Biomedical Engineering Reference
In-Depth Information
dimethylamino instead of nitro groups, precludes the formation of the charge
transfer, and no color develops in the presence of fluoride [
22
]. From these results,
it is possible to conclude that the electronic effect of substituents on the biphenyl
moiety influences complex stability and sensing properties.
In order to determine whether the presence of the cavity controls the complexa-
tion and sensing capacity, a related open compound 21 was prepared [
36
].
NO
2
NO
2
O
O
N
N
O
N
O
H
H
COO
OOC
COOCH
3
H
3
COOC
21
NO
2
NO
2
22
R =
S
O
N
R
N
N
H
H
H
H
O
N
N
N
N
O
N
R
N
H
24
S
23
R =
O
The complexation studies done with different halides, such as TBA salts, show a
similar behavior to that previously described for ligands 18 and 19 but, in this case,
the sensing response is faster. The solution changes in color from pale yellow to
dark red in the presence of fluoride, immediately after addition. This change is due
to a deprotonation reaction promoted by this anion's strong basicity.
The difference in kinetics is likely to be due to the greater flexibility of ligand 21,
which allows the anion easy access to the amide hydrogens. The sensitivity of 21
(0.001 M) is 1.5 mM for the naked eye and 0.1 mM for UV conditions. These
detection limits are similar to those described in the literature for other fluoride
chemosensors [
37
]. As observed with ligands 18 and 19, even though coordination
occurs also with bromide or chloride, no color changes are noted in these cases
given the lower basicity of these anions, which are unable to produce ligand
deprotonation.
Bipyridine-based ligands, compounds 22-24, have been demonstrated to be
suitable as anion sensors. Thioureas and amide derivatives were used as the binding
site for dicarboxylate recognition [
41
,
42
].
2,2
0
-Bipyridine is one of the most widely used samples of organic linkers.
Furthermore, the chemical modification of the 2,2
0
-bipyridine ligands by
introducing an additional functionality at the 4-, 5-, and 6- positions is well
known, and the coordination chemistry of such compounds has been actively
explored. However, fewer 3,3
0
-disubstituted ones are available and the coordination
chemistry of only a low number of them has been studied. These ligands are very
interesting because they present two coordination points; thus, they are able to
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