A pure red luminescent β-carboline-substituted biphenylmethyl radical: Photophysics, stability and OLEDs

Article abstract of DOI:10.1039/c8tc04298e

Luminescent radicals, whose emission comes from the doublet exited state, have potential application in the field of organic optoelectronics. However, few radicals show luminescence at room temperature. Herein, a new pure red-emissive biphenyl-type (N-pyrido[3,4-b]indolyl)bis(2,4,6-trichlorophenyl)methyl radical (PyID-BTM) is designed and synthesized, in which the carbazole moiety of CzBTM (a previously reported biphenylmethyl radical by our group) is successfully replaced by β-carboline. Its photophysical properties, including the ground and excited states, and the radiative and non-radiative processes are systematically investigated. The photoluminescence quantum efficiency ( = 19.5%) of PyID-BTM is ten times higher than that of CzBTM ( = 2.0%) in cyclohexane. The crystal structure, magnetic properties, photostability and electroluminescence performance of PyID-BTM are studied. An optimized OLED device using PyID-BTM as an emissive dopant showed pure red emission with CIE coordinates of (0.649,0.317) and a maximum EQE of 2.8%, and the formation ratio of doublet excitons was up to 70%. This study provides a new approach for designing high-performance luminescence biphenylmethyl radicals for applications in organic electroluminescence.

Full text of DOI:10.1039/c8tc04298e

View Article Online
View Journal
Journal of
M at erials Chemistry C
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: A. Abdurahman, Y.
Chen, X. Ai, O. Ablikim, Y. Gao, S. Dong, B. Li, B. Yang, M. Zhang and F. Li, J. Mater. Chem. C, 2018, DOI:
1
0.1039/C8TC04298E.
Volume
4
Number
1
7
January 2016 Pages 1–224
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
Journal of
Materials Chemistry C
Materials for optical, magnetic and electronic devices
www.rsc.org/MaterialsC
Accepted Manuscripts are published online shortly after
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
author guidelines.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the ethical guidelines, outlined
in our author and reviewer resource centre, still apply. In no
event shall the Royal Society of Chemistry be held responsible
for any errors or omissions in this Accepted Manuscript or any
consequences arising from the use of any information it contains.
ISSN 2050-7526
PAPER
~
Nguyên T. K. Thanh, Xiaodi Su et al.
Fine-tuning of gold nanorod dimensions and plasmonic properties using
the Hofmeister effects
rsc.li/materials-c

Page 1 of 8
J Po lue ran sael od fo Mn aot te rai ad l jsu Cs th em mai rs gt ri ny sC
View Article Online
DOI: 10.1039/C8TC04298E
Journal Name
COMMUNICATION
A Pure Red Luminescent β-Carboline-Substituented Biphenylmethyl
Radical: Photophysics, Stability and OLEDs
Received 00th January 20xx,
Accepted 00th January 20xx
Alim Abdurahman, Yingxin Chen, Xin Ai, Ablikim Obolda, Yu Gao, ShengZhi Dong, Bao Li, Bing
Yang, Ming Zhang and Feng Li*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Luminescent radicals, whose emission comes from the doublet harvesting of triplet exciton is circumvented in radical-based
exited state, have potential application in the field of organic OLEDs.13
optoelectronics. However, there are few radicals show
To date, there are few radicals show luminescence at
luminescence at room-temperature, Herein, a new pure red- room-temperature, typical examples are triphenylmethyl
emissive biphenyl-type (N-pyrido[3,4-b] indolyl) bis(2,4,6- radicals, such as the perchloro triphenylmethyl (PTM)-cored
trichlorophenyl) methyl radical (PyID-BTM) is designed and radicals obtained by Lambert and co-workers;14,15 the tris(2,4,6-
synthesized, in which the carbazole moiety of CzBTM trichlorophenyl) methyl (TTM) series radicals reported by Juliá
1
6,17
(
previously reported biphenylmethyl radical by our group) is et al.;
successfully replaced by β-carboline. Its photophysical synthesized by Nishihara and co-workers.
properties, including the ground and excited state, radiative It is an important and challenging task to develop a new
and non-radiative processes are systematically investigated. stable room-temperature luminescent radical system for
Photoluminescence quantum efficiency ( =19.5%) of PyID- extending the application of radicals in organic optoelectronics.
BTM is ten times higher than that of CzBTM ( =2.0%) in To our delight, we have recently reported a novel stable room-
cyclohexane. The crystal structure, magnetic property, temperature luminescent biphenylmethyl radical, (N-carbazolyl)
, pyridyl-containing triphenylmethyl radicals (PyBTM)
1
8,19


20
photostability and electroluminescent performance of PyID- bis (2,4,6-trichlorophenyl)-methyl radical (CzBTM), which has
BTM are studied. The optimized OLED device using PyID-BTM opened a new page of the luminous radical system. However,
as an emissive dopant shows a pure red emission with CIE although CzBTM has high thermal and photostability, the
coordinates of (0.649, 0.317), maximum EQE of 2.8% and the fluorescence quantum yield in cyclohexane is only 2.0%.
formation ratio of doublet excitons is up to 70%. This study Through calculation, we found that the non-radiative decay of
provides a new approach to design high-performance CzBTM is relatively large, which is probably due to strong
luminescence biphenylmethyl radicals for application on vibronic coupling between the carbazole and the central radical.
organic electroluminescence.
Since the carbazole is a medium electron-donating group, what
would happen if the weak electron-donating group attached to
bis(2,4,6-trichlorobennyl) methyl instead of carbazole? Can this
change reduce the vibronic couplings of molecules and thus
Introduction
Owing to their unique chemical and physical properties, open- improve the photophysical properties of the biphenylmethyl
shell organic radicals have been extensively studied for versatile radicals? Because the reaction mechanism for synthesis of
1
-4
5-7
application in organic spintronics, organic magnetism,
biphenylmethyl radical is rather special, many of the donor
moieties we tried cannot react with 2,2´- (bromomethylene)
8
9
10
energy storage, non-linear optics, catalysts and biological
imaging agents. Recently, luminescent radicals have attracted bis(1,3,5-trichlorobenzene) (HBTM-Br) by this reaction
more attentions due to their emission from the spin-allowed mechanism. We have found that the success of this reaction
11
1
2
doublet exited state (D
1
),
D
1

D
0
transition. Hence, the could be closely related to the activity of the hydrogen at the
nitrogen in the substituent group.
β-carboline moiety is composed of conjugated pyridine
and indole unit and has better electron transporting properties
State Key Laboratory of Supramolecular Structure and Materials, College of
Chemistry, Institute of Theoretical Chemistry, Jilin University, Qianjin Avenue
than carbazole due to electron deficiency of the pyridine unit.
The good electron deficiency of the β-carboline could balance
the holes and electrons, and improves the quantum efficiency
2699, Changchun, 130012, P. R. China. E-mail: (lifeng01@jlu.edu.cn)
†
Electronic Supplementary Information (ESI)available. See DOI: 10.1039/
x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
Please do not adjust margins

J Po lue ran sael od fo Mn aot te rai ad l jsu Cs th em mai rs gt ri ny sC
Page 2 of 8
COMMUNICATION
Journal Name
2
1
of the OLEDs. In this work, we successfully reacted β-carboline radical PyID-BTM is paramagnetic with weak spin-spin
View Article Online
with HBTM-Br and obtained pure red-emissive biphenyl-type interactions. The measured value of the CDuOriI:e1c0o.1n03s9ta/Cn8tTCC=004.239885E
radical (N-pyrido[3,4-b] indolyl) bis(2,4,6-trichlorophenyl) is in good agreement with the expected value of 0.375 emu mol
-
1
methyl (PyID-BTM). To our knowledge, PyID-BTM is the first
example of the biphenylmethyl radical that successfully links
the weak electron-donating group, and the secondly reported
biphenylmethyl-type radical. A comparative photophysical
studies on PyID-BTM and CzBTM reveals that PyID-BTM shows
ten times higher photoluminescence quantum efficiency
for a S=1/2 organic radical.
(=19.5%) than that of CzBTM (=2.0%) in cyclohexane. We also
studied the magnetic and photostability of PyID-BTM. It is worth
noting that the OLED based on PyID-BTM (played as an emissive
dopant) exhibits pure red color index of (0.667, 0.330) and
maximum EQE of 2.8%.
Result and Discussion
Synthesis and Characterization
Scheme 1 shows the synthesis route of intermediates and target
radical PyID-BTM. The synthetic details of compounds were
provided in the supporting information. The 1,2,3,4-
Tetrahydro-β-carboline (M1) was obtained by Pictet-Spengler
cyclization of
tryptamine with formaldehyde. Then the (M1) was oxidized to
β-carboline (PyID) by refluxing with Pd-C in xylenes. The HBTM-
Br attained by tow step reaction with high yield according to our
2
0
previous report. Then, PyID-BTM was synthesized via same
reaction mechanism with CzBTM and was characterized by
Fig. 1 a) Molecular structure of crystalline PyID-BTM with thermal ellipsoids set at 50%
probability. b) EPR spectrum of PyID-BTM in cyclohexane solution at 298 K. c) The weak
interactions in the crystal.
Scheme 1 Synthesis rout of biphenylmethyl radical PyID-BTM.
m
Fig. 2 Observed (circle) and fitted (line)  -T curve of PyID-BTM.
MALDI-TOF mass spectrometry, elemental analysis, and Fourier
transform infrared spectroscopy (FTIR) (see in ESI†). The
molecular structure of radical was determined by single crystal
X-ray diffraction, (Fig. 1a; Table S1 ESI†). The existence of spin
S=1/2 confirmed by Electron Paramagnetic Resonance (EPR)
spectroscopy at room temperature (Fig. 1b). In addition, the
magnetic susceptibility measurements for PyID-BTM powder
carried out with a superconducting quantum interference
device (SQUID) magnetometer over the range 2-300K using a
X-ray Crystallographic Analysis
A single crystal of PyID-BTM was obtained by slow evaporation
from the chloroform/ethanol solution at room temperature.
The single crystal X-ray diffraction shows that the central methyl
2
carbon atoms are sp hybridized and shielded sterically by four
halogen atoms (Fig. 1a), and C7 and its three neighboring atoms
N1, C6 and C8 lie in the same plane. The three aromatic rings
are held in a propeller-like conformation due to steric hindrance
from the chlorine atoms. The bond angles and dihedral angles
are similar to those of CzBTM (Table S2†). The crystal is
1
000 Oe field strength. Fig. 2 shows the plot of M-T curve of
PyID-BTM. The susceptibility reasonably follows a curie-Weiss
law, the best fit giving Weiss constant θ=- 4.8 K, and a Curie
-
1
constant C= 0.385 emu mol , the result indicates that the
2
| J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 3 of 8
J Po lue ran sael od fo Mn aot te rai ad l jsu Cs th em mai rs gt ri ny sC
Journal Name
COMMUNICATION
constructed by multiple CH···π, CH···N and CH···Cl interactions, orbitals (LUMO;137α and 137β) are mainly delocalized over the
View Article Online
DOI: 10.1039/C8TC04298E
which are summarized in Fig. 1c.
two benzene rings which are similar to those of CzBTM.
However, the α and β- highest occupied molecular orbitals
(HOMO 135α and 135β) are delocalized over the two benzene
rings and carboline moieties, which is different from CzBTM
Thermal and Electrochemical Properties
The thermal property of the PyID-BTM was carried out with where the HOMOs are mainly delocalized over the carbazole
thermogravimetric analysis (TGA) under a nitrogen atmosphere moiety. The DFT results of the decreasing in energy levels after
(
see Fig. S3 in ESI
corresponding to 5% weight loss) for PyID-BTM is 241
which is higher than that of CzBTM (230 ). The good thermal the photophysical properties of the radical.
†
). The decomposition temperatures (Td replacement of β-carboline are in agreement with the
℃
，
electrochemical results. We expect that these could improve
℃
stability of PyID-BTM could be caused by the rigid molecular
structure and strong intermolecular interaction imparted by Photophysical Properties.
nitrogen atoms at the pyridine unit.
The energy level of the frontier orbitals (α-SOMO and β-SUMO) Fig. 4a shows the UV/Vis absorption and emission spectra of
of PyID-BTM is examined by cyclic voltammetry (CV) in CH
2
Cl
2
PyID-BTM in cyclohexane. For comparison, the photophysical
with 0.1 M TBAPF as a supporting electrolyte. The results of CV properties of the CzBTM were also measured. The summarized
6
measurements are shown in Fig. S4. The energy level of the α- photophysical data of PyID-BTM and CzBTM are listed in Table
SOMO of PyID-BTM is obtained to be -4.83 eV from the 1 PyID-BTM exhibits three main absorptions bands, a strong
oxidation potential, which is lower than the α-SOMO level of absorption band at 260 nm caused by β-carboline is 30 times
CzBTM (-4.71 eV). From the reduction potential of CV curves, higher than the molar extinction coefficient () of the band at
the β-SUMO level of PyID-BTM and CzBTM obtained to be -3.76 284 nm caused by carbazole in CzBTM, and also has a blue shift
eV and -3.71 eV respectively. These data indicate that the of 24 nm; medium absorption band at 383 nm is a blue shift of
replacement of β-carboline leads to lowering the frontier 4 nm and 2.3 times higher  compared with those of CzBTM, and
energy levels of the radical PyID-BTM. These results were a weak and broad absorption band with two peaks at 510 nm
further confirmed by DFT calculation which is shown in the next and 550 nm, which are blue-shifted by 4 nm compared to the
section. We also performed 20 cycles of CV scan on PyID-BTM two peaks of CzBTM at 514 nm and 554 nm, and
 are also 2.3
and there is no any change that can be found in the redox peaks and 2.1 times higher, respectively. Time-dependent DFT
(
also see Fig. S5 in ESI
†).
calculation (TD-DFT) results shows that the medium (383 nm)
and weak absorption bands (510 and 554 nm) could be
attributed to the transition from 133β to 136β, 134β to 136β
and 135β to 136β, respectively. PyID-BTM in cyclohexane
displayed one emission band (664 nm), upon excitation at 550
nm, which is blue-shifted by 33 nm compared with those of
CzBTM (697 nm). Absolute photoluminescence quantum yield
(
) of PyID-BTM in cyclohexane solution is 19.5%, which is ten
times higher than that of CzBTM ( =2.0%). We also performed

the transient PL decay measurements of the PyID-BTM in
cyclohexane solution (Fig. 4b). The measurement results
revealed that PyID-BTM showed a single exponential decay with
a lifetime ( of 12.8 ns, which was three times longer than that
of CzBTM (4.0 ns), which leads to improved photoluminescence
1
9
quantum yield. According to the results of
radiative rate constants (k ) and non-radiative rate constants
nr) of PyID-BTM and CzBTM are calculated using basic
photophysical eqn (1) and (2), (Table 1).
= 풌 /(풌 + 풌 )
 and , the
Fig. 3 Frontier orbitals of PyID-BTM and CzBTM using DFT methods (UB3LYP/6-31G(d,p)).
r
(
k
Density Functional Theory Simulation

(1)
풓
풓
풏풓
흉 = ퟏ/(풌 + 풌 )
풓 풏풓
(2)
Frontier molecular orbitals (MOs) were calculated using DFT
methods (UB3LYP/6-31G(d,p)). The spin density distributions of
PyID-BTM are similar to those of CzBTM (see Fig. S6 in ESI†). The
calculated MOs results show (Fig. 3) that the α-SOMO and β-
SUMO (136α and 136β) of PyID-BTM and CzBTM are mainly
delocalized on the central carbon atom and extended to the
aromatic rings. The energy level of the α-SOMO of PyID-BTM (-
The calculation results show that the k
three times higher than and one quarter of those of the CzBTM,
respectively. These two factors can explain why PyID-BTM
r
and knr of PyID-BTM are
exhibits good photoluminescence efficiency. The increase of k
of PyID-BTM is due to the higher molar extinction coefficient
oscillator strength) at the excited transition (Fig. 4a, Table 1
and Fig. S7, S8 in ESI ). The TD-DFT calculation results also
r
(
†
5
.15 eV) is lower than that of CzBTM (-4.99 eV) and the β-SUMO
proved that there is the higher oscillator strength in PyID-BTM.
The decrease of knr of PyID-BTM can be explained by energy gap
law. The replacing the carbazole moiety by an electron-
of PYID-BTM (-3.13 eV) is also lower than that of CzBTM (-3.01
eV). In PyID-BTM, the α and β- lowest unoccupied molecular
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
Please do not adjust margins

J Po lue ran sael od fo Mn aot te rai ad l jsu Cs th em mai rs gt ri ny sC
Page 4 of 8
COMMUNICATION
Journal Name
View Article Online
DOI: 10.1039/C8TC04298E
-
6
Fig. 4 a) Absorption and emission spectra of PyID-BTM and CzBTM in cyclohexane solution (10 M) (the inset shows the photographs of the PyID-BTM (left) and CzBTM (right) in
-
5
cyclohexane solution (10 ) under irradiation (365 nm) with an UV lamp at room temperature. b) PL decay of PyID-BTM in cyclohexane at ambient condition. c) Normalized
absorption and emission spectra of PyID-BTM in various solvents (10-5 M) at room temperature. d) Linear correlation of orientation polarization (Δ) of solvent media with the

f
Stokes shift   ; a: absorbed light; f: fluorescence) for PyID-BTM (see Table S3 for the solvents and corresponding data).
withdrawing unit β-carboline, leads to increasing the D
0
-D
1
photoluminescence (PL) are measured in various solvents with
energy gap of PyID-BTM, which was confirmed by the DFT different polarities. As displayed in Fig. 4c and Table S3, the UV-
calculation, absorption, and emission spectra. The large energy vis absorption does not change much upon increasing solvent
gap of the PyID-BTM could reduce the Frank-Condon factors, polarity, thus indicating a rather small dipolar change at the
2
2
which indicates a decrease of internal conversion, as a result, ground state in different polar solvents. However, the PL
nr decreases. In order to further support this conclusion, spectra displays obviously bathochromic shift and decreases the
internal conversion of the radicals is calculated by the MOMAP emission intensity as the solvent polarity is increased. To further
k
(
Molecular
Materials
Property
Prediction
Package) understand the effect of solvent polarity on the excited state of
2
3,24
program.
The calculation results show that the internal PyID-BTM, we used the Lippert-Mataga model. Fig. 4d shows
11 -1
conversion of PyID-BTM (2.60 ×10 S ) is two times slower than the plot of the orientation polarizability (Δ
that of CzBTM (5.16 ×10 S ).

) versus the Stokes
). The linear fitting results showed that, PyID-BTM
state) display two section linear relations in the low-polarity solvents
and <0.15) and high-polarity solvents ( >0.15). The dipole
1
1
-1
shift 
f

To investigate the frontier orbital (D
0
and D
1
properties
of
PyID-BTM,
UV-vis
absorption
(


Table 1 Photophysical parameters of PyID-BTM and CzBTM in cyclohexane.
a)
-1
-1
a)
b)
c)
6
-1
6
-1
molecul
CzBTM
λ
abs /nm (ε/ M cm )
λ
pL (nm)
Φ
f
τ (ns)
k
r
(×10 s )
K
nr(×10 s )
4
3
284(1.42×10 )/387(8.99×10 )
14(2.24×10 )/554(2.47×10 )
3
3
697
664
0.020
0.195
4.0
5.0
245
64
5
5
4
PyID-BTM
260(4.36×10 )/383(2.11×10 )
3 3
12.8
13.9
5
10(5.17×10 /550(5.29×10 )
a)
Measured in cyclohexane at room temperature. b) Relative PLQYs in were measured using measured by integrating spectrophotometer. c) The lifetimes in cyclohexene were
measured using Edinburgh fluorescence spectrometer (FLS980) under the laser excitation at 378.8 nm and a pulse width of 68.9 ps.
4
| J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 5 of 8
J Po lue ran sael od fo Mn aot te rai ad l jsu Cs th em mai rs gt ri ny sC
Journal Name
COMMUNICATION
moment calculated to be 5.3 D and 10.6 D for the low and high
regions, respectively. Due to the small dipole moment and
higher photoluminescence quantum yield are appeared in the
low Δ regions, we believed that the emission of the PyID-BTM
View Article Online
DOI: 10.1039/C8TC04298E
Δ

mainly comes from the locally excited (LE ) state in the low-
polarity solvents. The large dipole moment and low
photoluminescence quantum yield of PyID-BTM in the region of
high Δ indicating that a charge transfer (CT) state is dominant
in the high-polarity solvents.
Photostability of radicals
Photostability, one of the important property of luminescent
radicals, was measured by recording the decay of fluorescence
intensity in cyclohexane solution with irradiation of a pulse laser
2
at 355 nm with a power density of 366.4 kW/cm . The results
show that (Fig. 5 a), PyID-BTM displays excellent photostability
as same as CzBTM and 160 times more stable than that of TTM.
To best of our knowledge, the estimated half-life (t1/2) of PyID-
BTM is the outstanding value to date for luminescent radicals
with fluorescence quantum yield up to 20%.25,26
Fig. 5 a) Comparison of fluorescence decay of TTM, CzBTM and PyID-BTM in
cyclohexane under irradiation with a 355 nm pulse laser (power density: 366.4
2
kW/cm , pulse width: 8 ns, frequency: 10 Hz). b) Photographs of TTM (left cuvette in
each image) and PyID-BTM (right cuvette) in cyclohexane solution (5×10-5 M) under
irradiation (365 nm) with an UV lamp at room temperature under ambient
conditions.
Electroluminescent Properties
To evaluate the potential application of the PyID-BTM in light-
emitting, OLEDs with the following architecture were fabricated:
1
,1′-biphenyl (CBP): PyID-BTM (x wt.%, 40 nm)/ 2,4,6-tris[3-
(diphenylphosphoryl)phenyl]-1,3,5-triazine
(PO-T2T)
(70
indium tin oxide (ITO)/MoO
3
(3 nm)/ 4,4′-cyclohexylidenebis [N,
nm)/LiF (0.8 nm)/Al(100 nm). In these devices, ITO was the
anode and Al was the cathode, TAPC played the role of hole-
N-bis(p-tolyl) aniline] (TAPC) (70 nm)/ 4,4′-bis(N-carbazolyl)-
Fig. 6 a) The energy-level diagram and the schematic representation of the OLED architecture. b) The EL spectrum (5 V) of the devices with three different doping concentrations
of PyID-BTM in CBP (the inset shows the photograph of the (CBP: PyID-BTM 5 wt%) device operating at 7 V and PL spectra of PyID-BTM doped film 5 wt% in CBP excited at 333
nm). (c) Current density-voltage-luminance (J-V-L) characteristics of the devices. (d) External quantum efficiency versus current density.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 5
Please do not adjust margins

J Po lue ran sael od fo Mn aot te rai ad l jsu Cs th em mai rs gt ri ny sC
Page 6 of 8
COMMUNICATION
Journal Name
Table 2 Electroluminescent performance of devices.
View Article Online
DOI: 10.1039/C8TC04298E
a)
b)
c)
d)
e)
f)
2
g
device
I
wt (%)
Φ
PL (%)
V
on [V]
EQE (%)
λEL
CE (cd/A)
L
max (cd/m )
PE (Im/W)
1.2
CIE(x,y)
2.5
13.7
3.5
4.0
4.5
2.8
2.0
1.3
660
666
672
1.4
0.9
0.5
21.2
31.8
22.5
(0.649, 0.317)
II
5
9.4
0.6
(0.667, 0.330)
III
7.5
7.3
0.3
(0.671, 0.326)
a)
Doping concentration of PyID-BTM in CBP. b) PL quantum yield of the thin film (50 nm) measured by integrating spectrophotometer. c) Turn-on voltage. d) The maximum external
quantum efficiency. e) the maximum EL Wavelength. f) Maximum current efficiency. g) Maximum power efficiency.
transporting layer (HTL), while PO-T2T served as an electron- We prepared a new biphenylmethyl-type stable, luminescent
transporting layer (ETL). MoO and LiF were used to improve the radical PyID-BTM. PyID-BTM exhibited ten times higher
3
charge injection. The PyID-BTM doped into CBP is used as the photoluminescent quantum yield than previously reported
emitting layer (EML), where x is equal to 2.5, 5 and 7.5%. The CzBTM in cyclohexane solution. The preferable luminescent
three different concentrations of the devices are defined as property of PyID-BTM is attributed to the higher molar
device I, device II and device III, respectively. The energy-level extinction coefficient and slower internal conversion resulting
diagram and the schematic representation of the OLED from β-carboline substituent instead of carbazole moiety. PyID-
architecture are shown in Fig. 6a. Important EL data and partial BTM performed excellent photostability, which is comparable
performance curves of devices are presented in Fig. S9 and with CzBTM. The doped OLEDs based on PyID-BTM showed pure
Table 2.
red emission. It is noteworthy that the device with 2.5% doping
As can be seen from Fig. 6b, the three devices display pure concentration of PyID-BTM in CBP host achieved a maximum
red emission, electroluminescence (EL) wavelength peaks at EQE of 2.8% and the formation ratio of doublet excitons of
6
60 nm, 666 nm, and 672 nm, respectively, demonstrating little device is up to 70%. Our study provides a new pathway for
redshift of the EL spectra as increasing the doping concentration desiging high-perfomance luminescence biphenylmethyl
of PyID-BTM in CBP host. The EL spectrum matched well with radicals in organic electroluminescence application.
the PL spectrum of the CBP: PyID-BTM (5wt%) thin film (Fig. 6b),
indicating the EL emission from the device comes from the
radiative decay of the doublets of PyID-BTM. Normalized EL
spectra of device II and III almost keep unchanged as driving
Conflicts of interest
voltage increased from 4.5V to 16 V for devices, however, there
is a tiny wide emission band was arising in the blue-green region
There are no conflicts to declare.
(400 ~550 nm) (Fig. S10), which is relatively obvious in device I.
The blue-green emission could be come from the emission of Acknowledgments
exciplex between CBP and PO-T2T.2
6,27
We are grateful for the financial support from National Natural
The Commission Internationale de L’Eclairage (CIE)
coordinate values of device I is (0.649, 0.317) at 5 V, and those
of device II and III are (0.667, 0.330), and (0.671, 0.326) at 7V,
respectively. which are very close to the red standard CIE
coordinate (0.670, 0.330) requirements defined by National
Television System Committee (NTSC). The current density-
voltage-luminance (J-V-L) characteristics are shown in Fig. 6c.
The turn-on voltages of the three doping concentration devices
is 3.5 V, 4 V, 4.5 V respectively. The results showed that the turn
on voltage depends on the doping concentration.
As can be seen from Fig. 6d, the highest EQE value is
observed from device I (2.8%). The maximum EQE for device II
and III is 2.0%, 1.3%, respectively. If the out-coupling efficiency
is assumed to be 30%, according to spin statistics, the formation
ratio of doublet excitons of the three devices are estimated to
be 68%, 72%, 59%, respectively. Fig. S11 shows the EPR
spectrum of PyID-BTM powder before and after evaporation.
The EPR results indicate that evaporation does not wreak havoc
with the unpaired electrons of PyID-BTM.
Science Foundation of China (Grant No. 51673080) National Key
R&D Program of China (Grant No. 2016YFB0401001) and
National Key Basic Research and Development Program of
China (973 program
MOST.
，Grant No. 2015CB655003) Founded by
Notes and references
1
2
3
4
A. R. Rocha, V. M. Garcia-Suarez, S. W. Bailey, C. J. Lambert, J.
Ferrer, S. Sanvito, Nat. Mater. 2005, 4, 335.
Y. W. Son, M. L. Cohen, S. G. Louie, Phys. Rev. Lett. 2006, 97,
216803.
M. Mastorrent, N. Crivillers, C. Rovira, J. Veciana, Chem. Rev.
2
011, 112, 2506
R. Frisenda, R. Gaudenzi, C. Franco, M. Mastorrent, C. Rovira,
J. Veciana, I. Alcon, S. T. Bromley, E. Burzuri, H. V. D. Zant,
Nano Lett. 2015, 15, 3109.
5
6
K. Awaga, Y. Maruyama, J. Chem. Phys. 1989, 91, 2743.
A. J. Banister, N. Bricklebank, I. Lavender, J. M. Rawson, C. I.
Gregory, B. K. Tanner, W. Clegg, M. R. J. Elsegood, F. Palacio,
Angew. Chem., Int.Ed. 1996, 108, 2648
7
8
M. Chikamatsu, T. Mikami, J. Chisaka, Y. Yoshida, R. Azumi, K.
Yase, A. Shimizu, T. Kubo, Y. Morita, K. Nakasuji, Appl. Phys.
Lett. 2007, 91, 605.
Y. Morita, S. Nishida, T. Murata, M. Moriguchi, A. Ueda, M.
Satoh, K. Arifuku, K. Sato, T. Takui, Nat. Mater. 2011, 10, 947.
Conclusion
6
| J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 7 of 8
J Po lue ran sael od fo Mn aot te rai ad l jsu Cs th em mai rs gt ri ny sC
Journal Name
COMMUNICATION
9
K. Kamada, K. Ohta, T. Kubo, A. Shimizu, Y. Morita, K. Nakasuji,
R. Kishi, S. Ohta, S. Furukawa, H. Takahashi, Angew. Chem.,
Int.Ed. 2007, 46, 3544.
View Article Online
DOI: 10.1039/C8TC04298E
1
1
0 Q. Cao, L. M. Dornan, L. Rogan, N. L. Hughes, M. J. Muldoon,
Chem. Commun. 2014, 50, 4524.
1 M. A. Sowers, J. R. Mccombs, Y. Wang, J. T. Paletta, S. W.
Morton, E. C. Dreaden, M. D. Boska, M. F. Ottaviani, P. T.
Hammond, A. Rajca, Nat. Commun. 2015, 5, 5460.
2 J. Han, Y. Jiang, A. Obolda, P. Duan, F. Li, M. Liu, J. Phys. Chem.
Letters 2017, 38, 5865.
1
1
1
3 Q. Peng, A. Obolda, M. Zhang, F. Li, Angew. Chem., Int.Ed.
2
015, 54, 7091.
4 A. Heckmann, C. Lambert, M. Goebel, R. Wortmann, Angew.
Chem., Int. Ed. 2004, 43, 5851.
1
1
5 A. Heckmann, C. Lambert, J. Am. Chem. Soc. 2007, 129, 5515.
6 V. Gamero, D. Velasco, S. Latorre, F. López-Calahorra, E.
Brillas, L. Juliá, Tetrahedron Lett. 2006, 47, 2305
1
1
1
2
7 D. Velasco, S. Castellanos, M. López, F. López-Calahorra, E.
Brillas, L. Juliá, J. Org. Chem. 2007, 72, 7523.
8 Y. Hattori, T. Kusamoto, H. Nishihara, Angew. Chem., Int. Ed.
2
014, 53, 11845
9 Y. Hattori, T. Kusamoto, H. Nishihara, Angew. Chem., Int.Ed.
015.1, 54, 3731.
0 X. Ai, Y. Chen, Y. Feng, F. Li, Angew. Chem., Int.Ed.
2
2
018,57,2869.
2
2
1 Y. Im, J. Y. Lee, Synth. Met. 2015, 209, 24.
2 N. J. Turro, Modern Molecular Photochemistry, University
Science Books, 1991.
2
2
3 Y. Niu, W. Li, Q. Peng, H. Geng, Y. Yi, L. Wang, G. Nan, D. Wang,
Z. Shuai, Mo. Phys. 2018, 116, 1078
4 D. Fan, Y. Yi, Z. Li, W. Liu, Q. Peng, Z. Shuai, J. Phys. Chem. A
2
015, 119, 5233.
2
2
5 Y. Gao, A. Obolda, M. Zhang, F. Li, Dyes Pigm. 2017, 139, 644.
6 Y. Gao, W. Xu, H. Ma, A. Obolda, W. Yan, S. Dong, M. Zhang,
F. Li, Chem. Mater. 2017, 29, 6733.
2
7 A. Obolda, X. Ai, M. Zhang, F. Li, Acs Appl. Mat. Interfaces
2
016, 8, 35472.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 7
Please do not adjust margins

Journal of Materials Chemistry C
Page 8 of 8
View Article Online
DOI: 10.1039/C8TC04298E
Table of Contents Entry
A novel stable room-temperature luminescent βꢀcarboline-substituented biphenylmethyl
radical with high luminescent efficiency and excellent OLED device performance.