Tris-heteroleptic Iridium Complexes Based on Cyclometalated Ligands with Different Cores

Article abstract of DOI:10.1021/acs.inorgchem.7b01307

A series of tris-heteroleptic iridium complexes of the form [Ir(C^N¹)(C^N²)(acac)] combining 2-phenylpyridine (ppy), 2-(2,4-difluorophenyl)pyridine (dFppy), 1-phenylpyrazole (ppz), and 1-(2,4-difluorophenyl)pyrazole (dFppz) as the C^

Full text of DOI:10.1021/acs.inorgchem.7b01307

Article
pubs.acs.org/IC
Tris-heteroleptic Iridium Complexes Based on Cyclometalated
Ligands with Different Cores
Yanouk Cudre,† Felipe Franco de Carvalho,‡ Gregory R. Burgess,† Louise Male,† Simon J. A. Pope,§
́
Ivano Tavernelli,∥ and Etienne Baranoff*,†
†School of Chemistry, University of Birmingham, Edgbaston, B15 2TT Birmingham, U.K.
‡
́
́ ́
Ecole Polytechnique Federale de Lausanne, CH-1015 Lausanne, Switzerland
§School of Chemistry, Main Building, Cardiff University, Park Place, CF10 3AT Cardiff, U.K.
∥Zurich Research Laboratory, IBM Research GmbH, 8803 Ruschlikon, Switzerland
̈
S
* Supporting Information
ABSTRACT: A series of tris-heteroleptic iridium complexes
of the form [Ir(C^N1)(C^N2)(acac)] combining 2-phenyl-
pyridine (ppy), 2-(2,4-difluorophenyl)pyridine (dFppy), 1-
phenylpyrazole (ppz), and 1-(2,4-difluorophenyl)pyrazole
(dFppz) as the C^N ligands have been synthesized and fully
characterized by NMR, X-ray crystallography, UV−vis
absorption and emission spectroscopy, and electrochemical
methods. It is shown that “static properties” (e.g., absorption
and emission spectra and redox potentials) are primarily
dictated by the overall architecture of the complex, while
“dynamic properties” (e.g., excited-state lifetime and radiative
and nonradiative rate constants) are, in addition, sensitive to
the specific positioning of the substituents. As a result, the two
complexes [Ir(dFppy)(ppz)(acac)] and [Ir(ppy)(dFppz)(acac)] have the same emission maxima and redox potentials, but their
radiative and nonradiative rate constants differ significantly by a factor ∼2. Then acetylacetonate (acac) was replaced by
picolinate (pic), and two pairs of diastereoisomers were obtained. As expected, the use of pic as the ancillary ligand results in
blue-shifted emission, stabilization of the oxidation potential, and improvement of the photoluminescence quantum yield, and
only minor differences in the optoelectronic properties are found between the two diastereoisomers of each pair.
the highest occupied molecular orbital (HOMO) is stabilized
more than the lowest unoccupied molecular orbital (LUMO),
an increased HOMO−LUMO gap is obtained, which generally
translates into emission at higher energy. However, stabilizing
the HOMO also means an increase of the oxidation potential.
As such, the complex fac-[Ir(ppy)3] (ppy = 2-phenylpyridine)
emits at 510 nm in dichloromethane (CH2Cl2) at room
temperature and has an oxidation potential Eox = 0.31 V versus
ferrocene/ferrocenium (Fc+/Fc), while the complex fac-[Ir-
(dFppy)3] [dFppy = 2-(2,4-difluorophenyl)pyridine] emits at
468 nm and has Eox = 0.78 V vs Fc+/Fc.50 It is only recently
that rationalization of the effect of the substituents on the
HOMO and LUMO energies allows for tuning of the emission
color independently of the energy of the HOMO using
electron-donating groups.51
INTRODUCTION
■
Cyclometalated iridium complexes are widely studied for a
range of applications including solar energy conversion,1−8
organic light-emitting diodes,9−25 light-emitting electrochemical
cells,26−34 sensing,35−39 imaging,40−43 and synthetic method-
ologies.44,45 For more than 2 decades, these complexes have
been particularly appealing because of the excellent perform-
ance of their excited states, specifically their relatively short
triplet excited-state lifetimes, wide color tunability, and
generally high phosphorescence quantum yield.46−49
The photophysical and electrochemical properties of the
complexes are crucial for the performance of the materials for a
given application, and different applications may require
different sets of properties. Interestingly, these properties can
be precisely adjusted through engineering of the chemical
structure of the complexes. However, because these properties
are ultimately all linked to the same chemical structure,
modifications to tune a given attribute also cause undesired
variations in many others. For example, the most common
strategy to blue shift the emission of cyclometalated iridium
complexes is to introduce electron-withdrawing substituents, in
particular fluorine, on the orthometalated phenyl group. When
The radiative and nonradiative rate constants, respectively kr
and knr, make for another set of parameters crucial to the
performance of the emissive complexes because they dictate the
photoluminescence quantum yield (PLQY, Φ) and lifetime of
Received: May 22, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.7b01307
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a
Scheme 1. Chemical Structures of the Complexes Studied
a
Only the Λ isomers are shown, while all complexes have been isolated as a mixture of Δ and Λ isomers.
the excited state (τ). Across a series of complexes, kr is generally
rationalized based on the amount of metal character in the
emitting excited state and the energy gap between the singlet
and triplet states, while knr is explained by the intramolecular
vibronic coupling and thermal accessibility of metal-centered
(MC) states.52−56 A number of design strategies have been
used to control the rate constants to improve the properties of
the emitting state. A major drawback of these strategies is that
they invariably result in the concomitant variation of the
emission color and/or redox potentials of the complexes
because of the intimate interplay between the chemical
structure and properties.57−59 Therefore, distinguishing the
exact effect of a structural modification on specific properties is
still a major challenge.
the core family of ligands (e.g., “ppy” specifically represents the
nonsubstituted 2-phenylpyridine ligand, while “phenylpyridine”
represents any 2-phenylpyridine ligand with or without
substituents).
First, different experimental conditions have been tested to
evaluate the possibility of improving the yields of the reactions.
Then all complexes have been isolated and characterized and
their electrochemical and photophysical properties measured.
Complexes 1−3 with both phenylpyridine-based ligands have
been previously reported. As expected, complexes 4−6 with
both phenylpyrazole-based main ligands are not emissive at
room temperature. Complexes 7−10 with mixed phenyl-
pyridine/phenylpyrazole main ligands display emission proper-
ties most similar to those of the phenylpyridine ligand used,
ppy or dFppy. As such, the phenylpyrazole ligand acts as an
ancillary ligand, perturbing the excited state on the phenyl-
pyridine ligand. As a result, for complexes containing at least
one chromophoric phenylpyridine ligand, the redox potentials
and emission maxima are dictated by the overall number of
fluorine substituents on the complexes, while kr and knr, hence,
Φ and τ, are dictated by the presence or absence of pyrazole
and, for complexes with only two fluorine atoms, their position
on either phenylpyridine or phenylpyrazole. In a second part,
the acac ligand of complexes 3 and 9 is replaced with picolinate
(pic). Contrary to acac, pic is an asymmetric ligand and two
diastereoisomers are obtained for each complex, with one
having the pyridine of pic trans to the phenyl of dFppy and one
having the oxygen of pic trans to dFppy (Scheme 1). Isolating
these diastereoisomers, we show that the properties of the two
diastereoisomers are very similar, which is confirmed by
theoretical calculations. Furthermore, theoretical calculations
show that the increased efficiency of nonradiative deactivation
processes with phenylpyrazole is mainly due to changes in the
vibronic coupling, not to thermally accessible MC states.
Herein, a design approach allowing variation of the radiative
and nonradiative rate constants independently of the emission
maximum and redox potential is demonstrated. Our strategy is
based on tris-heteroleptic complexes using cyclometalated
ligands (C^N) having different cores. Compared to tris-
homoleptic [Ir(C^N)3] and bis-heteroleptic [Ir(C^N)2(La)]
(La = ancillary ligand) complexes, tris-heteroleptic complexes
[Ir(C^N1)(C^N2)(C^N3 or La)] form a new family of
complexes attracting increased attention.60−70 Although often
challenging to synthesize and purify, they offer unparalleled
opportunities for fine-tuning of the properties. However, to
date, only C^N ligands with the same chemical core but
different substitution patterns have been used to prepare tris-
heteroleptic complexes. Phenyltriazole with adamantyl and
methyl substituents and phenylpyridine with, for example,
fluorine, methyl, benzo, and carbonyl substituents have been
reported.
We have prepared a series of [Ir(C^N1)(C^N2)(acac)]
complexes (acac = acetylacetonate), with C^N1 and C^N2
being ppy, dFppy, 1-phenylpyrazole (ppz), and 1-(2,4-
difluorophenyl)pyrazole (dFppz) making six tris-heteroleptic
and four bis-heteroleptic complexes (Scheme 1). Note that the
abbreviation is used for the specific ligand and the full name for
B
DOI: 10.1021/acs.inorgchem.7b01307
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1
Table 1. H NMR Yields of the Reactions
yield %
yield %
yield % [Ir(C^N1)
a
2
̂
entry
C^N1
ppy
C^N2
condition
[Ir(C^N1)2(acac)]
[Ir(C^N2)2(acac)]
(CN )(acac)]
overall yield %
1
dFppy
A
B
C
A
B
C
A
B
C
A
B
C
A
B
C
A
B
C
1
1
1
2
2
4
18
12
16
13
23
11
9
2
4
5
4
5
5
19
12
15
30
4
3
30
67
62
73
57
70
45
59
38
41
71
55
55
68
61
53
68
24
35
2
38
42
14
43
32
28
16
19
30
18
36
29
32
24
36
14
20
3
4
ppy
ppz
7
5
6
2
7
ppy
dFppz
ppz
22
3
10
9
8
19
15
22
33
17
25
24
24
22
4
9
7
10
11
12
13
14
15
16
17
18
dFppy
dFppy
ppz
19
4
2
dFppz
dFppz
14
5
8
5
10
6
6
4
11
a
Condition A: IrCl3·xH2O, ethoxyethanol/water, 130 °C, 12 h. Condition B: [Ir(COD)Cl]2, ethoxyethanol, 130 °C, 3 h. Condition C:
[Ir(COD)Cl]2, xylene, 130 °C, 3 h.
generated by the reaction (seven dimers are possible). As
expected, the H NMR signals from the crude are a perfect
RESULTS AND DISCUSSION
■
1
Synthesis. All tris-heteroleptic acac complexes 3 and 6−10
were synthesized in two steps starting with heating of the
selected ligands in the presence of an iridium source, IrCl3·
xH2O or [Ir(COD)Cl]2 (COD = cyclooctadiene), to obtain a
mixture of chloro-bridged dimers, followed by the reaction with
sodium acetylacetonate (acacNa) in a dichloromethane/
methanol solvent mixture overnight. This results in a mixture
of complexes containing the target tris-heteroleptic acac
complex with the corresponding bis-heteroleptic complexes,
[Ir(ppy)2(acac)] (1), [Ir(dFppy)2(acac)] (2), [Ir(ppz)2(acac)]
(4) or [Ir(dFppz)2(acac)] (5).
combination of signals from the purified products (Figures S1−
S6), which gives access to the relative amount of each complex
from integration of the signals. An average molar mass was
calculated from the ratio obtained, which rapidly gives access to
an approximate yield for each complex without the need for
purification. Isolated yields of target complexes were judged as
less appropriate for this discussion because of the possible
partial degradation of the complexes on acidic silica during
purification.60
The results summarized in Table 1 allow for a qualitative
discussion of the reactivity of the ligands. In terms of both the
overall yields and yields of tris-heteroleptic complexes, it
appears that condition A was the most generally applicable.
Importantly, the tris-heteroleptic complex is the major species,
except for entries 4, 8, and 11 of Table 1. Assuming a stepwise
coordination of the two ligands to form the final dimer, a two-
step mechanism can be hypothesized as follows: in step 1, the
iridium starting material, “IrCl”, reacts with a first ligand to give
an intermediate “Ir(C^N)Cl” species that reacts with another
ligand in step 2 to give the chloro-bridged dimer. Under
condition A using an iridium(III) starting material, both steps
can be considered to proceed through electrophilic substitution,
while with conditions B and C using an iridium(I) starting
material, the first step is ascribed to an oxidative addition,
followed by an electrophilic substitution. The yield of bis-
phenylpyrazole complexes is always low with conditions B and
C (entries 5, 6, 8, 9, 11, 12, and 14−18), suggesting that
phenylpyrazole is disfavored during the oxidative addition.
However, this is balanced by the higher reaction rate of the
fluorinated ligands.72 On the other hand, in the case of
electrophilic substitution, the reaction rate is disfavored by the
presence of electron-withdrawing groups on the ligand,72 while
the resulting “Ir(C^N)Cl” intermediate complex would be
more reactive because of the decreased electron density on the
metal. These antagonistic effects explain the absence of clear
and meaningful trends, and the results will depend largely on
Because the relative amount of the target tris-heteroleptic
complex compared to bis-heteroleptic complexes is embedded
in the mixture of chloro-bridged iridium dimers, different
reaction conditions for the first step have been explored as a
possible means to improve the yield of tris-heteroleptic
complexes. Three sets of conditions were tested: (condition
A) IrCl3·xH2O as the iridium source with ethoxyethanol/water
as the solvent and heating at 130 °C for 12 h; (condition B)
[Ir(COD)Cl]2 as the iridium source with ethoxyethanol as the
solvent and heating at 130 °C for 3 h; (condition C)
[Ir(COD)Cl]2 as the iridium source with xylene as the solvent
and heating at 130 °C for 3 h. Condition A is the most
commonly used condition for the synthesis of chloro-bridged
iridium dimers. Condition B was previously used for synthesis
of the tris-heteroleptic complex 3, where the use of an
iridium(I) starting material tremendously decreased the
reaction time from 12 to 3 h.60 Condition C was chosen to
briefly explore the effect of the solvent and was slightly
modified from a previously reported methodology.71
Because the reaction of the iridium chloro-bridged dimers
1
with acacNa is quasi-quantitative in the conditions used, H
NMR analysis of the mixture resulting from such a reaction is a
reasonable proxy for the composition of the mixture of dimers.
Direct analysis of the mixture of dimers would have been an
unnecessarily tedious task because of their poor solubility in
most solvents and because of the high number of species
C
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Figure 1. X-ray crystal structures of 3a, 3b, 9a, and 9b with ellipsoids drawn at the 50% probability level.
Table 2. Comparison between the Experimental and Theoretical Ground-State Geometries for 3a, 3b, 9a, and 9b
bond lengths (Å)
3a
3b
9a
9b
DFT
X-ray
DFT
X-ray
DFT
X-ray
DFT
X-ray
Ir−Npic
2.16
2.05
2.00
2.18
2.00
2.00
2.132(4)
2.034(4)
2.037(4)
2.162(3)
2.001(4)
1.990(3)
2.16
2.05
2.04
2.04
2.01
1.99
2.125(3)
2.036(3)
2.033(3)
2.152(3)
2.005(4)
1.988(4)
2.16
2.03
2.04
2.17
2.01
2.00
2.137(4)
2.026(4)
2.032(4)
2.150(3)
2.008(5)
1.979(5)
2.15
2.03
2.04
2.18
2.03
1.99
2.120(4)
2.028(4)
2.033(4)
2.159(3)
2.009(5)
1.996(5)
Ir−NC^N
Ir−NdFppy
Ir−Opic
Ir−CC^N
Ir−CdFppy
bond angles (deg)
3a
3b
9a
9b
DFT
X-ray
DFT
X-ray
DFT
X-ray
DFT
X-ray
CdFppy−Ir−CC^N
NdFppy−Ir−NC^N
CC^N−Ir−NC^N
CdFppy−Ir−NC^N
CdFppy−Ir−NdFppy
CdFppy−Ir−Opic
89.53
89.6(3)
175.7(2)
80.9(2)
95.3(2)
80.7(2)
95.1(2)
89.56
175.20
80.27
89.7(2)
174.7(1)
80.5(2)
95.1(2)
81.0(1)
174.7(1)
89.09
174.71
79.86
95.50
80.44
96.77
88.8(2)
176.0(2)
80.9(2)
96.5(2)
80.7(2)
96.8(2)
89.33
174.75
79.49
89.6(2)
174.1(2)
80.3(2)
95.2(2)
81.0(2)
175.7(2)
175.28
80.52
95.52
80.35
96.85
97.09
96.84
80.62
80.71
173.99
173.24
the types of core ligands and substituents as well as the
combination of ligands used. In conclusion, using an iridium(I)
starting material for the synthesis of heteroleptic dimers leads
to the best results in several cases (entries 2, 3, 5, 6, 12, and
14), but the use of iridium(III) is more likely to give
reasonable-to-high yields of tris-heteroleptic complexes (only
entry 4 shows a low yield).
The acac complexes can be purified using standard
chromatography techniques with silica gel (see Figure S7 for
thin-layer chromatography analyses), using triethylamine to
limit the degradation induced by the acidic silica. The pic
complexes were obtained by reacting 3 and 9 with BF3 in
acetonitrile to give the bis-acetonitrile complexes, followed by
the reaction with picolinic acid in the presence of a base. The
diastereoisomers have been purified by high-performance liquid
chromatography (HPLC). All isolated complexes have been
characterized by NMR (1H, 13C, 19F, COESY, NOESY, and
HMQC), high-resolution mass spectrometry, and Fourier
transform infrared (FT-IR) spectroscopy, and their purity was
evaluated with analytical HPLC and elemental analysis (Figures
S8−S17).
structures of 3a, 3b, 9a, and 9b are shown in Figure 1, and
selected crystallographic data are provided in Tables 2 and 3.
The details for all other structures are given in the Supporting
Information.
As expected, all complexes have a slightly deformed
octahedral coordination geometry around the iridium center
with the usual cis-C,C-trans-N,N chelate configuration.
Electrochemistry. The oxidation and reduction potentials
were measured by cyclic voltammetry in degassed acetonitrile
containing 0.1 M tetrabutylammonium hexafluorophosphate
(TBAPF6). The data are summarized in Table 4; the
voltammograms are shown in Figure 2 for the acac complexes
and in Figure 3 for the pic complexes.
All pyridine-containing complexes show quasi-reversible
oxidation and reduction potentials. As expected, the reduction
potentials of the bis-pyrazole complexes 4−6 are outside the
limit of the system and are therefore not measurable. Because
the oxidation potentials correspond to the oxidation of the
iridium center, the values are primarily controlled by the overall
number of fluorine substituents on the complexes with a minor
effect of the presence of pyrazole, with about 30 mV per
pyrazole unit replacing a pyridine unit. As such, complexes
without fluorine atoms have Eox values ranging from +0.42 to
+0.49 V vs Fc+/Fc, while the addition of two fluorine
substituents shifts the range to +0.58 to +0.64 V vs Fc+/Fc
and four fluorine substituents shift the range further to +0.74 to
X-ray Crystal Structures. X-ray crystal structures were
obtained for all new tris-heteroleptic complexes, and they
confirm the chemical structure of isomers 3a, 3b, 9a, and 9b.
Single crystals have been grown by the slow diffusion of hexane
into a dichloromethane solution of the complexes. The
D
DOI: 10.1021/acs.inorgchem.7b01307
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Table 3. Crystallographic Data for 3a, 3b, 9a, and 9b
3a
3b·3/2CH2Cl2·0.5H2O
1552223
9a
9b·3/2 CH2Cl2
1511803
CCDC
1552222
1511802
empirical formula
C28H18F2IrN3O2
C29.5H22Cl3F2IrN3O2.5
795.05
C26H17F2IrN4O2
647.63
C27.5H20Cl3F2IrN4O2
775.02
fw (g mol−1
temp (K)
cryst syst
space group
a (Å)
)
658.65
100.01(10)
100.01(10)
monoclinic
P21/c
99.97(10)
monoclinic
P21/n
100.01(10)
monoclinic
P21/c
orthorhombic
Pna21
15.0856(2)
16.7317(5)
12.3412(2)
15.5940(4)
90
9.2743(3)
14.7104(4)
16.5135(5)
90
16.3715(4)
12.2266(2)
15.6859(4)
90
b (Å)
16.3361(2)
c (Å)
9.47782(13)
α (deg)
β (deg)
γ (deg)
90
90
117.541(4)
90
93.301(3)
90
118.521(3)
90
90
volume (Å3)
2335.71(6)
2855.09(15)
4
2249.18(11)
4
2758.76(13)
4
Z
4
ρcalcd (g cm−3
)
1.873
1.850
1.913
1.866
μ (mm−1
)
5.764
5.006
11.923
12.455
F(000)
1272.0
1544.0
1248.0
1500.0
cryst size (mm3)
0.1932 × 0.1488 × 0.1233
4.968−54.958
0.2164 × 0.1215 × 0.0362
5.892−50.688
0.2106 × 0.0601 × 0.0459
8.056−136.498
0.4159 × 0.1754 × 0.0943
6.144−144.228
θ range for data collection
(deg)
reflns collected
indep reflns
24233
14404
12463
16925
5197 [Rint = 0.0312; Rσ =
0.0267]
5204 [Rint = 0.0269; Rσ =
0.0321]
4103 [Rint = 0.0353; Rσ =
0.0347]
5406 [Rint = 0.0310; Rσ =
0.0253]
data/restraints/param
GOF on F2
5197/1/325
5204/0/415
4103/0/316
5406/0/397
1.057
1.025
1.105
1.078
final R indexes [I ≥ 2σ(I)]
final R indexes (all data)
R1 = 0.0178; wR2 = 0.0348
R1 = 0.0214; wR2 = 0.0364
R1 = 0.0258; wR2 = 0.0606
R1 = 0.0314; wR2 = 0.0637
R1 = 0.0314; wR2 = 0.0803
R1 = 0.0361; wR2 = 0.0840
R1 = 0.0350; wR2 = 0.0841
R1 = 0.0356; wR2 = 0.0845
Table 4. Photophysical and Photochemical Properties of Complexes 1−10, 3a, 3b, 9a, and 9b
kr
(×105 (×105
s−1 s−1
knr
λem (nm),
d
d
a
b
b
b
c
Eox (V)
Ered (V)
λabs, nm (ε × 103, L mol−1 cm−1
)
RT
τ (ns), RT
ΦPL
)
)
c
c
c
c
c
c
c
c
c
1
2
0.43 (0.41)
0.73 (0.76)
−2.57 (−2.60)
−2.45 (−2.44)
260 (39.15), 340 (9.15), 405 (4.02), 459 (2.88), 489 (1.32) 519 (520)
1511 (1227)
0.47
0.63
3.83
7.22
4.32
4.24
c
c
c
c
253 (45.72), 328 (11.44), 389 (4.72), 435 (2.23),
466 (0.82)
482 (484)
898 (872)
c
c
c
c
c
c
c
3
4
0.58 (0.57)
0.49
−2.51 (−2.52)
256 (42.97), 334 (10.34), 398 (4.29), 449 (2.44),
479 (0.79)
503 (503)
1413 (1224)
0.69
5.64
2.53
252 (34.04), 297 (15.15), 330 (8.57), 377 (2.77),
418 (0.19)
5
6
0.79
0.64
253 (31.19), 321 (8.06), 359 (2.23), 400 (0.29)
251 (33.18), 269 (25.53), 294 (13.89), 325 (8.61),
370 (2.21), 408 (0.15)
7
0.45
0.75
0.61
0.60
0.73
0.72
0.76
−2.58
−2.44
−2.50
−2.52
−2.33
−2.34
−2.35
256 (36.36), 295 (17.76), 335 (8.81), 374 (4.14),
405 (2.35), 456 (1.27), 487 (0.38)
519
1592
619
0.26
0.22
0.32
0.30
0.49
0.43
0.42
1.63
3.55
3.82
1.90
2.65
2.76
4.18
4.65
8
253 (40.40), 289 (18.86), 325 (10.349), 361 (3.95),
385 (2.52), 430 (0.95), 461 (0.31)
483
12.60
8.12
4.44
2.76
3.66
5.78
9
254 (35.00), 291 (16.91), 330 (8.85), 369 (3.63),
392 (2.21), 437 (1.01), 469 (0.37)
503
837
10
3a
3b
9a
257 (35.63), 295 (17.39), 328 (9.17), 360 (4.28),
396 (2.36), 430 (1.46), 478 (0.32)
502
1575
1850
1559
1004
261 (37.55), 289 (20.99), 323 (10.15), 345 (5.81),
390 (3.87), 431 (2.33), 462 (0.27)
489, 515
485, 512
474, 499
259 (32.69), 289 (18.54), 305 (13.67), 322 (9.37),
345 (5.39), 389 (3.66), 430 (2.14), 470 (0.23)
253 (36.20), 283 (20.92), 303 (14.62), 321 (10.91),
341 (6.86), 383 (3.28), 415 (1.65), 432 (0.93),
460 (0.24)
9b
0.73
−2.33
258 (31.18), 281 (16.35), 302 (12.57), 322 (7.66),
341 (4.18), 384 (2.34), 414 (1.42), 431 (0.84),
458 (0.19)
476sh, 498 971
0.38
3.91
6.39
a
b
c
d
Aerated dichloromethane. Degassed dichloromethane. According to ref 60. Degassed acetonitrile/TBAPF6 (0.1 M), vs Fc+/Fc at 1 V s−1.
+0.79 V vs Fc+/Fc. The effect of fluorine substituents can also
be seen on the reduction potentials (no fluorine atoms, −2.57
to −2.58; two fluorine atoms, −2.50 to −2.52; four fluorine
atoms, −2.44 to −2.45 V vs Fc+/Fc), with virtually no effect of
E
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rate constants are largely influenced by the position of the 2,4-
difluorination (see below). In addition, these two complexes
display redox potentials very similar to those of the complex
[Ir(dFppy)(ppy)(acac)] (3) besides a small 20 mV shift to
lower oxidation potential with 3 due to the absence of the
pyrazole unit. This demonstrates that tris-heteroleptic com-
plexes provide the possibility for different complex designs that
have the same HOMO and LUMO energies and the same
emission maxima (see below), while each of these designs can
exhibit different values for kr and knr.
Because the two pairs of diastereoisomers 3a, 3b, 9a, and 9b
contain at least one pyridine group, they also show quasi-
reversible oxidation and reduction potentials (Figure 3). The
principal effect of replacing acac with pic is to anodically shift
both oxidation and reduction potentials by ∼150 mV without
changing the electrochemical gap (3.06−3.11 eV compared to
3.09−3.10 eV for 3 and 9). The four complexes display very
similar redox potentials, demonstrating further that these
energy levels are primarily dependent on the overall structure
and composition of the complexes and not on the particularities
of the chemical structures. This is confirmed by theoretical
calculations (see the Supporting Information), showing that the
localizations of the HOMOs and LUMOs are similar for the
four complexes. Furthermore, the orbitals of these complexes
are similar to those of FIrpic, another well-studied iridium
complex, which is not surprising considering the similarity
between these molecules.73−78 In their ground state, 3a, 3b, 9a,
and 9b isomers all have HOMOs composed of significant
Ir(5d) character, at around 50%. All of the occupied orbitals
have some degree of Ir(5d) character and some ligand π-orbital
character, with the maximum Ir(5d) contribution appearing in
HOMO−1 at 61% for 3a and 3b, in HOMO−2 at 67% for 9a,
and in HOMO−1 at 60% for 9b. The lower-lying unoccupied
orbitals have much lower Ir(5d) character (at around 1%) and
are mostly ligand-centered (LC).
Figure 2. Cyclic voltammograms of complexes 1−10: (black )
complexes with no fluorine atoms; (blue ) complexes with four
fluorine atoms; (red ) complexes with two fluorine atoms.
Photophysical Properties. The UV−vis electronic
absorption and luminescence spectra of complexes 1−10 are
shown in Figure 4, and the spectra for complexes 3a, 3b, 9a,
and 9b are shown in Figure 5. Data are summarized in Table 4.
As for other cyclometalated iridium complexes, the
absorption spectra are dominated by strong absorption bands
at 254−260 nm [ε ∼ (30−45) × 103 M−1 cm−1] attributed to
1
LC ππ* transitions followed by weaker transitions [ε ∼ (5−
20) × 103 M−1 cm−1] of charge-transfer (CT) character
between 300 and 440 nm corresponding to transitions from the
iridium center to the ligand (metal-to-ligand charge transfer,
MLCT). The last weak transitions (ε < 103 M−1 cm−1)
observed at lowest energy are attributed to the direct
population of the emitting triplet state. As expected, 4−6
show a blue-shifted absorption compared to the phenylpyridine
complexes with onset absorptions below 430 nm, coherent with
nonmeasurable reduction potentials, suggesting a high-energy
LUMO. The absorption profiles in the MLCT regions of 7−10
are very similar to those of their corresponding phenylpyridine-
based bis-heteroleptic complex 1 or 2 with roughly halved
molar extinction coefficients, due to the presence of only one
phenylpyridine ligand. The higher-energy transitions between
260 and 350 nm exhibit some more phenylpyrazole-like
character such as the 297 nm peak of 4 being visible on 7
(Figure 4) or the 294 and 325 nm peaks of 6 also appearing in
9 and 10 (Figure 4).
Figure 3. Cyclic voltammograms of complexes 3a, 3b, 9a, and 9b:
(black ) isomers with N-pic trans to dFppy; (red ) isomers with
O-pic trans to dFppy.
replacing a pyridine with a pyrazole due to the reduction being
localized on the phenylpyridine ligand.
For the complexes [Ir(dFppy)(ppz)(acac)] (9) and [Ir-
(ppy)(dFppz)(acac)] (10), the data are particularly interesting
because they show comparable values for the oxidation and
reduction potentials. This suggests that the effect of 2,4-
difluorination of the phenyl ring is independent of the ligand
being fluorinated. In contrast, the radiative and nonradiative
The bis-pyrazole complexes 4−6 are not emissive at room
temperature. At 77 K in frozen tetrahydrofuran, they display a
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Figure 5. Absorption (left) and emission (right) spectra measured at
room temperature in dichloromethane (abs) and in degassed
dichloromethane (em) of (top) 3a and 3b and (bottom) 9a and 9b.
for these acac complexes, the difference between the electro-
chemical and optical gaps is fairly constant and independent of
the presence or absence of a pyrazole unit. Combined with the
UV−vis profiles and redox potentials, these clues strongly
suggest that the excited state is localized on the phenylpyridine
ligand.
As seen above, replacing a pyridine with a pyrazole in
complexes 1−3 has virtually no effect on the redox potentials
and emission spectra. However, such a structural modification
has a significant impact on the radiative and nonradiative rate
constants. The excited-state lifetimes and quantum yields have
been measured (see Table 4) and used to calculate radiative (kr
= Φ/τ) and nonradiative [knr = (1−Φ)/τ] rate constants,
assuming unitary intersystem crossing. Compared to the bis-
phenylpyridine complexes 1−3, the phenylpyridine/phenyl-
pyrazole mixed complexes 7−10 display distinctly lower Φ with
τ dictated by the chosen phenylpyridine. As such, 7 and 10 that
contain ppy have τ = 1.60 and 1.58 μs, respectively, very close
to 1.52 μs of 1, while 8 and 9 that contain dFppy have τ = 0.62
and 0.84 μs, respectively, very close to 0.90 μs of 2. Similar τ
with lower Φ results in lower (down to about half) kr and
higher (up to 3 times) knr for the phenylpyridine/phenyl-
pyrazole mixed complexes compared to the bis-phenylpyridine
complexes without changing the emission spectra or the redox
potentials. This is particularly striking for complexes 9 and 10,
which differ only by the position of the 2,4-difluorination: their
optoelectronic properties differ only in their largely different
excited-state lifetimes.
Figure 4. Absorption (left) and emission (right) spectra measured at
room temperature in dichloromethane (abs) and in degassed
dichloromethane (em) of (top) 1, 4, and 7, (middle) 2, 5, and 8,
and (bottom) 3, 6, 9, and 10.
broad emission band with emission maxima at 538, 515, and
503 nm for 4, 6, and 5, respectively (Figure S32), and, hence,
are blue-shifted as the number of fluorine increases. Such bands
have been previously attributed to metal−ligand-to-ligand
charge transfer (3MLLCT).79,80 The emission profiles of the
tris-heteroleptic complexes follow the trend of typical iridium
complexes with phenylpyridine-based ligands, with a blue-
shifted emission as the overall number of fluorine substituents
is increased. Replacing a pyridine with a pyrazole induces only a
minor broadening (∼100 cm−1) of the spectra without
changing the emission maxima (1 vs 7; 2 vs 8; 3 vs 9 and
10). This is in line with the HOMO−LUMO gap also not
being affected by the structural change and demonstrates that,
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Figure 6. Comparison between the experimental and computed absorption spectra. The vertical bars represent the electronic transitions calculated
with LR-TDDFT, with their heights being proportional to the oscillator strengths. The dashed lines were obtained by convoluting the experimental
results with a Gaussian broadening function of full width at half-maximum of 20 nm.
The reduction of kr and increase of knr induced by the
presence of a phenylpyrazole ligand can be rationalized by
invoking a perturbative participation of the nonchromophoric
phenylpyrazole ligand to the excited state localized on the
phenylpyridine ligand. Indeed, studies of [Ir(ppz)3] and
[Ir(dFppz)3] have pointed to these complexes having low kr
on the order of 104 s−1, high knr on the order of 1012 s−1, and an
excited-state energy of about 3 eV.
When comparing the a and b diastereoisomers, no significant
differences can be found: 3a and 9a are very similar to 3b and
9b, respectively. The only statement that can be made without
exercising too much speculation is that isomers a with the
pyridine of the pic trans to the dFppy ligand have a slightly
higher (about 10%) absorption coefficient throughout the
absorption spectra and that their excited-state properties are
more favorable because of a slightly lower knr. This close
similarity between the two isomers is confirmed by theoretical
calculations.
between the isomers. The same results are obtained with
density differences calculated at the LR-TDDFT-optimized T1
geometries (Figure 7). The density differences also show
particularly well that the first triplet has a combination of
MLCT and LC character because we can distinguish the
electron (blue color) and hole (red color) contributions to the
state. The similarity between the electronic structures, observed
in both the spin densities and the density differences, of the two
isomers excludes the possibility that it causes a difference in
The absorption spectra at the ground-state equilibrium
geometry were calculated using linear-response time-dependent
density functional theory (LR-TDDFT). A total of 45 (singlet)
states for each complex were computed in order to cover the
experimental absorption spectral range. Details of the
transitions with the largest oscillator strengths are given in
Table S11. The spectra obtained with the calculations are in
good agreement with experiments (Figure 6). The first excited
state (S1) of both isomers has an MLCT character and a large
HOMO → LUMO contribution, which is, however, less than
50% in all four cases.
The estimates we obtained for the triplet emission peaks,
with the U-DFT calculations, were 2.35 eV (527 nm) for 3a,
2.36 eV (524 nm) for 3b, 2.49 eV (498 nm) for 9a, and 2.48 eV
(500 nm) for 9b. Experimental peaks are at 2.54 eV (489 nm),
2.56 (485 nm), 2.48 eV (500 nm), and 2.49 eV (498),
respectively. For 3a and 3b, our results are slightly red-shifted
but well within the expected error of the method.
The spin densities calculated at the U-DFT-optimized
geometries in the T1 state are shown in Figure S41. These
transitions have large Ir(5d) contributions and are mostly
localized on the ppy ligands for 3a and 3b and on the dFppy
ligands for 9a and 9b, with minor contributions on the pic
ligand, and we observe that there is very little difference
Figure 7. Density differences of 3a, 3b, 9a, and 9b at the LR-TDDFT-
optimized T1 geometries. The blue parts represent the positive parts of
the density (the electron), while the red parts represent the negative
parts (the hole).
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broadness observed in the emission spectra. This suggests that
this difference is very likely due to vibronic couplings. These
vibronic couplings may also be responsible for the nonradiative
deactivation of the excited states.54,81 Indeed, MC states are, in
all four cases, over 2 eV higher than T1 (see the Supporting
Information). Activation energies into the MC state are
therefore expected to also be relatively high, and thermal
activation into these quenching states is not very efficient,
although only a complete sampling of the configuration space
would answer this question.82,83 Another deactivation pathway
can be due to transitions into poorly emissive metal−ligand
(ppy)-to-ligand (pic) charge-transfer states.82
Because of the similarities between the isomers, we will
briefly discuss the pairs in comparison to their parent acac
complexes 3 and 9. Compared to 3, 3a and 3b show ∼0.06 eV
blue-shifted absorption and emission spectra, and 9a and 9b
show ∼0.06 eV blue-shifted absorption and ∼0.14 eV blue-
shifted emission spectra compared to 9. So, while both 3 and 9
display emission maxima in green at 502 nm, 3a and 3b emit at
485−489 nm and 9a and 9b at 475 nm, both in the bluish-
green region, with 9a and 9b coming close to FIrpic (468 nm)
in spite of having only two fluorine substituents.
A change of the ancillary ligand is a common strategy to tune
the emission color of cyclometalated iridium complexes either
because La is nonchromophoric and affects the energy of the
HOMO much more than the energy of the LUMO57 or
because La is chromophoric, which can result in the ancillary
ligand being the emitting ligand.84,85 In our case, pic is a
nonchromophoric ligand; therefore, changing acac for pic was
expected to result in a blue shift due to larger stabilization of
the HOMO. As was just seen, the emission is indeed blue-
shifted in complexes 3a, 3b, 9a, and 9b compared to 3 and 9,
respectively. However, this cannot be attributed to a larger
stabilization of the HOMO because the electrochemical gap is
constant within experimental error upon going from acac to pic
(see the Electrochemistry section). Therefore, the reasons for
the observed blue shift have to be attributed to the additional
term differentiating the electrochemical gap with the optical
gap.86
of the substituents. As a result, it is possible to vary kr and knr
without significantly impacting other properties.
When pic was used as the ancillary ligand, it was found that
both diastereoisomers have very similar photophysical and
electrochemical properties. It may therefore be possible to use
the mixture of isomers as materials for application without the
need for tedious purification.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b01307.
Full experimental procedures and characterization
(including NMR spectra, X-ray crystallography, FT-IR
spectra, and HPLC traces) and additional computational
data (PDF)
Accession Codes
CCDC 1511801−1511806 and 1552222−1552224 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.
AUTHOR INFORMATION
Corresponding Author
*E-mail: EB.Chimie@gmail.com.
■
ORCID
Ivano Tavernelli: 0000-0001-5690-1981
Etienne Baranoff: 0000-0003-3780-0733
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors acknowledge the European Union (HetIridium,
Grant CIG322280) for financial support, the School of
Chemistry, University of Birmingham, for a studentship, and
the Strategic Equipment Priority Fund from the University of
Birmingham.
CONCLUSION
■
In summary, a series of tris-heteroleptic cyclometalated
iridium(III) complexes, Ir(C^N1)(C^N2)(acac) (C^N1, C^N2
= ppy, dFppy, ppz, dFppz), and the corresponding bis-
heteroleptic complexes have been synthesized and fully
characterized. In addition, the use of picolinate as the ancillary
ligand with [Ir(dFppy)(ppy)] and [Ir(dFppy)(ppz)] cores
provided two pairs of diastereoisomers that have been fully
separated.
REFERENCES
■
(1) Baranoff, E.; Yum, J. H.; Jung, I.; Vulcano, R.; Gratzel, M.;
Nazeeruddin, M. K. Cyclometallated Iridium Complexes as Sensitizers
for Dye-Sensitized Solar Cells. Chem. - Asian J. 2010, 5, 496−499.
(2) Bezzubov, S. I.; Kiselev, Y. M.; Churakov, A. V.; Kozyukhin, S. A.;
Sadovnikov, A. A.; Grinberg, V. A.; Emets, V. V.; Doljenko, V. D.
Iridium(III) 2-Phenylbenzimidazole Complexes: Synthesis, Structure,
Optical Properties, and Applications in Dye-Sensitized Solar Cells. Eur.
J. Inorg. Chem. 2016, 2016, 347−354.
The exploration of various reaction conditions to obtain the
chloro-bridged iridium dimer showed that both iridium(I) and
iridium(III) can be used as starting materials to obtain tris-
heteroleptic complexes and that the most suitable conditions
depend on the core and substituents of the ligands used. If the
reactivity of the ligands is known, this opens up opportunities
for devising an effective stepwise synthesis with the potential to
greatly increase the yield of the tris-heteroleptic complex.
Photophysical and electrochemical studies showed that
absorption and emission wavelengths and redox properties
are governed primarily by the overall structure of the
complexes, while dynamic properties such as radiative and
nonradiative constants are controlled by the specific placement
(3) Fleetham, T. B.; Wang, Z.; Li, J. Exploring cyclometalated Ir
complexes as donor materials for organic solar cells. Inorg. Chem. 2013,
52, 7338−7343.
́ ́
(4) Gennari, M.; Legalite, F.; Zhang, L.; Pellegrin, Y.; Blart, E.;
Fortage, J.; Brown, A. M.; Deronzier, A.; Collomb, M.-N.; Boujtita, M.;
Jacquemin, D.; Hammarstrom, L.; Odobel, F. Long-Lived Charge
̈
Separated State in NiO-Based p-Type Dye-Sensitized Solar Cells with
Simple Cyclometalated Iridium Complexes. J. Phys. Chem. Lett. 2014,
5, 2254−2258.
(5) Huang, J. A.; Yu, J. S.; Guan, Z. Q.; Jiang, Y. D. Improvement in
open circuit voltage of organic solar cells by inserting a thin
I
DOI: 10.1021/acs.inorgchem.7b01307
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
phosphorescent iridium complex layer. Appl. Phys. Lett. 2010, 97,
143301.
Pentafluoride-Functionalized Cationic Iridium(III) Complexes. Inorg.
Chem. 2015, 54, 5907−5914.
(6) Mayo, E. I.; Kilsa, K.; Tirrell, T.; Djurovich, P. I.; Tamayo, A.;
Thompson, M. E.; Lewis, N. S.; Gray, H. B. Cyclometalated
iridium(III)-sensitized titanium dioxide solar cells. Photochem. Photo-
biol. Sci. 2006, 5, 871−873.
(22) Tokito, S. Recent progress in phosphorescent materials for
organic light-emitting devices. J. Photopolym. Sci. Technol. 2004, 17,
307−313.
(23) Tsuzuki, T.; Tokito, S. Highly efficient and low-voltage
phosphorescent organic light-emitting diodes using an iridium
complex as the host material. Adv. Mater. 2007, 19, 276−280.
(24) Wang, R. J.; Liu, D.; Zhang, R.; Deng, L. J.; Li, J. Y. Solution-
processable iridium complexes for efficient orange-red and white
organic light-emitting diodes. J. Mater. Chem. 2012, 22, 1411−1417.
(25) Xia, D.; Wang, B.; Chen, B.; Wang, S.; Zhang, B.; Ding, J.;
Wang, L.; Jing, X.; Wang, F. Self-host blue-emitting iridium dendrimer
with carbazole dendrons: nondoped phosphorescent organic light-
emitting diodes. Angew. Chem., Int. Ed. 2014, 53, 1048−1052.
(26) Bolink, H. J.; Cappelli, L.; Coronado, E.; Parham, A.; Stossel, P.
Green light-emitting solid-state electrochemical cell obtained from a
homoleptic iridium(III) complex containing ionically charged ligands.
Chem. Mater. 2006, 18, 2778−2780.
(27) Bolink, H. J.; Coronado, E.; Costa, R. D.; Orti, E.; Sessolo, M.;
Graber, S.; Doyle, K.; Neuburger, M.; Housecroft, C. E.; Constable, E.
C. Long-Living Light-Emitting Electrochemical Cells - Control
through Supramolecular Interactions. Adv. Mater. 2008, 20, 3910−
3913.
(28) Chen, H. F.; Wu, C.; Kuo, M. C.; Thompson, M. E.; Wong, K.
T. Anionic iridium complexes for solid state light-emitting electro-
chemical cells. J. Mater. Chem. 2012, 22, 9556−9561.
(29) Costa, R. D.; Orti, E.; Bolink, H. J. Recent advances in light-
emitting electrochemical cells. Pure Appl. Chem. 2011, 83, 2115−2128.
(30) Costa, R. D.; Orti, E.; Bolink, H. J.; Graber, S.; Schaffner, S.;
Neuburger, M.; Housecroft, C. E.; Constable, E. C. Archetype Cationic
Iridium Complexes and Their Use in Solid-State Light-Emitting
Electrochemical Cells. Adv. Funct. Mater. 2009, 19, 3456−3463.
(31) Evariste, S.; Sandroni, M.; Rees, T. W.; Roldan-Carmona, C.;
Gil-Escrig, L.; Bolink, H. J.; Baranoff, E.; Zysman-Colman, E. Fluorine-
free blue-green emitters for light-emitting electrochemical cells. J.
Mater. Chem. C 2014, 2, 5793−5804.
(32) Mydlak, M.; Bizzarri, C.; Hartmann, D.; Sarfert, W.; Schmid, G.;
De Cola, L. Positively Charged Iridium(III) Triazole Derivatives as
Blue Emitters for Light-Emitting Electrochemical Cells. Adv. Funct.
Mater. 2010, 20, 1812−1820.
(33) Su, H. C.; Fang, F. C.; Hwu, T. Y.; Hsieh, H. H.; Chen, H. F.;
Lee, G. H.; Peng, S. M.; Wong, K. T.; Wu, C. C. Highly efficient
orange and green solid-state light-emitting electrochemical cells based
on cationic Ir-III complexes with enhanced steric hindrance. Adv.
Funct. Mater. 2007, 17, 1019−1027.
(34) Zhang, J.; Zhou, L.; Al-Attar, H. A.; Shao, K.; Wang, L.; Zhu, D.;
Su, Z.; Bryce, M. R.; Monkman, A. P. Efficient Light-Emitting
Electrochemical Cells (LECs) Based on Ionic Iridium(III) Complexes
with 1,3,4-Oxadiazole Ligands. Adv. Funct. Mater. 2013, 23, 4667−
4677.
(7) Tan, G.; Liu, P.; Wu, H.; Yiu, S.-C.; Dai, F.; Feng, Y.-H.; Liu, X.;
Qiu, Y.; Lo, Y. H.; Ho, C.-L.; Wong, W.-Y. New iridium(III)
cyclometalates with extended absorption features for bulk hetero-
junction solar cells. J. Organomet. Chem. 2016, 812, 280−286.
(8) Wang, D.; Dong, H.; Wu, Y.; Yu, Y.; Zhou, G.; Li, L.; Wu, Z.;
Gao, M.; Wang, G. Effect of diphenylamine substituent on charge-
transfer absorption features of the iridium complexes and application
in dye-sensitized solar cell. J. Organomet. Chem. 2015, 775, 55−59.
(9) Adachi, C.; Baldo, M. A.; Thompson, M. E.; Forrest, S. R. Nearly
100% internal phosphorescence efficiency in an organic light-emitting
device. J. Appl. Phys. 2001, 90, 5048−5051.
(10) Forrest, S. R.; Baldo, M. A.; Thompson, M. E. High-efficiency
fluorescent organic light-emitting devices using a phosphorescent
sensitizer. Nature 2000, 403, 750−753.
(11) Chiu, Y. C.; Hung, J. Y.; Chi, Y.; Chen, C. C.; Chang, C. H.;
Wu, C. C.; Cheng, Y. M.; Yu, Y. C.; Lee, G. H.; Chou, P. T. En Route
to High External Quantum Efficiency (similar to 12%), Organic True-
Blue-Light-Emitting Diodes Employing Novel Design of Iridium (III)
Phosphors. Adv. Mater. 2009, 21, 2221−2225.
(12) Henwood, A. F.; Bansal, A. K.; Cordes, D. B.; Slawin, A. M. Z.;
Samuel, I. D. W.; Zysman-Colman, E. Solubilised bright blue-emitting
iridium complexes for solution processed OLEDs. J. Mater. Chem. C
2016, 4, 3726−3737.
(13) Ho, C. L.; Wong, W. Y.; Wang, Q.; Ma, D. G.; Wang, L. X.; Lin,
Z. Y. A multifunctional iridium-carbazolyl orange phosphor for high-
performance two-element WOLED exploiting Exciton-Managed
fluorescence/phosphorescence. Adv. Funct. Mater. 2008, 18, 928−937.
(14) Jou, J.-H.; Peng, S.-H.; Chiang, C.-I.; Chen, Y.-L.; Lin, Y.-X.;
Jou, Y.-C.; Chen, C.-H.; Li, C.-J.; Wang, W.-B.; Shen, S.-M.; Chen, S.-
Z.; Wei, M.-K.; Sun, Y.-S.; Hung, H.-W.; Liu, M.-C.; Lin, Y.-P.; Li, J.-
Y.; Wang, C.-W. High efficiency yellow organic light-emitting diodes
with a solution-processed molecular host-based emissive layer. J.
Mater. Chem. C 2013, 1, 1680−1686.
(15) Kim, T. H.; Lee, H. K.; Park, O. O.; Chin, B. D.; Lee, S. H.;
Kim, J. K. White-light-emitting diodes based on iridium complexes via
efficient energy transfer from a conjugated polymer. Adv. Funct. Mater.
2006, 16, 611−617.
(16) Lamansky, S.; Djurovich, P. I.; Abdel-Razzaq, F.; Garon, S.;
Murphy, D. L.; Thompson, M. E. Cyclometalated Ir complexes in
polymer organic light-emitting devices. J. Appl. Phys. 2002, 92, 1570−
1575.
(17) Li, C. L.; Su, Y. J.; Tao, Y. T.; Chou, P. T.; Chien, C. H.; Cheng,
C. C.; Liu, R. S. Yellow and red electrophosphors based on linkage
isomers of phenylisoquinolinyliridium complexes: Distinct differences
in photophysical and electroluminescence properties. Adv. Funct.
Mater. 2005, 15, 387−395.
(18) Lo, S. C.; Namdas, E. B.; Burn, P. L.; Samuel, I. D. W. Synthesis
and properties of highly efficient electroluminescent green phosphor-
escent iridium cored dendrimers. Macromolecules 2003, 36, 9721−
9730.
(19) Nasr, G.; Guerlin, A.; Dumur, F.; Beouch, L.; Dumas, E.;
Clavier, G.; Miomandre, F.; Goubard, F.; Gigmes, D.; Bertin, D.;
Wantz, G.; Mayer, C. R. Iridium(iii) soft salts from dinuclear cationic
and mononuclear anionic complexes for OLED devices. Chem.
Commun. 2011, 47, 10698−10700.
(20) Sasabe, H.; Takamatsu, J.-I.; Motoyama, T.; Watanabe, S.;
Wagenblast, G.; Langer, N.; Molt, O.; Fuchs, E.; Lennartz, C.; Kido, J.
High-efficiency blue and white organic light-emitting devices
incorporating a blue iridium carbene complex. Adv. Mater. 2010, 22,
5003−5007.
(21) Shavaleev, N. M.; Xie, G.; Varghese, S.; Cordes, D. B.; Slawin, A.
M.; Momblona, C.; Orti, E.; Bolink, H. J.; Samuel, I. D.; Zysman-
Colman, E. Green Phosphorescence and Electroluminescence of Sulfur
(35) Mak, C. S. K.; Pentlehner, D.; Stich, M.; Wolfbeis, O. S.; Chan,
W. K.; Yersin, H. Exceptional Oxygen Sensing Capabilities and Triplet
State Properties of Ir(ppy-NPh2)3. Chem. Mater. 2009, 21, 2173−
2175.
(36) Marin-Suarez, M.; Curchod, B. F. E.; Tavernelli, I.;
Rothlisberger, U.; Scopelliti, R.; Jung, I.; Di Censo, D.; Gratzel, M.;
Fernandez-Sanchez, J. F.; Fernandez-Gutierrez, A.; Nazeeruddin, M.
K.; Baranoff, E. Nanocomposites Containing Neutral Blue Emitting
Cyclometalated Iridium(III) Emitters for Oxygen Sensing. Chem.
Mater. 2012, 24, 2330−2338.
(37) Xie, Z. G.; Ma, L. Q.; deKrafft, K. E.; Jin, A.; Lin, W. B. Porous
Phosphorescent Coordination Polymers for Oxygen Sensing. J. Am.
Chem. Soc. 2010, 132, 922−923.
(38) You, Y.; Cho, S.; Nam, W. Cyclometalated iridium(III)
complexes for phosphorescence sensing of biological metal ions.
Inorg. Chem. 2014, 53, 1804−1815.
J
DOI: 10.1021/acs.inorgchem.7b01307
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
(39) You, Y. M.; Park, S. Y. A Phosphorescent Ir(III) Complex for
Selective Fluoride Ion Sensing with a High Signal-to-Noise Ratio. Adv.
Mater. 2008, 20, 3820−3826.
(40) Liu, J.; Liu, Y.; Liu, Q.; Li, C.; Sun, L.; Li, F. Iridium(III)
Complex-Coated Nanosystem for Ratiometric Upconversion Lumi-
nescence Bioimaging of Cyanide Anions. J. Am. Chem. Soc. 2011, 133,
15276−15279.
(41) Lo, K. K.-W.; Zhang, K. Y. Iridium(iii) complexes as therapeutic
and bioimaging reagents for cellular applications. RSC Adv. 2012, 2,
12069−12083.
(42) Tang, Y.; Yang, H.-R.; Sun, H.-B.; Liu, S.-J.; Wang, J.-X.; Zhao,
Q.; Liu, X.-M.; Xu, W.-J.; Li, S.-B.; Huang, W. Rational Design of an
“OFF-ON” Phosphorescent Chemodosimeter Based on an Iridium-
(III) Complex and Its Application for Time-Resolved Luminescent
Detection and Bioimaging of Cysteine and Homocysteine. Chem. -
Eur. J. 2013, 19, 1311−1319.
(43) Zhang, K. Y.; Liu, H. W.; Tang, M. C.; Choi, A. W.; Zhu, N.;
Wei, X. G.; Lau, K. C.; Lo, K. K. Dual-Emissive Cyclometalated
Iridium(III) Polypyridine Complexes as Ratiometric Biological Probes
and Organelle-Selective Bioimaging Reagents. Inorg. Chem. 2015, 54,
6582−6593.
(44) Nguyen, J. D.; D’Amato, E. M.; Narayanam, J. M.; Stephenson,
C. R. Engaging unactivated alkyl, alkenyl and aryl iodides in visible-
light-mediated free radical reactions. Nat. Chem. 2012, 4, 854−859.
(45) Nguyen, J. D.; Tucker, J. W.; Konieczynska, M. D.; Stephenson,
C. R. J. Intermolecular atom transfer radical addition to olefins
mediated by oxidative quenching of photoredox catalysts. J. Am. Chem.
Soc. 2011, 133, 4160−4163.
control of excited-state properties in cyclometalated Ir(III) complexes
using ancillary ligands. Inorg. Chem. 2005, 44, 1713−1727.
(58) Treadway, J. A.; Loeb, B.; Lopez, R.; Anderson, P. A.; Keene, F.
R.; Meyer, T. J. Effect of delocalization and rigidity in the acceptor
ligand on MLCT excited-state decay. Inorg. Chem. 1996, 35, 2242−
2246.
(59) Takizawa, S. Y.; Shimada, K.; Sato, Y.; Murata, S. Controlling
the excited state and photosensitizing property of a 2-(2-pyridyl)-
benzo[b]thiophene-based cationic iridium complex through simple
chemical modification. Inorg. Chem. 2014, 53, 2983−2995.
(60) Baranoff, E.; Curchod, B. F.; Frey, J.; Scopelliti, R.; Kessler, F.;
Tavernelli, I.; Rothlisberger, U.; Gratzel, M.; Nazeeruddin, M. K. Acid-
induced degradation of phosphorescent dopants for OLEDs and its
application to the synthesis of tris-heteroleptic iridium(III) bis-
cyclometalated complexes. Inorg. Chem. 2012, 51, 215−224.
(61) Edkins, R. M.; Wriglesworth, A.; Fucke, K.; Bettington, S. L.;
Beeby, A. The synthesis and photophysics of tris-heteroleptic
cyclometalated iridium complexes. Dalton Trans. 2011, 40, 9672−
9678.
(62) Felici, M.; Contreras-Carballada, P.; Smits, J. M. M.; Nolte, R. J.
M.; Williams, R. M.; De Cola, L.; Feiters, M. C. Cationic Heteroleptic
Cyclometalated Iridium(III) Complexes Containing Phenyl-Triazole
and Triazole-Pyridine Clicked Ligands. Molecules 2010, 15, 2039−
2059.
(63) Lepeltier, M.; Graff, B.; Lalevee, J.; Wantz, G.; Ibrahim-Ouali,
M.; Gigmes, D.; Dumur, F. Heteroleptic iridium (III) complexes with
three different ligands: Unusual triplet emitters for light-emitting
electrochemical cells. Org. Electron. 2016, 37, 24−34.
(46) Flamigni, L.; Barbieri, A.; Sabatini, C.; Ventura, B.; Barigelletti,
F. Photochemistry and photophysics of coordination compounds:
Iridium. Top. Curr. Chem. 2007, 281, 143−203.
(64) Liao, J. L.; Chi, Y.; Sie, Z. T.; Ku, C. H.; Chang, C. H.; Fox, M.
A.; Low, P. J.; Tseng, M. R.; Lee, G. H. Ir(III)-Based Phosphors with
Bipyrazolate Ancillaries; Rational Design, Photophysics, and Applica-
tions in Organic Light-Emitting Diodes. Inorg. Chem. 2015, 54,
10811−10821.
(47) Jiang, J. X.; Yang, W.; Cao, Y. Recent advances in organometallic
polymers as highly efficient electrophosphorescent emitters. J. Inorg.
Organomet. Polym. Mater. 2007, 17, 37−55.
(65) Park, G. Y.; Kim, Y.; Ha, Y. Iridium complexes containing three
different ligands as white OLED dopants. Mol. Cryst. Liq. Cryst. 2006,
462, 179−188.
(48) Ulbricht, C.; Beyer, B.; Friebe, C.; Winter, A.; Schubert, U. S.
Recent Developments in the Application of Phosphorescent Iridium-
(III) Complex Systems. Adv. Mater. 2009, 21, 4418−4441.
(49) You, Y.; Park, S. Y. Phosphorescent iridium(III) complexes:
toward high phosphorescence quantum efficiency through ligand
control. Dalton Trans. 2009, 1267−1282.
(66) Tordera, D.; Delgado, M.; Orti, E.; Bolink, H. J.; Frey, J.;
Nazeeruddin, M. K.; Baranoff, E. Stable Green Electroluminescence
from an Iridium Tris-Heteroleptic Ionic Complex. Chem. Mater. 2012,
24, 1896−1903.
(50) Tamayo, A. B.; Alleyne, B. D.; Djurovich, P. I.; Lamansky, S.;
Tsyba, I.; Ho, N. N.; Bau, R.; Thompson, M. E. Synthesis and
characterization of facial and meridional tris-cyclometalated iridium-
(III) complexes. J. Am. Chem. Soc. 2003, 125, 7377−7387.
(51) Frey, J.; Curchod, B. F. E.; Scopelliti, R.; Tavernelli, I.;
Rothlisberger, U.; Nazeeruddin, M. K.; Baranoff, E. Structure-property
relationships based on Hammett constants in cyclometalated iridium-
(III) complexes: their application to the design of a fluorine-free
FIrPic-like emitter. Dalton Trans. 2014, 43, 5667−5679.
(52) Abe, T.; Miyazawa, A.; Konno, H.; Kawanishi, Y. Deuteration
isotope effect on nonradiative transition of fac-tris (2-phenyl-
pyridinato) iridium (III) complexes. Chem. Phys. Lett. 2010, 491,
199−202.
(67) Tordera, D.; Serrano-Perez, J. J.; Pertegas, A.; Orti, E.; Bolink,
H. J.; Baranoff, E.; Nazeeruddin, M. K.; Frey, J. Correlating the
Lifetime and Fluorine Content of Iridium(III) Emitters in Green
Light-Emitting Electrochemical Cells. Chem. Mater. 2013, 25, 3391−
3397.
(68) Xu, X.; Yang, X.; Dang, J.; Zhou, G.; Wu, Y.; Li, H.; Wong, W.
Y. Trifunctional Ir(III) ppy-type asymmetric phosphorescent emitters
with ambipolar features for highly efficient electroluminescent devices.
Chem. Commun. 2014, 50, 2473−2476.
(69) Lepeltier, M.; Dumur, F.; Graff, B.; Xiao, P.; Gigmes, D.;
Lalevee, J.; Mayer, C. R. Tris-cyclometalated Iridium(III) Complexes
with Three Different Ligands: a New Example with 2-(2,4-
Difluorophenyl)pyridine-Based Complex. Helv. Chim. Acta 2014, 97,
939−956.
(70) Tamura, Y.; Hisamatsu, Y.; Kumar, S.; Itoh, T.; Sato, K.;
Kuroda, R.; Aoki, S. Efficient Synthesis of Tris-Heteroleptic Iridium-
(III) Complexes Based on the Zn2+-Promoted Degradation of Tris-
Cyclometalated Iridium(III) Complexes and Their Photophysical
Properties. Inorg. Chem. 2017, 56, 812−833.
(53) Haneder, S.; Da Como, E.; Feldmann, J.; Lupton, J. M.;
Lennartz, C.; Erk, P.; Fuchs, E.; Molt, O.; Munster, I.; Schildknecht,
̈
C.; Wagenblast, G. Controlling the Radiative Rate of Deep-Blue
Electrophosphorescent Organometallic Complexes by Singlet-Triplet
Gap Engineering. Adv. Mater. 2008, 20, 3325−3330.
(54) Harding, R. E.; Lo, S. C.; Burn, P. L.; Samuel, I. D. W. Non-
radiative decay mechanisms in blue phosphorescent iridium(III)
complexes. Org. Electron. 2008, 9, 377−384.
(71) Bottcher, H. C.; Graf, M.; Sunkel, K.; Mayer, P.; Kruger, H.
[Ir(acac)(eta(2)-C8H14)(2)]: A precursor in the synthesis of
cyclometalated iridium(III) complexes. Inorg. Chim. Acta 2011, 365,
103−107.
(72) Albrecht, M. Cyclometalation Using d-Block Transition Metals:
Fundamental Aspects and Recent Trends. Chem. Rev. 2010, 110, 576−
623.
(55) Karatsu, T.; Ito, E.; Yagai, S.; Kitamura, A. Radiative and
nonradiative processes of meridional and facial isomers of heteroleptic
irldium-trischelete complexes. Chem. Phys. Lett. 2006, 424, 353−357.
(56) Zhou, X.; Burn, P. L.; Powell, B. J. Bond Fission and Non-
Radiative Decay in Iridium(III) Complexes. Inorg. Chem. 2016, 55,
5266−5273.
(73) Baranoff, E.; Curchod, B. F. E.; Monti, F.; Steimer, F.; Accorsi,
(57) Li, J.; Djurovich, P. I.; Alleyne, B. D.; Yousufuddin, M.; Ho, N.
N.; Thomas, J. C.; Peters, J. C.; Bau, R.; Thompson, M. E. Synthetic
G.; Tavernelli, I.; Rothlisberger, U.; Scopelliti, R.; Gratzel, M.;
̈
Nazeeruddin, M. K. Influence of Halogen Atoms on a Homologous
K
DOI: 10.1021/acs.inorgchem.7b01307
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Series of Bis-Cyclometalated Iridium(III) Complexes. Inorg. Chem.
2012, 51, 799−811.
(74) Baranoff, E.; Curchod, B. F. E. FIrpic: archetypal blue
phosphorescent emitter for electroluminescence. Dalton Trans. 2015,
44, 8318−8329.
(75) Park, N. G.; Choi, G. C.; Lee, Y. H.; Kim, Y. S. Theoretical
studies on the ground and excited states of blue phosphorescent
cyclometalated Ir (III) complexes having ancillary ligand. Curr. Appl.
Phys. 2006, 6, 620−626.
(76) Gu, X.; Fei, T.; Zhang, H. Y.; Xu, H.; Yang, B.; Ma, Y. G.; Liu, X.
D. Theoretical studies of blue-emitting iridium complexes with
different ancillary ligands. J. Phys. Chem. A 2008, 112, 8387−8393.
(77) Li, H. F.; Winget, P.; Risko, C.; Sears, J. S.; Bredas, J. L. Tuning
the electronic and photophysical properties of heteroleptic iridium(III)
phosphorescent emitters through ancillary ligand substitution: a
theoretical perspective. Phys. Chem. Chem. Phys. 2013, 15, 6293−6302.
(78) Tsai, H.-R.; Lu, K.-Y.; Lai, S.-H.; Fan, C.-H.; Cheng, C.-H.;
Chen, I. C. Metal−Ligand Bonding Strength of Fluoro-Substituted
Cyclometalated Iridium(III) Complexes from Raman and Infrared
Spectra. J. Phys. Chem. C 2011, 115, 17163−17174.
(79) Batagoda, B. K. T.; Djurovich, P. I.; Brase, S.; Thompson, M. E.
̈
Synthesis and characterization of phosphorescent cyclometalated Ir
and Pt heteroleptic complexes using cyclophane-based chelates.
Polyhedron 2016, 116, 182−188.
(80) Fei, T.; Gu, X.; Zhang, M.; Wang, C.; Hanif, M.; Zhang, H.; Ma,
Y. Optical and electronic properties of phosphorescent iridium(III)
complexes with phenylpyrazole and ancillary ligands. Synth. Met. 2009,
159, 113−118.
(81) Pomarico, E.; Silatani, M.; Messina, F.; Braem, O.; Cannizzo, A.;
Barranoff, E.; Klein, J. H.; Lambert, C.; Chergui, M. Dual
Luminescence, Interligand Decay, and Nonradiative Electronic
Relaxation of Cyclometalated Iridium Complexes in Solution. J.
Phys. Chem. C 2016, 120, 16459−16469.
(82) Escudero, D.; Thiel, W. Exploring the Triplet Excited State
Potential Energy Surfaces of a Cyclometalated Pt(II) Complex: Is
There Non-Kasha Emissive Behavior? Inorg. Chem. 2014, 53, 11015−
11019.
(83) Younker, J. M.; Dobbs, K. D. Correlating Experimental
Photophysical Properties of Iridium(III) Complexes to Spin−Orbit
Coupled TDDFT Predictions. J. Phys. Chem. C 2013, 117, 25714−
25723.
(84) You, Y.; Kim, K. S.; Ahn, T. K.; Kim, D.; Park, S. Y. Direct
spectroscopic observation of interligand energy transfer in cyclo-
metalated heteroleptic iridium(III) complexes: A strategy for
phosphorescence color tuning and white light generation. J. Phys.
Chem. C 2007, 111, 4052−4060.
(85) You, Y. M.; Park, S. Y. Inter-ligand energy transfer and related
emission change in the cyclometalated heteroleptic iridium complex:
Facile and efficient color tuning over the whole visible range by the
ancillary ligand structure. J. Am. Chem. Soc. 2005, 127, 12438−12439.
(86) Vlcek, A. A.; Dodsworth, E. S.; Pietro, W. J.; Lever, A. B. P.
Excited-State Redox Potentials of Ruthenium Diimine Complexes -
Correlations with Ground-State Redox Potentials and Ligand
Parameters. Inorg. Chem. 1995, 34, 1906−1913.
L
DOI: 10.1021/acs.inorgchem.7b01307
Inorg. Chem. XXXX, XXX, XXX−XXX