Chemistry Reference
In-Depth Information
d
n
1
r
3
n
g
|
7
Scheme 5.1
Structures of organogelator 1 and guest compounds.
molecules that are not interacting with the gel network remain unaffected as if
they were in pure solvent.
17
For instance, we studied the NMR of compounds
2, a nonpolar molecule, and 3, bearing two H-bonding groups, in the presence
of a gel formed by 1 in C
6
D
6
(Scheme 5.1). We could observe that compound 3,
capable of H-bonding interaction with the gel phase shows a decrease of T
2
values, meaning a slower tumbling rate than the corresponding solution in the
absence of the gel (T
2sol
¼
1.77 s, T
2gel
¼
1.20 s). On the contrary, T
2
values for
compound 2, noninteracting with gel phase and well solvated by C
6
D
6
were
unaffected by the presence of the gel (T
2sol
¼
T
2gel
¼
4.95 s).
A relevant example of noninteracting species is represented by the ortho-
gonal self-assembly of gelators - the so-called ''self-sorting'' - in which non-
mutually interacting molecules may develop independent networks of fibres or
even other self-assembled systems such as micelles or vesicles.
18,19
In summary, molecular gels may behave as compartmentalised sample
holders that physically entrap solutions or aggregates of noninteracting mol-
ecules. However, they may be also chemically active and, as we shall see in the
following sections, have a strong influence in relevant processes such as the
stabilisation of proteins and drug crystals, the selective molecular recognition
of guests and catalysis among others.
Here, we will discuss all these phenomena in depth through a selection of
recent illustrative examples. The chapter will be divided into three sections:
1) host-guest noncovalent effects, 2) reactive gels that form covalent bonds and
3) catalytic gels.
3
.
5.3 Host-Guest Noncovalent Effects
Guest compounds loaded in the gel network may interact with gelator mole-
cules by all kind of noncovalent interactions (H-bonding, van der Waals
interactions, p-stacking, solvophobic interactions, etc.). As these weak inter-
actions are the same that sustain the gel network several competitive equilibria
will be established and depending on which one dominates the system will be
driven to either the incorporation of the guest into the gel phase or the
weakening and disassembly of the gel network. This mutual influence has to be
carefully considered when a given application is sought. For instance, the in-
corporation of the guest into the gel phase is convenient for the use of this
