A highly selective chemosensor for mercury(II) cations based on cyclometalated iridium(III) complex

Article abstract of DOI:10.1016/j.ica.2012.05.017

A cyclometalated iridium(III) complex, Ir(dpci)₂(dtc) (Ir1, dpci = 3,4-diphenylcinnoline, dtcH = diethyl dithiocarbamic acid), was synthesized from the reaction of the iridium complex [Ir(dpci)₂(Cl)]₂ and dtcNa in methanol/CH₂Cl₂, and characterized by FT-IR, ¹H NMR and mass spectroscopies. The photoluminescence spectrum of Ir1 shows emission maximum at 686 nm with a lifetime of 0.77 μs and PL quantum yields is ca. 0.021. Moreover, [Ir(dpci)₂(dtc)], containing a dithiocarbamate ancillary ligand, can serve as a highly selective turn-on chemodosimeter for Hg²⁺, which is associated with the dissociation of the dithiocarbamate ancillary ligand from the complex.

Full text of DOI:10.1016/j.ica.2012.05.017

Inorganica Chimica Acta 391 (2012) 15–19
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
A highly selective chemosensor for mercury(II) cations
based on cyclometalated iridium(III) complex
Bihai Tong a,b, Qunbo Mei c, Mangeng Lu a,
⇑
a Guangzhou Institute of Chemistry, Chinese Academy of Sciences, Guangzhou 510650, PR China
b College of Metallurgy and Resources, Anhui University of Technology, Ma’anshan, Anhui 243002, PR China
c Key Laboratory for Organic Electronics & Information Displays (KLOEID), Institute of Advanced Materials (IAM), Nanjing University of Posts & Telecommunications (NUPT),
Nanjing 210046, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A cyclometalated iridium(III) complex, Ir(dpci)2(dtc) (Ir1, dpci = 3,4-diphenylcinnoline, dtcH = diethyl
dithiocarbamic acid), was synthesized from the reaction of the iridium complex [Ir(dpci)2(Cl)]2 and dtcNa
in methanol/CH2Cl2, and characterized by FT-IR, 1H NMR and mass spectroscopies. The photolumines-
Received 11 November 2011
Received in revised form 25 April 2012
Accepted 2 May 2012
cence spectrum of Ir1 shows emission maximum at 686 nm with a lifetime of 0.77 ls and PL quantum
Available online 28 May 2012
yields is ca. 0.021. Moreover, [Ir(dpci)2(dtc)], containing a dithiocarbamate ancillary ligand, can serve
as a highly selective turn-on chemodosimeter for Hg2+, which is associated with the dissociation of the
dithiocarbamate ancillary ligand from the complex.
Keywords:
Cyclometalated iridium(III) complex
Phenylcinnoline
Chemosensor
Ó 2012 Elsevier B.V. All rights reserved.
Mercury(II)
1. Introduction
complexes for the detection of heavy and transition metal ions. The
sulfur atoms are introduced to these cyclometalated ligands by the
Mercury(II) is one of the most toxic ions known that lack any vi-
tal or beneficial effects. Accumulation of Hg2+ over time in the
bodies of humans and animals can lead to serious debilitating
illnesses [1,2]. Therefore, the development of increasingly selective
and sensitive methods for the detection of Hg2+ is currently receiv-
ing considerable attention [3–5]. Luminescent chemosensors for
Hg2+ provide an effective and convenient approach [6–9]. The
fluorescence enhancement (off–on signal) induced by complexa-
tion with heavy and transition metal ions is more desirable than
fluorescence quenching (on–off signal) in terms of increased sensi-
tivity and selectivity [10]. However, the development of off–on
fluorescent probes for heavy and transition metal ions remains a
significant challenge, because many of these ions are typical fluo-
rescence quenchers by reason of their paramagnetic nature and
the heavy atom effect [11].
‘‘receptor-conjugated signaling unit approach’’ and the final irid-
ium(III) complexes show a selective recognition for Hg2+ with
multisignaling optical–electrochemical response [12–16]. A new
sensory system based on the energy-transfer process between rho-
damine and iridium complex has been used for the detection of
Hg2+ [17]. A phosphorescent chemodosimeter for Hg2+ was also
realized by an anionic iridium(III) complex containing the thiocya-
nate group [18]. However, the performance of these iridium(III)
complex-based chemosensors is still limited to the ‘‘turn-off’’ re-
sponse to Hg2+. This issue necessitates the development of new
cyclometalated iridium(III) complex sensor that can selectively
respond to Hg2+ with ‘‘turn-on’’ phosphorescent signals.
Iridium(III) bis-cyclometalated complexes with dithionate
ligands have been synthesized by several research groups, and also
been applied as phosphors in organic light emitting diodes (OLEDs)
[19–22], but their potential application for heavy and transition
metal ions detection has never been explored. In our previous re-
ports, iridium(III) complexes based on dpci ligand exhibited good
photoluminescent and chemiluminescent properties [23,24]. Here-
in, we report on a ‘‘turn-on’’ phosphorescent chemosensor for Hg2+
based on a neutral iridium(III) complex, Ir(dpci)2(dtc) (Ir1), con-
taining dithiocarbamate ancillary ligands. The interaction of Hg2+
with the dithiocarbamate ancillary ligand can induce the dissocia-
tion of the ligand from the complex Ir1, which provides a new
A cyclometalated iridium(III) complex-based sensor is a promis-
ing alternative to detect Hg2+ for its excellent properties, such as
their relatively short excited state lifetime, high photolumines-
cence efficiency and excellent color tuning [12]. Recently, Li and
co-workers have exploited and demonstrated a series of iridium(III)
⇑
Corresponding author. Tel./fax: +86 20 85232978.
E-mail address: mglu@gic.ac.cn (M. Lu).
strategy for the detection of Hg2+
.
0020-1693/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ica.2012.05.017

16
B. Tong et al. / Inorganica Chimica Acta 391 (2012) 15–19
1H NMR (CDCl3, 300 MHz) d 1.31 (t, J = 7.1 Hz, 6H), 3.28–3.35 (q,
2. Experimental
J = 7.1 Hz, 2H), 4.02–4.09 (q, J = 6.8 Hz, 2H), 6.34–6.40 (m, 6H), 6.55
(t, J = 7.3 Hz, 2H), 7.44 (d, J = 6.9 Hz, 2H), 7.49 (d, J = 6.6 Hz, 2H),
7.54–7.63 (m, 10H), 7.75 (t, J = 7.3 Hz, 2H), 8.48 (d, J = 8.5 Hz,
2H). Anal. Calc. for C45H36N5S2Ir: C, 59.84; H, 4.02; N, 7.75. Found:
C, 59.74; H, 3.99; N, 7.81%. MS ((+)-ESI): m/z = 904 (M+H+).
2.1. Materials and characterization
The solvents were purified by routine procedures and distilled
under an atmosphere of dry nitrogen before use. All reagents, un-
less otherwise specified, were purchased from Aldrich and were
used as received. 3,4-Diphenylcinnoline was prepared according
to procedures described in the literature [25]. The infrared spec-
trum was recorded on a Digilab FTS-40 spectrophotometer with
use of pressed KBr pellets. UV–Vis absorption spectra were re-
corded on a Shimadzu UV-2501 PC spectrophotometer. Positive-
ion ESI mass spectra were recorded on a Perkin Elmer Sciex API
365 mass spectrometer. 1H NMR spectra were recorded on a Varian
Inova 300NB spectrometer using CDCl3 as solvent. Photolumines-
cence (PL) spectra were measured with a Shimadzu RF-5301PC
fluorescence spectrophotometer. Fluorescence quantum yield of
the complexe was calculated using [Ru(bpy)3]Cl2 in aqueous solu-
2.3. Metal ions titration of Ir1
The UV–Vis absorption and luminescence emission spectropho-
tometric titrations were performed on 100
lM solutions of Ir1 in
DMF (N,N-dimethyl formamide). Typically, the aqueous solutions
of fresh cations (Ag+, Cu2+, Co2+, Cr3+, Zn2+, Mg2+, Pb2+, Ni2+, Mn2+
,
Fe2+, Fe3+, Cd2+) and THF solution of Hg2+ were used to evaluate
the metal ion binding property and selectivity of Ir1. Hg(ClO4)2,
AgNO3, FeSO4 and chlorides of the other cations were used as the
cation source. The luminescence emission spectra were deter-
mined with excitation at 450 nm.
tion (U = 0.042) as a standard [26]. Luminescence lifetime was
determined on an Edinburgh FL920 time-correlated pulsed sin-
gle-photon-counting instrument with excitation at 405 nm and
the emission was monitored at 686 nm.
3. Results and discussion
3.1. Photophysical properties
2.2. Synthesis of Ir(dpci)2(dtc) (dpci = 3, 4-diphenylcinnoline,
dtcH = diethyl dithiocarbamic acid) (Ir1)
UV–Vis absorption spectrum of the iridium(III) complex Ir1 is
shown in Fig. 1(a). The band around 294 nm is assigned to a typical
The synthetic route to Ir(dpci)2(dtc) is given in Scheme 1.
To a round-bottomed flask (25 mL), 2-ethoxyethanol (9 mL),
3,4-diphenylcinnoline (0.5 g, 2.0 mmol), IrCl3Á3H2O (0.2 g,
0.56 mmol) and water (3 mL) were added sequentially. The mix-
ture was stirred under nitrogen at 120 °C for 24 h and cooled to
room temperature. The precipitate was collected and washed with
ethanol and acetone, and then dried in vacuum to give a dark red
spin-allowed 1p p⁄ transition of the ligands, and band at 424 nm
–
correspond likely to spin-allowed singlet metal-to-ligand charge-
transfer (1MLCT). On the other hand, the moderately intense
absorptions at ca. 500–600 nm can be assigned to a spin-forbidden
triplet metal-to-ligand charge-transfer (3MLCT).
The room temperature photoluminescence spectra of the irid-
ium(III) complex Ir1 in CH2Cl2 solution are shown in Fig. 1(b and
c). The excitation spectrum is dominated by a broad band centered
at 373 nm, which is the absorption arising from the 1MLCT and
3MLCT states. The iridium(III) complex Ir1 emit intense luminnes-
cence with emission wavelength at 686 nm, corresponding to deep
red light emitting. The excited state lifetime of the iridium(III)
complex Ir1 was determined to be 77 ns, remarkably shorter than
those of [Ir(Flpy)3] (H(Flpy) = 2-(9,9-dimethylfluoren-2-yl)pyri-
cyclometalated IrIII-
l-chloro-bridged dimer (0.36 g, 81.3%).
In a round-bottomed flask, (0.08 g, 0.05 mmol) of dichloro-
bridged iridium dimer and (0.03 g, 0.2 mmol) of sodium N,N-
diethyl-dithiocarbamate (Nadtc) were mixed together in CH2Cl2/
methanol (5:5 mL). The mixture was stirred at room temperature
for 1 h. The product was precipitated from water and dried in
desiccator. Dark red solid of Ir(dpci)2(dtc) (Ir1) was obtained from
column chromatography on a silica column using CH2Cl2 and
hexane (1:1) as eluent, 0.071 g, yield: 79%.
dine) (1.2 ls) [27] and most of the other reported neutral iridium
complexes. The phosphorescence quantum efficiency in CH2Cl2
N
N
Cl
Cl
IrCl
3
Ir
Ir
Ethoxyethanol/H2O = 3/1(V)
N
N
N
N
2
2
N
S
S
dtcNa, CH Cl /CH OH, r. t.
2
2
3
N
Ir
N
N
N
Ir1
Scheme 1. Synthetic route to [Ir(dpci)2(dtc)] (Ir1).

B. Tong et al. / Inorganica Chimica Acta 391 (2012) 15–19
17
Mn2+, Co2+, Ni2+, Zn2+, Mg2+, Cr3+, Pb2+ and Cd2+) was investigated
with UV–Vis absorption spectroscopy (Fig. 2(a)). As shown in
Fig. 2(a), Hg2+ has a pronounced effect on the spectra of Ir1, the
peak at 424 nm blue shifting to 400 nm. Except Ag+ which has a
week influence, resulting in a small blue shift, the other metal ions
gave nearly no disturbance to the UV–Vis absorption spectra of Ir1.
294 nm
35
30
25
20
15
10
5
ex 373 nm
em 686 nm
b
a
c
These results indicate Ir1 has a high selectivity to Hg2+
.
In order to explore the quantitative interrelation of Ir1 with
Hg2+, the changes in UV–Vis absorption spectra (Fig. 2(b)) were
investigated by titration experiments. Upon addition of Hg2+ to
Ir1, the absorption bands of Ir1 at ca. 421–600 nm gradually de-
creased, and a new band centered at 400 nm start to develop with
a distinct isosbestic point at 421 nm, indicating strong interactions
between Ir1 and the Hg2+. The stoichiometry of Ir1 is given by the
variation of A450nm with respect to equivalents of Hg2+ added
(Fig. 2(b) inset). Effectively, A450nm decreases continuously until
the addition of 1 equiv. of Hg2+. Further addition of Hg2+ induces
only very minor changes in A450nm, indicating that Ir1 has a 1:1
358 nm
424 nm
0
200
300
400
500
600
700
800
900
Wavelength (nm)
Fig. 1. UV–Vis absorption (a) and luminescence (b, ex.; c, em.) spectra of Ir1 in
CH2Cl2.
interaction with Hg2+
.
3.3. The luminescence emission spectroscopy response of Ir1 to Hg2+
and other metal ions
solution is ca. 0.021 used an aqueous solution of [Ru(bpy)3]Cl2
(U = 0.042) as the standard solution [26].
3.2. The UV–Vis absorption spectroscopy response of Ir1 to Hg2+ and
other metal ions
The luminescence emission spectroscopy is more sensitive to
the interaction between chemosensor and analyte than absorption
spectroscopy in general [11]. Photoluminescence responses of Ir1
to two equivalent different metal cations (Hg2+, Ag+, Cu2+, Fe2+
,
To evaluate the metal ion-selective nature of Ir1, the influence
of 2 equiv. of different metal cations (Hg2+, Ag+, Cu2+, Fe2+, Fe3+
,
140
Hg2+
Ag+
(a)
(a) 3.5
Hg2+
120
100
80
60
40
20
0
Cd2+
Co2+
Cr3+
Cu2+
Fe2+
Mg2+
Ni2+
3.0
Blank, Mg2+, Co2+, Cu2+, Fe2+, Fe3+,
2.5
Ni2+, Zn2+, Cr3+, Pb2+, Cd2+, Mn2+
Hg2+
2.0
Ag+
1.5
1.0
0.5
0.0
Pb2+
Zn2+
Blank
500
600
700
800
900
300
400
500
600
700
Wavelength (nm)
Wavelength (nm)
140
120
100
80
(b)
(b)
1.6
1.5
1.4
1.3
1.2
1.1
1.0
0.9
0.8
400 nm
670 nm
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
421 nm
1 eq.
0.0
0.2
0.4
0.6
0.8
1.0
E. q. of Hg2+
0 Hg2+
60
0 Hg2+
500 nm
40
1 eq.
20
0
300
400
500
Wavelength (nm)
600
700
500
600
700
800
900
Wavelength (nm)
Fig. 2. UV–Vis absorption spectra of Ir1 in the presence of 2 equiv. of different
metal ions (a) and the changes in the UV–Vis absorption spectra of Ir1 with various
amounts of Hg2+ (b).
Fig. 3. Luminescence emission spectra of Ir1 in the presence of 2 equiv. of different
metal ions (a) and the changes in the emission spectra of Ir1 with various amounts
of Hg2+ (b).

18
B. Tong et al. / Inorganica Chimica Acta 391 (2012) 15–19
Co2+, Ni2+, Zn2+, Mg2+, Cr3+, Pb2+ and Cd2+) show that Hg2+ induce
the strongest emission changes, and the luminescence intensity
in 686 nm of Ir1 increased to ca. 474%, and the max emission
wavelength blue shift to 670 nm. But no discernible change is ob-
complex probe (10À7 M) [12]. In addition, the reaction was fairly
fast and the emission intensity was changeless within 3 min. Gen-
erally, cyclometalated iridium(III) complexes have the character of
lower cytotoxicity [28], so this probe is potentially suitable for the
real-time tracking of Hg2+ in organisms.
served for all other metal ions, i.e. Ag+, Cu2+, Fe2+, Co2+, Ni2+, Zn2+
,
Mg2+, Cr3+, Pb2+ and Cd2+ (Fig. 3(a)). The photoluminescence re-
To explore practical applicability of Ir1 as a Hg2+ selective
chemosensor, a competition experiment was done. As shown in
Fig. 4 (Bars represent the final luminescence intensity at 686 nm.
White bars represent the addition of 15 equiv. of metal ions (Ag+,
Fe2+, Co2+, Cd2+, Cr3+, Cu2+, Zn2+, Pb2+, Ni2+, Mg2+ and blank) to the
Ir1 solution. Black bars represent the subsequent addition of 2
equiv. of Hg2+ to the solution.), in the absence and in the presence
of competitive cations, Ir1 showed similar luminescence changes
to Hg2+ ions, but the large excessive of Ag+ had also increased
the luminescence intensity of Ir1 to ca. 225%. All these results indi-
cate that the selectivity of Ir1 for Hg2+ over other cations was high.
The proposed mechanism for the selective and sensitive re-
sponse of Ir1 to Hg2+ is shown in Scheme 2. In the presence of hea-
vy atom substituted quenchers, the internal heavy atom effects
(due to the presence of S atom in the Ir1) are responsible for the
weak luminescence intensity. Therefore the distinct emission
enhancement of iridium(III) complex Ir1 after the addition of
Hg2+ can be deduced that a desulfurization reaction have occured.
This phenomenon could be explained by the properties that Hg2+
sponses of Ir1 upon addition indicate a fairly high selectivity and
sensitivity for Hg2+
.
The emission spectra titration of Ir1 with Hg2+ was also mea-
sured. It can be seen from Fig. 3(b) that the emission increases
continuously until the addition of 1 equiv. of Hg2+ and the lumin-
nescence intensity in 686 nm of Ir1 increased to ca. 474%, and the
maximum emission wavelength blue shift to 670 nm. Further addi-
tion induces only very minor change, which is consistent with the
UV–Vis absorption result. The linear response of the fluorescence
emission intensity toward [Hg2+] was obtained in Hg2+ concentra-
tion range of 0 to 10 Â 10À5 M, indicating the probe Ir1 can detect
quantitatively relevant concentrations of Hg2+. The linear equation
was found to be y = 0.84x + 25.92 (R = À0.988), where y is the fluo-
rescence at 686 nm measured at a given Hg2+ concentration (
lM)
and x is the concentration of Hg2+ added. The limit of detection
defined here as the concentration equivalent to a signal of blank
plus three times the standard deviation of the blank was calculated
to be 1.6
lM, which was close to the reported values of iridium
600
500
400
300
200
100
0
Ag+ Fe2+ Co2+ Cd2+ Cr3+ Cu2+ Zn2+ Pb2+ Ni2+ Mg2+ Blank
Fig. 4. Luminescence responses of Ir1 to various metal ions in DMF solution (kex = 450 nm).
+
+
solv
Hg
S
Hg2+
N
Ir
S
S
N
N
Ir
N
solv
N
N
+
S
N
N
N
N
weak phosphoresence
non-emissive
strong phosphoresence
Scheme 2. Proposed mechanism of the sensing reaction.

B. Tong et al. / Inorganica Chimica Acta 391 (2012) 15–19
19
147.8
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
422.1
73.8
[Hg(dtc)(H2O)]+
[Ir(dpci)2(ACN)]+ [Ir(dpci)2(ACN)2]+
559.6
366.1
273.8
836.9
796.3
100
200
300
400
500
600
700
800
900
1000
1100
1200
1300
1400
m/z
Fig. 5. ESI mass spectra of the mixture of Ir1 and Hg(ClO4)2 in ACN solution.
the design of novel phosphorescent probes based on iridium(III)
complexes.
Acknowledgments
This project was supported by Guangdong-Hongkong Technol-
ogy Cooperation Finding (Project No. 2009A091300012), the Na-
tional Natural Science Foundations of China (Grant Nos.
50903001 and 50803027) and the Fund of Anhui University of
Technology for Youth (No. QZ200907).
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4. Conclusions
In conclusion, we have demonstrated for the first time that a
neutral cyclometalated iridium(III) complex, Ir(dpci)2(dtc), con-
taining dithiocarbamate ligand can serve as a highly selective ‘‘turn
on’’ chemosensor for Hg2+. The interaction between the S atom of
dithiocarbamate leaving group and Hg2+ is responsible for the
highly selective and sensitive phosphorescent senor for Hg2+. This
work showed that new phosphorescent probes based on irid-
ium(III) complexes could be realized by using the auxiliary ligand
with specific leaving ability. This work would be very useful to
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