Highly efficient red thermally activated delayed fluorescence materials based on a cyano-containing planar acceptor

Article abstract of DOI:10.1039/c9tc05349b

Two highly emissive materials (W1 and W2) were designed and synthesized, where two ortho-triphenylamine (TPA) groups were used as electron-donating units and one dibenzo[a,c]phenazine group was used as an electron-withdrawing motif. In W1, two additional

Full text of DOI:10.1039/c9tc05349b

Journal of
Materials Chemistry C
View Article Online
View Journal | View Issue
PAPER
Highly efficient red thermally activated delayed
fluorescence materials based on a
cyano-containing planar acceptor†
Cite this: J. Mater. Chem. C, 2019,
7, 15301
Yuan-Yuan Wang,‡a Yuan-Lan Zhang, ‡a Kaining Tong,a Lei Ding,b
ac
ac
Jian Fan
*
and Liang-Sheng Liao
*
Two highly emissive materials (W1 and W2) were designed and synthesized, where two ortho-
triphenylamine (TPA) groups were used as electron-donating units and one dibenzo[a,c]phenazine group
was used as an electron-withdrawing motif. In W1, two additional cyano groups were attached to the
dibenzo[a,c]phenazine unit to reinforce the electron-accepting strength. As a consequence, remarkable
bathochromic shifts were observed in both the UV-vis absorption and photoluminescence spectra of
W1 relative to those of W2. Furthermore, the incorporation of cyano groups into W1 led to a significant
separation of the frontier molecular orbitals (FMOs), resulting in a small singlet–triplet splitting energy
(DEST) and a strong intramolecular charge transfer (ICT) state. Notably, W1 possessed higher photolumi-
nescence quantum yields (PLQYs) and better device performance than W2. The organic light-emitting
diodes (OLEDs) based on W1 with a doping ratio of 10 wt% achieved a maximum external quantum
(EQE) efficiency of 24.97%.
Received 29th September 2019,
Accepted 9th November 2019
DOI: 10.1039/c9tc05349b
rsc.li/materials-c
100% internal quantum efficiency (IQE), TADF OLEDs can achieve
over 30% external quantum efficiencies (EQEs) with the host-
Introduction
Following the invention of organic light-emitting diode (OLED) dopant dispersion strategy.20 However, efficient red TADF OLEDs
technology by C. W. Tang and S. A. VanSlyke in 1987,1 OLEDs are still highly desirable since red emitters generally suffer from
have attracted extensive attention in both academia and severe non-radiative decay due to their small band gaps.
industry.2–6 Currently, OLEDs have demonstrated a wide range
Although Hatakeyama et al. successfully demonstrated a
of applications in both display and lighting. So far, much progress novel design strategy based on multi-resonance induced TADF
has been made in the development of efficient emissive materials emitters with coplanar structures,21 emitters with twisted donor–
from fluorescent molecules,7–9 phosphorescent complexes,10–13 or acceptor (D–A) structures are the mainstream TADF materials
thermally activated delayed fluorescent (TADF) compounds.14–18 owing to the easy realization of large separation of the lowest
Recently, Yersin et al. reported the fourth generation OLEDs based unoccupied molecular orbitals (LUMOs) and highest occupied
1
on rigidified molecules with an extremely small CT–3CT energy molecular orbitals (HOMOs), thus resulting in a small singlet–
difference of around 2 meV.19 However, the development of TADF triplet energy difference (DEST). The twisted structure is a
emitters is currently a hot research topic because of their particular spontaneous outcome of steric hindrance between the D and
properties, such as the utilization of both singlet and triplet A units. It is notable that the rotation of the s bond between D and
excitons, diversity in their structural modification and functionality, A usually leads to a broad electroluminescence (EL) spectrum
and absence of noble metals. Since TADF emitters can realize a and the variation in the overall electronic structure of a TADF
molecule.22 Herein, we designed and synthesized two TADF
molecules, where two ortho-TPA units are attached to the planar
electron-withdrawing dibenzo[a,c]phenazine unit. Consequently,
a Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory
for Carbon-Based Functional Materials & Devices, Soochow University, Suzhou,
the rotation of the D–A s bond in W1 and W2 is highly limited
due to the increased repulsion interactions between D and A. In
addition, the TPA unit intrinsically shows a certain degree of
rotation flexibility relative to other rigidified donor units (for
example, acridine), which will also lead to non-radiative decay.
The crowded arrangement of these two ortho-TPA units in
W1 and W2 will reduce the rotation flexibility greatly. In W1,
Jiangsu 215123, China. E-mail: lsliao@suda.edu.cn, jianfan@suda.edu.cn
b College of Electrical and Information Engineering, Shaanxi University of Science
and Technology, Xi’an, Shaanxi, 710021, China
c Institute of Organic Optoelectronics (IOO) of the Jiangsu Industrial Technology
Research Institute (JITRI), Suzhou, Jiangsu 215212, China
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c9tc05349b
‡ These two authors contributed equally to this work.
This journal is ©The Royal Society of Chemistry 2019
J. Mater. Chem. C, 2019, 7, 15301--15307 | 15301

View Article Online
Paper
Journal of Materials Chemistry C
there are two cyano groups grafted onto the dibenzo[a,c]- 9.52 (s, J = 2.2 Hz, 2H), 8.38 (m, J = 5.9 Hz, 4H), 7.90 (s, 2H), 7.29
phenazine unit to enhance the electron-accepting capability of (m, J = 8.1 Hz, 8H), 7.22 (d, J = 8.4 Hz, 4H), 7.15 (d, J = 8.0 Hz,
the whole configuration, therefore tuning the emission into the 8H), 7.06 (t, J = 7.3 Hz, 8H).
deep red region. Xu et al. applied the gradient multi-inductive
Synthesis of 11,12-bis(4-(diphenylamino)phenyl)dibenzo[a,c]-
1
effect to achieve efficient CT radiation by enhancing the over- phenazine-2,7-dicarbonitrile (W1). 4,40-(2,7-Dibromodibenzo[a,c]-
lap of the frontier molecular orbitals.23 In comparison with W2, phenazine-11,12-diyl)bis(N,N-diphenylaniline) (520 mg, 0.56 mmol)
the introduction of multiple electron-withdrawing groups into and CuCN (125 mg, 1.4 mmol) were dissolved in 1-methyl-2-
W1 leads to substantial CT character in both the singlet (S1) and pyrrolidinone (20 mL). This reaction mixture was heated at 180 1C
triplet (T1) states, thus leading to a small DEST, which could be by microwave irradiation for 2 hours. After removing the solvent, the
favorable for the reverse intersystem crossing (RISC). Notably, residue was purified by column chromatography with PE/DCM
W1-based OLEDs demonstrate a highly efficient EL performance (2/3,v/v) as the eluent to afford the compound W1 (288 mg,
1
with an EQE of 23.24% (620 nm) at the doping ratio of 15 wt%, and 63%). H NMR (400 MHz, chloroform-d): d 9.73 (s, J = 1.8 Hz,
24.97% (608 nm) at the doping concentration of 10 wt%.
2H), 8.64 (d, J = 8.5 Hz, 2H), 8.37 (s, 2H), 8.03 (d, J = 8.4 Hz, 2H),
7.29 (m, J = 7.8 Hz, 8H), 7.22 (d, J = 8.3 Hz, 4H), 7.16 (d, J =
8.0 Hz, 8H), 7.07 (t, J = 7.4 Hz, 8H). 13C NMR (400 MHz, CDCl3): d
147.54, 147.50, 145.19, 142.12, 140.18, 133.61, 133.05, 132.08,
131.68, 130.91, 130.81, 129.62, 129.41, 124.80, 124.47, 123.32,
122.50, 118.39, 112.95. MALDI-TOF-MS: m/z calcd for C58H36N6 :
Experimental
General information
All chemicals and reagents were used as received without further 816.300, found: 816.591. Elemental analysis (%) for C58H36N6:
1
purification. H NMR and 13C NMR spectra were recorded on a C, 85.27; H, 4.44; N, 10.29; found: C, 85.43; H, 4.41; N, 10.34.
Bruker 400 spectrometer at room temperature. Matrix-assisted
Synthesis of 4,40-(dibenzo[a,c]phenazine-11,12-diyl)bis(N,N-
laser desorption/ionization time of flight mass spectrometry diphenylaniline) (W2). The synthesis procedure of compound
(MALDI-TOF-MS) was performed on a BRUKER ultrafleXtreme W2 was similar to that of compound 2. 1H NMR (400 MHz,
MALDI-TOF spectrometer. UV-vis absorption spectra were chloroform-d): d 9.41 (d, J = 7.8, 1.6 Hz, 2H), 8.58 (d, J = 8.0 Hz,
recorded on a PerkinElmer Lambda 750 spectrophotometer. 2H), 8.36 (s, 2H), 7.85–7.78 (t, 2H), 7.76 (t, J = 7.7 Hz, 2H), 7.35–
PL spectra and phosphorescence spectra were recorded on a 7.26 (m, 8H), 7.22 (d, J = 8.4 Hz, 4H), 7.14 (d, J = 7.9 Hz, 8H),
Hitachi F-4600 fluorescence spectrophotometer. Differential 7.05 (m, J = 7.9, 4.9 Hz, 8H). 13C NMR (400 MHz, CDCl3): d
scanning calorimetry (DSC) and thermogravimetric analysis 147.58, 146.99, 143.24, 142.53, 141.54, 134.48, 131.97, 130.83,
(TGA) were performed on a TA DSC 2010 unit and a TA SDT 130.42, 130.17, 129.67, 129.29, 127.90, 126.19, 124.53, 123.01,
2960 instrument, respectively, at a heating rate of 10 1C minꢀ1 122.90, 122.72. MALDI-TOF-MS: m/z calcd for C56H38N4:
under nitrogen. The glass transition temperatures (Tg) were 766.310, found: 766.591. Elemental analysis (%) for C56H38N4:
determined from the second heating scan. Cyclic voltammetry C, 87.70; H, 4.99; N, 7.31; found: C, 87.88; H, 4.818; N, 7.34.
(CV) was performed on a CHI600 voltammetric analyzer at room
temperature with ferrocenium–ferrocene (Fc/Fc+) as the internal
standard at a scan rate of 100 mV sꢀ1
.
Results and discussion
Synthesis of N4,N4,N40 0 0,N400-tetraphenyl-[1,10:20,100-terphenyl]-
4,40,400,50-tetraamine (1). A mixture of 4,5-dibromobenzene-1,2- Scheme 1 describes the synthetic routes of W1 and W2. The
diamine (1.33 g, 5 mmol), 4-(diphenylamino)phenylboronic acid intermediate 1 was synthesized through the Suzuki coupling
(3.63 g, 12.5 mmol), Pd(PPh3)4 (288.75 mg, 0.25 mmol), K2CO3 reaction between 4,5-dibromobenzene-1,2-diamine and (4-(diphenyl-
(2.76 g, 20 mmol), 1,4-dioxane (100 mL) and H2O (10 mL) was amino)phenyl)boronic acid. The following condensation reactions
heated at 85 1C for 24 hours under nitrogen. After cooling to of diamine 1 with 2,7-dibromophenanthrene-9,10-dione and
room temperature, the solution was poured into water and phenanthrene-9,10-dione under acidic conditions produced 2
extracted with DCM. The organic layer was dried over Na2SO4 and W2, respectively. The substitution reaction of compound 2
and concentrated under reduced pressure. The residue was with CuCN yielded the final product W1. The compounds W1
1
purified by column chromatography with PE/EA (2/1, v/v) to and W2 were fully characterized via H/13C NMR spectroscopy
afford the compound 1 (2.88 g, 96%). 1H NMR (400 MHz, (Fig. S1–S4, ESI†) and mass spectrometry.
DMSO-d6): d 7.23 (t, J = 7.7 Hz, 8H), 6.98 (dd, J = 13.6, 5.2 Hz,
The thermal stability of W1 and W2 was investigated by
4H), 6.93 (d, J = 9.6 Hz, 4H), 6.83 (d, J = 8.2 Hz, 2H), 6.59 (s, 1H). thermo-gravimetric analysis (TGA) measurements. As shown in
Synthesis of 4,40-(2,7-dibromodibenzo[a,c]phenazine-11,12- Fig. 1, both W1 and W2 exhibit excellent thermal stability with
diyl)bis(N,N-diphenylaniline) (2). Compound 1 (654.5 mg, the decomposition temperature of 521 1C and 480 1C, respectively.
1.1 mmol) and 2,7-dibromophenanthrene-9,10-dione (366 mg, The temperature at 5% weight loss is defined as the decom-
1 mmol) were dissolved in AcOH (30 mL) and heated at 125 1C position temperature. This result would be beneficial for device
overnight under nitrogen. After cooling down to room tempera- fabrication via a vacuum deposition method. Furthermore, the
ture, the mixture was subjected to filtration and washed with differential scanning calorimetry (DSC) curves indicate that
water. The crude product was re-crystallized in DCM to afford both compounds have a high glass transition temperature of
compound 2 (0.73 g, 79%). 1H NMR (400 MHz, Chloroform-d): d 209 1C, which could help maintain the morphology of the light
15302 | J. Mater. Chem. C, 2019, 7, 15301--15307
This journal is ©The Royal Society of Chemistry 2019

View Article Online
Journal of Materials Chemistry C
Paper
Scheme 1 Synthetic procedure of W1 and W2. (i) Pd(PPh3)4, K2CO3, 1,4-dioxane/H2O, 85 1C, 24 h; (ii) AcOH, 125 1C, overnight; (iii) CuCN, NMP, 180 1C,
MW, 2 h.
Fig. 1 TGA and DSC (inset) curves of W1 (left) and W2 (right).
emitting layer at the device working stage. The electrochemical that of W2 (ꢀ5.21 eV), which could be due to the p-conjugation
properties of W1 and W2 were studied by cyclic voltammetry. between the donor and the strong acceptor in W1. The LUMO of
The HOMO energy levels are determined from the onsets of the W1 is deeper than that of W2 (Table 1) because of the introduction
oxidation potentials (Fig. S5 and S6, ESI†), and the LUMO of the additional electron-withdrawing cyano group in W1, which
energy levels are calculated by the difference between the is consistent with the simulation results (vide infra).
HOMO and the corresponding band gap (Eg). It is interesting
The UV-vis absorption and PL spectra of W1 and W2 are
to note that the HOMO of W1 (ꢀ5.08 eV) is slightly higher than shown in Fig. 2. The strong absorption bands at around 300 nm
Table 1 Experimental properties of W1 and W2
Absorptiona (nm) PLa (nm) Fb (%)
S1/T1 (eV) DEST (eV) Tg/Td (1C)
Eg (eV) HOMO/LUMOg (eV)
f
tp (ns)
td (ms)
c
d
e
h
h
W1 344, 484
W2 317, 444
580
524
57.1/93.7 2.18/1.94
33.7/65.4 2.39/2.10
0.24
0.29
209/521
209/480
2.29
2.53
ꢀ5.08/ꢀ2.79
ꢀ5.21/ꢀ2.68
29.42
23.63
67.62
0.82
a
b
Measured in toluene solution at room temperature. Absolute PL quantum yield in neat film/in doped film (10% for W1 and 15% for W2).
Singlet/triplet energies (S1/T1) estimated from onsets of the fluorescence and phosphorescence spectra in toluene at 77 K. DEST = S1 ꢀ T1. Tg:
c
d
e
f
glass transition temperature; Td: decomposition temperature. Eg: optical band gap energies were calculated from the corresponding absorption
g
h
onsets in a toluene solution. HOMO levels were calculated from CV data; LUMO levels were calculated from the HOMOs and Egs. tp/td: prompt
& delayed exciton lifetime for W1 10 wt%/W2 15 wt% doped in CBP host film.
This journal is ©The Royal Society of Chemistry 2019
J. Mater. Chem. C, 2019, 7, 15301--15307 | 15303

View Article Online
Paper
Journal of Materials Chemistry C
using the Gaussian 09 B3LYP/6-31G(d) software package. As
shown in Fig. 3, the HOMO and LUMO in both W1 and W2 are
distributed over the ortho-TPA units and the planar acceptors,
respectively. The electrostatic potential map of W1 shows that
the electron-negative region is predominantly located on the
cyano groups, while in W2 it is mainly around the phenazine N
atoms (Fig. S11, ESI†). The introduction of cyano groups leads
to a major separation of the relevant wave functions, resulting
in a small DEST in W1, which is consistent with the photo-
physical results. Nevertheless, there is a partial overlap between
the HOMO and LUMO on the phenazine group for both
compounds. W1 and W2 have similar HOMO energy levels
since they have the same electron-donating groups. The LUMO
energy level of W1 (ꢀ2.75 eV) is much deeper than that of W2
(ꢀ2.16 eV), which derives from the cyano groups with strong
electron-withdrawing ability. The natural transition orbital
(NTO) calculation reveals that in W1, the S1 has substantial
Fig. 2 Normalized UV-vis absorption and PL spectra of W1 and W2.
are assigned to the p–p* transition, while the absorption bands CT character and T1 has a combination of a local excitation (LE)
observed at longer wavelengths (4400 nm) are attributed to the state and CT state. In W2, S1 predominantly exhibits CT character
charge transfer transition from triphenylamine to the planar and T1 is mainly a LE state. For organic TADF materials, the spin–
acceptor unit. As expected, W1 shows a red-shift in its UV-vis orbital coupling (SOC) also determines the rate of RISC from T1 to
absorption spectrum relative to that of W2 due to the presence S1, although this coupling is relatively weak due to the absence
of a strong acceptor unit in W1. In W2, the CT absorption band of heavy atoms in most of them.27 It is notable that the different
displays a maximum molar extinction coefficient that is higher spatial orbital occupation of S1 and T1 will result in a certain
than that in W1, indicating the larger HOMO/LUMO overlap degree of SOC, hence contributing to the RISC process of W1
and higher CT transition probability in W2. Based on their and W2.
absorption spectra, large oscillator strengths ( f )24 at around 0.2
To explore the electroluminescence properties of W1 and
are achieved for both the compounds, probably by virtue of W2, OLEDs based on these dopants were fabricated with a
their unique ortho-donor-planar-acceptor conjugation pattern. normal sandwiched configuration: ITO/HAT-CN (10 nm)/TAPC
W1 and W2 manifest intense photoluminescence (PL) emission (40 nm)/TCTA (10 nm)/CBP: emitter (2–15 wt%, 20 nm)/TmPyPB
at 580 nm and 524 nm, respectively. In addition, W1 possesses (55 nm)/Liq (2 nm)/Al (120 nm). HAT-CN (1,4,5,8,9,11-hexa-
a Stokes shift that is about 96 nm larger than that of W2 azatriphenylene-hexanitrile) and TAPC (1,1-bis[4-[N0,N0-di( p-tolyl)-
(80 nm), suggesting that a strong intramolecular 1CT state amino]-phenyl]cyclohexane) were utilized as the hole-injecting layer
exists in W1. Currently, one promising design strategy of (HIL) and hole-transporting layer (HTL), respectively. TCTA
efficient TADF materials is to introduce obvious CT character (4,4,4-tris(N-carbazolyl)triphenyl-amine) served as the exciton-
into the excitation state on account of the radiative nature of blocking layer, and TmPyPB (1,3,5-tri(m-pyrid-3-yl-phenyl)-
Frenkel excitons with strong Coulombic interactions.25 Both W1 benzene) and Liq were used as the electron-transporting layer
and W2 reveal solvent polarity-dependent emission properties. (ETL) and electron-injecting layer (EIL), respectively. In addition,
As depicted in Fig. S7 (ESI†), significant red-shifts are observed in the emitting layer (EML), W1 and W2 were doped into the host
in their PL spectra when the solvent polarities are increased, (CBP) with 2/5/10/15 wt%. Fig. S12 (ESI†) shows the energy level
which indicates that the singlet states exhibit substantial CT diagram and corresponding molecular structures.
character. In addition, their broad and featureless PL emission
The EQE–current density curves of W1 at different doping
spectra imply that the CT excitation state is present. Both W1 ratios are depicted in Fig. 4. A maximum EQE of 24.97% at the
and W2 are also highly emissive in neat films with photo- doping ratio of 10 wt% with an emission wavelength peak at 608 nm
luminescence quantum yields (PLQYs) of 57.1% and 33.7%, and corresponding Commission Internationale de L’Eclairage (CIE)
respectively (Fig. S8, ESI†). The singlet/triplet energy levels of coordinates of (0.61, 0.39) was achieved (Table 2). When the doping
W1 (2.18/1.94 eV) and W2 (2.39/2.10 eV) are estimated from the ratio was increased to 15 wt%, the emission wavelength red-shifted
onsets of the fluorescence and phosphorescence spectra in toluene to 620 nm with CIE coordinates of (0.63, 0.37) and a maximum EQE
at 77 K (Fig. S9 and S10, ESI†). As a result, W1 (0.24 eV) has a of 23.24% was obtained. For the W2-based device, a maximum EQE
smaller DEST than W2 (0.29 eV). Meanwhile, the transient PL of 8.69% was achieved at the doping ratio of 10% with the emission
decays are characterized to confirm the TADF feature, and the wavelength peak at 556 nm, corresponding to yellowish green light
detailed data are presented in Table 1. The difference in delayed with CIE coordinates of (0.43, 0.55). The superior device efficiency of
lifetime between W1 and W2 may result from the reduction in the W1 is attributed to its high PLQY (Table 1) as compared with that of
energy transfer rate from hosts to guests.26
W2. The inset of Fig. 4 shows the doping-ratio-dependent emission
To investigate the electronic structures of W1 and W2, spectra of W1. The higher the doping ratios, the longer the emission
density functional theory (DFT) calculations were performed wavelengths are. This doping ratio-dependent red-shift is caused by
15304 | J. Mater. Chem. C, 2019, 7, 15301--15307
This journal is ©The Royal Society of Chemistry 2019

View Article Online
Journal of Materials Chemistry C
Paper
Fig. 3 Energy levels simulated by DFT calculations and natural transition orbital (NTO) results.
the emission spectra of W1 (inset of Fig. 4) relative to that of W2
agrees well with the fact that W1 has a larger polarity than W2. The
absence of host CBP emission around 400 nm in the EL spectra
indicates the efficient energy transfer from the host to the dopant.
Current density–voltage–luminance ( J–V–L) characteristics of W1
are shown in Fig. 5. The turn-on voltage is measured at
100 cd mꢀ2. As the doping ratio is increased from 10 wt% to
15 wt%, the turn-on voltages show a slight decrease from 4.14 V
to 4.07 V, which could be due to the enhancement of carrier
capture. As shown in Fig. 6, a maximum current efficiency (CE)
of 40 cd Aꢀ1 and power efficiency (PE) of 38.27 lm Wꢀ1 could be
achieved. This device performance based on W1 is among the
best reported results with similar emission wavelengths.29
The electron and hole transporting properties of W1/W2 are
shown in Fig. S16 and S17 (ESI†), respectively. W1 and W2
demonstrate similar hole transport behaviors. W1 shows
better electron transport capability than W2, which could be
Fig. 4 EQE–current density curves of W1 (inset: EL spectra at 5 mA cmꢀ2).
the large ground-state dipole moment of W1.28 The polarity of EML due to the lower LUMO energy level and larger conjugation
changes as the doping ratio increases. The large redshift observed in system of W1.
This journal is ©The Royal Society of Chemistry 2019
J. Mater. Chem. C, 2019, 7, 15301--15307 | 15305

View Article Online
Paper
Journal of Materials Chemistry C
Table 2 Electroluminance performance of W1 doped device
V a (V)
CEb [cd Aꢀ1
]
PEb [lm Wꢀ1
]
EQEb [%]
CIEc (x, y)
lEL [nm]
d
Doping conc. [wt%]
10
15
4.14
4.07
40/15/7
30/14/6
38.3/11.6/3.9
39.6/10.9/3.3
24.97/8.73/3.88
23.24/9.85/4.06
0.61, 0.39
0.63, 0.37
608
620
a
b
c
Voltage at 100 cd mꢀ2
.
Maximum/luminance at 100/luminance at 1000 cd mꢀ2
. Commission Internationale de L’Eclairage coordinates.
d
Measured at 5 mA cmꢀ2
.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
The authors are grateful for the financial support from the
National Key R&D Program of China (No. 2016YFB0400703),
National Natural Science Foundation of China (No. 61575136
and 21871199), Key Research and Development Plan of Shaanxi
Province (No. 2017ZDCXL-GY-06-03), and Natural Science
Foundation of Jiangsu Province for Youth (No. BK20180288).
This work was also supported by the Collaborative Innovation
Centre of Suzhou Nano Science and Technology (Nano-CIC),
Priority Academic Program Development of Jiangsu Higher
Education Institutions (PAPD), and ‘‘111’’ Project of The State
Administration of Foreign Experts Affairs of China.
Fig. 5 Current density–voltage–luminance (J–V–L) characteristics of the
doped OLEDs based on W1.
References
1 C. W. Tang and S. A. VanSlyke, Appl. Phys. Lett., 1987, 51, 913.
2 Y. Sun, N. C. Giebink, H. Kanno, B. Ma, M. E. Thompson
and S. R. Forrest, Nature, 2006, 440, 908.
3 K. T. Kamtekar, A. P. Monkman and M. R. Bryce, Adv.
Mater., 2010, 22, 572.
4 P.-K. Chow, G. Cheng, G. S. M. Tong, W.-P. To, W.-L. Kwong,
K.-H. Low, C.-C. Kwok, C. Ma and C.-M. Che, Angew. Chem.,
2015, 127, 2112.
5 S. Reineke, F. Lindner, G. Schwartz, N. Seidler, K. Walzer,
B. Lu¨ssem and K. Leo, Nature, 2009, 459, 234.
6 H. Sasabe, Y. Seino, M. Kimura and J. Kido, Chem. Mater.,
2012, 24, 1404.
7 S.-W. Wen, M.-T. Lee and C. H. Chen, J. Disp. Technol., 2005,
1, 90.
Fig. 6 Power/current efficiency vs. luminance relationships of the
W1-based device.
8 S. Zhang, L. Yao, Q. Peng, W. Li, Y. Pan, R. Xiao, Y. Gao,
C. Gu, Z. Wang, P. Lu, F. Li, S. Su, B. Yang and Y. Ma, Adv.
Funct. Mater., 2015, 25, 1755.
9 X. Qiu, Y. Xu, C. Wang, M. Hanif, J. Zhou, C. Zeng, Y. Li,
Q. Jiang, R. Zhao, D. Hu and Y. Ma, J. Mater. Chem. C, 2019,
7, 5461.
Conclusions
In summary, two highly emissive materials (W1 and W2) with
planar electron-withdrawing dibenzo[a,c]phenazine groups
were synthesized. With the introduction of cyano groups into
W1, its PLQY in the doped film could reach 93.7%. In contrast,
the PLQY is only 65.4% for W2 without multiple acceptor units. 10 (a) M. A. Baldo, D. F. O’Brien, Y. You, A. Shoustikov, S. Sibley,
Moreover, the partial localization of LUMO on the cyano groups
in W1 leads to a large separation of FMOs and a small DEST
M. E. Thompson and S. R. Forrest, Nature, 1998, 395, 151;
(b) S.-J. Yeh, M.-F. Wu, C.-T. Chen, Y.-H. Song, Y. Chi, M.-H. Ho,
S.-F. Hsu and C. H. Chen, Adv. Mater., 2005, 17, 285; (c) D. Xia,
B. Wang, B. Chen, S. Wang, B. Zhang, J. Ding, L. Wang, X. Jing
and F. Wang, Angew. Chem., Int. Ed., 2014, 53, 1048; (d) D. Ma,
L. Duan, Y. Wei, L. He, L. Wang and Y. Qiu, Chem. Commun.,
2014, 50, 530; (e) Y.-C. Zhu, L. Zhou, H.-Y. Li, Q.-L. Xu,
.
As a result, W1 showed superior device performance with a
maximum EQE of 23.24% and emission wavelength peak at
620 nm, and 24.97% at 608 nm. So, the incorporation of a cyano-
containing planar acceptor into CT molecules demonstrates great
promise for the design of efficient red TADF molecules.
15306 | J. Mater. Chem. C, 2019, 7, 15301--15307
This journal is ©The Royal Society of Chemistry 2019

View Article Online
Journal of Materials Chemistry C
Paper
M.-Y. Teng, Y.-X. Zheng, J.-L. Zuo, H.-J. Zhang and X.-Z. You, Adv.
Mater., 2011, 23, 4041; ( f ) L. Ying, C.-L. Ho, H. Wu, Y. Cao and
W.-Y. Wong, Adv. Mater., 2014, 26, 2459.
J. Zhao, X. Du, H. Yang, P. Zhong, C. Zheng, H. Lin, S. Tao and
X. Zhang, J. Mater. Chem. C, 2019, 7, 11806.
17 (a) Y.-F. Wang, H.-Y. Lua, C. Chen, M. Li and C.-F. Chen,
Org. Electron., 2019, 70, 71; (b) X. Lv, R. Huang, S. Sun,
Q. Zhang, S. Xiang, S. Ye, P. Leng, F. B. Dias and L. Wang,
ACS Appl. Mater. Interfaces, 2019, 11, 10758.
11 (a) C.-L. Ho and W.-Y. Wong, Coord. Chem. Rev., 2013,
257, 1614; (b) T. Fleetham, G. Li, L. Wen and J. Li, Adv.
Mater., 2014, 26, 7116; (c) D. A. K. Vezzu, J. C. Deaton,
J. S. Jones, L. Bartolotti, C. F. Harris, A. P. Marchetti, 18 (a) L.-S. Cui, Y.-L. Deng, D. P.-K. Tsang, Z.-Q. Jiang, Q. Zhang,
M. Kondakova, R. D. Pike and S. Huo, Inorg. Chem., 2010,
49, 5107.
12 (a) K. Li, G. S. M. Tong, Q. Wan, G. Cheng, W.-Y. Tong,
L.-S. Liao and C. Adachi, Adv. Mater., 2016, 28, 7620; (b) Y. Yuan,
Y. Hu, Y.-X. Zhang, J.-D. Lin, Y.-K. Wang, Z.-Q. Jiang, L.-S. Liao
and S.-T. Lee, Adv. Funct. Mater., 2017, 27, 1700986.
W.-H. Ang, W.-L. Kwong and C.-M. Che, Chem. Sci., 1653, 19 H. Yersin, L. Mataranga-Popa, R. Czerwieniec and Y. Dovbii,
2016, 7; (b) K. Li, X. Guan, C.-W. Ma, W. Lu, Y. Chen and Chem. Mater., 2019, 31, 6110.
C.-M. Che, Chem. Commun., 2011, 47, 9075; (c) S. C. F. Kui, 20 (a) K.-W. Tsai, M.-K. Hung, Y.-H. Mao and S.-A. Chen, Adv.
P. K. Chow, G. S. M. Tong, S.-L. Lai, G. Cheng, C.-C. Kwok,
K.-H. Low, M. Y. Ko and C.-M. Che, Chem. – Eur. J., 2013,
19, 69; (d) S. C. F. Kui, P. K. Chow, G. Cheng, C.-C. Kwok,
C. L. Kwong, K.-H. Low and C.-M. Che, Chem. Commun.,
2013, 49, 1497.
Funct. Mater., 2019, 1901025; (b) T.-A. Lin, T. Chatterjee,
W.-L. Tsai, W.-K. Lee, M.-J. Wu, M. Jiao, K.-C. Pan, C.-L. Yi,
C.-L. Chung, K.-T. Wong and C.-C. Wu, Adv. Mater., 2016,
28, 6976; (c) T.-L. Wu, M.-J. Huang, C.-C. Lin, P.-Y. Huang,
T.-Y. Chou, R.-W. Chen-Cheng, H.-W. Lin, R.-S. Liu and
C.-H. Cheng, Nat. Photonics, 2018, 12, 235.
13 (a) T. Fleetham, G. Li and J. Li, ACS Appl. Mater. Interfaces,
2015, 7, 16240; (b) T. Fleetham, L. Huang and J. Li, Adv. 21 T. Hatakeyama, K. Shiren, K. Nakajima, S. Nomura,
Funct. Mater., 2014, 24, 6066.
14 (a) Q. Zhang, S. Sun, W. J. Chung, S. J. Yoon, Y. Wang,
S. Nakatsuka, K. Kinoshita, J. Ni, Y. Ono and T. Ikuta, Adv.
Mater., 2016, 28, 2777.
R. Guo, S. Ye, J. Y. Lee and L. Wan, J. Mater. Chem. C, 2019, 22 I. Lee and J. Y. Lee, Org. Electron., 2016, 29, 160.
7, 12248; (b) J.-X. Chen, W.-W. Tao, W.-C. Chen, Y.-F. Xiao, 23 H. Wang, B. Zhao, P. Ma, Z. Li, X. Wang, C. Zhao, X. Fan,
K. Wang, C. Cao, J. Yu, S. Li, F.-X. Geng, C. Adachi, C.-S. Lee
L. Tao, C. Duan, J. Zhang, C. Han, G. Chen and H. Xu,
and X.-H. Zhang, Angew. Chem., Int. Ed., 2019, 58, 14660;
J. Mater. Chem. C, 2019, 7, 7525.
(c) X. Song, D. Zhang, Y. Lu, C. Yin and L. Duan, Adv. Mater., 24 Y. Sagara, K. Shizu, H. Tanaka, H. Miyazaki, K. Goushi,
2019, 1901923. H. Kaji and C. Adachi, Chem. Lett., 2015, 44, 360.
15 (a) Z.-G. Wu, H.-B. Han, Z.-P. Yan, X.-F. Luo, Y. Wang, 25 X. Cai, Z. Qiao, M. Li, X. Wu, Y. He, X. Jiang, Y. Cao and
Y.-X. Zheng, J.-L. Zuo and Y. Pan, Adv. Mater., 2019, S.-J. Su, Angew. Chem., Int. Ed., 2019, 58, 13522.
31, 1900524; (b) W. Li, B. Li, X. Cai, L. Gan, Z. Xu, W. Li, 26 G. Mehes, K. Goushi, W. J. Potscavage, Jr. and C. Adachi,
K. Liu, D. Chen and S.-J. Su, Angew. Chem., Int. Ed., 2019, Org. Electron., 2014, 15, 2027.
58, 11301; (c) T. Ohsawa, H. Sasabe, T. Watanabe, K. Nakao, 27 X.-K. Chen, D. Kim and J.-L. Bredas, Acc. Chem. Res., 2018,
´
´
R. Komatsu, Y. Hayashi, Y. Hayasaka and J. Kido, Adv. Opt.
Mater., 2018, 1801282.
51, 2215.
´
28 A. D’Aleo, M. H. Sazzad, D. H. Kim, E. Y. Choi, J. W. Wu,
16 (a) H. Uoyama, K. Goushi, K. Shizu, H. Nomura and
C. Adachi, Nature, 2012, 492, 234; (b) F.-M. Xie, H.-Z. Li, G.-L.
G. Canard, F. Fages, J.-C. Ribierre and C. Adachi, Chem.
Commun., 2017, 53, 7003.
¨
Dai, Y.-Q. Li, T. Cheng, M. Xie, J.-X. Tang and X. Zhao, ACS 29 Y.-L. Zhang, Q. Ran, Q. Wang, Y. Liu, C. Hanisch,
Appl. Mater. Interfaces, 2019, 11, 26144; (c) Z. He, C. Wang,
S. Reineke, J. Fan and L.-S. Liao, Adv. Mater., 2019, 1902368.
This journal is ©The Royal Society of Chemistry 2019
J. Mater. Chem. C, 2019, 7, 15301--15307 | 15307