Stable luminescent iridium(III) complexes with bis(N-heterocyclic carbene) ligands: Photo-stability, excited state properties, visible-light-driven radical cyclization and CO2 reduction, and cellular imaging

Article abstract of DOI:10.1039/c5sc04458h

A new class of cyclometalated Ir(iii) complexes supported by various bidentate C-deprotonated (C^N) and cis-chelating bis(N-heterocyclic carbene) (bis-NHC) ligands has been synthesized. These complexes display strong emission in deaerated solutions at roo

Full text of DOI:10.1039/c5sc04458h

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Stable luminescent iridium(III) complexes with bis(N-
heterocyclic carbene) ligands: photo-stability, excited
state properties, visible-light-driven radical
Cite this: DOI: 10.1039/x0xx00000x
cyclization and CO2 reduction, and cellular imaging†
Received 00th January 2012,
Accepted 00th January 2012
Chen Yang,ab Faisal Mehmood,a Tse-Lung Lam,c Sharon Lai-Fung Chan*c, Yuan
Wu,a Chi-Shun Yeung,a Xiangguo Guan,a Kai Li,ab Clive Yik-Sham Chung,a
Cong-Ying Zhou,ab Taotao Zou,a and Chi-Ming Che*ab
DOI: 10.1039/x0xx00000x
www.rsc.org/
A new class of cyclometalated Ir(III) complexes supported by various bidentate C-
deprotonated (C^N) and cis-chelating bis(N-heterocyclic carbene) (bis-NHC) ligands has been
synthesized. These complexes display strong emission in deaerated solutions at room
temperature with photoluminescence quantum yields up to 89% and emission lifetimes up to
96 μs. A photo-stable complex containing C-deprotonated fluorenyl-substituted C^N shows
no significant decomposition even upon irradiation for over 120 h by blue LEDs (12 w). These,
together with the strong absorptions in the visible region and rich photoredox properties, allow
the bis-NHC Ir(III) complexes to act as good photo-catalysts for reductive C
–C bond
formation from C(sp3/sp2)
–
Br bonds cleavage using visible-light irradiation (λ > 440 nm). A
water-soluble complex, with glucose-functionalized bis-NHC ligand, catalyzed visible-light-
driven radical cyclization for synthesis of pyrrolidine in aqueous media. Also, the bis-NHC
Ir(III) complex in combination with cobalt catalyst can catalyze visible-light-driven CO2
reduction, with excellent turnover number (> 2400) as well as selectivity (CO over H2 in gas
phase: > 95%) can be achieved. Additionally, this series of bis-NHC Ir(III) complexes are
found to localize in and stain endoplasmic reticulum (ER) of various cell lines with high
selectivity, and exhibit high cytotoxicity towards cancer cells, revealing their potential uses
as bioimaging and/or anti-cancer agents.
by triplet metal-to-ligand-charge-transfer (3MLCT) excited state
of [Ru(bpy)3]2+ generated upon visible-light irradiation.
Introduction
Luminescent organometallic complexes of 3rd row transition
Subsequently, Stephenson and co-workers unfolded reductive
dehalogenation of activated alkyl halides catalysed by
[Ru(bpy)3]2+,44 and reductive dehalogenation of alkyl, alkenyl
and arly iodides by using fac-Ir(ppy)345 as photo-redox catalyst.
Compared to [Ru(bpy)3]2+ and fac-Ir(ppy)3, the application of
luminescent platinum(II) on photocatalysis is still nascent.
Recently, our group and Wu’s group demonstrated that pincer
Pt(II) complexes are capable of catalysing light induced C-C
bond formation.46, 47
The important features that endow luminescent transition
metal complexes to act as useful photo-redox catalysts or photo-
sensitizers for light induced reactions include: 1) the long
lifetime of their electronic states, thus allowing bimolecular
reaction to proceed in solutions, 2) their electronic excited states
are both a strong reducing and oxidizing reagent with reduction
potentials systematically varied by the auxilliary ligands.41 For
the photo-catalysis to have practical interests, the design of new
metals, such as Ir,1-5 Pt,6-11 Au12-19 are currently receiving
burgeoning interest due to their profound applications in
materials science,20-22 biology,23,
organic synthesis.25-27 In
24
particular, the favourable emission properties of cyclometalated
Ir(III) complexes have been harnessed for applications by many
research groups with examples such as the development of high
performance OLEDs by Thompson and co-workers 3-5, 28-31 and
the design of bioimaging and cellular probes by Lo and co-
workers.32-36 Over the past decade, as a result of the extensive
works most notably by MacMillan,37,
Stephenson and co-workers,
Yoon,27,
and
38
39-41
42-45
luminescent cyclometalated
iridium(III) complexes are widely used in photo-redox catalysts,
which has been shown to have useful applications in organic
synthesis.
In 2008, Yoon and co-workers40 reported [2+2] enone
cycloadditions, as well as MacMillan and co-workers38
published alkylation of aldehydes, both of which were catalyzed
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highly stable photo-redox catalysts having very long-lived
electronic excited states in solution is desirable.
Chart 1. Iridium(III) complexes in this work. Complexes 1a 73,74 73 1d28 and 2d31 have been reported in literature.
1b,
,
Our endeavour to develop transition metal photo-catalysis is the resultant photo-active transition metal complexes such as
to use visible light for activation of small molecules such as CO2 their solubility in various solvents including water. NHC ligands
described in this work and for C-X bond functionalization. A functionalized with carboxylate, sulfonate, amine/ammonium,
major consideration is to ultilize the visible-light, which falls alcohol motif have been reported for the development of water-
within the solar spectrum and avoids deterious high-energy UV- soluble transition metal catalysts for Suzuki coupling,
49
initialized photochemical side reaction(s).48,
In literature, hydrosilylation, hydrogenation, olefin metathesis and CO2
platinum group metal complexes and semi-conductors are reduction.65, 71
usually used as a photo-redox catalyst for photochemical
Compared with Ir(III) complexes supported by the bidentate
reduction of CO2.50 Earlier examples of transition metal photo- acetylacetonate28 and/or 2,2’-bypyridine1 ligands, the
catalyst used for photochemical reduction of CO2 include cobalt photophysical and application studies of the related bis-NHC
porphyrins,51 Re(bpy)(CO)3Cl52, 53 and Ir(terpy)(ppy)Cl.54 More Ir(III) complexes are relatively scarce. In 2010, cationic bis-
recently, system comprising of fac-Ir(ppy)3 in conjunction with NHC Ir(III) complexes were reported by De Cola and co-
[Ni(Prbimiq1)]2+,55 Fe(porphrin),56 [Co(TPA)Cl]Cl,57 (TPA = workers72 to have application in blue-light emitting
tris(2-pyridylmethyl)amine) and [Co(N5)]2+ (N5 = 2,13- electrochemical cells; subsequently, a number of bis-NHC Ir(III)
dimethyl-3,6,9,12,18-pentaazabicyclo-[12.3.1]octadeca-
1(18),2,12,14,16-pentaene)58 were reported for photochemical studies.65, 67, 72-74 In this work, a series of strongly luminescent
reduction of CO2. Ir(III) complexes (Chart 1) containing bis-NHC ligands and
complexes were reported for photophysical and biological
The stability of photo-catalysts is an important issue for visible light absorbing C-deprotonated (C^N) ligands was
practical application of transition metal photochemistry. synthesized and their photophysical and electrochemical
Numerous studies revealed that the photochemically active properties were examined. These complexes display high photo-
excited states of [Ru(bpy)3]2+,59-61 [Ir(ppy)2(bpy)]+,60, 62 and fac- stability and are strongly emissive with long lifetimes of up to 96
55, 57, 63
Ir(ppy)3
are not stable under light irradiation for a long μs in solutions at room temperature. The water-soluble
period of time as a result of dissociation of coordinated ligand(s) luminescent Ir(III) complexes, containing the glucose-
presumably via low lying d-d excited state(s). To address the functionalized NHC ligand, were found to be active photo-
photo-stability issue, we are attracted to the use of N- catalysts for radical cyclization leading to the formation of 5-
heterocyclic carbene (NHC) ligands which have been receiving membered ring pyrrole in aqueous media with high substrate
burgeoning attention in coordination chemistry due to their conversions and yields. One of the photo-stable Ir(III) complexes
strong σ-donor strength to develop robust metal photo-sensitizer was utilized as a photo-sensitizer and in conjuction with a
and photo-catalysts.64-70 Also, the N-substituent of NHC ligands recently reported catalyst [Co(TPA)Cl]Cl to convert CO2 into
can be used to tune both the physical and chemical properties of CO with a turnover number (TON) > 2400, selectivity in gases
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1
phase > 95% and yield of 5.6% (1 mL out of 18 mL of CO2 was photolysis, if any, was monitored by H NMR spectroscopy. As
converting into CO at 5 , less than 5% of the
M concentration of Co complex). Some depicted in Fig. 1a, in the case of 4Me
of the complexes were also demonstrated as potential bioimaging complex was observed to undergo photochemical decomposition
μ
b
and/or anti-cancer agents.
even after irradiation of 120 h, revealing its outstanding photo-
stability. Under the same conditions, Ru(bpy)3Cl2, fac-Ir(ppy)3
and [(dFCF3ppy)2Ir(dtbbpy)]PF6 (dFCF3ppy = 3,5-difluoro-2-[5-
(trifluoromethyl)-2-pyridinyl]phenyl; dtbbpy = 4,4’-bis(1,1-
dimethylethyl)-2,2’-bipyridine)) were found to decompose after
Results
Synthesis, characterization and photo-stability of
[(C^N)2Ir(NHC)]X complexes
1
10 h irradiation as revealed by the changes of their H NMR
spectra (Fig. 1c).
Interestingly, irradiation of 6Hb using blue LEDs for 10 h led
to a clean and quantitative conversion to a new species which did
not show further changes upon subsequent irradiation for another
90 h (Fig. 1b, this species is noted as cis-6Hb). The yield of scale
synthesis of cis-6Hb from the irradiation of degassed MeCN
solution of 6Hb (100mg in 4 mL MeCN) was 94%. cis-6Hb was
found to be stable upon standing in solution under dark for
The structures of 18 bis-NHC Ir(III) complexes synthesized in
this work (1c and 2a ), together with previously reported
Ir(III) complexes 1a 73,74 1b 73 1d28 and 2d31, are depicted in
Chart 1. Complexes 1–9 were synthesized in good yields by
,
2b
,
3
−
9
,
,
refluxing [(C^N)2Ir(μ-Cl)]2 with bis(imidazolium) salts in the
presence of silver(I) oxide in 2-methoxylethanol. Details of
synthesis and characterization data of ligands and complexes are
1
provided in the ESI
†
. Notably, H NMR spectra of 7b and 7c
another 40 h, or exposed to air for another 20 h (Fig. S2, ESI†).
To verify that the transformation of 6Hb to cis-6Hb was caused
show poorly resolved peaks in the aromatic region (6.7
at ambient temperture, and hence 1H NMR spectra are recorded
from 238 K to 300 K (Fig. S1, ESI ) in order to assure the purity
of 7b and 7c
Complexes 1a b–9b with N-methyl or N-
4Hex
–8.2 ppm)
by visible-light irradiation, a negative control experiment was
conducted by keeping 6Hb in deuterated MeCN in dark (Fig. S2,
†
1
.
ESI
†
), and no structural changes was detected by H NMR
–
4Me
a
,
1b
–
4Me
b
,
spectroscopy.
UV-vis absorption, emission and excitation spectra of cis-6Hb
measured in DCM solution at 298 K are different from those of
6Hb with hypersochromic shifts in peak maxima (Fig. 1e). The
emission band for cis-6Hb displays vibronic spacings of 1245
cm-1 while that of 6Hb shows spacings of 908 cm-1. Taken into
butyl substituent on bis-NHC ligands are soluble in most
common aprotic solvents, but not in protic solvents e.g. methanol
(MeOH) or water. Complexes 1c, c, , 7c and 9c with
4Me 6Hc
glucose functionalized bis-NHC ligand are soluble in MeOH,
ethanol (EtOH) and water.
consideration of ESI-MS and spectroscopic data of cis-6Hb
, cis-
The photostability of these Ir(III) complexes with bis-NHC
ligands was examined by using 4Me and 6Hb as representative
b
6Hb is likely a structural isomer for 6Hb. The exact structure of
cis-6Hb has been determined by X-ray crystallography.
examples. 4Me
b
and 6Hb in degassed deuterated MeCN were
irradiated using blue light (12 w blue LEDs) for 5 days. The
a)
b)
c)
6Hb
cis-6Hb
d)
e)
Photoinduced
465 nm
Ir
C
N
S
N trans to N
N cis to N
Fig. 1. 1H NMR spectra for a) 4Me ; b) 6Hb; c) Ru(bpy)3Cl2, fac-Ir(ppy)3 and [(dFCF3ppy)2Ir(dtbbpy)]PF6 in deuterated MeCN solution for irradiating by blue light (12 W,
b
λmax = 462 nm)46 (all solutions are degased by nitrogen gas for 10 min); d) crystal structure diagrams showing the photoinduced transformation of coordination for Ir
metals in 6Hb and cis-6Hb (butyl group on bis-NHC ligand and hydrogen atoms are omitted for clarity); e) UV/Vis absorption (dotted line), excitation (dashed line) and
emission (solid line) spectra of solutions of 6Hb and cis-6Hb (concentration of 2.0
×
10-5 M) in degassed DCM at 298 K.
slow diffusion of diethyl ether into DCM (6Hb), MeCN (4Me
b
and 7b) and chloroform (cis-6Hb) solution of these complexes,
respectively. Perspective views of 6Hb and cis-6Hb are shown in
X-ray crystallography
b ,
Crystals of 4Me (with PF6- counter anion), 6Hb cis-6Hb and 7b
suitable for X-ray crystallographic analysis were obtained by
Fig. 1d (those of 4Me
b, 7b are shown Fig. S3 in ESI†). Selected
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bond lengths and angles are compiled in Table S1 (ESI
†
). literature values of Ir-CNHC bonds trans to phenyl groups.72 The
Similar to the reported examples,72 6Hb adopts a distorted CNHC-Ir-CNHC bite angles of 4Me (84.7(3) o), 6Hb (85.10(9)o), 7b
b
octahedral geometry (Fig. 1), the iridium atom is coordinated by (85.58(6)o) and that of reported [(dfppy)2Ir(NHCMe)]PF6
two cyclometalated C^N ligands and one bis-NHC ligand. The (85.28(15)o) (dfppy
4,6-difluoro-phenylpyridine) are
and 7b are in line with that
=
two C^N ligands adopt a mutually eclipsed configuration, with consistent.72 The structures of 4Me
b
the two N atoms (N1 and N2) trans to each other and with Ir-N of 6Hb. Interestingly, for cis-6Hb the N atoms from the two C^N
bond length of 2.067(2) and 2.071(2) Å. The two substituted ligand are cis to each other (Fig. 1d), indicating that 6Hb and cis-
phenyl rings are oriented cis to each other with Ir-C bond length 6Hb are structural isomers. The crystallographic refinement
of 2.054(2) and 2.077(2) Å. The CNHC atoms of bis-NHC ligand parameters for 4Me
b, ,
6Hb cis-6Hb and 7b are summarized in
are trans to two C atoms from C^N ligands. The Ir-CNHC Table S2 (ESI
†).
distances of 2.105(2) Å and 2.128(2) Å are comparable to the
Table 1. Photophysical data of complexes 1–9.
Complex
Mediuma
Absorption
Emission
Φ/%d
λmax/nm) (10-3ε/M-1cm-1)
λem /nm (

/μs)
1a
1b
1c
2a
3a
CH2Cl2
CH2Cl2
H2O
255 (26.8), 267 (24.9), 311 (10.6), 342 (6.1), 380 (3.7), 416 (1.2)
254 (36.8), 266 (35.5), 311 (15.5), 342 (9.1), 381 (5.82), 416 (2.16)
254 (36.8), 266 (35.5), 311 (15.5), 342 (9.1), 381 (5.82), 416 (2.16)
252 (13.5), 290 (21.7), 332 (12.6), 369 (6.07), 405 (4.92)
470 (2.1), 499, 534
470 (2.1), 500, 534
469 (2.0), 498, 534
530, 547, 570 (3.03)
472, 674 (4.8), 824
524, 564 (28.6), 614
525, 566 (28.7), 614
527, 568 (32.7), 616
576 (96.1), 625, 683
531, 560(6.4)
89
89
89
11.1
0.2
65
75
82
22.6
78
66
68
36
32
3
CH2Cl2
CH2Cl2
252 (15.5), 282 (15.1), 313 (20.5), 335 (23.1), 375 (21.9), 389 (20.9), 440 (20.9)
260(31.4), 297 (30.0), 316 (33.5), 334 (32.3), 360 (22.4), 378 (16.8), 421 (10.5)
267(52.2), 297 (46.8), 316 (50.9), 335 (49.0), 358 (36.3), 376 (27.3), 421 (16.2)
268(42.1), 299 (42.3), 320 (50.4), 338 (49.2), 364 (32.6), 379 (25.6), 421 (14.6)
260 (54.1), 273 (42.5), 310 (32.9), 323 (34.8), 377 (67.2), 398 (97.0), 436 (71.6)
271 (23.0), 321 (25.0), 377 (9.57), 407 (6.81), 436 (3.38)
b
4Me
4Me
a
b
CH2Cl2
b
CH2Cl2
b
4Hex
5b
b
CH2Cl2
c
CH2Cl2
6Hb
6Hc
6Fb
7b
CH2Cl2
H2O
CH2Cl2
CH2Cl2
H2O
CH2Cl2
CH2Cl2
H2O
268 (21.4), 321 (22.1), 377 (8.06), 407 (5.49), 436 (2.42)
270 (23.0), 323 (32.1), 383 (10.7), 415 (7.88), 447 (4.08)
296 (26.8), 333 (18.5), 388 (9.31), 437 (7.76), 467 (4.60)
293 (25.9), 330 (17.4), 386 (9.63), 432 (6.92), 464 (3.71)
536, 560 (5.0)
530, 558(3.2)
582, 623 (6.2)
583, 622 (5.9)
618, 659(5.2)
617, 667(7.4)
620 (4.5), 668
7c
8b
9b
9c
260 (80.1), 290 (48.5), 315 (35.4), 376 (34.5), 429 (7.52), 485 (3.47), 494 (2.88)
313 (24.2), 334 (23.8), 384 (12.7), 428 (10.7), 457 (8.20), 485 (3.47)
315 (18.4), 380 (9.69), 420 (7.6), 454 (4.95), 475 (2.15)
13
9
a Measured in degassed CH2Cl2 and water (2.0×10-5 M) at 298 K. b 4Mea, 4Meb and 4Hexb were measured at the concentration of 5×10-6 M. c 5b was measured
at the concentration of 1×10-6 M. d Phosphorescence quantum yields were measured by using [Ru(bpy)3](PF6)2 (Ф = 0.062 in MeCN) as standard.
electron-withdrawing group of CF3 on the cyclometalated ligand
Electronic absorption and emission spectroscopy
which lowers the C^N based LUMO level.
Photophysical data of 1–9 including their UV/Vis absorption and
4x104
3x104
2x104
emission maxima, emission lifetimes and photo-luminescent
quantum yields are tabulated in Table 1. All the complexes show
intense high-energy absorptions ranging from 250 to 380 nm and
a)
1a
2a
3a
1d
2d
lower-energy absorptions at 416
– 494 nm (Fig.2, 3 and S4 in
1
1x104
0
ESI ). The high-energy absorptions are attributed to the π
†
→π*

transition of the ligands, whereas the low energy absorption
bands should be ascribed to the admixture of metal-to-ligand
charge transfer (MLCT) transitions and ligand-ligand charge
transfer (LLCT) transitions.72 The charge transfer (CT)
assignment is further supported by the energy trend observed in
the complexes with different C^N ligands. For example, the low-
energy absorptions of 8b and 5b reveal significant red-shifts
300
400
600
Wavelength (nm)500
b)
1.0
0.8
0.6
0.4
0.2
0.0
from those of 1b and 4Me
by the extended π-conjugation of the C^N ligand in 8b and 5b
b respectively. This can be rationalized
,
400
500
600
700
800
900
leading to the lowering of the π*(C^N) orbitals and hence
decreases in CT energy. These red-shifts in the UV-vis
absorptions, together with the large molar absorptivities in the
Wavelength (nm)
Fig. 2. UV-Vis absorption (top) and emission (bottom) spectra of solutions of 1a
3a and 1d 2d in degassed DCM (concentration of 2.0
10-5 M) at 298 K.
–
–
×
visible region (e.g., 5b: 7.2
×
104 M-1cm-1), allow the complexes
to show strong absorptions in the visible region. This is believed
to be crucial for harvesting visible-light in solar-spectrum as well
as avoiding UV-initiated side reactions due to the use of UV in
the photo-catalysis by the Ir(III) complexes. As revealed by the
On the other hand, the N-alkyl substituents on bis-NHC ligand
are found to have only minor effect, if any, on the UV/Vis
absorption spectra of the Ir(III) complexes, as revealed by the
UV absorption spectra of 6Fb and 6Hb (Fig. S4b, ESI
†), a
overlaid spectra of the group of 1a, 1b, 1c (Fig. S4a, ESI†).
bathochromic shift about 500 cm-1 are observed for the lower-
energy absorption band(s). These could be attributed to the
Complex 1a absorb weakly at the wavelength from 430 nm to
500 nm with molar absorptivity less than 500 M-1cm-1 in contrast
to the high values of 2500 M-1cm-1 for 1d having same C^N
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luminophore, respectively. The calculated wavelength for
ground stated HOMO LUMO (S0 S1 transition) is 369 nm
and 414 nm for 1a and 1d, respectively (Fig. S4e S4f, ESI ).
These calculated values are in reasonable agreement with the
corresponding experimental absorption λmax values 410 and 460
nm respectively.
planar C^N ligands, leading to reduced triplet-triplet annihilation
and a higher photoluminescence quantum yield can be achieved.
In addition, 5b shows a significantly longer emission lifetime
→
→
–
†
(96.1 μs) than 4Me
b (28.7 μs). This might be owing to the reduced
metal character in the electronic excited state of 5b, and hence
slower triplet radiative decay. Similarly, with a smaller parentage
of metal character in the frontier molecular orbitals, 6Hb (6.4 μs)
shows a longer emission lifetime than 6Fb (3.2 μs). These long-
lived triplet excited states allow the Ir(III) NHC complexes to
undergo a variety of photochemical reactions, notably for
visible-light-driven photo-catalytic reductive CBr bond
cleavage and CO2 reduction which will be illustrated later.
a)
1.2
6.0x104
4.0x104
2.0x104
UV of 4Me
b
Ex of 4Meb (em 524 nm)
Em of 4Meb (ex 420 nm)
1.0
0.8
0.6
0.4
0.2
0.0

300
400
500
600
700
800
900
Wavelength (nm)
b)
Electrochemistry
1.0x105
8.0x104
6.0x104
4.0x104
2.0x104
1.0
0.8
0.6
0.4
0.2
0.0
UV of 5b
Ex of 5b (em 576 nm)
Em of 5b (ex 437 nm)
Table 2. Electrochemical dataa of bis-NHC Ir(III) complexes.
Complex
Epab/V
0.87
0.64
0.74
0.79
0.74
0.72
0.98
1.04
0.69
0.62
0.62
E
pcc/V

1b
2a
-2.52
-2.24
-2.41
-2.40
-2.44
-2.25
-2.14
-1.94
-2.09
-1.94
-2.16
4Me
4Me
a
b
300
400
500
600
700
800
900
Wavelength (nm)
Fig. 3 UV/Vis absorption (black dashed line), excitation (red solid line) and
emission (green solid line) spectra of solutions of a) 4Me 10-6 M); b) 5b (1.0
(5.0
10-6 M) in degassed DCM at 298 K.
4Hex
5b
b
b
×
6Hb
6Fb
7b
×
Complexes 1–9 display strong phosphorescence in deaerated
8b
9b
solution at room temperature. (Fig. 2, 3 and S4 (ESI†); Table 1)
All the complexes show vibronic-structured emissions, and their
emission lifetimes are found to be in microsecond regime. For
b exhibits structured emission bands with
vibrational spacings of ~ 1380 cm-1 and long emission lifetime
a Supporting electrolyte: 0.1 M nBu4NPF6 in MeCN and values are
recorded vs Ag/AgNO3 (0.1 M) in MeCN; Cp2Fe+/0 occurs at the
range of 0.05–
0.08 (V) vs Ag/AgNO3. b Values refer to oxidation
example, 4Me
peak potential (Epa) at 25 oC for irreversible couples at a scan rate of
100 mV s-1. c Values refer to reduction peak potential (Epc) for the
irreversible reduction waves.
of 28.2 μs. Only small negative solvatochromic effect on the
emission at 524 nm is found (± 5 nm; Fig. S4d, ESI†). These
findings, together with TD-DFT calculations, suggest that the
photoluminescence of the complexes is derived from triplet
metal-perturbed ligand-centred (3LC) π–π* excited states. On the
other hand, the structureless emission of 1d (the acetylacetonate
(acac) analogous of 1a) at 516 nm should be ascribed to the
3MLCT/LLCT emission.2 Interestingly, complex 3a (Φ = 0.2%;
τ = 4.8 μs) displays dual emission (Fig. 2) in contrast to the single
The electrochemical data of 1b
– 9b are summarized in Table 2
(all values versus Ag/AgNO3, scan rate of 100 mV s-1, 0.1 M
nBu4NPF6 in MeCN as supporting electrolyte) and Table S3
(values vs Cp2Fe+/0, ESI
irreversible oxidative wave at Epa = 0.62
irreversible reductive wave at Epc = -2.52
†
). These complexes display one
1.04 V and one
-1.94 V (vs
–
–
Ag/AgNO3). The cyclic voltammograms of 1a have been
reported elsewhere74 with the oxidation potential of +1.16 V (vs
Ag/AgNO3) in CH2Cl2. By comparing with the electrochemical
data of 1d and 2d2 both of which have ancillary acac ligand, the
first oxidation wave of 1a should originate from Ir(III)-
centred/C^N ligand-centred oxidation. This assignment is further
supported by the observation of more positive oxidation potential
of 6Fb (+1.04 V) than that of 6Hb (+0.98 V). The electron-
withdrawing CF3 group of 6Fb will lower the energy levels of
d(Ir) and π(C^N) orbitals, leading to the more positive oxidation
emission of the acac analogous 3d 75
, suggesting the possibility
of modulation of photophysical properties of Ir(III) complexes
by the NHC ligands.
Changes in chemical structure of the cyclometalated ligands
also result in profound effect on the photophysical properties of
the Ir(III) complexes. For example, 5b displays a significant red-
shift in emission maximum (576 nm) when compared to that of
4Me
b b (525 and 527 nm respectively). This can be
and 4Hex
rationalized by the extended π-conjugation of the C^N ligands,
leading to the decrease in energies of metal-perturbed 3LC
potential of 6Fb. On the other hand, 4Me
b and 5b have more
extended π-conjugated C^N ligands than 1b. This will result in
emission. Moreover, for 4Me 4Me and 4Hex
a, b b, their
photoluminescence quantum yields in solutions are affected by
the substituents on fluorenyl moiety as well as the N-alkyl groups
an increase in the energy level of π(C^N) and/or d(Ir) orbitals,
thus 4Me
b and 5b can undergo oxidation more readily than 1b
of the bis-NHC ligands, e.g. complex 4Hex
b
exhibits higher
and hence less positive oxidation potential are found.
photoluminescence quantum yield than 4Me
a
and 4Me
b
. This is
72
Compared with 1b, [(dfppy)2Ir(bis-NHCBu)]PF6 displays a
probably attributed to the fact that the long hexyl and N-butyl
chains disfavour intermolecular stacking interactions among the
more anodic oxidation potential of Eox= 1.04 V and the similar
reduction potential of Ere= -2.37 V (vs Cp2Fe+/0), which is
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organic solvent and the other in water. The tr-abs and tr-em
attributed to stabilization of HOMOs as a result of the presence
of highly electron-withdrawing F substitution on C^N ligand
(dfppy). For the irreversible reduction wave, a less negative
reduction potential is found for 6Hb (Epc = -2.14 V vs Ag/AgNO3,
Table 2, Fig. S4), as compared to that of 1b (Epc = -2.52 V vs
Ag/AgNO3, Table 2, Fig. 4) which has a less extended π-
conjugation of the C^N ligand. The reductive wave is further
anodically shifted in the case of 6Fb (Epc = -1.94 V vs
spectra of the other bis-NHC Ir(III) complexes e.g. 5b in MeCN
(Fig. S6a, ESI
recorded and the results could be found in ESI
The time-resolved absorption and emission spectra of 4Me
† †) in water were also
), and 6Hc (Fig. S6b, ESI
†
.
b
recorded at various time intervals after excitation at 355 nm are
depicted in Fig. 5. Kinetic decay analysis of bleaching of ground-
state of 4Me
b
(τ350 nm =15.4 μs, Fig. 5a, left insert) matches well
with growth of the absorption of triplet excited state (τ480 nm
=
15.2 μs, Fig. 5a, right insert) as well as the emission lifetime (τ524
nm = 16.5 μs, Fig. 5b, right insert) in MeCN at 298 K.
Ag/AgNO3, Table 2, Fig. S5a, ESI†). Therefore, the reduction
process should be localized on the C^N ligands.
Laser energy
a)
b)
5 mJ
0.10
0.05
0.00
17 mJ
50 mJ
78 mJ
1b
2a
300
400
500
600
700
800
900
4Me
b
Wavelength (nm)
Conc. of Acetone
0 M
Laser energy: 50 mJ (beam area = 0.5 cm2)
0.10
0.05
0.00
0.68 M
1.36 M
2.04 M
2.72 M
5b
6Hb
720 nm
800
495 nm
300
400
500
600
700
900
-2.6 -2.4 -2.2 -2.0 -1.8 -1.6
0.2 0.4 0.6 0.8 1.0 1.2 1.4
Wavelength (nm)
Potentials (V) vs Ag/AgNO3
Fig. 4 Cyclic voltammograms of 1b 2a
4Me 5b and 6Hb in MeCN with nBu4NPF6
(0.1 M) as supporting electrolyte. Conditions: glass-carbon, working electrode,
scan rate: 100 mV s-1.
c)
d)
,
,
b,
0.08
4Mec only
 = 22s
[Acetone]

720
0.06
0.04
0.02
0.00
0 M 142 ns
0.68 M
2.72 M
46 ns
n.d.
4Mec + acetone(2.72 M)
 = 22s
Excited states properties
0
50000
100000 150000 200000
1000
2000
3000
4000
0.30
0.25
0.00
(a)
Time (ns)
Decay of tr-abs (480 nm)
Time (ns)
0.4
0.3
0.2
0.1
0.0
-0.05
-0.10
-0.15
Decay fit
0.20
0.15
0.10
0.05
0.00
Decay of tr-abs (350 nm)
Fig. 6 Transient absorption spectra recorded a) under specified laser energy; b) in
presence of specified concentration of acetone of a degassed aqueous solution of
Decay fit
20 40 60 80 100
Time (s)
0
4Me
c
(about 1
×
10-5 M); kinetic studies of c) λtr-abs (495 nm) and d) λtr-abs (720 nm)
under absence/presence and specified concentrations of acetone.
0
40
80 100
20 Time6(0s)
The transient absorption spectra of 4Me
c in aqueous solution
recorded at different energies of laser beams (355 nm) reveal
different spectral changes. As depicted in Fig. 6a, in addition to
300
400
500
600
700
800
Wavelength (nm)
0.025
(b)
1 0.00 s
the growth of the absorption of triplet excited state of 4Me
c from
Decay of em (524 nm)
2 8.00 µs
3 16.00 µs
4 24.00 µs
5 32.00 µs
6 40.00 µs
7 48.00 µs
8 56.00 µs
0.020
0.015
0.010
0.005
0.000
Decay fit
380 to 700 nm, an emergence of absorption band from 650 to
730 nm was observed as laser pulse energy ≥ 50 mJ (beam area:
0.5 m2). This absorption band could be quenched upon addition
of acetone (Fig. 6b and Fig. S7a and S7b, ESI†), but no
0
20 40 60 80 100
quenching of transient absorption at 594 nm and emission at 524
nm were observed in the presence of acetone (Fig. 6c and S7,
Time (s)
450
500
550
600
650
700
750
800
ESI† c (Fig.
). The decay rate constant monitored at 720 nm of 4Me
Wavelength (nm)
a) tr-abs (inserts: decay of tr-abs at
nm and 480 nm); b) tr-em (decay of tr-em at
6d, absence of acetone) was much faster than that measured at
495 nm in Fig. 6c/S7c and decay of emission at 524 nm (Fig. S7d,
Fig. 5 Time-resolved spectra of 4Me
b
λ
= 350
λ
= 524 nm) spectra recorded at
ESI
†). In view of these different kinetic behaviours, the transient
specified time after laser pulse excitation (355 nm) in degassed MeCN at 298 K.
absorption from 650 to 730 nm depicted in Fig. 6b might
originate from hydrated electrons eaq
the photo-induced ionization of 4Me
,
c
- 76, 77 which were formed by
in aqueous solution upon
Nano-second transient absorption and emission spectroscopic
(tr-abs and tr-em) measurements were undertaken for the highly
emissive bis-NHC Ir(III) complexes with long-lived electronic
excitation with high energy laser beams. This is in line with a
reported photoionization of [Pt2(POP)4]4-,78 as well as the
findings in the studies of solvated electron with acetone.79, 80
Similarly, complex 6Hc in aqueous solution was also observed to
undergo photo-ionization as revealed by the increase in transient
excited states i.e. 4Me (Fig. 5) and 4Me
b c (Fig. 6a) in MeCN and
water respectively. These two complexes have the same
lumophore but different functionalized NHC groups, leading to
the distinguishing solubility in solvents, such as one is soluble in
6 | Chem. Sci., 2016, 00, 1-3
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ARTICLE
absorption in the region of 600 to 700 nm (Fig. S8b, ESI
4Me
, its transient absorption spectra monitored at high laser
pulse energy in MeCN exhibits similar profile as that of lower
energy (Fig. S8b−S8c, ESI ). However, newly generated long-
lived species has been observed at high laser pulse energies after
100 μs (Fig. S8b S8e, ESI ), revealing that photo-ionization of
4Me with the generation of [4Me ]+ (Ir(IV)) is likely occurred.
†). For
b
Table 3. Screening bis-NHC Ir(III) complexes for photo-catalysis.*
†
–
†
b
b
The accompanying solvated electrons is not observed in this case
due to readily quenching by MeCN.81
Based on the electrochemical data and the determination of E0-
Entrya
PC
1b
2b
Conversion/%b
Yield/%b
1
2
3
4
5
6
7
8
9
93
1
59
0
from the spectroscopic measurements, the photoredox
0
properties of the bis-NHC Ir(III) complexes can be estimated
(Table S3, ESI†). The triplet excited states of the complexes are
4Me
5b
b
90
90
93
80
49
11
0.1
59
52
59
44
24
7.3
0
found to be powerful oxidants and reductants, and some of them
are even more reactive towards photoredox reactions than
6Hb
6Fb
7b
8b
9b
[Ru(bpy)3]2+ and fac-Ir(ppy)3. A representative example is 4Me
b
(E(IrIV/III*) = -1.51 V vs SCE), which is a stronger reductant than
[Ru(bpy)3]2+ (E(RuIII/II*) of -0.81 vs SCE).37, 82 As a result, it is
anticipated that the bis-NHC Ir(III) complexes described herein,
upon photoexcitation in the visible-light region, can catalyse a
number of reactions which are not feasible by the widely used
[Ru(bpy)3]2+.
*Complex 3a was not tested because of the low quantum yield (0.2
%, see Table 1). a Procedure: Substrate 50 μmol, PC (1 mol%),
DIPEA (5 equiv.), HCOOH (2.5 equiv.) in 4 mL MeCN solution
was degassed by nitrogen, and irradiated by blue light (12 W, λmax
462 nm) at ambient temperature for 4 h. b Determined by 1H NMR
spectroscopy by adding internal standard of 5,5’-dimethyl-2,2’-
bipyridine.
=
Table 4. Visible-light-induced C-C bond formation for aryl halide.
Entrya
1
2
3
4
PC (Substrate)
Amines
DIPEA
DIPEA
DIPEA
DIPEA
DIPEA
DIPEA
TEA
TEA
TMEDA
DBU
DIPEA
DIPEA
-
DIPEA
DIPEA
DIPEA
Conversion/%b Yield/%b
Chart 2. General photocatalytic reactions by cyclometalated complexes. EWG =
electron-withdrawing group.
1b
(
A1
A1
A1
A2
A2
A2
)
97
97
96
98
99
84
97
91
52
96
85
62
0
67
64
64
54
<15
51
79
64
32
72
54
28
0
4Me
b
(
)
6Hb
(
)
4Me
1b
b
(
)
Photocatalysis
5
(
(
)
)
6
6Hb
With good photo-stability, long excited state lifetime,
favourable absorption in the spectral region of blue LED and
tunable photo-redox properties, these bis-NHC Ir(III)
complexes have been investigated for photo-redox reactions
(Chart 2), examples of which are described in the following
section.
7c
4Me
4Me
4Me
4Me
4Me
4Me
4Me
4Me
b
b
(
A3
A1
A1
A1
A1
A1
A1
A1
)
)
)
)
)
)
)
)
8
9
(
(
(
(
(
(
(
b
b
b
b
b
b
10
11d
12e
13f
14g
15
16
We are attacted to a recent work by Lee and co-workers83
on visible-light-induced reductive cyclization of aromatic
0
0
Ru(bpy)3Cl2 (A1
fac-Ir(ppy)3 (A1
)
)
0
0
27
16
iodides
and
bromides
to
form
indoline
using
a Procedure: Substrate 50 μmol, PC (2 mol%), amine (5 equiv.),
HCOOH (2.5 equiv.) in 4 mL aqueous solution was degassed by
[(ppy)2Ir(dtbbpy)]PF6 as photo-catalyst (PC). Yet, aryl
bromides are found to be less reactive than the iodides.84
Barriault and co-workers addressed this issue by using
dinuclear gold(I) complexes ([Au2(dppm)2]OTf2) as the photo-
catalyst.85 However, this gold complex only shows absorption
at high-energy UV region (λ < 300 nm), which may result in
destructive effect on the products and/or lead to undesired side
reactions.
nitrogen, and irradiated by blue light (12 W, λmax = 462 nm) at
ambient temperature for 10 h. b Determined by 1H NMR spectroscopy
by adding internal standard of 5,5’-dimethyl-2,2’-bipyridine. c
Irradiated for 4 h. d Absence of HCOOH. e Presence of air (no
degassing). f Absence of amine. g Absence of light.
As the present bis-NHC Ir(III) complexes show strong
absorption at λ > 400 nm, they were used to perform photo-
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catalysis of the reductive cyclization of aryl bromides using blue yield of cyclization of B1 was improved to up to 90% (entry 5,
LEDs. Among the bis-NHC Ir(III) complexes examined, 1b
Table 3) when formic acid was added and 6Hb was used as a
4Me and 6Hb displayed good photo-catalytic activity for the photo-catalyst (PC). When 2,2,6,6-tetramethyl-1-piperidinyloxy
photo-catalytic reductive cyclization of aryl bromide (A1) in (TEMPO) was added, the reaction was totally inhibited,
,
b
terms of both substrate conversion and product yield (Table 3).
In the case of another aryl bromide substrate A2
4Me
showed both good substrate conversion and product yield (Table
indicating the involvement of radical intermediate in the reaction
(entry 12).
The photo-catalytic reaction could be initiated from the
,
b
4, entries 4 − 6). It was chosen as photo-sensitizer for oxidative quenching of 4Meb* with aryl/alkyl halide. This is
optimization of the reaction conditions. A number of control because the excited state reduction potential of 4Me
experiments were performed, no reaction was observed in the (E(IrIV/III*
pc = -1.51 V (vs SCE), Table S3, Fig. S9) can allow a
absence of amine or light (entries 13 – 14). Lowering the loading direct one electron reduction of aryl/alkyl halide by 4Me
*,
b*
)
b
of PC (Table 3, entry 4), the absence of HCOOH or exposure to leading to carbon-halogen (σ*(C-X)) bond cleavage to give alkyl
air (entries 11 – 12) were observed to decrease the substrate radical in the case of sp3 carbon or radical anion intermediate for
conversion of this reaction. Interestingly, using 1,8- sp2 carbon.86 Subsequent reactions of the alkyl radical or radical
diazabicyclo[5.4.0]undec-7-ene (DBU, entry 10, Eonset+/0 = 0.60 anion intermediate with C(sp2)-H bond lead to C-C bond
V vs Cp2Fe+/0), also led to excellent substrate conversion and formation. An aminium radical cation generated from the
+83, 85
good product yield comparable to that obtained with oxidation of amine by [4Me
b
]
could serve as an electron
tetramethylethylenediamine (TMEDA, entry 9, Eonset+/0 = 0.11 V donor to complete the reductive process. In the case of
vs Cp2Fe+/0, Fig.S9, ESI†). In contrast, the widely used photo- [(ppy)2Ir(dtbbpy)]PF6,83
its triplet excited state
catalysts [Ru(bpy)3]Cl2 and fac-Ir(ppy)3 (Table 4, entries 15 and ([(ppy)2Ir(dtbbpy)]+*) reacts with DIPEA via a reductive
16) showed no or little conversion under similar conditions.
quenching mechanism to generate Ir(II) species (E(IrIII/II
)
= -
pc
1.51 V (vs SCE)1, Fig. S9, ESI
†), which initiates the subsequent
Table 5. Visible-light-induced C-C bond formation of alkyl
bromide.
reducing catalytic reaction.
Table 6. Visible-light-induced radical cyclization in aqueous solution.
Entrya
PC
Amines Conversion/%b Yield/%b
Entrya
Solvents
Reductant
Conversion
/%c
Yield
/%c
1
2
Ru(bpy)3Cl2
fac-Ir(ppy)3
1b
DIPEA
DIPEA
DIPEA
DIPEA
DIPEA
TEA
19
90
99
99
99
99
84
99
0
5
72
85
76
90
72
55
64
0
(H2O/MeOH)b
1
2d
3
4d,e
5d
6
3/1
3/1
3/1
3/1
3/1
1/1
1/3
0/1
Ascorbic Acid
Ascorbic Acid
DIPEA
23
20
98
90
79
99
99
99
10
14
49
21
31
64
87
66
3
4
4Me
6Hb
b
5
DIPEA
6
4Me
4Me
4Me
4Me
4Me
4Me
4Me
4Me
b
DIPEA
7
b
b
b
b
b
b
b
TMEDA
DBU
DIPEA
8
7
DIPEA
9
-
8
DIPEA
10c
11d
12e
13f
DIPEA
DIPEA
DIPEA
DIPEA
27
0
15
0
a
Procedure: Substrate 50 μmol, 6Hc (2 mol%), reductant (5 equiv.),
nBu4NCl (5 equiv.) in 4 mL aqueous solution was degassed by nitrogen,
0
0
o
b
and irradiated by blue light (12 W, λmax = 462 nm) at 25 C. Solvent
99
59.1
c
system is used as water/methanol in volume ratio (v/v). Determined
by 1H NMR spectroscopy by adding internal standard of 5,5’-dimethyl-
2,2’-bipyridine. d Absence of HCOOH. e Absence of nBu4NCl.
a Entry 1
–11: R = H (substrate A1); Procedure: Substrate 50 μmol,
PC (2 mol%), amine (5 equiv.) and HCOOH (2.5 equiv.) in 4 mL
MeCN solution was degassed by nitrogen, and irradiated by blue
light (12 W, λmax = 462 nm) at 25 oC. b Determined by 1H NMR
spectroscopy by adding internal standard of 5,5’-dimethyl-2,2’-
bipyridine. c Absence of HCOOH (20 equiv.). d Absence of light. e
Presence of TEMPO (radical trapping reagent, 2 equiv.) f Entry 13:
R = Me (substrate A2).
Interestingly, modifying N-substituent of bis-NHC ligand
from alkyl group to glucose moiety renders photo-catalyst 6Hc
soluble in aqueous media. At outset, we examined the 6Hc
-
catalysed reductive cyclization of B1 in a mixture of H2O/MeOH
(3:1) with ascorbic acid as reductant, but both the substrate
conversion and product yield were low. The use of
diisopropylethylamine (DIPEA) as reductant instead of ascorbic
acid and addition of tetrabutylammonium chloride improved the
The radical cyclization of the alkyl bromides, B1 and B2
4Me or 6Hb proceed smoothly, with reasonable
catalysed by 1b, b
to excellent substrate conversion and product yields also. The
,
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ARTICLE
conversion and yield to 98% and 49%, respectively. Increasing
vol% of methanol in aqueous solution to 75% leads to 99%
conversion and 87% product yield. To the best of our knowledge,
this is the first example of visible-light-driven radical cyclization
for synthesis of pyrrolidine in aqueous media.
(a) 450
90
H2
Conc. of [Co] = 0.05 mM
CO
400
350
300
250
200
150
100
50
80
Conc. of [Ir]
2.5 mM
2.5 mM
70
0.5 mM
0.05 mM
0.5 mM
0.05 mM 60
50
40
30
20
10
0
Visible-light-driven CO2 reduction
There is a surge of interest to develop photo-catalytic CO2
reduction using earth abundant metal complexes as catalyst and
luminescent cyclometalated Ir(III) complexes particularly fac-
Ir(ppy)3 as PC. Nevertheless, several recent reports drawn out the
attention of the instability of fac-Ir(ppy)3, which is a challenge
for achieving efficient light-driven CO2 reduction in a long
0
0
5
10 15 20 25 30 35 40 45
Time (h)
run.55, 57, 63 In view of the good photo-stability of 4Me
b, we have
(b)
been prompted to investigate the visible-light-driven CO2
1400
Conc. of [Ir] = 0.5 mM
reduction by utilizing 4Me
b in combination with the recent
1200
1000
800
600
400
200
0
reported [Co(TPA)Cl]Cl complex.57
H2
2.5 mM
0.5 mM
0.05 mM
0.005 mM
A CO2-saturated MeCN/triethylamine solution (4:1, v:v; 4 mL)
CO
Conc. of [Co]
2.5 mM
containing catalytic amount of 4Me
b and [Co(TPA)Cl]Cl was
irradiated by blue LEDs (12 w) for a specified time period, and
the evolved gases were separated and identified by a GC-TCD
equipped with a molecular sieve column. The volume of H2 and
CO gases were calculated by using CH4 as the internal standard.
As shown in Fig. 7a and Fig. 7b, the amount of CO and H2
generated from the reaction mixture is found to show strong
0.5 mM
0.05 mM
0.005 mM
0
5
10
15
20
25
dependence on the concentrations of 4Me
Particularly, the highest TON value of over 5000 can be
accomplished at 0.5 M of Co(II), while no generation of gases
is found at low concentration of Co(II) (50 nM; Fig. S11, ESI ).
b and Co(II) catalysts.
Time (h)
(c)
μ
3000
60
50
40
30
20
10
0
†
H2
2500
2000
1500
1000
500
Similarly, only negligible amount of product gases can be
detected after irradiation for 24 h when [Ir] (0.005 mM) is lower
than [Co] (0.05 mM). With the representative system containing
CO
4Me
b (0.5 mM) and Co(II) (0.005 mM), (Fig. 7c) the visible-
light-driven CO2 reduction in three parallel reaction runs gives
TON (CO) > 2400 (conversion of about 18 mL of CO2 into 1 mL
of CO) with excellent selectively in generating CO over H2 in
gaseous phase (>95%) after reaction for 72 h. This result is better
than that of the system utilizing fac-Ir(ppy)3 as PC, which reveals
only TON (CO) > 900 and selectivity (CO) of 85% under similar
reaction conditions.57
0
0
10
20
30
40
50
60
70
80
Time (h)
In order to confirm the roles of catalysts in photo-driven CO2 Fig. 7 TON value and amounts of gases (CO and H2) generation from CO2 in a CO2-
saturated MeCN/TEA (4/1, v/v, 4 mL in total) solution as a function of irradiation
reduction, several control experiments were performed (Table S5,
ESI†). Firstly, in the absence of Co(II) complex, no CO gas was
time: concentration dependence of a) PC 4Me
b
and b) catalyst [Co(TPA)Cl]Cl; c) a
solution containing 0.005 mM [Co(TPA)Cl]Cl, 0.5 mM 4Me
b
was irradiated using
observed in a CO2-saturated MeCN/TEA (4:1, V:V; 4 mL)
solution after irradiation for 24 h. On the other hand, in the
absence of light, sacrificial amine or PC, the reaction mixture
only gives negligible amount of CO. To ascertain the catalytic
role of Co(II) complex in the reaction, mercury was added to the
reaction mixture in order to exclude the possibility of CO
generation from heterogeneous Co nanoparticles. To specify,
29.2 μmol of CO (0.714 mL, TON 146) could be generated from
the solution with [Ir] (0.5 mM), [Co] (0.05 mM) and elementary
Hg (1 mL) after irradiation for 18h, and this result is comparable
blue LEDs (12 w) based on the averaged results from three parallel reaction runs.
Cellular imaging and cell viability assay
We have a long-standing interest on luminescent transition
metal complexes particularly those supported by NHC ligands
16, 87-90
that display anti-cancer activities.9,
The luminescence
would allow direct monitoring of cellular uptake and tracking of
cellular location in cancer cells by fluorescence microscopy, and
such properties of transition metal NHC complexes have been
demonstrated to be useful in elucidation of anti-cancer
mechanism of action. In this work, in view of their favourable
photophysical properties, the in vitro cytotoxicity of the bis-
with that using the solution mixture without Hg (28.4
CO formed, TON 142).
μmol of
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NHC Ir(III) complexes against HeLa cells was investigated by localized in endoplasmic reticulum (ER), which is stained by
MTT assay. As shown in Table S6 (ESI high Pearson’s
), the Ir(III) complexes red-emissive ER-specific ER-TrackerEM
†
;
display potent cytotoxicity, with IC50 values ranging from 0.5 to correlation coefficient (R) between the emissions of complexes
56.1 μM depending on the lipophilicity and structures of the and ER-TrackerEM is found (for example, 6Hb shows a high R
Ir(III) complexes. Those complexes with butyl groups on the bis- value of 0.80). Consistently, these complexes do not accumulate
NHC ligand are more cytotoxic than those having NHC ligands in other organelles such as lysosome or mitochondria, as shown
with glucose unit.
by the poor co-localization of the emissions of the complexes
Since complexes 1b
,
4Me
b
,
6Ha
,
6Hb and 6Hc demonstrate with the emission of Lyso-Tracker® and Mito-Tracker®
outstanding photophysical properties i.e. high quantum yield respectively (Fig. S13, ESI†
). Noticeably, 1b+ (both the counter
with long-lived electronic excited state, cellular imaging of the anions of triflate and chloride) in our laboratory, were found to
entitled complexes in HeLa cells were performed. After be specifically localized in ER of cancer cells (Fig.S14), but not
treatment of human cervical cancer cells (HeLa) with the Ir(III) as reported into mitochondria73. With the consideration of
complexes for 15 min, strong green/yellow luminescence were specific accumulation of the complexes in ER, the cytotoxic
observed in cytoplasm (but not nucleus) of cancer cells, as properties of the complexes may originate from the induction of
revealed by fluorescence microscopy (Fig. 8). Co-localization ER stress88 and immunogenic cell death.91
analyses indicate that the emission of these complexes are mainly
Fig. 8 Fluorescence microscopy images of HeLa cancer cells incubated with the Ir(III) NHC complexes: merged (top), ER-trackerEM (middle) and complexes (bottom).
Complexes were excited at 340 nm using an emission filter of 510 nm. ER-trackerEM was excited at 546 nm using an emission filter of >580 nm.
less significant for NHC metal complexes. These behaviours are
similar to the coordination characteristics of phosphines, but
Discussion
NHCs are in general better electron-donors than phosphines.
Thus, the stronger metal ligand interaction renders NHC metal
coordination less labile than metal phosphine bonding and the
Herein, this work reveals the potential impact and usefulness of
bis-NHC ligands for the development of robust metal photo-
catalysts. Compared with the well-known complexes fac-
Ir(ppy)3, this series of bis-NHC Ir(III) complexes exhibits: i)
outstanding photo-stability under visible light irradiation; ii)
good photo-catalytic performance for several photo-
electrochemically reactions; and iii) long-lived emissive excited
states.
–
–
–
NHC complexes are more thermally and oxidatively stable.92
The distinct electronic properties and coordination chemistry of
NHCs can also lead to improved catalytic activity of the metal
complexes, owing to the increased catalyst stability and
consequently lower rates of catalyst decomposition.68 In
previous section, the studies on the photo-stability of 4Me
b and
Stability of NHC metal complexes
fac-Ir(ppy)3 have revealed the outstanding stabilization
contribution from bis-NHC carbene ligands. Thus this might be
one reason that explains the better performance for photo-
catalytic reactions using the present bis-NHC Ir(III) complexes
as PC.
The most important feature of metal-ligand bonding between
transition metals and NHCs, can be rationalized as the σ-
coordination (sp2-hybridized lone pair electron) from NHCs. The
contribution of both π-back-bonding into carbene p-orbital and
π-donation from the carbene p-orbital might be considered as
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Role of bis-NHC ligands in tuning emission energy and lifetimes
In addition to the photo-stability and strong absorptivity in
visible light region, the long-lived triplet excited states especially
its propensity to loss an electron, to a large extent, are believed
to be crucial for the photo-catalytic properties of the complexes.
A comparison between the photophysical data of bis-NHC Ir(III)
complexes with two notable Ir(III) complexes (fac-Ir(ppy)3 and
(ppy)2Ir(acac) (1d)) will be helpful in understanding the role of
NHC on the strong absorptivities, high emission efficiencies and
long lifetimes properties of the present bis-NHC Ir(III)
complexes. On the basis of DFT calculations on 1a as the
The long triplet excited state lifetime of 4Me
b is beneficial for bi-
molecular photochemical reactions allowing the electron-
transfer pathways to have sufficient time to take place prior to
the decay of the excited state to the ground state.
Considering the fact that the excited-state of [Ru(bpy)3]2+
(E(RuII*/I) = 0.77 V vs SCE) is a more powerful oxidative
potential than that of 4Me (E(IrIII*/II) = 0.51 V vs SCE, Table S3)
b
representative example (Fig. S4, ESI
absorption bands of 1a 1d and fac-Ir(ppy)3 are ascribed to the
HOMO LUMO transitions, and the energy of which is
†), the lowest-energy
,

sensitive to the charge of ancillary ligands. The transition energy
of fac-Ir(ppy)3 and 1d with the negatively charge ancillary
ligands (C^N and acac) are quite close with wavelength of 416
and 414 nm respectively. However, for 1a with the neutral
and no radical cyclization products is obtained when using
[Ru(bpy)3]2+, the photo-catalytic route via reductive quenching
cycle is not feasible. The photo-catalytic reaction would possibly
be initialized via oxidative quenching cycle of excited state of
photo-catalyst. By carefully examining transient-absorption
ancillary ligand (bis-NHC), the HOMO
LUMO transition is
markedly hypsochromically shifted to 369 nm, which is
consistent with the experimental observations. Fig. S10 (ESI†)
spectra of triplet state of 4Me
b in MeCN, newly generated long-
lived species by using higher energy laser beams were observed
(Fig.S8b-S8g). This long-lived species was found to be increased
in the presence of substrate A1, and could be quenched by
DIPEA. This long-lived species might be Ir(IV), which is
shows the surfaces and the energies of HOMO and LUMO of 1a
,
fac-Ir(ppy)3 and 1d. On the whole, HOMO contains comparable
components of iridium and C^N ligand, and LUMO is mainly
localized on the C^N ligand. Thus, the transition can be assigned
as admixtures of metal-to-ligand charge transfer (MLCT) and
ligand centered (LC) π–π* transition, which is in accordance
with the assignments reported in literatures.
generated by single electron transfer from [4Me
calculated excited-state reduction potential of 4Me
-1.51 V vs SCE, Table S3) reveals that 4Me
is not a stronger
photo-reductant than fac-Ir(ppy)3 (E(IrIV/III*) = -1.73 V vs
SCE).37, 93 However, performance of 4Me
in visible-light-driven
radical cyclization and CO2 reduction prove to be better than that
of fac-Ir(ppy)3. For example, the conversion of substrates A1 B1
to indoline/pyrrolidine by 4Me
(97%/99%) are higher than that
by fac-Ir(ppy)3 (27%/90%). Therefore, there should be other
reasons to account for the good performance of 4Me
in photo-
catalysis. Plausible reasons could be: i) the generated Ir(IV)
4Me ]+ (E(IrIV/III) = 0.96 V vs SCE, Table S3) are more easily
reduced by amines than [fac-Ir(ppy)3]+ (E(IrIV/III) = 0.77 V vs
b
b
]* to A1. The
(E(IrIV/III*) =
b
The different amounts of metal participations into the frontier
molecular orbitals of 1a, 1d and Ir(ppy)3, as deduced by TD-DFT
b
calculations, can account for the photophysical properties and
long emission lifetimes of the Ir(III) NHC complexes. As shown
in Fig. S10 (ESI†), fac-Ir(ppy)3 and 1d have similar energy
levels in HOMO and LUMO (around -4.5 eV and -2.0 eV). By
simply substitution of the negatively charge ancillary ligands
(C^N and acac) in Ir(ppy)3 and 1d by neutral NHC ligand, 1a is
found to show lower energy levels of HOMO and LUMO (about
-5.2 eV and -2.4 eV). This can be explained by the less electron-
donating effect of the neutral NHC ligand than that of the
negatively charged auxiliary ligands, as well as certain
contribution of the stabilization of dπ(Ir) by the π-acceptor
/
b
b
[
b
SCE); and ii) the higher photo-stability of 4Me
b compared with
that of fac-Ir(ppy)3.
Consequently, the excellent photo-stability, strong
absorptivity, long-lived lifetimes and the photo-ionization
behaviours of our bis-NHC Ir(III) complexes possibly make the
photolysis of long-lived excited state of Ir(III)* more easily to
Ir(IV) and thus promote the radical cyclization to proceed in a
catalytic cycle.
orbitals of NHC ligand of 1a 68
. As a result, the energy level of
HOMO of 1a is 700 mV lower that of 1d, which coincides with
the experimental observation that the oxidation potential of 1a is
410 mV more positive than that of 1d. On the other hand, the
transition energy of HOMO
LUMO in 1a is estimated to be 0.3
eV larger than that in Ir(ppy)3 and 1d, which may account for the
blue shift of low-energy absorption of 1a (369 nm vs 410 nm),
as compared to that of 1d and Ir(ppy)3, in UV-vis absorption
spectra. Notably, TD-DFT calculation reveals that frontier
molecular orbitals of 1a shows a smaller metal character than
those of 1d (Ir character in HOMO of 1a and 1d are found to be
~30 and ~40% respectively), and hence the triplet excited states
of 1a are likely to have smaller metal character. This would
probably slow down the spin-orbit coupling, resulting in slower
radiative and non-radiative T1S0 decay for 1a. As a result, 1a
Diversities of biological activities of Ir(III) complexes with
different N-substituents on NHC ligands
Although N-substituents of NHC ligands are found to show
negligible effects on the luminescent properties of the bis-NHC
Ir(III) complexes, they have a determinant role on the physical
properties and hence the anti-cancer activities of the complexes.
For example, complexes with N-butyl substituents on the bis-
NHC ligands generally display lower IC50 values than those with
N-methyl substituents (Table S6 in ESI†). This is probably
attributed to the increase in lipophilicity of the complexes,
resulting in better permeability through cellular membrane and
higher accumulation of the complexes in cancer cells. The fast
cellular uptake of the complexes is supported by the strong
shows a longer emission lifetime than that of 1d
.
Triplet excited states for photo-catalytic reactivity
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c
luminescence observed in HeLa cells after incubation of the cells
with the complexes for 15 min (Fig. 8 and S12 in ESI†). On the
other hand, the glucose-functionalized NHC ligands render the
Department of Applied Biology and Chemical Technology, The Hong
Kong Polytechnic University, Hung Hom, Hong Kong.
E-mails: sharonlf.chan@polyu.edu.hk; cmche@hku.hk
complexes good aqueous solubility and more hydrophilic in † Electronic Supplementary Information (ESI) available: additional
nature, leading to likely slower cellular uptake as well as the experimental details, figures and tables. CCDC 1428476
reduced cytotoxicity toward cancer cells. It is noteworthy that other electronic format see DOI: 10.1039/b000000x/
the complexes accumulate in cellular ER as revealed by
–1428479 and
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Acknowledgements
This work was supported by the National Key Basic Research
Program of China (No.2013CB834802), the University Grants
Committee of the HKSAR Area of Excellence Scheme (AoE/P-
03/08), and the CAS-Croucher Foundation Funding Scheme for
Joint Laboratories. C. Yang acknowledges the support by the
postgraduate studentships from the University of Hong Kong and
Miss. Yingshuo Zhang for synthesis of part of substrates. S. L.-
F. Chan thanks the financial support of Hong Kong Research
Grants Council (PolyU 253038/15P) and the National Science
Foundation of China (21401157). Cellular imaging data were
acquired using equipment maintained by the University of Hong
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