Molecular conformational effects on co-assembly systems of low-symmetric carboxylic acids investigated by scanning tunneling microscopy

The assembly behaviors of two low-symmetric carboxylic acid molecules (5'-(6-carboxynaphthalen-2-yl)[1,1':3',1''-triphenyl]-3,4'',5-tricarboxylic acid (CTTA) and 3',5'-bis(6-carboxynaphthalen-2-yl)-[1,1'-biphenyl]3,5-dicarboxylic acid (BCBDA)) containing naphthalene rings on graphite surfaces have been investigated using scanning tunneling microscopy (STM). The transformation of nanostructures induced by the second components (EDA and PEBP-C4) have been also examined. Both CTTA and BCBDA molecules self-assemble at the 1-heptanoic acid (HA)/HOPG interface, forming porous network structures. The dimer represents the most elementary building unit due to the formation of double hydrogen bonds. Moreover, the flipping of naphthalene ring results in the isomerization of BCBDA molecule. The introduction of carboxylic acid derivative EDA disrupts the dimer, which subsequently undergoes a structural conformation to form a novel porous structure. Furthermore, upon the addition of pyridine derivative PEBP-C4, N-H & ctdot;O hydrogen bonds are the dominant forces driving the three coassembled structures. We have also conducted density functional theory (DFT) calculations to determine the molecular conformation and analyze the mechanisms underlying the formation of nanostructures.