Design approach of exciplexes enhancing the singlet and triplet energy by managing electron transport type host

Article abstract of DOI:10.1016/j.orgel.2018.11.010

A high singlet energy and triplet energy exciplex host was developed by synthesizing an electron transport type host with a controlled lowest unoccupied molecular orbital (LUMO) level and intermolecular interaction. A diphenyltriazine derived host was des

Full text of DOI:10.1016/j.orgel.2018.11.010

Accepted Manuscript
Design approach of exciplexes enhancing the singlet and triplet energy by managing
electron transport type host
Sang Kyu Jeon, Ji Gwang Yu, Wook Song, Ha Lim Lee, Jun Yeob Lee
PII:
S1566-1199(18)30580-9
DOI:
https://doi.org/10.1016/j.orgel.2018.11.010
ORGELE 4973
Reference:
To appear in: Organic Electronics
Received Date: 3 September 2018
Revised Date: 2 November 2018
Accepted Date: 5 November 2018
Please cite this article as: S.K. Jeon, J.G. Yu, W. Song, H.L. Lee, J.Y. Lee, Design approach of
exciplexes enhancing the singlet and triplet energy by managing electron transport type host, Organic
Electronics (2018), doi: https://doi.org/10.1016/j.orgel.2018.11.010.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Design approach of exciplexes enhancing the singlet and triplet energy by
managing electron transport type host
Sang Kyu Jeon, Ji Gwang Yu, Wook Song, Ha Lim Lee, Jun Yeob Lee*
School of Chemical Engineering, Sungkyunkwan University
2066, Seobu-ro, Jangan-gu, Suwon, Gyeonggi, 440-746, Korea
* Corresponding author
E-mail : leej17@skku.edu
Abstract
A high singlet energy and triplet energy exciplex host was developed by synthesizing an
electron transport type host with a controlled lowest unoccupied molecular orbital (LUMO)
level and intermolecular interaction. A diphenyltriazine derived host was designed by
substituting t-butyl functional moieties to the electron transporting core structure to make the
LUMO shallow for high exciplex emission energy. An exciplex host with a singlet energy of
3.49 eV and a triplet energy of 3.02 eV was derived by mixing the t-butyl modified
diphenyltriazine type host with a hole transport type host. The comparison of the newly
developed t-butyl modified host with a previous host without the t-butyl group clarified that
the t-butyl group had a singlet and triplet energy boosting effect. A high external quantum
efficiency of 17.3% was demonstrated in the blue phosphorescent device using the high
triplet energy exciplex host.
Keywords : exciplex, host, singlet energy, triplet energy, efficiency

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Introduction
An exciplex is a kind of material class which forms an intermolecular charge transfer
complex after excitation by absorbing light. Generally, a hole transport type material is mixed
with an electron transport type material to make the exciplex. In the exciplex emission
process, electrons are excited from a ground state to an excited state by light absorption in the
hole transport type material and then the excited electrons are transferred to an adjacent
electron transport type material, which radiatively decays to the ground state of the hole
transport type material. Therefore, the emission energy of the exciplex is dependent on the
highest occupied molecular orbital (HOMO) of the hole transport type material and the
lowest unoccupied molecular orbital (LUMO) of the electron transport type material, and
exciton binding energy between holes and electrons in the exciplex.[1-2]
The exciplex has been widely used as the host of phosphorescent organic light-emitting
diodes (OLEDs) because of high efficiency and low driving voltage merited by the exciplex
host compared to conventional host. However, the exciplex host development was mostly
focused on red and green phosphorescent OLEDs[3-14] rather than blue phosphorescent
OLEDs[15-22] because it was difficult to increase the singlet and triplet energy of the exciplex
host. Until now, only several high triplet energy exciplex hosts compatible with blue triplet
emitters are available. Kido group reported a blue exciplex of a carbazole host and
diphenylsulfone host[15], Kim group demonstrated a blue exciplex of a carbazole host and a
diphenylphosphine oxide host[17, 19], Lee group developed an exciplex of a carbazole host and
a diphenylphosphine oxide modified triazine host[22], and our group explored an exciplex of a
carbazole modified diphenylamine host and a diphenyltriazine host[20]. Other than these, no
blue exciplex has been described in the literature. Therefore, it is essential to develop an
exciplex for use in blue organic light-emitting diodes.

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In this work, a design approach to increase the singlet and triplet energy of an exciplex host
was explored by synthesizing a diphenyltriazine type electron transport type host which has
an adjusted energy level and steric hindrance for high energy exciplex formation by sterically
hindered t-butyl modification. It was described that the exciplex derived using the t-butyl
modified diphenyltriazine type host enabled energy transfer from the exciplex host to blue
emitters and harvested singlet and triplet excitons of blue emitters.
Results and discussion
The electron transport type host designed for high singlet and triplet energy was 2,8-bis(4,6-
bis(3,5-di-tert-butylphenyl)-1,3,5-triazin-2-yl)dibenzo[b,d]furan (DBFtTrz) which has a
dibenzofuran core structure, triazine electron transport unit, and a t-butyl modified phenyl
unit surrounding the triazine unit. DBFtTrz was different from a previous 2,8-bis(4,6-
diphenyl-1,3,5-triazin-2-yl)dibenzo[b,d]furan (DBFTrz)[25] host in that two t-butyl units were
substituted at 3 and 5 positions of phenyl unit in the diphenyltriazine moiety. The role of the
t-butyl units would be to shift the LUMO level of the host due to the electron donating
function of the t-butyl unit and to interrupt intermolecular interaction by protecting aromatic
building blocks. The shifted LUMO level would increase the emission energy of the exciplex
host by enlarging HOMO-LUMO offset and the suppressed intermolecular interaction may
have the same effect on the emission energy by decreasing exciton binding energy.
The synthesis of DBFtTrz was carried out by Suzuki coupling reaction between 2,8-
bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)dibenzo[b,d]furan and 2-chloro-4,6-bis(3,5-
di-tert-butylphenyl)-1,3,5-triazine prepared by Grignard reaction of 1,3-di-tert-butyl-5-
chlorobenzene and cyanuric chloride. The synthesis scheme of DBFtTrz is shown in Scheme
1. Recrystallization and column chromatography purification of the crude product provided a

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pure product at a production yield of 81% and vacuum sublimation produced a highly pure
final product.
Ultraviolet-visible (UV-vis) absorption and photoluminescence (PL) emission behaviors
of DBFtTrz were measured for basic photophysical analysis. Figure 1 shows UV-vis
absorption spectra, fluorescence at room temperature and phosphorescence at 77 K. The
phosphorescence was observed from 426 nm and the triplet energy was 2.91 eV from the first
phosphorescence peak. Peak position of fluorescence was 387 nm, which corresponded to a
singlet energy of 3.20 eV. UV-vis absorption spectra estimated the HOMO-LUMO gap from
the edge of the absorption spectra. The HOMO-LUMO gap of the DBFtTrz was 3.59 eV.
Energy levels of DBFtTrz were determined from the HOMO gathered from electrochemical
cyclic voltammetry (CV) measurement and UV-vis absorption gap. The UV-vis and CV data
of DBFtTrz offered the HOMO/LUMO of -6.59/-3.00 eV, which was relatively shallow
compare to -6.64/-3.07 eV of DBFTrz without the t-butyl blocking groups. The t-butyl
blocking groups made the energy level shallow due to electron donating character of the t-
butyl functional group. Therefore, the DBFtTrz would generate high energy exciplex as an
electron transport type host compared to DBFTrz because the HOMO-LUMO offset between
two hosts in the exciplex would be increased.
As the DBFtTrz host was designed as an electron transport type host for exciplex host, it
was mixed with a hole transport type 9,10-Dihydro-9,9-dimethyl-10- (9-phenyl-9H-carbazol-
3-yl)-acridine (PCZAC)[26] host to develop a high emission energy exciplex. The exciplex
emission of PCZAC:DBFtTrz host was compared with that of PCZAC:DBFTrz to understand
the effect of t-butyl modification on the exciplex emission energy. Fluorescence and
phosphorescence spectra of PCZAC:DBFtTrz and PCZAC:DBFTrz are shown in Figure 2.
The fluorescence spectra of each material included in the exciplexes were added in Figure S1
in supporting information. Singlet energy and triplet energy were obtained from the onset

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wavelength of the PL spectra at room temperature (fluorescence) and 77 k (phosphorescence).
Overall, the onset wavelength of the PL spectra was shifted to short wavelength in the
PCZAC:DBFtTrz exciplex. Singlet energy/triplet energy values of PCZAC:DBFtTrz and
PCZAC:DBFTrz were 3.05/2.98 eV and 2.95/2.95 eV, respectively. Both singlet energy and
triplet energy of the exciplex were increased by adopting the t-butyl modified DBFtTrz host
instead of DBFTrz host. This can be correlated with the HOMO-LUMO offset and
geometrical structure of the exciplex. The HOMO-LUMO offset of the exciplex is increased
by the LUMO level shift in the DBFtTrz, which lead to large emission energy in the exciplex.
In addition, the geometrical structure separating the aromatic units of PCZAC and DBFtTrz
weakens the binding of two molecules, which decreases the exciton binding energy and
increases the emission energy.
Molecular dynamics (MD) simulation was conducted with PCZAC: DBFTrz and PCZAC:
DBFtTrz. After performing a MD simulation with a cubic structure consisting of two hosts,
the trajectory distribution was visualized by DFT calculation. MD and DFT simulation results
are shown in Figure 3. With the introduction of the t-butyl group, the distance between the
HOMO dominant unit (acridine) of the donor forming the two exciplexes and the LUMO
dominant unit (triazine) of the acceptor was affected. The distance between the acridine and
triazine of PCZAC: DBFTrz was 3.74 Å, while that of PCZAC: DBFtTrz was 4.54 Å. This
result suggests that the t-butyl group reduces the attractive interaction between the two host
molecules constituting the exciplex, reduces the exciton binding of the exciplex and then
shifts the emission wavelength to a short wavelength.
The singlet and triplet energy boosting effect of t-butyl modified DBFtTrz was validated by
monitoring the emission spectrum of blue emitter doped exciplex. A common fluorescent
emitter, TBPe, was a blue emitter to study the Forster energy transfer from the exciplex to the
fluorescent emitter. The emission spectrum of the blue emitter doped PCZAC:DBFtTrz and

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PCZAC:DBFTrz exciplexes is shown in Figure 4. It is clear from the emission spectrum that
the emission of the TBPe doped PCZAC:DBFTrz is a mixed emission of exciplex and TBPe
due to incomplete energy transfer, while that of TBPe doped PCZAC:DBFtTrz reflects TBPe
emission due to efficient energy transfer. As the singlet energy of the PCZAC:DBFtTrz was
higher than that of PCZAC:DBFTrz, the energy transfer from the exciplex host to the blue
emitter was efficient in the DBFtTrz based exciplex.
The effect of the increased triplet energy in the DBFtTrz derived exciplex was confirmed
using tris(4-carbazoyl9-ylphenyl)amine (TCTA):DBFtTrz exciplex. TCTA:DBFTrz was a
reference
exciplex
and
fac-tris(3-(1-(2,6-diisopropylphenyl)-1H-imidazol-2-
yl)benzonitrile)Iridium (Ir(CNpi)3) was a blue phosphorescent emitter to examine the effect
of DBFtTrz. Emission spectra of the Ir(CNpi)3 doped TCTA:DBFtTrz and TCTA:DBFTrz
exciplexes are shown in Figure 5. As can be expected from the emission spectra, the emission
of the Ir(CNpi)3 doped TCTA:DBFtTrz is a pure emission of the phosphorescent emitter, but
that of the Ir(CNpi)3 doped TCTA:DBFTrz is a mixture of exciplex emission and Ir(CNpi)3
emission, indicating incomplete energy transfer from the TCTA:DBFTrz exciplex to the
Ir(CNpi)3 emitter due to low triplet energy (2.90 eV at onset energy)[23]. This was also
confirmed by the emission spectra of the TCTA:DBFTrz:Ir(CNpi)3 and
TCTA:DBFtTrz:Ir(CNpi)3 devices in Figure S2. Pure Ir(CNpi)3 emission was observed in the
TCTA:DBFtTrz:Ir(CNpi)3 device, but complex emission of exciplex and Ir(CNpi)3 was
detected in the TCTA:DBFTrz:Ir(CNpi)3 device. The efficiency was also reduced by the
incomplete energy transfer. The low triplet energy issue was overcome in the
TCTA:DBFtTrz exciplex, which incurred pure Ir(CNpi)3 emission by energy transfer. The
singlet energy/triplet energy values of the TCTA:DBFTrz and TCTA:DBFtTrz were
2.95/2.81 eV and 3.17/2.92 eV, respectively. The singlet energy and triplet energy of the
exciplexes are summarized in Table S1 in supporting information.

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Encouraged by the complete triplet energy transfer from the DBFtTrz derived exciplex to a
blue phosphorescent emitter, blue phosphorescent OLEDs were developed using high triplet
energy exciplex hosts. Two exciplexes with the DBFtTrz platform, TCTA:DBFtTrz and 3,5-
di(9H-carbazol-9-yl)- N,N-diphenylaniline (DCDPA):DBFtTrz, were applied as the hosts of
Ir(CNpi)3. The singlet energy/triplet energy values of the TCTA:DBFtTrz and
DCDPA:DBFtTrz exciplexes were 3.17/2.92 eV and 3.49/3.02 eV, respectively, as projected
from the fluorescence and phosphorescence spectra in Figure 6. The emission energy was
relatively high in the DCDPA:DBFtTrz exciplex. The two exciplexes showed rather large
singlet energy-triplet energy splitting possibly due to weak binding of the two materials in the
exciplexes, suggesting that they are not thermally activated delayed fluorescence type
exciplexes. All PL spectra of the exciplexes are shown in Figure S3.
Device analysis data of the exciplex host based blue phosphorescent OLEDs are summarized
in Figure 7. The current density (J) of the exciplex device was high in the TCTA:DBFtTrz
device because of better hole injection property of TCTA than that of DCDPA as predicted
from the energy level diagram in Figure 7 (a). Luminance (L) of the device also showed the
same trend.
External quantum efficiency (EQE) data of the exciplex devices are shown in Figure 7 (c).
The maximum EQEs of the exciplex devices were 12.3% and 17.3% in the TCTA:DBFtTrz
and DCDPA:DBFtTrz devices, respectively. At 1,000 cd/m2, the EQEs were 9.1% and 12.0%
in the TCTA:DBFtTrz and DCDPA:DBFtTrz devices, respectively. The EQE was rather high
in the DCDPA:DBFtTrz exciplex device due to high triplet energy harvesting all triplet
excitons of Ir(CNpi)3. This can be confirmed by the relatively low PL quantum yield of the
Ir(CNpi)3 doped TCTA: DBFtTrz (0.42) compared to that of the Ir(CNpi)3 doped DCDPA:
DBFtTrz (0.53). Moreover, balanced carrier density in the DCDPA:DBFtTrz device also
contributed the relatively high EQE in the DCDPA:DBFtTrz device.

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Device emission spectra at a luminance of 1,000 cd/m2 are presented in Figure 7 (d). Exactly
the same spectra were observed in the two exciplex devices because of energy transfer. The
color coordinates of the Ir(CNpi)3 doped TCTA:DBFtTrz and DCDPA:DBFtTrz devices
were (0.16,0.24) and (0.15,0.23). The PL and EL emission spectra of the
DCDPA:DBFtTrz:Ir(CNpi)3 were quite similar as shown in Figure S4. Table 1 summarized
the device results of the blue phosphorescent OLEDs.
Conclusions
A singlet and triplet energy boosting host in the exciplex, DBFtTrz, was synthesized and
emission behavior as the exciplex and the host in the blue phosphorescent OLEDs was
investigated. The DBFtTrz based exciplex showed higher singlet and triplet energy than a
similarly designed DBFTrz by the t-butyl functional unit and performed well as the electron
transport type host in the exciplex host. Therefore, the t-butyl functional group based electron
transport type host design would be useful to develop high energy exciplex host for blue
OLEDs.
Experimental
General information
Cyanuric chloride and 1-bromo-3,5-di-tert-butylbenzene were purchased from TCI. Co. Mg
turning was purchased from Sigma-Aldrich, Inc. Tetrakis(triphenylphosphine)palladium(0)
(Pd(pph3)4) was purchased from Sunfine global Co. Ltd. Dichloromethane (MC) for high
performance liquid chromatography and potassium carbonate (K2CO3) were purchased from
Duksan Sci. Co. Tetrahydrofuran (THF) was purchased from Samchun pure chemical Co.
Ltd and distilled with sodium and calcium hydride.

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The ultraviolet- visible (UV-Vis) spectra were measured to analyze absorption of compounds
using a sample dissolved in refined THF. The photoluminescence (PL) was measured to
analyze PL spectra of compounds dissolved in refined THF. Low temperature PL
measurement was carried out at 77 k to measure triplet energy of compounds. The 1H and 13C
nuclear magnetic resonance (NMR) spectra were recorded to identify the compounds.
1
13
Samples were dissolved in deuterated dimethyl sulfoxide (DMSO-d6) for H and C NMR
measurement. The mass data were recorded by liquid chromatography-mass spectrometer
(LC-MS) to measure the molecular weight of compounds.
Synthesis
2-Chloro-4,6-bis(3,5-di-tert-butylphenyl)-1,3,5-triazine (1)
2-Chloro-4,6-bis(3,5-di-tert-butylphenyl)-1,3,5-triazine was synthesized according to the
synthetic method of 2-chloro-4,6-bis(3,5-di-tert-butylphenyl)-1,3,5-triazine[24] using 8.00 g
(35.6 mmol) of 1-bromo-3,5-di-tert-butylbenzene. Purification by column chromatography
(dichloromethane : n-hexane (1:4)) and drying afforded a white powder as a product (8.13 g,
72 % yield).
2,8-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)dibenzo[b,d]furan (2)
2,8-Diboronic ester dibenzofuran was synthesized according to the synthetic method of
previous research[25]
.
Crude product was purified by column chromatography
(dichloromethane:n-haxane (1:4) and complete drying afforded a white product (12.2 g, 64.4%
yield).

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2,8-Bis(4,6-bis(3,5-di-tert-butylphenyl)-1,3,5-triazin-2-yl)dibenzo[b,d]furan (DBFtTrz)
Compound 1 (4.92 g, 10.0 mmol), Compond 2 (2.0 g, 4.8 mmol), Pd(pph3)4 (0.17 g, 0.144
mmol) and K2CO3 (2.0 g, 14.4 mmol) were stirred in a two-neck round-bottomed flask filled
with THF (100 ml) under a nitrogen atmosphere. The solution was refluxed overnight.
Cooling of the mixture followed by extraction using dichloromethane and distilled water,
separation of the dichloromethane layer, and evaporation of the solvent afforded a crude
product. Crude product was purified by column chromatography (dichloromethane:n-haxane
(1:2)) and vacuum sublimation, yielding a white product (4.2 g, 81% yield) with a purity of
99.5% from high performance column chromatography analysis.
1H NMR (500 MHz, dimethylsulfoxide-d6): δ 9.324 (s, 2H), 8.964-8.943 (d, 2H, J=10.5 Hz),
13
8.682 (s, 8H), 7.855-7.837 (d, 2H, J=9.0 Hz), 7.721 (s, 4H), 1.468 (s, 72H). C NMR (125
MHz, dimethylsulfoxide-d6): δ172.50, 172.41, 159.66, 151.44, 135.88, 132.82, 129.77,
127.12, 124.86, 123.36, 122.28, 112.27, MS (API-) m/z: 1080.5 [(M + H)-].
Molecular dynamics simulations
The MD simulation and minimization process to investigate the geometry between the two
molecules was performed using the desmond tool and the jaguar tool, which are the edodes of
the schrodinger material science suite. In addition, Equilibration simulations are performed
on the NPT ensemble via the Nose-Hoover chain and the Martyna-Tobias-Klein method. The
simulation box was set as a buffer with a boundary distance of 10 Å and cut-off when it
exceeded.
Device fabrication and characterization

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All organic materials except for poly(3,4-ethylenedioxythiophene):poly (styrene sulfonate
(PEDOT: PSS) used in OLED devices were vacuum deposited at a pressure of 1.0 × 10-7 Torr.
The device structure is indium tin oxide (50 nm) / PEDOT:PSS (40 nm)/1,1-bis(4-
bis(4methylphenyl)aminophenyl)-cyclohexane
(TAPC)
(20
nm)/
1,3-bis(N-
carbazolyl)benzene (mCP) (10 nm)/ exciplex hosts:Ir(CNpi)3 (30 nm:5 wt%)/diphenyl-4-
triphenylsilylphenyl-phosphine oxide (TSPO1) (5 nm)/1,3,5-tris(1-phenyl-1Hbenzimidazol-
2-yl)benzene (TPBi) (30 nm)/LiF (1 nm)/Al (200 nm). The ratio of the two hosts constituting
the exciplex was 50:50. Chemical structures of the materials are displayed in Figure S5 in
supporting information. Electrical and optical analysis was performed using a Keithley 2400
source measurement tool and a CS2000 spectroradiometer.
Acknowledgements
This work was supported by Nano Materials Research Program (2016M3A7B4909243)
through the National Research Foundation of Korea (NRF) funded by Ministry of Science,
ICT and Future Planning.

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Fully Utilizing Exciton Energy for Highly Efficient Organic Light Emitting Diodes with Low
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List of table
Table 1. Summary of the device results of the blue phosphorescent OLEDs.

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Table 1. Summary of the device results of the blue phosphorescent OLEDs.
Driving Voltage*
[V]
QE*
[%]
QEMAX
CIE x* CIE y*
[%]
5.5
5.9
9.1
12.3
0.16
0.15
0.24
0.23
TCTA:DBFtTrz
12.0
17.3
DCDPA:DBFtTrz
QE: external quantum efficiency
* : Data at a luminance of 1000 cd/m2

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List of figures
Scheme 1. Synthesis scheme of DBFtTrz
Figure 1. UV-vis absorption spectra and PL spectra at room temperature and at low
temperature (77 K) of DBFtTrz.
Figure 2. Fluorescence and phosphorescence spectra of PCZAC:DBFTrz (a) and
PCZAC:DBFtTrz (b).
Figure 3. MD simulation and DFT visualization of two exciplexes.
Figure 4. The emission spectrum of TBPe doped PCZAC:DBFTrz (a) and PCZAC:DBFtTrz
(b).
Figure 5. Emission spectra of the Ir(CNpi)3 doped TCTA:DBFtTrz (a) and TCTA:DBFTrz
exciplexes (b).
Figure 6. Fluorescence and phosphorescence spectra of TCTA:DBFtTrz (a) and
DCDPA:DBFtTrz (b) exciplexes
Figure 7. Device configuration (a), current density-voltage-luminance curves (b), EQE-
current density curves (c), and EL spectra of the exciplex devices.

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Scheme 1.

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Figure 1.

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Figure 2.

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Figure 3.

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Figure 4.

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Figure 5.

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Figure 6.

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Figure 7.

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Supporting information
Design approach of exciplexes enhancing the singlet and triplet energy by
managing electron transport type host
Sang Kyu Jeon, Ji Gwang Yu, Wook Song, Ha Lim Lee, Jun Yeob Lee*
School of Chemical Engineering, Sungkyunkwan University
2066, Seobu-ro, Jangan-gu, Suwon, Gyeonggi, 440-746, Korea
* Corresponding author
E-mail : leej17@skku.edu

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Table S1. Summary of photophysical properties of exciplexes.
PL,peak (nm)
λ
S1 (eV)
T1 (eV)
483
469
505
477
409
2.95
3.05
2.95
3.17
3.49
2.95
2.98
2.81
2.92
3.02
PCZAC:DBFTrz
PCZAC:DBFtTrz
TCTA:DBFTrz
TCTA:DBFtTrz
DCDPA:DBFtTrz
* Singlet energy and triplet energy were calculated on the basis of spectrum onset.

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Figure S1. PL spectra of PCZAC, TCTA and DCDPA films