J.-c. Qin et al. / Inorganica Chimica Acta 435 (2015) 194–199
195
Scheme 1. Reagents and conditions: (a) HMTA, glacial acetic acid, reflux, 6 h; (b) EtOH, N2H4ꢂH2O, reflux, 20 h; (c) EtOH, reflux, 12 h.
1 cm path length. The melting point was determined on a Beijing
XT4-100x microscopic melting point apparatus.
Stock solutions of various cations (5 mM) were prepared using
nitrate salts. A stock solution of HL (5 mM) was prepared, and then
the stock solution was diluted 100 times.
3.2. UV–Vis analysis
The binding ability of L-M (Mg2+, Zn2+) was initially evaluated
by the UV–Vis analysis upon addition of different amounts of metal
ions in acetonitrile and in ethanol–water (v/v, 4:1), respectively.
Firstly, the interaction of HL and Mg2+ was investigated as a func-
tion of the concentration of Mg2+. As shown in Fig. 1, the spectrum
of the free HL showed a maximum absorption band at 375 nm,
2.2. Synthesis of the sensor (HL)
which can be assigned to
p?
p⁄ transition of naphthalene group
2-Hydroxy-1-naphthaldehyde (Fig. S1) and rhodamine 6G
hydrazide (Fig. S2) were synthesized according to the method
reported [24–25]. The synthetic route of the sensor was shown in
Scheme 1. An ethanol solution of rhodamine 6G hydrazide
(0.43 g, 1 mmol) was added to another ethanol containing 2-hy-
droxy-1-naphthaldehyde (1 mmol, 0.172 g), and then the mixture
was stirred and refluxed for 12 h. After that, the final product
was allowed to filtered, washed 3 times with 10 mL hot ethanol
and dried under reduced pressure, the reaction afforded yellow
solid, Yield: 65%. 1H NMR (400 MHz; DMSO-d6) (Fig. S3)
d (ppm) = 12.24 (s, H1), 9.78 (s, H10), 8.03 (m, H11), 7.79 (d, H3,
J = 8.6 Hz), 7.65 (m, H4, H7), 7.53 (m, H6, H13), 7.43 (m, H12), 7.33
(m, H14), 7.11 (m, H2, H5), 6.46 (s, H8, H15), 6.35 (s, H20, H21), 3.48
(s, H17, H22), 3.15–3.21 (q, 2H18, 2H23, J = 7.1 Hz), 1.82 (s, 3H9,
3H16), 1.16 (t, 3H19, 3H24, J = 7.1 Hz). ESI-MS (Fig. S4): [M+1]+:
583.25. IR (KBr, cmꢀ1) (Fig. S5): 3429.13, 1683.75, 1620.79.
Elemental Anal. C37H34N4O3: Calc. C, 76.27; H, 5.88; N, 9.62.
Found: C, 76.13; H, 5.91; N, 9.70.
[29–32]. With the increasing concentration of Mg2+, the absorption
band at 375 nm gradually decreased and a new absorption band
appeared at 439 nm with increasing intensity. Moreover, a clear
isosbestic point at 400 nm was observed, which clearly indicated
the presence of new complex in equilibrium with the receptor.
When the UV–Vis analysis was carried out in aqueous media, the
metal ion was changed from Mg2+ to Zn2+, as shown in Fig. 2, the
trend of change of the latter was similar to the former, the only dif-
ferences was the extent of variation of the absorption at 439 nm.
In addition, we could observed that the absorbance band from
500 nm to 550 nm not appeared, which indicated the rhodamine
core was in the ring closed isomeric form both in the presence of
Mg2+ and Zn2+ [33–34]. It meant that the interaction of HL with
Mg2+/Zn2+ could not trigger the ring-opened reaction of the rho-
damine spirolactam. Therefore, the reasons for all these changes
3. Results and discussion
3.1. General information
The binding constant values were determined from the emis-
sion intensity data following the modified Benesi–Hildebrand
equation [26–27].
1
1
1
¼
þ
KðFmax ꢀ FminÞ½Mnþꢁ
F ꢀ Fmin
Fmax ꢀ Fmin
where Fmin, F, and Fmax were the emission intensities of the organic
moiety considered in the absence of metal ion, at an intermediate
metal ion concentration, and at
a concentration of complete
interaction, respectively, and where K was the binding constant
concentration.
The detection limits were calculated with the following equa-
tion: detection limit 3
r/k, based on the fluorescence titration.
Where was the standard deviation of blank measurements, and
r
k was the slope between intensity versus sample concentration
[28].
Fig. 1. Changes in the absorption spectra of HL (20
lM) in acetonitrile at room
temperature as a function of added Mg2+ (0–1.0 equiv.).