Syntheses, photoluminescence and electroluminescence of two novel platinum(ii) complexes

Article abstract of DOI:10.1039/C6DT03991J

Two new platinum(ii) cyclometalated complexes with 2-(4-trifluoromethyl)phenylpyridine (4-tfmppy) as the main ligand and tetraphenylimidodiphosphinate (tpip) (Pt-tpip) and tetra(4-fluorophenyl)imidodiphosphinate (ftpip) (Pt-ftpip) as ancillary ligands wer

Full text of DOI:10.1039/C6DT03991J

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Efficient extraction of sulfate from water using
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ARTICLE
Syntheses, photoluminescene and electroluminescene of two
novel platinum(II) complexes
Cite this: DOI:
10.1039/x0xx00000x
Guang-Zhao Lu,a Yan Li,a Yi-Ming Jing,a You-Xuan Zhenga,b,c
*
Two new platinum(II) cyclometallated complexes with (2ꢀ(4–trifluoromethyl)phenyl)pyridine (4ꢀ
tfmppy) as main ligand and tetraphenylimidodiphosphinate (tpip) (Pt-tpip) and tetra(4ꢀ
fluorophenyl)imidodiphosphinate (ftpip) (Pt-ftpip) as ancillary ligands have been developed. All
complexes are green phosphors with photoluminescence quantum efficiency yields of 71.5% and
79.2% in CH2Cl2 solutions at room temperature, respectively. The organic lightꢀemitting diodes
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/
with the double emissive layers structure of ITO
/
TAPC (1,1ꢀbis(4ꢀ(diꢀpꢀ
tolylamino)phenyl)cyclohexane, 40 nm / Pt-tpip or Pt-ftpip : TcTa (4,4’,4’’ꢀtri(9ꢀcarbazoyl)ꢀ
triphenylamine) (5 wt%, 10 nm) / Pt-tpip or Pt-ftpip : 2,6DCzPPy (2,6ꢀbis(3ꢀ(carbazolꢀ9ꢀ
yl)phenyl)pyridine) (5 wt%, 10 nm) / TmPyPB (1,3,5ꢀtri(mꢀpyridꢀ3ꢀylꢀphenyl)benzene, 40 nm) /
LiF (1 nm) / Al (100 nm) showed good performances. Particularly, device based on complex Pt-
ftpip with 5 wt% doped concentration showed superior performances with low drive voltage of 3.3
V, a maximum current efficiency of 48.3 cd Aꢀ1, a maximum external quantum efficiency of 14.0%
and a maximum power efficiency of 35.7 lm Wꢀ1, respectively. Even at the brightness of 1000 cd mꢀ
2, a current efficiency of 47.0 cd Aꢀ1 can still be obtained suggesting the ancillary ligands tpip and
ftpip can be employed well in Pt(II) complexes, which would have potential applications in OLEDs.
Furthermore, the good electron mobility of the dopants is also
important for efficient OLEDs in view of the hole mobility of
Introduction
Interest in cyclometalated complexes based on iridium(III)1 and
platinum(II)2 has soared over the past decade following the
demonstration of their potential in the manufacture of high
efficient lightꢀemitting devices (OLEDs). The spinꢀorbit
coupling exerted by the heavy metal effect allows the harvest of
both singlet and triplet electroꢀgenerated excitons in devices,
leading to a theoretically achievable 100% internal quantum
efficiency.3 Compared with Ir(III) complexes, the squareꢀplanar
hole transport material is higher than the electron mobility of
electron transport material. Many groups reported efficient
devices based complexes with nitrogen, oxygen, phosphor,
sulphur and boronꢀheterocyclic derivatives as ligands to
increase the electron mobility.5 In our former publications,
efficient
Ir(III)
complexes
and
devices
with
tetraphenylimidodiphosphinate (tpip) derivatives have been
reported due to the stronger polar P=O bonds, which may
improve the electron mobility of the complexes and benefit its
OLEDs performances.6 Herein, two heteroleptic platinum
Pt(II) complexes particularly have
a great tendency for
aggregation through platinumꢀplatinum and π–π interactions,
resulting in detrimental effects on photoluminescence quantum
yields and color purity.4 It is well understood that the
introduction of steric hinderance into the structure can hinder
the intermolecular interactions. Photoluminescence quantum
efficiencies of the complexes have been significantly improved
by the incorporation of bulky substituents. In this work, the
bulky ancillary ligands tetraphenylimidodiphosphinate (tpip)
and tetra(4ꢀfluorophenyl)imidodiphosphinate (ftpip) were
introduced in which the four bulky phenyl groups can lead to a
larger spatial separation of the neighboring molecules of the
platinum(II) complexes to suppress the selfꢀquenching
behaviour.
complexes
were
synthesized
using
2ꢀ(4ꢀ
trifluoromethyl)phenylpyridine as main ligand and tpip or ftpip
(tetra(4ꢀfluorophenyl)imidodiphosphinate) as ancillary ligands.
The synthesis, structural characterization, thermal stability,
photophysical properties, electrochemical property and
theoretical calculation and OLEDs performances of these two
Pt(II) complexes were investigated in details.
Experimental section
Materials and measurements
All reagents and chemicals were purchased from commercial
1
Additionally, tpip derivatives have stronger polar P=O bonds,
which may improve the electron mobility of Pt(II) complexes.
sources and used without further purification. H NMR spectra
were measured on a Bruker AM 500 spectrometer. Mass
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spectrometry (MS) spectra were obtained on an electrospray found 835.015 [M+2]. Anal. Calcd for C36H27F3N2O2P2Pt: C,
ionization (ESI) mass spectrometer (LCQ fleet, Thermo Fisher 51.87; H, 3.26; N, 3.36. Found: C, 51.83; H, 3.73; N, 3.28.
Scientific) for ligands and a matrixꢀassisted laser desorption
Pt-ftpip. Yield: 25.0%. 1H NMR (500 MHz, CDCl3, Fig. S3) δ
ionization timeꢀofꢀflight (MALDIꢀTOF) mass spectrometer 9.13ꢀ8.99 (m, 1H), 7.91ꢀ7.87 (m, 8H), 7.78 (t, J = 7.83 Hz, 1H),
(Bruker Daltonic Inc.) for complexes. Elemental analyses for C, 7.37ꢀ7.34 (m, 10H), 7.13ꢀ7.07 (m, 2H), 6.85 (ddd, J = 2.37, 9.24,
H, and N were performed on an Elementar Vario MICRO 11.84 Hz, 1H). MALDIꢀTOF (Fig. S4), m/z: calcd for
analyzer. TGꢀDSC measurements were carried out on a DSC C36H23F7N2O2P2Pt, 905.080 [M]; found 905.379 [M]. Anal. Calcd
823e analyzer (METTLER). Absorption and photoluminescence for C36H23F7N2O2P2Pt: C, 47.75; H, 2.56; N, 3.09. Found: C,
spectra were measured on a UVꢀ3100 spectrophotometer and a 46.74; H, 2.59; N, 3.03.
Hitachi
Fꢀ4600
photoluminescence
spectrophotometer,
respectively. The decay lifetimes were measured with an
Edinburgh Instruments FLSꢀ920 fluorescence spectrometer in
X-ray crystallography
degassed CH2Cl2 solution at room temperature. Cyclic The single crystals of Pt-tpip and Pt-ftpip complexes were
voltammetry measurements were conducted on MPIꢀA carried out on a Bruker SMART CCD diffractometer using
a
multifunctional electrochemical and chemiluminescent system
(Xi’an Remex Analytical Instrument Ltd. Co., China) at room
temperature, with a polished Pt plate as the working electrode,
platinum thread as the counter electrode and AgꢀAgNO3 (0.1 M)
in CH3CN as the reference electrode, tetraꢀnꢀbutylammonium
perchlorate (0.1 M) was used as the supporting electrolyte, using
Fc+/Fc as the internal standard, the scan rate was 0.1 V/s. The
luminescence quantum efficiencies were calculated by
comparison of the emission intensities (integrated areas) of a
standard sample (facꢀIr(ppy)3) and the unknown sample.7
monochromated Mo Kα radiation (λ = 0.71073 Å) at room
temperature. Cell parameters were retrieved using SMART
software and refined using SAINT8 on all observed reflections.
Data were collected using a narrowꢀframe method with scan
widths of 0.30° in ω and an exposure time of 10 s/frame. The
highly redundant data sets were reduced using SAINT and
corrected for Lorentz and polarization effects. Absorption
corrections were applied using SADABS9 supplied by Bruker.
The structures were solved by direct methods and refined by fullꢀ
matrix leastꢀsquares on F2 using the program SHELXSꢀ97.10 The
positions of metal atoms and their first coordination spheres were
located from directꢀmethods Eꢀmaps; other nonꢀhydrogen atoms
were found in alternating difference Fourier syntheses and leastꢀ
squares refinement cycles and, during the final cycles, refined
anisotropically. Hydrogen atoms were placed in calculated
position and refined as riding atoms with a uniform value of Uiso.
Syntheses
The syntheses routes of the ligands and complexes were listed in
Fig. 1. Solvents were carefully dried and distilled from
appropriate drying agents prior to use for synthesis of ligands.
The 2ꢀ(4ꢀtrifluoromethyl)phenylpyridine, tpip and ftpip ligands
were synthesized with the methods according to our previous
publications.6 All reactions were performed under nitrogen.
OLEDs fabrication and measurement
All OLEDs were fabricated on the preꢀpatterned ITOꢀcoated glass
substrate with a sheet resistance of 15 Ω/sq. The deposition rate
General synthesis of platinum complexes Pt-tpip and Pt-ftpip.
for
tolylamino)phenyl)cyclohexane, TcTa (4,4’,4’’ꢀtri(9ꢀcarbazoyl)ꢀ
triphenylamine), 2,6DCzPPy (2,6ꢀbis(3ꢀ(carbazolꢀ9ꢀ
organic
compounds
(TAPC
(1,1ꢀbis(4ꢀ(diꢀpꢀ
The K2PtCl4 (0.415 g, 1 mmol) and 2.5 equivalent of 2ꢀ(4ꢀ
trifluoromethyl)phenylpyridine (2.5 mmol, 0.475 g) were mixed
in a 3 : 1 mixture of 2ꢀethoxyethanol and water. The mixture was
o
yl)phenyl)pyridine), TmPyPB (1,3,5ꢀtri(mꢀpyridꢀ3ꢀylꢀphenyl)
benzene)) is 1 Å/s. The phosphors and the host TcTa or
2,6DCzPPy were coꢀevaporated to form emitting layer from two
separate sources. The cathode of LiF and Al were deposited with
deposition rates of 0.1 and 3 Å/s, respectively. The characteristic
curves of the devices were measured with a computer which
controlled KEITHLEY 2400 source meter with a calibrated
silicon diode in air without device encapsulation. On the basis of
the uncorrected PL and EL spectra, the Commission
Internationale de l’Eclairage (CIE) coordinates were calculated
heated at 80 C for 16 h and the Ptꢀdimer was precipitated in
water. The resulting yellowꢀgreenish powder was filtered and was
subsequently reacted with Ktpip or Kftpip without further
purification. A stirred mixture of Ptꢀdimer (0.33 mmol, 0.30 g)
and Ktpip (0.99 mmol, 0.45 g) or Kftpip (0.99 mmol, 0.53 g) in
o
2ꢀethoxyethanol (25 mL) was heated at 80 C for 16 h. After
cooling to room temperature, the solution was concentrated under
reduced pressure. The resulting residue was subjected to column
chromatography (silica gel, CH2Cl2/petroleum ether 1 : 2 (v/v)) to
give product as yellow powders, which were further purified by
sublimation in vacuum.
using
a
test program of the Spectra scan PR650
1
spectrophotometer.
Pt-tpip. Yield: 40.0%. H NMR (500 MHz, CDCl3, Fig. S1) δ
9.08 (d, J = 5.13Hz, 1H), 7.92ꢀ7.77 (m, 10H), 7.56 (d, J = 8.02
Hz, 1H), 7.38ꢀ7.31 (m, 14H), 7.11 (t, J = 6.63 Hz, 1H). MALDIꢀ
TOF (Fig. S2), m/z: calcd for C36H27F3N2O2P2Pt, 833.115 [M];
Results and discussion
Preparation and characterization of compounds
2 | J. Name., 2012, 00, 1-3
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As shown
in
Fig. 1, the main
ligand 2ꢀ(4ꢀ Cl)]2 with tpip or ftpip potassium salt to give the products Pt-tpip
Pt-ftpip. Purification of the mixture by
trifluoromethyl)phenylpyridine was synthesized using a Suzuki and
coupling reaction from 4ꢀtrifluoromethylphenyl boronic acid and silica gel chromatography provided crude products, which were
2ꢀbromopyridine. The tpip and ftpip ancillary ligands further purified by vacuum sublimation. All the new compounds
and their potassium salt were prepared according to our previous were fully characterized by 1H NMR spectrometry and the crystal
publications.6 The platinum complexes were prepared by the structures further confirmed the identity of both complexes.
reaction of corresponding chlorideꢀbridged dimmers [(C^N)Pt(µꢀ
Si
R
R
R
R
R
R
NH
R
K+
N
H
H
Si
N
N
H2O2
KOH
P
P
P
P
Cl
P
P
P
O
O
O
O
R
R
CF3
R
R
R
R
R
R
R
EtOEtOH
Ktpip/Kftpip
O P
Htpip/Hftpip
N
Pt
CF3
Br
CF3
O P
CF3
N
N
N
K2PtCl4
Cl
Cl
R
Pt
R
Pt
EtOCH2CH2OH
H2O
Pd(PPh3)4
B(OH)2
N
N
Pt-tpip: R=H
Pt-ftpip: R=F
CF3
Fig. 1 The synthetic routes of ligands, Pt-tpip and Pt-ftpip.
constructed with O1, O2, and Pt atoms, the C1 and N1 atoms’
displacement values from the mean plane are 0.089 and 0.004 Å
in Pt-tpip and those are 0.074 and 0.099 Å in Pt-ftpip,
respectively (Fig. S5 and S6). The discussion above suggests that
they maintain a good squareꢀplanar coordination geometry with
the two O atoms opposite to the C, N atoms of the 4ꢀtfmppy
respectively. The bond lengths and angles around the central
platinum atom found for these new complexes are in good
agreement with the few previously published structures of this
type.11
Thermal stability
Fig. 2 Oka Ridge Thermal Ellipsoidal plot (ORTEP) diagrams of Pt-tpip
(left, CCDC No. 1509840) and Pt-ftpip (right, CCDC No. 1509843)
complexes with the atomꢀnumbering schemes. Hydrogen atoms are
omitted for clarity. Ellipsoids are drawn at 30% probability level.
100
(b)
6
8
(a)
90
80
70
60
50
40
Pt-ftpip
Pt-tpip
Pt-ftpip
Pt-tpip
10
12
14
16
Single crystals of Pt-tpip and Pt-ftpip were obtained from
CH3OH and CH2Cl2 solutions. Fig. 2 shows the Oak Ridge
thermal ellipsoidal plot (ORETP) diagrams of Pt-tpip and Pt-
ftpip given by Xꢀray analysis. Selected parameters of the
molecular structures and tables of atomic coordinates were
collected in the Table S1 and Table S2. As is shown in Fig. 2, for
two complexes, the PtꢀC bond lengths are 1.939 and 1.947 Å,
respectively. The PtꢀO bonds are all over 2Å. The average of the
dihedral angles between planes [C1ꢀPtꢀtpipꢀN1], [N1ꢀPtꢀtpipꢀO2]
and [O2ꢀPtꢀtpipꢀO1] is 178.58° in Pt-tpip and that is 177.60°
between planes [C1ꢀPtꢀftpipꢀN1], [N1ꢀPtꢀftpipꢀO2] and [O2ꢀPtꢀ
ftpipꢀO1] in Pt-ftpip Additionally, when the mean plane is
composed of C1, N1,and Pt atoms, the O1 and O2 atoms’
displacement values from the mean plane are 0.005 and 0.097 Å
in Pt-tpip and those are 0.095 and 0.075 Å in Pt-ftpip,
respectively. Correspondingly, referring to the mean plane
50
100
150
250
0
100
200
300
400
500
600
700
Temperature2(00
°C)
Temperature (oC)
Fig. 3 The DSC (a) and TG (b) curves of Pt-tpip and Pt-ftpip complexes.
The thermal stability of the emitters is important for the stability
of OLEDs. In this case, the thermal properties of the two Pt(II)
complexes were characterized by differential scanning
calorimetry (DSC) and thermogravimetric (TG) measurements
under a nitrogen steam. From the DSC curves in Fig. 3(a) it can
be observed the melting points of Pt-tpip and Pt-ftpip are as
o
high as 260 and 225 C, respectively. The TG curves give the
decomposition temperatures (5% loss of weight) as 328 oC for Pt-
tpip and 304 oC for Pt-ftpip, respectively, indicating that the
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fluorine atoms at the tpip ligand will reduce the melting point respectively, indicating that the platinum complexes have
effectively and also affect their decomposition temperature potential application in the OLEDs. Furthermore, the lifetimes are
obviously. And the high decomposition temperature of the in the range of microseconds for two complexes (1.5 ꢁs for Pt-
complexes suggested they are suitable for application in OLEDs.
tpip and 1.7 ꢁs for Pt-ftpip, respectively) (Fig. S7). The short
lifetimes would improve the spinꢀstate mixing and suppress the
excitons annihilation.
Photophysical property
The absorption spectra of two complexes show broad and intense
bands below 300 nm, assigned to the spinꢀallowed intraligand
1LC (π→π*) transition of cyclometalated main ligands and tpip
ligands. The weak bands last to 450 nm can be assigned to spinꢀ
allowed metalꢀligand charge transfer band (1MLCT) and spin
forbidden 3MLCT transition bands caused by the large spin
orbital coupling (SOC) that was introduced by platinum center
indicating an efficient spinꢀorbit coupling that is prerequisite for
phosphorescent emission.12 The complexes Pt-tpip and Pt-ftpip
show green emission at 510/542 nm and 508/540 nm,
respectively, suggesting that fluorine atom on ftpip groups has no
obvious effect on the emission wavelength of Pt(II) complexes.
Both complexes based on tpip or ftpip show high quantum
efficiency (Ф) as 79.2% and 71.5% for Pt-tpip and Pt-ftpip,
Pt-tpip
Pt-tpip
(a)
(b)
Pt-ftpip
Pt-ftpip
250
300
350
400
450
500
450
500
550
600
650
700
Wavelength(nm)
Wavelength(nm)
Fig. 4 The UVꢀvis absorption (a) and emission spectra (b) of complexes
Pt-tpip and Pt-ftpip in degassed dichloromethane (5 × 10ꢀ5 M) at room
temperature.
Table 1. Photophyscial date of platinum complexes.
Compound
Absorption a)
Emissiona)
Eox
(V)
HOMO/LUMOc)
(eV)
a)
τ298 K
Φb)
λ (nm)
λ
max (nm), 298K
(µs)
(%)
278/320/369/445
278/301/365/441
510/542
508/540
1.5
1.7
79.2
71.5
0.74
0.72
ꢀ5.39/ꢀ2.55
ꢀ5.37/ꢀ2.53
Pt-tpip
Pt-ftpip
Tm: Absorption, emission spectra and lifetime were taken in degassed CH2Cl2;bΦ: emission quantum yields were measured relative to Ir(ppy)3(Φ = 0.4)
in degassed CH2Cl2 solution; c From the onset of oxidation potentials of the cyclovoltammety (CV) diagram using ferrocene as the internal standard and
the optical band gap from the absorption spectra in degassed CH2Cl2. HOMO(eV)=ꢀ(EoxꢀE1/2,Fc)ꢀ4.8, LUMO(eV)=HOMO+Ebandgap
.
Electrochemical properties and theoretical calculation
The highest occupied molecular orbital (HOMO) and lowest
6
5
Pt-tpip
Pt-ftpip
LUMO
unoccupied molecular orbital (LUMO) energy levels of the
dopants are important for the design of the OLEDs. In order to
determine the HOMO/LUMO values of the Pt-tpip and Pt-
ftpip, the electrochemical properties were investigated by cyclic
voltammetry in deaerated CH2Cl2 (Fig. 5 (Left)). The cyclic
voltammograms of the complexes Pt-tpip and Pt-ftpip in the
positive range show strong oxidation peaks, while the reduction
peaks are not obvious, demonstrating that the redox process of
the complexes is not reversible completely, which is rarely
observed in related Ir(III) complexes containing tpip units. From
the Table 1, it is observed that the difference of the
HOMO/LUMO values of the two complexes is neglectable.
This means that the introduction of fluorine atom on tpip groups
has no obvious effect on electrochemical properties of Pt(II)
complexes, which match the DFT calculation results below very
well.
4
-1.95 eV
-5.85 eV
-1.97 eV
-5.91 eV
3
2
-3.91 eV
-3.92 eV
1
0
-1
-2
HOMO
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
Pt-tpip
Pt-ftpip
Potential (V)
Fig. 5 Cyclic voltammogram curves (Left) and Molecular orbital
diagram for the HOMOs/LUMOs and their energy levels of Pt-tpip
and Pt-ftpip calculated in CH2Cl2(Right).
In order to gain insights into the electronic states and the
orbital distribution, the density functional theory (DFT)
calculations13 for both Pt(II) complexes were conducted
employing Gaussian09 software with B3LYP function.14 Plots
of the HOMO/LUMO and the molecular orbital energy levels
are presented in Fig. 5 (right). The basis set used for C, H, N, O,
F and P atoms was 6ꢀ31G(d, p) while the LanL2DZ basis set
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was employed for Pt atoms.15 The solvent effect of CH2Cl2 was
taken into consideration using conductor like polarizable
continuum model (CꢀPCM). All these calculations were
performed with Gaussian09. For these complexes, the HOMOs
correspond to a mixture of 4ꢀtfmppy (36.36ꢀ36.48%) and Pt d
orbitals (53.85ꢀ52.20%) with minor contribution from the tpip,
ftpip ligands (9.81ꢀ9.33%), while the LUMOs are mainly
localized on the main ligands (93.09ꢀ92.89%) with minor
contribution from Pt d orbitals (5.70ꢀ5.65%) and tpip, ftpip
ligands (1.21ꢀ1.47%). The calculation results indicate that the
orbital distributions of HOMO and LOMO for Pt-tpip and Pt-
ftpip have more contributions from Pt d orbitals and 4ꢀtfmppy
ligand, less contribution from tpip, ftpip ligands. Therefore,
replacing hydrogen atoms with fluorine atoms in tpip ligand
almost makes no difference to both the HOMO and the LUMO
energy of Pt-tpip and Pt-ftpip.
Pt-tpip or Pt-ftpip : 2,6DCzPPy (5 wt%, 10 nm) / TmPyPB (40
nm) / LiF (1 nm) / Al (100 nm) were investigated to evaluate the
electroluminescence (EL) performances. The energy diagram of
the devices as well as the molecular structure of the materials
used are shown in Fig. 6. TAPC was used as hole
transport/electron block layer (HTL/EBL) due to its high hole
mobility (1×10ꢀ2 cm2 Vꢀ1 sꢀ1) and highꢀlying LUMO level (ꢀ1.8
eV), while TmPyPB was used as hole block/electron transport
layer (HBL/ETL) due to its lowlying HOMO level (ꢀ6.7 eV) and
high electron mobility (1×10ꢀ3 cm2 Vꢀ1 sꢀ1).16 pꢀType material
TcTa and bipolar material 2,6DCzPPy were chosen as host
materials of EML1 and EML2, respectively.
Theoretically speaking, the stepwise HOMO levels of TAPC
(ꢀ5.5 eV), TcTa (ꢀ5.7 eV), and 2,6DCzPPy (ꢀ6.1 eV) are
beneficial for the injection and transport of holes, while the
stepwise LUMO levels of TmPyPB (ꢀ2.7 eV), 2,6DCzPPy (ꢀ2.6
eV) and TcTa (ꢀ2.4 eV) are beneficial for the injection and
transport of electrons. Therefore, balanced distribution of
carriers (holes and electrons) and wide recombination zone
could be expected. More importantly, the LUMO level of TAPC
is 0.6 eV higher than that of TcTa while the HOMO level of
TmPyPB is 0.6 eV lower than that of 2,6DCzPPy; thus, holes
and electrons are well confined within EMLs and and the triplet
excitons quenching of will be effectively avoided. Since the
HOMO and LUMO levels of Pt-tpip or Pt-ftpip (ꢀ5.39/ꢀ5.37
eV and ꢀ2.55/ꢀ2.53 eV, respectively) are within those of TcTa
and 2,6DCzPPy, carrier trapping is conceived to be the
dominant EL mechanism of these devices. In addition, the
triplet energies of TcTa (2.83 eV) and 2,6DCzPPy (2.71 eV) are
higher than that of Pt-tpip or Pt-ftpip (2.43 eV), which
effectively avoids the reverse energy transfer from dopant to
host molecules. In this device, the optimized Pt(II) complexes
with 5 wt% doped concentration in TcTa and 2,6DCzPPy,
respectively, were used as the two emitting layers. In most
cases, double emitting layers will broad the holeꢀelectron
recombination zone which can increase the unilization ratio of
the holeꢀelectron causing the improved efficiency and impressed
efficiency rollꢀoff.
OLEDs performance
-1.8 eV
F
-2.4 eV
-2.6 eV
CF3
N
CF3
-2.7 eV
O
O
P
N
P
F
F
O
O
P
N
P
Pt
Pt
-4.3 eV
LiF/Al
N
-5.1 eV
ITO
F
Pt-tpip
Pt-ftpip
-5.5 eV
-5.7 eV
N
N
N
-6.1 eV
N
N
-6.7 eV
TAPC
N
N
N
N
N
N
TcTa
TmPyPB
2,6DCzPPy
N
Fig. 6 Energy level diagram of HOMO and LUMO levels (relative to
vacuum level) for materials investigated in this work and their
molecular structures.
To verify our conjecture, devices G1 and G2 using Pt-tpip and
Pt-ftpip as the emitters, respectively, with the structure of ITO /
TAPC (40 nm) / Pt-tpip or Pt-ftpip : TcTa (5 wt%, 10 nm) /
Table 2. EL performances of the devices G1 and G2.
a)
d)
e)
Device
Emitter
Vturnꢀon
Lmax (Voltage)b)
ηc(voltage)c)
(cd Aꢀ1(V))
ηc, L1000
EQEmax
ηp(voltage)f)
(lm Wꢀ1(V))
CIE
(V)
(cd mꢀ2 (V))
(cd Aꢀ1)
38.0
(%)
(x,y)
3.3
3.3
25544(7.7)
40.6(4.1)
48.3(5.4)
12.0
14.0
30.1(4.0)
35.7(4.4)
(0.27,0.62)
(0.31,0.60)
G1
G2
Pt-tpip
17014(11.2)
47.0
Pt-ftpip
a)
V
turnꢀon
: Turnꢀon voltage recorded at a luminance of 1 cd mꢀ2; b) Lmax: maximum luminance; c)
η
c : maximum current efficiency; d)ηc, L1000: current
efficiency at 1000 cd mꢀ2; e) EQEmax: maximum external quantum efficiency; f)
ηp : maximum power efficiency.
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ARTICLE
3.0x104
(b)
(a)
G1
G2
2.5x104
2.0x104
1.5x104
1.0x104
G1
G2
5.0x103
0.0
0
1
2
3
4
5
6
7
8
9
10
11
400
500
600
700
Wavelength (nm)
Voltage (V)
100
10
1
100
10
1
(d)
(c)
G1
G2
G1
G2
0.1
0
0.1
2000
4000
6000
8000
10000
0
2000
4000
6000
cd m-2
)
8000
10000
Luminance ( )
cd m-2
Luminance
(
Fig. 7 Characteristics of devices with configuration ITO / TAPC (40 nm) / Pt-tpip or Pt-ftpip: TcTa (10 nm, 5 wt%) / Pt-tpip or Pt-ftpip: 2,6DCzPPy
(10 nm, 5 wt%) / TmPyPB (40 nm) / LiF (1 nm) / Al (100 nm): (a) EL spectra , (b) luminance(L) versus voltage (V), (c) current efficiency (ηc) as a
function of luminance (L) and (d) power efficiency (ηp) as a function of luminance (L) for G1 and G2.
The EL spectra, luminance (L) versus voltage (V), current
efficiency (ηc) versus luminance (L) and power efficiency (ηp)
versus luminance (L) characteristics of each device are shown in
Fig. 7. The key EL data are summarized in Table 2. Devices G1
and G2 all emit green light with the EL emission peaks at
505/540 and 507/540 nm and the Commission Internationale de
l'Eclairage (CIE) coordinate for G1 and G2 is (0.27, 0.62) and
(0.31, 0.60), respectively. Additionally, the emission spectra are
almost invariant of the current density and also do not show any
concentration dependent. As is shown in Fig. 7(a), the EL
spectra are very close to the PL spectra of the complexes in
CH2Cl2 solution indicating that the EL emission of the device
originates from the triplet excited states of the phosphors.
Furthermore, the residual emission from the host 2,6DCzPPy for
each device is not obvious suggesting that the energy and/or
charge transfer from the host exciton to the phosphor is nearly
complete upon electrical excitation.
exhibits a little higher efficiency with a ηc,max of 48.3 cd Aꢀ1, an
EQEmax of 14.0% and a ηp,max of 35.7 lm Wꢀ1, respectively.
Furthermore, both devices show low efficiency rollꢀoff. For
example, even at the brightness of 1000 cd mꢀ2, the current
efficiencies can still be kept at 38.0 and 47.0 cd Aꢀ1 for devices
G1 and G2, respectively.
The good EL properties should be firstly due to the
application of tpip and ftpip as the ancillary ligand, which can
improve the electron mobility of the complexes. Secondly, the
LUMO levels of the dopants are low and they are just in the
range of the ones of two host materials, which will benefit
electron transport properties. Thirdly, we have designed and
fabricated the high performances green EL devices by doping
Pt-tpip or Pt-ftpip into host materials with stepwise energy
levels, which is beneficial for the injection and transport of both
holes and electrons. Finally, double lightꢀemitting layers are
adopted in this device, which displayed higher EL efficiency,
slower rollꢀoff of efficiency, and higher brightness attributed to
better balance of holes and electrons, broadening recombination
zone and improved trapping of holes and electrons.17
Furthermore, compared with Ptꢀtpip, four fluorine atoms are
introduced in the four phenyl of tpip ligand in Pt-ftpip, which
can result in more superior electron mobility of Pt-ftpip than
The OLEDs based on complexes Pt-tpip or Pt-ftpip exhibit
encouraging EL performances. Both devices G1 and G2 display
low turnꢀon voltages (Vturnꢀon) (3.3 V). Moreover, the device G1
shows a maximum current efficiency (ηc,max) of 40.6 cd Aꢀ1, a
maximum external quantum efficiency (EQEmax) of 12.0% with
a maximum power efficiency (ηp,max) of 30.1 lm Wꢀ1. Device G2
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DOI: 10.1039/C6DT03991J
that of Pt-tpip. Therefore, it can be observed that the current
efficiency of Pt-ftpip is higher than that of Pt-tpip.
Che, K.ꢀK. Cheung and N. Zhu, Chem. Eur. J., 2001, 7, 4180; (f) J.
Brooks, Y. Babayan, S. Lamansky, P. I. Djurovich, I. Tsyba, R. Bau and
M. E. Thompson, Inorg. Chem., 2002, 41, 3055; (g) Z. M. Hudson, C.
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Conclusions
In conclusion, two cyclometalated platinum complexes Pt-tpip
and Pt-ftpip with tetraphenylimidodiphosphinate and tetra(4ꢀ
fluorophenyl)imidodiphosphinate as the ancillary ligands have
been successfully prepared and applied in the OLEDs. EL
devices with the double emissive layers configuration of ITO /
TAPC (40 nm) / Pt-tpip or Pt-ftpip (5 wt%) : TcTa (10 nm) /
Pt-tpip or Pt-ftpip (5 wt%) : 2,6DCzPPy (10 nm) / TmPyPB
(40 nm) / LiF (1 nm) / Al (100 nm) showed good performances
(a ηc,max of 48.3 cd Aꢀ1, a peak EQE of 14.0% and a peak ηp of
35.7 lm Wꢀ1) with low efficiency rollꢀoff. Even at the brightness
of 1000 cd mꢀ2, a current efficiency of 47.0 cd Aꢀ1 can still be
obtained. This study suggested the ancillary ligands tpip and
ftpip can be employed well in Pt(II) complexes, which would
have potential applications in OLEDs.
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Acknowledgements
This work was supported by the National Natural Science
Foundation of China (21371093, 91433113), the Major State
Basic Research Development Program (2013CB922101), the
Natural Science Foundation of Jiangsu Province (BY2016075ꢀ
02) and State Key Laboratory of Luminescence and
Applications.
Notes and references
a
State Key Laboratory of Coordination Chemistry, Collaborative
Innovation Center of Advanced Microstructures, School of Chemistry
and Chemical Engineering, Nanjing University, Nanjing 210093, P. R.
China, eꢀmail: yxzheng@nju.edu.cn
b
State Key Laboratory of Luminescence and Applications, Changchun
Institute of Optics, Fine Mechanics and Physics, Chinese Academy of
Science, Changchun 130033, P. R. China
Fig
†Electronic Supplementary Information (ESI) available: the
photoluminescence lifetime curve, the crystallographic data, selected
1
bonds and angles, H NMR spectra, the MALDIꢀTOF spectra, atoms’
displacement values spectra and current density (J) versus voltage (V)
spetra of complexes Pt-tpip and Pt-ftpip.
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DOI: 10.1039/C6DT03991J
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Graphical abstract
R
R
CF3
10
O P
N
Pt
O P
N
R
R
1
0
2000 4000 6000 8000 10000
Luminance (cd m-2)
Highly efficient green OLEDs based on two platinum complexes show a peak current
efficiency of 48.3 cd A-1 with low efficiency roll-off, and even at the brightness of
1000 cd m-2, a current efficiency of 47.0 cd A-1 can still be obtained.