Non-doped thermally activated delayed fluorescent organic light-emitting diodes using an intra- and intermolecular exciplex system with a: Meta -linked acridine-triazine conjugate

Article abstract of DOI:10.1039/c8tc02590h

We report OLEDs utilizing non-doped TADF emitters, 10-(3-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-9,9-dimethyl-9,10-dihydroacridine (AmT) and 10-(3′-(4,6-diphenyl-1,3,5-triazin-2-yl)-[1,1′-biphenyl]-3-yl)-9,9-dimethyl-9,10-dihydroacridine (AmmT). These two emitters are composed of electron donor (acridine) and acceptor (triphenyltriazine) moieties, which are connected at the meta positions so that intra- and intermolecular charge transfer (CT) states can be induced through exciplex-like interactions between the same emitters. A neat film of AmmT forms intermolecular CT state-induced exciplexes more favorably than AmT because of the packing benefits and relatively shorter radiative lifetime of AmmT. Electroluminescent devices using AmmT as an emitter exhibit an external quantum efficiency (EQE) over 18%, which is much better than the EQE value for devices using AmT. This is the first example of a non-doped TADF system using a single molecule-based exciplex as a dominant exciton source.

Full text of DOI:10.1039/c8tc02590h

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J. K. Kim, D. Jang and J. Hong, J. Mater. Chem. C, 2018, DOI: 10.1039/C8TC02590H.
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ISSN 2050-7526
PAPER
~
Nguyên T. K. Thanh, Xiaodi Su et al.
Fine-tuning of gold nanorod dimensions and plasmonic properties using
the Hofmeister effects
rsc.li/materials-c

tꢈꢊꢇꢂꢊ !ꢄ ꢆꢄꢍ ꢇ!ꢃꢅꢂꢍ ꢏꢇ "ꢁꢆꢂ
Journal of Materials Chemistry C
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DOI: 10.1039/C8TC02590H
Wꢀꢁꢂꢃꢄꢅ ꢀ aꢄꢆꢇꢂꢈꢄꢅꢉ /ꢊꢇꢋꢈꢉꢆꢂꢌ /
t!t9w
bꢀꢃꢔ ꢀ!ꢇ Çꢊꢇꢂꢋꢄꢅꢅꢌ !ꢑꢆꢈ#ꢄꢆꢇ 5ꢇꢅꢄꢌꢇ Cꢅꢁꢀꢂꢇꢉꢑꢇꢃꢆ hꢂꢒꢄꢃꢈꢑ
[ꢈꢒꢊꢆꢔꢇꢋꢈꢆꢆꢈꢃꢒ 5ꢈꢀ ꢇꢉ Üꢉꢈꢃꢒ ꢄꢃ Lꢃꢆꢂꢄꢔ ꢄꢃ Lꢃꢆꢇꢂꢋꢀꢅꢇꢑꢁꢅꢄꢂ 9*ꢑꢈ!ꢅꢇ*
{ꢌꢉꢆꢇꢋ ꢏꢈꢆꢊ ꢄ aꢇꢆꢄꢔꢅꢈꢃ,ꢇ !ꢑꢂꢈ ꢈꢃꢇꢔÇꢂꢈꢄ-ꢈꢃꢇ /ꢀꢃ.ꢁꢒꢄꢆꢇ
wꢊꢌꢊꢁ#ꢊ! ꢑꢑꢍꢀ Wꢇꢆꢅꢇ ꢋ ꢐꢑꢒꢒꢔ
!ꢌꢌꢊ&ꢍꢊ! ꢑꢑꢍꢀ Wꢇꢆꢅꢇ ꢋ ꢐꢑꢒꢒ
{ꢊꢄꢆ"ꢃꢁꢆ Wꢊꢄꢆ"ꢔ òꢄꢅꢆ"ꢆꢇꢏ [ꢊꢊꢔ Wꢄꢄꢆ Yꢁ Yꢁꢏꢔ 5ꢅꢗWꢊꢄꢆ Wꢇꢆ" ꢇꢆ! Wꢄꢆ"ꢗLꢆ Iꢄꢆ"1
5hL* ꢕꢑꢓꢕꢑꢖ+,ꢒꢑꢒꢒꢑꢑꢑꢑꢑꢒ
ꢏꢏꢏꢐꢂꢉꢑꢐꢀꢂꢒꢓ
íꢊ ꢊ&ꢄ ꢍ h[95ꢂ ꢅꢍꢁꢈꢁ4ꢁꢆ" ꢆꢄꢆꢗ!ꢄ&ꢊ! Ç!5C ꢊꢏꢁꢍꢍꢊ ꢂꢔ ꢕꢑꢗ6ꢖꢗ67ꢔ8ꢗ!ꢁ&ꢀꢊꢆꢋꢈꢗꢕꢔꢖꢔ9ꢗꢍ ꢁꢇ4ꢁꢆꢗꢐꢗꢋꢈ:&ꢀꢊꢆꢋꢈ:ꢗ+ꢔ+ꢗ!ꢁꢏꢊꢍꢀꢋꢈꢗ+ꢔꢕꢑꢗ
!ꢁꢀꢋ! ꢄꢇꢌ ꢁ!ꢁꢆꢊ 6!ꢏÇ: ꢇꢆ! ꢕꢑꢗ6ꢖ;ꢗ67ꢔ8ꢗ!ꢁ&ꢀꢊꢆꢋꢈꢗꢕꢔꢖꢔ9ꢗꢍ ꢁꢇ4ꢁꢆꢗꢐꢗꢋꢈ:ꢗ<ꢕꢔꢕ;ꢗ=ꢁ&ꢀꢊꢆꢋꢈ>ꢗꢖꢗꢋꢈ:ꢗ+ꢔ+ꢗ!ꢁꢏꢊꢍꢀꢋꢈꢗ+ꢔꢕꢑꢗ!ꢁꢀꢋ! ꢄꢇꢌ ꢁ!ꢁꢆꢊ
6!ꢏꢏÇ:ꢓ Çꢀꢊꢂꢊ ꢍ?ꢄ ꢊꢏꢁꢍꢍꢊ ꢂ ꢇ ꢊ ꢌꢄꢏ&ꢄꢂꢊ! ꢄꢎ ꢊꢈꢊꢌꢍ ꢄꢆ !ꢄꢆꢄ 6ꢇꢌ ꢁ!ꢁꢆꢊ: ꢇꢆ! ꢇꢌꢌꢊ&ꢍꢄ 6ꢍ ꢁ&ꢀꢊꢆꢋꢈꢍ ꢁꢇ4ꢁꢆꢊ: ꢏꢄꢁꢊꢍꢁꢊꢂꢔ ?ꢀꢁꢌꢀ
ꢇ ꢊ ꢌꢄꢆꢆꢊꢌꢍꢊ! ꢇꢍ ꢍꢀꢊ ꢏꢊꢍꢇ &ꢄꢂꢁꢍꢁꢄꢆꢂ ꢂꢄ ꢍꢀꢇꢍ ꢁꢆꢍ ꢇꢗ ꢇꢆ! ꢁꢆꢍꢊ ꢏꢄꢈꢊꢌꢅꢈꢇ ꢌꢀꢇ "ꢊ ꢍ ꢇꢆꢂꢎꢊ 6/Ç: ꢂꢍꢇꢍꢊꢂ ꢌꢇꢆ =ꢊ ꢁꢆ!ꢅꢌꢊ! ꢍꢀ ꢄꢅ"ꢀ
ꢊꢒꢌꢁ&ꢈꢊꢒꢗꢈꢁ@ꢊ ꢁꢆꢍꢊ ꢇꢌꢍꢁꢄꢆꢂ =ꢊꢍ?ꢊꢊꢆ ꢍꢀꢊ ꢂꢇꢏꢊ ꢊꢏꢁꢍꢍꢊ ꢂꢓ ! ꢆꢊꢇꢍ ꢎꢁꢈꢏ ꢄꢎ !ꢏꢏÇ ꢎꢄ ꢏꢂ ꢁꢆꢍꢊ ꢏꢄꢈꢊꢌꢅꢈꢇ /Ç ꢂꢍꢇꢍꢊꢗꢁꢆ!ꢅꢌꢊ!
ꢊꢒꢌꢁ&ꢈꢊꢒꢊꢂ ꢏꢄ ꢊ ꢎꢇ#ꢄ ꢇ=ꢈꢋ ꢍꢀꢇꢆ !ꢏÇ =ꢊꢌꢇꢅꢂꢊ ꢄꢎ ꢍꢀꢊ &ꢇꢌ@ꢁꢆ" =ꢊꢆꢊꢎꢁꢍꢂ ꢇꢆ! ꢊꢈꢇꢍꢁ#ꢊꢈꢋ ꢂꢀꢄ ꢍꢊ ꢇ!ꢁꢇꢍꢁ#ꢊ ꢈꢁꢎꢊꢍꢁꢏꢊ ꢄꢎ !ꢏꢏÇꢓ
9ꢈꢊꢌꢍ ꢄꢈꢅꢏꢁꢆꢊꢂꢌꢊꢆꢍ !ꢊ#ꢁꢌꢊꢂ ꢅꢂꢁꢆ" !ꢏꢏÇ ꢇꢂ ꢇꢆ ꢊꢏꢁꢍꢍꢊ ꢊꢒꢀꢁ=ꢁꢍ ꢇꢆ ꢊꢒꢍꢊ ꢆꢇꢈ Aꢅꢇꢆꢍꢅꢏ ꢊꢎꢎꢁꢌꢁꢊꢆꢌꢋ 69v9: ꢄ#ꢊ ꢕCDꢔ ?ꢀꢁꢌꢀ ꢁꢂ
ꢏꢅꢌꢀ =ꢊꢍꢍꢊ ꢍꢀꢇꢆ ꢍꢀꢊ 9v9 #ꢇꢈꢅꢊ ꢎꢄ !ꢊ#ꢁꢌꢊꢂ ꢅꢂꢁꢆ" !ꢏÇꢓ Çꢀꢁꢂ ꢁꢂ ꢍꢀꢊ ꢎꢁ ꢂꢍ ꢊꢒꢇꢏ&ꢈꢊ ꢄꢎ ꢇ ꢆꢄꢆꢗ!ꢄ&ꢊ! Ç!5C ꢂꢋꢂꢍꢊꢏ ꢅꢂꢁꢆ" ꢇ ꢂꢁꢆ"ꢈꢊ
ꢏꢄꢈꢊꢌꢅꢈꢊꢗ=ꢇꢂꢊ!
ꢊꢒꢌꢁ&ꢈꢊꢒ
ꢇꢂ
ꢇ
!ꢄꢏꢁꢆꢇꢆꢍ
ꢊꢒꢌꢁꢍꢄꢆ
ꢂꢄꢅ ꢌꢊꢓ
approach can provide 100% internal quantum efficiency (IQE)
because the triplet can be converted to a singlet because of the
small energy difference between the singlet and triplet states
(;EST), which leads to efficient reverse intersystem crossing
(RISC). In TADF molecules, donor and acceptor moieties have
limited electronic interactions, by their orthogonal arrangement
as a result of steric hindrance, or by increasing the distance
between them. Therefore, TADF molecules have small ;EST
and emit light by charge transfer (CT) processes with a high
IQE.10,11 This makes it possible to overcome the efficiency
problems of fluorescent materials.
However, TADF molecules can easily aggregate through <œ
< interactions that arise from their innate hydrophobicity and
rigid planar structures and lead to aggregation-caused
quenching (ACQ) and exciton concentration quenching in the
solid state.11-13 These phenomena fatally weaken the OLEDs
using solid-state emission. To overcome these problems, a
doping system was introduced into the EML of TADF OLED
devices, despite the aforementioned drawbacks.14 An
alternative method to avoid ACQ is to use aggregation-induced
emission (AIE).15,16 An external quantum efficiency (EQE) of
approximately 10% was obtained in an OLED device using
non-doped EML materials that caused AIE and TADF.17
However, there are a few reports of highly efficient OLED
devices using AIE-TADF systems.
ꢎꢐ Lꢃꢆꢂꢀ ꢁꢑꢆꢈꢀꢃ
Since the 1987 report by Tang and Vanslyke, organic light-
emitting diodes (OLEDs)1 have continued to draw attention
because of their many possible applications in future displays,
from smart phones to large flat panel displays.2 The vast
majority of previous fluorescent emitting layers (EML) of
OLED devices were constructed using a host-dopant system
(doping system).3,4,5 Controlling intermolecular interactions
through the doping system made it possible to achieve high-
efficiency luminescence characteristics by preventing
concentration quenching and controlling the color purity.
However, the device performance of the doping system would
seriously deteriorate during operation because of inherent phase
separation.4 In addition, a scrupulous design of the appropriate
host is required based on the energy gap and charge-
transporting characteristics of the dopant material. Therefore,
developing non-doped (or host-free) emitters is desirable
because the non-doped system has a simple device structure
that employs only a single emitting material without using a
complicated host-dopant co-deposition process.6,7
Recently, there have been continuous efforts to overcome
the limitations of fluorescence-based OLED devices by
harvesting long-lived triplet excitons through thermally
activated delayed fluorescence (TADF) utilizing additionally
generated singlet excitons without using heavy atoms.8,9 This
Çꢀꢁꢂ ꢃꢄꢅ ꢆꢇꢈ ꢁꢂ ꢉ Çꢀꢊ wꢄꢋꢇꢈ {ꢄꢌꢁꢊꢍꢋ ꢄꢎ /ꢀꢊꢏꢁꢂꢍ ꢋ ꢐꢑꢒꢒ
Wꢀ bꢁꢂꢃꢓꢔ ꢐꢑꢕꢖꢔ ꢍꢍꢔ ꢕꢗꢖ ꢘ ꢎ
tꢈꢊꢇꢂꢊ !ꢄ ꢆꢄꢍ ꢇ!ꢃꢅꢂꢍ ꢏꢇ "ꢁꢆꢂ

tꢈꢊꢇꢂꢊ !ꢄ ꢆꢄꢍ ꢇ!ꢃꢅꢂꢍ ꢏꢇ "ꢁꢆꢂ
Journal of Materials Chemistry C
Page 2 of 7
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DOI: 10.1039/C8TC02590H
t!t9w
Wꢀꢁꢂꢃꢄꢅ ꢀ aꢄꢆꢇꢂꢈꢄꢅꢉ /ꢊꢇꢋꢈꢉꢆꢂꢌ /
Since the report by Kim et al. in 2012, there have been
many reports on TADFs using intermolecular CT complexes
based on two different materials (exciplex system).18,19 One can
easily achieve appropriate charge balance and reduce the
quenching process by lowering the charge carrier and exciton
density in the EML using exciplex systems.20,21 Above all,
exciplex systems should provide a small ;EST. However,
Cꢁ"ꢓ ꢕꢓ aꢄꢈꢊꢌꢅꢈꢇ ꢂꢍ ꢅꢌꢍꢅ ꢊꢂꢔ Ihah ꢇꢆ! [Üah !ꢁꢂꢍ ꢁ=ꢅꢍꢁꢄꢆꢂ ꢇꢆ! ꢌꢇꢈꢌꢅꢈꢇꢍꢊ! ꢂꢁꢆ"ꢈꢊꢍ ꢇꢆ!
ꢍ ꢁ&ꢈꢊꢍ ꢊꢆꢊ "ꢋ ꢈꢊ#ꢊꢈꢂ ꢎ ꢄꢏ ꢍꢁꢏꢊꢗ!ꢊ&ꢊꢆ!ꢊꢆꢍ !ꢊꢆꢂꢁꢍꢋ ꢎꢅꢆꢌꢍꢁꢄꢆꢇꢈ ꢍꢀꢊꢄ ꢋ ꢌꢇꢈꢌꢅꢈꢇꢍꢁꢄꢆꢂ ꢇꢍ ꢍꢀꢊ
.ꢖ[òt,8ꢗꢖꢕD6!: ꢈꢊ#ꢊꢈꢓ
exciplex systems require careful consideration of the highest
occupied molecular orbital (HOMO) and lowest unoccupied
molecular orbital (LUMO) energies of the donor and acceptor,
respectively. In addition, it is important to have the optimum
ratio of donor and acceptor materials when fabricating the
device, because slight deviation in the optimum donor/acceptor
molar ratio can cause a significant decrease in the device
performance by breaking the charge carrier balance of the
device.22,23 Therefore, we developed a non-doped TADF
intermolecular distances in the neat film. In addition, AmT and
AmmT show DF. Photophysical and single crystal X-ray
structural analyses confirmed that intermolecular CT-induced
DF can occur more effectively in AmmT because AmmT
molecules are more uniformly and tightly packed in the neat
film than AmT. OLEDs using AmmT as an emitter without a
host showed an EQE above 18%, which is better than the EQE
value of AmT-based OLEDs, because of the more efficient
formation of intermolecular exciplexes of AmmT. This is the
first example of TADF molecules utilizing intermolecular CT
excitons (exciplex) between donor-acceptor-type emitter
molecules of a single species.
system based on
a single molecule exciplex with the
advantages of the exciplex system and the non-doped TADF.
In this study, we designed two emitting materials that have
delayed fluorescence (DF) character by their intra- and
intermolecular interactions; 10-(3-(4,6-diphenyl-1,3,5-triazin-2-
yl)phenyl)-9,9-dimethyl-9,10-dihydroacridine (AmT) and 10-
(3'-(4,6-diphenyl-1,3,5-triazin-2-yl)-[1,1'-biphenyl]-3-yl)-9,9-
dimethyl-9,10-dihydroacridine (AmmT). AmT and AmmT
consist of an acridine (Ac) donor and a triphenyltriazine (TRZ)
acceptor, and have phenyl and biphenyl linkers that are
connected to the donor and the diphenyltriazine moiety of the
acceptor, respectively, through a meta linkage. Calculated
structures of AmT and AmmT show that the electron
distributions of the HOMO and LUMO are almost separated,
providing small exchange energies and almost the same singlet
and triplet energy levels. Because of the meta-positioned
phenyl and biphenyl moieties, ICT-induced absorption bands of
AmT and AmmT were not observed, and the resulting low
oscillator strength should lead to poor luminescence. However,
a strong emission with a large Stokes shift was observed in the
neat film of AmT or AmmT; it originated from the
intermolecular CT state (like exciplex) corresponding to the
difference between the HOMO of the emitter donor moiety and
the LUMO of the emitter acceptor moiety. This phenomenon
results from the more favorable formation of an intermolecular
exciplex in the neat film than in solution because of the reduced
/ꢐ wꢇꢉꢁꢅꢆꢉ ꢄꢃ ꢈꢉꢑꢁꢉꢉꢈꢀꢃ
2.1 Synthesis of AmT and AmmT
The synthetic routes of AmT and AmmT are illustrated in
Scheme 1. 9,9-Dimethyl-9,10-dihydroacridine (Ac, 1) and 2-(3-
bromophenyl)-4,6-diphenyl-1,3,5-triazine (4) were synthesized
according to previous methods.24,25 10-(3-Iodophenyl)-9,9-
dimethyl-9,10-dihydroacridine (2) was obtained using the
Ullmann cross coupling reaction between compound 1 and 1,3-
diiodobenzene.
(3-(9,9-dimethylacridin-10(9H)-yl)phenyl)-
boronic acid (3) was synthesized by reacting compound 2 with
trimethyl borate at -78 I . AmT and AmmT were prepared by
Buchwald cross coupling between compounds 1 and 4 and by
Suzuki cross coupling using compounds 3 and 4, respectively.
1
AmT and AmmT were characterized by H NMR, 13C NMR,
and low- and high-resolution mass spectrometry. These two
compounds were purified further by train sublimation (twice)
under a reduced pressure (<10-4 Torr).
/ ꢘ Wꢀ bꢁꢂꢃꢓꢔ ꢐꢑꢕꢐꢔ ꢍꢍꢔ ꢕꢗꢖ
Çꢀꢁꢂ ꢃꢄꢅ ꢆꢇꢈ ꢁꢂ ꢉ Çꢀꢊ wꢄꢋꢇꢈ {ꢄꢌꢁꢊꢍꢋ ꢄꢎ /ꢀꢊꢏꢁꢂꢍ ꢋ ꢐꢑꢒꢒ
tꢈꢊꢇꢂꢊ !ꢄ ꢆꢄꢍ ꢇ!ꢃꢅꢂꢍ ꢏꢇ "ꢁꢆꢂ

tꢈꢊꢇꢂꢊ !ꢄ ꢆꢄꢍ ꢇ!ꢃꢅꢂꢍ ꢏꢇ "ꢁꢆꢂ
Journal of Materials Chemistry C
Page 3 of 7
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DOI: 10.1039/C8TC02590H
t!t9w
Wꢀꢁꢂꢃꢄꢅ ꢀ aꢄꢆꢇꢂꢈꢄꢅꢉ /ꢊꢇꢋꢈꢉꢆꢂꢌ /
The photophyscial properties of AmT and AmmT were
analyzed using ultraviolet-visible (UV-Vis) and
photoluminescence (PL) spectroscopies, and transient PL decay
measurements. At room temperature (300 K), each compound
shows almost the same UV-Vis absorbance around 270 nm,
which is attributed to the <-<* absorption band from the Ac
donor and the TRZ acceptor in the dichloromethane solution
(Fig. 3(a)).27,28 Both materials do not exhibit any ICT-induced
absorption bands above 300 nm because of the limited
electronic interactions by the meta-positioned donor and
acceptor moieties. However, the two compounds exhibit dim,
broad emission with a large Stokes shift (187 nm and 256 nm
for AmT and AmmT, respectively) in toluene. The triplet
energy levels of AmT and AmmT were estimated by the
emission onset of the PL spectra at 77 K to be 2.94 and 2.97
eV, respectively (Fig. 3(b)). The low temperature fluorescence
and phosphorescence (20 ms time delay) spectra of AmT show
Rmax values at 514 and 484 nm, respectively. AmmT shows the
same low temperature fluorescence and phosphorescence
spectra at 468 nm. The structureless fluorescence and
phosphorescence emission spectra of AmT and AmmT at low
temperature indicate that their emissions are dominated by the
CT state, which means that the T1 states of AmT and AmmT
originate from the 3CT state. We examined the PL
solvatochromic shift of AmT and AmmT using various solvents
at room temperature (Fig. S2). Both materials show that the PL
spectra are red-shifted, and the PL intensities decrease as the
dielectric constant of the solvents increases. This indicates that
their emissive S1 states are dependent on the dielectric constant
of the solvent.
2.2 DFT Calculations and Crystallography
Fig. 1 depicts the molecular structures and theoretical
simulation of AmT and AmmT. The optimized geometries and
electron density distributions of the frontier molecular orbitals
were investigated using time-dependent density functional
theory calculations at the B3LYP/6-31G(d) level using
Gaussian ‘09. The HOMOs of AmT and AmmT are located on
the Ac donors, whereas the LUMOs are distributed over the
TRZ acceptors. The HOMO of AmT is extended slightly to the
neighboring phenyl of the TRZ acceptor while its LUMO is
extended over TRZ. There is a small orbital overlap between
the HOMO and LUMO of AmT in the connecting phenyl
moiety of TRZ, as shown in Fig. 1. In contrast, the calculated
structure of AmmT shows totally separated electron
distributions of the HOMO and LUMO because of an
additional meta-linked phenylene moiety. Thus, AmT shows a
small ;EST value of 0.015 eV, whereas that of AmmT is even
lower (0.002 eV).
Single crystal X-ray diffraction analysis of AmT and
AmmT (grown from a dichloromethane-cyclopentane mixed
solvent) reveals that the torsion angles between the Ac donor
and the phenyl linker of AmT and AmmT are 79° and 81°,
respectively. In addition, the dihedral angle between the two
benzene rings of the biphenyl group in AmmT is 31°, providing
additional twisting (Fig. S1).26 Details of the single-crystal
structures of AmT and AmmT are given in Table S1.
2.4 Intra- and intermolecular exciplex formation
Fig. 2 shows the packing patterns of AmT and AmmT in
single crystals. For AmT, alternating pairs of adjacent TRZ
moieties are partially stacked with distances of 3.455-3.512 Å.
However, all pairs of adjacent planar TRZ moieties in AmmT
Because of the extremely low luminescence of the dilute
CH2Cl2 solution (10 TM) of AmT and AmmT at room
temperature, we wanted to investigate the luminescence
characteristics according to the state of aggregation in
tetrahydrofuran/water mixtures (Fig. S3).29 Particularly, the PL
intensities of AmmT are almost similar when the water fraction
are parallel, and their
ꢀ-surfaces partially overlap with
alternating distances of 3.452 and 3.494 Å; these properties
would be beneficial for charge transport. Closely packed
AmmT molecules in the crystal structure are expected to
(
fw) is less than 70%. When the fw is larger than 70%, the PL
provide
a more efficient environment for intermolecular
intensities increase, indicating AIE-like behavior.
interactions, thereby increasing the efficiency of exciplex
(exciton) formation.
As shown in Fig. 3(a), AmT and AmmT do not show ICT
absorption bands. However, the large solvatochromic shift
observed in AmT and AmmT is likely due to CT from the Ac
donor to the TRZ acceptor. This was supported by additional
photophysical experiments using Ac and 9,9-dimethyl-10-
phenyl-9,10-dihydroacridine (PhAc) as the donors of AmT and
2.3 Photophysical properties in solution
Çꢀꢁꢂ ꢃꢄꢅ ꢆꢇꢈ ꢁꢂ ꢉ Çꢀꢊ wꢄꢋꢇꢈ {ꢄꢌꢁꢊꢍꢋ ꢄꢎ /ꢀꢊꢏꢁꢂꢍ ꢋ ꢐꢑꢒꢒ
Wꢀ bꢁꢂꢃꢓꢔ ꢐꢑꢕꢖꢔ ꢍꢍꢔ ꢕꢗꢖ ꢘ 0
tꢈꢊꢇꢂꢊ !ꢄ ꢆꢄꢍ ꢇ!ꢃꢅꢂꢍ ꢏꢇ "ꢁꢆꢂ

tꢈꢊꢇꢂꢊ !ꢄ ꢆꢄꢍ ꢇ!ꢃꢅꢂꢍ ꢏꢇ "ꢁꢆꢂ
Journal of Materials Chemistry C
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t!t9w
Wꢀꢁꢂꢃꢄꢅ ꢀ aꢄꢆꢇꢂꢈꢄꢅꢉ /ꢊꢇꢋꢈꢉꢆꢂꢌ /
AmmT, respectively, and TRZ as the acceptor. Fig. 4 shows PL
spectra of Ac:TRZ (1:1 molar ratio, film) and PhAc:TRZ (1:1
molar ratio, film) along with those of Ac, PhAc, and TRZ films.
The PL spectra of Ac, PhAc, and TRZ are almost the same with
a maximum wavelength of 400 nm. The PL spectrum of the 50
mol% Ac:TRZ film is similar to that of the Ac or TRZ film;
however, that of the 50 mol% PhAc:TRZ film is significantly
red-shifted compared to that of the PhAc or TRZ film. In
particular, the PL spectrum of the 50 mol% PhAc:TRZ film is
similar to that of the AmmT neat film. Cyclic voltammetry
(CV) measurements revealed that the HOMO energy levels of
Ac and PhAc are 5.12 and 5.30 eV, respectively, and the
LUMO energy level of TRZ is 2.48 eV (Fig. S4). These results
indicate that the emission of AmT results from the
intramolecular exciplex-like formation between the HOMO of
Ac and the LUMO of TRZ, and the emission of AmmT comes
mostly from the intermolecular exciplex-like system between
the HOMO of PhAc and the LUMO of TRZ.
doped film indicate that the CT exciton formed in the neat film
is better than that of the doped film because of the decreased
intermolecular distances in the neat film. This suggests that a
crucial factor that affects the luminance of AmT and AmmT is
the formation of CT excitons by intermolecular interactions, not
the AIE phenomenon by restricting the intramolecular
rotation.30 The higher PL quantum yield of AmT in the doped
film than that of AmmT in the doped film indicates that the
intramolecular exciplex formation of AmT is better than that of
AmmT. However, the PL quantum yield of AmmT in the neat
film is much higher than that of AmT in the neat film because
formation of an intermolecular exciplex is more favorable for
AmmT than for AmT.
2.5 Photophysical properties in films
Fig. 5 (c) and (d) also depict the transient PL decay curves
of the neat and doped films of AmT and AmmT at room
temperature, and show double-exponential decay profiles,
confirming the existence of the DF. The prompt component is
assigned to an intra- and intermolecular exciplex, and the
delayed component is attributed to (identical) DF occurring via
the RISC process. The neat and doped films of AmT have
prompt fluorescence decay times (ꢀPF) of 93 and 79 ns,
The photophysical properties of AmT and AmmT in neat
and doped films (20 wt% in bis[2-(diphenylphosphino)phenyl]
ether oxide (DPEPO)) were also investigated at room
temperature (Fig. 5). The Rmax absorption peaks of AmT are 276
and 283 nm in the neat and doped films, respectively, and those
of AmmT appear at 283 and 284 nm, respectively. The
estimated emission onset of the PL spectra of AmT in both the
neat and doped films was 456 nm, which corresponds to the
optical bandgap of 2.72 eV. In contrast, the measured emission
onset values of the PL spectra of AmmT in the neat and doped
films were 443 and 427 nm, respectively, which correspond to
optical bandgaps of 2.80 and 2.91 eV, respectively. The LUMO
energy levels of AmT and AmmT in the neat film are 2.81 and
2.68 eV, respectively, which were calculated from the optical
bandgap and the HOMO energy levels of 5.53 and 5.48 eV for
AmT and AmmT, respectively, estimated from the CV in a
dichloromethane solution (Fig. S4(a)). The LUMO energy
levels were obtained from CV measurements (Fig. S4(b)). The
PL Rmax values of AmT in the neat and doped films are 519 and
522 nm, respectively. The PL Rmax values of AmmT in the neat
and doped films are 509 and 480 nm, respectively. The PL
quantum yields of the neat and doped films (measured using
integrating spheres) are 0.52 and 0.27 (AmT), respectively, and
0.69 and 0.20 (AmmT), respectively. The higher PL quantum
yields of AmT and AmmT in the neat film than those in the
respectively, and the delayed fluorescence decay time (ꢀDF) of
5.80 and 4.32 Ts, respectively. The neat and doped films of
AmmT exhibit ꢀPF values of 59 and 72 ns, respectively, and ꢀDF
values of 1.76 and 4.01 Ts, respectively. The ꢀPF and ꢀDF values
of the neat film of AmmT are smaller than those of the doped
film of AmmT, suggesting a more efficient radiative decay in
the neat film than in the doped film. The PL quantum yields of
AmmT contributed by a prompt component (ꢁPF) and delayed
component (ꢁDF) were estimated from the decay lifetime and
its amplitude (Table S2) as 0.16 and 0.53 in the neat film,
respectively, while the values of ꢁPF and ꢁDF of AmmT are
0.04 and 0.16, respectively, in the doped film. However, the
values of ꢁPF and ꢁDF of the AmT neat film are 0.16 and 0.36,
respectively, and those of the AmT doped film are 0.08 and
0.19, respectively. Regardless of prompt or delayed
fluorescence, the PL quantum yields of the AmmT neat film are
3 to 4 times higher than those of the AmmT doped film because
of the decreased intermolecular distances, whereas the AmT
1 ꢘ Wꢀ bꢁꢂꢃꢓꢔ ꢐꢑꢕꢐꢔ ꢍꢍꢔ ꢕꢗꢖ
Çꢀꢁꢂ ꢃꢄꢅ ꢆꢇꢈ ꢁꢂ ꢉ Çꢀꢊ wꢄꢋꢇꢈ {ꢄꢌꢁꢊꢍꢋ ꢄꢎ /ꢀꢊꢏꢁꢂꢍ ꢋ ꢐꢑꢒꢒ
tꢈꢊꢇꢂꢊ !ꢄ ꢆꢄꢍ ꢇ!ꢃꢅꢂꢍ ꢏꢇ "ꢁꢆꢂ

tꢈꢊꢇꢂꢊ !ꢄ ꢆꢄꢍ ꢇ!ꢃꢅꢂꢍ ꢏꢇ "ꢁꢆꢂ
Journal of Materials Chemistry C
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t!t9w
Wꢀꢁꢂꢃꢄꢅ ꢀ aꢄꢆꢇꢂꢈꢄꢅꢉ /ꢊꢇꢋꢈꢉꢆꢂꢌ /
neat film showed PL quantum yields that were twice the value
of those for the doped film of AmT.
2.6 TADF properties
We investigated the temperature dependence of the transient
PL decay of AmT and AmmT measured from 78 to 300 K. The
DF increases with temperature, indicating that AmT and
AmmT exhibit strong TADF characters in both the neat and
doped films (Fig. S5). The RISC rate values (kRISC) were
estimated using the equations provided in the supplementary
notes, and the calculated ;EST values in the neat and doped
films were 33.6 and 34.4 meV for AmT, and 56.5 and 31.7
meV for AmmT, respectively, from the Arrhenius plots of kRISC
between 125 and 250 K, as shown in Fig. S6. The values of
kRISC for the doped films of AmmT did not show a clear
temperature dependence. This is because formation of the
intermolecular (exciplex) CT excitons for the doped film of
AmmT is less efficient than that of the neat film (because of the
relatively long distances between AmmT molecules in the
doped film).
and AmmT-doped devices show maximum EQEs of 4.19% and
3.28%, respectively, supporting that the formation of an
intermolecular CT exciton is a major contributor to the high
efficiency of non-doped devices. The EQE of AmmT-based
non-doped devices is about 3.5 times higher than that of AmT-
based devices, and about 3.6-5.4 times higher than the
theoretical maximum EQE of approximately 3.45% to 5.18%
(out-coupling efficiency = 0.2Þ0.3), assuming that the EL
quantum efficiency is the same as the PL quantum efficiency of
0.69. Fig. 6(d) shows normalized EL spectra obtained using
non-doped devices. Inherent EL emission Rmax values of the
AmT- and AmmT-based devices were observed at 524 and 504
nm, respectively, without any other emission from the adjacent
layers.
Donor-acceptor-type TADF materials do not generally emit
fluorescence by a Förster resonance energy transfer (FRET)
mechanism because they have a small overlap between
absorption and emission (large Stokes shift).31 Therefore, the
effect of concentration quenching by FRET (of singlet excitons
in the luminescence process) of AmT and AmmT is also
negligible. In contrast, concentration quenching by electron-
exchange interactions (for triplet excitons) is partially
considered. Recently, Yasuda and co-workers reported that
concentration quenching of TADF molecules was dominated by
electron-exchange interactions for triplet excitons using the
Dexter energy-transfer (DET) model.32 The maximum EQE of
18.66% in the AmmT-based device suggests that the exciton
quenching in the neat film of AmmT is mainly caused by the
DET process, but it is insignificant compared to the CT
excitons generated by intermolecular interactions, as shown by
the experimental results of the neat and doped films.
2.7 Device performances
Non-doped devices were fabricated using AmT and AmmT
as emitters in the EML with the following device configuration:
glass substrate/indium tin oxide/N,N'-di(1-naphthyl)-N,N'-
diphenyl-(1,1-biphenyl)-4,4'-diamine (NPB) (30 nm)/4,4'-
cyclohexylidenebis[N,N-bis(4-methylphenyl)benzenamine]
(TAPC) (30 nm)/AmT or AmmT (20 nm)/DPEPO (10
nm)/2,2',2''-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-
benzimidazole) (TPBI) (40 nm)/LiF (1 nm)/Al (100 nm). The
following components were deposited on the substrate in
sequential order: NPB and TAPC as hole transporting layers,
DPEPO with a deep HOMO energy level and high triplet
energy (3.1 eV) as a hole blocking and an exciton confinement
layer, TPBI as an electron transporting layer, an LiF as an
electron injection layer, and Al cathode. Two different OLEDs
were fabricated, and the performance of each device was
evaluated (Table S3). Fig. 6(a) shows the current densityœ
voltage characteristics of each device with AmT and AmmT.
The current density of the device with AmT is higher than that
of the device with AmmT, whereas the luminance values and
turn-on voltage of each device are similar at the same driving
voltage as shown in Fig. 6(b). Although AmmT-based devices
show worse current density-voltage characteristics than AmT-
based devices, the similar luminance values and turn-on
voltages of the two devices indicate that AmmT-based devices
generate intermolecular exciplexes more efficiently because of
the high packing density of AmmT molecules in the non-doped
environment; thus, AmmT-based devices produce light
emission better than AmT-based devices. Quantum efficiencyœ
luminance characteristics of two devices were plotted in Fig.
6(c). AmmT-based devices show a maximum EQE of 18.66%,
which is much higher than that of AmT based-devices (5.30%).
To understand the reasons for high efficiency of non-doped
devices, we fabricated additional devices using 20 wt% AmT
and AmmT doped into DPEPO in the EML (Fig. S7). AmT-
0ꢐ /ꢀꢃꢑꢅꢁꢉꢈꢀꢃꢉ
In conclusion, we developed TADF molecules (AmT and
AmmT) for highly efficient non-doped OLEDs utilizing intra-
and intermolecular CT excitons, which are composed of meta-
Çꢀꢁꢂ ꢃꢄꢅ ꢆꢇꢈ ꢁꢂ ꢉ Çꢀꢊ wꢄꢋꢇꢈ {ꢄꢌꢁꢊꢍꢋ ꢄꢎ /ꢀꢊꢏꢁꢂꢍ ꢋ ꢐꢑꢒꢒ
Wꢀ bꢁꢂꢃꢓꢔ ꢐꢑꢕꢖꢔ ꢍꢍꢔ ꢕꢗꢖ ꢘ 2
tꢈꢊꢇꢂꢊ !ꢄ ꢆꢄꢍ ꢇ!ꢃꢅꢂꢍ ꢏꢇ "ꢁꢆꢂ

tꢈꢊꢇꢂꢊ !ꢄ ꢆꢄꢍ ꢇ!ꢃꢅꢂꢍ ꢏꢇ "ꢁꢆꢂ
Journal of Materials Chemistry C
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DOI: 10.1039/C8TC02590H
t!t9w
Wꢀꢁꢂꢃꢄꢅ ꢀ aꢄꢆꢇꢂꢈꢄꢅꢉ /ꢊꢇꢋꢈꢉꢆꢂꢌ /
14 H. Kaji, H. Suzuki, T. Fukushima, K. Shizu, K. Suzuki, S.
Kubo, T. Komino, H. Oiwa, F. Suzuki, A. Wakamiya, Y.
Murata, C. Adachi, Nat. Commun., 2015, 6, 8476.
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Lo, S. W. Liu, Y. Zhang, A. Lien, J. R. Xu, Angew. Chem.
Int. Ed., 2015, 54, 874-878.
16 K. Y. Sun, W. Jiang, X. X. Ban, B. Huang, Z. H. Zhang, M.
Y. Ye, Y. M. Sun, RSC Adv., 2016, 6, 22137-22143.
17 R. Furue, T. Nishimoto, I. S. Park, J. Lee, T. Yasuda, Angew.
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linked Ac donor and TRZ acceptor molecules. Intermolecular
CT excitons of AmT and AmmT are generated more favorably
under non-doped conditions than in solution or under doped
conditions because of their spatial proximity. This means that
efficient formation of CT excitons results from intermolecular
interactions, rather than by AIE. AmT and AmmT are the first
TADF molecules based on intermolecular CT exciplexes
between donor-acceptor-type emitter molecules of a single
species. While AmT shows more efficient intramolecular CT
exciton formation than AmmT, the neat film of AmmT forms
an intermolecular CT state-induced exciplex better than AmT
because of the packing benefits and relatively shorter radiative
lifetime of AmmT. As a result, EL devices that use AmmT as
an emitter without a host exhibit an EQE over 18%. A single
molecule exciplex-based TADF system using intra- and
intermolecular CT excitons could be useful for developing high
efficiency non-doped OLEDs.
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/ꢀꢃ ꢅꢈꢑꢆꢉ ꢀ ꢈꢃꢆꢇꢂꢇꢉꢆ
There are no conflicts to declare.
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A. Ahn, H. S. Kim, C. B. Kim, ACS Appl. Mater. Interface
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This work was supported by the NRF grant
(2018R1A2B2001293) funded by the MSIP. We thank the
Samsung Display Co., Ltd. for partial financial support.
Equipments for low temperature PL experiments were
supported by CSOM (Prof. S. Y. Park). We are also grateful to
Ms. B. Sim (Prof. J. Kim) for measurements of absolute PL
quantum yields.
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3 ꢘ Wꢀ bꢁꢂꢃꢓꢔ ꢐꢑꢕꢐꢔ ꢍꢍꢔ ꢕꢗꢖ
Çꢀꢁꢂ ꢃꢄꢅ ꢆꢇꢈ ꢁꢂ ꢉ Çꢀꢊ wꢄꢋꢇꢈ {ꢄꢌꢁꢊꢍꢋ ꢄꢎ /ꢀꢊꢏꢁꢂꢍ ꢋ ꢐꢑꢒꢒ
tꢈꢊꢇꢂꢊ !ꢄ ꢆꢄꢍ ꢇ!ꢃꢅꢂꢍ ꢏꢇ "ꢁꢆꢂ

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