Both compounds self-assemble into molecular fibers in a water/THF mixture
(7:3 v/v), as shown by cryo-TEM, complying with the predesigned pairwise
stacking motif. Thus, the fibers assembled from 4 display tubular structures with
a diameter of 3.8 Æ 0.4 nm. The diameter of the interior lower-contrast part is
2.0 Æ 0.2 nm, and the higher-contrast wall thickness is 0.9 Æ 0.2 nm. The length of
the fibers is 183 Æ 96 nm. The identical fibrous structures assembled from 4 were
also observed in pure water. The fiber dimensions and fine structure are in excellent
agreement with the molecular model based on pairwise stacking, and, as expected,
in the case of 4, all the molecules in the fibril structure interact through the aromatic
surfaces of PDI units (Fig. 7a) located on both sides of the benzene core.
The thermodynamics of supramolecular polymerization of 4 was studied using
fluorescence spectroscopy, revealing strong association constants (~10
9 M
À1 ). The
thermodynamic parameters of the process indicate that it is both enthalpically
(ΔH ¼ À8.5 kcal/mol) and entropically driven (TΔS ¼ 3.6 kcal/mol), with the
dominant enthalpic contribution that arises due to the large hydrophobic surfaces
involved in polymer formation. Photonic studies using ultrafast spectroscopy reveal
that the supramolecular polymers based on aromatic pairwise interactions differ
significantly from the continuous stack systems, enabling exciton confinement
(trapped excimers) that leads to localized emission, rather than the exciton hopping
that is typical of most aromatic stacks. Directional pairwise hydrophobic interactions regulated by scaffold geometry serve as a convenient strategy for rational
design of supramolecular polymers in aqueous noncovalent synthesis.
3.5 Code: Anisotropic Hierarchical Hydrophobic
Interactions. 2D Crystalline Arrays.
Crystallization is an especially important area of organic self-assembly, enabling
ultimate long-range ordering. Understanding of and control over crystallization of
organic molecules is a long-standing challenge of fundamental importance for
organic materials, pharmaceuticals, and other fields. For example, crystalline aromatic structures constitute the core components of organic electronic devices,
whose optimal performance requires a high degree of order [56]. The simplest
view of an aromatic organic crystal involves an array where 1D aromatic stacks
interact to form an ordered periodic structure. We note that soluble 2D crystalline
nanosheets represent a very interesting target because they can mimic the properties
of thin films and self-assembled monolayers, covering large areas with ordered
nanometer-thick material.
We noted that in several cases (see Sects. 3.1, 3.2, and 4.1) our supramolecular
polymers have complex segmented morphologies, where individual segments are
aromatic stacks that interact via alkyl “edges.” This prompted us to consider a
design in which the extended “segments” will interact to form a highly ordered
crystalline array (Fig. 8). We chose to make PDI bolaamphiphiles with smaller
hydrophilic groups (carboxylic acids, 5 and 6; Fig. 8) or larger hydrophobic cores
Aqueous Supramolecular Polymers Based on Aromatic Amphiphiles: Rational. . .
375
(7:3 v/v), as shown by cryo-TEM, complying with the predesigned pairwise
stacking motif. Thus, the fibers assembled from 4 display tubular structures with
a diameter of 3.8 Æ 0.4 nm. The diameter of the interior lower-contrast part is
2.0 Æ 0.2 nm, and the higher-contrast wall thickness is 0.9 Æ 0.2 nm. The length of
the fibers is 183 Æ 96 nm. The identical fibrous structures assembled from 4 were
also observed in pure water. The fiber dimensions and fine structure are in excellent
agreement with the molecular model based on pairwise stacking, and, as expected,
in the case of 4, all the molecules in the fibril structure interact through the aromatic
surfaces of PDI units (Fig. 7a) located on both sides of the benzene core.
The thermodynamics of supramolecular polymerization of 4 was studied using
fluorescence spectroscopy, revealing strong association constants (~10
9 M
À1 ). The
thermodynamic parameters of the process indicate that it is both enthalpically
(ΔH ¼ À8.5 kcal/mol) and entropically driven (TΔS ¼ 3.6 kcal/mol), with the
dominant enthalpic contribution that arises due to the large hydrophobic surfaces
involved in polymer formation. Photonic studies using ultrafast spectroscopy reveal
that the supramolecular polymers based on aromatic pairwise interactions differ
significantly from the continuous stack systems, enabling exciton confinement
(trapped excimers) that leads to localized emission, rather than the exciton hopping
that is typical of most aromatic stacks. Directional pairwise hydrophobic interactions regulated by scaffold geometry serve as a convenient strategy for rational
design of supramolecular polymers in aqueous noncovalent synthesis.
3.5 Code: Anisotropic Hierarchical Hydrophobic
Interactions. 2D Crystalline Arrays.
Crystallization is an especially important area of organic self-assembly, enabling
ultimate long-range ordering. Understanding of and control over crystallization of
organic molecules is a long-standing challenge of fundamental importance for
organic materials, pharmaceuticals, and other fields. For example, crystalline aromatic structures constitute the core components of organic electronic devices,
whose optimal performance requires a high degree of order [56]. The simplest
view of an aromatic organic crystal involves an array where 1D aromatic stacks
interact to form an ordered periodic structure. We note that soluble 2D crystalline
nanosheets represent a very interesting target because they can mimic the properties
of thin films and self-assembled monolayers, covering large areas with ordered
nanometer-thick material.
We noted that in several cases (see Sects. 3.1, 3.2, and 4.1) our supramolecular
polymers have complex segmented morphologies, where individual segments are
aromatic stacks that interact via alkyl “edges.” This prompted us to consider a
design in which the extended “segments” will interact to form a highly ordered
crystalline array (Fig. 8). We chose to make PDI bolaamphiphiles with smaller
hydrophilic groups (carboxylic acids, 5 and 6; Fig. 8) or larger hydrophobic cores
Aqueous Supramolecular Polymers Based on Aromatic Amphiphiles: Rational. . .
375