Twist to Boost: Circumventing Quantum Yield and Dissymmetry Factor Trade-Off in Circularly Polarized Luminescence

Article abstract of DOI:10.1021/acs.inorgchem.1c00070

Circularly polarized luminescence (CPL) enables promising applications in asymmetric photonics. However, the performances of CPL molecules do not yet meet the requirements of these applications. The shortcoming originates from the trade-off in CPL between the photoluminescence quantum yield (PLQY) and the photoluminescence dissymmetry factor (gPL). In this study, we developed a molecular strategy to circumvent this trade-off. Our approach takes advantage of the strong propensity of [Pt(N^C^N)Cl], where the N^C^N ligand is 1-(2-oxazoline)-3-(2-pyridyl)phenylate, to form face-to-face stacks. We introduced chiral substituents, including (S)-methyl, (R)- and (S)-isopropyl, and (S)-indanyl groups, into the ligand framework. This asymmetric control induces torsional displacements that give homohelical stacks of the Pt(II) complexes. X-ray single-crystal structure analyses for the (S)-isopropyl Pt(II) complex reveal the formation of a homohelical dimer with a Pt···Pt distance of 3.48 ?, which is less than the sum of the van der Waals radii of Pt. This helical stack elicits the metal-metal-to-ligand charge-transfer (MMLCT) transition that exhibits strong chiroptical activity due to the electric transition moment making an acute angle to the magnetic transition moment. The PLQY and gPL values of the MMLCT phosphorescence emission of the (S)-isopropyl Pt(II) complex are 0.49 and 8.4 × 10-4, which are improved by factors of ca. 6 and 4, respectively, relative to the values of the unimolecular emission (PLQY, 0.078; gPL, 2.4 × 10-4). Our photophysical measurements for the systematically controlled Pt(II) complexes reveal that the CPL amplifications depend on the chiral substituent. Our investigations also indicate that excimers are not responsible for the enhanced chiroptical activity. To demonstrate the effectiveness of our approach, organic electroluminescence devices were fabricated. The MMLCT emission devices were found to exhibit simultaneous enhancements in the external quantum efficiency (EQE, 9.7%) and the electroluminescence dissymmetry factor (gEL, 1.2 × 10-4) over the unimolecular emission devices (EQE, 5.8%; gEL, 0.3 × 10-4). These results demonstrate the usefulness of using the chiroptically active MMLCT emission for achieving an amplified CPL.

Full text of DOI:10.1021/acs.inorgchem.1c00070

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Twist to Boost: Circumventing Quantum Yield and Dissymmetry
Factor Trade-Off in Circularly Polarized Luminescence
Sumin Lee, Yongmoon Lee, Kyungmin Kim, Seunga Heo, Dong Yeun Jeong, Sangsub Kim,
Jaeheung Cho,* Changsoon Kim,* and Youngmin You*
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ABSTRACT: Circularly polarized luminescence (CPL) enables promising applications in
asymmetric photonics. However, the performances of CPL molecules do not yet meet the
requirements of these applications. The shortcoming originates from the trade-off in CPL
between the photoluminescence quantum yield (PLQY) and the photoluminescence
dissymmetry factor (gPL). In this study, we developed a molecular strategy to circumvent
this trade-off. Our approach takes advantage of the strong propensity of [Pt(N^C^N)Cl],
where the N^C^N ligand is 1-(2-oxazoline)-3-(2-pyridyl)phenylate, to form face-to-face
stacks. We introduced chiral substituents, including (S)-methyl, (R)- and (S)-isopropyl, and
(S)-indanyl groups, into the ligand framework. This asymmetric control induces torsional
displacements that give homohelical stacks of the Pt(II) complexes. X-ray single-crystal
structure analyses for the (S)-isopropyl Pt(II) complex reveal the formation of a homohelical
dimer with a Pt···Pt distance of 3.48 Å, which is less than the sum of the van der Waals radii of
Pt. This helical stack elicits the metal−metal-to-ligand charge-transfer (MMLCT) transition
that exhibits strong chiroptical activity due to the electric transition moment making an acute angle to the magnetic transition
moment. The PLQY and gPL values of the MMLCT phosphorescence emission of the (S)-isopropyl Pt(II) complex are 0.49 and 8.4
× 10−4, which are improved by factors of ca. 6 and 4, respectively, relative to the values of the unimolecular emission (PLQY, 0.078;
gPL, 2.4 × 10−4). Our photophysical measurements for the systematically controlled Pt(II) complexes reveal that the CPL
amplifications depend on the chiral substituent. Our investigations also indicate that excimers are not responsible for the enhanced
chiroptical activity. To demonstrate the effectiveness of our approach, organic electroluminescence devices were fabricated. The
MMLCT emission devices were found to exhibit simultaneous enhancements in the external quantum efficiency (EQE, 9.7%) and
the electroluminescence dissymmetry factor (gEL, 1.2 × 10−4) over the unimolecular emission devices (EQE, 5.8%; gEL, 0.3 × 10−4).
These results demonstrate the usefulness of using the chiroptically active MMLCT emission for achieving an amplified CPL.
developed by Fuchter and co-workers.13 In a more recent study,
Chen and co-workers have developed CPEL-active helical dyads
INTRODUCTION
■
Circularly polarized luminescence (CPL) refers to the differ-
that consist of two thermally activated delayed fluorescence
ential emission of left- and right-handed circularly polarized
units.23
light.1 CPL has particular advantages for electroluminescence
Despite such pioneering research, materials that exhibit
appreciable CPEL remain rare. This paucity is due to the trade-
applications.2 First, CPL emissions are not impaired by the
antiglare films of electroluminescence devices, which signifi-
off between PLQY and gPL in luminescent molecules. Although
cantly improves brightness.3 In addition, the two-handedness of
the magnitude of PLQY obeys the relationship PLQY ∝ |μ|2 (μ
CPL creates a binocular disparity, which enables three-
is the electric transition dipole moment of emission), the
dimensional (3D) displays.4 Furthermore, circularly polarized
magnitude of gPL follows the relationship |gPL| ∝ (|m|/|μ|) × |cos
electroluminescence (CPEL) devices can be integrated into
θ1| (m is the magnetic transition dipole moment of emission, and
spintronic circuits to transmit spin information remotely.5,6
θ1 is the angle between μ and m).1 An increase in the PLQY in
The exploitation of these advantages requires CPL-active
materials. The key parameters of CPL emitters are the
most systems results in a decrease in |gPL| due to their opposing
photoluminescence quantum yield (PLQY) and the photo-
luminescence dissymmetry factor (gPL). The latter is quantified
Received: January 11, 2021
by the equation gPL = 2 × (ILCPL − IRCPL)/(ILCPL + IRCPL), where
ILCPL and IRCPL are the intensities of the left- and right-handed
CPL, respectively. Recent efforts have created CPL-active
materials for CPEL devices.7−22 Notable examples include
phosphorescent cycloplatinates bearing helicene, which were
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dependences on |μ|.12−14,24−65 Therefore, we require molecular
designs that afford a large |m| and a parallel disposition between
μ and m (i.e., a large |cos θ1|). These design rules are, however,
hard to meet because the majority of luminescent molecules,
except lanthanide complexes,66 exhibit small |m| and orthogonal
μ and m (i.e., |cos θ1| = 0).
An alternative approach to creating CPL emitters makes use
of the asymmetric exciton coupling between vicinal lumino-
phores. This approach is reminiscent of electronic circular
dichroism (ECD), which has found wide utility in the study of
biological systems. If two identical luminophores are helically
positioned, CPL arises with |gPL| ∝ (|μ|2 × |sin θ2|)/|r|2, where θ2
and |r| are the angle and distance, respectively, between the two
μ of the luminophores.67 Note that the relationship in this case
between |gPL| and μ is different from that discussed above (i.e., |
gPL| ∝ (|m|/|μ|) × |cos θ1|); here, the |gPL| of a dyad increases at a
large |μ|, a small interluminophore distance (i.e., small |r|), and a
perpendicular disposition between two μ (i.e., |sin θ2| = 1).
Although this strategy has been widely employed,18,23,26,68−88 its
versatility is limited by an antagonistic effect on electro-
luminescence: the close proximity of the luminophores (i.e.,
small |r|) frequently leads to a biexcitonic annihilation that
lowers the electroluminescence efficiency.89
We propose that a square-planar Pt(II) complex can serve as a
platform that circumvents this trade-off. Pt(II) complexes
exhibit a unique propensity to form face-to-face stacks in
which the Pt-centered 5dz2 orbitals are perturbed to generate the
highest occupied molecular orbital (HOMO) with the
intermolecular σ* character, whereas the lowest unoccupied
molecular orbital (LUMO) remains unimolecular with the π*
character of a ligand.90−96 Our earlier study of amphiphilic
molecules predicted that the stacks of Pt(II) complexes will elicit
a metal−metal-to-ligand charge-transfer (MMLCT) transition
with nonorthogonal μ and m (i.e., |cos θ1| ≠ 0).17 On the basis of
this earlier study, we reason that helically stacked Pt(II)
complexes exhibiting the MMLCT transition could achieve
CPEL with an improved electroluminescence dissymmetry
factor (gEL = 2 × (ILCPL − IRCPL)/(ILCPL + IRCPL), where ILCPL
and IRCPL are the intensities of left- and right-handed CPEL,
respectively), without attenuating external quantum efficiency
(EQE).
To test this proposal, we chose a cyclometalated Pt(II)
complex with a tridentate N^C^N pincer ligand (i.e., [Pt-
(N^C^N)X], X = Cl). The [Pt(N^C^N)X] complexes have
electroluminescence applications because their superior struc-
tural rigidity yields a strong phosphorescence emission.97−114
Moreover, the high-field anionic N^C^N ligand destabilizes the
nonradiative metal-centered ligand-field state, which also
facilitates strong emissions.115,116 Taking advantage of the
phosphorescence properties of [Pt(N^C^N)X], the groups of
Williams, Yam, and Li have demonstrated high-efficiency
electroluminescence.103,111,114,117−121
Figure 1. Molecular strategy for circumventing the trade-off between
PLQY and gPL in circularly polarized luminescence.
was varied to optimize the torsional displacement and the
intermolecular distance in the stack. In this paper, we describe
the design, syntheses, photophysical properties, and electro-
luminescence applications of these Pt(II) complexes. Our
investigations revealed that both the PLQY and the dissymmetry
factor improve upon the formation of helical Pt(II) complex
aggregates. This approach was then tested further by fabricating
multilayer CPEL devices. We believe that our results provide
valuable insights into the development of new CPEL materials.
RESULTS AND DISCUSSION
■
Syntheses of Pt(II) Complexes. The syntheses of our
Pt(II) complexes involve 1-(2-oxazoline)-3-(2-pyridyl)benzene
as the N^CH^N proligand. The 2-oxazoline moiety bears a
chiral center at its 4-position. We varied the steric bulk around
the chiral center by introducing methyl, isopropyl, or indanyl
groups (Scheme 1). This steric control aims to optimize the
intermolecular distance and torsional displacement between
vicinal Pt(II) complexes in the condensed state. The chiral
N^CH^N ligands were prepared with two-step syntheses
involving the Pd(0)-catalyzed Stille reaction between 3-
bromobenzaldehyde and 2-(tributylstannyl)pyridine, followed
by a dehydrative condensation reaction of the Stille product with
chiral ethanolamine. Cycloplatination of the chiral N^CH^N
ligand and the in situ-generated [PtCl2(NCPh)2] afforded the Pt
complexes [Pt(MeOppy)Cl], [Pt(iPrOppy)Cl], and [Pt-
(IdOppy)Cl] in yields of 47−82% (Oppy: 2-(3-(4,5-dihydroox-
azol-2-yl)phenyl)pyridine Id: indanyl). In each complex, the
stereocenter is in the (S)-configuration. An enantiomeric
[Pt(iPrOppy)Cl] (i.e., (R)-[Pt(iPrOppy)Cl]) and a control
complex devoid of a chiral alkyl unit (i.e., [Pt(Oppy)Cl]) were
also synthesized to assess the effectiveness of our helical control.
The Pt(II) complexes were found to be soluble in polar organic
solvents, such as CH2Cl2. The molecular structures of the
complexes and their precursors were fully characterized with
One challenge in this molecular strategy is to induce a helical
torque during the formation of the Pt(II) complex stacks.
[Pt(N^C^N)X] usually forms antiparallel head-to-tail molec-
ular stacks, due to its large permanent dipole mo-
ment.113,117,119,122−126 This mutually orthogonal disposition is
not favorable to chiroptical activity, because there is no helical
sense. In order to ensure torsional displacement in the pair, we
introduced the chiral oxazoline moiety into the N^C^N
framework (Figure 1). It was anticipated that this modification
would exert a helical torque that produces homohelical
[Pt(N^C^N)X] stacks. The alkyl unit around the chiral center
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Scheme 1. Syntheses and Chemical Structures of the Pt(II) Complexes
1
standard spectroscopic methods, including H and 13C{1H}
NMR spectroscopy, high-resolution mass spectrometry, and
elemental analyses. The synthetic details and the character-
ization data are summarized in the Experimental Details and the
Supporting Information.
bands are found for (S)- and (R)-[Pt(iPrOppy)Cl], respectively.
The spectral positions of the ECD bands coincide with the CT
transition regions (i.e., less than 430 nm), which suggests that
the CT transitions are chirogenic. The ECD spectra of
[Pt(iPrOppy)Cl] aggregates are very similar to the solution
spectra, except in the reversal of their signs. However, careful
measurements for increased concentrations of [Pt(iPrOppy)Cl]
reveal ECD signaling in the region greater than 500 nm
(Supporting Information, Figure S3). Note that the low-energy
ECD bands indicate the formation of new chiroptically active
species. The magnitude of the Kuhn dissymmetry factor (gabs),
defined as gabs = 2 × (εL − εR)/(εL + εR) (εL and εR are the molar
absorbance of the left- and right-handed circularly polarized
light), of the low-energy band (488 nm) is 9.9 × 10−4 for (R)-
[Pt(iPrOppy)Cl]. This absolute value is greater than that of the
CT transition (388 nm) by a factor of 2 (Table 1). The results
collectively indicate the dominant role of the new low-energy
species over asymmetric exciton coupling in the chiroptically
active electronic transition.
As shown in Figure 3, the photoluminescence spectra of 10
μM (S)- and (R)-[Pt(iPrOppy)Cl] (Ar-saturated THF) contain
weak emission bands with a peak wavelength at 489 nm. The
photoluminescence spectra exhibit distinct vibronic structures
with a progression of 1500 cm−1, which indicates that the
emission contains the π−π* transition character of the N^C^N
ligand. PLQYs were determined to be 0.078 0.012 and 0.080
0.016 for the (S)- and (R)-[Pt(iPrOppy)Cl], respectively,
relative to the 9,10-diphenylanthracene standard. Similar to the
absorption spectra, significant bathochromic shifts are found in
Photophysical Properties. Figure 2 compares the UV−vis
absorption spectra of 10 μM (R)- and (S)-[Pt(iPrOppy)Cl]
recorded in tetrahydrofuran (THF). The spectrum of [Pt-
(iPrOppy)Cl] contains a strong absorption band with a peak
wavelength of 388 nm (molar absorbance (ε) = 8.4 × 103 M−1
cm−1). The 388 nm absorption bands are bathochromically
shifted relative to those (<320 nm) of free ligands (i.e., (R)- and
(S)-iPrOppyH). The spectral shift indicates that the 388 nm
absorption band is attributed to the metal-to-ligand charge-
transfer (MLCT) and the ligand-to-ligand charge-transfer
(LLCT) transitions. The transitions are mixed significantly
with the ligand-localized π−π* transition, as they exhibit
negligible solvatochromism (Supporting Information, Figure
S1). The 430−500 nm shoulder bands are likely due to spin-
forbidden transitions. Significantly red-shifted bands are
observed upon aggregation of [Pt(iPrOppy)Cl]; the diffuse
reflectance spectra contain strong absorption bands with a peak
wavelength of 488 nm, as shown in the Kubelka−Munk function
(F(R) = (1 − R)2/2R; R is the reflectance) in Figure 2. The
emergence of the bathochromically shifted bands suggests the
formation of low-energy species in an aggregated state.
The ECD spectra of 10 μM solutions (THF) of (S)- and (R)-
[Pt(iPrOppy)Cl] have a mirror relationship (see the bottom
panel in Figure 2). Positive and negative monosignate ECD
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Figure 2. UV−Vis absorption (top) and ECD (bottom) spectra of 10
μM solutions (THF) and aggregates of (R)- and (S)-[Pt(iPrOppy)Cl].
Absorption spectra of 10 μM free ligands ((R)- and (S)-iPrOppyH) are
included for comparison. See the Supporting Information, Figure S2,
for the spectra of the Kuhn dissymmetry factors (gabs).
Figure 3. Photoluminescence (top) and CPL (bottom) spectra of
solutions (THF) and aggregates of (R)- and (S)-[Pt(iPrOppy)Cl].
The CPL spectra of the 10 μM [Pt(iPrOppy)Cl] solutions are
barely discernible, even after a thorough deaeration (see the
bottom panel in Figure 3). In sharp contrast, the [Pt(iPrOppy)-
Cl] aggregates produce appreciable CPL signals. We carefully
validated the CPL signals of the aggregates. The corresponding
the photoluminescence spectra of the aggregates. The emission
bands centered at 611 nm are structureless, which suggests that
they have significant charge-transfer character. The photo-
luminescence lifetime (τobs) is as long as 1.9 μs for both
enantiomers. The PLQYs determined absolutely using an
integrating sphere are as high as 0.49 0.02 and 0.50 0.02
for (S)- and (R)-[Pt(iPrOppy)Cl], respectively. These PLQY
values are an order of magnitude higher than the solution values.
The PLQY improvements indicate that the Pt(II) complexes
belong to the family of aggregation-induced emission (AIE)
luminophores.
g
PL values are 8.4 × 10−4 for the (S)-[Pt(iPrOppy)Cl] aggregates
and −7.8 × 10−4 for the (R)-[Pt(iPrOppy)Cl] aggregates at a
wavelength of 610 nm. These values are higher than those of the
solutions ((S)-[Pt(iPrOppy)Cl], 2.5 × 10−4; (R)-[Pt-
(iPrOppy)Cl], −1.8 × 10−4). The brightness of the CPL
(BCPL), which is defined to be BCPL = ε × PLQY × |gPL|/2 by
Arrico et al., amounts to 1.73 and 1.76 M−1 cm−1 for the
aggregates of (S)-[Pt(iPrOppy)Cl] and (R)-[Pt(iPrOppy)Cl],
Table 1. Photophysical Properties of the Pt(II) Complexes
a
b
solution /aggregates
c
d
e
f
g
h
λabs (nm) (ε (103 M−1 cm−1))
λem (nm)
τobs (μs)
PLQY
gabs (10−4
)
gPL (10−4
)
(S)-[Pt(iPrOppy)Cl]
(R)-[Pt(iPrOppy)Cl]
[Pt(Oppy)Cl]
[Pt(MeOppy)Cl]
[Pt(IdOppy)Cl]
388 (8.4) /488
388 (9.0) /488
387 (5.2) /488
388 (8.6) /487
390 (7.9) /483
489/611
489/611
501/602
489/601
504/534
0.17/1.87
0.16/1.89
0.13/0.89
0.27/1.14
0.010/0.34
0.078 0.012/0.49 0.02
0.080 0.016/0.50 0.02
0.032 0.006/0.02 0.005
0.058 0.002/0.28 0.001
0.055 0.001/0.10 0.001
5.8 /−7.4
−4.8/9.9
N.D.i
3.7 /−2.9
14/2.4
2.5/8.4
−1.8 /−7.8
N.D.i
4.3/5.2
11/4.8
a
b
c
10 μM in deaerated THF. Powders or vacuum-evaporated thin films on glass substrates. Absorption peak wavelength (molar absorbance).
d
e
Emission peak wavelength. Photoluminescence lifetime determined by nonlinear least-squares fits of the photoluminescence decay traces
monitored after picosecond pulsed laser photoexcitation at 377 nm. Detection wavelengths (solution/aggregates): 490/611 nm, (R)-/(S)-
f
[Pt(iPrOppy)Cl]; 502/600 nm, [Pt(Oppy)Cl]; 489/601 nm, [Pt(MeOppy)Cl]; 504/534 nm, [Pt(IdOppy)Cl]. Photolumunescence quantum
yields determined relatively for the solutions by using the 9,10-diphenylanthracence standard (PLQY = 1.00, toluene)127 and photoluminescence
quantum yields determined absolutely for the aggregates by using an integrating sphere under an Ar condition. The measurements were performed
g
h
i
in triplicate. Kuhn dissymmetry factor determined at a wavelength of 388 nm. Photoluminescence dissymmetry factor determined at λem. Not
determined, because the g values are less than 10−4.
D
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respectively.128 These BCPL values reach the median value of
those of the state-of-art CPL performance Pt(II) complexes.128
With these results taken together, it is concluded that the
simultaneous increases in PLQY and |gPL| demonstrate the
effectiveness of our molecular strategy.
This finding encouraged us to assess the chiroluminescence
properties of the series of Pt(II) complexes, [Pt(Oppy)Cl],
[Pt(MeOppy)Cl], (R)- and (S)-[Pt(iPrOppy)Cl], and [Pt-
(IdOppy)Cl]. As shown in Figure 4, the |gPL| value of the
preserve an orbital overlap between two adjacent complexes,
without decreasing the helical torque.
There is a qualitatively identical trend in the absolute PLQY
values (see the bottom panel in Figure 4). For the aggregates, the
PLQY increases in the order [Pt(Oppy)Cl] (0.02 0.005) <
[Pt(MeOppy)Cl] (0.28 0.001) < [Pt(iPrOppy)Cl] (0.49
0.02 and 0.50 0.02) and is lower for the [Pt(IdOppy)Cl]
aggregates (0.10 0.001). Obviously, increased steric hindrance
yields greater PLQYs, except in the case of [Pt(IdOppy)Cl].
The aggregation-induced enhancements in the PLQY may be
attributed to the suppression of exciton−vibration coupling.129
The aggregation-induced formation of the MMLCT transition
state is vital for the PLQY improvements, because N,N′-
dicarbazolyl-3,5-benzene (mCP) films doped with a low
concentration (5 wt %) of [Pt(iPrOppy)Cl] exhibit PLQY
values ((R)-[Pt(iPrOppy)Cl], 0.19 0.003; (S)-[Pt(iPrOppy)-
Cl], 0.21
0.002) smaller than those of their aggregates.
[Pt(Oppy)Cl] does not contain any steric substituent and
exhibits the smallest PLQY value probably due to short
intermolecular distances that facilitate a multiexcitonic
annihilation. Table 1 and the Supporting Information, Table
S1, list the photophysical and electrochemical parameters of the
Pt(II) complexes.
The above results demonstrate that the trade-off between the
PLQY and |gPL| does not apply to the Pt(II) complex aggregates.
In order to elucidate the structural origin of this outcome, we
performed photophysical and structural investigations. It is
possible that the square-planar [Pt(N^C^N)Cl] readily forms
(i) excimers or (ii) oligomeric stacks that promote the MMLCT
transition. The former would produce a monomer-like photo-
luminescence excitation (PLE) spectrum, whereas the latter has
a PLE spectrum distinct from that of the monomer. Figure 5a
shows that the PLE spectrum of the (S)-[Pt(iPrOppy)Cl]
aggregates (observation wavelength = 611 nm) differs
significantly from that of the solution (observation wavelength
= 489 nm). Identical behaviors are also found for the other
Pt(II) complexes (Supporting Information, Figure S8). This
result indicates that excimers are not responsible for the strong
CPL emission. The same conclusion can be drawn from the
transient photoluminescence measurements: the photolumi-
nescence decay traces of (S)-[Pt(iPrOppy)Cl] aggregates
recorded at a wavelength of 611 nm are devoid of delayed rise
profiles (see the Supporting Information, Figure S9, of the decay
traces for the other Pt(II) complexes). If the 611 nm emission
has an excimeric origin, a delayed rise component should be
observed due to the time required for an exciton migration to
form excimers. Thus, ground-state aggregates with the MMLCT
transition state are responsible for the improved CPL activity.
Our 195Pt{1H} NMR (107 MHz, THF-d8) experiments also
show that there is a downfield shift from −3546 ppm to −3539
ppm upon aggregation, which indicates the presence of Pt···Pt
interactions required for the MMLCT transition (Supporting
Information, Figure S10).130
Figure 4. Magnitudes of gPL (top) and PLQY (bottom) for the
solutions (Ar-saturated THF; black bars) and the aggregates (red bars)
of the Pt(II) complexes. gPL for each complex was determined at the
emission peak wavelength.
aggregates increases with increases in the steric hindrance
around the stereocenter: [Pt(Oppy)Cl], less than 10−4;
[Pt(MeOppy)Cl], 5.2 × 10−4; (R)- and (S)-[Pt(iPrOppy)Cl],
7.8 × 10−4 and 8.4 × 10−4. There is a similar trend in |gabs|, which
indicates that the chiroptical control has a ground-state origin
(Supporting Information, Figure S4). This trend may be
explained based on the relationship |gPL| ∝ (|m|/|μ|) × |cos θ1|;
the more sterically bulky unit would attenuate |μ| of the
MMLCT transition due to a weaker Pt···Pt interaction,
increasing |gPL|. Our TD-B3LYP calculations at various Pt···Pt
distances support this notion (Supporting Information, Figure
S5). |gPL| for [Pt(IdOppy)Cl] deviates from this trend, which is
probably due to the reduction in aggregation that results from
the bulky indane unit. This claim is supported by the UV−vis
absorption and photoluminescence spectra of the [Pt(IdOppy)-
Cl] aggregates, which are very similar to its solution spectra
(Supporting Information, Figures S6 and S7). These findings
suggest a strategy to improve the |gPL| value: Locating chiral units
away from the coordination sphere of a Pt(II) complex would
A single-crystal analysis can be used to provide compelling
evidence for the CPL-active MMLCT transition (Figure 6).
Crystals suitable for X-ray crystallography were grown from a
150 mM CH3CN solution of (S)-[Pt(iPrOppy)Cl]. It was found
that (S)-[Pt(iPrOppy)Cl] adopts a distorted square-planar
geometry with a sum of the four bond angles around the Pt
center of 359.94°. The bond distance of Pt−Cl is 2.42 Å, which
is longer than the other bonds due to the strong trans influence
exerted by the anionic phenyl ring. The molecular packing
geometry can be characterized with the presence of a helical
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Figure 5. (a) Comparisons of the UV−vis absorption (solid lines) and the photoluminescence excitation (dashed lines, λobs = 489 (solution) and 611
nm (aggregates)) spectra of a 10 μM solution (THF; blue) and aggregates (red) of (S)-[Pt(iPrOppy)Cl]. (b) Photoluminescence decay traces of (S)-
[Pt(iPrOppy)Cl] aggregates (red circles) recorded at a wavelength of 611 nm after a picosecond pulsed laser excitation at 377 nm. The black circles
correspond to the instrumental response function (IRF). (inset) A magnified view in the early region (<30 ns). The coincidence between the rise
profiles of (S)-[Pt(iPrOppy)Cl] and the IRF signals attests to the absence of delayed rise components.
Figure 6. (a) ORTEP drawing at 50% probability for the X-ray crystal structure of (S)-[Pt(iPrOppy)Cl]. (b) Molecular packing structure. The
interplane distance is 3.41 Å. The Pt···Pt distance is 3.48 Å. (c) Top view of a helical dimer. The labeled angle corresponds to the dihedral angle
between two Pt−C bonds in the top (marked in blue) and bottom (marked in red) molecules. (d) Simulated (TD-B3LYP/6-311+G(d,p):LANL2DZ)
ECD spectra of a single molecule (blue) and a helical dimer (black) of (S)-[Pt(iPrOppy)Cl]. See the Supporting Information, Table S2, for the details
of the calculation results. (e) Photoluminescence spectra of a 10 μM solution (THF), single crystals, and aggregates of (S)-[Pt(iPrOppy)Cl]. The top
photograph shows the photoluminescence emissions under illumination at a wavelength of 365 nm.
dimer pair of (S)-[Pt(iPrOppy)Cl]. The Pt···Pt distance in the
pair is estimated to be 3.48 Å. This distance is shorter than the
sum of the van der Waals radii of Pt (3.5 Å),131 which is
consistent with an MMLCT transition. The two (S)-[Pt-
(iPrOppy)Cl] molecules are coplanar with an interplane
distance of 3.41 Å. The most exciting feature is the homohelical
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Figure 7. (a) Schematic energy-level diagram of the materials used in CP OLEDs. (b) Electroluminescence spectra of the CP OLEDs employing the
(R)- and (S)-[Pt(iPrOppy)Cl] emitters. (c) J−V−L characteristics. (d) EQE−J characteristics. (e) ΔI (top) and gEL (bottom) averaged over 10
measurements. Error bars represent standard deviations. ΔI is the output of the current amplifier proportional to the magnitude of the differential CP
emission.
torsional displacement of the two (S)-[Pt(iPrOppy)Cl]
molecules. When quantified by using the two Pt−Cl bonds,
the torsional angle is 53.0°. The formation of this unique helical
structure benefits from two factors, (i) the helical torque
provided by the chiral isopropyl moieties and (ii) the
intermolecular adhesive forces due to the π···π and metallophilic
interactions. A balance between the two factors is essential:
[Pt(IdOppy)Cl] has a large helical torque but weak
intermolecular forces and is present in the head-to-tail packed
structure with a significant offset (Supporting Information,
Figure S11).
To increase our theoretical understanding of this chiroptical
activity, quantum-chemical calculations (TD-B3LYP/6-311+G-
(d,p):LANL2DZ) were performed for the dimer geometry of a
single crystal of (S)-[Pt(iPrOppy)Cl] (see the Supporting
Information, Tables S2, for the details of the calculation results).
Our calculations show that the MMLCT transition is the lowest-
energy transition of the helical pair of (S)-[Pt(iPrOppy)Cl].
This MMLCT transition possesses appreciable μ and m with an
angle of 49°. The nonparallel orientation between μ and m is
beneficial for chiroptical activity. In sharp contrast, the
unimolecular transition with mixed π−π*/MLCT transition
character possesses orthogonal μ and m with an angle of 91°. As
shown in Figure 6d, the simulated ECD spectrum contains a
strong band in the region of 430−500 nm, where the PLE
spectrum of the aggregates of (S)-[Pt(iPrOppy)Cl] emerges
(Figure 5a). This ECD band is absent from the simulated
spectrum of the monomeric geometry. Therefore, our
calculations support the claim that the helical dimer yields the
chiroptically active MMLCT transition. This claim is also
corroborated by the appreciable CPL signals with a |gPL| value of
1.6 × 10−3 at λem = 620 nm from (S)-[Pt(iPrOppy)Cl] single
crystals (Supporting Information, Figure S12). Although the
PLQY of a single crystal could not be determined due to a
difficulty in focusing the photoexcitation beam, the perfect
match between the photoluminescence spectra of the aggregates
and single crystals provides direct evidence for the presence of
helical dimers in aggregates (Figure 6e). The close resemblance
of the experimental powder X-ray diffraction (XRD) spectrum
and that simulated from the X-ray single-crystal results supports
this conclusion (Supporting Information, Figure S13).
Circularly Polarized Electroluminescence. In the final
stage of our study, we fabricated circularly polarized organic
light-emitting diodes (CP OLEDs) employing [Pt(iPrOppy)Cl]
and characterized their CPEL performance. A 40 nm thick layer
of N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)benzidine
(NPB) and a 10 nm thick layer of 4,4′-cyclohexylidenebis-
(N,N-bis(4-methylphenyl)benzenamine) (TAPC) were used as
a hole-transport layer and an electron-blocking layer,
respectively, while a 50 nm thick layer of 1,3,5-tri((3-pyridyl)-
phen-3-yl)benzene (TmPyPB) was used as an electron-
transport and hole-blocking layer. The light-emitting layer
(EML) was 20 nm in thickness and composed of mCP doped
with 5 wt % (R)- or (S)-[Pt(iPrOppy)Cl], or a neat film of (R)-
or (S)-[Pt(iPrOppy)Cl]. A schematic energy-level diagram of
the CP OLEDs is shown in Figure 7a. The energy levels of the
Pt(II) complexes were determined electrochemically (Support-
ing Information, Figure S14). For both types of CP OLEDs (the
doped- and neat-EML devices), the electroluminescence
spectrum of the device with (R)-[Pt(iPrOppy)Cl] is almost
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identical to that with (S)-[Pt(iPrOppy)Cl] (Figure 7b). The
doped-EML devices exhibit monomeric emission characteristics
similar to those observed in the photoluminescence spectrum of
the 10 μM [Pt(iPrOppy)Cl] solution (Figure 6e), with more
pronounced vibronic features. The electroluminescence spectra
of the neat-EML devices resemble those of the single crystal and
the aggregates shown in Figure 6e, with the peaks occurring at
longer wavelengths. The current density−voltage−luminance
(J−V−L) characteristics of the devices are shown in Figure 7c.
The differences in characteristics between the devices of the
same type with the (R)- and (S)-[Pt(iPrOppy)Cl] dopants are
very likely not of intrinsic origin but due to variations in film
purity during repeated device fabrication. Notably, the neat-
EML devices exhibit higher EQEs than the 5 wt %-doped-EML
devices throughout the entire current density region (Figure
7d). The maximum EQEs of the neat-EML and the 5 wt
%-doped-EML devices are 9.7% and 5.8%, respectively. These
results are consistent with the fact that the (R)- and (S)-
[Pt(iPrOppy)Cl] aggregates have higher PLQYs than the
solutions (Figure 4, bottom). Increases in the doping
concentrations (3, 10, 20, and 50 wt %) of the (S)-
[Pt(iPrOppy)Cl]-doped devices result in a gradual increase in
the EQE due to the transition from monomeric to aggregate
emission (Supporting Information, Figure S15). The 50 wt
%-doped-EML CP OLED with the (S)-[Pt(iPrOppy)Cl]
dopant exhibits the highest EQEs at all current densities, with
a maximum of 10.9% (Supporting Information, Figure S16). We
also fabricated 5 wt %-doped- and neat-EML devices employing
the [Pt(MeOppy)Cl] and [Pt(IdOppy)Cl] control dopants,
with device structures identical to those of the [Pt(iPrOppy)Cl]
devices. Both the [Pt(MeOppy)Cl] and [Pt(IdOppy)Cl]
devices exhibit monomer-to-aggregate transitions in their
electroluminescence spectra that are similar to those observed
for the [Pt(iPrOppy)Cl] devices (Supporting Information,
Figures S17 and S18). The [Pt(MeOppy)Cl] and [Pt(IdOppy)-
Cl] neat-EML devices have peak EQEs of 6.7 and 5.9%,
respectively, which are smaller than those of the [Pt(iPrOppy)-
Cl] neat-EML devices; these lower EQEs are probably due to
the formation of exciton traps or the insufficient formation of the
MMLCT-emissive aggregates, consistent with the PLQY
comparison (Figure 4, bottom).
the results support the validity of our strategy for increasing the
chiroptical performances of Pt(II) complexes with helical
stacking. The |gEL| values (∼10−4) of our CP OLEDs are one
order of magnitude smaller than the |gPL| values (∼10−3) of the
(R)- and (S)-[Pt(iPrOppy)Cl] aggregates, which is very likely
due to the reversal of the handedness of CPL upon reflection at
the metal electrode.133 The gEL spectra of the [Pt(MeOppy)Cl]
and [Pt(IdOppy)Cl] devices are difficult to accurately record, as
can be seen in the large extent of the scatter in cos(Δθ)
(Supporting Information, Figures S21 and S22). Nevertheless,
the polarity of gEL at many wavelengths and the magnitude of gEL
near the emission peaks were reliably determined. The
comparison of the |gEL| values of the CP OLED employing the
Pt(II) complexes at their respective emission peaks show that
the neat-EML devices with the [Pt(iPrOppy)Cl] emitters have
higher |gEL| values than the corresponding devices with
[Pt(MeOppy)Cl] and [Pt(IdOppy)Cl] (Figure 8), which is
consistent with the |gPL| comparison (Figure 4, top). The CPEL
and EQE characteristics of the OLEDs are summarized in the
Supporting Information, Table S7.
The CPEL characteristics of the OLEDs were measured by
using a homemade CPL spectroscopic system described in the
Experimental Details.1 For both the 5 wt %-doped- and neat-
EML [Pt(iPrOppy)Cl] devices, the spectra of ΔI, which is
proportional to the CP intensity of the electroluminescence,
clearly exhibit a mirror-image feature (Figure 7e, top).132 On the
one hand, at λem = 490 nm, where the monomeric emission peak
is located, |gEL| is negligible (Figure 7e, bottom). On the other
hand, the |gEL| values of the doped devices increase as λem
increases from 490 nm, which is probably due to the increased
contribution of the MMLCT emission. Furthermore, the |gEL|
values (ca. 1.2 × 10−4) of the neat-EML devices at their
MMLCT emission peak (λem = 650 nm) is greater than those
(ca. 3 × 10−5) of the doped devices recorded at their monomeric
emission peak (λem = 490 nm) (the full data are shown in the
Supporting Information, Figure S19). There are similar trends in
the gEL spectra of the (S)-[Pt(iPrOppy)Cl]-doped OLEDs with
the other doping concentrations (Supporting Information,
Figure S20). When compared at the same wavelength of ca.
650 nm, the doped devices have higher |gEL| values than the neat-
EML devices, suggesting that there exists an optimum aggregate
size, beyond which |gEL| decreases. Despite the moderate values,
Figure 8. Magnitudes of the EQE (top) and gEL values (bottom) of the
CP OLEDs employing the Pt(II) complex emitters.
SUMMARY AND CONCLUSIONS
■
There is a trade-off between the PLQYs and dissymmetry factors
of conventional CPL molecules. In this study, we have
established a molecular strategy to circumvent this intrinsic
limitation. Our approach was based on the strong tendency of
square-planar Pt(II) complexes to form face-to-face stacks. The
presence of point chirality in the complexes induces torsional
displacement in the molecular stacks. The resulting helical stacks
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stirred for an additional 3 h. The reaction mixture was filtered through
diatomaceous earth, and the filtrate was washed with an aqueous
solution saturated with sodium carbonate (120 mL) and water (130
mL). The organic layer was collected, dried over anhydrous MgSO4,
and concentrated in vacuo. A purification with silica gel column
chromatography was performed by using EtOAc/hexane = 1:2 (v/v) as
the eluent. A pale yellow oil was obtained in a 26% yield. Rf = 0.23
(EtOAc/hexane = 1:2, v/v). 1H NMR (300 MHz, CD2Cl2) δ (ppm):
0.94 (d, J = 6.6 Hz, 3H), 1.04 (d, J = 6.6 Hz, 3H), 1.84 (septet, J = 6.6
Hz, 1H), 4.04−4.18 (m, 2H), 4.45 (td, J = 8.0, 1.2 Hz, 1H), 7.27 (ddd, J
= 6.7, 4.8, 1.8 Hz, 1H), 7.52 (td, J = 7.8, 0.6 Hz, 1H), 7.75−7.84 (m,
2H), 7.97 (dt, J = 7.8, 1.5 Hz, 1H), 8.15 (ddd, J = 7.8, 1.8, 1.2 Hz, 1H),
8.57 (t, J = 1.5 Hz, 1H), 8.69 (ddd, J = 4.8, 1.7, 1.2 Hz, 1H). 13C{1H}
NMR (126 MHz, CD2Cl2) δ (ppm): 18.61, 19.23, 33.56, 70.93, 73.40,
120.90, 123.01, 127.03, 129.02, 129.24, 129.84, 137.31, 140.03, 150.27,
156.80, 163.33. HR MS (FAB, m-NBA): Calcd for C17H19N2O ([M +
H]+), 267.1492; found: 267.1496.
produce |gPL| values fourfold greater than those of dilute
solutions. In addition, the PLQY also improves by a factor of 6
upon the formation of the aggregates that contain the helical
stacks. These simultaneous increases in |gPL| and PLQY are
attributed to the facilitation of the MMLCT transition in the
helical stack by the short Pt···Pt distance of 3.48 Å. The
MMLCT transition is chiroptically active because its electric (μ)
and magnetic (m) transition moments are nonorthogonal with
an angle of 49°. The PLQY and |gPL| values for a series of Pt(II)
complexes with controlled steric hindrance around the chiral
center were quantified. Our results demonstrate that the
luminescence and chiroptical performances of the complexes
are sensitive to their chemical structures. For instance, a
sterically bulky bicyclic indanyl unit hampers the generation of
chiroptically active helical stacks. A CPEL device that employs a
neat film of Pt(II) complexes as the emitting layer was found to
exhibit strong MMLCT electroluminescence with an EQE of
9.7%. This EQE value is ca. twofold higher than that of CPEL
devices with emitting layers doped with 5 wt % Pt(II)
complexes. Notably, this improvement accompanies an increase
in |gEL| by a factor of 4. Collectively, these CPEL results
demonstrate the validity of our molecular strategy. Although our
dissymmetry factors are lower than state-of-the-art values, this
approach to enhancing both the PLQY and the dissymmetry
factor is an important step forward.
Synthesis of (R)-iPrOppyH. (R)-iPrOppyH was prepared by
following the method identical to the synthesis of (S)-iPrOppyH,
except the use of D-valinol in place of L-valinol. A pale yellow oil was
obtained in a 34% yield. Rf = 0.23 (EtOAc/hexane = 1:2, v/v). 1H NMR
(300 MHz, CD2Cl2) δ (ppm): 0.94 (d, J = 6.9 Hz, 3H), 1.04 (d, J = 6.9
Hz, 3H), 1.84 (septet, J = 6.6 Hz, 1H), 4.05−4.18 (m, 2H), 4.45 (td, J =
8.0, 1.2 Hz, 1H), 7.27 (ddd, J = 6.7, 4.8, 1.8 Hz, 1H), 7.52 (td, J = 7.8,
0.6 Hz, 1H), 7.75−7.85 (m, 2H), 7.97 (dt, J = 7.8, 1.5 Hz, 1H), 8.15
(ddd, J = 7.8, 1.8, 1.2 Hz, 1H), 8.57 (t, J = 1.5 Hz, 1H), 8.69 (ddd, J =
4.8, 1.7, 1.2 Hz, 1H). 13C{1H} NMR (126 MHz, CD2Cl2) δ (ppm):
18.60, 19.24, 33.55, 70.92, 73.39, 120.91, 123.02, 127.04, 129.03,
129.25, 129.87, 137.32, 140.04, 150.28, 156.81, 163.36. HR MS (FAB,
m-NBA): Calcd for C17H18N2O ([M]+), 266.1419; found: 266.1417.
Synthesis of OppyH. OppyH was prepared by following the
method identical to the synthesis of (S)-iPrOppyH, except the use of 2-
aminoethanol in place of L-valinol. Purification with silica gel column
chromatography was performed by using EtOAc/hexane = 4:1 (v/v) as
the eluent. A sticky yellow liquid was obtained in a 22% yield. Rf = 0.31
(EtOAc/hexane = 4:1, v/v). 1H NMR (300 MHz, CD2Cl2) δ (ppm):
4.05 (t, J = 9.0 Hz, 2H), 4.45 (t, J = 9.0 Hz, 2H), 7.27 (ddd, J = 6.6, 4.8,
1.8 Hz, 1H), 7.53 (td, J = 7.8, 0.6 Hz, 1H), 7.75−7.84 (m, 2H), 7.98 (dt,
J = 7.7, 6.3, 1.5 Hz, 1H), 8.17 (ddd, J = 7.8, 1.8, 1.2 Hz, 1H), 8.57 (t, J =
1.8 Hz, 1H), 8.69 (ddd, J = 4.7, 1.5, 1.2 Hz, 1H). 13C{1H} NMR (126
MHz, CD2Cl2) δ (ppm): 55.54, 68.28, 120.91, 123.04, 126.97, 128.96,
129.29, 129.96, 137.34, 140.04, 150.25, 156.74, 164.62. HR MS (FAB,
m-NBA): Calcd for C14H12N2O ([M]+), 224.0950; found: 224.0947.
Synthesis of MeOppyH. MeOppyH was prepared by following the
method identical to the synthesis of (S)-iPrOppyH, except the use of
(S)-(+)-2-amino-1-propanol in place of L-valinol. Purification with
silica gel column chromatography was performed by using EtOAc/
hexane = 4:1 (v/v) as the eluent. A brown liquid was obtained in a 38%
EXPERIMENTAL DETAILS
■
Materials and General Methods. Commercially available
chemicals were purchased and used without further purification. 2-
(Tributylstannyl)pyridine (85%), tetrakis(triphenylphosphine)-
palladium(0) (99%), N-bromosuccinimide (99%), (S)-(+)-2-amino-
1-propanol (98%), platinum(II) chloride (≥99.9%), benzonitrile
(≥99%), 2-ethoxyethanol (≥99%), dichloromethane (≥99.8%), and
toluene (≥99.8%) were purchased from Sigma-Aldrich. 3-Bromoben-
zaldehyde (>98%), D-valinol (>98%), L-valinol (>97%), 2-amino-
ethanol (>99%), and (1S,2R)-(−)-1-amino-2-indanol (>98%) were
purchased from Tokyo Chemical Industry. H and 13C{1H} NMR
1
spectra were collected with Bruker, Avance III 300 or 500 NMR,
spectrometers. Chemical shifts were referenced to (CH3)4Si. High-
resolution mass spectra (HR MS; positive mode, fast atom bombard-
ment (FAB)) were obtained by employing a JEOL, JMS-700 mass
spectrometer. C, H, and N analyses were performed on a Thermo
Fisher Scientific, Flash2000 elemental analyzer.
Synthesis of 3-(2-Pyridinyl)benzaldehyde. 3-Bromobenzalde-
hyde (18.0 g, 97.3 mmol), 2-(tributylstannyl)pyridine (42.9 g, 117
mmol), and tetrakis(triphenylphosphine)palladium(0) (11.2 g, 9.73
mmol) were dissolved in anhydrous toluene (800 mL) in a 1000 mL
one-necked round-bottom flask equipped with a magnetic stir bar. The
resulting mixture was stirred at 110 °C for 43 h under an Ar atmosphere.
After it cooled to room temperature, the solution was concentrated
under a reduced pressure. A silica gel column purification was
performed by increasing the polarity of the eluent from ethyl acetate
(EtOAc)/hexane = 1:3 (v/v). A yellow liquid was obtained in a 90%
1
yield. Rf = 0.34 (EtOAc/hexane = 4:1, v/v). H NMR (300 MHz,
CD2Cl2) δ (ppm): 1.34 (d, J = 3.3 Hz, 3H), 3.97 (t, J = 7.8 Hz, 1H),
4.32−4.44 (m, 1H), 6.04 (dd, J = 9.3, 7.8 Hz, 1H), 7.26 (ddd, J = 6.9,
4.8, 1.8 Hz, 1H), 7.52 (td, J = 7.8, 0.6 Hz, 1H), 7.75−7.84 (m, 2H), 7.96
(dt, J = 7.7, 1.5 Hz, 1H), 8.16 (ddd, J = 7.8, 1.8, 1.2 Hz, 1H), 8.57 (t, J =
1.5 Hz, 1H), 8.68 (ddd, J = 4.8, 4.7, 0.9 Hz, 1H). 13C{1H} NMR (126
MHz, CD2Cl2) δ (ppm): 21.75, 62.71, 74.71, 120.90, 123.02, 127.02,
128.98, 129.28, 129.92, 137.32, 140.04, 150.27, 156.75, 163.42. HR MS
(FAB, m-NBA): Calcd for C15H14N2O ([M]+), 238.1106; found:
238.1106.
1
yield. Rf = 0.17 (EtOAc/hexane = 1:3, v/v). H NMR (300 MHz,
CD2Cl2) δ (ppm): 7.30 (ddd, J = 6.6, 4.8, 2.1 Hz, 1H), 7.66 (t, J = 7.8
Hz, 1H), 7.79−7.87 (m, 2H), 7.93 (dt, J = 7.8, 1.5 Hz, 1H), 8.33 (ddd, J
= 7.8, 2.0, 1.5 Hz, 1H), 8.54 (td, J = 1.8, 0.6 Hz, 1H), 8.71 (dt, J = 4.8,
1.5 Hz, 1H), 10.11 (s). 13C{1H} NMR (126 MHz, CD2Cl2) δ (ppm):
120.94, 123.37, 128.64, 129.97, 130.14, 133.05, 137.51, 140.82, 150.37,
156.19, 192.68. HR MS (FAB, m-nitrobenzyl alcohol (m-NBA)): Calcd
for C12H10NO ([M + H]+), 184.0757; found: 184.0759.
Synthesis of IdOppyH. IdOppyH was prepared by following the
method identical to the synthesis of (S)-iPrOppyH, except the use of
(1S,2R)-(−)-1-amino-2-indanol in place of L-valinol. A purification
with silica gel column chromatography was performed by using EtOAc/
hexane = 1:1 (v/v) as the eluent. A white powder was obtained in a 38%
1
yield. Rf = 0.23 (EtOAc/hexane = 1:1, v/v). H NMR (300 MHz,
CD2Cl2) δ (ppm): 3.38 (dd, J = 18.0, 1.2 Hz, 1H), 3.54 (dd, J = 18.0, 6.6
Hz, 1H), 5.52 (ddd, J = 7.8, 6.8, 1.8 Hz, 1H), 5.76 (d, J = 8.1 Hz, 1H),
7.24−7.31 (m, 4H), 7.47−7.56 (m, 2H), 7.74−7.82 (m, 2H), 7.95 (dt, J
= 7.8, 1.5 Hz, 1H), 8.03 (ddd, J = 7.8, 1.8, 1.2 Hz, 1H), 8.55 (t, J = 1.5
Hz, 1H), 8.68 (dt, J = 4.8, 1.2 Hz, 1H). 13C{1H} NMR (126 MHz,
CD2Cl2) δ (ppm): 40.37, 77.68, 83.91, 120.87, 123.02, 125.87, 125.99,
Synthesis of (S)-iPrOppyH. 3-(2-Pyridinyl)benzaldehyde (4.60 g,
25.1 mmol) and L-valinol (2.85 g, 27.6 mmol) were added into a 250
mL one-necked round-bottom flask. The reaction mixture was stirred in
dichloromethane (120 mL) in the presence of a molecular sieve (4 Å)
for 19 h at room temperature. N-Bromosuccinimide (2.14 g, 25.1
mmol) was delivered into the solution, after which the solution was
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127.09, 127.82, 128.91, 129.09, 129.24, 129.97, 137.31, 139.98, 140.56,
142.76, 150.25, 156.71, 164.02. HR MS (FAB, m-NBA): Calcd for
C21H16N2O ([M]+), 312.1263; found: 312.1261. Anal. Calcd for
C21H16N2O: C, 80.75; H, 5.16; N, 8.97. Found: C, 80.62; H, 5.19; N,
8.93%.
= 5.7, 0.9 Hz, JPt−H = 42.8 Hz, 1H). 13C{1H} NMR (126 MHz, CD2Cl2)
δ (ppm): 21.16, 59.06, 78.54, 120.11, 122.91, 123.98, 126.19, 128.31,
139.33, 141.91, 151.34, 161.78, 166.06, 180.39. HR MS (FAB, m-
NBA): Calcd for C15H13ClN2OPt ([M]+), 467.0364; found: 467.0368.
Anal. Calcd for C15H13ClN2OPt: C, 38.51; H, 2.80; N, 5.99. Found: C,
38.29; H, 2.79; N, 5.78%.
Synthesis of (S)-[Pt(iPrOppy)Cl]. PtCl2 (1.50 g, 5.63 mmol) was
added to a 100 mL two-necked round-bottom flask. Anhydrous
benzonitrile (10 mL) was delivered into the flask via a glass syringe
under an Ar atmosphere. The reaction mixture was heated at 160 °C for
10 min, and the temperature was lowered to 140 °C. A mixture of 25 mL
of benzonitrile and 20 mL of 2-ethoxyethanol containing 1.50 g of (S)-
iPrOppyH (5.63 mmol) was added slowly to the reaction mixture, after
which the solution was heated for an additional 4.5 h at 140 °C. After
the reaction mixture was cooled to room temperature, it was poured
onto EtOAc. The diluted solution was washed thoroughly with water
(400 mL × five times). The organic layer was collected, dried over
anhydrous MgSO4, and concentrated in vacuo. A yellow precipitate
formed when the concentrated solution was poured into hexane, which
was filtered and washed with hexane. The filter cake was recovered by
dissolving in CH2Cl2. Silica gel column chromatography was performed
by increasing the polarity of the eluent from CH2Cl2 to CH2Cl2/
CH3OH = 99:1 (v/v) to afford an orange powder in a 56% yield. Rf =
0.23 (CH2Cl2/CH3OH = 49:1, v/v). 1H NMR (300 MHz, CD2Cl2) δ
(ppm): 0.81 (d, J = 6.9 Hz, 3H), 0.97 (d, J = 7.2 Hz, 3H), 2.82 (d, J = 7.0
Hz, 1H), 4.34 (septet, J = 3.3 Hz, 1H), 4.73−4.82 (m, 2H), 7.24 (t, J =
7.8 Hz, 1H), 7.31−7.39 (m, 2H), 7.59 (dd, J = 7.8, 0.9 Hz, 1H), 7.74
(ddd, J = 8.1, 1.4, 0.6 Hz, 1H), 8.01 (td, J = 7.7, 1.5 Hz, 1H), 9.34 (ddd
with 195Pt satellites, J = 5.8, 1.5, 0.9 Hz, JPt−H = 42.9 Hz, 1H). 13C{1H}
NMR (126 MHz, CD2Cl2) δ (ppm): 14.14, 18.89, 29.45, 67.57, 72.05,
120.11, 122.95, 123.97, 126.26, 127.99, 139.36, 141.96, 151.38, 161.74,
166.07, 180.42. HR MS (FAB, m-NBA): Calcd for C17H17ClN2OPt
([M]+), 495.0677; found: 495.0677. Anal. Calcd for C17H17ClN2OPt:
C, 41.18; H, 3.46; N, 5.65. Found: C, 41.36; H, 3.49; N, 5.54%.
Synthesis of (R)-[Pt(iPrOppy)Cl]. (R)-[Pt(iPrOppy)Cl] was
prepared by following the method identical to the synthesis of (S)-
[Pt(iPrOppyH)Cl], except the use of (R)-iPrOppyH in place of (S)-
iPrOppyH. An orange powder was obtained in a 47% yield. Rf = 0.23
Synthesis of [Pt(IdOppy)Cl]. [Pt(IdOppy)Cl] was prepared
following the method identical to the synthesis of (S)-[Pt(iPrOppy)-
Cl], except the use of IdOppyH in place of (S)-iPrOppyH. Silica gel
column chromatography was performed by increasing the polarity of
the eluent from CH2Cl2 to CH2Cl2/CH3OH = 99:1 (v/v). A yellow
powder was obtained in an 82% yield. Rf = 0.11 (CH2Cl2). 1H NMR
(300 MHz, CD2Cl2) δ (ppm): 3.52−3.68 (m, 2H), 5.76 (d, J = 7.8 Hz,
1H), 5.96 (t, J = 3.0 Hz, 1H), 7.19 (t, J = 7.5 Hz, 1H), 7.30−7.37 (m,
5H), 7.54 (d, J = 7.2 Hz, 1H), 7.72 (d, J = 7.5 Hz, 1H), 8.01 (td, J = 7.8,
1.8 Hz, 1H), 8.42 (m, 1H), 9.37 (d with 195Pt satellites, J = 5.1 Hz, JPt−H
= 43.8 Hz, 1H). 13C{1H} NMR (126 MHz, deuterated dimethyl
sulfoxide (DMSO-d6)) δ (ppm): 38.12, 54.88, 71.15, 89.64, 120.61,
122.69, 124.23, 125.16, 125.75, 126.63, 127.12, 127.29, 128.98, 139.64,
139.91, 141.08, 150.31, 160.51, 164.64, 179.49. HR MS (FAB, m-
NBA): Calcd for C21H15ClN2OPt ([M]+), 541.0521; found: 541.0521.
Circularly Polarized Luminescence Measurements. We
employed an OLIS, CPL Solo spectrometer to obtain a quick survey
of the CPL signals. Ten micromolar solutions in THF (beam path
length = 1.0 cm) and powders immobilized on the quartz plates were
employed in the measurements. The quartz plates were placed on a
solid sample stage (OLIS), and the incidence angle of the excitation
beam (380 nm) was kept at 35° to the normal of the sample. The CPL
measurements were repeated 20 times for solutions and five times for
powders to obtain reliable results. We validated our CPL measurements
with the spectrometer by reproducing the CPL dissymmetry spectrum
of 5.5 mM europium tris[3-(trifluoromethylhydroxymethylene)-
(+)-camphorate] ([Eu(facam)3]) diluted in DMSO, which is widely
used as a standard.66 Measurement conditions: scan rate, 20 nm min−1;
detection range, 450−720 nm for solutions and 450−800 nm for
aggregates. The gEL spectra were recorded by using a homemade setup
consisting of a photoelastic modulator (PEM-100, Hinds Instruments)
with a modulation frequency of 47 kHz and a linear polarizer
(LPVISE100-A, Thorlabs), with their axes aligned at 45°.1 The light
emitted from a sample and then modulated by the PEM was detected
with a photomultiplier tube (PMT, R9182-01, Hamamatsu) after the
light passed through a spectrometer (SR303i, Shamrock). The electrical
current from the PMT was amplified and converted to a voltage signal
with a preamplifier (C11184, Hamamatsu). The dc component of the
voltage signal (I) was measured with a digital multimeter (34410A,
Agilent), while the peak-to-peak value of the ac component oscillating
at 47 kHz (ΔI) was obtained by using a lock-in amplifier (SR830,
Stanford Research System). The phase difference Δθ between the 47
kHz ac signal and the reference signal from the PEM was also recorded.
The CP OLEDs, packaged in a N2 atmosphere, were driven by a
constant current source (2400, Keithley) at J = 5 mA cm−2 for the gEL
measurements. The full width at half-maximum resolution of the
spectrometer was set at 13 nm.
1
(CH2Cl2/CH3OH = 49:1, v/v). H NMR (300 MHz, CD2Cl2) δ
(ppm): 0.81 (d, J = 6.9 Hz, 3H), 0.97 (d, J = 6.9 Hz, 3H), 2.82 (d, J = 6.9
Hz, 1H), 4.34 (septet, J = 3.3 Hz, 1H), 4.73−4.82 (m, 2H), 7.24 (t, J =
7.8 Hz, 1H), 7.31−7.39 (m, 2H), 7.59 (dd, J = 7.5, 0.9 Hz, 1H), 7.75
(ddd, J = 8.0, 1.5, 0.6 Hz, 1H), 8.01 (td, J = 7.5, 1.5 Hz, 1H), 9.34 (ddd
with 195Pt satellites, J = 5.7, 1.4, 0.6 Hz, JPt−H = 42.3 Hz, 1H). 13C{1H}
NMR (126 MHz, CD2Cl2) δ (ppm): 14.14, 18.89, 29.45, 67.57, 72.05,
120.11, 122.95, 123.97, 126.26, 127.99, 139.36, 141.95, 151.38, 161.74,
166.06, 180.42. HR MS (FAB, m-NBA): Calcd for C17H17ClN2OPt
([M]+), 495.0677; found: 495.0672. Anal. Calcd for C17H17ClN2OPt:
C, 41.18; H, 3.46; N, 5.65. Found: C, 41.42; H, 3.69; N, 5.68%.
Synthesis of [Pt(Oppy)Cl]. [Pt(Oppy)Cl] was prepared following
the method identical to the synthesis of (S)-[Pt(iPrOppyH)Cl], except
the use of OppyH in place of (S)-iPrOppyH. An orange powder was
1
obtained in a 65% yield. Rf = 0.09 (CH2Cl2). H NMR (300 MHz,
CD2Cl2) δ (ppm): 4.10 (t, J = 9.6 Hz, 2H), 4.94 (t, J = 9.6 Hz, 2H), 7.24
(t, J = 7.8 Hz, 1H), 7.31−7.40 (m, 2H), 7.59 (dd, J = 7.8, 0.9 Hz, 1H),
7.75 (dt, J = 8.1, 1.5 Hz, 1H), 8.01 (td, J = 7.8, 1.5 Hz, 1H), 9.28 (ddd
with 195Pt satellites, J = 5.8, 1.5, 0.6 Hz, JPt−H = 42.2 Hz, 1H). 13C{1H}
NMR (126 MHz, CD2Cl2) δ (ppm): 14.43, 30.24, 72.61, 120.19,
123.02, 124.12, 126.28, 128.10, 130.17, 139.45, 151.75. HR MS (FAB,
m-NBA): Calcd for C14H11ClN2OPt ([M]+), 453.0208; found:
453.0208.
Synthesis of [Pt(MeOppy)Cl]. [Pt(MeOppy)Cl] was prepared by
following the method identical to the synthesis of (S)-[Pt(iPrOppyH)-
Cl], except the use of MeOppyH in place of (S)-iPrOppyH. Silica gel
column chromatography was performed by increasing the polarity of
the eluent from CH2Cl2 to CH2Cl2/CH3OH = 199:1 (v/v). A yellow
powder was obtained in a 68% yield. Rf = 0.26 (CH2Cl2/CH3OH =
49:1, v/v). 1H NMR (300 MHz, CD2Cl2) δ (ppm): 1.55 (t, J = 6.0 Hz,
3H), 4.37−4.54 (m, 2H), 5.00 (t, J = 8.7 Hz, 1H), 7.23 (t, J = 7.8 Hz,
1H), 7.30−7.38 (m, 2H), 7.57 (d, J = 7.5 Hz, 1H), 7.74 (dd, J = 7.4, 0.9
Hz, 1H), 8.01 (td, J = 7.7, 1.5 Hz, 1H), 9.32 (ddd with 195Pt satellites, J
Single-Crystal X-ray Crystallography. The X-ray crystallo-
graphically available crystals were obtained from acetonitrile solutions
of 150 mM (S)-[Pt(iPrOppy)Cl] and 3 mM [Pt(IdOppy)Cl]. Single
crystals were picked from the solution with a nylon loop (Hampton
Research Co.) at room temperature and mounted on a goniometer
head in an N2 cryostream. Data from single crystals were collected by
using a Bruker SMART APEX II CCD diffractometer equipped with a
monochromator and a Mo Kα (λ = 0.710 73 Å) incident beam. The
charge-coupled device (CCD) data were integrated and scaled by using
the Bruker-SAINT software package, and the structure was solved and
refined with SHEXTL ver. 6.12. Hydrogen atoms were placed on the
calculated positions. Nonhydrogen atoms were refined with anisotropic
thermal parameters. Crystal data for (S)-[Pt(iPrOppy)Cl]:
C17H17ClN2OPt, orthorhombic, P212121, Z = 8, a = 11.0487(10), b =
16.292(3), c = 17.4514(16) Å, V = 3141.3(6) Å3, μ = 9.106 mm−1, ρcalcd
= 2.097 g cm−3, R1 = 0.0137, wR2 = 0.0296 for 7792 unique reflections,
401 variables. Crystal data for [Pt(IdOppy)Cl]: C21H15ClN2OPt,
J
https://doi.org/10.1021/acs.inorgchem.1c00070
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
orthorhombic, P212121, Z = 4, a = 6.8224(7), b = 13.940(3), c =
19.916(3) Å, V = 1894.1(5) Å3, μ = 7.560 mm−1, ρcalcd = 1.900 g cm−3,
R1 = 0.0205, wR2 = 0.0683 for 4675 unique reflections, 235 variables.
The crystallographic data with the selected bond distances and angles
for (S)-[Pt(iPrOppy)Cl] and [Pt(IdOppy)Cl] are also summarized in
the Supporting Information, Tables S3−S6. CCDC 2046058 for (S)-
[Pt(iPrOppy)Cl] and 2046057 for [Pt(IdOppy)Cl] contain the
supporting crystallographic data for this paper. Additional crystallo-
graphic information is available in the Supporting Information.
CP OLED Fabrication and Characterization. Prepatterned
indium tin oxide (ITO)-coated glass substrates were sequentially
cleaned with detergent, deionized water, acetone, and isopropyl alcohol
in an ultrasonic bath, followed by a UV-ozone treatment before organic
layer deposition. All organic depositions were performed by thermal
evaporation in vacuum (∼10−7 Torr) onto the substrates. The
deposition rates of the Pt(II) complexes were precisely controlled in
the range of 0.1−0.2 Å s−1, and those of the other materials were fixed at
1 Å s−1 except for LiF (0.1 Å s−1). Square-shaped active device areas of 3
× 3 mm2 were defined by overlapping regions between 3 mm wide
bottom ITO lines and 3 mm wide top Al lines. The electroluminescence
spectra of the CP OLEDs were obtained at a constant J of 1 mA cm−2
using a spectrometer (SR303i, Shamrock) with a charge-coupled device
(iDUS 401, Andor). The J−V−L characteristics were obtained using a
source measure unit (2400, Keithley) and a Si photodiode (818-SL,
Newport) connected to an optical power meter (1928-C, Newport).
AUTHOR INFORMATION
Corresponding Authors
■
Jaeheung Cho − Department of Emerging Materials Science,
DGIST, Daegu 42988, Republic of Korea; Department of
Chemistry, UNIST, Ulsan 44919, Republic of Korea;
orcid.org/0000-0002-2712-4295; Email: jaeheung@
unist.ac.kr
Changsoon Kim − Graduate School of Convergence Science and
Technology, and Inter-University Semiconductor Research
Center, Seoul National University, Seoul 08826, Republic of
Korea; orcid.org/0000-0002-3825-6293;
Email: changsoon@snu.ac.kr
Youngmin You − Division of Chemical Engineering and
Materials Science, and Graduate Program in System Health
Science and Engineering, Ewha Womans University, Seoul
03760, Republic of Korea; orcid.org/0000-0001-5633-
6599; Email: odds2@ewha.ac.kr
Authors
Sumin Lee − Division of Chemical Engineering and Materials
Science, and Graduate Program in System Health Science and
Engineering, Ewha Womans University, Seoul 03760, Republic
of Korea
Yongmoon Lee − Graduate School of Convergence Science and
Technology, and Inter-University Semiconductor Research
Center, Seoul National University, Seoul 08826, Republic of
Korea
ASSOCIATED CONTENT
■
sı
* Supporting Information
Kyungmin Kim − Department of Emerging Materials Science,
DGIST, Daegu 42988, Republic of Korea; orcid.org/0000-
0002-3362-128X
Seunga Heo − Division of Chemical Engineering and Materials
Science, and Graduate Program in System Health Science and
Engineering, Ewha Womans University, Seoul 03760, Republic
of Korea
Dong Yeun Jeong − Division of Chemical Engineering and
Materials Science, and Graduate Program in System Health
Science and Engineering, Ewha Womans University, Seoul
03760, Republic of Korea
Sangsub Kim − Graduate School of Convergence Science and
Technology, and Inter-University Semiconductor Research
Center, Seoul National University, Seoul 08826, Republic of
Korea
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.1c00070.
Experimental details; lists of the photophysical and
electrochemical properties, the TD−DFT calculations
results, and crystallographic data and selected geometric
parameters for (S)-[Pt(iPrOppy)Cl] and [Pt(IdOppy)-
Cl]; displays of variable-solvent UV−vis absorption
spectra of (S)-[Pt(iPrOppy)Cl], the gabs spectra and the
photoluminescence and ECD spectra obtained for various
concentrations of (S)-[Pt(iPrOppy)Cl], the magnitude of
gabs, calculated rotatory strength values for (S)-[Pt-
(iPrOppy)Cl], the UV−vis, ECD, photoluminescence,
and CPL spectra of the Pt(II) complexes, the absorption
and photoluminescence excitation spectra of Pt(II)
complexes, the photoluminescence decay traces, the
195Pt{1H} NMR spectra of the aggregates and a 2.0 mM
solution of [Pt(MeOppy)Cl], the ORTEP drawing and
packing structure of a [Pt(IdOppy)Cl] single crystal, the
photoluminescence and CPL spectra of a (S)-[Pt-
(iPrOppy)Cl] single crystal, a comparison of experimen-
tal and simulated powder XRD spectra, the cyclic and
differential pulse voltammograms of Pt(II) complexes, the
electroluminescence spectra and the external quantum
efficiencies of CP OLEDs with various concentrations of
(S)-[Pt(iPrOppy)Cl], the electroluminescence character-
istics and the CPEL performances of CP OLEDs, and the
1H and 13C{1H} NMR spectra (PDF)
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.1c00070
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by a grant from the Samsung Research
Funding Center for Future Technology (SRFC-MA1602-03).
J.C. acknowledges financial support from the NRF
(2019R1A2C2086249).
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