A pure red organic light-emitting diode based on a luminescent derivative of tris(2,4,6-trichlorotriphenyl)methyl radical

Article abstract of DOI:10.1016/j.dyepig.2016.12.063

A novel derivative of Tris(2,4,6-trichlorotriphenyl)methyl radical was synthesized and used as the emitter of an organic light-emitting diode. Stable electroluminescent (EL) spectra at the full-range driving voltages and pure red color were obtained throu

Full text of DOI:10.1016/j.dyepig.2016.12.063

Accepted Manuscript
A pure red organic light-emitting diode based on a luminescent derivative of
Tris(2,4,6-trichlorotriphenyl)methyl radical
Yingchang Gao, Ablikim Obolda, Ming Zhang, Feng Li
PII:
S0143-7208(16)31093-2
10.1016/j.dyepig.2016.12.063
DYPI 5690
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 1 November 2016
Revised Date: 20 December 2016
Accepted Date: 26 December 2016
Please cite this article as: Gao Y, Obolda A, Zhang M, Li F, A pure red organic light-emitting diode
based on a luminescent derivative of Tris(2,4,6-trichlorotriphenyl)methyl radical, Dyes and Pigments
(2017), doi: 10.1016/j.dyepig.2016.12.063.
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Graphical abstract:

ACCEPTED MANUSCRIPT
A pure red organic light-emitting diode based on a luminescent derivative
of Tris(2,4,6-trichlorotriphenyl)methyl radical
Yingchang Gao, Ablikim Obolda, Ming Zhang and Feng Li*
State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin
University, Qianjin Avenue 2699, Changchun, 130012, P. R. China
E-mail:lifeng01@jlu.edu.cn
Abstract: A novel derivative of Tris(2,4,6-trichlorotriphenyl)methyl radical was synthesized
and used as the emitter of an organic light-emitting diode. Stable electroluminescent (EL)
spectra at the full-range driving voltages and pure red color were obtained through optimizing
the device structure. The Commission Internationale de L' Eclairage (CIE) coordinate of the
device at 7 V is (0.667, 0.301), which is very close to the red standard CIE coordinate (0.670,
0.330) of NTSC. The maximum external quantum efficiency of the device is 2.4 %, and the
formation ratio of doublet exciton is nearly 100 %.
1. Introduction
Organic light emitting diodes (OLEDs) are being used as the flat displays of high-end cell
phones and Televisions and are also being expected to play an important role for future lighting
sources due to their special features such as easy processing, light weight, high contrast, and
flexibility [1-4]. During the development of OLEDs, many fluorescent and phosphorescent
materials have been reported [5-8]. Although the internal quantum efficiency (IQE) of
phosphorescent OLEDs has achieved almost 100 %, practically useful phosphorescent
materials are mainly concentrated to the expensive Ir and Pt complexes. In contrast, fluorescent
materials are cost-effective, but the theoretical upper limit of internal quantum efficiency (IQE)
of fluorescent OLEDs is only 25 % according to the spin statistics [9]. To overcome the 25 %
IQE upper limit of fluorescent OLEDs, some methods have been realized including thermally
activated delayed fluorescence (TADF) [10-13], hybridized local and charge-transfer (HLCT)
emission [14-16], triplet-polaron-interaction (TPI) [17] and triplet-triplet-annihilation (TTA)
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[18-22] induced upconversion from triplet to singlet.
Different from the previous OLEDs in which the emitters are all closed-shell molecules, a
new kind of OLEDs using an open-shell molecule (neutral π radical) as the emitter was reported
recently [23, 24]. The emission of that kind of OLEDs comes from the radiative decay of
doublet exciton which are spin-allowed. Thus the transition problem of triplet exciton is
circumvented. However, the electroluminescent (EL) spectra of OLEDs in that work are not
stable and the blue component will appear and become stronger as the driving voltage increases.
So the device structure needs further optimization. In addition, although there is a little of
reported light-emitting neutral π radicals in solutions [25-32], their EL properties are lacking.
Moreover, the reported light-emitting radicals are almost all deep red / near infrared materials
and the pure red radicals which is important to display and lighting are rare, e.g.
[4(N-indolyl)-2,6-dichlorophenyl]bis(2,4,6-trichlorophenyl)-methyl
radical
(TTM-1ID)
reported by Juliá and et al [25,27]. In this work, a novel pure red neutral π radical,
bis[4-(N-indolyl)-2, 6-dichlorophenyl](2, 4, 6-trichlorophenyl)methy radical (TTM-2ID) was
synthesized and the red OLEDs with stable spectra based on it were obtained.
2. Experimental section
2.1 General Information
All chemicals and solvents for the synthesis and measure, unless otherwise stated, were
purchased from commercial suppliers and used without further purification. THF and CHCl3
1
were redistilled before use. The H-NMR spectra was recorded at 298 K with a Bruker
AVANCZ 500 spectrometer, using deuterated dimethyl sulfoxide (DMSO) as solvent. Mass
spectra were measured on a Thermo Fisher ITQ1100 GC-MS mass detector. Elemental analysis
was carried on a CHNS-O elemental analysis instrument. Thermogravimetric analysis (TGA)
was performed on a TAQ500 instrument at heating rate of 10 K·min-1 under nitrogen
atmosphere. Ultraviolet-visible (UV-vis) absorption spectrum was recorded on a shimadzu
UV-2550 spectrophotometer. Fluorescence spectrum was measured with a RF-5301PC
spectrophotometer. Lifetime in n-hexane was measured with a FLS-980 apparatus. EPR spectra
of solid radicals were recorded with JES-FA200 at room temperature. Photostability of
TTM-2ID was performed under the excitation of a pulsed laser at 355 nm with power density
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of 98.1 kw·cm-2 (pulse width: 8 ns, frequence: 10 Hz), the emission spectra were detected
using a Maya 2000 Pro fiber spectrometer.
The cyclic voltammetry (CV) measurement was performed using an electrochemical
analyzer (CHI660C, CH Instruments, USA). A glass carbon disk was used as the working
electrode. A platinum wire acted as the counter electrode and Ag/Ag+ acted as the reference
electrode together with the redox couple ferrocenium/ferrocene as the internal standard at the
rate of 50 or 100 mV·s-1. Tetrabutylammonium hexafluorophosphate (TBAPF6) in anhydrous
dichloromethane (0.1 M) were used as the supporting electrolyte for CV scan. The HOMO
levels are calculated according to the following formula:
HOMO = - ( Eox vs. Ag/Ag+ - E1/2+ vs. Ag/Ag+ + 4.8) eV
+
where the Eox vs. Ag/Ag+ is oxidation onset potential relative to Ag/Ag+ electrode. E1/2 vs.
Ag/Ag+ is half wave potential of Fe+/Fe obtained from positive CV scan.
2.2 Synthesis
αHTTM and the dehydrogenation of it were synthesized according to the literature [33].
The detailed synthetic procedures and some characterizations of TTM-1ID have been reported
by Luis Juliá’s group [27]. And we used the homologous strategy and conditions to obtain
TTM-2ID. The corresponding NMR and FTIR spectra were shown in the supporting
information (Figure S1 and S2).
αHTTM-2ID:
The mixtures of TTM-1ID (1.27 g, 2.0 mmol), indole (1.17 g, 10.0 mmol), Cs2CO3 (0.55 g,
1.7 mmol), and DMF (50 mL) were added into a 100 ml 2-neck flask together, stirring at 160 0C
under N2 atmosphere and in the dark for 12 hours. When the reaction accomplished, stop
heating and let it cool to room temperature. The resulting mixture was poured into dilute
hydrochloric acid, leached with buchner funnel and suction flask. The precipitate was washed
with purified water and dissolved in dichloromethane. After evaporation of the solvent, the
crude product was purified through silica gel column chromatography using the mixture of
petroleum ether and dichloromethane as eluent to give αHTTM-2ID as a pea green solid (0.34 g,
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24 %). Owing to the stereo-hindrance effect, we can say the chloride atom is mainly
substituted by indole group at the p-carbon of TTM. Even though there is the possible that the
chloride at the o-carbon can be substituted, it can’t be the major product. This is also
1
suggested by the similar molecules reported in ref 26, 27. HNMR (500 MHz, DMSO), δ
(ppm): 7.89 (dd, J = 25.5, J = 11.3, 4 H), 7.79 (s, 1 H), 7.75 (s, 1 H), 7.72 – 7.64 (m, 6 H), 7.28
(s, 2 H), 7.19 (t, J=7.1, 2 H), 6.86 (s, 1 H), 6.77 (s, 2 H). Elemental Analysis: calculated for
C35H19Cl7N2: C, 58.74; H, 2.68; N, 3.91; found: C, 58.37; H, 2.69; N, 3.81, and MS (ESI, m/z):
(M+H)+ calcd for: C35H19Cl7N2, 715.71, found, 714.35.
TTM-2ID:
In a 50 ml round flask with αHTTM-2ID (0.72, 1.0 mmol) in anhydrous THF (20ml) , a
tetrabutylammonium hydroxide solution in methanol (2.0 M) (0.75 mL, 1.5 mmol) was added
under N2 atmosphere and stirred in the dark for 5 hours. Then tetrachloro-p-benzoquinone (0.70
g, 2.8 mmol) was added and stirred for another 1.5 hours at room temperature. The crude
product was dried under reduced pressure and purified through silica gel column
chromatography using the mixture of petroleum ether and dichloromethane as eluent to afford
the final product TTM-2ID radical (0.49 g, 68 %). FT-IR (KBr, ν·cm-1): 3070 (Ar, C-H), 2918,
1597 (Ar, C=C), 1575, 1544, 1518 (Ar, C=C), 1468, 1330 (ArN), 1208, 1133, 859, 796, 761,
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738, 715, and MS (ESI, m/z): (M+H)+ calcd for: C35H18Cl7N2, 714.70, found, 713.85.
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
a
b
CHCl3
Cl
Cl
Cl
CCl l
Cl
Cl
CCl l
Cl
TTM
HTTM
α
Cl
N
N
Cl
Cl
Cl
Cl
c
d
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
N
H
Cl
CCl
l
Cl
Cl
Cl
CCl l
Cl
CCl l
Cl
TTM-1ID
HTTM-1ID
α
TTM
N
N
N
Cl
Cl
Cl
Cl
e
f
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
N
H
Cl
Cl
Cl
Cl
N
Cl
Cl
N
Cl
CCl l
Cl
HTTM-2ID
α
TTM-2ID
TTM-1ID
～
0
Reagents and conditions: (a) AlCl3, 80 C, 2.5 h; (b) Bu4NOH, anhydrous THF, rt, stirring for 5 h, then adding
tetrachloro-p-benzoquinone, stirring for another 1.5 h; (c) Cs2CO3, DMF, 160 0C, reflux for 4 h; (d) the same as b;
(e) Cs2CO3, DMF, 160 0C, reflux for 12 h; (f) the same as b.
Scheme 1. Synthesis route of radicals based on TTM molecule.
2.3. OLEDs fabrication and testing
Ready-made indium tin oxide (ITO) glass substrates were purchased and cleaned with
ethanol, acetone, toluene and isopropyl alcohol. After dried with N2, they were treated with UV
irradiation for 20 min and next transferred to a vacuum deposition system with the gas pressure
of 3-4×10-6 mbar. The MoO3 layer was deposited at a rate of 0.2 Å·s-1. All the organic layers
were deposited at 0.4-0.6 Å·s-1. The evaporation rate of cathode LiF and Al metal layer were
0.1 Å·s-1 and 1-4 Å·s-1 respectively. The current-voltage-bright (I-V-B) characteristics were
measured using a Keithley 2400 programmable electrometer combined with a Photo Research
spectroradiometer (PR-650). The EL spectra and CIE coordination were measured using a
Photo Research spectroradiometer (PR-650) at room temperature under ambient conditions.
3. Results and discussion
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The synthesis routes of target products are shown in scheme 1. TTM-2ID consists of a
tris(2,4,6-trichlorotriphenyl)methyl radical (TTM) core and two indolyl moieties connected
outside, and its molecular structure is shown in Figure 1 a). The electron paramagnetic
resonance (EPR) spectra of TTM-2ID powders before and after the vacuum thermal
evaporation are shown in Figure 1 b), and the results indicate that there is an unpaired electron
in TTM-2ID throughout the evaporation process. Thermogravimetric analysis (TGA) of
TTM-2ID (shown in Figure 2) suggests that the 5 % weight-loss temperature is 326 0C, which
is far exceeding the temperature of vacuum-deposition (approximately 170 0C). The results of
EPR after evaporation process and TGA verify that TTM-2ID is stable enough to endure the
process of vacuum thermal evaporation to fabricate OLEDs.
Figure. 1. a) The molecular structure of radical TTM-2ID. b) The EPR spectra of TTM-2ID
powder measured at room temperature before (up) and after (down) evaporation.
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Figure. 2. TGA curve of TTM-2ID.
～
To know about the electrochemical properties and the singly occupied molecular orbital
(SOMO) and the lowest singly unoccupied molecular orbital (SUMO) levels of TTM-2ID, the
test of cyclic voltammetry (CV) was performed. The curves are displayed in Figure 3 a).
From the onset of the oxidation curve and ferrocenium/ferrocene (Fe+/Fe) redox couple, the
ionization potential (IP) value of this radical was calculated to be 5.13 eV, and the electron
affinity (EA) value can be determined from the equation: EA (eV) = Ip - Eg. Eg represents the
optical gap, which can be calculated from the absorption onset of TTM-2ID. IP and EA are
corresponding to the SOMO and SUMO of the frontier orbitals in an open-shell molecule. To
know about the stability of gaining and losing electrons of TTM-2ID which reflects the
stability of TTM-2ID under electrical excitation to some degree, we further scanned 20 cycles
of the CV curves as displayed in Figure 3 b). The multicycle CV curves show that either the
shape, peak position or the onset does not change, which implies the decent stability of
TTM-2ID in the process of oxidation and reduction.
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Figure. 3. a) Cyclic voltammogram of TTM-2ID in 0.1 M TBAPF6/CH2Cl2, scan rate: 50
mV·s-1, room temperature. b) The multicycle CV curves of TTM-2ID with scan rate of 100
mV·s-1, the inset is the partial enlarged detail.
We also investigated the photostability of TTM-2ID. We used a pulsed laser at 355 nm
with power density of 98.1 kw·cm-3 as the excitation light source and measured the decay of
fluorescence intensity in diluted n-hexane solution under ambient conditions. For comparison,
we also tested the photostability of TTM, which is absent of the peripheral protection groups.
As shown in Figure 4, the PL intensity of TTM decayed much faster than that of
TTM-2ID.The decay lifetimes (1/t1/2) of 63 s for TTM and 2400 s for TTM-2ID were
obtained, respectively. The decay lifetime of TTM-2ID is about 40 times longer than that of
TTM, which is comparable to a stable PyBTM radical [31].
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Figure. 4. PL intensity decay of TTM and TTM-2ID in n-hexane solution irradiated by a
pulsed laser at 355 nm with power density of 98.1 kw·cm-2 (pulse width: 8 ns, frequence: 10
Hz) under ambient conditions in air.
Figure 5 shows the energy levels and electron-cloud distributions of SOMO and SUMO of
TTM-2ID, respectively, which were calculated by the density functional theory (DFT) using
Gaussian 09 series of programs with the B3LYP/6-31G(d) basis set [34]. As can be seen, the
SUMO is mainly localized on the TTM and there is a little electron cloud extending to the
pyrrole of the outer indolyl moieties, while the SOMO is delocalized on the whole molecule.
ESOMO is calculated to be -5.31 eV, which is consistent with the result of -5.13 eV from Cyclic
Voltammetry measurement. ESUMO is calculated to be -3.27 eV. Thus the energy gap is 2.04 eV
according to the DFT calculation.
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Figure. 5. The SOMO/SUMO and corresponding orbital energy levels calculated at the
B3LYP/6-31G(d).
Figure 6 a) shows UV-vis absorption and photoluminescence (PL) spectra of TTM-2ID in
diluted n-hexane solution. The absorption onset of the compound is 617 nm corresponding to
the optical gap of 2.01 eV, which agrees well with the calculated result. From the PL spectrum,
we can see TTM-2ID has a emission band at 620 nm with a weak shoulder at longer
wavelength, which has a little red shift (10 nm) compared to the reported TTM-1ID (TTM core
connected with one indolyl moiety outside). The PL quantum yiled (Φf) of TTM-2ID in
n-hexane is measured to be 0.26 at the excitation wavelength of 420 nm by using rhodamine
6G (R6G) as the standard [35]. We also performed the transient PL measurement of the
compound in n-hexane solution. A single exponential decay is observed and the PL lifetime (τ)
is 34.5 ns, as shown in Figure 6 b). Based on the results of Φf and τ, the radiative (kr) and
non-radiative rate constant (knr) were obtained to be 0.75 x 107 s-1 and 2.15 x 107 s-1,
respectively. kr is only one third of knr, which may be detrimental to acquiring high PL
quantum yield. The decay lifetime of doublet exciton is longer than that of singlet exciton
(generally from several hundred picosecond to several nanosecond) but much shorter than
that of triplet exciton (longer than microsecond), which suggests the transition of doublet
exciton to ground state is not totally allowed and needs further study.
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Figure. 6. a) The normalized UV-vis spectrum (black line) and PL spectrum (red line) in
n-hexane solution. The inset is the photograph of TTM-2ID irradiated under 365 nm of UV
lamp. b) Transient PL spectrum of TTM-2ID in dilute n-hexane solution with the fitted curve.
The EL characteristics of TTM-2ID acting as an emitter were explored by fabricating
multilayer OLEDs under vacuum thermal evaporation. The structure of the fabricated device is
ITO / MoO3 (3 nm) / TAPC (70 nm) / TTM-2ID: CBP (5 wt %, 40 nm) / B3PyMPM (10 nm) /
PO-T2T (70 nm) / LiF (0.8 nm) / Al (100 nm). MoO3, 4,4’-cyclohexylidenebis[N,N-bis(p-tolyl)
aniline](TAPC),
4,4’-Bis(N-carbazolyl)-1,1’-biphenyl(CBP),
2,4,6-Tris(3-
(diphenylphosphoryl)phenyl)-1,3,5-triazine (PO-T2T) and LiF were used as hole-injecting
layer, hole-transporting layer, doping-host material, electron-transporting layer and
electron-injecting
layer,
respectively.
The
4,6-Bis(3,5-di(pyridin-3-yl)
phenyl)-2-methylpyrimidine (B3PyMPM) layer with 10 nm thickness was inserted between
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emitting layer and PO-T2T layer to effectively confine excitons / holes in emitting layer and
avoid the influence of PO-T2T on the emitting layer. The device architecture and the
corresponding energy level diagram of the materials [36-38] are depicted in Figure 7 a).The
molecular structures of TAPC, CBP, B3PyMPM and PO-T2T used in this work are shown in
Figure 7 b).
Figure. 7. a) The schematic diagram of the structure in this device and the energy levels of each
material. b) Molecular structures used in this work.
Figure 8 presents the EL spectrum of this device at 7 V and the inset shows the normalized
EL spectra from 5 V to 11 V. As can be seen, the emission maximum is at 648 nm and the
spectra are almost unchanged at the voltages ranging from 5 V to 11 V. The Commission
Internationale de L' Eclairage (CIE) coordinate at 7 V is (0.667, 0.301), which is very close to
the red standard CIE coordinate (0.670, 0.330) of National Television System Committee
(NTSC). So this device has the potential as the red pixels of display screens. The energy barrier
of holes at the interface between CBP (HOMO level: -5.9 eV ) and B3PyMPM (HOMO level:
-7.0 eV) is 1.1 eV, which is large enough to prevent holes from entering into PO-T2T layer. The
existence of B3PyMPM layer also blocks the formation of exciplex between CBP and PO-T2T,
whose emission can be seen in the device without B3PyMPM layer, as shown in Figure S3.
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Thus the inserted thin B3PyMPM layer between emitting layer and PO-T2T layer is the reason
of stable EL spectra of this device.
Figure. 8. The normalized EL spectrum of the OLED at 7 V. The inset shows the EL spectra at
different voltages.
The current density-voltage-luminance (J-V-L) characteristics of the OLED are shown in
Figure 9 a). The turn-on voltage of the device is around 3.8 V. Figure 9 b) shows the external
quantum efficiency (EQE) and current efficiency of the OLED as a function of voltage. The
maximum EQE is 2.4 % (0.05 mA·cm-2), which is a modest value among the typical red
non-phosphorescent OLEDs (listed in Table S1). The efficiency roll-off of this device seems
not pronounced. The EQE value keeps 2.1 % at the current density of 0.5 mA·cm-2 and 1.3 %
at 5.0 mA·cm-2, which is superior to many traditional closed-shell TADF molecules and
phosphorescent materials [12, 39-43]. Taking ref 39 as an example, the EQE in that work
reaches a high value of 10.1 % at a very low current density of 3.3 x 10-4 mA·cm-2, while it
decreases rapidly with increasing current density. The EQE value declines to ꢀ2.0 % at 0.5
mA·cm-2 and even less than 1.0 % at 5.0 mA·cm-2. The improved efficiency roll-off in this
work may be attributed to the much shorter lifetime of the doublet exciton (order of
nanosecond) than that of triplet exciton (order of microsecond). The triplet-triplet annihilation
(TTA) at higher current density due to the long lifetime of triplet exciton is believed to be the
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reason of inferior efficiency roll-off. The EQE of the OLEDs can be expressed by the
following equation [44]:
(1)
wherein Ф is the EQE, χD is the ratio of doublet excitons to the total excitons, ϕPL is the PL
quantum yield of the emitting layer, here is 0.11 measured in an integrating sphere at ambient
temperature, ηr is the fraction of injected charge carriers that form excitons and can be regarded
to be unit in our devices, ηout is the light out-coupling efficiency which is considered to be
20-30 % [45,46]. According to equation (1), we obtained the formation ratio of doublet exciton
(χD) as 74-110 %. This means that the device structure has reached optimum and the
nonradiative transitions of the radical molecule have to be suppressed to enhance the PL
quantum yield if the EQE of the device wants to be further increased.
Figure. 9. a) The current density–voltage–luminance (J–V–L) characteristics of the OLED. b)
The external quantum efficiency (ηEQE) and current efficiency (CE) as a function of current
density.
4. Conclusions
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In summary, we reported an efficient pure red OLED based on a novel open-shell radical
molecule. The EL spectra at different driving voltages are stable and the CIE coordinate of
(0.667,0.301) is very close to the red standard CIE coordinate (0.670, 0.330) of NTSC. The
maximum EQE of the device is 2.4 %, and the formation ratio of doublet exciton in this device
is up to 100 %, which indicates that the IQE of OLEDs using neutral radicals as emitters has
the potential to be 100 % provided the nonradiative pathways of neutral radicals are suppressed.
Our work enriched the luminescent molecular system and we also expected to design other
TTM-based adducts with blue-shifted EL through replacing the peripheral indole moiety with
some electron-withdrawing groups.
Acknowledgements
This work was supported by the National Key Basic Research and Development
Program of China (Grant No. 2016YFB0401001 and 2015CB655003) Founded by MOST,
National Natural Science Foundation of China (No.51673080).
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Highlight:
ꢀ A novel luminescent Tris(2,4,6-trichlorotriphenyl)methyl radical with two
indole moieties connected outside was synthesized;
ꢀ A pure red OLED with the emission from doublet excitons was firstly
reported;
ꢀ The maximum EQE of the device was 2.4 %, and the formation ratio of
doublet exciton is up to 100 %.