assemble into uniform micron-long, ~3-nm-thick nanofibers in aqueous medium
(Fig. 2a).
Upon addition of sodium dithionite, 1 was reduced to 1
3À and the fibers underwent fission, leading to the formation of ~8-nm spherical assemblies (Fig. 2b). This
drastic change in morphology upon reduction was due to the decreased hydrophobicity (enhanced solvation) of the anionic species and their mutual electrostatic
repulsion. Upon exposure to air, the anions were oxidized back to the neutral PDI
state and the supramolecular fibers were restored (Fig. 2). The reduction/oxidation
sequence can be performed several times, without altering the observed structures.
Electrochemical reduction also leads to reversible fission. The sequence represents
reversible supramolecular depolymerization–polymerization in situ, which is
accompanied by a change in rheological behavior and altered electronic and
photonic properties of the assemblies. The latter is manifested by efficient exciton
movement in the fibers, which is switched off upon reduction and restored upon
oxidation. In general, long nanofibers (nanowires) are often utilized for functional
(signal transduction) and physical connectivity, and their reversible disassembly is
important for switching on and off such functionality. In polymers based on 1,
simple redox encoding leads to adaptive behavior involving reversible structural
and functional transformations in situ.
The system showed predesigned robustness and adaptivity, but we realized that
the fiber structure is substantially more complex than that expected from our initial
design. Thus, cryo-TEM studies revealed that the fibers are not built from single
molecular PDI stacks but instead are ribbons with a complex segmented structure
(Fig. 2a), resulting from hierarchical hydrophobic interactions due to the presence
of two distinct hydrophobic moieties: the aromatic system and alkyl groups. In our
later work, we were able to take advantage of this intrinsic anisotropy of PDIs by
using it to encode crystalline-like structures in water (see Sect. 3.5).
3.2 Code: Metal Coordination. Diversity via Coordination
Chemistry
A typical strategy for noncovalent synthesis is based on the paradigm “one assembling
unit forms a single assembly,” whereby the structure of a primary building block
elicits the assembly structure. Apparently, this strategy can be significantly expanded
if a single generic covalent building block can generate multiple structural and
functional motifs.
In order to achieve such diversity-oriented noncovalent synthesis, we focused on
designing a primary covalent unit with built-in functionality that can be expediently
modified, leading to changes in the overall structure. The reversible supramolecular
depolymerization described in the previous section utilizes this concept, yet it is
based on a single type of input, the reduction of a PDI unit, implying the use of
simple binary on/off switching. To achieve a high degree of diversity (i.e., a system
allowing multiple inputs) we aimed at two levels of self-assembly encoded in a
Aqueous Supramolecular Polymers Based on Aromatic Amphiphiles: Rational. . .
369
(Fig. 2a).
Upon addition of sodium dithionite, 1 was reduced to 1
3À and the fibers underwent fission, leading to the formation of ~8-nm spherical assemblies (Fig. 2b). This
drastic change in morphology upon reduction was due to the decreased hydrophobicity (enhanced solvation) of the anionic species and their mutual electrostatic
repulsion. Upon exposure to air, the anions were oxidized back to the neutral PDI
state and the supramolecular fibers were restored (Fig. 2). The reduction/oxidation
sequence can be performed several times, without altering the observed structures.
Electrochemical reduction also leads to reversible fission. The sequence represents
reversible supramolecular depolymerization–polymerization in situ, which is
accompanied by a change in rheological behavior and altered electronic and
photonic properties of the assemblies. The latter is manifested by efficient exciton
movement in the fibers, which is switched off upon reduction and restored upon
oxidation. In general, long nanofibers (nanowires) are often utilized for functional
(signal transduction) and physical connectivity, and their reversible disassembly is
important for switching on and off such functionality. In polymers based on 1,
simple redox encoding leads to adaptive behavior involving reversible structural
and functional transformations in situ.
The system showed predesigned robustness and adaptivity, but we realized that
the fiber structure is substantially more complex than that expected from our initial
design. Thus, cryo-TEM studies revealed that the fibers are not built from single
molecular PDI stacks but instead are ribbons with a complex segmented structure
(Fig. 2a), resulting from hierarchical hydrophobic interactions due to the presence
of two distinct hydrophobic moieties: the aromatic system and alkyl groups. In our
later work, we were able to take advantage of this intrinsic anisotropy of PDIs by
using it to encode crystalline-like structures in water (see Sect. 3.5).
3.2 Code: Metal Coordination. Diversity via Coordination
Chemistry
A typical strategy for noncovalent synthesis is based on the paradigm “one assembling
unit forms a single assembly,” whereby the structure of a primary building block
elicits the assembly structure. Apparently, this strategy can be significantly expanded
if a single generic covalent building block can generate multiple structural and
functional motifs.
In order to achieve such diversity-oriented noncovalent synthesis, we focused on
designing a primary covalent unit with built-in functionality that can be expediently
modified, leading to changes in the overall structure. The reversible supramolecular
depolymerization described in the previous section utilizes this concept, yet it is
based on a single type of input, the reduction of a PDI unit, implying the use of
simple binary on/off switching. To achieve a high degree of diversity (i.e., a system
allowing multiple inputs) we aimed at two levels of self-assembly encoded in a
Aqueous Supramolecular Polymers Based on Aromatic Amphiphiles: Rational. . .
369