Iridium complexes of N-heterocyclic carbene ligands: Investigation into the energetic requirements for efficient electrogenerated chemiluminescence

Article abstract of DOI:10.1021/om500076w

A series of five heteroleptic Ir(III) complexes of the general form Ir(ppy)₂(C^∧C:) have been prepared (C^∧C represents a bidentate cyclometalated phenyl-substituted imidazolylidene ligand). The five complexes arise from the

Full text of DOI:10.1021/om500076w

Article
pubs.acs.org/Organometallics
Iridium Complexes of N‑Heterocyclic Carbene Ligands: Investigation
into the Energetic Requirements for Efficient Electrogenerated
Chemiluminescence
Bradley D. Stringer,† Linh M. Quan,† Peter J. Barnard,* David J. D. Wilson, and Conor F. Hogan*
Department of Chemistry, La Trobe Institute for Molecular Science, La Trobe University, Bundoora 3086, Victoria, Australia
S
* Supporting Information
ABSTRACT: A series of five heteroleptic Ir(III) complexes of
the general form Ir(ppy)2(C∧C:) have been prepared (C∧C
represents a bidentate cyclometalated phenyl-substituted
imidazolylidene ligand). The five complexes arise from the
cyclometalated phenyl ring of the NHC ligand being
unsubstituted or having electron-donating (OMe and Me) or
electron-withdrawing (Cl and F) groups at the 2- and 4-
positions of the ring. The synthesized phenyl-substituted
imidazole precursors, imidazolium salts, and Ir(III) complexes
have been characterized by elemental analysis, NMR spectros-
copy, cyclic voltammetry, and electronic absorption and
emission spectroscopy. The molecular structures for two imidazolium salts and two Ir(III) complexes were determined by
single-crystal X-ray diffraction. Each of the Ir(III) complexes exhibited intense photoluminescence in acetonitrile solution at
room temperature with quantum yields (ϕp) ranging from 42% to 68% and excited-state lifetimes on the order of 2 μs.
Voltammetric experiments revealed one formal metal-based oxidation process and two ligand-based reductions for each complex.
All complexes gave moderate to intense annihilation electrochemiluminescence (ECL); however, only the fluorinated complex
produced significant coreactant ECL. The combined electrochemical, spectroscopic, and theoretical investigations offer insights
into the reasons for this behavior and suggest useful strategies for the design of ECL emitters. A plot of oxidation potential versus
emission color is proposed as a convenient reference guide to aid in the prediction of energy sufficiency in ECL reactions.
tris-cyclometalated Ir(III)−NHC complexes has also been
reported.10 The emission colors of a series of heteroleptic
Ir(III) complexes with either cyclometalated benzimidazoliny-
INTRODUCTION
■
The popularity of N-heterocyclic carbenes (NHCs) has grown
rapidly in recent years, such that they have become one of the
most widely utilized ligand types in modern organometallic
chemistry.1 NHCs are particularly attractive, as they provide
excellent synthetic framework flexibility and NHC-containing
complexes often show good stability to air, moisture, and heat.2
Although NHCs have probably received the most attention for
the preparation of homogeneous catalysts,3 there has been
growing interest in the use of these ligands for the development
of novel luminescent materials.4 Luminescent NHC−metal
complexes have been prepared for a range of metal ions,
including Ru(II),5 Pt(II),6 Re(I),7 and Au(I).8
Blue-emitting complexes are sought for applications in light-
emitting devices such as organic light-emitting diodes (OLED)
and light-emitting electrochemical cells (LECs). In an early
study, Forest and co-workers reported the synthesis of
homoleptic, tris-cyclometalated Ir(III) complexes of imidazo-
linylidene- and benzimidazolinylidene-based NHC ligands via a
one-pot procedure, which displayed photoluminescence in the
near-UV region (∼380 nm).9 These researchers suggested that
the use of strong-field ligands, such as carbenes, should result in
an increase of the blue phosphorescence efficiency due to a
destabilization of thermally accessible nonemissive states. An
efficient two-step procedure for the synthesis of homoleptic,
lidene-11 or imidazolinylidene-based12 NHC ligands were tuned
̂
through modification of the chelating NN: ancillary ligand.
Cationic Ir(III) complexes with two cyclometalated 2-(phenyl)-
pyridine ligands and one chelating NHC−pyridine ancillary
ligand, which have a range of emission wavelengths, have been
reported.13
Electrochemiluminescence or electrogenerated chemilumi-
nescence (ECL), where luminescent emission is initiated
electrochemically, is an area of growing importance in analytical
science.14 In ECL reactions the excited state of the
luminophore is populated when an electron is transferred
from a powerful reductant to the lowest unoccupied molecular
orbital (LUMO; usually a ligand-based π* orbital) of the
oxidized complex. The reductant may be derived from either
the reduction of the metal complex itself, referred to as
annihilation ECL, or from a sacrificial reagent, in which case it
is called coreactant ECL. The application of coreactant ECL as
Special Issue: Organometallic Electrochemistry
Received: January 24, 2014
© XXXX American Chemical Society
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the basis for highly sensitive immunoassays has been
predominantly based on the use of tris(2,2′-bipyridine)-
ruthenium(II) ([Ru(bpy)3]2+) and its derivatives, the vast
majority of which emit in a narrow wavelength range around
600 nm.14c,15 In contrast, relatively few green and blue ECL
emitters are available, and the most effective design of blue-
emitting complexes for ECL detection is an important area of
current research. Since the early work of Wightman,16
Richter,17 and Kapturkiewicz,18 interest in the ECL of
cyclometalated iridium complexes has grown steadily due to
their efficient luminescence and tunable emission properties.
We are interested in the development of new ECL-based
sensing materials with varying emission and redox character-
istics for their potential application in multiplexed sensing. N-
heterocyclic carbene based complexes are potentially useful in
this regard: a wide variety of ligands can be readily prepared
due to the framework flexibility of the NHC moiety. Moreover,
the electronic properties of NHC ligands can be easily tuned
through the choice of the azole precursor as well as the wing-tip
substituents.
Scheme 2. Synthesis of 2,4-Substituted Azolium Salts 6−10
imidazoles 1−5 with iodomethane in acetonitrile. The
unsubstituted imidazolium salt precursor 1 has been described
previously.14d
The Ir(ppy)2(C∧C:) complexes 11−15 (the bidentate
imidazolylidene ligands are derived from the imidazolium
salts 6−10, respectively) were prepared by heating the chosen
imidazolium salt with the chloro-bridged Ir(III) dimeric
precursor compound [Ir(ppy)Cl]2 and Ag2O in 1,2-dichloro-
ethane (Scheme 3). Early attempts to carry out this reaction at
In the search for new luminescent and ECL active materials
we have undertaken a combined electrochemical, spectroscopic,
and theoretical investigation of a series of green-/blue-emitting
Ir(III) complexes. The synthesized Ir(III) complexes were of
the general form Ir(ppy)2(C∧C:) (where ppy represents
cyclometalated 2-(phenyl)pyridine and C∧C: represents a
cyclometalated phenyl-substituted imidazolylidene ligand).
The NHC ligands were derived from a series of 2,4-disubsituted
phenylimidazolium salts, where the substituents on the phenyl
ring were modified to provide both electron-donating and
electron-withdrawing character. Electrochemical and spectro-
scopic studies show how the electron density at the metal
center was modulated by these structural modifications to the
cyclometalated phenyl group of the NHC ligand. We report for
the first time a description of the ECL properties of NHC-
based iridium complexes. The results of these studies,
combined with theoretical studies, provide insight not only
into the electronic structure of these new complexes but also
into the energetic requirements for efficient ECL in general.
Scheme 3. Synthesis of Iridium(III) Complexes 11−15
∼60 °C in solvents commonly used for NHC/Ag(I) trans-
metalation reactions (e.g., CH2Cl2 and CH3OH) produced very
low yields (<2%) of the desired complexes, with the starting
materials being isolated. Employing the higher boiling solvent
1,2-dichloroethane in combination with an increased reaction
temperature (95 °C) produced complexes 11−15 in good
yields.
RESULTS AND DISCUSSION
■
Synthesis. The 2,4-disubsituted phenylimidazoles 1−5 were
obtained in moderate to good yield via a two-step procedure
involving initial reaction of an equimolar mixture of glyoxal
with the chosen disubstituted aniline, followed by the addition
of NH4Cl and ring closure with formaldehyde (Scheme 1).14d
Formation of the desired imidazolium salts 6−10 (Scheme 2)
was achieved via alkylation of the 2,4-disubsituted phenyl-
Characterization. The structures of the 2,4-disubstituted
phenylimidazoles, the imidazolium salts and the Ir-
(ppy)2(C∧C:) complexes were confirmed by 1H and 13C
NMR spectroscopy and, in the case of the 9, 10, 12, and 15, by
X-ray crystallography. The imidazolium salts gave relatively
1
Scheme 1. Synthesis of 2,4-Substituted Phenylimidazoles 1−
5
simple H NMR spectra, all with a characteristic downfield
resonance for the strongly deshielded pro-carbenic proton,
which occurred at 9.75, 10.09, 9.97, 10.19, and 10.36 ppm for
6−10, respectively.
The synthesized Ir(ppy)2(C∧C:) complexes 11−15 were
uncharged, with two cyclometalated 2-(phenyl)pyridine ligands
in addition to the 2,4-disubstituted phenyl imidazolylidene
ancillary ligand coordinated to the Ir(III) center. The predicted
1
number of signals were obtained in the H and 13C NMR
spectra for the Ir(ppy)2(C∧C:) complexes, consistent with the
low-symmetry structures. As is generally observed for systems
of this type,19 the mutually trans disposition of the pyridyl
groups of the ppy ligands found in the precursor compound
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[Ir(ppy)Cl]2 was retained in the Ir(ppy)2(C∧C:) complexes
that formed. As expected, upon deprotonation and coordina-
tion of the NHC group the signal for the imidazolium salt pro-
1
carbenic proton was absent from the H NMR spectra of the
complexes. In addition a characteristic downfield chemical shift
was observed for the carbenic carbon atom, occurring at 176.21,
178.36, 174.82, 178.47, and 176.32 ppm for complexes 11−15,
respectively.
Structural Studies. Crystallographic data for the imidazo-
lium salts 9 and 10 and the Ir(III) complexes 12 and 15, are
given in Table S1 (Supporting Information) with selected bond
distances collated in Table 1. The X-ray crystal structures for 9
Figure 2. ORTEP20 representations of the X-ray crystal structures of
the Ir(III) complexes (a) 12 and (b) 15. The hydrogen atoms and
solvents of crystallization have been omitted for clarity. Thermal
ellipsoids are shown at 50% probability.
Table 1. Selected Bond Distances (Å) from the X-ray
Structures of Imidazolium Salts 9 and 10 and Iridium(III)
Complexes 12 and 15
Absorption Spectroscopy. The UV−visible absorbance
spectra for compounds 11−15 (Figure 3 and Table 2) show
9
10
12
15
C1−N1
C1−N2
Ir1−C1
Ir1−C4
Ir1−C11
Ir1−C22
Ir1−N3
Ir1−N4
1.334(3)
1.327(3)
1.331(3)
1.320(3)
1.377(5)
1.355(5)
2.053(4)
2.106(4)
2.038(4)
2.065(4)
2.039(3)
2.045(3)
1.367(4)
1.356(4)
2.063(3)
2.091(3)
2.036(3)
2.059(3)
2.040(2)
2.050(2)
and 10 are illustrated in Figure 1. For compound 9, a short
hydrogen-bonding interaction of 2.7572(2) Å is observed
between the weakly acidic pro-carbenic proton (C11) and the
iodide counterion (I2).
The X-ray crystal structures of the Ir(III) complexes 12 and
15 are illustrated in Figure 2. Both complexes display a slightly
distorted octahedral coordination geometry about the Ir(III)
centers with two cyclometalated 2-(phenyl)pyridine ligands in
combination with the cyclometalated phenyl-substituted
imidazolylidene unit. As expected for the ppy ligands, the
pyridyl groups and phenyl groups adopt mutually trans and cis
dispositions, respectively. The Ir−Ccarbene (Ir1−C1) bond
distances are similar for each complex, being 2.053(4) and
2.063(4) Å for 12 and 15, respectively. Comparable Ir−Ccarbene
bond distances have been reported previously for imidazolyli-
dene-containing complexes. For example, the Ir−Ccarbene bond
distance for the Ir(III) complex of a pyridyl-substituted NHC
ligand in combination with two cyclometalating 2-(2,4-
difluorophenyl)pyridine ligands is 2.060(5) Å.13c Shorter
Ir(III)−Ccarbene bond distances of 1.91 and 1.97 Å were
reported for a homoleptic Ir(III) complex of a cyclometalated
imidazolylidene-based NHC ligand.10
Figure 3. Absorbance spectra of 10−5 M solutions of complexes 11−
15 (bottom to top) dissolved in acetonitrile. Spectra are offset for
clarity.
intense absorption bands below 300 nm due to spin-allowed π
→ π* ligand-centered (LC) transitions originating on the NHC
and phenylpyridine ligands. The low-intensity bands between
500 and 300 nm can be assigned to both allowed and spin-
forbidden metal-to-ligand charge transfer (MLCT) transitions.
Strong spin−orbit coupling of the metal center promotes
mixing of these charge-transfer transitions with the higher
energy spin-allowed ligand-centered transitions.21 For com-
plexes 14 and 15 the strongly electron withdrawing groups on
Figure 1. ORTEP20 representations of the X-ray crystal structures of the imidazolium salts (a) 9 and (b) 10. Thermal ellipsoids are shown at 50%
probability.
C
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a
Table 2. Spectroscopic Properties of Ir(III) Complexes 11−15 in Comparison with Those of [Ru(bpy)3]2+
radiative rate
photoluminescence quantum
const kr/105
nonradiative rate
b
c
d
e
λmax/nm (ε/M−1 cm−1
)
λmax/nm
yield ϕp
lifetime τp/μs
s−1
const knr/105 s−1
11
12
13
14
15
237 (43100), 271 (37900), 334 (8500), 379 (6100), 408
(4500 sh), 450 (2200 sh)
529
0.498
1.98
2.5
2.6
3.2
3.4
2.5
3.6
1.9
1.6
240 (45700), 270 (38500), 337 (8200), 382 (6300), 408
(4700 sh), 450 (2100 sh)
533
525
507
0.420
0.621
0.683
1.59
1.97
2.03
255 (48000), 271 (39000 sh), 380 (4900), 409 (3400 sh),
450 (1100 sh)
215 (51500), 244 (48400), 273 (41600), 368 (11100 sh),
401 (8800), 445 (5200 sh)
f
243 (31100), 267 (28300), 353 (4900 sh), 384 (3600)
487
620
0.531
0.087
2.06 (0.64)
2.6 (8.3)
1.0
2.3 (7.4)
10.7
[Ru(bpy)3]
[PF6]2
208 (46300 sh), 244 (27000), 253 (23300), 286 (82400),
324 (9300), 353 (5400), 424 (11000 sh), 450 (13600)
0.85
a
b
All complexes 1 × 10−5 M in acetonitrile. λmax is the position of the peak or shoulder in the absorbance or emission profile, and ε is the molar
c
d
absorptivity. Emission corrected for variation in detector sensitivity with wavelength. Absolute quantum yields. The 95% confidence intervals (n =
e
6) were no larger than 0.0035. Radiative rate constant kr = ϕp/τp = 1/τ0 and nonradiative rate constant knr = (1 − ϕp)/τp, where the intrinsic
f
lifetime τ0 = τp/ϕp. Double-exponential decay (longer lifetime component 60%).
the auxiliary NHC ligand result in the onset of these MLCT
bands being blue-shifted (10−50 nm) relative to the other
complexes.
Photoluminescence. The photoluminescence from the
Ir(III) complexes arises from the excitation of an electron from
the HOMO, which is partially metal based and partially located
on the phenyl moieties of the two phenylpyridine ligands, to a
ligand-based π* antibonding orbital. The excited state is
therefore best described as an admixed metal−ligand/intra-
ligand charge transfer (MLCT/LLCT). Initial excitation is
followed by intersystem crossing to the lowest triplet state,
from which emission occurs.
Each of the iridium(III) complexes exhibited intense
photoluminescence in dilute acetonitrile solution at room
temperature, with quantum yields (ϕp) ranging from 42% to
68%, increasing in the order 14 > 13 > 15 > 11 > 12 (Table 2
and Figure 4a). The kinetic parameters for luminescence decay
(Table 2) suggest that the differences in quantum yield
between the complexes may be due to differences in the rate of
a nonradiative reaction, as knr follows the opposite trend to ϕp
and, with the exception of 15, obeys the energy gap law (Figure
S2, Supporting Information). The deactivation route process
3
may also comprise thermal deactivation to a dissociative MC
state, as suggested by Thompson et al. for other tris-
cyclometalated iridium complexes.22
As is often observed for other cyclometalated iridium
complexes,22 the photoluminescence intensities were found to
be extremely sensitive to the presence of oxygen even in minute
amounts. This necessitated a very high level of care to maintain
anaerobic conditions during quantum yield and lifetime
determinations. Photoluminescence decay profiles are shown
in Figure S1 (Supporting Information). The excited-state
lifetimes for the iridium complexes, calculated from these
transients, were all between 1.6 and 2.1 μs, which is
approximately twice as long as that measured for [Ru(bpy)3]2+.
This is one factor which undoubtedly contributes to the high
sensitivity to oxygen quenching, in comparison with the
ruthenium complex.
Figure 4. (a) Photoluminescence spectra for 10−6 M solutions of the
iridium complexes in acetonitrile: (red) 11; (green) 12; (black) 13;
(pink) 14; (blue) 15. The integrated emission intensity in each case
has been normalized for the absorbance of the solution at the
wavelength of excitation (345 nm) reflecting relative photolumines-
cence quantum efficiencies. (b) ECL spectra produced via annihilation
between the oxidized and reduced forms of the complexes. All iridium
complexes are 10−4 M solutions in acetonitrile containing 0.1 M
[Bu4N][PF6]. Intensities have been normalized to reflect relative ECL
quantum efficiencies.
The corrected photoluminescence spectra for the iridium
complexes in deaerated CH3CN at room temperature are
shown in Figure 4. For complexes 11−13 the emission color
shows little variation. However, as shown by the data presented
in Table 2, the two halogenated complexes, 14 and 15, are blue-
shifted significantly (by about 25 and 50 nm, respectively).
These observations may be accounted for by considering
electron resonance and inductive effects. The electron-with-
drawing halogen groups are positioned meta to the metal
coordination site and therefore inductively stabilize the
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Table 3. Electrochemical and Electrochemiluminescence Properties of Iridium Complexes 11−15 in Comparison with Those of
a
[Ru(bpy)3]2+
b
c
anodic process
cathodic process
E°ox
−
106D /cm2 λmax ECL / rel annihilation ECL intensity rel coreactant ECL intensity
d
e
E°ox/V
E°red/V
E°red/V
s−1
nm
ϕECL
ϕECL
11
12
13
14
0.24
0.21
0.25
0.36
−2.72, −2.98
−2.73, −3.00
−2.72, −2.98
2.96
2.95
2.98
2.97
8.43
7.79
7.44
532
537
531
524
31
10
48
17
0
0
0
(−2.61), −2.70,
10.4
0.1
−2.97
15
0.66
0.89
−2.57, −2.82
3.23
2.62
5.54
7.90
490
618
103
100
20
100
[Ru(bpy)3]2+
−1.73, −1.92,
−2.16
a
b
0.2 mM/0.1 M (Bu4N)PF6 in acetonitrile. Potentials referenced to Fc/Fc+. Diffusion coefficients calculated from plots of peak current vs square
root of scan rate. A representative Randles−Sevci̧ k plot is shown in the Supporting Information (Figure S3). Spectra from annihilation ECL
experiments. Relative ECL efficiency via annihilation. Relative coreactant ECL efficiency.
c
d
e
HOMO, which is substantially based on the iridium center (see
Figure 6). The effect is to increase the HOMO−LUMO gap
and thus the emission energy. Therefore, a blue shift in 14 and
15 is observed when chlorine or fluorine groups are substituted
at the 2- and 4-positions of the phenyl group on the auxiliary
ligand. On the other hand, no shift is observed in the case of
the methoxy-substituted complex 13 because this group exerts a
resonance effect and would therefore need to be situated
ortho/para to the coordination site to exert an influence on the
HOMO.
Electrochemistry. The electrochemical properties of 11−
15 were studied using cyclic voltammetry, and the results are
summarized in Table 3. Figure 5 shows the cyclic voltammo-
grams (CVs) of 11−15 and [Ru(bpy)3]2+ in oxygen-free
acetonitrile. When scanned anodically, each complex exhibited
a one-electron-oxidation process, which can be formally
assigned to the IrIII/IrIV redox couple, though it should be
noted that the HOMO is also delocalized over the phenyl
moieties of the phenylpyridine ligands (see Figure 6). In the
case of 15 this oxidation couple was fully chemically reversible,
with forward and reverse peaks the same magnitude (ip,ox
=
ip,red) at all scan rates tested (0.1−5.0 V s−1), whereas the
responses for 11 and 13 were reversible only at higher scan
rates (≥1.0 V s−1) and the responses for 12 and 14 exhibited
only semireversible behavior regardless of scan rate.
Scanning in the cathodic region typically resulted in two
reversible, one-electron waves which are assigned to the
stepwise reduction of the two ppy ligands on the basis of
previous results for similar compounds such as Ir(ppy)3 (which
has a reduction potential of −2.67 V vs Fc).23 This conclusion
is also supported by the theoretical data presented later (see
Figure 6). Complex 14 also displayed a third irreversible peak
at −2.61 V, the shape of which suggests that it is related to
adsorption of the complex on the electrode.
Figure 5. Cyclic voltammetric responses for (top to bottom) 15, 14,
13, 12, 11, and [Ru(bpy)3]2+ using a glassy-carbon-disk working
electrode (3 mm diameter) at a scan rate of 1.0 V s−1. The
concentration was 0.2 mM in each case for the samples dissolved in
acetonitrile containing 0.1 M [Bu4N][PF6] as supporting electrolyte.
The CVs in Figure 4 indicate that the oxidation and
reduction potentials of the Ir(III) complexes are all
considerably more negative than those of [Ru(bpy)3]2+.
While the oxidation potential (E°ox) and reduction potential
(E°red) for 11−13 are all quite similar, substitution of
electronwithdrawing groups on the ancillary ligand caused a
positive shift in E°ox: by 120 mV in the case of the dichloro-
substituted 14 and by 420 mV in the case of the difluoro-
substituted 15. With halogen substitution at the NHC ligand
the effect on reduction potentials was comparatively modest,
resulting in a positive shift in E°red by 20 mV for 14 and 150
mV for 15.
It is notable that the electrochemical HOMO−LUMO gap
(E°ox − E°red) (see Table 3) correlates very well with the
emission energies of the complexes. The electrochemical results
therefore provide a basis for understanding the luminescence
data presented in Table 2. For example, in light of the oxidation
potential data, the blue shifts observed for the luminescence
maxima of 14 and 15 can be understood as resulting from a
widening of the HOMO−LUMO gap caused by stabilization of
the HOMO energy levels of these complexes. In both
complexes, the HOMO stabilization effect dominates the
relatively small LUMO stabilizations evident in the reduction
potentials.
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Theoretical Calculations. The optimized geometries of
complexes 11−15 maintain a common framework with little
variation due to the R substituent. For R = OMe, a
conformational search was carried out in order to identify the
lowest energy structure. Both M06-2X and mPW1PW91 DFT
methods yielded equivalent geometries. Geometrical parame-
ters are consistent with the X-ray structures (Table 1):
mPW1PW91/def2-SVP calculated bond distances for 15 are
Ir1−C1 = 2.102 Å, C1−N1 = 1.367 Å, and C1−N2 = 1.355 Å,
in comparison to 2.063, 1.367, and 1.356 Å, respectively, from
the X-ray structure. Similarly, for 12 the calculated and X-ray
derived structures are in excellent agreement. The
mPW1PW91/def2-SVP geometries are provided in the
Supporting Information as a text file of all computed molecule
Cartesian coordinates in .xyz format for convenient visual-
ization.
Molecular orbitals (MOs) were calculated with the inclusion
of solvent effects and with the def2-TZVP basis set, which
yielded much better agreement with experimental trends than
gas-phase results. Only solvent-corrected results are presented.
There is significant DFT functional dependence for the MO
energies and HOMO−LUMO gaps, although the relative
trends always remain consistent. The mPW1PW91 and PBE
functionals yielded MO energies within 0.01 eV of each other.
In comparison, the B3LYP and BP86 functionals consistently
predicted smaller HOMO−LUMO gaps than did mPW1PW91:
0.4 eV lower with B3LYP and 1.7−1.8 eV with BP86. M06-2X
and ωB97XD functionals yielded larger HOMO−LUMO gaps
than mPW1PW91: 1.8−1.9 eV larger with M06-2X and 3.2 eV
with ωB97XD. The Hartree−Fock calculated HOMO−LUMO
gaps were 5.6−5.7 eV higher in energy, resulting from much
lower HOMO energies and much higher LUMO energies.
These trends are consistent with those we recently reported for
a series of blue-emitting Ir(III) complexes.24
orbital and the phenylpyridine ligand, while with R = OMe
(15) there is additional contribution from the auxiliary NHC
ligand. The HOMO-1 has contributions from the metal d
orbital and the auxiliary ligand. The nature of the frontier MOs
was further characterized from a Mulliken population analysis
as a function of molecular fragments (metal, phenylpyridine,
and auxiliary NHC ligand). As with the MO energies, the
nature of the frontier MOs also exhibited a functional
dependence, although it was limited to differences in the
nature of the HOMO. Both BP86 and mPW1PW91 functionals
produced similar trends; however, it is notable that the BP86
functional consistently predicts a 10% larger metal contribution
to the HOMOs for the iridium complexes. All methods predict
that the LUMO for all of the iridium complexes is entirely ppy
based (>95%). Plots of the fragment contribution (metal, ppy,
or NHC-based ligand) to the LUMO and HOMO are included
in Figures S5 and S6 (Supporting Information).
TDDFT calculations confirm the assignment of UV−visible
bands; the most intense bands below 300 nm are principally
transitions from low-lying ligand-based π orbitals (typically
HOMO-6) to the LUMO (π*) centered on the ppy ligand.
Electrochemiluminescence. All five complexes exhibited
intense to moderately intense ECL via the annihilation
mechanism.14b As illustrated in Figure 4b, the ECL emission
profiles are essentially identical with the photoluminescence
spectra, indicating that the same excited state for each complex
is populated regardless of whether an optical or electrochemical
excitation method is employed. To accurately quantify the
ability of these complexes to produce ECL, their relative
annihilation ECL efficiencies were measured from potential
step experiments where the reduced and oxidized forms were
generated sequentially at the working electrode according to
[Ir(ppy)2(L)] → [Ir(ppy)2(L)]+ + e−
(1)
[Ir(ppy)2(L)] + e− → [Ir(ppy)2(L)]−
Basis set effects were further investigated with the addition of
diffuse functions to def2-TZVP. Augmenting with diffuse
functions altered mPW1PW91 frontier MO energies by only
0.03 eV and the HOMO−LUMO gap by 0.01 eV, which is not
significant. For comparison with experiment and subsequent
analysis of MO energies only mPW1PW91/def2-TZVP
solvent-corrected results are considered.
Contour plots of the MOs indicate very similar properties for
the Ir(III) complexes (see Figure 6 and Figure S4 in the
Supporting Information). For all iridium complexes the LUMO
(and LUMO+1 to LUMO+3) is located on the phenylpyridine
ligand. The HOMO is predominantly centered on the metal d
(2)
[Ir(ppy)2(L)]− + [Ir(ppy)2(L)]+
→ [Ir(ppy)2(L)]* + [Ir(ppy)2(L)]
(3)
[Ir(ppy)2(L)]* → [Ir(ppy)2(L)] + hv
(4)
where L is the bidentate C∧C: imidazolylidene-based NHC
ligand. As indicated in Table 3, 15 produced the highest
annihilation ECL intensity, slightly exceeding that of [Ru-
(bpy)3]2+. The annihilation ECL efficiencies of the other
compounds varied from 10 to 48% relative to the standard.
In the case of coreactant ECL using tripropylamine (TPA),
the excited state is generated via the following mechanism, after
generation of [Ir(ppy)2(L)]+ via reaction 1:
TPA → TPA•+ + e−
(5)
•
TPA•+ → TPA + H+
(6)
[Ir(ppy)2(L)]+ + TPA → [Ir(ppy)2(L)]* + P
•
(7)
followed by de-excitation via reaction 4
Interestingly, despite the strong annihilation ECL observed
for the complexes, only 15 produced significant emission via
the coreactant pathway; the ECL emission efficiency was 20%
for this complex with respect to the ruthenium-based standard
These observations provide an interesting insight into the
importance of energetic considerations in the design of novel
electrochemiluminophores, and in particular the interplay of
Figure 6. HOMO and LUMO for 11. Hydrogen atoms are omitted for
clarity. The complete set of orbitals for each complex can be seen in
the Supporting Information (Figure S4).
F
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Figure 7. Energy diagram for coreactant ECL (reaction 7 in the text): (route A) energy-sufficient coreactant ECL system where E°ox > Ecritical; (route
B) energy-insufficient coreactant ECL system where E°ox < Ecritical
.
spectroscopic and electrochemical properties in such systems.
For example, the free energy (ΔG) associated with annihilation
reaction (3) above may be estimated using the relationship
eq 9 shows that the ECL reaction (eq 7) for the cases of 11−13
does not have sufficient energy to populate the excited state
while that for 14 is only marginally energy sufficient. This
explains why the last four complexes (11−14) give little or no
coreactant ECL emission, whereas 15, for which the reaction is
energy sufficient by about 0.2 eV according to eq 9, gives
relatively intense emission.
These observations are in accord with those of Kapturkiewicz
et al., who gave a detailed account of how the efficiency of
Ir(ppy)3 ECL depended on the available energy supplied by
reaction with various electroreduced aromatic nitriles and
ketones.18a On the basis of eq 9 it is clear that, when seeking to
design new electrochemiluminophores for applications such as
ECL-based assays, it is necessary to optimize the energetics of
the ECL reaction (7) so as to achieve maximum ECL efficiency
and therefore enhance sensitivity. However, unlike the case
with the model systems investigated by Kapturkiewicz et al.,18a
there is little scope to vary the reducing power of the coreactant
radical. This is because, in order to function in aqueous-based
assays, the reductant must be capable of being produced
oxidatively as in eqs 5 and 6. Therefore, the best strategy is to
focus on increasing the oxidizing ability of the luminophore.
Apart from optimizing sensitivity, an increasingly important
goal in the field of ECL detection is the development of
°
°
− E + EMLCT
ΔG = E
(8)
red
ox
where EMLCT is the spectroscopic energy of the excited state in
eV, which is best taken from the λmax value of the low-
temperature emission spectrum (but may be derived from
room-temperature data to a first approximation). As the values
for EMLCT for complexes 11−15 vary in the range 2.35−2.55
eV, it is clear that the annihilation reaction (3) between the
reduced and oxidized species leading to the excited state is
energy sufficient in each case, with 15 having the most negative
value of ΔG and exhibiting the greatest ECL efficiency.
In the case of the coreactant ECL pathway the electro-
reduced complex is replaced by a reducing radical species
derived from the coreactant (TPA•), and the ΔG value of the
reaction may be estimated from
•
°
°
ΔG = E (TPA ) − E + EMLCT
(9)
ox
where E°(TPA•), the reduction potential of the radical, has
previously been found to have a value of −2.1 V vs Fc.23,25
Substituting the values of E°ox from Table 3 and the values of
EMLCT derived from the emission peak maxima in Table 2 into
G
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electrochemiluminophores with varying emission wavelengths
for applications such as multiplex analysis.26 An important
consideration here is that higher energy emitters place greater
demands on the energy requirements of the ECL reaction. In
other words, the bluer the emission, the greater the amount of
energy that must be available from reaction 7 in order to
populate the excited state. Figure 7 illustrates this point clearly:
for a given emission color (and EMLCT value), the oxidation
potential must be sufficiently positive (and the reduction
potential sufficiently positive) such that the electron transfer
from the HOMO of TPA• to the LUMO of the complex is
energetically favorable. Note that, according to Marcus−Hush
theory, the more positive the value of E°ox, the more favorable
will be the kinetics of the reaction leading to the excited state
(normal region) and the less favorable will be the kinetics of the
reaction leading to the ground state (inverted region).24,27
Figure 7 suggests that (for a given coreactant) there is a
critical value of E°ox for each emission color, necessary for ECL
to be observed (denoted Ecritical). Figure 8 illustrates the
most unfavorable location on the graph from the point of view
of ECL.
It should be acknowledged that there are other determinants
of ECL efficiency that are not taken into account by the graph,
such as photoluminescence quantum yield and the kinetic
stability of the oxidized form of the luminophore. However, this
plot is proposed as a useful “ready reckoner” for predicting the
likely ECL ability of compounds in the search for new
electrochemiluminophores with differing emission colors.
CONCLUSION
■
A novel series of heteroleptic iridium(III) complexes with two
cyclometalated 2-(phenyl)pyridine ligands in combination with
a cyclometalated phenylimidazolylidene unit were synthesized.
The phenyl group of the bidentate imidazolylidene unit was
substituted at the 2- and 4-positions with electron-withdrawing
and -donating substituents in an effort to modulate the electron
density at the metal center. The precursor 2,4-disubsituted
phenylimidazoles were prepared using a two-step procedure
from the appropriate disubstituted aniline and glyoxal, followed
by the addition of NH4Cl and formaldehyde. Each of the
iridium(III) complexes exhibited intense photoluminescence in
acetonitrile solution at room temperature with quantum yields
(ϕp) ranging from 42% to 68% and excited-state lifetimes on
the order of 2 μs. These properties suggest possible applications
in electroluminescent devices, as lifetimes are long enough for
triplet exciton harvesting, yet short enough that triplet−triplet
annihilation and quantum efficiency rolloff would be unlikely to
diminish device performance.
Substitution at the phenyl ring of the auxiliary ligand can be
effectively used to modulate the energy of the substantially
metal based HOMO. For example, fluorine substituents at the
2- and 4-positions of this ligand bring about a 50 nm shift in
emission wavelength. Similarly, the oxidation potentials of the
complexes are sensitive to substitution on the auxiliary ligand
but the reduction potentials are not. It can be surmised
therefore that substitution at this position represents a good
strategy for stabilizing HOMO levels while having a lesser or no
effect on the LUMO, which is most remote from the
substitution point in these complexes. DFT calculations
confirm that the HOMO is delocalized over the iridium center
and the phenyl rings of the phenylpyridine ligands and that the
LUMO is largely localized on the pyridine moieties of the
phenylpyridines.
Figure 8. ECL wall of energy sufficiency for tripropylamine (TPA).
The dashed line represents the critical oxidation potential required for
ECL for a given emission color when TPA is the coreactant. Reactions
of the TPA radical with compounds falling to the right of this line will
be energy sufficient for excited state formation; reactions with
compounds falling to the left of the line will not be energy sufficient.
All five complexes exhibited moderate to intense electro-
chemiluminescence (ECL) via the annihilation mechanism;
however, only 15 produced significant coreactant ECL with
tripropylamine (TPA). This can be understood as being a
consequence of the relative oxidizing abilities of the complexes
and by considering the energetics of the ECL reaction leading
to the excited state. Generally it can be surmised that bluer
emitters require more positive oxidation potentials in order to
be coreactant ECL active. To this end, a plot of oxidation
potential versus emission energy is proposed as a convenient
guide to aid in the prediction of energy sufficiency in ECL
reactions.
It is clear that when seeking to design electrochemilumino-
phores with bluer emission (larger HOMO−LUMO gap) the
only fruitful strategies will involve stabilization of the HOMO
levels (rather than destabilization of LUMO). Fortunately,
because of the delocalized nature of the HOMO in iridium
complexes of this type (as opposed to their ruthenium-based
counterparts which have HOMOs almost exclusively localized
location of the luminophores investigated in this study relative
to these critical values, plotted as a line transecting a graph of
oxidation potential versus emission color. We have termed the
critical line the ECL “wall”, as only complexes positioned to the
right of the line will have reactions which are energy sufficient
and thus exhibit ECL. The plot in Figure 8 well illustrates the
issues discussed above: 15 gave the most intense ECL of the
iridium complexes and is positioned relatively far from the
“wall”; 14 gave very low ECL and is adjacent to the wall; 11−
13 all fall on the energy-deficient side of the wall. For
comparison, Ru(bpy)32+, the benchmark ECL emitter, enjoys
the position most remote from the “wall” on the energy-
sufficient side, whereas Ir(pmi)3 (tris(1-phenyl-3-methylimida-
zolin-2-ylidene-C,C2′)iridium(III)), which is known from a
previous study24 to give no ECL despite a photoluminescence
quantum yield approaching unity, has an oxidation potential of
0.22 V and an emission maximum of 405 nm and occupies the
H
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sonicated in acetonitrile for 10 s followed by a final rinse in acetonitrile
and dried with a stream of N2. Potentials were referenced to the
ferrocene/ferrocenium couple measured in situ (0.2 mM) in each case.
Stock solutions of the complexes were prepared at a concentration of
0.2 mM in freshly distilled acetonitrile, and [Bu4N][PF6] was added to
give a concentration of 0.1 M of supporting electrolyte. Oxygen was
removed from the solutions by bubbling vigorously with N2 for 10 min
followed by lightly blanketing the solution with N2 during the
experiments. Scan rates ranging from 0.01 to 0.5 V s−1 were used in
cyclic voltammetry to evaluate the diffusion coefficients using the
on the metal), proven strategies exist for independently
modifying the frontier orbital energies. For example, in the
series of compounds studied here, the HOMO levels could be
selectively stabilized by substitution of strongly electron
withdrawing groups on the phenyl rings of the phenylpyridine
ligands. In principle this should result in a more positive
oxidation potential and greater ECL efficiency. We intend to
explore this avenue in future work.
Randles−Sevci
̧ k equation (see Figure S3 in the Supporting
EXPERIMENTAL SECTION
■
Information).
General Considerations. All reagents were purchased from
Sigma-Aldrich or Alfa Aesar and were of analytical grade or higher
and were used without further purification unless otherwise stated. Dry
acetonitrile was distilled from CaH2. NMR spectra were recorded
Electrochemiluminescence (ECL). Relative ECL efficiencies
(ϕECL) were evaluated by comparison of the ECL spectra
(annihilation) or PMT signal intensity (coreactant) with that of 0.2
mM [Ru(bpy)3]2+ in acetonitrile containing 0.1 M [Bu4N][PF6].28
Solutions were prepared with deoxygenated acetonitrile (freeze/
pump/thaw three cycles) in a light-tight custom-made N2 filled
glovebox. Annihilation ECL was generated using chronoamperometry,
where the cathodic and anodic potentials were stepped for 1.0 s
sequentially for one cycle. An overpotential of 0.1 V was used to
generate the 1+ and 1− forms of the Ir(III) complexes in the
annihilation reactions. ECL spectra were obtained using a fiber optic
(feed through from glovebox) to either an Ocean Optics CCD Model
QE65 Pro (or QE6500 with the spectra smoothed) with a HR 4000
Breakout box trigger in conjunction with a μ-Autolab type II
electrochemical station potentiostat (MEP Instruments, North Ryde,
NSW, Australia).
Coreactant ECL was generated with a 3 mm diameter glassy-carbon
electrode in a 0.2 mM solution of the complex containing 0.1 M
[Bu4N][PF6] supporting electrolyte and 10 mM tripropylamine
(TPA) as the coreactant. The potential was pulsed 0.1 V beyond
the complex’s oxidation potential for 1.0 s. ECL intensity was detected
using a photomultipler tube (Model H7826-01 No. 62340001
Hamamatsu Japan), biased by an input control voltage of 0.3 V
using a power supply with a variable resistor. The output signal of the
photomultiplier was acquired using the auxiliary channel of the
potentiostat via a TA-GI-74 Ames Photonics Inc. amplifier (Model
D7280).
using a Bruker ARX-300 (300.14 MHz for H, 75.48 MHz for 13C)
1
NMR spectrometer and were referenced to solvent resonances.
Microanalyses were performed by the Microanalytical Laboratory at
the ANU Research School of Chemistry, Canberra, Australia. All
compounds were prepared in air unless otherwise specified.
Photophysical Measurements. UV−visible spectra were col-
lected using a Cary Series UV−visible spectrophotometer (Agilent)
with a 1 cm path length quartz cuvette, a spectral bandwidth of 1 nm, a
signal averaging time of 0.1 s, a data interval of 0.25 nm, and a scan
rate of 150 nm/min. Steady-state emission spectra were collected on a
Nanolog (HORIBA Jobin Yvon IBH) spectrometer using a 1 cm
quartz cuvette, a band pass of 2 nm, an increment of 1 nm, and an
integration time of 0.2 s. One micromolar solutions in an airtight four-
sided quartz cuvette were degassed with Ar in a N2 glovebox for 10
min. A 450 W xenon-arc lamp was used to excite the complexes using
a 1200 g/mm grating blazed at 330 nm excitation monochromators, a
1200 g/mm grating blazed at a 500 nm emission monchromator, and a
thermoelectrically cooled TBX picosecond single-photon detector.
Emission and excitation spectra were corrected for source intensity,
gratings, and detector response. Lifetimes (5 μM solutions) were
measured using the time correlated single photon counting (TCSPC)
option on the spectrometer and correlated by a time-to-amplitude
converter (TAC) in forward TAC mode. Nanoled 340 (344 nm) and
Nanoled 460 (451 nm) lasers were pulsed at a 100 kHz repetition rate,
and the emission bandwidth was set to 5 nm. Signals were collected
using a FluoroHub counter and the data analyzed using DAS6 software
(HORIBA Jobin Yvon IBH). Spectra for absolute quantum yields were
Solution-phase ECL efficiencies (ϕECL) were measured relative to
2+
Ru(bpy)3 taken as 100% and were evaluated using the equation
φECL = IQ f /Q fIs
where I and Is are the integrated spectra (or PMT responses) for the
sample and standard, respectively, and Qf and Qfs are the total Faradaic
charges passed during the forward step for the sample and standard,
respectively.
s
(11)
measured at room temperature (22
2 °C) with a Quanta-phi
HORIBA Scientific 6 in. diameter integrating sphere connected to the
Nanolog via optical fibers. The complexes were excited using a 450 W
xenon lamp and detected with a liquid nitrogen cooled Symphony II
(Model SII-1LS-256−06) CCD. Absolute quantum yields were
calculated by the four-plot method using Fluoressence v3.5 software
following the equation
Theoretical Calculations. Density functional theory (DFT)
calculations were carried out within the Gaussian 09 suite of
programs.29 Ground state geometries were optimized in the absence
of solvent with the mPW1PW9130 functional in conjunction with the
def2-SVP basis set and associated core potential.31 The mPW1PW91
functional has previously been demonstrated to yield reliable results
for such systems.15f,32 M06-2X33 optimized geometries were
equivalent. Single-point energy calculations were carried out with the
def2-TZVP basis set and core potential.31 The polarizable continuum
model (PCM)34 self-consistent reaction field (SCRF) was used to
model solvent effects at the gas-phase optimized geometries with a
solvent of acetonitrile, consistent with the experimental system.
Frontier MO energies were calculated using DFT MOs with
mPW91PW91, PBE,35 B3LYP,36 BP86,36a,37 M06-2X,33 and
wB97XD.38 Diffuse basis functions for the def2-TZVP basis set were
generated in an even-tempered manner; one additional function of
each orbital angular momentum was generated using a ratio of 2.4 to
the exponent of the most diffuse function in the original basis set. An
SCF convergence criteria of 10−8 au was employed throughout, with
the exception of the augmented basis set calculations that required a
reduced threshold of 10−6 au (test calculations indicate the effect of
reduced convergence criteria was in the order of 10−4 au in MO
energies). TD-DFT calculations were carried out at the mPW1PW91/
photons out
photons in
(Ec − Ea)/A
La − Lc
φ =
=
p
(10)
where Ec is the integrated luminescence of the sample, Ea is the
integrated luminescence of the blank, A is the area factor due to CCD
integration time, La is the integrated excitation from the blank, and Lc
is the integrated excitation from the sample. Spectral measurements
were averaged from at least six replicates. The 95% confidence
intervals for the quantum yield measurements were no more than
0.0035. For example, the quantum yield for Ru(bpy)32+ was found to
be 0.087 0.002 at the 95% level of confidence (n = 6).
Electrochemistry. Electrochemical experiments were performed
using a PGSTAT12 AUTOLAB electrochemical potentiostat (MEP
Instruments, North Ryde, NSW, Australia) with Nova 1.8 software. A
conventional three-electrode cell configuration was used, consisting of
a silver-wire quasi reference electrode, a platinum-wire auxiliary
electrode, and a 3 mm diameter glassy-carbon-disk working electrode
shrouded in Teflon (CH Instruments, Austin, TX, USA). The working
electrode was polished with 0.3 μm and then 0.05 μm alumina slurry
on a felt pad, rinsed with Milli-Q water followed by acetone, and then
I
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def2-TZVP level of theory with the SCRF PCM solvent field of
acetonitrile; 30 singlet states were modeled. Molecular orbital analysis
was carried out with the AOMix program.39
After purification the product was obtained as a dark brown oil (yield:
1.08 g, 53%). H NMR (d6-DMSO): δ 3.80 (s, 3H, OCH3), 3.81 (s,
1
3H, OCH3), 6.62 (dd, 1H, 3JHH = 8.68 Hz, 4JHH = 2.64 Hz, Haryl), 6.77
(d, 1H, 3JHH = 2.60 Hz, Haryl), 7.00 (t, 1H, 3JHH = 1.81 Hz, Himi), 7.28
(s, 1H, Haryl), 7.30 (t, 1H, 3JHH = 3 Hz, Haryl), 7.76 (t, 1H, 3JHH = 1.08
Hz, NCHN). 13C NMR (d6-DMSO): δ 55.58 (OCH3), 55.96 (OCH3),
99.72 (Caryl), 105.06 (Caryl), 119.36 (Cq), 120.86 (Cimi),126.52 (Caryl),
128.05 (Cimi), δ 137.68 (NCN), 153.45 (Cq), 159.92 (Cq). Anal.
Found: C, 61.89; H, 6.29; N, 13.20. Calcd for C11H12N2O2·0.5H2O: C,
61.96; H, 6.15; N, 13.14.
X-ray Crystallography. Single crystals of imidazolium salts 9 and
10 were grown by slow evaporation of methanol solutions of each
compound. Single crystals of the Ir(III) complexes 12 and 15 suitable
for X-ray diffraction studies were grown by slow evaporation of
acetonitrile solutions of each compound. Crystallographic data for all
structures determined are given in Table 1 and Table S1 (Supporting
Information). For all samples, crystals were removed from the
crystallization vial and immediately coated with Paratone oil on a glass
slide. A suitable crystal was mounted in Paratone oil on a glass fiber
and cooled rapidly to 173 K in a stream of cold N2 using an Oxford
low-temperature device. Diffraction data were measured using an
Oxford Gemini diffractometer mounted with Mo Kα (λ = 0.71073 Å)
and Cu Kα radiation (λ = 1.54184). Data were reduced and corrected
for absorption using the CrysAlis Pro program.5b The SHELXL2013-
Compound 4. This compound was prepared as described for 1,
from 2,4-dichloroaniline (1.62 g, 0.01 mol), 40% glyoxal (4.2 mL, 0.1
mol), CH3OH (7 mL), NH4Cl (1.07 g, 0.02 mol), 37% formaldehyde
(0.5 mL, 0.02 mol), and H3PO4 (0.15 mL, 85%). After purification the
product was obtained as a dark brownish red oil (yield: 0.91 g, 43%).
3
4
1H NMR (d6-DMSO): δ 7.11 (dd, 1H, JHH = 1.2 Hz, JHH = 1 Hz,
Himi), 7.45 (dd, 1H, 3JHH = 1.28 Hz, 4JHH = 1.32 Hz, Himi), 7.59−7.60
(m, 2H, Haryl), 7.90−7.91 (m, 2H, Haryl). 13C NMR (d6-DMSO): δ
121.40 (Cimi), 128.95 (Caryl), 129.38 (Cimi), 130.01 (Caryl), 130.07 (Cq),
130.44 (Caryl), 134.17 (Cq), 134.39 (Cq), 138.24 (NCN). Anal. Found:
C, 50.63; H, 2.79; N, 13.02. Calcd for C9H6Cl2N2: C, 50.74; H, 2.84;
N, 13.15.
2
40 program was used to solve the structures with direct methods, with
refinement by full-matrix least-squares refinement techniques on F2.
The non-hydrogen atoms were refined anisotropically, and hydrogen
atoms were placed geometrically and refined using the riding model.
Coordinates and anisotropic thermal parameters of all non-hydrogen
atoms were refined. All calculations were carried out using the
program Olex2.41 Images were generated by using ORTEP-3.20
Further XRD details are provided in the Supporting Information.
CCDC 982477−982480 contain supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif
Compound 5. This compound was prepared as described for 1,
from 2,4-difluoroaniline (1.29 g, 0.01 mol), 40% glyoxal (4.2 mL, 0.1
mol), CH3OH (7 mL), NH4Cl (1.07 g, 0.02 mol), 37% formaldehyde
(0.5 mL, 0.02 mol), and H3PO4 (0.15 mL, 85%). After purification on
silica the product was obtained as a purple crystalline solid, and
subsequent recrystallization from ether and hexane resulted in a
1
colorless crystalline solid (yield: 0.89 g, 49%). H NMR (d6-DMSO):
δ 7.12 (dd, 1H, 3JHH = 1.2 Hz, 4JHH = 1 Hz, Himi), 7.25−7.30 (m, 1H,
Synthesis. These compounds were prepared using a modified
literature procedure.14d
Haryl), 7.53 (dd, 1H, 3JHH = 3 Hz, 4JHH = 1.4 Hz, Himi), 7.56−7.61 (m,
3
3
Compound 1. A mixture of aniline (9.13 mL, 0.1 mol), 40% glyoxal
(4.2 mL, 0.1 mol), and CH3OH (50 mL) was stirred for 18 h at room
temperature to give a brown solution. NH4Cl (10.7 g, 0.2 mol) was
added followed by 37% formaldehyde (16 mL, 0.2 mol), and the
mixture was diluted with CH3OH (400 mL). The resulting mixture
was refluxed for 1 h, and then H3PO4 (14 mL, 85%) was added
dropwise over 30 min and the reflux was continued for a further 10 h.
The solvent was then removed under reduced pressure, and ice-cold
water (300 mL) was added followed by 40% aqueous KOH to raise
the pH to 9. The mixture was extracted with ethyl acetate (5 × 150
mL), the organic extracts were washed with brine (100 mL) and dried
with Mg2SO4, and the solvent was removed under reduced pressure.
After purification on silica, with ethyl acetate/hexane (4/1 v/v) as the
eluent, the product was obtained as a brown oil (yield: 9.01 g, 64%).
1H, Haryl), 7.71 (td, 1H, JHH = 5.9 Hz, JHH = 8.9 Hz Haryl), 7.80 (s,
1H, NCHN). 13C NMR (d6-DMSO): δ 105.21 (Cq), 105.48 (Caryl),
112.30 (Caryl), 112.53 (Cq), 120.33 (Cimi), 121.88 (Cq), 127.37 (Caryl),
129.17 (Cimi), 137.43 (NCN). Anal. Found: C, 60.00; H, 3.26; N,
15.46. Calcd for C9H6F2N2: C, 60.00; H, 3.36; N, 15.55.
Compound 6. A mixture of 1 (0.5 g, 3.47 mmol) and CH3I (0.74 g,
5.2 mmol) in CH3CN (20 mL) was refluxed for 18 h under N2. The
solvent was removed under reduced pressure, the crude product was
recrystallized from acetone and ether, and the product was obtained as
a pale cream-colored solid (yield: 0.79 g, 80%). 1H NMR (d6-DMSO):
3
δ 3.95 (s, 3H, NCH3), 7.60 (t, 1H, JHH = 8.0 Hz, Haryl), 7.67 (t, 2H,
3
3JHH =13.5 Hz, Haryl), 7.76 (d, 2H, JHH = 8.5 Hz, Haryl), 7.95 (t, 1H,
3
3JHH = 1.8 Hz, Himi), 8.29 (t, 1H, JHH = 1.9 Hz, Himi), 9.75 (s, 1H,
NCHN). 13C NMR (d6-DMSO): δ 36.14 (CH3), 120.98 (Cimi), 121.81
(2 × Caryl), 124.42 (Cimi), 129.75 (Caryl), 130.20 (2 × Caryl), 134.73
(Cq), 135.96 (NCN). Anal. Found: C, 40.89; H, 3.80; N, 9.33. Calcd
for C10H11IN2·0.5H2O: C, 40.70; H, 4.10; N, 9.49.
3
4
1H NMR (DMSO): δ 7.12 (dd, 1H, JHH = 2.24 Hz, JHH = 1.2 Hz,
3
3
Himi), 7.35 (t, 1H, JHH = 7.4 Hz, Haryl), 7.51 (t, 2H, JHH = 7.52 Hz,
3
3
Haryl), 7.64 (d, 2H, JHH = 8.60 Hz, Haryl), 7.74 (dd, 1H, JHH = 2.72
Hz, JHH = 1.36 Hz, Himi), 8.26 (t, 1H, JHH = 1.08 Hz, NCHN). 13C
NMR (DMSO): δ 118.44 (Cimi), 120.79 (2 × Caryl), 127.31 (Caryl),
130.30 (2 × Caryl), 130.36 (Cimi), 135.97 (NCN), 137.40 (Cq). Anal.
Found: C, 70.17; H, 5.95; N, 18.16. Calcd for C9H8N2·0.5H2O: C,
70.57; H, 5.92; N, 18.29.
4
4
Compound 7. This compound was prepared as described for 6,
from 2 (0.5 g, 2.9 mmol) and CH3I (0.62 g, 4.4 mmol) in CH3CN (20
mL). The crude product was recrystallized from acetone and ethyl
acetate, and the product was obtained as a white crystalline solid
1
(yield: 0.78 g, 86%). H NMR (CDCl3): δ 2.26 (s, 3H, CCH3), 2.39
3
Compound 2. This compound was prepared as described for 1,
from 2,4-dimethylaniline (1.21 g, 0.01 mol), 40% glyoxal (4.2 mL, 0.1
mol), CH3OH (7 mL), NH4Cl (1.07 g, 0.02 mol), 37% formaldehyde
(0.5 mL, 0.02 mol), and H3PO4 (0.15 mL, 85%). After purification the
product was obtained as a dark brownish red oil (yield: 0.82 g, 48%).
1H NMR (CDCl3): δ 2.13 (s, 3H, CH3), 2.37 (s, 3H, CH3), 7.03 (t,
(s, 3H, CCH3), 4.31 (s, 3H, NCH3), 7.16 (d, 1H, JHH = 8.04 Hz,
3
Haryl), 7.19 (s, 1H, Haryl), 7.31 (t, 1H, JHH = 1.81 Hz, Himi), 7.34 (d,
3
3
1H, JHH = 8 Hz, Haryl), 7.71 (t, 1H, JHH = 1.72 Hz, Himi), 10.09 (s,
1H, NCHN). 13C NMR (CDCl3): δ 17.91 (CCH3), 21.14 (CCH3),
37.67 (NCH3), 123.21 (Cimi), 123.98 (Cimi), 126.10 (Caryl), 128.37
(Caryl), 131.15 (Cq), 132.61 (Caryl). 132.83 (Cq), 137.76 (NCN),
141.59 (Cq). Anal. Found: C, 45.95; H, 4.90; N, 9.09. Calcd for
C10H16IN2: C, 45.73; H, 5.12; N, 8.89.
1H, 3JHH = 1.28 Hz, Himi), 7.06−7.11 (m, 2H, Haryl), 7.4 (t, 1H, 3JHH
=
3
1.52 Hz, Haryl), 7.20 (t, 1H,, JHH = 1.00 Hz, Himi), 7.60 (s, 1H,
NCHN). 13C NMR (CDCl3): δ 17.51 (CH3), 21.04 (CH3), 120.74
(Cimi), 126.36 (Caryl), 127.48 (Caryl), 128.83 (Cimi), 131.92 (Caryl),
133.58 (Cq), 134.05 (Cq), 137.50 (NCN), 138.96 (Cq). Anal. Found:
C, 72.46; H, 7.22; N, 15.33. Calcd for C11H12N2·0.6H2O: C, 72.18; H,
7.27; N, 15.3.
Compound 8. This compound was prepared as described for 6,
from 3 (0.5 g, 2.45 mmol) and CH3I (0.52 g, 3.68 mmol) in CH3CN
(20 mL). The crude product was recrystallized from acetone and ethyl
acetate, and the product was obtained as a brown crystalline solid
1
(yield: 0.65 g, 77%). H NMR (CDCl3): δ 3.86 (s, 3H, OCH3), 3.89
Compound 3. This compound was prepared as described for 1,
from 2,4-dimethoxyaniline (1.53 g, 0.01 mol), 40% glyoxal (4.2 mL,
0.1 mol), CH3OH (7 mL), NH4Cl (1.07 g, 0.02 mol), 37%
formaldehyde (0.5 mL, 0.02 mol), and H3PO4 (0.15 mL, 85%).
(s, 3H, OCH3), 4.27 (s, 3H, NCH3), 6.60−6.62 (m, 2H, Haryl), 7.44 (t,
3
3
1H, JHH = 1.8 Hz, Himi), 7.56 (d, 1H, JHH = 7.14 Hz, Haryl), 7.60 (t,
1H, 3JHH = 1.76 Hz, Himi), 9.97 (s, 1H, NCHN). 13C NMR (CDCl3): δ
37.61 (CH3), 56.00 (OCH3), 56.48 (OCH3), 100.03 (Caryl), 105.45
J
dx.doi.org/10.1021/om500076w | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
(Caryl), 116.52 (Cq), 123.17 (Cimi), 123.46 (Cimi), 126.90 (Caryl), 137.55
(NCN), 153.31 (Cq), 162.44 (Cq). Anal. Found: C, 41.72; H, 4.78; N,
8.14. Calcd for C12H16IN2O2: C, 41.52; H, 4.65; N, 8.07.
NMR (d6-DMSO): 36.79, 54.72, 55.65, 92.32, 116.21, 119.26, 119.39,
119.71, 119.84 (2× C), 121.21, 122.57, 123.13, 124.37, 124.73, 128.98,
129.26, 129.99, 130.53, 132.87, 135.20, 135.77, 143.99, 144.53, 147.48,
152.50, 152.78, 157.80, 160.46, 168.53, 169.65, 172.80, 173.03, 174.82.
Anal. Found: C, 56.08; H, 4.38; N, 7.08. Calcd for C34H29IrN4O2·
CH3OH: C, 56.06; H, 4.44; N, 7.47.
Compound 9. This compound was prepared as described for 6,
from 4 (0.45 g, 2.1 mmol) and CH3I (0.45 g, 3.18 mmol) in CH3CN
(20 mL). The crude product was recrystallized from acetone and
ether, and the product was obtained as a white crystalline solid (yield:
Complex 14. This complex was prepared as described for 11, from
9 (0.15 g, 0.42 mmol), Ag2O (0.149 g, 0.65 mmol), and [Ir(ppy)2Cl]2
(0.23 g. 0.22 mmol) in 1,2-dichloroethane (15 mL). The crude
product was recrystallized from in CH2Cl2 and CH3CN, and the
product was obtained as a yellow solid (yield: 0.267 g, 86%). 1H NMR
(d6-DMSO): δ 3.08 (s, 3H, CH3), 6.16 (d, 1H, J = 6.24 Hz), 6.43−
6.46 (m, 2H), 6.74−6.88 (m, 4H), 6.98−7.44 (m, 3H), 7.23 (d, 1H, J
= 2.20 Hz), 7.72−7.79 (m, 3H), 7.81−7.86 (m, 3H), 8.08 (dd, 2H, J =
7.84 Hz, J = 7.32 Hz), 8.50 (d, 1H, J = 2.16 Hz). 13C NMR (d6-
DMSO): δ 37.09, 117.63, 119.65, 119.80, 120.33, 120.44, 122.65,
123.07, 123.40, 123.62, 124.79, 124.90, 129.26, 129.82, 129.95, 130.29,
132.48, 136.00, 136.12, 136.36, 143.30, 143.84, 144.32, 152.33, 152.91,
165.01, 168.25, 169.37, 170.61, 171.74, 178.47. Anal. Found: C, 51.09;
H, 3.15; N, 7.34. Calcd for C32H23Cl2IrN4·1.5H2O: C, 50.99; H, 3.48;
N, 7.43.
Complex 15. This complex was prepared as described for 11, from
10 (0.1 g, 0.31 mmol), Ag2O (0.11 g, 467 mmol), and [Ir(ppy)2Cl]2
(0.166 g, 0.155 mmol) in 1,2-dichloroethane (15 mL). The crude
product was recrystallized in CH3CN, and the desired product was
obtained as a greenish yellow solid (yield: 0.08 g, 38%). 1H NMR (d6-
DMSO): δ 3.08 (s, 3H, CH3), 6.08 (dd, 1H, J = 5.24 Hz, J = 2.48),
6.21 (d, 1H, J = 6.88 Hz), 6.50 (d, 1H, J = 7.00 Hz), 6.66 (t, 1H, J =
10.42 Hz), 6.75−6.88 (m, 4H), 6.99 (t, 2H, J = 5.80 Hz), 7.21 (d, 1H,
J = 1.92 Hz), 7.70−7.78 (m, 3H), 7.81−7.87 (m, 4H), 8.07 (dd, 2H, J
= 9.52 Hz, J = 8.28 Hz). 13C NMR (d6-DMSO): δ 36.81, 118.60,
118.78, 118.93, 119.09, 119.54, 119.65, 120.26, 120.28, 122.97, 123.25,
123.30, 123.54, 124.65, 124.85, 129.19, 12.64, 130.39, 131.14, 132.59,
135.86, 136.27, 144.02, 144.45, 152.37, 152.97, 164.64, 168.31, 169.46,
170.31, 171.08, 176.32. Anal. Found: C, 55.43; H, 3.45; N, 8.27. Calcd
for C32H23F2IrN4: C, 55.40; H, 3.34; N, 8.08.
1
0.59 g, 79%). H NMR (CDCl3): δ 4.27 (s, 3H, NCH3), 7.45 (t, 1H,
3
3JHH = 1.84 Hz, Himi), 7.48 (d, 1H, JHH = 2.2 Hz, Haryl), 7.60 (s, 1H,
3
3
Haryl), 7.67 (t, 1H, JHH = 1.8 Hz, Himi), 7.96 (d, 1H, JHH = 6 Hz,
Haryl), 10.19 (s, 1H, NCHN). 13C NMR (CDCl3): δ 37.92 (CH3),
123.38 (Cimi), 124.00 (Cimi), 129.35 (Caryl), 129.61 (Caryl), 130.15 (Cq),
130.51 (Cq), 130.97 (Caryl), 138.19 (Cq), 138.38 (NCN). Anal. Found:
C, 33.88; H, 2.60; N, 8.09. Calcd for C10H10Cl2IN2: C, 33.74; H, 2.83;
N, 7.87.
Compound 10. This compound was prepared as described for 6,
from 5 (0.5 g, 2.77 mmol) and CH3I (0.59 g, 4.17 mmol) in CH3CN
(20 mL). The crude product was recrystallized from acetone and
ether, and the product was obtained as a purplish white crystalline
1
solid (yield: 0.73 g, 82%). H NMR (CDCl3): δ 4.27 (s, 3H, NCH3),
3
4
7.05−7.14 (m, 2H, Haryl), 7.56 (dd, 1H, JHH = 3.76 Hz, JHH = 1.88
Hz, Himi), 7.70 (t, 1H, 3JHH = 1.76 Hz, Himi), 8.10 (td, 1H, 3JHH = 8.46
Hz, 4JHH = 5.36 Hz, Haryl), 10.36 (s, 1H, NCHN). 13C NMR (CDCl3):
δ 37.83 (CH3), 105.75 (Cq), 106.01 (Caryl), 106.25 (Cq), 113.48 (Caryl),
113.71 (Cq), 122.80 (Cimi), 124.25 (Cimi), 128.16 (Caryl), 137.73
(NCN). Anal. Found: C, 37.40; H, 2.90; N, 8.79. Calcd for
C10H10F2IN2: C, 37.17; H, 3.12; N, 8.67.
Complex 11. A mixture of 6 (0.15 g, 0.52 mmol), Ag2O (0.182 g,
0.79 mmol), and [Ir(ppy)2Cl]2 (0.28 g, 0.26 mmol) in 1,2-
dichloroethane (15 mL) was heated in the dark at 100 °C under
N2. The hot reaction mixture was filtered through Celite, and the
solvent was removed under reduced pressure. The crude product was
recrystallized from CH3CN and CH2Cl2, and the product was obtained
as a yellow solid (yield: 0.30 g, 87%). 1H NMR (d6-DMSO): δ 3.06 (s,
3H, CH3), 6.24 (d, 1H, J = 6.2 Hz), 6.55−6.64 (m, 3H), 6.73−6.78
(m, 2H), 6.80−6.95 (m, 3H), 6.93 (dd, 2H, J = 6.1 Hz, J = 1.24 Hz),
7.16 (d, 1H, J = 1.9 Hz), 7.38 (d, 1H, J = 7.7 Hz), 7.64−7.74 (m, 3H),
7.80 (d, 1H, J = 7.5 Hz), 7.88 (d, 1H, J = 5.5 Hz), 7.97−8.00 (m, 3H),
8.05 (d, 1H, J = 8.1 Hz). 13C NMR (d6-DMSO) δ 36.59, 111.27,
116.34, 119.27, 119.35, 119.66, 119.88, 121.85, 122.49, 122.79, 123.13,
124.45, 124.73, 125.30, 129.01, 129.31, 130.53, 132.88, 135.14, 135.81,
138.54, 143.88, 144.64, 148.91, 152.59, 152.87, 156.15, 168.46, 169.77,
172.23, 172.66, 176.21. Anal. Found: C, 58.33; H, 3.62; N, 8.32. Calcd
for C32H25IrN4: C, 58.43; H, 3.83; N, 8.52.
ASSOCIATED CONTENT
■
S
* Supporting Information
Text, a table, figures, and CIF and .xyz files giving further
information on X-ray crystallography, X-ray crystallographic
data for 9, 10, 12, and 15, photoluminescence and electro-
chemistry data, optimized geometry plots of molecular orbitals
and plots of fragment contributions to frontier MOs and all
computed molecule Cartesian coordinates in a format for
convenient visualization. This material is available free of charge
via the Internet at http://pubs.acs.org.
Complex 12. This complex was prepared as described for 11, from
7 (0.15 g, 0.48 mmol), Ag2O (0.166 g, 0.72 mmol), and [Ir(ppy)2Cl]2
(0.256 g, 0.24 mmol) in 1,2-dichloroethane (15 mL). The crude
product was recrystallized from CH3CN, and the product was
1
obtained as a brownish orange solid (yield: 0.24 g, 73%). H NMR
(CDCl3, 500 MHz): δ 2.07 (s, 3H, CH3), 2.63 (s, 3H, CH3), 3.13 (s,
3H, CH3), 6.37 (d, 1H, J = 7.0 Hz), 6.52 (s, 1H), 6.63−6.73 (m, 4H),
6.79−6.89 (m, 4H), 7.46 (t, 1H, J = 7.5 Hz), 7.51 (t, 1H, J = 7.5 Hz),
7.65 (dd, 2H, J = 8 Hz, J = 4 Hz), 7.76−7.82 (m, 4H), 8.09 (d, 1H, J =
6 Hz). 13C NMR (CDCl3 - 500 MHz): δ 20.83, 21.98, 37.18, 118.19,
118.73, 118.89, 119.37, 119.43, 120.15, 120.18, 121.34, 121.61, 123.86,
123.98, 126.87, 129.00, 129.40, 130.40, 132.97, 133.65, 134.13, 134.23,
138.39, 143.18, 144.14, 145.37, 152.30, 153.19, 158.02, 169.15, 170.13,
173.36, 173.72, 178.36. Anal. Found: C, 58.72; H, 4.64; N, 8.07. Calcd
for C34H29IrN4·0.5H2O: C, 58.77; H, 4.35; N, 8.06.
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail for P.J.B.: p.barnard@latrobe.edu.au.
*E-mail for C.F.H.: c.hogan@latrobe.edu.au.
Author Contributions
†These authors contributed equally.
Complex 13. This complex was prepared as described for 11, from
8 (0.1 g, 0.29 mmol), Ag2O (0.1 g, 0.43 mmol), and [Ir(ppy)2Cl]2
(0.155 g, 0.15 mmol) in 1,2-dichloroethane (15 mL). The crude
product was recrystallized from hot CH3CN, and the product was
Notes
The authors declare no competing financial interest.
1
ACKNOWLEDGMENTS
obtained as a greenish yellow solid (yield: 0.088 g, 42%). H NMR
■
(d6-DMSO, 400 MHz): δ 3.04 (s, 3H, CH3), 3.42 (s, 3H, CH3), 3.85
(s, 3H, CH3), 5.76 (d, 1H, J = 2.5 Hz), 6.15 (d, 1H, J = 2.5 Hz), 6.21
(dd, 1H, J = 6.4 Hz, J = 1 Hz), 6.54 (dd, 1H, J = 6.3 Hz, J = 0.9 Hz),
6.73−6.77 (m, 2H), 6.78−6.83 (m, 2H), 6.93−6.97 (m, 2H), 7.05 (d,
1H, J = 2.0 Hz), 7.65−7.32 (m, 3H), 7.81 (d, 2H, J = 6.5 Hz), 8.00 (d,
2H, J = 7.3 Hz), 8.05 (d, 1H, J = 8.4 Hz), 8.11 (d, 1H, J = 2.0 Hz). 13C
This work was funded by the Australian Research Council
(LE120100213 and DP1094179). We thank the Victorian
Partnership for Advanced Computing (VPAC), the National
Computational Infrastructure National Facility (NCI-NF), and
La Trobe University for computing resources.
K
dx.doi.org/10.1021/om500076w | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Spiccia, L. Inorg. Chem. 2012, 51, 3302−3315. (h) Joshi, T.; Barbante,
G. J.; Francis, P. S.; Hogan, C. F.; Bond, A. M.; Spiccia, L. Inorg. Chem.
2011, 50, 12172−12183.
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