Rod and Helical Organic Fiber Structures Revealing Lamellar and Rosette Ordering Pathways in Self-Assembly of Barbiturate Oligothiophene Derivatives

The self-assembled structures of a hydrogen-bonding oligo­(thiophene) molecule functionalized with a barbiturate unit is investigated at different hierarchical levels. Atomic force microscopy observations show that the nanostructures formed upon drop-casting the solution adopt either a nonhelical rodlike or a helical nanofiber. This suggests the existence of two distinct molecular self-assembly pathways. This is confirmed by scanning tunneling microscopy (STM) investigations at the solid–liquid interface. STM reveals that the molecule can either adopt a lamellar or a hexameric macrocycle two-dimensional structure depending on the solute concentration. These arrangements are attributed to tape and rosette motifs stabilized by double hydrogen bonds between barbiturate groups. On the basis of the previous observation of two compounds that exclusively form rosette-based rodlike nanofibers and tape-based helical fibers, we propose the current molecule undergoes two specific hierarchical self-assembly pathways governed by tape and rosette hydrogen-bonding motifs leading the formation of rod and helical fibers, respectively.