Organic light-emitting diodes using a neutral π radical as emitter: The emission from a doublet

Article abstract of DOI:10.1002/anie.201500242

Abstract Triplet harvesting is a main challenge in organic light-emitting devices (OLEDs), because the radiative decay of the triplet is spin-forbidden. Here, we propose a new kind of OLED, in which an organic open-shell molecule, (4-N-carbazolyl-2,6-dich

Full text of DOI:10.1002/anie.201500242

Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201500242
German Edition: DOI: 10.1002/ange.201500242
Light-Emitting Diodes
Hot Paper
Organic Light-Emitting Diodes Using a Neutral p Radical as Emitter:
The Emission from a Doublet**
Qiming Peng, Ablikim Obolda, Ming Zhang, and Feng Li*
Abstract: Triplet harvesting is a main challenge in organic
light-emitting devices (OLEDs), because the radiative decay of
the triplet is spin-forbidden. Here, we propose a new kind of
OLED, in which an organic open-shell molecule, (4-N-
carbazolyl-2,6-dichlorophenyl)bis(2,4,6-trichlorophenyl)-
methyl (TTM-1Cz) radical, is used as an emitter, to circumvent
the transition problem of triplet. For TTM-1Cz, there is only
one unpaired electron in the highest singly occupied molecular
orbital (SOMO). When this electron is excited to the lowest
singly unoccupied molecular orbital (SUMO), the SOMO is
empty. Thus, transition back of the excited electron to the
SOMO is totally spin-allowed. Spectral analysis showed that
electroluminescence of the OLED originated from the electron
transition between SUMO and SOMO. The magneto-electro-
luminescence measurements revealed that the spin configura-
tion of the excited state of TTM-1Cz is a doublet. Our results
pave a new way to obtain 100% internal quantum efficiency of
OLEDs.
harvest both singlet and triplet excitons,[2d] and a very high
efficiency has been reported.[2f]
In this work, different from the fluorescent, phosphor-
escent, and TADF-based materials, all of which are closed-
shell molecules, an open-shell organic molecule was used as
an emitter of OLEDs. For a ground-state closed-shell
molecule, there are two electrons in the highest occupied
molecular orbital (HOMO). When the molecule is excited,
one electron stays in the HOMO and the other electron
transits to the lowest unoccupied molecular orbital (LUMO).
The spin configuration of the two electrons is either singlet or
triplet, as shown in Figure 1a.[4] According to the Pauli
O
rganic light-emitting diodes (OLEDs) have been
expected to be the next flat panel displays and lighting
sources because of their special features, such as easy
processing, lightweight, and flexibility.[1] During the evolution
of OLEDs over the past two decades, many fluorescent and
phosphorescent materials have been developed.[1,2] Although
fluorescent materials are cost-effective, the theoretical upper
limit of internal quantum efficiency (IQE) of fluorescent
OLEDs is only 25% according to the spin statistics.[3] In
contrast, the IQE of phosphorescent OLEDs has achieved
almost 100%.[2a–c] However, practically useful phosphores-
cent materials are mainly concentrated to the expensive Ir
and Pt complexes. Recently, thermally activated delayed
fluorescence (TADF)-based OLEDs have been proposed to
Figure 1. Schematic diagram of the spin configuration of the excited
states. a) For closed-shell molecules, the spin configuration of the
excitons can be either singlet or triplet. b) For open-shell molecules,
the spin configuration is doublet.
exclusion principle, the transition of a triplet exciton to the
ground state is forbidden.[3a] However, if the emitting
materials are open-shell molecules, the problem of triplets
can be circumvented. Because for an ground-state open-shell
molecule, there is only one electron in the highest molecular
orbital, that is, the singly occupied molecular orbital
(SOMO).[5] When this electron is excited to the lowest
singly unoccupied molecular orbital (SUMO), the SOMO is
empty, as shown in Figure 1b. Thus, transition back of the
excited electron to the SOMO is spin-allowed. So that the
upper limit of internal quantum efficiency (IQE) of the
OLEDs using open-shell molecules as emitter is theoretically
100%. Because one electron has two spin states, the excited
state of the open-shell molecules is called doublet.
[*] Q. Peng,[+] A. Obolda,[+] Prof. Dr. F. Li
State Key Laboratory of Supramolecular Structure and Materials
Institute of Theoretical Chemistry, Jilin University
Qianjin Avenue 2699, Changchun, 130012 (P. R. China)
E-mail: lifeng01@jlu.edu.cn
Prof. Dr. M. Zhang
State Key Laboratory of Supramolecular Structure and Materials
College of Chemistry, Jilin University
Qianjin Avenue 2699, Changchun, 130012 (P. R. China)
[+] These authors contributed equally to this work.
[**] This work was supported by the the Ministry of Science and
Technology of China (grant number 2015CB655000) and the
National Natural Science Foundation of China (grant numbers
61275036, 21221063, and 91233113) and Graduate Innovation Fund
of Jilin University (project number 2014012).
Neutral radicals are one typical kind of open-shell
molecules.[6] Generally, neutral radicals are quite unstable.
However, through the molecular design, stable neutral
radicals can be obtained and they are able to withstand
oxygen and light for an extremely long time at room
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201500242.
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temperature.[7] They are commonly studied in the fields of
photophysics,[8] molecular magnetic materials,[9] spintron-
ics,[10] and so forth, because the unpaired electron can easily
take part in physical processes and chemical reactions.[6] Here,
for the first time, we demonstrate a stable neutral p radical,
(4-N-carbazolyl-2,6-dichlorophenyl)bis(2,4,6-trichlorophe-
nyl)methyl (TTM-1Cz), as an emitter of OLEDs. For the
details of the synthesis, characterization and purity of the
material, please see the Supporting Information.
The chemical structure of TTM-1Cz is shown in Figure 2a.
Figure 2b shows the steric configuration of the molecule
optimized by DFT calculation. As can be seen, TTM-1Cz is
stabilized by the six chlorine atoms surrounding the radical
center carbon. The electron paramagnetic resonance (EPR)
spectrum (Figure 2c) shows that there is an unpaired electron
tion. In addition, we carried out EPR and nuclear magnetic
resonance (NMR) measurements of the deposited sample.
The EPR and NMR spectra of the deposited sample show
that the chemical structure of TTM-1Cz was unchanged after
vacuum deposition (Figures S2 and S3 in the Supporting
Information).
Energy levels of the frontier molecular orbitals (i.e.
HOMO, SOMO, SUMO, and LUMO) were identified by
density functional theory (DFT) calculations using the
Gaussian 09 series of programs.[11] As presented in Figure 3a,
Figure 3. Molecular orbital energy levels and spectra analysis. a) The
energy levels (left panels) and contour plots (right panels) of the
molecular orbitals (LUMO, SUMO, SOMO, and HOMO) of TTM-1Cz.
b) The UV/Vis Abs and PL spectra of TTM-1Cz in chloroform solution.
the energy levels of HOMO, SOMO, SUMO, and LUMO
were calculated to be ꢀ5.94, ꢀ5.47, ꢀ3.37, and ꢀ1.43 eV,
respectively. The absorption (Abs) and photoluminescence
(PL) spectra of TTM-1Cz in chloroform solution are shown in
Figure 3b. In the Abs spectrum, there is a band centered at
approximately 600 nm corresponding to the energy gap of
2.07 eV, which is assigned to the electronic transition from
SOMO to SUMO. The shorter-wavelength bands of the Abs
spectrum are attributed to the transitions from SOMO/
HOMO to higher-energy orbitals. The PL spectrum (centered
at 660 nm) demonstrated that the emission was originated
from the transition of SUMO to SOMO, that is, the radiative
decay from the doublet excitons.
Figure 2. Material characteristics. a) The chemical structure of TTM-
1Cz. b) The steric configuration of TTM-1Cz calculated by DFT. The
unpaired electron is surrounded by six chlorine atoms. c) The EPR
spectrum of TTM-1Cz powder measured at room temperature. d) The
TGA spectrum of TTM-1Cz.
The thin film of pure TTM-1Cz is not emissive, because of
the aggregation-caused quenching. Thus the only way to make
TTM-1Cz emissive is doping it into some matrix. We then
fabricated OLEDs using the structure of indium tin oxide
in TTM-1Cz. One fundamental issue that needs to be
addressed is the stability of the molecule. In our OLED,
thin films of TTM-1Cz are fabricated using the method of
organic molecular beam deposition. During fabrication the
organic powder sample is heated in a high vacuum chamber
until it sublimes. Most standard molecular semiconductors are
thermally stable and do not chemically decompose during the
deposition procedure. However, organic radicals are compa-
ratively less stable, and one cannot take for granted that
TTM-1Cz remains chemically unchanged upon heating under
high vacuum. In other words, it must be experimentally
proved that the chemical structure of TTM-1Cz in thin films
after deposition should be the same as that drawn in
Figure 2a. In view of these, we carried out thermogravimetric
analysis (TGA) of the pristine material (see Figure 2d), which
shows that TTM-1Cz can be vacuum-deposited (at approx-
imately 908C) to fabricate OLEDs without any decomposi-
(ITO)/N,N’-di-1-naphthyl-N,N’-diphenylbenzidine
(30 nm)/TTM-1Cz:4,4-bis(carbazol-9-yl)biphenyl
(NPB)
(CBP)
(5wt%, 40 nm)/1,3,5-tri(phenyl-2-benzimidazolyl)-benzene
(TPBi) (35 nm)/lithium fluoride (LiF) (0.8 nm)/aluminum
(100 nm), as schematically depicted in Figure 4a, where
NPB, CBP, and TPBi serve as the hole-transporting, doping-
host, and electron-transporting materials, respectively. Fig-
ure 4b shows the electroluminescence (EL) spectrum of the
OLED (at 7 V) accompanied by the PL spectra of the TTM-
1Cz:CBP (5wt%) thin film. It can be found from Figure 4b
that the PL and EL come from the radiative decay of the
doublets of TTM-1Cz. The EL is broader compared to the PL,
because of some effects from the electrical excitation. There is
a tiny emission band in the blue region (< 500 nm) of the EL
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a conventional fluorescent material, NPB.
The device structure of the OLED is ITO/
NPB (30 nm)/CBP (10 nm)/TTM-1Cz:CBP
(5wt%, 40 nm)/TPBi (35 nm)/LiF (0.8 nm)/
Aluminum (100 nm). In this OLED, the
CBP layer of 10 nm inserted between the
two emission layers was used to avoid the
influence of the triplets of NPB on TTM-
1Cz. A short-wavelength-pass filter chopped
at 540 nm and long-wavelength-pass filter
chopped at 550 nm were used to separate the
blue emission of NPB and red emission of
TTM-1Cz. The EL spectra of the total
emission, the blue emission, and the red
emission are shown in Figure 5a. As the
magnetic field increased from 0 to 200 mT
with a step of 20 mT (Figure 5b), the EL
intensity of NPB and TTM-1Cz was mea-
sured. Figure 5c shows the blue emission
from NPB when the magnetic field was
applied. An obvious MEL can be detected,
indicating the existence of triplet excitons in
the NPB layer. Whereas no MEL was
observed for the red emission from TTM-
1Cz, as shown in Figure 5d. The results
demonstrate that there is no triplet exciton
in the TTM-1Cz layer. The spin configura-
tion of the excited states of TTM-1Cz in the
OLED is neither singlet nor triplet, but
doublet.
Figure 4. Device performance. a) The schematic diagram of the structure of TTM-1Cz-
based OLEDs. b) The EL spectrum (7 V) of the OLEDs accompanied by the PL spectra of
the doped thin film. The inset shows a photograph of the TTM-1Cz-based OLEDs operating
at 7 V. c) The J–V–L characteristics of the OLEDs. d) The hEQE of the OLEDs as a function
of voltage. The inset shows the lifetime of the excited states of TTM-1Cz in toluene
solution.
spectrum, which originates from the emission of CBP and
NPB. The intensity of the blue band increased with the
voltage (Figure S4). A photograph of the OLED at 7 V is
given as an inset of Figure 4b. For photographs of the OLED
at other voltages, please see the Supporting Information
(Figure S5). The current density–voltage–luminance (J–V–L)
characteristics of the OLED are shown in Figure 4c. Fig-
ure 4d shows the external quantum efficiency (hEQE) of the
OLED as a function of voltage. The maximum hEQE is 2.4%,
which lies in the same level of most deep-red/near-infrared
OLEDs (Table S2). For the device performance of the TTM-
1Cz-based OLED with other structure, please see the
Supporting Information (Figures S6–S11 and Table S1).
In an effort to confirm that the emission of TTM-1Cz
originated from the radiative decay of the doublets, we
carried out the magneto-electroluminescence (MEL) mea-
surement. The MEL describes the change of the EL of an
OLED when an external magnetic field of tens of millitesla
was applied. There are several mechanisms underlying the
MEL in OLEDs, such as the electron–hole pair mechanism
and the triplet–triplet annihilation mechanism.[13] However,
each of the mechanism relates to the triplet excitons.[13] Thus,
if the spin configuration of the excited states of TTM-1Cz is
the same as that of the conventional closed-shell organic
Figure 5. MEL measurements. a) The EL spectrum of the OLED used
in the MEL experiments. The inset shows the separated blue emission
of NPB and red emission of TTM-1Cz. b) The magnetic field applied in
the MEL experiments. c) The EL intensity of the blue part of the OLED
emission at 7 V when the magnetic field was applied. d) The EL
intensity of the red part of the OLED emission at 7 V when the
magnetic field was applied.
fluorescent molecules, that is, the singlet and triplet excitons,
the MEL should be detected. We can verify whether the
emission of the OLED comes from the radiative decay of
a doublet or a singlet through the MEL measurement. Herein
we fabricated a well-designed OLED in which the electro-
luminescence contains the emission from TTM-1Cz and
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Besides TTM-1Cz, another neutral p radical, (TTM-2Cz),
was also synthesized. The parallel electroluminescent experi-
ments of TTM-2Cz were performed and similar results were
obtained, please see the Supporting Information (Figures S12
and S13).
to high performance organic OLEDs based on open-shell
compounds.
Experimental Section
TGA measurement: Thermal gravimetric analysis (TGA) was carried
out on the Pyris1 TGA thermal analysis system at heating rate of
208Cminꢀ1 in nitrogen.
EPR measurements: EPR spectrum of the powder of TTM-1Cz
was measured using the JES-FA200 EPR spectrometer at ambient
temperature.
DFT calculations: The DFT calculations were performed with the
Gaussian09 series of programs using the B3LYP hybrid functional
and 6-31G(d) basis set.
Spectral measurements: For the Abs and PL measurements,
TTM-1Cz was dissolved at a concentration of 1 ꢀ 10ꢀ5 molLꢀ1. Then
the spectra were measured using a UV/Vis spectrophotometer
(Shimadzu UV-2550) and a spectrofluorophotometer (Shimadzu
RF-5301PC).
Lifetime measurements: For the lifetime measurements, an
Edinburgh fluorescence spectrometer (FLS980) was used. Then the
lifetime of the excited states was measured by the time-correlated
single-photon-counting method (detected at the peak of the PL)
under the laser excitation at 375 nm and a pulse width of 50 ps.
OLED fabrication and characterization: The OLEDs were
fabricated by the multiple source organic molecular beam deposition
method at 2 ꢀ 10ꢀ4 Pa. The current density–voltage (J–V) character-
This study paves a new way to obtain 100% IQE of an
OLED. However, the following aspects of open-shell mole-
cules still need to be improved to achieve high efficient and
stable OLEDs. I) The stability: It was reported that by
carefully designing the molecular structure, some open-shell
molecule can maintain its properties for almost 30 years.[14]
II) The PL efficiency: For TTM-1Cz, the PL lifetime is 25.2 ns
(see the inset in Figure 4d), which is relatively long thus the
radiative transition may be not totally allowed. For a mole-
2
cule, the probability of the radiative transition, Rij , can be
expressed as shown in Equation (1).[3a]
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢀ
ꢁ
ꢀ
ꢀ
2
2
2
R2ij / jhyeijMjyej jhcvijcvj jhysijysj
ð1Þ
Here yei and yej denote the initial and final electronic-state
wavefunctions, respectively. M is the dipole moment operator.
cvi and cvj denote the initial and final vibrational state
wavefunctions, respectively. ysi and ysj are the spin wave-
functions of the initial and final states, respectively. In the
right side of Equation (1), the first term refers to the dipole
selection rule. For a dipole allowed transition, yei and yej
should be of opposite symmetry with respect to the inversion
operator; the second term refers to the Franck–Condon
factor, which depends on the vibrational overlap integrals; the
third term refers to the spin selection rule. Spin-allowed
transition can only occur between states with the same spin
quantum number. If any of the three terms is small, the
transition could be partially forbidden. For TTM-1Cz, the
transition is clearly spin-allowed, thus the relatively slow
transition rate is attributed to the first and second terms,
which correlate with the molecular structure. Therefore, to
enhance the PL efficiency of open shell molecules, research-
ers should make efforts to rationally design the molecular
structure.
istics were measured by
a Keithley 2400 source meter. The
luminance–voltage (L–V) characteristic and the EL spectrum were
measured by a PR650 spectroradiometer. For measuring the spectrum
of the blue emission of the OLED used in the MEL measurements,
a high-sensitivity Maya2000 Pro Spectrometer was used.
MEL measurements: After fabrication, OLEDs were immedi-
ately placed on a Teflon stage between the poles of an electromagnet
with the magnetic field perpendicular to the current. A Keithley 2612
sourcemeter was used to provide a constant voltage from channel A.
The emission of the devices was collected by an optic fiber (2 m)
connected to a Hamamatsu photomultiplier (H10721-20). The photo-
multiplier was connected to channel B of the Keithley 2612 to record
the EL intensity. In order to eliminate the interference of the
magnetic field on the photomultiplier, we placed the photomultiplier
far away from the electromagnet.
Keywords: doublet · luminescence · open-shell molecules ·
In summary, we have fabricated OLEDs using stable
neutral p radicals, TTM-1Cz and TTM-2Cz, as emitters.
There is only one electron in the SOMO of the two open-
shell molecules. This feature renders the excited state of the
open-shell molecules to doublet. The key issue of harvesting
the triplet energy in an OLED is circumvented, as the
radiative decay of the doublet is totally spin-allowed. In the
TTM-1Cz- and TTM-2Cz-based OLED, the emission was
confirmed to be from the electronic transition from SUMO to
SOMO by the spectral analysis. The spin configuration of
excited states of TTM-1Cz is doublet according to the
magneto-electroluminescence measurements. The maximum
hEQE of the TTM-1Cz-based OLED was achieved to be of
2.4%, which is comparable to most deep-red/near-infrared
OLEDs. We believe the device performance will be further
improved when better emitting open-shell molecules and
more effective device structures are found. Nevertheless,
using neutral p radicals as emitter to fabricate OLED paves
a new way to obtain 100% IQE of OLEDs. We anticipate that
our work will be a starting point for further research, leading
organic light-emitting diodes · radicals
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Chen, F. Li, Y. Ma, Adv. Opt. Mater. 2013, 1, 362 – 366.
[14] O. A. Rakitin, Russ. Chem. Rev. 2011, 80, 647 – 659.
Received: January 11, 2015
Revised: April 7, 2015
Published online: && &&, &&&&
Angew. Chem. Int. Ed. 2015, 54, 1 – 6
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

.
Angewandte
Communications
Communications
Light-Emitting Diodes
Organic electronics: An organic open-
shell molecule (TTM-1Cz) was used as an
emitter in organic light-emitting diodes
(OLEDs). There is only one electron at the
highest occupied molecular orbital (see
picture), leading to the excited state of
TTM-1Cz to be a doublet. The key issue of
how to harvest the triplet energy in an
OLED is thus circumvented, because the
radiative decay of the doublet is totally
spin-allowed.
Q. Peng, A. Obolda, M. Zhang,
F. Li*
&&&& — &&&&
Organic Light-Emitting Diodes Using
a Neutral p Radical as Emitter: The
Emission from a Doublet
6
www.angewandte.org
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2015, 54, 1 – 6
These are not the final page numbers!