Two-channel emission controlled by a conjugation valve for the color switching of thermally activated delayed fluorescence emission

Article abstract of DOI:10.1039/c9tc02618e

The development of two-channel emission featuring deep-blue thermally activated delayed fluorescent (TADF) emitters is highly challenging due to their unavoidably extended conjugated structures. Herein, we propose a simple and effective strategy involving engineering of the interconnection position and formation of a "conjugation valve" to control their emission colors, including enabling emission of deep-blue light with an uncompromised external quantum efficiency (EQE). Two novel TADF emitters, namely 2,3′,4,5′-tetra(9H-carbazol-9-yl)-[1,1′-biphenyl]-3,4′-dicarbonitrile (34CzBN) and 3,3′,5,5′-tetra(9H-carbazol-9-yl)-[1,1′-biphenyl]-4,4′-dicarbonitrile (44CzBN), were developed to produce a two-channel-emission path by directly connecting two identical TADF units in two different ways to produce different degrees of conjugation. The non-conjugative mode of connection in 34CzBN resulted in severe steric hindrance at the central core, which interrupted the extensive conjugation and resulted in significant blue-shifts of the absorption and emission compared to those of its isomer 44CzBN. The two-channel-emission process provided a four-times higher EQE than the single-channel-emission process. The 34CzBN-based TADF device demonstrated deep-blue emission with color coordinates of (0.14, 0.14) due to a closed conjugation channel, and displayed an EQE of 15.2%. In contrast, the 44CzBN device showed green emission with color coordinates of (0.20, 0.44), due to the open conjugation channel, and an EQE of 13.8%. Additionally, the emission peak in the spectrum of the 34CzBN-based device became narrow (full width at half-maximum of ～63 nm) due to restricted molecular motion resulting from the interlocked carbazole donor at the central core.

Full text of DOI:10.1039/c9tc02618e

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Materials for optical, magnetic and electronic devices
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Materials for optical, magnetic and electronic devices
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Page 1 of 25
Journal of Materials Chemistry C
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DOI: 10.1039/C9TC02618E
Two Channel Emission Controlled by Conjugation Valve for
Color Switching of Thermally Activated Delayed Fluorescence
Emission
Rajendra Kumar Konidena, Kyung Hyung Lee, and Jun Yeob Lee*
School of Chemical Engineering, Sungkyunkwan University, 2066 Seobu-ro, Jangan-gu,
Suwon, Gyeonggi 440-746, Korea
Email: leej17@skku.edu
Abstract
Development of two channel emission featured deep blue thermally activated delayed
fluorescent (TADF) emitters is highly challenging due to their unavoidable extended
conjugated structures. Herein, we propose a simple and effective interconnection position
engineering strategy to control their emission color in deep blue region through a conjugation
valve with uncompromised external quantum efficiency (EQE). Two novel TADF emitters
named 2,3',4,5'-tetra(9H-carbazol-9-yl)-[1,1'-biphenyl]-3,4'-dicarbonitrile (34CzBN) and
3
,3',5,5'-tetra(9H-carbazol-9-yl)-[1,1'-biphenyl]-4,4'-dicarbonitrile (44CzBN) were developed
to utilize two channel emission path by directly connecting two identical TADF units to have
different degree of conjugation. The non-conjugative mode of connection in 34CzBN resulted
severe steric hindrance at central core, which interrupted the extensive conjugation and resulted
in significant blue-shift in absorption and emission compared to its isomer 44CzBN. The two
channel emission process provided four times higher EQE than the single channel emission
process. The 34CzBN based TADF device demonstrated deep blue emission with a color
coordinate of (0.14, 0.14) due to closed conjugation channel and EQE of 15.2%. Whereas, the

Journal of Materials Chemistry C
Page 2 of 25
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DOI: 10.1039/C9TC02618E
4
4CzBN device showed green emission with a color coordinate of (0.20, 0.44) due to opened
conjugation channel and EQE of 13.8%. Additionally, the emission spectrum of the 34CzBN
based device became narrow (full width at half-maximum ~ 63 nm) due to restricted molecular
motion by interlocked carbazole donor at central core.
Key words : two channel emission, blue, TADF, efficiency
Introduction
During the past three decades, organic light emitting diodes (OLEDs) received immense
interest from both academia and industry due to their distinct advantages over the inorganic
light-emitting diodes such as light weight, improved picture quality and contrast, flexibility,
1
,2
fast response times and low power consumption. In the OLEDs, the recombination of charge
carriers produces 25% singlet and 75% triplet excitons in accordance with the spin statistics.27
According to spin selection rules, fluorescent emitters can utilize only one quarter of excitons
for light emission, which limits their internal quantum efficiency to 25%.1-3 Meanwhile,
phosphorescent emitters can harness both the singlet and triplet excitons for light emission to
3
achieve 100% internal quantum efficiency. However, the use of expensive rare metals and
short lifetime of precious metal-based deep-blue phosphorescent emitters for OLEDs are
2
challenging issues. To overcome these issues, metal-free thermally activated delayed
fluorescence (TADF) based organic emitters attracted significant attention due to their 100%
theoretical exciton utilization for light emission by harvesting non-emissive triplet excitons
through favorable reverse inter system crossing (RISC) from triplet excited state (T ) to singlet
1
excited state (S1).2,4 In principle, small singlet-triplet energy splitting (ΔE ) is essential to
ST
2
,4
ensure efficient RISC. It has been demonstrated that the donor-acceptor (D-A) type molecular

Page 3 of 25
Journal of Materials Chemistry C
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DOI: 10.1039/C9TC02618E
design with twisted geometry is highly desirable to reduce the ΔE through well separation of
ST
the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital
2
(
LUMO). In addition to the high RISC, high photoluminescence quantum yield (PLQY) is
also an important parameter of the TADF emitter and several strategies have been successful
to reach the high PLQY value in the TADF emitters. For instance, multiple donor based
molecular design, HOMO dispersing molecular design, and multiple TADF units based
molecular design improved the PLQY of the TADF emitters.5,6
Among all the colors of TADF emitters, efficient deep blue TADF emitters with CIEy ≤ 0.15
rarely reported in the literature, because as mentioned above the basic design of TADF emitter
adopts D-A type molecular structure, which always leads to broad (FWHM ~ 70-100 nm) and
red-shifted emissions.7-28 Moreover, simultaneous achievement of small ΔE , deep blue
ST
emission and high PLQY is highly challenging task.7-28 Therefore, the development of efficient
deep blue TADF emitters became one of the grand challenges to the researchers. It has been
demonstrated that the utilization of two channel emission is an effective strategy to boost the
6
EQE of TADF emitters compared to single channel emission. However, this approach was
unsuccessful for the development of deep blue emitters (CIEy ≤ 0.15) because of their
6
increased degree of conjugation and strong charge transfer interactions. Therefore, the new
molecular design for the development of two channel emission featured efficient deep blue
TADF emitters is desirable. Recently, Adachi et al. reported high triplet energy deep blue
emitter (DCzBN1) with good color purity (CIEy ~ 0.05), but the emitter demonstrated poor
external quantum efficiency (EQE) of 2.5% due to the large ΔEST (0.31 eV) and inefficient
1
9
RISC. Here, we intended to utilize this backbone structure to develop two channel emission
based TADF emitters. We expect that the connection of two identical high triplet energy deep
blue TADF units can open two channel emission and the control of the degree of conjugation
can manage the emission color. The coupling of two emitting cores can improve the PLQY and

Journal of Materials Chemistry C
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DOI: 10.1039/C9TC02618E
can effectively utilize the two channel emission process for boosting the overall EQE.
In this contribution, we developed two novel TADF emitters named 2,3',4,5'-tetra(9H-
carbazol-9-yl)-[1,1'-biphenyl]-3,4'-dicarbonitrile (34CzBN) and 3,3',5,5'-tetra(9H-carbazol-9-
yl)-[1,1'-biphenyl]-4,4'-dicarbonitrile (44CzBN) by connecting two identical emitting units
through different positions. The interconnect position played a role of a conjugation valve by
opening and closing the conjugation channel through steric hindrance, which switched the
emission color between green and deep blue colors. The non-conjugative mode of linkage
between two emitting units of 34CzBN produced severe steric hindrance at central core with
large distortion, which interrupted the effective degree of conjugation and resulted in
significant blue-shift of absorption and emission profiles compared to its structural isomer
4
4CzBN. Moreover, the two channel emission via the two TADF units of 34CzBN and
4CzBN improved the EQE of the devices by four times compared with the single TADF unit
4
CzBN. A high EQE of 15.8% with a deep blue color coordinate of (0.14, 0.14) and an EQE of
1
3.8% with a green color coordinate of (0.20, 0.44) were demonstrated for the 34CzBN and
4CzBN based devices, respectively. We believe that our molecular design would provide
4
fundamental insights to design efficient two channel emission featured deep blue TADF
emitters.
CN
F
F
O
O
O
O
CN
CN
B
B
NC
F
F
F
N
N
N
F
F
F
F
NC
Br
N
H
F
CN
O
N
CN
Cl
B
Pd(PPh ) , K CO
3
Cs2CO3, DMF, 150 oC, 2h
KOAc, Pd(dppf)Cl2,
,4-dioxane, 100 oc, 6h
3
4
2
O
THF+H O, 80 oC, 10h
1
2
2
3
1
34CzBN
CN
F
F
O
O
O
O
CN
CN
B
B
F
F
F
F
F
N
N
N
N
N
H
F
F
Br
NC
CN
NC
CN
Pd(PPh3)4, K2CO3
THF+H2O, 80 oC, 10h
Cs CO , DMF, 150 oC, 2h
2 3
KOAc, Pd(dppf)Cl2,
1,4-dioxane, 100 oc, 6h
B
F
O
O
Br
6
4
44CzBN

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DOI: 10.1039/C9TC02618E
Scheme 1. Synthetic protocol of the compounds
Results and Discussions
Molecular design and synthesis
Fig. 1 Design concept of two channel emission featuring TADF materials.
Molecular design concept of this work is that the connection of two identical high triplet
energy deep blue emitting cores in a non-conjugative mode through a phenyl linker with severe
steric hindrance at central core can disrupt the planarity and hinder the effective conjugation
thought the molecule (Figure 1). As a result, the emission color can fall in deep blue region
with small ΔE and high PLQY. In addition, two channel emission process can be effectively
ST
utilized to promote the overall EQE of deep blue TADF emitters. Whereas, the connection of
two TADF units in a conjugative mode without steric hindrance can extend the conjugation
and the emission color can be shifted to long wavelength while maintaining the small ΔEST,
high PLQY, and high EQE due to two channel emission process. Therefore, high efficiency

Journal of Materials Chemistry C
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DOI: 10.1039/C9TC02618E
TADF emitters with different emission colors can be developed using the same building blocks.
The interconnect position can play a role of conjugation valve for opening and closing the
conjugation channel and managing the emission color.
Two novel TADF emitters, 34CzBN and 44CzBN, with different degree of conjugation were
developed to rationalize the design concept. The synthetic procedure to synthesize the target
materials is illustrated in Scheme 1. The key aryl fluoride intermediates such as 3 and 6 were
prepared in two step synthetic protocol starting from the boronate ester formation of 1 and 4
2
9
using palladium-catalyzed reaction conditions. Subsequently, the intermediates 2 and 5 were
treated with 4 under palladium-catalyzed Suzuki-Miyaura cross-coupling reaction conditions
3
0
to yield aryl fluoride intermediates 3 and 6 in good yield, respectively. Finally, the fluoride
intermediates 3 and 6 were allowed to react with 4.0 eq. of 9H-carbazole using cesium
carbonate mediated nucleophilic substitution reaction to obtain the desired 34CzBN and
4
4CzBN TADF emitters in reasonable yields. The compound 2,6-di(9H-carbazol-9-
1
9
yl)benzonitrile (CzBN) were synthesized as per the given procedure in the literature. We also
tried to synthesize the additional isomer in the series by connecting two CzBN cores via 3,3'-
positions of phenyl linker using various synthetic methods, but the synthesis was unsuccessful
due to the large steric hindrance. The chemical structures of the 34CzBN and 44CzBN
compounds were confirmed by the spectroscopic analysis such as nuclear magnetic resonance
spectroscopy ( H NMR and 13C NMR), elemental analysis and mass spectroscopy. The
materials were purified by the vacuum train sublimation procedure before application in the
OLED devices.
1

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DOI: 10.1039/C9TC02618E
Fig. 2 Optimized geometries and FMO distributions of the compounds estimated using DFT
computations
Theriotical Caleculations
To understand the effect of interconnection position of the two channel emission featured
TADF materials on their frontier molecular orbital distributions (HOMO and LUMO) and ΔEST,
density functional theory (DFT) computations were implemented on the TADF compounds
employing M062X/6-31G(D) basis set on Gaussian 16 program. The optimized geometries and
their frontier molecular orbital (FMO) distributions of the compounds are shown in Figure 2.
The dihedral angle between directly connected two emitting cores is dependent on the linking

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position. The dihedral angles (α) between the two phenyl units of biphenyl core of 34CzBN
o
and 44CzBN are 55 and 36 , respectively. The two phenyl units of 34CzBN is largely distorted
due to the large steric hindrance by the interlocked carbazole donor at ortho-position of the
o
phenyl linker with a large dihedral angle (β) of 71 . As a result, the effective π-conjugation in
the 34CzBN emitter can be interrupted. Whereas, the small dihedral angle between two phenyl
units of 44CzBN extends the π-conjugation. The connection of the two TADF CzBN units via
C4 & C4 position opens the π-conjugation channel, while that via C3 & C4 position closes the
π-conjugation channel. The dihedral angle (γ) between the carbazole donor and phenyl linker
of 34CzBN and 44CzBN is analogues to that of the CzBN. The interconnection position and
dihedral angle also affected the HOMO and LUMO distributions. In the case of 44CzBN, the
HOMO is uniformly dispersed over the all four carbazole units because of the existence of
extensive conjugation between all the carbazole donors through a small dihedral angle (α)
between two emitting cores. Whereas, in the case of 34CzBN, the HOMO was non-uniformly
distributed due to the electronically separated carbazole donor units by large steric hindrance
at the biphenyl unit. In the two compounds, the LUMO was mainly localized on the biphenyl
linker and cyano acceptor. Although the FMO distribution is different in these compounds, the
well separated HOMO and LUMO orbitals with partial overlap on the biphenyl unit fulfilled
the requirement of TADF emitters. The calculated singlet (S )/triplet energies (T )/ΔE of the
1
1
ST
compounds were 3.84/3.58/0.26 eV for CzBN, 3.58/3.27/0.28 eV for 34CzBN and
.41/3.18/0.23 eV for 44CzBN. The closed π-conjugation channel of the 34CzBN emitter by
large twisting angles is responsible for the high singlet and triplet energies compared to
4CzBN with opened π-conjugation channel.
3
4

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160000
140000
120000
100000
CzBN
34CzBN
44CzBN
80000
60000
40000
20000
0
250
300
350
400
450
500
Wavelength (nm)
Fig. 3 Absorption spectra of the compounds recorded in THF solution.
Photophysical Properties
To understand the correlation between the molecular structure and absorption and emission
properties of the materials, photophysical properties were investigated by ultraviolet-visible
(UV-vis) absorption and fluorescence spectrometer. The absorption spectra of the compounds
collected in tetrahydrofuran (THF) solution are shown in Figure 3, and the detailed data are
listed in Table 1. The compounds showed multiple absorption bands in the range of 270-450
nm. The short wavelength bands between 250 and 340 nm are attributed to the localized π-π*
electronic transitions of the aromatic segments. The long wavelength absorption band above
3
50 nm stems from the intramolecular charge transfer (ICT) from the carbazole donor to cyano
acceptor. In the UV-vis absorption data, the molar extinction coefficient of the 34CzBN and
4CzBN emitters increased significantly compared to that of the CzBN because of intensified
4

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light absorption by the two light absorbing TADF units. In particular, the 44CzBN emitter
showed even higher molar extinction coefficient at short wavelength region (< 300 nm) than
3
4CzBN because of extended conjugation throughout the molecular structure with small
twisting angle (α). The comparison of the absorption profiles of the compounds revealed that
the absorption edge depends on the number of chromophores and their interconnect position.
The ICT band was red-shifted in the 34CzBN and 44CzBN emitter relative to that of CzBN
due to the increased charge transfer interactions between carbazole donor and cyano acceptor
and extended π-conjugation. Indeed, the 34CzBN showed blue-shifted absorption compared to
4
4CzBN due to the interrupted conjugation between the two emitting units.
The photoluminescence (PL) spectra of the compounds recorded using 10 wt% emitter doped
bis[2-(diphenylphosphino)phenyl] ether oxide (DPEPO) films are shown in Figure 4. The trend
of emission profiles of the compounds is in good agreement with the absorption profile,
confirming the effect of interconnect position and dihedral angle. The increasing order of
emission wavelengths of the compounds are CzBN (403 nm) < 34CzBN (453 nm) < 44CzBN
(490 nm). The 44CzBN and 34CzBN displayed 50 and 87 nm red-shifted emission compared
to CzBN. As expected, the large dihedral angle of 34CzBN allowed the deep-blue emission by
minimizing the degree of conjugation, while the small dihedral angle of 44CzBN leads to the
green emission.
Furthermore, the nature of the excited state of the compounds was investigated by
solvatochromic study. The emission profiles of the compounds were progressively red-shifted
by increasing the solvent polarity from non-polar cyclohexane (CH) to polar dichloromethane
3
1
(
MC) (Figure 5). Generally, the red-shifted broad emission in polar solvents indicates charge
3
1
transfer (CT) excited state of the compounds. The 34CzBN emitter displayed weak CT in the
excited state as evident from the small bathochromic-shift (Δλ(CH-ACN) ~ 39 nm) compared to

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its isomer 44CzBN (Δλ(CH-ACN) ~ 75 nm) and even the CzBN (Δλ(CH-ACN) ~ 45 nm). This is
attributed to the limited conjugation between the two emitting units with large dihedral angle
3
1
in 34CzBN, which restricted the strong CT in the excited state. The phosphorescence spectra
of the compounds recorded in frozen THF at 77 k exhibited the similar tendency to the
fluorescence spectra. The S and T energies of the CzBN, 34CzBN and 44CzBN were
1
1
estimated from their onset wavelengths of fluorescence and phosphoresce spectra and were
3
.40/3.10 eV, 3.18/2.96 eV and 2.95/2.80 eV, respectively. As expected, the T energy of the
1
3
4CzBN increased significantly compared to 44CzBN due to the interrupted π-conjugation by
large dihedral angle. As a result, the ΔE values of the CzBN, 34CzBN and 44CzBN were
ST
calculated to be 0.30 eV, 0.22 eV and 0.15 eV, respectively.
1.05
0.90
0.75
0.60
0.45
0.30
0.15
0.00
(a)
CzBN
34CzBN
44CzBN
350
400
450
500
550
600
650
Wavelength (nm)
1.05
0.90
0.75
0.60
0.45
0.30
0.15
0.00
(b)
CzBN
34CzBN
44CzBN
390
440
490
540
590
640
Wavelength (nm)
Fig. 4 a) Fluorescence spectra of the compounds collected using 10 wt% emitter doped DPEPO

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films and b) phosphorescence spectra of the compounds recorded at 77 k in frozen THF.
45 nm
39 nm
(a)
CH
1
.0
.8
1.0
0.8
0.6
0.4
0.2
0.0
TOL
THF
MC
(b)
CH
TOL
THF
MC
0
0.6
0.4
0.2
0.0
350
375
400
425
450
475
500
525
550
380
420
460
500
540
580
620
660
Wavelength (nm)
Wavelength (nm)
75 nm
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
(c)
CH
TOL
THF
MC
385
430
475
520
565
610
655
Wavelength (nm)
Fig. 5 Emission spectra of the compounds a) CzBN, b) 34CzBN and c) 44CzBN collected in
different solvents (CH = cyclohexane, TOL = toluene, THF = tetrahydrofuran, MC =
dichloromethane).
The TADF behavior of these materials was analyzed by transient PL measurements using
1
0 wt% emitter doped DPEPO films at room temperature (Figure 6). The CzBN displayed
strong prompt with lifetime of 4.7 ns and very weak delayed fluorescence. The weak delayed
component of CzBN could be attributed to its large ΔE and inefficient RISC process. The
ST
3
4CzBN and 44CzBN compounds showed clear prompt and delayed components with

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DOI: 10.1039/C9TC02618E
corresponding lifetimes of 6.6 ns/7.9 µs and 9.0 ns/5.8 µs, respectively. The delayed
fluorescence component was unambiguously assigned to the thermal up-conversion of triplet
excitons from T to S via RISC process followed by radiative transition from S to S . The
1
1
1
0
relatively short delayed fluorescence lifetime (t ) of 44CzBN is attributed to its small ΔE .
d
ST
The absolute PLQY of the doped films was measured using an integrating sphere method under
nitrogen at room temperature. The PLQYs of the compounds were 20% for CzBN, 58% for
3
4CzBN, 51% for 44CzBN. The superior PLQY of 34CzBN and 44CzBN compared to CzBN
is attributed to the utilization of two channel emission process and efficient contribution of
triplet excitons for light emission through RISC, which is evident from the high PLQY of
delayed fluorescence component of 34CzBN (46%) and 44CzBN (40%) compared to CzBN
(9%). The relatively high PLQY of 34CzBN compared to its isomer 44CzBN may be due to
molecular rigidity by the sterically hindered carbazole unit in the biphenyl unit. Furthermore,
RISC rate constants (KRISC) of the materials were calculated using PLQY and transient PL data
according to the literature procedure. The 34CzBN and 44CzBN showed KRISC of 5.51 × 105
-1
5
-1
s and 7.0 × 10 s . These results indicate that the 34CzBN and 44CzBN have potential as
TADF emitters.

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Fig. 6 Transient PL decay profiles of the 10wt% emitter doped DPEPO films.
Electrochemical and Thermal Properties
The electrochemical behavior of these compounds was analyzed by cyclic voltammetry (CV)
measurements in dilute dichloromethane solution using 0.1 M tetrabutylammonium perchlorate
electrolyte. The corresponding HOMO and LUMO energy levels of the materials were
determined from their onset potentials of oxidation and reduction waves relative to ferrocene
(
-4.80 eV) and were -5.87 eV and -2.92 eV for CzBN, -5.87 eV and -3.25 eV for 34CzBN and
5.84 eV and -3.29 eV for 44CzBN. The HOMO energy levels of the compounds were almost
-
unchanged, whereas the LUMO levels of 34CzBN and 44CzBN were deeper than that of CzBN
due to the increased number of cyano acceptor units in former compounds. The 44CzBN
emitter exhibited relative small HOMO-LUMO gap owing to the increased π-conjugation
between the two emitting units.
Thermal stability of the compounds was determined by thermogravimetric analysis (TGA)
o
under nitrogen at a heating rate of 10 C/min. The corresponding TGA traces are shown in
Figure S1 and the data are tabulated in Table 1. The two materials exhibited marked thermal
stability with high decomposition temperatures corresponding to 5% weight loss (T ) of 448 °C
d
for 34CzBN and 485 °C for 44CzBN compared to 354 °C of CzBN. The extended -
conjugation through small dihedral angle thermally stabilized the 44CzBN molecule.

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Journal of Materials Chemistry C
Table 1. Photophysical, electrochemical and thermal data of the compounds
g
h
i
j
Compound
λ , nm (ε , M- λem (nm), ES/E
ΔEST
(eV)
HOMO/LUMO
(eV)
Eg (eV)f t /t (ns/μs) Φ /Φ /Φ
T
( C)
354
abs
-1
max
3 a
T
c
p
d
p
d
PL
onset
1
b
d
e
o
k
cm ×10 )
350 (6.3), 332 403
13.4), 289 (32.3)
364 (19.9), 331 453
41.0), 287 (93.7)
392 (11.0), 316 490
sh), 289 (153.8)
(film)
(eV)
CzBN
3.40/3.10 0.30
3.18/2.96 0.22
2.95/2.80 0.15
5.87/2.92
5.87/3.25
5.84/3.29
2.95
2.62
2.55
4.7/-
0.11/0.09/0.20
0.12/0.46/0.58
0.11/0.40/0.51
(
3
4
4CzBN
4CzBN
6.6/7.9
9.0/5.8
448
485
(
(
a
, b
c
Collected in THF solution Collected for 10wt% doped film in DPEPO. Calculated from the onset of fluorescence and phosphorescence spectra.
Calculated singlet-triplet energy gap. HOMO and LUMO estimated from the cyclic voltammograms. Electrochemical band gap. Prompt (t )and
delayed (t ) fluorescence lifetimes. Prompt (Φ ) and delayed (Φ ) PLQY determined from the total PLQY and the proportion of the integrated
area of individual components in the transient PL spectra to the total integrated area. Absolute PLQY of the doped films in DPEPO measured
d
e
f
g
p
h
i
d
p
d
j
k
using an integrating sphere under nitrogen atmosphere at room temperature. Temperature corresponds to 5% weight loss (Tonset)

Journal of Materials Chemistry C
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DOI: 10.1039/C9TC02618E
Electroluminescence Properties
To evaluate the electroluminescence (EL) performance of the emitters, we fabricated
vacuum processed multilayer OLED devices by employing them as emitting dopants in the
DPEPO host. The fabricated device configuration is as follows: ITO (50 nm)/PEDOT:PSS(60
nm)/TAPC(20 nm)/mCP(10 nm)/EML (25 nm:10 wt%)/TSPO1(5 nm)/TPBi(20
nm)/LiF(1.5)/Al(200 nm). Here, diphenylphosphine oxide-4-(triphenylsilyl)-Phenyl (TSPO1)
used as electron transporting type exciton blocking layer, mCP used as hole transporting type
exciton blocking layer. Di-[4-(N,N-ditolyl-amino)phenyl]cyclohexane (TAPC) and LiF served
as hole transport and electron injection layers. The emitting layers (EML) were CzBN or
3
4CzBN or 44CzBN (10wt%) doped DPEPO layer. The device structure and energy level
diagram of device are shown in Figure 7. The results of the device performance of the
compounds including current density (J)-voltage (V)-luminance (L), external quantum
efficiency (EQE) versus luminance plots, EL spectra and CIE 1931 color chromaticity diagram,
are shown in Figure 8 and the final device parameters were summarized in Table 2.
Fig. 7 Device structure and energy level alignment of the materials.

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DOI: 10.1039/C9TC02618E
The main objective of the two channel emission molecular design was to enhance the EQE
and to control the emission color by managing the degree of conjugation through
interconnection position engineering. As intended, the 34CzBN device exhibited deep-blue
emission with a peak maximum of 458 nm and CIE color coordinate of (0.14, 0.14), whereas
its structural isomer 44CzBN showed green emission with a peak maximum of 500 nm and
CIE color coordinate of (0.20, 0.44). The simple engineering of the interconnect position
greatly changed the EL emission spectra by switching the conjugation channel. The deep-blue
emission of 34CzBN is attributed to the interrupted π-conjugation between the two emitting
cores by their non-conjugative mode of connection. Also, the increased molecular rigidity of
3
4CzBN with the interlocked carbazole unit in the biphenyl core sharpened the emission
spectrum, resulting in small FWHM of 63 nm compared to 76 nm of its isomeric derivative
4CzBN. The emission spectrum of 34CzBN was even narrower than that of the CzBN
FWHM ~ 72 nm). The absence of any additional emission peaks in the EL spectra from the
4
(
adjacent layers and the close resemblance of the EL spectra of the compounds with their PL
spectra indicate the efficient exciton confinement and complete energy transfer from the host
to dopant.
The maximum EQEs (EQEmax) of the 34CzBN and 44CzBN devices were 15.2 and 13.8%
compared with 3.4% of the CzBN device. The 34CzBN and 44CzBN devices showed more
than four-times higher EQE than the CzBN device. This is mainly attributed to the high PLQY
through two channel emission process and fast RISC through small ΔEST of 34CzBN and
4
4CzBN compared to CzBN. In particular, the device performances of the 34CzBN device is
comparable to the reported deep blue TADF emitters with CIEy ≤ 0.15.7-28 The results
demonstrate that the simple conjugation isolation strategy of the two channel emission based
TADF emitters by careful interconnect position engineering is an effective approach to develop
high EQE and deep-blue emitting TADF OLEDs.

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Fig. 8 a) I-V-L plots, b) EQE vs luminance, c) EL spectra and d) CIE color chromaticity diagram
of the devices.

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Journal of Materials Chemistry C
Table 2. EL properties of the compounds
Dopant
Voltage (V)
EQE (%)
Current efficiency (cd/A)
Power efficiency (lm/W)
EL
peak CIE (x, y)
Max. @100 cd/m2 Max.
@100 cd/m
2
Max
@100 cd/m
2
(nm)/FWHM
CzBN
4.4
4.3
4.4
3.4
15.2
13.8
0.6
7.3
9.1
1.4
17.4
35.2
0.3
8.5
22.8
1.1
12.1
26.6
0.1
4.4
11.7
426/72
458/63
500/76
0.16, 0.08
0.14, 0.14
0.20, 0.44
3
4
4CzBN
4CzBN

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DOI: 10.1039/C9TC02618E
Conclusion
In conclusion, we developed two TADF emitters, 34CzBN and 44CzBN, with different
degree of conjugation by directly coupling two identical TADF units through different
interconnect positions. The change of the interconnection position controlled the degree of
conjugation and managed emission properties of the TADF emitters. The non-conjugative
mode of connection in 34CzBN limited the extensive conjugation and enabled deep blue
emission. Whereas, the extended conjugation of 44CzBN witnessed green emission. Two-
channel emission process of 34CzBN and 44CzBN benefited more than four-fold improved
device performance with EQEmax of 15.2% and 13.8% compared to single channel emission
process in CzBN (3.4%). Therefore, we believe that our results provide fundamental insights
to design two channel emission based TADF emitters over a wide color gamut with
uncompromised EQE.
Acknowledgement
This work was supported by the Basic Science Research Program (Grant No.
2
016M3A7B4909243 and 2018R1D1A1B07048498) through the National Research
Foundation (NRF) of Korea funded by the Ministry of Science, ICT, and Future Planning.
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Table of Content