Efficient and stable deep-blue narrow-spectrum electroluminescence based on hybridized local and charge-transfer (HLCT) state

Article abstract of DOI:10.1016/j.dyepig.2021.109482

Two phenanthroimidazole-acridine derivatives (DPM and TDPM) were designed and synthesized for deep-blue organic light-emitting diode (OLED). Twisted combined rigid structures with hybridized local and charge-transfer (HLCT) state properties enable them to

Full text of DOI:10.1016/j.dyepig.2021.109482

Dyes and Pigments 193 (2021) 109482
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Efficient and stable deep-blue narrow-spectrum electroluminescence based
on hybridized local and charge-transfer (HLCT) state
,
Shengbing Xiaoa, Shi-Tong Zhanga **, Ying Gaoa, Xinqi Yanga, Haichao Liua, Weijun Lib,
,
*
Bing Yanga
a State Key Laboratory of Supramolecular Structure and Materials, Institute of Theoretical Chemistry, College of Chemistry, Jilin University, Changchun, China
b State Key Laboratory Breeding Base of Green Chemistry Synthesis Technology, College of Chemical Engineering, Zhejiang University of Technology, Hangzhou, 310014,
China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Two phenanthroimidazole-acridine derivatives (DPM and TDPM) were designed and synthesized for deep-blue
organic light-emitting diode (OLED). Twisted combined rigid structures with hybridized local and charge-
transfer (HLCT) state properties enable them to achieve excellent OLED performance. Non-doped OLEDs
based on DPM and TDPM show decent deep-blue narrow-spectrum emission with Commission International de
Phenanthroimidazole-acridine derivatives
HLCT
Deep-blue OLED
´
L’Eclairage (CIE) coordinates of (0.157, 0.053) and (0.158, 0.045), as well as maximum external quantum ef-
ficiency (EQEmax) of 4.0% and 2.6%, respectively. More importantly, OLEDs based on TDPM exhibit a smaller
efficiency roll-off (12%) than that of DPM (35%) at high brightness (820 cd mꢀ 2 for DPM and 893 cd mꢀ 2 for
TDPM, respectively). Overall, our work provides a molecular design strategy of HLCT materials for efficient deep-
blue OLED with high color purity using purely organic emitter.
1. Introduction
blue electro-fluorescent materials: electro-exciton utilization and color
purity [15]. On one hand, the most crucial drawback of the pure organic
Organic light-emitting diode (OLED) has attracted much attention
for its potential commercial application in flat-panel display and solid-
state lighting [1–4]. Over the past few years, great efforts have been
made to improve OLED efficiency and stability through the exploration
on new material systems and the optimization of device structures
[5–9]. In the past two decades, the green- and red-emissive phospho-
rescent materials have been developed and commercialized very well
[10–12]. However, despite that there have been many reports on the
blue electro-phosphorescent OLED (PhOLED), efficient deep-blue phO-
LEDs are still rare compared with blue and sky-blue phOLEDs [13,14].
Additionally, noble-metals are essential to the current organometallic
complexes phosphors, which are potential pollutants and expensive.
Therefore, the cheap and eco-friendly pure organic molecules are more
promising to be the next-generation OLED materials, and it is necessary
to explore the molecular design methods based on the structure-property
relationship of them, especially the high-performance blue-emissive
materials.
electro-fluorescent materials is that they can only utilize 25% of the total
electro-exciton (that is, the singlet electro-exciton) in OLED due to the
spin-statistics rule [16]. In recent years, some novel solutions have been
proposed to the above electro-triplet utilization problem in pure organic
electro-fluorescent materials, such as triplet-triplet annihilation (TTA)
[17–19], the thermally activated delayed fluorescence (TADF) [20–23]
and the hybridized local and charge-transfer (HLCT) state [24–26].
These materials can convert the electro-triplet excitons into the
spin-transition-allowed singlet excitons in different paths, and achieve
far better electroluminescence (EL) performance than the traditional
fluorescent materials. Among these mechanisms, despite that some
anthracene and pyrene derivatives have already been commercialized
for the replacement of phosphorescent materials, the upper limit of
exciton utilization efficiency (EUE) such TTA materials are only 62.5%,
which is obviously not an ideal solution [27]. Besides, although the
TADF OLEDs can harvest up to 100% electro-exciton through reverse
intersystem crossing (RISC) of T1→S1, they often suffer a serious effi-
ciency roll-off at high current density due to the inefficient RISC
There are two key issues to be addressed for the ideal next-generation
* Corresponding author.
** Corresponding author.
E-mail addresses: stzhang@jlu.edu.cn (S.-T. Zhang), yangbing@jlu.edu.cn (B. Yang).
https://doi.org/10.1016/j.dyepig.2021.109482
Received 11 March 2021; Received in revised form 11 May 2021; Accepted 11 May 2021
Available online 27 May 2021
0143-7208/© 2021 Elsevier Ltd. All rights reserved.

S. Xiao et al.
Dyes and Pigments 193 (2021) 109482
Scheme 1. Previous work and our work.
processes and accumulated of T1 excitons. An effective method to deal
with problem is to apply such TADF materials as an assistant host,
are far from the ideal one of <50 nm (Scheme 1). Therefore, it is
necessary to develop efficient deep-blue OLEDs based on PM derivatives
with reasonable molecular design.
¨
passing on energy to the emitters through a Forster resonance energy
transfer (FRET) process [28,29]. Therefore, the application of TADF
materials in OLED relies heavily on the doping technology, which in-
creases the cost and may hinder the successful commercialization to
some extent. The HLCT state is a linear combination of excited state with
In order to obtain efficient deep-blue emission materials with narrow
FWHMs, appropriate CT components in HLCT state should be finely
controlled in molecular design, together with a rigid donor unit to
suppress molecular vibrations for both increased PLQY and narrow
spectra. In this work, we synthesize two compounds: 2-2-(4-(9,9-dime-
thylacridin-10-(9H)-yl)phenyl)-1-phenyl-1H-phenanthro[9,10-d]im-
idazole (DPM) and 1-4-(tert-butyl)phenyl)-2-(4-(9,9-dimethylacridin-10
(9H)-yl)phenyl)-1H-phenanthro-[9,10-d]imi-dazole (TDPM), in which
the planar and rigid function group 9,9-dimethyl-9,10-dihydroacridine
(DMAC) serves as electron donor (D) and the PM moiety acts as elec-
tron acceptor (A). Compared with non-rigid donor (TPA), rigid donor
(DMAC) could efficient suppress the molecular vibrations and rotations,
which is beneficial to construct efficient deep-blue lighting materials.
Tertbutyl (ttBu) is also introduced as a chemical modification to inhibit
the aggregation caused quenching (ACQ) and improve emission color
relative to the counterpart. As expected, both two materials display
decent deep-blue emission with narrower FWHMs compared with TPA-
substituted PM derivatives. Non-doped OLEDs based on DPM and TDPM
share maximum external quantum efficiency (EQE) of 4.0% and 2.6%
with CIE coordinates of (0.157, 0.053) and (0.158, 0.045) and low ef-
ficiency roll-off of 35% and 12% at high brightness (820 cd mꢀ 2 for DPM
and 893 cd mꢀ 2 for TDPM), as well as narrow FWHMs of 52 nm and 49
nm, respectively.
a
certain proportion between locally-excited (LE) state and
charge-transfer (CT) excited state, corresponding to high photo-
luminescent (PL) efficiency and high EUE, respectively [30,31].
Different from TADF mechanism, HLCT materials can realize a
high-efficiency electroluminescence from the fast RISC process along
high-lying excited state channels (Tm→Sn), demonstrating as no delayed
component in the time-resolved PL measurement [32]. Meantime, the
fast RISC can effectively suppress the accumulation of electro-triplet
exciton and improve the stability of OLED at high voltage, which en-
ables the practical application owing to the easy processing of
non-doped OLED.
On the other hand, the color purity is also a very important index for
the blue-emissive materials, since the good color purity of emitters have
the optimal color-gamut and avoid unnecessary energy loss from the
additional light filter in full-color display. Although TADF materials
have successfully solved the spin-statics rule problem, the strong CT
excited states usually lead to poor color purity due to the broad PL
spectra [33,34]. More recently,
a new class of boron- and
nitrogen-containing heterocyclic TADF materials are reported with
decent color purity, as a result of a well suppressed vibronic coupling by
the so-called multi-resonance mechanism [35–37]. Nevertheless, their
synthesis, especially the scale-up synthesis are still a very difficult task.
Overall, HLCT state is still an effective strategy to realize blue-emission
with good color purity, since the linear combination of LE excited state
can reduce the spectral broadening of pure CT excited state to a certain
degree.
2. Experimental section
2.1. General information
1H NMR and 13C NMR were recorded on a Bruker AVANCZ 500
spectrometer at 500 MHz using deuterated dimethyl sulfoxide (DMSO)
as the solvent for 1H NMR and CDCl3 as the solvent for 13C NMR at 298
K. Tetramethylsilane (TMS) was used as internal standards. The MALDI-
TOF mass spectra were recorded by an AXIMA-CFRTM plus instrument.
The Flash EA 1112, CHNS–O elemental analysis instrument was chosen
to characterize these compounds. Diffraction experiments were carried
Phenanthroimidazole (PM) group have been widely used to
construct blue-emissive materials, because of its bipolar transport
capability, high photoluminescence quantum yields (PLQY) and good
thermal stability [39,40]. The earliest and the most representative works
on the PM-based HLCT blue-emissive materials are a series of highly
efficient triphenylamine (TPA)-substituted PM derivatives, such as
TPA-PPI, TBPMCN [41,42]. However, their planar D-A configuration
out on a Rigaku R-AXIS RAPID diffractometer equipped with a Mo-K
α
and control Software using the RAPID AUTO at 293 (±2) K. The dif-
ferential scanning calorimetry (DSC, DSC Q100) PerkinElmer thermal
analysis system was set at a heating rate of 10 ◦C minꢀ 1 and a nitrogen
flow rate of 50 mL minꢀ 1. Thermal gravimetric analysis (TGA) was
performed on a Perkin-Elmer thermal analysis system from 30 ◦C to
with large π-conjugated characteristics make it difficult for them to
achieve a deep-blue emission, and non-doped OLEDs based on them
both exhibit pure blue emission with CIEy > 0.1. Meanwhile, their full
width at half-maximum (FWHM) are still too large (about 75 nm), which
2

S. Xiao et al.
Dyes and Pigments 193 (2021) 109482
Scheme 2. Synthesis route of DPM and TDPM.
800 C with a heating rate of 10 C minꢀ 1. UV–vis absorption spectra
and Fluorescence spectra were recorded by the UV-3100 spectropho-
tometer and RF-5301PC, respectively. Corrected PLQY is obtained by
manual operation using an integrating sphere apparatus. Transient
photoluminance decay characteristics were measured using an Edin-
burgh Instruments F980 spectrometer. Cyclic voltammetry (CV) was
carried out using BAS 100B/W electrochemical analyzer with standard
one-compartment, three-electrode electrochemical cell. A glass-carbon
disk electrode was selected as a working electrode. Pt wire was per-
formed as a counter electrode. Ag/Ag+ was used as a reference electrode
with Ferrocene/ferrocenium (Fc/Fc+) redox couple was used as the in-
ternal standard.
2.3.2. TPM-Br
◦
◦
A mixture of 9,10-phenanthrenequinone (4.16 g, 20 mmol), 4-tert-
butylanilineaniline (12.80 mL, 80 mmol), 4-Bromobenzaldehyde
(3.70 g, 20 mmol) ammonium acetate (7.7 g, 100 mmol), acetic acid
◦
(50 mL) was heated to 120 C, and stirred for 4 h under nitrogen at-
mosphere. After cooling to room temperature, the mixture was filtered
to give a yellow solid and washed with a small amount of acetic acid and
water, then dried under vacuum. Crude products were purified by col-
umn chromategraphy using to obtain a white solid (7.42 g, yield =
74%). Eeluent: CH2Cl2. Crude products were used directly used for the
next reaction. MS:MW 504.06, m/z = 504.12. (M+).1H NMR (500 MHz,
DMSO) δ 8.93 (d, J = 8.2 Hz, 1H), 8.88 (d, J = 8.4 Hz, 1H), 8.69 (d, J =
7.9 Hz, 1H), 7.78 (t, J = 7.2 Hz, 1H), 7.74–7.66 (m, 3H), 7.63 (d, J = 8.4
Hz, 2H), 7.57 (dd, J = 12.0, 5.2 Hz, 3H), 7.51 (d, J = 8.6 Hz, 2H), 7.33 (t,
J = 7.7 Hz, 1H), 7.08 (d, J = 7.7 Hz, 1H), 1.42 (s, 9H).
2.2. Device fabrication and performances
ITO-coated glass was used as the substrate with a sheet resistance of
20 Ω squareꢀ 1. The ITO glass was cleaned by ultrasonic cleaner with
deionized water, isopropyl alcohol, acetone and chloroform. The evap-
oration rate was controlled to be 0.03–0.1 nm/s for organic layers, 0.01
nm sꢀ 1 for the LiF layer was and 0.3 nm/s for the Al layer. The PR650
spectra scan spectrometer was used to record EL spectrum and
luminance-current density-voltage was measured by spectrometer with
a Keithley model 2400 programmable voltage-current source. Evapo-
rated films were also produced under same conditions.
2.3.3. DPM
A mixture of PM-Br (1.48 g, 3.3 mmol), 9,9-dimethyl-9,10-dihydroa-
cridine (627 mg, 3.0 mmol), sodium tert-butoxide (t-BuONa; 2.24 g, 20
mmol), tri-tert-butylphosphine solution (P(t-Bu)3; 1.0 mL, 0.86 g), tris
(dibenzylideneacetone)dipalladium (Pd2(dba)3, (120 mg)) was dis-
solved in dry toluene (30 mL), and purged three times by nitrogen/
vacuum cycle. Then the reaction was heated to 110 ◦C and refluxed for
36 h. After cooling to room temperature, the mixture was washed with
20 mL water and extracted with CH2Cl2 for several times. The organic
phase was dried over sodium sulfate (Na2SO4). The products were pu-
rified by chromatography using the mixture of CH2Cl2: petroleum ether
1:2 as eluent to give a white solid. The products were further purified by
sublimation. (1.45 g, yield = 84%). MS:MW 577.22, m/z = 576.87.
(M+). 1H NMR (500 MHz, DMSO) δ 8.97 (d, J = 8.4 Hz, 1H), 8.92 (d, J =
8.4 Hz, 1H), 8.74 (d, J = 8.0 Hz, 1H), 7.88 (d, J = 8.4 Hz, 2H), 7.82 (dd,
J = 11.3, 3.7 Hz, 3H), 7.77–7.69 (m, 4H), 7.59 (t, J = 7.2 Hz, 1H), 7.50
(dd, J = 7.7, 1.4 Hz, 2H), 7.38 (t, J = 7.8 Hz, 3H), 7.15 (d, J = 7.7 Hz,
1H), 7.04–6.96 (m, 2H), 6.95–6.89 (m, 2H), 6.12 (dd, J = 8.2, 1.0 Hz,
2H), 1.62 (s, 6H). 13C NMR (126 MHz, CDCl3) δ 150.11 (s), 141.76 (s),
140.71 (s), 138.70 (s), 137.58 (s), 131.86 (s), 131.28 (s), 130.50–129.77
(m), 129.15 (s), 127.34 (d, J = 24.3 Hz), 126.40 (s), 125.84 (s), 125.21
(d, J = 15.7 Hz), 124.23 (s), 123.25 (s), 123.24–123.13 (m), 122.95 (d, J
= 25.4 Hz), 120.88 (d, J = 21.8 Hz), 120.77–120.49 (m), 114.14 (s),
77.21 (dd, J = 54.7, 22.8 Hz), 36.05 (d, J = 6.5 Hz), 31.22 (d, J = 7.4
Hz), 29.77 (s). Element Analysis: Calculate for C42H31N3: C, 87.32; H,
5.41; N,7.27; Found: C, 88.08; H, 5.26; N, 7.32.
2.3. Synthetic procedures
All reagents and solvents were purchased from commercial source
and used directly unless otherwise specified. All reactions were per-
formed under nitrogen atmosphere. Column chromatography was per-
formed using silica gel (200–300). Detail synthesis route is shown in
Scheme 2.
2.3.1. PM-Br
A mixture of 9,10-phenanthrenequinone (4.16 g, 20 mmol), aniline
(7.4 mL, 80 mmol), 4-Bromobenzaldehyde (3.70 g, 20 mmol) ammo-
nium acetate (7.7 g, 100 mmol), acetic acid (50 mL) was heated to
120 ◦C, and stirred for 4 h under nitrogen atmosphere. After cooling to
room temperature, the mixture was filtered to give a yellow solid and
washed with a small amount of acetic acid and water, then dried under
vacuum. Crude products were purified by column chromatography to
obtain a white solid. (7.71 g, yield = 86%). (eluent: dichloromethane
(CH2Cl2)). The products were not further purified and directly used for
1
the next reaction. MS:MW 448.06, m/z = 447.84 (M+). H NMR (500
2.3.4. TDPM
MHz, DMSO) δ 8.94 (d, J = 8.3 Hz, 1H), 8.89 (d, J = 8.4 Hz, 1H), 8.69
(dd, J = 7.9, 1.1 Hz, 1H), 7.78 (q, J = 6.9 Hz, 1H), 7.76–7.68 (m, 6H),
7.61–7.54 (m, 3H), 7.54–7.48 (m, 2H), 7.35 (dd, J = 11.3, 4.1 Hz, 1H),
7.09 (d, J = 7.5 Hz, 1H).
A mixture of TPM-Br (1.66 g, 3.3 mmol), 9,9-dimethyl-9,10-dihy-
droacridine (627 mg, 3.0 mmol), t-BuONa (2.24 g, 20 mmol), P(t-Bu)3
(1.0 mL, 0.86 g), Pd2(dba)3 (120 mg) was dissolved in dry toluene (30
mL), and purged three times by nitrogen/vacuum cycle. Then the re-
action was heated to 110 ◦C and refluxed for 36 h. After cooling to room
temperature, the mixture was washed with 20 mL water and extracted
3

S. Xiao et al.
Dyes and Pigments 193 (2021) 109482
Fig. 1. (a) Molecular geometries (b) The S1 NTOs of DPM and TDPM, respectively. f: oscillator strength.
with CH2Cl2 for several times. The organic phase was dried over Na2SO4.
The crude products were purified by chromatography using the mixture
of CH2Cl2: petroleum ether 3:4 as eluent to give a white solid. The
products were further purified by sublimation. (1.22 g, yield = 64%).
MS:MW 633.12, m/z = 633.13. (M+). 1H NMR (500 MHz, DMSO) δ 8.96
(d, J = 8.5 Hz, 1H), 8.91 (d, J = 8.4 Hz, 1H), 8.73 (d, J = 6.8 Hz, 1H),
Fig. 2. Photophysical properties of DPM and TDPM in different solutions. (a) Normalized UV absorption spectra. (b) Normalized PL spectra. (c) Lippert-Mataga
models (d) Normalized transient PL spectra. All measurements were performed in solutions (1*10ꢀ 5 M) at room temperature.
4

S. Xiao et al.
Dyes and Pigments 193 (2021) 109482
Fig. 3. Thermal properties. (a) Thermal gravimetric analysis (TGA), Td: decomposition temperatures (5% weight loss). (b) Differential scanning calorimetry (DSC),
Tg: glass transition temperatures.
7.86 (d, J = 8.4 Hz, 2H), 7.81 (t, J = 7.5 Hz, 1H), 7.76–7.67 (m, 5H),
7.59 (t, J = 7.7 Hz, 1H), 7.50 (dd, J = 7.6, 1.5 Hz, 2H), 7.37 (dd, J =
15.2, 7.9 Hz, 3H), 7.21 (d, J = 8.2 Hz, 1H), 7.02–6.87 (m, 4H), 6.10 (dd,
J = 8.1, 1.2 Hz, 2H), 1.62 (s, 6H), 1.41 (s, 9H). 13C NMR (126 MHz,
CDCl3) δ 153.55 (s), 150.34 (s), 141.62 (s), 140.74 (s), 137.48 (s),
135.88 (s), 131.88 (s), 131.18 (s), 130.69 (s), 130.14 (s), 129.42 (s),
128.50 (d, J = 19.9 Hz), 127.47 (s), 127.46–126.80 (m), 126.40 (s),
125.59 (d, J = 46.6 Hz), 125.20 (d, J = 25.9 Hz), 125.00–124.71 (m),
124.19 (s), 123.21 (d, J = 9.0 Hz), 123.01 (d, J = 39.8 Hz), 121.04 (s),
120.79 (s), 114.11 (s), 77.41 (s), 77.16 (s), 76.90 (s), 36.05 (s), 35.11 (s),
31.38 (d, J = 26.3 Hz), 31.21–30.89 (m). Element Analysis: Calculate for
(UV) absorption and PL spectra of these two materials were measured in
diluted solutions with different solvent polarities at room temperature.
Consistent with the theoretical results, they share very similar photo-
physical properties. As shown in Fig. 2a, DPM and TDPM display a
similar absorption behavior, and their strong absorption band around
260 nm originates from the
attached to the nitrogen of imidazole [10,47]. The medium absorption
band around 280–350 nm can be assigned to the * transition of the
π-π* transition of the isolated benzene ring
π-π
PM and TPM moieties. The weak absorption band at 365 nm can be
assigned to the intramolecular CT transition. Interestingly, the vibronic
structures disappear with the increase of solvent polarity, and the
obvious CT spectral characteristics (smooth and broadened PL, large
solvatochromic redshift) can be observed in high-polarity solvents such
as tetrahydrofuran (THF) and acetonitrile, while their PL spectra both
exhibit LE state character with vibronic structures in the low-polarity
solvent hexane (Fig. 2b). Lippert-Mataga model of these two materials
both demonstrate clear linear relation between Stokes shift and solvent
polarity factor f, reflecting a quasi-equivalent HLCT excited states from
both LE and CT excited states (Fig. 2c). Furthermore, the dipole moment
C
46H39N3: C, 87.17; H, 6.20; N,6.63; Found: C, 87.64; H, 6.17; N, 6.57.
3. Results and discussion
3.1. Theoretical calculations
To deeply understand the molecular design, density functional the-
ory (DFT) and time-dependent density functional theory (TD-DFT) at
M062X/6-31g (d, p) level were carried out to calculate electronic
structures using Gaussian 09 D.01 package [44–46]. As shown in Fig. S1,
the close energy level of the highest occupied molecular orbital (HOMO)
between DMAC and PM (or TPM) can avoid the formation of strong
intramolecular CT excited state, and maintain a high PLQY. The opti-
mized molecular geometries (Fig. 1a) show that the D-A twist angle of
the two molecules are both near 90◦. The side group (phenyl or 4-tert--
butylphenyl) and imidazole ring also display large dihedral angles of 71◦
and 73◦ for DPM and TDPM, respectively. This largely twisted structure
of DPM and TDPM can prevent the formation of excimer and exciplex
species, leading to the emission quenching and color-impurities. More-
over, the largely twisted D-A structures also bring about the separation
of HOMO and LUMO distributions (Fig. S2). The HOMO and LUMO are
mainly located on the DMAC and PM units, respectively. Furthermore,
we characterized their electrochemical properties by cyclic voltammetry
(CV) method (Fig. S3). According to the CV analysis, the HOMO energy
levels of DPM and TDPM are calculated to be ꢀ 5.39 eV and ꢀ 5.16 eV VS
ferrocene. The LUMO energy levels are calculated to be ꢀ 2.44 eV and
ꢀ 2.53 eV. Consistent with the theoretical results, it shows that the two
materials both have decent capability of carrier injection. The natural
transition orbitals (NTOs) of the S1 excited state also demonstrate their
CT-dominated excited state characters (Fig. 1b).
(
μe) of S1 excited state of these two materials can be estimated as 14.7 D
and 14.8 D, respectively, which are relative moderate values comparing
to those of traditional strong CT materials, indicating that the large twist
structure will not significantly affect the PLQY and color-purity. Lifetime
measurements also confirm the HLCT character of these two materials:
the nanosecond-scale single exponential decay and the gradually
extended lifetime with increasing solvent polarity (Fig. 2d). In addition,
the energy gap (ΔEST) between S1 and T1 were calculated to be 0.56 eV
and 0.59 eV according to their fluorescence emission peaks and phos-
phorescent emission peaks (Fig. S5). Therefore, TADF mechanism with
reverse intersystem crossing (RISC) process from T1 to S1 seems to be
impossible in these two materials. Besides, we also do not observe the
long lifetime from transient PL spectra of their evaporated film in air at
room temperature. (Fig. S6). In short, twisted and rigid structures,
appropriate CT components, and bulk t-Bu substitution make the two
materials display a deep-blue emission, narrow FWHM and high PLQY
[48–50].
3.3. Thermal stabilities
Thermal gravimetric analysis (TGA) and differential scanning calo-
rimetry (DSC) were performed to investigate the thermal stability of
DPM and TDPM under nitrogen atmosphere (Fig. 3). Both of them
demonstrate high thermal stability owing to the rigid structures. The
decomposition temperatures of these two materials (Td, 5% weight loss)
are 437 ◦C for DPM and 433 ◦C for TDPM, respectively. What is more,
they also exhibit glass transition temperatures (Tg) as high as 134 ◦C and
133 ◦C respectively, which are beyond the 120 ◦C criterion of OLED
industrial application [12].
3.2. Photophysical properties
From above theoretical calculations, tBu-substitution does not make
any difference on the electron transition essence of excited state in these
two materials. We then carried out photophysical characterizations on
them for the purpose of comparison. Firstly, both ultraviolet–visible
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S. Xiao et al.
Dyes and Pigments 193 (2021) 109482
Table 1
Non-doped OLED performances of DPM and TDPM.
Emitter
V1on (V)
λ2EL (nm)
CE3max (cd/A)
PEmax4 (lm/w)
CIE5 (x, y)
EQE6 (%)
FWHM7 (nm)
EUE8 (%)
DPM
3.1
3.1
428
424
1.77
0.99
1.39
0.95
(0.157,0.053)
(0.158,0.045)
4.0
2.6
52
49
61
65
TDPM
V1on: Opening voltage at the brightness of 1 cd mꢀ 2; λ2max: EL peak at the voltage of 6 V; CEm3 ax: Maximum current efficiency; PE4max: Maximum power efficiency. CIE5 (x,
y) at the voltage of 5 V. EQE6: Maximum external quantum efficiency; FWHM7: Full-width at half-maximum. EUE8: exciton utilization efficiency.
Fig. 4. OLED structures and key performances. (a) Energy level diagram of device structures. (b) (c) EL spectra at different voltages. (d) Current density-voltage-
luminance characteristics. (d) Current efficiency-luminance-power efficiency characteristics. (e) EQE versus luminance plots.
3.3.1. OLED performances
light out-coupling fraction, which is estimated as ~ 20% for the glass
substrates [51]. For DPM and TDPM, the EUEs are calculated to be 61%
and 65% respectively, which both break through the upper limit 25% of
spin statistics. The exceeding EUE of these two materials can be un-
derstood from their excited state energy levels and NTOs (Fig. 5). On one
hand, although the energy gaps (ΔEST) between S1 and T1 are too large
for conventional TADF mechanism, their S1 and T5 energy gaps are small
enough (0.01eV for DPM and 0.04eV for TDPM, respectively) to afford
efficient RISC. On the other hand, the NTOs of S1 and T5 are of similar
HLCT transition character, which is in accordance with the “hot exciton”
mechanism between high-lying excited states (Tm→Sn) [52].
Considering the decent deep-blue emission in evaporated films and
high thermal stability of these two materials, their non-doped bottom-
emitting OLEDs have been fabricated with the structure of the structure
of ITO/HATCN (5 nm)/TAPC (20 nm)/TCTA (10 nm)/emitting layer
(20 nm)/TPBI (40 nm)/LiF (1 nm)/Al (100 nm) (DPM) and ITO/HATCN
(5 nm)/TAPC (25 nm)/TCTA (10 nm)/emitting layer (20 nm)/TPBI (35
nm)/LiF (1 nm)/Al (100 nm) (TDPM). Benefiting from their HLCT
excited state, both these two materials achieve efficient and stable
electro-fluorescence emission in OLED (Fig. 4Table 1). Both EL of DPM
and TDPM exhibit stable deep-blue emissions of 428 nm and 424 nm,
corresponding to the CIE of (0.157, 0.053) and (0.158, 0.045) respec-
tively, which are in good agreement with their evaporated films.
(Fig. S6). It is noteworthy that their FWHMs were effectively narrowed
(~50 nm) compared with those of the PM-based HLCT blue-emissive
materials in the previous works, especially in terms of the CIE y-coor-
dinate. In addition, DPM and TDPM OLEDs show low turn-on voltages
of about 3.1 V, maximum EQEs of 4.0% and 2.6%, respectively. Espe-
cially, the efficiency roll-off of TDPM is only 13% at the brightness of
892 cd mꢀ 2, which indicates OLEDs based on TDPM display excellent
OLED stability. More importantly, based on the above OLED structures,
the thickness of emitter layers appears to have little effect on the EL
spectrum and efficiency as shown in Fig. S7 and Fig. S8.
The last but not the least, tBu substitution usually induces an increase
of PLQY and EQE in OLED devices. However, the PLQY of TDPM is only
20% comparing to the 33% of DPM in evaporated films (Fig. S6), which
directly brings down its OLED performance. We then prepared their
single crystals to clarify this issue by investigating their structural dif-
ferences and intermolecular interactions (Fig. 6). Similar to their opti-
mized geometries, both conformations of DPM and TDPM display large
D-A twist, as well as the dihedral angle between the side group (phenyl
or 4-tert-butylphenyl) and imidazole ring in single crystals. However,
their intermolecular interactions are quite different. The ttBu in TDPM
produces an extra C–H⋅⋅⋅
side group and the PM -plane of an adjacent TDPM molecule, which
also causes obvious long-range ordered stacking of PM moieties that
π interactions between the 4-tert-butylphenyl
π
Furthermore, the EUE of OLEDs were estimated according to the
equation as below:
π-π
can affect the PLQY (31% for DPM crystal and 19% for TDPM crystal). In
fact, the non-radiative transition rate (knr) of TDPM is 0.84 times that of
DPM crystal, while the radiative transition rate (kr) of TDPM is only
0.43 times that in DPM crystal, indicating that H-aggregation results in
the fluorescence quenching in a certain degree (Fig. S10, Table S3).
EQE = (γ × EUE × ΦPL) × ηout
where γ is the recombination efficiency of the injected electron and hole
(generally identified as 100%); ΦPL is the PLQY of the emitter; ηout is the
6

S. Xiao et al.
Dyes and Pigments 193 (2021) 109482
Fig. 5. Excited state energy diagram and NTOs of S1, T1 and T5 for DPM (a, b) and TDPM (c, d), respectively. f: oscillator strength.
Fig. 6. Crystal structures and the main intermolecular interactions of DPM (a) and TDPM (b), respectively. (Crystal DPM was obtained by slow volatilization of
dichloromethane and methanol with a volume of 2:1 (c < 10ꢀ 5 mol/L), and the single crystal of TDPM was prepared by sublimation at the temperature of 270–280 ◦C
under vacuum environment.).
More importantly, the ttBu induced extra C–H⋅⋅⋅
π
interactions can surely
and TDPM have been designed and synthesized for high-performance
deep-blue OLEDs. Benefiting from the twisted and rigid structures
with HLCT properties, non-doped OLEDs using DPM and TDPM as
emitters achieve the excellent deep-blue emissions peaking at 428 nm
and 424 nm, maximum EQEs of 4.0% and 2.6%, along with narrow
FWHMs of 52 nm and 49 nm, respectively. Although the introduction of
optimize the efficiency roll-off in TDPM, which implies that ttBu sub-
stitution can be a potential strategy for the new-generation electro-
fluorescent materials with nice color purity and stable
electroluminescence.
tBu leads a poorer PLQY and EQE, it can induce extra C–H⋅⋅⋅
teractions and ordered stacking, which favors the EL stability than
that of DPM. The tBu substitution is still a promising strategy for the
π in-
4. Conclusion
π-π
In summary, two phenanthroimidazole-acridine derivatives DPM
7

S. Xiao et al.
Dyes and Pigments 193 (2021) 109482
[15] Zhang S-T, Bai X, Li X, Bai Q, Wang M, Liu H, et al. Assistant acceptor induced
hybrid local and charge transfer blue-emissive electro-fluorescent materials based
on locally excited triphenylamine-phenanthroimidazole backbone. Organic
Electronics 2019;75:105404.
molecule design to realize the efficient and stable electro-fluorescent
materials with good color purity. Overall, our work is very helpful to
design efficient deep-blue OLED in the future.
[16] Segal M, Baldo MA, Holmes RJ, Forrest SR, Soos ZG. Excitonic singlet-triplet ratios
in molecular and polymeric organic materials. Phys Rev B 2003;68. 075211.
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in light emitting diodes based on poly[2-methoxy-5-(2′-ethyl- hexyloxy)-1,4-
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Author contribution statement
S. Xiao, Y. Gao, X. Yang performed the experiment and S. Xiao wrote
the original manuscript.
[18] Tang X, Bai Q, Shan T, Li J, Gao Y, Liu F, Liu H, Peng Q, Yang B, Li F, Lu P. Efficient
nondoped blue fluorescent organic light-emitting diodes (OLEDs) with a high
external quantum efficiency of 9.4% @ 1000 cd mꢀ 2 based on
H. Liu and W. L: Methodology, Formal analysis.
S. Zhang and B. Yang supervised the whole work and revised the
manuscript.
phenanthroimidazole-anthracene derivative. Adv Funct Mater 2018;28:1705813.
[19] Xing L, Zhu Z-L, He J, Qiu Z, Yang Z, Lin D, et al. Anthracene-based fluorescent
emitters toward superior-efficiency nondoped TTA-OLEDs with deep blue emission
and low efficiency roll-off. Chem Eng J 2020:127748. https://doi.org/10.1016/j.
cej.2020.127748.
Declaration of competing interest
[20] Uoyama H, Goushi K, Shizu K, Nomura H, Adachi C. Highly efficient organic light-
emitting diodes from delayed fluorescence. Nature 2012;492:234–8.
[21] Yang S-Y, Wang Y-K, Peng C-C, Wu Z-G, Yuan S, Yu Y-J, et al. Circularly polarized
thermally activated delayed fluorescence emitters in through-space charge transfer
on asymmetric spiro skeletons. J Am Chem Soc 2020;142. 17756–11765.
[22] Zhang D, Song X, Gillett AJ, Drummond BH, Jones STE, Li G, He H, Cai M,
Credgington D, Duan L. Efficient and stable deep-blue fluorescent organic light-
emitting diodes employing a sensitizer with fast triplet upconversion. Adv Mater
2020;32:1908355.
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Acknowledgements
[23] Liu H, Zeng J, Guo J, Nie H, Zhao Z, Tang BZ. High-performance non-doped OLEDs
with nearly 100 % exciton use and negligible efficiency roll-off. Angew Chem Int
Ed 2018;57:9290–4.
The authors thank the support from the National Natural Science
Foundation of China (51803071, 91833304 and 51603185), the Na-
tional Basic Research Program of China (2016YFB0401001), the Post-
doctoral Innovation Talent Support Project (BX20180121), the China
Postdoctoral Science Foundation (2018M641767), and JLUSTIRT
(2019TD-33).
[24] Chen L, Zhang S, Li H, Chen R, Jin L, Yuan K, Li H, Lu P, Yang B, Huang W.
Breaking the efficiency limit of fluorescent OLEDs by hybridized local and charge-
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A multifunctional blue-emitting material designed via tuning distribution of
hybridized excited-state for high-performance blue and host- sensitized OLEDs.
Adv Funct Mater 2020;30:2002323.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.dyepig.2021.109482.
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