Effect ofortho-biphenyl substitution on the excited state dynamics of a multi-carbazole TADF molecule

Article abstract of DOI:10.1039/d0tc02627a

We report a new thermally activated delayed fluorescence (TADF) molecule developed by theortho-biphenyl substitution of a multi-carbazole TADF molecule. The new TADF molecule,4mCzBN-BP, is composed of four dimethylcarbazole donors, a benzonitrile acceptor core and anortho-biphenyl triplet scaffold.4mCzBN-BPexhibits a bluish-green emission with a peak wavelength at 491 nm and a very high photoluminescence quantum yield (PLQY) of 95% in an mCBP host. To investigate the effect of theortho-biphenyl substitution on the excited-state dynamics and photophysical properties of the TADF molecule, we performed time dependent density functional theory (TD-DFT) calculations of4mCzBN-BPand two multi-carbazole TADF molecules with similar molecular structures. A local excited triplet state (³LE) with an energy (2.81 eV) close to the lowest singlet charge-transfer state (¹CT) can be formed at the biphenyl of4mCzBN-BP. However, due to the planarization of the biphenyl at the triplet state and a large steric hindrance caused by adjacent carbazole donors,³LE at the biphenyl can only be formed by a subset of possible conformations of4mCzBN-BPmolecules in the solid mCBP host, leading to a multiexponential decay of the delayed fluorescence. Color coordinates of (0.196, 0.452), a high external quantum efficiency of 23.7% and a long operational lifetime (LT50) of 750 hours were achieved with the organic light emitting diodes based on4mCzBN-BP.

Full text of DOI:10.1039/d0tc02627a

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Materials for optical, magnetic and electronic devices
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ISSN 2050-7526
PAPER
Nosang V. Myung, Yong-Ho Choa et al.
A
noble gas sensor platform: linear dense assemblies of
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single-walled carbon nanotubes (LACNTs) in
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Effect of Ortho-Biphenyl Substitution on the Excited State
Dynamics of Multi-Carbazole TADF Molecule
Seung-Je Woo, Yeon-Hee Ha, Yun-Hi Kim, and Jang-Joo Kim*,a
a
b
*,b
Received 00th January 20xx,
Accepted 00th January 20xx
We report a new thermally activated delayed fluorescence (TADF) molecule developed by ortho-biphenyl substitution of a
multi-carbazole TADF molecule. The new TADF molecule, 4mCzBN-BP, is comprised of four dimethylcarbazole donors, a
benzonitrile acceptor core and a ortho-biphenyl triplet scaffold. 4mCzBN-BP exhibits a bluish-green emission of a peak
wavelength at 491 nm and a very high photoluminescence quantum yield (PLQY) of 95% in mCBP host. To investigate the
effect of ortho-biphenyl substitution on excited-state dynamics and photophysical properties of the TADF molecule, we
performed time dependent density functional theory (TD-DFT) calculations of 4mCzBN-BP and two multi-carbazole TADF
DOI: 10.1039/x0xx00000x
www.rsc.org/
3
molecules with similar molecular structures. A local excited triplet state ( LE) with an energy (2.81 eV) close to the lowest
1
singlet charge-transfer state ( CT) can be formed at the biphenyl of 4mCzBN-BP. However, due to the planarization of the
3
biphenyl at triplet state and a large steric hindrance caused by adjacent carbazole donors, LE at the biphenyl can only be
formed by a subset of possible conformations of 4mCzBN-BP molecules in the solid mCBP host, leading to a multiexponential
decay of the delayed fluorescence. A color coordinate of (0.196, 0.452), a high external quantum efficiency of 23.7% and a
long operational lifetime (LT50) of 750 hours were achieved with the organic light emitting diodes based on 4mCzBN-BP.
3
3
process: vibronic coupling between CT and LE states and SOC
3
1
20
1. Introduction
between the LE and CT states. This strategy has been
adopted to TADF emitters and greatly improved the operation
Thermally activated delayed fluorescence (TADF) materials
8-9
stability. Recently, Adachi et al. reported that intermediate CT
states of multi-donor/acceptor TADF molecules participate in
RISC process and emphasized the contribution of partial
molecular structures. The conformational distribution of the
TADF molecules doped in solid organic films are also critical to
RISC rate as the CT energies and excited state configurations are
have been researched and developed intensively in the field of
organic light emitting diodes (OLEDs) to replace iridium
1-10
phosphorescent emitters and blue fluorescent emitters.
Many breakthroughs have been made since the first
2
1
11
development of TADF OLEDs. Still, the operational lifetime,
efficiency, and colour purity of TADF OLEDs are not suitable for
practical applications.3
To utilize nonradiative triplet states, TADF molecules
generally incorporate a highly tilted donor-acceptor (D-A)
structure to minimize the singlet-triplet energy gap (ΔEST).
However, small ΔEST do not guarantee an efficient reverse
22-35
affected by the molecular geometry.
In solvents, TADF
-14
molecules at excited state relax to a certain geometry to
minimize potential energy. In solid organic thin films,
conformational distribution of TADF molecules are formed by
the thermal deposition and the relaxation of TADF molecules
are limited by surrounding solid host materials, leading to the
energy distribution of the CT, CT and LE states. Consequently,
the RISC rate of the molecules in each conformation differs and
results in the multiexponential decay of delayed fluorescence.
TADF molecules with a structure of multi-carbazole donors
intersystem crossing (RISC) due to the small spin-orbit coupling
1
3
3
1
(
SOC) of singlet charge transfer ( CT) and of triplet charge
3
8,15-19
transfer ( CT) states.
Therefore, additional triplet states
with different configuration of transition orbitals and similar
1
3
energy levels compared to the CT and CT states should exist
within the TADF molecules to utilize efficient RISC process.
Monkman et al. have shown that the energy alignment of locally
linked to a benzonitrile acceptor core are the most promising
TADF emitters in terms of long operational lifetime.4
,8-9,36
Due
to the multi-carbazole structure, the singlet and triplet state
configurations and energies can be easily tuned by modifying
the number and position of carbazoles or by substituting
3
excited triplet states ( LE) formed at the donor or acceptor can
enhance the efficiency and rate of RISC process via second order
1
,8-9,37-42
moieties such as phenyls and alkyls.
However, only the
3
CT states and LE states of donors and acceptors were
considered in the RISC mechanism. Moreover, the excited state
dynamics were mostly discussed in the solution phase,
overlooking the conformational distribution of the TADF
a.
Organic Photonics Laboratory, Department of Materials Science and Engineering,
Seoul National University, Seoul 151-742, South Korea.
Department of Chemistry and RIGET, Gyeongsang National University, Jinju 660-
b.
701, South Korea. † Footnotes relating to the title and/or authors should appear
here.
8
,19,21,43-49
Electronic Supplementary Information (ESI) available: [details of any supplementary molecules in solid organic films.
information available should be included here]. See DOI: 10.1039/x0xx00000x
Herein, we developed a new multi-carbazole TADF molecule
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DOI: 10.1039/D0TC02627A
1
by substituting ortho-biphenyl. The newly designed 4mCzBN-BP The intermediates and final products were characterized by H-
incorporates four dimethylcarbazole (mCz) as donors, a NMR, 13C-NMR, and electron ionization (EI) mass spectroscopy.
benzonitrile (BN) as an acceptor, and a biphenyl as a local triplet Thermogravimetric analysis (TGA) and differential scanning
scaffold (Fig. 1). In addition, 4mCzBN, which is a TADF molecule calorimetry (DSC) confirmed a thermal stability up 390°C for all
without a biphenyl, was also synthesized to compare the
excited state dynamics with that of 4mCzBN-BP through time-
dependent density functional theory calculations and various
photoluminescence (PL) experiments. We show that the wide
distribution of the delayed decay rate of 4mCzBN-BP originates
from the possibility of formation of 3LE in biphenyl which
molecules.
.2 Theoretical Calculations and Photophysical Properties
2
Density functional theory (DFT) and time-dependent DFT (TD-
DFT) calculations were performed to investigate the ground and
excited state geometries, HOMO-LUMO distribution, natural
transition orbitals (NTOs), and transition energies of the TADF
molecules. Optimization of the ground and excited state
geometries were performed with Gaussian09.50 Single point
calculations for transition energies and NTOs of excited states
were performed with Jaguar (Materials Science Suite,
Schrödinger)51 and Gaussian09. For the ground state
calculations, the B3LYP functional was employed. The CAM-
B3LYP functional was used for the geometry optimization of
depends on the conformation of 4mCzBN-BP due to the
3
planarization of the biphenyl at LE state.
2. Results and Discussion
2
.1 Synthesis and Characterization
Detailed synthetic procedure and characterization of 4mCzBN
and 4mCzBN-BP are shown in the supplementary information.
1
singlet states ( CT). The long-range corrected functional wB97X-
2
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D with a modified range separation parameter (
used for the single point calculations of the CT states.
ω
=0.16) was
1
52,53
The
M06-2X functional was used for the geometry optimization and
single point calculations of the hybrid local and charge-transfer
3
21
triplet ( HLCT) states. For all calculations, the 6-31G(d,p) basis
set was used.
The molecular structures, geometries, and NTOs of 4CzBN,
4
mCzBN, and 4mCzBN-BP are shown in Fig. 1. The donor
moieties of the TADF molecules are highly tilted against the
acceptor core due to the steric hindrance of adjacent donors.
The dihedral angles of the donors in 4CzBN and 4mCzBN are all
6
5° in the ground state. In the case of 4mCzBN-BP, the ortho-
linked biphenyl is tilted towards the mCz-2 moiety and the mCz-
moiety is pushed down to a dihedral angle of 62° whereas the
2
mCz-3 moiety is pushed up to a dihedral angle of 75° (Fig. 2 (a)).
The natural dihedral angle between biphenyl and benzonitrile
without the steric hindrance of any other moieties is 49° in the
ground state (Fig. S1). Similarly, the natural dihedral angle
between carbazole and benzonitrile is 52° in the ground state. moieties in the optimized S
0
geometries differ, the change of
Therefore, the substitution of ortho-biphenyl gives a great steric dihedral angle was used for the x-axis of the relative energy
hindrance among the moieties linked to acceptor core of plot. The energy curve of mCz-1s of 4mCzBN and 4mCzBN-BP
4
mCzBN-BP. To further investigate the steric hindrance in the are similar. Contrarily, the energies of mCz-2 and mCz-3 of
TADF molecules, the relative total energies of 4mCzBN and 4mCzBN-BP are higher than those of 4mCzBN at the same
mCzBN-BP as a function of dihedral angle of mCz-1,2,3 and dihedral angle change due to the substitution of ortho-biphenyl.
4
biphenyl (BP) was obtained from the DFT calculation using the In particular, the relative energy of 4mCzBN-BP increases
B3LYP functional (Fig.2 (b)). Since the dihedral angles of the
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rapidly by the dihedral angle change of mCz-2 and BP because
the biphenyl is
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Table 1. Photophysical properties of the molecules (10wt in mCBP).
a
b
c
d
e
8
f
5
g
5
h
8
i
6
j
Emitter
λ
PL (nm)
PLQY (%)
Ф
p
(%)
Ф
d
(%)
k
p
(10 /s)
k
d,2.5 (10 /s)
k
d,150 (10 /s)
k
ISC (10 /s)
k
RISC (10 /s)
Θ (%)
4
4
4
CzBN
460
470
491
70
92
95
6.3
6.6
9.8
93.7
93.4
90.2
3.03
2.78
1.36
0.532
1.08
2.24
0.223
0.282
0.185
2.84
2.60
1.23
0.75
1.62
70
78
80
mCzBN
mCzBN-BP
2.28
a
Photolumincescence (PL) peak. b PL quantum yield. Prompt portion of total emission. Delayed portion of total emission. e Prompt decay rate. Delayed decay rate at
c
d
f
.5 μs. g Delayed decay rate at 150 μs. h ISC rate. RISC rate calculated from kd,2.5. horizontal emitting dipole ratio.
i
j
2
tilted towards the mCz-2 moiety.
In addition, the C-N bond length between the acceptor core and
(S ) to 1.425
(1CT), which is
The HOMOs of the TADF molecules are dispersed over the the donors increase from 1.415
Å
0
Å
four donors (Fig. S2). However, the hole distribution of NTOs of a result of energy minimization of the CT state. Although the
lowest singlet charge transfer state ( CT) is localized in a single TADF molecules have similar 1CT transition orbital
1
1
1
Cz-1 or mCz-1 donor. In the CT state, the dihedral angles of Cz- configurations, their CT energies and emission spectra are
and mCz-1 moieties increase to angles about 75° while the clearly different (Fig. 3). The methyl substituted at the carbazole
1
other donors’ dihedral angle remaining unchanged. This of 4mCzBN has an electron donating property that raise the
geometry relaxation is similar to a twisted intramolecular HOMO level of the carbazole and results in the redshift of the
1
charge transfer (TICT) and results in the different configurations 4CzBN’s CT energy of 2.99 eV to 2.88 eV of 4mCzBN in toluene.
between the HOMOs and hole NTOs of the TADF molecules.54 On the other hand, the biphenyl substituted in 4mCzBN-BP
4
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forms a slight conjugation which can be seen from the LUMO
and electron NTO distribution and results in the lowest 1CT
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energy (2.80 eV) among the TADF molecules in toluene. The
phosphorescence spectra (Fig. 3) of the TADF molecules are
redshifted more than 0.1 eV from their fluorescence spectra,
indicating decreased charge transfer character and increased
conjugation of the lowest triplet state. At the optimized
geometries of the T states, the dihedral angles of the donors
1
decrease to 50°~56° (Fig. 1). Furthermore, the C-N bond lengths
between the acceptor and the donors that participate in the
hole NTOs of T
decreased to 1.39 Å from that of 1.415-1.425 Å at the S
1
state, which is mCz-2(Cz-2) and mCz-4(Cz-4), are
and S
0
1
states. The decreased tilt angle and bond length induce larger
3
conjugation between the donor and acceptor resulting a HLCT
state and an energy gap larger than 0.1 eV.21,47
The transient PL decay of the TADF molecules are shown in
Fig. 4. In toluene, all molecules exhibit a clear biexponential
decay with constant delayed component decay rates (k
d
) of
5
5
5
0
.114×10 /s, 0.285×10 /s, and 0.194×10 /s for 4CzBN, 4mCzBN,
and 4mCzBN-BP, respectively. However, the transient PL decay
of the molecules doped in mCBP films show multiexponential
decays which cannot be fitted with a single delayed component
5
(
1
Fig. S3). The k
d
of the TADF molecules at 2.5 μs is 0.532×10 /s,
5
5
.08×10 /s, and 2.24×10 /s for 4CzBN, 4mCzBN, and 4mCzBN-
BP respectively and decrease gradually to become similar to
their k in toluene solution at 150 μs. The multiexponential
d
decay is not caused by processes such as energy transfer from
the host to the TADF dopant because the similar
multiexponential decay is also observed from the TADF
molecules doped in DPEPO which does not absorb the 337 nm
laser excitation (Fig. S4). This phenomenon is frequently
observed from TADF molecules in solid thin film which is caused
by the conformational distribution and incomplete geometry
relaxation at the excited states.26-35,55-58 Generally,
multiexponential decay of a TADF molecule in a thin film is
accompanied by a spectral shift of time-resolved spectra.
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However, the molecules in this study do not show any spectral
shift from 8 ns to 200 μs (Fig. S5). The spectral shift is only
observed at early part of the prompt decay from 3 ns to 8 ns (Fig.
S6). These experimental results indicate that conformational
disorder formed by the thermal deposition is preserved at the
delayed emission time region due to the incomplete structural
relaxation of the triplet states of the TADF molecules in mCBP. triplet states of the TADF molecules were investigated through
1
On the other hand, the structural relaxation at the CT state phosphorescence spectra at 77 K (Fig. 3, Fig 5, Fig. S7) and TD-
3
occurs rapidly within a few nanoseconds so that the spectral DFT calculation (Fig. 6, Fig. S8). The CT energies of the TADF
shift is only observed at prompt decay. Otherwise, there should molecules were determined from the onset energies of the
be a spectral shift at the delayed decay. The fast geometry phosphorescence spectra of toluene solutions at 77 K at the
1
3
relaxation of the CT states and slow geometry relaxation of delay time of 100 ms. The measured CT energies were 2.80 eV,
triplet states in mCBP can be explained with the optimized 2.76 eV, and 2.69 eV for 4CzBN, 4mCzBN, and 4mCzBN-BP,
1
geometries obtained from TD-DFT calculations. At the CT state, respectively. However, we cannot specify that this
only the dihedral angle of the mCz-1(Cz-1) relaxed. On the phosphorescence is from a single CT state because the TADF
contrary, at the HLCT states, all the donors relax to a less tilted molecules are in conformational distribution due to the
geometry accompanied by the shortening of the C-N bond hindered structural relaxation by frozen toluene matrix.6 The
3
3
2,63
3
length of the donor moieties that participate in the HLCT onset energy of phosphorescence from 4mCzBN-BP decreases
transition. In addition, mCz-1 moiety is relatively easier to from 2.69 eV at 100 ms to 2.65 eV at 2000 ms (Fig. 3). In addition,
rotate compared to mCz-2 and mCz-3 in 4mCzBN-BP (Fig. 2(b)). the phosphorescence of 4mCzBN-BP in toluene (77 K) shows
The change of k
d
in the time range of 2.5-100 μs is smallest in multiexponential decay (Fig. S9). The larger the CT character of
CzBN and largest in 4mCzBN-BP. The smallest change of k in the triplet state, the higher the energy and the faster the decay
CzBN is due to the absence of bulky methyl attached to the rate. Contrarily, the larger the HLCT character of the triplet state,
the lower the energy and the slower the decay rate. These are
4
4
d
donors which hinders the tilt angle relaxation. The change of k
d
of 4mCzBN-BP is the largest because of the high steric hindrance the reasons of the redshift of phosphorescence spectrum over
among the ortho-linked biphenyl and adjacent donor moieties. time. Furthermore, a small shoulder exists at 425-450 nm in the
On the contrary, the clear biexponential decay of the TADF phosphorescence spectrum of 4mCzBN which makes it
molecules in toluene indicates that the delayed emission comes ambiguous to determine the onset. These experimental results
from a singlet state upconverted from a single triplet clearly show the conformational distribution of the TADF
configuration which is the result of the complete structural molecules in frozen toluene. Hence, it should be noted that
relaxation in a freely moving toluene solution. The decay rate of triplet states that are in higher energies or in lower energies
3
delayed component is directly affected by the RISC rate. We compared to the CT energy shown in Fig. 5 exist.
3
calculated the RISC rate of the TADF molecules in mCBP (Table.
The lowest LE states of 4CzBN and 4mCzBN is localized at
1
) from the kd,2.5, using the method previously reported from the Cz and mCz moiety and their energies are 3.03 eV and 2.97
58-61
several researches.
As expected from the k
d
, the RISC rate eV respectively (note that these triplet energies are redshifted
at the early time region of the delay components was the fastest from the triplet states of carbazole and dimethylcarbazole, (Fig.
for 4mCzBN-BP and the slowest for 4CzBN. The RISC rate is
S7)). As the triplet states of phenyl-cored multi-carbazole
molecules are constant regardless of the number of
closely related to the energy levels and configuration of the
1
3
triplet states. It is known that the close energy level of CT, CT
8,19
8
carbazoles and as the T
1
energy of benzonitrile (3.32 eV) is
3
3
(
HLCT), and LE is critical for a fast RISC process. Hence, the
3
very high, only the LE of Cz and mCz could be involved in the
6
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and the biphenyl. For the same reason, triplet states of [1,1'-
biphenyl]-4-carbonitrile and o-terphenyl can be neglected.
The energy levels of the TADF molecules are arranged in Fig.
3
3
5
. The LE- CT energy gap which determines the RISC rate is 0.23
eV and 0.21 eV for 4CzBN and 4mCzBN, respectively. For
3
3
3
4
3
mCzBN-BP, there are two LE- CT energy gaps: 0.12 eV ( LEBP-
3
3
CT) and 0.28 eV ( LEmCz- CT). The former is the smallest and the
latter is the largest among the LE- CT gaps of all molecules.
Hence, it can be inferred that the fastest k and RISC rate of
3
RISC process. In 4mCzBN-BP, there is an additional LE state
localized in the biphenyl (Fig. 6). As the lowest triplet state of
3
3
d
3
3
4
mCzBN-BP is CT or HLCT, the lowest triplet energy localized
4
mCzBN-BP at 2.5 μs originate from the molecules in the
at the biphenyl (3LEBP
)
is not observed from the
conformation that vibronic coupling of LEBP-3CT can occur. As
3
phosphorescence spectra. Therefore, the 3LEBP energy was
estimated from the phosphorescence of biphenyl and energies
calculated from TD-DFT (Fig. S8). The LEBP of 4mCzBN-BP was
estimated to be 2.81 eV, which is slightly lower than triplet of
isolated biphenyl due to the slight conjugation with the
benzonitrile core (Fig. 1). Biphenyl has a dihedral angle of 38° at
the decay of 4mCzBN-BP progresses, only the molecules in the
conformation that LEBP cannot be formed (which are highly
3
3
3
3
3
likely to be the HLCT states) are left. The LE- CT gap of these
conformers are the largest among all TADF molecules and
5
results in the slowest kd,150 of 0.185×10 /s. Over 60% of total
radiative excitons are emitted in the time region from 2.5 μs to
the ground state and becomes planar at the T
1
state.64-66 When
5
d
0 μs, in which the k significantly changes (Fig. S11). Therefore,
3
the LEBP is formed at the ortho-biphenyl, the dihedral angle
between the benzonitrile and the biphenyl must be increased
up to 77° to avoid overlapping with the mCz-2 moiety. Hence,
it is important to control conformational distribution to
minimize the average lifetime of the total fluorescence. Not as
much as 4mCzBN-BP, but also 4CzBN and 4mCzBN shows
3
the formation of LEBP in 4mCzBN-BP must overcome a great
change of k
d
due to the conformational distribution of the
3
steric hindrance, especially from the HLCT state geometry due
3
lowest triplet states. The fast kd,2.5 will originate from the CT
states which are higher than the CT energies determined from
to the closer distance between the biphenyl and mCz moieties
3
(
Fig. 2(a)). The relative energy of 4mCzBN-BP as a function of
the phosphorescence (Fig. 3) and the slow kd,150 will originate
from 3HLCT states which are lower than the 3CT energies
determined from the phosphorescence. On the other hand, all
triplet states of the TADF molecules in toluene at 300 K relax to the
phenyl-phenyl dihedral angle of the BP at S
0
and 3HLCT
optimized geometries are shown in Fig. 2(c). About twice the
3
energy is required for the planarization of biphenyl at the HLCT
3
geometry than at the S
0
geometry. Therefore, the LEBP that
3
geometries of HLCT states which leads to the clear biexponential
3
could be coupled with CT to promote RISC can be only formed
by a subset of possible conformations of the 4mCzBN-BP
molecules in mCBP. In toluene, the 4mCzBN-BP molecules at
triplet state will completely relax to HLCT geometry and the
3LEBP state can hardly be formed. In addition, the triplet state of
d
decay and constant k values.
Schematic representation of the excited state dynamics after the
1
formation of CT in 4mCzBN-BP in toluene and mCBP is shown in Fig.
3
1
7
. In toluene, all triplet state 4mCzBN-BP formed by ISC from the CT
3
state relax to the HLCT geometry. Only a single conformation of
[
1,1':2',1''-terphenyl]-4-carbonitrile (BN-BP) was investigated
by phosphorescence measurement and TD-DFT (Fig. S10). The
energy of BN-BP was 2.7 eV but this state cannot be formed
3
HLCT exists after relaxation and the RISC by direct spin-orbit
coupling (SOC)67 occurs in a single rate. After the RISC process, all CT
states relax to a single geometry and delayed emission takes place.
In mCBP, conformers of triplet states are formed after the ISC from
1
T
1
in 4mCzBN-BP due to the steric hindrance among the donors
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Journal Name
molecules. The experimental EQEmax of 4mCzBNV-ieBwPArwticalesOntlhinee
Table 2. Electroluminescence properties of OLEDs.
DOI: 10.1039/D0TC02627A
closest to the EQEs calculated from the optical simulations,
EQEmaxa
EQE400b
(%)
λ
c
L
e
LT50f
(hr)
EL
max
Emitter
CIEd
indicating that the RISC efficiency of 4mCzBN-BP in the emitting
2
(%)
(nm)
(cd/m )
layer was the highest among the TADF molecules. The
(
0.153,
4
4
4
CzBN
12.7
19.0
23.7
8.99
13.7
17.6
464
11000
40
2
operational lifetime (LT50, L
0
=400 cd/m ) of the device
0
.184)
0.151,
.266)
0.196,
.452)
incorporating 4mCzBN-BP as the emitter was 750 hours which
was 5.4-fold higher than that of device incorporating 4mCzBN.
The longest operational lifetime of the device incorporating
4mCzBN-BP is the result of enhanced RISC rate and red-shifted
spectra caused by the substitution of ortho-biphenyl. The long
lifetime of excited states is thought to be the reason of the short
operational lifetime of TADF OLEDs as the degradation can
occur at excited states.3,8-9 Therefore, each conformer of the
TADF molecules in OLEDs will exhibit different degradation
(
mCzBN
472
14100
138
0
(
mCzBN-BP
496
22400
750
0
a
Maximum external quantum efficiency (EQE). b EQE at 400 cd/m . Maximum
Electroluminescence (EL) peak.
Internationale de L’Eclairage (CIE) 1931 coordinates at 400 cd/m . Maximum
2 c
d
e
power efficiency (PE).
Commission
2
e
f
2
luminance. LT50 Operational lifetime (Initial luminance = 400 cd/m ).
1
the CT states. The relaxation of triplet states is restricted, and some
conformers of 4mCzBN-BP are in high energies with large CT
character and some conformers of 4mCzBN-BP are in low energies
with HLCT character. In the conformers with large CT character, Cz
and mCz donors are highly tilted and LEBP can couple with CT which
leads to a fast RISC. In the conformers with HLCT character, LEBP
cannot be formed due to the small spacing between the biphenyl and
adjacent donors which results in a slow RISC. Finally, the upconverted
CT states relax to a single geometry, followed by a delayed
lifetime according to their k
slow k will degrade faster and become an electron/hole trap or
an exciton quencher that accelerates the degradation of the
OLED. Therefore, minimizing the TADF conformers with slow k
d
. For example, the conformers with
d
3
3
d
3
may improve the operational lifetime of the OLEDs, which will
be our future work.
1
fluorescence.
3. Conclusion
2
.3 Electroluminescent Properties
In summary, we developed new multi-carbazole TADF molecule
with an ortho-substituted biphenyl and investigated its excited
state dynamics and photophysical properties through TD-DFT
calculations and various experimental methods. The newly
synthesized TADF molecule, 4mCzBN-BP, exhibited the fastest
RISC rate in mCBP host at the early decay time of the delayed
component. We showed through TD-DFT calculations that the
triplet excited state localized at the ortho-biphenyl exists which
cannot be observed from the phosphorescence spectra. This
triplet state promotes the RISC process and results in the fastest
decay rate at the early decay time of the delayed component.
Prior to the fabrication of electroluminescent (EL) devices, angle
dependent PL measurement and optical simulations were carried out
(Fig. S12-13). The proportion of horizontally oriented emitting dipole
(Θ) of the TADF molecules were determined to be 70%, 78%, and
8
0%for 4CzBN, 4mCzBN, and 4mCzBN-BP, respectively. The device
structure of the EL device was ITO (75 nm)/ HATCN (10 nm)/ NPB
(20 nm)/ Tris-PCz (10 nm)/ mCBP (5 nm)/ TADF 20 wt%: mCBP
(
30 nm)/ Liq 30 wt%:DPPyA68 (30 nm)/Liq (2 nm)/ Al (100 nm)
(Fig. S14). Calculated maximum EQEs from the optical
simulation of the device structure was 18.6%, 25.4% and 25.3%
for 4CzBN, 4mCzBN, and 4mCzBN-BP, respectively. The device
characteristics are shown in Fig. 8 and Table 2. Note that the
structures of fabricated devices are not optically optimized for
each emitter. Instead, similar device structures from reports of
long-lifetime TADF OLEDs were adopted. Hence, the simulated
EQEs (18.6%, 25.4%, and 25.3% for 4CzBN, 4mCzBN, and
3
However, the formation of LEBP state is accompanied by
planarization of the biphenyl moiety that must overcome a
3
large steric hindrance. Hence, the LEBP that promotes RISC can
only be formed by a subset of possible conformations of
4
mCzBN-BP in solid mCBP matrix. At the early region of the
delayed decay, k of 4mCzBN-BP is the fastest among the three
d
4
mCzBN-BP respectively) for the fabricated devices are lower
TADF molecules. At the time region where the delayed decay
than the values of simulated EQEmax (19.6%, 27.2%, and 29.0%
for 4CzBN, 4mCzBN, and 4mCzBN-BP respectively). The device
incorporating 4mCzBN-BP as the emitter showed the highest
EQE of 23.7%. This value was close to the simulated EQE of
3
-3LE
from LE-formable 4mCzBN-BP conformers are over, the T
1
gap of the leftover conformers are the largest among the three
TADF molecules, leading to the slowest decay rate of 4mCzBN-
BP at the time over 100 μs. Furthermore, we observed the
redshift of phosphorescence spectrum of 4mCzBN-BP over time
and showed that the triplet energies obtained from low-
temperature phosphorescence do not represent all
conformations of TADF molecules that exist in frozen matrix.
Our results emphasize the importance of controlling
geometrical structure and minimizing conformational
distribution of TADF molecules in solid organic films for the
development of efficient and stable TADF emitters.
2
5.3% which is obtained by assuming charge balance factor
exciton formation efficiency) and RISC yield is unity. Hence, the
charge balance factor and RISC yield of the device incorporating
mCzBN-BP is higher than 0.94 (23.7%/25.3%). As the device
structures for 4CzBN and 4mCzBN are the same as that for
(
4
4
4
mCzBN-BP, the charge balance factor of devices based on
CzBN and 4mCzBN should be similar to that of 4mCzBN-BP
device. Thus, the difference of experimental EQEmax of the
fabricated OLEDs stems from the RISC yield of each TADF
8
| J. Name., 2012, 00, 1-3
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Journal Name
ARTICLE
4. Experimental Section
Acknowledgements
View Article Online
DOI: 10.1039/D0TC02627A
4
.1 Fabrication of organic thin film and OLEDs
This work was supported by Industrial Strategic Technology
Development Program (10079671) funded by the Ministry of
Trade, Industry, and Energy (MOTIE, Korea) and NRF
(2018R1A2A1A05078734).
Organic thin films for PL measurements were fabricated by
thermal deposition onto pre-cleaned fused silica substrates at a
base pressure of < 5 × 10-7 Torr. OLEDs were fabricated by
thermal deposition onto electrode-patterned ITO glass
-7
substrates at a base pressure of < 5 × 10 Torr. Before the
deposition of the organic layers, the ITO substrates were pre-
cleaned with acetone and isopropyl alcohol and then exposed
to UV-ozone for 15 minutes. Samples for photoluminescence
and electroluminescence measurements were encapsulated in
a nitrogen-filled glove box.
Supplementary Information
Synthesis and characterization of the materials, experimental
methods, and supplementary figures are included in the
supplementary information.
4
.2 Measurements of absorption, photoluminescence, and
electroluminescence
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