The assembly of p-aryl triazole foldamers into double and other super- helical structures

Abstract

The assembly of poly(para-aryltriazoles) (pPATs), synthesized via Cu(I)-catalysed azide-alkyne cycloaddition, into highly ordered structures is investigated. Firstly, the assembly of the pPATs into double helical structures, as a function of solvent quality and side chain chirality, was studied. The solvents employed (DMF and water) induced changes in van der Waals forces and surface free energy thus influencing the order of the pPATs’ random coils into double helical structures. The observed double helical structures, assembly that was analyzed using ultraviolet-visible (UV-Vis) specroscopy, circular dichroism (CD) spectroscopy, scanning transmission electron microscopy (STEM) and confocal fluorescence imaging microscopy upon addition of 0 – 80% water into the pPATs’ random coils, exhibit stable morphologies stabilized by π-π stacking and hydrogen bonding at 80% water content. The pPATs exist as random coils at 10% water in DMF. At 40% water in DMF, the pPATs’ strands were observed to exist in a side-by-side orientation. The adjacent strands, side by side, intertwine into double helices and eventually stack as the amount of water is increased to 80%. The obtained results present a facile strategy for the fabrication of polymeric double helical structures with stable morphologies. The average diameter of the resulting one-handed and opposite handed double helical structures is about 200 nm, a pitch of 170 nm and an overall length of several micrometres. The assembly of the pPATs into ordered structures in the presence of a neutral organic template and anions was also assessed. Hydrophobic poly(γ-benzyl-L-glutamate) (PBLG) template was introduced at various concentrations and transition region of the pPATs (10% water in DMF). At this stage the PBLG template is CD inactive. The template modified the assembly mechanism to afford structures, which cannot be achieved in its absence. It disallows the organization of the pPATs into double helical structures. The pPATs strands thread around the template and stack into long tubules of up to 10 microns and irregular diameter. The irregular diameter is attributed to uneven threading of pPATs strands around the template at some sections. The sensitivity and binding ability of the pPAT system to halide ions such as F-, Cl- and Br- , which involves re-organization of the aryl-triazole bonds, is explored using NMR and UV-Vis and CD spectroscopies. Br- which induces the highest shift of the triazole C-H proton signal in the NMR analysis also shows the highest dynamic quenching of the pPATs’ UV-Vis and CD spectra. The UV-Vis and CD signals are linearly dependent on the concentration of the anions. This confirms non-aggregation assembly in the presence of anions. Conclusively, the pPATs interact with the bromide anion in aqueous solution, which consequently prevents the aggregation of the foldamers. Finally, using PATs with different amounts of chiral side chains, co-operativity among side chains that leads to transfer, propagation and amplification of chirality is confirmed. A non-linear dependence of the CD signal on the amount of chiral side chains was observed. Chiral amplification was observed as low as 1% of the chiral side chains. However, approximately 20% of the chiral side chains are needed to obtain half the intensity of the cotton effect exhibited by the homochiral pPAT