J.-c. Qin et al. / Tetrahedron Letters xxx (2015) xxx–xxx
3
filtered, and washed 3 times with 10 mL hot ethanol. After drying
under reduced pressure, the reaction afforded HL as a yellow solid.
Yield: 60.4%. 1H NMR (400 MHz; CDCl3) (Fig. S4) d (ppm) = 11.83 (s,
1H), 8.72 (s, 1H), 7.99 (d, J = 7.2 Hz 1H), 7.76–7.82 (m, 2H), 7.37–
7.45 (m, 3H), 7.01 (d, J = 7.1 Hz 1H), 6.62 (dd, J = 9.0 Hz,
J = 2.1 Hz, 1H), 6.45 (s, 1H), 6.38 (s, 2H), 6.32 (s, 2H), 3.54 (s, 2H),
3.44 (q, J = 7.1 Hz, 4H), 3.24–3.29 (q, J = 7.1 Hz, 4H), 1.87 (s, 6H),
1.29–1.32 (t, J = 7.1 Hz, 6H), 1.22 (t, J = 7.1 Hz, 6H). 13C NMR
(CDCl3, 100 MHz) (Fig. S5): 206.68, 165.85, 162.52, 160.13,
157.99, 153.68, 153.22, 150.93, 149.10, 148.65, 147.75, 142.73,
134.09, 131.38, 128.19, 123.81, 123.37, 118.16, 110.37, 108.69,
105.39, 97.67, 96.57, 65.92, 45.13, 38.48, 16.85, 14.78, 12.51. IR
(KBr, cmꢁ1) (Fig. S6): 3415, 1702, 1619, 1591, ESI-MS: [M+1]+:
726.27. [M+Na]+: 748.25. (Fig. S7) Elemental analysis:
C42H43N7O5, Found: C, 69.84.; H, 5.79; N, 13.76. Calcd: C, 69.50;
H, 5.97; N, 13.51.
Results and discussion
Figure 2. Fluorescence spectra of HL (10
l
M) upon the addition of metal salts
Absorption spectroscopic studies on metal ions
(20.0 equiv) of Li+, Na+, K+, Ca2+, Mg2+, Cu2+, Co2+, Mn2+, Ni2+, Zn2+, Ba2+, Fe2+, Cd2+
,
Hg2+, Pb2+, Cr3+, and Fe3+ in ethanol–water solutions (9:1, v/v, Tris–HCl, pH = 7.4).
Excitation wavelength was 450 nm, slit = 3 nm/3 nm.
The optical behavior of HL was initially studied using UV–Vis as
a function of the concentration of Fe3+ in ethanol–water solutions
(9:1, v/v, Tris–HCl, pH = 7.4). As shown in Figure 1, in the absence
of Fe3+, the receptor only exhibited a maximum absorption wave-
length at 439 nm which corresponded to the absorption band of
coumarin moiety.32–34 Moreover, the characteristic absorption
band of Rhodamine 6G at about 530 nm was not observed, demon-
strating its existence in spirolactam form.35,36 However, upon addi-
tion of Fe3+ to an aqueous solution of HL, the absorption band at
425 nm slightly decreased while the absorption intensity exhibited
significantly increased in the range of 385–475 nm. Besides the
change of coumarin’s absorption band, a new absorption band
appeared at 525 nm with increasing intensity which was ascribed
to ring opened rhodamine moieties. This indicated that the interac-
tion of HL with Fe3+ could trigger the formation of the ring-opened
form of HL from the spirolactam form.
pH = 7.4). As shown in Figure 2, the free sensor displayed fluores-
cence emission at 475 nm, attributable to the characteristic signals
of coumarin and the characteristic emission of Rhodamine 6G at
550 nm not appeared, which also indicated the rhodamine core
was in the ring closed isomeric form (Ex = 450 nm). On addition
of various metal ions including Li+, Na+, K+, Ca2+, Mg2+, Cu2+, Co2+
,
Mn2+, Ni2+, Zn2+, Ba2+, Fe2+, Cd2+, Hg2+, Pb2+, Cr3+ and Fe3+ to aque-
ous solution of HL, there was no significant change in its fluores-
cence spectrum except in the presence of Fe3+. Upon the addition
of Fe3+, the receptor exhibited remarkable fluorescence enhance-
ment at 550 nm at the expense of the fluorescent emission at
475 nm. The change reflected on the fluorescent colors, one was
green, the other was yellow. These phenomena further confirmed
the addition of Fe3+ promoted the ring-opened reaction of the
Rhodamine 6G spirolactam.
Selectivity studies and effects of metal ions
The effect of Fe3+ on the fluorescence properties of HL was also
investigated in ethanol–water solutions (9:1, v/v, Tris–HCl,
The complexation of HL with Fe3+
In order to further validate the stoichiometry of HL and Fe3+
Job’s method for absorbance measurement was carried out.
The total concentration of HL and Fe3+ was 400
lM.
XL = ([HL]/([Mn+] + [HL]. As shown in Figure 3, the maximum point
appeared at a mole fraction of 0.5. The result indicated that they
are a 1:1 stoichiometry of the binding mode of HL with Fe3+ which
was further confirmed by the appearance of a peak at m/z 815.81
assignable to [HL+Fe3++ClꢁꢁH+]+ (Fig. S8) in the ESI/MS. Since the
formation of 1:1 ligand–metal complex was confirmed by Job’s plot
analysis and ESI/MS, in combination with the fluorescence titration
(Fig. 4), the binding constant values (KL–Fe = 5.2 ꢀ 104) were deter-
mined based on the modified Benesi–Hildebrand equation
(Fig. S9).37,38
1=ðFx ꢁ F0Þ ¼ 1=ðFmax ꢁ F0Þ þ ð1=K½CꢃÞð1=ðFmax ꢁ F0ÞÞ
where F0, Fx, and Fmax are the emission intensities (at 550 nm) of the
organic moiety considered in the absence of metal ion, at an inter-
mediate metal ion concentration, and at a concentration of com-
Figure 1. Changes in the absorption spectra of HL (20 lM) in ethanol–water
plete interaction, respectively, and where
constant concentration.
K is the binding
solutions (9:1, v/v, Tris–HCl, pH = 7.2) at room temperature upon addition of
different amounts of Fe3+ ions.