Molecule length directed self-assembly behavior of tetratopic oligomeric phenylene-ethynylenes end-capped with carboxylic groups by scanning tunneling microscopy

A series of tetratopic C2-symmetric quasi-planar oligomeric phenylene-ethynylenes bearing carboxylic groups (OPE-COOHs), including the shortest 1,4-di((3,5-dicarboxyphenyl)ethynyl)benzene (TCB), midlength 4,4′-di((3,5-dicarboxyphenyl)ethynyl)biphenyl (TCBP), and the longest 4,4″-di((3,5-dicarboxyphenyl)ethynyl)-p-terphenyl (TCTP), were synthesized via Pd(PPh₃)₄-CuI-catalyzed Sonogashira coupling reaction. Their molecular structures were characterized by nuclear magnetic resonance spectroscopy (NMR), matrix-assisted laser desorption-ionization time-of-flight mass spectroscopy (MALDI-TOF-MS), and element analyses. The molecule length effect of OPE-COOHs on two-dimensional (2D) assemblies at the octanoic acid-highly ordered pyrolytic graphite (HOPG) interface was investigated by scanning tunneling microscopy (STM) technique at ambient temperature and simulated by molecular modeling. The patterns and domain sizes of OPE-COOHs strongly depend on their molecule length with Kagomé network only for TCB and parallel network for TCBP and TCTP. TCTP with the longest rigid-rod OPE backbone exhibits clearly larger ordered domain size than that of TCB and TCBP. Besides, TCBP exhibits tight boundaries transition among different orientation domains via the acute or obtuse V-shaped chevron arrangements. Those different two-dimensional (2D) assembly behaviors will be favorable to get a further understanding of the condensed architectures in conjugated organic semiconductors.