Dipyrrolylquinoxaline-bridged Schiff bases: A new class of fluorescent sensors for mercury(II)

Article abstract of DOI:10.1039/b505676d

Novel 2,3-bis(1H-pyrrol-2-yl)quinoxaline-functionalized Schiff bases were prepared and characterized as new fluorescent sensors for mercury(II) ion. The X-ray crystal structures of compounds 4, 5, 4a and 5a were determined. The binding properties of 4 and 5 for cations were examined by UV-vis and fluorescence spectroscopy. The UV-vis and fluorescence data indicate that a 1: 1 stoichiometric complex is formed between compound 4 (or 5) and mercury(II) ion, and the association constant is (3.81 ± 0.7) × 10⁵ M^-1 for 4 and (3.43 ± 0.53) × 10⁵ M ^-1 for 5. The recognition mechanism between compound 4 (or 5) and metal ion was discussed based on their chemical construction and the fluorescence quenching effect when they interact with each other. Competition experiments revealed that compound 4 (or 5) has a highly selective response to mercury(II) ion in aqueous solution. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b505676d

View Article Online / Journal Homepage / Table of Contents for this issue
F U L L P A P E R
Dipyrrolylquinoxaline-bridged Schiff bases: a new class of
fluorescent sensors for mercury(II)
Lei Wang,a Xun-Jin Zhu,b Wai-Yeung Wong,b Jian-Ping Guo,c Wai-Kwok Wong*b and
Zao-Ying Li*a
a Department of Chemistry, Wuhan University, Wuhan, Hubei, People’s Republic of China
b Department of Chemistry and Centre for Advanced Luminescence Materials, Hong Kong
Baptist University, Hong Kong, People’s Republic of China
c Department of Chemistry, Shanxi University, Taiyuan, Shanxi, People’s Republic of China.
E-mail: zyliwuc@whu.edu.cn; Fax: 86-27-68754067; Tel: 86-27-87219084
Received 25th April 2005, Accepted 29th July 2005
First published as an Advance Article on the web 26th August 2005
Novel 2,3-bis(1H-pyrrol-2-yl)quinoxaline-functionalized Schiff bases were prepared and characterized as new
fluorescent sensors for mercury(II) ion. The X-ray crystal structures of compounds 4, 5, 4a and 5a were determined.
The binding properties of 4 and 5 for cations were examined by UV-vis and fluorescence spectroscopy. The UV-vis
and fluorescence data indicate that a 1 : 1 stoichiometric complex is formed between compound 4 (or 5) and
mercury(II) ion, and the association constant is (3.81 0.7) × 105 M−1 for 4 and (3.43 0.53) × 105 M−1 for 5. The
recognition mechanism between compound 4 (or 5) and metal ion was discussed based on their chemical
construction and the fluorescence quenching effect when they interact with each other. Competition experiments
revealed that compound 4 (or 5) has a highly selective response to mercury(II) ion in aqueous solution.
the cation binding can be visualised by changes in the UV-vis
absorption spectrum and fluorescence spectrum.
Introduction
Contamination with heavy metal ions may have severe effects on
human health and the environment.1 Mercury contamination is
Experimental
widespread and arises from a variety of natural sources, such
as oceanic and volcanic emission, as well as anthropogenic
sources,2 such as gold mining and the combustion of solid waste
and fuels. The development of an easy mercury determination
system for monitoring its concentration in polluted areas has
attracted considerable attention in the field of supramolecular
chemistry.3 Although HTM (heavy and transition metal) ions are
relatively easy to be chelated and detected in organic solvents, it
is rather difficult to recognize HTM in an aqueous environment
due to their strong hydration.4 In the reported small-molecule
sensors, only a few compounds can selectively detect Hg2+ ion
in aqueous solutions.5
In recent years, there has been an emerging interest in
the application of macrocyclic ligands as effective metal ion
chelators.6 In contrast to the other analytical methods detecting
cations, the fluorescent sensors offer distinct advantages in
terms of sensitivity, selectivity and response time.7 A fluorescent
sensing system generally consists of two parts. The first part is a
cation binding site, which is affected by the ligand topology, the
characteristic of the cation (ionic radius, charge, coordination
number, hardness, etc.) and the nature of the solvent (e.g. pH).
The other part deals with the fluorophore, which converts the
binding process or recognition phenomena to optical signals.8
The dipyrrolylquinoxaline (DPQ) derivatives have been re-
ported as anion receptors,9 but to the best of our knowledge,
they have never been employed as cation receptors. In this
article, we wish to develop a new fluorescent sensor by using
dipyrrolylquinoxaline as the basic fluorophore, pyrrole and the
imine as the recognition site. Due to the change in the electronic
density of the chromophore induced by complexation, when
the ionophore moiety is complexed to a metal ion, remark-
able changes in the fluorescence intensity and the absorption
spectra appear.10 Since the recognition site is now part of the
fluorophore, when it is conjugated with the cation in solution, the
analytical potential of the system should be greatly enhanced.
The metal ion binding affinity is adequately controlled by the
ring nitrogen atoms. As expected, the compounds synthesized
have the advantage of detecting Hg2+ ion in aqueous solutions,
General information
All starting materials and solvents were purchased from Aldrich
Chemical Company, and used without further purification
unless otherwise stated. Pyrrole was distilled under N2 prior
to use. DMF and 1,2-dichloroethane were distilled from CaH2
and stored over activated molecular sieves prior to use. Pyridine
was distilled from NaOH and stored over activated molecular
sieves prior to use. Methanol and dichloromethane were distilled
from CaH2 under N2. The M(OAc)2 (M = Cu2+, Ni2+, Pb2+, Mn2+,
Zn2+, Cd2+, Co2+), FeCl2 and CaCl2 were purchased from Aldrich
Chemical Company. The infrared spectra were measured on
a Nicolet 670 FT-IR spectrophotometer. 1H NMR and 13C
NMR spectra were recorded on a JEOL EX 270 spectrometer,
or Varian Mercury-VX 600 spectrometer, and low-resolution
mass spectra on a Finnigan MAT SSQ-710 in FAB (positive)
mode. Fluorescence spectra were obtained on a Schimadzu RF-
5301 luminescence spectrometer. UV-vis spectra were taken on
a TU-190 spectrometer. Elemental analyses were determined by
a Finnigan FLASH 1112 SERIES EA elemental autoanalyzer.
Syntheses
1,2-Bis(1H-pyrrole-2-yl)ethane-1,2-dione(1). This compound
was prepared according to the published procedure.9a Yellow
powder (320 mg, 24%). 1H NMR (270 MHz, DMSO-d6): d 12.28
(br, 2H, NH), 7.30 (s, 2H, pyrrole), 6.89 (d, J = 3.8 Hz, 2H,
pyrrole), 6.26 (t, J = 2.4 Hz, J = 3.5 Hz, 2H, pyrrole); FAB-MS:
m/z 189 (M + H)+.
2,3-Bis(1H-pyrrol-2-yl)quinoxaline (2). It was prepared by
the literature method.9b Yellow powder (416 mg, 94%). 1H NMR
(270 MHz, DMSO-d6): d 11.56 (br, 2H, NH), 7.91–7.95 (dd,
J = 3.5 Hz, J = 3.2 Hz, 2H, quinoxaline), 7.68–7.72 (dd, J =
3.5 Hz, J = 3.2 Hz, 2H, quinoxaline), 6.97 (br, 2H, pyrrole),
6.11–6.20 (m, 4H, pyrrole); IR (KBr, cm−1): 3206(s), 1541(s),
1417(s), 1338(s), 1102(s), 1045(s), 750(s), 672(s); FAB-MS: m/z
261 (M + H)+.
T h i s j o u r n a l i s
T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 5
D a l t o n T r a n s . , 2 0 0 5 , 3 2 3 5 – 3 2 4 0
3 2 3 5
©

View Article Online
2,3-Bis(5ꢀ-formylpyrrol-2ꢀ-yl)quinoxaline (3). POCl3 (240 lL,
2.57 mmol) was added to DMF (454 lL, 5.86 mmol) at 0 ◦C
under N2. The mixture was then allowed to warm to room
temperature and stirred for 10 min before 1,2-dichloroethane
(3 mL) was added. To this mixture was then added a solution of
compound 2 (260 mg, 1.00 mmol) in 1,2-dichloroethane (15 mL)
over a period of 10 min. The resulting mixture was refluxed for
30 min before being cooled to 0 ◦C. Saturated aqueous NaOAc
(3 mL) was then added, and the mixture was refluxed for a
further 30 min. Upon cooling, the mixture was washed with
CH2Cl2, and the combined organic phases were then washed
with water and brine, dried over anhydrous Na2SO4, filtered and
evaporated to dryness. The residue was then subjected to chro-
matography over silica gel, MeOH/CH2Cl2(1 : 100) as eluent,
and recrystallized from CH2Cl2/hexane to afford 3 (250 mg,
79%) as a yellow solid. 1H NMR (270 MHz, DMSO-d6): d 12.71
(br, 2H, NH), 9.68 (s, 2H, CHO), 8.11–8.15 (dd, J = 3.5 Hz, 2H,
quinoxaline), 7.88–7.91 (dd, J = 3.5, 2H, quinoxaline), 7.03 (t,
J = 2.7 Hz, 2H, pyrrole), 6.14 (br, 2H, pyrrole); IR (KBr, cm−1):
3252(s), 1657(s), 1498(s), 1338(s), 1258(s), 1194(s), 805(s), 769(s),
731(s); FAB-MS: m/z 317 (M + H)+.
(KBr, cm−1): 2962–2850(m), 1596(s), 1547(s), 1473(s), 1261(s),
1094(s), 1031(s), 802(s), 768(s), 678(s); FAB-MS: m/z 411, (M +
H)+; Anal. calcd for C21H16N6Ni: C, 61.36; H, 3.92; N, 20.44.
Found: C, 61.32; H, 3.93; N, 20.40.
Compound 5a. Similar to the preparation of 4a, compound
5 (71.8 mg, 0.2 mmol) was used in place of 4 in this reaction
1
to afford complex 5a as a red powder (80 mg, 97%). H NMR
(270 MHz, CDCl3): d 8.03–8.06 (dd, J = 3.5 Hz, J = 3.2 Hz,
2H, quinoxaline), 7.61–7.65 (dd, J = 3.5 Hz, J = 3.2 Hz, 2H,
=
quinoxaline), 7.37 (s, 2H, –CH N), 7.75 (d, J = 4.1 Hz, 2H,
=
pyrrole), 6.87 (d, J = 4.1 Hz, 2H, pyrrole), 3.34 (br, 4H, N–
CH2–CH2), 1.86 (br, 4H, N–CH2–CH2). 13C NMR(270 MHz,
=
CDCl3): d160.75, 149.44, 142.18, 140.16, 139.92, 128.76, 128.41,
119.63, 117.30, 54.30, 25.90; IR (KBr, cm−1): 2960–2850(m),
1598(s), 1474(s), 1261(s), 1087(s), 1036(s), 807(s), 787(s), 679(s);
FAB-MS: m/z 424 (M + H)+; Anal. calcd for C22H18N6Ni: C,
62.16; H, 4.27; N, 19.77. Found: C, 62.10; H, 4.28; N, 19.67.
X-Ray crystallography
Pertinent crystallographic data and other experimental details
are summarized in Table 1. Intensity data for 4, 5, 4a and 5a were
collected at 293 K on a Bruker Axs SMART 1000 CCD area-
detector diffractometer using graphite-monochromated Mo-Ka
Compound 4. A solution of 2,3-bis(5ꢀ-formylpyrrol-2ꢀyl)-
quinoxaline (3) (63.4 mg, 0.2 mmol) and triethylamine (80 lL) in
dry methanol (50 mL) was stirred for 30 min at reflux. After that,
1,3-diaminopropane (15 mg, 0.2 mmol) in dry methanol (2 mL)
was added dropwise to the solution. The resulting solution
was refluxed for 2 h to give a yellow solid. The residue was
subjected to chromatography over silica gel (eluent, CH2Cl2),
and recrystallized from CH2Cl2/CH3OH to furnish a yellow
˚
radiation (k = 0.71073 A). The collected frames were processed
with the software SAINT11 and an absorption correction was
applied (SADABS)12 to the collected reflections. The space
groups of each crystal were determined from the systematic
absences and Laue symmetry checks and confirmed by suc-
cessful refinement of the structures. For 5, the other possible
centrosymmetric alternative Pbcm was tried but did not give
any reasonable solution. The structures of all compounds were
solved by direct methods (SHELXTL)13 and refined against F2
by full-matrix least-squares analyses. All non-hydrogen atoms
were refined anisotropically. The inner hydrogen atoms on N(3)
and N(5) for 4 as well as those on N(2) and N(4) for 5 were
located from the difference Fourier maps and they were found
to be disordered over other N-atom sites. The positions of these
hydrogen atoms were subsequently fixed in the refined model,
which give reasonable bond parameters. All other hydrogen
atoms were generated in their idealized positions and allowed to
ride on their respective parent carbon atoms.
1
solid (65 mg, 92%). H NMR (270 MHz, CDCl3): d 8.04–8.07
(dd, J = 3.5 Hz, 2H, quinoxaline), 7.65–7.68 (dd, J = 3.5 Hz,
=
J = 3.2 Hz, 2H, quinoxaline), 7.86 (s, 2H, –CH N), 7.59 (d,
J = 3.8 Hz, 2H, pyrrole), 6.96 (d, J = 4.1 Hz, 2H, pyrrole), 3.97
=
=
(t, 4H, J = 5.4 Hz, N–CH2–CH2), 2.14 (br, 2H, N–CH2–
CH2);13C NMR (150 MHz, CDCl3): d 150.09, 145.94, 144.53,
140.28, 131.58, 129.20, 128.74, 121.59, 117.46, 55.16, 29.70; IR
(KBr, cm−1) 2937–2832(m), 1647(s), 1604(s), 1469(s), 1299(s),
1015(s), 805(w), 770(s), 685(s); FAB-MS: m/z 355 (M + H)+;
Anal. calcd for C21H18N6: C, 71.17; H, 5.12; N, 23.71. Found: C,
71.10; H, 5.14; N, 23.67.
Compound 5. This compound was prepared similarly as
for 4 except that 1,4-diaminobutane (17.6 mg, 0.2 mmol) was
used in place of 1,3-diaminopropane to give a yellow power
(66 mg, 89.4%). 1H NMR (270 MHz, CDCl3): d 8.01–8.05 (dd,
J = 3.5 Hz, 2H, quinoxaline), 7.64–7.67 (m, 4H, quinoxaline
CCDC reference numbers 256611 (4), 254888 (5), 254150 (4a)
and 254149 (5a).
See http://dx.doi.org/10.1039/b505676d for crystallographic
data in CIF or other electronic format.
=
and pyrrole), 7.89 (s, 2H, –CH N), 6.97 (d, J = 3.8 Hz, 2H,
=
=
pyrrole), 3.72 (br, 4H, N–CH2–CH2), 2.04 (br, 4H, N–CH2–
CH2);13C NMR (270 MHz, CDCl3): d 151.56, 146.36, 144.23,
140.25, 131.11, 129.06, 128.62, 123.04, 117.47, 56.01, 28.83; IR
(KBr, cm−1): 2832–2937(m), 1647(s), 1604(s), 1469(s), 1299(s),
1015(s), 805(s), 770(s), 685(s); FAB-MS: m/z 369 (M + H)+;
Anal. calcd for C22H20N6: C, 71.72; H, 5.47; N, 22.81. Found: C,
71.06; H, 5.48; N, 22.76.
Results and discussion
Synthesis and characterization of compounds
The synthetic routes for compounds 4, 5, 4a and 5a are
depicted in Scheme 1. The new fluorescent sensor 4 (or 5) was
synthesized by refluxing the methanol solution of 2,3-bis(5ꢀ-
formylpyrrol-2ꢀ-yl)quinoxaline and an equivalent molar amount
of 1,3-diaminopropane (or 1,4-diaminobutane) in the presence
of triethylamine. The product were obtained as a yellow powder
Compound 4a. Compound 4 (69 mg, 0.2 mmol) was
dissolved in dry methanol (30 mL) and a solution of
(CH3CO2)2Ni·4H2O (49.8 mg, 0.2 mmol) in dry methanol
(15 mL) was added. The mixture was stirred for 30 min at room
temperature. The colour of the solution turned from yellow to
red. The mixture was evaporated to dryness. The residue was
subjected to chromatography over silica gel (eluent, CH2Cl2),
and recrystallized from CH2Cl2/CH3OH to give a red solid
1
in high yield and characterized by IR, H NMR, 13C NMR,
FAB-MS and elemental analysis.
Single crystals of 4, 5, 4a and 5a suitable for X-ray analysis
were obtained from their dichloromethane–methanol solution
by slow evaporation at room temperature. Perspective drawings
of compounds 4, 5, 4a and 5a are shown in Figs. 1–4, respectively.
For 4 and 5, the two inner hydrogen atoms were located by
Fourier difference syntheses, one of which was shown to be on
the pyrrole N atom and the other on the imino N atom in the
refined solid-state model. The structures of 4 and 5 revealed
the presence of intramolecular hydrogen bonding between the
N–H group and the imino N atom of the Schiff base with the
1
(78 mg, 96%). H NMR (270 MHz, CDCl3): d 7.99–8.02 (dd,
J = 3.5 Hz, 2H, quinoxaline), 7.58–7.61 (dd, J = 3.5 Hz, J =
=
3.2 Hz, 2H, quinoxaline), 7.57 (s, 2H, –CH N), 7.76 (d, J =
4.3 Hz, 2H, pyrrole), 6.89 (d, J = 4.1 Hz, 2H, pyrrole), 3.35
=
=
(t, 4H, J = 4.9 Hz, N–CH2–CH2), 1.97 (br, 2H, N–CH2–
CH2); 13C NMR (150 MHz, CDCl3): d 159.53, 149.41, 141.56,
140.30, 140.04, 128.93, 128.59,119.59,117.59, 53.04, 31.26; IR
˚
˚
N · · · H distances of 1.744–1.899 A for 4 and 1.735–2.148 A for
3 2 3 6
D a l t o n T r a n s . , 2 0 0 5 , 3 2 3 5 – 3 2 4 0

View Article Online
Table 1 Crystallographic data for compounds 4, 5, 4a and 5a
Compound
4
5
4a
5a
Formula
C21H18N6
354.41
0.30 × 0.20 × 0.15
Monoclinic
P21/c
9.2969(8)
11.0235(10)
16.9434(15)
90
90.496(2)
90
1736.4(3)
744
4
1.356
0.085
C22H20N6
368.44
0.34 × 0.10 × 0.10
Orthorhombic
Pca21
25.4999(12)
4.9181(2)
28.3899(13)
90
90
90
3560.4(3)
1560
8
2(C21H16N6Ni)·CH2Cl2
907.10
0.10 × 0.15 × 0.30
Monoclinic
P21/c
16.3090(18)
26.440(3)
9.1440(10)
90
104.061(2)
90
3824.9(7)
1864
4
1.575
1.176
C22H18N6Ni
425.13
0.30 × 0.10 × 0.10
Orthorhombic
P212121
13.9137(7)
16.3936(9)
24.5367(13)
90
90
90
5596.7(5)
2640
12
Formula weight
Crystal size/nm
Crystal system
Space group
˚
a/A
˚
b/A
˚
c/A
a/◦
b/◦
c /◦
V/A
F(000)
3
˚
Z
D
calc/g cm−3
1.375
0.086
16235
1.514
1.062
28363
Absorption coefficient/mm−1
Reflections collected
8447
19083
Independent reflections (Rint
)
3048 (0.0374)
1834
0.0479, 0.1167
0.0937, 0.1418
1.016
3193 (0.0383)
2859
0.0437, 0.1163
0.0503, 0.1227
1.052
6734 (0.0412)
4357
0.0590, 0.1504
0.0961, 0.1733
1.054
5459 (0.0632)
3960
0.0369, 0.0742
0.0684, 0.0856
1.047
Observed reflections (I > 2r(I))
R1, wR2 (I > 2r(I))
R1, wR2 (all data)
Goodness-of-fit on F2
Fig. 2 ORTEP drawing of compound 5.
Scheme 1 Synthesis of ligands and complexes. Reagents and condi-
tions: (a) 1,2-diaminobenzene, AcOH, reflux, 94%; (b) POCl3, DMF,
0
◦C for 10 min/1,2-dichloroethane, reflux for 0.5 h, 79%; (c) Et3N,
CH3OH, 1,3-diaminopropane, 92%, (or 1,4-diaminobutane, 89%); (d)
Ni(OAc)2·4H2O, CH3OH, CH2Cl2, room temperature, 0.5 h (4a, 96%;
5a, 97%).
Fig. 3 ORTEP drawing of compound 4a.
Fig. 1 ORTEP drawing of compound 4.
5. Worthy of note here, the solid-state structures shown in Fig. 1
and 2 represent only one of the possible canonical forms for 4
and 5, respectively, and the detailed resonance structures for 4
are depicted in Chart 1. Although crystals of the mercury(II)
complex of 4 (or 5) could not be obtained, the binding mode of
4 (or 5) towards the Hg(II) ion was revealed by X-ray structures
Fig. 4 ORTEP drawing of compound 5a.
D a l t o n T r a n s . , 2 0 0 5 , 3 2 3 5 – 3 2 4 0
3 2 3 7

View Article Online
Fig. 5 pH-fluorescence profiles for 1.0 × 10−5 mol L−1 solutions of
compounds 4 and 5 (kex = 390 nm). Emission was measured at the
emission maximum centred near 506 nm. pH was adjusted by using the
0.2 M NaOAc–HOAc buffering solution.
4 and 5 (1.0 × 10−5 mol L−1) at different pH buffer solutions.
In acidic buffer solution, the variation of fluorescence intensity
of the receptor is not distinct. Then ligands 4 and 5 are able
to act as a fluorescent cation sensor in low pH buffer solution.
Ligand 5, which has a longer hydrocarbon chain and a larger
ring size, is more flexible and, thus, easier to be protonated at
both imino groups than that of ligand 4. As a result, 5 appears to
have a slightly higher tolerance towards high pH than 4. All the
metal ion detections were operated in the 0.2 M NaOAc–HOAc
buffering solution. The fluorescence quantum yields of 4 and 5
were determined by using quinine sulfate in 0.1 M H2SO4 (U =
0.546) as a reference.16 As expected, 4 and 5 showed a very weak
fluorescence (for 4, U = 0.006, kmax (em) = 507 nm; for 5, U =
0.007, kmax (em) = 507 nm) in the 0.2 M NaOAc–HOAc buffering
solution.
Chart 1 Different resonance forms of compound 4.
of the Ni(II) complexes 4a and 5a (Fig. 3 and 4, respectively).
Selected bond lengths and bond angles of 4a and 5a are given
in Table 2. There are three independent molecules of 5a in the
asymmetric unit, only one of these is shown in Fig. 4. In each
of the structures of 4a and 5a, the nickel atom is bound to
four nitrogen atoms, two from the deprotonated pyrroles and
two from the adjacent imines, and is almost coplanar to the
respective N4 plane (average mean deviation from the N4 plane
˚
˚
of ca. 0.024 A for 4a and 0.062 A for 5a). The geometry at each
nickel centre is square planar. The Ni–Npyr (pyrrole nitrogen)
distances for 4a and 5a are identical; whereas the Ni–Nim (imino
˚
nitrogen) distances for 4a (1.855–1.887 A) are slightly shorter
Fluorescence titration of 4 and 5 with mercury(II) and other
metal ions
˚
than that of 5a (1.909–1.922 A). Furthermore, the Nim–Ni–Nim
angle for 4a [N(3)–Ni–N(4) 93.12◦] is smaller than that of 5a
[N(4)–Ni–N(5) 94.9◦]. These slight deviations may be as a result
of the ring size. The structure of a smaller ring, such as 4, will
be more rigid than a larger ring, such as 5. Thus, the interaction
between 4 and the cation is slightly stronger than that between
5 and the cation.
The selective and sensitive signal response of compounds 4
and 5 toward Hg2+ were carried out in the buffer solution. As
shown in Figs. 6a and 6b, with a gradual increase of Hg2+ ion
concentration, the fluorescence intensity of 4 and 5 at 506 nm
was gradually reduced. Meanwhile, a blue shift was distinctly
observed for each of 4. The fluorescence intensity decreased
linearly (0.0–1.0 × 10−5 M, linearly dependent coefficient R2 =
0.9951 for 4 and 0.9905 for 5) with an increase in Hg2+ ion
concentration. When the amino group (the electron donor)
interacts with Hg2+ ion, the latter reduces the electron-donating
character of this group due to the reduction of conjugation, and
a decrease of the extinction coefficient was expected.17 Careful
analysis of the evolution of the emission spectra revealed that
a 1 : 1 complex was formed between Hg2+ and compound 4 (or
5). However, there was virtually no change in the fluorescence
intensity when other cations (e.g. Cu2+, Ni2+, Pb2+, Mn2+, Zn2+,
Cd2+, Co2+, Ca2+ and Fe2+) were used instead of Hg2+. In other
words, the ligands 4 and 5 have good selective fluorescence
responses to Hg2+.
pH Effect on the fluoresence intensity of 4 and 5
The fluorescence of this type of sensor is often pH-sensitive,14
and the observed dependence of fluorescence on pH is in line
with the intramolecular amine quenching mechanism that has
been described previously.15 When the N atom binds to a proton,
the amine lone pairs become involved in hydrogen bonding
and are unable to donate electrons to the quinoxaline, the
fluorescence intensity will change with the pH of the solution.
Fig. 5 shows the fluorescence emission spectra of the receptors
◦
˚
Table 2 Selected bond lengths (A) and bond angles ( ) of 4a and 5a
Failure of nickel(II) binding by 4 and 5 in buffer solution
is somehow quite surprising since the corresponding Ni(II)
complexes have been isolated in methanol and their crystal
structures were elucidated through X-ray diffraction studies.
In order to explore the role of the solvent, we carried out
similar titration experiments in acetonitrile. In acetonitrile, the
fluorescence is too weak to be detected. When Ni(II) ion was
added, we can observe a colour change of the methanol solution
of 4 and 5 from yellow to red with the naked eye; when Zn2+
or Cd2+ ion was added, the fluorescence intensity of 4 and 5 at
kmax (em) = 554 nm was gradually enhanced with a gradual increase
in the concentration. It showed that the nature of the solvent is
important for the ligand’s selectivity.
Compound 4a
Compound 5a
Ni(1)–N(2)
Ni(1)–N(3)
Ni(1)–N(4)
Ni(1)–N(5)
1.881(4)
1.885(4)
1.887(4)
1.883(4)
Ni(1)–N(3)
Ni(1)–N(4)
Ni(1)–N(5)
Ni(1)–N(6)
1.881(4)
1.922(5)
1.909(5)
1.883(4)
N(2)–Ni(1)–N(5)
N(2)−Ni(1)–
96.04(18)
85.43(19)
N(3)–Ni(1)–N(6)
N(3)–Ni(1)–N(5)
95.51(18)
174.1(2)
N(3)
N(2)–Ni(1)–N(4)
N(5)–Ni(1)–N(4)
N(5)–Ni(1)–N(3)
N(3)–Ni(1)–N(4)
178.45(18)
85.44(18)
176.14(17)
93.12(19)
N(6)–Ni(1)–N(5)
N(3)–Ni(1)–N(4)
N(6)–Ni(1)–N(4)
N(5)–Ni(1)–N(4)
84.9(2)
85.30(19)
173.81(19)
94.9(2)
3 2 3 8
D a l t o n T r a n s . , 2 0 0 5 , 3 2 3 5 – 3 2 4 0

View Article Online
Fig. 6 (a) Fluorescence spectra of 4 (1.0 × 10−5 mol L−1) upon the
addition of various amounts of Hg2+ in AcOH–NaOAc solution (0.2 M,
pH = 4.5), kex = 390 nm. The inset shows the fluorescence intensity
at kmax (em) = 506 nm as a function of Hg2+ ion concentration. (b)
Fluorescence spectra of 5 (1.0 × 10−5 mol L−1) upon the addition of
various amounts of Hg2+ in AcOH–NaOAc solution (0.2 M, pH = 4.5),
Fig. 7 (a) UV absorbance spectra of 4 (1.0 × 10−5 mol L−1) upon the
addition of various amounts of Hg2+ AcOH–NaOAc solution (0.2 M,
pH = 4.5). The inset shows the absorbance intensity at kmax = 309 nm
as a function of mercury concentration. (b) UV absorbance spectra
of 5 (1.0 × 10−5 mol L−1) upon the addition of various amounts of
Hg2+ in AcOH–NaOAc solution (0.2 M, pH = 4.5). The inset shows
the absorbance intensity at kmax = 309 nm as a function of mercury
concentration.
kex = 390 nm. The inset shows the fluorescence intensity at kmax (em)
=
506 nm as a function of Hg2+ ion concentration.
Absorbance change of 4 and 5 with mercury(II) and other metal
ions
As shown in Fig. 7, a broad absorption of ligand 4 (or 5)
appeared near 309 nm in aqueous solution, and their absorption
intensity also decreased linearly with an increase in Hg2+ ion
concentration (0.0–1.0 × 10−5 M, R2 = 0.9955 and 0.9995 for
4 and 5, respectively). With Hg2+/ligand mole ratio >1 : 1, a
clear red shift (up to 15 nm) was observed. Isosbestic point
was observed at 415 nm. As more and more Hg2+ solution was
added to the ligand solution, the dilution effect becomes more
significant and causes the isosbestic point at 415 nm to become
unclear. These results showed the formation of a complex with
a 1 : 1 stoichiometry for 4 (or 5) and Hg2+.
The competition experiments
To explore the utility of 4 and 5 as an ion-selective fluores-
cence chemosensor for Hg2+, the competition experiments were
conducted in the presence of Hg2+ at different concentrations
that is also mixed with Cu2+, Ni2+, Pb2+, Mn2+, Zn2+, Cd2+,
Co2+, Ca2+, and Fe2+ at 5.0 × 10−5 M. As shown in Fig. 8,
ligand 4 has no response with metal ions such as Cu2+, Ni2+,
Pb2+, Mn2+, Zn2+, Cd2+, Co2+, Ca2+ and Fe2+. Furthermore, no
obvious interference was observed in its fluorescence spectrum
while performing the titrations with Hg2+ in the mixtures of these
Fig. 8 Fluorescence spectra of 4 (1.0 × 10−5 M) in AcOH–NaOAc
solution (0.2 M, pH = 4.5) in the presence of different metal ions (e.g.
Cu2+, Ni2+, Pb2+, Mn2+, Zn2+, Cd2+, Co2+ and Ca2+, 5.0 × 10−5 M).
metal ions. Ligand 5 shows similar properties in this context. The
above results indicate that the selectivity of 4 and 5 for Hg2+ ions
were remarkable.
D a l t o n T r a n s . , 2 0 0 5 , 3 2 3 5 – 3 2 4 0
3 2 3 9

View Article Online
Table 3 Association constants Kass of receptors 4 and 5 with guest cationsa
Cation
Hg(II)
Receptor 4 Kass/M−1
(3.81 0.7) × 105
R
Receptor 5 Kass/M−1
(3.43 0.53) × 105
R
0.9956
0.9958
a The data were calculated from the absorbance titration at pH 4.5 in AcOH–AcONa buffer solution.
Determination of the association constants (Kass) of complexes
5 (a) A. B. Descaloz, R. Martines-Manes, R. Radeglia, K. Rurack and
J. Sato, J. Am. Chem. Soc., 2003, 125, 3418; (b) E. M. Nolan and S. J.
Lippard, J. Am. Chem. Soc., 2003, 125, 14270; (c) M. J. Choi, M. Y.
Kim and S. K. Chang, Chem. Commun., 2001, 1664; (d) L. Prodi, C.
Bargossi, M. Montalti, N. Zaccheroni, N. Su, J. S. Bradshaw, R. M.
Izatt and P. B. Savage, J. Am. Chem. Soc., 2000, 122, 6769.
6 (a) D. S. Kumar and V. Alexander, Inorg. Chim. Acta, 1995, 238, 63;
(b) Y. J. Mikata, M. Wakamatsu and S. Yano, Dalton Trans., 2005,
545; (c) T. Lazarides, T. A. Miller, J. C. Jeffery, T. K. Ronson, H.
Adams and M. D. Ward, Dalton Trans., 2005, 528.
7 (a) A. P. Silva, H. Q. N. Gunaratne, T. Gunnlaugsson, A. J. M.
Huxley, C. P. McCoy, J. T. Rademacher and T. E. Rice, Chem. Rev.,
1997, 97, 1515; (b) L. Fabbrizzi and A. Poggi, Chem. Soc. Rev., 1995,
24, 197.
For a metal complex of 1 : 1 stoichiometry, the association
constant (Kass) can be estimated according to the following
relation:18
X = X0 + (Xlim − X0)/2c0 {cH + cG + 1/Kass
− [(cH + cG + 1/Kass)2 − 4cHcG]1/2
}
Where X represents the absorbance intensity, cH and cG are
the corresponding concentration of the host and guest. The
association constants obtained by a non-linear least-square
analysis (the insets of Fig. 6 and 7) of X vs. cH and cG are
listed in Table 3. The data shows that receptors 4 and 5 have an
excellent selectivity for Hg2+ ion.
8 (a) J. L. Wu, Y. B. He, Z. Y. Zeng, L. H. Wei, L. Z. Meng and T. X.
Yang, Tetrahedron, 2004, 60, 4309; (b) F. Unon, Z. Asfari and J. Vicen,
Tetrahedron Lett., 1998, 29, 2951; (c) R. Bergonzi, L. Fabbrizzi, M.
Lichelli and C. Mangano, Coord. Chem. Rev., 1998, 170, 31.
9 (a) T. Mizuno, W. H. Wei, L. R. Eller and J. L. Seller, J. Am. Chem.
Soc., 2002, 124, 1134; (b) P. Anzenbacher, Jr., A. C. Try, H. Miyaji,
V. M. Lynch, M. Matquez and J. L. Seller, J. Am. Chem. Soc., 2000,
122, 10268; (c) C. B. Black, B. Andrioletti, A. C. Try, C. Ruiperez and
J. L. Seller, J. Am. Chem. Soc., 1999, 121, 10438; (d) S. V. Shevchuk,
V. M. Lnych and J. L. Sessler, Tetrahedron Lett., 2004, 60, 11283.
10 (a) H. Sakamoto, J. Ishikawa, S. Nakao and H. Wada, Chem.
Commun., 2000, 2395; (b) H. F. Ji, R. Dabestani, G. M. Brown and
R. L. Hettich, Photochem. Photobiol., 1999, 69, 513.
Conclusion
In summary, two novel, water-soluble chemosensors 4 and 5
for the recognition of Hg2+ ion were successfully designed and
synthesized. Both compounds display selective and sensitive
fluorescence quenching responses toward Hg2+ ion in aqueous
solution. Our findings will help to improve the direct detection
of Hg2+ ion in the environment.
Acknowledgements
11 SAINT, Reference manual, Siemens Energy and Automation, Madi-
son, WI, 1994–1996.
The authors would like to thank Hong Kong Baptist University
(FRG/03-04/II-52), Hong Kong Research Grants Council
(HKBU 2038/02P) and National Natural Science Foundation
of China (No: 29972035) for financial support.
12 G. M. Sheldrick, SADABS, Empirical Absorption Correction Pro-
gram, University of Go¨ttingen, Germany, 1997.
13 G. M. Sheldrick, SHELXTLTM Reference manual, version 5.1,
Siemens, Madison, WI, 1997.
14 (a) S. C. Burdette, G. K. Walkup, B. Spingler, R. Y. Tsien and S. J.
Lippard, J. Am. Chem. Soc., 2001, 123, 7831; (b) Y. J. Mikata, M.
Wakamatsu and S. Yano, Dalton Trans., 2005, 545; (c) A. Bencini,
E. Berni, A. Bianchi, A. Bianchi, P. Fornasari and C. Giorgi, Dalton
Trans., 2004, 2180.
15 (a) A. P. de Silva and T. E. Rice, Chem. Commun., 1999, 163; (b) X. F.
Guo, X. H. Qian and L. H. Jia, J. Am. Chem. Soc., 2004, 126, 2272.
16 (a) Z. K. Chen, W. Huang, L. H. Wang, E. T. Kang and S. T. Lee,
Macromolecules, 2000, 33, 9015; (b) S. J. Hima, J. Ramin and T.
Yitzhak, Angew. Chem, Int. Ed., 1999, 38, 2722.
References
1 (a) F. M. M. Morel, A. M. L. Kraepiel and M. Amyot, Annu. Rev.
Ecol. Syst., 1998, 29, 543; (b) R. von Burg, and R. M. Greenwood,
in Metals and Their Compounds in the Environment, ed. E. Merian,
VCH, Weinheim, 1991, p. 1045.
2 (a) A. Renzoni, F. Zino and E. Franchi, Environ. Res., 1998, 77, 68;
(b) O. Malm, Environ. Res., 1998, 77, 73.
3 (a) Q. Y. Chen and C. F. Chen, Tetrahedron Lett., 2005, 46, 165; (b) B.
Liu and H. Tian, Chem. Commun., 2005, 3156.
4 K. Ruaack, M. Killmannsberger, U. Resch-Genger and J. Daub,
J. Am. Chem. Soc., 2000, 122, 968.
17 (a) B. Valeur and I. Leray, Coord. Chem. Rev., 2000, 205, 3; (b) V.
Alexander, Chem. Rev., 1995, 95, 273.
18 (a) J. Valeur, J. Pouger and J. Bourson, J. Phys. Chem., 1992, 96, 6545;
(b) Y. Liu, B. Li and C. You, J. Org. Chem., 2001, 66, 225.
3 2 4 0
D a l t o n T r a n s . , 2 0 0 5 , 3 2 3 5 – 3 2 4 0