Mono- and di-nuclear iridium(III) complexes. Synthesis and photophysics

Article abstract of DOI:10.1039/b300704a

This paper reports the synthesis and photophysical characterization of heteroleptic mono- and di-nuclear iridium(III) complexes. The complexes contain two ortho-metalating ligands, 2-phenylpyridine, with a bipyridine derivative as the third chelating unit. In the case of the dinuclear complexes the two iridium moieties are connected by a conjugated bridging ligand containing three or four phenyl units. All the complexes emit at room temperature and steady state and time resolved spectroscopy demonstrates that the lowest excited state is a metal-to-ligand charge transfer involving the bipyridine ligand. The Royal Society of Chemistry 2003.

Full text of DOI:10.1039/b300704a

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Mono- and di-nuclear iridium(III) complexes.
Synthesis and photophysics†
Edward A. Plummer,a,b Johannes W. Hofstraata,b and Luisa De Cola*a
a University of Amsterdam, Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlands.
E-mail: ldc@science.uva.nl
b Philips Research, Philips Natuurkundig Laboratorium, Prof. Holstlaan 4, 5656 AA Eindhoven,
The Netherlands
Received 17th January 2003, Accepted 17th February 2003
First published as an Advance Article on the web 23rd April 2003
This paper reports the synthesis and photophysical characterization of heteroleptic mono- and di-nuclear iridium()
complexes. The complexes contain two ortho-metalating ligands, 2-phenylpyridine, with a bipyridine derivative as the
third chelating unit. In the case of the dinuclear complexes the two iridium moieties are connected by a conjugated
bridging ligand containing three or four phenyl units. All the complexes emit at room temperature and steady state
and time resolved spectroscopy demonstrates that the lowest excited state is a metal-to-ligand charge transfer
involving the bipyridine ligand.
pyridine, bpy-ph-Br], and the dinuclear iridium compounds
Introduction
containing a conjugated bridging ligand comprising three or
Iridium() complexes have recently attracted a lot of interest
four phenyl units, [Ir-ph3-Ir] and [Ir-ph4-Ir] respectively, are
as potential triplet emitters in electronic devices1 and in bio-
reported (see Fig. 1).
logical applications as luminescent and electrochemilumines-
cent labels.2 Despite their difficult synthesis many homo- and
hetero-leptic ortho-metalated compounds have been reported
together with their photophysical properties.3–6 In fact iridium
complexes as the more well-known ruthenium polypyridine
analogues show room temperature emission (from an excited
triplet state) with long excited state lifetime and rather high
emission quantum yields.7,8 Also terpyridine compounds have
been described by several authors, since contrary to the isolec-
tronic ruthenium complexes they show strong low energy (in the
visible region) emission with long excited state lifetimes.9,10 The
main attractive features of the iridium complexes are related to
the possibility to tune the colors of their emission and their
excited state lifetime by simply changing the chelating lig-
ands.3,11 Such diversification in their behavior is related to their
excited state properties since often the ligand centered state and
the metal-to-ligand excited state are so close in energy that the
lowest excited state can be of different nature depending on
the nature of the coordinated ligands employed.4,6,12
Polypyridine compounds are often characterized by long
lived excited states due to a mixing of metal-to-ligand charge
transfer, MLCT and ligand centered, LC states.4,6,13 Replacing
one or two of the diimine ligands with the structurally similar
ortho-metalating chelates such as 2-phenylpyridine, leads to
higher metal centered, MC, excited state due to the strong
ligand field of the CN versus the NN coordinating ligands.
Furthermore their strong σ-donation causes a decrease of the
energy of the MLCT state increasing the energy gap between
the MLCT and LC states. This results in a better absorption of
visible light, a lower energy emission and shorter excited state
lifetimes.
Fig. 1 Schematic formulae of the complexes investigated in this paper.
Results and discussion
More recently the rich redox chemistry of iridium complexes
and the possibility to create the excited state by charge
recombination reactions has led to the development of several
types of simple devices based on iridium complexes.14
In this paper we discuss the synthesis, characterization, and
photophysical behavior of mixed-ligand mono- and di-nuclear
iridium() complexes. A mononuclear species, [Ir(bpy-ph-Br)],
containing two ortho-metalating ligands (2-phenylpyridine,
ppy), and a bipyridine derivative [4-(p-bromo)phenyl-2,2Ј-bi-
Synthesis and characterisation
The synthesis of iridium complexes has received a lot of atten-
tion in the last few years3,6,15 and many new synthetic methods
including microwave preparation have been reported.16 In our
case the complexes have been prepared using the ‘complex as
precursor’ strategy17 as shown in Scheme 1. The key point is
the completion of the desired iridium coordination sphere
before the entire complex is assembled via coupling through
covalent bonding. The dinuclear complexes have been prepared
from the mononuclear building block and coupled together
through a phenyl bridge. The phenyl bridge is constructed by
† Based on the presentation given at Dalton Discussion No. 5, 10–12th
April 2003, Noordwijkerhout, The Netherlands.
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Table 1 Absorption dataa
Complex
Absorption feature/nm
103 ε/MϪ1 cmϪ1
[Ir(bpy)]
465
455
410
375
335
310
265
0.6
3
5
—
16
36
—
[Ir(bpy-ph-Br)]
[Ir-ph3-Ir]
450
405
375
310
290
2
5
10
30
50
450
355
330
295
270
250
4
60
65
69
83
90
[Ir-ph4-Ir]
450
355
340
295
265
255
4
60
61
59
70
75
Scheme
1
Synthetic route employed to prepare the iridium
a Air equilibrated acetonitrile.
compounds.
separated by 145 mass units which can be attributed to the loss
of one then two hexafluorophosphate anions.
the reaction of the aryl bromide containing complex and the
appropriate aromatic bisboronic acid in a standard Suzuki
protocol.18
Photophysical properties
Ϫ
All the complexes have been obtained as PF6 salts. The
complexes have been purified by column chromatography and
characterized by H NMR and mass spectrometry. The ligand
The absorption spectra in acetonitrile of the complexes are
displayed in Fig. 2. The complexes exhibit certain features
found also for the complex [Ir(bpy)].22,23
1
4-(p-bromo)phenyl-2,2Ј-bipyrydine19 and the iridium dichloro-
bridged dimer20 were synthesised as already reported. Reaction
between the iridium dichloride precursor and the bipyridine
ligand in a methanol–dichloromethane mixture produced the
complex in the well documented bridge splitting reaction,21 in
which the bridging chlorides are replaced with the bipyridine
ligand.
To produce the dimeric species two equivalents of the com-
plex [Ir(bpy-ph-Br)] as the hexafluorophosphate salt were
reacted with one equivalent of the appropriate bisboronic acid
in the presence of a catalytic amount of tetrakistriphenyl-
phosphine and an excess of potassium carbonate. Initially the
reaction was carried out in dimethylformamide producing
reasonable yields, but latterly dimethoxyethane–water mixtures
were used due to the high solubility of the complexes in less
polar solvents; in this solvent mixture not only was a slightly
higher yield possible, but the purification procedure was greatly
simplified due to the reduced number of side reactions. The
dimeric complexes were purified over silica gel and eluted with
dichloromethane, in both cases the complex eluted as the
middle of three bands, the first monomer starting material and
the third mostly comprising of catalyst and catalyst by-prod-
Fig. 2 Absorption spectra in acetonitrile solutions.
The spectra show moderately intense absorption in the visible
region between 380–500 nm (See Table 1 for absorption maxi-
ma and molar absorbance) that in the dinuclear species are
twice as intense compared with the [Ir(bpy-ph-Br)] mono-
nuclear compound. Such a transition can be assigned to metal-
1
ucts. The H NMR of the complexes is consistent with those
found for similar complexes.22 For the complex [Ir(bpy-ph-Br)]
the pyridyl resonances can be distinguished from the ppy reson-
ances due to their half equivalent integration and their familiar
coupling constants and splitting patterns. The assignment of
the resonances for the phenyl bridged dimers becomes more
difficult due to the superimposed peaks, however, the reson-
ances assigned to the pyridyls and the phenyl are shifted with
respect to the complex [Ir(bpy-ph-Br)]. The mass spectrometric
data for the complex [Ir(bpy-ph-Br)] show only the cationic
species, lacking the hexafluorophosphate anions, whereas the
two dimeric complexes show cationic and dicationic species
to-ligand charge transfer, MLCT, bands (Ir
bpy) both spin
allowed and therefore more intense and at higher energy, than
the spin forbidden that appear as shoulder in the 420–500 nm
region. In fact for iridium complexes it is expected that a strong
spin–orbit coupling is induced by the presence of the heavy
metal and therefore the singlet
triplet transitions become
partially allowed. The intense peaks between 265 and 300 nm
can be assigned to spin allowed ligand centered, LC, transition
involving ppy and bpy, respectively.
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Table 2 Emission data
Luminescence, 298 K
Luminescence, 77 K
Complex
λmax a/(nm
τa/ns
τb/ns
Φb
Φa
λmax c/nm
τc/µs
[Ir(bpy)]d
606
611
608
606
—
76
72
75
337
334
390
411
—
—
532
522
535
537
5.2
4.5
6.0
6.2
[Ir(bpy-ph-Br)][PF6]
0.127
0.094
0.175
0.024
0.022
0.023
[Ir-ph3-Ir][PF6]2
[Ir-ph4-Ir][PF6]2
a In air equilibrated acetonitrile. b In deoxygenated acetonitrile. c In butyronitrile glass. d Data taken from ref. 22.
For the dinuclear [Ir-ph4-Ir] and [Ir-ph3-Ir] complexes the
peak with a maximum at 340 and 330 nm, respectively, can be
assigned to the phenyls of the bridging ligand. The maximum
of this band changes according to the number of phenyls pres-
ent in the bridge since the energy decreases with increasing
number of units until the effective conjugation length is
reached. The room temperature emission spectra recorded in
acetonitrile solution are shown in Fig. 3 and the data summar-
ised in Table 2.
Fig. 4 Emission spectra at 77 K in butyronitrile glass, λex = 405 nm.
all the complexes are mono exponentials at any wavelength and
quite similar to the MLCT transitions reported for other Ir()
ortho-metalated complexes containing bpy or phen as the third
coordinating ligand.9,12,25
A further proof of our assignment comes from time resolved
transient spectroscopy. The transient absorption spectra of the
reference complex and of the mono- and di-nuclear complexes
synthesised, have been measured in acetonitrile at room
temperature. The spectra are strikingly different.
Fig. 3 Emission spectra at 295 K in acetonitrile, λex = 405 nm.
The monomeric complexes, [Ir(bpy-ph-Br)] and [Ir(bpy)]
have similar features, but differ from the dimeric complexes. For
the monomeric complexes an absorption band appears in the
region 400–550 nm, and a weaker band forms within the laser
pulse (2 ns) in the region 650–800 nm. Both bands decay mono-
exponentially with a lifetime of around 60 ns. Such bands are
due to the population of the MLCT state, with the lowest
The luminescence of all the investigated complexes show
broad, structureless, rather intense bands at about 610 nm
attributed to MLCT transition involving the coordinated bi-
pyridine ligand. The emission properties are very similar for all
the investigated compounds and the maxima are almost identi-
cal, independent both from the number of iridium units as well
as the number of phenyls present in the bridging ligand.
3
26
energy band due to the formation of the radical anion bpyϪ
.
ؒ
The lowest excited state localized on the substituted bi-
pyridine is therefore very little perturbed by the substitution
and further more the metal moieties show very weak electronic
interaction. Also the excited state lifetime and emission quan-
tum yield are in agreement with such an assignment (see Table
2). The complex [Ir(bpy-ph-Br)] is slightly red-shifted with
respect to the two dimeric complexes, and this can be attributed
to the electron withdrawing nature of the bromine lowering the
LUMO level.24 The quantum yields of the emissions are highly
sensitive to oxygen which also confirms that the emission arises
from a triplet state, which seems, in these cases, to be a pure
MLCT state and not a mixing of the LC and MLCT states as
often reported for polypyridine complexes possessing very long
excited state lifetimes even at room temperature. MLCT states
are known to be strongly solvent and temperature dependent.
In butyronitrile glass at 77 K the charge transfer transitions
However, the dinuclear complexes do not appear to show the
band between 400–550 nm although this could be hidden under
the very intense and broad absorption at lower energy. They
exhibit a band that stretches the entire region (400–850 nm), see
Fig. 5. This absorption band can be assigned to the ph3 and ph4
substituted bpyϪ . The conjugated units connected with the
ؒ
chelating ligand favor the delocalisation of the charge pro-
moted on the bpy by light excitation. By comparison, the
unsubstituted [Ir(bpy)] or monophenyl [Ir(bpy-ph-Br)] substi-
tuted bpy complexes have different transient absorption spectra
(see Fig. 5). The lifetimes of the decays are directly related to
the emission decays suggesting that the emission is derived from
the absorbing species seen in the transient spectra.
Conclusions
3
move to higher energy and the LC state could become the
Mixed mono- and di-nuclear Ir() complexes have been pre-
pared and their photophysical properties investigated. All the
complexes exhibit a single emission at room and at low temper-
ature. The lowest excited state is a 3MLCT state and involves the
substituted bipyridine. The dinuclear complexes are character-
ized by a weak electronic coupling between the metal units and
lowest excited state. Our experimental data at 77 K show blue-
shifted emission with additional structure when compared to
the room temperature spectra (Fig. 4 and Table 2). The struc-
tured emission could indicate the presence of different emissive
excited states. However, the excited state lifetimes observed for
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Fig. 5 Transient absorption spectra and decay kinetics in acetonitrile for [Ir(bpy)] (left) and [Ir-Ph4-Ir] (right).
time resolved transient spectroscopy clearly shows that the low-
equipped with 150 or 600 g mmϪ1 grating and tunable slit
est excited state is a significantly delocalized state involving the
phenyl units of the bridging ligand. The complexes are interest-
ing materials for electroluminescent devices and studies are
in progress to evaluate their performance as light emitting
diodes.27
(1–500 µm) resulting in a 6 or 1.2 nm maximum resolution,
respectively. The data were collected with a system containing a
gaited intensified CCD detector (Princeton Instruments ICCD-
576G/RB-EM) and an EG&G Princeton Applied Research
Model 9650 digital delay generator. I and I0 were measured
simultaneously using a double 8 kernel 200 µm optical fiber
with OMA-4 setup. WINSPEC (version 1.6.1, Princeton
Instruments) used under Windows, programmed and accessed
the setup.
Experimental
Solvents and starting materials
All reagents used were obtained from available commercial
sources and used without additional purification unless other-
wise indicated. Commercial deuterated solvents were used as
received for the characterization of the compounds. Solvents
used for spectroscopy (acetonitrile, butyronitrile), were freshly
distilled from CaH2 before use.
Synthesis
[Ir(bpy-ph-Br)][PF6]. Ir2Cl2ppy2 (0.13 g, 0.123 mmol) and
4-(p-bromo)phenyl-2,2Ј-bipyridine (0.06 g, 0.194 mmol) were
heated to reflux in a dichloromethane–methanol (3 : 1, 20 cm3)
mixture under a nitrogen atmosphere for 3 hours. The volume
of the solution was reduced to 5 cm3 and methanol added
(10 cm3). An excess of saturated methanolic ammonium
hexafluorophosphate was added. The resulting precipitate was
filtered off and washed with ether (20 cm3) to yield [Ir(bpy-ph-
Br)][PF6] as a bright yellow solid (0.141 g, 60%). 1H NMR (300
MHz, MeCN) δ: 8.76 (1H, s, H3Ј), 8.73 (1H, d, J = 8.1 Hz, H),
8.19 (1H, dt, J = 7.8, 1.5 Hz), 8.11 (2H, d, J = 8.1 Hz), 8.06 (1H,
d, J = 5.1 Hz), 8.01 (1H, d, J = 5.7 Hz), 7.92–7.76 (9H, m), 7.68
(2H, t, J = 6 Hz), 7.56 (1H, t, J = 5.4 Hz), 7.08 (4H, m), 6.97
(2H, t, J = 7.2 Hz), 6.34 (2H, d, J = 7.5Hz). ESI-MS (MeCN)
m/z: 811.1 (100%) [Ir(bpy-Ph-Br)]؉.
Instrumentation
1H NMR spectra were recorded on a Varian Mercury 300
(300.13 MHz for 1H). Mass spectra were recorded on a Bruker
FTMS 4.7T BioAPEX II in acetonitrile.
Spectroscopy
The emission quantum yields were measured by the method of
Demas and Crosby28 with [Ru(bpy)3](PF6)2 in air equilibrated
acetonitrile solution as the standard (Φ = 0.016). Deareated
samples were prepared by use of the freeze–pump–thaw
method. At least three cycles were used. The UV-Vis absorption
spectra were recorded on a Hewlett-Packard diode array
8453 spectrophotometer. Recording of the emission spectra
was done with a SPEX Fluorolog spectrofluorometer. Low-
temperature (77 K) emission spectra for glasses and solid-state
samples were recorded in 5 mm diameter quartz tubes which
were placed in a liquid nitrogen Dewar equipped with quartz
walls. The emission spectra were corrected for monochromator
and photomultiplier efficiency and for xenon lamp stability.
Sample and standard solutions were degassed with at least three
freeze–pump–thaw cycles. Lifetimes were determined using a
Coherent Infinity Nd:YAG-XPO laser (1 ns pulses FWHM)
and a Hamamatsu C5680–21 streak camera equipped with a
Hamamatsu M5677 low-speed single-sweep unit. Transient
absorption spectroscopy was performed by irradiation of the
sample with a Coherent Infinity Nd:YAG-XPO laser (1 ns
pulses FWHM). The sample was probed by a low-pressure,
high-power EG&G FX-504 Xe lamp. The passed light was dis-
persed by an Acton SpectraPro-150s imaging spectrograph
[Ir-ph3-Ir][PF6]2. [Ir(bpy-ph-Br)][PF6] (0.025 g, 0.026 mmol),
K2CO3 (20 mg, excess) in H2O (5 cm3) and phenyl bisboronic
acid (0.0021 g, 0.013 mmol) in anhydrous DME (30 cm3) were
degassed three times using the freeze–pump–thaw technique.
Palladium tetrakis(triphenylphosphine) (0.3 mg, 0.00026
mmol) was then added. The mixture was heated to reflux under
a nitrogen atmosphere for 18 hours. The DME was removed
under vacuum. The resulting solid was washed with water (3 ×
20 cm3), methanol (20 cm3) and diethyl ether (20 cm3) before
being dissolved in dichloromethane and applied to a silica
column eluted with dichloromethane to yield [Ir-ph3-Ir][PF6]2
1
as a yellow–orange solid (0.011 g, 42%). H NMR (300 MHz,
MeCN) δ: 8.85 (2H, s), 8.79 (2H, d, J = 7.8 Hz), 8.22–7.86 (32H,
m), 7.70 (4H, t, J = 5.4 Hz), 7.56 (2H, t, J = 6.6 Hz), 7.09 (8H,
m), 6.97 (4H, t, J = 6.3 Hz) 6.34 (4H, d, J = 7.2 Hz).
[Ir-ph4-Ir][PF6]2. [Ir(bpy-ph-Br)][PF6] (0.035 g, 0.036 mmol),
K2CO3 (0.04 g, excess) in H2O (5 cm3) and biphenyl bisboronic
acid (0.004 g, 0.018 mmol) in anhydrous DMF (35 cm3) was
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degassed three times using the freeze–pump–thaw technique.
Palladium[tetrakis(triphenylphosphine)] (0.4 mg, 0.00036
mmol) was then added. The mixture was heated to reflux under
a nitrogen atmosphere for 18 hours. The DME was removed
under vacuum. The resulting solid was washed with water (3 ×
30 cm3), methanol (30 cm3) and diethyl ether (30 cm3) and then
dissolved in dichloromethane and applied to a silica column
eluted with dichloromethane to yield [Ir-ph4-Ir][PF6]2 as a
yellow–orange solid (0.017 g, 52%). 1H NMR (300 MHz,
MeCN) δ: 8.84 (2H, s), 8.77 (2H, d, J = 7.8 Hz), 8.22 (2H,
t, J = 7.8 Hz), 8.17–7.82 (38H, m), 7.71 (4H, t, J = 5.1 Hz),
7.56 (2H, t, J = 6.6 Hz), 7.09 (8H, m), 6.97 (4H, t, J = 6.6 Hz),
6.34 (4H, d, J = 7.8 Hz). MS-ESI (MeCN) m/z: 732 (95%)
[Ir-ph4-Ir]2؉, 1609 (1%) [Ir-ph4-Ir][PF6]؉.
Acknowledgements
E. A. Plummer wishes to thank the EU for financial support,
project number HPRN-CT-2000–00024 MIPA.
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manuscript in preparation.
28 J. N. Demas and G. A. Crosby, J. Phys. Chem., 1971, 75, 991.
D a l t o n T r a n s . , 2 0 0 3 , 2 0 8 0 – 2 0 8 4
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