Coordination mode-induced isomeric cyclometalated [Ir(tpy)(nbi)Cl](PF6) complexes: Distinct luminescence, self-assembly and cellular imaging behaviors

Article abstract of DOI:10.1039/c7dt03523c

Two isomeric Ir(iii) complexes Ir-O and Ir-R arising from the different coordination mode of a naphthalene-containing ligand, show distinct luminescence, self-assembly ability and cellular imaging behaviors.

Full text of DOI:10.1039/c7dt03523c

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COMMUNICATION
Coordination
mode-induced
isomeric
cyclometalated
[Ir(tpy)(nbi)Cl](PF6) complexes: distinct luminescence, self-assembly
and cellular imaging behaviors
†
Tai-Bao Gao,a§ Run-Qi Yan,b§ Alexander J. Metherell,c Deng-Ke Cao,a* De-Ju Yeb* and Michael
5
D. Wardc,d*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
Two isomeric Ir(III) complexes Ir-O and Ir-R arising from
the different coordination mode of a naphthalene-containing
10 ligand, show distinct luminescence, self-assembly ability and
cellular imaging behaviors.
imaging of mitochondrial dynamics in living cells (Scheme S2).7c
They found that the C^N ligands ppy, pq and pbt with increasing
π-electron conjugation could make the complexes emit at 551,
50 578 and 626 nm, respectively. So far, all reported
[Ir(N^N^N)(C^N)X]+ complexes have only incorporated
benzene-based C^N ligands, such as ppy, pq and pbt (Scheme S2),
which use their cyclometalating carbon atoms from benzene rings
Cyclometalated Ir(III) complexes possess many advantageous
characteristics, such as good physicochemical stability, high
photoluminescence quantum yields and facile color tuning,1
15 which make them useful for a range of applications such as
chemosensors,2 phosphorescent emitters in organic light-emitting
diodes (OLEDs),3 cellular imaging,4 information storage and
security protection,5 and catalysis.6 Various Ir(III) complexes with
different functional properties have been synthesized through
20 incorporating one, two or three bidentate C^N cyclometalating
ligands, thus forming [Ir(N^N^N)(C^N)X]+,7-9 Ir(C^N)2(L^X),10
to coordinate with Ir(III) ions. There is still
55 [Ir(N^N^N)(C^N)X]+ complexes incorporating naphthalene-
based C^N ligands. From the viewpoint of structure,
a lack of
a
naphthalene-based C^N ligand could provide more
cyclometalating carbon atoms (e.g., C2 and C8 atoms in nbiH,
Scheme 1) with respect to a benzene-based C^N ligand (e.g. 2-
60 phenyl-pyridine). Moreover, the naphthalene-based C^N ligand
could provide more π-electron conjugation, thus resulting in a
cyclometalated Ir(III) complex with lower energy emission.12
11
or Ir(C^N)3 complexes (where N^N^N = terpyridine (tpy) and
its derivatives; C^N = cyclometalating ligands; X = anionic
monodentate ligands; L^X = neutral/anionic bidentate ancillary
25 ligands).
Among
these,
complexes
of
the
type
[Ir(N^N^N)(C^N)X]+ are of particular interest as the
incorporation of tridentate N^N^N ligands (e.g. tpy) can
dramatically enhance their chemical stability.7a Moreover, their
luminescence and photo-catalyzing properties can be easily tuned
30 by using different C^N ligands [e.g., 2-phenyl-pyridine (ppyH)
and its derivatives] and/or anionic monodentate ligands X (e.g.,
Cl- and CN-),7 which facilitates their applications in photo-
catalysis and cellular imaging.7b,c,8 For example, Sato et al. first
reported complex [Ir(tpy)(ppy)Cl](PF6) (λem = 541 nm in
35 CH3CN), which served as an efficient photo-catalyst for
CO2 reduction with a quantum yield of 0.21.8a Inspired by this
work, Bernhard’s group later reported [Ir(tpy)(ppy)(CN)](PF6),7a
which showed a blue-shifted emission at 496 nm, indicating that
different ligands X (Cl- vs. CN-) can result in distinct emission
40 wavelengths. They further found that the emission wavelengths
and the photo-catalyzing ability of the [Ir(ttbutpy)(C^N)Cl](PF6)
complexes [Scheme S1, C^N = a series of fluorinated 5-methyl-
2-phenylpyridine (mppy-)] could be precisely optimized via using
differently fluorinated mppy- ligands. 7b Very recently, Long and
45 Chao et al. synthesized three complexes [Ir(ttpy)(ppy)Cl](PF6),
[Ir(ttpy)(pq)Cl](PF6) and [Ir(ttpy)(pbt)Cl](PF6) for fluorescence
Scheme 1 Syntheses of complexes Ir-O and Ir-R. Reaction
65 conditions: (a) Ir(tpy)Cl3, EtOEtOH, 190 or 135 oC, 24 hours, Ar;
(b) KPF6.
In this paper, we reported a cyclometalating ligand 2-
naphthalen-1-yl-1H-benzoimidazole (nbiH, Scheme 1), in which
besides the imidazole nitrogen atom, the naphthalene unit is
70 expected to coordinate with an Ir(III) ion through either of its
carbon atoms C2 or C8. Thus, the ligand nbi- could chelate an
Ir(III) ion in either a five- or six-membered ring chelating mode
(Schemes 1 and S3). Based on nbiH coordinating in these two
modes, we have obtained two isomeric [Ir(tpy)(nbi)Cl](PF6)
75 complexes, namely Ir-O and Ir-R. To the best of our knowledge,
these are the first examples of a matched pair of isomeric
[Ir(N^N^N)(C^N)X]+ complexes. Herein, we discuss their
syntheses, and the significant differences in luminescence, self-
assembly and cellular imaging behaviors arising from their
80 different structures.
As shown in Scheme 1, complexes Ir-O and Ir-R were
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synthesized together through the reaction of nbiH and Ir(tpy)Cl3 50 naphthalene unit in Ir-O are at the same plane (Fig. S11 top). In
at 190 ºC or 135 ºC for 24 hours, followed by the anion exchange
of Cl- with PF6- (see ESI for synthesis details). Their mixture was
purified by silica column chromatography, sequentially obtaining
Ir-O and Ir-R in 24% and 43% yields, respectively, for the
contrast, the naphthalene unit in Ir-R shows a significant twist
(Fig. S11 bottom), with a 14.2(1)o dihedral angle between the
plane defined by atoms C5-C7 and the plane defined by C1-C4
and C9 and C10 atoms. In addition, the dihedral angle between
5
reaction temperature of 190 oC, and in 19% and 47% yields, 55 benzoimidazole unit and benzene ring containing C1 atom (Fig.
o
respectively, for 135 C. This indicates that reaction temperature
can influence the yields of two complexes. However, it is not
clear why the yield of Ir-R is always higher that of Ir-O,
S11) in Ir-R is 29.0(1)o, which is significantly larger than that in
Ir-O [10.2(1)o]. Five-membered chelate ring, like that seen for
ligand nbi- in Ir-O, are often observed in Ir(III) complexes
incorporating naphthalene-based C^N ligands (Scheme S4).16-21
10 although the molecular structure of Ir-R is more hindered or
strained than that of Ir-O. Their chemical structures were 60 However, the six-membered chelate ring seen in Ir-R is quite
1
characterized by H NMR, IR, ESI-MS and elemental analysis
(see ESI), revealing the formation of isomeric complexes
[Ir(tpy)(nbi)Cl](PF6). To further clarify the structures of Ir-O and
15 Ir-R, the single crystals of Ir-O∙Et2O∙CH3COCH3 and Ir-R were
unusual, and has not been reported so far for a naphthalene-based
C^N ligand.
Complexes Ir-O and Ir-R possess dramatically different
supramolecular interactions. First, the [Ir(tpy)(nbi)Cl]+ cation and
-
grown from an acetone/diethyl ether and CHCl3/CH3OH 65 its counter anion PF6 in Ir-O show a hydrogen-bonding
respectively, and measured by X-ray crystallography. Their
crystallographic data are shown in Table S2, and the selected
bond distances and bond angles are listed in Tables S3 and S4.
interaction between them [N2-H∙∙∙F6 = 2.897(1) Å]22 (Fig. 1a),
while no such hydrogen-bonding interaction occurs in Ir-R (Fig.
1b). Second, both Ir-O and Ir-R reveal supramolecular chain
structure (Figs. 1c and 1d). Neighboring [Ir(tpy)(nbi)Cl]+ cations
70 in this chain are associated through π⋅⋅⋅π stacking interactions
between two pyridine rings of tpy ligands from adjacent
[Ir(tpy)(nbi)Cl]+ units [plane–plane distance: 3.37 Å]23 in Ir-O,
whereas there a weak hydrogen bond [N2-H∙∙∙Cla = 3.206(1) Å,
symmetry code a = -x + 3/2, y, z + 1/2]22 in Ir-R.
20
75
Fig. 1 Molecular structures of Ir-O (a) and Ir-R (b), and
supramolecular chains in Ir-O (c) and Ir-R (d) (yellow and
magenta dot lines showing hydrogen bonds and π⋅⋅⋅π stacking
25 interactions, respectively).
Fig. 2 (a) UV-vis absorption spectra and (b) luminescence spectra
of Ir-O, Ir-R, nbiH and tpy in CH3CN (λex = 398 nm). Inset: the
photographs under room light or 365 nm light.
As shown in Fig. 1a, the Ir(III) ion in Ir-O shows a distorted
octahedral coordination geometry. Four of the six coordination
sites are occupied by one Cl- ion, and three nitrogen atoms (N3,
30 N4 and N5) from the tpy ligand. The remaining two coordination
sites are filled with atoms C2 and N1 from the ligand nbi-.
Around the Ir(III) ion, atom C2 is at the trans position of Cl- (Fig.
1a). The molecular structure of Ir-R is clearly different from that
of Ir-O, in which an Ir(III) ion is coordinated by atom C8 instead
35 of atom C2 from the ligand nbi- (Fig. 1b). The Ir-Cl bonds in both
Ir-O [2.447(4) Å] and Ir-R [2.4638(13) Å] are longer than those
in Ir(tpy)Cl3 [2.347-2.370 Å],13 due to the strong σ donating
ability of the Ir-C2/C8 bond.14 The Ir-N4 bonds in both Ir-O
[1.959(12) Å] and Ir-R [1.960(4) Å] are significantly shorter than
40 the other Ir-N bonds [2.047(12)-2.063(4) Å], which is attributed
to geometric constraints arising from the tridentate binding of
tpy.15
Complexes Ir-O and Ir-R are isomeric owing to the different
coordination modes of their nbi- ligands. As shown in Figs. 1a
45 and 1b, the nbi- ligand in Ir-O chelates an Ir(III) ion through a
five-membered ring chelating mode, while a six-membered
chelate ring occurs in Ir-R. This difference in coordination mode
results in a significantly different extent of twist within the nbi-
ligand between Ir-O and Ir-R. All ten C atoms belonging to a
80
Ir-O and Ir-R have different optical spectroscopic properties.
Their UV-vis spectra in CH3CN at room temperature are in Fig.
2a and Table S5. The high-energy absorption bands in Ir-O
appear at 269, 314 and 325 nm, while at 280 and 316 nm in Ir-R.
85 These absorption bands can be assigned to spin-allowed ligand-
centered (1LC) transitions (nbi- and tpy ligands), which are
comparable to the π→π* absorption bands of nbiH (312 nm) and
tpy (279 nm). The low-energy broad absorption band of Ir-O
(maximum at 384 nm) reveals a slight red shift compared to that
90 of Ir-R (maximum at 376 nm), probably due to the fact that the
nbi- ligand in Ir-O less twisted and therefore has better π-
conjugation (Fig. S11) than that in Ir-R. These low-energy
absorption bands in two complexes are mainly attributed to spin-
allowed metal-to-ligand charge transfer (1MLCT) because the
95 isolated ligands nbiH and tpy have no obvious absorption around
380 nm. The weaker absorption tails towards 570 for Ir-O and Ir-
R correspond to 3MLCT absorption.9a,24 The UV-vis spectra of Ir-
O and Ir-R were also measured in phosphate buffer solution
(PBS, pH = 7.4, 10% DMSO) (Fig. S14, Table S5), showing an
100 increase in MLCT absorption intensity at ~450 nm probably due
2
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to the increasing solvent polarity from CH3CN to PBS.
Complexes Ir-O and Ir-R show different luminescence 60 and Ir-R could cause distinct intermolecular interactions, thus
that different coordination modes of ligands nbi- between Ir-O
properties in CH3CN at room temperature (Fig. 2b, Table S5).
Compared to Ir-O with a strong emission at 581 nm, complex Ir-
R displays a weaker and red-shifted emission at 612 nm (Fig. 2b).
This could be due to the fact that the six-membered chelate ring
resulting in different photophysical properties in solution.
To evaluate the properties of Ir-O and Ir-R in cellular
imaging, we first tested their cytotoxicity against human cervical
cancer HeLa cells using 3-(4,5-dimethylthiazol-2-yl)-2,5-
5
in Ir-R, and the resultant substantial twist of the nbi- ligand might 65 diphenyltetrazolium bromide (MTT) assay. No adverse effect
not only destabilize the highest occupied molecular orbitals
(HOMOs), but also promote additional non-radiative processes.
10 The luminescence quantum yields (Φ) and lifetimes (τ) of Ir-O in
towards HeLa cells was observed when incubated with 20 μM Ir-
O or Ir-R for 24 hours (Fig. S22), demonstrating that both
complexes had good biocompatibility for the cellular imaging
applications. We then incubated HeLa cells with either 20 μM Ir-
air equilibrated CH3CN were found to be ~2.1%, and 165 ns
(93%) and 296 ns (7%), which are both significantly larger 70 O or Ir-R for different times (0, 1, 3, and 6 h), and the
compared to Ir-R [Φ = 0.56% and τ = 86 ns (91%) and 169 ns
(9%)]. Such dual-exponential decay for Ir-O and Ir-R could be
15 attributed to molecular aggregation in solution through π⋅⋅⋅π
stacking interactions and/or hydrogen interactions (see
luminescence images were acquired using a Confocal laser
scanning microscopy (CLSM). Our experiment results showed
that both Ir-O and Ir-R could enter into HeLa cells and generate
time-dependent enhanced intracellular luminescence (Fig. S23).
experiments below).25 The broad and unstructured emissions of 75 After incubation for 3 h, the luminescence in HeLa cells stained
Ir-O and Ir-R indicate that their emissive excited states have
predominantly 3MLCT character. This was confirmed by their
20 luminescence spectra measured at 77 K,10b exhibiting clearly
blue-shifted emissions at 552, 599 and 655 nm for Ir-O, and 581
with Ir-O was brighter than that with Ir-R, presumably owing to
the higher luminescence quantum yield of Ir-O compared to Ir-R
(Table S5). In addition, the enhanced lipophilicity might also
enhance its uptake into HeLa cells, contributing to the higher
and 633 nm for Ir-R (Fig. S15, Table S5). The luminescence 80 intracellular luminescence in cells relative to Ir-R. These results
spectra of Ir-O and Ir-R were further measured in PBS (pH = 7.4,
10% DMSO) at room temperature (Fig. S16). The two complexes
25 exhibited broad emission bands, 580 nm for Ir-O and 610 nm for
Ir-R, which are comparable to their emission bands in CH3CN
suggested that complex Ir-O is more appropriate for cellular
imaging studies compared to Ir-R. To further elucidate the
intracellular locations of Ir-O, HeLa cells were incubated with
Ir-O (20 μM) for 3 hours, and then co-stained with lysosomal
(581 nad 612 nm, respectively), indicating that the solvents 85 tracker (Lyso Tracker Green DND-26). As shown in Fig. 3, HeLa
(CH3CN and PBS) only slightly influence their excited state
energies. Moreover, both Ir-O and Ir-R in CH3CN showed
30 acid/base-induced luminescence switch between on state and off
state. Upon addition of tetrabutylammonium hydroxide (TBAOH)
cells incubated with Ir-O showed bright intracellular
luminescence (pseudo red color), which overlaid very well with
the green fluorescence from the lyso tracker, implying that Ir-O
localized mainly in the lysosomes of HeLa cells. These results
(Figs. S17 and S18), both complexes revealed gradual decrease in 90 indicated that Ir-O had a high capacity to accumulate and stain
luminescence intensity due to the deprotonation of the imidazole
units in their nbi- ligands (i.e. the formation of neutral species).
35 Both Ir-O and Ir-R recovered their original luminescent states
after adding some TFA to neutralize the involved TBAOH (Figs.
S19 and S20).
the lysosomes of live cells, which was also revealed in certain
cyclometalated Ir(III) complexes.26
We subsequently compared the self-assembly ability of Ir-O
and Ir-R in PBS buffer. Dynamic light scattering (DLS) analysis
40 showed that Ir-O (20 µM) could easily self-assemble into mono-
dispersed nanoparticles, with a mean hydrodynamic diameter of ~
100 nm (Fig. S21). This was also confirmed by transmission
electron microscopy (TEM) analysis, showing spherical
nanoparticles with an average size of ~90 nm (Fig. S21). In
45 contrast, there were no nanoparticles observed for Ir-R under the
same conditions. This difference in aggregation properties in
solution can be correlated with the differences in crystal packing
between Ir-O and Ir-R. Complex Ir-O possesses strong
intermolecular π⋅⋅⋅π stacking interactions as revealed by its
50 crystal structure (Fig. 1c), in which the molecules are orderly
packed into superstructures, facilitating the self-assembling into
nanoparticles. However, Ir-R possesses a more twisted structure
and lacks intermolecular π⋅⋅⋅π stacking interactions in the
crystalline state (Fig. 1d), and shows no evidence for formation of
55 nanoparticles in solution. As such, Ir-R can be easily dissolved in
PBS buffer to form a clear solution. This was also reflected in the
much smaller oil-water partition coefficient (logP, -0.717) of Ir-
R compared to Ir-O (-0.220) (Table S6). These results revealed
Fig. 3 Confocal laser scanning microscopy images of Hela cells
95 stained with Ir-O (20 μM, λex = 405 nm, pseudo red color) (left),
and Lyso Tracker Green DND-26 (1 μM, λex = 514 nm, pseudo
green color). Scale bar: 25 μm.
In conclusion, two isomeric [Ir(tpy)(nbi)Cl](PF6) complexes,
100 Ir-O and Ir-R, have been synthesized using the ambidentate
naphthalene-based C^N ligand nbiH (Scheme 1). Their crystal
structures indicate that an {Ir(tpy)Cl}2+ unit is chelated by a nbi-
ligand using a carbon atom from naphthalene unit and an
imidazole nitrogen atom (atoms C2 and N1 in Ir-O, and atoms
105 C8 and N1 in Ir-R, Fig. 1a and 1b). Thus, complexes Ir-O and
Ir-R are isomeric, which contain five- and six-membered chelate
rings, respectively. The nbi- ligand in Ir-R shows significant
torsional distortion, which is confirmed by the non-coplanarity of
all ten C atoms belonging to its naphthalene unit, and the large
110 dihedral angle between benzoimidazole unit and naphthalene unit.
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DOI: 10.1039/C7DT03P52a3gCe 4 of 5
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70
5
80
10 work, we will explore the possibility of using this approach to
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This work was supported by the National Natural Science
15 Foundation (NSF) of China (21505070 and 21632008), the NSF
of Jiangsu Province, China (BK 20141314).
85
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Two isomeric Ir(III) complexes Ir-O and Ir-R arising from the different coordination mode of a naphthalene-containing ligand, show
distinct luminescence, self-assembly ability and cellular imaging behaviors.
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