Unraveling the versatility of aromatic polyamides: A comprehensive exploration of structuring from solution to bulk

Abstract

The functions of biological macromolecules heavily rely on their higher-order structures, which are stabilized by a combination of amino acid sequences (primary structure) and non covalent interactions such as hydrogen bonding, π-stacking, and Van der Waals forces. This has motivated synthetic chemists to imitate natural folding processes by creating non-natural macromolecules capable of folding into higher-order conformations. Among various folding motifs, aromatic polyamides stand out as exceptionally promising scaffolds. The precise placement of non-covalently interacting motifs within the aromatic polyamide chains directs the formation of well-defined folded higher-order structures. This thesis investigates the folding of aromatic polyamide scaffolds in both solution and bulk phases, highlighting the functional applications of these folded higher-order structures. The design and synthesis of periodically grafted aromatic polyamides were demonstrated that fold into a rare intrachain β sheet structure in solution. This intricate folding process is driven by cooperative π-stacking (intramolecular) and H-bonding (intramolecular) interactions. Moreover, the structural stability and integrity were remarkably enhanced by incorporating guest molecules with flat aromatic π-surfaces. Subsequently, the selective host-guest complexation between the folded aromatic polyamide scaffold (β-sheet, host) and various polycyclic aromatic guest molecules was investigated, utilizing anthracene photo-dimerization as a probe to evaluate the selectivity. The folded secondary structure of the aromatic polyamides (P1PEG) in solution can be effectively translated to the bulk phase, thereby mimicking the higher-order structure while maintaining the integrity of the secondary structure. The well-structured bulk assemblies comprise folded π-domains, enabling efficient ambipolar charge transport (TSCT) across the ordered π surfaces. The orientation of these π-domains, along with the efficiency of TSCT, can be modulated by substrate surface chemistry, underscoring the potential of these aromatic polyamides for electronic applications through strategic substrate selection. Finally, a novel design strategy is demonstrated for inducing helical twisting in the aromatic polyamide backbone and strategically manipulating its folded secondary structure through host-guest interactions. The light-triggered conformation changes of the guest molecule (planner→nonplanner) enabled dynamic control of the polymer's secondary structure, allowing for reversible twisting → untwisting → retwisting of the aromatic polyamide backbone. Furthermore, comprehensive investigations utilizing model compounds elucidated the critical role of noncovalent interactions and their synergistic effects in stabilizing the secondary structure of the aromatic polyamides. This interesting class of aromatic polyamides possesses a unique chiral pocket that selectively encapsulates regioisomeric amino pyrenes and exhibits enantioselective encapsulation of chiral aromatic amines.