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rigidochromism'' [33]. Here, the long-lived triplet MLCT excited
state is apparently raised in energy compared to the ground-state
molecule, owing to the restricted ability of the solvent molecules
to reorient and stabilize the excited-state dipole moment [30
−
34].
Several optical transitions are possible in diimine rhenium(I)
tricarbonyl complexes, such as ligand field (LF), MLCT, ligand-to-
ligand charge transfer (LLCT),
σ
-bond-to-ligand charge transfer
), and intraligand (IL) excited states, which can be realized by
tuning the diimine or ancillary ligands.
Different topological shapes of Re(I) tricarbonyl
(
σ→π
*
−
based
metallacycles including triangles, squares, and rectangles have
been the most common reported homonuclear systems [36
−
54].
Heteronuclear molecular squares
contain two Re(I) and two
Pd(II)/Pt(II) complexes in opposite corners have also been reported
[49,50]. The spacers bridging the metal centers are mostly based
on the ligands containing pyridine functionalities on the opposite
ends of an organic linker. Other bridging ligands including diimines
[44,45], alkoxides
6
and
7
[39,40] have
also been reported. Bridging ligands containing metal complexes
resulting in additional metal centers in the squares
1
[36,37], and hydroxyquinones
8
4m, 5m
,
7
, and
12
were also studied [48,50
−
56]. Cage-like hexanuclear rhenium
−
molecular prism
14
has also been reported [59
62].
was
located near 385 and 600 nm, respectively, and its emission intensity
was dependent on the solvent [36]. The quantum efficiency and
lifetime increased from 0.39
The MLCT band and emission maximum of rectangle
1c
−
3
×
10
and 0.39 ns in CH
CN to 6.54
3
×
−
3
O (1:9 in v/v) mixture, owing to
the aggregation in the presence of water. The rectangles
10
and 212 ns in a CH
CN:H
3
2
1
having
linkers R = H, CH
OH were not emissive in solution at room
temperature [36,37]. The intense absorption maxima observed at
∼
, CH
CH
3
2
2
, which is ascribed to a spin-allowed
MLCT transition from the Re (d
350 nm for the rectangle
2
π
*
orbital of the
ligand, and the high energy band ca. 260 nm is assigned to the ligand-
centered transition. Emission maxima for rectangle
n
) orbital to the
were
located near 600 nm and the emission lifetimes varied from 86 to
495 ns for
2
in CH
Cl
2
2
2c
,
2d
, and
2e
[38]. The exceptionally long lifetime for
π
2e
-system of
the bridging ligand that brings in strong character of ligand-based
emission. The electronic absorption of square
is probably due to the involvement of the extended
3
contained with
π→π
*
two main features of bridging-ligand localized
and MLCT
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