Chemistry Reference
In-Depth Information
OR
R
O
O
O
H
3
H
3
RO
H
3
H
3
Dendrimer
generation
H
3
N
3
H
Terminal group
H
3
OR
HN
3
O
O
H
2
Glu
3
OR
O
H
2
PdP-(Glu
n
OR)
4
O
H
3
O
H
2
Glu
2
H
3
Ga
N
2
H
OR
O
O
N
3
H
Glu
1
H
1
H
3
O
H
1
R = H
O
H
2
HN
2
N
1
H
H
2
Gb
Dendrimer MW Number of Number of
generation carboxyls glutamates
H
1
H
H
H
2
O
H
m
O
N
3
H
Glu
O
(core)
0
894 4
0
N
H
o
O
OR
H
3
1
1410
8
4
H
3
Pd
N
N
2
2442
16
12
H
3
N
3
4506
32
28
O
4
8634
64
60
OR
PdP
FIGURE 14.5
Polyglutamic Pd porphyrin dendrimers, based on PdTCPP (abbreviated as
PdP). Molecular data are shown for carboxylic acid terminated dendrimers (n
ΒΌ
0-4) [65].
throughout the text we will use similar abbreviations to designate porphyrin den-
drimers of different types and generations, terminated by different groups.
Pd porphyrin dendrimers shown in Figure 14.5 exhibit strong phosphorescence in
deoxygenated aqueous solutions (
s). Considering
large separation between the carboxyl groups on the core porphyrin (
l
max
690 nm,
t
0
700-800
m
18A
across the porphyrin macrocycle) and relatively small size of the glutamic
monomer, the dendrimers were assumed to adopt quite open architectures up
to rather high generations (n
>
6), still allowing for access of oxygen to the core
and substantial quenching of the phosphorescence. Indeed, in DMF there was
practically no difference in the values of the quenching constants k
q
(Figure 14.6a) between the unprotected core (PdTCPP) and all the dendrimers
in the series. Oxygen quenching in this case was still presumably limited by the
rate of oxygen diffusion through the solvent, and deviations caused by the
dendritic shells were negligible (
500mmHg
1
s
1
)ascomparedtotheabsolute
8000-10,000mmHg
1
s
1
).
In contrast, in water k
q
values dropped significantly with increase in the
dendrimer size (Figure 14.4b), approximately 1000-1500mmHg
1
s
1
per gen-
eration. Overall, from gen 0 (PdTCPP) to gen 4 dendrimer, the absolute k
q
values
decreased 4-5 times.
Large variation between shielding capabilities of dendrimers in water and DMF
suggested that depending on the solvent properties, the polyglutamic branches
adopted either sparse or compact conformations, altering the barriers of oxygen
diffusion to the porphyrin core to a different extent (Figure 14.6). It was therefore
reasonable to conclude that the interaction of the dendrimer with the solvent is one of
the key factors determining the degree of protection that the dendrimer offers to the
core porphyrin. In much the same way as proteins fold in water, hiding their
hydrophobic residues inside the macrostructure, more hydrophobic dendrimers fold
values (k
q
