Polymerization in nanometer-sized fibers: Molecular packing order and polymerizability

The bolaform 1-glucosamide 1 and its deacetylated derivative 2 (N,N'-bis(2, 3,4,6-tetra-O-acetyl-beta -D-glucopyranosyl)deca-4,6-diyne 10-dicarboxamide and N,N'-bis(beta -D-glucopyranosyl)deca-4,6-diyne-1,10-dicarboxamide, res pectively) have been synthesized and their self-assembled structures invest igated. The hydrophobic peracetylated derivative 1 formed an organogel from solution in boiling ethyl acetate/n-hexane mixtures on cooling to room tem perature. In contrast, 2 produced fibrous assemblies from aqueous solution on addition of THF via vapor diffusion. Energy filtering transmission elect ron microscopy (EF-TEM) revealed the formation of nanometer-sized fibers (n anofibers) with widths of between 6 and 20 nm and more than 50 nm for 1 and 2, .respectively. The amide and sugar hydroxyl groups in 2 allow formation of stronger, multiple hydrogen bond networks in the nanofiber than for 1. Powder X-ray diffraction studies revealed that fibers derived from 2 displa yed a higher molecular packing order than those from 1. Exposure of a dispe rsion of either nanofiber to UV light resulted in the appearance of a red c oloration. While the UV-irradiated nanofibers of I were readily soluble in CHCl3, those of 2 required peracetylation to allow dissolution. Subsequent gel permeation chromatography for the UV-irradiated nanofibers displayed pe ak-top molecular weights corresponding to 18-mer and 64-mer for 1 and 2, re spectively. On the basis of EF-TEM observation, no significant morphologica l changes in the fibers were found after W-initiated polymerization. On the basis of these findings, we suggest that the polymerization proceeds effic iently within the nanofiber morphologies with higher molecular packing orde r leading to a higher degree of polymerization to give pi -conjugated polym er chains.