Molecular Engineering of Iridium Blue Emitters Using Aryl N-Heterocyclic Carbene Ligands

Article abstract of DOI:10.1002/ejic.201600971

The synthesis of a new series of neutral bis[2-(2′,4′-difluorophen-2′-yl)pyridine][1-(2′-aryl)-3-methylimidazol-2-ylidene]iridium(III) complexes is reported. Each complex has been characterized by NMR spectroscopy, UV/Vis spectrophotometry, and cyclic vol

Full text of DOI:10.1002/ejic.201600971

A Journal of
Accepted Article
Title: Molecular Engineering of Iridium Blue Emitters Using Aryl N-Heterocyclic Car-
bene Ligands
Authors: Mohammad Khaja Nazeeruddin; Sadig Aghazada; Aron Joel Huckaba; An-
tonio Pertegas Ojeda; Azin Babaei; Giulia Grancini; Iwan Zimmermann; Henk
Bolink
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Object Identifier (DOI) given below. The VoR will be published online in Early View as
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201600971
Link to VoR: http://dx.doi.org/10.1002/ejic.201600971

European Journal of Inorganic Chemistry
10.1002/ejic.201600971
FULL PAPER
Molecular Engineering of Iridium Blue Emitters Using Aryl N-
Heterocyclic Carbene Ligands
Sadig Aghazada,[a] ‡ Aron J. Huckaba,[a]‡ Antonio Pertegas,[b] Azin Babaei,[b] Giulia Grancini,[b] Iwan
Zimmermann,[a] Henk Bolink,[b] Mohammad Khaja Nazeeruddin,*[a]
Abstract: The synthesis of a new series of neutral Iridium (III) bis(2-
(2’,4’-difluorophenylpyridine)(1-(2’-aryl)-3-methylimidazol-2-ylidene)
complexes is reported. Each complex has been characterized by
NMR, UV-Vis, cyclic voltammetry, and the photophysical properties
deeply explored. Furthermore, two of the complexes were
characterized by single crystal X-ray diffraction. By systematically
modifying the cyclometallating aryl group on the N-heterocyclic
carbene (NHC) ligand from 2,4-dimethoxyphenyl to 4-methoxy-2-
methyl-3-pyridine the energy levels of the Ir complexes were
modified to produce new blue emitters with increased HOMO and
triplet level energies. OLED devices fabricated with these emitters
showed external quantum efficiencies (EQE) in the range of 2.3-
3.2% with low turn-on values (2.7-2.9 V) and efficacies up to 6.3%..
PLQY.16,17 Charged complexes have also been shown to exhibit
high-energy emission upon excitation.18–21 As for cyclometallated
ligands in general, few homoleptic complexes have been
reported as “true” blue emitters, with the few cases being mainly
constrained to arylNHC or 2-arylimidazole complexes.16,17,22–24
Heteroleptic complexes of the form (dfppy)2Ir(L)25–27 (where
dfppy = 2-(2’,4’-difluorophenyl)pyridine and L = a bidentate
ligand) have been developed and explored as blue emitters, as
have
a range of fluorine free 2-(3’-pyridyl)pyridine-based
emitters.25,26,28,29 Blue emissive complexes of the structure
(dfppy)2Ir(L)2, where L = isocyanide have also been very recently
reported.30
The strong electron withdrawing nature of the dfppy ligand
stabilizes the highest occupied molecular orbital (HOMO) much
more than the lowest unoccupied molecular orbital (LUMO).31
Such energy level tuning widens the HOMO-LUMO gap. As was
thoroughly described by Zuo et al. very recently for the
(dfppy)2Ir(arylNHC) scaffold (and by others for other ligand
classes),31–33 the ancillary ligand also greatly impacts
photophysical properties.32 As a practical example, starting from
(ppy)2Ir(acac)2 and changing the ancillary ligand can result in
Introduction
Reducing the energy consumption of light fixtures and
display panels would significantly lower humanity’s carbon
footprint. Organic light emitting diodes (OLED) offer efficient
illumination in a cost-effective manner, the performance of which
has improved rapidly due to development of heavy atom
emitters. Third row transition metals especially exhibit strong
spin-orbit coupling (SOC), which allows fast intersystem
crossing from excited singlet to triplet state and theoretical
device efficiency near unity.1,2 Among them, iridium is an
excellent candidate for inclusion in OLEDs, due to its ability to
emit at room temperature and its strong SOC that allows for
efficient phosphorescence from its 3MLCT state instead of the S1
state.3,4,5,6 Furthermore, Ir(III) complexes normally exhibit long
-state lifetimes5,7 and high photoluminescence
either a red-shift (as in 2,2’-bipyridine34 or phenanthrene35) or
31,36
blue-shift (as in (CN)2
or bis(1-methylimidazol-3-yl-2-
idene)methane, Im2CH218, or 2-picolinic acid)37 of the observed
quantum yields (PLQY).5,8
Many Ir (III) emitters have been synthesized to date, but
far fewer complexes have been shown to emit in the blue or
violet region of the visible spectrum.5 Since the first report of an
isolable N-heterocyclic carbene (NHC) ligand, their use has
spread far and wide.9–12 Many cyclometallated Ir(III) complexes
have also been reported.13–15 Neutral [Ir(arylNHC)3] complexes
have been reported to exhibit high-energy emission with high
a:Group for Molecular Engineering of Functional Materials, Ecole Polytechnique
Federale de Lausanne Valais Wallis, Rue de l’Indutrie 17, 1950 Sion,
Valais, Switzerland E-mail: mdkhaja.nazeeruddin@epfl.ch
b: Instituto de Ciencia Molecular, Universidad de Valencia, c/Catedrático J.
Beltrán, 2, 46980 Paterna, Spain
‡: These authors contributed equally.
Supporting information for this article is given via a link at the end of the
document.
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European Journal of Inorganic Chemistry
10.1002/ejic.201600971
FULL PAPER
emission (Figure 1).
Figure 1. Illustration of ancillary ligand impact on emitter energy level position.
HOMO and LUMO energy level values were taken from the respective
references: Ir(ppy)2bpy,34 Ir(ppy)2acac,2 and Ir(ppy)2(Im2CH2),18 FIrPic.37 An
estimate of the E(T1) value was made by the following conversion E(T1) =
E(S+/S) – (1240/em onset). em
was estimated from the intercept of an
onset
emission curve tangent line with the baseline of the high energy side of the
emission curve. Values in NHE obtained by converting first to SCE (Fc = 0.40
V vs SCE in 0.1 M NBu4PF6 MeCN) and then to NHE (SCE = 0.24 V vs
NHE).43 All NHE values converted to vacuum scale (fermi level of NHE = -4.5
eV vs vacuum).38
Scheme 1. Synthetic route to reach Ir complexes Ir1-Ir4.
Crystal Structures of Ir2 and Ir3
Single crystals of Ir2 and Ir3 were obtained either by vapor
diffusion of hexanes into CH2Cl2 or concentration of a saturated
CDCl3 solution (See Figure 3, Supporting Information).
Structures are shown in Figure 3, and other relevant structural
data are listed in Tables S2, S3 of the Supporting Information.
Both complexes exhibit a distorted octahedral geometry around
the Ir center, with the pyridines trans to each other. All bonds
involving Ir are > 2Å in length and the Ir-C4 (NHC) bond length is
in the range 2.056-2.099 Å for both complexes. The Ir-NHC
bond distance (Ir-C4, Table S2) is similar to those described for
other Ir(III) NHC cyclometallated14,17,18,32,34,41 and non-
cyclometallated complexes.42 The Ir-NHC is shorter in both
cases than the Ir-Pyr bond, indicating a stronger bonding
interaction.
A recent study by Zuo et al. showed that by using
progressively more withdrawn arylNHC ancillary ligands the
emission energy could be increased to that of the sky blue FIrPic
emitter by selective stabilization of the HOMO energy level32 as
is also the case for FIrPic.27 Because of the success of Zuo et al.,
we sought to better understand the effect of NHC ancillary
ligands bearing cyclometallating moieites with electron donating
functionalities. In order to ascertain the influence of NHC ligand
-donor strength, the NHC ligands chosen were imidazole (Ir1-
Ir3) or 1,2,4-triazole (Ir4). In order to better understand how the
electron donating ability to both the Ir metal center and NHC
ligand effects emission energy, aryl wingtip groups of different
donating and accepting strengths were used as cyclometallating
ligands (Figure 2).6,17,18,32
I
NHCCH
+
N
Ir
CNHC
CAr
N
Ir
N
C
Ag2O
C
C
C
C
Cl
Cl
Ir
C
ArCH
N
N
N
OMe
OMe
OMe
MeO
OMe
C^N =
F
N
N
NHCC
N
MeO
MeO
H3C
=
ArC
Figure 3. Drawings of complexes Ir2 (left, measured at 100 K) and Ir3 (right
measured at 303 K) at 50% probability. Hydrogen atoms are omitted for clarity.
Selected bond distances and angles shown in Tables S2, S3 in the Supporting
Information.
N
N
N
N
N
N
N
N
N
F
dfppy
Ir1
Ir2
Ir3
Ir4
Optical and Electronic Properties
Figure 2. Structures of Neutral Iridium Complexes Used in this Study and the
General Reaction to Reach Them.
Once the complexes were synthesized, they were
characterized by cyclic voltammetry to better understand the
structure-function relationship (see Table 1, Figure 4a). Upon
obtaining oxidation and reduction potentials in 0.1M NBu4PF6
MeCN, we observed the Ered for each of the complexes was
nearly constant throughout the series and similar (+30 to -140
mV difference) to that observed for FIrPic. Instead of selectively
modifying one energy level over the other, as was observed for
the withdrawn arylNHC ligands,32 both HOMO and LUMO levels
were modified (Figure 4b). The complex with the highest ground
state energy, (most destabilized HOMO, 1.14 V vs NHE) E(S+/S)
value, was Ir1 due to the dimethoxybenzene cyclometallating
ligand and imidazole chelating group. The effect of -donation
on E(S+/S) level is evidenced by the comparison of energy level
values for Ir1 and Ir4.
Results and Discussion
Complex Synthesis
Each of the aryl NHC ligands was synthesized in two steps
by Cu catalyzed coupling of the appropriate commercially
available aryl bromide with imidazole or triazole,39,40 followed by
alkylation with methyl iodide (Scheme 1). The yields for the initial
couplings were lower in this series than for aryls without
methoxy substituents, likely due to reactant instability in the
reaction conditions.
One-pot metallation and subsequent
transmetallation was achieved through reaction of each ligand
with Ag2O and [(dfppy)2IrCl]2.16 Chromatographic separation
and/or crystallization yielded each of the four complexes as a
single isomer with the exception of Ir4.
Table 1. Electrochemical Properties of complexes Ir1-Ir4 and FIrPic.
Property
E(S+/S)b
Ir1
Ir2
Ir3
Ir4
FIrPic
1.56d
1.14
1.24
1.24
1.43
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European Journal of Inorganic Chemistry
10.1002/ejic.201600971
FULL PAPER
wavelength to 375nm (Figure 6a). The most red-shifted emission
was observed from Ir1 (max = 497 nm), while the most blue-
shifted emission was observed from Ir3 (max = 464 nm). Taking
the em onset (Figure 5, vide infra) as an estimate for the T1 energy
level, more conclusions can be drawn about the structure-
function relationship between the different ancillary ligands.
Changing the NHC from imidazole to triazole resulted in less
energy lost between E(S/S-) and T1 (300 mV for Ir4 and 390 mV
for Ir1). Surprisingly, while the HOMO and LUMO levels for Ir2
and Ir3 were identical, the emission energy was slightly different,
with Ir2 having a more stabilized E(T1) energy level.
E(S/S-)c
-2.0(irr)
-2.0 (irr)
3.24
-2.0 (irr)
3.24
-1.64 (irr)
3.07
-1.65d
3.21
Ered
3.14
a: Measured in a 0.1 M Bu4NPF6 in deoxygenated CH3CN solution with glassy
carbon working electrode, Pt wire reference and counter electrodes with
Ferrocene/Ferrocenium (Fc/Fc+) as an internal standard. E0(Fc+/Fc) was taken
as 0.64 V vs NHE and all values are converted and reported versus NHE.43 b:
E(S+/S) taken as E(ox) = 1/2(Epa + Epc). c: Estimated from the peak of the
irreversible reduction in reference to half wave potential of Fc+/Fc; d: Taken
from Ref. 37, originally reported in reference to (Fc/Fc+) in 0.1M NBu4PF6
MeCN, converted to NHE by converting first to SCE (Fc = 0.40 V vs SCE in
0.1 M NBu4PF6 MeCN) and then to NHE (SCE = 0.24 V vs NHE).43
The PL decay was registered at the PL maximum (see
Table 2) for each of the complexes and Figure 6a shows the
normalized PL spectra of the complexes synthesized along with
the Ir(ppy)3 as a reference. The PL dynamics are presented in
Figure 6b. The complexes exhibit a mono-exponential decay in
the sub-microsecond time window with similar time constant
(PL) of few hundreds of ns (see details in Table 2). From the
retrieved constants, PL, we could extract the radiative (krad) and
non-radiative (knrad) rates according to the following equation:
The less-donating triazole ligand contributed to
stabilization of both E(S+/S) and E(S/S-) values. Furthermore, Ir4
exhibited the most stabilized E(S+/S) value (1.43 V vs NHE).
Substituting benzene with 3-pyridyl (complexes Ir2 and Ir3)
translated to a 100mV stabilization of the E(S+/S) energy level
(1.24 V vs NHE) compared to Ir1, and substituting one methoxy
for methyl on the pyridyl ring did not affect either the E(S+/S) or
E(S/S-) energy level.
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
Ir1
Ir2
Ir3
Ir4
300
350
400
450
500
550
600
Wavelength, (nm)
Figure 5. Normalized absorption (solid line) and emission (dashed line)
Spectra for Complexes Ir1 (“1”, black), Ir2 (“2”, red), Ir3 (“3”, blue), Ir4 (“4”,
teal) measured in deoxygenated DCE.
Figure 4. a: Cyclic voltammograms of complexes Ir1 (“1”, black), Ir2 (“2”, red),
Ir3 (“3”, blue), Ir4 (“4”, teal) measured in 0.1 M NBu4PF6 CH3CN solution. b:
Energy level diagram of complexes Ir1-Ir4. E(S+/S) and E(S/S-) values
obtained from CV in 0.1 M NBu4PF6 CH3CN solution using Fc/Fc+ as internal
reference, Pt wire reference and counter electrodes, with glassy carbon disk
working electrode. An estimate of the E(T1) value was made by the following
conversion E(T1) = E(S+/S) – (1240/em onset). em
was estimated from the
onset
intercept of an emission curve tangent line with the baseline of the high energy
side of the emission curve. Values for FIrPic taken from Ref. 37, originally
reported in reference to (Fc/Fc+) in 0.1M NBu4PF6 MeCN, values in NHE
obtained by converting first to SCE (Fc = 0.40 V vs SCE in 0.1 M NBu4PF6
MeCN) and then to NHE (SCE = 0.24 V vs NHE).43 All NHE values converted
to vacuum scale (NHE = -4.5 eV vs vacuum).38
Figure 6 a. Photoluminescence spectra at 375 nm excitation for the samples
indicated in the legend. b. PL time decay at 490 nm wavelength for the series
as indicated in the legend of a. Solid lines represent the fits of the data (open
symbols) obtained using a monoexponential function of the form y=A exp(-x/t).
PL lifetimes obtained are listed in Table 2.
The photophysical behavior of the dissolved complexes
has been observed by steady state and time resolved
photoluminescence (PL) measurements in the sub-microsecond
time window. The steady state PL spectra have been recorded
in deoxygenated dichloroethane (DCE) by tuning the excitation
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European Journal of Inorganic Chemistry
10.1002/ejic.201600971
FULL PAPER
hole transport layer (HTL) or electron blocking layer (EBL). The
emissive layer (30 nm) (EML) was deposited from spin-coating
using a host-guest system, where either Ir2 (device 1) or Ir3
(device 2) are dopants into TcTa (hole transport material) and
OXD7 (electron transport material) cohost system. A thin layer of
TSPO1 (10 nm) was thermally evaporated on top of the EML in
order to efficiently confine excitons into the emissive layer,
whereas a BmPymPb (40 nm) was evaporated as electron
transport layer (ETM). Finally, Barium coated with silver was
used as cathode. The OLED energy diagram is described in the
Figure 7a. The energy levels reported were found in the
literature.47–50
PLQY

%

 krad

krad  knrad

The PLQY(%) have been obtained comparing the
measured PL spectra of each complex to the reference Ir(ppy)3
having an established PLQY = 89% in deoxygenated DCE (see
Supporting Information).44 Considering that the PL lifetime is
defined as the inverse of the sum of the rate constants of all the
deactivating
pathways
of
the
excited
states
as
 PL  1 krad  knrad


45
,
the rates were calculated and listed in
Table 3. While the PL constants were similar for each of the
complexes, Ir2 had the highest PLQY (0.66, Table 2) and Ir3
displayed a lower PLQY (0.31, PLQY). Similarly, the krad value
for Ir2 was the highest (and knrad the lowest), with Ir1 and Ir2
having similar values for both (Table 2). The PLQY for complex
Ir4 was much lower than the rest (0.01 vs >0.31 for the others),
likely because of quenching by isomeric impurities or by
nonradiative processes. The extremely low observed PLQY also
hindered PL measurement. Unfortunately, this hypothesis could
not be tested, as efforts to purify Ir4 were not successful. The
PLQY values observed are similar to those for other
(dfppy)2Ir(ArNHC) complexes, however, the excited-state
lifetimes we observed were substantially lower.32 We calculated
the Commission Internationale de l'Eclairage (CIE) coordinates
for each of the emission spectra and observed that Ir2 and Ir3
displayed spectra that were modestly similar to the CIE
coordinates for “Blue 476” of (0.1153,0.104) with values of
(0.155,0.306) for Ir2 and (0.157,0.294) for Ir3 (Supporting
Information).
a)
b)
2.2
2.4
5.4
2.5
2.6
2.8
Ba / Ag
BmPyPhB
TSPO1
EML
XHT-314
HIL: S-P3MEET:PHOST
5.6
Glass + ITO
6.5
6.8
6.8
EML
c)
Table 2. PL peak wavelength, PLQY, PL lifetime, calculated radiative (krad
)
and non-radiative (knrad) rates for Ir1-Ir4.
Property
Ir1
Ir2
Ir3
Ir4
Figure 7. (a) Energy diagram and (b) device architecture for the OLEDs
prepared using either Ir2 or Ir3 together with the (c) chemical structures of the
emitting host materials and electron transport materials. The energy levels
were obtained from the literature.
PL
Peak
497
467
464
496
Wavelength
(λp) (nm)
PLQYa
0.34
0.46
0.66
0.45
0.31
0.38
0.01
nd
Figure 8 displays the OLED performance represented as
the current density and luminance versus voltage (Figure 8a and
b, respectively) and the efficacy and power efficiency versus
luminance (Figure 8c and d, respectively). The current density
profiles show typical diode behavior with a turn-on voltage
around 3 volts. The profiles are very similar for the two OLEDs
using the different emitting complexes. This implies that the
charge transport is not significantly affected by the type of
emitter used. In view of the similar device architecture and
comparable energy levels of the two emitters, this is expected.
The luminance starts at virtually the same turn-on voltage as the
current density, yet for the OLED using complex 2 the luminance
values are higher, which is in line with the higher PLQY
observed for this complex. As a result, the efficacy (Figure 8c),
power efficiency (Figure 8d) and external quantum efficiency are
higher for the device 2, which achieved 8.0 cd A-1 (4.2 V), 6.3 lm
W-1 (3.8 V) and 3.2% (4.2V) see Table 3.
PL
lifetime
(τPL) (μ
s)
krad
0.74
1.47
0.82
nd
(106s-1)
1.43
0.75
1.81
nd
knrad
(106s-1)
CIE
Values
(x,y)
0.187,0.430
0.155,0.306
0.157,0.294
0.174,0.380
a: Measurements taken in Ar saturated dichloroethane and compared against
Ir(ppy)3 to obtain a relative PLQY value. See Figure S2 in the supporting
information for further details. b: Mixture of isomers (see Supporting
Information).
Both OLEDs exhibit similar blue electroluminescence (EL)
spectra (Figure 9) with a maximum peak at 489 nm and two
shoulders at 471 nm and 530 nm. However, device 2 exhibits a
more pronounced intensity for the second shoulder, which
extends the EL spectrum to a slightly wider emission range. The
CIE coordinates for the devices, therefore, are substantially
further from “Blue 476” (0.1153,0.104) than devices
incorporating FIrPic (0.18, 0.33),51 with device 1 (0.238, 0.407)
performing slightly better than device 2 (0.259, 0.409). The
precise reason for the red-shifted shoulder (530 nm) is not
known at this time. The highest energy peak observed in
solution (467 nm for Ir2 and 464 nm for Ir3) shifts to 471 nm and
OLED Device Characteristics
Because Ir2 and Ir3 exhibited the most blue-shifted
emission spectra in solution, we chose to investigate their
performance as OLED emitters. They were tested in the
multilayer OLED shown in Figure 7b. S-P3MEET:PHOST (40
nm) acts as hole injection layer (HIL)46 and XHT314 (20 nm) as
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European Journal of Inorganic Chemistry
10.1002/ejic.201600971
FULL PAPER
takes on the appearance of a shoulder instead of a true peak
upon inclusion in device EML. This shift is likely due to
aggregation-based energy losses as evidenced by the difference
in the device, drop-cast amorphous film, x-ray quality crystal,
crystal, Figure S1 Supporting Information), the peak is lost
entirely and the highest energy transition is centered at 490 nm.
and solution emission spectra (Figure
9 and Figure S1
Supporting Information). In the solid state (amorphous film and
Figure 8. (a) J-V, (b) L-V, (c) Eff-V and (d) PE-V scan for the OLEDs containing either Ir2 (red open circles) or Ir3 (blue open squares).
102
1000
a)
b)
c)
100
10
101
100
1
0.1
0.01
1E-3
1E-4
1E-5
Device 1
Device 2
Device 1
Device 2
10-1
102
1E-6
10000
d)
1000
100
10
101
100
10-1
1
0.1
0.01
Device 1
Device 2
Device 1
Device 2
1E-3
-2
-1
0
1
2
3
4
5
6
7
8
9
10
1
10
100
1000
Luminance (cd m-2)
Voltage (V)
Table 3. Device performance under J-V-L characterization of the OLEDs
containing either Ir2 or Ir3 as emitters.
Device 1
Device 2
Devi
ce
Lumm
Efficacy
PEmax
EQE
Emitt
er
max
[nm]
CIE
[x,y]
Von
[V]
[cd
[cd
[lm W-
ax
max
A-1]
max
m-2]
]
[%]
1
1
2
Ir2
Ir1
489,
471(sh),
530(sh)
2.7
2.9
1116
5.0
3.8
6.3
2.0
3.2
0.238,
0.407
489,
471(sh),
541(sh)
1587
8.0
0.259,
0.408
400
500
600
700
800
Wavelength (nm)
of OLED devices to near unity. Changing the ancillary ligand of
the common Ir (III) complex (dfppy)2IrL from a weakly donating
picolinate ligand to a strongly donating aryl NHC ligand allowed
for the destabilization of the E(S+/S) and E(T1) value while
maintaining a wide band-gap. Blue-emitting OLED devices were
constructed utilizing the emitters and exhibited EQE up to 3.2%
and turn on values as low as 3V.
Figure 9. Electroluminescence spectrum for the OLEDs containing either Ir2
(red open circles, device 1) or Ir3 (blue open squares, device 2). Inset: Picture
of the device 2.
Conclusions
In the quest to lower energy consumption by developing
more efficient ways to produce low-cost lighting, Ir (III) emitters
are an attractive option. They are emissive ambient-air stable
complexes capable of emitting at room temperature in many
cases. Due to strong spin-orbit coupling, the complexes emit
from a long-lived triplet state that enhances theoretical efficiency
Because the ancillary ligand plays such an important role
in fine-tuning the photophysics of Ir (III) emissive complexes,
judicious ligand selection is important to developing optimal
emitters. Our future efforts into the development of new
heteroleptic Ir (III) blue emitters will focus on proper “main”
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European Journal of Inorganic Chemistry
10.1002/ejic.201600971
FULL PAPER
ligand to “ancillary” ligand matching in order to most efficiently
OXD-7 and the emitter at 10 wt% (solids). Subsequently, a thin
10 nm layer of TSPO1 and 40 nm ETL of BmPyPhB were
thermally evaporated. Finally, the barium (5 nm) and silver
(70nm) top electrode was thermally evaporated.
tune emission to higher energies.
Experimental Section
Device Characterization: The devices were characterized in an
inert atmosphere without encapsulation. The current versus
voltage (J-V) and luminance versus voltage (L-V) curves were
obtained using a Keithley 2400 source meter and a photodiode
coupled to a Keithley 6485 picoampmeter using a Minolta LS100
to calibrate the photocurrent. The electroluminescent spectra
were measured using an Avantes AvaSpec-2048 Fiber Optic
Spectrometer.
General Information: All commercially obtained reagents and
OLED materials were used as received. Sulfonated
poly{thiophene-3-[2-(2-methoxyethoxy)ethoxy]-2,5-diyl}:poly(4-
hydroxystyrene) (SP3MEET:PHOST) and XHT314 (commercial
crosslinkable hole transport material) were supplied by Solvay.
4,4',4"-Tris(carbazol-9-yl)triphenylamine (TcTa), 1,3-Bis[2-(4-
tert-butylphenyl)-1,3,4-oxadiazo-5-yl]benzene
(OXD-7),
Diphenyl-4-triphenylsilylphenyl-phosphine oxide (TSPO1) and
1,3-Bis[3,5-di(pyridin-3-yl)phenyl]benzene (BmPyPhB) were
supplied by Luminescence technology Corporation (Lumtec).
Unless otherwise noted, all reactions were performed under a N2
atmosphere. Thin-layer chromatography (TLC) was conducted
with Sigma T-6145 pre-coated TLC Silica gel 60 F254 aluminum
sheets and/or visualized with UV and potassium permanganate
staining. Flash column chromatography was performed as
described by Still using Silicycle P60, 40-63 m (230-400
mesh).52 1H NMR spectra were recorded on a Bruker AVIII-HD
(400 MHz), and are reported in ppm using solvent as an internal
standard (CDCl3 at 7.26 ppm). Data reported as: s = singlet, d =
doublet, t = triplet, q = quartet, p = pentet, m = multiplet, b =
broad, ap = apparent; coupling constant(s) in Hz; integration.
UV-Vis spectra were measured with an LS-55 spectrometer.
Cyclic voltammetry was measured with a Biologic S-200 cyclic
voltammeter. Mass spectra were recorded on a Bruker Microflex
MALDI-TOF or ESI mass spectrometer. IR were recorded on a
Perkin Elmer UATR Two FT-IR spectrometer.
Synthetic Procedures to Reach Ir Emitters Ir1-Ir4:1-(2’,4’-
dimethoxyphenyl)imidazole (1).19 An oven-dried Schlenk tube
equipped with a stirring bar was charged with 1-bromo-2,4-
dimethoxybenzene (2.2 g, 10 mmol), imidazole (0.95 g, 14
mmol), K3PO4 (4.24 g, 20 mmol). Anhydrous DMF (15 mL) was
added and the mixture was deoxygenated with nitrogen for 20
minutes. Then CuI (0.38 mg, 2 mmol) was added and the
reaction mixture was refluxed for 24 h. Afterward, the reaction
was cooled down to room temperature, excess of
dichloromethane was added and the mixture was washed with
water. Organic phase was dried over MgSO4, filtered and
concentrated to a small amount. Flash chromatography on silica
gel and with DCM:EtOAc (4:1) as eluent resulted in a pale
1
yellow oil (650 mg, 32 %). H NMR (400 MHz, Chloroform-d) δ
7.72 – 7.43 (broad signal, 1H), 7.25-6.28 (broad signal, 1H)
overlapping with 6.98 (d, J = 8.6 Hz, 1H), 6.42 (d, J = 2.6 Hz,
1H), 6.34 (dd, J = 8.6, 2.6 Hz, 1H), 3.64 (s, 3H), 3.59 (s, 3H).
1-(2’,4’-dimethoxyphenyl)-3-methylimidazolium iodide (2). In a
round-bottom flask equipped with a stirring bar and condenser, 1
(0.64 g, 3.1 mmol) was dissolved in acetonitrile (20 mL). Then
CH3I (0.67 g, 4.7 mmol) was added and the reaction was
refluxed for 12 hours. Afterwards, a product was precipitated
with the excess of hexane and filtered. The pure product (0.99 g,
92 %) was obtained as white crystals after recrystallization from
Photoluminescence (PL) Experimental Details: The time-
resolved
PL
experiments
were
performed
with
a
spectrophotometer (Gilden Photonics) coupled with a white lamp
for steady state measurements or with the output of an optical
parametric amplifier coupled with a Ti:sa picosecond laser (200
m spot diameter, density of excitation around 30 J/cm2) and
the signal was recorded at the selected wavelength using a Si-
based photomultiplier detection technique with a time resolution
of 1 ns.
1
acetone:hexane. H NMR (400 MHz, Acetonitrile-d3) δ 9.50 (s,
1H), 7.84 (d, J = 1.8 Hz, 1H), 7.81 (d, J = 1.8 Hz, 1H), 7.55 (d, J
= 8.7 Hz, 1H), 6.78 (d, J = 2.6 Hz, 1H), 6.64 (dd, J = 8.8, 2.5 Hz,
1H), 4.12 (s, 3H), 3.86 (s, 2H), 3.82 (s, 2H). IR (neat, cm-1):
3105.3, 3083.1, 3056.2, 3006.8, 2987.8, 2946.4, 1611.1, 1555.7,
1503.6, 1295.1, 1211.6, 1035.2, 774.6, 614.7. HRMS (MALDI-
TOF-MS): Theo. for C12H15N2O2, [M]+:220.1212, observed:
220.143.
Device Fabrication: The electroluminescent devices were
prepared using patterned indium-tin-oxide (ITO) glass substrates
sequentially cleaned with soap, de-ionized water, isopropanol
and UV-O3 lamp for 20 minutes. The hole injection layer (HIL),
hole transport layer (HTL) and emissive layer (EML) were
coated by spin-coating while the electron transport layer (ETL)
and top electrode were thermally evaporated using an Edwards
Auto500 evaporator integrated into a glovebox. The thickness of
films was determined with an Ambios XP-1 profilometer. First, a
40 nm S-P3MEET:PHOST HIL was coated and annealed
(180ºC) in air. Then, the substrates were transferred to inert
atmosphere where XHT-314 HTL (20 nm) and the EML (30 nm)
were coated from 1wt% toluene and chlorobenzene solution
respectively. The emissive layer consisted of a mixture of TcTa,
1-(2’,6’-dimethoxy-3’-pyridine)imidazole (3). The synthetic
procedure is analogous to the synthesis of 1-(2’,4’-
dimethoxyphenyl)-imidazole.
3-bromo-2,6-dimethoxypyridine
(4.5 g, 20.6 mmol), imidazole (1.97 g, 29 mmol), K3PO4 (8.48 g,
40 mmol) and CuI (0.76 g, 4 mmol) in dry DMF (20 mL) were
used. Flash chromatography on silica gel with acetone as eluent
provided the product as a pale yellow oil (1.55 g, 37 %). 1H NMR
(400 MHz, Chloroform-d) δ 7.73 – 7.51 (broad signal, 1H), 7.36
This article is protected by copyright. All rights reserved

European Journal of Inorganic Chemistry
10.1002/ejic.201600971
FULL PAPER
(d, J = 8.2 Hz, 1H), 7.18 – 6.92 (broad signal, 2H), 6.26 (d, J =
chromatography on silica gel with DCM - acetone (20 : 1) as
eluent provided a product as a pale yellow oil (245 mg, 15 %).
1H NMR (400 MHz, Chloroform-d) δ 8.60 (s, 1H), 8.06 (s, 1H),
7.59 (d, J = 8.5 Hz, 1H), 6.59 (m, 2H), 3.86 (s, 3H), 3.85 (s, 3H).
1-(2’,4’-dimethoxyphenyl)-4-methyl1,2,4-triazol- iodide (8). 1-
(2’,4’-dimethoxyphenyl)-1,2,4-triazole (245 mg, 1.2 mmol), CH3I
(400 mg, 2.8 mmol) and THF (4 mL) were mixed in high
pressure flask equipped with a stirring bar. The flask was
8.2 Hz, 1H), 3.85 (s, 3H), 3.84 (s, 3H).
1-(2’,6’-dimethoxy-3-pyridine)-3-methyl-imidazolium iodide (4).
The synthetic procedure is analogous to the synthesis of 1-(2’,4’-
dimethoxyphenyl)-3-methylimidazolium iodide. 3 (1.46 g, 7.2
mmol), CH3I (1.53 g, 10.8 mmol) and acetonitrile (30 mL) were
used. Recrystallization from acetone-hexane afforded the
product (2.15 g, 86 %) as a pale violet crystals. 1H NMR (400
MHz, Acetonitrile-d3) δ 8.90 (s, 1H), 7.81 (d, J = 8.5 Hz, 1H),
7.62 (d, J = 1.9 Hz, 1H), 7.55 (d, J = 1.8 Hz, 1H), 6.55 (d, J = 8.4
Hz, 1H), 4.04 (s, 3H), 4.01 (s, 3H), 3.97 (s, 3H). IR (neat, cm-1):
3102.2, 3065.9, 3013.2, 2991, 1594.1, 1478.7, 1319.6, 1003.7,
816.6, 734.6. HRMS (MALDI-TOF-MS): Theo. for C11H14N3O2,
[M]+:221.1164, observed:221.132.
o
capped and mixture was heated to 105 C for 3 hours. After 20
minutes a white precipitate formed. Afterwards, the mixture was
cooled down, an excess of THF was added, a white precipitate
filtered and washed with an excess of THF and dried under high
1
vacuum (302 mg, 72 %). H NMR (400 MHz, Acetonitrile-d3) δ
9.94 (s, 1H), 8.76 (s, 1H), 7.73 (d, J = 8.9 Hz, 1H), 6.81 (d, J =
2.5 Hz, 1H), 6.73 (dd, J = 9.0, 2.5 Hz, 1H), 4.00 (s, 3H), 3.99 (s,
3H), 3.89 (s, 3H). HRMS (MALDI-TOF-MS) Theo. for
C11H14N3O2,
[M]+:221.1164,
observed:
221.151.
(2-(4’,6’-difluorophen-2’-yl)pyridine)2Ir(1-(4’,6’-dimethoxyphen-2’-
ylene)-3-methyl-imidazol-2-ylidene) (Ir1). In an aluminium foil
protected 25 mL round bottom flask equipped with a stirring bar
[(dfppy)2IrCl]2 (0.2 g, 0.16 mmol) and 1-(2,4-dimethoxyphenyl)-
3-methyl-1H-imidazol-3-ium iodide (0.12 g, 0.35 mmol) were
dissolved in dichloroethane (15 mL). The solution was
deoxygenated with nitrogen for 20 minutes and then Ag2O (0.11
g, 0.49 mmol) was added and the flask was sealed with a
stopper and electrical tape. The reaction mixture was stirred at
reflux for 18 hours and then hot-filtered through celite. The
1-(6-methyl-2-methoxy-3-pyridine)imidazole
(5).
A
flask
equipped with a stirbar and reflux condenser was charged with
imidazole (1.19 g, 17.44 mmol), K2CO3 (3.01 g, 21.8 mmol), 5-
bromo-6-methyl-2-methoxypyridine (2.94 g, 2.0 mL, 14.5 mmol),
CuO (0.29 g, 3.63 mmol), and DMSO (20 mL). The reaction
mixture was then warmed to 150ºC open to air, allowed to stir,
1
and monitored by H NMR. After 16h, the reaction mixture was
cooled to room temperature, diluted with CH2Cl2 (100 mL), the
crude mixture passed through celite, washed with H2O (3 x 50
mL), and dried with MgSO4. Filtered, concentrated, and passed
the crude mixture through an SiO2 plug (75 mL) first using 20%
CH2Cl2: Hexanes (0.56g of unreacted starting material in this
fraction), CH2Cl2, and EtOAc (product in this fraction) to yield a
pale orange solid (1.35 g, 46% yield). 1H NMR (400 MHz, CDCl3)
filtrate
was
concentrated
and
purified
by
column
chromatography on silica gel and with hexane:dichloromethane
(1:3) as eluent. The product (40 mg, 14 %) was obtained as a
yellow powder. 1H NMR (400 MHz, Chloroform-d) δ 8.27 – 8.12
(m, 3H), 8.08 (d, J = 5.9 Hz, 1H), 7.78 (d, J = 5.2 Hz, 1H), 7.55
(dt, J = 16.8, 7.7 Hz, 2H), 6.79 – 6.67 (m, 2H), 6.66 (d, J = 1.9
Hz, 1H), 6.36 (dtd, J = 11.7, 9.2, 2.4 Hz, 2H), 6.17 (d, J = 2.5 Hz,
1H), 6.06 (dd, J = 7.9, 2.4 Hz, 1H), 5.98 (d, J = 2.4 Hz, 1H), 5.77
(dd, J = 8.5, 2.4 Hz, 1H), 3.90 (s, 3H), 3.56 (s, 3H), 3.15 (s, 3H).
19F NMR (376 MHz, Chloroform-d) δ -110.03 (q, J = 9.1 Hz), -
110.40 (q, J = 9.0 Hz), -110.81 (ddd, J = 12.1, 9.2, 2.8 Hz), -
111.59 (ddd, J = 12.7, 9.4, 2.8 Hz). HRMS (ESI-MS): Theo. for
C34H26F4IrN4O2, [MH]+: 791.1621, observed: 791.0087.
(2-(4’,6’-difluorophen-2’-yl)pyridine)2Ir(1-(2,6-dimethoxypyridin-3-
yl)-3-methyl-1H-imidazol-3-ium) (Ir2). The synthetic procedure
analogous for ((2-(4’,6’-difluorophen-2’-yl)pyridine)2Ir(1-(4’,6’-
dimethoxy-5’-pyridin-2’-ylene)-3-methyl-imidazol-2-ylidene) was
carried out. [(dfppy)2IrCl]2 (0.4 g, 0.33 mmol), 1-(2’,6’-
dimethoxypyridin-3’-yl)-3-methylimidazolium iodide (0.24 g, 0.69
mmol) and Ag2O (0.23 g, 0.98 mmol) in dichloroethane (15 mL)
were used. Column chromatography on silica gel with
hexane:DCM (1:2) as eluent afforded the product as a yellow
powder (95 mg, 16 %). 1H NMR (400 MHz, Chloroform-d) δ 8.26
– 8.14 (m, 2H), 8.10 (t, J = 1.4 Hz, 1H), 8.06 (d, J = 5.9 Hz, 1H),
7.72 (d, J = 5.9 Hz, 1H), 7.57 (dt, J = 16.0, 7.9 Hz, 2H), 6.78 –
6.70 (m, 2H), 6.69 (s, 1H), 6.45 – 6.28 (m, 2H), 6.05 (dd, J = 8.0,
2.3 Hz, 1H), 5.85 (d, J = 1.0 Hz, 1H), 5.73 (dd, J = 8.5, 2.3 Hz,
1H), 4.04 (s, 3H), 3.76 (s, 3H), 3.15 (s, 3H). 19F NMR (376 MHz,
Chloroform-d) δ -109.52 (q, J = 9.0 Hz), -110.07 (q, J = 9.1 Hz),
-110.62 (ddd, J = 12.6, 9.5, 2.8 Hz), -111.14 (ddd, J = 12.7, 9.6,
2.9 Hz). HRMS (ESI-MS): Theo. for C33H25F4IrN5O2, [MH]+:
δ
J = 8 Hz, 1H), 7.26 (bs, 1H), 7.04 (bs,
1H), 6.66 (d, J = 8 Hz, 1H), 3.97 (s, 3 H), 2.28 (s, 3H). 13C NMR
(100 MHz, CDCl3) δ
20.3. IR (neat, cm-1): 3117.0, 3098.2, 2988.8, 2941.9, 1580.3,
1499.1, 1474.7, 1301.3, 1030.35, 657.5. HRMS (ESI-MS): Theo.
for
C10H12N3O,
[M]+:
190.0980,
observed:190.0842.
1-(2’-methyl-6’-methoxypyridine)-3-methylimidazolium iodide (6).
A flask equipped with a stirbar was charged with 1-(2’-methyl-4’-
methoxy-3-pyridine)imidazole (0.50 g, 2.61 mmol), MeI (0.33 mL,
5.23 mmol), and MeCN (2 mL). The reaction mixture was then
1
allowed to stir at 50ºC, and monitored by H NMR. After 4h, the
reaction mixture was concentrated, and recrystallized from
CH2Cl2/Hexanes to yield an off-white solid (0.87 g, 96% yield).
1H NMR (400 MHz, CDCl3) δ
1H), 7.88-7.66 (m, 1H), 7.4-7.26 (m, 1H), 6.74 (d, J = 8 Hz, 1H),
4.31 (s, 3H) 4.00 (s, 3 H), 2.44 (s, 3H). 13C NMR (100 MHz, d6-
DMSO) δ
109.3, 54.4, 36.7, 20.3. IR (neat, cm-1): 3117.0, 3098.2, 2988.8,
2941.9, 1580.3, 1499.1, 1474.7, 1301.3, 1030.35, 657.5. IR
(neat, cm-1): 3137.1, 3072.2, 2971.9, 1583.5, 1481.6, 1307.0,
812.5, 619.2. HRMS (ESI-MS): Theo. for C11H15N3O, [M-I]+:
J = 8 Hz,
215.1215,
observed:
215.1500.
1-(2’,4’-dimethoxyphenyl)-1,2,4-triazole (7).53 The synthetic
procedure is analogous to the synthesis of 1-(2’,4’-
dimethoxyphenyl)imidazole. 1-iodo-2,6-dimethoxybenzene (2.5
g, 9.5 mmol, 1.2 equiv), 1,2,4-triazole (545 mg, 7.9 mmol, 1
equiv), K3PO4 (3.5 g, 16.6 mmol, 2.1 equiv) and CuI (75 mg, 0.4
792.1574,
observed:
792.0132.
(2-(4’,6’-difluorophen-2’-yl)pyridine)2Ir(1-(6’-methyl-4’-methoxy-
mmol,
5 %) in dry DMF (15 mL) were used. Flash
This article is protected by copyright. All rights reserved

European Journal of Inorganic Chemistry
10.1002/ejic.201600971
FULL PAPER
5’-pyridin-2’-yl)-3-methylimidazol-2-ylidene)
(Ir3).
A
flask
Acknowledgements
equipped with a stirbar was charged with 6 (0.15 g, 0.45 mmol),
and 2-ethoxyethanol (5 mL). After degassing by vigorous N2
sparging for 15 min [(dfppy)2IrCl]2 (0.25 g, 0.21 mmol), Ag2O
(0.42 g, 1.80 mmol) added and the flask sealed with a stopper
and electrical tape. The reaction mixture allowed to stir at 120ºC,
This work was supported by the Swiss Federal Office for Energy
(Energy program fund 563074 managed by Prof. Andreas Züttel)
and
the
European
Commission
H2020-ICT-2014-1,
SOLEDLIGHT project, grant agreement N°: 643791 and the
Swiss State Secretariat for Education, Research and Innovation
(SERI). We thank Prof. Hubert Girault for providing laser
facilities. This work was supported by the Spanish Ministry of
Economy and Competitiveness (MINECO) (MAT2014-55200
and MDM-2015-0538) and the Generalitat Valenciana
(Prometeo/2012/053). We also thank Solvay OLED/Plextronics
by provide the hole injection material and the hole transport
material.
1
and monitored by H NMR. After 18h, the reaction mixture was
concentrated, suspended in CH2Cl2 (100 mL), passed through
celite, and concentrated. The crude product was submitted to
flash chromatography using a long SiO2 column (700 mL),
gradient of solvents from 33% CH2Cl2 : Hexanes with increasing
polarity to 100% CH2Cl2 and continuing with increasing polarity
to 100% EtOAc. The product was isolated as a 3:1 mixture of
isomers (0.090 g, 18% yield). Selected key NMR data for the
mixture: 1H NMR (400 MHz, CDCl3) δ Minor isomer: 8.31 (d, J =
8 Hz, 1H) 7.91 (d, J = 7.5 Hz, 1H), 7.31 (d, J = 8 Hz, 1H), 3.13 (s,
3H), 2.73 (s, 3H). Major isomer: 8.26-8.16 (m, J = 8 Hz, 2H) 8.01
(d, J = 7.5 Hz, 1H), 7.76 (s, 1H), 3.17 (s, 3H), 2.75 (s, 3H). 19F
{1H} NMR (376 MHz, Chloroform-d) δ -109.46 (d, J = 9.1 Hz), -
109.76 (d, J = 9.2 Hz), -110.03 (d, J = 9.3 Hz), -110.74 (d, J =
9.3 Hz). Single isomers were obtained after repetitive
crystallization. HRMS (ESI-MS): Theo. for C33H24F4IrN5O,
[MH]+:775.1546, observed: 775.0011.
Keywords: Emissive Materials • Ir Complexes • Blue Emitters •
OLED emitters • Phosphorescent Materials
(1)
(2)
(3)
(4)
(5)
Baldo, M. A.; Lamansky, S.; Burrows, P. E.; Thompson, M. E.;
Forrest, S. R. Appl. Phys. Lett. 1999, 4 (75).
Adachi, C.; Baldo, M. A.; Thompson, M. E.; Forrest, S. R. J.
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Alternate synthesis of Ir3: A flask equipped with a stirbar was
charged
with
5-(3’-methylimidazolium-1’-yl)-6-methyl-2-
methoxypyridine iodide (0.25 g, 0.79 mmol), and chlorobenzene
(50 mL). After degassing by vigorous N2 sparging for 15 min,
Ag2O (0.18 g, 0.79 mmol), NEt3 (0.12 mL, 0.79 mmol),
[(dfppy)2IrCl]2 (0.44 g, 0.36 mmol), Ag2O (0.42 g, 1.80 mmol)
added, and a reflux condenser attached. The reaction mixture
was allowed to stir at reflux, and monitored by 1H NMR. After
18h, the reaction mixture was concentrated and submitted
directly to flash chromatography using a long SiO2 column (700
mL), gradient of solvents starting from CH2Cl2 with increasing
polarity to 10% acetone: CH2Cl2. The product was isolated as a
>15:1 mixture of isomers (0.40 g, 72% yield). analytical data
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Thompson, M. MRS Bull. 2007, 32 (September), 694–701.
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(8)
(9)
(10)
(11)
(12)
(13)
(14)
(15)
identical
to
that
of
the
above
isolated
Ir3.
(2-(4’,6’-difluorophen-2’-yl)pyridine)2Ir(1-(4’,6’-dimethoxyphen-2’-
yl)-4-methyltriazol-2-ylidene) (Ir4). The synthetic procedure was
analogous to that of ((2-(4’,6’-difluorophen-2’-yl)pyridine)2Ir(1-
(4’,6’-dimethoxyphen-2’-yl)-3-methylimidazol-2-ylidene).
[(dfppy)2IrCl]2 (0.3 g, 0.25 mmol), 1-(2’,4’-dimethoxyphenyl)-4-
methyltriazolium iodide (0.19 g, 0.54 mmol) and Ag2O (0.17 g,
0.74 mmol) in dichloroethane (15 mL) were used. Column
chromatography on silica gel with DCM:EtOAc (4:1) as eluent
afforded the product as a yellow powder (55 mg, 14 %).
(16)
(17)
1
(18)
Selected key NMR data for the mixture: H NMR (400 MHz,
Chloroform-d) δ for major isomer 9.64 (dd, J = 6.1, 1.5 Hz, 1H),
8.60 (d, J = 5.8 Hz, 1H), 8.30 (d, J = 8.5 Hz, 1H), 8.07 (s, 1H),
8.03 – 7.98 (m, 1H), 5.40 (dd, J = 9.6, 2.3 Hz, 1H), 5.13 (dd, J =
8.4, 2.3 Hz, 1H), 4.29 (s, 3H), 3.77 (s, 3H), 3.37 (s, 3H); and for
minor isomer 9.58 (d, J = 5.9 Hz, 1H), 8.67 (d, J = 5.8 Hz, 1H),
7.64 (t, J = 7.8 Hz, 1H), 7.06 (t, J = 6.7 Hz, 1H), 5.65 (dd, J = 9.4,
2.3 Hz, 1H), 4.96 (dd, J = 8.6, 2.3 Hz, 1H), 4.29 (s, 3H), 3.77 (s,
3H), 3.25 (s, 3H). HRMS (ESI-MS): Theo. for C33H25F4IrN5O2,
[M+H]+: 792.1574, observed: 792.0132.
(19)
(20)
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Destabilized but not Diminished. N-
Heterocyclic Carbenes (NHCs) were
investigated as ancillary ligands in
Ir(dfppy)2L complexes, where dfppy = 2-
(2’,4’-difluorophenyl)pyridine. By
utilizing arylNHC ligands of different
electron accepting and donating
strengths, the energy levels of the
celebrated FIrPic were destabilized while
maintaining blue emission. Blue OLED
devices incorporating these emitters
showed efficiencies up to 3.2% and turn
on voltages as low as 2.7V.
Sadig Aghazada,[a] ‡ Aron J.
Huckaba,[a]‡ Antonio Pertegas,[b] Azin
Babaei,[b] Giulia Grancini,[b] Iwan
Zimmermann,[a] Henk Bolink,[b]
Mohammad Khaja Nazeeruddin,*[a]
Molecular Engineering of Iridium
Blue Emitters Using Aryl N-
Heterocyclic Carbene Ligands
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