Identifying\nTarget Molecule and Trace Amount of the\nByproduct by Two-Dimensional Self-Assembly with Different Solution\nConcentrations

Scanning tunneling microscopy (STM) is a powerful way\nto realize\nthe recognition of self-assembled nanostructures on the atomic scale.\nIn this article, dihexadecyl 6,9-bis((4-(hexadecyloxy)phenyl)ethynyl)\nphenanthro[9,10-<i>c</i>]thiophene-1,3-dicarboxylate (D-PT)\nand dihexadecyl 6-bromo-9-((4-(hexadecyloxy) phenyl)ethynyl)phenanthrol[9,10-<i>c</i>]thiophene-1,3-dicarboxylate (S-BrPT) with different substituents\nwere chosen as the target system. D-PT with four side chains as the\ntarget molecule and S-BrPT with three side chains and a bromine substituent\nas the byproduct were mixed in a molar concentration ratio of 20:1.\nThe effect of solution concentration on the molecular self-assembly\nof the mixture was investigated by STM at the hexadecane/HOPG interface.\nAt high concentrations, only D-PT molecules formed a dimer pattern\nresulting from the intermolecular van der Waals force and self-adaption.\nFurther diluting the solution, D-PT formed the coexisting dimer and\nlinear structures, in which the linear pattern was formed via solvent\ncoadsorption. At low concentrations, S-BrPT molecules forming N-shaped\ndimers appeared and filled the linear structure fabricated by D-PT\nmolecules. With further decrease in the concentration, S-BrPT molecules\nformed N-shaped dimers covering almost half of the surface area, resulting\nfrom the C−Br···π and Br···H−C\nbonds. At very low concentrations, S-BrPT molecules formed N-shaped\ndimers to arrange the matrix architecture due to the coadsorption\nof more hexadecane molecules. Density functional theory (DFT) calculations\ndemonstrated that the stronger intermolecular C−Br···π\nand Br···H−C bonds were significant factors\nin determining the formation of N-shaped dimers and the stability\nof this nanostructure. This work enriches the diversity of self-assembled\nmotifs and provides a strategy to characterize different symmetric\nmolecules with trace amounts in a mixed system by STM.