Conversion from ILCT to LLCT/MLCT excited state by heavy metal ion binding in iridium(III) complexes with functionalized 2,2'-bipyridyl ligands

Article abstract of DOI:10.1021/om900334f

Iridium(III) complexes with two ppy (ppy = 2-phenylpyridine) and one functionalized 2,2'bipyridyl ligand containing a bidentate or terdentate binding group were synthesized, and their photophysical and luminescence sensing properties to heavy metal ions w

Full text of DOI:10.1021/om900334f

Organometallics 2009, 28, 5603–5611 5603
DOI: 10.1021/om900334f
Conversion from ILCT to LLCT/MLCT Excited State by Heavy Metal
Ion Binding in Iridium(III) Complexes with Functionalized 2,20-Bipyridyl
Ligands
Na Zhao,† Yu-Hui Wu,† Hui-Min Wen,† Xu Zhang,† and Zhong-Ning Chen*,†,‡
†State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter,
Chinese Academy of Sciences, Fuzhou, Fujian 350002, People’s Republic of China, and ‡State Key
Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of
Sciences, Shanghai 200032, People’s Republic of China
Received April 29, 2009
Iridium(III) complexes with two ppy (ppy = 2-phenylpyridine) and one functionalized 2,20-
bipyridyl ligand containing a bidentate or terdentate binding group were synthesized, and their
photophysical and luminescence sensing properties to heavy metal ions were studied. Complex 1 was
also structurally characterized by single-crystal X-ray crystallography. These complexes display a
low-energy absorption band with the maximum at 410-430 nm ascribed to an intraligand charge
transfer (ILCT) transition from the HOMO (π) residing on the fragment CtC-C6H4-R to the LUMO
(π*) localized on the 2,20-bipyridyl (bpy) in the functionalized 2,20-bipyridyl ligand bpyCtC-C6H4-R,
as revealed from DFT calculations. Upon irradiation at 350 nm < λex < 430 nm, they show weak
3
emission at ca. 618 nm since ILCT transition is the lowest lying excited state. The ion-binding
properties of these iridium(III) complexes to heavy metal ions were investigated by UV-vis and
emission spectroscopy. Upon complexation with metal ions through the binding sites in the
functionalized 2,20-bipyridyl ligand, the absorption band at 410-430 nm is significantly blue-shifted
to 350-390 nm, resulting most likely from a conversion of ILCT to LLCT/MLCT transition as the
lowest energy excited state, as verified from DFT calculations. Interestingly, a significant lumines-
cence enhancement was detected in these iridium(III) complexes upon binding to specific metal ions
3
3
due to a conversion of the lowest energy state from ILCT to the highly emissive LLCT/3MLCT
triplet excited state. The sensing properties of these iridium(III) complexes to heavy metal ions were
modulated by modifying the binding sites in the functionalized 2,20-bipyridyl ligand.
Introdution
considerable interest because of their advantages such as
significant Stoke shifts, long emissive lifetimes, and large
emission shifts from changes in the local environment com-
pared with those of organic emitters.1 Transition-metal com-
plexes including those of platinum(II), ruthenium(II), and
rhenium(I) have beenextensively utilized asprobesfor anions,
metal ions, and biomolecules.2 It has been well demonstrated
thatthe luminescence propertiesof iridium(III) complexescan
be fine-tuned by ligand substituents, resulting in distinct
emission color tuning so that they are adapt to highly sensitive
chemosensors.3 Although someiridium(III) compound-based
chemosensors for anion,4 oxygen,5 and biological labeling
reagents6 have been described, the chemosensors to heavy
metal ions are still less explored.7 On the other hand, heavy
metal ions play a vital role in various domains such as
In recent years, studies on chemosensors using transition-
metal complexes as luminescence signal groups have attracted
*To whom correspondence should be addressed. E-mail: czn@fjirsm.
ac.cn.
(1) (a) Lamansky, S.; Djurovich, P.; Murphy, D.; Abdel-Razzaq, F.;
Lee, H. E.; Adachi, C.; Burrows, P. E.; Forrest, S. R.; Thompson, M. E.
J. Am. Chem. Soc. 2001, 123, 4304–4312. (b) Thomas, K. R. J.; Velusamy,
M.; Lin, J. T.; Chien, C. H.; Tao, Y. T.; Wen, Y. S.; Hu, Y. H.; Chou, P. T.
Inorg. Chem. 2005, 44, 5677–5685. (c) Chan, S. C.; Chan, M. C. W.; Wang,
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r
2009 American Chemical Society
Published on Web 09/09/2009
pubs.acs.org/Organometallics

5604 Organometallics, Vol. 28, No. 19, 2009
Zhao et al.
Scheme 1. Structures of the Iridium(III) Complexes
3LLCT and 3[d(Ir)fπ*(bpyCtCC6H4-R)] 3MLCT triplet
states, which are highly luminescent, inducing a remarkable
luminescence enhancement. Most interestingly, complex 3
shows an emissive off-on-off effect to copper(II) ion.
Experimental Section
General Procedures and Materials. The manipulations were
carried out under an argon atmosphere using Schlenk
techniques and vacuum-line systems. The solvents were
dried, distilled, and degassed prior to use, except that those
for spectroscopic measurements were of spectroscopic grade.
(Trimethylsilyl)acetylene (Me3SiCtCH), copper(I) iodide
(CuI), 4-iodophenylamine, 4-chloromethylpyridine hydrochlor-
ide, 2-thenyl chloride, thiophen-2-ylacetaldehyde, hexadecytri-
methylammonium chloride, and hydrate iridium(III) chloride
analytical chemistry and biological and environment
sciences.8 It is well known that Zn2þ, Fe3þ, and Cu2þ are
the three most abundant essential heavy metal ions in the
human body and participate in diverse biological processes.
Hg2þ and Cd2þ are also considered as significant pollutants,
which can cause serious environmental and health problems.9
Therefore, the design and development of potential chemo-
sensors for the detection of heavy metal ions are very im-
portant.
Aiming at exploring the responses toward various heavy
metal ions, we designed four iridium(III) complexes
(Scheme 1) with two ppy (ppy = 2-phenylpyridine) and
one functionalized 2,20-bipyridyl ligand, where the cyclome-
talated Ir(ppy)2(bpy) chromophore served as a signal emitter
and the pyridyl/thiophene-containing moiety as a binding
receptor for the metal ion. Absorption and luminescence
sensing behavior of these iridium(III) complexes can be
modulated by variations in the binding sites of the functio-
nalized 2,20-bipyridyl ligand. These iridium(III) complexes
exhibit a lower energy absorption band arising from intra-
ligand change transfer (ILCT) centered on the functionalized
2,20-bipyridyl ligand.7a,10 Upon binding to specific metal ions
through the binding sites possessing pyridyl/thiophene donors,
the absorption band at 410-430 nm is blue-shifted to 350-
390 nm. Meanwhile, the weakly emissive lowest energy 3ILCT
excited state changes to 3[π(ppy)fπ*(bpyCtCC6H4-R)]
(IrCl3 6H2O) were commercially available and used as received.
3
12
5-Ethynyl-2,20-bipyridine (bpyCtCH)11 and Ir2(ppy)4Cl2
were synthesized by the literature procedures. Metal salts of
perchlorate hydrate were all purchased from Alfa Aesar.
Caution: Perchlorate salts are potentially explosive. All com-
pounds containing perchlorates should be handled with great care
and in small amounts.
(4-Iodophenyl)pyridin-2-ylmethylamine. This compound was
synthesized by modification of the literature procedures.13
To an aqueous (5 mL) solution of 4-chloromethylpyridine
hydrochloride (1.64 g, 10 mmol) were added 4-iodophenyla-
mine (1.75 g, 8 mmol), NaOH (5 M, 4 mL), and hexadecytri-
methylammonium chloride (20 mg). After stirring at ambient
temperature for 12 h, the mixture was extracted with CH2Cl2,
which was washed with H2O and dried with MgSO4.
The product was then purified by silica gel column chroma-
tography using dichloromethane-ethyl acetate (v/v = 5:1)
1
as eluent. Yield: 70%. ESI-MS (m/z): 310.2 [M]þ. H NMR
(400 MHz, CD3CN, 298 K): 8.51 (d, J = 6.2 Hz, 1H), 7.76 (t,
J = 10.5 Hz, 1H), 7.35 (d, J = 11.4 Hz, 2H), 7.2 (d, J = 7 Hz,
1H), 7.25 (t, J = 7 Hz, 1H), 6.44 (d, J = 11.5 Hz, 2H), 5.36
(s, 1H), 4.36 (s, 2H).
(4-Iodophenyl)bispyridin-2-ylmethylamine. To a solution of
(4-iodophenyl)pyridin-2-ylmethylamine (310 mg, 1 mmol) in
dry THF (40 mL) were added NaH (96 mg, 4 mmol) and
2-chloromethylpyridine (152 mg, 1.2 mmol). After the mixture
was refluxed for 6 h, ethanol (5 mL) was added to quench the
reaction. The solvents were removed in vacuo, and the residue
was then poured into 100 mL of water, which was extracted with
dichloromethane and dried with MgSO4. The product was
purified by chromatography on a silica gel column using di-
chloromethane-acetone (v/v = 5:1) as eluent. Yield: 82%. ESI-
MS (m/z): 402.4 ([M þ H]þ). 1H NMR (400 MHz, CD3CN, 298
K): 8.56 (d, J = 8 Hz, 2H), 7.70 (t, J = 3.2 Hz, 2H), 7.37 (d,
J = 8 Hz, 2H), 7.29 (d, J = 8 Hz, 2H), 7.24 (t, J = 4 Hz, 2H),
6.52 (d, J = 7.6 Hz, 2H), 4.83 (s, 4H).
(4-Iodophenyl)thiophen-2-ylmethylamine. A mixture of 4-io-
dophenylamine (219 mg, 1 mmol) and thiophen-2-ylacetalde-
hyde (126 mg, 1 mmol) in ethanol (30 mL) was refluxed for 5 h,
producing a yellow precipitate after cooling down. The solid was
filtered off and washed with cold ethanol. The residue was
redissolved in ethanol (30 mL), followed by addition of NaBH4
with stirring for 4 h. After the solvent was removed, the pro-
duct was purified by chromatography on a silica gel column
using dichloromethane-acetone (v/v = 10:1) as eluent. Yield:
70%. ESI-MS (m/z): 315.2 ([M]þ). 1H NMR (400 MHz,
CD3CN, 298 K): 7.41 (d, J = 8.4 Hz, 2H), 7.28 (d, J = 8 Hz,
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Article
Organometallics, Vol. 28, No. 19, 2009 5605
1H), 7.03 (t, J = 8.2 Hz, 1H), 6.98 (d, J = 3.2 Hz, 1H), 6.51
(d, J = 8 Hz, 2H), 5.13 (s, 1H), 4.50 (s, 2H).
1H), 7.09 (d, J = 1.2 Hz, 1H), 7.01 (t, J = 3.6 Hz, 1H), 6.82
(d, J = 9.2 Hz, 2H), 4.99 (s, 2H), 4.76 (s, 2H). IR (KBr, cm-1):
ν 2209m (CtC).
(4-Iodophenyl)pyridin-2-ylmethylthiophen-2-ylmethylamine.
To a solution of (4-Iodophenyl)pyridin-2-ylmethylamine (310.2
mg, 1 mmol) in dry toluene (40 mL) was added sodium amide
(39 mg, 1 mmol). After the mixture was refluxed for 2 h, a
toluene (10 mL) solution of 2-thenyl chloride (132 mg, 1 mmol)
was dropwise added. The mixture was refluxed for 4 h and then
the solvent evaporated in vacuo. The residue was dissolved in
dichloromethane and chromatographed on a silica gel column
using dichloromethane-acetone (v/v = 10:5) as eluent to give
the product as a yellow solid. Yield: 79%. ESI-MS (m/z): 406.9
([M]þ). 1H NMR (400 MHz, CD3CN, 298 K): 8.55 (d, J = 8 Hz,
1H), 7.69 (t, J = 7.6 Hz, 1H), 7.42 (d, J = 8 Hz, 2H), 7.30 (d, J =
7.6 Hz, 1H), 7.25 (t, J = 4 Hz, 1H), 7.22 (d, J = 9.6 Hz, 1H), 7.02
(t, J = 7.2 Hz, 1H), 6.95 (d, J = 8 Hz, 1H), 6.62 (d, J = 8 Hz,
2H), 4.89 (s, 2H), 4.69 (s, 2H).
(4-[2,20]Bipyridinyl-5-ylethynylphenyl)pyridin-2-ylmethyla-
mine (L1). (4-Iodophenyl)pyridin-2-ylmethylamine (310 mg,
1 mmol) and bpyCtCH (180 mg, 1 mmol) were dissolved in
dry THF (45 mL) and triethylamine (15 mL) with stirring. To
the solution were then added Pd(PPh3)2Cl2 (3 mol %) and CuI
(1 mol %). After refluxing for 1 day, the mixture was filtered and
the filtrate evaporated to dryness. The residue was dissolved in
dichloromethane and chromatographed on the silica gel column
using dichloromethane-acetone (v/v = 2:1) to give a yellow
product. Yield: 50% ESI-MS (m/z): 363.3 ([M þ H]þ). 1H NMR
(400 MHz, d-DMSO, 298 K): 8.92 (s, 1H), 8.70 (d, J = 4 Hz,
1H), 8.55 (d, J = 3.2, 1H), 8.43 (d, J = 4 Hz, 1H), 8.40 (d, J =
2.8 Hz, 1H), 8.01 (d, J = 3.2 Hz, 1H), 7.97 (t, J = 4 Hz, 1H), 7.77
(t, J = 3.6 Hz, 1H), 7.48 (d, J = 4 Hz, 1H), 7.40 (t, J = 1.6 Hz,
1H), 7.34 (t, J = 3.2 Hz, 2H), 7.28 (t, J = 4 Hz, 1H), 6.95 (s, 1H),
6.63 (d, J = 8.8 Hz, 2H), 4.42 (s, 2H). IR (KBr, cm-1): 2209m
(CtC).
(4-[2,20]Bipyridinyl-5-ylethynylphenyl)bispyridin-2-ylmethyla-
mine (L2). Synthetic procedures of this compound were the same
as that of (4-[2,20]bipyridinyl-5-ylethynylphenyl)pyridin-2-yl-
methylamine (L1) except for the use of (4-iodophenyl)bispy-
ridin-2-ylmethylamine instead of (4-iodophenyl)pyridin-2-yl-
methylamine. Yield: 52%. ESI-MS (m/z): 454.2 ([M þ H]þ). 1H
NMR (400 MHz, d-DMSO, 298 K): 8.74 (s, 1H), 8.73 (d, J = 0.8
Hz, 1H), 8.57 (d, J = 4.4, 2H), 8.39 (d, J = 1.2 Hz, 1H), 8.37
(d, J = 1.6 Hz, 1H), 7.98 (t, J = 8.8 Hz, 2H), 7.96 (s, 1H), 7.76 (t,
J = 1.6 Hz, 2H), 7.46 (t, J = 3.2 Hz, 1H), 7.33 (d, J = 4 Hz, 2H),
7.31 (d, J = 1.6 Hz, 2H), 7.29 (t, J = 4 Hz, 1H), 6.70 (d, J = 8.8
Hz, 2H), 4.902 (s, 4H). IR (KBr, cm-1): 2208m (CtC).
(4-[2,20]Bipyridinyl-5-ylethynylphenyl)thiophen-2-ylmethyla-
mine (L3). Synthetic procedures of this compound were the same
as that of (4-[2,20]bipyridinyl-5-ylethynylphenyl)pyridin-2-yl-
methylamine (L1) except for the use of (4-iodophenyl)thio-
phen-2-ylmethylamine instead of (4-iodophenyl)pyridin-2-
ylmethylamine. Yield: 48%. ESI-MS (m/z): 368.2 ([M þ H]þ).
1H NMR (400 MHz, d-DMSO, 298 K): 8.76 (s, 1H), 8.70 (d, J =
8 Hz, 1H), 8.40 (d, J = 8 Hz, 1H), 8.38 (d, J = 8 Hz, 1H), 8.00 (d,
J = 4 Hz, 1H), 7.96 (t, J = 8 Hz, 1H), 7.47 (t, J = 4.8 Hz, 1H),
7.39 (d, J = 1.2 Hz, 1H), 7.31 (d, J = 8.4 Hz, 2H), 7.06 (d, J =
4 Hz, 1H), 6.98 (d, J = 4 Hz, 1H), 6.87 (t, J = 4 Hz, 1H), 6.68 (d,
J = 8 Hz, 2H), 4.51 (s, 2H). IR (KBr, cm-1): 2208m (CtC).
(4-[2,20]Bipyridinyl-5-ylethynylphenyl)pyridin-2-ylmethylthio-
phen-2-ylmethylamine (L4). Synthetic procedures of this com-
pound were the same as that of (4-[2,20]bipyridinyl-5-yl-
ethynylphenyl)pyridin-2-ylmethylamine (L1) except for the use
of (4-iodophenyl)pyridin-2-ylmethylthiophen-2-ylmethylamine
instead of (4-iodophenyl) pyridin-2-ylmethylamine. Yield: 55%.
ESI-MS (m/z): 459.5 ([M þ H]þ). 1H NMR (400 MHz,
d-DMSO, 298 K): 8.76 (s, 1H), 8.70 (d, J = 4 Hz, 1H), 8.58
(d, J = 4 Hz, 1H), 8.40 (d, J = 1.2 Hz, 1H), 8.38 (d, J = 1.2 Hz,
1H), 8.01 (d, J = 8 Hz, 1H), 7.98 (t, J = 1.2 Hz, 1H), 7.74 (t, J =
2 Hz, 1H), 7.45 (t, J = 0.8 Hz, 1H), 7.41 (d, J = 1.2 Hz, 1H), 7.36
(d, J = 8.8 Hz, 2H), 7.29 (t, J = 4 Hz, 1H), 7.24 (d, J = 8 Hz,
[Ir(ppy)2(L1)]PF6 (1). A mixture of Ir2(ppy)4Cl2 (267.7 mg,
0.25 mmol) and L1 (181.6 mg, 0.5 mmol) in methanol-
dichloromethane (30 mL, v/v = 1:2) was refluxed with exclusion
of light for 4 h. After cooling to ambient temperature, to the
solution was added a methanol solution (2 mL) of potassium
hexafluorophosphate (93 mg) with stirring for 30 min. The
solution was evaporated to dryness and the residue was dis-
solved in dichloromethane (5 mL). The solution was then
chromatographed on a silica gel column using dichlorometha-
ne-methanol (v/v = 2:1) as eluent to give an orange product.
Yield: 82%. Anal. Calcd for C46H34IrN6F6P: C, 54.81; H, 3.40;
N, 8.34. Found: C, 54.51; H, 3.15; N, 8.56. ESI-MS (m/z): 863.3
([M - PF6]þ). 1H NMR (400 MHz, CD3CN, 298 K): 8.61
(s, 1H), 8.50 (t, J = 4.2 Hz, 2H), 8.07-8.17 (m, 5H), 7.94 (d,
J = 2 Hz, 1H), 7.93 (s, 1H), 7.82-7.88 (m, 5H), 7.72-7.74 (m,
3H), 7.63 (d, J = 9.6 Hz, 1H), 7.50 (t, J = 1.6 Hz, 1H), 7.36 (d,
J = 7.6 Hz, 1H), 7.064-7.091 (m, 4H), 6.94-6.96 (m, 2H), 6.64
(d, J = 12 Hz, 2H), 6.33 (d, J = 12 Hz, 2H), 4.47 (s, 2H), 4.06
(s, 1H). IR (KBr, cm-1): 2213m (CtC).
[Ir(ppy)2(L2)]PF6 (2). This compound was prepared by the
same synthetic procedures as that of 1 except using L2 instead of
L1. Yield: 82%. Anal. Calcd for C52H39IrN7F6P: C, 56.82; H,
3.58; N, 8.82. Found: C, 56.40; H, 3.57; N, 8.72. ESI-MS (m/z):
954.2 ([M - PF6]þ). 1H NMR (400 MHz, CD3CN, 298 K): 8.60
(d, J = 4.2 Hz, 2H), 8.50 (t, J = 3.2 Hz, 2H), 8.07-8.15 (m, 4H),
7.98 (d, J = 5.2 Hz, 1H), 7.91 (s, 1H), 7.81-7.87 (m, 7H), 7.71
(d, J = 4.8 Hz, 1H), 7.61 (d, J = 8 Hz, 1H), 7.50 (t, J = 2.4 Hz,
1H), 7.36 (d, J = 8 Hz, 2H), 7.33 (t, J = 3.2 Hz, 2H), 7.21 (d, J =
8.4 Hz, 2H), 7.06-7.19 (m, 5H), 6.94 (t, J = 4 Hz, 2H), 6.30 (m,
2H), 4.89 (s, 4H). IR (KBr, cm-1): 2213m (CtC).
[Ir(ppy)2(L3)]PF6 (3). This compound was prepared by the
same synthetic procedures as that of 1 except using L3 instead of
L1. Yield: 75%. Anal. Calcd for C45H33IrN5F6PS: C, 53.35; H,
3.28; N, 6.91. Found: C, 53.10; H, 3.15; N, 7.21. ESI-MS (m/z):
868.0 ([M - PF6]þ). 1H NMR (400 MHz, CD3CN, 298 K): 8.47
(d, J = 3.2 Hz, 2H), 8.09-8.12 (m, 4H), 7.94 (d, J = 1.6 Hz, 1H),
7.87 (d, J = 4.2 Hz, 1H), 7.84-7.88 (m, 4H), 7.74-7.76 (m, 2H),
7.63-7.65 (m, 2H), 7.50 (t, J = 4 Hz, 1H), 7.30 (d, J = 1.2 Hz,
1H), 7.24 (d, J = 8.8 Hz, 2H), 7.04-7.09 (m, 5H), 6.99 (d, J = 2
Hz, 1H), 6.66 (d, J = 8 0.8 Hz, 2H), 6.30 (m, 2H), 5.56 (s, 1H),
4.09 (s, 2H). IR (KBr, cm-1): 2213m (CtC).
[Ir(ppy)2(L4)]PF6 (4). This compound was prepared by the
same synthetic procedures as that of 1 except using L4 instead of
L1. Yield: 79%. Anal. Calad for C51H38IrN6F6PS: C, 55.48; H,
3.47; N, 7.61. Found: C, 55.10; H, 3.15; N, 7.86. ESI-MS (m/z):
959.2. ([M - PF6]þ). 1H NMR (400 MHz, CD3CN, 298 K): 8.51
(d, J = 4.6 Hz, 2H), 8.49 (t, J = 5.6 Hz, 2H), 8.07-8.13 (m, 5H),
7.99 (d, J = 5.2 Hz, 1H), 7.93 (s, 1H), 7.82-7.86 (m, 6H), 7.72
(d, J = 5.2 Hz, 1H), 7.62 (d, J = 5.6 Hz, 1H), 7.50 (t, J = 3.6 Hz,
1H), 7.30 (t, J = 1.2 Hz, 2H), 7.25 (d, J = 8.8 Hz, 2H),
7.05-7.09 (m, 4H), 6.95 (t, J = 4 Hz, 1H), 6.93 (d, J = 6.4
Hz, 1H), 6.80 (d, J = 8.8 Hz, 2H), 6.30 (m, 2H), 4.79 (s, 2H), 4.98
(s, 2H). IR (KBr, cm-1): 2214m (CtC).
Physical Measurements. 1H NMR spectra were measured on a
Bruker Avance III (400 MHz) spectrometer with SiMe4 as the
internal reference. Electrospay ion mass spectra (ESI-MS) were
performed on a Finnigan LCQ mass spectrometer using dichlor-
omethane-methanol as the mobile phase. Elemental analyses
(C, H, N) were recorded on a Perkin-Elmer model 240C auto-
matic instrument. UV-vis absorption spectra were measured
on a Perkin-Elmer Lambda 25 UV-vis spectrometer. IR spectra
were recorded on
a Magna 750 FT-IR spectrophoto-
meter with KBr pellets. Emission and excitation spectra were
recorded on a Perkin-Elmer LS55 luminescence spectrometer
with a R928 red-sensitive photomultiplier. The cyclic voltam-
mograms (CV) were made with a potentiostat/galvanostat
model 263A in acetonitrile solutions containing 0.1
M

5606 Organometallics, Vol. 28, No. 19, 2009
Zhao et al.
(Bu4N)PF6 as the supporting electrolyte. The CV was per-
formed at a scan rate of 200 mV s-1. Platinum and glassy
graphite were used as the counter and working electrodes,
respectively, and the potential was measured against a Ag/AgCl
reference electrode.
Table 1. Crystallographic Data for 1
empirical formula
temp, K
C46H34F6IrN6P
293(2)
space group
˚
˚
b, A
˚
c, A
P21/n
a, A
17.081(5)
12.896(4)
18.368(5)
98.918(5)
3997(2)
4
1.675
3.451
0.71073
0.0562
0.1473
1.136
Crystal Structural Determination. Crystals of 1 were obtained
by diffusion of n-pentane into a dichloromethane solution of 1.
Data collection was performed on a SATURN70 CCD diffract-
ometer using the ω scan technique at room temperature using
β, deg
3
˚
V, A
Z
˚
graphite-monochromated Mo KR (k = 0.71073 A) radiation.
F
calcd, g/cm-3
μ, mm-1
Absorption corrections by SADABS were applied to the inten-
sity data. The structures were solved by direct methods. The
heavy atoms were located from E-maps, and the rest of the non-
hydrogen atoms were found in subsequent Fourier maps. All
non-hydrogen atoms were refined anisotropically, and the
hydrogen atoms were generated geometrically and refined with
isotropic thermal parameters. The structures were refined on F2
by full-matrix least-squares methods using the SHELXTL-97
program package. The crystallographic data are summarized in
Table 1.
˚
radiation (λ, A)
R1(Fo)a
wR2(Fo)b
GOF
P
P
P
P
a R1 = |Fo - Fc|/ Fo. b wR2 = [w(Fo2 - Fc2)2]/ [w(Fo2)]1/2
.
atoms. All calculations were performed using the Gaussian 03
program package.20
Theoretical Calculations. The computational method used
was the density functional theory (DFT),14 where the gradient-
corrected correlation functional PBE1PBE15 was employed for
the calculations. The geometry structures of complex 1 and Zn2þ
binding species 1/Zn2þ were optimized as isolated molecules
from the solvent phase at the DFT level of theory. We performed
geometry optimization from initial geometry of C1 symmetry
at first from the X-ray structure. Therefore, 60 singlet absorp-
tions and six triplet emissions were obtained to determine the
vertical excitation energies for 1 and the simplified Zn2þ binding
species 1/Zn2þ in acetonitrile using the time-dependent DFT
(TD-DFT)16 calculations, respectively. The polarized conti-
nuum model method (CPCM)17 with acetonitrile as solvent
was used to calculate all the electronic structures in solution.
In the calculations, a “double-ξ” quality (SDD) basis set con-
sisting of effective core potentials (ECPs) for the iridium(II)
atom proposed by Hay and Walt18 was used to precisely describe
the molecular properties. One additional f-type polarization
function is implemented for the iridium(III) atom (R =
0.938),19 and the 6-31G(p,d) basis set was used for the remaining
Results and Discussion
Preparations and Characterization. Syntheses of (4-iodo-
phenyl)bispyridin-2-ylmethylamine and (4-iodophenyl)pyridin-
2-ylmethylthiophen-2-ylmethylamine need the use of strong
bases such as NaNH2 and NaH because of the difficulty
in removal of the amino proton of aniline when one proton in
N-aniline is substituted by a 2-methylpyridine group. The
functionalized 2,20-bipyridyl ligands L1-L4 were prepared by
reaction of 5-ethynyl-2,20-bipyridine with 4-iodophenylamine
derivatives according to the Sonogashira-Hagihara cross-
coupling method.21 Cyclometalated iridium(III) complexes
were synthesized by reaction of the corresponding functiona-
lized 2,20-bipyridyl ligand (L1-L4) with Ir2(ppy)4Cl2 in
CH2Cl2-CH3OH (v/v = 2:1) under reflux, followed by me-
tathesis with KPF6.22 The desired iridium(III) complexes 1-4
(Scheme 1) were obtained as deep orange solids in good yields
through chromatographic purification on silica gel columns.
They were characterized by elemental analyses, positive ion
ESI-MS spectrometry, and 1H NMR and IR spectroscopy, and
by X-ray crystallography for complex 1.
(14) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
(15) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1997, 78,
1396.
(16) (a) Casida, M. E.; Jamorski, C.; Casida, K. C.; Salahub, D. R. J.
Chem. Phys. 1998, 108, 4439. (b) Stratmann, R. E.; Scuseria, G. E.; Frisch,
M. J. J. Chem. Phys. 1998, 109, 8218.
(17) (a) Cossi, M.; Regar, N.; Scalmani, G.; Barone, V. J. Comput.
Chem. 2003, 24, 669–681. (b) Barone, V.; Cossi, M. J. Chem. Phys. A 1998,
102, 1995–2001. (c) Cossi, M.; Barone, V. J. Chem. Phys. 2001, 115, 4708–
4717.
A perspective view of complex 1 is depicted in Figure 1.
The iridium(III) center exhibits a distorted octahedral geo-
metry composed of C2N4 donors from cyclometalated
ppy and chelating 2,20-bpy ligands. The C donors are cis-
oriented, whereas the N donors from cyclometalated ppy are
trans-arranged, as revealed by previous structural studies on
iridium(III) complexes containing ppy ligands.23 The Ir-N
0
(18) (a) Wadt, W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284. (b) Hay,
P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299.
˚
distances [2.152(6) and 2.162(5) A] for 2,2 -bipyridyl are
˚
obviously longer than those [2.017(7) and 2.062(6) A] for
€
(19) Ehlers, A. W.; Bohme, M.; Dapprich, S.; Gobbi, A.; Hollwarth,
A.; Jonas, V.; Kohler, K. F.; Stegmann, R.; Veldkamp, A.; Frenkig, G.
€
€
ppy due most likely to the remarkable trans influence of the
C donors for the former.
Chem. Phys. Lett. 1993, 208, 111.
(20) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.;
Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.;
Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson,
G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.;
Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai,
H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken,
V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev,
O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.;
Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakr-
zewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.;
Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.;
Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov,
B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.;
Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.;
Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.;
Gonzalez, C.; Pople, J. A. Gaussian 03, revision C.02; Gaussian, Inc.:
Wallingford, CT, 2004.
Photophysical and Electrochemical Properties. The absorp-
tion and emission spectral data of iridium(III) comp-
lexes measured in acetonitrile solutions are summarized in
Table 2. The UV-vis spectra of 1-4 are featured by intense
(21) Okubo, J.; Shinozaki, H.; Koitabashi, T.; Yomura, R. Bull.
Chem. Soc. Jpn. 1998, 71, 329.
(22) Neve, F.; Crispini, A.; Campagna, S.; Serroni, S. Inorg. Chem.
1999, 38, 2250.
(23) (a) Waern, J. B.; Desmarets, C.; Chamoreau, L. M.; Amouri, H.;
Barbieri, A.; Sabatini, C.; Ventura, B.; Barigellettit, F. Inorg. Chem.
2008, 47, 3340–3348. (b) Lo, K. K.-W.; Chung, C. K.; Lee, T. K.-M.; Lui, L.-
H.; Tsang, K. H.-K.; Zhu, N. Inorg. Chem. 2003, 42, 6886. (c) Neve, F.; La
Deda, M.; Crispini, A.; Bellusci, A.; Puntoriero, F.; Campagna, S. Organo-
metallics 2004, 23, 5856–5863. (d) Neve, F.; Crispini, A.; Campagna, S.;
Serroni, S. Inorg. Chem. 1999, 38, 2250.

Article
Organometallics, Vol. 28, No. 19, 2009 5607
localized on the 2,20-bipyridyl in the functionalized 2,20-
bipyridyl ligand, as confirmed by TD-DFT calculations
(vide infra). It has been suggested that the lowest energy
absorption of iridium(III) complexes containing two ppy
and one diimine ligand with a π-conjugated or amino-sub-
stituted system is always dominated by the intraligand ππ*
transition centered on the diimine ligand.7a,e,27,28 This lowest
energy absorption is quite useful for chemical sensing due to
the high sensitivity to ion binding. As presented in Table 2,
the corresponding low-energy absorptions of 1-4 exhibit a
slight distinction due to the electronic effects of the diverse
substituents in the corresponding functionalized bpy ligands
L1-L4. In contrast to the iridium(III) complexes, the free
ligands L1-L4 display a corresponding ICT band with the
maximum around 360 nm (Figure S1, Supporting In-
formation). A red-shift of about 60 nm in the iridium(III)
complexes compared with that in the free ligands may be
related to the increased electron-accepting properties of the
2,20-bipyridyl π* orbitals upon coordination to an iridium-
(III) ion.7e,28a
Figure 1. ORTEP drawing of the cation of 1 with atomic
labeling scheme. Hydrogen atoms are omitted for clarity. Ther-
mal ellipsoids are shown at the 30% probability level.
Table 2. UV-Vis Absorption and Luminescence Data for 1-4
medium
(T/K)
λ
abs/nm
(ε/dm3 mol-1 cm-1
complex
)
λem/nm
The iridium(III) complexes 1-4 display weak emission at
615-650 nm in both the solid state and solution at ambient
temperature. Similar patterns of emission spectra of 1-4
suggest that diverse substituents of the corresponding func-
tionalized bpy ligands L1-L4 exert inappreciable influence
on the luminescence properties. With reference to previous
spectroscopic studies on a series of cyclometalated iridium-
(III) complexes with an amino-substituted or π-extended
diimine ligands,27a,28a,c,d the weak emission in 1-4 is likely
due to the lowest energy 3ILCT exited state, as verified from
DFT calculations (vide infra). At 77 K, the emission maxima
of 1-4 show a small blue-shift in both the solid state and
frozen glasses compared with those measured at ambient
temperature, suggesting that the emission originates from
3ILCT, which was observed in cyclometalated iridium(III)
diimine complexes.6a,d
1
MeCN (298)
254 (44 571), 267 (46 497),
287 (45 869), 416 (25 009)
618
MeCN (77)
solid (298)
solid (77)
605, 652
621
610, 659sh
616
2
3
4
MeCN (298)
259 (43 415), 294 (41 241),
421 (24 643)
MeCN (77)
solid (298)
solid (77)
604, 647
634
611, 660sh
617
MeCN (298)
252 (29 510), 267 (30 403),
285 (29 772), 407 (16 760)
MeCN (77)
solid (298)
solid (77)
605, 648sh
648
524, 599sh
617
MeCN (298)
255 (48 056), 292 (45 481),
416 (25 902)
MeCN (77)
solid (298)
602, 649sh
637
The electrochemical properties of 1-4 were studied by
cyclic voltammetry (CV) in acetonitrile solutions, and the
data are summarized in Table 3. They have similar eletro-
chemical behavior independent of the R groups attached to
aniline amine (Scheme 1). They exhibit an irreversible oxida-
tion wave in the anodic side and a quasi-reversible reduction
wave in the cathodic region. With reference to the previous
studies on electrochemical behavior of a series of iridium(III)
diimine complexes,6d,23c,d,29,30 the oxidation wave at ca. þ1.0
V vs Fc in the anodic region can be assigned as an iridium-
(III)-centered oxidation process, whereas the reduction
waves at ca. -1.6 V vs Fc originate likely from reduction
of the 2,20-bipyridyl ligand.
absorptions at 255-350 nm together with a low-energy
absorption band with the maximum at 410-430 nm. With
reference to previous studies on iridium(III) diimine com-
plexes,24-26 the high-energy bands arise most probably from
spin-allowed intraligand transition from ppy and functiona-
lized bpy ligands. The low-energy absorption band with the
maximum at 410-430 can be assigned to an intraligand
charge transfer (ILCT) character from the HOMO (π)
residing on the fragment CtC-C6H4-R to the LUMO (π*)
(24) (a) Colombo, M. G.; Hauser, A.; Gudel, H. U. Inorg. Chem.
1993, 32, 3088–3092. (b) Wilkinson, A. J.; Puschmann, H.; Howard, J. A. K.;
Foster, C. E.; Williams, J. A. G. Inorg. Chem. 2006, 45, 8685–8699.
(c) Dixon, I. M.; Collin, J. P.; Sauvage, J. P.; Flamigni, L.; Encinas, S.;
Barigelletti, F. Chem. Soc. Rev. 2000, 29, 385–391. (d) Didier, P.; Ortmans,
I.; Kirsch-De Mesmaeker, A.; Watts, R. J. Inorg. Chem. 1993, 32, 5239–
5245. (e) Calogero, G.; Giuffrida, G.; Serroni, S.; Ricevuto, V.; Campagna, S.
Inorg. Chem. 1995, 34, 541–545.
(25) (a) Juris, A.; Balzani, V.; Barigelletti, F.; Campagna, S.; Belser,
P.; von, Zelewsky, A. Coord. Chem. Rev. 1988, 84, 85–277. (b) Didier, P.;
Ortmans, I.; Kirsch-De Mesmaeker, A.; Watts, R. J. Inorg. Chem. 1993, 32,
5239–5245. (c) Wilde, A. P.; King, K. A.; Warrts, R, J. J. Phys. Chem. 1991,
95, 629–634.
(26) (a) Tsuboyama, A.; Iwawaki, H.; Furugori, M.; Mukaide, T.;
Kamatani, J.; Igawa, S.; Moriyama, T.; Miura, S.; Takiguchi, T.;
Okada, S.; Hoshino, M.; Ueno, K. J. Am. Chem. Soc. 2003, 125,
12971–12979. (b) Tamayo, A. B.; Garon, S.; Sajoto, T.; Djurovich, P. I.;
Tsyba, I. M.; Bau, R.; Thompson, M. E. Inorg. Chem. 2005, 44, 8723–8732.
(c) Yang, C.-H.; Li, S.-W.; Chi, Y.; Cheng, Y.-M.; Yeh, Y.-S.; Chou, P.-T.; Lee,
G.-H.; Wang, C.-H.; Shu, C.-F. Inorg. Chem. 2005, 44, 7770–7780.
UV-Vis Absorption Response to Metal Ion Binding. The
metal ion binding properties of 1-4 were investigated using
the corresponding perchlorate salts in acetonitrile solutions.
(27) (a) Flamigni, L.; Barbieri, A.; Sabatini, C.; Ventura, B.; Bar-
igelletti, F. Top. Curr. Chem. 2007, 281, 143–230. (b) Colombo, M. G.;
Hauser, A.; G€udel, H. U. Top. Curr. Chem. 1994, 171, 143–171.
(28) (a) Leslie, W.; Poole, R. A.; Murray, P. R.; Yellowlees, L. J.;
Beeby, A.; Williams, J. A. G. Polyhedron 2004, 23, 2769–2777. (b) Glusac,
K. D.; Jiang, S.; Schanze, K. S. Chem. Commun. 2002, 21, 2504–2505.
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Trans. 2004, 623–631.
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3464–3471.

5608 Organometallics, Vol. 28, No. 19, 2009
Zhao et al.
Table 3. Cyclic Voltammetric Data for 1-4 in 0.1 M (Bu4N)(PF6)
Acetonitrile Solutionsa
complex
Ea (V)
Ec1/2 (V)
1
2
3
4
þ0.97b
þ1.07b
þ1.01b
þ1.08b
-1.65c
-1.66c
-1.64c
-1.58c
a Potential data in volts vs Fcþ/Fc are from single scan cyclic
voltammograms recorded at 25 °C in 0.1 M acetonitrile solution of
(Bu4N)(PF6). Detailed experimental conditions are given in the Experi-
mental Section. b Irreversible waves. c Quasi-reversible couples.
Figure 3. UV-vis spectral changes for complex 2 (20 μM) upon
addition of Hg2þ (0-1.0 equiv) in CH3CN. Inset: plot of
UV-vis titration data (points), showing the fitting to a 1:1
binding isotherm (solid line) at 421 nm.
Complex 2 exhibits remarkable spectroscopic response to
Zn2þ, Hg2þ, and Cd2þ in a similar way to that described for
complex 1 to Zn2þ and Cd2þ. Figure 3 shows the UV-vis
absorption spectral changes of 2 by addition of up to 1.0
equiv of Hg2þ. With gradual decrease of the absorbance at
421 and 306 nm, a new band at 355 nm appeared and
increased progressively. A metal ion binding species with
1:1 complexation ratio was supported by a good agreement
of the experimental data with the theoretical fitting (Figure 3,
inset). Similar UV-vis spectroscopic changes were also
observed by titration of complex 2 with Zn2þ or Cd2þ
(Figures S4 and S5, Supporting Information) in acetonitrile.
The binding constants (log K) calculated from the absor-
Figure 2. UV-vis spectral changes for complex 1 (20 μM) upon
addition of Zn2þ (0-50 equiv) in CH3CN. Inset: the Job’s plot
for determining the stoichiometric ratio between complex 1 and
Zn2þ, where the variations of absorbance at 352 nm were
measured as a function of molar ratio Xm ([Zn2þ]/[Zn2þ] þ [1])).
bance changes of complex 2 upon addition of Zn2þ, Hg2þ
,
and Cd2þ are 8.18 ( 0.04, 8.01 ( 0.1, and 7.6 ( 0.06,
respectively.
Figure 2 shows the changing UV-vis spectroscopic curves of
1 by titration with Zn2þ. Upon addition of up to 50 equiv of
Zn2þ to an acetonitrile solution of 1, the absorption bands at
287 and 421 nm decrease gradually with appearance of new
absorptions at 267 and 358 nm. Well-defined isosbestic
points were observed at 378, 318, and 275 nm, indicating
that only one new absorbing species was produced during the
titration. Job’s equilibrium plot analysis suggested the for-
mation of a stoichiometric ratio of 1:2 between Zn2þ and 1,
as shown in the inset of Figure 2. A plot of absorbance versus
Xm ([Zn2þ]/[Zn2þ] þ [1])) shows that the absorbance goes
through a maximum at a molar fraction of ca. 0.33. The
binding constants log K1 and log K2 for 1:1 and 2:1 binding
ratios between 1 and Zn2þ were estimated to be 4.21 ( 0.16
and 4.18 ( 0.05, respectively, by a nonlinear least-squares
fitting using the absorbance versus the concentration
of the metal ions.31,32 Analogous to that of Cd2þ, addition
up to 50 equiv of Cd2þ to acetonitrile solutions of 1 gave rise
to obvious UV-vis spectroscopic changes (Figure S2,
Supporting Information) because of the similar electronic
and coordination character between Zn2þ and Cd2þ. Bind-
ing constants log K1 and log K2 for 1:1 and 2:1 binding ratios
between 1 and Cd2þ are 3.3 ( 0.15 and 3.05 ( 0.03,
respectively. They are obviously lower than those between
1 and Zn2þ, suggesting that 1 displays a better selective
To further investigate the influence of variations in accep-
tors on metal ion binding properties of cyclometalated
iridium(III) complexes, a thiophene-containing moiety was
introduced as the binding sites.33 As the bonding character of
thiophene S donor is quite different from that of the pyridine
N donor, it is expected that thiophene-containing complexes
3 and 4 would cause unique sensing properties to some metal
ions. With addition of up to 1.5 equiv of Cu2þ into an
acetonitrile solution of complex 3, the absorption band at
ca. 410 nm decreased gradually with the appearance of a new
band at 385 nm, where well-defined isosbestic points were
observed at 273, 315, and 396 nm (Figure 6a). Nevertheless, if
more than 1.5 equiv of Cu2þ was added to an acetonitrile
solution of 3, the new absorption band at 385 nm was
obviously reduced with gradual addition of up to 50 equiv
of Cu2þ (Figure 6b). In order to determine the stoichiometric
ratio of the formed species during titration of 3 with Cu2þ
,
Job’s method was also applied to the absorbance versus Xm
([Cu2þ]/[Cu2þ] þ [1]). The stoichiometric ratio of the
species formed between Cu2þ and complex 3 was estimated
as 1:2 (Figure S10, Supporting Information). Due to the ab-
normal changes in the process of spectroscopic titration,
the binding constants could not be exactly estimated by
the method mentioned above. The result is consistent with
binding to Zn2þ
.
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Article
Organometallics, Vol. 28, No. 19, 2009 5609
the assumption that each Cu2þ is surrounded by a square-
planar N2S2 chromophore from two molecules of complex 3,
but no new species is formed when more than 1.5 equiv of Cu2þ
is added. Distinct UV-vis spectroscopic changes were unob-
served upon addition of other metal ions except that Fe3þ gave
rise to a faint effect to the UV-vis spectrum of 3, suggesting
that complex 3 displays a highly selective binding to Cu2þ
.
The UV-vis spectrum of complex 4, containing both
pyridine and thiophene moieties, displayed significant
changes upon binding to Cu2þ, Hg2þ, or Fe3þ ion. By
addition of up to 1.0 equiv of Cu2þ ion to an acetonitrile
solution of 4, a new band at 375 nm appeared and increased
progressively with gradual decrease of the absorbance at 416
and 310 nm (Figure S12, Supporting Information). A plot of
the absorbance at 416 nm of 4 versus the concentration of
Cu2þ is consistent with the theoretical fitting and suggests
that the stoichiometric ratio of the formed species is 1:1
(Figure S12, inset, Supporting Information). The UV-vis
spectroscopic changing curves of 4 in acetonitrile solutions
upon titration with Hg2þ or Fe3þ were depicted in Figure S13
or S14 (Supporting Information), respectively. The binding
constants (log K) calculated from the absorbance changes of
complex 4 upon addition of Cu2þ, Hg2þ, and Fe3þ are 7.24 (
0.35, 7.42 ( 0.1, and 7.5 ( 0.13, respectively.
Figure 4. Emission spectral changes (λex = 360 nm) for 1
(20 μM) upon addition of Zn2þ (0-50 equiv) in CH3CN. Inset:
Luminescence images in CH3CN solution before (a) and after
(b) addition of 1 equiv of Zn2þ for complex 1.
Upon binding to specific metal ions, the new observed
bands around 350-380 nm in the absorption spectra of
iridium(III) complexes 1-4 arise most probably from
π(ppy)fπ*(bpyCtCC6H4-R) LLCT and d(Ir)fπ*(bpyCt
CC6H4-R) MLCT transitions, whereas the low-energy ILCT
band is significantly weakened, as revealed from DFT
calculations (vide infra). Once the binding sites in 1-4 are
bonded to a specific metal ion, the ILCT process is signifi-
cantly suppressed, resulting in the LLCT/MLCT transition
as the lowest- energy state. In the case of 3, an obvious
decrease in the LLCT/MLCT absorption at 385 nm with
addition of more than 1.5 equiv of Cu2þ implies that the
LLCT/MLCT absorption band could be perturbed by
a paramagnetic Cu2þ ion with a high concentration.34
Luminescence Sensing Properties. The luminescence sensing
properties of iridium(III) complexes 1-4 were investigated by
emission spectral titration in acetonitrile solutions. With ex-
citation at 350 nm e λex e 430 nm, which spans the whole
absorption region of both LLCT/MLCT and ILCT states, 1-4
Figure 5. Emission responses of complex 1 (20 μM) at 617 nm
as a function of various added metal ions (50 equiv).
1 before and after titration with Zn2þ. This luminescence
enhancement is well consistent with the corresponding
UV-vis spectral changes during titration of 1 with Zn2þ
,
where the Zn2þ ion binding results in a conversion of the
3
3
lowest energy excited state from ILCT to LLCT/3MLCT
triplet character. The emission response behavior at 617 nm
for complex 1 (20 μM) as a function of 50 equiv of various
metal ions is shown in Figure 5, revealing that 1 displays high
binding selectivity to Zn2þ and Cd2þ ions.
3
are weakly luminescent at ca. 617 nm because the ILCT
transition is mostly populated as the lowest energy excited
state. As predicted from TD-DFT calculations (vide infra),
once the binding sites are bound to a specific metal ion, highly
Titration of complex 2 with 1 equiv of Zn2þ, Cd2þ
,
and Hg2þ in acetonitrile solutions resulted in a 30-, 27-, and
24-fold increase (I - I0/I0) in emission intensity (Figure S9,
Supporting Information), respectively. This indicates that
complex 2 displays a remarkable switching on luminescence
sensing effect to Zn2þ, Cd2þ, and Hg2þ. The titration data
fit to a 1:1 binding ratio between 2 and the correspond-
ing Zn2þ, Cd2þ, or Hg2þ ion (Figures S6-S8, Supporting
Information), coinciding well to that from UV-vis absorp-
tion spectral titration.
3
3
luminescent LLCT and MLCT states are thus populated
as the lowest energy excited states, resulting in a signi-
ficant luminescence enhancement for 1-4 in response to
metal ion binding. The switching behavior from 3ILCT
3
to LLCT/3MLCT triplet excited state in 1-4 upon binding
to a specific metal ion and the corresponding luminescence
off-on effect are very helpful for heavy metal ion detection and
identification.
Figure 4 depicts the emission spectral changes of complex
1 as well as a naked-eye detectable luminescence response to
Zn2þ binding. The luminescence intensity of 1 (20 μM) shows
a 25-fold (I - I0/I0) enhancement upon addition of 50 equiv
of Zn2þ, where I0 and I are the emission intensity of complex
As demonstrated in UV-vis spectroscopic titration
(Figure 6a and b), complex 3 exhibits specific binding
properties to Cu2þ apart from a weak binding to Fe3þ
.
Figure 7 shows the emission response of complex 3
(20 μM) at 617 nm as a function of various added metal ions
(1.5 equiv), suggesting that complex 3 exhibits high binding
selectivity toward Cu2þ. Addition of up to 1.5 equiv of
Cu2þ to an acetonitrile solution of complex 3 induced
7-fold emission enhancements (Figure 6c), but the emission
(34) (a) Manrtinez, R.; Zapata, F.; Caballero, A.; Espinosa, A.;
Tarraga, A.; Monlia, P. Org. Lett. 2006, 8, 3235–3238. (b) Varnes, A.
V.; Dodson, R. B.; Whery, E. L. J. Am. Chem. Soc. 1972, 94, 946–950.

5610 Organometallics, Vol. 28, No. 19, 2009
Zhao et al.
Figure 8. Emission spectral changes (λex = 360 nm) for 4
(20 μM) upon addition of Cu2þ (0-1.0 equiv) in CH3CN. Inset:
plot of the fluorescence titration data (points) showing the fit to
a 1:1 binding isotherm (solid line) at 421 nm.
Figure 6. UV-vis spectral changes of complex 3 (20 μM) in
CH3CN upon addition of 0-1.5 equiv (a) and 1.5-5 equiv (b) of
Cu(ClO4)2, and emission spectral titration of complex 3 (20 μM)
upon the addition of 0-1.5 equiv (c) and 1.5-5 equiv (d) of
Cu(ClO4)2.
enhancement, respectively. Figure 8 depicts the emission
spectral changes of complex 4 upon addition of Cu2þ, and
the inset shows the changes of the emission intensity as a
function of the Cu2þ ion concentration. The enhanced
luminescence response for complex 4 at 617 nm as a function
of various metal ions is shown in Figure S17 (Supporting
Information).
TD-DFT Calculation. To further understand the nature of
absorption and emissive transition characters of complexes
1-4 and the effect of metal ion titration on the photophysical
properties, calculations based on TD-DFT for 1 and simpli-
fied Zn2þ binding species 1/Zn2þ were performed. The
calculated frontier orbitals for the low-lying transitions
revealed dramatic differences between 1 (Tables S1 and S2,
Supporting Information) and 1/Zn2þ (Tables S3 and S4,
Supporting Information). As shown in Figure 9, the HOMO
(π) for 1 mainly resides on the fragment CtC-C6H4-R and
the LUMO (π*) is localized on the 2,20-bipyridyl (bpy) in the
functionalized 2,20-bipyridyl ligand bpyCtC-C6H4-R.
Thus, the calculated lowest singlet at 466 nm is featured by
a HOMOfLUMO intraligand charge transfer (ILCT) tran-
sition for 1. The calculated absorptions at 378 and 346 nm
are from HOMO-2fLUMO and HOMO-1fLUMO
states with LLCT/MLCT character (Table S2, Supporting
Information). In strikingly contrast, the HOMO (Figure 9)
resides on both ppy (51.9%) and Ir(5d) (44.1%) for Zn2þ
binding species 1/Zn2þ (Table S3, Supporting Information),
although the LUMO is still predominately located on 2,20-
bipyridyl (79.6%) and the CtC-C6H4-R fragment (16.4%)
of the bpyCtC-C6H4-R ligand. The HOMO-2 is contrib-
uted by the bpyCtC-C6H4-R ligand (62.6%) and Ir(5d)
(25.9%). The HOMO-3 is mainly composed of ppy
(47.7%), Ir(5d) (24.5%), and the bpyCtC-C6H4-R ligand
(27.8%). The lowest lying absorption (HOMOfLUMO
transition) at 529 nm is typical of a LLCT/MLCT state,
but the oscillator strength is too weak (0.0002) to be experi-
mentally detected. The calculated excitations of absorption
at 359 nm from HOMO-3fLUMO (54.9% contribution)
and HOMO-2fLUMO (30.0% contribution) transitions
are thus ascribed as an admixture of LLCT/MLCT/ILCT
and ILCT/MLCT character, respectively. The calculation of
emission properties of 1 and 1/Zn2þ was also carried out by
TD-DFT. The compositions of frontier orbitals (HOMO
and LUMO) are comparable to the character of those
from the singlet manifold mentioned above. The emission
Figure 7. Emission responses of complex 3 (20 μM) at 617 nm
as a function of various added metal ions (1.5 equiv).
intensity was then gradually reduced when more Cu2þ was
added (Figure 6d). Under UV light irradiation at 365 nm, the
luminescence variation of complex 3 in response to different
amounts of Cu2þ was naked-eye detectable, as indicated in
Figure S11 (Supporting Information). This luminescence
off-on-off behavior during the titration is indeed interest-
ing and can be reasonably elucidated. The luminescence
switch-on effect during titration of 3 with Cu2þ is most likely
due to blocking of the quenching pathway upon Cu2þ
binding to the amine N and thiophene S donors. In con-
trast, when more than 1.5 equiv of Cu2þ is added to the aceto-
nitrile solution of 3, the paramagnetism from excess
3
Cu2þ would exert severe influence on the LLCT/3MLCT
emissive state, resulting in some quenching of the lumines-
cence of 3.34 This explanation was further confirmed by the
Job’s curve, which indicated that only one species existed
during titration of 3 with Cu2þ (Figure S10 Supporting
Information).
Complex 4, containing N2S binding donors, displays a
remarkable luminescence response to Cu2þ, Hg2þ, or Fe3þ
.
The emission titration data were fitted to a 1:1 binding
model, suggesting the metal ion binding to amine N, pyridyl
N, and thiopene S donors. Upon titration of complex 4 by
addition of equimolar Cu2þ, Hg2þ, or Fe3þ ion in acetonitrile
solutions, the emission intensity exhibits 20-, 18-, or 15-fold

Article
Organometallics, Vol. 28, No. 19, 2009 5611
Figure 9. Frontrier orbitals of complex 1 and Zn2þ ion binding species 1/Zn2þ
.
3
character of 1 changes from ILCT to 3LLCT/3MLCT
character. On the one hand, the low-energy absorption at
410-430 nm in these iridium(III) complexes is significantly
blue-shifted to around 360 nm upon metal ion binding. On
the other hand, a 9-30-fold luminescence enhancement was
detected upon binding to specific metal ions because of the
conversion from 3ILCT excited state to highly emissive
3LLCT/3MLCT triplet state. Complex 1 shows selective
luminescence recognition to Zn2þ with a 25-fold lumines-
cence enhancement. Particularly, complex 3 acts as a specific
luminescence chemosensor to Cu2þ, which exhibits an unique
off-on-off luminescence switching effect.
transition for 1/Zn2þ upon Zn2þ ion binding.
Conclusions
Four cyclometalated iridium(III) complexes of functiona-
lized 2,20-bipyridyl ligands with a N,N or N,S, N,N,N, or N,
N,S binding donors were synthesized and characterized, and
their binding properties to the metal ions investigated. The
UV-vis and emission spectral titrations demonstrated that
the ion-binding properties of 1-4 could be modulated
through modification of the binding sites by introducing
one or two pyridyl/thiophene as binding sites. Complexes
1-4 display a low-energy absorption at 410-430 nm, arising
from an ILCT transition from the HOMO (π) residing on the
fragment CtC-C6H4-R to the LUMO (π*) localized on the
2,20-bipyridyl (bpy) in the functionalized 2,20-bipyridyl ligand
bpyCtC-C6H4-R as revealed from TD-DFT calculations.
They are weakly luminescent in both solid state and solution
at ambient temperature. Upon binding to specific metal
ions, complexes 1-4 displayed remarkable UV-vis and
emission spectral changes, arising from a conversion of the
lowest energy excited state from ILCT to LLCT and MLCT
Acknowledgment. We thank the NSFC (20625101,
20773128, and 20821061), the 973 project (2007CB815304)
from MSTC, and NSF of Fujian Province (2008I0027 and
2008F3117) for financial support.
Supporting Information Available: Figures giving additional
UV-vis and emission spectral titration curves, tables and
figures regarding the TD-DFT calculations, and an X-ray
crystallographic file in CIF format for the structure determina-
tion of compound 1. This material is available free of charge via
the Internet at http://pubs.acs.org.