Side-Chain Symmetry Effect on Self-Assembled Structures of Thienophenanthrene Derivatives

Alkyl side chains of molecules play a crucial role in molecular self-assembly on highly oriented pyrolytic graphite (HOPG), and their symmetry serves as a determinant factor in controlling the self-assembled patterns. Herein, the self-assembly behaviors of TPTD-Nap1 bearing symmetric side chains and TPTD-Nap2 with asymmetric side chains were investigated at the liquid/solid interface by scanning tunneling microscopy (STM). At the 1-octanoic acid/HOPG interface, TPTD-Nap1 self-assembled into a large-scale linear structure accompanied by a small fraction of zigzag structure, whereas TPTD-Nap2 exclusively formed a zigzag structure. In contrast to the self-assembled polymorphism TPTD-Nap1 at the n-tetradecane/HOPG interface, TPTD-Nap2 self-assembled into only one type of zigzag structure. We found that shape complementarity and dipole-dipole interactions between the conjugated cores thermodynamically favored the antiparallel packing of these molecules, and the selective interdigitation of asymmetrical side chains restricted the self-assembled structural polymorphism of TPTD-Nap2. Consequently, tailoring the side-chain symmetry emerges as a promising strategy for regulating the self-assembly of conjugated molecules with well-defined geometries.