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were coordinated to the core-side layer (A). After the filling of layer A, the
coordination site moved to the next layer (B) until that layer was filled (1
<
[M]
0
/
[dendrimer]
0
<
3). Again it moved to layer C (3
<
[M]
0
/[dendrimer]
0
<
7) and
finally to D (7
15). The coordination process was
completely stepwise; therefore, most metal ions (99%) would be trapped within
the inner three layers if 5M amounts of the free metal ions were added to the
dendrimer (Table 10.1). The dendrimer model assumed in Case 3 can distribute
metal ions to core-side layers with higher priority. In principle, this should result
in the monodispersity of the number of metal ions distributed to each dendrimer
molecule because the statistical factor becomes negligible at each section of the
molar ratio between the dendrimer and the metal ion.
<
[M]
0
/[dendrimer]
0
<
10.2.3 Fine-Controlled Metal Assembling in DPAs
As shown in the previous section, the complexation randomness within the dendritic
architecture should be removed if the constants of the metal coordination could be
gradated by layers. A design strategy for such an unusual system is to utilize the
unique dense-shell property of rigid dendrimers [69,70]. All dendrimers reported
as ligands for multimetal ion assemblies have flexible backbones composed of single
covalent (
) bonds. However, they are not suitable for precise metal assembly because
their conformational changes and back folding with thermal vibrations would prevent
the precise discrimination of the coordination sites [71]. We reported some dendritic
ligands that have fully aromatic
s
conjugating backbones [72,73]. The dendritic
phenylazomethines (DPAs) shown in Figure 10.4 behave as unique dendritic ligands,
in which various metal ions were coordinated to the imine nitrogen (C
¼
N) from the
core-side layer in an orderly fashion.
An example of the stepwise complexation about the simplest DPA (
p
)
with SnCl
2
is explained below [39]. SnCl
2
acts as a Lewis acid and forms a 1:1
complex with each imine (C
Ph-DPA G4
¼
N) in the
Ph-DPA G4
. This is confirmed by the model
compound (
) bearing only one imine unit. Because the complexation
accompanies a significant change in the
Ph-DPA G1
p
transition energy, it can be monitored by
measurements of the UV-vis absorption spectra upon the addition of SnCl
2
. The
spectral change saturating up to the addition of 1M amount of SnCl
2
means a 1:1
complexation between the phenylazomethine unit and SnCl
2
. It can also be deter-
mined by the Job titrationmethod, showing themaximumabsorption change at the 1:1
mixing ratio. A similar UV-vis titration was also applied to the
p
-
bearing
15 imines within 1 dendrimer (Figure 10.5). Thismaterial showed a significant change
in the UV-vis absorption upon the addition of SnCl
2
. The spectrum of
Ph-DPA G4
Ph-DPA G4
gradually changed with an isosbestic point at 371 nmup to the addition of 1 equimolar
amount versus the molar concentration of the dendrimer. The isosbestic point then
shifted upon the further addition of SnCl
2
, and appeared at 368 nm between 1 and
3 equimolar amounts. During the addition of SnCl
2
from 3 to 7 equimolar amounts,
the isosbestic point appeared at 363 nm, and finally it moved to 360 nm with the
addition of more than 7 equimolar amounts. Overall, the isosbestic point shifted
about 11 nm from 371 to 360 nm.
