Two dimensional self-assembly of bis-acylureas having various functional end groups.

We present the synthesis and morphology study of thirteen bis-acylurea molecules with various functional end groups. The bis-acylureas have two acylurea groups, -NH-CO-NH-CO-, divided by a pentamethylene spacer, -(CH(2))(5)-, and two symmetric functional end groups, such as, aliphatic, benzyl, mono- and bi-thiophenyl, sulfur-containing, and propargyl (HC[triple bond]CCH(2)-) moieties. The bis-acylureas were synthesized by the coupling reactions of ureas with pimeloyl chloride or pimelic acid. Upon cooling from hot isotropic solutions, the bis-acylureas spontaneously form supermolecules. In the cases of aliphatic, benzyl, mono- and bi-thiophenyl functional groups, two dimensional supramolecular structures with molecularly flat surfaces were formed. Single crystal X-ray diffraction results demonstrate that each bis-acylurea molecule forms biaxial hydrogen bonds with four adjacent molecules forming a grid-like crystalline structure. Among the bis-acylureas with sulfur-containing end groups, those with sulfide (-S-) and disulfide (-SS-) moieties self-organize into two dimensional superstructures, while those with thiol (-SH) and bulky protecting group (-S-trityl) precipitate with irregular shapes. The bis-acylurea with propargyl end groups form superstructures of low order possibly due to the steric effect between the rigid moieties. However, the derivatives prepared by click reactions have lamellar molecular packing similar to those of bis-acylureas forming multilayered nanosheet structures.