A novel blue fluorescent polymer for solution-processed fluorescent-phosphorescent hybrid WOLEDs

Article abstract of DOI:10.1039/c4tc02899f

Two blue-emitting fluorescent polymers PTPATPPO and PTPATPP with small singlet-triplet splitting comprising triphenylamine and triphenylphosphine/triphenylphosphine oxide moieties have been designed and synthesized. An appropriate overlap between the HOMO

Full text of DOI:10.1039/c4tc02899f

View Article Online
View Journal
Journal of
Materials Chemistry C
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: L. Li, J. Wang, X.
Xu, Y. Tian, C. Yao and R. Liu, J. Mater. Chem. C, 2015, DOI: 10.1039/C4TC02899F.
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
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
Information for Authors.
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 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.
www.rsc.org/materialsC

Page 1 of 29
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C4TC02899F
Novel Blue Fluorescent Polymer for Solution-Processed
Fluorescence-Phosphorescence Hybrid WOLEDs
*
Jinshan Wang, Xinjun Xu , Yuan Tian, Chuang Yao, Ronghua Liu and Lidong Li
*
Two blue-emitting fluorescent polymers PTPATPPO and PTPATPP with small
singlet-triplet splitting comprising triphenylamine and
triphenylphosphine/triphenylphosphine oxide moieties have been designed and synthesized.
An appropriate overlap between the HOMO and LUMO in these compounds was realized. This
design strategy endows the two blue-emitting polymers with high triplet energy of 2.45 and 2.46
eV, a shallow HOMO level of -5.21 and -5.23 eV, and bipolar feature to act as good host materials.
Monochromic organic light-emitting devices (OLEDs) using these polymers as emitters show
sky-blue emissions with Commission Internationale de l’Eclairage (CIE) coordinates of (0.24,
0
.32) and (0.24, 0.31) together with a maximum current efficiency of 3.63 cd/A. Moreover, the
single-emitting-layer two-element fluorescence-phosphorescence (F−P) hybrid white OLEDs based
on PTPATPPO or PTPATPP as both hosts and blue-emitting fluorophors were fabricated by a
solution process. Among the two polymers, PTPATPPO-based F-P hybrid white OLEDs shows a
better performance with a maximum current efficiency of 10.5 cd/A, a maximum external quantum
−
efficiency of 6.1%, CIE coordinates of (0.40, 0.34), and a maximum luminance of 11962 cd m .
2
_
_____________________________________________
1

Journal of Materials Chemistry C
Page 2 of 29
View Article Online
DOI: 10.1039/C4TC02899F
State Key Lab for Advanced Metals and Materials, School of Materials Science and Engineering,
University of Science and Technology Beijing, Beijing 100083, P. R. China. E-Mail:
lidong@mater.ustb.edu.cn; xuxj@mater.ustb.edu.cn Fax: +86-10-82375712.
†
Electronic supplementary information (ESI) available: PL in different solvents, Fluorescence and
phosphorescence spectra at 77K, TGA and DSC. See DOI: 10.1039/x0xx00000x
2

Page 3 of 29
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C4TC02899F
1
Introduction
Among all the possibilities to achieve white organic light-emitting diodes (WOLEDs), fluorescence
and phosphorescence (F-P) hybrid devices have drawn considerable attention. Generally, in F-P
hybrid WOLEDs, a blue fluorescent emitter is used for singlet excitons, while two-element
1−4
green/red or single-element orange phosphors are used for high-efficiency triplet emissions. Due
to the large possibility of complete utilization of both singlet and triplet excitons in F-P hybrid
5
WOLEDs, an internal quantum efficiency close to 100% can be realized. Besides, compared with
fully phosphorescent WOLEDs, F-P hybrid devices have the unique advantages because the blue
fluorescent emitters are typically more stable than blue phosphors and there is nearly no exchange
energy loss on the way from exciton generation on the host to radiative decay on the phosphors by
6
,7
avoiding the use of a large-gap host material. Therefore, the approach to achieve white light
emission by F-P hybrids is considered to be an ideal solution to obtain high-performance
8
−10
WOLEDs.
Despite the F-P hybrid WOLEDs have the advantages mentioned above, their development is
still subject to many challenges. One of the most important key issues is the separation and
respective utilization of singlet and triplet excitons. Commonly, an extra interlayer was introduced
in the emissive layers (EMLs) between the fluorophor and phosphor layer to separate singlet and
11−14
triplet excitons.
However, such multi-EML structures with the interlayer not only limit the
1
5
device efficiency, but also make the fabrication process be complicated. Moreover, the
multi-EML structure is inconvenient to obtain by solution process which is the most ideal process
for low-cost and large-area manufacturing. To solve the problem and avoid the undesirable triplet
energy transfer from the phosphorescent dopant to the blue fluorophor (i.e. the reverse triplet energy
transfer), recently, compounds with small singlet-triplet splitting (ꢀE ) have been utilized to serve
ST
3

Journal of Materials Chemistry C
Page 4 of 29
View Article Online
DOI: 10.1039/C4TC02899F
7
,16
as the blue fluorescent emitter in F-P hybrid WOLEDs.
However, most of the reported F-P
hybrid WOLEDs are based on small-molecule compounds as hosts fabricated by vacuum thermal
deposition. As a result, blue fluorescent polymers with small ꢀEST are highly desired for their
convenience to construct F-P hybrid WOLEDs through solution-processed method which is
compatible with the large-area roll-to-roll manufacturing.
Due to the absence of suitable blue emitting polymer, polyfluorenes have been commonly used
as a host in F-P hybrid WOLEDs. However, their large ꢀE value (~0.9 eV) is disadvantageous to
ST
1
prevent the undesirable reverse triplet energy transfer, thus resulting in a low device efficiency of
7
1
8,19
WOLEDs.
To achieve a small ꢀE , we tend to construct blue fluorescent polymers by
ST
connecting the electron donor (D) and acceptor (A) groups. In such a molecular structure, the
highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO)
would be located mainly on the D and A parts, respectively. This will lead to a small ꢀE because
ST
7
,20,21
it is inversely proportional to the spatial overlap between HOMO and LUMO.
Conversely,
6
,16,22
since the fluorescence quantum yield is proportional to such an overlap,
ꢀE should not be too
ST
small and there is a trade-off between them to maintain good fluorescence efficiency in devices.
Based on these concepts, we designed and synthesized two sky-blue fluorescent polymers
PTPATPP and PTPATPPO using triphenlyamine as the donor group and
triphenylphosphine/triphenylphosphine oxide as the acceptor group.
2
2
Experimental sections
.1 Materials and equipments
Commercially available materials and reagents were purchased from Sigma-Aldrich, J&K and Alfa
1 13
aesar and used without purification unless otherwise stated. H NMR and C NMR spectra were
4

Page 5 of 29
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C4TC02899F
determined on a 400 MHz AC Bruker spectrometer with chemical shifts reported as ppm. Mass
spectrum data were obtained with a Bruker Daltonics BIFLEX III MALDI-TOF analyzer using the
matrix-assisted laser desorption ionization spectrometer time-of-flight (MALDI) mode. Elementary
analysis (EA) was measured on a FLASH EA1112 element analyzer. Ultraviolet-visible (UV)
absorption and photoluminescence (PL) emission spectra were recorded with a Hitachi U-3900H
spectrophotometer and a Hitachi F-7000 fluorescence spectrometer, respectively. Cyclic
voltammetry was performed on a CH1860D electrochemical analyzer using a Pt button working
electrode, a platinum-wire counter electrode and an Ag/AgCl reference electrode in a 0.1 M
tetrabutylammonium hexafluorophosphate (Bu NPF ) solution in dichloromethane/tetrahydrofuran
4
6
under argon gas protection at room temperature. Thermo-gravimetric analysis (TGA) was
undertaken with a Diamond TG/DTA instrument under a nitrogen atmosphere by measuring the
−1
weight loss during heating from 25 to 650 °C at a rate of 10 °C min . Differential scanning
−
calorimetry (DSC) was performed on a Diamond DSC unit at a heating rate of 10 °C min under an
1
argon atmosphere. The glass transition temperature (T ) was determined from the second heating
g
scan. The molecular weights of the polymers were measured by the gel permeation chromatography
(GPC) method with polystyrene as a standard by using tetrahydrofuran as the eluent.
2
.2 Theoretical calculations
The geometry and electronic properties of the compounds were carried out by Gaussian 09 D.01
2
program. The geometry of the ground state was optimized by density functional theory (DFT).
3
There is no imaginary frequency for the optimized structures. These were accomplished by the
2
4
B3LYP (Becke three parameters hybrid functional with Lee-Yang-Perdew correlation functional)
5

Journal of Materials Chemistry C
Page 6 of 29
View Article Online
DOI: 10.1039/C4TC02899F
functional with the 6-31G(d) atomic basis set. The spatial distribution of the HOMO and LUMO
were depicted by GaussView.
2
.3 Device Fabrication and Measurements
Patterned indium tin oxide (ITO) glass with a sheet resistance of 10 ꢁ per square was used as the
substrate. Before device fabrication, the ITO glass substrates were cleaned with detergent,
deionized water, acetone, and alcohol, and then dried in an oven at 120 °C followed by treating with
UV-ozone. The hole inject layer (PEDOT:PSS), emissive layer and electron transfer layer (SPPO13)
were fabricated by spin-coating in a nitrogen filled glove box (H O < 0.1 ppm, O < 0.1 ppm). The
2
2
−
4
cathode (Ca/Al) was deposited by thermal evaporation under a vacuum of 5.0 × 10 Pa. The
current density−luminance−voltage (J−L−V) and efficiency−current density (η−J) curves were
measured using a Keithley 2612B source-measurement unit and a silicon photodiode that was
calibrated by a PR-655 Spectrascan spectrophotometer.
2
.4 Synthesis and Characterization
The solvents were dried using standard procedures. All other reagents were used as received from
commercial sources unless otherwise stated.
Bis(4-bromophenyl)(phenyl)phosphine
(1).
Under
an
argon
atmosphere,
1
,4-dibromobenzene (14.2 g, 60 mmol) was dissolved in 100 mL dry tetrahydrofuran (THF), then
n-BuLi (26.25 mL, 1.6 M in hexane) was added dropwise at −78 °C and the mixture was kept at this
temperature for 1 h. Then dichloro(phenyl)phosphine (5.9 g, 4.5 mL) was added in one portion.
After stirred for 1 h, the reaction mixture was warmed to room temperature and stirred overnight.
The reaction mixture was extracted with dichloromethane (DCM), then the combined organic
solution was dried over anhydrous magnesium sulfate. The crude product was purified by column
6

Page 7 of 29
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C4TC02899F
1
chromatography to give 1 (10 g, 79.3%) as a white solid. H NMR (400 MHz, CDCl ): δ(ppm) 7.46
3
1
d, J = 7.7, 4H), 7.35 (d, J = 4.1, 3H), 7.27 (d, J = 7.5, 2H), 7.13 (t, J = 7.6, 4H). C NMR (101
3
(
MHz, CDCl ) δ (ppm) 135.98, 135.92, 135.86, 135.28, 135.07, 133.77, 133.57, 131.86, 131.79,
3
1
29.25, 128.83, 128.76, 123.69.
Phenylbis(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)
phenyl)phosphine
(2).
Compound 1 (8.0 g, 18.0 mmol) was dissolved in 100 mL dry tetrahydrofuran and cooled to −78 °C.
Under an argon atmosphere n-BuLi (26.25 mL, 1.6 M in hexane) was added dropwise and the
mixture
was
kept
at
this
temperature
for
1
h.
Then,
2
-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (9.8 mL, 45 mmol) was added and stirred for
another 1 h. The reaction mixture was warmed to room temperature and stirred overnight. The
reaction mixture was extracted with dichloromethane, and then the combined organic solution was
dried over anhydrous magnesium sulfate. The crude product was purified by column
1
chromatography to give 2 (7.2 g, 77.8%) as a white solid. H NMR (400 MHz, CDCl ) δ (ppm) 7.86
3
(d, J = 6.5, 0.6H), 7.73 (d, J = 7.6, 3H), 7.68 – 7.58 (m, 1H), 7.55 – 7.47 (m, 0.3H), 7.42 (t, J = 7.8,
1
3
0
1
.3H), 7.31 – 7.25 (m, 7.8H), 1.31 (s, 24H). C NMR (101 MHz, CDCl ) δ (ppm) 140.65, 140.53,
3
34.64, 134.58, 134.04, 133.85, 133.01, 132.82, 128.87, 128.56, 128.45, 83.81, 24.81. MS
+
MALDI-TOF): m/z calculated for C H B O P 512.2; found 513.2 (M +1).
(
3
0
37
2
4
Phenylbis(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl) phenyl)phosphine oxide (3).
Compound 2 (5.13 g, 10 mmol), FeCl ⋅6H O (305.4 mg, 0.791 mmol), Fe(SCN) (230.64 mg, 2.37
3
2
3
mmol) and iodine (catalytic amount) were added to 60 mL acetonitrile. The mixture was stirred for
0 min at 80 C under vigorous bubbling of oxygen. After removal of the solvent, the reaction
3
°
mixture was extracted with dichloromethane, and then the combined organic solution was dried
over anhydrous magnesium sulfate. The crude product was purified by column chromatography to
7

Journal of Materials Chemistry C
Page 8 of 29
View Article Online
DOI: 10.1039/C4TC02899F
1
give 3 (5.0 g, 94%) as a white solid. H NMR (400 MHz, CDCl ) δ (ppm) 7.85 (d, J = 5.7, 4H), 7.70
3
1
7.56 (m, 6H), 7.53 – 7.47 (m, 1H), 7.42 (d, J = 6.7, 2H), 1.32 (s, 24H). C NMR (101 MHz,
3
–
CDCl ) δ (ppm) 135.57, 134.60, 134.55, 134.48, 132.14, 132.04, 131.97, 131.25, 131.15, 128.55,
3
1
28.43, 84.20, 24.87. MS (MALDI-TOF): m/z calculated for C H B O P 530.2; found 531.2
30 37 2 5
+
M +1).
(
4
-(bis(4-bromophenyl)amino)phenol (4). Tris(4-bromophenyl)amine (10 g, 21 mmol) was
dissolved in 100 mL dry tetrahydrofuran and cooled to −78 °C. Then n-BuLi (13.1 mL, 1.6 M in
hexane, 21 mmol) was added dropwise over 30 min under an argon atmosphere. The solution was
stirred at −78 °C for 1 h, and then triisopropyl borate (4.96 mL, 21 mmol) was added to the mixture
and kept at −78 °C for 1 h. The mixture was stirred overnight at ambient temperature. Then the
reaction mixture was extracted with dichloromethane and the organic layer was dried over
magnesium sulfate. After removal of the solvent, the crude was directly used in the next step
without purification. To the mixture of the crude product, KOH (2.5 g, 45 mmol) and H O (50 mL),
2
H O (30%, 7.6 g, 6.9 mL) was added dropwise at 0 °C. The mixture was heated to 50 °C for 3 h.
2
2
The pH of the aqueous solution was adjusted to 2 with hydrochloric acid and extracted with
dichloromethane. The organic layer was dried over magnesium sulfate and the solvent was removed.
The crude product was purified by column chromatography to give 4 (5.0 g, 58%) as a solid and
was directly used in the next step.
4
-bromo-N-(4-bromophenyl)-N-(4-(octyloxy)phenyl)aniline (5). Compound 4 (3.5 g, 8.35
mmol), 1-bromooctane (2.2 mL, 12.5 mmol) and K CO (3 g, 22 mmol) were added to 50 mL
2
3
acetone. The reaction mixture was refluxed for 24 h under an argon atmosphere. After the solvent
was removed, the crude was extracted with dichloromethane and the combined filtrate was
1
concentrated and purified on silica gel column to afford 5 (3.0 g, 67.7%) as a colorless oil. H NMR
8

Page 9 of 29
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C4TC02899F
(
400 MHz, CDCl ) δ (ppm) 7.28 (1 H, d, J = 8.6), 7.00 (1 H, d, J = 8.6), 6.88 (1 H, d, J = 8.6), 6.83
3
(1 H, d, J = 8.7), 3.92 (1 H, t, J = 6.4), 1.86 – 1.70 (1 H, m), 1.45 (1 H, t, J = 7.0), 1.30 (2 H, s, J =
1
3
1
1
0.8), 0.88 (1 H, t, J = 6.3). C NMR (101 MHz, CDCl ) δ (ppm) 156.40, 146.86, 139.49, 132.16,
3
27.44, 124.24, 115.60, 114.50, 68.30, 31.85, 29.39, 29.34, 29.27, 26.10, 22.69, 14.14. MS
+
MALDI-TOF): m/z calculated for C H Br NO 531.3; found 531.1(M ).
(
2
6
29
2
General Procedure of the Suzuki Polymerization: To a mixture of compound 5 and 2 (or 3)
with the corresponding feed ratios, Aliquat 336 (two drops) and Pd(PPh ) (1 mol %) were added.
3
4
The mixture was degassed for 30 min, and then 5 mL of degassed aqueous K CO (2 M) and
2
3
degassed toluene (20 mL) were added under argon. The mixture was heated to 90 °C and stirred for
8 h in the dark. Then, the end-capping reagents phenylboronic acid (100 mg) in 2 mL degassed
4
toluene and bromobenzene (0.5 mL) were added sequentially into the reaction mixture, after which
the mixture was stirred for another 12 h for each addition. After that, the mixture was poured into
methanol. The precipitate was collected by filtration and then extracted with CH Cl followed by
2
2
washing and drying with anhydrous sodium sulfate. Then, the solution was concentrated to an
approximate volume and poured into methanol to obtain polymer fibers. The polymer was purified
by hexane and methanol to remove the catalyst and residual monomers. Thereafter, the resulting
polymer was subjected to Soxhlet extraction with acetone for 24 h. Finally, the precipitation
procedure in chloroform/methanol was repeated for two times.
PTPATPP: Compound 2 (0.7826 g, 1.417 mmol) and 5 (0.75 g, 1.417 mmol) were used in the
1
polymerization, and a yellow fiber was obtained (350 mg, 39%). H NMR (400 MHz, CDCl ): δ
3
7
.75 – 7.67 (4 H, m), 7.64 (3 H, s), 7.54 (2 H, d, J = 8.0), 7.48 – 7.41 (5 H, m), 7.30 (2 H, d, J = 8.7),
.14 – 7.10 (2 H, m), 7.05 (3 H, d, J = 8.2), 6.94 (2 H, d, J = 8.7), 6.85 (2 H, t, J = 9.5), 3.92 (2 H, t,
7
1
J = 6.1), 1.82 – 1.65 (2 H, m), 1.44 (2 H, s), 1.28 (8 H, d, J = 11.6), 0.86 (3 H, d, J = 6.7). C NMR
3
9

Journal of Materials Chemistry C
Page 10 of 29
View Article Online
DOI: 10.1039/C4TC02899F
(101 MHz, CDCl3): δ 156.43, 147.92, 146.84, 144.15, 139.54, 133.13, 132.69, 132.59, 132.19,
1
1
8
32.15, 132.10, 131.94, 128.60, 128.48, 127.92, 127.66, 126.57, 126.45, 124.70, 122.89, 122.48,
15.59, 68.43, 31.81, 29.36, 29.32, 29.23, 26.08, 22.65, 13.97. Anal. Calcd. for (C H NOP) : C,
4
4
44
n
3.38; H, 7.0; N, 2.21, found: C, 83.95; H, 6.70; N, 2.15. GPC: M = 6.9 K, PDI = 1.09.
w
PTPATPPO: Compound 3 (0.953 g, 1.794 mmol) and 5 (0.951 g, 1.794 mmol) were used in
1
the polymerization, and a yellow fiber was obtained (400 mg, 34.3%). H NMR (400 MHz, CDCl ):
3
δ (ppm) 7.89 (0.5 H, d, J = 5.8), 7.71 (4 H, d, J = 8.5), 7.66 (4 H, d, J = 7.2), 7.56 (1.5H, d, J =7.6),
7
.52 – 7.39 (5 H, m), 7.30 (1 H, d, J = 8.7), 7.15 – 7.09 (3H, m), 7.05 (2H, d, J = 8.3), 6.94 (1 H, d,
J =8.6), 6.85 (2H, t, J =9.2), 3.92 (2 H, t, J = 5.6), 1.82 – 1.72 (2H, m), 1.46 – 1.40 (2 H, m), 1.34 –
1
3
1
1
1
.26 (8 H, m), 0.86 (3 H, d, J =6.7). C NMR (101 MHz, CDCl ) δ (ppm) 156.45, 147.95, 146.77,
3
44.49, 139.39, 134.56, 132.82, 132.30, 131.16, 127.96, 127.69, 126.71, 126.59, 124.71, 122.90,
22.42, 115.66, 114.71, 105.43, 68.40, 31.76, 29.09, 26.00, 24.87, 22.56, 14.08. Anal. Calcd. for
(C H NO P) : C, 81.33; H, 6.83; N,2.16, found : C, 81.44 ; H, 6.72; N 1.97. GPC: M = 4.97 K,
44 44 2 n w
PDI = 1.02.
3
3
Results and discussion
.1 Synthesis and characterization
The synthetic route of PTPATPP and PTPATPPO is outlined in Scheme 1. Triphenylamine and
triphenylphosphine/triphenylphosphine oxide were designed as hole and electron transport units,
respectively, for charge balancing in the emission zone. 1,4-Dibromobenzene was treated with
n-BuLi and dichloro(phenyl)phosphine to produce bis(4-bromophenyl)(phenyl)phosphine (1), then
it was converted into boronic esters. After that, it was oxidized to afford the monomer for coupling
under the presence of FeCl , KSCN and I . 4-(Bis(4-bromophenyl)amino)phenol (4) and
3
2
1
0

Page 11 of 29
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C4TC02899F
4
-bromo-N-(4-bromophenyl)-N-(4-(octyloxy)phenyl)aniline (5) were prepared according to
2
previously reported procedures. Finally, the polymer was synthesized by suzuki coupling reaction
5
of 5 with phenylbis(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)phosphine (2)/
phenylbis(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)phosphine oxide
(3). The
1
13
chemical structures of the compounds were fully characterized by H NMR, and C NMR
spectroscopy, mass spectrometry and elemental analysis.
3
.2 Photophysical properties
The ultraviolet-visible (UV) absorption and photoluminescence (PL) emission spectra of the two
polymeric fluorophors are shown in Fig. 1 and Table 1. The absorption peak around 270 nm in
solution is associated with the n-π* transition in triphenylamine, while the peak with longer
wavelength around 370 nm can be attributed to the intramolecular charge transfer (ICT) transition
from the electron-rich triphenylamine moiety to the relatively electron-deficient
triphenylphosphine/triphenylphosphine oxide group. Both of the polymers show blue emissions in
toluene at room temperature. In their solid states, the UV and PL spectra were slightly red shifted
by 10−15 nm and 30−40 nm compared with those in solutions, respectively. Large shifts of PL
emission peaks are observed in polar solvents (Fig. S1, ESI†), which are typically associated with
6
compounds that have strong ICT character. The triplet energy of PTPATPPO and PTPATPP
determined by the shortest-wavelength phosphorescence peaks of their phosphorescence spectra at
7
7 K (Fig. S2, ESI†) in 2-methyltetrahydrofuran were 2.45 and 2.46 eV, respectively. From their
fluorescence and phosphorescence emissions in 2-methyltetrahydrofuran at 77 K, the ꢀE values of
ST
PTPATPPO and PTPATPP were determined to be 0.49 and 0.47 eV, respectively. Such values are
1
7,26
apparently smaller than that of other blue emissive polymers.
The fluorescent quantum yields
1
1

Journal of Materials Chemistry C
Page 12 of 29
View Article Online
DOI: 10.1039/C4TC02899F
(
Φ ) of PTPATPPO and PTPATPP in cyclohexane solution were determined to be 0.59 and 0.44
f
using 1,4-Bis(5-phenyl-2-oxazolyl)benzene (Φ = 0.97 in cyclohexane solution) as a standard.
f
2
Because the phosphine oxide moiety does not contribute to the conjugation, there is no significant
7
difference in the photophysical properties between the two polymers.
3
.3 Thermal properties
The thermal properties of PTPATPP and PTPATPPO were investigated by TGA and DSC. TGA
measurement reveals their high thermal-decomposition temperatures (T , corresponding to 5%
d
°
°
weight loss) of 220 C (PTPATPP) and 271 C (PTPATPPO) (Fig. S3, ESI†). The DSC curves
°
exhibit distinct glass-transition temperatures (T ) of 121 C (PTPATPP) and 127 C (PTPATPPO)
°
g
during the second heating scans (the inset of Fig. S3, ESI†). These results imply that the phosphine
2
oxide group improves the compound’s thermal stability. As expected, such high T values are
7
g
beneficial to the stability of OLEDs.
3
.4 Electrochemical properties and theoretical calculations
The density functional theory at the B3LYP/6-31G(d) level using the Gaussian 09 program
was employed to demonstrate the electronic structure of these polymers. For computational
simplification, the backbones were simplified to two repeating units. Fig. 2 shows the frontier
molecular orbitals (FMOs) of PTPATPPO and PTPATPP. The two compounds show different
optimization molecular conformations. The calculated energy differences between HOMO and
HOMO-1 and between LUMO and LUMO+1 were extremely small (PTPATPPO: 0.02 and 0.17
eV; PTPATPP: 0.03 and 0.16 eV). Therefore, considering the interchange between HOMO and
HOMO-1 and between LUMO and LUMO+1 when the molecules vibrate, these orbitals can all
contribute to the electronic properties of the compounds. The HOMO and HOMO-1 of the two D-A
1
2

Page 13 of 29
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C4TC02899F
molecules mainly locates at the electron-rich triphenylamine unit, whereas the LUMO and
LUMO+1 primarily populates on the biphenyl which joins the triphenylamine and
triphenylphosphine/triphenylphosphine oxide units. Furthermore, compared with PTPATPP, less
electron density was found on phosphine atom and the benzene ring of triphenylphosphine in
LUMO and LUMO+1 of PTPATPPO. This may be attributed to the poorer conjugate of
phosphine-oxygen with the backbones than that of phosphine atom. In addition, the spatial overlap
between HOMO and LUMO in PTPATPPO and PTPATPP molecules is limited on the
biphenyl-π-conjugated bridge. Reversible anodic oxidation behaviors of PTPATPPO and PTPATPP
occurred during the potential sweep (see Fig. 3), indicating the stable electrochemical doping
processes. However, the polymers displayed irreversible reduction behavior during the cathodic
sweep. The HOMO energy levels of PTPATPPO and PTPATPP were calculated using ferrocene as
an internal reference to be -5.21 and -5.23 eV, respectively. Meanwhile, the LUMOs were -2.47
and -2.44 eV, respectively, measured from the cyclic voltammograms. Such HOMO levels of the
two polymers suggest that there are almost no hole-injection barriers for them when employing the
commonly-used poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) (5.2 eV)
as a hole-injecting layer in OLEDs. The electron-deficient phosphine-oxygen group increases the
electron affinity of PTPATPPO which leads to the slightly lower LUMO level. This is consistent
2
7
with the conclusion in the literature. It allows electrons transport easier from the cathode or the
electron transport layer to PTPATPPO than to PTPATPP. The smaller energy barriers for hole and
electron injection make PTPATPPO be better to balance holes and electrons than PTPATPP inside
the emitting layer.
3
.5 Electroluminescence properties
1
3

Journal of Materials Chemistry C
Page 14 of 29
View Article Online
DOI: 10.1039/C4TC02899F
Before application in F-P hybrid WOLEDs, the performance of PTPATPPO and PTPATPP as blue
fluorescent emitters was evaluated. Devices using the structure of ITO/PEDOT:PSS (40 nm)/ EML
(60 nm) /SPPO13 (70 nm)/Ca (10 nm)/Al (100 nm) were fabricated, where EML is either
PTPATPPO
or
PTPATPP
in
neat
which
film,
and
SPPO13
as
is
2
,7-bis(diphenylphosphoryl)-9,9'-spirobifluorene
was
used
the
electron-transporting and hole-blocking layer for its low HOMO/LUMO energy levels beneficial
28
for blocking holes and facilitating electron injection. The nondoped devices based on the neat
PTPATPPO and PTPATPP films exhibit stable sky-blue fluorescent spectra with emission peaks at
4
0
73 and 471 nm (Fig. 4) and Commission Internationale de L’Eclairage (CIE) coordinates of (0.24,
.32) and (0.24, 0.31), respectively. The EL spectra are broader than the PL spectrum, indicating
that the EL emission is not generated exclusively from the emitter itself. The EL emission was
thought to be caused by both the emitting material and the EML/electron transport layer
2
9,30
interface.
blue emissive devices are 3.63 and 1.56 cd/A, while the maximum external quantum efficiency
EQEs) are 1.9% and 0.76%, respectively (Fig. 4 and Table 2). Since triphenylphosphine oxide has
The maximum current efficiencies (CEs) of the PTPATPPO and PTPATPP based
(
3
1
stronger electron-accepting ability than triphenylphosphine group, PTPATPPO is expected to
show a better bipolar property than PTPATPP. Consequently, the maximum EQE and CE in
PTPATPPO-based sky-blue OLED were increased by about 1.5-fold compared to the
PTPATPP-based one, which may be attributed to the balanced charge transport in the EML
benefited from the bipolar nature of PTPATPPO.
To further distinguish the bipolar transporting properties of the two polymers, hole-only and
electron-only devices were fabricated. The hole-only devices have a configuration of
ITO/PEDOT:PSS (40 nm)/ PTPATPPO or PTPATPPO (70 nm)/Au (60 nm), while the
1
4

Page 15 of 29
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C4TC02899F
electron-only devices have a structure of ITO/Al (100 nm)/ PTPATPPO or PTPATPPO (70 nm)/Ca
10 nm)/Al (100 nm). Fig. 5 shows the current density-voltage characteristic curves of these
(
devices. The electron current density of the PTPATPPO was higher than that of the PTPATPP,
indicating better electron-injection and transport properties of the PTPATPPO. Meanwhile, the
difference of current density between the hole-only and electron-only devices based on PTPATPPO
is much smaller compared to PTPATPP at the same voltage. Fig. 5 also shows that the hole density
of PTPATPPO was lower than that of PTPATPP. This proves that the electron transport property is
improved and that the hole transport ability is reduced by incorporating the phenyl phosphine oxide
3
unit into the polymer backbone. These results clearly show that the hole and electron fluxes are
2
more balanced in PTPATPPO than in PTPATPP.
Finally, single-EML F-P hybrid WOLEDs using PTPATPPO and PTPATPP as the blue
fluorescent emitters and hosts for phosphors have been fabricated based on a simple configuration
of
ITO/PEDOT:PSS
(40
nm)/blue-emitting
polymer:
bis(2-phenylquinoline)-(acetylacetonate)iridium (Ir(2-phq) acac) (60 nm) (2 wt%)/SPPO13 (70
2
nm)/ Al (100 nm). The orange-red emitter Ir(2-phq) acac was used as a complementary colour for
2
sky-blue to achieve white emission. Such two-element WOLEDs simplified the preparation
procedure relative to the three-element (red-green-blue) WOLEDs. All the organic layers including
PEDOT:PSS, EML, and electronic-transporting layer (SPPO13) were deposited by a solution
process. The structurally-simplified single-emissive-layer WOLEDs based on PTPATPPO and
PTPATPP have avoided the introduction of intermediate layer. Normalized EL spectra of the two
F-P hybrid WOLEDs at different driving voltages are shown in Fig. 6a and 6b. The white light
emission covering the range of 420−750 nm, and there are two distinct emission peaks, each from
the fluorescent polymer and Ir(2-phq) acac. This implies PTPATPPO and PTPATPP successfully
2
1
5

Journal of Materials Chemistry C
Page 16 of 29
View Article Online
DOI: 10.1039/C4TC02899F
executed the functions of sky-blue fluorescent emitter and orange-red phosphorescence host. The
CIE coordinates show only small shifts from (0.47, 0.34) to (0.40, 0.34) and from (0.42, 0.35) to
(0.44, 0.35) with increasing the driving voltage for PTPATPPO (8−14 V) and PTPATPP (8−12 V)
based WOLEDs, respectively. The blue component was gradually enhanced along with increase of
the voltage. This probably results from the slow decay triplets concentration being high under high
5
driving voltages, leading to increased T-T annihilation under a high voltage. However, the
emission color always sustain in the white light area. The F-P hybrid WOLEDs have a maximum
−
1
−1
CE and EQE of 10.5 cd A and 6.1% for PTPATPPO based device, while 2.0 cd A and 1.2% for
PTPATPP based device (Fig 6c and 6d). Again, in white emission devices, PTPATPPO showed
better luminous efficiency than PTPATPP which can also be ascribed to its balanced carrier
transport mentioned above. It should be noted that the PTPATPPO based devices show stable
−1
efficiency roll-off. For example, its current efficiency decreased from the maximum 10.5 cd A to
−
0.4 cd A at 1000 cd m . The turn-on voltages of PTPATPPO and PTPATPP based WOLEDs
1
−2
1
were obtained from the current density-luminance-voltage characteristics (Fig. 7) and showed low
values of 4.9 and 5.5 V, respectively. In these WOLEDs, singlet excitons are formed in the
polymers for producing blue fluorescence, whereas the triplet excitons generated in the polymers
will transfer to the phosphorescent dopant to give off red phosphorescence, resulting in white
emission (Fig. 8).
4
Conclusions
In conclusion, two ICT transition based sky-blue fluorescent polymers, PTPATPPO and PTPATPP,
have been designed and synthesized using a D-A structural strategy. They have been proved as
good sky-blue emissive fluorescent polymers and suitable hosts for orange phosphor to achieve
1
6

Page 17 of 29
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C4TC02899F
solution-processed single-EML WOLEDs. An appropriate overlap between the HOMO and LUMO
on triphenylamine and triphenylphosphine/triphenylphosphine oxide moieties in the two polymers
simultaneously provided an efficient blue fluorescent emission, appropriate HOMO/LUMO energy
levels, high T energy of 2.45 and 2.46 eV, and small ꢀE of 0.49 and 0.47 eV for PTPATPPO and
1
ST
PTPATPP, respectively. Among the two sky blue-emissive polymers, PTPATPPO based
single-EML F-P WOLEDs fabricated though a facile solution process showed a high device
−
1
performance with a maximum CE of 10.5 cd A and a maximum EQE of 6.1%, which are much
higher than those of devices with the commonly-used polyfluorenes as both blue fluorescent
emitters and phosphorescent hosts.
Acknowledgements
This work is supported by the National Natural Science Foundation of China (51273020,
5
1373022), the Fundamental Research Funds for the Central Universities (FRF-SD-12-005B) and
Research Fund for the Doctoral Program of Higher Education of China (20130006110007). We
acknowledge National Supercomputing Center in Shenzhen for providing the computational
resources and Gaussian 09 (D.01).
Notes and references
1
Y. Sun, N. C. Giebink, H. Kanno, B. Ma, M. E. Thompson, S and R. Forrest, Nature 2006, 440,
08.
9
2
3
4
G. Schwartz, M. Pfeiffer. S. Reineke, K. Walzer and K. Leo, Adv. Mater, 2007, 19, 3672.
Y. Tao, Q. Wang, Y. Shang, C. Yang, L. Ao, J. Qin and Z. Shuai, Chem. Comm. 2009, 77.
M. E. Kondakova, J. C. Deaton, T. D. Pawlik, D. J. Giesen, D. Y. Kondakov, R. H. Young and
1
7

Journal of Materials Chemistry C
Page 18 of 29
View Article Online
DOI: 10.1039/C4TC02899F
J. D. Shore, J. Appl. Phys. 2010, 107, 014515.
J. Ye, C. J. Zheng, X. M. Ou, X. H. Zhang, M. K. Fung and C. S. Lee, Adv. Mater. 2012, 24,
410.
X. K. Liu, C. J. Zheng, M. F. Lo, J. Xiao, Z. Chen, C. L. Liu and X. H. Zhang, Chem. Mater.
013, 25, 4454.
G. Schwartz, S. Reineke, T. C. Rosenow, K. Walzer and K. Leo, Adv. Fun. Mater. 2009, 19,
319.
5
6
7
3
2
1
8
9
F. Zhao, N. Sun, H. Zhang, J. Chen and D. Ma, J. Appl. Phys. 2012, 112, 084504.
W. Y. Hung, L. C. Chi, W. J. Chen, Y. M. Chen, S. H. Chou, and K. T. Wong, J. Mater. Chem.
2010, 20, 10113.
1
0 X. Yang, S. Zheng, R. Bottger, H. S. Chae, T. Tanaka, S. Li, A. Mochizuki and G. E. Jabbour, J.
Phys. Chem. C 2011, 115, 14347.
1
1 C. L. Ho, W. Y. Wong, Q. Wang, D. Ma, L. Wang and Z. Lin, Adv. Fun. Mater. 2008, 18, 928.
2 B. P. Yan, C. C. Cheung, S. C. Kui, H. F. Xiang, V. A. L. Roy, S. J. Xu and C. M. Che, Adv.
Mater. 2007, 19, 3599.
1
1
1
1
1
3 J. H. Seo, I. H. Park, G. Y. Kim, K. H. Lee, M. K. Kim, S. S. Yoon and Y. K. Kim, Appl. Phys.
Lett. 2008, 92, 183303.
4 U. S. Bhansali, H. Jia, M. A. Quevedo-Lopez, B. E. Gnade, W. H. Chen and M. A. Omary,
Appl. Phys. Lett. 2009, 94, 203501.
5 N. Sun, Q. Wang, Y. Zhao, Y. Chen, D. Yang, F. Zhao, J. Chen and D. Ma, Adv. Mater. 2014,
26, 1617.
6 C. J. Zheng, J. Wang, J. Ye, M. F. Lo, X. K. Liu, M. K. Fung, X. H. Zhang and C. S. Lee, Adv.
Mater. 2013, 25, 2205.
1
8

Page 19 of 29
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C4TC02899F
1
1
7 H. D. Burrows, J. Seixas de Melo, C. Serpa, L. Arnaut, M. d. G. Miguel, A. Monkman, I.
Hamblett and S. Navaratnam, Chem. Phys. 2002, 285, 3.
8 X. Gong, W. Ma, J. C. Ostrowski, G. C. Bazan, D. Moses and A. J. Heeger, Adv. Mater. 2004,
16, 615.
19 Y. Xu, J. Peng, J. Jiang, W. Xu, W. Yang and Y. Cao, Appl. Phys. Lett. 2005, 87, 193502.
20 H. Uoyama, K. Goushi, K. Shizu, H. Nomura and C. Adachi, Nature 2012, 492, 234.
21 H. Wang, L. Xie, Q. Peng, L. Meng, Y. Wang, Y. Yi and P. Wang, Adv. Mater. 2014, 26, 5198
22 A. Endo, K. Sato, K. Yoshimura, T. Kai, A. Kawada, H. Miyazaki and C. Adachi, Appl. Phys.
Lett. 2011, 98, 083302.
2
3 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G.
Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P.
Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota,
R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A.
Montgomery, Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V.
N. Staroverov, T. Keith, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant,
S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V.
Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R.
Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth,
P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, O. Farkas, J. B. Foresman, J. V. Ortiz,
J. Cioslowski, and D. J. Fox, Gaussian 09 Revision D.01, Gaussian, Inc., Wallingford CT, 2013.
4 A. D. Becke, J. Chem. Phys. 1993, 98, 5648.
2
2
2
5 M. Wang, C. Li, A. Lv, Z. Wang, Z. Bo and F. Zhang, Polymer 2012, 53, 324.
6 A. Köhler and D. Beljonne, Adv. Funct. Mater. 2004, 14, 11.
1
9

Journal of Materials Chemistry C
Page 20 of 29
View Article Online
DOI: 10.1039/C4TC02899F
27 S. O. Jeon, K. S. Yook, C. W. Joo and J. Y. Lee, Adv. Funct. Mater. 2009, 19, 3644.
28 K. S. Yook, S. E. Jang, S. O. Jeon and J. Y. Lee, Adv. Mater. 2010, 22, 4479.
29 H. Lee, Y. I. Park, B. Kim, J.-H. Lee and J. Park, Opt. Mater. Express 2014, 4, 924.
30 M. Mazzeo, D. Pisignano, F. Della Sala, J. Thompson, R. Blyth, G. Gigli, R. Cingolani, G.
Sotgiu and G. Barbarella, Appl. Phys. Lett. 2003, 82, 334.
3
3
1 L. Pehlivan, E. Métay, D. Delbrayelle, G. Mignani and M. Lemaire, Tetrahedron 2012, 68,
3151.
2 S. O. Jeon, S. E. Jang, H. S. Son and J. Y. Lee, Adv. Mater. 2011, 23, 1436.
2
0

Page 21 of 29
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C4TC02899F
Scheme 1 Synthesis of PTPATPP and PTPATPPO
2
1

Journal of Materials Chemistry C
Page 22 of 29
View Article Online
DOI: 10.1039/C4TC02899F
Table 1 Summary of physical properties of the two fluorophors
c
c
d
e
f
g
h
i
Compound
λmax,abs
λmax,PL λmax,f
λmax,p
ET
ꢀEST
Φf
Eg
HOMO
LUMO
(
nm)
(nm)
(nm)
(nm)
508
(eV)
2.45
(eV)
0.49
%
(eV)
2.86
(eV)
(eV)
a
a
PTPATPPO
PTPATPP
362
435
422
59
-5.21
-2.47
b
a
b
b
3
70
361
68
462
a
434
423
505
2.46
0.47
44
2.85
-5.23
-2.44
b
3
473
a
b
Measured in toluene solution at room temperature. Measured in neat film. Measured in 2-MeTHF solution at 77 K.
c
d
e
f
Calculated from the Phos emission peaks at 77 K. Singlet−triplet energy difference, ꢀE =S −T . Fluorescence quantum yields
ST
1
1
measured in dilute cyclohexane solution, using 1,4-bis(5-phenyl-2-oxazolyl)benzene (Φ = 0.97 in cyclohexane solution) as a
f
g h,i
reference. Calculated from the absorption spectrum of film. Deduced from cyclic voltammetry.
Table 2. Electroluminescence Characteristics of the Devices
2
maximum
@1000cd/m
Emission layer
Von
Lmax
CE
EQE
%
PE
(lm/W)
2.2
CE
(cd/A)
3.6
EQE
PE
CIE
2
V) (cd/m ) (cd/A)
(
%
1.5
-
(lm/W)
1.43
-
(x,y)
(
(
0.24,0.32)
PTPATPPO
PTPATPP
7.8
7.2
3105
971
3.63
1.56
10.5
2.0
1.9
0.76
6.1
1.2
0.24,0.31)
0.98
7.2
-
(
(
0.40,0.34)@14V
0.44,0.35)@12V
PTPATPPO:2% Ir(2-phq) acac 4.9
2
11962
4789
10.4
1.3
6.0
0.8
6.9
PTPATPP:2% Ir(2-phq) acac
2
5.5
0.86
0.86
2
2

Page 23 of 29
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C4TC02899F
Fig. 1 UV absorption (Abs) and PL/phosphorescence (Phos) spectra of PTPATPPO
a) and PTPATPP (b) in solution and film.
(
23

Journal of Materials Chemistry C
Page 24 of 29
View Article Online
DOI: 10.1039/C4TC02899F
Fig. 2 HOMO and LUMO distributions of PTPATPPO (a) and PTPATPP (b)
calculated by using DFT with the B3LYP 6-31G(d) level.
24

Page 25 of 29
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C4TC02899F
Fig. 3 Cyclic voltammograms (CV) of PTPATPPO and PTPATPP with 0.1 M
tetrabutylammonium hexafluorophosphate (Bu NPF ) as a supporting electrolyte,
Ag/AgCl as a reference electrode, Pt/C as a working electrode, and a scan rate of 50
4
6
-1
+
mV s . Ferrocene/ferrocinium (Fc/Fc ) was used as an internal reference. Oxidation
CV was performed in DCM and reduction CV was performed in THF.
25

Journal of Materials Chemistry C
Page 26 of 29
View Article Online
DOI: 10.1039/C4TC02899F
Fig. 4 EL spectra of the PTPATPPO (a) and PTPATPP (b) based sky-blue fluorescent
OLEDs under different voltages. (c) Current density-voltage-luminance
characteristics of the PTPATPPO and PTPATPP based sky-blue fluorescent OLEDs.
(d) Efficiency−current density curves of the PTPATPPO and PTPATPP based
sky-blue fluorescent OLEDs.
Fig. 5 Characteristics of current density−voltage in the hole-only and electron-only
devices of PTPATPPO and PTPATPP.
26

Page 27 of 29
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C4TC02899F
Fig. 6 EL spectra of PTPATPPO (a) and PTPATPP (b) based F-P hybrid WOLEDs at
different voltages. CE-Luminance-EQE characteristics of the PTPATPPO (c) and
PTPATPP (d) based F-P hybrid WOLEDs. The insets in (a) is the photographs of an
OLED under testing at the corresponding voltage.
27

Journal of Materials Chemistry C
Page 28 of 29
View Article Online
DOI: 10.1039/C4TC02899F
Fig. 7 Current density-Luminance-Voltage curves of PTPATPPO (a) and PTPATPP
b) based F-P hybrid WOLEDs at different driving voltages.
(
Fig. 8 Energy level diagrams in PTPATPPO or PTPATPP based F-P hybrid
WOLEDs together with the proposed energy transfer and emission mechanism. Solid
lines correspond to HOMO and LUMO energy levels; circles and triangles refer to the
exciton (S , S and T ) energies for the polymers (blue) and phosphorescent dopant
0
1
1
(red).
28

Page 29 of 29
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C4TC02899F
Table of Contents
Blue-emitting polymers with small singlet-triplet splitting (ꢀEST < 0.5 eV) as host
materials for solution-processed fluorescence-phosphorescence hybrid WOLEDs were
achieved.
29