Multifunctional terpyridine/diphenylamine derivatives as highly efficient blue fluorescent emitters and red phosphorescent hosts

Article abstract of DOI:10.1039/c4tc02459a

Three terpyridine (TPY)/diphenylamine (DPA) derivatives, with DPA functioning as the electron donor and TPY as the electron acceptor, were designed and synthesized. By switching the position of the nitrogen atom in the substituted pyridine of TPY acceptor

Full text of DOI:10.1039/c4tc02459a

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Multifunctional terpyridine/diphenylamine derivatives as highly efficient blue
fluorescent emitters and red phosphorescent hosts
a,b
a
a
a,c
b
Chuan-Lin Liu, Cai-Jun Zheng,* Xiao-Ke Liu, Zhan Chen, Ji-Ping Yang,* Fan
a
a
a,d
Li, Xue-Mei Ou, Xiao-Hong Zhang*
Three terpyridine (TPY)/diphenylamine (DPA) derivatives, with DPA functioning as
the electron donor and TPY as the electron acceptor, were designed and synthesized.
By switching position of the nitrogen atom in the substituted pyridine of TPY
acceptors, we can adjust the electron-drawing strength of TPY group, hence, further
modify the fluorescence, lowest unoccupied molecular orbital energy levels, carrier
transporting properties of three compounds, but barely influence triplet energy levels.
Three compounds satisfy the requirements of multifunctional blue fluorophors and are
successfully used as highly efficient blue fluorescent emitters and red phosphorescent
hosts in organic light-emitting devices (OLEDs). Non-doped blue fluorescent OLEDs
that use TPY22DPA, TPY33DPA, and TPY44DPA as emitters exhibit maximum
external quantum efficiencies (EQEs) of 4.9%, 3.8%, and 2.7%, respectively.
Meanwhile, red phosphorescent OLEDs that use TPY22DPA, TPY33DPA, and
TPY44DPA as host materials exhibit maximum EQEs of 19.1%, 20.9%, and 17.2%,
respectively. These results are among the best reported multifunctional blue
fluorophor efficiencies.
Introduction
Organic light-emitting devices (OLEDs) have attracted considerable attention because
of their potential applications in next-generation full-color flat-panel displays and
1
solid lighting sources. During the past two decades, the exploitation of new OLED
2
emitters has been greatly successful, particularly for noble metal complex
3
phosphorescent emitters. However, given the difficulty of obtaining wide energy gap
phosphors, reports regarding highly efficient blue phosphors that can exhibit deep
4
blue emission (yCIE < 0.15, xCIE + yCIE < 0.30) are insufficient. In addition, blue
phosphorescent devices are typically unstable, which impedes their practical
applications. To address these issues, researchers have refocused their attention on
5
blue fluorophors, in which stability and high color purity are readily realized.

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Moreover, given the wide energy gaps, blue fluorophors can further realize high
triplet energy levels (E s), hence, have the potential to be used as host materials for
T
6
red phosphorescent dopants or even orange ones. Considering this multifunctional
property, blue fluorophors can play an important role in reducing the production cost
7
and simplifying the manufacturing process of full-color displays and white lightings.
For instance, Chen et al. synthesized a series of benzimidazole/amine derivatives. By
using them as the blue emitters and red phosphorescent hosts, they fabricated blue
fluorescent and red phosphorescent OLEDs with maximum EQEs of 1.6% and 7.8%,
7
a
respectively. Based on one of their multifunctional hosts, a simplified white OLED
WOLED) with a maximum EQE of 3% was successfully developed. Meanwhile,
Wang et al. reported two metal complexes, namely, Zn(PPI) and Be(PPI) , and
(
2
2
demonstrated their multifunction by using them as blue emitters and green/red
8
phosphorescent hosts, respectively. However, as multifunctional blue fluorophors are
still rarely reported, blue fluorescent and red phosphorescent devices both perform
4
a,7,8
poorly based on currently available multifunctional blue fluorophors.
The
efficiencies of these devices are significantly lower than the 5% and 20% theoretical
4
a,7–9
limits for fluorescent and phosphorescent devices, respectively.
Therefore, further
development on new highly efficient multifunctional blue fluorophors is highly
desirable.
In this work, three terpyridine (TPY)/diphenylamine (DPA) derivatives, namely,
4
-[4-(2,2’:6’,2’’-terpyridinyl)]phenyltriphenylamine (TPY22DPA), 4-[4-(3,2’:6’,3’’-
terpyridinyl)]phenyltriphenylamine (TPY33DPA), and 4-[4-(4,2’:6’,4’’-terpyridinyl)]
phenyltriphenylamine (TPY44DPA), were synthesized as multifunctional blue
fluorophors. In these compounds, the well-known hole-transporting moiety DPA was
chosen as the electron-donor unit, whereas TPY moieties were used as the electron-
acceptor units, considering that Kido et al. proved the outstanding electron-
1
0
transporting property of the pyridine group. By linking DPA and TPY moieties with
a size-appropriate biphenyl-π conjugated bridge, all of the three compounds exhibit
1
1
efficient blue fluorescence, high E s, and bipolar transporting properties. As shown
T
in Scheme 1, the only difference among the three TPY/DPA derivatives is the various
locations of the nitrogen (N) atoms in the substituted pyridine ring. According to our
detailed study, the position of the N atom in TPY acceptors can affect the electron-
drawing strength of TPY group, thus further modify the fluorescence, lowest
unoccupied molecular orbital (LUMO) energy levels, and carrier transporting

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properties of three compounds, but barely influence E s. All three compounds satisfy
T
the requirements of multifunctional blue fluorophors and are successfully used as
highly efficient blue fluorescent emitters and red phosphorescent hosts in various
devices. Considering performances of both blue and red devices simultaneously, blue
fluorescent and red phosphorescent devices based on TPY22DPA can achieve highest
efficiencies with maximum EQEs of 4.9% and 19.1%, respectively. These results are
among the best reported performance of multifunctional blue fluorophors.
Experimental section
General Procedures. Nuclear magnetic resonance (NMR) spectral data were
obtained using a Bruker Advance-400 spectrometer with chemical shifts reported in
ppm. Mass spectral data were obtained using a Finnigan 4021C gas chromatography
(GC)–mass spectrometry (MS) instrument. Absorption and photoluminescence (PL)
spectra were obtained using Hitachi ultraviolet−visible (UV−Vis)
a
spectrophotometer U-3010 and a Hitachi fluorescence spectrometer F-4600,
respectively. Moreover, differential scanning calorimetry (DSC) measurements were
–1
acquired using a NETZSCH DSC204 instrument at a heating rate of 10 °C min from
0 °C to 420 °C in N atmosphere. Thermogravimetric analysis (TGA) measurements
2
2
were acquired using a TA Q500 thermogravimeter by measuring the weight loss of
–
1
the specimens while heating them at a rate of 10 °C min from 25 °C to 800 °C in N
2
atmosphere. Lastly, cyclic voltammetry (CV) was performed using a CHI660E
electrochemical analyzer.
Synthesis. All commercially available reagents and chemicals were used without
further purification. The synthetic routes of TPY22DPA, TPY33DPA, and
TPY44DPA are outlined in Scheme 1.
4
’-(4-Bromophenyl)-2,2’:6,2’’-terpyridine (TPY22-Ph-Br). This compound was
1
2
synthesized according to a modified literature procedure. In a 500 mL round-bottom
flask with a magnetic stir bar, NaOH (0.80 g, 20 mmol) and aqueous NH OH (100
4
mL of a 28 wt% solution, 80 mmol) were added into a stirring solution of 4-
bromobenzaldehyde (3.70 g, 20 mmol) and 2-acetylpyridine (4.85 g, 40 mmol) in
ethanol (150 mL). The resulting mixture was stirred in ambient atmosphere at room
temperature for 4 h, during which a precipitate formed. The mixture was then filtered
through a Buchner funnel, and the collected precipitate was washed with ethanol (50
mL) and water (100 mL) before being vacuum dried, which yielded a light yellow
1
solid TPY22-Ph-Br (3.21 g) (41%). H NMR (400 MHz, CDCl ) δ (ppm) 8.85 (d, J =
3

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3
2
4
5.8 Hz, 6H), 8.05 (s, 2H), 7.90 (d, J = 7.9 Hz, 2H), 7.66 (d, J = 8.6 Hz, 2H), 7.52 (s,
DOI: 10.1039/C4TC02459A
+
H). MS (EI, m/z): [M] calcd for C H BrN , 387.03; found, 387.02.
2
1
14
3
-[4-(2, 2’: 6’, 2’’-terpyridinyl)]phenyltriphenylamine (TPY22DPA). In a double-
necked 150 mL round-bottom flask with a magnetic stir bar, reflux condenser, and N2
atmosphere, toluene (12 mL), ethanol (6 mL), and 2 M aqueous Na CO (9 mL) were
2
3
added into a mixture of 4’-(4-bromophenyl)-2,2’:6’,2’’-terpyridine (0.71g, 2 mmol),
-(diphenylamino)phenylboronic acid (0.58 g, mmol), and
4
2
tetrakis(triphenylphosphine)platinum (0.23 g, 0.2 mmol). The mixture was refluxed
for 12 h in N inlet. After the mixture cooled to room temperature, the reaction
2
mixture was extracted using dichloromethane and distilled water. The
dichloromethane phase was dried over Na SO and then filtered. After the solvent was
2
4
removed under reduced pressure, the crude product was purified through silica gel
column chromatography using dichloromethane/methanol (20:1) as eluant. Further
purification was achieved via sublimation, which produced a white compound of
1
TPY22DPA (0.84 g) (76%). H NMR (400 MHz, D -acetone) δ (ppm) 8.92 (s, 2H),
6
8
.81 – 8.77 (m, 4H), 8.09 – 8.02 (m, 4H), 7.95 (d, J = 8.5 Hz, 2H), 7.76 (d, J = 8.7 Hz,
H), 7.51 (ddd, J = 7.5, 4.7, 1.3 Hz, 2H), 7.40 – 7.35 (m, 4H), 7.20 – 7.10 (m, 8H).
2
1
3
C NMR (101 MHz, CDCl ) δ 156.31, 155.95, 149.80, 149.13, 147.65, 147.59,
3
1
41.32, 136.90, 136.71, 134.16, 129.35, 127.78, 127.72, 127.05, 124.57, 123.87,
+
1
23.75, 123.10, 121.41, 118.67. MS (EI, m/z): [M] calcd for C H N , 552.23;
38
27
4
found, 552.23.
4
’-(4-Bromophenyl)-3,2’:6’,3’’-terpyridine (TPY33-Ph-Br). The same approach for
synthesizing TPY22-Ph-Br was followed to obtain the compound of TPY33-Ph-Br
1
(
3.62 g) (46%). H NMR (400 MHz, CDCl ) δ 9.40 (ppm) (d, J = 2.0 Hz, 2H), 8.74
3
(dd, J = 4.9, 1.5 Hz, 2H), 8.63 (d, J = 8.0 Hz, 2H), 7.96 (s, 2H), 7.71 (d, J = 8.5 Hz,
+
2
3
4
H), 7.65 – 7.56 (m, 4H). MS (EI, m/z): [M] calcd for C H BrN , 387.03; found,
21
14
3
87.02.
-[4-(3,2’:6’,3’’-terpyridinyl)]phenyltriphenylamine (TPY33DPA). The same
approach for synthesizing and purifying TPY22DPA was followed to obtain the white
1
compound of TPY33DPA (0.92 g) (83%). H NMR (400 MHz, CDCl ) δ (ppm) 9.45
3
(s, 2H), 8.77 – 8.70 (m, 4H), 8.07 (s, 2H), 7.83 (d, J = 8.5 Hz, 2H), 7.78 (d, J = 8.4 Hz,
2
H), 7.66 (d, J = 5.2 Hz, 2H), 7.56 (d, J = 8.6 Hz, 2H), 7.32 – 7.28 (m, 4H), 7.17 (t, J
1
3
=
7.2 Hz, 6H), 7.07 (t, J = 7.3 Hz, 2H). C NMR (101 MHz, CDCl ) δ 155.41, 150.40,
3
1
50.21, 148.45, 147.85, 147.55, 141.89, 136.29, 134.72, 134.56, 133.56, 129.39,

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1
27.76, 127.54, 127.38, 124.70, 123.65, 123.59, 123.26, 117.45. MS (EI, m/z): [M]
calcd for C H N , 552.23; found, 552.23.
3
8
27
4
4
’-(4-Bromophenyl)-4,2’:6’,4’’-terpyridine (TPY44-Ph-Br). The same approach for
synthesizing TPY22-Ph-Br was followed to obtain the compound of TPY44-Ph-Br
1
(
3.49 g) (46%). H NMR (400 MHz, CDCl ) δ 8.82 (ppm) (dd, J = 4.6, 1.6 Hz, 4H),
3
8
2
4
.15 (d, J = 5.8 Hz, 4H), 8.04 (s, 2H), 7.72 (d, J = 8.5 Hz, 2H), 7.63 (d, J = 8.5 Hz,
+
H). MS (EI, m/z): [M] calcd for C H BrN , 387.03; found, 387.02.
2
1
14
3
-[4-(4,2’:6’,4’’-terpyridinyl)]phenyltriphenylamine (TPY44DPA). The same
approach for synthesizing and purifying TPY22DPA was followed to yield the green
1
compound of TPY44DPA (0.91 g) (82%). H NMR (400 MHz, CDCl ) δ (ppm) 8.83
3
(s, 4H), 8.20 (d, J = 5.2 Hz, 4H), 8.14 (s, 2H), 7.80 (dd, J = 20.6, 8.4 Hz, 4H), 7.55 (d,
J = 8.6 Hz, 2H), 7.32 – 7.27 (m, 4H), 7.17 (t, J = 7.2 Hz, 6H), 7.07 (t, J = 7.3 Hz, 2H).
1
3
C NMR (101 MHz, CDCl ) δ 155.30, 150.68, 150.56, 147.94, 147.52, 146.10,
3
1
1
42.08, 135.98, 133.41, 129.40, 127.76, 127.54, 127.41, 124.73, 123.54, 123.32,
+
21.22, 118.68. MS (EI, m/z): [M] calcd for C H N , 552.23; found, 552.23.
3
8
27
4
Quantum Calculation. The theoretical calculations of the TPY/DPA derivatives
were performed using the Gaussian-09 program. Density functional theory (DFT)
B3LPY/6-31G (d) was used to determine and optimize the structures.
Device Fabrication and Measurement. Indium tin oxide (ITO)-coated glasses with a
sheet resistance of 30 Ω per square were used as substrates. Before the device was
fabricated, the substrates were first cleaned with acetone, ethanol, and deionized water.
Then, they were oven-dried at 120 °C and treated with UV-ozone in ambient
condition for 5 min. Finally, the cleaned glasses were transferred to a vacuum
−
6
deposition system at approximately 1 × 10 Torr. Thermally evaporated organic
−
1
layers were sequentially grown onto the ITO substrates at a rate of 1−2 Å s . The
cathode was completed via the thermal deposition of lithium fluoride (LiF) at a rate of
−
1
−1
0
.1 Å s and then covered with aluminum (Al) metal deposited at a rate of 10 Å s .
Electroluminescence (EL), CIE color coordinates, and spectra were obtained using a
Spectrascan PR650 photometer, and the current−voltage characteristics were
determined with a computer-controlled Keithley 2400 SourceMeter in ambient
atmosphere.
Results and discussion
Synthesis. Scheme 1 illustrates the synthetic routes and molecular structures of
TPY22DPA, TPY33DPA, and TPY44DPA. These compounds were synthesized via

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the Suzuki cross-coupling reaction between 4-(diphenylamino) phenylboronic acid
and 4-bromophenylterpyridine, which were synthesized using acetylpyridine and 4-
bromobenzaldehyde. The chemical structures of the three new compounds were fully
1
13
characterized and confirmed via H, C NMR spectroscopy, and MS. Moreover, the
TPY/DPA compounds were further purified by sublimation before the device was
fabricated.
Thermal Analysis. The thermal properties of the TPY/DPA derivatives were
determined through DSC and TGA measurements. As shown in Figure 1 and Table 1,
the decomposition temperatures of the three compounds increased in the following
order: TPY22DPA (384 °C) < TPY33DPA (403 °C) < TPY44DPA (408 °C), which
indicates the increasing polarity and molecular interactions from TPY22DPA to
TPY33DPA and TPY44DPA. The glass transition temperature (T ) of TPY22DPA
g
was observed at 100 °C, which is considerably higher than those of common host
13
materials such as CBP and mCP. Although no T was detected in TPY33DPA and
g
TPY44DPA, both compounds were ensured to have high thermal stabilities because
they have similar molecular weights and nearly the same chemical structures with
TPY22DPA.
Theoretical Calculations. To gain insight into the structure–property relationship of
the TPY/DPA derivatives at the molecular level, DFT calculations were performed in
TPY22DPA, TPY33DPA, and TPY44DPA, as shown in Figure 2. The calculations
suggest that the three TPY/DPA compounds have similar distributions at the highest
occupied molecular orbital (HOMO) and LUMO. HOMOs are primarily located at the
DPA unit and the biphenyl π bridge because of the strong electron-donating character
of the DPA unit. In LUMOs, however, electron distributions are mostly located at the
electron-deficient TPY moieties and the biphenyl π bridge. The separation between
these two orbitals maintains the individual electronic properties of the functional
groups and benefits efficient hole- and electron-transporting molecular properties.
Moreover, a suitable LUMO–HOMO overlap can provide an efficient intramolecular
1
4
charge transfer (ICT) transition emission and high E s to the three compounds.
T
Electrochemical Properties. The electrochemical properties of the TPY/DPA
derivatives were investigated via CV in dimethylformamide. HOMO/LUMO energy
levels were calculated based on ferrocene (with a Fermi level of 5.10 eV according to
1
5
literature ). As shown in Figure 3, the three TPY/DPA compounds presented
irreversible oxidation curves, which possibly originated from the electron-rich DPA

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unit. Based on the estimated onset potentials of the oxidation curves, the HOMO
energy levels of TPY22DPA, TPY33DPA, and TPY44DPA were −5.54, −5.52, and
5.50 eV, respectively. Similar HOMO levels can also confirm the occurrence of
−
oxidation in the DPA units of the three compounds. During the reduction process,
quasi-reversible curves were observed in the three compounds, which can be
attributed to the electron-deficient TPY moieties. The LUMO energy levels of
TPY22DPA, TPY33DPA, and TPY44DPA were reduced to −2.83, −2.86, and −2.93
eV from the onset potentials of the reduction curves. From TPY22DPA to
TPY33DPA and TPY44DPA, the lower and lower LUMO energy levels indicate that
the positions of the N atoms in the substituted pyridine rings can affect the electron-
deficient level of TPY moiety. The 4, 4’’-positioned TPY moiety can draw electrons
most effectively.
Photophysical Properties. The UV−vis absorption and PL spectra of the three
TPY/DPA derivatives in the diluted ethyl acetate solution are shown in Figure 4a. The
three compounds exhibited similar absorption behaviors in the ethyl acetate solution.
The absorption peaks at 270 nm are associated with the DPA-centered n–π*
16
transition, whereas the long wavelength peaks at approximately 360 nm are
attributed to ICT transition from the electron-rich DPA moiety to the electron-
deficient TPY core. Unlike their absorption behaviors, the three TPY/DPA derivatives
displayed slightly red-shifted blue emissions with peaks at 460, 470, and 487 nm for
TPY22DPA, TPY33DPA, and TPY44DPA in ethyl acetate solution, respectively. The
red-shifted fluorescence from TPY22DPA to TPY33DPA and TPY44DPA can be
attributed to the different electron-deficient levels of TPY moieties in molecules,
which can be verified through the HOMO/LUMO energy levels of the three
compounds. As shown in Figure 4b, the shift trend was maintained in the PL spectra
of the three compounds in thin solid films, wherein the spectrum peaks were at 455,
4
65, and 475 nm for TPY22DPA, TPY33DPA, and TPY44DPA, respectively. The
respective fluorescent quantum yields (Φ ) of TPY22DPA, TPY33DPA, and
f
TPY44DPA were 0.76, 0.67, and 0.59 in the ethyl acetate solution, which were
1
7
measured using quinine bisulfate (Φ = 0.546 in 1 M H SO ) as the standard. In neat
f
2
4
thin solid film, these values still remain at high levels of 0.45, 0.42, and 0.42 for
TPY22DPA, TPY33DPA, and TPY44DPA respectively. The high values of Φf
indicate that our TPY/DPA derivatives are excellent blue fluorescent emitters. The
phosphorescent spectra of the three TPY/DPA derivatives were investigated in 2-

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methyltetrahydrofuran glass at 77 K. As shown in Figure 4c, the nearly similar
phosphorescent spectra of the derivatives indicate that the T states of the three
1
1
8
compounds are located mostly in the electron-donating DPA moiety. Based on the
peaks of the phosphorescent spectra, E s were estimated as 2.44, 2.43, and 2.43 eV
T
for TPY22DPA, TPY33DPA, and TPY44DPA, respectively. The similar triplet
energy results indicate that the positions of the N atoms in the substituted pyridine
rings do not influence the T states of the three compounds. The high E s are
1
T
1
9
sufficient for sensitizing phosphorescent dopants with E s that are below 2.40 eV.
T
Therefore, our novel TPY/DPA derivatives are applicable as blue emitters and red
phosphorescent hosts.
Electroluminescence Performance. Non-doped blue fluorescent OLEDs were
initially fabricated using a device structure of ITO/TAPC (50 nm)/TCTA (5
nm)/TPY/DPA (30 nm)/TmPyPB (45 nm)/LiF (1 nm)/Al (120 nm). Devices B1, B2,
and B3 represent OLEDs that use TPY22DPA, TPY33DPA, and TPY44DPA as the
emitting layer (EML), respectively. In these devices, ITO and LiF/Al are the anode
and
cathode,
respectively.
TAPC
(4,4’-
cyclohexylidenebis[N,N2bis(42methylphenyl)benzenamine]) is the hole-transporting
layer. TCTA (4,4’,4’’-tris(N-carbazolyl)triphenylamine) is the exciton-blocking layer.
TmPyPB (1, 3, 5-tri[(32pyridyl)-phen-3-yl]benzene) is the electron-transporting, hole-
blocking, and exciton-blocking layers. The non-doped blue devices based on
TPY22DPA, TPY33DPA, and TPY44DPA exhibit stable blue fluorescent spectra
with different luminances; peaks at 448, 460, and 476 nm; and CIE coordinates of
(0.15, 0.11), (0.15, 0.17), and (0.17, 0.26), respectively. As shown in Figure S1,
devices B1, B2, and B3 exhibit significantly low turn-on voltages of 3, 2.6, and 2.7 V,
respectively, which nearly reach the energy limit of blue OLEDs. An important reason
for such low turn-on voltages is the suitable HOMO and LUMO energy levels of the
three compounds. As shown in Figure 7, only small carrier injection barriers exist in
the holes and electrons because of the bipolar transporting property of blue
fluorophors. To further prove this property, we fabricated hole-only and electron-only
devices using the ITO/MoO (12 nm)/TPY/DPA (60 nm)/MoO (12 nm)/Al and
3
3
ITO/Al (60 nm)/TPY/DPA (60 nm)/Al structures, respectively. As shown in Figure 6,
all non-doped carrier-only devices based on the three compounds exhibited
remarkable current densities, which proved the bipolar transporting property of the
three TPY/DPA derivatives. Moreover, with the increasing electron-deficient level of

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TPY moieties, the electron mobilities of the three compounds increased rapidly from
TPY22DPA to TPY33DPA and TPY44DPA, whereas hole mobilities exhibited the
opposite result. Accordingly, TPY22DPA displayed the best ambipolar transporting
property with nearly similar current densities for holes and electrons. Therefore, as
shown in Figure 5a, device B1 exhibited the highest efficiency and lowest efficiency
roll-off. As shown in Table 2, device B1 exhibited maximum efficiencies of 4.4 lm
−1
−1
W for power efficiency (PE), 5.6 cd A for current efficiency (CE), and 4.9% for
EQE. Such high efficiencies are also attributed to the highest Φ of TPY22DPA in
f
neat thin films. Considering the 5% theoretical limit for fluorescent devices,
9
b
TPY22DPA is deemed as an outstanding blue fluorophor.
To verify the host properties of the three compounds for red phosphors,
phosphorescent devices were constructed based on the TPY/DPA derivatives. The
devices have similar configurations of ITO /TAPC (50 nm)/TCTA (5 nm)/TPY/DPA:
2
% Ir(MDQ) acac (30 nm)/TmPyPB (45 nm)/LiF (1 nm)/Al (120 nm). Devices R1,
2
R2, and R3 were used with TPY22DPA, TPY33DPA, and TPY44DPA, respectively.
The Ir(MDQ) acac used in this study was iridium(III)bis(2-methyldibenzo[f,h]
2
quinoxaline)(acetylacetonate). As shown in Figure S2, devices R1, R2, and R3 have
low turn-on voltages of 2.9, 2.4, and 2.5 V, respectively. These turn-on voltages,
which are lower than those for non-doped blue devices, can be attributed to the
trapping effect of the red dopant Ir(MDQ) acac. As shown in Table 2, the maximum
2
-1
-1
EQE, PE and CE were 19.1%, 40.2 lm W and 38.5 cd A , respectively, for device
-1
-1
R1; 20.9 %, 53.6 lm W and 42.7 cd A , respectively, for device R2; as well as
-1
-1
1
7.2 %, 37.2 lm W and 35.5 cd A , respectively, for device R3. The outstanding
results can be ascribed to the perfect energy transfer from the hosts to the dopants,
which has been demonstrated by the high Φ s of the host films doped with a
f
phosphorescent emitter, i.e. 0.89 for 2% Ir(MDQ) acac doped in TPY22DPA, 0.86 for
2
2
% Ir(MDQ) acac doped TPY33DPA, and 0.85 for 2% Ir(MDQ) acac doped
2 2
TPY44DPA, respectively. Considering the 20% theoretical limit for EQE in
9
a
phosphorescent devices, the three devices exhibited extremely high EQEs, which
indicate that the TPY/DPA derivatives are not only efficient blue fluorescent emitters,
but also perform remarkably for red phosphors. According to Figure 5c, device R2
exhibited the highest efficiency and lowest efficiency roll-off, which is in contrast to
those of non-doped blue fluorescent devices. To understand this result, doped carrier-
only devices were constructed based on the three hosts. Through the trapping effect of

Journal of Materials Chemistry C
Page 10 of 25
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DOI: 10.1039/C4TC02459A
the red dopant Ir(MDQ) acac, the carrier transporting properties of doped EMLs
2
significantly differed from those of non-doped EMLs. As shown in Figure 6, the
Ir(MDQ) acac of doped TPY33DPA exhibited an ambipolar transporting property,
2
and thus, achieved the best performance. Considering all the results of non-doped blue
fluorescent and red phosphorescent devices, TPY22DPA performed the best among
the three derivatives and achieved the best results for reported multifunctional blue
fluorophors (Table S1).
To further prove the superiority of our multifunctional blue fluorophors, a
WOLED (device W) was fabricated based on TPY22DPA using the configuration of
ITO/TAPC (50 nm)/TCTA (5 nm)/TPY22DPA:0.1% Ir(MDQ) acac (30
2
nm)/TmPyPB (45 nm)/LiF (1 nm)/Al (120 nm). In this device, based on appropriate
doping concentration, the singlet and triplet excitions generated in EML can be
successfully separated and respectively utilized by TPY22DPA and red
phosphorescent dopants. As a result, TPY22DPA had dual roles as both an efficient
blue emitter and an excellent red phosphor host. The device exhibited a warm white
–
2
emission with a CIE value of (0.43, 0.34) at 1000 cd m . Moreover, the max forward-
−1
viewing EQE of 19.3% and the PE of 38.6 lm W were achieved in this device
Figure 8). The single-doped device structure is considerably simpler than the stacked,
(
multi-emissive layer or multi-doped WOLEDs. This white device proved that our
multifunctional blue fluorophor TPY22DPA can reduce production cost and simplify
the manufacturing process for such devices, particularly for WOLEDs.
Conclusion
In summary, a series of multifunctional TPY/DPA derivatives, namely, TPY22DPA,
TPY33DPA, and TPY44DPA, were designed and synthesized. By switching position
of the N atom in the substituted pyridine of TPY acceptors, we can adjust the
electron-drawing strength of TPY group, thus, further modify the fluorescence,
LUMO energy levels, and carrier transporting properties of three compounds, but
barely affect triplet energy levels. The non-doped blue devices based on TPY22DPA,
TPY33DPA, and TPY44DPA exhibited high maximum EQEs of 4.9%, 3.8%, and
2
.7%, respectively, whereas the red phosphorescent device hosts based on
TPY22DPA, TPY33DPA, and TPY44DPA exhibited high maximum EQEs of 19.1%,
0.9%, and 17.2%, respectively. These results illustrate that the TPY/DPA derivatives
2
are the most excellent blue fluorescent emitters and are outstanding host materials for
red phosphorescent dopants. Furthermore, our work also successfully presents a

Page 11 of 25
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DOI: 10.1039/C4TC02459A
simple design strategy for further and precisely adjusting the properties of
multifunctional blue fluorophors.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (Grant
No. 51373190, 51033007, 51103169, 21174009), the Beijing Nova Program (Grant
No. Z14110001814067) and the Instrument Developing Project of the Chinese
Academy of Sciences (Grant No. YE201133), P. R. China.
Notes and reference
a
Nano-organic Photoelectronic Laboratory and Key Laboratory of Photochemical
Conversion and Optoelectronic Materials, Technical Institute of Physics and
Chemistry, Chinese Academy of Sciences, Beijing 100190, P.R. China. Fax: +86-10-
6
4879375; Tel: +86-10-82543510; E-mail: xhzhang@mail.ipc.ac.cn,
zhengcaijun@mail.ipc.ac.cn
b
Key Laboratory of Aerospace Advanced Materials and Performance, Ministry of
Education, School of Materials Science and Engineering , Beihang University,
Beijing 100191, P.R. China. E-mail: jyang@buaa.edu.cn
c
University of Chinese Academy of Sciences, Beijing 100039, P.R. China.
d
Functional Nano & Soft Materials Laboratory (FUNSOM) Jiangsu Key Laboratory
for Carbon-Based Functional Materials & Devices and Collaborative Innovation
Center of Suzhou Nano Science and Technology, Soochow University Suzhou, Jiangsu
2
15123 (P. R. China)
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Scheme 1. Synthetic Routes and Chemical Structures of TPY22DPA, TPY33DPA, and
TPY44DPA.

Page 15 of 25
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DOI: 10.1039/C4TC02459A
a) 0.5
TPY22DPA
1
00 ℃
0.0
-
-
0.5
1.0
50
100
150
200
Temperature(℃)
b)
0
-
10
20
TPY22DPA
TPY44DPA
TPY33DPA
-
200
250
300
Temperature(℃)
c)
TPY22DPA
TPY33DPA
TPY44DPA
1.0
0.8
0.6
0.4
0.2
0.0
200
300
400
500
600
Temperature(℃)

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Figure 1. DSC and TGA thermograms of TPY22DPA, TPY33DPA, and TPY44DPA.
HOMO
LUMO
TPY22DPA
TPY33DPA
TPY44DPA
Figure 2. Calculated spatial distributions of the HOMO/LUMO of TPY22DPA,
TPY33DPA, and TPY44DPA.

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TPY44DPA
TPY33DPA
TPY22DPA
-
2.5
-2.0
-1.5
0.0
0.5
1.0
1.5
+
Potential (V vs Ag/Ag )
Figure 3. Cyclic voltammograms of TPY22DPA, TPY33DPA, and TPY44DPA in
DMF.

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a) 1.2
1.2
1.0
0.8
0.6
0.4
0.2
0.0
TPY22TPA
TPY33TPA
TPY44TPA
1.0
0.8
0.6
0.4
0.2
0.0
300
350
400
450
500
550
600
650
Wavelength (nm)
b)
1.0
0.8
0.6
0.4
0.2
0.0
TPY22DPA-film
TPY33DPA-film
TPY44DPA-film
400
450
500
550
600
Wavelength(nm)
C)
TPY44DPA
3
TPY33DPA
TPY22DPA
2
1
0
450
500
550
600
650
700
Wavelength (nm)

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Figure 4. a) UV-vis absorption and PL spectra of TPY22DPA, TPY33DPA, and
TPY44DPA in ethyl acetate solution at room temperature. (b) PL spectra of
TPY22DPA, TPY33DPA, and TPY44DPA in solid film at room temperature. c) The
phosphorescence spectra of TPY22DPA, TPY33DPA, and TPY44DPA in 2-
methyltetrahydrofuran at 77K.
Table 1 Summary of the Physical Properties of TPY22DPA, TPY33DPA, and
TPY44DPA
a
a
b
c
d
d
λabs
nm)
λPL
λPL
ET
LUMO
(eV)
HOMO
(eV)
Tg
Tm
Td
a
Materials
Φf
(
(nm) (nm)
(eV)
2.44
2.43
2.42
(°C)
(°C)
(°C)
TPY22DPA
TPY33DPA
TPY44DPA
281,350
272,356
270,360
460
470
487
455
465
475
0.76
0.67
-2.83
-2.86
-2.93
-5.54
-5.52
100
241
253
284
384
403
408
e
-
e
0.59
-5.5
-
a
-5
b
c
Measured in Ethyl acetate solution (10 M); Measured in thin solid film; Triplet energy measured in 2-MeTHF
-5
d
e
at77K(10 M); LUMO and HOMO was measured from the onset of reduction and oxidation potentials; Not
observed.

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a)
TPY22DPA
TPY33DPA
TPY44DPA
10
100
10
1
1
100
1000
-
2
Luminance(cd m )
b) 1.0
TPY22DPA
TPY33DPA
TPY44DPA
0.8
0.6
0.4
0.2
0.0
400
500
600
700
Wavelength (nm)

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c)
TPY22DPA
TPY33DPA
TPY44DPA
100
10
10
1
1
1
10
100
1000
10000
-
2
Luminance(cd m )
Figure 5. a) PE-EQE-Luminance plots of device B1, B2, and B3. b) The EL spectra
of device B1, B2, and B3. c) PE-EQE-Luminance plots of device R1, R2, and R3.
Table 2. The Electroluminescent Properties of the Devices
-
2
Max
PE
1000 cd m
Von
V)
Device
CE
EQE
(%)
4.9
CE
PE
EQE
(%)
4.7
(
CIE
-1
-1
-1
-1
(
cd A ) (lm W )
(cd A ) (lm W )
B1
B2
B3
R1
R2
R3
3.0
2.6
2.7
2.9
2.4
2.5
5.6
5.3
4.4
5.6
5.3
4.5
3.7
3.5
(0.15,0.11)
(0.15,0.17)
(0.16,0.26)
(0.60,0.40)
(0.59,0.40)
(0.59,0.40)
3.8
2.8
3.2
1.3
5.0
5.2
2.3
1.1
38.5
42.7
35.5
40.2
53.6
37.2
19.1
20.9
17.2
36.2
35.9
30.5
28.4
32.2
23.9
17.9
17.6
15.0

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a)
TPY22DPA-electron
TPY22DPA-hole
4
00
00
00
00
TPY22DPA-Ir(MDQ) acac-electron
2
TPY22DPA-Ir(MDQ) acac-hole
2
3
2
1
0
2
4
6
8
10
Voltage (V)
600
450
300
150
0
b)
TPY33DPA-electron
TPY33DPA-hole
TPY33DPA-Ir(MDQ) acac-electron
2
TPY33DPA-Ir(MDQ) acac-hole
2
0
2
4
6
8
10
Voltage (V)
1200
c)
TPY44DPA-electron
TPY44DPA-hole
TPY44DPA-Ir(MDQ) acac-electron
2
900
600
300
TPY44DPA-Ir(MDQ) acac-hole
2
0
2
4
6
8
10
Voltage (V)

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Figure 6. Current density versus voltage characteristics of the hole-only and
electron-only devices for a) TPY22DPA and TPY22DPA with 2%Ir(MDQ) acac; b)
2
TPY33DPA and TPY33DPA with 2%Ir(MDQ) acac; c) TPY44DPA
and
2
TPY44DPA with 2%Ir(MDQ) acac.
2
Figure 7. Energy level diagram of the devices based on the TPY22DPA, TPY33DPA,
and TPY44DPA.
a)
1
200
150
100
50
0
0000
1000
100
10
1
3
4
5
6
7
Voltage (V)

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100
b)
10
10
PE
EQE
1
1
1000
10000
-
2
Luminance (cd m )
-
2
1
.0
.8
.6
.4
.2
.0
100 cd m
c)
-
2
-
1
000 cd m
2
0
0
0
0
0
1
0000 cd m
400
500
600
700
800
Wavelength(nm)
Figure 8. a) Current density-Luminance-Voltage characteristics of device W; b) PE-
EQE-Luminance plots of device W; c) The EL spectra of device W.

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A series of multifunctional fluorophors as highly efficient blue fluorescent emitters
and red phosphorescent hosts.