A t-butyl modification approach of acceptor moiety for stable deep blue emission in thermally activated delayed fluorescent devices

Article abstract of DOI:10.1016/j.dyepig.2016.11.036

A t-butyl modification method of an acceptor moiety was developed as a molecular design approach of blue emitters to shift the emission color to deep blue region and to stabilize the emission color according to doping concentration. A phenyl unit of triphenyltriazine acceptor moiety of blue thermally activated delayed fluorescent emitter was modified with either one or two t-butyl units to study the effect of the t-butyl unit on the electroluminescence emission of the blue thermally activated delayed fluorescent emitters. It was found that two t-butyl units introduced in the phenyl unit of the triphenyltriazine shifted the emission color to deep blue emission while keeping the deep blue color over wide doping concentration range.

Full text of DOI:10.1016/j.dyepig.2016.11.036

Accepted Manuscript
A t-butyl modification approach of acceptor moiety for stable deep blue emission in
thermally activated delayed fluorescent devices
Yu Jin Kang, Jun Yeob Lee
PII:
S0143-7208(16)30925-1
10.1016/j.dyepig.2016.11.036
DYPI 5605
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 9 October 2016
Revised Date: 18 November 2016
Accepted Date: 20 November 2016
Please cite this article as: Kang YJ, Lee JY, A t-butyl modification approach of acceptor moiety for stable
deep blue emission in thermally activated delayed fluorescent devices, Dyes and Pigments (2016), doi:
10.1016/j.dyepig.2016.11.036.
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ACCEPTED MANUSCRIPT
A t-butyl modification approach of acceptor moiety for stable deep blue emission in
thermally activated delayed fluorescent devices
Yu Jin Kang, Jun Yeob Lee*
School of Chemical Engineering, Sungkyunkwan University
2066, Seobu-ro, Jangan-gu, Suwon-si, Gyeonggi-do, 440-746, Korea
Fax: (+) 82-31-299-4716
E-mail: leej17@skku.edu
* To whom correspondence should be addressed
Abstract
A t-butyl modification method of an acceptor moiety was developed as a molecular design approach of
blue emitters to shift the emission color to deep blue region and to stabilize the emission color
according to doping concentration. A phenyl unit of triphenyltriazine acceptor moiety of blue thermally
activated delayed fluorescent emitter was modified with either one or two t-butyl units to study the
effect of the t-butyl unit on the electroluminescence emission of the blue thermally activated delayed
fluorescent emitters. It was found that two t-butyl units introduced in the phenyl unit of the
triphenyltriazine shifted the emission color to deep blue emission while keeping the deep blue color
over wide doping concentration range.
Key words : deep blue, thermally activated delayed fluorescent emitter, color stability, t-butyl
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Introduction
For the last several years, there have been tremendous efforts to explore thermally activated delayed
fluorescent (TADF) emitters as high efficiency emitters because of strong need for high efficiency blue
emitters overcoming the low theoretical quantum efficiency (QE) of conventional blue fluorescent
emitters[1-9]. Currently, the maximum QE of the blue TADF emitters is as high as 25 % by molecular
design utilizing all generated excitons by electrical injection for radiative transition process.
Although several classes of blue TADF emitters have been reported, triphenyltriazine type TADF
emitters are promising as the blue TADF emitters when both QE and lifetime of the TADF devices are
considered. In fact, one of the best lifetime and QE data of the blue TADF devices were realized using
tiphenyltriazine acceptor based TADF emitters. The lifetime of the stable blue TADF device was about
50 h up to 80% of initial luminance at 500 cd/m2 using 9,9',9'',9'''-((6-phenyl-1,3,5-triazine-2,4-
diyl)bis(benzene-5,3,1-triyl))tetrakis(9H-carbazole) emitter[10], while the QE of the blue TADF device
was 25% using 9,9',9''-(5-(4,6-diphenyl-1,3,5-triazin-2-yl)benzene-1,2,3-triyl)tris(9H-carbazole)
emitter[11].
It is true that a great progress of the lifetime and QE of the triazine type blue TADF devices was
made by rational molecular design utilizing all excitons for light-emission process, but the TADF
devices still suffer from red shift of emission color according to the increase of TADF doping
concentration by strong intermolecular interaction originated by donor-acceptor type chemical
structure[13-17]. In particular, planar structure of the diphenyltriazine acceptor facilitated
intermolecular interaction of the triazine based TADF emitters, which shifted the emission wavelength
of the TADF devices to long wavelength[12]. Until now, electron transport type bis[2-
(diphenylphosphino)phenyl]ether oxide (DPEPO) is the most popular host material for blue TADF
devices and the device performances were optimized at a relatively high doping concentration above
10%. Therefore, the emission color of the blue TADF devices was red-shifted compared to the original
photoluminescence (PL) emission, which made it difficult to develop deep blue TADF devices.
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In this work, a molecular design blocking an acceptor moiety with a t-butyl substituent was
proposed as a method of suppressing red-shift of emission color according to doping concentration of
TADF emitters. The number of t-butyl units in the molecular structure was controlled, which
demonstrated that the increase of t-butyl units could stabilize the emission spectrum of TADF devices
according to TADF doping concentration and shift the emission spectrum to short wavelength.
Results and discussion
The t-butyl modification of the diphenyltriazine acceptor unit was the color stabilizing design
approach because intermolecular interaction would be suppressed by the t-butyl blocking group. The t-
butyl unit was used to modify the acceptor unit because t-butyl modification of donor unit would
strengthen the donor character and lead to red shift of emission color[13]. Donor moiety of the TADF
emitters in this work was carbazole and only the acceptor moiety was substituted with t-butyl modified
phenyl unit.
Preparation method of two t-butyl modified TADF emitters, 9,9'-(5-(4,6-bis(4-(tert-butyl)phenyl)-
1,3,5-triazin-2-yl)-1,3-phenylene)bis(9H-carbazole)
(tDCzTrz)
and
9,9'-(5-(4,6-bis(3,5-di-tert-
butylphenyl)-1,3,5-triazin-2-yl)-1,3-phenylene)bis(9H-carbazole) (DtDCzTrz), is explained in Scheme
1. The t-butyl modification method of the diphenyltriazine acceptor unit was Grignard reaction of
mono- or di- butylated bromobenzene with 2,4,6-trichlorotriazine. Two butylated phenyl units were
reacted with 2,4,6-trichlorotriazine by the Grignard reaction, which produced tDCzTrz with mono- t-
butylated phenyl unit and DtDCzTrz with di- t-butylated phenyl unit. Work-up and purification
procedures of the two TADF emitters were similar.
The effect of the t-butyl modification on the electronic orbital distribution of the TADF emitters was
investigated although it may not change the electronic state of the compounds. Molecular orbital
distribution were determined using B3LYP/6-31G* basis set of Gaussian 09 program. The ground state
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simulation results of the TADF emitters in Figure 1 suggested that the t-butyl modification of the
diphenyltriazine acceptor had little effect on the electronic orbital represented by highest occupied
molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). As the diphenyltriazine
is a strong acceptor, the weakly electron donating t-butyl unit did not alter the HOMO and LUMO
irrespective of the number of t-butyl units. The carbazole based donor moieties and diphenyltriazine
based acceptor moiety were the HOMO and LUMO determining parts of the TADF emitters. t-butyl
modification effect on the HOMO and LUMO values was examined by indirectly measuring the
ionization potential (IP) and electron affinity (EA) of the TADF emitters by electrochemically driven
cyclic voltammetry (CV). Oxidative voltage scan and reductive voltage scan in Figure 2 allowed the
determination of the IP and EA of the compounds using Ferrocene as the standard material. The IP of
the two compounds was not different as inferred from the electronic orbital dispersion. As the t-butyl
modification was applied to the acceptor moiety, the IP was not changed in the TADF emitters. The t-
butyl modification only changed the EA of the TADF emitters by weak electron donating character of
the t-butyl unit. The EA of DtDCzTrz with two t-butyl units became shallow by the electron donating
effect of the t-butyl unit. This indicates that the t-butyl substitution of the diphenyltriazine acceptor can
control the EA of the TADF emitters. The IP-EA gap of the TADF emitters was widened in the
DtDCzTrz emitter as can be confirmed in Table 1.
The ultraviolet-visible (UV-vis) absorption, room temperature fluorescent emission and low
temperature phosphorescent emission of the two TADF emitters were described in Figure 3. The UV-
vis absorption spectra of the TADF emitters had nothing to do with the t-butyl substituent because main
chromophore of the tDCzTrz and DtDCzTrz was the same. There was slight red-shift of the UV-vis
absorption edge in the DtDCzTrz emitter. Considering that the IP-EA gap widening effect of the t-butyl
modification, the t-butyl modification affected the PL behaviour of the TADF emitters. Comparing
tDCzTrz and DtDCzTrz, the addition t-butyl unit of DtDCzTrz slightly shifted the emission wavelength
to short wavelength by the weakened electron acceptor character of the diphenyltriazine acceptor. The
fluorescent peak wavelengths of tDCzTrz and DtDCzTrz emitters were 423 nm with a shoulder at 450
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nm and 425 nm, respectively. However, the phosphorescent peak appeared at a similar wavelength
because the t-butyl modifier had little effect on the degree of conjugation of the TADF compounds.
Peak wavelengths of the phosphorescence were 449 nm and 450 nm in the tDCzTrz and DtDCzTrz,
respectively. The singlet and triplet energy of the TADF materials were estimated from the onset point
of fluorescent and phosphorescent emission peaks and the extra t-butyl unit in the DtDCzTrz slightly
increased the singlet-triplet energy splitting. Therefore, the role of the extra t-butyl unit in the
synthesized TADF emitters can be summarized as blue shift of emission spectrum and slight increase
of singlet-triplet energy splitting.
PL behaviour of the TADF compounds was further studied by analysing PL decay at different
temperatures. PL measurement results of tDCzTrz and DtDCzTrz at 10, 100, 200 and 300 k are
displayed in Figure 4. From the similar PL decay data irrespective of temperature, it can be noticed
that the t-butyl modification had little influence on the radiative decay of the TADF emitters. At 10 k
and 100 k, the delayed PL component was very weak and the temperature did not alter the delayed PL
behaviour of tDCzTrz and DtDCzTrz. However, the weak delayed fluorescence was intensified at 200
k and 300 k by increased TADF emission. Intense delayed emission and quick PL decay were apparent
in the PL data, which were generally observed in other TADF emitters.
Two TADF emitters were independently evaluated in the device structure designed for high QE
using DPEPO high triplet energy host material. Doping concentration of each emitter was a variable to
monitor the change of the emission spectrum. Basic device performances of the blue TADF devices
were plotted in Figure 5. In all TADF devices, gradual increase of current density and luminance at the
same driving voltage was observed by increased doping of the TADF emitters due to hole injection and
transport function of the TADF emitters in the poor hole conducting DPEPO host. Although the TADF
emitter itself is a hole trapping dopant in the DPEPO host, heavily doped TADF emitters played a role
of hole transport medium by hole hopping effect.
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External quantum efficiency (EQE) of each TADF device was presented according to emitter doping
content in Figure 6. Optimum doping concentration of the TADF emitters for high EQE was different
in the tDCzTrz and DtDCzTrz devices, and it was 10% in the tDCzTrz and 20% in the DtDCzTrz
devices. Increase of the optimum doping concentration by the additional t-butyl unit was observed in
the TADF devices. This result can be interpreted by intermolecular interaction blocking role of the t-
butyl unit. When DPEPO was the host material, there are two major effects caused by the change of
doping concentration, which are concentration quenching and charge balance[16]. High doping
concentration is favoured for charge balance due to more hole density, but low doping concentration is
preferred for less concentration quenching. If the concentration quenching effect is dominant, optimum
doping concentration would be decreased, and vice versa. In the case of the DtDCzTrz, charge
balancing effect is more dominant than concentration quenching effect by the additional t-butyl unit,
which optimized the EQE at 20%.
The t-butyl unit also exhibited emission spectra stabilizing and blue-shifting function in the doped
TADF devices. The electroluminescence (EL) spectra of tDCzTrz and DtDCzTrz devices at 10, 20, 30
and 50% doping concentrations in Figure 7 apparently testified blue shift of EL emission at the same
doping concentration and less red shift of EL emission at high doping concentration by the extra t-butyl
unit of DtDCzTrz. The color coordinates of the tDCzTrz devices at 10% and 50% doping
concentrations were (0.15, 0.12) and (0.16, 0.19), while those of the DtDCzTrz devices were (0.15,
0.09) and (0.15, 0.13) at the same doping concentrations. Compared with the EL spectra change of the
DCzTrz, there was much less change of the EL sepctra according to doping concentration[10].
Moreover, the EL spectra were not affected by driving voltage. Deep blue emission color could be
demonstrated by the t-butyl modification approach which suppressed intermolecular interaction
between emitters and weakened electron accepting character of the acceptor moiety. Moreover, the
suppression of intermolecular interaction between DPEPO and DtDCzTrz may assist the deep blue
emission of DtDCzTrz. All device performances of the tDCzTrz and DtDCzTrz devices are shown in
Table 2.
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Experimental
General information
1,3,5-Tribromobenzene, potassium acetate and 9H-carbazole were purchased from Aldrich Chem. 9H-
carbazole was used after purification by recrystallization. 1-Bromo-4-(tert-butyl)benzene, 1-bromo-3,5-
di-tert-butylbenzene, and cyanuric chloride purchased from Alfa Aesar Co. were used without
purification. Tetrakis(triphenylphosphine)palladium(0) and 1,1ꢀ-bis(diphenylphosphino)ferrocene were
received from P&H Tech Co. Potassium carbonate, magnesium sulfate, anhydrous 1,4-dioxane, sodium
hydride and n-hexane were products of Duksan Sci. Co. These chemicals were used as received.
Tetrahydrofuran and toluene was distilled over sodium and calcium hydride. General chemical analysis
was reported our previous paper[13-15].
Synthesis
9,9'-(5-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-1,3-phenylene)bis(9H-carbazole)
The intermediate material was synthesized according to previous literature[10].
2,4-bis(4-(tert-butyl)phenyl)-6-chloro-1,3,5-triazine (1) A solution of 1-bromo-4-(tert-butyl)benzene
(4.0 g, 18.7 mmol) in anhydrous THF (10 ml) was added dropwise to a suspension of iodine-activated
magnesium (2.3 g, 93.8 mmol) in anhydrous THF (5 ml). After that the reaction mixture was refluxed
for 3 h and then cooled to room temperature. The resulting Grignard solution was slowly added to a
solution of cyanuric chloride (2.3 g, 12.4 mmol) in anhydrous toluene (50 ml) at room temperature. The
mixture was refluxed overnight and was cooled down to room temperature. The reaction mixture was
quenched with distilled water and extracted by methylene chloride. The crude product was purified by
column chromatography on silica gel using a n-hexane/methylene chloride eluent and a white powder
was obtained (2.5 g, yield 62 %).
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2-Chloro-4,6-bis(3,5-di-tert-butylphenyl)-1,3,5-triazine (2) A solution of 1-bromo-3,5-di-tert-
butylbenzene (5.0 g, 18.5 mmol) in anhydrous THF (15 ml) was added dropwise to a suspension of
iodine-activated magnesium (2.2 g, 92.8 mmol) in anhydrous THF (10 ml). After that the reaction
mixture was refluxed for 3 h and then cooled to room temperature. The resulting Grignard solution was
slowly added to a solution of cyanuric chloride (1.0 g, 5.4 mmol) in anhydrous Toluene (30 ml) at room
temperature. The mixture was refluxed for overnight. Work-up procedure of 2 was the same as that of 1
and the product was obtained as a white powder (1.3 g, yield 53 %).
9,9'-(5-(4,6-Bis(4-(tert-butyl)phenyl)-1,3,5-triazin-2-yl)-1,3-phenylene)bis(9H-carbazole)
(tDCzTrz) 2,4-Bis(4-(tert-butyl)phenyl)-6-chloro-1,3,5-triazine (1) (0.8 g, 2.1 mmol) and 9,9'-(5-
(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-1,3-phenylene)bis(9H-carbazole) (1.3 g, 2.3 mmol) were
dissolved in anhydrous THF (30 ml). After 30 min, tetrakis(triphenylphosphine)palladium (15.0 mg,
0.1 mmol) and 2M aqueous potassium carbonate (6.9 g, with distilled water 15 ml) was added in above
mixture continuously. The residue was extracted by methylene chloride and distilled water. And then
solvent was removed from the resulting mixture under reduced pressure. The crude product was
purified by column chromatography on silica gel using n-hexane/methylene chloride. The product was
1
sublimed and obtained as a greenish yellow powder (1.2 g, yield 76 %). H NMR (500 MHZ, CDCl3):
9.10 (s, 2H), 8.65-8.63 (d, 4H, J = 5.0 Hz), 8.21-8.19 (d, 4H, J = 3.7 Hz), 8.04-8.03 (t, 1H, J = 1.3 Hz),
7.64-7.62 (d, 4H, J = 4.0 Hz), 7.55-7.53 (d, 4H, J = 4.2 Hz), 7.50-7.46 (t, 4H, J = 5.5 Hz), 7.36-7.35 (t,
4H, J = 2.5 Hz), 1.36 (s, 18H) MS (APCI) miz 752.9 [(M)+]. Elemental Analysis calculated for
C53H45N5 : C, 84.65; H, 6.03; N, 9.31. Found : C, 84.64; H, 6.24; N, 8.75;
9,9'-(5-(4,6-bis(3,5-di-tert-butylphenyl)-1,3,5-triazin-2-yl)-1,3-phenylene)bis(9H-carbazole)
(DtDCzTrz) 2-chloro-4,6-bis(3,5-di-tert-butylphenyl)-1,3,5-triazine (2) (1.0 g, 2.0 mmol) and 9,9'-(5-
(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-1,3-phenylene)bis(9H-carbazole) (1.2 g, 2.2 mmol) were
dissolved in anhydrous THF (30 ml). After 30 min, tetrakis(triphenylphosphine)palladium (15.0 mg,
0.1 mmol) and 2M aqueous potassium carbonate (6.9 g, with distilled water 15 ml) was added in above
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mixture continuously. Work-up procedure of DtDCzTrz was the same as that of tDCzTrz and the
product was obtained as a yellowish green powder after sublimation (1.1 g, yield 63 %). 1H NMR (500
MHZ, CDCl3): 9.0 (s, 2H), 8.65-8.65 (d, 4H, J = 0.7 Hz), 8.20-8.18 (d, 4H, J = 4.0 Hz), 8.12-8.11 (t, 1H,
J = 1.3 Hz), 7.76-7.75 (d, 4H, J = 4.0 Hz), 7.70-7.69 (t, 2H, J = 1.1 Hz), 7.50-7.46 (t, 4H, J = 5.5 Hz),
7.36-7.33 (t, 4H, J = 5.1 Hz), 1.41 (s, 36H) MS (APCI) miz 865.1 [(M)+]. Elemental Analysis
calculated for C61H61N5: C, 84.78; H, 7.11; N, 8.10. Found : C, 84.75; H, 6.98; N, 7.87;
Device Fabrication
All devices reported in this work were fabricated using a thermal evaporator with a vacuum pressure
of 1.0 × 10−6 torr. To investigate the electroluminescence (EL) performance of the two compounds, we
fabricated devices with the optimized structures of indium tin oxide(ITO)/ poly(3,4-
ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS, 60 nm)/ 4,4′-cyclohexylidenebis[ N,N -
bis(4-methylphenyl) aniline] (TAPC, 20 nm)/ 1,3-bis(N-carbazolyl)benzene (mCP, 10 nm)/
DPEPO:tDCzTrz, or DPEPO:DtDCzTrz (25 nm)/ diphenylphosphine oxide-4-(triphenylsilyl)phenyl
(TSPO1, 5 nm)/ 1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene (TPBi, 30 nm)/ LiF (1.5 nm)/ Al (200
nm). Doping concentrations of tDCzTrz and DtDCzTrz in the DPEPO host were 10%, 20%, 30% and
50%, respectively.
Conclusions
In conclusion, a study on the effect of t-butyl modification of an acceptor of TADF emitters on the
light emission of blue emitters demonstrated blue-shifting and spectrum stabilizing function of the t-
butyl unit. DtDCzTrz TADF emitter with two t-butyl units included phenyl unit showed a deep blue
color coordinate of (0.15,0.11) by the blue-shifting effect of the t-butyl unit. Therefore, the t-butyl
modification approach of the acceptor would be effective to develop deep blue TADF emitters.
.
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List of tables
Table 1. Measured photophysical properties of tDCzTrz and DtDCzTrz.
Table 2. Summarized of device performance of tDCzTrz and DtDCzTrz devices at different doping
concentrations.
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Table 1. Measured photophysical properties of tDCzTrz and DtDCzTrz.
Singlet
energy
(eV)
Triplet
energy
(eV)
IP
EA
excited state lifetime
ΔEST
(μs)
(eV)
-6.17
(eV)
-3.40
(eV)
13.9
12.9
tDCzTrz
3.14
3.18
2.95
3.01
0.19
0.17
DtDCzTrz
-6.17
-3.35
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Table 2. Summarized of device performance of tDCzTrz and DtDCzTrz devices at different doping
concentrations.
Doping
Concentration
(%)
Maximum
Quantum efficiency
(%)
Maximum
Color
Current efficiency Coordinate
(cd/A)
10
20
14.5
10.7
5.0
13.8
12.8
8.9
0.15,0.12
0.15,0.14
0.16,0.19
0.20,0.24
tDCzTrz
50
100
2.6
6.8
10
20
8.8
14.7
6.8
6.7
14.1
7.7
0.15,0.09
0.15,0.11
0.15,0.13
0.16,0.14
DtDCzTrz
50
100
2.0
3.5
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List of figures
Scheme 1. Synthetic scheme of tDCzTrz and DtDCzTrz,
Figure 1. HOMO and LUMO distribution of tDCzTrz and DtDCzTrz calculated using Gaussian 09
program
.
Figure 2. Electrochemical oxidation and reduction curves of tDCzTrz and DtDCzTrz.
Figure 3. UV-Vis and fluorescence and phosphorescence spectra of the tDCzTrz and DtDCzTrz.
(Fluorescence spectra are prompt emission of solid film at room temperature and phosphorescence
spectra are delayed emission spectra at 77 K.)
Figure 4. Transient PL decay curves of the 10 wt % tDCzTrz (a) and 20 % DtDCzTrz (b) doped
DPEPO films.
Figure 5. Current density-voltage-luminance curves of tDCzTrz (a) and DtDCzTrz (b) devices
according to doping concentration of dopant materials.
Figure 6. Quantum efficiency–luminance curves of tDCzTrz and DtDCzTrz devices at optimized
doping concentration.
Figure 7. Electroluminescence spectra of tDCzTrz (a) and DtDCzTrz (b) devices according to doping
concentration of dopant materials.
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Scheme 1. Synthetic scheme of tDCzTrz and DtDCzTrz.
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tDCzTrz
DtDCzTrz
HOMO
Figure 1. HOMO and LUMO distribution of tDCzTrz and DtDCzTrz calculated using Gaussian 09
program
LUMO
.
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Figure 2. Electrochemical oxidation and reduction curves of tDCzTrz and DtDCzTrz.
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(a)
(b)
Figure 3. UV-Vis, fluorescence and phosphorescence spectra of the tDCzTrz (a) and DtDCzTrz (b).
(Fluorescence spectra are prompt emission of solid film at room temperature and phosphorescence
spectra are delayed emission spectra at 77 K.)
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(b)
Figure 4. Transient PL decay curves of the 10 wt % tDCzTrz (a) and 20 w% DtDCzTrz (b) doped
DPEPO films.
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(b)
(a)
Figure 5. Current density-voltage-luminance curves of tDCzTrz (a) and DtDCzTrz (b) devices
according to doping concentration of dopant materials.
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Figure 6. Quantum efficiency–luminance curves of tDCzTrz and DtDCzTrz devices at optimized
doping concentration.
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(b)
(a)
Figure 7. The electroluminescence (EL) spectra of tDCzTrz (a) and DtDCzTrz (b) devices according to
doping concentration of dopant materials.
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-
-
-
A t-butyl modification of a triazine based acceptor moiety as an approach to manage the
light-emitting properties of blue emitters
Deep blue emission color in thermally activated delayed fluorescent organic light-emitting
diodes by t-butyl substituent
Stable emission spectrum according to the change of doping concentration