New iridium cyclometallated complexes with potential application in OLED

Article abstract of DOI:10.1016/j.poly.2011.08.010

The photophysical and electrochemical properties of cyclometallic cationic iridium complexes that also contain bipyridine type ligands have been investigated in this work. The effect on the photophysical properties of the complexes on the presence of aliphatic long chain or branched substitution was analyzed. The complexes show photoluminescence maxima in the green-blue region of the visible spectrum, with acceptable quantum yields and lifetime, showing in this way potentiality to be tested in OLED devices.

Full text of DOI:10.1016/j.poly.2011.08.010

Polyhedron 30 (2011) 2863–2869
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
New iridium cyclometallated complexes with potential application in OLED
⇑
S. Salinas, M.A. Soto-Arriaza, B. Loeb
Facultad de Química, Pontificia Universidad Católica de Chile, Casilla 306, Correo 22, Santiago, Chile
a r t i c l e i n f o
a b s t r a c t
Article history:
The photophysical and electrochemical properties of cyclometallic cationic iridium complexes that also
contain bipyridine type ligands have been investigated in this work. The effect on the photophysical
properties of the complexes on the presence of aliphatic long chain or branched substitution was ana-
lyzed. The complexes show photoluminescence maxima in the green-blue region of the visible spectrum,
with acceptable quantum yields and lifetime, showing in this way potentiality to be tested in OLED
devices.
Received 30 May 2011
Accepted 10 August 2011
Available online 25 August 2011
Keywords:
Iridium cyclometallated
OLED
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
the metal-based HOMO. Electron-donating substituents in (N^N)
destabilize the ligand-based LUMO, ultimately leading to increased
A concept introduced in electroluminescence devices proposes
the use of ionic transition metal complexes (iTMS) [1]. These are
based on single-layer electroluminescent devices that may also
operate at low voltage with high work function air-stable elec-
trodes, due to the high electric fields produced by ions accumulat-
ing at the anode and cathode interfaces, which favor charge
injection. Cationic iridium (III) cyclometallated complexes have
achieved great interest because of their high phosphorescence
quantum yields, and the possibility to effectively tune their emis-
sion maxima over a large spectral range by an appropriate combi-
nation of ligands and substituents [2]. These properties permit
their use as emitting materials in OLED devices.
HOMO–LUMO gaps and emission energies.
Finally, considering the possibility for Ir complexes to be used in
OLED devices, it is worth mentioning that is has been proposed [8]
that the incorporation of long aliphatic chains on the bipyridine
ligand helps to the improvement of the device by e.g. minimizing
self quenching effects.
In this paper complexes of iridium with cyclometallated ligands
of general formula [Ir(C^N)2(N^N)]+, with (C^N) = 7,8-benzoquino-
line (bzq), and (N^N) = 4-40diphenylethyl-2,20-bipyridine (DPhB) or
4,40diterbutyl-2,20-bipyridine (tBuB), were synthesized and charac-
terized by spectroscopic and electrochemical techniques. These
complexes possess bipyridine type ligands substituted with
aliphatically connected substituents, or with directly bound
branched aliphatic groups. The photochemical behavior of these
complexes is studied and compared to complexes of similar struc-
ture previously synthesized or reported in literature [4,8,23]. These
types of complexes should be free of electronic effects, and should
show only structural effects. The potentiality of the complexes in
OLED devices is discussed.
The luminescent efficiency and the emission wavelength of ionic
iridium (III) complexes are affected significantly by the nature of
the organic ligands [3,4]. The ligands present in the complexes
are frequently derivatives of phenylpyridine coordinated to the me-
tal center via the formation of Ir–N and Ir–C bonds, combined with
bipyridine bidentate type ligands. The type of emission observed
(originating in d–d, d–p or p–
p⁄ excited states) for metals such as
iridium (III), will depend on the chemical and electronic properties
of the substituents on the ligands [5,6]. Specifically, studies devel-
oped by Gratzel’s and others [7] show the modulation of the emis-
sion color of iridium complexes, based on the selective stabilization
of the HOMO and/or the selective destabilization of the LUMO in the
complex. The ligands in complexes of type [Ir(C^N)2(N^N)+]n+, with
(C^N) a cyclometallating ligand and (N^N) a bipyridine substituted
ligand, act independently in order to achieve the desired color set-
ting. In general, electron-withdrawing substituents in the (C^N)
ligand decrease the donation to the metal and, therefore, stabilize
2. Experimental
Reagents and solvents were purchased from Sigma–Aldrich,
with the exception of IrCl3 hydrate, which was purchased from Pre-
cious Metals Online, PMO Pty LTD, Australia. The cyclometallated Ir
(III) chloro-bridged dimmer [Ir(bzq)4Cl2] (bzq = 7,8-benzoquino-
line) was synthesized according to the literature [9]. 1H NMR spec-
tra were obtained on a Bruker AC/200 (200 MHz) or a Bruker
400 MHz instrument, using CDCl3 as solvent and TMS as reference.
IR spectra were carried out in a Bruker Vector-22 FTIR spectrome-
ter using KBr pellets. Elemental analyses were performed on a
Fisons Instruments Analyzer, model EA 1108/CHNS-O with PC
⇑
Corresponding author.
E-mail addresses: spsalina@uc.cl (S. Salinas), bloeb@uc.cl, bloeb@puc.cl (B. Loeb).
0277-5387/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2011.08.010

2864
S. Salinas et al. / Polyhedron 30 (2011) 2863–2869
NCR system 3225. Mass Spectra were recorded with an LCQ Duo
Ion Trap Mass Spectrometer (Thermo Finnigan, USA).
H0 ), 5,05 (t, H6, H0 ), 3,10 (d, H4, H0 , H5, H0 ), 2,44 (s, H7, H0 ). FTIR
2 6 4 5 7
(cmꢀ1) (KBr): 3356 (OH stretch, OH group), 1597 (stretching C@C
pyridine), 1555 (stretching C@N pyridine), 700 (monosubstituted
aromatic deformation). Anal. Calc. for C26H24N2O2: C, 78.76; H,
6.10; N 7.07. Found: C, 75.82; H, 8.17; N, 6.83%.
2.1. Synthesis
2.1.1. Ligands
The ligand 4,40diterbutyl-2,2-bipyridine ([tBuB]), was commer-
cially obtained through Sigma Aldrich. The ligand 4-40diphenyl-
ethyl-2, 20-bipyridine ([DPhB]) was synthesized by slightly
modified procedures reported by Juris [10] for similar compounds,
with the use of acetic acid as dehydrating agent, instead of sulfuric
acid, to convert 4,40-bis[2-hydroxy-2-(phenyl)ethyl]-2,20-bipyri-
2.1.1.2. 4,40-Bis( -estirene)-2,20-bipyridine. RMN 1H (200 MHz,
a
CDCl3) d: 8,67 (d, H1, H0 ), 8,56 (s, H3, H0 ), 7,48 (d, H4, H0 ), 7,37
1
3
4
(d, H2, H02), 7,15 (d, H5, H05). FTIR, (cmꢀ1) (KBr): 1634 (stretching
C@C, alkene chromophoric unit), 1584 (stretching C@C, pyridine),
1541 (stretching C@N pyridine), 692 (monosubstituted aromatic
deformation). Anal. Calc. for C26H20N2: C, 86.64; H, 5.59; N, 7.77.
Found: C, 87.45; H, 5.56; N, 7.27%.
dine to 4,40-bis(
a
-estirene styrene)-2,20-bipyridine. Instead,
4,40-bis( -estirene)-2,20-bipyridine dehydrogenation was carried
a
out using a catalyst of Pd/C in the presence of ethanol, as shown in
Scheme 1.
2.1.1.3. 4,40-Diphenylethyl-2,20-bipyridine. RMN 1H (200 MHz,
CDCl3) d: 8,30 (s, H1, H0 ), 8,57 (d, H2, H0 ),7,11 (d, H3, H0 ), 3,01
1
2
3
2.1.1.1. 4,40-Bis[2-hydroxy-2-(phenyl)ethyl]-2,20-bipyridine. RMN 1H
(200 MHz, CDCl3) d: 8,52 (d, H1, H0 ), 8,31 (s, H3, H0 ), 7,13 (d, H2,
(s, H4, H0 , H5, H0 ), 8,75 (m, Ph). Anal. Calc. C26H24N2: C, 85.68; H,
4 5
6.64; N, 7.69. Found: C, 85.43; H, 7.05; N, 7.72%.
1
3
CH3
1) THF, cooling baths EtOH/N 2(I)
N
N
N
N
OH
OH
2) LDA, 1h
3) Benzaldehyde, 1 night
4) H2O, extaction CHCl3
CH3
[Ldiol]
N
N
OH
OH
N
CH3COOH
18 h
N
[LH]
1) Pd/C 10%, CH3COOH/CH3CH2OH
2) H2
N
N
N
N
3) Extraction CH3Cl
[DPhB]
Scheme 1.

S. Salinas et al. / Polyhedron 30 (2011) 2863–2869
2865
2.1.2. Complexes
aliquots of 50 mL diethyl ether. The aqueous layer was heated to
70 °C for 10 min using a heat gun to remove residual organic sol-
vents. The vessel was then placed on ice, and 10 mL of an aqueous
ammonium hexafluorophosphate solution (1.0 g in 10 mL of deion-
ized water) was slowly added to the mixture, yielding a colored
suspension. The precipitate was collected by suction filtration
and allowed to air-dry overnight. The product was recrystallized
by acetonitrile/diethyl ether vapor diffusion to yield the pure prod-
uct (yield: 75%).
Cationic complexes of iridium, [Ir(bzq)2L][PF6] (bzq = 7,8-
benzoquinoline, L = 4-40diphenylethyl-2,20-bipyridine or 4-40diter-
butyl-2,20-bipyridine) were prepared based on
a
literature
procedure reported by Lowry et al. [9], using the iridium precursor
tetrakis-(C^N)-
-(dichloro)-diiridium (III) ([Ir(bzq)4Cl2]) in the
l
presence of the respective [DPhB] or [tBuB] pyridine ligand in a
solvent such as ethylene glycol. To the reaction vessel an aqueous
solution of ammonium hexafluorophosphate was added and the
corresponding cationic species obtained. They will be referred as
complex [C1] and complex [C2], respectively.
1H NMR (400 MHz, CDCl3) d: 8.64 (s, 2H), 8.26 (d, J = 7.60 Hz,
2H), 7.85 (d, J = 7.6 Hz, 2H), 7.69 (d, J = 5.6 Hz, 2H), 7.47 (d,
J = 8 Hz, 2H), 7.44 (d, J = 8 Hz, 2H), 7.15 (d, J = 7.6 Hz, 2H), 6.98 (d,
J = 5.6 Hz, 2H), 6.30 (d, J = 7.2 Hz, 2H), 3.48 (m, J = 7.2 Hz, 2H),
3.19 (m, J = 7.6 Hz, 2H). MS for [C52H40IrN4]+ PF6ꢀ: m/z: 912.09
[M+].
2.1.2.1. Complex [C1] [Iridium bis (7,8-benzoquinoline)(4,40-diphenyl-
ethyl-2,20-bipyridine)]PF6. The dichloro bridged dimer (0.1 mmol)
of iridium [Ir(bzq)4Cl2] was reacted with the appropriate neutral
ligand (0.22 mmol) in ethylene glycol (5.0 mL) under reflux
(150 °C) with constant stirring for 18 h. Upon cooling to room tem-
perature, the mixture was transferred to a separator funnel by
adding three aliquots of 30 mL water, and washed with three
2.1.2.2. Complex [C2] [Iridium bis (7,8-benzoquinoline) (4,40-diterbu-
tyl-2,20-bipyridine])PF6. The synthetic route for this complex is
analogous to that described for complex [C1]. The product was
CH3
PF6
CH3
PF6
N
Ir
CH3
N
N
N
N
Ir
N
N
N
CH3
CH3
CH3
(a)
(b)
Fig. 1. (a) [Iridium bis (7,8-benzoquinoline)(4,40-diphenylethyl-2,20-bipyridine)]PF6, [C1]; (b) [Iridium bis (7,8-benzoquinoline) (4,40-diterbutyl-2,20-bipyridine])PF6, [C2].
c
d
e
h
b
a
3
4
PF6
f
N
Ir
2
5
i
N
g
1
h'
g'
i'
1'
N
N
c'
f'
5'
a'
b'
2'
3'
4'
e'
d'
9.5
9.0
8.5
8.0
7.5
7.0
6.5
6.0
5.5
5.0
4.5
f1 (ppm)
4.0
3.5
3.0
Fig. 2. NMR 1H cationic complex [C1] (a) and [DPhB] ligand (b).

2866
S. Salinas et al. / Polyhedron 30 (2011) 2863–2869
Table 1
Assignment of 1H NMR signals for [C1] complex.
-0.000004
Protons
Signals [C1] ppm
Signals [DPhB] ppm
Signals [Bzq] ppm
a,a0
b,b0
c,c0
d,d0
e,e0
f,f0
8.26
7.47
7.85
7.69
6.98
7.44
7.15
6.30
9.03
7.52
-0.000002
8.20
7.49
7.94
8.15
7.67
7. 77
9.34
0.000000
0.000002
0.000004
g,g0
h,h0
i,i0
1,10
2,20
3,30
4,40
5,50
Ph
8.64
7.83
7.65
3.48
3.19
8.59
8.30
8.57
7.11
3.01
3.01
8.75
1000
500
0
-500
-1000
-1500
Potential,mV (vs.Fc+/Fc)
-0.000006
-0.000004
-0.000002
0.000000
0.000002
0.000004
0.000006
0.000008
recrystallized by acetonitrile/diethyl ether vapor diffusion to yield
the pure product (yield: 70%).
1H NMR (400 MHz, CDCl3) d: 8.01 (d, 2H), 7.54 (d, J = 7.60 Hz,
2H), 7.86 (d, J = 7.6 Hz, 2H), 7.78 (d, J = 5.6 Hz, 2H), 7.14 (d,
J = 8 Hz, 2H), 7.43 (d, J = 8 Hz, 2H), 8.40 (d, J = 7.6 Hz, 2H), 6.33 (d,
J = 5.6 Hz, 2H), 8.27 (d, J = 7.2 Hz, 2H), 7.68 (d, J = 7.2 Hz, 2H),
7.64 (d, J = 7.6 Hz, 2H), 1.40 (s, 18H). MS for [C48H38IrN4]+PF6
:
ꢀ
m/z: 962.04 [M+].
2.2. UV–Vis and photophysical measurements
2.2.1. UV–Vis
UV–Vis spectra were recorded on a Shimadzu, UV-3101 PC
spectrophotometer.
1500
1000
500
0
-500
-1000 -1500
Potential,mV (vs. Fc+/Fc)
2.2.2. Fluorescence spectroscopic measurements
Emission measurements were carried out in a Perkin Elmer
spectrofluorimeter model SP 8765 at room temperature in acetoni-
trile, with previous degassing with N2(g) for approximately 20 min.
Emission quantum yields were determined by the method
Fig. 3. Cyclic voltammogram of complexes [C1] and [C2] vs. Fc (Ag/Ag+), measured
in acetonitrile in the presence of 0.1 M tetrabutylammonium hexafluorophosphate
as supporting electrolyte, with a 100 mV/s scan speed.
+2
described by Caspar and Meyer [11] using Ru(bpy)3 in CH3CN
as reference (/ = 0.062).
+
Steady-state and time-resolved fluorescence measurements
were performed on a K2 multifrequency phase and modulation
spectrofluorometer (ISS, Champaign, IL, USA). The instrument was
equipped with Glan-Thompson polarizers. Excitation light was
obtained by the modulable ISS 375 nm LED laser at frequencies
between 10 and 150 MHz. The emission was measured through
Schott KV-399 and WG-420 long band-pass filters. Lifetime mea-
surements were made with the polarizers oriented in the ‘‘magic
angle’’ at 54.7° condition [12a,12b]. In the multifrequency phase
and modulation technique the intensity of the exciting light is mod-
ulated, and the phase shift and relative modulation of the emitted
light are determined. Phase and modulation values were obtained
as previously described [12c,12d]. Dimethyl-POPOP (1,4-bis[2]4-
N
N
Ir
N
N
Methyl-5-phenyloxazoly benzene) in ethanol (s = 1.45 ns) was
used as a reference for intensity decay. In all measurements, data
were taken until the standard deviation of the phase and modula-
tion measurements were, at each modulation frequency, smaller
than 0.2° and 0.004°, respectively.
Scheme 2.
Fluorescence spectra were obtained employing a 300 W Xe
lamp and slits were set at 20 nm (excitation) and 5 nm (emission).
was a discrete mode and a discrete component, fixed at 0.01 ns,
in order to account for scattered light [12d].
2.2.3. Data analysis
The results were analyzed according to the Global Program
(Global Unlimited software package, Laboratory for Fluorescence
Dynamics, University of Illinois at Urbana-Champaign, Urbana, IL,
USA) [12e]. The fitting function for the lifetime measurements
2.3. Electrochemical measurements
The cyclic voltammetric (CV) measurements were carried out
using
a Bas CV-50W-2,3-MF-9093 equipment with a three

S. Salinas et al. / Polyhedron 30 (2011) 2863–2869
2867
electrode arrangement with a Pt or glassy platinum working elec-
trode, a Pt gause counter electrode, and a Ag/Ag+ reference elec-
trode. Experiments were carried out under argon atmosphere,
working in acetonitrile (CH3CN, Merck) with 0.1 M tetrabutylam-
monium hexafluorophosphate (Fluka, puriss electrochemical
grade) as the supporting electrolyte.
3. Results and discussion
3.1. Synthesis and characterization
3.1.1. Ligands
The [Ldiol], [LH] and [DPhB] ligands depicted in Scheme 1 were
synthesized by minor modifications to the procedures reported by
Juris and others [13]. For example, acetic acid instead of sulfuric
acid was used as a drying agent in the conversion of [Ldiol] to
[LH], in order to use milder conditions for a longer period of time.
Regarding the synthesis of the [DPhB ligand], when literature pro-
cedures where strictly used, [10] low reaction yields were
obtained. By rising the temperature until reflux (about 120 °C)
and allowing more reaction time (24 h), significant enhancements
of the reaction yield and the purity of the product were achieved.
The [tBuB] ligand was obtained commercially; however, selective
spectroscopic analyses of this free ligand were performed in order
to compare with the spectra of the corresponding iridium complex.
Fig. 5. Emission spectra for complexes [C1] (-) and [C2] (——), in acetonitrile.
Table 2
Photophysical data of complexes [C1] and [C2] measured at 298 K in acetonitrile.
Complex
Absorption
kmax (nm)
Emission
kmax (nm)
Lifetime
(ns)
/
3.1.2. Complexes
s
The synthetic method used to obtain the cationic iridium com-
plexes [C1] and [C2] depicted in Fig. 1(a and b), respectively,
worked out straight forward and relatively fast, with yields over
70%. Analyses carried out by 1H NMR spectroscopy showed clearly
the formation of complexes [1] and [2] by the synthetic route
reported in this work. By comparison of the 1H NMR spectra of
complex [C1] (Fig. 2a) and ligand [DPhB] (Fig. 2b), clear evidence
for the formation of the cationic complex can be inferred. The
CH2–CH2 fragment that bridges the pyridine and phenyl groups
in [DPhB], appears as a singlet at 3.01 ppm for the free ligand,
while for the complex, this signal shifts to lower fields, appearing
as two multiplets at 3.19 and 3.48 ppm. On the other hand, the
coordination of the cyclometallated fragment in complex [C1] is
also evidenced by 1H NMR, mainly by the set of signals in the aro-
matic region, between ca. 7 and 9 ppm. The assignment of the
signals is given in Table 1. The spectrum of free benzoquinoline
in CDCl3 shows signals at lower field, between 7.49 and
9.34 ppm. As an example, the shift of the h and h0 protons, from
7.77 ppm in the free ligand to 6.30 ppm in the [C1] complex is
assigned to the coordination to the Iridium metal center, which
[C1]
[C2]
250, 305, 417
264, 305, 423
385
554
559
555
582
68
60
0.082
0.071
Ir(bzq)2(bpy)+ [25]
Ir(ppy)2(bpy)+ [26]
10900a
385
250, 310, 390
0.073
a
Lifetime of Ir(III) complex in 4:1 ethanol/methanol at 298 K.
increases the electronic density on these protons, displacing them
to higher fields. It can also be observed that the signal at 9.34 ppm
in the free ligand, assigned to the i,i0 protons, is absent in the [C1]
complex spectrum, evidencing coordination.
Similar effects on the 1H NMR pattern were observed when
comparing the spectra of the free ligand 4,40diterbutyl-2,2-bipyri-
dine ([tBuB]) and complex [C2].
3.2. Electrochemical measurements
The cyclic voltammogram for complexes [C1] and [C2] show a
similar pattern, Fig. 3, with an irreversible couple at 0.91 V versus
Fig. 4. Absorption spectra for complexes [C1] (-) and [C2] (——), in acetonitrile.

2868
S. Salinas et al. / Polyhedron 30 (2011) 2863–2869
Fc, assigned to the oxidation of iridium (III/IV) together with a min-
or contribution from the cyclometallated phenyl fragment of bzq.
An irreversible reduction peak around ꢀ1.4 V, associated with
the reduction of the substituted bipyridine, is also observed. These
results compare well with those reported for similar compounds
[23]. Studies carried out earlier in our group [22a,22b] with an
analogous cyclometallated iridium complex, but containing a more
rigid and conjugated ligand (Scheme 2) also show irreversible oxi-
dation peaks at about 0.96 V, with the reduction of the ligands dis-
placed to ca. ꢀ1.3 V. The unsaturated bridge present in this case in
the substituted bipyridine, favors ligand reduction, due to the high-
er electronic density and delocalization on the complex.
responsible for the vibronic structure observed even at RT on the
emission spectra.
As expected, due to the saturated bridge, the emission of [C1]
and [C2] is very close in energy to that observed for the corre-
sponding bpy analogue, Table 2. In the same table, spectroscopic
data for the phenylpyridine (ppy) analogue is also reported.
The lower lifetime for [C1] and [C2] in regard to [Ir(ppy)2(bpy)]+
and [Ir(bzq)2(bpy)]+ can be understood because of an increase in
non radiative deactivation pathways in the formers, due to the free
rotating substitution on the bpy ligands. It should be mentioned
that the reported value for [Ir(bzq)2(bpy)]+ was measured in
another media than the rest of the series. Additionally, the quan-
tum yield values observed for [C1] and [C2] compare well with
the reported one for [Ir(ppy)2(bpy)]+.
3.3. UV–Vis and photophysical measurement
As a whole, the substitution on the bpy ligands does not change
considerable the energy (color) and the quantum yield of the emis-
sion, but affects the emission lifetime due to non-radiative
processes. Nevertheless, this eventual disadvantage should be
overcome by the advantage of avoiding ordering (that favors deac-
tivation of the excited state) when used in a device, justifying the
testing of these substrates in OLED devices.
The UV–Vis spectra of the cationic complexes [C1] and [C2] in
acetonitrile show three main absorption regions, at 240–270 nm,
300–350 nm and 390–480 nm, Fig. 4. Cyclometallated cationic irid-
ium complexes are reported to show an intense absorption band
about 250 nm in the UV, assigned to spin allowed (p ?
p⁄) transi-
tions on the cyclometallating ligands [2]; bands in the 300–380 nm
range are assigned to spin allowed metal–ligand charge transfer
transitions (1MLCT). Finally, the bands appearing in the proximity
of 420 nm are attributed to spin-forbidden 3MLCT transitions
[14]. It is discussed in literature [15] that this information indicates
an efficient spin–orbital coupling, which favors the emission phos-
phorescence, see below.
Acknowledgment
The financial assistance to this work by FONDECYT, through
project No. 1070799, is gratefully acknowledged.
The emission spectra of complexes [C1] and [C2] are shown in
Fig. 5. Complex [C1] emits at room temperature at kem = 554 nm,
while the maximum for complex [C2] appears at 559 nm. The kmax
values correlate well with those reported for this type of cyclomet-
allated complexes [16]. The vibronic substructure observed can be
related to a contribution of
MLCT processes, as discussed by Dragonetti et al. [17] and He et al.
[18].
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p ?
p⁄ LC transitions, in addition to the
Table 2 sums up the photophysical data for the complexes
reported in this work. Quantum yields of / = 0.082 and / = 0.071,
and lifetimes of
s = 68 ns and s = 60 ns, were observed for [C1]
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–
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[Ir(bzq)2(bpy)]+, i.e. to the analogous complex with 2,20-bipyridine
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4. Conclusions
The objective of this study was to analyze the photophysical
properties of new iridium cyclometallated complexes, in order to
infer their potentiality as materials of iTMC type to be used in
OLED devices. The complexes were designed to possess a bulky
substitution on the bpy ligand, in order to avoid ordering, when
eventually used in an OLED device [8]. At the same time, the sub-
stituents were chosen to possess saturated chains, which should
‘‘disconnect’’ electronically bpy with the substituent.
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According to literature information on similar compounds,
the substitution on the bipyridine ligands is not responsible for
the photochemical characteristics of these complexes, but rather
the cyclometallated ligand [24]. This latter ligand would also be

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2869
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S.M. Peng, G.H. Lee, Inorg. Chem. 44 (2005) 1344.
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