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- β - d -glucopyranosyl)deca-4,6-diyne-1,10-dicarboxamide and N,N ‘-bis( β - d -glucopyranosyl)deca-4,6-diyne-1,10-dicarboxamide, respectively) have been synthesized and their self-assembled structures investigated. The hydrophobic peracetylated derivative 1 formed an organogel from solution in boiling ethyl acetate/ n -hexane mixtures on cooling to room temperature. In contrast, 2 produced fibrous assemblies from aqueous solution on addition of THF via vapor diffusion. Energy filtering transmission electron microscopy (EF-TEM) revealed the formation of nanometer-sized fibers (nanofibers) 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 displayed a higher molecular packing order than those from 1 . Exposure of a dispersion of either nanofiber to UV light resulted in the appearance of a red coloration. While the UV-irradiated nanofibers of 1 were readily soluble in CHCl 3, those of 2 required peracetylation to allow dissolution. Subsequent gel permeation chromatography for the UV-irradiated nanofibers displayed peak-top molecular weights corresponding to 13-mer and 64-mer for 1 and 2, respectively. On the basis of EF-TEM observation, no significant morphological changes in the fibers were found after UV-initiated polymerization. On the basis of these findings, we suggest that the polymerization proceeds efficiently within the nanofiber morphologies with higher molecular packing order leading to a higher degree of polymerization to give π-conjugated polymer chains.