Using substituted cyclometalated quinoxaline ligands to finely tune the luminescence properties of iridium(III) complexes

Article abstract of DOI:10.1021/ic301853t

The syntheses of five new heteroleptic iridium complexes [Ir(L ^1-4)₂(Diobpy)]PF₆ (where Diobpy = 4,4′-dioctylamido-2,2′-bipyridine) and [Ir(L³) ₂(bpy)]PF₆ (where L = para-substituted 2,3-diphenylquinoxaline cyclometalating ligands; bpy = 2,2′-bipyridine) are described. The structures of [Ir(L³)₂(Diobpy)]PF ₆ and [Ir(L³)₂(bpy)]PF₆ show that the complexes each adopt a distorted octahedral geometry with the expected trans-N, cis-C arrangement of the cyclometalated ligands. Electrochemical studies confirmed subtle perturbation of the Ir^III/IV redox couple as a function of ligand variation. Luminescence studies showed the significant contribution of ³MLCT to the phosphorescent character with predictable and modestly tunable emission wavelengths between 618 and 636 nm. DFT studies provided approximate qualitative descriptions of the HOMO {located over the Ir(5d) center (11-42%) and the phenylquinoxaline ligand (54-87%)} and LUMO {located over the ancillary bipyridine ligands (ca. 93%)} energy levels of the five complexes, confirming significant MLCT character. TD-DFT calculations indicate that UV-vis absorption and subsequent emission has substantial MLCT character, mixed with LLCT. Predicted absorption and emission wavelengths are in good general agreement with the UV-vis and luminescence experiments.

Full text of DOI:10.1021/ic301853t

Article
pubs.acs.org/IC
Using Substituted Cyclometalated Quinoxaline Ligands To Finely
Tune the Luminescence Properties of Iridium(III) Complexes
Emily E. Langdon-Jones, Andrew J. Hallett, Jack D. Routledge, David A. Crole, Benjamin D. Ward,
James A. Platts, and Simon J. A. Pope*
School of Chemistry, Main Building, Cardiff University, Park Place, Cardiff CF10 3AT, United Kingdom
S
* Supporting Information
ABSTRACT: The syntheses of five new heteroleptic iridium complexes [Ir-
(L1−4)2(Diobpy)]PF6 (where Diobpy = 4,4′-dioctylamido-2,2′-bipyridine) and
[Ir(L3)2(bpy)]PF6 (where L = para-substituted 2,3-diphenylquinoxaline cyclo-
metalating ligands; bpy = 2,2′-bipyridine) are described. The structures of
[Ir(L3)2(Diobpy)]PF6 and [Ir(L3)2(bpy)]PF6 show that the complexes each adopt
a distorted octahedral geometry with the expected trans-N, cis-C arrangement of the
cyclometalated ligands. Electrochemical studies confirmed subtle perturbation of the
IrIII/IV redox couple as a function of ligand variation. Luminescence studies showed the
3
significant contribution of MLCT to the phosphorescent character with predictable
and modestly tunable emission wavelengths between 618 and 636 nm. DFT studies
provided approximate qualitative descriptions of the HOMO {located over the Ir(5d)
center (11−42%) and the phenylquinoxaline ligand (54−87%)} and LUMO {located
over the ancillary bipyridine ligands (ca. 93%)} energy levels of the five complexes,
confirming significant MLCT character. TD-DFT calculations indicate that UV−vis absorption and subsequent emission has
substantial MLCT character, mixed with LLCT. Predicted absorption and emission wavelengths are in good general agreement
with the UV−vis and luminescence experiments.
aromatic ring (to give phenyl quinoxaline) will further decrease
the band gap and red-shift the π−π* and MLCT emission
(Figure 1). To exploit this bathochromic shift of emission,
INTRODUCTION
■
3
3
Iridium(III) cyclometalated complexes have attracted increas-
ing attention over the past 10 years due to their use in a range
of photonic applications, such as phosphorescent organic light
emitting diodes (OLEDs).1,2 Typically, strong heavy metal
mediated spin−orbit coupling results in efficient intersystem
crossing from singlet to triplet excited states, resulting in high
phosphorescent efficiencies. In reality, the effective commerci-
alization of OLEDs requires emitters that address the breadth
of the visible light spectrum from blue through green to red
light.3 Whereas green emitters, such as [Ir(ppy)3], have been
well developed,4 the lower band gap inherent to red emitters
often results in lower luminescence quantum yields.5 While the
majority of reports of cationic Ir(III) complexes have been
concerned with electroluminescent devices,4,5 due to mobile
counterions facilitating charge transport across films, there are
several reports of their uses as dopants in OLEDs.6
Figure 1. Structures of phenylpyridine, phenylquinoline, phenyl-
pyrazine, and phenylquinoxaline based complexes showing increasing
red-shifted emission.
Most of the early examples of iridium(III) cyclometalated
complexes were based on the phenylpyridine orthometalating
ligand,7 although there have been more recent reports of
myriad variations in the cyclometalating ligand.3b,4d,8 Impor-
tantly, the replacement of one CH group in the pyridyl
fragment of phenylpyridine by an electronegative nitrogen atom
(giving the phenyl pyrazine analogue) has been reported to
significantly decrease the LUMO energy level while the
HOMO (located over the phenyl group) remains energetically
unaltered, in particular in neutral acac complexes such as
[Ir(ppy)2(acac)].3b Increasing the π-conjugation by adding an
there have been several reports of 2,3-disubstituted quinoxaline
derivatives as cyclometalating ligands with various ancillary
ligands.9 In addition, we have previously reported the synthesis
and optical properties of amido-substituted phenylquinoline
complexes as cyclometalating ligands10 and substituted
hydroxyquinoxalinato complexes as ancillary ligands.11 It is
noteworthy that many applications of solid-state devices require
Received: August 24, 2012
© XXXX American Chemical Society
A
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Scheme 1. Synthetic Route to Ligands LHn, Ir(III) Dimers [(Ln)2Ir(μ-Cl)2Ir(Ln)2], and Complexes [Ir(L3)2(bpy)]PF6 and
[Ir(Ln)2(Diobpy)]PF6 (n = 1−4)
the formation of good quality thin films from the light-emitting
component. This is often achieved via vapor deposition or from
solutions with the use of alkyl chains on the periphery of the
ligand architectures, since these prevent aggregation of
complexes.
acetic acid (Scheme 1). The characterization data were
consistent with the published results for these compounds.19
The ancillary 4,4′-dioctylamido-2,2′-bipyridine (Diobpy) ligand
was prepared as previously reported.12a The precursor iridium
chlorobridged dimers [(Ln)2Ir(μ-Cl)2Ir(Ln)2] were synthesized
according to established literature conditions for bridged-
chloride dimers of this type and used without further
purification.20 Each of the [(Ln)2Ir(μ-Cl)2Ir(Ln)2] dimeric
compounds readily reacted with 4,4′-dioctylamido-2,2′-bipyr-
idine (Diobpy) in 2-methoxyethanol; workup and counteranion
exchange yielded the crude monometallic complexes as
[Ir(Ln)2(Diobpy)]PF6 (Scheme 1). For comparison, the
analogous 2,2′-bipyridine complex [Ir(L3)2(bpy)]PF6 was
similarly prepared. Further purification of each complex was
achieved using column chromatography (silica). After elution of
unreacted starting materials with CH2Cl2, the product was
eluted as the first band (appearing red) with CH2Cl2/MeOH
(9:1) giving the complexes as red powders, in good yields of
>77%. The resultant complexes are soluble in a range of
common organic solvents and were characterized in the
Following consideration of a number of these design criteria,
a class of iridium(III) complexes have been developed,
incorporating a range of cyclometalated 2,3-disubstituted
quinoxaline ligands, wherein the electronic nature of the
aromatic substituent is varied. The ancillary 2,2′-bipyridine
ligand has also been functionalized with alkyl amide groups in
the 4,4′-positions (which has also been shown to red-shift the
absorption of, in particular, ruthenium(II) complexes, via
12
1
attenuation of MLCT transitions), which should aid with
solubility and be well suited to film formation and future
applicability. This Article presents the syntheses and structural
and spectroscopic, including photophysical, characterization of
the complexes, together with detailed theoretical studies
revealing a class of emissive complexes with tunable electronic
properties.
1
solution state using H NMR, 13C{1H} NMR, IR, UV−vis,
RESULTS AND DISCUSSION
■
and luminescence spectroscopies, and cyclic voltammetry.
Additional analysis was provided through X-ray crystallographic
studies, ES mass spectrometry (including HR), which revealed
the parent cations [M − PF6]+ for each complex, and elemental
analysis.
X-ray Crystallography. Single crystals suitable for X-ray
diffraction studies were isolated using vapor diffusion of Et2O
into CHCl3 or MeCN solutions of the complexes [Ir-
(L3)2(Diobpy)]PF6 and [Ir(L3)2(bpy)]PF6 over a period of
48 h at room temperature. The parameters associated with the
data collection are presented in the Supporting Information
Synthesis and Characterization of the Ligands and
Complexes. It is noteworthy that many of the literature
reports on functionalized quinoxaline compounds describe
biological activities behaving as anticancer,13 antiviral,14
antibacterial,15 and activity as kinase inhibitors.16 In addition
to medicinal applications, quinoxaline derivatives have also
found applications as dyes17 and as components of light
emitting diodes (LEDs).18 For our purposes, the cyclo-
metalating quinoxaline, ligands LHn (n = 1−4), were prepared
by a simple condensation reaction of 1,2-phenylenediamine
with the appropriate diketone in hot ethanol with catalytic
B
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−
Figure 2. (a) Ortep representation of [Ir(L3)2(bpy)]+ (50% probability ellipsoids, solvent molecules, PF6 anion, and hydrogen atoms have been
omitted for clarity) and (b) representation showing nonbonding contact interactions between chelating ligands.
(Table S1) with selected bond lengths and angles (principally
involving the coordination sphere) shown in Table S2.
respect to the phenazine fragment of the quinoxaline moiety,
with the largest distortions on the cyclometalating tolyl groups.
Density Functional Theory (DFT) Studies. DFT
calculations (computed using the B3PW91 hybrid functional)
were performed to assess the frontier orbitals and provide
qualitative descriptions of the HOMO (highest occupied
molecular orbital) and LUMO (lowest unoccupied molecular
orbital) energy levels.
For the phenyl-substituted quinoxaline complexes described
here the energy levels of both the HOMO and LUMO are
sufficiently different (ΔE > 0.2 eV) from the other MOs to be
considered independent. In each case the HOMO of
[Ir(Ln)2(Diobpy)]+ is located on the metal 5d(Ir) center and
the cyclometalated phenylquinoxaline (Figure 3). For [Ir-
The structures obtained for the two complexes [Ir-
(L3)2(Diobpy)]PF6 (Figure S1, SI) and [Ir(L3)2(bpy)]PF6
(Figure 2a) confirmed the suggested formulations from
solution state spectroscopic analyses and, even given the
disorder along the alkyl chains of [Ir(L3)2(Diobpy)]PF6, show
that the Ir(III) ion in both complexes is octahedrally
coordinated by the three chelating ligands (Tables S1 and S2,
SI). The diimine always coordinates trans to the cyclometalated
rings, therefore retaining the cis-C,C and trans-N,N chelating
disposition of the original chloro-bridged dimer, as reported in
related examples.21 Bond lengths and angles for both complexes
are consistent with the only structurally characterized Ir
quinoxaline diimine complex [Ir(dpbq)2(bpy)]+ (Hdpbq
=2,3-diphenyl benzo[g]quinoxaline) {Ir−N1 2.062(6) Å; Ir−
N3 2.076(5) Å; Ir−C1 1.992(7) Å; N1−Ir−N3 173.7(2)°}9n as
well as related neutral quinoxaline complexes.3b,9d,u
For both complexes, the uncoordinated and cyclometalated
tolyl groups are not coplanar with [Ir(L3)2(bpy)]PF6 showing
twisting angles of 59.2° and 72.6° for each L3, respectively. In
addition, as with [Ir(dpbq)2(bpy)]+,9n there is a distortion
within the quinoxaline moiety caused by steric interactions
between the chelating ligands (Figure 2b). Most of the
interligand C···C and C···N nonbonding contact distances
(Table S2) are shorter than 3.4 Å (i.e., the sum of the van der
Waals radii of the atoms). This results in the tolyl groups
showing deformation angles of 10.10−14.71° with respect to
the phenazine fragment of the quinoxaline moiety.
The bond lengths and angles of both [Ir(L3)2(Diobpy)]PF6
and [Ir(L3)2(bpy)]PF6 were compared with the optimized
values calculated from density functional theory studies (see
DFT Studies section and Table S2, SI). In general, a reasonable
agreement is obtained between the theoretical and exper-
imentally observed bond lengths, although some small
differences are found. The calculated Ir−Nbipyridine bond lengths
are, respectively, 0.038 and 0.054 Å longer than the
experimental values for [Ir(L3)2(bpy)]PF6 of Ir−N(5) (2.186
Å) and Ir−N(6) (2.169 Å). In the case of the cyclometalated
bonds, the calculated value is 1.997 Å and comparable to Ir−
C(39) 1.999(5) Å for [Ir(L3)2(bpy)]PF6, but 0.010 Å longer
than the value obtained by X-ray crystallography for Ir−C(17);
the Ir−Nquinoxline bonds are similar, where the calculated values
are 0.023 and 0.016 Å longer. Again, there are C···C and C···N
interactions between the ligands, although the contact distances
are larger. This results in lower deformation angles compared to
experimental values for [Ir(L3)2(bpy)]+ of 8.88−13.09° with
Figure 3. Graphical representation of the frontier orbitals of
[Ir(Ln)2(Diobpy)]+ (n = 1−4 left to right): bottom, HOMO; top,
LUMO.
(L1)2(Diobpy)]+ and [Ir(L3)2(Diobpy)]+ there is little or no
contribution from the uncoordinated substituted phenyl
moieties to the HOMO coverage. However, for [Ir-
(L2)2(Diobpy)]+ and [Ir(L4)2(Diobpy)]+ there is contribution
with one cyclometalated ligand (Ln 1) showing greater coverage
that the second ligand (Ln 2) (Figure 3, Table 1).
The nature of the pendant R-group imparts an influence on
the overall energy level of the HOMO. This is unsurprising,
given that one of the R-groups is bound directly to a
cyclometalating phenyl moiety on each ligand Ln. Of the
complexes [Ir(Ln)2(Diobpy)]+ (n = 1−4), the electron
withdrawing para-Br ligands of [Ir(L2)2(Diobpy)]+ induced
the lowest HOMO energy level (E = −8.19 eV), whereas the
C
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Table 1. Percentage Distribution of HOMO and LUMO in [Ir(Ln)2(Diobpy)]+ (n = 1−4) and [Ir(L3)2(bpy)]+
[Ir(L1)2(Diobpy)]+
[Ir(L2)2(Diobpy)]+
[Ir(L3)2(Diobpy)]+
[Ir(L4)2(Diobpy)]+
[Ir(L3)2(bpy)]+
HOMO
LUMO
HOMO
LUMO
HOMO
LUMO
HOMO
LUMO
HOMO
LUMO
Ir
42.4
3.3
3.3
28.8
2.2
3.3
38.4
3.0
3.4
11.4
1.0
3.5
37.6
2.9
2.8
bpy
Ln 1
Ln 2
93.2
1.7
93.3
1.7
93.2
1.7
93.3
1.6
93.9
1.7
26.5
27.9
37.9
31.1
30.6
28.1
61.7
25.9
27.9
31.6
1.8
1.7
1.7
1.6
1.6
Table 2. Absorption and Electrochemical Properties of [Ir(Ln)2(Diobpy)]PF6 (n = 1−4) and [Ir(L3)2(bpy)]PF6
a
b
c
d
e
complex
λmax/nm (ε/M−1 cm−1
)
Eox/V
HOMO/eV
Ebandgap/eV
LUMO/eV
[Ir(L1)2(Diobpy)]PF6
[Ir(L2)2(Diobpy)]PF6
[Ir(L3)2(Diobpy)]PF6
[Ir(L4)2(Diobpy)]PF6
[Ir(L3)2(bpy)]PF6
294 (60 400), 386 (28 600), 447 (10 000)
293 (41 600), 386 (17 200), 447 (6000)
2.00
2.02
1.99
1.97
1.52
−6.35
−6.37
−6.34
−6.32
−5.87
2.12
2.10
2.12
2.10
2.09
−4.23
−4.27
−4.22
−4.22
−3.78
296 (40 400), 388 (21 600), 466 (5900)
305 (44 500), 398 (19 300), 415 (19 100), 458 (11 300)
298 (39 800), 388 (19 600), 471 (4900)
a
b
Absorption spectra measured in MeCN solutions (5 × 10−5 mol dm−3). Oxidation potentials measured in CH2Cl2 solutions at 200 mV s−1 with
c
0.1 M [NBu4][PF6] as supporting electrolyte calibrated with Fc/Fc+. The HOMO energy level was calculated using the equation −EHOMO (eV) =
d
e
+
Eox − EFc/Fc + 4.8. Ebandgap was determined from the absorption edge of the iridium complexes. The LUMO energy level was calculated using the
equation ELUMO (eV) = EHOMO + Ebandgap
.
complex of the electron donating para-OMe groups has the
highest (E = −7.51 eV). The LUMO energies also reflect the
variation of pendant R-group (para-Br ELUMO = −5.28 eV;
para-OMe ELUMO = −4.84 eV). This calculated data suggests
that variation of the para-substituent can lead to a degree of
tunable optical properties within this series of complexes.
Unlike our previously reported substituted phenylquinoline10
and quinoxalinato11 Ir(III) complexes, the calculated HOMO−
LUMO band gaps do show a significant variance (2.67−2.91
eV). The LUMOs for all complexes are almost entirely
delocalized over the 2,2′-bipyridine ligand. Population analyses
revealed that the distribution of the frontier orbitals over the
various ligands and metal varies substantially for the HOMOs
of [Ir(Ln)2(Diobpy)]+ (n = 1−4) (HOMO: 11.4−42.4% Ir;
1.0−3.3% bpy; 26.5−61.7% Ln 1; 25.9−31.1% Ln 2) but for the
LUMOs is essentially identical (LUMO: 3.3−3.5% Ir; 93.2−
93.3% bpy; 1.6−1.7% Ln 1; 1.6−1.8% Ln 2) (Table 1).
influence on the electron density at iridium. The EHOMO values
were determined using the relevant equations,23 and the
resultant values (Table 2) fall in the narrow range −6.37 to
−6.32 eV. The electronic nature of the bipyridine ligand has a
larger effect on the oxidation potential: removal of the electron
withdrawing alkylamido groups in the 4,4′-positions of
[Ir(L3)2(Diobpy)]PF6 to give [Ir(L3)2(bpy)]PF6 results in a
lowering of the Ir3+/4+ oxidation potential by +0.47 to +1.52 V,
and a resultant EHOMO value of −5.87 eV. This is contradictory
to DFT calculations which essentailly show no difference
between the location and energy levels for the HOMO of both
complexes. Each complex also showed one or two partially
reversible or irreversible reduction waves, assigned to ligand-
centered processes involving both the bipyridine and quinoxa-
line ligands.
Electronic Properties of the Complexes. The cationic
complexes each absorb in the UV−vis regions (Table 2, Figure
4) with variations in the positioning of the lowest energy
shoulder absorption. Ligand-centered transitions clearly dom-
inate at shorter wavelengths while the presence of charge
transfer (CT) bands accounts for the visible absorption
characteristics with large absorption coefficients (∼104 M−1
cm−1). With reference to the DFT calculations, it is clear that
both metal-to-ligand CT (MLCT) and ligand-to-ligand CT
(LLCT) transitions can contribute to the low-energy profiles of
the absorption spectra, and it should be noted that iridium-
mediated spin−orbit coupling can faciliate some
3MLCT/3LLCT character as well. Since the HOMO possesses
significant coverage of the substituted quinoxaline ligands it is
likely that the nature of the substituent influences the
absorption characteristics, albeit subtly.
The bpy complex [Ir(L3)2(bpy)]+ (EHOMO −7.76 eV, ELUMO
−4.92 eV) showed no significant difference from the Diobpy
analogue. The HOMO is located over the iridium center
(37.6%) and the cyclometalating ligands (27.9 and 31.6%) with
little or no contribution from the uncoordinated phenyl ring
(Figure S2). The LUMO is again almost entirely delocalized
over the 2,2′-bipyridine ligand (93.9%).
Electrochemical Studies. The electrochemical character-
istics of the four [Ir(Ln)2(Diobpy)]PF6 (n = 1−4) complexes
and [Ir(L3)2(bpy)]PF6 were studied in deoxygenated CH2Cl2.
The HOMO energy levels (EHOMO) can be determined from
the ionization potential of the first oxidation (Ir3+/4+) by direct
correlation with the redox couple of FeCp20/1+. The cyclic
voltammograms, measured at a platinum disc electrode (scan
rate υ = 200 mV s−1, 1 × 10−3 M solutions, 0.1 M [NBu4][PF6]
as a supporting electrolyte), each showed one nonfully
reversible oxidation (Table 2), over a range very close to the
edge of the solvent window (+1.97 to +2.02 V), for the 4,4′-
dioctylamido-2,2′-bipyridine complexes. The extent of the
irreversibility can be ascribed to the varying contribution of
the cyclometalating ligands to the electron density of the
HOMO,22 which in this case is calculated to be 54−87%. The
subtle differences in potential are probably due to the para-
phenyl substituent on the cyclometalating ligand having a small
Nonrelativistic TD-DFT calculations (SI) in simulated
MeCN on [Ir(L3)2(bpy)]+ support the assignment of these
bands as having substantial MLCT and LLCT character. The
lowest energy absorption with significant intensity consists of
excitation from the HOMO (metal + quinoxaline) into π*
orbitals on both bpy and quinoxaline. This is predicted to lie at
501 nm (oscillator strength = 0.05 au), and so coincides
reasonably closely with the lowest energy peak seen in Figure 4.
A set of stronger bands centered around 410 nm (424 nm, 0.12
au; 408 nm, 0.10 au; 397 nm, 0.15 au) are also predicted, again
D
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Table 4. Photophysical Properties of [Ir(Ln)2(Diobpy)]PF6
(n = 1−4) and [Ir(L3)2(bpy)]PF6
τ/
kr/s−1
×
knr/s−1
×
a,
b
a
a
complex
λem/nm
ns
Φem
104
106
[Ir(L1)2(Diobpy)]
628 (686)
618 (699)
631 (677)
636 (699)
633 (688)
359
420
317
298
288
0.024
0.018
0.027
0.022
0.021
6.7
4.3
8.5
7.4
7.3
2.7
2.3
3.1
3.3
3.4
PF6
[Ir(L2)2(Diobpy)]
PF6
[Ir(L3)2(Diobpy)]
PF6
[Ir(L4)2(Diobpy)]
PF6
[Ir(L3)2(bpy)]PF6
a
b
Recorded in aerated MeCN (λex = 425 nm). Values in parentheses
are the emission maxima obtained on solid samples.
quinoxaline ligands, with electron donating groups (Me and
OMe; L3 and L4, respectively) shifting λem to lower energy. The
quinoxaline based complexes here show emission maxima ca.
3000 cm−1 lower energy than the analogous phenylpyridine
complex [Ir(ppy)2(bpy)]+.7b,c A graphical plot (Figure S4, SI)
of E versus knr revealed a good linear correlation, suggesting
that the series of 4,4′-dioctylamido-2,2′-bipyridine complexes
obey the energy gap law.
The expected sensitivity of the emission wavelengths to
different solvent polarities confirmed that the excited states are
characterized with significant CT character, wherein more polar
solvents bathochromically shift λem. The complexes also showed
a somewhat unusual solvent-dependent variance in emission
lifetime. From the related time-resolved deoxygenated measure-
ments it was possible to approximate the rate of quenching by
oxygen, kq(O2), for a given solvent. These calculated values
revealed that the rate of quenching in chloroform and toluene
(∼2 × 106 s−1) was an order of magnitude greater than in
acetonitrile or dimethyl sulfoxide (∼8 × 105 s−1). Typically,
luminescent complexes of the type [Ru(N∧N)3]2+ and
[Ir(ppy)2(N∧N)]+ possess a lower rate of quenching by oxygen
in lower polarity media.24 However, the quinoxaline-derived
complexes here do not adhere to that precedent.
Measurements obtained on solid films deposited from the
slow evaporation of solvent revealed broad, bathochromically
shifted emission maxima. In all cases, obtained lifetime decay
profiles were sensitive to the wavelength of detection and best
fitted to two exponential components (Table S4, SI). This
suggests that (at least) two emitting species coexist in the solid
state and contribute to the emission profile, presumably due to
packing influences, the differences in the local environment of
the luminophore, and therefore difference in quenching (cf.
comments regarding O2 sensitivity above) associated with those
environments. However, the dominant contribution in each
case was the retention of a long-lived species and is therefore
consistent with a phosphorescence and the observations in
solution. The luminescent lifetimes of these species were
generally longer than those in solution for the octylamido
appended complexes.
Figure 4. UV−vis spectra of [Ir(Ln)2(Diobpy)]PF6 (n = 1−4) and
[Ir(L3)2(bpy)]PF6 in MeCN solutions (5 × 10−5 M).
in good agreement with Figure 4. These bands consist of
varying combinations of metal and L3 π orbitals excited into L3
π* orbitals, but have no contribution from the bpy π* orbitals,
suggesting MLCT and IL contributions. The largest intensity
bands are shown in Table 3 (with the dominant excitation in
Table 3. Calculated Excitation Wavelengths for
[Ir(L3)2(bpy)]+ from Scalar DFT Studies Showing the
Dominant Contribution to Each Transition in Bold
a
λ (nm)
f
character
501
0.0491
HOMO → LUMO + 1
HOMO → LUMO + 2
HOMO − 3 → LUMO
HOMO − 1 → LUMO
HOMO − 4 → LUMO
HOMO − 2 → LUMO
HOMO − 1 → LUMO + 1
HOMO − 3 → LUMO
HOMO − 2 → LUMO + 1
HOMO − 1 → LUMO
424
408
0.1218
0.1015
397
0.1487
a
Excitation wavelength.
bold), and a full breakdown of all orbitals involved, with a
predicted absorption spectrum, is shown in the Supporting
Information (Table S3 and Figure S3).
The complexes are visibly luminescent in a variety of solvents
(Tables 4 and 5) with corresponding lifetime values (τ) in the
range 288−420 ns, and modest quantum yields (Φ) which are
in line with related species. Comparison of the excitation
spectra for the complexes [Ir(L3)2(Diobpy)]PF6 and [Ir-
(L3)2(bpy)]PF6 (Figure 5) highlight the differences in the
visible part (400−550 nm) of the spectral profile through
simple substitution of the bipyridine ligand. The variation of
emission wavelengths in MeCN reveals the subtle, but effective,
influence of the remote aryl substituents of the cyclometalated
Nonrelativistic TD-DFT calculations on [Ir(L3)2(bpy)]+ in
MeCN again support these experimental observations. The
lowest energy triplet state is found to lie 2.05 eV, or 604 nm
above the ground state, at the optimal geometry of the triplet
state, in good agreement with the experimental value (633 nm
= 1.96 eV). This transition is dominated by the HOMO and
LUMO (76%), and hence, we can surmise that the emission of
E
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Table 5. Photophysical Properties of [Ir(Ln)2(Diobpy)]PF6 (n = 1−4) and [Ir(L3)2(bpy)]PF6 in Various Air-Equilibrated
Solvents
λem/nm (τ/ns)
complex
toluene
CHCl3
MeCN
DMSO
[Ir(L1)2(Diobpy)]PF6
[Ir(L2)2(Diobpy)]PF6
[Ir(L3)2(Diobpy)]PF6
[Ir(L4)2(Diobpy)]PF6
[Ir(L3)2(bpy)]PF6
619 (454)
612 (435)
622 (389)
628 (424)
632 (336)
622 (511)
615 (591)
628 (459)
634 (504)
632 (432)
628 (359)
618 (420)
631 (317)
636 (298)
633 (288)
634 (939)
622 (992)
639 (744)
642 (788)
640 (636)
out at the EPSRC National Mass Spectrometry Service at Swansea
University. UV−vis studies were performed on a Jasco V-570
spectrophotometer in MeCN solutions (5 × 10−5 M). IR spectra
were recorded on a Thermo Scientific Nicolet iS5 spectrometer fitted
with an iD3 ATR attachment. Photophysical data were obtained on a
JobinYvon−Horiba Fluorolog spectrometer fitted with a JY TBX
picoseconds photodetection module. Emission spectra were un-
corrected, and excitation spectra were instrument corrected. The
pulsed source was a Nano-LED configured for 372 nm output
operating at 500 kHz. Luminescence lifetime profiles were obtained
using the JobinYvon−Horiba FluoroHub single photon counting
module and the data fits yielded the lifetime values using the provided
DAS6 deconvolution software. Quantum yield measurements were
obtained on aerated MeCN solutions of the complexes, using
[Ru(bpy)3](PF6)2 in aerated MeCN as a standard (Φem = 0.016).25
Electrochemical studies were carried out using a Parstat 2273
potentiostat in conjunction with a three-electrode cell. The auxiliary
electrode was a platinum wire and the working electrode a platinum
(1.0 mm diameter) disc. The reference was a silver wire separated
from the test solution by a fine porosity frit and an agar bridge
saturated with KCl. Solutions (10 mL CH2Cl2) were 1.0 × 10−3 mol
Figure 5. Excitation (−−−) and emission () spectra of
[Ir(Ln)2(Diobpy)]PF6 (n = 1−4) and [Ir(L3)2(bpy)]PF6 recorded
in MeCN solutions.
this complex (and its analogues in the series) is indeed of
dm−3 in the test compound and 0.1 mol dm−3 in [NBun ][PF6] as the
4
3
3MLCT and/or LLCT character.
supporting electrolyte. Under these conditions, E0′ for the one-
electron oxidation of [Fe(η-C5H5)2], added to the test solutions as an
internal calibrant, is 0.46 V.26 Unless specified, all electrochemical
values are at υ = 200 mV s−1. Microanalyses were performed by Medac
Ltd., U.K.
CONCLUSIONS
■
Aryl disubstituted quinoxaline-type chromophores have been
cyclometalated to iridium(III) forming mixed ligand species of
the general form [Ir(Ln)2(bpy)]PF6 where the ancillary
bipyridine can incorporate octyl chains, facilitating excellent
solubility properties upon the complexes. The complexes have
been comprehensively characterized through a variety of
experimental and theoretical studies. The substituents impart
a subtle modulation upon the electronic character of the
complexes, which generally possess excellent visible region
molar absorption coefficients. In the excited state this
perturbation manifests itself in varied emission wavelengths
Data Collection and Processing. Diffraction data for [Ir-
(L3)2(bpy)]PF6, and [Ir(L3)2(Diobpy)]PF6 were collected on a
Nonius KappaCCD using graphite-monochromated Mo Kα radiation
(λ = 0.710 73 Å) at 150 K. Software package Apex 2 (v2.1) was used
for the data integration, scaling, and absorption correction. CCDC
reference numbers 890952 and 890953 contain the 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.
Structure Analysis and Refinement. The structure was solved
by direct methods using SHELXS-97 and was completed by iterative
cycles of ΔF-syntheses and full-matrix least-squares refinement. All
non-H atoms were refined anisotropically, and difference Fourier
syntheses were employed in positioning idealized hydrogen atoms and
were allowed to ride on their parent C-atoms. All refinements were
against F2 and used SHELX-97.27
3
and lifetimes, which are best described by a significant MLCT
character; this luminescence was also sensitive to the nature of
the solvent. DFT calculations confirm that orbital energies can
be tuned by variation of substituent, and also support the
assignment of lowest energy absorption and emission spectra as
3
mixed MLCT/3LLCT character. Future work will investigate
DFT Studies. Nonrelativistic calculations were performed on the
Gaussian 03 program.28 Geometry optimizations were carried out
without constraints using the B3PW91 functional. The LANL2DZ29
basis set was used for the Ir centers, and was invoked with
pseudopotentials for the core electrons, a 6-31G(d,p)30 basis set for
all coordinating atoms with a 6-31G31 basis set for all remaining atoms.
All optimizations were followed by frequency calculations to ascertain
the nature of the stationary point (minimum or saddle point). TD-
DFT studies were performed using the functional, but with 6-31G(d)
on all nonmetal atoms, and also included a simulated MeCN
environment using the polarized continuum model (PCM)
approach.32 For prediction of absorption spectra, the geometry used
to calculate orbital and other properties was used without
modification. For prediction of emission, however, the triplet state
the potential application of these complexes in electro-
luminescent devices with a primary focus upon the quality of
film formation.
EXPERIMENTAL DETAILS
■
All reactions were performed with the use of vacuum line and Schlenk
techniques. Reagents were commercial grade and were used without
1
further purification. H and 13C{1H} NMR spectra were recorded on
an NMR-FT Bruker 400 MHz or Joel Eclipse 300 MHz spectrometer
1
and recorded in CDCl3. H and 13C{1H} NMR chemical shifts (δ)
were determined relative to internal tetramethylsilane, Si(CH3)4, and
are given in ppm. Low-resolution mass spectra were obtained by the
staff at Cardiff University. High-resolution mass spectra were carried
F
dx.doi.org/10.1021/ic301853t | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
was allowed to relax to its optimal geometry using unrestricted
B3PW91 in the gas phase, prior to solvated TD-DFT.
7.9 Hz), 7.11−6.96 (6H, m), 6.56 (2H, d, 3JHH = 8.4 Hz), 6.19 (2H, s),
3.44−3.28 (4H, m), 2.48 (6H, s), 1.49 (6H, s), 1.60−1.46 (4H, m),
1.31−1.09 (20H, m), 0.75 (6H, t, 3JHH = 7.0 Hz) ppm. 13C{1H} NMR
(75 MHz, CDCl3) δC = 163.3, 162.9, 155.9, 153.9, 152.4, 148.5, 145.9,
142.1, 141.3, 140.6, 140.4, 139.9, 136.6, 135.1, 131.7, 131.5, 130.5,
130.3, 130.1, 128.8, 127.5, 124.1, 123.2, 121.4, 41.0, 31.9, 29.3, 29.2,
29.0, 27.0, 22.7, 21.8, 21.7, 14.2 ppm. UV−vis (MeCN): λmax (ε/dm3
mol−1 cm−1) 296 (40 350), 388 (21 550), 466 (5900) nm. IR (ATR):
Synthesis. The cyclometalating ligands LHn (n = 1−4) were
prepared according to the literature procedures.33 4,4′-Dioctylamido-
2,2′-bipyridine (Diobpy) was prepared as previously reported.12a
General Procedure for the Synthesis of Ir(III) μ-Chloro-
bridged Dimers. Cyclometalated Ir(III) μ-chlorobridged dimers
[(C∧N)2Ir(μ-Cl)2Ir(C∧N)2] were synthesized according to the
Nonoyama route.20 IrCl3·xH2O (0.200 g, 0.68 mmol) and LHn (2.5
equiv) in 2-methoxyethanol (9 mL) and distilled water (3 mL) were
heated at 120 °C for 48 h. The mixture was allowed to cool, and the
product precipitated on addition of distilled water (30 mL). The red
solids were collected by filtration, washed with distilled water, and
dried in an oven. The Ir(III) dimers were used in subsequent reactions
without purification or characterization.
υ ( C O )
=
1 6 7 1 c m − 1
.
A n a l . C a l c d ( % ) f o r
C72H76N8O2IrPF6·0.5CH2Cl2: C, 59.44, H, 5.30, N, 7.65. Found: C,
59.50, H, 5.38, N, 7.49. ES MS found m/z 1277.6, calculated m/z
1277.6 for [M − PF6]+. HR MS found m/z 1275.5695, calculated m/z
1275.5692 for [C72H76N8O2191Ir]+.
[Ir(L4)2(Diobpy)]PF6. This compound was prepared similarly from
[(L4)2Ir(μ-Cl)2Ir(L4)2] (0.084 g, 0.046 mmol) and 4,4′-dioctylamido-
2,2′-bipyridine (0.045 g, 0.096 mmol). Yield 0.112 g (82%). 1H NMR
(400 MHz, CDCl3) δH = 8.60 (2H, d, 3JHH = 5.7 Hz), 8.55 (2H, br s),
8.11 (2H, d, 3JHH = 5.6 Hz), 7.87 (2H, d, 3JHH = 6.8 Hz), 7.68 (4H, d,
[Ir(L1)2(Diobpy)]PF6. [(L1)2Ir(μ-Cl)2Ir(L1)2] (0.085 g, 0.054
mmol) and 4,4′-dioctylamido-2,2′-bipyridine (0.053 g, 0.114 mmol)
were heated at 120 °C in 2-methoxyethanol (10 mL) for 16 h. The
solvent was then removed in vacuo and the crude product dissolved in
MeCN (4 mL). An excess of KPF6 (1.10 g, 5.976 mmol) in distilled
water (2 mL) was added and the solution stirred for 10 min. Distilled
water (30 mL) was then added and the product extracted with CH2Cl2
(2 × 20 mL). The combined organic phases were washed with water
(30 mL) and brine (30 mL) before being dried over MgSO4. The
solution was filtered and the solvent removed in vacuo. The product
was purified by column chromatography (silica, CH2Cl2) and was
eluted as the first red fraction with CH2Cl2/MeOH (9:1). The solvent
was lowered in volume (to ca. 3 mL) and the product precipitated by
the slow addition of Et2O (25 mL), filtered, and dried in vacuo. Yield =
3
3
3JHH = 8.2 Hz), 7.43 (2H, app. t, JHH = 7.2 Hz), 7.19 (2H, d, JHH
=
8.4 Hz), 7.11−6.98 (8H, m), 6.36 (2H, d, 3JHH = 8.6 Hz), 5.87 (2H, s),
3.88 (6H, s), 3.38 (6H, s), 3.36−3.24 (4H, m), 1.60−1.48 (4H, m),
1.28−1.04 (20H, m), 0.75 (6H, t, 3JHH = 6.8 Hz) ppm. 13C{1H} NMR
(75 MHz, CDCl3) δC = 162.9, 162.7, 161.3, 161.1, 156.0, 154.5, 153.3,
148.6, 145.9, 140.2, 139.7, 136.8, 133.7, 131.8, 131.4, 130.4, 129.9,
127.5, 123.0, 121.6, 119.9, 115.1, 114.9, 108.4, 55.7, 54.9, 41.0, 31.9,
29.3, 29.2, 29.1, 27.0, 22.7, 14.2 ppm. UV−vis (MeCN): λmax (ε/dm3
mol−1 cm−1) 305 (44 500), 398 (19 300), 415 (19 050), 458 (11 250)
nm. IR (ATR): υ(CO) = 1672 cm−1. Anal. Calcd (%) for
C72H76N8O6IrPF6·0.5CH2Cl2: C, 56.95, H, 5.08, N, 7.33. Found: C,
57.12, H, 5.25, N, 7.16. ES MS found m/z 1341.5, calculated m/z
1341.6 for [M − PF6]+. HR MS found m/z 1339.5461, calculated m/z
1339.5488 for [C72H76N8O6191Ir]+.
1
3
0.128 g, 87%. H NMR (400 MHz, CDCl3) δH = 8.61 (2H, d, JHH
=
5.9 Hz), 8.49 (2H, br s), 8.13 (2H, d, 3JHH = 5.8 Hz), 7.96 (2H, d, 3JHH
= 7.9 Hz), 7.75−7.71 (4H, br d), 7.60−7.55 (4H, m), 7.52 (2H, app. t
3
[Ir(L3)2(bpy)]PF6. This compound was prepared similarly from
[(L3)2Ir(μ-Cl)2Ir(L3)2] (0.085 g, 0.050 mmol) and 2,2′-bipyridine
{coincident dd}, JHH = 7.4 Hz), 7.45 (2H, br), 7.18−7.06 (6H, m),
6.72 (2H, app. t, 3JHH = 7.9 Hz), 6.66 (2H, app. t, 3JHH = 7.9 Hz), 6.40
3
1
(2H, d, JHH = 7.9 Hz), 3.47−3.38 (4H, m), 1.61−1.52 (4H, m),
(0.017 g, 0.108 mmol). Yield 0.091 g (79%). H NMR (400 MHz,
1.27−1.05 (20H, m), 0.77 (6H, t, 3JHH = 7.3 Hz) ppm. 13C{1H} NMR
(75 MHz, CDCl3) δC = 163.1, 162.8, 155.9, 154.0, 152.0, 148.3, 146.0,
143.9, 140.6, 140.0, 139.3, 134.6, 131.9, 131.8, 131.3, 130.9, 130.6,
130.4, 129.5, 128.9, 127.5, 123.4, 122.9, 121.7, 40.9, 31.9, 29.6, 29.2,
29.0, 27.0, 22.7, 14.1 ppm. UV−vis (MeCN): λmax (ε/dm3 mol−1
cm−1) 294 (60 350), 386 (28 550), 447 (9950) nm. IR (ATR): υ(CO)
= 1672 cm−1. Anal. Calcd (%) for C68H68N8O2IrPF6·0.5CH2Cl2: C,
58.39, H, 4.94, N, 7.95. Found: C, 58.37, H, 5.13, N, 7.89. ES MS
found m/z 1221.5, calculated m/z 1221.5 for [M − PF6]+. HR MS
found m/z 1219.5077, calculated m/z 1219.5066 for
[C68H68N8O2191Ir]+.
CDCl3) δH = 8.56 (2H, d, 3JHH = 5.5 Hz), 8.22 (2H, d, 3JHH = 8.1 Hz),
3
3
8.00 (2H, app. t, JHH = 7.0 Hz), 7.89 (2H, d, JHH = 8.3 Hz), 7.62
(6H, m), 7.43 (2H, app. t, 3JHH = 8.0 Hz), 7.36 (4H, d, 3JHH = 7.9 Hz),
3
3
7.17 (2H, d, JHH = 9.0 Hz), 7.09−6.94 (4H, m), 6.56 (2H, d, JHH
=
8.3 Hz), 6.21 (2H, s), 2.46 (6H, s), 1.87 (6H, s) ppm. 13C{1H} NMR
(75 MHz, CDCl3) δC = 163.5, 155.7, 153.9, 153.0, 147.9, 142.0, 141.4,
140.8, 140.6, 140.4, 140.2, 136.7, 135.5, 131.6, 131.2, 130.3, 130.2,
130.1, 128.8, 128.1, 125.1, 124.0, 123.7, 21.8, 21.7 ppm. UV−vis
(MeCN): λmax (ε/dm3 mol−1 cm−1) 298 (39 800), 388 (19 600), 471
(4900) nm. Anal. Calcd (%) for C54H42N6IrPF8: C, 58.32, H, 3.81, N,
7.55. Found: C, 58.08, H, 3.95, N, 7.41. ES MS found m/z 967.3,
calculated m/z 967.3 for [M − PF6]+. HR MS found m/z 965.3069,
calculated m/z 965.3071 for [C54H42N6191Ir]+.
[Ir(L2)2(Diobpy)]PF6. This compound was prepared similarly from
[(L2)2Ir(μ-Cl)2Ir(L2)2] (0.088 g, 0.040 mmol) and 4,4′-dioctylamido-
2,2′-bipyridine (0.040 g, 0.086 mmol). Yield 0.108 g (81%). 1H NMR
3
(400 MHz, CDCl3) δH = 8.50 (4H, d, JHH = 5.7 Hz), 8.12 (2H, d,
ASSOCIATED CONTENT
* Supporting Information
■
3
3
3JHH = 5.6 Hz), 7.96 (2H, d, JHH = 8.3 Hz), 7.72 (4H, d, JHH = 8.5
S
3
Hz), 7.67−7.52 (6H, m), 7.13 (2H, app. t, JHH = 7.2 Hz), 7.02 (4H,
app. t, 3JHH = 9.2 Hz), 6.96 (2H, d, 3JHH = 8.7 Hz), 6.51 (2H, s), 3.48−
3.35 (4H, m), 1.60−1.46 (4H, m), 1.29−1.01 (20H, m), 0.75 (6H, t,
3JHH = 7.2 Hz) ppm. 13C{1H} NMR (75 MHz, CDCl3) δC = 162.7,
162.1, 155.9, 152.8, 152.7, 148.4, 146.6, 142.7, 140.9, 139.8, 137.6,
136.7, 132.9, 132.8, 131.5, 130.9, 130.6, 130.5, 127.7, 127.3, 126.8,
125.3, 122.8, 122.2, 41.0, 31.9, 29.3, 29.2, 29.0, 27.0, 22.7, 14.2 ppm.
UV−vis (MeCN): λmax (ε/dm3 mol−1 cm−1) 293 (41 550), 386 (17
150), 447 (5950) nm. IR (ATR): υ(CO) = 1673 cm−1. Anal. Calcd
(%) for C68H64N8Br4O2IrPF6: C, 48.55, H, 3.84, N, 6.66. Found: C,
48.28, H, 3.95, N, 6.52. ES MS found m/z 1537.1, calculated m/z
1537.1 for [M − PF6]+. HR MS found m/z 1531.1450, calculated m/z
1531.1486 for [C68H64N8Br4O2191Ir]+.
Parameters associated with the single crystal diffraction data
collection, selected bond lengths and angles, ortep representa-
tion of [Ir(L3)2(Diobpy)]+, TD-DFT data, and luminescence
spectroscopy data. This material is available free of charge via
the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: popesj@cardiff.ac.uk. Phone: +44 (0) 29 20879316.
Fax: +44 (0) 29 20874030.
■
Notes
[Ir(L3)2(Diobpy)]PF6. This compound was prepared similarly from
[(L3)2Ir(μ-Cl)2Ir(L3)2] (0.080 g, 0.047 mmol) and 4,4′-dioctylamido-
2,2′-bipyridine (0.048 g, 0.103 mmol). Yield 0.104 g (77%). 1H NMR
(400 MHz, CDCl3) δH = 8.52 (2H, d, 3JHH = 5.7 Hz), 8.41 (2H, br s),
8.08 (2H, d, 3JHH = 5.8 Hz), 7.91 (2H, d, 3JHH = 7.8 Hz), 7.59 (4H, d,
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
Cardiff University and EPSRC are thanked for funding. Dr.
Robert Jenkins, Mr. Robin Hicks, and Mr. David Walker are
■
3
3
3JHH = 7.8 Hz), 7.48 (2H, app. t, JHH = 7.0 Hz), 7.33 (4H, d, JHH
=
G
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Article
Lee, T. K-M.; Lui, L.-H.; Tsang, K. H-K.; Zhu, N. Inorg. Chem. 2003,
42, 6886−6897. (h) Lau, J. S-Y.; Lee, P.-K.; Tsang, K. H-K.; Ng, C. H-
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708−718. (i) Baranoff, E.; Fantacci, S.; De Angelis, F.; Zhang, X.;
Scopelliti, R.; Graetzel, M.; Nazeeruddin, Md. K. Inorg. Chem. 2011,
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Cui, X.; Horton, P. N.; Coles, S. J.; Zhao, J.; Pope, S. J. A. Chem.
Commun. 2012, 48, 10838−10840. (l) Li, X.; Minaev, B.; Ågren, H.;
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thanked for running low resolution mass spectra. Dr. Benson
Kariuki is thanked for obtaining the raw diffraction data on
[Ir(L3)2(Diobpy)]PF6 and [Ir(L3)2(bpy)]PF6. The staff of the
EPSRC MS National Service at the University of Swansea are
also gratefully acknowledged.
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