Wrapping of self-organized fluorescent nanofibers with a silica wall

Amphiphilic compounds 1 and 2, composed of an aromatic pyrene core and an amphiphilic three-branched unit, were synthesized and investigated for their self-organizing process in solution by UV-vis, fluorescence spectra, Fourier transform infrared (FT-IR), X-ray diffraction (XRD), and fluorescence microscopes. While 2 formed spherical objects in a mixed solvent of methanol and water, 1 assembled into long, flexible, and fluorescent fibers through pi-pi stacking of pyrene cores and hydrogen bonding among amide groups. The fluorescence spectra and morphologies strongly depended on the concentration and solution temperature. The fibrous assemblies were wrapped with an ultrathin silica wall by the acidic sol-gel polymerization of tetraethoxysilane (TEOS). A transmission electron microscopy (TEM) image after the sol-gel polymerization showed discrete fibrous structures with a uniform diameter of 3.5 nm and several micrometers in length. The thickness of the silica wall and the inner diameter of one fiber were estimated to be 0.5 nm and 2.5 nm, respectively. The observed inner diameter of the fiber was almost compatible with the width of the cylindrical assembly made of 1. The pyrene unit in 1 can interact with the sidewall of single-walled carbon nanotubes (SWNTs) through pi-pi interaction, and the adsorption of 1 onto the surface of SWNTs could disrupt the formation of bundles. The accumulation of oligomeric silica species at the hydrophilic surface created organic-inorganic nanoscopic fibers containing electronic conductive SWNTs.