pyrene-based aggregates are common to promote excimer formation through p–p
stacking interaction, causing aggregation-caused quenching (ACQ) [24, 25].
Decreasing the p–p coupling is the general method to avoid ACQ, and balancing
with strengthening other interactions such as the H bonds of gelator 3 (Scheme 2.2
and Fig. 2.4) [25, 26].
The reversibility of the sol–gel transitions of supramolecular gels can be exploited
in diverse applications, such as supports for catalytic particles, enzymes, quantum
dots or controlling crystallization nanofabrication processes [27–30]. In the liquidand gas-processes, the supramolecular gels as temporary catalyst supports have been
successfully prepared in situ growth of gold, palladium and platinum nanoparticles
[31, 32]. For example, Rodríguez-Llansola and co-workers demonstrated that gelator
4 can be used as catalyst for Henry nitroaldol reactions with high efficiency only in
the gel state. Upon raising the temperature to dissolve the gel, both the efficiencies of
the conversion and selectivity are decreased. Proton transfer between proline residues in the gel promotes deprotonation of the nitroalkane reagent, which may
subsequently react with an aldehyde to yield a nitroaldol product. In solution, the
catalyst is considerably less basic and thus acts as a soft nucleophile, directly
attacking the aldehyde to form an imine (Fig. 2.5) [33].
Upon heating, the thermoresponsive gels also exhibit variable physical properties before reverting to sol state. The intrinsic size and shape of gel networks as well
as the interactions with solvent molecules are likely affected by the raising temperature. And the changes cause substantial amounts of liquid to enter or leave the
material, further producing swelling and deswelling effects [34]. And the
supramolecular interactions present in the gel are dependent on the heating.
Typically, upon raising temperature the gels forming of hydrophobic gelators and a
polar solvent undergo deswelling process, due to the entropic disfavour of ordering
of the solvent molecules around the gel networks. Conversely, gel fibres comprising
hydrogen bonds exist swelling effects, because the hydrogen bonding is weakened
by heating, inducing uptake of solvent molecules [35, 36].
A simple method of inducing a deswelling response is to remove solvent molecules from the gel, generally, by heating or decreased pressure. As for avoiding the
collapse of frameworks of gels, supercritical drying is the most common method to
prepare porous materials from small-molecule gels. Shi and co-workers obtained the
aerogels by dealing the bis(urea) gelators with perfluorinated end groups can gel
with supercritical drying with carbon dioxide directly at concentrations of 1–6 wt%
and a pressure of 300 atm. The target xerogel exhibiting microsized pores shows
ideal supports for heterogeneous catalysis or molecular sensing applications [37, 38].
Generally, the expulsion of solvent molecules in the gel is linked to two kinds of
variation, including reducing solubility of the gelator or increased aggregation of
the gel fibres. Krieg and co-workers reported a kind of swelling supramolecular
gelator 5 based on a perylene diimide chromophore (PDI) decorated with polyethylene glycol groups. Gelator 5 forms gels in the mixture of tetrahydrofuran and
water. In the formation process of elongated fibres, p–p stacking of the planar
pyrene moieties plays a pivotal role. The supramolecular self-assembly gels are
stable even at 100 °C. At higher temperature, the conformational flexibility of
2.1 Heat/Temperature Responsive Gels
13
stacking interaction, causing aggregation-caused quenching (ACQ) [24, 25].
Decreasing the p–p coupling is the general method to avoid ACQ, and balancing
with strengthening other interactions such as the H bonds of gelator 3 (Scheme 2.2
and Fig. 2.4) [25, 26].
The reversibility of the sol–gel transitions of supramolecular gels can be exploited
in diverse applications, such as supports for catalytic particles, enzymes, quantum
dots or controlling crystallization nanofabrication processes [27–30]. In the liquidand gas-processes, the supramolecular gels as temporary catalyst supports have been
successfully prepared in situ growth of gold, palladium and platinum nanoparticles
[31, 32]. For example, Rodríguez-Llansola and co-workers demonstrated that gelator
4 can be used as catalyst for Henry nitroaldol reactions with high efficiency only in
the gel state. Upon raising the temperature to dissolve the gel, both the efficiencies of
the conversion and selectivity are decreased. Proton transfer between proline residues in the gel promotes deprotonation of the nitroalkane reagent, which may
subsequently react with an aldehyde to yield a nitroaldol product. In solution, the
catalyst is considerably less basic and thus acts as a soft nucleophile, directly
attacking the aldehyde to form an imine (Fig. 2.5) [33].
Upon heating, the thermoresponsive gels also exhibit variable physical properties before reverting to sol state. The intrinsic size and shape of gel networks as well
as the interactions with solvent molecules are likely affected by the raising temperature. And the changes cause substantial amounts of liquid to enter or leave the
material, further producing swelling and deswelling effects [34]. And the
supramolecular interactions present in the gel are dependent on the heating.
Typically, upon raising temperature the gels forming of hydrophobic gelators and a
polar solvent undergo deswelling process, due to the entropic disfavour of ordering
of the solvent molecules around the gel networks. Conversely, gel fibres comprising
hydrogen bonds exist swelling effects, because the hydrogen bonding is weakened
by heating, inducing uptake of solvent molecules [35, 36].
A simple method of inducing a deswelling response is to remove solvent molecules from the gel, generally, by heating or decreased pressure. As for avoiding the
collapse of frameworks of gels, supercritical drying is the most common method to
prepare porous materials from small-molecule gels. Shi and co-workers obtained the
aerogels by dealing the bis(urea) gelators with perfluorinated end groups can gel
with supercritical drying with carbon dioxide directly at concentrations of 1–6 wt%
and a pressure of 300 atm. The target xerogel exhibiting microsized pores shows
ideal supports for heterogeneous catalysis or molecular sensing applications [37, 38].
Generally, the expulsion of solvent molecules in the gel is linked to two kinds of
variation, including reducing solubility of the gelator or increased aggregation of
the gel fibres. Krieg and co-workers reported a kind of swelling supramolecular
gelator 5 based on a perylene diimide chromophore (PDI) decorated with polyethylene glycol groups. Gelator 5 forms gels in the mixture of tetrahydrofuran and
water. In the formation process of elongated fibres, p–p stacking of the planar
pyrene moieties plays a pivotal role. The supramolecular self-assembly gels are
stable even at 100 °C. At higher temperature, the conformational flexibility of
2.1 Heat/Temperature Responsive Gels
13