Axially Chiral Bis-Cycloplatinated Binaphthalenes and Octahydro-Binaphthalenes for Efficient Circularly Polarized Phosphorescence in Solution-Processed Organic Light-Emitting Diodes

Article abstract of DOI:10.1021/acs.inorgchem.1c01861

A new series of axially chiral binuclear Pt(II) complexes with bridging ligands of binaphthalenes and octahydro-binaphthalenes and auxiliary ligands of β-diketones were designed and prepared. These complexes, identified by spectral and electrochemical met

Full text of DOI:10.1021/acs.inorgchem.1c01861

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Article
Axially Chiral Bis-Cycloplatinated Binaphthalenes and Octahydro-
Binaphthalenes for Efficient Circularly Polarized Phosphorescence in
Solution-Processed Organic Light-Emitting Diodes
Lei Wang,§ Hui Xiao,§ Lang Qu, Jintong Song, Weilan Zhou, Xiangge Zhou, Haifeng Xiang,*
and Zong-Xiang Xu*
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ABSTRACT: A new series of axially chiral binuclear Pt(II) complexes
with bridging ligands of binaphthalenes and octahydro-binaphthalenes and
auxiliary ligands of β-diketones were designed and prepared. These
complexes, identified by spectral and electrochemical methods and single-
crystal X-ray diffraction, emit an orange-red phosphorescence with a
quantum yield up to 21% and 70% in solution and solid, respectively, due
to the effect of steric hindrance from bridging ligands and the 2,3-position
extension of chiral axis planes. They can be used as emitters in solution-
processed organic light-emitting diodes to achieve luminance efficiency,
asymmetry factor, and external quantum efficiency up to 5.4 cd A−1, 3.0 ×
10−3, and 3.1%, respectively. Moreover, the essential relationships between
their chemical structures and luminescence quantum efficiency and
asymmetry factor are discussed, which affords explicit insights for
designing circularly polarized luminescent materials and devices.
Therefore, the development of new chiral emitters for high-
performance CP-OLEDs is still urgent and significant.
INTRODUCTION
■
Recently, circularly polarized organic light-emitting diodes
(CP-OLEDs) have received growing attention due to their
efficient ability to generate a circularly polarized luminescence
(CPL) directly and extensive applications in optical
spintronics, optical data storage, and three-dimensional (3D)
displays.1−5 Since Meijer et al. demonstrated the first CP-
OLEDs in 1997,6 various organic polymers and small
molecules have been successively employed as emitters for
the application in CP-OLEDs. On the one hand, except a few
thermally activated delayed fluorescence organic emitters,7
most of them are fluorescent emitters, and consequently their
OLEDs usually are inefficient with an intrinsic internal
quantum efficiency (IQE) of 25%.8,9 On the other hand,
because of the McClure heavy-atom spin−orbit coupling
effect,10 phosphorescent transition-metal complexes, such as
Ir(III) and Pt(II) complexes, can harvest not only singlet but
also triplet excitons to realize a theoretical 100% IQE for
OLEDs.1−5 Although considerable progress has been made,
few high-performance CP-OLEDs with both a high electro-
luminescent (EL) efficiency and asymmetry factor (gEL) have
been reported. In general, there is a trade-off between
improving EL efficiency and increasing gEL. For example,
Ln(III) complexes-based CP-OLEDs have a very high gEL up
to 1.41 but a relatively low external quantum efficiency (EQE)
of 4.2 × 10−3 %.11 On the contrary, Ir(III) complexes have an
extra high EQE of 23.6% but a low gEL of 5 × 10−4.12
In the past two decades, the class of square-planar
cyclometalated Pt(II) complexes has been extensively inves-
tigated because of their interesting phosphorescence na-
ture,13−19 but there are only a finite number of chiral Pt(II)
complexes for the applications in CP-OLEDs.20−27 In 2016,
Brandt et al. used helicene-based Pt(II) complexes [Figure 1a,
(C^N)*Pt(O^O), C^N = (P)/(M)-pyridinyl-helicene, O^O =
β-diketones] to construct solution-processed CP-OLEDs,
which have a high gEL (+0.22 and −0.38) but a low power
efficiency (ηP = 0.23 lm W−1).20 After that Yan et al.
introduced −F and −CF3 moieties at a pyridinyl-helicene
ligand to increase the volatility and emission intensity for
vacuum-evaporated CP-OLEDs (EQE = 18.8%, gEL = 1.6 ×
10−3).21,22 Han et al.23 [(N^C^N)Pt(≡R)*, N^C^N = 1,3-
bis(2-pyridyl, ≡R = (R)/(S)-(+)-α-methylbenzylisocyanide,
solution-processed CP-OLEDs, luminance efficiency ηL = 0.84
cd A−1, gEL ≈ 10−4], Lee et al.24 [(N^C^N)*Pt(Cl), N^C^N =
Received: June 21, 2021
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N^C = (R)/(S)-binaphthalene-derived N^C ligands, EQE =
2.15%, gEL = 1.1 × 10−3]. However, all the above Pt(II)
complexes are mononuclear Pt(II) complexes. In comparison
with mononuclear Pt(II) complexes, multinuclear Pt(II)
complexes may have some unique luminescence proper-
ties,17,33−37 but they usually might suffer the problem of low
solubility and aggregation-caused quenching (ACQ) through
intramolecular and/or intermolecular interactions.
We are increasingly concerned about the preparation, optics,
and sensing properties of axially chiral binaphthalenes
(BINA).36−41 Because of a restriction in the intramolecular
rotations27,37 or motions36 of BINA, the reported BINA-based
Pt(II) complexes usually have aggregation-induced emission
(AIE)-active CPL. Moreover, since the dihedral angle of BINA
is ∼90°, BINA derivates are not planar molecules and have
good solubility consequently. In the present work, a series of
novel neutral and chiral BINA- or octahydro-BINA-bridging
binuclear Pt(II) complexes [(O^O)Pt(μ-BINA-C^N)*Pt-
(O^O) or (O^O)Pt(μ-H8-BINA-C^N)*Pt(O^O), molecular
weight up to 1342] (Figure 1d) were designed and prepared
for efficient solution-processed CP-OLEDs. Because of the
effect of a steric hindrance from bridging ligands and the 2,3-
position extension in chiral axis planes, the binuclear Pt(II)
complexes emit an orange-red phosphorescence in both
solution [emission quantum yield (Φ) up to 21% in degassed
CH2Cl2] and solid [Φ up to 14% and 70% in powder and
4,4′,4″-tris(carbazol-9yl)triphenylamine (TCTA) host, respec-
tively]. Furthermore, the interrelationships between their
chemical structures and Φ/EQE/g are explored, which afford
useful tips for the design of CPL materials and devices. As far
as we are aware, this is the first example of binuclear Pt(II)
complexes for the application in CP-OLEDs.
Figure 1. Selected reported Pt(II) complexes for CP-OLEDs (a−c)
and binuclear Pt(II) complexes (d) in this work.
(R)/(S)-1-(2-oxazoline)-3-(2-pyridyl)phenylate, vacuum-
evaporated CP-OLEDs, EQE = 9.7%, gEL = 1.2 × 10−4], and
Fu et al.25 [Figure 1b, (C^N)Pt(N^O)*, C^N = 1-(benzo[b]-
thiophen-2-yl)-isoquinoline, N^O = (R)/(S)-Schiff base
ligands; solution-processed CP-OLEDs, EQE = 0.93%, gEL
≈
10−3] adopted point-chirality-ligand-based Pt(II) complexes to
fabricate CP-OLEDs. Qian et al. further utilized a liquid crystal
to improve the EL performance of point-chirality-ligand-based
Pt(II) complexes [(N^C)*Pt(O^O), N^C = (R)/(S)-2-
phenylpyridine derivate, solution-processed CP-OLEDs, EQE
= 11.3%, gEL = 0.02].26 Recently, Jiang et al.27 reported that
axial-chirality-ligand-based Pt(II) complexes bearing aggrega-
tion-induced emission28−32 are suitable for fabricating
solution-processed CP-OLEDs [Figure 1c, (N^C)*Pt(O^O),
RESULTS AND DISCUSSION
■
Synthesis and Characterization. The synthetic methods
of binuclear Pt(II) complexes are illuminated in Figure 2 and
described in detail in the Supporting Information. The
complexes have a good solubility in 1,4-dioxane, CH2Cl2,
Figure 2. Synthesis of binuclear Pt(II) complexes.
B
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CHCl3, benzene, toluene, ethyl acetate (EtOAc), and
tetrahydrofuran (THF) but a poor solubility in H2O, ether,
hexane, ethanol, and CH3CN. Ten milliliters of toluene would
dissolve 0.5 g of (R)-[tmd] (tmd = tetramethyl-l,2-dioxetane)
and (R)-[H8-tmd] at least. The single crystals of (R)-[tmd]
(CCDC No. 2086886) and (S)-[H8-acac] (CCDC No.
2086879) were obtained by a slow diffusion and evaporation
of a CH3CN/toluene solution.
Photophysical Properties. The UV/visible absorption
and phosphorescence data of all binuclear Pt(II) complexes at
room temperature are listed in Table S1 and Figures 3−6 and
Figure 3. Absorption spectra of (R)-binuclear Pt(II) complexes in
CH2Cl2 (5.0 × 10−6 mol dm−3).
Figure 5. (a) Emission spectra of (R)-binuclear Pt(II) complexes in
powder and TCTA (10%). Corresponding photos (b, c) in degassed
CH2Cl2 (5.0 × 10−6 mol dm−3), (d) in PVK (5.0%), and (e) in
powder under sunlight and 360 nm UV lamp.
Figure 6. Emission spectra (5.0 × 10−6 mol dm−3 in CH2Cl2) of (R)-
[tmd]. (inset) Calculated T1 single spin density of (R)-[tmd].
methyl-l,2-dioxetane (tmd). The similar results were observed
in the previous work on simple (C^N)Pt(O^O) com-
plexes.42,43 Since the H8−BINA-based Pt(II) complexes have
a smaller π-conjugation system than BINA-based Pt(II)
complexes, H8−BINA-based Pt(II) complexes exhibit blue-
shifted absorption spectra (λabs = 445 nm).
To understand the mechanism of the excited states and
transitions, density functional theory (DFT) and time-
dependent (TD) DFT were performed by the Gaussian 09
program package. In a dilute CH2Cl2 solution, both absorption
spectra bands and molar absorption coefficients (ε) of (R)-
[tmd] are predicted well by a theoretical calculation (Figure
4). The low-energy absorption band (λabs = 460 and 453 nm
for experiment and calculation, respectively) is contributed by
Figure 4. Experimental and calculated absorption spectra (a) and
energy-level diagram and frontier molecular orbitals (b) of (R)-[tmd]
(in CH2Cl2).
(S1−S3 Supporting Information). Except for the circular
dichroism (CD) and CPL properties, the optical properties of
five pairs of (S)/(R) enantiomers are similar (Figures S1−S3),
as in our previous reports.35−41 As shown in Figure 3, the
BINA-based Pt(II) complexes have similar absorption spectra
with the same low-energy absorption band at λabs = 460 nm,
even though they bear different O^O auxiliary ligands of
acetylacetone (acac), dibenzoylmethane (dbm), and tetra-
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the highest occupied molecular orbital (HOMO) → lowest
unoccupied molecular orbital (LUMO) (12.3%), HOMO →
LUMO+1 (39.2%), and HOMO−1 → LUMO (40.7%)
transitions with an oscillator strength (f OSC) of 0.135. The
energy-level diagram and frontier molecular orbitals of (R)-
[tmd] (Figure 4) reveal that the electron clouds of HOMO−1,
HOMO, LUMO, and LUMO+1 are symmetrically distributed
over the two sides of the BINA. For the HOMO−1 and
HOMO, the electron clouds are mainly located at electron-rich
naphthalene rings and Pt(II) ions, but they are primarily
contributed from not only naphthalene rings but also electron-
deficient quinoline rings for LUMO and LUMO+1. Thereby,
the low-energy absorption band can mainly be ascribed to both
a singlet intraligand charge transfer (1ILCT) in a BINA
bridging ligand and a singlet metal-to-ligand charge transfer
(1MLCT) from Pt(II) ions to a BINA bridging ligand. Similar
transitions are found for (R)-[dbm] (Figure S4) and (S)-[H8-
acac] (Figure S5) as well. Clearly, for different energy levels of
(R)-[tmd], (R)-[dbm], and (S)-[H8-acac], the π-conjugated
skeletons of O^O ligands have a small contribution to the
electron clouds, but R groups (CH3, t-Bu, and Ph) have no
contribution. This might be reason that BINA-based (λabs
=
460 nm) and H8-BINA-based (λabs = 445 nm) Pt(II)
complexes show alike low-energy absorption bands, even if
they have different R groups.
As the reported BINA-based Pt(II) complexes usually have
AIE-active photoluminescence (PL),27,36,37 it is unexpected
that all of them are highly luminescent in degassed CH2Cl2 and
powder (Table S1 and Figures 5, S2, and S3).41 In a dilute
solution, all the BINA-based binuclear Pt(II) complexes show
similar emission spectra (λem = 620 nm, Φ = 7.5−8.2%). At the
same time, all the emission spectra of H8-BINA-based
binuclear Pt(II) complexes blue shift to λem = 610 nm with
higher Φ (19−21%). Compared with solution samples, powder
samples give a bit of a red shift in the emission band (λem
=
635−643 and 625−630 for BINA- and H8-BINA-based
binuclear Pt(II) complexes, respectively) along with a slight
emission quenching (Φ = 3.2−14%). If the binuclear Pt(II)
complexes are doped in the host of polyvinylcarbazol (PVK) or
TCTA, their Φ values would sharply increase into 51% and
70%, respectively, through a host−guest energy transfer
(Figure S6). Among all the binuclear Pt(II) complexes, tmd-
based complexes have the highest Φ, because of their biggest
effect of t-Bu steric hindrance.
Figure 7. X-ray single-crystal structures of (R)-[tmd] (a, c) and (S)-
[H8-acac] (b).
structures, which are normal for alkyl groups.34,46,47 No
racemization is observed in the X-ray single-crystal structures
of (R)-[tmd] and (S)-[H8-acac], revealing that the (R) and
(S) axial chirality is stable enough for a chemical synthesis.
Except the peripheral atoms of methyl groups and cyclohexyl
for (R)-[tmd] and (S)-[H8-acac], respectively, all their other
atoms are almost in two chiral axis planes, because the
figuration of the 4-coordinated Pt(II) ion is square-planar. This
is much different from the previous results27,36,37 and would
help to improve the CPL by extending axially chiral planes and
achieve a strong emission in solution. These molecules have
propeller-type structures with a dihedral angle within the BINA
and H8-BINA of 90.8 and 90.5° for (R)-[tmd] and (S)-[H8-
acac], respectively, and thus there are no intramolecular π−π
stacking and Pt−Pt interactions. However, for (R)-[tmd],
strong face-to-face intermolecular π−π stacking interactions
(3.33 Å) are observed between the closest molecules, which
might contribute to its low Φ in a powder. The shortest
intermolecular Pt−Pt distance is 8.01 Å,17,35 revealing that
there is no intermolecular Pt−Pt interactions for (R)-[tmd].
On the contrary, (S)-[H8-acac] molecules have strong
intermolecular Pt−Pt interactions (3.50 Å) but weak face-to-
face π−π stacking interactions. The comprehensive effect of
Because of the long emission decay lifetimes (0.50−7.9 μs)
and the large Stokes shifts (∼160 nm) between the absorption
and emission band (Table S1 and Figures S7 and S8), the
emission of binuclear Pt(II) complexes originates from triplet
excited states (T). This can be further verified by the fact that
the phosphorescence of a degassed solution can be efficiently
quenched by oxygen (Figure 6). A single-electron spin density
that is the density difference between α- and β-electrons is
useful to describe a triplet electron transition.44,45 As depicted
in Figures 6 and S9, the spin density distribution of the T1 state
for (R)-[tmd] and (S)-[H8-acac] is dominated by the upper
side of the BINA bridging ligand and the upper Pt(II) ion,
indicating that its phosphorescence originates mainly from
3
3ILCT and MLCT.
X-ray Single-Crystal Structures. The chemical structures
and arrangements of organic molecules play a key in their
emission characters. The X-ray single-crystal structures and
packing of (R)-[tmd] and (S)-[H8-acac] are shown in Figure
7. There are some disorder problems in the single-crystal
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two factors would result in (S)-[H8-acac] having a higher Φ.
Moreover, with the help of chirality and strong intermolecular
interactions, (R)-[tmd] can self-assemble to form helix-like
structures in a crystal.36,37 No intramolecular π−π stacking and
Pt−Pt interactions combine the fact that the π-conjugated
system of binaphthalenes is broken by an axial chirality, which
leads to the similar photophysical properties between
binaphthalenes-based mononuclear Pt(II) complexes (Figure
1c) and binuclear Pt(II) complexes.
Chiroptical Properties. The chirality properties of all
binuclear Pt(II) complexes in a dilute CH2Cl2 solution (5.0 ×
10−6 mol dm−3) are investigated by CD spectra (Figure S10).
As an example, the CD spectra of (R)/(S)-[tmd] exactly
mirrored each other (Figure 8), revealing that they are a pair of
Figure 9. PL (top) and CPL spectra (middle) and gPL (bottom) of
dye-doped PVK films (5.0%) based on (a) (R)-[tmd] and (b) (R)-
[H8-tmd].
CP-OLEDs Properties. Combining high emission quantum
yields and asymmetry factors, (R)/(S)-[tmd] and (R)/(S)-
[H8-tmd] were used as dopants in an emitting layer (EML) to
fabricate cost-effective and large-area CP-OLEDs by a wet
method. To avoid the triplet−triplet annihilation of phosphor-
escent complexes, TCTA and 1,3-di-9-carbazolylbenzene
(mCP) were selected as hole-transport host material, and
1,3-bis(5-(4-(tert-butyl)phenyl)-1,3,4-oxadiazol-2-yl)benzene
(OXD-7) was selected as electron-transport host materials,
which possess high triplet state energy (2.5−3.3 eV) to ensure
an efficient energy transfer from the host to phosphorescent
complexes.48−53 Besides, mixed hole-transport and electron-
transport host materials could effectively promote the carrier
balance in OLEDs. The HOMO and LUMO energy levels of
binuclear Pt(II) complexes lie between the host materials
TCTA, mCP, and OXD-7, which further indicates that they
can be used as host materials in CP-OLEDs. The device
architecture and chemicals used in CP-OLEDs are listed in
Figure 10. The optimal device structure is indium tin oxide
(ITO) glass/poly(2,3-dihydrothieno-1,4-dioxin)-poly-
(styrenesulfonate) (PEDOT:PSS) (30 nm)/blended host
materials: Pt(II) complex (50 nm)/1,3,5-tris(1-phenyl-1H-
benzo[d]imidazol-2-yl)benzene (TPBI) (30 nm)/LiF (1 nm)/
Al (100 nm). PEDOT:PSS, TPBI, and LiF served as a hole-
injection layer (HIL), electron-transporting layer (ETL)/hole-
blocking layer, and electron-injecting layers, respectively. The
EMLs were prepared through spin-coating from a chlor-
obenzene solution of Pt(II) complex (10% in weight) in the
mixed host of TCTA/mCP/OXD-7 (1,1:1).
In general, OLED layers are amorphous. The morphology of
the spin-coated HIL and EML were characterized by an atomic
force microscope (AFM). (R)-[tmd]- and (R)-[H8-tmd]-
containing EMLs were deposited on the HIL of PEDOT:PSS.
The HIL of PEDOT:PSS has a rough surface with a root-
mean-square (RMS) roughness of 1.31 nm (Figure S13). If
further depositions of EML were made, the RMS surface
roughness would reduce to 0.61 and 0.47 nm for (R)-[tmd]
and (R)-[H8-tmd], respectively. It is evident that the EML
would help to generate a smothered surface morphology with
fewer islandlike features. This would prevent the formation of
electrical shorts and nonemissive dark spots and, eventually,
improve the performance of solution-processed OLEDs.
Figure 8. Experimental and computational CD spectra of (R)/(S)-
[tmd] and (R)/(S)-[H8-tmd] in CH2Cl2 (5.0 × 10−6 mol dm−3).
enantiomers. The computed CD spectrum of (R)-[tmd] in
dilute CH2Cl2 is almost identical to the experimental spectrum,
revealing that the absolute (R) stereochemistry of (R)-[tmd] is
reserved in a dilute solution. For (R)/(S)-[H8-tmd], similar
results are observed. On the basis of previous work,36−41 the
high-energy (<350 nm) and lower-energy (>350 nm) CD
bands can be mainly assigned to chiral binaphthyl itself and
1
chiral binaphthyl-induced MLCT/1ILCT, respectively. It is
noteworthy that the CD signals at 465 nm of BINA-based
Pt(II) complexes are much bigger than those of H8-BINA-
based Pt(II) complexes, meaning that BINA-based Pt(II)
complexes have much bigger absorption asymmetry factors
[gabs = 4.3 × 10−3 and 1.2 × 10−3 for (R)-[tmd] and (R)-[H8-
tmd], respectively, Table S1].
(R)-[tmd] and (R)-[H8-tmd] were doped in PVK to
examine their CPL properties, because their cast films have
high Φ values up to 51% (Figure 9). The dissymmetric factors
of gPL are determined up to −4.2 × 10−3 and −1.9 × 10−3 for
(R)-[tmd] and (R)-[H8-tmd], respectively, which are in
agreement with the CD absorption data.
Electrochemical Properties. Electrochemical spectra
were measured by cyclic voltammetry (CV) in CH2Cl2
solutions to examine the redox properties of (R)-[tmd]
(Figure S11) and (R)-[H8-tmd] (Figure S12). The energy
levels of the HOMO and LUMO in the corresponding
complexes were calculated by the first oxidation potential and
the onset of the absorption spectra. The band gap and energy
levels of HOMO and LUMO are −2.44, −5.46, & −3.02 and
−2.53, −5.50, & −2.97 eV for (R)-[tmd] and (R)-[H8-tmd],
respectively. These data are a bit higher than the TD-DFT
computational data (Figure 4), but their trends are coincident.
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Figure 10. Device architecture (a, c) and chemical structures (b) of the materials used in CP-OLEDs.
The EL, current density, ηL, and EQE curves of CP-OLEDs
are shown in Figure 11. The EL spectra of (R)/(S)-[tmd] are
almost identical and have a good agreement with their PL
spectra. (R)/(S)-[tmd]-based CP-OLEDs emit red light (λem
= 605 and 642 nm) with the 1931 Commission Internationale
de l’Eclairage (CIE_1931) coordinates of (0.62, 0.38), which
are comparable to those of a pure red emission (0.65, 0.35).
Their maximum EQE, ηL, ηP, and brightness are 1.4%, 2.9 cd
A−1, 1.7 lm W−1, and 1470 cd m−2, respectively. For (R)/(S)-
between the PL/EL efficiency and the asymmetry factor.
Therefore, we believe that the present work might potentially
provide a new way to design chiral multinuclear complexes for
CPL applications.
EXPERIMENTAL SECTION
■
Materials and Instrumentation. All reagents were obtained
commercially and utilized directly. 1H NMR (400 MHz) spectra were
measured in CDCl3 and C6D6. Tetramethylsilane was used as an
internal standard to measure chemical shifts in parts per million. UV/
Vis absorption spectra were done by a U-5100 (Hitachi)
spectrophotometer with quartz cuvettes of 1 cm path length. An F-
7000 fluorescence spectrophotometer (Hitachi) was used to measure
fluorescence spectra at room temperature; slit width = 5.0 nm, photon
multiplier voltage = 400 V. CD spectra were done by a Chirascan plus
qCD (Applied Photophysics) at room temperature. Emission decay
lifetime spectra were measured by a Horiba Tempro-01 (Horiba
Scientific). The CPL spectra were performed by a JASCO CPL-300
spectro-fluoropolarimeter at room temperature. High-resolution mass
spectra (HR-MS) were obtained with a Shimadzu LCMS-IT-TOF
(ESI). The synthetic routes of binuclear Pt(II) complexes were
described in detail in the Supporting Information.
[H8-tmd]-based CP-OLEDs, the EL spectra blue shift to λem
=
597 nm (CIE_1931: 0.59, 0.40) with higher EL performances
(EQE, ηL, ηP, and brightness up to 3.1%, 5.4 cd A−1, 3.4 lm
W−1, and 3070 cd m−2, respectively). Compared with those of
achiral mononuclear analogues (vacuum-evaporated
OLEDs),54 the EL performances of the binuclear Pt(II)
complexes (solution-processed OLEDs) still have much room
for further improvement.
The circularly polarized electroluminescence properties of
OLEDs were studied as well. As displayed in Figure 12, (R)/
(S)-[tmd] and (R)/(S)-[H8-tmd]-based CP-OLEDs emit
strong and broad mirror-image CPEL signals (500−750 nm)
with the maximum |gEL| of 3.0 × 10−3 and 2.0 × 10−3,
respectively. These gEL data are in the same region with those
of the most reported CP-OLEDs.1−5 In addition, a self-
assembled supramolecule technology is widely used for
chirality transfer and amplification.55−57 However, this stage
might be not so efficient for CP-OLEDs. The EMLs of OLEDs
usually are amorphous layers, and thus the electrons and holes
can efficiently recombine in an EML to form excitons.
Islandlike crystalline materials acting as dark spots would
lead to electrical shorts and poor performances in OLEDs.58
Measurement of Cyclic Voltammetry. CV was measured by a
CHI 620E electrochemical analyzer in degassed CH2Cl2 solutions
with n-Bu4NPF6 as the supporting electrolyte. The CV cell consisted
of a glassy carbon electrode, a Pt wire counter electrode, and an Ag/
AgCl reference electrode. The ferrocene/ferrocenium (Fc/Fc+) was
used as a reference. The HOMO energy-level calculations of the
corresponding complexes were performed using the first oxidation
potential, and LUMO energy levels were calculated according to the
equations LUMO (eV) = Eg + HOMO.
OLED Device Fabrication and Characterization. Patterned
indium−tin oxide (Lumtec) substrates were cleaned by a sonication
in detergent, ethanol, acetone, and isopropyl alcohol continuously,
followed by the UV−O3 treatment for 30 min. The hole injection
layer (PEDOT: PSS, Al4083) was spin-coated on the preprocessed
ITO substrates through a 0.22 μm filter at a spin-coating rate of 4500
rpm for 40 s and oven-dried at 120 °C for 30 min to form a film of 30
nm thickness. The emitting layer was spin-coated through a filter with
blend host materials and Pt(II) complex in chlorobenzene solution
(20 mg/mL) and dried at 95 °C for 20 min. The thicknesses of spin-
coated layers were examined on a KLA-TENCOR D-100 profiler.
After that, the substrates were transferred to a vacuum chamber. A 50
nm portion of an electron-transporting layer (TPBI), 1 nm of LiF, and
100 nm of Al were thermally evaporated consecutively under a
vacuum (<1 × 10−6 Torr, Mbraun MB200). The layer thickness was
measured by quartz crystal monitors during the deposition. All
CONCLUSIONS
■
In conclusion, a novel series of chiral BINA and H8-BINA-
bridging binuclear Pt(II) complexes was designed and
prepared for solution-processed CP-OLEDs. Because of the
effect of steric hindrance from bridging ligands and 2,3-
position extension in chiral axis planes, the binuclear Pt(II)
complexes are highly phosphorescent in both solution and
solid states. Moreover, on the one hand, BINA-bridging
complexes would have a higher asymmetry factor but a lower
PL/EL efficiency. On the other hand, H8-BINA-bridging
complexes would have a contrary tendency. There is a trade-off
F
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Figure 11. (a) Current density and luminance; (b) ηL and EQE; (c) EL of (R)/(S)-[tmd]. (d) Current density and luminance; (e) ηL and EQE;
(f) EL of (R)/(S)-[H8-tmd]. (g) Photograph of CP-OLED.
OLEDs were encapsulated by a UV adhesive under N2. The
current−voltage−luminance data were done by Keithley 2400 and
Photo Research PR680.
Measurement of Atomic Force Microscopy. Film morpholo-
gies were measured by an MFP3D-Stand Alone scanning probe in the
tapping mode. The thin films were fabricated in a similar way to that
of OLEDs fabrication.
Measurement of Fluorescence Quantum Yield (Φ). The Φ of
degassed solution was done by the optical dilute method of Demas
and Crosby.40 The Φ of powder and film was performed by an
integrating sphere.
X-ray Crystallographic Analysis. The method of X-ray
crystallographic analysis was similar to that of the previous work.40
Computational Details. DFT and TD-DFT calculations (PBE0/
SDD/6-31G) were done by Gaussian 09 software. The ground-state
geometry was optimized by DFT based on the X-ray single-crystal
structure of Pt(II) complexes. After that, the optimized structure was
used to calculate the UV/vis absorption and CD spectra by TD-DFT
(CH2Cl2 as the solvent, pcm method, 100 singlet−singlet transitions).
Figure 12. CPEL properties of (R) (orange) /(S)-[tmd]-based
(black) (a) and (R) (orange) /(S)-[H8-tmd]-based (black) (b) CP-
OLEDs.
G
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Measurement of CP-EL Spectra. CP-EL Spectra of CP-OLEDs
were obtained by fixing the encapsulated OLED substrates in the
groove of a JASCO circular polarization spectrophotometer (CPL-
300) and applying a 6−7 V voltage under a scanning speed of 200 nm
min−1 with the “Continuous” mode. The measurement adopts the
“slit” mode with an Em slit width of 3000 μm and multiple
accumulations (three cycles).
and CD spectrometer. We appreciate D.-b. Luo and Y. Xie for
their help with the single-crystal and CD measurements.
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* Supporting Information
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AUTHOR INFORMATION
■
Corresponding Authors
Haifeng Xiang − College of Chemistry, Sichuan University,
Chengdu 610041, China; orcid.org/0000-0002-4778-
2455; Email: xianghaifeng@scu.edu.cn
Zong-Xiang Xu − Department of Chemistry, Southern
University of Science and Technology, Shenzhen 518000,
China; Email: xu.zx@sustech.edu.cn
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Authors
Lei Wang − College of Chemistry, Sichuan University,
Chengdu 610041, China
Hui Xiao − Department of Chemistry, Southern University of
Science and Technology, Shenzhen 518000, China
Lang Qu − College of Chemistry, Sichuan University, Chengdu
610041, China
Jintong Song − College of Chemistry, Sichuan University,
Chengdu 610041, China
Weilan Zhou − College of Chemistry, Sichuan University,
Chengdu 610041, China
Xiangge Zhou − College of Chemistry, Sichuan University,
Chengdu 610041, China; orcid.org/0000-0001-5327-
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ACKNOWLEDGMENTS
■
The present work was supported by the National Natural
Science Foundation of China (No. 21871192) and Major
Program of Guangdong Basic and Applied Research (No.
2019B030302009). We thank the Analytical & Testing Center
of Sichuan University for the X-ray single-crystal diffractometer
H
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