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luminescence intensity was decreased by ca. 25-fold after the square
formation. This quenching of luminescence was attributed to the
Pd(II) fragment. The square
containing azp as bridging ligand
shows high-energy bands below 300 nm, which is assigned to azp-
localized
7
π→π
*
transitions and a broad shoulder at ca. 380 nm is
*
assigned to d
(azp) MLCT [50]. The very weak absorption
band that tails into the visible region comprises azp-localized
n
π
(Re)
→π
→π
*
, Pd-centered LF and Fe-centered LF absorptions. The 4,4
′
-
ethenylbipyridine (ebp) linked square
7
show bands characteristics
of both
square shows band at
ca. 300 nm that extends into the visible region while the
cis
- and
trans
-ebp. The
trans
-ebp-
7
7
shows a blue-shifted band at ca. 290 nm [50]. No luminescence was
observed for square
cis
-bpe-
because the proximity of close-lying Pd- and
Fe-localized LF bands to the lowest excited state(s) provide rapid
nonradiative decay to ground state.
7
CO
PPh
2
CO
PPh
2
X
N
Re
CO
CO
CO
P
Ph
2
M N
X
Cl
N
P
Ph
2
Pd N
N
Re
CO
N
N
Cl
N
7
X
6
X
X
M = Pd, Pt
X = CH, N
X
N
Re N
N
N
Cl
Ph
2
P
N
Re N
Cl
OC
N
Pd
PPh
2
X
N
M
OC
X
OC
CO
Ph
2
P
Ph
2
P
OC
CO
The gondola-shaped molecular rectangle
8
displayed an intense
absorption band in the region 230
395 nm and a weak shoulder
at ca. 420 nm, which are ascribed to
−
π→π
*
and MLCT transitions,
respectively [40]. It also shows weak intraligand absorptions at
ca. 585 and 632 nm, originating from anthraquinone-bridging
ligands. A set of structured emissions appeared at ca. 438 nm with
high quantum yields of 0.719 for
8a
and 0.379 for
8b
. The small
Stokes shift and short lifetime of
8a
indicate that the emission is
π→π
*
originated from a singlet
state. Solid-state emission centered
*
at ca. 448 and 518 nm for
8a
is attributed to the decay of the
π→π
excited state of the bridging benzoxazolylthiophene (bzt) ligand and
d
π
(Re)
→π
*
(bzt) excited state, respectively [40].
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