Phosphorescent Tetradentate Platinum(II) Complexes Containing Fused 6/5/5 or 6/5/6 Metallocycles

Article abstract of DOI:10.1021/acs.inorgchem.0c02569

A series of phenylpyridine (ppy)-based 6/5/5 N*C^N^O and biphenyl (bp)-based 6/5/6 N*C^C*N Pt(II) complexes employing tetradentate ligands with nitrogen or oxygen atoms as bridging groups have been developed. Ligand structural modifications have great inf

Full text of DOI:10.1021/acs.inorgchem.0c02569

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Article
Phosphorescent Tetradentate Platinum(II) Complexes Containing
Fused 6/5/5 or 6/5/6 Metallocycles
Guijie Li,* Gang Shen, Xiaoli Fang, Yun-Fang Yang,* Feng Zhan, Jianbing Zheng, Weiwei Lou,
Qisheng Zhang, and Yuanbin She*
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ABSTRACT: A series of phenylpyridine (ppy)-based 6/5/5
N*C^N^O and biphenyl (bp)-based 6/5/6 N*C^C*N Pt(II)
complexes employing tetradentate ligands with nitrogen or oxygen
atoms as bridging groups have been developed. Ligand structural
modifications have great influences on the electrochemical,
photophysical, and excited-state properties, as well as photo-
stabilities of the Pt(II) complexes, which were systematically
studied by experimental and theoretical investigations. The time-
dependent density functional theory calculations and natural
transition orbital analyses reveal that Pt(bp-6), Pt(bp-7), and
Pt(bp-8) have dominant ligand-centered (3LC) mixed with small
metal-to-ligand charge-transfer (3MLCT) characters in T1 states, resulting in relatively low quantum efficiencies (ΦPL) of 5−33%
3
and 12−32% in dichloromethane solution and PMMA film, respectively. By contrast, Pt(ppy-1) possesses much more MLCT
character in the T1 state, enabling a high ΦPL of 95% in dichloromethane and 90% in DPEPO film, and large radiative decay rates.
The strength of the Pt−N1 coordination bond plays a critical role in the photostability. Pt(ppy-1)- and Pt(bp-6)-doped polystyrene
films demonstrate long photostability lifetimes of 150 min for LT97 and LT98.5, respectively. A Pt(ppy-1)-based green OLED using
26mCPy as host realized a peak EQE of 18.5%, which still maintained an EQE of 10.4% at 1000 cd/m2, and an Lmax of over 40 000
cd/m2 was achieved. This study should provide a valuable reference for the further development of efficient and stable
phosphorescent Pt(II) complexes.
in the full-color display; thus, all stable and efficient RGB light-
emitting materials are highly desired.
INTRODUCTION
■
An organic light-emitting device (OLED) is a key technology
for full-color display application in high-end electronics.1,2 The
design and development of light-emitting materials play a
critical role in this field and have been attracting great attention
in both academia and industry.3−9 Heavy metal complexes-
based phosphorescent materials have a potential ability to
harvest both electrogenerated singlet and triplet excitons to
achieve unity internal quantum efficiency (IQE) and act as an
important kind of light-emitting material in OLED fabrica-
tions.3,4 With the efforts of the past three decades, many
phosphorescent transition metal complexes, such as Ir(III),
Pt(II), Au(III), Pd(II), Rh(III), and Ru(II) complexes,10−18
have been developed, and OLEDs using the phosphorescent
metal complexes as emitters have demonstrated good device
performances with high external quantum efficiencies
(EQEs),19−25 high color purities,14,15,26−30 or long operational
lifetimes,9,27,29,31−50 indicating the potential applications of the
phosphorescent metal complexes in full-color display and
solid-state lighting. However, the phosphorescent OLEDs that
can meet the commercial application for electronics are scarce.
In general, red, green, and blue are three essential components
The structure-property relationship, especially the effect of
molecular structure on the excited-state property, is an
essential research topic. Typically, the lowest triplet excited
state (T1) of the phosphorescent metal complexes originates
from a mixed interligand charge-transfer (3ILCT) state with a
metal-to-ligand charge-transfer (1MLCT/3MLCT) charac-
ter.51−61 The degree of the spin−orbital coupling (SOC)
depends on the nature of the excited state. The phosphor-
escent metal complexes with more MLCT character enhance
both the intersystem crossing (ISC) rates of S1 → T1 and
radiative decay rate of T1 → S0, and usually realize a short
triplet state lifetime (τ) and high quantum efficiency (ΦPL).28
Meanwhile, the complexes typically show broad unstructured
emission spectra, resulting in low color purity. By contrast,
Received: August 28, 2020
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Figure 1. (a) Chemical structures of previously reported tetradentate 6/5/6 Pt(II) complexes with bp-containing ligands. (b) Chemical structures
of newly developed tetradentate 6/5/5 and 6/5/6 Pt(II) complexes with ppy- or bp-containing ligands. The bond length of the Pt−N1 from DFT
calculation based on optimized S0 geometry for each Pt(II) complex is marked. The ppy and bp represent conjugated phenylpyridine and biphenyl
group, respectively. Py, pyridyl group; Ac, acridinyl group; Ph, phenyl group; AAc, aza acridinyl group; ACz, aza carbazolyl group; Cz, carbazolyl
group; ACz, aza carbazolyl group.
through rational design, the ligand-centered (3LC) dominant
phosphorescent complexes30,62 can exhibit narrow emission
spectra and achieve ultra high color purity with a full-width at
half-maximum (fwhm) of about 20 nm,26−28,30,62 but their
decay lifetimes are relatively long, which make the devices have
a large efficiency roll-off. Thus, the tuning of the excited-state
property of metal complexes showing both a short decay
lifetime and narrow emission spectrum remains a great
challenge, and the structure-property relationship research
provides an in-depth understanding of the complexes and
facilitates to solve this issue.
It has been well documented that a rigid structural scaffold
can minimize the excited-state structural deformations of
Pt(II) complexes and reduce nonradiative decays, enabling
improved emission quantum efficiencies.47,63 Hence, compared
to OLEDs doped with bidentate and tridentate cyclometalated
Pt(II) complexes, tetradentate Pt(II) complexes-based OLEDs
typically demonstrated better device performances in efficien-
cies17,18,20,26−30 and stabilities.29,30,32,34−37,41−44 Recently, we
reported a new series of biphenyl (bp)-based Pt(II) complexes
with fused 6/5/6 metallocycles employing tetradentate ligands
with only a nitrogen atom as bridging group (Figure 1a).61 The
Pt(II) complexes exhibited high ΦPL in both dichloromethane
solution and doped PMMA film and also showed excellent
photostability in doped polystyrene film, which were attributed
to their rigid molecular structures and chemically stable
biphenyl- and pyridine-based ligands.61 In this work, another
series of phenylpyridine (ppy)-based 6/5/5 and biphenyl (bp)-
based 6/5/6 Pt(II) complexes with new tetradentate ligands
using nitrogen or oxygen atoms as bridging groups are
designed and synthesized (Figure 1b). The bridging nitrogen
atoms are designed as an acridinyl group (Ac) for Pt(ppy-1)
and Pt(bp-6), and as a more rigid carbazolyl group (Cz) and
aza carbazolyl group (ACz) for Pt(bp-7) and Pt(bp-8). The
two Pt−N coordination bonds are incorporated at the para-
position of the Pt(II) ion in Pt(ppy-1), and at the ortho-
position in the other three Pt(II) complexes. Their electro-
chemical, photophysical, and excited-state properties are
systematically investigated through experimental and theoreti-
cal studies. Our study reveals that the ligand structures
significantly affect the excited-state properties, and the bond
length of the Pt−N1 coordination bonds have a great influence
on their photostabilities.
EXPERIMENTAL SECTION
■
The synthesis and structural characterization of the 6/5/5 and 6/5/6
Pt(II) complexes are provided in the Supporting Information. The
theoretical calculations were performed using Gaussian 09 according
to a previous report.61 Electrochemical measurements were performed
on a CH1760E electrochemical analyzer. The absorption spectra were
measured on an Agilent 8453 UV−vis spectrometer. Steady-state
emission experiments and decay lifetime measurements were
performed on a Horiba Jobin Yvon FluoroLog-3 spectrometer.
B
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Scheme 1. Synthesis of Pt(ppy-1)
Scheme 2. Synthesis of Ligand L(bp-6)
Scheme 3. Synthesis of Ligands L(bp-7) and L(bp-8)
Low-temperature (77 K) emission spectra and decay lifetimes were
measured in 2-MeTHF cooled with liquid nitrogen. See the
Supporting Information for details.
Scheme 5. Improved Synthesis of Pt(bp-7)
RESULTS AND DISCUSSION
Synthesis and Characterization. The synthetic routes of
the Pt(II) complexes are shown in Schemes 1−4. The ligand
■
(MBP) catalyzed by Pd(PPh3)4, and hydrolysis reaction of 1-
OCH3. Then metalation of the L(ppy-1) with K2PtCl4 in acetic
acid and chloroform under nitrogen gave Pt(ppy-1) in 70%
yield66 (Scheme 1). CuI-catalyzed C−O bond coupling of 3-
bromophenol with 2-bromopyridine afforded 2-Br, which was
borated to afford 2-B, and then 2-B coupled with 1-Br to give
L(bp-6) (Scheme 2). L(bp-7) could be easily made through
direct Suzuki coupling of 3-Br67 with 2-B, and L(bp-8) was
obtained through boronation of 3-Br and then a Suzuki
coupling reaction (Scheme 3). Pt(bp-6), Pt(bp-7), and Pt(bp-
8) were synthesized by metalation with PtCl2 in benzonitrile at
180 °C or K2PtCl4 in acetic acid to give a mixture of Pt(II) and
Pt(IV) complexes,68,69 and the Pt(IV) complex in the mixture
was reduced by t-BuOK to form the desired Pt(II) complex
with 29%, 8%, and 9% isolated yield, respectively (Scheme
4).61,69 The low yields of Pt(bp-7) and Pt(bp-8) were
attributed the long distance between the Py and Py or ACz
in L(bp-7) and L(bp-8), resulting in weak coordinations with
the Pt(II) ion and easy decomposition in harsh reaction
conditions. Notably, direct metalation of L(ppy-7) or L(ppy-8)
Scheme 4. Synthesis of Pt(bp-6), Pt(bp-7), and L(bp-8)
L(ppy-1) could be facilely synthesized through boronation of
1-Br61 using 4,4,4′,4′,5,5,5′,5′-octamethyl-2,2′-bi(1,3,2-dioxa-
borolane) (OMBDB) catalyzed by Pd(dppf)Cl2, Suzuki
coupling reaction of 1-B with methyl 6-bromopicolinate
C
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Figure 2. ORTEP drawings of X-ray single crystal diffraction structures and crystal packing views of (a) Pt(bp-6) (CCDC 2036928) and (b) Pt(bp-
7) (CCDC 2036927). Solvent molecules were omitted for clarity. Ellipsoids are shown at the 50% probability level.
Table 1. Electrochemical Properties and Energy Levels of Tetradentate Pt(II) Complexes
a
b
c
d
e
complex
Eox [V]
Ered [V]
HOMO /LUMO [eV]
ΔEg [eV]
ET1 [eV]
calcd HOMO/LUMO [eV]
Pt(ppy-1)
Pt(bp-6)
Pt(bp-7)
Pt(bp-8)
0.51
0.27
0.24
0.20
−2.10
−2.61
−2.51
−2.34
−5.31/−2.70
−5.07/−2.19
−5.04/−2.29
2.61
2.88
2.75
2.54
2.44
2.48
2.24
2.23
−5.40/−1.89
−4.67/−1.28
−4.65/−1.42
−4.61/−1.61
−5.00/−2.46
a
b
c
d
HOMO = −(Eox + 4.8) eV. LUMO = −(Ered + 4.8) eV. ΔEg = LUMO − HOMO. Triplet energies estimated from the peak of the
phosphorescent emission spectrum at 77 K in 2-MeTHF. From DFT calculations at the B3LYP/6-31G(d)/LANL2DZ level based on optimized
S0.
e
the reaction time of the metalation, and employing SnCl2 as
reductant (Scheme 5). The synthetic details are provided in
the Supporting Information. All the new intermediates and
newly developed 6/5/5 and 6/5/6 Pt(II) complexes were
characterized by 1H and 13C NMR spectroscopy, high
resolution mass spectrum (HRMS), and elemental analysis.
Theoretical Investigation, X-ray Structure, and Elec-
trochemistry. Density functional theory (DFT) calcula-
tions47,61 reveal that Pt(ppy-1) has a significantly planar
molecular geometry with small distortion and a very small
dihedral angle of only 16.6° between the terminal pyridine and
carboxyl planes in the S0 state (Tables S1 and S2). By contrast,
the molecular geometries of Pt(bp-6), Pt(bp-7), and Pt(bp-8)
are profoundly twisted in S0 states with large dihedral angles of
48.9−52.6° between the two terminal aryl planes in S0 states
1
(Tables S1 and S2). H NMR spectra of the two methyl
groups on the 9-position of the acridinyl moiety show two
Figure 3. Absorption spectra of the Pt(II) complexes in dichloro-
methane at room temperature. Their T1 absorption transitions are
shown in the inset.
single peaks of 1.34 and 1.88 ppm for Pt(ppy-1) and 1.37 and
1.84 ppm for Pt(bp-6), respectively; similar results are also
observed in their 13C NMR spectra, which reveal their
unsymmetric chemical environments and twisted molecular
geometries (see Supporting Information). All the Pt(II)
complexes exhibit smaller dihedral angles in T1 states
compared to S0 states, and the structural deformation in
Pt(ppy-1) (0.1°) is much smaller than those of the other three
with K2PtCl4 in acetic acid afforded very little desired Pt(II)
complexes, which were difficult to purify by column
chromatography to get pure products. However, the isolated
yield of Pt(bp-7) could be significantly improved by shortening
D
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Table 2. Calculated Excitation Energy (E), Wavelength (λ), Oscillator Strength (f), Main Orbital Contribution, and Charge
a
Characters of the T1 and S1 States of Pt(II) Complexes
complex
state
E [eV]
λ [nm]
f
orbital contribution (>10%)
assignment
Pt(ppy-1)
T1
S1
2.472
2.716
2.472
2.669
2.249
2.519
2.196
502
457
502
465
551
492
565
0
HOMO → LUMO (87%)
HOMO → LUMO (96%)
HOMO → LUMO (80%)
HOMO → LUMO (97%)
HOMO → LUMO (72%)
HOMO → LUMO (97%)
HOMO → LUMO (57%)
HOMO → LUMO+1 (21%)
HOMO → LUMO (94%)
3MLCT(πppyCOOdPt → πPy*)
1
0.0196
1LC(πppy → πppy*), MLCT(πppyCOOdPt → πPy*)
Pt(bp-6)
Pt(bp-7)
Pt(bp-8)
T1
S1
0
3MLCT(πbpdPt → πPy*)
1
0.0212
1LC(πbp → πbp*), MLCT(πbpdPt → πPy*)
T1
S1
0
3MLCT (πbpdPt → πPy*)
1
0.0073
0
1LC(πCzPh → πCzPh*), MLCT(πbpdPt → πPy*)
T1
3MLCT(πbpdPt → πACz*)
1
S1
2.407
515
0.0338
1LC(πCzPh → πCzPh*), MLCT(πbpdPt → πACz*)
a
Calculated by the TD-B3LYP method with a basis set of SV for H, SVP for C, F, and N atoms, and a def2-TZVP basis set for the Pt atom based on
optimized S0 geometries.
complexes (1.6−2.6°) (Table S2). Notably, the coordination
bonds Pt−N1 in Pt(bp-7) (2.232 Å) and Pt(bp-8) (2.237 Å)
are significantly longer than those in Pt(ppy-1) (2.047 Å),
Pt(bp-6) (2.187 Å), and the previously reported Pt(bp-1),
Pt(bp-2), and Pt(bp-3) (2.182−2.186 Å)61 (Figure 1); this is
attributed to that the rigid Cz segment in Pt(bp-7) and Pt(bp-
8) makes its 9-position pyridine away from the central Pt(II)
ion. This is also supported by the X-ray analyses that the bond
length of Pt−N1 in Pt(bp-7) (2.1460(19) Å) is significantly
longer than that of Pt(bp-6) (2.115(2) Å), and the Ac moiety
in Pt(bp-6) is a seriously twisted chair configuration. In
contrast, the Cz moiety in Pt(bp-7) is a planar configuration
(Figure 2a,b Tables S2−S4). Additionally, intermolecular C−
H···π interactions are observed in Pt(bp-6) between the methyl
group and the pyridine ring (Figure 2a), and intermolecular
π−π interactions are observed in Pt(bp-7) between the two
pyridine rings with a distance of 3.396 Å (Figure 2b).
Pt(bp-6), Pt(bp-7), and Pt(bp-8) show similar highest
occupied molecular orbital (HOMO) distributions predom-
inantly on the πAcPh, πCzPh orbitals and the dPt centers, and the
lowest unoccupied molecular orbital (LUMO) distributions
dominantly occupy on the two πPy for Pt(bp-6) and Pt(bp-7),
and exclusively on πACz for Pt(bp-8) (Figure S1), indicative of
a stronger electron-withdrawing ability of the ACz compared
to Py. Pt(ppy-1) has significantly different HOMO and LUMO
distributions. The HOMO is mainly on the πAc-pPt‑O-πOC
orbitals, and the LOMO is dominantly on the electron-
deficient two πPy moieties (Figure S1). Additionally, Pt(ppy-1)
exhibits stabilized HOMO and LUMO levels (−5.40 and
−1.89 eV, respectively) compared to the bp-based Pt(II)
complexes, which are −4.67, −4.65, and −4.61 eV for HOMO
levels, and −1.28, −1.42, and −1.61 eV for HOMO levels of
Pt(bp-6), Pt(bp-7), and Pt(bp-8), respectively (Figure S1),
because the electron-withdrawing ability of the ppy is much
stronger than that of the bp moiety. This result can be also
supported by the electrochemical study that Pt(ppy-1) has a
greatly larger oxidation potential of 0.51 eV than those of the
bp-based Pt(II) complexes of 0.20−0.27 eV, and less negative
reduction potential of −2.10 eV compared to the other Pt(II)
complexes of −2.61 to −2.34 eV (Table 1). These significant
differences reveal that the redox processes in ppy-based Pt(ppy-
1) are different with those in the bp-based Pt(II) complexes.
The oxidation process of Pt(ppy-1) is mainly on the Ac-Pt
moiety, and the reduction process is dominantly on the
electron-deficient two Py rings. Pt(bp-6) has the oxidation
process on the bp-Pt moiety, and reduction process on the Py
ring. Pt(bp-7) and Pt(bp-8) have the oxidation processes on
the CzPh-Pt moieties; however, the reduction processes are
much different, mainly on the Py ring and ACz moiety,
respectively. Moreover, the HOMO and LUMO levels
calculated from the redox values are in good agreement with
the trend from the DFT calculations (Table 1). All the Pt(II)
complexes exhibit irreversible redox processes except the
reduction process of Pt(ppy-1) (Figure S2). These results
demonstrated that the distribution and energy levels of the
frontier orbitals can be effectively regulated through rational
ligand design.
Photophysical and Excited-State Properties. The UV−
vis absorption spectra of the Pt(II) complexes and their
corresponding ligand in dichloromethane (DCM) solutions at
room temperature (RT) are illustrated in Figures 3 and S3. For
all the Pt(II) complexes, the intense absorption bands below
325 nm are assigned as spin-allowed ligand-centered (1LC) π
→ π* transitions from localized ppy in Pt(ppy-1), localized bp
in Pt(bp-6), and from localized CzPh in Pt(bp-7) and Pt(bp-8)
on the basis of the time-dependent density functional theory
(TD-DFT) calculations47,61 (Tables 2, S5−S8) and natural
transition orbital (NTO) analyses61,64 (Figure S4). Notably,
Pt(ppy-1) exhibits much weaker π → π* transitions than those
of the other Pt(II) complexes because of less aryl groups in
Pt(ppy-1) (Figure 3). The absorptions at the region of 340−
475 nm are attributed to spin-allowed metal-to-ligand charge-
transfer (1MLCT) transitions, which are involved with both
the center Pt(II) ion and the cyclometalating ligand (Figure 3).
3
Pt(ppy-1) has a very weak unstructured MLCT transition
involving πppyCOOdPt → πPy* (Figure S4). However, Pt(bp-6),
3
Pt(bp-7), and Pt(bp-8) show well-resolved MLCT transitions
at about 498, 550, and 555 nm, respectively, involving πbpdPt →
πPy* for Pt(bp-6) and Pt(bp-7), and πbpdPt → πACz* for Pt(bp-
8) (Figure S4); these 3MLCT transitions are in good
agreement with the T1 absorptions by the TD-DFT
calculations of 502, 551, and 565 nm, respectively (Table 2).
The emission spectra of the Pt(II) complexes in various
conditions are illustrated in Figures 4 and S5, and the data are
recorded in Table 3. Ligand modifications have a great
influence on the photophysical properties. At 77 K in 2-
methyltetrahydrofuran (2-MeTHF), all the Pt(II) complexes
show well-resolved vibronic emission spectra, indicating the
30,62
3
dominant LC characters in their T1 states.
Pt(ppy-1) and
Pt(bp-6), both of which have PyAc-containing ligands, show a
dominant emission peak in high energy region at 508 and 500
nm, respectively (Figure 4). By contrast, PyCz-based Pt(bp-7)
and Pt(bp-8) have a dramatic red shift of about 50 nm and
show a dominant emission peak at 553 and 557 nm,
E
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Figure 5. Photoluminescence spectra of (a) Pt(ppy-1) and (b) Pt(bp-
7) in various thin films.
respectively (Figure 4). Especially, Pt(bp-7) has a red shift of
about 53 nm compared to Pt(bp-6), although they have similar
ligands; this is attributed to the extended conjugation of Cz
compared to Ac, resulting in a significantly stabilized LUMO
for Pt(bp-7) and a small energy gap (ΔEg) between the
HOMO and LUMO (Figure S1). Additionally, the Pt(II)
complexes have excited lifetimes (τ) of 5.3−11.9 μs. PyAc-
based Pt(bp-6) (τ = 11.9 μs) has a similar excited lifetime with
PyAc-based Pt(bp-1), Pt(bp-2), and Pt(bp-3) (τ = 11.9, 11.4,
and 12.2 μs, respectively)61 and exhibits very little blue-shift
emission spectrum, 500 nm for Pt(bp-6) and 504, 512, and 505
nm for Pt(bp-1), Pt(bp-2), and Pt(bp-3), respectively61 (Figure
1). This indicates that the bridging atoms of nitrogen or
oxygen have a small influence on their excited photophysical
properties. However, PyCz-based Pt(bp-7) and Pt(bp-8) have
significantly shortened τ of 5.7 and 5.3 μs, respectively, which
are only half that of the PyAc-based Pt(bp-1), Pt(bp-2), Pt(bp-
3), and Pt(bp-6), indicating the great influence of the Cz
Figure 4. Photoluminescence spectra of the tetradentate Pt(II)
complexes (a) at 77 K in 2-MeTHF (dash-dotted lines), (b) at room
temperature in dichloromethane solution (solid lines), and (c) at
room temperature in PMMA film (solid-ball lines).
a
Table 3. Photophysical Properties of Tetradentate Pt(II) Complexes
emission at 77 K in
emission at RT in 5 wt %
PMMA
2-MeTHF
emission at RT in DCM
complex
λmax [nm] τ [μs] λmax [nm] τ [μs] ΦPL [%] kr [104 s−1
]
knr [104 s−1
]
λmax [nm] τ [μs] ΦPL [%] kr [104 s−1
]
knr [104 s−1
]
Pt(ppy-1)
Pt(bp-6)
Pt(bp-7)
Pt(bp-8)
508
500
553
557
7.8
11.9
5.7
516
506
557
563
6.4
8.2
4.0
3.5
95
33
10
5
14.8
4.0
2.5
0.8
8.2
22.5
28.6
516
506
558
564
5.3
9.1
4.2
3.8
68
32
12
14
12.8
3.5
2.9
6.0
7.4
21.0
22.6
5.3
1.4
3.7
a
DCM, dichloromethane; 2-MeTHF, 2-methyltetrahydrofuran; PMMA, poly(methyl methacrylate).
F
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states by NTO analyses (Figure S4). These analyses also can
be supported by the fact that Pt(bp-6), Pt(bp-7), and Pt(bp-8)
exhibit much lower ΦPL and smaller kr compared to Pt(ppy-1)
(Table 3). Another reason for the low ΦPL of Pt(bp-6), Pt(bp-
7), and Pt(bp-8) is their large structural deformations in T1
states, especially the long and weak Pt−N1 coordination bonds
in Pt(bp-7) and Pt(bp-8) (Figure 1), which are revealed by the
TD-DFT calculations discussed above. Additionally, Pt(bp-6),
Pt(bp-7), and Pt(bp-8) exhibit nearly identical emission
spectra in both dichloromethane solution and PMMA film
(Figure S5), also revealing the dominant 3LC mixed with small
3MLCT characters in their T1 states.
Table 4. Photophysical Properties of Tetradentate Pt(II)
Complexes in Various Thin Films
a
thin film
λmax [nm] τ [μs] ΦPL [%] kr [104 s−1
]
knr [104 s−1
]
5 wt % Pt(ppy-1) in
516
516
519
518
558
562
562
558
5.3
68
12.8
6.0
PMMA
5 wt % Pt(ppy-1) in
mCBP
6.2
62
10.0
6.1
10 wt % Pt(ppy-1)
in mCBP
5 wt % Pt(ppy-1) in
DPEPO
5.0
4.2
5.0
90
12
7
18.0
2.9
2.0
21.0
18.6
5 wt % Pt(bp-7) in
PMMA
Notably, the intensity of the vibronic v0‑1 sideband of
Pt(ppy-1) with a N*C^N^O-coordinated ligand is extremely
high and merges with the v0‑1 vibrational peak to form a broad
emission spectrum (Figure 4), because the strong δ-donating
5 wt % Pt(bp-7) in
mCBP
1.4
10 wt % Pt(bp-7)
in mCBP
5 wt % Pt(bp-7) in
DPEPO
4.6
17
3.7
18.0
3
ability of the O− enhances the MLCT character in T1 states.
a
However, N*C^C*N-coordinated Pt(bp-6) with smaller
3MLCT character shows a much vibronically featured
spectrum and lower v0‑1 vibrational peak (Figure 4). Increasing
the molecular rigidity through employing Cz to replace Ac of
Pt(bp-6), Pt(bp-7) exhibited a further smaller v0‑1 vibrational
peak (Figure 4). Pt(bp-8), which adopts ACz as nitrogen
brigding atom in the ligand design (Figure 1), possesses the
most rigid molecular skeleton and shows a significantly
decreased v0‑1 sideband in various conditions (Figure 4), also
indicative of dominant 3LC mixed with small 3MLCT
characters in the T1 state. The decreased intensities of the
vibronic sidebands increase the color purity of the Pt(II)
complexes-based phosphorescent emitters and facilitate their
applications in the OLED field. Additionally, the spectrometer
also shows influence on the spectrum. The v0‑1 vibrational
peaks of Pt(bp-7) and Pt(bp-8) measured by the HITACHI F-
7000 spectrometer are much smaller than those by the Horiba
Jobin Yvon FluoroLog-3 spectrometer (Figure S6). This is
attributed to the different gratings, as well as the detectors have
different sensitivities to the light with the same wavelength.
The photophysical properties of Pt(ppy-1) and Pt(bp-7) in
various thin films were also investigated (Figure 5 and Table
4). Compared to the doped PMMA film, 5 wt % Pt(ppy-1)
doped CBP film shows similar τ, and a ΦPL PL spectrum with a
smaller sideband at about 550 nm. Increasing the concen-
tration to 10 wt %, and no excimer emission can be observed,
although Pt(ppy-1) has a planar molecular geometry with a
very small dihedral angle of only 16.6° between the terminal
pyridine and carboxyl planes in the S0 state (Tables S1 and
S2), indicating no Pt···Pt interaction in the excited states. The
ΦPL of 5 wt % Pt(ppy-1) doped DPEPO film is significantly
enhanced to 90%, which is attributed to the high ET of DPEPO
(3.0 eV) to result in efficient energy transfer from the host to
Pt(bp-1). Similar results are also found for the Pt(bp-1) in
various thin films. No significant solvatochromic behavior is
observed for Pt(bp-7), which is in agreement with its excited-
DPEPO, bis[2-(diphenylphosphino)phenyl]ether oxide; mCBP, 3,3-
di(9H-carbazol-9-yl)biphenyl.
Figure 6. Photostability comparison of the previously reported Pt(bp-
2) and the newly developed Pt(ppy-1), Pt(bp-6), Pt(bp-7), and Pt(bp-
8). The photostabilities of the Pt(II) complexes were assessed using
the complex-doped polystyrene film (5 wt %) excited by UV light of
375 nm at 500 W/m2.
moiety of the ligands on the excited lifetimes of the Pt(II)
complexes.
At RT in dichloromethane solution and PMMA film, all four
Pt(II) complexes show broadened emission spectra with a little
red shift compared to the cryogenic emission spectra, where
Pt(ppy-1) exhibits an extremely broad featureless spectrum
merged from the vibrational peaks of v0‑0 and v0‑1 (Figures 4
and S5). The four Pt(II) complexes show a quantum efficiency
(ΦPL) of 5−95% and 12−68%, and τ of 3.5−8.2 μs and 3.8−
9.1 μs in dichloromethane solution and PMMA film,
respectively (Table 3). Pt(ppy-1) exhibits broad structureless
spectra, high ΦPL of 95% and 68%, and large radiative rate (kr)
3
state property of a dominant LC(πCzPh → πCzPh*) character
discussed above.
of 14.8 × 104 and 12.8 × 104 s−1 in dichloromethane and
3
PMMA, respectively, revealing more MLCT(πppyCOOdPt
→
Photostability Studies. High photostability of the Pt(II)
complexes is essential to be used as emitters for device
fabrication, and also critical to device operational lifetime
improvement. However, the photostability study of Pt(II)
complexes is scarce.61,65 The photostability of the newly
developed Pt(II) complexes was assessed according the
previously reported method using UV irradiation, which was
the 5 wt % Pt(II) complexes-doped polystyrene film excited by
πPy*) character in T1 states (Table 3). By contrast, Pt(bp-6),
Pt(bp-7), and Pt(bp-8) still show well-resolved vibronic
3
emission spectra, indicative of dominant LC(πbp → πbp*)
mixed with small 3MLCT(πbpdPt → πPy*) characters for Pt(bp-
3
3
6), LC(πCzPh → πCzPh*) mixed with small MLCT(πbpdPt
→
πPy*) characters for Pt(bp-7), and 3LC(πCzPh → πCzPh*) mixed
3
with small MLCT(πbpdPt → πACz*) for Pt(bp-8) in their T1
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Figure 7. (a) Energy level diagram and chemical structures of the materials used for the Pt(ppy-1)-doped OLEDs. (b) EL spectra, (c) current
density−voltage (J−V) characteristics, (d) external quantum efficiency (EQE) versus luminance plots, and (e) power efficiency−luminance (P−L)
characteristics of the Pt(ppy-1)-doped OLEDs. HTL: hole-transporting layer; EML: emissive layer; ETL: electron-transporting layer.
UV light of 375 nm at 500 W/m2 (Figure 6).61 Pt(ppy-1)
demonstrated a photostability lifetime of 150 min, LT97, at
97% of initial luminance, which was significantly superior to
that of our previously reported Pt(bp-2) with LT85, at 85% of
initial luminance, of 125 min under identical conditions
(Figure 6).61 The photostability can be further improved by
using an oxygen-bridged ligand; Pt(bp-6) achieved an
estimated photostability lifetime of 150 min, about LT98.5, at
98.5% of initial luminance (Figure 6). However, the PyCz-
based Pt(bp-7) and Pt(bp-8) exhibited relatively poor
photostabilities, which decreased to LT95 and LT86 for a
lifetime of 150 min, respectively (Figure 6). This is attributed
to the long and weak coordination bonds Pt−N1 in Pt(bp-7)
(2.232 Å) and Pt(bp-8) (2.237 Å) (Figure 1), resulting in
rapid decomposition under the excitation of the UV light. This
photostability study reveals the importance of the integrated
design of the ligand to well coordinate with the center Pt(II)
ion.
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a
Table 5. EL Performance of Pt(ppy-1)-Doped OLEDs
EQE (%)
EML
Von (V)
λEL (nm)
fwhm (nm)
peak
100 cd/m2
1000 cd/m2
Lmax
CIE (x, y)
10% Pt(ppy-1):DPEPO
10% Pt(ppy-1):mCBP
10% Pt(ppy-1):26mCPy
6.4
3.9
2.4
518
518
516
80
66
58
3.9
7.2
18.5
1.9
6.7
12.3
0.8
7.2
10.4
1165
43709
40979
(0.358, 0.560)
(0.349, 0.602)
(0.298, 0.634)
a
Von: turn-on voltage at 1 cd/m2. fwhm: full width at half-maximum. EQE: external quantum efficiency. Lmax: maximum brightness. CIE:
Commission Internationale de l′Eclairage. 26mCPy: 2,6-di(9H-carbazol-9-yl)pyridine.
Electroluminescence (EL) Properties. On the basis of
the high of the ΦPL (90%) of 5 wt % Pt(ppy-1) doped DPEPO
film, Pt(ppy-1)-based OLED was fabricated using DPEPO as
host material with the device structure of ITO/HATCN (10
nm)/TAPC (30 nm)/Pt(ppy-1):DPEPO (10%, 35 nm)/
TmPyPB (55 nm)/LiF (1 nm)/Al (100 nm) (structure I).
HATCN was used as hole injection material, and TAPC and
TmPyPB were employed as hole- and electron-transporting
materials, respectively (Figure 7). The device performance is
shown in Table 5. It was unexpected that the device exhibited a
large turn-on voltage of 6.4 V and an extremely low peak EQE
of only 3.9% with a CIE coordinates of (0.358, 0.560) (Figure
7, Table 5). This could be attributed to the poor charge-
transfer ability of the DPEPO, which also resulted in low
current density and small luminance (Figure 7c). The device
performance could be significantly improved employing mCBP
as host material (structure II), which showed a decreased turn-
on voltage of 3.9 V and a peak EQE of 7.2% with very small
roll-off (Figure 7d, Table 5). Notably, a Lmax of 43 709 cd/m2
could be achieved because of the improved balance of charge
carriers (Figure 7c). Moreover, OLED using 26mCPy as host
material realized a peak EQE of 18.5% (structure III), which
still maintained an EQE of 10.4% at 1000 cd/m2, and an Lmax
of over 40 000 cd/m2 was achieved. The EL spectrum was
much narrower than those using mCBP or DPEPO as host
materials. It is believed that the device performance can be
further improved by optimizing the device structure with
charge blocking materials.
Figure 8. Chemical structures of selected ppy-based tetradentate
DISCUSSION
Pt(II) complexes with N^C^N^O ligands and their photophysical
properties in dichloromethane solution at room temperature reported
by Che’s group.70−74
■
In 2013, Che and co-workers first reported a series of highly
robust phosphorescent tetradentate Pt(II) complexes with
N^C^N^O ligands containing 5/5/6 metallocycles, Pt-2-F2,
Et-Pt-2-F2, and their analogues (Figure 8).70,71 The extremely
planar molecular geometries of the Pt-2-F2 and Et-Pt-2-F2
enabled both monomer and excimer emissions in high
concentrations, making them ideal emitters for single-doped
white OLEDs. Our previous study demonstrated the dihedral
angle of only 0.6° between the terminal pyridine and phenyl
planes for Et-Pt-2-F2 in the S0 state by DFT calculation,47
which is much smaller than that of Pt(ppy-1) (16.6°). Pt-2-F2-
doped white OLED achieved an EQE of 16.5% with CIE (0.33,
0.42) and CRI (Color Rendering Index) of 77. Further studies
demonstrated that incorporating an electron-donating large
steric group (3,5-di-tert-bulylphenyl) into the ligands, or
breaking the conjugation with a N- or C-linker could further
enhance the ΦPL, tune the emission color of the N^C^N^O
Pt(II) complexes, and meanwhile avoid the excimer emission
(Figure 8).72−74 Compared to Pt-2-F2 (λ = 482 nm) and Et-Pt-
2-F2 (λ = 480 nm), Y-Pt (λ = 553 nm), Pt-1-Pt-5 (λ = 517−
551 nm) and the newly developed Pt(ppy-1) (λ = 516 nm)
show a significant red shift in dichloromethane solution at RT,
making them good emitters for high performance green and
yellow OLEDs.72−74 The Pt-1-based green OLED demon-
strated a peak EQE of 18.2% with efficiency stability (2.4%
roll-off at 1000 cd/m2), and CIE coordinates of (0.282, 0.657)
using a similar device structure of the Pt(ppy-1)-based
OLED.72 Pt-4- and Pt-5-based OLEDs demonstrated peak
EQEs over 20% and realized maximum power efficiencies of
118 and 126 lm/W.74 All the N^C^N^O Pt(II) complexes
show broad and less featured or Gaussian-type emission
3
spectra, revealing a large percent of MLCT character in their
exciter states, which is also supported by the fact of the high
ΦPL and short τ. Moreover, the HOMO distributions of Pt-4
and Pt-5 are mainly on the πPh-pPt‑O-πPh orbitals,74 which are
similar to the Pt(ppy-1) on the πAc-pPt‑O-πOC orbitals discussed
above. These studies suggested that the N^C^N^O Pt(II)
complexes could act as efficient emitters for OLED
fabrications.
CONCLUSIONS
In summary, we developed a series of ppy-based 6/5/5 and bp-
based 6/5/6 Pt(II) complexes employing tetradentate ligands
■
I
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Article
Accession Codes
with nitrogen or oxygen atoms as bridging groups. Exper-
imental and theoretical studies reveal that ligand structural
modifications have great influences on the electrochemical,
photophysical, and excited-state properties and photostabilities
of the Pt(II) complexes. All the Pt(II) complexes strongly emit
in various conditions and show ΦPL and τ of 5−95% and 3.5−
8.2 μs in dichloromethane solution at RT, and 12−68% and
3.8−9.1 μs in PMMA film at RT. The TD-DFT calculations
and NTO analyses reveal that Pt(bp-6), Pt(bp-7), and Pt(bp-8)
have dominant 3LC mixed with small 3MLCT characters in T1
states, resulting in relatively low ΦPL, but emission spectra with
decreased vibronic sidebands and high color purities. By
CCDC 2036927 and 2036928 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Authors
Guijie Li − College of Chemical Engineering, Zhejiang
University of Technology, Hangzhou, Zhejiang 310014,
People’s Republic of China; orcid.org/0000-0002-0740-
2235; Email: guijieli@zjut.edu.cn
Yun-Fang Yang − College of Chemical Engineering, Zhejiang
University of Technology, Hangzhou, Zhejiang 310014,
People’s Republic of China; Email: yangyf@zjut.edu.cn
Yuanbin She − College of Chemical Engineering, Zhejiang
University of Technology, Hangzhou, Zhejiang 310014,
People’s Republic of China; orcid.org/0000-0002-1007-
1852; Email: sheyb@zjut.edu.cn
3
contrast, Pt(ppy-1) possesses much more MLCT character in
the T1 state, enabling high ΦPL and large kr. All the Pt(II)
complexes show high photostabilities in 5 wt % doped
polystyrene films; especially, PyAc-based Pt(ppy-1) and
Pt(bp-6) demonstrate long photostability lifetimes of 150
min for LT97 and LT98.5, respectively. The strength of Pt−N1
coordination bonds has a great influence on their photo-
stabilities. This work suggests that rational and integrated
design of the ligand should be taken into account to well
coordinate with the center Pt(II) ion. Host materials
significantly affected the Pt(ppy-1)-based green OLEDs.
Using 26mCPy as host, the OLED demonstrated a peak
EQE of 18.5% with CIE coordinates of (0.298, 0.634), which
still maintained an EQE of 10.4% at 1000 cd/m2, and an Lmax
of over 40 000 cd/m2 was achieved. This study should provide
a valuable reference for the further development of efficient
and stable phosphorescent Pt(II) complexes for display and
lighting applications.
Authors
Gang Shen − College of Chemical Engineering, Zhejiang
University of Technology, Hangzhou, Zhejiang 310014,
People’s Republic of China
Xiaoli Fang − College of Chemical Engineering, Zhejiang
University of Technology, Hangzhou, Zhejiang 310014,
People’s Republic of China
Feng Zhan − College of Chemical Engineering, Zhejiang
University of Technology, Hangzhou, Zhejiang 310014,
People’s Republic of China
Jianbing Zheng − College of Chemical Engineering, Zhejiang
University of Technology, Hangzhou, Zhejiang 310014,
People’s Republic of China
Weiwei Lou − College of Chemical Engineering, Zhejiang
University of Technology, Hangzhou, Zhejiang 310014,
People’s Republic of China
Qisheng Zhang − MOE Key Laboratory of Macromolecular
Synthesis and Functionalization, Department of Polymer
Science and Engineering, Zhejiang University, Hangzhou
310027, People’s Republic of China; orcid.org/0000-
0002-0899-6856
EXPERIMENTAL SECTION
■
The detailed synthesis and structural characterization of all the
tetradentate Pt(II) complexes are provided in the Supporting
Information. Diffraction data of Pt(bp-6) and Pt(bp-7) were collected
on a Bruker SMART APEX diffractometer with Mo KR radiation (λ)
0.71073 Å at 170 K. Cyclic voltammetry and different pulsed
voltammetry were carried out using a CH1760E electrochemical
analyzer. The theoretical calculations were performed using Gaussian
09. The absorption spectra of the Pt(II) complexes were performed
on an Agilent 8453 UV−vis spectrometer, and steady-state emission
and decay lifetime experiments were measured on a Horiba Jobin
Yvon FluoroLog-3 spectrometer. All devices were fabricated by
vacuum thermal evaporation and then were tested outside the
glovebox after encapsulation. Please see the Supporting Information
for details.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c02569
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c02569.
■
sı
Notes
The authors declare no competing financial interest.
DFT calculations of Pt(II) complexes (Tables S1, S2,
S5−S8, Figure S1); crystal data and structure refinement
for Pt(bp-6) and Pt(bp-7) (Tables S3, S4); cyclic
voltammograms of the Pt(II) complexes (Figure S2);
comparisons of absorption spectra of the Pt(II)
complexes and their ligands (Figure S3); NTO analyses
(Figure S4); comparisons of luminescence spectra of the
Pt(II) complexes at 77 K in 2-MeTHF, at RT in DCM
solution, and at RT in PMMA film (Figure S5);
luminescence spectra comparison measured by different
spectrometers (Figure S6); detailed synthetic proce-
ACKNOWLEDGMENTS
■
We thank the National Natural Science Foundation of China
(grant no. 21878276) and the Fundamental Research Funds
for the Provincial Universities of Zhejiang (RF-A2019013) for
financial support. We also thank Prof. Xiao-Chun Hang of the
Institute of Advanced Materials (IAM), Nanjing Tech
University (NanjingTech), for the support of the photo-
physical data measurements.
REFERENCES
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