Tailoring Excited State Properties and Energy Levels Arrangement via Subtle Structural Design on D-π-A Materials

Article abstract of DOI:10.1002/cjoc.201700211

The donor-π-conjugated-acceptor (D-π-A) structure is an important design for the luminescent materials because of its diversity in the selections of donor, π-bridge and acceptor groups. Herein, we demonstrate two examples of D-π-A structures capable to finely modulate the excited state properties and arrangement of energy levels, TPA-AN-BP and CZP-AN-BP, which possess the same acceptor and π-bridge but different donor. The investigation of their photophysical properties and DFT calculation revealed that the D-π-A structure with proper donor, π-bridge and acceptor can result in separation of frontier molecular orbitals on the corresponding donor and acceptor with an obvious overlap on the π-bridge, resulting in a hybridized local and charge-transfer (HLCT) excited state with high photoluminescent (PL) efficiencies. Meanwhile, their singlet and triplet states are arranged on corresponding moieties with large energy gap between T₂ and T₁, and a small energy gap between S₁ and T₂, which favor the reverse intersystem crossing (RISC) from high-lying triplet levels to singlet levels. As a result, the sky-blue emission non-doped OLED based on the TPA-AN-BP reached maximum external quantum efficiency (EQE) of 4.39% and a high exciton utilization efficiency (EUE) of 77%. This study demonstrates a new strategy to construct highly efficient OLED materials.

Full text of DOI:10.1002/cjoc.201700211

FULL PAPER
DOI: 10.1002/cjoc.201700211
Tailoring Excited State Properties and Energy Levels
Arrangement via Subtle Structural Design on D-π-A Materials
a
a
a
a
,a
Xiaoming Liang, Zhiheng Wang, Liangxuan Wang, Muddasir Hanif, Dehua Hu,*
a
a
b
b
,a
Shijian Su, Zengqi Xie, Yu Gao, Bing Yang, and Yuguang Ma*
a
Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and
Devices, South China University of Technology, Guangzhou, Guangdong 510640, China
b
State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun,
Jilin 130012, China
The donor-π-conjugated-acceptor (D-π-A) structure is an important design for the luminescent materials be-
cause of its diversity in the selections of donor, π-bridge and acceptor groups. Herein, we demonstrate two examples
of D-π-A structures capable to finely modulate the excited state properties and arrangement of energy levels,
TPA-AN-BP and CZP-AN-BP, which possess the same acceptor and π-bridge but different donor. The investigation
of their photophysical properties and DFT calculation revealed that the D-π-A structure with proper donor, π-bridge
and acceptor can result in separation of frontier molecular orbitals on the corresponding donor and acceptor with an
obvious overlap on the π-bridge, resulting in a hybridized local and charge-transfer (HLCT) excited state with high
photoluminescent (PL) efficiencies. Meanwhile, their singlet and triplet states are arranged on corresponding moie-
ties with large energy gap between T and T , and a small energy gap between S and T , which favor the reverse
2
1
1
2
intersystem crossing (RISC) from high-lying triplet levels to singlet levels. As a result, the sky-blue emission
non-doped OLED based on the TPA-AN-BP reached maximum external quantum efficiency (EQE) of 4.39% and a
high exciton utilization efficiency (EUE) of 77%. This study demonstrates a new strategy to construct highly effi-
cient OLED materials.
Keywords D-π-A, organic light emitting diode, exciton utilization, hybridized local and charge-transfer, reverse
intersystem crossing
Introduction
the π bridge can also effectively tune the photophysical
properties of D-π-A materials, which not only extends
the π conjugation (between the donor and acceptor) but
also modulates the coupling of the donor and acceptor
The donor-π-conjugated-acceptor (D-π-A) structural
motif has attracted much attention because of optoelec-
tronic materials applications, such as nonlinear optical
[36,37]
[1-4]
to change the ICT state.
Especially for the blue
(
NLO) materials,
dye-sensitized solar cells (DSSCs)
[5-10]
fluorophores, the directly linked donor-acceptor (D-A)
structural motif usually causes a strong ICT between the
donor and the acceptor and leads to the emission shift to
materials.
Particularly, emissive D-π-A type fluoro-
phores are considered as promising candidates for emit-
ting layer in the organic light emitting diodes
[
38-40]
[
11-25]
The insertion of π-bridge in the
long wavelengths.
(OLEDs).
Generally, the nature of donor and ac-
D-π-A structural motif can also weaken the ICT process
ceptor in the D-π-A architectures usually endows the
materials with impressive bipolar charge-transporting
properties, reducing turn-on voltage and increasing
electroluminescence (EL) efficiencies of the devic-
[41]
and maintains higher optical energy gap. As a result,
the use of D-π-A structural motif can be an effective
strategy to design blue emissive fluorophores with
available modification of excited state properties.
[
26-30]
es.
More importantly, the diverse and independent
selections of donor, π-bridge and acceptor groups allow
Recently, our group reported a series of donor-
acceptor (D-A) material with a hybridized local and
charge-transfer (HLCT) excited state by appropriate
for facile modulation of the electronic structure and
[
31,32]
photophysical properties.
For example, the strength
[
42-46]
of the donor and acceptor determines the excited intra-
molecular charge-transfer (ICT) state, resulting in a va-
molecular design.
In these HLCT materials, the
co-existed local state (LE) and charge-transfer state (CT)
components confer two advantages to the excited state:
[33-35]
riety of emission and PL efficiencies.
In addition,
*
E-mail: msdhhu@scut.edu.cn, ygma@scut.edu.cn; Tel.: 0086-020-22236311; Fax: 0086-020-87110606
Received March 25, 2017; accepted April 29, 2017; published online XXXX, 2017.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cjoc.201700211 or from the author.
Chin. J. Chem. 2017, XX, 1—10
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1

Liang et al.
FULL PAPER
the LE component provides a high PL efficiency as a
consequence of a high radiative transition rate, whereas
the CT component possesses a small singlet-triplet
energy splitting. Moreover, the energy level distribution
of some HLCT materials has two characteristics: (1)
solution and film were measured with Hamamatsu
Quantaurus-Tau C11367. The cyclic voltammetry (CV)
was performed with a BAS 100W Bioanalytical
Systems, using a glass carbon disk (Φ＝3 mm) as the
working electrode, a platinum wire as the auxiliary
＋
energetic closeness between T
m
and S
n
; (2) large energy
This energy levels
arrangement makes the reverse intersystem crossing
RISC) from T to S competitive with internal
conversion (IC) from T to T . Thus, the devices based
electrode with a porous ceramic wick, Ag/Ag as the
[
47,48]
gap between T
m
and T
1
.
reference electrode, standardized for the redox couple
ferricinium/ferrocene. All solutions were purged with a
nitrogen stream for 10 min before measurement. The
procedure was performed at room temperature and a
nitrogen atmosphere was maintained over the solution
during measurements. Thermal gravimetric analysis
(TGA) was undertaken on a Perkin-Elmer thermal
analysis system from 30 ℃ to 850 ℃ at a heating rate
of 10 K/min and a nitrogen flow rate of 80 mL/min.
Differential scanning calorimetry (DSC) analysis was
carried out using a NETZSCH (DSC-204) instrument
from 30 ℃ to 380 ℃ at a heating rate of 10 K/min
while flushing with nitrogen.
(
m
n
m
1
on these HLCT materials can reach a high external
quantum efficiency (EQE) and exciton utilization effi-
ciency (EUE) by harvesting both singlet and triplet
excitons. However, the design of HLCT materials with
effective RISC on D-A structure is very difficult, e.g.
both the strength of and the twist angles between donor
and acceptor affect the formation of HLCT state, the
energy levels arrangement of the excited state cannot be
tuned independently. Therefore, new materials design
strategy should be found to solve these problems.
Synthesis
In this report, we adopt the D-π-A structural motif to
design the HLCT materials, namely TPA-AN-BP and
CZP-AN-BP, in which triphenylamine (TPA) and
All the reagents and solvents used for the synthesis
and characterization were purchased from Aldrich and
Acros companies and used without further purification.
4-(10-Bromoanthracen-9-yl)-N,N-diphenylaniline
9
-phenyl-9H-carbazole (CZP) are selected as the elec-
tron donors, benzophenone (BP) and anthracene (AN)
are used as the electron acceptor and π-bridge respec-
tively. TPA and CZP are good hole-transporting groups
and their different electron donating ability can adjust
ICT state, while the large π-conjugated AN-bridge can
increase the overlap of frontier molecular orbitals to
introduce the LE component. Thus, the combined effect
of these two factors will result in an HLCT state.
Moreover, anthracene has low triplet energy of 1.82
(TPA-AN-Br)
N,N-Diphenyl-4-(4,4,5,5-tetramethyl-
1,3,2-dioxaborolan-2-yl)aniline (M1) (2.00 g, 5.38
mmol), 9,10-dibromoanthracene (2.71 g, 8.07 mmol,
2 3
1.50 equiv.), 15 mL aqueous K CO solution (2.0
1
mol•L , 4.15 g), 15 mL methanol, and 30 mL toluene
were placed in a 100 mL round-bottom flask. After the
3 4
mixture was degassed under liquid nitrogen, Pd(PPh )
(0.19 g, 0.16 mmol, 0.03 equiv.) was added under ni-
trogen. The mixture was heated at 90 ℃ for 36 h under
nitrogen. After cooling to room temperature, the mixture
was washed with 50 mL water 3 times and extracted
with dichloromethane. The organic solution was dried
over Mg SO , and then the solvent was evaporated. The
[
49]
eV,
which may act as the lowest triplet of the de-
signed materials. In contrast, BP unit has high triplet
[
49]
energy level around 3.02 eV, which is close to the T
of anthracene. Thus, no matter the T of the materials is
distributed on BP or anthracene, there should be a large
energy gap between T and T . And if the materials
possess a blue or more red-shift emission, the T of the
materials should be close to or above the S . Therefore,
2
2
2
4
2
1
residue was purified by column chromatography eluting
with petroleum ether-dichloromethane mixtures to give
2
1
1
a pale green solid. Yield: 65%, 1.75 g. H NMR (500
an effective RISC from T
2
to S
1
can be expected.
MHz, DMSO) δ: 8.52 (d, J＝8.8 Hz, 2H, ArH), 7.78－
7
.68 (m, 4H, ArH), 7.60－7.51 (m, 2H, ArH), 7.44－
7.34 (m, 4H, ArH), 7.32－7.26 (m, 2H, ArH), 7.25－
.16 (m, 6H, ArH), 7.15－7.08 (m, 2H, ArH). LC-MS
Experimental
7
General information
(
M): m/z 500.8.
1
13
The H NMR spectra and C NMR spectra were
recorded on a Bruker AVANCE 500 spectrometer at
9-(4-(10-Bromoanthracen-9-yl)phenyl)-9H-carba-
zole (CZP-AN-Br) 9-(4-(4,4,5,5-Tetramethyl-1,3,2-
dioxaborolan-2-yl)phenyl)-9H-carbazole (M2) (2.00 g,
5.41 mmol), 9,10-dibromoanthracene (2.73 g, 8.12
mmol, 1.50 equiv.), 15 mL aqueous K CO solution (2.0
mol•L , 4.15 g), 15 mL methanol, and 30 mL toluene
were placed in a 100 mL round-bottom flask. After the
mixture was degassed under liquid nitrogen, Pd(PPh3)4
(0.19g, 0.16 mmol, 0.03 equiv.) was added under
nitrogen. The mixture was heated at 90 ℃ for 36 h
under nitrogen. After cooling to room temperature, the
mixture was washed with 50 mL water 3 times and
5
00 MHz and 125 MHz, respectively, using tetrameth-
ylsilane (TMS) as the internal standard and CDCl or
3
DMSO as the solvent. The MALDI-TOF-MS mass
spectra were measured using an AXIMA-CFRTM plus
instrument. The UV-vis absorption spectra were
recorded on a Shimadzu UV-3100 spectrophotometer.
Fluorescence measuremennts were recorded on
HORIBA FLUOROMAX-4. PL efficiencies in solvents
and films were measured with Hamamatsu Quantaurus-
QY C11347-11. The transient PL decay spectra in
2
3
1
2
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Tailoring Excited State Properties and Energy Levels Arrangement
extracted with dichloromethane. The organic solution
MHz, CDCl ) δ: 8.22 (d, J＝7.8 Hz, 2H, ArH), 8.13－
3
was dried over Mg
evaporated. The residue was purified by column chro-
matography eluting with petroleum ether-dichloro-
2
SO
4
, and then the solvent was
8.08 (m, 2H, ArH), 8.01 (dt, J＝8.4, 1.6 Hz, 2H, ArH),
7.91－7.82 (m, 4H, ArH), 7.18－7.64 (m, 9H, ArH),
7.59 (td, J＝7.1, 3.6 Hz, 2H, ArH), 7.55－7.41 (m, 6H,
1
3
methane mixtures to give a palegreen solid. Yield: 60%,
ArH), 7.36 (dt, J＝11.7, 2.4 Hz, 2H, ArH); C NMR
(125 MHz, CDCl ) δ: 195.53, 142.74, 139.84, 136.98,
1
1
6
7
7
7
.62 g. H NMR (500 MHz, DMSO) δ: 8.58 (d, J＝14.4,
.0 Hz, 2H, ArH), 8.35－8.29 (m, 2H, ArH), 7.94－
.89 (m, 2H, ArH), 7.84－7.77 (m, 4H, ArH), 7.76－
.72 (m, 2H, ArH), 7.70－7.60 (m, 4H, ArH), 7.57－
.51 (m, 2H, ArH), 7.39－7.33 (m, 2H, ArH). LC-MS
3
136.63, 136.18, 135.84, 135.65, 135.15, 131.76, 131.56,
130.40, 129.21, 128.92, 128.63, 127.42, 125.91, 125.69,
125.03, 124.51, 122.55, 119.41, 119.13, 108.90.
MALDI-TOF-MS (M): m/z 599.163. Elemental analysis
(
M): m/z 498.6.
4-(10-(4-(Diphenylamino)phenyl)anthracen-9-yl)-
phenyl)(phenyl)methanone (TPA-AN-BP) 4-(10-
calculated [%] for C45H29NO: C 90.12, H 4.87, N 2.34,
O 2.67; found C 91.66, H 4.45, N 2.19.
(
Bromoanthracen-9-yl)-N,N-diphenylaniline (TPA-AN-
Br) (1.50 g, 3.00 mmol), phenyl(4-(4,4,5,5-tetrame-
thyl-1,3,2-dioxaborolan-2-yl)phenyl)methanone (M3)
Results and Discussion
Synthesis
(
1.11 g, 3.60 mmol, 1.20 equiv.), 15 mL aqueous K
2
CO
3
Scheme 1 shows molecular structures and synthetic
route of the TPA-AN-BP and CZP-AN-BP. The two
molecules are composed of three components: triphe-
nylamine (TPA) or 9-phenyl-9H-carbazole (CZP) acting
as the electron donor, benzophenone (BP) as the accep-
tor, and the anthracene (AN) as the π-bridge to link the
donor and acceptor units. Firstly, the corresponding bo-
ron esters of the TPA, CZP and BP were synthesized
(S-II, supporting information). Secondly, the precursors
TPA-AN-Br and CZP-AN-Br were synthesized by the
Suzuki coupling reaction of 9,10-dibromoanthracene
with the boron esters (TPA and CZP). Finally, the
TPA-AN-Br and CZP-AN-Br were reacted with the bo-
ron ester of the BP through the Suzuki coupling reaction
to get the products. The final products were purified
using silica gel column chromatography, preparative
HPLC and vacuum sublimation in sequence. The mo-
1
solution (2.0 mol•L , 4.15 g), 15 mL methanol, and 30
mL toluene were placed in a 100 mL round-bottom flask.
After the mixture was degassed under liquid nitrogen,
3 4
Pd(PPh ) (0.11 g, 0.09 mmol, 0.03 equiv.) was added
under nitrogen. The mixture was heated at 90 ℃ for 36
h under nitrogen. After cooling to room temperature, the
mixture was washed with 50 mL water 3 times and ex-
tracted with dichloromethane. The organic solution was
dried over Mg SO , and then the solvent was evaporated.
2 4
The residue was purified by column chromatography
eluting with petroleum ether-dichloromethane mixtures
to give an off-white solid. Yield: 62%, 1.12 g. H NMR
(
8
7
7
1
500 MHz, CDCl
3
) δ: 8.08 (d, J＝8.0 Hz, 2H, ArH),
.01－7.96 (m, 2H, ArH), 7.91－7.85 (m, 2H, ArH),
.72－7.55 (m, 7H, ArH), 7.44－7.27 (m, 16H, ArH),
1
3
3
.12－7.06 (m, 2H, ArH); C NMR (125 MHz, CDCl )
1
δ: 195.55, 146.75, 146.24, 142.95, 136.65, 135.71,
lecular structures are fully confirmed by the H NMR,
1
3
1
1
1
34.50, 131.52, 131.30, 131.03, 130.43, 129.13, 128.62,
28.38, 127.39, 126.22, 125.51, 124.39, 124.06, 123.72,
22.06. MALDI-TOF-MS (M): m/z 601.157. Elemental
C NMR, mass spectrometry and elemental analysis.
Theoretical calculations
analysis calculated [%] for C45
N 2.33, O 2.66; found C 90.92, H 4.67, N 2.18.
0-(4-(9H-Carbazol-9-yl)phenyl)anthracen-9-yl)-
phenyl)(phenyl)methanone (CZP-AN-BP) (4-(10-
H
31NO: C 89.82, H 5.19,
Figure 1 shows the density functional theory (DFT)
calculated frontier molecular orbital (ground state)
structures at the B3LYP/6-31G*. For both the TPA-AN-
BP and CZP-AN-BP, the highest occupied molecular
orbitals (HOMOs) are delocalized over the donor (TPA
and CZP) and AN units, while the lowest unoccupied
molecular orbitals (LUMOs) are mainly distributed on
the acceptor (BP) and AN. The comparison of orbital
structure reveals that the AN acts as an important bridge
to overlap the HOMOs and LUMOs. This observation
indicates that the electronic transition from the HOMOs
to LUMOs of the materials is not a pure CT or LE tran-
sition but an intercrossed transition of CT and LE tran-
sition (HLCT). Moreover, because of the stronger elec-
tronic donating properties of TPA as compared to the
CZP, the HOMO is more delocalized on the donor and
AN unit in the TPA-AN-BP. This can cause better inter-
crossing of the CT and LE transition in the TPA-AN-BP.
1
(
(
4-(9H-Carbazol-9-yl)phenyl)anthracen-9-yl)phenyl)-
phenyl)methanone (CZP-AN-Br) (1.50 g, 3.01 mmol),
phenyl(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-
phenyl)methanone (M3) (1.11 g, 3.61 mmol, 1.20
1
equiv), 15 mL aqueous K
.15 g), 15 mL methanol, and 30 mL toluene were
placed in a 100 mL round-bottom flask. After the mix-
ture was degassed under liquid nitrogen, Pd(PPh
0.11 g, 0.09 mmol, 0.03 equiv.) was added under nitro-
2 3
CO solution (2.0 mol•L ,
4
3 4
)
(
gen. The mixture was heated at 90 ℃ for 36 h under
nitrogen. After cooling to room temperature, the mixture
was washed with 50 mL water 3 times and extracted
with dichloromethane. The organic solution was dried
2 4
over Mg SO , and then the solvent was evaporated. The
residue was purified by column chromatography eluting
with petroleum ether-dichloromethane mixtures to give
an off-white solid. Yield: 60%, 1.08 g. H NMR (500
Photophysical properties
1
The ultraviolet-visible (UV-vis) and PL spectra of
Chin. J. Chem. 2017, XX, 1—10
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3

Liang et al.
FULL PAPER
Scheme 1 Chemical structures and synthetic route of TPA-AN-BP and CZP-AN-BP
O
O
O
O
O
O
N
B
N
B
B
O
M3
M1
M2
M1
M3
N
Br
N
i
i
O
TPA-AN-Br
TPA-AN-BP
Br
Br
M2
M3
N
Br
N
i
i
O
CZP-AN-Br
i) Pd(PPh ) , toluene, methanol, 2 mol/L K CO (2:1:1), 90 ℃, 36 h
CZP-AN-BP
(
3
4
2
3
Figure 1 The frontier molecular orbitals (HOMO and LUMO) in the ground state of TPA-AN-BP and CZP-AN-BP.
Table 1 Thermal, photophysical and energy levels of the TPA-AN-BP and CZP-AN-BP
Solution (DCM)
Film
PL/nm
Molecule
T /T (℃)
LUMO/HOMO (eV)
g
d
UV/nm
PL/nm
258, 360, 378, 397 511
260, 357, 376, 396 480
Φ
fl
UV/nm
Φ
fl
TPA-AN-BP
CZP-AN-BP
111/420
0.68
0.57
263, 360, 383, 402
260, 361, 381, 402
468
449
0.28
0.12
2.72/5.26
2.75/5.57
/397
the TPA-AN-BP and CZP-AN-BP were recorded in di-
absorption spectra of donor, π-bridge and acceptor units
are shown in Figure S1 for comparison. Obviously, the
absorption of TPA-AN-BP broadened with a long tail in
low energy range, which indicates certain CT
components are introduced in the electron transition
process. This is because the TPA unit has stronger
electron donating ability than CZP unit, resulting in
higher CT component, consistent with the DFT calcu-
lations. Thus, it is reasonable to consider that TPA-AN-
BP has more effective hybridization of the LE and CT
chloromethane (10 mol•L , DCM) to reveal the pho-
tophysical properties. The two materials exhibit similar
absorption properties (Figure 2a) in DCM. The high
energy band around 260 nm is attributed to the benzene
ring absorption. The absorption bands around 300 nm
can be assigned to the π-π* and n-π* transitions of the
TPA or CZP unit. The well-resolved vibronic fine
structure absorption bands from 350 nm to 450 nm are
the π-π* transitions of the anthracene unit. The
5
1
4
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Tailoring Excited State Properties and Energy Levels Arrangement
components. The PL emission spectra of the TPA-AN-
BP and CZP-AN-BP in DCM solution are structureless
and broad with λmax at 511 nm and 480 nm, respectively,
due to CT emission. The PL quantum efficiencies
ity. TPA-AN-BP exhibits a large red-shift of 94 nm with
increasing solvent polarity (PL λmax＝446 nm for hex-
ane and 540 nm for acetonitrile). The CZP-AN-BP
showed relatively smaller red-shift of 79 nm (PL λmax
＝
(
integrating sphere) of the TPA-AN-BP and CZP-AN-
428 nm in hexane and 507 nm in acetonitrile). In order
to estimate the dipole moments of the lowest excited
BP in DCM solutions are 0.68 and 0.57 respectively.
The relatively high efficiency of TPA-AN-BP can be
attributed to its different arrangement of singlet and
triplet which may lead to different ISC process and will
be discussed in the theoretical calculation section later.
The absorption and PL spectra (Figure 2b) of the TPA-
AN-BP and CZP-AN-BP were also measured for the
film (vacuum-evaporated). The film absorption spectra
of the two materials are similar to those measured in
DCM. The PL spectra show blue-shift to 468 nm and
state (S
1
), the data points of the Stokes shift (ν
a
f
ν ) ver-
sus the orientation polarizability f(ε,n) were fitted for
the TPA-AN-BP and CZP-AN-BP according to the Lip-
[50,51]
pert-Mataga equation
(Figure 4, Table S1 and Table
S2). The two materials display two linear lines (Figure 4)
with different slopes in low-polarity solvents and high-
polarity solvents. This observation implies two different
excited states. In low-polarity solvents, the estimated
e
excited state dipole moments (μ ) of TPA-AN-BP and
4
49 nm for the TPA-AN-BP and CZP-AN-BP
CZP-AN-BP are 11.27 D and 5.99 D, respectively.
Generally, the dipole moments of common LE materials
are 4－5 D, and the dipole moments of common CT
respectively, which is likely caused by the change of
medium from the polar DCM to the pristine film. The
PL quantum efficiencies of the TPA-AN-BP and
CZP-AN-BP were lower (0.28 and 0.12 respectively)
due to the aggregation induced quenching.
[52]
materials are 15－20 D or higher. Clearly, the μ
TPA-AN-BP in low-polarity solvents is much larger
than the μ of common LE materials but smaller than
that of typical CT materials. This excited state with
moderate μ can be treated as the HLCT state, in which
e
of
e
e
the energy of CT components is close to that of LE
components, resulting in effective hybridization of the
two components. In contrast, the μ of CZP-AN-BP in
e
low-polarity solvents is more LE-like state, because
CZP is a weaker electron donor. In this case, the CT
a
446 nm
540 nm
TPA-AN-BP
1
.0
.8
.6
.4
.2
.0
Hexane
Triethylamine
Butylether
0
Ethylether
Ethylacetate
Dichloromethane
Acetone
0
Acetonitrile
0
0
0
400 450 500 550 600 650 700 750
Wavelength/nm
b
428 nm
507 nm
CZP-AN-BP
1
.0
.8
.6
.4
0.2
.0
Hexane
Triethylamine
Butylether
0
Ethylether
Ethylacetate
Dichloromethane
Acetone
0
Acetonitrile
Figure 2 (a) UV spectra and PL spectra of TPA-AN-BP and
0

5
1
CZP-AN-BP in DCM solution (10 mol•L ). (b) UV spectra and
PL spectra of TPA-AN-BP and CZP-AN-BP in vacuum-evapo-
rated films.
0
To further investigate the photophysical properties,
we measured the PL spectra (Figure 3) of the two mate-
rials in different solvents. The two materials exhibit ob-
vious solvatochromism in solvents with different polar-
400 450
500 550 600
Wavelenth/nm
650 700
Figure 3 PL spectra of TPA-AN-BP (a) and CZP-AN-BP (b) in
different solvents with increasing polarity.
Chin. J. Chem. 2017, XX, 1—10
© 2017 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.cjc.wiley-vch.de
5

Liang et al.
FULL PAPER
5
10
CZP-AN-BP
7
k
a
In Me-THF
Low-polarity: 5.99 D
High-polarity: 22.41 D
4
6k
10
TPA-AN-BP
CZP-AN-BP
TPA-AN-BP
5k
Low-polarity: 11.27 D
High-polarity: 21.61 D
3
0
1


4k
2
10
IRF
3k
1
10
2k
0
20
40
60
0.0
0.1
0.2
0.3
Time/ns
f
Figure 4 Linear correlation of the orientation polarization (f) of
5
10
b
a f
solvent media with the Stokes shift (ν ν ); a: absorption; f: fluo-
rescence for the three materials.
4
10
Film
state is energetically higher than that of LE state, there-
fore less likely to mix with each other. In high-polarity
solvents, the excited state μ of TPA-AN-BP and CZP-
e
AN-BP increased to 21.61 D and 22.41 D, respectively,
which are CT-like states. This is because increasing
solvent polarity can stabilize polarized CT states by re-
orienting the solvent molecules around the molecules so
as to lower the energy.
3
TPA-AN-BP
CZP-AN-BP
10
2
0
1
IRF
1
0
1
0
100
200
300
We also measured the transient PL decay spectra of
TPA-AN-BP and CZP-AN-BP in solution with medium
Time/ns

5
polarity (2-methyltetrahydrofuran, Me-THF, 10
Figure 5 Transient PL decay spectra of TPA-AN-BP and
CZP-AN-BP in Me-THF (a) and in film states (b).
1
mol•L ) and in film states (Figure 5). The transient PL
decay spectra in solution and film state show two decay
trends and can be fitted by double exponential. The life-
time of the two molecules is listed in Table 2. In
Me-THF, the fluorescent lifetimes of TPA-AN-BP are
Electrochemical and thermal properties
The electrochemical properties of TPA-AN-BP and
CZP-AN-BP were studied by the cyclic voltammetry
analysis. The oxidation and reduction processes were
2
.98 ns (τ
cent lifetimes of CZP-AN-BP are 1.49 ns (τ
ns (τ ), respectively. The shorter lifetime τ
1
) and 7.82 ns (τ
2
), respectively. The fluores-
) and 6.69
can be as-
1
performed in a mixed solution of CH
2
Cl
NPF
2
and aceto-
6
was used
2
1
1
nitrile (ACN), while 0.1 mol•L n-Bu
4
cribed to the prompt decay lifetime (τprompt), while the
longer lifetime can be ascribed to the delayed decay
lifetime (τdelayed) which undergoes the RISC process
from high laying excited states. In film state, the fluo-
as the supporting electrolyte and ferrocene (Fc) was
used as the internal standard. The CV curves (Figure 6)
of the two materials indicate positive (oxidation) and
negative scans (reduction). The oxidation scan process,
rescent lifetimes of TPA-AN-BP are 1.05 ns (τ
.56 ns (τ ), respectively. The fluorescent lifetimes of
CZP-AN-BP are 0.95 ns (τ ) and 4.07 ns (τ ), respec-
1
) and
6
2
1
2
tively. The lifetimes in film state decreases comparing to
the lifetime in solution because of aggregation. The
transient PL decay spectra also reveal that the decay of
HLCT state is a fast transition process.
TPA-AN-BP 0.60 V
a
Table 2 Fluorescent lifetime of TPA-AN-BP and CZP-AN-BP
CZP-AN-BP
0.91 V
Compd.
Condition Wavelength
τ/ns
1 2
τ /ns τ /ns
TPA-AN-BP Me-THF
CZP-AN-BP Me-THF
490 nm
445 nm
468 nm
449 nm
3.05 2.98 7.82
1.79 1.49 6.69
2.59 1.05 6.56
1.40 0.95 4.07
-2.0 -1.5 -1.0 -0.5
Potential/V
0.0
0.5
1.0
TPA-AN-BP
CZP-AN-BP
Film
Film
Figure 6 Cyclic voltammogram of TPA-AN-BP and CZP-AN-
a
τ is the average lifetime.
BP.
6
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© 2017 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Chin. J. Chem. 2017, XX, 1—10

Tailoring Excited State Properties and Energy Levels Arrangement
shows the oxidative onset potential of 0.60 V for TPA-
AN-BP and 0.91 V (positively shift) for CZP-AN-BP.
This observation shows that CZP unit has weaker
electronic donating ability when compared with the TPA.
The oxidation potentials were used to calculate the
HOMO energy levels (5.26 eV and 5.57 eV) of the
TPA-AN-BP and CZP-AN-BP. Since the two molecules
have the same acceptor, the reductive onset potentials
and 397 ℃, respectively. These results indicate these
two materials meet the thermal stability requirement for
the organic light emitting diodes (OLEDs).
Electroluminescence properties
In order to explore the EL properties of the two
compounds, optimized non-doped OLEDs were fabri-
cated with a frequently used multilayered structure: ITO
(95 nm)/NPB (30 nm)/TCTA (10 nm)/EML (20 nm)/
TPBi (30 nm)/LiF (1 nm)/Al (80 nm), in which NPB
and TCTA act as hole transporting and buffer layers, and
TPBi as an electron-transporting and hole-blocking lay-
er (NPB: N,N'-bis(1-naphthalenyl)-N,N'-bisphenyl-(1,1'-
biphenyl)-4,4'-diamine; TPBi: 1,3,5-tris(1-phenyl-1H-
benzimidazol-2-yl)benzene; TCTA: 4,4',4''-tri(9-carba-
zoyl)triphenylamine). The electroluminescence charac-
teristics of the devices are listed in Table 3. The turn-on
voltages (OLED devices) for the TPA-AN-BP and
CZP-AN-BP are 3.5 V and 3.9 V, respectively, indicat-
ing small injection barriers between the emitters and
transporting layers. Notably, the devices based on
TPA-AN-BP have a lower turn-on voltage than that of
CZP-AN-BP because there is a smaller injection barrier
between the TPA-AN-BP and hole transporting layers
due to higher HOMO level. The TPA-AN-BP and
CZP-AN-BP based non-doped devices exhibit sky-blue
and standard-blue emissions (Figure 8a) at a current of 1
(1.94 V and 1.91 V) were almost the same for the
TPA-AN-BP and CZP-AN-BP. The reduction potentials
were used to calculate the LUMO energy levels (2.72
eV and 2.75 eV) of the TPA-AN-BP and CZP-AN-BP.
The thermal properties of the two materials were
measured using differential scanning calorimetry (DSC)
and thermogravimetric analysis (TGA) under nitrogen
atmosphere (Figure 7). The heating scan curves of the
TPA-AN-BP (Figure 7a) showed a glass-transition tem-
perature (T
temperature (T
an exothermic process (crystallization, T
g
) of 111 ℃, without observable melting
) until 380 ℃. The CZP-AN-BP shows
) at 254 ℃.
m
c
Further increase in temperature revealed an endothermic
process at 340 ℃, assigned to the melting transition,
without observable glass-transition between the temper-
ature ranges. Figure 7b shows the thermogravimetric
curves of the two materials, TPA-AN-BP and CZP-AN-
BP exhibited both high thermal decomposition temper-
d
ature (T , corresponding to 5% weight loss) of 420 ℃
2
mA•cm with λmax at 488 nm and 460 nm and the cor-
responding CIE coordinates are (0.165, 0.377) and
(
0.147, 0.155). Their EL spectra exhibit a little red-shift
as compared with the emission in the vacuum-evapo-
rated films state which can be explained by the space
charge induced spectral shift, originating from the space
charge redistribution as response to the dipole moment
change in electronic excitation. Moreover, the EL
spectrum of CZP-AN-BP based device displays a
certain contribution from TCTA around 400 nm, which
indicates the unbalanced hole and electron transporting
due to the relatively lower HOMO level of CZP-AN-BP.
The EQE (ηext) versus luminance characteristics of
the two materials based devices (Figure 8b) show that
the TPA-AN-BP based device exhibits a much larger
maximum EQE of 4.39% and a maximum luminance of

2
1
5883 cd•m , while the CZP-AN-BP based device
showed a maximum EQE of 0.66% and a maximum
2
luminance of 4368 cd•m . Comparing to TPA-A re-
[
53]
ported by our group before,
the devices based on TPA-AN-BP is improved from 3%
TPA-AN, CIE (0.15, 0.23)) to 4.39% by adopting the
the maximum EQE of
(
D-π-A structural motif to design the efficient HLCT
materials. As expected, the performance of TPA-AN-BP
based device is much superior when compared to the
CZP-AN-BP based device. This is likely due to two
reasons: firstly, the TPA-AN-BP has a high PL efficien-
cy in film than that of CZP-AN-BP; and secondly, the
Figure 7 (a) Third heating DSC traces of TPA-AN-BP and
CZP-AN-BP with a heating rate of 10 K/min under nitrogen. (b)
Thermogravimetric curves of TPA-AN-BP and CZP-AN-BP un-
der nitrogen.
s
different EUE. The EUE of these devices (η ) can be
determined from the following EQE equation:
η
ext＝ηout•ηrec•ηPL•η
s
Chin. J. Chem. 2017, XX, 1—10
© 2017 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.cjc.wiley-vch.de
7

Liang et al.
FULL PAPER
Table 3 Electroluminescence characteristics of the devices
at 100 cd•m
2
at 1000 cd•m2
Lmax
cd•m )
/
CEmax
(cd•A ) [lm/W]
/
PEmax
/
Device
V
on/V
EQEmax/%

2
1
(
V/V
5.3
6.6
EQE/%
2.74
CIE [x, y]
V/V EQE/%
TPA-AN-BP
CZP-AN-BP
3.5
3.9
15883
4368
4.39
0.66
9.74
0.77
8.28
0.32
(0.165, 0.377)
(0.147, 0.155)
7.1
9.6
2.16
0.54
0.34
10
a
460 nm 488 nm
TPA-AN-BP
CZP-AN-BP
b
TPA-AN-BP
CZP-AN-BP
1
.0
.8
.6
.4
.2
.0
0
0
0
0
0
1
0.1
0
1
10
2
10
3
10
4
10
400 450 500 550 600 650 700 750
10
-
2
.
Wavelength/nm
Luminance/(cd m )
2
Figure 8 (a) The EL spectra of TPA-AN-BP and CZP-AN-BP based non-doped devices at a current density of 1 mA/cm . (b) The ex-
ternal quantum efficiency-luminance curves of TPA-AN-BP and CZP-AN-BP based non-doped devices.
where the ηout is light out-coupling efficiency (~20%),
η
rec is the recombination efficiency of injected holes and
electrons (ηrec could reach 100% under balanced and
complete recombination), ηPL is the intrinsic PL
efficiency. The maximum η can be estimated to be 77%
s
and 26% for TPA-AN-BP and CZP-AN-BP based
devices, respectively. Here, it should be noted that the
ηrec of CZP-AN-BP based device, should be below
1
00% according to its unbalanced recombination,
concluded from the observation of TCTA emission.
Therefore, η of CZP-AN-BP based device should be
s
underestimated.
To explain the different EUE values (77% for
TPA-AN-BP and 26% for CZP-AN-BP) observed for
the OLED devices, we have analyzed their natural tran-
sition orbitals (NTOs) of the singlet and triplet states
and the energy landscape on the basis of TD-DFT re-
sults with M06-2X/6-31G(d,p). In a single electron
transition process, the electron transition can be consid-
ered that the electron of the molecule migrates from one
area to another. The area before the electron transition is
called the “hole”, while the area after the electron tran-
sition is called “particle”. The transition from “hole” to
Figure 9 Natural transition orbitals of (a) TPA-AN-BP and (b)
CZP-AN-BP. The percentage represents the probability of the
transition.
3
particle localized on the AN. The T state shows an
HLCT character as the hole is delocalized over both AN
and BP unit, while the particle is mainly localized on the
BP unit. The NTOs of CZP-AN-BP (Figure 9b) show
1
“
particle” reveals the excited states properties. The
NTOs of TPA-AN-BP (Figure 9a) showed that the T1
state is mainly located on AN. For the S state, the hole
the distribution of T state similar to the TPA-AN-BP,
exhibiting an LE character. However, for the S state of
1
1
CZP-AN-BP, the hole is localized on AN unit, and the
is mainly located on the donor TPA and the π-bridge AN
while the particle is distributed on the AN and acceptor
BP unit, which implies substantial contribution of CT
particle is delocalized over both AN unit and BP unit.
This is more like LE state although a certain CT com-
ponent involved. The T state exhibits an LE character
2
component (HLCT). The T state also shows a CT
because both the hole and particle are localized on BP
2
character with hole mainly localized on the TPA and the
3
unit. The T state shows a CT character, the hole is
8
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© 2017 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Chin. J. Chem. 2017, XX, 1—10

Tailoring Excited State Properties and Energy Levels Arrangement
mainly localized on AN while the particle is localized
on BP unit. As we have mentioned above, the natural
design to construct highly efficient luminescent material
with an HLCT state in a D-π-A framework, in which the
anthracene π-bridge can act as an overlap site of the
frontier molecular orbitals. Based on this molecular de-
sign (same acceptor (BP) and π-bridge (AN) but differ-
ent donor), two D-π-A materials: TPA-AN-BP and
CZP-AN-BP, were designed, synthesised, characterized
and applied to make OLEDs. The comparison of pho-
tophysical properties and the DFT calculation of the two
compounds, reveal that the TPA-AN-BP with TPA as
donor is more in line with the expected excited state
properties, and the separated HOMO and LUMO are
mainly distributed on the corresponding donor and ac-
ceptor but overlap with the π-bridge to form well mixed
HLCT state. Moreover, its energy level is well alligned
with a large energy gap between T and T state, and a
1 2 3
transition orbitals (S , T , T ) of the two molecules are
different except the T
1
state which is likely an origin of
different photo-physical and electroluminescence prop-
erties. The energy landscapes for singlet and triplet ex-
cited states of TPA-AN-BP and CZP-AN-BP (Figure 10)
were calculated from the B3LYP method. There is a
very large energy gap of 1.14 eV and 1.19 eV between
T
2
and T
tively. This could slow down the interconversion IC
T ) process. We found that the energy gap between
the S and T for TPA-AN-BP and CZP-AN-BP was
1
for the TPA-AN-BP and CZP-AN-BP respec-
(T
2
1
1
1
also large (1.15 eV and 1.29 eV), which means the tri-
plet excitons could not transform to singlet excitons
through the TADF path. It is noted that the ΔES1-T2 of
TPA-AN-BP is only 0.01 eV and 10 times smaller than
that of CZP-AN-BP (0.1 eV). Thus, the RISC process
2
1
small energy gap between S and T , which can favor
1
2
the RISC from T to S . Therefore, a nondoped device
2
1
2 1
from the T to S in TPA-AN-BP should be more effec-
with TPA-AN-BP as the emitter reached a maximum
EQE of 4.39% and a high EUE of 77%. These results
suggest that the D-π-A framework is a promising mo-
lecular design to construct high efficiency luminescent
materials for the OLED applications.
tive than that of the CZP-AN-BP. This is likely the rea-
son for the higher EUE of 77% observed for the
TPA-AN-BP based OLED devices. Although the
CZP-AN-BP has a small energy gap between T
and it seems that the RISC process from the T
should be effective, the small energy gap between T
and T
3
and S
1
1
3
3
to S
Acknowledgement
2
can make IC process a dominant path. Therefore,
the RISC process from the high triplet levels to the sin-
glet levels in CZP-AN-BP becomes ineffective. Moreo-
The authors express their thanks to the National Nat-
ural Science Foundation of China (21334002, 51521002,
ver, the S
the S of CZP-AN-BP is located above its T
may lead to that the CZP-AN-BP has effective S
1
of TPA-AN-BP is located below its T
, which
-T
2
, while
5
1403063), the Ministry of Science and Technology of
1
2
China (2013CB834705, 2015CB655003), the Funda-
mental Research Funds for the Central Universities
1
2
ISC process. Thus, the PL efficiency of CZP-AN-BP is
lower than that of TPA-AN-BP, despite of the oscillator
strength of S state of CZP-AN-BP to be larger than that
1
of TPA-AN-BP (0.1385 vs. 0.0867). It should be noted
that we tried to measure the phosphorescence of
TPA-AN-BP and CZP-AN-BP at low temperature, even
using phosphorescent sensitizer to determine the T state,
1
but no unambiguous phosphorescence emission from
TPA-AN-BP and CZP-AN-BP was observed.
(
2015ZP001, 2015ZM042), Introduced Innovative R&D
Team of Guangdong (201101C0105067115), Major
Science and Technology Project of Guangdong Province
(
2015B090913002), Foundation of Guangzhou Science
and Technology Project (201504010012), China Post-
doctoral Science Fund (Grant No. 2014M562174) for
their support.
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