Cationic Iridium Complexes with 5-Phenyl-1 H-1,2,4-triazole Type Cyclometalating Ligands: Toward Blue-Shifted Emission

Article abstract of DOI:10.1021/acs.inorgchem.9b01433

Four cationic iridium complexes with 5-phenyl-1H-1,2,4-triazole (phtz) type cyclometalating ligands (C^N) and different ancillary ligands (N^N), namely, [Ir(dphtz)₂(bpy)]PF₆ (1), [Ir(dphtz)₂(pzpy)]PF₆ (2), [Ir(M

Full text of DOI:10.1021/acs.inorgchem.9b01433

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Cationic Iridium Complexes with 5‑Phenyl‑1H‑1,2,4-triazole Type
Cyclometalating Ligands: Toward Blue-Shifted Emission
Xiaoxiang Wang,† Shirun Wang,† Fangfang Pan,† Lei He,*,† and Lian Duan*,‡
†College of Chemistry, Key Laboratory of Pesticide and Chemical Biology of Ministry of Education, Hubei International Scientific
and Technological Cooperation Base of Pesticide and Green Synthesis, International Joint Research Center for Intelligent
Biosensing Technology and Health, Central China Normal University, Wuhan 430079, People’s Republic of China
‡Key Lab of Organic Optoelectronics and Molecular Engineering of Ministry of Education, Department of Chemistry, Tsinghua
University, Beijing 100084, People’s Republic of China
S
* Supporting Information
ABSTRACT: Four cationic iridium complexes with 5-phenyl-
1H-1,2,4-triazole (phtz) type cyclometalating ligands (C^N)
and different ancillary ligands (N^N), namely, [Ir-
(dphtz)2(bpy)]PF6 (1), [Ir(dphtz)2(pzpy)]PF6 (2), [Ir-
(Mephtz)2(pzpy)]PF6 (3), and [Ir(Mephtz)2(dma-pzpy)]PF6
(4), have been designed, synthesized, and fully characterized
(dphtz = 1-(2,6-dimethylphenyl)-3-methyl-5-phenyl-1H-1,2,4-
triazole, Mephtz = 1,3-dimethyl-5-phenyl-1H-1,2,4-triazole;
bpy = 2,2′-bipyridine, pzpy = 2-(1H-pyrazol-1-yl)pyridine,
dma-pzpy = 4-dimethylamino-2-(1H-pyrazol-1-yl) pyridine).
In solution, complex 1 emits efficient yellow light (λmax = 547
nm), which is blue-shifted by nearly 40 nm (or by 1187 cm−1)
compared with that from the archetypal complex [Ir-
(ppy)2(bpy)]PF6 (Hppy = 2-phenylpyridine), owing to the stabilization of the highest occupied molecular orbital by the
phtz-type C^N ligand. In the lightly doped rigid films, complex 1 emits green light with a high luminescent efficiency of 0.89.
Although complexes 2−4 with electron-rich N^N ligands are weakly emissive or nearly nonemissive in the solution, they emit
relatively strong deep-blue light peaked around 435 and 461 nm in the lightly doped films, which is among the bluest reported
for cationic iridium complexes. Theoretical calculations reveal that for complex 1, the emission always comes from the charge-
transfer (CT) (Ir/C^N → N^N) state; for complexes 2 and 3, the 3CT and C^N-centered 3π−π* states lie close in energy and
the emission could originate from either or both of them; for complex 4, the emission comes predominantly from the C^N-
3
centered π−π* state. For blue-emitting complexes 2−4, metal-centered states play an active role in the nonradiative
deactivation of the emitting triplet states. Solid-state light-emitting electrochemical cells (LECs) based on complexes 1−3 show
yellow-green, blue, and blue-green electroluminescence, respectively, with the yellow-green LEC affording a peak current
efficiency of 21.5 cd A−1.
and 2,2′-bipyridine (bpy) as the ancillary ligand (N^N), emits
orange-red light (λmax = 585 nm in CH3CN).13−15 Because
blue light is indispensable for lighting applications, extensive
research has been conducted for blue-shifting the emission of
complex R through widening its energy gap.16−18 For example,
electron-deficient substituents (such as F) have been placed at
the phenyl rings of ppy to stabilize the highest occupied
molecular orbital (HOMO),19−22 and electron-rich groups
have been attached at bpy to destabilize the lowest unoccupied
molecular orbital (LUMO).23 In addition, using N^N ligands
with strong ligand field strength stabilizes the HOMO to a
large extent,24−26 whereas employing electron-rich or non-
conjugated N^N ligands destabilizes the LUMO signifi-
INTRODUCTION
■
Phosphorescent iridium complexes have aroused significant
research interest in the past years, owing to their wide
applications in biological imaging or sensing,1 photocatalysis,2
and optoelectronic devices,3 to mention but a few. While
neutral iridium complexes have been extensively investigated,
cationic ones have drawn increasing attention in recent years,
owing to their desirable features such as intrinsic ionic nature,
good solubility in polar solvent, and rich redox properties. In
particular, cationic iridium complexes have been employed as
emitting materials in solid-state light-emitting electrochemical
cells (LECs),4−12 which, as simple, low-cost and efficient thin-
film devices, represent one promising technology toward next-
generation solid-state lighting.
The archetypal phosphorescent cationic iridium complex,
namely, [Ir(ppy)2(bpy)]PF6 (complex R in Chart 1), with 2-
phenylpyridine (Hppy) as the cyclometalating ligand (C^N)
Received: May 16, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.9b01433
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Chart 1. Archetypal Examples of Cationic Iridium Complexes (R1-R7) with C^N Ligands That Contain Five-Membered
a
Nitrogen-Rich N-Heterocycles
a
Bn is short for benzyl group. The archetypal complex (R) with conventional ppy C^N ligands is also shown.
Chart 2. Complexes 1−4 Reported in This Work
cantly.16,27−30 All these approaches enlarge the energy gap of
the complex, leading to blue-shifted emission.17
the requirement for such a molecular design. Moreover, phtz-
3
type C^N ligands possess high π−π* triplet energy, which is
It has recently been shown that replacing the pyridine ring in
the ppy-type C^N ligand with a five-membered nitrogen-rich
heterocycle, such as pyrazole,31 triazole,32−34 tetrazole,35,36 or
oxidazole,37,38 can largely blue-shift the emission of the
complex by stabilizing the HOMO in most cases31,34−38 or
by destabilizing the LUMO in some cases.32,33 Chart 1
summarizes these complexes (complexes R1−R7) together
with their emission maximums and luminescent efficiencies in
degassed CH3CN solution. This approach for blue-shifting the
emission is advantageous over the commonly used fluorine-
substitution at the phenyl ring of ppy, because fluorine-
substitution degrades the electrochemical and thermal stability
of the complex.39−41 We note that 5-phenyl-1H-1,2,4-triazole
(phtz), with a five-membered nitrogen-rich triazole ring, fulfills
suitable for constructing blue-emitting complexes.42−44 How-
ever, phtz derivatives have so far not been tested as C^N
ligands for constructing blue-emitting cationic iridium
complexes.
Herein, we report a series of cationic iridium complexes with
phtz-type C^N ligands, i.e., [Ir(dphtz)2(bpy)]PF6 (1), [Ir-
(dphtz)2(pzpy)]PF6 (2), [Ir(Mephtz)2(pzpy)]PF6 (3), and
[Ir(Mephtz)2(dma-pzpy)]PF6 (4; Chart 2). Here 1-(2,6-
dimethylphenyl)-3-methyl-5-phenyl-1H-1,2,4-triazole (dphtz)
and 1,3-dimethyl-5-phenyl-1H-1,2,4-triazole (Mephtz) are
employed as the phtz-type C^N ligands, while bpy, 2-(1H-
pyrazol-1-yl)pyridine (pzpy), and 4-dimethylamino-2-(1H-
pyrazol-1-yl)pyridine (dma-pzpy) are selected as the N^N
ligands. Single-crystal structures have been determined to
B
DOI: 10.1021/acs.inorgchem.9b01433
Inorg. Chem. XXXX, XXX, XXX−XXX

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ClCH2CH2Cl/CH3CH2OH (10/10 mL). The mixture was refluxed
overnight under an argon atmosphere. To the solution, NH4PF6 (648
mg, 3.9 mmol) was added. After being stirred at room temperature for
1 h, the reaction mixture was purified by column chromatography on
silica gel (200−300 mesh) with acetone/CH2Cl2 (1:20) as the eluent,
affording a yellow solid (327 mg, 0.32 mmol). Yield: 80%. Mp > 360
°C (dec.). 1H NMR (acetone-d6, 400 MHz, δ [ppm]): 8.94 (d, J = 8.0
Hz, 2H), 8.38 (t, J = 7.6 Hz, 2H), 8.27 (d, J = 5.2 Hz, 2H), 7.86−7.76
(m, 2H), 7.54 (t, J = 7.6 Hz, 2H), 7.42 (d, J = 7.6 Hz, 4H), 7.03 (t, J
= 7.6 Hz, 2H), 6.83 (t, J = 7.6 Hz, 2H), 6.75 (d, J = 7.2 Hz, 2H), 6.47
(d, J = 7.6 Hz, 2H), 2.13 (s, 6H), 1.93 (s, 6H), 1.71 (s, 6H). 13C
NMR (acetone-d6, 101 MHz, δ [ppm]): 165.10, 158.26, 156.96,
151.05, 149.29, 139.80, 136.15, 135.92, 135.16, 133.14, 131.36,
131.28, 131.12, 129.22, 129.13, 128.56, 124.90, 123.81, 122.90, 28.99,
28.80, 28.60, 28.41, 16.68, 16.35, 11.04. HRMS (ESI, m/z): 873.2941
[M − PF6]+ (calcd. 873.3000). Anal. Found: C, 52.11; H, 4.11; N,
10.94. Anal. Calcd for C44H40F6IrN8P: C, 51.91; H, 3.96; N, 11.01.
Synthesis of [Ir(dphtz)2(pzpy)]PF6 (2). The synthesis of [Ir-
(dphtz)2(pzpy)]PF6 followed a procedure similar to that for
[Ir(dphtz)2(bpy)]PF6, except that pzpy replaced bpy, affording an
verify the chemical structures of the four complexes.
Photophysical and electrochemical characteristics of the
complexes have been systematically investigated with the aid
of theoretical calculations. The use of the complexes as
emitting materials in LECs has also been evaluated.
EXPERIMENTAL SECTION
■
General Information. All solvents and reactants were commer-
cially obtained and used as received. NMR spectra, mass
spectrometry, and elemental analysis were measured with a BRUKER
400/500 NMR spectrometer, an LTQ-ORBITRAP-ETD mass
spectrometer, and an EA3000 elemental analyzer (Eurovector,
Italy), respectively. Absorption spectra, photoluminescence (PL)
spectra, and the PL transient lifetimes were measured with a
Shimadzu UV-2450 spectrophotometer, a HITACHI F-4600
spectrometer, and a transient spectrofluorimeter (Edinburgh Instru-
ments, FLSP920), respectively. The photoluminescent quantum
yields (PLQYs) in solution were measured relative to quinine sulfate
(Φp = 0.545 in 1 M H2SO4). The PLQYs in thin films were measured
with a Hamamatsu C11347 PLQY measurement system equipped
with an integrating sphere. Cyclic voltammetry was conducted with a
RST5200 voltammetric analyzer in dichloromethane (for oxidation of
complexes 1−4 and reduction of complexes 1−3) or N,N-
dimethylformamide (for reduction of complex 4) solutions (10−3
M), and the solutions were degassed with argon before measure-
ments. The cyclic voltammetry was performed at a scan rate of 100
mV s−1 by using a glass-carbon plate, a silver wire, and a platinum wire
as the working, pseudoreference, and counter electrodes, respectively.
Ferrocene and tetrabutylammonium hexafluorophosphate (0.1 M)
were employed as the internal standard and the supporting electrolyte,
respectively.
1
off-white solid. Yield: 72%. Mp > 350 °C (dec.). H NMR (acetone-
d6, 400 MHz, δ [ppm]): 9.22 (d, J = 2.8 Hz, 1H), 8.58 (d, J = 8.0 Hz,
1H), 8.43 (t, J = 7.2 Hz, 1H), 8.07 (d, J = 5.2 Hz, 1H), 7.66 (t, J = 6.8
Hz, 1H), 7.61−7.51 (m, 3H), 7.47−7.38 (m, 4H), 7.04−6.94 (m,
3H), 6.87−6.71 (m, 4H), 6.46 (t, J = 7.6 Hz, 2H), 2.16−2.10 (m,
6H), 2.00 (s, 3H), 1.94 (s, 3H), 1.83 (s, 3H), 1.79 (s, 3H). 13C NMR
(acetone-d6, 101 MHz, δ [ppm]): 165.07, 164.97, 158.16, 158.15,
150.30, 149.53, 148.00, 144.90, 143.97, 141.86, 136.16, 136.14,
135.90, 135.18, 135.16, 133.38, 133.26, 132.31, 131.52, 131.48,
131.29, 131.10, 130.89, 129.21, 129.13, 125.24, 123.77, 123.44,
122.97, 122.78, 113.63, 111.42, 16.67, 16.41, 16.36, 11.26, 11.04.
HRMS (ESI, m/z): 862.2906 [M − PF6]+ (calcd. 862.2952). Anal.
Found: C, 50.31; H, 4.14; N, 12.41. Anal. Calcd for C42H39F6IrN9P:
C, 50.09; H, 3.90; N, 12.52.
Synthesis. pzpy and dma-pzpy were synthesized following
reported procedures.16,45 The chloro-bridged iridium dimers, [Ir-
(dphtz)2Cl]2 and [Ir(Mephtz)2Cl]2, were prepared by reacting dphtz
or Mephtz with IrCl3·nH2O in refluxed water/2-ethoxylethanol (1:3)
under an argon atmosphere according to a reported procedure.46
Synthesis of 1-(2,6-Dimethylphenyl)-3-methyl-5-phenyl-1H-
1,2,4-triazole (dphtz). Benzoyl chloride (2.67 g, 19 mmol) and
ethyl acetimidate hydrochloride (2.35 g, 19 mmol) were dissolved in
chloroform (50 mL). To the solution, triethylamine (3.84 g, 38
mmol) was added dropwise under stirring. After being stirred at room
temperature for 18 h under an argon atmosphere, the reaction mixture
was washed with water and dried over anhydrous sodium sulfate. The
solvent was removed under a vacuum, and the residual was dissolved
in chloroform (50 mL). To the solution, 2,6-dimethylphenylhydrazine
hydrochloride (4.32 g, 25 mmol) and triethylamine (9.60 g, 95 mmol)
were added. The mixture was refluxed overnight under an argon
atmosphere. The solvent was removed under a vacuum, and the
residue was purified by column chromatography on silica gel (200−
300 mesh) with ethyl acetate/petroleum ether (1:10) as the eluent,
affording a white solid (2.34 g, 8.8 mmol). Yield: 46%. Mp: 96−97
°C. 1H NMR (CDCl3, 400 MHz, δ [ppm]): 7.48 (d, J = 7.6 Hz, 2H),
7.39−7.27 (m, 4H), 7.16 (d, J = 7.6 Hz, 2H), 2.55 (s, 3H), 1.98 (s,
6H). 13C NMR (CDCl3, 101 MHz, δ [ppm]): 160.98, 154.45, 136.95,
135.85, 129.99, 129.80, 128.74, 128.69, 127.54, 127.25, 17.62, 14.03.
HRMS (ESI, m/z): 264.1504 [M + H]+ (calcd. 264.1495). Anal.
Found: C, 77.78; H, 6.77; N, 16.08. Anal. Calcd for C17H17N3: C,
77.54; H, 6.51; N, 15.96.
Synthesis of 1,3-Dimethyl-5-phenyl-1H-1,2,4-triazole (Mephtz).
The synthesis of Mephtz is similar to that of dphtz, except that
methylhydrazine sulfate replaced 2,6-dimethylphenylhydrazine hydro-
chloride. Yield: 43%. Mp: 72−73 °C. 1H NMR (CDCl3, 500 MHz, δ
[ppm]): 7.69−7.61 (m, 2H), 7.54−7.46 (m, 3H), 3.93 (s, 3H), 2.43
(s, 3H). 13C NMR (CDCl3, 101 MHz, δ [ppm]): 159.80, 154.95,
129.96, 128.79, 128.55, 127.99, 36.59, 13.81. HRMS (ESI, m/z):
174.1036 [M + H]+ (calcd. 174.1026). Anal. Found: C, 69.65; H,
6.64; N, 24.31. Anal. Calcd for C10H11N3: C, 69.34; H, 6.40; N, 24.26.
Synthesis of [Ir(dphtz)2(bpy)]PF6 (1). [Ir(dphtz)2Cl]2 (300 mg,
0.20 mmol) and bpy (65 mg, 0.42 mmol) were dissolved in
Synthesis of [Ir(Mephtz)2(pzpy)]PF6 (3). The synthesis of [Ir-
(Mephtz)2(pzpy)]PF6 followed a procedure similar to that for
[Ir(dphtz)2(bpy)]PF6, except that [Ir(dphtz)2Cl]2 and bpy were
replaced by [Ir(Mephtz)2Cl]2 and pzpy, respectively, yielding an off-
1
white solid. Yield: 80%. Mp > 350 °C (dec.). H NMR (acetone-d6,
500 MHz, δ [ppm]): 9.15 (d, J = 3.0 Hz, 1H), 8.47 (d, J = 8.4 Hz,
1H), 8.33 (td, J = 8.0, 1.5 Hz, 1H), 7.91 (d, J = 5.0 Hz, 1H), 7.82 (d, J
= 7.5 Hz, 1H), 7.80 (d, J = 7.5 Hz, 1H), 7.53 (t, J = 6.0 Hz, 1H), 7.37
(d, J = 1.5 Hz, 1H), 7.17−7.04 (m, 2H), 7.03−6.89 (m, 3H), 6.55 (d,
J = 7.5 Hz, 1H), 6.52 (d, J = 7.5 Hz, 1H), 4.34 (s, 3H), 4.33 (s, 3H),
1.64 (s, 3H). 1.60 (s, 3H). 13C NMR (acetone-d6, 101 MHz, δ
[ppm]): 164.40, 164.33, 156.08, 150.09, 149.83, 147.33, 144.20,
144.00, 141.51, 133.42, 133.34, 132.08, 132.04, 131.94, 130.40,
130.00, 125.12, 124.99, 124.78, 122.62, 122.46, 113.20, 111.14, 37.47,
10.92, 10.71. HRMS (ESI, m/z): 682.2026 [M − PF6]+ (calcd.
682.2013). Anal. Found: C, 40.85; H, 3.52; N, 15.03. Anal. Calcd for
C28H27F6IrN9P: C, 40.68; H, 3.29; N, 15.25.
Synthesis of [Ir(Mephtz)2(dma-pzpy)]PF6 (4). The synthesis of
[Ir(Mephtz)2(dma-pzpy)]PF6 followed a procedure similar to that of
[Ir(dphtz)2(bpy)]PF6, except that [Ir(dphtz)2Cl]2 and bpy were
replaced by [Ir(Mephtz)2Cl]2 and dma-pzpy, respectively, yielding an
1
off-white solid. Yield: 72%. Mp > 350 °C (dec.). H NMR (acetone-
d6, 500 MHz, δ [ppm]): 9.15 (d, J = 3.0 Hz, 1H), 7.81−7.76 (m, 2H),
7.53 (d, J = 2.5 Hz, 1H), 7.29−7.25 (m, 2H), 7.10−7.00 (m, 2H),
6.99−6.89 (m, 2H), 6.84−6.81 (m, 1H), 6.69 (dd, J = 6.8, 2.6 Hz,
1H), 6.53 (d, J = 7.6 Hz, 2H), 4.34−4.33 (m, 6H), 3.22 (s, 6H), 1.72
(s, 3H), 1.63 (s, 3H). 13C NMR (acetone-d6, 101 MHz, δ [ppm]):
164.67, 164.40, 156.50, 155.94, 155.91, 150.14, 148.29, 147.71,
145.70, 142.88, 133.71, 133.33, 132.31, 132.06, 130.81, 130.17,
129.82, 124.91, 124.65, 122.17, 122.07, 110.57, 107.69, 93.34, 39.01,
37.42, 37.40, 11.18, 10.63. HRMS (ESI, m/z): 725.2413 [M − PF6]+
(calcd. 725.2435). Anal. Found: C, 41.23; H, 3.84; N, 16.01. Anal.
Calcd for C30H32F6IrN10P: C, 41.42; H, 3.71; N, 16.10.
X-ray Crystallography. The X-ray experiments on single crystals
of complexes 1 and 2 were performed with a Bruker Apex Duo CCD
diffractometer at 150 K. The X-ray experiments on single crystals of
C
DOI: 10.1021/acs.inorgchem.9b01433
Inorg. Chem. XXXX, XXX, XXX−XXX

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Scheme 1. Synthesis of C^N Ligands and Complexes 1−4
complexes 3 and 4 were performed with a Rigaku CCD Saturn 724 +
diffractometer at 153 K. The diffractometers were equipped with
graphite monochromatized Mo Kα radiation (λ = 0.71073 Å). The
positions of non-hydrogen atoms were produced by direct phase
determination, which were subjected to anisotropic refinement.
Hydrogen atoms were theoretically produced, riding on their parent
atoms in the refinement. Data analysis was conducted by using the
OLEX2 program.47
Quantum Chemical Calculations. Calculations on the ground
and excited states were conducted with the density functional theory
(DFT) at the B3LYP level.48,49 “Double-ξ” quality basis sets were
used (6-31G** for C, H, N and LANL2DZ for Ir).50 The inner core
electrons of Ir(III) were replaced by an effective core potential
(ECP), and the outer core (5s)2(5p)6 electrons and the (5d)6 valence
electrons of Ir(III) were left.51 Optimization of the geometries of
singlet ground states (S0) were conducted without any symmetry
constraints. Time-dependent DFT (TD-DFT) was used to calculate
the first 10 triplets at the optimized S0 geometries. Optimization of
the geometries of selected triplet states was conducted at the spin-
unrestricted B3LYP level with a spin multiplicity of three. All
calculations were done in the presence of CH3CN solvent. The self-
consistent reaction field (SCRF) theory employing the polarized
continuum (PCM) model was used for the solvent effect.52 The
Gaussian 09 software was used for the above calculations.53 The
Multiwfn program was used for natural transition orbital (NTO)
analysis of triplet transitions.54
organic films at 8−10 Å/s. A Keithley 4200 semiconductor
characterization system was used to measure the current−time−
brightness curves under constant driving voltages. A Photo Research
PR705 spectrophotometer was used to measure the electro-
luminescent (EL) spectra. All devices were sealed in the glovebox
before measurements.
RESULTS AND DISCUSSION
■
Synthesis and X-ray Crystal Structures. Scheme 1
depicts the synthetic pathway to dphtz and Mephtz and
complexes 1−4. dphtz and Mephtz were prepared through a
procedure similar to that reported in the literature.42
Complexes 1−4 were prepared by cleavages of the chloro-
bridged iridium dimers, [Ir(dphtz)2Cl]2 or [Ir(Mephtz)2Cl]2,
with bpy, pzpy, or dma-pzpy N^N ligands, followed by anion-
exchange reactions from Cl− to PF6 . The complexes were
−
purified by column chromatography, and their chemical
1
structures were verified by H and 13C NMR spectroscopies
(Figures S1−S6), high-resolution mass spectrometry, and
elemental analysis (see Experimental Section).
Single crystals of complexes 1−4 suitable for X-ray
experiments were grown by slow diffusion of hexane into
CH2Cl2 solution of complexes 1−4. Figure 1 displays the
single crystal structures. X-ray crystallographic data and
selected crystallographic parameters are presented in Tables
S1 and S2 in the Supporting Information. Analogous to
reported cationic iridium complexes,15,16,31,33−35 complexes
1−4 display distorted octahedral geometries, and the two C^N
ligands adopt a C,C-cis,N,N-trans configuration. The Ir−C and
Ir−N bond lengths and the ligand bite angles fall within the
normal ranges reported for similar cationic iridium com-
plexes.15,16,31,33−35 For complexes 1 and 2, the pendant 2,6-
Fabrication and Characterization of LECs. Indium−tin−oxide
(ITO) substrates (15 Ω/□) were ultrasonically cleaned in detergent
and then in deionized water. Before film depositions, the ITO
substrates were subjected to the UV zone for 10 min. The
PEDOT:PSS buffer layer was spin-coated onto the ITO substrate
and baked at 200 °C on a hot plate for 10 min. The emissive layer was
spin coated on top of PEDOT:PSS from the CH3CN solution in a
nitrogen-filled glovebox and then baked at 70 °C on a hot plate for 30
min. The film-coated substrate was transferred into a vacuum
chamber, and the Al cathode was thermally evaporated onto the
D
DOI: 10.1021/acs.inorgchem.9b01433
Inorg. Chem. XXXX, XXX, XXX−XXX