(7; Fig. 9) because both are anticipated to promote interaction via aromatic surfaces, leading to the formation of long stacks. This was implemented in the
synthesis of compounds 5–7 (Figs 8, 9) that have PDI cores functionalized with
two alkyl groups (ethyl propyls attached at imide positions), and two hydrophilic
groups (PEG or carboxyl, attached to the aromatic core) [57].
Compounds 5–7 assemble into crystalline-like 2D arrays, as revealed by cryoTEM and AFM measurements, and largely preserve their structure upon drying.
According to the concept presented in Fig. 8, this assembly motif is generated by a
hierarchical mode of two distinct hydrophobic interaction types induced by an
aromatic core and alkyl groups, suggesting that a simple design strategy can be
used to obtain crystalline organic assemblies in water [57]. Importantly, photonic
properties of these assemblies resemble those of solid-state crystals, as manifested
by spectral broadening and large red shifts, typical of PDI crystals. The studies on
exciton dynamics using femtosecond transient absorption also revealed that 6 and
7 exhibit fast exciton diffusion. Remarkably, compound 7 exhibits exciton diffusion
constants that are even slightly higher than those of solid-state crystalline perylene
tetracraboxylic acid dianhydride (PTCDA), a benchmark exciton diffusion system.
The light absorption properties, the ordered 2D morphology, and the nanoscale
thickness appear to be useful for fabricating light-harvesting systems. Remarkably,
relatively simple molecular systems can be designed to undergo self-assembly into
extended ordered arrays by simply putting them in aqueous solutions, thus allowing
facile, cost-efficient, and environmentally friendly fabrication, and the processing
of water-based photonic and electronic materials.
4 Water-Based Noncovalent Polymeric Materials
4.1 Adaptive Hydrogel
The crystalline systems described in the previous section represent an entry into
photonic and electronic materials. Hydrogels constitute another interesting class of
systems relevant to the area of water-based functional materials. Hydrogels are
composed of water and a small amount (usually 0.1–10 wt%) of gelator molecules
that create a water-entrapping 3D network over the entire bulk of the material,
resulting in solid-like viscoelastic behavior. In the case of water-based supramolecular systems, gelation may occur when supramolecular fibers (polymers)
interact, forming a network [58]. Thus, in order to design a supramolecular gel,
one needs to introduce an additional level of interaction - the entanglement
of fibrous assemblies. This proved to be a difficult task because the rational design
of gelator molecules is extremely challenging, and most hydrogels have been
discovered serendipitously[58].
In our pursuit of metal-coordinating supramolecular polymers, we prepared
compound 8 so that it has a bipyridyl (bipy) linker connecting two PDI moieties.
One of the consequences of having bipy is the completely flat geometry of the
Aqueous Supramolecular Polymers Based on Aromatic Amphiphiles: Rational. . .
377
synthesis of compounds 5–7 (Figs 8, 9) that have PDI cores functionalized with
two alkyl groups (ethyl propyls attached at imide positions), and two hydrophilic
groups (PEG or carboxyl, attached to the aromatic core) [57].
Compounds 5–7 assemble into crystalline-like 2D arrays, as revealed by cryoTEM and AFM measurements, and largely preserve their structure upon drying.
According to the concept presented in Fig. 8, this assembly motif is generated by a
hierarchical mode of two distinct hydrophobic interaction types induced by an
aromatic core and alkyl groups, suggesting that a simple design strategy can be
used to obtain crystalline organic assemblies in water [57]. Importantly, photonic
properties of these assemblies resemble those of solid-state crystals, as manifested
by spectral broadening and large red shifts, typical of PDI crystals. The studies on
exciton dynamics using femtosecond transient absorption also revealed that 6 and
7 exhibit fast exciton diffusion. Remarkably, compound 7 exhibits exciton diffusion
constants that are even slightly higher than those of solid-state crystalline perylene
tetracraboxylic acid dianhydride (PTCDA), a benchmark exciton diffusion system.
The light absorption properties, the ordered 2D morphology, and the nanoscale
thickness appear to be useful for fabricating light-harvesting systems. Remarkably,
relatively simple molecular systems can be designed to undergo self-assembly into
extended ordered arrays by simply putting them in aqueous solutions, thus allowing
facile, cost-efficient, and environmentally friendly fabrication, and the processing
of water-based photonic and electronic materials.
4 Water-Based Noncovalent Polymeric Materials
4.1 Adaptive Hydrogel
The crystalline systems described in the previous section represent an entry into
photonic and electronic materials. Hydrogels constitute another interesting class of
systems relevant to the area of water-based functional materials. Hydrogels are
composed of water and a small amount (usually 0.1–10 wt%) of gelator molecules
that create a water-entrapping 3D network over the entire bulk of the material,
resulting in solid-like viscoelastic behavior. In the case of water-based supramolecular systems, gelation may occur when supramolecular fibers (polymers)
interact, forming a network [58]. Thus, in order to design a supramolecular gel,
one needs to introduce an additional level of interaction - the entanglement
of fibrous assemblies. This proved to be a difficult task because the rational design
of gelator molecules is extremely challenging, and most hydrogels have been
discovered serendipitously[58].
In our pursuit of metal-coordinating supramolecular polymers, we prepared
compound 8 so that it has a bipyridyl (bipy) linker connecting two PDI moieties.
One of the consequences of having bipy is the completely flat geometry of the
Aqueous Supramolecular Polymers Based on Aromatic Amphiphiles: Rational. . .
377