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nm and formation of a new emission band at 490 nm were observed
when titrated with Hg
2+
. The metallacycle forms a 1:1 host
−
guest
∆
I
−
1
complex, determined by plotting 1/
vs. [G]
at 495 nm, and the
3
−
1
2+
binding constant was determined as 1.3
. Notably,
the binding constant did not change with different counter anions.
Lu and coworkers reported photoluminescence electron-transfer
quenching of the
×
10
M
for Hg
3
MLCT state of Re(I)-based rectangles
2c, 2d,
and
2e
by several amines [38]. The amines with lower oxidation potential
showed a higher quenching constant suggesting that it is an electron-
transfer process. This is further supported by a linear plot of log
K
q
vs. oxidation potential. The binding constants for larger amines such
as
N
,
N
,
N
′
,
N
′
-tetramethylphenylene diamine to the three different
×
4
×
2
rectangles
2c, 2d,
and
2e
were found to be 2.3
10
, 2.6
10
, and
1
−
1
6.4
×
10
M
, respectively. These values suggest that the rectangle
2c
is a better host for the larger amines. They also reveal the formation
of an amine radical in the
3
MLCT excited-state electron-transfer
reaction in the Re(I) rectangle supramolecules with amine.
10.4
Photophysics and Photochemistry
of Metallacyclic Supramolecules and
Molecular Recognition
One of the first metallacyclic supramolecules,
16
, was reported to
act as a host by Maverick
et al.
as early as in 1986 [68]. The binuclear
copper(II) cycle
acts as a host for various nitrogen bases such
as pyrazine, pyridine, quinuclidine, and diazabiclo-[2,2,2]octane
(DABCO). The olive green solution of
16
changes to turquoise on
addition of nitrogen bases. The binding constants of the nitrogen
bases
16
with
16
were
determined
spectrophotometrically
in
chloroform at room temperature. The binding constants for
16
with
−
1
mono- and bifunctional Lewis bases such as pyridine (
K
= 0.5 M
),
a
−
1
−
1
quinuclidine (
K
= 7 M
), pyrazine (
K
= 5 M
), and DABCO (
K
a
a
a
−
1
= 220 M
) in chloroform solutions reveal that the larger binding
constant for the bidentate coordinating ligands were consistent
with the coordination inside the cavity. This was also supported
by the crystal structure of the inclusion complex of DABCO and
16
. The cyclic trimetallic copper complex
17
has been known for
−
some time [69] but its luminescence and host
guest chemistry was
first reported by Provencher and Harvey in 1996 [70], which emits
strongly both in solid and solution. The solid-state emission (
l
=
max
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