Mercury(ii)-mediated formation of imide-Hg-imide complexes

Article abstract of DOI:10.1039/c0dt01118e

Three different kinds of cyclic imides with imido protons are chosen to interact with Hg(ii): succinimide, the simplest cyclic imide (five-membered ring); phthalimide (conjugated five-membered ring); and 1,8-naphthalimide (conjugated six-membered ring). Based on the results of MS, ¹H-NMR, XPS, IR spectroscopy and fluorescence response analyses, it is suggested that N-unsubstituted cyclic imides react specifically with Hg(ii) and form imide-Hg-imide complexes through an imido proton-metal exchange process. The reaction is reversible and occurs rapidly at moderate to high pH. This discovery expands the comprehension of the specific interaction of Hg(ii) with the nucleobase thymine, and may open up new possibilities in designing novel ligands for sensing and removing Hg(ii) based on cyclic imides. The Royal Society of Chemistry 2011.

Full text of DOI:10.1039/c0dt01118e

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PAPER
Mercury(II)-mediated formation of imide-Hg-imide complexes†
Can-liang Fang,a,b Jin Zhou,a,b Xiang-jun Liu,a Ze-hui Caoa and Di-hua Shangguan*a
Received 27th August 2010, Accepted 5th November 2010
DOI: 10.1039/c0dt01118e
Three different kinds of cyclic imides with imido protons are chosen to interact with Hg(II): succinimide,
the simplest cyclic imide (five-membered ring); phthalimide (conjugated five-membered ring); and
1,8-naphthalimide (conjugated six-membered ring). Based on the results of MS, 1H-NMR, XPS, IR
spectroscopy and fluorescence response analyses, it is suggested that N-unsubstituted cyclic imides react
specifically with Hg(II) and form imide-Hg-imide complexes through an imido proton–metal exchange
process. The reaction is reversible and occurs rapidly at moderate to high pH. This discovery expands
the comprehension of the specific interaction of Hg(II) with the nucleobase thymine, and may open up
new possibilities in designing novel ligands for sensing and removing Hg(II) based on cyclic imides.
phthalocyanine have also been developed for the selective
Introduction
adsorption and detection of Hg(II).4,5 Therefore this specific
recognition of Hg(II) is regarded as a feature of thymine and has
been widely exploited for the construction of Hg(II) sensors and
Hg(II) sorbents. However, we recently observed that the highly
selective binding to Hg(II) ions was not unique to thymine but
also common to N-unsubstituted cyclic imides.
Based on the highly specific interaction of Hg(II) with the
nucleobase thymine (T), a variety of T-rich DNA sensors for
the selective detection of Hg(II) ions in aqueous solutions have
been developed recently.1,2 It has been demonstrated that Hg(II)
binds to two thymine residues and forms a stable neutral T-Hg-T
complex (Scheme 1) through an imido proton–metal exchange
process.3 Guided by this theory, thymine-modified polymers and
To demonstrate this finding, three different kinds of cyclic
imides with imido protons were chosen to interact with Hg(II):
succinimide (1), the simplest cyclic imide (five-membered ring);
phthalimide (2) (conjugated five-membered ring); and 1,8-
naphthalimide (3) (conjugated six-membered ring) (Scheme 1).
The Hg(II)-mediated formation of imide-Hg-imide complexes was
1
characterized by mass spectrometry (MS), H-nuclear magnetic
resonance spectroscopy (1H-NMR), X-ray photoelectron spec-
troscopy (XPS), infrared spectroscopy (IR) and luminescence
responses.
Experimental
Materials and general procedures
All starting materials were obtained from commercial suppli-
ers and used without further purification. Succinimide (99%)
and phthalimide (99%) were purchased from Acros. 4-Bromo-
1,8-naphthalic anhydride (98%) was purchased from Liaoning
Liangang Dyes Chemical Co. Ltd., China. Ammonia (25%),
Ethanolamine (99%), Glycine (99%), 1,2-Diaminoethane (99%)
and other reagents were received from Sinopharm Chemical
Reagent Co. Ltd., China.
All 1H-NMR spectra were measured in DMSO-d6 or CD3OD on
a Bruker AVANCE-400 NMR spectrometer (400 MHz) with TMS
as an internal reference. X-Ray photoelectron spectroscopy data
were obtained using an ESCALab220i-XL electron spectrometer
from VG Scientific using 300 W Al-Ka radiation. The base
pressure was about 3 ¥ 10-9 mbar. Binding energies were referenced
Scheme 1 Compounds containing the imide moiety (red), the T-Hg-T
complex and the formation of imide-Hg-imide complexes.
aBeijing National Laboratory for Molecular Sciences, Key Laboratory of
Analytical Chemistry for Living Biosystems, Institute of Chemistry, Chinese
Academy of Sciences, Beijing, 100190, China. E-mail: sgdh@iccas.ac.cn
bGraduate School of the Chinese Academy of Sciences, Beijing, 100039,
China
† Electronic supplementary information (ESI) available: Characterization,
absorption, mass spectra, NMR and fluorescence spectral data. See DOI:
10.1039/c0dt01118e
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to the C1s line at 284.8 eV of adventitious carbon. FTIR spectra
were measured with a Bruker Tensor27 spectrometer using KBr
discs. All the fluorescence measurements were acquired on a
SpectraMax M5 instrument (Molecular Devices Corporation,
USA).
(10 equiv.) and DMSO (30 mL) were added to a flask. The flask
was then sealed after filling with N2 and the mixture heated to
90 ◦C. After stirring at this temperature for 10 h, the heterogeneous
mixture was cooled to room temperature and diluted with
dichloromethane. The resulting solution was washed three times
with water and dichloromethane, then directly filtered through a
pad of silica gel and concentrated to yield the product, which was
purified by silica gel chromatography (4 : 1 ethyl acetate–MeOH)
to yield 3a as a yellow solid. EI-MS m/z 256 (M). 1H-NMR (400
MHz, DMSO-d6) (d, ppm): 3.47 (q, J = 5.6 Hz, 2H), 3.70 (q, J =
5.2 Hz, 2H), 4.87 (t, J = 5.2 Hz, 1H), 6.80 (d, J = 8.4 Hz, 1H), 7.66
(m, 2H), 8.20 (d, J = 8.4 Hz, 1H), 8.38 (d, J = 7.2 Hz, 1H), 8.69 (d,
J = 8.4 Hz, 1H), 11.22 (s, 1H). 13C-NMR (400 MHz, DMSO-d6)
(d, ppm): 46.6, 59.9, 104.7, 109.2, 121.6, 123.5, 124.9, 129.6, 131.0,
131.9, 134.4, 151.9, 164.8, 165.6.
Syntheses
The syntheses of compounds 3 and 4.
General. The syntheses of 3 and 4 were achieved in a two-step
procedure (Scheme 2). Firstly, the intermediates were synthesized
by reacting 10 equiv. of ammonia or ethanolamine in refluxing
ethanol with 4-bromo-1,8-naphthalic anhydride.6 The second step
involved stirring the intermediates with the corresponding amine
in DMSO at 90 ◦C for 10 h under N2, then washing with water and
DCM to yield the product as a yellow solid after purification by
column chromatography on silica (ethyl acetate–MeOH 90–80%).7
3b. 4-Bromo-1,8-naphthalimide (4 mM,
1 equiv.), Cu2O
(0.2 equiv.), potassium carbonate (0.5 equiv.), 1,2-diaminoethane
(10 equiv.) and DMSO (30 mL) were added to a flask. The flask
was then sealed after filling with N2 and the mixture heated to
90 ◦C. After stirring at this temperature for 10 h, the heterogeneous
mixture was cooled to room temperature and diluted with
dichloromethane. The resulting solution was washed three times
with water and dichloromethane, then directly filtered through a
pad of silica gel and concentrated to yield the product, which was
purified by silica gel chromatography (4 : 1 ethyl acetate–MeOH)
to yield 3b as a yellow solid. EI-MS m/z 255 (M). 1H-NMR (400
MHz, DMSO-d6) (d, ppm): 2.86 (t, J = 6.4 Hz, 2H), 3.38 (t, J = 6.4
Hz, 2H), 6.80 (d, J = 8.4 Hz, 1H), 7.66 (m, 2H), 8.20 (d, J = 8.4 Hz,
1H), 8.38 (d, J = 7.2 Hz, 1H), 8.69 (d, J = 8.4 Hz, 1H). 13C-NMR
(400 MHz, DMSO-d6) (d, ppm): 46.1, 62.6, 104.2, 108.6, 121.0,
122.8, 124.7, 129.2, 130.5, 131.2, 133.9, 151.3, 164.3, 165.0.
Scheme 2 The synthesis of the 1,8-naphthalic derivatives: (i) ammonia
or ethanolamine, reflux in ethanol, 2 h; (ii) K2CO3, Cu2O, DMSO, 90 ◦C,
10 h.
4a. 4-Bromo-N-(2-hydroxyethyl)-1,8-naphthalimide (0.6 mM,
1 equiv.), Cu2O (0.2 equiv.), potassium carbonate (0.5 equiv.),
ethanolamine (10 equiv.) and DMSO (6 mL) were added to a
flask. The flask was then sealed after filling with N2 and the
mixture heated to 90 ◦C. After stirring at this temperature for
10 h, the heterogeneous mixture was cooled to room temperature
and diluted with dichloromethane. The resulting solution was
washed three times with water and dichloromethane, then directly
filtered through a pad of silica gel and concentrated to yield the
product, which was purified by silica gel chromatography (4 : 1
ethyl acetate–MeOH) to yield 4a as a yellow solid. ESI-MS m/z
4-Bromo-1,8-naphthalimide. 4-Bromo-1,8-naphthalic anhyd-
ride (2.7 g, 10 mmol) was suspended in 300 mL of absolute ethanol
and the solution heated at 85 ◦C under stirring. Ammonia (5 mL)
was then dropped into the flask. After 1 h of reflux, the mixture
was cooled to room temperature. Then, the solid was filtered and
washed with absolute ethanol, and dried under vacuum at 40 ◦C.
EI-MS m/z 275 (M). 1H-NMR (400 MHz, DMSO-d6) (d, ppm):
7.94 (t, J = 7.6 Hz, 1H), 8.15 (d, J = 7.6 Hz, 1H), 8.23 (d, J = 7.6
Hz, 1H), 8.48 (m, 2H), 11.80 (s, 1H).
1
301.2 (M + H), 323.1 (M + Na), 339.1 (M + K). H-NMR (400
MHz, DMSO-d6) (d, ppm): 3.45 (q, J = 5.6 Hz, 2H), 3.57 (q, J =
6.4 Hz, 2H), 3.68 (q, J = 5.6 Hz, 2H), 4.10 (t, J = 6.4 Hz, 2H), 4.76
(t, J = 5.6 Hz, 1H), 4.86 (t, J = 5.6 Hz, 1H), 6.80 (d, J = 8.4 Hz,
1H), 7.66 (m, 2H), 8.25 (d, J = 8.4 Hz, 1H), 8.42 (d, J = 7.2 Hz,
1H), 8.69 (d, J = 8.4 Hz, 1H).13C-NMR (400 MHz, DMSO-d6)
(d, ppm): 41.9, 46.0, 58.5, 59.3, 104.3, 108.2, 120.6, 122.4, 124.7,
129.0, 130.0, 131.1, 134.7, 151.3, 163.6, 164.4.
4b. 4-Bromo-N-(2-hydroxyethyl)-1,8-naphthalimide (0.4 mM,
1 equiv.), Cu2O (0.2 equiv.), potassium carbonate (0.5 equiv.),
glycine (10 equiv.) and DMSO (4 mL) were added to a flask.
The flask was then sealed after filling with N2 and the mixture
heated to 90 ◦C. After stirring at this temperature for 10 h,
the heterogeneous mixture was cooled to room temperature
and diluted with dichloromethane. The resulting solution was
washed three times with water and dichloromethane, then directly
filtered through a pad of silica gel and concentrated to yield the
4-Bromo-N-(2-hydroxyethyl)-1,8-naphthalimide. 4-Bromo-1,8-
naphthalic anhydride (0.56 g, 2 mmol) was suspended in 50 mL of
absolute ethanol and the solution heated at 85 ◦C under stirring.
Ethanolamine (3.6 mL) was then dropped into the flask. After
1 h of reflux, the mixture was cooled to room temperature. Then,
the solid was filtered and washed with absolute ethanol, and
◦
1
dried under vacuum at 40 C. EI-MS m/z 319 (M). H-NMR
(400 MHz, DMSO-d6) (d, ppm): 3.60 (q, J = 6.0 Hz, 2H), 4.13
(t, J = 6.4 Hz, 2H), 4.78 (t, J = 6.0 Hz, 1H), 7.98 (t, J = 7.6 Hz,
2H), 8.20 (d, J = 7.6 Hz, 1H), 8.32 (d, J = 7.6 Hz, 1H), 8.55
(t, 2H). 13C-NMR (400 MHz, DMSO-d6) (d, ppm): 42.4, 58.2,
122.2, 123.1, 128.5, 129.1, 130.0, 130.1, 131.3, 131.7, 131.9, 132.9,
163.38, 163.43.
3a. 4-Bromo-1,8-naphthalimide (4 mM,
1 equiv.), Cu2O
(0.2 equiv.), potassium carbonate (0.5 equiv.), ethanolamine
900 | Dalton Trans., 2011, 40, 899–903
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product, which was purified by silica gel chromatography (4 : 1
1H-nuclear magnetic resonance
ethyl acetate–MeOH) to yield 4b as a yellow solid. ESI-MS m/z
313.1 (M - H). H-NMR (400 MHz, DMSO-d6) (d, ppm): 3.57
1
The H-NMR spectra of the complex of Hg(II) with 3a show a
complete disappearance of the imido proton peak (11.2 ppm),
considerable downfield shifts of the two naphthyl proton peaks
(8.2 and 8.4 ppm) (Dd ª 0.1 ppm), and moderate downfield shifts
of the other naphthyl protons and aromatic N–H peaks compared
to the spectrum of 3a (Fig. 2). Similar changes were also found
1
(q, J = 6.0 Hz, 2H), 4.10 (t, J = 6.4 Hz, 2H), 4.16 (d, J = 5.6 Hz,
2H), 4.76 (t, J = 5.6 Hz, 1H), 6.64 (d, J = 8.4 Hz, 1H), 7.70 (t, J =
7.6 Hz, 1H), 8.03 (t, J = 5.6 Hz, 1H), 8.25 (d, J = 8.4 Hz, 1H),
8.44 (d, J = 7.2 Hz, 1H), 8.63 (d, J = 8.4 Hz, 1H), 12.40 (s, 1H).
13C-NMR (400 MHz, DMSO-d6) (d, ppm): 40.8, 41.9, 58.5, 104.8,
109.2, 120.6, 122.5, 125.1, 128.8, 129.8, 131.2, 134.4, 150.9, 163.6,
164.4, 171.8.
1
in the H-NMR spectra of the complex of Hg(II) with 1 (loss of
imido proton peak and considerable downshift of the methylene
protons) (Fig. S2†). These H-NMR data suggest that the imido
1
protons of compounds 3a and 1 are displaced by Hg(II).
Synthesis of imide-Hg-imides8
1-Hg-1. The complex was prepared by adding HgCl2 salt
(0.5 equiv. in water) to a solution of 1 (1 mmol, 2 mL basic
water solution). The resulting solid was washed with water and
methanol, and dried under vacuum (130 mg, 65%).
2-Hg-2. 2 (0.5 mmol) was dissolved in 6 mL methanol–DMF.
Triethylamine (TEA) (1 equiv.) and a HgCl2 solution (0.5 equiv. in
methanol) were then added. The resulting solid was washed with
water and methanol, and dried under vacuum (83 mg, 67%).
3a-Hg-3a. 3a (0.13 mmol) was dissolved in 2 mL DMSO.
Then, TEA (1 equiv.) and a HgCl2 solution (0.5 equiv. in methanol)
were added. The resulting solid was washed with DMSO and
methanol, and dried under vacuum (35 mg, 75%).
Fig. 2 1H-NMR spectra (400 MHz, DMSO-d6) of 3a (A), coexisted 3a
and 3a-Hg-3a (B); 3a-Hg-3a (C).
Results and discussion
X-ray photoelectron spectroscopy
Mass spectrometry
The XPS survey spectra of complexes of Hg(II) with 3a and 2 ex-
hibit four characteristic peaks corresponding to C1s, O1s, N1s and
Hg 4f (Fig. 3, S3†). No signals attributed to Cl2p are found. The
measured atom ratios of C : O : N : Hg are 28.00 : 5.77 : 3.89 : 0.93
and 15.73 : 1.89 : 4.00 : 1.15 for the two complexes, which are in
good agreement with the theoretical ratios of complexes 3a-Hg-3a
(28 : 6 : 4 : 1) and 2-Hg-2 (16 : 2 : 4 : 1) respectively (Table S1†). The
high-resolution N1s XPS spectra show that the N1s binding energy
The electrospray ionization-MS (ESI-MS) analysis of the precipi-
tates of Hg(II) with 1, 2 and 3a exhibited a [2(1 - H) + Hg + Na]
peak at m/z 421.0, a [2(2 - H) + Hg + Na] peak at m/z 517.1
and a [2(3a - H) + Hg + H] peak at m/z 713.5, respectively. The
electron impact-MS (EI-MS) analysis of the complexes of Hg(II)
with 1 and 2 also exhibited molecular ion peaks at m/z 398 [2(1 -
H) + Hg] and 494 [2(2 - H) + Hg] (Fig. 1, S1†), respectively9
The isotopic pattern of these peaks supports the presence of Hg
in the molecular ions. The MS data suggest the formation of 1 : 2
complexes of Hg with the imides.
Fig. 3 XPS spectrum of 3a (blue) and 3a-Hg-3a (red) (a: survey spectrum;
Fig. 1 The EI-MS of complex 2-Hg-2.
b: high resolution Hg 4f spectrum; c: high resolution N 1 s spectrum).
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of these complexes shifts mildly towards a lower value than that of
3a (from 399.7 to 399.3 eV) and 2 (from 400.0 to 399.4 eV) (Fig.
S3†), which is consistent with a smaller Pauling electronegativity
for Hg (2.00) than for H (2.20).10 The XPS results suggest Hg–N
bond formation and a 2 : 1 stoichiometry of ligand/Hg.
Infrared spectroscopy
The IR spectra of the complexes of Hg(II) with 1, 2 and 3a
also exhibit changes compared with those of ligands 1, 2 and 3a
alone (Fig. 4, S4†). The peaks in the range 3250–3050 cm-1 (nN–H
)
disappear, which may be due to the displacement of the imido
protons. The strong absorption bands of nC (imide I, 1650–
O
1750 cm-1)11 shift to a lower wavenumber by ca. 70 cm-1, which
is consistent with that reported for platinum-imide complexes.11
The absorption bands of nC–N–C (imide III, 1050–1200 cm-1)11 shift
to a higher wavenumber by ca. 50 cm-1. These changes in the IR
spectra suggest that the formation of the N–Hg bond may result
in a reduced bond order of the C O bond and an enhanced bond
order of the N–C bond.
Fig. 5 (A) The emission spectra (excitation at 440 nm) of 3a and 3a +
Hg(II) in phosphate buffer at pH 7.50; (B) the fluorescence titration of 3a
(5 mM) with Hg(II). RFQ = relative fluorescence quenching.
Fig. 4 IR spectra of 1 (green) and 1-Hg-1 (red).
The above results demonstrate that Hg(II) substitutes the imido
proton to form a stable neutral imide-Hg(II)-imide complex,
as shown in Scheme 1. Since crystals of these complexes for
X-ray analysis have not been obtained yet, we cannot confirm
whether there are any interactions between the carbonyl oxygen
and Hg atoms. As far as we know, only one cyclic imide, 3,3-
dimethylglutarimidato (six-membered ring, unconjugated)12 has
been report to form a two-coordinate imide-Hg-imide complex be-
sides two thymine derivatives: 1-methylthymine3d and thymidine.13
The reported crystal data3d,12 show no interaction between the car-
bonyl oxygen and Hg atoms. Based on all the information we have,
it can be deduced that the formation of a two-coordinate imide-
Hg-imide complex is common to all N-unsubstituted cyclic imides.
of Hg(II) to the imide N-atom through imido proton–Hg exchange,
but not to other groups on the naphthalimides. The quenched
fluorescence could be restored by adding Na2S or HCl (Fig. S7†),
indicating that the reaction of Hg(II) with imides is reversible.
The fluorescence titration curve of 3a with Hg(II) (Fig. 5b) shows
that the level of fluorescence quenching of 3a reached plateau with
1
the addition of about equiv. of Hg(II) and that excess Hg(II) did
2
not increase the quenching any further. This result implies that
a complex of 2 : 1 stoichiometry (3a-Hg(II)) was formed. When
5.0 mM of 3a was employed in water or phosphate buffer, its
fluorescence intensity decreased linearly with the concentration of
Hg(II) between 0.25 and 2.5 mM (R2 = 0.9985) (Fig. S8†), implying
that 3a can be used as a fluorescence sensor for Hg(II).
Since two protons are released when the 3a-Hg-3a complex
forms, the effect of pH on the formation of the 3a-Hg-3a complex
is worth investigating. In the absence of Hg2+, the fluorescence
intensity of 3a remains constant in the pH range 2.9–11.4 (Fig.
S9†). As shown in Fig. 6, upon adding Hg(II), the fluorescence
of 3a was quenched quickly at pH values higher than 4.5, and no
significant change of fluorescence was observed at pH values lower
than 4.05. At pH 4.25, the fluorescence intensity of 3a decreased
gradually over time and reached an equilibrium after 800 min. The
reaction equilibrium constant can be calculated as:
The fluorescence properties of 3a and 4a
1,8-Naphthalimide derivatives are well-known fluorescent dyes
with a strong fluorescence, high photostability and a large Stokes
shift. Therefore, they have been widely used for the development
of fluorescence sensors.14–16 Complex formation of Hg2+ with
3a was found to quench the fluorescence of 1,8-naphthalimides
in aqueous solution (Fig. 5a), which provides an easy way to
further understand the interactions between Hg2+ and imides.
Three naphthalimides, 3b, 4a and 4b, were synthesized for further
investigation. Hg(II) was able to quench the fluorescence of 3a
and 3b by up to 90% but did not quench the fluorescence of
N-substituted naphthalimides 4a and 4b (Fig. S5 and S6†), which
suggests that the fluorescence quenching is the result of the binding
K = [H+]2[3a-Hg-3a]/([3a]2[Hg2+]).
2
3
At equilibrium, the fluorescence decrease is about
of the
maximum quenching observed; therefore [3a-Hg-3a] should equal
902 | Dalton Trans., 2011, 40, 899–903
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exhibit a fluorescence turn-off property upon binding to Hg(II),
which may limit their practical utility in highly sensitive detection.
It is feasible to design highly sensitive sensors by combining cyclic
imides with other fluorescence molecules (for example pyrene),
polymers or nanomaterials, in which imides are only used as the
recognition unit. Cyclic imides may also be used as ligands for
preparing a sorbent for Hg(II) removal. This sorbent can possess
high selectivity and be regenerated by acidic solutions.
Conclusions
In summary, besides the nucleobase thymine (T), other N-
unsubstituted cyclic imides have been found to react specifically
with Hg(II) to form imide-Hg-imide complexes through an imido
proton–metal exchange process. This specific reaction is reversible
and occurs rapidly at moderate to high pH values. Given that the
toxicity of mercury is well documented, and that Hg(II) is a main
and stable form of mercury pollutants in the environment,1,2 the
discovery of this highly specific binding feature of N-unsubstituted
imides to Hg(II) expands the comprehension of T-Hg-T interac-
tions, opening up new possibilities in designing novel and potent
ligands for sensing and chelating Hg(II) based on cyclic imides.
Fig. 6 Fluorescence decay curves of 3a in the presence of Hg(II) at
different pH values (5 mM 3a, 50 mM Hg(II) in 20 mM phosphate buffer).
[3a] at this point. Since the amount of added Hg(II) is in a large
excess, [Hg2+] is approximately equal to its initial concentration.
Considering all of the above, we can calculate the apparent
equilibrium constant to be:
Kapp = (10-4.25)2 ¥ (31 ¥ 5 ¥ 10-6)/((31 ¥ 5 ¥ 10-6)2 ¥ (50 ¥ 10-6))
= 1.2 ¥ 102.
Although this equilibrium constant is not high enough, the
concentration of 3a-Hg-3a is inversely proportional to the square
of the concentration of H+ according to the equation:
Acknowledgements
We are grateful for financial support from the NSF of China
(Grants 20775082 and 20805049), the 973 Program (Grant
2007CB935601) and the 863 Program (Grant 2008AA02Z206).
[3a-Hg-3a] = K[3a]2[Hg2+]/[H+]2.
This means that the formation of 3a-Hg-3a can begreatly increased
by increasing the pH of the solution.
Notes and references
The specificity of the formation of imide-Hg-imide was inves-
tigated by measuring the fluorescence of 3a in the presence of
cations such as Pb(II), Cu(II), Ge(II), Co(II), Mn(II), Fe(II), Zn(II),
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