Noncovalent Catalyst-cum-Inhibitor Directed Supramolecular Pathway Selection and Asymmetry Amplification by Aggregate Cross-Nucleation.

Abstract

The key to any controlled supramolecular polymerization (CSP) process lies in controlling the nucleation step, which is typically achieved by sequestering monomers in a kinetically trapped state. However, kinetic traps that are shallow cannot prevent spontaneous nucleation, thus limiting the applicability of the CSP in such systems. We use a molecular additive to overcome this limitation by modifying the energy landscape of a competitive self-assembly process and increasing the kinetic stability of an otherwise short-lived trap state. The additive achieves this by simultaneously catalyzing OFF-pathway nucleation and inhibiting ON-pathway aggregation. In the process, it guides the molecular assembly exclusively toward the OFF-pathway aggregate analogue. The mechanisms of OFF-pathway catalysis and ON-pathway inhibition are elucidated. By specifically targeting the nucleation step, it was possible to achieve pathway selection at an extremely low additive-to-monomer ratio of 1:100. The generality of our approach is also demonstrated for other related molecular systems. Finally, removing the additive triggers the cross-nucleation of the ON-pathway aggregate on the surface of a less stable, OFF-pathway aggregate analogue. The resultant supramolecular polymer not only exhibits a more uniform morphology but more importantly, a marked improvement in the structural order that leads to an amplification of chiral asymmetry and a high absorption dissymmetry factor (gAbs) of ∼0.05.