Carboranes Tuning the Phosphorescence of Iridium Tetrazolate Complexes

Article abstract of DOI:10.1002/chem.201404743

New iridium tetrazolate complexes containing o-, m-, or p-carboranyl substitution in different positions of a phenylpyridine ligand have been prepared. The carborane isomers and the effect of their substitution position in the tuning of optical properties

Full text of DOI:10.1002/chem.201404743

DOI: 10.1002/chem.201404743
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&
Phosphorescence |Hot Paper|
Carboranes Tuning the Phosphorescence of Iridium Tetrazolate
Complexes
Chao Shi,[a] Deshuang Tu,[a] Qi Yu,[b] Hua Liang,[b] Yahong Liu,[b] Zhihong Li,[a] Hong Yan,*[a]
Qiang Zhao,*[b] and Wei Huang*[b]
Abstract: New iridium tetrazolate complexes containing o-,
m-, or p-carboranyl substitution in different positions of
a phenylpyridine ligand have been prepared. The carborane
isomers and the effect of their substitution position in the
tuning of optical properties have been examined. The neu-
tral complexes with the carboranyl substituent on the
phenyl ring in meta position relative to the metal exhibit
redshifted emission bands in contrast to blueshifts for those
with carboranyl in para position. All cationic complexes dis-
play evidently blueshifted dual-peak emission compared
with the carborane-free complex (c-TZ) with a broad single-
peak emission. Introduction of carborane leads to a blueshift
over 70 nm relative to c-TZ. Carboranes also significantly im-
prove phosphorescence efficiency (FP) and lifetime (t), that
is, FP =0.64 versus 0.21 (c-TZ) and t=880 ns versus 241 ns
(c-TZ). The unique hydrophilic nido-carborane-based IrIII com-
plex nido-o-1 shows the largest phosphorescence efficiency
(abs FP =0.57) among known water-soluble iridium com-
plexes, long emission lifetime (t=4.38 ms), as well as varying
emission efficiency and lifetime with O2 content in aqueous
solution. Therefore, nido-o-1 has been used as an excellent
oxygen-sensitive phosphor for intracellular O2 sensing and
hypoxia imaging.
Introduction
ting diodes (OLEDs). 2-Pyridyltetrazole is generally chosen as
a N^N ligand to construct blue-emitting iridium complexes be-
cause it can both improve luminescence and lead to blueshift-
ed emission of the target complexes.[31] Besides, tetrazoles
have been used as multidentate ligands that allowed to switch
neutral to cationic complexes (Scheme 1), thus significantly
tuning the optical properties of complexes.[35] Moreover, hyp-
oxia-sensitive imaging[37–41] has attracted attention because
hypoxia is a common feature of many diseases. However, most
of the existing hypoxia probes are made from organic fluores-
cence dyes and phosphorescent transition-metal com-
plexes.[37–41] Hydrophobicity of these compounds is still a big
problem. Phosphorescent iridium complexes have been exten-
sively used in chemical sensing[42–47] and living-cell imag-
ing,[48–52] because of the high luminescent efficiency, long phos-
phorescent lifetime, and tunable emission wavelength over the
whole visible range. Therefore, to develop hydrophilic phos-
phorescent probes for detection of intracellular O2 levels is of
great significance for biological studies.
Icosahedral closo-carboranes including 1,2-, 1,7- and 1,12-
C2B10H12 (i.e., o-, m- and p-carborane) possess unusual three-di-
mensional pseudoaromatic geometric structures, which
behave as bulky groups and exhibit unique push–pull electron-
ic properties and thermal stability.[1–5] On the basis of these ad-
vantages, recently, carboranes have been widely incorporated
into organic[6–15] and polymeric[16–23] luminescent materials and
have successfully tuned emission wavelength and lumines-
cence efficiency. However, introduction of carboranes into
phosphorescent transition-metal complexes has been less re-
ported[24–30] and mechanistic issues, such as how carboranes in-
fluence luminescence, have not been well addressed. We need
to clarify the functions of carborane units on optical properties
of metal complexes for further molecular design.
Current development of high-performance blue phosphores-
cent materials[31–36] is urgently required for organic light-emit-
In this work, carboranes were introduced to pyridyltetrazo-
late iridium complexes TZ and c-TZ to synthesize neutral (o-1,
m-1, p-1 and o-2, m-2, p-2) and cationic (c-o-1, c-m-1, c-p-
1 and c-o-2, c-m-2, c-p-2) iridium derivatives containing o-, m-
or p-carboranyl units in 4- or 5-position of phenyl ring of the
cyclometalated C^N ligand (ppy) as shown in Scheme 1. We fo-
cused on revealing the effects of carboranes on the photo-
physical properties of the new Ir complexes through the fol-
lowing aspects: 1) the effect of different carborane isomers;
2) the effect of the carboranyl group at different sites (4 or 5
position) of the C^N ligand; 3) the effect of the carboranyl
[a] C. Shi, D. Tu, Z. Li, Prof. Dr. H. Yan
State Key Laboratory of Coordination Chemistry
Nanjing University, Nanjing 210093 (P. R. China)
E-mail: hyan1965@nju.edu.cn
[b] Q. Yu, H. Liang, Y. Liu, Prof. Dr. Q. Zhao, Prof. Dr. W. Huang
Key Laboratory for Organic Electronics and Information Displays
and Institute of Advanced Materials
Nanjing University of Posts and Telecommunications
Nanjing 210046 (P. R. China)
E-mail: iamqzhao@njupt.edu.cn
iwei-huang@njupt.edu.cn
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201404743.
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tion). The two types of chloro-
bridged dimers [(4-R-ppy)2Ir(m-
Cl)]2 and [(5-R-ppy)2Ir(m-Cl)]2
were prepared as reported by
Nonoyama.[57] Then a series of
neutral carborane-based iridium
tetrazolate complexes o-1, m-1,
p-1 and o-2, m-2, p-2 were ob-
tained by using 2-pyridyltetra-
zole as N^N ligand under mild
conditions (at room temperature
for 6-8 h). Reactions of the neu-
tral complexes with stoichiomet-
ric amounts of methyltriflate led
to the corresponding cationic
complexes c-o-1, c-m-1, c-p-
1
and c-o-2, c-m-2, c-p-2
(Scheme 1 and S1 in the Sup-
porting Information). The two
nido-carborane-functionalized
iridium complexes nido-o-1 and
nido-o-2 were obtained by de-
Scheme 1. The structures of closo-carborane-functionalized neutral and cationic iridium tetrazolate complexes
(top: synthesis of cationic complex c-TZ from the neutral complex TZ through methylation).
group on different structural type (neutral and cationic) of
complexes. Carboranes can remarkably tune photophysical
properties of these complexes. o-Carborane has the strongest
influence, because it has the strongest electron-withdrawing
ability among the three carborane isomers. The emission in c-
o-2 is blueshifted over 70 nm compared with the carborane-
free complex (c-TZ), thus the emission color changes from
yellow to blue. Phosphorescence efficiency (FP) and lifetime (t)
have been improved 3–4 times (FP =0.64 for o-1 relative to
0.21 for TZ and t=880 ns for c-o-1 versus 241 ns for c-TZ). The
carborane substitution site and the structural type of iridium
complexes also have different impact on the photophysical
properties, depending on the degrees of carborane-involved in
excited state, as demonstrated by DFT studies. Also, hydrophil-
ic nido-carborane-functionalized iridium complexes nido-o-
1 and nido-o-2 have been prepared; the former shows the
highest phosphorescence efficiency, with an absolute value of
FP =0.57 in aqueous solution, among all known water-soluble
iridium complexes and long phosphorescence lifetime (t=
4.38 ms).[29,53,54] The photophysical parameters are also quite
sensitive to oxygen in both aqueous solution and in living
cells. Thus, this nido-carborane-functionalized iridium complex
might be promising in intracellular O2 sensing and hypoxia
imaging for biological system.
boronation from the corresponding closo derivatives o-1 and
o-2 in boiling ethanol solution in quantitative yields
(Scheme S2 in the Supporting Information).
All the new iridium complexes were identified by 1H, 13C,
11B NMR spectroscopy, MS, and single-crystal X-ray crystallogra-
phy. As shown in the solid-state structures of o-1 and o-2
(Figure 1), the iridium center adopts a distorted octahedral co-
ordination geometry with cis-metalated carbons and trans-phe-
nylpyridine nitrogen atoms, the same as previously observed
Results and Discussion
Synthesis and structure
Carborane-based cyclometalated ligands (4-R-ppy and 5-R-ppy)
(ppy=phenylpyridine) were synthesized through the Suzuki–
Miyaura cross-coupling reactions,[55,56] by using 2-(6-phenyl-
1,3,6,2-dioxazaborocan-2-yl)pyridine as a boron reagent, from
the corresponding carborane-based precursors (4-R-Br and 5-R-
Br) in yields of 80–85% (Scheme S1 in the Supporting Informa-
Figure 1. The crystal structures of o-1 and o-2.
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in other complexes containing the analogous cyclometalated
C^N ligands.[35] The IrꢀN(N^N) distances (2.095–2.174 ꢁ) are
slightly longer than in IrꢀN(C^N) (2.031–2.078 ꢁ) because of
the trans influence of the metalated carbon atoms (Figure S1
and S2 in the Supporting Information). Note that the two car-
borane cages are in the para position to the iridium atom in o-
2, whereas in the meta position in o-1, which causes different
polarity and electronic effects, as discussed latter by quantum
chemical calculations.
Photophysical properties
The absorption and emission spectra of all complexes were
measured at room temperature in dichloromethane (Figures 2,
Figure 3. The emission spectra of cationic complexes a) c-o-1, c-m-1,
c-p-1 and b) c-o-2, c-m-2, c-p-2, and c-TZ in CH2Cl2 and the corresponding
luminescence photographs.
All the neutral complexes show dual emission peaks and
also reflect the same trend in the excited-state energy
(Figure 2 and Table 1). The emission peaks of o-1 (490 and
522 nm), m-1 (488 and 520 nm), and p-1 (488 and 520 nm) ex-
hibit redshifts of 5–12 nm relative to those of TZ (483 and
510 nm). In contrast, o-2 (476 and 507 nm), m-2 (480 and
510 nm), and p-2 (480 and 510 nm) show blueshifts of 3–7 nm
relative to those of TZ. The o-carborane substitution site mod-
ulates more evident changes in emission wavelength, about
14 nm from o-1 to o-2,thus the emission color tunes from
green to blue-green (Figure 2). Moreover, carborane-substitu-
tion remarkably improves phosphorescence efficiency (FP)
owing to the rigidity of the carborane groups (Table 1), for ex-
ample, FP =0.64 for o-1 versus 0.21 for TZ.
In the cationic complexes, the introduction of carboranes
has little effect on the absorption spectra (Figure S4 in the
Supporting Information). However, the emission spectra are
different. Firstly, all carborane-based cationic complexes display
vibronic dual-peak emission in contrast to the broad single
peak of the control complex c-TZ (Figure 3 and Table 1), dem-
Figure 2. a) The luminescence photographs of all neutral complexes and
b) the corresponding emission spectra in CH2Cl2.
3
3 and Table 1, see also the Supporting Information, Figures S3,
S4, and Table S2). All carborane-based neutral complexes fea-
ture the intense absorption bands in the region of 250–
onstrating that the ligand-centered p!p* state (3LC) is more
involved in the excited state of the carborane-based cationic
complexes than it is in c-TZ. Secondly, all new cationic com-
plexes exhibit an evident blueshift (29-71 nm) compared to c-
TZ (542 nm), for example, c-o-2 (471 and 501 nm) has reached
blueshifts of up to 71 nm. Correspondingly, the emission color
changes from yellow for c-TZ to sky blue for c-o-2. Moreover,
the carborane-substitution at C5 of the ppy ligand (see
Scheme 1) leads to a slightly larger blueshift than that at C4
(Table 1). Thirdly, the carborane-based cationic complexes ex-
hibit obvious blueshifts in comparison with the corresponding
neutral complexes (that is, c-o-1 (481 and 513 nm) vs o-1 (490
and 522 nm)) in spite of a redshift from the neutral model
complex TZ (483, 510 nm) to the cationic model complex c-TZ
(542 nm). Complexes c-o-1, c-m-1, and c-p-1 display identical
1
360 nm, assigned to p!p* transition of the ligand (1LC) and
show approximate redshift of 5 nm relative to the model com-
plex TZ, due to the participation of carboranes in the extend-
ing p-conjugation (see calculated HOMO and LUMO in Fig-
ure S6 in the Supporting Information). The weak bands in the
range of 350–450 nm are assigned to the metal-to-ligand
3
charge-transfer transitions (1MLCT and MLCT; Figure S3 in the
Supporting Information). MLCT bands for o-1, m-1 and p-1 ex-
hibit the obvious redshift in contrast to a blueshift for o-2, m-2
and p-2 (both referred to those in TZ). This indicates that the
position of carborane substitution has influence on the
ground- or excited-state energy.
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emission wavelengths at 481 and 513 nm, but different relative
intensities of the two peaks (Figure 3a), however, complexes c-
o-2, c-m-2, and c-p-2 show different emission wavelengths and
intensities of the two peaks (Figure 3b). Furthermore, the
peaks at about 510 nm of all cationic carborane-based com-
plexes are broad in the order c-o-1<c-m-1<c-p-1 and c-o-2<
c-m-2<c-p-2, and the latter series of complexes show broader
peaks. Therefore, the results demonstrate that carboranes can
evidently tune emission spectra of pyridyltetrazolate iridium
complexes in position, shape, phosphorescence efficiency, and
lifetime (Table 1). Carborane isomers, carborane substitution
sites, and structural types of complexes also have different
impact on photophysical properties.
Table 1. Selected photophysical and electrochemical data of the IrIII
complexes.
[a]
ox
onset
PL[a]
[nm]
Fp
t[a]
E
Eg[b]
[eV]
HOMO/LUMO
[eV][b]
[ns]
[eV]
o-1
m-1
p-1
o-2
m-2
p-2
490, 522
488, 520
488, 520
476, 507
480, 510
480, 510
483, 510
481, 513
481, 513
481, 513
471, 501
475, 507
476, 509
542
0.64
0.47
0.43
0.38
0.36
0.31
0.21
0.32
0.16
0.15
0.25
0.22
0.21
0.19
747
556
479
360
338
309
207
880
410
328
530
510
495
241
0.91
0.84
0.83
0.90
0.83
0.82
0.68
1.08
1.02
1.01
1.07
1.00
0.99
0.86
2.66
2.68
2.68
2.71
2.70
2.70
2.69
2.69
2.68
2.68
2.71
2.70
2.70
2.62
ꢀ5.71/ꢀ2.99
ꢀ5.64/ꢀ2.96
ꢀ5.63/ꢀ2.95
ꢀ5.70/ꢀ2.99
ꢀ5.63/ꢀ2.93
ꢀ5.62/ꢀ2.92
ꢀ5.48/ꢀ2.79
ꢀ5.88/ꢀ3.19
ꢀ5.82/ꢀ3.14
ꢀ5.81/ꢀ3.13
ꢀ5.87/ꢀ3.16
ꢀ5.80/ꢀ3.10
ꢀ5.79/ꢀ3.09
ꢀ5.66/ꢀ3.04
TZ
c-o-1
c-m-1
c-p-1
c-o-2
c-m-2
c-p-2
c-TZ
The electrochemical properties of all complexes were exam-
ined by cyclic voltammetry in acetonitrile solutions (Figure 4
[a] in CH2Cl2. [b] HOMO(eV)=ꢀe(Eoonxset +4.8), Eg=1240/l, LUMO(eV)=
Eg+HOMO.
introduction of a hydrophilic nido-o-carboranyl unit to prepare
the water-soluble organometallic complexes nido-o-1 and nido-
o-2 (Figure 5a and Scheme S2 in the Supporting Information).
The 1H NMR spectra show the characteristic broad BꢀHꢀB reso-
nance of a nido-o-carboranyl unit. The 11B NMR show less over-
lapped signals in a wider range, typical for a nido-o-carborane
cage. The ESI-MS clearly demonstrate two peaks of [MꢀK+]ꢀ
and [Mꢀ2K+]2ꢀ. Both complexes display intense absorption
Figure 4. Cyclic voltammograms of a) neutral and b) cationic iridium
tetrazolate complexes. Scan rate=100 mVs<M->1
.
and Table 1). All complexes have reversible oxidation waves
with potentials in the range of 0.6–1.3 V. The oxidation poten-
tials of the cationic complexes show a positive shift of about
0.17 V in comparison with the corresponding neutral com-
plexes, demonstrating that the latter are more easily oxidized.
The values of oxidation potentials follow the order: o-1>m-
1>p-1>TZ, o-2>m-2>p-2>TZ, c-o-1>c-m-1>c-p-1>c-TZ,
and c-o-2>c-m-2>c-p-2>c-TZ, consistent with the order of
electron-withdrawing ability of the carborane isomers. The re-
sults reveal that carboranes can increase oxidation potentials
and reduce the energy level of HOMO (Table 1), as found in
our previous reports.[28,29] Carborane substitution in different
positions does not affect the oxidation potential (i.e., Eonset
0.91 V for o-1 and Eonset =0.90 V for o-2).
In attempt to explore potential applications of carborane-
functionalized iridium complexes for biological systems, we
modified the hydrophobic iridium tetrazolate complexes TZ by
=
Figure 5. a) The structures of nido-carborane-functionalized iridium
tetrazolate complexes and b) the absorption and PL spectra in aqueous
solution (1.0ꢂ10ꢀ5 m). The inset shows the corresponding luminescence
photographs.
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bands below 400 nm, assigned to 1p!p* transitions of the
ligand (1LC), but nido-o-1 shows one additional peak at
270 nm. The weak bands in the range of 350–450 nm are likely
attributed to metal-to-ligand charge-transfer transitions
m-2>p-2 or c-o-2>c-m-2>c-p-2, indicating that the different
carboranes and the substitution position cause evident differ-
ence in dipole moment of complexes led by the polarity com-
petition between tetrazolate and carboranyl units. The data for
the orbital analysis for all complexes are summarized in
Figure 7, and in the Supporting Information in Figures S6, S7
(1MLCT, LLCT and MLCT, LLCT; Figure 5b). Unexpectedly, the
luminescent properties of both complexes are evidently differ-
ent. Complex nido-o-1 shows a vibronic dual-peak emission
(489 and 520 nm) in contrast to a broader single-peak emission
at 520 nm for nido-o-2 (Figure 5b and Table S3 in the Support-
1
3
3
3
ing Information), demonstrating that the LC state is more in-
volved in excited-state in nido-o-1 than in nido-o-2, as con-
firmed below by DFT calculations.
To our delight, nido-o-1 exhibits exceptionally high phos-
phorescence efficiency with an absolute value of FP =0.57, the
highest reported for water-soluble iridium complexes[29,53,54]
and long phosphorescence lifetime (t=4.38 ms) in aqueous so-
lution in sharp contrast to the very weak emission (FP =0.08)
and shorter lifetime (t=0.40 ms) for nido-o-2 (Table S3 in the
Supporting Information). This indicates that the nido-carborane
substitution site has quite an impact on the emission. On the
other hand, nido-o-1 is highly sensitive to dioxygen in aqueous
solutions (Figure 6). The emission intensity (I) and lifetime dra-
Figure 7. The optimized geometries of o-1, o-2, c-o-1, and c-o-2 at the
lowest triplet excited state (T1).
and Tables S5–S7. The lowest energy absorption of all com-
plexes is mainly characterized by a HOMO!LUMO transition.
The HOMO is located on the iridium center and the phenyl
ring of the C^N ligand, but LUMO distribution depends on the
complex types. In neutral complexes the LUMO is mainly lo-
cated on the pyridyl and phenyl rings of the C^N ligand,
whereas in the cationic complexes, the LUMO distributes on
the whole N^N ligand (2-pyridyltetrazole). HOMO and LUMO
energy levels descend with the incorporation of carboranes,
and follow the order: o-1<m-1<p-1<TZ, c-o-1<c-m-1<c-p-
1<c-TZ, and o-2<m-2<p-2<TZ, c-o-2<c-m-2<c-p-2<c-TZ,
which is in agreement with the tendency revealed by experi-
mental data.
Figure 6. The change of emission spectra of nido-o-1 (1ꢂ10ꢀ5 m) in aqueous
solution with different dioxygen concentrations.
matically reduces as the oxygen content increases, owing to
energy transfer from the triplet excited state of nido-o-1 to the
triplet ground state of dioxygen. For instance, emission intensi-
ty and phosphorescence lifetime are decreased from I=
135822, t=2.83 ms (2.0% O2) to I=8437, t=1.18 ms (21% O2).
The neutral and cationic complexes with carborane substitu-
tion in different positions show interesting optimized geome-
tries of the lowest triplet excited state (T1). Taking o-carborane
derivatives as examples (Figure 7), the T1 of both o-1 and o-2
are dominated by the HOMO!LUMO transition described as
Theoretical calculations
To elucidate the photophysical properties of all complexes dis-
cussed above, quantum chemical calculations were performed
to gain insight into the structure–property relationships, in-
cluding the functions of carboranes. As shown in Table S4 and
Figure S5 in the Supporting Information, the calculated dipole
moments of the ground states of all the complexes decrease
as follows: o-1>p-1>m-1 or c-o-1>c-p-1>c-m-1, and o-2>
3
3
3MLCT, ILCT, and LC. Interestingly, the o-carborane cage par-
tially participates in the LUMO (T1), but is not involved in the
HOMO for o-1. In o-2, however, the carborane unit is not in-
volved in the LUMO (T1), but partially participates in the HOMO
(T1). Thus the energy gap in o-2 is larger than that in o-1, ex-
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plaining well why o-2 shows an evident blueshifted emission
compared with o-1. In the case of cationic complexes, the T1
state of c-o-1 is dominated by HOMO!LUMO transitions with
the character of 3MLCT and 3LLCT, but by HOMO!LUMO+
1 transitions in c-o-2 with the same character. The HOMO
energy level (T1) of c-o-2 is stabilized by a contribution from o-
carborane, in contrast, no such contribution is found for c-o-1;
thus the emission is more blueshifted in c-o-2 than in c-o-1.
Similar trends were also observed in m- and p-carborane-based
complexes (Table S7 in the Supporting Information).
HepG2 liver cancer cells was evaluated by the MTT assay. After
treatment of the cancer cells with different concentrations of
nido-o-1 for 48 h, the cellular viabilities were estimated to be
more than 80% even at 100 mm, and no cytotoxicity was ob-
served at 10 mm (Figure S8 in the Supporting Information),
which is the concentration that was used for imaging experi-
ments in this study.
We then used nido-o-1 as a probe to detect intracellular
oxygen levels in HepG2 cells, which were cultured with 21% or
2.5% O2 concentration for 24 h at 378C. Complex nido-o-
1 (10 mm) was added to the medium and the cells were incu-
bated for two hours after washing with phosphate-buffered
saline (PBS) in a 21% or 2.5% O2 atmosphere. The excitation
wavelength was 405 nm. Figure 9A shows the emission
In addition, the T1 state of nido-o-1 originates from
HOMOꢀ1!LUMO+1 (0.58) and HOMOꢀ1!LUMO (0.15) tran-
3
3
sitions with the respective character of LLCT and LC (Table S8
in the Supporting Information). The HOMOꢀ1 is located on
the C^N ligand, including the whole nido-carborane unit,
whereas the LUMO only includes parts of the carborane unit.
LUMO+1 resides on the whole N^N ligand (2-pyridyltetrazole;
Figure 8). Complex nido-o-2, however, shows different T1
Figure 9. A) Phosphorescent images (a,d), bright-field images (b,e), overlay
images (c,f) and B) phosphorescence lifetime images (a, b) of HepG2 live
cells incubated with nido-o-1 (10 mm) in PBS at 378C with different O2
content (21 and 2.5%) and excitation at 405 nm.
Figure 8. The optimized geometries of nido-o-1 and nido-o-2 at the lowest
images of the living cells observed in the wavelength range
450–550 nm. We found that nido-o-1 was efficiently taken up
by HepG2 cells. From overlays of bright-field images and con-
focal luminescence images, the luminescence was localized in
the cytoplasm at the nucleus and the membrane (Figure 9A
(c,f)), indicating that nido-o-1 is internalized into the cells. The
image is brighter for cells exposed to 2.5% O2 than for those
exposed to 21% O2, demonstrating that nido-o-1 may be used
to indicate intracellular oxygen concentrations. Additionally,
considering the long phosphorescence lifetime, which is sensi-
tive to dioxygen, lifetime-imaging experiments with both 21%
and 2.5% O2 concentrations were also conducted. As a result,
high-quality long emission lifetime signals were observed (Fig-
ure 9B (a,b)) and the lifetime was significantly increased from
260 ns (21% O2) to 380 ns (2.5% O2) along with a visually dis-
tinct color change from green to yellow. Thus nido-o-1 might
be a potential agent for hypoxia imaging by using both emis-
sion intensity and lifetime as sensing signals.
triplet excited state (T1).
states, which arises from the transitions: HOMO!LUMO+
1 (0.37) and HOMOꢀ1!LUMO+1 (0.35) with one character of
3LLCT (Table S8 in the Supporting Information). Both HOMO
and HOMOꢀ1 reside on the whole C^N ligand and LUMO+
1 is located on the whole N^N ligand (2-pyridyltetrazole;
3
Figure 8). It demonstrates that the LC state is more involved
in the excited state in nido-o-1 than in nido-o-2, consistent
with the individual spectra.
Bioimaging
Because of the unique properties, such as excellent water solu-
bility, high phosphorescence efficiency (FP =0.57), long phos-
phorescence lifetime (t=4.38 ms), and emission sensitivity to
dioxygen in aqueous solution, nido-o-1 is promising applica-
tions in hypoxia imaging. Firstly, the cytotoxicity towards
Chem. Eur. J. 2014, 20, 16550 – 16557
www.chemeurj.org
16555
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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Quantum chemical calculations: The geometries of all iridium(III)
complexes were optimized with the DFT method. The electronic
transition energies, including electron correlation effects, were
computed by the time-dependent (TD)-DFT method using the
B3LYP functional (TD-B3LYP). The LANL2DZ basis set was used to
treat the iridium atom, whilst the 6–31G(d) basis set was used to
treat all other atoms. All calculations described here were per-
formed by using the Gaussian 09 program.[58]
Conclusion
Carboranes have been successfully introduced into phosphor-
escent iridium tetrazolate complexes; this efficiently tuned the
photophysical properties of this type of complexes. The photo-
physical properties depend on structural variations of the car-
boranes, the substitution site of the carboranes, and the struc-
ture of the iridium complexes. The maximum emission wave-
length can be tuned by 71 nm with the emission color chang-
ing from yellow to blue. The phosphorescence efficiency and
lifetime can be improved approximately 3–4 times. Incorpora-
tion of a hydrophilic nido-carborane leads to the water-soluble
organometallic phosphorescent iridium complex nido-o-1,
which has excellent phosphorescence efficiency (FP =0.57),
long phosphorescence lifetime (t=4.38 ms), variable phosphor-
escence efficiency and lifetime with O2 content in aqueous so-
lution, and low cytotoxicity. Thus, nido-o-1 was used for O2
sensing and bioimaging in living cells. These results are helpful
for further design of desired carborane-functionalized, phos-
phorescent, metal complexes for optoelectronic and biological
applications.
Confocal luminescence imaging and phosphorescence lifetime
imaging (PLIM): Confocal luminescence imaging was carried out
on an Olympus IX81 laser scanning confocal microscope equipped
with a 40 immersion objective lens. A semiconductor laser served
as the excitation source of the HepG2 liver cancer cells incubated
with nido-o-1 at 405 nm. The one-photon emission was collected
at 450–550 nm for the HepG2 liver cancer cells incubated with
nido-o-1. Complex nido-o-1 was added to RPMI 1640 to yield
a 10 mm solution. The HepG2 liver cancer cells were incubated with
the solution of nido-o-1 with 20% or 2.5% O2 for 2 h at 378C.
The FLIM image setup was integrated with an Olympus IX81 laser
scanning confocal microscope. The fluorescence signal was detect-
ed by the system of the confocal microscope and correlative calcu-
lation of the data was performed with professional software, which
was provided by PicoQuant. The light from the pulse diode laser
head (PicoQuant, PDL 800-D), with an excitation wavelength of
405 nm and a frequency of 0.5 MHz, was focused onto the sample
with a 40x/NA 0.95 objective lens for single-photon excitation. The
emitted fluorescence signal was collected at 450–550 nm.
Experimental Section
General: All air- and moisture-sensitive reactions were carried out
under an argon atmosphere. Dry 1,2-dimethoxyethane and pyri-
dine were obtained by refluxing and distilling over CaH2 under ni-
trogen. Dry THF was distilled from sodium/benzophenone. nBuLi
(2.4m in hexanes, Amethyst) was used as supplied. [(4-R-ppy)2Ir(m-
Cl)]2 (R=o-, m- or p-carboranyl)[28] were synthesized according to
Acknowledgements
Financial support from the National Natural Science Founda-
tion of China (21271102), the Natural Science Foundation of
Jiangsu Province of China (BK20130038 and BK20130054), the
Major State Basic Research Development Program of China
(2013CB922100) and the High-Performance Computational
Centre of Nanjing University is acknowledged.
1
literature procedures. The H, 13C and 11B NMR spectra were mea-
sured at room temperature with a Bruker DRX-500 or a Bruker
DRX-600 spectrometer. Mass spectra were measured with a Micro-
mass GC-TOF for EI-MS (70 eV) and an ESI-MS (LCQ Fleet, Thermo
Fisher Scientific). Infrared spectra were determined by a Nicolet
NEXUS870 FTIR spectrometer. Electrochemical measurements were
performed with an IM6ex instrument (Zahner). Phosphorescence
spectral measurements were carried out by using a Hitachi F-4600
fluorescence spectrophotometer. Electronic absorption spectra
were recorded with Shimadzu UV-2550 spectrophotometer. Phos-
phorescence lifetimes were determined by an Edinburgh instru-
ment laser impulse fluorimeter with picosecond time resolution. El-
emental analyses for C, H, and N were performed on a Vario
MICRO elemental analyzer.
Keywords: bioimaging
phosphorescence
·
carboranes
·
iridium
·
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Received: August 5, 2014
Published online on October 28, 2014
Chem. Eur. J. 2014, 20, 16550 – 16557
www.chemeurj.org
16557
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim