CN-Modified Imidazopyridine as a New Electron Accepting Unit of Thermally Activated Delayed Fluorescent Emitters

Article abstract of DOI:10.1002/chem.201903877

Two efficient thermally activated delayed fluorescent (TADF) emitters were developed by utilizing CN-modified imidazopyridine as an acceptor unit. The CN-modified imidazopyridine acceptor was combined with either an acridine donor or a phenoxazine donor through a phenyl linker to produce two TADF emitters, Ac-CNImPy and PXZ-CNImPy. The acridine-based Ac-CNImPy emitter exhibited sky-blue emission with a CIE coordinate of (0.18, 0.38), whereas the phenoxazine-donor-based PXZ-CNImPy showed greenish-yellow emission with a CIE coordinate of (0.32, 0.58). A high photoluminescence quantum yield of 80 % was observed for the PXZ-CNImPy emitter compared with 40 % for the Ac-CNImPy emitter. Organic light-emitting diodes based on the PXZ-CNImPy emitter demonstrated high external quantum efficiency of 17.0 %. Hence, the CN-modified imidazopyridine unit can be considered as a useful electron acceptor for the future design of highly efficient TADF emitters.

Full text of DOI:10.1002/chem.201903877

A Journal of
Accepted Article
Title: CN-modified imidazopyridine as a new acceptor of thermally
activated delayed fluorescent emitter
Authors: Shantaram Kothavale, Kyung Hyung Lee, and Jun Yeob Lee
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CN-modified imidazopyridine as a new acceptor of thermally activated
delayed fluorescent emitter
Shantaram Kothavale, Kyung Hyung Lee, 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
Two efficient thermally activated delayed fluorescent (TADF) emitters were developed utilizing
CN-modified imidazopyridine as an acceptor unit. The CN-modified imidazopyridine acceptor
was combined with either an acridine donor or a phenoxazine donor through a phenyl linker to
produce two TADF emitters of Ac-CNImPy and PXZ-CNImPy. The acridine donor based Ac-
CNImPy emitter exhibited sky blue emission with a CIE coordinate of (0.18, 0.38), while the
phenoxazine donor based PXZ-CNImPy showed greenish-yellow emission with a CIE coordinate
of (0.32, 0.58). High photoluminescence quantum yield of 80 % was observed for the PXZ-
CNImPy emitter compared to 40% of the Ac-CNImPy emitter. Organic light emitting diode based
on the PXZ-CNImPy emitter demonstrated high external quantum efficiency of 17.0 %. Hence,
the CN-modified imidazopyridine unit can be considered as useful electron acceptor for the future
design of highly efficient TADF emitters.
Key words: Thermally activated delayed fluorescence, imidazopyridine, organic light emitting
diode, high external quantum efficiency.
Introduction
Thermally activated delayed fluorescence (TADF) organic light-emitting diodes (OLEDs) have
recently emerged as an efficient triplet harvesting device to compete or replace conventional
fluorescent and phosphorescent OLEDs due to its ability to exhibit 100 % internal quantum
efficiency. They are similar to phosphorescent OLEDs in terms of efficiency and have advantages
in that they can be built on easily available organic materials rather than expensive noble metals
used in the phosphorescent OLEDs 1–10. However, there are still some challenges limiting their
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practical applications such as short device lifetime, long delayed fluorescence lifetime, and broad
emission spectra and so on. To date, a lot of TADF emitters have been explored, but only a few
can truly demonstrate high EQE and excellent stability for OLEDs 11,12
In the TADF process, triplet excitons generated after electrical charge injection are up-converted
into the singlet excitons through reverse intersystem crossing (RISC) by thermal activation of the
triplet excitons for light emission in the singlet state13. The RISC process can be enhanced by the
reduced singlet-triplet energy gap (ΔEST) by designing materials with distorted backbone structure
between the electron donor (D) and electron acceptor (A) for the highest occupied molecular
orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) separation14,15. At the same time,
a large oscillator strength is also desired for efficient radiative decay from singlet excited state (S1)
to the ground state (S0) to reach high external quantum efficiency (EQE)16,17
.
Therefore, new molecular design strategies as well as new materials with efficient RISC are
desired for highly efficient and stable TADF based OLED devices by compensating the two
contradictory requirements18–23. In the last several years, a number of organic molecules with
different electron-donating and accepting groups bridged via a π-linker and/or σ-spacer have been
reported to exhibit TADF properties24,25 Electron donating moieties such as carbazole,
triphenylamine, acridine, phenoxazine, phenazine,26–30 and acceptor moieties including triazine,
pyrimidine, ketone, sulfone, phosphine oxide, dicyanobenzene, pyridine and triarylboron31–37 were
employed as successful donors/acceptors in the efficient TADF molecular emitter design.
However, for their commercial applications, TADF emitters still require new design of efficient
donor/acceptor moieties with improved RISC process. As the emission color of the TADF emitters
is closely related with the energy level difference between the HOMO and LUMO, weak electron
acceptors with shallow LUMO levels are generally promising to realize blue emission. Among
several acceptors, imidazole unit has proved potential as the electron acceptor for blue emitters
due to its shallow LUMO level and high triplet energy38. Recently, imidazole based sky-blue and
green TADF emitters have been developed39, but the device performances were not superb because
of weak electron accepting character of the imidazole unit. It is expected that the device
performances of the imidazole derived TADF emitters would be enhanced by strengthening the
acceptor character of the imidazole acceptor.
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In this work, we report CN-modified imidazopyridine as an electron acceptor of the TADF emitters.
The 3 and 6 positions of imidazo[1,2-a]pyridine were substituted with cyano groups to improve
the electron accepting ability and further deepen the LUMO level of the imidazo[1,2-a]pyridine
core, and a donor modified phenyl linker was connected via the 2-position of imidazo[1,2-
a]pyridine. Strong electron donors such as 9,9-dimethyl-9,10-dihydroacridine and 10H-
phenoxazine were attached at 4-position of phenyl ring to develop 2-(4-(9,9-dimethylacridin-
10(9H)-yl)phenyl)imidazo[1,2-a]pyridine-3,6-dicarbonitrile (Ac-CNImPy) and 2-(4-(10H-
phenoxazin-10-yl)phenyl)imidazo[1,2-a]pyridine-3,6-dicarbonitrile (PXZ-CNImPy) as new
TADF emitters. Photophysical analysis, molecular simulation, and device application of the two
emitters were carried out. Among the two TADF emitters, PXZ-CNImPy emitter with the
phenoxazine donor demonstrated high EQE of 17.0 %. Therefore, the CN-modified imidazo[1,2-
a]pyridine unit can be used as the effective acceptor for the future design of TADF materials.
Results and discussion
Synthesis and theoretical calculations
The electron acceptor of the two emitters, imidazo [1, 2-a] pyridine-3, 6-dicarbonitrile (CNImPy),
was designed to have strong electron accepting character by introducing pyridine and two CN units
in the well-known benzimidazole moiety. The replacement of the benzene unit of benzimidazole
with pyridine would intensify the electron deficiency and further substitution of two CN units can
produce the strongly electron deficient CNImPy acceptor. Scheme 1 represents synthetic route of
the Ac-CNImPy and PXZ-CNImPy materials. A commercially available 2-aminopyridine was
used as a main synthetic precursor. Iodination of 2-aminopyridine was carried out by following
the literature method to obtain intermediate 1, which was reacted with 4-bromophenacyl bromide
in ethanol using sodium bicarbonate as a base to obtain imidazopyridine intermediate 2. Further
3-position iodination of intermediate 2 was carried out using N-iodosuccinimide in acetonitrile
solvent to obtain 3, 6-diiodoimidazopyridine intermediate 3. Selective cyanation of intermediate 3
o
was carried out by reacting it with copper cyanide at 100 C for 24 h to obtain an important
intermediate 4. The target compounds, Ac-CNImPy and PXZ-CNImPy, were synthesized by
reacting intermediate 4 with 9, 9-dimethyl-9, 10-dihydroacridine and 10H-phenoxazine,
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respectively, according to Buchwald–Hartwig reaction condition in moderate yields. Pure products
were obtained by column chromatography purification and they were characterized by 1H-NMR,
13C-NMR and mass spectroscopy. The detailed synthetic procedures and analysis are given in the
experimental section. The two emitters were further purified by sublimation process under a
vacuum condition to obtain extra-pure materials.
Time dependent density functional (TD-DFT) theoretical calculations at B3LYP/6-31G (d) level
was carried out to find out plausible molecular configuration of Ac-CNImPy and PXZ-CNImPy
and deep understanding of their electronic properties such as HOMO-LUMO distribution,
oscillator strength, singlet and triplet energies, etc. Figure 1 represents optimized structures of Ac-
CNImPy and PXZ-CNImPy, their HOMO-LUMO distribution and energy levels. The effect of
CN-substitution on imidazopyridine acceptor was clearly visible, as both HOMO and LUMO
energy levels were deepened by 1.23 and 1.44 eV, respectively. For Ac-CNImPy and PXZ-
CNImPy compounds, the HOMO was mainly located on the acridine or phenoxazine donor, while
LUMO was distributed over imidazopyridine acceptor as well as phenyl linker between the donor
and acceptor. Due to the stronger electron donating capability of phenoxazine, the HOMO-LUMO
energy gap of PXZ-CNImPy was further decreased to 2.30 eV as compared to the 2.56 eV for
Ac-CNImPy. The dihedral angles calculated between acridine/phenoxazine donor and phenylene
o
o
linker were 89.8 and 76.6 for Ac-CNImPy and PXZ-CNImPy respectively. Such a large
dihedral angle between the donor and acceptor unit confirmed their highly distorted molecular
structures leading to spatial HOMO-LUMO separation and TADF characteristics. Further,
calculation results of the CN free imidazopyridine acceptor (ImPy) were compared with those of
CN-substituted imidazopyridine acceptor (CNImPy) for acridine donor substituted Ac-ImPy
compound (Figure S8 in SI). Comparatively large singlet-triplet energy gap (ΔEST) was observed
for the CN free ImPy based TADF material. Therefore, imidazopyridine acceptor was modified
with two CN units to synthesize CNImPy acceptor based Ac-CNImPy and PXZ-CNImPy TADF
emitters.
Photophysical properties
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The ultraviolet-visible (UV-vis) absorption of Ac-CNImPy and PXZ-CNImPy was measured in
toluene solvent as shown in Figure 2. Detailed photophysical parameters are summarized in Table
1. High energy absorption peaks from 300 to 350 nm observed for both compounds can be
attributed to the π−π* transitions of the CNImPy moiety, while low energy absorption peaks from
385 to 410 nm can be considered as intramolecular charge transfer transition from acridine or
phenoxazine donor unit to the CNImPy acceptor. Comparatively broad and red-shifted absorption
peak by 25 nm was observed for phenoxazine donor based PXZ-CNImPy compound due to its
strong electron donating ability. Fluorescence and phosphorescence spectra of the two compounds
were measured using 1 wt % emitter doped polystyrene film and toluene solution as shown in
Figure 2. Similar to the absorption spectra, the photoluminescence (PL) spectra of PXZ-CNImPy
were shifted to long wavelength relative to those of Ac-CNImPy. Peak wavelengths of the
fluorescence were 495 and 464 nm in the PXZ-CNImPy and Ac-CNImPy, respectively. Singlet
and triplet energies calculated from the onset of their fluorescence and phosphorescence spectra
were estimated to be 3.13 and 2.77 eV for Ac-CNImPy, respectively, while they were 2.85 and
2.66 eV for PXZ-CNImPy. The ΔEST calculated for Ac-CNImPy and PXZ-CNImPy were 0.36
and 0.19 eV, respectively. PL quantum yields (PLQYs) of Ac-CNImPy and PXZ-CNImPy under
nitrogen were measured to be 42 and 80 %, respectively. From the small ΔEST value and high
PLQY, efficient TADF process can be predicted for the PXZ-CNImPy compound. To confirm
charge transfer (CT) feature, PL spectra of Ac-CNImPy and PXZ-CNImPy in different polarity
organic solvents was measured (Figure S1 in SI). With the increasing solvent polarity, fluorescence
emission of Ac-CNImPy as well as PXZ-CNImPy was highly red shifted, suggesting stabilization
of their excited state in polar solvents to show strong CT emission characteristics.
Transient PL decay data of Ac-CNImPy and PXZ-CNImPy were measured to further investigate
the TADF characteristics. Figure 3 represents transient PL decay curves of two emitters with
nanosecond-order prompt decay and microsecond-order delayed decay components under vacuum.
Prompt decay with short lifetime component was attributed to the exciton relaxation from S1 to S0,
while the delayed decay with long lifetime can be assigned to the TADF emission involving
intersystem crossing from S1 to T1, RISC from T1 to S1, and then emission from S1 to S0. The
lifetimes of Ac-CNImPy for the prompt and delayed components were estimated to be 9.5 ns and
107.5 s, respectively, while those of PXZ-CNImPy were 15.6 ns and 57.9 s, respectively (Table
1). Strong electron donating ability of phenoxazine and small ΔEST allowed comparatively short
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delayed lifetime for PXZ-CNImPy. The detailed photophysical parameters including prompt
decay PLQY (ΦF), delayed decay PLQY (ΦTADF), total PLQY(ΦPL), and rate constants (kp and kd)
were calculated using the formulae in the SI and summarized in Table 2. Comparatively large
ΦTADF (43 %) and kd (1.73 × 104 s-1) were calculated for PXZ-CNImPy due to its smaller ΔEST
value than Ac-CNImPy, which was further reflected in the large kRISC (3.19 × 104 s-1) and small
Knr (5.47 × 103 s-1) for the PXZ-CNImPy emitter.
Thermal and Electrochemical properties
Thermal stability of the newly synthesized compounds, Ac-CNImPy and PXZ-CNImPy, was
checked by thermogravimetric analysis (TGA) as represented in Figure 4. Thermal decomposition
temperatures of Ac-CNImPy and PXZ-CNImPy at 5% weight loss were found at 362 and 376 oC,
respectively, confirming their high thermal stability for device application. To study their
electrochemical properties for the evaluation of HOMO-LUMO energy levels, cyclic voltammetry
study was carried out using a conventional three electrode system (Figure S2 in SI). Onset
oxidation and reduction potential calculated for Ac-CNImPy were found at 0.89 and -1.23 eV,
while 0.69 and -1.28 eV for PXZ-CNImPy respectively. HOMO/LUMO energy levels were
calculated from the onset oxidation/reduction potential and found at -5.71/-3.58 eV Ac-CNImPy
and -5.51/-3.53 eV for PXZ-CNImPy, respectively. As expected, shallow HOMO level was
detected by the strong donor and deep LUMO level was observed by the strong electron accepting
properties of the CNImPy acceptor.
Electroluminescence Performance
To find out the electroluminescence properties of Ac-CNImPy and PXZ-CNImPy, multilayer
OLEDs were fabricated by doping the emitters in 3-[3-(9H-carbazol-9-yl)phenyl]furo[2,3-b:5,4-
b']dipyridine(mCP-PFP) host material. The device configurations was ITO (50 nm)/PEDOT:PSS
(60 nm)/TAPC (20 nm)/ mCP (10 nm)/mCP-PFP:Ac-CNImPy or PXZ-CNImPy (25 nm, x
wt%)/TSPO1 (5 nm)/ TPBi (40 nm)/LiF(1.5 nm)/Al(200 nm); where indium tin oxide (ITO) was
used as anode, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as hole-
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injection layer, 4,4′-cyclohexylidenebis[N,N-bis(4-methylphenyl)aniline] (TAPC) as hole-
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. 2,2′,2′′-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) was
used as an electron transporting layer. The current density–voltage–luminance (J–V–L)
characteristics, external quantum efficiency (EQE)-luminescence curves and electroluminescence
(EL) spectra of the 10 wt % emitter doped devices are shown in Fig. 5, 6 and 7 respectively. In the
J-V-L data, the J was relatively low in the PXZ-CNImPy device because of comparatively strong
hole trapping by the shallow HOMO level. However, the L was high in the PXZ-CNImPy device
because of red-shifted emission color and high EQE of the PXZ-CNImPy device. Figure S3-S8
in SI represents current density–voltage–luminance (J–V–L) characteristics, external quantum
efficiency (EQE)-luminescence curves and electroluminescence (EL) spectra of Ac-CNImPy and
PXZ-CNImPy in different doping concentrations. Both devices exhibited high J and L values at
high doping concentrations. The EL spectra of the TADF devices showed red shifted emission
with increasing doping concentration.
The Ac-CNImPy emitter based OLED device exhibited bluish green color with CIE coordinate of
(0.18, 0.38) and PXZ-CNImPy emitter based device showed greenish yellow color with CIE
coordinate of (0.32, 0.58). The EL spectrum of the PXZ-CNImPy device appeared at long
wavelength compared to that of AC-CNImPy, agreeing with the trend of the PL spectra. The use
of the highly polar mCP-PFP host material in the device structure comparatively red-shifted the
EL peaks (495 and 526 nm) from their respective PL peaks (464 and 495 nm) in dilute toluene
solution for Ac-CNImPy and PXZ-CNImPy, respectively. Consistent with its high PLQY (80 %),
short delayed fluorescence lifetime (57.9 us) and small ΔEST (0.19 eV), strong donor substituted
PXZ-CNImPy emitter based device exhibited high maximum EQE of 17.0 %, clearly crossing
theoretical EQE limit (5 %) of fluorescent emitters. This confirms the utilization of triplet excitons
in addition to the singlet excitons for EL emission through RISC process. Assuming that the light
extraction efficiency of the device is 25%, the exciton utilization efficiency of the PXZ-CNImPy
device was 68%. From the device results, the CNImPy unit can be considered as an useful acceptor
for the further design of efficient TADF OLEDs.
Conclusions
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In conclusion, we designed and synthesized two thermally activated delayed fluorescent emitters,
Ac-CNImPy and PXZ-CNImPy, using CNImPy as a new acceptor. Due to the attachment of the
CN unit to the imidazopyridine core, electron accepting ability of CNImPy acceptor was further
increased. High PLQY, small ΔEST, and short delayed fluorescence lifetime were exhibited by the
phenoxazine donor based PXZ-CNImPy emitter. As a result, the TADF device using the PXZ-
CNImPY emitter demonstrated high EQE of 17.0 %. Therefore, the potential of the CNImPy unit
as an acceptor of the TADF emitter was confirmed for the future design of efficient TADF emitters.
Experimental Section
General information
2-Aminopyridine and 4-bromophenacyl bromide (Tokyo Chemical Industry Co., LTD.), N-iodo
succinimide and copper cyanide (Alfa Aesar Co., Inc.), 9,9-dimethyl-9,10-dihydroacridine, 10H-
phenoxazine, Pd2(dba)3, tri-tert-butylphosphine (P&H tech.), caesium carbonate, sodium hydrogen
carbonate, anhydrous o-xylene, anhydrous ethanol, dimethyl sulfoxide, and N,N-dimethyl
formamide (Duksan Sci. Co.) were used without further purification. Solvent used for NMR
analysis was deuterated chloroform (CDCl3). The ultraviolet-visible (UV-vis) absorption spectra
were measured using UV-vis spectrophotometer (JASCO, V-730), and the photoluminescence (PL)
spectra were recorded on a fluorescence spectrophotometer (PerkinElmer, LS-55). The HOMO
and LUMO levels were estimated from the onset of oxidation and reduction potential using a cyclic
voltammetry (Ivium Tech., Iviumstat). The 1H and 13C nuclear magnetic resonance (NMR) spectra
were recorded on a Avance-500 (Bruker, 500 MHz) spectrometer. The mass spectra were recorded
using a Advion, ExpresionL CMS spectrometer in APCI mode.
Synthesis of 2-(4-bromophenyl)-6-iodoimidazo[1,2-a]pyridine (2)
To
a
solution of 5-iodopyridin-2-amine (4 g, 18.18 mmol) and 2-bromo-1-(4-
bromophenyl)ethanone (6.06 g, 21.81 mmol) in anhydrous ethanol (40 mL), NaHCO3 (2.29 g,
27.27 mmol) was added and the reaction mixture was refluxed for 12 hrs. After cooling to room
temperature, the resulting mixture was concentrated on rotary evaporator to remove ethanol and
then water was added in the reaction mixture. The resulting solid precipitated out was filtered,
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washed with excess water, dried well and recrystallized in dichloromethane (DCM)-hexane
1
mixture to get a pure product (4.4 g, 60.68 %). MS (FAB) m/z 398 [(M + H)+]. H NMR (500
MHz, CDCl3) δ 8.37 (s, 1H), 7.78-7.81 (m, 3H), 7.55-7.57 (d, J = 8.5 Hz, 2H), 7.40-7.42 (d, J =
8.5 Hz, 1H), 7.33-7.36 (dd, J = 9.5 and 1.5 Hz, 1H).
Synthesis of 2-(4-bromophenyl)-3,6-diiodoimidazo[1,2-a]pyridine (3)
2-(4-Bromophenyl)-6-iodoimidazo[1,2-a]pyridine (4 g, 10.02 mmol) was dissolved in anhydrous
acetonitrile (50 mL). N-iodosuccinimide (2.71 g, 12.02 mmol) was added and resulting mixture
was refluxed for 12 hrs. After cooling to room temperature, water was added to the reaction
mixture and the mixture was extracted with ethyl acetate (100 mL) three times. Organic layer was
washed with saturated solution of sodium bisulfate, dried over Mg2SO4 and evaporated on rotary
evaporator. Crude product obtained was further recrystallized in DCM-hexane mixture to get a
pure product (4.2 g, 79.84 %). MS (FAB) m/z 525 [(M + H)+]. 1H NMR (500 MHz, CDCl3) δ 8.46
(s, 1H), 7.93-7.95 (d, J = 8.5 Hz, 2H), 7.60-7.62 (d, J = 8.5 Hz, 2H), 7.39-7.46 (m, 2H).
Synthesis of 2-(4-bromophenyl)imidazo[1,2-a]pyridine-3,6-dicarbonitrile (4)
2-(4-Bromophenyl)-3,6-diiodoimidazo[1,2-a]pyridine (4 g, 7.62 mmol) was dissolved in
dimethylformamide (50 mL), copper cyanide (2.38 g, 26.67 mmol) was added and the resulting
mixture was stirred at 100 o C for 24 hrs. After cooling to room temperature, water was added to
the reaction mixture and the mixture was extracted with ethyl acetate. The organic layer was dried
over MgSO4, concentrated on rotary evaporator and purified by column chromatography (1:1 ethyl
acetate-hexane mixture) to afford a pure product (1.05 g, 42.68 %). MS (FAB) m/z 324 [(M +
H)+]. 1H NMR (500 MHz, CDCl3) δ 9.53 (s, 1H), 8.04-8.06 (d, J = 8.5 Hz, 2H), 7.89-7.91 (dd, J
= 9 and 1 Hz, 2H), 7.82-7.84 (d, J = 8.5 Hz, 2H).
Synthesis of 2-(4-(9,9-dimethylacridin-10(9H)-yl)phenyl)imidazo[1,2-a]pyridine-3,6-
dicarbonitrile (Ac-CNImPy)
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2-(4-Bromophenyl)imidazo[1,2-a]pyridine-3,6-dicarbonitrile (0.3 g, 0.92 mmol) was reacted with
9,9-dimethyl-9,10-dihydroacridine (0.23 g, 1.11 mmol), Cs2CO3 (0.60 g, 1.85 mmol), Pd2(dba)3
(0.08 g, 0.09 mmol) and tri-tert-butylphosphine (0.02 mL, 0.09 mmol) in anhydrous o-xylene (20
mL). The resulting mixture was stirred at 140 oC for 12 h. After cooling to room temperature, the
reaction mixture was extracted with DCM. Organic layer was dried over anhydrous MgSO4 and
was concentrated on rotary evaporator. The crude product obtained was further purified by column
chromatography (DCM : n-hexane (2:1)) to obtain a greenish-yellow solid as a pure product (0.24
g, 57 %). MS (FAB) m/z 452 [(M + H)+]. 1H NMR (500 MHz, CDCl3): δ 8.82 (s, 1H), 8.45-8.47
(d, J = 8.5 Hz, 2H), 7.87-7.89 (d, J = 9.5 Hz, 1H), 7.61-7.63 (dd, J = 9.5 and 1.5 Hz, 1H), 7.53-
7.55 (d, J = 8.5 Hz, 2H), 7.47-7.49 (dd, J = 7.5 and 1.5 Hz, 2H), 6.94-7.01 (m, 4 H), 6.33-6.35 (d,
J = 8 Hz, 2H), 1.71 (S, 6H). 13C NMR (125 MHz, CDCl3). δ 31.3, 36.2, 95.5, 101.7, 111.5, 114.3,
115.3, 119.5, 121.2, 125.5, 126.6, 129.2, 130.1, 130.3, 130.5, 131.3, 132.4, 140.7, 144.1, 146.3,
154.5.
Synthesis of 2-(4-(10H-phenoxazin-10-yl)phenyl)imidazo[1,2-a]pyridine-3,6-dicarbonitrile
(PXZ-CNImPy)
Similar procedure as described for the synthesis of Ac-CNImPy was used except that 10H-
phenoxazine was used as the reactant instead of 9,9-dimethyl-9,10-dihydroacridine, yielding a
1
yellow solid (0.21 g, 54 %). MS (FAB) MS (FAB) m/z 426 [(M + H)+]. H NMR (500 MHz,
CDCl3): δ 8.82 (s, 1H), 8.42-8.44 (d, J = 8.5 Hz, 2H), 7.88-7.90 (d, J = 9.5 Hz, 1H), 7.62-7.64 (dd,
J = 9.5 and 1.5 Hz, 1H), 7.54-7.56 (d, J = 8.5 Hz, 2H), 6.60-6.73 (m, 6 H), 6.00-6.02 (d, J = 8 Hz,
2H). 13C NMR (125 MHz, CDCl3) δ 101.7, 111.5, 113.5, 115.3, 115.8, 119.5, 121.9, 123.5, 129.2,
130.3, 130.4, 131.3, 132.0, 134.0, 141.7, 144.1, 146.3, 154.3.
Device fabrication and measurements
The Ac-CNImPy and PXZ-CNImPy emitters were evaluated in the device structure of indium
tin oxide (ITO, 50 nm)/poly(3,4 ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS,
60 nm)/4,4′-cyclohexylidenebis[N,N-bis(4-methylphenyl)aniline] (TAPC, 20 nm)/1,3-bis(N-
carbazolyl)benzene
(mCP,
10
nm)/
3-[3-(9H-carbazol-9-yl)phenyl]furo[2,3-b:5,4-
b']dipyridine(mCP-PFP):Ac-CNImPy or PXZ-CNImPy (25 nm, x wt%)/diphenylphosphine
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oxide-4-(triphenylsilyl)phenyl (TSPO1, 5 nm)/2,2′,2′′-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-
benzimidazole) (TPBi) (40 nm)/LiF(1.5 nm)/Al(200 nm). All layers of the device structure were
deposited by vacuum thermal evaporation under a high pressure of 5.0 × 10−7 Torr. The device
was encapsulated with a glass lid to protect from moisture and oxygen. The optical characterization
was carried out using a CS 2000 spectroradiometer and electrical characterization of the devices
was performed using a Keithley 2400 source meter.
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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List of table
Table 1. Photophysical parameters of Ac-CNImPy and PXZ-CNImPy
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Table 2. Transient PL related photophysical parameters of Ac-CNImPy and PXZ-CNImPy
Table 3. Electroluminescence performance of Ac-CNImPy and PXZ-CNImPy
Table 1. Photophysical parameters of Ac-CNImPy and PXZ-CNImPy
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c)
c)
e)
f)
Emitter λabs λPL
ΦPL τ p
τ d
HOMO LUMO ES / ET
ΔE ST
b)
[nm] [nm]
[ns]
[μs]
d) [eV]
-5.71
-5.51
d) [eV] [eV]
[eV]
0.36
0.19
sol a) sol a) [%]
Ac-
385 464 42
9.5
107.5
57.9
-3.58 3.13/2.77
-3.53 2.86/2.67
CNImPy
PXZ-
410 495
80 15.6
CNImPy
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) energies
were obtained from the onset of fluorescence and phosphorescence emission spectra, respectively; f) ΔEST = ES – ET.
Table 2. Transient PL related photophysical parameters of Ac-CNImPy and PXZ-CNImPy.
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Emitter
ΦF
ΦTADF
kp
kd [×104
kISC
[×107
s−1]
kRISC
kRISC/kISC
kr [×107
knr
[×103
s−1]
[%]
[%]
[×107
s−1]
s−1]
[×104 s−1]
s−1]
Ac-CNImPy
0.36
0.056
0.431
10.5
6.40
0.93
1.73
6.67
4.04
0.23
3.19
0.0000
0.0008
3.81
2.36
8.48
5.47
PXZ-CNImPy 0.37
Table 3. Electroluminescence performance of Ac-CNImPy and PXZ-CNImPy
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Emitter
λ EL
Voltagea
[V]
CEmax
[cd A-1]
13.8
PEmax
[lm W-1]
14.5
EQEmax
[%]
CIE (x , y )
[nm]
495
Ac-CNImPy
PXZ-CNImPy
a At 1000 cd m-2
5.8
5.2
(0.18, 0.38)
(0.32, 0.58)
526
4.9
56.9
58.1
17.0
List of Figures
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Fig. 1. Chemical structures, HOMO and LUMO distribution, HOMO and LUMO levels,
HOMO-LUMO gap (Eg), and oscillator strength (f) of the imidazopyridine, CN-modified
imidazopyridine, Ac-CNImPy and PXZ-CNImPy.
Scheme 1. Synthetic route for Ac-CNImPy and PXZ-CNImPy.
Fig. 2. UV–vis absorption and PL spectra of 1 wt% Ac-CNImPy and PXZ-CNImPy-doped
polystyrene films.
Fig. 3. Transient PL decay curves of 1 % Ac-CNImPy and PXZ-CNImPy doped films in
polystyrene at room temperature.
Fig. 4. Thermogravimetric analysis (TGA) curves of Ac-CNImPy and PXZ-CNImPy.
Fig. 5. Current density–voltage–luminance curves of Ac-CNImPy and PXZ-CNImPy based
OLED devices.
Fig.6. Quantum efficiency–luminance curves of the Ac-CNImPy and PXZ-CNImPy based
OLED devices.
Fig.7. EL spectrum of the Ac-CNImPy and PXZ-CNImPy.
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Fig. 1. Chemical structures, HOMO and LUMO distribution, HOMO and LUMO levels,
HOMO-LUMO gap (Eg), and oscillator strength (f) of the imidazopyridine, CN-modified
imidazopyridine, Ac-CNImPy and PXZ-CNImPy.
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Scheme 1. Synthetic route for Ac-CNImPy and PXZ-CNImPy.
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Fig. 2. UV–vis absorption and PL spectra of 1 wt% Ac-CNImPy and PXZ-CNImPy-doped
polystyrene films.
1.0
0.8
0.6
0.4
0.2
0.0
Ac-CNImPy-UV-Vis
PXZ-CNImPy-UV-Vis
Ac-CNImPy-PL
PXZ-CNImPy-PL
Ac-CNImPy-LTPL
PXZ-CNImPy-LTPL
300
400
500
600
700
Wavelength (nm)
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Fig. 3. Transient PL decay curves of 1 % Ac-CNImPy and PXZ-CNImPy doped films in
polystyrene at room temperature.
10000
Ac-CNImPy
PXZ-CNImPy
1000
100
10
1
0
250
500
750
1000
1250
1500
Time (uS)
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Fig. 4. Thermogravimetric analysis (TGA) curves of Ac-CNImPy and PXZ-CNImPy.
100
Ac-CNImPy
PXZ-CNImPy
90
80
70
60
100
200
300
400
500
600
o
Temperature ( C)
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Fig. 5. Current density–voltage–luminance curves of Ac-CNImPy and PXZ-CNImPy based
OLED devices.
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Fig.6. Quantum efficiency–luminance curves of the Ac-CNImPy and PXZ-CNImPy based
OLED devices.
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Fig.7. EL spectrum of the Ac-CNImPy and PXZ-CNImPy.
1.0
Ac-CNImPy
0.8
PXZ-CNImPy
0.6
0.4
0.2
0.0
400
500
600
700
Wavelength (nm)
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Table of Contents
CN-modified imidazopyridine unit was developed as novel acceptor for the highly efficient
thermally activated delayed fluorescent emitter. Detail photophysical, thermal, electrochemical,
and electroluminescence properties of two emitters were investigated and correlated with the
theoretical calculation results.
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