regarding the molecular design, TADF emitter normally
possesses a twisted intramolecular charge transfer (TICT) framework with a very small spatial
overlap between the highest occupied molecular orbit (HOMO) and the lowest unoccupied
molecular orbit (LUMO). Employing this strategy, the research efforts have focused on the
design of donor-acceptor (D-A) typed TADF emitters for realizing highly efficient TADF
OLEDs.6-9 Various donors and acceptors have been applied by considering their electron-
donating or electron-withdrawing capabilities.9-23 Among theses acceptors, however,
3

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pyrimidine (PM), which is the two nitrogen-contained six-memberedheterocycle and
DOI: 10.1039/C7TC05746F
possesses an asymmetric molecular structure with the rather strong electron-withdrawing
characteristic and the intrinsic high triplet (T ) energy level, has received less attentions in
1
constructing the TADF emitters. The current PM-based TADF emitters only possess the
external luminescence quantum efficiency (ηext) of less than 25% in TADF OLEDs mainly
due to the relatively low photoluminescence quantum yield induced by the inappropriated
molecular orientations and packing behaviors.4, 8, 24-26 Therefore, the rational molecular design
is highly necessary and also challenged with the aim of improving the device performances of
PM-based TADF emitters.
In this work, we designed and synthesized three highly efficient blue TADF materials
2
SPAc-HPM, 2SPAc-MPM and 2SPAc-PPM (Scheme 1), which are based on a highly
twisted donor-acceptor-donor (D-A-D) structures with pyrimidine (PM), 2-methylpyrimidine
MPM) and 2-phenylpyrimidine (PPM) as the acceptor unit and 10H-spiro[acridan-9,9'-
(
fluorene] (SPAc) as the donor moieties, respectively. The bulky, rigid and steric SPAc
structure was selected because it could provide the improved photoluminescence quantum
yields (PLQYs), strongly horizontally oriented emitting dipoles and even a blue-shifted
6
emission compared to the 9,10-dihydroacridine. 2SPAc-HPM, 2SPAc-MPM and 2SPAc-
PPM exhibit the obvious TADF phenomena with ΔEST of 0.15 eV, 0.19 eV, and 0.16 eV,
respectively. The electroluminescence (EL) of three molecules from their TADF OLEDs
showed the emission peak (λem) of 479-489 nm. The device performances varied with the
substitutes of pyrimidine under same device conditions. The highest ηext of these blue TADF
OLEDs based on 2SPAc-HPM, 2SPAc-MPM and 2SPAc-PPM are as high as 25.56%,
2
4.34% and 31.45%, respectively. To our best knowledge, the device performance of 2SPAc-
PPM is the best among the pyrimidine TADF emitters, and is also comparable with those of
the state-of-the-art triazine-based TADF and phosphorescent OLEDs.27, 28
4

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O
View Article Online
O
B
O
DOI: 10.1039/C7TC05746F
Br
I
O
B
O
NH
N
Br
N
-
,
,
tert BuONa CuI
THF, n-B
u
Li
,
-
enan roline, dioxane
1
10 ph
th
1
2
3
N
N
N
N
4
Cl
Cl
,
,
o uene
Pd(PPh )4 K CO
T
l
/H O
2
N
N
3
2
3
-
2
SPAc HPM
CH3
N
N
N
N
5
Cl
Cl
,
,
o uene
N
N
-
SPAc MPM
Pd(PPh )4 K CO
T
l
/H O
2
3
2
3
2
N
N
N
N
6
Cl
Cl
N
N
-
SPAc PPM
2
,
,
o uene
T l /H O
Pd(PPh )4 K CO
3
2
3
2
Scheme 1. The synthetic routes toward 2SPAc-HPM, 2SPAc-MPM and 2SPAc-PPM.
Fig. 1. The HOMO and LUMO distributions for 2SPAc-HPM, 2SPAc-MPM and 2SPAc-PPM.
Results and Discussions
5

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The synthetic rotues for 2SPAc-HPM, 2SPAc-MPM and 2SPAc-PPM are shown in
DOI: 10.1039/C7TC05746F
Scheme 1. The 10H-spiro[acridan-9,9'-fluorene] boronic ester (3, SPAc-Bpin) was prepared
8
via four steps according to the literature (Scheme S1). The TADF molecules 2SPAc-HPM,
2
SPAc-MPM and 2SPAc-PPM were then synthesized through Suzuki-Miyaura coupling
reaction of 3 with 4,6-dichloropyrimidine (HPM-2Cl, 4), 4,6-dichloro-2-methylpyrimidine
8
(
MPM-2Cl, 5) and 4,6-dichloro-2-phenylpyrimidine (PPM-2Cl, 6), respectively. Due to the
limited solubilities of these TADF molecules in common organic solvents, they were first
purified by recrystallization, and further sublimated by temperatures-gradient sublimation
under vacuum to give the purity of more than 99%. The thermal properties were investigated
by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) under
nitrogen atmosphere. As shown in Fig. S1, 2SPAc-HPM, 2SPAc-MPM and 2SPAc-PPM
o
displays the decomposition temperature (T
is no obvious glass transition temperature (T
SPAc-PPM. Such excellent thermal properties provide good potential thermal stability for
the organic electroluminescent device.
d
) of 518, 487 and 535 C, respectively. While there
g
) found for 2SPAc-HPM, 2SPAc-MPM, and
2
Table 1. HOMO, LUMO, E
g
opt, E
S
, E and ΔEST values of three TADF molecules.
T
a)
HOMOb) LUMOc)
opt d)
e)
f)
ΔESTg)/ ΔESTh)
Compounds
T
m
/T
d
E
g
E
S
E
T
(oC)
(eV)
-5.46
-5.45
-5.45
(eV)
(eV)
2.86
2.91
2.88
(eV)
2.94
2.98
2.95
(eV)
2.79
2.79
2.79
(eV)
2
SPAc-HPM
SPAc-MPM
SPAc-PPM
432/518
426/487
461/535
-2.60
-2.54
-2.57
b)
0.15/0.030
0.19/0.018
0.16/0.015
2
2
a)
T
m
was measured by DSC; T
LUMO were calculated from ELUMO=EHOMO - E
normalized absorption spectrum. Estimated from the onset of fluorescence spectrum in toluene (5 × 10
M) at 300 K. Estimated from the onset of phosphorescence spectrum in toluene (5 × 10 M) at 77 K.
Singlet-triplet energies splitting determined experimentally using ΔEST=E
splitting calculated using Berberan-Santos plot.
d
was measure by TGA. HOMO was calculated from the oxidation potential.
c)
opt d)
opt
g
.
E
g
was obtained from the absorption edge of
e)
-5
f)
-5
g)
h)
S
–E . Singlet-triplet energies
T
To better understand the molecular orbital distribution of these TADF molecules, we
utilized the density function theory (DFT) calculation at the B3LYP/6-31G(d) level to
6

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investigate the geometric and electronic structures of 2SPAc-HPM, 2SPAc-MPM and 2SPAc-
DOI: 10.1039/C7TC05746F
PPM. As shown in Fig. 1, it can find that the optimized molecular geometrics for the TADF
o
molecules are apparently twisted and show the large dihedral angle of near 90 between the
SPAc moiety and the phenyl spacer. The HOMO and LUMO delocalizations of the three
TADF molecules are very similar since they have almost identical chemical structures. The
HOMO are mainly located on the SPAc moiety with a small portion spreading on the phenyl
spacer, while the LUMO are mainly delocalized on the pyrimidine unit with partial extending
to the phenyl spacer. It therefore leads to a small frontier molecular orbitals (FMOs) overlap
between the HOMO and LUMO delocalizations on the phenyl spacer for each TADF
molecule, which can facilitate the radiative decay of 2SPAc-HPM, 2SPAc-MPM and 2SPAc-
29
PPM. The experimental HOMO and LUMO energy levels of the three TADF molecules are
measured by cyclic voltammetry (CV). It can find that each TADF molecule exhibited a
reversible oxidation wave (Fig. S2). The HOMO energy levels were calculated from the onset
of oxidation potential and were found to be -5.46 eV, -5.45 eV and -5.45 eV, respectively, for
2
SPAc-HPM, 2SPAc-MPM and 2SPAc-PPM. The almost identical HOMO energy levels are
the result of the same SPAc moiety in the three TADF molecules, which is also consistent
with the theoretical DFT calculations. The LUMO energy levels of 2SPAc-HPM, 2SPAc-
MPM and 2SPAc-PPM were at -2.60 eV, -2.54 eV and -2.57 eV, respectively, which were
opt, where E
opt
is the optical band-gap
calculated from the equation of ELUMO = EHOMO- E
g
g
(Table 1). The slight varies on the LUMO energy levels of these TADF molecules are
believed to be from the different substitute on the 2-poistion of pyrimidine unit. The slight
higher LUMO energy levels of 2SPAc-MPM and 2SPAc-PPM compared to that of 2SPAc-
HPM are mainly induced by the electron-donating properties of the methyl and phenyl groups.
These results indicated that the substitutes on the donor and acceptor moieties play an
important role on tuning the HOMO and LUMO energy levels of such TADF molecules,
7

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which are crucial for facilitating the hole and electron injections in the organic
DOI: 10.1039/C7TC05746F
electroluminescent device.
Fig. 2. UV-Vis absorption and emission spectra of 2SPAc-HPM, 2SPAc-MPM and 2SPAc-PPM in toluene
-5
solution (10 M) at room temperature.
The UV-Vis absorption and emission spectra of 2SPAc-HPM, 2SPAc-MPM and 2SPAc-
-5
PPM in dilute toluene (10 M) are shown in Fig. 2. It can be found that these TADF
molecules exhibit similar absorption properties and has a higher energy absorption band
below 350 nm, which is attributed to the π-π* absorption of the conjugated aromatic moieties,
where the much weaker absorption band at 350-425 nm with various peak is assigned to the
intramolecular charge transfer (ICT) peak between the pyrimidine unit and the SPAc moiety,
which also can be affirmed by Fig. S3. Upon the photoexcition of the ICT absorption, the
photoluminescence (PL) spectra of 2SPAc-HPM, 2SPAc-MPM and 2SPAc-PPM showed the
intense blue emission with peaks at 469 nm, 460 nm and 464 nm, respectively. The blue-
shifted emission peaks of 2SPAc-MPM and 2SPAc-PPM compared with that of 2SPAc-HPM
and the slight changes on the ICT absorption are due to the different electron-donating
properties of the hydrogen, methyl and phenyl substitutes on the pyrimidine units.4, 26 For
2
SPAc-PPM, the conjugation between pyrimidine and the phenyl substitute may also
contribute the effect on the optical property. It is known that the solvent polarity has an effect
8

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on the optical properties of the TADF materials. As shown in Fig. S4, the UV-Vis absorption
DOI: 10.1039/C7TC05746F
and PL spectra of the three TADF molecules in various polar solvents were investigated. The
absorption spectra of 2SPAc-HPM, 2SPAc-MPM and 2SPAc-PPM have no obvious solvent
3
0
effect in various polar solvents as a result of the nonpolar ground states. In contrast, the PL
spectra of the three TADF molecules exhibited strong dependence on the solvent polarity (Fig.
S4, Table S1). The PL spectra become broad with the full width at half maximum (FWHM)
changing from ~55 nm (n-hexane, ε=1.90) to ~130 nm (DMF, ε=37.00) and bathochromically
shifted ~100 nm with increasing polarity of solvents from n-hexane to DMF. These results
clearly indicate that the emissions of the three TADF molecules are from the ICT states and
show the larger transition of the dipole moment from the ground state to the excited state. To
further make a quantitative analysis of the fluorescence solvachromism for the three TADF
molecules, we plotted the curve of the stokes shift (ΔνFl-Abs) versus the orientation
polarizability (f) and then calculated the change value of dipole moment (Δμe-g) according to
31
the equations S1-S3. As shown in Fig. S5, a well-fitted linear correlation between the stoke
shifts and the orientation polarizability for all three TADF molecules. It demonstrates that the
dependence of PL spectra on solvent polarity mainly ascribed to the dipole-dipole interactions
between the molecule and solvent. Furthermore, the Δμe-g values were calculated to be 4.29 D,
4
.22 D and 4.34 D for 2SPAc-HPM, 2SPAc-MPM and 2SPAc-PPM, respectively (Table S2).
SPAc-PPM possess larger Δμe-g than that of two other compounds, indicated that 2SPAc-
2
PPM possess a more stable polarized excited state in the high polar solvents.
To further investigate the photophysical and TADF properties of 2SPAc-HPM, 2SPAc-
MPM and 2SPAc-PPM. Fluorescence and phosphorescence spectra of the three TADF
molecules are measured in toluene at 300 K and 77 K, respectively, as shown in Fig. S6. The
S
1
(E
S
) and T
1
(E ) energies are determined from the onsets of fluorescence and
T
phosphorescence spectra, respectively, and are summarized in Table 1. The ΔEST values are
obtained to be 0.15 eV, 0.19 eV and 0.16 eV for 2SPAc-HPM, 2SPAc-MPM and 2SPAc-
9

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PPM, respectively. The small ΔEST values support that the three TADF molecules can
DOI: 10.1039/C7TC05746F
effectively harvest triplet excitons by the RISC process. It is known that oxygen can
effectively quench the triplet state emission, and has no effect on the singlet state emission.32
To study the delayed fluorescence (DF), the PL spectra in degassed and aerated toluene
solution are investigated. As shown in Fig. S7, fluorescence intensity become much stronger
in the absence of oxygen for 2SPAc-HPM, 2SPAc-MPM and 2SPAc-PPM than that in the
aerated solution, indicating a strong fluorescence quenching by oxygen as a result of
suppressing the RISC process. These results exhibited that 2SPAc-HPM, 2SPAc-MPM and
2
SPAc-PPM could possess TADF property..
In order to further confirm the delayed fluorescence behavior of 2SPAc-HPM, 2SPAc-
MPM, and 2SPAc-PPM in solid film, the transient PL delay curves of these three compounds
in DPEPO host were measured at 300 K. As shown in Fig. 3a, all curves can be divided into
the prompt and delayed decay components. The prompt decay component (Fig. 3b) with a
transient decay time of 16.67, 13.56 and 16.80 ns were observed for 2SPAc-HPM, 2SPAc-
MPM and 2SPAc-PPM, respectively, corresponding to the relaxation of excitons from S
1
to
3
3
ground state (S
0
). While the delayed decay component with the lifetimes (τ
d
) of 52.23, 53.29,
and 56.94 μs for 2SPAc-HPM, 2SPAc-MPM and 2SPAc-PPM, respectively were estimated
.33, 34 Moreover,
and may be rationalized as the thermal up-conversion of excitons from T
1
to S
1
the transient PL decay curves were also investigated under vacuum (Fig. S8) and exhibited
that the delayed decay component is extremely sensitive to oxygen, which is different with
prompt decay component and shows strong TADF characteristics. In addition, the temperature
dependent transient PL decays of 2SPAc-HPM, 2SPAc-MPM, and 2SPAc-PPM were also
investigated from 100 to 300 K (Fig. S9). It is found that the prompt decay curves showed no
temperature dependence, and the delayed decay components exhibit pronounced temperature
dependence. The ratio of delayed component were also gradually increased with increasing
1
0

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temperature, which further demonstrates the presence of TADF features in the three PM-
DOI: 10.1039/C7TC05746F
based TADF emitters.
Fig. 3. a, b)Transient PL characteristics of 10 wt% 2SPAc-HPM, 2SPAc-MPM, and 2SPAc-PPM doped in
DPEPO film at 300 K. c) Temperature dependence of the ΦPL(squares), ΦPF (circle), and ΦDF (triangles)
for 2SPAc-HPM (black), 2SPAc-MPM (red), and 2SPAc-PPM (blue). d) Energy diagram and
photoluminescence process of 2SPAc-PPM.
Table 2. Physical properties of TADF molecules-doped DPEPO films.
a)
PF/ΦDFb)
c)
PF
d)
DF
Φ
PL
Φ
τ
Φ
T
τ
k
PF
k
ISC
k
nr
k
r
Compounds
7
-1
7
-1
5
-1
5
-1
(%)
(%)
(ns)
(%)
(μs)
(10 s )
(10 s )
(10 s ) (10 s )
2
SPAc-HPM
SPAc-MPM
SPAc-PPM
96.60
82.20
97.00
1.54/47.26
0.53/43.25
1.06/61.40
16.67
13.56
16.80
99.4
99.4
99.9
52.23
53.29
56.94
6.00
7.37
5.95
5.96
7.33
5.94
2.05
2.25
0.38
1.95
1.75
0.63
2
2
a)
b)
Absolute PLQYs measured using an integration sphere in nitrogen atmosphere. Estimated according
c)
to the prompt and delayed components in transient delay curve. PL lifetimes of prompt decay component.
d)
PL lifetimes of delayed decay component.
The PLQYs of the doped films in DPEPO host were measured at 300 K and listed in Table
1
1

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2
. The PLQYs (ΦPL) of 2SPAc-HPM, 2SPAc-MPM and 2SPAc-PPM at 20wt% doping ratios
DOI: 10.1039/C7TC05746F
are found to be 96.60%, 82.20% and 97.00%, respectively (Table S3), measured at nitrogen
atmosphere under 320 nm excitation. It can find that 2SPAc-PPM with the phenyl substitute
on PM unit possesses the highest PLQY, where 2SPAc-MPM with the methyl substitute on
PM unit exhibits the lowest PLQY compared to that of 2SPAc-HPM. The highest PLQY of
2
SPAc-PPM is believed to associate with the restricted torsion of phenyl ring induced by the
hydrogen bonding of N-H between PM and phenyl spacer, and also the increased
intermolecular distances, which will reduce the self-quenching of 2SPAc-PPM. It is known
that the PLQYs are related to the oscillator strength (F) and the transition dipole moment (Q)
for fluorescence, and the large F and Q values are necessary for achieving high quantum
efficiency. Recent study has shown that the Strickler-Berg equations can also applied in the
35
TADF molecules (equation. S5-S10, Fig. S10). In our cases, the F values for 2SPAc-HPM,
SPAc-MPM and 2SPAc-PPM were calculated to be 0.029, 0.027 and 0.033, respectively,
where the Q values are 1.62 D, 1.56 D and 1.72 D (Table S4), respectively, for 2SPAc-HPM,
SPAc-MPM and 2SPAc-PPM. In addition, the temperature dependence of PLQYs for the
2
2
prompt (ΦPF) and delayed (ΦDF) TADF components are also calculated from ΦPL and the PL
transient curves (Fig. 3c and Table 2). For all three TADF emitters, the ΦPFs are kept almost
same at various temperatures and the slight increase with the decreased temperature indicates
the suppression of the non-radiative decay from the S
1
to S . However, the ΦDFs of all three
0
TADF emitters are significant higher than ΦPFs and also show obviously dependence on
temperature. The ΦDFs increased as the temperature increases, which is the typical
phenomenon for TADF emitter and indicates the faster thermodynamically RISC process at
higher temperature. Furthermore, the large ΦDF/ΦPL ratio of ~97-99% were also observed for
the three TADF emitters from 77K to 300K (Table S5), indicating that a large portion of
excited singlets and triplets underwent the efficient ISC and RISC processes between T
1
and
36
S
1
states. The triplet formation efficiencies (Φ
T
) were estimated to be 99.4%, 99.4% and
1
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9
9.9% for 2SPAc-HPM, 2SPAc-MPM and 2SPAc-PPM, respectively, according to Berberan-
DOI: 10.1039/C7TC05746F
Santos equation (equation S4, Fig. S11). The almost 100% Φ
T
indicates that most of S
1
excitons are converted to T
1
excitons via ISC process. It is known that the up-conversion from
T
1
to S
1
was dominated by the Arrhenius process.36, 37 Therefore, ΔESTs can also be estimated
to be 0.030, 0.018 and 0.015 eV for 2SPAc-HPM, 2SPAc-MPM and 2SPAc-PPM,
respectively (Table 1) based on Berberan-Santos equation. These small ΔEST will ensure the
high efficient up-conversion from T
1
excitons to S excitons in these doped films, especially
1
for 2SPAc-PPM. The experimental rate constants of higher kPF and kISC and smaller knr and k
r
also further proved the efficient ISC and RISC processes (Table 2). An energy diagram and
photoluminescence process of three TADF emitters are shown in Fig. 3d. The high ΦPL and
small ΔEST in these TADF emitters, especially for 2SPAc-PPM, will make them to be the
promising candidates for achieving high efficient blue TADF OLEDs.
Fig. 4. a) The energy level diagrams, b) OLED device structure, and c) the chemical structure of the
materials used in the TADF devices.
In order to evaluate the device performances of 2SPAc-HPM, 2SPAc-MPM and 2SPAc-
PPM, multilayer OLEDs were fabricated by doping the three TADF emitters into the high
1
3

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triplet energy DPEPO host as the emissive layer (EML). The doping concentration was
DOI: 10.1039/C7TC05746F
optimized for each TADF emitters (see the supporting information, Fig. S12-14, Table S6).
The optimized device has the architecture of ITO/HATCN (5 nm)/TAPC (30 nm)/mCP (10
nm)/TADF emitter:DPEPO (x) wt% (20 nm)/DPEPO (5 nm)/TPBi (40 nm)/LiF (0.9 nm)/Al
(100 nm). Among them, 2,3,6,7,10,11-hexacyano-1,4,5,8,9,12-hexaazatriphenylene (HATCN)
and di-[4-(N,N-ditoly-amino)-phenyl]cyclohexane (TAPC) were used as a hole injection
layer (HTL) and a hole transporting layer (HTL), respectively. 1,3,5-tris(N-
phenylbenzimidazol-2-yl)benzene (TPBi) and LiF were the electron transporting layer (ETL)
and an electron injection layer (EIL), respectively. N,N-Dicarbazolyl-3,5-benzene (mCP) and
(
oxybis(2,1-phenylene))bis(diphenylphosphine oxide) (DPEPO) were inserted as excitons
blocking layers (EBLs). The E levels of neighboring mCP and DPEPO were 3.00, and 3.30
eV, respectively, which are higher than the E of the three TADF emitters (E =2.79 eV) and
T
T
T
can efficiently suppress the triplet excitons quenching and confine the triplet excitons within
the EML. The energy level diagrams, OLED device structure, and the chemical structures of
the materials used in the TADF devices are illustrated in Fig. 4.
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DOI: 10.1039/C7TC05746F
Fig. 5. a) EL spectra of the TADF OLEDs at 8 V. b) Current density and luminance versus voltage (J-V-L)
characteristics. c) External quantum efficiency (ηext) versus current density plots. d) Current efficiency-
luminance-power efficiency.
The EL spectra of the multi-layer OLEDs are exclusively from the three TADF emitters
without the emission from the neighboring layers (Fig. 5a and Fig. S12-14). The devices of
2
SPAc-HPM, 2SPAc-MPM and 2SPAc-PPM exhibited bright blue EL emission with the
peaks at 487 nm, 479 nm and 484 nm, respectively, which are consistent with their PL spectra
and suggested that all devices possessed the efficient charge carrier injection, transport and
recombination in the EML. Futhermore, the EL spectra at different voltages are also provided
evaluate the device stability as shown in Fig. S15. The device performances of all TADF
OLEDs are summarized in Table 3. Fig. 5b shows the J-V-L characteristics of all TADF
OLEDs. The turn-on voltages were found at 3.5-3.6 V for the devices, indicating the similar
carrier injection barrier as a result of the similar energy levels. Among the three TADF
devices, 2SPAc-PPM device at 30% doping concentration achieved the highest EL
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Journal of Materials Chemistry C
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-1
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efficiencies with the best ηext of 31.45%, the current efficiency (η ) of 68.77 cd A and the
c
DOI: 10.1039/C7TC05746F
-1
power efficiency of 56.85 lm W at the low current densities in the absence of light out-
-2
coupling enhancement. The maximum luminance of 2SPAc-PPM device was 5600 cd m .
) of 57.50 cd A-1
For 2SPAc-HMP device, the best ηext of 25.56%, the current efficiency (η
c
-1
and the power efficiency of 51.60 lm W were obtained under same device conditions at 20%
doping concentration. It is noted that the maximum luminance of 2SPAc-HMP device at 40%
-2
doping concentration could reach to 11900 cd m (Table S6). The device performances of
2
SPAc-MPM, however, exhibited a lower ηext of 24.34%, the current efficiency (η ) of 50.00
c
-1
-1
cd A and the power efficiency of 42.45 lm W compared to that of 2SPAc-PPM and 2SPAc-
HPM devices. The 2SPAc-PPM-based TADF OLEDs in this report is one of the very few
blue TADF emitters with ηext of more than 30%. This excellent performance further affirmed
the efficient up-conversion of triplet excitons from T
1
to S through RISC process, which is
1
also consistent with the above transient PL measurements.
Table 3. Summarized device performance of the 2SPAc-HPM, 2SPAc-MPM and 2SPAc-PPM devices.
Va)
V)
η
b)
η
b)
/
η
b)
c
Luminance
ext max/100/1000
p
max
100/1000
max/100/1000
Emitter
CIE c)
-1
(cd A-1)
(cd m-2)
(
(%)
(lm W )
2
SPAc-HPM
3.5
3.6
3.6
25.56(24.58)/18.86/11.94
51.60(49.62)/26.65/12.98
42.45(37.00)/20.03/7.68
57.50(55.28)/42.42/26.85
50.00(46.30)/33.15/18.82
68.77(62.34)/40.11/23.71
7200
(0.18,0.34)
(0.17,0.29)
(0.18,0.32)
2
SPAc-MPM
24.34(22.54)/16.14/9.16
31.45(28.50)/18.34/10.84
4100
5600
2
SPAc-PPM
56.85(51.54)/23.78/9.55
-2
a)
Turn-on voltage at the luminance of 1 cd m ; b)
η
ext, η
p
, and η is external quantum efficiency, power
efficiency, and current efficiency, respectively, averaged efficiency embedded in brackets; Commission
c
c)
-2
Internationale deL’Eclairage (CIE) coordinates at 1000 cd m .
Conclusion
In summary, we synthesized three pyrimidine based TADF materials 2SPAc-HPM, 2SPAc-
MPM and 2SPAc-PPM with 10H-spiro[acridan-9,9'-fluorene] as the donor moieties and
pyrimidine, 2-methylpyrimidine and 2-phenylpyrimidine as the acceptor unit, respectively.
These three molecules with highly twisted D-A-D structure endow small ΔEST of 0.15, 0.19,
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and 0.16 eV, respectively, supporting the efficient TADF emission with higher PLQY in co-
DOI: 10.1039/C7TC05746F
doped films. As a result, the maximum ηext of these blue TADF OLEDs based on 2SPAc-
HPM, 2SPAc-MPM and 2SPAc-PPM are as high as 25.56%, 24.34%, and 31.45%,
respectively. Especially, the 2SPAc-PPM based TADF OLEDs also is one of the very few
blue TADF emitters with the external quantum efficiency of more than 30%. To our best
knowledge, the device performance is comparable with those of the state-of-the-art triazine-
based TADF OLEDs and phosphorescent OLEDs. These confirm that 2SPAc-HPM, 2SPAc-
MPM and 2SPAc-PPM are promising for candidates in the organic flat panel displays and
solid-state lightings.
Electronic supplementary information (ESI) available: The Full experimental
details, additional equations and Figures for the characterizations of the optical properties, and
the detailed devices performances.
Acknowledgements
The authors thank the supports from the National Key Research and Development Plan (Grant
No. 2016YFB0401004) and the National Science Foundation of China (Grant Nos.
6
1420106002 and 51373189). Y. Zhang thanks the supports from the Open Fund of Beijing
National Laboratory for Molecular Sciences (BNLMS) (Grant No. BNLMS20160131) and the
Fundamental Research Funds for the Central Universities (Harbin Institute of Technology).
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DOI: 10.1039/C7TC05746F
TOC
Highly Efficient Blue Organic Light-Emitting Diodes from
Pyrimidine-Based
Thermally
Activated
Delayed
Fluorescence Emitters
*
*
Bowen Li, Zhiyi Li, Taiping Hu, Yong Zhang , Ying Wang , Yuanping Yi, Fengyun Guo, Liancheng
Zhao
2
0