Chemistry Reference
In-Depth Information
to a CH
2
Cl
2
/TBAH solution of dendrimer
in the presence of 10-fold excess of
HSO
4
,Cl
, and Br
results to be closely similar to that recorded for
5
in the presence
of H
2
PO
4
alone. In a like manner, HSO
4
can be detected, unambiguously, in
the presence of Cl
and Br
. The results obtained indicated that the dendrimer displays
the selectivity trend H
2
PO
4
>
5
Br
. The presence of Si-NH groups
in the dendrimers plays a significant role in the electrochemical recognition of anions
that is highlighted by the behavior observed for dendrimers
HSO
4
>
Cl
>
, which do not
contain Si-NH groups (see Figure 8.1) [28]. In fact, when 2 equiv. of H
2
PO
4
per
branch are added to CH
2
Cl
2
/0.1MTBAH solutions of dendrimers
2
and
8
, no splitting
is observed and the cathodic shifts of the Fc oxidation wave are smaller. Moreover,
dendrimer
2
and
8
displayed no change in the electrochemical response upon the addition of
2equiv. of Cl
per dendrimer branch, ruling out the possibility of the cathodic
perturbations observed for
2
being caused only by ion-pairing interactions.
An especially interesting approach for the preparation of electrochemical
sensory devices is the immobilization of redox-responsive receptor systems on
electrode surfaces [55,56]. Electrodes modifiedbyelectrooxidationofaminofer-
rocenyl dendrimer
5
resulted sensitive to the presence and concentration of anions
and constituted the first example of molecular recognition with dendritic receptors
confined on an electrode surface [30,35]. The electroactive dendrimer film behaved
almost ideally and showed rapid charge-transfer kinetics. In the presence of H
2
PO
4
a new peak appears at a more negative potential, while the original wave is observed
only as a reminiscent weak shoulder (Figure 8.4). Increasing values of
5
D
E can be
measured in the H
2
PO
4
concentration range from 10
4
to 2
10
3
M.
Neutral amide-substituted ferrocene receptors have been widely used to bind and
electrochemically recognize guest anions combining hydrogen-bonding effects and
electrostatic effects [48-52,57,58]. Amidoferrocenyl dendrimers have also been used
as redox sensors as the acidic amide hydrogen atom can form a hydrogen bond with an
oxygen atom of oxoanions [34,59-61]. Astruc et al. have investigated the behavior of
the trimetallic ferrocenyl compound
5
-C
5
H
5
)
10
, that of the monometallic ferrocene (
Z
5
-C
5
H
4
CONHCH
2
CH
2
OPh) and those of dendrimers
Fe(
(Figure 8.5), in
the electrochemical recognition of small inorganic anions (H
2
PO
4
, HSO
4
,Cl
,Br
,
and NO
3
) and reported a strong positive dendritic effect (i.e., the sensing and
recognition ability of the ferrocenyl host increases as the dendrimer generation is
higher) [61]. The dendritic core topology as well as the nature of the solvent are
critical on the recognition by the amidoferrocenyl groups.
Astruc and coworkers have also undertaken anion recognition studies with
diaminobutane-based (DAB) poly(propyleneimine) amidoferrocenyl [34] and
pentamethylamidoferrocenyl dendrimers [59-61] (Figure 8.6) for comparison. The
electron-releasing methyl groups at the dendrimer periphery stabilize the oxidized
ferrocenium 17-electron form and decrease the hydrophilicity and therefore, the
pentamethylated series shows much less adsorption and no chemical irreversibility
that prevent a clean recognition in the nonmethylated series. In addition, recognition
and titration with the pentamethylated series are subjected to a modest dendritic effect
in DMF whereas no dendritic effect is noted in CH
2
Cl
2
, which contrasts with the large
dendritic effect found with dendrimers
Z
11
and
12
11
12
and
.
