Novel multifunctional fluorene-phenanthroimidazole hybrid materials: Non-doped near-ultraviolet fluorescent emitter and host for green phosphorescent OLEDs

Article abstract of DOI:10.1016/j.dyepig.2020.109019

Deep-blue-emissive host materials are crucial to achievement of high-performance full-color organic light-emitting diodes (OLEDs), simplifying the manufacturing process and reducing production costs. To develop efficient deep-blue-emissive host materials is still challenging. By incorporating 9,9-diphenylfluorene moiety into phenanthroimidazole via the insulating N1 position of imidazole, two fluorene-phenanthroimidazole hybrid near-ultraviolet-emitting materials FL-PI and FL-BPI were designed and synthesized. The photophysical property, thermal and amorphous ability, electrochemical property and electron structure could be effectively regulated by molecular design strategy. Both FL-PI and FL-BPI exhibited moderate near-ultraviolet emission (0.50/0.63), high triplet energy level (2.58/2.57eV), high thermostability (T_d: 371/542 °C; T_g: 134/210 °C) and carrier-transporting nature. Non-doped near-ultraviolet OLEDs adopting FL-BPI emitter showed the maximum efficiencies of 5.4%, 3.8 cd A^?1 and 2.8 lm W^?1 with the CIE coordinates of (0.15, 0.05). In addition, FL-PI/FL-BPI-hosted green phosphorescent OLEDs demonstrated the maximum efficiencies of 16.3%, 28.8 cd A^?1, 21.1 lm W^?1 and 21.1%, 75.3 cd A^?1, 27.8 lmW^?1, respectively. Moreover, the external quantum efficiencies of FL-PI/FL-BPI-hosted green phosphorescent devices remained as high as 16.1% and 21.0% at 1000 cd m^?2. This work demonstrates that the strategy for the development of near-ultraviolet high-emissive host materials is feasible.

Full text of DOI:10.1016/j.dyepig.2020.109019

Dyes and Pigments xxx (xxxx) xxx
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: http://www.elsevier.com/locate/dyepig
Novel multifunctional fluorene-phenanthroimidazole hybrid materials:
Non-doped near-ultraviolet fluorescent emitter and host for green
phosphorescent OLEDs
,
Hui-Min Guana b, Yong-Xu Hua, Dong-Dong Xiea, Hai-Jun Chia, Guo-Yong Xiaoa, Yan-Li Lva,
,
,
,***
Xiao Lia *, Dong-Yu Zhangc **, Zhi-Zhi Hua
a School of Chemical Engineering, University of Science and Technology Liaoning (USTL), Anshan, 114051, People’s Republic of China
b College of Chemistry, Chemical Engineering and Environment Engineering, Liaoning Shihua University, Fushun, 113001, People’s Republic of China
c Suzhou Institute of Nano-Tech and Nano-Bionics (SINANO), Chinese Academy of Sciences, Suzhou, 215123, People’s Republic of China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Deep-blue-emissive host materials are crucial to achievement of high-performance full-color organic light-
emitting diodes (OLEDs), simplifying the manufacturing process and reducing production costs. To develop
efficient deep-blue-emissive host materials is still challenging. By incorporating 9,9-diphenylfluorene moiety into
phenanthroimidazole via the insulating N1 position of imidazole, two fluorene-phenanthroimidazole hybrid
near-ultraviolet-emitting materials FL-PI and FL-BPI were designed and synthesized. The photophysical property,
thermal and amorphous ability, electrochemical property and electron structure could be effectively regulated by
molecular design strategy. Both FL-PI and FL-BPI exhibited moderate near-ultraviolet emission (0.50/0.63), high
triplet energy level (2.58/2.57eV), high thermostability (Td: 371/542 ◦C; Tg: 134/210 ◦C) and carrier-
transporting nature. Non-doped near-ultraviolet OLEDs adopting FL-BPI emitter showed the maximum effi-
ciencies of 5.4%, 3.8 cd Aꢀ 1 and 2.8 lm Wꢀ 1 with the CIE coordinates of (0.15, 0.05). In addition, FL-PI/FL-BPI-
hosted green phosphorescent OLEDs demonstrated the maximum efficiencies of 16.3%, 28.8 cd Aꢀ 1, 21.1 lm Wꢀ 1
and 21.1%, 75.3 cd Aꢀ 1, 27.8 lmWꢀ 1, respectively. Moreover, the external quantum efficiencies of FL-PI/FL-BPI-
hosted green phosphorescent devices remained as high as 16.1% and 21.0% at 1000 cd mꢀ 2. This work dem-
onstrates that the strategy for the development of near-ultraviolet high-emissive host materials is feasible.
Phenanthroimidazole
Fluorene
Near-ultraviolet emission
Host
OLEDs
1. Introduction
significant for achievement of high-performance full-color organic
light-emitting diodes (OLEDs) via energy transfer to emissive dopant,
The achievement of deep-blue-emissive organic optoelectronic ma-
terials integrated with strong emission, high triplet energy level (ET) and
suitable carrier-transporting ability remains a persistent challenge.
Deep-blue-emissive materials usually possess wide bandgap, which
affect carrier migration to some extent [1–3]. For blue bipolar fluo-
rophores, the interaction of intramolecular donor-acceptor unit typically
increases the conjugation and thus lowers the ET. Moreover, classical
materials with strong fluorescent emission always exhibit low carrier
mobility [4–6]. All those facts seem to be a contradiction for obtaining
such type of optoelectronic materials. However, deep-blue-emissive
materials with strong emission, high ET and carrier mobility are
simplifying the manufacturing process and reducing production costs.
Phenanthroimidazole scaffold, which possesses the incomparable
features with high PLQY (photoluminescence quantum yield) in solution
and film, both electron- and hole-transporting capability [7,8], excellent
thermolability [9], simple synthesis (one-pot method) [10,11] and facile
structure regulation, shows great potential as host and/or blue fluo-
rophore in OLEDs [10–17]. The emergence of such materials provides
the possibility to address above-mentioned problems. For example,
Wang’s group demonstrated phenanthroimidazole-based host materials
in succession: diarylborane-phenanthroimidazole hybrid p-BPPI and
m-BPPI bipolar host materials for high-performance red (EQE: 13.5%),
* Corresponding author.
** Corresponding author.
*** Corresponding author.
E-mail addresses: lixiao@ustl.edu.cn (X. Li), dyzhang2010@sinano.ac.cn (D.-Y. Zhang), huzhizhi@163.com (Z.-Z. Hu).
https://doi.org/10.1016/j.dyepig.2020.109019
Received 1 September 2020; Received in revised form 17 November 2020; Accepted 17 November 2020
Available online 23 November 2020
0143-7208/© 2020 Elsevier Ltd. All rights reserved.
Please cite this article as: Hui-Min Guan, Dyes and Pigments, https://doi.org/10.1016/j.dyepig.2020.109019

H.-M. Guan et al.
Dyes and Pigments xxx (xxxx) xxx
Fig. 1. Chemical structures of FL-PI and FL-BPI.
yellow (EQE: 14.5%) and green (EQE: 18.2%) electrophosphorescent
devices were prepared in 2014 [14]; efficient green phosphorescent
OLEDs (EQE: 26.6%, PE: 92.7 lm Wꢀ 1) were also elaborated with a
twisted phenanthroimidazole-based host BP-DPPI [15]. Tao and
coworker reported efficient blue emitters with phenan-
with Edinburgh FLS1000 Series of Fluorescence Spectrometers.
2.2. Synthesis of compounds
2.2.1. Synthesis of 9-phenyl-9H-fluoren-9-ol
troimidazole-π-indolizine system for application in OLEDs, which
Magnesium 0.264 g (11 mmol) was placed into the flask with dried
THF (10 mL) accompanied by trace iodine as initiator, and then bro-
mobenzene 1.57 g (10 mmol) dissolving in THF solution was dripped
into the flask. After dropping, the reaction solution was refluxed
continuously for 2 h. Subsequently, 9H-fluoren-9-one 1.802 g (10 mmol)
dissolving in THF solution was added into flask, and further stirred for
12 h. Hydrolysis, extraction, drying and column chromatography were
performed successively to obtain the target product of 2.1 g. Yellow-
white solid. Yield: 80%. FTIR (Neat, cmꢀ 1): 3054, 1696, 1658, 1440,
1290.1H NMR (CDCl3, 500 MHz) δ 7.65 (d, J = 7.3 Hz, 4H), 7.53–7.48
(m, 5H), 7.47 (d, J = 7.5 Hz, 2H), 7.29 (d, J = 7.3 Hz, 3H). HRMS (APCI):
Found: [M ꢀ H]- 257.0975; Molecular formula C19H14O requires [M ꢀ
H]- 257.0972.
showed the maximum EQE of over 6% at ultrahigh brightness and
deep-blue emission with the CIE of (0.151, 0.076) [18]. High perfor-
mance of the OLEDs was mainly attributed to the novel-designed blue
fluorophore with hybridized local and charge-transfer features. How-
ever, it is still rare and challenging to develop high-emissive phenan-
throimidazole-based blue fluorophore which acts simultaneously as host
of low-energy phosphorescent or TADF material (thermally activated
delayed fluorescence) in OLEDs [19–25]. Liu et al. reported the phe-
nanthroimidazole derivatives PPI-PPITPA and PPI-PPIPCz with small
singlet-triplet splitting features and dual carrier-transporting charac-
teristics, which were applied as emitters and hosts to realize highly
efficient non-doped deep-blue devices and RYG (red/yellow/green)
phosphorescent OLEDs [24]. Tong and Lee demonstrated an ideal blue
phenanthroimidazole-based host emitter coupled with new electron
acceptor [1,2,4] triazolo[1,5-a]pyridine (TP-PPI) for high-efficiency
single-emissive-layer (TP-PPI + PO-01) hybrid white OLEDs [25]. As a
matter of fact, efficient deep-blue host emitters especially
near-ultraviolet emitters are still greatly limited up to now [26–29].
In this study, two multifunctional phenanthroimidazole materials
integrated into 9,9-diphenyl-fluorene through non-conjunction N1 po-
sition of imidazole part (FL-PI and FL-BPI, showed in Fig. 1) were
designed and synthesized as near-ultraviolet emitter and host for
application in OLEDs. Here, 9,9-diphenyl-fluorene plays a bridge be-
tween two phenanthroimidazoles and fluorene unit has high PLQY/ET,
good carrier transporting ability and film-forming property [21,30]. The
2.3. Synthesis of 4-(9-phenyl-9H-fluoren-9-yl)aniline
Under nitrogen protection, 0.2583 g (1 mmol) 9-phenyl-9H-fluoren-
9-ol dissolving in chloroform solution was dipped into the flask with
methanesulfonic acid 0.065 mL (1 mmol), acetaniline 0.1352 g (1 mmol)
and chloroform solution, and then the reaction solution was refluxed for
3 h. Crude products were obtained by extraction and drying. Subse-
quently, 0.53 mL water and 0.53 mL HCl were drip into it and then
continued to react for 12 h at 140 ◦C. Finally, the intermediate was
neutralized by sodium hydroxide to acquire the target product of 0.26 g.
Light white solid. Yield: 77%. FTIR (Neat, cmꢀ 1): 3441, 3060, 2350,
1450, 1250.1H NMR (CD2Cl2, 500 MHz) δ 7.57 (d, J = 7.5 Hz, 2H),
7.50–7.44 (m, 2H), 7.20 (t, J = 7.4 Hz, 2H), 7.07 (d, J = 11.8 Hz, 6H),
6.99–6.92 (m, 2H), 6.77 (dd, J = 14.9, 7.2 Hz, 3H), 5.24 (s, 2H). HRMS
(APCI): Found: [M+H]+ 334.1591; Molecular formula C25H19N requires
[M+H]+ 334.1590.
purpose
of
the
material
design
is
to
guarantee
deep-blue/near-ultraviolet emission, mild PLQY, high ET and efficient
carrier injection and transport. As it turned out, the FL-BPI-based
non-doped OLEDs illustrated the outstanding performances (5.4%, 3.8
cd Aꢀ 1 and 2.8 lm Wꢀ 1) with the CIE1931 coordinates of (0.15, 0.05).
Furthermore, highly efficient green phosphorescent OLEDs utilizing
FL-PI (16.3%, 28.8 cd Aꢀ 1) and FL-BPI (21.1%, 75.3 cd Aꢀ 1) as hosts
were expounded, respectively.
2.4. 4,4’-(9H-fluorene-9,9-diyl)dianiline
Under nitrogen protection, 0.1802 g (1 mmol) 9H-fluoren-9-one,
0.06 mL (1 mmol) methanesulfonic acid and 0.2703 g (2 mmol) aceta-
niline were placed into the flask and reacted for 24 h at 160 ◦C.
Following, HCl aqueous solution was added and continued to react for
12 h. The intermediate was neutralized with saturated NaOH to get the
target product of 0.17 g. Grey-green solid. Yield: 50%. FTIR (Neat,
cmꢀ 1): 3323, 3024, 2362, 1611, 1506, 1437, 1250.1H NMR (DMSO‑d6,
500 MHz) δ 7.85 (d, J = 7.5 Hz, 2H), 7.36–7.30 (m, 4H), 7.26 (t, J = 7.4
Hz, 2H), 6.82–6.71 (m, 4H), 6.48–6.32 (m, 4H), 4.94 (s, 4H). HRMS
(APCI): Found: [M+H]+ 349.1707; Molecular formula C25H20N2
2. Experimental section
2.1. General information
The raw materials and reagents involved in this work can be pur-
chased conveniently via the Reagent Company. Testing, characteriza-
tion and theoretical calculation of FL-PI and FL-BPI are consistent with
our recent publication [10]. In addition, the decay lifetime is obtained
2

H.-M. Guan et al.
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Scheme 1. Synthetic routes of FL-PI/FL-BPI.
requires [M+H]+ 349.1699.
2.6. Synthesis of FL-BPI
2.5. Synthesis of FL-PI
The synthesized procedure was similar to that of FL-PI. Yellowish
solid of 0.56 g. Yield: 62%. Mp: 348 ◦C. FTIR (Neat, cmꢀ 1): 3050, 2358,
1506, 1457, 1378.1H NMR (CDCl3, 500 MHz) δ 8.78 (d, J = 8.4 Hz, 2H),
8.71 (d, J = 8.4 Hz, 2H), 7.93 (d, J = 7.5 Hz, 2H), 7.76 (t, J = 7.5 Hz,
2H), 7.68 (t, J = 7.7 Hz, 2H), 7.62–7.55 (m, 8H), 7.53 (d, J = 7.3 Hz,
8H), 7.47 (d, J = 8.0 Hz, 6H), 7.34 (d, J = 7.3 Hz, 2H), 7.30 (s, 2H), 7.27
(d, J = 5.0 Hz, 2H), 7.20 (d, J = 7.4 Hz, 2H), 7.15 (d, J = 8.3 Hz, 2H)⋅13C
NMR (CDCl3, 126 MHz) δ 149.58, 139.71, 128.99 (d, J = 13.4 Hz),
128.63, 127.70 (d, J = 25.1 Hz), 126.86, 125.64, 125.27, 123.56,
122.45, 120.17. Elemental Analysis: Anal. Cacld. for C67H42N4: C,
89.11%; H, 4.69%; N, 6.20%. Found: C, 89.02%; H, 4.61%; N, 6.31%.
HRMS (APCI): Found: [M+H]+ 903.3487; Molecular formula C67H42N4
requires [M+H]+ 903.3482.
In a three-neck flask, 0.208 g (1 mmol) phenanthrene-9,10-dione, 1
mL (1 mmol) benzaldehyde, 0.33 g (1 mmol) 4-(9-phenyl-9H-fluoren-9-
yl)aniline, 18.87 g (24.5 mmol) ammonium acetate and glacial acetic
acid (10 mL) were added in turn and then were heated to reflux for 24 h
under the atmosphere of nitrogen. The flask was cooled naturally, and
the black-yellow mixture was then poured into methanol. The raw solid
obtained via suction was further purified by column chromatography.
Grey solid of 0.24 g. Yield: 40%. Mp: 241 ◦C. FTIR (Neat, cmꢀ 1): 3060,
2358, 1509, 1477, 1437, 1378.1H NMR (CDCl3, 500 MHz) δ 8.88 (d, J =
8.0 Hz, 1H), 8.77 (d, J = 8.4 Hz, 1H), 8.71 (d, J = 8.4 Hz, 1H), 7.85 (d, J
= 7.6 Hz, 2H), 7.75 (t, J = 7.6 Hz, 1H), 7.66 (t, J = 7.9 Hz, 1H), 7.58 (d,
J = 7.6 Hz, 2H), 7.55–7.39 (m, 8H), 7.37 (d, J = 9.3 Hz, 4H), 7.32–7.25
(m, 7H), 7.22 (d, J = 7.3 Hz, 1H), 7.18 (d, J = 8.3 Hz, 1H)⋅13C NMR
(CDCl3, 126 MHz) δ 150.06, 147.67, 144.51, 139.63, 136.58, 129.17,
128.80, 128.64, 128.25, 128.19, 127.87, 127.62, 127.49, 127.36,
127.29 (d, J = 2.7 Hz), 126.64, 126.39, 125.53, 125.46, 124.96, 124.20,
123.45, 122.44, 122.14, 120.26, 119.80, 64.77. Elemental Analysis:
Anal. Cacld. for C46H30N2: C, 90.46%; H, 4.95%; N, 4.59%. Found: C,
90.32%; H, 4.89%; N, 4.63%. HRMS (APCI): Found: [M+H]+ 611.2510;
Molecular formula C46H30N2 requires [M+H]+ 611.2482.
2.7. OLED fabrication and measurements
Pre-patterned ITO glass with the resistance of 15 Ω cm ꢀ 2 are nor-
mally rinsed by organic solvents and deionized water. The substrate is
dried under following nitrogen and exposed to ultraviolet light for 15
min. The ITO glass, functional-layer materials including Liq and Al are
immediately transferred into the deposition chamber which is then kept
in the pressure of more than 6 × 10ꢀ 4 Pa for vacuum deposition.
Current-luminance/current-voltage characteristics are performed by a
Keithley 2400 source meter. Electroluminescence (EL) spectra were
achieved with PR650 spectrometer. All devices are not encapsulated and
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Fig. 2. TGA curves of FL-PI and FL-BPI. Inset: the corresponding DSC curves.
Fig. 3. The UV–Vis absorption and PL spectra of FL-PI and FL-BPI in dilute CH2Cl2 solution, neat film at room temperature and in 2-Me-THF at 77 K.
the tests are implemented under an ambient atmosphere.
3. Results and discussion
9,9-diyl)dianiline were synthesized from industrial-grade fluorenone by
electrophilic substitution in advance, respectively. Then, the target
products were prepared by one-pot Debus reaction with isolated yield of
40–62%. All intermediates and target compounds were adequately
confirmed by high-resolution mass spectrometry (HRMS) and NMR
spectrometry.
3.1. Synthesis and characterization
The molecular structures and synthetic scheme of FL-PI/FL-BPI are
shown in Fig. 1 and Scheme 1. Two key fluorenyl-arylamine in-
termediates 4-(9-phenyl-9H-fluoren-9-yl)aniline and 4,4’-(9H-fluorene-
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Fig. 4. The transient PL spectra of FL-PI and FL-BPI films at room temperature.
3.2. Thermal properties
Thermal properties of FL-PI and FL-BPI were investigated by the
thermogravimetric analysis (TGA) and differential scanning calorimetry
(DSC) tests at a heating rate of 10 ◦C/min under N2 atmosphere, as
shown in Fig. 2 and Fig. S1. The principal thermal parameters including
the thermal decomposition temperature (Td, weight loss of 5%), glass
transition temperature (Tg) and melting point (Tm) of FL-PI and FL-BPI
were systematically studied. Both FL-PI and FL-BPI displayed excellent
thermal properties. As the molecular weight increased, the corre-
sponding Td (370–542 ◦C) and Tg (134–210 ◦C) of FL-PI and FL-BPI
gradually ascended. As inset of Fig. 2, no phase transition or crystalli-
zation was observed for FL-PI/FL-BPI upon heating beyond Tg. High Tg is
conducive to the formation of amorphous film to prevent phase sepa-
ration. Moreover, the bulky and twisted configuration from the link
through SP3–C9/N1 of fluorene and imidazole could prevent dense
molecular packing, which was also favorable to the formation of
amorphous film. Such excellent thermal property is desirable for the
stability and lifetime of OLEDs.
In addition, conventional glass molecular normally abides by the
Boyer-Kauzmann rule: (g = Tg +273)/(Tm +273), and the “g” value is
usually ca. 0.7 [31]. The unexpectedly high “g” values of FL-PI (0.79)
and FL-BPI (0.78) illustrated strong intermolecular interaction among
Fig. 5. CV curves of FL-PI and FL-BPI in anhydrous CH2Cl2 solution with 0.1 M
TBAPF6 as supporting electrolyte.
Table 1
Photophysical, thermal and electrochemical properties of FL-PI and FL-BPI.
e
f
Compound
λaabs (nm)
λbem (nm)
Φc
τ
(ns)
ETd(eV)
Td/Tg/Tm (◦C)
HOMOf (eV)
LUMOf (eV)
Eg (eV)
FL-PI
261, 308, 361
261, 309, 361
398/403
396/406
0.50/0.38/0.55
0.63/0.52/0.61
1.2
1.8
2.58
2.57
371/134/241
542/210/348
5.58
5.57
2.35
2.34
3.23
3.23
FL-BPI
a
Measured in CH2Cl2 solution with 1 × 10ꢀ 5 mol Lꢀ 1
.
b
c
Fluorescence emission measured in CH2Cl2 solution and neat film, respectively.
The quantum yields measured in CH2Cl2 using 9,10-diphenylanthrecene (Φ = 0.90) as the standard, in neat film and 10% Ir(ppy)3 doped in FL-PI/FL-BPI film,
respectively.
d
Triplet energy level measured in 2-MeTHF at 77K, ET = 1240.8/λPh
.
e
f
Decomposition temperature of 5% weight loss, glass transition temperature and melting point, respectively.
EHOMO (eV) = -e(Eox
+ 4.4), Eg = 1240.8/λ, ELUMO (eV) = Eg + EHOMO.
onset
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Fig. 6. The optimized structures and the frontier molecular orbitals of FL-PI.
Fig. 7. (a) Device configuration; (b) Device energy-level diagram; (c) Molecular structures of the involved materials in OLEDs.
their glass state [32].
absorption spectrometry. As shown in Fig. 3, both FL-PI and FL-BPI
exhibited almost identical emission and absorption in CH2Cl2, which
was also consistent with those of 1,2-diphenyl-1H-phenanthro[9,10-d]
imidazole (shown in Fig. S2). The strong absorption bands of ca. 260 nm
3.3. Photophysical properties
were attributed to π-π* transition of N1-benzol on imidazole [33], while
The photophysical properties of FL-PI and FL-BPI were studied with
the relatively weak absorption bands of ca. 360 nm mainly resulted from
photoluminescence (PL) emission and ultraviolet–visible (UV–Vis)
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respectively, which was high enough to trigger green phosphorescent
dyes.
Table 2
Summary of EL properties of non-doped devices with FL-BPI emitter.
The transient PL spectra of FL-PI and FL-BPI films at room temper-
ature were surveyed in Fig. 4. Both FL-PI and FL-BPI exhibited the
monoexponentially decay with the lifetimes of 1.2 and 1.8 ns, respec-
tively, demonstrating that the emissions were solely derived from the
radiation decay of the singlet excited state.
Emitter
V
Lmax (cd
EQE
CE (cd
PE (lm
CIE (x,
b
c
c
(V)a
mꢀ 2
)
(%)c
Aꢀ 1
)
Wꢀ 1
)
y)d
FL-BPI
a
4.0
10,252
5.4/
3.8/2.9/
1.8
2.8/1.7/
0.7
(0.15,
0.05)
5.2/4.4
Voltage at 10 cd mꢀ 2
.
b
c
Maximum luminance.
3.4. Electrochemical properties
Order of measured efficiency values: maximum, then values at 100/1000 cd
mꢀ 2for device.
d
Commission International de I’Eclairage (CIE) 1931 coordinate measured at
The redox characteristics of FL-PI and FL-BPI were examined by the
cyclic voltammetry (CV) measurement in Fig. 5 and the corresponding
electrochemical data were generalized in Table 1. Both FL-PI and FL-BPI
displayed reversible oxidation potential. The first onset oxidation po-
tentials (Eoxonset) of FL-PI and FL-BPI were observed to 1.18 and 1.17 V,
respectively, and then the HOMO levels of 5.58 and 5.57 eV were figured
7 V.
π-π
* transition from the substituted benzol of C2-position on imidazole
and phenanthroimidazole. For emission spectra, FL-PI and FL-BPI
demonstrated similar and intense violet-blue emission peaking at ca.
400 nm with high PLQYs of 0.50 for FL-PI and 0.63 for FL-BPI in CH2Cl2
solution. Both emissions in film were slightly red-shifted and the cor-
responding PLQYs of FL-PI (0.38) and FL-BPI (0.52) were still high. High
PLQYs have also been acquired to be 0.55 and 0.61 of 10% Ir(ppy)3--
doped FL-PI and FL-BPI film, respectively. The interrupted conjunction
resulting from molecular modification on N1 position of imidazole of
out by the formula EHOMO (eV) = -e(Eox
+ 4.4). The LUMO levels
could be determined to be 2.35 and 2.34oenVsetvia the difference between
the optical absorption bandgap and the HOMO levels. The almost
identical HOMO/LUMO energy levels of FL-PI and FL-BPI suggested that
the introduction of another phenanthroimidazole unit on the base of FL-
PI had virtually no impact on the orbitals of origin core, which was
further validated by the subsequent DFT (Density Functional Theory)
calculations.
FL-PI and FL-BPI enabled that
π-electron transfer from one unit to
another unit in one molecular system became difficult and then caused
violet-blue emission and broad spectral band.
The absorption bandgaps of both FL-PI and FL-BPI were 3.23 eV from
the onset energy of the UV–Vis absorption spectra. The PL spectra from
frozen 2-Me-THF solution at 77 K showed phosphorescence emission at
481 and 482 nm corresponding to the ET of 2.57 and 2.58 eV,
3.5. Theoretical calculations
Aiming at further understanding the relationship between property
and structure of FL-PI and FL-BPI at the molecular level, the Gaussian 16
Fig. 8. Characterizations of non-doped FL-BPI-based OLED: (a) EL spectra at 9 V; (b) J-V-L (Current-Voltage-Luminance) characteristic curves; (c) Current/Power
Efficiency-Luminance characteristic curves; (d) EQE-Luminance curves.
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Fig. 9. Characterizations of green phosphorescent OLEDs with FL-PI/FL-BPI as host: (a) EL spectra at 8 V; (b) J-V-L characteristic curves; (c) Current/Power
Efficiency-Luminance characteristic curves; (d) EQE-Luminance curves. (For interpretation of the references to color in this figure legend, the reader is referred to the
Web version of this article.)
(2 nm)/Al (100 nm), in which FL-BPI was screened as emitter, were
Table 3
firstly fabricated and investigated. TPBi (1,3,5-Tri(1-phenyl-1H-benzo
Summary of EL properties of Ir(ppy)3-doped OLEDs with FL-PI and FL-BPI as
[d]imidazol-2-yl)phenyl) functioned as the hole-blocking/electron-
host.
transporting layer, and TAPC (4,4’-(cyclohexane-1,1-diyl)bis(N,N-di-p-
Device
GPD-1
V
Lmax (cd
EQE (%)c
CE (cd
PE (lm
CIE (x,
y)d
tolylaniline)) acted as the hole-transporting layer. The OLEDs structure,
energy-level diagram and chemical structures of involved materials are
presented in Fig. 7, and the corresponding key EL parameters are also
listed in Table 2.
b
c
c
(V)a
mꢀ 2
)
Aꢀ 1
)
Wꢀ 1
)
3.8
30,330
16.3/
16.1/
14.8
28.8/
28.4/
26.4
21.1/
(0.29,
0.60)
14.4/11.2
As shown in Fig. 8, the non-doped device with FL-BPI emitter emitted
the near-ultraviolet light with the peaked emission of 405 nm. The ac-
quired CIE1931 coordinates of (0.15, 0.05) at 9 V matched well with the
deep-blue standard (CIEy < 0.06) of European Broadcasting Union
(EBU) [37–39]. And there was almost no change in color purity with the
variation of voltages (Supplementary Information Fig. S3). It is known to
all that difficulty of carrier injection universally roots in OLEDs with
wide-bandgap emitter to thus result in the high turn-on voltage. Here,
the voltage of the non-doped OLEDs was down to 4.0 V at 10 cd mꢀ 2 and
low operating voltage was obtained to be 5.5 V at 100 cd mꢀ 2, which
was ascribed to the negligible energy barrier (HOMO) of 0.07 eV be-
tween TAPC and FL-BPI. The non-doped OLEDs exhibited the
outstanding EL performances with the maximum efficiencies of 5.4%
(EQE), 3.8 cd Aꢀ 1 (current efficiency) and the maximum luminance of
10,252 cd mꢀ 2 at 12 V. Moreover, the FL-BPI-based non-doped device
GPD-2
6.2
43,620
21.1/
21.0/
19.0
75.3/
74.8/
69.0
27.8/
(0.30,
0.63)
27.3/20.8
a
b
c
Voltage at 10 cd mꢀ 2
.
Maximum luminance.
Order of measured efficiency values: maximum, then values at 1000/5000
cd mꢀ 2for device.
d
CIE1931 coordinate measured at 9 V.
software at B3LYP/6-31G (d, p) level was adopted to perform DFT cal-
culations. The spatial distributions of HOMO/LUMO for FL-PI and FL-
BPI were plotted in Fig. 6. For FL-PI and FL-BPI, their electron den-
sities of HOMO/LUMO levels were almost localized on one phenan-
throimidazole moiety and C2-linked benzene, and the distributions of
their LUMO levels were further spread over the N1-linked benzene. The
adequate overlap between the HOMO and LUMO for FL-PI and FL-BPI
would be conducive to highly efficient luminescence [34–36]. No any
electron distributions (HOMOs and LUMOs) on fluorene and another
phenanthroimidazole moiety for FL-PI and FL-BPI were detected, which
endowed them with the nearly uniform HOMO/LUMO energy levels
either for theoretical or experimental values.
still achieved high EQEs of 5.2% and 4.4% at 100 and 1000 cd mꢀ 2
,
illustrating that the device had slow efficiency roll-off with the elevated
luminance. Compared with the recently reported performance of the
non-doped near-ultraviolet OLEDs (CIEy < 0.06, Supplementary Infor-
mation Table S1), the current device performance had also reached the
state-of-the-art level.
Furthermore, the single-carrier devices based on FL-BPI with the
structure: ITO/TAPC (10 nm)/FL-BPI (60 nm)/TAPC (10 nm)/Al (100
nm) (hole-only device) and ITO/TPBi (10 nm)/FL-BPI (60 nm)/TPBi (10
To validate the promising EL performance of FL-PI and FL-BPI as
near-ultraviolet light-emitting materials, non-doped OLEDs following
the structure of ITO/TAPC (30 nm)/emitter (100 nm)/TPBi (50 nm)/Liq
8

H.-M. Guan et al.
Dyes and Pigments xxx (xxxx) xxx
nm)/LiF (1 nm)/Al (100 nm) (electron-only device) were fabricated to
investigate its carrier-transporting characteristics. As indicated in
Fig. S4, both current densities of the hole-only device and electron-only
device increased with the voltage, which meant ambipolar feature of FL-
BPI.
4. Conclusions
In summary, two new near-ultraviolet emitting materials FL-PI and
FL-BPI were designed and synthesized by the twisted fluorene-
phenanthroimidazole hybrid molecular strategy. Influence of 9,9-diphe-
nylfluorene moiety on the photophysical characteristics of FL-PI and FL-
BPI molecules was thoroughly investigated. Both FL-PI and FL-BPI
showed excellent thermal stability, high PLQY and sufficiently high
ET. FL-PI and FL-BPI functioned as both near-ultraviolet fluorescent
material to fabricate non-doped OLEDs and green phosphorescent
sensitized materials to construct high-efficiency phosphorescent de-
vices. More importantly, the devices with FL-BPI as pure emitter
demonstrated near-ultraviolet emission with the CIE1931 coordinates of
(0.15, 0.05) and the maximum EQE of 5.4%. Furthermore, green phos-
phorescent OLEDs using FL-PI and FL-BPI as the hosts realized the
maximum EQEs of 16.3% and 21.1% with negligible efficiency roll-off.
Those remarkable results can be clearly attributed to 9,9-diphenylfluor-
ene unit, which endowed FL-PI/FL-BPI with promising photophysical
properties. This work will be a great asset for the development of near-
ultraviolet high-emissive materials.
Considering the high ET of FL-PI and FL-BPI, their potential appli-
cations as hosts which sensitized green phosphorescent material were
further investigated in OLEDs following the uniform structure: ITO/
TAPC (40 nm)/Host: Ir(ppy)3 (30 nm)/TPBi (30 nm)/Liq (2 nm)/Al
(100 nm). Among them, the optimized 10%-doped Ir(ppy)3 in FL-PI
(marked as GPD-1) or FL-BPI (marked as GPD-2) worked as emissive
layer. The corresponding phosphorescent device performances are out-
lined in Fig. 9 and Table 3.
The pure electrophosphorescent spectra of Ir(ppy)3 were observed in
Fig. 9(a), suggesting that forward energy transfer from FL-PI/FL-BPI to Ir
(ppy)3 occurred without a hitch in GPD-1 and GPD-2 and there was no
exciton leakage. Combining with PL spectra of FL-PI/FL-BPI and ab-
sorption spectra of Ir(ppy)3 in CH2Cl2 (Supplementary Information
Fig. S5), sufficient overlaps between them further confirmed exothermic
energy transfer from host FL-PI/FL-BPI to guest Ir(ppy)3.
GPD-1 and GPD-2 demonstrated the promising EL performances. For
GPD-1, the peak efficiencies (16.3%, 28.8 cd Aꢀ 1, 21.1 lm Wꢀ 1), and the
maximum brightness of 30,330 cd mꢀ 2 at 11 V were presented. In
comparison, GPD-2 exhibited the peak efficiencies of 21.1%, 75.3 cd
Aꢀ 1, 27.8 lm Wꢀ 1 and the maximum brightness of 43,620 cd mꢀ 2 at 13.5
V. Moreover, these values are superior to those of the mostly reported
devices based on blue-emitter host [21–24,40–43].
CRediT authorship contribution statement
Hui-Min Guan: designed and synthesized the materials, and Xiao Li
wrote the manuscript, with input from all authors. Yong-Xu Hu: and.
Dong-Dong Xie: participated in synthesis and CV, TGA and DSC ex-
periments. Hai-Jun Chi: performed HRMS and NMR measurements.
Guo-Yong Xiao: performed the photophysical measurements. Yan-Li
Lv: performed the quantum chemical calculations. . Xiao Li: optimized
the, and. Dong-Yu Zhang: and, and. Zhi-Zhi Hu: initiated, design,
Supervision.
Negligible efficiency roll-off at higher luminance was achieved
simultaneously in these devices. For example, the EQEs of GPD-1 and
GPD-2 remained as high as 16.1% and 21.0% at 1000 cd mꢀ 2. Notably,
FL-BPI-hosted green phosphorescent OLEDs (GPD-2) demonstrated the
higher EL performance than that of FL-PI-hosted device (GPD-1),
although they had quite similar frontier orbital energy levels and ET. The
possible explanation for this was that for FL-BPI host material except for
better thermal stability and amorphous characteristics, its higher
Declaration of competing interest
The authors declared that they have no conflicts of interest to this
work. We declare that we do not have any commercial or associative
interest that represents a conflict of interest in connection with the work
submitted.
distortion geometry could decrease
high ET value and hinder the extinction of excitons by reducing inter-
molecular interaction [44,45].
π-conjugation degree to maintain
π-π
All those results expound that the strategy of molecular design for
realizing multifunctional optoelectronic materials which are applied as
both emitter and host is doable. The EL performances of devices in which
FL-PI/FL-BPI act as emitter and host are bound to be further boosted by
means of the optimized device structure. The EL mechanism of non-
doped OLEDs with FL-PI/FL-BPI emitter need to be clarified. All those
corresponding works are ongoing.
Acknowledgements
This research was funded through Liaoning Provincial Natural Sci-
ence Foundation (Grant No. 2020-MS-282) and University of Science
and Technology Liaoning Talent Project Grants (201603).
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.org/10.1016/j.dyepig.2020.109019.
9

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