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DEUTSCHE BUNSEN-GESELLSCHAFT
UNTERRICHT
Figure 19: Self-aggregation of functionalized polyphenylene dendrimers with a) short (n < 16) and b) rather long (n > 20) peptide side chains. Right) representative
RE-REDOR sideband pattern analysis: the continuous drop of the local order parameter S indicates the mobility gradient.
In this case, local segmental dynamics of both the dendritic
sidechains and the aromatic core were determined in detail
by means of 1 H- 13 C and 1 H- 1 H NMR. Within the solid phase, the
aromatic core forms well-ordered, rather rigid columnar stacks
(p-stacks) that possess a particularly high photoconductivity. 76
After preassembly, self-organisation occurs in the liquid crystalline
phase, in which the core is mobilized. Depending on the
actual dendron, this is refl ected by a more or less reduced (local)
dynamic order parameter and can be attributed to either
small angle librations or out-of-plane motions. In any case,
the hexagonal columnar stacks are maintained. The structure
formation is governed by dendritic sidechains und their local
packing rendering stabilization of the columnar stacks due to
p-interactions rather negligible. Dendritic building blocks of
this kind therefore constitute an example where self-organisation
of molecules into functional aggregates can be driven
by fl uorophobic effects. The most prominent feature of such
stacks, however, is their (rather rare) ability to self-repair intracolumnar
structural defects as demonstrated by 1 H- 1 H doublequantum
2D-NMR. 75
Self-organisation of building blocks can also be dominated
by shape-persistent moieties such as polyphenylene dendrimers.
78 While their size is tunable on a nanometer-scale by appropriate
choice of dendrimer generations, self-organisation of
the columnar stacks can be highly infl uenced by attachment
of poly-L-lysine side chains. In case of short peptide chains
(n < 16), the hexagonal ordering of the polyphenylene cores
is essentially retained. Longer chains, however, perturb the
order of the cores, where 13 C-CPMAS data indicates a rather
well-defi ned secondary structure of the sidechains. It could be
shown that peptide chains which are confi ned by the packing
of the polyphenylene cores can adopt unprecedented secondary
structure motifs not known for their linear analoga. On
one hand, the effective dipolar order parameters derived from
1 H- 13 C RE-REDOR measurements, indicate “frozen” polyphenylene
cores. On the other hand, a mobility gradient was found
ranging from the rather rigid peptide backbone to the fl exible
sidechains (fi gure 19). 77
3.4. SURFACES
The high sensitivity of 1 H NMR facilitates the study of defect
sites 79 and molecules adsorbed at surfaces (e.g. of nanoparticles).
80 As in supramolecular systems, the structures of defects
are rather diffi cult to characterize in industrially important microporous
solids such as zeolites, 81 because defects that possibly
present in the compounds are typically not ordered in the
crystallographic sense and, therefore, are not easily amenable
to analysis by diffraction methods. Both rotor-encoded doublequantum
(RE-DQ) and triple-quantum (TQ) 1 H MAS NMR on siliceous
as-made ZSM-12 independently revealed that at least
three silanol groups are engaged in hydrogen bonding within
the defect site, which in fact has important implications concerning
the nucleation mechanism of high-quality zeolites. 79
Figure 20: 2D- 1 H DQ spectra of HO 2 C(CH 2 ) 2 PO 3 H 2 (a) in the bulk state and (b)
adsorbed in the nano-ZrO 2 . Note that the strong AC-DQ cross-peak reveals
the formation of a hetero-dimer. 80
Self-assembled monolayers are widely used to introduce different
chemical functionalities on metals or metal oxides.
High-resolution 1 H MAS NMR was used to characterize surface
hydrogen-bonding interactions in adsorbed carboxyalkylphosphonates
of varying chain lengths on zirconia and
71