JOURNAL OF CHEMICAL RESEARCH 2012 147
Fig. 2 Fluorescence emission spectra of compound 3a (10 µM)
Fig. 4 Job’s plot for determining the stoichiometry of receptor
2+
2+
2+
upon addition of Hg and Cu (20 µM) in DMF and H
2
O (v/v=1/1).
3a and Hg ion in DMF and H
2
O (v/v=1/1), I and I
0
are the
fluorescence intensity of 3a in the presence and absence of
2+
2+
Hg , respectively, the total concentration of 3a and Hg ion is
.1 mM (λex = 400 nm).
2
+
2+
The fluorescence enhancement effects of Hg and Cu ions
on compound 3a in DMF aqueous media were investigated as
show in Fig. 2.
0
In the fluorescence spectrum, compound 3a exhibited a
complexing properties of compound 3a, which can selectively
2+
weak fluorescence emission at 445 nm in DMF aqueous media.
recognise Hg are described. Compound 3a may provide a
2+
2+
When Hg was added to 3a, the maximum fluorescence wave-
length shifts 31 nm bathochromically and the fluorescence
intensity is enhanced remarkably. The change of fluorescence
spectra observed for 3a is attributed to the formation of the
potential fluorescent probe for Hg .
Experimental
9
3-methyl-4-amino-5-mercapto-1,2,4-triazole, 2-methyl-5-mercapto-
2+
3
a–Hg complex as a result of which the PET from sulfur
10
11
1
,3,4-thiadiazole, (E)-4,4′- bismethyl azobenzene, (E)-4,4′-bisbro-
atoms to the pyrimidine moiety is suppressed, resulting in the
wavelength red shifts and the intensity increase. Addition of
12
momethyl azobenzene were prepared according to procedures
described in the literature; other reactants and chemicals were pur-
chased from Aladdin. Solvents were of analytical grade. H NMR
2+
1
Cu also resulted in a modest emission enhancement.
2
+
Titration of compound 3a by Hg with an excitation at
λex = 400 nm, given in Fig. 3 as an example, exhibited an increase
of its emission intensity at 476 nm. From the titrations, we
spectra were recorded at room temperature on a Bruker Avance-
500 NMR spectrometer. DMSO-d6 and CDCl were used as solvents
3
and tetramethylsilane (TMS) as internal standard. Mass spectral data
were obtained on an Agilent 1100 LC/MS. IR spectra were obtained
with a Perkin-Elmer spectrophotometer. Elemental analyses were
obtained with a Thermo Finnigan CHN Analyzer. UV-Vis absorption
spectra were recorded at room temperature using a Lambda-900 spec-
trometer. The fluorescence emission spectra were measured using a
LS-55 spectrometer.
5
−1
determined the association constant of 3a (Ka = 2.1 × 10 M )
2+
for Hg .
Fitting the changes in fluorescence spectra of compound 3a
2+
with Hg ions, we applied the method of continuous variation
Job’s plot) to derive the complexation ratio between 3a and
(
2+
Hg ions. As shown in Fig. 4, the maximum point at the mole
fraction of 0.67 indicates the complexation ratio of 3a and
Hg is 1:2.
In conclusion, we have synthesised four novel fluorescent
azobenzene derivatives with different heterocyclic units, the
Preparation of 3a–d; general procedure
2+
(
E)-4,4′-Bisbromomethyl azobenzene 2 (0.8 mmol) was added to
a mixture of the corresponding sulfhydryl substituted heterocyclic
(1.6 mmol), KOH (1.6 mmol) in CH Cl (25 mL) and the mixture was
2
2
stirred for 24 h at room temperature, the mixture was filtered and
washed with water, the resulting residue was then dried. The crude
product was purified on a silica gel column using petroleum ether/
ethyl acetate (from 15/1 to 5/1, v/v). The product was obtained as
different coloured solid.
4
,4′-Bis(4,6-dimethyl-pyrimidine-2-ylmethylthio)azobenzene (3a):
Yield 70.3%; m.p. 184–186 °C (dec); FT-IR(KBr): 3058, 2942, 2869,
1
1
7
4
593, 1497, 1388, 1242, 845, 759; H NMR (500 MHz, CDCl ): δ
3
.810 (d, 4H, PhH), 7.647 (d, 4H, PhH), 6.969 (S, 2H, pyrimidine-H),
13
.462 (s, 4H, CH ), 2.371(s, 12H, CH ); C NMR (500 MHz, CDCl ):
2
3
3
δC (ppm) 170.39, 167.04, 151.72, 141.59, 129.88, 122.76, 115.83,
+
3
4.89, 23.80; MS, m/z: 487 (M + H ); Anal. Calcd for C H N S : C,
26 26 6 2
6
4.17; H, 5.38; N, 17.27; S, 13.18. Found: C, 64.13; H, 5.40; N, 17.31;
S, 13.16%.
,4′-Bis(3-methyl-4-amino-1,2,4-triazole-5-ylmethylthio)azobenzene
4
−
1
(
2
3b): Yield 70.5%; m.p. 236–238 ºC; IR (KBr, cm ): 3337, 3101,
984, 1593, 1498, 1432, 1244, 1099, 840, 745; H NMR (500 MHz,
1
DMSO-d6): δ 7.825 (d, 4H, PhH), 7.624(d, 4H, PhH), 5.891 (s, 4H,
13
NH ), 4.457 (s, 4H, CH ), 2.295(s, 6H, CH ); C NMR (500 MHz,
2
2
3
DMSO-d6): δ (ppm) 159.46, 158.32, 148.27, 139.15, 130.51, 123.02,
C
+
3
5.72, 10.24; MS, m/z: 467 (M + H ); Anal. Calcd for C H N S :
Fig. 3 Fluorescence emission spectra of compound 3a (10 µM)
20 22 10 2
2
+
for Hg ion titration in a mixture of DMF/H
2
O (v/v=1/1) (λex
=
C, 51.49; H, 4.75; N, 30.02; S, 13.74. Found: C, 51.44; H, 4.78; N,
30.07; S, 13.71%.
400 nm).