components. The mechanical structures and switchablitity of MIMs are being
exploited in applications that encompass catalysis [32–35], drug delivery [36–38],
and molecular electronics [39–41], to name but a few examples. Polymeric MIMs
[42–47] are of special interest for new applications because they have the capacity
to scale [31, 48] the concerted actuation of bistable MIMs to a macroscopic size
regime.
The (supra)molecular recognition motifs that are common to mechanostereochemistry and foldamers include metal–ligand coordination [49–51], ion-pairing
[52–56], hydrogen bonding [6–12, 57–59], solvophobic forces [60–62], anion
binding [63–66], and the distinctive [67, 68] van der Waals interactions that arise
in stacked planar π-conjugated systems. Over two decades of research [69] in our
group has been focused on the development of MIMs templated by donor–acceptor
[70] (D–A) interactions between π-electron rich and π-electron poor aromatic
recognition units. Strategies that utilize π-associated D–A interactions to create
foldamers include D–A copolymers with flexible backbones that give “pleated”
secondary structures (also known as aedamers) [71–82], as well as the stabilization
of serpentine-like aromatic oligomers resulting from the regular intercalation of
aromatic tweezer molecules [83–88].
Although many similarities exist between their noncovalent bonding motifs,
foldamers and MIMs typically utilize weak inter- and intramolecular interactions
in fundamentally different ways. Whereas MIMs leverage noncovalent interactions
to interlace cyclic and acyclic components in the templation of mechanical bonds
and then control relative intramolecular motions, foldamers use noncovalent bonds
to render well-defined macromolecular conformations and three-dimensional
geometries. Recent work in our laboratories, however, has shown that these two
paradigms are not mutually exclusive. We have developed a family of mechanically
interlaced polyelectrolytes that adopt well-defined folded secondary structures,
both in the solid state (Sect. 2) and in solution (Sect. 3). The proclivity of these
macromolecules to form [C–H Á Á Á O] hydrogen bonds and extended π-associated
D–A stacks has allowed us to elaborate a genre of pseudorotaxanes and rotaxanes
with compact three-dimensional geometries that embody the traits of foldamers.
Here, we review our findings related to the construction of D–A rotaxane and
pseudorotaxane foldamers.
2 D–A Pseudorotaxane Foldamers in the Solid State
The unique packing motifs of π-associated organic donor and acceptor molecules in
the solid state is being investigated [89–96] by the organic electronics community
in order to create materials with novel optoelectronic and multiferroic properties.
Whereas segregated stacks of crystalline donors and acceptors express remarkably
high conductivities [89–91] or photoconductivies [92], for example, mixed-stack
crystals of alternating donors and acceptors can exhibit ferroelectric behavior below
certain transition temperatures [93–95]. Recently, we demonstrated [96] that
Mechanically Interlaced and Interlocked Donor–Acceptor Foldamers
273
exploited in applications that encompass catalysis [32–35], drug delivery [36–38],
and molecular electronics [39–41], to name but a few examples. Polymeric MIMs
[42–47] are of special interest for new applications because they have the capacity
to scale [31, 48] the concerted actuation of bistable MIMs to a macroscopic size
regime.
The (supra)molecular recognition motifs that are common to mechanostereochemistry and foldamers include metal–ligand coordination [49–51], ion-pairing
[52–56], hydrogen bonding [6–12, 57–59], solvophobic forces [60–62], anion
binding [63–66], and the distinctive [67, 68] van der Waals interactions that arise
in stacked planar π-conjugated systems. Over two decades of research [69] in our
group has been focused on the development of MIMs templated by donor–acceptor
[70] (D–A) interactions between π-electron rich and π-electron poor aromatic
recognition units. Strategies that utilize π-associated D–A interactions to create
foldamers include D–A copolymers with flexible backbones that give “pleated”
secondary structures (also known as aedamers) [71–82], as well as the stabilization
of serpentine-like aromatic oligomers resulting from the regular intercalation of
aromatic tweezer molecules [83–88].
Although many similarities exist between their noncovalent bonding motifs,
foldamers and MIMs typically utilize weak inter- and intramolecular interactions
in fundamentally different ways. Whereas MIMs leverage noncovalent interactions
to interlace cyclic and acyclic components in the templation of mechanical bonds
and then control relative intramolecular motions, foldamers use noncovalent bonds
to render well-defined macromolecular conformations and three-dimensional
geometries. Recent work in our laboratories, however, has shown that these two
paradigms are not mutually exclusive. We have developed a family of mechanically
interlaced polyelectrolytes that adopt well-defined folded secondary structures,
both in the solid state (Sect. 2) and in solution (Sect. 3). The proclivity of these
macromolecules to form [C–H Á Á Á O] hydrogen bonds and extended π-associated
D–A stacks has allowed us to elaborate a genre of pseudorotaxanes and rotaxanes
with compact three-dimensional geometries that embody the traits of foldamers.
Here, we review our findings related to the construction of D–A rotaxane and
pseudorotaxane foldamers.
2 D–A Pseudorotaxane Foldamers in the Solid State
The unique packing motifs of π-associated organic donor and acceptor molecules in
the solid state is being investigated [89–96] by the organic electronics community
in order to create materials with novel optoelectronic and multiferroic properties.
Whereas segregated stacks of crystalline donors and acceptors express remarkably
high conductivities [89–91] or photoconductivies [92], for example, mixed-stack
crystals of alternating donors and acceptors can exhibit ferroelectric behavior below
certain transition temperatures [93–95]. Recently, we demonstrated [96] that
Mechanically Interlaced and Interlocked Donor–Acceptor Foldamers
273