New approach way using substituent group at core chromophore for solution process blue emitter

Article abstract of DOI:10.1166/jnn.2015.9299

Comparing the conventional vapor desposition process for OLEDs, the solution process using small molecules has merits of low production cost because of many reasons. For the solution process blue flourescent material, tertiary butyl (T) and anthracene (A) were first introduced as substituents to TAT core part, 2-tert-butyl-9,10-bis(3a?3,5a?3-diphenylbiphenyl-4a?2-yl)anthracene (T-TAT) and 2-(9-anthracenyl)-9,10-bis(3a?3,5a?3-diphenylbiphenyl-4a?2-yl)anthracene (A-TAT). All three materials indicated typical absorption band of anthracene in the range of 350 to 400 nm. T-TAT exhibited similar optical properties to TAT, but A-TAT has longer absorption and PL emission compared to other two compounds. In case of spin-coated film, A-TAT exhibited absorption maximum value of 408 nm and photoluminescence maximum value of 469 nm. T-TAT and A-TAT can be applicable to solution process as a blue fluorescence material.

Full text of DOI:10.1166/jnn.2015.9299

Article
Journal of
Nanoscience and Nanotechnology
Vol. 15, 1850–1854, 2015
Copyright © 2015 American Scientific Publishers
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New Approach Way Using Substituent Group at Core
Chromophore for Solution Process Blue Emitter
Sunmi Lee, Seungho Kim, Jaehyun Lee, Beomjin Kim, and Jongwook Park∗
Department of Chemistry, The Catholic University of Korea, Bucheon, 420-743, Korea
Comparing the conventional vapor desposition process for OLEDs, the solution process using
small molecules has merits of low production cost because of many reasons. For the solution
process blue flourescent material, tertiary butyl (T) and anthracene (A) were first introduced as
substituents to TAT core part, 2-tert-butyl-9,10-bis(3ꢀꢀ,5ꢀꢀ-diphenylbiphenyl-4ꢀ-yl)anthracene (T-TAT)
and 2-(9-anthracenyl)-9,10-bis(3ꢀꢀ,5ꢀꢀ-diphenylbiphenyl-4ꢀ-yl)anthracene (A-TAT). All three materials
indicated typical absorption band of anthracene in the range of 350 to 400 nm. T-TAT exhibited
similar optical properties to TAT, but A-TAT has longer absorption and PL emission compared to
other two compounds. In case of spin-coated film, A-TAT exhibited absorption maximum value of
408 nm and photoluminescence maximum value of 469 nm. T-TAT and A-TAT can be applicable to
solution process as a blue fluorescence material.
Keywords: OLEDs, Solution Process, Blue Fluorescent Emitter.
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1. INTRODUCTION
material characteristics. Therefore, development of small
molecule material that is suitable for a solution process is
necessary. A research on small molecule emitting materials
which is available for solution process is very important
for implementation of OLED.
As for the solution process, emitting materials are
required to be soluble, which means that compound should
break packing property inside of molecule itself.
In this study, we propose new approach method
to prevent packing of chromophores by introducing
alkyl or aromatic moiety into main chromophore. In
our previous paper, 9,10-bis(3ꢀꢀ,5ꢀꢀ-diphenylbiphenyl-4ꢀ-
yl)anthracene (TAT),13 a blue fluorescent emitter was
shown to have high efficiency through the deposition pro-
cess. In here, we changed main core part using tertiary
butyl or another anthracene group as shown in Figure 1
and characterized new compounds.
Small organic molecules based organic light emitting
diodes (OLEDs) are under many intense researches in
view of their promising future for large full color dis-
play applications such as TV and advertising displays.
OLED has been under active researches so far with its
overlooking high potential for use in liquid crystal display
(LCD) backlight and lighting field due to its low power
consumption.1–4
Small molecule based OLED device (SMOLED) has
been already commercialized in mobile phone due to its
high efficiency compared to polymer based OLED device
(PLED).
Nevertheless, SMOLED has a problem of high cost
as it utilizes a deposition method, which involves high
cost for deposition equipment and large consumption of
material.5–8 Thus, to solve high cost and large size panel
production problem, OLED is widely researched by using
a solution process such as ink-jet printing, nozzle printing,
and spin-coating that uses a simple device structure and
consumes less material.9–12
2. EXPERIMENTAL DETAILS
2.1. General Experiment
1H-NMR spectra were recorded on Bruker, Advance 300,
while Fast atom bombardment (FAB) mass spectra were
recorded by JEOL, JMS-AX505WA, HP5890 series II.
The elementary analysis (EA) was checked by EA1110
and EA1112 of CE Instruments. The optical absorp-
tion spectra were obtained by HP 8453 UV-VIS-NIR
Materials applied to SMOLED have been mainly devel-
oped for a deposition process so far. However, a deposi-
tion method and a solution process require very different
∗Author to whom correspondence should be addressed.
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J. Nanosci. Nanotechnol. 2015, Vol. 15, No. 2
1533-4880/2015/15/1850/005
doi:10.1166/jnn.2015.9299

Lee et al.
New Approach Way Using Substituent Group at Core Chromophore for Solution Process Blue Emitter
1H-NMR (300 MHz, CDCl3ꢀꢁ(ppm): 8.01–7.94 (m,
8 H), 7.87 (t, 2 H), 7.83–7.79 (m, 6 H), 7.77 (s, 5 H),
7.73 (d, 1 H), 7.64–7.60 (m, 4 H), 7.55–7.48 (m, 9 H),
7.45–7.39 (m, 4 H), 7.38–7.25 (m, 2 H), 1.29 (s, 9 H).
TAT
Fab+-MS 963 m/z. Anal. Calcd for C66H50: C, 94.02; H,
5.98 Found: C, 93.98; H, 5.88.
2.5. Synthesis of Compound 3
The method of synthesis for Compound 3 in Scheme 1 is
published in Angew. Chem.15
T-TAT
A-TAT
Figure 1. Chemical structures of TAT, T-TAT and A-TAT.
2.6. Synthesis of Compound 4
The method of synthesis for Compound 4 in Scheme 1 is
spectrometer. Perkin Elmer luminescence spectrometer
LS50 (Xenon flash tube) was used for photolumines-
cence (PL) spectroscopy. HOMO values were measured by
RIKEN Surface analyzer AC-2. The melting temperatures
(Tm) and glass-transition temperatures (Tg) of the com-
pounds were measured by carrying out differential scan-
ning calorimetry (DSC) under a nitrogen atmosphere using
a DSC4000 (Perkin Elmer).
published in J. Mater. Chem. C.13
2.7. Synthesis of Compound 5
In a 500 mL round-bottomed flask, Compound 3
(0.5 g, 1.76 mmol), Compound 4 (0.58 g, 2.64 mmol),
Pd(OAc)2 (11.8 mg, 0.05 mmol) and tricyclohexylphos-
phine (29.5 mg, 0.1 mmol) were placed with ꢁAnhy-
drous Toluene and Ethanol. It was raised to 110 C and
(Et)4NOH was added to reaction flask. After reaction is
completed, it is extracted by Chloroform and water. After
removing water from the mixture, it was filtered by using
MgSO4. The mixture was columned under MC:Hexane
(1:4) mixed solvent. The product was concentrated under
Thin film was prepared by spin-coating of the synthe-
sized compound solution (1.5 wt%, toluene) with 15 sec-
onds of 1000 rpm and 15 seconds of 2000 rpm. The film
ꢁ
was baked for 30 min at 70 C by using a hot plate to
remove the solvent. All deposited films were thermally
evaporated on the substrates in a vacuum of less than 1×
10−6 torr at an evaporation rate of 1 Å/s. Film thickness
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reduced pressure and re-precipitated with methanol to
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obtain pure solid material. The yield was 59%.
was measured by using an Alpha-step 500 surface profiler
Copyright: American Scientific Publishers
(KLA Tencor, Mountain View, CA).
2.8. Synthesis of Compound 6
The method of synthesis for Compound 1 in Scheme 1 is
2.2. Synthesis of Compound 1
The method of synthesis for Compound 1 in Scheme 1 is
published in Chem. Mater.14
published in Chem. Mater.14
2.9. Synthesis of 2-(9-anthracenyl)-9,10-Bis(3ꢀꢀ,5ꢀꢀ-
diphenylbiphenyl-4ꢀ-yl)Anthracene (A-TAT)
2.3. Synthesis of Compound 2
In a 250 mL round-bottomed flask, compound 6 (1.4 g,
2.10 mmol), compound 2 (1.876 g, 5.26 mmol), Pd(OAc)2
(113.5 mg, 0.17 mmol), and tricyclohexylphosphine
(94 mg, 0.34 mmol) were placed with anhydrous THF
solvent. It was raised to 80 ꢁC and (Et)4NOH was
added to reaction flask. After reaction is completed, it is
extracted by chloroform and water. After removing water
from the mixture, it was filtered by using MgSO4. The
mixture was columned under EA:Hexane (1:10) mixed
solvent. The product was concentrated under reduced pres-
sure and re-precipitated with methanol to obtain pure
solid material in light yellow color. The final step yield
was 49%.
The method of synthesis for Compound 2 in Scheme 1 is
published in J. Mater. Chem. C.13
2.4. Synthesis of 2-Tert-Butyl-9,10-Bis(3ꢀꢀ,5ꢀꢀ4-
diphenylbiphenyl-4ꢀ-yl)Anthracene (T-TAT)
Compound 1 (3.0 g, 5.5 mmol) and compound 2
(3.3 g, 12.1 mmol) were added to Pd(OAc)2 (50 mg,
0.22 mmol), tris(2-methylphenyl)phosphine (155 mg,
0.44 mmol), K3PO4 (9.4 g, 44 mmol), and solvent
(dimethoxyethane:water = 4:1) in a 500 mL round-
bottomed flask under
a
nitrogen atmosphere. The
ꢁ
temperature was raised to 100 C. When the reaction was
completed, extraction of the product was performed with
water and chloroform. The organic extract was dried with
added MgSO4 and then filtered. The solvent was removed
under vacuum. The remaining crude mixture was passed
through a column of silica with MC:Hexane (1:5) and then
re-crystallized from chloroform to obtain a yellow solid.
The final step yield was 73%.
1H-NMR (300 MHz, THF) ꢁ(ppm): 8.53–8.50 (s, 1 H),
8.18–8.12 (d, 2 H), 8.12–8.08 (d, 2 H), 8.08–8.00 (m,
3 H), 7.98–7.93 (m, 3 H), 7.93–7.86 (m, 6 H), 7.86–7.82
(m, 4 H), 7.79–7.68 (m, 8 H), 7.67–7.62 (d, 2 H),
7.53–7.46 (m, 5 H), 7.46–7.36 (m, 10 H), 7.34–7.28 (m,
4 H). Fab+-MS 963 m/z. Anal. Calcd for C76H50: C,
94.77; H, 5.23 Found: C, 94.67; H, 5.23.
J. Nanosci. Nanotechnol. 15, 1850–1854, 2015
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New Approach Way Using Substituent Group at Core Chromophore for Solution Process Blue Emitter
Lee et al.
Br
Br
O
O
O
O
B
1. n-BuLi/THF, –78 C
Br
Br
2. KI, NaHPO2H2O/AcOH
(2)
(1)
Pd(OAc)2, Tris(2-methylphenyl)phosphine, K3PO4
Dimethoxyethane/H2O
T-TAT
O
O
O
O
O
Br
Pd(OAc)2, (Et)4NOH
Toluene/EtOH
NH2
t-BuONO/ CH3CN
CuBr2, THF
B(OH)2
O
(5)
(3)
(4)
O
O
O
O
Br
Br
B
1. n-BuLi/THF, –78 C
Br
Br
2. KI, NaHPO2H2O/AcOH
(2)
(5)
(6)
Pd(OAc)2, Tricyclohexylphospine, (Et)4NOH
THF
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Scheme 1. Synthetic routes of T-TAT and A-TAT.
3. RESULTS AND DISCUSSION
The absorption edge for A-TAT starts at longer wave-
lengths compared with TAT or T-TAT. Therefore, the band
gap for A-TAT is reduced compared with that of TAT or
T-TAT. This is caused by an increase in conjugation length
from the anthracene introduced as a substituent. As shown
in the PL spectrum of solution state, TAT, T-TAT and
A-TAT have maximum values at 436 nm, 439 nm and
451 nm, respectively. As for A-TAT, since the conjugation
length is increased by substituted anthracene to main core,
emission value is located at longer wavelength compared
to TAT or T-TAT.
In blue fluorescence compound, it generally has core and
side moieties. For example, anthracene is core part and
triphenyl group is two side groups in TAT chemical struc-
ture as shown in Figure 1. TAT can basically emits from
core part and it can be tuned by side moiety.13 As we men-
tioned in introduction part, we modified chemical struc-
tures at #2 position of antracence core part in order to
break molecule packing and be available for solution pro-
cess by using two big size chemical groups such as tertiary
butyl (t-butyl) and anthracene.
From Figure 2(b) data, similar results were obtained, but
A-TAT had vivid excimer band at 525 nm in PL spectrum.
It means that A-TAT compound can produce another inter-
action between molecules when it was evaporated due to
the second anthracene moiety at core part. Thus, in evapo-
ration case, it can cause partially limitation and disadvan-
tage to introduce another aromatic group although it has
large size because excimer band decrease the efficiency
and color purity.
Figure 2(c) and Table I exhibit the spin coating film
data. PL maximum values of TAT, T-TAT and A-TAT are
451 nm, 452 nm, and 469 nm. A-TAT has red-shifted
data value. Since the film condition is closer in separation
Figure 2 shows UV-visible (UV-Vis.) absorption and PL
spectra of three compounds in solution state and film state.
Film was prepared by evaporation (Fig. 2(b)) and spin
coating (Fig. 2(c)) methods.
Optical properties of two new synthesized materials
were different. T-TAT exhibited similar property with un-
modified compound of TAT but A-TAT showed relatively
longer PL maximum wavelength.
Those data are summarized in Table I. Absorption
in solution state of three compounds shows the typi-
cal anthracene absorption band in the range of 350 to
400 nm and major peaks of 376 nm, 376 nm and 386 nm,
respectively.
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J. Nanosci. Nanotechnol. 15, 1850–1854, 2015

Lee et al.
New Approach Way Using Substituent Group at Core Chromophore for Solution Process Blue Emitter
Table I. Optical properties of synthesized materials.
1.2
(a)
1.2
1.0
0.8
0.6
0.4
0.2
0.0
TAT
T-TAT
A-TAT
Solutiona
UVmax
Filmb
PLmax UVmax
Filmc
1.0
0.8
0.6
0.4
0.2
0.0
••
PLmax
(nm)
UVmax PLmax
(nm)
Compound
(nm)
(nm) (nm)
(nm)
TAT
T-TAT
A-TAT
358, 376, 396 436 383, 403
359, 376, 395 439 383, 402
369, 386, 400 451 390, 406 464, 525 391, 408 469
451
452
382, 400 451
382, 402 452
Notes: aChloroform solution (1×10−5 M); bdeposited film; cspin coating film.
Table II. Electrical and thermal properties of the synthesized
compounds.
300
350
400
450
500
550
600
650
Wavelength (nm)
HOMO
(eV)
LUMO
(eV)
Band gap
(eV)
Tg
Tm
(ꢁC)
(ꢁC)
1.2
1.0
0.8
0.6
0.4
0.2
0.0
1.2
1.0
0.8
0.6
0.4
0.2
0.0
Compound
(b)
TAT
T-TAT
A-TAT
TAT
T-TAT
A-TAT
5.62
5.62
5.65
2.68
2.71
2.84
2.94
2.91
2.81
150
163
194
340
–
–
respectively. It was verified that when tertiary butyl or
anthracene substituent group added to the TAT, HOMO
values would not significantly change. LUMO values of
2.68, 2.71, and 2.84 eV for the TAT, T-TAT, and A-TAT
were calculated from the band gap energy. We carried out
DSC analyses to determine the thermal properties of the
synthesized compounds. All of TAT, T-TAT and A-TAT
300
400
500
600
700
Wavelength (nm)
ꢁ
have high Tg values above 150 C. TAT exhibited Tm of
1.2
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1.2
1.0
0.8
0.6
0.4
0.2
0.0
340 ꢁC and T-TAT and A-TAT didn’t show Tm values.
(c)
TAT
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T-TAT
Those data are summarized in Table II.
1.0
Copyright: American Scientific Publishers
A-TAT
0.8
4. CONCLUSION
Tertiary butyl and anthracene groups were introduced as
substituents to the anthracene core part of TAT in order to
be available for solution process blue emitter.
T-TAT exhibited similar optical properties with TAT.
However, A-TAT showed relatively longer absorption and
emission in solution as well as film state. In case of
A-TAT, excimer band was shown in deposition film, but no
excimer band at spin-coated film. It may be prevented to
interact anthracene each other in spin-coated film. T-TAT
and A-TAT can be applicable to solution process as a blue
fluorescence material.
0.6
0.4
0.2
0.0
300
400
500
600
700
Wavelength (nm)
Figure 2. UV-visible absorption and PL spectra of TAT (square), T-TAT
(circle) and A-TAT (triangle): (a) solution in chloroform (1ꢂ0×10−5 M),
(b) evaporation film, (c) spin-coated film.
Acknowledgments: This work was supported by
the National Research Foundation of Korea (NRF)
grant funded by the Korea government (MEST) (No.
2012001846).
of distances between molecules compared to the solution
state, it can be seen that the UV absorption peaks have
red-shifted approximately 4∼8 nm and the emission peaks
between 13∼18 nm. Interestingly, A-TAT does not show
excimer band at 525 nm in spin-coated film. It may be
prevented to interact anthracene each other in spin-coated
film.
CIE values of the devices including TAT, T-TAT, and
A-TAT emitting layer were (0.150, 0.097), (0.147, 0.097),
and (0.180, 0.250) at 20 mA/cm2.
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J. Nanosci. Nanotechnol. 15, 1850–1854, 2015
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New Approach Way Using Substituent Group at Core Chromophore for Solution Process Blue Emitter
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Received: 14 June 2013. Accepted: 24 September 2013.
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J. Nanosci. Nanotechnol. 15, 1850–1854, 2015