4510 Zhou et al.
Asian J. Chem.
1H NMR (CD3SOCD3, δ ppm): 2.33 (s, 3H); 3.65 (s, 6H); 5.26
(s, 3H); 5.96 (s, 1H); 6.44 (d, J = 9.6 Hz, 1H); 7.56 (d, J = 9.6
Hz, 1H); 8.79 (d, J = 12.4 Hz, 1H). 13C NMR (CD3SOCD3, δ
ppm): 18.53, 60.38, 65.89, 103.15, 104.92, 106.41, 120.39,
131.16, 154.82, 156.46, 158.18, 159.76, 178.27. MS m/z: 306.2
[M-H]+. Found, %: C, 58.68; H, 5.50; N, 4.59. Calculated for
C15H17NO6: C, 58.63; H, 5.58; N, 4.56.
of Mg2+ and Zn2+ into the acetonitrile solution of sensor L, a
new emission peak at about 450 nm appeared and the intensity
was dramatically increased. When the concentration of Mg2+
and Zn2+ arrived at 100 µM, the fluorescence intensity reached
the maximum and showed 17.6-fold and 24-fold enhancement,
respectively.
250
a
NH2
OH
HO
200
HO
O
O
OH
Hexamine
100 µM
N
HO
O
O
HO
O
O
OH
150
CHO
HO
OH
L
100
Scheme-I: Synthetic route of compound L
0
50
0
RESULTS AND DISCUSSION
We examined the chemosensing behaviour of the fluore-
scent sensor L by fluorescence measurement in the presence
of various metal ions in acetonitrile by comparing the fluore-
scence intensities of the solutions before and after addition of
10 equiv. of the following metal ions as: Li+, Na+, K+, Ca2+,
Ba2+, Cu2+, Fe3+, Ag+, Hg2+, Al3+, Co2+, Cd2+, Ni2+, Pb2+, Mg2+
and Zn2+. As shown in Fig. 1, the fluorescence intensity of
free L (10 µM) alone was weak at 424 nm when it was excited
at 341 nm, due to isomerization of the C=N double bond in
Schiff base. Compound with an unbridged C=N structure is
often nonfluorescent due to the C=N isomerization, but it may
be inhibited by complexation with special ion14. In the presence
of Mg2+ and Zn2+, L showed large fluorescence enhancement.
In addition, Mg2+ and Zn2+ resulted in an obvious red-shift of
the λem of L from 424 to 455 nm. The sensor gave slight fluore-
scence enhancement with Hg2+, Al3+, Fe3+ and Cr3+. Upon
addition of Li+, Na+, K+, Ca2+, Ba2+, Cu2+, Ag+, Hg2+, Cd2+, Ni2+
and Pb2+, the emission intensity of L only slightly increased
and the λem was red shifted to near 455 nm.
350
400
450
500
550
600
Wavelegth (nm)
350
300
250
200
150
100
50
b
100 µM
0
0
350
400
450
500
550
600
Wavelength (nm)
Fig. 2. Fluorescence spectra of L (10 µM, λex = 341 nm) upon addition of
(a) 0-100 µM of Mg2+ and (b) 0-100 µM of Zn2+ in acetonitrile
Zn2+
300
250
In order to further test the interference of other metal ions
on the determination of Mg2+ and Zn2+, competition experi-
ments were performed in which L was added to a solution of
Mg2+ and Zn2+ in the presence of other metal ions. As shown
in Fig. 3a, addition of Hg2+ resulted in relative large increase
of the fluorescence intensity of Mg2+ complex. Co2+, Cr3+ and
Cu2+ showed only slight interference on fluorescent enhance-
ment upon the subsequent addition of Mg2+. The other metal
ions showed little influence on the determination of Mg2+. As
shown in Fig. 3b, the fluorescence intensity of Zn2+ complex
increased with the addition of Hg2+, Mg2+, Ag+, K+, Cd2+, Li+
and Na+. With the addition of Ba2+, Ca2+, Cr3+, Al3+, Pb2+ and
Fe3+, the fluorescence intensity of Zn2+ complex decreased,
but it still had strong fluorescence intensity. Co2+, Ni2+, espe-
cially Cu2+ could quench fluorescence of L-Zn2+ complex via
energy or electron transfer15.
Mg2+
200
150
Li+, Na+, K+, Ca2+, Cd2+, Cu2+,
Ba2+, Hg2+, Co2+, Pb2+
Hg2+, Al3+
Fe3+, Cr3+
,
100
50
0
L
350
400
450
500
550
600
Wavelength (nm)
Fig. 1. Fluorescence spectra of L (10 µM, λex = 341 nm) with addition of
various metal ions (10 equiv, respectively) in acetonitrile
To further investigate the selectivity of L to Mg2+ and Zn2+,
we performed fluorescence titrations of L in acetonitrile upon
excitation at 341 nm.As shown in Fig. 2, upon gradual addition
Detailed investigations were carried out to understand the
coordinating behaviour of L with Mg2+ and Zn2+. The stoichi-
Zhou
