Double-twist pyridine-carbonitrile derivatives yielding excellent thermally activated delayed fluorescence emitters for high-performance OLEDs

Article abstract of DOI:10.1039/c9tc05340a

Possessing high photoluminescence quantum yield (PLQY) and fast reverse intersystem crossing (RISC) process are critical for obtaining efficient thermally activated delayed fluorescence (TADF) emitters. Herein, two donor-spacer-acceptor molecules, namely, 4-(4-(9,9-dimethylacridin-10(9H)-yl)phenyl)-2,6-dimethylpyridine-3,5-dicarbonitrile (Me-DMAC) and 4-(4-(10H-phenoxazin-10-yl)phenyl)-2,6-dimethylpyridine-3,5-dicarbonitrile (Me-PXZ), were developed via a double-twist design strategy. The large hindrance induces a twisted geometry, leading to small ΔE_ST values and fast RISC processes. The time-resolved photophysical measurements revealed the TADF emissions of these pyridine-3,5-dicarbonitrile-based molecules in doped thin films. High external quantum efficiency (EQE) values of 25.8% and 21.1% were achieved in organic light-emitting diodes (OLEDs) using green Me-DMAC and yellow Me-PXZ dyes, respectively, as emitters.

Full text of DOI:10.1039/c9tc05340a

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Materials Chemistry C
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Double-twist pyridine–carbonitrile derivatives
yielding excellent thermally activated delayed
fluorescence emitters for high-performance
Cite this: J. Mater. Chem. C, 2020,
8, 602
OLEDs†
b
Jiafang Li,a Wen-Cheng Chen,
He Liu, *a Zhanxiang Chen,c Danyang Chai,a
Chun-Sing Lee*b and Chuluo Yang
*
ac
Possessing high photoluminescence quantum yield (PLQY) and fast reverse intersystem crossing (RISC)
process are critical for obtaining efficient thermally activated delayed fluorescence (TADF) emitters.
Herein, two donor–spacer–acceptor molecules, namely, 4-(4-(9,9-dimethylacridin-10(9H)-yl)phenyl)-
2,6-dimethylpyridine-3,5-dicarbonitrile (Me-DMAC) and 4-(4-(10H-phenoxazin-10-yl)phenyl)-2,6-dimethyl-
pyridine-3,5-dicarbonitrile (Me-PXZ), were developed via a double-twist design strategy. The large hindrance
induces a twisted geometry, leading to small DEST values and fast RISC processes. The time-resolved
photophysical measurements revealed the TADF emissions of these pyridine-3,5-dicarbonitrile-based
molecules in doped thin films. High external quantum efficiency (EQE) values of 25.8% and 21.1% were
achieved in organic light-emitting diodes (OLEDs) using green Me-DMAC and yellow Me-PXZ dyes,
respectively, as emitters.
Received 29th September 2019,
Accepted 17th November 2019
DOI: 10.1039/c9tc05340a
rsc.li/materials-c
S (S1) and T (T1) excited states, yielding an efficient reverse inter-
system crossing (RISC) process. This is referred to as thermally
Introduction
Manipulating the excited states to completely utilize the electri- activated delayed fluorescence (TADF).17,18 By effecting an
cally generated excitons, including singlet (S) and triplet (T) intricate molecular design, RISC pathways can also be estab-
excitons in the ratio 1 : 3,1 has always been a formidable challenge lished between the higher excited states (Tm–Sn), which is
in the history of organic light-emitting diodes (OLEDs).2–14 One of described by the ‘‘hot exciton’’ mechanism featuring neglect-
the most attractive ways is to manage pathways between the S and able delayed fluorescence.19,20 Among all these methods, TADF
T excited states.15–20 Among the efforts made by worldwide has attracted considerable attention recently. One reason for
researchers, several mechanisms that can completely harvest both this is that TADF emitters can be easily constructed at a lower
S and T excitons have been proposed. By enhancing the spin–orbit cost since the selection of a pure organic donor and acceptor
coupling by introducing heavy metal elements, the lowest excited building blocks are infinite and inexpensive. At an early stage,
state (T1) can radiatively decay via phosphorescence, leading to TADF emitters have already broken the 20% external quantum
100% internal quantum yield in OLEDs.15,16 With regard to pure efficiency limit of ordinary fluorescent OLEDs.18 Recently, an
organic molecules, an alternative pathway can be opened by ultrahigh external quantum efficiency exceeding 38% in the blue
minimizing the energy difference (DEST) between the lowest region was achieved via the combination of boron acceptors and
indolocarbazole donor,21 which further demonstrates its potential.
a Shenzhen Key Laboratory of Polymer Science and Technology,
Guangdong Research Center for Interfacial Engineering of Functional Materials,
College of Materials Science and Engineering, Shenzhen University,
Shenzhen 518060, People’s Republic of China. E-mail: clyang@szu.edu.cn,
liuhe001@szu.edu.cn
b Center of Super-Diamond and Advanced Films (COSDAF) and Department of
Chemistry, City University of Hong Kong, Kowloon, Hong Kong SAR 999077,
P. R. China. E-mail: C.S.Lee@cityu.edu.hk
In general, a small DEST value is required to ensure effective
RISC from electrogenerated triplet excitons to S (usually S1).
Meanwhile, a sufficiently high photoluminescence quantum
yield (PLQY) of S1 is also needed to maximize the conversion of
S excitons into photons. The rate constant of RISC (kRISC) is
expressed as kRISC = BA ꢀ exp(ꢁDEST/kBT), where A is the
preexponential factor, kB is the Boltzmann constant, and T is
the temperature.22 Therefore, a high kRISC value can be achieved
by achieving a sufficiently small DEST value. In conventional
molecular design, a reduced DEST value can be achieved by
minimizing the overlap between the highest occupied
c Department of Chemistry, Hubei Key Lab on Organic and Polymeric Optoelectronic
Materials, Wuhan University, Wuhan 430072, People’s Republic of China.
E-mail: clyang@whu.edu.cn
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c9tc05340a
602 | J. Mater. Chem. C, 2020, 8, 602--606
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molecular orbital (HOMO) and lowest unoccupied molecular could induce large hindrance that could resulted in more
orbital (LUMO) by connecting the electron donor and electron twisted geometry. Further, a benzene ring introduced as a
acceptor with a large torsion angle. However, small overlaps spacer between the donor and acceptor could further enlarge
between the HOMO and LUMO generally tend to decrease the the overlap between the HOMO and LUMO. The double twists
value of the radiative decay rate (kr) and oscillator strength between the donor–spacer and acceptor–spacer can ensure
between the S1 and ground state (S0),23 thereby limiting PLQY. small DEST. The rational designs of Me-DMAC and Me-PXZ
Consequently, the external quantum efficiency (EQE) of devices can yield excellent OLEDs with maximum EQE values of 25.8%
can be seriously decreased.24 The tradeoff between high kr/PLQY and 21.1%, respectively.
and fast RISC process demands a careful molecular design and
smarter choices of donor and acceptor units.
Results and discussion
Synthesis and thermal properties
The cyano group (CN) with a strong electron-withdrawing
effect and short conjugation length has been a popular acceptor
building block since the early design of TADF emitters. The
Me-DMAC and Me-PXZ were synthesized through Buchwald–
star-shape 2,4,5,6-tetra(9H-carbazol-9-yl)isophthalonitrile (4CzIPN)
Hartwig coupling reactions between 4-(4-bromophenyl)-2,6-
molecule is the first reported green TADF emitters, reaching EQE
dimethylpyridine-3,5-dicarbonitrile (Me-Br) and DMAC or PXZ
higher than 20%.18 Extensive research has been conducted after-
in good yields (Scheme S1, ESI†). The compounds were fully
ward, leading to EQE as high as 31.2%.25 The derivatives of 4CzIPN
identified by 1H NMR, 13C NMR spectroscopy, MALDI-TOF, and
as well as other materials containing CN-benzene building blocks
elemental analysis. Thermal gravimetric analysis (TGA) showed
also exhibit excellent electroluminescent (EL) performance.26–30 As
the decomposition temperatures (Td) at about 282 1C and
compared to the CN-benzene unit, the CN-pyridine group has
298 1C with 5% loss of weight (Fig. S1(a), ESI†). In addition, the
drawn lesser attention for application in TADF emitters, but most
glass transition temperatures were not found for both these
of the reported compounds perform well in EL devices. Huang’s
materials during the second heating (differential scanning calori-
group reported a 4-CN-pyridine-based green emitter, namely,
metry curve, Fig. S1(b), ESI†), which demonstrates their good
4CzCNPy, with a structure similar to that of 4CzIPN, achieving
thermal stability for evaporating deposition OLED devices.
EQE value of 19.7%.31 Wong’s,32 Kido’s,33 and Zhang’s34 groups
developed CN- and diCN-pyridine derivatives, reaching EQE values
of 29.2%, 22.7%, and 21.2%, respectively. Su and coworkers
Theoretical simulations
To validate our design, density functional theory (DFT) simula-
tion was performed under the B3LYP-D3(BJ)/def2-SVP and
developed 3,5-diCN-pyridines, possessing high exciton utilization
efficiency even with large DEST.35 Recently, our group reported
PBE0/def2-SVP levels to optimize their ground and excited
3,5-diCN-pyridine derivatives, exhibiting the maximum EQE of
20%.36 Both high kr/PLQY and fast RISC process can be estab-
states, respectively. The oscillator strengths ( f ) were calculated
by using time-dependent DFT (TDDFT) calculations at the
lished in these systems. Further exploring CN-pyridine-based
PBE0/def2-SVP level with the nuclear ensemble approach.37
TADF emitters would facilitate insights into and the construction
As shown in Fig. 1a, LUMOs of both the molecules were entirely
of more practical materials.
located on the acceptor unit. This may be attributed to two
Herein, two molecules containing donor–spacer–acceptor
reasons. First, the large hindrance of CN breaks the conjuga-
tion. Second, the strong electron-withdrawing effect of CN pins
LUMO on the acceptor unit. Meanwhile, the HOMOs are mainly
double-twist structure called 4-(4-(9,9-dimethylacridin-10(9H)-
yl)phenyl)-2,6-dimethylpyridine-3,5-dicarbonitrile (Me-DMAC)
and 4-(4-(10H-phenoxazin-10-yl)phenyl)-2,6-dimethylpyridine-3,5-
spread on the donor unit with a small portion remaining on the
dicarbonitrile (Me-PXZ) with 9,9-dimethyl-9,10-dihydroacridine
central benzene. The electron-donating effects of nitrogen
shares a part of the electron with the adjacent benzene. The
(DMAC) and 10H-phenoxazine (PXZ) (Fig. 1a) as the corres-
ponding donors were designed. Considering the requirement of
complete separation of HOMO and LUMO ensures a small DEST
a fast RISC process, 3,5-diCN-pyridine connected via the 4
value, which is also proven in the natural transition orbital
position was adopted as the acceptor. Two adjacent CN groups
(NTO) analysis, as shown in Fig. 1b. Both Me-DMAC and
Me-PXZ possess small DEST values of 0.01 and 0.05 eV, respec-
tively. Such small DEST values facilitate efficient RISC. The
calculated f values of Me-DMAC and Me-PXZ are 0.007230
and 0.006414 (Table S1, ESI†), respectively, which suggest their
high PLQY values.
Photophysical properties
To experimentally validate the above speculation, UV-vis absorp-
tion, fluorescence (Fl), and phosphorescence (Ph) tests were
performed. The spectra for 1,3-bis(N-carbazolyl)benzene (mCP)
Fig. 1 Molecular structures and DFT simulation of ground and excited
states of Me-DMAC and Me-PXZ. (a) Molecular structures and distribution
film (10 wt%) and in dilute toluene are shown in Fig. 2 and Fig. S3
(ESI†), respectively, and detailed data are summarized in Table 1.
of HOMO and LUMO. (b) NTO distributions of S1 and T1 of Me-DMAC and
Me-PXZ.
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Fig. 2 Fluorescence and phosphorescence spectra (at 77 K) in mCP-
doped film (10 wt%) of (a) Me-DMAC and (b) Me-PXZ obtained via spin-
coating. Transient decay spectra of (c) Me-DMAC and (d) Me-PXZ in
mCP-doped film (10 wt%) under oxygen-free condition.
Fig. 3 (a) Energy diagram of the doped device. (b) Chemical structures of
the materials used in the device, fabrication. (c) EL spectra of the devices
based on Me-DMAC and Me-PXZ. (d) EQE–L characteristics of the devices.
UV-vis spectra in dilute toluene exhibit weak charge transfer (CT)
absorptions at around 381 and 412 nm for Me-DMAC and
Me-PXZ, respectively. As demonstrated in the CV results (Fig. S4, levels of S1 and T1 calculated from the onset of the spectra are
ESI†), PXZ possesses stronger electron-donating ability than that 2.85 (2.73) and 2.64 (2.55) eV for Me-DMAC (Me-PXZ), respectively.
of DMAC. Therefore, a more efficient CT excited state is formed Therefore, the DEST values for Me-DMAC and Me-PXZ were
when connected with an acceptor, leading to redshifts in the estimated to be 0.12 and 0.09 eV, respectively. When compared
absorption and emission. Dual emissions are found in these two with the theoretical results, the slightly larger DEST values prove
materials. Taking Me-DMAC as an example, emissions with short that a certain degree of HOMO–LUMO overlap does exist. In
(444 nm) and long (542 nm) wavelengths may be attributed to the addition, both Me-DMAC and Me-PXZ possess high absolute
so-called ‘‘quasi-axial’’ (QA) and ‘‘quasi-equatorial’’ (QE) confor- PLQY values of 96% and 71% in mCP-doped films, respectively.
mations, respectively.38,39 The QE conformation possesses more Such high PLQY values improve device efficiency.
planar geometry that leads to low-lying energy and efficient CT.
To further understand the photophysical properties, transient
The theoretical geometries of QA and QE conformations of photoluminescence (PL) measurement was carried out in mCP
Me-DMAC and Me-PXZ were calculated, as shown in Fig. S5 (10 wt%)-doped films and in toluene (10ꢁ4 M). As shown in
(ESI†). The Fl and Ph spectra were also obtained in doped mCP Fig. 2c, d, and Fig. S3 (ESI†), the PL lifetimes of Me-DMAC and
films (10 wt%) to evaluate the properties in the solid state toward Me-PXZ under Ar atmosphere clearly exhibit two components: one
device applications. The Fl emissions are located at 499 and is the prompt portion from direct S1 to S0 transition and the other
529 nm for Me-DMAC and Me-PXZ, respectively. The shoulder is the delayed portion related to recursive S1 to S0 transition. The
due to QA configuration is found in Me-PXZ films. The energy delayed PL portion in dilute toluene is quenched after bubbling
Table 1 Photophysical and electrochemical data of Me-DMAC and Me-PXZ
HOMOa,b (eV)
LUMOa,c (eV)
S1 (eV)
T1 (eV)
DEST (eV)
a,d
a,d
a,d
Me-DMAC
Me-PXZ
ꢁ5.20/ꢁ5.28
ꢁ4.97/ꢁ5.09
ꢁ2.38/ꢁ2.57
ꢁ2.41/ꢁ2.61
2.54/2.85
2.29/2.64
2.53/2.73
2.24/2.55
0.01/0.12
0.05/0.09
a
Results from theoretical simulations under the B3LYP-D3(BJ)/def2-SVP and PBE0/def2-SVP levels for the ground and excited states, respectively.
Calculated from the onset of cyclic voltammetry (Fig. S4, ESI). Calculated from the following equation: LUMO = HOMO + Eg, where Eg denotes
b
c
d
the estimated value from absorption in dilute toluene. Estimated from the onset of fluorescence and phosphorescence spectra of the mCP-doped
film (10 wt%).
Table 2 Photophysical data of Me-DMAC and Me-PXZ in the doped film (10 wt% in mCP) at room temperature
tp (ns)
td (ms)
FPL (%)
Fp (%)
Fd (%)
kr (107 sꢁ1
)
kISC (107 sꢁ1
)
kRISC (106 sꢁ1
)
Me-DMAC
Me-PXZ
22.9
24.8
16.0
2.7
1.0
4.6
96
71
82
20.0
47.8
—
76.0
23.2
—
0.90
1.95
0.63
3.57
1.34
5.60
1.70
1.47
2.03
4CzIPNa
tp: lifetime of the prompt component; td: lifetime of the delayed component; FPL: PLQY in the mCP-doped film; Fp: quantum yield of the prompt
component; Fd: quantum yield of the delay component; kr, kISC, and kRISC represent the rate constant of radiative, ISC, and RISC, respectively.
a
Results of 4CzIPN was tested in CBP (4,40-N,N0-dicarbazole-biphenyl) (6 wt%) by Masui et al.40
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Table 3 Electroluminescence characteristics of the devices
CIEb (x, y)
Current eff.b,c (cd Aꢁ1
)
Power eff.b,c (lm Wꢁ1
)
EQEb,c (%)
a
b
Emitter
Von (V)
lEL (nm)
Me-DMAC
Me-PXZ
3.2
3.0
517
562
(0.28, 0.51)
(0.44, 0.53)
59.3, 53.6
54.4, 53.5
46.6, 39.2
48.2, 37.2
25.8, 18.2
21.1, 18.1
a
b
c
Voltage at 1 cd mꢁ2
.
Measured at 1000 cd mꢁ2 Efficiencies at maximum and 1000 cd mꢁ2, respectively.
.
with O2, indicating this process involves RISC from the T excited the shorter lifetime of the delayed component could alleviate
state. As the PL transient decay curves can be fitted well with efficiency roll-off.
double-exponential functions, the prompt (tp) and delayed (td)
The excellent EL performances of both Me-DMAC and
fluorescent lifetimes together with the PLQY of prompt (Fp) and Me-PXZ demonstrate that high PLQY together with fast RISC
delayed (Fd) components can be determined. The rate constants process are critical for fabricating efficient TADF emitters. This
of prompt (kp) and delayed (kd) fluorescence, radiative decay (kr), also proves the rationality of the double-twist design strategy
and ISC (kISC) and RISC (kRISC) processes can be estimated and the suitable choice of donor and acceptor. We hope that
(equations are listed in the ESI†). The rate constants of different these results can provide guidelines for further molecular
kinetic processes are listed in Table 2. High kr and kRISC values design.
of B107 and B106 could be achieved. Both these compounds
possessed kRISC values of the same order of magnitude as those
Conclusions
of 4CzIPN.19,40 Considering the high utilization of T excitons of
4CzIPN, our molecules should perform in a similar manner.
In short, these photophysical properties suggest that Me-DMAC
and Me-PXZ possess high PLQY and exhibit a fast RISC process.
In summary, we have developed two donor–spacer–acceptor-
type double-twist TADF emitters for Me-DMAC and Me-PXZ.
The underlying TADF properties of these materials were inves-
tigated by time-resolved photophysical measurements, together
with TDDFT calculations. The two emitters of Me-DMAC and
Me-PXZ exhibited TADF emissions with very high PLQY values
of up to 96% in doped thin films. As a result, devices doped
with the mCP host yielded EQEmax values of 25.8% and 21.1%
(for Me-DMAC and Me-PXZ, respectively), which are among the
best available results. The excellent device performance demon-
strates that the double-twist design strategy is effective in
yielding desirable TADF emitters.
EL properties
As theoretically predicted and experimentally revealed, both
Me-DMAC and Me-PXZ could yield excellent device perfor-
mances. OLED devices with a structure of ITO/TPAC (40 nm)/
TCTA (10 nm)/mCP:10 wt% emitters (20 nm)/3TPYMB (55 nm)/
LiF (1 nm)/Al (100 nm) were fabricated to seek validation. Here,
1,1-bis[4-[N,N0-di(p-tolyl)amino]phenyl]cyclohexane (TAPC) and
4,40,400-tri(9-carbazoyl)triphenylamine (TCTA) were used as the
hole-transporting layer, and mCP served as the host material
for Me-DMAC and Me-PXZ. Further, tri[3-(3-pyridyl)mesityl]borane
(3TPYMB) was employed as the electron-transporting layer. The
energy level diagram of the devices, EL spectrum, external
quantum efficiency–luminescence (EQE–L), and current
density–voltage–luminescence (J–V–L) curves are shown in
Fig. 3 and Fig. S6 (ESI†). The key data of the devices are listed
in Table 3. The single-peak emission of the EL spectrum
indicates the absence of the QA configuration in the vacuum
deposition process and efficient energy transfer from the host
to the TADF emitters. Both these devices exhibit high current
efficiency (CE), power efficiency (PE), and EQE. Me-PXZ
endowed yellow devices with maximum CE (CEmax), PE (PEmax),
and EQE (EQEmax) values of 54.4 cd Aꢁ1, 48.2 lm Wꢁ1, and
21.1%, respectively. Further, Me-DMAC performed better,
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (No. 51803125 and 91833304), Shenzhen
Peacock Plan (KQTD20170330110107046), Science and Technology
Innovation Commission of Shenzhen (JCYJ20180507182244027)
and Natural Science Foundation of SZU (No. 2018002). The
numerical calculations in this paper have been done on the
supercomputing system in the Supercomputing Center of
Wuhan University.
where the CEmax, PEmax, and EQEmax values were 59.3 cd Aꢁ1
,
46.6 lm Wꢁ1, and 25.8%, respectively. Neither of the devices
employs the light out-coupling process. The utilizations of exci-
tons were estimated from the equation UE = EQE/(PLQYꢂZout),
where Zout is the out-coupling efficiency (B0.3); the efficiencies
were calculated as 90% and 99%. Such high efficiencies for these
devices can be attributed to the excellent PLQY, fast RISC process,
and intelligent selection of the host material and design of the
device structures. Me-PXZ-based devices show lower roll-off of
14% at 1000 cd mꢁ2 than that of Me-DMAC (29%), indicating that
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