Biomedical Engineering Reference
In-Depth Information
e
-
e
-
III
L
·+
(dihedral)
(L
2+
)*(
planar
)
L
I
II
e
-
(L
·+
)*
(planar)
Scheme 2 Mechanism of oxidation of ligand 4
All three Ia/Ic, IIa/IIc, and IIIa/IIIc pairs can be described as essentially revers-
ible one-electron-transfer processes, as judged from the variation of the anodic-to-
cathodic peak potential separation with the Epa-Epc potential scan rate.
Thus, parent neutral ligand, L, is reversibly oxidized to the corresponding radical
cation L
.+
and dication, L
2+
in two successive one-electron-transfer steps. These
correspond to the Ia/Ic and IIa/IIc pairs. The presence of an additional IIIa/IIIc pair
is rationalized if we consider that the overall oxidation process is accompanied by a
significant stereochemical modification: a transition takes place from the dihedral
neutral molecule to the planar dication. Accordingly, the first electron-transfer step
yields a nonplanar cation radical (L
.+
), which undergoes a relatively slow pre-
organization process to some extent (Scheme
2
). Under similar conditions, ligand 5
shows the Ia/Ic (0.62 V) and IIa/IIc (0.91 V) pairs, but not the IIIa/IIIc pair. The
absence of these peaks suggests that the oxidation process of ligands exclusively
occurs through the nonplanar dication. In addition, a new pair (IVa/IVc) around 1.5 V
appears due to the oxidation of
o
-phenylenediamine (Fig.
5b
).
In the presence of Cd
2+
or Zn
2+
, ligand 4's preferred form of oxidation is through
the nonplanar radical cation. Thus, peak III totally disappears and overlaps peaks II
and I, as observed (Fig.
6
). This behavior is due to the small size of this ligand
which precludes rotation toward the planar radical cation. In contrast, ligand 5's
larger size allows the transition metal cations to be located close to the softer
nitrogen atoms of the
o
-phenylenediamine moiety and then oxidation through
planar geometry is made possible.
These results indicate that both compounds 4 and 5 can be used as electrochemi-
cal sensors, their electrochemical response being directly related to possibility of
each ligand being able to adopt a coplanar conformation under oxidation
conditions. Unfortunately,
these ligands are unable to distinguish between
Zn
2+
and Cd
2+
.
On the other hand, compound 6, which has a related structure, is able to act as a
selective fluorescent chemosensor for Hg
2+
. This ligand shows an emission band at
372 nm, yet a new emission band at 464 nm appears in the presence of Hg
2+
.This
band was attributed to intermolecular excimer formation. Selectivity experiments
carried out with mixtures of Zn
2+
,Cd
2+
, and Hg
2+
demonstrate that the presence of
Zn
2+
and Cd
2+
does not bring about any changes in the response (Fig.
7
).
Additionally, the fluorescence studies carried out with compound 7 demonstrate
that this compound is able to distinguish between different mercury salts. Thus,
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