Cationic Iridium Complexes with 3,4,5-Triphenyl-4 H-1,2,4-Triazole Type Cyclometalating Ligands: Synthesis, Characterizations, and Their Use in Light-Emitting Electrochemical Cells

Article abstract of DOI:10.1021/acs.inorgchem.0c00645

Cationic iridium complexes that show blue-shifted emission and high phosphorescent efficiency have been pursued for their optoelectronic applications. Five cationic iridium complexes with 3,4,5-triphenyl-4H-1,2,4-triazole (tPhTAZ) type cyclometalating ligands (C^N) and 2,2′-bipyridine or 2-(pyridin-2-yl)-1H-benzo[d]imidazole type ancillary ligands (N^N) have been designed and synthesized. Their structures have been confirmed by X-ray crystallography, and their photophysical and electrochemical properties have been comprehensively characterized. In solution and thin films, the complexes afford efficient yellow to blue-green emission. The highest occupied molecular orbitals (HOMOs) of these complexes are delocalized over the C^N ligand and the iridium ion, and compared with the conventional 2-phenylpyridine (Hppy) ligand, the tPhTAZ ligand largely shifts the emission of the complex toward blue by over 40 nm through stabilizing the HOMO. Moreover, the peripheral phenyl rings in tPhTAZ provide steric hindrance to the complexes, which suppresses phosphorescence concentration-quenching of the complexes, leading to high luminescent efficiencies in neat films. Theoretical calculations have shown that the emission of the complexes originates from either the charge-transfer state (Ir/C^N → N^N) or the C^N/N^N-centered 3π-π? state, depending on the local surrounding of the complex. The complexes exhibit good electrochemical stability with reversible oxidation and reduction processes in solution. Solid-state light emitting electrochemical cells (LECs) using the complexes afford yellow to blue-green emission, with peak current efficiencies of up to 34.7 cd A-1 and maximum brightness of up to 256 cd m-2 at 3.0 V, which are among the highest for LECs based on cationic iridium complexes reported so far, indicating the great potential for the use of tPhTAZ-type C^N ligands in construction of cationic iridium complexes for LEC applications.

Full text of DOI:10.1021/acs.inorgchem.0c00645

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Cationic Iridium Complexes with 3,4,5-Triphenyl‑4H‑1,2,4-Triazole
Type Cyclometalating Ligands: Synthesis, Characterizations, and
Their Use in Light-Emitting Electrochemical Cells
Xianwen Meng, Mengzhen Chen, Rubing Bai, and Lei He*
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ABSTRACT: Cationic iridium complexes that show blue-shifted
emission and high phosphorescent efficiency have been pursued for
their optoelectronic applications. Five cationic iridium complexes
with 3,4,5-triphenyl-4H-1,2,4-triazole (tPhTAZ) type cyclometalat-
ing ligands (C^N) and 2,2′-bipyridine or 2-(pyridin-2-yl)-1H-
benzo[d]imidazole type ancillary ligands (N^N) have been
designed and synthesized. Their structures have been confirmed
by X-ray crystallography, and their photophysical and electro-
chemical properties have been comprehensively characterized. In
solution and thin films, the complexes afford efficient yellow to
blue-green emission. The highest occupied molecular orbitals
(HOMOs) of these complexes are delocalized over the C^N ligand
and the iridium ion, and compared with the conventional 2-
phenylpyridine (Hppy) ligand, the tPhTAZ ligand largely shifts the emission of the complex toward blue by over 40 nm through
stabilizing the HOMO. Moreover, the peripheral phenyl rings in tPhTAZ provide steric hindrance to the complexes, which
suppresses phosphorescence concentration-quenching of the complexes, leading to high luminescent efficiencies in neat films.
Theoretical calculations have shown that the emission of the complexes originates from either the charge-transfer state (Ir/C^N →
3
N^N) or the C^N/N^N-centered π−π* state, depending on the local surrounding of the complex. The complexes exhibit good
electrochemical stability with reversible oxidation and reduction processes in solution. Solid-state light emitting electrochemical cells
(LECs) using the complexes afford yellow to blue-green emission, with peak current efficiencies of up to 34.7 cd A−1 and maximum
brightness of up to 256 cd m−2 at 3.0 V, which are among the highest for LECs based on cationic iridium complexes reported so far,
indicating the great potential for the use of tPhTAZ-type C^N ligands in construction of cationic iridium complexes for LEC
applications.
quantum efficiency of up to 100%.2,7−14 Among all the metal
complexes used for LECs, cationic iridium complexes have
drawn the most interest because of their short triplet lifetimes,
INTRODUCTION
■
Solid-state light-emitting electrochemical cells (LECs) are one
of the simplest light-emitting devices that comprise a single
high phosphorescent efficiencies, and tunable emission color
active layer (∼100 nm) sandwiched between two electrodes.1,2
across the entire visible spectrum.7−11,13,14 In general, cationic
The active layer of a LEC comprises a luminescent material
iridium complexes can be expressed as [Ir-
and an ionic additive.1 The mobile ions in the active layer,
(C^N)2(N^N)]+PF6 , where C^N is the cyclometalating
−
which migrate and redistribute under an external electrical
ligand (e.g., 2-phenylpyridine (Hppy)) and N^N is the
field, allow the LEC to work efficiently under low operating
ancillary ligand (e.g., 2,2′-bipyridine (bpy)), with the typical
one, [Ir(ppy)2(bpy)]PF6, emitting orange-red light.15,16
Because of the indispensability of blue emission for fabricating
white LECs, tuning the emission color of the complex (thus
voltages.1,3−5 Compared to traditional organic electrolumines-
cence (EL) devices such as organic light-emitting diodes
(OLEDs), LECs exhibit many attractive features such as a
simple device structure, an easy fabrication by a solution
process, and an inert-metal cathode (e.g., Al or Au).
Altogether, these make them very promising emissive thin-
film devices for next-generation, low-cost, large-area solid state
lighting.6
Received: March 2, 2020
In recent years, LECs made with ionic transition metal
complexes have been extensively researched, because they can
harvest both singlet and triplet excitons, affording an internal
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Scheme 1. Synthesis of the C^N Ligands and Complexes 1−5
the LEC) toward blue has been highlighted in the past decade,
which has been accomplished by engineering the C^N and
N^N ligands, such as substitution of an electron-withdrawing
group (e.g., − F, − CF3) at the phenyl ring of the C^N
ligand14,17−22 or use of electron-rich or high ligand-field N^N
ligands.14,23−27 Moreover, LECs using iridium complexes suffer
from severe phosphorescence concentration quenching in the
emissive layer. Reduction of the phosphorescence concen-
tration quenching to increase the overall performances of LECs
has also been emphasized, which can be realized by
introducing sterically bulky groups to the complexes28−32 or
adopting a host−guest device configuration.33,34
Recently, C^N ligands that have nitrogen-rich, five-
membered heterocycles, such as phenyl-triazole, phenyl-
tetrazole, and phenyl-oxadiazole type ligands, have been widely
explored,35−41 because they can blue-shift the emission of the
complex in comparison with the ppy ligand. By using these
C^N ligands, cationic iridium complexes with largely blue-
shifted emission have been successfully prepared, but their
performances in LECs are still unsatisfactory, largely because
of their strong phosphorescence concentration quenching and/
or reduced electrochemical stability.35−42 It is noted that 3,4,5-
triphenyl-4H-1,2,4-triazole (tPhTAZ; Scheme 1) belongs to
the phenyl-triazole type C^N ligand, which can be readily
prepared and possesses a wide energy band gap and high triplet
energy;43,44 more importantly, tPhTAZ carries two peripheral
phenyl rings which introduce large steric hindrance to the
complex and thus suppress phosphorescence concentration
quenching of the complex in the solid state.28−32 Cationic
iridium complexes using tPhTAZ-type cyclometalating ligands
would afford blue-shifted emission and high phosphorescent
efficiency in the solid state, which appear to be suitable
emitting materials for LECs but have never been reported.
Herein, we report for the first time a series of yellow to blue-
green-emitting cationic iridium complexes with tPhTAZ-type
C^N ligands and bpy or 2-(pyridin-2-yl)-1H-benzo[d]-
imidazole type ancillary ligands (Scheme 1) and their use in
LECs. 2-(Pyridin-2-yl)-1H-benzo[d]imidazole type ancillary
ligands have been used because they are capable of enhancing
the stability of the complex in operating LECs as compared to
bpy.45 The photophysical and electrochemical properties of the
complexes have been comprehensively investigated, and
theoretical calculations have been done to gain insight into
these properties. Compared with the ppy ligand, the tPhTAZ
ligand largely shifts the emission of the complex toward blue by
over 40 nm. In neat films, the complexes show remarkably
suppressed phosphorescence concentration-quenching and
thus high luminescent efficiencies. Moreover, the complexes
exhibit good electrochemical stability with reversible oxidation
and reduction in solution. LECs based on the complexes afford
B
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(tPhTAZ)2Cl]2, except that FtPhTAZ replaced tPhTAZ. The crude
product was used for following reactions without further purification.
Synthesis of [Ir(tPhTAZ)2(bpy)]PF6 (1). [Ir(tPhTAZ)2Cl]2 (0.8 g,
0.42 mmol) and bpy (0.14 g, 0.92 mmol) were dissolved in 1,2-
dichloroethane/ethanol (10/10 mL). The mixture was refluxed under
nitrogen for 12 h. The solvent was removed under reduced pressure.
The residual was extracted with methanol (100 mL), and the
insoluble solid was removed through filtration. To the methanol
solution, NH4PF6 (1.05 g, 6.47 mmol) was added under stirring. The
precipitate was collected and purified by column chromatography on
silica gel with CH2Cl2 as the eluent, yielding a yellow powder (581
yellow to green-blue EL, with high efficiencies of up to 34.7 cd
A−1 and maximum brightness of up to 256 cd m−2 at 3.0 V,
which are the highest for LECs based on complexes with
phenyl-triazole/tetrazole type C^N ligands and also among the
highest for LECs based on cationic iridium complexes reported
so far, indicating the great potential for the use of tPhTAZ-type
C^N ligands in construction of cationic iridium complexes for
optoelectronic applications.
EXPERIMENTAL SECTION
1
■
mg, 0.54 mmol). Yield: 64%. Mp > 360 °C (dec.). H NMR (400
General Information. Reactants and solvents for synthesis or
measurements were bought from commercial sources and used as
received. Mass spectrometry and elemental analysis were recorded on
a LTQ-ORBITRAP-ETD mass spectrometer and an EA3000
MHz, CDCl3, δ [ppm]): 8.59 (d, J = 8.0 Hz, 2H), 8.20 (d, J = 4.4 Hz,
2H), 8.14 (td, J = 7.8, 1.5 Hz, 2H), 7.71−7.61 (m, 6H), 7.52 (d, J =
7.2 Hz, 2H), 7.44−7.41 (m, 4H), 7.35−7.20 (m, 10H), 6.98 (td, J =
7.5, 1.1 Hz, 2H), 6.75 (t, J = 7.2 Hz, 2H), 6.70 (d, J = 7.6 Hz, 2H),
6.44 (d, J = 7.2 Hz, 2H). 13C NMR (151 MHz, DMSO-d6, δ [ppm]):
163.64, 156.72, 153.91, 152.04, 151.27, 139.63, 133.83, 133.55,
131.82, 131.19, 131.13, 131.02, 130.96, 129.12, 128.96, 128.92,
128.65, 128.37, 125.68, 124.41, 123.47, 122.26. HRMS (ESI, m/z):
941.2649 [M−PF6]+ (calcd 941.2687). Anal. found: C, 55.62; H,
3.48; N, 10.45. Anal. calcd for C50H36F6IrN8P: C, 55.30; H, 3.34; N,
10.32.
1
elemental analyzer (Eurovector, Italy), respectively. H NMR spectra
and 13C NMR spectra were measured on a BRUKER 400/500 NMR
spectrometer and a BRUKER AMX 600-MHz NMR spectrometer,
respectively. Absorption and photoluminescence (PL) spectra were
measured with a UV−vis spectrophotometer (UV-1800, Shanghai
Macy Instrument Co., Ltd.) and a fluorospectrophotometer (F-320,
Tianjin Gangdong Sci. and Tech. Development Co., Ltd.),
respectively. Transient lifetimes for PL were measured with a
steady/transient state spectrofluorometer (FLS1000, Edinburgh
Instruments). Photoluminescent quantum yields (PLQYs) in
degassed solution were measured with quinine sulfate (PLQY =
0.545 in 1 M H2SO4) as the reference. The absolute PLQYs in doped
or neat films were determined on the steady/transient state
spectrofluorometer with an integrating sphere (FLS1000, Edinburgh
Instruments). Experimental details for cyclic voltammetry, X-ray
crystallography, theoretical calculations, and fabrication and character-
ization of LECs have been presented in the Supporting Information.
Synthesis. 2,5-Diphenyl-1,3,4-oxadiazole was purchased from Alfa
Aesar. 2,5-Bis(4-fluorophenyl)-1,3,4-oxadiazole, 1-methyl-2-(pyridin-
2-yl)-1H-benzo[d]imidazole (mepybi), and 1-phenyl-2-(pyridin-2-yl)-
1H-benzo[d]imidazole (phpybi) were synthesized by reported
procedures.45−47
Synthesis of [Ir(tPhTAZ)2(mepybi)]PF6 (2). The synthesis of
complex 2 followed a procedure similar to that for complex 1, except
that bpy was replaced with mepybi. Yield: 38%. Mp > 360 °C (dec.).
1H NMR (400 MHz, acetone-d6, δ [ppm]): 8.87 (d, J = 7.6 Hz, 1H),
8.48 (d, J = 4.4 Hz, 1H), 8.35 (t, J = 8.0 Hz, 1H), 7.88 (d, J = 8.44 Hz,
1H), 7.81−7.65 (m, 10H), 7.65−7.65 (m, 1H), 7.49 (t, J = 7.6 Hz,
1H), 7.45−7.20 (m, 10H), 7.10 (t, J = 8.0 Hz, 1H), 7.03−6.95 (m,
3H), 6.87 (d, J = 7.2 Hz, 1H), 6.82−6.75 (m, 2H), 6.68 (d, J = 8.0
Hz, 1H), 6.55−6.47 (m, 2H), 4.64 (s, 3H). 13C NMR (151 MHz,
DMSO-d6, δ [ppm]): 163.81, 163.67, 153.89, 153.79, 153.35, 153.27,
152.16, 148.34, 147.76, 140.59, 139.63, 136.74, 134.55, 133.81,
133.78, 133.42, 131.77, 131.44, 131.41, 131.16, 131.11, 131.05,
130.98, 130.94, 130.89, 130.28, 129.07, 129.04, 128.96, 128.82,
128.69, 128.65, 128.35, 125.72, 125.67, 125.65, 124.94, 123.54,
123.35, 122.18, 122.09, 118.15, 112.82, 33.64. HRMS (ESI, m/z):
994.2888 [M−PF6]+ (calcd 994.2952). Anal. found: C, 56.07; H,
3.57; N, 11.03. Anal. calcd for C53H39F6IrN9P: C, 55.88; H, 3.45; N,
11.07.
Synthesis of 3,4,5-Triphenyl-4H-1,2,4-triazole (tPhTAZ). 2,5-
Diphenyl-1,3,4-oxadiazole (2.0 g, 9.0 mmol) and aniline hydro-
chloride (5.8 g, 4.5 mmol) were suspended in N-cyclohexyl-2-
pyrrolidone (5 mL) in a sealed tube. The mixture was stirred at 160
°C under nitrogen for 12 h. To the solution, HCl solution (pH = 2−
3) was added. The precipitate was collected and then recrystallized
from ethanol, affording the pure product as a white solid (2.1 g, 7.1
mmol). Yield: 79%. Mp: 203−204 °C. 1H NMR (500 MHz, DMSO-
d6, δ [ppm]): 7.50−7.35 (m, 15 H). 13C NMR (151 MHz, CDCl3, δ
[ppm]): 154.72, 135.10, 129.99, 129.77, 129.69, 128.86, 128.45,
127.82, 126.69. HRMS (ESI, m/z): 298.1348 [M + H]+ (calcd
298.1339).
Synthesis of [Ir(tPhTAZ)2(phpybi)]PF6 (3). The synthesis of
complex 3 followed a procedure similar to that for complex 1, except
that bpy was replaced with phpybi. Yield: 52%. Mp > 360 °C (dec.).
1H NMR (400 MHz, DMSO-d6, δ [ppm]): 8.33 (d, J = 5.2 Hz, 1H),
8.05 (t, J = 8.0 Hz, 1H), 7.91−7.80 (m, 4H), 7.76−7.60 (m,12H),
7.43−7.26 (m, 12H), 7.2−27.14 (m, 2H), 7.09−7.01 (m, 2H), 6.91
(d, J = 7.6 Hz, 1H), 6.85 (t, J = 7.6 Hz, 1H), 6.82−6.71 (m, 2H), 6.57
(d, J = 8.4 Hz, 1H), 6.39 (d, J = 7.6 Hz,1H), 6.30 (d, J = 8.0 Hz, 1H).
13C NMR (151 MHz, DMSO-d6, δ [ppm]): 163.76, 163.69, 153.93,
153.89, 153.35, 153.02, 151.37, 147.95, 147.37, 140.57, 139.41,
137.69, 134.70, 134.37, 133.84, 133.79, 133.35, 131.90, 131.82,
131.78, 131.46, 131.21, 131.10, 131.03, 130.98, 130.92, 130.34,
129.13, 129.10, 129.02, 129.01, 128.81, 128.77, 128.73, 128.67,
128.48, 128.27, 126.82, 125.73, 125.70, 125.60, 124.49, 123.57,
123.41, 122.27, 118.47, 112.59. HRMS (ESI, m/z): 1056.3031 [M−
PF6]+ (calcd 1056.3109). Anal. found: C, 58.29; H, 3.58; N, 10.56.
Anal. calcd for C58H41F6IrN9P: C, 57.99; H, 3.44; N, 10.49.
Synthesis of [Ir(FtPhTAZ)2(bpy)]PF6 (4). The synthesis of complex
4 followed a procedure similar to that for complex 1, except that
[Ir(tPhTAZ)2Cl]2 was replaced with [Ir(FtPhTAZ)2Cl]2. Yield: 33%.
Synthesis of 3,5-Bis(4-fluorophenyl)-4-phenyl-4H-1,2,4-triazole
(FtPhTAZ). The synthesis of FtPhTAZ followed the same procedure
as that for tPhTAZ, except that 2,5-bis(4-fluorophenyl)-1,3,4-
oxadiazole replaced 2,5-diphenyl-1,3,4-oxadiazole. Mp: 220−221 °C.
1H NMR (500 MHz, DMSO-d6, δ [ppm]): 7.55−7.37 (m, 9H), 7.23
(t, J = 7.8 Hz, 4H). 13C NMR (151 MHz, DMSO-d6, δ [ppm]):
164.02, 162.37, 154.00, 135.04, 131.37, 131.31, 130.41, 128.78,
124.01, 123.99, 116.18, 116.04. HRMS (ESI, m/z): 334.1161 [M +
H]+ (calcd 334.1150).
Synthesis of [Ir(tPhTAZ)2Cl]2. The chloro-bridged iridium dimers
were prepared by following reported procedures.48,49 tPhTAZ (1.0 g,
3.4 mmol) and IrCl3·nH2O (0.5 g, 1.5 mmol) were suspended in 2-
ethoxyethanol/H2O (24/8 mL). The mixture was stirred at 110 °C
under nitrogen for 24 h. The solvent was removed under reduced
pressure. The residual was washed with ethanol (50 mL) under
ultrasonication, affording the crude product as a yellow solid (1.4 g),
which was used for the following reactions without further
purification.
1
Mp > 370 °C (dec.). H NMR (400 MHz, CD3CN, δ [ppm]): 8.51
(d, J = 8.0 Hz, 2H), 8.29−8.22 (m, 2H), 8.16 (t, J = 7.6 Hz, 2H),
7.79−7.62 (m, 8H), 7.61 (d, J = 7.0 Hz, 2H), 7.51 (d, J = 7.6 Hz,
2H), 7.35−7.25 (m, 4H), 7.02 (t, J = 8.0 Hz, 4H), 6.65−6.55 (m,
4H), 6.47 (t, J = 7.0 Hz, 2H). 13C NMR (151 MHz, DMSO-d6, δ
[ppm]): 164.39, 164.37, 162.72, 162.69, 156.59, 154.63, 154.60,
153.22, 152.09, 139.96, 133.46, 132.00, 131.45, 131.39, 131.29,
131.16, 128.86, 128.69, 128.62, 127.70, 125.51, 125.44, 124.49,
122.16, 122.14, 120.10, 119.99, 116.44, 116.29, 109.88, 109.72.
Synthesis of [Ir(FtPhTAZ)2Cl]2. The synthesis of [Ir-
(FtPhTAZ)2Cl]2 followed the same procedure as that for [Ir-
C
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Figure 1. Crystal structures of (a) complex 1, (b) complex 3, and (c) complex 4. For clarity, PF6 counteranions, solvent molecules, and all
hydrogen atoms were omitted. Thermal ellipsoids were drawn at 35% probability.
HRMS (ESI, m/z): 1013.2229 [M−PF6]+ (calcd 1013.2310). Anal.
found: C, 52.03; H, 2.99; N, 9.75. Anal. calcd for C50H32F10IrN8P: C,
51.86; H, 2.79; N, 9.68.
found: C, 52.87; H, 3.28; N, 9.57. Anal. calcd for C52H36F10IrN8P: C,
52.66; H, 3.06; N, 9.45.
Synthesis of [Ir(FtPhTAZ)2(dmebpy)]PF6 (5). The synthesis of
complex 5 followed a procedure similar to that for complex 1, except
that [Ir(tPhTAZ)2Cl]2 and bpy were replaced with [Ir-
(FtPhTAZ)2Cl]2 and 4,4′-dimethyl-2,2′-bipyridine (dmebpy), respec-
tively. Yield: 33%. Mp: > 370 °C (dec.). 1H NMR (400 MHz,
CD3CN, δ [ppm]): 8.35 (s, 2H), 8.04 (d, J = 5.6 Hz, 2H), 7.75−7.60
(m, 8H), 7.51 (d, J = 7.2 Hz, 2H), 7.38 (d, J = 7.4 Hz, 2H), 7.34−
7.25 (m, 4H), 7.03 (t, J = 8.0 Hz, 4H), 6.60−6.50 (m, 4H), 6.45 (t, J
= 7.0 Hz, 2H), 2.58 (s, 6H). 13C NMR (151 MHz, DMSO-d6, δ
[ppm]): 164.39, 164.35, 162.74, 162.67, 156.30, 155.04, 155.00,
153.24, 151.52, 151.20, 133.44, 131.96, 131.60, 131.54, 131.27,
131.13, 129.08, 128.89, 128.67, 127.72, 125.47, 125.40, 125.04,
122.16, 122.14, 120.09, 119.98, 116.43, 116.28, 109.75, 109.59, 21.38.
HRMS (ESI, m/z): 1041.2545 [M−PF6]+ (calcd 1041.2623). Anal.
RESULTS AND DISCUSSION
■
Synthesis and Structural Characterizations. As shown
in Scheme 1, tPhTAZ and FtPhTAZ can be readily synthesized
by condensation reactions between phenyl-oxadiazole pre-
cursors and aniline hydrochloride. The chloro-bridged iridium
dimer intermediates, [Ir(tPhTAZ)2Cl]2 or [Ir(FtPhTAZ)2Cl]2,
were synthesized by following typical procedures. Complexes
1−5 were then readily synthesized by cleaving the dimer with
selected N^N ligands and exchanging the counteranion from
Cl− to PF6 . Purification of the complexes were performed by
−
column chromatography and following recrystallization from
CH2Cl2/hexane. The complexes were characterized by high-
D
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Figure 2. Selected packing patterns of complex 1 in the single crystal. (a) C^N/C^N packing and (b) N^N/N^N packing. For clarity, the PF6
counteranions, hydrogen atoms, and solvent molecules were omitted.
resolution mass spectrometry, NMR spectroscopy (Figures
S1−S7), and elemental analysis.
Suitable single crystals of complexes 1, 3, and 4 were grown
from slow diffusion of hexane into dichloromethane solution,
slow evaporation of acetonitrile solution, and slow evaporation
of mixed acetone/CH3OH solution, respectively, and were
subjected to X-ray diffraction experiments. Figure 1 depicts the
single-crystal structures. Similar to analogous com-
plexes,35,38,41,50 the iridium cations of complexes 1, 3, and 4
present distorted octahedral geometries, with the two C^N
ligands exhibiting C,C-cis and N,N-trans configurations. The
pertinent bond lengths and ligand bite angles fall within the
ranges reported for similar complexes (Table S1).
As shown in Figure 2, in the single crystal, complex 1 can
pack together either from the C^N ligand side or from the
N^N ligand side. In the C^N/C^N packing pattern (Figure
2a), the shortest Ir-to-Ir distance is 10.2 Å, whereas in the
N^N/N^N packing pattern (Figure 2b), the Ir-to-Ir distance is
8.0 Å. These large Ir-to-Ir distances indicates that the
complexes are largely separated from each other in solid
state.28,29,32 In the N^N/N^N packing pattern, the two bpy
N^N ligands exhibit an offset π−π stacking interaction, with a
centroid-to-centroid distance of 3.7 Å and a centroid-to-plane
distance of 3.5 Å. Because electron-transport between cationic
iridium complexes in films occurs through electron-hopping
between neighboring electron-deficient N^N ligands, this π−π
stacking interaction benefits the electron-transport in LECs
using complex 1. In the single-crystal of complex 4, similar
large Ir-to-Ir distances (9.6 and 8.7 Å) and an offset π−π
stacking interaction between bpy N^N ligands were observed
(Figure S8). In the single crystal of complex 3, even larger Ir-
to-Ir distances (13.7 and 13.0 Å) were observed, largely
because of the additional steric hindrance provided by the
peripheral phenyl ring in the phpybi ancillary ligand (Figure
S9). The single crystal structures show that the bulky tPhTAZ-
type ligands effectively increase the intercomplex distances and
thus reduce intercomplex interactions in solid state owing to
the steric hindrance provided by the peripheral phenyl rings in
the ligands.
Figure 3. Absorption and PL spectra of complexes 1−5 in CH3CN
solution.
transfer), 3LLCT, 3MLCT, and ligand-centered 3π−π*
transitions,15,50,51 as supported by theoretical calculations
(vide infra). The spin-forbidden triplet transitions are partially
permitted owing to the heavy atom effect of iridium atom.
In CH3CN solution, complex 1 affords a yellow emission
centered at 543 nm, which is blue-shifted by 42 nm compared
to that (λmax = 585 nm) of the archetypal complex
[Ir(ppy)2(bpy)]PF6.15,16 This emission blue-shift reveals that
the tPhTAZ ligand can blue-shift the emission of the complex
as compared to the ppy ligand, which is similar to the blue-
shifting effect (by 25−65 nm) observed for other phenyl-
triazole type C^N ligands.35,36,41,52 Complexes 2 and 3 with
pyridine-benzo[d]imidazole N^N ligands (mepybi and phpy-
bi) also emit yellow light centered at 547 and 556 nm,
respectively, which agrees with the previous report that the
pyridine-benzo[d]imidazole N^N ligand resembles bpy in
terms of tuning the emission color of the complex.47 With
FtPhTAZ as the C^N ligand, complex 4 emits green light
peaked at 518 nm, which is blue-shifted by 25 nm compared
with that of complex 1 because fluorination of the C^N ligand
always results in blue-shift in emission.14,18,20,21 Complex 5
emits green-blue light peaked at 508 nm, which is further blue-
shifted by 10 nm compared with that from complex 4, because
of the use of more electron-rich dmebpy as the ancillary
ligand.23,24,53 As shown in Figure 3, complexes 1−5 all show
almost featureless PL spectra, which indicates strong charge-
transfer character involved in their emitting triplet states in the
CH3CN solution.15,50,51 At 77 K in the CH3CN glass, the PL
spectra become highly structured (Figure S10), which implies
Photophysical Characterizations. Figure 3 presents the
absorption and PL spectra of complexes 1−5 in CH3CN
solution, and Table 1 summarizes detailed photophysical
characteristics. Complexes 1−5 all show intensitve absorption
bands (ε > 104 M−1 cm−1) in the ultraviolet region (<350 nm),
1
which can be ascribed to ligand-centered π−π* transitions of
3
both C^N and N^N ligands. In addition, they afford weak
that at low temperature, considerable ligand-centered π−π*
absorption bands (ε < 104 M−1 cm−1) that extend to the visible
character develops in the emission as a result of the remarkable
destabilization of charge-transfer state in the frozen, rigid
matrix.35,50,54,55 It is further noted that the emission at 77 K
1
region. These weak bands can be assigned to LLCT (ligand-
to-ligand charge-transfer), 1MLCT (metal-to-ligand charge-
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Table 1. Photophysical and Electrochemical Characteristics of Complexes 1−5 in solution
b
c
d
PL at room temperature
PL at 77 K
electrochemical data
a
absorption λ [nm] (ε [×104 M−1 cm−1])
λ [nm]
Φp
τ [μs]
kr [105 s−1
]
knr [105 s−1
]
λ [nm]
Eox [V]
Ered [V]
1
2
3
4
5
248 (4.41), 358 (0.94), 382 (0.59)
260 (3.85), 314 (2.68), 379 (0.78)
243 (4.03), 315 (2.69), 383 (0.77)
246 (4.11), 345 (0.88), 362 (0.71)
249 (4.72), 345 (1.00), 358 (0.88)
543
547
556
518
508
0.25
0.23
0.19
0.39
0.45
1.24
2.32
1.93
1.58
2.0
1.0
1.0
2.5
3.1
6.1
3.3
4.2
3.9
3.8
508, 539
525, 563
554, 575
489, 517
483, 510
0.93
0.86
0.83
1.13
1.01
−2.00
−2.13
−2.08
−1.91
−2.13
1.45
b
a
In acetonitrile solution (10−5 M). ε denotes molar extinction coefficient. In degassed acetonitrile solution (10−5 M); kr and knr were calculated
c
d
based on the equations τ = 1/(kr + knr) and Φp = kr/(kr + knr). In acetonitrile glass. Potentials were quoted referred to Fc+/Fc (Fc represents
ferrocene).
Table 2. Photophysical Characteristics of Complexes 1−5 in Thin Films
PL in 2 wt % doped PMMA films
neat film
a
a
λ [nm]
Φp
τ [μs]
kr [105 s−1
]
knr [105 s−1
]
λ [nm]
Φp
τ [μs]
1
2
3
4
5
501
0.81
0.80
0.83
0.80
0.76
1.20
2.26
2.00
1.29
1.55
6.8
3.5
4.2
6.2
4.9
1.6
0.9
0.8
1.6
1.6
540
551
559
512
502
0.56
0.58
0.60
0.62
0.57
0.34 (12%), 0.98 (88%)
0.46 (13%), 1.25 (87%)
0.42 (13%), 1.12 (87%)
0.30 (13%), 0.91 (87%)
0.34 (19%), 0.85 (81%)
523, 541
527, 551
472, 490
466, 484
a
kr and knr were calculated based on the equations τ = 1/(kr + knr) and Φp = kr/(kr + knr).
only exhibits a small to moderate rigidochromic blue-shift (by
2−35 nm) compared to that at room temperature (Table 1).
This further suggests that the emission of complexes 1−5 at 77
K involves considerable ligand-centered 3π−π* character,
because charge-transfer emission, such as that from [Ir-
(ppy)2(bpy)]PF6, usually exhibits a quite large rigidochromic
blue-shift (by >60 nm).56
addition to the charge-transfer character, considerable ligand-
centered π−π* character is involved in their emitting triplet
3
states. As shown in Table 2, complexes 1−5 in the doped
PMMA films afford high luminescent efficiencies at 0.76−0.83,
owing to the significant suppression of nonradiative
deactivations in the rigid solid films. Similar to that observed
in the solution, in the doped PMMA films, complexes 2 and 3
show relatively longer excited-state lifetimes (2.26 and 2.00 μs,
respectively) and smaller kr values (3.5 × 105 s−1 and 4.2 × 105
s−1, respectively) than complex 1 (1.20 μs and 6.8 × 105 s−1),
which further suggests considerable ligand-centered 3π−π*
character in the emitting triplet states of complexes 2 and 3.
Figure 4 shows the PL spectra of complexes 1−5 in neat
films. Complexes 1−5 in neat films all show almost featureless
In the degassed CH3CN solution, complexes 1−5 afford
PLQY values of 0.25, 0.23, 0.19, 0.39, and 0.45, respectively,
and excited-state lifetimes of 1.24, 2.32, 1.93, 1.58, and 1.45 μs,
respectively. From these parameters, the radiative (kr) and
nonradiative (knr) decay rates were calculated and listed in
Table 1. Although complexes 1−3 all show yellow emission
and similar PLQY values, complexes 2 and 3 show much
longer excited-state lifetimes (2.32 and 1.93 μs, respectively)
than complex 1 (1.24 μs). Compared to complex 1, complexes
2 and 3 show smaller kr and knr values. For complexes 2 and 3,
their relatively long excited-state lifetimes and small kr values
(1.0 × 105 s−1) suggest that their emitting triplet states should
contain considerable ligand-centered 3π−π* character.54
Compared to complex 1, complexes 4 and 5 show higher
PLQY, larger kr and smaller knr values (Table 1). The smaller
knr values of complexes 4 and 5 agree with the energy gap law,
which predicts that the blue-shift in emission is accompanied
with the reduction of nonradiative deactivation among a series
of similar transition-metal complexes.57,58
For evaluating the potential use of the complexes in light-
emitting devices, emission properties of the complexes in thin
films were characterized. Table 2 summarizes detailed emission
characteristics in films. In the 2 wt % doped poly(methyl
methacrylate) (PMMA) films, complex 1 affords green-blue
emission peaked at 501 nm. Complexes 2 and 3 emit green
light peaked at 523 and 527 nm, respectively, and complexes 4
and 5 emit blue-green light peaked at 490 and 484 nm,
respectively (Figure S11). In the doped PMMA films, complex
1 exhibits an almost featureless PL spectrum (Figure S11),
consistent with the predominant charge-transfer character in its
emitting triplet state; complexes 2−5 afford PL spectra with
small vibronic structures (Figure S11), which indicates that in
Figure 4. PL spectra of complexes 1−5 in neat films.
PL spectra centered at 540, 551, 559, 512, and 502 nm,
respectively, which are red-shifted by 20−40 nm compared
with those recorded in doped PMMA films (Table 2). The
luminescent efficiencies of the complexes in the neat films
remain as high as 0.56−0.62, which indicates that the
intercomplex interactions and phosphorescence concentra-
tion-quenching are significantly suppressed, owing to the steric
hindrance provided by the peripheral phenyl rings in tPhTAZ
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or FtPhTAZ ligands.28−30 The reduction of phosphorescence
concentration-quenching benefits the application of the
complexes in LECs.
Electrochemical Characterizations. Cyclic voltammetry
was used to measure electrochemical properties of complexes
1−5 in solution. Figure 5 depicts the cyclic voltammograms.
lize the HOMO level of the complex.36,38−41,50 As a result,
complex 1 exhibits an enlarged energy gap with respect to
[Ir(ppy)2(bpy)]PF6, which results in the blue-shifted emission
for complex 1. Compared with complex 1, complex 4 exhibits a
further stabilized HOMO level (by 0.2 eV) and thereby a
further enlarged energy gap and further blue-shifted emission,
owing to the substitution of electron-withdrawing fluorine at
the cyclometalated phenyl ring of the C^N ligand.18,21,53
Because of the use of more electron-rich dmebpy N^N ligand,
complex 5 exhibits a destabilized (by 0.22 eV) LUMO level
and thus further blue-shifted emission compared to complex
4.23,24,53,59
Theoretical Calculations. To obtain deep understanding
into the photophysical and electrochemical properties,
quantum chemical calculations based on density functional
theory (DFT) were performed on complexes 1−5 in CH3CN
solvent (see experimental details in Supporting Information).
Figure 6 depicts energy levels and surface distributions of
frontier molecular orbitals for complexes 1−5 at optimized S0
geometries. The geometrical parameters of optimized S0
structures were compiled in Table S1. For complexes 1, 3,
and 4, these parameters accord well with those found in single
crystal structures (Table S1). As shown in Figure 6, the
HOMO orbitals predominantly distribute on the C^N ligands
and the iridium ions, whereas the LUMO orbitals reside almost
exclusively on the N^N ligands. Surface distributions of other
molecular orbitals (including HOMO−3, HOMO−2,
HOMO−1, LUMO+1, LUMO+2, and LUMO+3) were
presented in Table S2. The HOMO−1, HOMO−2, and
HOMO−3 orbitals of complexes 1−5 are mainly delocalized
over the iridium ions and the C^N ligands, except that the
HOMO−3 orbitals of complexes 2 and 3 show partial
delocalization on the N^N ligands. For complexes 1−3 and
5, the LUMO+1 and LUMO+2 orbitals mainly distribute on
the C^N ligands and the LUMO+3 orbitals predominantly
distribute on the N^N ligands. For complex 4, the LUMO+1
and LUMO+3 orbitals mainly distribute on the C^N ligands,
and the LUMO+2 mainly distributes on the N^N ligand.
Time-dependent DFT (TDDFT) calculations were carried
out at the optimized S0 geometries to reveal the character of
excited states. The singlet states were first calculated and
compiled in Table S3. The absorption spectra were then
Figure 5. Cyclic voltammograms for complexes 1−5 in solution. The
small peaks at about −1.4 V arose from some unknown impurities in
the electrolytes, and similar peaks appeared in the blank test.
Redox potentials are summarized in Table 1. As shown in
Figure 5, complex 1 exhibits a quasi-reversible oxidation
process, with an oxidation potential at 0.93 V; complexes 2−4
exhibit reversible oxidation processes, with oxidation potentials
at 0.86, 0.83, 1.13, and 1.01 V, respectively. Complexes 1−5 all
show reversible reduction processes, with reduction potentials
at −2.00, −2.13, −2.08, −1.91, and −2.13 V, respectively.
According to EHOMO = −(4.8 + Eox) eV and ELUMO = −(4.8 +
Ered) eV, HOMO levels of complexes 1−5 were calculated at
−5.73, −5.66, −5.63, −5.93, and −5.81 eV, respectively, and
LUMO levels were calculated at −2.80, −2.67, −2.72, −2.89,
and −2.67 eV, respectively.
From reported electrochemical data,59 the HOMO and
LUMO levels of the archetypal complex [Ir(ppy)2(bpy)]PF6
were calculated at −5.63 eV and −2.80 eV, respectively.
Compared with [Ir(ppy)2(bpy)]PF6, complex 1 exhibits a
stabilized (by 0.1 eV) HOMO level, which agrees with
previous reports that, compared to the ppy ligand, most C^N
ligands with nitrogen-rich, five-membered heterocycles stabi-
Figure 6. Energy levels and surface distributions of HOMO (lower images) and LUMO (upper images) orbitals at optimized S0 geometries
calculated for complexes 1−5.
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Table 3. Characteristics of Selected Triplet States of Complexes 1−5 Calculated by the TDDFT Approach
a
b
state E (eV)
dominant transitions
dominant nature
dπ(Ir)-π(tPhTAZ) → π*(bpy)
dπ(Ir)-π(tPhTAZ) → π*(bpy)
1
2
3
4
T1
T2
T3
T1
T2
T3
T1
T2
T3
T1
T2
T3
T4
T1
T2
T3
2.64
2.94
2.95
2.61
2.73
2.95
2.61
2.73
2.94
2.74
2.99
3.00
3.01
2.80
3.00
3.01
H→L (96%)
H-2→L (48%), H-10→L (23%), H-4→L (13%)
H→L+1 (43%), H-1→L+2 (20%)
H→L (56%), H-3→L (13%), H-1→L (12%)
H→L (42%), H-1→L (25%), H-3→L (14%), H-2 →L (10%)
H→L+1 (47%), H-1→L+2 (12%)
H→L (64%), H-3→L (12%)
H→L (33%), H-1→L (24%), H-3→L (18%), H-2→L (12%)
H→L+1 (47%), H-1→L+2 (14%)
H→L (92%)
H-8→L (26%), H-2→L (23%), H-4→L (12%)
H-1→L (67%)
H→L+1 (30%), H-1→L+3 (18%), H-2→L (10%)
H→L (88%)
dπ(Ir)-π(tPhTAZ) → π*(tPhTAZ)
dπ(Ir)-π(tPhTAZ) → π*(mepybi), dπ(Ir)-π(mepybi) → π*(mepybi),
dπ(Ir)-π(tPhTAZ) → π*(mepybi), dπ(Ir)-π(mepybi) → π*(mepybi),
dπ(Ir)-π(tPhTAZ) → π*(tPhTAZ)
dπ(Ir)-π(tPhTAZ) → π*(phpybi), dπ(Ir)-π(phpybi) → π*(phpybi)
dπ(Ir)-π(tPhTAZ) → π*(phpybi), dπ(Ir)-π(phpybi) → π*(phpybi)
dπ(Ir)-π(tPhTAZ) → π*(tPhTAZ)
dπ(Ir)-π(FtPhTAZ) → π*(bpy)
dπ(Ir)-π(FtPhTAZ) → π*(bpy)
dπ(Ir)-π(FtPhTAZ) → π*(bpy)
dπ(Ir)-π(FtPhTAZ) → π*(FtPhTAZ)
5
dπ(Ir)-π(FtPhTAZ) → π*(dmebpy)
dπ(Ir)-π(FtPhTAZ) → π*(FtPhTAZ)
dπ(Ir)-π(FtPhTAZ) → π*(FtPhTAZ)
H→L+1 (41%), H-1→L+2 (22%)
H-1→L+1 (32%), H→L+2 (28%), H-1→L (11%)
a
b
Vertical excitation energies. Dominant monoexcitations that have contributions (within parentheses) larger than 10%; H and L represent the
HOMO and LUMO, respectively.
Figure 7. Optimized geometries and unpaired-electron spin-density contours (0.002e bohr−3) calculated for the T1 states of (a) complex 1, (b)
complex 2, (c) complex 3, (d) complex 4, and (e) complex 5.
simulated based on the calculated singlet states, which accord
well with the experimentally measured absorption spectra
(Figure S12). For complex 1, the low-lying S1, S2, S3, S6, and S7
states arise from HOMO−3/HOMO−2/HOMO−1/HOMO
→ LUMO/LUMO+3 transitions, thus exhibiting predominant
1MLCT/1LLCT (Ir/tPhTAZ → bpy) character, and the S4, S5
states arise from HOMO → LUMO+1/LUMO+2 transitions,
thereby exhibiting 1MLCT (Ir → tPhTAZ) and tPhTAZ-
character. The photophysical characterizations have revealed
that complex 1 affords emission with dominant charge-transfer
character in both solution and films, whereas complexes 2 and
3 afford emission with charge-transfer as well as some ligand-
centered π−π* character in both solution and films. These
experimental results, together with the TDDFT calculation
3
results, indicate that for complex 1, the emission arises from
3
the T1 state, i.e., mixed MLCT (Ir → bpy)/3LLCT (tPhTAZ
1
→ bpy) state, and for complexes 2 and 3, the emission also
arises from the T1 state, i.e., mixed 3MLCT (Ir →
N^N)/3LLCT (tPhTAZ → N^N)/3π−π* (N^N-centered)
state.
centered π−π* character. Similar conclusions can be drawn
for the low-lying singlet states of complexes 2−5. These results
indicate that the low-energy absorption bands in the
experimentally measured absorption spectra include a notable
1
contribution from the MLCT/1LLCT transitions (Figure 3).
For complex 4, the T1, T2, and T3 states show dominant
3MLCT (Ir → bpy)/3LLCT (FtPhTAZ → bpy) character,
whereas the T4 state shows dominant 3MLCT (Ir →
FtPhTAZ)/3π−π* (FtPhTAZ-centered) character. For com-
plex 5, the T1 state exhibits dominant 3MLCT (Ir →
dmebpy)/3LLCT (FtPhTAZ → dmebpy) character; however,
the T2 and T3 states exhibit dominant 3MLCT (Ir →
FtPhTAZ)/3π−π* (FtPhTAZ-centered) character. It is noted
The low-lying triplet states were then calculated, and the
calculation results are summarized in Table 3. It can be seen
from Table 3 that the low-lying triplet states (E < 3.0 eV or λabs
> 400 nm) exhibit mixed 3MLCT/3LLCT/ligand-centered
3π−π* character. These triplet transitions become partially
allowed because of the strong spin−orbital coupling on the
heavy iridium atom, which can contribute to the low-energy
absorption band as observed in the experimentally measured
absorption spectra (Figure 3).
3
that for complexes 4 and 5, the lowest-lying MLCT/3LLCT
3
state (T1) and the higher-lying MLCT/3π−π* state (T4 for
For complex 1, the T1 and T2 states both exhibit dominant
3MLCT (Ir → bpy)/3LLCT (tPhTAZ → bpy) character,
whereas the T3 state shows dominant 3MLCT (Ir →
tPhTAZ)/3π−π* (tPhTAZ-centered) character. For com-
complex 4 and T3 for complex 5) lie close (within 0.21 and
0.24 eV for complexes 4 and 5, respectively) in energy; thus,
the emission could arise from either of them, depending on the
local surrounding of the complex.39,40,60,61 Photophysical
measurements have shown that complexes 4 and 5 afford
emission with dominant charge-transfer character in solution
and with charge-transfer as well as some ligand-centered
3π−π* character in doped films. Therefore, for complexes 4
3
plexes 2 and 3, their T1 and T2 states both exhibit MLCT
(Ir → N^N)/3LLCT (tPhTAZ → N^N) as well as some N^N-
centered 3π−π* character, whereas the T3 state shows
dominant 3MLCT (Ir → tPhTAZ)/3π−π* (tPhTAZ-centered)
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and 5, the emission in solution should mainly arise from the T1
3
state, i.e., mixed MLCT (Ir → N^N)/3LLCT (FtPhTAZ →
N^N) state, and that in doped films could partially occur from
the higher-lying 3MLCT (Ir → FtPhTAZ)/3π−π* (FtPhTAZ-
centered) state.
To get insight into the emitting triplet states of complexes
1−5, the lowest-lying triplet states were further optimized for
the complexes. Figure 7 presents the optimized T1 geometries
and spin-density distributions on the T1 states for complexes
1−5. Geometrical parameters for optimized T1 structures were
listed in Table S1. As shown in Figure 7, for complexes 1, 4,
and 5, the spin-densities of the T1 states distribute over the
C^N ligands (0.48e, 0.47e, and 0.41e, respectively), the iridium
ions (0.50e, 0.48e, and 0.48e, respectively) and the N^N
ligands (1.02e, 1.05e, and 1.11e, respectively), which agrees
well with the predominant 3MLCT (Ir → N^N)/3LLCT
(C^N → N^N) nature assigned to these T1 states by the
TDDFT calculations (Table 3). For complexes 2 and 3, the
spin-densities of the T1 states mainly distribute on the iridium
ions (0.33e and 0.34e, respectively) and the N^N ligands
(1.59e and 1.56e, respectively), and slightly distribute on the
C^N ligands (0.08 e and 0.10 e, respectively). These spin-
density distributions indicate that the T1 states of complexes 2
and 3 show predominant 3MLCT (Ir → N^N)/3π−π* (N^N-
3
centered) character as well as slight LLCT (C^N → N^N)
character, which is consistent with the TDDFT calculations.
LECs. The high luminescent efficiencies in neat films and
the good electrochemical stability in solution indicate that
complexes 1−5 are suitable emitting materials for LECs. LECs
were thus fabricated with a structure of indium−tin-oxide
(ITO)/PEDOT:PSS (40 nm)/complex:BMIMPF6 (molar
ratio 1:0.5) (100 nm)/Al (100 nm), where poly(3,4-
ethylenedioxythiophene):poly(styrenesulfonate) (PE-
DOT:PSS) was inserted as a buffer layer to smoothen the
anode surface and the ionic liquid 1-butyl-3-methylimidazo-
lium hexafluorophosphate (BMIMPF6) was added into the
emissive layer to accelerate the device response.62 Figure 8
Figure 9. (a) Current-density versus time and (b) brightness versus
time curves for LECs using complexes 1−5 driven at 3.0 V.
typical characteristic for LECs.1 Under the external electrical
−
bias, the PF6 anions migrate toward the anode and
accumulate at the anode interface; however, the immobile
large iridium complex cations and 1-butyl-3-methylimidazo-
lium cations are excessive at the cathode interface. Interfacial
electrical fields are thus formed at the electrode/emissive layer
interfaces, which facilitate carrier injection and followed
electrochemical doping and establishment of the p−i−n
junction within the emissive layer, leading to the increase of
current and brightness.1,3−5 Nevertheless, further growth of
doped zones (i.e., over doping) in the emissive layer finally
results in the decrease of brightness,63−65 as shown in Figure
9b. Under a low operating voltage of 3.0 V, the LECs took
2.9−6.0 min to turn on and 87−170 min to reach the
maximum brightness. The LECs based on complexes 1−3
showed a higher current and brightness as compared to the
LECs based on complexes 4 and 5. One reason for this
phenomenon could be that the enlarged energy gaps for
complexes 4 and 5 hinder the carrier injection in LECs,
because of the increased energy barriers at the electrode/
emissive layer interfaces and thus hindered establishment of a
p−i−n junction in the emissive layers. As shown in Figure 9a
and b, the LECs based on complexes 2 and 3 exhibit higher
current and brightness than the LEC based on complex 1,
although complexes 1−3 show similar energy levels and
luminescent efficiencies in films. This suggests that compared
to bpy, 2-(pyridin-2-yl)-1H-benzo[d]imidazole type ancillary
ligands could better promote carrier-injection/transport in the
LECs.
Figure 8. EL spectra of LECs using complexes 1−5 as the emitting
material.
shows the EL spectra of LECs. The LECs showed yellow to
blue-green emission with the EL spectra centered at 540, 552,
567, 512, and 500 nm for complexes 1−5, respectively, which
resembled the PL spectra of the emissive layers (Figure S13).
Figure 9 shows current-density and brightness versus time
curves for LECs under a constant driving voltage of 3.0 V.
Detailed EL characteristics were summarized in Table 4. As
shown in Figure 9a and b, the current and brightness gradually
increase with time until they reach the maxima, which is a
At 3.0 V, the green-yellow LEC based on complex 1 afforded
a maximum brightness of 124 cd m−2 and a peak current
efficiency of 26.1 cd A−1. When the concentration of ionic
liquid in the emissive layer was increased (molar ratio between
complex 1 and BMIMPF6 was changed from 1:0.5 to 1:0.75),
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Table 4. EL Characteristics of LECs Using Complexes 1−5 at 3.0 V
a
b
c
d
e
f
g
molar ratio
ton (min)
tmax (min)
Bmax (cd m−2
)
ηc,max (cd A−1
)
EQEmax (%)
t1/2 (min)
EL λmax (nm)
CIE (x, y)
1
1:0.5
1:0.75
1:0.5
1:0.5
1:0.5
1:0.5
3.8
1.4
3.8
2.9
6.0
5.8
170
28
140
168
87
124
134
187
256
40
26.1
34.7
15.6
25.0
24.6
12.5
7.9
11
5.1
8.7
7.9
5.5
1130
138
1230
1600
1050
540
540
542
552
567
512
500
(0.38, 0.58)
(0.38, 0.56)
(0.43, 0.55)
(0.48, 0.51)
(0.27, 0.58)
(0.21, 0.51)
2
3
4
5
110
27
a
b
c
Molar ratio between the complex and ionic liquid. Time for the brightness to reach 1 cd m−2. Time for the brightness to reach the maximum.
d
e
f
g
Maximum brightness. Maximum current efficiency. Maximum external quantum efficiency. Time for the brightness to decrease from the
maximum to the half value.
the peak current efficiency was boosted to 34.7 cd A−1 (Table
4), which is among the highest for green-yellow LECs reported
so far11,66 and is the highest for LECs based on complexes with
CΛN ligands containing five-membered, nitrogen-rich triazole/
tetrazole/oxadiazole heterocycles.37−42 Increasing the amount
of ionic liquid in the emissive layer would promote the carrier
injection for improved carrier injection/recombination bal-
ance, leading to enhanced device efficiency.16,67,68 The yellow
LECs using complexes 2 and 3 showed maximum brightness/
peak current efficiency of 187 cd m−2/15.6 cd A−1 and 256 cd
m−2/ 25.0 cd A−1, respectively. The green LEC using complex
4 and the blue-green LEC using complex 5 showed a lower
brightness of 40 and 27 cd m−2, respectively, but they still
afforded good peak efficiencies of 24.6 and 12.5 cd A−1,
respectively. These high efficiencies should be related to the
increase of intercomplex distance and suppression of
phosphorescence concentration-quenching in the emissive
layers. For the LECs based on complexes 2−5, further
increasing the amount of ionic liquid in the emissive layer
led to decreased device performances largely because of
increased leaking currents upon increasing the content of ionic
liquid.
It is noticed that the peak efficiencies were reached at low
luminance levels (5−20 cd m−2), and the efficiencies decreased
rapidly with time (Figure S14), i.e., the LECs exhibited severe
efficiency roll-offs during the operation. Efficiency roll-offs
have always been observed for LECs operated under constant
driving voltages, which can be caused by the migration of the
carrier recombination zone toward the electrode, deteriorated
carrier recombination balance, or intrinsic degradation of the
complexes during the electrical operation.8,14,67 The LECs
based on complexes 1−5 showed varied external quantum
efficiencies (EQEs) (5.1−11%), although the complexes show
similar luminescent efficiencies in films (Table 4). This
indicates that the carrier recombination balance largely differs
among the LECs, presumably due to the different energy levels
and carrier-transporting properties of the complexes.67,68
The LECs showed half lifetimes at 540−1600 min. The
relatively short half lifetimes should be ascribed to (i) relatively
high doping concentration of ionic liquid in the emissive layer
and (ii) the constant voltage-driving mode used for the
tests.11,14 It has been shown that the response time and
stability of LECs are directly correlated with the doping
concentration of ionic liquid, i.e., higher doping concentration
of ionic liquid results in faster response but lower stability.16,62
Moreover, doping concentration of ionic liquid affects carrier
recombination balance and thus the device efficiency.16,67,68 In
this study, the concentration of ionic liquid in the emissive
layer was selected to balance the response time, efficiency, and
stability of LECs driven under a constant voltage. Reducing the
doping concentration of ionic liquid and meanwhile driving the
LEC with a pulsed-current mode would largely enhance the
stability and meanwhile maintain fast response for the
device.19,69 As shown in Table 4, the LECs using complexes
2 and 3 showed better stability than the LEC using complex 1,
as revealed by their higher brightness and longer half lifetimes.
This aligns with a previous report that 2-(pyridin-2-yl)-1H-
benzo[d]imidazole type ancillary ligands largely enhance the
stability of LECs as compared to the bpy ancillary ligand.45 It is
further noted that the LECs based on complexes 4 and 5
showed much lower stability (lower brightness and half
lifetimes) as compared with the LEC based on complex 1,
which can be related to (i) unstable C−F bonds introduced in
complexes 4 and 570 and (ii) off-centered recombination zones
in the LEC based on complexes 4 and 5 as a result of
unbalanced carrier injection/transport.14,67,68 By further ligand
control and device engineering, LECs based on the complexes
should afford even higher performances.
CONCLUSION
■
We reported for the first time a series of yellow to blue-green-
emitting cationic iridium complexes with tPhTAZ-type C^N
ligands. Compared with the typical ppy C^N ligand, the
tPhTAZ ligand can blue-shift the emission of the complex by
over 40 nm through stabilizing the HOMO level. The
bulkiness of tPhTAZ provides large steric hindrance that
separates the complexes from each other and therefore
remarkably decreases the phosphorescence concentration-
quenching of the complex in film. These complexes exhibit
good electrochemical stability in solution which benefits their
use in LECs. LECs using the complexes showed yellow to blue-
green EL and the yellow-green LEC afforded a high peak
current efficiency of 34.7 cd A−1. It is shown here that
tPhTAZ-type ligands are promising C^N ligands for
construction of cationic iridium complexes with blue-shifted
emission, suppressed phosphorescence concentration-quench-
ing, and good electrochemical stability for advanced optoelec-
tronic applications.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c00645.
■
sı
Experimental details on cyclic voltammetry, X-ray
crystallography, theoretical calculations, and fabrication
1
of LECs; H and 13C NMR spectra (Figures S1−S7),
packing patterns of complexes 3 and 4 in single crystals
(Figures 8 and 9), PL spectra of the complexes at 77 K
(Figure S10), PL spectra of the complexes in doped
films (Figure S11), calculated absorption spectra (Figure
J
https://dx.doi.org/10.1021/acs.inorgchem.0c00645
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
S12), PL spectra of the emissive layers of LECs (Figure
pubs.acs.org/IC
Article
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Metalated Ir(Iii) Complex. J. Am. Chem. Soc. 1987, 109, 1589−1590.
(16) Costa, R. D.; Orti, E.; Bolink, H. J.; Graber, S.; Schaffner, S.;
Neuburger, M.; Housecroft, C. E.; Constable, E. C. Archetype
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3463.
S13), EQE versus current-density curves of LECs
(Figure S14), selected geometrical parameters in crystal
structures and theoretically calculated structures (Table
S1), surface distributions of other molecular orbitals
(Table S2), calculated singlet states (Table S3), and
Cartesian coordinates of optimized geometries (Table
S4) (PDF)
Accession Codes
CCDC 1984641−1984643 contain the supplementary crys-
tallographic data for this paper.
AUTHOR INFORMATION
Corresponding Author
■
Lei He − 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, Central China Normal University, Wuhan
430079, People’s Republic of China; orcid.org/0000-0003-
2106-740X; Email: helei@mail.ccnu.edu.cn
Authors
Xianwen Meng − 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, Central China Normal
University, Wuhan 430079, People’s Republic of China
Mengzhen Chen − 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, Central China Normal
University, Wuhan 430079, People’s Republic of China
Rubing Bai − 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, Central China Normal
University, Wuhan 430079, People’s Republic of China
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c00645
(17) Lowry, M. S.; Hudson, W. R.; Pascal, R. A.; Bernhard, a. S.
Accelerated Luminophore Discovery through Combinatorial Syn-
thesis. J. Am. Chem. Soc. 2004, 126, 14129−14135.
Notes
(18) Lowry, M. S.; Goldsmith, J. I.; Slinker, J. D.; Rohl, R.; Pascal, R.
A.; Malliaras, G. G.; Bernhard, S. Single-Layer Electroluminescent
Devices and Photoinduced Hydrogen Production from an Ionic
Iridium(Iii) Complex. Chem. Mater. 2005, 17, 5712−5719.
(19) Shavaleev, N. M.; Scopelliti, R.; Graetzel, M.; Nazeeruddin, M.
K.; Pertegas, A.; Roldan-Carmona, C.; Tordera, D.; Bolink, H. J.
Pulsed-Current Versus Constant-Voltage Light-Emitting Electro-
chemical Cells with Trifluoromethyl-Substituted Cationic Iridium(Iii)
Complexes. J. Mater. Chem. C 2013, 1, 2241−2248.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the National Natural Science Foundation of China
(Grant No. 51773074), the Program of Introducing Talents of
Discipline to Universities of China (111 Program, B17019),
and the Fundamental Research Funds for the Central
Universities (Innovation Funding Project; Grant No.
2019CXZZ026) for financial support.
(20) Shavaleev, N. M.; Xie, G.; Varghese, S.; Cordes, D. B.; Slawin,
A. M. Z.; Momblona, C.; Orti, E.; Bolink, H. J.; Samuel, I. D. W.;
Zysman-Colman, E. Green Phosphorescence and Electrolumines-
cence of Sulfur Pentafluoride-Functionalized Cationic Iridium(Iii)
Complexes. Inorg. Chem. 2015, 54, 5907−5914.
(21) Pal, A. K.; Cordes, D. B.; Slawin, A. M. Z.; Momblona, C.; Orti,
E.; Samuel, I. D. W.; Bolink, H. J.; Zysman-Colman, E. Synthesis,
Properties, and Light-Emitting Electrochemical Cell (Leec) Device
Fabrication of Cationic Ir(Iii) Complexes Bearing Electron-With-
drawing Groups on the Cyclometallating Ligands. Inorg. Chem. 2016,
55, 10361−10376.
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