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physiologically important metal ions such as Li
+
,K
+
,Mg
2+
,andCa
2+
,forthe
formation of pseudorotaxanes from the molecular cage 25 and squaraines 23a-23c
(Fig.
11
)[
63
,
64
]. The complexation and decomplexation of the pseudorotaxane
complexes in solution are observable with the naked eye. The encircling molecular
cage protects the squaraine moiety from polar solvents, thereby substantially enhanc-
ing its quantum yield, and increasing its stability toward chemical attack.
Table
7
lists the photophysical properties of the squaraine 23a and the rotaxane
[23a
Na
2
][2ClO
4
] in acetonitrile. Although both the absorption and emis-
sion maxima of the rotaxane are shifted to slightly shorter wavelengths relative to
those of squaraine 23a, the quantum yield is increased substantially. The authors
explained this effect by the fact that the relatively nonpolar molecular cage partially
protected the squaraine dye from solvation by polar CH
3
CN molecules, thereby
reducing the degree of nonradiative relaxation of the squaraine counterpart. This
hypothesis is supported by observation that increasing the amount of CH
3
CN in a
solution of rotaxane [23a
25
25
Na
2
][2ClO
4
] in CHCl
3
/CH
3
CN decreases the
emission intensity noticeably.
A substantial increase of the stability of squaraine dye 23a toward nucleophilic
attack after embedding it in a rotaxane system was demonstrated in the example of
the reaction with octanethiol in acetonitrile. At the same time, the rotaxane [23a
25
Na
2
][2ClO
4
] was destroyed rapidly in a solution of CH
3
CN/H
2
O (9:1) after
addition of a large excess of cysteine. Addition of Na
+
ions significantly increases
stability of the rotaxane towards cysteine. Other important metal ions such as Li
+
,
K
+
,Mg
2+
, and Ca
2+
do not affect instability of the rotaxane system, which is in good
agreement with the observation [
64
-
66
] that the formation of the rotaxane is
templated by Na
+
ions in a highly selective manner.
Rotaxane [23a
Na
2
][2ClO
4
] was not investigated as a fluorescent probe
for biological species and fluorescent imaging. Apparently, it can be used to
25
R
N
R
O
O
O
O
O
O
O
O
O
O
Na
+
O
O
O
23
O
O
Na
+
O
O
O
O
O
+2Na
+
O
O
O
O
O
O
R
N
2ClO
-
R
25
[
23
⊂
25
⊃
Na
2
][2ClO
4
]
Fig. 11 Synthesis of squaraine rotaxane [23a
25
Na
2
][2ClO
4
][
63
,
64
]
Table 7 Spectral characteristics of squaraine 23a and rotaxane [23a
25
Na
2
][2ClO
4
]in
acetonitrile [
63
]
Dye
l
max
Abs (nm)
log
e
l
max
Em (nm)
F
M
F
23a
640
5.57
656
0.13
[23a
25
Na
2
][2ClO
4
]
634
5.04
643
0.38
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