Thiophene-based fluorescent mercury-sensors

Article abstract of DOI:10.1080/00958972.2016.1164851

Coordination chemistry of thiophene rings is poorly understood, despite their common use in organic electronic materials. The absorption and emission responses to transition metal ions of three thiophene-based ligands containing pyridine chelating groups are examined. These ligands, 2-(2′-thienyl)-pyridine (L1), 2,5-bis(2-pyridyl)thiophene (L2), and 2,6-bis(2-thienyl)pyridine (L3), show a ratiometric fluorescence response in the presence of Hg(ClO₄)₂ with reasonable selectivity against many transition metal ions in acetonitrile. ¹H NMR data support S,N chelation of L1 and L2 to the Hg(II) center, while L3 coordinates through cyclometalation via one carbon of thiophene. DFT calculations suggest thiophene coordinates to Hg(II) in a bent geometry. Our results indicate that thiophene could offer selectivity for Hg(II) towards the design of fluorescent sensors.

Full text of DOI:10.1080/00958972.2016.1164851

Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: http://www.tandfonline.com/loi/gcoo20
Thiophene-based fluorescent mercury-sensors
Austin K. Shigemoto, Carolyn N. Virca, Samuel J. Underwood, Lauren R.
Shetterly & Theresa M. Mccormick
To cite this article: Austin K. Shigemoto, Carolyn N. Virca, Samuel J. Underwood, Lauren R.
Shetterly & Theresa M. Mccormick (2016): Thiophene-based fluorescent mercury-sensors,
Journal of Coordination Chemistry, DOI: 10.1080/00958972.2016.1164851
To link to this article: http://dx.doi.org/10.1080/00958972.2016.1164851
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Date: 22 March 2016, At: 23:53

Publisher: Taylor & Francis
Journal: Journal of Coordination Chemistry
DOI: http://dx.doi.org/10.1080/00958972.2016.1164851
Thiophene-based fluorescent mercury-sensors
AUSTIN K. SHIGEMOTO, CAROLYN N. VIRCA, SAMUEL J. UNDERWOOD, LAUREN R. SHETTERLY and
THERESA M. MCCORMICK*
Department of Chemistry, Portland State University, Portland, Oregon, USA
Coordination chemistry of thiophene rings is poorly understood, despite their common use in
organic electronic materials. The absorption and emission responses to transition metal ions of
three thiophene-based ligands containing pyridine chelating groups are examined. These ligands,
2
-(2’-thienyl)-pyridine (L1), 2,5-bis(2-pyridyl)thiophene (L2) and 2,6-bis(2-thienyl)pyridine
(
L3), show a ratiometric fluorescence response in the presence of Hg(ClO ) with reasonable
4
2
1
selectivity against many transition metal ions in acetonitrile. H NMR data support S,N chelation
of L1 and L2 to the Hg(II) center while L3 coordinates through cyclometalation via one carbon
of thiophene. DFT calculations suggest thiophene coordinates to Hg(II) in a bent geometry. Our
results indicate that thiophene could offer selectivity for Hg(II) towards the design of fluorescent
sensors.
Keywords: Thiophene; Mercury; Coordination modes; Fluorescence; Absorption; Sensors
1
. Introduction
The coordination chemistry of heterocyclic aromatic ligands has been studied for a wide variety
of applications including catalysts, photosensitizers, and molecular sensors [1-5]. Ligands such
as pyridine or N-heterocyclic carbenes have been studied extensively [6-9]. In contrast,
thiophene coordination chemistry is poorly understood. The few examples of thiophene
*Corresponding author. Email: t.m.mccormick@pdx.edu
1

coordination in the literature focus on metal-catalyzed hydrodesulfurization [10]. The photo-
electronic properties of polythiophenes and thiophene containing small molecules are extensively
studied for use in organic photovoltaic devices (OPVs) [11-14], yet the ability of sulfur in
thiophene to coordinate to metals has not been fully explored. To increase efficiency in OPV
applications it is necessary to increase device absorption in the red to near-infrared (NIR) region
[11, 15-20]. Metal coordination provides a potential avenue to red-shift absorption and enhance
the electronic properties of these materials. Also, as a molecular sensor, sulfur of thiophene
could offer selectivity for the detection of soft heavy metals such as mercury(II) or lead(II).
Existing crystal structures of coordinated thiophene ligands display a number of different
1
1
2
coordination modes including η (S) sulfur-coordination, η (C) carbon-coordination, η through
4
5
two carbon atoms, η through four carbon atoms, or η through all atoms in the cyclic ring
10, 21-27]. To the best of our knowledge carbon coordination in the 2-position of the thiophene
[
is the most commonly observed coordination mode for thiophene ligands [26-37]. In contrast,
sulfur-coordination is rarely observed, likely due to resonance effects placing a positive charge
along the sulfur and delocalizing a negative charge along the carbons [21-23, 38]. The
coordination of thiophene is dependent on both the functionalization of the ring as well as the
transition metal [10, [38-40]. The effect of thiophene coordination on the photophysics of the
ligand has not been reported.
Herein we report several thiophene-based ligands substituted with pyridine groups to
facilitate sulfur-coordination (figure 1). We speculate that pyridine should enhance sulfur
coordination both through chelation and increasing the electron density on the thiophene. Metal
coordination has been studied using absorption, emission and NMR spectroscopy. Based on
Density Functional Theory (DFT) calculations, we propose that mercury coordinates through S-
N chelation for these ligands. Additionally, the ligands show selectivity towards mercury as
evidenced by their absorption and emission responses, making them potentially viable as
fluorescent mercury sensors.
2
2
. Experimental
.1. Materials and methods
Reagents were obtained from Sigma Aldrich and TCI America Chemicals and used without
purification. The ligand 2-(2’-thienyl)-pyridine (L1) was obtained from TCI Chemicals and
2

further purified by recrystallization in hexanes. Acetonitrile (CH CN), acetone, hexanes,
3
tetrahydrofuran (THF), dichloromethane, diethyl ether (Et O), chloroform, ethyl acetate, and
2
metal salts were obtained from Sigma Aldrich. 1,4-Dioxane was obtained from TCI America
1
Chemicals. The identity of organic compounds was confirmed by H NMR spectroscopy
performed with a Bruker-Avance III 400 MHz spectrometer with a 9.4 Tesla magnet and a 5 mm
BBO probe. Mass spectra were recorded on a ThermoElectron LTQ-Orbitrap Discovery high
resolution mass spectrometer. Emission spectra and quantum yields were obtained using a PTI
Quantamax 300 Phosphorimeter with a Xe-flash lamp and PMT detector. Quantum yields were
obtained using a PTI K-sphere ‘petite’ integrating sphere. Absorption spectra were collected on a
UV-3600 Shimadzu UV-Vis NIR spectrophotometer.
2
2
2
4
.2. Synthetic methods
.2.1. Preparation of 2,5-bis(2-pyridyl)thiophene (L2). A flask containing
,5-thiophenediylbisboronic acid (0.402 g, 2.33 mmol, 1 eq), cesium carbonate (1.63 g,
.99 mmol, 2 eq) and tetrakis(triphenylphosphine)palladium(0) (0.020 g, 0.017 mmol, 0.01 eq)
was purged with N gas before the addition of 2-bromopyridine (0.500 mL, 5.24 mmol, 2.2 eq),
2
1
,4-dioxane (10 mL) and water (5 mL). Solvents and 2-bromopyridine were purged of air prior
to addition by 3 freeze-pump-thaw cycles. The yellow solution was heated to 105 °C and left
stirring overnight. After the reaction cooled to room temperature, 10 mL of water was added.
2 2
The product was extracted in CH Cl (3×40 mL). The organic layers were combined and dried
with Na SO . The filtered solvent was evaporated in vacuo to afford a yellow-orange solid. The
2
4
crude product was purified by silica gel column chromatography, eluted with 50:50 ethyl
acetate:hexanes. The final product was obtained as a yellow solid (0.207 g, 37.1% yield) and
1
13
characterized via H and C NMR spectroscopy (CD CN 450 MHz) δ(ppm): δ 8.60 (d, J = 4.0,
3
1
3
1
H), 7.74-7.65 (m, 2H), 7.64 (s, 1H), 7.15-7.18 (dd, J = 6.4, 4.9 Hz, 1H) C NMR (101 MHz,
CD CN) δ 151.85, 149.20, 146.03, 136.63, 125.47, 122.16, 118.44. FTMS + p ESI calcd for
C H N S, 239.06374; found, 239.06560.
3
14
10
2
2
.2.2. Synthesis of 2,6-bis(2-thienyl)pyridine (L3). A flask containing 2,6-dibromopyridine
(0.482 g, 2.03 mmol), thiophene-2-boronic pinacol ester (0.882 g, 4.19 mmol), potassium
fluoride dihydrate (0.351 g, 6.04 mmol) and tetrakis(triphenylphosphine)palladium(0) (0.082 g,
3

0
.064 mmol) was purged with N . A solution of 1,2-dimethoxyethane (20 mL) and water (15
2
mL) was degassed by 3 freeze-pump-thaw cycles and added. The flask was sealed and heated to
reflux for 4 h. The organic layer was then removed in vacuo and 50 mL of water was added. The
product was extracted in Et O (3×50 mL), the organic layers combined, and then washed with
2
2 4
water (1×50 mL). The organic layer was dried over Na SO and the solvent was evaporated to
afford an orange oil. The crude product was then purified by column chromatography, eluted
with 50:50 CHCl :hexanes. The final product was obtained as a white solid (0.0814 g,
3
1
13
1
1
6.4% yield) and characterized by H and C NMR spectroscopy. H NMR (CD
3
CN 450 MHz)
δ 7.79 (dd, J = 8.4, 7.3 Hz, 1H), 7.71 (d, J = 3.7 Hz, 2H), 7.65 (d, J = 7.7 Hz, 2H), 7.52 (d,
1
3
J = 5.1 Hz, 2H), 7.16 (dd, J = 5.1, 3.7 Hz, 2H). C NMR (101 MHz, CD CN) δ 151.84, 137.83,
3
1
28.17, 128.07, 125.03, 116.79. FTMS + p ESI calcd for C13
9 2
H NS , 244.02492; found,
2
44.02667.
2
.3. General procedure for absorption/emission titrations
-5
Ligand solutions of 1×10 M L1, L2 and L3 were prepared in CH CN. The metal salts solutions
3
4 3 4 2
of Fe(ClO ) and Hg(ClO ) were prepared fresh the day of the experiment at 0.02 M
-5
concentration in 1×10 M L1 solution to preserve the concentration of ligand throughout the
titration. Metal salt solutions for titrations with L2 and L3 were prepared at 0.20 M
-5
concentration in 1×10 M solution of either L2 or L3. After an initial spectrum of the ligand was
obtained, increasing molar equivalents of the metal salts were added to the solution, via
micropipette, until no change in the spectra was observed.
1
2
1
.4. General procedure for H NMR titrations
-3
1
H NMR titrations were performed by preparing 4×10 M ligand solutions in CD CN. Initial H
3
and COSY NMR spectra of the ligand were obtained prior to addition of a metal salt. Increasing
1
equivalents of the metal salt were added to the solution. H NMR spectra were obtained. COSY
spectra were used to assign the peaks of the complexes.
2
.5. General procedure for metal competition
Solutions of silver(I), cadmium(II), cobalt(II), copper(II), iron(II), iron(III), magnesium(II),
manganese(II), sodium(I), nickel(II), lead(II), and zinc(II) perchlorates (0.02 M) and L1-L3
4

-5
(
1×10 M) were prepared in CH CN. An emission spectrum was obtained for the ligand
3
-5
solution. Metal perchlorate (5 eq) was added to a 10 mL aliquot of 1×10 M ligand solution and
an emission spectrum was obtained after 5 minutes of stirring. Then 5 eq of Hg(ClO was
added to each solution and stirred for 5 minutes before another emission spectrum was obtained.
4 2
)
2
.6. Computational chemistry
DFT calculations were run using Gaussian 09 [41] with the B3LYP [42, 43]/ 6-311+G(d) for all
atoms but Hg, which was run with the LanL2DZ basis set. Optimizations and TD-DFT
calculations were performed for Hg(L1)(ClO ) , L1, L2, and L3. The full width at half max for
4
2
Gaussian functions used in the generated UV-vis spectra was 0.3 eV.
3
3
. Results and discussion
.1. Ligand synthesis and structure
Three ligands with thiophene and pyridine coordination sites were studied. L1 has one thiophene
and one pyridine (figure 1). We hypothesize that pyridine will coordinate to transition metals and
hold the thiophene in close proximity to the metal, thereby facilitating sulfur coordination. L2
(central thiophene with two flanking pyridine groups) and L3 (central pyridine with two flanking
thiophene groups) were synthesized using modified literature procedures [44, 45] and were
designed to have several coordination sites in a larger π-conjugated system to increase sensitivity
and red-shift the absorption and emission properties (figure 1). Previous attempts to coordinate
L3 to chromium(II), cobalt(II), nickel(II), copper(II), silver(II) and zinc(II) were unsuccessful
[44]. To the best of our knowledge these ligands have not been shown to coordinate to
mercury(II).
1
13
The structures of all three ligands were verified by H, C NMR and MS. Density
Functional Theory calculations were performed on each of the ligands. Optimized structures of
the ligands show that they are planar compounds. The dihedral angles measured from the
optimized structures of the ligands are nearly planar (angles: L1 – S-C-C-N, 179.98°; L2 – N-C-
C-S, 179.98°; L3 – S-C-C-N, 179.99°). The heteroatoms in each of the rings rotate as shown in
figure 2 to prevent ortho-hydrogen interactions.
5

3
.2. Photophysical properties of ligands
-5
Absorption and emission spectra of L1-L3 were obtained with 1×10 M solutions in CH CN
3
(
figure 3). Absorption spectra of both L2 and L3 are red-shifted relative to L1 due to extended
conjugation from the additional ring. The lowest energy absorption maximum of L1 is at
98 nm; L2 and L3 have peaks at 342 nm and 335 nm, respectively (table 1). Likewise the
2
emission of both L2 (λmax = 394, Φ = 0.56) and L3 (λmax = 376, Φ = 0.34) are red-shifted
compared to that of L1 (λmax = 356 nm, Φ = 0.43). Extending the conjugation to three rings
pushes the emission into the visible region, providing easy detection of metal binding through
visual inspection. The lower energy absorption and emission of L2 relative to L3 is likely due to
the more bent structure of L3 caused by the geometry of the central pyridine (figure 2).
Measured absorbance spectra are consistent with excitation energies calculated by
TD-DFT (see Supporting Information). The calculated lowest energy absorption for L1 is at
3
38 nm and the calculated lowest energy absorption for L2 is red shifted to 355 nm. For both L1
and L2 the lowest energy excitation is HOMO to LUMO in nature. The calculated lowest energy
peak in calculated absorption spectra of L3 comprises three transitions at 332 nm, 314 nm and
2
90 nm. All three excitations are combinations of the HOMO-1 and HOMO to the LUMO and
LUMO+1 (depicted in the Supporting Information). Likewise, three excitations are seen in the
measured absorption spectra of L3.
3
3
.3. Interaction with mercury in solution
.3.1. Photophysical response to mercury. In the presence of mercury ions (Hg(ClO ) ), the
4
2
absorption and emission spectra of all three ligands red-shift. With increasing equivalents of
Hg(II), the ligand based absorption decreases and a new, red-shifted absorption grows in. For L1,
the absorbance at 298 nm decreases while a peak at 322 nm grows until 50 equivalents of Hg(II)
have been added (figure 4a). An isosbestic point is observed at 314 nm. Likewise, the emission
of L1 at 356 nm is quenched with addition of Hg(II) and a new peak at 411 nm (Φ = 0.79)
appears (figure 4b).
Although L2 displays a similar red-shift in both absorption and emission, several distinct
features are noted. With increasing amounts of Hg(II), λmax of the absorption at 342 nm
decreases as a new peak at 368 nm increases until 20.0 eq of Hg(II) have been added (figure 4c).
Similarly, a new emission at 446 nm (Φ = 0.55) is observed (figure 4d). Further additions of
6

Hg(II) past 20.0 eq. cause a blue shift in absorption to 360 nm and in emission to 421 nm (Φ =
.22) (see Supporting Information). A clear isosbestic point is not observed for L2 suggesting the
0
formation of more than one new species in solution.
Addition of Hg(II) to L3 also causes a red-shift in both the absorption and emission,
however 100 eq of metal are required to reach saturation (figure 4e,f). An isosbestic point at
344 nm indicates that no intermediate forms during the reaction. For all three compounds,
addition of excess Hg(II) is required to completely quench ligand emission. The emission of the
complexes also have higher quantum yield than the free ligand. Likely, this results from
suppression of nonradiative pathways caused by decreased rotation around the single bond
connecting the rings due to metal chelation. The bathochromic shift is likely due to a
conformational change of the ligand to a more rigid structure.
3
.3.2. NMR titration for structure determination. In order to elucidate the structure of the
1
L1-L3/Hg(II) complexes, formation of the complexes were monitored using H NMR during the
metal titrations into solutions of each ligand (figure 5a-c). COSY spectra were obtained at the
end of each titration to assign peaks. Downfield shifts for ligand peaks were attributed to
deshielding of the protons due to metal coordination [46]. The positive charge is delocalized on
the rings such that the protons para to the Hg(II) coordination site are most deshielded. Initial
addition of Hg(II) to all ligand solutions caused downfield shifts for all peaks in each spectrum.
E
In the case of L1 the para proton on the pyridyl group (H ) (figure 5a) shifts 0.717 ppm
with addition of 2 eq of Hg(II), suggesting nitrogen coordination. H in the 3-position of
C
thiophene was shifted 0.457 ppm relative to the free ligand suggesting possible sulfur-
coordination to Hg(II). These results are consistent with S,N chelation of L1 to Hg(II).
Since two new products are formed with addition of Hg(II) to L2, emission spectroscopy
1
was used in conjunction with H NMR to differentiate the two products. The symmetry of the
molecule was retained during the titration, as evidenced by retention of the singlet peak from HE
on the thiophene (figure 5b). After 1.00 eq of Hg(II) was added, emission spectroscopy
confirmed the identity of the first product (λmax = 435 nm). A low downfield shift (Δppm =
0
E
.083 ppm) is observed for the protons at the 3,4-positions of thiophene (H ), while a 0.471 ppm
shift was observed for the proton para to the pyridyl nitrogen (H ) (figure 5b). After 5 eq. an
C
emission at 420 nm indicated the second product had formed in solution. A greater downfield
7

shift for all peaks was observed. Though it is evident that pyridine chelation occurs, the identity
of the products formed in solution between L2 and Hg(II) is unclear. The minimal shift observed
for the singlet from the proton on the thiophene would indicate sulfur coordination does not
occur until the second product is formed in solution. Further investigation into the structures of
the two products formed for L2 are currently underway.
During the titration of L3 with Hg(II), a broadening of all peaks was observed until
5
.00 eq of Hg(II) were added and the product was formed. Unlike titrations with L1 and L2, the
total number of aromatic protons decreased by one after the addition of Hg(II). Another distinct
feature is an apparent break in symmetry observed by an increase in the number of proton
1
environments (figure 5c). Following addition of 5 eq of Hg(II), the H NMR spectrum suggests a
proton at the 3-position of one of the thiophenes had been removed from the ligand. Removal of
this proton indicates S,N,C-chelation by L3 to Hg(II). This is the only ligand that appears to
coordinate through a carbon rather than sulfur.
3
.3.3. Calculated structure. DFT structure optimization was used to explore the coordination
geometry of Hg(II) with L1. The optimized structure shows S,N-chelation to Hg(II) with the
sulfur on the thiophene coordinating in a side on manner (see Supporting Information). This
causes L1 to twist so that the N and S are on the same side with a N-C-C-S dihedral angle of
4
1.2°. The lowest energy excitation for the calculated absorption spectrum from the optimized
geometry is red shifted to 349 nm compared to L1 (λmax = 338 nm), consistent with experiment.
The orbitals involved in the absorption of the complex formed between L1 and Hg(ClO4)2
involve a mix of metal centered and ligand centered orbitals (see Supporting Information).
3
.4. Selectivity towards mercury
The selectivity of the fluorescent response was tested against other metal perchlorate salts:
Hg(II), Ag(I), Cd(II), Co(II), Cu(II), Fe(II), Fe(III), Mg(II), Mn(II), Na(I), Ni(II), Pb(II) and
Zn(II). The emissions of solutions with 5 eq of each metal salt were measured. For L1, only
addition of Hg(II), Cu(II) and Fe(III) resulted in an emission response. The presence of other
metal ions in solution did not affect the emission response to Hg(II), with the exception of Fe(III)
which showed an enhanced emission (figure 6a). Similar selectivity was observed for L2. There
is a bathochromic shift for 5 eq of Fe(III) (λmax = 420 nm) relative to the free L2. However,
8

addition of Hg(II) alone results in a different emission (λmax = 446 nm) (figure 6b). Notably, L3
emission was only minorly quenched in response to 5 eq of all metal salts, including Hg(II).
Fe(III) addition caused a new emission at 457 nm (see Supporting Information) (figure 6c).
These results indicate that thiophene containing fluorescent sensors can display selectivity for
Hg(II).
4
. Conclusion
We have developed three new thiophene-based ligands with a fluorescence response to Hg(II)
via chelation to the metal center. The addition of pyridyl groups enhances coordination of
thiophene via chelation, as is evident through the decreased amount of Hg(II) needed to reach
equilibrium with L2, which contains two pyridines, relative to the amount needed to reach
equilibrium with L1 and L3, which both contain only one pyridine. This suggests that the sulfur
1
of thiophene is not a strong coordinating group. H NMR suggests a S,N,C-chelation of L3 to
Hg(II). Our results indicate that thiophene ligands offers selectivity for Hg(II). A unique
coordination chemistry is observed for L2 and Hg(II), forming two distinct products dependent
on the amount of Hg(II) in solution. Currently our lab is researching the identity of these
products towards the design of thiophene-based molecular sensors. In conclusion, we have
shown that thiophene containing ligands display selectivity for Hg(II) giving a visible,
ratiometric fluorescence response.
Acknowledgements
We would like to thank Horiba for the PTI Equipment Grant.
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12

Table 1. Summary of photophysical properties of L1, L2, L3, and the complexes formed in
solution with Hg(ClO ) .
4
2
-
1
4
-1
λ
abs (nm)
λ
ex (nm)
λ
em (nm)
Φ (%)
42
ε (×10 , M , cm )
L1
298
335
342
369
284
380
1.50
1.71
2.90
2.27
2.00
1.78
304
344
343
374
291
377
356
410
394
435
376
457
Hg(L1)(ClO
L2
4
4
4
)
)
)
2
2
2
79
56
Hg(L2)(ClO
L3
54
34
Hg(L3)(ClO
48
13

Figure captions
Figure 1. Structural depiction of L1-L3.
Figure 2. Optimized structures of a) L1, b) L2, and c) L3 (B3LYP/ 6-311+G(d)).
Figure 3. a) Absorption spectra of L1 (solid), L2 (dots) and L3 (dashed); b) Emission spectra of
L1 (λ = 297 nm, solid), L2 (λ = 340 nm, dots), and L3 (λ = 297 nm, dashed) in CH CN.
ex
ex
ex
3
Figure 4. Change in a) absorption of L1 (isobestic point = 314 nm), b) absorption of L2 to
0.0 eq, c) absorption of L3 (isobestic point = 344 nm), d) emission of L1 (λ = 297 nm),
2
ex
e) emission of L2 (λ = 340 nm) to 20.0 eq, f) emission of L3 (λ = 297 nm) with increasing
ex
ex
amounts of Hg(ClO ) in CH CN.
4
2
3
1
Figure 5. Change in H NMR spectrum with increasing amounts of Hg(ClO ) for a) L1, b) L2,
4
2
and c) L3. Proposed structures with labelled protons are overlaid for each spectrum.
Figure 6. Emission of a) L1, b) L2, and c) L3 at 410, 435, and 457 nm, respectively, after the
addition of 5 mol. eq. of various metal ions (grey bars) and then subsequent addition of 5 mol.
eq. of Hg(ClO ) (black bars) in CH CN. Ligand emission alone is indicated by white bars.
4
2
3
Excitation wavelengths for L1 = 297 nm, L2 = 361 nm, and L3 = 297 nm.
14

Figure 1. Structural depiction of L1-L3
1x4mm (300 x 300 DPI)
2

Figure 2. Optimized structures of a) L1, b) L2, and c) L3 (B3LYP/ 6-311+G(d))
5x11mm (300 x 300 DPI)
3

Figure 3. a) Absorption spectrum of L1 (solid), L2 (dots) and L3 (dashed) b) Emission spectrum of L1
λex = 297 nm, solid), L2 (λex = 340 nm, dots), and L3 (λex = 297 nm, dashed) in CH3CN
7x17mm (300 x 300 DPI)
(
4

Figure 4. Change in a) absorption of L1 (isobestic point = 314 nm) b) absorption of L2 to 20.0 eq c)
absorption of L3 (isobestic point = 344 nm) d) emission of L1 (λex = 297 nm) e) emission of L2 (λex = 340
nm) to 20.0 eq f) emission of L3 (λex = 297 nm) with increasing amounts of Hg(ClO
4 2 3
) in CH CN.
7
8x37mm (300 x 300 DPI)

1
Figure 5. Change in H NMR spectrum with increasing amounts of Hg(ClO
)
4 2
for a) L1, b) L2, and c) L3.
Proposed structures with labelled protons are overlaid for each spectrum.
3x17mm (300 x 300 DPI)
5

Figure 6. Emission of a) L1, b) L2, and c) L3 at 410, 435, and 457 nm respectively after the addition of 5
mol. eq. of various metal ions (grey bars) and then subsequent addition of 5 mol. eq. of Hg(ClO (black
bars) in CH CN. Ligand emission alone is indicated by white bars. Excitation wavelengths for L1 = 297 nm,
L2 = 361 nm, and L3 = 297 nm.
1x10mm (300 x 300 DPI)
4 2
)
3
4