Synthesis and property of new blue emitting materials with bulky side group

Article abstract of DOI:10.1080/15421406.2015.1076296

Two emitting compounds, 9,10-bis-[1,1′;3′,1″]terphenyl-5′-yl-1,5-di-o-tolyl-anthracene [TP-DTA-TP] and 9,10-bis-phenyl[1,1′;3′,1′′]triphenyl-5′-yl-1,5-di-o-tolyl-anthracene [TPB-DTA-TPB] based on new twisted core moiety were synthesized through boration, Suzuki reaction, and Sandmeyer reactions. EL performance was improved by varying the chemical structure of the side group. Physical properties such as optical, electrochemical, and electroluminescent properties were investigated. Synthesized compounds were used as an EML in OLED device: ITO / 2-TNATA (60nm) / NPB (15nm)/ TP-DTA-TP or TPB-DTA-TPB (35 nm) / Alq3 (20nm) / LiF (1nm) / Al (200nm). It was found that TPB-DTA-TPB showed higher luminance efficiency and better C.I.E. value than TP-DTA-TP device.

Full text of DOI:10.1080/15421406.2015.1076296

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Synthesis and Property of New Blue Emitting
Materials with Bulky Side Group
Seokwoo Kang, Hayoon Lee, Beomjin Kim, Hyeonmi Kang & Jongwook Park
To cite this article: Seokwoo Kang, Hayoon Lee, Beomjin Kim, Hyeonmi Kang & Jongwook Park
(2015) Synthesis and Property of New Blue Emitting Materials with Bulky Side Group, Molecular
Crystals and Liquid Crystals, 618:1, 66-73, DOI: 10.1080/15421406.2015.1076296
To link to this article: http://dx.doi.org/10.1080/15421406.2015.1076296
Published online: 07 Oct 2015.
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Date: 11 October 2015, At: 07:23

Mol. Cryst. Liq. Cryst., Vol. 618: pp. 66–73, 2015
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2015.1076296
Synthesis and Property of New Blue Emitting
Materials with Bulky Side Group
SEOKWOO KANG, HAYOON LEE, BEOMJIN KIM,
HYEONMI KANG, AND JONGWOOK PARK∗
Department of Chemistry, The Catholic University of Korea, Bucheon, Korea
Two emitting compounds, 9,10-bis-[1,1ꢀ;3ꢀ,1ꢀꢀ]terphenyl-5ꢀ-yl-1,5-di-o-tolyl-anthra
cene [TP-DTA-TP] and 9,10-bis-phenyl[1,1ꢀ;3ꢀ,1ꢀꢀ]triphenyl-5ꢀ-yl-1,5-di-o-tolyl-
anthracene [TPB-DTA-TPB] based on new twisted core moiety were synthesized through
boration, Suzuki reaction, and Sandmeyer reactions. EL performance was improved by
varying the chemical structure of the side group. Physical properties such as optical,
electrochemical, and electroluminescent properties were investigated. Synthesized com-
pounds were used as an EML in OLED device: ITO / 2-TNATA (60nm) / NPB (15nm)/
TP-DTA-TP or TPB-DTA-TPB (35 nm) / Alq3 (20nm) / LiF (1nm) / Al (200nm). It was
found that TPB-DTA-TPB showed higher luminance efficiency and better C.I.E. value
than TP-DTA-TP device.
Keywords OLEDs; Blue materials; Bulky Side group; Anthracene
Introduction
Since electroluminescence (EL) result in small molecular organic materials (Tang et al.)
was announced in 1987, studies on organic semiconductors such as organic light emitting
diodes (OLED) [1–3], organic thin film transistors (OTFT) [4] and organic photovoltaics
(OPVs) have been actively conducted [5]. Many efforts have been made in recent years
to improve the performance and stability of OLEDs because of their potential application
in flat panel displays [6–11]. In order to make full color OLED display, we need high
performance red, green and blue materials with high EL efficiency, pure color coordinate
(C.I.E.) value and long life time, as well as material thermal property. Host and dopant
methods in fluorescence and phosphorescence systems have been used to get efficiency of
OLED, and red and green emitters have been developed and improved to commercialize
full color OLED. However, there are many problems to be solved, such as blue emitting
material, device lifetime and luminous efficiency, etc. Many kinds of blue materials in-
cluding diarylanthracene, di(styryl)arylene, fluorene and pyrene have been intensively used
and studied on improved EL property [12]. The recent research of Pei et al. has shown
that spirotruxene derivatives have excellent color coordinates (0.16, 0.09) but maximum
external quantum efficiency (EQE) of only 2.9% [13]. Shu et al. have reported anthracene
∗Address correspondence to Jongwook Park, Department of Chemistry, The Catholic University
of Korea, 43-1, Yeokgok, Wonmi, Bucheon, Korea(ROK). E-mail: hahapark@catholic.ac.kr
Color versions of one or more of the figures in the article can be found online at
www.tandfonline.com/gmcl.
[276]/66

Synthesis and Property of New Blue Emitting Materials
[277]/67
derivatives with color coordinates of (0.14, 0.12) and maximum EQE of 5.3% [14]. Jenekhe
et al. have described oligoquinoline derivatives with high EQE of 6.56%, which at the time
was the highest reported value for a non-doped blue-light emitter. Unfortunately, however,
the color coordinates of this material are (0.15, 0.16), with the value on y-axis larger than
0.1 [15].
In recent papers, we have reported new high performance blue-light emitters based
on tetraphenylethylene- and anthracene-cored derivatives [16–18]. Of these synthesized
compounds, 9,10-bis(4-(1,2,2-triphenylvinyl)phenyl)anthracene [BTPPA] has a structure
with mixed anthracene and tetraphenylene units, which are non-alkylated bulky side groups.
This compound has better thermal properties, better luminance efficiency, lower operating
voltage, higher power efficiency, and deeper blue CIE x,y coordinates (0.159, 0.135) than
other tetraphenylethylene derivatives. The EL performance of these synthesized blue-light
emitters can be optimized and improved by varying the chemical structures of the side
groups.
Based on these results, we designed new blue-light emitters. First, anthracene moiety
with high photoluminescence quantum efficiency was introduced at the center of molecular
structure. Second, bulky aromatic rings were attached to anthracene 9, 10-position to
improve color properties. Lastly, bulky aromatic rings, m-terphenyl and triphenylbenzene,
were used instead of alky groups that induce red-shift of PL and EL and increase in operating
voltage. Meta-positioned aromatic ring with short conjugation length was used. This has
an advantage of reducing y CIE coordinate value of the emitter [19].
The synthesized compounds are expected to exhibit better than those in our previous
studies.
Experimental
Measurements
The 1H NMR spectra were recorded on Bruker Avance 300. The FAB+ mass were recorded
on JMS-AX505 WA. The optical absorption spectra were obtained by HP 8453 UV-VIS-
NIR spectrometer. Perkin Elmer luminescence spectrometer LS50 (Xenon flash tube) was
used for photo-luminescence (PL) spectroscopy. Thermal degradation temperature (Td)
of the compounds were measured by thermo-gravimetric analysis (TGA) using SDP-
TGA2960 (TA instrument). The current–voltage-luminance (I-V-L) characteristics of the
fabricated EL devices were obtained by Keithley 2400 electrometer, and light intensity was
obtained by Minolta CS-1000A.
Synthesis
Synthesis of 1,5-dibromoanthraquinone (1). 1,5-Diaminoanthraquinone (10.0 g,
42.0 mmol) and CuBr2 (21.2 g, 94.6 mmol) were suspended in CH3CN (200 mL). Tert-butyl
nitrite (12.5 mL, 90%) was added under vigorous stirring and the mixture was heated to
65◦C for 2.5 h. The mixture was cooled to room temperature (rt), and hydrochloric acid
(3 M, 100 mL) and H2O (100 mL) were added. The solid was filtered after stirring the
mixture for additional 20 min, washed with water (4 × 100mL) and EtOH (50 mL), and
dried in vacuum. Subsequent filtration of the material through a plug of silica gel using

68/[278]
S. Kang et al.
CHCl3 as eluent and drying in vacuum yielded 13.7 g of 1,5-dibromoanthraquinone as an
orange powder. (88% yield)
1H-NMR (300MHz, CDCl3) δ(ppm) : 8.31(dd, 2H), 8.01(dd, 2H), 7.58(t, 2H)
Synthesis of 1,5-dibromoanthracene (2)
10 g (27.1 mmol) of 1,5-bromoanthraquinone was added in a 3-neck round flask, and
400 mL of acetic acid, 50 mL of agitate hydrogen bromide and 82 mL of hypophosphorous
acid were sequentially added. The solution was stirred for 4 days at 120◦C. After the
reaction ended, the solvent was removed by distillation. Subsequent filtration of the material
through a plug of silica gel using CHCl3 as eluent and drying in vacuum yielded 4.2 g of
1,5-dibromoanthracene as a yellow powder. (46% yield)
1H-NMR (300MHz, CDCl3) δ(ppm) : 8.82(s, 2H), 8.07(dd, 2H), 7.84(dd, 2H), 7.36(dd,
2H),
Synthesis of 1,5-di-o-tolyl-anthracene (3). 0.2 g (0.59 mmol) of 1,5-dibromoanthracene,
0.18 g (1.36 mmol) of 2-methylphenylboronic acid and 0.07 g (0.059 mmol) of
tetrakis(triphenylphosphine)palladium(0) were added in a 3-neck round flask, and 20 mL
of anhydrous toluene was sequentially added. 2mL of 2M NaOH was added. The solution
was stirred for 6 hours at 120◦C. After the reaction ended, the solution was extracted using
chloroform and H2O. Small amount of H2O remaining in the chloroform layer was removed
using MgSO4. After obtaining solid by evaporation of product in the chloroform layer, col-
umn refinement was performed under chloroform : hexane = 1 : 5 condition. The product
obtained by column was dissolved in small amount of chloroform and re-precipitated using
methanol before filtering. 0.17 g of white solid was obtained by evaporating the filtered
solid. (79% yield)
1H-NMR (300MHz, THF) δ(ppm) : 8.07(s, 2H), 7.86(d, 2H), 7.45(t, 2H), 7.39-7.23(m,
10H), 2.05 (d, 6H). Fab+-MS: 358 m/z
Synthesis of 9,10-dibromo-1,5-di-o-tolyl-anthracene (4). 1.1 g (3.06 mmol) of 1,5-di-o-
tolyl-anthracene was added in a 3-neck round flask, and 100mL of chloroform was sequen-
tially added before slowly adding bromine 0.34 mL (6.75 mmol). The solution was stirred
for 6 hours at 50◦C. After the reaction ended, the solution was extracted using chloroform
and H2O. Small amount of H2O remaining in the chloroform layer was removed using
MgSO4. After obtaining solid by evaporation of product in the chloroform layer, column
refinement was performed under chloroform : hexane = 1 : 3 condition. The product ob-
tained by column was dissolved in small amount of chloroform and re-precipitated using
methanol before filtering. 0.63 g of yellow solid was obtained by evaporating the filtered
solid. (40% yield)
1H-NMR (300MHz, CDCl3) δ(ppm) : 8.57-8.56 (d, 2H), 7.91-7.88(d, 2H), 7.43-7.25(m,
10H), 2.05(d, 6H).
Synthesis of 9,10-Bis-[1,1ꢀ;3ꢀ,1ꢀꢀ]terphenyl-5ꢀ-yl-1,5-di-o-tolyl-anthracene (TP-DTA-TP).
0.7 g (1.35 mmol) of 9,10-dibromo-1,5-di-o-tolyl-anthracene, 1.11 g (3.11 mmol) of 4,4,5,5-
tetramethyl-2-[1,1ꢀ;3ꢀ,1ꢀꢀ]terphenyl-5ꢀ-yl-[1,3,2]dioxaborolane and 0.15 g (0.135 mmol) of

Synthesis and Property of New Blue Emitting Materials
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tetrakis(triphenylphosphine)palladium(0) were added in 3-neck round flask, and 80 mL
of anhydrous toluene was added. 10 mL of 2M NaOH was added at 60◦C. The solution
was stirred overnight. After completion of the reaction, the solution was extracted using
chloroform and H2O. Small amount of H2O remaining in the chloroform layer was removed
using MgSO4. After obtaining solid by evaporation of product in the chloroform layer,
column refinement was performed under chloroform : hexane = 1 : 3 condition. The product
obtained by column was dissolved in small amount of chloroform and re-precipitated using
methanol before filtering. 0.15 g of yellow solid was obtained by evaporating the filtered
solid. (13% yield)
1H-NMR (300MHz, DMSO) δ(ppm) : 8.44(d, 2H), 7.92-7.79(m, 14H), 7.75-7.73(d,
2H), 7.59-7.48(m, 12H), 7.43-7.41(d, 2H), 7.19-7.13(t, 2H), 7.05-7.01(t, 2H), 6.87-6.79(m,
2H), 6.66-6.59(t, 2H), 2.05(d, 6H). Fab+-MS: 814 m/z
Synthesis of 9,10-bis-phenyl[1,1ꢀ;3ꢀ,1ꢀꢀ]triphenyl-5ꢀ-yl-1,5-di-o-tolyl-anthracene
(TPB-DTA-TPB)
0.2 g (0.38 mmol) of 9,10-dibromo-1,5-di-o-tolyl-anthracene, 0.36 g (0.85 mmol)
of 4,4,5,5-tetramethyl-2-phenyl[1,1ꢀ;3ꢀ,1ꢀꢀ]triphenyl-5ꢀ-yl-[1,3,2]dioxaborolane and 0.05 g
(0.038 mmol) of tetrakis(triphenylphosphine)palladium(0) were added in 3-neck round
flask, and 30 mL of anhydrous toluene was added. 3 mL of 2M NaOH was added at 60◦C.
The solution was stirred overnight. After completion of the reaction, the solution was ex-
tracted using chloroform and H2O. Small amount of H2O remaining in the chloroform
layer was removed using MgSO4. After obtaining solid by evaporation of product in the
chloroform layer, column refinement was performed under chloroform : hexane = 1 : 3
condition. The product obtained by column was dissolved in small amount of chloroform
and re-precipitated using methanol before filtering. 0.09 g of yellow solid was obtained by
evaporating the filtered solid. (24% yield)
1H-NMR (300MHz, THF) δ(ppm) : 8.64(d, 2H), 8.17(s, 6H), 8.12-8.02(m, 12H),
7.87-7.84(t, 4H), 7.82-7.75(m, 10H), 7.67-7.62(t, 6H), 7.54-7.49(m, 8H), 2.05(dd, 6H).
Fab+-MS: 966 m/z
Results and Discussion
Scheme 1 shows the chemical structures of the synthesized materials. All compounds were
purified with the silica column method, producing pure powders that were characterized by
NMR and FAB-MS analysis. The synthesis routes of the compounds are shown in Scheme 1.
Boration and Suzuki aryl-aryl coupling reactions were used in overall syntheses. The
bulky side groups, [1,1ꢀ;3ꢀ,1ꢀꢀ]terphenyl and phenyl[1,1ꢀ;3ꢀ,1ꢀꢀ]terphenyl were substituted
by Suzuki reactions. Thus, the side groups can be extensively applied for incorporating
them in other core molecular structures UV and PL spectra of the synthesized compounds
in the CHCl3 solution state and the thin-film state are shown in Table 1 and Figure 1. In the
CHCl3 solution state, all of the synthesized materials exhibit UVmax value of 372–412 nm
and PLmax values in the range of 441–450 nm. However, in the film state, the PL spectra
of all these materials are red-shifted. Especially, TP-DTA-TP substituted with m-terphenyl
was observed with an excimer peak. However, excimer peak was not observed in TPB-
DTA-TPB substituted with bulky triphenylbenzene group. Accordingly, as the side group
becomes bulkier, excimer is prevented by reducing intermolecular interaction and emission
width is narrowed. In order to accurately interpret the EL efficiency of each material, the
relative QE values of the side groups were measured [20]. The following QE results were

70/[280]
S. Kang et al.
Table 1. Optical properties of synthesized materials
Solution[a]
Film
Compounds
UVmax (nm)
PLmax (nm)
UVmax (nm)
PLmax (nm)
TP-DTA-TP
TPB-DTA-TPB
372, 391, 412
392, 410
441
450
394, 414
397, 413
459, 515
460
[a]: Chloroform solution (1.0 × 10−5 M)
found for the solution state: m-terphenyl (0.10) < triphenylbenzene (0.17). Thus there
are close correlations between the side group PL QE and the EL efficiency of the final
compounds.
The HOMO and LUMO electron densities of TP-DTA-TP, TPB-DTA-TPB were de-
termined with DFT/B3LYP/6-31G∗ calculations by carrying out optimization and time
Figure 1. (a) UV&PL spectra of TP-DTA-TP and TPB-DTA-TPB in solution state (Chloroform
10−5M), (b) UV&PL spectra of TP-DTA-TP and TPB-DTA-TPB in film state.

Synthesis and Property of New Blue Emitting Materials
[281]/71
Scheme 1. Chemical structures of the synthesized compounds.
dependent DFT (TDDFT) using Gaussian 03. The synthesized materials were shown to
have HOMO and LUMO electrons located in anthracene in Figure 2.
We fabricated EL devices with the synthesized compounds in the following configura-
tion: ITO/2-TNATA (60 nm)/NPB (15 nm)/synthesized materials (35 nm)/Alq3 (20 nm)/LiF
(1 nm)/Al (200 nm). EL performance of the devices is summarized in Table 2. Electrochem-
ical cyclic voltammetry (CV) was performed to determine the highest occupied molecular
orbital (HOMO) energy levels of the synthesized materials. The lowest unoccupied molec-
ular orbital (LUMO) energy levels were obtained from the HOMO levels and the optical
band gaps. The luminance efficiency (cd/A), power efficiency (lm/W) of the devices in-
crease in the following order: TP-DTA-TP < TPB-DTA-TPB. Interestingly, this order is
Figure 2. HOMO and LUMO levels of (a) TP-DTA-TP, (b) TPB-DTA-TPB calculated at the
DFT/B3LYP/6-31G∗ for optimization and Time Dependent DFT (TDDFT) using Gaussian 03.

72/[282]
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Table 2. EL performances of multilayered devices with the synthesized compound at
10 mA/cm2 and the optical, electrical and thermal properties of the synthesized materials
HO LU
Volt[a] C.E.[b] P.E.[c] C.I.E[d]
ELmax
(nm)
MO MO Eg
Td
[e]
[f]
Emitting
Materials
(V)
(cd/A) (lm/W) (x, y)
(eV) (eV) (eV) (◦C)
TP-DTA-TP
TPB-DTA-TPB 8.71 4.85
9.27 3.58
1.34
1.96
0.36, 0.53 521, 552 5.59 2.75 2.84 452
0.17, 0.23 475 5.60 2.78 2.82 468
[a]: turn on voltage, [b]: current efficiency, [c]: power efficiency, [d]: Commission International de
lꢀEclairage, [e]: band gap, [f]: TGA at a heating rate of 10 ◦C /min
identical to the order of the PLQE values for the side groups. Moreover, it is interesting that
the operating voltages of the devices decrease under the same current density conditions
as the side group changes from m-terphenyl to triphenylbenzene (Figure 3(a)). In other
words, the changes in the side groups of the compounds increase the power efficiency and
Figure 3. (a) I-V-L characteristics of TP-DTA-TP, TPB-DTA-TPB, (b) Normalized EL spectra of
TP-DTA-TP, TPB-DTA-TPB.

Synthesis and Property of New Blue Emitting Materials
[283]/73
luminescence efficiency properties. The normalized EL spectra of the fabricated EL devices
are shown in Figure 3(b). As for the PL spectra, excimer peak was also disappeared from
the EL spectra.
Conclusion
We have synthesized new anthracene derivatives with EL luminescence properties that
were tuned in detail by varying the bulky side groups. As the luminescence efficiency of the
side groups increases, the performance of the synthesized blue light emitters also improves
substantially. TP-DTA-TP device showed C.I.E. value of (0.36, 0.53) and luminance effi-
ciency of 3.58 cd/A at 10 mA/cm2. TPB-DTA-TPB device exhibited C.I.E. value of (0.17,
0.23) and high luminance efficiency 4.85 cd/A at 10 mA/cm2. TPB-DTA-TPB was found
to show better luminance efficiency and C.I.E. value than TP-DTA-TP device because it
had increased size of the side group.
Funding
This research was supported by a grant from the Fundamental R&D Program for Core
Technology of Materials funded by the Ministry of Trade, Industry and Energy, Republic
of Korea (Project No. 10050215).
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