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low-energy shoulder is assigned to be MLCT in character and the
band at 383 nm is assigned to the ligand localized
π→π
*
transitions
and no luminescence were observed from
11
in CH
Cl
solution.
2
2
The square assembly of phorphyrins
12a
and
12b
is highly soluble
but insoluble in water and highly chromophoric in the
phorphyrin soret region ca. 400
in CH
Cl
2
2
420 nm [51]. The square formation
induces a bathochromic shift of ca. 6 nm in the Soret region because
of rhenium
−
−
pyridine coordination. Near-UV fluorescence excitation
studies suggest that rhenium corners serve only a structural rather
than a direct photophysical role within the square framework.
Single photon counting experiments yielded excited state lifetimes
of 3.0 ns for
12a
and 2.4 ns for
12b
. The squares
12c, d, e
display
−
broad and intense visible absorptions in the region 400
600 nm,
which are assigned to metal (Fe, Ru, or Os)-to-ligand (pytpy) MLCT
transitions [52]. Square
exhibits an additional weak band
at ca. 676 nm, which is assigned to an Os-
12e
3
MLCT band. The bands
centered between 279 and 377 nm are assigned to ligand
π−π
*
bands
and the Re-based MLCT band. Although the corresponding corner
Re(CO)
′
′
′
″
(pytpy) (pytpy = 4
-pyridyl-2,2
;6
,2
-terpyridine) exhibits
3
3
MLCT luminescence at ca. 530 nm in CH
CN with a lifetime of 646
3
ns, the squares
do not have any detectable luminescence
at room temperature. The lack of luminescence from squares
12c
and
12d
12c
and
is attributed to the existence of metal-centered (MC) states
lying in close proximity to the MLCT states. The square
12d
12e
exhibits
room temperature luminescence in deoxygenated CH
CN solution at
ca. 748 nm with a shorter lifetime of ca. 42 ns and lower quantum
yield of 4.2
3
×
−
4
10
compared with its bridging unit Os(pytpy)
.
2
N
N
Bridging ligand (-L-M-L-)
CO
CO
CO
Cl
CO
OC
Re
L
M
L
Re
Cl
OC
L
L
N
M
n
Bu
n
Bu
tBu
N
N
12
N
Zn
O
NO
M
M
R
N
N
L
N
N
L
tBu
C
O
N
Cl
M
N
N
OC
Re
L
M L
Re
CO
N
N
Cl
OC
CO
CO
n
Bu
n
Bu
N
M = H,
a
;
Zn,
b
N
R =
M = Fe,
c
;
Ru,
d
;
Os,
e
f
g
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