Molecular Gels and their Fibrillar Networks - Soft Fibrillar Materials: Fabrication and Applications - page 42

NBGE hot solution

O O

O O

HN

O

O

O

H

N

O

H

O

Precipitate Organogel

63

O O

Ultrasonication

(a) (b)

500 nm

500 nm

(c) (d)

Figure 1.39 (a) Structure of sonogela-
tor ( 63 ). (b) Stable organogels can be
formed by ultrasonic treatment of a hot
solution/sol or of the precipitate in a

liquid. (c,d) TEM images of the xerogels
from hexane (c) and water (d). Reprinted
with permission from Ref. [121]. Copyright
2007 Wiley.

spectra of dried samples, it was concluded that a larger percentage of the ala-
nine residues is involved in 90%) than in the vesicles

(∼ 50%). Because the CD spectra of the two aggregates are very different, there
must be significant differences in the orientation of the bipyridine groups and
in the packing of the peptide side chains. These data led to the model shown
in Figure 1.40b,c. In it, ultrasonication induces conformational changes in the
bipyridine linker which lead to changes in the nature of packing of the pep-
tide sections (loose bilayer vs tight bilayer) and the mode of supramolecular
aggregation.
An interesting example of ultrasound-induced changes in the aggregate mor-
phology of the zinc coordination polymer of compound 65 is shown in Figure 1.41a.
Apparently, ultrasonication here alters the coordination mode of the metal
[123]. Non-acoustic processing yields coordination polymer particles (CPPs,
Figure 1.41b,c) or single crystals, while sonication enforces gel formation
(Figure 1.41d). These gels are very stable thermally - their gel-sol phase transition
temperatures exceed the boiling points of the liquid components (MeOH, EtOH,
or CH 3 CN). The crystal structure (assuming the CPP and single crystals are
analogous) shows that the Zn is tetrahedrally coordinated in the CPPs. Also,
solid state (CP-MAS)

β -sheets in gel fibers ( ∼

13 C NMR spectral data indicate a difference between the