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transitions [39
the lowest energy band
is solely of MLCT character, while the other squares
−
43]. In squares
3a
and
3b
3c
−
f
π→π
*
, the
and MLCT bands are substantially overlapping. Square
3d
gave the
3
characteristic triplet MLCT (
Cl
complexes located at 635 nm with a short emission lifetime of 39 ns
and a low emission quantum yield of 8.5
MLCT) emission of Re(diimine)(CO)
3
−
4
×
10
[42,43]. The MLCT
−
1
band of square
3d
is bathochromically shifted by 1385 cm
to that of
its corner complex because of extended
conjugation. The shorter
lifetime of the molecular square compared to its corner complex is
attributed to an enhanced nonradiative decay rate of the molecular
square owing to the lower energy of the excited state and more
effective vibronic coupling [31]. The square
n
3g
shows an additional
low-energy n
→π
*
transitions that originates from the trans-azp
′
ligand (azp = 4,4
-azobipyridine), which is overlapped with a MLCT
transition [41]. Square
3
MLCT emission at ca. 635 nm,
which is independent of excitation wavelength with an emission
lifetime of 39 ns in THF at ambient temperature. In contrast, the
square
3d
exhibits
is nonluminescent owing to the effective intramolecular
energy transfer from the initially formed
3g
1,3
MLCT excited states to
→π
*
the lowest nonemitting n
state. The luminescent metallacycle
3h
exhibits electronic absorption bands at ca. 245, 305, and 360 nm in
the UV region originating from ligand
π→π
*
transitions and the band
at ca. 325 nm, which is probably due to the MLCT transition [44]. The
square
shows two luminescence peaks ca. 412 and 536 nm in THF
when excited at 360 nm. The emission peak at 412 nm can be assigned
to a ligand-localized
3h
*
π→π
excited state, while the weaker emission
3
at 536 nm is assigned to a
MLCT excited state. Molecular rectangle
4a
shows a solvatochromic absorption at 490, 484, and 450 nm in
π
→π
*
H
O, DMSO, and CH
CN, respectively, owing to d
(Re)
(bpm)
2
3
′
charge-transfer transitions (bpm = 2,2
-bipyrimidine) [45]. These
rectangles are not emissive at room temperature in agreement with
results reported for the monomer [Re(bpm)(CO)
Cl], which is also
3
nonluminescent [46]. The UV
shows
a band in the high-energy region, which is absent in the dinuclear
bisbenzimidazole (biz), Re
−
vis spectrum of rectangle
5b
(CO)
(biz)
complex. This band is
2
6
2
*
ascribed to the allowed singlet d
π
(Re)
→π
(bpy) charge transfer
[47]. Unlike
luminesces
in solution at ca. 617 nm in tetrahydrofuran and in the solid state at
ca. 572 nm. The emission lifetime varies in the range 26 ns (CHCl
4b
or other pyrimidine-edged rectangle,
5b
)
to 238 ns (THF/MeOH) depending on the solvent. Owing to its
3
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