Spirobifluorene-based oligopyridine derivatives as electron-transporting materials for green phosphorescent organic light-emitting diodes

Article abstract of DOI:10.1016/j.orgel.2019.105498

The electron-transporting materials (ETMs), 2,7-bis(3,5-di (pyridin-3-yl)phenyl)-9,9′-spirobi [fluorene] (3-4PySF) and 2,7-bis(3,5-di (pyridin-4-yl)phenyl)-9,9′-spirobi [fluorene] (4-4PySF) were designed and synthesized by combining spirobifluorene moiety

Full text of DOI:10.1016/j.orgel.2019.105498

Journal Pre-proof
Spirobifluorene-based oligopyridine derivatives as electron-transporting materials for
green phosphorescent organic light-emitting diodes
Xuan Guo, Fang Lv, Zifeng Zhao, Jiannan Gu, Bo Qu, Lixin Xiao, Zhijian Chen
PII:
S1566-1199(19)30525-7
DOI:
https://doi.org/10.1016/j.orgel.2019.105498
ORGELE 105498
Reference:
To appear in: Organic Electronics
Received Date: 5 September 2019
Revised Date: 24 September 2019
Accepted Date: 10 October 2019
Please cite this article as: X. Guo, F. Lv, Z. Zhao, J. Gu, B. Qu, L. Xiao, Z. Chen, Spirobifluorene-based
oligopyridine derivatives as electron-transporting materials for green phosphorescent organic light-
emitting diodes, Organic Electronics (2019), doi: https://doi.org/10.1016/j.orgel.2019.105498.
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Spirobifluorene-based oligopyridine derivatives as
electron-transporting materials for green phosphorescent organic
light-emitting diodes
Xuan Guoa, Fang Lva, Zifeng Zhaob, Jiannan Gua, Bo Qua, Lixin Xiaoa * and Zhijian
Chena *
a State Key Laboratory for Artificial Microstructures and Mesoscopic Physics,
Department of Physics, Peking University, 100871, Beijing, People’s Republic of
China;
b Beijing National Laboratory for Molecular Sciences, State Key Laboratory of Rare
Earth Materials Chemistry and Applications, College of Chemistry and Molecular
Engineering, Peking University, Beijing 100871, P. R. China.
*Email: xiao66@pku.edu.cn, zjchen@pku.edu.cn,
Abstract:
The
electron-transporting
materials
(ETMs),
and
2,7-bis(3,5-di(pyridin-3-yl)phenyl)-9,9'-spirobi[fluorene]
(3-4PySF)
2,7-bis(3,5-di(pyridin-4-yl)phenyl)-9,9'-spirobi[fluorene] (4-4PySF) were designed
and synthesized by combining spirobifluorene moiety with di(pyridine-3-yl)phenyl
and di(pyridine-4-yl)phenyl, respectively. The spirobifluorene moiety improves
materials’ rigid twisted structure to ensure the morphological stability of amorphous
film, and pyridine acts as electron acceptor to enhance electron-transporting ability of
materials. The dependence of electron-transporting property on the position of
substituted pyridine rings was studied. The melting point (Tm) of 4-4PySF is estimated
to be 41 ℃ higher than that of 3-4PySF. And the higher current density in the

electron only devices exhibited by 4-4PySF revealed the effect of nitrogen atom
position on the charge-transporting properties. Green PhOLEDs based on
bis(2-phenylpyridine)iridium(III)(2,2,6,6-tetramethylheptane-3,5-diketonate)
(Ir(ppy)2tmd) as the emitter and 3-4PySF, 4-4PySF and 1,3,5-tris(N-phenylbenzimid
azol-2-yl-benzene (TPBi) as ETMs were fabricated. Compared to the device based on
the conventional ETM TPBi, the devices based on new ETMs exhibited a higher
maximum external quantum efficiency (EQE) of 20.5% and a lower turn-on voltage
(Von) of 2.6 V.
Keywords:
electron-transporting material; organic light-emitting diode; oligopyridine;
spirobifluorene; intermolecular interaction.
1. Introduction
Since the milestone work of OLEDs in 1987 by Tang and VanSlyke, organic
light-emitting diodes (OLEDs) have experienced rapid development due to its merits
in large-area and flexible devices.[1-4] Efficiency of OLEDs depends on the
transporting and recombination effectively of electron and hole. By reason that
electron mobility in organic materials are usually several orders of magnitude lower
than hole mobility,[5] many researchers are devoted to developing
electron-transporting materials with high electron mobility.[6, 7] Pyridine-based
compounds have been synthesized because of strong electron-accepting character of
pyridine, and the ability to change the electrical properties of the molecule by
changing the substitution sites.[8, 9] Kido et al. reported that changing the position of

nitrogen atoms on oligopyridine derivatives can change the intermolecular weak
hydrogen bonds, thereby adjusting the horizontal orientation in amorphous organic
semiconductor films and significantly modulating electron mobility.[10, 11] However,
these materials often own a planar structure, and excessive π–π stacking between the
molecules during deposition leads to a rough surface.[12] On the other hand, the
spirobifluorene has rigid twisted structure that provides a high triplet energy (T1) and
a high glass transition temperature (Tg), which is beneficial to maintaining the
morphological stability.[13, 14] It is promising to synthesize electorn-transporting
materials by combining oligopyridine and spirobifluorene.
In this study, we synthesized 3-4PySF and 4-4PySF by combining
di(pyridine-3-yl)phenyl and di(pyridine-4-yl)phenyl with spirobifluorene, respectively.
The only difference between 3-4PySF and 4-4PySF is the substitution of the nitrogen
atoms on the pyridine. The effect of nitrogen position of the pyridine on
photophysical properties, thermal properties and the carrier transport were studied.
Compared to 3-4PySF, 4-4PySF possesses a higher melting point, indicating stronger
intermolecular forces between molecules. By electron only devices, it was found that
the electron mobility of 4-4PySF is higher than that of 3-4PySF. Green PhOLEDs
based on 3-4PySF and 4-4PySF were fabricated, and a maximum EQE of 20.5%, a
lowest turn-on voltage of 2.6 V were achieved, which were superior to TPBi-based
comparison device.
2. Experimental
2.1. General

Electrospray ionization mass spectroscopy in positive ion mode was performed on a
Bruker Solarix XR Fourier Transform Ion Cyclotron Resonance Mass Spectrometer.
1
The H NMR and 13C NMR spectra were recorded on Bruker 500 MHz NMR
spectrometer. Elemental analysis was measured on a VARIO elemental analyzer from
Elementar Analysensysteme GmbH. DSC was performed on a PerkinElmer Diamond
DSC Pyris instrument while TGA was recorded on a SEIKO EXSTAR 6000 TG/DTA
6200 unit under nitrogen atmosphere at a heating rate of 10℃/min. UV-vis absorption
spectra were measured by using a Perkin-Elmer Lambda 35 spectrometer. Emission
spectra were recorded by using Hitachi F7000 fluorescence spectrophotometer. The
surface roughness of the film was performed on a conductive atomic force microscope
(CAFM, SEIKO SPA300HV). Cyclic voltammetry was measured by using a
CHI600C voltammetric analyzer with tetrabutylammonium hexafluorophosphate
(TBAPF6, 0.1 M) as the supporting electrolyte and ferrocene as an internal standard,
in a nitrogen-purged dichloromethane (CH2Cl2) or DMSO solution at room
temperature. The conventional three electrodes were a platinum working electrode, a
platinum wire auxiliary electrode, and a Ag/AgCl wire pseudo reference electrode.
HOMO level in film state was estimated by using ultraviolet photoelectron
spectroscopy (AXIS Supra) from Kratos Analytical Ltd. The current density and
voltage characteristics of OLEDs were obtained by using a computer controlled
Keithley 2611 Sourcemeter. The luminance characteristics of OLEDs were recorded
on a SRC-600 spectroradiometer (EVERFINE Corporation).
2.2. Materials

dipyrazino[2,3-f:2’,3’-h]quinoxaline-2,3,6,7,10,11-hexacarbonitrile
1,1-bis[4-[N,N-di(p-tolyl)-amino]phenyl]-cyclohexane
tris(4-(9H-carbazol-9-yl)phenyl)-amine
(HAT-CN),
(TAPC),
(TCTA),
2,4-diphenyl-6-bis(12-phenylindolo)[2,3-a]carbazole-11-yl)-1,3,5-triazine (DIC-TRZ),
bis(2-phenylpyridine)iridium(III)(2,2,6,6-tetramethylheptane-3,5-diketonate)
(Ir(ppy)2tmd), and 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBi) were
commercially available and used without further purification.
2.3. Synthesis and characterization
Synthesis of 3,3'-(5-chloro-1,3-phenylene)dipyridine (1):
3,5-dibromochlorobenzene (18.5
A
mixture of
mmol),
3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)pyridine (39 mmol) and K2CO3 (93
mmol) were mixed in a flask containing nitrogen-saturated 1,4-dioxane (350 mL)
before 1,1'-bis(diphenylphosphino)ferrocene]dichloropalladium(II) (PdCl2(dppf))
(0.38 g) was added. The resulting mixture reacted for 8.0 h and then purified by
silica-gel column chromatography.
Synthesis
of
3,3'-(5-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-1,3-phenylene)dipyridine
(2):
Compound (1) (48.7 mmol), bis(pinacolato)diboron (54 mmol) and KOAc (122 mmol)
were mixed in a flask containing nitrogen-saturated 1,4-dioxane (130 mL) before
phenylallylchloro[1,3-bis(diisopropylphenyl)imidazole-2-ylidene]palladium(II)
(Pd(IPr)(cin)Cl) (0.35 g) was added. The resulting mixture reacted for 3.0 h and then
purified by silica-gel column chromatography.

Synthesis of 3-4PySF: Compound (2) (34.1 mmol), 2,7-dibromo-9,9 ′
-spirobifluorene (15.8 mmol) and K2CO3 (79 mmol) were mixed in a flask containing
nitrogen-saturated 1,4-dioxane (120 mL) before PdCl2(dppf) (0.3 g) was added. The
resulting mixture reacted for 2.0 h and then purified by silica-gel column
chromatography. 1H NMR (500 MHz, CDCl3): δ 8.86 (d, J = 2.0 Hz, 4H), 8.62 (dd, J
= 4.8, 1.5 Hz, 4H), 8.02 (d, J = 7.9 Hz, 2H), 7.89 (ddd, J = 8.7, 5.5, 3.4 Hz, 6H), 7.75
(dd, J = 8.0, 1.6 Hz, 2H), 7.63 (dd, J = 3.9, 1.4 Hz, 6H), 7.39 (ddd, J = 12.7, 7.7, 2.8
Hz, 6H), 7.15 (td, J = 7.5, 0.8 Hz, 2H), 7.01 (d, J = 1.4 Hz, 2H), 6.86 (d, J = 7.6 Hz,
2H). 13C NMR (126 MHz, CDCl3) δ 150.29, 148.87, 148.39, 148.25, 142.91, 141.90,
141.14, 140.37, 139.32, 136.26, 134.64, 128.08, 128.03, 127.36, 125.84, 125.03,
124.18, 123.61, 122.84, 120.72, 120.34, 66.24. HRMS (ESI, m/z): [M + H]+, calcd
for C57H36N4, m/z = 777.301274, found m/z = 777.298694; Anal. Calc. for C57H36N4:
C, 88.12; H, 4.67; N, 7.21. Found: C, 88.09; H, 4.66; N, 7.16.
Synthesis of 4-4PySF: Compound 4-4PySF was synthesized by the same
1
procedure as described above for 3-4PySF using pyridin-4-ylboronic acid. H NMR
(500 MHz, CDCl3): δ 8.68 (d, J = 5.3 Hz, 12H), 8.03 (d, J = 7.9 Hz, 3H), 7.89 (d, J =
7.6 Hz, 3H), 7.72 (ddd, J = 10.6, 8.6, 1.4 Hz, 12H), 7.51 (d, J = 5.5 Hz, 12H), 7.41 (t,
J = 7.5 Hz, 3H), 7.16 (t, J = 7.5 Hz, 3H), 7.00 (d, J = 1.2 Hz, 3H), 6.87 (d, J = 7.6 Hz,
3H). 13C NMR (126 MHz, CDCl3) δ 150.33, 150.30, 148.23, 147.83, 143.07, 141.93,
141.19, 140.26, 139.82, 128.08, 128.06, 127.46, 126.55, 124.72, 124.22, 122.84,
121.87, 120.79, 120.34, 66.28. HRMS (ESI, m/z): [M + H]+, calcd for C57H36N4, m/z
= 777.301274, found m/z = 777.301608; Anal. Calc. for C57H36N4: C, 88.12; H, 4.67;

N, 7.21. Found: C, 87.81; H, 4.66; N, 7.17.
2.4. Device fabrication
The indium-tin-oxide (ITO) anode with a sheet resistance of 14 Ω-1 was commercially
available. Before the deposition of organic layers, the ITO substrate was cleaned
successively with deionized water, acetone and ethanol. The organic and cathode
layers were evaporated in same chambers, the base pressure of which is lower than 8
× 10-4 Pa. The active area for all devices was 3.14 mm2. The organic and Al layers
were thermally evaporated at a rate of 0.1–0.2 nm/s, while that of LiF layer was 0.01
nm/s.
3. Results and discussion
3.1. Synthesis and characterization
Two different types of electron-transporting materials were synthesized as shown in
Scheme 1. The difference between 3-4PySF and 4-4PySF is the positions of the
nitrogen atom on the pyridine rings, 3-4PySF is a meta-substituted nitrogen atom, and
4-4PySF is a para-substituted nitrogen atom. Two materials were characterized by
1
13
electrospray ionization mass spectroscopy (ESI-MS), H NMR, C NMR, elemental
analysis and single-crystal analysis (as shown in supporting information).

Scheme 1. The synthesis process of the 3-4PySF and 4-4PySF.
3.2. Theoretical calculations
As shown in Figure 1, the highest occupied molecular orbital (HOMO) distribution
and the lowest unoccupied molecular orbital (LUMO) distributions of 3-4PySF and
4-4PySF were carried out by Gaussian 09 software with the 6-31G(d, p) basis set. The
electronic calculation results of 3-4PySF and 4-4PySF showed few differences. The
HOMO was dispersed over the electron-rich spirofluorene moiety, while the LUMO
was more distributed on the phenyl moiety, suggesting high electron mobility. After
structural optimization, the corresponding energy levels were calculated by using the
same theory. As a result, the HOMO energy levels of the 3-4PySF and 4-4PySF were
calculated at -5.7 and -5.8 eV, respectively. The LUMO energy levels were calculated
to -1.6 eV for 3-4PySF and -1.7 eV for 4-4PySF. To further explore the electron

injection ability, the total self-consistent field density of 3-4PySF and 4-4PySF were
performed by using the electrostatic potential (ESP) method by Singh and
Kollman.[15, 16] As illustrated in Figure 1, the high electronegativity region was
concentrated on the nitrogen atom of pyridine, which was conducive to forming the
coordination effect with metals, thereby efficiently enhancing the electron injection
from the cathode.[17]
Figure 1. The HOMO, LUMO distributions of 3-4PySF and 4-4PySF. The isosurface
of 3-4PySF ESP and 4-4PySF ESP.
3.3. Photophysical properties and electrochemical properties
The room-temperature ultraviolet-visible (UV-Vis) absorption spectra and
photoluminescence (PL) spectra of 3-4PySF and 4-4PySF in chloroform (CHCl3)
(10-5 M) and film state were measured to investigate their photophysical properties, as
shown in Figure 2a and Table 1. In solution, 3-4PySF exhibited two absorption peaks
at 310 nm and 334 nm, respectively, corresponding to the π–π* transition of the
fluorene units in spirobifluorene[18] and indicating the presence of electronic
coupling between the spiobifluorene and the phenyl group.[19, 20] Compared with

the absorption spectrum in solution, the absorption spectrum in neat film showed a
small red shift. The two absorption peaks moved to 312 nm and 336 nm, respectively,
indicating the twisted structure of spirobifluorene inhibited the formation of π–π
stacking of 3-4PySF. The absorption peaks of 4-4PySF in solution and film state were
located at 309 nm, 333 nm, 313 nm and 339 nm, respectively, which were close to the
absorption peaks of 3-4PySF. But 4-4PySF exhibited a bigger red shift in absorption
from solution to thin film compared to 3-4PySF. The band gap Eg determined from the
band edge of the UV-vis spectra were 3.34 eV for 3-4PySF and 3.29 eV for 4-4PySF.
As for the PL spectra, the fluorescence peaks of 3-4PySF in CHCl3 and the film
state were 369 nm and 401 nm, respectively. The fluorescence peaks of 4-4PySF were
identical to that of 3-4PySF, which were also located at 369 nm and 401 nm,
respectively. Compared to their similar absorptions in solution and film state, the thin
film fluorescence emission showed significant red-shift comparing those in solution,
indicating the existence of the interaction between the molecules in the solid state. [21,
22] For the purpose of studying the interaction between molecules, single crystal
structural analysis of the 3-4PySF was performed using single crystal X-ray
diffractometer. As shown in Figure S1 (Supplementary Information), the
intermolecular weak H-bonding interactions with a distance of 0.260 nm were
confirmed. The interaction between molecules made the molecules pile up tightly.
In order to confirm the triplet energy (T1) of 3-4PySF and 4-4PySF, the
phosphorescence spectra of compounds in a frozen 2-methyl-tetrahydrofuran (10-5 M)
at 77 K were measured with a 1 ms delay. From those low-temperature emission

spectra, we obtained T1 of 2.44 eV for both materials (e.g. 3-4PySF and 4-4PySF).
This result suggested that both materials have potential as electron-transporting
materials in green phosphorescent devices.
For obtaining the relative energy levels of 3-4PySF and 4-4PySF, the
oxidation/reduction potentials were measured in the dichloromethane/
dimethylsulfoxide (DMSO) solution using cyclic voltammetry (CV), as depicted in
Figure 2b. The HOMO and LUMO energy levels of the compounds could be
estimated by comparing the difference between the onset potentials for oxidation and
reduction of the materials and the ferrocene (4.8 eV below vacuum).[23] Using an
ꢉ
ꢋ
ꢌ
empirical formula (ꢀꢁꢂꢃꢂ = −ꢄꢀꢅꢆ − ꢀꢇꢈ ꢇꢈ ꢊ eV + −4.8 eV), the HOMO of
⁄
3-4PySF and 4-4PySF could be calculated to be -5.9 eV and -6.0 eV, respectively.
And the LUMO levels of 3-4PySF and 4-4PySF were figured to be -2.3 eV and -2.4
ꢉ
ꢋ
ꢌ
eV, respectively, by using (ꢀꢍꢎꢃꢂ = −ꢄꢀꢏꢐ − ꢀꢇꢈ ꢇꢈ ꢊ eV + −4.8 eV). Meanwhile,
⁄
the HOMO energy levels of two materials in film were estimated by ultraviolet
photoelectron spectroscopic (UPS) measurements (Figure S2 and S3, Supplementary
Information), both at -5.9 eV. The LUMO energy levels determined via the
subtraction of the Eg from the HOMO energy levels were -2.6 eV for both compounds.
The difference between the energy level in the solution state and the film state could
be due to the interaction between molecules in the film state.

(a)
(b)
Figure 2. (a) Absorption, fluorescence, and phosphorescence spectra of 3-4PySF and
4-4PySF. (b) The CV curves of 3-4PySF and 4-4PySF at a scan rate of 100 mV s-1.
Table 1. Physical properties of 3-4PySF and 4-4PySF.
a)
b)
a)
b)
Material λabs
[nm]
λabs
[nm]
λPL
λPL
Tm
HOMOc)/
HOMOd)
[eV]
LUMOc)/
LUMOd)
[eV]
ET
[nm] [nm] [℃]
[eV]
3-4PySF 310, 334 312, 336 369
4-4PySF 309, 333 313, 339 369
401
401
383
424
-5.9/-5.9
-5.9/-6.0
-2.6/-2.3
-2.6/-2.4
2.44
2.44
a)Measured in CHCl3; b)Measured in neat film; c)Measured using ultraviolet
photoelectron spectroscopic, LUMOc) = HOMOc) + Eg; d)Measured using CV.
3.4. Thermal properties and film morphology
The thermal properties of 3-4PySF and 4-4PySF were estimated by thermogravimetric

analysis (TGA) and differential scanning calorimetry (DSC) under nitrogen
atmosphere (Figure S4-S7, Supplementary Information). The decomposition
temperature (Td) were observed to be 462 ℃ and 472 ℃ for 3-4PySF and 4-4PySF,
respectively, indicating a high thermal stability. The melting (Tm) of 4-4PySF was
measured to be 424 ℃, which was 41 ℃ higher than that of 3-4PySF (Figure 3).
The apparent difference in Tm between two materials may be mainly due to the
difference in H-bonding interactions between two materials in the solid state,[11]
suggesting that 4-4PySF possessed stronger intermolecular H-bonding with respect to
3-4PySF. The glass transition temperature (Tg) of 3-4PySF was 176 ℃, which was
higher than those commonly used electron-transporting materials such as TPBi (Tg =
124 ℃). The high Tg is derived from the rigid and twisted molecular structure, which
helps to maintain the morphological stability of amorphous film during thermal
evaporation and contribute to enhancing the device lifetime.
Figure 3. DSC plots of 3-4PySF and 4-4PySF.
As shown in Figure 4, the morphology of films of 3-4PySF and 4-4PySF were
measured by atomic force microscopy (AFM) in tapping mode. The 50 nm thick films

of 3-4PySF and 4-4PySF deposited on silica glass exhibited root-mean-square
roughness (Rrms) of only 0.337 nm and 0.307 nm, respectively. Amorphous smooth
stable films, which were resulted from the rigid twisted spirobifluorene moiety
hindering the π–π stacking between the molecules, helped to the preparation of stable
OLEDs.[24]
(b)
(a)
Figure 4. 3D topography images and Rrms values of 50 nm thick films of (a) 3-4PySF
(Rrms 0.337 nm), and (b) 4-4PySF (Rrms 0.307 nm).
3.5. Device characteristics
To further study the compound’s electron-transporting properties, three single-carrier
devices were fabricated. They had the device structure of indium tin oxide (ITO)/BCP
(10 nm)/3-4PySF or 4-4PySF or TPBi (30 nm)/LiF/Al, respectively. Here, the purpose
of BCP in electron only devices was to block the injection of hole carriers from the
ITO electrode. Figure 5 shows the current density-voltage curves of three single
charge device. Current density of the 4-4PySF based device was higher than that of
3-4PySF and TPBi based devices at the same voltage. Since 3-4PySF and 4-4PySF
were extremely similar in terms of optical properties and energy levels, the difference

in electron-transporting ability was likely to be caused by different molecular
packings during deposition. It’s worth noting that the only difference between
derivatives is the position of the nitrogen atom on peripheral pyridine rings.
Compared with the nitrogen atom in the meta-position, the nitrogen atom in the
para-positon could provide stronger intermolecular H-bonding in the film state, hence
increasing the horizontal molecular orientation in the film state, and finally leading to
an enhancement in electron-transporting ability, as suggested by Kido et al.[10, 12]
Therefore, 4-4PySF possessing the para-position nitrogen atom exhibited strongest
electron-transporting capability in three single-carrier devices.
Figure 5. Current density-voltage characteristics of electron-dominant device:
ITO/BCP (10 nm)/3-4PySF or 4-4PySF or TPBi (30 nm)/LiF/Al.
To verify the performance of 3-4PySF and 4-4PySF as ETMs, Ir(ppy)2tmd-based
green phosphorescent OLEDs were fabricated. The device based on the conventional
electron-transporting material TPBi was also prepared for reference. Figure 6 shows
the chemical structures and energy diagram of materials used in devices. The device

structures are listed as follows:
Device A: ITO/HAT-CN
nm)/DIC-TRZ:Ir(ppy)2tmd (7 wt%, 20 nm)/3-4PySF (40 nm)/LiF/Al
Device B: ITO/HAT-CN (2 nm)/TAPC (60
nm)/DIC-TRZ:Ir(ppy)2tmd (7 wt%, 20 nm)/4-4PySF (40 nm)/LiF/Al
Device C: ITO/HAT-CN (2 nm)/TAPC (60
nm)/DIC-TRZ:Ir(ppy)2tmd (7 wt%, 20 nm)/TPBi (40 nm)/LiF/Al
(2
nm)/TAPC
(60
nm)/TCTA
nm)/TCTA
nm)/TCTA
(5
(5
(5
where HAT-CN was used to modify ITO to enhance hole injection from ITO to TAPC,
TAPC and TCTA were used as hole-transporting materials. The green phosphorescent
emitter Ir(ppy)2tmd co-deposited with the TADP hose material DIC-TRZ was chosen
to be emitting materials.
As shown in Figure 7 and Table 2, the turn-on voltages (Von) of 2.6 V, 2.8 V
obtained from the devices based on 3-4PySF and 4-4PySF, respectively, were lower
than those of the device based on TPBi (2.9 V), indicating superior electron injection
abilities of both materials. Among the three devices, device A possessed the best
performance. The maximum current efficiency (ηCE, max) was 76.6 cd/A for device A,
this efficiency data corresponded to 20.5% of EQE. Since the superior electron
injection capability, the maximum power efficiency (ηPE, max) of device A was 83.6
lm/W. However, it may be due to the high electron-transporting property of 4-4PySF
that caused carrier imbalance in the device, the device B did not perform as well as
device A. But, the parameters of device B were comparable to those of TPBi-based
comparison device. The ηCE, max and ηPE, max were 68.8 cd/A and 62.7 lm/W for device

B, 69.8 cd/A and 59.5 lm/W for device C, respectively, and correspond to 18.3% and
18.5% of EQEs for device B and C, respectively. Based on these results, we believe
that both 3-4PySF and 4-4PySF could provide fairly good device performances
because of their superior electron injection and electron-transporting properties.
Figure 6. Chemical structures, and energy diagram of materials.

Figure 7. (a) Current density-voltage and luminance-voltage, (b) power
efficiency-luminance, (c) current efficiency-luminance, (d) external quantum
efficiency- luminance curves and (e) electroluminescence (EL) spectra obtained from
Device A, Device B and Device C.

Table 2. List of device performances
a)
b)
b)
b)
Von
[V]
2.6
EQE, max/1000/10 000
[%]
CIEc)
ηCE, max/1000/10 000
[cd A-1]
ηPE, max/1000/10 000
[lm W-1]
(x,y)
A
B
C
76.6/75.1/68.2
68.8/67.5/55.7
69.8/68.1/56.4
83.6/47.9/26.2
62.7/31.1/18.0
59.5/29.1/17.1
20.5/20.1/18.2
18.3/17.9/14.8
18.5/18.0/15.0
(0.35, 0.62)
(0.35, 0.62)
(0.34, 0.62)
2.8
2.9
a)At 1 cd m-2; b)Maximum, at 1000 cd m-2, at 10 000 cd m-2; c)At 100 cd m-2.
4. Conclusions
In conclusion, we synthesized two compounds with electron-withdrawing pyridine
and rigid twisted spirobifluorene moiety, 3-4PySF and 4-4PySF. The structural
difference between 3-4PySF and 4-4PySF is the position of substituted pyridine rings.
Through a series of characterizations, the effect of nitrogen atom positon on the
photophysical properties, energy levels, thermal properties, electron-transporting
properties and device performances was demonstrated. It was found that the
electron-transporting ability of materials could be adjusted according to substitution
position of nitrogen atom in the pyridine. The difference in the Tm of 3-4PySF and
4-4PySF suggested that the difference in electrical properties may be due to the
different intermolecular interaction in the film state. Green PhOLEDs with 3-4PySF
and 4-4PySF as ETM were fabricated, which exhibited a maximum EQE of 20.5%
and a lowest turn-on voltage of 2.6 V, superior to TPBi-based comparison device.
These results manifest that the integration of pyridine and spirobifluorene is a
promising way to design an ideal ETM.
Conflicts of interest

There is no conflict to declare.
Acknowledgements
This work was supported by the National Key Basic Research and Development
Program of China (No. 2016YFB0401000) and the National Natural Science
Foundation of China (11574009, U1605244, 61575005, 11574013, 61775004,
11527901).
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Highlights
ò Design and synthesis of two oligopyridine derivatives with different nitrogen
positions.
ò Adjusting the electron-transporting ability of materials by changing the
substitution positon of pyridine.
ò High external quantum efficiency over 20% in the green phosphorescent organic
light-emitting diodes.