Highly Stable and Efficient Light-Emitting Electrochemical Cells Based on Cationic Iridium Complexes Bearing Arylazole Ancillary Ligands

Article abstract of DOI:10.1021/acs.inorgchem.7b01167

A series of bis-cyclometalated iridium(III) complexes of general formula [Ir(ppy)₂(N^∧N)][PF₆] (ppy^- = 2-phenylpyridinate; N^∧N = 2-(1H-imidazol-2-yl)pyridine (1), 2-(2-pyridyl)benzimidazole (2), 1-methyl-2-pyridin-2-yl-1H-benzimidazole (3), 2-(4′-thiazolyl)benzimidazole (4), 1-methyl-2-(4′-thiazolyl)benzimidazole (5)) is reported, and their use as electroluminescent materials in light-emitting electrochemical cell (LEC) devices is investigated. [2][PF₆] and [3][PF₆] are orange emitters with intense unstructured emission around 590 nm in acetonitrile solution. [1][PF₆], [4][PF₆], and [5][PF₆] are green weak emitters with structured emission bands peaking around 500 nm. The different photophysical properties are due to the effect that the chemical structure of the ancillary ligand has on the nature of the emitting triplet state. Whereas the benzimidazole unit stabilizes the LUMO and gives rise to a ³MLCT/³LLCT emitting triplet in [2][PF₆] and [3][PF₆], the presence of the thiazolyl ring produces the opposite effect in [4][PF₆] and [5][PF₆] and the emitting state has a predominant ³LC character. Complexes with ³MLCT/³LLCT emitting triplets give rise to LEC devices with luminance values 1 order higher than those of complexes with ³LC emitting states. Protecting the imidazole N-H bond with a methyl group, as in complexes [3][PF₆] and [5][PF₆], shows that the emissive properties become more stable. [3][PF₆] leads to outstanding LECs with simultaneously high luminance (904 cd m^-2), efficiency (9.15 cd A^-1), and stability (lifetime over 2500 h).

Full text of DOI:10.1021/acs.inorgchem.7b01167

Article
pubs.acs.org/IC
Highly Stable and Efficient Light-Emitting Electrochemical Cells
Based on Cationic Iridium Complexes Bearing Arylazole Ancillary
Ligands
†
́ ́ ́
Marta Martínez-Alonso, Jesus Cristina Momblona, Antonio Pertegas
Cerda,‡ ,‡
‡
‡
†
§
,†
́ ́ ́
Jose M. Junquera-Hernandez, Aranzazu Heras, Ana M. Rodríguez, Gustavo Espino,*
,‡
,‡
Henk Bolink,* and Enrique Ortí*
†
Departamento de Química, Facultad de Ciencias, Universidad de Burgos, Plaza Misael Ban
̃
uelos s/n, 09001 Burgos, Spain
‡
Instituto de Ciencia Molecular, Universidad de Valencia, Catedrat
́
ico Jose
́ ́
Beltran 2, 46980 Paterna, Spain
§
Departamento de Química Inorganica, Organica y Bioquímica, Facultad de Químicas, Universidad de Castilla-La Mancha, Avda.
́
́
Camilo J. Cela 10, 13071 Ciudad Real, Spain
S Supporting Information
*
ABSTRACT: A series of bis-cyclometalated iridium(III)
∧
−
complexes of general formula [Ir(ppy) (N N)][PF ] (ppy
=
2
6
∧
2-phenylpyridinate; N N = 2-(1H-imidazol-2-yl)pyridine
(
1), 2-(2-pyridyl)benzimidazole (2), 1-methyl-2-pyridin-2-yl-
H-benzimidazole (3), 2-(4′-thiazolyl)benzimidazole (4), 1-
1
methyl-2-(4′-thiazolyl)benzimidazole (5)) is reported, and
their use as electroluminescent materials in light-emitting
electrochemical cell (LEC) devices is investigated. [2][PF6]
and [3][PF ] are orange emitters with intense unstructured
6
emission around 590 nm in acetonitrile solution. [1][PF6],
[
4][PF ], and [5][PF ] are green weak emitters with
6 6
structured emission bands peaking around 500 nm. The
different photophysical properties are due to the effect that the
chemical structure of the ancillary ligand has on the nature of the emitting triplet state. Whereas the benzimidazole unit stabilizes
3
3
the LUMO and gives rise to a MLCT/ LLCT emitting triplet in [2][PF ] and [3][PF ], the presence of the thiazolyl ring
6
6
3
produces the opposite effect in [4][PF ] and [5][PF ] and the emitting state has a predominant LC character. Complexes with
MLCT/ LLCT emitting triplets give rise to LEC devices with luminance values 1 order higher than those of complexes with
6
6
3
3
3
LC emitting states. Protecting the imidazole N−H bond with a methyl group, as in complexes [3][PF ] and [5][PF ], shows
6
6
that the emissive properties become more stable. [3][PF ] leads to outstanding LECs with simultaneously high luminance (904
6
−2
−1
cd m ), efficiency (9.15 cd A ), and stability (lifetime over 2500 h).
INTRODUCTION
with iridium(III) complexes due to their efficient spin−orbit
coupling, which ideally allows harvesting of 100% of the
excitons generated in the device.
■
Energy-efficient devices for lighting and display applications are
highly demanded. During the last few years, organic light-
emitting diodes (OLEDs)
efficient displays, but their high manufacturing costs make them
16
1
−3
The optical and emitting properties of Ir-iTMCs can be
easily modulated by chemically modifying the structure of the
ligands attached to the central Ir(III) core. Nowadays, a wide
variety of complexes has been designed and used to
manufacture LEC devices covering the whole visible
have been widely used in energy-
unsuitable for wide use in lighting. Light-emitting electro-
4
,5
chemical cells (LECs) have emerged as potential candidates
for light-emitting devices with promising energy savings and
high-quality lighting. LECs are composed of a single active layer
of an ionic organic or organometallic semiconductor placed
between two electrodes. Due to their simple single-layer
structure and their easy processing from solution, LECs are
16−19
20−22
23−25
spectrum,
with high efficiencies,
top lifetimes,
26−28
and short turn-on times.
However, there is still to be
found a complex that allows the fabrication of a device which
unites all of these characteristics and is suitable for real-life
lighting applications.
6
compatible with low-cost manufacturing and constitute a good
alternative to OLEDs for lighting. A wide range of organic
materials has been used in the manufacture of LECs.
However, the best efficiencies have been achieved with ionic
transition-metal complexes (iTMCs)
7
−11
Received: May 10, 2017
12−15
and, in particular,
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.7b01167
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Inorganic Chemistry
Article
One desirable key feature for LEC devices is a long lifetime,
which should be in the range of a few thousands of hours if
LECs have to compete with commonly used lighting devices. A
long device lifetime is related to a high complex stability. The
most common strategy to achieve such high stability has been
the inclusion of bulky hydrophobic ligands to protect the
24,29
coordination center.
However, bulky groups are known to
slow down the migration of the ions in the device, inducing
3
0,31
long turn-on times.
Hence, there is still room to explore
new strategies and ligands in the search for higher perform-
ances and stabilities.
In this work, we introduce a series of new complexes with
arylazole-based ancillary ligands without bulky groups and use
them to prepare LEC devices trying to reach high stabilities
without affecting the turn-on times. Arylazoles present some
advantages as ancillary ligands to prepare iridium iTMCs. They
are either commercially available or are easily prepared and
offer a great deal of structural and chemical diversity, since a
variety of aryl and azole entities (imidazole, oxazole, thiazole,
etc.) can be used. In addition, the enhanced electron-donating
nature of the azole rings, due to the presence of one additional
heteroatom with one or two lone electron pairs (N−H, O, or
S), tends to destabilize the lowest unoccupied molecular orbital
Figure 1. Arylazole ligands used in this work and chemical structures
of the synthesized iridium(III) complexes [1][PF ]−[5][PF ].
6
6
(LUMO), enlarging the optical gap and blue-shifting the light
emitted in comparison to complexes bearing classical 2,2′-
bipyridine (bpy)-type ligands. Arylazole ligands can therefore
be used to fine tune the photophysical and chemical properties
properties was investigated and further analyzed with the help
of density functional theory (DFT) calculations. The electro-
luminescence properties of the complexes were investigated in
3
2
of Ir-iTMCs. In particular, the imidazole ring can furthermore
be functionalized through the reactive N−H bond.
LEC devices operated under pulsed driving conditions.
44
Complex [2][PF ] was previously known, but it has not yet
6
The electroluminescence properties of Ir-iTMCs bearing
arylazole units as cyclometalating ligands have been widely
been used in LEC devices. Complex [3][PF ] and its use in
6
LECs under voltage scan (current density and luminance) were
reported but, to the best of our knowledge, no investigation of
33−37
investigated in LECs.
In contrast, few examples of Ir-
iTMCs with arylazole-based ancillary ligands have been tested
40
the device performance vs time was performed.
3
8
in LECs. He et al. described LEC devices using a family of Ir-
iTMCs incorporating functionalized imidazole-type ancillary
ligands with peak external quantum efficiencies of up to 6.1%, a
RESULTS AND DISCUSSION
Synthesis and Characterization. The complexes depicted
in Figure 1 were prepared following the Nonoyama protocol
■
−2
maximum luminance of 395 cd m , and different emission
colors depending on the complex. One year later, the same
group reported a blue-green LEC with a pyrazole-type ancillary
ligand and a peak external quantum efficiency of 7.6%, although
45
in two steps. In the first step, the chloro-bridged dimeric
iridium complex [Ir(μ-Cl)(ppy)2]2 was isolated from the
reaction between IrCl ·xH O and 2-phenylpyridine (Hppy).
−2
39
3
2
at a much lower luminance of less than 20 cd m . In both
cases, the devices exhibited long turn-on times from several
minutes to hours and a poor device stability under constant
voltage operation. More recently, Choe and co-workers have
reported and characterized LECs based on arylimidazole
ancillary ligands exhibiting high brightness and color tunability,
although no study of the time-response characteristics was
∧
In the second step, the corresponding ancillary N N ligand was
introduced in the coordination sphere of Ir(III) through a
bridge-splitting reaction, and then an anion exchange protocol
was applied to isolate the PF6− salt, as outlined in Scheme 1.
The complexes were isolated in good yields as racemic mixtures
(
Δ, Λ) of yellow to orange solids that are soluble in common
organic solvents.
4
0−43
reported.
All of the complexes were characterized by multinuclear
NMR, IR, and mass spectroscopy and by elemental analysis. 2D
In this work, the complexes [1][PF ]−[5][PF ] shown in
6
6
Figure 1 have been synthesized, characterized, and employed to
develop new LEC devices. Their general formula is [Ir-
Scheme 1. Second Step in the Synthetic Route for
∧
∧
∧
(
C N) (N N)][PF ], where the C N ligand always corre-
2 6
Complexes [1][PF ]−[5][PF ]
−
∧
6
6
sponds to 2-phenylpyridinate (ppy ) and the ancillary N N
ligand is 2-(1H-imidazol-2-yl)pyridine (pyim), 2-(2-pyridyl)-
benzimidazole (pybim), 1-methyl-2-pyridin-2-yl-1H-benzimida-
zole (pyMebim), 2-(4′-thiazolyl)benzimidazole (thiabendazole,
tbz), and 1-methyl-2-(4′-thiazolyl)benzimidazole (Metbz). The
complexes were selected to study (i) the importance that the
protection of the N−H bond of the imidazole has on the
stability of the complexes and (ii) the influence that the
substitution of the pyridyl group by the more electron donating
thiazolyl group has on the photophysical properties. The effect
of the ancillary ligand on the electrochemical and photophysical
B
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Inorganic Chemistry
Article
NMR experiments were conducted to assign all the signals in
1
the H (Figures S1 and S2 in the Supporting Information) and
1
3
1
C{ H} NMR spectra.
The following main features are inferred from the analysis of
the NMR spectra recorded in DMSO-d at 25 °C.
6
(
1) The asymmetric nature of the synthesized compounds
(
C symmetry) renders the two phenylpyridinate ligands in
1
1
every complex inequivalent and thus the corresponding H and
1
3
1
C{ H} NMR spectra show two different sets of signals for the
∧
C N donors.
(2) The 2D NOESY spectra of some complexes exhibit
interligand and inter-ring cross peaks very useful in the
assignation of all the signals (Figure S2 in the Supporting
Information).
(
3) Complexes [1][PF ], [2][PF ], and [4][PF ] exhibit a
6 6 6
broad peak between 14 and 15 ppm for their N−H proton,
whereas complexes with N−Me groups display a sharp singlet
at 4.45 and 4.31 ppm for [3][PF ] and [5][PF ], respectively.
6
6
The 13C{ H} NMR spectra are fully consistent with the
1
1
structural features ascertained by H NMR. In the FAB+ MS
spectra, the characteristic sets of peaks for the cationic
∧
∧
+
∧
+
fragments [Ir(C N) (N N)] and [Ir(C N) ] are found,
2
2
with isotopic patterns compatible with the presence of iridium,
whereas in the FT-IR spectra typical diagnostic bands are
−
observed for the PF6 counteranion (see the Supporting
Information).
The thermal stability of complexes [1][PF ]−[5][PF ] was
6
6
investigated by thermogravimetric analysis (TGA) under
nitrogen flow. All of the complexes are thermally stable with
decomposition temperatures (T , corresponding to 5% weight
d
loss) of over 300 °C (Figure S3 in the Supporting
Information). Interestingly, complexes with N−Me groups are
more stable than those with N−H groups (for instance, T =
d
3
06 and 386 °C for [2][PF ] and [3][PF ], respectively).
6 6
Crystal Structures by X-ray Diffraction. Single crystals
suitable for X-ray diffraction analysis were obtained for
4][PF ] and [5][PF ] by slow diffusion of n-hexane into a
Figure 2. ORTEP diagrams for the asymmetric unit of compounds
[
6
6
(
Λ)-[4][PF ] (top) and (Λ)-[5][PF ] (bottom). Thermal ellipsoids
6
6
solution of [4][PF ] in CH CN/CH Cl and by slow
6
3
2
2
are shown at the 30% probability level.
evaporation of a solution of [5][PF ] in CH Cl . Both
6
2
2
complexes crystallize in the orthorhombic Pbca space group.
The ORTEP diagrams for the asymmetric units are shown in
Figure 2. Selected bond lengths and angles with estimated
standard deviations are gathered in Table S1 in the Supporting
Information, and relevant crystallographic parameters are given
in Table S2 in the Supporting Information. For both
complexes, the corresponding unit cell consists of four pairs
of the two possible enantiomers (Λ and Δ), which result from
the helical chirality inherent to tris-bidentate octahedral metal
complexes. Indeed, the cationic complexes adopt a pseudo-
octahedral geometry around the iridium center with th−e
expected cis-C,C and trans-N,N disposition for the ppy
complexes. Conspicuously, these π−π interactions are ham-
pered by the presence of the N−Me group in [5][PF ] and
6
hence they are very weak relative to the analogous interactions
in [4][PF ], where the centroid−centroid distance is shorter,
6
3.58 vs 3.87 Å (see Table S3 and Figures S5 and S6 in the
Supporting Information).
Electrochemical Behavior. The redox potentials of
complexes [1][PF ]−[5][PF ] were experimentally determined
6
6
n
in acetonitrile solution by cyclic voltammetry using [ Bu N]-
4
[PF ] as supporting electrolyte and glassy carbon as the
6
working electrode. Potentials were defined with respect to the
∧
+
ligands. The ancillary N N ligands show N−Ir−N bite angles
ferrocene/ferrocenium (Fc/Fc ) couple. All of the electro-
of 76.1(1)° (for tbz in [4][PF ]) and 75.9(1)° (for Metbz in
chemical experiments were performed under an argon
atmosphere. The cyclic voltammogram of every PF6− salt
shows a unique quasi-reversible oxidation peak in the anodic
potential region between +0.74 and +0.79 V (see Table 1 and
Figure 3), which is ascribed to the oxidation of the ppy-Ir(III)
environment as discussed below.
6
[
5][PF ]), in agreement with those previously reported for five-
6
46,47
membered iridacycles in similar complexes.
The cyclo-
∧
metalated C N ligands also have standard values (∼80°) for the
C−Ir−N bite angles. In both compounds, the 3D crystal
architecture is built on the foundation of hydrogen-bonding
−
contacts and anion−π interactions in which the PF anions
The analysis of the cathodic region allowed us to establish
the following trends: (a) complexes with N−H groups in the
ancillary ligand ([1][PF ], [2][PF ], and [4][PF ]) show
6
occupy a central position (Figure S4 in the Supporting
Information). Furthermore, both crystal structures show
dimeric entities derived from π−π stacking interactions
involving the benzimidazole units of two vicinal cation
6
6
6
totally irreversible peaks at −2.20, −1.95, and −2.14 V,
respectively, indicating the instability of the reduced species;
C
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Inorganic Chemistry
Table 1. Cyclic Voltammetric Data Referenced to Fc/Fc in
Article
+
−3
a
Acetonitrile Solution (10 M)
compound
Eox (V)
Ered (V)
ΔE1/2 (V)
1
/2
1/2
[
[
[
[
[
1][PF6]
2][PF6]
3][PF6]
4][PF6]
5][PF6]
+0.74 (qr)
+0.77 (qr)
+0.79 (qr)
+0.76 (qr)
−2.20 (ir)
−1.95 (ir)
−1.91 (qr)
−2.14 (ir)
−2.27 (ir)
2.94
2.72
2.70
2.90
3.04
+0.77 (qr)
a
n
Measured using 0.1 M [ Bu N][PF ] as supporting electrolyte and a
scan rate of 0.15 V s (qr = quasi-reversible, ir = irreversible).
4
6
−1
Figure 4. UV−vis absorption spectra of complexes [1][PF ]−
6
−5
[
5][PF ] recorded in acetonitrile solution (5 × 10 M).
6
layer of LEC devices (iTMC mixed with 1-butyl-3-methyl-
imidazolium hexafluorophosphate in a 4:1 molar ratio).
Photoluminescence spectra are shown in Figure 5, and
emission maxima wavelengths (λ ), photoluminescence
em
quantum yields (Φ ), excited-state lifetimes (τ), and radiative
PL
Figure 3. Cyclic voltammograms of complexes [1][PF ]−[5][PF ] in
(k ) and nonradiative (k ) rate constants are summarized in
r
nr
6
6
−
3
n
acetonitrile solution (10 M), using 0.1 M [ Bu N][PF ] as
Table 2. In solution, at room temperature, [2][PF ] and
4
6
6
−
1
supporting electrolyte. Scan rate: 0.15 V s . The symbol ● indicates
the starting (E ) and final potentials (E ); E = E = −0.46 V (vs Fc/
[3][PF ] show broad and unstructured emission bands,
6
i
f
i
f
suggesting a large charge-transfer (CT) character for T in
1
+
Fc ), with a clockwise scan.
these complexes. The CT character of the emitting state is
furthermore supported by the hypsochromic shift observed in
the emission bands of these complexes when the spectra are
48−50
(
b) complex [3][PF ], where the reactive N−H group of pybim
registered at 77 K (rigidochromic effect).
In contrast,
6
has been replaced with a methyl group, displays a quasi-
reversible reduction peak at −1.91 V, which supports the higher
stability of [3][PF ] versus [2][PF ]; (c) complex [5][PF ],
complexes [1][PF ], [4][PF ], and [5][PF ] show emission
6
6
6
profiles with some vibrational structure at room temperature
that becomes more marked at 77 K and present no significant
rigidochromic shift (Figure 5a,b). This behavior is typical of
6
6
6
also with a N−Me group, shows a reduction peak at −2.27 V
giving rise to the largest electrochemical gap (3.04 V), and its
irreversible nature suggests that the reduction mechanism is
3
51,52
emissive states with a large LC character.
The emission color of the complexes in solution goes from
different from that of [3][PF ]. These results anticipate a very
yellow-orange ([2][PF ] and [3][PF ]) to blue-green ([1]-
6
6
6
good stability for complex [3][PF ] under the operation
[PF ], [4][PF ], and [5][PF ]), and the peak emission
6
6 6 6
conditions used in LEC devices owing to the reversible nature
of the oxidation/reduction processes in which the complex is
involved in those devices.
Photophysical Properties. The UV−vis absorption
spectra recorded in acetonitrile solution are displayed in Figure
wavelengths follow the trend [4][PF ] (489, 508 nm) ≈
6
[5][PF ] (486, 508 nm) < [1][PF ] (516 nm) < [2][PF ] (583
6
6
6
nm) < [3][PF ] (596 nm) (Table 2). Hence, when the
6
ancillary ligand is changed from pyim ([1][PF ]) to pybim
6
([2][PF ]), the emission experiences a red shift, whereas the
6
4
. The complexes show intense absorption bands in the UV
replacement of pybim ([2][PF ]) or pyMebim ([3][PF ]) by
6
6
with maxima in the range 250−350 nm, which are mainly
ascribed to spin-allowed π → π* transitions of the ligands.
Lower energy bands above 350 nm correspond to spin-allowed
tbz ([4][PF ]) or Metbz ([5][PF ]) leads to a blue shift that is
6
6
comparable in magnitude. The substitution of the N−H group
with a methyl in [3][PF ] produces a small red shift (13 nm)
6
1
metal to ligand ( MLCT) and ligand to ligand charge transfer
on the emission peak with respect to the unsubstituted complex
1
(
LLCT) excitations, whereas the low-intensity tails above 450
[2][PF ] that is not observed on passing from [4][PF ] to
6
6
3
3
nm arise from spin-forbidden MLCT, LLCT, and ligand-
centered ( LC) transitions. In comparison with complex
[5][PF6].
3
Complexes [4][PF ] and [5][PF ] exhibit weak emission
6 6
[
1][PF ], taken as a reference, complexes [2][PF ] and
(Φ = 9.0% and 4.4%, respectively), in good agreement with
6
6
PL
3
[3][PF ] exhibit a more intense absorption centered around
the LC character predicted for the emissive state in these
6
3
25 nm due to the benzimidazole unit. Substitution of the
complexes. In comparison, complexes [2][PF ] and [3][PF ]
6
6
∧
pyridine by a thiazole ring in the N N ligand of complexes
feature significantly higher Φ values (38.5% and 39.4%,
PL
3
[4][PF ] and [5][PF ] blue-shifts this absorption to 300 nm.
respectively), in accord with the expected MLCT character of
6
6
The photoluminescence properties of complexes [1][PF ]−
the emissive state that enhances the spin−orbit coupling and
6
[
5][PF ] were investigated by excitation of both deoxygenated
therefore favors the intersystem crossing. The Φ recorded for
6
PL
−5
40
acetonitrile solutions (5 × 10 M, room temperature and 77
[3][PF ] is similar to that previously reported by Sunesh et al.
6
K) and thin films with the same composition used in the active
The ΦPL value of complex [1][PF ] (20.4%) is between both
6
D
DOI: 10.1021/acs.inorgchem.7b01167
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Inorganic Chemistry
Article
extremes, doubling the values registered for the LC-type
complexes and being about half of the values recorded for the
MLCT-type complexes. This suggests that the electronic
description of the emissive state for [1][PF ] requires a deeper
6
analysis.
Excited-state lifetimes measured in deoxygenated acetonitrile
solution under an argon atmosphere are on the order of
hundreds of nanoseconds (Table 2). The longest lifetime (485
ns) is recorded for complex [3][PF ], and similar lifetimes of
6
around 200 ns are obtained for [2][PF ], [4][PF ], and
6
6
[
5][PF ]. For [1][PF ], a biexponential fit was used to
6 6
determine an average lifetime value of 316 ns (see Table S4
in the Supporting Information for details). The lower Φ and
PL
shorter τ values recorded for [4][PF ] and [5][PF ] determine
6
6
the higher nonradiative decay rate constants obtained for these
two complexes.
The emission spectra were also recorded for amorphous thin
films of the complexes with the composition used in LEC
devices (Figure 5c). The spectra show a similar shape for all the
complexes, and the emission maxima follow the same trend
observed in solution: [4][PF ] (507 nm) < [5][PF ] (514 nm)
6
6
<
[1][PF ] (542 nm) < [2][PF ] (574 nm) < [3][PF ] (581
6 6 6
nm). Interestingly, complexes with CT emitting states
(
[2][PF ] and [3][PF ]) exhibit their emission maxima slightly
6
6
shifted to shorter wavelengths in comparison to solution (Table
), whereas those with LC emitting states ([4][PF ] and
2
6
[
5][PF ]) are red-shifted. [1][PF ] undergoes the most drastic
6
6
change in the emission maximum in passing from solution to
thin film, pointing again to the existence of differences between
the electronic nature of the emitting state in this complex in
comparison with complexes [4][PF ] and [5][PF ]. The thin-
6
6
film emission spectra present almost no vibrational structure,
probably caused by the rigidity of the solid state, and only
[
5][PF ] keeps observable vibrational bands.
6
It is worth noting that, in contrast to the results observed in
solution, where substitution of the N−H group with a methyl
has no systematic effect, the presence of the methyl group
produces an increase of the quantum yield in thin film. The Φ
PL
increases from 32.1% in [2][PF ] to 44.6% in [3][PF ] and
6
6
from 3.3% in [4][PF ] to 6.0% in [5][PF ]. The gain in Φ is
6
6
PL
attributed to the fact that, as discussed above, the methyl group
prevents the aggregation of the complex in the solid state and
reduces self-quenching processes.
Theoretical Calculations. Density functional theory
DFT) and time-dependent DFT (TD-DFT) calculations
(
Figure 5. Photoluminescence spectra recorded (a) at room temper-
were performed on the cations of complexes [1][PF ]−
−5
6
ature and (b) at 77 K in deaerated acetonitrile solution (5 × 10 M)
by excitation at 360 nm. (c) Photoluminescence spectra registered in
thin films at room temperature by excitation at 320 nm.
[
5][PF ] to get a better understanding of the electrochemical
6
and photophysical properties discussed above. Calculations
were carried out at the B3LYP/(6-31G**+LANL2DZ) level
−5
Table 2. Photophysical Properties of Complexes [1][PF ]−[5][PF ] in Solution (5 × 10 M) and as Thin Films
6
6
solution (77 K)
a,b
solution (room temperature)
λem (nm)
thin film
a
,
b
a
,
b
a
,
c
k /10 (s−1)d
5
k /10 (s−1)d
5
e
e
complex
λem (nm)
ΦPL (%)
τ (ns)
λem (nm)
542
ΦPL (%)
r
nr
f
[
1][PF6]
2][PF6]
3][PF6]
4][PF6]
5][PF6]
516
583
596
20.4
38.5
39.4
9.0
316
6.5
17.3
8.1
25.2
27.7
12.5
48.9
47.8
478, 511
515, 551
519, 557
477, 512
486, 518
5.5
32.1
44.6
3.3
[
[
[
[
222
485
186
200
574
581
507
489, 508
486, 508
4.8
4.4
2.2
495 (sh), 514
6.0
a
b
c
d
e
f
Deoxygenated acetonitrile solution. λexc 360 nm. λ 365 nm. k = Φ /τ and k = (1 − Φ )/τ. λ 320 nm. Average lifetime estimated using
exc
r
PL
nr
PL
exc
a biexponential fit (Table S4 in the Supporting Information).
E
DOI: 10.1021/acs.inorgchem.7b01167
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
including solvent effects (see the Experimental Section for
details).
destabilization is due to the strong electron-donating character
of the thiazole ring resulting from the presence of the sulfur
atom, which contributes with a lone electron pair to the
aromatic π system of the five-membered ring. Methylation of
the imidazole ring has a very small effect, destabilizing the
LUMO by around 0.02 eV. The changes predicted for the
energy of the LUMO justify the less negative reduction
potentials measured for [2][PF ] (−1.95 V) and [3][PF ]
Calculations predict a near-octahedral structure for cations
+
+
[
1] −[5] in their ground electronic state (S ), in good
0
agreement with the results observed for similar complexes in
3
6,53−56
previous studies
and the X-ray structures reported above
for complexes [4][PF ] and [5][PF ] (Figure 2). Both the
6
6
cyclometalating and the ancillary ligands display coplanarity for
their constituting rings, as is usually found for iTMC complexes
possessing five-atom chelate rings and without bulky
6
6
(
−1.91 V) in comparison with [1][PF ] (−2.20 V) and the
6
increase in the potential in passing to [4][PF ] (−2.14 V) and
6
32
substituents in their molecular structure. The largest deviation
[
5][PF ] (−2.27 V). Upon reduction, the extra electron fully
6
∧
from coplanarity is indeed found for the pyMebim N N ligand
∧
enters in the N N ligand, as illustrated by the unpaired-electron
spin densities shown in Figure S7 in the Supporting
Information for the radical species [3] and [5] . It should
(
7.3°) in [3][PF ] due to the methyl group introduced in the
6
interannular region. The values predicted for the bite angles C−
•
•
+
+
+
Ir−N (∼80.1°) and N−Ir−N (∼74.5° for [1] , [2] , and [4]
•
be noted that for [5] a high spin density of 0.47e is located on
+
+
and ∼74.0° for [3] and [5] ) are very close to those obtained
the C2 atom of the thiazole ring (see Figure 2 for atom
numbering). The highly localized nature of the unpaired
electron could explain the instability of the reduced species of
from the X-ray structures of [4][PF ] and [5][PF ].
6
6
Figure 6 displays the isovalue contours calculated for the
highest occupied (HOMO) and lowest unoccupied molecular
[
5][PF ] despite the fact that the N−H group of the pybim
6
unit is protected with a methyl group as in [3][PF ].
6
The size of the HOMO−LUMO energy gap is therefore
mainly determined by the position of the LUMO and increases
+
+
+
+
+
along the series [2] ≈ [3] < [1] < [4] ≈ [5] . Obviously, if
a direct correlation is established between the HOMO−LUMO
gap and the emission energy, bluer emissions have to be
expected along this series, which is indeed in agreement with
the experimental trend observed for the emission maxima
(
Table 2). However, the picture is not as simple since, as
discussed in the previous section, experimental evidence points
to a different electronic nature of the emitting triplet state.
Therefore, we cannot directly presume that emission originates
from the HOMO → LUMO transition for all of the complexes.
TD-DFT calculations will help us to elucidate the nature of the
emitting state and to rationalize the experimental observations.
TD-DFT calculations were performed for the lowest-energy
+
+
Figure 6. Energy diagram showing the isovalue contours (±0.03 au)
triplet (T ) excited states of cations [1] −[5] . The results
n
and the energy values calculated for the HOMO, LUMO, and LUMO
obtained for the four lowest triplets (vertical excitation
energies, description in terms of orbital monoexcitations, and
electronic nature) are detailed in Table S5 in the Supporting
Information and are summarized in a graphical way in Figure 7.
For each cation, two triplet states of LC nature with some
MLCT character are found, which are mainly described by the
HOMO → LUMO+1 and HOMO → LUMO+2 excitations.
+
+
+
+
+
1 of complexes [1] −[5] . The MOs of [3] and [5] are identical
+ +
with those of [2] and [4] and are not displayed.
+
+
orbital (LUMO) of cations [1] −[5] . The LUMO+1 is also
displayed because it plays a relevant role in the photophysical
properties (see below). As is usually found for ppy-based
5
,54,57
+
cyclometalated Ir-iTMCs,
[
the HOMO of cations [1] −
∧
+
As all these orbitals are located on the C N ligands with some
5] results from a mixture of d orbitals of Ir(III) and phenyl π
π
participation of the metal (Figure 6), they remain unchanged
orbitals, with some contribution from the pyridine rings of the
3
∧
along the series and the resulting LC(C N) states present
almost the same energy (2.75 and 2.79 eV) for all of the
complexes (Figure 7). The HOMO → LUMO excitation
implies an electron transfer from the ppy-Ir environment,
cyclometalating ligands. As the five complexes share the same
∧
C N ligand, the HOMO has almost identical energies for all of
ox
them. This is in good agreement with the experimental E 1
/2
values (Table 1), which remain almost constant and imply the
oxidation of the ppy-Ir environment. The same trend is found
for the LUMO+1, which is centered on the cyclometalating
ligands and presents almost the same energy for all of the
complexes (Figure 6).
∧
where the HOMO is located, to the N N ligand, where the
LUMO resides (Figure 6), and therefore gives rise to a triplet
3
3
state of mixed MLCT/ LLCT character. The state is calculated
+
as the lowest triplet at 2.73 eV for [1] , very close in energy to
3
∧
+
the LC(C N) states, and stabilizes to 2.51 eV for [2] and
In contrast to the HOMO, the LUMO is fully located over
+
∧
[
3] (Figure 7), in accord with the stabilization of the LUMO
the ancillary N N ligand and its energy varies upon modifying
+
+
the identity of the ligand. The condensation of a benzene ring
to the imidazole ring to produce a benzimidazole unit leads to
the stabilization of the LUMO by ∼0.3 eV due to the extended
conjugation. In contrast, the exchange of the pyridine ring by a
thiazole ring in [4]+ and [5]+ compensates the effect of
benzene condensation and leads to a strong destabilization of
when pybim is used as the ancillary ligand. For [4] and [5] ,
3
3
the MLCT/ LLCT state increases in energy to 2.95 and 2.97
eV, respectively, because of the LUMO destabilization induced
by the replacement of the pyridyl ring with the thiazole ring,
and it is no longer the lowest triplet. It appears as T (Figure 7).
4
3
∧
3
3
A fourth state of mixed LC(N N)/ MLCT/ LLCT character
+
+
the LUMO (∼0.35 eV) in comparison to [2] and [3] . This
mainly resulting from the HOMO-1 → LUMO or HOMO-2 →
F
DOI: 10.1021/acs.inorgchem.7b01167
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 7. Energy diagram showing the energy values calculated for the
+
+
lowest-energy triplet excited states (T ) of complexes [1] −[5] . The
n
different electronic natures of the T states are denoted by using
Figure 8. Unpaired-electron spin-density contours (0.002 au)
n
+
different colors.
calculated for fully relaxed T and T states of [1] and T states of
1
2
1
+
+
[
2] and [4] . The nature of the states is indicated within parentheses.
LUMO excitations is also computed within the low-energy
triplets.
contribution from the metal (∼0.3e). UB3LYP calculations
3
3
The electronic nature of the lowest-lying T triplet therefore
therefore confirm the MLCT/ LLCT character of the emitting
1
+
+
+
+
+
3
changes along the series [1] −[5] (Figure 7). For [2] and
3] , the HOMO → LUMO MLCT/ LLCT state is lower in
triplet for [2] and [3] and the predominant LC nature of
+
3
3
+
+
+
[
that triplet for [4] and [5] . For [1] , the T and T states
1 2
3
∧
energy than the LC(C N) states and it clearly corresponds to
the emitting state. This assignment supports the CT character
proposed for this state in [2][PF ] and [3][PF ] on the basis of
remain mostly isoenergetic after relaxation (ΔE = 2.59 and 2.61
eV, respectively, calculated as the difference between the total
energies of S and T /T at their respective minimum energy
6
6
0
1
2
3
3
the broad and unstructured emission bands observed
structures), the MLCT/ LLCT state being slightly more
stable. Thus, emission could take place from both T and T .
+
+
experimentally (Figure 5a). For [4] and [5] , the destabiliza-
1
2
3
3
3
∧
tion of the MLCT/ LLCT state makes the LC(C N) triplets
the lowest-lying states. The LC character of the emitting state
explains the different photophysical behavior observed for
complexes [4][PF ] and [5][PF ], which present structured
The calculated emission energies, estimated as the vertical
energy difference between T1 and S0 at the optimized
minimum-energy geometry of T , follow closely the exper-
1
imental values of the emission maxima. The values predicted for
6
6
+
+
emission bands with no significant rigidochromic shift at low
[2] (590 nm) and [3] (593 nm) are in good accordance with
+
temperatures (Figure 5a,b). For [1] , TD-DFT calculations
the experimental values (583 and 596 nm, respectively),
3
3
3
∧
+
+
predict that the MLCT/ LLCT state and the LC(C N)
triplets are very close in energy and they can compete as
emitting states. Experimental evidence actually points in this
whereas those obtained for [1] (534 nm), [4] (527 nm), and
+
[5] (528 nm) slightly overestimate the experimental values
(516, 489−508, and 486−508 nm, respectively).
direction since the emission spectrum of [1][PF ] shows a
Light-Emitting Electrochemical Devices. Light-emitting
electrochemical cells (LECs) were fabricated with complexes
[1][PF ]−[5][PF ] in a double-layer architecture consisting of
6
nearly unnoticeable vibrational structure and appears inter-
mediate between those of [2][PF ] and [3][PF ] and those of
6
6
6
6
[
4][PF ] and [5][PF ] (Figure 5).
To get additional information about the emitting state, the
a poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)
(PEDOT:PSS) layer and the emissive layer sandwiched
between indium tin oxide (ITO) and aluminum electrodes.
The active layer contained the Ir-iTMC mixed with the ionic
liquid (IL) 1-butyl-3-methylimidazolium hexafluorophosphate
6
6
+
+
geometries of the lowest triplet state of [1] −[5] were
optimized using the spin-unrestricted UB3LYP approach. For
+
3
3
3
∧
[1] , both the T ( MLCT/ LLCT) and T ( LC(C N)) triplets
1
2
were calculated at the UB3LYP level. Figure 8 displays the+
([Bmim][PF ]) at a 4:1 (iTMC:IL) molar ratio. The IL was
added to shorten the turn-on time of the LEC by increasing the
6
unpaired-electron spin density distributions calculated for [1]
+
+
(
T and T ), [2] , and [4] at their optimized geometries.
concentration of ionic species and the ionic mobility in the
1
2
+
+
27,58
Those computed for [3] and [5] are identical with those
shown for [2] and [4] , respectively (Figure S8 in the
light-emitting layer.
After deposition of the emissive layer,
+
+
the devices were transferred to an inert-atmosphere glovebox
(<0.1 ppm of O and H O). The time dependence of the
+
+
Supporting Information). For [2] and [3] , T features a spin
1
2
2
density distribution spreading across the ppy-Ir environment
luminance and average voltage of the LECs prepared were
evaluated using a pulsed-current driving mode. LECs were
operated by applying a block-wave pulsed current (frequency, 1
kHz; duty cycle, 50%), where the peak current density of the
∧
∧
and the N^N ligand (Ir 0.55e, C N ligands 0.43e, N N ligand
.02e) that perfectly matches the topology of the HOMO →
1
+
+
LUMO MLCT/LLCT excitation. In contrast, for [4] and [5]
−2
the spin-density distribution is mainly centered on one of the
cyclometalating ligands (∼1.7 unpaired electrons) with a small
pulse was 200 A m and the average current density was 100 A
−2
m . Pulsed-current driving has been demonstrated to lead to
G
DOI: 10.1021/acs.inorgchem.7b01167
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
LEC devices with longer lifetimes and faster response in
comparison to fixed-current/-voltage driving.
The dependence of the luminance and average voltage with
time is depicted in Figure 9, and key parameters derived from
luminance increases until a maximum is reached and then starts
to decrease. The parameters used to quantify the response of
the device are the time to reach this maximum luminance (tmax)
24
and the time to reach a luminance value of 100 cd m− (t ).
2
on
The ton for devices with [1][PF ] and [5][PF ] was not
6
6
established because these LECs did not achieve the threshold of
−
2
1
00 cd m , yet they started up very rapidly and reached
−2
luminance levels of 18 and 51 cd m within 4 s, respectively.
The absence of light emission for [4][PF ] and the low
6
luminance observed for [1][PF ] and [5][PF ] can be related
6
6
to the nature of the emitting state and, in the case of [4][PF6]
and probably [1][PF ] as well, with the observed formation of
6
dimeric entities in the crystal that favor self-quenching
processes. As discussed above, the emitting state of these
3
complexes has a predominant LC character with small
participation of the iridium center. This limits the intersystem
crossing, and as a consequence, low Φ values are obtained for
PL
[
1][PF ] (5.5%), [4][PF ] (3.3%), and [5][PF ] (6.0%) in thin
6 6 6
film (Table 2). In contrast, [2][PF ] and [3][PF ] have
6
6
3
3
emitting states of MLCT/ LLCT nature with higher Φ
values (32.1 and 44.6%, respectively), and their LECs feature
much higher luminances (Lummax= 460 and 906 cd m ,
respectively), comparable to those of devices with the highest
luminance maxima available in the literature for this wavelength
The higher luminance registered for the [3][PF ]
device combines with a top efficiency (efficacy of 9.2 cd A
PL
−2
11,25
range.
6
−
1
and external quantum efficiency of 3.0%) and is in good
agreement with the higher Φ recorded for this complex in
PL
thin film. In addition, the LECs with [2][PF ] and [3][PF ]
6
6
show a fast response, the longest ton value (6,9 s) being found
for the LEC with [3][PF6].
The stability of the devices was estimated as the decay time
required for reaching half of the maximum luminance (t1/2
)
after the tmax is attained. The values obtained for t1/2 (Table 3)
indicate that the condensation of a benzene to the imidazole
ring, which leads to increased luminescence, does not improve
the device lifetime. In fact, the device made with the pybim-
containing complex [2][PF ] features a short t of only 0.30
6
1/2
h, shorter than that of the pyim-containing [1][PF ] device
6
(
2.25 h). These low device stabilities are in line with the
instability of the reduced species observed electrochemically,
which is attributed to the high reactivity of the N−H bond. The
presence of this bond therefore is likely to be the origin of the
limited device stability of [1][PF ] and [2][PF ] and also
6
6
[
4][PF ], which emits no light. However, when the imidazole
6
N−H bond is substituted with a methyl group, the LEC
stability improves dramatically, and t1 increases up to 200 h in
Figure 9. Luminance and average voltage for ITO/PEDOT:PSS/
/2
iTMC:[Bmim][PF ] (4:1 molar ratio)/Al LEC devices measured by
applying a block-wave pulsed current of 100 A m at a frequency of 1
[5][PF ] and above 2500 h in [3][PF ]. The luminance of the
6
6
6
−2
[
3][PF ] device decreases only by 12% after 650 h (27 days) of
6
kHz and duty cycles of 50%: (a) iTMC = [1][PF ] and [5][PF ]; (b)
6
6
continuous operation, and the linear extrapolation of the time
dependence of luminance provides the value t1/2 = 2700 h,
which is a remarkable result for a LEC (Figure 9). The value of
iTMC = [2][PF ] and [3][PF ].
6
6
the graphs are collected in Table 3. All of the devices present a
fast initial decrease of the voltage until a minimum stable value
t1/2 extrapolated for [3][PF ] is among the best values available
6
11
in the bibliography for LECs. In a recent review, Henwood
and Zysman-Colman concluded that the most stable emitter
reported to date under the same pulsed current operation of
(
2.3−2.7 V) is reached. This behavior is typical under the
pulsed-current driving applied due to the operation mechanism
of LECs. The voltage required to maintain the current density
decreases versus time due to the formation of p- and n-doped
regions, which reduces the resistance of the active layer. The
fact that the voltage remains in all cases close to the minimum
along the device operation time reveals that there are no signs
of charge transport issues or chemical degradation. Except for
100 A m− was presented by Tordera et al., who described a
2
59
LEC device based on an imidazole-including complex ([Ir-
(ppy) (imp)]; imp = 1H-imidazo[4,5-f ][1,10]phenanthroline)
2
that featured high lifetime and luminescence and low turn-on
times. The results obtained here for the device manufactured
with complex [3][PF ] are even better than those of the device
6
the device with [4][PF ], which does not emit any light, the
with [Ir(ppy) (imp)][PF ]. The device stability obtained when
6
2
6
H
DOI: 10.1021/acs.inorgchem.7b01167
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Table 3. Performance Parameters Obtained for ITO/PEDOT:PSS/Active Layer/Al LEC Devices Measured by Applying a
−2
a
Block-Wave Pulsed Current of 100 A m at a Frequency of 1 kHz and Duty Cycles of 50%
Lum0 (cd m−2
b
)
Lummax (cd m−2
c
)
ton (s)
d
tmax (min)
e
t1/2 (h)
f
efficacy (cd A−1)
g
EQE (%)
h
i
PE (lm W−1)
complex
[
1][PF6]
2][PF6]
3][PF6]
5][PF6]
18
143
44
25
460
904
61
15
18
2.25
0.30
0.3
4.6
9.2
0.6
0.1
1.4
3.0
0.2
1.3
2.5
5.7
0.3
[
[
[
<4
j
6.9
1680
1
2700
51
200
a
d
g
b
c
Active layer: ([1][PF ], [2][PF ], [3][PF ], and [5][PF ]):[Bmim][PF ] at a 4:1 molar ratio. Initial luminance. Maximum luminance reached.
6
6
6
6
6
−2
e
f
Time to reach 100 cd m luminance. Time to reach the maximum luminance. Time to reach half of the maximum luminance after t is attained.
Maximum efficacy (ratio luminance/average current). Maximum external quantum efficiency reached. Maximum power efficiency reached.
max
h
i
j
Extrapolated value.
using [3][PF ] is longer (t = 2700 vs 2000 h), the turn-on
time is shorter (6.9 vs 45 s), the maximum luminance is higher
luminescence spectra for [1][PF ], [2][PF ], and [3][PF ]
6 6 6
(Tables 2 and 4). However, the emission is red-shifted by 47
6
1/2
−
2
(
904 vs 684 cd m ), and the maximum current efficiency is
nm in the device of complex [5][PF ], which presents a
6
−1
also higher (9.2 vs 6.5 cd A ). The values obtained here are
also in excess of those for the most stable yellow-orange
emitting LEC (t1/2 ≈ 4000 h, 3.6 cd A ), which was operated
under different pulsed current conditions (30% of duty
shoulder at 493 nm that was also observed in the PL spectrum
in thin film (Table 2). The shift of the emission could be due to
−1
3
the electric field present in the device that stabilizes the LC
emitting state of the complex, whereas the shoulder would
correspond to the emission from a close triplet state (Table S5
in the Supporting Information) not affected by the electric field.
LEC devices emit green light for [1][PF ] and [5][PF ] and
2
4
cycle). They are also competitive with the top peak
−1
efficiencies (18.6 cd A ) obtained by He et al. under constant
voltage operation that leads to higher peak luminances and
higher currents accompanied by a strong reduction in
lifetime. Interestingly, the exceptional efficiency found for
the [3][PF ] LEC (9.2 cd A ) is much higher than the
6
6
yellow light for [2][PF ] and [3][PF ].
6
6
3
8
−1
6
CONCLUSIONS
■
A series of [Ir(ppy) (N N)][PF ] complexes with 2-phenyl-
2 6
efficiency previously obtained for this complex under voltage
∧
−1
40
scan (1.5 cd A ).
∧
pyridinate cyclometalating ligands and different N N arylazole
Figure 10 shows the electroluminescence (EL) spectra of the
four LECs, and Table 4 summarizes the key parameters
ancillary ligands has been prepared and fully characterized. X-
ray structural data are reported for complexes [4][PF ] and
6
[
5][PF ] incorporating thiazolyl rings.
The complexes show significantly different photolumines-
6
cence properties in solution depending on the chemical
structure of the ancillary ligand. Complexes [2][PF ] and
6
[
3][PF ], bearing 2-(2-pyridyl)benzimidazole (pybim) and 1-
6
∧
methyl-2-pyridin-2-yl-1H-benzimidazole (pyMebim) N N li-
gands, respectively, exhibit broad unstructured emission bands
with maxima around 590 nm and experience a notable
rigidochromic effect when recorded at 77 K. In contrast,
complexes [4][PF ] and [5][PF ], in which the pyridyl ring of
the N N ligand is substituted by a thiazolyl ring, present
vibrationally structured emission bands peaking at around 500
nm with no significant rigidochromic shift. DFT calculations
fully justify the different photophysical behavior. The presence
of the benzimidazole unit in [2][PF ] and [3][PF ] strongly
6
6
∧
6
6
stabilizes the LUMO (located on the ancillary ligand) and
3
3
determines a MLCT/ LLCT character for the lowest-energy
emitting triplet state. In contrast, the incorporation of the
thiazolyl ring destabilizes the LUMO and thereby the HOMO
Figure 10. Normalized electroluminescence (EL) spectra of LECs
based on complexes [1][PF ], [2][PF ], [3][PF ], and [5][PF ].
6
6
6
6
3
3
→
LUMO MLCT/ LLCT state in [4][PF ] and [5][PF ],
6 6
3
and the emitting triplet has a predominant LC nature. The
different nature of T1 explains the low PLQYs (<10%)
measured for [4][PF ] and [5][PF ]. Complex [1][PF ],
obtained from the EL spectra. The position of the emission
maxima is similar to that recorded in the thin-film photo-
6
6
6
∧
bearing a 2-(1H-imidazol-2-yl)pyridine N N ligand, displays an
Table 4. Electroluminescence Parameters of LECs under
intermediate behavior due to close energy of the
−2
Block-Wave Pulsed Current of 100 A m at a Frequency of
kHz and Duty Cycles of 50%
3
3
3
MLCT/ LLCT and LC states.
1
Light-emitting electrochemical cells prepared with complexes
[
1][PF ]−[5][PF ] also present very different performances, in
6
6
complex
λmax,EL (nm)
CIE coordinates
line with the photophysical and electronic structure differences
mentioned above. LECs prepared with [2][PF ] and [3][PF ]
( MLCT/ LLCT emissive state) feature yellow color, high
luminances (460 and 904 cd m ) and efficiencies (4.61 and
9.15 cd A ), and low turn-on times (a few seconds). In
[
1][PF6]
2][PF6]
3][PF6]
5][PF6]
558
573
(0.40, 0.56)
(0.48, 0.51)
(0.49, 0.50)
(0.41, 0.52)
6
6
[
[
[
3
3
576
−2
567 (493 sh)
−1
I
DOI: 10.1021/acs.inorgchem.7b01167
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
contrast, LECs prepared with complexes [1][PF ] and
TGA measurements were performed on a TGA Q50 V20.10
6
3
instrument from 25 to 800 °C under a flowing mixture of nitrogen/
[
5][PF ], for which the emissive state has an important LC
6
−1
air (60/40 mL/min) at a heating rate of 10 °C·min .
character, exhibit green color and low luminances (25 and 61
−2
X-ray Crystallography. A summary of crystal data collection and
refinement parameters for compounds [4][PF ] and [5][PF ] is given
cd m ). LECs with complexes bearing unprotected imidazole
6
6
N−H bonds show short lifetimes ([1][PF ] and [2][PF ]) or
6
6
in Table S1 in the Supporting Information. Single crystals of [4][PF6]
emit no light ([4][PF ]). The LEC based on [3][PF ] exhibits
6
6
and [5][PF ] were mounted on a glass fiber and transferred to a
6
record performance, simultaneously presenting high luminance
and efficiency, short turn-on time, and high stability, reaching
lifetimes of over 2500 h.
The use of the benzimidazole unit in the ancillary ligand
without incorporation of bulky groups is therefore confirmed as
an efficient strategy to obtain LECs with long lifetimes, short
turn-on times, and high luminances, provided that the N−H
group of the imidazole ring is conveniently protected with a
methyl substitution.
Bruker X8 APEX II CCD-based diffractometer equipped with a
graphite-monochromated Mo Kα radiation source (λ = 0.71073 Å).
66
The highly redundant data sets were integrated using SAINT and
corrected for Lorentz and polarization effects. The absorption
correction was based on the function fitting to the empirical
transmission surface as sampled by multiple equivalent measurements
6
7
with the program SADABS.
6
8
The software package SHELXTL (version 6.10) was used for
space group determination, structure solution, and refinement by full-
2
matrix least-squares methods based on F . A successful solution by
direct methods provided most non-hydrogen atoms from the E map.
The remaining non-hydrogen atoms were located in an alternating
series of least-squares cycles and difference Fourier maps. All non-
hydrogen atoms were refined with anisotropic displacement
coefficients. Hydrogen atoms were placed using a “riding model”
and included in the refinement at calculated positions. The CCDC
EXPERIMENTAL SECTION
■
General Methods and Starting Materials. Starting Materials.
IrCl ·xH O was purchased from Johnson Mattey and used as received.
3
2
[
Ir(μ-Cl)(ppy) ] (Hppy = 2-phenylpyridine) was prepared according
2 2
45,60,61
to literature procedures.
The salt [NH ][PF ] and the ligands 2-
4 6
(
(
2-pyridyl)benzimidazole (pybim) and 2-(4′-thiazolyl)benzimidazole
reference numbers for [4][PF ] and [5][PF ] are 1519073 and
6 6
thiabendazole, tbz) were purchased from Sigma-Aldrich and used
1519074, respectively.
The thermal ellipsoid molecular structures depicted in Figure 2
without further purification. The ligands 2-(1H-imidazol-2-yl)pyridine
62,63
64
(
pyim),
1-methyl-2-pyridin-2-yl-1H-benzimidazole (pyMebim),
were plotted using the ORTEP-III program for crystal structure
6
9
and 1-methyl-2-(4′-thiazolyl)benzimidazole (Metbz) were prepared
according to literature procedures or via adapted literature protocols.
Deuterated solvents were obtained from SDS (CD SOCD ) and Carlo
illustrations.
Electrochemical Measurements. Electrochemical measurements
were performed using a customized SPELEC (DropSens) equipment,
a commercial fully integrated synchronized spectroelectrochemical
device, that includes a bipotentiostat/galvanostat controlled by
DropView (DropSens). All experiments were carried out using a
three-electrode cell using a glassy-carbon disk with a diameter of 3 mm
as the working electrode, a platinum wire as the auxiliary electrode,
and a silver-wire pseudoreference electrode. Oxygen was removed
from the solution by bubbling argon for 10 min and keeping the
current of argon along the whole experiment. The formal potentials
were determined by cyclic voltammetry (CV), scanning the potential
3
3
Erba or Euriso-top (CDCl ). Acetonitrile (HPLC, Prolabo) and
3
n
tetrabutylammonium hexafluorophosphate ([ Bu N][PF ], Acros)
4
6
were used to prepare solutions for electrochemical measurements.
Synthesis Method and Complex Characterization. All synthetic
manipulations were carried out under an atmosphere of dry, oxygen-
free nitrogen using standard Schlenk techniques. The solvents were
dried and distilled under a nitrogen atmosphere before use. Elemental
analyses were performed with a LECO CHNS-932 Elemental
́
Microanalyzer (from Universidad Autonoma de Madrid). The
−1
analytical data for the new compounds were obtained from crystalline
samples when possible. In some cases the data were reasonably
accurate, but in others the agreement of calculated and found values
for carbon was >0.4%, so that solvent molecules were introduced in
the molecular formulas to improve agreement. IR spectra were
at a scan rate of 0.15 V s . All potentials were referred to the redox
+
pair ferrocene/ferrocenium (Fc/Fc ) taken as internal standard. All
voltammetric experiments were started and finished at a potential of
−0.46 V and performed in a clockwise direction. Acetonitrile solutions
−3
of the complexes (10 M) were used in the presence of 0.1 M
−1
n
recorded on a Jasco FT/IR-4200 spectrophotometer (4000−400 cm
[ Bu N][PF ] as supporting electrolyte.
4
6
range) with a Single Reflection ATR Measuring Attachment. FAB
mass spectra (position of the peaks in DA) were recorded with an
Autospec spectrometer. The isotopic distribution of the heaviest set of
peaks matched very closely that calculated for the formulation of the
complex cation in every case. NMR samples were prepared under a
nitrogen atmosphere by dissolving a suitable amount of the compound
in 0.5 mL of the respective oxygen-free deuterated solvent, and the
spectra were recorded at 298 K (unless otherwise stated) on a Varian
Photophysical Characterization. The UV−vis absorbance
spectra were measured with a Secomam XT5 UVi Light spectrometer.
Quantum yields and PL emission spectra were recorded using a
Hamamatsu C9920 absolute quantum yield instrument, whereas
excited-state lifetimes were determined using a Hamamatsu C11367
Compact fluorescence lifetime spectrometer (Quantaurus-Tau).
Computational Details. Density functional calculations (DFT)
were carried out with the D.01 revision of the Gaussian 09 program
1
31
70
Unity Inova-400 (399.94 MHz for H; 161.9 MHz for P; 376.28
package. Becke’s three-parameter B3LYP exchange-correlation
MHz for 19F; 100.6 MHz for C). Typically, 1D H NMR spectra
13
1
71,72
functional
was used, together with the 6-31G** basis set for C,
7
3
were acquired with 32 scans into 32 k data points over a spectral width
H, N, and S and the “double-ζ” quality LANL2DZ basis set for the Ir
1
13
1
74
of 16 ppm. H and C{ H} chemical shifts were internally referenced
atom. Relativistic effects were accounted for by means of an effective
1
13
to TMS via the residual H and C signals of the corresponding
core potential (ECP), which was used to replace the inner core
electrons of Ir. The geometries of both the singlet ground electronic
state (S ) and the lowest-energy triplet state (T ) were fully optimized
solvents, CDCl (δ 7.26 and 77.16 ppm) and (CD ) SO (δ 2.50 and
3
3 2
6
5
3
9.52 ppm), according to the values reported by Fulmer et al.
0
1
+
+
•
Chemical shift values are reported in ppm and coupling constants (J)
for cations [1] −[5] . In addition, the radical neutral species [1] −
1
•
+
+
in hertz. The splitting of proton resonances in the reported H NMR
[5] resulting from the reduction of [1] −[5] were also optimized in
data are defined as s = singlet, d = doublet, t = triplet, st =
a doublet electronic state configuration (D ). No symmetry
0
+
+
pseudotriplet, q = quartet, sept = septet, m = multiplet, and bs = broad
restrictions were imposed. The geometries of [1] −[5] in the T
state and of [1] −[5] in the D state were calculated at the spin-
1
1
1
1
1
•
•
singlet. 2D NMR spectra such as H− H gCOSY, H− H NOESY,
0
1
13
1
13
H− C gHSQC, and H− C gHMBC were recorded using standard
unrestricted UB3LYP level using spin multiplicities of 3 and 2,
respectively. All calculations were performed in the presence of the
solvent (acetonitrile). Solvent effects were considered within the self-
consistent reaction field (SCRF) theory using the polarized continuum
pulse sequences. The probe temperature (±1 K) was controlled by a
standard unit calibrated with methanol as a reference. All NMR data
processing was carried out using MestReNova version 10.0.2.
Characterization data have been placed in the Supporting Information.
7
5−77
78−80
model (PCM) approach.
Time-dependent DFT (TD-DFT)
J
DOI: 10.1021/acs.inorgchem.7b01167
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
calculations of the lowest-lying five triplet states of the complexes were
performed in the presence of the solvent at the minimum-energy
geometry optimized for the ground state. The geometries of the first
and second triplet excited states of [1] were first optimized at the
TD-DFT level and then reoptimized at the UB3LYP level to compare
Notes
The authors declare no competing financial interest.
+
ACKNOWLEDGMENTS
■
with the results obtained for T from DFT calculations for all of the
1
Financial support by the Spanish Ministry of Economy and
Competitiveness (MINECO) of Spain (projects CTQ2014-
58812-C2-1-R, MAT2014-55200, CTQ2014-55583-R,
CTQ2014-61914-EXP, CTQ2015-71955-REDT, CTQ2015-
70371-REDT, CTQ2015-71154-P, and Unidad de Excelencia
other complexes.
Device Preparation. LECs were prepared on top of a patterned
−1
indium tin oxide (ITO, 15 Ω square ) coated glass substrate (www.
naranjosubstrates.com) previously cleaned as follows: (a) sonication
with soap, (b) deionized water, (c) isopropyl alcohol, and (d) UV-O3
lamp for 20 min. The thickness of the films was determined with an
Ambios XP-1 profilometer. Prior to the deposition of the emitting
layer, 80 nm of poly(3,4-ethylenedioxythiophene):poly-
Marıa
(CTQ2015-71154-P), Obra Social “la Caixa” (OSLC-2012-
07), Junta de Castilla y Leon (BU033-U16), and Generalitat
́
de Maeztu MDM-2015-0538), European Feder funds
0
́
(
styrenesulfonate) (PEDOT:PSS) (CLEVIOS P VP AI 4083, aqueous
Valenciana (Prometeo2016/135) is gratefully acknowledged.
We are also indebted to A. Colina for his inspiring comments
and generosity and to P. Castroviejo and M. Mansilla, from PCI
of the University of Burgos, for technical support. C.M. thanks
the MINECO for a predoctoral grant.
dispersion, 1.3−1.7% solid content, Heraeus) was coated in order to
avoid the formation of pinholes and to increase the reproducibility of
the cells. The emitting layer (100 nm) was prepared by spin-coating of
a MeCN solution consisting of the iTMC with the addition of the
ionic liquid (IL) 1-butyl-3-methylimidazolium hexafluorophosphate
(
(
[Bmim][PF ]; >98.5%, Sigma-Aldrich) in a 4:1 molar ratio
iTMC:IL). The devices were then transferred to an inert-atmosphere
6
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*
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Corresponding Authors
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Bernhard, S.; Malliaras, G. G. Solid-State Electroluminescent Devices
E-mail for G.E.: gespino@ubu.es.
E-mail for H.B.: henk.bolink@uv.es.
E-mail for E.O.: enrique.orti@uv.es.
Based on Transition Metal Complexes. Chem. Commun. 2003, 2392−
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ORCID
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Henk Bolink: 0000-0001-9784-6253
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