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was expected that this molecule would act as a host for anions because
of the cationic nature of the inorganic cyclophane and the addition
of ClO
did indeed increase the luminescence
intensity (see Fig. 10.2) and excited-state lifetime. The enhancement
effect was believed to originate from anion encapsulation
to the solution of
6
4
induced
inhibition of oxidative quenching by Pd(II) sites. However, addition of
BF
1
, which binds more strongly than ClO
(
K
= 6000 M
compared
4
4
a
1
to 900 M
), induces a larger enhancement of luminescence, while
addition of trifluoromethanesulfonate, which has a binding constant
of intermediate value (3000 M
1
), enhances the luminescence to a
degree between that observed for BF
and ClO
(see Fig. 10.2).
4
4
Figure 10.2
Left: structure of complex
15
. Right: Relative emission
intensity (
l
= 420 nm,
l
= 625 nm; average of four
ex
em
experiments) for
M) in acetone as a function
of added tetraethylammonium perchlorate (TEAP)
concentration. Reproduced with permission from Ref. [49].
6
(5
×
10
5
Hupp and coworkers studied the affinities of the
rhenium
bipyridine rectangles
4b
and
4f
[45]. The dimension of the
×
cavity in the metallacyclic rectangle
4b
was 5.89
11.55 Å. Proton
NMR studies for the rectangles
with bis-sodium salt of
2,6-naphthalenedisulfonic acid gave association constants of 2.3
4b
and
4f
×
3
×
3
1
10
and 2.4
10
M
, respectively, assuming the formation of a 1:1
host
guest complex. The roughly spherical tetraphenylborate anion
showed no binding with
4f
.
Lu and coworkers
reported gondola-shaped
tetra-rhenium
metallacyclic compounds,
[40]. The metallacycles consist
of a hydrophobic cavity of dimensions 5.6
8a
and
8b
17.8 Å and show
strong luminescence. The rectangle showed selectivity for Hg
×
7.0
×
2+
over
other inorganic cations. A decrease in the emission maxima at 438
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