Molecular Gels and their Fibrillar Networks - Soft Fibrillar Materials: Fabrication and Applications - page 10

COO H 2 N

O 2 N

Xerogel (Benzene)

8

a

1-D

Xerogel (p-Xylene)

Gelator

(a)

Bulk solid

O 2 N COO H 2 N

Simulated

9

5 10 15 20 25 30 35

0-D (b)

/ o 2 θ

Nongelators

(c)

Figure 1.7 (a) The structure of gelator
8 and 1D HB network of ion pairs in the
crystal structure of 8 . (b) XRD patterns un-
der various conditions for 8 .(c)Thestruc-
ture of nongelator 9 and 0D HB network

of ion pairs in the crystal structure of 9 .
Reproduced from Ref. [58] with permis-
sion of The Royal Society of Chemistry.
http://dx.doi.org/10.1039/B504969E

the single crystal since the XRD patterns from the gel was difficult to obtain due to
the strong scattering from the solvent. Importantly, salt

was unable to form a gel,
and only 0D HB networks were identifiable in the single crystal (Figure 1.7c).
Additional spectroscopic tools, including nuclear magnetic resonance (NMR),
Fourier-transform infrared (FT-IR), UV-vis absorption, fluorescence (FL), and
circular dichroism (CD), are available to monitor the changes in physical properties
of aggregates during gelation. These techniques are able to identify different aspects
of intermolecular interactions which contribute to gelation. CD spectroscopy,
limited to chiral gelators or liquids, is discussed in Section 1.2.2.
A comprehensive description of NMR investigations of gels has been presented
in a recent review by Shapiro [59]. Upon transition from a sol to a gel phase, proton
resonances in 1 H NMR spectra experience significant broadening or disappear
completely due to limited molecular motion [53d, 60]. For example, aromatic,
vinylic, and some aliphatic protons of compound

9

cannot be observed in gel state
spectra, but are clearly seen in the solution/sol phase spectra at high temperature
where the system is a solution/sol (Figure 1.8) [60b].
Gels where solvent molecules are incorporated within fibers do allow more proton
signals from gelator molecules to be observed, although some line broadening and
shifts in proton resonances occur [52b, 61]. For example, a gel of

10

(Figure 1.9a)
[61c] in toluene- d 8 exhibited a downfield shift of the N-H protons (H a and H b )
in the gel state, indicating the presence of HB in the fibers (Figure 1.9b). The
aromatic H c signal appeared as a doublet in the solution/sol state and as two
overlapping doublets in the gel state due to their inequivalence as packed in the
fibers (Figure 1.9c). The spectra also indicate significant

11

π - π stacking in the gel

fibers.