congregate and precipitate that can lead to dramatic color
modulation and fluorescence quenching.6 These phenom-
ena have been successfully applied for designing sensors.
For example, Ajayaghosh et al. designed a sensor based on
the cation inducing aggregation of the dye. When squar-
aine dye podands bind with Ca2þ, they form folded or
sandwich inclusion complexes, which are similar to the
“H”-aggregates, leading to the solution color change from
light-blue to intense purple and fluorescence quenching.7
Squaraines possess four potential binding sites: two
oxides at the electron-deficient cyclobutene ring and two
nitrogen atoms of anilino moieties; their coordination with
metal ions can cause a color modulation of the dyes, i.e.,
absorption spectral changes. For example, a squaraine dye
can consecutively bind two protons or metal ions causing a
hypsochromic shift of the peak and then complete bleach-
ing, respectively. This has been inferred reasonably as
sequential coordination at the amine nitrogen atoms.8
Scheme 1. Synthesis of the Squaraine Dye SQ-1
known that squaraines are susceptible to nucleophilic
attack which would be inhibited by acidic media. Our
experiments show that in acetic acid SQ-1 is robust show-
ing good spectral reproducibility without the interference
of the proton. It shows a fairly strong and symmetrical
absorption band at 644 nm and an emission band at 661
nm, both of which arise from its donor-acceptor-donor
type of charge transfer and the extensive conjugation
structure. With increasing water % in the solution, the
644 nm absorption peak decreases and a new absorption
peak at ca. 548 nm emerges, reaching a maximum at about
60% water (see Figure S5 on p S4 in the Supporting
Information). The spectral change is interpreted, in ana-
logy to the previous report,12 as being the equilibrated
formation of a “H”-aggregate (face-to-face dimer) and
SQ-1 monomer wherein the shorter new absorption arises
from the “H”-aggregate at 60% water concentration
ꢀ~
´
nez-Manez and Rurack have reported a symmetrical
Martı
and aggregation reversible squaraine dye hitched with a
dithia-dioxa-aza crown, which is completely bleached on
binding with an excess of Hg2þ; the monomer-aggregate
absorption bands are reduced without affecting the band
shape. In contrast, a similar binding with Agþ shifts the
color to green and also causes regaining of the monomer
absorption band with a concomitant decrease in the dimer
band. This is a nice sensor to discriminate Hg2þ over other
metal ions except Agþ in mixture samples but fails where
Agþ ions are present.9
Herein, we report a new squaraine dye that can specifi-
cally probe for the Hg2þ cation in complex aqueous
samples containing common metal ions, even thiophillic
cation Agþ and Pb2þ, by means of colorimetric and
fluorescent monitors. To the best of our knowledge, this
is one of the very few squaraine-based “turn on” fluores-
cent sensors.10
The fact that dithiocarbamate (DTC) is an effective
ligand for heavy metal ions11 prompted us to select
(phenylazanediyl)-bis-(ethane-2,1-diyl)-bis-diethyl-carba-
modithioate as a binding arm to synthesize SQ-1 by the
reaction with squaric acid in i-PrOH with triethyl ortho-
formate as an in situ homogeneous absorbent as shown in
Scheme 1 (also see pp S1-S4 in the Supporting
Information). SQ-1 is highly soluble in CH2Cl2, CHCl3,
and AcOH and also stable especially in AcOH without an
absorption spectrum change over several months. It is
Figure 1. Color changes upon addition of different metal ions
(10 equiv) to SQ-1 (2 μM) in AcOH-H2O (40:60, v/v) solution.
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´
(6) (a) Ros-Lis, J. V.; Martınez-Manez, R.; Soto, J. Org. Lett. 2005, 7,
2337–2339. (b) Li, J.-R.; Li, B.-F.; Li, X.-C.; Tang, J.-A.; Jiang, L. Thin
Solid Films 1996, 287, 247–251.
(7) (a) Ajayaghosh, A.; Arunkumar, E.; Daub, J. Angew. Chem., Int.
Ed. 2002, 41, 1766–1769. (b) Arunkumar, E.; Ajayaghosh, A.; Daub, J.
J. Am. Chem. Soc. 2005, 127, 3156–3164.
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ꢀ
(8) Ros-Lis, J. V.; Martı
´
nez-Manez, R.; Sancenon, F.; Soto, J.;
Spieles, M.; Rurack, K. Chem.;Eur. J. 2008, 14, 10101–10114.
ꢀ~ ꢀ
(9) Ros-Lis, J. V.; Martınez-Manez, R.; Rurack, K.; Sancenon, F.;
´
Soto, J.; Spieles, M. Inorg. Chem. 2004, 43, 5183–5185.
(10) (a) Avirah, R. R.; Jyothish, K.; Ramaiah, D. Org. Lett. 2007, 9,
ꢀ~
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121–124. (b) Ros-Lis, J. V.; Marcos, M. D.; Martınez-Manez, R.;
Rurack, K.; Soto, J. Angew. Chem., Int. Ed. 2005, 44, 4405–4407. (c)
Gassensmith, J. J.; Matthys, S.; Lee, J.-J.; Wojcik, A.; Kamat, P. V.;
Smith, B. D. Chem.;Eur. J. 2010, 16, 2916–2921.
Figure 2. Absorption spectra of SQ-1 (2 μM) upon addition of
Hg2þ (0-27 μM) in AcOH-H2O (40:60, v/v) solution.
(11) Cheung, S.-M.; Chan, W.-H. Tetrahedron 2006, 62, 8379–8383.
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