Synthesis and properties of a dendritic FRET donor-acceptor system with cationic iridium(iii) complex core and carbazolyl periphery

Article abstract of DOI:10.1039/c1dt11716e

In order to enhance the photoluminescence of cyclometalated iridium(iii) complexes, which are potentially useful for biolabeling and bioimaging, a series of benzyl ether branched dendritic moieties with carbazolyl termini were introduced to the cyclometal

Full text of DOI:10.1039/c1dt11716e

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Dalton
Transactions
An international journal of inorganic chemistry
www.rsc.org/dalton
Volume 41 | Number 9 | 7 March 2012 | Pages 2529–2836
ISSN 1477-9226
COVER ARTICLE
Li, Liu and Zhou
Synthesis and properties of a dendritic FRET donor–acceptor system with
cationic iridium(III) complex core and carbazolyl periphery

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PAPER
Synthesis and properties of a dendritic FRET donor–acceptor system with
cationic iridium(III) complex core and carbazolyl periphery†
Yanfang Li,a,b Yang Liu*a,c and Ming Zhou*a,c
Received 10th September 2011, Accepted 8th November 2011
DOI: 10.1039/c1dt11716e
In order to enhance the photoluminescence of cyclometalated iridium(III) complexes, which are
potentially useful for biolabeling and bioimaging, a series of benzyl ether branched dendritic moieties
with carbazolyl termini were introduced to the cyclometalating C^N ligands of the heteroleptic Ir(III)
complexes. The complexes also contain a bidentate bipyridine ligand with a carboxyl group for further
bioconjugation or functionalization. The dendritic benzyl ether moieties with carbazolyl peripheral groups
have demonstrated a dual function as both a Förster resonance energy transfer (FRET) donor and an
oxygen shield to the Ir(III) complex core. The peripheral carbazolyl groups absorb UV light more
intensively and transfer energy efficiently to the Ir(III) complex core via the FRET effect, and thus the
photoluminescence of the Ir(III) complex at around 560 nm is significantly enhanced. Furthermore, the
benzyl ether dendrimers containing carbazolyl termini can shield the Ir(III) complex core to weaken the
oxygen quenching effect, which leads to a further enhancement of the PL of the Ir(III) complex.
complexes. In addition, the Ir(III) complexes can be made electri-
Introduction
cally negative (anionic35), neutral and positive (cationic com-
plexes36), offering versatile application possibilities.
Transition metal complexes, such as Ir(III), Ru(II), Os(II) and Re
(I) complexes, are characterized by their metal to ligand change
transfer (MLCT) transition leading to phosphorescent emission.
Among them, cyclometalated iridium(III) complexes are of
special interest as promising photoluminescent (PL) substances
widely used as dopants for organic light-emitting diodes
(OLEDs),1–8 luminescent bio-labeling reagents,9–16 photocataly-
sis17 and chemosensors for oxygen,18–21 heavy metal ions,22–25
anions26–28 and amino acids.29,30 In addition to the general fea-
tures of transition metal complexes, such as the long lifetime of
the excited state and the large Stokes shift for easy separation of
excitation and emission wavelengths, cyclometalated Ir(III) com-
plexes normally have high photochemical stability due to the
chelating bonds of the cyclometalating ligands with the metal
atoms, high quantum efficiencies31,32 and a broad range of tun-
ability33,34 of emission properties that allow their luminescence
to span from NIR to blue light. The largely tunable photophysi-
cal properties of Ir(III) complexes have been realized by varying
the ligands of either the homoleptic or heteroleptic Ir(III)
In biological and medical fields, ruthenium(II) polydiimine
complexes and lanthanide (Eu(III), Tb(III) etc.) cryptates
have been successfully used as bio-labels in commercial
electrochemiluminescence37 and time-resolved fluorescence
immunoassays,38,39 respectively. Interest in developing lumines-
cent metal complexes and exploiting their bio-applications has
grown rapidly since the successful industrial introduction of
metal complex-based bio-analytical methodologies. Cyclometa-
lated Ir(III) complexes are among the most important families
of these complexes, which may potentially be developed to
a level of commercial value.40 It has been reported that the
luminescence from Ir(III) complexes can be generated by both
photo-excitation and electrochemical excitation.41 Luminescence
generated by photon excitation is simpler in some applications
but often suffers from insufficient absorption cross section,
even though the quantum yields are acceptably high. A mechan-
ism of enhancing the absorption and thus the emission of
luminophores is to utilize Förster resonance energy transfer
(FRET),42 by which a donor, which has a large absorption
extinction coefficient, absorbs light energy and efficiently trans-
fers the energy to the nearby acceptor, which then becomes
excited and emits luminescence (i.e., the signal of detection
value).
aSuzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of
Sciences, 398 Ruoshui Road, Suzhou Industrial Park, Jiangsu, 215123,
P. R. China. E-mail: mzhou2007@sinano.ac.cn, yliu2007@sinano.ac.
cn; Fax: +86-512-62872181; Tel: +86-512-62872558
bGraduate School of the Chinese Academy of Sciences, Beijing, 100049,
P. R. China
It was our objective in this work to construct such a system in
Ir(III) complexes, the applications of which have been largely
biased towards OLEDs so far, to demonstrate the intramolecular
energy transfer that helps to gain more intense emission from the
luminophores. A strategy of introducing as many donor moieties
as possible has been demonstrated through the design and the
cSunaTech Inc. bioBAY, Suzhou Industrial Park, Jiangsu, 215123,
P. R. China
†Electronic supplementary information (ESI) available: Extra absorp-
1
tion, emission, H and 13C NMR and mass spectra, and cyclic voltam-
mograms. See DOI: 10.1039/c1dt11716e
2582 | Dalton Trans., 2012, 41, 2582–2591
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successful syntheses of
a
series of dendritic scaffolds
bio-conjugatable system provides possibilities for further appli-
cations that require covalent incorporation of the luminophores
with biomolecules, a target recognizable moiety or other biologi-
cally active substances.
Reported here are the synthesis and properties of a series of cat-
ionic bis-cyclometalated Ir(III) complex (acceptor) cored bio-con-
jugatable compounds having peripheral carbazolyl moieties
(donor) and a carboxylic acid functionalized bipyridine (N^N
ligand) derivative as an ancillary ligand. The compounds of
different generations of dendritic scaffolds contain 0, 4, 8 and 16
carbazolyl groups. The photophysical properties and the emission
enhancement as combined results of both the FRET and the
dendrimer shielding effect on oxygen quenching will be discussed.
(Scheme 1). The donor used in this work is the carbazolyl
group, which possesses well studied photo- and electrochemical
properties, such as relatively intense luminescence43 and revers-
ible oxidation processes.44 The energy level of the emission of
carbazole (∼360 nm) matches well with the MLCT absorption
bands of many Ir(III) complexes.45–48 As perfectly branched
polymeric moieties with a high density of surface functionaliz-
able sites, the dendritic scaffolds used in the FRET molecular
structures are not novel.49,50 The introduction of dendrimers into
metal complexes was first reported by Serroni et al.51 in 1994. It
is worth noting that Li et al. have synthesized Ir(III)-cored den-
drimers with carbazole peripherally functionalized β-diketone
dendritic ligands and observed remarkable FRET phenomena.45
Introducing the FRET donors to the β-diketonato ligand provides
a strategy for easily synthesizing a large number of Ir(C^N
)2(acac), which could share the carbazole functionalized β-dike-
tonato. Similarly, Kwon and coworkers reported dendrimer light-
harvesting systems and a high intramolecular energy transfer
efficiency greater than 90% based on FIrpic (iridium(III)bis[(4,6-
difluorophenyl)pyridinato-N,C2′]picolinate) with dendritic picoli-
nate ligands containing carbazolyl peripheral groups.46 This
system was further modified to include a biotin functional group
for potential bioanalytical application.48
Results and discussion
Synthesis and characterization
Dendritic donor precursors (G1-Br, G2-Br, and G3-Br) were
prepared by the convergent strategy according to the literature46
(Scheme 1). Phenylpyridine (ppy) derivatives, (2-phenylpyridin-
4-yl)methanol (4) and (4-(pyridin-2-yl)phenyl)methanol (8),
were synthesized from 4-methyl-2-phenylpyridine (1) and 2-p-
tolylpyridine (5) (Scheme 2). Conjugation of benzyl bromide of
the dendritic donor precursors with the benzyl alcohol groups of
4 and 8 gave dendritic C^N ligands, LGn and LGn′, respectively
(Scheme 3).
Scheme 1 Dendritic donor precursors.
In the above mentioned work, the donor moieties at the den-
dritic peripheries are built on just one ligand, i.e., the ancillary
ligand of the heteroleptic Ir(III) complexes. We intended to intro-
duce more donor groups without having to extend the branch
lengths or to increase the number of the dendrimer generation.
Most importantly, we intended to build a system with a bio-con-
jugatable capability based on the usual amide chemistry, as it is
generally employed in bio-labeling and bio-conjugation tech-
niques.52 Therefore, unlike other previous systems, the donor-
bearing dendritic scaffolds were based on the two cyclometalat-
ing C^N ligands, rather than the heteroleptic ancillary ligand,
such as acac45 and picolinate.46 This strategy of introducing
more donors could be more synthetically challenging than in the
cases of dendronizing acac or picolinate, because the bulky den-
dronized C^N ligand make the cyclometalation reaction during
the Ir(III) dimer formation more difficult. In addition, the Ir(III)
compounds in this work have a positive charge (cationic lumino-
phores) due to the charge imbalance arising from replacement
one of the usual carbon–metal bonds (between the iridium(III)
and the C^N ligands) with the coordination bonds (between
the metal and the bio-conjugatable diimine ligand). Such a
Scheme 2 Synthetic route to phenylpyridine derivatives.
Scheme 3 Synthetic route to dendritic ligands.
The common synthetic route for the cationic iridium complexes
involves two steps as illustrated in Scheme 4. The chloro-bridged
cyclometalated Ir(III) dimers, (C^N )2Ir(μ-Cl)2Ir(C^N )2, were syn-
thesized by heating iridium chloride trihydrate (IrCl3·3H2O) with
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Scheme 4 Synthetic route to dendritic Ir(III) complexes IrLGn and IrLGn′.
an excess of the dendritic C^N ligands in a mixture of 2-
ethoxyethanol/water (3 : 1, v/v) according to the literature.53 Then
the cationic Ir(III) complexes were prepared by refluxing the as-
prepared dichloro-bridged dimers and the ancillary bidentate N^N
ligand, bipy-COOH, in CH2Cl2/methanol solution overnight. The
introduction of the carboxyl functionalized ancillary ligand makes
the Ir(III) complexes bioconjugatable, providing opportunities to
be utilized as luminescent probes for bioanalysis or imaging.
When using dendritic ligands, LGn, the yield of the final com-
plexes decreased with the increase of the dendrimer. The first- to
third-generation of Ir(III)-cored dendrimers (IrLG1–IrLG3) were
obtained with yields of 71%, 40%, and 20%, respectively. For
and THF, as well. However, IrLG1, IrLG2, and IrLG3 are soluble
only in solvents such as CH2Cl2, chloroform and THF.
The compounds synthesized in this work were structurally
verified with 1H and 13C NMR spectroscopies and MALDI-TOF
mass spectra (see Fig. S16, S18, S20 and S22, ESI†).
1
As shown in Fig. 1, the H NMR spectra of the dendrimers
become very complicated with the growth of dendrimer gener-
ation. However, the chemical shifts at 1.8–3.0 ppm, assigned to
the aliphatic proton of the bipy-COOH ligand, are very
the dimerization, because the solubility of LGn and LGn
′
becomes smaller with the increase of the generation number (n),
certain amounts of THF were added to the reaction mixtures.
The difference between LGn and LGn′ lies in the position at
which the dendritic scaffolds are rooted. Such a steric difference
seems to have a fundamental impact on the reactivity of the LGn
during the dimerization process. For the LGn′ series, only LG1
′
′
could finally form the metal complex IrLG1′, while LG2′ failed
to form the dichloro-bridged cyclometalated Ir(III) dimer. Com-
paring the meta position of the carbon atom that forms the C–Ir
bond in the phenyl ring of LG2′ with the para position of the
nitrogen atom of LG2, the former has a greater steric hindrance,
which most likely prevents the formation of the Ir(III) complex
dimer. Such a steric effect and the challenge to the synthesis had
been envisaged when starting this work. Fortunately, by switch-
ing the C^N ligands from LGn′ to LGn, we successfully prepared
the target compounds with designed functionalities and were
able to conduct further characterization.
Among these metal complexes, IrLG0 and IrLG0′ are readily
soluble in common polar solvents, such as acetonitrile, methanol
and ethanol, and non-polar solvents, such as CH2Cl2, chloroform
1
Fig. 1 The H NMR spectra of IrLGn (n = 0–3) in CDCl3 (* denotes
signals arising from the active proton.).
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Fig. 2 (a) UV/Vis spectra of Ir(III) complex-cored dendrimers IrLGn (n = 0–3) in CH2Cl2 solutions (5 μM) at 298 K. The inset shows the fine absorp-
tion details in the range from 300 nm to 480 nm and the emission spectrum (dashed line) of 5-methyl-N,N′-dicarbazolyl-1,3-benzene (G1); (b) A com-
parison between the absorption and the emission spectra of IrLG0–IrLG0′ and IrLG1–IrLG1′.
characteristic and indicative of the proper coordination of the
N^N ligand with the Ir(III) atom. The integration of the aliphatic
500 nm, a featureless, much weaker absorption band also
appeared due to the MLCT transitions of the Ir(III) complexes.
With the increase of the number of carbazolyl groups from the
IrLG1 dendrimer to the IrLG3 dendrimer, the molar extinction
coefficient of the π–π* transitions of carbazolyl groups increased
gradually, but the MLCT bands of IrLGn (n = 0–3) remained
almost unchanged. From the comparison of the absorption
spectra between IrLG0 and IrLG0′, and between IrLG1 and
IrLG1′, we found that introducing the dendritic branches bearing
peripheral carbazolyl groups at the phenyl ring and at the pyridi-
nyl ring of the cyclometalating ppy ligand has almost the same
effect on the absorption of the final dendritic cyclometalated Ir
(III) complexes (Fig. 3). The results are consistent with other pre-
viously reported metal complex systems with dendritic
attachments.54–56
An emission spectrum of carbazolyl groups in G1 is also
demonstrated in the inset of Fig. 2(a). With a maximum at
360 nm, the emission spans from about 350 to 450 nm; this
range perfectly matches the MLCT absorption band of IrLG0
and the dendritic Ir(III) complexes IrLGn (n = 1–3), suggesting
an efficient energy transfer that could likely occur from the car-
bazolyl groups to the Ir(III) complex core.
protons from the bipy-COOH ligand (IrLG1, IrLG2, and IrLG3
)
and from the additional methyl groups of phenylpyridine (only
for IrLG0) match well with the integration of the proton signals
within the aromatic region. The signals of the methylene protons
of the benzyl ether structure located between 4–5.5 ppm provide
further evidence for the structural confirmation. For instance,
two sets of overlapped signals at 4.8 ppm were observed for
complex IrLG1, which was assigned to the only two kinds of
ether methylene. On the other hand, consistent with its four ether
methylene groups, four sets of signals with an integral ratio of
16 : 8 : 4 : 4 in the range from 4.4 to 5.2 ppm were observed for
IrLG3
.
The absorption spectra, which will be discussed in detail in
the following section, provide extra evidence to support the
structures of IrLG1–IrLG3, which have 4, 8 and 16 peripheral
carbazolyl groups, respectively. Corresponding to the linear
increase of the number of carbazolyl groups, we observed the
same linear increase in the relative absorbance (at e.g., 339 nm)
of the carbazolyl groups (Fig. 2).
The photoluminescence (PL) emission spectra (Fig. 3(a)) of
IrLGn (n = 0–3) obtained in degassed CH2Cl2 solutions show
negligible differences when excited at 380 nm, which falls into
the MLCT absorption band of the Ir(III) complexes. The blue
shift in the emission maximum from IrLG0 to IrLGn (n = 1–3) is
only marginally significant. The same blue shift has also been
observed from the emission spectrum of IrLG0′ to that of
IrLG1′. Similar to the iridium(III) complexes, ruthenium(III) poly-
diimine complexes show also the blue shift when an ether substi-
tution moiety is attached to the ligand ring.57
The relative PL quantum yields (Φ) of the Ir(III) complexes are
almost constant with the increase of the generation of carbazolyl
dendrons except for IrLG3, which possesses a relatively lower
quantum efficiency. It indicates that tethering the benzyl ether
dendrons containing carbazolyl termini to the Ir(III) complex
core does not noticeably affect the PL properties of the Ir(III)
complexes. A similar phenomenon was also observed by Lo
et al.,55 Ding et al.58 and Li et al.45
Photophysical and electrochemical properties
As a common solvent, CH2Cl2 was chosen as the solvent for the
photophysical characterization of the dendritic metal complexes.
The UV/Vis absorption spectra of IrLGn (n = 0–3) and IrLGn′ (n
= 0–1) in CH2Cl2 solutions are depicted in Fig. 2.
The complex IrLG0 presents very broad and overlapped
absorption bands, which are assigned to the typical intra-ligand
charge transfer (ILCT) transition (<350 nm) and the metal-to-
ligand charge transfer (MLCT) transition (>350 nm) of cyclome-
talated Ir(III) complexes. The Ir(III) complex-cored dendrimers
IrLGn (n = 1–3) exhibit very similar absorption spectra in
CH2Cl2 at room temperature. Each of them has clear absorption
maxima at 292, 325 and 339 nm. Compared with the absorption
spectrum of G1, the absorption bands below 350 nm are
obviously originating from the spin allowed π–π* transitions of
carbazolyl groups and overlapped with the ILCT band of the Ir
(III) complex core. In the lower energy region from 350 to
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Fig. 3 Emission spectra of a series of Ir(III) complex-cored compounds measured in degassed CH2Cl2 solutions (10 μM) when excited by UV light
at 380 nm (a) and when excited at 339 nm (b).
However, when the same systems were excited with light at
339 nm, a dramatic change in the emission intensity was
observed as demonstrated in Fig. 3(b). By incorporating carbazo-
lyl groups into the Ir(III) complex, we observed a monotonic
increase of the emission from IrLG0 to IrLG3. Between the data
in Fig. 3(a) and in Fig. 3(b), the distinctive difference lies in the
excitation wavelength. As shown in the absorption spectra
(Fig. 2), the wavelength of 339 nm corresponds to the first
absorption peak of carbazolyl-containing compounds and excit-
ing these compounds with the 339 nm light leads to an emission
of 360 nm from carbazolyl groups. The remarkable emission
enhancement of the IrLGn with the increase of the dendrimer
generation n, and thus the number of carbazolyl groups, is
indicative of the FRET effect. The effect of the excitation wave-
length is also demonstrated in the excitation spectra (Fig. S3,
ESI†), from which we can see the excitation in the window of
about 315–340 nm (corresponding to the carbazole absorption)
becomes progressively more efficient with the increase of the
dendrimer generation. This is another clear indication of the
FRET effect.
In Fig. 3(b), the 360 nm emissions of the carbazolyl groups in
IrLGn are also depicted. IrLG3, which has sixteen carbazolyl
groups, showed a much lower emission than a single dendron
(G1), which contains only two carbazolyl groups, suggesting an
efficient energy transfer. The existence of this peak suggests that
not all the emissive energy was transferred to Ir(III) luminophores.
Carbazole has been chosen as an energy donor to Ir(III) com-
plexes because of their higher singlet and triplet emission ener-
gies and the perfect match to the MLCT absorption;45–48 our
work further proves that the energy transfer from carbazolyl
groups to the Ir(III) complexes is efficient. Increasing the number
of carbazolyl groups at both the C^N ligands can be a plausible
strategy for achieving a meaningful emission enhancement, but
complete energy transfer can hardly be achieved as evidenced by
the residual emission at 360 nm (in Fig. 3(b)).
in this work, and thus the function of the emission intensity vs.
the number of carbazolyl groups or vs. the number of dendrimer
generation is hardly available based on the limited data from
only IrLG0, IrLG1, IrLG2 and IrLG3
.
An ideal FRET should fulfill several key requirements: the
coexistence of donor–acceptor, the spectral match, and a suitable
distance between the donor and acceptor. According to Förster’s
equation, the FRET rate constant correlates with r−6 (in the range
of 10–100 Å) where r represents the distance between the donor
and acceptor.59 According to a simple structural simulation,60 we
have estimated the donor–acceptor distances to be 9.17 Å,
13.38 Å and 27.4 Å in IrLG1, IrLG2 and IrLG3, respectively. We
assumed that the elongation of the donor–acceptor distance in
higher generations of dendritic complexes, such as IrLG3, could
diminish the energy transfer efficiency and result in a smaller PL
enhancement effect. Unfortunately, the limited number of data
points could not enable a conclusion to be drawn.
We have also performed electrochemical characterization by
cyclic voltammetry to acquire the redox properties of these com-
plexes. The cyclic voltammograms of IrLGn (n = 0–3) are dis-
played in Fig. S4, ESI.† All these complexes displayed a one-
electron reduction wave between −1.94 V and −1.84 V with a
good reversibility. Based on the studies on some 2,2′-bipyridine
2+61
metal complexes, for example, Ru(bipy)3
and (C^N )2Ir(dtb-
bipy),62 the reduction wave is associated with a ligand (bipy-
COOH) centered process. But, except for IrLG0, the carbazolyl
functionalized IrLGn (n = 1–3) did not show well-defined oxi-
dation waves with an observable reversibility. For this reason, the
oxidation potentials for these complexes are hard to determine.
The available electrochemical data are summarized in Table 1.
Oxygen quenching/shielding
The photophysical studies discussed above were experimentally
based on the employment of fully deaerated solutions. In air
saturated CH2Cl2 solutions, under excitation at 380 nm, which
does not cause a FRET effect, we have found a large variation of
the emission intensities of IrLGn (n = 1–3) as shown in Fig. 4.
The emission spectra, which almost overlap with each other in
the deaerated CH2Cl2 solutions (Fig. 2(a)), are largely separated
in the air saturated solutions (Fig. 4). We believe that oxygen in
A quantitative description of the emission intensity versus the
number of carbazole units from the light-harvesting antenna
demonstrates that the luminescent intensity was enhanced by
more than 7 times when comparing IrLG3 with IrLG0. However,
due to synthetic difficulties, higher generation dendrimers
bearing larger numbers of carbazolyl groups were not available
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Table 1 Photophysical and electrochemical properties of dendritic Ir(III) complexes IrLGn (n = 0–3)
Absorbance
Emissiona
Redox Potentialsb
Complex
λ (ε, ×104 M−1cm−1
)
λmax (nm)
Φ
E1/2Ox(V)
E1/2red(V)
IrLG0
IrLG1
IrLG2
IrLG3
255(2.6)
566
555
557
558
0.31
0.30
0.30
0.24
0.75d
0.80c
0.72c
0.76c
−1.94d
−1.84d
−1.93d
−1.87d
293(4.9), 324(1.5), 339(1.5)
293(9.9), 325(2.7), 339(2.8)
293(17.0), 324(4.3), 339(4.6)
a Measured in degassed CH2Cl2 solution and calculated by using Ir(ppy)3 (Φ = 0.90 in dichloroethane, a very high quantum efficiency of Ir(ppy)3
reported by Adachi et al. recently32) as standard reference; b Measured in DMF with IrLGn concentration of 0.4 mM; Potential values referenced to
Fc/Fc+ c irreversible wave; d reversible wave.
IrLGn (n = 1–3) is a result of different shielding degrees, which
is related to the complexity of dendritic structure surrounding the
Ir(III) complex core. A quantitative description of the oxygen
quenching or shielding was conducted based on the studies of
the luminophores exposed to the oxygen at different levels.
From the oxygen-free solutions to the oxygen-saturated sol-
utions, the four IrLGn (n = 0–3) exhibited a PL intensity
decrease with factors of 12.04, 11.65, 9.89 and 9.00-fold,
respectively. Detailed in Fig. 5 are the PL emission spectra of
IrLG3 exposed to five different oxygen levels (other complexes
showed similar trends and are reported in Fig. S5, ESI†).
Fig. 4 Emission spectra of a series of Ir(III) complex-cored dendrimers
measured in air saturated CH2Cl2 solutions (10 μM) when excited by
UV light at 380 nm.
the air and the shielding effect of the dendritic scaffolds play
roles in the observed emission features.
It is known that the cyclometalated Ir(III) complexes are sensi-
tive to molecular oxygen and their phosphorescent emission can
be readily quenched.63,64 The mechanism of this quenching
effect involves the long-lived triplet excited state of Ir(III) com-
plexes undergoing efficient energy transfer to the triplet ground
state of molecular oxygen and results in nonradiative relaxation
of the Ir(III) luminophore and the generation of singlet oxygen.
Actually, Fig. 4 not only shows the separation of the PL from
different metal complexes that have almost the same emission
Fig. 5 Emission spectra of 10 μM IrLG3 in DMF recorded under
different oxygen partial pressures (pO ) at room temperature. Excitation
2
wavelength 380 nm, at which no FRET could possibly take place.
properties in deaerated solvent, but discloses a reduction of all
the PL intensities. It is interesting to note that IrLG0 underwent
the biggest change in PL intensity while IrLG3 was least
influenced by molecular oxygen. A plausible assumption is that
the benzyl ether dendrimers with carbazolyl termini act as an
outer shield of oxygen and protect the excited state of the Ir(III)
complex core from being quenched by oxygen.
A linear correlation between I0/I and pO was plotted in Fig. 6
2
according to the Stern–Volmer equation I0/I = 1+KSVpO , from
2
which the quenching constants, KSV, were deducted. They are
10.60, 9.81, 8.48 and 7.30 atm−1 for IrLGn (n = 0–3), respect-
ively. A gradual lowering of KSV indicates reduced sensitivity of
the luminophores to molecular oxygen as the dendritic scaffold
becomes larger.20,69
The oxygen sensitivity of luminophores can be utilized to
develop oxygen sensors for various applications including those
in bio-imaging65 and environmental analysis.66 It is, however, a
drawback when using Ir(III) complexes for some other purposes
in which the emission intensity is correlated to a physical or
chemical quantity of interest.
The shielding effect of dendritic aromatic branches surround-
ing a ruthenium complex core67,68 has been reported. There is
little doubt that the different effects of oxygen on the PL of
Conclusions
A series of bioconjugatable, cationic cyclometalated Ir(III) com-
plexes containing a dendritic carbazole energy donor on the
cyclometalating C^N ligands was successfully synthesized and
thoroughly characterized. Photophysical studies revealed that the
cationic Ir(III) complex core and the carbazolyl terminated
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were measured in degassed CH2Cl2 solutions by using Ir(ppy)3
(Φ = 0.90 in dichloroethane solution32) as a reference compound
at room temperature. Cyclic voltammetry (CV) measurements
were performed using a PARSTAT 2263 Advanced Electroche-
mical System with PowerSUITE software. Each sample was pre-
pared by dissolving 2 μmol of Ir(III) complex in 5 mL anhydrous
DMF with 0.1 M tetrabutylammonium hexafluorophosphate as
the electrolyte and all measurements were conducted at room
temperature. A platinum disk (1 mm in diameter) sealed in a
PTFE rod was used as the working electrode. A piece of plati-
num wire and a piece of silver wire were used as the counter
electrode and the quasi-reference electrode, respectively. The
potentials were calibrated with the ferrocene/ferrocenium Fc/Fc+
redox couple (E1/2 = 0.35 V vs. Ag/AgCl). The potential scan
rate in all the experiments was kept at 100 mV s−1
.
Fig. 6 Stern–Volmer plots for the oxygen quenching of 10 μM of
IrLGn (n = 0–3) in DMF at room temperature.
Oxygen quenching test
All PL measurements were conducted in anhydrous DMF sol-
ution with 10 μM of Ir(III) complexes at room temperature. The
volume concentration of oxygen in the gas mixture was con-
trolled by adjusting the flow rates of the oxygen and the nitrogen
streams flowing into a mixing chamber. The Ir(III) complex sol-
utions were purged with the mixed gas stream from the mixing
chamber for 30 min prior to the PL measurement. The oxygen
quenching effects were evaluated by the Stern–Volmer equation:
dendrimers constructed an efficient donor–acceptor energy trans-
fer system, in which the dendritic carbazole moiety could
efficiently transfer energy via a FRET pathway leading to a
remarkable PL enhancement of the Ir(III) complexes. In addition
to the FRET effect, the dendritic structure, including the branch
and the peripheral carbazolyl groups, form a shield that prevents
the luminophore core from being quenched by molecular oxygen
under the photo excitation. The dual functions, i.e. energy donor
and oxygen shield, of the designed structures suggest further
exploitation of this system, in particular because of the bioconju-
gatability, for applications in biological and biochemical areas.
I0/I = 1+KSVpO , where I0 and I are the luminescence intensities
2
for the oxygen-free solutions and the solutions flushed by gas
flow with different oxygen partial pressure (pO ), respectively.
2
The Stern–Volmer quenching constant, KSV, was obtained from a
linear plot of (I0/I) versus pO .
2
Experimental section
Synthesis and characterization
Materials
2-Phenyl-4-carboxypyridine (2). This compound was pre-
pared according to the literature procedure (Scheme 2).70 4-
methyl-2-phenylpyridine (1) (2.1 g, 12.4 mmol) and SeO2
(10.8 g, 97.3 mmol) were refluxed in pyridine (100 mL) for 24 h
under an argon atmosphere. After the reaction, the mixture was
then filtered through Celite while hot. The solid was obtained
after washing with water (60 mL). The resulting light brown
solid was suspended in a mixture of methanol (70 mL) and
water (50 mL) and made basic by addition of an aqueous NaOH
solution to pH 9–10. After filtration of insoluble materials, the
filtrate was then acidified with concentrated HCl to pH 2–3. The
organic solution of methanol was evaporated and the formed pre-
cipitate was filtered, washed with water and finally dried in
vacuo to afford 2.0 g (yield: 81%) of the titled compound as a
All solvents were of analytical reagent grade and purified accord-
ing to standard procedures. Trans-1,2-diaminocyclohexane was
purchased from Aldrich. Carbazole, N-bromosuccinimide,
IrCl3·3H2O, 2-p-tolylpyridine (5), 1,3-dibromo-5-methylbenzene
and 3,5-dihydroxybenzyl alcohol were commercial products and
used without further purification. The carbazolyl-type dendritic
benzyl bromides G1-Br and G2-Br and G3-Br were prepared
according to the procedures developed by Kwon.46 4-methyl-2-
phenylpyridine (1) and bipy-COOH (4-(4′-methyl-2,2′-bipyridin-
4-yl)butanoic acid) were obtained from SunaTech Inc. (>99%)
and used without further purification.
Measurements
1
slightly brown solid. H NMR (δ (ppm), DMSO-d6, 400 MHz):
13.75 (s, 1H), 8.87 (d, J = 5.2 Hz, 1H), 8.29 (s, 1H), 8.14–8.11
(m, 2H), 7.79 (d, J = 5.2 Hz, 1H), 7.55–7.46 (m, 3H).
1H and 13C NMR spectra were acquired on a VARIAN 400 MHz
magnetic resonance spectrometer. The UV-Vis absorption
spectra and emission spectra were obtained on a Perkin Elmer
Lambda 25 UV-Vis spectrophotometer and a HITACHI F-4600
spectrofluorophotometer, respectively. Mass analysis was carried
out on an Agilent TOF spectrometer (ESI-MS). Matrix assisted
laser desorption/ionization time-of-flight mass spectroscopic
(MALDI-TOF-MS) data were obtained by a Bruker APEX-II
spectrometer at an acceleration voltage of 19 000 V with
α-cyano-4-hydroxycinnamic acid as the matrix. Relative photolu-
minescence quantum efficiencies of the synthesized compounds
Methyl 2-phenyl-4-carboxypyridine (3). This compound was
prepared according to the literature procedure (Scheme 2).67 2-
phenyl-4-carboxypyridine (2) (2.0 g, 10.0 mmol) was refluxed
in methanol (50 mL) and H2SO4 (3 mL) for 12 h. After the reac-
tion, the organic solvent was evaporated and water (100 mL)
was added. The mixture was neutralized with saturated NaHCO3
solution to pH ∼7. The resulting solution was extracted twice
with CH2Cl2 (2 × 100 mL) and dried over anhydrous MgSO4.
2588 | Dalton Trans., 2012, 41, 2582–2591
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General synthetic route for the C^N ligands bearing carbazolyl
at the dendritic peripheries LGn (n = 1–3) and LGn′ (n = 1–2)
The crude compound was purified by silica gel column chrom-
atography eluting with CH2Cl2 to give 3 (2.1 g, 99%) as a color-
less oil. H NMR (δ (ppm), CDCl3, 400 MHz): 8.84 (d, J = 5.2
1
The cyclometalating ligands have the general structures shown
in Scheme 3. LGn (n = 1–3) series have their dendritic moieties
at the pyridine ring and the LGn′ (n = 1–2) series at the phenyl
ring. Each series shares the same starting materials, i.e., (2-phe-
nylpyridin-4-yl)methanol 4 or (4-(pyridin-2-yl)phenyl)methanol
8 respectively.
For n = 1, the corresponding phenylpyridine derivative (4 or
8) (1.0 equiv.) was dissolved in anhydrous THF. After NaH (2
equiv.) was added, the reaction mixture was stirred for 30 min in
an ice-bath. Then a solution of G1-Br (1.5 equiv.) in anhydrous
THF was added. After stirring for 4 h under an argon atmos-
phere, the reaction was quenched by slowly pouring the mixture
into water. After most of the THF was evaporated, the water
phase was extracted with CH2Cl2. The organic layer was dried
over anhydrous sodium sulfate and evaporated in vacuo. The
crude product was obtained and purified by silica gel column
chromatography eluting with CH2Cl2. Yield: 67–78%.
Hz, 1H), 8.30 (s, 1H), 8.07–8.04 (m, 2H), 7.77 (d, J = 5.2 Hz,
1H), 7.52–7.42 (m, 3H), 3.99 (s, 3H).
2-Phenylpyridyl-4-methanol (4). A suspension of compound 3
(0.9 g, 4.7 mmol) and sodium borohydride (1.0 g, 26.4 mmol) in
50 mL of ethanol was refluxed for 12 h. After reaction, the mixture
was cooled to room temperature, and then 80 mL of a saturated
ammonium chloride aqueous solution was added to decompose
the excess sodium borohydride. Most of the ethanol was removed
by rotary evaporation. The resulting solution was extracted three
times with ethyl acetate (3 × 50 mL) and dried over anhydrous
sodium sulfate. After the solvent was removed in vacuo, the crude
product was subsequently purified by silica gel column chromato-
graphy eluting with CH2Cl2/EtOAc (1 : 1, v/v) to give 4 (0.7 g,
1
80%) as a colorless oil. H NMR (δ (ppm), CDCl3, 400 MHz):
8.62 (d, J = 5.2 Hz, 1H), 7.99–7.96 (m, 2H), 7.71 (s, 1H),
7.49–7.39 (m, 3H), 7.20 (d, J = 5.2 Hz, 1H), 4.78 (s, 2H).
For n = 2 and 3, because of the poor solubility of G2-Br and
G3-Br in THF, DMF was used as the reaction solvent. After the
reaction was complete, it was terminated with water. The light
yellow precipitate was filtered over Celite and purified by silica
gel chromatography eluting with CH2Cl2/hexane (from 3 : 1 to
1 : 3, v/v). Yield: 75–85%.
2-(4-(Dibromomethyl)phenyl)pyridine (6). To a mixture of 2-
p-tolylpyridine (5) (5.0 g, 29.5 mmol) and N-bromosuccinimide
(NBS) (11.6 g, 65.0 mmol) in CCl4 (100 mL) was added azobi-
sisbutyronitrile (AIBN) (80.0 mg, 4.8 mmol). After refluxing for
36 h, the reaction mixture was cooled to room temperature and
filtered to remove precipitates. The filtrate was evaporated and
purified by silica gel column chromatography eluting with
CH2Cl2/hexane/EtOAc (10 : 30 : 1, v/v/v) to give 6 (7.3 g, 75%)
as a white powder. 1H NMR (δ (ppm), CDCl3, 400 MHz):
8.71–8.70 (m, 1H), 8.01 (d, J = 8.4 Hz, 2H), 7.79–7.72 (m, 2H),
7.68 (d, J = 8.4 Hz, 2H), 7.28–7.25 (m, 1H), 6.71(s, 1H).
L
G1: (Yield: 78%). 1H NMR (δ (ppm), CDCl3, 400 MHz): 8.67
(d, J = 4.8 Hz, 1H), 8.15 (d, J = 7.6 Hz, 4H), 7.97–7.95 (m, 2H),
7.79–7.74 (m, 4H), 7.54 (d, J = 8.0 Hz, 4H), 7.44–7.40 (m, 7H),
7.33–7.25 (m, 5H), 4.85 (s, 2H), 4.79 (s, 2H). 13C NMR (δ (ppm),
CDCl3, 100.65 MHz): 157.71, 149.64, 147.88, 141.77, 140.48,
139.55, 138.93, 129.11, 128.76, 126.97, 126.17, 124.41, 124.36,
123.63, 120.47, 120.41, 120.32, 118.67, 109.63, 71.92, 71.28.
4-(Pyridin-2-yl)benzaldehyde (7). Compound
6
(4.5 g,
1
LG2: (Yield: 82%). H NMR (δ (ppm), CDCl3, 400 MHz):
13.8 mmol) was transferred to a 250 mL flask and mixed
thoroughly with 20.0 g (0.2 mol) of powdered calcium carbon-
ate. About 100 mL of water was added and the mixture was
heated cautiously and then refluxed for 15 h to enable efficient
hydrolysis. After subsequent cooling and filtration, the filtrate
was extracted with CH2Cl2 and dried over anhydrous Na2SO4.
After evaporation in vacuo, a crude product was obtained and
purified by silica gel column chromatography eluting with
CH2Cl2 to give pure aldehyde 7 (2.0 g, 80%). 1H NMR (δ
(ppm), CDCl3, 400 MHz): 10.09 (s, 1H), 8.75–8.74 (m, 1H),
8.19–8.17 (m, 2H), 8.00–7.98 (m, 2H), 7.82–7.80 (m, 2H),
7.34–7.29 (m, 1H).
8.64–8.63 (m, 1H), 8.12 (d, J = 7.6 Hz, 8H), 7.89 (d, J = 8.0
Hz, 2H), 7.76 (s, 6H), 7.69–7.65 (m, 1H), 7.58 (d, J = 8.0 Hz,
1H), 7.50 (d, J = 8.0 Hz, 8H), 7.42–7.38 (m, 10H), 7.30–7.25
(m, 8H), 7.19–7.16 (m, 1H), 6.74 (d, J = 2.0 Hz, 2H), 6.68 (t, J
= 2.0 Hz, 1H), 5.30 (s, 4H), 4.60 (s, 2H), 4.56 (s, 2H). 13C
NMR (δ (ppm), CDCl3, 100.65 MHz): 159.58, 156.97, 149,58,
141.31, 140.81, 140.40, 139.53, 138.81, 138.74, 138.67, 128.09,
126.91, 126.46, 124.29, 124.18, 123.59, 122.05, 120.42, 120.37,
109.66, 107.00, 101.79, 71.87, 71.78, 69.07.
1
L
G3: (Yield: 85%). H NMR (δ (ppm), CDCl3, 400 MHz):
8.54 (d, J = 5.2 Hz, 1H), 8.07 (d, J = 7.6 Hz, 15H), 7.92–7.9
(m, 2H), 7.71–7.69 (m, 12H),7.58 (s, 1H), 7.45 (d, J = 8.0 Hz,
15H), 7.37–7.33 (m, 19H), 7.25–7.21 (m, 18H), 7.05 (d, J = 4.4
Hz, 1H), 6.71 (d, J = 2.0 Hz, 4H), 6.63 (t, J = 2.0 Hz, 2H), 6.55
(d, J = 2.0 Hz, 2H), 6.51 (d, J = 2.0 Hz, 1H), 5.22 (s, 8H), 4.93
(s, 4H), 4.42 (s, 2H), 4.39 (s, 2H). MALDI-TOF-MS: m/z
(4-(Pyridin-2-yl)phenyl)methanol (8). Sodium borohydride
(0.4 mg, 10.6 mmol) was added to a solution of aldehyde 7
(2.0 g, 10.9 mmol) in 80 mL of methanol. The mixture was
stirred for 30 min in an ice-bath and for 2.5 h at room tempera-
ture. After reaction, 40 mL of saturated ammonium chloride
aqueous solution was added to decompose the excess sodium
borohydride. Most of the methanol was removed by rotary evap-
oration. The resulting solution was extracted with CH2Cl2 and
dried over anhydrous sodium sulfate. After the solvent was
removed in vacuo, the target product was obtained (1.9 g, 92%).
1H NMR (δ (ppm), CDCl3, 400 MHz): 8.69–8.67 (m, 1H),
7.97–7.95 (m, 2H), 7.78–7.70 (m, 2H), 7.47–7.43 (m, 2H),
7.25–7.22 (m, 1H), 4.75 (s, 2H).
2233.0 [M−Cl]+, 2256.0 [M + Na]+.
1
L
G1′: (Yield: 67%). H NMR (δ (ppm), CDCl3, 400 MHz):
8.70–8.68 (m, 1H), 8.15 (d, J = 7.6 Hz, 4H), 8.00–7.98 (m, 2H),
7.77–7.70 (m, 5H), 7.56–7.51 (m, 6H), 7.46–7.42 (m, 4H),
7.33–7.29 (m, 4H), 7.25–7.21 (m, 1H), 4.78 (d, J = 6.0 Hz, 4H).
13C NMR (δ (ppm), CDCl3, 100.65 MHz): 156.98, 149.62,
142.37, 140.47, 139.33, 138.97, 138.48, 136.73, 128.13, 127.05,
126.12, 124.53, 124.11, 123.54, 122.12, 120.47, 120.40, 120.29,
109.70, 72.52, 71.22.
This journal is © The Royal Society of Chemistry 2012
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1
LG2′: (Yield: 75%). H NMR (δ (ppm), CDCl3, 400 MHz):
2.51 (br), 2.47 (s, 3H), 2.25 (t, J = 6.4 Hz, 2H), 2.03–1.96 (m,
2H). 13C NMR (δ (ppm), CDCl3, 100.65 MHz): 177.85, 167.71,
167.52, 159.71, 156.58, 155.81, 154.98, 151.82, 150.81, 150.71,
149.84, 149.40, 149.14, 148.16, 147.87, 143.37, 143.18, 140.58,
140.37, 140.16, 140.14, 139.53, 131.67, 131.59, 130.61, 130.48,
128.34, 126.87, 126.11, 124.63, 124.58, 124.39, 124.20, 123.56,
122.28, 122.23, 121.22, 120.87, 120.42, 120.38, 117.05, 116.92,
109.94, 109.55, 107.38, 107.35, 101.90, 101.87, 73.10, 69.82,
69.11, 36.43, 34.97, 26.57, 21.41. MALDI-TOF-MS: m/z
2742.3 [M]+.
8.67 (d, J = 4.0 Hz, 1H), 8.13 (d, J = 7.6 Hz, 7H), 7.76 (s, 6H),
7.61 (d, J = 8.4 Hz, 1H), 7.51–7.49 (m, 8H), 7.43–7.30 (m,
10H), 7.30–7.25 (m, 11H), 6.74 (d, J = 2.0 Hz, 2H), 6.68 (t, J =
2.0 Hz, 1H), 5.31 (s, 4H), 4.60 (s, 2H), 4.57 (s, 2H).
General synthetic route for IrLGn (n = 0–3) and IrLGn′ (n = 0–2)
IrCl3·3H2O and 2.5 equiv. of cyclometalating ligand (LGn or
L
Gn′) were added into a mixture of 2-methoxyethanol and water
1
(3 : 1, v/v), and the reaction mixture was refluxed for 36 h to
yield the corresponding dichloro-bridged dimer complexes.
After cooling to room temperature, the solution was filtered
through Celite and the yellow precipitate was washed with water,
ether and hexane and dried in vacuo. The obtained dimer and 2.2
equiv. of bipy-COOH were added to CH2Cl2 (CH2Cl2/methanol
for IrLG0 and IrLG0′), and the reaction mixture was refluxed
under an inert atmosphere for 24 h. After cooling to room temp-
erature, the crude product was purified by silica gel column
chromatography with CH2Cl2/methanol (various ratios based on
the mixture properties) eluent to give the pure product. Yield:
84–20%.
IrLG3: (Yield: 20%). H NMR (δ (ppm), CDCl3, 400 MHz):
8.74 (d, J = 3.2 Hz, 2H), 8.02 (d, J = 8.0 Hz, 31H), 7.73–7.17
(m, 129H), 6.96 (d, J = 5.2 Hz, 1H), 6.87–6.50 (m, 26H), 6.91
(m, 2H), 5.17 (s, 16H), 4.89 (s, 8H), 4.46 (s, 4H), 4.40 (s, 4H),
2.85 (t, J = 6.0 Hz, 2H), 2.42 (s, 3H), 2.37 (br), 2.30 (t, J = 6.4
Hz, 2H), 2.98–1.96 (m, 2H). 13C NMR (δ (ppm), CDCl3,
100.65 MHz): 177.23, 167.72, 167.44, 160.00, 159.62, 156.03,
155.69, 155.27, 151.96, 150.92, 149.97, 149.48, 149.11, 148.27,
147.94, 143.55, 143.32, 140.68, 140.39, 139.82, 139.78, 139.53,
139.04, 131.84, 131.67, 130.62, 130.45, 128.76, 128.45, 128.28,
126.88, 126.15, 125.31, 124.70, 124.29, 124.13, 123.59, 123.28,
123.14, 122.27, 121.33, 120.91, 120.41, 120.39, 116.96, 116.77,
113.12, 111.06, 109.60, 106.91, 102.02, 101.47, 73.11, 69.80,
69.73, 69.08, 35.63, 34.93, 26.16, 21.40. MALDI-TOF-MS: m/z
4917.0 [M]+.
1
IrLG0: (Yield: 84%). H NMR (δ (ppm), CDCl3, 400 MHz):
8.85–8.84 (m, 2H), 7.74–7.63 (m, 6H), 7.34–7.32 (m, 2H), 7.23
(d, J = 5.6 Hz, 2H), 7.17 (d, J = 5.6 Hz, 1H), 7.00 (t, J = 7.6 Hz,
2H), 6.90–6.86 (m, 3H), 6.82 (d, J = 6.0 Hz, 1H), 6.32–6.29 (m,
1H), 4.28 (br), 2.94 (t, J = 7.2 Hz, 2H), 2.62 (s, 3H), 2.51 (d, J
= 5.6 Hz, 6H), 2.42–2.33 (m, 2H), 2.10–2.04 (m, 2H). 13C
NMR (δ (ppm), CDCl3, 100.65 MHz): 177.42, 167.30, 167.04,
156.02, 155.81, 155.42, 151.94, 150.96, 150.88, 149.66, 149.58,
149.23, 148.00, 147.71, 143.67, 143.49, 131.86, 131.76, 130.46,
130.33, 128.49, 128.26, 126.32, 125.98, 124.47, 124.42, 124.35,
124.18, 122.26, 122.19, 120.24, 120.07, 35.65, 34.92, 26.32,
21.50, 21.37, 21.34. TOF-MS (ESI): Calcd. for [M−Cl]+
785.2468; found, 785.2461.
1
IrLG0′: (Yield: 83%). H NMR (δ (ppm), CDCl3, 400 MHz):
8.77 (d, J = 7.8 Hz, 2H), 7.86–7.82 (m, 2H), 7.74–7.69 (m, 4H),
7.55 (d, J = 8.0 Hz, 2H), 7.47–7.44 (m, 2H), 7.25–7.23 (m, H),
7.20–7.18 (m, H), 7.02–6.94 (m, 2H), 6.84–6.82 (m, 2H), 6.08
(s, 2H), 3.30 (br), 2.94 (t, J = 7.2 Hz, 2H), 2.60 (s, 3H),
2.35–2.28 (m, 2H), 2.12 (s, 6H), 2.09–2.04 (m, 2H). 13C NMR
(δ (ppm), CDCl3, 100.65 MHz): 177.95, 167.80, 167.68,
156.21, 155.59, 155.25, 151.84, 150.77, 149.33, 148.29, 148.14,
140.80, 140.72, 140.66, 137.74, 137.70, 132.40, 132.36, 128.56,
128.20, 125.92, 125.55, 124.50, 124.46, 123.46, 123.42, 122.63,
122.49, 119.07, 118.96, 35.96, 34.80, 26.45, 21.71, 21.38.
TOF-MS (ESI): Calcd. for [M−Cl]+ 785.2468; found, 785.2475.
1
IrLG1: (Yield: 71%). H NMR (δ (ppm), CDCl3, 400 MHz):
8.74 (s, 1H), 8.64 (s, 1H), 8.15 (d, J = 7.6 Hz, 8H), 7.93 (s, 2H),
7.79 (t, J = 2.0 Hz, 2H), 7.73 (d, J = 2.0 Hz, 4H), 7.68 (d, J =
5.6 Hz, 2H), 7.60–7.57 (m, 2H), 7.54 (d, J = 8.0 Hz, 8H),
7.44–7.40 (m, 10H), 7.33–7.29 (m, 8H), 7.16 (d, J = 5.6 Hz,
1H), 7.07 (d, J = 5.6 Hz, 1H), 7.03 (d, J = 6.0 Hz, 1H), 6.98 (d,
J = 6.0 Hz, 1H), 6.94–6.88 (m, 2H), 6.86–6.80 (m, 2H), 6.29 (t,
J = 1.2 Hz, 1H), 6.27 (t, J = 1.2 Hz, 1H), 4.92–4.81 (m, 8H),
2.90 (t, J = 6.8 Hz, 2H), 2.75 (br), 2.54 (s, 3H), 2.26 (t, J = 6.0
Hz, 2H), 2.07–2.00 (m, 2H). 13C NMR (δ (ppm), CDCl3,
100.65 MHz): 177.81, 167.96, 167.79, 156.69, 155.87, 155.03,
151.88, 150.85, 150.75, 149.51, 149.27, 148.35, 148.07, 143.34,
143.14, 141.40, 141.38, 140.43, 139.61, 131.77, 131.68, 130.76,
130.64, 128.48, 126.19, 124.79, 124.50, 124.31, 123.61, 122.45,
122.39, 121.19, 120.84, 120.50, 120.45, 116.88, 116.73, 109.56,
72.41, 70.51, 36.52, 35.05, 26.64, 21.49. MALDI-TOF-MS: m/z
1657.7 [M−Cl]+.
1
IrLG1′: (Yield: 60%). H NMR (δ (ppm), CDCl3, 400 MHz):
8.96 (s, 1H), 8.79 (s, 1H), 8.18 (d, J = 8.0 Hz, 8H), 7.73–7.72
(m, 2H), 7.60–7.58 (m, 2H), 7.52–7.42 (m, 20H), 7.35–7.31 (m,
10H), 7.07 (d, J = 5.2 Hz, 1H), 6.98 (d, J = 5.6 Hz, 1H),
6.88–6.64 (m, 9H), 6.49 (t, J = 6.8 Hz, 1H), 6.19 (s, 2H),
4.61–4.34 (m, 8H), 2.94 (br), 2.85 (t, J = 6.0 Hz, 2H), 2.55–2.41
(m, 5H), 2.07–2.04 (m, 2H). 13C NMR (δ (ppm), CDCl3,
100.65 MHz): 176.73, 167.09, 166.71, 155.55, 155.41, 155.36,
152.15, 150.84, 150.78, 149.55, 149.09, 148.17, 143.06, 142.84,
142.47, 142.41, 140.53, 140.51, 139.49, 139.19, 139.16, 137.66,
130.21, 129.94, 128.45, 128.16, 126.28, 126.01, 124.62, 124.35,
123.28, 122.80, 121.69, 120.59, 120.55, 120.50, 120.47, 119.12,
118.89, 109.78, 109.73, 72.69, 71.21, 34.84, 30.96, 25.74,
21.43. TOF-MS (ESI): Calcd. for [M−Cl]+ 1657.5619; found,
1657.5631.
1
IrLG2: (Yield: 40%). H NMR (δ (ppm), CDCl3, 400 MHz):
8.71 (s, 1H), 8.61 (s, 1H), 8.09 (d, J = 7.6 Hz, 16H), 7.77 (s,
2H), 7.72–7.70 (m, 12H), 7.59 (d, J = 5.2 Hz, 2H), 7.47 (d, J =
8.4 Hz, 18H), 7.38–7.30 (m, 18H), 7.27–7.23 (m, 17H), 7.05 (d,
J = 5.6 Hz, 1H), 6.95 (d, J = 5.6 Hz, 1H), 6.91 (d, J = 6.0 Hz,
1H), 6.86–6.77 (m, 3H), 6.75–6.69 (m, 8H), 6.18 (d, J = 7.2 Hz,
2H), 5.26 (s, 8H), 4.63–4.62 (m, 8H), 2.85 (t, J = 6.8 Hz, 2H),
Acknowledgements
The financial support of this work from National Science Foun-
dation of China (NSFC) under Project Nos. 20904038 and
21072218 is gratefully acknowledged.
2590 | Dalton Trans., 2012, 41, 2582–2591
This journal is © The Royal Society of Chemistry 2012

View Article Online
35 M. K. Nazeeruddin, R. Humphry-Baker, D. Berner, S. Rivier,
L. Zuppiroli and M. Graetzel, J. Am. Chem. Soc., 2003, 125, 8790.
36 Q. Zhao, S. J. Liu, M. Shi, C. M. Wang, M. X. Yu, L. Li, F. Y. Li, T. Yi
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