Probing Pb2+ cation via the iridium based phosphorescent dye

Article abstract of DOI:10.1016/j.poly.2007.06.029

We demonstrate the concept of Pb²⁺ cation sensing using the emissive Ir(III) complex (1) based on the associated decrease of room-temperature phosphorescence upon forming the 1:1 adduct 1-Pb²⁺. Complex 1 bears two cyclometalated N-ph

Full text of DOI:10.1016/j.poly.2007.06.029

Polyhedron 26 (2007) 4886–4892
www.elsevier.com/locate/poly
Probing Pb2+ cation via the iridium based phosphorescent dye
a
a
a
a,
a,
*
Mei-Lin Ho , Yi-Ming Cheng , Lai-Chin Wu , Pi-Tai Chou *, Gene-Hsiang Lee
,
b
b,
*
Fang-Chi Hsu , Yun Chi
a
Department of Chemistry, National Taiwan University, Taipei 106, Taiwan
Department of Chemistry, National Tsing Hua University, Hsinchu 300, Taiwan
b
Received 11 April 2007; accepted 21 June 2007
Available online 6 July 2007
Abstract
We demonstrate the concept of Pb2+ cation sensing using the emissive Ir(III) complex (1) based on the associated decrease of room-
temperature phosphorescence upon forming the 1:1 adduct 1-Pb2+. Complex 1 bears two cyclometalated N-phenyl pyrazoles with pyr-
azoles residing at the mutual trans dispositions as well as one 3,5-di(pyridyl) pyrazolate chelate. X-ray structural analyses on the adduct
1-Pb2+ confirm the key function of 3,5-di(pyridyl) pyrazolate as it forms chelate interaction with the metal analytes, while quenching of
phosphorescent emission is probably due to the Pb2+ induced perturbation, which increases the intersystem crossing to another lower-
lying triplet state for the host chromophore via an enhanced spin–orbit coupling.
ꢀ 2007 Elsevier Ltd. All rights reserved.
Keywords: Iridium; Phosphorescence; Pyridyl pyrazolate; Pb2+; Cation sensor
1. Introduction
fluorescent sensor for Pb2+ based on the dansyl-tetrapep-
tide framework [4]. In another approach, Czarnik et al.
synthesized 9-anthracene derivatives bearing N-meth-
ylthiohydroxamate ligand, which is capable of quenching
fluorescence in the uncomplexed form with certain selectiv-
ity [5]. Yoon et al. reported a new fluorescent sensor for
Pb2+ based on rhodamine B, which displayed a good selec-
tivity for Pb2+ in acetonitrile solution compared with other
metal ions examined [6]. Although much work has been
devoted in developing various fluorescent chemosensors
for metal ions [4–6], there have been relatively few reports
on the respective phosphorescence chemosensors [7,8].
Recently, extension was made to systems with third-row
transition–metal complexes, among which the majority of
these sensors comprised of a highly efficient phosphores-
cent entity, together with at least one crown ether substitu-
ent that allowed selective binding to the designated metal
cations [9].
Owing to their toxicity to human life, the identification
of heavy metal ions in the environment remained to be
an important issue. Among these, lead should be one of
the most serious environmental concerns and health
threats. It has been known that the toxicity of lead affects
the kidneys, reproductive system, and nervous systems
[1]. The FDA limit for lead in drinking water is
0.015 mg/L (7.2 · 10ꢀ8M) [2]. Development of optical sen-
sors for Pb2+ would thus help to clarify the Pb2+ ions
in vivo as well as to monitor its concentration in the con-
taminated sources [3]. Recently, Chen et al. [1] used a cou-
marine derivative bearing an azacrown binding site as a
new ratiometric fluorescent sensor for Pb2+ in acetonitrile,
for which the specificity is attributed to the carbonyl lariat
formation. Godwin et al. also reported a new ratiometric
Typical signaling involves the switching of the non-emis-
sive intra-ligand (3IL) pp* and metal-to-ligand charge
transfer (3MLCT) states [8,10] such that positive lumines-
cent response versus concentrations of metal cations, e.g.
*
Corresponding authors. Tel.: +886 3 5712956; fax: +886 3 5720864
(Y. Chi).
E-mail addresses: chop@ntu.edu.tw (P.-T. Chou), ychi@mx.nthu.
edu.tw (Y. Chi).
0277-5387/$ - see front matter ꢀ 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2007.06.029

M.-L. Ho et al. / Polyhedron 26 (2007) 4886–4892
4887
Ca2+, Hg2+ or Pb2+, can be obtained. Moreover, it may
allow the design of ‘‘light-controlled ion switches’’, for
which the cation ejection from, e.g. the crown ether can
be triggered by the effective reduction of electron donation
at the receptor on excitation of the nearby chromophore
[11]. As many of heavy metals are known as fluorescence
quenchers via enhanced spin–orbit coupling [12], it is thus
intriguing and of both fundamental and application impor-
tance to examine the interaction between environmental
hazard metals such as Pb2+ (or Hg2+) and heavy metal
complexes that mainly exhibit room-temperature phospho-
rescence in solution due to the strong spin–orbit coupling
effect.
and 13C NMR spectra were recorded on a Varian Mercury
400 instrument; chemical shifts are quoted with respect to
internal standard Me4Si. All synthetic manipulations were
performed under N2 atmosphere, while solvents were used
as received. Synthesis of 3,5-di(2-pyridyl) pyrazole
(py2pz)H, 3-phenyl-5-(2-pyridyl) pyrazole (ppypz)H and
1-phenyl-3,5-dimethyl pyrazole (pdpz)H follows the proce-
dures documented in the literature [17]. Treatment of
(pdpz)H with IrCl3 Æ nH2O in refluxing methoxyethanol
afforded the chloride bridged dimer [(pdpz)2IrCl]2 in 75%
yield [14]; it was then used for subsequent reactions without
further purification.
Aiming at this goal, we present a novel phosphorescence
sensor, in which an 3,5-di(pyridyl)pyrazole (py2pz) moiety
is attached to the heteroleptic Ir(III) complex (1). This
design provides three inherent advantages [13]. First, the
Ir(III) metal atom forms highly stable, octahedral coordi-
nated structure and inherits strong phosphorescent emis-
sion due to the heavy atom effect induced by the Ir(III)
metal center. Moreover, the ancillary cyclometalated N-
phenyl pyrazole ligands, for which their pp* energy levels
are far greater than those of the respective MLCT and
other ligand-centered pp* excited states [14], enable both
the HOMO and LUMO to reside predominantly on the
ancillary py2pz segment. Thirdly, the py2pz fragment is
attached to the Ir(III) metal center and then forming a neu-
tral metal complex. Such a charge-neutral characteristic is
similar to that of the Re(I) and Pt(II) based sensor com-
plexes, but is in sharp contrast to most of the Ru(II) based
polypyridyl sensors [15], for which the net cationic charge
on the overall metal complex is expected to reside, in part,
at the ligand site, giving a much reduced sensitivity in rec-
ognizing metal cations [16]. Finally, the nitrogen atoms of
the py2pz moiety would act as the effective donor site,
allowing the direct recognition of the incoming cationic
analytes. In other words, the py2pz segment acts as crab
chela and can potentially be used for distinguishing metal
ions, simplifying the design strategy (see Scheme 1).
2.2. Preparation of [(pdpz)2Ir(py2pz)] (1)
A mixture of [(pdpz)2IrCl]2 (150 mg, 0.13 mmol), 3, 5-
di(2-pyridyl) pyrazole (py2pzH, 64 mg, 0.29 mmol) and
Na2CO3 (93 mg, 0.88 mmol) in 2-methoxyethanol
(25 mL) was heated to reflux for 6 hours. Excess of water
was added after cooling the solution to room temperature.
The precipitate was collected by filtration and subjected to
silica gel column chromatography, eluting with a mixture
of ethyl acetate and methanol (9:1). Pale yellow crystals
of [(pdpz)2Ir(py2pz)] (1) were obtained from repeated
recrystallization using a mixture of THF and pentane at
room temperature (49 mg, 0.065 mmol, 50%).
Spectral data of 1: MS (FAB, 193Ir): m/z 756 (M+). H
1
NMR (400 MHz, DMSO-d6, 298 K): d 8.43 (d, 1H,
JHH = 4.8 Hz), 7.99 (d, 1H, JHH = 7.6 Hz), 7.84 (t, 1H,
JHH = 7.8 Hz), 7.63 (t, 1H, JHH = 7.6 Hz), 7.60 (t, 1H,
JHH = 8.0 Hz) 7.53 (d, 1H, JHH = 5.6 Hz), 7.47 (d, 1H,
JHH = 8.0 Hz), 7.42 (d, 1H, JHH = 8.4 Hz), 7.39 (s, 1H),
7.07 (t, 2H, JHH = 6.4 Hz), 6.95 (t, 1H, JHH = 8.0 Hz),
6.86 (t, 1H, JHH = 8.0 Hz), 6.75 (t, 1H, JHH = 7.6 Hz),
6.65 (t, 1H, JHH = 7.6 Hz), 6.28 (d, 1H, JHH = 7.2 Hz),
6.23 (d, 1H, JHH = 8.0 Hz), 6.20 (s, 1H), 6.16 (s, 1H),
2.75 (s, 3H, Me), 2.72 (s, 3H, Me), 1.50 (s, 3H, Me), 1.47
(s, 3H, Me). Anal. Calc. for C35H31IrN8: C, 55.61; N,
14.82; H, 4.13. Found: C, 55.37; N, 14.46; H, 4.20%.
2. Experimental
2.3. Preparation of [(pdpz)2Ir(ppypz)] (2)
2.1. General information and materials
A mixture of [(pdpz)2IrCl]2 (200 mg, 0.17 mmol), 3-phe-
nyl-5-(2-pyridyl) pyrazole (ppypzH, 85 mg, 0.38 mmol) and
Na2CO3 (214 mg, 0.21 mmol) in 2-methoxyethanol
(25 mL) was heated to reflux for 6 h. Excess of water was
added after cooling the solution to room temperature.
The precipitate was collected by filtration and subjected
to silica gel column chromatography, eluting with a mix-
ture of ethyl acetate and methanol (9:1). Pale yellow crys-
tals of [(pdpz)2Ir(ppypz)] (2) were obtained from repeated
recrystallization using a mixture of THF and pentane at
room temperature (68 mg, 0.009 mmol, 53%).
Elemental analyses and Mass spectra (operating in FAB
mode) were carried out at the NSC Regional Instrument
Centre at National Chiao Tung University, Taiwan. H
1
Pb2+
N
N
N
N
N
N
N
N
N
Ir
N
N
Ir
N
Ir
N
N
N
N
N
N
N
N
N
N
N
1
Spectral data of 2: MS (FAB, 193Ir): m/z 755 (M+). H
NMR (400 MHz, DMSO-d6, 298 K): d 7.91 (d, 1H,
JHH = 7.6 Hz), 7.82 (t, 1H, JHH = 7.2 Hz), 7.59 (d, 1H,
JHH = 7.2 Hz), 7.52 (d, 1H, JHH = 6.2 Hz), 7.46 (d, 1H,
(1)
(1-Pb2+
)
(2)
Scheme 1.

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M.-L. Ho et al. / Polyhedron 26 (2007) 4886–4892
ꢀ
ꢁꢂ
ꢃ
A0
A0 ꢀ A
eM
1
JHH = 8.0 Hz), 7.40 (d, 1H, JHH = 8.0 Hz), 7.24 (t, 2H,
JHH = 8.0 Hz), 7.19 (s, 1H), 7.08 (t, 1H, JHH = 7.2 Hz),
7.04 (t, 1H, JHH = 6.2 Hz), 6.93 (t, 2H, JHH = 7.6 Hz),
6.84 (t, 1H, JHH = 8.0 Hz), 6.74 (t, 1H, JHH = 7.6 Hz),
6.63 (t, 1H, JHH = 7.6 Hz), 6.27 (d, 1H, JHH = 7.2 Hz),
6.22 (d, 1H, JHH = 7.2 Hz), 6.19 (s, 1H), 6.15 (s, 1 H),
2.77 (s, 3H, Me), 2.74 (s, 3H, Me), 1.52 (s, 3H, Me), 1.50
(s, 3H, Me). Anal. Calc. for C36H32IrN7: C, 57.28; N,
12.99; H, 4.27. Found: C, 57.14; N, 13.01; H, 4.28%.
¼
þ 1
ð1Þ
eM ꢀ ep KaCg
where Cg is the added guest (e.g. Pb2+) concentration. A0
(eM) and A (ep) denote the absorbance (molar extinction
coefficient) of free 1, and solution after adding, e.g. Pb2+
respectively, at a selective wavelength. Eq. (1) can be fur-
ther extended to the emission titration experiment ex-
pressed as
,
ꢀ
ꢁ
F 0
UMeM
1
¼
þ 1
ð2Þ
2.4. Preparation of [(pdpz)2Ir(py2 pz)Pb(ClO4)2]-
(1-Pb2+
F 0 ꢀ F ðUMeM ꢀ UpepÞ KaCg
)
where F0(UM) and F(Up) denote the photoluminescence
(quantum yield) of free 1, and that of solution after adding
Pb2+, respectively, at a selective wavelength. Details of
nanosecond lifetime measurements have been described in
previous publication [19].
Pb(ClO4)2 Æ H2O (18.7 mg, 0.046 mmol) was dissolved in
6 mL acetonitrile and then added into a glass tube with
solid sample of 1 (17.4 mg, 0.023 mmol). The light green-
yellow crystal suitable for single crystal X-ray diffraction
was grown by ether diffusion for several days.
3. Results and discussions
2.5. X-ray diffraction study
As shown in Fig. 1, ion-free 1 in CH3CN exhibits a weak
426 nm (e = 235 Mꢀ1 cmꢀ1) absorption band, which is
assigned to the spin-forbidden 3pp* (3ILCT) transition per-
Single crystal X-ray diffraction data were measured on a
Bruker SMART Apex CCD diffractometer using (Mo Ka)
3
haps mixed with a small amount of the MLCT transition
˚
radiation (k = 0.71073 A). The data collection was exe-
to the py2pz ligands. The absorption band around 300 nm
cuted using the SMART program. Cell refinement and data
reduction were performed with the SAINT program. The
structure was determined using the SHELXTL/PC program
and refined using full-matrix least squares.
1
(e ꢁ 104 Mꢀ1 cmꢀ1) is attributed to the pp* transition,
while transitions associated with 1MLCT are relatively
1
weak and thus are possibly hidden underneath the pp*
bands. In degassed or N2 purged CH3CN, 1 exhibits a
485 nm emission with moderate intensity (U ꢁ 0.04,
s ꢁ 70 ns). Upon aeration, the emission yield is reduced
to 0.01 (s ꢁ 21 ns). The strong O2 quenching and a calcu-
lated long radiative lifetime of ꢁ2.6 ls lead us to conclude
that the emission unambiguously originates from
phosphorescnece.
Crystal data for 1-Pb2+: C35H31Cl2IrN8O8Pb, M =
1161.97, monoclinic, space group P21/n, T = 150 K, a =
˚
19.0572(12), b = 9.9577(6), c = 19.2354(12) A, b = 91.354
3
ꢀ1
˚
(2)ꢁ, U = 3649.2(4) A , Z = 4, l (Mo Ka) = 8.462 mm
,
24109 reflections collected, 8360 unique (Rint = 0.0604),
final R1[I > 2r(I)] = 0.0391, wR2 (all data) = 0.0898.
The absorption and luminescence spectra recorded for 1
upon titrated with Pb2+ are also depicted in Fig. 1. Note
that the titration was performed in the aerated solution.
2.6. UV–Vis and phosphorescence titration of compounds 1
with Pb2+ ions
The stock solution of compound 1 (2 · 10ꢀ5 M) was pre-
pared by using spectroscopic grade CH3CN. The solution
of Pb2+ ions (1 · 10ꢀ3 M) was prepared by using Pb(ClO4)2
hydrate dissolved in CH3CN. The solutions of 1 in the
titration range exhibited a linear dependence of absorption
and emission. The absorption and emission spectrum of 1
(3.2 mL of 2 · 10ꢀ5 M stock solution) in a quartz cell
(1 cm width) were recorded at 298 K. The absorption titra-
tion of 1 with Ca2+ and Hg2+ ions were carried out by a
procedure similar to that for Pb2+ ion.
120000
0.8
0.6
80000
0.4
40000
0.2
0.0
250 300 350 400 450 500 550 600 650
0
2.7. Measurements
wavelength (nm)
Steady-state absorption and emission spectra were
recorded by a Hitachi (U-3310) spectrophotometer and
an Edinburgh (FS920) fluorimeter, respectively. The asso-
ciation constant Ka of 1:1 1/guest complex formation cal-
culated by the UV–Vis absorption method is obtained by
the following equation [18]:
Fig. 1. The absorption and emission spectra of 1 (2.0 · 10ꢀ5 in aerated
CH3CN) upon addition of increasing amounts of Pb2+ at (a) 0, (b) 0.07,
(c) 0.13, (d) 0.20, (e) 0.33, (f) 0.47, (g) 0.60, (h) 0.80, (i) 1.07, (j) 1.27, (k)
1.43, (l) 1.60, (m) 1.77, (n) 1.93, (o) 2.10, (p) 2.27, (q) 2.43, (r) 2.60, (s) 2.77,
(t) 2.93, (u) 3.10, (v) 3.27, (w) 3.43, (x) 3.60, (y) 3.77, (z) 3.93, (aa) 4.10,
(ab) 4.27 lM of 1, kex: 339 nm.

M.-L. Ho et al. / Polyhedron 26 (2007) 4886–4892
4889
Evidently, the addition of Pb2+ caused a significant change
of the absorption spectral profile with an isosbestic
point appearing at ꢁ340 nm, indicating the existence of
ground-state equilibrium between two species, namely the
free and Pb2+ linked 1-Pb2+ complexes. Concomitantly,
upon excitation at the isosbestic point, the 485 nm phos-
phorescence was gradually red shifted toward 505 nm,
accompanied by the drastic decrease of the emission inten-
sity. Taking the emission peak intensity of 1 and 1-Pb2+ to
be F0 and F, respectively, a straight line plot of F0/(F0 ꢀ F)
versus 1/[Pb2+] at, e.g. 485 nm is depicted in Fig. 2, sup-
porting the 1:1 complexation ratio. Additional evidence
was provided by the Job’s plot analysis [20], in which
decreases of the 485 nm were plotted against molar frac-
tions of 1 and Pb2+ under a condition of the constant total
concentration. As such, the concentration of 1-Pb2+
reached a turning point when the molar fraction of [1]/
([Pb2+]+[1]) was about 0.5 (see insert of Fig. 2). Accord-
ingly, the association constant Ka was deduced to be as
Fig. 3. ORTEP diagram of 1-Pb2+ with thermal ellipsoids shown at 50%
probability level; Ir–N(6) = 2.023(5) Ir–N(8) = 2.028(5), Ir–N(1) =
2.167(6), Ir–N(2) = 2.082(5), Ir–C(25) = 1.997(7), Ir–C(14) = 2.002(7),
large as (7.0 0.3) · 105 Mꢀ1
.
˚
Pb–N(3) = 2.415(6), Pb–N(4) = 2.471(5) A.
The key-role of crab chela made by the py2pz segment
can be envisaged by the synthesis of complex 2, in which
the pyridyl lariate was replaced by the essentially inert phe-
nyl group. As a result, negligible spectral changes were
observed during the Pb2+ titration for complex 2 up addi-
tion to 10ꢀ3 M, providing a preliminary spectral evidence
for the high activity of py2pz functional group in 1. More-
over, the definitive support for the 1-Pb2+ structure is ren-
dered by the single crystal X-ray structural analyses, for
which the single crystals of 1-Pb2+ adduct were obtained
from a slow diffusion of Et2O into a concentrated acetoni-
trile solution of a 1:1 mixture of 1 and Pb(ClO4)2. Its
ORTEP diagram was depicted in Fig. 3, showing the typi-
cal pseudo-octrahedral coordination arrangement around
the Ir(III) center. Similar to those of related cyclometalated
Ir(III) complexes [21], the nitrogen atoms of the pdpz
ligands resided at the mutual trans-disposition, while the
coordinated N atoms of py2pz ligands occupied the posi-
tions trans to the phenyl group of the pdpz ligands. All
the Ir–C and Ir–N bond distances are unremarkable, as
they falled in the range typical for these heteroleptic Ir(III)
complexes reported in literatures. On the other hand, it is
notable that the Pb2+ ion is linked to the outer pocket of
py2pz ligand with Pb-N distances being 2.415(6) and
˚
2.471(5) A, which fall within the literature value [22]. Such
a bridged py2pz bonding is akin to those of homodinuclear
py2pz complexes [23] and hybid complex containing lantha-
nide and transition metal elements such as [(hfac)3Ln-
(py2pz)Cr(acac)2] [24].
Competitive experiments have been performed using the
same titration method for other metal cations, including
alkali, alkaline earth metals and certain heavy metals.
While there is apparently no spectral changes for alkali
metal ions, the associated absorption spectra revealed a
small amount of changes for alkaline and Hg2+ ions (see
Figs. 4 and 5 for Ca2+ and Hg2+titration). However, except
for Hg2+, none of them revealed appreciable changes in
terms of emission profile and intensity in the range of
10ꢀ6–10ꢀ3 M (also see Fig. 6). Addition of Hg2+ exhibited
an emission quenching effect but with a smaller Ka. Based
on a similar approach with that used for the Pb2+ titration,
the association constant was determined to be (5.9 0.3) ·
104 Mꢀ1 and (1.0 0.2) · 104 Mꢀ1 for Hg2+ and Ca2+
(absorption titration), respectively. The association con-
stant for those selected metal ions are all less than that
for Pb2+ by more than one order of the magnitude. This
can also be illustrated by the phosphorescence intensity
ratio of 1 (2 · 10ꢀ5 M in acetonitrile) in the presence of
selected metal ion (10ꢀ3 M) before and after adding
10ꢀ3 M of Pb2+ shown in Fig. 6. The highly selective effect
can be qualitatively rationalized by the amount of charge
100000
100
80000
90
60000
80
40000
70
20000
60
0
0.2
0.4
0.6
0.8
1.0
50
40
30
20
10
0
[Pb2+]/([Pb2+]+[1])
0
2
4
6
8
10
12
14
16
1/Cg(x10-6)( M-1)
Fig. 2. The plot of F0/F0 ꢀ F against 1/Cg at 486 nm. Insert: Job’s plot of
a 1:1 complex of 1 and Pb2+ cation, where the quenches of emission
intensity at 484 nm was plotted against the mole fraction of Pb2+
.

4890
M.-L. Ho et al. / Polyhedron 26 (2007) 4886–4892
10
8
1.0
0.8
0.6
0.4
0.2
0.0
6
4
2
1
2
3
4
5
6
7
8
9
-5
-1
1/C (x10 )(M
)
g
250
300
350
400
450
wavelength (nm)
Fig. 4. The absorption spectra of 1 (2.0 · 10ꢀ5 in aerated CH3CN) upon
Fig. 6. (Grey) The phosphorescence intensity ratio of ion-free 1 (F0,
2 · 10ꢀ5 M in acetonitrile) versus 1 in the presence of the selected metal
ions (10ꢀ3 M). The phosphorescence intensity ratio of ion-free 1 vs. 1 in
the presence of the selected metal ions (10ꢀ3 M), followed by addition of
10ꢀ3 M of Pb2+ ion (Slash-Grey).
addition ꢀo5f increasing amounts of Ca2+ at (a) 0, (b) 1.16 · 1ꢀ0ꢀ5 5, (c)
1.69 · 10
,
,
(d) 2.22 · 10ꢀ5
,
,
(e) 2.75 · 10ꢀ5
,
(f) 2.97 · 10
,
(g)
(k)
3.50 · 10ꢀ5
(h) 4.03 · 10ꢀ5
(i) 4.56 · 10ꢀ5
,
(j) 5.09 · 10ꢀ5
,
5.63 · 10ꢀ5, (l) 5.89 · 10ꢀ5 M of 1. Insert: the plot of A0/(A0 ꢀ A) against
1/Cg at 306 nm.
Fig. 7. The color of the sensor developed on a TLC plate with 1 in
CH3CN (left) and with Pb2+ solution in H2O (right) was illuminated under
365 nm (UV lamp) for visualization (see text for the experimental detail).
(For interpretation of the references to colour in this figure legend, the
reader is referred to the web version of this article.)
Fig. 5. The absorption spectra of 1 (2.0 · 10ꢀ5 in aerated CH3CN) upon
addition of increasing amounts of Hg2+ at (a) 0, (b) 0.31, (c) 0.63, (d) 0.94,
(e) 1.25, (f) 1.47, (g) 1.88, (h) 2.19, (i) 2.50, (j) 2.81, (k) 3.13, (l) 3.44, (m)
3.75, (n) 4.06, (o) 4.38, (p) 4.61, (q) 5.00, (r) 5.31, (s) 5.63 (t) 5.94, (u) 6.25,
(v) 6.56, (w) 6.87, (x) 7.19, (y) 7.50, (z) 7.81, (aa) 8.13, (ab) 8.44, (ac) 8.75,
(ad) 9.06, (ae) 9.38 lM of 1. Insert: the plot of A0/(A0 ꢀ A) against 1/Cg at
306 nm.
ꢁ1.0 at 350 nm. Then we transfer a drop (ꢁ10 lL) of an
aqueous solution (pH ꢁ 7.0) containing ꢁ10ꢀ4 M of
Pb(ClO4)2 solution onto the plate, and then vacuum-dried
to remove water. With the use of a commercially available
UV-lamp (365 nm), photograph demonstrates a salient
change of emission from blue–green (complex 1) to a nearly
dark state upon complexation with Pb2+ ion (see Fig. 7),
implying the feasibility of 1 in sensing Pb2+ in aqueous
solution down to nanomolar scales.
density, q, specified as q = g/(4/3pr3) where g and r are the
corresponding formal charge (+1 or +2) and radius,
respectively. For example, the reciprocal of r3 (the Shannon
effective ionic radius with coordination number of 8:
Pb2+ ꢁ 1.29, Hg2+ ꢁ 1.16, and Ca2+ ꢁ 1.12) [25] correlates
well with the trend of the association constants.
4. Conclusions
We have also examined whether a similar recognition
capability can be applied in the heterogeneous solid film
in aqueous solution. Since 1 is insoluble in water, to sim-
plify the process, a silica-based TLC plate was used as a
solid support to soak 1 in CH3CN so that a complex 1
coated TLC plate was prepared with an optical density of
In conclusion, we apply complex 1 to demonstrate the
concept of sensing Pb2+ ion based on the rare metal–ion
sensitive phosphorescence. The ligated chromophores act
as both recognition and signal-transmitting units,
while the heavy metal, i.e. both Ir(III) ion and guest
Pb2+, serve as perturbing bases to enhance and quench

M.-L. Ho et al. / Polyhedron 26 (2007) 4886–4892
4891
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1
cally designed for the pp* (i.e. fluorescence) chromoph-
ores. In view of oxygen quenching, in a steady state
approach, one can then saturate the solution with N2 so
that the enhanced phosphorescence can also serve as an
additional signaling to distinguish it from the fluorescence
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We thank the National Science Council of Taiwan,
ROC for the financial Support.
Appendix A. Supplementary material
CCDC 643498 contains the supplementary crystallo-
graphic data for 1-Pb2+. These data can be obtained free
of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.
html, or from the Cambridge Crystallographic Data Centre,
12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44)
1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk. Supple-
mentary data associated with this article can be found, in
the online version, at doi:10.1016/j.poly.2007.06.029.
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