Cationic iridium dendrimers: Synthesis and photophysical properties

Article abstract of DOI:10.1071/CH11143

Two dendrimers, D1 and D2, containing the cationic iridium complexes (C1 and C2) as cores and truxene-functionalized chromophores as the branches, have been developed by a convergent synthetic strategy. The cationic complexes employ 3-(pyridin-2-yl)-1H-1,

Full text of DOI:10.1071/CH11143

RESEARCH FRONT
Full Paper
CSIRO PUBLISHING
www.publish.csiro.au/journals/ajc
Aust. J. Chem. 2011, 64, 1211–1220
Cationic Iridium Dendrimers: Synthesis
and Photophysical Properties
A
B C
,
A C
,
Bin Du, Si-Chun Yuan,
and Jian Pei
A
The Key Laboratory of Bioorganic Chemistry and Molecular Engineering of the Ministry
of Education, College of Chemistry and Molecular Engineering, Peking University,
Beijing 100871, China.
B
Department of Fundamental Science, Beijing University of Agriculture,
Beijing 102206, China.
C
Corresponding authors. Email: ysc2007@bua.edu.cn, jianpei@pku.edu.cn
Two dendrimers, D1 and D2, containing the cationic iridium complexes (C1 and C2) as cores and truxene-functionalized
chromophores as the branches, have been developed by a convergent synthetic strategy. The cationic complexes employ
3-(pyridin-2-yl)-1H-1,2,4-triazole and 2-(pyridin-2-yl)-benzimidazole derivatives as the ancillary ligands. To avoid the
change in emission colour arising from the iridium complex, the conjugation between the dendron and the ligand is
decoupled by separating them using the alkyl chain. An investigation of their photoluminescent features reveals that
efficient energy transfer happens from the dendrons to the core in the solid state. Likewise, the charged dendritic structure
is demonstrated to be an efficient method to improve the compatibility between the polar charged iridium complexes and
typical hydrophobic hosts with the additional benefit of excellent solution processability. Both dendrimers exhibit strong
solvatochromic behaviours in solvents and exclusive green and yellow-orange light in the solid state.
Manuscript received: 13 April 2011.
Manuscript accepted: 27 May 2011.
Published online: 27 July 2011.
devices.[23] Some metal complexes[24–26] using 3-(pyridin-2-yl)-
1H-1,2,4-triazole and 2-(pyridin-2-yl)-benzimidazole deriva-
tives have shown intriguing properties as the active materials
for optoelectronics. Moreover, the strong s-donor property of
the triazole, together with the p-accepting ability of the second
pyridyl fragment, provides a synergism of the electron delocali-
zation so that the electron density is transferred from triazole to
the metal ion and back to the pyridyl side of the ligand, thus
enhancing the chelating interaction.[27] So far, very few cationic
iridium complexes utilizing these ligands have been developed.
Meanwhile, the truxene unit, a heptacyclic polyarene, can act as a
desirable building block for larger polyarenes and bowl-shaped
fragments of fullerenes, liquid crystals, and C3 tripod materials,
owing to its unique three-dimensional topology and readily
available functionality.[28] Notably, Ru-based p-conjugated den-
drimers based on truxene units exhibit potential applications as
light harvesting materials.[29]
In this contribution, we have developed a new phosphores-
cent dendrimer system in which cationic phosphorescent accep-
tors at the core sites are covalently attached to a donor at the
peripheral site by a non-conjugated bridge, as shown in Fig. 1.
The dendrimers utilize 5-methyl-3-(pyridin-2-yl)-1,2,4-triazole
and 2-(pyridin-2-yl)-benzimidazole as the diimine ligands, as
well as truxene-based units as the branches. The alkyl linker was
introduced to isolate the photophysical properties of both the
donor and the acceptor. In this system, the truxene-functionalized
donors not only prevent intermolecular self-aggregation but also
donate energy to the cationic iridium complexes, which provide
us a good model for the upcoming research of intramolecular
Introduction
The development of transition metal complexes has attracted
considerable attention because of their potential for applications
in organic optoelectronic devices.[1–7] A substantial amount of
the earlier work focussed on the development of neutral iridium
complexes in the field of optoelectronics, owing to easy puri-
fication, and their lack of counterions leading to time delay
between bias and light emission in display-related devices.
However, cationic iridium complexes also showed unique
virtues as the active materials for optoelectronics, such as
excellent electrochemical and photochemical properties, good
charge-transporting properties, long-lived excited states, and
high photoluminescence (PL) efficiency.[8–15] Efforts have
recently been devoted to the design and synthesis of cationic
iridium(III) complexes with various types of ligands and to gain
more understanding of their associated photophysical properties
for applications in material science. Indeed, some cationic
iridium complexes with a higher volatility and a lack of internal
defects exhibited good performance in the fabrication of
optoelectronic devices.[16–21] Despite considerable attention
towards charged iridium complexes, there are few reports on
dendrimers[22] containing cationic iridium complexes, the bulky
substituents of which can enhance the steric hindrance of the
molecules and suppress the quenching of luminescence
effectively.
As most cationic IrIII diimine-containing complexes employ
bipyridyl or phenanthroline units as ancillary ligands, the
emissive metal–ligand charge transfer (MLCT) excited state
limits the utilization of these complexes in light-emitting
Ó CSIRO 2011
10.1071/CH11143

RESEARCH FRONT
1212
B. Du, S.-C. Yuan, and J. Pei
N
N
N
N
N
N
Ir
PF6
N
N
N
N
N
N
N
N
N
Ir
PF6
N
N
N
N
D1
C1
N
N
N
N
N
N
Ir
PF6
N
N
N
N
N
N
N
N
Ir
PF6
N
N
N
D2
C2
Fig. 1. Chemical structures of cationic Ir dendrimers D1 and D2 and complexes C1 and C2.
singlet–singlet (S-S) and triplet–triplet (T-T) energy transfer
(ET) mechanisms in addition to the advantages of a typical
conjugated dendrimer. Furthermore, Holmes et al.[30] has
reported that the triplet energy back transfer, which will reduce
the triplet population at the iridium complexes, can be avoided
by the incorporation of a spacer between the host and polymer
phosphorescent guest to achieve higher efficiencies in electro-
phosphorescent devices for which the triplet energy levels of the
host and guest are similar. It is envisioned that this new family of
dendritic architectures will be helpful to further understand the
structure–property relationship of charged iridium complexes
by investigating their physical and chemical properties.
Results and Discussion
Synthesis and Characterization
The synthetic routes to prepare the target dendrimers D1 and D2
and model complexes C1 and C2 are presented in Scheme 1.
5-Methyl-3-(pyridin-2-yl)-1H-1,2,4-triazole was prepared
according to the literature procedure[31] by the reaction between

RESEARCH FRONT
Cationic Iridium Dendrimers
1213
Br
Br
N
H
N
2
N
(i) 65 %
Br
Br
Br
Br
N
N
N
H
(i) 60 %
N
N
3
ϩ
N
N
Br
Br
1
N
(i) 35 %
N
N
N
Br
H
N
N
4
N
N
N
N
N
N
C6H13
C6H13
C6H13
C6H13
2
(ii) 45 %
C6H13
C6H13
13 C6H13
C6H
C6H13
C6H13
C6H13
C6H13
C6H13
C6H13
C6H13
C6H13
(ii) 56 %
ϩ
3
C6H13
C6H13
N
B OH
HO
5
(ii) 38 %
4
R
N
N
N
N
N
N
R ϭ
N
N
N
8
7
6
(i) NaOH, DMF, reflux; (ii) Pd(OAc)2, Cy3P, THF, Na2CO3, 80ЊC
Scheme 1. Synthetic routes to cationic iridium dendrimers D1 and D2, and model complexes C1 and C2.
2-pyridinecarboxamidrazone and acetic anhydride in acetic acid
and D2 involved directly heating iridium chloride trihydrate
with an excess of the requisite dendritic ligand in a 3 : 1
mixture of 2-methoxyethanol and water. However, we did
not obtain the corresponding intermediate iridium dimers
using this typical method,[16] owing to the poor solubility of
6 in a highly polar solvent. With a modified approach using
DMF as the solvent, the intermediate, denoted C^N2Ir
(m-Cl)2IrC^N2, could be successfully obtained, which was
directly used for the next step without further purification. A
general preparation of the mixed ligand cationic orthometal-
lated iridium(III) dendrimers D1 and D2 was afforded by the
bridge-splitting reaction between C^N2Ir(m-Cl)2IrC^N2 and an
excess of ligand 7 or 8 in THF under reflux in argon atmo-
sphere, followed by a counterion exchange process in the
presence of a saturated methanolic solution of NaPF6. The
charged iridium dendrimers D1 and D2 were purified in
,10 % yield by silica column chromatography in two steps
to give rise to the air-stable green and yellow-orange solids.
The complexes were characterized by 1H and 13C NMR
spectroscopy, matrix-assisted laser desorption ionization
time-of-flight (MALDI-TOF) MS, and elemental analysis.
at 08C with 30 % yield. 3,6-Dibromo-9-(60-bromohexyl)carba-
zole[32] (1) wasobtainedthrough the classical nuleophilic reaction
between the 3,6-dibromocarbazole and 1,6-dibromohexane in
the presence of sodium hydride in 75 % yield. 2-(20-Phenyl)-1H-
benzimidazole, 2-(20-pyridyl)-1H-benzimidazole, or 5-methyl-
3-(pyridin-2-yl)-1H-1,2,4-triazole can undergo a nuleophilic
reation with 1 in the presence of sodium hydroxide as a base to
produce 2, 3, and 4 in 60, 65, and 35 % yield, respectively.
Likewise, oily compounds 9, 10, and 12 can be prepared
according to the same procedure as used to produce 2.
Monobromo-5,5,10,10,15,15-hexahexyltruxene was lithiated
using n-BuLi followed by reacting with trimethyl borate and
then acidic hydrolysis to afford boronic acid 5 in 56 % yield.[28]
Suzuki coupling reaction between 2 and 5 catalyzed by Pd(OAc)2
and Cy3P afforded 6 as a white solid in 45 % yield. Following the
same procedures, we also obtained 7 and 8 in 56 and 38 % yields,
respectively.
Cationic iridium dimers D1 and D2 were formed from
iridium chloride trihydrate using a two-step procedure. Origi-
nally, the first step towards the dendritic iridium dimers D1

RESEARCH FRONT
1214
B. Du, S.-C. Yuan, and J. Pei
6
DMF, IrCl3
C6H13
C6H13
C6H13
C6H13
C6H13
C6H13
C6H13
C6H13
Cl
C6H13
C6H
C6H13
C6H13
C6H13
C6H13
Ir
Ir
N
5
N
N
N
N
N
5
Cl
13 C6H13
C6H13
C6H13
C6H13
C6H13
C6H13
C6H13
C6H13
C6H13
C6H13
7 or 8
THF
2
2
10 %
D1 or D2
N
9
N
N
N
C1
THF, reflux, 70 %
Cl
N
N
H2O, IrCl3, 2-ethoxyethanol
75 %
N
N
N
N
Ir
Ir
11
10
THF, reflux, 63 %
N
Cl
2
2
C2
N
N
12
Scheme 1. (Continued)
Likewise, for comparison, we prepared the corresponding
model complexes C1 and C2. The bridged iridium dimer 11 can
be produced by the reaction between iridium chloride trihydrate
with an excess of ligand 10 in a 3:1 mixture of 2-methoxyethanol
and water in high yield. The cationic model C1 was obtained
through the bridge-splitting reaction between 11 and 9 in 70 %
yield. C2 was generated using the same procedure to that of C1
with a yield of 63 %.
The signals assigned to the molecular ions of D1 and D2 in
the MALDI-TOF mass spectra also directly verified the pres-
ence of the desired dendrimers, as shown in Fig. 3. For D1, the
clear molecular ion signal peaked at 6554.6 Da (calc.: 6554.7),
which corresponds to the signal after loss of one PFꢀ6 unit. For
D2, the MALDI-TOF MS signal for the molecular ion exhibited
at 6589.6 Da (calc.: 6589.7). Interestingly, both dendrimers D1
and D2 showed different behaviours in solubility relative to a
dispersion of C1 and C2 in the dendrons (6, 7, or 8). All
dendrimers exhibited good solubility in common organic
solvents, such as hexane, THF, and dichloromethane, as well
as facile film forming properties. However, C1 and C2 are
insoluble in non-polar solvents. Therefore, the strategy used
here for the construction of cationic dendrimers could also be
regarded as an efficient solution to improve the compatibility
between the polar charged iridium complexes and typical
hydrophobic hosts with the additional benefits of excellent
Successful synthesis of dendrimers D1 and D2 could be
directly probed by the appearance of the characteristic reso-
nances for the protons assigned to C1 and C2, and through the
1
increased complexity of the aromatic region in the H NMR
spectra of D1 and D2, as shown in Fig. 2. In comparison with the
1
1H NMR spectra of C1 and C2 some H resonances of the Ir
complex in the D1 and D2 dendrimers, such as the peaks with a
red mark in Fig. 2 in the range of 5.5 to 7.2 ppm are undoubtedly
assigned to the protons of C1 and C2.

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Cationic Iridium Dendrimers
1215
(a)
8.50
8.00
7.50
7.00
6.50
6.00
(b)
8.50
8.00
7.50
7.00
6.50
6.00
Fig. 2. 1H NMR spectrum of D1 (a) and D2 (b).
(a) 1500
1000
500
(b)
1500
6557.6
6592.6
6593.6
6554.6
6558.6
6556.6
6591.6
6594.6
6589.6
1000
500
0
6559.6
6590.6
6555.6
6554.6
6595.6
6596.6
6560.6
6561.6
6589.6
6588 6590 6592 6594 6596 6598
6554 6556 6558 6560 6562
m /z
m /z
0
5833
6250
6667
m /z
7083
7500
7917
5500
6000
6500
7000
7500
m /z
Fig. 3. MALDI-TOF MS spectra of D1 (a) and D2 (b).
solution processability in the upcoming application of optoelec-
tronic devices. Thermogravimetric analyses (TGA) indicated
that dendrimers D1 and D2 had a thermal stability with a
decomposition temperature, corresponding to a 5 % weight loss,
at 380 and 3878C, respectively.
the absorption bands from dendron units and the iridium com-
plexes in the cores. The emission features of D1 exhibited a
emission peak at 399 nm assigned to truxene-functionalized
branches, and at 548 nm assigned to the iridium core (C1) at
room temperature. A strong phosphorescence band (lmax
:
569 nm) assigned to the acceptor (C2) was observed for D2, with
the donor emission peak at 397 nm. In comparison with the
phosphorescence of D1, D2 exhibited a relatively red-shifted
luminescence, which should be ascribed to the different auxil-
iary ligand. As expected, this donor–acceptor system did not
Optical Properties
The absorption and emission spectra of D1 and D2 in solution
are shown in Fig. 4. Their absorption features showed broad
bands in the range of 300 to 400 nm, which are assigned to both

RESEARCH FRONT
1216
B. Du, S.-C. Yuan, and J. Pei
1.2
1
1.2
1
1.2
1
D1-Absorb.
D2-Absorb.
D2-PL
D2
D1
D1-PL
0.8
0.6
0.4
0.2
0
0.8
0.6
0.4
0.2
0
0.8
0.6
0.4
0.2
0
300
400
500
600
700
800
300
400
500
600
700
800
Wavelength [nm]
Wavelength [nm]
Fig. 4. The absorption and emission spectra of D1 and D2 in dichlor-
omethane at 298 K, with the excitation wavelength of 310 nm.
Fig. 6. The emission spectra of both dendrimers D1 and D2 in the solid
state.
1.2
1
1.2
1
C1-Absorb.
C2-Absorb.
6-PL
C2-PL
C1-PL
benzimidazole acceptor orbital compared with that of 1,2,4-
triazole. Thus, it is possible to control the energy of the
lowest excited state by deliberately adjusting the energy of the
metal and ligand orbital, which can be achieved by changing
the ligand parent structure entirely (e.g., 1,2,4-triazole versus
benzimidazole).[14]
0.8
0.6
0.4
0.2
0
0.8
0.6
0.4
0.2
0
As observed from the emission features of D1 and D2, the
blue-light emission from the truxene-functionalized dendrons
was almost completely quenched after excitation at the absorp-
tion maximum of the branches, with emission peaks at only 526
and 560 nm, indicating the very effective energy transfer from
the truxene-based branches to the emissive cationic iridium
complexes in the solid state, as shown in Fig. 6. The phospho-
rescent emission spectrum of D1 and D2 blue-shifted (,19 and
11 nm) relative to the emission of the parent complexes C1 and
C2 (545 nm for C1 and 571 nm for C2) in the solid state,
respectively, because of the decreased intermolecular interac-
tions of the iridium cores, attributable to such large sizes of the
dendrons with multiple hexyl substituents and truxene moieties.
Therefore, it would be expected that the presence of giant
truxene-functionalized branches probably can hinder a close
overlapping among emissive metal cores, thus decreasing the
possibility of non-radiative intermolecular carrier recombina-
tion as a result of steric hindrance and rendering the iridium
complexes more hydrophobic to reduce the degradation of
devices as a consequence of a reaction with water which usually
leads to EL quenching of light-emitting electrochemical cells[30]
based on cationic transition metal complexes in our upcoming
investigation of optoelectronic devices.
The reason for the complete ET in the film as opposed to in
solution may be attributable to the shorter distance for the
intermolecular ET between the donor and the acceptor, as well
as a less flexible film phase structure, which leads to reduced
non-radiative decay.[33]
The phosphorescence spectra of both D1 and D2 were
observed to be dependent on solvent polarity. In the case of
D1, the wavelength of the emission peak was red-shifted by
48 nm by changing the solvent from hexane (520 nm) to toluene
(568 nm), as shown in Table 1 and Fig. 7a. Likewise, a remark-
ably large red-shifted emission was observed for the phospho-
rescence of D2. Since the absorption band of the cationic iridium
cores (C1 and C2) has an significant overlap with that of the
truxene-functionalized branches in the range of 280 to 400 nm, it
300
400
500
600
700
800
Wavelength [nm]
Fig. 5. The absorption and emission of complexes C1, C2, and 6 in the
solid state.
exhibit a good ET from the donor to the acceptor by both the S-S
and the T-T ET mechanisms and the strong remaining emission
arising from donors can be observed in solution, because of the
absence of conjugation and long distance between donors and
acceptors.
The steady state photophysical properties of 6, C1, and C2 in
a thin film were investigated at room temperature. As shown in
Fig. 5, compound 6 showed the PL features of the functionalized
truxene unit with a maximum emission peak at 410 nm. The
absorption spectra of C1 and C2 showed two distinct absorption
bands ranging from 320 to 425 nm. With reference to the
previous work[14] on IrIII complexes, the intense absorptions at
260–350 nm most likely originate from phenyl- and pyridyl-
based ligand-centred p–p* transitions. The absorption band at
350–450 nm was identified as the MLCT transitions from the dp
orbital of the Ir-metal centre to the unoccupied p* orbital of
ligands, mixed with some intra-ligand charge transfer transition
from ligands. None of the complexes absorbed significantly at
wavelengths longer than 500 nm. Importantly, the absorption
features of C1 and C2 over 375 nm (1MLCT and LC region)
3
had a large overlap with the emission band of compound 6,
which ensured the energy transfer from the branches to the cores
(C1 and C2) in the solid state. The emission energy of C2 is
lower than that of C1 in the solid state, with a red-shift of around
26 nm, which was attributed to the lower energy of the

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Cationic Iridium Dendrimers
1217
Table 1. Solvatochromic behaviour of dendrimers
max emission [nm]
these systems, the phosphorescence is not only sensitive to
solvent polarity, but also to other factors such as temperature
and concentration.
Solvent
l
Core of D1
Core of D2
Conclusion
Hexane
520
568
548
552
539
545
575
569
573
571
In conclusion, two cationic iridium dendrimers (D1 and D2)
built from functionalized-truxene have been synthesized in a
convergent strategy. PL results revealed that energy could be
transferred efficiently from the dendrons to the core in the solid
state. The dendrimers exhibited strong phosphorescence solva-
tochromism in different solvents. Evidence showed that the
dendritic structure could be regarded as an efficient solution to
improve the compatibility between the polar charged iridium
complexes and typical hydrophobic hosts with the additional
benefits of excellent solution processability. Further investiga-
tion of the optical properties of the dendrimers, such as elec-
trophosphorence, are still in progress.
Toluene
DCM
THF
Methanol/DCM (4/1)
(a) 1000
Hexane
THF
Methanol:DCM 4:1
DCM
Toluene
800
600
400
200
0
Experimental
General Methods
1H and 13C NMR spectra were recorded on a Varian Mercury
plus 300 MHz and Bruker ARX-400 (400 MHz) spectrometers
using CDCl3 as solvent. All chemical shifts were reported in
1
parts per million (ppm), H NMR chemical shifts were refer-
enced to TMS (0 ppm) or CHCl3 (7.26 ppm), and 13C NMR
chemical shifts were referenced to CDCl3 (77.00 ppm). Cz:
carbazole, Tr: truxene, Ir: iridium complex, Ar: phenyl.
Absorption spectra were recorded on a Perkin–Elmer Lambda
705UV-vis Spectrometer. PL spectra were carried out on
Perkin–Elmer LS55 Luminescence Spectrometer. MALDI-TOF
mass spectra were recorded on a Bruker BIFLEX III TOF mass
spectrometer (Bruker Daltonics, Billerica, MA, USA) using a
337 nm nitrogen laser with dithranol as matrix. Elemental
analyses were carried out on an Elementar Vario EL (Germany).
360
400
440
480
520
560
600
640
Wavelength [nm]
(b) 30
DCM
25
20
15
10
5
Hexane
THF
Toluene
Methanol:DCM 4:1
Synthesis
Chemicals were purchased and used as received. All air and
water sensitive reactions were performed under a nitrogen
atmosphere. Tetrahydrofuran was distilled over sodium. Com-
pounds 3, 4, and 5 were prepared according to the previous
literature procedures.[31]
0
360
400
440
480
520
560
600
640
Wavelength [nm]
Compound 2
Fig. 7. The photoluminescence spectra of D1 (M) (a) and D2 (1 ꢁ 10ꢀ6 M)
(b) in different solutions. The emission spectra were obtained with an
excitation wavelength of 310 nm.
A mixture of 3,6-dibromo-9-(60bromohexyl)-carbazole (4.88 g,
10 mmol), 2-(20-phenyl)-benzimidazole (1.95 g, 10 mmol), sodium
hydroxide (0.44 g, 11 mmol), and DMSO (30 mL) were heated
at 1308C for 24 h under an argon atmosphere. After the reaction
was cooled to room temperature, the mixture was poured into ice
water (100 mL). The mixture was extrated with chloroform and
the organic layer was washed with water and brine, and dried
over anhydrous MgSO4. After removal of the solvents under
reduced pressure, the residue was purified by silica gel column
chromatography using pentane/ethyl acetate, 2/1, as eluent to
give 2 as a white solid (3.90 g, 65 %). dH 8.13–8.12 (2H, d, J 2.1,
Cz-H), 7.85–7.82 (1H, m, Ar-H), 7.69–7.66 (2H, m, Ar-H),
7.55–7.48 (5H, m, Cz-H and Ar-H), 7.42–7.30 (3H, m, Ar-H),
7.27–7.15 (2H, m, Cz-H), 4.21–4.10 (4H, m, N-CH2), 1.78–1.71
(4H, m, CH2), 1.18–1.16 (4H, m, CH2). dC 153.5, 142.9, 138.9,
135.3, 130.5, 129.6, 129.1, 128.8, 128.6, 123.2, 123.0, 122.6,
122.3, 119.8, 111.8, 110.1, 109.9, 44.1, 43.6, 29.1, 28.3,
26.3, 26.0. m/z (ESI) Calc. for C31H27Br2N3: 599.06. Found:
600.08 [Mþ].
is difficult to determine the red-shifted change of the absorption
spectra of D1 and D2. In addition, because C1 and C2 are totally
insoluble in hexane and toluene, the absorption spectra for C1
and C2 alone in these solvent are unavailable. Based on the
above, we cannot investigate the phosphorescence change of C1
and C2 in different solvents.
According to the literature,[34] the large red-shifted phospho-
rescence observed for D1 and D2 should probably be assigned to
a large change in the dipolar vector of the T1 state. The solvent
dipolar relaxation takes place during the T1–S0 transition,
leading to an obvious phosphorescence solvatochromism.
This viewpoint can be supported by the previous solvatochro-
mism studies of osmium(II) complexes, [Ru(CN)4(bpy)] and
[W(CO)4(bpy)] (bpy ¼ 2,20-bipyridine),[35–37] which are the
most significant solvatochromic MLCT emissive systems. In

RESEARCH FRONT
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B. Du, S.-C. Yuan, and J. Pei
Compound 6
(30 mL) were heated at 1308C for 24 h under an argon atom-
sphere. The reaction was then cooled to room temperature and
the mixture was subsequently poured into ice water (100 mL).
The mixture was extracted with chloroform, and the organic
layer was washed with water and brine, and dried with anhy-
drous Na2SO4. After removal of the solvents under reduced
pressure, the residue was purified by silica gel column chromato-
graphy using pentane/ethyl aceate (10/1) to afford 2-(1-hexyl-
5-methyl-[1,2,4-triazol]-3-yl)-pyridine (9) as an oily product
(0.98 g, 40 %). dH 8.66–8.64 (1H, m, Py-H), 8.17–8.13 (1H, m,
Py-H), 7.84–7.78 (1H, m, Py-H), 7.34–7.30 (1H, m, Py-H),
4.75–4.70 (2H, t, J1 7.5, N–CH2), 2.40 (3H, s, CH3), 1.85–0.85
(11H, m, CH2 and CH3).
To a solution of 2 (1.25 g, 2.0 mmol) and boronic acid 5
(5.34 g, 6.0 mmol) in toluene (20 mL), was added Pd(OAc)2
(1.0 mol-%), Cy3P (2.0 mol-%), and 2 M aqueous KOH solution
(12 mL). The mixture was degassed and stirred at 908C for 24 h,
and the mixture was poured into a saturated solution of NH4Cl
and extracted with ethyl acetate. The combined organic layers
were washed with brine and dried over MgSO4. After removal of
the solvents under reduced pressure, the residue was purified by
column chromatography using pentane/ethyl acetate (4/1) as
eluent to give 6 as a white solid (1.90 g, 45 %). dH 8.60–8.58 (2H,
m, Cz-H), 8.50–8.40 (6H, m, Tr-H), 7.90–7.72 (11H, m, Ar-H,
Cz-H and Tr-H), 7.55–7.25 (18H, m, Ar-H and Tr-H), 4.37–4.24
(4H, m, N-CH2), 3.04–3.01 (12H, m, CH2), 2.26–2.09 (12H, m,
CH2), 1.91–1.82 (4H, m, CH2), 1.39–1.28 (4H, m, CH2), 0.63–
0.53 (132H, m, CH2 and CH3). dC 154.4, 153.5, 153.1, 144.9,
144.6, 140.4, 140.3, 139.7, 138.9, 138.3, 138.0, 134.9, 132.7,
130.1, 129.3, 128.8, 126.2, 125.9, 125.4, 125.1, 124.9. 124.6,
123.6, 123.1, 122.9, 122.1, 120.6, 119.6, 118.9, 110.1, 108.9,
55.7, 55.5, 44.5, 36.9, 36.9, 31.5, 31.4, 29.5, 29.4, 28.9, 26.7,
26.4, 23.9, 23.8, 22.2, 13.8, 13.8. m/z (ESI) Calc. for
C157H205N3: 2132.61. Found: 2133.60 [Mþ]. Anal. Calc. for
C157H205N3: C 88.35, H 9.68, N 1.97. Found: C 88.59, H 9.37,
N 1.89 %.
Compound 10
1-Bromohexane (2.46 g, 15 mmol) and 2-(20-phenyl)-
benzimidazole (1.95 g, 10 mmol), and sodium hydroxide
(0.44 g, 11 mmol), were dissolved in anhydrous DMSO
(30 mL) and were heated at 1308C overnight under an argon
atomsphere. After cooling to room temperature the mixture was
poured into ice water (100 mL). The mixture was extracted with
chloroform, the organic layer was washed with water and brine,
and dried with anhydrous Na2SO4. After removal of the solvents
under reduced pressure, the residue was purified by silica gel
column chromatography using pentane/ethyl aceate (20/1) to
afford 10 as an oily product (2.1 g, 75 %). dH 7.84–7.81 (1H, m,
Ar-H), 7.71–7.69 (2H, m Ar-H), 7.52–7.51 (2H, m, Ar-H), 7.42–
7.40 (1H, m, Ar-H), 7.32–7.10 (3H, m, Ar-H), 4.25–4.20 (2H, t,
J1 8.4, J2 6.9, N–CH2), 1.89–0.83 (11H, m, CH2 and CH3).
Compound 7
This compound was prepared by the same procedure as used
to produce 6 albeit using compound 3 in place of 2. Yield: 56 %.
dH 8.64–8.59 (3H, m, Cz-H and Py-H), 8.53–8.45 (7H, m, Py-H
and Tr-H), 7.96–7.57 (8H, m, Cz-H, Py-H and Tr-H), 7.42–7.25
(18H, m, Ar-H and Tr-H), 4.91–4.88 (2H, m, N–CH2), 4.50–
4.44 (2H, m, N–CH2), 3.04–3.01 (12H, m, –CH2), 2.24–198
(12H, m, CH2), 1.91–1.82 (4H, m, CH2), 1.39–1.28 (4H, m,
CH2), 0.63–0.53 (132H, m). dC 154.4, 153.6, 150.7, 149.8,
148.6, 145.0, 144.7, 142.7, 140.4, 139.9, 138.9, 138.4, 136.7,
136.6, 132.8, 126.3, 125.9, 125.4, 125.2, 124.9, 124.6, 123.7,
123.3, 122.5, 122.2, 120.7, 120.1, 118.9, 115.8, 110.1, 109.1,
55.8, 55.6, 45.3, 43.2, 36.9, 31.5, 29.9, 29.5, 29.0, 26.9, 26.7,
24.0, 22.8, 13.9. m/z (ESI) Calc. for C157H205N4: 2133.61.
Found: 2134.63 [Mþ]. Anal. Calc. for C156H204N4: C 87.75,
H 9.63, N 2.62. Found: C 87.92, H 9.49, N 2.66 %.
Tetrakis(1-hexyl-2-phenylbenzimidazole N,C20)
(m-chlorobridged) Diiridium (11)
Iridium trichloride hydrate (1.318 g, 3.8 mmol) and 10 (2.6 g,
9.4 mmol) were dissolved in a mixture of 2-ethoxyethanol and
water (3:1, 20 mL), and the mixture was refluxed for 24 h under
argon atomsphere. The mixture was allowed to cool to room
temperature, and the yellow precipitate was collected on a glass
filter. The precipitate was washed with ethanol and ethyl ether to
obtain a yellow power (2.7 g, 75 %), which was used directly for
the next step without purification.
Complex C1
Compound 8
In a round-bottomed flask, complex 11 (1.0 g, 0.64 mmol)
and compound 9 (0.32 g, 1.3 mmol) were mixed together in
THF (25 mL) and the mixture was refluxed overnight under an
argon atmosphere. After cooling to room temperature, excessive
NaPF6 in methanol was added dropwise and the mixture was
stirred for an additional 4 h. After the counterion exchange from
Clꢀ to PFꢀ6 was accomplished, the solvent was removed under
reduced pressure. The residue was dissolved in CH2Cl2 (20 mL),
filtered, and precipitated in hexane. The product was purified by
recrystallization using ethanol as solvent (0.88 g, 70 %). dH
8.46–8.38 (2H, m), 8.07–8.06 (1H, d, J 4.8 ), 7.70–7.76
(2H, m), 7.51–7.25 (5H, m), 7.10–6.83 (4H, m), 6.80–6.72
(2H, m), 6.32–6.28 (2H, m), 6.11–6.08 (1H, m), 5.59–5.56
(1H, m), 4.75–4.64 (6H, m), 2.02–1.89 (5H, m), 1.70 (3H, s),
1.48–1.19 (16H, m), 0.90–0.77 (8H, m). dC 162.5, 159.9, 155.5,
152.3, 151.7, 149.1, 145.3, 140.5, 139.1, 138.8, 135.2, 135.1,
133.8, 133.3, 130.6, 130.0, 128.1, 124.9, 124.6, 124.5, 124.3,
123.6, 123.5, 122.5, 122.16, 113.7, 113.6, 110.9, 110.7, 52.2,
45.0, 31.3, 31.1, 29.6, 29.5, 29.3, 26.3, 25.7, 22.3, 22.3, 13.9,
12.5. m/z (ESI) Calc. for [M – PFꢀ6 ]þ: 991.4. Found: 991.5.
This compound was prepared by the same procedure as used
to produce 6 albeit using compound 4 in place of 2. Yield: 38 %.
dH 8.60–8.58(2H, m, Cz-H), 8.49–8.41 (7H, m, Py-H and Tr-H),
8.20–8.18 (1H, m, Py-H), 7.93–7.78 (9H, m, Py-H, Cz-H and
Tr-H), 7.55–7.40 (13H, m, Tr-H and Py-H), 4.76–4.21 (4H, m,
N–CH2), 3.04–3.01 (12H, m, CH2), 2.40 (3H, s, CH3), 2.12–1.93
(12H, m, CH2), 1.66–0.61 (140H, m). dC 159.3, 154.4, 153.6,
151.6, 148.9, 148.0, 145.0, 144.7, 140.4, 139.9, 138.9, 138.4,
138.2, 136.9, 132.7, 126.3, 125.4, 125.2, 124.9, 124.6, 123.9,
123.7, 122.2, 120.6, 118.9, 109.1, 55.8, 55.6, 50.4, 43.2, 36.9,
31.5, 30.0, 29.5, 29.5, 29.0, 26.9, 26.3, 23.9, 22.3, 13.9, 13.9. m/z
(ESI) Calc. for C152H203N5: 2098.60. Found: 2099.60 [Mþ].
Anal. Calc. for C152H203N5: C 86.92, H 9.74, N 3.33. Found:
C 87.25, H 9.82, N 3.29 %.
Compound 9
A mixture of 1-bromohexane (2.46 g, 15 mmol) and 2-(5-
methyl-1H-[1,2,4-triazol]-3-yl)-pyridine (1.60 g, 10 mmol),
sodium hydroxide (0.44 g, 11 mmol), and anhydrous DMSO

RESEARCH FRONT
Cationic Iridium Dendrimers
1219
Complex C2
second step dH 8.68–8.68 (1H, m, Ir-H), 8.61–8.58 (6H, m, Cz-
H), 8.50–8.39 (19H, m, Tr-H and Ir-H), 8.18–8.16 (1H, d, J 5.1,
Ir-H), 7.95–7.83 (18H, m, Cz-H and Tr-H), 7.77–7.75 (1H, d,
J 8.3, Ir-H), 7.70–7.68 (1H, d, J 7.7, Ir-H), 7.57–7.29 (39H, m,
Tr-H and Ir-H), 7.19–7.14 (4H, m, Ir-H), 7.09–7.07 (2H, m,
Ir-H), 6.97–6.95 (2H, m, Ir-H), 6.82–6.75 (3H, m, Ir-H), 6.51–
6.50 (1H, d, J 7.6, Ir-H), 6.41–6.39 (1H, d, J 7.5, Ir-H), 6.34–6.32
(1H, d, J 8.4, Ir-H), 5.90–5.87 (1H, d, J 8.2, Ir-H), 5.70–5.67
(1H, d, J 8.3, Ir-H), 4.86–4.32 (12H, m, N-CH2), 3.04–3.02
(36H, m, CH2), 2.22–2.20 (36H, m, CH2), 2.07–0.57 (420H, m,
CH2 and CH3). dC 162.5, 160.0, 155.7, 154.5, 153.7, 153.6,
152.2, 151.9, 149.3, 145.3, 145.1, 145.0, 144.8, 144.7, 140.5,
140.4, 140.0, 139.8, 139.2, 139.1, 138.8, 138.4, 138.2, 138.1,
135.1, 134.9, 134.1, 133.8, 133.4, 133.0, 132.6, 130.7, 130.1,
128.2, 126.4, 126.0, 125.5, 125.2, 124.9, 124.6, 123.7, 123.6,
122.6, 122.2, 120.7, 119.0, 118.8, 113.9, 113.8, 110.7, 110.5,
109. 3, 109.0, 55.8, 55.6, 52.2, 44.8, 43.2, 37.1, 31.9, 31.6, 31.5,
29.7, 29.6, 29.5, 29.2, 28.9, 27.2, 26.8, 25.9, 25.5, 23.9, 23.8,
22.7, 22.3, 22.1, 14.1, 13.9, 13.8, 13.8, 13.7, 12.6. m/z (MALDI-
TOF) Calc. for C470H612IrN10: [M – PFꢀ6 ]þ: 6589.7. Found
[M – PFꢀ6 ]þ:6589.6. Anal. Calc. for C470H612F6IrN10P ꢂ 2H2O:
C 83.32, H 9.16, N 2.07. Found: C 83.49, H 9.51, N 2.32 %.
The procedure followed was the same as for the synthesis of
C1 albeit using compound 12 in place of 9 (yield: 63 %). dH
8.54–8.51 (d, 1H, J 8.4 ), 8.38–8.33 (m, 1H), 8.17–8.15 (d, 1H,
J 5.4 ), 7.80–7.55 (m, 2H), 7.51–7.43 (m, 5H), 7.26–6.95
(m, 6H), 6.90–6.72 (m, 2H), 6.48–6.28 (m, 3H), 5.91–5.88
(d, 1H, J 8.1), 5.68–5.65 (d, 1H, J 8.4), 4.82–4.57 (m, 6H),
2.07–1.86 (m, 6H), 1.49–0.72 (m, 27H); dC 162.6, 162.5, 153.4,
152.4, 151.9, 149.5, 148.1, 140.1, 139.9, 139.0, 138.7, 136.1,
135.1, 135.0, 134.5, 134.3, 133.9, 133.6, 130.4, 129.7, 127.5,
125.9, 125.1, 124.9, 124.6, 124.6, 124.1, 123.9, 123.4, 123.2,
122.3, 122.1, 118.5, 114.3, 113.6, 111.4, 110.6, 45.8, 44.9, 44.7,
31.3, 31.2, 31.2, 29.8, 29.5, 29.4, 26.3, 26.2, 26.0, 22.4, 22.3,
22.2, 13.9, 13.8, 13.8. m/z (ESI) Calc. for [M – PFꢀ6 ]þ: 1026.5.
Found: 1026.5.
Dendrimer D1
A mixture of 6 (5.7 g, 2.47 mmol), iridium chloride trihydrate
(174 mg, 0.50 mmol), and DMF (13 mL) was heated (bath
temperature: 1308C) under argon for 48 h. After the reaction
was cooled to room temperature, the mixture was extracted with
dichloromethane and washed with water and brine. The organic
layer was dried over MgSO4. After removal of the solvents
under reduced pressure, the residue was filtered through a short
silica column using pentane as eluent to give C^N2Ir
(m-Cl)2IrC^N2 as a yellow solid powder . The chloro-bridged
iridium dimer C^N2Ir(m-Cl)2IrC^N2 was used for the next step
without further purification. A mixture of C^N2Ir(m-Cl)2IrC^N2
(1.3 g, 0.15 mmol) and compound 7 (1.0 g, 0.47 mmol) was
heated in THF at 758C under an argon atmosphere for 3 days.
After cooling to room temperature, excess NaPF6 in methanol
was added dropwise and the mixture was stirred for an additional
4 h. After the counterion exchange from Clꢀ to PF6ꢀ was
accomplished, the solvent was removed under reduced pressure
and the mixture was extracted with chloroform. The organic
layer was washed with brine and dried over anhydrous MgSO4.
The product was purified by silica gel column chromatography
using petroleum ether and ethyl acetate (20:1) as eluents (yield:
10 %) in two steps.
dH 8.60–8.58 (6H, m, Cz-H), 8.63–8.43 (20H, m, Ir-H and
Tr-H), 8.10–8.07 (1H, m, Ir-H), 7.95–7.72 (24H, m, Cz-H
and Tr-H), 7.72–7.70 (2H, m, Ir-H) 7.50–7.26 (41H, m, Tr-H
and Ir-H), 7.07–7.02 (4H, m, Ir-H), 6.81–6.75 (2H, m, Ir-H),
6.38–6.32 (2H, m, Ir-H), 6.13–6.11 (1H, d, J 8.0 , Ir-H), 5.64–
5.62 (1H, d, J 8.1, Ir-H), 4.77–4.38 (12H, m, N-CH2), 3.01–2.99
(36H, m, CH2), 2.22–2.04 (36H, m, CH2), 1.70 (3H, s, CH3),
1.56–0.53 (420H, m, CH2 and CH3). dC 162.7, 162.5, 154.4,
153.8, 153.7, 153.6, 152.3, 149.7, 148.1, 145.0, 144.9, 144.8,
144.7, 144.6, 140.5, 140.4, 140.3, 139.9, 139.8, 139.7, 139.1,
139.0, 138.9, 138.8, 138.4, 138.3, 138.2, 138.1, 138.0, 136.0,
134.9, 134.8, 134.5, 133.9, 133.7, 132.6, 130.9, 130.6, 129.8,
128.8, 127.5, 126.3, 125.9, 125.5, 125.2, 124.9, 124.6, 123.7,
123.6, 122.2, 120.6, 119.0, 118.8, 114.5, 113.9, 111.5, 110.4,
109.3, 109.0, 55.8, 55.6, 37.1, 37.0, 36.9, 31.6, 31.5, 29.7, 29.6,
29.5, 24.0, 23.9, 22.3, 14.0, 13.9, 13.8. m/z (MALDI-TOF) Calc.
for C466H611IrN11 [M – PFꢀ6 ]þ: 6554.7. Found [M – PF6ꢀ]þ:
6554.6. Anal. Calc. for C466H611F6IrN11P: C 83.49, H 9.19, N
2.30. Found: C 83.72, H 9.53, N 2.48 %.
Acknowledgement
This work was supported by the Major State Basic Research Development
Program (No. 2009CB623601) from the Ministry of Science and Technol-
ogy, China, and National Natural Science Foundation of China.
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