1.2 Driving Force
Intermolecular forces are responsible for the assembled way of discrete molecular
building blocks. Although gelation is the result of a subtle balance between a
multitude of non-covalent interactions, one or some interactions may be identified
to be crucial as the driving force(s). In gels, the driving forces range from strong
chemical bonds to weak intermolecular interactions (Fig. 1.1).
Traditional covalent bond is irreversible, and the reactions are performed under
kinetic control. For supramolecular gelation, “error correction” and “proof-reading”
should be allowed in the self-assembly of gelator molecules. The corresponding
reactions should be reversible, and the products should be under thermodynamic
control. Dynamic covalent chemistry has been developed based on reversible
reactions, such as imine-type exchange and disulphide exchange reactions [11, 12].
Its advantages include the combination of robust covalent chemical bonds and the
reactions under thermodynamic control. To realize a dynamic covalent process,
catalyst is usually needed. Dynamic covalent exchange is generally slower than
other dynamic processes. However, switching a dynamic covalent system from
dynamic to inert is possible upon activation or deactivation of the catalyst.
Metal–organic bonds are acid–base interactions based on the Lewis theory of
acid–base interactions of electron pair donation and acceptance. Their stability may
be explained by the hard–soft interaction principle. Soft electron donors (e.g. S, P)
form more stable complexes with soft metal ions such as Ag
+ or Pt
2+ . Metal ions
having high charge and small size (e.g. Al
3+ , Fe
3+ , Cr
3+ ) are hard Lewis acids and
bond best to hard electron donors (e.g. H 2 O, NH 3 ). The chelate compound is more
stable, called the chelate effect. The chelate effect is an entropy effect. Moreover,
chelate rings having five or six members are generally more stable than those of
other sizes.
Hydrogen bonding, abbreviated as X–H…A (X, donor; A, acceptor), is the most
popular non-covalent interactions using for construction of the gels. Conventional
hydrogen bonding is often described as an electronic dipolar–dipolar interaction, and
contribution from the Coulomb energy is important. The energy (10–120 kJ mol
−1
)
is much stronger than that of van der Waal interaction, but weaker than that of
covalent bonds. Charge-assisted interactions are usually stronger than hydrogen
bonding involving neutral species. More importantly, owing to its two characteristics, directional and saturable property, hydrogen bonding is usually chosen as
driven force to construct supramolecular architecture. Carboxylic acid dimer is one
of the most well-known hydrogen-bonding motifs. Besides conventional hydrogen
bonding, weaker hydrogen bonding is also known, including X–H/p, C–H/n and
C–H/p interactions (2–20 kJ mol
−1
).
Another important driving force is p–p interaction, which exits among aromatic
and other p-delocalized moieties. Substitution of electron-donating or
electron-withdrawing atoms or groups leads to p-delocalized molecules with
electron-donor and electron-acceptor properties. Combination of donors and
acceptors may lead to partial charge transfer from donor to acceptor. Offset
1.2 Driving Force
3
Intermolecular forces are responsible for the assembled way of discrete molecular
building blocks. Although gelation is the result of a subtle balance between a
multitude of non-covalent interactions, one or some interactions may be identified
to be crucial as the driving force(s). In gels, the driving forces range from strong
chemical bonds to weak intermolecular interactions (Fig. 1.1).
Traditional covalent bond is irreversible, and the reactions are performed under
kinetic control. For supramolecular gelation, “error correction” and “proof-reading”
should be allowed in the self-assembly of gelator molecules. The corresponding
reactions should be reversible, and the products should be under thermodynamic
control. Dynamic covalent chemistry has been developed based on reversible
reactions, such as imine-type exchange and disulphide exchange reactions [11, 12].
Its advantages include the combination of robust covalent chemical bonds and the
reactions under thermodynamic control. To realize a dynamic covalent process,
catalyst is usually needed. Dynamic covalent exchange is generally slower than
other dynamic processes. However, switching a dynamic covalent system from
dynamic to inert is possible upon activation or deactivation of the catalyst.
Metal–organic bonds are acid–base interactions based on the Lewis theory of
acid–base interactions of electron pair donation and acceptance. Their stability may
be explained by the hard–soft interaction principle. Soft electron donors (e.g. S, P)
form more stable complexes with soft metal ions such as Ag
+ or Pt
2+ . Metal ions
having high charge and small size (e.g. Al
3+ , Fe
3+ , Cr
3+ ) are hard Lewis acids and
bond best to hard electron donors (e.g. H 2 O, NH 3 ). The chelate compound is more
stable, called the chelate effect. The chelate effect is an entropy effect. Moreover,
chelate rings having five or six members are generally more stable than those of
other sizes.
Hydrogen bonding, abbreviated as X–H…A (X, donor; A, acceptor), is the most
popular non-covalent interactions using for construction of the gels. Conventional
hydrogen bonding is often described as an electronic dipolar–dipolar interaction, and
contribution from the Coulomb energy is important. The energy (10–120 kJ mol
−1
)
is much stronger than that of van der Waal interaction, but weaker than that of
covalent bonds. Charge-assisted interactions are usually stronger than hydrogen
bonding involving neutral species. More importantly, owing to its two characteristics, directional and saturable property, hydrogen bonding is usually chosen as
driven force to construct supramolecular architecture. Carboxylic acid dimer is one
of the most well-known hydrogen-bonding motifs. Besides conventional hydrogen
bonding, weaker hydrogen bonding is also known, including X–H/p, C–H/n and
C–H/p interactions (2–20 kJ mol
−1
).
Another important driving force is p–p interaction, which exits among aromatic
and other p-delocalized moieties. Substitution of electron-donating or
electron-withdrawing atoms or groups leads to p-delocalized molecules with
electron-donor and electron-acceptor properties. Combination of donors and
acceptors may lead to partial charge transfer from donor to acceptor. Offset
1.2 Driving Force
3