Molecular Design Strategy of Thermally Activated Delayed Fluorescent Emitters Using CN-Substituted Imidazopyrazine as a New Electron-Accepting Unit

Article abstract of DOI:10.1002/asia.201901311

Thermally activated delayed fluorescence (TADF)-based organic light-emitting diodes (OLEDs) have attracted enormous attention recently due to their capability to replace conventional phosphorescent organic light-emitting diodes for practical applications. In this work, a newly designed CN-substituted imidazopyrazine moiety was utilized as an electron-accepting unit in a TADF emitter. Two TADF emitters, 8-(3-cyano-4-(9,9-dimethylacridin-10(9H)-yl)phenyl)-2-phenylimidazo[1,2-a]pyrazine-3-carbonitrile (Ac-CNImPyr) and 8-(3-cyano-4-(10H-phenoxazin-10-yl)phenyl)-2-phenylimidazo[1,2-a]pyrazine-3-carbonitrile (PXZ-CNImPyr), were developed based on the CN-substituted imidazopyrazine acceptor combined with acridine and phenoxazine donor, respectively. A CN-substituted phenyl spacer was introduced between the donor and acceptor for a sufficiently small singlet-triplet energy gap (ΔE_ST) and molecular orbital management. Small ΔE_ST of 0.07 eV was achieved for the phenoxazine donor-based PXZ-CNImPyr emitter. As a result, an organic light-emitting diode based on the PXZ-CNImPyr emitter exhibited a high external quantum efficiency of up to 12.7 %, which surpassed the EQE limit of common fluorescent emitters. Hence, the CN-modified imidazopyrazine unit can be introduced as a new acceptor for further modifications to develop efficient TADF-based OLEDs.

Full text of DOI:10.1002/asia.201901311

CHEMISTRY
AN ASIAN JOURNAL
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Accepted Article
Title: Molecular design strategy of thermally activated delayed
fluorescent emitter using CN-substituted imidazopyrazine as a
new electron accepting unit
Authors: Shantaram Kothavale, Kyung Hyung Lee, and Jun Yeob Lee
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Molecular design strategy of thermally activated delayed fluorescent emitter
using CN-substituted imidazopyrazine as a new electron accepting unit
Dr. Shantaram Kothavale, Kyung Hyung Lee, Prof. Jun Yeob Lee*
School of Chemical Engineering, Sungkyunkwan University
2066, Seobu-ro, Jangan-gu, Suwon, Gyeonggi, 440-746, Korea
E-mail: leej17@skku.edu
Abstract
Thermally activated delayed fluorescence (TADF) based organic light emitting diodes (OLEDs)
have attracted enormous attention recently due to their capability to replace conventional
phosphorescent organic light emitting diodes for practical applications. In this work, newly
designed CN-substituted imidazopyrazine moiety was utilized as an electron accepting unit in
TADF emitter. Two TADF emitters, 8-(3-cyano-4-(9,9-dimethylacridin-10(9H)-yl)phenyl)-2-
phenylimidazo[1,2-a]pyrazine-3-carbonitrile (Ac-CNImPyr) and 8-(3-cyano-4-(10H-phenoxazin-
10-yl)phenyl)-2-phenylimidazo[1,2-a]pyrazine-3-carbonitrile (PXZ-CNImPyr), were developed
based on the CN-substituted imidazopyrazine acceptor combined with acridine and phenoxazine
donor, respectively. CN-substituted phenyl spacer was introduced between the donor and
acceptor for sufficiently small singlet-triplet energy gap (ΔEST) and molecular orbital
management. Small ΔEST of 0.07 eV was achieved for the phenoxazine donor based PXZ-
CNImPyr emitter. As a result, organic light emitting diode based on PXZ-CNImPyr emitter
exhibited high external quantum efficiency up to 12.7 %, which surpassed the EQE limit of
common fluorescent emitters. Hence, the CN-modified imidazopyrazine unit can be introduced
as a new acceptor for further modifications to develop efficient TADF OLEDs.
Key words: TADF, acceptor, imidazopyrazine, efficiency, OLED
Introduction
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Thermally activated delayed fluorescence (TADF) has been proved as an efficient triplet
harvesting mechanism to realize high external quantum efficiency (EQE) in organic light
emitting diodes (OLEDs) and hence can be considered as a replacement for the
phosphorescence1–9. In general, the TADF molecules are small organic compounds based on the
donor-acceptor architecture with small singlet-triplet energy gap (ΔEST). They can exhibit 100 %
internal quantum efficiency like phosphors without use of any precious metals such as iridium or
platinum10–13. Recently, a lot of TADF materials demonstrated very good performances in the
OLEDs. Nevertheless, TADF OLEDs still require further improvement in material and device
parameters such as short device lifetimes, long delayed fluorescence lifetime, and large
efficiency roll-off for their commercial applications14,15. New molecular design strategies for
efficient reverse intersystem crossing (RISC) are indispensable to improve these parameters16–20
.
According to Tamm–Dancoff approximation theory21, both singlet-triplet energy gap (ΔEST) and
transition dipole moment are directly proportional to the highest occupied molecular orbital
(HOMO)–lowest unoccupied molecular orbital (LUMO) overlap. As the oscillator strength
depends on the transition dipole moment; the oscillator strength and ΔEST are complementary to
each other. In the TADF material design, these two complementary parameters need to be
matched in a single molecule for high performance OLEDs. Small ΔEST is necessary to facilitate
the triplet excited state (T1) to singlet excited state (S1) reverse intersystem crossing (RISC)
process in TADF emitters and is obtained by separating the wave function distributions of the
HOMO and LUMO9,18. The HOMO and LUMO can be largely separated by inducing
perpendicular orientation between the donor and acceptor. Therefore, distorted backbone
structure between the electron donor and electron acceptor for the HOMO-LUMO separation is
required for small ΔEST. At the same time, effective HOMO-LUMO overlap for the large
oscillator strength with efficient radiative decay from S1 to the ground state (S0) is also necessary
for high EQE22,23
.
In the last several years, many electron donating/accepting moieties were tested for the high
performing TADF OLEDs. Electron donating moieties including carbazole, triphenylamine,
acridine, phenoxazine, and phenazine24–28, and electron accepting moieties including triazine,
pyrimidine, ketone, sulfone, phosphine oxide, dicyanobenzene, pyridine and triarylboron29–35
were employed as the donors and acceptors in the TADF emitters. Among the electron acceptors,
the imidazole unit has been proved as an electron acceptor to realize blue emission due to its
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shallow LUMO level and high triplet energy36. Until now, only a few TADF emitters have been
developed based on the imidazole unit due to its weak electron acceptor strength26,37
.
In this work, we successfully designed TADF OLEDs based on the 2-phenyl imidazopyrazine
(ImPyr) as an acceptor unit. Due to the presence of one extra nitrogen atom in the acceptor unit,
the ImPyr unit can be considered as a stronger acceptor than the well-known 2-phenyl
imidazopyridine unit. Furthermore, the 3-position of ImPyr was substituted with a CN unit to
increase the electron accepting ability and deepen the LUMO level. Finally, 9,9-dimethyl-9,10-
dihydroacridine and 10H-phenoxazine electron donors were attached at 8-position of the 3-cyano
2-phenyl imidazopyrazine (CNImPyr) unit through a CN-substituted phenyl linker to construct
8-(3-cyano-4-(9,9-dimethylacridin-10(9H)-yl)phenyl)-2-phenylimidazo[1,2-a]pyrazine-3-
carbonitrile
(Ac-CNImPyr)
and
8-(3-cyano-4-(10H-phenoxazin-10-yl)phenyl)-2-
phenylimidazo[1,2-a]pyrazine-3-carbonitrile (PXZ-CNImPyr) as new TADF emitters. The newly
developed CNImPyr acceptor unit was successfully employed as a TADF acceptor to reach EQE
up to 12.7 %. Due to the feasible synthetic approach, further modification of this acceptor unit is
possible for superior OLED device performances.
Results and discussion
Theoretical calculations and synthesis
To deeply understand molecular configuration of Ac-CNImPyr and PXZ-CNImPyr, and their
electronic properties such as HOMO-LUMO energy gap, singlet and triplet energies, oscillator
strength, etc; time dependent density functional theory (TD-DFT) calculations were carried out
at B3LYP/6-31G (d) level. Figure 1 represents optimized structures, HOMO-LUMO
distributions, energy gap, and oscillator strength of the TADF emitters. The HOMO and LUMO
energy levels of the CNImPyr acceptor were deepened by 0.47 and 0.50 eV, respectively, as
compared to that of ImPyr acceptor without the CN unit. In the case of Ac-CNImPyr and PXZ-
CNImPyr, the HOMO was mainly located on the electron donors of acridine or phenoxazine,
while the LUMO was mainly located on the CNImPyr acceptor and the CN-substituted phenyl
linker between the donor and acceptor. The phenoxazine substituted PXZ-CNImPyr exhibited
smaller HOMO-LUMO energy gap (2.16 eV) and higher oscillator strength (0.0245) than the
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acridine substituted Ac-CNImPyr (2.40 eV and 0.0001). The dihedral angles between the donor
unit and phenyl spacer attached to the acceptor of Ac-CNImPyr and PXZ-CNImPyr were 88.3
o
o
and 74.2 , respectively; suggesting twisted molecular structure for HOMO-LUMO separation
and TADF characteristic of these compounds.
Fig. 1. Chemical structures, HOMO-LUMO distribution andenergy gap (Eg), oscillator strength
(f) of the imidazopyrazine, CN-substituted imidazopyrazine acceptor, Ac-CNImPyr and PXZ-
CNImPyr.
For sufficient electron accepting character, ImPyr unit was utilized as a new electron accepting
unit of the TADF emitters and it was further modified by the incorporating cyano group at 3-
position. Scheme 1 represents synthetic route for Ac-CNImPyr and PXZ-CNImPyr.
Commercially available 3-chloro pyrazine 2-amine was reacted with phenacyl bromide to obtain
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a cyclized imidazopyrazine intermediate 1, which was further converted into important 3-cyano
substituted intermediate 3 through iodination and cyanation reactions. At the same time, 5-bromo
2-fluorobenzonitrile was reacted with acridine or phenoxazine in NaH/dimethylformamide
(DMF) reaction condition to obtain intermediates 4 and 5 (scheme S1 in supporting information),
which were further converted into intermediate 6 and 7, respectively, by the common boronate
ester formation reaction. They were reacted with previously synthesized intermediate 3 by
Suzuki coupling reaction to acquire the final products of Ac-CNImPyr and PXZ-CNImPyr.
1
The TADF materials were confirmed by mass and H/13C nuclear magnetic resonance analysis
and further purified by sublimation process under a vacuum condition to obtain extra pure
materials for the device study.
Scheme 1: synthetic route for Ac-CNImPyr and PXZ-CNImPyr.
Photophysical properties
To estimate their charge transfer features and singlet/triplet energies, photophysical properties of
Ac-CNImPyr and PXZ-CNImPyr such as ultraviolet-visible (UV-vis) absorption, low
temperature fluorescence (FL) and phosphorescence (Ph) were measured in toluene solution as
depicted in Figure 2. Their detailed photophysical parameters are summarized in Table 1. In the
absorption spectra of Ac-CNImPyr and PXZ-CNImPyr, high energy absorption peaks observed
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between 300 and 350 nm can be assigned to the π−π* transitions of the CNImPyr acceptor
derived backbone structure and the low energy absorption peaks at 430 and 458 nm are attributed
to the charge transfer (CT) transitions from acridine or phenoxazine donor to the CNImPyr
acceptor. Due to its strong electron donating ability, phenoxazine donor based PXZ-CNImPyr
exhibited red shifted absorption and PL peaks compared to the acridine donor based Ac-
CNImPyr. Fluorescence and phosphorescence spectra were measured at 77 K in toluene solvent
under a nitrogen atmosphere. Low temperature phosphorescence was discriminated from the
fluorescence by measuring it after 2 ms delay time because the TADF is not observed at low
temperature due to deactivation of the TADF by little thermal energy. Singlet/triplet energies of
Ac-CNImPyr and PXZ-CNimPyr were calculated from the onset energy of the fluorescence
and phosphorescence spectra and were measured to be 2.69/2.40 eV for Ac-CNImPyr, and
2.47/2.38 eV for PXZ-CNImPyr. The ΔEST values of Ac-CNImPy and PXZ-CNImPy were
0.29 and 0.09 eV, respectively. Photoluminescence quantum yields (PLQYs) of Ac-CNImPyr
and PXZ-CNImPyr emitters were 40 and 52 %, respectively. Comparatively high PLQY and
small ΔEST were noted in the PXZ-CNImPyr emitter, suggesting relatively good TADF
characteristic to show improved device performances for this material. To confirm their CT
properties, PL spectra of Ac-CNImPyr and PXZ-CNImPyr were measured in different organic
solvents as represented in Figure 3. As expected, the fluorescence emission spectra of them were
red shifted by increasing solvent polarity from non-polar to polar due to CT excited state of these
compounds.
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1.0
0.8
0.6
0.4
0.2
Ac-CNImPyr-UV
PXZ-CNImPyr-UV
Ac-CNImPyr-FL
PXZ-CNImPyr-FL
Ac-CNImPyr-Ph
PXZ-CNImPyr-Ph
0.0
300
400
500
600
700
800
Wavelength (nm)
Fig. 2. Normalized UV–vis absorption spectra (measured in toluene solution at 300 K),
fluorescence and phosphorescence spectra (measured in toluene solution at 77 K) of Ac-
CNImPyr and PXZ-CNImPyr emitter.
Table 1. Photophysical parameters of Ac-CNImPyr and PXZ-CNImPyr
b)
c)
c)
e)
f)
Emitter
λabs λPL
ΦPL
τ p
τ d
HOMO
d) [eV]
LUMO
d) [eV]
ES / ET
[eV]
ΔEST
[nm] [nm] [%]
sol a) sol b)
[ns]
[μs]
[eV]
Ac-CNImPyr
430
458
524
568
40
52
10.1
15.1
729.1
86.3
-5.82
-5.56
-3.57
-3.59
2.69/2.40
2.47/2.38
0.29
0.09
PXZ-CNImPyr
a) Measured in toluene solution; b) PLQY measured in 1 wt % doped polystyrene film under oxygen-free conditions
at room temperature; c) PL lifetimes of prompt (τp) and delayed (τd) decay components; d) Estimated from cyclic
voltammetry measurements through onset oxidation and reduction potentials; e) Singlet (Es) and triplet (ET)
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energies were obtained from the onset of fluorescence and phosphorescence emission spectra, respectively; f) ΔEST
= ES – ET.
(b)
(a)
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
n-hexane
Toluene
Chloroform
THF
n-hexane
Toluene
Chloroform
THF
Acetonitrile
Acetonitrile
400
500
600
700
800
900
500
600
700
800
900
Wavelength (nm)
Wavelength (nm)
Fig. 3. PL spectra of Ac-CNImPyr (a) and PXZ-CNImPyr (b) in different organic solvents.
In addition, TADF characteristics of these emitters were further confirmed by transient PL
measurements. Figure 4 represents transient PL data of Ac-CNImPyr and PXZ-CNImPyr at
room temperature, which show typical TADF decay with nanosecond-order prompt decay and a
microsecond order delayed decay under a degassed condition. The short lifetime prompt
components (τp) can be assigned to S1→S0 transition, while the long lifetime delayed
components (τd) are associated with TADF based intersystem crossing (ISC) and RISC
processes. The prompt /delayed lifetimes were measured to be 10.1 ns/729 s and 15.1 ns/86.1
s for Ac-CNImPyr and PXZ-CNImPyr, respectively. In consistent with their experimental
ΔEST values, strong phenoxazine donor based PXZ-CNImPyr emitter demonstrated shorter
delayed fluorescence lifetime than the acridine donor based Ac-CNImPyr emitter.
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Fig. 4. Delayed transient PL decay curves of 1 % Ac-CNImPyr and PXZ-CNImPyr doped
films in polystyrene at room temperature.(Inset: Prompt PL decay curves of 1 % Ac-CNImPyr
and PXZ-CNImPyr doped films in polystyrene at room temperature).
Thermal and electrochemical properties
Thermogravimetric analysis (TGA) study was performed to study thermal stability of Ac-
CNImPyr and PXZ-CNImPyr at high temperature. As represented in Figure 5, thermal
decomposition temperatures of Ac-CNImPyr and PXZ-CNImPyr were found to be 423 and
o
410 C, respectively, at 5 % weight loss. Both compounds were stable at high temperature and
suitable for vacuum evaporation process. Additionally, cyclic-voltammetry study of Ac-
CNImPyr and PXZ-CNImPyr was carried out using conventional three electrode system for the
calculation of their HOMO and LUMO energy levels (Figure 6). Onset oxidation potentials of
Ac-CNImPyr and PXZ-CNImPyr were found to be 1.01 and 0.85 eV respectively, while their
onset reduction potentials were -1.24 and -1.22 eV respectively. The HOMO/LUMO energy
levels of Ac-CNImPyr and PXZ-CNImPyr were calculated from the respective onset potentials
of oxidation and reduction, and were found to be -5.82/-3.57 eV for Ac-CNImPyr and -5.56/ -
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3.59 eV for PXZ-CNImPyr. The shallow HOMO level of PXZ-CNImPyr can be attributed to
the comparatively strong electron donating ability of the phenoxazine donor. The LUMO levels
were similarly deep by the strongly electron accepting CNImPyr acceptor shared in the two
TADF emitters.
110
Ac-CNImPyr
100
PXZ-CNImPyr
90
80
70
60
423oC
410oC
50
100
200
300
400
500
600
o
Temperature ( C)
Fig. 5. Thermogravimetric analysis (TGA) curves of Ac-CNImPyr and PXZ-CNImPyr.
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8
6
Ac-CNImPyr
PXZ-CNImPyr
4
0.85 eV
1.03 eV
2
0
-2
-4
-6
-8
-1.24 eV
-1.22 eV
-3
-2
-1
0
1
2
Voltage
Fig. 6. Cyclic voltammograms of TADF emitters Ac-CNImPyr and PXZ-CNImPyr.
Electroluminescence Performance
The electroluminescence (EL) performances of Ac-CNImPyr and PXZ-CNImPyr were
evaluated by doping them into 2-phenyl-4,6-bis(12-phenylindolo[2,3-a]carbazole-11-yl) -1,3,5-
triazine (PBICT) host material. Figure 7 shows the energy level diagram and chemical structures
of the materials used in different layers of the OLED devices. Figure 8, 9 and 10 represent
current density–voltage–luminance (J–V–L) characteristics, EQE-L curves and EL spectra of the
10 wt % emitter doped devices, respectively. The turn-on voltage of the two devices was quite
similar, but the driving voltage at 1,000 cd/m2 was dissimilar. The PXZ-CNImPyr emitter based
device exhibited lower driving voltage (4.9 V) than the Ac-CNImPyr emitter based device (5.8
V) possibly due to good hole transport through the Ac-CNImPyr emitter. EL spectrum of the
PXZ-CNImPyr device was red-shifted compared to that of the Ac-CNImPyr device, which
followed the same trend of the PL spectra. EL peaks of the PXZ-CNImPyr and Ac-CNImPyr
(541 and 570 nm) devices were red-shifted relative to their respective PL peaks (524 and 565
nm) due to the use of comparatively polar PBICT host material. Figure S1-S6 in supporting
information represents J–V–L characteristics, EQE-L curves and EL spectra of the Ac-CNImPyr
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and PXZ-CNImPyr devices at different doping concentrations. The EL spectra of the devices
were slightly red-shifted by increasing doping concentration from 1 to 10 % mainly due to the
interaction between dopant molecules at high doping concentration. The color coordinates of the
Ac-CNImPyr and PXZ-CNImPyr devices were (0.37, 0.55) and (0.50, 0.49), respectively.
Relatively high EQE of 12.7 % was exhibited by PXZ-CNImPyr. The device result was
consistent with comparatively high PLQY (52 %), short delayed fluorescence lifetime (86.3 s)
and small ΔEST (0.07 eV) of the PXZ-CNImPyr emitter. The EQE values of the devices were
decreased at high doping concentration mainly due to concentration quenching effect. Low EQE
(5.6 %) of the Ac-CNImPyr device is due to low PLQY (40 %). The 12.7 % EQE value of PXZ-
CNImPyr emitter is higher than the theoretically estimated value of ~5 % for the conventional
fluorescent emitter38–40 (this is not an absolute limit, but rather based on estimates of the
refractive index difference). Therefore, it was confirmed that triplet excitons as well as singlet
excitons were utilized through RISC process for the EL emission of the PXZ-CNImPyr emitter
based OLED device, demonstrating the potential of the CNImPyr acceptor based TADF emitters
as the efficient emitters in the future.
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Fig. 7. Energy-level diagram and material chemical structures of the Ac-CNImPyr and PXZ-
CNImPyr emitter doped devices.
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Fig. 8. Current density–voltage–luminance curves of Ac-CNImPyr and PXZ-CNImPyr based
OLED devices.
Fig.9. Quantum efficiency–luminance curves of the Ac-CNImPyr and PXZ-CNImPyr based
OLED devices.
1.0
Ac-CNImPyr
PXZ-CNImPyr
0.8
0.6
0.4
0.2
0.0
400
500
600
700
800
Wavelength (nm)
Fig.10. EL spectra of the Ac-CNImPyr and PXZ-CNImPyr devices.
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Table 3. Electroluminescence performances of the Ac-CNImPyr and PXZ-CNImPyr devices.
Emitter
λ EL Voltagea
CEmax
[cd A-1]
36.9
PEmax
[lm W-1]
38.6
EQEmax
[%]
CIE (x , y )
[nm]
541
[V]
5.8
4.9
Ac-CNImPy
PXZ-CNImPy
a At 1000 cd m-2
5.6
(0.37, 0.55)
(0.50, 0.49)
570
32.3
29.1
12.7
Conclusions
In conclusion, we developed two TADF emitters based on the newly introduced CN-substituted
2-phenyl imidazopyrazine acceptor named CNImPyr. The CNImPyr acceptor was combined
with either acridine or phenoxazine donor. Among the two emitters, the strong phenoxazine
donor based PXZ-CNImPyr emitter showed comparatively high PLQY, short delayed
fluorescence lifetime and small ΔEST, resulting in the device EQE of 12.7 %. This result
confirmed efficient TADF process in the PXZ-CNImPyr emitter and capability of CNImPyr
unit as an efficient electron acceptor for TADF emitters.
Experimental
Intermediate 1 and 2 were synthesized by following the reported procedure41 . General
information about measurement and calculation is in supporting information.
Synthesis of 8-chloro-2-phenylimidazo[1,2-a]pyrazine-3-carbonitrile (3)
8-Chloro-3-iodo-2-phenylimidazo[1,2-a]pyrazine (2) (1.3 g, 3.65 mmol) was dissolved in
dimethylformamide (50 mL) and copper cyanide (1.63 g, 18.28 mmol) was added in the reaction
o
mixture. The resulting mixture was stirred at 150 C for 12 h. Reaction mixture was cooled to
room temperature and water was added. A solid precipitate was filtered, dried well and purified
by column chromatography (60 % DCM in hexane) to afford a pure product (1.05 g, 42.68 %).
MS (FAB) m/z 255 [(M + H)+]. 1H NMR (500 MHz, CDCl3) δ 8.21-8.27 (m, 3H), 7.96-7.98 (d,
J = 7 Hz, 1H), 7.53-7.56 (m, 3H).
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Synthesis of 8-(3-cyano-4-(9,9-dimethylacridin-10(9H)-yl)phenyl)-2-phenylimidazo[1,2-
a]pyrazine-3-carbonitrile (Ac-CNImPyr)
8-Chloro-2-phenylimidazo[1,2-a]pyrazine-3-carbonitrile (0.3 g, 1.17 mmol) and 2-(9,9-
dimethylacridin-10(9H)-yl)-5-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzonitrile (0.61 g,
1.41 mmol) were dissolved in 1, 4 dioxane (20 mL). The reaction mixture was degassed with
argon for half an hour. tetrakis(triphenylphosphine)palladium (0.14 g, 0.12 mmol) was added
followed by K2CO3 (6 mL, 0.2 M solution in water) and the reaction mixture was stirred at 100 o
C for 12 h. After cooling to room temperature, water was added to the reaction mixture followed
by extraction with ethyl acetate three times (50 mL). The organic layer was dried over MgSO4,
concentrated on rotary evaporator and purified by column chromatography (40 %
dichloromethane in hexane) to afford a pure product (0.55 g, 88.42 %). MS (FAB) MS (FAB)
1
m/z 529 [(M + H)+]. H NMR (500 MHz, CDCl3): δ 9.58 (d, J = 2 Hz, 1H), 9.41-9.43 (dd, J =
8.5 and 2 Hz, 1H), 8.35-8.38 (m, 2H), 8.31-8.32 (d, J = 7 Hz, 2H), 7.75-7.76 (d, J = 8.5 Hz, 1H),
13
7.52-7.59 (m, 5H), 7.00-7.02 (m, 4 H), 6.24-6.26 (m, 2H), 1.76 (S, 6H). C NMR (125 MHz,
CDCl3): δ 31.3, 36.4, 111.5, 113.6, 116.2, 116.8, 118.3, 121.9, 125.8, 126.8, 127.9, 129.5, 130.3,
130.9, 131.3, 131.9, 133.9, 135.8, 135.9, 136.9, 139.9, 146.5, 146.9, 153.8.
Synthesis of 8-(3-cyano-4-(10H-phenoxazin-10-yl)phenyl)-2-phenylimidazo[1,2-a]pyrazine-
3-carbonitrile (PXZ-CNImPyr)
Similar procedure as described for the synthesis of Ac-CNImPyr was used except that 10H-
phenoxazine was used as the reactant instead of 9,9-dimethyl-9,10-dihydroacridine, to afford a
yellow solid (0.28 g, 71.06 %). MS (FAB) MS (FAB) m/z 503 [(M + H)+]. 1H NMR (500 MHz,
CDCl3): δ 9.55 (d, J = 2 Hz, 1H), 9.37-9.40 (dd, J = 8.5 and 2 Hz, 1H), 8.35-8.39 (m, 2H), 8.30-
8.32 (d, J = 7 Hz, 2H), 7.76-7.78 (d, J = 8.5 Hz, 1H), 7.54-7.61 (m, 3H), 6.73-6.79 (m, 4H),
13
6.64-6.68 (m, 2H), 5.97-5.99 (d, J = 8 Hz, 2H ). C NMR (125 MHz, CDCl3): δ 111.3, 113.2,
115.7, 116.2, 116.6, 118.1, 122.6, 123.4, 127.6, 129.3, 130.1, 131.1, 131.6, 132.6, 132.8, 135.8,
136.3, 136.6, 139.6, 143.9, 144.0, 146.4, 153.6.
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10.1002/asia.201901311
Chemistry - An Asian Journal
Device fabrication and measurements
The device configuration was ITO (50 nm)/PEDOT:PSS (60 nm)/TAPC (20 nm)/ mCP (10 nm)/
PBICT:Ac-CNImPyr or PXZ-CNImPyr (25 nm, X wt%)/TSPO1(5 nm)/TPBi (40 nm)/LiF(1.5
nm)/Al(200 nm); where poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)
was used as a hole injection layer; 4,4′-cyclohexylidenebis[N,N-bis(4-methylphenyl)aniline]
(TAPC) as a hole transporting layer and 2,2′,2′′-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-
benzimidazole) (TPBi) as an electron transporting layer. N,N-Dicarbazolyl-3,5-benzene (mCP)
and diphenyl-4-triphenylsilylphenyl phosphine oxide (TSPO1) were employed as hole- and
electron-transport type exciton blocking layer, respectively. Vacuum evaporation process was
used to produce the devices and the fabricated devices were encapsulated with a glass cover to
protect the devices at ambient condition. The devices were characterized using CS2000
spectroradiometer and Keithley 2400 source measurement unit.
Conflicts of interest
There are no conflicts of interest to declare.
Acknowledgements
This work was supported by the Basic Science Research Program (NRF
2018R1D1A1B07049240) through the National Research Foundation of Korea (NRF) funded by
the Ministry of Education.
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Table of Contents
Molecular design strategy utilizing imidazopyrazine as new electron accepting unit for two
thermally activated delayed fluorescence (TADF) emitters (Ac-CNImPyr and PXZ-
CNImPyr) was proposed. Especially TADF emitter with strong phenoxazine donor (PXZ-
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10.1002/asia.201901311
Chemistry - An Asian Journal
CNImPyr) exhibited sufficiently small ΔEST to initiate RISC process and realize high
external quantum efficiency up to 12.7 %. This new CN-modified imidazopyrazine
(CNImPyr) unit was successfully tested as proficient electron acceptor for the future
TADF emitter design.
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