Controlled fabrication of organic nanotubes via self-assembly of non-symmetric bis-acylurea

We present novel non-symmetric bis-acylurea organogelators that self-assemble into hollow tubular nanostructures upon cooling in solutions. The bis-acylureas have aliphatic end groups of different lengths divided by a spacer group [?NHCONHCO?(CH₂)₅?CONHCONH?, C5] with two hydrogen bonding sites. Due to the intermolecular biaxial hydrogen bonding, the molecules crystallize into 2D thin layers at first, and then their wrapping ultimately results in nanotubes. On the contrary, symmetric bis-acylureas form multilayered nanosheets which are stabilized by the van der Waals interaction between the stacked layers. The size and shape of the nanotubes can be controlled by varying the difference of the alkyl chain lengths. When the difference is big, for example, eight methylene units (BuC5DD, butyl (Bu) and dodecyl (DD)), uniform nanotubes of 65-nm mean outer diameter are obtained, while a non-symmetric bis-acylurea with one methylene unit difference (UDC5DD, undecyl (UD)) forms a mixture of nanosheets and nanotubes. Template-assisted formation of nanotubes was successfully performed via gelation in inorganic nanopores. We also synthesized a thiol-functionalized non-symmetric bis-acylurea, HS-UDC5Bu (thiol (HS)), which was used as a tubular template for gold nanoparticles.