Solution-processed white organic light-emitting diodes with blue fluorescent and orange-red thermally activated delayed fluorescent dendritic luminogens

Article abstract of DOI:10.1016/j.dyepig.2019.107650

In this work, dendritic luminogens, namely, 3ICz-Tr as a blue emitter and Tri-tNID as an orange-red emitter, were synthesized and used for the application of white organic light-emitting diodes (WOLEDs). These molecules have appropriately high molecular weights, which are favorable for the fabrication of thin films with fine surface morphologies. Interestingly, the blue emitting 3ICz-Tr displays fluorescent character and the orange-red emitting Tri-tNID exhibited thermally activated delayed fluorescence, which was confirmed by steady state and transient photoluminescence (TRPL) spectroscopy. Eventually, WOLED devices could be realized by mixing the two molecules and optimizing their mixture composition. Among the devices we fabricated, the device with Tri-tNID and 3ICz-Tr (0.5:99.5 wt ratio) without additional host material demonstrated nearly pure white emission, with the maximum EQE of 8.32percent and CIE coordinate of (0.32, 0.30). Furthermore, by changing the composition of the two dendritic molecules, the emission color can be adjusted from a warm white to a cool white in the corresponding WOLED.

Full text of DOI:10.1016/j.dyepig.2019.107650

DyesandPigments170(2019)107650
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Solution-processed white organic light-emitting diodes with blue fluorescent
and orange-red thermally activated delayed fluorescent dendritic
luminogens
Jiwon Yoona, Suna Choia, Cheol Hun Jeonga, Seong Keun Kimb, Hyuna Leeb, Youngseo Kima,
Jang Hyuk Kwonb, Sungnam Parka, Min Ju Choa,∗∗, Dong Hoon Choia,∗
a Department of Chemistry, Research Institute for Natural Sciences, Korea University, 145, Anam-ro, Sungbuk-gu, Seoul, 02841, South Korea
b Department of Information Display, Kyung Hee University, 26, Kyungheedae-ro, Dongdaemun-gu, Seoul, 02447, South Korea
A R T I C L E I N F O
A B S T R A C T
Keywords:
Dendritic molecule
Blend film
White organic light-emitting diodes
Solution-process
In this work, dendritic luminogens, namely, 3ICz-Tr as a blue emitter and Tri-tNID as an orange-red emitter,
were synthesized and used for the application of white organic light-emitting diodes (WOLEDs). These molecules
have appropriately high molecular weights, which are favorable for the fabrication of thin films with fine surface
morphologies. Interestingly, the blue emitting 3ICz-Tr displays fluorescent character and the orange-red emit-
ting Tri-tNID exhibited thermally activated delayed fluorescence, which was confirmed by steady state and
transient photoluminescence (TRPL) spectroscopy. Eventually, WOLED devices could be realized by mixing the
two molecules and optimizing their mixture composition. Among the devices we fabricated, the device with Tri-
tNID and 3ICz-Tr (0.5:99.5 wt ratio) without additional host material demonstrated nearly pure white emission,
with the maximum EQE of 8.32% and CIE coordinate of (0.32, 0.30). Furthermore, by changing the composition
of the two dendritic molecules, the emission color can be adjusted from a warm white to a cool white in the
corresponding WOLED.
1. Introduction
host.
When using a blue emitter that exhibits phosphorescence or TADF
Organic light-emitting diodes (OLEDs) are considered as the most
promising technology for the next generation of flat-panel displays due
to their light weight, low cost, and easy processability among other
properties [1–3]. In particular, white OLEDs (WOLEDs) have recently
attracted attention for their use in solid-state-lighting applications such
as indoor lighting and backlights for liquid crystal displays. To realize
white light emission, many studies have been conducted, primarily
addressing a two- or three- color blend systems [4–12] and tandem
systems [13–19]. Amongst the foregoing systems, a two-color blend
system is the simplest process, due to the minimal number of compo-
nents in the emitting layer (EML).
To achieve efficient WOLEDs in a two-color blend system, various
combinations of emitters were have been proposed such as fluorescent/
fluorescent [20], fluorescent/phosphorescent [21], phosphorescent/
phosphorescent [22–26], fluorescent/thermally activated delayed
fluorescence (TADF) [27], TADF/phosphorescent [28–31], and TADF/
TADF [5–7,32,33] emitters, which were embedded into appropriate
characteristics for the implement efficient WOLED devices. When using
a mixture of two-color emitters as an emitting layer, the stability of the
device could be impaired by the triplet-polaron annihilation (TPA) and
triplet-triplet annihilation (TTA) processes, due to the long triplet ex-
citon lifetime. Hence, the use of a fluorescent blue emitting molecule is
favorable and efficient for the implementation of proper WOLED de-
vices. A WOLED can be fabricated with a mixture of a blue fluorescent
emitter capable of transferring energy to a green or red light emitting
material, and a red-emitting TADF material displaying prompt emission
in a singlet state and a delayed emission through a reverse intersystem
crossing (RISC) from a triplet state to a singlet state. Correspondingly,
the device efficiency is expected to be higher than that of a device
having red fluorescent materials in the emitting layer.
With respect to the device fabrication process, the orange-red
emitter is mixed with the blue emitter and used in a very small amount
(less than 1 wt%), which makes it difficult to control the doping ratio
using a vacuum-phase process. Compared with the vacuum process, the
∗ Corresponding author.
∗∗ Corresponding author.
E-mail addresses: chominju@korea.ac.kr (M.J. Cho), dhchoi8803@korea.ac.kr (D.H. Choi).
https://doi.org/10.1016/j.dyepig.2019.107650
Received 30 April 2019; Received in revised form 18 June 2019; Accepted 18 June 2019
Availableonline21June2019
0143-7208/©2019PublishedbyElsevierLtd.

J. Yoon, et al.
DyesandPigments170(2019)107650
solution process enables easier preparation of the blend solutions with
the desired composition. In addition, it is favorable for low-cost and
large-area devices. On the other hand, for the realization of a high-
efficiency WOLED, the complex manufacturing process should be sim-
plified, and the dopant concentration to be added to the host should be
optimized precisely.
using silica-gel column chromatography (eluent: DCM/hexane (1:2 v/
v)) to obtain a pure compound 4, which was a white-green solid (0.3 g);
yield 88%. IR (KBr, cm−1) ν: 2957, 1604, 1516, 1440, 1368, 1219, 772,
692. 1H NMR (CDCl3, 500 MHz, ppm): δ 9.08–9.07 (2H, d, J = 8.6 Hz),
8.85–8.83 (4H, dd, J = 8.4, 1.4 Hz), 8.41 (1H, s), 8.34 (1 H, d,
J = 1.8 Hz), 7.87–7.86 (1H, d, J = 7.6 Hz), 7.84–7.83 (2H, d,
J = 8.6 Hz), 7.68–7.61 (6H, m), 7.53 (1 H, s), 7.51 (1H, d, J = 1.9 Hz),
7.46–7.44 (1H, d, J = 7.7 Hz), 7.42–7.39 (2H, t, J = 7.7 Hz), 7.33–7.30
(1H, td, J = 7.3, 0.9 Hz), 1.54–1.52 (6H, d, J = 8.3 Hz). 13C NMR
(CDCl3, 125 MHz, ppm): δ 171.87, 170.85, 154.11, 141.22, 141.10,
139.24, 136.07, 135.37, 133.24, 132.75, 130.82, 129.03, 128.74,
128.47, 127.18, 126.82, 126.68, 125.75, 123.03, 122.61, 122.22,
In this study, two dendritic molecules (i.e. 3ICz-Tr and Tri-tNID)
were synthesized successfully and a two-component EML was prepared
for the high-performance WOLEDs. 3ICz-Tr containing 5-(4-(4,6-di-
phenyl-1,3,5-triazin-2-yl)phenyl)-7,7-dimethyl-5,7-dihydroindeno[2,1-
b]carbazole exhibits blue fluorescent emission, and Tri-tNID containing
2-(4-(tert-butyl)phenyl)-6-(7,7-dimethyl-13,13-diphenyl-7,13-dihydro-
5H-indeno[1,2-b]acridin-5-yl)-1H-benzo[de]isoquinoline-1,3(2H)-
dione exhibits orange-red emission based on TADF behavior. Owing to
their relatively high molecular weight, the solubility and film-forming
abilities of both molecules are excellent. The unique PL characteristics
of the blue and orange-red-emitting dendritic molecules enabled the
fabrication of a device using a white light emitting EML, which allowed
for the optimization of the composition of 3ICz-Tr and Tri-tNID; thus, a
pure white OLED was realized. Moreover, because the ratio of the two
molecules could be controlled, the emission color was precisely tuned
from warm white to cool white in the corresponding WOLEDs.
119.51, 113.21, 111.54, 111.40. 104.31, 27.91; Uv–Vis (CHCl3) λmax
/
nm (ε/105 mol−1 dm3 cm−1): 368 (0.21); Found: [M+H]+ 668.1479;
molecular formula C42H29BrN4 requires [M+H]+ 668.1576.
2.1.3. Synthesis of 3ICz-Tr (5)
Compound 4 (0.50 g, 0.75 mmol), 1,3,5-tris(4,4,5,5-tetramethyl-
1,3,2-dioxaborolan-2-yl)benzene (0.10 g, 0.22 mmol), tris(dibenzyli-
dene acetone)dipalladium (40 mg, 0.04 mmol), tri-(o-tolyl)phosphine
(60 mg, 0.20 mmol), and potassium carbonate (0.21 g, 1.52 mmol) were
dissolved in a mixed solvent of distilled toluene (18 mL), water (6 mL),
and methanol (2 mL). The reaction mixture was refluxed under N2 at
100 °C for 12 h. After cooling to 25 °C, the reaction mixture was ex-
tracted using DCM and water. The extracted DCM layer was dried over
anhydrous sodium sulfate. After filtering the mixture, the residue was
purified by silica-gel column chromatography using DCM/hexane (1:2
v/v) as the eluent to obtain a white-green solid (0.20 g); yield 48%. Td
541 °C. IR (KBr, cm−1) ν: 2957, 1604, 1516, 1444, 1370, 772, 694. 1H
2. Experimental
2.1. Materials
All chemical reagents used in this study were purchased from
Sigma-Aldrich, TCI, Alfa Aesar and Acros. They were used without
further purification. Compounds 1, 2, 6, and 7 used for the synthesis of
the two dendritic luminogens were synthesized according to the
methods in the published literature [34–37]. Moreover, 1,3,5-tris
(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzene was synthesized
in accordance with the reported procedures [38,39].
NMR (CDCl3, 500 MHz, ppm):
δ 9.12–9.11 (6H, d, J = 8.6 Hz),
8.86–8.85 (12H, d, J = 7.5 Hz), 8.70 (3H, s), 8.61 (3H, s), 8.18 (3H, s),
7.97–7.94 (9H, t, J = 8.1 Hz), 7.91–7.89 (3H, d, J = 7.6 Hz), 7.73–7.72
(3H, d, J = 8.6 Hz), 7.68–7.61 (21H, m), 7.47–7.45 (3H, d, J = 7.3 Hz),
7.40–7.37 (3H, t, J = 7.5 Hz), 7.32–7.29 (3H, t, J = 7.4 Hz), 1.58 (18H,
s). 13C NMR (CDCl3, 125 MHz, ppm): δ 171.80, 170.92, 153.55, 153.37,
143.05, 141.71, 141.23, 140.49, 139.53, 136.11, 134.99, 134.18,
133.03, 132.68, 130.77, 129.03, 128.72, 127.13, 126.74, 126.46,
125.64, 124.98, 124.70, 123.55, 122.55, 119.53, 119.09,
111.68 110.31, 104.27, 46.87, 27.97; Uv–Vis (CHCl3) λmax/nm (ε/
105 mol−1 dm3 cm−1): 367 (0.62); Found: [M+H]+ 1843.5632; mo-
lecular formula C132H90N12 requires [M+H]+ 1842.7411.
2.1.1. Synthesis of ICz-Tr (3)
Compound 1 (0.60 g, 2.12 mmol), compound 2 (0.9 g, 2.32 mmol),
tripotassium phosphate (0.89 g, 4.19 mmol), copper iodide (8 mg,
0.04 mmol), and trans-1,2-diaminocyclohexane (0.21 g, 1.48 mmol)
were dissolved in distilled toluene (40 mL), and the reaction mixture
was refluxed at 100 °C in N2 for 12 h. Thereafter, the reaction mixture
was cooled to 25 °C and filtered through Celite. It was concentrated
under vacuum. The residue was purified by silica gel column chroma-
tography using d ichloromethane (DCM)/hexane (1:5 v/v) as the eluent
to give a pure compound 3, which was a white-green solid (1.13 g);
yield 90%. mp 255.6 °C. IR (KBr, cm−1) ν: 2957, 1604, 1516, 1452,
1370, 1219, 838, 772, 692. 1H NMR (CDCl3, 500 MHz, ppm): δ
9.02–9.01 (2H, d, J = 8.5 Hz), 8.82–8.80 (4H, dd, J = 8.3, 1.5 Hz), 8.43
(1H, s), 8.20–8.19 (1H, d, J = 7.6 Hz), 7.85–7.84 (1H, d, J = 7.7 Hz),
7.83–7.81 (2H, dd, J = 6.7, 1.8 Hz), 7.62 (1H, s), 7.61–7.60 (1H, t,
J = 1.7 Hz), 7.60–7.58 (3H, d, J = 7.3 Hz), 7.57–7.56 (1H, t,
J = 1.7 Hz), 7.54–7.53 (2H, d, J = 8.3 Hz), 7.44–7.40 (2H, m),
7.38–7.35 (1H, td, J = 7.5, 1.1 Hz), 7.34–7.31 (1H, t, J = 7.5 Hz),
7.29–7.26 (1H, td, J = 7.4, 1.1 Hz), 1.53 (6H, s). 13C NMR (CDCl3,
125 MHz, ppm): δ 170.91, 153.36, 153.31, 141.71, 140.86, 140.76,
139.56, 136.10, 134.94, 132.80, 132.67, 130.70, 129.01, 128.70,
127.07, 126.82, 126.40, 125.81, 124.01, 123.32, 122.56, 120.43,
120.26, 119.41, 111.41, 109.92, 104.07, 46.81, 27.94; Uv–Vis (CHCl3)
2.1.4. Synthesis of tNID (8)
A toluene solution (45 mL) of compound 6 (1.70 g, 3.78 mmol) and
compound 7 (1.40 g, 3.43 mmol), potassium tert-butoxide (0.77 g,
6.86 mmol),
tris(dibenzylideneacetone)dipalladium
(63 mg,
0.07 mmol), and tri-tert-butylphosphine (0.14 g, 0.69 mmol) was re-
fluxed in N2 at 120 °C for 24 h. After cooling to room temperature, the
mixture was extracted using water and DCM, and then the DCM layer
was dried over anhydrous sodium sulfate. After filtration, the collected
DCM filtrate was evaporated under vacuum. The residue was purified
by silica-gel column chromatography using DCM/hexane (3:2 v/v) as
the eluent to obtain an orange solid (1.21 g); yield 45%. Mp 358.1 °C. IR
(KBr, cm−1) ν: 2961, 1713, 1672, 1586, 1452, 1363, 1236, 1190, 754,
702. 1H NMR (CDCl3, 500 MHz, ppm): δ 8.88–8.87 (1H, d, J = 7.6 Hz),
8.56–8.54 (1H, dd, J = 7.2, 1.1 Hz), 7.89–7.88 (1H, d, J = 7.6 Hz),
7.60–7.58 (2H, d, J = 8.9 Hz), 7.45–7.44 (1H, d, J = 7.2 Hz), 7.38 (3H,
s), 7.33–7.29 (6H, m), 7.28 (1H, s), 7.23–7.21(2H, d, J = 7.8 Hz),
7.21–7.18 (3H, t, J = 6.3 Hz), 7.02–6.96 (6H, m), 6.43 (1H, s),
6.26–6.24 (1H, d, J = 8.7 Hz), 1.40 (9H, s), 1.31 (3H, s), 1.18 (3H, s).
13C NMR (CDCl3, 125 MHz, ppm): δ 164.24, 163.96, 153.17, 153.12,
151.56, 143.38, 141.97, 141.51, 139.10, 132.41, 132.28, 131.81,
131.17, 130.70, 130.42, 130.07, 129.26, 128.07, 127.91, 127.84,
126.50, 126.24, 123.34, 123.05, 122.38, 121.79, 121.00, 119.23,
113.90, 107.87, 57.28, 46.72, 34.79, 31.39, 27.61, 26.95; Uv–Vis
(CHCl3) λmax/nm (ε/105 mol−1 dm3 cm−1): 490 (0.01); Found: [M
λ
max/nm (ε/105 mol−1 dm3 cm−1): 362 (0.24); Found: [M+H]+
590.2468; molecular formula C42H30N4 requires [M+H]+ 590.2470.
2.1.2. Synthesis of ICz-Tr-Br (4)
Compound 3 (0.30 g, 0.51 mmol) and NBS (0.09 g, 0.51 mmol) were
dissolved in DCM (25 mL). The reaction mixture was stirred con-
tinuously in N2 at room temperature. After stirring the mixture for 16 h,
it was precipitated using methanol. The crude product was purified
2

J. Yoon, et al.
DyesandPigments170(2019)107650
+H]+ 777.3288; molecular formula C56H44N2O2 requires [M+H]+
776.3403.
2.1.5. Synthesis of tNID-Br (9)
Compound 8 (0.50 g, 0.64 mmol) and N-bromosuccinimide (0.13 g,
0.73 mmol) were dissolved in DCM (13 mL), and the reaction mixture
was stirred under N2 at room temperature. After stirring the mixture for
12 h, it was concentrated in vacuo, and then precipitated into methanol.
After filtration, the residue was purified by silica-gel column chroma-
tography using DCM as the eluent to give a pure compound 9, which
was an orange-red solid (0.48 g); yield 87%. Mp 369.8 °C. IR (KBr,
cm−1) ν: 2961, 1713, 1674, 1584, 1446, 1361, 1236, 1190, 756, 702.
1H NMR (CDCl3, 500 MHz, ppm): δ 8.88–8.86 (1H, d, J = 7.6 Hz),
8.57–8.55 (1H, dd, J = 7.0, 0.9 Hz), 7.86–7.84 (1H, d, J = 7.6 Hz),
7.60–7.58 (2H, d, J = 8.6 Hz), 7.44–7.40 (4H, m), 7.37–7.31 (4H, m),
7.29–7.27 (2H, d, J = 8.6 Hz), 7.24–7.20 (4H, m), 7.17–7.15 (2H, t,
J = 3.7 Hz), 7.13–7.09 (2H, td, J = 8.9, 2.3 Hz), 6.99–6.97 (2H, d,
J = 7.3 Hz), 6.93–6.91 (1H, dd, J = 8.6, 0.9 Hz), 6.43 (1H, s),
6.12–6.11 (1H, d, J = 8.6 Hz), 1.40 (9H, s), 1.31(3H, s), 1.18(3H, s).
13C NMR (CDCl3, 125 MHz, ppm): δ 164.13, 153.38, 153.09, 151.60,
142.78, 141.15, 141.04, 138.87, 132.59, 132.37, 131.79, 130.56,
127.87, 126.50, 123.41, 121.81, 119.29, 115.52, 113.66, 107.93,
57.26, 46.75, 34.78, 31.37, 26.91; λmax/nm (ε/105 mol−1 dm3 cm−1):
490 (0.01); Found: [M+H]+ 855.2507; molecular formula
Fig. 1. Chemical structures of 3ICz-Tr and Tri-tNID.
the Suzuki coupling method between three equivalents of ICz-Tr-Br
(compound 4) and 1,3,5-tris(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-
yl)benzene.
For preparing Tri-tNID, tNID (8) was synthesized using 7,7-di-
methyl-13,13-diphenyl-7,13-dihydro-5H-indeno[1,2-b]acridine
(DPIAC, 6) as an electron donor unit, and N-(4-tert-butylphenyl)-1,8-
naphthalimide (NAI, 7) as an electron accepting unit, in accordance
with a previously reported method [36,37]. Moreover, NAI was se-
lected as the acceptor moiety for its planar structure and strong elec-
tron-accepting ability [36,40].
C
56H43BrN2O2 requires [M+H]+ 854.2508.
The Tri-tNID that contained the tNID was synthesized via the
Suzuki coupling reaction between tNID-Br, which was prepared by the
bromination of tNID (8), and 1,3,5-tris(4,4,5,5-tetramethyl-1,3,2-di-
oxaborolan-2-yl)benzene, in accordance with a previously reported
method40−41. By designing a highly rigid molecular structure and
highly pre-twisted charge transfer (CT) state from DPIAC to NAI, Tri-
tNID displayed orange-red emissions. In the structure of the dendritic
luminogens 3ICz-Tr and Tri-tNID, the three emitters as donor-acceptor
type molecules were attached to 1-, 3-, and 5-positions of a benzene
ring for the formation of a robust film. The structures of the desired
molecules, namely Tri-tNID and 3ICz-Tr, were analyzed using 1H and
13C NMR spectroscopy and mass spectrometry.
The thermal properties of Tri-tNID and 3ICz-Tr were obtained by
DSC. Both molecules did not exhibit thermal transition temperatures
less than 320 °C (Fig. S1). A thermogravimetric analysis (TGA) was
conducted to confirm the thermal stability of the two luminogens. The
primary thermal decomposition temperatures of 3ICz-Tr and Tri-tNID
were observed at 541 °C and 501 °C, respectively. From these results, it
was confirmed that the thermal stabilities of the two materials were
excellent (Fig. S2).
2.1.6. Synthesis of Tri-tNID (10)
Compound 9 (0.30 g, 0.35 mmol), 1,3,5-tris(4,4,5,5-tetramethyl-
1,3,2-dioxaborolan-2-yl)benzene (53 mg, 0.12 mmol), tris(dibenzyli-
dene acetone)dipalladium (20 mg, 0.02 mmol), tri-(o-tolyl)phosphine
(60 mg, 0.20 mmol), and potassium carbonate (0.10 g, 0.72 mmol) were
dissolved in a mixture of distilled toluene (12 mL), water (4 mL), and
methanol (4 mL). The mixture was refluxed in nitrogen at 120 °C for
14 h. After cooling the mixture to room temperature, it was extracted
with DCM and water, and then the organic layer was then dried over
anhydrous Na2SO4. After filtering the mixture, the filtrate was pre-
cipitated with methanol. The crude product was purified by column
chromatography on silica gel using DCM/Hexane (4:1 v/v) as the eluent
to obtain an orange-red solid (0.17 g); yield 61%. Td 501 °C. IR (KBr,
cm−1) ν: 2961, 1713, 1674, 1586, 1485, 1446, 1415, 1361, 1308, 1236,
1188, 756, 702. 1H NMR (CDCl3, 500 MHz, ppm): δ 8.92–8.90 (3H, d,
J = 7.6 Hz), 8.58–8.56 (3H, d, J = 7.0 Hz), 7.93–7.91 (3H, d,
J = 6.4 Hz), 7.61–7.59 (6H, d, J = 8.6 Hz), 7.45–7.43 (3H, d,
J = 7.1 Hz), 7.34–7.29 (27H, m), 7.25–7.23(12H, t, J = 3.7 Hz),
7.22–7.18 (12H, t, J = 7.4 Hz), 7.14–7.13 (3H, d, J = 7.3 Hz),
7.02–7.01 (9H, d, J = 7.0 Hz), 6.43 (3H, s), 6.33–6.31 (3H, s), 1.41
(27H, s), 1.32 (9H, s), 1.19 (9H, s). 13C NMR (CDCl3, 125 MHz, ppm): δ
164.22, 163.98, 153.27, 153.11, 151.62, 146.33, 143.17, 141.78,
141.59, 141.33, 139.00, 132.65, 132.38, 131.83, 131.11, 130.76,
130.65, 130.59, 130.17, 129.96, 129.21, 128.16, 127.95, 127.91,
126.91, 126.74, 126.52, 126.34, 125.99, 123.39, 123.18, 121.82,
119.27, 114.24, 107.93, 57.40, 46.74, 34.80, 31.39, 29.70, 27.61,
26.93; λmax/nm (ε/105 mol−1 dm3 cm−1): 498 (0.03); Found: [M+H]+
2402.8821; molecular formula C174H132N6O6 requires [M+H]+
2401.0208.
The morphologies of the spin-coated films of 3ICz-Tr, Tri-tNID, and
0.5 wt% of Tri-tNID added into 3ICz-Tr were investigated using atomic
force microscopy (AFM) (Fig. S3). The blend film of 3ICz-Tr and Tri-
tNID (0.5 wt%) exhibited a root mean square (RMS) roughness of
0.16 nm, which is comparable to those of the as-cast films of the two
dendritic luminogens. An ideally smooth surface morphology could
reduce occurrence of non-radiative processes in the quench centers and
therefore be favorable for WOLED performance.
3.2. Theoretical calculation
3. Results and discussion
For the evaluation of the relationship between the frontier mole-
cular orbitals (FMOs) and the material properties, and its effect on the
electronic structures of Tri-tNID and 3ICz-Tr, time-dependent density
functional theory (TD-DFT) calculations were carried out at the B3LYP/
6-31G* basis level. DFT calculations were carried out to evaluate the
FMOs and electronic states of ICz-Tr and tNID, to determine the FMOs
and expected electronic states of 3ICz-Tr and Tri-tNID.
3.1. Synthesis and characterization
As shown in Fig. 1, the 3ICz-Tr and Tri-tNID molecules were de-
signed as dendritic luminogens and synthesized for their application to
solution-processable WOLEDs. The detailed synthetic procedures are
presented in Scheme 1. First, 3ICz-Tr was designed to contain ICz-Tr
units, known to be donor-acceptor (D-A) moieties, which were tethered
into the 1,3,5-position of a benzene ring. 3ICz-Tr was prepared using
As shown in Fig. S4, the highest occupied molecular orbital (HOMO)
and lowest unoccupied molecular orbital (LUMO) distributions of the
3

J. Yoon, et al.
DyesandPigments170(2019)107650
Scheme 1. Synthetic procedures for dendritic lu-
minogens, 3ICz-Tr and Tri-tNID: (i) 2, CuI, trans-
1,2-diaminocyclohexane, K3PO4, toluene, 15 h, re-
flux; (ii) NBS, DCM, 16 h, rt.; (iii) Pd2(dba)3, P(o-
tolyl)3, 1,3,5-tris(4,4,5,5-tetramethyl-1,3,2-dioxa-
borolan-2-yl)benzene, toluene, MeOH, 2 M K2CO3,
24 h, reflux; (iv) Pd2(dba)3, P(t-Bu)3, KOtBu, to-
luene, overnight, reflux; (v) NBS, DCM, 48 h, rt.;
(vi) Pd2(dba)3, P(o-tolyl)3, 1,3,5-tris(4,4,5,5-tetra-
methyl-1,3,2-dioxaborolan-2-yl)benzene, toluene,
MeOH, 2 M K2CO3, 14 h, reflux.
two molecules (e.g. ICz-Tr and tNID) are highly separated in the per-
ipheral moieities. The HOMO state of the tNID is mainly localized on
the indenoacridine units, whereas the LUMO state is primarily localized
on the naphthalimide moiety, accompanied with a large twisted angle
between the indenoacridine and NAI moieties (θ2 = 82.9° for tNID).
Such a separation of the FMOs results in a tNID with small calculated
△EST values of 0.0076 eV, which potentially enables the effective re-
verse intersystem crossing (RISC) process from T1 to S1 in the tNID.
Moreover, the HOMO of ICz-Tr was localized into the 7,7-dimethyl-5,7-
dihydroindeno[2,1-b]carbazole (ICz) moieties. The LUMO was found to
be localized on the triazine acceptor unit. However, given that the ICz-
Tr contained less twisted geometries composed of donor and acceptor
characters, ICz-Tr exhibited relatively large calculated ΔEST values of
0.2424 eV. Thus, 3ICz-Tr may be defined as a conventional fluores-
cence luminogen that does not exhibit delayed fluorescence, which was
experimentally verified using transient photoluminescence (TRPL)
spectroscopy.
1.92 eV, respectively.
Given that the absorption spectra of the two materials were dif-
ferent, their corresponding PL spectra were significantly different. In
particular, the PL intensity maxima for 3ICz-Tr and Tri-tNID in toluene
solutions appeared at wavelengths of 446 and 613 nm, respectively. In
the film states, the emission maxima of 3ICz-Tr and Tri-tNID were
observed at 474 and 617 nm, respectively. A large shift in the emission
band at 617 nm in the Tri-tNID film was observed, although the pri-
mary absorption band was observed at 355 nm. The origin of the
emission band at 617 nm was the weak absorption band at 486 nm
(inset figure), and not the absorption band at 355 nm, which was due to
em
the intramolecular CT band. The larger Stokes shift (Δλ = λmax
–
λmaxabs = 120–131 nm) of Tri-tNID can be attributed to the larger
difference between the ground and excited state dipole moment in the
polar environment.
Fig. 2c and d present the fluorescence and phosphorescence spectra
of the two molecules in solution states recorded at 77 K. The single-
triplet energy gap (ΔEST) values of the two molecules were calculated as
0.28 eV for 3ICz-Tr and 0.08 eV for Tri-tNID by the difference between
the emission peaks in the fluorescence spectra (3ICz-Tr: S1 = 3.06 eV;
Tri-tNID: S1 = 2.35 eV) and those of the phosphorescence spectra
(3ICz-Tr: T1 = 2.78 eV; Tri-tNID: T1 = 2.27 eV), which was de-
termined using methods from the published literature [43–45].
Since we have to use a blended film of the two luminogens as an
emitting layer in the fabrication of WOLEDs, we investigated the PL
spectroscopic properties of the blend films with two luminogens. In this
case, the thin films were prepared by varying the concentration of Tri-
tNID and their PL spectra were measured. Since the CT band of Tri-
tNID showed very low absorption at around 486 nm, the PL intensity is
not so significant due to energy transfer from 3ICz-Tr (Fig. S5).
The changes in the geometries of 3ICz-Tr and Tri-tNID were ex-
pected to promote the CT/dipole-dipole interactions in a polar solvent
environment. Fig. S6 presents the PL spectra of the samples in which
the two materials were dissolved in five different solvents with different
polarities. In particular, Tri-tNID exhibited significant solvatochromic
effects up to a red-shift of 85 nm, due to the enhanced push-pull in-
teractions between the donor and acceptor units in its excited state.
Likewise, a spectral shift of approximately 60 nm was also observed in
the 3ICz-Tr solution as the solvent polarity was increased.
3.3. Photophysical properties of 3ICz-Tr and Tri-tNID
Fig. 2 presents the absorption and PL spectra of 3ICz-Tr and Tri-
tNID in films and solutions. The spectroscopic data are summarized in
Table 1. As shown in Fig. 2, 3ICz-Tr exhibited strong absorption bands
at approximately 314 nm in solution and film, which was due to the π-
π∗ transition of the locally excited (LE) state. 3ICz-Tr also exhibited
high-intensity absorption bands at wavelengths of 368 and 385 nm in
the solution state, which can be ascribed to the intramolecular charge-
transfer (ICT) transition from the ICz donor unit to the 2,4,6-triphenyl-
1,3,5-triazine (Trz) acceptor moiety. The absorption maxima of the thin
film were observed at 369 and 387 nm, which are almost the same as
those observed from the solution, which indicates that negligible in-
termolecular interactions occurred due to the three-armed dendritic
amorphous structure.
Compared with 3ICz-Tr, the Tri-tNID compound exhibited a π–π*
transition band at 354 and 355 nm in the solution and film states, re-
spectively, which can be attributed to the π- π∗ transition of the locally
excited (LE) state of the NAI moiety [36,41,42]. Unique ICT absorption
peaks were observed at 493 and 486 nm in the solution and film states
of Tri-tNID, respectively, with low intensity due to the large dihedral
angle between the indenoacridine and NAI moieties. The optical
bandgaps (Egopt) of the 3ICz-Tr and Tri-tNID films, as estimated from
their absorption edges (443 and 645 nm respectively), were 2.80 eV and
To investigate the relationship between the molecular structure and
photophysical characteristics of the two new luminogens, the TRPL
spectra of the 3ICz-Tr and Tri-tNID films were obtained. Fig. 3 presents
4

J. Yoon, et al.
DyesandPigments170(2019)107650
Fig. 2. Absorption and photoluminescence spectra of (a) 3ICz-Tr and (b) Tri-tNID (i, iii) in toluene solution and (ii, iv) neat film states. (Inset: UV–Vis absorption
spectra of Tri-tNID at wavelengths from 400 to 600 nm). Fluorescence and phosphorescence spectra (measured at 77 K) of (c) 3ICz-Tr and (d) Tri-tNID in solutions.
the transient PL decay behavior at room temperature for the 3ICz-Tr
and Tri-tNID films. 3ICz-Tr and Tri-tNID demonstrated significantly
different PL decay behavior.
tNID. The prompt lifetime measured from the decaying curve is 15.0 ns,
and the delayed life time is 0.04 μs. In this case, the prompt lifetime of
the mixed film was measured to be slightly longer than that of the single
thin film of Tri-tNID, which is presumably due to energy transfer
process from 3ICz-Tr to Tri-tNID.
The 3ICz-Tr exhibited typical conventional fluorescence (FL) be-
havior with a small prompt lifetime. The TRPL signal was fitted by a
double-exponential decaying function. The prompt lifetime component
(τp) of the 3ICz-Tr film was measured as 1.5 ns with a negligible de-
layed exciton lifetime (τd). On the other hand, the Tri-tNID exhibited
delayed decaying behavior. As shown in the decaying curve of Tri-
tNID, τp of 14.4 ns was clearly observed for Tri-tNID. τd was calculated
as 1.33 μs for the single-component film. Based on the TADF behavior
in a similar molecule reported in the previous literature [36,37], it was
confirmed that the Tri-tNID synthesized in this study also exhibited the
TADF behavior.
After observing the TRPL spectroscopic behavior of the single
component films prepared with 3ICz-Tr and Tri-tNID, we investigated
whether the similar behavior can be observed in the mixed films (3ICz-
Tr and Tri-tNID = 99.5 : 0.5 wt ratio) of two molecules (Fig. 3c). As a
result, the prompt lifetime obtained from the decaying curve at 477 nm
was measured at 2.0 ns and no delaying component was observed.
Additionally, the decaying curve measured at 626 nm showed prompt
component and delayed component like single component film of Tri-
3.4. Electrochemical properties
The electrochemical behavior of 3ICz-Tr and Tri-tNID were ob-
served by conducting cyclic voltammetry (CV). The first oxidation po-
tentials were determined as +1.19 and + 1.03 V for 3ICz-Tr and Tri-
tNID, respectively, as shown in Fig. 4. The HOMO levels of the two
molecules 3ICz-Tr and Tri-tNID were −5.61 and −5.45 eV in the film
states, respectively. The LUMO levels were determined in accordance
with a method published in the literature, as presented in Table 1. The
appropriate HOMO and LUMO levels of 3ICz-Tr and Tri-tNID can ef-
fectively induce the energy transfer, charge carrier injection and re-
combination of holes and electrons in the EML made with the mixture
of 3ICz-Tr and Tri-tNID, which can improve the WOLED performance.
Table 1
Optical, photophysical, and electrochemical properties of 3ICz-Tr and Tri-tNID.
opt
onset,
c
f
max
abs
max
Eg
Eox
Energy levels
ФPL (%)
S1/T1 (eV)
ΔEST (eV)
λ
(nm)
λ
(nm)
PL
(eV)
(eV)
a
a
d
e
Sol.
Film
Sol.
Film
HOMO (eV)
LUMO (eV)
b
b
b
b
b
b
3ICz-Tr
Tri-tNID
314, 368
354, 493
313, 369
355, 486
446
613
474
617
2.80
1.92
1.19
1.03
−5.61
−5.45
−2.81
−3.53
71.0
7.2
3.06/2.78
2.35/2.27
0.28
0.08
b
b
a
Toluene solution.
Film.
b
c
d
e
f
The values were obtained from cyclic voltammograms; *sample: film on Pt electrode and solution on CH2Cl2.
onset
HOMO (eV) = -e (4.8 V + Eox
- Eferrocene, ox).
opt
HOMO (eV) - Eg (eV).
Photoluminescence quantum yield (PLQY) measured in toluene solution at N2 atmosphere.
5

J. Yoon, et al.
DyesandPigments170(2019)107650
Fig. 3. The transient PL decay spectra of films of (a) 3ICz-Tr (detection wavelength = 477 nm), (b) Tri-tNID (detection wavelength = 626 nm), (c) 3ICz-Tr doped
with 0.5 wt% Tri-tNID (detection wavelength = 477 and 626 nm), measured at 25 °C; the excitation wavelength is 365 nm.
3.5. Performance of WOLEDs made with the mixture of 3ICz-Tr or Tri-
tNID
Finally, the performances of the WOLEDs with EMLs composed of
the blend film made of 3ICz-Tr or Tri-tNID with the composition were
evaluated. (Fig. 5 and Table 2). The two-component EML-based WOLED
device configuration is glass/ITO/PEDOT:PSS (50 nm)/PVK (15 nm)/
3ICz-Tr:
x wt% Tri-tNID (35 nm)/TPBi (40 nm)/LiF (0.8 nm)/Al
(100 nm), (where x = 1.0, 0.75, 0.5, and 0.25). The blend films made of
3ICz-Tr and Tri-tNID exhibited maximum EQE values of 3.8–8.32% at
doping concentrations of 0.25–1.0 wt% of Tri-tNID. Depending on the
desired white chromaticity, an appropriate concentration of Tri-tNID in
the blue host and emitter, 3ICz-Tr, should be optimized precisely. As
expected, when 0.25 wt% of Tri-tNID was doped into 3ICz-Tr, the
white color chromaticity was on the left side of (0.33, 0.33), which
indicates the emission of cool white light. With 0.75 and 1.0 wt% of Tri-
Fig. 4. Cyclic voltammograms of 3ICz-Tr and Tri-tNID measured in film states.
Fig. 5. (a) Device configurations, (b) J–V–L characteristics dependent on the concentration of Tri-tNID added to 3ICz-Tr, (c) EL spectra of WOLEDs at 1000 cd/m2,
(d) EQE–J curves, and (e) CE–J–PE curves. The images of white light emission were included.
6

J. Yoon, et al.
DyesandPigments170(2019)107650
Table 2
Device performances of WOLEDs with concentration of Tri-tNID added to 3ICz-Tr.
a
b
c
d
e
f
g
Tri-tNID
Von (V)
CEmax
(cd/A)
PEmax
Lmax
EQEmax
(%)
At 100 cd/m2
EQE (%)
At 1000 cd/m2
EQE (%)
CIE (x,y)
CIE (x,y)
(lm/W)
(cd/m2)
1.0 wt%
0.75 wt%
0.5 wt%
0.25 wt%
5.1
4.8
4.6
4.6
5.68
7.22
14.47
7.07
3.93
4.53
9.09
2.44
3379
3196
5818
2656
3.83
4.00
8.32
4.09
2.80
2.79
5.33
2.11
1.99
1.89
3.35
1.50
(0.44, 0.36)
(0.41, 0.35)
(0.31, 0.30)
(0.30, 0.30)
(0.41, 0.34)
(0.38, 0.33)
(0.32, 0.30)
(0.27, 0.29)
a
Turn-on voltage at 1 cd/m2.
b
c
d
e
f
CEmax = maximum current efficiency.
PEmax = maximum power efficiency.
Lmax = maximum luminance.
EQEmax = external quantum efficiency.
At 100 cd/m2.
g
At 1000 cd/m2.
Fig. 6. EL spectra of WOLEDs with a Tri-tNID dopant concentration of (a) 1.0 wt%, (b) 0.75 wt%, (c) 0.5 wt%, and (d) 0.25 wt%.
tNID in 3ICz-Tr, the CIE coordinates shifted to the right side of (0.33,
0.33), which indicates the emission of warm white light. As a result, the
highest EQE (∼8.32%) was observed at a concentration of 0.5 wt% of
Tri-tNID, and the CIE coordinates were determined as (0.32, 0.30),
which indicates the emission of almost pure white light. The CIE co-
ordinates could be changed according to the Tri-tNID concentration
change; thus, the composition of the mixtures could be changed to
realize various white color chromaticities.
The EL spectra and CIE values of the white OLEDs under different
voltages are presented in Fig. 6. As mentioned above, all the spectra
were normalized to compare the changes in the red emission peaks. All
four devices exhibited a slightly blue-shifted color chromaticity with an
increase in the applied bias, as the blue emitters were excited by the
higher energy at a higher electric field [46,47].
4. Conclusions
In this study, two different three-armed luminogens used as emitters
were synthesized and applied to two components based WOLEDs. The
WOLEDs with the mixture of blue fluorescent and orange-red TADF
dendritic luminogens were successfully fabricated using solution pro-
cessing. By optimizing the composition of two luminogens, the emission
of nearly pure white light (0.32, 0.30) could be achieved with the blend
films made of 3ICz-Tr and Tri-tNID (99.5:0.5 wt ratio) accompanied
with the external quantum efficiency up to 8.32%. With the composi-
tion of the blend film, the white emission color could be tuned between
warm white, pure white and cool white. Due to their intrinsic PL
characteristics, the two blue and orange-red emitting dendritic lumi-
nogens exhibited great solution-processed WOLED performances.
When the applied bias increased from 6 V to 9 V in the device with
0.5 wt% of Tri-tNID, a slight change in the CIE coordinates of the de-
vice from (0.32, 0.30) to (0.31, 0.30) was observed. The total variation
in the CIE coordinates was Δ (x, y) ≤ (0.01, 0.00), which reveals that
the doped device with 0.5 wt% Tri-tNID exhibited the most stable color
chromaticity in comparison with the other devices.
Acknowledgments
This research was supported by National Research Foundation of
Korea (NRF-2019R1A2C2002647and 2019R1A6A1A11044070).
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://
7

J. Yoon, et al.
DyesandPigments170(2019)107650
doi.org/10.1016/j.dyepig.2019.107650.
High-efficiency single emissive layer white organic light-emitting diodes based on
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