Tuning Supramolecular Polymer Assembly through Stereoelectronic Interactions.

The supramolecular polymerization of 2,11-dithia[3.3]paracyclophanes through self-complementary intermolecular and transannular amide hydrogen bonding is presented. An n → π* interaction between the amide hydrogen bonding units and the central bridging atom results from the single-point exchange of a carbon atom for a sulfur atom. This orbital donor-acceptor interaction can be strengthened by oxidizing the sulfide to a sulfone which acts to shorten the donor···acceptor distance and increase orbital overlap. Experimental signatures of the increased n → π* interaction include larger isodesmic polymerization elongation constants in solution, changes in characteristic bond stretching frequencies, and geometric/structural changes evaluated by X-ray crystallography. The experimental data are supported by extensive computational investigations of both assembling and nonassembling 2,11-dithia[3.3]paracyclophanes as well as a rationally designed model system to confirm the role of stereoelectronic effects on supramolecular polymer assembly.

Influence of Amide Connectivity on the Hydrogen-Bond-Directed Self-Assembly of [n.n]Paracyclophanes.

Reported here is the synthesis and self-assembly characterization of [n.n]paracyclophanes ([n.n]pCps, n=2, 3) equipped with anilide hydrogen bonding units. These molecules differ from previous self-assembling [n.n]paracyclophanes ([n.n]pCps) in the connectivity of their amide hydrogen bonding units (C-centered/carboxamide vs. N-centered/anilide). This subtle change results in a ≈30-fold increase in the elongation constant for the [2.2]pCp-4,7,12,15-tetraanilide ([2.2]pCpNTA) compared to previously reported [2.2]pCp-4,7,12,15-tetracarboxamide ([2.2]pCpTA), and a ≈300-fold increase in the elongation constant for the [3.3]pCp-5,8,14,17-tetraanilide ([3.3]pCpNTA) compared to previously reported [3.3]pCp-5,8,14,17-tetracarboxamide ([3.3]pCpTA). The [n.n]pCpNTA monomers also represent the reversal of a previously reported trend in solution-phase assembly strength when comparing [2.2]pCpTA and [3.3]pCpTA monomers. The origins of the assembly differences are geometric changes in the association between [n.n]pCpNTA monomers-revealed by computations and X-ray crystallography-resulting in a more favorable slipped stacking of the intermolecular π-surfaces ([n.n]pCpNTA vs. [n.n]pCpTA), and a more complementary H-bonding geometry ([3.3]pCpNTA vs. [2.2]pCpNTA).

Self-Assembling [n.n]Paracyclophanes: A Structure-Property Relationship Study.

Reported here is the synthesis, characterization, and isodesmic supramolecular polymerization of [3.3]paracyclophane-5,8,14,17-tetracarboxamide ([3.3]pCpTA). The self-assembling monomer, a bridge-expanded homolog of [2.2]paracyclophane-4,7,12,15-tetracarboxamide ([2.2]pCpTA), forms homochiral assemblies in nonpolar solution and the solid state through double-helical intermolecular and transannular hydrogen bonding. The additional methylene unit in the [3.3]paracyclophane bridge results in a weakened supramolecular assembly for [3.3]pCpTA compared to [2.2]pCpTA in solution. Likely origins of the change in assembly strength, revealed through X-ray crystallography, computational analysis, and solution-phase spectroscopy, are an increase in (a) the intramolecular and intermolecular deck-to-deck spacing compared to [2.2]paracyclophane resulting from larger amide dihedral angles accompanying transannular hydrogen bonding in the [3.3]paracyclophane and (b) monomer entropy associated with the scissoring motion of the [3.3]paracyclophane bridge.

Differentiating the mechanism of self-assembly in supramolecular polymers through computation.

The mechanism by which monomers in solution, beyond a certain concentration or below a certain temperature, self-assemble to form one dimensional supramolecular polymers determines much of the bulk properties of the polymer. The two distinct pathways of assembly, namely isodesmic and cooperative, can be experimentally identified using spectroscopy and in simulations via a determination of the dependence of the association constant on the oligomer size. Employing large scale free energy calculations, we have been able to show the independence of the free energy change of oligomerization on size in the self-assembly of a [2.2]paracyclophane-tetracarboxamide ([2.2]pCpTA) derivative ([2.2]pCpTA-hex), which is experimentally shown to follow the isodesmic pathway. In contrast, simulations show the free energy change in the case of benzene-1,3,5-tricarboxamide (BTA) to depend on the oligomer size which is a signature of its cooperative nature of self-assembly. These observations are rationalized through the development of a macrodipole moment in BTA oligomers and lack thereof in the [2.2]pCpTA system.