A selective colorimetric Hg2+ probe featuring a styryl dithiaazacrown containing platinum(II) terpyridine complex through modulation of the relative strength of ICT and MLCT transitions

Article abstract of DOI:10.1021/ic101908p

A series of platinum(II) terpyridine complexes featuring an aminostilbene donor-acceptor framework was synthesized. The complex with a dithiaazacrown moiety exhibits a highly sensitive and selective colorimetric response to a Hg²⁺ cation throug

Full text of DOI:10.1021/ic101908p

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pubs.acs.org/IC
A Selective Colorimetric Hg2þ Probe Featuring a Styryl
Dithiaazacrown Containing Platinum(II) Terpyridine
Complex through Modulation of the Relative Strength of
ICT and MLCT Transitions
Sung-Kuang Chung, Yong-Ren Tseng, Chan-Yu Chen, and Shih-Sheng Sun*
Institute of Chemistry, Academia Sinica, 115 Nankang, Taipei, Taiwan, Republic of China
S Supporting Information
b
Scheme 1. Synthetic Procedures for the Ligands and Com-
plexesa
ABSTRACT: A series of platinum(II) terpyridine com-
plexes featuring an aminostilbene donor-acceptor frame-
work was synthesized. The complex with a dithiaazacrown
moiety exhibits a highly sensitive and selective colorimetric
response to a Hg2þ cation through modulation of the relative
strength of ICT and MLCT transitions. The results from 1H
NMR titration suggest the existence of a weak PtII HgII
3 3 3
metallophilic interaction at low Hg2þ concentration.
he toxicity of certain metal ions to human and other living
T
organisms has been a constant cause of environmental
concern. The sensitive detection of trace amounts of heavy-
metal ions in the environment is of utmost importance.1 Among
these heavy-metal ions, mercury ion (Hg2þ) is one of the most
troublesome to the environment and living systems because it is
highly toxic and easily accumulates in living organisms.2,3 In
recent years, numerous efforts have been directed at the design of
chemosensors capable of specifically detecting Hg2þ.4 One of the
most common Hg2þ binding motifs is the thioether-containing
macrocycles, which have been proven to be effective hosts for
Hg2þ because of the high affinity of Hg2þ for a sulfur atom.5
Although a number of methods are available for determining
the presence of metal ions in the environment, there is a growing
need for rapid on-site analysis with chemosensors capable of
detecting metal ions on a real-time basis. In this regard, the ability
to operate a naked-eye colorimetric response to the analyte is an
attractive approach to the design of sensing molecules. Recently,
a series of platinum(II) terpyridyl complexes imparted with
metal-binding units developed by Yam and co-workers have
been reported that display colorimetric and/or luminescent
responses toward various protons and cations.6a-6c Typically,
these metal complexes possess multiple optical transitions such as
metal-to-ligand charge transfer (MLCT) and ligand-to-ligand charge
transfer in the visible region. The binding of protons or cations
perturbs the charge density in the binding unit and subsequently
induces optical signal switching between different energy states to
achieve the desired optical responses. Related proton- or cation-
induced excited-state switching in platinum(II) terpyridyl complexes
with azacrown moieties has also been reported by Tung et al.6d
The design principle here is to integrate two chromophores,
platinum(II) terpyridine and dithiaazacrown-functionalized stilbene,
with charge-transfer dipoles in opposite directions and optical
transitions in different absorption regions. With analyte recognition
a Experimental conditions: (i) NaH, DMF, ice bath, 1 h; (ii) aldehyde,
reflux 24 h; (iii) MeOH, reflux 8 h; (iv) Bu4NClO4, DMF, reflux 6 h.
motifs conjugated to the ligand framework, the recognition event
of a specific analyte can be conveniently monitored by analyte-
binding-triggered switching of optical transitions by utilizing a
transition-metal complex possessing a MLCT transition in
conjunction with a ligand-based optical transition such as an
internal charge-transfer (ICT) transition to achieve such a dual-
channel sensing mode.5i,7
The synthetic procedures for the terpyridyl ligands and their
platinum(II) complexes are depicted in Scheme 1. Terpyridyl ligands
L1-L3 were synthesized in N,N-dimethylformamide (DMF) via
a standard Hornet-Emmons reaction with moderate-to-excellent
yields.8 Complexes Pt1-Pt3 were obtained by the reaction of
Received: September 17, 2010
Published: January 26, 2011
r
2011 American Chemical Society
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Inorganic Chemistry
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Figure 1. Left: Absorption spectral changes of Pt3 (2.0 ꢀ 10-5 M)
upon the addition of Hg(ClO4)2 [(0-1.4) ꢀ 10-3 M] in DMF. Right:
Changes of absorbance at 500 nm (red curve) and 422 nm (blue curve)
versus equivalents of Hg2þ addition.
Pt(COD)Cl2 (COD = 1,5-cyclooctadiene) and terpyridyl ligands
L1-L3 in MeOH. Complexes Pt4 and Pt5 were also synthesized
to serve as model compounds.
The UV-visible spectra of L2 and L3 in a CH3CN solution
showed intense bands centered at 388 and 394 nm, respectively.
These low-energy bands showed great solvatochromic effects
and are assigned as ICT [diphenylamine (π) to terpyridine (π*)
for L2 and thiaazacrown (π) to terpyridine (π*) for L3] transi-
tions. Dissolution of Pt2 and Pt3 in DMF gave pinkish solutions
that consisted of intense absorption bands at 300-400 nm and
broad low-energy bands at 464 and 500 nm for Pt2 and Pt3,
respectively (Figures S1-S3 in the Supporting Information, SI).
By comparison to the spectra of Pt1 and Pt4 and relevant
platinum(II) complexes in the literature,9 the high-energy ab-
sorption bands are assigned to ligand-localized π-π* transitions,
whereas the low-energy absorption is assigned to an ICT transi-
tion overlapping with a MLCT (Ptdπ-tpyπ*) transition for Pt2
and Pt3. All three terpyridyl ligands are highly emissive at room
temperature. However, Pt1 and Pt2 are only luminescent in the
solid state at 77 K, and Pt3 is nonluminescent even at 77 K. The
photophysical properties are summarized in Table S1 in the SI.
Figure 1 illustrates the absorption spectral changes of Pt3 upon
the addition of Hg(ClO4)2 in a DMF solution. The absorbance of
the ICT band at 500 nm gradually diminished, accompanied by
an increase of the MLCT band centered at 422 nm. The original
absorption profile of Pt3 can be restored by extracting the mix-
ture with excess ethylenediaminetetraacetic acid. The addition of
other biologically and/or environmentally important metalcations
Figure 2. Plots of 1H NMR spectra of Pt3 (26.3 mM) showing aromatic
(top) and aliphatic (bottom) regions upon the addition of Hg(ClO4)2 in
DMF-d7.
equivalents reveals that there was no apparent optical response
until an addition of Hg2þ of over 2 equiv. Indeed, the intensity of
the ICT band at 500 nm was slightly increased between the
addition of 1 and 2 equiv of Hg2þ before diminishing with higher
concentrations of Hg2þ (Figure S6 in the SI). A similar minor
increase of absorbance was observed upon titration of Hg2þ to a
DMF solution of Pt2 (Figure S7 in the SI). The possibility of
displacing Pt2þ by Hg2þ during titration is excluded by compar-
ing the model complex L3HgCl2 with the titration spectra
between Pt3 and Hg2þ (Figure S8 in the SI). Furthermore, Job’s
plot did not show the expected 1:1 stoichiometry but yielded a
complicated pattern (Figure S9 in the SI), which indicates the
existence of possible multiple equilibria between Hg2þ and Pt3.
A 1H NMR titration experiment was performed with the aim
to gaining a further understanding of the interaction between the
(Liþ, Naþ, Kþ, Csþ, Mg2þ, Ca2þ, Cr3þ, Co2þ, Ni2þ, Cu2þ
,
Zn2þ, Cd2þ, and Pb2þ) to a DMF solution of Pt3 resulted in no
apparent spectral and solution color changes. No luminescence
was detected with the addition of Hg2þ ions. The specific
colorimetric responses clearly demonstrated that Pt3 is highly
selective toward a Hg2þ ion. The control experiments carried out
by titration of Hg2þ to a DMF solution of either Pt2 or Pt1
resulted in only very minor spectral responses under the same
conditions. Pt5 as the perchlorate salt of Pt3 was titrated with
Hg2þ to access the counterion effect. The titration profile of the
absorption spectra (Figures S4 and S5 in the SI) is essentially
identical with the one shown in Figure 1 that excludes the
possible interference between Hg2þ and Cl- during titration.
It is intuitive to rationalize the spectral response of Pt3 upon
the addition of Hg2þ to be simply the result of encapsulation of
Hg2þ to the dithiaazacrown moiety via HgII-S,N bonding,
which suppressed the ICT process (diminishing of the ICT band
at 500 nm) but enhanced the MLCT transition (increasing of the
MLCT band at 422 nm). However, careful scrutiny of the plot of
the absorbance changes as a function of the addition of Hg2þ
1
complex Pt3 and Hg2þ cation. Figure 2 shows the H NMR
spectra for titration of a DMF-d7 solution of Pt3 with Hg2þ. It is
surprising to see that, upon the addition of Hg2þ of up to 1 equiv,
the proton signals of the expected interacting dithiaazacrown
virtually did not shift at all, but only the protons in the region
between 8.2 and 9.3 ppm associated with the terpyridine moiety
(H1-H7) showed a slight shift. After the addition of Hg2þ of
over 1 equiv, the proton signals from dithiaazacrown (H13 and
H14) became broad and started to shift in the downfield
direction. Control experiments carried out by titration of Hg2þ
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Inorganic Chemistry
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’ AUTHOR INFORMATION
Corresponding Author
*E-mail: sssun@chem.sinica.edu.tw.
Figure 3. Proposed mechanism for HgII sensing by probe Pt3.
’ ACKNOWLEDGMENT
into a DMF-d7 solution of Pt1 or Pt2 resulted in a similar upfield
shift of the proton signals associated with the terpyridine moiety
(Figures S10 and S11 in the SI). Apparently, the Hg2þ ion
interacted with the platinum terpyridyl moiety first, followed by
encapsulation of Hg2þ to the dithiaazacrown moiety. Although
attempts to grow good quality single crystals of a Hg2þ-Pt3
We thank the National Science Council of Taiwan and
Academia Sinica for support of this research.
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in the electrospray ionization mass spectrum (Figure S12 in the SI).
Recent studies have suggested that the dispersive forces
responsible for the formation of metallophilic interactions between
closed-shell atoms would be magnified by relativistic effects,
especially when these interactions involve a Hg2þ ion.10 Exam-
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3 3 3
3 3 3
and HgII PdII 13 bonds have been well documented. 195Pt
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NMR spectra of Pt3 and Pt3 upon the addition of 1 equiv of
Hg2þ were separately recorded. Although the low solubility of Pt3 in
DMF-d7 precluded the observation of 199Hg satellites, the 10 ppm
upfield shift of the 195Pt signal of Pt3 in the presence of 1 equiv of
Hg2þ indicates the existence of a weak metallophilic interaction
between Pt2þ and Hg2þ (Figures S13 and S14 in the SI). Thus, we
tentatively suggest that the initial interaction of Pt3 and the incoming
Hg2þ involves the formation of a weak PtII HgII metallophilic
3 3 3
interaction, followed by HgII-dithiaazacrown interaction.
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thiaazacrown to terpyridine may reinforce the possible donor
(Pt2þ) to acceptor (Hg2þ) interaction. The mechanism of optical
response of Pt3 upon the addition of Hg2þ is summarized in
Figure 3. It is expected to observe a minor enhancement of the
ICT transition upon formation of the PtII HgII interaction at
3 3 3
low Hg2þ concentration. Indeed, both Pt2 and Pt3 exhibited such
an enhancement in the absorption spectra with the initial addi-
tion of a Hg2þ ion. Upon the addition of more than 1 equiv of Hg2þ
,
encapsulation of Hg2þ to the dithiaazacrown moiety occurred,
which shuts down ICT absorption and turns on the optical output
from MLCT absorption. The binding constants based on the two-
step equilibrium shown in Figure 3 were calculated to be 1.33 ꢀ 104
M-1 (K1) and 1.64 ꢀ 103 M-1 (K2), and the detection limit for
HgII is estimated to be on the order of micromolar concentration.
In conclusion, we have designed a simple yet highly selective
optical probe for Hg2þ. The optical output can be modulated by
switching off the ICT transition upon Hg2þ binding and con-
verted to a colorimetric response from MLCT transition. The
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possible dispersive PtII HgII interaction is proposed to ratio-
3 3 3
nalize the observed unusual spectral properties at low concentra-
tions of Hg2þ in solution. This result may contribute not only to
the design of new HgII probes but also to the development of the
model system to explore the metallophilic interactions.
’ ASSOCIATED CONTENT
S
Supporting Information. Synthetic procedures, charac-
b
terization data, and titration spectra. This material is available free
of charge via the Internet at http://pubs.acs.org.
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