Biomedical Engineering Reference
In-Depth Information
80E+05
13
13+Ni
2+
13+Cd
2+
13+Pb
2+
60E+05
40E+05
20E+05
0
320
340
360
380
400
420
440
460
Wavelength (nm)
Fig. 14 Fluorescence spectra of 13 after addition of ca. 4.5 equiv. of Ni
2+
,Cd
2+
, and Pb
2+
at 20
C,
l
exc
¼
300 nm. The initial concentration of ligand was ca. 5.6
10
6
MinCH
3
CN
only 2 equiv. of Cu
2+
. The results obtained with ligand 13 and copper are reflected
in Fig.
15
. These new bands are apparently due to intermolecular excimers. So after
adding 2.93 equiv. of Cu
2+
, the excitation spectrum of 14 at
l
¼
372 nm resembles
the corresponding UV spectrum, while a shift in the maximum of the corresponding
excitation spectrum at
494 nm is obtained. This finding, therefore, reveals that
the species emitting at these two wavelengths are not similar in nature. Even though
dependence between complex concentration and the intensity of the new band is
observed, intensity values do not directly relate to the expected square of concen-
tration for intermolecular excimer formation [
24
].
l
¼
NO
2
N
O
O
O
O
N
O
N
O
O
O
O
O
O
O
O
O
O
O
O
O
O
N
O
N
O
O
O
O
17
16
N
NO
2
Compound 16 is a bis-crown ether deriving from TMB, which combines the
possibility of forming clamp complexes with the macrocyclic effect, and prove
most important in complexation processes [
25
]. This compound is able to recognize
both Zn
2+
and Cd
2+
with a 1:1 stoichiometry. The UV-visible absorbance spectrum
of ligand 16 in acetonitrile displays strong absorbance in the UV region, centered at
295 nm (e
¼
29,600) and with a shoulder at 333 nm (e
¼
5,680). Upon excitation at
340 nm, compound 16 exhibits an intense fluorescence emission band centered
at 474 nm with a quantum yield of 0.08 (Fig.
16
). These values compare with a
measured quantum yield of 0.04 for tetramethylbenzidine under the same
conditions, indicating significantly enhanced fluorescence when these substituents
are introduced.
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