Tuning the Excited State of Tetradentate Pd(II) Complexes for Highly Efficient Deep-Blue Phosphorescent Materials

Article abstract of DOI:10.1021/acs.inorgchem.0c01907

Deep-blue-light-emitting materials are urgently desired in high-performance organic light-emitting diodes (OLEDs) for full-color display and solid-state lighting applications. However, the development of stable and efficient deep-blue emitters remains a great challenge. Herein, a series of stable and efficient tetradentate Pd(II)-complex-based deep-blue emitters with rigid 5/6/6 metallocycles and no F atom were designed and synthesized. These deep-blue emitters employ various isoelectronic five-membered heteroaryl-ring-containing ligands to exhibit extremely narrow emission spectra peaking at 439-443 nm with a full width at half-maximum (fwhm) of only 22-38 nm in 2-methyltetrahydrofuran at room temperature. In particular, the design of an intramolecular hydrogen bond enabled the 1-phenyl-1,2,3-trazole-based Pd(II) complexes to achieve CIEy < 0.1 (0.069-0.078; CIE is Commission Internationale de L'Eclairage). Theoretical calculation and natural transition orbital analysis reveal that these deep-blue materials emit light exclusively from their ligand (carbazole)-centered (3LC) states. Moreover, the triplet excited-state property can be efficiently regulated through ligand modification with isoelectronic oxazole and thiazole rings or pyridine rings, resulting in sky-blue-to-yellow materials, which emit light originating from an admixture of metal-to-ligand charge-transfer (3MLCT) and intraligand charge-transfer states. The newly developed Pd(II) complexes are strongly emissive in various matrixes with a quantum efficiency of up to 51% and also highly thermally stable with a 5% weight-reduction temperature (ΔT5%) of up to 400 °C. Deep-blue OLEDs with CIEy < 0.1 employing Pd(II) complexes as emitters were successfully fabricated for the first time. This study demonstrates that the Pd(II) complexes can act as excellent phosphorescent light-emitting materials through rational molecular design and also provide a valuable method for the development of Pd(II)-complex-based efficient and stable deep-blue emitters.

Full text of DOI:10.1021/acs.inorgchem.0c01907

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Tuning the Excited State of Tetradentate Pd(II) Complexes for
Highly Efficient Deep-Blue Phosphorescent Materials
Guijie Li,* Jianbing Zheng, Xiangdong Zhao, Tyler Fleetham, Yun-Fang Yang,* Qunmin Wang,
Feng Zhan, Wenyue Zhang, Kun Fang, Qisheng Zhang, and Yuanbin She*
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ABSTRACT: Deep-blue-light-emitting materials are urgently de-
sired in high-performance organic light-emitting diodes (OLEDs) for
full-color display and solid-state lighting applications. However, the
development of stable and efficient deep-blue emitters remains a
great challenge. Herein, a series of stable and efficient tetradentate
Pd(II)-complex-based deep-blue emitters with rigid 5/6/6 metallo-
cycles and no F atom were designed and synthesized. These deep-
blue emitters employ various isoelectronic five-membered heteroaryl-
ring-containing ligands to exhibit extremely narrow emission spectra
peaking at 439−443 nm with a full width at half-maximum (fwhm)
of only 22−38 nm in 2-methyltetrahydrofuran at room temperature.
In particular, the design of an intramolecular hydrogen bond enabled
the 1-phenyl-1,2,3-trazole-based Pd(II) complexes to achieve CIEy <
0.1 (0.069−0.078; CIE is Commission Internationale de L’Eclairage). Theoretical calculation and natural transition orbital analysis
reveal that these deep-blue materials emit light exclusively from their ligand (carbazole)-centered (3LC) states. Moreover, the triplet
excited-state property can be efficiently regulated through ligand modification with isoelectronic oxazole and thiazole rings or
pyridine rings, resulting in sky-blue-to-yellow materials, which emit light originating from an admixture of metal-to-ligand charge-
transfer (3MLCT) and intraligand charge-transfer states. The newly developed Pd(II) complexes are strongly emissive in various
matrixes with a quantum efficiency of up to 51% and also highly thermally stable with a 5% weight-reduction temperature (ΔT5%) of
up to 400 °C. Deep-blue OLEDs with CIEy < 0.1 employing Pd(II) complexes as emitters were successfully fabricated for the first
time. This study demonstrates that the Pd(II) complexes can act as excellent phosphorescent light-emitting materials through
rational molecular design and also provide a valuable method for the development of Pd(II)-complex-based efficient and stable deep-
blue emitters.
the development of deep-blue-light-emitting materials with
CIEy < 0.1 (CIE = Commission Internationale de L’Eclairage)
meeting the requirement of the National Television Standards
Committee (NTSC) standard blue of (0.14, 0.08), remains a
great challenge.16−20
The strong spin−orbit coupling (SOC) enables phosphor-
escent Ir(III)21,22 and Pt(II)2,19,23−32 complexes to have
efficient intersystem crossing between singlet and triplet states
and fast radiative decay (kr) from the lowest triplet excited
state (T1) to the ground state (S0), which results in short
emission lifetimes (τ = 0.1−10 μs) and high photo-
luminescence quantum yields (PLQYs; ΦPL).33,34 By contrast,
1. INTRODUCTION
Since the first practical organic light-emitting diode (OLED)
was demonstrated by Tang and VanSlyke in 1987,1 OLEDs as
one of the most appealing technologies have attracted
tremendous attention in both academia and industry for
their application in the next-generation full-color display and
solid-state lighting.2−12 Light-emitting materials play a pivotal
role in this technology.2,3,5−9 Over the past 3 decades, several
kinds of light-emitting materials capable of harvesting all
electrogenerated singlet and triplet excitons have been
demonstrated in device settings, such as phosphorescent,2
hybridized local and charge-transfer,10 thermally activated
delayed fluorescent,5,11,12 and metal-assisted delayed fluores-
cent (MADF) materials.13−15 Thanks to the pioneering work
of Thompson, Forrest, and co-workers,2 up to now, the Ir(III)-
based phosphorescent red and green materials meet
commercial requirements and have been successfully utilized
in commercial consumer electronic devices. However, the
development of efficient and stable blue emitters, especially for
Received: June 27, 2020
https://dx.doi.org/10.1021/acs.inorgchem.0c01907
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Figure 1. Synthesis and chemical structures of the tetradentate 5/6/6 Pd(II) complexes studied in this work.
the SOC constant of Pd(II) (ξ = 1412 cm−1)35 is significantly
smaller than those of Ir(III) (ξ = 3909 cm−1)8,36 and Pt(II) (ξ
= 4481 cm−1).8,36 This results in less metal-to-ligand charge-
transfer (MLCT) character in the T1 state of the Pd(II)
complexes and relatively long τ in the hundreds of micro-
seconds range.13−15,36,37 Up to now, several types of efficient
luminescent Pd(II) complexes in the sky-blue-to-near-infrared
spectral region have been developed,13−15,35,37 and most of
them exhibit broad emission spectra. However, efficient blue
and deep-blue Pd(II) complexes with high color purity are rare
and highly desired. Our previous study demonstrated that the
Pd(II) complex showed a blue-shift emission spectrum
compared with the Pt(II) complex with the same cyclo-
metalated ligand.14 This provides a new avenue to develop
blue and deep-blue phosphorescent materials by incorporating
a Pd(II) ion into the cyclometalated ligand, which also has an
advantage of much higher abundance of the Pd ion than of the
Ir ion (>15 times).38 In this work, we designed and developed
a series of tetradentate phosphorescent Pd(II) complexes with
rigid 5/6/6 metallocycles by adopting various five- and six-
membered heteroaryl (Het) ligands, such as 1-phenylpyrazole
(ppz), 2-phenyl-1,2,3-triazole (2-ptz), 1-phenyl-1,2,3-triazole
(1-ptz), 1-methyl-2-phenyl-1H-imidazole (piz), 2-phenylpyr-
idine (ppy), 2-phenyloxazole (poz), and 2-phenylthiazole
(pthz) (Figure 1). The Pd(II) complexes are highly thermally
stable with 5% weight-reduction temperature (ΔT5%) of up to
400 °C. Several deep-blue-light-emitting materials with narrow
emission spectra are developed, and the emission color can be
easily tuned by modifying the Het group of the ligand. The
Pd(II) complexes are all highly emissive with ΦPL of up to 51%
in solution and 22% in doped poly(methyl methacrylate)
(PMMA) film. They also exhibit relatively short τ of less than
100 μs in both solution [except Pd(piz)] and PMMA film.
Their photophysical nature and excited-state properties are
systematically investigated through theoretical calculations and
natural transition orbital (NTO) analyses. Deep-blue OLEDs
with CIEy < 0.1 employing Pd(II) complexes as emitters were
successfully fabricated for the first time.
2. RESULTS AND DISCUSSION
Molecular Design, Synthesis, and Characterization.
Compared with the extensively studied Ir(III) and Pt(II)
complexes, the investigation of the photophysical properties of
the Pd(II) complexes is less explored because of their low ΦPL.
This is primarily attributed to the low radiative rates of Pd(II)
complexes but may also be affected by the small ligand-field
splitting of the Pd(II) ion, which results in facile population of
2
2
the antibonding 4dx −y orbital; therefore, primarily non-
radiative decay occurs by significant molecular geometry
distortion of the excited state.35,37−41 Thus, traditionally,
bidentate and tridentate cyclometalated Pd(II) complexes
show nearly no emission in solution at room temperature
(RT),40,42,43 and only very weak emission can be observed
even at 77 K.44−46 These low PLQYs have slowed the
development of Pd(II) complexes as emitters in OLED
application. When Schiff base47 or porphyrin48 is employed
as the tetradentate ligand, efficient red-to-near-infrared (λmax
=
668−882 nm) phosphorescent Pd(II) complexes with ΦPL of
1%−45% were reported.47,48 The high efficiencies are
attributed to both the increased molecular rigidity and lower
energies of T1 states to inhibit access to the metal-centered
(MC) d−d excited states.35 To develop efficient phosphor-
escent Pd(II) complexes with higher T1 energy to cover the
whole visible region, one strategy is to utilize strong-field
ligands to destabilize the d−d excited states by introducing
strong δ-donating atoms or groups to the ligands, such as C
atoms or alkyl substituents. Another strategy is to employ the
tetradentate ligand to afford rigid molecular scaffold,
minimizing the molecular geometry distortion of the excited
state to reduce the nonradiative decay. On the basis of these
molecular design principles, significant progress has been made
in recent years. Several types of green-to-yellow (λmax = 498−
B
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Figure 2. X-ray molecular structures and ORTEP drawing of the crystal packing of Pd(1-ptz) (CDCC 2003072) and Pd(poz) (CDCC 1981640).
H atoms are omitted for clarity in the crystal packing. The solid double arrows show the distance between the two ptz ligands in Pd(1-ptz) and the
pyridine/oxazole planes in Pd(poz). The dotted line shows the distance between two Pd atoms. Ellipsoids are shown at the 50% probability level.
Table 1. Selected Bond Lengths (Å), Angles (deg), and Dihedral Angle (deg) for Pd(1-ptz) and Pd(poz) Based on X-ray
Crystallographic Analysis and DFT Calculation
a
complex
Pd−N1
Pd−C1
Pd−C2
Pd−N2
N1−Pd−C2
C1−Pd−N2
dihedral angle
Pd(1-ptz)_X-ray
Pd(1-ptz)_S0
Pd(1-ptz)_T1
Pd(poz)_X-ray
Pd(poz)_S0
2.150(2)
2.19
2.16
2.166(2)
2.25
2.22
1.975(2)
1.99
2.00
1.970(2)
2.00
1.99
1.977(2)
2.00
2.00
1.962(2)
1.99
1.99
2.128(2)
2.18
2.20
2.1066(19)
2.18
2.18
168.79(10)
168.62
169.55
168.44(9)
167.96
168.54
176.60(9)
173.56
175.30
173.50(9)
171.08
170.13
18.2
21.3
20.3
35.3
37.6
37.4
Pd(poz)_T1
a
Dihedral angle between the terminal five-membered Het plane and Py plane.
543 nm) phosphorescent Pd(II) complexes with ΦPL up to
22% in solution35 and sky-blue (λmax = 456−466 nm) Pd(II)
complexes with ΦPL up to 64% in PMMA film37 have been
developed. Our recent study also demonstrated that ΦPL of the
tetradentate Pd(II) complex could be increased by 8-fold by
introducing δ-donating substituents to the ligand.14 The third
strategy is to develop Pd(0) complexes, which have d10
electronic structures with full-filled 4d orbitals to enable no
accessible d−d states. Expectedly, ΦPL of 69.8% was achieved
for the Pd(0) complex with λmax of 608 nm by using two strong
δ-donating N-heterocyclic carbene ligands.38 However, the
Pd(0) complexes are highly reactive toward molecular oxygen
to afford palladium(II) peroxo adducts and not stable in air,38
limiting their further application in OLED. Additionally, the
recently developed Pd(II)-complex-based MADF is also an
effective design strategy.13−15 By using the rigid tetradentate
ligand with a donor−acceptor system, yellow (λmax = 522 nm)
MADF material realized a ΦPL of 72% and demonstrated a
peak external quantum efficiency (EQE) of 20.9% in the device
setting with good operational lifetime.13 These studies indicate
the promising future of the phosphorescent Pd(II) complexes
for OLED applications.
suppresses the nonradiative decay, which is helpful in
increasing ΦPL. As illustrated in Figure 1, to obtain
fundamental and comprehensive insight into the influence of
the Het variation on the photophysical and excited-state
properties of the 5/6/6 Pd(II) complexes, a series of
isoelectronic five-membered Het rings, such as pyrazole (pz),
1,2,3-triazole (tz), imidazole (iz), oxazole (oz), and thiazole
(thz), and a six-membered heteroaryl pyridine (Het−Py) were
employed. Additionally, substituents and their positions effects
were also investigated, such as Pd(ppz) versus Pd(ppzDM)
and also Pd(1-ptz) versus Pd(1-ptzMe) and Pd(1-ptzPh).
The tetradentate ligands were synthesized through a cross-
coupling reaction of phenol derivative (1) with 2-bromo-9-
(pyridin-2-yl)-9H-carbazole (2-BrCbPy) or its functionalized
derivative (2) catalyzed by Cu(I).49 The Pd(II) complexes
could be easily obtained by direct metalation of the
corresponding ligand with palladium acetate (Pd(OAc)2) in
acetic acid at 105−120 °C (Figure 1).14,50 All of the newly
developed Pd(II) complexes are air-stable and can be easily
purified through column chromatography on silica gel (see the
Supporting Information). The molecular structures of the
Pd(II) complexes were confirmed by 1H and 13C NMR
spectroscopies and high-resolution mass spectrometry
(HRMS).
Employing rigid tetradentate configurations reduces molec-
ular deformation from the ground state (S0) to T1 and
C
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Figure 3. (a) TGA curves of the unsublimated tetradentate Pd(II) complexes with 5% weight-reduction temperature (ΔT5%). (b) TGA and DSC
curves of sublimated Pt(1-ptz).
Table 2. Electrochemical Properties and Energy Levels of the Newly Developed Pd(II) Complexes
a b
,
c
d
1
complex
Pd(ppz)
Eox/Ered [V]
HOMO/LUMO/ΔEg [eV]
ET [eV]
calcd HOMO/LUMO/ΔEg [eV]
0.64 (ir)/−2.59 (ir)
0.62 (ir)/−2.61 (ir)
0.67 (ir)/−2.54 (ir)
0.69 (ir)/−2.57 (ir)
0.64 (ir)/−2.58 (r)
0.64 (ir)/−2.52 (qr)
0.57 (ir)/−2.67 (ir)
0.62 (ir)/−2.36 (r)
0.61 (ir)/−2.55 (ir)
0.58 (ir)/−2.45 (r)
−5.44/−2.21/3.23
−5.42/−2.19/3.23
−5.47/−2.26/3.21
−5.49/−2.23/3.26
−5.44/−2.22/3.22
−5.44/−2.28/3.16
−5.37/−2.13/3.24
−5.42/−2.44/2.98
−5.41/−2.25/3.16
−5.38/−2.35/3.03
2.87
2.88
2.84
2.86
2.86
2.85
2.77
2.45
2.66
2.68
−4.93/−1.33/3.60
−4.85/−1.31/3.54
−5.03/−1.48/3.55
−5.01/−1.43/3.58
−4.98/−1.39/3.59
−5.01/−1.44/3.57
−4.77/−1.19/3.58
−4.95/−1.38/2.57
−4.95/−1.64/3.31
−4.86/−1.51/3.35
Pd(ppzDM)
Pd(2-ptz)
Pd(1-ptz)
Pd(1-ptzMe)
Pd(1-ptzPh)
Pd(piz)
Pd(poz)
Pd(pthz)
Pd(ppy)
a
Obtained from DPV and measured in degassed anhydrous DMF under a nitrogen atmosphere; glassy carbon, platinum wire, and silver wire were
b
used as the working, counter, and pseudoreference electrodes, respectively; the scan rate is 300 mV s−1. Irreversible (ir); reversible (r); quasi-
reversible (qr). HOMO = −(Eox + 4.8) eV; LUMO = −(Ered + 4.8) eV; ΔEg = LUMO − HOMO. The lowest triplet energies (ET ) were
c
d
1
estimated from the phosphorescent spectra of the Pd(II) complexes in a 2-MeTHF solution at 77 K.
The structures of Pd(1-ptz) and Pd(poz) were further
confirmed by X-ray crystallographic analyses (Figures 2 and
S1). Notably, an intramolecular hydrogen bond between Hα of
the Py rings and N3 of the triazole rings in Pd(1-ptz) was
observed because of the small steric resistance between the
triazole and pyridine rings (Figure 2a), resulting in a small
dihedral angle of only 18.2° (Table 1). In contrast, Pd(poz)
exhibits a distorted configuration with a dihedral angle of 35.3°
between the terminal oxazole and pyridine planes (Figure 2c).
The bond lengths of covalent Pd−C (1.962−1.977 Å) are
much shorter compared to coordination Pd−N (2.1066−2.166
Å; Table 1). The metal−C and metal−N bonds in Pd(1-ptz)
are longer than those in Pt(1-ptz).51 This is attributed to the
smaller atomic radius of Pd compared to Pt. Both Pd(1-ptz)
and Pd(poz) molecules stack in crystals as head-to-head
dimers with a distance between the two ptz planes of 3.21 Å
and the pyridine/oxazole planes of 3.38 Å, respectively (Figure
2b,d). This indicates the existence of moderate π−π
interactions. The shortest distance between two Pd atoms in
Pd(poz) is 4.59 Å, which is much shorter than that in Pd(1-
ptz) (6.17 Å; Figure 2b,d). This reveals a metal−metal
interaction in Pd(poz).
investigate the thermal property of the Pd(II) complexes
without sublimation (Figure 3a). All of the Pd(II) complexes
exhibit high thermal stability with ΔT5% of 304−400 °C except
Pd(piz) (162 °C). Particularly, ΔT5% of Pd(ppz) (400 °C) is
comparable to the sublimated PtON1 (445 °C)14 with a
similar ligand. The low ΔT5% of Pd(piz) is attributed to some
solvent with a high boiling point in the sample. However, the
ΔT5% values of the Pd(II) complexes are high enough for
sublimation, which are usually below 300 °C.14,31 The
sublimated Pd(1-ptz) shows a high glass transition temperature
(Tg) of 252 °C from the differential scanning calorimetry
(DSC) analysis (Figure 3b). This study reveals that the five-
membered Het rings are thermally stable enough to act as
functional groups in Pd(II)-complex-based phosphorescent
materials.
Electrochemistry. The reversibilities of the redox of
Pd(II) complexes were determined by cyclic voltammetry
(CV; Figure S2), and the values of the redox potentials were
measured through differential pulse voltammetry (DPV) and
listed in Table 2. All of the electrochemical data were measured
in anhydrous N,N-dimethylformamide (DMF) under a nitro-
gen atmosphere relative to an internal reference of
ferrocenium/ferrocene (Cp2Fe+/Cp2Fe0).52 All of the Pd(II)
complexes exhibit an irreversible oxidation process, and both
the oxidation potentials (0.57−0.69 V) and estimated highest
occupied molecular obital (HOMO; between −5.37 and −5.49
eV) are in a very narrow range, indicative of small perturbation
Thermal Property. The thermal stability of the newly
developed Pd(II) complexes is one of the critical parameters
for the application in OLED because of the great effect on the
morphological stability and device performances. In this
regard, thermogravimetric analysis (TGA) was carried out to
D
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Figure 4. DFT-computed frontier orbitals of the S0 state and the spin densities of the T1 states for Pd(ppz), Pd(2-ptz), Pd(1-ptz), Pd(piz),
Pd(poz), and Pd(pthz) containing five-membered heteroaromatic rings.
by the various five- or six-membered Het rings. This reveals
that the oxidation processes typically occur on the palladium
phenylcarbazolyl moieties, which is in good agreement with
the electrochemical properties of the related tetradentate
Pd(tzp-1) and its derivatives, which have been thoroughly
studied.14 The irreversible oxidation processes are mainly
attributed to the potential reactivity of the square-planar
Pd(III) metal centers with the surrounding solvent mole-
cules.53,54
By contrast, the reduction potentials of the Pd(II)
complexes are strongly molecular-structure-dependent. Most
complexes also show a first irreversible reduction process;
however, Pd(1-ptzMe), Pd(poz), and Pd(ppy) exhibit a first
reversible reduction process, and Pd(1-ptzPh) has a first quasi-
reversible reduction process (Figure S2). The Pd(II)
complexes have a significantly wide region from −2.36 to
−2.67 V for the reduction potentials (Table 2), and the
estimated energy of the lowest unoccupied molecular obital
(LUMO) is between −2.13 and −2.44 eV. Introducing methyl
(−Me) groups to the pyrazole ring of Pd(ppz) nearly has a
small effect on the reduction potential of Pd(ppzDM) (−2.59
vs −2.61 V), indicating that the reduction processes mainly
take place on the electron-deficient pyridyl fragments in
Pd(ppz) and Pd(ppzDM). Moreover, modifying the 4-position
of the pyridyl ring with an electron-donating group (−Me) in
Pd(1-ptz) also hardly changes the reduction potential of Pd(1-
ptzMe) (−2.57 vs −2.58 V); however, Pd(1-ptzPh) has a
significantly more positive value (−2.52 V) by adding a phenyl
(−Ph) group to the 4-position of the triazole ring. This result
reveals that the reduction processes mainly occur on the
triazole ring because of its strong electron-withdrawing ability.
Pd(poz), Pd(pthz), and Pd(ppy) also have more positive
values of the reduction potentials compared with Pd(ppzDM)
(Table 2), demonstrating that the reduction processes mainly
occur on the Het groups linked to the −Ph group in Pd(poz),
Pd(pthz), and Pd(ppy). These analyses are in agreement with
the LUMO distributions by density functional theory (DFT)
calculations, which will be discussed below. Overall, the five-
and six-membered Het groups greatly affect the first reduction
potentials and LUMO levels of the Pd(II) complexes, which
largely depends on the degree of electron deficiency of the Het
groups.
The force between the atomic nucleus and the electron in
the 4d orbital of the Pd(II) complex is bigger compared with
that in the 5d orbital of the corresponding Pt(II) complex.
Thus, the Pd(II) complex is difficult to oxidize. Compared
with the 5/6/6 Pt(II) complexes with similar tetradentate
ligands,14,51 the corresponding 5/6/6 Pd(II) complexes exhibit
larger oxidation values and more negative reduction values,
resulting in a larger energy gap (ΔEg = 2.98−3.26 eV) between
HOMO and LUMO, e.g., Pd(1-ptz) of 3.26 eV versus Pt(1-
ptz) of 2.89 eV.49
Theoretical Calculation. DFT calculations were per-
formed, and the S0 and T1 states of the 5/6/6 Pd(II)
complexes were optimized at the B3LYP/6-31G(d)/
LANL2DZ level using Gaussian 09.14,54 All of the Pd(II)
complexes exhibit twisted molecular geometries in both the S0
and T1 states. In general, the bond lengths of the covalent Pd−
C are in the region of 1.98−2.00 Å, and the weaker
coordination Pd−N bonds are relatively longer from 2.13 to
E
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Table 3. Photophysical Properties of the Tetradentate Pd(II) Complexes
absorption, DCM, RT
2-MeTHF, 77 K/RT
DCM, RT
a
b
c
λ [nm]/τ [μs]//λ [nm] (fwhm
λ [nm] (fwhm [nm])/τ [μs]/ΦPL
kr [×103
knr [×103
complex
Pd(ppz)
Pd(ppzDM) 250 {344}, 302 {351}, 350 {151}
λabs [nm] {ε × 10−2, cm−1 M−1
}
[nm])
[%]
s−1
]
s−1
]
250 {530}, 294 {341}, 345 {153}
432/614/440(38)
431/505/439(31)
436/704/443(36)
434/358/440(22)
433/362/439(30)
439(64)/35/21
438(45)/68/22
443(63)/59/12
441(48)/37/16
441(48)/19/19
6.0
3.2
2.0
4.3
10.0
5.9
1.6
7.7
4.4
5.5
22.6
11.5
14.9
22.7
42.6
12.6
6.5
24.5
10.8
5.3
Pd(2-ptz)
Pd(1-ptz)
251 {558}, 307 {350}, 347 {162}
251 {489}, 307 {265}, 350 {144}
Pd(1-ptzMe) 251 {369}, 307 {204}, 348 {117}
Pd(1-ptzPh) 254 {453}, 309 {221} ,351 {112}
435/258/442(26)
445(61)/54/32
Pd(piz)
Pd(poz)
Pd(pthz)
Pd(ppy)
250 {447}, 292 {290}, 345 {116}
256 {338}, 315 {207}, 347 {129}
250 {339}, 307 {214}, 353 {95}
255 {457}, 315 {247}, 360 {117}
448,479/347/452,480(93)
506,545/195/512,547(106)
467,502/188/468,501(73)
462,495/299/467,492(64)
452,483(89)/123/20
513,546(91)/31/24
468,501(83)/66/29
469,497(73)/92/51
a
b
c
Absolute quantum efficiency under oxygen-free conditions using an integrating sphere. kr = ΦPL/τ. knr = (1 − ΦPL)/τ.
Figure 5. Absorption spectra of the 5/6/6 Pd(II) complexes in DCM at RT.
2.25 Å, which are in good agreement with the X-ray
crystallographic analyses (Tables 1 and S3). For all of the
Pd(II) complexes, the bond angles of N1−Pd−C2 and C1−
Pd−N2 are larger for the T1 states compared to the S0 states
except C1−Pd−N2 for Pd(pthz) and Pd(poz) (Table S4),
revealing that the molecular geometries of the T1 states are
more planar. This analysis can be supported by the fact that the
dihedral angles of the T1 states are smaller than those of the S0
states for most of the Pd(II) complexes (Table S4). Notably,
variations of the dihedral angles between the S0 and T1 states
are very small (0.2−2.8°), indicating weakly distorted T1 states
and the strong rigidities of the Pd(II) complex scaffolds. This
facilitates suppression of the nonradiative decay resulting from
molecular distortion to achieve high quantum efficiency.
Besides Pd(1-ptz), DFT calculations also indicate the existence
of intramolecular hydrogen bonds in Pd(1-ptzMe) and Pd(1-
ptzPh) (Figure S3). Thus, they have small dihedral angles of
21.3°, 27.4°, and 28.1° for the S0 states and 20.3°, 27.2°, and
28.4° for the T1 states, which are smaller than 37.4° for the S0
and T1 states of the other Pd(II) complexes (Table S4).
As illustrated in Figures 4 and S4, the HOMO and
HOMO−1 distributions of all of the Pd(II) complexes are
almost identical, which are mainly localized on dPd−πPh−
pO−πCb and dPd−πPy−πCb orbitals, respectively. The HOMO
energy levels are in a narrow region from −4.77 to −5.03 eV.
By contrast, the LUMO and LUMO+1 distributions are
strongly structure-dependent. The LUMO distributions are on
the pure πPy orbitals for Pd(ppz), Pd(ppzDM), and Pd(piz),
most πPy orbitals with small πPh−Het orbitals for Pd(2-ptz) and
Pd(poz), and dominant πPh−Het with small πPy orbitals for
Pd(1-ptz), Pd(1-ptzMe), Pd(1-ptzPh), Pd(pthz), and Pd-
(ppy). Compared to Pd(ppz), electron-deficient triazole-based
Pd(2-ptz), Pd(1-ptz), Pd(1-ptzMe), and Pd(1-ptzPh) and
large conjugated 4-phenylpyridine-based Pd(ppy) remarkably
stabilize the LUMO and LUMO+1 orbitals. Methylation of the
pyrazole ring in Pd(ppz) results in destabilization of both the
LUMO and LUMO+1 orbitals. Moreover, both Pd(poz) and
Pd(pthz) display deeper LUMO and LUMO+1 levels relative
to Pd(piz). Furthermore, the incorporation of a methyl group
into the 4-position of the Py ring destabilizes all four orbitals
[Pd(1-ptz) vs Pd(1-ptzMe)], by comparison, the introduction
of a phenyl group to the 4-position of the pyrazole ring hardly
affects the energy levels [Pd(1-ptz) vs Pd(1-ptzPh)] (Figure
S4). Hence, the LUMO energy levels are in a wide region from
−1.19 to −1.64 eV. This reveals that the energy levels of
LUMO and LUMO+1 can be readily regulated through the
careful choice of Het rings. Both the HOMO and LUMO
levels are consistent with the experimental trend but a little
different in value because the solvent was not considered in the
DFT calculations (Table 2).
The large energy gaps between the HOMO and HOMO−1
orbitals (0.30−0.41 eV) result in the HOMO−1 orbitals of the
Pd(II) complexes having small contributions to the T1 and S1
states (Tables S5). By contrast, the energy gaps between the
LUMO and LUMO−1 orbitals are much different (0.20−0.52
eV). Hence, the S0 → S1 processes are dominant from the
HOMO → LUMO transitions (95−98%), and the spin-
forbidden S0 → T1 processes are more complicated, involving
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Figure 6. Emission spectra of deep-blue emitters for (a) Pd(ppz), (b) Pd(ppzDM), (c) Pd(2-ptz), (d) Pd(1-ptz), (e) Pd(1-ptzMe), and (f) Pd(1-
ptzPh) in 2-MeTHF at 77 K (dash-dot-dotted line) and RT (solid line). The chemical structure and CIE coordinates (RT) of each Pd(II) complex
are inset.
LUMO+1, LUMO+3, and even LUMO+4 orbitals, although
the HOMO → LUMO transitions (58−88%) are also the
major contributions (Tables S5). The small gap (0.20 eV)
between LUMO and LUMO+1 for Pd(poz) enables the 25%
HOMO → LUMO+1 transition in the S0 → T1 process
(Tables S5). This result indicates that structural modification
can effectively manipulate the excited-state properties of the 5/
6/6 Pd(II) complexes.
transitions involving both the Pd(II) ions and cyclometalating
ligands, which are from πPhdPd → πPy* for Pd(ppz),
Pd(ppzDM), Pd(2-ptz), Pd(piz), and Pd(poz) and from
πCbdPd → πHet* for Pd(1-ptz), Pd(1-ptzMe), Pd(1-ptzPh),
Pd(ppy), and Pd(pthz). Because of the heavy-metal-induced
SOC effect, the spin-forbidden 3MLCT absorptions, which are
very weak (ε < 100 cm−1 M−1) bands with structureless
profiles, can also be observed at longer wavelength region
(>400 nm). Structural modification to the Het moiety results
Photophysical Properties. The data of UV−vis absorp-
tion of the Pd(II) complexes in dichloromethane (DCM) at
RT are summarized in Table 3, and the absorption spectra are
illustrated in Figures 5 and S5. For all of the 5/6/6 Pd(II)
complexes, the strong absorption bands (ε > 1.00 × 104 cm−1
M−1) below 325 nm are contributed mainly by allowed
intraligand π → π* transitions from πCb → πPy* for Pd(ppz),
Pd(ppzDM), and Pd(piz) and from both πCb → πPy* and πPh
→ πHet* (Het = five- or six-membered heteroaryl rings) for all
other complexes by time-dependent DFT (TD-DFT) calcu-
lations (Figures 4 and S4 and Table S5).14,54 This analysis is
also consistent with the NTO analysis (discussed below), in
which Pd(ppz), Pd(ppzDM), Pd(2-ptz), and Pd(piz) show a
strong intraligand charge transfer (1ILCT) from πCb → πPy*
for the S0 → S1 process because of their extremely high
electron densities on the πPy* orbitals in the S1 electron. This
analysis was also supported by the fact that these four
complexes have stronger absorption bands at about 300 nm
compared with the other complexes (Figure 5). The
broadened absorption bands centered at around 350 nm (ε
1
3
in significant energy changes to the MLCT and MLCT
transitions. Introducing a −Ph group to the triazole ring
[Pd(1-ptzPh)] or employing pyridine [Pd(ppy)] and oxazole
1
[Pd(poz)] significantly reduces the energy of the MLCT and
3MLCT transitions (Figure 5). Moreover, these Pd(II)
1
complexes exhibit a remarkable blue shift for both MLCT
3
and MLCT absorptions compared with the corresponding
3
Pt(II) complexes, most of which show well-resolved MLCT
absorption profiles.14,54
Structural modification of the Het moieties significantly
affects the emission spectra, luminescent lifetimes, and
quantum efficiencies of the newly designed 5/6/6 Pd(II)
complexes (Table 3 and Figures 6−8 and S6 and S7). All of
the Pd(II) complexes are highly emissive in 2-methyltetrahy-
drofuran (2-MeTHF) at 77 K. The pyrazole- and triazole-
based Pd(II) complexes exhibit very similar-structured
emission spectra with one dominant peak and two well-
resolved vibronic side bands (Figure 6), indicative of a
dominant ligand-centered (3LC) with small 3MLCT-based
emitting state character, which arises mostly from the carbazole
1
> 5 × 103 cm−1 M−1) are assigned to spin-allowed MLCT
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molecular hydrogen bond between the pyridine and triazole
rings discussed above, resulting in weaker v0−1 vibrational
bands (Figures 6d−f). Additionally, the emission colors of the
Pd(II) complexes could be easily tuned. Structural modifica-
tion of the Het moieties using imidazole [Pd(piz)], pyridine
[Pd(ppy)], thiazole [Pd(pthz)], and oxazole [Pd(poz)] rings
significantly red-shifts and broadens the emission spectra (λ =
452−547 nm; fwhm = 64−106 nm; Table 3 and Figure 8b),
3
indicating more MLCT character in the T1 states.
The solvent effect on the photophysical properties of the
Pd(II) complexes was also investigated. In a DCM solution, all
of the Pd(II) complexes keep nearly identical emission
wavelengths (Table 3); however, the vibrational bands are
much stronger compared with those in 2-MeTHF (Figures 7a
and 8c). This is attributed to the bigger dielectric constant of
DCM (ε = 7.6) relative to 2-MeTHF (ε = 6.97),51 which
favors the stabilization of more polar excited molecules.
Moreover, all of the Pd(II) complexes have notably shorter
luminescent lifetimes of less than hundreds of microseconds (τ
= 19−92 μs) with the exception of Pd(piz) (τ = 123 μs). They
are all strongly emissive, with quantum efficiencies of 12−51%
(Table 3), which are much higher than those of the reported
[1,2,4]triazole[4,3-a]pyridine-based tetradentate Pd(II) com-
plexes with 5/6/6 metallocycles (ΦPL = 0.21−2.88%).14 The kr
values of the Pd(II) complexes are between 1.6 × 103 and 1.00
× 104 s−1; by contrast, the knr values are in a much larger range
from 5.3 × 103 to 4.26 × 104 s−1, indicating that both the kr
and knr values are also structure-dependent.
The photophysical properties of the Pd(II) complexes in
doped PMMA films were investigated (Figures 7b and 8d and
Table 4). Compared to the spectra in DCM, the dominant
emission peaks of the spectra in PMMA almost unchanged;
however, the vibrational bands are weaker, resulting in
narrower spectra (fwhm = 42−90 nm). This is attributed to
the suppression of vibration in the solid surrounding
environment. Pd(poz) shows almost identical emission spectra
in 2-MeTHF, DCM, and PMMA, indicating similar excited-
state character. Their phosphorescent decay lifetimes were
measured in the region of 37−61 μs (Table 4), which are
comparable with the lifetimes in DCM at RT (Table 3). The
quantum efficiencies are relatively lower (ΦPL = 6−22%)
because of the long transient times, leading to increased
concentration quenching at higher doping and rapid non-
radiative processes of the T1 states (knr > 1.39 × 104). A higher
quantum efficiency (ΦPL = 19%) was obtained for Pd(1-ptzPh)
in a low (1 wt %) doped PMMA film.
Ground- and Excited-State Properties. Generally, the
excited states of the phosphorescent Ir(III)21,22 and Pt-
(II)23,51,54,55 complexes are characterized as admixed LC
states with MLCT character by SOC and achieve high
quantum efficiency. For Pd(II) complexes, the excited-state
properties are more complicated because of the existence of
the low-lying MC d−d states, which typically lead to thermally
accessible luminescence quenching in solution at RT.35
Therefore, highly phosphorescent Pd(II) complexes are
relatively sparse, and studies about their excited-state proper-
ties are limited.35,37 To have an insightful understanding of the
excited-state properties of the newly developed 5/6/6 Pd(II)
complexes, NTO analyses54,56 of the S1 and T1 states were
performed by TD-DFT calculations based on the optimized S0
and T1 geometries (Figures 9 and S8), and the charge
transitions of the T1 → S0 process are summarized in Table S6.
Figure 7. Emission spectra of deep-blue emitters of Pd(ppz),
Pd(ppzDM), Pd(2-ptz), Pd(1-ptz), Pd(1-ptzMe), and Pd(1-ptzPh)
in DCM (a) and PMMA film (b) at RT.
(Cb) moiety of the Pd(II) complexes. Both Pd(piz) and
Pd(ppy) show highly vibronic emission spectra typical of
complexes with a C−C bond in the chelation ring. Pd(pthz)
and Pd(poz) are also highly vibronic but have a second band of
vibronic emission peaks at lower energy (Figure 8a), indicative
of the existence of two excited states with similar energy levels.
All of the Pd(II) complexes have luminescent lifetimes over
hundreds of microseconds (τ = 188−704 μs). The long
luminescent lifetimes also indicate small structural distortion in
the T1 states resulting in slow radiative decay from T1 → S0.37
However, the luminescent lifetimes of the Pd(II) complexes
are much shorter than those of the reported [1,2,4]triazole-
[4,3-a]pyridine-based Pd(II) complexes (τ = 1.98−2.36 ms)
with similar tetradentate 5/6/6 ligands.14
Employing pyrazole [Pd(ppz) and Pd(ppzDM)] or triazole
[Pd(2-ptz), Pd(1-ptz), Pd(1-ptzMe), and Pd(1-ptzPh)] rings,
a series of deep-blue emitters with high-color-quality and
triplet energy (ET ) of 2.84−2.88 eV were successfully
1
developed. They exhibit dominant emission peaks at 439−
443 nm merged with thermally enhanced ν0−1 vibrational
bands in 2-MeTHF at RT. All of the emission spectra are
extremely narrow with full width at half-maximum (fwhm) of
3
only 22−38 nm (Figure 6), indicating also dominant LC
mixed with small 3MLCT character in the T1 states.
Particularly, 1-phenyl-1,2,3-triazole-based Pd(II) complexes,
Pd(1-ptz), Pd(1-ptzMe), and Pd(1-ptzPh), achieve CIEy < 0.1.
The further narrowing of the emission spectrum for the Pd(1-
ptz) complexes is attributed to the existence of an intra-
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Figure 8. Emission spectra of the Pd(II) complexes in various conditions: (a) in 2-MeTHF at 77 K; (b) in 2-MeTHF at RT; (c) in DCM at RT;
(d) in 5 wt % PMMA film at RT.
DCM at RT (Table 3). Two different kinds of 3MLCT
Table 4. Photophysical Properties of the Tetradentate
Pd(II) Complexes in 5 wt % Doped PMMA Films
transitions exist (Table S6). One is from πPy* to dPdπPh for
Pd(ppz), Pd(ppzDM), and Pd(piz), which have large electron
distributions on the Py moieties and a very small portion on
the Het−Ph moieties in electron T1, and the other kind is from
πHet* to dPdπCb in all other Pd(II) complexes, which show
electron-distribution-rich Het moieties and electron-distribu-
tion-poor Cb moieties (Figures 9 and S8). All of the Pd(II)
a
b
c
fwhm
[nm]
τ
[μs]
ΦPL
[%]
kr [×103 knr [×103
complex
Pd(ppz)
Pd(ppzDM) 437
Pd(2-ptz)
Pd(1-ptz)
Pd(1-ptzMe) 439
Pd(1-ptzPh) 443
λ [nm]
s−1 s−1
]
]
438
44
43
50
45
42
52
61
9
1.5
4.3
2.8
4.0
1.4
3.5
14.9
15.3
16.0
19.8
18.6
23.5
51 22
53 15
42 17
50
37 13
(19 )
49 11
440
440
3
complexes exhibit ILCT transitions of both πHet* → πPh and
7
πPy* → πCb in various percentages (Table S6). Importantly, a
large portion of the 3LC transition (πCb* → πCb; 16.2−40.2%)
is observed in all of the Pd(II) complexes except Pd(pthz) and
Pd(ppy) (Figures 9 and S8). These analyses are consistent
with the results from the spin densities of the T1 states
d
Pd(piz)
Pd(poz)
Pd(pthz)
Pd(ppy)
452,
482
513,
546
469,
500
466,
496
84
90
87
71
2.2
1.5
1.1
3.7
18.2
15.2
16.8
13.9
60
56
9
6
3
(Figures 4 and S4). Notably, no low-lying MC state was
observed in the photophysical studies.
57 21
On the basis of the photophysical behavior and NTO
analyses, the Pd(II) complexes are subdivided into three
classes. Class I ppz- and ptz-based deep-blue Pd(II) complexes
a
Absolute quantum efficiency under oxygen-free conditions using an
b
c
d
3
integrating sphere. kr = ΦPL/τ. knr = (1 − ΦPL)/τ. ΦPL measured in
a 1 wt % doped PMMA film.
(Figure 6) emit exclusively from the Cb-centered LC state at
3
77 K and predominantly from the same LC state with a little
perturbation by the temperature-sensitive 3MLCT state
(Figure 10a). This is because the Pd(II) complexes are hard
In the spin-forbidden T1 → S0 process, the 3MLCT
transition plays a critical role in the efficient radiation decay
from the T1 to S0 state to achieve a high phosphorescent
quantum efficiency. The NTO analyses reveal that all of the
Pd(II) complexes involve 7.8−14.7% 3MLCT transitions
(Figures 9 and S8 and Table S6), which is in good agreement
with their intense emission in various matrixes, e.g., in 2-
MeTHF at 77 K and in DCM and PMMA at RT. In particular,
Pd(ppy) achieves a high quantum efficiency of up to 51% in
3
to oxidize and therefore destabilize the ILCT state, so that
only the Cb-localized 3LC state is emitting to display extremely
structured narrow spectra at both 77 K and RT. This is also
supported by the fact that structural modification of the
pyridine [Pd(1-ptzMe)] and triazole [Pd(1-ptzPh)] rings of
Pd(1-ptz) has little effect on the triplet energies and also for
Pd(ppz) compared to Pd(ppzDM) (Table 2). Moreover, with
3
an increase in the ratio of the LC character in the deep-blue
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Figure 9. NTO comparison of the S1 and T1 states for five-membered Het-based Pd(ppz), Pd(2-ptz), Pd(1-ptz), Pd(piz), Pd(poz), and Pd(pthz).
Figure 10. (a−c) Potential energy surface diagram of the Pd(II) complexes. Solid and dashed lines represent processes at RT and 77 K,
respectively. (d) Emission spectral comparison of Pd(ppy) and PdNON in 2-MeTHF at 77 K and in DCM at RT. The chemical structures of
Pd(ppy) and PdNON and the attributions of each emission peak are inset.
J
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Figure 11. (a) Photoluminescent spectral comparison for Pd(1-ptz) and Pt(1-ptz) under various conditions. (b) Electroluminescent spectra of
Pd(1-ptz)- and Pd(1-ptzMe)-based OLEDs. Their CIE coordinates on the CIE 1931 chromaticity diagram are inset. (c) Current density and
luminance versus voltage characteristics. (d) EQE versus luminance.
3
emitters (Figures 9 and S8), the ν0−1 vibrational bands of the
emission spectra of the doped PMMA films decrease (Figure
8d).
the emission from the MLCT (πPy* → dPdπCb) state, which
3
merges with the normal vibronic progression of the ILCT
emission to make the second peak appear larger than the first
peak. This analysis is consistent with the broad Gaussian-type
emission peak (λ = 497 nm) of PdNON at RT, both of which
are from the transitions of πPy* → dPdπCb. Similar emission
spectra are also observed in Pd(poz) and Pd(pthz), all of
which have lower-energy 3ILCT (πHet* → πPh) states. Notably,
this can explain the fact that Pd(poz), with large portions of
electron distributions on the Cb moieties (16.2%; Figure 9), is
Class II, as shown in Figure 8, Pd(piz) exhibits a high energy
3
peak at 452 nm in various matrixes originating from the LC
(Cb* → Cb) transition, and also a stronger emission peak at
3
about 480 nm is assigned to the stabilized MLCT (πPy* →
dPdπPh) transition. Thus, Pd(piz) has a strongly mixed
3MLCT/3LC character (Figure 10b), which is agreement
with NTO analysis (Table S6).
3
not a deep-blue emitter because of stabilization of the ILCT
Class III Pd(poz), Pd(pthz), and Pd(ppy) all show broad
well-resolved spectra with two strong peaks in various matrixes
at RT. This indicates strong admixed 3MLCT/3ILCT character
in the T1 states because of the stabilized 3ILCT (πHet* → πPh)
states by extended conjugation of the Het−Ph moieties
(Figure 10c). To gain deep insight into the photophysical
behavior of the Class III complexes, PdNON with a symmetric
Py−Cb-containing ligand was remade,50 and the emission
spectral comparison of Pd(ppy) and PdNON is shown in
Figure 10d. PdNON exhibits a pure 3LC (πCb* → πCb)
dominated spectrum with vibronically structured features at 77
K, whose sharp peaks are identical with that of PtNON−OMe
3
(πHet* → πPh) state. The energy levels of the ILCT and
3MLCT states can be slightly perturbed by the matrixes
because of the tiny change of the wavelength in various media
3
(Tables 3 and 4). The above studies suggest that the LC and
3MLCT/3ILCT character in the T1 states of the Pd(II)
complexes can be effectively tuned by simple modification of
the Het rings.
Pd(II) versus Pt(II) Complexes. How do the intrinsically
different electronic configurations of Pd(II) 4d8 and Pt(II) 5d8
ions determine the electrochemical and photophysical proper-
ties of their metal complex? A fundamental study of this
question can promote the design and development of new
materials. Generally, the oxidation process typically localizes
on the metal−phenyl portion of the metal complex and the
reduction process mainly occurs on the electron-deficient
ligand moiety. Therefore, the metal ion can seriously affect the
oxidation potential and keep the reduction potential less
changed. Compared to the Pt(II) complex, the corresponding
Pd(II) complex is harder to oxidize and has a much deeper
57,58
3
with the same LC character (Figure S9),
and the peak
wavelength (λ = 436 nm) is in good agreement with those of
the Class I complexes (λ = 431−436 nm; Table 3). By
comparison, no emission peak was observed below 450 nm for
Pd(ppy) at 77 K, indicating that the 3ILCT (πPy* → πPh) state
is lower in energy than the 3LC (πCb* → πCb) state in
Pd(ppy), resulting in the dominant peak redshifting to 462 nm.
The second peak at 492 nm of Pd(ppy) at RT is identified as
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HOMO level, resulting in a larger HOMO−LUMO energy gap
and a higher triplet energy. This enables the Pd(II) complex
typically to have a significant blue shift (Figures 11a and S10−
S15). That is, EHOMO = −5.49 and −5.20 eV, ΔEg = 3.26 and
based deep-blue OLEDs with CIEy < 0.1. This study
demonstrates that the Pd(II) complexes can serve as stable
and efficient emitters for deep-blue OLEDs, and it is believed
that the device performance can be further improved through
optimization of the device architecture and the design of a
Pd(II) complex with short τ.
2.89 eV, and ET = 2.86 and 2.56 eV for Pd(1-ptz) and Pt(1-
1
ptz),51 respectively. Pd(1-ptz) displays blue shifts of 22 and 28
nm relative to Pt(1-ptz) in 2-MeTHF at 77 K and RT,
respectively (Figure 11a). Moreover, the Pd(II) complex
possesses a significantly narrower emission spectrum through
rational regulation of the triplet state unperturbed by the
3MLCT state, such as Pd(1-ptz), Pd(1-ptzMe), Pd(1-ptzPh),
Pd(2-ptz), and Pd(ppzDM), compared to their corresponding
Pt(II) complexes (Figures 11a and S10−S13). The extremely
narrow spectrum of Pd(1-ptz) (fwhm = 22 nm) enables the
majority of the emission in the deep-blue region and eliminates
color contamination from the green region,19 making it an
ideal deep-blue-light-emitting material (Figure 11a). Further-
more, the existence of an intramolecular hydrogen bond
enables the 1-phenyltriazole-based Pt(II) and Pd(II) com-
plexes to have small dihedral angles between the terminal
triazole and pyridine rings, 17.4−22.8° and 18.2−28.1° for the
Pt(II) and Pd(II) complexes, respectively. This enables the
Pt(II) complexes, e.g., Pt(1-ptz), to generate excimer emission
at about 560 nm in both 2-MeTHF and PMMA film (Figure
11a). However, the excimer emission was not observed for
Pd(1-ptz), even in solution with increased concentrations,
because of the extremely weak Pd···Pd interaction in Pd(1-ptz)
discussed above (Figure 2). On the contrary, the vibrational
ν0−1 bands can be weakened by the intramolecular hydrogen
bonds to make the emission spectra more narrow with CIEy <
0.1 in a 2-MeTHF solution. In summary, Pd(II) complexes
possess blue-shifted and narrow emission spectra, no excimer
emission, and weaker vibrational ν0−1 bands, all of which are
beneficial to the design and development of blue and deep-blue
emitters. Importantly, all of the phosphorescent deep-blue
Pd(II) complexes have no F atom in their structures, which is
also beneficial to the device stability. It is well-known that an
aryl F atom facilitates degradation of the OLED devices.
Pd(II) Complexes as Emitters in Deep-Blue OLEDs.
OLEDs incorporating Pd(1-ptz) and Pd(1-ptzMe) as emitters
were successfully fabricated by vapor deposition with a device
architecture of ITO/HATCN (10 nm)/NPB (30 nm)/TCTA
(10 nm)/mCBP (10 nm)/emitter:A3 (7%, 30 nm)/PPT (5
nm)/Bepp2:Li2CO3 (5%, 5 nm)/Li2CO3 (1 nm)/Al (100
nm), where the host material A3 was [(perfluoropropane-2,2-
diyl)bis(4,1-phenylene)]bis(diphenylphosphine oxide);59 see
Figure S16 for the detailed device architecture, energy-level
diagrams, and molecular structures of the used materials. Both
Pd(1-ptz)- and Pd(1-ptzMe)-based OLEDs demonstrated
narrow electroluminescent spectra peaking at 445 and 442
nm with fwhm values of 44 and 42 nm, respectively, enabling
deep-blue color with CIE coordinates of (0.149, 0.079) and
(0.150, 0.073) (Figure 11b). These CIE coordinates are very
close to the standard for blue (0.14, 0.08) defined by NTSC
(Figure 11b). The deep-blue OLEDs also exhibited low turn-
on voltages of less than 4.0 eV and reasonable luminances
(Figure 11c). A maximum EQE of 2.0% was achieved for the
Pd(1-ptzMe)-based OLED (Figure 11d). The roll-off in the
device efficiency is attributed to the relatively long τ of the
Pd(II) complexes, which results in increased triplet−polaron
and triplet−triplet annihilation at higher current density.8 As
far as we know, this is the first report of the Pd(II)-complex-
3. CONCLUSION
In summary, a series of tetradentate 5/6/6 Pd(II) complexes
employing various isoelectronic five-membered Het rings and
Py-containing ligands were designed and synthesized. Their
thermal, electrochemical, and photophysical properties were
systematically studied and significantly affected by the Het
moieties. The pyrazole- and triazole-based Pd(II) complexes,
with no F atom in the structures, emit deep-blue light in all
matrixes and exhibit extremely narrow emission spectra with
dominant peaks of 439−443 nm and fwhm values of only 22−
38 nm in 2-MeTHF at RT. In particular, Pd(1-ptz), Pd(1-
ptzMe), and Pd(1-ptzPh) achieve CIEy < 0.1, which enabled
the existence of an intramolecular hydrogen bond. All of the
deep-blue materials show radiative decay exclusively from the
3
Cb-centered LC states, which is also strongly supported by
the TD-DFT and NTO analyses. The Het moiety plays a
pivotal role in the efficient regulation of the triplet-state
property. Structural modification of the ligands with
isoelectronic imidazole, oxazole, and thiazole rings or a
3
pyridine ring destabilizes the LE state and results in sky-
blue-to-yellow materials with an admixture of MLCT/3ILCT
3
states. Moreover, all of the newly developed 5/6/6 Pd(II)
complexes are highly thermally stable with ΔT5% of up to 400
°C, which is comparable to the corresponding Pt(II)
complexes. Furthermore, the Pd(II) complexes are strongly
emissive in various matrixes and achieve quantum efficiencies
of up to 51%. Therefore, these 5/6/6 Pd(II) complexes can act
as good phosphorescent light-emitting materials. Compared to
the Pt(II) complexes, the Pd(II) complexes typically possess
blue-shifted and narrow emission spectra because of their
stabilized HOMO levels and less 3MLCT character. The
rational design of an intramolecular hydrogen bond in the 1-
phenyl-1,2,3-trazole-based Pd(II) complexes significantly
weakens the vibrational ν0−1 band to realize satisfied deep-
blue light emission rather than facilitating the excimer emission
observed in the Pt(II) complexes. Deep-blue OLEDs with
CIEy < 0.1 employing Pd(1-ptz) and Pd(1-ptzMe) as emitters
were successfully fabricated. This study provides a valuable
method for the design and development of Pd(II)-complex-
based deep-blue-light-emitting materials.
4. EXPERIMENTAL SECTION
The detailed synthesis and molecular structural characterization of the
newly developed 5/6/6 Pd(II) complexes are described in the
Supporting Information. The diffraction data of Pd(1-ptz) and
Pd(poz) were collected at 273(2) K on a Bruker SMART APEX
diffractometer with Mo Kα radiation (λ = 0.71073 Å). Electro-
chemical measurements (CV and DPV) were carried out on a
CH1760E electrochemical analyzer. Theoretical calculations were
performed using Gaussian 09 according to a previous report.14 The
absorption spectra were measured using an Agilent 8453 UV−vis
spectrometer. Steady-state emission experiments and decay lifetime
measurements were performed on a Horiba Jobin Yvon FluoroLog-3
spectrometer. Cryogenic emission spectra (77 K) and decay lifetimes
were measured in 2-MeTHF cooled with liquid nitrogen. See the
Supporting Information for details.
L
https://dx.doi.org/10.1021/acs.inorgchem.0c01907
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
and Engineering, Zhejiang University, Hangzhou 310027,
Zhejiang, P. R. China
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c01907.
Kun Fang − State Key Laboratory Breeding Base of Green
Chemistry−Synthesis Technology, College of Chemical
Engineering, Zhejiang University of Technology, Hangzhou
310014, Zhejiang, P. R. China; orcid.org/0000-0003-
4449-0219
Qisheng Zhang − MOE Key Laboratory of Macromolecular
Synthesis and Functionalization, Department of Polymer Science
and Engineering, Zhejiang University, Hangzhou 310027,
Zhejiang, P. R. China; orcid.org/0000-0002-0899-6856
X-ray molecular structure, crystal data and structure
refinement for Pt(1-ptz) and Pt(poz), cyclic voltammo-
grams, selected bond lengths, bond angles, and dihedral
angles of the Pd(II) complexes, DFT calculations,
absorption and photoluminescence spectra, comparison
of the photoluminescence spectra of the Pd(II) and
corresponding Pt(II) complexes, detailed synthetic
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c01907
1
procedures, H and 13C NMR and HRMS spectra of
all of the newly developed Pd(II) complexes, and
Cartesian coordinates of the optimized structures (PDF)
Notes
The authors declare no competing financial interest.
Accession Codes
CCDC 1981640 and 2003072 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.
ACKNOWLEDGMENTS
■
The authors gratefully acknowledge the National Natural
Science Foundation of China (Grants 21878276, 21602198,
and 21808204) and the Fundamental Research Funds for the
Provincial Universities of Zhejiang (Grant RF-A2019013) for
supporting this work. We thank Prof. Xiao-Chun Hang at the
Institute of Advanced Materials, Nanjing Tech University, for
support of the partial photophysical data measurements.
AUTHOR INFORMATION
■
Corresponding Authors
Guijie Li − State Key Laboratory Breeding Base of Green
Chemistry−Synthesis Technology, College of Chemical
Engineering, Zhejiang University of Technology, Hangzhou
310014, Zhejiang, P. R. China; orcid.org/0000-0002-
0740-2235; Email: guijieli@zjut.edu.cn
Yun-Fang Yang − State Key Laboratory Breeding Base of Green
Chemistry−Synthesis Technology, College of Chemical
Engineering, Zhejiang University of Technology, Hangzhou
310014, Zhejiang, P. R. China; Email: yangyf@zjut.edu.cn
Yuanbin She − State Key Laboratory Breeding Base of Green
Chemistry−Synthesis Technology, College of Chemical
Engineering, Zhejiang University of Technology, Hangzhou
310014, Zhejiang, P. R. China; orcid.org/0000-0002-
1007-1852; Email: sheyb@zjut.edu.cn
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Xiangdong Zhao − State Key Laboratory Breeding Base of
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