Chemistry Reference
In-Depth Information
The coupling chemistry used in the original implementation was based on
CDMT/NMM coupling (CDMT, 2-chloro-4,6-dimethoxy-1,3,5-triazine; NMM,
N-methylmorpholine) [90]. It allows synthesis of pure gen 2 dendron in high yield
(84%), but HBTU/DIPEA chemistry was found generally more effective, especially
at higher generations. Using HBTU/DIPEA, gen 3 AG dendron could be isolated
without chromatographic purifications in 97%yield. Overall, the developed approach
is inexpensive, has a broad scope and allows modification of the dendrimer periphery,
interior, focal functionality and/or internal topology, without changing the basic
chemistry. For example, if primary amines R
0
-NH
2
are used instead of ammonia in the
synthesis of building blocks in Figure 14.11, groups R
0
will automatically become
included in the dendrimer interior.
AG dendrons were found to be well suited for encapsulation of metalloporphyrins,
giving rise to a new generation of fully protected phosphorescent oxygen probes [89].
The optimized synthetic assembly of the dendritic probes, consisting of the attach-
ment of dendrons to the core porphyrins, hydrolysis of the peripheral ester groups and
modification of the peripheral carboxyls with polyethyleneglycol residues, is shown
in Figure 14.13. The structure of a selected probe molecule, PdTBP-(AG
2
OPEG)
8
is
also shown in the figure.
Each step of the sequence was carefully optimized in order to ensure maximal
yields and monodispersity of the probes. Porphyrins modified with AG
1
and AG
2
dendrons could be isolated in high purity, but modification of porphyrins with AG
3
dendrons proved extremely challenging, always yieldingmixtures of dendrimers with
six, seven, and eight dendritic branches. The peripheral ester groups were hydrolyzed
using a special two-step procedure [89], which in parallel destroys unreacted
dendrons, making it possible to entirely avoid chromatographic purifications. The
resulting peripheral carboxyl groups on the dendrimers are esterified with PEG
residues of the desired length, and simple reprecipitation from THF upon addition of
diethyl ether gives pure PEGylated dendrimers.
Photophysical properties (Table 14.1) of AG dendrimers practicallymatch those of
the unprotected cores (Figure 14.2). However, both absorption and emission spectra
revealed some degree of aggregation in gen 0 and gen 1. In aqueous solutions, the
phosphorescence lifetimes (
t
0
) and the quantum yields (
f
) of the probes somewhat
increase with an increase in the dendrimer generation.
Phosphorescence quantum yields depend on the rates of singlet internal conver-
sion, intersystem crossing, radiative and nonradiative relaxation of the triplet state,
and intra- and intermolecular quenching processes. Since in Pt and Pd porphyrins
S
1
!
T
1
intersystem crossing is extremely fast (10
11
-10
12
s
1
) [25], the yield of the
triplet state is unlikely to be influenced by the substituents. Likewise, the radiative rate
of the triplet emission is intrinsic to the chromophore [91] and should not dependent
dramatically on the dendrimer generation. In contrast, nonradiative processes are
subject to changes induced by the substituents and the environment, which may
influence porphyrin's vibrational flexibility and its accessibility to various quenching
species. Due to aggregation of gen 0 and gen 1 dendrimers, triplet-triplet self-
quenchingmay be a result of their lower than expected phosphorescence yields. In gen
2 and especially gen 3 dendrimers, although the latter were not ideal, cores are much
