H.-C. Hung et al. / Tetrahedron Letters 50 (2009) 302–305
303
pyrene units in an intramolecular
p
ꢀ
p
stacking under UV
screened revealed two types of binding modes: (1) for Cu2+, Hg2+
,
Cr3+, Pb2+, and Ni2+ ions, the monomer and excimer emissions were
both quenched, and (2) for Cd2+ and Zn2+ ions, the monomer emis-
sion was enhanced but the excimer emission was quenched (see
irradiation.2 The changes of fluorescence intensities of 7ꢀ10
(10 lM) in the presence of 15 metal perchlorates (10 equiv) were
studied and the results are summarized in Table S1. The fluores-
cence of compound 7 was found to be selectively quenched by
the addition of Cu2+, Hg2+, and Cr3+ ions. Such a selective quenching
by the same group of metal ions (Cu2+, Hg2+, and Cr3+) was also ob-
served on the mono-triazole model compound 11. The results sug-
gest that metal ions may be complexed by both the triazole
group(s) and the oxygen atom(s) of 7 and 11, which then quenched
the fluorescence intensity of the pyrenes. The quenching of mono-
mer emission in 7 and 11 can be explained as a reverse PET as well
as a heavy atom effect;8 that is, the pyrene units behaved as a PET
donor and the metal ion bound triazole groups behaved as an elec-
tron acceptor. The quenching of excimer emission resulted from
the conformational change caused by the two outward-facing tria-
zole groups that turned inward upon binding with metal ions. Job
plot experiments were carried out for 7 which revealed a 1:1 com-
plex with Cu2+, Hg2+, and Cr3+, respectively. The association con-
stants for complexation of 7 with Cu2+, Hg2+, and Cr3+ in MeCN,
using Stern–Volmer equation at low guest concentration
(0ꢀ1.0 equiv), were calculated to be 1.2 ꢁ 105, 1.6 ꢁ 105, and
2.1 ꢁ 105 Mꢀ1, respectively. Similar fluorescence quenching by
Cu2+, Hg2+, and Cr3+ was also observed for compounds 8–10, how-
ever, the emission of 9 was further quenched by Pb2+ and the emis-
sion of 10 was further quenched by Pb2+ and Ni2+ ions (see Table
S1). The fluorescence of compound 10 toward the 15 metal ions
Figs. 1a and b). The association constants of 10 with Pb2+, Ni2+
,
Cd2+, and Zn2+ ions in MeCN using Stern-Volmer plots were deter-
mined to be 1.2 ꢁ 105, 5.1 ꢁ 103, 3.2 ꢁ 103, and 2.9 ꢁ 103 Mꢀ1
,
respectively.
In order to gain insights into the complexation modes of these
receptors with metal ions, we compared the 1H NMR spectra of
10 (5 mM) in the absence and presence of various metal cations
in CDCl3/CD3CN = 3/1 at 25 oC (see Figs. 2a–d). The 1H NMR spectra
of 10 with Cu2+, Cr3+, and Ni2+ were broadened due to the high spin
states of these metal ions, therefore, they were not analyzed. In the
titration of 10 with Hg2+, precipitate was observed which de-
creased the 1H NMR signals substantially. Nevertheless, the
methine protons Ha of triazoles disappeared in the presence of
Hg2+ and the methylene protons (Hb, Hc, Hd, He, and Hf) exhibited
substantial up-field shifts (
D
d = ꢀ0.20, ꢀ0.43, ꢀ0.57, ꢀ0.33, and
ꢀ0.47 ppm) upon complexation with 1 equiv of Hg2+. The com-
plexation of 10 with Hg2+ is expected to reduce the electron den-
sity of the coordination sites and induce a down-field shift of the
nearby proton signals; however, up-field shift was observed for
all the methylene protons. Thus, we believe that there must be a
change of conformation which causes the methylene protons to
be shielded by the pyrene rings. A similar change in proton chem-
ical shifts of 10 was observed when it was titrated with Pb2+ ion. In
the complexation of 10 with Cd2+, the signals of Hb and Hc were
slightly up-field shifted by 0.02 and 0.11 ppm, however, the signals
of Hd, He, and Hf were more up-field shifted by 0.17, 0.14, and
0.36 ppm, respectively. The fact that the chemical shift of Hb of
compound 10 was more affected by Pb2+ than by Cd2+ implies that
10 complexed with these two metal ions in a different mode (vide
infra). On the one hand, compounds 7ꢀ9 did not show any fluores-
cence change upon adding Cd2+ or Zn2+ probably because their bis-
triazoles could not find a proper conformation to accommodate
either one of them. On the other hand, compound 10 did form
complexes with Cd2+ and Zn2+ because the cavity formed by the
methylene chains and the two triazoles was just big enough to
accommodate them.
a
Host 10 + Pb2+
7
host + Pb 2+
6
0 eq.
0.2 eq.
0.4 eq.
5
0.6 eq.
0.8 eq.
1.0 eq.
4
2 eq.
4 eq.
6 eq.
3
8 eq.
10 eq.
15 eq.
2
20 eq.
30 eq.
Based on the titration results of 1H NMR and fluorescence spec-
trometry on 10, we proposed that 10 shows a dual-mode recogni-
tion toward transition metal ions (see Scheme 2): (a) Ni2+ and Pb2+
were bound between the oxygen atoms of the ether groups and the
triazoles 30-N which distorted the original conformation and
1
0
350
400
450
500
550
600
650
700
Wavelength (nm)
Host 10 + Cd2+
@
b
X
#
#
b
c
a
d
f
f
e
e
6
(d)
(c)
X
host + Cd2+
0 eq.
a
@
@
5
4
3
2
1
0
d
X
1 eq.
b
c
6 eq.
X
10 eq.
30 eq.
50 eq.
100 eq.
200 eq.
#
b
b
(b)
(a)
c
a
d
e
f
X
a
@
f
d
#
c
e
8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 2.0 1.5 1.0
350
400
450
500
550
600
650
700
Figure 2. 1H NMR of 10 (5 mM) in CDCl3/CD3CN = 3/1 (a) and in the presence of
5 mM of various metal ions (b) Hg2+, (c) Pb2+, and (d) Cd2+. Where @ denotes
internal CHCl3, # denotes external CHCl3, and X denotes impurity upon addition of
metal ion.
Wavelength (nm)
Figure 1. Changes in fluorescence spectra (kex = 312 nm) of 10 (10
lM) in MeCN
solution at 298 K with the addition of (a) Pb2+, and (b) Cd2+ ions.