Bulky 9-phenylfluorene fuctionalized 2,6-bis(N-carbazolyl)- pyridine with high triplet energy level as host for blue thermally activated delayed fluorescence devices

Article abstract of DOI:10.1016/j.dyepig.2019.108127

A new 9-phenylfluorene functionalized 2,6-bis(N-carbazolyl)-pyridine compound 2,6-bis(3-(9-phenyl-fluoren- 9-yl)-carbazol-9-yl)pyridine (2,6-PFCzPy) with large steric hindrance and high triplet energy level has been successfully prepared by introducing 3-

Full text of DOI:10.1016/j.dyepig.2019.108127

Journal Pre-proof
Bulky 9-phenylfluorene fuctionalized 2,6-bis(N-carbazolyl)- pyridine with high triplet
energy level as host for blue thermally activated delayed fluorescence devices
Xiang-Hua Zhao, Chun-Miao Han, Yue Li, Ming-Gui Bai, Jin-Can Yang, Hui Xu, Shun-
Dong Yuan, Ling-Hai Xie, Zhi-Jie Xu
PII:
S0143-7208(19)32217-X
DOI:
https://doi.org/10.1016/j.dyepig.2019.108127
DYPI 108127
Reference:
To appear in: Dyes and Pigments
Received Date: 18 September 2019
Revised Date: 29 November 2019
Accepted Date: 10 December 2019
Please cite this article as: Zhao X-H, Han C-M, Li Y, Bai M-G, Yang J-C, Xu H, Yuan S-D, Xie L-H, Xu
Z-J, Bulky 9-phenylfluorene fuctionalized 2,6-bis(N-carbazolyl)- pyridine with high triplet energy level as
host for blue thermally activated delayed fluorescence devices, Dyes and Pigments (2020), doi: https://
doi.org/10.1016/j.dyepig.2019.108127.
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Bulky 9-phenylfluorene fuctionalized 2,6-bis(N-carbazolyl)-
pyridine with high triplet energy level as host for blue
thermally activated delayed fluorescence devices
a
b
a
a
Xiang-Hua Zhao* , Chun-Miao Han , Yue Li , Ming-Gui Bai , Jin-Can
a
b
c
d
c
Yang , Hui Xu* , Shun-Dong Yuan , Ling-Hai Xie , Zhi-Jie Xu*
The first 2,6-bis(N-carbazolyl)-pyridine-based compound with
large steric hindrance was employed as host for blue TADF
OLEDs with stable EL spectrum under different driving voltages
at doping concentration as high as 30%.

Bulky 9-phenylfluorene fuctionalized 2,6-bis(N-carbazolyl)-
pyridine with high triplet energy level as host for blue
thermally activated delayed fluorescence devices
a
b
a
a
Xiang-Hua Zhao* , Chun-Miao Han , Yue Li , Ming-Gui Bai , Jin-Can
a
b
c
d
c
Yang , Hui Xu* , Shun-Dong Yuan , Ling-Hai Xie , Zhi-Jie Xu*
a
College of Chemistry and Chemical Engineering, Xinyang Normal University,
Xinyang 464000, Henan, China.
b
Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education,
Heilongjiang University, Harbin, Heilongjiang 150080, China
c
College of Science, China University of Petroleum, Qingdao 266580, China.
d
Key Laboratory for Organic Electronics and Information Displays & Institute of
Advanced Materials (IAM), Jiangsu National Synergetic Innovation Center for
Advanced Materials (SICAM), Nanjing University of Posts & Telecommunications, 9
Wenyuan Road, Nanjing 210023, China.
Corresponding author: E-mail:4773zxh@163.com;
E-mail: hxu@hlju.edu.cn; E-mail: xuzj@upc.edu.cn.

Bulky 9-phenylfluorene fuctionalized 2,6-bis(N-carbazolyl)-
pyridine with high triplet energy level as host for blue
thermally activated delayed fluorescence devices
Abstract:
A
new
,6-bis(N-carbazolyl)-pyridine compound 2,6-bis(3-(9-phenyl-fluoren-
-yl)-carbazol-9-yl)pyridine (2,6-PFCzPy) with large steric hindrance
9-phenylfluorene
fuctionalized
2
9
and high triplet energy level has been successfully prepared by
introducing 3-(9-phenyl-fluoren-9-yl)-carbazole unit into the 2,6-position
of pyridine group via Ullmann reaction. Cyclic voltammetry (CV) was
used to study the electrochemical properties of the compound and obtain
HOMO (-5.85 eV), LUMO (-2.09 eV), and energy gap (E , 3.57 eV). The
g
2
,6-CzPy-based compound was employed as host to fabricate blue
thermally activated delayed fluorescence (TADF) organic light-emitting
diodes (OLEDs) for the first time with the maximum EQEs of 6.2% at the
doping concentration as high as 30%. The electroluminescent (EL)
spectra show almost no changes along with the driving voltage ranging
from 5.1 to 15.1 V at high doping concentration, which indicates the
2
,6-CzPy-based three dimensional bulky compound could successfully
suppress the excimer emission of the guest in high doping ratio.
Key words: Host, 2,6-bis(N-carbazolyl)-pyridine, steric hindrance,

thermally activated delayed fluorescence
1
. Introduction
Organic light-emitting diodes (OLEDs) have developed rapidly in the
light of both organic semiconductors and device technologies in academic
and industry in the past three decades since the innovative discovery
reported by Tang and VanSlyke in 1987 [1], because of their particular
advantages such as low cost, light weight, ultra thinner, low
driving-voltage, flexibility and fast response in the application of flat
panel display and solid-state lightings [2-3]. Despite the successful
application of high-efficient phosphorescent organic light-emitting diodes
(PhOLEDs) due to their nearly 100% theoretical internal quantum
efficiencies via harvesting both triplet and singlet excitons caused by
spin-coupling [4], high cost and rarely global resources of scarce metals
restrict their commercial application in OLEDs [5-7]. Moreover, the
phosphor emitters often encounter instability in practical applications for
full-color display and solid-state lighting [8]. Therefore, metal-free
emitters with high efficiency are vital for realizing high-efficient and
inexpensive device in large scale production and commercialization.
According to this point of view, purely organic thermally activated
delayed fluorescence (TADF) metal-free luminophores are the most
prospective candidates due to their ability of upconverting the lowest

triplet (T ) excitons to the lowest singlet-state (S ) through reverse
1
1
intersystem crossing (RISC), thus generating ideal 100% IQE [9].
However, TADF emitters are similar to phosphor luminophores that often
suffer self-quenching and triplet-triplet (TT) annihilation in solid-state at
high concentration [10]. Consequently, TADF dyes are also necessary to
be dispersed into a suitable host matrix to control intermolecular
interactions for avoiding the de-excitation pathways (such as triplet-triplet
annihilation and/or concentration quenching) of emission excitations,
which could greatly enhance the device performance [11-13]. Moreover,
the singlet excitations with microsecond and/or millisecond liftimes are
prospectively discovered due to part of them originated from the triplet
state, which often leads to the fast decay of device efficiency at high
current density [14]. Thus, it is urgent to explore excellent host materials
for TADF emitters, especially for blue-emitting device.
On the other hand, the host should possess both higher singlet and
triplet states than the singlet states of the TADF emitters due to all the
emissive excitons of the TADF emitters derived from their singlet state
[15]. Otherwise the excitonic energy of TADF emitter could
exothermically transfer to the host [14]. In the case of blue TADF devices,
it is a great challenge to realize a host with high T above 2.8 eV to
1
ensure energy effectively transfer from host to blue TADF emitter. In
pursuit of this requirement, the π-conjugation extent of the host should be

3
minimized by meta-/ortho-lingkage, sp carbon, other insulating linkages
(
such as triarylphosphine and tetraaryl silane) to destroy π conjugation
16]. For example, Ye et al. manifested that the host materials of
[
fluorene-coupled D-σ-A type molecules with high triplet energy levels
T1 > 2.80 eV) could obtain the maximum EQEs of 17% [15]; while Xu
(
and his group reported a series of dibenzothiophene-based phosphine
oxide as host materials through ortho- and/or meta-linkage for blue TADF
OLEDs with maximum EQEs and PE (power efficiency) of 21.6% and
-1
3
3.7 lm W [17]. Huang and Tao demonstrated a series of
carbazole/pyridine hybrids by one-step approach with ortho- and/or
meta-linkage to achieve triplet energy level (T > 2.90 eV), using
1
2
,6-bis(N-carbazolyl)pyridine
carbazolyl)pyridine (2,4,6-CzPy) as hosts with the maximum EQEs of
8.0% for blue PhOLEDs [18]. Although both of the synthesis method
(2,6-CzPy)
and
2,4,6-tri(N-
1
and molecular structure of 2,6-CzPy is convenient and simple owning
high T , which has been successfully used as host for high perfromance of
1
PhOLEDs [18], the 2,6-CzPy and/or 2,6-CzPy-based compounds have
seldom been employed as host for TADF OLEDs [19a], especially for
blue TADF OLEDs. Therefore, it is prospective to obtain high
performance devices with low cost by exploring 2,6-CzPy-based
compounds as hosts for blue TADF OLEDs due to its simple synthesis
method and molecular structure as well as high T . The planar molecular
1

configuration of 2,6-CzPy can construct ordered and compact packing
due to short intermolecular distance, which can not suppress
concentration quenching and triplet-triplet annihilation of guest at high
doping ratio. In addition, the low morphological stability of 2,6-CzPy (Tg,
o
7
5 C) would impair the performance of device [18]. Therefore, the
research of 2,6-CzPy-based novel hosts with bulky steric hindrance and
stable morphology is anticipated to inhibit concentration quenching and
triplet-triplet annihilation of guest in solid state for high performance
device. One of the effective methods is design and synthesis of
2
,6-CzPy-based compounds with bulky 9-arylfluoren-9-ol moiety (AFM)
to solve this problem, which is available for good solubility, high thermal
and morphological stability, controllable intermolecular interaction and
promising carrier transport ability [19b]. Herein, 9-phenylfluoren-9-ol
3
moiety (PFM) has been introduced into 2,6-CzPy via the sp -hybridized
carbon linkage by Friedel-Crafts reaction to enhance its thermal and
o
morphological stability, which is demonstrated by TGA (T , 334 C) and
d
DSC curves (No detectable melting point and glass transition temperature
o
ranging from 40 to 150 C) with the desirable thermal and morphological
3
stability. Meanwhile, the sp atom lingkage between PFM and 2,6-CzPy
limits the π-conjugation in the whole molecule retaining high T (2.83 eV)
1
and controllable electrochemical properties, which has been demonstrated
by the density functional theory (DFT) calculations and triplet energy

levels of 2,6-CzPy and 2,6-PFCzPy [18]. The novel compound
,6-bis(3-(9-phenyl-fluoren-9-yl)-carbazol-9-yl)pyridine (2,6-PFCzPy)
2
with 3D bulky steric hindrance is used as host for blue TADF OLEDs for
the first time.
2
2
. Experimental Section
.1 Materials and Methods
All chemical reagents and solvents were bought from Xi'an Polymer
Light Technology Corp or Alfa Aesar and used directly without further
purification. 3-(9-Phenyl-fluoren-9-yl)-carbazole (PFCz) was prepared
according to the facile synthesis of complicated 9,9-diarylfluorenes
reported by Xie and Huang [20]. The reaction was carried out under
1
13
nitrogen atmosphere. H and C NMR spectra were recorded on a
JNM-ECS 600 and 150 MHz spectrometer, respectively, in deuterated
chloroform relative to tetramethylsilane as internal standard.
MALDI-TOF MS spectrum was executed on a reflective mode. The
ultraviolet-visible (UV-vis) absorption spectra were studied by using a
Shimadzu UV-3600. The photoluminescence (PL) spectra were measured
in frozen dichloromethane at 77 k with Edinburgh FLS 920 fluorescence.
Cyclic voltammetric measurements (CVs) were performed by using a
traditional three-electrode construction and an electrochemical
workstation (CHI660C, China) by a glass carbon working electrode

dripped with thin film of 2,6-PFCzPy, a Pt wire counter electrode, a
+
reference electrode (Ag/Ag , referenced against ferrocene/ferrocenium
-1
(
FOC)) in degassed acetonitrile solution with 0.1 mol·L of Bu NPF at
4
6
room temperature. The HOMO, LUMO, and HOMO-LUMO energy gap
+
(
E ) of the compound were calcuated with ferrocene (Fc/Fc , 0.03 V)
g
+
according to the formulas: HOMO = -[E - E(Fc/Fc ) + 4.8] eV, LUMO
ox
+
=
-[E - E(Fc/Fc ) + 4.8] eV, where oxidation potential is set at -4.8 V
red
against zero vacuum level referenced to the ferocenium/ferrocene using
ferrocene as internal standard [21a]. Thermogravimetric analysis (TGA)
and differential scanning calorimetry (DSC) curves were performed on a
Perkin Elmer DSC 8500 unit and a TA Q2000 instrument, respectively,
o
-1
under nitrogen flow at a heating rate of 10 C min . The molecular
structure optimization and electronic properties of the compound were
performed by density functional theory (DFT) calculations with Gaussian
0
9 program package at B3LYP/6-31G (d, p) level [21b], and the
corresponding HOMO and LUMO spatial distributions were also
obtained. Vibrational frequency analysis was also performed at this level
of theory. This result showed that the optimized structure corresponded to
a minimum of potential energy surface with no imaginary frequency.
2
.2 Synthesis of 2,6-bis(3- (9-phenyl-fluoren-9-yl)-carbazol-9-yl)pyridine
2,6-PFCzPy)
(

2
,6-Dibromopyridine (0.24 g, 1 mmol), 3-(9-phenyl-fluoren-9-yl)-
carbazole (1.02 g, 2.5 mmol), potassium carbonate (3.45 g, 25 mmol),
cuprous iodide (0.05 g, 0.25 mmol) and 18-crown-6 (0.03 g, 0.13 mmol)
and 50 mL 1,2-dichlorobenzene were added successively into a dry
three-necked flask under nitrogen atmosphere and away from light and
refluxed for 24h. After cooling to room temperature, the solvent was
o
removed by decompressing distillation around 140 C. The residue was
washed by water and extracted with CH Cl for several times. The
2
2
organic phase was collected and dried by anhydrous Na SO . Then, the
2
4
organic solution was condensed by a rotary evaporation apparatus under
reducing pressure and purified by column chromatography on silica gel
with the eluents of ethyl acetate and petroleum ether (ratio of volume,
+
8
:1) to obtain white powder; yield: 44%. LC-MS (EI) m/z 890.3537 [M ];
1
H NMR (600 MHz, CDCl , ppm): δ 8.03 (t, J = 7.80 Hz, 1H), 7.91 (t, J
3
=
7.50 Hz, 4H), 7.87 (d, J = 2.10 Hz, 2H), 7.82 (d, J = 8.50 Hz, 2H), 7.78
(d, J = 7.80 Hz, 4H), 7.53 (d, J = 7.80 Hz, 2H), 7.46 (d, J = 7.80 Hz, 4H),
7
.35 (t, J = 7.80 Hz, 4H), 7.32 (t, J = 8.40 Hz, 2H), 7.26 (m, 10H), 7.21
1
3
(
m, 8H). C NMR (150 MHz, CDCl ): δ (ppm) 151.75, 151.54, 146.52,
3
1
1
1
40.41, 140.18, 139.73, 139.03, 138.35, 128.28, 127.78, 127.52, 127.14,
26.68, 126.34, 124.51, 124.40, 121.16, 120.27, 119.32, 114.72, 112.06,
11.73.

3
3
. Results and discussions
.1 Synthesis and thermal stability
The molecular structure and synthetic route of 2,6-bis(3-(9-phenyl-
fluoren-9-yl)-carbazol-9-yl)pyridine (2,6-PFCzPy) are displayed in
Scheme 1. The raw material 3-(9-phenyl-fluoren-9-yl)-carbazole (PFCz)
was synthesized by the previous work reported by Xie and Huang [20].
The product 2,6-PFCzPy was prepared by introducing two bulky PFCz
units into the 2,6-position of pyridine moiety via Ullmann reaction under
nitrogen ambience and avoiding light. Two PFCz units were linkaged at
the 2,6-position of pyridine moiety to ensure the high T (2.83 eV) of the
1
compound, and the bukyl 9-phenyl-fluoren-9-yl is to construct steric
hindrance. It is noteworthy that most host materials of carbazole/pyridine
hybrides are derived from 2,6-CzPy just with different linkage position or
midified with distinct groups, whereas the carbazole/pyridine hybrides
with 3D steric hindrance effects have rarely been reported. The chemical
1
13
structure of the compound is fully characterized by H NMR, C NMR,
LC-MS, which has good solubility in common solvents such as, Dimethyl
sulfoxide, N,N-dimethylformamide, Dichloromethane, and Chloroform,
et al. The compound shows good thermal stability that is proved through
o
its high decomposition temperature (T , 334 C; corresponding to 5%
d
weight loss) characterized by thermogravimetric curve (Fig. 1a), which
shows higher thermal stability than that of 2,6-CzPy is probably due to

the increased molecular weight [18]. The glass transition temperature was
determined through differential scanning calorimetry, which exhibited no
detectable crystallization and glass transition phenomena in the heat
ranging from 40 to 150 °C (Fig. 1b). Compared to the glass temperature
of 2,6-CzPy (T , 75 °C), the enhanced glass temperature of 2,6-PFCzPy
g
is probably ascribed to the twisted 3D bulky steric hindrance effect.
3
.2 Optical properties
The photophysical properties of the 2,6-PFCzPy were elaborately
studied by ultraviolet-visible (UV-vis) absorption and photoluminescence
PL) measurements in diluent dichloromethane (DCM) solutuion at room
(
temperature (Fig. 2). As can be seen from Fig. 2, 2,6-PFCzPy displays
five main absorption peaks at about 231, 240, 252, 300 and 350 nm. The
strongest absorption peak around 231 nm could be ascribed to the π-π*
transition of phenyl unit; The middle absorption bands around 240, 252
and 300 nm are assigned to the the π-π* transition of pyridine, fluorene
and carbazole, respectively. The weakest absorption peak around 340 nm
is belonged to the n-π* transition of carbazole moiety. In order to evaluate
the potential appliaction of 2,6-PFCzPy as host material for TADF
OLEDs, the phosphorescent spectrum was measured in frozen
dichloromethane. The T (2.83 eV) of the compound was obtained from
1
the highest-energy vibronic subband of phosphorescent spectrum (Fig.
S1), which was higher than the general triplet energy level (2.80 eV) of

blue TADF emitters to restrict the triplet excitons in the emitting layer
15]. This indicates 2,6-PFCzPy can be used as host for blue TADF
[
OLEDs. The singlet energy level (S , 3.47 eV) is calculated by the
1
threshold value of absorption spectrum in DCM solution, while the
energy gap (∆E ) is obtained from the difference between T and S
ST
1
1
(0.64 eV).
3
.3 The frontier orbital and electrochemical properties of the compound
DFT calculation was carried out to further understand the structure and
molecular orbitals (MOs) of the compound. As shown in Table 1,
although 2,6-CzPy was functionalized with bukyl PFCz, the highest
occupied molecular orbital (HOMO, -5.36 eV) is still located on
carbazole moiety with slightly extended on part of pyridyl group, whereas
the lowest unoccupied molecular orbital (LUMO, -1.07 eV) is mainly
distributed on the pyridyl group. The HOMO and LUMO energy levels
are almost no change compared to the corresponding energy levels of
3
2
,6-CzPy [18], which is probably ascribed to the sp linkage between
carbazole and PFM. The controllable energy levels is beneficial to
enhancing the thermal and morphological stability of the host by bulky
PFM group for stable device. In addition, the little spatial overlap
between the HOMO and LUMO of 2,6-PFCzPy is also helpful to achieve
high-efficient device [22].

To research the electrochemical characteristics of 2,6-PFCzPy, cyclic
voltammetry measurements were performed to estimate the value of
HOMO (-5.85 eV ) and LUMO (-2.09 eV ) of the compound (Table 2).
The LUMO energy level of 2,6-PFCzPy is high in accord with that of
2
,6-CzPy, while the HOMO of 2,6-PFCzPy is slightly lower 0.22 eV than
that of 2,6-CzPy (-5.63 eV ) [18]. The slim variations of HOMO and
LUMO levels between 2,6-PFCzPy and 2,6-CzPy further proved the
3
minute influence caused by the sp linkage between carbazole and PFM.
The energy gap (E , 3.76 eV) was obtained from the difference between
g
the HOMO and LUMO energy levels, which was approximate to the
result (E , 3.47 eV) calculated from the threshold of absorption spectrum.
g
4
. Electroluminescent properties
In view of its good thermal stability and stable morphology as well as
high T of the hybrid, blue TADF OLEDs (device A and B) were
1
fabricated using both 2,6-PFCzPy and 2,6-CzPy as host materials to
investigate the influence of steric hindrance with the following
configuration: ITO/MoO (6 nm)/NPB(50 nm)/mCP (5 nm)/ EML (30
3
wt%, 20 nm)/ DPEPO (35 nm)/Bphen (40 nm)/LiF (1 nm)/Al (100 nm)
(shown in Fig. 3). In the devices, 2,6-PFCzPy (device A) and 2,6-CzPy
(device B) were employed as hosts with DMAC-DPS
(bis[4-(9,9-dimethyl-9,10-dihydroacridine)-phenyl]sulfone) as guest for
blue TADF OLEDs, respectively. MoO and LiF are served as hole and
3

electron-injecting layers, with ITO and Al as the anode and cathode,
respectively. mCP (N,N'-dicarbazole-3,5-benzene) and Bphen
(
bathophenanthroline) act as exciton blocking (EBL) and electron
transporting layers (ETL), respectivly, with NPB
4,4'-bis[N-(1-naphthyl)- Nphenylamino]biphenyl) as hole-transporting
(
layer (HTL). Bis(2-(diphenylphosphino)phenyl)ether oxide (DPEPO)
was used to restrict excitons inside the emitting layer (EML). The
luminance-voltage-current density (L-V-C), current efficiency-voltage,
EQE-Luminance curve and EL spectra of the 2,6-PFCzPy and 2,6-CzPy
hosted blue TADF OLEDs are shown in Fig. 4 and Fig. S3. All the
relevant device data are summarized in Table 3. As illustrated in Fig. 4a,
the device A displays turn-on voltage at about 4.7 V (defined as the
-2
voltage at L = 1 cd m ) that is lower than that of device B (V = 5.0 V),
on
which is probably originated from the more balanced energy levels
between EML and EBL of device A compared with that of device B,
prompting the charge trapping and exciton recombination on
DMAC-DPS ascendant in collecting charger carriers. This is confirmed
by the higher luminance of device A than that of device B when the
driving voltage ranged from 4.0 to 12.0 V, which is further demonstrated
by the V-J curves that displayed higher J of device A compared to that of
device B as the operation voltage aggrandized from 4.0 to 13.0 V. The
higher luminance and J value of 2,6-PFCzPy-based device A is in

accord with higher exciton concentration due to larger and well balanced
excitons in its EML compared to that of 2,6-CzPy-based device B,
which indicates the introducing bulky PFM into 2,6-CzPy could
favourably suppress concentration quenching and triplet-triplet
annihilation of guest at high doping concentration. As can be seen in Fig.
4
and Fig. S3, the maximum luminance, current efficiency, and EQE of
-2
-1
devices A and B are 1356/6459 cd m , 14.4/21.0 cd A , and 6.2/10.36%,
respectively. Although the maximum performance of device A is lower
than that of device B, device A possesses better characteristics at low
driving voltage. In addition, the luminance of the device A is 1000 cd
-2
m when the driving voltage of device is at 10.8 V, which implies the
applicability of the device in portable equipment. The big driving
voltage of device A is probably ascribed to large barrier carrier injection
of the LUMO energy levels between host and guest, which is further
confirmed by the driving voltage of device B with the luminance of
-2
1
000 cd m at about 12.0 V. Meanwhile, the electroluminescence (EL)
spectra of the device of A and B exhibit electroluminescence
characteristics with emission peaks around 486 and 478 nm, which is
bathochromic-shift around 24 and 16 nm, respectively, compared with
DPEPO-based devices reported by Xu [23], probably due to the
difference of polarity between the hosts [24]. The change of EL spectra
of device A is negligible as the driving voltages increases from 5.1 to

1
5.1 V with the doping concentration of 30% w.t., which indicates
excellent color stability. This result demonstrates the bukyl steric
hindrance of the host could supress excimer emission of the guest
successfully. The maximum EQE of the device A descends to one half of
-2
itself as the luminance increases from 0.88 to 336 cd m (Fig. 4c). The
rapid roll-off is probably caused by the unbalanced excitons and/or the
insufficient energy transfer from host to guest. The high-efficient TADF
OLEDs based on bukyl steric hindrance functionalized 2,6-CzPy
derivatives is on-going by optimizing the molecular structure with
improved charge carriers balance and device configuration.
5
. Conclusions
In this work, a novel 2,6-bis(N-carbazolyl)pyridine (2,6-CzPy) with
large steric hindrance was successfully synthesized by combining two
-(9-phenyl-fluoren-9-yl)-carbazole groups with pyridine unit via
3
Ullmann reaction. TGA and DSC curves manifest the bulky steric
hindrance effect is beneficial for improving the thermal and
morphological stability compared to that of 2,6-CzPy. The blue TADF
OLEDs was first fabricated using 2,6-CzPy-based compounds as host
with maximum EQE of 6.2% at doping concentration as high as 30%
without any change of EL spectra as the driving voltages increased from
5
.1 to 15.1 V, which suggests the 3D bulky steric hindrance of the

compound could suppress the excimer emission of the guest at high
doping concentration successfully. The high performance TADF OLEDs
is expected to be achieved by modifying the molecular structure of
2
,6-PFCzPy and optimizing the device conformation on the basis of the
aforementioned guidelines in this work.
Acknowledgements
For the financial support, we thank the National Key Basic Research
and Development Program of China (No. 61405170), and the Nanhu
Scholars Program for Young Scholars of XYNU.
Appendix A. Supplementary material
Supplementary data associated with this article can be found in the
online version at http://dx.doi.org/.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
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Scheme 1 Synthetic route to 2,6-PFCzPy

Fig.1 TGA (a) and DSC (b) curves of 2,6-PFCzPy

Fig. 2 The UV-vis and PL spectra of the compound in DCM solution.
Table 1. HOMO and LUMO surfaces from DFT calculations
Molecule Structure
HOMO
LUMO
N
N
N
-5.36 eV
-1.07 eV
Table 2. Electrochemical properties and triplet energy levels of the host
HOMO
eV)
LUMO
(eV)
E
g
T
1
Compound
(
(eV)
3.76
(eV)
2.83
2
,6-PFCzPy
-5.85
-2.09

Table 3. Device performance of the blue TADF OLEDs
a
-2
a
-1
a
Device
Host
L (cd m )
CE (cd A )
EQE (%)
A
B
2,6-PFCzPy
2,6-CzPy
1356
14.4
6.2
6459
21.0
10.36
a
The efficiencies of L, CE and EQE of prototype devices at maximum values
Fig. 3 (a) Schematic configuration of the OLEDs and the corresponding molecular
structures of the main organic materials, (b) Energy-band diagrams of the OLEDs.

Fig. 4 (a) Luminance-voltage-current density of devices A (squares) and B (spheres)
with black color for luminance and blue color for current density, (b) Current
efficiency-Voltage of devices A (black squares) and B (red spheres), and (c)
EQE-Luminance curves and EL spectra (inset) of device A
.

·
The first 2,6-bis(N-carbazolyl)-pyridine-based compound with large
steric hindrance was used as host for blue TADF OLEDs
·
Stable blue EL spectra were achieved under different driving voltages at
high doping concentration of 30%.
·
The enhanced thermal and morphological stability was achieved with
high triplet energy level (T , 2.83 eV) and controllable electrochemical
1
properties.

The authors declare that they have no conflicts of interest.

Xiang-Hua Zhao and Hui Xu conceived the project; Xiang-Hua Zhao performed the experiments
and wrote the paper; Chun-Miao Han fabricated the devices; Yue Li, Ming-Gui Bai and Jin-Can
Yang provided the data of cyclic voltammogram, UV, PL, TGA and DSC; Shun-Dong Yuan and
Ling-Hai Xie analysed the data of devices; Zhi-Jie Xu calculated the compound.