Highly sensitive colorimetric phosphorescent chemodosimeter for Hg 2 + based iridium(III) complex with (Ph2PS)2N auxiliary ligand

Article abstract of DOI:10.1016/j.inoche.2012.11.014

A colorimetric phosphorescent iridium(III) complex chemodosimeter (Ir1) for Hg^{2 +} has been prepared and confirmed by NMR, MS, and crystal data, which displays a high selectivity and antidisturbance for Hg^{2 +} detection among relevant metal ions. Phosphorescent studies show that the luminescence intensity at 598 nm decreased to ca. 12%, while the luminescence intensity in 441 nm increased to ca. 195%. The ratio of Ir1 responding to Hg^{2 +} was determined to be 1:1 by UV-vis absorption and phosphorescent emission measurements. Further study demonstrates that the detection limit on phosphorescent response of the sensor to Hg^{2 +} is down to 10^{- 6} M range. The mechanism study shows that the interaction between the S atom of ancillary ligand and Hg^{2 +} is responsible for the highly selective and sensitive phosphorescent senor for Hg^{2 +}.

Full text of DOI:10.1016/j.inoche.2012.11.014

Inorganic Chemistry Communications 28 (2013) 31–36
Contents lists available at SciVerse ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Highly sensitive colorimetric phosphorescent chemodosimeter for Hg2+ based
iridium(III) complex with (Ph2PS)2N auxiliary ligand
b
a
c
a
Bihai Tong a, , Ye Zhang , Zhao Han , Dongyang Chen , Qian-Feng Zhang
⁎
a
College of Metallurgy and Resources, Institute of Molecular Engineering and Applied Chemistry, Anhui University of Technology, Ma'anshan, Anhui 243002, China
Department of Chemistry and Engineering Technology, Guilin Normal College, Guilin 541001, China
Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16802, USA
b
c
a r t i c l e i n f o
a b s t r a c t
A colorimetric phosphorescent iridium(III) complex chemodosimeter (Ir1) for Hg2+ has been prepared and
confirmed by NMR, MS, and crystal data, which displays a high selectivity and antidisturbance for Hg2+ de-
tection among relevant metal ions. Phosphorescent studies show that the luminescence intensity at 598 nm
decreased to ca. 12%, while the luminescence intensity in 441 nm increased to ca. 195%. The ratio of Ir1
responding to Hg2+ was determined to be 1:1 by UV–vis absorption and phosphorescent emission measure-
ments. Further study demonstrates that the detection limit on phosphorescent response of the sensor to
Hg2+ is down to 10−6 M range. The mechanism study shows that the interaction between the S atom of an-
Article history:
Received 4 October 2012
Accepted 15 November 2012
Available online 23 November 2012
Keywords:
Cyclometalated iridium(III) complex
Bis(diphenylthiophosphoryl)amide
Chemodosimeter
cillary ligand and Hg2+ is responsible for the highly selective and sensitive phosphorescent senor for Hg2+
.
© 2012 Elsevier B.V. All rights reserved.
Mercury(II)
Mercury(II) is one of the most toxic ions known that lack any vital
or beneficial effects. Accumulation of Hg2+ over time in the bodies of
humans and animals can lead to serious debilitating illnesses and cen-
tral nervous system damages [1,2]. Therefore, the development of in-
creasingly selective and sensitive methods for the detection of Hg2+ is
currently receiving considerable attention [3–6]. Comparing to cold
vapor atomic absorption spectroscopy or inductively coupled plasma
mass spectroscopy, optical methods are more amenable to the on-site
analysis of mercury with fewer resources [7]. Numerous fluorescent
chemosensors and chemodosimeters have been reported in the litera-
ture [8,9]. Some of them were successfully applied to real samples and
in biological settings with low concentrations of mercury ions [10–12].
A cyclometalated iridium(III) complex-based sensor is a promising
alternative to detect Hg2+ for its excellent properties, such as their rel-
atively short excited state lifetime, high photoluminescence efficiency
and excellent color tuning [13]. Recently, Li et al. have exploited and
demonstrated a series of iridium(III) complexes for the detection of
heavy and transition metal ions. The sulfur atoms are introduced to
these cyclometalated ligands by the “receptor-conjugated signaling
unit approach” and the final iridium(III) complexes show a selective
recognition for Hg2+ with multisignaling optical–electrochemical re-
sponse [13–17]. A new sensory system based on the energy-transfer
process between rhodamine and iridium complex has been used for
the detection of Hg2+ [18]. A phosphorescent chemodosimeter for
Hg2+ was also realized by an anionic iridium(III) complex containing
the thiocyanate group [19]. However, there are still needs to develop
simple and effective iridium(III) complex-based chemosensors with
different methodologies.
In this article, a new iridium(III) bis-cyclometalated complex with
bis(diphenylthiophosphoryl)amide auxiliary ligand has been synthe-
sized and its application for mercury ion detection has been explored.
Because mercury is a soft Lewis acid, the interaction of Hg2+ with the
bis(diphenylthiophosphoryl)amide ancillary ligand can induce the
dissociation of the ancillary ligand from the complex Ir1, which pro-
vides a new strategy for the detection of Hg2+
.
The neutral cyclometalated iridium(III) complex Ir1 was readily
prepared from the IrIII-μ-chloro-bridged dimer and K[(Ph2PS)2N] at
room temperature (Scheme 1). This complex was characterized by
the MS and NMR spectroscopy. According to our previous research,
the cyclometalated ligands hydrolyzed partially during the refluxing
reaction of the ligands and IrCl3·3H2O [20].
The structure of Ir1 has been further confirmed by X-ray crystallog-
raphy. As shown in Fig. 2, the complex has distorted octahedral coordi-
nation geometry around iridium center by two cyclometalated ligands
and one S,S-bidentate ligand with cis-C–C and trans-N–N dispositions.
The Ir\C bond lengths – ranging from 2.019(6) to 2.029(6) Å – are
close to the Ir\N bond distances spanning from 2.020(5) to 2.031(5)
Å. The bond lengths between Ir center and the S,S-bidentate ligand are
ranging from 2.4776(19) to 2.492(2) Å, which are longer than that be-
tween the Ir center and the cyclometalated ligands. It is because of
stronger donating and back-bonding interactions between aryl groups
and the iridium atom. The conformation of the six-membered IrS2P2N
ring is described as a twisted boat imposed by the non-parallel orienta-
tion of the two P\S bonds [21]. Furthermore, the C\C and C\N bond
lengths and angles are also within normal ranges and are in agreement
⁎
Corresponding author. Tel./fax: +86 555 2311573.
E-mail address: tongbihai@163.com (B. Tong).
1387-7003/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.inoche.2012.11.014

32
B. Tong et al. / Inorganic Chemistry Communications 28 (2013) 31–36
S P
S P
1, IrCl3, 110oC
2, K[(Ph2PS)2N], r. t.
N
N
HO
N
N
N
N
Ir
O
N
O
Ir1
Scheme 1. Synthesis route to iridium(III) complex Ir1.
with corresponding parameters described for similarly constituted com-
plexes [22,23].
Ni2+, Zn2+, Mg2+, Cr3+, Pb2+ and Cd2+) was investigated with UV–
vis absorption spectroscopy (Fig. 3(a)). As shown in Fig. 3(a), Hg2+
has a pronounced effect on the spectra of Ir1, the peak at 357 nm
blue shifting to 334 nm. The other metal ions gave inconsiderable dis-
turbance to the UV–vis absorption spectra of Ir1. These results indi-
cate that Ir1 has a high selectivity to Hg2+. At the same time, the
color of the solution changed from orange to light yellow (Fig. 3(a)
inset) as soon as Hg2+ was added, but no change occurred in the
presence of other metal ions, indicating that Ir1 can serve as a sensi-
Fig. 1 shows the absorption and photoluminescence spectra of the
iridium(III) complex Ir1 in CH3CN solution at room temperature. The
strong absorption bands between 220 nm and 330 nm in the ultraviolet
region areas are assigned to the spin-allowed π–π* transition of the
ligands, and the broad absorption bands spanning from 330 nm to
570 nm can be attributed to the typical spin-allowed and spin-
forbidden metal-to-ligand charge transfer bands (MLCT).
On irradiation with 385 nm light, the PL spectrum of Ir1 exhibits an
orange emission band at 600 nm and a weak blue emission at 441 nm.
The orange emission can be assigned to the emission of cyclometalated
iridium(III) center and the blue emission to ancillary ligand. This implies
that energy transfer from the ancillary ligand to the cyclometalated
iridium(III) center is not complete.
tive “naked-eye” indicator for Hg2+
In order to explore the quantitative interrelation of Ir1 with Hg2+
.
,
the changes in UV–vis absorption spectra (Fig. 3(b)) were investigated
by titration experiments. Upon addition of Hg2+ to Ir1, the absorption
bands of Ir1 at longer than 355 nm gradually decreased, and a new
band centered at 334 nm starts to develop with a distinct isosbestic
point at 355 nm, indicating strong interactions between Ir1 and the
3.3. The UV–vis absorption spectroscopy response of Ir1 to Hg2+ and
other metal ions.
Hg2+. The stoichiometry of Ir1 is given by the variation of A390
nm
with respect to equivalents of Hg2+ added (Fig. 3(b) inset). Effectively,
A390
decreases continuously until the addition of 1 equivalent of
nm
To evaluate the metal ion-selective nature of Ir1, the influence of 2
Hg2+. Further addition of Hg2+ induces only very minor changes in
equivalents of different metal cations (Hg2+, Ag+, Cu2+, Fe3+, Co2+
,
A390 nm, indicating that Ir1 has a 1:1 interaction with Hg2+
.
The luminescence emission spectroscopy is more sensitive to the
interaction between chemodosimeter and analyte than absorption
spectroscopy in general [11]. Photoluminescence responses of Ir1 to
two equivalent different metal cations (Hg2+, Ag+, Cu2+, Fe3+, Co2+
,
Ni2+, Zn2+, Mg2+, Cr3+, Pb2+ and Cd2+) show that Hg2+ induces
the strongest emission changes (Fig. 4(a)), and the luminescence inten-
sity in 598 nm decreased to ca. 12%, while the luminescence intensity in
441 nm increased to ca. 195%. However, no discernible change is ob-
served for all other metal ions, i.e., Ag+, Cu2+, Fe3+, Co2+, Ni2+
,
Zn2+, Mg2+, Cr3+, Pb2+ and Cd2+. The photoluminescence responses
of Ir1 upon addition indicate a fairly high selectivity and sensitivity
chemodosimeter for Hg2+
.
The emission spectra titration of Ir1 with Hg2+ was also measured.
It can be seen from Fig. 4(b) that the emission increases continuously
until the addition of 1 equivalent of Hg2+ and the luminescence inten-
sity in 598 nm decreased to ca. 12%, while the luminescence intensity in
441 nm increased to ca. 195%. Further addition induces only very minor
change, which is consistent with the UV–vis absorption result. The lin-
ear response of the fluorescence emission intensity in 598 nm toward
[Hg2+] was obtained in Hg2+ concentration range of 0 to 100 μM, indi-
cating that the probe Ir1 can detect quantitatively relevant concentra-
tions of Hg2+. The linear equation was found to be y=−8.79x+989
(R=−0.988), where y is the fluorescence at 598 nm measured at a
given Hg2+ concentration (μM) and x is the concentration of Hg2+
added. The limit of detection defined here as the concentration equiva-
lent to a signal of blank plus three times the standard deviation of the
blank was calculated to be 1.2 μM, which was close to the reported
values of iridium complex probe (10−7 M) [13]. In addition, the reac-
tion was fairly fast and the emission intensity was changeless within
2 min.
Fig. 1. Perspective view of Ir1 with selected displacement ellipsoids drawn at the 25%
probability level. H atoms omitted.

B. Tong et al. / Inorganic Chemistry Communications 28 (2013) 31–36
33
1000
900
800
700
600
500
400
300
200
100
0
UV
PL
250
300
350
400
450
500
550
600
650
700
750
Wavelength (nm)
Fig. 2. UV–vis absorption and photoluminescence spectra of the complex Ir1.
To explore practical applicability of Ir1 as an Hg2+ selective
chemodosimeter, a competition experiment was done. As shown in
Fig. 5, in the absence and in the presence of competitive cations, Ir1
showed similar luminescence changes to Hg2+ ions except that the ex-
istence of Ni2+ slightly increased the last luminescence intensity ratio
of 441 nm to 598 nm to ca. 130% than blank and the existence of
Cu2+ slightly decreased the ratio to ca. 80%. As a whole, these results in-
dicate that the selectivity of Ir1 for Hg2+ over other cations was high
and Ir1 had the potential capacity in application.
Appendix A. Supplementary material
Supplementary data to this article can be found online at http://
dx.doi.org/10.1016/j.inoche.2012.11.014.
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The proposed mechanism for the selective and sensitive response
of Ir1 to Hg2+ is shown in Scheme 2. The distinct orange emission de-
crease and the blue emission enhancement of Ir1 after the addition of
Hg2+ can be deduced that ligand exchange reactions have occurred.
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bis(diphenylthiophosphoryl)amide ligand can serve as a highly selective
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34
B. Tong et al. / Inorganic Chemistry Communications 28 (2013) 31–36
Ag+
(a)
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
Cd2+
Cr3+
Cu2+
Hg2+
blank
Ni2+
Blank
Co2+
Mg2+
Pb2+
Zn2+
Hg2+
Hg2+
250
300
350
400
450
500
550
600
650
700
Wavelength (nm)
0.75
0.70
0.65
0.60
0.55
0.50
0.45
0.40
0.35
0.30
(b)
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
355 nm
0.0
0.2
0.4
0.6
0.8
1.0
Eq. of Hg2+
0 Hg2+
1 eq.
250
300
350
400
450
500
550
600
Wavelength (nm)
Fig. 3. The UV–vis absorption spectra of Ir1 in the presence of 2 equivalents of different metal ions (a) and changes on addition of Hg2+ (b).
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B. Tong et al. / Inorganic Chemistry Communications 28 (2013) 31–36
35
Blank, Co2+, Mg2+, Ag+,
(a)
1000
800
600
400
200
0
Cu2+, Fe3+, Ni2+, Zn2+,
Cr3+, Pb2+, Cd2+
Hg2+
Cu2+
450
500
550
600
650
700
Wavelength (nm)
1000
800
600
400
200
0
(b)
1000
800
600
400
200
0
0 Hg2+
1 eq.
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2
Eq. of Hg2+
450
500
550
600
650
700
Wavelength (nm)
Fig. 4. Luminescence emission spectra of Ir1 (λex=385 nm) in the presence of 2 equivalents of different metal ions (a) and the changes in the emission spectra of Ir1 with various
amounts of Hg2+ (b).

36
B. Tong et al. / Inorganic Chemistry Communications 28 (2013) 31–36
14
12
10
8
6
4
2
0
2+
2+
2+
Ag+ Cd2+ Co Cr2+ Cu2+ Fe3+ Mg2+ Ni2+ Pb Zn Blank
Fig. 5. Luminescence responses of Ir1 to various metal ions in DMF solution (λex
=
385 nm). Bars represent the final luminescence intensity at 598 nm. White bars repre-
sent the addition of 2 equivalents of metal ions (Ag+, Fe2+, Co2+, Cd2+, Cr3+, Cu2+
,
Zn2+, Pb2+, Ni2+, Mg2+ and blank) to the Ir1 solution. Black bars represent the subse-
quent addition of 2 equivalents of Hg2+ to the solution.
+
+
S
S
P
P
S
S
P
P
solv
solv
Hg2+
N
N
HO
N
N
Ir
Hg
N
N
HO
N
N
+
Ir
O
N
O
N
strong phosphoresence
blue-emitting
weak phosphoresence
Scheme 2. Proposed mechanism of the sensing reaction.