Chemistry Reference
In-Depth Information
TABLE 5.1 Comparison of Radii of Endcapped TPM-G and TPM-leG from the
Center to the Outer Hydrogen from Geometry Optimizations by Semiempirical
AM1 Calculations
Dendrimers TPM-G
Radius (nm)
Dendrimers TPM-leG
Radius (nm)
G1
1.40
leG1
2.1
G2
2.20
leG2
4.7
G3
3.15
leG3
6.9
G4
3.99
leG4
9.1
G5
4.85
leG5
11.2
leG6
13.4
leG7
15.5
Some additional shoulders occurred in the size exclusion chromatography (SEC)
as demonstrated in Figure 5.5, which indicated even higher molecular weight
fractions, and which were been further examined. It was concluded that they must
stem from interlocked dimers, covalently linkedmoieties, or higher aggregates. These
intermolecular connectedmolecules could be assumed by some possible cross-linking
of the reactive triple bonds upon intra- and intermolecular couplings. The higher
aggregates, nonetheless, also occurred even after repeated synthesis under higher
dilutions, such that entanglements of side arms before the growth reaction could
also be responsible. Light scattering experiments of the partially separated higher
molecular weight fractions provided further proof of their dimer nature [42,43].
Concerning the shape persistence of those extended PPDs, these results con-
firmed that the flexibility of the scaffold is strongly enhanced with increasing length
of the branching units. The fact that the four dendrons clearly reached and finally
exceeded the persistence length of a polyphenylene chain supported the idea of their
loss of rigidity. The applied atomic force microscopy to the dendrimers,
TPM-leG5
and
, demonstrated their partial flattening on surfaces. Two effects
responsible for this flattening were discussed and have to be considered (i)
deformation from strict linearity with increasing number of phenylene units and
(ii) some collapse against the surface used for AFM, which does not exist in solution.
Bothfactorsseemedtocontributetosuchflatteningeffectsonsurfacesandin
addition it should be considered that size extension beyond the persistence length of
conjugated dendrons finally break the rigidity of any conjugated branch of a
dendrimer or a conjugated polymer. Nevertheless, it should be highlighted in this
context that discrete monodisperse macromolecules became available exceeding
molecular weights of 500 kg/mol still being soluble in common organic solvents
such as THF and chloroform.
For the introduction of further functionalities, however, the standard PPDs of
generations 2-4 as
TPM-leG6
are perfectly suited, since they prevent attached or
incorporated functional units from aggregation. This is important since aggregation
causes a red shift of absorption with strong quenching of the fluorescence and
excimer formation.
19
or
20
